Paleomagnetic constraints on Siluro-Devonian Laurentian margin tectonics from northern Appalachian volcanics

Abstract

Paleomagnetic analyses of Silurian mafic volcanics from the overstep sequence of sedimentary and volcanic rocks deposited over the Central Mobile Belt of the northern Appalachians provide insight into the mid-Paleozoic tectonic history of the Laurentian margin. Stepwise thermal demagnetization of the subaerial mafic volcanics of the Ludlovian Fivemile Brook Formation of northwestern Maine reveals two ancient high-temperature, dual-polarity remanences. A tilt-corrected mean direction yields an inclination of 41° (σ I = 3.9°, N = 4 sites) and the magnetization is interpreted as a near-primary magnetization acquired well before the end of Early Devonian deformation. An in situ mean direction ( D = 165°, I = 35°, α 95 = 6°, k = 65, N = 10 entries from 8 sites), is interpreted as a secondary overprint with a pole (22°S, 306°E) near the Early Carboniferous segment of the North American apparent polar wander path (APWP). Conglomerate and fold tests of the high-temperature characteristic remanence preserved in Wenlockian subaerial mafic volcanics of the Bryant Point Formation and a red bed of the South Charlo Formation, both of the Chaleur Group of northeastern New Brunswick, constrain paleolatitude and deformational age. The inclination-only fold test peaks at 50% unfolding with a mean inclination of −35° (σ I = 8.5°, n = 148). Synfolding acquisition of magnetization is Wenlockian, based on a negative conglomerate test at the base of the section and a positive conglomerate test at the top of the section. Clockwise streaking of site mean directions away from a predominantly northerly declination is consistent with post-middle Wenlockian dextral shear in the Chaleur Bay region. Comparison of a locus of paleomagnetic pole positions with the North American APWP supports a Silurian age of the magnetization of the Chaleur Group. The age and synfolding nature of this remanence furthermore requires that deformation associated with the Acadian orogeny in New Brunswick began by mid-Silurian times. Moreover, inclinations from these units indicate that, within paleomagnetic resolution, there has been no significant latitudinal displacement with respect to stable North America since the Silurian.