Date of Award

2018

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Geophysics (PhD)

Administrative Home Department

Department of Geological and Mining Engineering and Sciences

Advisor 1

Aleksey Smirnov

Committee Member 1

Elisa J. Piispa

Committee Member 2

Gregory P. Waite

Committee Member 3

Thomas Oomen

Abstract

Understanding the geomagnetic field direction and strength (e.g., paleomagnetism and paleointensity, respectively) recorded by Precambrian rocks is essential to obtain insight into the nature and evolution of the Earth’s early geodynamo and for constraining models of planetary evolution. Major milestones of our planet’s history, such as beginning of plate tectonics, development of the atmosphere and life, took place during the first four billion years. However, the available data on the Earth’s magnetic field in the Precambrian are very limited, especially the information about the field intensity which represents one of the most challenging aspect of paleomagnetic research. Many Precambrian rocks are not suitable for paleointensity determinations with the most commonly used Thellier double-heating method because of their geological and experimental alteration. Complementing the Precambrian paleomagnetic and paleointensity database and improving alternative paleointensity techniques is essential to better our understanding of the Earth. This dissertation presents the results of investigations of the Precambrian geomagnetic field direction and strength, and the results of a methodological study of the pseudo-Thellier experimental protocol for absolute paleointensity determination. Detailed paleomagnetic and rock magnetic observations were performed on rocks emplaced ~1100 Ma during the formation of the North-American Mid-Continent Rift. The identification of three independent paleomagnetic directions in the Baraga and Marquette dikes and their comparison to the Mesoproterozoic Apparent Polar Wonder Path (the so-called Logan loop) permitted the estimation of the relative timescale for their intrusion (Chapter 2). The Shaw paleointensity results on the dikes from Baraga and Marquette areas yielded consistent intensity values of ~14.4 μT and ~20.4 μT corresponding to a VDM of ~2 x 1022 Am2 and 3.3 x 1022 Am2, respectively. The detailed paleomagnetic analysis of the ~1094 Ma Greenstone flow, one of the largest large flows on the planet, was applied to evaluate the various scenarios of emplacement and cooling of the flow and to assess the paleosecular variation of the geomagnetic field (Chapter 3). The results of an experimental study using the combination of the pseudo-Thellier and Shaw experimental protocols on synthetic and natural magnetite-bearing samples suggest that the calibrated pseudo-Thellier method can

xii provide absolute paleointensity estimates for Precambrian rock equivalent to the results from conventional heating methods (Chapter 4). The paleointensity values (2-3 x 1022Am2) obtained from the Baraga and Marquette dikes are low in comparison to the average field strength value (~8.0 x 1022 Am2) for the last 10 Myr but agree with some low paleointensity determinations from other Precambrian rocks. These results suggest that the Baraga-Marquette dikes intruded during a period where the geomagnetic field was weak. However, a possibility of underestimating the field strength due to the presence of thermochemical remanent magnetization cannot be ruled out.

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