Middle Jurassic orogeny in the northern North China block

Multiple shortening took place during the Mesozoic in the periphery of the North China block (NCB), a major tectonic grain of East Asia, but no consensus exists as to the timing, intensity, and causes of individual events. This work focuses on the first phase shortening in Jurassic, which represents the initiation of Yanshanian orogeny, by investigating the structural evolution of the Yanshan belt in the middle segment of the northern NCB. The first-phase shortening used to be regarded as a weak compressive event for it is recorded by parallel or low-angle unconformities between Middle and Upper Jurassic strata in some places. Our recent field investigations, however, identified some pronounced angular unconformities separating the Upper Jurassic strata from the underlying highly folded units in the northern Yanshan belt. The new observations demonstrate that the first-phase shortening was actually a strong contractional event. The co-existence of parallel and angular unconformities in the southern and northern Yanshan belt can be explained by different thrusting processes. It is argued that lateral variations of initial basement dips must have played a crucial role in controlling structural evolution of the overlying pre-Middle Jurassic sedimentary succession. It is shown that the first-phase thrusting and folding system was thin-skinned and predominantly verge northwestward. Deformational intensity also weakened toward the north and northwest. The first-phase shortening is constrained to occur in a short time span from 170 to 165 Ma based on new radiometric ages of strata above and below the unconformity, and presumably took place simultaneously in the northern NCB. The first-phase shortening was likely driven by horizontal tectonic push from the southeast. It is suggested that low-angle subduction of the paleo-Pacific plate might be responsible for the Middle Jurassic orogeny in the northern NCB and other parts of East Asia.