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GeoPRISMS Draft Implementation Plan

Appendix F. Rift Initiation and Evolution Implementation

Workshop Whitepapers

RIE Implementation Workshop: Submitted White Papers , listed alphabetically

1. Understanding Rift Initiation Mechanisms: East African Rift System as a focus site?Estella Atekwana, Eliot Atekwana, Rob Evans, Juan Pablo Canales, Alison Shaw, Dan Lizarralde, Mark Behn, Roger Buck, Mohamed Abdelsalam, John Hogan, Steve Gao, and Kelly Liu, Alan Jones, Damien Delvaux, Elisha Shemang, Kebabonye Laletsang, Moidaki Mokwathai, Tiyapo Ngwisanyi, Remmy Phiri, Daniel Mutamina, Lostina Chapola1 and Reidwel Nyirenda

2. Rift Initiation and Evolution Within an Active Plate Boundary Zone: The Woodlark Rift of Papua New GuineaSuzanne Baldwin , Paul Fitzgerald , Daniel Curewitz, Paul Mann, Brad Hacker, Laura Webb, Geoff Abers, Tim Little, Laura Wallace, Colin Devey, Kaj Hoernle, Romed Speckbacher, Jan Behrmann

3. The Afar R-R-R Triple Junction, a Focus Area for MARGINS RIE, Plate Boundary Deformation and GeodynamicsRebecca Bendick, Robert Reilinger, Katie Keranen, Lucy Flesch, Shimelles Fisseha

4. The Gulf of California-Walker Lane Belt: A Natural Laboratory to Study Rift Initiation that Culminates in Seafloor SpreadingCathy Busby, Graham Kent, Neal Driscoll, Glenn Biasi, Ken Smith, John Louie, Bill Hammond, Alistair Harding, Geoff Blewitt, Corne Kreemer, Danny Brothers, Fred Phillips, Jared Kluesner, Peter Lonsdale, Keith Putirka, Debi Kilb, Pat Cashman, Paul Umhoefer, Gary Fuis, Dan Lizarralde, Jeff Babcock

5. Active Faulting and Magmatic Processes: Fundamental Constraints on Passive Margin FormationCynthia Ebinger

6. Opportunities to investigate early-stage rifting with comparative studies in the Western Rift of the East Africa Rift SystemJames B. Gaherty, Donna J. Shillington, Cornelia Class, Scott L. Nooner, Cynthia J. Ebinger, Andrew A. Nyblade, Christopher A. Scholz, Matthew E. Pritchard, Leonard Kalindekafe, Winstone Kapanje, Patrick R.N. Chindandali, Reidwel Nyrienda, Richard Wambura Ferdinand, Shukrani Manya, Nelson Boniface, Fredrick M. Mangasini, Abdul Mruma

7. Addressing RIE objectives at rifted margins: some applications to US marginsJames B. Gaherty, Donna J. Shillington, Daniel Lizarralde, Harm Van Avendonk

8. Continental Breakup and Formation of Rifted Margins: The Gulf of Mexico as a Natural LaboratoryD. Harry, R. Stern, E. Anthony, G. R. Keller, I. Norton, J. van Wijke

9. 3D CONSTRAINTS ON SLOPE FAILURE ON A PASSIVE MARGINMatthew J. Hornbach, Peter Flemings, Rob Harris, Brandon Dugan

10. Is the Wabash Valley Seismic Zone related with the ancient Reelfoot Rift?Yevgeniy A. Kontar, Fred Boadu, Philip J. Carpenter, Robert S. Nelson, Abdelmoneam Raef, Ramesh Singh, Michael S. Zhdanov

11. Evolution of a continental rift: the Rio Grande RiftJames Ni, Rich Aster, Steve Grand, Jolante van Wijk, Scott Baldridge, and Ristra Team.

12. Rifting, LIPs, and Life (RLL)Paul Olsen, Martha Withjack, Roy Schlische, Dennis Kent, Donna Shillington, Daniel Lizarralde Samule Bowring, Mohammed Et-Touhami, Mike Waddell, James Knapp, Dave Goldberg

13. Rifting through transtension: The role of oblique shear in breaking continental lithosphere, an example from the Gulf of CaliforniaChristina Plattner, Falk Amelung, Timothey H. Dixon, Rocco Malservisi, Peter Lonsdale, Dave Chadwell, Don Forsyth, Jim Gaherty, Danielle Sumy, Francisco Suarez-Vidal, Javier Gonzalez-Garcia, Raul Castro

14. Advances and Ongoing Challenges in Interdisciplinary Rifting ResearchT. O. Rooney, K. Keranen, M. H. Benoit

15. Oblique rifted margins: Lena trough as an archetype.Jonathan Snow and Henry Dick

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Understanding Rift Initiation Mechanisms: East African Rift System as a focus site?

US Scientists Estella Atekwana1, Eliot Atekwana1, Rob Evans2, Juan Pablo Canales2, Alison Shaw2, Dan Lizarralde2, Mark Behn2, Roger Buck3, Mohamed Abdelsalam4, John Hogan4, Steve Gao4 and Kelly Liu4

International Collaborators Alan Jones5, Damien Delvaux6, Elisha Shemang7, Kebabonye Laletsang7, Moidaki Mokwathai7, Tiyapo Ngwisanyi8, Remmy Phiri9, Daniel Mutamina10, Lostina Chapola11 and Reidwel Nyirenda12 1Oklahoma State University, 2 Woods Hole Oceanographic Institution, 3Lamont-Doherty Earth Observatory, 4 Missouri University of Science & Technology, 5Dublin Institute of Advanced Studies, 6Central African Museum, Belgium, 7University of Botswana, 8Botswana Department of Geological Survey, 9University of Zambia, 10Department of Geological Survey, Zambia, 11Catholic University, Malawi, 12Geological Survey of Malawi

Although investigations of well-developed rifts worldwide have resulted in increased understanding of advanced stages of continental rifting, almost no research has focused on the processes that initiate and control the earliest stages of continental rifting. Key controversies exist regarding models explaining the initiation of continental rifting and several unsolved process-oriented questions about global continental rifting remain, such as where and why do continental rifts initiate? This is a key question posed by the GeoPRISMS Rift Initiation and Evolution Initiative. To fully understand continental rift formation and its eventual development into rifted continental margins, all evolutionary stages of rifting from the incipient to the more advanced rift stages must be investigated jointly. The East Africa Rift System (EARS) provides an excellent geodynamic setting for addressing these questions, exhibiting a strong gradient in rift evolution along its length (from the very early stage Okavango Rift Zone at its southernmost terminus, where classic geomorphic rift features are just beginning to emerge (e.g., Modisi et al., 2000; Kinabo et al., 2007; 2008) to the Afar in the north where continental break up is just beginning to occur (e.g., Keranen et al., 2004; Ebinger, 2005)). This provides a unique opportunity to investigate the processes that drive rift initiation and control early rift localization as well as to test models proposed for rift initiation. For example, studies by Buck (2004, 2006) have highlighted the important role that magmatism plays in the initiation of rifting supported by recent diking events in Afar and northern Tanzania (Wright et al., 2006, Ayele et al., 2007; Calais et al., 2008). However, this model does not appear to be representative for all of the rift basins within the EARS as some rift basins do not appear to fit this model. For example, the less evolved rifts (e.g., L. Malawi rift) is characterized by a well expressed topographic rift basin with a fully developed border fault system but lacks any expression of sub-aerial magmatism (Ebinger et al., 1987). Instead, the only volcanic center is within the Rungwe volcanic province found at the northern termination of L. Malawi. The same is true for the Okavango rift. The lack of any surface expression of magmatism within the aforementioned rift basins raises the question as to the role of magmatic processes in the early rift stage for these and other similar rifts. Thus, strain partitioning between

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faulting and magmatism during the different stages of rifting, as well as the role for magma injection as a driving force remains poorly documented and poorly understood. This could be addressed by comparative studies between more evolved components of the rift where magmatism is clearly playing a role with less evolved components of the rift where the role of magmatism is questionable.

Other rift initiation and evolution models such as edge-driven asthenospheric-mantle convection (EDC) (van Wijk et al., 2008) can also be tested here. Edge-driven convection is induced by lateral thermal gradients associated with thicker lithosphere below the cratons. Intriguingly, many of the rift basins of the EARS occur within Proterozoic mobile belts rimming the cratons. For example, the Okavango rift zone is developing with the Proterozoic Damarian mobile belt bounded by the Kapvaal craton to the south and the Congo/Angolan craton to the north. Recent magnetotelluric results (SAMTEX) clearly show variations in lithospheric thickness between the cratons and the Proterozoic mobile belts. The EDC hypothesis thus seems a plausible rifting mechanism at the southernmost tip of the EARS -Okavango Rift Zone.

We believe that several key questions related to rift initiation can be addressed at key locations within the EARS. In addition, NSF has already made some significant scientific infrastructural investments in the region (the data from these projects should be available to the larger scientific community) including: Kenya Rift International Seismic Project (KRISP), Ethiopian Afar Geoscientific Lithospheric Experiment (EAGLE) (partnership between EU and US); Africa Array, The Southern African MagnetoTelluric Experiment (SAMTEX), Kapvaal Craton Project and our recently approved CD project (Collaborative Research: Integrated Studies of Early Stages of Continental Extension: From Incipient (Okavango) to Young (Malawi) Rifts). In addition, several other new projects are being proposed such as the Malawi Rift project (e.g., see white paper by Shillington et al.) and a new MT project by scientists at the Dublin Institute for Advanced Studies (The Central African Magnetotelluric Experiment (CAMTEX)). Hence newly acquired data can be integrated with existing geophysical transects such as a newly funded transect in the Afar depression (funded by Statoil Hydro) in order to provide a comprehensive understanding of the along-axis time-transgressive propagation and evolution of continental rifts.

Broader Impact: There is a great potential for partnership with industry and the governments in this part of Africa because of the strong exploration focus for natural resources for many of the countries transected by the rift. For example the SAMTEX project had significant investments from the mining companies and also local buy in by the governments of the different countries. Additionally, continental rift zones are prolific hydrocarbon producers. Thus the discovery of hydrocarbons in the Albertine graben in Uganda has stimulated a flurry of exploration activity for some of the other EAR basins (e.g., recent seismic acquisition over the Rukwa Rift in Tanzania). There is also the opportunity here to build new international academic partnerships for U.S. institutions in sub-Saharan Africa, provide international research field experience for US students, promoting global engagement of U.S. scientific workforce in sub-Saharan Africa, and build capacity in the different countries transected by the rift. Our results should provide scientific input into strategic planning for sustainable resource management for natural resources (oil, gas, water, geothermal energy) and environmental hazards mitigation (volcanism, earthquakes).

References


Ayele, A., E. Jacques, M. Kassim, T. Kidane, A. Omar, S. Tait, A. Nercessian, J-B. deChabalier, and G. King (2007), The volcano-seismic crisis in Afar, Ethiopia, starting September 2005, Earth Planet. Sci. Lett., 255, 177-187.

Buck, W.R. (2004), Consequences of the asthenospheric variability on continental rifting. In: Karner G.D., Taylor B., Driscoll N.W., and Kohlstedt D.L. (eds.) Rheology and Deformation of the Lithosphere at Continental Margins, New York, Columbia Univeristy Press, p. 1-31.

Buck, W.R. (2006), The role of magma in the development of the Afro-Arabian rift system, Yirgu, G., Ebinger, C.J. and Maguire, P.K.H. (eds), The Afar Volcanic Province within the East African Rift System, Geological Society, London, Special Publications, 259, 43-54.

Calais, E., N. d’Oreye, J. Albaric, A. Deschamps, D. Delvaux, J. De´verche`re, C. Ebinger, R. W. Ferdinand, F. Kervyn, A.S. Macheyeki, and A.Oyen, (2008), Strain accommodation by slow slip and dyking in a youthful continental rift, East Africa, Nature, 456, 783-787, doi:10.1038/nature07478, 2008.

Ebinger, C., (2005), Continental rifting and break-up process: Insights from East Africa. International Conference, In: Atekwana et al., (eds.), Extended Abstracts on the International conference on the East African Rift System, Mbeya, Tanzania, August 16-18, 2005: University of Dar es Salaam, Geology and Geography Departments, p. 17-20.

Ebinger, C.J., B.R. Rosendahl, and D.J. Reynolds (1987), Tectonic model of the Malawi rift, Africa. Tectonophys., 141, 215– 235.

Keranen, K., S.L., Klemperer, R. Gloaguen, EAGLE Working Group, (2004), Imaging a proto-ridge axis in the Main Ethiopian Rift, Geology 32, 949-952.

Kinabo, B. D., J. P. Hogan, E. A. Atekwana, M. G. Abdelsalam, and M. P. Modisi (2008), Fault growth and propagation during incipient continental rifting: Insights from a combined aeromagnetic and Shuttle Radar Topography Mission digital elevation model investigation of the Okavango Rift Zone, northwest Botswana, Tectonics, 27, TC3013, doi:10.1029/2007TC002154.

Kinabo, B.D., E.A. Atekwana, J.P. Hogan, M.P. Modisi, and A.B. Kampunzu (2007), Early structural development of the Okavango rift zone, NW Botswana. J. Afr. Earth Sci., 48, 125-136.

Modisi, M.P., E.A. Atekwana, and A.B. Kampunzu (2000), Rift kinematics during the incipient stages of continental extension: Evidence from nascent Okavango rift basin, northwest Botswana. Geology, 28, 939-942.

Van Wijk, J., J. van Hunen, and S. Goes (2008), Small-scale convection during continental rifting: Evidence from the Rio Grande rift. Geology, 36, 575-578.

Wright, T.J., C. Ebinger, J. Biggs, A. Ayele, G. Yirgu, D. Keir and A. Stork, (2006), Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode, Nature 442, 291–294.

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Rift Initiation and Evolution Within an Active Plate Boundary Zone: The Woodlark Rift of Papua New Guinea

Authors & Affiliations: Syracuse University: Suzanne Baldwin <[email protected]>, Paul Fitzgerald <[email protected]>, Daniel Curewitz <[email protected]> University of Texas at Austin: Paul Mann <[email protected]> University of California at Santa Barbara: Brad Hacker <[email protected]> University of Vermont: Laura Webb <[email protected]> Lamont Doherty Earth Observatory of Columbia University: Geoff Abers <[email protected]> Victoria University of Wellington; Tim Little <[email protected]> GNS Science, New Zealand: Laura Wallace <[email protected]> IFM-GEOMAR, Kiel University: Colin Devey <[email protected]>, Kaj Hoernle <[email protected]>, Romed Speckbacher <[email protected]>, Jan Behrmann<[email protected]>

Summary: The relative roles of magmatism and pre-existing structures in rift initiation remain a topic of intense study. To date, many studies of rift initiation and evolution have focused on rifting of continental lithosphere (e.g., Gulf of California, East Africa) and subsequent development of passive continental margins. Few studies have focused on the initiation, propagation, and evolution of rifts in heterogeneous lithosphere. We propose a scientific implementation plan centered on the Woodlark Rift of Papua New Guinea for consideration as a RIE focus site where the spatial and temporal record of rift initiation and propagation can be examined. The youngest HP/UHP terrane on Earth is preserved in the Woodlark Rift, and has been exhumed by rifting associated with westward propagation of the Woodlark Basin sea floor spreading system. The Australia (AUS)/Woodlark (WDK) plate boundary zone provides a diachronous, along-strike record of 1) Tertiary northward subduction of thinned continental margin of AUS affinity beneath an oceanic arc terrane; 2) Late Miocene to present rifting of that arc-continent collision zone, and 3) since 6 Ma, lithospheric rupture of that collisional orogen. Our objectives are to understand the relative roles of plate boundary and mantle forces controlling the onset and evolution of rifting within compositionally heterogeneous lithosphere. The proposed scientific implementation plan will require integrated and interdisciplinary onshore/offshore field investigations, laboratory and theoretical studies involving teams of geologists, geophysicists, geochemists, petrologists, and modelers. Ancient rifted margins comprising heterogeneous lithosphere abound in the geological record; therefore comparative global studies could be incorporated into this initiative. Background: The composition and architecture of lithosphere subjected to rifting and ultimately rupture, along with variable geothermal gradients and strain rates, influence the composition and degree of partial melting that will affect the lithosphere during rifting. Partial melting, in turn, may dramatically decrease lithospheric strength by enhancing the potential for lateral or upward flow of lower continental crust in response to isostatic and/or tectonic stresses. However, feedback mechanisms between magmatism, lower crustal flow, and rift initiation and propagation are both uncertain and a worthy topic of further research. The composition and structure of “pre-rifted” lithosphere in the Woodlark Rift differs from that within many focus sites of rifts in “normal” continental lithosphere. In eastern Papua New Guinea, subduction of rifted Australian margin beneath an island arc built on Pacific lithosphere led to obduction of mafic and ultramafic rocks (i.e., the Papuan Ultramafic Belt) and regional metamorphism. The resultant lithosphere is characterized by inverted density structure, and hence a complex strength profile. Distinct zones of weakness whose geometry are determined by the tectonic history of the


