Deep-time tectono-thermal history of the northern us cordillera: understanding zircon (U-Th)/he thermochronology and its application to Idaho-Montana basement rocks

Kaempfer, Jenna Marie

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Description

Title

Deep-time tectono-thermal history of the northern us cordillera: understanding zircon (U-Th)/he thermochronology and its application to Idaho-Montana basement rocks

Author(s)

Kaempfer, Jenna Marie

Issue Date

2021-11-30

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

Guenthner, Willy R

Doctoral Committee Chair(s)

Guenthner, Willy R

Committee Member(s)

• Anders, Alison
• Liu, Lijun
• Marshak, Stephen

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Thermochronology
• Tectonics
• Zircon thermochronology

Abstract

The emerging application of the zircon (U-Th)/He thermochronometer to understand long-term (Precambrian to present) thermal histories provides new opportunities to integrate multiple thermochronometers in interpreting the long-term tectonic evolution of geologic regions. However, the continued development of the zircon (U-Th)/He deep-time thermochronometer requires a detailed understanding of zircon diffusion kinetics, a process dependent on the accumulation and zonation of radiation damage within the crystal. Here, I present results from central Idaho and southwestern Montana, where the deformation and exhumation associated with Laramide basement-cored uplifts overlap with that associated with the Sevier fold-and-thrust belt, and the tectono-thermal history of the region remains unresolved. I applied zircon (U-Th)/He (ZHe), and apatite (U-Th)/He (AHe), in combination with inverse modeling using the programs HeFTy and QTQt, to examine regional thermal trends such as the Rodinia supercontinent cycle (recorded in basement uplifts), and the Sevier and Laramide orogenies (observed in thrust belt rocks). I present n=108 new ZHe dates, and n=67 new AHe dates, collected along a transect from central Idaho to southwestern Montana. Deep-time inverse modeling of foreland basement-cored ranges indicates substantial cooling from >400 ˚C to near surface conditions between 800 and 510 Ma, coinciding with the initiation of Rodinia rifting and subsequent Snowball Earth glaciation. I interpret this late-Paleozoic exhumation pulse as the creation of the Great Unconformity surface in this region. Samples from the interior Sevier fold-thrust belt in Idaho yield ZHe dates and model ages ranging from 82 Ma to 34 Ma that I interpret to record exhumation related to Sevier-aged contractional tectonics as well as post-orogenic collapse. ZHe model dates from samples in the Laramide-style, basement-involved foreland uplifts span 68 to 53 Ma, which indicate that exhumation was concomitant with exhumation in the interior fold thrust belt. My data, integrated with independent timing constraints from foreland basin strata and previously published thermochronometric results, suggest that mid-Cretaceous uplift of basement-cored uplifts in southwestern Montana was low-magnitude and preceded tectonism along the classic Laramide “corridor” from southern California to Wyoming. In contrast, latest Cretaceous and Paleogene thrust-related exhumation was more significant and was largely complete by Eocene time. The basement-involved deformation was contemporaneous with and younger than along-strike Sevier belt thin-skinned thrusting in southeastern Idaho. Zircon crystals from the crystalline basement rocks used in these studies display strong internal zonation; heterogeneous concentrations of radioactive actinides, and resultant radiation damage, which changes the diffusion behavior of He and therefore affect radiation damage-based modeling systems. I observe complex zonation behavior (oscillatory zones, chaotic zones, and rim-core zones) in the majority of representative grains from the northern Madison Range, and therefore the majority of grains that could be used for (U-Th)/ He dating do not follow basic assumptions to the kinetics used in radiation damage based models. A thorough understanding of zircon zonation types observed in these rocks assists me in understanding the disparity between model predictions and observed dates, particularly for those grains with high concentrations of radioactive actinides.

Graduation Semester

2021-12

Type of Resource

Thesis

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Copyright 2021 Jenna Kaempfer

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