Three-dimensional Subduction Models: Implications for Plate-Mantle Coupling and Length-scales of Seismic Anisotropy

三维俯冲模型：对板块-地幔耦合和地震各向异性长度尺度的影响

• 批准号: 1316416
• 负责人: Margarete Jadamec
• 金额: $ 5.87万
• 依托单位: Brown University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2014-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1316416&HistoricalAwards=false
• 关键词: Three; dimensional; Subduction; Models; Implications

The theory of plate tectonics predicts that the outer layer of the Earth is comprised of lithospheric plates (0 - 250 km thick) that are in motion with respect to one another (at rates on the order of 1 - 20 cm/yr), with the majority of deformation concentrated at the plate boundaries. At the Earth's surface, this deformation is commonly manifested in the form of increased and localized seismicity, volcanism, and mountain building. The expression of plate boundary zone deformation underneath the plates, in the Earth's mantle, however, is not well understood. Geodynamic modeling is a valuable tool that can be used to predict the Earth's viscous mantle response to and interaction with the tectonic plates, as the mantle cannot be accessed directly. The proposed work will use high-resolution, three-dimensional geodynamic modeling as a tool to simulate how the Earth's viscous mantle responds to and interacts with the tectonic plates at subduction zones, regions where one tectonic plate slides beneath another and induces motion within the underlying viscous mantle. Recent seismological observations of shear wave splitting indicate the portion of the mantle wedged between the overriding plate and subducting plate, the mantle wedge, is commonly characterized by seismic fast axes oriented oblique to plate motion, indicative of a complex mantle flow field in many subduction zones. However, far from the plate boundary, the seismic fast axes are commonly oriented sub-parallel to plate motion, indicative of coupling between the plate interior and underlying mantle flow field. A two-dimensional subduction regime cannot explain the along strike and across strike variations in mantle flow implied by the shear wave splitting, thus requiring a three-dimensional framework. Previous three-dimensional numerical simulations of subduction zones predict that trench-parallel flow can be driven by pressure gradients in the mantle wedge and that toroidal flow can be generated around lateral slab edges due to steepening of the subducting plate or rollback in the trench. In addition models of small-scale convection within the mantle wedge predict complex mantle flow and seismic anisotropy in the mantle wedge. However, what controls the transition from complex mantle-overriding plate motion near the subduction zone to aligned mantle-overriding plate motion in the plate interiors has not been investigated. Furthermore, although recent work indicates the rheological flow law governing deformation in the mantle may be instrumental in controlling the magnitude and length-scales of the subduction induced viscosity reduction and complex mantle flow field, this has not been quantified in three-dimensional models of subduction. For this research, three-dimensional numerical models will be constructed, run, and analyzed, to systematically test the controls on the lateral extent of the slab driven viscosity reduction in the mantle wedge due a strain-rate dependent viscosity. The results have implications for the length-scales of complex seismic anisotropy observed in subduction zones and may place constraints on the magnitude of coupling between the mantle and overriding plate. Moreover, understanding how the rheology modulates the viscous flow of the mantle has important implications for understanding the rates of tectonic plate motion, the length-scales of plate boundary zone deformation, as well as the three-dimensional transport of geochemical signatures within the volcanic front.

板块构造理论预测，地球的外层是由岩石圈板块（0 - 250公里厚）组成的，这些板块彼此相对运动（速度约为1 - 20厘米/年），大部分变形集中在板块边界。在地球表面，这种变形通常表现为增加和局部地震活动，火山活动和造山运动。然而，在板块之下，在地幔中的板块边界带变形的表达还没有得到很好的理解。 地球动力学建模是一种有价值的工具，可用于预测地球的粘性地幔对构造板块的反应和相互作用，因为地幔不能直接进入。拟议的工作将使用高分辨率三维地球动力学建模作为一种工具，模拟地球的粘性地幔如何在俯冲带对构造板块作出反应并与之相互作用，在俯冲带，一个构造板块滑到另一个构造板块之下，并在下面的粘性地幔内引起运动。剪切波分裂的最新地震学观测表明，地幔楔夹在上覆板块和俯冲板块之间的部分，通常具有地震快轴倾斜于板块运动的特征，这表明许多俯冲带中存在复杂的地幔流场。然而，在远离板块边界的地方，地震快轴通常与板块运动近平行，这表明板块内部与下伏地幔流场之间存在耦合。二维俯冲机制不能解释剪切波分裂所隐含的地幔流沿沿着走向和跨走向的变化，因此需要一个三维框架。以前的俯冲带的三维数值模拟预测，沟槽平行流可以驱动的地幔楔的压力梯度和环形流可以产生周围的横向板边缘由于俯冲板块或回滚在沟槽变陡。此外，地幔楔内的小尺度对流模型预测了复杂的地幔流和地幔楔内的地震各向异性。然而，是什么控制着从俯冲带附近复杂的地幔覆盖板块运动到板块内部对齐的地幔覆盖板块运动的转变还没有被研究。此外，虽然最近的工作表明，在地幔变形的流变流动规律可能有助于控制的大小和长度尺度的俯冲引起的粘度降低和复杂的地幔流场，这还没有被量化的俯冲的三维模型。对于这项研究，三维数值模型将被构建，运行和分析，系统地测试控制板驱动的地幔楔中的粘度降低由于应变率依赖的粘度的横向范围。这些结果对在俯冲带中观察到的复杂地震各向异性的长度尺度具有影响，并可能对地幔和覆盖板块之间的耦合幅度产生限制。此外，了解流变学如何调制地幔的粘性流有重要的意义，了解构造板块运动的速率，板块边界区变形的长度尺度，以及火山前缘内的地球化学特征的三维运输。