Thrusting and exhumation around the margins of the western Tarim basin during the India‐Asia collision

Abstract

The Cenozoic collision between India and Asia has deformed a large part of central Asia. To the north of Tibet around the margins of the western Tarim basin, major basin‐vergent thrusting has uplifted and exhumed thick Jurassic to Neogene sedimentary sections; this presumably reflects the propagation of collision‐induced deformation into the basin. Apatite fission track data from five sections involved in this thrusting record strong late Oligocene to middle Miocene exhumation and cooling. On the northwest margin of the basin on the piedmont of the Tian Shan, a section exhumed by thrusting yields an exhumation age of 13.6±2.2 Ma (±1σ). Four Miocene sandstones from a second section 40 km to the east yield detrital source area cooling ages which decrease upsection from 25.0±3.9 to 13.1±2.2 Ma. Landsat imagery suggests that probable sediment source areas were dominated by Neogene thrusting, so these ages likely record progressive unroofing in Tian Shan thrust systems. Deformed Miocene to Pleistocene strata indicate that thrusting has continued and propagated basinward up until the present. Previously published apatite data from the Junggar basin on the northern flank of the Tian Shan yield a similar age of 24.7±3.9 Ma. On the southwest margin of Tarim on the piedmont of the western Kunlun Shan, three sections yield cooling ages of 19.8±0.9 Ma, 20.0±3.1 Ma, and roughly 20 Ma. Farther south at Kudi, previous work has yielded apatite cooling ages of 17±2 Ma and a zircon cooling age of 22±2 Ma. These similar cooling ages over a ≈250 km long belt in the western Kunlun Shan are associated with the transpressional Kumtag fault and the Main Pamir Thrust (MPT). Geologic relations within the western Kunlun Shan suggest that the MPT‐Kumtag fault system offsets the originally linear trend of the Paleozoic‐early Mesozoic Kunlun arc system by 200–300 km, accommodating much of the Neogene northward indentation of the Pamir block. We propose that the ≈20 Ma ages slightly postdate the initiation of this indentation and consequent crustal thickening. Taken together, the Tian Shan and Kunlun Shan results indicate that crustal thickening, in part accommodated by strike‐slip faulting, became the dominant mode of deformation by ≈25–20 Ma in a large region extending from the Pamir and west Kunlun Shan north to the Tian Shan.