Proterozoic magnetic overprinting and tectonics of the Laurentian Shield
Published:January 01, 1983
David J. Dunlop, Larry D. Schutts, 1983. "Proterozoic magnetic overprinting and tectonics of the Laurentian Shield", Proterozoic Geology: Selected Papers from an International Proterozoic Symposium, L. G. Medaris, Jr., C. W. Byers, D. M. Mickelson, W. C. Shanks
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The NRM (natural remanent magnetization) of a metamorphic rock is the vector sum of primary NRM and one or more secondary overprints. Magnetic overprinting is a thermal or chemical effect resulting from (1) burial and uplift (regional heating), (2) intrusive activity (local heating), or (3) hydrothermal supercritical solutions (local heating and/or alteration. We present a number of examples drawn from the Proterozoic of North America illustrating how paleomagnetists decipher magnetic overprints and use them to date and interpret tectonic events. Granites from the western Superior Province record the 2600 Ma Kenoran orogeny and an event at 1250 Ma, perhaps related to Mackenzie dike intrusion elsewhere in the Shield. Diabase dikes and country rocks in the Abitibi subprovince, particularly those near incoherent dike contacts and fault zones, bear thermal overprints whose probable ages are 1900 to 1700 Ma. The overprints agree in direction with primary NRMs of similar ages from the Churchill Province, implying that the Hudsonian orogeny involved little relative motion of Superior and Churchill cratons. Multiple overprints dating from slow uplift and cooling of the Grenville orogen between 1050 and 800 Ma postdate the collisional phase of the Grenvillian orogeny, but 1100-Ma-old surviving primary NRM of the Tudor gabbro appears to record precollisional divergence between “Grenvillia” and the rest of the Shield.