Emplacement and propagation mechanisms of magma fingers - TSG 2021 poster

Abstract

Tectonic Studies Group - Annual Group Meeting 2021Emplacement and propagation mechanisms of magma fingersMagma transport in large volcanic plumbing systems is often described to occur via networks of channel-like sheet intrusions (i.e., dykes and sills). In many cases, elongate, finger-like geometries emerge from the outer margin of these planar sheet intrusions during magma migration through the Earth’s crust. Previous field studies and both analogue and numerical experiments suggest that magma finger emplacement is dominated by either i) fracture propagation (mode I failure), ii) shear failure (mode II failure), or iii) viscous deformation (e.g., host rock fluidisation). In this study, we present field-based data collected at the outer margin of the Paleogene Shonkin Sag laccolith, Montana, USA, to evaluate how host rocks (Cretaceous Eagle sandstone) deform to accommodate magma finger emplacement. We combine unmanned aerial vehicle (UAV or drone) photogrammetry surveys with field observations to map and quantify host rock deformation and to assess whether or not magma finger emplacement can be described by a single end-member process. Our field observations show that magma finger emplacement at the outer margin of the Shonkin Sag laccolith was accommodated by all three proposed end-member models. We predominantly observe brittle deformation, folding and shear failure between adjacent magma fingers, whereas evidence for host rock fluidisation is mainly observed at the cross-sectional lateral tips of magma fingers and to a minor extent at the top and bottom intrusion-host rock contacts. Preliminary results of photogrammetric analyses suggest that overburden strata uplift due to magma finger inflation accommodates up to 20% of the intrusion thickness. It is important to note, that we often observe all three modes of deformation mechanism in the same outcrop at metre scale and in some cases associated with a single magma finger. This metre-scale variation of host rock deformation indicates that magma finger emplacement is likely accommodated by multiple deformation mechanisms. We conclude that host rock fluidisation in combination with brittle, ductile and elastic deformation accommodated magma emplacement at the outer margin of the Shonkin Sag laccolith. We further hypothesise magma fingers were potentially initiated due to host rock fluidisation and resulting viscous fingering. The propagation and growth in length, however, might be dominated by i) fracture propagation, ii) viscous indentation, and/or iii) host rock fluidisation, highlighting that emplacement mechanisms might change during magma emplacement. We relate brittle deformation, folding, and shear failure between adjacent magma fingers to a compressional regime that forms once fingers grow in width, resulting in host rock shortening and thickening.