THE TRANSITION FROM A VOLCANIC TO A NONVOLCANIC RIFTED MARGIN OFF EASTERN CANADA

Three multichannel seismic reflection profiles were collected on the rifted continental margin southeast of Nova Scotia, eastern Canada. The profiles cross the East Coast Magnetic Anomaly (ECMA), which parallels much of the margin of eastern North America south of the Grand Banks and which is usually associated with the transition from continental to oceanic crust. Studies to the south of the work reported here suggest that the ECMA may be related to the emplacement of large thicknesses of late rift stage or early drift stage igneous material which is characterized by seaward dipping reflections in basement and a high-velocity lower crustal layer. The seismic data show that seaward dipping reflections (SDR) continue northward into the study area and support the correlation between the SDR unit and the presence of a well-developed ECMA. Magnetic modellng confirms this association, although it does not rule out an additional contribution to the magnetic anomaly from an edge effect or suture. Just north of the study area the ECMA diminishes and is no longer well developed. The SDR unit also terminates and it is not observed over most of the Nova Scotian margin. If our understanding of the origin of these features is correct then their disappearance marks a transition from a volcanic margin in the south to a nonvolcanic margin in the north. The association of the transition with significant changes in the prerift fabric of the adjacent continental crust, in the trend of synrift extensional structures, and in the width of the zone of thinned continental crust below the margins must be clues to the deeper processes controlling the amount of volcanism produced. We suggest that these clues are consistent with small-scale convection as a mechanism for delivering large melt volumes to crustal depths during rifting.