B. Clark Burchfiel

The Geological Evolution of the Tibetan Plateau

The geological evolution of the Tibetan plateau is best viewed in a context broader than the India-Eurasia collision zone. After collision about 50 million years ago, crust was shortened in western and central Tibet, while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western Pacific and Indonesia. Cessation of rapid Pacific trench migration (∼15 to 20 million years ago) coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern Tibet. The latter appear to result from rapid eastward flow of the deep crust, probably within crustal channels imaged seismically beneath eastern Tibet. These events mark a transition to the modern structural system that currently accommodates deformation within Tibet.

Large‐scale crustal deformation of the Tibetan Plateau

The topography, velocity, and strain fields calculated from a three-dimensional Newtonian viscous model for large-scale crustal deformation are generally in good agreement with results from geological, geodetic and earthquake studies in and around the Tibetan Plateau, provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau (equivalent to a viscosity of 1012 Pa s within a 250-m-thick channel or 1018 Pa s within a 15-km-thick channel). Model studies and observations show a plateau at ∼5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins. Rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins (at rates ∼20 mm/yr). Model results show north-south shortening (∼10 mm/yr) in reasonable agreement with GPS data (5–8 mm/yr of north-south shortening across the northern two thirds of the plateau) and east-west stretching (10–15 mm/yr) across the eastern half of the high plateau, in reasonable agreement with seismic, geologic, and GPS data. Upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east. Clockwise rotation of material around the east Himalayan syntaxis (at rates up to ∼10 mm/yr) occurs partly as a result of dextral shear between Indian and Asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east. There is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust, except in southern and northern Tibet where margin-perpendicular extension occurs only for the case of an extremely weak lower crust. Our results suggest that the Tibetan Plateau is likely to have gone through a two-stage development. The first stage produced a long, narrow, high orogen whose height may have been comparable to the modern plateau. The second stage produced a plateau that grew progressively to the north and east. East-west stretching, eastward plateau growth and dextral rotation around the east Himalayan syntaxis probably did not begin until well into the second stage of plateau growth, perhaps becoming significant after ∼20 m.y. of convergence.

Geodetic measurement of crustal motion in southwest China

Global Positioning System measurements performed over a 2–4 yr period confirm the Xianshuihe-Xiaojiang fault system as one of the primary active structures in southwest China. Stations southwest of these faults show southerly motions of 5–15 mm/yr relative to the western Sichuan basin, and stations in a 200 km geodetic network located northwest of the basin move at only 0–5 mm/yr. These results are consistent with clockwise rotation of southwestern Sichuan and western Yunnan about the eastern Tibetan syntaxis, accommodated by left-lateral slip of 12 ± 4 mm/yr on the Xianshuihe-Xiaojiang fault system. The results imply that if eastward extrusion of crustal material from the plateau occurs at present, it is not accommodated by east-west shortening along the margin of the plateau and must involve wholesale eastward motion of low-lying regions to the east.

Quaternary Climate Change and the Formation of River Terraces across Growing Anticlines on the North Flank of the Tien Shan, China

Nested stream terraces, warped upward over actively growing anticlines along the north flank of the Tien Shan in western China, appear to record alternating phases of valley widening and incision. Differences of relative heights between remnants of four separate strath terraces along one river and between two such terraces along another reach 100 to 120 m over the crests of the anticlines. We infer that this spacing is due to alternating stages of valley widening and rapid incision associated with climate changes with a periodicity of 100 kyr. The crests of the anticlines appear to emerge from the aggrading flanks of the anticlines at an average rate of about 1 mm/a. The maximum heights of 25 and 35 ( ± 10) m for the lowest terraces above their projected initial profiles imply ages of roughly 25 kyr and 35 kyr ( ± 10 kyr). Hence, they suggest that flood plains, which were abandoned to form the terraces, developed adjacent to active stream beds during the last glacial period, when climates were relatively cold and dry. We presume that they were incised during deglacial periods when discharges and stream power increased. Apparent durations of exposure, obtained from

Neotectonics of the Min Shan, China: Implications for mechanisms driving Quaternary deformation along the eastern margin of the Tibetan Plateau

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Propagation of surface uplift, lower crustal flow, and Cenozoic tectonics of the southeast margin of the Tibetan Plateau

in quartz cobbles lying on the surface of the lower terrace from one anticline, concur with abandonment and deep (~150 m) incision of the flood plain during the last global deglaciation (ca. 20 to 13 kyr B.P.). A minimum carbon-14 date of 33.9 kyr B.P. from deposits on the lowest terrace sequence from the other anticline, however, implies that such abandonment and incision of this flood plain occurred before the most recent global glacial maximum, about 20 kyr B.P. We infer that incision of this second anticlines floodplain began during an earlier deglacial epoch within the last glacial period (between about 70 and 20 kyr, and perhaps near 35 kyr B.P.).

