The Late Mesozoic Intracontinental contraction–Extension Transition in the Beishan Fold‐Thrust Belt, Central Asia: Constraints From Structural Analysis and Apatite (U–Th)/He Thermochronology

Abstract

AbstractThe late Mesozoic Beishan fold‐thrust belt occupies a key position in central Asia, figuring importantly in our understanding of intracontinental deformation in response to far‐field plate‐boundary forces. To determine when and how the Beishan fold‐thrust belt was developed, we conducted detailed field mapping, structural analysis, and apatite (U–Th)/He (AHe) thermochronology in the Hongliudaquan area, southeastern Beishan Range. The study area exposes the Hongliudaquan fold and thrust system, consisting of the southern Hongliudaquan thrust nappe and autochthonous fold sets. The nappe experienced >16 km northward thrust‐induced translation. The autochthonous fold sets strongly deformed Lower–Middle Jurassic strata showing diverse trends of kilometer‐scale basins and ranges. We interpret that this fold interference pattern was produced in a constrictional deformation field, which underwent synchronous shortening in two orthogonal directions. This stain history was related to synchronous imbricate thrusting, duplex formation, and strike‐slip faulting at different structural levels. Restoration of the Hongliudaquan fold and thrust system yields a minimum of ∼27% (∼10.1 km) north–south shortening strain. Thrusting and nappe emplacement within the Beishan fold‐thrust belt may have started in the late Middle Jurassic and persisted until the Early Cretaceous (∼140–133 Ma). Our AHe data also record ∼124–115 Ma cooling related to footwall exhumation during subsequent normal faulting, indicative of the late Mesozoic intracontinental contraction–extension transition. The Beishan fold‐thrust belt initiated in central Asia via the late Mesozoic reactivation of the Paleozoic–early Mesozoic Beishan orogen as a result of the far‐field responses to multi‐directional plate convergence around Asia.