GEOLOGY AND LANDSCAPE EVOLUTION OF THE HELLAS REGION OF MARS

Hellas basin on Mars has been the site of volcanism, tectonism, and mo dification by fluvial, mass-wasting, and eolian processes over its mor e than 4-b.y. existence. Our detailed geologic mapping and related stu dies have resulted in the following new interpretations. The asymmetri c distribution of highland massifs and other structures that define th e uplifted basin rim suggest a formation of the basin by the impact of a low-angle bolide having a trajectory heading S60 degrees E. During the Late Noachian, the basin was infilled, perhaps by lava flows, that were sufficiently thick (>1 km) to produce wrinkle ridges on the fill material and extensional faulting along the west rim of the basin. At about the same time, deposits buried northern Malea Planum, which are interpreted to be pyroclastic flows from Amphitrites and Peneus Pater ae on the basis of their degraded morphology, topography, and the appl ication of a previous model for pyroclastic volcanism on Mars. Peneus forms a distinctive caldera structure that indicates eruption of massi ve volumes of magma, whereas Amphitrites is a less distinct circular f eature surrounded by a broad, low, dissected shield that suggests gene rally smaller volume eruptions. During the Early Hesperian, a similar to 1- to 2-km-thick sequence of primarily fined-grained, eolian materi al was deposited on the floor of Hellas basin. Subsequently, the depos it was deeply eroded, except where armored by crater ejecta, and it re treated as much as 200-300 km along its western margin, leaving behind pedestal craters and knobby outliers of the deposit. Local debris flo ws within the deposit attest to concentrations of groundwater, perhaps in part brought in by outflow floods along the east rim of the basin. These floods may have deposited similar to 100-200 m of sediment, sub duing wrinkle ridges in the eastern part of the basin floor. During th e Late Hesperian and Amazonian, eolian mantles were emplaced on the ba sin rim and floor and surrounding highlands. Their subsequent erosion resulted in pitted and etched plains and crater fill, irregular mesas, and pedestal craters. Local evidence occurs for the possible former p resence of ground ice or ice sheets similar to 100 km across; however, we disagree with a hypothesis that suggests that the entire south rim and much of the floor of Hellas have been glaciated. Orientations of dune fields and yardangs in lower parts of Hellas basin follow directi ons of the strongest winds predicted by a recently published general c irculation model (GCM). Transient frost and dust splotches in the regi on are, by contrast, related to the GCM prediction for the season in w hich the images they appear in were taken.