CENEZOIC DEFORMATION HISTORY OF QAIDAM BASIN AND ITS TECTONIC IMPLICATIONS

The Qaidam basin is located at the northern margin of the Tibetan Plateau and preserves a nearly complete Cenozoic sedimentary section that records the timing and kinematics of India-Eurasia collision in this region. Using seismic reflection data and surface geology, we define when and where the deformation initiated within the basin and how it has propagated over time. These insights, in turn, provide constraints on how the northeast Tibetan Plateau formed and evolved over geologic timescales.

The Qaidam basin is bounded by two major strike slip faults, the Altyn Tagh and Kunlun Faults, which accommodate eastward extrusion of crustal blocks that are driven by the India-Eurasian collision. The history of motions on these faults is recorded within the basin by deformation along a series of fault-related folds, generally underlain by blind-thrust faults. Focal mechanisms in the west and northeast of the basin indicate thrust fault activity, with some component of strike-slip motion. Our analysis shows that the basin was controlled by activity of thrust faults rooted on both sides of the basin that are related to motions of both the Altyn Tagh Fault and Kunlun Fault systems. These thrust faults typically form wedge structures that consist of deeply rooted thrust ramps and overlying roof thrusts that form anticlines. The upper portions of these structures generally consist of fault-propagation folds. Most of these structures contain syntectonic growth strata that record structural timing and kinematics. The growth strata indicate that deformation of the Qaidam basin may have started very early, at around 45 Ma, shortly after or coeval with the collision of the India Plate with Eurasia Plate. After a subsequent period of tectonic quiescence during Eocene, deformation became widespread across the basin in the Miocene, corresponding with activity of the Altyn Tagh and Kunlun Faults and the emergence of the Qiman Tagh and Qilian ranges. This led to the formation of numerous thrust sheets and anticlines within the basin, and the migration of the basin depocenter to the southeast. Deformation in the Qiadam basin thus records two phases of deformation in the northeast Tibetan Plateau, spanning the transition from north-south convergent to escape tectonics.