Research

My research focuses on understanding the kinetics of geochemical systems, and using these insights to describe the dates and rates of change in orogenic belts. A primary tool in this endeavor is (U-Th)/He low-temperature thermochronometry, a collection of chronometers in which the daughter He atoms are only retained below certain closure temperatures (from ~200 to ~40 °C depending on the mineral). This research field has greatly enhanced the study of uplift and erosion in orogenic systems, and has been integral in showing the links between surface and tectonic processes. The continued use of (U-Th)/He dating as a reliable thermochronometer relies on improving the accuracy of its predictions for a sample’s thermal history. Specifically, unresolved challenges include data interpretation and the proper characterization of diffusion kinetics. By combining laboratory experiments with field methods and modeling, I seek to address these issues, expand the method’s utility, and better demonstrate how the combined effects of temperature and time operate in earth systems from the crystal to the mountain belt scale.

CN-OC-RB HI RES bottom_adjusted

Helium Diffusion in Zircon: Radiation Damage Effects

A prominent unresolved problem in low-temperature thermochronology relates to the (U-Th)/He zircon (zircon He) thermochronometer. Given its ubiquity and longevity in crustal rocks, zircon is a versatile and frequently used mineral in both detrital and bedrock thermochronology studies. Zircons with the same thermal history sometimes display significant irreproducibility in their He dates. In certain cases, zircon He dates are either positively or negatively correlated with a proxy for the degree of radiation damage in each zircon, effective uranium (eU, a single value for parent concentration that combines the weighted daughter contributions of U and Th). Much of my recent research has been focused on using computational models, laboratory techniques, and a mechanistic understanding of atomic-scale processes like tortuosity and percolation to explain these correlations. I have combined these methods to create a model that explains He diffusivity in zircon in the context of radiation damage (Guenthner et al., 2013, AJS). Specifically, the model accounts for damage accumulation, damage annealing, and He diffusion and, given each grain’s eU concentration and a common time-temperature (t-T) path, can be used in a forward and inverse sense to constrain thermal histories. The model is providing fresh geologic insights for places such as the Longmen Shan (Guenthner et al., 2014, EPSL), and the Sevier belt in Utah (Guenthner et al., 2015, GSA Bulletin), and provides thermochronologists in general with a tool to interpret zircon He dates that were previously considered to be spurious results.

utahglacier

Exhumation Studies in Active and Ancient Orogenic Belts

Exhumation in the Sevier/Laramide Orogenies of the Western US: My co-authors and I just completed a study published in GSA Bulletin that focused on the Charleston-Nebo Salient (CNS) of central Utah (centered near Provo). Our CNS study showed complex zircon He results, which we were able to interpret with a new approach that combines radiation damage and He inheritance effects. I’m also part of a project led by Devon Orme (Arizona) using zircon (U-Th)/He dates to constrain the exhumation of Laramide ranges in Wyoming. Finally, Dave Pearson (Idaho State) and I are working on a new project to describe the long-term exhumation in the central and eastern Idaho fold-and-thrust belt and how it might relate to timing of earliest deformation in this portion of the Sevier belt, and the growth of major tectonomorphic provinces such as the Nevadaplano. Stay tuned for more details!

-Exhumation in the Antarctic Peninsula in Response to a Slab Window: As part of my tenure with the illustrious Team Barbeau, I collected a series of samples from the Antarctic Peninsula for apatite and zircon (U-Th)/He and fission track analysis (Guenthner et al., 2010). This study provided detailed evidence for major pulses of exhumation that coincided with the progressive opening of a slab window. Through the use of inverse modeling, I was able to quantify the degree to which a developing slab window influences the upper plate’s geothermal gradient and dynamic uplift history.

antarctica