LATE MESOPROTEROZOIC REVERSAL ASYMMETRY (OR LACK THEREOF)
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.
In conjunction with paleomagnetics labs at Yale and MIT,
Adam Maloof and I are reevaluating Keweenawan reversal
asymmetry with samples from Mamainse Point, Ontario, that
were collected in the context of rigorous
volcanostratigraphy through the basalt flows and their
associated autobreccias, sandstones and conglomerates.
The
preliminary 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 (-73 degrees to -53
degrees) leading up to a reversal with <10 degrrees of
inclination asymmetry. In contrast, comparing average
directions from throughout the reversed zone with
directions from the overlying normal zone shows the same
>20 degrees 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.
Adam Maloof, Catherine Rose and I will returned to the
field this past May for more work in the mid-continent
rift.