NEW EVIDENCE FOR TWO-STAGE EVOLUTION OF THE ALTYN TAGH FAULT AND ITS IMPLICATIONS FOR THE INDO-ASIAN COLLISION ZONE

New geological and geophysical evidence supports the two-stage model for the Altyn Tagh fault (ATF) of Yue and Liou (1999 Geology 27; 227-230), which predicted: 1. 400±50 km of left-lateral offset at the present northeastern end of the ATF; 2. low post-Early Miocene slip rate along its eastern segment; 3. mid-Miocene exhumation of northern Tibet; and 4. Cenozoic age for the Alxa-East Mongolia fault. First, Paleozoic and Tertiary piercing points along the ATF constrain the left-lateral offset around the eastern end of the fault at 375±25 km. Second, a correlation between granitic boulders in the upper Lower Miocene strata of the Xorkol basin and their source terrane across the fault indicates that post-Early Miocene slip rate along the eastern segment of the ATF is less than 10 mm/year. Third, apatite fission-track analysis of the northern Qilian Shan (George et al., 2001, Geology 29; 939-942) and sedimentary basin analysis of the Subei basin indicate that exhumation of northern Tibet began between 10 and 20 Ma. Fourth, interpretation of seismic profiles demonstrates that the East Mongolia fault, previously considered a Paleozoic or Jurassic structure, is post-Late Cretaceous (Johnson, 2002, Stanford Ph.D. Dissertation).

Not only do these new data support the two-stage model of the ATF, but they shed new light on the tectonics of the Indo-Asian collision zone as well. The amount of extrusion during the Oligocene and Early Miocene along the ATF (375±25 km) and Red River fault (500±200 km) is able to accommodate most or all of the synchronous convergence between India and Asia (680-850km), suggesting dominance of extrusion over crustal shortening. The cessation of extrusion along the Red River fault at about 17 Ma and slowing down of slip on the ATF at the same time suggest that distributed crustal thickening has become dominant since the Early/Middle Miocene boundary.