255-1 Integrated Multi-System Thermochronology Resolves the Deep-Time Evolution of the Great Unconformity Surface in the Upper Midwest, USA

Session: Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record

Presenting Author:

William Guenthner

Authors:

Sigat, Ryan O. A.¹, Guenthner, William R.², McDannell, Kalin T.³, Keller, C. Brenhin⁴, Zeitler, Peter K.⁵, Orme, Devon A.⁶, Marshak, Stephen⁷

(1) Department of Earth Science & Environmental Change, University of Illinois Urbana-Champaign, Urbana, IL, USA, (2) Department of Earth Science & Environmental Change, University of Illinois Urbana-Champaign, Urbana, IL, USA, (3) Department of Earth Sciences, Dartmouth College, Hanover, NH, USA, (4) Department of Earth Sciences, Dartmouth College, Hanover, NH, USA, (5) Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA, USA, (6) Department of Earth Sciences, Montana State University, Bozeman, MT, USA, (7) Department of Earth Science & Environmental Change, University of Illinois Urbana-Champaign, Urbana, IL, USA,

Abstract:

Deep-time thermochronology enables the reconstruction of thermal histories spanning over a billion years in Precambrian rocks, offering insights into mid- to upper-crustal geological processes occurring at temperatures below 350 °C. This approach has been used to infer the origin of the Great Unconformity surface, which separates lowermost Paleozoic strata from underlying Precambrian basement. Within the US Upper Midwest, exposures of Precambrian basement rocks present an opportunity to unravel the poorly understood Mesoproterozoic–Phanerozoic thermal history of the cratonic interior region. We present medium- to low-temperature thermochronometric datasets from Precambrian basement samples across the Upper Midwest USA, specifically in Wisconsin and the Upper Peninsula of Michigan. Our data include biotite ⁴⁰Ar/³⁹Ar (n=2), K-feldspar ⁴⁰Ar/³⁹Ar (n=2), zircon (n=124) and apatite (n=19) (U-Th)/He, and apatite fission track (n=3) samples. Thermal history models from Thermochron.jl inversions integrate data from all available thermochronometric systems for each sample location. In the Precambrian, these models show several late Mesoproterozoic and Neoproterozoic reheating and cooling events that broadly coincide. In the late Mesoproterozoic–early Neoproterozoic, our models suggest cooling pulse(s) (and by inference, exhumation) at ~1.1–1.0 Ga. We infer that this is associated with broad rift-margin uplift during the 1.1 Ga Midcontinent rifting and/or basement-cored uplifts due to continental-interior fault inversion driven by 1.09–0.98 Ga Grenville compression. Models show variable degrees of reheating and cooling in the Neoproterozoic, which we interpret as widespread Grenvillian sedimentation followed by exhumation due to Snowball Earth glacial erosion. Taken together, these late Mesoproterozoic–Neoproterozoic burial and erosional events represent a complex, non-monotonic evolution of the Great Unconformity surface in the US Upper Midwest. In the Phanerozoic, we attribute Paleozoic–Mesozoic reheating and cooling pulses to a combination of sedimentary burial and unroofing, and late Paleozoic basin-scale hot brine migration focused along high-permeability, Great Unconformity-parallel pathways. Overall, our work demonstrates that the US Upper Midwest cratonic interior has experienced late Mesoproterozoic to Phanerozoic burial, erosional, and thermal episodes driven by the localized effects of tectonic events subsequent to cratonization. We also demonstrate that an integrated multi-system thermochronometric approach is essential in resolving the dates and rates of cooling events in deep time.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6577

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