Rapid post-Pliocene crustal shortening in northern Tibet: Evidence from the Kumkuli Basin, Xinjiang Province, China

The Altyn Tagh and Kunlun strike-slip faults dominate the modern deformation of northern Tibetan Plateau. We present shortening magnitudes and rates from the deformed Kumkuli Basin (~4000 m a.s.l.), located near the intersection of these faults ~80 km south of the Altyn Tagh and ~200 km northwest of the termination of the Kunlun fault. Quaternary vertical uplift rates of ~2.5-3 mm/yr occur beneath range front faults and folding of Eocene - Pliocene sediments record a total of >26% of post-Pliocene shortening. Basin infilling initiated in Eocene time and continued through Pliocene time with deposition of conglomerates to fluvial and lacustrine sediments. Notable Miocene gypsum beds indicate an arid environment and likely act as a zone of structural weakness in ongoing deformation. Paleocurrent directions indicate northward paleo-flow that sourced material from the Hoh Xil Basin located 150 km to the south of the Kumkuli Basin depocenter. Subsequent basin closure in Miocene time resulted from the uplift of the Quimen Tagh range to the north. Post-Pliocene deformation of basin sediments form a ~ 1 km high mountain range where a net of >12 km shortening is estimated from balanced cross sections. Folded Quaternary alluvial fan surfaces indicate that crustal shortening is ongoing. We combine new 36Cl cosmogenic radionuclide dating of depth profiles at the peaks of two folds with geomorphic reconstructions to estimate ~550 m of vertical fault motion since ~200 ka on subsurface thrust faults. We relate upper crustal shortening of the Kumkuli Bsain with left-lateral slip along the nearby Altyn Tagh fault and continued upward growth of the plateau in a region between major strike-slip faults and the rigid basement material of the Qaidam Basin. Although localized, Quaternary rates presented here are among the highest shortening rates documented in northern Tibet. Rapid upper crustal shortening at high elevations compounded with an addition of lower crustal material at depth may provide a mechanism by which northern Tibet reached elevations of >5000 m.