The Olympus Mons Aureole: Formation by gravitational spreading

<jats:p>New observations of the aureole deposits of Olympus Mons are in agreement with previous suggestions that the aureole lobes could have been formed by movement under gravity away from the observed scarps. The characteristic corrugated morphology of the aureole is probably secondary, formed by erosion along structural inhomogeneities. Geometries of the corrugations and translational faults in the aureole deposits favor a mechanism of emplacement that involves material with substantial shear strength rather than viscous flow. Estimates of the aspect ratios of terrestrial landslides, nappes, and ash flows suggest that the aureole deposits are more similar in overall shape to slide‐emplaced bodies than to pyroclastic flows. Although Martian physical conditions permit the formation of ash flows, there is no positive evidence for their existence. A gravitational spreading mechanism is proposed that involves imbricate thrusting in distal regions, analogous with terrestrial thrust sheets. Corrugations in the aureole developed by erosion along imbricate or listric faults. Basal shear stresses on the decoupling surface may have been of the order of 10<jats:sup>6</jats:sup> Pa (10 bars). The nature of the decoupling surfaces in the proximal regions is uncertain but is likely to have been the topographic surface in distal parts. The aureole deposits were probably emplaced as a series of sheets by spreading under gravity from an ancestral Olympus Mons, which extended farther to the northwest than does the present volcano, as a result of the accumulation of lavas under the then prevailing stress conditions. Recent lava eruptions from the volcano have been concentrated in northeast and southwest sectors, indicating a changed stress regime since aureole emplacement.</jats:p>