Origin of Low-Angle Ductile/Brittle Detachments: Examples From the Cretaceous Linglong Metamorphic Core Complex in Eastern China

摘要

The origin of low-angle normal faults (LANFs) and shear zones is subject to debate, and the Linglong metamorphic core complex (MCC) in eastern China provides good examples of how they form. Here we present structural and geochronological data that shed light on the geometry, kinematics, and evolution of this MCC. The MCC is bound by the Zhaoping and Jiaojia detachments with an opposite vergence and appears as a bivergent MCC. Both detachments consist of low-angle ductile shear zones and parallel normal faults. A series of NNE-SSW-striking low-angle normal-sense shear zones, including ones that are subhorizontal, occur within the northern part of the MCC, and they show opposite tectonic vergence on either side of the MCC. Microstructures in the shear zones suggest that temperatures during deformation were mainly 300-400 degrees C, corresponding to the brittle-ductile transition. These shear zones are typically overprinted by brittle deformation structures represented by foliation-parallel normal faults, local LANFs, and widespread high-angle normal faults. Zircon U-Pb dating of intrusive rocks within these shear zones constrains the formation of the MCC to 137-108 Ma. Regional NW-SE extension was responsible for the development of the MCC. The two low-angle detachments were initiated along the boundary of the preexisting Linglong batholith. The low-angle shear zones within the MCC formed due to synchronous extension and arching, and their orientations were affected by synkinematic intrusion. Later LANFs developed mainly along the previous low-angle shear zones due to their high reactivation angles and the arching-induced flexural stresses.