The Mesozoic Tectonic Evolution Of South China

The South China block, featured by rich and complex patterns of Mesozoic intracontinental deformation and large igneous provinces, is one of the most optimal areas in the world to understand not also the Mesozoic geological architecture of East Asia, but also how the subduction or collision occurring in the plate boundaries controls the formation of intracontinental orogeny and deformation. In order to clarify the deformation features and decipher the tectonic evolution of South China during the Mesozoic period, detailed structural and geochronological studies have been conducted in the Dabashan, Yuanma Basin and Hengshan massif. Moreover, an overall review for the Cretaceous tectonics in South China by considering multiple lines of evidence is given to constrain the Cretaceous tectonic evolution, and to aid in directing future research of South China.Structural and40Ar/39Ar data from the mylonitic rocks of the North Dabashan zone (NDZ) document kinematic and tectonothermal characteristics of the Mesozoic collisional and intra-continental orogenesis in the northern segment of Yangtze Block. The NDZ underwent two deformational phases during the Mesozoic period. The earlier one is characterized by top-to-the-SW thrust ductile shearing along a NW-trending shear zone (DSZ-1), while the later one is featured by dextral strike-slip ductile shearing along another NNW-trending shear zone (DSZ-2). The timing of the two deformation events have been constrained to be245-189Ma and178-143Ma respectively, by using mica40Ar/39Ar geochronology It is proposed that the earlier deformation event was associated with the Middle Triassic-Early Jurassic collision between the North and South China Blocks, which generated the initial framework of the NDZ; and the later one was related to the Middle Jurassic to Early Cretaceous intra-continental orogeny in East Asia, which caused a significant eastward extrusion of the South Qinling and led to the formation of the SW-convex Dabashan foreland orocline. The distinguishing between these two deformation events sheds a new insight into the Mesozoic tectonic evolution of the Qinling orogenic belt. Apatite fission track (AFT) dating and thermal modeling were carried out on eight rock samples collected from the Fenghuangshan massif in the North Dabashan zone. The results show that the uplifting history of Fenghuangshan massif can be divided into two stages during the Cretaceous period:Early Cretaceous slow (135±5～95±5Ma) and Late Cretaceous rapid uplifting (95±5～65±5Ma). The former one represented a period of relative stability after the Late Jurassic intracontinental orogenesis within the Dabashan structural belt; while the latter one was concomitant with post-orogenic extension and collapse of the Dabashan structural belt, during which arrays of coeval rifted basins had been generated along the Qinling orogenic belt.The Yuanma Basin, situated in central South China with more than800km far away from the active plate boundaries, is a key region in deciphering the Late Mesozoic-Early Cenozoic tectonic evolution of South China. Based on a field analysis of fault-slip data collected from different rock units, we document polyphase tectonic stress fields and address the changes in the orientation of the principal stresses, and finally establish a five-phase stress evolution of the basin during the Late Mesozoic-Early Cenozoic. The earliest one with E-W compression and N-S extension occurred in the Mid-Late Jurassic and resulted in regional folding and west-directed thrusting along the eastern margin of the basin. This deformational event constrains the timing of the significant intracontinental orogeny in South China to be Mid-Late Jurassic. The subsequent NW-SE extensional regime was responsible for the initial opening and sedimentary infill of the basin during the earliest Cretaceous, which occurred coevally with the extensive Early Cretaceous magmatism, volcanism and extensional doming in South China. The tectonic regime then changed, in the late Early Cretaceous, to a compressional one with NW-SE compression and NE-SW extension, causing the inversion of this extensional basin. This compression affected the whole South China, leading to an apparent magmatic quiescence at～108-117Ma and sinistral strike-slip faulting on the NNE-trending faults. The N-S extension prevailed in the Late Cretaceous, and activated the east-trending normal faults that controlled the Late Cretaceous subsidence. This extensional phase seems to have lasted through the Early Paleogene and ended in the Late Paleogene by a compressional regime with NE-SW compression and NW-SE extension, as attested by the stratigraphic unconformity at the base of the Neogene in the Jianghan Basin. The temporary evolution of these tectonic stress fields documented in the Yuanma Basin provides a new insight into the influences of different plate tectonics exerting on the South China Block over the Late Mesozoic to Early Cenozoic time. We infer that the Yuanma basin was influenced, in the Middle Jurassic-Early Cretaceous, by the process of