The Mira-Bras d'Or terrane boundary in Cape Breton Island, Nova Scotia: Potential field and petrophysical investigations applied to tectonic analysis in the northern Appalachian orogen

The Mira and Bras d'Or terranes in Cape Breton Island have been recognized previously on the basis of geological, geochemical and isotopic data, and correlated with the Avalon and Central Composite terranes, respectively, of the northern Appalachian orogen. However, the location and orientation in three dimensions of the terrane boundary is for the most part equivocal due to water and/or sedimentary cover sequences.; In this study, petrophysical measurements on a large representative suite of samples from southern Cape Breton Island assisted the interpretation of available regional gravity and magnetic data. Potential field models constructed from these data sets were used to assess and compare the geophysical character of the Mira and Bras d'Or terranes and evaluate the geometry of the boundary between them. Map patterns and petrophysical data show systematic contrasts between the Mira and Bras d'Or terranes. Furthermore, derived physical properties and long wavelength map patterns suggest that deeper units defined by potential field models are lithologically distinct from exposed and near-surface units and from one another across the terrane boundary. These deeper components may represent the rocks on which the exposed units of the Mira and Bras d'Or terranes were deposited. The terrane boundary is sub-vertical at surface and shallows to 50–70° NW. Correlative surface faults may include the Georges River Fault and/or the Florence Fault zone. The Mira-Bras d'Or terrane boundary and associated structural features were reactivated and offset during several episodes of Devonian and younger tectonic activity. The terrane boundary appears to have been a focus for Carboniferous basin development.