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

Dykes, sills, and inclined sheets may intrude as continuous, planar sheets, but can also form by the amalgamation of discrete injections of magma. Describing the host rock deformation style around distinct geometric components within sheet intrusions can help us to determine emplacement mechanisms, how magma flow influences accumulation of economic resources, and how magma migration may translate into ground surface deformation prior to volcanic eruptions. Geometric components, such as ‘magma fingers’ and ‘segments’, form during sheet propagation by brittle and/or non-brittle emplacement mechanisms (i.e., shear failure, host rock fluidization), and fracture segmentation (i.e., elastic-brittle fractures), respectively. Seismic reflection data provide unique opportunities to map the 3D geometry of igneous sills over large areas and in high spatial detail, allowing us to identify these distinct geometric components and constrain magma flow patterns. However, limitations in the resolution of seismic reflection data mean we cannot discern the host rock deformation structures that allow the origin of these geometric components to be interpreted. Here, we use 3D seismic reflection data located offshore NW Australia to: i) introduce new terminology that defines the building blocks (i.e., elements) of sheet-like igneous intrusions and their connectors (i.e., H-type, S/Z-type), without tacitly linking their description to emplacement mechanisms; ii) quantify the 3D geometry of these elements and their connectors in two sills; and iii) discuss how element and connector geometries can be related to emplacement processes. For example, our measurements of connector heights along connectors exclude syn-emplacement rotation of principal stress axes as a primary segmentation mechanism for the host sills. Based on seismic attribute analyses and the 3D geometry of elements, we conclude that potential brittle and/or non-brittle magma emplacement processes led to sill segmentation and to the formation of magma fingers. We show thickness varies across sills, and across distinct elements, which suggests that they grew and inflated independently with individual flow kinematics. Our data of element geometries and their connectors: i) permit comparison between different subsurface and field datasets spanning a range of host rock types and tectonic settings; and ii) allow us to test predictions of numerical and physical analogue models, helping us better understand magma emplacement mechanisms.