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Abstract. This paper explores the suggestion that observations of large igneous crustal thickness at rifted volcanic margins may in part be explained by small-scale convection in the upper mantle. This may increase the delivery of magma to the overlying lithosphere without the need for anomalously high mantle temperatures. This concept is quantitatively assessed by numerical modeling of the flow, caused by divergent plate motions, in a viscous, temperature-and pressure-dependent, nonlinear fluid. Significant timedependent small-scale convection is generated at the lower model viscosities (and/or higher temperatures, and/or sharper rift geometries). These models can provide the required thick igneous crust observed at volcanic margins. However, they also give excessive variations in the thickness of igneous crust within the ocean basin and are therefore unacceptable. An additional factor we have explored is the recent suggestion that the oceanic mantle may become dehydrated when melt is generated and removed. This increases its viscosity by 2 to 3 orders of magnitude in the melting zone, above -80 km. The high-viscosity lid stabilizes the flow and provides a more uniform oceanic crustal thickness, while at the same time allowing for vigorous small-scale convection during the early history of the rift and the delivery of thick igneous crust against the margin. This factor, coupled to the models for flow described above, allow the main features of volcanic margins to be simulated. Besides the thick igneous crust predicted at the margins, these models suggest that time-dependent small-scale convection below the margin may persist for tens of million years following the onset of seafloor spreading and that there may be coupling between this flow at the margin and that at the ridge axis. The periodicity of this time-dependent convection is primarily dictated by the model viscosities; the models suggest that episodicities of tens of million years are reasonable. These episodicities may be reflected in the record of vertical motions at rifted margins.

Seismic reflection data were collected and processed to 20 s two-way travel time along four lines which cross the rifted continent-ocean boundary off the Grand Banks region of eastern Canada specifically to examine the origin, age, and nature of this fundamental boundary. This represents the first regional study of its kind. The most important result is the presence of landward dipping reflectors near the foot of the continental slope. These occur where oceanic crust appears to terminate against the continent. We suggest that the dipping reflectors mark the continent-ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary. Sedimentary basins lie just landward of the continent-ocean boundary. Their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting, and this may be an important stage leading to continental breakup. Rift basins formed further landward on the Grand Banks do not exhibit this thinning. Other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent-ocean boundary.

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