MESOZOIC STRUCTURAL EVOLUTION OF THE YANSHAN FOLD-THRUST BELT, NORTHEAST CHINA

The Yanshan fold-thrust belt is an exposed portion of an extensive orogenic system that occupied the eastern portion of the north China block during Mesozoic time. 150 km northeast of Beijing, the Yanshan belt consists of east-west trending folds, thrusts, and associated intermontane basins that are cut by northeast trending normal faults. We present results of geologic mapping in this region that reveals a complex structural history consisting of 1) south vergent, basement-involved folding in Late Triassic-Early Jurassic time; 2) bivergent thrust faulting in Late Jurassic time; and 3) Early Cretaceous northeast-trending normal faulting.

Evidence for the oldest phase of deformation includes the existence of south vergent, basement-cored folds and synformally folded Triassic-Early Jurassic strata that are unconformably overlain by Late Jurassic volcanic rocks. These structures were overridden by a major north vergent thrust system (the Xiabancheng thrust system) that cuts and overturns the older structures and Late Jurassic synorogenic strata that unconformably overlie them. Late Jurassic north vergent thrusting was contemporaneous with south vergent thrusting along the Gubeikou thrust to the south. Northeast trending normal faults of mid-Cretaceous age cut all older structures (and in some cases may reactivate them), and Cretaceous strata bounded by these faults unconformably overlie older units with profound angularity.

This sequence of events agrees well with the chronology of deformation that has been documented in the Yanshan by other workers. A key difference in our interpretation is the importance of northeast-trending normal faults that crosscut older east-west trending structures, exposing different levels of Yanshan structure along the strike of the thrust belt.