Syn-tectonic fluid-flow and fault-fluid interaction along bedding-parallel thrust faults: Evidence from the frontal zone of the Chuandong-Xiangexi fold-and-thrust belt in central South China

Abstract

Fluid-flow within tectonic belts and fault-fluid interaction are fundamental and controlling factors of large thrust belt evolutions, formation of mineral deposits in the upper crust and the ductile shear deformation in the middle crust. In central South China, a large Chuandong-Xiangexi fold-and-thrust belt and long-distance displaced thrust faulting are developed, but their formation mechanism is still in debate. In particular, the evolution of fluid-flow and fault-fluid interactions in this fold-and-thrust belt is still not well constrained. In this study, through detailed structural and microstructural observations together with stable isotope and fluid inclusion analyses, we aim to unravel the deformation characteristics and syn-tectonic fluid-flow along the detachment in the frontal zone of this fold-and-thrust belt. One approximately 30–60 m thick, NW-verging detachment thrust with brittle-ductile deformation and alternating the strongly and weakly deformed zones is identified in the Lower Permian bioclastic and argillaceous limestones. The strongly deformed zone is characterized by tectonic lenses, fault gouge, calcite veins, S–C fabric, and foliated fault rocks/gouge, while the weak deformation zone is marked by scattered fractures without calcite veins and weakly deformed tectonic lenses. The fluids within the thrust faults originate from formation waters (saline fluids or brines) in a closed system, evidenced by the δ18O values, the 87Sr/86Sr ratios, the hydrocarbon-bearing fluid inclusions characterized by moderate salinity (10–14 wt% NaClequiv.) and moderate-high homogenization temperatures (∼160–350 °C). In the hanging wall of the Huangyingshan fault, the fluids involved in the thrusting processes are most likely brines derived from Middle and Lower Triassic carbonate aquifers. While in the footwall, the fluids are likely brines derived from Cambrian evaporitic aquifers. These findings on the detachment thrust fault zone could provide valuable constraints on the conduit and distance of hydrocarbon secondary migration for exploring the potential of large-scale hydrocarbon accumulation in the Chuandong-Xiangexi fold-and-thrust belt.