Gravity Inversion on Crust Structures of the Shikoku Basin, Philippine Sea, and Its Implication to the Evolution Process
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摘要: 四国海盆是位于菲律宾海板块内由岛弧张裂形成的弧后盆地，其深部地壳结构对认识伊豆小笠原岛弧的裂解和弧后盆地的扩张过程有重要的意义.在反射多道地震剖面和深部海底地震（OBS）探测剖面的约束下，结合磁异常条带数据，利用两条横穿四国海盆的重力测线数据对海盆的地壳物性结构反演，对比重力反演剖面与深部探测剖面地壳厚度和密度特征，得到更加精细的四国海盆地壳结构.研究结果显示，四国海盆洋壳厚度自西向东逐渐增厚，在残留扩张脊处莫霍面深度迅速增加.根据地壳密度和厚度将四国海盆分为：洋壳减薄区、洋壳增厚区、后扩张洋壳增生区，分别对应初始慢速张裂、单翼快速扩张、对称慢速扩张3期扩张活动.南北测线不同构造分区得到的扩张速率与由磁异常条带得到扩张速率相同；洋壳减薄区下地壳均有高密度体，与OBS剖面中下地壳高速体相对应，可能是由于洋壳慢速扩张过程中强烈拆离作用，地幔蛇纹石化导致.Abstract: The Shikoku basin is a back-arc basin in the Philippine Sea which experienced arc rifting in Cenozoic. Study on deep structures plays an important role in understanding the break-up of the Izu-Bonin-Mariana arc and back-arc rifting history. With constraints of multi-channel seismic data and refraction seismic data, as well as the magnetic data, we carry out gravity inversion on two profiles crossing the whole Shikoku basin to study the density changes along the lines, and to reconstruct the deep structures. Results show that the oceanic crustin creases gradually from west to east, and the Moho depth increases sharply in the residual spreading ridge. According to the crustal density and thickness, we separate the Shikoku basin into three domains:the thin nedoceanic domain, the thickened oceanic domain, and the domain affected by post-spreading magmatism, which corresponds to th einitial slow spreading stage, single-wing fast spreading stage, and the symmetrical spreading stage, respectively. The spreading rate calculated from the different tectonic domains fits well with that from the magnetic omalies. High-density body exists under the oceanic crust with in thinned oceanic domain, which is correspondence with high-velocity layer in the refraction seismic velocity model. We suggest that this high-density body was formed by mantle serpentinization caused by active detachments during slow-spreading stage.
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图 1 研究区位置以及地球物理探测剖面分布
红线部分：两条重力反演线位置；背景是地形图
Fig. 1. Location of the study area and the distribution of geophysical exploration profiles in the Shikoku basin
图 4 四国海盆南部密度D(g/cm3)模型剖面(剖面AA')
Fig. 4. Vertical cross section of density (g/cm3) model through the southern Shikoku basin
图 5 四国海盆北部密度D(g/cm3)模型剖面(剖面BB')
Fig. 5. Vertical cross section of density (g/cm3) model through the northern Shikoku basin
图 6 四国海盆OBS速度剖面与密度D(g/cm3)模型地壳结构对比
图中黑色线为OBS探测的莫霍面，白色线为洋壳层2、层3分界面
Fig. 6. Comparison of crustal structure between refraction seismic velocity model and density model in the Shikoku basin
表 1 反演建模采用的密度参数
Table 1. Densities used in the modeling
模型块体 海水 新生代沉积 大洋层1 大洋层2 大洋层3 海山 岩浆侵入体 上地壳 中地壳 下地壳 密度(g/cm3) 1.03 1.96~2.30 1.96~2.30 2.38~2.81 2.81~3.05 2.50~2.69 2.54~2.62 2.40~2.70 2.69~2.78 2.80~2.95 
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表 2 迭代拟合反演密度
Table 2. Densities from iterative inversion
模型块体 海水 新生代沉积 大洋层1 大洋层2 大洋层3 海山 岩浆侵入体 上地壳 中地壳 下地壳 密度(g/cm3) 1.03 1.96~2.30 1.96~2.30 2.38~2.81 2.81~3.05 2.50~2.69 2.54~2.62 2.40~2.70 2.69~2.78 2.80~2.95
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