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，地球外层由岩石圈板块（0 - 250 公里厚）组成，这些板块之间相对运动（速度约为 1 - 20 厘米/年），其中大部分变形集中在板块边界处。在地球表面，这种变形通常表现为局部地震活动增加、火山活动和造山活动。然而，地幔中板块下方的板块边界带变形的表达尚不清楚。 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响应和相互作用，因为地幔无法直接进入。拟议的工作将使用高分辨率、三维地球动力学模型作为工具来模拟地球的粘性地幔如何响应俯冲带的构造板块并与之相互作用，俯冲带是一个构造板块在另一个构造板块下方滑动并引起底层运动的区域。粘性地幔。最近对剪切波分裂的地震观测表明，楔在上覆板块和俯冲板块之间的地幔部分（地幔楔）通常以与板块运动倾斜的地震快轴为特征，表明许多俯冲带存在复杂的地幔流场。然而，远离板块边界，地震快轴通常与板块运动近平行，表明板块内部和下面的地幔流场之间存在耦合。二维俯冲体系无法解释剪切波分裂所暗示的地幔流的沿走向和跨走向变化，因此需要三维框架。先前对俯冲带的三维数值模拟预测，海沟平行流可以由地幔楔中的压力梯度驱动，并且由于俯冲板块变陡或海沟中的回滚，可以在侧板边缘周围产生环形流。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。此外，尽管最近的工作表明控制地幔变形的流变流动定律可能有助于控制俯冲引起的粘度降低和复杂地幔流场的大小和长度尺度，但这尚未在俯冲的三维模型中得到量化。在这项研究中，将构建、运行和分析三维数值模型，以系统地测试对由于应变率相关粘度而导致地幔楔中板片驱动的粘度降低的横向范围的控制。这些结果对俯冲带中观测到的复杂地震各向异性的长度尺度有影响，并且可能对地幔和上覆板块之间的耦合程度施加限制。此外，了解流变学如何调节地幔的粘性流对于了解构造板块运动的速率、板块边界带变形的长度尺度以及火山前缘内地球化学特征的三维传输具有重要意义。