Study of the geodynamic evolution of the Chinese Tianshan metamorphic belt to unravel deep processes occurring at the plate interface.

Field-based studies of fossil plate interfaces can be used to provide constraints on the mechanical behaviour of subduction channel processes at depth. The Chinese Tianshan metamorphic belt (TMB) represents a suitable test area to study such processes since it contains fresh high-pressure rocks and well exposed structures in its central part. Lithologies composed of intercalated metasediments and volcanoclastics are likely derived from the trench during subduction with incorporation of first metapelitic-rich components and then increasing amounts of psammitic- and mafic-rich components and few carbonates. During burial, this material as well as pieces of the upper oceanic crust (pillow basalts) were scrapped off the slab and stacked to the upper plate and/or to the subduction channel at depth. During exhumation most of the deformation was accommodated by shearing and schistosity developed at blueschist-facies conditions (glaucophane lineations). A systematic sampling survey was carried out in the area through two N-S transects in order to reconstruct the P-T-t history of the central part of the TMB. Peak pressures and temperatures were estimated independently using Raman spectroscopy (quartz inclusions in garnet (P) and carbonaceous material (T)) and electron microprobe analysis (Zr-in rutile) on metasediments and metabasites. These yield coherent peak conditions of 530±30°C and 2.3±3 GPa. We combined these results with new geochronological Rb-Sr data to constrain the age of underplating and exhumation. We finally propose a tentative model for the geodynamic evolution of the TMB, based on field observations and P-T-t conditions. The TMB is interpreted to represent an erosive margin with accretion and stacking of tectonic slices at depths of 70 km, implying strong coupling/decoupling processes around this depth. During later exhumation this stack of detached material was brought back to the surface as a single unit undergoing shearing with minor internal displacement.