Anomalous upper mantle beneath the Central Andes : isostasy and Andean uplift

Four theoretical models isostatically compensated were prepared and compared between themselves in order to analyze either the isostatic equilibrium or the probable mechanisms for justifying the Central Andean uplift. They are: Model 1, or Airy Model: in this classical model the isostatic compensation would take place at the maximum Moho depth's level: 61.7 km. Model 2: this model justifies the Andean elevation by means of a combination of lithospherical thermal root and crustal root. In this case, the last one is diminished 5.3 km respect to crustal root's thickness of Model 1. Isostatic compensation takes place here at the bottom of thermal lithosphere: 140 km deep. Model 3: this model explains the elevation by means of a combination of the subsidence that the subducted Nazca Plate could produce, and the crustal root effect, that is 6 km thicker than the first model's root. The isostatic compensation could take place at a depth of 300 km. Model 4: this model involves Models 1, 2 and 3 mechanisms; the isostatic compensation takes place at a depth of 300 km. Crustal shortenings S are sensitive to the upper mantle's heterogeneity, varying from 17% to +18%, h as it is shown by the following values: S = 278 km (Model 1), S = 230 km (Model 2), S = 338 km (Model 3) and h h h S = 288 km (Model 4). h We also demonstrate that the model selected for evaluating the Andean isostatic equilibrium in this zone of the anomalous upper mantle is not critical. The analysis of the two EW gravity sections at 22°S and 24.5°S latitudes favors Model 2 as the most likely, since the crustal thickness found from it is clearly consistent with seismic data available at 24.5°S. Nevertheless, we can admit other gravimetric models.