Mantle Flow and the Development of Sub-Lithospheric Seismic Anisotropy

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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.