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Detailed analysis of whole-rock geochemistry, clay minerals, sedimentary color, and pollen in the Dahonggou section, northeast of the Qaidam Basin, are investigated, and the results suggest an intense weathering in the source area during the middle Eocene (~8.5-40.5 Ma), indicating a warm and humid condition. The distinct decrease of chemical weathering degree in source regions began at ~0.5 Ma, which is in agreement with the distinct decrease in redness of sedimentary sequences and the disappearance of thermophilic elements in pollen records. This 40.5 Ma cooling event extent demonstrated evidence for an intensification of central Asian aridification, which could be attributed to attainment of high elevations in southern-central Tibet and retreat of the Paratethys from central Asia in the late Eocene, reducing moisture transport to the Qaidam Basin. Une analyse detaillee de la geochimie de la roche entiere, des mineraux argileux, de la couleur des sediments et du pollen dans le secteur de Dahonggou, au nord-est du bassin Qaidam, a ete effectuee et les resultats montrent une alteration meteorique intense dans la region source au cours de l'Eocene moyen (~8,5-40,5 Ma), indiquant des conditions chaudes et humides. La decroissance sensible du degre d'alteration chimique dans les regions sources a debute vers 40,5 Ma, ce qui concorde avec la diminution sensible de la rougeur de la sequence sedimentaire et la disparition des elements thermophiles dans les enregistrements de pollen. Cet evenement de refroidissement a 40,5 Ma prouve une intensification de l'aridification de l'Asie Centrale, laquelle pourrait etre attribuee a l'atteinte de hautes elevations dans le centre-sud du Tibet et le retrait du Paratethys de l'Asie Centrale a l'Eocene tardif, reduisant le transport d'humidite vers le bassin Qaidam. [Traduit par la Redaction], Introduction In terms of Cenozoic climate history, the transition from the extreme greenhouse of the early Paleogene (~2 Ma) to the present-day icehouse is the most prominent change in Earth's [...]

Because the inertia of tectonic plates is negligible, plate velocities result from the balance of forces acting at plate margins and along their base (1). Observations of past plate motion derived from marine magnetic anomalies provide evidence of how continental deformation may contribute to plate driving forces (2-8). A decrease in convergence rate at the inception of continental collision is expected because of the greater buoyancy of continental than oceanic lithosphere (2,3), but post-collisional rates are less well understood. Slowing of convergence has generally been attributed to the development of high topography that further resists convergent motion (7-10); however, the role of deforming continental mantle lithosphere on plate motions has not previously been considered. Here I show that the rate of India's penetration into Eurasia has decreased exponentially since their collision. The exponential decrease in convergence rate suggests that contractional strain across Tibet has been constant throughout the collision at a rate of 7.03 x [10.sup.-16] [s.sup.-1], which matches the current rate. A constant bulk strain rate of the orogen suggests that convergent motion is resisted by constant average stress (constant force) applied to a relatively uniform layer or interface at depth. This finding follows new evidence that the mantle lithosphere beneath Tibet is intact (11), which supports the interpretation that the long-term strain history of Tibet reflects deformation of the mantle lithosphere. Under conditions of constant stress and strength, the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved., Marked slowing of the convergence rate of India with Eurasia determined from plate circuit motions (2,3) agrees broadly with the estimate of continental collision age from the stratigraphic record (12), [...]

The timing and causes of the >1.0 km elevation gain of the southern African Plateau since Paleozoic time are widely debated. We report the first apatite and titanite (U-Th)/He thermochronometry data for southern Africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau. The study area encompasses ~1500 m of relief within Archean basement of the Barberton Greenstone Belt region of the eastern Kaapvaal craton. Titanite dates are Neoproterozoic. Apatite dates are Cretaceous, with most results clustering at ca. 100 Ma. Thermal history simulations confirm Mesozoic heating followed by accelerated cooling in mid- to Late Cretaceous time. The lower temperature sensitivity of the apatite (U-Th)/He method relative to previous thermochronometry in southern Africa allows tighter constraints on the Cenozoic thermal history than past work. The data limit Cenozoic temperatures east of the escarpment to [less than or equal to] 35[degrees]C, and appear best explained by temperatures within a few degrees of the modern surface temperature. These results restrict Cenozoic unroofing to less than ~850 m, and permit negligible erosion since the Cretaceous. If substantial uplift of the southern African Plateau occurred in the Cenozoic as advocated by some workers, then it was not responsible for the majority of post-Paleozoic unroofing across the eastern escarpment. Significant Mesozoic unroofing is coincident with large igneous province activity, kimberlite magmatism, and continental rifting within and along the margins of southern Africa, compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events. doi: 10.1130/G30980.1

