zoback et al 1993 stresses in the lithosphere and sedimentary basin formation

8/13/2019 Zoback Et Al 1993 Stresses in the Lithosphere and Sedimentary Basin Formation

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Tectonophysics, 226 (1993) 1-13Elsevier Science Publishers B.V., Amsterdam

Stresses in the lithosphere and sedimentary basin formationM.D. Zoback a, R.A. Stephenson b, S. Cloetingh b, B.T. Larsen , B. Van Hoorn d,

A. Robinson e, F. Horvath f, C. Puigdefabregas B and Z. Ben-Avraham haStanford Uni uersit y, Stanford, USA

b Vrije Univ ersit eit , Amsterdam, The Netherlands Norsk Hydro, Oslo, Norw ay

d Shell I nternat ional , The Hague, The Netherla ndse Brit ish Petr oleum, London, Uni ted Ki ngdom

f Eotv os Uni versit y, Budapest, Hungaryg Geological Survey of Cataluny a, Barcelona, Spain

h University of Tel Av iv, Tel Aviv , Israel(Received October 20, 1992; revised version accepted June 15, 1993)

ABSTRACT

Seven fundamental questions on the relationship of lithospheric stresses and sedimentary basin origin and evolution areposed and elaborated. These are: (1) How is the evolution of sedimentary basins tied to lithospheric dynamics? (2) What arethe sources of stress that lead to formation of sedimentary basins? (3) How does the interaction between internally andexternally applied stresses affect the formation and evolution of sedimentary basins? (4) How do space and time variationsof rheology and pore pressure affect stress transmission, strain, vertical motions, and stratigraphy during basin evolution? (5)What factors influence spatial and temporal variations of rheology in, and around, different kinds of sedimentary basins? (6)What is the role of magmatism in basin formation and what deep-seated processes does it reflect? (7) What is therelationship between basin evolution, thermal history, and fluid flow? These fundamental questions can be investigated bydeveloping fully-dynamic models of processes at the basin and sub-basin scale. Models must incorporate or honour basinhistory and plate reconstructions; the intregration of structural and sedimentological data; direct observations of the physicalstate; and the synthesis of geological and geophysical data, including tectono/dynamic stratigraphy, petrological andgeochemical data, and geophysical data (gravity, magnetic+ heat flow, deep and shallow seismics). A number of type basins- in passive margin, foreland, rift, and strike-slip tectonic settings - suitable for modelling studies elucidating thefundamental questions are identified.

Introduction

To understand the origin and evolution ofsedimentary basins we must understand whatcontrols: (1) where and why sedimentary basinsdevelop, (2) the rates and duration of subsidenceand basin fill, (3) uplift and erosion, 4) structuralstyle and changes of structural style with time, (5)salt tectonics and shale diapirism, and (6) heattransfer, fluid flow and rock/water interaction.To quantitatively address these issues, the inter-play between lithospheric strength and stress iscritically important as an overall constraint onlithospheric deformation. The topic of litho-

spheric strength and rheology is the focus ofsection 2 of this special Tectonophysics volume.Fundamental questions

In the context of the relationship betweenlithospheric stresses and basin development weneed to address a series of fundamental ques-tions:1. How i s t he evol ut i on of sediment ary basi ns t i edto ithosphere dynamics?

Sedimentary basins provide the recorder forthe spatial and temporal evolution of the litho-0040-1951/93/ 06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved


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M D ZOHA K I I Al.

sphere. The formation of these basins reflects thelarge-scale tectonic processes operating in theplates as well as plate interactions (Ziegler, 1992).For example, the timing of initiation and abortionof rifts occurs in many cases in intervals too shortto be explained by thermal processes in theathenosphere/ lithosphere system, suggesting astrong control exerted by forces operating on thelithosphere plates themselves (Ziegler, 1992). Ex-amples include the abandonment of the Labradorspreading centre in conjunction with a plate tec-tonic reorganisation of the stress regimes as aresult of plate interactions. The formation ofMesozoic extensional basins in Central Africa(Ziegler, 1992) provides another example of basinformation controlled by the large-scale plate tec-tonic evolution of the opening Atlantic Ocean.The timing of inversion of these basins in LateCretaceous times also seems to reflect plate in-teraction events with an enhanced mechanicalcoupling of the African plate and the Eurasianplate (e.g., Van der Meer and Cloetingh, 1993-thisvolume). Similar patterns have been establishedfor the inversion events in northwest Europe,following closely the changes in plate motionsand reflecting the transmission of far-field com-pressional stresses induced in the Alpine collisionzones (Ziegler, 1990).

In other cases, e.g., the Tertiary East Africanrifts, other processes such as thermal perturba-tions in the upper mantle must have provided thedriving force for extensional basin formation (e.g.,Shudofsky et al., 1987). Rifted basin formation bygravitational collapse of overthickened crust (De-wey, 1988) is another example of a basin forma-tion process not directly linked to the plate mo-tions. Interactions with superimposed far-fieldstresses induced by plate motions may, however,strongly influence the effectiveness of crustal col-lapse as a basin formation mechanism. As widelyrealized (e.g., Bois, 1993-this volume), it is often acombination of these mechanisms that providesthe unique signature of the individual basin.

