Sources of magnetic and gravity anomalies on the Scotian Shelf southeast of Cape Breton Island, Nova Scotia, and onshore-offshore geological correlations using geophysical modeling.

摘要

The Scatarie Ridge Magnetic Anomalies (SRMA) form a prominent northeast-trending belt, 100 km long by 25 km wide, on the Scotian Shelf southeast of Cape Breton Island. Using the second vertical derivative magnetic map, the SRMA has been resolved into a linear anomaly trending east-northeast, and two large magnetic aureoles located north of the central part of the SRMA. Forward potential field models, constrained by seismic and well log data in the offshore and density and susceptibility measurements from onshore and some offshore units indicate that the southern margin of the linear anomaly is the northern margin of the Orpheus Graben. A south-dipping fault separates rocks of the Meguma terrane to the south from rocks of the Avalon terrane to the north of the graben. In the models, the Avalon terrane to a depth of about 15 km consists of five geological units with different magnetic susceptibility and density values. The source of the large linear anomaly is interpreted to be a belt of mafic volcanic rocks at a depth of 6--15 km. The large magnetic aureoles have associated gravity anomalies indicating that they are caused by granite plutons and magnetic rocks in the surrounding contact metamorphic aureoles. In the Gabarus Bay area, similar magnetic aureoles are associated with the Devonian Deep Cove granite and other intrusions. In contrast, two granitic bodies south of the Orpheus Graben in the Meguma terrane have densities and magnetic susceptibilities similar to Devonian plutons onshore in the Meguma terrane. Upper crustal units beneath the Avalon terrane offshore have higher susceptibility and lower density than units beneath the onshore Mira terrane, suggesting that the offshore area is a different part of the Avalon terrane, perhaps equivalent to the Antigonish Highlands or eastern Newfoundland.