Miocene to present activity along the Red River fault, China, in the context of continental extrusion, upper-crustal rotation, and lower-crustal flow

From at least 2–4 Ma to present, crust in the southeastern part of the Tibetan Plateau west of the convex-east Xianshuihe-Xiaojiang fault system has deformed internally and rotated clockwise around the eastern Himalayan syntaxis. The northwest-striking Ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78–100 km, of which ∼60 km is transferred to the Xianshuihe fault zone across a diffuse transfer zone, and ∼22–40 km is absorbed by bending of older structures and crustal shortening. Crustal shortening is expressed along and east of the eastern end of the Ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the Tibetan Plateau. A similar transfer of left slip into shortening occurs farther south across the Xianshuihe fault in the high mountains around and east of Gongga Shan (7556 m). The northwest-striking, convex-east, left-lateral Litang fault zone lies southwest of the Ganzi-Xianshuihe-Xiaojiang fault zone and appears to be less well developed but otherwise similar to the Ganzi fault zone. The Batang, Chenzhi, and other northeast-striking right-lateral faults of small displacement occur within the rotating crustal fragment. Together with the left-slip faults, they accommodate east-west shortening northeast of the eastern Himalayan syntaxis. South of this region of shortening, the crust is extending to form grabens within the Dali and southern Xiaojiang fault systems and in the Tengchong volcanic province. The progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern Himalayan syntaxis. The assemblage of structures in southwestern Sichuan geometrically resembles structures of Eocene to Miocene age in southern Yunnan that were positioned northeast of the eastern Himalayan syntaxis, similar to present-day southwestern Sichuan, at the time of their development. The similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past ∼50 m.y. Structures in Sichuan are less fully developed than older structures in southwestern Yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis. Deformation in these areas indicates that plateau formation has been complex, inhomogeneous, and diachronous at scales from 1000 km to less than 100 km.

Mesozoic large-scale lateral extrusion, rotation, and uplift of the Tongbai-Dabie Shan belt in east China

The Min Shan region, located along the eastern margin of the Tibetan Plateau north of the Sichuan Basin, provides an important natural laboratory in which to study the rates and patterns of deformation and their relationship to mountain building at the margin of the plateau. The topographic margin of the plateau is coincident with a north-trending mountain range, the Min Shan, that stands nearly 2 km above the mean elevation of the plateau (~3500 m in this region). We exploit the preservation of a series of variably deformed Quaternary sediments along the western flank of the range to investigate the Pleistocene-Holocene deformation field within the Min Shan region. Mapping and field observations of remnant alluvial fans of late Pleistocene age indicate that deformation within the Min Shan involved substantial (~10°), rapid, down-to-the-northwest tilting. The geometry of the deposits and the partial preservation of an erosion surface beneath the basin suggest that much of the modern relief of the Min Shan relative to the Tibetan Plateau is a consequence of this late Pleistocene tilting. Rates of tilting inferred from luminescence dating of interbedded loess have been remarkably rapid (~10 ‐8 rad/yr). Similarly rapid rates of Holocene differential rock uplift are inferred from tilted lacustrine sediments in the southwestern part of the range. The range is bounded on the west by the Min Jiang fault zone, an east-vergent reverse fault. However, Holocene alluvial terraces in headwaters of the Min River are preserved across the fault in several places, indicating that displacement rates on the Min Jiang fault are <1 mm/yr. Active faulting only occurs along the eastern foot of the range (Huya fault) for a short distance (~60 km), despite 3 km of relief on the eastern range front. The relationship between these structures and the tilting observed in the Min Jiang basin is enigmatic; the faults do not appear to exert a strong control on the rates and pattern of deformation within the basin. A simple flexural model demonstrates that rates of tilting on the western flank of the Min Shan are too high to be simply attributed to an isostatic response to surficial loading and unloading of the lithosphere. Present-day horizontal shortening across the Min Shan is geodetically determined to be less than 2‐3 mm/yr, suggesting that only a small part of the observed tilting can be attributed to horizontal shortening. Thus, tilting and concomitant differential rock uplift in the Min Shan appear to require an additional driving component. We suggest that Quaternary deformation along the western Min Shan may reflect the surface response to thickening of a weak lower crust at the margin of the Tibetan Plateau.