Structural signatures of igneous sheet intrusion propagation

• Published in: Journal of structural geology 2019-08, Vol.125, p.148-154
• Main Authors: Magee, Craig, Muirhead, James, Schofield, Nick, Walker, Richard J., Galland, Olivier, Holford, Simon, Spacapan, Juan, Jackson, Christopher A-L., McCarthy, William
• Format: Article
• Language: English
• Subjects: Dyke; Flow; Magma; Sheet intrusion; Sill; Structure
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2018.07.010

The geometry and distribution of planar igneous bodies (i.e. sheet intrusions), such as dykes, sills, and inclined sheets, has long been used to determine emplacement mechanics, define melt source locations, and reconstruct palaeostress conditions to shed light on various tectonic and magmatic processes. Since the 1970's we have recognised that sheet intrusions do not necessarily display a continuous, planar geometry, but commonly consist of segments. The morphology of these segments and their connectors is controlled by, and provide insights into, the behaviour of the host rock during emplacement. For example, tensile brittle fracturing leads to the formation of intrusive steps or bridge structures between adjacent segments. In contrast, brittle shear faulting, cataclastic and ductile flow processes, as well as heat-induced viscous flow or fluidization, promotes magma finger development. Textural indicators of magma flow (e.g., rock fabrics) reveal that segments are aligned parallel to the initial sheet propagation direction. Recognising and mapping segment long axes thus allows melt source location hypotheses, derived from sheet distribution and orientation, to be robustly tested. Despite the information that can be obtained from these structural signatures of sheet intrusion propagation, they are largely overlooked by the structural and volcanological communities. To highlight their utility, we briefly review the formation of sheet intrusion segments, discuss how they inform interpretations of magma emplacement, and outline future research directions. •Igneous sheet intrusions commonly comprise magma segments (e.g., magma fingers).•Segments connect via step and bridge structures, formed by brittle fracturing.•Brittle shear and flow, as well as viscous deformation, can accommodate intrusion.•Segment long axes form parallel to sheet propagation direction.•Identifying segments allows magma flow and host rock behaviour to be determined.