Proterozoic Geomagnetic Field Geometry

Abstract

Pre-Mesozoic continental reconstructions and paleoclimatic inferences
from paleomagnetism rely critically upon the assumption of a
time-averaged geocentric axial dipole (GAD) magnetic field. We have been
testing the GAD assumption and localized non-dipole components in a
different manner, by observing directional variations within the
Matachewan, Mackenzie and Franklin dyke swarms. Large dyke swarms,
commonly emplaced within a few million years, provide the necessary
broad areal coverage to perform a test of global geomagnetic field
geometry. Our analysis varies the quadrupole and octupole values of the
generalized paleolatitude equation to determine a minimal angular
dispersion and maximum precision of paleopoles from each dyke swarm. As
a control, paleomagnetic data from the central Atlantic magmatic
province (CAMP) show the sensitivities of our method to non-GAD
contributions to the ancient geomagnetic field. Within the
uncertainties, CAMP data are consistent with independent estimates of
non-GAD contributions derived from global tectonic reconstructions
(Torsvik & Van der Voo, 2002). Current results from the three
Proterozoic dyke swarms all have best fits that are non-dipolar, but
they differ in their optimal quadrupole/ octupole components. Treated
together under the hypothesis of a static Proterozoic field geometry,
the data allow a pure GAD geodynamo within the uncertainty of the
method. Current results were performed using Fisherian statistics, but
Bingham statistics will be included to account for the ellipticity of
data.