How much continent under the ocean?

A crustal attenuation model, initially proposed to explain the evolution of the Canadian margin of the Labrador Sea, is applicable to both intra-cratonic rift zones and passive continental margins. Examples examined are the East African rifts and the New Zealand Plateau, S. W. Pacific. In the model continental lithosphere is stretched and rifted over thermally and chemically expanding asthenosphere blisters. Magmas, abundantly generated in the zone of partial melting at the lithosphere-asthenosphere boundary, ascend and intrude into the fracturing crust. Upon cooling, these intrusives produce low amplitude magnetic anomalies. Sea-floor spreading begins when the asthenosphere finally breaks through en masse and it is thus only a late phase in a much longer cycle. Attenuated continental crust is often underlain by low velocity, low density, upper mantle, which is indicative of the blending of the crust and mantle. It is suggested that substantial parts of marginal quiet magnetic zones are continent-ocean transitions that formed by crustal attenuation. The attenuation model is used to explain the evolution of the Arctic region. It should help bridge the gap between fixists and mobilists.