Thermal and geodynamic contributions to the elevation of the Altiplano-Puna plateau

PREZZI, CLAUDIA BEATRIZ; IGLESIA LLANOS, MARÍA P.; GÖTZE, HANS-JÜRGEN; SCHMIDT SABINE,

PHYSICS OF THE EARTH AND PLANETARY INTERIORS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2014 vol. 237 p. 51 - 64

0031-9201

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. 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 (most of them derived from crustal melting): the Altiplano-Puna Volcanic Complex. In addition, the upper mantle structure changes along strike below the plateau. The upper mantle below the Puna becomes hotter, and the lithosphere becomes thinner and weaker. 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. The aim of this study is to estimate the elevation effect due to compositional variations and remove it by an isostatic adjustment, revealing the thermal and geodynamic contributions elevation. The effects of compositional and thickness variations within the crust are removed using the crustal density structure obtained for the Central Andes between 19°S and 30°S from 3D forward gravity modelling. The thermal isostatic relationship describing the thermal contributions to the elevation is determined. 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 1 km, the thermal contribution to the southern Puna elevation would be of 1.5 km. Previous works highlighted the fact that the Puna and the Altiplano have uniform average elevation in spite of showing great variation in the amount of structural shortening. Shortening estimates 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. Other authors suggested that thermal heating and crustal flow would explain the uniform altitude of the Altiplano-Puna, in coincidence with our results. The obtained results suggest that the thermal state of the lithosphere would play a significant role in the elevation of the Central Andes, and may be responsible of some of the geological differences displayed by the Altiplano and the Puna.