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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS017 Oral Presentation 2364
Altiplano-Puna Elevation and the role of thermal isostasy
Dr. Claudia Prezzi
Universidad de Buenos Aires - Ciencias Geolgicas Universidad de Buenos Aires - CONICET
IAGA
Hans-Jrgen Gtze, Sabine Schmidt
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 (e.g. extreme high conductivity zones,
broad low seismic velocity zones, etc.). 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. On the other hand, the forearc and the foreland basins have lower heat flow,
thinner crust, and lower altitude. 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 19S and 30S 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 (average crustal density: 2850 kg/m3,
average mantle density: 3350 kg/m3, crustal thickness: 40 km). 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. 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. Average elevation
adjustments range between 300 and 3000 m, with maximum values of approximately 6000 m. 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, with
an increase of around 3000 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. Our results suggest that while the thermal component of the
Altiplano elevation would be of 1 km, the thermal contribution to the Puna elevation would be of 2 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 22S, but are less than that
needed in the Puna south of 22S. Other authors suggested that thermal heating and crustal flow would
explain the uniform altitude of the Altiplano-Puna, in coincidence with our results. Moreover, it was
determined that the above mentioned partially molten zone would extend in the backarc region
between about 22 and 24 S and below the Altiplano-Puna Volcanic Complex area of caldera