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When continents rift, magmatism can produce large volumes of melt that migrate upwards from deep below the Earth’s surface. To understand the role of magmatism in rifting, it is critical to understand how much melt is generated and how it transits the crust. Estimating melt volumes and pathways is, however, difficult, particularly in the lower crust where the resolution of geophysical techniques is limited. New broadband seismic reflection data allow us to image the 3-D geometry of magma crystallized in the lower crust (17.5-22 km depth) of the northern North Sea, in an area previously considered a magma-poor rift. The sub-horizontal magmatic intrusion is ~97 km long (N-S), ~62 km wide (E-W), and comprises several irregular lobes. The significant areal extent of the intrusion (~2700 km^2), as well as presence of intrusive steps indicate widespread lateral magma transport in the lower crust. We estimate that 472±161 km^3 of magma was emplaced within this intrusion, suggesting that the northern North Sea is more magmatic than previously thought.

Host rock deformation in active volcanic settings can signal and be used to constrain magma emplacement. Yet it is difficult to evaluate the accuracy of intrusion parameters derived from inversion of deformation signals because we cannot test estimates by directly accessing the magma body. Physical modelling is thus critical to understanding how intrusion translates into host rock deformation, particularly surface uplift and/or subsidence, because we can use transparent materials or excavate models to view the actual intrusion geometry. However, few physical models have investigated how a heterogeneous, layered host material impacts magma emplacement, despite evidence suggesting the presence of weak layers can control intrusion style and geometry. We conduct several models that simulate emplacement of a felsic magma at ~6 km depth within a granular (sand) host rock; in two of our models we incorporate two, thin, weak microbead layers into the layered host material. We show that intrusion solely within the granular material is primarily accommodated by lateral contraction (compaction and folding) of the host material, resulting in a dyke-like intrusion that erupted. When the microbead layers were present, a cone sheet and saucer-shaped sill preferentially formed, without erupting, accommodated by forced folding. Furthermore, we demonstrate that surface deformation does not simply reflect the complexity of the intrusion geometry or internal host material deformation. Overall, our results indicate that physical models should further explore the role of host material heterogeneity on magma emplacement.

Layer-bound, low-displacement normal faults, arranged into a broadly polygonal pattern, are common in many sedimentary basins. Despite having constrained their gross geometry, we have a relatively poor understanding of the processes controlling the nucleation and growth (i.e., the kinematics) of polygonal fault systems. In this study we use high-resolution 3-D seismic reflection and borehole data from the northern North Sea to undertake a detailed kinematic analysis of faults forming part of a seismically well-imaged polygonal fault system hosted within the up to 1,000 m thick, Early Palaeocene-to-Middle Miocene mudstones of the Hordaland Group. Growth strata and displacement-depth profiles indicate faulting commenced during the Eocene to early Oligocene, with reactivation possibly occurring in the late Oligocene to middle Miocene. Mapping the position of displacement maxima on 137 polygonal faults suggests that the majority (64%) nucleated in the lower 500 m of the Hordaland Group. The uniform distribution of polygonal fault strikes in the area indicates that nucleation and growth were not driven by gravity or far-field tectonic extension as has previously been suggested. Instead, fault growth was likely facilitated by low coefficients of residual friction on existing slip surfaces, and probably involved significant layer-parallel contraction (strains of 0.01–0.19) of the host strata. To summarize, our kinematic analysis provides new insights into the spatial and temporal evolution of polygonal fault systems.

Borehole checkshot data used to derive a time���depth relationship for estimating seismic velocities and depth-converting measurements

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 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 3D, 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. Yet 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 3D seismic reflection data from offshore NW Australia, we quantify the 3D 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 3D geometry, we conclude that the mapped elements likely formed by non-elastic brittle and/or non-brittle deformation ahead of the advancing sill tip, implying 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 datasets, spanning 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.

Supplementary Figure S1: Uninterpreted version of figure 1

Dyke swarms are common on Earth and other planetary bodies, comprising arrays of dykes that can extend laterally for tens to thousands of kilometres. The vast extent of such dyke swarms, and their presumed rapid emplacement, means they can significantly influence a variety of planetary processes, including continental break-up, crustal extension, resource accumulation, and volcanism. Determining the mechanisms driving dyke swarm emplacement is thus critical to a range of Earth Science disciplines. However, unravelling dyke swarm emplacement mechanics relies on constraining their 3D structure, which is difficult given we typically cannot access their subsurface geometry at a sufficiently high enough resolution. Here we use high-quality seismic reflection data to identify and examine the 3D geometry of the newly discovered Exmouth Dyke Swarm, and associated structures (i.e. dyke-induced normal faults and pit craters). Dykes are expressed in our seismic reflection data as ∼335–68 m wide, vertical zones of disruption (VZD), in which stratal reflections are dimmed and/or deflected from sub-horizontal. Borehole data reveal one ∼130 m wide VZD corresponds to an ∼18 m thick, mafic dyke, highlighting that the true geometry of the inferred dykes may not be fully captured by their seismic expression. The Late Jurassic dyke swarm is located on the Gascoyne Margin, offshore NW Australia, and contains numerous dykes that extend laterally for > 170 km, potentially up to > 500 km, with spacings typically 3.5 km. The mapped dykes are distributed radially across a ∼39∘ wide arc centred on the Cuvier Margin; we infer that this focal area marks the source of the dyke swarm. We demonstrate that seismic reflection data provide unique opportunities to map and quantify dyke swarms in 3D. Because of this, we can now (i) recognise dyke swarms across continental margins worldwide and incorporate them into models of basin evolution and fluid flow, (ii) test previous models and hypotheses concerning the 3D structure of dyke swarms, (iii) reveal how dyke-induced normal faults and pit craters relate to dyking, and (iv) unravel how dyking translates into surface deformation.

Poster originally presented at the Tectonic Studies Group 2020 AGM in Hull, UK.