Modelling of dyke-induced fracturing and faulting for understanding graben and fracture system development across the Inner Solar System

Grabens are found on planetary bodies across the inner Solar System. They are elongated topographic depressions bounded by normal faults and fracture systems, and they attest to extension induced by tectonic and magmatic stresses in the upper rocky crust. The intrusion of dykes – tabular, subvertical magma bodies – in the upper five kilometers of Earth´s crust can induce the formation of normal faults and graben systems in the overlying crustal rocks. Therefore, graben-and-fracture systems on Mars and other planetary bodies, observed in association with volcanic eruption products, have been inferred as often induced by dyke intrusions. These systems are one of the few surface features where we can interrogate current and past subsurface stress states of the upper planetary crust and understand magmato-tectonic processes that form and deform the rocks.

Geometric and analytical assumptions have been combined with fundamental principles of rock mechanics to model the relationship between observed graben-and-fracture system geometry at the surface and dyke geometry, orientation, and depth in the subsurface. The existing models mostly assume an elastic response of a homogeneous upper planetary crust to dyke-induced stresses, which overlooks observations at geological outcrops on Earth that evidence the role of mechanical host rock heterogeneities and dynamic fracturing during dyke propagation. The implications for the uncertainties of existing model results remain unclear.

Within the DAGGER project funded by the National Science Centre of Poland, we have therefore implemented dyke propagation simulations using the Discrete Element Method (DEM). The DEM allows for simulating large strain concentrations and dynamic fracturing in a particle-based assemblage that is unfeasible or ignored in other finite element or analytical methods (Cundall & Strack, 1979; Potyondy & Cundall, 2004). Using the two-dimensional particle flow code (2D PFC) of Itasca Consultants Ltd, we have systematically varied 1/ the host rock strength for representative crustal rocks (rhyolitic ignimbrites, sandstones, basaltic lava, and fine-grained granite); 2/ the gravitational acceleration of Mars and Earth; 3/ the depth of the dyke top below the surface. Our 2D DEM modelling results allow quantitative comparison of the displacements, strains, and stresses within the crust and at the surface induced by an opening dyke, and highlight the differences between the surface displacement and fracturing patterns that can be expected on Mars versus Earth.

Our approach will allow further comparison of the dyke propagation mechanisms on other planetary bodies and prepare for interpretations of graben-and-fracture system observations that are expected to improve drastically with upcoming orbital missions to Mercury, Venus, and the Moon. Our results will therefore contribute to a better understanding of magma propagation processes and the accumulating mechanical damage of the upper crust over time on extraterrestrial rocky planetary bodies.

References:

Cundall, P.A. and Strack, O.D., 1979. A discrete numerical model for granular assemblies. geotechnique, 29(1), pp.47-65.

Potyondy, D.O. and Cundall, P.A., 2004. A bonded-particle model for rock. International journal of rock mechanics and mining sciences, 41(8), pp.1329-1364.