Cenozoic uplift of the East African Plateau has been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While this influence is widely accepted, the timing and the magnitude of plateau uplift have remained unclear. This uncertainty stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. Here, we document the existence of significant relief along the East African Plateau prior to rifting, as inferred from modeling the emplacement history of one of the longest terrestrial lava flows, the ~300-km-long Yatta phonolite flow in Kenya. This 13.5 Ma lava flow originated on the present-day eastern Kenya Rift flank, and utilized a riverbed that once routed runoff from the eastern rim of the plateau. Combining an empirical viscosity model with subsequent cooling and using the Yatta lava flow geometry and underlying paleotopography (slope angle), we found that the prerift slope was at least 0.2[degrees], suggesting that the lava flow originated at a minimum elevation of 1400 m. Hence, high paleotopography in the Kenya Rift region must have existed by at least 13.5 Ma. We infer from this that middle Miocene uplift occurred, which coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa. doi: 10.1130/G31022.1

The Early Cretaceous Ontong Java Plateau was emplaced at almost the same time as marine biotic changes that culminated in oceanic anoxic event 1 (OAE1a). A causative link between these events has been suggested, but direct evidence has been lacking until now. New Os isotope measurements across the Lower Aptian 'Selli Level' black shale deposited during OAE1a in central Italy reveal two negative excursions in marine [sup.187]Os/[sup.188]Os ratios within a period of 2 Ma starting above the Barremian-Aptian boundary and ending just above the Selli Levei horizon, suggesting an order-of-magnitude increase in the global flux of unradiogenic Os. The results are consistent with early and major phases of eruption of the Ontong Java Plateau. The latter phase is estimated to have been as short as ~1 Ma and may have induced widespread oceanic stratification that triggered OAE1a.

New magnetostratigraphic data from seven Western Ghats sections in the Deccan Volcanic Province are presented. These are combined with an established geochemically defined stratigraphy, and volcanological logs, to provide a correlated, chronological eruptive framework. We identify two magnetic polarities in five of the sections, and these are assigned to chrons 29r and 29n. Importantly, the reversal boundary represents an identifiable isochronous surface within the volcanic pile. This surface occurs at different elevations, as does the altitude of the geochemically defined formation boundary (i.e. Ambenali-Mahabaleshwar Fms), which defines a second isochronous surface. Inspection reveals significant differences in the number and thickness of lava units preserved between these two surfaces. This indicates that there was significant local topography (c. 80 m) across Deccan Volcanic Province lava fields during their development; an interpretation consistent with topographies observed across modern and historical examples (e.g. Hawaii, Iceland). These data also indicate that the geochemical stratigraphies of continental flood basalt provinces can mask local and sub-regional detail in lava stacking patterns when applied at smaller spatial scales (

Lacustrine strata of the Wudaoliang Group demonstrate that two vast lake complexes covered the central Tibetan Plateau during early Miocene time. The Wudaoliang Group, which is characterized by dolostone, limestone, and marl as thick as 310-350 m, lies horizontally or with a slight dip above Oligocene reddish-brown sandstone, and is covered by upper Miocene and Pliocene mudstone and sandstone. The extensive distribution of this group outlines interconnected lake basins of 5000-15,000 [km.sup.2] or larger in the north-central Tibetan Plateau, such as the Wudaoliang, Beiluhe, Tuotuohe, Tongtianhe, and East Wenquan Basins, and lake basins 2000 [km.sup.2] or larger in the south-central plateau, such as the Ando, Naqu, Bangoin, Lunpola, and Shuanghu Basins. These lake basins, separated by mountain ranges and islands, were linked by water passages in early Miocene time, when they apparently formed a huge lake complex that covered as much as 100,000 [km.sup.2] in the north-central Tibetan Plateau and another one >50,000 [km.sup.2] in the south-central part of the plateau. Such immense lakes existed for several million years between ca. 23.5 and ca. 13.5 Ma after the Tibetan Plateau rose, and their interconnected basins define the topography during early Miocene time. The fossils contained in these basins indicate a change from the warm, dry climate of the Oligocene to moderately cool and wet conditions at the beginning of the early Miocene, followed by progressive cooling and drying, which suggests continuous uplift during deposition of the Wudaoliang Group. Keywords: Wudaoliang Group, lacustrine strata, vast lakes, Miocene spores and pollen, paleoclimate, early Miocene, central Tibetan Plateau, Miocene topography.