Many questions still need to be answered aboutthe precise nature of the interplay of the stressesoperating on the lithosphere and the mechanicalstructure of the lithosphere. We believe interplay

can strongly influence the location, timing, andstyle of basin formation.2. What are the sources of stress that lead toform ati on qf sedi ment ary basin s

The sources of stress in the lithosphere can besubdivided into several categories defined below.These stresses are by their nature of variablemagnitude, ranging from several tens to as muchas a few hundred MPa, comparable to the strengthof the lithosphere. Stress concentration by geo-metrical focusing or local weakness zones can bea crucial factor in magnifying the level of thestresses to that comparable to lithosphericstrength.The question of whether lithospheric stressesremain relatively constant over time, as has beensuggested for the Proterozoic Australian litho-sphere, or varies episodically, is of key interestfor understanding the dynamics of basin forma-tion. Recent studies of large-scale plate tectonicprocesses complemented by structural geologicalfield studies on a much smaller scale havedemonstrated that in some places, lithosphericstresses undergo important temporal changes bothin orientation and in magnitude. In particular,studies of the northwest European and Mediter-ranean stress fields have revealed temporalchanges on a characteristic time interval of 2-5Ma (Philip, 1987).

Sources of stress in the lithosphere can becharacterized as follows (see for recent reviews,Whitmarsh et al., 1991; Zoback, 1992):

1) Loads applied at the upper surface of thelithosphere: (a) The weight of deposited sedi-ments (or the negative weight of eroded materi-als). (b) The weight of water/ice (at the surfaceor in pore space). (c> The weight of thrusted/displaced rock. (d) The weight of extruded rock.(e> Variations in topography of the upper surface.

(2) Stresses due to plate driving forces re-lated to: (a) Ridge push and slab pull. (b) Basaltractions from sub-lithospheric mantle convec-tion.

(3) Stresses due to Earths gravitational loads(i.e., body forces) associated with: (a) The weight


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STRESSES IN THE LITHOSPHERE ND SEDIMENT RY B SIN FORM TION 3

of overburden. (b) Lateral variations in densityand porosity. (c> Thermal loads from lateral tem-perature heterogeneities in the lithosphere andcanvective processes such as whole mantle plumesand/or thermal instabilities at the base of thelithosphere.

(4) Various stresses related to bending mo-ments in the lithosphere which include: (a) Flexu-ral stresses. (b> Membrane stresses related to thecurvature of the Earths surface. (c) Stresses fromthermal bending moments related to differentialcooling of a rheologically-layered lithosphere.

It should be noted that what is also of interestis the temporal changes in the stresses leading tothe dynamic evolution of basins. Applied forceschange because of changes in plate motion, con-ductive and convective cooling, sedimentation anddeposition, sea-level changes, folding and thrust-ing, etc. Important changes in the stress state alsooccur as a result of stress relaxation over time.Such relaxation, whether accompanying brittle orductile deformation, can lead to amplification ofstresses elsewhere in the lithosphere.3. How does t he int eracti on betw een i nternally andext ernal l y appl i ed st resses affect t he format i on andevol ut i on of sediment ary basi ns?

Time-varying interactions of internally and ex-ternally applied stresses are probably of key im-portance in the explanation of migrating rift axesof extensional basins (Cloetingh et al., 1992a;Pedersen, 1992) as well as migrating depocentresin the foredeeps of foreland fold-and-thrust belts(Meulenkamp et al., 1993). The actual magnitudeand the depth distribution of internally appliedbody forces and externally derived far-field tec-tonic stresses depends on the rheological layeringof the lithosphere. Stress concentrations causedby lateral variations in lithospheric rheology(Cloetingh and Banda, 1992) affect the spatialwavelengths involved in basin formation pro-cesses. This applies both to extensional basins(e.g., Lobkovsky and Kerchman, 1991) as well asto basins in a compressional setting (e.g., Burovand Diament, 1991; Burov et al., 1993-this vol-ume; Nikishin et al., 1993-this volume).Key areas to investigate the interaction of ex-

ternally and internally applied forces in conjunc-tion with rheological models are the Betic/Alboran Sea system and the Carpathian/Pannonian basins. For both areas an intensivedebate is going on regarding the relative impor-tance of internal stresses and body forces associ-ated with gravitational collapse versus far-fieldtectonic stresses induced by changes in plate con-figuration and subduction zone dynamics. Bothareas form sites of extensional basin fo~ation ina regime of overall convergence with a strongcontrol exerted by the dynamics of the downgoingAfrican lithosphere imaged by seismic tomogra-phy (Wortel and Spakman, 1992). In these sys-tems, a flexural coupling of foreland flexure in-duced by thrust sheets of the Carpathian arc andthe Betic nappes occurs with shoulder uplift ofthe ajacent Pannonian and Alboran extensionalbasins (e.g., Cloetingh et al., 1992b; Morley,1993-this volume), reflecting lateral variations inthe mode of extension along strike of the Beticand Carpathian arcs.4. How do space and time variations of rheologyand pore pressure a ff ect str ess t ransmissi on, stra i n,vert ical moti ons and str ati graphy duri ng basin evo-lution?