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region (e.g., serpentinite or phyllosilicates along relict shear zones) may have pre-conditioned the loci of rift initiation within this arc-continent collision zone such that exploitation of collisional structures led to extensional faulting during counter-clockwise rotation of the WDK microplate relative to AUS. GeoPRISMs RIE themes can be examined within the Woodlark Rift in regions of active rifting west of the propagating sea floor spreading rift tip. The pre-6 Ma record of rift initiation, evolution and lithospheric rupture can be investigated in rocks exhumed and preserved along the southern (Louisiade Archipelago) and northern rifted margins (Woodlark Rise). Implementation Plan: Investigation of RIE research themes outlined in the GeoPRISMS Draft Science Plan requires funding for onshore/offshore multidisciplinary studies including: 1. Active source seismic experiments to determine the architecture of the Woodlark Rift prior to, during, and post-lithospheric rupture. Along-strike variations in the offshore crustal structure within the Woodlark Rift will reveal how deformation has been partitioned (e.g., on low angle normal faults in the upper and/or middle crust), and how sedimentary basins have evolved over time. Seismic lines straddling the Trobriand Trough will help determine the geometry and role of this bathymetric feature (recently active southward dipping subduction zone?). 2. Passive seismic experiments will extend the existing network to assess lithospheric structure along strike and across strike, enable visualization of the extent of crustal thinning within the rift and may contribute to the assessment of earthquake and tsunami hazards that could threaten coastal populations. 3. Dredging/Drilling. The paucity of sediment cover makes possible direct sampling of rifted basement. The geographic extent of HP/UHP rocks offshore along the boundaries of the northern and southern-rifted margins, can be determined and submerged MCCs identified. Determination of the geographical extent of HP/UHP terranes (and dating of these terranes) will test the inferred fundamental relationship between HP/UHP exhumation, rift initiation and evolution. 4. Geodetic studies. Broadening and densification of the existing GPS campaign network will help refine our understanding of the kinematics and location of structures associated with the initiation of rifting, as well as the rift/seafloor spreading transition. 5. Geologic fieldwork on the Papuan Peninsula, on the southern rifted margin, and west of the active sea floor spreading rift tip will allow assessment of the structural architecture of rifted margins during and after breakup. 5. P-T-t-D studies. Structural analysis, petrogenesis, and thermochronology of onshore/offshore samples will refine understanding of the rheology of rifted heterogeneous lithosphere. Terrestrial cosmogenic nuclide studies can provide surface exposure ages, constrain paleoseismicity, fluvial incision rates, as well as erosion rates. 6. Fluid studies. Studies of hydrocarbons, precious metals, and geothermal areas (ie. mineral and energy resources) will assess mechanisms and consequences of fluid and volatile exchange between the lithosphere/asthenosphere during rifting, and will provide data to allow assessment of the possible importance of fluids for enabling slip along low-angle faults. 7. Modeling and Theoretical studies will provide a means to identify key areas for the testing of current hypotheses regarding the linkage between lithospheric architecture and the initiation of rifting. Integrated onshore and offshore data sets will provide initial and boundary conditions for 4-D models of rifting within heterogeneous lithosphere.

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Selected References Abers, G. A., et al. (2002), Mantle compensation of active

metamorphic core complexes at Woodlark rift in Papua New Guinea, Nature, 418, 862-865.

Baldwin, S. L., et al. (2004), Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea, Nature, 431, 263-267.

Baldwin, S. L., et al. (2008), Late Miocene coesite-eclogite exhumed in the Woodlark Rift, Geology, 36, 735-738.

Davies, H. L., and A. L. Jacques (1984), Emplacement of ophiolite in Papua New Guinea, Geol. Soc. London Spec. Pub., 13, 341-350.

Davies, H. L., and R. G. Warren (1988), Origin of eclogite-bearing, domed, layered metamorphic complexes ("core complexes") in the D'Entrecasteaux Islands, Papua New Guinea, Tectonics, 7(1), 1-21.

Ferris, A., et al. (2006), Crustal structure across the transition from rifting to spreading: the Woodlark rift system of Papua New Guinea, Geophyscial Journal International, 166, 622-634.

Goodliffe, A., et al. (1997), Synchronous reorientation of the Woodlark basin spreading center, EPSL, 146, 233-242.

Hegner, E., and I. E. N. Smith (1992), Isotopic compositions of late Cenozoic volcanics from southeast Papua New Guinea: Evidence for multi-component sources in arc and rift environments, Chemical Geology, 97, 233-250.

Hill, E. J. (1994), Geometry and kinematics of shear zones formed during continental extension in eastern Papua New Guinea, Journal of Structural Geology, 16(8), 1093-1105.

Hill, E. J., et al. (1995), Magmatism as an essential driving force for formation of active metamorphic core complexes in eastern Papua New Guinea, Journal of Geophysical Research, 100, 10441-10451.

Little, T. A., et al. (2007), Continental rifting and metamorphic core complex formation ahead of the Woodlark Spreading Ridge, D’Entrecasteaux Islands, Papua New Guinea, Tectonics, 26, TC1002, doi: 1010.1029/2005TC001911.

Martinez, F., et al. (2001), Metamorphic core complex formation by density inversion and lower-crust extrusion, Nature, 411, 930-934.

Monteleone, B. D., et al. (2001), Thermochronologic constraints for the tectonic evolution of the Moresby Seamount, Woodlark Basin, Papua New Guinea, Proceeding of the Ocean Drilling Project Leg 180.

Smith, I. E. M., and W. Compston (1982), Strontium isotopes in Cenozoic volcanic rocks from southeastern Papua New Guinea, Lithos, 15, 200-206.

Smith, I. E. M., and J. S. Milsom (1984), Late Cenozoic volcanism and extension in the eastern Papua, in Marginal Basin Geology, edited by B. P. Kokelaar and M. F. Howells, pp. 163-171.

Spencer, J. E. (2010), Structural analysis of three extensional detachment faults with data from the 2000 Space-Shuttle Radar Topography Mission, GSA Today, 20(8), 1-10.

Stolz, A. J., et al. (1993), Sr, Nd, and Pb isotopic compositions of calc-alkaline and peralkaline silicic volcanics from the D'Entrecasteaux Islands, Papua New Guinea, and their tectonic significance, Contributions to Mineralogy and Petrology, 47, 103-126.

Taylor, B., et al. (1995), Continental rifting and initial sea-floor spreading in the Woodlark Basin, Nature, 374(534-537).

Taylor, B., et al. (1999), How continents break up: Insights from Papua New Guinea, Journal of Geophysical Research, 104(7497-7512).

Taylor, B., and P. Huchon (2002), Active continental extension in the western Woodlark Basin: A synthesis of Leg 180 results, in Proceedings of the Ocean Drilling Program, edited by P. Huchon, et al., pp. 1-36.

Van Ufford, Q. A., and M. Cloos (2005), Cenozoic tectonics of New Guinea, AAPG Bulletin, 89, 119-140.

Wallace, L. M., et al. (2004), GPS and seismological constraints on active tectonics and arc-continent collision in Papua New Guinea: Implications for mechanics of microplate rotations in a plate boundary zone, Journal of Geophysical Research, 109(B05404).

Webb, L. E., et al. (2008), Can microplate rotation drive subduction inversion?, Geology, 36, 823-826.


TheAfarRRRTripleJunction,aFocusAreaforMARGINSRIE,PlateBoundaryDeformationandGeodynamicsRebeccaBendick,Dept.ofGeosciences,Univ.ofMontana<[email protected]>RobertReilinger,DEAPS,MIT,Cambridge,MA<[email protected]>KatieKeranen,SchoolofGeologyandGeophys.,OklahomaUniv.,<[email protected]>LucyFlesch,Dept.ofEarthandAtm.Sci.,PurdueUniv.,IN,<[email protected]>ShimellesFisseha,Geophy.Obs.,AAU,AddisAbaba,Ethiopia<[email protected]>TheAfarTripleJunction(ATJ)isdefinedbytheintersectionoftheRedSea(RS),GulfofAden(GA),andEastAfricaRift(EAR)system.Thesethreestructurescoverthefullrangeofriftingstagesandstylesfromtheearlieststagesofcontinentalbreakup(EAR)throughtheinitialstagesofoceanspreading(central/northernRedSea),tofull‐oceanriftingalongtheGA.DecipheringthekinematiccomplexitiesoftheATJanditstimeevolutionastheAdenriftpropagatedintotheNubianplatetoformthepresentAfarDepressionandDanakilBlockiscriticaltoimprovingunderstandingofthreetopicsingeodynamics:i.Whatarethecriticalparametersthatcontroltheinitiationofriftingofthecontinentallithosphere(therolesofpre‐existingweaknesses,thesub‐lithosphereerosion[hotspot],subduction/continentalcollision),ii.Howdoextensionalplateboundariesevolveovertime,andhowdorealR‐R‐Rtriplejunctionsdevelopunderfiniteextension(incontrasttothelimitofa0Dtriplejunctioninavectorsumapproximation),andiii.Howisthedistributionofsurfacedeformationrelatedtocrustal/lithosphericmechanicsandrheology?TherelativemotionbetweenAR,NU,andSO,aswellasthemotionsofARandNUwithrespecttoEurasia,arewellconstrainedbyplatetectonic,geologic,andgeodeticstudies–thesemotionsareremarkablefortheirsimplicityanduniformityintime.NU‐ARandSO‐ARmotion,andresultingRSandGAextension,developedin2stages,initial,slowextensionstartingat24±4Mathatacceleratedbyafactorof2at~11Ma,andhasremainedsteadytothepresenttime(ArRajehietal.,2010;McCluskyetal.,2010).NU‐SOmotionlikelyinitiatedsimultaneouslywiththatintheRSandGA(i.e.,24±4Ma;e.g.,GarfunkelandBeyth,2006),althoughtherateofextensionhasbeenslowerbyroughlyafactorof4–5thanthatintheRSandGA,andthereisnoclearevidenceforchangesinratesinceriftinitiation.Thesewell‐definedmotionsprovidesimpleboundaryconditionsfortheriftingprocessallowingfocusontheinfluenceoftherateandorientationofplatemotions,theinfluenceofpre‐existingstructures(lithosphericandcrustal),magmatism,andmantleflow,amongotherpossiblefactors.DespitetherecentresearchfocusonboththeATJandtheEthiopianRift,theavailabledatafortheregionislimitedandinsufficienttodeterminewhethertheclassicWilsoncycleconceptofriftingandmarginformationfitstheAfricanexample.IntheEthiopianRift,avarietyofactiveandpassivesourceseismicproductsimagethestructuralrift,buthaveverylimitedresolutionbeyondtheriftboundingfaults(Keranenetal.,2009;KeranenandKlemperer,2008;Bendicketal.,2006;Pasyanos,2005;Kendalletal.,2005;Bastowetal.,2005).Geodeticdataisalsomostdensewithinthestructuralrift.GeochemicalandstructuralstudiesextendfurtherintotheNubianandSomalianplates,butarestillsparse.IntheRedSeaandGulfofAdenrecentgeodeticresultsprovideboundsonthetotalextensionrates,butnotonhoworwhereextensionisaccommodated(e.g.ArRajehietal.,2010;Pallisteretal.,inpress).IntheAfarregionitself,geodeticdatahasbeencollectedmostlyinatransectalongasingle,sparsetransectfromtheEthiopianHighlandstotheRedSeacoast,withadditionalobservationsaroundtheDabbahudikeinjectionsite,supported

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bylimitedseismicdata(Figure1).Alloftheseobservationssufficetoplacesomeboundsonthekinematicsandmechanicsoftheregion,buthavealsostimulatednewquestions,especiallyabouthowcontinentalbreakupinitiatesandevolves,howfeedbacksbetweenstrainandtemperatureevolutionaffecttheprocess,andhowthemechanicsofcontinentalriftingchangeinspaceandtime.TheATJandassociatedriftstructurescontinuetobethefocusofabroadrangeofgeologicandgeophysicalstudiesbyUS,African,Arabian,andEuropeaninvestigators.SubstantialgeophysicalstudiesweresupportedundertheMARGINSRCLinitiative(seismic,geologic,geodetic)thatprovideastrongbasisforfuturestudies.SaudiArabiaisdedicatingmajorresourcestostudyriftingprocessesandassociatedhazardsalongtheRedSea,ARAMCOhasanexpandinginterestinseismicexplorationoftheRedSea,andEuropeaninvestigatorshavelongfocusedonMediterraneanactivetectonicsthatareadirectconsequenceofchangesinNU‐EUconvergence(JolivetandFaccenna,2000;McCluskyetal.,2010).ThedevelopmentofAFREF,aUSleadprojecttoinitiategeodeticinfrastructureinSub‐SaharanAfrica,willfacilitateallfutureactivetectonicstudies,aswilltheproliferationofnationalGPSnetworksinNAfrica(Morocco,Libya,Algeria,Egypt).EstablishingthisbroadregionasaGeoPRISMSfocussitewillprovidefurtheropportunitiesfortheacademicEarthsciencecommunitytogainaccesstotheseimportantdata.BroaderImpacts:DevelopmentofGeophysicalexpertiseinAfricaandArabiathroughdirectcollaborations.Constraintsfor,anddevelopmentof,basinmodelsforresourceexploration.Internationalcollaborations.Studentinvolvementininternationalresearch.Hazardmitigation(eqs,surfacefailures,volcanicevents,improvedweatherforecastingusingnationalGPSnetworks).Methodology:Possibleresearchtargetsfortheregionshouldincludeenhancedgeodesyfordeformationmonitoring(distributionofstrainacrossriftstructuresatdifferentstagesofevolutionandindifferenttectono‐magmaticsettings),ongoingstudyofexistinggeodeticassetstoimprovetemporalresolution,improvementofgeodeticresolutionwithintheATJwithnetworkexpansion,seismicstudiesforconstrainingcrustal,lithospheric,anddeepmantlestructure,faultgeometry,andbasinevolutionwithintheATJandextendingintotheNubianandSomalian“plates”,geodynamicsimulations,andgeochemistryandgeochronologyofregionalmagmaticactivity.ReferencesA.ArRajehi,etal.(2010)Geodeticconstraintsonpresent‐daymotionoftheArabianPlate:Implications for Red Sea and Gulf of Aden rifting, Tectonics, 29, TC3011,doi:10.1029/2009TC002482.I.Bastow,G.Stuart,J.Kendall,andC.Ebinger(2005)Upper‐mantleseismicstructureinaregionofincipientcontinentalbreakup:northernEthiopianrift,Geophys.J.Int.162:479‐493.Z.Garfunkel,Z.,andM.Beyth(2006)ConstraintsonthestructuraldevelopmentofAfarimposedbythekinematicsofthemajorsurroundingplates,inTheAfarVolcanicProvinceWithintheEastAfricanRiftSystem,editedbyG.Yirgu,C.J.Ebinger,andP.K.H.Maquire,Geol.Soc.Spec.Publ.,259,23–42.


L.JolivetandC.Faccenna,MediterraneanextensionandtheAfrica‐EurasiaCollision,Tectonics,19:1095‐1106,2000.J.Kendall,G.Stuart,C.Ebinger,I.Bastow,andD.Keir(2005)Magma‐assistedriftinginEthiopia,Nature433:146‐148.K.Keranen,S.Klemperer,J.Julia,J.Lawrence,andA.Nyblade(2009)LowlowercrustalvelocityacrossEthiopia:istheMainEthiopianRiftanarrowriftinahotcraton?G310:Q0AB01.K.KeranenandS.Klemperer(2008)DiscontinuousanddiachronousevolutionoftheMainEthiopianRift:Implicationsfordevelopmentincontinentalrifts,EarthPlanet.Sci.Lett.265:96‐111.S.McClusky, R. Reilinger, and 9 others (2010b) Kinematics of the southern Red Sea‐AfarTriple junction and implications for plate dynamics, Geophys. Res. Lett., 37, L05301,doi:10.1029/2009GL041127.S.McCluskyandR.Reilinger,Arabia/Africa/EurasiakinematicsandtheDynamicsofPost‐OligoceneMediterraneanTectonics,AGUFallMeeting,T23(abstract),2010.J.Pallister,W.McCausland,S.Jonsson,Z.Lu,H.Zahran,S.ElHadidy,A.Aburukbah,I.Stewart,P.Lundgren,R.White,andM.Moufti(2010)DykeintrusioncrisisinSaudiArabiaanddiscoveryofanewvolcanicearthquaketype,NatureGeosciences,inpress.M.Pasyanos(2005)AvariableresolutionsurfacewavedispersionstudyofEurasia,NorthAfrica,andsurroundingregions,J.Geophys.Res.110:B12301.

Figure1.Theproposedriftmechanicsstudyarea.TheapproximateEAGLEseismicexperimentfootprintistheblackbox.ExistingGPSsitesareopencircles.Theredboxesshowthetargetregionsfornewseismicandnumericalsimulationstudies;redcirclesarepotentialnewgeodeticobservationsites.BroadscalestudiesofthemechanicsofplatemotionswillconsiderthefullAR,NU,SO,EUplatesystem.

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The Gulf of California—Walker Lane Belt: A Natural Laboratory to Study Rift Initiation that Culminates in Seafloor Spreading

Cathy Busby (UCSB), Graham Kent (UNR), Neal Driscoll (SIO), Glenn Biasi (UNR), Ken Smith (UNR), John Louie (UNR), Bill Hammond (UNR), Alistair Harding (SIO), Geoff Blewitt (UNR), Corne Kreemer (UNR), Danny Brothers (USGS), Fred Phillips (NMIT), Jared Kluesner (SIO), Peter Lonsdale (SIO), Keith Putirka (Fresno St.), Debi Kilb (SIO), Pat Cashman (UNR), Paul Umhoefer (NAU), Gary Fuis (USGS), Dan Lizarralde (WHOI), Jeff Babcock (SIO) An ideal locale for the study of continental rifting and the initiation of sea-floor spreading would afford scientists the opportunity to address fundamental questions regarding rifting: (1) Why does rifting occur, specifically how do extensional forces exceed the yield strength of the lithosphere? (2) What controls the initial architecture and segmentation of rifts and how or does this affect the evolution and geometry of divergent margins? (3) How do rifting and rift-related processes evolve in time and space; what controls these feedbacks and interactions? Other attributes also need to be considered when selecting a study site: (1) Data infrastructure (e.g., base geologic mapping, digital topography, seismic and geodetic studies) so that rifting studies can focus immediately on the key questions. (2) Physical access (cost, safety, etc…). (3) An on-going history of active rifting so time-transgressive processes can be observed (e.g., northern Walker Lane may be a proxy for southern Gulf of California 8-12 Ma). (4) Preservation of rifting processes in sedimentary basins and surviving geologic exposures. (5) Opportunities for cost-effective experimentation to improve scientific return and broaden community access for primary data acquisition and analysis. Based on these considerations, the Gulf of California—Walker Lane corridor of eastern California and western Nevada is an ideal site to examine the initiation of rifting through to the culmination of sea-floor spreading, and underscores the suitability of this site for focused attention under the GeoPRISMS program. Building on the success of the first decade of MARGINS research (e.g., Lizarralde et al., 2007; Brothers et al., 2009) this coupled-site will allow us to focus our research efforts and employ a process-oriented approach to examine the initiation and evolution of rifts. Why does rifting occur? Continental rifting is a time-integrated consequence of extensional forces that exceed the yield strength of the lithosphere. By stating it in these terms, we are addressing the “Strength Paradox”; that is how do forces associated with rifting exceed the yield strength of lithosphere (e.g., importance of strain rate, lithospheric rheology, and temperature structure). Rifting in Walker Lane Belt began around 12 Ma as a terminal phase of Basin and Range province extension. Regional forces on the system include terminal phases of gravitational collapse of crust thickened in Sevier and Laramide orogenies. Local body forces include the strike-length loss of negative buoyancy beneath the Sierra Nevada, compressional and boundary shear forces on the southern and western margins, and less-well defined positive and negative contributions of mantle buoyancy. Initially resisting these forces was a thick section of Mesozoic intrusive and metamorphic rock and, locally, strong mantle lithosphere. The theoretical strength of this section was dramatically weakened by faulting during compressional phases of margin development, extensive hydrolytic weakening by subduction-related fluids, and significant volcanism. A fundamental goal of MARGINS/GeoPrisms is to understand how these processes act alone or in concert to weaken the strength of the lithosphere. To tease out how these processes impact the strength of the lithosphere spatially and temporally, we need excellent preservation of syngenetic sedimentary and volcanic deposits to establish a chronostragraphic framework. How do rifting and rift-related processes evolve in time and space, and what controls these relationships One answer to the question of temporal and spatial evolution of rifting in eastern California is already provided in the Gulf of California. In the GoC, on- and offshore geology records progressive thinning, basin infilling by volcanic sequences, and uplift of margins as differentiation and extension progressed. More careful examination shows that the rifting in the southern WLB is not focused on a single zone of weakness, but rather is distributed across 150 km in the Death Valley-Furnace Creek, Panamint, and Owens Valley fault systems. Intervening ranges remain topographically high. To the north, rifting is expressed in a complex interplay of NE and NW faults and basins, suggesting that significant lithospheric strength


heterogeneities need to be identified and understood. Detailed geologic mapping has only recently discovered the signals of continental rifting in this area (Busby et al., 2009 and in review). New rifting evidence includes: (1) discovery of large volcanic centers transtensional stepovers, (2) effusive eruptions along fault-controlled fissures, including intermediate-composition fissure eruptions of “flood lava” during an episode of “continental spreading”, and (3) displacement of ancient E-W drainage systems that flowed off the Nevadaplano since Paleocene time. We anticipate similar and even greater discoveries as GeoPRISMS investigators approach the GOC-WLB systematically and with a process and systems science viewpoint. What controls the architecture of rifts and rift margins? The GOC-WLB offers an opportunity to explore a variety of competing hypotheses for structural controls of rift architecture and evolution. South of about 38° N in the WLB, an east-west cross-section shows high peak elevations divided by deep valleys, consistent with gravitational collapse triggered by NW transtensional motion of the Sierran microplate. North of this, volcanism is more extensive and aligned along the range, valley base elevations are higher, and faulting is more diffuse. The Sierran microplate appears to serve as a stress guide on the west, but interior drainages have developed indicating significant crustal thinning to the east as well. These internal drainages preserve the sedimentary history of the region, such that evidence is still available to discern the relative importance of faulting, extension, volcanism, uplift, and subsidence in rift development. The lakes in the Walker Lane provide an important record into early rift development and evolution; an insight that will help us understand structures in the GOC and how early rift structures influence margin architecture and drainage patterns. By examining the GOC-WLB, we can define the development and evolution of the rift through time; as the northern portion of the system is in the early phase of rift development, whereas the southern section of the system is characterized by seafloor spreading. Lake systems are common during the early phase of extensional deformation (i.e., East African Rift lakes; Newark Basin, Culpepper Basin), which present several advantages. Stratigraphic preservation of event beds within relatively calm rift lakes (Kent et al., 2005; Dingler et al., 2009; Brothers et al., 2009) is far superior to an open ocean environment, where wave energy tends to rework and winnow sediment, thereby decreasing seismic reflectors and temporal resolution. The Walker Lane belt hosts no fewer than half dozen large rift lakes (e.g., Lake Tahoe and Pyramid, Walker, Mono, Honey and Eagle Lakes), and even larger regions occupied by paleolakes/shorelines (e.g., Lake Lahontan), where lacustrine stratigraphy of Pleistocene age is easily accessible to land geologists. Lake coring using portable systems such as the GLAD800 can provide a chronologic framework for understanding rift development over the past several million years. Taken together, the advantages of both high fidelity lake records, along with land-based techniques such as geology/geodesy/seismology (e.g., Blewitt et al., 2009), present an unheralded opportunity to understand early trans-tensional rift development in an arc setting. The insights gleaned from the WLB will provide enhanced understanding of the processes that shape early rift evolution and segmentation. Furthermore, as the WLB appears to be an analog for early rifting in the GOC, we can examine laterally adjacent areas of the system, which are undergoing different degrees of extension, to explore the link between early rift architecture and that along segments undergoing seafloor spreading. In summary, studies of the extensional deformation laterally along the GOC-WLB will allow us to understand the deformation building blocks through time and how they stack vertically to build a divergent margin. In addition, a large seismic reflection and refraction experiment is scheduled for February 2011 in the Salton Sea and will bridge an important gap between seafloor spreading to the south and rifting to the north (Figure 1). The Gulf of California and Sierra Nevada microplate—Walker Lane Belt system is of global importance because of the many important concepts developed there and exported to other parts of the world. Some of these features or concepts include: listric normal faults, detachment faults and metamorphic core complexes, “chaos” (large-scale landslide deposits), calderas and geothermal fields (e.g. Long Valley), maar volcanoes and Pluvial lake deposits, vertical axis rotations of crustal blocks, strain partitioning, thermochronologic dating of ancient landscape surfaces and tectonic tilting events, root delamination, and the emplacement of huge batholiths. An added benefit from the proposed study region is it will improve our understanding of geohazards and their recurrence interval along the GOC-WLB system.

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Figure 1: Map highlighting the Gulf of California—Walker Lane rift system. (1) Northern Walker Lane: Incipient rifting along the eastern edge of the Sierra Nevada Microplate. (2) Salton Trough: Transitional structures linking the Gulf of California to the south, with the Walker Lane Belt to the north. (3) Southern Gulf of California: Rifting that has culminated in seafloor spreading.


References

Blewitt, G., W.C. Hammond, and C. Kreemer (2009). Geodetic observation of contemporary strain in the northern Walker Lane:1, Semi-permanent GPS strategy. Late Cenozoic Structure and Evolution of the Great Basin – Sierra Nevada Transition, Vol. 447, pp. 1-15, Geological Society of America. doi: 10.1130/2009.2447(01). Brothers, D. S., N. W. Driscoll, G. M. Kent, A. J. Harding. J. M. Babcock and R. L. Baskin, New Constraints on the Salton Sea fault architecture and deformational history, Nature Geosciences, pp. 1-4, NGEO590, DOI:101038, 2009. Busby, C. J., Hagan, J. C., Koerner, A., Putirka, K., Pluhar, C., and Melosh, B., Birth of a Plate Boundary, Submitted to Geology, 2010. Dingler, J., G. Kent, N. Driscoll, G. Seitz, J. Babcock, A. Harding, B. Karlin, and C. Goldman, A high-resolution seismic CHIRP investigation of active normal faulting across the Lake Tahoe basin, California-Nevada, Bull. Geol. Soc. Am., pp. 1089-1107, v. 121, doi:10.1130/B26244.1 , 2009. Kent, G. M., J. A. Babcock, N. W. Driscoll, A. J. Harding, G. G. Seitz, J. A. Dingler, J. V. Gardner, C. R. Goldman, A. C. Heyvaert, P. Gayes, R. Karlin, L. A. Mayer, C. W. Morgan, L. A. Owen, R. C. Richards, “A 60 k.y. record of extension across the western boundary of the Basin and Range Province: Estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath Lake Tahoe”, Geology, 33, 365-368, 2005. Lizarralde, D., G. J. Axen, H. E. Brown, J. M. Fletcher, A. Gonzalez-Fernadez, A. J. Harding, W. S. Holbrook, G. M. Kent, P. Paramo, F. Sutherland and P. J. Umhoefer, Variations in styles of rifting in the Gulf of California, Nature, 466-469, doi:10.1038/nature06035, 2007.

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ActiveFaultingandMagmaticProcesses:FundamentalConstraintsonPassiveMarginFormation

CynthiaEbinger,UniversityofRochester

Thescarsoffailedcontinentalriftsandthefoldedstrataofancientpassivemarginsdecoratecontinentallithosphereworldwide,aswellasthesurfacesofthe‘2ndand4throcksfromthesun’.Althoughriftingprocesseshavebeenoperativesincetheformationoftheearliestcontinentallithosphere,thereremainslittleconsensusontheprocessescontrollingthegeometryandkinematicsofplaterupture,andontheroleofmagmatismininitiatingandmaintainingthefundamentally3Darchitectureofriftzonesworldwide.Therecordofbreakupisfullyrecordedalongpassivecontinentalmarginsworldwide,butwelackthespatialdrillingand3Dsubsurfacecoveragetounravelthestory,despitedecadesofacademicandindustryresearch.Windowsofopportunityarisefromrift‐relatedfaultingandmagmaintrusionevents,whichreceivelittleattentionowingtotheirdiminutiveearthquakemagnitudeincomparisontosubductionzonesettings.Theymay,however,havespectacularsurfacedeformationpatterns,with8mofopeninginoneriftingeventwithinazoneofcontinentalrupture(e.g.,Wrightetal.,2006;Grandinetal.,2009).Magmasupplyratestocratonicriftsandincipientspreadingzoneshavebeenonlycrudelyestimated,andrecentsatellite‐geodeticobservationshintatriftzonecrustalformationratesapproachingthoseofsomevolcanicarcsystems(e.g.,Biggsetal.,2009).AsdocumentedbyPallisteretal.(2010),eventheupliftedflanksofpassivemarginsmayexperiencedikeintrusions,aftertheinitiationofseafloorspreadingwithinthecentralriftzone.Theimplicationsofthesestudiesforvolcanicandseismichazardareprofound:dikescancausefaultslipsequivalenttoanorderofmagnitudemorethanthelargestearthquake(e.g.,Rowlandetal.,2007).TheRIEaimscouldbeachievedwithanimplementationplanthatenablesnimbleresearcherstoconstrainriftingprocesseswheneverandwhereverriftzonesexperienceML>5earthquakes,andparticularlyrepeatingswarmsof~Mw~3‐5earthquakessuggestiveofmagmaintrusionandassociatedfaulting.Seismicandgeodeticequipmentpoolsneedtorecognizetheinherentdifferencesinextensionalandcompressionalsystems,andfacilitaterapidresponseeffortsafterMw~5‐6eventsinriftzones.Programsofintegratedseismicity,highspatialresolutionInSAR,andhightemporalresolutionGPSdataacquisitioncanconstraintheprocessesinitiatingandmaintainingalong‐axissegmentation,thevolumeandrepeattimesofmagmaintrusion,thecontributionofmagmaintrusiontoriftarchitecture,lithosphericrheology,andtherelationshipbetweenmagmatismandborderfaultsegmentation(e.g.,Hamlingetal.,2009;Keiretal.,2009;Nooneretal.,2009;Calaisetal.,2009).Themagmatismlocallyincreasesthegeothermalgradient,andtogetherwiththeassociatedfluids,willleadtoreductionsinplatestrength,focusingstrain(e.g.,Buck,2004;Holtzmanetal.,inpress).Ebingeretal.(2008)mobilizedaseismicarray1monthafterasequenceofMw<5.5earthquakesinAfar,yetthey


wereabletoimagethedikeintrusionzoneandshallowmagmareservoirswithpost‐dikeseismicity.Insummary,thenewscienceplandrivingRIEimplementationstrategiesshouldclearlyoutlinestrategiestodirectlyobserveriftingprocessesfromactiveriftzonestoconstrainthenowinactiveprocessesrecordedinfailedriftsandpassivemargins.Buck, W.R. (2004), Consequences of asthenospheric variability on continental rifting. In Karner, G., B.

Taylor, N. Driscoll, and B. Kohlstedt (eds.), Rheology and deformation of the lithosphere at continental margins, Columbia Univ. Press, pp. 92-137.

Ebinger, C. J., D. Keir, A. Ayele, E. Calais, T. J. Wright, M. Belachew, J. O. S. Hammond, E. Campbell, and W. R. Buck (2008), Capturing magma intrusion and faulting processes during continental rupture: Seismicity of the Dabbahu (Afar) rift, Geophys. J. Int., 174, 1138 – 1152, doi:10.111/j.1365-246X.2008.03877.x.

Grandin, R., A. Socquet, R. Binet, Y. Klinger, E. Jacques, J.B. Chabalier, G.C.P. King, C. Lasserre, S. Tait, P. Tapponnier, A. Deloeme, and P. Pinzuti (2009), September 2005 Manda Hararo-Dabbahu rifting event, Afar (Ethiopia): Constraints provided by geodetic data, J. Geophys. Res., doi:10.1029/2009JB006883, in press.

Hamling, I. J., A. Ayele, L. Bennati, E. Calais, C. J. Ebinger, D. Keir, E. Lewi, T. J. Wright, and G. Yirgu (2009), Geodetic observations of the ongoing Dabbahu rifting episode: New dyke intrusions in 2006 and 2007, Geophys. J. Int., 178, 989 – 1003, doi:10.1111/j.1365-246X.2009.04163.x.

Holtzman, B., J. M. Kendall, D. Angus, Corroding a continent from below: Feedbacks among melt transport, melt-rock reaction and rheology active in the rifting process, J. Geophysical Research, in press, 2010.

Keir, D., I.J. Hamling, A. Ayele, E. Calais, C.J. Ebinger, T. Wright, E. Jacques, K. Mohamed, O.S.J. Hammond, M. Belachew, E. Baker, V.J Rowland, E. Lewi, L. Bennati (2009), Evidence for focused magmatic accretion at segment centers from lateral dike injections captured beneath the Red Sea rift in Afar. Geology 37:59--62, doi: 10.1130/G25147A.1.

Pallister, J., and 10 others, Broad accommodation of rift-related extension recorded by dyke intrusions. Nature Geosciences (2010).

Rowland J.V., E. Baker, C.J. Ebinger, D. Keir, T. Kidane, et al. (2007), Fault growth at a nascent slow spreading ridge: 2005 Dabbahu rifting episode, Afar. Geophys. J. Int. 171:1226–46.

Wright, T. J., C. Ebinger, J. Biggs, A. Ayele, G. Yirgu, D. Keir, and A. Stork (2006), Magma maintained ift segmentation at ontinental rupture in the 2005 Afar dyking episode, Nature, 442, 291 – 294, doi:10.1038/nature04978.

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Opportunities to investigate early-stage rifting with comparative studies in the Western Rift of the East Africa Rift System

U.S. Scientists James B. Gaherty1, Donna J. Shillington1, Cornelia Class1, Scott L. Nooner1, Cynthia J. Ebinger2, Andrew A. Nyblade3, Christopher A. Scholz4, Matthew E. Pritchard5

Malawi Scientists Leonard Kalindekafe6, Winstone Kapanje6, Patrick R.N. Chindandali6, Reidwel Nyrienda6 Tanzania Scientists Richard Wambura Ferdinand7, Shukrani Manya7, Nelson Boniface7, Fredrick M. Mangasini7, Abdul Mruma8

1Lamont-Doherty Earth Observatory, 2University of Rochester, 3Pennsylvania State University, 4Syracuse University, 5Cornell University, 6Geological Survey of Malawi, 7University of Dar Es Salaam, Tanzania, 8Geological Survey of Tanzania A core part of the GeoPRISMS RIE science plan concerns the initiation of rifting and the evolution of early-stage rifts. Key questions concern the origin and role of magma during earliest rifting, the formation and evolution of rift segmentation and its manifestation at depth in the crust and lithosphere, and the temporal style of deformation (episodic versus discrete). To address these core questions, we require information at a range of length and time scales on the distribution of deformation and magma throughout the lithosphere and along strike in an early-stage rift. On successfully rifted margins and mature rifts, extensive stretching, syn- and post-rift magmatism, and post-breakup sedimentation usually overwrite and bury the record of early-stage extension. The Western Branch of the East African Rift (EAR) System is an excellent locality to examine early-stage rifting at slow rates in strong, cold lithosphere. Only a small amount of stretching has occurred (<15%)[Ebinger, 1989], and extension is estimated to be proceeding relatively slowly at ~3.5 mm/yr [Stamps et al., 2008], but model constraints are very sparse. It exhibits pronounced tectonic segmentation, which is defined by ~100-km-long border faults [Ebinger et al., 1987]. The length scales of segments together with flexure associated with faulting [Ebinger et al., 1991], occurrence of deep seismicity [Jackson and Blenkinsop, 1993; Foster and Jackson, 1998] and inferred mafic lower crust [Nyblade and Langston, 1995] suggest that rifting is occurring in relatively strong, cold lithosphere. Very little volcanism is associated with rifting, providing a serious test for recent models that require intrusive magmatism to initiate rifting in cold, strong continental lithosphere [Buck, 2004]. Strikingly, the only surface expression of magmatism in this system occurs in an accommodation zone between segments rather than at segment centers [Furman, 2007], in clear contrast to MOR, mature rifts, and parts of the eastern branch of the EAR. This relationship is also observed in other early-stage rifts, but the 3D distribution of magma at depth and its role in extension are unknown in all cases. We are in the early stages of developing a multidisciplinary, multinational study focused on northern Lake Malawi, in the southern part of the Western Branch, that would include


imaging of the distribution of deformation and magmatism in the crust and mantle lithosphere along a couple of rift segments, quantification of temporal patterns in deformation, and evaluation of the source of magmas. We suggest that this rift system could be an excellent target for addressing RIE objectives in addition to rifts and rifted margins selected as ‘primary’ GEOPRISMS sites. Rifts along the Western Branch of the EAR constitute examples of rifting in strong, cold lithosphere at low strain rates. Basins in the Western Branch contain extensive syn-rift sedimentary sequences that record at exceptionally high temporal resolution, the interplay of orography and terrestrial climate evolution on continental scales (Cane and Molnar, 2001; Sepulchre et al. 2006). For example, the Albertine and Edward rift segments in the northern part of the Western Branch preserve signals of the rapid uplift of Ruwenzori Mountains, as well as long-term records of regional volcanism and the aridification of East Africa. By comparing the styles and spatiotemporal scale lengths of extension, magmatism, segmentation, and sedimentation (and the relationships between them) in this rift to those exhibited by, for example, other early-stage rifts in hotter, weaker lithosphere and/or with higher strain rates, we can capture the spectrum of rift parameters and assess their importance for controlling rift initiation and early development. Furthermore, by comparing regions that have undergone different amounts of extension (either within individual rift systems or between different rifts), our community can illuminate the evolution of extensional provinces from inception to breakup. For example, a funded CD program by Estella Atekwana and colleagues targets the extensional province beneath the Okavango delta, where rifting is just commencing (see Atekwana white paper). Comparing patterns of deformation and magmatism beneath the poorly known, weakly extended rift of the Okavango Delta and the Western Branch of the EARS with the well-studied, mature rifts in Ethiopia [e.g., Keranen et al., 2004] and Afar [Wright et al., 2006; Doubre et al., 2007] would reveal the progression of magmatism and deformation with increasing amounts of cumulative stretching, and provide vital constraints on the structure and rheology of extending continental lithosphere. Studies of early-stage rifts could also inform studies of more evolved rifts and rifted margins selected as primary sites. All of these comparisons would provide a critical test for numerical and conceptual models of early-stage rifting. GEOPRISMS research in regions like the Western Branch of the EARS would also facilitate a large range of much-needed and societally relevant activities. Populations along the Western Branch of the EAR and in other early-stage rifts face significant seismic and volcanic hazards [e.g., Biggs et al., 2009], but many countries have little capacity to monitor activity or mitigate risks. Scientific investigations of active rifts could simultaneously provide fundamental insights into rifting processes, critical new constraints on geohazards, and capacity building in developing countries. Biggs, J., E. Y. Anthony, and C. J. Ebinger (2009), Multiple inflation and deflation

events at Kenya volcanoes, East African Rift, Geology, 37, 979-982. Buck, W. R. (2004), Consequences of Asthenospheric Variability on Continental Rifting,

in Rheology and deformation of the lithosphere at continental margins, edited by G. D. Karner, et al., pp. 1-30, Columbia University Press, New York, NY, USA.

Cane, M.A. and Molnar, P., 2001. Closing of the Indonesia seaway as a precursor to east African aridification around 3-4 million years ago. Nature 411:157-162.

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Doubre, C., I. Manighetti, C. Dorbath, L. Dorbath, E. Jacques, and J. C. Delmond (2007), Crustal structure and magamato-tectonic processes in an active rift (Asal-Ghoubbet, Afar, East Africa): 1. Insights from a 5-month seismological experiment, J. Geophys. Res., 112(B5), B05405.

Ebinger, C. J., B. R. Rosendahl, and D. J. Reynolds (1987), Tectonic model of the Malaŵi Rift, Africa, Tectonophysics, 141, 215-235.

Ebinger, C. J. (1989), Tectonic development of the western branch of the East African rift system, GSA Bulletin, 101, 885-903.

Ebinger, C. J., G. D. Karner, and J. K. Weissel (1991), Mechanical strength of extended continental lithosphere: Constraints from the Western Rift System, East Africa, Tectonics, 10(6), 1239-1256.

Foster, A. N., and J. A. Jackson (1998), Source parameters of large African earthquakes: implications for crustal rheology and regional kinematics, Geophys. J. Int., 134, 422-448.

Furman, T. (2007), Geochemistry of East African Rift basalts: An overview, J. African Earth Sci., 48, 147-160.

Jackson, J., and T. Blenkinsop (1993), The Malawi Earthquake of March 10, 1989: Deep Faulting within the east African rift system, Tectonics, 12(5), 1131-1139.

Keranen, K., S. L. Klemperer, R. Gloaguen, and EAGLE Working Group (2004), Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian Rift, Geology, 32(11), 949-952.

Nyblade, A. A., and C. A. Langston (1995), East African earthquakes below 20 km and their implications for crustal structure, Geophys. J. Int., 121, 49-62.

Sepulchre, P., Ramstein, G., Fluteau, F., Schuster, M., Tiercelin, J.J. and Brunet, M., 2006. Tectonic uplift and Eastern Africa Aridification. Science 313:1419-1423.

Stamps, D. S., E. Calais, E. Saria, C. Hartnady, J.-M. Nocquet, C. J. Ebinger, and R. M. Fernandes (2008), A kinematic model for the East African Rift, Geophys. Res. Lett., 35, L05304, doi:05310.01029/02007GL032781.

Wright, T. J., C. Ebinger, J. Biggs, A. Ayele, G. Yirgu, D. Keir, and A. Stork (2006), Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode, Nature, 442, 291-294.


Addressing RIE objectives at rifted margins: some applications to US margins

James B. Gaherty1, Donna J. Shillington1, Daniel Lizarralde2, Harm Van Avendonk3

1Lamont-Doherty Earth Observatory of Columbia University, 2Woods Hole Oceanographic Institution, 3Institute for Geophysics, University of Texas

Active rift systems offer the opportunity to examine extension and magmatism in progress at a particular stage in rift development, while passive margins store the cumulative record of syn- and post-rift deformation, magmatism and sedimentation. Examination of passive margins thus provides unique, complementary information on several key aspects of the rifting process highlighted in the GeoPRISMS science plan, including the distribution of deformation and magmatism throughout the lithosphere and the emergence during rifting of primary characteristics of mid-ocean ridges (segment-centered magmatism focused at the ridge axis, tectonic segmentation, etc). Below we describe some aspects of rift evolution that can be best addressed at rifted margins, with examples of how they might be implemented on the US east coast.

1. Distribution of magmatism and deformation in the mantle lithosphere. The majority of existing constraints on the volume and bulk composition of magmatism during rifting are derived from seismic studies of the crust. These studies therefore miss a large part of the magmatic system recorded in the mantle lithosphere. The generation of melts during rifting leaves behind a depleted mantle that is stronger and more buoyant. Not only will rheological changes to the lithosphere caused by melt extraction influence ongoing development of the rift, they also have consequences for the stability of continental lithosphere long after rifting. It is also necessary to examine the mantle lithosphere to fully account for all of the magma generated during rifting. Studies of mid-ocean ridges suggest that the extraction of melts to form new crust can be incomplete in very slow-spreading systems (which have a thicker, colder lithosphere) and at very magmatic ridges, where the volume of magmas can overwhelm the melt extraction system. To fully tally up the magmas and assess the effect of depletion requires a close look at the lithosphere. Anisotropy of the mantle lithosphere also holds the record of pre-existing fabrics imparted prior to rifting and mantle deformation during rifting. Passive margins are excellent places to investigate all of these phenomena because the absence of melt and temperature anomalies associated with rifting make it possible to interpret geophysical parameters in terms of strain and composition. The breakup of Pangea to form the Atlantic margins was associated with one of the largest magmatic events in Earth’s history: the Central Atlantic Magmatic Province. The arrival of USArray on the east coast together with advances in ocean bottom seismology offer an excellent opportunity to understand the coupled magmatic and deformational processes in the mantle associated with magmatic rifting. 2. 3D rift structure: Existing 2D studies on rifted margins, including MARGINS investigations in the Gulf of California, hint at fundamental changes in magmatism and rifting style along-strike between and within segments in many rifts, but are insufficient to fully characterize it or its relationship to segmentation of the mid-ocean ridge or segmentation of the failed rift basins onshore and onshore. This along-strike variability is an important clue to the controls on segmentation and the influence of competing factors on the

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style of deformation and magmatism. Existing potential fields data and 2D seismic data on rifted margins provide an opportunity to design crustal surveys to study the 3D structure of key margin features (e.g., magmatic underplates, etc), and constrain how magmatism and deformation change within and between segments. Gravity and magnetic data reveal clear segmentation over thick magmatic crust of the east coast of the US at the 100-150 km length scale (similar to the Mid-Atlantic Ridge) as well as at longer wavelengths (300 km) (Behn and Lin, 2000). Segmentation of magma-rich margins is not necessarily expected because voluminous magmatism and high temperatures during and immediately after continental breakup might overwhelm focusing mechanisms and/or result in lower crustal flow that would work against segmentation. These questions and many others could be addressed with integrated geophysical studies designed to capture 3D structure of the crust and mantle lithosphere on passive margins under GeoPRISMS.

3. Full transition to seafloor spreading. Studies from both magma-rich and magma-poor margins provide evidence that the transition to mature seafloor spreading is often much more protracted than originally thought. Off-axis magmatism and deformation can persist for tens of millions of years after the formation of a new mid-ocean ridge. Almost no existing studies continue seaward far enough to capture the full transition. Passive margins record the full transition. By expanding investigations of margins farther seaward and comparing the style of accretion and deformation in the earliest oceanic crust (e.g., off the east coast of the US) with that produced at a the associated mid-ocean ridge (e.g., the Mid-Atlantic ridge), we can understand the full transition to mature spreading.

4. Relationship between failed rift basins and the successfully rifted margin. Existing observations indicate that continental breakup is often a protracted, multi-phase affair, such that one or more pulses of extension usually precede continental rupture. These earlier phases of extension are evidenced by failed rift basins that flank successfully rifted margins and by the stratigraphic and structural architecture of the rifted margin, itself. Nonetheless, it is not clear how much extension occurs during these earlier phases of rifting, if extension occurred simultaneously in these failed basins and in the location of the ultimately successful rift, the participation of magmatism throughout rifting, or what factors are most important in determining the location of the final rift. This uncertainty exists in part because there is a paucity of comprehensive constraints on crustal/mantle-lithospheric structure across an entire rift system, encompassing failed basins onshore and offshore, the successfully rifted margin and oceanic crust farther seaward. The east coast of the US is flanked by failed rift basins (e.g., Newark Basin, South Georgia basin, etc); comprehensive studies of these basins together with the associated rifted margin would significantly improve our understanding of these processes.

5. Postrift evolution. Rifted margins are modified by a number of processes long after the cessation of rifting as they cool and age. Furthermore, the contrast in thickness and rheology of the crust and mantle lithosphere at continental margins might have a profound effect on the response to later tectonic events and on the underlying mantle (i.e., small scale convection). The structure and stratigraphy of passive margins record their evolution after rifting in response to (or as drivers of) later tectonic and climatic events.


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Continental Breakup and Formation of Rifted Margins: The Gulf of Mexico as a Natural Laboratory

Dennis Harry (Colorado State University, [email protected]); Robert J. Stern (University Texas at Dallas, [email protected]); Elizabeth Anthony (University of Texas at El Paso, [email protected]); G. Randy Keller (University of Oklahoma, [email protected]); Ian Norton (University of Texas, [email protected]); Jolante van Wijke (University of Houston, [email protected]) Identifying Theme: Rifted Margins

Rifted continental margins capture the full pre-rift through post-rift process of continental breakup. As such, studying rifted continental margins should be a central focus of the MARGINS Successor Program (Stern and Klemperer, 2008). Based on four workshops held during the past year (GSA Southeast Section and Annual Meetings, AGU Fall Meeting, and Workshop for Earthscope Science Plan meeting), we argue on behalf of >100 participants that the Gulf of Mexico (GOM) is an ideal place for such studies (Fig. 1). The GOM formed during a brief (~25 m.y.) period in Late Jurassic time when Pangea broke up to form the Tethys seaway (Dickinson, 2009). Rifting exploited the Paleozoic Ouachita orogenic belt in the northern Gulf, which formed during the final stages of assembly of Pangea, and on the west involved an extensive Mesozoic magmatic arc system formed by subduction along the western North American plate boundary (Torres et al., 1999; Barboza-Gudiño et al., 2008 ). Prior events include formation of a segmented south Laurentian rift and transform margin during the Cambrian, which followed the trend of the Mesoproterozoic Grenville orogenic belt (Mosher 1998). During rifting, Yucatan separated from Texas-Louisiana and slid south along the Tehuatepec transform (Marton and Buffler, 1994) allowing variations on the Yucatan margin to be compared with those of the NW Gulf.

