曾母盆地西部岩石圈特性与有效弹性厚度:来自构造模拟的约束

LITHOSPHERIC CHARACTERISTICS AND EFFECTIVE ELASTIC THICKNESS OF WESTERN ZENGMU BASIN, SOUTH CHINA SEA: CONSTRAINTS FROM TECTONIC MODELING

曾母盆地是南海南部最大的新生代沉积盆地,保留了南海新生代共轭边缘演化历史以及性质的重要信息。岩石圈有效弹性厚度Te是了解大陆地壳张裂时岩石圈强度的一个关键指标。本文选取曾母盆地西部的三条典型地震剖面,在构造解释的基础上,按照挠曲均衡原理,通过对三条剖面的反演模拟,对Te进行了敏感性测试。在此基础上利用弹性梁模型对三条剖面进行了正演模拟,来模拟盆地的形态以及主要地层单元的展布。模拟结果与实测剖面的对比表明,Te值取3～5km较为合理,盆地西部的脆性/韧性地壳转换深度为15km。正演和反演模拟中的拉张因子β具有不同的构造含义,正演模拟拉张因子β代表了脆性上地壳的拉伸程度,反演模拟的拉张因子则表示整个岩石圈的拉张作用,拉张因子同时呈向北增大的趋势。

Zengmu basin is the largest Cenozoic sedimentary basin in southern South China Sea and study of this basin can provide some key information concerning the evolution and characteristics of the conjunct Cenozoic margin of South China Sea. In order to learn the lithospheric strength of the basin during the active extension, effective elastic thickness （Te） of the lithosphere will be an important parameter. In this study, the authors first interpret faults along their 3 seismic profiles starting from the western part of the basin. Sensitivity Te tests were carried out for all the three profiles based on the flexure isostasy model. This is followed by forward modeling using a flexural cantilever model to predict modern basin structure and the distribution of major strata in the basin. The modeled results support the adoption of a Te value of 3 - 5 km during the active extension, based on the degree of matchbetween modeled results with the observed ones. Modeling also shows that the depth of brittle/ductile transition in the region is 15km. The stretching factor β that increases northwards has different meanings in reverse and forward modeling. In the forward modeling, β reflects the degree of extension of the upper crust, while in the reverse modeling, it represents the degree of extension of the whole lithosphere.