Proc. Natl. Acad. Sci. USAVol. 76, No. 9, pp. 4201-4207, September 1979Geology

Structural geology of the Earth's exterior*(plate tectonics/lithosphere/asthenosphere/mantle/crustal deformation)

B. C. BURCHFIELMassachusetts Institute of Technology, Cambridge, Massachusetts 02139

Communicated by Preston Cloud, June 14, 1979

ABSTRACT Plate tectonics offers an explanation for thepresent motions and heterogeneity of the rocks that form theexternal part of the Earth. It explains the origin of the first-orderheterogeneity of oceanic and continental lithospheres. Fur-thermore, it explains the youth and simplicity of the oceaniclithosphere and offers the potential to explain the antiquity,complexity, and evolution of the continental lithosphere. Theframework of plate tectonics must be used carefully, becausethere are geological features within continents, particularly inthe more ancient rocks, that may require alternative explana-tions. The task of understanding lithospheric motions throughgeologic time must be focused on the continents, where themajor evidence for 95% of Earth history resides.

In interpreting earth motions from the geologic record, threeneeds seem paramount: (i) to develop a three-dimensional un-derstanding of the kinematics, dynamics, and thermal structureof modern plate boundary systems and at the same time to rec-ognize those geological and geophysical features that are un-related to plate interaction; (ii) to use this understanding toreconstruct the extent and evolution of ancient systems that formthe major elements of continental crust; and (iii) to determinethe dynamics and evolution of systems that have no modernanalogs. Decoupling along subhorizontal zones within thelithosphere may be widespread in all types of plate boundarysystems. Thus, in order to interpret the motion and dynamicsof the mantle correctly, it is important to know if upper litho-spheric motion within boundary systems is controlled directlyor indirectly by or is independent of deeper mantle motions.

THE PLATE TECTONIC FRAMEWORKThe rock masses that form the external part of the Earth are inrapid motion. This motion is so rapid that 65-70% of the Earth'scrust has been recycled into the Earth's mantle in the last 200million years, a period that represents only 5% of recordedEarth history. The most external part of the Earth involved inthese motions is the lithosphere, a relatively rigid outer skin ofthe Earth that consists of both continental and oceanic crust anda variable thickness of upper mantle. Oceanic and continentallithospheres make up the first-order heterogeneity of the ex-ternal part of the Earth and the differences between them ex-tend to depths of at least 200 km. The thickness of lithosphereis largely thermally controlled. Its base is marked by a zone ofshear at a transition from more rigid rocks above to more ductileand rapidly flowing rocks of the asthenosphere below.The asthenosphere extends to a depth of about 250 km and

is generally well defined in areas underlaid by oceanic crust.In oceanic regions, it begins only a few kilometers below thesurface at ridges that mark the site of sea-floor spreading anddeepens away from these ridges to about 100 km in depth be-neath the broad abyssal plains. Beneath continental crust, theasthenosphere is poorly defined, particularly beneath the moreancient parts of continents. In areas of modern continentalvolcanism, the top of the asthenosphere may be only a few tens

of kilometers deep. Estimates of depth to the asthenospherebeneath the more ancient parts of continents range from 200km to more than 400 km, and some workers have even doubtedits presence.

Data derived largely from the oceans show that the presentlithosphere consists of six large and five or six smaller movingplates. These plates include large areas of relatively undeformedlithosphere bounded by narrow interconnecting zones of de-formation marked by earthquake activity. Geophysical studiesshow three types of plate boundaries: (i) divergent, (ii) trans-form, and (iii) convergent. Along the first type of boundary,plate motion is horizontal and divergent. Within oceanic areas,new oceanic lithosphere is created along divergent boundariesat rates of up to 20 cm/yr. Within continents, these boundariesform zones of extensional rifting which, if continued, wouldlead to separation of continental fragments and formation ofnew oceanic crust between them. Along transform boundaries,lithospheric plates slide past each other horizontally. At con-vergent boundaries, one lithospheric plate commonly movesbeneath another. Because oceanic lithosphere is more densethan the asthenosphere, it slips baqk into the asthenosphere, bythe process of subddctibn. The subduction of oceanic lithosphereleads to the recyclink of oceanic lithosphere at such a rate thatno oceanic crust iii the present oceans is older than -200 Myr.The subduction of continental lithosphere is more difficult,because continental crust is less dense than the astheno-sphere.