Eastern Anatolia is the site of lithospheric thinning, plateau uplift, heating, and synconvergent extension. Using numerical geodynamic experiments, we test the hypothesis that these tectonic anomalies are all related and the consequence of delamination of the mantle lithosphere. Our findings indicate that delamination during plate convergence results in ~2-km-high plateau uplift. The removal of mantle lithosphere induces distinct regions of contraction and thickening, as well as extension and thinning of the crust. The latter occurs even within a regime of plate shortening, although it is muted with increasing plate convergence. Detachment of the delaminating slab results in minor surface topographic perturbation, but only above the delamination hinge. The plateau uplift and pattern of surface contraction and/or extension are consistent with a topographic profile at 42[degrees]E and geologically interpreted zone of synconvergent extension at eastern Anatolia. Keywords: Eastern Anatolian plateau, delamination, lithospheric thinning, synconvergent extension.

The Xianshuihe fault system is a highly active, left-lateral slip fault that has played an important role in accommodating late Cenozoic crustal deformation of the Eastern Tibetan Plateau. The left-lateral displacement on the western portion of the fault system, the Ganzi fault, is ~80 km, based on geologic and geomorphic markers. Left-lateral movement along the fault may have initiated in middle Miocene time (12 Ma), followed by the intensive, brittle, left-lateral movement in late Cenozoic time (2-4 Ma). Lying within the headwater regions of the Jinsha (Yangtze) River in the Yusbu region, the northwestern end of the Xianshuihe fault system consists of four branches. These are, from north to south--the Dangjiang fault, the Yushu fault, the Batang fault, and the Xialaxiu fault. Both the Yushu and Dangjiang faults display clear geologic and geomorphic evidence for active, left-lateral slip movement. Several offset markers of Triassic age along the Dangjiang fault suggest ~39 km of left-lateral slip, but the geomorphic evidence along the Dangjiang and Yushu faults suggests only ~16 km and ~25 km of left-lateral slip, respectively. Both the Batang and Xialaxiu faults demonstrate clear evidence for normal dip slip, by an associated series of E-W-trending grabens and horsts that absorb ~9 km of leftlateral movement in early to middle Pleistocene time. About 32 km of left-lateral movement on the fault system was estimated to be absorbed by NE-SW shortening along the rugged North Yushu block in late Neogene time. This is interpreted to have formed as a rhomb-shaped restraining bend, along which the left-lateral movement along the northwestern end of the fault system was partially transferred into crustal shortening. The westward continuation of the Dangjiang fault is marked by an E-W-trending fault, which can be traced westward for 150 km to central Tibet, where it joins the Fenghuo Shan thrust belt. This thrust belt is interpreted to represent the northwestern continuation of the Xianshuihe fault system, along which 39 km of the left-lateral movement was entirely transferred into the N-S shortening. It indicates that the left-lateral movement within the southeastern part of the Tibetan Plateau along the fault system, either caused by lateral extrusion or by clockwise rotation, is largely restricted to the northwestern end of the fault system. Quaternary extension occurred along the northwestern continuation of the Xianshuihe-Xiaojiang fault was coeval with extension along its southern end, indicating that the southeastern margin of the plateau underwent clockwise rotation.