The manner in which lithospheric stress isdistributed throughout a basin is a function of therheological properties of the basin and the sur-rounding crust. Within the brittle domain, themagnitude of pore pressure establishes a funda-mental control on the strength of the crust andchanges of pore pressure with time and space willhave a profound effect on basin evolution. Animportant example of why is related to the obser-vation that many basins are pervasively overpres-sured at depth (i.e., the pore pressure is essen-tially equal to the magnitude of the overburdenstress). In such cases the strength of the rock isessentially zero; compressional deformation, oreven basin inversion, can be induced by relativelyminor changes of applied stresses, and, as region-ally applied stresses could not be transmittedthrough the overpressured sections of the basins,they would be concentrated in stronger sectionsof the crust. Perhaps more importantly, apphca-


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tion of compressive stress will generate apprecia-ble fluid flow as pore pressure increases resultingfrom pore volume compression will result in natu-ral hydraulic fracturing and fluid escape.

At greater depths, within the semi-brittle orductile regimes of the crust, temperature, strainrate and rock type all effect the creep propertiesof the crust. Within this deformational domainthe therma evolution and rate of crustal defor-mation will profoundly effect the rate of subsi-dence, lower crustal strain, and flank uphfts, allof which reflect the interaction of lithosphericflow, isostasy and flexure.

The extremely low strength and viscosity ofsalt and some overpressured shales also representimportant examples of the manner in which rockrheology affects the evolution of deformation ofsedimentary basins. Low-angle normal faultingand detachments frequently occur along salt andshale layers, providing an important inffuence onthe style of defo~ation within basins.5. W hat factor s ~n~ue~ce spati al and t emporalvari ati ons of rheology i n, and around, diff erentki nds qf sediment ary basi ns?

The basic factors affecting rheology are tem-perature, lithology (perhaps more correctly, min-eralogy, including the state of hydration or dehy-dration), the state of stress, porosity, chemicalcomposition of pore fluids, and the possible pres-ence of melts. These factors change with time,especially as the result of vertical displacementsproducing temperature/ pressure and stress-re-lated effects.

Spatial variations of rheological properties arelikely to be extremely important in controlling thelocalization and style of deformation. Of course,the primary control on spatial variations of rheo-logical properties are the compositional changesassociated with the geologicai evolution of anarea. A widely observed geological phenomenonis the tendency for Iocalized deformation to re-peatedly occur in certain places over geologictime, often with different structural styles. Forexample, both the New Madrid rift and RhineGrabens are Paleozoic intraplate extensionalstructures that are currently being compression-

M.D. ZOBACK t-f I

ally reactivated with the predominant style ofcontemporary faulting in both areas being strike-slip.6. What i s the role of magmati sm in basin forma-t i on and w hat deep-seat ed processes does it refl ect?

Volcanism is rare inside actively forming sedi-mentary basins. But magmatism can occur asintrusions (dykes, sills and plutons) at differentdepths, at different locations, and at specific timesduring development of basins, When magmatismoccurs, however, it gives important informationabout the relationship between heat, magmapressure and the development of stresses in thebasin. This knowledge is only rarely utilized whenstudying the geological history of basins.

Anderson (1936, 1938) set the stage for discus-sion of the relationship between lithosphericstresses and magmatism. Roberts (19701, Naka-mura (19771, Nakamura and Uyeda (19801,Zoback and Zoback ~198~~, Francis (1982) andparticularly Shaw (1980) contributed significantlyto the topic, and presented a wide span of exam-ples. Pollard and Muller (1976) and Muller andPollard (1977) contributed to the problems con-cerning details in the stresses related to intrusionof dykes.

Three aspects in the relationship betweenmagmatism and stress in sedimentary basins seemto be important.

(1) Volcanism and magmatism are rare in sedi-mentary basins, but occur most frequently in riftbasins, and also in strike-slip related pull-apartbasin. Other types of sedimentary basins lackmagmatism related to the formation of the basin.Most likely this points directly to the fact thatstresses in rift basins are extensional, giving possi-bilities to the magma to ascend through the litho-sphere.

(2) It leads to that volcanic alignments, anddyke and sill intrusions, are excellent paleo-stressindicators, if occurring regionaily (Zoback andZoback, 1980). Stress directions, stress regime(extensional or compressional) can directly bededuced from the direction of sheeted intrusions.We presume that large dykes and sills propagatesuch that c7 is perpendicular to the dyke. Also,


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STRESSES IN THE LITHOSPHERE AND SEDIMENTARY BASIN FORMATION 5

the relative ratio between the magma pressureand the least principal stress a, can be deduced,and to ascend into a fracture the magma pressuremust be greater. The time of magmatism relativeto the formation of sediments in a basin mostlikely represents the peak in the tectonic episodes(stress) and also represents an important heatinflux event.

