Studies of Crust and Upper Mantle Structure, Mantle Flow, and Geodynamics of the Chile Ridge Subduction

洋脊俯冲作用是太平洋盆地新生代构造的一个重要过程。智利海脊是纳斯卡-南极板块的分界线，目前正沿着南纬46.5度的智利海沟俯冲到南美洲大陆之下，在台陶半岛附近形成一个板块三重结合部。智利海脊的扩展速度和形态以及海脊俯冲的历史是众所周知的：至少有三个智利南部海脊段已经俯冲到南美洲14、6和3英里以下。那是以前的事了。目前俯冲纳斯卡岩石圈南部的地震活动决定了纳斯卡板块在纳斯卡-南极洲-南美洲三重结合部以北的倾角，尽管很差。然而，在三重结点以南，地震活动明显减弱，南极板块倾角未知。尽管如此，根据扩张和会聚历史，以及南美洲相对于深部地幔的运动知识，可以预测俯冲的纳斯卡和南极实验室及其相关扩张脊的形态。因此，可以确定纳斯卡板块和南极板块-板块窗口的活动俯冲部分之间形成的缺口的轨迹和预期形状。板块窗口的想法隐含着这样的假设，即俯冲后俯冲海脊两侧之间的扩张继续，但俯冲后不形成新的岩石圈，导致逐渐扩大的缺口和前海脊两侧之间的软流圈地幔流动。智利山脊俯冲区剪切波分裂和地震衰减研究的初步结果表明，南美洲之下的上地幔流场可能具有很强的变异性。在智利边缘三交界区部署了大约50个被动震源地震台，提供了直接观测板窗的前景，并有可能确定在这一地球动力学独特的构造体制中地幔流场的形式和功能。对纳斯卡-南极洲-南美洲三联结区的地壳和上地幔结构、地幔各向异性、地震衰减和构造进行了研究，以获得板片缝隙的结构和通过这些缝隙的地幔流动。对地幔对山脊俯冲的响应进行有针对性的测试，包括确定研究区域下方地幔软流圈的存在和厚度，确定由于板条窗口岩浆作用和加强的构造侵蚀而可能不均匀的地壳和岩石圈结构，观察空区附近的地幔流动方向，以及估计地震衰减和有关地幔温度场的相关推断。

Subduction of active oceanic spreading ridges has been an important process in Cenozoic tectonics of the Pacific Basin. The Chile Ridge, the divergent Nazca-Antarctic plate boundary, is currently subducting beneath continental South America along the Chile Trench at ~46.5S, forming a plate triple junction in the vicinity of the Taitao Peninsula. Chile Ridge spreading rate and morphology, and the history of ridge subduction, are well-known: at least three southern Chile Ridge segments have subducted beneath South America 14, 6 and 3 m.y. ago. The current seismicity of the southern portion of subducting Nazca lithosphere defines the dip of the Nazca slab north of the Nazca-Antarctica-South America triple junction, albeit poorly. However, south of the triple junction seismicity diminishes markedly and Antarctic plate slab dip is not known. Nonetheless, given spreading and convergence histories, and knowledge of the motion of South America with respect to the deep mantle, the morphology of the subducted Nazca and Antarctic labs and their associated spreading ridge can be predicted. Thus, the loci and expected shapes of gaps formed between actively spreading subducted portions of the Nazca and Antarctic slabs - slab windows can be determined.Implicit in the slab window idea is the assumption that spreading between the two sides of the subducted ridge continues after subduction, but that no new lithosphere is formed after subduction, leading to formation of a progressively widening gap and asthenospheric mantle flow between the two edges of the former ridge. Preliminary results from studies of shear wave splitting and seismic attenuation in the Chile Ridge subduction region indicate that the upper mantle flow field beneath South America may be highly variable. Deployment of some 50 passive-source seismic stations in the Chile Margin triple junction area offers the prospect of direct observation of a slab window and the possibility that the form and function of the mantle flow field in this geodynamically unique tectonic regime can be determined. Studies of crust and upper mantle structure, mantle anisotropy, seismic attenuation, and tectonics of the Nazca-Antarctica-South America triple junction region are designed to yield the structure of slab gaps and mantle flow through those gaps. Targeted tests of mantle responses to the ridge subduction include establishment of the presence and thickness of mantle asthenosphere beneath the study region, determination of potentially heterogeneous crustal and lithospheric structure due to slab window magmatic processes and enhanced tectonic erosion, observations of mantle flow directions in the vicinity of the gaps, and estimation of seismic attenuation and associated inferences concerning mantle temperature fields.