HR: 1340h
AN: G43A-0795    [Abstracts]
TI: The Formation of the Thaumasia Highlands: Elastic Thickness Estimates From Line-of-Sight Accelerations
AU: * Williams, J
EM: jpierre@mars.ucla.edu
AF: University of California Los Angeles, Dept. Earth and Space Sciences, Los Angeles, CA 90095 United States
AU: Nimmo, F
EM: nimmo@ess.ucla.edu
AF: University of California Los Angeles, Dept. Earth and Space Sciences, Los Angeles, CA 90095 United States
AU: Nimmo, F
EM: nimmo@ess.ucla.edu
AF: University College London, Dept. Earth Sciences Gower St., London, WC1E 6BT United Kingdom
AU: Moore, W
EM: bmoore@ess.ucla.edu
AF: University of California Los Angeles, Dept. Earth and Space Sciences, Los Angeles, CA 90095 United States
AB: The Thaumasia region of Mars defines the southeastern most portion of Tharsis. It consists of a 4-5 km high, arcuate mountain belt, the Thaumasia Highlands, which bound a 2-4 km high interior plateau that includes Solis, Sinai, and Syria Planum. Exterior to this plateau in the cratered plains, a negative free-air gravity anomaly flanks the highlands. As there in no topographic expression associated with the gravity anomaly, it represents a density contrast in the crust. We interpret this feature to result from the burial of a flexural moat created by the formation of the highlands. Raw line-of-sight (LOS) spacecraft accelerations confirm the presence of this gravity anomaly and allows for an estimate to be made of the flexural wavelength and hence the elastic thickness at the time the highlands loaded the lithosphere. As the elastic strength of the Martian lithosphere is presumed to increases over time as the planet's heat flux diminishes, its value provides an estimate of the relative timing of loading. A best-fit to the spherical harmonic gravity signal provides and elastic thickness of $\sim$20 km implying the highlands are an ancient feature (Early Noachian). This is consistent with the preservation of Early Noachian terrains on the elevated topography of the highlands and by the infilling of the adjacent trench by middle and late Noachian units, the boundaries of which, to first order, correlate with the gravity anomaly. The admittances derived from the LOS accelerations for the entire Thaumasia region, containing Noachian and Hesperian features, yield an elastic thickness estimate of $\sim$70 km. Values $>$100 km are obtained for the northwestern portion of Tharsis containing largely Amazonian units. These values indicate Tharsis development progressed toward the northwest through the Hesperian and Amazonian, consistent with the mapped geology that reflects the same trend. This suggests there has been an alteration to the underlying convection since the Early Noachian resulting in a migration in volcanic activity away from Thaumasia and the preservation of Noachian terrains on the southeast margin. Further, the style of volcanic construction has changed over this time period, transitioning from plateau building in the Noachian to the formation of large shields in the Amazonian, perhaps reflecting the changing response of the lithosphere to volcanic loads.
DE: 5417 Gravitational fields (1227)
DE: 5480 Volcanism (8450)
DE: 6225 Mars
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting