cesses along the Antarctic Peninsula margin. Argon-40/ar-gon-39 age spectrum studies have already indicated a Paleozoicage for the blue schist metamorphism on Smith Island(Grunow, Dalziel, and Harrison 1987), but no ages older thanlate Mesozoic have been obtained from the Elephant Islandgroup. The seismic reflection profiles show a complex zone ofdeformation in the fore-arc region between Smith Island adja-cent to the termination of the Hero fracture zone. This could berelated to the localized uplift of the subduction complex rocksforming the island.

The field work in Cordillera Darwin conducted during May1987 by Daiziel and R. L. Brown of Carleton University, Ottawa,Ontario, Canada, provided strong evidence that the BeagleChannel marks the trace of an oceanward dipping low-anglenormal fault, the Beagle Channel detachment. The presence ofthe fault is indicated by ductile footwall fabrics and brittle hang-ing wall structures. Both sets deform the Upper CretaceousBeagle suite granites and an early Cenozoic age is likely for themain displacement. Thus Cordillera Darwin is a tectonicallydenuded metamorphic core complex similar to the Shushwapcomplex of British Columbia and other complexes along thePacific margin of the North American craton. It is, however,unique in the Andes (Dalziel and Brown 1987).

Laboratory studies have continued based on earlier fieldwork in the project. The first direct radiometric dates have beenobtained from the Rocas Verdes ophiolitic rocks of the south-ernmost Andes. Zircons from the Larsen Harbour Formation ofSouth Georgia collected on a cruise of iIv Polar Duke in 1985yield internally concordant lead-207/uranium-235 and lead-206/uranium-238 ages of close to 150 million years (Mukasa, Dalziel,and Brueckner 1987). Paleomagnetic data obtained from sam-ples collected on the same cruise indicate, as expected, thatSouth Georgia has not changed latitude since the mid- to Late

Cretaceous. The data do, however, suggest a counter clockwiserotation of 30±23 degrees since that time (Beaver, Dalziel, andKent 1987).

Finally, laboratory studies of material and data collected onearlier cruises have revealed crossite in the metamorphic rocksof the Diego Ramirez Islands, southernmost Chile. Thisprovides a link with the subduction complexes of the SouthShetland Islands (Wilson et al. 1987).

This research was supported by National Science Foundationgrant DPP 86-43441.

References

Beaver, D.C., I.W.D. Daiziel, and D.V. Kent. 1987. Palaeomagneticevidence bearing on the tectonic reconstruction of South Georgia andsouthern South America. Fifth international Symposium on AntarcticFart/i Sciences, Cambridge, United Kingdom, August, 1987.

Daiziel, I.W.D., and R.L. Brown. 1987. Andean core complex develop-ment related to marginal basin collapse: Implications for cordilleranorogenesis. Geological Society of America Abstracts with Programs, 19(7),634. 1987 Annual Meeting, Phoenix, Arizona.

Grunow, AM., I.W.D. Dalziel, and T.M. Harrison. 1987. Tectonic sig-nificance of new 40Ar/19Ar ages from the comparatively high P/low Tmetamorphic rocks of the Scotia arc. (Abstract.) Fifth InternationalSymposium on Antarctic Fart/i Sciences, Cambridge, United Kingdom,August, 1987.

Mukasa, SB., I.W.D. Daiziel, and H.K. Brueckner. 1987. Zircon U-Pbages on marginal basin and arc rocks from the northern Scotia arc.(Abstract.) Fifthinternational Symposiu in on Antarctic Earth Sciences,Cambridge, United Kingdom, August, 1987.

Wilson, T.J., A.M. Grunow, R.E. Hanson, and K.R. Schmitt. 1987.Subduction complex rocks on Diego Ramirez Islands, Chile. FifthInternational Syniposiurn on Antarctic Earth Sciences, Cambridge, UnitedKingdom, August, 1987.

Tectonic development of WestAntarctica and its relation to East

Antarctica: Joint U.S./U.K. program1986-1987

IAN W.D. DALZIEL

Institute for GeophysicsUniversity of Texas at Austin

Austin, Texas 78759

R.J. PANKHURST

British Antarctic SurveyCambridge, United Kingdom CB3 OFT

Field work in the Joint United States/United Kingdom WestAntarctic Tectonics Project during the 1986-1987 season waslimited to aeromagnetic flights flown by a British Antarctic

Survey Twin Otter aircraft with fuel supplied by usAI at SipleStation. The area of operation was between the BellingshausenSea and the Ronne Ice Shelf. Approximately 15,000 kilometersof track was flown with spacing of 20-25 kilometers. The result-ing data will allow further evaluation of the sub-ice geology andcrustal structure of the Antarctic Peninsula, Thurston Island,Ellsworth-Whitmore Mountains, and Haag Nunataks blocks ofWest Antarctica.

Reduction of data from earlier work, obtained as part of thesame project, has resulted in a regional magnetic anomaly con-tour map of the Ellsworth-Thiel Mountains sub-ice ridge con-toured at 25-nanoTesla intervals. The regional magnetic field ofthis area is generally flat reflecting the thick sequence of Pal-eozoic sediments exposed in the Ellsworth Mountains. Anoma-lies of 500 nanoTeslas amplitude and 30 kilometers wave lengthare present, however, extending from over 100 kilometers alongstrike. These anomalies attain their greatest expression overexposures of granite and gabbro in nunataks such as PaganoNunataks and the Hart Hills respectively, and are attributed tolarge bodies of mafic composition at mid-crustal depth (Garrett,Maslanyj, and Danaske in press). The field setting and geo-chemical data indicate that these igneous rocks are not related toeither subduction or continental collision but rather representthe products of a major thermal event associated with, and

50 ANTARCTIC JOURNAL

possibly a prime cause of, the initial break-up of the superconti-nent of Gondwana (Daiziel et al. 1987a and 1987b; Millar andPankhurst 1987; Storey and Daiziel 1987; Storey et al. in press aand in press b; Vennum and Storey 1987a and 1987b).

New paleomagnetic data from Lower Cretaceous rocks col-lected in 1984-1985 from the Thurston Island block indicate thaton the basis of paleomagnetic constraints the Antarctic Penin-sula, Ellsworth-Whitmore Mountains and Thurston Islandblocks define a single entity for which the name Weddellia hasbeen proposed. Some motion between these blocks is possiblewithin the limits of error of the paleomagnetic data, but be-tween the Middle Jurassic and the Early Cretaceous Weddelliaremained attached to West Gondwana (Africa-South America)while East Antarctica moved southward, i.e., dextrally withrespect to Weddellia (Grunow, Dalziel, and Kent 1987a and1987b). Weddellia rotated clockwise 300 and moved sinistrallyapproximately 2,500 kilometers relative to East Antarctica to itspresent-day position. This suggests that the main, if not initial,opening of the Weddell Sea occurred in the Early to mid-Cretaceous.

Finally, laboratory studies resulting from field work as part ofthe project in the Thiel Mountains in 1983-1984 have estab-lished a late Cambrian or Early Ordovician age for the ThielMountains porphyry, the shallow water volcaniclastic and vol-canic rocks of the Mount Walcott Formation, and the Reed Ridgegranites (Pankhurst et al. in press). The concluding field seasonof the present project will be undertaken in the PensacolaMountains in 1987-1988. The main objective will be to comparethe structure and orogenic history of the Paleozoic successionsof the Pensacola Mountains and the Ellsworth-WhitmoreMountains block.

This research was supported by National Science Foundationgrant DPI' 86-43404.

References

Dalziel, I.W.D., S.W. Garrett, A.M. Grunow, R.J. Pankhurst, B.C.Storey, and W.R. Vennum. 1987a. The Ellsworth-Whitmore Moun-tains crustal block: Its role in the tectonic evolution of West Ant-arctica. In D.H. Elliot, S. Haban, and G.D. McKenzie (Eds.), Gond-wana Six: Structure, tectonics, and geophysics, (Geophysical Monograph40). Washington, D.C.: American Geophysical Union.

Dalziel, I.W.D., B.C. Storey, S.W. Garrett, A.M. Grunow, L.D.B. Her-od, and R.J. Pankhurst. 1987b. Extensional tectonics and the frag-mentation of Gondwanaland. In M.P. Coward, J.F. Dewey, and P.L.Hancock (Eds.), Special Publication of Geological Society of London, Conti-nental Extensional Tectonics.

Garrett, SW., M.P. Maslanyj, and D. Danaske. In press. Interpretationof aeromagnetic data from the Ellsworth Mountains-Thiel MountainsRidge, West Antarctica.

Grunow, A.M., I.W.D. Dalziel'and DXKent. 1987a. Ellsworth-Whit-

more Mountains Crustal Block, Western Antarctica: New Paleomag-netic Results and their Tectonic Significance. In D.H. Elliot, S.M.Haban, and G.D. McKenzie (Eds.), Gondwana Six: Structure, tectonics,and geophysics, (Geophysical Monograph 40). Washington, D.C.:American Geophysical Union.

Grunow, AM., D.V. Kent, and I.W.D. Dalziel. 1987b. Mesozoic evolu-tion of West Antarctica and the Weddell Sea basin: New paleomag-netic constraints. Earth and Planetary Science Letters. 86, 16-26.

Millar, IL., and R.J. Pankhurst. 1987. Rb-Sr geochronology of the regionbetween the Antarctic Peninsula and the Transantarctic Mountains:Haag Nunataks and Mesozoic Granitoids. In D.H. Elliot, S.M.Haban, and G.D. McKenzie (Eds.), Gondwana Six: Structure, tectonics,and geophysics, (Geophysical Monograph 40). Washington, D.C.:American Geophysical Union.

Pankhurst, R.J., et al. In press. Cambro-ordovician magmatism in theTransantarctic Mountains and its implications for the Beardmoreorogeny.

Storey, B.C., and I.W.D. Dalziel. 1987. Outline of the structural andtectonic history of the Ellsworth Mountains-Thiel Mountains Ridge,West Antarctica. In D.H. Elliot, S.M. Haban, and G.D. McKenzie(Eds.), Gondwana Six: Structure, tectonics, and geophysics, (GeophysicalMonograph 40). Washington, D.C.: American Geophysical Union.

Storey, B.C., et al. In press a. West Antarctica in Gondwanaland: Crust-al blocks, reconstruction and break-up processes. Tectonophysics.

Storey, B.C., et al. In press b. Within plate granites in West Antarcticaand their bearing on the break-up of Gondwanaland.

Vennum, W.R., and B.C. Storey. 1987a. Petrology, geochemistry andtectonic setting of granitic rocks from the Ellsworth-Whitmore Moun-tains crustal block and Thiel Mountains, West Antarctica. In D.H.Elliot, S.M. Haban, and G.D. McKenzie, (Eds.), Gondwana Six: Struc-ture, tectonics, and geophysics, (Geophysical Monograph 40). Wash-ington, D.C.: American Geophysical Union.

Vennum, W.R., and B.C. Storey. 1987b. Correlation of gabbroic anddiabasic rocks from the Ellsworth Mountains, Hart Hills, and ThielMountains, West Antarctica. In D.H. Elliot, S.M. Haban, and G.D.McKenzie, (Eds.), Gondwana Six: Structure, tectonics, and geophysics,(Geophysical Monograph 40). Washington, D.C.: AmericanGeophysical Union.

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