Structural anatomy of the exhumation of high-pressure rocks: constraints from the Tongbai Collisional Orogen and surrounding units

The Tongbai Collisional Orogen and associated units, located between the Qinling and Dabie orogens, can be divided into nine lithotectonic units. From north to south, these are the southern tectonic unit of the North China Plate-the Maoji Group, the Erlangping tectonic unit, the Qinling Complex unit, the Guishan and Nanwan slice, the Xiaojiamiao melange unit, the northern high-pressure (HP) unit, the Tongbai high-grade Complex, the southern HP unit and the Suizhou tectonic belt. Based on structural analysis and available geochronological data, five principal episodes of deformation (D-1-D-5) are distinguished in this area following the Mesozoic HP metamorphism. The first two episodes of deformation developed in various tectonic units to the south of the Xiaojiamiao melange unit, and the subsequent episodes of deformation developed in all the units of this area. The D-1 deformation is correlated to ca. 255-238 Ma generating regional penetrative foliation and WNW-trending stretching lineation, and leading to the initial stage of eastward oblique extrusion of the HP rocks. The D-2 event is characterized by top-to-the-SSW thrusting which contributed to the further vertical uplift of the HP rocks sometime around 230-215 Ma. The D-3 event is defined by WNW-trending open folds, and must have taken place earlier than Lower Jurassic. The D-4 event is characterized by ductile strike-slip shearing, and during this episode the Tongbai high-grade Complex was extruded to the east at ca. 131-119 Ma. The D-5 event probably took place at ca. 100-80 Ma, and is marked by the NW and NE trending brittle faults which overprint all of the above mentioned structures. The exhumation of the Tongbai HP rocks illustrates the process of gradual extrusion and uplift, controlled by several episodes of deformation. Copyright (C) 2010 John Wiley & Sons, Ltd.