南海东部次海盆扩张期后火山作用的不对称性及其主控因素研究

Seamounts are the product of submarine volcanism. The volume and spatial distribution of seamounts directly reflect the tectonic-magmatic characteristics of associated plates and thus are the keys to understanding the dynamic evolution of the oceanic lithosphere. During the Cenozoic, the South China Sea experienced multiple episodes of seafloor spreading and asymmetric volcanism after the seafloor spreading stopped. It is therefore an ideal area for studying the interaction between tectonics and magmatism of the marginal basins. High-resolution multi-beam bathymetry data, seismic reflection profiles, gravity and magnetic data allow a comprehensive study of deep tectonic-magmatic activities and shallow topographic features. This project will focus on the East Subbasin of the South China Sea, characterized by an asymmetric distribution of seamounts. First, by applying a spatial filtering method based on the Top-hat transform, we will automatically identify the seamounts above the seafloor and calculate their volume distributions. Then, we will use correlation analysis to quantify the special correlation between the seamount topography, seamount volume distribution and pre-existing structures that formed during the seafloor spreading (i.e. fossil spreading ridges and deep fault zones). Finally, we will discuss the impact of important tectonic events, such as spreading ridge jumps and seafloor spreading re-orientation, on the development of the seamounts. This discussion will lead to the postulation of the dominant factors that cause the asymmetric distribution of the post-spreading volcanism. This project aims to clarify the quantitative relationship between post-spreading volcanism and the tectonic movement during seafloor spreading of the East Subbasin. As a consequence, the project will help to improve the dynamic evolution model of the marginal basin represented by the South China Sea.

海山是海底火山作用的产物。其发育规模和空间分布是板块构造—岩浆作用的直接表现，因此是理解大洋岩石圈动力学演化的关键。南海在新生代期间经历了多幕次的海底扩张和扩张期后非对称式的火山作用过程，是研究边缘海盆地构造—岩浆相互作用的理想地区。本项目基于高分辨率多波束探测和重磁震数据，将深部构造—岩浆活动与浅部地貌特征相结合，以海山不对称分布特征突出的南海东部次海盆作为主要研究区，运用“顶帽转换”新技术，精确统计出露海底的海山体积及分布特征；基于统计学原理，分析海山形态、体积分布规律与幕次扩张形成的先存构造（残余扩张脊和深大断裂带）之间的空间相关性；探讨洋脊跃迁、海底扩张变向等构造事件对海山发育的影响，揭示扩张期后火山作用不对称性的主控因素。本项目将从定量化角度，阐明东部次海盆扩张期后火山作用与扩张期构造运动之间的关系，有助于完善以南海为代表的边缘海盆地动力学演化模式。

海山的发育规模及分布规律是板块构造与岩浆作用的直接表现，高精度的海山分布信息是阐明大尺度的板块构造和地球动力学的重要基础。由于过去缺乏有效方法，前人有关海山发育规模的定量研究主要针对局部地区，且通常采用对单个海山建立椭圆体模型进行体积估算的方法。.申请者创新性地将图像滤波Tophat方法运用到海底构造分析，开发了精确计算海底火山发育规模及其分布的新方法，通过自动拾取海山地形并严格遵循其不规则的原始形态，高效且准确地计算海山体积结果及其空间变化规律，从而获得火山活动规模的精确结果。该新方法首先在南海海盆区进行尝试，首次获得了南海海盆海山空间分布精确的定量结果；同时，将深部构造—岩浆活动与浅部地貌特征相结合，发现了南海海盆的岩浆活动存在明显的不对称分布特征，即在海盆北翼发育更大规模的海山，且海山体积峰值与残留洋脊、岩石圈薄弱带的分布密切相关。由此提出，南海海盆的扩张过程对非对称式的岩浆活动具有明显的控制作用。.本项目共发表SCI论文5篇，包括Earth and Planetary Science Letters, Tectonics和Gondwana Research等国际期刊文章。海山的发育规模和分布规律可以直接反映板块的构造和岩浆活动特征，对海山喷出量的统计是衡量岩浆活动的关键指标，因此，精确计算海山体积的新方法为定量研究板块构造及地球动力学过程提供了有效手段。同时，对南海非对称式岩浆活动机制的揭示，增进了海底浅层构造和深部地幔熔融之间相互作用等前沿领域的认识，也是对基于开放大洋的板块构造理论的有效补充。