HR: 0800h
AN: T31A-0287    [Abstracts]
TI: The Andes: Geoid, Topography, and Flexure of the South American Plate
AU: * Chase, C G
EM: chase@geo.arizona.edu
AF: Dept. of Geosciences, University of Arizona, Tucson, AZ 85721-0077, United States
AU: Coblentz, D D
EM: coblentz@lanl.gov
AF: Geophysics, Los Alamos National Lab, Los Alamos, NM 87545, United States
AU: Sussman, A J
EM: spring@lanl.gov
AF: Geophysics, Los Alamos National Lab, Los Alamos, NM 87545, United States
AB: The topographic load exerted by the Andean orogenic system on the South American plate provides an opportunity to evaluate the relationship between topography, geoid, lithospheric flexure and orogenic curvature. The large aerial extent of topography in excess of 3 km between 5º S and 30º S latitude in the Andes loads the margin of an otherwise topographically "simple" continental plate, allowing us to interpret the geoid anomaly and flexure associated with loading. The Andean orogen is a curved continental/oceanic collision system for which the flexural forebulge is difficult to delineate due to the vegetative cover related to the Amazonian forest. We evaluate evidence for the presence and geometry of an Andean forebulge by integrating the global geoid anomaly with topography and flexural modeling. The GRACE global geopotential data up to degree and order 360 provides a geoid anomaly field with a minimum spatial resolution of about 100 km. The geoid signal from deep sub- lithospheric sources was removed using a long wavelength filter which passes information at shorter wavelengths with a cosine taper between degree and order 7 to 11 and ending at the maximum resolution of order/degree 360. This filter allows the evaluation of contributions to the geoid from mass distributions from depths less than 800 km. A number of features in the filtered geoid field are remarkable, including a close correlation between a geoid anomaly of 25 m over high topography, and a consistent geoid bulge of about 5 m present east of the Andes mountain chain that has no topographic expression. We successfully model this geoid signal with a topographic load on an elastic plate with an effective elastic thickness of 50 km. A good match for the geoid anomaly bulge east of the Andes is obtained between 5º and 35º S and provides a calibration of a single uniform elastic plate model. The 500 km wavelength of the bulge restricts its source to lithospheric depths. Calculations of the three- dimensional distribution of forebulge amplitudes and positions determined from our analysis suggest that two- dimensional flexural models do not correctly estimate the load and flexural parameter for curved orogens.
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 8122 Dynamics: gravity and tectonics
DE: 8138 Lithospheric flexure
DE: 9360 South America
SC: Tectonophysics [T]
MN: 2007 Fall Meeting