VENT STRUCTURES ON THE SUMMIT AND FLANKS OF PAVONIS MONS: IMPLICATIONS FOR THE VOLCANIC EVOLUTION OF A MAJOR SHIELD VOLCANO ON MARS

Small volcanic edifices in volcanic plains-type and rift-related settings of Mars were given much consideration recently, but only a few reports have addressed vent-related morphologies on the flanks and in the summit regions of the large Martian shields. Impact crater chronology has shown ages on the order of only few hundred million years for several of the shield calderas, despite the much higher ages obtained for the entire edifices. A key question for understanding the volcanic evolution concerns the age and structure of the flanks of the shields and their involvement in the volcanic activity through time.

Based on impact crater chronology using CTX data, we can demonstrate that Pavonis Mons showed 3-4 periods of emplacement of large volumes of volcanic deposits during the last ~0.5 Gyr. These were associated with caldera formation, but also with the formation of new rift apron deposits and with resurfacing of the flanks. We performed a systematic survey of vent structures and associated local deposits visible on the entire shield, and observe a great number of both on the flanks and inside the caldera complex. We have analyzed recently available high-resolution images (HRSC, CTX, HIRISE), as well as high-resolution DTMs derived from HRSC data and MOLA DTMs for morphometric characterization.

We mapped more than 300 vent features, the majority of which are rimless depressions at the head of rille-like channels which formed most likely by erosion by lava. Also a number of small shields characterized by a low crater/cone diameter ratio were formed by mainly effusive flank eruptions. Edifice heights and the diameters of craters and cones suggest a significant contribution of explosive eruptions for a second class of cone-shaped edifices. Lava ramparts and fissure vent associations are also observed. Deposition of volcanic material has produced a number of fan-shaped deposits with terrace-like morphology. Some of these are directly associated with flank vents, or the fan apex is linked to collapsed lava tubes or eroded lava channels. The flank fans show a similarly young age as the caldera and apron deposits. We suggest that the phases of high volcanic activity of Pavonis during the last ~0.5 Gyr were accompanied – in addition to the formation of rift aprons – by significant resurfacing due to effusive and explosive flank eruptions in other sectors.