Assessing Earth's Proterozoic Geomagnetic Field Geometry and Paleogeography Using Theoretical and Analytical Techniques.

摘要

Previous investigations of the Proterozoic geomagnetic field have yielded widely varying results ranging from a purely geocentric axial dipole (GAD) to large contributions of the quadrupolar and octupolar zonal harmonics, on the order of 15% and 30% strength relative to the primary dipole field, respectively. The geometry of the geomagnetic field has large implications for how the paleomagnetie record is interpreted to infer tectonic histories and geodynamic processes. Due to the limited geologic preservation of little-metamorphosed Proterozoic rocks, a solution is best determined by combining analytical and theoretical techniques.;One previously proposed test of Proterozoic geomagnetic field geometry compared paleomagnetic remanence directional variations across a large igneous province (LIP), to a model geometry with optimized axial dipole, quadrupole, and octupole terms. The present study modifies and expands this test: by changing the inferred geomagnetic field geometries, the spatial distributions of virtual geomagnetic poles (VGPs) are evaluated. Using the varying geometries and VGP positions, it is determined that a globally distributed paleomagnetic database can replicate the global geomagnetic field geometry, but regional features such as individual dike swarms or LIPs are not areally extensive enough to provide such constraints.;Independent of paleomagnetic observations, geodynasno simulations of Earth's outer core have also provided insight into the structure of the geomagnetic field. Though there are a large number of parameters that affect the results of the simulations, previous studies have shown that the heat-flow between the outer core and lower mantle and the core-mantle boundary has the largest influence. As a natural progression, the heat-flow models have evolved from simple spherical harmonic truncations to more realistic physical models based on seismic tomography observations. In this study, using realistic heat-flow boundaries yields geomagnetic field geometries with more diminished non-dipolar components than previous results, on the order of 5% and 8% for quadrupolar and octupolar zonal harmonics, respectively.;Even with fluctuating opinions on the long-term Proterozoic geomagnetic field geometry, more paleomagnetic data are needed to constrain interpretations of tectonic history and terrane formation. One prospective field locality is the subgreenschistmetamorphosed Sinclair terrane in Namibia, southwestern Africa, for which paleomagnetic data can yield insights into its relation to the adjacent Kalahari craton, as well as global-scale plate kinematics during the formation of the supercontinent Rodinia. This study's field collection and laboratory analysis of paleomagnetic data provides reliable pole positions that imply the Sinclair terrane was autochthonous to the Kalahari craton at ca. 1100 Ma, and Sinclair magmatism was coincident with the Kalahari-wide Umkondo LIP.;The incorporation of analytical and theoretical techniques will continue to illuminate Earth's Proterozoic tectonic and magnetic history, and provide insights into deeper time.