Seismological evidence of mantle flow driving plate motions at a palaeo-spreading centre
Seismological evidence of mantle flow driving plate motions at a palaeo-spreading centre
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DOI：
10.1038/ngeo2121
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发表时间：
2014-05-01
影响因子：
18.3
通讯作者：
Takahashi, Narumi
中科院分区：
文献类型：
--
作者：
Kodaira, Shuichi;Fujie, Gou;Takahashi, Narumi
关于大洋中层扩张中心的动力学，一个长期存在的问题是，地幔上涌是被动的还是主动的。在被动上升流过程中，上覆板块的运动有望驱动地幔流动(1，2)。相反，在活跃的上升流期间，地幔在扩张轴下方的减压熔融将导致低密度熔融，从而产生局部浮力，迫使地幔对流和驱动板块运动(3，4)。太平洋板块较老部分的地震图像显示，下地壳中的倾斜反射体可能是活动地幔流动(5-7)造成的剪切带，但几乎没有地震学证据表明地幔流动拖累了板块。在这里，我们使用有源地震数据来成像太平洋板块，该板块是在西北太平洋的一个古扩张中心形成的。在紧靠下地倾斜反射层下方的最上地幔中，我们发现S(-1)极高的P波速度和8.5-9.8%的强地震各向异性。我们认为，如果地幔流动比板块运动快得多，就可能产生如此强烈的地震各向异性，这是由于地幔流动响应于地幔流动而导致的橄榄石晶体的排列。我们的结论是，下地壳倾角反射体和各向异性都是由快速地幔流动引起的莫霍面上的阻力形成的，这为扩张中心的活动地幔流动提供了直接证据。
A long-standing question about the dynamics of mid-ocean spreading centres is whether mantle upwelling is passive or active. During passive upwelling, the motion of the overlying plate would be expected to drive mantle flow(1,2). In contrast, during active upwelling, decompression melting of the mantle beneath the spreading axis would result in a low-density melt that creates local buoyancy, forcing mantle convection and driving plate motion(3,4). Seismic images of older parts of the Pacific Plate reveal dipping reflectors in the lower crust that could be shear zones created by active mantle flow(5-7), but there is little seismological evidence to indicate that mantle flow drags the plate. Here we use active-source seismic data to image the Pacific Plate that was created at a palaeo-spreading centre in the northwest Pacific Ocean. We identify very high P-wave velocities of 8.5-8.6 km s(-1) and strong seismic anisotropy of 8.5-9.8% in the uppermost mantle, immediately below the lower-crustal dipping reflectors. We suggest that such strong seismic anisotropy, caused by the alignment of olivine crystals in response to mantle flow, could have been generated if mantle flow was much faster than plate motion. We conclude that both the lower-crustal dipping reflectors and the anisotropy were formed by a drag force at the Moho caused by rapid mantle flow, providing direct evidence for active mantle flow at a spreading centre.