Showing papers in "Tectonics in 1986"
TL;DR: In this paper, an arc migration model was proposed to explain the dynamic relationship between extension in the Tyrrhenian basin and compression in the Apennines, and the estimated contemporaneous (post-middle Miocene) amounts of extension and shortening in the apennines appear to be very similar.
Abstract: Previously proposed models for the evolution of the Tyrrhenian basin-Apenninic arc system do not seem to satisfactorily explain the dynamic relationship between extension in the Tyrrhenian and compression in the Apennines. The most important regional plate kinematic constraints that any model has to satisfy in this case are: (1) the timing of extension in the Tyrrhenian and compression in the Apennines, (2) the amount of shortening in the Apennines, (3) the amount of extension in the Tyrrhenian, and (4) Africa-Europe relative motion. The estimated contemporaneous (post-middle Miocene) amounts of extension in the Tyrrhenian and of shortening in the Apennines appear to be very similar. The extension in the Tyrrhenian Sea is mostly accomplished in an E-W direction, and cannot be straightforwardly related to the calculated N-S Africa-Europe convergence. A model of outward arc migration fits all these constraints. In a subducting system, the subduction zone is expected to migrate outward due to the sinking of the underthrusting plate into the mantle. The formation of a back-arc or internal basin, i.e. of a basin internal to the surrounding belt of compression, (in this case the Tyrrhenian Sea) is then expected to take place if the motion of the overriding plate does not compensate for the retreat of the subduction zone. The sediment cover will be stripped from the underthrusting plate by the outward migrating arc of the overriding plate, and will accumulate to form an accretionary wedge. This accretionary body will grow outward in time, and will eventually become an orogenic belt, (in this case the present Apennines) when the migrating arc collides with the stable continental foreland on the subducting plate. An arc migration model satisfactorily accounts for the basic features of the Tyrrhenian-Apennine system and for its evolution from 17 Ma to the present, and appears to be analogous to the tectonic evolution of other back-arc settings both inside and outside the Mediterranean region. An interesting implication of the proposed accretionary origin of the Apennines is that the problematic “Argille Scagliose” (scaly clays) melange units might have been emplaced as overpressured mud diapirs, as observed in other accretionary prisms, and not by gravity slides from the internal zones.
1,745 citations
TL;DR: In this paper, the authors present stratigraphic and petrologic data from Chile and Argentina between 29° and 33°S latitude that demonstrate the "suspect" nature of several major terranes, which they infer to have been accreted during the Paleozoic.
Abstract: The recognition of accreted terranes and their importance in orogenesis has spurred the search for allochthonous fragments along the western and southern margins of South America. Here we present stratigraphic and petrologic data from Chile and Argentina between 29° and 33°S latitude that demonstrate the “suspect” nature of several major terranes, which we infer to have been accreted during the Paleozoic. Three lower-middle Paleozoic terranes are described (from east to west): (1) the Pampeanas terrane, a Cambrian-Devonian magmatic and metamorphic province built on late Precambrian basement at the margin of South America, (2) the Precordillera terrane, a Cambrian-Devonian shelf-slope-oceanic basin assemblage bounded by melanges on both sides and bearing many stratigraphic similarities to the lower-middle Paleozoic of the Northern Appalachians, and (3) the “Chilenia” terrane, which has largely been obliterated by late Paleozoic magmatism and metamorphism. The distribution of Carboniferous continental, deltaic, and marine strata demonstrates that these three terranes were sutured together and part of South America by the end of the Devonian. Subsequent Permo-Carboniferous plate interactions more closely resembled the modern Andean margin, with eastward subduction, accretionary prism formation, and minor terrane emplacement exposed along the present coast of Chile and eastward migrating arc magmatism from the present coast of Chile to western Argentina.
507 citations
TL;DR: In this paper, the authors proposed a model in which the lower crust of the converging Arabian plate located beneath the Zagros is being shortened and thickened plastically between the seismogenic, rigid upper crust and a rigid uppermost mantle lid.
Abstract: The Zagros mountain belt region, a Neogene continental collisional zone between Arabia and Iran that is characterized by a suture, a folded belt, and a foreland basin, is unusual among active mountain belts in that its 6–12 km thick sedimentary cover is shortening predominantly by folding while the basement is apparently thickening along numerous high-angle reverse faults. We have used over 9000 gravity measurements in the region in conjunction with limited seismological observations and geological cross sections of the uppermost 10 km of crust to infer the deeper crustal structure beneath the region. Crustal models constrained by the Bouguer anomalies and other seismological and geological data are characterized by a Moho that dips about 1° to the northeast beneath the folded belt and increases in dip to about 5° near the Main Zagros Thrust (MZT); the Moho depth increases from 40 km beneath the leading edge of the foreland basin (the Mesopotamian foredeep and Persian Gulf) to as much as 65 km beneath the MZT. Alternative models that incorporate a subducted oceanic crust that is attached to the underthrusting Arabian crust deviate little in their Moho configurations from the above simpler models. Negative isostatic residual anomalies are interpreted to indicate local overcompensation beneath the foreland basin and near the folded belt and beneath the MZT. We show that underthrust sedimentary rocks of the converging margins along the suture zone may account for the negative isostatic residual anomalies near the MZT. However, to explain the rest of the isostatic anomalies requires other forces to be acting on this collisional plate boundary. For example, elastic flexure models required a combination of topography and subsurface loads or downward force (the origin of which is not clear) to approximate the Bouguer or isostatic residual anomalies associated with the foreland basin and the folded belt. The topography load of the Zagros mountain belt is insufficient to cause the required deflection of the underthrusting Arabian plate. A combination of isostatic, elastic flexure, and horizontal compression forces acting on the edge of the Arabian craton and the transitional lithosphere of central Iran appears to best model the crust of the Zagros region. We propose a model in which the lower crust of the converging Arabian plate located beneath the Zagros is being shortened and thickened plastically between the seismogenic, rigid upper crust and a rigid uppermost mantle lid. The mantle lid, probably decoupled from the lowermost crust, un der thrusts the Iranian crustal blocks. Thus confined from below, the horizontally compressed, plastic lower crust hydraulically depresses the Moho and raises the faulted and folded, brittle upper crust in isostatic equilibrium.
