Structural and geochronological evidence for Early Cretaceous orogen-parallel extension of the ductile lithosphere in the northern Dabie orogenic belt, east China
Abstract The WNW-trending North Dabie dome (NDD) in the northern Dabie orogenic belt is bounded by the Xiaotian-Mozitan ductile shear zone (XMSZ) in the north and the Wuhe-Shuihou shear zone (WSSZ) in the south. Fabric geometries and kinematics of both the XMSZ and the WSSZ are similar. They both show a top-to-NW or WNW sense of shear and display similar microstructures which indicate that the deformation temperature was about 600–650 °C. 40 Ar– 39 Ar dating of hornblende and biotite from the mylonites of the XMSZ suggests that the shear zone was developed prior to 142 Ma. U–Pb zircon LA-ICPMS dating of undeformed granite dikes that cut the XMSZ suggests that the ductile shearing occurred before 130 Ma. The WSSZ was also developed in the Early Cretaceous. The interior of the NDD is dominated by top-to-NW or WNW sense of shear in the migmatitic gneiss and deformed plutons of the earliest Early Cretaceous ages. We suggest that the XMSZ, the WSSZ, and the interior of the NDD were involved in the same Early Cretaceous deformation. The XMSZ and the WSSZ were originally a single connected, more flat-lying zone, which we call the East Dabie Detachment Zone (EDDZ). It developed at a crustal level of over 18 km in depth. Kinematics of the EDDZ suggest that the ductile crust and possibly part of the lithospheric mantle in the eastern Dabie orogen underwent pervasive orogen-parallel and ESE-ward extension at the beginning of the Early Cretaceous. Large-scale magmatic intrusions following this deformation and the resultant increase in buoyancy led to the exhumation of the NDD and the warping of the EDDZ. Erosion separated the EDDZ into two zones (the XMSZ and the WSSZ) as observed today. Many metamorphic core complexes of Early Cretaceous were developed in the eastern North China Craton, most showing top-to-NW or WNW sense of shear. We suggest that such a widespread uniform shear sense reflects a uniform flow direction of the ductile lithosphere in the East China that has also been documented in the Liaonan, Yiwulushan and Yumenshan metamorphic core complexes in the northeastern part of the North China Craton.