Paleomagnetism of Upper Triassic diabase from southeastern Pennsylvania: Further results

Abstract

Seventy-eight reliable sites in Upper Triassic diabase from southeastern Pennsylvania yield a paleomagnetic pole at 62.0°N, 104.5°E, about 0.5° from an earlier pole calculation based on a preliminary study of 20 sites. Site poles have a Fisherian distribution, with an angular standard deviation of 7.4°, considerably less than the dispersion predicted by models based on the present geomagnetic field. This distribution of site poles suggests that geomagnetic secular variation during the Late Triassic consisted mainly of dipole wobble, with little or no contribution from nondipole sources. No reversals of the field are recorded in Upper Triassic igneous rocks from eastern North America, although reversals are known elsewhere in the Triassic section. Available evidence indicates that, although North America was actively drifting during the Late Triassic and Jurassic, polar wandering during that time interval was minimal. A model based on work by E. Irving, A. Cox, P. Goldreich, and A. Toomre is used to predict a correlation between general convective overturn in the upper mantle, polar wandering, high intensities of the nondipole component of the geomagnetic field, and high frequencies of geomagnetic reversal.