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Late Cretaceous exhumation of the Little Belt Mountains and regional development of the Helena salient, west-central Montana, USA

Caden J. Howlett, Gilby Jepson, Barbara Carrapa, Peter G. DeCelles and Kurt N. Constenius
Late Cretaceous exhumation of the Little Belt Mountains and regional development of the Helena salient, west-central Montana, USA
Geological Society of America Bulletin (September 2023) Pre-Issue Publication

Abstract

The timing of deformation within and adjacent to the Helena salient of west-central Montana is poorly constrained relative to other segments of the Sevier fold-and-thrust belt. This study presents low-temperature thermochronology data from the Little Belt Mountains, a basement-cored Laramide uplift that is juxtaposed with the leading edge of the salient. We analyzed eight samples of Paleoproterozoic basement for apatite fission-track (AFT) and zircon (U-Th)/He (ZHe) thermochronology. Four samples yielded AFT ages ranging from ca. 80 Ma to 73 Ma and associated long, unimodal confined track lengths, indicating rapid cooling and exhumation of Little Belt Mountains basement during the Late Cretaceous. The other four samples are characterized by younger AFT ages (ca. 55 Ma), which suggest a combination of prolonged residence in the apatite partial annealing zone and postexhumation magmatic reheating. In total, 20 new ZHe dates range from ca. 236 Ma to 28 Ma and show a correlation between date and effective uranium. Forward model results for ZHe data are consistent with upper-crustal residence during the Proterozoic followed by Phanerozoic burial and rapid Late Cretaceous cooling. Cross sections across the Little Belt Mountains display the geometry of the Volcano Valley fault zone, an array of down-to-the-south Proterozoic normal faults that profoundly influenced the development of the Cordilleran thrust belt. Our new constraints from the Little Belt Mountains when integrated with published kinematic constraints from the Helena salient reveal significant out-of-sequence deformation in this portion of the thrust belt between ca. 80 Ma and 55 Ma. A kinematic model is proposed that involves Late Cretaceous (ca. 80 Ma) exploitation of rheologically incompetent units at the base of the Belt Supergroup within the Helena Embayment, facilitating early exhumation in the Little Belt Mountains. Our new data and synthesis are consistent with previous interpretations in which an inherited stratigraphic and structural architecture of Proterozoic ancestry was the predominant control on the development of the Helena salient during Cretaceous-early Eocene time.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: Pre-Issue Publication
Title: Late Cretaceous exhumation of the Little Belt Mountains and regional development of the Helena salient, west-central Montana, USA
Affiliation: University of Arizona, Department of Geosciences, Tucson, AZ, United States
Published: 20230929
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 198
Accession Number: 2023-071666
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., 2 tables, geol. sketch map
N46°34'60" - N46°34'60", W112°00'00" - W112°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2023, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 2023
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