ABSTRACT

In stable continents worldwide, a substantial velocity decrease has been detected at about 100 km depth (varying depending on region) and at an expected temperature of about 1000 degrees C. This decrease in velocity of roughly 3-5% or more, is called the mid-lithosphere discontinuity (MLD). The lithosphere (hard rocks) of a stable continent is expected to be old, and cold, therefore observations of a geological wide-spread discontinuity in seismic velocity internal to the lithosphere is puzzling. This has led to a variety of different, often contradictory, explanatory models for a wavespeed drop within stable lithosphere, e.g., partial melt, anisotropy, sub-solidus rheology transitions, and chemical stratification. This project will evaluate these proposed causative models against new geophysical and geological constraints, using EarthScope data, laboratory experiments and computer modelling. The project will focus on (1) variation in elastic and anelastic properties and electrical conductivity across the MLD, (2) a global presence of the MLD, regardless of geological history, (3) laboratory studies of the influence of water (hydration) on properties of rock that could cause the velocity to increase, and (3) composition and textures of mantle xenoliths, samples of solid mantle rock that hitch a ride with rising magma. This project will engage early career scientists, Ph.D. students, and undergraduate students. The project will also promote EarthScope's education and outreach goals, by presenting the science results and research opportunities at the IRIS minority recruitment speaker series and the Nifty Fifty science lectures to K-12 educators and students.

The project will: (1) extend the seismological observations using new receiver-function estimates and Bayesian methodology that can quantify the magnitude of anisotropy and the sharpness of the velocity drop over a more extensive footprint of seismic stations; (2) acquire measurements of surface wave amplitudes and Pg reverberation coda to identify whether there is a peak in attenuation around the MLD depth; and (3) jointly integrate magnetotelluric (MT) conductivity estimates with new mineral-physics and seismological constraints, to identify the presence of melt or hydration across the MLD. The investigators will focus the study on the stable Precambrian North American Craton, which was covered by the second half of the lower-48 deployment of the EarthScope Transportable Array. The project will also involve new lab experiments on how water influences grain-boundary mobility in mantle rocks. An improved understanding of the MLD is crucial for relating EarthScope results to the evolution of continents. The extension of the seismological observation and its integration with MT and mineral physics is a unique approach that will provide new insights into the origin of the MLD. These new strategies for processing seismic data and integrating MT data with seismology and mineral physics will be useful to the general geophysical community. With this interdisciplinary hypothesis-testing approach, the investigators propose to obtain a better understanding of the cause of the MLD that will extend the initial studies of USArray data to the structure and evolution of the North American continent, and by analogy, to other continents.

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.

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