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 an incomplete understanding of the spatial and temporal evolution of physical and chemical processes at 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 physical processes and observations relevant to rifted margin evolution. We synthesize results from geological observations of fossil OCTs preserved in ophiolites, a recent seismic experiment on the Ivorian margin, and numerical modeling 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 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 that a hot, upwelling, melt-rich asthenosphere is an important control on the local stress environment. During the exhumation phase, continentward-dipping shear zones couple with seaward-dipping detachment faults to exhume the subcontinental and formerly asthenospheric mantle. The mantle forms 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.