[1] Recent studies of the northeastern part of the Tibetan Plateau have called attention to two emerging views of how the Tibetan Plateau has grown. First, deformation in northern Tibet began essentially at the time of collision with India, not 10-20 Myr later as might be expected if the locus of activity migrated northward as India penetrated the rest of Eurasia. Thus, the north-south dimensions of the Tibetan Plateau were set mainly by differences in lithospheric strength, with strong lithosphere beneath India and the Tarim and Qaidam basins steadily encroaching on one another as the region between them, the present-day Tibetan Plateau, deformed, and its north-south dimension became narrower. Second, abundant evidence calls for acceleration of deformation, including the formation of new faults, in northeastern Tibet since~15 Ma and a less precisely dated change in orientation of crustal shortening since~20 Ma. This reorientation of crustal shortening and roughly concurrent outward growth of high terrain, which swings from NNE-SSW in northern Tibet to more NE-SW and even ENE-WSW in the easternmost part of northeastern Tibet, are likely to be, in part, a consequence of crustal thickening within the high Tibetan Plateau reaching a limit, and the locus of continued shortening then migrating to the northeastern and eastern flanks. These changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation.

No abstract

[1] Based on field investigations, aerial-photo morphological analysis, topographic profiling, and optically stimulated luminescence (OSL) dating of alluvial surfaces, we estimate vertical components of the slip rate along the South Heli Shan thrust fault, which lies on the northern margin of the Hexi Corridor and the northeastern edge of the Tibetan Plateau. The fault consists of three segments with scarp heights ranging from less than 1 m to more than 16 m. OSL dating indicates that most of the alluvial fans cut by fault scarps formed during the transition from the last glacial stage to the present interglacial stage from 19 to~9 ka along southern Heli Shan and from~27 ka to~22 ka along its northern margin. In addition, remnants of older alluvial fan have been abandoned after~67 ka. Scarp heights increase from west to east and reach a maximum of more than 16 m near the eastern end. Using three approaches, we calculate late Quaternary slip rates for each of the three fault segments along the southern margin and the fault on the northern flank. These approaches yield maximum vertical slip rates from 0.18 to 0.2 mm/a for the western segment, 0.3 to 0.43 mm/a for the central segment, 0.36 to 0.53 mm/a for the eastern segment, and 0.21 mm/a for the Wutongjing Fault, which lies on the north side of the Heli Shan. For a range of likely fault dips, these correspond to 0.1-0.2 mm/a of average horizontal shortening for the western segment, and increase to 0.4-0.5 mm/a across the eastern segment of the southern Heli Shan Fault. Combining the height of the eastern parts of the Heli Shan (Daqing Peak) above the Hei He (a major river that incised the western end of the range) and the vertical component of the slip rate of the eastern segment, we suggest that the Heli Shan was uplifted by motion on the South Heli Shan Fault beginning sometime between 1 and 4 Ma, most likely since~2 Ma. This age suggests that the Tibetan Plateau continues to grow northeastward across the Hexi Corridor.

No abstract

Significance The Songpan-Ganzi terrane lies in the central-east of the Tibetan Plateau, which was considered a stable block in some tectonic models. Its deformation mode is of crucial importance for understanding the evolutionary history and seismic hazard of the plateau. The recent Maduo earthquake occurred inside the terrane. We resolve a bilateral rupture process with distinct super- and subshear rupture modes for this event. We also find that pervasive folding structures that are aligned by shear deformation in the current Songpan-Ganzi terrane are responsible for the seismic wave anisotropy and shear strain orientation in its upper crust. Its deformation mode can be classified as distributed simple shear, which receives shear loads from side walls and produces internal earthquakes.