Discontinuum modelling of magma emplacement below surface doming on the Moon, Mars and Earth analogues from the Polish Sudetes

Surface deformation patterns on terrestrial planetary bodies have been interpreted as magma emplacement features due to their similarity with Earth analogues or their association with inferred eruptive products. Dome-shaped surface uplift features were recognized for example at floor-fractured impact craters on the Moon (Jozwiak et al., 2012), and at the Tharsis volcanic province on Mars (Farrand et al., 2011). The lack of subsurface geophysical data however precludes a clear observation of the characteristics and emplacement mechanisms of the inferred underlying magma intrusions.

On Earth, our insight into the characteristics of the magmatic plumbing systems that underly surface doming also remains cryptic due to the sparsity of observations. The most recent event was the hybrid explosive-effusive eruption at Cordón Caulle, Chile, in 2011. Pre- and syn-eruptive surface doming and pervasive ground fracturing observed there using satellite radar interferometry was interpreted there to result from the intrusion of a rhyolitic laccolith (Castro et al., 2016). Analytical and numerical models can use such geodetic observations to help estimate intrusion characteristics and assist in forecasting potential volcanic eruptions. The same models are used to infer shallow magma intrusion characteristics on terrestrial planetary bodies where the only observations available are surface topography and shallow ground-penetrating radar data.

Existing models generally assume that planetary crusts deform linearly-elastic, which leads to a simplification of the magmatic source geometry and the related displacement and stress fields. On Earth, however, host rocks around exposed solidified and exposed dykes, sills and laccoliths often show indicators of non-elastic deformation. Strain can accumulate along large-scale discontinuities in the overburden rocks, making the investigation of the emplacement mechanisms by traditional continuum models difficult.

To provide a more geologically realistic means of estimating magma intrusion characteristics and investigate the associated emplacement mechanisms, we have developed a numerical discontinuum model of laccolith emplacement within the framework of the “DeMo-Planet” project. We use the two-dimensional (2D) Discrete Element Method (DEM) model PFC2D (Itasca Consulting Group, Inc.) to represent the modelled medium as a particle-based network. We can use this model to indicate fracturing and highly discontinuous deformation, as well as visualizing the localization of subsurface strain and corresponding deformation.

Two stages of the model application are now available. In a first stage, a laccolith-shaped pressure source is inflated at the base of the host medium with varied depth of the magmatic source. In a second stage, particles are injected into the laccolith-shaped fluid body to simulate constant magma supply into a growing intrusion.

In this presentation, we will simulate laboratory tests to calibrate the mechanical behaviour and the relations between particle contact parameters of the numerical host rock and magma. To that end, we use the elastic property values (Young modulus) and tensile strength measured on host rocks estimated for the Moon and Mars (Heap et al., 2020). We then use elastic properties measured using uni-axial laboratory tests. The investigated rock samples were collected from exposed Permian sediments above Permian trachyandesite intrusions of the Intra-Sudetic Synclinorium in Poland. The calibration of the model with geological data then allows a comparison of the numerical results with field observations of the structural deformation in the intrusions’ overburden. In the future, we plan to analyze the effect of varying gravity, geometric and mechanical model parameters on the ratio of tensile to shear-mode fracturing, as well as investigating the magma-induced surface displacement in detail.

DeMo-Planet demonstrates a promising multidisciplinary approach of informing numerical models of complex, discontinuous magma emplacement processes in the heterogeneous and fractured shallow crust of terrestrial planetary bodies. The validation of this new modelling application with detailed structural information from representative Earth analogue sites will allow us to investigate the characteristics of the cryptic magma bodies underlying surface doming on the Moon and Mars. The application of our model to other terrestrial planetary bodies will simultaneously improve the interpretation of volcanic unrest signals on Earth.

References

Castro, J.M. et al. (2016) “Rapid laccolith intrusion driven by explosive volcanic eruption,” Nature Communications, 7, p. 13585. doi:10.1038/ncomms13585.

Farrand, W.H. et al. (2011) “Spectral evidence of volcanic cryptodomes on the northern plains of Mars,” Icarus, 211(1), pp. 139–156. doi:10.1016/j.icarus.2010.09.006.

Heap, M.J. et al. (2020) “Towards more realistic values of elastic moduli for volcano modelling,” Journal of Volcanology and Geothermal Research, 390, p. 106684. doi:10.1016/j.jvolgeores.2019.106684.

Jozwiak, L.M. et al. (2012) “Lunar floor-fractured craters: Classification, distribution, origin and implications for magmatism and shallow crustal structure,” Journal of Geophysical Research E: Planets, 117(11), pp. 1–23. doi:10.1029/2012JE004134.