Pole Positions from the Mamainse Point Lavas and their Bearing on a Keweenawan Pole Path and Polarity Sequence

Abstract

Thermal and alternating field studies of the remanent magnetization of the Mamainse Point lavas, on the east shore of Lake Superior, indicate stable magnetization. The flows form two groups of normally magnetized (N) flows, each overlying reversely magnetized (R) flows. It is suggested that the sequence consists of a single sequence of R flows overlain by N flows, the sequence being repeated by faulting. A mean pole position calculated from 15 N sites is 30 °N, 183 °E, (dp = 5, dm = 8), and the pole position from 12 R sites is 49 °N, 232 °E (dp = 14, dm = 16). Reasons are given for believing that this difference in pole position between the older (R) and younger (N) flows was caused by relative movement of the Keweenawan area and the rotation axis of the earth, and not to secondary components of magnetization acquired since the rocks cooled. Review of the Keweenawan paleomagnetic data reveals a consistent picture of an older reversely magnetized and younger normally magnetized magnetic field (if allowance is made for radiogenic errors). The time at which the field switched from reversed to normal polarity may provide a valuable time datum traceable in the rocks from continent to continent.