Study of the Salado Basin (Argentina) from a double inversion of gravity and geoid

IVÁN L. NOVARA1*, CAROLINA B. CROVETTO1, ANTONIO INTOCASO1

Santiago de Chile

Congreso; GEOSUR2007 INTERNATIONAL GEOLOGICAL CONGRESS ON THE SOUTHERN HEMISPHERE; 2007

INTERNATIONAL GEOLOGICAL CONGRESS ON THE SOUTHERN HEMISPHERE

In this work we analyse several theoretical models to reproduce the observed gravimetric and geoid anomalies on the Salado Basin, Buenos Aires province, Argentina. The study area is located between 32 - 39º South latitudes and 56 - 62º West longitudes, covering the whole basin; part of the basin is placed over the Argentinean continental shelf. The isostatic study that we propose here is based in a double inversion of gravity anomalies and geoid undulations, employing three-dimensional computations. We employed continental observed Bouguer anomalies from an Argentinean data base, and we extended them to the oceanic area employing oceanic free-air anomalies. To isolate the Basin wavelengths, the Bouguer anomalies were filtered by a planar tendency surface to eliminate long wavelengths, and by upward continuation to eliminate very short wavelengths features. Positive Bouguer anomalies were found, with 40 mGal of maximum amplitude. The effect of the sediments filling the basin was computed by three-dimensional integration from an isobaths’ chart constructed from seismic studies. The effect of the unknown underlying masses was then computed subtracting the sedimentary attraction from the residual Bouguer anomalies. Observed geoid undulations were obtained from two different sources: GPS-levelling (h-H) values and the global geopotencial model EIGEN-CG03C. Both charts show similar values, but the GPS-levelling chart is restricted to the continental area, while the geopotential chart covers the oceanic area also. Both charts were filtered to isolate short wavelengths from the basin employing a first degree tendency surface. Residual geoid undulations are similar between them, and present positive values of 1 m amplitude. A theoretical model was proposed considering Airy’s system, employing the basin isobaths and normal physical parameters. Bouguer anomalies and geoid undulations predicted by this model and computed by three-dimensional integrations are both negative. The comparison between them and the positive observed values indicates that the Salado Basin is unbalanced. Several theoretical over-balanced models were proposed, considering that the basin presents a thicker anti-root than the one predicted by Airy’s hypothesis, finding that only a strong over-balance could produce positive Bouguer anomalies and geoid undulations like the observed ones. The best fitting was found with an anti-root of almost twice the Airy thickness. Nevertheless, none of the models reproduce exactly the gravity and geoid effects; they present a wider wavelength than the observed ones and a difference in the location of the maximum values. This could be attributed to the isobaths employed, which have been digitized from an old planar representation. For the best over-balanced model found, the amount of future subsidence was computed, finding that more than 2 km subsidence is necessary to reach the local balance. Finally, in order to improve the model, a bi-dimensional study was performed over a profile transversal to the deeper part of the basin, where a more complex model was proposed.