These lithospheric motions define a global system-platetectonics-whose basic concepts are simple. Oceanic lithosphereis formed at divergent boundaries, slides past itself alongtransform boundaries, and plunges back into the mantle atconvergent boundaries along subduction zones. Plateboundaries are the locus of deformation and plate interiors arerelatively undeformed. Plates may contain continental oroceanic lithosphere or both. The usual mode of subduction isfor oceanic lithosphere to be returned to the mantle where itis partially or wholly assimilated. Continental lithosphere, be-cause of its less dense cap of continental crust, is more difficultto subduct.The ability of plate tectonics to explain a great many features

of the external Earth today and for the last few million years,coupled with its simplicity, global character, and quantitativeaspect, has led to rapid acceptance of the model by a largesegment of the earth science community. Plate tectonics inte-grates widely diverse types of data from several branches ofscience toward solutions of many geologic problems. The birthand development of plate tectonics during the past 15 years hasaffected a major scientific revolution.The revolution has begun, but we are a long way from real-

izing its full potential. We might compare the present stage ofplate tectonics to a time shortly after the presentation of Dar-win's theory of evolution; a framework has been erected butfundamental and exciting new observational ideas will still beadded to the edifice. As with any major scientific revolution,

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* Presented at the symposium "Earth Science and Earth Resources-ACentenary Salute to the U.S. Geological Survey," 23 April 1979, atthe Annual Meeting of the National Academy of Sciences of theUnited States of America.

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a tremendous spectrum of new areas of scientific researchpresents itself-a spectrum too richly dispersed to be discussedin this limited presentation. I will focus on an approach as wellas some aspects of plate tectonics that should yield informationconcerning lithospheric motion and the relevance of platetectonics during that 95% of Earth history for which we haveno remaining evidence on the ocean floors. The data must comefrom the continents where we find the evidence from whichto reconstruct geologic events covering nearly 4 billion years

of Earth history. Three general topics will be considered: (i)plate boundaries as dynamic systems, (ii) plate tectonics relatedto continental growth and evolution, and (iii) relationship ofplate motions to deeper mantle motions.

PLATE BOUNDARIES AS DYNAMIC SYSTEMSPlate tectonics is principally a kinematic description of platemotion. It has been assumed that plates are rigid and that rel-ative motion between them is taken up along narrow zones attheir boundaries. The validity of this assumption is supportedby the fact that the relative motion at one plate boundary can

be predicted by knowing the relative motion at two or more

other plate boundaries and it can be confirmed by independentgeophysical data. It has become clear, however, that platemotions are associated with a wide range of geologic phe-nomena, sometimes extending more than 3000 km from theplate boundary. The nature of some of these phenomena clearlydemonstrates nonrigid behavior, locally of a first-order mag-nitude. Equally important is the fact that many geologic fea-tures related to plate boundary activity, often far removed fromthe boundary, are all that remain to record ancient plate tec-tonic activity.

Plate boundaries should be regarded as dynamic systemsexpressed as zones of variable width of plate interaction thatgenerate a broad spectrum of geologic phenomena. The rangeof these phenomena is still unclear. Even for those features thatcan be directly related to plate boundaries, their genetic rela-tionships to those boundaries or to other associated phenomenaare poorly understood. Thus, it is necessary to study the work-ings of modern plate boundaries as completely as possible. Oncea boundary becomes inactive, some of its characteristics thatcould have yielded important dynamic and kinematic databecome lost (either rapidly, as by an end of earthquake activity,or more slowly, as by an end of volcanism and decay of thermalregime). Furthermore, at modern plate boundaries, the geologicfeatures and phenomena generated by plate interaction are

contemporaneous-that is, they are a simultaneous expressionof the active system. Studies in some areas have shown that plateboundary configurations may change rapidly, on the order ofa few million years. Thus, establishing contemporaneity ofevents is extremely important for understanding the relation-ships among kinematics, dynamics, and thermal structurewithin the plate boundary system. As we go back in geologictime, contemporaneity of events and features becomes more

difficult to establish, and our reliability in reconstructing olderplate boundaries deteriorates. Thus, research on modern plateboundaries, the most obvious approach to understanding thesedynamic systems, is an area of continuing intense study.

It is impossible in this brief space to review the completerange of plate boundary systems that confront us today. Mostof those systems within oceanic lithosphere are ultimatelydestined for destruction by subduction, even though they mayinfluence events in regions that will be preserved. The more

important systems are those that generate geological featuresthat remain in the geologic record. A few examples can bepresented that typify the three different types of modern plateboundaries and the extent of associated geologic activity thatform the dynamic plate boundary system.

