Regional thermal history trends from the Idaho-Montana fold thrust belt using multiple low-T thermochronometers

Low-temperature thermochronometric data can reveal the long-term evolution of erosion, uplift, and thrusting in fold-thrust-belts. We present results from central-Idaho and southwestern-Montana, where the close spatial overlap of the Sevier fold-thrust-belt and Laramide-style, basement-involved foreland uplifts signify a complex region with an unresolved, long-term tectono-thermal history. Inverse QTQt thermal-history modeling of new zircon (U-Th)/He (ZHe, n=106), and apatite (U-Th)/He dates (AHe, n=43) collected from hanging walls of major thrusts systems along a central-Idaho to southwestern-Montana transect, and apatite fission track (AFT) results from 6 basement samples, reveal regional thermal and spatial trends related to Sevier and Laramide orogenesis. Inverse modeling of foreland basement uplift samples suggest Phanerozoic exhumation initiated as early as ~80 Ma and continued through the early-Paleogene. Inverse modeling of interior Idaho fold-thrust-belt ZHe samples documents mid-Cretaceous cooling at ~125 Ma in the Lost River Range (western transect), and a younger cooling episode in the Lemhi Arch region (mid-transect) from as early as ~100 Ma through the late-Paleogene. This cooling in the Lemhi Arch temporally overlaps with cooling in southwestern-Montana’s basement-cored uplifts, which we interpret as roughly synchronous exhumation related to contractional tectonics and post-orogenic collapse. These data and models, integrated with independent timing constraints from foreland basin strata and previously published thermochronometric results, suggests that mid-Cretaceous deformation of southwestern-Montana’s basement-cored uplifts was low-magnitude and preceded tectonism along the classic California-Wyoming Laramide “corridor”. In contrast, late-Cretaceous and Paleogene thrust-related exhumation was more significant and largely complete by the Eocene. The basement-involved deformation was contemporaneous with and younger than along-strike Sevier belt thrusting in central-Idaho.