210 citations
TL;DR: In this article, the authors consider that there are two Proterozoic rifts of E-W trend in the northern margin of the North China Platform, and the Zhaertay Group and Baiyun'ebo Group of closed lagoon facies or neritic facies were deposited respectively in the two rifts.
Abstract: On the basis of recent investigations in southern Inner Mongolia the authors consider that there are two Proterozoic rifts of E–W trend in the northern margin of the North China Platform. The Zhaertay Group and Baiyun'ebo Group of closed lagoon facies or neritic facies were deposited respectively in the two rifts. The lead isotopic dating of carbonaceous limestone from the middle sequence of Baiyun'ebo Group yields an age of 1500 Ma. Along the rifts a series of alkaline volcanics and intrusions is scattered in the Yinshan Mountains and Yanshan Mountains. To the north of the rifts, four ophiolite belts and at least five accreted terranes of various ages have been recognized in the Paleozoic orogenic zone. The presence of the ophiolite suites and these accreted suspect terranes in Inner Mongolia indicates that probably there was an open ocean between Siberia and North China in Paleozoic times, when repeated subduction of the oceanic crust had occurred along both continental margins. The suture line between Cathaysia and Angaraland extends from Linxi to Solon Obo. In the late Permian, epicontinental mountain systems of the two converging continents collided with each other. From the analysis of the tectonic evolution of Inner Mongolia and adjacent areas we conclude that the evolution of the southern margin of the Siberian Platform is approximately the same as the history of activity of the trench-arc-basin system in the eastern Asian epicontinental areas during Mesozoic and Cenozoic times but the northern margin of the North China Platform is similar in evolution to the Cordilleran system in western North America.
192 citations
TL;DR: In this paper, the authors show that the Lhasa block was, in elevation as well as in magma type, an Andean margin by the end of the Cretaceous, and suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern Lhaa block.
Abstract: The strong Tertiary deformation of the northern margin of the Indian plate and of the Tsangpo suture zone is not matched by comparable deformation within the Lhasa block, yet the southern margin of the block was the site of large-scale intrusive activity (the Gangdese, or Transhimalayan, batholith) throughout the late Cretaceous and was, therefore, presumably weaker than its surroundings. This poses a problem for any approach that interprets the deformation of Tibet in terms of diffuse crustal thickening. However, there is evidence that the Lhasa block was, in elevation as well as in magma type, an Andean margin by the end of the Cretaceous, and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern Lhasa block. This suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast. When subjected to boundary conditions representing the collision between India and Asia, the sheet accommodates convergence by diffuse deformation over a region comparable in size with the Tibetan plateau; in the absence of an elevation contrast this deformation consists of approximately 100% thickening strain over the region. A precollision elevation contrast of 3000 m in the Lhasa block would have resulted in its experiencing roughly 40%, rather than 100%, postcollisional thickening; smaller elevation contrasts would have resulted in greater thickening, and vice versa. These calculations have implications for the mode of crustal thickening in northern Tibet: As the available evidence suggests that the topographic expression of the earlier Mesozoic tectonic activity in southern Asia was erased before the India-Asia collision, we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the Lhasa block during the Tertiary.
191 citations
TL;DR: The most useful criteria used for the determination of the sense of shear can be found in the huge Himalayan ductile shear zone known as M.C.T. as discussed by the authors.
Abstract: The most useful criteria used for the determination of the sense of shear can be found in the huge Himalayan ductile shear zone known as M.C.T. They allow the determination of the sense of shear in relation with an unequivocal southward-directed intracontinental thrust. The various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution. However, most of the observed shear-criteria are due to late nappe emplacement postdating the climax of the metamorphism. The most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt. It parallels everywhere the shear direction and thus indicates the transport direction of the nappes. The observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence.
174 citations
TL;DR: In this article, the authors proposed that back-arc spreading in the Okhotsk and Japan Seas was related to the movement of microplates relevant to the India-Eurasia collision.
Abstract: The India-Eurasia collision caused extensive deformation in the Eurasia Continent since late Eocene time. We propose that back-arc spreading in the Okhotsk and Japan Seas was related to the movement of microplates relevant to the India-Eurasia collision. The Kuril Basin of the Okhotsk Sea and the basins of the Japan Sea were formed as back-arc basins simultaneously. Their late Oligocene to middle Miocene age is constrained by sediment stratigraphy, basement depth, and heat flow data. The movements of two microplates, the Okhotsk and Amuria Plates, are significant for the tectonics of the Okhotsk and Japan Seas. Simultaneous retreat of both microplates from trench hinge lines caused back-arc spreading in the Kuril Basin and the Japan Sea. The Amuria Plate moved north-northeastward due to the India-Eurasia collision. This caused pull-apart opening of the Baikal Rift along a transform boundary between the Siberia and Amuria Plates and a collision along the Stanovoy Range at its northern margin. This movement triggered a clockwise rotation of the Okhotsk Plate with a component of dextral collision between the Okhotsk and Amuria Plates, which is well observed along the central zone of the Sakhalin-Hokkaido islands between the two back-arc plates. The clockwise rotation of the Okhotsk Plate is suggested by the southwestward fan-shaped opening of the Kuril Basin. The Kuril Basin narrows to the northeast and terminates south of the Kamchatka Peninsula, where a relative rotation pole between the Okhotsk and Kuril forearc plates is located. The Kamchatka Peninsula was a zone of collision on the other side of the clockwise rotating Okhotsk Plate during the opening of the Kuril Basin. The Sredinny Range in the Kamchatka Peninsula is one of the results of the collision between the Okhotsk Plate and the forearc plate of the Kuril Arc. The bending of the Japanese islands occurred in association with the back-arc spreading of the Yamato Basin of the Japan Sea.
