Multiple-process origin of Valles Marineris basins and troughs, Mars

An improved sequence of formation for Valles Marineris depressions is proposed. It involves three key stages; (a) dike emplacement radial to Syria Planum during Late Noachian to Early Hesperian time; (b) localized subsidence of crustal rocks during post-Early Hesperian time, forming ancestral basins such as Hebes Chasma; and (c) regional normal faulting that overprints the ancestral basins and forms the structural troughs, such as Coprates Chasma, principally during Amazonian time. Attainment of high topography at Syria Planum, completed by the Early Hesperian, was accompanied and/or followed by extrusion of ridged plains lavas, emplacement of subsurface dikes, and wrinkle-ridge deformation. The high topography contributed to a substantial hydrostatic head, leading to subsurface volume reduction, vertical displacement of crustal strata to from chaotic terrain and ancestral basins, and localized outbreaks of fluid as outflow channels. The dikes contributed a pervasive structural influence on the orientations of later nearsurface processes. Sometime after infilling of ancestral basins with their interior layered deposits, normal faults related to Tharsis centered stresses were superimposed on the relict high topography, ancestral basins, and dike network, forming large grabens. Recognition of the polygenetic origin of troughs such as Melas Chasma, and the disparate origins of irregular and rectangular troughs, provides the key to unraveling the conflicting accounts of trough origin and timing.