Strike-Slip Pull-Apart Process of the Jiyang Depression during the Yanshanian Tectonic Cycle and Its Response to Paleo-Pacific Plate Movement

Abstract The Yanshanian tectonic evolution of the Jiyang Depression was complex in Jurassic and Cretaceous, making it difficult to pin down the tectonic regime of that time. This study compared fault activity, lithology, and sedimentary-facies patterns across the depression using 3D seismic volumes, borehole data, and sedimentary facies data. This integrated dataset let us define the Yanshanian tectonic regime and subdivide the tectonic evolution together with the record of regional plate motions. In the Early Yanshanian Stage (J₁-₂), faults trended mainly NW and shew low activity. Sedimentation was dominated by stable continental deposits, including coal-bearing clastic rocks and dolomitic clastics. In the Mid Yanshanian Stage (J₃-K₁), NW-trending en echelon faults became more numerous and much more active, and braided-river to fan-delta systems developed in the depression. Volcanism was strong, producing tuffaceous clastics and volcanic rocks. In the Late Yanshanian Stage (K₂), compressional uplift removed the Upper Cretaceous by erosion. Taken together, these patterns point to a strike-slip pull-apart process along the pre-existing Tan-Lu Fault Zone. This process was driven by sinistral stress linked to the subduction of the Paleo-Pacific (Izanagi) Plate. In the Early Yanshanian Stage, low-angle subduction toward the NNW formed a small angle with the Tan–Lu Fault Zone, promoting left-lateral motion and the initial pull-apart basin. In the Mid Yanshanian Stage, subduction shifted to a high-angle NNW direction and sped up, which strengthened left-lateral slip on the Tan–Lu Fault, increased normal-faulting inside the depression, and triggered vigorous volcanism. In the Late Yanshanian Stage, subduction changed to a low-angle NWW direction, nearly perpendicular to the Tan–Lu Fault Zone trend, producing compression and uplift of the depression. Overall, the Yanshanian strike-slip pull-apart evolution in the Jiyang Depression shows a clear staged pattern. These two pull-apart stages partitioned the depositional environments and, as a result, controlled the type and richness of the source rocks.