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

Abstract

The rift-to-drift transition at rifted margins is an area of active investigation due to unresolved issues of the ocean-continent transition (OCT). Deep structures that characterize modern OCTs are often difficult to identify by seismic observations, while terrestrial exposures are preserved in fragments separated by tectonic discontinuities. Numerical modeling is a powerful method for contextualizing observations within rifted margin evolution. In this article, we synthesize geological observations from fossil ocean-continent transitions preserved in ophiolites, a recent seismic experiment on the Ivorian Margin of West Africa, and GeoFLAC models to characterize mantle deformation and melt production for magma-poor margins. Across varied surface heat fluxes, mantle potential temperatures, and extension rates our model results show important homologies with geological observations. We propose that the development of large shear zones in the mantle, melt infiltration, grain size reduction, and anastomosing detachment faults control the structure of OCTs. We also infer through changes in fault orientation that upwelling, melt-rich asthenosphere is an important control on the local stress environment. During the exhumation phase of rifting, continentward-dipping shear zones couple with seaward-dipping detachment faults to exhume the subcontinental and formerly asthenospheric mantle. The mantle forms into 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.