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A Thermal Profile across the Idaho-Montana Fold-Thrust Belt Reveals a Low-Relief Orogenic Wedge That Developed atop a Pre-Orogenic Basement High
Journal article   Open access   Peer reviewed

A Thermal Profile across the Idaho-Montana Fold-Thrust Belt Reveals a Low-Relief Orogenic Wedge That Developed atop a Pre-Orogenic Basement High

Stuart D. Parker, David M. Pearson and Emily S. Finzel
Lithosphere, Vol.2022(1), 9475780
11/01/2022
DOI: 10.2113/2022/9475780
url
https://doi.org/10.2113/2022/9475780View
Published (Version of record) Open Access

Abstract

Growing orogenic wedges cool rocks during exhumation of thrust hanging walls and heat them during burial of footwalls, leaving behind a resilient thermal record of earlier deformation in fold-thrust belts. In order to investigate early burial of deformed strata within the retroarc Idaho-Montana fold-thrust belt, we use Raman spectroscopy of carbonaceous material to construct a maximum temperature profile that constrains the thicknesses of eroded rocks structurally above the Lemhi arch, a pre -thrusting basement high. In the eastern portion of the study area, a sharp maximum temperature change of-120 degrees C occurs across the Johnson thrust, signifying that regional burial and heating predated late-stage faulting. West of here, cumulative exhumation is irregular, varying by up to 5 km over large (-75 km) wavelength folds; however, maximum temperatures in this same region are consistently-200 degrees C higher than correlative stratigraphic units in the adjacent foreland. The pre-thrusting, low-relief unconformity above the Lemhi arch, which served as the early decollement to the fold-thrust belt, was everywhere buried to at least-6.5 km depth, which is-1.5-5.0 km deeper than can be explained by stratigraphic burial. We hypothesize that between-145 and 80 Ma, a combination of Cretaceous deposition and folding and thrusting at higher structural levels buried the decollement of the Medicine Lodge-McKenzie thrust system to this depth. These results suggest that the early orogenic wedge had exceptionally low taper. We propose that thin strata over the low-relief Lemhi arch limited the availability of potential decollements, which restricted the maximum surface slope that could be constructed in a thin-skinned system. Subsequent growth of the orogenic wedge required activation of a much deeper decollement and a switch to a thick-skinned structural style, promoting a shift from burial to exhumation of the former decollement and the underlying Lemhi arch. This suggests that the growth of an orogenic wedge is dependent on the thicknesses of the preexisting strata and the availability of potential decollements, with sedimentation and burial heating potentially playing a key role.
 Geology Physical Sciences Geochemistry & Geophysics Science & Technology

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