Tectonic history of the Western United States
I am actively working on two research projects investigating the Cenozoic geologic history of the Basin and Range. Despite decades of study, basic aspects of this area's tectonic history remain controversial. The most geologically recent events—associated with Cenozoic Basin and Range extension—have provoked some of the most heated, persistent, and scientifically profound disagreements. When did extension begin? How thick and gravitationally unstable was the crust at the onset of extension? What is the geometry of normal faults below the upper, brittle crust, and how do they interact with mylonitic shear zones in the viscoplastic domain?
All of these questions in turn have implications for understanding present-day tectonic processes: What drives intraplate extension? How are stress and strain in the viscoplastic lower crust and mantle coupled with brittle faulting in the upper crust? What is the relative role of boundary conditions for dictating stress and strain? What implications do these questions have in turn for the earthquake cycle?
I recently completed a detailed geologic mapping study of the sedimentary and volcanic basin flanking the Ruby Mountains–East Humboldt Range metamorphic core complex on its west side with Elizabeth L. Miller of the Stanford Structural Geology and Tectonics Group. Previous work showed a thick succession of Eocene volcanic and volcaniclastic sedimentary rocks in this basin, above the hanging wall of the detachment fault that exposed the core complex, suggesting that extension began in Eocene time or earlier. The timing of core complex formation has been cited as evidence for an early (Eocene) onset of extension in the Basin and Range.
On the basis of our detailed geologic mapping, geochronology of sedimentary and volcanic rocks, and geochemical analysis, we found that the Eocene sedimentary deposits were actually very thin, and that, instead, a large succession of Miocene fluviolacustrine sediments were deposited during rapid uplift of the nearby core complex beginning in Middle Miocene time. The results add an important piece of evidence that the northern Basin and Range probably did not experience significant upper crustal extension until the Middle Miocene. My field map won 3rd place in the 2013 Geological Society of America Best Student Geologic Map Competition.
Our findings also have major implications for understanding the Cenozoic evolution of paleoelevation and climate across the Cordillera because we discovered that an important part of the sedimentary succession used for stable isotope paleoaltimetry in this part of the country is actually >20 Ma younger than previously thought. This means that some climate and elevation estimates made from Elko Basin rocks correspond to a significantly different time frame than expected.