EVOLUTION OF EFFUSIVE STYLE AT PAVONIS MONS, MARS, INFERRED FROM LAVA FLOW MAPPING OF MARS EXPRESS HIGH RESOLUTION STEREO CAMERA DATA

The abundances and temporal relationships of lava channels and tubes can be used to infer recent styles of effusive activity on a volcano, as they generally form from different eruptive conditions. The southern flank of Pavonis Mons (PM), as imaged by the High Resolution Stereo Camera (HRSC), displays a higher ratio of channels to tubes than Olympus Mons (OM), with few, if any, tubes located near the summit. This suggests that either PM never experienced abundant tube-forming eruptions, or that the volcano experienced a transition from tube- to channel-forming eruptions, as seen at OM. Hawaiian shields display a similar transition, where older shields have lower abundances of tubes. Following the emplacement of flank flows from the PM summit, NE oriented rifting developed a new source region midway down the flank, producing a volcanic apron-shaped terrain to the southwest. The rift apron lacks lava tubes and is dominated by channels that are less subdued or buried than the flank channels. This channel-dominated, rift style of effusion appears to have transitioned to plains-style volcanism, superposed over the southeastern portion of the rift apron. This plains-style lava field is composed of >31 low shields and fissure-fed flows. The plains flows were emplaced via a combination of lava tubes and channels, surrounding the low shields and maintaining a slope below 1 degree, while the channel dominated portion of the apron displays slightly higher slopes. This transition in effusive style likely represents a change from major shield building to the plains-style volcanism that helped form the eastern Tharsis Rise. Preliminary crater counts for one low shield yield a model age of 100-500 Ma, suggesting activity at PM until this time.