Early Inception of the Laramide Orogeny in Southwestern Montana and Northern Wyoming: Implications for Models of Flat‐Slab Subduction

Abstract

Timing and distribution of magmatism, deformation, exhumation, and basin development have been used to reconstruct the history of Laramide flat‐slab subduction under North America during Late Cretaceous‐early Cenozoic time. Existing geodynamic models, however, ignore a large (~40,000‐km2) sector of the Laramide foreland in southwestern Montana. The Montana Laramide ranges consist of Archean basement arches (fault‐propagation folds) that were elevated by thrust and reverse faults. We present new thermochronological and geochronological data from six Laramide ranges in southwestern Montana (the Beartooth, Gravelly, Ruby and Madison Ranges, and the Tobacco Root and Highland Mountains) that show significant cooling and exhumation during the Early to mid‐Cretaceous, much earlier than the record of Laramide exhumation in Wyoming. These data suggest that Laramide‐style deformation‐driven exhumation slightly predates the eastward sweep of magmatism in western Montana, consistent with geodynamic models involving initial strain propagation into North American cratonic rocks due to stresses associated with a northeastward expanding region of flat‐slab subduction. Our results also indicate various degrees of Cenozoic heating and cooling possibly associated with westward rollback of the subducting Farallon slab, followed by Basin‐and‐Range extension.

Orogenic Recycling of Detrital Zircons Characterizes Age Distributions of North American Cordilleran Strata

“…Upper Cretaceous foreland basin deposits record initiation of the northern Rocky Mountains arc (Figure a), regression of the Western Interior Seaway from western Montana (DeCelles, ), continued regional uplift of Laramide intraforeland structures (Carrapa et al, ), and subsidence of Laramide intraforeland basins, as well as a transition to localized sediment derivation from the Sevier‐Laramide orogen (Figure f). The northern Rocky Mountains arc (today represented by the Idaho‐Boulder batholith complex) and fold‐thrust belt respectively sourced abundant first‐ and poly‐cycle DZ to the foreland basin (e.g., Scarberry & Yakoklev, ).…”

“…The northern Rocky Mountains arc (today represented by the Idaho‐Boulder batholith complex) and fold‐thrust belt respectively sourced abundant first‐ and poly‐cycle DZ to the foreland basin (e.g., Scarberry & Yakoklev, ). Continued Maastrichtian‐Paleocene emergence of Laramide intraforeland structures (Carrapa et al, ) locally unroofed North American basement rocks as sources of first‐cycle DZ, recorded by the presence of metamorphic clasts in synorogenic conglomerates (Haley & Perry, ; Perry et al, ).…”

“…This sector of the Cordillera is unique in that much of the modern landscape is inherited from the latest Cretaceous to the Paleogene, with the current distribution of mountainous areas and valleys closely paralleling Paleogene topography (Carrapa et al, ; Schwartz & Schwartz, ). Paleocurrent and modal sediment provenance data from Paleogene deposits of southwestern Montana and adjacent regions (the Renova Formation and equivalents, ~48–19 Ma) further reveal that Paleogene highlands were composed of rock types similar to those exposed today.…”

A multi-proxy provenance approach and depositional age constraints for the Upper Cretaceous Beaverhead Group, Southwest Montana

“…The mechanism behind flat-slab subduction is still heavily debated, but it is most typically attributed either to the subduction of an oceanic plateau or aseismic ridge (e.g. Livacarri et al, 1981;Henderson et al, 1984;Saleeby, 2003;Liu et al, 2010), or varying interactions between the upper and lower plates based on local plate geometries and changes in plate kinematics (Bird, 1998;Engebretson et al, 1985;Jones et al, 2011), with the former providing the best and most widely supported models (Carrapa et al, 2019). A record of flat-slab subduction during the Laramide orogeny is absent within the study area in southwest Montana because it is outside of the predicted flat-slab trajectory, albeit adjacent to the well-studied "classic" Laramide region that falls within the trajectory.…”

“…DeCelles (1986) argued that the Laramide-style uplifts were exhuming early (~100 Ma) due to sedimentological observations made within the Kootenai Formation. Furthermore, recent but limited thermochronological data from various Laramidestyle uplifts in southwest Montana suggests that exhumation began as early as ~100 Ma, which would predate exhumation of uplifts within the well-defined flat-slab corridor (Carrapa et al, 2019).…”