Exhumation tectonics of the HP-UHP orogenic belt in Eastern China: New structural-petrological insights from the Tongcheng massif, Eastern Dabieshan

The Tongcheng massif, a Structure lying to the east of the central Dabieshan domain, previously considered a unique unit metamorphosed under Ultrahigh-Pressure (UHP) conditions, similar to those involved in the South Dabieshan and wrenched by the Tan-Lu fault during the Mesozoic. Detailed held survey and laboratory work in the areas of structural geology and petrology indicates that the Tongcheng massif is a tectonic stack of allochthonous sheets. From the bottom to the top, this massif is divided into three units: Lower, Middle and Upper Unit. The Lower Unit suffered an UHP metamorphism with a P-T condition of about 3.21-3.31 GPa and 590-651 degrees C. Eclogites in the Middle Unit underwent high pressure (HP) and medium temperature metamorphism at 2.46-2.64 GPa and 500-565 degrees C. In the Upper Unit. the peak metamorphic P-T conditions for eclogites were estimated as 1.54-2.36 GPa and 389-510 degrees C. These petrological results correspond well to the geometry of the held observation. Structural analysis indicates that the Tongcheng massif has experienced polyphase deformation. The earliest event (D-1) corresponds to top-to-north (northeast) shearing associated with the early exhumation stages of HP-UHP rocks during the late Triassic. The D-2 deformation overprints D, and coincides with a domal structure similar to a metamorphic core complex of early Cretaceous age. The D-3 deformation is expressed by a pervasive ENE-WSW trending, subvertical foliation and a subhorizontal lineation or penetrative slickenline with a sinistral sense of shear, probably related to the NE-SW contraction during the NW-SE subhorizontal extension, the strike slip deformation raised by the N-S compression during the early Cretaceous. The D-4 event is represented by a brittle NE striking, moderately to steeply dipping to the SE fault with down-dip slickenlines. Kinematic indicators show a normal displacement with the east side moving downward. Half-grabens of late Cretaceous to early Cenozoic age are widespread throughout Eastern China. Based Oil Structural analysis, mineral chemistry data and, in particular, the results of P-T calculations for eclogites in different units, a geodynamic scenario for the evolution of the HP-UHP orogenic belt of the Dabieshan is proposed, and the significance of the Tan-Lu fault is also discussed. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.