Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Evans, David

Abstract

Supercontinents embody perhaps the longest cyclic process on our planet, profoundly influencing the evolution of Earth’s biosphere, geosphere, and atmosphere on timescales of hundred-million years or more. Yet, the configurations of pre-Pangean supercontinents, Rodinia, Nuna, and/or Kenorland, as well as their transition processes remain the subjects of debate, hampering the understanding of the interactions between the global tectonics and Earth’s evolution in deep time. Compared to other approaches, paleomagnetism is the only quantitative method to reconstruct pre-Pangean supercontinents in an absolute paleogeographic framework. However, a recent summary of global paleomagnetic data of Precambrian age reveals two problems. First is that discordant paleomagnetic records exist in some cratons’ datasets, thus rendering paleogeographic interpretations difficult. Second is that some cratons (e.g., West African Craton) have heretofore essentially no reliable Precambrian paleomagnetic records, yielding large uncertainties in their paleogeography. This dissertation is dedicated to addressing these problems. Specifically, we studied three cases of discordant paleomagnetic records, one from Tonian data in Baltica, and a pair of Orosirian datasets from the Slave craton. By adding new data and carefully compiling and evaluating published results, we thoroughly discussed the possible causes of discordance in the paleomagnetic directions. For Baltica, using detailed laboratory demagnetization techniques on samples from the Dalarna-Blekinge dolerite dike suite, we attributed an abnormal direction within the Dalarna-Blekinge mafic dikes to unremoved overprints. In addition, by comparing our new Baltica poles around ~950 Ma with coeval poles from Laurentia, we proposed rapid latitudinal motions of these two cratons in early Tonian time. For the Slave craton, we proposed that although basin- or local-scale vertical-axis rotations could account for some discrepancies among time-correlative 2.02-1.88 Ga poles, the overall pattern of the Orosirian apparent polar wander path of the Slave craton is better explained by true polar wander. These findings are helpful for studying the dynamic movements of Rodinia and the amalgamation of Nuna. In addition, we reviewed the Precambrian paleomagnetic records of the West African Craton to understand the limitations of previous studies. We found that most of the low-quality published results from the West African Craton suffer from inadequate sampling, poor dating, and the lack of field tests. We conducted a paleomagnetic study on Proterozoic mafic dike swarms in the Anti-Atlas Belt, Morocco, combined with U-Pb geochronology. After detailed field and laboratory work, we provided two reliable paleomagnetic poles for the West African Craton, one at 2.04 Ga and the other at 1.4–1.36 Ga. These new poles help fill the large gaps in the paleomagnetic dataset of the West African Craton. Using the two poles, we proposed a new connection between the West African Craton and Amazonia before and within Nuna, of which the relative position between the two cratons is 180° different from their connection in Gondwana. Incorporating paleomagnetic and geological constraints from other major cratons, we revised the configuration of supercontinent Nuna. Our new reconstruction model sheds light on the plate motion pattern between neighboring cratons in deep time, as well as the style of Nuna-Rodinia supercontinental transition. To summarize, the outcome of this dissertation expands the reliable paleomagnetic constraints on configurations of pre-Pangean supercontinents, and promotes an understanding of the Earth’s evolution in a spatial perspective. The final establishment of a global paleogeographic framework in the Precambrian still awaits further integrations of robust paleomagnetic studies with geochronological, stratigraphical, geochemical, and paleontological constraints.

Download
COinS