Date of Award

Spring 1-1-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Anne F. Sheehan

Second Advisor

Paul A. Bedrosian

Third Advisor

Craig H. Jones

Fourth Advisor

Lang Farmer

Fifth Advisor

Shijie Zhong

Abstract

Knowledge of the physical and chemical properties of the lithosphere plays an important role in determining how deep geodynamic processes that drive plate tectonics are expressed at the surface of the Earth. Magnetotellurics (MT), an electromagnetic geophysical imaging technique, allows us to map variations in electric resistivity at great depth, providing a unique window into the modern-day condition of the crust and upper mantle. The Rio Grande rift is a mid- to late-Cenozoic age continental rift system in the southwestern United States that is located at the boundary between the actively deforming western U.S. and the stable continental interior. Previous work in the rift has revealed an apparent discrepancy in the vertical distribution of extensional rift structure in that an exceptionally broad low velocity zone in the upper mantle appears to underlie a series of narrow axial rift basins. I present the results of a wide-aperture, high-resolution, deep sensing magnetotelluric survey across the Rio Grande rift that was designed to reconcile this discrepancy through targeted imaging of the lower crust and uppermost mantle. Two-dimensional modeling of broadband and long period magnetotelluric data along three ~450 km profile lines straddling the Rio Grande rift at latitudes of 39.2ºN, 36.2ºN and 32.0ºN reveals anisotropic electrical resistivity structure along the rift axis to depths >150 km. A consistently broad (200 km wide) zone of high electrical conductivity is present within the lower crust along the entire length of the rift axis. This feature is interpreted to be the result of recent tectonic activity, including a recent supply of partial melt and/or saline fluids to the mid- to lower-crust from a broad zone of high-temperature upper mantle. Imaging of the upper mantle reveals a zone of hydrous modification within the sub-continental lithospheric mantle that extends east of the Rocky Mountain Front. A focused magnetotelluric study of the Jemez Mountains Volcanic Field, a major volcanic center on the margin of the rift in northern New Mexico, reveals local perturbations to the regional scale resistivity structure with implications for the most recent episodes of magmatic activity in the Valles caldera.

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