南海东北部东沙隆起带下地壳高速体的成因及其构造意义

The magmatism and structural feature in northeastern South China Sea (SCS) is an important foundation to understand the extensional rifting evolution and seafloor spreading process of the SCS. Furthermore, some special tectonic bodies such as the high gravity belt and the high magnetic anomaly belt, Mesozoic strata and magnetic quiet zone make it a perfect site to study the tectonic evolution of SCS from late Mesozoic to early Cenozoic. The origin of the high-velocity layer developed in lower crust remains controversial, which leads to the disputations of the tectonic attribution of continental margin of SCS, and the north boundary of continent-ocean transition (COT). In order to reveal the characteristics of the high velocity body, we construct P wave and S wave velocity models utilizing the profiles DS2014-NS2 and DS2014-NS5, which pass through Dongsha uplift, Chaoshan depression and reach to the COT. Based on these results and combined with previous results, we reveal the spatial distribution of the high velocity bodies and discuss the nature difference between the Dongsha uplift and COT based on the petrophysical information. On these bases, geological and geophysical data of northeast continental shelf and adjacent area will be incorporated to analyze the origin of the lower crustal high velocity body beneath Dongsha uplift, and its tectonic significances will be discussed.

南海东北部陆缘岩浆活动和构造特征是解决南海张裂演化和海底扩张过程的重要基础，高重、磁异常带、中生代地层和磁静区等特殊构造体是研究南海晚中生代-新生代构造体制转化的重要载体。南海东北部下地壳高速体的发现使陆缘构造属性备受争议，同时前人对东沙隆起带和洋陆过渡带下方高速体的构造成因因缺乏有力证据也各持己见。为了获得更多东沙隆起带和洋陆过渡带下地壳高速体特征，本项目将利用2014年在东沙海域完成的穿越东沙隆起带、潮汕坳陷延伸到洋陆过渡带的DS2014-NS2和DS2014-NS5两条深地震测线，通过正、反演计算获得精细二维纵、横波速度地壳结构和岩石物性信息，并结合前人的研究成果，揭示南海东北部下地壳高速体构造特征和空间分布情况；通过岩石物性信息并综合利用该区域及邻区的地质与地球物理资料，分析东沙隆起带下地壳高速体的成因，探讨其相关的构造意义。

本项目主要利用横波速度模型和纵横波速比对南海北部沉积层特征、下地壳高速体性质和岩浆活动特征做了一系列研究，研究结果得出①新生代沉积厚度在模型45-100 km范围最薄，可能与该区域新生代岩浆活动导致的基底隆升有关；中生代沉积层具有高Vp低Vp/Vs的特征，反映出成岩固结程度较高，与长期沉积压实和古俯冲的高水平应力作用以及后期岩浆活动的影响有关；②横波及波速比模型显示，下地壳高速体成分是镁铁质的，其与裂后期岩浆作用过程中的岩浆底侵有关。洋陆过渡带下地壳具有更高的Vp/Vs值，说明该区域内岩浆侵入更强烈；③洋陆过渡带的地壳拉张减薄剧烈，拉张因子可达3-5，深部岩浆更易于向上运移。因此，强烈岩浆侵入导致了下地壳Vp/Vs更高，也导致了下地壳高速体在陆侧厚洋薄。该项目的研究结果对进一步了解南海的形成演化过程具有重要的意义。