基于磁力资料的南海及邻区磁性基底和居里面深度特征研究

The magnetic basement and Curie surface depth in the South China Sea and adjacent areas derived from magnetic data

磁性基底和居里面是研究地壳和岩石圈的地质构造和热演化过程的两个重要磁性界面.为了研究南海及邻区磁性基底和居里面所反映的深部构造及其热活动的地质效应,本文在对磁异常进行化极处理的基础上,采用最小曲率位场分离方法,获得了磁性基底和居里面引起的化极磁异常,利用双界面模型快速反演方法,反演了南海及邻区的磁性基底和居里面深度,研究了磁性基底、居里面深度及其分布特征,讨论了磁性基底、居里面与新生界深度之间相关性特征及其地质意义.研究表明,磁性基底深度5~20km,洋盆南北两侧磁性基底走向分别以NE、NEE向为主,中南半岛周缘磁性基底呈NW、NNW走向.居里面深度15~32km,宏观表现为"洋壳浅、周缘深"及周缘"北浅南深"的特征,洋盆地区居里面深度呈现"西南浅、东部深",洋壳与陆壳接触带在居里面深度上表现为梯级带特征.新生界深度与磁性基底深度相关性(Correlation between the depth of magnetic basement and Cenozoic,CDMBC)多以不规则形状分布,在盆地的沉积中心呈现正相关;新生界深度与居里面深度相关性(Correlation between the depth of Curie surface and Cenozoic,CDCSC)多呈NE、NEE向带状正相关分布,走向与盆地走向一致;莺歌海盆地、琼东南盆地、万安盆地南部和曾母盆地CDMBC呈正相关、CDCSC呈负相关,莺歌海相关性特征推测为:居里面随岩石圈变形隆起而抬升,磁性基底张裂下沉,发生大规模沉降引起;琼东南盆地相关性特征推测为:居里面随岩石圈变形下坳而下降,沉积中心与磁性基底下沉方向一致;万安盆地和曾母盆地相关性特征推测为:深部流体沿南海西缘断裂直接进入地壳,引起该处居里面深度变浅.

The magnetic basement and the Curie surface are two important interfaces for studying geological structure and thermal evolution of crust and lithosphere.This work focused on the geologic effects of the deep structures and thermal activities reflected by these two interfaces of the South China Sea and adjacent areas.The minimum-curvature method was used to obtain the reduction-to-the pole magnetic anomalies produced by the magnetic basement and the Curie surface.Based on study of inversion in a dual-interface model,the depths of both he magnetic basement and Curie surface were determined.Then we have analyzed the depths of the magnetic basement and Curie surface and their distribution characteristics,and discussed the correlation between the magnetic basement,Curie surface and the depth of the Cenozoic and its geological implications.The results show that the magnetic basement depth of the study area is 5~20 km,dominated by NE and NEE strikes on the south and north sides of the ocean basin,and NW and NNW in the periphery of the Indo-China peninsula,respectively.The depth of the Curie surface is 15~32 km,and is characterized by“shallow in ocean crust and deep in periphery”and shallow in north and deep in south-south in the rim.While in the ocean basin Curie is“shallow in the southwest and deep in the east”.In the contact zone between the oceanic and the continental Curie surface shows a a step fashion.Correlation between the depths of magnetic basement and Cenozoic(CDMBC)is mostly distributed in an irregular shape,showing a positive correlation in the sedimentary center of the basin.The depths of Curie surface and Cenozoic(CDCSC)are mostly positively correlated trending in NE and NEE,consistent with the basin′s trend.CDMBC is positively correlated and CDCSC is negatively correlated in the Yinggehai,Qiongdongnan,southern Wan'an and Zengmu basins.The correlation feature of Yinggehai is presumed to be that the inside is uplifted with the deformation of the lithosphere,while the magnetic basement is subsided,causing large-scale subsidence.The correlation feature of the Qiongdongnan basin is presumed to be that the Curie surface declines with downward warping deformation of the lithosphere,and the sedimentary center is consistent with the direction of the magnetic basement sinking.The correlation between the Wan′an and Zengmu basins is presumed to be that the deep fluid directly enters the crust along the fault of the western margin of the South China Sea,causing the rise of the Curie surface.