Two-stage Triassic exhumation of HP–UHP terranes in the western Dabie orogen of China: Constraints from structural geology

Abundant exposures of widely-distributed HP–UHP metamorphic rocks in the western part of the Dabie orogen enable us to study the tectonic evolution of HP–UHP terranes associated with the world's largest preserved continental subduction zone. Previous tectonic models for the Dabie orogen were based largely on metamorphic studies, most of them lacking significant structural constraints. We present a comprehensive structural analysis based on detailed structural geology. The results suggest that syn-UHP (D0 at 241–231Ma) and syn-HP (D1 at 225–215Ma) southeast-vergent thrusting formed a series of stacked structural slices. This was followed by southeast-vergent folding under amphibolite facies conditions (D2 at 215–205Ma); then a third generation of flexural folding occurred at shallow levels (D3 at 200–184Ma). This leads us to proposes a two-stage Triassic exhumation model in which initially rapid vertical extrusion (D0–D1) from UHP to HP conditions to lower crustal levels is followed by slow southeastward extrusion (D3) from lower crustal levels to the Earth's surface. The tectonic model combines the early southeastward vertical extrusion with the later southeastward lateral extrusion, revealing two different stages and thus different types of Triassic extrusion for the exhumation of HP–UHP rocks in the Dabie orogen. The first stage extrusion occurred in the Middle Triassic, whereas the second stage extrusion lasted from the Late Triassic to Early Jurassic. These two extrusion episodes correlate with the two stages of Triassic exhumation of the Dabie HP–UHP rocks, respectively, during continental collision.