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A theoretical similarity derived for spreading volcanoes remains valid for over three orders of magnitude in size, from small slumping volcanoes to large ocean plates. This similarity means that Mount Etna, Sicily, can be used as a terrestrial analogue of Tharsis Rise on Mars. A quantitative comparison of surface structures suggests that Tharsis has been spreading outward as a result of movement of plates at a planetary scale, producing rifting through the summit of the Rise, folding around its periphery, and with radial tear-fault systems connecting the summit rifts to the basal folds. The tear-fault systems form the fossae, of which Valles Marineris is the largest, and decouple the various plates of the Rise, so Tharsis appears to be analogous to a terrestrial mid-ocean ridge, but without any corresponding subduction zone. Instead, like volcanoes, the Tharsis plates are obducted over the Martian crust. The maximum viscosity at the bottom of the Martian lithosphere, derived from our spreading analysis, is in the order of 1021 Pa s, so if the mantle of Mars is hot and ductile, we suggest that the spreading of Tharsis could still be occurring.

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