Tectonic Evolution And Structural Styles Of The Tongbai Collisional Orogen And Its Neighbors

In the past twenty years, based on petrology, geochemistry, structural geology and geochronology, different models have been proposed to explain the exhumation process of Qinling-Dabie-Sulu high/ultrahigh-pressure (HP-UHP) terranes. Most of the above studies focused on the Dabie-Sulu orogens. Recently, in the Tongbai orogen which is located at the junction between the Qinling orogen without Triassic UHP rocks and the Dabie orogen with Triassic UHP rocks, detailed petrological, geochemical and geochronological investigations identified two Mesozoic low-temperature and high-pressure eclogite zones separated by the Tongbai high-grade complex. In this contribution, structural data are reported for the Tongbai collisional orogen and the surrounding units in order to provide structural constraints for the exhumation process of the Tongbai HP rocks. We further evaluate the tectonic evolution of the Tongbai collisional orogen and its neighbours based on our detailed field data and available geochronological results.The Tongbai collisional orogen and its Neighbors can be divided into nine tectonic units. They are, from north to south, the southern margin of the North China Craton tectonic unit-the Kuanping group, the Erlangping tectonic unit with the nature of back-arc basin, the Qinling complex unit with the nature of island arc, the Guishan group and the Nanwan group composed of leading-edge wedge of the subduction zone, the tectonic melange zone, the northern High-pressure (HP) unit, the Tongbaishan high-grade complex, the southern HP unit, the Suizhou tectonic deformation zone.According to structural analysis and new geochronological evidences, five distinct episodes of deformation (D1-D5) are distinguished in this area since the Mesozoic. The former two-episode of deformation developed in the various tectonic units south of tectonic melange zone, and the latter three-episode of deformation developed in all units of this area. The time of the D1 episode is about 255-238Ma. D1 formed regional penetrative foliations and the WNW-oriented stretching lineations. D2 maybe occurred at some time about 230-215Ma, characterized with the southward-directed thrust. D3 should be earlier than lower Jurassic, and nearly WNW trending open folds as the main characteristics of this episode. D4 occurred in roughly between 135-120Ma, characterized with the activities of the ductile shear zones. D5 occurred in 120-80Ma, characterized with the activities of NW and NE trending brittle faults which cut all of the above structural features.The D1-D5 deformation events that occurred after the peak HP metamorphism, record the exhumation process of the Tongbai HP rocks. The first three stages of deformation (D1-D3) can be summarized as syn-collisional deformation in Indosinian, during which the HP slices were extruded and uplifted:1) the early stage of extrusion of the high-pressure rocks from west to east is constrained by the L1 stretching lineations.2) The southward-directed thrust (D2) makes the high-pressure rocks further vertical uplift.3) The Tongbaishan high-grade complex and the high-pressure rock units are synchronously uplifted during D3. The first two stages of deformation (D1-D2) are accommodated by the first fast stage of exhumation, and the D3 deformation is associated with the late slow stage of exhumation. Subsequently, the regional structural framework was overprinted by Yanshanian granites, strike-slip ductile shear zone (D4), and brittle faults (D5), resulting from the post-collisional extensional deformation.