165 citations
TL;DR: In this paper, Griffin et al. show that during early Devonian times bouyancy forces resulting from the extreme crustal thickening initiated a period of extension which led to the development of a major westerly dipping extensional fault affecting the whole thickness of the imbricated margin.
Abstract: Imbrication of the north-west margin of the Baltic Shield during the mid-Silurian continental collision caused continental “subduction” as recorded by the eclogites of the Western Gneiss Region of western Norway (Griffin et al., 1986). During early Devonian times bouyancy forces resulting from the extreme crustal thickening initiated a period of extension which led to the development of a major westerly dipping extensional fault affecting the whole thickness of the imbricated margin. As the footwall of this fault was uplifted the erosional products were deposited on the hanging wall to form a series of basins containing thick sequences of coarse clastic continental sediments. Similarities between this extensional regime and that of the Basin and Range province of the western United States are discussed.
163 citations
TL;DR: In this article, the magnetic lineation of the magnetic susceptibility ellipsoid was analyzed on sedimentary sequences in Northwestern Greece and Taiwan, in regions which have been submitted to large scale compressional events.
Abstract: We present the results of magnetic fabric analysis obtained on samples of marine clays from sedimentary sequences in Northwestern Greece and Taiwan, in regions which have been submitted to large scale compressional events. The lithology of the sediments is mainly composed of unmetamorphosed marls, siltstones and sandstones. These formations are folded and faulted at different scales. The magnetic fabric is largely sedimentary. However both at single sites (Taiwan) and regionally (Greece) the magnetic lineation is systematically perpendicular to the main compressive stress σ1 inferred from previous fault tectonic analysis. This is an indication that the fabric is also partly of tectonic origin. In Northwestern Greece the orientation of the magnetic susceptibility ellipsoid is constant over distances exceeding 200 km, suggesting that the orientation of the principal compressive strain relative to the trend of the main structures lias been constant since Lower Oligocene.
161 citations
TL;DR: In this paper, a rotating block model was proposed to account for a number of structural styles characteristic of strike-slip deformation in California, including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults, domains of evenly-spaced antithetic faults that terminate against major fault boundaries, continued development of bends in faults with large lateral displacements; anomalous focal mechanisms; and differential uplift in areas otherwise expected to experience extension and subsidence.
Abstract: The pattern of seismicity in southern California indicates that much of the activity is presently occurring on secondary structures, several of which are oriented nearly orthogonal to the strikes of the major through-going faults. Slip along these secondary transverse features is predominantly left-lateral and is consistent with the reactivation of conjugate faults by the current regional stress field. Near the intersection of the San Jacinto and San Andreas faults, however, these active left-lateral faults appear to define a set of small crustal blocks, which in conjunction with both normal and reverse faulting earthquakes, suggests contemporary clockwise rotation as a result of regional right-lateral shear. Other left-lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data. Many of these structures predate the modern San Andreas system and may control the pattern of strain accumulation in southern California. Geodetic and paleomagnetic evidence confirm that block rotation by strike-slip faulting is nearly ubiquitous, particularly in areas where shear is distributed, and that it accommodates both short-term elastic and long-term nonelastic strain. A rotating block model accounts for a number of structural styles characteristic of strike-slip deformation in California, including: variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults; domains of evenly-spaced antithetic faults that terminate against major fault boundaries; continued development of bends in faults with large lateral displacements; anomalous focal mechanisms; and differential uplift in areas otherwise expected to experience extension and subsidence. Since block rotation requires a detachment surface at depth to permit rotational movement, low-angle structures like detachments, of either local or regional extent, may be involved in the contemporary strike-slip deformation of southern California. A block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge-bounding faults, but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction. This behavior, coupled with the presence of possible regional detachments, accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges. Although fault displacements along secondary structures associated with block rotations remain small, they may still influence the nucleation and the characteristic rupture length of large earthquakes. A more complete description of what these structures are, and how they interact, may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard.
160 citations
TL;DR: In this article, the thermal consequences of stacking-up thrust slices in collision zones are investigated using simple one dimensional thermal models, and it is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone.
Abstract: The thermal consequences of stacking-up thrust slices in collision zones are investigated using simple one dimensional thermal models. The metamorphic evolution (geotherms and (P,T, time) paths) of a given tectonic unit belonging to a crustal stacking wedge made of more than two units is governed by three effects: cooling and pressure decrease associated with erosion, cooling by the lowest units (screen effect), heating by the upper units (cover effect). It is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone. The metamorphic evolutions are very sensitive to the following tectonic parameters: the number of thrusted units involved in thickening, the time delay between each thrust and finally the mode of stacking of the different units (over and understacking). It appears that, for a given depth of burial, the temperature increase during uplift is less important in a crust thickened by three or four units than in the case of two units. The screen effect during understacking is more efficient for a short time delay (∼ 10 Ma) between each thrust.Overstacking leads to higher temperatures before uplift when compared to understacking. It is also shown that the thermal perturbation induced in an intermediate unit of the pile is more efficiently recorded when its thickness is rather small (∼ 10 km). It is shown that, for an erosion-controlled uplift, the shape of the (P,T,t) path depends on the position of the rocks within a given unit and on the unit position within the pile. Finally, the general metamorphic evolutions during crustal thickening (HP-LT metamorphism and subsequent overprint) are discussed in terms of the previously mentioned parameters. In the case of the Western Alps, it appears that the more or less efficient greenschist overprint of the units involved in thickening can be explained by different time intervals between the thrust events.
