HR: 17:00h
AN: G44A-05    [Abstracts]
TI: Modelling the salar de Uyuni, Bolivia as an equipotential surface of Earth's gravity field
AU: * Borsa, A A
EM: aborsa@ucsd.edu
AF: Institute of Geophysics and Planetary Physics, University of California at San Diego, La Jolla, CA 92093-0225 United States
AU: Bills, B G
EM: bbills@ucsd.edu
AF: NASA, Goddard Space Flight Center, Geodynamics Branch, Greenbelt, MD 20771 United States
AB: The salar de Uyuni is a massive dry salt lake that lies at the lowest point of an internal-drainage basin in the Bolivian Altiplano. Its topography is remarkable for its extraordinary flatness over almost a full degree of latitude and longitude, despite constant topographic forcing due to faulting and local isostatic rebound. We surveyed a 54 x 45 km region of the salar with kinematic GPS in September, 2002 and found a topographic range of only 80 cm over the surveyed area. Furthermore, the survey revealed distinct surface features with wavelengths between 5 and 40 km. Some of these appear to be aligned with orographic features that intersect the salar, leading us to conjecture that they are the surface expression of high-density massifs that have been buried by low-density basin sediments. Over the oceans, a similar correspondence between basin bathymetry and surface topography is exploited to map the seafloor using sea-surface satellite altimetry measurements, with the sea surface following geoid undulations due to the underwater mass distribution. On the salar, annual flooding creates a shallow seasonal lake whose surface is likewise an equipotential shaped by the distribution of underlying mass. The dissolution and redeposition of salt by the lake waters appears to push the system toward an equilibrium of constant water depth so that the salt surface itself closely approximates the local equipotential surface. To test our hypothesis about the origin of the surface features on the salar, we compare our GPS survey elevations with the equipotential surface estimated from a combined analysis of local gravity measurements and the EGM96 global geopotential model. 50% of the variance of the GPS elevations can be explained by equipotential surface undulations from the EGM96 model alone. An additional 40% is explained by the shorter-wavelength equipotential surface derived from local gravity. The elevation residual is remarkably consistent with our independent estimate (from remote-sensing analysis of water depths) of the departure of the surface from an equipotential, suggesting that it is the signature of unmodelled secondary surface processes. We believe that the salar de Uyuni is one of the few terrestrial locations where potential fields are able to play a major role in shaping local ($<$ 100 km) topography, overcoming the typical dominance of tectonics and erosion in geomorphology on this scale.
DE: 8045 Role of fluids
DE: 1219 Local gravity anomalies and crustal structure
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting