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Date of Award

2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Geology (PhD)

College, School or Department Name

Department of Geological and Mining Engineering and Sciences

First Advisor

Aleksey V. Smirnov

Abstract

Paleomagnetic data measured from the Precambrian rocks are fundamental in deciphering the early history of our planet. Paleo-geographic reconstructions based on the paleomagnetic record provide important insight into the Precambrian geodynamics and plate tectonics as well as form a useful framework for interpretation of geological and geochemical data. In addition, data on the long-term behavior and configuration of the Earth’s magnetic field during the Precambrian are crucial in understanding the origin and nature of Earth’s early geodynamo. These data are also important for investigating potential causative links between the geomagnetic field evolution and the evolution of other components of the Earth system. For example, a weak or unstable field of a primordial geodynamo could result in weaker magnetosphere shielding and, hence, a stronger effect on the atmosphere and biosphere from solar and cosmic radiation. The long-term trends in the strength, geometry, and stability of geomagnetic field also reflect the thermal evolution of our planet and therefore may provide important insight into the timing of some important transitions in the Earth’s interior such as the formation and initial growth of the solid inner core. In the absence of strict theoretical constraints, paleomagnetic data become a principal source of information about the behavior of Earth’s magnetic field in the Precambrian. However, our knowledge of the field history during the first four billion years of Earth history remains very limited. This especially concerns the database on the field strength (paleointensity) which contains only a handful of reliable data points.

This dissertation presents the results of investigations of the strength, morphology and stability of the Proterozoic geodynamo as well as Precambrian geodynamics and rifting processes. These results are based on detailed paleointensity and paleomagnetic analyses of several prominent mafic dike swarms in Peninsular India and North America. New high quality paleomagnetic data have been obtained from the ~1140 Ma Lamprophire dyke swarm exposed around Lake Superior (North America) and from the ~2.37 Ga and 2.18 Ga swarms in the Dharwar craton (Southern India), the ~1.99 Ga swarm in the Bundelkhand craton (North-West India), the ~1.89 Ga swarm in the southern Bastar and Dharwar craton (East and South India), and ~0.75 Ga swarm of the Malani Igneous Suite in the Marwar Terrain (India). In addition, new paleointensity determinations have been obtained from the ~0.75 Ga Malani and ~1.99 Ga Bundelkhand dyke swarms using the multispecimen and quasi-perpendicular Thellier method. The results provide new datapoints to the sparse Precambrian database and therefore contribute to a synoptic view of the Proterozoic geodynamo. In addition, we attemp a preliminary interpretation of the long term trends affecting the geodynamo. A connection between the strength of the magnetic field and the supercontinent cycle is suggested to be in effect during the Precambrian. The new data from India provides additional constraints on the configuration of the Mesoproterozoic Nuna/Columbia supercontinent confirming amalgamation of the Dharwar and Bastar cratons by ~2.37 Ga. The data from North America provide a strong support for the mantle (super-)plume origin of the Midcontinent Rift and suggest a standstill of the Laurentia plate before the start of its fast southward motion.

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