Episodic evolution of intracontinental orogenic deformation: Insight from the southern Bogda fold-and-thrust belt of the Tianshan Mountains, NW China

Abstract

The Mesozoic-Cenozoic Tianshan orogenic belt, a key area for studying intracontinental orogenic deformation, experienced multiple uplift episodes due to the far-field effects from a series of collision events along the southern margin of Eurasia. Focusing on the fold-and-thwrust belt at the southern margin of the Bogda Mountain, a branch of the eastern Tianshan orogenic belt, we integrated 2D seismic data, borehole and outcrop data to construct a balanced and sequentially restored cross-section across the central part of this belt. The activities and mechanical evolution of the major faults were analyzed by using the discrete-element modeling (DEM) method, offering new insights into the tectonic transformation of the Turpan-Hami Basin influenced by episodic uplifts of the Bogda Mountain in an intracontinental setting. The studied fold-and-thrust belt can be divided into two deformation systems: (i) the piedmont deformation system, comprising a southward basement thrust system and a thin-skinned thrust system on Jurassic detachment, and (ii) the thrust front belt, exhibiting an en-echelon pattern in the basin center and featured by fault-propagation folds along the lower Jurassic coal unit. Stratigraphic and reported thermochronological analyses pinpoint four key contractional deformation periods: Late Jurassic, Late Cretaceous, Paleocene-Pliocene, and Pleistocene-present. The DEM numerical simulation results indicate a critical conical Coulomb wedge model governed by the episodic activities of the southern Bogda fold-and-thrust belts from the Mesozoic to the Cenozoic. Surface processes (syntectonic erosion/deposition) significantly impacted differential activities between the orogenic belt and the basin's thrust structures. We propose that periodic arid climates in the Tianshan Mountains reduced erosion rates, promoting the expansion of the southern Bogda fold-and-thrust belts toward the basin and the widening of the orogenic belt.