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°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.
Many metamorphic core complexes (MCCs) of Early Cretaceous age are documented in the northern part of the North China Craton (NCC), which formed in a backarc extensional setting. However, whether or not the MCCs are also present in the southern part of the NCC, and where the western boundary of backarc extension lies, remain unclear. We present new structural and geochronological data to show that Early Cretaceous structures in the Xiaoqinling region (China) lying in the southern part of the central NCC represent a Cordilleran-type MCC. The NW-dipping detachment zone on the northwestern edge of the Xiaoqinling MCC is a ductile extensional shear zone that is overprinted by a later brittle detachment fault. The footwall (lower plate) consists of Archean metamorphic rocks and Mesozoic plutonic rocks, and was cut by a series of ductile normal sense shear belts and later brittle normal faults that strike predominantly NE-SW. Both the ductile and brittle structures indicate that NW-SE extension was responsible for the development of the MCC. Geochronological data suggest that the MCC initiated at 138 Ma and lasted until 100 Ma, recording a protracted extensional history. The MCC experienced an early phase of crustal-scale normal faulting (138–126 Ma) and later isostatic doming (125–100 Ma), consistent with the “rolling-hinge” model. The Xiaoqinling MCC shows similar features and a similar evolution to other intraplate MCCs in the northern and southeastern parts of the NCC, and shows that the southern part of the NCC was also involved in intense backarc extension and magmatism. Distribution of these intraplate MCCs indicates synchronous backarc extension over a length of around 1800 km. Delamination of a flat oceanic slab during roll-back is consistent with such large-scale, synchronous extension in the overriding plate.
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