Alba Patera, Mars: Topography, structure, and evolution of a unique late Hesperian–early Amazonian shield volcano

[1] New spacecraft data, especially MOLA topography, reveal important characteristics of Alba Patera that were either unknown or poorly resolved in previous data sets. The major and consistent breaks of slope and the specific pattern of lava flows within a broad region around Alba Patera permit division of this area into two major parts that appear to be related to different episodes of volcanism at Alba Patera: a low-lying apron of lava flows that broadly surrounds the edifice, a high-standing main construct, consisting of a central edifice, western and eastern flanking lobes, and a summit area characterized by a summit plateau and a summit dome topped by a caldera complex. Volcanic activity apparently began as massive effusive eruptions that formed the broad apron of lava flows, followed by more focused effusive eruptions that built the central construct. The construct spread gravitationally in a northerly direction to favor E-W dike propagation, leading to the formation of Tharsis-Montes-like flanking lobes. The final stages of volcanic activity at Alba Patera were concentrated in the summit area, forming a summit dome 400 km in diameter, a caldera complex, and two small shields near and inside the caldera, which underwent late-stage tilting and deformation; the load of the newly documented summit dome may have led to the development of Alba Fossae, reactivation of graben within Tantalus Fossae, and tilting of the summit dome to the east. A clear sequence in eruptive style is observed (early massive sheet flows, later tube-fed flows, and late, shorter-length and lower-volume sheet flows). Dendritic valley networks on the northern flanks of Alba Patera have been interpreted to be evidence of easily erodable material of pyroclastic origin, but new data suggest that these friable deposits and the valley networks may have formed instead during Amazonian climate excursion periods. The extensive apron of lava flows, combined with the flanking lava lobes and the Tharsis-Montes-scale summit dome, make Alba Patera much more similar to the Tharsis Montes than to the Hesperian-aged paterae. The Alba Patera lava apron was emplaced on early Hesperian ridged plains largely during the middle to late Hesperian; stratigraphic relationships suggest that effusive eruptions from Alba Patera interacted with ice-rich deposits in the northern lowlands. Alba Patera flanking lobes and the summit dome were emplaced in the earliest Amazonian, accompanied by late-stage loading, graben formation, and edifice tilting.