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

智利洋脊俯冲的地壳和上地幔结构、地幔流动和地球动力学研究

• 批准号: 0509225
• 负责人: Raymond Russo
• 金额: --
• 依托单位: University of Florida
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-06-30 至 2007-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0509225&HistoricalAwards=false
• 关键词: Studies; Crust; Upper; Mantle; Structure

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.

活动洋脊俯冲是太平洋盆地新生代构造的一个重要过程。 智利海岭是纳斯卡-南极板块的分叉边界，目前正沿着南纬46.5度的智利海沟俯冲到南美洲大陆下方，在泰涛半岛附近形成板块三重交界处。 智利海岭的扩张速度和形态，以及海脊俯冲的历史，都是众所周知的：至少智利南部的三个海岭部分已在 14、6 和 3 m.y. 俯冲到南美洲以下。前。 目前俯冲纳斯卡岩石圈南部的地震活动定义了纳斯卡-南极洲-南美洲三重交界点以北纳斯卡板片的倾角，尽管效果很差。 然而，三重交界处以南地震活动明显减弱，南极板块板片倾角未知。 尽管如此，考虑到扩张和汇聚的历史，以及南美洲相对于深部地幔的运动知识，俯冲的纳斯卡和南极实验室的形态及其相关的扩张脊是可以预测的。因此，可以确定纳斯卡和南极板片的活跃扩展的俯冲部分之间形成的间隙的轨迹和预期形状 - 板片窗口。板片窗口思想中隐含的假设是俯冲山脊两侧之间的扩展在俯冲后继续进行，但俯冲后没有形成新的岩石圈，导致在前脊的两个边缘之间形成逐渐扩大的间隙和软流圈地幔流。 智利海岭俯冲区剪切波分裂和地震衰减研究的初步结果表明，南美洲下方的上地幔流场可能变化很大。在智利边缘三联结区部署约 50 个被动源地震台，提供了直接观测板片窗口的前景，并有可能确定这一地球动力学独特构造体系中地幔流场的形式和功能。 对地壳和上地幔结构、地幔各向异性、地震衰减和纳斯卡-南极洲-南美洲三重交界区的构造的研究旨在产生板片间隙和通过这些间隙的地幔流的结构。 地幔对山脊俯冲响应的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、确定由于板片窗岩浆过程和增强的构造侵蚀而导致的潜在异质地壳和岩石圈结构、观测地幔流动方向。间隙附近、地震衰减估计和有关地幔温度场的相关推论。