LATE CRETACEOUS EXHUMATION OF THE LITTLE BELT MOUNTAINS IN RELATION TO REGIONAL DEVELOPMENT OF THE HELENA SALIENT, WEST-CENTRAL MONTANA, USA

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. The Little Belt Mountains (LBM) are an intraforeland uplift that is juxtaposed with the eastern edge of the thrust belt and situated along the northern Belt Supergroup basin margin within the Helena Embayment. The conjunction of these tectonic elements makes the LBM an ideal setting for assessing the timing of different tectonic events and the relative contribution of each structural element on Phanerozoic tectonic development. In this study, we present eight samples of Paleoproterozoic basement that were analyzed for apatite fission track (AFT) and zircon (U-Th)/He (ZHe) thermochronology. Four samples produced AFT ages ranging from ca. 80 to 73 Ma and associated long, unimodal confined track lengths, consistent with rapid cooling and exhumation of basement rocks in the Late Cretaceous. The other four samples are characterized by younger AFT ages (ca. 55 Ma), representing a combination of prolonged residence in the apatite partial annealing zone and post-exhumation magmatic reheating. A total of twenty new ZHe dates range from ca. 28 to 236 Ma and show a correlation between ZHe date and effective Uranium, enabling an investigation of the “deep-time” thermal history of the LBM basement. Cross sections across the LBM display the geometry of the Volcano Valley Fault Zone, a down-to-the-south Proterozoic normal fault that was reactivated as an oblique reverse fault during development of the Cordilleran thrust belt. Our new constraints from the LBM when compared to a synthesis of existing kinematic constraints from the Helena salient reveal significant out-of-sequence deformation in this portion of the thrust belt between ca. 80-55 Ma. A kinematic model is proposed that involves a Late Cretaceous (ca. 80 Ma) exploitation of incompetent units at the base of the Belt supergroup within the Helena Embayment, facilitating exhumation in the LBM. Our data and synthesis are consistent with previous interpretations that an inherited stratigraphic and structural architecture of Proterozoic ancestry was the predominant control on the development of the Helena salient in Cretaceous-early Eocene time.