INVESTIGATING LITHOSPHERIC FLEXURE OF MARTIAN VOLCANOES THROUGH TOPOGRAPHIC MAPPING

Large shield volcanoes on Mars are structurally characterized by summit caldera complexes, pit craters, graben, and flank terraces. Terraces occur as topographically subtle, laterally expansive convexities arrayed on some or all sectors of a volcano in an imbricate pattern. Analog and numerical models demonstrate that terraces are contractional tectonic structures, the result of flank shortening caused by compression of a volcano as it down-flexes its supporting basement. Mapping of a large terraced case-study volcano, Ascraeus Mons, indicates that its development has been heavily influenced by lithospheric flexure. The identification of flank terraces on a given volcano therefore has key implications for its structural and volcanic evolution. Such identification is hindered by terraces’ low relief, however, so that photogeologic images alone cannot be used to determine if, or the extent to which, a volcano is terraced. Slope maps derived from Mars Orbiter Laser Altimeter (MOLA) topographic data are of sufficient resolution to demarcate terraces on Mars’ larger shields but cannot resolve them on smaller volcanoes. The recent acquisition of stereophotogrammetric topography at resolutions greater than MOLA, however, means that terracing of smaller shields can now be investigated. Slope maps from High Resolution Stereo Camera (HRSC) data show that Ceraunius Tholus, a mid-sized shield in the northeast of the Tharsis Rise, is terraced. This finding requires that the geologic history of Ceraunius be appraised within the context of volcano-induced flexure, which acts to inhibit late-stage summit volcanism as compression of the edifice squeezes shut magma conduits. Interestingly, neighboring Uranius Tholus, ~14% the volume of Ceraunius, does not appear terraced even on HRSC data. This observation helps constrain a dimensionless parameter for predicting volcano-induced flexure, developed for Earth but equally applicable to Mars, which relates a volcano’s weight to the flexural rigidity of its basement. If the threshold for flexure lies between Ceraunius and Uranius Tholi then, shields smaller than Uranius—including Biblis, Jovis, and Ulysses Tholi—should not be terraced either, an inference that can be tested as improved topographic coverage of Mars’ smaller volcanoes becomes available.