Extension and uplift at Alba Patera, Mars: Insights from MOLA observations and loading models

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Abstract
High‐resolution Mars Orbiter Laser Altimeter (MOLA) profiles of the large shield volcano Alba Patera call for a critical reexamination of models for the growth and evolution of this volcanic edifice. An extensive system of graben cuts across Alba Patera, extending from Ceraunius Fossae to the south to Tantalus Fossae to the northeast. In the vicinity of the edifice, the otherwise generally north‐south orientations of these graben are deflected to circumferential directions. MOLA topography reveals that the circumferential graben are located well up the flanks of Alba Patera. Both the type (extensional) and location (midflank) of faulting on Alba Patera are inconsistent with the state of stress predicted for purely surface‐loaded flexure models. To constrain the conditions governing the evolution of Alba Patera, we employ finite element models for the volcano and lithospheric stress field under a combination of loading mechanisms. Buoyant sublithospheric loads (representing underplated magma, low‐density mantle residuum, or dynamic support from mantle convection) and intralithospheric displacements (representing sills “trapped” by horizontal compressive stresses in the upper lithosphere) can generate the observed midflank slope breaks and circumferential extensional fault zones at radial distances comparable to that of the subsurface load's characteristic radial extent. Both mechanisms moderate stresses in the upper lithosphere in favor of continued magma ascent. However, bottom loading requires a disk‐like load geometry and an elastic lithosphere thickness Te sufficiently small to allow the load's shape to be apparent in the surface deformation (Te ≤ 32 km). Such low Te is inconsistent with Mars Global Surveyor (MGS) gravity and topography data. In contrast, intralithospheric sill complexes can produce the observed topography and faulting at Te values consistent with those inferred from gravity and topography (50 ≤ Te ≤ 100 km). For these conditions, principal stress orientations on the lower flanks are also consistent with the orientations of lower‐flank graben, provided that faulting was induced by dike intrusion or tensile failure rather than shear failure.

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