Lithological controls on shallow-level magma emplacement
Craig Magee1, Christopher A-L Jackson1, Nick Schofield2 & Freddie Briggs1
1
Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London,
SW7 2BP, UK
2
School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston,
Birmingham, B15 2TT, UK
The emplacement of magma within the upper crust requires space to be generated by the
deformation or assimilation of the host rock. Intrusion morphologies, magma reservoir
locations and the architecture of interconnecting magma conduits are therefore strongly
influenced by the behaviour of the host rock during emplacement. Importantly, monitoring
host rock deformation affects (e.g., surface uplift) can provide invaluable insights into the
potential timing, location and magnitude of future volcanic eruptions.
This has led to significant advances in the inversion of host rock deformation patterns,
acquired from geophysical and geodetic data, to elucidate sub-volcanic plumbing systems.
However, the link between the shape and size of intrusion and the style and magnitude of the
ground deformation is non-unique. While numerical and physical models have been
developed to test plausible intrusion-deformation scenarios, they cannot explicitly incorporate
complex host rock stratigraphies, temperature-driven intrusion-host rock interactions or
brittle faulting. We advocate that three-dimensional seismic reflection data, which provide
unparalleled images of entire volcanic plumbing systems, can be used to enhance our
understanding of the intrusive networks and to test hypotheses concerning syn-emplacement
host rock deformation.
We use 3D seismic reflection data from the Exmouth Sub-basin, offshore NW Australia, to
examine the link between a saucer-shaped sill and an overlying, dome-shaped fold developed
at the contemporaneous palaeosurface. Our results highlight a disparity in size (e.g., areal
coverage, thickness/amplitude) between the sill and fold, which we attribute to the initial
accommodation of magma by fluid expulsion from the poorly consolidated claystone host
rock, prior to a period of (forced) folding. This is supported by field observations, which
indicate ‘triggered’ or ‘thermal’ fluidisation of the host rock may occur during sill
emplacement. In such cases, intrusion-induced uplift of the overburden did not occur.
Furthermore, we suggest the final phase of magma ascent, i.e. the transgression of the saucershaped sill limbs, did not produce seismically resolvable uplift but was instead
accommodated by plastic deformation and fluidisation of the host rock.
Our results suggest that, due to complex sill-host rock interactions, initial sill intrusion and
the final phase of sill emplacement produced little, if any, ground deformation, at least at the
resolution afforded by seismic data (i.e. c. 20 m). While geodetic data can discern small-scale
ground movements on the order of centimetres to metres, this study highlights the influence
that host rock lithology may have on the emplacement of an igneous intrusion and the style of
any associated ground deformation at the decametre-scale. Implicitly, ground deformation
models used in volcanic hazard assessment, that do not take host rock lithology into account,
likely provide incorrect magma volume estimates and, hence, should consider complex
intrusion geometries and the impact of lithology and emplacement depth on host rock
behaviour.