Onset activity of the Haiyuan fault by the late Miocene: Evidence from magnetostratigraphy in the Laolongwan Basin, northeastern Tibetan Plateau

As the boundary fault of the northeastern margin of the Tibetan Plateau, the Haiyuan fault plays a crucial role in understanding the growth of the Tibetan Plateau to its northern margins during the Cenozoic. Despite numerous studies of the Haiyuan fault, the Cenozoic activity history of the fault remains controversial. We performed stratigraphic analysis, magnetostratigraphy, and cosmogenic nuclide dating in the Laolongwan Basin, which is a pull-apart basin within the Haiyuan fault zone. Our findings identify two pivotal deformation events: (1) an unconformity and widespread northwest-southeast−trending folds within the basin that likely reflect northeast-directed thrusting of the basin boundary faults at ca. 13.6 Ma and (2) growth strata associated with Haiyuan fault motion, together with near north-south−trending folds, that occur adjacent to the Haiyuan fault and seem to mark the onset of left-lateral strike-slip along the fault at ca. 3.2 Ma. These phenomena clearly show that the Haiyuan fault underwent northeast-directed thrusting followed by left-lateral strike-slip since the late Miocene.