TECTONICS OF MARS

On Earth, plate tectonics has controlled the level of greenhouse gases in the atmosphere modulating and maintaining clement conditions and liquid oceans for most of geologic time and thus has been important in the evolution of life. Similarly on Mars, the development and growth of the Tharsis province (the largest single tectonic entity in the solar system) may have been responsible for the wet and likely warm early climate. Tharsis is an enormous elevated volcanic and tectonic bulge that rises ~10 km above the datum and is topped by 4 giant shield volcanoes that together with a system of outlying tectonic features encompasses the entire western hemisphere of the planet. It is surrounded by radial extensional rifts and grabens and concentric compressional wrinkle ridges that extend thousands of kilometers from its center. Mapping of structures in the established stratigraphic framework of Mars shows that they formed during 5 main stages, with about half forming during the Noachian >3.7 Ga, indicating that tectonic activity peaked early and generally decreased with time. Lithospheric deformation models show that loading over the scale of Tharsis (large relative to the radius of the planet) produces the concentric extensional stresses around the periphery and the radial compressional stresses closer in that are needed to explain the radial grabens and rifts and concentric wrinkle ridges. Because models based on present day gravity and topography can explain the observed distribution and strain of radial and concentric tectonic features, the basic lithospheric structure of the province has probably changed little since ~3.7 Ga. This enormous load appears to have produced a flexural moat around it, which shows up most dramatically as a negative gravity ring, and an antipodal dome that explains the first order topography and gravity of the planet. Many ancient fluvial valley networks, which likely formed during an early wetter and likely warmer period on Mars, flowed down the present large-scale topographic gradient, further arguing that Tharsis loading was very early. If the load is composed of magmatic products as suggested by fine layers within Valles Marineris, water released with the magma would be equivalent to a global layer up to 100 m thick, which might have enabled the early warm and wet Martian climate.