1. Divergent Plate Boundary Systems. Plate divergence ismost widely developed at oceanic spreading ridges, but wheredivergence begins within continental lithosphere, its geologicresults have the best chance to be preserved. The Afar andsurrounding regions of northeast Africa are areas of moderndivergent plate motion. They are characterized by extensionalfaulting, lithospheric thinning, alkalic volcanism, high heatflow, and a characteristic suite of sedimentary rocks. Earth-quakes are all shallow (generally 10 km or less), suggesting thatthe high heat flow has rendered the lower crust ductile. Surficialfaults and volcanic and sedimentary rocks extend for more than100 km from the boundary and can be studied at the surface.Our three-dimensional knowledge of these features, however,is poor. Brittle faults are believed to flatten at depth to mergewith horizontal ductile flow at a zone of decoupling, but con-vincing data to support this idea are lacking.

Continued divergence moves these ruptured parts of conti-nents away from the direct effects of the plate boundary, butcontinued cooling of the lithosphere takes place causing sub-sidence and eventual formation of a continental shelf as oceaniclithosphere is formed at the divergent boundary. The long-termthermal evolution of divergent margins has been modeled butis still poorly understood. The key evidence is to be found in thesedimentary history of the margin and the thermal history ofthose rocks as shown in the maturation of hydrocarbons. Thedeeper structure of crust in modern margins can be sampleddirectly by deep drilling or indirectly by geophysical methods.If the lower crust has deformed ductilely, a metamorphic fabricshould develop in the rocks which might generate velocityanisotropies that could be measured by seismic refraction or bea source of seismic reflections. Finally, ancient deeply erodedcontinental margins will yield exposures of deeper crustal levelsfrom which the nature of extensional deformation can be ex-amined.

Divergent boundaries should have a wide range of geologicalcharacteristics that extends for tens or even hundreds of kilo-meters from the zone of rapid divergence that defines the plateboundary. Such a terrane would be better treated as a broaddynamic system which evolves and develops a characteristictemporal and spatial suite of geologic features. These featurescan thus be used to identify ancient divergent margins evenwhen their characteristic sedimentary packages are missing.It may be that structures formed at depth within divergentsystems have been unrecognized and erroneously assigned tothe effects of other types of plate boundary systems, particularlywhere divergent systems have been subject to later overprintingby convergent systems.

2. Transform Boundary Systems. At transform boundaries,two lithospheric plates slide horizontally past one another.Within oceanic lithosphere, transform boundaries tend to benarrow and well defined. However, where transformboundaries separate continental lithospheres, they often becomebroad zones of plate interaction with various associated features.The broader and more diffuse character of plate boundarieswithin continental lithosphere, in contrast to that of oceaniclithosphere, seems to be a general rule.The Alpine fault in New Zealand has been interpreted as a

transform boundary between the Pacific and Indian plates.Horizontal movement is not restricted to displacement only onthe fault. Mesozoic structures are bent in sympathy with therelative motion at the boundary, recording 500-600 km ofdifferential motion. This ductile response to the plate motiondominates the Cenozoic structural pattern of New Zealand,affects the entire exposed continental crust, and accounts formore relative motion than displacement on the Alpine fault.Work by Walcott (1, 2) on the present strain pattern within

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New Zealand and its implications for the recent history of theregion has shown that the plate boundary is a 200-km-wide zoneof pervasive deformation. His interpretation is that, in someparts of the zone, relative motion is taken up more by faultingthan by aseismic ductile flow whereas, along other parts of thezone, ductile flow is the dominant strain mechanism. The dif-fuse nature of earthquake activity also supports the idea of abroad zone of plate interaction. In addition, in tracing the rel-ative motion of the two plates for the past 40 Myr, Walcott hasshown the rapid evolution of the boundary. For the early partof its history, the motion on the Alpine fault was nearly purelytransform but, as the pole of rotation between the two platesmigrated, the motion became oblique, with components of bothtransform and compressional motion. This change in motionhas resulted in deformation and bending of early fault segmentsof the transform boundary and a present complex arrangementof oblique transform and compressive deformation along theentire zone and regions of nearly pure compression. This clearlyexemplifies the need to relate contemporaneous features inorder to understand the geologic evolution of such a boundarysvstem.The San Andreas transform system of the southwestern

United States has evolved somewhat differently. Transformmotion has extended across a zone more than 500 km from theCalifornia borderland to western central Nevada with variousfaults active at different times. Along its western margin, theoil-rich basins of western California have evolved during de-velopment of the system. These basins show a very complexpattern of subsidence and compression which can in a generalway be related to the evolution of the transform boundary (3),but the detailed dynamics of the system are as yet poorly un-derstood.