(3) It follows further that magmatism in itselfcan be a stress modifier in a basin (Parsons andThompson, 1993) and can rapidly change anddiminish the basis for the regional transmission oflithospheric stresses in an efastic plate by chang-ing its rheology and strength. Local ascending oflarge volumes of hot magma into a shallower partof the lithosphere locally alters the rheology andcreates thermal stresses. Also when the magmabodies cool the local stresses will change. Oftenthese effects are local and the dimension of localstresses is directly related to the dimension of theactual magmatism. Highly volcanic areas withthousands of km3 lava are more affected thanother areas.

~agmatism is dependent on the partial melt-ing in the mantle and/or the crust, and thecomposition is further dependent of the type ofmaterial melted and contaminated, and the de-gree of melting. The degree of melting is thenfurther dependent of the temperature, pressureand the composition of the parent material.Petrological studies can reveal much detail aboutthe parent material, the crustal contamination,the temperature and the degree of melting inmost magmatic provinces worldwide.

The time of melting, the amount of heat, thedensity of the magma, the magma pressure (thebuoyancy), the viscosity, the type of magmatism(extrusions in volcanoes, dykes, sills and plutonicintrusions) depends on the composition and thevolume and depth of the magma, but also verymuch on the conditions in the host lithosphere.Of these physical extra-magma-body parameters,stresses in the lithosphere are most likely one ofthe main factors. The fact that magmatism ismore frequent in rifted basins is direct evidenceof stresses being a major necessary factor forshallow intrusions and extrusions in sedimentarybasins, and that extensional stress regimes leads

to volcanism whereas compressional ones do not.Sill intrusions are more likely to form in compres-sional regimes. If so, basins where there are sillsin the margin and extrusions in the centre, mostlikely reflect the details in the lateral stresschanges across the basin at the time of magma-tism. Observations in some rifted basins are thatthe extrusives often occur aligned with the centralaxis of the basin, and sills along the margins.Early Cretaceous magmatism at Svalbard andFranz Josephs land, and the volcanic rifted mar-gin of mid-Norway are examples of this.

In the Oslo Graben the sills predate the timeof greatest volcanic activity, consistent with asetting of increasing extensional stress, with highmagma pressure (Sundvoll et al., 1992). Also inthe Oslo Graben, the large extrusions of plateaulavas mostly predate the major normal faultingactivity. This is an example of magma pressuredeclining as the extensional stress is still active.

In an area like the Faero islands sills occurinside a huge pile of plateau basalts. This can beinterpreted as recording a decreasing extensionalstress as the magma pressure is still kept high,such as when the continental margin changesfrom rifting to drifting.In a number of rift related basins the followingfeatures are observed:

(1) Extrusives often occur aligned to the cen-tral axis of the basin.

(2) Sill intrusions occur along the margins ofbasins.

(3) Vertical dykes often, but not always, occurin the centre of the basin.

(4) Vertical dykes occur in swarms and areparallel.

(5) Sill intrusions often predate and sometimespostdate extrusions,

(6) Both low-density (felsic) dykes and high-density sills are intruded at the same time and atthe same stratigraphic depth.

(7) Sills occur less frequently and at deeperstratigraphic levels away from the aligned vol-canic extrusion centres if they occur.

(8) Plutonic intrusions often postdate extru-sions in the same magmatic province.

(9) Plutonic intrusions often are more silicicthan the extrusions (often basalts).


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(10) Volcanism and dyke intrusions often pre-date strong normal fault episodes in a rift.

11) In some cases the width of the magmaticprovince is wider than the rift structure.

Many of these observations are directly relatedto the thermal history of the rifted basin. Butmany of the observations cannot fully be ex-plained by this alone. Can the stress history ofthe specific basin be the other main cause thatcan explain what cannot be explained by temper-ature and amount of heat alone?

More examples of worldwide relationships withmagmatism and stress in rifted basins, pull-apartbasins and passive margins are needed. Fre-quently, the use of magmatism as stress indicatoris overlooked by geologists working in such areas.It should be noted that magmatism is in generaleasily datable and can also provide importantadditional information on the heat budget.

7. What is the relationship between basin ersolutionthermal history and fluid flow?

In any model of basin evolution heat plays akey role. The idea of lithospheric stretching, ei-ther uniform or non-uniform, implies that attenu-ation of the crust and mantle lithosphere pro-vides a major heat impulse into crustal dynamics.After termination of active rifting, basin evolu-tion is controlled by thermal contraction of thelithosphere. The stratigraphic record suggests.however, that this simple process of basin evolu-tion can be strongly modified by other forces likevariation of intraplate stress (Cloetingh and Kooi,1992; Kooi and Cloetingh, 1992; Van Balen et al.,1993).

