ACCRETIONARY IMPLICATIONS OF PALEOMAGNETIC OVERPRINTS IN PERI-GONDWANAN TERRANES OF SOUTHEASTERN NEW ENGLAND AND CAPE BRETON ISLAND, NOVA SCOTIA

Paleopoles from primary Ediacaran magnetization directions established the Gondwanan origin of northern Appalachian Avalonian terranes, but magnetic overprints in the same rocks also provide useful tectonic information. Multi-vectorial magnetizations in pilot samples from the Bras d'Or and Mira terranes of Cape Breton Island are compared here with previous results from SE New England. These data, representing both Ganderian and Avalonian elements transferred from Gondwana, bear on the accretionary history of these terranes before, during and after reaching Laurentia.

As in SE New England, secondary remanences are identified in Cape Breton Island as consistent magnetization directions in rocks of differing ages. The S- to SSE-trending and gently downward pointing direction reported in 1985 by Johnson and Van der Voo in Middle Cambrian sedimentary rocks of the Bourinot Group (Bras d’Or terrane), for example, is also present in the 563 Ma Main à Dieu Formation and in 620 Ma Chisholm Brook Granite and East Bay Hill rhyolite (Mira terrane). This magnetization matches the C component already found in 595 Ma Lynn-Mattapan volcanic rocks and 490-488 Ma Nahant Gabbro around Boston, MA. The resulting virtual geomagnetic poles (VGPs) in both areas occupy Mid-Paleozoic positions on Van der Voo’s 1990 North American apparent polar wander path (APWP) that are consistent with a Neo-Acadian overprint, possibly related to the docking of the Meguma terrane against previously accreted Avalonia. Magnetic B components (S- to SSE–trending and moderately downward pointing) in Boston-area rocks give rise to VGPs that fall along the Late Silurian to Lower Devonian segment of the APWP corresponding to the interval when Avalonia is generally thought to have docked with Laurentia.

Other overprint directions from our preliminary Cape Breton Island results yield VGPs that do not coincide with the North American APWP, and thus appear to reflect tectonic interactions en route to Laurentia. The most intriguing of these is a low-latitude cluster of VGPs lying near the Late Ordovician overprint VGPs from Swedish limestones (after rotation into a North American reference frame). Other workers have previously interpreted these Baltic VGPs to reflect the collision between East Avalonia and Baltica during closure of the Tornquist Sea.