Paleomagnetism of Middle--Late Triassic plutons in southern Maine

Abstract

A paleomagnetic study of two small syenitic to alkali-granitic plutons, Abbott and Agamenticus (southern Maine), yields paleopoles at 48[deg]N, 92[deg]E, and 48 [deg]N, 99 [deg]E, with K-Ar ages of 221 +/- 8 and 228 +/- 5 Ma, respectively. A third pluton, the Litchfield, was much more weathered and did not contain stable magnetizations. Dual-polarity magnetizations, a dike contact test, and excellent statistical parameters obtained after detailed thermal and alternating field demagnetization support our claim that the magnetizations and paleopoles are representative of the Middle to Late Triassic paleomagnetic field. However, the two paleopoles fall about 10[deg] to the southwest of previously published Triassic apparent polar wander path segments for cratonic North America. Unless this small deviation can be ascribed to post-Triassic tilt of the plutons, this requires a modification of the polar wander path. To bring the poles into agreement, a tilt correction of some 15-20[deg] needs to be applied about a strike which is perpendicular to the area's geologic trends and the Main coast line, which appears to be highly unlikely given the stabilized tectonic setting of this area in post-Triassic time. A small but significant revision of the North American cratonic apparent polar wander path may therefore be suggested by these results, but this needs further corroboration. The more southerly paleolatitudes indicated by these Triassic results for North America would imply lesser displacements than previously proposed on the basis of Triassic paleomagnetism for exotic terranes in the western Cordillera.