[1] Thousands of phyllosilicate-rich outcrops, mainly iron or magnesium-rich are exposed on Noachian terrains in the Martian southern highlands. We analyzed 90 CRISM observations and more than a hundred HiRISE images located on the plateaus surrounding Valles Marineris. We mapped an extensive Al-and Fe/Mg-phyllosilicate-rich formation covering at least $197,000 km 2 , for which we introduce the name "Plateau Phyllosilicates." Tens of meters in thickness, this light-toned formation crops out at various elevations on top of the Noachian units Npl 1 and Npl 2 , as flat exposures on plateaus and along scarps such as valley walls, chasma walls, pit walls and impact crater rims. The Fe/Mg-phyllosilicate-rich lower member of the formation is composed of Fe/Mg-smectites (nontronite, saponite) and vermiculite. The Al-phyllosilicate-rich upper member of the formation contains Al-smectites (montmorillonite, beidellite) and locally kaolinite and/or halloysite. We suggest that the Plateau Phyllosilicates were mainly formed by pedogenesis related to the weathering of the Noachian bedrock by percolation of meteoric water or melted snow under a temperate and subarid climate during the Noachian Epoch in an alkaline to neutral environment. Kaolinite and/or halloysite may have formed in areas of more intense drainage at the surface under slightly acidic environments during the Noachian and Hesperian Epochs. Fluvial activity and deuteric alteration may have locally contributed to the genesis of phyllosilicates. This study suggests that the alteration of the Noachian basement of the plateaus surrounding Valles Marineris was widespread during the Noachian Epoch, and was still active during the Hesperian Epoch even though the water availability was limited.

Abstract. Some of the most spectacular instances of deep-seated gravitational slope deformation (DSGSD) are found on Mars in the Valles Marineris region. They provide an excellent opportunity to study DSGSD phenomenology using a scaling approach. The topography of selected DSGSD scarps in Valles Marineris and in the Tatra Mountains is investigated after their likely similar postglacial origin is established. The deformed Martian ridges are larger than the deformed terrestrial ridges by 1 to 2 orders of magnitude with, however, a similar height-to-width ratio of ∼0.24. The measured horizontal spreading perpendicular to the ridges is proportionally 1.8 to 2.6 times larger for the Valles Marineris ridges than the Tatra Mountains and vertically 2.9 to 5.1 times larger, suggesting that starting from two different initial conditions, with steeper slopes in Valles Marineris, the final ridge geometry is now similar. Because DSGSD is expected to now be inactive in both regions, their comparison suggests that whatever the initial ridge morphology, DSGSD proceeds until a mature profile is attained. Fault displacements are therefore much larger on Mars. The large offsets imply reactivation of the DSGSD fault scarps in Valles Marineris, whereas single seismic events would be enough to generate DSGSD fault scarps in the Tatra Mountains. The required longer activity of the Martian faults may be correlated with a long succession of climate cycles generated by the unstable Martian obliquity.