The building blocks of igneous sheet intrusions: insights from 3D seismic reflection data

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.