Two end member models of how the high elevations in Tibet formed are (i) continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and (ii) time-dependent, localized shear between coherent lithospheric blocks. Recent studies of Cenozoic deformation, magmatism, and seismic structure lend support to the latter. Since India collided with Asia ~55 million years ago, the rise of the high Tibetan plateau likely occurred in three main steps, by successive growth and uplift of 300- to 500-kilometer-wide crustal thrust-wedges. The crust thickened, while the mantle, decoupied beneath gently dipping shear zones, did not. Sediment infilling, bathtub-like, of dammed intermontane basins formed flat high plains at each step. The existence of magmatic belts younging northward implies that slabs of Asian mantle subducted one after another under ranges north of the Himalayas. Subduction was oblique and accompanied by extrusion along the left lateral strike-slip faults that slice Tibet's east side. These mechanisms, akin to plate tectonics hidden by thickening crust, with slip-partitioning, account for the dominant growth of the Tibet Plateau toward the east and northeast., Tibet is Earth's largest and highest plateau, with a flat interior disrupted by active normal faulting (1-6) (Fig. 1). How such topography formed and is maintained has profound implications on [...]

The term tessera has been used to describe regions of deformed venusian crust exhibiting two or more intersecting sets of structural elements; however, tessera includes terrains formed by a variety of spatially and temporally discrete tectonic processes. Tessera fabric characterizes highland plateau structure, and thus understanding the nature of this deformation is critical to understanding the mode of highland plateau formation. Many tessera fabrics include ribbons, folds, and late graben. In this paper, we refine the geometry of ribbons through geologic mapping and radargrammetric analysis of type ribbon structures at southwestern Fortuna Tessera; we extend our findings to ribbon fabrics at Thetis Regio. Any model of ribbon formation must account for the following constraints on ribbon geometry. (a) Ribbon-forming lineaments exhibit sharp contrasts relative to adjacent materials. (b) Ribbon-bounding lineaments form a distinct pattern alternating between radar-dark and radar-bright, which represent trough walls oriented away from and toward the satellite, respectively. (c) Ribbons form long, narrow troughs that alternate with parallel, narrow ridges; ridges and troughs display extreme length: width aspect ratios. (d) Trough walls are near vertical. (e) Troughs are shallow with consistent shallow depth along individual troughs and in adjacent troughs. (f) In some cases (e.g., southwest Fortuna Tessera) trough walls are parallel and matched and would exhibit a close fit if the trough was closed; in these cases trough walls merge laterally forming V-shaped terminations. Trough floors are smooth and flat, lacking small-scale interior lineaments. (g) In other cases (e.g., Thetis Regio) ribbons display (a)-(d), but trough walls are defined by a series of subparallel lineaments including local interior lineaments, and trough floors ramp up to join trough walls displaying parallel rather than V-shaped terminations. Trough walls would not display a close fit if closed. We propose two member types of ribbons, tensile-fracture ribbons and shear-fracture ribbons. Tensile-fracture ribbons display features (a)-(f) and formed by the opening of tensile fractures of a thin brittle layer above a ductile substrate. They require a near fracture-free shallow crust and very shallow depth to the brittle-ductile transition (BDT) ( Key Words: boudins; geological processes; radar; Venus; Venus, surface.

Fractures in oriented Middle Devonian shale cores from the Appalachian Plateau province provide evidence for multiple deformation events associated with the Alleghanian orogeny post-orogenic stress relaxation, and neotectonic (?) stresses. The orientation, distribution, and mineral paragenesis of these fractures are used to establish the timing, regional extent, and stress orientation of each deformation event. During the Alleghanian orogeny, the Middle Devonian shale section was a regional decollement zone in the central Appalachian Plateau province. The decollement extends from the Appalachian Structural Front to northwestern Pennsylvania and east-central Ohio. It is defined by abundant joints, veins, and slickensided fractures localized within multiple zones of organic-rich brown and black shale. These zones are distributed over tens to hundreds of meters of the Middle Devonian shale section. The shale cores record a continuous counterclockwise rotation of the maximum compressive stress direction in the west-central Appalachians during the Alleghanian orogeny. Three stages are recognized: stage 1 - early Alleghanian tectonic jointing and slip along the Middle Devonian shale decollement with shortening directed 340 [degrees] to 360 [degrees] stage 2 - 'main phase' Alleghanian tectonic jointing and detachment that is pre- to syn-folding, with shortening directed 300 [degrees] to 335 [degrees]; stage 3-late Alleghanian tectonic jointing, detachment, and coal joint formation, that is generally post-folding with shortening directed 270 [degrees] to 295 [degrees]. Local Stage 3 shortening directed 255 [degrees] to 270 [degrees] resulted in jointing in southwestern Pennsylvania and northern West Virginia. Discrete post-Alleghanian deformation events include stage 4-post-Alleghanian release jointing associated with regional stress relaxation and a 030 [degrees]-oriented shale fabric anisotropy, and stage 5-neotectonic (?) jointing related to unloading in the 060 [degrees]-to 070 [degrees]-oriented contempory stress field.

