Integrated 3D model of the Central Andes density structure

CLAUDIA PREZZI; HANS-JÜRGEN GÖTZE; SABINE SCHMIDT

Santiago, Chile

Congreso; 2005 International Association of Seismology and Physics of the Earth’s Interior (IASPEI) General Assembly; 2005

The Central Andes are a subduction related orogenic belt, which hosts the Altiplano-Puna plateau. The plateau has an average elevation of approximately 4 km. The processes leading to its formation are still subject of analysis. Numerous geophysical and geological studies carried out in this part of the Andean chain produced a large amount of data, which can be used to construct well constrained models. We developed a 3D density model of the continental crust, the subducted plate and the upper mantle of the Central Andes between 20-29°S and 76-61°W. The goal of this project is to integrate the available information (geophysical, geological, petrological, geochemical) in a single model to gain insight on the lithospheric structure and the geodynamics of the area. The Bouguer anomaly was forward modelled using the software IGMAS. IGMAS permits to model potential field anomalies interactively in 3D, including constraining data through GIS functions. The geometry of our model is defined and constrained by hypocenters location, reflection and refraction on and off-shore seismic lines, travel time and attenuation tomography, receiver function analysis, magnetotelluric studies, thermal models and balanced structural cross sections. The densities allocated to the different bodies were calculated considering petrological, geochemical and heat flow data for the continental lithosphere and astenosphere; and taking into account petrological assumptions and the age of the Nazca plate for the oceanic lithosphere and astenosphere. The model consists of 31 parallel E-W vertical planes extending between 12-35°S and 57-79°W. The continental crust comprises distinct bodies, which represent the morfotectonic units of the Central Andes: Coastal Cordillera, Precordillera, Western Cordillera, Altiplano-Puna, Eastern Cordillera, Subandean Ranges and Chaco. We included a partial melting zone at midcrustal depths under the Altiplano-Puna (Low Velocity Zone) and considered the presence of a rheologically strong block beneath the Salar de Atacama basin, according to recent seismic studies. Contour maps of the depth of the continental Moho, the depth to the top of the subducted slab and the depth to the top of the astenosphere below South America were produced. We compared the modelled continental Moho to regional and local isotatic Mohos calculated with and without considering internal loads. The internal loads were obtained from our gravity model. The possible percentage of partial melt in the Central Andes Low Velocity Zone was estimated. Different residual anomalies were calculated by subtracting from the Bouguer anomaly the gravimetric effect of the modelled subducted slab and of the modelled and the calculated isostatic Mohos. In this study we have demonstrated how 3D gravity modelling, integrating geophysical, geological and petrological information, can help to reveal the geodynamic processes playing a role in the building of Central Andes.