Microstructures documenting Cenozoic extension processes in the northern continental margin of the South China Sea

In order to investigate the thinning process of the northern continental margin of the South China Sea, petrographic and microstructural analysis were carried out on 20 greenschistfacies mylonite samples, which were obtained from Site U1504 of IODP Expedition 367/368 in the Outer Margin High of the region. The mineral assemblage of the greenschist-facies mylonite is chlorite + epidotite + albite (Ab = 94.7–99.9) + quartz, which contains 10-30% gravel components. Microstructural analysis indicates that the greenschist-facies mylonite experienced two episodes of deformation：early ductile deformation followed by a later stage of brittle deformatio. Both episodes of deformation suggest an extensional environment. The extensive development of bulging recrystallization (BLG) of quartz, microscopic fractures and fine granulation of albite suggest that the temperature of ductile deformation is about 300-400°C, compatiable with a ductile shearing at shallow crust levels (~5-10 km). Petrographic features suggest that the greenschist-facies mylonite might originate from volcanic sedimentary rocks or sedimentary rocks affected by the intrusion of mafic magma. Combined with seismic interpretation, we propose that the greenschist-facies mylonite might be formed by crustal exhumation after thick Mesozoic sediments were denuded by a major extension.

为探究南海北部大陆边缘的地壳减薄过程，研究人员对采自该区域外边缘高地综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）367/368航次U1504站位的20件绿片岩相糜棱岩样品开展了岩石学与微观结构分析。该绿片岩相糜棱岩的矿物组合为绿泥石+绿帘石+钠长石（Ab=94.7–99.9）+石英，含10%~30%的砾石组分。微观结构分析结果显示，该绿片岩相糜棱岩经历两期变形作用：早期韧性变形与后期脆性变形，两期变形均指示伸展构造环境。石英的凸起重结晶（bulging recrystallization, BLG）广泛发育，辅以显微裂隙与钠长石细粒化现象，表明韧性变形的温度约为300~400℃，与浅地壳深度（~5~10 km）下的韧性剪切作用特征相符。岩石学特征表明，该绿片岩相糜棱岩可能源自火山沉积岩或受基性岩浆侵入影响的沉积岩。结合地震解释成果，本研究提出该绿片岩相糜棱岩可能形成于中生代厚层沉积物经大规模伸展作用剥蚀后的地壳剥露过程。