70-4 Examining the timing, magnitude, and mechanisms of rock exhumation and Earth surface deflection in the Montana fold-thrust belt and foreland basin

Abstract:

The Cordillera of Montana and southern Canada represents one of the best-studied orogenic systems on Earth. Uplift and subsidence in this region have been primarily attributed to fold-thrust belt shortening, associated crustal thickening, and topographic development; and flexural subsidence in the adjacent foreland basin. However, the undeformed foreland has undergone >1 km of surface uplift and ~2–5 km of erosion and rock exhumation, as recorded by Upper Cretaceous marine strata now at ~1–1.5 km elevations and peak temperature data from basin stratigraphy. These patterns of rock exhumation and lithospheric deflection cannot be explained by supracrustal deformation or eustasy, and require the influence of deeper crustal, subducted slab-related, or mantle dynamic processes during evolution of the fold-thrust belt and foreland basin. We present new low-temperature thermochronology and peak temperature data spanning the Montana hinterland, fold-thrust belt (Sawtooth Range) and proximal-distal foreland basin, interpreted alongside preliminary flexural thermokinematic models, to 1) quantify the timing and magnitude of supracrustal deformation, structural/sedimentary burial, exhumation, and inferred topographic uplift, and 2) evaluate potential drivers of deep deflection. Apatite fission track (AFT) and apatite and zircon (U-Th)/He analyses (AHe, ZHe) yield partially to fully reset dates and constrain Late Cretaceous–Miocene burial heating and exhumation-driven cooling. Fully reset AFT and ZHe dates align with independent constraints on the timing of thrust motion in the Sawtooth Range, and require post-depositional heating to at least ~100–140°C or ~4–6 km depth (given a ~20–25°C/km geothermal gradient). These findings are consistent with flexural thermokinematic models that predict rocks currently exposed in the thrust belt were buried beneath >5–8 km of “phantom,” proximal foredeep deposits and the southernmost extent of the Lewis thrust sheet prior to exhumation. AHe dates are fully reset across the study area and span ~60–20 Ma, with the youngest dates (<40–20 Ma) collocated within the frontal fold-thrust belt and proximal foreland basin. The magnitude, distribution, and spatial progression of overburden removal indicated by these results reinforces thermal maturity trends defined by vitrinite reflectance data, and suggest that, while uplift and exhumation of the foreland may have initiated in response to subducted slab rollback, slab window subduction, and associated mantle convection (~55 to 40 Ma), foreland deflection continued to <20 Ma and potentially present day.

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