The GOM basin presents several opportunities to study fundamental processes associated with continental breakup. We briefly outline 8 of these opportunities:

1) Tectonic Inheritance. Rift style and location are strongly influenced by preexisting lithospheric fabric, with continental breakup commonly occurring along the trend of the last orogen (Dunbar and Sawyer, 1989; Thomas, 2006). The GOM provides an opportunity to study the influence of two end-member cases: a “soft” collisional orogen in the central and eastern Gulf (eastern Ouachitas - thin-skinned deformation and minimal syn-orogenic telescoping of the crust, which results in relatively shallow mantle and a strong lithosphere), and a “hard” collision in the western Gulf (western Ouachitas - thick skinned deformation and a great deal of crustal telescoping, which generates both thick crust and a lithospheric weakness (Harry and Londono, 2004). In the GOM, we can evaluate how these two tectonic fabrics affect subsequent rifting under similar lithologies, thermal conditions, and extension rates.

2) Rift Segmentation. What controls whether transitional crust is broad (among the broadest in the world in the eastern GOM) or narrow (as in the western GOM)? How do along-strike variations in the nature of transitional crust and lithosphere segmentation impact the subsidence history and thermal evolution of the GOM margins? Rifted margins also vary from magma-rich (volcanic rifted margins, e.g. Norway) to magma poor (e.g. Iberia). Such variations are inferred along strike in the central Gulf of Mexico, from magma-rich beneath the Texas Gulf Coast (Mickus et al., 2009) to magma-poor beneath Louisiana-Mississippi (Harry and Londono, 2004). Do magma supply variations control the width of transitional crust, or do these variations reflect control by inherited tectonic fabrics? Although much of the syn-rift magmatic evidence in the GOM is deeply buried under late Mesozoic and younger sediments, a possibly unique opportunity to examine the GOM rift magmatic record is presented by xenoliths recovered from salt diapirs, some of which contain 160 Ma syn-rift alkalic lavas (Ren et al., 2009) picked up from underlying rift-related lavas.

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3) Mantle Fabrics. What is the nature of mantle lithosphere and how does this change from continent, across the transitional crust, and into the center of the basin? Mantle xenoliths from central Texas (Young and Lee, 2009) indicate that the mantle lithosphere here is composed of slightly depleted spinel peridotite. Limited studies indicate strong, margin-parallel shear-wave splitting beneath the Texas Gulf coastal plain (Gao et al., 2008). Do the shear wave studies reflect crystal orientations developed during Jurassic rifting, or are they remnants of older events (e.g., the Ouachita orogeny)? This question speaks to how long mantle fabrics are preserved, and to whether orogeny or rifting may dominate mantle fabrics. Shear wave splitting beneath oceanic crust is generally perpendicular to the spreading ridge; because the spreading ridge is thought to have trended ~E-W, associated mantle fabrics would be oriented ~N-S, perpendicular to that observed beneath the coastal plain. How does the transition from rift-parallel to rift-normal fabric occur, and what does this signify about lithospheric evolution beneath the rift?

4) Lithospheric Reactivation. The Gulf coastal plain was magmatically and tectonically reactivated after the Jurassic, as revealed by Cretaceous low-degree asthenospheric melts (Griffin et al., 2010) and ongoing faulting. The landward extent of Cenozoic faulting and Cretaceous magmatism is generally associated with the landward limit of the Louann Salt. Is this reactivation a result of regional or plate-scale stresses? Local salt movement and/or eustatic rebound?

5) Sedimentation, Eustasy, and Coastal Subsidence. Sediments accumulated around the GOM range up to 18 km in thickness and are among the thickest of any continental margin. The NW GOM receives an extraordinary load of sediments from rivers draining southern Canada and the continental U.S. from east of the Rockies to west of the Appalachians as well as eastern central Mexico. This presents an opportunity to study source to sink depositional systems at a continental scale over a wide range of depositional environments, how rift architecture controls subsidence and thus sediment accumulation, and how sedimentation influences rift margin evolution. Evolution of this thick sedimentary section is also reflected in migration of the GOM shoreline. Position of the shoreline records the interplay between eustasy, sediment supply, tectonic subsidence and flexure. Understanding these variables has broad societal implications, from possible coastal flooding associated with sea level rise, to predictions of relative subsidence in important population centers like New Orleans.

6) Salt Tectonics. The GOM provides an ideal opportunity to study the connections between active faulting and salt movement on the scale of the coastal plain to continental slope, as well as more detailed studies of salt tectonic movements in environments ranging from shallow burial near the landward pinchout, deep burial beneath the shelf, and extrusion onto the abyssal plain. The GOM basin also provides world-class examples of a variety of salt tectonic styles (salt domes, ridges, welds, minibasins, etc.).

7) Fluid Evolution and Migration. The GOM is a factory for generating a wide variety of fluids: CO2, brines, and a wide range of hydrocarbons. These are generated in different ways, including mantle flux (CO2), sediment compaction (brines), biologic activity (biogenic methane), and diagentic/metamorphic reactions (fresh water, oil, thermogenic methane). The GOM provides an opportunity for understanding how these fluids form, migrate, and interact.

8) Synergies: The MARGINS successor program provides a timely opportunity to advance understanding of rifted margins in collaboration with other NSF initiatives as well as industry. These include EARTHSCOPE, which will conduct onshore broadband seismic, GPS, and magnetotelluric studies adjacent to rifted margins of the U.S. in the next decade; the Computational Infrastructure for Geodynamics (CIG), which provides computational tools to examine geodynamic problems related to continental breakup; and the Oceans Observatory Initiative, which will install a wide range of seafloor sensors. Participation by the hydrocarbon industry also is likely and should be encouraged. We expect that the MARGINS Successor Program will continue to stress geoscientific studies that cross the shoreline and will be well-positioned to lead the effort to study GOM evolution.


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REFERENCES Barboza-Gudino, J.R., Orozco-Esquivel, M.T., Gómez-Anguiano, M., Zavala-Monsiváis, A., 2008. The Early Mesozoic volcanic arc

of western North America in northeastern Mexico. J. S. Amer. Earth Sci. 25, 49-63. Dickinson, W.R., 2009. Research Focus – The Gulf of Mexico and the Southern Margin of Laurentia. Geology 37, 479-480. Dunbar, J.A., Sawyer, D.S., 1989. How preexisting weaknesses control the style of continental breakup. Jour. Geophys. Res., 94,

7278-7292. Gao, S. S., Liu, K.H., Stern, R.J., Keller, G.R., Hogan, J. P., Pulliam, J., and Anthony, E. Y., 2008. Characteristics of mantle fabrics

beneath the southern-central United States: Constraints from shear-wave splitting measurements. Geosphere 4, 411-417 Griffin, W.R., Foland, K. A., Stern, R.J., Leybourne, M.I., 2010. Geochronology of Bimodal Alkaline Volcanism in the Balcones

Igneous Province, Texas: Implications for Cretaceous Intraplate Magmatism in the Northern Gulf of Mexico Magmatic Zone. Journal of Geology 118, 1-21.

Harry, D.L., and Londono, J., 2004. Structure and evolution of the central Gulf of Mexico continental margin and coastal plain, southeast United States. Geol. Soc. America Bull., 116, 188-199.

Martön, G., Buffler, R.T., 1994. Jurassic reconstruction of the Gulf of Mexico Basin. International Geology Review, 36, 545-586. Mickus, K. Stern, R.J., Keller, G.R., and Anthony, E.Y., 2009. Potential field evidence for a volcanic rifted margin along the Texas

Gulf Coast. Geology 37, 387-390 Mosher, S., 1998. Tectonic evolution of the southern Laurentian Grenville orogenic belt. GSA Bull. 110, 1357-1375. Pindell, J. and Kennan, L. 2010 (in press). Tectonic evolution of the Gulf of Mexico, Caribbean and northern South America: an

update. James, K., Lorente, M. A. & Pindell, J. (eds) The Geology and Evolution of the Region between North and South America, Geological Society of London, Special Publication.

Ren, M., Stern, R., Lock, B., Griffin, R., Anthony, E., and Norton, I., 2009 Origin of igneous rock fragments from South Louisiana salt domes. 5 9 T H Gulf Co a s t A sso c i a t io n o f G eo lo g i c a l So c ie t i e s and th e Gu l f Co as t S e c t i o n o f S E P M , S e p t e mb e r 2 7 -2 9 .

Stern, R.J. and Klemperer, S. L., 2008. U.S. Passive Margins: Are we missing an Important Opportunity? Eos 89, 7, 64-65 Thomas, W.A., 2006. Tectonic inheritance at a continental margin. GSA Today 16, doi: 10.1130/1052-

5173(2006)016<4:TIAACM>2.0.CO;2 Torres, R., Ruiz, J., Patchett, P.J., and Grajales, J.M., 1999. A Permo–Triassic continental arc in eastern Mexico: Tectonic

implications for reconstructions of southern North America, in Bartolini, C., et al., eds., Mesozoic sedimentary and tectonic history of north-central Mexico: Geological Society of America Special Paper 340, 191–196.

Young, H. P., and Lee, C.-T. A., 2009. Fluid-metaomatized mantle beneath the Ouachita belt of southern Laurentia: Fate of lithospheric mantle in a continental orogenic belt. Lithosphere 1, 370-383.

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GEOPRISMS WHITEPAPER

TITLE: 3D CONSTRAINTS ON SLOPE FAILURE ON A PASSIVE MARGIN AUTHORS: Matthew J. Hornbach1, Peter Flemings1, Rob Harris2, Brandon Dugan3

1University of Texas at Austin (email: [email protected]; phone: (512) 636-5030), 2Oregon State University, 3Rice University

Proposed Site: Cape Fear Slide Themes Addressed: 3.1 (Origin and Evolution of Continental Crust)

3.3 (Climate/Surface/Tectonic Feedbacks)

I. Summary A fundamental theme of the emerging GeoPRISMS program is understanding continental margins dynamic where crust is created, destroyed, or modified. Retrogressive submarine slide occur occurring on all of Earth’s passive margins. These repetitive back-stepping failures record long-term instability (e.g., Micallef et al., 2008). These slides are wide-spread; they impact margin erosion and evolution, and they are a societal risk because of their potential to generate tsunamis. Their repetitive failure cycles make them both conducive for understanding failure conditions and for testing slope stability models and establishing what influences the size and rate of failure.

Why retrogressive submarine slides occur remains controversial (e.g., Dugan and Flemings, 2000, 2002; Maslin et al., 2004). Hypotheses for their occurrence invoke external drivers such as infrequent, strong earthquakes (e.g., Kvalstad et al., 2005). Others hypothesize that hydrate dissociation driven by sea-level fall or ocean warming drives slumping (Paull et al. 1991; Rothwell et al. 1998; Maslin et al., 2004; Liu and Flemings, 2009)). Conversely, other studies hypothesize that sediment strength, slope geometry, and depositional history drive retrogressive slope failure (Dugan and Flemings, 2000; Lee, 2009; Locat et al., 2009) such that external drivers including sea-level fall or earthquakes are not necessary.

We propose an interdisciplinary multi-stage field-based study of slope stability focused on integrating in situ pore pressure measurements with high-resolution 3D seismic images and 3D fluid-flow/heat-flow observations and models to constrain key factors that cause instability at a retrogressive slide. Slope failure and associated tsunamis are a recognized geohazard for the East Coast of the United States and several studies link slope failure with climate change. Many studies have relied on empirical observations and correlations to estimate causes of slope failure on continental margins. We will test these hypotheses by providing a process-oriented understanding of failure based on direct observations. Very few studies have pursued a fundamental quantitative approach to analyze the process and consequences of submarine failure. This study will elucidate the geotechnics of slope failure to understand how unloading due to slope failure can lead to a characteristic timescale of regressive failure. This timescale will be influenced in-part by the geotechnical properties of the slope. This study will also directly address whether steady-state or dynamic (and therefore unstable) gas-hydrate stability conditions exist at a slide, and therefore, if methane hydrates are presently contributing to instability at these sites. Our study has the potential to define slide re-occurrence time-scales, slide size, and the process of slope failure by direct measurements of in situ time-dependent variables (e.g. pore pressure, stress creep), and how these variables affect margin stability with time. Our study is focused on the Upper Cape Fear Slide but the


approach and expected results will be broadly applicable to retrogressive submarine failures around the globe. II. Study Area: The Upper Cape Fear Slide (CFS) The Cape Fear Slide (CFS), perhaps the largest slide complex on the U.S. Atlantic margin, is located ~200 km southeast of Cape Fear, North Carolina, just seaward of the Carolina trough (Figures 1 and 2). Initial studies (Cashman and Popenoe, 1985) suggested the CFS may consist of only a few large slides. However, more recent multibeam studies have identified at least five (but likely many more) moderately sized (all >1 km2) slide events [(Hornbach et al., 2007; Paull et al., 1996; Popenoe et al., 1993; Rodriguez and Paull, 2000; Schmuck and Paull, 1993)].

The upper headwall of the CFS has a crown-shaped morphology, is ~10 km long and ~20 m high (figures 3, 4). It is likely one of the youngest slides in the complex; old single-channel seismic lines indicate no other up-slope debris obscures the scarp and associated features (Carpenter, 1981). As the most landward component of a retrogressive slide, it also represents an area where future failure will likely occur.

Figure 1.(A) Multibeam data collected at the Cape Fear slide complex during reconnaissance work on 2003 NOAA Ocean Exploration cruise (adapted from Hornbach (2007). (B) Single chirp seismic line collected across the headwall of Upper CFS. A continuous, variable amplitude reflector tracks across the section and may represent both the base of overpressure and base of the slide. No sediments onlap the sidewall or headwall, suggesting recent failure.

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Carpenter, G., 1981, Coincident sediment slump/clathrate complexes on the US Atlantic continental Slope:

Geo-Marine Letters, v. 1, p. 29-32. Cashman, K.V., and Popenoe, P., 1985, Slumping and shallow faulting related to the presence of salt on the

Continental Slope and Rise off North Carolina: Marine and Petroleum Geology, v. 2, p. 260-271. Dillon, W.P., and Popenoe, P., 1988, The Blake Plateau Basin and Carolina Trough in Sheridan, R.E., and A., G.J., eds., The Atlantic continental margin, U.S. V. I-2 of The geology of North

America: Boulder, GSA, p. 291-328. Dillon, W.P., Popenoe, P., Grow, J.A., Klitgord, K.D., Swift, B.A., Paull, C.K., and Cashman, K.V., 1982,

Growth faulting and salt diaprism: their relationship and control in the Carolina Trough, Eastern North America, in Watkins, J.S., and Drake, C.L., eds., Studies of Continental Margin Geology, AAPG Memoirs, p. 21-46.

Dugan, B., and Flemings, P.B., 2000, Overpressure and fluid flow in the New Jersey continental slope: Implications for slope failure and cold seeps: Science, v. 289, p. 288-291.

Hornbach, M.J., Lavier, L.L., and Ruppel, C.D., 2007, Triggering Mechanism and Tsunamogenic Potential of the Cape Fear Slide Complex, U.S. Atlantic Margin: Gechemistry, Geophysics, Geosystems, v. 8.

Kvalstad, T.J., Andresen, L., Forsberg, C.F., Berg, K., Bryn, P., and Wangen, M., 2005, The Storegga slide: evaluation of triggering sources and slide mechanics: Marine and Petroleum Geology, v. 22, p. 245-256, doi: 10.1016/j.marpetgeo.2004.10.019

Lee, H.J., 2009, Timing of occurrence of large submarine landslides on the Atlantic Ocean Margin: Marine Geology, v. 264, p. 53-64.

Liu, X., and Flemings, P., 2009, Dynamic response of oceanic hydrates to sea level drop: Geophys. Res. Lett., v. 36.

Locat, J., Lee, H.J., ten Brink, U.S., Twichell, D., Geist, E., and Sansoucy, M., 2009, Geomorphology, stability and mobility of the Currituck slide: Marine Geology, v. 264, p. 28-40.

Maslin, M., Owen, M., Day, S., and Long, D., 2004, Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis: Geology, v. 32, p. 53-56.

Paull, C.K., Buelow, W.J., Ussler, W., and Borowski, W.S., 1996, Increased continental-margin slumping frequency during sea-level lowstands above gas hydrate-bearing sediments: Geology, v. 24, p. 143-146.

