PALEOMAGNETIC CONSTRAINTS ON SILURO-DEVONIAN LAURENTIAN MARGIN TECTONICS FROM NORTHERN APPALACHIAN VOLCANICS

Paleomagnetic analyses of Silurian mafic volcanics from the overstep s equence of sedimentary and volcanic rocks deposited over the Central M obile Belt of the northern Appalachians provide insight into the mid-P aleozoic tectonic history of the Laurentian margin. Stepwise thermal d emagnetization of the subaerial mafic volcanics of the Ludlovian Fivem ile Brook Formation of northwestern Maine reveals two ancient high-tem perature, dual-polarity remanences. A tilt-corrected mean direction yi elds an inclination of 41 degrees (sigma(I) = 3.9 degrees, N = 4 sites ) and the magnetization is interpreted as a near-primary magnetization acquired well before the end of Early Devonian deformation. An in sit u mean direction (D = 165 degrees, I = 35 degrees, alpha(95) = 6 degre es, k = 65, N = 10 entries from 8 sites), is interpreted as a secondar y overprint with a pole (22 degrees S, 306 degrees E) near the Early C arboniferous segment of the Noah American apparent polar wander path ( APWP). Conglomerate and fold tests of the high-temperature characteris tic remanence preserved in Wenlockian subaerial mafic volcanics of the Bryant Point Formation and a red bed of the South Charlo Formation, b oth of the Chaleur Group of northeastern New Brunswick, constrain pale olatitude and deformational age. The inclination-only fold test peaks at 50% unfolding with a mean inclination of -35 degrees (sigma(I) = 8. 5 degrees, n = 148). Synfolding acquisition of magnetization is Wenloc kian, 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 i n 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 t his remanence furthermore requires that deformation associated with th e Acadian orogeny in New Brunswick began by mid-Silurian times. Moreov er, inclinations from these units indicate that, within paleomagnetic resolution, there has been no significant latitudinal displacement wit h respect to stable North America since the Silurian. (C) 1998 Elsevie r Science B.V. All rights reserved.