EAR-PF:Quantifying Scale of Lower Crust and Mantle Heterogeneities Beneath the Continental United States: Bridging Seismology, Mineral Physics, Petrology, and Magnetotellurics

Dr. William Shinevar has been granted an NSF EAR Postdoctoral Fellowship to carry out research and education activities at University of Colorado Boulder under the mentorship of Dr. Vera Schulte-Pelkum.

The goal of this project is to improve interpretations of lower crustal and mantle seismic anomalies in terms of temperature, pressure, major element chemistry, hydration, and melt fraction by integrating constraints from the measurement of Earth’s electrical resistivity (magnetotellurics), and the directional dependence of the seismic wavespeeds (seismic anisotropy). Constraining the scale of lower crustal and mantle thermal and chemical heterogeneities is fundamental to determining the rheology and stress state of the lithosphere.

This, in turn, is crucial to understanding seismic hazard, as well as how the continental crust and mantle compositionally evolved over Earth’s history, which is important to better understand the formation of natural resources. Dr. Shinevar’s education plan focuses on mentoring an undergraduate student on a research project associated in order to introduce a student to interdisciplinary approaches for understanding Earth processes.

In addition, he will participate in public outreach and K-12 education activities in the Boulder area.

Seismic wave speed on the lithospheric-scale is dependent on the (an)elastic moduli of the minerals present as well as the distribution and elastic moduli of fluids if present. The minerals present mainly depend on major element composition, water content, pressure, and temperature. Seismic wavespeeds of lower crustal and mantle rocks have been recently calculated using databases of whole-rock compositions, thermodynamic modelling, and grain-scale experimental observations of mineral moduli and anelasticity.

The interpretation of seismic wavespeeds is non-unique. Applying these new calculations, this project will assess the scale and magnitude of chemical and thermal heterogeneities compatible with the most current seismic tomographic models of the lower crust and upper mantle, thus quantifying interpretation uncertainty. However, not all wavespeeds from these tomographic models can be interpreted using these calculations.

For these anomalous regions, two other datasets will be used: seismic anisotropy can estimate uncertainty in isotropic tomographic models, and magnetotelluric measurements, which are highly sensitive to fluids and silicate melts, can better constrain mantle melt fraction or hydration. With these independent constraints, tomographic models will be edited to include expected seismic wavespeeds in anomalous regions due to melt or hydrous phases.

Seismic delay times will be forward modeled to validate these calculations. This synthesis will increase our understanding of the evolution of mantle lithosphere from the Archean to the present as well as allow for a direct link between large-scale geophysical observations and small-scale laboratory measurements.

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.