Mid-Tertiary transition from plate-like to continuous deformation in NE Tibet

Whether Cenozoic India-Eurasia convergence has been accommodated by plate-like extrusion or distributed shortening within the Tibet Plateau has been a long-standing debate. Geodetic studies by numerous groups suggest that shortening is accommodated continuously across Tibet and that slip rates on major strike-slip faults are slow, consistent with predictions of distributed shortening models. However, longer-term geologic records of translation on the Altyn Tagh fault (ATF) as well as shortening and uplift on the NE margin of the Tibet Plateau, suggest that rates and styles of deformation changed in the mid-Miocene. Tertiary basins on the ATF are offset from their sources in the Qilian Shan by slip on the ATF. These sediment-source matches provide Oligocene, Early Miocene, mid-Miocene and Pliocene piecing points that demonstrate that most of the 375±25 km of total displacement on the ATF accumulated in the Oligocene and earliest Miocene, and that relatively small amounts of offset have occurred since the Early Miocene. These offsets, suggest a Late Oligocene-Early Miocene slip rate for the ATF in excess of 17 mm/year and post-Early Miocene slip rates of <10 mm/year. Thermochronologic and stratigraphic evidence also support mid-Tertiary shortening and uplift of the Kunlun and Qilian thrust belts, with rapid exhumation and propagation of thrust belts occurring in the Miocene and younger. Finally, we have identified strike-slip faults that extend northeast of the Tibet Plateau that potentially have slip histories that are very similar to the ATF. These faults, with large pre-Miocene offsets, and little post-Miocene activity, may extend to the Pacific margin. We interpret these changes in rates and styles of deformation as a reorganization of intraplate tectonic style in the Miocene, from plate-like to distributed accommodation of India-Eurasia convergence. Evidence for rapid strike-slip on the ATF before the mid-Miocene, and mapping of pre-Miocene structures that extend beyond the plateau and could have accommodated extrusion of Tibet are consistent with an early phase of plate-like extrusion. Initiation of rapid shortening and propagation of thrust belts in the Miocene of the Qilian Shan, Altun Shan, and Kunlun Shan suggest that deformation was more widely distributed in the later Tertiary and that deformation had become more continuous across the region. The change in tectonic style may have been related to changes in India-Eurasia convergence, mechanics of Tibet lithosphere, or confinement along the free-face at the Pacific margin.