Modelling the contrasting tectonic and magmatic evolution of rifted and transform margins and subsequent oceanic spreading

The links between tectonics, surface processes and magmatism govern the evolution of rifted and transform margins. Quantifying the control of surface and deep Earth processes, lithosphere rheology and plate kinematics is challenging because of their non-linear interactions. We designed and conducted systematic 3D magmatic-thermo-mechanical numerical experiments coupled with surface processes modelling to better understand the formation of rifted and transform continental margins. Oceanic transform faults are formed by either the opposite polarity of oceanic detachment faults or their formation is linked to the gradual interaction between two propagating rift and spreading centers. Lower divergence velocities, faster crustal and slower mantle thinning, lower surface processes (i.e. erosion and sedimentation) rates, and lower mantle potential temperature lead to the formation of magma-starved continental margins, mantle exhumation and eventually the formation of a stable transform fault zone with a magma-starved, deep transform valley. Suppressed melting and small-scale mantle instabilities govern the along-ridge variation of magmatic and non-magmatic segments, often leading to V-shaped zero-offset oceanic fracture zones. In contrast, faster divergence, lithospheric mantle inherited weak zones, enhanced erosion and sedimentation, result in enhanced mantle melting, and rift magmatism and the formation of a spreading center in the transform zone. Models simulating the temporal increase of divergence velocities show the evolution from an initial magma-poor to a final magma-rich oceanic basin. In models without simulating mantle melting, enhanced surface processes lead to delayed break-up linked to a longer continental hyper-extended stage. However, enhanced surface processes and a more localized and accelerated lithospheric mantle thinning can promote earlier mantle melting and the formation of magma-chambers beneath the crust.