Heat transfer in sedimentary basins is a com-plex process and varies strongly in space andtime. There are two mechanisms which should betaken into consideration: thermal conduction andconvection. Most models of basin evolution ne-glect thermal convection and reconstruct thermalhistory by assuming a purely conductive tempera-ture field. Even in this case the following factorsshould be taken into account:

(1) The variation of background (i.e., base-ment) heat flow,

(2) Thermal blanketing of fast sedimentation,and

(3) Change of thermal conductivity of differentlithologies with progressive compaction.

Also, the conductive thermal field is oftenstrongly overprinted by fluid flow systems in sedi-mentary basins. Fluid flow in sedimentary basinsis controlled (e.g., Burrus and Audebert, lY91) bya combination of basin development and sedi-mentation through the agency of three main pro-cesses:

(1) Expulsion of pore fluids by compaction ofsediments (including overpressured zones),

(2) Gravity instability created by temperaturedifferences in permeable rocks, and

(3) Hydraulic head differences created by vari-ations in the height of the water table.Sedimentary processes, themselves strongly in-

fluenced by tectonics, largely determine perme-ability distribution and hence exert the initialcontrol on the location of overpressured zonesand permeable pathways. Basin tectonics mayaffect fluid flow in a number of ways (e.g.. VanBalen and Cloetingh, 1993). Rates of subsidenceexert the second control on overpressuring in finegrained rocks and determine the pattern of fluidexpulsion along more permeable sedimentaryunits. Faulting will affect flow paths by creatingimpermeable barriers or permeable conduitswhich did not exist in the unfaulted sequence andmay also release overpressures. Margin uplift anderosion will subject portions of basins to artesianflow of meteoric water. Although these featuresof the evolution of sedimentary basins will have amajor influence on fluid flow. the details of howthey affect flow rates and paths remain poorlyunderstood.

The problem may be approached from twocomplementary directions:

(1) Analysis of the consequences of fluid lowand the products of its interaction with rockssuch as mineral precipitation, may be timed andmapped and thus related to basin development.

(2) Modelling the responses of fluid low tosignificant features of basin development.

Cementation of elastic rocks appears to bemainly a time- rather than a depth-related pro-cess in many basins and is likely to reflect a


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significant episode of fluid flow late in basinhistory. Such an episode must itself be a reflec-tion of a change in basin development. Progresstowards understanding the nature of these links ismost likely to come from empirically establishingtemporal connections between sediment diagene-sis and features of basin evolution in a number ofareas and then by modelling the effects thesephenomena might have on fluid flow.Required activities

Fully-dynamic models at basin and sub-basinscale are required to address these questions.Numerical modelling studies have demonstratedthe key role intraplate stresses play in shaping thestratigraphic record of sedimentary basins.Changes in stress level in the post-basin forma-tion stage induce rapid changes in subsidence andproduce differential vertical motions within sedi-mentary basins. These stress-induced rapid verti-cal motions affect estimates of crustal extensionderived from the post-rift subsidence history ofrifted basins and are also of importance for di-apirism and fluid flow within the basins.

In extensional tectonic environments neckingand faulting will take place when stresses exceedlithosphere strength. Numerically modelling ofthese processes requires the integration of theinterplay of the various factors that control therecord of rifting-induced vertical motions. This iseven more true on a somewhat longer time-scale,dependent on, for example, the rheological layer-ing and the formation of decoupling zones,Strengthening and healing of the lithospheremight take place during its cooling. Numericalmodelling has also demonstrated the role playedby the depth of necking in the process of basinformation. Future work must address the interre-lation of lateral variations in rheology and thevariation in depth of necking (Kooi et al., 1992).

In a compressional tectonic setting, the effectof stress on foreland basin flexure can be impor-tant in particular when stresses are building up toa threshold at which thrusting takes place (Peperet al., 1992). Future numerical modelling shouldfocus on the interplay between short wavelengthbrittle tectonics and long wavelength flexural dy-

namics of the underlying lithosphere. The effectof stresses may be quite important in modulatingand enhancing the magnitude of the flexural pe-ripheral foreland bulges and might also affect theflexural coupling between foreland basins andneighbouring.These models require attention to the follow-ing four themes:1. Basin fist ed and pl at e reconst ~cti ~ns

It is generally accepted that plate tectonic pro-cesses have determined the evolution of the Earththroughout Phanerozoic times and possibly alsoduring the Proterozic. These processes involvecontinental extension and the opening of newoceanic basins, as well as the subduction ofoceanic lithosphere and the collision of continen-tal cratons leading to the formation of erogenicbelts. As a result of this continuous history ofplate movements and plate interactions, geody-namic processes dete~ining the origin and evo-lution of sedimentary basins have changedthrough time and space. These changes may bereflected in the nature of the stratigraphic recordand in distinct phases of basin deformation. It isimportant, therefore, to document and predictthe spatial relations of plate interactions andassociated intraplate paleo-stresses using com-puter programmes that permit the construction ofglobal palinspastic maps. For this purpose usecould be made of the results of the PaleomapProject, a joint IUGG-IUGS programme whosegoal is to produce a plate tectonic and paleogeo-graphic synthesis of the Earth during thePhanerozoic, both through publications and bydevelopment of appropriate software packages.2. Integration of structural and sedimentologicaldata

