The transition from a volcanic to a nonvolcanic rifted margin off eastern Canada

Abstract

Three multichannel seismic reflection profiles were collected on the riffed continental margin southeast of Nova Scotia, eastem Canada. The profiles cross the East Coast Magnetic Anomaly (ECMA), which parallels much of the margin of eastem 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 riff stage or early driff 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 preriff fabric of the adjacent continental crust, in the trend of synriff 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 riffing.

Architecture of rifted continental margins and break-up evolution: insights from the South Atlantic, North Atlantic and Red Sea–Gulf of Aden conjugate margins

“…Moving northeastward along strike in Nova Scotia the volcanic features are less evident in the seismic profiles, especially east of 62°N up to the Orpheus Graben and the terrainbounding Cobequid-Chedabucto Fault Zone (Keen & Potter 1995). Offshore Newfoundland the margin becomes magma-poor (Manatschal & Bernoulli 1998;Péron-Pinvidic & Manatschal, 2008;), although several authors have interpreted oceanic basalts in the transitional crust basinward of the Flemish Cap (e.g., Enachescu 1988).…”

“…The evaporites are almost coeval with emplacement of tholeiitic volcanic rocks (i.e., CAMP) in both onshore and offshore basins (Roy & Piqué 2001;McHone et al 2005;Brown et al 2008;Luheshi et al 2012). The southwestern Scotian Basin and the entire U.S. East Coast margin are magma-rich, being characterized by massive wedges of seaward-dipping reflectors (Keen & Potter, 1995;Talwani & Abreu 2000).…”

Relative timing of CAMP, rifting, continental breakup, and basin inversion: Tectonic significance

“…The wedge lies near the continent-ocean boundary, and formed during the transition from rifting to drifting [Hinz, 1981;Benson and Doyle, 1988;Austin et al, 1990]. A wedge is not observed on the passive margin of southeastern Canada [Keen and Potter, 1995]. The exact age of the wedge (and, by inference, the onset of seafloor spreading) is unknown and, in fact, may vary along the margin [e.g., Oh et al, 1995].…”

Small-scale convection beneath oceans and continents

“…Only the Sohm plain is not tracked by obvious hotspots [44]. Its northern part lies offshore of a nonvolcanic margin [43]. Its depth is that predicted by the half-space model.…”

“…A volcanic margin exists from the Bahamas into southern Canada [43]. The New England, Corner, and Great Meteor seamounts are a long-lived track.…”