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突出问题之一。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规模上与主要板块边界下的板块运动相互作用。最近在地幔各向异性结构的地震成像方面的进展有可能对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而产生的，因此提供了对地幔流动方向的直接估计。到目前为止，大多数研究比较的seismicanisotropy的观测地幔流的预测集中在广泛的模式ofanisotropy与全球地幔对流或在板块边界的区域研究，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动与全球对流流动的相互作用比现有模型所能解释的更为复杂。在这项研究中，PI将构建一系列3-D数值模拟，以研究全球对流和与走滑断层（如圣安德烈亚斯断层）相关的区域流之间的相互作用。首先，全球流动模型将被构建为在一个粗分辨率的网格上结合板块运动和地幔密度不均匀性驱动的流动。由这些模型得到的流场将作为以断裂带为中心的高分辨率区域尺度模型的边界条件。预测的各向异性来自这些详细的区域模型将计算使用一系列代理的发展的晶格优选取向（LPO）在地幔岩石和比较的面波和横波分裂数据instrike-slip环境。这一研究将加深我们对地幔流、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性是很重要的。这个项目的结果直接关系到许多NSF赞助的计划，如边缘，脊2000，CSEDI，和地球镜。

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.