东海OBS深部地学探测断面及其地质意义

A profile study of OBS deep geological detect in the East China Sea

利用主动源海底地震仪(ocean bottom seismometer,简称OBS)进行东海地壳深部结构地震探测,对探讨东海大地构造演化意义重大.采用立体气枪阵列延迟激发震源新技术,完成了一条横跨东海陆架盆地-琉球岛弧的OBS深部地学地震探测测线(OBS-2015),获得了地壳内部主要地层界面和莫霍面的地震反射/折射震相.对OBS台站记录经预处理、震相识别与拾取后,通过射线追踪、走时拟合等正反演处理,得到了沿测线的二维地壳速度结构剖面.剖面的地质解释与综合研究认为,东海陆架中-新生代沉积盆地沉积层厚度大、分布广,中-新生界最大叠合厚度达13 km左右;东海陆架区地壳是大陆地壳向海区的延伸,莫霍面起伏小、埋藏深度在30 km左右.钓鱼岛隆褶带到琉球岛弧区域莫霍面起伏剧烈,呈由西向东快速抬升的趋势.冲绳海槽区地壳厚度只有东海陆架盆地的一半左右,并呈中轴线最薄(13 km左右)、向两侧快速加厚(19 km左右),莫霍面上方发育速度达7.1 km/s的下地壳高速地质体,东部坳陷地区莫霍面已出现明显抬升,与盆地存在明显镜像关系,推测为盆地拉张作用所致.冲绳海槽的纵向速度结构与陆壳差异大,与洋壳的速度结构较为接近.综合以上因素并结合区域地质背景分析认为,南冲绳海槽进入了海底扩张的初始阶段,地壳具有非典型洋壳的特征.本次探测结构为东海深部地壳结构研究及区域地质演化研究提供了重要的基础资料.

The use of ocean bottom seismometer(OBS)is of great significance to explore the tectonic evolution of the East China Sea.To study the characteristics of the deep geological structure from the East China Sea shelf basin to the Ryukyu trench uplifting fold belt,the NW-SE active source deep seismic experiment(OBS-2015)was carried out in the East China Sea.By using the delayed fire source of multilayer gun array,wide-angle seismic reflection and refraction signals up to the Moho can be obtained in the continental shelf and Okinawa Trough,and various seismic phases such as Ps,Pg and Pm P can be recognized.The data acquisition results of OBS deep geoscience seismic exploration in the East China Sea show that the effective seismic phase up to Moho surface can also be obtained by using the three-dimensional array source composed of medium and small capacity air guns,which provides a new way for the selection of excitation source for OBS deep exploration.After data preprocessing,identifying and picking up seismic phases,a 2 D velocity structure profile was obtained by ray tracing and travel time forward and inversion modeling.Geological interpretation and comprehensive study show that the East China Sea shelf basin is a Mesozoic-Cenozoic sedimentary basin with the maximum superimposed Mesozoic-Cenozoic thickness of 13 km.The crust of the East China Sea shelf area is the extension of the continental crust to the sea.The Moho surface has small fluctuation and the buried depth is about 30 km.From the Diaoyu Island fold to the Ryukyu Island Arc,the Moho Surface fluctuates sharply,rising rapidly from west to East.The crustal thickness of the Okinawa Trough area is only about half the thickness of in the East China Sea shelf basin.It is a high velocity geological body with the thinnest central axis of approximately 13 km and rapid thickening to both sides(approximately 19 km).The high velocity(up to 7.1 km/s)abnormal body develops above the Moho surface in the Okinawa Trough area.The Moho surface in the eastern depression has uplifted significantly.This shows an obvious mirror image relationship with the extensional basin.The vertical velocity structure of the Okinawa Trough is quite different from that of the continental crust and is close to that of the oceanic crust.It is speculated that the thick high-speed body developed on the Moho surface is the manifestation of a large amount of upwelling of mantle derived materials during the back arc stretching process.Combined with the phenomena of low-speed abnormal magma chamber in the shallow part and the penetration of magmatic rocks into the sedimentary basement layer,it is believed that the crust in the southern Okinawa Trough along the survey line has ruptured and entered the initial stage of sea-floor spreading,and the initial oceanic crust has appeared in the southern Okinawa Trough and the crust has atypical oceanic crust characteristics.This experiment provides important basic data for studying the deep crustal structure and regional geological evolution of the East China Sea.