西太平洋大塔穆火山的重力导纳分析以及对其形成机制的启示

Gravity admittance analysis and its implications for the formation of the Tamu Massif in the west Pacific Ocean

本文利用卫星重力数据和海底地形数据对大塔穆火山开展详细的重力导纳分析。结果显示大塔穆火山的岩石圈有效弹性厚度是1~3 km,指示火山形成于洋中脊之上,符合低重力异常和洋中脊三联点的构造背景。Airy均衡模型和岩石圈挠曲均衡模型推算的大塔穆火山的平均地壳厚度是11~17 km,最厚处拥有一个约30 km的地壳根,与实际地震观测结果基本一致。大塔穆火山的超厚洋壳不同于正常洋中脊,目前地幔柱与洋中脊相互作用是比较合理的成因模式。研究还发现大塔穆火山底下存在一个质量缺失的低密度区,这个区域可能是残留岩浆房造成的结果,与火山中心的地球化学特征、地震波速异常以及广泛的后期火山活动相吻合。另外,这个低密度区提供浮力支撑火山中心隆起,可能导致火山侧翼因差异性沉降而产生正断层。

The west Pacific Ocean has the most underwater volcanoes on Earth,and among them the Tamu Massif within the world’s third largest oceanic plateau is proved to be the Earth’s biggest single volcano.Studies on the Tamu Massif provide insights to oceanic crustal formation and submarine volcanism.This paper uses satellite gravity and seafloor topography data to conduct admittance analysis in details.Results indicate that the lithospheric effective elastic thickness of the Tamu Massif is 1~3 km,implying its formation on mid-ocean ridges,which is consistent with small gravity anomaly and triple junction settings.The estimates from the Airy and flexural isostasy theoretical models show that the mean crustal thickness of the Tamu Massif is 11~17 km with a deep crustal root of~30 km thick,similar to the seismic observation across the volcano.The extraordinary thick crust beneath the Tamu Massif requires a different geodynamic mechanism rather than normal mid-ocean ridges,and plume-ridge interaction is the possible explanation.Moreover,a low-density area or mass deficit is found under the Tamu Massif,likely because of the remnant of magma chamber,which is in agreement with the MgO geochemical signature and low seismic velocity at the volcano center as well as wide-distributed,late-stage volcanism on the volcano surface.Meanwhile,the buoyancy induced by this low-density anomaly may support more to the middle of volcano and increase subsidence on its flanks,leading to the formation of many normal faults.