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Claire Harrigan Dissertation Defense

June 6 @ 3:00 pm - 4:00 pm MDT

Claire Harrigan

Petrochronology and statistical analysis to integrate different types of data to solve complex Earth systems problems

Abstract: Complex Earth systems problems, like reconstructing orogens and calibrating the geologic time scale, require investigations that link time to geologic processes. To use time as a means of organizing geologic evidence, geochronometric dates must be contextualized by integrating with different data types. This is the work of petrochronology—linking mineral ages to geochemical, textural, or other geologic information. The U-Pb isotopic system as preserved in the minerals zircon (ZrSiO4) and titanite (CaTiSiO5) can be used in a petrochronological context to date geologic events including the age of granitoid pluton emplacement, the age of rock fabric formation in deformed granitoids, and the age of volcanic ash beds.

One focus of my dissertation was to use petrochronology to investigate high-temperature crustal strain partitioning and localization on the micro- to macro-scale using the western Idaho shear zone (WISZ), west-central Idaho. The WISZ is a crustal-scale structure that localized arc magmatic process and deformation related to terrane accretion and translation along the North American Cordillera. I used a WISZ orthogneiss to examine how fabric develops during high temperature deformation on the micro-scale. By integrating the geochronometric, geochemical, and microstructual titanite record using statistical and petrologically-relevant visualizations, I document the local preservation of titanite related to magmatic and subsolidus processes. Importantly, this petrochronological workflow results in a date for the onset of deformation in the WISZ, confirming tectonic interpretations of WISZ deformation as a separate event from local terrane suturing.

I expanded this work to the macro-scale with a suite of samples that transect the WISZ near McCall, ID to track the spatial-temporal patterns of pluton emplacement and deformation. My tandem zircon and titanite petrochronology results show that 1) the propensity of titanite to (re)crystallize in response to changes in pressure and temperature makes titanite petrochronology a useful approach for documenting subtle subsolidus fabric development, 2) strain localizes in time and space in response to local intrusions, and 3) WISZ fabric development is observed further east than previously mapped, causing the model of a west-to-east younging of pluton emplacement and deformation to be updated in favor of a model in which deformation focuses young magmatism within the center of the shear zone.

In a second focus of my dissertation work, I integrated geochronology and statistical modeling to recalibrate and refine the Devonian time scale. The Devonian is a period in Earth history with significant biologic, climatic, and tectonic events. I dated Devonian ash beds using high-precision zircon geochronology and used those dates with a Bayesian age-depth model as the statistical framework to relate geochronometric and astrochronologic data to biostratigraphic data. I produced an updated Devonian time scale with new stage boundary ages with robust uncertainty estimates. This integrated stratigraphic approach is broadly applicable to time scale modeling.

This work is united under a theme of using petrochronology and statistical modeling to link time to geologic processes including magmatism, deformation, and stratigraphic accumulation. Time constraints on the initiation and duration of geologic processes can deepen our understanding of the evolution of complex Earth systems.

Advisor: Mark Schmitz

Co-Advisors: Dorsey Wanless, CJ Northrup, Jim Crowley

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