Popenoe, P., Schmuck, E.A., and Dillon, W.P., 1993, The Cape Fear landslide: slope failure associated with salt diapirism and gas hydrate decomposition, in Schwab, W.C., Lee, H.J., and Twitchell, D.C., eds., Submarine Landslides--Selected Studies in the U.S. Exclusive Economic Zone, U.S. Geological Survey Bulletin, p. 40-53

FIGURE 2(A) Basemap (same orientation as Figure 1) showing the multibeam data obtained in 2003 on the R/V Atlantis. Chirp lines are shown as thin black lines. (B) Proposed coring and seismic lines at the Upper headwall. (C) idealized 2D cross section of seismic data with core site locations in red. Chirp images (figure 4B) and previous coring results near this area indicate long cores should penetrate below the proposed detachment surface.


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Is the Wabash Valley Seismic Zone related with the ancient Reelfoot Rift?

Yevgeniy A. Kontar1, Fred Boadu2, Philip J. Carpenter3, Richard Harrison4, Maureen D. Long5, Robert S. Nelson6, Scott M. Olson7, Abdelmoneam Raef8, Ramesh Singh9, Michael S. Zhdanov10

1. Illinois State Geological Survey, Institute of Natural Resource Sustainability, University of Illinois

at Urbana-Champaign, Champaign, IL, U.S.A. 2. Department of Civil and Environmental Engineering, Duke University, Durham, NC, U.S.A. 3. Department of Geology and Environmental Geosciences, Northern Illinois University, De Kalb, IL,

U.S.A. 4. Eastern Geology and Paleoclimate Program, U.S. Geological Survey, Reston, VA, U.S.A 5. Department of Geology and Geophysics, Yale University, New Haven, CT, U.S.A 6. Department of Geography-Geology, Illinois State University, Normal, IL, U.S.A. 7. Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign,

Champaign, IL, U.S.A. 8. Department of Geology, Kansas State University, Manhattan, KS, U.S.A. 9. Department of Physics, Computational Sciences, and Engineering, Chapman University, Orange,

CA, U.S.A. 10. Department of Geology and Geophysics, University of Utah, University of Utah, salt Lake City, UT,

U.S.A. Introduction

The multitude of active faults in the Illinois Rift System poses a significant hazard to people and infrastructure. A sustainable earthquake disaster mitigation strategy requires compilation of base maps of known faults, as well as efforts to detect possible unknown faults. It is also necessary to build interactive databases of high-risk areas and integrate these with population distribution, seismic history, and vulnerability to hazards and disasters. In order to advance seismic research, it is necessary to develop cooperation among existing institutions and networks. Furthermore, collaboration is required with geotechnical and civil engineers and the disaster management community. It is proposed that several pilot studies be carried out to build capacity in integrated seismic hazard assessment. This is one of these projects.

Objectives Illinois is largely a stable intra-plate region characterized by a relatively low level of seismic activity, with earthquakes randomly distributed in space and time. The only part of Illinois that does not display the characteristics of an intra-plate region is the Wabash Valley, where earthquakes are associated with active fault zones. The Wabash Valley seismic zone (WVSZ) is centered on a series of high angle NNE trending normal faults, the so-called Wabash Valley fault system (Figure 1). This fault system covers an area of about 90 km by 50 km (Nelson, 1991) and is located in the center of the United States, to the north of the New Madrid seismic zone (NMSZ). Over the past 40 years, this fault system is even more seismically active than the NMSZ: Six medium-sized earthquakes up to magnitude of 5.6 took place in this area reaching a depth of 20 km or so, including one and its aftershock occurred last year. The WVSZ has been hypothesized to be an extension of the ancient Reelfoot rift. Identification of a high velocity zone in the lower crust, which could be linked to magmatic activity during ancient rifting, would help to confirm this hypothesis. This project will address the following primary questions:

Question 1 Why do earthquakes take place in this region down to 20 km? Is this seismic zone related with New Madrid Seismic Zone, the ancient Reelfoot rift? Braile et al. (1982) propose that the Wabash

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Valley seismic zone could be the north-extending arm of the ancient Reelfoot rift based on the correlation of the seismicity. To test the hypothesis that the WVSZ is a branch of the ancient rift, the most direct evidence is the high velocity in the lower crust, which can be linked to the magmatic activities during the ancient rifting. The previous studies on seismic velocity structures in the NMSZ or nearby region either did not cover the WVSZ or the resolution is low. Catchings (1999) presents a high-velocity lower crust under the NMSZ yet the results do not go south of St. Louis. A recent study by Liang and Langston (2009) suggests the existence of a triple junction-like high velocity body centered on the new Madrid and the Wabash valley seismic zones with the Reelfoot rift, the Ozark uplift, and the Nashville dome on its southwestern, northwestern, and southeastern arms, respectively. Nevertheless, this study is aiming for the whole eastern half of North America and the cell size of the inversion is 3o × 3o, which is much larger than the size of the WVSZ and therefore can not resolve the high velocity under the WVSZ. Conducting this study we would try to answer the following two more questions.

Figure 1. Seismicity in the New Madrid Seismic Zone (NMSZ) and the Wabash Valley Seismic Zone (WVSZ), adopted from Kim (2003).

Question 2

How does shallow structure in the Wabash Valley relate to deep structure? The Wabash Valley fault system is a group of predominantly high-angle normal faults that trend north-south to north-northeast along both sides of the lower Wabash River. Displacements in shallow (Mississippian and Pennsylvanian) strata range up to nearly 150 m. Because these faults control oil production and impact deep mining of coal, they have been mapped in considerable detail, but only at relatively shallow depths of less than 1,000 m (Bristol and

Northern Extension of Reelfoot Rift?


3

Treworgy, 1979). Actual exposures of fault surfaces are limited to underground coal mines. Shallow faults are post-Pennsylvanian and are presumed to have formed in an extensional stress regime, most likely during the Mesozoic (Kolata and Nelson, 1991). Braile et al. (1982 and 1986) proposed that the Wabash Valley was part of a “triple junction” or “quadruple junction” with the Reelfoot Rift and Rough Creek Graben. Although this theory has been largely discounted, the relationship of the Wabash Valley fault system to other structures in the region is still poorly understood. Bear et al. (1997) used seismic reflection data to demonstrate continuity of shallow Wabash Valley faults with faults of larger displacement (up to 600 m) in the Precambrian basement. A Precambrian ancestry thus is implied, but timing and tectonic ancestry of deep faults are unknown. Seismic lines published by Bear et al. (1997) and others indicate pre- Mt. Simon layered rocks in and near the Wabash Valley, but these rocks have never been reached by the drill. We then propose to carry out an experiment with a dense, “cross” shaped array of broadband seismometers covering the WVSZ (Figure 2), which will provide in situ higher resolution results.

Figure 2. Schematic map of the proposed seismic and MT arrays, high resolution 3D seismic survey area (light blue box) and a close up indicting design bin fold of coverage; inlines Northwest and X-lines (Northeast), and 2D seismic reflection profile (pink line) in and near the WVSZ. Small black triangles show permanent broad-band seismic stations, solid green triangles with black borders indicate tentative locations for the proposed temporary seismic arrays, solid brown diamonds indicate tentative locations of the MT stations. Blue Xs indicate locations of previous MT stations during the USGS measurements.

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Question 3 Is the Wabash Valley fault system seismically active? The Wabash Valley seismic zone is rather diffuse, and historic events have been much less frequent and smaller in magnitude than those in the New Madrid seismic zone. To date, most earthquakes that were large enough to generate data useful for focal-plane solutions lay outside the bounds of the Wabash Valley fault system. Preliminary analysis of the earthquake of April 2008 near Mt. Carmel, Illinois suggests that the focus lies along a buried extension of one of the mapped Wabash Valley faults. This is the first event in the region that may be tied to a mapped, shallow fault. A related question is whether mapped faults in the valley are oriented favorably to slip under the current stress regime. Nelson and Bauer (1987) showed that current stress is prone to produce or reactivate north-trending thrust faults rather than high-angle normal faults. However, steeply dipping faults that strike northeast or northwest may be capable of strike-slip under the current stress field. Through a combination of receiver functions, surface wave dispersion and regional body wave travel time analysis, we will measure the seismic velocity structure for the crust and determine whether the velocity of the lower crust beneath the WVSZ is high or low. Other passive seismic techniques, including transverse shear wave splitting and teleseismic body wave tomography will be used to probe the mantle beneath the WVSZ. The results of seismic and magnetotelluric vertical profiles will provide subsurface characteristics of fluids and sediments, as well as information about the presence of fluids or sediments at mid- or lower-crustal depths. The proposed integrated approach will give us an opportunity to examine the level of interseismic microseismicity and its relation with the deep subsurface structures. Furthermore, active and passive shear wave velocity measurements will be used to estimate site amplification functions (site response), critical for characterizing seismic hazards in the region.

References Bear, G.W., J.A. Rupp, and A.J. Rudman (1997). Seismic interpretation of the dep structure of

the Wabash Valley fault system. Seismological Research Letters. 68, 4, 624-640. Braile, L.W., G.R. Keller, W.J. Hinze, and E.G. Lidiak (1982). An ancient rift complex and its

relation to contemporary seismicity in the New Madrid seismic zone, Tectonics, 1(2), 225-237.

Braile, L.W., W.J. Hinze, G.R. Keller, E.G. Lidiak, and J.L. Sexton (1986). Tectonic development of the New Madrid rift complex, Mississippi Embayment, North America: Tectonophysics, 131, 1-21.

Bristol, H.M. and J.D. Treworgy (1979). The Wabash Valley fault system in southeastern Illinois: Illinois State Geological Survey, Circular 509, 19 p.

Catchings, R.D. (1999). Regional Vp, Vs, Vp/Vs, and Poisson’s Ratios across earthquake source zones from Memphis, Tennessee, to St. Louis, Missouri, Bulletin of the Seismological Society of America, 89(6), 1591-1605.

Kim, W.-Y. (2003). The 18 June 2002 Caborn, Indiana, earthquake; reactivation of ancient rift in the Wabash Valley seismic zone, Bulletin of the Seismological Society of America, 93 (5), 2201-2211.

Kolata, D.R. and W.J. Nelson (1991). Tectonic history of the Illinois Basin: in M.W. Leighton, D.R. Kolata, D.F. Oltz, and J.J. Eidel, eds., Interior Cratonic Basins: American Association of Petroleum Geologists, Memoir 51, 263-285.

Liang, C. and C.A. Langston (2009). Three-dimensional crustal structure of eastern North America extracted from ambient noise, Journal of Geophysical Research, 114, doi:10.1029/2008JB005919

Nelson, J. (1991). Structural styles of the Illinois Basin. Interior Cratonic Basins, 209-243, American Association of Petroleum Geologists, Tulsa.

Nelson, W.J. and R.A. Bauer (1987). Thrust faults in southern Illinois Basin - result of contemporary stress? Geological Society of America Bulletin, 98, 302-307.


1

Evolution of a Continental Rift: the Rio Grande Rift

by James Ni and the La Ristra team

We propose a multi-disciplinary study to investigate the evolution of the southern Rio Grande Rift (RGR). The RGR lies at the far-inboard edge of one of Earth’s great continental plateaus and is one of the major continental rifts in the world. Thus, results from its study have implications for rifts worldwide. The targeted study would include: 1) a high-density broadband seismological deployment for 3-d imaging, 2) geological investigations on the location and age of faulting, and 3) petrological analysis of late Cenozoic, post rifting basaltic magma. This study builds on results from the LA RISTRA project and other experiments in the region (e.g. Seidcar, CREST, CD-ROM), which have recorded teleseismic earthquake data across the southwestern U.S (Figure 1). The P-wave and S-wave velocity anomalies (Figure 1B) imply uniform stretching across the RGR and small-scale mantle convection on the eastern and western edges of the Colorado Plateau (CP) and the boundary between the RGR and Great Plains (GP) (Figure 1B) [van Wijk et al., 2008; van Wijk et al., 2010], with dramatic along-strike variations. These observations are significant in determining dynamic processes in the development and ongoing evolution of this geologically active terrain. The rather symmetrical structure of the central RGR lithosphere imaged in RISTRA rules out deep lithospheric detachment models [e.g., Wilson et al., 2004]. However, LA RISTRA was a 1-d deployment, not broad enough to detail the rift’s relationship to the obliquely crossing Jemez Volcanic Lineament (JVL). Because of its orientation, the JVL is widely conjectured to be a pre-existing structure that separated the 1.8 Ga province from the 1.6 Ga province. Another possibility, however, is that small-scale mantle convection between the southern RGR and the CP may be associated with the genesis and/or present mantle state of the JVL. During the last 10 Ma, the RGR was characterized by north-south trending high-angle normal faulting and relatively minor alkalic basaltic volcanism, where tholeiitic basalts erupted from central volcanic complexes from 13 Ma to the present. Beginning about 5 Ma, basaltic volcanism became widespread throughout the southeastern transition zone of the CP (e.g. Mt. Taylor and the Valles Caldera) along the JVL and its intersection with the RGR (Figure2).

Small-scale mantle convection modeling predicts a magmatic pulse at the CP and GP edges as the lithosphere edge is heated by lateral heat conduction and mantle lithosphere thinned by convective removal, resulting in a hot asthenosphere replacing lithosphere. The amount of modeled melt depends on initial thickness of the RGR lithosphere, the extension rate, and the amount of convective removed mantle lithosphere. The JVL appears to follow a mantle low-velocity structure; however, it is not clear why the rift did not form along the JVL. Indeed, previous investigators have noted that sub-lithospheric low-velocity anomalies lie directly below the JVL and they suggst a large magmatic source beneath the JL but not beneath the RGR, however, the existing seismic data (predominantly the 1-d La Ristra line and the sparse USArray Transportable Array is insufficient to make such a distinction on a 3-d basis with sufficient resolution to resolve the probable small-scale convective or general state of the upper mantle. Likely consequences of small-scale mantle convection between the southern RGR boundaries (CP and GP) are: 1) migration of normal faulting toward the CP and GP interior, and 2) time and spatial distribution of basaltic magma from lithospheric and, later, asthenospheric sources. The proposed study will determine the parameters and physical properties that can be related to the

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process of continental evolution as well as to further understanding of both mineral deposits and basin formation in continental rift regions.

References:

Sine,C.R.,D.Wilson,W.Gao,S.P.Grand,R.Aster,J.Ni,andW.S.Baldridge,2008,MantlestructurebeneaththewesternedgeoftheColoradoPlateau,GeophysicalResearchLetters,v.35,L10303,doi:10.1029/2008GL033391.

Van Wijk, J., J. van Hunen, and S. Goes, 2008, Small-scale convection during continental rifting: Evidence from the Rio Grande rift, Geology, v. 36, p. 575-578, doi: 10.1130/G24691A.1

Van Wijk, J.W., W.S. Baldridge, J. van Hunen, S. Goes, R. Aster, D.D. Coblentz, S.P. Grand, and J. Ni, Small-scale convection at the edge of the Colorado Plateau: Implications for topography, magmatism, and evolution of Proterozoic lithosphere.

Wilson, D., Aster, R., West, M., Ni, J., Grand, S., Gao, W., Baldridge, W.S., Semken, S., Lithospheric Structure of the Rio Grande Rift, Nature, 433, doi:10.1038/nature03297, 2005.

Figure1A:MapoftheCPandsurroundingregions.BlackandwhitetrianglesshowthelocationofLARISTRAtemporarystations.Reddotsrepresentseismicitybetween1973‐2000.BRP,Basinandrangeprovince;CP,ColoradoPlateau;RGR,RioGrandeRift;GP,Great


3

Plains.B:S‐wavevelocitytomography[Sinceetal.,2008].Highvelocityanomalies,a,bandcareinterpretedasdownwellingoflithosphericmantle.C:IgneousrockagesinNewMexico.D:Distributionandtimingofmagmatismsince26MaintheCPregion.Arrowsindicateyoungingdirectionofindividualvolcanicfields.PGC,Pahranagat‐SanRaphaelandgrandCanyonvolcanicbelt;SF,SanFranciscovolcanicfield;S,Springervillevolcanicfield.

Figure2TypeandageofthebasalticmagmatisminthesoutheasternColoradoPlateauandsouthernRioGrandeRift.

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GeoPRISMS White Paper Olsen et al.

