The Western U.S.: Overview

The West is one of the broadest elevated areas on Earth. If you took intro geology, you might think this was because everything in the West was shortened (and so the crust thickened) from about 200 to 50 million years ago, and then this was modified as the thick crust thinned out under its own weight in places like the Basin and Range. Were this the case, there would be little to do. But it is more complex.

Origins of the Sierra Nevada

I've spent much of my career trying to understand the Sierra Nevada. This range's crust was built by the end of the Cretaceous, more than 65 million years ago, yet there are strong arguments that the elevation of the range is young (though this is disputed). This uplift is accompanied by normal faulting on the east side of the Sierra and occured behind a transform plate boundary. I've deployed seismometers in the range many times (1988,1993,1994, 1997, 2005-2007) to try and constrain the earth structure supporting these elevations. Results from these experiments suggests the high Sierra is supported in part by lower density crust and in part by sitting over low density mantle.

That the mantle is low wavespeed under the high Sierra opens the question of just what has happened under the Sierra. A number of observations suggest that an old high density and high wavespeed mafic lower crust has fallen off the bottom of the Sierra, but where is it now? A good candidate is a high wavespeed body (the Isabella anomaly) in the mantle under the southwestern part of the range and the adjoining San Joaquin Valley, but the physical mechanism by which this material moved from under the eastern Sierra to its present position is poorly understood.

Ongoing seismological work seeks to resolve one of the oldest seismological observations in the Sierra and in the process gain some constraints on how material might be falling away from the range. We are trying to reproduce the seismic waveforms recorded on the east side of the Sierra from earthquakes to the west; these seismic arrivals are anomalously late, yet existing seismological models do not predict such arrivals. Hopefully this work will clarify just what is happening near the Moho under the westernmost Sierra.

Having spent many years in the Sierra and been asked many times why study it, I've written a book, The Mountains that Remade America published by UC Press in the fall of 2017.

Raising the Rockies and High Plains

The wide extent of the American Cordillera is unexpected for an orogen that was not created by continental collision. The earliest uplifts of modern terrain dated to the end of the Cretaceous when some mountains began to rise out of the seaway and coastal plain that existed at the time. This event, the Laramide Orogeny, has long been attributed to a subducting plate scraping the bottom of North America at this time, but the best physical mechanism--a strong coupling of the two plates--makes predictions at odds with observations. Some years ago we suggested an alternative where subsidence of the Colorado and Wyoming region was key in localizing high stresses so far from the continent's edge. Some ongoing work explores whether such a process will reproduce the observed stresses and early strains in the Rockies.

But at a broader scale, the Rockies are just the deformed part of a broad uplift that includes the High Plains of Wyoming, Colorado, western Kansas, and New Mexico. The absence of deformation of the High Plains limits the processes that could be active. We have suggested that this reflects a novel process where originally garnet-rich lower crust has been altered by water rising through the lithosphere to lower density minerals. If true, this challenges our usual notions of the fate of water rising off a subducting plate and adds a new mechanism to those already known to create high topography. Ongoing work is looking to quantify the amount of topography unaccounted for by any other process (such as sedimentation); longer term goals include finding ways of testing this hypothesis seismically and (in collaboration with colleagues in Geological Sciences) determining the age and extent of any hydration event.

Mojave "Waistland"

Over the past 20+ years, most explanations for the cause of the Laramide orogeny ran through the Mojave Desert. Patterns of metamorphism, intrusionm and deformation preserved to the north and south of the Mojave are badly disrupted in the Mojave. This has widelly been attributed to the subduction of an oceanic plateau or exceptionally shallow subduction, which in turn are posited to be the cause of the Laramide Orogeny. Over the years, I've come to feel that the geology seen from the Late Cretaceous (~85-70 Ma) is better thought of as the collapse of a very thick crust in a very narrow part of the Cordilleran orogen (thus the "waistland"). This material has now been collected in a paper within a GSA Special Paper in 2025. One outcome of this work is to suggest what parts of the geological history of that area are reflecting changes in the subduction regime, which might refocus how scientists approach this unusual part of the Cordilleran orogen.

Discerning heat from melt from composition in the deep earth: exploring a new approach to measuring seismic attenuation