DECOMPRESSION MELTING AT RIFTED MARGINS COMPARISON OF MODEL PREDICTIONS WITH THE DISTRIBUTION OF IGNEOUS ROCKS ON THE EASTERN CANADIAN MARGIN

Estimates of the distribution of igneous rocks generated by decompression melting of the mantle during extension are presented for the rifted continental margin off eastern Canada. Melting is assumed to be caused by upwelling of mantle material as a result of stretching and thinning of the lithosphere during rifting. The amount of lithospheric stretching and associated melting are computed from the post-rift and the syn-rift subsidences, which are mapped across the continental margins, from the northern Labrador margin to the margin off Nova Scotia. Model results are constrained by comparing predictions with seismic observations of depth to Moho and thickness of a high-velocity lower crustal layer which may result from magmatic underplating, The predicted melt volumes are sensitive to a number of input parameters specifying the initial physical state of the lithosphere, of which the most important is temperature at the base of the lithosphere. An equally important factor in predicting the magmatic history of rifted margins is the time-dependence of deformation during rifting, and the model results imply, for example, that a significant delay between the onset of rifting and of igneous activity may occur. The mapped distribution of melt layer thickness does not always match observations of the distribution of igneous rocks in the region, which suggests that other processes may also be important. As plumes do not appear to play a major role, we suggest that lateral migration of melt, perhaps over several hundred kilometres and/or small-scale convection may be important controls in the distribution of volcanism in this region.