Abstract

A paleomagnetic study involving alternating field (AF) and thermal cleaning, baked contact tests, and the measurement of hysteresis and other magnetic properties has been carried out on 21 Keweenawan diabase dikes (age ~1.1 Ga) from Baraga and Marquette Counties, northern Michigan. The main results are as follows.(1) The dikes exhibit a steep, negatively inclined remanence, which is shown to be a thermoremanent magnetization (TRM) acquired parallel to the ambient field at the time of initial magma cooling.(2) A secondary component, more resistant to magnetic cleaning than the primary TRM, and with west-northwest declination and positive inclination is occasionally found in the dikes, especially those from Baraga County.(3) Despite an early pole position obtained by Graham, which suggested that the dikes in Michigan were potentially important in defining the depth of the Logan Loop, most of them, from Marquette County, yield a pole that lies on the loop's western arm. The region of the projected apex of the loop thus still remains without reliable data.(4) The pole position of the Marquette dikes (48.4°N, 213.5°E; dp = 5.2°; dm = 6.2°) obtained from 14 sites is virtually identical to that obtained from 17 reversely magnetized dikes from the Thunder Bay district in Ontario, but appears to be distinct from poles derived from other Keweenawan units. Both dike swarms are thus interpreted to represent parts of the same magmatic pulse, and one that occurred in the centre of Lake Superior during an early opening phase of the Keweenawan rift system.

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