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Abstract

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 margins, the outer continental margin collapses to tilt basinward (“outer marginal collapse”) to depths up to 3-3.5 km sub-sea at the continent-ocean transition, so that it normally lies deeper than the adjacent oceanic crust (accreted later by seafloor spreading). We use the term “collapse” because of the rapidity of the deepening, which seems to be <3 Ma, and possibly <1 Ma. Salt deposition or rapid clastic (deltaic) sedimentation at non-magmatic margins, or SDRs and less organised magmatism at magmatic margins, may accompany the collapse, with salt thicknesses rapidly reaching 5 km and volcanic piles reaching 15-20 km, as controlled by isostasy. This mechanism of rapid salt deposition allows some megasalt basins (e.g., Gulf of Mexico) to be deposited on synrift sections at global sea level, although other salt basins may form in air-filled depressions below sea level. Outer marginal collapse is “postrift” from the perspective of the continental crust, but of tectonic, and not of thermal, origin, owing to shear on outer marginal detachments. We examine how outer marginal collapse can migrate diachronously along strike, much like the onset of seafloor spreading, as is required in models of ocean creation. We suggest that backstripping estimates of lithospheric thinning (beta) at outer continental margins may be excessive, because they probably attribute outer marginal collapse to thermal subsidence.

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