Dynamo-thermal subsidence and sag–salt section deposition as magma-rich rifted margins move off plume centres along incipient lines of break-up

Abstract

Magma-rich rifting generally occurs over mantle plumes or rising convection cells; magma-poor rifting does not. Plumes produce up to 2 km dynamic elevation, outcompeting synrift subsidence in active and ancient magma-rich rifts, as evidenced by top-rift unconformities. Magma-rich margins show anomalously fast and large post-tectonic subsidence, evidenced by thick, short-lived little-faulted sag–salt sections. Syndepositional thermal subsidence cannot be responsible alone, and lack of faulting argues against tectonic subsidence. We argue that dissipation of dynamic topography (dynamic subsidence) is the additional component with thermal subsidence. We depict the formation of the magma-rich central South Atlantic and Gulf of Mexico salt-bearing margins relative to former plumes, showing that sag–salt deposition occurs as the margins migrate off plume flanks while supra-plume rifting continues. However, the pre-sag–salt basement must first be extended or eroded during dynamic elevation to fall below its pre-uplift elevation. ‘Dynamo-thermal subsidence’ creates ‘dynamo-thermal accommodation’, the sum and combined result of dynamic plus thermal subsidence, respectively. It is fast enough to negate the need for deep, sub-sea-level, subaerial depressions for accumulation of thick sag–salt sections. Also, differences in dynamic elevation during salt deposition explain differences in autochthonous salt thickness. Supplementary material : Supplementary figures, seismic data, structural restorations, a compilation of igneous activity and an extended figure caption are available at https://doi.org/10.6084/m9.figshare.c.5908896