3D Modelling and Interpretationof Gravity in the Central Andes

CLAUDIA PREZZI; HANS-JÜRGEN GÖTZE; SABINE SCHMIDT

Potsdam, Alemania

Simposio; 19th Colloquium on Latin American Geosciences; 2005

Our key motivation to study the Central Andes (between 20-29°S and 76-61°W) is that this mountain belt is a subduction related cordillera, which accommodates the Earth’s largest plateau formed without continent-continent collision. Moreover, a wealth of geophysical and geological information has been produced during the last years, probably making the Central Andes the geophysically most densely covered convergent continental margin, and providing a huge amount of data to adequately constrain our model. The major goal of this model is to contribute to an integrated understanding of the Central Andes geodynamic features and processes. We present a forward modelling of the Bouguer anomaly through a 3D model of densities distribution up to a depth of 1200 km. We used the interactive modeling tool IGMAS, which provides a wide range of GIS functions in 3D space. The final 3D density model shows a very good fit between the measured and modelled gravity fields. The geometry of our model is constrained by seismic reflection and refraction profiles, receiver function analysis, hypocenter locations, magnetotelluric data, different tomographic studies, thermal models, numerous structural balanced cross sections, etc. The density values assigned to the different bodies forming the model were constrained based on information and assumptions about the chemical and/or mineralogical composition and pressure-temperature conditions expected for each body. We included a partial melting zone at midcrustal depths under the Altiplano-Puna and considered the presence of a rheologically strong block beneath the Salar the Atacama basin, according to recent seismic studies. On the base of our model, we produced a contour map of the upper surface of the subducting slab below the continental margin, which is in good accordance with the slab geometries proposed previously by other authors. We also compared the modelled Moho with airy and flexural isostatic Mohos calculated from the actual topography and with the Moho depths obtained from receiver functions analysis, discussing the observed differences. Residual gravity maps were prepared considering the gravity effect of the subducting slab and of the isostatic and modelled Mohos, to gain insight on the compensation mechanisms. The upper and lower limits of the possible amount of partial melting occurring in the Andean Low Velocity Zone were estimated.   In this study we have demonstrated how 3D gravity modelling, integrating geophysical and geological information, can help to reveal the geodynamic processes playing a role in the building of Central Andes.