Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31e1740g&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31E-1740
Mathematics
Logic
[5417] Planetary Sciences: Solid Surface Planets / Gravitational Fields, [5418] Planetary Sciences: Solid Surface Planets / Heat Flow, [5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Tyrrhena Patera is a low-relief, central-vent volcano located in the southern highlands of Mars, northeast of the Hellas impact basin. The main edifice contains few primary lave flow features and the flanks of the volcano are heavily eroded, indicating that they are composed of friable material which could have been formed by pyroclastic flows. The volcano itself was emplaced in the Noachian, but was subsequently modified during the Hesperian period, with episodes of resurfacing - probably driven by erosion - stretching well into the Amazonian period. Resurfacing of the caldera rille floor and upper shield probably mark the cessation of volcanic activity at Tyrrhena Patera around 800 Ma ago, and the geological evidence suggests that activity at Tyrrhena Patera transitioned from dominantly explosive to dominantly effusive eruptions. In summary, Tyrrhena Patera is generally thought to be predominantly composed of multi-layered, compacted ignimbrite deposits. The Tyrrhena Patera volcano is associated with a well localized positive free-air gravity anomaly and a good correlation exists with the features topography. We have used the latest gravity field model for Mars expanded up to degree and order 110 to model the localized admittance spectrum at Tyrrhena Patera considering surface as well as subsurface loading. We use a spherical cap localization window with a cap diameter of 7 degrees and a spherical harmonic bandwidth of 37. Ignoring the lowest degree terms that may be influenced by the Tharsis signal, we analyze the localized admittance in the degree range 42 to 57. The observed admittance is then compared to a forward model which is localized in the same manner as the data. In this way, we have quantified the range of admissible load densities as well as the admissible magnitude of a potentially present subsurface load. Modelling suggests that load densities need to be between 3290 and 3450 kg/m3 if no subsurface loads are present. If subsurface loads in the form of, e.g., solidified magma are present at depth, the lower limit of admissible load densities is extended to 3070 kg/m3. The degree of welding of an ignimbrite deposit depends on the emplacement temperature, the kind of pyroclasts involved, and the load of overlying material. Densest welding occurs in the lowest parts of the cooling unit, which may be many tens of meters thick, and typical densities encountered in ignimbrite deposits range from 1500 to 2500 kg/m3. This is considerably smaller than the load densities required by the gravity field observations, indicating that the bulk of the edifice must be composed of well compacted material of negligible porosity. Furthermore, the required high densities give no indication of more silicate rich, evolved magma compositions. Densities between 3250 and 3450 kg/m3 fall within the range of pore-free densities estimated for the Martian meteorites, and it seems likely that the bulk of Tyrrhena Patera is basaltic in composition. Furthermore, low density ignimbrite deposits must be a volumetrically minor component of the volcano and can only be of shallow extent.
Grott Matthias
Wieczorek Mark A.
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