Rapid change from compression to extension in the North China Craton during the Early Cretaceous: Evidence from the Yunmengshan metamorphic core complex

Abstract

The Yunmengshan metamorphic core complex (YMMCC) is situated in the northern part of the eastern North China Craton (NCC). The main structures in the YMMCC area include the arcuate Sihetang shear zone (SHTSZ) and the NE-striking Shuiyu shear zone (SYSZ). We present structural and geochronological evidence to show that the SHTSZ was a top-to-the-SSW thrusting zone formed between 140 and 137Ma. The SYSZ is an extensional, top-to-the-SE shear zone developed between 131 and 114Ma, in association with intense magmatism and the development of the Huairou rifted basin in the hanging wall. Ductile shearing on the SYSZ was followed by isostatic rebound from 114Ma to the end of the Early Cretaceous and then was replaced by brittle normal faulting. The normal displacement on the SYSZ and subsequent isostatic rebound led to the formation of the YMMCC. The mechanism of formation is consistent with a rolling-hinge model of detachment faulting and core complex formation. The structural evolution of the Yunmengshan area, in common with the whole eastern NCC, thus records a rapid switch from a NNE–SSW shortening regime to a NW–SE extension one in the earliest Cretaceous. The intense NW–SE extension of the Early Cretaceous, as recorded by the YMMCC, was associated with peak lithospheric thinning in the eastern NCC. We attribute the shortening to the final closure of the Mongol–Okhotsk Ocean between the Siberian Craton and the North China–South Mongol block, and the extension to the back-arc deformation related to the subduction of the Izanagi Plate in the Pacific Ocean. We suggest that the Yunmengshan area illustrates how back-arc compression can be driven by the overriding plate motion, while connection of the overriding plate to a larger continent favors subduction-related back-arc extension.