The hengshan low-angle normal fault zone: Structural and geochronological constraints on the late mesozoic crustal extension in South China

摘要

We document in this paper a significant low-angle normal fault, namely the Hengshan detachment fault, which extends more than 150km long in NNE orientation and bounds an elongated Early Cretaceous basin in Hunan Province, central South China. Detailed structural and geochronological analyses have been conducted along its southern segment, where a well exposed, ~3km thick, flat ductile shear zone develops along the western margin of the Hengshan granitic massif. This zone is featured by shallow-dipping foliations of varied trend from NE to NW, which bear penetrative stretching lineations varying from NW to SW trend. Shear sense criteria indicate top-to-the-NW and top-to-the-SW motions along its northern and southern parts, respectively. Quartz c-axis orientations of mylonitic rocks from the shear zone exhibit asymmetric single or crossed girdle patterns, and the distributions of fabric point maxima suggest a simultaneous operation of basal and prism slips, indicating a deformation temperature ranging from 400°C to 550°C. One zircon U-Pb age of the sheared albitite and three muscovite ~(40)Ar/~(39)Ar ages of the mylonitic rocks indicate that the ductile shearing initiated at 136Ma and lasted till 97Ma. The zircon U-Pb dating results of five granitic samples from the Hengshan granitic pluton yield two phases of crystallization ages of 232-228Ma and 150-151Ma respectively, indicating two preceding magmatic events prior to the ductile shearing in the lower plate. Our structural and geochronological data allow defining the Hengshan massif as an extensional dome, rather than a metamorphic core complex as previously predicted, due to the lack of syn-tectonic plutonism and the origin of detachment fault associated with the reactivation of the inherited thrust. This study provides crucial evidence for understanding how the mid-upper crust deformed during the process of extension, which testifies to a coeval occurrence of non-coaxial deformation of the ductile flow in the middle crust and brittle faulting in the upper crust. It also sheds new insights into the timing of the regional crustal extension in South China, and permits to constrain its onset time to be ca. 136Ma.