Refolded syn-ultrahigh-pressure thrust sheets in the south Dabie complex, China: Field evidence and tectonic implications

Abstract

The exhumation mechanisms of ultrahigh-pressure (UHP) rocks have been a controversial issue ever since the discoveries of coesite and diamond in metamorphic rocks from several orogenic belts. The lack of consensus principally reflects the limited number of combined structural, petrologic, and geochronologic studies that have been carried out in great detail within these terrains. In this study, we derive a complex structural history for the UHP South Dabie terrain based on detailed (1:10000) mapping in the eastern Dabie Shan, and use this to place new constraints on the exhumation process. Structural analysis demonstrates that syn-UHP south-vergent thrusting (D 1 ) was followed by north-vergent folding under amphibolite facies conditions (D 2 ) and a third generation of folding at shallower level (D 3 ). This supports a two-stage exhumation model in which initially rapid exhumation from mantle depth to lower crustal levels is followed by slower exhumation from the lower crust level to the Earth surface. The remarkable structural concordance between lithological units containing UHP mineral assemblages and the enclosing gray gneiss units implies that the apparent difference in peak pressures recorded by different lithologies is not a consequence of tectonic juxtaposition, but a result of differential reequilibration during decompression.