The full in~~oration of data on the mechan-ics of thrusting and faulting in stratigraphic mod-els is important as well as the collection of dataon erosion rates and sediment transport in fore-land basins. This is a must for a better under-standing of the role of tectonics versus climate inthese basins. The fill of foreland basins is charac-


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x M D ZOBACK ET ALterized by a geometry and facies distributionwhich is continuously controlled by the structuraldevelopment of the adjacent mountain chain (e.g.,Puigdefabregas et al., 1991). Plate collision, thrustsheet emplacement, and thrust propagation aredirectly responsible for the basin geometry, strati-graphic architecture, sequential partitioning,stepwise outward displacement of depocentresand facies distribution. Further development offoreland basin models with better predictive ca-pabilities on geometry and lithofacies require databases on facies patterns and paleocurrents, theconstruction of reliable local time scales, bios-tratigraphic and paleomagneti~ data, sequencestratigraphy and construction of balanced cross-sections. Integration of these data forms an im-portant step in the construction of a time-stepmodel of the basin fill where thrust load, subsi-dence, sediment input and eustasy are incorpo-rated (e.g., Zoetemeijer et al., 1993-this volume).

Similarly, the sedimentary fill of extensionalbasins is to a large extent influenced by the typeand geometry of the basement structure. Thecontact between the syn-rift fill and the basementand the lateral extent of infilling sequences isoften fault-controlled. Sequence boundaries inextensional basins often involve erosion of thebasin margins due to uplift of rift shoulders andreliable estimates of eroded thickness are, there-fore, crucial in quantitative modelling. Integrateddata bases on seismic stratigraphy, mapping offacies distribution, mapping and dating of exten-sional faults, good time control on the existingand missing record and accurate determination ofthe sequence-stratigraphic subdivision of the basinfill is required.3. Di rect observat i ons of physical stat e

Data on the state of stress, pore pressure,temperature and heat flow are critical for animproved understanding of the origin and evolu-tion of sedimentary basins in two important ways.Knowledge of the state of stress and pore pres-sure provides insight into the nature of deforma-tion and temperature and heat flow provide criti-cal information about the nature of heat transferand mechanisms driving fluid flow. These types of

data provide fundamental information for bothinput and testing of dynamic models,

In most commercially-developed sedimentarybasins (where sub-surface data from wells shouldbe widely available) data on stress, pore pressure,temperature and heat flow are available in vary-ing amount. Unfortunately, many opportunitiesto obtain more complete and better data are nottaken advantage of and relatively few integratedand complete data sets exist. A principal goal of adetailed study and modelling of sedimentarybasins should be optimization of this type ofinformation.4. Synt hesis of geol ogical and geophysical dat a

1) Tectono /dynam i c strat i graphy

Regional stratigraph~c analyses. IvIodern tech-niques like sequence stratigraphy have provided abetter genetic understanding of the evolution ofbasin fill. The relative contributions of subsidencerate, eustacy and sediment supply can be studiedby a careful analysis of regional seismic lines, welllogs and outcrop data. These techniques enable amore precise differentiation between rheologicalbehaviour of the Earths crust and its response toextensional and compressional processes, thermalchanges, the imposition of sedimentary loading,and other factors influencing the nature of thestratigraphic record.

(2) Pet rol ogical and geochemi cal dat aAs exploration and production activities

progress in a sedimentary basins, geophysical welllogs, cores and cuttings and interpretation ofseismic reflection profiles lead to an appreciableset of data on lithology and physical properties ofmany reservoir rocks. As part of detailed studiesof sedimentary basins, it is crucial to integrateand synthesize these data for input into realisticmodels.

3) Geophysical dat a gravi t y, magnet i cs, heatfl ow , deep and shall ow sei smics)

Geophysical data are the primary source ofdetermining the structure and properties of thepart of the lithosphere which lies beyond drilling


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STRESSES IN THE LITHOSPHERE AND SEDIMENTARY 3AStN FORMATION v

depth. The geometry and interrelationships ofstructures at depth, within sedimentary basinsand below them, and their fundamental physicalproperties, seismic wave velocities and attenua-tion, density, magnetic susceptibil~, gross texture(in terms of seismically-imaged fabric) can besignificantly constrained by geophysical data.

