151-3 U-Pb Geochronology of Syn-Kinematic Calcite from the Sawtooth Mountains, Montana – Implications for the Temporal and Kinematic History of the NW Montana Disturbed Belt

Session: Chronology of Orogenesis: Unlocking the Timelines of Mountain Building (Posters)

Presenting Author:

Authors:

Abstract:

Progression of deformation in fold and thrust belts is commonly constrained through correlation with foreland basin deposition. While this approach provides indirect timing constraints, it lacks the resolution required to discern the precise timing and sequence of thrusting events. In the Disturbed Belt of Montana, broad temporal constraints have traditionally been provided by relating thrust related deformation to both the foreland basin stratigraphy and cross-cutting igneous intrusions. The Sawtooth Mountains of Northwestern Montana are a world class example of an imbricate stack of early Paleozoic carbonate thrust sheets, which are thought to have formed largely by in-sequence thrusting. Further North, the Lewis Range in the Glacier National Park area is home to the dramatic Lewis Thrust that places Mesoproterozoic rocks over Mesozoic foreland basin strata. Despite their prominence as classic examples of thrust faulting, the timing of displacement along the Lewis Thrust and the imbricate stacking in the Sawtooth Mountains remain ambiguous. For this study, we systematically documented brittle structures associated with thrust fault emplacement and sampled syn-kinematic calcite for U-Pb geochronology. We targeted calcite slickenfibers on major and subsidiary thrust faults, recrystallized calcite in fault cataclasites, and calcite-filled tension gashes tied to fault-bend folding in the Sun Canyon and Glacier NP areas to directly constrain fault activation and refine regional kinematic models, thrust imbrication, out-of-sequence motion, and Cenzoic extensional overprinting. Despite analytical challenges posed by low U content and incorporation of common Pb, preliminary U-Pb data yielded promising results that constrain Late Cretaceous to Paleocene contractional deformation and Cenozoic extensional overprinting, suggesting a more complex kinematic history. Field relationships and these early geochronologic results hint at episodes of out-of-sequence thrusting and significant Eocene and younger extension. By integrating calcite U-Pb dating with micro and macro-structural analysis, this research is refining the tectonic evolution of Montana’s Disturbed Belt, offering insights into the temporal dynamics of thrust belt formation. This study illustrates the overall power of an integrated structural geochronology approach applied to brittle deformation to elucidate the temporal and kinematic evolution of carbonate-dominated fold and thrust belts and the potential to understand fundamental linkages between progressive structural thrust belt evolution and foreland basin deposition and basin architecture.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10206

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.