Styles of Volcanism, Tectonic Associations, and Evidence for Magma-Water Interactions in Eastern Hellas, Mars

Four of the martian highland paterae (low-relief, areally-extensive features with central calderas and radial channels and ridges [1,2]) are associated with inferred rings of the Hellas basin [3,4] (Figure 1). Their morphometry [5] and the erosional characteristics of Tyrrhena Patera [2] have been used as evidence that they are composed of ash. Analyses of the energetics of eruption and flow processes have shown that the distribution of units at Hadriaca and Tyrrhena Paterae is consistent with an origin by the emplacement of gravity-driven pyroclastic flows and that explosive eruptions driven by magmatic volatiles or groundwater could provide sufficient energy [6-91. The supply of groundwater for hydromagmatic eruptions has been considered, large volumes of water could accumulate rapidly at flow rates determined for Mars [7,10-111. The present investigation synthesizes the previous studies of Tyrrhena Patera and results of modeling martian volcanic processes with current photogeologic investigations of the Hadriaca Patera region [6,12] in order to examine the evolution of volcanism in the eastern Hellas region. In eastern Hellas, Hadriaca and Tyrrhena Paterae are observed in association with the ridged plains of Hesperia Planum. Zones of weakness generated by the Hellas impact event may have produced conduits for the magmas forming these deposits [4]. The structure of Hellas has clearly influenced the shapes of the paterae, which are elongated downslope toward the basin, as well as the positions of the volcanic source vents. The orientations of 243 ridges have been mapped in the Hadriaca [260-275" W, 27.5-42.5' S] and Tyrrhena [247.5-265" W, 15-30" S] regions. The 219 ridges contained within volcanic units have a wide range of orientations but exhibit trends which approximate radial (NE-SW) and concentric (NW-SE) patterns to Hellas. Although orthogonal and parallel alignments between ridges and the regional slope and lava flow directions in the flank flow unit at Tyrrhena are also observed, basin-related tectonism is considered the primary factor governing ridge orientation because the regional slope is a consequence of the basin and has controlled the emplacement of the volcanic units. The NE-SW and NW-SE ridge orientations measured agree with previously documented trends for Hesperia Planum [13] and indicate the effects of the Hellas impact event over a wide region of the southern highlands and the persistence of the tectonic signature of the basin over geologic time. Photogeologic studies indicate that Tyrrhena Patera consists of basal and summit shield units, the surfaces of which contain channels radiating from the summit caldera region [6-7,121. Layering is observed in these units, the distal margins of which are highly dissected and form isolated erosional remnant mesas. The large flank flow unit composed of well-defined lava flow lobes and leveed channels, the first definitive evidence for effusive volcanic activity associated with the paterae, extends from near the summit of Tyrrhena for over 1000 km to the SW adjacent to Hadriaca. Hadriaca Patera also exhibi~ channeled flanks, although the channels are smaller and more closely spaced. Multiple layers and erosional remnants are also observed. The morphologies of these volcanoes suggest that their surfaces have been modified by fluvial erosion (sapping? ), and these erosional characteristics in combination with similar morphometry to large terrestrial ash sheets [5] and the absence of primary lava flow features (except in the flank flow unit) suggest that the highland paterae are composed primarily of ash [6,7]. Cross-cutting relationships determined from geologic mapping of eastern Hellas indicate that the ridged plains of western Hesperia Planum and the flank flow unit at Tyrrhena Patera are younger than the basal and summit shield units [7,12], suggesting a transition from explosive to effusive volcanism. The erosion of the flanks of Hadriaca pre-dates the erosion of the channeled plains to the south. The erosion on the plains (presumably by runoff) and the eventual formation of the Dao Vallis outflow system continues through the formation of the flank flow. Thus, the erosional processes affecting Hadriaca and Tyrrhena Paterae and the plains were probably active, at least in some areas, after the emplacement of the flank flow. This relationship supports the interpretation of the dissected units at Hadriaca and Tynhena as ash rather than eroded lavas because the flank flow unit should have been subjected to similar degradational processes, at least