THERMAL HISTORY OF THE CRAZY MOUNTAINS IN THE SOUTH-CENTRAL MONTANA FORELAND: INSIGHTS FROM INVERSE THERMAL HISTORY MODELING OF APATITE (U-TH)/HE AND FISSION TRACK DATA

The Crazy Mountains expose igneous, sedimentary, and contact metamorphosed rocks of the foreland basin of south-central Montana. The range contains some of the highest elevations and greatest topographic relief of this segment of the North American Cordillera, yet is located east (craton-ward) of the fold-thrust belt and has not experienced significant uplift due to shortening and crustal thickening. We present new low-temperature thermochronometry data and inverse thermal history modeling results to quantify the timing, magnitude, and rates of heating and cooling and interrogate the mechanisms of uplift and relief development. We specifically exploit the apatite (U-Th)/He (AHe) and fission track (AFT) systems, which record mineral cooling below ~40–80ºC and ~60–120ºC, respectively. AFT results for sedimentary rocks of the Paleocene (60–64 Ma) Ft. Union Formation are generally younger than sample depositional age and show a positive cooling age-elevation trend, with the oldest AFT ages at highest elevations and the youngest ages from well samples collected from ~1800 m depth. AFT results from the Eocene Big Timber Stock are within error of the 49–51 Ma emplacement age, while AHe data are younger than AFT and geochronological dates obtained for the same samples and define another positive age-elevation relationship. Inverse thermal history models based on the Ft. Union Formation AFT data confirm that our sample locations did not experience significant (<140ºC) heating due to stock emplacement or sedimentary burial. Models based on the Big Timber Stock AFT and AHe data further indicate that stock samples were emplaced at depths shallower than the AFT closure threshold and possibly experienced a two-stage thermal history involving initial, relatively rapid cooling from 51 to ~42 Ma at rates of 24–42ºC/Myr, followed by protracted cooling at rates of 0.13–1.20ºC/Myr to present day. Late-stage cooling rates for the Big Timber Stock are consistent with rates for a similar time interval calculated for the Ft. Union Formation samples and are attributed to post-tectonic erosion and landscape evolution.