A plume head melting under a rifting margin

A large igneous province (LIP), in the form of a long narrow band of thickened oceanic crust, runs along the Atlantic margin of North America abutting the rifted continental shelf. We propose that this, like many other LIPs, has a mantle plume origin. There is evidence that when the central Atlantic Ocean opened the rift was underlain by the flattened head of a mantle plume, and that the rift site had drifted away from the plume tail by the time of the rifting, so that the tail took little part in the formation of the LIP. We carried out numerical simulations in which we rifted the lithosphere over various model plume heads and calculated the volumes of melt produced. We found that the thickness and width of the resulting thickened oceanic crust is very sensitive to the thermal structure directly under the rift and the structure of the lithosphere. To fit observations of the LIP, a thin flat plume head is required. Such a plume head results when a mantle plume with temperature-dependent viscosity passes through a significant step reduction in the background mantle viscosity at 660 km depth. However, an extensive layer of low viscosity under the rift results in a region of thickened crust much wider than the layer is deep, by decoupling the mantle flow from the lithosphere. To avoid decoupling, we propose that there must be significant topography on the lithosphere, and the rift site is a region of thinned lithosphere. Very thick crust next to the margin can be explained by lithospheric necking and the resulting fast initial upflow under the rift.