Deep-time thermochronology of the US Upper Midwest cratonic interior

Sigat, Ryan Owen Anak

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https://hdl.handle.net/2142/130175 Copy

Description

Title

Deep-time thermochronology of the US Upper Midwest cratonic interior

Author(s)

Sigat, Ryan Owen Anak

Issue Date

2025-07-14

Director of Research (if dissertation) or Advisor (if thesis)

Guenthner, William

Doctoral Committee Chair(s)

Guenthner, William

Committee Member(s)

• Anders, Alison
• Lundstrom, Craig
• Marshak, Stephen

Department of Study

Earth Sci & Environmental Chng

Discipline

Geology

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• thermochronology
• deep time
• craton
• Great Unconformity
• zircon
• diffusion
• radiation damage
• Precambrian geology

Abstract

Thermochronology has been extensively used to study the timing and/or rate of cooling associated with a wide range of geologic processes such as tectonic exhumation, sedimentary basin evolution, and hydrothermal fluid migration. More recently, thermochronometers such as the zircon (U-Th)/He (ZHe) have been used, at times in concert with other low- to medium-temperature systems, to resolve deep-time (>> 1-billion-year) thermal histories. However, extracting useful thermochronometric information from the ZHe method relies on a robust understanding of the diffusion kinetics of ⁴He for zircon. Here, I present results investigating the deep-time thermal history of Precambrian rocks in the US Upper Midwest cratonic interior from an integrated medium- to low-thermochronometric approach and the diffusion kinetics of highly radiation-damaged zircon to improve our understanding of the ZHe system. The Mesoproterozoic–Phanerozoic thermal histories of Precambrian basement rocks in the US Upper Midwest, specifically in Minnesota (the Minnesota River Valley), Wisconsin, and the upper peninsula of Michigan, remain poorly constrained since the most recent tectonic episodes within the region (i.e., the ~1.47 Ga Baraboo orogeny and the failed ~1.1 Ga Midcontinent rift). These thermal histories have important implications for understanding the long-term (in)stability of this cratonic interior region. In my study, I used deep-time thermochronology to resolve timing of cooling events, and by inference, mid- to upper-crustal processes within the US Upper Midwest over the past > ~1 billion years. I present new medium- to low-temperature thermochronometric datasets from Precambrian basement samples across the US Upper Midwest. My data include new ages and dates from biotite and K-feldspar ⁴⁰Ar/³⁹Ar, zircon and apatite (U-Th)/He (ZHe and AHe, respectively), and apatite fission track (AFT) analyses. I present time-temperature histories from model inversions that integrate all available thermochronometric systems for each sample location. In the Precambrian, thermal history models suggest diverse late Mesoproterozoic–early Neoproterozoic cooling histories of basement rocks in Wisconsin and the upper peninsula of Michigan. Specifically, they show prominent late Mesoproterozoic cooling signals likely controlled by the ca. 1.1 Ga Midcontinent Rift and/or the far-field shortening effects of the 1090–980 Ma Grenville orogeny. In the Neoproterozoic, my models show exhumation, and possibly burial, of varying duration and magnitude that suggest possible Grenvillian sedimentation and subsequent unroofing that may be driven by Snowball Earth glacial erosion. The thermal history models of the Paleoarchean–Neoarchean rocks of the Minnesota River Valley, however, suggest an earlier pulse of primary cooling at ~1.9 Ga, followed by less well-resolved time-temperature (t-T) paths in the Mesoproterozoic and the Neoproterozoic. Overall, the thermal history models likely represent a protracted and composite formation of the Great Unconformity surface in the US Upper Midwest. My models also suggest significant reheating and cooling event(s) in the late Paleozoic–early Mesozoic. I attribute these to a combination of sedimentary burial and unroofing, and Paleozoic basin-scale hot brine migration coeval with the Paleozoic Appalachian orogenies. My study demonstrates the instability of cratonic interior rocks of the US Upper Midwest due to the presence of significant vertical displacements and thermal perturbations across deep-time, influenced by the Midcontinent Rift and far-field orogenic events. Although t-T modeling with an integrated multi-system dataset can provide excellent constraints on deep-time thermal histories of rocks, my Minnesota River Valley dataset shows that complexities may arise due to factors such as data dispersions and imperfect kinetics. I performed various approaches to the treatment of the kinetics and date uncertainties in model inversions to identify the best approach to improve model predictions. I found that for my dataset, performing empirical Bayes resampling of the date uncertainties, allowing for diffusion kinetics resampling, and using the K-feldspar ⁴⁰Ar/³⁹Ar ages as a t-T box constraints yield model predictions that best honor the general trend in our observed dates and the ZHe diffusivities from previous empirical studies. This work highlights that a robust understanding of diffusion and/or annealing kinetics is crucial in ensuring the geologic utility of thermochronometric techniques. The zircon radiation damage accumulation and annealing model (ZRDAAM) for the ZHe system relies on the parameterized relationship between accumulated radiation damage and diffusivity fitted to empirical diffusion data. For zircon grains with well-characterized thermal histories, a zircon’s calculated alpha dose can be related to its level of radiation damage. However, ⁴He diffusion kinetic data for zircon with high levels of radiation damage, above the range of alpha dose where diffusivity transitions from decreasing to increasing with dose (i.e., the damage-diffusivity rollover threshold), is scarce. My study presents new diffusion kinetic data for c-axis-oriented slabs of Sri Lankan zircon with high dose (> ~1–2 × 10¹⁸ ⍺/g), derived from step-heating experiments. I report new diffusion kinetic parameter values that correlate well with observed changes in activation energy, Ea, and frequency factor, D0, with alpha dose from previous studies on well-characterized natural zircon samples. However, my study also shows unique behavior for highly-damaged zircons (dose ≥ ~3 × 10¹⁸ ⍺/g) that may be characterized by multipath diffusion through an amorphous matrix and isolated crystalline remnants, presently unaccounted for in ZRDAAM. My study shows that complex behavior at very high damage levels involving mostly amorphous zircons require a revision of the current model for a continued use of the ZHe method, especially in deep-time studies.

Graduation Semester

2025-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/130175

Copyright and License Information

Copyright 2025 Ryan Sigat

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