TL;DR: Fission track (FT) thermochronology studies on igneous and metamorphic rocks from the Ruby Mountains-East Humboldt Range metamorphIC core complex provide important constraints on the timing and nature of major mid-Tertiary extension in northeast Nevada as mentioned in this paper.
Abstract: Fission track (FT) thermochronology studies on igneous and metamorphic rocks from the Ruby Mountains-East Humboldt Range metamorphic core complex provide important constraints on the timing and nature of major mid-Tertiary extension in northeast Nevada. Rocks from within the Ruby-East Humboldt detachment (brittle-ductile normal-sense shear) zone were analyzed; the dominant lithology studied was variably mylonitic mafic orthogneiss. Nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the Oligocene (circa 36 Ma) Harrison Pass pluton were also studied. FT ages on apatite, zircon, and sphene (except one 18.4-Ma apatite from the basal portion of the detachment zone) are concordant and range in age from 26.5 Ma to 23.6 Ma; all ages overlap at 1 σ between 25.4 Ma and 23.4 Ma. These data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above ∼285°C (sphene closure temperature) to below ∼70°C (lower temperature limit of track stability in apatite) near the beginning of the Miocene (minimum cooling rate of 40°C/Ma). The length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly (Fish Canyon ash flow tuff). The lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval, but apparently only cooled to a temperature within the zone of partial annealing of apatite (∼70°–130°C). Residence of these rocks in this zone led to a partial retention of fission tracks, thus resulting in a reduced age. Confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history (remained hotter longer) than structurally higher levels of the zone. FT data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23.4 Ma. Rapid cooling of the region is considered to reflect large-scale tectonic denudation (intracrustal thinning), the vertical complement to crustal extension. Rocks originating in the middle crust (10–15 km) were quickly brought near the surface along the Ruby-East Humboldt detachment fault (brittle-ductile simple shear zone) and juxtaposed against brittlely extended rocks deformed under upper crustal conditions.
TL;DR: In this paper, the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity was modeled and the boundary element displacement discontinuity method was used to solve this problem.
Abstract: Overlapping spreading centers (OSC’s) are a fundamental aspect of accretionary processes at intermediate and fast-spreading centers and typically occur at deep points along the axial depth profile. They have a characteristic geometry consisting of two en echelon overlapping, curving ridges separated by an elongated depression. The length to width ratio of this overlap basin is typically 3∶1. We have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity. A boundary element displacement discontinuity method was used to solve this problem. Our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region. For small OSC’s, there is a very good agreement between calculations and observations but, for large ones, the overlap basin tends to be longer than our predicted results indicate. This suggests that the assumptions made in the model (i.e., perfectly elastic, isotropic and homogeneous medium) are perhaps valid for the brittle lid above the magma chamber that underlies OSC’s with small offsets (< 2 km) but oversimplified for OSC’s with large offsets. Our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach. The deflection will be the greatest for small misalignments of the fracture systems, thus even minor misalignments of the spreading centers may result in the development of OSC’s. Where the misalignment is less than the width of the cracking front, the fracture systems may meet head-on creating a saddle point along the axial depth profile. Our results support the hypothesis suggested by Macdonald et al. [1984] in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned.
TL;DR: In this article, the authors show that strike-slip faults are an expected response to north-oblique subduction of the Farallon and/or Kula plates.
Abstract: Cretaceous and early Tertiary arc and forearc rocks found along the coast in southern California and Baja California have been shown by paleomagnetic measurements to have originated many hundreds, or even several thousands, of kilometers south of their current locations. Northward transport also is found in Cretaceous batholithic rocks near the edge of the continent in Washington and British Columbia. The consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of North America have been translated along the coast by strike-slip faulting. Strike-slip faults are shown to be an expected response to north-oblique subduction of the Farallon and/or Kula plates. Faulting and subduction probably were concurrent activities; i.e., the arc and forearc grew and were displaced simultaneously. Because displacement was dominantly parallel to subduction-related lithic belts, arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts. Subduction-related strike-slip faulting is shown to be favored by high angles of oblique convergence, shallow dip of the subducting slab, relatively easy slip on potential faults within the arc or forearc, and the existence of a place for the moving sliver to go. It is suggested that these conditions are not difficult to satisfy and that subduction-related strike-slip faulting may thus be a common feature of convergent orogens. The location of the San Andreas fault inboard from the continental margin may be attributable to localization of the Pacific-North American transform by a system of precollision subduction-related faults.
TL;DR: In this article, a kinematic model for southern California, based on late-Quaternary slip rates and orientations of major faults in the region, is proposed and the velocities of the blocks are calculated along several paths in southern California that begin in the Mojave Desert and end off the California coast.
Abstract: A kinematic model for southern California, based on late-Quaternary slip rates and orientations of major faults in the region, is proposed. Internally consistent motions are determined assuming that these faults bound rigid blocks. Relative to North America, most of California west of the San Andreas fault is moving parallel to the San Andreas fault through the Transverse Ranges and not parallel to the motion of the Pacific plate. The velocities of the blocks are calculated along several paths in southern California that begin in the Mojave Desert and end off the California coast. A path that crosses the western Transverse Ranges accumulates the accepted relative North America-Pacific plate velocity, whereas paths to the north and south result in a significant missing component of motion, implying the existence of a zone of active deformation in southern California.
