Gravity, isostasy and Andean crustal shortening between latitudes 30 and 35°S

Abstract

We started the work by analyzing an E-W crustal section at 33°S latitude, which extends from the western Chilean Trench, crosses the Andean Belt and goes inland into Argentina. A two-layer gravity-seismic model was prepared here. It provided a maximum Moho depth of 62 km beneath the Andean axis. This model is consistent with a one-layer standard crustal model, with “normal” thickness T = 33 km and densities of 2.9 g/cm 3 and 3.3 g/cm 3 for the crust and the upper mantle, respectively. Then we analyzed the Andean isostatic behaviour on four Andean gravimetric sections at latitudes of 30, 32, 33 and 35°S, based on one-layer simple models. These results show antiroots near the Chilean Trench and crustal thickenings beneath the Andean Belt which locate the Moho discontinuity at depths of 70 km (at 30°S), 65 km (at 32 and 33°S) and 57 km (at 35°S). From the crustal gravimetric models, the following provisional charts for the sector were prepared and compared: (1) Topography; (2) Bouguer gravity; (3) Isostatic corrections; (4) Moho contours. These charts are consistent with each other and they point out that the compensation of the Andean Belt trends approximately to isostasy in the Airy concept. Other results also confirm this: (1) the remarkable decrease of the isostatic anomalies in each of the analyzed sections, as regards the Bouguer anomalies; and (2) the relationships between the exceeding areas and the corresponding crustal-root areas which are consistent with the Airy numerical relations as to the perfect isostatic equilibrium. Thus, the Bouguer anomalies are mainly controlled by the M discontinuity. Finally, shortenings in the Andean sector were fixed based on the crustal thicknesses found from gravity inversion. These are 150 km at 30°S, decreasing southwards and reaching only 90 km at 35°S. These shortenings explain the Cenozoic Andean uplift without requiring other mechanisms, such as magmatic intrusions.