--> Segmentation and Mechanism of Differential Extension in the Continental Marginal Basins of the Northern South China Sea

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Segmentation and Mechanism of Differential Extension in the Continental Marginal Basins of the Northern South China Sea

Abstract

Although the South China Sea margin is generally regarded as magma-poor rifted margin, the intricate rift architecture preserved in the continental marginal basins of the Northern South China Sea arise debate over extension model during this process. We employ detailed seismic reflection data to investigate spatial-temporal differences of structure style and examine distribution of extension in the crust. Our results show geometry, kinematics of extensional fault systems in Paleogene rift zone of Pearl River Mouth Basin (PRMB) and Qiongdongnan Basin (QDNB) change greatly along strike, either basin is composed of NE- and EW- trending normal faults. Furthermore, concealed regional/local NW-trending strike-slip/normal faults with associated deformation bands constitute transfer zones between extensional faults systems. NE-trending normal faults are characterized as “Non-detachment” fault system, where crust is less depth-dependent extension with low upper (<1.8), lower (<5), and whole (<3) crust stretching factor. Whilst EW- trending normal faults sole into the Moho as detachment fault system, where crust is significant depth-dependent extension with upper (>2.0), lower (>15), whole (>4) crust stretching factor spikes. Combining with major faults slip rate, we identify three rift stages:1) Eocene initial stretching, (half) grabens controlled by distributed high-angle faults reactive from NE-/EW-orientated pre-existing fabrics respectively under uniform pure shear; 2) Early Oligocene focused thinning under the continental slope necking zone, thereinto development of NWW-/EW- trending detachment fault systems with large horizontal displacement in westmost Ledong sag (QDNB) and eastern Baiyun sag (PRMB) caused extension discrepancy by simple shear transition, whereas the central transfer zone in PRMB formed along with inversion to accommodate NE-trending faults with inherited activity by pure shear. 3) Late Oligocene, development of NWW-/EW- trending detachment fault systems in eastern Changchang sag (QDNB) caused extension discrepancy by simple shear transition, and the central transfer zone in QDNB formed along with inversion and flower structures to accommodate all the rest subdued faults activity. The hyper-extended crust zones are next to either intra-basin transfer zones near COT or Red River fault zone, which suggests that superposition of local sinistral shear stress field and different extension amount in segmented basins result from strike-slip movement along the NW-trending pre-existing deep-seated faults are the mechanism for differential extension. The causes of significant depth-dependent extension arise from intensive lower crust thinning that are associated with the large-scale low-angle detachment systems by transition from pure shear to simple shear model and mantle exhumation predates seafloor spreading.