Within the San Andreas transform system, numerous anas-tomosing strike-slip faults have cut the continental crust intomany long narrow slivers, particularly in its western part.Movement on these slivers has greatly disrupted the earliergeologic framework generated by older plate boundary systems.Recent work by Kamerling and Luyendyk (4) has shown thatone of the slivers has been not only translated but also rotatedabout 600 clockwise. Evidence from other workers has sug-gested that other slivers may have rotated also. Rotations ofsmall (150 X 50 km) slivers within the transform system mightindicate that they are not directly reflecting movement withinthe mantle below. If so, it would require that the slivers aredecoupled at some level within or at the base of the litho-sphere.The problem of decoupling along subhorizontal surfaces may

be very important. It has already been mentioned with regardto divergent systems and will be mentioned again for conver-gent systems. If decoupling is a widespread phenomenon," asI suspect may be true, it would require a complete under-standing of the entire boundary system before ancient motionsof large plates could be derived.

3. Convergent Boundary Systems. Of the three fundamentaltypes of plate boundary systems, convergent systems are themost complex and have the broadest effects. Subduction ofoceanic lithosphere beneath either oceanic or continentallithosphere is the most usual configuration at a convergentboundary. Away from the subduction zone in the overridingplate, the following sequence of geologic elements is commonlypresent: (i) an accretionary wedge of thrusted sedimentaryrocks, (ii) an outer topographic high formed from the most el-evated parts of the accretionary wedge, (iii) a forearc basinfilled with relatively undeformed sedimentary rocks lyingbetween the outer arc high and a volcanic arc, and (iv) a vol-canic arc. If the overriding plate is underlaid by oceanic

lithosphere, these geologic elements form an island arc, but ifit is underlaid by continental lithosphere, the geologic elementsform a continental margin volcanic arc.

Behind the volcanic arc, the overriding plate may undergoextension or compression or may be relatively passive. Whereit is in extension, structures similar to those at divergent marginsmay form, leading to the development of oceanic lithospherebehind the arc. At places where the overriding plate is conti-nental, a broader zone of extension may thin the lithosphere byboth ductile and brittle mechanisms, forming regions like theAegean Sea of the eastern Mediterranean or the Basin andRange terrane of the western United States. In this latter case,the effects of the extension may extend for several hundredkilometers behind the arc and be associated with bimodal vol-canism (basalt-rhyolite volcanism) that is different from thecalc-alkaline volcanism of the volcanic arc. Where the over-riding plate is in compression, belts of folds and thrust faults arepresent behind the arc. In the modern Andes, such features arepresent more than 800 km east of the outcrop of the subductionzone. Where the overriding plate is not in compression or ex-tension, the back arc region may be relatively passive.Our present level of understanding is such that we cannot

adequately explain the origin of the three different strain fieldsin the overriding plate. In addition, we lack the basic data toconstruct accurate three-dimensional geometries for the variousgeologic features within the overriding plate, their relationshipsto one another, and their relationships to the activity at thesubduction zone. The knowledge required for solving many ofthese problems can be obtained, but this requires a concertedmultidisciplinary effort. The state of stress can be determinedat and near the surface by geologic and geodetic studies and atgreater depths by study of intraplate seismicitv. Geologic re-lationships among structures, sedimentation, ansd volcanism canbe determined at the surface and projected to depth by geo-physical techniques and deep drilling. An example of thetechniques presently available to us is seen in the work of theConsortium for Continental Reflection Profiling (COCORP),whose deep reflection studies have shown the continuation ofthe Wind River thrust fault in Wyoming to near the M-dis-continuity and revealed the presence and configuration of amagma body in the crust beneath the Rio Grande rift in NewMexico (5). Similar studies directed toward the more complexactive and ancient plate boundary systems are necessary toconstruct accurate three-dimensional geometries, so that theentire complex can be treated as a single integrated dynamicsystem.

Inevitably, plate convergence leads to collision of lithosphericplates containing island arcs or continental crust. Both of thesecrustal elements are less dense than oceanic crust andl thus resistsubduction. The result is the addition of island arc terranes tocontinents, to produce continental growth, or the suturing oftwo continents, to cause the rearrangement of continentalmaterial. Normally, the subclucting plate reflects little geologicactivity related to the active convergent margin but, duringcollision, the plate boundarv system may extend far into thesubducting plate, greatly enlarging the convergent system.Two examples of arc-continent collisions showv one type of

possible evolutionary sequence at a convergent boundary. Thenorthwestern part of the Australian continent has just enteredthe north dipping subduction zone along the southeasternmargin of the Asian plate. The island of Timor contains an as-semblage of thrust sheets that represent the southernmost ele-ments related to the overriding Asian plate (6). Sedimentaryrocks of the Australian plate are being detached as separatethrust sheets as the convergent system broadens to include therocks of the subducting plate. Continued convergence leads to

