What brought them up? Exhumation of the Dabie Shan ultrahigh-pressure rocks

Abstract

Metamorphic coesite and diamond in the Dabie Shan, eastern China, testify to subduction of continental crust to >100 km depth. Exhumation of these ultrahigh-pressure rocks through the crust encompassed two stages. (1) South-dipping foliation, southeast-plunging stretching lineation, lineation-parallel isoclinal folds, and boudins indicate extreme subhorizontal shortening and subvertical extension during top-to-northwest shearing at 200–180 Ma. Syntectonic recrystallization occurred at eclogite and amphibolite facies temperatures and at pressures below coesite stability. (2) Northwest-southeast subhorizontal extension from 133 to 122 Ma was concentrated within an asymmetric structural dome in a magmatic complex that forms the northern half of the Dabie Shan. Pluton cores at deep structural levels have weak hypersolidus fabrics, and pluton carapaces are mylonitic gneisses formed at upper amphibolite facies conditions. Deformation is concentrated in greenschist facies mylonites and ultramylonites along the Xiaotian-Mozitang detachment fault at the northern topographic limit of the Dabie Shan. Our preferred exhumation model involves two stages: Triassic indentation—vertical extrusion and erosion—followed by Cretaceous plate margin transtension.