TL;DR: The Witwatersrand Basin of South Africa is the best known of Archean sedimentary basins and contains some of the largest gold reserves in the world as discussed by the authors, however, the geologic evidence does not agree with earlier suggestions that the basin were deposited in a rift or half-graben.
Abstract: The Witwatersrand Basin of South Africa is the best-known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch-type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These features indicate that the Witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an Andean-type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes above zones of shallow subduction. It is suggested that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision-related plateau, now represented by the Limpopo Province. Similarities are seen between this phase of Witwatersrand Basin evolution and that of active basins north of the Tibetan Plateau. The geologic evidence does not agree with earlier suggestions that the Witwatersrand strata were deposited in a rift or half-graben.
TL;DR: In this paper, a seismotectonic map of the lower Cheliff basin (scale: 1/500,000) has been synthesized from known geologic, tectonic and seismological work on the region, and from satellite photos.
Abstract: The 1980 El Asnam earthquake should be understood within its tectonic environment. A seismotectonic map of the lower Cheliff basin (scale: 1/500,000) has been synthesized from known geologic, tectonic and seismological work on the region, and from satellite photos. The data set was integrated by a computer in order to handle different scales and projections. Several important tectonic features divide the region into 5 domains. Active faults and recent folds are consistent with a NNW-SSE direction of compression. The Cheliff basin is limited to the south by the Relizane fault. The basin is also crossed by three broken folds, the eastern one being the one associated with the El Asnam fault. Historical and instrumental seisinicity seems to be located at both ends of Relizane fault. It is suggested that the sites corresponding to folds and reverse faulting crossing the basin are relevant to seismic risk. A simple tectonic model is proposed.
TL;DR: A tectonic reconstruction of pre-Red Sea Afro/Arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide.
Abstract: The young continental margin of the western Arabian Peninsula is uplifted 3.5 to 4 km and is well exposed. Rift-related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18° N and its intensity increases gradually from east to west. Extension is negligible near the crest of the Arabian escarpment, but it reaches a value of 8 to 10% in the western Asir, a highly dissected mountainous region west of the escarpment. There is an abrupt increase in extensional deformation in the foothills and pediment west of the Asir (about 40 km inland of the shoreline) where rocks in the upper plate of a system of low-angle normal faults with west dips are extended by 60 to 110%. The faults were active 23 to 29 Ma ago and the uplift occurred after 25 Ma ago. Tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 Ma old are concentrated in the foothills and pediment as well. The chemistry of the dikes suggests (1) fractionation at 10 to 20 kbar, (2) a rapid rise through the upper mantle and lower crust, and (3) differentiation and cooling at 1 Atm to 5 kbar. Structural relations between dikes, faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval, but that most of the extension preceded the expansion. A tectonic reconstruction of pre-Red Sea Afro/Arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide. Steep Moho slopes probably developed during rift formation as indicated by published gravity data, two published seismic interpretations and the surface geology.
TL;DR: The northeast trending Kapuskasing uplift transects the east-west belts of the central Superior Province over a distance of some 500 km as mentioned in this paper, which is attributed to an early Proterozoic compressional event by modelling of geophysical anomalies to determine dip and by geobarometry of garnet-orthopyroxene-plagioclase-quartz assemblages to determine vertical displacement.
Abstract: The northeast trending Kapuskasing uplift transects the east-west belts of the central Superior Province over a distance of some 500 km. Granulite to upper amphibolite facies rocks of the uplift form three distinct geological-geophysical entities: from south to north, the Chapleau, Groundhog River, and Fraserdale-Moosonee blocks. Uplift of the granulites along a moderately northwest dipping crustal-scale thrust fault is attributed to an early Proterozoic compressional event. Major northeast-striking faults that bound the Kapuskasing zone on the west were examined by modelling of geophysical anomalies to determine dip and by geobarometry of garnet-orthopyroxene-plagioclase-quartz assemblages to determine vertical displacement. Granulites in the Kapuskasing zone have 7- to 9-kbar signatures whereas those in the Quetico belt to the west indicate metamorphic pressure of 4–6 kbar. Individual calibrations of the barometer yield consistent pressure differences of 2–3 kbar, suggesting 7–10 km of west-side-down movement on the faults. Modelling of gravity and aeromagnetic gradients indicates westerly dips of 60°–65°, with west-side-down offset of up to 14 km. These major normal faults probably formed as collapse structures in response to crustal thickening which occurred during the preceding compressional uplift stage. Differences in the configuration of individual blocks of the Kapuskasing zone can be related to variable fault slip and intersection angles between normal and reverse faults. Thus the Groundhog River and southern Fraserdale-Moosonee blocks are perched thrust tips analogous to the Sangre de Cristo Range of the Laramide uplift province, whereas the southern Chapleau block is a tilted slab with similar configuration to the Laramide Wind River Range. Pop-up geometry deduced for the northern Fraserdale-Moosonee block resembles the structure of the Laramide Uinta Mountains. A normal fault crosses the surface trace of the basal thrust fault between the Groundhog River and Fraserdale-Moosonee blocks and causes a 65-km “gap” without granulites. This article contains supplementary material.
TL;DR: In this paper, a dextral strike-slip fault in the southwest Virginia Piedmont is estimated to have a displacement of 17 km from rotated foliation measurements in the Melrose Granite, where an S-C mylonite was produced by dynamic recrystallization of all constituent minerals.
