Regional Thermal History Trends From the Idaho-Montana Fold-Thrust Belt Using Multiple Low-T Thermochronometers

Abstract

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 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 Early Cretaceous cooling at ∼125 Ma in the Lost River Range (western transect), and a younger cooling episode in the Lemhi Arch region (mid-transect) at ∼90–80 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 middle Cretaceous deformation of southwestern Montana's basement-cored uplifts was low magnitude and preceded tectonism along the classic Arizona-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.