Subsurface Geometry and Emplacement Conditions of a Giant Dike System in Elysium Fossae, Mars

Abstract

Elysium Fossae is a NW-SE, 350 km-long linear graben system located in the eastern flank of Elysium Mons, Mars. Graben widths, lengths, and topographic data were used to model the thicknesses, depths, heights, and excess pressures of a possible dike system underneath these grabens. Area balance on topographic profiles across the structures reveals dikes at depths of 241–2,330 m below the surface, with average thicknesses <60 m, and heights of 15–20 km. These heights are approximately three times larger than terrestrial dikes intruded in the shallow crust. The subsurface properties suggest a teardrop-shape geometry for the dikes, which has been observed in many theoretical and analog dike models. Additionally, the aspect ratios (thicknesses and lengths) of the inferred dikes are consistent with sublinear scaling, characteristic of fluid-induced fractures. The obtained critical stress intensity factors (Kic) are between 3.14 and 15.15 GPa·m1/2 and are slightly higher than those of terrestrial dikes intruded in similar lithologies. The maximum excess pressures calculated from stress intensity factors are between 31 and 276 MPa. These are also consistent with pressures calculated for terrestrial dikes despite the greater aperture and height of the Elysium dikes. Theoretical calculations of excess pressures for variable dike heights support both the values calculated from empirical observations, and that Martian dike heights may consistently be three times the heights of terrestrial dikes. In summary, this work supports: (1) the existence of a dike system underneath Elysium and (2) that dike heights in rocky planets may scale inversely with gravitational acceleration.