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a deeper, more extended involvement of the subducted plate,as exemplified in Papua, New Guinea (7). Here, convergenceafter collision of an island about 15 Myr ago has continued, anddisruption of the northern edge of the Australian plate nowextends more than 300 km south of the original site of collision.The collisional system consists of all the geologic features of theoverriding island arc system plus those now affecting the sub-ducted plate. This system is superposed on and has disruptedthe ancient divergent margin of northern Australia and hasgreatly altered the overriding plate. Study of such modern arccollisions offers a chance to understand the workings of thesecomplex dynamic systems, their effects on the lithosphere, andtheir relationship to mantle processes.

Continent-continent collisions are currently active in manyplaces along the Alpine-Himalayan chain where the Indian,Arabian, and African plates collide with the Eurasian plate. Inthe eastern Mediterranean region, the active collisional systemis more than 500km wide. The geologic evolution of the regionsuggests that several small fragments of continental lithospherewere swept together between two large converging plates. Theoceanic tracts were subducted, while the buoyant continentalblocks remained at the surface. Continued convergence hasdeformed both the small continental fragments and the marginsof the larger plates, so that the collisional system now extendsacross a very broad zone. Because convergence has taken placealong very irregular boundaries, the disruption of continentallithosphere has resulted in complex motions of small fragments.Motions along different fragment boundaries are divergent,transform and convergent (8). Relative motions between somefragments is at right angles to the overall convergent motionof the two large plates. It is clear from such examples that, inorder to decipher the relative motion of the large plates, thecomplex motions and geologic features of the entire dynamicsystem must be understood.The most spectacular example of an active broad convergent

boundary system is in eastern Asia. About 50 Myr ago, con-vergence between the Indian and Asian plates produced acontinent-continent collision between the continental litho-sphere of India and Asia. After collision, continued convergenceof perhaps more than 2000 km has been absorbed principallyby disruption of the former Asian plate (9); additionally, thenorthern edge of the Indian plate has been deformed bysouth-directed thrust faults that involve a considerable thicknessof the Indian continental crust. Asia has absorbed massive in-tracontinental deformation by strike-slip, thrust, and extensionalfaulting. Simplistically, Asian crust has shortened longitudinallyand extended latitudinally to accommodate the northwardmovement of India. Faulting within Asia extends nearly 3000km from the former collision boundary and, coupled with de-formation of the Indian plate, forms a wide, complex conver-gent system. The pattern of faulting has been considered byMolnar and Tapponnier (9) to result from a stress regime similarto that produced by a rigid body (India) indenting a plasticmedium (Asia). Their analysis offers a solution that integratesthe complex deformation within the dynamic system. It is aninteresting hypothesis, and one that will require considerabletesting and further investigation. If true, the hypothesis predictsthat extension at Lake Baikal is related to the same system thatis producing thrust faulting in the southern Himalaya.Even though these broad zones of deformation and related

geologic activity can be assigned to convergent collisional ac-tivity, the answers to many fundamental questions still areunknown. The magnitude, distribution, and origin of stresseswithin these systems remain uncertain. Studies of the presentdistribution of earthquake activity, earthquake focal mecha-nisms, and active faults have been used to define the relative

motion of small plates in attempts to describe the tectonic ac-tivity within the system. This approach assumes that the be-havior of the small plates is similar to that of- the large ones.Geologic studies in some areas suggest that, given long periodsof time, the activity within the convergent systems is betterdescribed in terms of nonrigid penetrative or semipenetrativedeformation rather than as the relative motion between smallrigid plates. Earthquake studies can yield important data onthe instantaneous picture of stress and localization of defor-mation, but it is still unclear how such a picture relates to thelong-term activity within the convergent systems. The rela-tionship of surficial and shallow lithosphere activity to deeperlithosphere and asthenosphere motion remains unknown. Mostcollisional systems, particularly in continent-continent colli-sions, contain only shallow earthquake activity; thus, directevidence for deep lithosphere or asthenosphere motion anddynamics is lacking. It seems possible that in collisional zones,and perhaps in all convergent zones, subhorizontal decouplingwithin the lithosphere is present, creating even greater difficultyin correlating shallow lithosphere motions with those deeperwithin the Earth.

