利用SS前驱波研究南海及周缘地区的上地幔不连续面结构

南海及其周缘地区具有复杂的地质构造和地球动力学特征，例如，海南岛及其附近被观测到有地幔热柱的存在；南海周缘存在多个俯冲带，且整个区域下方存在多个古俯冲板片。这对回答南海及周缘地区的热点是源于热柱还是源于俯冲系统以及热柱起源深度等一系列科学问题提出了挑战。而上地幔不连续面携带着重要的地幔热结构和动力学信息，因此获得其精细结构对于南海及周缘地区构造演化的研究具有重要的意义。鉴于南海海盆缺少地震台站的分布，采用接收函数和三叠波等方法研究地幔精细结构困难重重，因此我们采用SS前驱波的方法。通过收集全球的地震和台站，SS地表反射点在研究区域实现了较高密度的覆盖，因此可以很好地探测到南海及周缘地区下方的地幔热结构。

我们收集了震中距在100-165°、震级大于5.5Mw和震源深度小于75公里的SS反射点位于研究区域的地震记录，并进行人工挑选最终得到了包含10853个高信噪比地震波形的SS地震数据集。我们将研究区域划分为169个间隔为2.5°，半径为5°的面元.对每个面元内的地震波形进行了预处理、走时校正和动校正等相关工作，并通过线性叠加得到了SS前驱波的精确到时，然后通过时深转换方法得到了研究区域内上地幔不连续面的起伏和地幔转换带（MTZ）厚度的横向变化。

我们的结果显示，在海南岛及其周缘下方410公里不连续面（d410）和660公里不连续面（d660）均出现下沉，但下沉更为明显的d410导致MTZ减薄约10公里，我们推测该区域下方存在起源下地幔的地幔热柱，d660的形成受到后石榴石相变的控制。在婆罗洲西北部地区、南海北部及菲律宾北部地区、印支半岛地区和苏禄海西北部的下方MTZ出现增厚，为这些区域下方存在俯冲板片提供了直接的证据，并且推断引起MTZ增厚的相应俯冲板片分别是南向古南海俯冲板片、北向古南海板片、印度板片和菲律宾板片。在南海北侧和印支半岛东侧，我们观测到MTZ减薄和火山活动的发生，结合SS前驱波振幅信息，我们推断这可能是由北向古南海板片和印度板片在其边缘促进了地幔上涌导致。该地幔上升流携带了这两个板片脱离的水分和其他分离的板片物质。此外，在菲律宾南部地区观测到MTZ出现微弱减薄的现象，我们推断可能是由菲律宾板片携带了一层薄的较热的软流圈物质下沉并在其下方产生了地幔上涌导致的。

我们的观测结果还显示在台湾岛及其东北部地区、南海北部地区和印支半岛东北方向与中国大陆的交界地区下方有X不连续面的存在，通常认为其形成与有含有二氧化硅的榴辉岩中的柯石英到斯石英的相变有关。对于榴辉岩在上地幔的富集，我们推测台湾岛及其东北部是由携带古洋壳物质的菲律宾板片的俯冲所导致的，后两个区域则是俯冲板片诱导产生的地幔上涌携带了下沉至深部的古洋壳物质上升所导致的。

The South China Sea (SCS) and its periphery have complex geological structures and geodynamics characteristics. For example, the existence of mantle plumes has been observed beneath Hainan Island and its vicinity, and there are subduction zones around the SCS, and there are many proto subducted slabs beneath the whole region. These characteristics pose a major challenge to addressing a series of scientific questions, such as whether the hot spots in the SCS and its periphery originate from the mantle plume or the subducted slab and where the origin depth is for the Hainan mantle plume. The upper mantle discontinuities carry significant thermal and dynamic information, so obtaining the fine structure of these discontinuities is of great significance for studying the tectonic evolution of the SCS and its periphery. Given the lack of seismic stations in the SCS basins, it is difficult to detected the fine structure of upper mantle discontinuities using the methods of receiver function and Triplicated waves, so we adopt SS precursor method. Through collecting the global earthquakes and stations, the SS bounced points realize high-density coverage in our research region, ensuring that the mantle thermal structure beneath the SCS and its periphery can be well detected.

We have collected seismic records with SS bounced points located in the research region. Their epicentral distances are between 100° and 165°, magnitudes are greater than 5.5 Mw, and focal depths are less than 75 km. Then, we obtain the high-quality SS seismic data set including 10853 seismic waveforms through manual selection. The research region is divided into 169 bins with an interval of 2.5° and a radius of 5°. Preprocessing, travel time correction and move-out correction are carried out for the seismic waveforms in each bin. The exact arrival time of SS precursor is obtained using a linear stack. Then, the topography of upper mantle discontinuity and the lateral variation of mantle transition zone (MTZ) thickness in our research region are obtained using time-to-depth conversion.

Our results show that the 410 km discontinuity (d410) and 660 km discontinuity (d660) both have a subsidence beneath Hainan Island and its periphery, and the more obvious subsidence of the d410 resulted in the thinning of MTZ by about 10 km. We speculate that a mantle plume originates from the lower mantle beneath this region, and the post garnet phase transition controls the formation of the d660. The thickening of MTZ is observed beneath the northwestern Borneo, the northern SCS, the northern Philippines, the Indochina region and the northwestern Sulu Sea region, which provides direct evidence for the existence of subducted slabs. it is inferred that the corresponding slabs causing MTZ thickening beneath these regions are southward proto SCS sea slab, northward proto SCS sea slab, Indian slab and Philippine slab, respectively. The thinning of MTZ and volcanic activities are observed in the north side of the SCS and the east side of Indochina, combined with the amplitude information of the SS precursors, it is inferred that this might be caused by the mantle upwelling induced by the northward proto-SCS slab and the Indian slab respectively. The mantle upwelling carries the water dehydrated from these two slabs and other separated slab materials. In addition, a weak weakening of MTZ was observed beneath the southern Philippines, which might be caused by the mantle upwelling beneath the Philippine slab. This mantle upwelling was induced by the Philippine slab, because it might carry a thin layer of hot asthenosphere material and sink deep into the lower mantle.

Our observations also show that there exist X discontinuities beneath Taiwan Island and its northeast side, the north side of SCS and the border region between the northeastern Indochina and the Chinese mainland, the formation of which is related to the phase transformation of coesite to stishovite in eclogitic materials within the oceanic crust. For the eclogite enrichment in the upper mantle, we speculate that the former was caused by the subduction of the Philippine slab carrying the paleo-oceanic material, the latter two are caused by the mantle upwelling induced by the subducted slab, which carries the paleo-oceanic crust material sinking to the deep mantle.

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