Abstract

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 Yushu 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 left-lateral 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.

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