RAPID OUTER MARGINAL COLLAPSE AT THE RIFT-DRIFT TRANSITION: GULF OF MEXICO CASE EXAMPLE

Thick (5 km) rapid (<3 Ma) salt deposition is problematic for basin modelling because such accommodation cannot be thermal, yet GoM salt (Callovian-E Oxfordian) appears to be post-rift (most salt overlies syn-rift section at planar base-salt unconformities). One possible solution is that the GoM rift was a deep (~2 km) air-filled depression where basement had already subsided tectonically, and thus could receive up to 5 km of salt, roughly the isostatic maximum on exhumed mantle, hyper-thinned continent, or new ocean crust. Seismic data along W Florida and NW Yucatán show that 1) mother salt was only 1 km thick in these areas, 2) that these areas were depositionally connected to areas of thicker deposition, and 3) top of all salt was near global sea level, and hence the base-salt unconformity along Florida and Yucatán was only 1 km sub-sea level at end of salt deposition. These observations fit the air-filled chasm model; however, two further observations make that mechanism highly improbable: 1) basinward limits of base-salt unconformities along Florida/Yucatán are deeper than top of adjacent ocean crust emplaced at ~2.7 km subsea (shown by backstripping), and 2) deepest abyssal sediments over ocean crust onlap the top of distal salt, demonstrating that the salt itself was rapidly drowned after deposition. Global ION seismic datasets demonstrate the process of “rapid outer marginal collapse” at most margins, which we believe is achieved by low-angle detachment on deep, landward-dipping, Moho-equivalent surfaces such that outer rift margins comprise crustal scale hanging walls over rising mantle footwalls. The tectonic accommodation space produced (2-3 km, < 3 Ma) can be filled by ~5 km of sag/salt sequences with little apparent hanging wall rifting. When salt deposition ends during outer marginal collapse, deep-water settings over thin salt result. We suggest that this newly identified, “outer marginal detachment” rift phase, normally separates the traditional rift from drift stages during continental margin creation. Importantly, this 2-3 km of subsidence presently is neither treated as tectonic nor as thermal in traditional subsidence analysis; Beta estimates may thus be excessive at outer margins. Outer marginal collapse was probably eastwardly diachronous along with initiation of spreading in the GoM.