westward subduction of paleo-Pacific plate; and in the Late Cretaceous-Early Paleogene by a combined effect of the subduction along the west pacific margin and in the Tethyan domain; and finally in the Late Paleogene by the far-field effects of India-Asia collision.A significant low-angle normal fault, namely the Hengshan detachment fault, has been documented in the Jiangnan orogenic belt, which extends more than150km long in NNE orientation and bounds an elongated Early Cretaceous basin in Hunan Province, central South China. Detailed structural and geochronological analyses have been conducted along its southern segment, where a well exposed,～3km thick, flat ductile shear zone develops along the western margin of the Hengshan granitic massif. This zone is featured by shallow-dipping foliations of varied trend from NE to NW, which bear penetrative stretching lineations varying from NW to SW trend. Shear sense criteria indicate top-to-the-NW and top-to-the-SW motions along its northern and southern parts, respectively. Quartz c-axis orientations of mylonitic rocks from the shear zone exhibit asymmetric single or crossed girdle patterns, and the distributions of fabric point maxima suggest a simultaneous operation of basal<a> and prism<a> slips, indicating a deformation temperature ranging from400℃to550癈.℃One zircon U-Pb age of the sheared albitite and three muscovite40Ar/39Ar ages of the mylonitic rocks indicate that the ductile shearing initiated at136Ma and lasted till97Ma. The zircon U-Pb dating results of five granitic samples from the Hengshan granitic pluton yield two phases of crystallization ages of232-228Ma and150-151Ma respectively, indicating two preceding magmatic events prior to the ductile shearing in the lower plate. Our structural and geochronological data allow defining the Hengshan massif as an extensional dome, rather than a metamorphic core complex as previously predicted, due to the lack of syn-tectonic plutonism and the origin of detachment fault associated with the reactivation of inherited thrust. This study provides crucial evidence for understanding how the mid-upper crust deformed during the process of extension, which testifies to a coeval occurrence of non-coaxial deformation of the ductile flow in the middle crust and brittle faulting in the upper crust. It also sheds new insights into the timing of the regional crustal extension in South China, and permits to constrain its onset time to be ca.136Ma.The Cretaceous tectonics of South China is characterized by widespread extensional basins, voluminous magma intrusion/eruption and associated polymetallic mineralization, all of which are of world-wide interests and have stimulated the attention of geologists for more than half a century. The controversies regarding the nature and origin of these features rise from a lack of comprehensive understanding of the regional tectonic evolution and geodynamics of South China. This paper attempts to make a review by synthesizing existing structural, petrological, geochronological and geochemical data of the Cretaceous basins and magmatism, and to propose a chronology of six-stage tectono-thermal evolutionary history of South China during the Cretaceous. The earliest Cretaceous compression (145-137Ma) led to the formation of adakitic rocks and associated Cu-Au deposits in the Lower Yangtze River Belt and the gneissic granites in the coastal area, formation of them were possibly related to the midocean ridge subduction of the Paleo-Pacific and Izanagi plates. The middle Cretaceous (136-118Ma) was predominated by the NW-SE extensional regime, which led to significant taphrogenesis manifested by large-scale extensional basins and voluminous magma intrusion/eruption, formation of them might have resulted from a combination of ridge subduction and rollback of the subducted oceanic slab of the Paleo-Pacific plate. This extensional stage was followed during the latest Early Cretaceous (117-108Ma) by a NW-SE-oriented compressional one, which led to cessation of the active marginal magmatism and caused tectonic inversion of previous rift basins and sinistral strike-slip faulting along the NE-striking faults. This compression is attributed to the collision between the eastern Asian margin and the West Philippine block. The collision-induced crustal contraction was quickly terminated by a WNW-ESE extensional regime during the Mid-Late Cretaceous (107-86Ma), which led to a new phase of basin subsidence and magmatism particularly along the coastal area of South China. Then a WNW-ESE compressional regime settled down in South China during the Late Cretaceous, which resulted in a second tectonic inversion of the Cretaceous basins. The changes in tectonic regime may be due to the break-off or changing geometries of the subducted oceanic slab beneath South China. Plate kinematics seems to have changed dramatically during the late Cretaceous when a N-S extensional stress regime prevailed over South China, which might result from a far-field effect produced by northward subduction of the Neo-Tethys oceanic plate beneath the southwestern margin of South China.