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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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Evidence for the Neoproterozoic Rifting of Rodinia in the Rocky Mountain Front Range
Abstract Recent advances in low‐temperature thermochronology enable the recovery of deep‐time thermal histories from Precambrian crystalline rocks shaped by multiple tectonic events, offering unprecedented opportunities to test tectonic hypotheses and links to significant biologic and climatic episodes. In particular, the late Neoproterozoic breakup of supercontinent Rodinia profoundly shaped the western margin of Laurentia, leaving a geologic record along the Cordilleran hingeline that temporally associates continental rifting with biological change at the Ediacaran‐Cambrian transition and may explain the unusual eastern extent of the Laramide orogeny. However, sedimentary evidence east of the Cordilleran hingeline is lacking, leaving postulated links untested. Here we interpret Neoproterozoic to recent tectonic histories from the Colorado Front Range using thermal history modeling of zircon (U‐Th)/He (ZHe) ages (50–607 Ma), which vary with grain U‐Th composition. These models are constrained by geologic records that place basement rocks near Earth's surface at ca. 700, 500, and 300 Ma, and they resolve late Neoproterozoic heating to 240–285°C followed by cooling. Sensitivity tests confirm this heating signal depends on fitting Mesoproterozoic40Ar/39Ar ages and a ZHe data set that includes high‐U‐Th grains with reproducible 61 ± 7.5 Ma ages that correspond to Colorado Mineral Belt magmatism and Laramide exhumation. We interpret the Neoproterozoic heating as direct evidence that intracontinental rifting in the Front Range region drove kilometer‐scale burial coeval with global glaciation and the fragmentation of Rodinia. The magnitude and duration of reheating are well constrained, but resolving subsequent cooling during Neoproterozoic‐Paleozoic time strongly depends on surface constraints from the geologic record.
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- Award ID(s):
- 2140480
- PAR ID:
- 10567743
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Tectonics
- Volume:
- 44
- Issue:
- 1
- ISSN:
- 0278-7407
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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