Mantle Flow and the Development of Sub-Lithospheric Seismic Anisotropy

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

The style of convection beneath the Earth's tectonic plates is one of the outstanding questions in solid earth geophysics. The lack of direct constraints on mantle flow makes it difficult to determine how and on what scale global convective flow interacts with plate motions beneath major plate boundaries. Recent advances in seismic imaging of the mantle's anisotropic structure have the potential to provide important new constraints on these flow patterns. Seismic anisotropy results from the preferred alignment of olivine crystals in the sub-lithospheric flow field, and thus provides a direct estimate of the direction of mantle flow. To date, most studies comparing observations of seismic anisotropy to predictions of mantle flow have focused on either broad patterns of anisotropy associated with global mantle convection or on regional studies at plate boundaries that neglect the global flow field. However, abundant seismic evidence indicates that regional flow associated with local plate motions and lithospheric structure interacts with global convective flow in a more complicated way than can be explained by current models. In this study the PIs will construct a series of 3-D numerical simulations to investigate the interaction between global convective flow and regional flow associated with strike-slip faults, such as the San Andreas. First, global flow models will be constructed that incorporate flow driven by plate motions and mantle density heterogeneity on a coarse resolution grid. The flow field derived from these models will be used as boundary conditions on high-resolution regional scale models centered on the fault zone. The predicted anisotropy derived from these detailed regional models will be calculated 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 in strike-slip environments. This study will improve our understanding of the relationship between mantle flow, LPO, and seismic anisotropy. Understanding the pattern of mantle flow beneath major strike-slip faults is important in order to quantify the tectonic forces along strike-slip plate boundaries, as well as to determine seismic hazards associated with these faults. The results of this project are directly relevant to many NSF sponsored programs such as Margins, Ridge2000, CSEDI, and Earthscope.