南海西南次海盆后扩张期轴部海山的三维地壳结构及其形成演化模式

Post-spreading magmatism was extremely strong in the South China Sea, which leaded to the formation of post-spreading seamounts in the basin. Previous studies of the post-spreading seamounts concentrated on morphology, lithology and age, and the knowledge of the crustal structure and formation mechanism of them is still limited. This proposed study is mainly based on the active-source wide-angle seismic reflection/refraction data collected from the Southwest-Sub basin, South China Sea to determine the 3D detailed crustal structure and physical properties of the post-spreading seamount by using a joint P/S wave inversion method. In addition, we will simultaneously use teleseismic receiver functions to inverse the deep structure beneath the seamount based on the passive seismic data. All these work are conducted for a systematic study on the magma and tectonic processes of the post-spreading seamount at the fossil spreading center in the Southwest-Sub basin. Moreover a comparison of the seamounts between the Southwest-Sub basin and the East-Sub basin will be made. And finally we will present an evolutionary model of the post-spreading seamount of the Southwest-Sub basin. This proposed study will increase the knowledge of the post-spreading magmatism and evolution process of the South China Sea.

南海存在着强烈的后扩张期岩浆事件，并在海盆中形成了大量的海山。前人研究主要集中在海山的形态、岩性和形成时代上，对于其内部结构及形成机制的认识尚未深入。本申请通过在南海西南次海盆获得的主动源广角反射/折射地震数据，利用纵、横波联合反演技术对该海盆残留中脊附近海山的3D精细内部结构及物性特征进行刻画；同时开展被动源地震数据的接收函数研究，获得海山深部和下方的速度结构，系统探索后扩张期海山的构造-岩浆过程，并与东部次海盆区后扩张期海山进行比较，最终建立西南次海盆后扩张期海山的形成演化模式。该项目有助于增进南海后扩张期岩浆活动和演化过程的认识。

后扩张期海山是一种特殊类型的海山，通常形成在残留扩张中心附近，广泛分布于太平洋区域，如东太平洋、南海、日本海、菲律宾海等。过去几十年里，科学界对于此类型海山的认识主要依赖于岩石地球化学数据。由于缺乏相关地震的调查，我们对于这类海山的内部结构及其反应的演化过程知之甚少。南海西南次海盆在扩张停止后仍然存在着强烈的岩浆活动，形成了大量的后扩张期的海山。其中龙南海海山处已经开展了海底地震的调查，获得了相关的地震数据。我们基于地震数据，通过正反演模拟的方法获得了海山的内部结构，为研究后扩张期海山的岩浆加载和形成演化提供了窗口。我们的结果表明：（1）南海西南次海盆龙南海山是一个喷发占主导的海山，其喷发/侵入边界（~6.0km/s）在形态上呈现出下凹状态。通过计算海山体积获得海山的喷发/侵入比为3.0；（2）通过与全球其他地区相同规模、相同（不同）起源海山的结构特征的比较，建立了2种可以解释海山结构差异的演化模型。这2种模型均表明海山的喷发/侵入比和地壳增厚形式的差异源于岩浆供应量和岩浆供应时间的差异；海山侵入核的缺失指示其起源与热点/地幔柱无直接的联系；（3）通过与南海东部次海盆海山（黄岩-珍贝海山）结构的对比，发现其喷发/侵入比和地壳增厚形式存在差异，这种差异与南海洋脊跃迁导致的异常热地幔相关。