The paleomagnetism of Keweenawan dikes from Baraga and Marquette Counties, northern Michigan

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.

Paleomagnetism of Late Precambrian Keweenawan igneous and baked contact rocks from Thunder Bay district, northern Lake Superior

“…This paper and a similar one on dikes from Northern Michigan (Pesonen and Halls 1979) are part of a much larger study on the paleointensity of Keweenawan dikes and their baked host rocks (Pesonen 1978;Pesonen and Halls, in preparation). The demonstration of a TRM is an essential prerequisite for paleointensity determinations.…”

“…Beck 1970;Palmer 1970;Pesonen and Halls 1977). The Keweenawan data define a prominent anticlockwise movement of the pole known as »The Great Logan Paleomagnetic Loop» (Robertson and Fahrig 1971;Fig.…”

“…A major problem in Keweenawan paleomagnetism is the existence of two asymmetric (i.e. not 180 degrees) reversals (Pesonen 1978): the reversed remanence direction always has a much steeper (upward) inclination than the normal (downward) one . Of particular concern in the interpretation of Keweenawan paleomagnetism is whether the asymmetry is caused by a secondary magnetization component (Palmer 1970), by an apparent polar wander (e.g.…”

“…Of particular concern in the interpretation of Keweenawan paleomagnetism is whether the asymmetry is caused by a secondary magnetization component (Palmer 1970), by an apparent polar wander (e.g. Beck 1970;Robertson and Fahrig 1971;Pesonen and Halls 1977) or by a fundamental difference in the source region of the geomagnetic field between normal and reversed polarities (Wilson 1972;Pesonen 1978). Previous demagnetization studies of in situ Keweenawan igneous rocks have failed to detect any evidence of a regional secondary component which could explain the asymmetry of the reversal (e.g.…”

“…The significance of such loops or »hairpins» (Irving and Park 1972) (Rove shale): R27, R28, R40, R42, R43, R44, R46, R47, R48 and N67, N68 (Sibley red bed series). For references see Pesonen (1978).…”