PLATE MOTIONS AND CONTINENTALEVOLUTION

Most of our data on Earth evolution must be derived from thecontinents, because their rocks contain the only remainingrecord of the first 95% of the Earth history that we can sampleand study directly. Examination of continental geology showsthat they are largely composed of broad belts of deformed andvariously metamorphosed rocks intruded by igneous robks andlocally overlaid by a thin veneer of younger sedimentary rocks.The age belts are thus a direct reflection of the kinematic,thermal, and dynamic events that affected these rocks and forma first-order heterogeneity of continental crust. Each belt hasevolved over a specific span of time that distinguishes it fromadjacent belts. Within these age belts, some of the tocks andassociated structures are similar to those formed by rmodernplate boundary systems; however, some are not.

Studies of some terranes within the different age belts hasdemonstrated that island arc-continent collisions have beenimportant in the net growth of continental crust, as in westernNorth America (10) or in western Saudi Arabia (11). Many ofthese arc terranes formed on oceanic crust, and their volcanicpiles are derived largely from the mantle. Thus, after collision,they represent a net increase in continental crust. Older collidedisland arc systems within continental crust are often deeplyeroded and expose rocks and structures similar to those that lieat depth in modern convergent systems. By careful examinationof ancient island arcs and associated rocks, the limits of theconvergent system can be defined and its three-dimensionalgeometry constructed. Establishment of contemporaneity ofigneous and sedimentary rock associations, structures, andmetamorphism is difficult within convergent systems but es-sential in order to trace their evolution and to distinguish pre-collisional, collision, and postcollision events from eachother.

Because island arc systems are surrounded by oceaniclithosphere, after collision they remain bounded by oceaniclithosphere on one side and are inevitably subject to reworkingby later plate boundary systems. In western North America,continental margin convergent arc systems and transformsystems have been superposed on ancient collided arcs (10, 12).Only continent-continent or additional arc-continent collisionscan remove them from later plate boundary system effects.However, they may never be far enough removed to ensurefinal stability. Many of the active faults in China that are part

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of the modern 3000-km-wide continent-continent collisionsystem follow ancient arc-continent and continent-continentcollisional systems.

Continent-continent collisions are well known in the geologicrecord of continents. The expression of these dynamic systemsis present, for example, in the Urals (12), several Paleozoicmountain chains in China (13), and the Caledonides of Scan-dinavia and Greenland (14). Such collisions rearrange conti-nental rocks but, with the exception of remnants of oceaniclithosphere and island arcs caught between collided continentsand igneous activity of mantle origin, they do not represent netgrowth of continents. The dynamic boundary systems that areassociated with the collisional events may profoundly reworkalready formed continental rocks, superimposing new struc-tural, magmatic, and metamorphic trends on older ones. Theeffects of collisional events must affect the entire litho-sphere.

Even though the results of plate interactions similar to thosepresent today can be recognized, the relative positions of thedifferent continents through time cannot be completely re-constructed. The major unknowns are the width of oceaniclithosphere that has been subducted and the nature of the plateboundaries that may have existed within the oceanic litho-sphere. Paleomagnetic studies of continental rocks can give thepaleolatitude for ancient continents at the time of formationof their remnant magnetism and provide data demonstratingrotation of continental blocks, but they cannot give paleolon-gitude. Ziegler et al. (15) attempted, by study of ancient biotalassemblages as well as paleoclimatology and oceanic paleocir-culation, to place some constraints on the longitudinal separa-tion of ancient continents, but the uncertainties are large (16).No approach yet tried seems to offer a solution to thisproblem.

Continents also contain belts of rocks whose geologic char-acteristics are so different from those of modern plateboundaries that they may be a result either of plate boundarysystems unlike any modern ones or of tectonism unrelated toplate boundaries. It is clear that modern plate motions and as-sociated boundary systems do not represent a complete spec-trum. The spatial and temporal superposition and variationsin intensity of the effects of boundary systems can produce awide spectrum of structural, magmatic, and metamorphicevolution and final crustal architecture. For example, duringthe early Paleozoic era, a nearly 1000-km-wide belt of rocks wasadded to the eastern margin of Australia. There is, however,no agreement on an evolutionary model for eastern Australiaand those models that have been proposed seem to have no di-rect modern analogs (e.g., see ref. 17).The Limpopo belt of southern Africa (18) is a zone of highly

deformed rocks 300 km wide. Geologic evidence suggests thatmotion within the northern part of this belt was largely hori-zontal, and the amphibolite-to-granulite grade of metamor-phism indicates that the rocks now exposed were deformed atdeep crustal levels. Because rocks of similar grade wouldpresently be at depths of 20-30 km, its modern analog wouldhave a very different surficial expression. I suggest that theLimpopo belt could be the deep-seated equivalent of a trans-form boundary system such as that of New Zealand's Alpinefault described earlier. This example emphasizes the point thatthe three-dimensional nature of modern plate boundaries mustbe understood before their deep-seated equivalents can berecognized in the geologic record.Some age belts within continents have been difficult to in-

terpret as plate boundary systems analogous to modern ones.This is particularly true of some late Precambrian belts (0.6-2.5billion years) and all of the Archean (2.5-3.8 billion years).

