Volcanic Spreading and Lateral Variations in the Structure of Olympus Mons, Mars: Insights from Particle Dynamics Simulations

The Olympus Mons volcano on Mars is notable not only for its immense height and width, but also for substantial asymmetries in its structure. The gently sloped northwest (NW) flank extends to a much greater distance from the central caldera complex than the more steeply sloped southeast (SE) flank. Furthermore, the NW flank exhibits lower flank extensional faults, whereas the (SE) shows upper flank compressional terraces and lower flank upthrust blocks. However, both the NW and SE flanks exhibit characteristic concave-upward profiles and steep bounding scarps, in contrast to other sectors. These NW-SE asymmetries are aligned with the regional slope from the Tharsis Rise, but an understanding of the underlying causes has remained elusive. We use Particle Dynamics models of growing, spreading volcanoes to demonstrate that these flank structures could reflect the properties of the basement materials underlying Olympus Mons. We find that basal slopes alone are insufficient to produce the observed concave-upward slopes and asymmetries in flank extent and deformation style that are observed at Olympus Mons; instead, lateral variations in basal friction are required. These variations most likely result from high pore fluid (water) pressures in a basal sediment layer, transported and preferentially accumulated downslope from the Tharsis Rise. Sediments consisting of clay-type minerals could provide the low hydraulic diffusivity required to build up high pore pressures. Such sediments are present on Mars: the ancient (Noachian epoch) phyllosilicates (clays) recently discovered by the Mars Express mission. Our basal detachment model implies the sustained presence of liquid water at depth beneath Olympus Mons, perhaps even to the present day. This reservoir, sustained by thermal and chemical fluxes, would be a favored environment for the development and maintenance of hyperthermophile organisms, particularly given the role attributed to clay minerals as pre-biotic catalysts.