The feedback between dyke and sill intrusions and the evolution of stresses within volcanic systems is poorly understood, despite its importance for magma transport and volcano instability. Long-lived ocean island volcanoes are crosscut by thousands of dykes, which must be accommodated through a combination of flank slip and visco-elastic deformation. Flank slip is dominant in some volcanoes (e.g., Kilauea), but how intrusions are accommodated in other volcanic systems remains unknown. Here we apply digital mapping techniques to collect > 400,000 orientation and aperture measurements from 519 sheet intrusions within Volcán Taburiente (La Palma, Canary Islands, Spain) and investigate their emplacement and accommodation. We show that vertically ascending dykes were deflected to propagate laterally as they approached the surface of the volcano, forming a radial dyke swarm, and propose a visco-elastic model for their accommodation. Our model reproduces the measured dyke-aperture distribution and predicts that stress accumulates within densely intruded regions of the volcano, blocking subsequent dykes and causing eruptive activity to migrate. These results have significant implications for the organisation of magma transport within volcanic edifices, and the evolution and stability of long-lived volcanic systems.

We investigate the conditions under which saucer‐shaped sills form and segment in the upper crust. We performed a series of scaled laboratory experiments that employ visco‐elastic‐plastic Laponite RD® (LRD) gels to model upper crustal rocks, and Newtonian paraffin oil as the magma analog. Saucer‐shaped sills always formed in experiments with a two‐layer upper crust. These experiments show sharp transitions from an inner flat sill to outer inclined sheets, which are characterized by non‐planar margins. The results show that: (a) the transition from an inner flat sill to an outer inclined sheet occurs when the sill radius to overburden depth ratio is between 0.5 and 2.5; (b) outer inclined sheets dip angles vary from 15° to 25°; (c) this transition is controlled by the ratio of the Young's modulus between the host material layers; (d) irregular finger‐like and/or lobate segment geometries form at the propagating tip of the experimental sills; and (e) the evolution and geometry of marginal segments and their connectors are different within the inner and outer sill. The results also suggest that there is no strict requirement for high horizontal stresses (>5 MPa) to form natural saucer‐shaped sill geometries. We conclude that analog experiments of magma emplacement into layered visco‐elastic‐plastic upper crustal analogs reproduce the complexity of natural saucer‐shaped sills and their marginal segmentation. The behavior of the experimental sills is compatible with brittle‐elastic fracture mechanisms operating at the intrusion scale, while marginal lobes and finger‐like segments are most likely linked to small‐scale visco‐plastic instabilities occurring at the crack tip scale, possibly aided by the low fracture surface energy of the host material.

The propagating margins of igneous sills (and other sheet intrusions) may divide into laterally and/or vertically separated sections, which later inflate and coalesce. These components elongate parallel to and thus record the magma flow direction, and they can form either due to fracture segmentation (i.e., “segments”) or brittle and/or non-brittle deformation of the host rock (i.e., “magma fingers”). Seismic reflection data can image entire sills or sill-complexes in 3-D, and their resolution is often sufficient to allow us to identify these distinct elongate components and thereby map magma flow patterns over entire intrusion networks. However, seismic resolution is limited, so we typically cannot discern the centimeter- to meter-scale host rock deformation structures that would allow the origin of these components to be interpreted. Here, we introduce a new term that defines the components (i.e., “elements”) of sheet-like igneous intrusions without linking their description to emplacement mechanisms. Using 3-D seismic reflection data from offshore NW Australia, we quantify the 3-D geometry of these elements and their connectors within two sills and discuss how their shape may relate to emplacement processes. Based on seismic attribute analyses and our measurements of their 3-D geometry, we conclude that the mapped elements likely formed through non-elastic-brittle and/or non-brittle deformation ahead of the advancing sill tip, which implies they are magma fingers. We show that thickness varies across sills, and across distinct elements, which we infer to represent flow localization and subsequent thickening of restricted areas. The quantification of element geometries is useful for comparisons between different subsurface and field-based data sets that span a range of host rock types and tectonic settings. This, in turn, facilitates the testing of magma emplacement mechanisms and predictions from numerical and physical analogue experiments.

Magma emplacement is commonly accommodated by uplift of the overburden and free surface. By assuming this deformation is purely elastic, we can invert the shape and kinematics of ground deformation to model the geometry and dynamics of underlying intrusions. However, magma emplacement can be accommodated by viscoelastic and/or inelastic processes. We use 3D seismic reflection data to reconstruct how elastic bending and inelastic processes accommodated emplacement of a Late Jurassic sill offshore NW Australia. We restore syn-emplacement ground deformation and compare its relief to sill thickness, showing that: (i) where they are equal, elastic bending accommodated intrusion; but (ii) where sill thickness is greater, inversion of a pre-existing fault and overburden compaction contributed to magma accommodation. Our results support work showing inelastic processes can suppress ground deformation, and demonstrate magmatism can modify fault displacements. Reflection seismology is thus powerful tool for unravelling links between magma emplacement, ground deformation, and faulting.

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