Magmatically driven antithetic faulting: structural and numerical insights

This study identifies magmatically driven antithetic faulting as a previously unrecognised mechanism responsible for dike-related graben formation. It investigates the structural and mechanical factors that control graben geometry associated with dike intrusion in a key site of the Fremrinamar Rift, in the Northern Volcanic Zone of Iceland. A N-S graben is entirely developed across a topographic high, corresponding to a subglacial pillow lava/ hyaloclastite cone, without affecting the surrounding lava plateau, as the ground is covered by the most recent lava flows. The graben faults display marked asymmetry: the eastern fault, aligned with the rift border fault, exhibits a vertical offset up to one order of magnitude larger than the western fault. Numerical models reproduce this pattern, showing that an inclined dike propagating along a pre-existing fault, and the mechanical contrast between the lava substratum and the weaker subglacial cone, strongly control the stress distribution and the resulting deformation, favouring the formation of antithetic faults. Additionally, such a dike did not produce any eruption on the pillow lava/hyaloclastite cone, instead erupting just north and south of it. This suggests that the dike arrested its upward propagation towards the surface as a result of a combination of factors, including mechanical contrast, gravitational loading, interaction with pre-existing faults, and stress release associated with the nucleation of the antithetic fault. Overall, our results highlight the combined influence of pre-existing structures, topography, and lithological contrasts in shaping dike-induced deformation and graben fault asymmetry in volcanic rift zones.