Dissertation Defence: Determining the primary controls on in situ Rb-Sr mica geochronology at varying conditions and its applicability for determining the timing of shear zone movement and other tectonic processes

Kyle Tollefson, supervised by Dr. Kyle Larson, will defend their dissertation titled “Determining the primary controls on in situ Rb-Sr mica geochronology at varying conditions and its applicability for determining the timing of shear zone movement and other tectonic processes” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Earth and Environmental Sciences.

An abstract for Kyle Tollefson’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Registration is not required for in-person exams.

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Abstract

Geochronology is essential for determining the timing and rates of various geologic processes. The information recorded by chronometers largely depends on how temperature, deformation, and fluid activity influence the resulting dates. For some chronometers, the effects of these factors are well understood, allowing for straightforward interpretation (e.g., U-Pb zircon geochronology typically records crystallisation). Other chronometers, however, exhibit more complex behaviour and may require additional data, such as temperature, deformation, and fluid conditions, for coherent interpretations.

Recent advancements in instrumentation have enabled in situ Rb-Sr dating of mica, a common rock-forming mineral, making it a viable technique for quantifying the timing and rates of geological processes. This method is less time and cost-intensive than conventional mica geochronology. It is, however, relatively new, and as such, further research is needed to outline the details, limitations, and possibilities of the method.

The work presented herein investigates: 1) the applicability of in situ Rb-Sr geochronology for recording the timing of deformation, 2) how different shear zone conditions may influence the in situ Rb-Sr mica geochronology record, and 3) how P-T-t-D (pressure-temperature-time-deformation) paths can be informed by in situ Rb-Sr mica geochronology to develop more complete tectonic models.

Through combined in situ Rb-Sr geochronology and microstructural analysis of two shear zones, this study demonstrates that Rb-Sr dates can record multiple processes, including crystallization, exhumation/cooling, fluid flow, and deformation. Further analysis reveals that, in rocks deformed under greenschist facies conditions (<450 °C), deformation was the dominant control on the Rb-Sr dates. In contrast, rocks deformed under amphibolite facies conditions (>450 °C) yielded Rb-Sr dates primarily influenced by temperature and/or fluid activity, regardless of the finite deformation. Those findings informed interpretations of in situ Rb-Sr dates as part of a larger metamorphic petrologic and geochronologic study, and illustrated how they can be used to better inform P-T-t-D paths and contribute to the generation of robust tectonic models.