Emplacement of giant radial dikes in the northern Tharsis region of Mars

Three distinct sets of graben are associated with the volcano Alba Patera on Mars. One set, approximately circumferential to the edifice, has long been accepted to have formed as a tectonic response to an extensional stress regime associated with the evolution of the Alba Patera edifice. A second set includes mainly linear structures interpreted by many workers to have formed in response to very large‐scale regional stresses. We infer that the third set of graben, all of which are relatively linear, none of which are strictly parallel to members of the second set, and many of which contain numerous pit craters, formed above long (∼1000 km), laterally propagating regional dikes emanating from a volcanic center located to the south within the Tharsis region. The expected geometries of such dikes (several hundred meters depth to dike top, ∼20 km depth to dike base, mean dike width ∼30–90 m) are modeled on the assumption that they were fed from a shallow magma reservoir centered on a neutral buoyancy horizon, expected to be present at a depth of ∼10 km on Mars. The volumes of magma in the dikes are consistent with a reservoir similar in size to those inferred to be present under the Tharsis shield volcanoes provided that the dikes were emplaced during caldera collapse episodes. The sizes of the graben associated with these dikes are consistent with the relaxation, during or immediately after dike emplacement, of preexisting regional extensional stresses of a few tens of MPa.