Effects of 3-D Mantle Wedge Flow and Crystal Preferred Orientation on Shear-Wave Splitting in Subduction Zones

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

The Earth’s mantle is solid, but it flows very slowly under high pressure and temperature, like a thick fluid. This flow affects important geological processes such as earthquakes and volcanic eruptions, but it occurs too far below the Earth’s surface to be observed directly. In subduction zones, where oceanic plates descend below continents, mantle flow patterns are thought to be very complex. Though these patterns can be estimated using computer models, we need real data to check their accuracy. One way to indirectly obtain mantle flow data is to use seismic waves that are generated by earthquakes. Seismic waves that travel through the mantle are affected by fine layering that forms when mantle minerals line up in response to flow. Recordings of such waves at the Earth’s surface can be interpreted to identify mantle regions where minerals are lined up, showing what the flow patterns look like in 3D (at least, in parts of the mantle that many of the recorded seismic waves have passed through). In this study, Dr. Wada and her group will develop computer models of mantle flow in selected subduction zones, and predict how mantle minerals should line up. Next, they will use different software to predict what seismic waves travelling through this model mantle should look like when they reach the Earth’s surface. By comparing these predictions with data from seismometers, they can test the model and fully describe the geometry of mantle flow. The project will provide training for a postdoctoral scholar and an undergraduate student in seismology and scientific computer programming. The software and scientific results will be used in an undergraduate geophysics course at Minnesota and will be freely and publicly shared. One of the indirect observations that are used to infer mineral orientations and mantle flow directions are shear-wave splitting (SWS) observations. Minerals in the mantle, in particular olivine, become aligned relatively to the mantle flow direction, developing crystal preferred orientations (CPO). Shear waves that are generated by earthquakes travel through the upper mantle and become polarized into two orthogonal components that travel at different speeds. Measurements of the polarization direction of the fast component and the delay time between the two components provide information about the CPO of minerals in the mantle and thus the mantle flow pattern. However, the interpretation of the SWS parameters (i.e., the fast direction and the delay time) in terms of CPO and mantle flow directions is not trivial, and large uncertainties remain, particularly in subduction zones with oblique plate convergence that induces complex mantle wedge flow patterns. The previously funded project generated a suite of 3-D steady-state coupled kinematic-dynamic models for generic subduction systems with a range of subduction obliquity to examine its effect on the mantle wedge flow pattern. The model-predicted variation in mantle wedge flow pattern with the local subduction obliquity allowed prediction of along-margin variations in the mantle wedge flow pattern based on the local subduction obliquity, for subduction zones worldwide. Using calculated steady-state mantle velocity fields, the evolution of olivine and pyroxene CPOs in the mantle wedge was simulated. The calculated distributions of CPO were fed into the forward modeling of SWS. The results highlighted the complex evolution of CPOs in the mantle wedge for different CPO types and their impact on the SWS parameters. The proposed study is aimed to fill the gaps in the previous work through the following three activities: (1) Quantifying the variations in the CPO distribution and SWS parameters due to along-margin variations in the margin strike and the slab geometry. (2) Testing the model-predicted SWS parameters against observed SWS parameters in NE Japan where sufficient detailed SWS analyses have been performed. (3) Investigating the relative importance of the impact of transtensional and transpressional strains in the mantle wedge during slab rollback and advance on the CPO evolution and SWS. The results provide foundational understanding of the impacts of subduction zone geometry and slab motion on deformation and CPO evolution in the mantle wedge and the dependence of SWS parameters on the CPO distribution. The synthesis of the results with those from the previously funded project will be used to address the observed variations in SWS parameters within and among subduction systems globally. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.