DID THE ALTYN TAGH FAULT EXTEND BEYOND THE TIBETAN PLATEAU?

The pre-mid-Miocene termination of Altyn Tagh fault (ATF) is a critical outstanding problem for understanding the mechanics of Cenozoic deformation resultant from the Indo-Asian collision and mechanisms of Tibetan Plateau formation. Recent fault slip-rate data for the central-eastern ATF suggest Oligocene- mid-Miocene extrusion. Structures beyond the widely-accepted NE end of the Altyn Tagh fault, near the town of Yumen, are needed in order to accommodate the large amount of Oligocene-Early Miocene slip on the ATF (at least 310 km near Yumen— beyond the frontal Qilian Shan thrust fault along the northern margin of the Qilian Shan); but structures with the necessary slip magnitudes and histories have not been identified. We report on a series of newly-recognized and documented E to ENE-striking faults within the Alxa block, NE of the Tibetan Plateau, that are visible on remotely-sensed images and confirmed by field studies. These structures are demonstrably left-lateral faults based on offset geology and kinematic indicators. Although total pre-Miocene left-lateral slip on these structures is unknown, it may be much greater than 150 km based upon the juxtaposition of numerous different Lower Cretaceous sedimentary basins and disparate basement units across the structures. Based on documented timing relationships for the southern left-lateral faults in the Alxa region, most of this slip occurred prior to the Miocene. Furthermore, the extremely low estimates of Miocene and younger slip documented in our study suggest that large-scale strike-slip faulting within the Alxa region was effectively restricted to the post-Cretaceous, pre-mid-Miocene. The left-lateral faults in the Alxa region may represent the pre-mid-Miocene continuation of the ATF system beyond the Tibetan Plateau. These Tertiary structures provide a mechanism to accommodate extrusion of crustal blocks beyond the Tibet and must be considered in the Cenozoic tectonic framework of Asia.