Mantle Flow and the Development of Sub-Lithospheric Seismic Anisotropy

地球构造板块下的对流形式是固体地球物理学中的一个突出问题。由于缺乏对地幔流动的直接约束，很难确定全球对流如何以及在多大程度上与主要板块边界下的平台相互作用。地幔各向异性结构地震成像的最新进展有可能为这些流动模式提供重要的新约束。地震各向异性是由于岩石圈下流场中橄榄石晶体的优先排列，从而提供了对地幔流动方向的直接估计。迄今为止，大多数比较地震各向异性观测与地幔流预测的研究都集中在与全球地幔对流相关的各向异性的广泛模式上，或者集中在板块边界的区域研究上，而忽略了全球流场。然而，大量的地震证据表明，与局部板块运动和岩石圈结构相关的区域流与全球对流流的相互作用比当前模型所能解释的更为复杂。在这项研究中，pi将构建一系列三维数值模拟来研究全球对流流动和与走滑断层（如圣安德烈亚斯断层）相关的区域流动之间的相互作用。首先，将在粗分辨率网格上构建包含板块运动和地幔密度非均质驱动的全球流动模型。由这些模型得到的流场将用作以断裂带为中心的高分辨率区域尺度模型的边界条件。从这些详细的区域模型中得到的预测各向异性将使用一系列代理来计算地幔岩石中晶格优选取向（LPO）的发展，并与走滑环境中的面波和横波分裂数据进行比较。这项研究将有助于我们对地幔流动、LPO和地震各向异性之间关系的理解。了解主要走滑断层下的地幔流模式对于量化沿走滑板块边界的构造力以及确定与这些断层相关的地震危险是很重要的。该项目的研究结果与许多NSF资助的项目直接相关，如边际、山脊2000、CSEDI和地球范围。

The style of convection beneath the Earth's tectonic plates is one of the outstandingquestions in solid earth geophysics. The lack of direct constraints on mantle flow makesit difficult to determine how and on what scale global convective flow interacts with platemotions beneath major plate boundaries. Recent advances in seismic imaging of themantle's anisotropic structure have the potential to provide important new constraints onthese flow patterns. Seismic anisotropy results from the preferred alignment of olivinecrystals in the sub-lithospheric flow field, and thus provides a direct estimate of thedirection of mantle flow. To date, most studies comparing observations of seismicanisotropy to predictions of mantle flow have focused on either broad patterns ofanisotropy associated with global mantle convection or on regional studies at plateboundaries that neglect the global flow field. However, abundant seismic evidenceindicates that regional flow associated with local plate motions and lithospheric structureinteracts with global convective flow in a more complicated way than can be explainedby current models. In this study the PIs will construct a series of 3-D numericalsimulations to investigate the interaction between global convective flow and regionalflow associated with strike-slip faults, such as the San Andreas. First, global flow modelswill be constructed that incorporate flow driven by plate motions and mantle densityheterogeneity on a coarse resolution grid. The flow field derived from these models willbe used as boundary conditions on high-resolution regional scale models centered on thefault zone. The predicted anisotropy derived from these detailed regional models will becalculated using a series of proxies for the development of lattice preferred orientation(LPO) in mantle rocks and compared to surface wave and shear-wave splitting data instrike-slip environments. This study will improve our understanding of the relationshipbetween mantle flow, LPO, and seismic anisotropy. Understanding the pattern of mantleflow beneath major strike-slip faults is important in order to quantify the tectonic forcesalong strike-slip plate boundaries, as well as to determine seismic hazards associated withthese faults. The results of this project are directly relevant to many NSF sponsoredprograms such as Margins, Ridge2000, CSEDI, and Earthscope.