Multi-Kinetic Apatite Fission Track Thermochronology Applied to Resolving Thermal Histories of Sedimentary Basins and Crystalline Bedrock

Abstract

Understanding mountain building processes requires insight into the mechanisms and timing of exhumation. The integration of innovative radiometric dating techniques with field-based structural observations has significantly improved our ability to constrain the rates and timescales at which different exhumation processes operate.

This thesis investigates exhumation histories in two tectonically contrasting orogens: the Peel Plateau, adjacent to the Mackenzie Mountains which are actively undergoing crustal shortening in the Canadian Cordillera, and the Tauern Window in the Eastern Alps, where regional strain evolved from N-S compression to E-W extension during the Cenozoic. Apatite-fission track (AFT) dates are traditionally interpreted as cooling ages. For AFT dates resolved from samples that have undergone protracted cooling or comprise of apatite grains with variable annealing kinetics the ages resolved can instead represent a partial annealing date or reflect mixed age-populations and may not have be geologically significant. Herein, apatite grain-specific chemistry was used to calculate the kinetic proxy rₘᵣ₀, and was incorporated into thermal history models to test whether this innovative approach could reveal new information regarding the study area's upper crustal exhumation histories. Additional work in the Tauern Window documented an earlier stage of exhumation, from middle to upper crustal levels, which focussed on detailed structural analysis and deformation dates resolved via white mica ⁴⁰Ar/³⁹Ar geochronology.

Overall, results from the Peel Plateau suggest that although erosion remains the dominant unroofing mechanism, spatially, the localized cooling can be explained by the reactivation of underlying structures. In the Tauern Window, results suggest exhumation began much earlier than previously proposed and was facilitated by tectonic unroofing along a newly discovered detachment system. Upper crustal exhumation in the core of the Tauern Window is mostly driven by erosion, whereas results from deeper structural units record an earlier onset of cooling which may be associated to tectonic unroofing. This work demonstrates that rₘᵣ₀ captures the variability of apatite's annealing kinetics within AFT samples, and incorporating this kinetic proxy into thermal models produces more refined results.

Collectively, this research exemplifies the complex interplay between different exhumation processes and underscores the value of integrating advanced radiometric techniques with detailed structural analysis.