南海深部构造特征及其地质意义:来自重磁位场反演的认识

Characteristics of deep structure in the South China Sea and geological implications:Insights from gravity and magnetic inversion

莫霍面和居里面是认识深部过程重要的地质与地球物理界面.为了进一步理解南海深部构造活动与洋盆扩张的关系,本文以OBS剖面和深反射地震剖面作为约束,对卫星测高重力异常进行海水、沉积层影响校正,采用最小曲率位场分离方法消除局部密度体的重力影响,获取了反映莫霍面起伏的重力异常,并利用双界面模型重力场快速反演方法计算得到了南海地区莫霍面深度值.通过与居里面起伏的对比研究,发现南海莫霍面和居里面整体均表现为"洋盆浅、周缘深"的菱形特征,两者在洋陆转换区呈现明显的窄梯级带特征,反映了南海扩张期岩石圈的强烈伸展减薄、南北向构造拉张作用等深部构造过程.洋盆莫霍面和居里面的西南向楔形形态是对南海由东向西渐进式扩张的深部构造响应.洋盆南部莫霍面浅于北部,这与扩张中心逐渐向南迁移的特征一致,而洋盆居里面南深北浅的特征则可能与洋盆的简单剪切扩张方式以及洋盆北部的岩浆活动更活跃有关.南海地区莫霍面和居里面呈现交错叠置关系,南、北陆缘表现为明显的深部构造差异,说明南海为非对称式扩张.北部陆缘区居里面深度浅于莫霍面,而洋盆区和南部陆缘区居里面深于莫霍面,这与南、北陆缘性质的差异和南部陆缘复杂的中-新生代俯冲碰撞等构造演化相关,而洋盆区居里面深于莫霍面的现象推测与大洋上地幔橄榄岩蛇纹石化导致的岩石磁性增强有关.

The Moho and Curie surfaces are significant geological and geophysical interfaces in understanding the evolution of the deep earth.Constrained by OBS profiles and deep reflecting seismic profiles,Bouguer gravity anomalies which reflect the undulation of the Moho were acquired by correcting the gravity effect of sea water and shallower sediments from satellite altimetry gravity data,and by eliminating the effect of gravity caused by the local density bodies through the application of the minimum-curvature method.Also,based on study of gravity field inversion in a dual interface model,the Moho depth of the South China Sea(SCS)was calculated,and a comparative study with the Curie surface was carried out.The results show that the Moho and Curie surface of the SCS are shallow in the ocean basin and deep in the margins,and both of the two surfaces in the continent-ocean transition zone show the distinct narrow gradient bands,which reflect the lithospheric extension-thinning and NS tectonic extension during the expansion of the SCS.The typical wedge-shaped Moho and Curie surfaces in the ocean basin generally corresponds to the gradual expansion from east to west of the SCS.The Moho of the ocean basin is shallower in the south than that in the north,consistent with the gradual southward migration of the expansion center.While the Curie surface of ocean is deeper in the south and shallower in the north,which is probably related to the simple shear expansion of the ocean basin and occurrence of more active magmatic activity in the north of the ocean basin.The Moho and Curie surfaces exhibit staggered overlapping relationships in the SCS.The south and north continental margins show obvious deep structural differences,indicating that the SCS is an asymmetric expansion.The Curie surface is shallower than Moho in the north,but deeper in the ocean basin and south margin.These differences are closely related with the different continental marginal natures between south and north margins of the SCS,and also with the complicated Meso-Cenozoic tectonic evolution of the southern margin of the SCS.The Curie surface is deeper than Moho in the ocean basin,which is presumably related with magnetic enhancement of lithosphere,caused by the oceanic peridotite serpentinization in the upper mantle.