Tectonic evolution of Kevin Dome Northwestern Montana, evidence from new Apatite thermochronology (AHe) results.
Abstract
Kevin Dome is a 320-km long, north plunging structural culmination along the Sweetgrass Arch in Northwest Montana that hosts an estimated 85 bcm natural CO2 gas. We present preliminary apatite U-Th/He data bearing on the timing of exhumation and thermal evolution of the dome. This new study will provide a framework for understanding the relationship between the structural evolution of Kevin Dome and emplacement of the CO2 resource. Here, we test two competing tectonics and structural evolution models for Kevin Dome evolution namely (1) differential lithospheric upwarp related to isostatic equilibrium and lithospheric stress relaxation between an ancestral Rocky Mountain basin on the west and the Williston Basin to the east, or (2) an evolved forebulge related to Late Cretaceous-Paleocene emplacement of the Rocky Mountain overthrust. These hypotheses were based mainly on evidence of strata distribution, erosion or non-deposition but are now constrained with Low-temperature thermochronology, integrating U-Th/He and Fission-track dating of apatite and zircons thermochronometers. Preliminary U-Th/He cooling ages of apatite recovered from drill cuttings including the Precambrian basement, Three Forks, Bakken, Kootenai and Bow Island strata at depths of 400-1430 m fall into two distinct populations: (1) Paleogene, 68 - 25 Ma (Late Cretaceous - Oligocene), and (2) Neogene, 10 - 23 Ma. The data clearly show that all units sampled reached temperatures above 70° C (apatite closure T) and cooled during the Tertiary, implying up to 3-km of overburden have been removed since the Late Cretaceous. Furthermore, the Paleogene cooling histories would be consistent with (1) the forebulge dome evolution model related to Late Cretaceous-Paleocene emplacement of the Rocky Mountain overthrust, (2) post Eocene uplifts related to late Laramide orogeny model, (3) evidence of uniform eastward tilting of the proximal Sweetgrass Hills, Bearpaw Uplift, Little Rockies Mountains and Highwood Mountains similar to the Sweetgrass Arch during Paleocene to early Eocene. The Neogene cooling ages might also indicate a structural offset on the Sweetgrass Arch anticline ancestral hinge by the Pendroy Fault during Oligocene-Miocene (or younger ages) which moved a bulge on the arch's crest (~48-km) eastward to form the present-day Kevin Dome.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T41G0332A
- Keywords:
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- 8125 Evolution of the Earth;
- TECTONOPHYSICS;
- 8169 Sedimentary basin processes;
- TECTONOPHYSICS;
- 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS;
- 8177 Tectonics and climatic interactions;
- TECTONOPHYSICS