Lin Cheng-Horng, Shih Min-Hung, Lai Ya-Chuan
Institute of Earth Sciences, Academia Sinica, Nankang, PO Box 1-55, Taipei, Taiwan.
National Center for Research on Earthquake Engineering, National Applied Research Laboratories, Taipei, Taiwan.
Sci Rep. 2021 Jan 15;11(1):1561. doi: 10.1038/s41598-021-81357-7.
It is conventionally believed that magma generation beneath the volcanic arc is triggered by the infiltration of fluids or melts derived from the subducted slab. However, recently geochemical analyses argue the arc magma may be formed by mélange diapirs that are physically mixed by sediment, altered oceanic crust, fluids, and mantle above the subducted slab. Further numerical modeling predicts that the mantle wedge diapirs have significant seismic velocity anomalies, even though these have not been observed yet. Here we show that unambiguously later P-waves scattered from some obstacles in the mantle wedge are well recorded at a dense seismic array (Formosa Array) in northern Taiwan. It is the first detection of seismic scattering obstacles in the mantle wedge. Although the exact shape and size of the scattered obstacles are not well constrained by the arrival-times of the later P-waves, the first order approximation of several spheres with radius of ~ 1 km provides a plausible interpretation. Since these obstacles were located just beneath the magma reservoirs around depths between 60 and 95 km, we conclude they may be mantle wedge diapirs that are likely associated with magma generation beneath active volcanoes.
传统观点认为，火山弧下方的岩浆生成是由俯冲板块衍生的流体或熔体的渗透引发的。然而，最近的地球化学分析认为，弧岩浆可能是由混杂岩底辟形成的，这些底辟是由俯冲板块上方的沉积物、蚀变洋壳、流体和地幔物理混合而成的。进一步的数值模拟预测，地幔楔底辟具有显著的地震速度异常，尽管尚未观测到这些异常。在此，我们表明，从地幔楔中的一些障碍物散射的清晰的晚P波在台湾北部的一个密集地震台阵（福尔摩沙台阵）中得到了很好的记录。这是首次在地幔楔中探测到地震散射障碍物。尽管晚P波的到达时间不能很好地约束散射障碍物的确切形状和大小，但几个半径约为1千米的球体的一阶近似提供了一个合理的解释。由于这些障碍物位于深度约60至95千米的岩浆库正下方，我们得出结论，它们可能是地幔楔底辟，可能与活火山下方的岩浆生成有关。
