Fissure eruptions in Tharsis, Mars : implications for eruption conditions and magma sources.

Wilson, Lionel and Mouginis-Mark, Peter J. and Tyson, Shelly and Mackown, Jennifer and Garbeil, Harold (2009) Fissure eruptions in Tharsis, Mars : implications for eruption conditions and magma sources. Journal of Volcanology and Geothermal Research, 185 (1-2). pp. 28-46. ISSN 0377-0273

Abstract

The Tharsis region has been the focus of many studies of volcanism on Mars. Not only are the largest volcanoes on the planet located here, but also many of the youngest volcanic features are found within Tharsis. Comparatively little attention has been given to understanding the origin of the smaller volcanoes within Tharsis, and these smaller structures may provide important information on both the regional tectonics of the area, and the availability of magma from the shallow or deep crust. We have identified six separate, but physically close, vent systems in eastern Tharsis just to the east of the volcano Jovis Tholus. 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. The maximum length of individual lava flows from these fissures is ~ 30 km, but more typical lengths are 15–20 km. Mapping of the vent complexes reveals that smooth material, interpreted to be spatter from fire fountaining, is located on the rims of some of the fissures. This spatter then formed either short (< 5 km) flows or merged close to the fissure to form the longer flows. 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 find evidence of frozen lava ponds and channelized flows from central vents. Photoclinometric profiles are constructed across one of the longer flows to confirm an unusual attribute, originally identified by Mouginis-Mark and Christensen [Mouginis-Mark, P.J., Christensen, P.R., 2005. New observations of volcanic features on Mars from the THEMIS instrument. J. Geophys. Res. 110: E8, doi: 10.1029/2005JE002421], of the flows from the central vent complex: they are all < 5 m thick, which is a factor of ~ 8–15 thinner than flows elsewhere on Mars. 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, the subsurface conditions that may have led to these eruptions, and the volatile content of the magma. We find lava viscosities and yield strengths to be ~ 100 Pa s and ~ 100 Pa, respectively, eruption rates to be ~ 5000 m3 s− 1, flow speeds to be 1–2 m s− 1, durations of emplacement of individual flow units to be ~ 5 h, and the equivalent magma water content to be 0.1–0.2 mass%. These eruption conditions are consistent with a wide range of possible depths of the magma reservoirs feeding the 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. It is therefore possible that additional searches of other volcanic areas (Syrtis Planum, Elysium Planitia, and Hesperia Planum) may also show greater diversity of activity than is currently accepted.