Abstract: The northern Appalachian, dextral fault system of late Paleozoic age is also present in the central and southern Appalachians. An example of a dextral strike-slip fault is the 4 km wide Brookneal shear zone in the southwest Virginia Piedmont. The shear zone is, in part, superimposed on the Melrose Granite where an S-C mylonite was produced by dynamic recrystallization of all constituent minerals. The Arvonia metasedimentary and Charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent +24° ± 3° to the shear zone boundary. A minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the Melrose Granite. The age of movement on the Brookneal shear zone has been constrained by isotopic dating to between 324 and 300 Ma. Other faults in the southern Appalachians, including the Nutbush Creek and Modoc zones show similar ages and relative offsets. Possible plate tectonic models that could account for the late Paleozoic dextral fault system throughout the Appalachians include: (1) tectonic escape resulting from the collision of a plate with North America to the north of the Canadian Appalachians, (2) postcollision interplate readjustments involving counterclockwise rotation of Africa relative to North America, and (3) oblique convergence of eastern North America with an oceanic plate moving west.
TL;DR: COCORP seismic reflection profiling in the western and northern Mojave Desert of southern California has revealed the presence of numerous major low-angle reflecting horizons within the crust, and as such they place important constraints on the tectonic evolution of the region.
Abstract: COCORP seismic reflection profiling in the western and northern Mojave Desert of southern California has revealed the presence of numerous major low-angle reflecting horizons within the crust. These complex, though laterally continuous, horizons are interpreted to represent major southwesterly dipping crustal fault zones, and as such they place important constraints on the tectonic evolution of the region. The upper-most horizon is interpreted to be the Rand thrust, which, where exposed, places Precambrian and Late Cretaceous crystalline rocks over possibly younger Pelona-Rand-Orocopia Schist. This reflecting horizon extends for 25 km southwest of the Rand Mountains where it appears to be truncated at a depth of about 7.4 km by another horizon, which may be a later low-angle normal fault. The other reflecting horizons are not traceable to the surface, and so greater ambiguity remains in their interpretation. The most prominent of these horizons occurs at midcrustal depths (15±6 km), exhibits a “ramp and flat” geometry, and extends over the northern area of the Mojave survey into the Basin and Range Province. A lower horizon, at depths of 20–30 km in the northern part of the survey area, is antiformal and appears to terminate above a flat and relatively continuous Moho-depth horizon. The crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s (33 km) in the north of the survey and at about 8–9 s (26–29 km) in the south. These reflections are offset in the vicinity of the town of Mojave. The deep intracrustal fault zones inferred from the COCORP survey may represent (1) the deep crustal continuation of the system of Mesozoic thrusts which crops out in southern Nevada and southeastern California, (2) Late Cretaceous to Early Cenozoic, northeast-vergent thrusts related to the uplift of the Pelona-Orocopia-Rand Schist, or (3) low-angle normal faults related to Early Miocene, northeast-southwest directed crustal extension. The COCORP survey also traversed the major strike-slip faults that bound the Mojave block. The San Andreas fault zone appears to truncate reflectors at depths of 6, 8 and 20 km within the Mojave basement, suggesting that it is a major vertical feature which extends to at least 20 km depth. Conversely, the Garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature.
TL;DR: In-plane lithospheric stress, though known to exist, has generally been ignored in the quantitative modeling of basin stratigraphy as discussed by the authors, however, low levels of intraplate lateral stress can induce observable plate deformations (10-100 m of vertical motion) if the lithosphere contains a preexisting deformation (such as a sedimentary basin).
Abstract: In-plane lithospheric stress, though known to exist, has generally been ignored in the quantitative modeling of basin stratigraphy. However, low levels of intraplate lateral stress can induce observable plate deformations (10–100 m of vertical motion) if the lithosphere contains a preexisting deformation (such as a sedimentary basin). The importance of in-plane stress in modifying the stratigraphy of a sedimentary basin has been assessed using an elastic plate model for the lithosphere. A compressive in-plane stress generally induces basin subsidence with peripheral uplift (shoreline regression), while a tensile in-plane stress induces basin uplift and peripheral subsidence (shoreline transgression). These simple results are complicated by variations in crustal thickness, as now two interfaces are involved, that is, the sediment/basement interface of the sedimentary basin and the Moho topography; the isostatic state of a sedimentary basin therefore ultimately controls the resultant deformation induced by in-plane stress. The implications of in-plane stress modification of basin stratigraphy are profound: regionally correlatable transgressions and regressions of basin interiors (e.g. cyclothems) and passive continental margins (the third-order variations of the Vail et al. [1977a] coastal onlap curve), may be tectonically produced by the interaction of stress-induced base level changes and a basin tectonic driving subsidence. In-plane stress, its generation, magnitude, and variation, is considered a consequence of plate boundary reconfigurations during continental collisions, rifting, and subduction/obduction.
TL;DR: A combined K-Ar and 40Ar/39Ar geochronological study of the Ruby Mountains-East Humboldt Range, Nevada, a Cordilleran metamorphic core complex, has revealed a complex tectonothermal history as mentioned in this paper.
