Exhumation of an ultrahigh-pressure metamorphic terrain in the Dabie Shan, China.

摘要

This dissertation is a field and laboratory based study of the exhumation of the Dabie Shan ultrahigh-pressure (UHP) metamorphic terrain, China. Large-scale structural mapping in the UHP terrain revealed a history that involves syn-UHP metamorphic juxtaposition of tectonolithologic units (D₁/M ₁), N-vergent top-to-the-N shearing and folding under amphibolite facies conditions (D₂/M₂), and refolding at shallow-levels. The UHP metamorphism occurred on a truly regional scale, as indicated by the remarkable structural concordance between the units containing UHP mineral assemblages and the enclosing quartzofeldspathic orthogneiss units, close similarity of the U-Pb zircon ages of metamorphism in UHP eclogites and quartzofeldspathic host-gneiss, and oxygen isotopes. The Huwan Detachment, a N-dipping, top-to-the-N shear zone discovered between the UHP footwall in the S and low-grade hanging-wall in the N, accounts for ca. 130 km of displacement in the Late Triassic and Early Jurassic. Detailed petrologic studies and structural mapping also led to the recognition of the Taihu Detachment, a S-dipping, top-to-the-S, greenschist facies ductile shear zone between the Hualiangting eclogite zone and northern Susong Complex, which was the locus of ca. 20 km of down-dip displacement at a relatively shallow level.; The Dabie massif, a previously subducted continental slab, is thus bounded by two N-dipping fault systems, the Huwan Detachment above and the foreland thrusts below. The exhumation of the slab was made possible by simultaneous displacement along these faults, aided by extension at the top of the orogenic system (as represented by the Taihu Detachment) and synkinematic erosion. P-T-t history indicates that the majority of exhumation of the UHP/HP metamorphic terrain had been accomplished during the Late Triassic at rates of ca. 3–5 mm/yr, before subsequent slow (at ca. 0.5–0.8 and then ca. 0.1–0.2 mm/yr) denudation finally exposed the UHP terrain in the middle Jurassic.