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Graduate Defense: Michael Mohr

March 10 @ 10:00 am - 12:00 pm MST

square graphic which says "Dissertation Defense"Dissertation Information

Title: A Tour of High-Precision U-Pb geochronology: Novel and Timely Methods, Applications, and Innovations for Super-eruption Petrochronology, Chronostratigraphy, and Linking Ancient Large Igneous Provinces

Program: Doctor of Philosophy in Geosciences

Advisor: Dr. Mark Schmitz, Geosciences

Committee Members: Dr. James Crowley, Geosciences, Dr. CJ Northrup, Geosciences, and Dr. Dorsey Wanless, Geosciences


The perpetual and persistent forward march of time is a geoscientist’s reference frame for understanding Earth’s natural changes that manifest in all scales of natural materials (e.g., minerals, fossils, glaciers, rocks, mountain belts, cratons). While indicators of relative timing can be abundant in natural materials (e.g., stratigraphic superposition, cross-cutting relationships, crystal growth zonation), numerical timing constraints are required to contextualize and constrain the ordering, duration, and rate of Earth system processes. For investigating Earth processes in deep time, radioisotopic ages provide a temporal foundation for deciphering the clues from an often fragmented and incomplete rock record. Among the radioisotope chronometers, high-precision U-Pb geochronology via isotope dilution thermal ionization mass spectrometry is the premier technique for obtaining accurate and precise radioisotopic ages and has exceptional utility for decoding the many complex puzzles of the Earth system.

In this dissertation, I present results and inferences from three different applications of high-precision U-Pb geochronology, which are outlined in the first chapter. The second chapter applies high-precision U-Pb geochronology in petrochronologic context (integrating petrologic and geochronologic data) to investigate the dichotomy between crystal-poor and crystal-rich rhyolites and the magma dynamics that occurred in the caldera magmatic system of the Miocene Superstition Volcanic Field (central Arizona) leading up to a super-eruption. The third chapter integrates high-precision U-Pb geochronology with stratigraphic data to refine the chronostratigraphic framework for an important rock archive of changes in terrestrial biota and ecosystems in the Pacific Northwest from the Eocene to Miocene, the John Day Formation. The fourth chapter showcases the abilities of high-precision U-Pb geochronology in deeper time (Mesoproterozoic) with an investigation of ancient (ca. 1.1 Ga) mafic magmatism preserved as voluminous sills in California and Arizona and the Cardenas basalts in Grand Canyon, which form the Southwestern Laurentia Large Igneous Province. The new precise age constraints for these mafic rocks result in a geodynamic hypothesis that could link two large igneous provinces across Laurentia. Altogether, these chapters demonstrate how high-precision U-Pb geochronology can elucidate a diversity of Earth processes that occur over a multitude of timescales.