The Early Cretaceous extensional deformation in the southeastern Beishan Range, central Asia: Insights from 2D seismic reflection profiles and apatite fission track thermochronology

To better understand the timing, processes, and dynamic mechanisms of the enigmatic late Mesozoic tectonic transition from contraction to extensional deformation in the southern Central Asian Orogenic Belt (CAOB), detailed structural analysis and apatite fission track thermochronology were conducted in the southeastern Beishan Range. Field observations show that Lower-Middle Jurassic strata have been strongly deformed by numerous thrusts and folds. 2D seismic reflection profiles reveal two NE- to NEE-striking normal faults. The Suosuojing fault is a SE-dipping low-angle listric normal fault, and the Wudaoming fault is a NW-dipping high-angle normal fault. The normal faults cut the early-formed fold and thrust system, indicative of the contraction-extension transition. The Suosuojing and Wudaoming faults border the Early Cretaceous Zongkouzi Basin to the northwest and to the southeast, respectively. The Zongkouzi Basin has a graben geometric, in which Lower Cretaceous strata display typical growth-strata relationships, suggesting that the normal faults were active during the late Early Cretaceous. Thermal history modelling results by apatite fission track data show that the Suosuojing fault footwall experienced rapid cooling during 132~110 Ma. This phase of rapid cooling was intimately related to footwall exhumation during normal slip of the Suosuojing fault. We argue that the late Mesozoic intracontinental contraction-extension transition in the southeastern Beishan Range may have occurred between ~133 Ma and ~129 Ma in the late Early Cretaceous. Collapse of thickened crust and coupled mantle upwelling triggered the Early Cretaceous extensional deformation in the southern CAOB.