Archean belts are characterized by the extensive developmentof high-metamorphic-grade gray gneisses and sodic graniticrocks accompanied by sublinear to irregular discontinuousterranes of greywackes and greenstones of low metamorphicgrade. Even though some rocks in these belts are similar to thoseof modern convergent systems, their arrangement and structureare dissimilar. Archean terranes also lack many rock types andstructures associated with modern convergence. Interpretationof Archean terranes has generally taken one of two views: (i)that they are the result of lithospheric motions whose geometryand intensity were very different from modern activity and thatlithosphere thickness was less and its ductility greater than atpresent; (ii) that plate tectonics was not operative in the Archeanand that mechanisms not now observed must be invoked.Clearly, the Earth is an evolving body whose thermal structurecontrols the motions, thickness, and ductility of the lithosphereas well as the generation of igneous and metamorphic rocks.Thermal gradients were very probably higher in the Archeanand have decreased to their present state. It is probable thatmotion of the lithosphere was more rapid than at present.However, until these terranes are better understood geologicallyand the variables that affect modern plate boundary systemsare better known, the Archean and younger terranes, unlikethose developed along modern plate boundaries, will remaina major challenge to the reconstruction of ancient lithosphericmotions.

Structures and configurations of rock masses formed by plateboundary systems or systems not related to a plate boundarywithin the deformed belts of continents are extremely complex.However, seismic refraction data have suggested that thefirst-order heterogeneity of the continental crust is a subhori-zontal layering. More recently, deep seismic reflection studiesby COCORP have recorded numerous subhorizontal reflectionswithin the crust and at very shallow levels in the upper mantle.Even though geologists accept a general and perhaps gradualgeochemical and petrological zonation within the upper con-tinental lithosphere, it remains difficult for anyone who hasmapped the geology of deep crustal rocks to imagine the rela-tionship between their configurations and structures and asubhorizontal geophysical layering. There appears to be a dif-ficulty in reconciling the two types of first-order heterogeneityof continental crust-age belts with complex internal structureand subhorizontal geophysical layering. They may in part re-flect two different geological phenomena: (i) heterogeneitycaused by the cumulative effects of plate boundaries; (ii) het-erogeneity superimposed on the first by interplate modificationsof continental crust through a protracted thermal, hydrother-mal, geochemical, and structural evolution. A solution to thisproblem can be obtained by following refraction and reflectionlayers to the surface, deep drilling, or tracing these geophysicalsurfaces across ancient plate boundaries.

RELATIONSHIP OF LITHOSPHERIC ANDASTHENOSPHERIC MOTIONS

Even if relative motion between external parts of the litho-sphere can be reconstructed from the geology of continents,motion of the deeper lithosphere and mantle may not becompletely resolvable. For regions within large plates unaf-fected by plate boundary deformations, sublithospheric mantlemotions may be resolvable but, within the limits of plateboundary systems, decoupling on subhorizontal surfaces maymake the resolution of deeper mantle motions difficult or im-possible. Hadley and Kanomori (19) have shown that the SanAndreas fault in southern California does not affect an east-trending ridge-like body of high-velocity mantle extendingfrom depths of 40 to perhaps 100km. Thus, the fault apparently

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is confined to the upper 40 km of the continental lithosphere.The ridge of high-velocity upper mantle extends 150 km eastof the San Andreas fault before ending. They suggest that theupper lithosphere beneath southern California is decoupledalong a surface of horizontal shear, perhaps marked by lowvelocity, and that the plate boundary at depth is further eastthan that at the surface. Thus, the surface motions are not adirect reflection of the deeper motions.

Reexamination of the present motions of the small conti-nental fragments in the eastern Mediterranean area couldsuggest that the motion of these fragments is partly or whollyrestricted to the lithosphere, or even only the shallow part ofthe lithosphere, whereas the motion of the deeper mantle isquite different. The relative motion of these fragments is de-fined by earthquake studies and locally substantiated by studiesof active faults. These data are largely restricted to the upperfew tens of kilometers of the lithosphere. The continentalfragments have a lateral dimension of a few hundred kilometersand have such diverse relative motions in relation to the relativemotions of the large plates that it is conceivable that the frag-ments are decoupled and that their motion is independent ofthe deeper mantle. Perhaps this motion of continental frag-ments is caused by shallow stress fields generated within con-tinental lithosphere during collisional events. Similar decouplingmight also be possible beneath even broader terranes such aswithin the Asian convergent system discussed above.

