Transition From Reverse to Left‐Slip on the Eastern Haiyuan Fault, NE Tibetan Plateau, From the Structure and Age of the Ganyanchi Pull‐Apart Basin.

Because the active, left‐slip Haiyuan fault is a first‐order structure along the NE margin of the Tibetan Plateau, its tectonic evolution provides insight into deformation processes along this margin over time. Late Cenozoic deformation in the area of the eastern Haiyuan fault initiated as thrust faulting, followed by left‐lateral strike slip displacement. However, the time of this kinematic change has been long debated. Here we use the structural evolution and the age of the Ganyanchi basin to date the onset of this kinematic transition. Seismic reflection data, integrated with published structural mapping, indicate that the basin is a pull‐apart controlled by strands of the eastern Haiyuan fault. Previously reported magnetostratigraphy of a core from the basin interior indicates deposition started at ∼2.8 Ma. However, this age likely postdates initiation of the bounding strike‐slip faults. We estimate that an additional 0.2–1.0 Myr of lateral slip occurred before the dated section formed. Thus, we find that the age of the eastern Haiyuan fault is at least ∼3.5 Ma, which is significantly older than early estimates of less than 2 Ma. Integrating our new data with prior work reveals that the kinematic change from NE‐SW shortening to left‐slip along the Haiyuan fault youngs to the east, which we interpret to result from growth of the Haiyuan strike‐slip fault via progressive lateral propagation from west to east. Miocene‐Pliocene onset of the Haiyuan strike‐slip fault coincides with initiation ages of other major strike‐slip faults in NE Tibet, likely implying that the NE margin of the Tibetan Plateau had switched to a strike‐slip dominated mode of deformation by this time. Key Points: Seismic reflection data indicate that the Ganyanchi basin is an asymmetric pull‐apart filled with more than 900 m of growth strataWe find that the age of the eastern Haiyuan fault is at least ∼3.5 Ma, which is significantly older than early estimates of less than 2 MaKinematic change along the Haiyuan fault shows a west‐to‐east younging trend, implying the left‐slip motion propagated eastward over time [ABSTRACT FROM AUTHOR]