Insights from integrated deep-time thermal history modeling; Constraining the burial, exhumation, and long-term stability of the Fennoscandian Shield

Isaacs, Joshua M.

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https://hdl.handle.net/2142/125837 Copy

Description

Title

Insights from integrated deep-time thermal history modeling; Constraining the burial, exhumation, and long-term stability of the Fennoscandian Shield

Author(s)

Isaacs, Joshua M.

Issue Date

2024-07-17

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

Guenthner, William R

Committee Member(s)

Lundstrom, Craig C

Department of Study

Earth Sci & Environmental Chng

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Thermochronology
• craton stability
• Fennoscandia
• Great Unconformity
• deep-time

Abstract

Cratons, the oldest and thickest sections of the Earth’s lithosphere, are traditionally thought to have stable freeboard and minimal internal deformation over billion-year timescales. However, recent thermochronometric insights and geodynamic predictions suggest that cratons can undergo large vertical motions throughout their existence. Here, I investigate questions related to craton stability with a thermochronologic study of the bedrock of the Fennoscandian Shield in southeastern Finland. This region has a striking lack of sedimentary cover, which previous studies have used, along with inferred paleosurfaces, impact craters, and negligible relief to argue that Fennoscandia has experienced minimal sedimentation since accretion during the Orosirian. However, field observations cannot distinguish strata that never existed from the effects of erosion. By leveraging temperature as a sign of burial, thermochronometry can identify stratigraphy now lost, and this makes it indispensable in deciphering whether cratonic cores are stable across deep time. Cratonic stability has implications for how we plan to dispose of hazardous materials, understand the relationship between tectonics, geodynamics, and erosion, and envision the histories of Earth’s oldest crust. Furthermore, recent work has suggested this area is key to tracing the longest record of habitable conditions for Earth’s deep biosphere. This emerging paleomicrobiology topic requires a thermochronologic perspective to validate the interpretation of long-term habitability. This study presents new zircon (U-Th)/He dates from depths of a single borehole and new apatite (U-Th-Sm)/He dates from one borehole depth in southeastern Finland. Effective uranium (eU) is a proxy for radiation damage, which determines the diffusivity of He in individual grains. The observed negative relationships between date and eU in zircons at all depths indicate prolonged residence above annealing temperatures. Ages of the shallowest sample (35.9 m deep) span 961.1 ± 18.9 to 7.44 ± 0.17 Ma, and the deepest sample (806.1 m) preserves ages from 745.0 ± 12.4 to 12.71 ± 0.18 Ma. Significant overlap in date-eU space demonstrates that the temperature offset across ∼ 70 m vertical separation is not enough to record drastically different thermal information. Inverse thermal history modeling requires well-constrained heating to between 70 and 90 °C in the past 100 Ma while suggesting residence <60 °C in the Mesozoic and much of the Paleozoic. The models also necessitate one older phase of reheating to more than 150 °C before 900 Ma and most likely between 1350 and 1220 Ma. The lack of magmatism in southern Finland from the Mesoproterozoic onwards means that reheating must be the result of burial and exhumation equivalent to several kilometers of vertical variation. Model results independently predict observations and events described throughout Fennoscandian history that significantly increase confidence in the magnitude and timing of results. These findings recontextualize the sub-Cambrian peneplain as resulting from multiple episodes of denudation punctuated by sedimentary burial and suggest that cratonic interiors are anything but stable.

Graduation Semester

2024-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/125837

Copyright and License Information

Copyright 2024 Joshua M.Isaacs

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