对黄海—东海研究区深部结构的一些新认识

Some new understandings on deep structure in Yellow Sea and East China Sea

综合地震层析成像与重磁数据的处理结果,选择26°N～36°N,120°E～130°E的范围作为研究区,讨论了黄、东海研究区的深部结构特点及其与周边各地质单元的相互关系,完成了研究区两条剖面的密度结构反演,认为东海陆架地区地壳厚度变化与大陆地区相比并不明显,显著减薄开始于冲绳海槽地区,中地壳消失;琉球岛弧处地壳厚度明显再度增加,特别是上地壳的厚度增加最大,推断其原因应与俯冲作用及俯冲带附近板块与地幔的运动速率之差有关.地球物理场“东西分带”是黄海—东海地区壳内结构从西向东变化的反映,但随着深度的增加,研究区的岩石层结构出现以近EW向为优势的构造格局.因此推断深部近EW向的异常是三叠纪时期南北板块碰撞、挤压所致,浅部的NE向条带异常是后期构造运动在岩石层较浅部位构造效应的反映.黄海—东海地区岩石层结构存在浅部与深部优势构造方向不协调的现象.层析成像结果证实了南黄海东缘断层的存在,还勾绘出绍兴—十万大山碰撞带为以40°左右的倾角向NW方向倾斜的高速带,另一条倾向基本相同的高速带则是南、北扬子块体结合带在深部的反映.

Yellow Sea and part of East China Sea are chosen as the research region （26°N - 36°N,120°E - 130°E） to discuss the deep structure and the relationship with adjacent geological units via tomography and gravity, magnetic inversion. Two density structure profiles are obtained after inversion. In East China Sea Continental Shelf region, the crust thickness doesn＇ t change obviously. The thickness of crust begins to thin evidently in Okinawa Trough and middle crust disappears there. In Ryukyu Island Arc region, the crust thickness increases again and upper crust increases most obviously. We deduce that it is relative with the movement difference betwegn plate and mantle around subduction zone. ＂Belt geophysical anomaly in longitude direction＂ is obvious symbol in research area and it is also the reflection of crust structure from West to East. With the increase of depth, tectonics in EW direction in lithosphere becomes the preponderance tectonic system. It is deduced that they are from collision and compression process of plates in Triassic but anomaly belts in NE direction are the reflection of later movement in shallow part of lithosphere. Unharmonious phenomena between deep and shallow preponderance tectonics exist in lithosphere of research region. The seismic tomography results from plane and profile not only approve the existence of East Marginal Fault Zone of South Yellow Sea but also give the attitude of that Shaoxing-Shiwandashan collision belt is a high velocity belt and it inclines to NW direction with 40° dip angle. Another high velocity belt with almost the same direction of dip is the image of the boundary between Southern and Northern Yangtze Block in deep.