In terms of imaging the lithosphere, crustalgeophysical methods are generally limited to di-rect methods such as seismic reflection profilingand indirect methods such as gravity and aero-magnetics, magnetotellurics, and seismic refrac-tion profiling. Reflection profiling allows a directrepresentation of underlying structure (albeit withsome distortion and limited resolution of steeplydipping structures). The indirect methods requirea phase of modelling prior to their geologicalinterpretation. While reflection profiling providesthe most direct information of geometrical rela-tionships, the indirect methods provide supe-rior information on the physical properties of thecrust and upper mantle. New methods, involvingseismic tomography, attenuation and shear wavestudies, are rapidly evolving and will also provideimportant information on physical properties thatare relevant to the rheology of the lithosphere.Key areas /natural laboratories

For the quantification of the effects of litho-spheric stresses on basin evolution a number ofareas have been selected for integrated modellingand basin analysis focusing both on the basin filland the underlying lithosphere.1. Passive mar g~ basins

Norw egian and Greenl and margi nsThe refection seismic database of the Mid

Norway margin is large and of excellent quality,and more than 60 exploration wells have beendrilled. The Mid Norway margin extends alongstrike for about 1100 km, and is divided intoseveral very different parts: the More basin, theVoring basin, the Halten terrace, the TrondelagPlatform, and the Lofoten area. Generally theMid Norway margin evolved in a long-lived exten-sional stress setting with two distinct rift episodes,

one in the Permo-Carboniferous, and the next inthe Late Jurassic-earliest Cretaceous. The differ-ent parts of the margin were developed differ-ently, and the long and pulsating extensional tec-tonism ended in rifting and breakup in earliestEocene, From then on the passive margin wasformed, and the stress regime most likely changedto weakly compressive. The margin is a volcanicpassive continental margin, and is today one ofthe type examples of such a margin (Skogseid andEidholm, 1989). Sill intrusions can be seen in theseismics east of the volcanic high, and these sillsintrude deeper to the east, reflecting a lateralchange in stresses. Both folding and doming, andlandward uplift and seaward tilting are excel-lently expressed, and the diversity makes it agood area for modelling studies. A strong cou-pling seems to exist between the lithosphere andcrustal deformation, with melting occurring overalmost the entire width of the rift, more than 300km, taking also the northeastern Greenland mar-gin into account (Skogsheid et al., 1992). A num-ber of studies on the Norwegian margin carriedout by research teams participating in the work ofthe Task Force Origin of Sedimentary Basinswill be published in a forthcoming issue ofTectonophysics (Cloetingh et al., submitted).

Western M edi t erranean ~t ens~onal basi ns andt he Pannokan Basin

The western Mediteranean extensional basinsand the Pannonian Basin offer unique naturallaboratories to study the dynamics of extension ina regime of overall convergence of lithosphericplates. Excellent geological and geophysical datasets exist both for the Gulf de Lions margin (e.g.,Burrus and Audebert, 1991), the Valencia Trough(Banda and Santanach, 1992) and the AlboranSea (Maldonado, 1992). Phases of rapid subsi-dence have been documented, deviating frompredictions from stretching models (e.g., Burruset al., 1987) as well as important structural con-trol on the scale of subbasins (e.g., Maldonado,1992). In conjunction to these studies of westernMediterranean extensional basins, a parallel pro-ject has been initiated on the dynamical mod-elling of the Pannonian Basin building on previ-ous work (see for a summary, Royden and Hor-


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1 M D ZOBACK ET Al

vath, 1988) and on recently collected data pre-sented in the papers on the Pannonian Basin andsurrounding areas published in the present vol-ume. Recent evidence (e.g., Horvath and Cloet-ingh, 1993) exists for anomalous subsidence anduplift patterns in the Pannonian basin, possiblyreflecting changes in the stress regime.2. Foreland basins

Pyr enean and Bet i c foreland basi nsThe western Mediteranean Pyrenean and Betic

foreland basins are well suited for the testing anddevelopment of predictive models linking basinformation processes to basin structure and basinfill, using high-quali~ stratigraphy and extensivegeophysical data sets as constraints.

The Pyrenean foreland basin has been studiedin detail by deep seismic reflection profiling car-ried out in the framework of the ECORSroject.As a result, good control is available on thedeeper structure while intensive field work(Puigdefabregas et al., 1991) has resulted in firmconstraints on the timing of the tectono-strati-graphic evolution and the geometry of the near-surface structural configuration. The quality ofthe Pyrenean foreland has recently led to theselection of this basin as the site for a Penrosemeeting on foreland basins of the GeologicalSociety of America.

Refraction and reflection seismic lines and asubstantial geological data base are available toconstrain models of the Betic foreland (Peper etal., 1992).

The Pyrenean and Betic foreland basins,formed at the sites of preexisting rift basins withlow rigidities (~etemeijer et al,, 1990; Desegaulxet al,, 1991; Van der Beek and Cloetingh, 19921,offer unique prospects to quantify the interplayof stresses with thermo-mechanical effects of therifting-inherited lithospheric heterogeneity offoreland basin evolution, At the same time, thelocation of these foreland basins adjacent to off-shore extensional basins in the Valencia Troughand Alboran Sea allows the quantification of theeffects of coeval flexural interaction betweenforeland basins and rifts (Cloetingh et al., 1992)on basin stratigraphy.

