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

Three multichannel seismic reflection profiles were collected on the r ifted continental margin southeast of Nova Scotia, eastern Canada. The profiles cross the East Coast Magnetic Anomaly (ECMA), which parallel s 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 sugge st that the ECMA may be related to the emplacement of large thicknesse s of late rift stage or early drift stage igneous material which is ch aracterized by seaward dipping reflections in basement and a high-velo city lower crustal layer. The seismic data show that seaward dipping r eflections (SDR) continue northward into the study area and support th e correlation between the SDR unit and the presence of a well-develope d ECMA. Magnetic modellng confirms this association, although it does not rule out an additional contribution to the magnetic anomaly from a n edge effect or suture. Just north of the study area the ECMA diminis hes 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 unders tanding of the origin of these features is correct then their disappea rance marks a transition from a volcanic margin in the south to a nonv olcanic margin in the north. The association of the transition with si gnificant changes in the prerift fabric of the adjacent continental cr ust, in the trend of synrift extensional structures, and in the width of the zone of thinned continental crust below the margins must be clu es to the deeper processes controlling the amount of volcanism produce d. We suggest that these clues are consistent with small-scale convect ion as a mechanism for delivering large melt volumes to crustal depths during rifting.