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 and n = 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.
Original language | English (US) |
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Article number | e2021TC006744 |
Journal | Tectonics |
Volume | 40 |
Issue number | 11 |
DOIs | |
State | Published - Nov 2021 |
Keywords
- HeFTy
- Laramide tectonics
- great unconformity
- thermochronology
- zircon (U-Th)/He
ASJC Scopus subject areas
- Geophysics
- Geochemistry and Petrology