Magnetostratigraphyof theNeogeneChakabasinand its implications formountainbuildingprocesses in the north-easternTibetan Plateau

Magnetostratigraphy of sedimentary rock deposited in the Chaka basin (north-easternTibetan Plateau) indicates a lateMiocene onset of basin formation and subsequent development of the adjacent Qinghai Nan Shan. Sedimentation in the basin initiated at 11Ma. In the lower part of the basin ¢ll, a coarsening-upward sequence starting at 9Ma, as well as rapid sedimentation rates, and northward paleocurrents, are consistent with continued growth of the Ela Shan to the south. In the upper section, several lines of evidence suggest that thrust faulting and topographic development of the Qinghai Nan Shan began at 6.1Ma. Paleocurrent indicators, preserved in the basin in the proximal footwall of theQinghaiNan Shan, show a change from northward to southward £ow between 6.5 and 3.8Ma. At the same location, sediment derived from the Qinghai Nan Shan appears at 6.1Ma. Finally, the initiation of progressively shallowing dips observed in deformed basin strata and a change to pebbly, £uvial deposits at 6.1Ma provide a minimum age for the onset of slip on the thrust fault that dips north-east beneath the Qinghai Nan Shan.We interpret a decrease in sediment accumulation rates since 6Ma to indicate a reduction in Chaka basin accommodation space due to active faulting and folding along theQinghaiNanShan and incorporation of the basin into the wedge-top depozone. Declination anomalies indicate the beginning of counter-clockwise rotation since 6.1Ma, whichwe associatewith local deformation, not regional block rotation.The emergence of theQinghaiNanShan near the end of theMiocene Epoch partitioned the once contiguous Chaka-Gonghe and Qinghai basin complex. In a regional framework, our study adds to a growing body of evidence that points to widespread initiation and/or reactivation of fault networks during the lateMiocene across the northeasternTibetan Plateau.