[1] Deformation measured by regional GPS networks in continental plateaus reflects the geologic and tectonic variability of the plateaus. For two collisional plateaus (Tibet and Anatolia) and one noncollisional (the Altiplano), we analyze the regional strain and rotation rate by inverting GPS velocities to calculate the full two-dimensional velocity gradient tensor. To test the method, we use gridded velocities determined from an elastic block model for the eastern Mediterranean/ Middle East region and show that to a first order, the deformation calculated directly from the GPS vectors provides an accurate description of regional deformation patterns. Principal shortening and extension rate axes, vertical axis rotation, and two-dimensional (2-D) volume strain (dilatation) are very consistent with long-term geological features over large areas, indicating that the GPS velocity fields reflect processes responsible for the recent geologic evolution of the plateaus. Differences between geological and GPS descriptions of deformation can be attributed either to GPS networks that are too sparse to capture local interseismic deformation, or to permanent deformation that accrues during strong earthquakes. The Altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2-D dilatation everywhere. Vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau. In contrast, the collisional plateaus have large regions of quasi-rigid body rotation bounded by strike-slip faults with the opposite rotation sense from the rotating blocks. Tibet and Anatolia are the mirror images of each other; both have regions of positive dilatation on the outboard sides of the rotating blocks. Positive dilatation in the Aegean correlates with a region of crustal thinning, whereas that in eastern Tibet and Yunnan province in China is associated with an area of vertical uplift. Rollback of the Hellenic trench clearly facilitates the rotation of Anatolia; rollback of the Sumatra-Burma trench probably also enables rotation about the eastern syntaxis of Tibet. Citation: Allmendinger, R. W., R. Reilinger, and J. Loveless (2007), Strain and rotation rate from GPS in Tibet, Anatolia, and the Altiplano, Tectonics, 26, TC3013, doi:10.1029/ 2006TC002030.

East-west extension in the Tibetan Plateau is generally assumed to have resulted from gravitational collapse following thickening and uplift. On the basis of this assumption, several studies have dated east-west extensional structures to determine when the plateau attained its current high elevation. However, independent estimates of elevation are needed to determine whether extension occurred before, during, or after the plateau achieved its current elevation. Because the isotopic composition of meteoric water decreases with increasing elevation, significant change in local elevation throughout the Thakkhola graben depositional history should be recorded by change in [Delta][sup.18]O values of fluvial and lacustrine carbonates. The [Delta][sup.18]O values of-16% to -23% of Thakkhola graben carbonates reflect meteoric water values similar to modern values and suggest that the southern Tibetan Plateau attained its current elevation prior to east-west extension. Initiation of Thakkhola graben extension is constrained between 10 and 11 Ma, based on magnetostratigraphy of the older Tetang Formation. The [Delta][sup.13]O values of soil carbonates suggest an age younger than 8 Ma for the base of the Thakkhola Formation. Keywords: Tibetan Plateau, paleoelevation, oxygen isotopes.

[1] High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperature-time paths inferred from [sup.40]Ar/[sup.39]Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar [sup.40]Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (30 [degrees] - 50 [degrees] C/m.y.) coincident with exhumation from inferred depths of ~8-10 km, at denudation rates of 1-2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period (~3 [degrees] C/m.y.), suggesting limited exhumation (1-2 km). However, these samples record a slight increase in cooling rate (from