FISSURE ERUPTIONS IN THE THARSIS REGION OF MARS

Compared to the largest volcanoes in the Tharsis region of Mars (e.g., Olympus Mons), comparatively little attention has been given to understanding the origin of the smaller volcanoes within the region, despite the fact that these smaller structures may provide important information on both the regional tectonics of the region, and the availability of magma from the shallow or deep crust. THEMIS VIS, HiRISE and CTX images have allowed us to investigate six separate vent systems in eastern Tharsis east of the volcano Jovis Tholus (between 2.5S to 25.0N, 220.0E to 259.0E). These vents are typically linear fissures a few to ~20 km in length that have built small shields rising to ~50 – 85 m above the level of the surrounding topography. We find some remarkable similarities with Hawaiian fissure eruptions such as those that built Mauna Iki between 1919 – 1920; deposits from the Jovis Tholus eruptions included spatter from fire fountain eruptions, evidence that the fissures were once full and “leaked” lava onto the surrounding plain to build elongate shield volcanoes, and drain-back into the vents. This spatter subsequently coalesced to form either short (<5 km) lava flows or merged close to the fissure to form the longer (>20 km) flows. The maximum length of individual lava flows from these fissures is ~30 km, but more typical lengths are 15 – 20 km; these flows are all very thin (<5 m), which is a factor of ~8 – 15 thinner than flows elsewhere on Mars. There appears to have been some temporal evolution of the flow field, as the oldest parts of the basement at each center are built from a series of compound flows that cannot be subdivided into individual flows. We also find evidence that both frozen lava ponds and channelized flows were associated with the central vents. To investigate the unusual eruption conditions (limited total volume of the construct and of individual flows) we model the lava rheology, the duration of emplacement, and the subsurface conditions that may have led to these eruptions, and compare these durations to typical Hawaiian eruptions. Small vents such as the ones studied here have also been identified in other parts of eastern Tharsis, so that the eruptions described here may characterize a common style of volcanism on Mars that can only be identified now that image spatial resolution in the range 1 – 20 m/pixel is available.