南海西南次海盆剪刀式扩张

Spreading pattern with scissors of Southwest sub-basin in the South China Sea

南海西南次海盆的平面形态比较独特,是一个往北东开口、尖角指向西南的“V”字型海盆。正确认识其扩张运动学特征是研究南海新生代扩张与构造演化的关键一环。本文利用反射地震剖面、重力异常和磁异常资料、IODP大洋钻孔资料等,详细分析了西南次海盆的地质构造特征。研究结果揭示,次海盆边缘扩张期的上超层序自北东而西南特征相同,是海盆同一时间开启形成的沉积;作为西南次海盆与东部次海盆的构造分界,中南-礼乐断裂带并非完全呈S—N向延伸,而是一条南、北两端均朝西弯曲、略呈弧形的断裂带,此为西南次海盆扩张期间呈扇形推移所导致;西南次海盆西部一条断层面西倾的断裂带在此分隔了扩张海盆区与过渡壳区,两区之上发育一套扩张期间沉积的层序,覆盖在扩张区的洋壳之上和过渡壳区的古近系之上,证实两区经历了同步拉张的过程。由此证实,西南次海盆的扩张运动学模式为“剪刀式扩张”。即海盆的扩张自北东而西南同时开启,扩张速率却自北东而西南逐渐减慢,导致越往西洋壳区越窄,使得海盆西部尖角区域成为有拉张而无扩张、面积较窄的过渡壳区,因而形成呈“V”字型展布的西南次海盆。

Southwest sub-basin(SWSB)in the South China Sea(SCS)is“V”shaped with unfold curving towards northeast and pointing to southwest.Fully understanding the opening kinematics of the SWSB is crucial to study the tectonic evolution of the SCS in Cenozoic.Data of reflection seismic profiles,gravity and magnetic anomalies,chrono-stratigraphy from IODP 349 are used to study the geological and tectonic characteristics in this paper.Results showed that the features of the onlap sequences at the northeastern and southwestern edges are almost the same,which reveals these sequences were deposited in the same period of sea-floor opening,and the trend of the Zhongnan-Liyue fault zone as the boundary between SWSB and East sub-basin(ESB)is not completely straight from north to south but curves to west on both tails,resulting in an arc-fault zone,resulting from the fanshaped migration of sea-floor spreading of the SWSB.At western side of SWSB,there is another fault zone with its fault-face tilting to west,separating oceanic crust from transitional crust areas.A set of sedimentary sequence cover both oceanic crust area and transitional crust area with Paleogene sediment formed during the extensive stage,which exhibits the synchronous extension of the two areas.All the geological and tectonic appearances mentioned above verified that the sea-floor spreading pattern of SWSB had the scissors type opening.This basin opened from northeast to southwest at the same time but the rate of spreading was very slow,forming a large area of sea-basin with oceanic crust at the east and a small area with transitional crust at the west in the SWSB.Thus,the plane“V”shape of SWSB has been taken shape.