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Abstract Crystalline basement rocks of southwestern Montana have been subjected to multiple tectonothermal events since ∼3.3 Ga: the Paleoproterozoic Big Sky/Great Falls orogeny, Mesoproterozoic extension associated with Belt‐Purcell basin formation, Neoproterozoic extension related to Rodinia rifting, and the late Phanerozoic Sevier‐Laramide orogeny. We investigated the long‐term (>1 Ga), low‐temperature (erosion/burial within 10 km of the surface) thermal histories of these tectonic events with zircon and apatite (U‐Th)/He thermochronology. Data were collected across nine sample localities (n = 55 zircon andn = 26 apatite aliquots) in the northern and southern Madison ranges, the Blacktail‐Snowcrest arch, and the Tobacco Root uplift. Our zircon (U‐Th)/He data show negative trends between single aliquot date and effective uranium (a radiation damage proxy), which we interpreted with a thermal history model that considers the damage‐He diffusivity relationship in zircon. Our model results for these basement ranges show substantial cooling from temperatures above 400°C to near surface conditions between 800 and 510 Ma. Subsequent Phanerozoic exhumation culminated by ∼75 Ma. Late Phanerozoic cooling is coincident with along‐strike Sevier belt thin‐skinned thrusting in southeastern Idaho, and older than exhumation in basement‐involved uplifts of the Wyoming Laramide province. Our long‐term, low‐temperature thermal record for these southwestern Montana basement ranges shows that: (a) these basement blocks have experienced multiple episodes of upper crustal exhumation and burial since Archean time, possibly influencing Phanerozoic thrust architecture and (b) the late Phanerozoic thick‐skinned thrusting recorded by these rocks is among the earliest thermochronologic records of Laramide basement‐involved shortening and was concomitant with Sevier belt thin‐skinned thrusting.
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Pre‐Thrusting Stratigraphic Control on the Transition From a Thin‐Skinned to Thick‐Skinned Structural Style: An Example From the Double‐Decker Idaho‐Montana Fold‐Thrust Belt
Abstract Continental fold‐thrust belts display a variety of structural styles, ranging from thin‐skinned thrusts following weak lithologic contacts to thick‐skinned thrusts that deform mechanical basement. The common practice of splitting fold‐thrust belts into thin‐skinned and thick‐skinned map domains has not yielded a predictive model of the primary controls on structural style. Within the Mesozoic‐Paleogene Idaho‐Montana fold‐thrust belt (44°N‐45°N, 112°W‐114°W), we identify crosscutting thin‐skinned and thick‐skinned thrusts within an otherwise thin‐skinned map domain. This transition occurs within a thin (∼2.5 km) portion of the western Laurentian passive margin, where lower strata pinch out over a prominent basement high (Lemhi arch). Early fold‐thrust belt shortening of sedimentary cover rocks was accommodated through detachment folding, followed by east‐directed, thin‐skinned thrusting along regional‐scale faults (Thompson Gulch and Railroad Canyon thrusts). Later, basement and cover rocks were tilted toward the southeast and a basement‐involved normal fault was reactivated during thick‐skinned thrusting (Radio Tower‐Baby Joe Gulch‐Italian Gulch thrusts), which accommodated shortening at an oblique angle to and truncated the basal detachment of the older thin‐skinned thrusts. This progression from thin‐skinned to thick‐skinned thrusting occurred >50 km from the foreland, coincident with a regional basement high. Thus, the Idaho‐Montana fold‐thrust belt is a double‐decker system, with upper thin‐skinned and lower thick‐skinned domains. This double‐decker model is applicable to other fold‐thrust belts and predicts that the transition from thin‐skinned to thick‐skinned thrusting occurs where the growing critically tapered wedge can no longer fit within the sedimentary cover rocks and the basal detachment steps down into the structurally lower mechanical basement.
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- Award ID(s):
- 1728563
- PAR ID:
- 10449136
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Tectonics
- Volume:
- 40
- Issue:
- 5
- ISSN:
- 0278-7407
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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