Reversal Transition Records from Intrusions: Implications for the Reversal Process.

The nature of reversals of the geomagnetic field and the details of the transition fields remain controversial. However, reversal records from the Agno batholith and Tatoosh intrusion confirm the suggestion of Valet et al., (2012) from studies of lava records, that there is a threefold division in reversal transition directions. In the Agno, the first phase, or precursor, consists of a CCW loop of the VGP moving from high southerly latitude reverse poles to reach North America. The second phase takes the VGP along a half CCW loop from the tip of South America to northern latitudes at the intensity minimum. The third phase, or rebound is a smaller CCW loop and the main intensity recovery begins. The first and third phases appear to be paleosecular variation loops analogous to present London-Paris secular variation loops. The Tatoosh intrusion gives a similar, but less complete record with the VGPs again confined to the East Pacific and the Americas. Away from the reversal region, secular variation loops in the Tatoosh were shown to be comparable in duration to the precursor in the transition record, consistent with the first phase being a paleosecular variation loop in the Agno. Using westward drift estimates from the present field, this should last about1800 years. This gives ~3300 for phase 2, in an intensity low of >16,000 years. A feature of R to N reversal field models is a low latitude magnetic field flux concentration of the same sign as the polar vortex of the south geographic pole. This is followed by northward flux flow, e.g. Shao et al., (1999). The reversal is achieved by northward motion of this flux feature. The feature is locked in longitudinal mantle coordinates and similarly the VGPs in the Agno and Tatoosh records are confined to the longitudes of the eastern Pacific and the Americas. Whether we are approaching a reversal remains to be seen, although judging by these intrusion records the field intensity would need to decrease much further before major directional changes should take place. In the Agno record, these changes start near to the intensity minimum. In the Tatoosh record, the field decreases by an order of magnitude before the major directional changes take place.