Subsurface Geometry And Emplacement Conditions Of A Giant Dike System In Elysium Fossae, Mars

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 (K-ic) are between 3.14 and 15.15 GPa.m(1/2) 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.Plain Language Summary Elysium Fossae is a set of long linear troughs, or grabens, in the eastern flank of the Elysium Mons volcano, Mars. Such depressions are hypothesized to host giant dike systems underneath them, which are tall, long, and narrow magmatic intrusions. The lengths, widths, and topography of these grabens were used to infer the apertures, depths, heights, and pressure of these possible dikes. Interestingly, the obtained heights (15-20 km) are on average three times larger than those of dikes emplaced in the terrestrial crust (2-10 km). Furthermore, the apertures and lengths of these dikes match well with those expected in fractures caused by fluid pressure. The pressures which formed the dikes are like those in terrestrial dikes despite the greater apertures and heights of the Elysium dikes. Therefore, similar conditions produced larger structures on Mars than on Earth. This likely results from the smaller Martian gravitational acceleration, which is one third of the terrestrial. Theoretical calculations suggest that these pressures are possible for dikes of the estimated heights. Therefore, this work supports the presence of a giant dike system underneath Elysium Fossae, and that the height of dikes in rocky planets may be directly dependent on gravitational acceleration, and planet size.Key PointsKey>Area balance calculations on long linear graben can be used to make estimates of the apertures, depths and heights of dikes underneath themDike aspect ratios are consistent with fluid-induced fractures and suggest stress intensity factors between 3.14 and 15.45 GPa.m(1/2)Calculations of dike heights and their excess pressures suggest that the former may scale inversely with gravitational acceleration