Eakin Caroline M, Rychert Catherine A, Harmon Nicholas
Research School of Earth Sciences The Australian National University Canberra ACT Australia.
Ocean and Earth Science National Oceanography Centre Southampton, University of Southampton Southampton UK.
J Geophys Res Solid Earth. 2018 Feb;123(2):1736-1751. doi: 10.1002/2017JB015176. Epub 2018 Feb 26.
Mantle anisotropy beneath mid-ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries. Observations are sparse, however, given the remoteness of the oceans and the difficulties of seismic instrumentation. To overcome this, we utilize the global distribution of seismicity along transform faults to measure shear wave splitting of over 550 direct S phases recorded at 56 carefully selected seismic stations worldwide. Applying this source-side splitting technique allows for characterization of the upper mantle seismic anisotropy, and therefore the pattern of mantle flow, directly beneath seismically active transform faults. The majority of the results (60%) return nulls (no splitting), while the non-null measurements display clear azimuthal dependency. This is best simply explained by anisotropy with a near vertical symmetry axis, consistent with mantle upwelling beneath oceanic transforms as suggested by numerical models. It appears therefore that the long-term stability of seafloor spreading may be associated with widespread mantle upwelling beneath the transforms creating warm and weak faults that localize strain to the plate boundary.
大洋中脊和大洋转换断层下方的地幔各向异性是我们理解海底扩张和离散板块边界潜在动力学的关键。然而，由于海洋偏远以及地震仪器部署困难，观测数据稀少。为克服这一问题，我们利用沿转换断层的全球地震活动分布，来测量全球56个精心挑选的地震台站记录的550多个直接S波震相的剪切波分裂。应用这种震源侧分裂技术可以对上地幔地震各向异性进行表征，进而直接描绘出地震活跃转换断层正下方的地幔流动模式。大部分结果（60%）显示为零值（无分裂），而有非零测量结果的则表现出明显的方位依赖性。最简单的解释是，其各向异性具有近乎垂直的对称轴，这与数值模型所表明的大洋转换断层下方的地幔上涌一致。因此，海底扩张的长期稳定性似乎可能与转换断层下方广泛的地幔上涌有关，这种上涌形成了温暖且脆弱的断层，将应变集中在板块边界处。
