Paleomagnetic and geological implications of magnetic properties of the Triassic diabase of southeastern Pennsylvania

Abstract

Preliminary measurements of magnetic properties of Triassic diabase in southeastern Pennsylvania imply a Triassic geomagnetic pole in central Siberia, at 62°N, 105°E. This pole is virtually identical to that obtained from paleomagnetic studies of rocks of similar age in New Jersey and Arizona. A North American Triassic pole, the mean of six independent determinations, is located at 62°N, 101°E. Dispersion in directions of magnetization, examined to estimate the intensity of Triassic geomagnetic secular variation, is less than half that predicted by Creer (1962) from an analysis of the present field, which may indicate an episode of relative geomagnetic ‘quiet’ during the Late Triassic. These magnetic measurements also define certain linear distributions of site-mean directions of magnetization, probably caused by differential rotations of diabase units about the regional geologic strike. A preliminary tabulation of magnetic properties suggests several correlations with lithology, including the observation that the strongest, but least stable, magnetic moment is associated with the late-stage granophyric diabase facies. The effective average ratio of remanent to induced magnetization for these intrusions probably falls in the range 1.60 to 1.89.