Structural geometry and kinematics of a strike-slip fault zone in an intracontinental thrust system: A case study of the No. 15 fault zone in the Fuling area, eastern Sichuan Basin, southwest China

Integrated geological and geophysical methods have been used to describe the geometric characteristics and kinematic evolution of the No. 15 fault zone (ES15FZ) in the eastern Sichuan Basin (ESB). The geometric evidence of strike-slip fault activity includes a single subvertical fault and a composite flower structure with deeper positive relief and shallower negative relief in cross-sectional view, as well as a transpressional stepover between fault segments located above the principal displacement zone (PDZ) in map view. The structural relief amplitude and vertical displacement of the ES15FZ along strike provide insights into the formation and multistage development of the fault zone. The ES15FZ originated in a convergent setting, resulting in positive flower structures near the upper Cambrian strata. At shallower depths, the northern ES15FZ is composed of two large-scale negative relief regions and two small-scale positive relief regions, and the southern ES15FZ is composed of one large-scale positive relief region and one small-scale negative relief region. After progressive deformation, the ES15FZ exhibits composite flower structures with a cross-sectional pattern of deeper positive and shallower negative relief and a map-view pattern of northern negative, central positive, and southern negative relief. From the Late Ordovician to the Early Triassic, the transpressional setting and the subsequent transtensional setting were consistent with the tectonic evolution history and stress field environment of the study area. In cross-sectional view, the stress reduction effect of the ductile detachment layer caused the tectonic activity after the Middle Triassic to have little impact on the pre-existing strike-slip faults.