The formation of Valles Marineris: 3. Trough formation through super-isostasy, stress, sedimentation, and subsidence

Despite the enormous size of the Valles Marineris chasmata on Mars, the mechanism responsible for the formation of these unique troughs remains unknown. Previous studies proposed mechanisms of trough formation through extensional tectonism, vertical collapse into subsurface voids, or some combination of the two. Recent work in a pair of companion papers demonstrated that the troughs must have formed dominantly by vertical subsidence with little net horizontal extension, and that the modest extension required likely arose from the effect of the buried dichotomy boundary beneath Tharsis. This study now proposes a new mechanism of Valles Marineris formation, with a focus on the large-scale geodynamic and tectonic processes. Prior to the formation of Valles Marineris, the lithosphere within Tharsis was maintained in a super-isostatic state by the membrane-flexural support of the rise. Local moderate extension at Valles Marineris controlled subsequent intrusive activity, resulting in the emplacement of long parallel dikes. This intrusive activity weakened the lithosphere, effectively removing the flexural support from long lithospheric blocks. Left unsupported, these blocks would have subsided approximately 0.49 ± 0.32 km to their isostatic level. The total subsidence would have been increased to 8.6 ± 5.6 km by the effects of contemporaneous sedimentary loading within the troughs and viscous deformation at the base of the crust. This mechanism successfully predicts the observed depths of the Valles Marineris chasmata, and is consistent with all geological and geophysical observations. The unique nature of Valles Marineris is explained as a result of the unique geodynamic and tectonic environment of Tharsis.