Collaborative Research: Mapping Upper-Mantle Anisotropy in Western U.S.: Constraints on Crust-Mantle Coupling

地球范围计划的一个主要目标是更好地了解控制大陆组装和演化的过程。其中许多过程很可能与上地幔结构和动力学有关--例如，地幔浮力的变化驱动了地表的变形。由此产生的构造作用的性质不仅取决于浮力的性质，还取决于传递这些力的地壳和地幔岩石圈的强度。地幔岩石中组构产生的地震各向异性提供了绘制地幔变形图的手段，而地质学和大地测量观测提供了地表变形的图景。使用数值模型将两者结合起来，科学家可以得出关于地幔流动、岩石圈强度和控制地表变形的机制的新限制条件。在这个项目中，一个综合的地震-地球动力学分析正在发展，并应用于美国西部地幔流动和地壳变形之间的几个关键问题，包括：(1)加利福尼亚州圣安德烈亚斯断层上地幔组构的变化能否用地壳和/或地幔强度从北到南的变化来解释？(2)中央盆地和山脉之下的复杂地幔组构是由围绕增厚的岩石圈的地幔流动产生的，还是更可能是该地区下方浮力驱动的上升流？具体地说，地震学家正在测量美国阵列和现有宽带地震台记录的地震波的有限频率旅行时，并使用普遍各向异性结构的真实敏感核来反演三维上地幔模型的旅行时，在该模型中，地幔组构受到约束。与此同时，地球动力学家正在开发区域数值流动模型，这些模型是根据地质约束和地表运动学观测得出的。运动学流动模型为各种变形情景提供了对地幔组构发展的估计，然后对这些组构估计与地震观测的兼容性进行了定量评估。除了约束沿美国西部板块边界的地幔流动和地壳变形之间的耦合外，该项目还将为大陆的区域尺度各向异性模拟提供一般方法。

0545777GahertyA principal goal of the EarthScope program is to develop a better understanding of the processes that control the assembly and evolution of the continents. It is likely that many of these processes are associated with upper-mantle structure and dynamics -- mantle buoyancy variations drive surface deformation, for example. The character of the resulting tectonism is dependent not only on the nature of the buoyancy forces, but also on the strength of the crust and mantle lithosphere that transmit those forces. Seismic anisotropy produced by fabric in mantle rocks provides a means to map deformation in the mantle, and geology and geodetic observations provide a picture of surface deformation. Using numerical models to combine the two, scientists can derive new constraints on mantle flow, lithospheric strength, and the mechanisms that control surface deformation. In this project, an integrated seismic-geodynamic analysis is being developed and applied to several key questions regarding the coupling between mantle flow and crustal deformation in the western US, including: (1) Can variations in mantle fabric along the San Andreas Fault in California be explained by changes in the strength of the crust and/or mantle from north to south? (2) Is complex mantle fabric inferred beneath the central Basin and Range produced by mantle flow around a thickened lithosphere, or is it more suggestive of buoyancy-driven upwelling beneath this region? Specifically, seismologists are measuring finite-frequency travel-times from seismic waves recorded at USArray and existing broad-band seismic stations, and inverting these travel-times for 3-D upper-mantle models in which mantle fabric is constrained using realistic sensitivity kernels for generally anisotropic structure. Simultaneously, geodynamicists are developing regional numerical flow models that are derived from geological constraints and observations of surface kinematics. The kinematic flow models provide estimates of mantle fabric development for a variety of deformation scenarios, and these fabric estimates are then quantitatively evaluated for compatibility with the seismic observations. In addition to constraining the coupling between mantle flow and crustal deformation along the western US plate boundary, the project will provide a general methodology for regional-scale anisotropy modeling for continents.