The shallow transport of magma occurs through dikes causing surface deformation. Our understanding of the effects of diking at the surface is limited, especially on the long term, for repeated intrusive episodes. We use analogue models to study the upper crustal deformation induced by dikes. We insert metal plates within cohesive sand with three setups: in setup A, the intrusion rises upward with constant thickness and in setups B and C, the intrusion thickens at a fixed depth, with final rectangular (setup B) or triangular (setup C) shape in section. Setup A creates a doming delimited by reverse faults, with secondary apical graben, without close correspondence in nature. In setups B and C, a depression flanked by two uplifted areas is bordered by inward dipping normal faults propagating downward and, for deeper intrusions in setup B, also by inner faults, reverse at the surface; this deformation is similar to what is observed in nature, suggesting a consistent physical behavior. Dikes in nature initially propagate developing a mode I fracture at the tip, subsequently thickened by magma intrusion, without any host rock translation in the propagation direction (as in setup A). The deformation pattern in setups B and C depends on the intrusion depth and thickness, consistently to what is observed along divergent plate boundaries. The early deformation in setups B and C is similar to that from a single rifting episode (i.e., Lakagigar, Iceland, and Dabbahu, Afar), whereas the late stages resemble the structure of mature rifts (i.e., Krafla, Iceland), confirming diking as a major process in shaping divergent plate boundaries.

Recent studies highlight the importance of annual‐scale dike‐induced rifting episodes in developing normal faults and graben along the active axis of magmatic divergent plate boundaries (MDPB). However, the longer‐term (102–105 years) role of diking on the cumulative surface deformation and evolution of MDPB is not yet well understood. To better understand the longer‐term normal faults and graben along the axis of MDPB, we analyze fissure swarms in Iceland and Ethiopia. We first focus on the simplest case of immature fissure swarms, with single dike‐fed eruptive fissures; these consist of a <1 km wide graben bordered by normal faults with displacement up to a few meters, consistent with theoretical models and geodetic data. A similar structural pattern is found, with asymmetric and multiple graben, within wider mature fissure swarms, formed by several dike‐fed eruptive fissures. We then consider the lateral termination of normal faults along these grabens to detect their upward or downward propagation. Most faults terminate as open fractures on flat surface, suggesting downward fault propagation; this is consistent with recent experiments showing dike‐induced normal faults propagating downward from the surface. However, some normal faults also terminate as open fractures on monoclines, which resemble fault propagation folds; this suggests upward propagation of reactivated buried faults, promoted by diking. These results suggest that fault growth and graben development, as well as the longer‐term evolution of the axis of MDPB, may be explained only through dike emplacement and that any amagmatic faulting is not necessary.

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Recent rifting episodes highlight the role of magmatic systems with propagating dikes on crustal spreading. However, our knowledge of magmatic systems is usually limited to surface observations and geophysical data. Eastern Iceland allows direct access to extinct and eroded deeper magmatic systems. Here we collected field structural and AMS (anisotropy of magnetic susceptibility) data on 187 and 19 dikes, respectively, in the 10–12 Ma old Alftafjordur magmatic system. At a paleodepth of ~1.5 km, the extension due to diking is at least 1–2 orders of magnitude larger than that induced by regional tectonics, confirming magmatism as the key mechanism for crustal spreading. This magma‐induced extension, inferred from the aspect ratio of the magmatic system, was of ~8 mm/yr, lower than the present one. AMS data suggest that most of dikes have geometrically normal fabric, at least at the margins, consistent with prevalent subvertical magma flow and propagation.

Recent diking episodes along slow spreading boundaries included the generation of normal faults, showing that extension is accommodated, on a scale of a few years or less, by both magma intrusion and fault movement. Here we aim to define how diking may affect the overall rift structure on the longer term (=100 yr). We first summarize the main features of the transient diking episodes as obtained from geological, geophysical, geodetic, and modeling studies. We then put these episodes into a broader context, considering the overall longer term shallow and deep structure of the plate boundaries. The synthesis of the data shows that in Iceland crustal extension at depth largely occurs by means of dikes, with negligible normal faulting; faults focus toward the surface (<1 km depth), forming dike-induced grabens commonly propagating downward; along-rift diking episodes (1 in ~200 yr) may induce all the observed surface deformation. The close similarities with transitional (Afar Rift) and magmatic continental rifts (East African Rift System) suggest that repeated diking induces most of the surface deformation along slowly spreading (≤2 cm/yr) magmatic plate boundaries. The frequency of diking and the induced strain may not allow extension to be accommodated amagmatically, through creep or seismic or aseismic faulting. This implies that a diking episode locks any amagmatic faulting until the strain is released (centuries), when the subsequent diking episode occurs, with the cumulative result of controlling and shaping the evolution of slow spreading magmatic plate boundaries. This process appears independent of the stage of magmatic rifting