Abstract: A combined K-Ar and 40Ar/39Ar geochronological study of the Ruby Mountains-East Humboldt Range, Nevada, a Cordilleran metamorphic core complex, has revealed a complex tectonothermal history. The three structural subdivisions of the core complex (migmatitic core, mylonitic zone, and cover) share magmatic and deformational histories in part, but also display important contrasts in structural style, metamorphic paragenesis, and thermal history. During the Late Jurassic (ca 160 Ma), miogeoclinal rocks were polyphase-deformed, metamorphosed under amphibolite facies conditions, and pervasively intruded by peraluminous granitic magmas, thereby forming an igneous and metamorphic complex. Late Jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks. Late Cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved Cretaceous incremental-release 40Ar/39Ar hornblende spectra and by U-Pb monazite ages from pelitic schist exposed in the northern Ruby Mountains. Between ca 45 Ma and 20 Ma, the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low-angle, normal-sense, simple shear zones. As a result, rocks metamorphosed at depth in the Mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals. Near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation. However, it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 Ma metamorphism or if distinct, locally superposed late Mesozoic and/or middle Tertiary thermal overprints had affected the cooling history. A major extensional simple shear zone is marked by an approximately 1-kilometer-thick mylonite zone that experienced late kinematic, superposed brittle deformation. A geometric consequence of the development of low-angle extensional fault zones is the eventual translation of colder rocks over hotter rocks. Such a history is supported by the 40Ar/39Ar data. For example, mylonitic mafic orthogneisses from a major allochthon record 40Ar/39Ar hornblende plateau ages between 44 and 48 Ma and biotite plateau ages between 32 and 33 Ma, and are structurally positioned above mylonitic rocks of the northern Ruby Mountains which record biotite cooling ages of ca 22–24 Ma. Such relations suggest either multiple episodes of Tertiary mylonitization or a protracted, multiphase history of mylonitization. However, after ca 20 Ma, all of the presently exposed mylonitic rocks were maintained below 100°C and have only experienced upper crustal brittle deformation. This article contains supplementary material.
TL;DR: In this article, the authors reconstruct the kinematics of the Western Alpine Arc between the Mercantour and Aar Massifs using geochronological data and P-T conditions, estimated from metamorphic mineral reactions.
Abstract: We attempt to reconstruct the kinematics (deformation and motion) of the Western Alpine Arc between the Mercantour and Aar Massifs. First, we compile a map showing principal extension (X) direction within three major structural units. In the external zone, X directions are broadly radial to the Alpine Arc; in the internal zone, they are uniform or smoothly varying in some domains; whereas in the intermediate zone they often show much scatter. Next, we use geochronological data and P-T conditions, estimated from metamorphic mineral reactions, to time the deformation. On each of a series of interval maps, we show X-directions for strain accumulated during a chosen time interval. We argue that these incremental X-directions are approximately equivalent to segments of particle paths, showing displacements relative to stable Europe. For 120–100 Ma, the displacement direction was about N150 and coeval with eclogite facies metamorphism; it is interpreted as a direction of overthrusting by the African Plate. Around 40 Ma, the displacement in the south part of the arc became westerly. Finally, between 25 and 15 Ma, displacement directions were generally radial to the arc.
TL;DR: The age and kinematics of this structural reworking have been approached using U/Pb zircon dating in the Tamanrasset region as discussed by the authors, which provides a time span of 30-40 m.y.y between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes.
Abstract: Discovery of large-scale deep-seated thrusts in Central Hoggar, with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation, has led us to reconsider the Pan-African tectonic and metamorphic history in that region. Two areas are described, and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust. The age and kinematics of this structural reworking have been approached using U/Pb zircon dating in the Tamanrasset region. Despite the difficulty of estimating the age of the initiation of the assumed intracontinental A-type subduction, the results provide a time span of 30–40 m.y. between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes. Some key ages were determined: (1) 2075 ± 30 Ma is the age given by granulite facies remnants which escaped from the refoliation, the corresponding lower intercept at 530 ± 70 Ma confirms the Pan-African imprint; (2) 615 ± 5 Ma reflecting the age of syntectonic to late-tectonic granitoids emplaced in reworked gneisses and in preserved granulites; (3) 580 Ma, the concordant age of sphenes and monazites from the same granitoids, which is interpreted as corresponding to the end of medium-grade conditions. No evidence has been found of a ∼1000 Ma age: a Kibaran event does not appear to exist in Central Hoggar. The age similarity between the observed deep intracontinental evolution of Central Hoggar and the collision-related tectonics of Western Hoggar and Iforas suggests a common origin for both phenomena.
TL;DR: In this article, age and structural data have been used for reconciliation of previously conflicting models for Late Jurassic genesis of the Josephine, Smartville and Coast Range ophiolites, and the Nevadan orogeny in the Klamath Mountains and Sierra Nevada.
Abstract: Recently published age and structural data allow the reconciliation of previously conflicting models for Late Jurassic genesis of the Josephine, Smartville and Coast Range ophiolites, and the Nevadan orogeny in the Klamath Mountains and Sierra Nevada. The resulting model is consistent with the mode of initiation, location and geometry of the Great Valley forearc basin, and with the lack of a significant forearc basin west of the Klamath Mountains. The Coast Range ophiolite formed by backarc spreading west of an east-facing intraoceanic arc. Soon thereafter, a remnant arc was calved off the west side of this arc, and the Smartville ophiolite formed by backarc (interarc) spreading. During this time, the Sierran phase of the Nevadan orogeny began as the intraoceanic arc encountered the west-facing continental-margin arc of North America. An east-west-trending calcalkaline dike swarm in the Sierra Nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision. Simultaneously, a new subduction zone was initiated west of the collision (suture) zone, and this new trench propagated southward, thus trapping the Coast Range ophiolite in the new forearc area south of the Klamath area. Intense deformation in the Sierran region resulted from this collision, and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted. Continued westward relative movement of the North American arc was permitted north of the Sierra Nevada owing to the lack of a colliding intraoceanic arc. The result was the westward rifting of the continental-margin arc by intraarc spreading, which formed the Josephine ophiolite in the Klamath area. The Klamath phase of the Nevadan orogeny resulted from contraction of the west-facing intraoceanic arc and Josephine backarc basin beneath the continental margin. Basal sediments of the Great Valley forearc basin were derived primarily from the sutured arc/ophiolite terranes, and were deposited on top of the Coast Range ophiolite, the southern edge of the Klamaths, and the western side of the Sierra Nevada. A new (late Mesozoic) magmatic arc was superposed across the previously accreted terranes, and formed the primary sediment source for the Cretaceous forearc basin.
