南海西南次海盆与超慢速扩张西南印度洋中脊地壳结构对比

Comparison of crustal structure between the Southwest Sub-basin, South China Sea and the ultraslow-spreading Southwest Indian Ridge

近年来对西南印度洋中脊的研究显示,超慢速扩张(全扩张率:12~18mm·yr^(-1))的西南印度洋中脊包含岩浆增生型和非岩浆增生型两种截然不同的地壳结构。岩浆增生型中脊段表现为轴向的海底隆起,通常具有较低的地幔布格重力异常和较强的磁性,地壳厚度较大;非岩浆增生型中脊段通常水深较深,缺乏转换断层,发育拆离断层和高角度正断层,具有较高的地幔布格重力异常和微弱的磁性,大量蛇纹石化的地幔橄榄岩出露海底,火成岩地壳较薄甚至不存在。南海西南次海盆具有较慢速扩张率(全扩张率:50~35mm·yr^(-1)),其接近消亡洋中脊中央部分的地壳厚度也较薄,也有可能存在蛇纹石化地幔,具有超慢速扩张脊非岩浆增生段的特点。

Recent investigations of the ultraslow-spreading(full spreading rate:12~18mm·yr-1)Southwest Indian Ridge revealed two kinds of crustal structure:Magmatic and amagmatic accretionary crust.Magmatic accretionary segments appear as the axial rise,relatively low mantle Bouguer gravity anomaly,strong magnetization and thick crust.Amagmatic accretionary segments feature detachments and abundant high-angle normal faults,lack of transform faults,deep water,relatively high mantle Bouguer gravity anomaly and weak magnetization.There are also significant amount of serpentinized peridotites exposed on the seafloor,and the igneous crust is thin,even absent.The Southwest Sub-basin of the South China Sea(SWSB)has relatively slow-spreading rates(full spreading rate:50~35mm·yr-1).The central part of SWSB also presents thin crust and there might be some serpentinized peridotites in the basin area,which are similar to the characteristics of the amagmatic accretionary crust in the ultraslow-spreading Southwest Indian Ridge.