The Mesozoic Amdo micro-block and East Asian superconvergent tectonic system

Abstract

The assembly of East Asia involved multi-directional convergence of several micro-blocks during the Mesozoic. However, the geodynamic and tectonic elements associated with the southwestern convergent tectonic system of East Asia involving the Bangong-Nujiang subduction-collision system remain unclear. The Bangong-Nujiang Suture Zone in central Tibet shows a prominent east–west-trending differentiation divided by the Amdo micro-block. Here we evaluate the structural architecture of the Amdo micro-block, together with stratigraphic and paleomagnetic data, geochronological and geochemical data on the multi-stage magmatic and metamorphic rock suites with a view to trace the subduction and closure history of the Bangong-Nujiang Ocean as well as the collisions among micro-blocks. We also present new data from outcrop structural analysis which suggest that the micro-block has undergone five episodes of deformation (abbreviated as D1 to D5), which are represented by respectively NW-striking isoclinal tight folds, NE-striking tight folds, E-trending asymmetric folds, V-type conjugate strike-slip shear zones, and top-to-the-southwest thrust faults. In the Early Jurassic, initial active collision occurred between the Amdo micro-block and the Qiangtang Block followed by rifting and back-arc extension of the Bangong-Nujiang Ocean south of Amdo and passive collision between the Amdo micro-block and the Qiangtang Block in the late Early Jurassic. In the Middle-Late Jurassic, the Bangong-Nujiang Oceanic lithosphere underwent flat subduction beneath the Amdo micro-block resulting in slab tearing along the western border of the micro-block, large-scale intracontinental orogeny and thrusting. The slab tearing induced eastward mantle and crustal flows, and along with the N-S-directed compression, V-shaped conjugate shear zones formed in the interior of the Amdo micro-block. The intracontinental destruction and deep mantle lithospheric delamination in East Asia triggered the coeval convergent drops of deep materials in late Mesozoic, which drove further collision between the Lhasa Block and the East Asian continent in the Early Cretaceous.