Magmatic intrusions and deglaciation at mid-latitude in the northern plains of Mars
Résumé
On Earth, igneous rocks are generally affected by magmatic differentiation and the volume of intrusive bodies is considered to be 5-10 times larger than the volume of volcanic rocks. In contrast, in the case of Mars, the occurrence of olivine-phyric basalts at Gusev crater (McSween, H.Y. [2006]. J. Geophys. Res. (Planets) 111(E10), E02S10), the primitive nature of at least four picritic shergottites (Grott, M. [2013]. Planet. Space Sci. 174, 49-111) and the chemical composition of major volcanic provinces (Baratoux, D., Toplis, M.J., Monnereau, M., Gasnault, O. [2011]. Nature 472, 338-341) suggest that primary melts of the mantle may have commonly reached the surface. These observations, together with sparse morphologic evidence of intrusions in comparison with other terrestrial planets, raise the issue of the conditions for the formation and occurrence of sub-surface zones of magma storage on Mars. In this respect, a series of two types of young (Amazonian) domes located at mid-latitudes in western Arcadia Planitia, East of Phlegra Montes, previously interpreted as surface expressions of intrusive bodies (Farrand, W.H., Lane, M.D., Edwards, B.R., Yingst, R.A. [2011]. Icarus 211, 139-156), are examined here. A fraction of the domes consist of upraised plain material, whereas others, distinguished by a massive core and a large aspect ratio, could be partially exhumed intrusions or extrusions of viscous differentiated magma. The latter interpretation is however not compatible with the mafic compositions inferred from orbital visible and near infrared spectroscopic observations. The thicknesses and radii of both types of domes appear to be related through a power-law relationship with exponents close to 1 or 5/4. These exponent values are not compatible with extrusive morphologies but are characteristic of intrusions magma below an elastic layer (Michaut, C. [2011]. J. Geophys. Res. 10.1029/2010JB008108). We conclude that all types of domes are intrusive bodies. The large aspect ratios of some of these features may be reconciled with a mafic composition and a shallow emplacement (a few hundred meters) as well as a relatively large injection rate. Magma emplacement below an ice-rich horizon that was subsequently removed offers a plausible scenario for their partial exhumation. This scenario is supported by independent signs of ice-removal and deflation events in the northern plains. The formation of these intrusions appear therefore to be related to an unusual context of magma ascent below a thick and low-density ice-rich horizon.