Paleomagnetic results from the Snake River Plain: Contribution to the time‐averaged field global database

Abstract

This study presents paleomagnetic results from the Snake River Plain (SRP) in southern Idaho as a contribution to the time‐averaged field global database. Paleomagnetic samples were measured from 26 sites, 23 of which (13 normal, 10 reverse) yielded site mean directions meeting our criteria for acceptable paleomagnetic data. Flow ages (on 21 sites) range from 5 ka to 5.6 Ma on the basis of 40Ar/39Ar dating methods. The age and polarity for the 21 dated sites are consistent with the Geomagnetic Reversal Time Scale except for a single reversely magnetized site dated at 0.39 Ma. This is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records. Combining the new data from the SRP with data published from the northwest United States between the latitudes of 40° and 50°N, there are 183 sites in all that meet minimum acceptability criteria for legacy and new data. The overall mean direction of 173 normally magnetized sites has a declination of 2.3°, inclination of 61.4°, a Fisher concentration parameter (κ) of 58, and a radius of 95% confidence (α95) of 1.4°. Reverse sites have a mean direction of 182.4° declination, −58.6° inclination, κ of 50, and α95 of 6.9°. Normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95% confidence level. Virtual geomagnetic pole dispersion was found to be circularly symmetric, while the directional data were elongate north‐south. An updated and corrected database for the northwestern U.S. region has been contributed to the Magnetics Information Consortium (MagIC) database at http://earthref.org.