Interpretation of ION-GXT long-offset 2D depth-imaged and other seismic data suggests that outer continental margins collapse and tilt basinward rapidly as rifting yields to seafloor spreading. The outer continental margin, already thinned by rifting processes, can be viewed as the hanging wall of a mega-half-graben associated with a landward dipping shear zone. The footwall of the shearmega is the serpentinised or serpentinising sub-continental mantle that commonly becomes exhumed as it rises from beneath the thinned and embrittling continental margin. We call this shear zone between the continental crust and the rising subcontinental mantle the “outer marginal detachment.” At magma-poor...