Investigating the composition of lower crust on a continental scale with Transportable Array data

University Of Texas At Dallas, Richardson TX

Investigators

Abstract

The Moho is a first-order feature of the Earth, the boundary where the Earth’s crust meets the mantle. Since the lower crust and upper mantle are difficult to directly sample, debate on the composition of the lower continental crust continues. In particular, questions remain on how its composition might vary across a continent and the physical state of the mantle underneath the Moho. Seismic techniques provide an opportunity to study the deep crust by analyzing the world-class seismic datasets that span the United States. The researchers will map this boundary on a continental scale to find new insights into the origin of the continents, the drivers of high topography, and the tectonic history of North America. This project will support a post-doctoral scholar and graduate student who will be trained to apply novel computational approaches for constraining the detailed structure of the crust-to-mantle transition. Comparing and contrasting results from the distinct tectonic domains that make up North America can provide a better understanding of the Moho discontinuity. This project will constrain seismic velocities above, within, and below the Moho transition zone across the continental United States from the joint analysis of converted phases and surface wave velocities. A novel inverse scheme will incorporate the highest frequency data possible to obtain the most detailed model that existing data allows. With this tool, the researchers can investigate how the continental crust is stratified, and how this stratification may or may not relate to age. By pushing the limits of resolution with the relevant data, the team will explore potential processes that may modulate the sharpness of the Moho transition zone and ultimately target the role of variations in lower crustal composition in supporting modern high topography. A key parameter of interest is the shear-wave velocities in the lower crust, which correlates with compositional variations (ranging from quartz-rich to quartz-poor) that control crustal density. The good match between observed shear-wave velocities at some test sites and predicted shear-wave velocities for the Kapuskasing uplift – a site in southeastern Canada with one of the few known exposures of lower crust at the surface – implies more quartz-rich compositions that would be inconsistent with formation directly from mantle melting processes. Constraints on crustal composition can also help explain regions where high topography and crustal thickness are out of balance; first in the Appalachian Mountains where very high shear-wave velocities would be consistent with the hypothesis that metamorphic reactions densify the lower crust and thus drag down the height of the mountains, and then in the Basin and Range, where high elevation appears to require a reduced density for the lower crust. 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.