HR: 1340h
AN: P23B-0191 INVITED     [Abstracts]
TI: Interrelated Glacial, Volcanic and Hydrologic Processes on the Tharsis Montes, Olympus Mons and Hecates Tholus, Mars
AU: * Head, J W
EM: James_Head@brown.edu
AF: Brown Univ, Geol Sci, Providence, RI 02912 United States
AU: Marchant, D R
P23B-0191 AF: Boston Univ, Earth Sci, Boston, MA 02215 United States
AU: Shean, D E
P23B-0191 AF: Brown Univ, Geol Sci, Providence, RI 02912 United States
AU: Fassett, C I
P23B-0191 AF: Brown Univ, Geol Sci, Providence, RI 02912 United States
AU: Wilson, L
P23B-0191 AF: Lancaster Univ, Env Sci Dept, Lancaster, LA1 4YQ United Kingdom
AB: Volcanic, hydrological and glacial processes are prominent in the geological history of Mars and assessment of areas where their relationships can be established provides important information on their nature and intensity. An effort to examine these interrelationships is motivated by the uncertainties that exist in atmospheric general circulation models concerning the homogeneous or heterogeneous distribution of water ice during periods of high obliquity and resulting transport of polar volatiles equatorward. Geological evidence for distinctive and very large tropical mountain glacial deposits on the NW flanks of the Tharsis Montes shows that the emplacement of ice in the equatorial regions is heterogeneous and is intimately linked to the presence of large volcanic edifices on the broad Tharsis rise. Accumulation of ice and formation of glaciers is likely caused by adiabatic cooling of water-laden polar air masses and precipitation and accumulation on the NW volcano flanks (F. Forget, personal communication). The resulting glacial deposits show interesting relationships to volcanic deposits formed prior to, during and subsequent to glaciation. Evidence that the glacial deposits formed from cold-based glaciation comes from the lack of modification of delicate structures associated with underlying lava flows (seen in detrended altimetry data). Evidence that volcanism occurred during glaciation is five-fold: 1) narrow linear ridges in the glacial deposits radial to the volcano are interpreted as dikes that were intruded into the glacier, rapidly melting the adjacent ice and collapsing, as has been proposed for englacial dike intrusions in Iceland; 2) broad, steep-sided, thick lobate flow-like features are interpreted to represent sill-like subglacial lava flows at the volcano-glacial interface; 3) circular donut-like annuli surround vent-like craters suggesting localized subglacial explosive eruptions; 4) steep asymmetric lava flows at glacial deposit margins are interpreted to be formed by the cooling and banking up of lava against the glacier terminus; 5) collapsed lava flows and local chaotic terrain are interpreted to be places where lava flows descended directly over glacial accumulation zones. Evidence for post-glacial volcanic emplacement is seen in: 1) edifice and flanking rift zone flows that invade and are superposed on the fan-shaped deposits, but show no evidence for associated melting and 2) the surface manifestation of a dike in excess of a hundred km in length that formed rows of tephra and spatter cones and localized lava flows a few to more than 10 km in length, all superposed on the Arsia fan-shaped glacial deposit. Emplacement of tropical mountain glacial deposits on the flanks of the Tharsis volcanoes suggests that there might be meltwater produced from one of several sources: 1) climate-induced (top-down) melting of surface snow in the tropics, 2) thickening of the ice sufficiently to produce basal melting, 3) conduction of magmatic heat to the glacier on the edifice flanks, 4) direct contact of the magma/lava with glacial ice. We find only limited evidence for meltwater processes at the Tharsis Montes, most likely related to dike emplacement and subglacial intrusion/flow events. In contrast, Hecates Tholus may provide an example of a volcanic edifice where summit ice accumulating occurred and basal melting was sufficient to produce the radial valleys that are observed. Thus, comparison of closely associated volcanic, fluvial and glacial deposits can be synergistic and provide new insight into the processes shaping the surface of Mars.
DE: 5416 Glaciation
DE: 5419 Hydrology and fluvial processes
DE: 5480 Volcanism (6063, 8148, 8450)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: Fall Meeting 2005