CONNECTING CAPE BRETON ISLAND AND SOUTHERN NEW BRUNSWICK, CANADA: DEVONIAN STITCHING PLUTONS IN THE SOUTHERN GULF OF ST. LAWRENCE

Tectonostratigraphic terranes in Cape Breton Island appear to be offset to southern New Brunswick by a major dextral structure which has been named the Canso Fault. The position of this fault in the southern Gulf of St. Lawrence area is uncertain, in large part due to the presence of the East Point Magnetic Anomaly (EPMA). This 50 km-wide and 150 km-long magnetic anomaly, on the order of 500 nT in magnitude, trends east-west across the area, and obscures magnetic signatures associated with terranes and terrane boundaries. Using the second vertical derivative magnetic anomaly map, the EPMA has been divided into four magnetic aureoles (WMA, CMA, EMA, and MHMA), attributed to four separate plutonic sources at depth. These sources were evaluated by forward-modeling of magnetic and gravity data, with additional constraints from seismic and petrophysical data and on-land geology in western Cape Breton Island. This procedure established that the source of the easternmost anomaly (MHMA) is the ca. 375 Ma Port Ban Diorite, part of the Mabou Highlands of western Cape Breton Island. The diorite extends off shore for 12 km beneath the Carboniferous and younger sedimentary cover, and appears to have a steep intrusive contact at a depth of about 4 km with the EMA pluton to the west. The EMA, CMA, and WMA plutons have densities and magnetic susceptibilities consistent with granitic bodies which are progressively deeper, denser, and more magnetic toward the west. The unusually high magnetic susceptibility suggests correlation with the highly magnetic ca. 390 Ma Gaytons Quartz Monzonite and associated Lower Coverdale Gabbro/Anorthosite in the Brookville terrane of southern New Brunswick.

Basements at depths of ca. 3-7 km on the north and ca. 5-6 km on the south of the EPMA appear to be different, mainly on the basis of density characteristics. The northern basement has characteristics consistent with those determined in southern New Brunswick for Ganderia, whereas the southern basement is a better match for Brookville-Bras d'Or terrane than for Avalon terrane. The four EPMA source bodies may be "stitching plutons" emplaced along the Canso Fault.