DEFORMATION AND EXHUMATION HISTORY OF NORTHEASTERN TIBET

Early compressional deformation in northern Tibet implies that the plateau has grown narrower in time commensurate with decreasing plate convergence speed, which produces a constant bulk strain rate since collision equal to the modern geodetic rate. Regional deformation at or near collision time across northern Tibet is shown by a growing dataset that includes large-magnitude paleomagnetic rotations, onset of basin deposition, increased erosion rates in hanging-wall rocks, and dated fault gouge. After this period of NE-NNE directed compression in northeastern Tibet, many new faults initiate (< 20 Ma) in roughly the same location but in various orientations that accommodate both left- and right-lateral fault motion as well as thrusting. Cooling histories from transpressional ranges along the Kunlun left-lateral fault suggest initiation at ~ 20 Ma along the central fault segment. By 12 Ma, strike-slip faulting expanded westward by > 300 km, as well as eastward and northward to the Haiyuan Fault. New right-lateral and reverse subsidiary faults located between the Kunlun and Haiyuan faults initiate between 15 and 6 Ma without spatial progression (Elashan, Riyueshan, Dulan-Chaka, Gonghe Nan Shan, Jishi Shan, Qinghai Nan Shan). Finally, the Kunlun and Haiyuan faults expanded to their eastward terminations by 8 Ma, coincident with initiation of subsidiary east-west directed thrust faulting and folding, as well as extension in the northern foreland (Yinchuan graben). The distribution of fault timing between major strike-slip faults and intervening, smaller structures suggests that the northeastern plateau is a broad step-over zone of deformation resulting from the concurrent Kunlun and Haiyuan strike-slip faults beginning at ca. 15 Ma. The cause of such a kinematic shift -within northeastern Tibet and other plateau margins is controversial. Reasons may include eastward propagation by lower crustal flow, mantle foundering beneath northern Tibet, or the interaction with the Pacific subduction zone. A growing number of fault and erosion studies, however, suggest that this plateau-wide kinematic shift may be more gradual, rather than pulsed as previously thought. The Miocene shift, therefore, may relate to the evolutionary processes of orogen growth, rather than to an abrupt tectonic event.