Rifting, LIPs, and Life (RLL)

Paul Olsenaˇ†, Martha Withjackb, Roy Schlischeb, Dennis Kentb,a, Donna Shillingtona, Daniel Lizarraldec, Samuel Bowringd, Mohammed Et-Touhamie, Mike Waddellf, James Knappg, Dave Goldberga

a Lamont-Doherty Earth Observatory, Palisades, NY 10968; b Department of Earth & Planetary Science, Rutgers University, Piscataway, NJ 08854; c Woods Hole Oceanographic Institution, Woods Hole, MA 02543; d Dept. of Earth, Atmospheric, & Planetary Sciences, MIT, Cambridge, MA 02139; e Département de Géologie, Faculté des Sciences, Université Mohammed Premier, Oujda 60 000, Morocco; f ESRI-University of South Carolina, Columbia, SC 29208; g Department of Geological Sciences, University of South Carolina, Columbia, SC 29208. † [email protected]

GeoPRISMS seeks to address fundamental Earth System problems at continental margins, a major part of which concerns rifting and the exchange of materials among magma, the oceans, and atmosphere. The world’s largest rift system and aerially most extensive large-igneous province (LIP) have their pre-drift locus on the southeastern North American-West African conjugate margins. Here, a linked offshore-onshore project could explore the patterns and processes connecting rifting and passive-margin formation, magmatism, and biotic turnover during the birth of the Atlantic Ocean.

The past thirty years has seen a growing appreciation of the possible correlation between emplacement of LIPs and mass extinctions (1) (Fig. 1), and between LIPs and continental rifting and breakup (2-4). The three largest Phanerozoic mass extinctions —end-Permian, end-Triassic, and Cretaceous-Paleogene — are associated with the three largest continental LIPs — the Siberian Traps, Central Atlantic Magmatic Province (CAMP), and the Deccan Traps, respectively. The Paleocene-Eocene Thermal Maximum (PETM) has been linked with the North Atlantic LIP and a major terrestrial faunal and floral turnover as well as benthic marine extinctions (e.g., refs. 5-7). The common threads through most of these events are rifting, LIPs, and life. The relationship among these three areas involves some of the largest geodynamic and biotic issues, including such fundamental processes as mantle-plume behavior, continental breakup and ocean-crust formation, fluxes of mantle liquids and gasses, the nature of major drivers of biotic change, and planetary habitability. Better understanding of LIPs and their relationship to climate and extinction is crucial to determine whether they are drivers of biotic turnover, triggers of tipping points for already compromised ecosystems, or leave no record of biological effects.

Drilling, geophysical experiments, and surface studies have contributed in recent decades to our understanding of the relationship among rifting, seaward-dipping reflectors (SDRs), LIPs, and biotic events (especially the PETM) on continental margins. But the Central Atlantic conjugate margin, once the best known of all, has lagged, and is now among the least known, especially along the eastern USA. Even the most basic questions are unanswered or poorly constrained: When did seafloor spreading begin? What is the age of the SDRs and their relationship to CAMP? Were there multiple episodes of SDR formation? What is the observable heritage of the driving mantle processes? Was a mantle plume involved? And, most surprising, what is the relationship between continental rifting and the timing of breakup and seafloor spreading?

The southeastern USA margin is where these themes of rifting, LIPs, and life come together at the largest scale. The Earth’s largest known rift system bisected Pangea in the Late Permian through Early Jurassic, extending from Greenland through eastern North America, western Europe, Iberia, West Africa, and South America (Fig. 2). After 50 million years of non-volcanic rifting, CAMP — covering an area of 11x106 km2, with a volume of 2.4 x106 km3: ref. 8) — was created in apparently less than a million years (9,10). Significantly, this coincided with massive inputs of CO2 (11) with consequent drawdown by carbonation of basalt and ocean assimilation (12). The ensuing climatic and ocean chemistry catastrophes contributed to or drove the end-Triassic mass extinction, which ironically appears to account for the rise of dinosaurs to ecological dominance (13). A giant dike swarm (~6000 km x 1000 km) of CAMP has its apparent center on south Florida and the Bahamas at the southern limit of the East Coast Magnetic Anomaly and associated SDRs (Fig. 2). Conventionally the age of the SDRs is thought to be about 180 Ma (14), which would be 17 m.y. after the end of continental rifting in the southeastern USA (15);



however, it is plausible that the SDRs are actually part of CAMP1 emplaced when rifting was still active, and in fact accelerating in the north (16) The nexus of radial dikes perhaps marks the point of arrival of a mantle plume2 (17), which is itself controversial (18,19). What do we really know about the origin of the Central Atlantic Ocean, rifting, and drifting in the face of these remarkable desiderata?

The time has come to resolve these outstanding issues using an aggressive program of modern subsurface sampling and geophysical strategies including, but not limited to, the following (Fig. 3): 1) direct sampling by core including: a) use of a suitable drilling platform, such as the Chikyu or a spar3, to sample the SDRs off South Carolina or Georgia, thereby directly testing the hypothesis that they are part of CAMP by high-precision 40Ar/39Ar and/or U-Pb dating, paleomagnetic stratigraphy, and petrology; b) a shallower-water transect to test the hypothesized relationship between basalt flows and extensions of the SDRs toward the onshore rift basins; and c) an onshore transect to test if rifting persisted to the time of emplacement of the lava flows purportedly connected to the SDRs; 2) use of state-of-the-art marine and land reflection seismic experiments to image the geometry of the SDRs, their landward-extending lava flows, and rift-basin stratigraphy and structure in South Carolina, Georgia, and Florida, particularly in the South Georgia rift; 3) deployment of the EarthScope transportable array or dedicated new array to image present mantle structure for testing the hypothesized passage of a plume head with ambient noise tomography in Georgia, Florida, and the Bahamas or alternative hypotheses.

This proposed GeoPRISM RLL initiative addresses all four broad questions within the Rift Initiation and Evolution component of the science plan. The proposed work also: 1) leverages ongoing DOE-funded basin-characterization studies for carbon sequestration in the South Georgia rift and industry interest in exploration around Florida and the Bahamas, and 2) complements ongoing projects in the North Atlantic exploring the generality of the largest-sale geodynamic processes involving rifting, magmatism, and biotic turnover. Comparisons between the North Atlantic and Central Atlantic projects would address the following questions. What is the duration of breakup, from the cessation of rifting to the onset of seafloor spreading, for a classic 'volcanic' margin with SDRs? Does this differ from the duration of breakup for classic 'non-volcanic' margins without SDRs (e.g., Newfoundland, Iberia)? Can a classic 'volcanic' margin with SDRs develop if seafloor spreading rates are slow to ultra-slow? Does shortening occur on 'passive' margins during breakup? If so, what are the potential causes of this shortening? Finally, because pulses of CO2 over human times scales (hundred of years) seem to have caused the end-Triassic mass-extinction that took tens of millions of years for recovery, this initiative has clear resonance with societal concerns and should provide needed deep time context and a range of natural experiments for understanding anthropogenic environmental change. References: 1, Courtillot VE. & Renne PR. 2003 C. R. Geosci. 335:113; 2, Campbell IH. & Griffiths RW. 1990. Earth Planet. Sci. Lett. 99:79; 3, White RS. & McKenzie DP. 1989 JGR 94:7685; 4) Harry DL. & Sawyer DS. 1992. Geology 20:207; 5, Kennett JP. & Stott, LD. Nature 353, 225 (1991); 6, Svensen H. et al., 2004, Nature 429: 542-545; 7, Wing SL. et al., 2003, Geol. Soc. Am. Spec. Pap. 369; 8, McHone JG. 2003, AGU Mon. 136:241; 9, Olsen PE. et al. 1996. MNMH Bull. 60:11; 10, Marzoli A. et al. 1999. Science 284:616; 11, McElwain JC. et al. 1999. 285:1386; van de Schootbrugge B. et al. 2007. PPP 244:126; 13, Olsen PE. et al. 2002. Science 296:1305; 14, Holbrook WS. & Kelemen PB. Nature 364:433; 15, Olsen PE. 1997. Ann. Rev. Earth Planet. Sci. 25:337; 16, Olsen PE. 2003. AGU Mon. 136:7; 17, Ernst RE. & Buchan KL. 2003. Ann. Rev. Earth Planet. Sci. 31:469; 18, Anderson DL. 2000. Geophys. Res. Lett. 27:3623; 19, Coltice, N. et al. 2007. Geology 35:391; 20, McBride JH. et al. 1989. GSA Bull. 101:512; 21, Schettino A. & Turco E. 2009. Geophys. J. Int. 178:1078; 22, Whiteside JH. et al. 2007. PPP 262:194.

1 The Clubhouse Crossroad Basalt overlies at least part of the South Georgia Rift and based on seismic reflection profiles seems to extend seaward connecting with the SDRs (20). 2 The apparent absence of a hot spot track in the Central Atlantic is mitigated by the fact that Pangea translated rapidly north (21), not west, in the Early Jurassic and the plume tail would be under South America for nearly the entire Jurassic and Early Cretaceous (where there are in fact basalts of appropriate age), intersecting the South Atlantic in the Late Cretaceous, and surviving today as the Cape Verde hot spot (3). 3 A spar is a type of is a type of floating drilling platform tethered to the sea floor.

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Figure 1

Figure 2

Figure 3

Figure 1: Correlation between large igneous provinces and extinction events. The end-Permian, end-Triassic and Cretaceous-Paleogene are shown in red. Modified from (1 and http://www.largeigneousprovinces. org/frontiers.html#tim). Figure 2. CAMP igneous rocks in central Pangea. Note radial pattern of dikes. Possible plume head shown in green and yellow is at focus of radial dike swarm is at location of the south Florida volcanic province. Modified from (22). Figure 3. Possible GeoPRISMS RLL focus areas: Red box, high-resolution ambient noise tomography area; Light Blue Box, seismic profile area; Green Box, core transect area. Specific targets will be determined later in conjunction with community analysis. Abbreviations are: SDRs, seaward dipping reflectors; SFVP, south Florida volcanic province; SGR, South Geogia rift. Google Earth Image


Riftingthroughtranstension:theroleofobliqueshearinbreakingcontinentallithosphere,anexamplefromtheGulfofCalifornia.

Christina Plattner1, Falk Amelung1, Timothey H. Dixon1, Rocco Malservisi2, Peter Lonsdale3, Dave Chadwell3, Don Forsyth4, Jim Gaherty5, Danielle Sumy5, Francisco Suarez-Vidal6, Javier Gonzalez-Garcia6, Raul Castro6

1) University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Marine Geology and Geophysics, 4600 Rickenbacker Causeway, Miami, FL, 33149, USA ([email protected], [email protected], [email protected]).

2) University of South Florida, Dept. of Geology, 4202 E. Fowler Ave, SCA 528, Tampa, FL, 33620, USA ([email protected]).

3) Brown University, Dept. of Geological Sciences, Box 1846, Providence, RI, 02912, USA ([email protected])

4) Scripps Institution of Oceanography, UCSD 9500 Gilman Drive La Jolla CA, 92093, USA ([email protected], [email protected])

5) Lamont Doherty Earth Observatory, Columbia University, 61 Rt 9W, Palisades, NY, 10964, USA ([email protected], [email protected])

6) División Ciencias de la Tierra, Centro de Investigación Cientifica y de Educación Superior de Ensenada, Carretera Ensenada-Tijuana No. 3918, Zona Playitas, C.P. 22860, Ensenada, B. C. México ([email protected], [email protected], [email protected]).

Research motivation:

The Gulf of California served as the primary focus site for the MARGINS initiative, and we feel

that it remains an excellent focus site for the GeoPRISMS RIE initiative as well. The oblique

opening of the Gulf of California (GOC) is accommodated along transcurrent faults connected by

deep basins (Lonsdale, 1989). The timing between extension and shear in the GOC, and the role

of shear in breaking continental lithosphere is not completely understood (e.g. Fletcher et al.,

2007). Accurately characterizing this spatio-temporal evolution of the rifting is one of the central

questions of the RIE draft science plan (Q. 5.2). It has been shown that in a strain weakening

regime shear can provide the necessary strain to localize deformation and can lead to

asymmetric basin formation (Petruin and Sobolev, 2008). Numerical modeling of breaking

continental lithosphere requires reliable estimates of the mechanical and rheological properties.

The geographical location of the plate boundary in the central part of the GOC allows to constrain

these parameters from different geological and geophysical observations and to study the

processes that control the rupture of continental lithosphere in space and time in highly oblique

plate boundaries.

Project description:

In the central GOC, the main plate boundary fault of this oblique-rift system runs along the

Ballenas Transform, bounded to the north and south by pull-apart basins (Figure 1). This fast

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moving fault (approx. 45.9 ± 0.7 mm/yr) lies within a narrow (~40km) oceanic channel adjacent to

the east of the Baja California peninsula and west of several islands (Figure 1). This location

provides the unique opportunity to study the kinematics and dynamics of the GOC rift:

• Space geodetic data from Global Positioning System (GPS) (Figure 1) and Interferometric

Satellite Aperture Radar (InSAR) (Figure 2) reach back to 2003/2004 (straddling the August

2009 Mw 6.9 earthquake event at the Ballenas Transform) and provide spatially dense

information on the surface-displacement field during the full earthquake cycle (inter-, co- and

postseismic) (Plattner et al., 2009).

• High-resolution bathymetry and seabed geodesy data allows locating the submarine fault

trace, and partitioning of shear (Ballenas Transform) and extension (Sal si Puedes basin)

(Figure 2).

• Seismological observations interpreted with full-waveform, 3-D modelling can provide 3D

resolution of crustal and lithospheric velocity structure (Wang et al., 2009), which can be

interpreted in terms of thermal structure and lithospheric strength (Figure 1).

• Seismicity data can provide information on the earthquake cut-off depth, focal mechanisms,

and can constrain the presence/absence of fault creep.

Figure1:

TheBallenasTransformispartoftheGuaymasFaultSystem,whichisboundedtothe

northandsouthbylargeextensional

basins.

TheinterseismicvelocityfieldfromGPSdatashows

strainaccumulationfromtheBallenas

Transform(Plattneretal.,

2009).

PhasevelocityanomaliesalongstriketheGuaymasFaultSystem(Wang

etal.,2009).

Combining these data in numerical modeling, it is possible to constrain the rheological and

mechanical properties, their spatial variation, and provide knowledge on the current kinematics

and dynamics of this rift. The underlying hypothesis is that heat flow, rock properties, and strain


history affect the contemporary strain field. The main question to be addressed will be how

extensional and shear deformation is partitioned in space and time during breakage of continental

lithosphere, and how shear, lateral heterogeneities (including melt), and preexisting features

influenced the rift formation.

Additional comments:

The Gulf of California rift system has been a focus site of MARGINS Rupture Continental

Lithosphere Program, resulting in the availability of various geophysical data sets and specific

knowledge (http://www.nsf-margins.org/Nuggets_Public/nuggets_public.html#RCL).

The GeoPRISMS research strategy emphasizes

the contributions from GPS geodesy (rift

kinematics in the GOC) made to the past

MARGINS RCL program and clearly states that

“geodesy could play a much larger role in making

new discoveries along active continental margins

in GeoPRISMS”. Current efforts from UNAVCO to

initiate a permanent GPS network throughout

Mexico (including stations in Baja California and

Sonora) could help to facilitate this project.

References:

Fletcher, J.M., Grove, M., Kimbrough, D., Lovera, O. and

Gehrels, G.E., 2007. Ridge-trench interactions and the

Neogene tectonic evolution of the Magdalena shelf and

southern Gulf of California: insights from detritical zircon U-Pb

ages from the Magdalena fan and adjacent aeras. Geol. Soc.

Am. Bull., 119, 1313–1336.

Lonsdale, P., 1989, Geology and tectonic history of the Gulf of

California, in Winterer, E.L., et al., eds., The Eastern Pacific

Ocean and Hawaii: Boulder, Colorado, Geological Society of

America, Geology of North America, v. N, p. 499–521.

Petruin, A.G. and S.V. Sobolev, 2008, Three-dimensional

numerical models of the evolution of pull-apart basins. Physics of the Eartha dn Planetary Interiors, 171, 387-399,

doi:10.1016/pepi.2008.08.017.

Plattner, C., Malservisi, R., Amelung, F., (2009), Deformation in the Central Gulf of California from Geodetic data (Invited),

Eos Trans. AGU, 90(52), Fall Meet. Suppl., Abstract T33E-06.

NSF-MARGINS Decadal Review, MARGINS (and MARGINS-related) nuggets: http://www.nsf-

margins.org/Nuggets_Public/nuggets_public.html#RCL

Wang, Yu; Forsyth, D.W.; Savage, B., 2009, Convective upwelling in the mantle beneath the Gulf of California, Nature,

462, 499-501, doi:10.1038/nautre08552.

Figure2:CoseismicdisplacementfromInSARdataspanningBajaCaliforniaandIslaAngeldelaGuarda(Plattneretal.,2009).BathymetrydatashowthesubmarinefaulttraceoftheBallenasTransformandtheSalsiPuedesbasin(collectedbyP.Lonsdale).

