COULD THERMAL ISOSTASY CONTRIBUTE TO ALTIPLANO-PUNA ELEVATION?

The most remarkable feature of the Central Andes is the Altiplano-Puna plateau. This plateau is characterized by 3.5 km average elevation, approximately 70 km crustal thickness and very high heat flow. On the other hand, the forearc and the foreland basins have lower heat flow, thinner crust, and lower altitude. Such correlation suggests that thermal isostasy could play a role in the compensation of the Andean chain.

The thermal contribution to continental elevation is difficult to asses, because several other factors can mask it. The effects of compositional and thickness variations within the crust were removed using the crustal density structure obtained for the Central Andes between 19°S and 30°S from 3D forward gravity modelling. The gravity model is very well constrained by a huge amount of geophysical, geological, petrological and geochemical data. The elevation was adjusted for compositional buoyancy by calculating the density-thickness product from the 3D gravity model, relative to an isostatically compensated reference crustal section (average density: 2830 kg/m3, thickness: 40 km). We considered Airy and Vening-Meinesz models to isostatically adjust the observed elevation. The GTOPO30 digital elevation model was used to estimate the actual topography. The heat flow data base considered in this study includes new values recently published.

Average elevation adjustments range between 300 – 3000 m, with maximum values of aproximately 6000 m. It is observed that no correlation exist between the actual elevation and the corresponding heat flow values. In contrast, the isostatically adjusted elevation shows direct correlation with heat flow, with an increase of around 6000 m elevation between low and high heat flow zones. The forearc and the foreland basins areas are characterized by lower heat flow and lower elevation adjustments, whereas the Altiplano-Puna plateau, the Western Cordillera and the Eastern Cordillera, show higher heat flow and higher elevation adjustments. This fact agrees with the 3D gravity model, which shows the presence of shallow asthenosphere below the Altiplano-Puna. The obtained results would suggest that the thermal state of the lithosphere could contribute to the elevation of the Central Andes.