M olasse BasinThe Molasse Basin is a classical Cenozoic fore-

land basin which extends from France throughSwitzerland, Germany to Austria, a distance ofapproximately 700 km. In this basin the thicknessof the Cenozoic series ranges from a few metresalong its northern margin to over 3000 m alongalong the Alpine deformation front. In easternSwitzerland, Bavaria and Austria the basin isrealtively undisturbed and rests upon a Meso-zoic/ Early Cenozoic authochtonous forelandsubstratum. In western Switzerland, however,Alpine deformation has extended into the Jurausing a Triassic salt layer as a decollement sur-face, thereby detaching the basin from its crys-talline basement. The Molasse Basin represents amajor zone of subsidence linked to the adjacentorogen through loading and iithospheric fl exuringduring the Oligocene and Early Miocene. Hun-dreds of wells and dense grids of reflection seis-mic, coupled to a number of numerical modellingstudies, provide a unique background to furtherinvestigations of its structural and statigraphicevolution.

Alberta BasinThis basin is considered a classic foreland basin

that formed in Mesozoic time in response toempIacement of massive thrust sheets comprisingthe Rocky Mountains. Extensive drilling and de-velopment of this basin provide an excellent database on the structure and evolutional history ofthis basin as well as the current state of stress(Adams and Bell, 1991).

Central A ustr ali an BasinsCentral Austraha is the site of a series ofProterozoic-Paleozoic sedimental basins with

intervening basement complexes, which togetherproduce some of the largest gravity anomaliesseen on the continents. Deep seismic reflectionprofiling and teleseismic travel-time studies, inconjunction with the exposed geology and thegravity field, indicate that whole crustal faulting,possibly with large-scale folding as a precursor,has significantly displaced the Moho. These struc-tures have apparently existed since around theend of the Paleozoic, when the last major oro-


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genie event took place. The creation of suchstructures, as well as, very importantly, their per-severance and apparent resistence to relaxationhas obvious implications for crustal rheology andstresses.3. Rifted basins

North SeaThe North Sea is a Mesozoic rift system which

came into existence during the Early Triassic buthad its main phase of extension during the LateJurassic-earliest Cretaceous. Crustal extensiondecayed subsequently and terminated altogetherduring the Paleocene when a broad thermal sagbasin developed. The North Sea is unique interms of its data availability. Its geological evolu-tion is well constrained by thousands of wells,reflection and refraction seismic, and gravity datawhich provide information for quantitative analy-sis of subsidence, crustal thickness, stratigraphicresponse and structural deformation. One of themajor unresolved problems is the important dis-crepancy between upper- and lower-crustal atten-uation accros the rift which may suggest destabi-lization and upward displacement of the Mohodiscontinuity during rifting (Ziegler, 1992).

Sverdrup BasinThis is a major, thick, rift basin that has sub-

sided in a fairly straight-forward way during muchof the Mesozoic with the initiating rifting phaseoccurring in the Carboniferous and Permian. Thehigh-quality geophysical and subsurface databasesare completely available and easily accessible.The geology is well-exposed in the east where ithas been uplifted and mildly deformed during theEocene Eurekan Orogeny.

There are several unique geological eventsduring the evolution of the Sverdrup Basin thatare fairly well documented geologically and arevery well-suited to fully dynamic modelling exer-cises. These include (1) what appears to be anexample of elastic strain recovery immediately atthe end of the syn-rift episode, resulting in mildcompressional deformations (Stephenson et al.,1992) in the syn-rift depositional sequences alongsome of the basin margins (perhaps depending on

orientation to the prevailing stress field), and 2)what appears to be a rapid folding of the entirecrustal section, with a wavelength of 200 km,during the intracontinental shortening associatedwith the Eurekan Orogeny (Stephenson et al.,1990). Additionally, the geometry and geologicalrelationships of third-order stratigraphic cyclesare very well-documented and this could be anexcellent place to test competing ideas about theorigin of third-order cycles in basins (Embry,1990).

4. St ri ke-sl i p rel at ed basins

San Andreas faul t syst emCenozoic sedimentary basins along the San

Andreas fault system were formed in a transformenvironment in response to the divergent relativemotion between the Pacific plate and NorthAmerica and the low frictional strength of thetransform plate boundary @back and Zoback,1991). The transtensional deformation resultingfrom this divergent motion abruptly ended 4 Mawhen a change in Pacific plate motion occurredand there was an abrupt onset of convergentrelative motion and transpressional deformation.Uplift and compressional deformation is cur-rently observed throughout these basins and it isgenerally correct to say that these are currentlybeing inverted (Ben-Avraham and Zoback, 1992).

Factors which make these basins appropriatefor detailed study include the facts that the platetectonic setting is so well known and there isextensive seismic profiling and other geophysicaldata available. Similar projects are possible alongthe Dead Sea, El-Pilar, Sumatra, and otheronce-divergent transform plate boundaries.

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