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

“地球范围”项目的一个主要目标是更好地了解控制大陆组合和演化的过程。这些过程很可能与上地幔结构和动力学有关——例如，地幔浮力变化驱动地表变形。由此产生的构造运动的特征不仅取决于浮力的性质，而且还取决于传递这些力的地壳和地幔岩石圈的强度。地幔岩石结构产生的地震各向异性提供了绘制地幔形变图的手段，地质和大地测量观测提供了地表形变图。利用数值模型将两者结合起来，科学家们可以得出地幔流动、岩石圈强度和控制地表变形机制的新约束条件。在这个项目中，一个综合的地震-地球动力学分析正在被开发，并应用于美国西部地幔流动和地壳变形之间耦合的几个关键问题，包括：(1)加利福尼亚圣安德烈亚斯断层的地幔结构变化是否可以用地壳和/或地幔从北向南的强度变化来解释？(2)从盆地和山脉中部推断出的复杂地幔结构是由加厚岩石圈周围的地幔流动产生的，还是更能说明该地区的浮力驱动的上升流？具体来说，地震学家正在测量USArray和现有宽带地震台站记录的地震波的有限频率传播时间，并将这些传播时间反演为3-D上地幔模型，其中地幔结构使用一般各向异性结构的真实灵敏度核。与此同时，地球动力学家正在开发基于地质约束和地表运动学观测的区域数值流动模型。运动学流动模型提供了各种变形情景下地幔结构发展的估计，然后定量评估这些结构估计与地震观测的兼容性。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，该项目还将为区域尺度的大陆各向异性建模提供一般方法。

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.