Deformation history of the Hengshan Complex: Implications for the tectonic evolution of the Trans-North China Orogen

摘要

Structural analysis indicates that the Hengshan Complex underwent five distinct episodes of deformation (D_1-D_5). The D_1 episode formed small isoclinal folds (F_1), penetrative axial planar foliations (S_1) and mineral stretching lineations (L_1). D_1 fabrics were reworked by following D_2 deformation. Associated with D_2 was the development of NW-verging asymmetric folds and accompanying thrust faults. Both D_1 and D_2 resulted from crustal thickening and coherent with prograde and peak metamorphism. D_2 deformation was followed by transpressional dextral shearing (D_3), including top-to-NW oblique-slip shearing and NNE-SSW dextral strike-slip shearing. Ongoing collision led to development of the Zhujiafang ductile shear zone (D_4), a near E-W trending high strain belt across the Hengshan Complex. D_5 deformation is characterized by F_5 open folds and associated normal faults, probably related to exhumation of the complex. Structural patterns of the Hengshan Complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the Eastern and Western Blocks underwent eastward-directed subduction beneath the western margin of the Eastern Block, and closure of this ocean finally led to collision between the two blocks to form the coherent North China Craton.