Mesozoic Tectonic Evolution Of The Southern Boundary Faults Of Tongbai-Dabie Shan Orogen With Implications For Orogen Extrusion Processes

The Tongbai-Dabie Orogen Belt (DOB) in eastern central China is a600km long, NW-SE trending continental-continental collisional orogen and it was formed by Triassic northward subduction of the Yangtze craton beneath the Sino-Korean craton. The DOB contains one of Earth’s three giant UHP terranes (>10,000km2). Geologists have made great efforts on revealing the ultra-high pressure (UHP) exhumation processes. The dynamic mechanism concerning the exact exhumation paths can be mainly divided into four types:buoyancy, disintegration, syn-collisional extrusion and plate detachment. Most of these models are based on petrology, isotope geochemistry and geochronology with only few studies based on structural work. Furthermore, most of the Triassic syn-collisional structures of the DOB have been destroyed by post-collisional extension, igneous intrusion and large-offset strike-slip faulting, which makes it difficult to understand the syn-collisional deformation processes of the DOB.Several seismic profiles across the DOB have revealed its three-dimensional architecture.The DOB is composed of several fault-bounded slices with continuously changing metamorphic and deformation intensities. These tectonically stacked terranes were extruded upward along their boundary detachment faults. It is widely recognized that systematic studies on kinematics and evolution of these boundary faults can provide firsthand information on the exhumation paths of the orogen. Structural geologists did much work on the Huwan fault and the Mozitan-xiaotian fault in the northern margin, the Tanlu fault along the eastern margin of the DOB. However, the southern boundary faults of the DOB are still poorly understood.The southern DOB includes two major boundary faults, the Xiangfan-Guangji fault (XGF) and the Xincheng-Huangpi fault (XHF). The XGF separates the DOB from the Yangtze craton and it is the lower boundary detachment of the orogen. Various extrusion models of the DOB give birth to various deformation mechanisms of the X-GF, such as a Jurassic continental-continental subduction zone, eastern extension of the Mianlue suture, S-dipping detachment surface of the Dabieshan dome, lower detachment surface of the UHP terrane. Multi-explanations of the XGF prove that the XGF played an important part in this evolution, but is still porrly understood. The XHF is the other major boundary fault in the southern DOB. It separates the southern DOB greenschist/blueschist unit (BU) from the DOB UHP complex (UC). The UC is characterized by extensively out-cropping coesite-bearing UHP rocks and is intruded by voluminous Cretaceous plutons, while the BU is composed of HP blueschists and greenschists with rare Cretaceous intrusions. This major fault was even considered as the suture zone between the South China Block and the North China Block.This study presents integrated macro-and micro structural data constraints on the architecture and kinematics of the two boundary faults in the southern DOB.40Ar/39Ar and U/Pb zircon geochronology is used to determine the deformation time of the faults. We then discuss the implications of the two boundary faults combined with other major boundary faults for the exhumation processes of the DOB. We got following major conclusions.1. Because of differential deformation and later reconstruction, the structural characters of the western part of the XGF are significantly different from the eastern part. The eastern X-GF trends NWN with15°-60°dip angles to the WSW. The fault is represented by incohesive fault breccia and fault gouge showing brittle deformation characters. Drag folds, fault rock fabrics and slickenlines show reverse fault characters, while the deep pattern of the X-GF represents as a low angle S-Directed thrust. The western part of the X-GF is composed of several NW-SE trending, NE dipping reverse faults. The general attitude of the fault belt is25-60°∠15°～65°. Several Fault-bounded Proterozoic albite-quartz-sericite schist klippes that belong to the DOB were found upon the Paleozoic unmetamorphosed rocks in the Banqiaodian area about30km south from the X-GF. These klippes reveal that the X-GF must have reached a further southern position (at least30km) than present and then it was eroded. The Dengxian-Nanzhang seismic reflection profile across the X-GF reveals that the X-GF is a listric continental crustal subduction fault along which the Yangtze craton subducted beneath the DOB. The dip angle of X-GF becomes gentler with depth and then it is combined to a subhorizontal detachment along the bottom of the upper crust at a depth of about20km.2. Five distinct episodes of deformation (designated as D1-D5), distinguished on the basis of cross-cutting relationships and deformation overprints, have been recognized in the X-GF. The Dl of the X-GF was characterized by the formation of greenschist facies mylonitic gneisses and it was the main deformation episode. The greenschist/blueschist facies rocks of DOB thrusted onto the Yangtze craton along the X-GF during this stage, the deformation developed in the temperature range from350℃to450℃during234-231Ma and it was a little earlier than the "main" UHP metamorphic event. The succedent D2deformation was characterized by brittle phase NE-dipping reverse faults, conjugated kink zones and buckle folds and it showed NE45°-trending coaxial compression during200-190Ma. The D3deformation is characterized by shallow brittle thrusting and tectonic rotation during-150-140Ma. the western and the eastern parts of the X-GF experienced different kinematic processes during this stage. The D3deformation was subsequently overprinted by a strong N-directed compression (D4deformation) during the Jiangnan intraplate orogeny. Field outcrops reveal that the former N-dipping faults and fold axial planes were reversed to be S-dipping (Fig.6, f). However, the middle-lower crust present early N-dipping reflectors. The stress field from the Jiangnan orogen reached to Wuhan to the north and the western part of the X-GF was scarcely influenced. The D5deformation of X-GF is characterized by local thrusts in a dominantly extensional stage. The Upper Cretaceous red beds locally overlapped on Paleozoic strata. This small thrust of the XGF indicates that the XGF was still active in the late Cretaceous.3. The Xincheng-Huangpi fault (XHF) is a1-2km wide, NW-trending belt extending for more than500km and is characterized by widely exposed mylonites and ultramylonites. The southern contact of the XHF is cut by later brittle normal faults parallel to the mylonite foliation. Along the ductile XHF a1to2km wide belt of protomylonites, mylonites and locally ultramylonites are exposed. To the north, the XHF gradually transitions into the granulite facies DOB UHP complex.4. The Xincheng-Huangpi fault (XHF) experienced two episodes of shearing during Mesozoic. It exhumed to middle crustal levels during234-195Ma with exhumation rates of ca.1.5mm/yr. It was contemporary with the Huwan shear zone along the northern boundary of the DC. Coeval thrust-slip and normal sense shear zones along the southern and northern margins of the Tongbai-Dabie UHP complex would have caused southward vertical extrusion of the UHP slab. The UHP metamorphosed rocks then exhumed to the surface by erosion. The later NW-trending dextral strike-slip shearing occurred during145-140Ma.5. The UHP terrane of Tongbai-Dabie orogen experienced two stage extrusion during Mesozoic. The UHP terrane extruded upperward around234Ma and exhumed to middle crustal levels during220-195Ma with an average exhumation rates of ca.1.5mm/yr. The UHP rocks then exhumed to the surface by erosion. Later NW-SE trending dextral strike-slip shearing of the XHF occurred between145-140Ma and was generally contemporaraneous with sinistral-oblique slip of the Xiaotian-Mozitan fault along the northern margin of the UC. Coeval dextral and sinistral-oblique shearing along the southern and northern margins of the UC would have caused southeastern lateral extrusion. This lateral extrusion has no relationship with the UHP exhumation.