61-2 Geo/Thermochronologic Records of Late Stage Mountain Building, Basin Accumulation, Magmatism, and Foreland Exhumation in The Crazy Mountains, Montana

Abstract:

Paleogene sedimentary and igneous rocks preserved within the Crazy Mountains of Montana provide a unique record of the final stages of Cordilleran Mountain building, foreland basin deposition, magmatism, and subsequent landscape evolution. These preserved foreland basin deposits, comprised of both marine and nonmarine strata, are currently exposed at elevations of ~1.5–3 km and are the youngest in the North American Cordillera. Their preservation is owed to contact metamorphism associated with emplacement of the Eocene Big Timber Stock. Constraining the timing and rates of sedimentation, basin exhumation, and topographic development is essential for (1) reconstructing the transition from active orogenesis to orogenic collapse and (2) understanding the processes driving topographic evolution. This work aims to clarify the regional history of sedimentary burial, magmatic heating, and post-intrusion exhumation with low temperature thermochronology and inverse thermal history modeling. We present apatite (U-Th)/He (AHe) and apatite fission-track (AFT) thermochronology from 13 sedimentary and igneous foreland samples. Comparing ages from multiple thermochronometer systems helps to distinguish short-lived thermal perturbations associated with Big Timber Stock emplacement from longer-term heating and cooling driven by sedimentary burial, regional uplift, and exhumation. AFT ages from Paleocene (~66-56 Ma) Fort Union Formation sedimentary rocks that are generally younger than depositional ages, whereas AFT ages from the Big Timber Stock overlap its ~49–51 Ma emplacement age within analytical uncertainty. In contrast, AHe ages from the stock are younger than corresponding AFT ages and exhibit spatial patterns consistent with erosion-driven exhumation following cessation of magmatism. Thermal history models of the Fort Union Formation AFT data indicate peak post-depositional temperatures below 120°C, consistent with limited heating from sedimentary burial and/or emplacement of the Big Timber Stock. Models incorporating AFT and AHe data from the stock suggest emplacement at depths shallower than the AFT closure temperature, followed by relatively rapid cooling from ~51 to 42 Ma and slower cooling to present-day conditions. These preliminary results constrain the extent and duration of magmatic heating, provide a framework for quantifying burial and exhumation, and highlight the need to jointly evaluate magmatic and erosional processes when interpreting low-temperature thermochronology data.

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