Evolution of magma-poor continental margins from rifting to seafloor spreading

The rifting of continents involves faulting (tectonism) and magmatism, whic h reflect the strain-rate and temperature dependent processes of solid-stat e deformation and decompression melting within the Earth(1,2). Most models of this rifting have treated tectonism and magmatism separately, and few nu merical simulations have attempted to include continental break-up and melt ing, let alone describe how continental rifting evolves into seafloor sprea ding. Models of this evolution conventionally juxtapose continental and oce anic crust. Here we present observations that support the existence of a zo ne of exhumed continental mantle, several tens of kilometres wide, between oceanic and continental crust on continental margins where magma-poor rifti ng has taken place. We present geophysical and geological observations from the west Iberia margin(3-7), and geological mapping of margins of the form er Tethys ocean now exposed in the Alps(8-13). We use these complementary f indings to propose a conceptual model that focuses on the final stage of co ntinental extension and break-up, and the creation of a zone of exhumed con tinental mantle that evolves oceanward into seafloor spreading. We conclude that the evolving stress and thermal fields are constrained by a rising an d narrowing ridge of asthenospheric mantle, and that magmatism and rates of extension systematically increase oceanward.