Tectonic evolution of the Tianshan Akeyazi metamorphic complex (NW China)

Abstract

The South Tianshan orogen (northwest China) comprises a thick volcanoclastic HP-UHP metamorphic belt, the Akeyazi (high-pressure) metamorphic complex (AMC), exhibiting well-preserved eclogite- and blueschist-facies rocks and pervasive deformation throughout all units. In order to clarify the tectonic evolution of this fossil subduction zone, which remains controversial due to the lack of systematic field observations combining structural and petrological data, we herein present the results of extensive mapping of lithologies, deformation patterns and petrological evolutions.The AMC is composed of a high-pressure (HP) to ultra-high-pressure (UHP) central domain – the HP-UHP unit – separated in the North by a major detachment from a greenschist-facies domain. A gradational transition in lithologies (i.e., interlayered micaschists, volcanoclastic, and mafic rocks) is observed within the HP-UHP unit, from more pelitic in the North to more mafic in the South. Tectonic patterns and contrasting lithologies allow to recognize five distinct sub-units stacked at HP to UHP conditions. Pervasive ductile deformation revealed by penetrative schistosities, folding, shear planes and boudinage structures is characterized by successive deformation stages which are attributed to burial (Dn-1), peak deformation accompanying detachment from the slab and/or deep nappe stacking (Dn, Dn+1) and exhumation (Dn+2, Dn+3). During exhumation under blueschist-facies conditions, the HP-UHP nappe stack underwent pervasive shearing, in particular across km-scale extensional shear zones with consistent top to the north shear senses reworking inherited thrust contacts. The entire HP-UHP unit reached peak-metamorphic conditions at ∼2.5 GPa and ∼540 °C. Peak pressures of 2.52 ± 0.25 GPa, consistent with the widespread occurrence of coesite in the HP-UHP unit, suggest a maximum burial depth of ∼80 km which is thought to reflect a transition between mechanical coupling and decoupling at the plate interface. This tectono-metamorphic evolution is set back in a tentative geodynamic model.