南海北部磁场特征及其构造意义

Magnetic field of the northern South China Sea: its characteristics and tectonic significance

根据南海北部的地磁场数据及其化极异常特征,该区由北向南可分为复杂异常区、高磁异常区、陆坡磁异常平静区(磁静区)和海盆磁条带区四大构造特征区。其中,磁静区可为内磁静区1、内磁静区2和外磁静区三部分。该区磁性基底反演结果表明,外磁静区磁性基底深度为6—7km,介于内磁静区(8—10km)和海盆区(4—5km)之间,可能是前新生代残留古洋壳。外磁静区和下地壳高速层相对应,指示其可能是在裂前或裂间由底侵作用形成的。陆架坡折带附近的F2断裂是内磁静区的北侧边界,指示了南海北部陆壳向过渡壳的转变分界;位于下陆坡与海盆的交界处的F4断裂为外磁静区的南侧边界,F3断裂可能指示了洋陆分界线的位置。

According to the magnetic field of the northern of the South China Sea and its ＂reduction-to-the-pole＂-anomaly,the area could be divided into four zones with different tectonic features from north to south.They are complex magnetic anomaly zone and high magnetic anomaly belt in the shelf,magnetic quiet zone in the slope,and magnetic lineation zone in the ocean ba-sin.The magnetic quiet zone could be divided further into three parts for inside magnetic quiet zone 1,inside magnetic quiet zone 2,and outside magnetic quiet zone.The depth of magnetic basement in the outside magnetic quiet zone is 6-7 km,between the inside magnetic quiet zone（8-10 km） and the lineation zone in the ocean basin（4-5 km）.It may be the residue oceanic crust before the Cenozoic.Outside the magnetic quiet zone there is a high velocity layer in the lower crust,which implies that the magnetic quiet zone is formed by under plating during rifting or pre-rifting.F2 fault is located in the shelf slope break,which is the northern border of the inside magnetic quiet zone.The fault may indicate the transformational border from continental crust to transitional crust in the northern South China Sea.F4 fault,located at the junction between lower continental slope and ocean basin,is the south border outside the magnetic quiet zone.F3 fault may indicate the location of continent–ocean boundary.