A PLUME HEAD MELTING UNDER A RIFTING MARGIN

A large igneous province (LIP), in the form of a long narrow band of t hickened oceanic crust, runs along the Atlantic margin of North Americ a abutting the rifted continental shelf. We propose that this, like ma ny other LIPs, has a mantle plume origin. There is evidence that when the central Atlantic Ocean opened the rift was underlain by the flatte ned head of a mantle plume, and that the rift site had drifted away fr om the plume tail by the time of the rifting, so that the tail took li ttle part in the formation of the LlP. We carried out numerical simula tions in which we rifted the lithosphere over various model plume head s and calculated the volumes of melt produced. We found that the thick ness and width of the resulting thickened oceanic crust is very sensit ive 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 t emperature-dependent viscosity passes through a significant step reduc tion in the background mantle viscosity at 660 km depth. However, an e xtensive layer of low viscosity under the rift results in a region of thickened crust much wider than the layer is deep, by decoupling the m antle 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. (C) 1998 Elsevier Science B.V. All rig hts reserved.