REEVALUATING KEWEENAWAN REVERSAL ASYMMETRY: NEW PALEOMAGNETIC DATA FROM MAMAINSE POINT, ONTARIO

1.1 billion year old igneous and volcanic rocks of the Keweenawan large igneous province form part of a mid-continent rift system (MCR) in North America, spanning 3000 km and emplaced between 1110 and 1090 Ma. For years, paleomagnetic data from the MCR has been interpreted to reveal strong inclination asymmetry during polarity reversals. This asymmetry has been interpreted as being a result of (a) rapid motions of the North American plate between reversals, (b) incomplete removal of a magnetic overprint, or (c) the presence of a significant non-dipole component to the surface geomagnetic field.

For this study, samples for paleomagnetic analysis were collected in the context of rigorous volcanostratigraphy through approximately thirty basalt flows and their associated autobreccias, sandstones and conglomerates at Mamainse Point. After initial low-field AF steps, and low-temperature thermal cycling for a subset of the samples, directions were determined through detailed thermal demagnetization. This allows for an evaluation of the similarity of magnetite and hematite components and their relative importance. The resulting data set, which includes multiple reversals, indicates that the actual asymmetry across reversals is less than has been previously reported. In the lower reversed zone younging stratigraphy is accompanied by a progressive inclination shallowing (-73º to -53º) leading up to a reversal with <10º of inclination asymmetry. In contrast, comparing average directions from throughout the reversed zone with directions from the overlying normal zone shows the same >20º of inclination asymmetry apparent in the literature. The uppermost reversal in the section is symmetric with the normal and reversed directions antiparallel to one another. This suggests that the directions of the characteristic remnant magnetization (ChRM) are not strongly affected by incompletely removed magnetic overprints or large non-dipolar components in the geomagnetic field.