TL;DR: In this article, the authors used the deeply-towed instrument package of the Scripps Institution of Oceanography to study the fine-scale structure of an overlapping spreading center (OSC) system: the 9°03'N OSC on the East Pacific Rise (EPR).
Abstract: The deeply-towed instrument package of the Scripps Institution of Oceanography was used to study for the first time the fine scale structure of an overlapping spreading center (OSC) system: the 9°03’N OSC on the East Pacific Rise (EPR). The eastern and western limbs of this OSC system which are 8 km apart and overlap by 27 km show marked differences. Away from the tips, the spreading centers are highly tectonized and differ greatly from the ridge tips. The western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area. In both cases, the ridge tip fissures are 10°–15° oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis, indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively. The eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through-going trace of the East Pacific Rise. In contrast, the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis. Some of the very few tectonic features observed in the overlap basin may reflect the existence of a short-lived shear couple due to the interaction of the two overlapping spreading centers. Sediment cover over the whole area and especially in the overlap basin is relatively thick, supporting the idea of a time-averaged deficit in the magmatic budget at this OSC. We suggest that the 9°03’N OSC developed where two magmatic pulses, independent in space and time and propagating along the strike of the East Pacific Rise away from the loci of melt emplacement, failed to meet. Misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing OSC geometry. Independent lines of evidence derived from the three-dimensional inversion of the magnetic field at the 9°03’N OSC (Sempere et al., 1984), from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field (Sempere and Macdonald, 1986) and from geochemical data (Langmuir et al., 1986) support our interpretation of the Deep-Tow data. We suggest that deviations from axial linearity of the spreading centers (DEVALs; Langmuir et al., 1986) and small nonoverlapping offsets (SNOOs; Batiza and Margolis, 1986) are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on. OSCs and some saddle points (i.e., DEVALs, SNOOs) are all small, rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells beneath the East Pacific Rise.
TL;DR: The late Cenozoic episode of crustal shortening in the South Island, New Zealand, which has caused present day uplift rates of greater than 10 mm/yr and formed the Southern Alps, may have also caused clockwise rotation of the Alpine Fault by up to 20° as discussed by the authors.
Abstract: The late Cenozoic episode of crustal shortening in the South Island, New Zealand, which has caused present day uplift rates of greater than 10 mm/yr and has formed the Southern Alps, may have also caused clockwise rotation of the Alpine Fault by up to 20°. In contrast to the northern end of the Alpine Fault, which appears to have remained fixed relative to the Australian plate, increasing westward movement of the Fault trace has occurred southward along the fault. This westward movement may exceed 100 km where the Fault crosses the coastline in northern Fiordland. Most shortening adjacent to the southern end of the Alpine Fault has occurred within Australian plate continental crust which has been thrust beneath Pacific plate continental crust. This zone of continental underthrusting merges with the Fiordland subduction zone further south, where Australian plate oceanic crust is being subducted. Gravity modelling of the continental collision zone beneath the Southern Alps indicates that the leading edge of underthrust Australian plate crust may be close to a relatively sharp 10 km change in Moho depth. Inward dipping thrust faults on both plates mark the outer limits of the collision zone, which is over 300 km wide near the southern end of the Alpine Fault. The probable increase in crustal thickness due to the late Cenozoic shortening suggests that the amount of crustal thickening is similar in size to the amount of eroded crust.
TL;DR: In this article, the effect of deformation on the remanent magnetization vector was investigated using passive marker theory, and the results showed that the remanence distribution was dominated by the shortening axis directions of the magnetic anisotropy ellipsoid.
Abstract: Paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector. Deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis. However, the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known. Anisotropy of magnetic susceptibility (AMS) provides a quantitative basis for strain determination. The principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid. A linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types. If the correlation method used is valid, AMS observations can be used to establish the magnitudes and directions of the principal strains. Strain modifies bedding planes and realigns the grains that carry magnetic remanence. Compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain. The effects of strain on bedding can be corrected using passive marker theory. Attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive. During deformation linear and planar elements deflect away from the axis of maximum shortening. The passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter. Tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions. The passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix.
TL;DR: The Transantarctic rift, an extensional continental rift valley, formed between East and West Antarctica during latest Early and Middle Jurassic time and is represented today by the high Transantcarctic Mountains, which contain large volumes of continental flood basalt, diabase, and gabbro as mentioned in this paper.
Abstract: The Transantarctic rift, an extensional continental rift valley, formed between East and West Antarctica during latest Early and Middle Jurassic time and is represented today by the high Transantarctic Mountains, which contain large volumes of continental flood basalt, diabase, and gabbro. Transantarctic rifting marked the beginning of the breakup of Gondwanaland; it was contiguous and synchronous with continental rifting between East Antarctica-India and Africa as represented by the continental basalt and diabase of Queen Maud Land and the Karroo of southern Africa. During Late Jurassic time, about 150 Ma or slightly earlier, East and West Gondwanaland separated and new oceanic crust of the earliest Indian Ocean formed between East Antarctica-India and Africa. If, as assumed, West Antarctica and South America remained fixed through a tip-to-tip join between the Antarctic Peninsula and Tierra del Fuego, then this seafloor spreading required major right-lateral transform faulting of 500 to 1000 km on the Transantarctic rift system between East and West Antarctica. The Transantarctic Mountains were elevated at about the same time in Late Jurassic; such uplifts are characteristic of active rift margins worldwide. During Cenozoic time, extensional block faulting, independent of the Jurassic rifting, further disrupted large areas of West Antarctica. During the same time, the Transantarctic Mountains were further uplifted.