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Advances and Ongoing Challenges in Interdisciplinary Rifting Research T. O. Rooney Dept. Of Geological Sciences, Michigan State University ([email protected]) K. Keranen School of Geology & Geophysics, University of Oklahoma ([email protected]) M. H. Benoit School of Science, The College of New Jersey ([email protected])

Over the past decade, investigations focused on understanding processes associated with continental extension have generated a framework for understanding the primary morphology of rifts, their seismic and volcanic activity, the architecture of the deep crust and mantle, and the thermal state of the lithosphere in specific extensional settings that went beyond the MARGINS focus sites:

A) Understanding the transition from continental to oceanic domains Studies that have focused on the transition from continental rifting to seafloor spreading in actively forming rifts have led to a model of progressive evolution (e.g. Ebinger and Casey, 2001) in the basic mechanisms of migration of strain from rift border faults towards zones of focused magmatism (Keranen et al., 2004; Casey et al., 2006). Along with these advances in understanding crustal deformation processes, recent studies have also revealed the 3-D architecture of the lithosphere and sublithosperic mantle (Bastow et al., 2005;2008; Benoit et al., 2006; Van Avendonk et al., 2006; Keir et al., 2006; 2009a; 2009b; Rooney et al., 2007; Rooney, 2010) beneath transitional rift systems. Magmatism is increasingly recognized as a key component in these transitional systems. Its influence on the rifting process is best addressed through multi-disciplinary research because of its role in accommodating extension, triggering seismicity, and revealing the composition of the evolving mantle. We suggest that the selection of a transitional region where magmatism can be traced from its generation (mantle tomography; isotope geochemistry), to intrusion into the crust (e.g. MT, seismicity, geodesy – GPS and InSAR, crustal tomography, major and trace element geochemistry), and eventual eruption is imperative in ongoing rift studies.

B) Magmatism and extension An association between dike swarms and flood basalts with continental rifting and breakup remains the orthodoxy for volcanic margins. Even so, the precise role of plumes at breakup and throughout rift evolution remains uncertain. Research to this point has been largely focused on rift valleys themselves, with less recent research effort dedicated to exploring the margins of rifts where early volcanism and distributed extensional features may still be preserved. The work that has been completed, even within rifts, has indicated that extension and magmatism may be broadly distributed at intermediate stages of rifting (Bendick et al., 2006; Keranen et al., 2009). One limitation in studying their role is the intense erosion that frequently destroys evidence in older volcanic provinces. Another significant limitation is that key flood basalt and related volcanic sequences are subsequently buried beneath rift-related sediments, younger basalts and eventually seawater. Studies completed in regions where lengthy time histories of rift- or flood basalt-related volcanism is preserved can best characterize the role of these different phases of volcanism, its spatial and temporal distribution, and its role in the evolution of extensional processes. The results of these studies provide a strong framework for a series of second-generation questions that will probe extensional processes through a new series of investigations. Among the questions and research priorities raised by the community related to rift initiation, rift evolution and feedbacks, the architecture of rifted margins and the role of volatiles, two broad themes have emerged as very promising for continued focused research efforts: 1) what is the thermo-chemical state of the sub-rift mantle, and 2) how does strain manifest and localize in the continental lithosphere?

1) What is the thermo-chemical state of the upper mantle?

Recent studies have made significant advances in understanding the relationship between thermal and chemical heterogeneity in the upper mantle and its relationship with rifting processes. Heterogeneity in rifting style and magmatism over short length scales have been attributed to variations in the composition the upper mantle (Lizarralde et al., 2007; Shillington et al., 2009). Previously melted regions of the upper mantle are less likely to

Page 1F-41GeoPRISMS IP - March 2011

be as productive in comparison to fertile areas and juxtaposition of these heterogeneities may be central to explaining short-length scale variations in melt distribution (Lizarralde et al., 2007). Similarly, the effect of arc enrichment and depletion processes on the mantle and lithospheric mantle may be relevant in controlling mantle melt productivity (Muntener and Manatschal, 2006; Shillington et al., 2009). Focused, high resolution studies of variations in mantle potential temperature and chemical heterogeneity in the sub-rift the mantle are key goals in addressing the new frontiers facing rifting research. Plume-influenced rifting environments possess important physical tracers that chronicle the distribution of plume material that may extend along an expanding rift system (e.g. Schilling et al., 1992; Leroy et al., 2010). Renewed focus on these environments may provide much needed fine-scale constraints on the effect of thermo-chemical mantle heterogeneity on rift development. Some key questions related to this concept include: What is the nature of the volatile exchange that takes place between asthenospehric, lithospheric and atmospheric reservoirs during rifting?; Does mantle plume material continue to contribute to upper mantle heterogeneity and melt production throughout rift evolution?; Does the lithosphere melt during rifting?; What is the mass exchange between asthenospheric and lithospheric reservoirs during rifting?

2) How does strain manifest and localize in the continental lithosphere?

The presence of melt within the lithosphere significantly modifies its thermal structure and also accommodates strain (Buck, 2006; Lizarralde et al., 2007; Bialas et al., 2010). Since syn-breakup magmatism is generally inactive by the passive margin stage, regions which preserve the mode of melt-lithosphere interactions remain a central focus of ongoing rifting research. The intrusion of melt in the lithosphere facilitates lithospheric rupture at much lower tectonic stresses than if magmatism is absent (e.g. Buck et al., 2006), but the time and length scales of magma generation, migration, storage, and emplacement are not well constrained and are therefore not considered in most numerical modeling of extension/continental rifting. The erosional removal of rift-related large igneous provinces and the burial of early rifting magmatic products presents a significant difficulty for probing early rift development. The role of melt during the mature stages of rifting where the transfer of strain from thinning to intrusion occurs represents another important focus in ongoing studies (e.g. Wright et al., 2006). Questions remain as to where melt is distributed within the lithosphere and how it may focus (Michael et al., 2003; Keranen et al., 2004; Whaler and Hautot, 2006; Rooney et al., 2007) in addition to constraining the amount of magma available and its the spatial variation (e.g. Wang et al., 2009).

While much of the previous research focus has been on hypothetical ‘homogenous’ lithosphere, the presence of pre-existing heterogeneities in the form of faults, metamorphic fabrics, terranes, and other discontinues may, however, control the development and mechanical segmentation of rifting (Corti, 2008; Keranen and Klemperer, 2008). Key remaining questions include: How does melt intrude the lithosphere?; What controls the length scale of diking?; Why does magmatism and faulting occur distinctly beyond the limits of the main border fault systems in early and middle stages of extension, and how do these peripheral features evolve? How do lithospheric discontinuities interact with magmatism to control rifting?; What is the relationship between strain migration and magmatic plumbing systems?; How is strain partitioned between faulting, stretching, and magma intrusion? Summary statement To address the key questions we have outlined, a focus on a site which provides the following characteristics is necessary:

1. Young, relatively uneroded flood basalt province 2. Preserves a key continental-oceanic transition 3. Rift system should include/span a wide range of morphologies 4. Pre-existing lithospheric heterogeneity is well-characterized 5. Thorough initial scientific (geological, geophysical, geochemical, geodetic) framework established. 6. Crossing the shoreline; accessible to a broad range of researchers

The African-Arabian rifting province exhibits all of these characteristics and therefore represents an unparalleled natural laboratory in which the next generation of rift-related research may be undertaken. By conducting new, focused research in this region, and leveraging existing datasets with both independent and collaborative efforts by international partners in Africa, the Middle-East, and Europe, we can most effectively advance our understanding of fundamental rift processes.

Page 2F-42

REFERENCES Bastow, I.D., Nyblade, A.A., Stuart, G.W., Rooney, T.O., and Benoit, M.H., 2008, Rifting at the Edge of the African Low Velocity Anomaly: G3, doi:10.1029/2008GC002107. Bastow, I.D., Stuart, G.W., Kendall, J.M., and Ebinger, C.J., 2005, Upper-mantle seismic structure in a region of incipient continental breakup: northern Ethiopian rift:

Geophysical Journal International, 162, 479-493. Bendick, R., McClusky, S., Bilham, R., Asfaw, L., and Klemperer, S., 2006, Distributed Nubia-Somalia relative motion and dike intrusion in the Main Ethiopian Rift: Geophysical

Journal International, 165, 303-310. Benoit, M.H., Nyblade, A.A., and VanDecar, J.C., 2006, Upper Mantle P-Wave Speed Variations Beneath Ethiopia and the Origin of the Afar Hotspot: Geology, 34, 329-332. Bialas, R.W., Buck, W.R., and Qin, R., 2010, How much magma is required to rift a continent?: Earth and Planetary Science Letters, 292, 68-78. Buck, W.R., 2006, The role of magma in the development of the Afro-Arabian rift system, in Yirgu, G., Ebinger, C., and Maguire, , eds., The Afar Volcanic Province within the

East African Rift System, 259, Geological Society, London, 43-54. Casey, M., Ebinger, C., Keir, D., Gloaguen, R., and Mohamed, F., 2006, Strain accommodation in transitional rifts: Extension by magma intrusion and faulting in Ethiopian rift

magmatic segments, in Yirgu, G., Ebinger, C., and Maguire (eds.), The Afar Volcanic Province within the East African Rift System, 259: Geological Society, London,143-164. Corti, G., 2008, Control of rift obliquity on the evolution and segmentation of the main Ethiopian rift: Nature Geoscience, 1, 258-262. Ebinger, C.J., and Casey, M., 2001, Continental breakup in magmatic provinces: An Ethiopian example: Geology, 29, 527-530. Keir, D., Bastow, I., Whaler, K., Daly, E., Cornwell, D.G., and Hautot, S., 2009a, Lower-crustal earthquakes near the Ethiopian rift induced by magma injection: G3:

doi:10.1029/2009GC002382. Keir, D., Ebinger, C.J., Stuart, G.W., Daly, E., and Ayele, A., 2006, Strain accommodation by magmatism and faulting as rifting proceeds to breakup: Seismicity of the northern

Ethiopian rift: Journal of Geophysical Research-Solid Earth, 111, doi:10.1029/2005jb003748. Keir, D., Hamling, I.J., Ayele, A., Calais, E., Ebinger, C., Wright, T.J., Jacques, E., Mohamed, K., Hammond, J.O.S., Belachew, M., Baker, E., Rowland, J.V., Lewi, E., and

Bennati, L., 2009b, Evidence for focused magmatic accretion at segment centers from lateral dike injections captured beneath the Red Sea rift in Afar: Geology, 37, 59-62. Keranen, K., Klemper, S.L., Gloaguen, R., and Grp, E.W., 2004, Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian rift: Geology, v. 32, 949-952. Keranen, K., and Klemperer, S.L., 2008, Discontinuous and diachronous evolution of the Main Ethiopian Rift: Implications for development of continental rifts: Earth and

Planetary Science Letters, 265, 96-111. Keranen, K.M., Klemperer, S.L., Julia, J., Lawrence, J.F., and Nyblade, A.A., 2009, Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot

craton?: G3, doi:10.1029/2008GC002293 Leroy, S., d'Acremont, E., Tiberi, C., Basuyau, C., Autin, J., Lucazeau, F., and Sloan, H., 2010, Recent off-axis volcanism in the eastern Gulf of Aden: Implications for plume-

ridge interaction Earth and Planetary Science Letters, 293, 140-153. Lizarralde, D., Axen, G.J., Brown, H.E., Fletcher, J.M., Gonzalez-Fernandez, A., Harding, A.J., Holbrook, W.S., Kent, G.M., Paramo, , Sutherland, F., and Umhoefer, J., 2007,

Variation in styles of rifting in the Gulf of California: Nature, 448, 466-469. Michael, J., Langmuir, C.H., Dick, H.J.B., Snow, J.E., Goldstein, S.L., Graham, D.W., Lehnert, K., Kurras, G., Jokat, W., Muhe, R., and Edmonds, H.N., 2003, Magmatic and

amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean: Nature, 423, 956-U1. Muntener, O., and Manatschal, G., 2006, High degrees of melt extraction recorded by spinel harzburgite of the Newfoundland margin: The role of inheritance and consequences

for the evolution of the southern North Atlantic: Earth and Planetary Science Letters, 252, 437-452. Rooney, T., Furman, T., Bastow, I.D., Ayalew, D., and Gezahegn, Y., 2007, Lithospheric modification during crustal extension in the Main Ethiopian Rift: Journal of Geophysical

Research, B, Solid Earth and Planets, 112, doi:10.1029/2006JB004916. Rooney, T.O., 2010, Geochemical evidence of lithospheric thinning in the southern Main Ethiopian Rift: Lithos, 117, 33-48. Rooney, T.O., Bastow, I.D. and Keir, D., 2010. Insights into Extensional Processes During Magma Assisted Rifting: Evidence from Aligned Scoria Cones and Maars Journal of

Volcanology and Geothermal Research, doi:10.1016/j.jvolgeores.2010.07.019 Schilling, J.G., Kingsley, R.H., Hanan, B.B., and Mccully, B.L., 1992, Nd-Sr-Pb isotopic variations along the Gulf of Aden - Evidence for Afar Mantle plume continental

lithosphere interaction: Journal of Geophysical Research-Solid Earth, 97, 10927-10966. Shillington, D.J., Scott, C.L., Minshull, T.A., Edwards, R.A., Brown, J., and White, N., 2009, Abrupt transition from magma-starved to magma-rich rifting in the eastern Black

Sea: Geology, 37, 7-10. Van Avendonk, H.J.A., Holbrook, W.S., Nunes, G.T., Shillington, D.J., Tucholke, B.E., Louden, K.E., Larsen, H.C., and Hopper, J.R., 2006, Seismic velocity structure of the

rifted margin of the eastern Grand Banks of Newfoundland, Canada: Journal of Geophysical Research-Solid Earth, 111, doi 10.1029/2005jb004156. Wang, Y., Forsyth, D.W., and Savage, B., 2009, Convective upwelling in the mantle beneath the Gulf of California: Nature, 462, 499-502. Whaler, K., and Hautot, S., 2006, Magnetotelluric studies of the northern Ethiopian rift., in Yirgu, G., Ebinger, C., and Maguire, , eds., The Afar Volcanic Province within the East

African Rift System, Volume 259, Special Publication of the Geological Society, London, 293-306. Wright, T.J., Ebinger, C., Biggs, J., Ayele, A., Yirgu, G., Keir, D., and Stork, A., 2006, Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking

episode: Nature, 442, 291-294.

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Oblique rifted margins: Lena trough as an archetype. Jonathan Snow (University of Houston) and Henry Dick (Woods Hole Oceanographic Institution). The classic rift geometry proceeds with basement faulting perpendicular to the direction of spreading. Increasingly, it has become clear that oblique rifting is an important second endmember in the overall spectrum of rift types. This is because plate boundaries almost never run parallel to the great circle through the euler pole (at Gakkel Ridge, for example they do), therefore nearly all margins have some oblique component. At the oblique end of the spectrum, 90 degree obliquity is identical to a transform plate margin, of which there are many. People tend to ignore plate boundaries in between orthogonal and transform, or call them “leaky transforms”. We argue that this ignores significant structural differences between oblique rifts and pure orthogonal rifts. In an oblique rift, the plate boundary and major basement faulting run at a steep angle to the spreading direction. An example of this is the North Brazil-Cote d’Ivoire conjugate margin in the equatorial Atlantic. Both extensive subsurface mapping of basement faults and the marine magnetic anomaly record in this region show a very clear obliquity to spreading, rather than transform-stepped rift segments. The basement structure is very different from well-known orthogonal rifts, resulting in a very narrow OCT with extensive oblique transtensional faulting, steep basement ridges, and a region of seafloor with subdued oblique anomalies offshore that may be dominated by mantle peridotite. Another good example is the eastern margin of the Great Bight in Australia, which shows a similar set of features. The resulting structures both in nearshore and offshore have seen significant recent hydrocarbon development. Lena Trough in the Arctic Ocean is an example of an early oblique rift whose basement faults are exposed at the seafloor and whose basement rocks are directly accessible for study. Dredging on the central tectonic ridge of Lena Trough (“Lucky Ridge”) has returned 100% mantle peridotite, that shows indications of being derived from both subcontinental and asthenospheric sources. The Western (Greenland) block of the conjugate margin exposes both mantle peridotites and rare potassic alkalic volcanics with a geochemical and heavy isotopic signature consistent with melting of lower continental crust, but a noble gas signature identical to MORB. These rocks document the transition of the margin in the last 15 million years from an active continental shear zone to the oblique spreading occurring today. Lena Trough provides the opportunity to examine both oblique rifting and lower crust/mantle exposure in the rift of a young margin, is relatively unsedimented, and is free of political encumbrance. The logistical challenges of working in the ice margin are not insuperable, as shown by numerous recent expeditions, and actually work to the advantage of many types of geophysical investigation including seismic, magnetic gravity and sampling operations.

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