What happens when a tectonic plate plunges back into the Earth?s interior at a subduction zone? Surprisingly, this simple question remains one of the fundamental unsolved problems in earth science. The flow pattern around the downgoing plate (often called the slab) is typically studied using seismic anisotropy, a property of the mantle that can be detected by recording seismic waves. In particular, measurements of shear wave splitting can be used to characterize seismic anisotropy in subduction zones. It is increasingly clear from such measurements that the simplest model of two-dimensional corner flow above the slab and entrained flow beneath the slab is likely incorrect. However, consensus on an alternative model has not been forthcoming. In this project, we will undertake a global survey of shear wave splitting observations with the goal of understanding what controls the mantle flow field in subduction zone regions. To do this, we will identify parameters that describe subduction zone dynamics that appear to exert a first-order control on shear wave splitting. Preliminary work has identified systematic variations in anisotropy both above and below the slab linked with the magnitude of trench migration velocity. This has led to the hypothesis that 3-D flow dominates beneath the slab and interacts with 2-D corner flow in the mantle wedge. In addition to a systematic evaluation of seismic anisotropy in subduction zones, we will construct laboratory and numerical models of mantle flow above and below the slab to identify diagnostic features of the flow field in shear wave splitting measurements and to explore the implications of our model for mantle dynamics.
This project constitutes an interdisciplinary effort to understand and characterize the character of the mantle flow field that accompanies subduction using observations of seismic anisotropy and geodynamical modeling. With the increasing popularity of shear wave splitting as a tool for mapping mantle flow, a copious amount of data from subduction zones is now available. It is timely, therefore, to undertake a global survey of splitting observations with the goal of understanding which subduction parameters (such as convergence velocity, trench migration and curvature, age and spreading history of the downgoing plate, slab dip and morphology, seismicity, arc length, overriding plate thickness and stress, and volcanic production) appear to control the subduction zone flow field. From a preliminary survey, we hypothesize that 3-D flow dominates beneath the slab and that this flow field interacts with 2-D corner flow in the mantle wedge. We will complement our primary observational seismology goals with laboratory and numerical modeling studies. This forward modeling work will be used to validate the predictions of our working model, formulate alternative hypotheses, identify any second-order effects on the flow field, and explore the implications of our working model for larger-scale mantle dynamics. The availability of constraints on anisotropy from many regions around the globe and the combination of seismological observations and laboratory and numerical modeling suggest that a solution to the fundamental problem of interaction between downgoing slabs and the surrounding mantle is within reach.
