Early magmatic processes on Mars

This thesis investigates tectonic, volcanic and magmatic processes on Mars during the Noachian (< 3.7 Ga) and Hesperian (< 3.0 Ga) periods with the help of combined remote sensing techniques, high-resolution images and reflectance spectroscopy. A survey of magmatic dikes in eastern VallesMarineris has been conducted in order to constrain their number and extent in the Tharsis region. Magmatic dikes have been proposed to have weakened and fractured the crust, allowing the formation of Valles Marineris. Hence, this survey helped to understand the relationship of dike formation with Valles Marineris formation. Over a hundred dikes were identified. Estimation of magma eruption rates suggest that dikes alone may have fed the volcanic plains by successive fissure eruptions. Three distinct dike orientations are recorded: 70±, 90± and 110±, the latest being subparallel to VallesMarineris main trend. An east-west dichotomy is observed delimited by the Coprates Rise. 90± and 70± groups are observed to the east, group 110± is observed to the west. Surprisingly, other features have similar orientations, turning from 110± to 90± from west to east: pits chains and elongated troughs. This suggests they formed above dikes and/or under a similar stress regime. Ganges Chasma and a 400 km long wall segment of Coprates Chasma located east of the Coprates Rise are oriented 90±. It remains unclear if dikes controlled Valles Marineris opening but their formation appears to be closely related. The Coprates Rise is likely a limit between two crustal blocs that have undergone distinct and successive tectonic stress fields that led to dike emplacement, Chasmata opening and other small features formations. Impact crater central peaks were used as natural probes to constrain the Tharsis dome geometry, volume and age of formation of the bulk of the edifice. With HiRISE high-resolution images, two lithologies were identified in central peaks: dark-toned layers likely to represent stack of lava flows emplaced during Tharsis formation, and lighter-toned massive rocks, interpreted to be the lower magmatic early-Noachian crust. A loading model gives an elastic thickness (Te) of the lithosphere of 49 to 56 km to match the 1.3 degrees dip. Proposed values of Te for the Noachian are below 40 km but increase with planetary cooling during the Hesperian to Amazonian era, from 50 to 150 km. Our observations suggest that a post-Noachian bulk emplacement is probable as recently suggested by a previous study, but improvements in the model are needed. The planetary distribution of the massive and light-toned lower crust observed in Valles Marineris and beneath the Tharsis dome was investigated in detail. This enigmatic lower crust is considered key to understand magmatic processes involved during the early-Noachian, but also during the earliest stages of other rocky planets in general. This lower crust is not restricted to the Tharsis Dome but is found to be widespread in the Southern Highlands, linked to a global crustal formation process. Its morphologies, relatively higher albedo, and its spectral signature dominated by low-calcium pyroxene (LCP) suggest a magmatic origin and an enrichment in feldspar. A similar morphology but with a distinct signature of olivine is observed in the vicinity of large impact basins and in the Northern Plains. This lithology could be impact-derived rocks, younger than the Highlands, or come from a deeper part of the crust. This survey suggests that the early Martian crust is made of gabbro-noritic-like rocks, contrasting with the predominant basaltic upper crust. This lithology, less dense than the upper crust explains the inferred low-density anomaly of the Southern Highlands. Its nature, primary or secondary is still unclear and discussed in this PhD.