Within many deformed belts, thrust faults commonly containonly the upper 10-15 km of the continental crust; and most suchthrust faults are the result of convergence within plate boundarysystems. Where displacements are greater than 100 km, as inthe Alpine-Himalayan or Caledonian mountain systems, theymay represent shallow decoupling within continental crust.Once decoupling of this scale is admitted, then the motion ofthe lower plate can be independent of that of the upperplate.

Shallow crustal decoupling suggests that the lower crust mayremain attached to the lower part of the lithosphere and par-ticipate in its motion. Thus, during collision or perhaps even innoncollisional settings, the lower part of the continental or islandarc crust may be subducted with the rest of the lithosphere. Insuch cases, the upper crust and lower crust are decoupled andthe attachment of part of the lower crust to the rest of thelithosphere is not sufficient to inhibit subduction (e.g., see ref.20). This might be the reason why convergence continues aftermany continent-continent or arc-continent collisions. It is in-teresting that, in the original conception of subduction byAmpferer (21), which he called Verschluckung, it was proposedthat continental crust was engulfed into the mantle duringcrustal convergence.

In order to evaluate deeper mantle motions, it is importantto know whether lithospheric or upper lithospheric motion iscontrolled directly or indirectly by or is independent of deepermantle motions. Geologic studies have suggested that de-coupling may be widespread in deformed rocks within thelithosphere. As suggested here, decoupling may be present inall three types of plate boundary relationships. If decouplingis as widespread as proposed here, then it becomes imperativethat reconstructions of ancient zones of plate interaction betreated as broad dynamic systems, because relative motionsbetween lithospheric fragments within them cannot alone berelied upon to yield valid information on large plate motionsor on ancient mantle dynamics.

It is well known that upper mantle seismic velocity is aniso-tropic near oceanic divergent margins. Seismic velocities area few percent higher for paths perpendicular to the boundaryrelative to paths parallel to the boundary. Laboratory studies

on the ductile flow of mantle rocks has shown that such varia-tions in seismic velocity can be related to the crystallographicorientation of the minerals in rocks produced during ductileflow. Similar, but admittedly more complicated, seismic studiesin crustal and mantle rocks in collisional convergent systems.might yield critical data demonstrating the presence of thickzones of rocks within the lithosphere with well-developed fabricthat mark decoupling layers. Teleseismic studies such as thoseby Hadley and Kanomori (19) or studies of earthquake distri-bution and focal mechanism such as those by Armbruster et al.(22) might also demonstrate decoupling and provide the groundrules to apply to ancient plate boundary dynamics.

CONCLUSIONSThe tasks ahead seem clear: (i) to develop a three-dimensionalunderstanding of the kinematics, dynamics, and thermalstructure of modern plate boundary systems and at the sametime to recognize those geological and geophysical features thatare unrelated to plate interaction; (ii) to use this understandingto reconstruct the extent and evolution of ancient systems thatform the major elements of continental crust; and (iii) to de-termine the dynamics and evolution of systems that have nomodern analogs. The task of understanding the Earth's motionsthrough geologic time must be focused on the continents, wherethe only evidence for 95% of Earth history resides. This cannotbe considered to be a purely academic pursuit; it is extremelypragmatic. A deeper understanding of plate boundary andnon-plate-related systems forms a framework within which tounderstand the evolution of continents. Continents are the partof the Earth on which we live and are the terranes from whichwe derive and will continue to derive the bulk of our naturalresources. We are subject to natural hazards that are both thedirect result of modern plate motions such as earthquakes andvolcanic activity and the indirect result of earth motions suchas the controls placed on earthquake and volcanic activity byolder structures within the continents.

Plate tectonics offers an explanation for the present hetero-geneity of the external part of the Earth. It explains the originof the first-order heterogeneity of oceanic and continentallithosphere. Furthermore, it explains the youth and simplicityof oceanic lithosphere and offers the potential to explain theantiquity, complexity, and evolution of the continental litho-sphere. The framework of plate tectonics must be used care-fully, because there are geological features, particularly in themore ancient part of the record, that may require alternativeexplanations. The continents display an internal heterogeneitythat is still poorly understood and that represents an obviouschallenge to those who are interested in the structure of theexternal part of the Earth. They are complex but not incom-prehensible. The study of continents and their evolution mustbe interdisciplinary and international. I think we are on thethreshold of significant advances in our understanding of thecontinents. These advances may rival those that have alreadycome with the understanding of modern Earth motions and thestructure and evolution of oceanic lithosphere.

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