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Dike intrusions commonly generate normal faulting and graben structures in volcanic rift zones, but distinguishing magma-driven deformation from regional tectonics remains challenging, especially where pre-existing faults, topography, and lithological contrasts coexist. Here we document a previously unrecognised mechanism of magmatically driven antithetic faulting, based on an integrated field and numerical study from the Fremrinámur Rift, Northern Iceland.We investigate a N–S-trending graben developed entirely on a Late Glacial subglacial pillow lava–hyaloclastite cone, without deformation of the surrounding lava plateau. High-resolution UAV photogrammetry combined with detailed field mapping reveals a strongly asymmetric graben geometry: the eastern fault, aligned with the rift-border fault, displays vertical offsets up to one order of magnitude larger than the western fault. Eruptive fissures at the northern and southern base of the cone suggest a single dike intrusion event that failed to propagate to the cone summit.To explore the controlling mechanisms, we performed 2D finite-element numerical models simulating dike-induced stress and surface deformation under varying dike dip, intrusion depth, interaction with a pre-existing fault, and host-rock rheology. The models show that an inclined dike propagating along a pre-existing rift-border fault, combined with a strong mechanical contrast between the competent basaltic substratum and the weaker subglacial cone, produces pronounced stress and displacement asymmetry. In this configuration, von Mises shear stresses concentrate within the hanging-wall block, promoting the formation of an antithetic fault, while tensile stresses above the dike tip are significantly reduced, favouring dike arrest within the cone.These results highlight the combined role of fault inheritance, topography, and lithological heterogeneity in controlling dike-induced deformation, fault asymmetry, and intrusion arrest in volcanic rift environments.