Mantle Deformation Processes during the Rift-to-Drift Transition at Magma-Poor Margins

The rift-to-drift transition at rifted margins is an area of active investigation due to the unresolved issues of the the ocean-continent transition. The deep structures that characterize present-day OCTs are often difficult to identify by seismic observations, while terrestrial exposures are preserved only in fragments separated by tectonic discontinuities, such as at some ophiolites. Numerical modeling is a powerful method for contextualizing observations within rifted margin evolution. In this article, we synthesize geological observations from various fossil ocean-continent transitions preserved in ophiolites and from a recent seismic experiment on the Ivorian Margin of West Africa with a novel formulation of GeoFLAC to characterize mantle deformation and melt production for magma-poor margins. Across varied surface heat fluxes, mantle potential temperatures, and extension rates our numerical modeling results show important homologies with geological observations. We propose that the development of large shear zones in the subcontinental mantle, melt infiltration, grain size reduction, and anastomosing detachment faults control the structure of the ocean-continent transition. We also infer, through changes in fault orientation, that the active push of upwelling, melt-rich asthenosphere is an important control on the local stress environment. During the exhumation phase of rifted margin evolution, continentward-dipping shear zones couple with seaward-dipping crustal detachment faults to partially exhume the subcontinental and former asthenospheric mantle. The mantle and crust form lithospheric boudinage that creates core-complex-like domes of peridotite at or near the surface. The faults that exhume these peridotite bodies are largely anastomosing and exhibit magmatic accretion in their footwalls. A combination of magmatic accretion and volcanic activity derived from the shallow melt region constructs the oceanic lithosphere in the footwalls of the out-of-sequence, continentward-dipping detachment faults in the oceanic crust and subcontinental mantle