Collaborative Research: Seismic Imaging of Mid-Mantle Reflectors Associated with Geodynamical Processes and Compositional Heterogeneity

New Mexico State University, Las Cruces NM

Investigators

Abstract

The Earth is composed of several distinct layers: the crust at the surface, the mantle which comprises the bulk of our planet, and the metallic core at its center. Heat released from the core drives slow convection of the hot mantle. This process results in the movement of tectonic plates, produces volcanism, and reshapes continents. Recent geophysical studies have suggested that the pattern of convection changes in the mid-mantle, from 800-1300 km depth. The upwelling plumes and subducting slabs which dominate mantle convection appear in seismic tomography images to become deflected in the mid-mantle. It is unknown whether this is due to composition or viscosity. The upper mantle is known to have similar seismic discontinuities that are thought to be caused by variations in chemistry or mineral phase changes as the mantle gets hotter and pressure increases with depth. The goal of this research is to use new methods to understand the large reflectors in the mid-mantle by developing comprehensive maps of the small-scale seismic properties, and comparing them to characteristic signatures for different types of geodynamic flow domains, such as known slab or upwelling regions. This project will support both graduate and undergraduate student research. The project will foster new international collaborations between New Mexico State University, a Hispanic-serving institution, and universities in Europe and Australia. The work will benefit various other disciplines across the wider Earth Sciences community, and promote Geophysics to minority students in New Mexico. This project is jointly funded by the Geophysics Program, and the Established Program to Stimulate Competitive Research (EPSCoR). This project will determine the varied origins of mid-mantle seismic reflectors, ascertain their spatial relationship to flow patterns in the mantle, such as downwelling cold slabs or upwelling plumes, and investigate potential formative processes and compositional origins. The work will generate comprehensive regional-scale seismic maps of reflectors from 800-1300 km depth in the mantle. Novel and established analytical methods will be applied to compile and evaluate complementary datasets (SS and PP precursors; ScS and PcP reverberations; receiver functions), to constrain reflector geometry, apparent impedance contrast, sharpness, and heterogeneity across multiple length-scales of lateral resolution (100-1,000 km). These observables will be tested and linked via forward modelling of synthetic seismograms. Results will be analyzed in the context of global seismic tomography and geodynamical models, using seismic velocity as a proxy for geodynamical flow. This research will establish the characteristic signatures of mid-mantle reflectors for several well-sampled velocity domains representative of diverse geodynamical settings, incorporating statistical correlations and clustering analysis. Reflector properties from each domain type will be assessed within the framework of global geodynamical numerical simulations, and testing against the influence of different scenarios of convection history. Subsequently, this will inform regarding likely formative mechanisms and thermochemical origins for the reflectors. Synthetic seismic data generated from different geodynamical models will provide a direct comparison to seismic observations. This multidisciplinary approach will inform regarding the influence of seismic transitions on mantle convection and vice versa and elucidate the role of flow for generating physical structures. The project will help to map the seismic signatures of global mid-mantle circulation, characterize thermochemical exchange between lower and upper mantle, and provide insight into the history and style of mantle mixing and dynamics. The results will provide valuable inputs for the next generation of geodynamical simulations and mineral physics studies in the mantle, and have wide implications ranging from the mapping of primordial unmixed mantle reservoirs, to tracing the deep origins of continental volcanism. 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.