Displacement and fracturing around laccolith intrusions affected by gravity on Mars versus the Moon, and Earth
Authors: Poppe Sam , Cornillon Anne , Morand Alexandra , Harnett Claire E. , Mège Daniel 
Published: July 2024

INTRODUCTIONDome-shaped uplifted and fractured terrain has been observed at the surface of volcanic regions on Mars, amongst other terrestrial planets and moons in our Solar System. Such dome-shaped ground deformation features include, for example, domes in Elysium Planitia and floor-fractured craters (e.g., Farrand et al., 2011; Jozwiak et al., 2012). These features are inferred to have formed by the emplacement and inflation of sill- and laccolith-shaped magma bodies in the shallowest 1-2 km of the crust (e.g., Michaut et al. 2013). This interpretation is informed by monitored magma intrusion events and geological observations at eroded outcrops of volcanic plumbing systems on Earth. On Mars and other planetary bodies, however, only the final surface deformation features can be observed from orbit. Besides studying analogue outcrops on Earth, analytical and numerical models are used instead to understand the emplacement dynamics and the deformation of the planetary crust&#180;s rocks. A mismatch exists, however, between the oversimplified intrusion geometry and the linearly elastic response to magma intrusion assumed by most numerical models, and the complex intrusion geometries and mechanical response of host rocks to magma-induced stresses observed on Earth in exposed volcanic plumbing systems. Strain can accumulate along large-scale discontinuities in the overburden rocks, making the investigation of the emplacement mechanisms by traditional continuum models difficult. To investigate the ill-understood effect on magma-induced deformation of the lower gravity on Mars, and the Moon, due to their smaller mass compared to Earth, we compare simulations of laccolith inflation in the Discrete Element Method (DEM) under gravity of the Moon, Mars and Earth.METHODWe used the two-dimensional (2D) DEM particle flow code PFC2D (Itasca Consulting Group, Inc.) to simulate the inflation of a half-ellipsoid laccolith in a rock medium represented as a particle-based assemblage. Previously, we have shown how this model can indicate fracturing and highly discontinuous deformation, as well as visualise the localization of subsurface strain and corresponding deformation (for details see Morand et al., 2024). To simulate laccolith inflation, the laccolith-shaped pressure source is inflated by increasing the area of the 2D particles (Figure 1). We ran inflation simulations in rocks of different toughness and stiffness representing a range of realistic crustal strengths, under the gravity of the Moon (g = 1.62 m.s-2), Mars (g = 3.71 m.s-2), and Earth (g = 9.81 m.s-2).&#160;Figure 1. General view of the DEM model in PFC2D with rock particles (grey) and the initial particle-based, laccolith-shaped magmatic body (red) at 1 km depth. The subsets display the magma reservoir area in its initial state before inflation and in its final state after 25% of inflation.RESULTS AND DISCUSSIONFor equal rock stiffness and amounts of intruded magma, our model results show that we can expect the same volume of magma injected at the same depth to induce more vertical surface displacement on Mars, due to the lower gravity there compared to Earth. Nevertheless, rock toughness and rock stiffness control the amount of fracturing more than gravity does. These findings are even better expressed under Lunar gravity. Our finding implies that both gravity and realistic crustal rock strength are essential parameters to account for in modelling efforts of magma-induced deformation on moons and planets such as Mars with a size and mass that is significantly different from that of Earth.Our model results will induce a better understanding of the emplacement and architecture of shallow magmatic intrusions below magma-induced uplifted terrain and floor-fractured craters on Mars. Verification of this new modelling application with detailed structural information from analogue magma intrusions on Earth, such as Permian trachy-andesite intrusions in the Intra-Sudetic Synclinorium in SW Poland, allow us to investigate the characteristics of the cryptic magma bodies underlying surface doming on Mars and the Moon, while simultaneously improving the interpretation of volcanic unrest signals on Earth.REFERENCESFarrand, W.H. et al. (2011) &#8220;Spectral evidence of volcanic cryptodomes on the northern plains of Mars.&#8221; Icarus, 211(1), 139&#8211;156. https://doi.org/10.1016/j.icarus.2010.09.006Jozwiak, L.M. et al. (2012) &#8220;Lunar floor-fractured craters: Classification, distribution, origin and implications for magmatism and shallow crustal structure.&#8221; Journal of Geophysical Research E: Planets, 117(11), 1&#8211;23. https://doi.org/10.1029/2012JE004134Michaut, C., et al. (2013) &#8220;Magmatic intrusions and deglaciation at mid-latitude in the northern plains of Mars.&#8221; Icarus, 225(1), 602-613. https://doi.org/10.1016/j.icarus.2013.04.015Morand, A. et al. (2024) &#8220;Fracturing and dome-shaped surface displacements above laccolith intrusions: Insights from Discrete Element Method modeling.&#8221; Journal of Geophysical Research: Solid Earth, 129, e2023JB027423. https://doi.org/10.1029/2023JB027423