Role of tectonic stress and topography on repeated lateral dikes: case studies from the 1975–1984 Krafla and 2023–2025 Svartsengi rifting episodes in Iceland

The behavior of temporally sequenced lateral dike intrusions into rift zones depend on intrinsic and extrinsic factors including the pressure build-up in the magma source and the local stress regime influenced by tectonic stress and topography. To reexamine the effects of these factors, we revised the simplified elastic model of rifting by Buck et al. (2006). In the revised model, topographic gradients and tectonic stress, in addition to magma accumulation, contribute to the driving pressure of dike propagation. In our model, dikes follow the positive gradient of driving pressure and open in the segment of the rift zone, where the local maximum driving pressure occurs, while available tectonic stress controls individual and total openings. A case study of the 1975–1984 Krafla rifting episode indicates repeated dike intrusions can be explained by a single magma inlet into the rift zone, located ~2–4 km north of the Krafla caldera center. An inferred magma pressure ~1–10 MPa above lithostatic stress at the inlet prior to the rifting episode generated the first and largest dike intrusion in the entire rifting episode, supported by >20 MPa of driving pressure from tectonic stress and topography. The case study indicates that the magma pressure at the initiation of the first dike is larger than that for later dikes by a factor of 2. The lower magma pressure to initiate later dikes, together with tectonic stress and magma compressibility, permits dike initiations when magma pressure at the inlet is below lithostatic. The model is also adapted to fit spatial distribution of dike openings in the 2023–2025 Svartsengi rifting episode in SW Iceland. In this case, inferred tectonic stress and significant magma buoyancy effects (~15 MPa) enable dike initiations with the magma pressure at the inlet below lithostatic, while topographic effects contribute ~2 MPa more of driving pressure to the southern propagation of the first dike intrusion. Tectonic stress also inhibited eruption from the initial dike of the Svartsengi rifting episode. Our findings demonstrate that tectonic stress and topographic effects are critical factors driving lateral dike propagation in an extensional plate boundary, allowing magma flow into dikes or eruptions under relatively low magma pressure, including magma pressure below lithostatic.