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

活动洋扩张脊的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-南极板块的分叉边界，目前在~46.5S沿智利海沟向南美大陆俯冲，在太涛半岛附近形成板块三联结。智利海脊的扩张速度和形态以及海脊俯冲的历史是众所周知的：至少有三个南智利海脊段在14、6和3亿年前俯冲到南美洲之下。俯冲的纳斯卡岩石圈南部目前的地震活动性确定了纳斯卡板块在纳斯卡-南极洲-南美洲三重交界处以北的倾角，尽管不太确定。然而，三交界以南的地震活动明显减弱，南极板块板块倾角不详。尽管如此，考虑到扩张和收敛的历史，以及对南美洲深部地幔运动的了解，俯冲的纳斯卡和南极实验室及其相关的扩张脊的形态可以预测。这样，就可以确定纳斯卡板块和南极板块积极扩张的俯冲部分之间形成的间隙的位置和预期形状。板块窗理论隐含的假设是，俯冲后在俯冲脊两侧之间继续扩张，但俯冲后没有形成新的岩石圈，导致在前山脊两侧之间形成一个逐渐扩大的缺口和软流圈地幔流。智利海脊俯冲区的横波分裂和地震衰减研究的初步结果表明，南美洲的上地幔流场可能是高度可变的。在智利边缘三交界区部署了50个被动震源地震台站，提供了直接观测板块窗口的前景，并为确定这种地球动力学上独特的构造状态下地幔流场的形式和功能提供了可能性。研究纳斯卡-南极洲-南美洲三联结地区的地壳和上地幔结构、地幔各向异性、地震衰减和构造学的目的是产生板块裂缝的结构和通过这些裂缝的地幔流动。对脊向俯冲的地幔响应的目标测试包括确定研究区域下地幔软流圈的存在和厚度，确定由于板块窗岩浆作用和增强的构造侵蚀而可能存在的非均匀地壳和岩石圈结构，观察裂缝附近的地幔流动方向，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.