Thermal evolution of the Hengshan extensional dome in central South China and its tectonic implications: New insights into low-angle detachment formation

Abstract

The Hengshan massif is an exhumed, mid-crustal, plutonic–metamorphic dome formed during Cretaceous crustal extension in the Jiangnan orogenic belt, central South China. Multiple thermochronometers (mica 40Ar/39Ar, apatite fission track and zircon (U–Th)/He) are applied to its footwall along a slip-parallel transect to quantify its thermal history and cooling rate, and the slip magnitude, rate, initial geometry and kinematic evolution of the low-angle Hengshan detachment fault. Our thermochronological data, in conjunction with previous ages, indicate that (1) footwall rocks cooled from ~700°C to ~60°C in less than 60Myr (136–80Ma) at variable rates ranging from ~50°C/Myr to ~13°C/Myr, (2) the Hengshan detachment fault accommodated ~8–12km of total slip at variable slip rates from 0.14 to 1mm/yr during tectonic exhumation, (3) the footwall has been tilted ~26°–50° to the east since slip began, indicating that the low-angle Hengshan detachment fault initiated at a steep dip and was passively rotated to a more gentle orientation during subsequent normal slip. This study provides compelling evidence supporting that the low-angle detachment fault in the extensional dome can be generated by the reactivation and passive rotation of an initially steep reverse fault during normal slip. In addition, our thermochronological data constrain the time of extension in the Hengshan dome between 136 and 80Ma, which implies that the back-arc extension within South China associated with the rollback of the Paleo-Pacific slab might have lasted until at least 80Ma.