NEOGENE TECTONIC EVOLUTION OF THE NE PAMIR MARGIN, NW CHINA

Deformation along the NE Pamir margin has propagated northward since the early Miocene in response to the Indo-Eurasian collision. To reconstruct the deformation along the NE Pamir margin from the Miocene through the Quaternary, we synthesize structural mapping, stratigraphy, magnetostratigraphy, and cosmogenic burial dating of basin fill and growth strata with geodetic surveys and optically stimulated luminescence dating of deformed fluvial terraces and fans. We focus on the Bieertuokuoyi piggyback basin, located in the hanging wall of the Pamir Frontal Thrust (PFT), which provides a unique record of deformation along the NE Pamir margin. Stratigraphic and magnetostratigraphic data, in combination with field observations and cosmogenic burial ages, show the basin was filled after the initiation of the Pamir Frontal Thrust ~5-6 Ma.

The onset of shortening along the Main Pamir Thrust (MPT) on the NE Pamir margin began in the Miocene (~20 Ma), and deformation propagated basinward during the Middle Miocene to Pliocene. Synchronous deformation on the MPT, PFT and other thrust faults accommodated shortening throughout the Miocene to Recent. During the Late Quaternary, most shortening across the region is accommodated on structures within a narrow zone along the northeastern edge of the Pamir, with deformation distributed between several faults and folds. In contrast to the apparently unidirectional, southward propagation of deformation in the Tian Shan, deformation on both hinterland and basinward structures appeared to play an important role in accommodating shortening along the NE Pamir margin until the Late Quaternary.

Our data support a change in the regional kinematics around the Miocene-Pliocene boundary (~5-6 Ma). Rapid exhumation of NE Pamir domes, coupled with cessation of the Kashgar-Yecheng Transfer System on the eastern margin of the Pamir, accelerated southward propagation of the Kashi-Atushi fold and thrust belt, and outward propagation of the northeastern Pamir margin. This coeval deformation signifies the coupling of the Pamir and Tarim plates, leading to a transfer of shortening and accelerated deformation on both the Pamir frontal faults and the southern Tian Shan Kashi-Atushi fold-and-thrust system.