Triassic and Jurassic Tectonics in the Eastern Yanshan Belt, North China: Insights from the Controversial Dengzhangzi Formation and Its Neighboring Units

Abstract

The Jurassic geology of the Niuyingzi–Dengzhangzi area of western Liaoning province south of Lingyuan has challenged geologists for many years. Its most controversial structural element is a narrow (0.1–1.5 km), subvertical, fault-bounded panel of Middle Proterozoic Changzhougou quartzite and, locally, higher Changcheng System units. The 35 km-long, NNE-striking panel is bordered on the west by the subvertical Daqizi fault (new name) and the steep, largely east-overturned Late Triassic–Early Jurassic Dengzhangzi Formation. It is bordered on the east by the subvertical Zhuzhangzi fault (new name) and west-dipping Middle Jurassic Guojiadian Formation strata. Previous structural interpretations have favored ideas that the Daqizi fault is a steep normal fault with Dengzhangzi side down, and that the Zhuzhangzi fault is a steep reverse fault with Guojiadian side down. We disagree with both interpretations and present evidence that both faults have formed during previously unrecognized Jurassic tectonic events. The Daqizi fault is interpreted as a late Early Jurassic to early Middle Jurassic west-directed thrust fault placing Changcheng strata across lower-plate Dengzhangzi strata. Both upper and lower plates were then folded. The Zhuzhangzi fault was originally a normal fault bounding a Guojiadian half-graben that formed during an episode of previously unrecognized Middle Jurassic extension. The fault was localized along the folded, east-dipping contact between Changzhougou quartzites and overlying shales of the Chuanlinggou Formation. As the fault developed, detached hanging wall Chuanlinggou and higher Changcheng strata above the Changzhougou quartzite were dropped down and buried beneath the half-graben fill. The geometry of the Dengzhangzi–Changzhougou–Guojiadian structural assemblage was subsequently modified by late Middle Jurassic or early Late Jurassic contraction. Among its effects were (1) reverse fault inversion of the Zhuzhangzi normal fault and (2) eastward Yangzhangzi–Wafangdian thrusting of Jixian strata across the Changcheng fault-bounded panel and its neighboring Jurassic units. Steepening and eastward overturning of Dengzhangzi strata occurred at this time. The results of our study indicate that the Jurassic tectonic history of the western Liaoning area was considerably more complicated than believed earlier.