Altiplano-Puna elevation budget.

PREZZI, CLAUDIA BEATRIZ; GÖTZE, HANS-JÜRGEN; SCHMIDT SABINE,

Santiago de Chile

Congreso; Geosur 2007; 2007

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. Furthermore, below the Altiplano-Puna the existence of a partial melting zone at mid-crustal depth has been established by a number of independent observations. This interpretation is strongly supported by the presence of a huge concentration of Neogene ignimbrites: the Altiplano-Puna Volcanic Complex. These features suggest that thermal isostasy could play a role in the compensation of the Altiplano-Puna. Thermal isostasy is the geodynamic process whereby regional variations in the lithospheric thermal regime cause changes in elevation. Elevation changes result from variations in rock density in response to thermal expansion. However the thermal contribution to continental elevation is difficult to asses, because variations in crustal density and thickness can mask it. This study estimates the elevation effect due to compositional variations and removes it by an isostatic adjustment, revealing the thermal and geodynamic effects on elevation. 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 large amount of geophysical, geological, petrological and geochemical data. The elevation was adjusted for compositional buoyancy by calculating the density-thickness product from our 3D gravity model, relative to a reference crustal section. The heat flow data base considered includes new values recently published. The thermal isostatic relationship describing the thermal contributions to the elevation was determined using a reference geotherm corresponding to a surface heat flow of 30 mW/m2 and assigning a lithosphere having this thermal estate an elevation of 0 km. It is observed that no correlation exists between the actual elevation and the corresponding heat flow values. In contrast, the compositionally adjusted elevation shows direct correlation with heat flow. Our results suggest that while the thermal component of the Altiplano elevation would be of 0.5 km, the thermal contribution to the Puna elevation would be of 1.5 km. Previous works highlighted the fact that the Puna has higher elevation than the Altiplano in spite of showing lower amount of structural shortening and thinner crust. Shortening values are sufficient to account for crustal cross sectional area in the Altiplano north of 22°S, but are less than that needed in the Puna south of 22°S. Our estimates of the thermal contribution to the Altiplano and Puna elevation would explain these features. Moreover, our 3D gravity model shows the presence of shallower asthenosphere below the Puna than below the Altiplano, suggesting a possible relationship between the depth to the top of the asthenosphere and the higher heat flow, the greater thermal contribution to the elevation, the existence of a mid-crustal partially molten zone and of the Altiplano-Puna Volcanic Complex in the Puna. The obtained results would suggest that the thermal state of the lithosphere could play a significant role in the elevation of the Central Andes.