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

摘要

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 Ar-40/Ar-39, 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 similar to 700 degrees C to similar to 60 degrees C in less than 60 Myr (136-80 Ma) at variable rates ranging from similar to 50 degrees C/Myr to similar to 13 degrees C/Myr, (2) the Hengshan detachment fault accommodated similar to 8-12 km of total slip at variable slip rates from 0.14 to 1 mm/yr during tectonic exhumation, (3) the footwall has been tilted similar to 26 degrees-50 degrees 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 80 Ma, 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 80 Ma. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.