Syn-rift magmatism and spreading initiation controlled by rift obliquity: insights from 3D thermo-mechanical modelling and observations

Continental rifting is often oblique, influenced by the strain localization effects of the various structural, compositional and thermal heterogeneity zones pre-existing in the lithosphere. Oblique rifting generates strain partitioning and leads to the along-strike segmentation of the rift structure, including the development of strike-slip transfer zones and en echelon fault geometries. While previous modelling studies have explored the relation between the rift obliquity and crustal fault patterns, its effects on the syn-rift magmatism and the oceanic spreading initiation have remained underexplored.

In this study, we conducted a series of high resolution 3D numerical models using the I3ELVIS-FDSPM numerical code to compare the continental rift evolution and spreading initiation in orthogonal and oblique rift settings. The code handles visco-plastic rheologies, staggered finite differences and marker-in-cell techniques to solve the mass, momentum and energy conservation equations for incompressible media. Oblique rifting is linked to strain localization along a pre-defined hydrated weak zone in the mantle lithosphere that simulates an inherited suture zone, while the applied two-way coupling between the thermo-mechanical and surface processes models allows for the quantification of the dynamic feedbacks between rift obliquity, crustal strain patterns, magmatism, and the erosion-sedimentation processes.

The models show that oblique rifting delays the onset of melting and continental break-up. Due to the feedbacks between crustal deformation, thermal evolution and melting, increasing rift obliquity leads to the non-linear reduction of the crustal melt supply, while at higher rift obliquity (α>30°), the en echelon arrangement of the elongated magma chambers in the crust suggests a strong structural control over the spatial distribution of crustal melts. When the rift evolution enters the spreading stage, first continental break-up occurs along the offset sub-orthogonal rift segments, and the individual embryonic oceanic segments are subsequently merged by the two-directional along-strike propagation of the incipient spreading ridges. The rate of this propagation changes in space and time, driven by the variable efficiency of strain localization. Above 30° obliquity, deformation along the offset spreading ridges is accommodated by oceanic transform faults that develop spontaneously, without a precursory lithospheric inhomogeneity in their place during the latest stage of spreading initiation. These inferences are in line with observations from the Woodlark Basin and Main Ethiopian Rift.