Metamorphic Precambrian basement is exposed in the Chilean Pre-Cordillera in the form of a narrow horst, and also in the Argentinian Puna. Miogeosynclinal pelites and psammites of Devonian age dominate in the western part of the Andes, while early Palaeozoic clastics occur in the E. Insignificant Triassic sediments were followed by wide-spread shallow-marine Jurassic deposition. Cretaceous and Tertiary strata are predominantly terrestrial deposits. Whereas the Precambrian and Palaeozoic series were intensely deformed by folding, block-faulting dominates in the volcano-sedimentary cover.

Magmatic activity has been significant since the Precambrian. While S-type intrusives and extrusives occurred mainly in the Palaeozoic, I-type magmatic rocks predominated in the Mesozoic. The Cenozoic volcanism in the Cordillera Occidental originated in the lower and upper crust. Magnetotelluric and seismic investigations on the traverse show extremely low electric resistivity values coinciding with a strong absorption of seismic waves below the Chilean Pre-Cordillera and the Cordillera Occidental. This phenomenon can best be explained by the existence of partial melts at depths of more than 10 km. The Bouguer values decrease by about -430 mgal from the coast to the Cordillera Occidental.

In the Precambrian and Palaeozoic, the continent apparently extended farther to the W. The change from a passive to an active continental margin took place at the Palaeozoic/Mesozoic boundary with the break-up of Pangea. In order to grasp better the tectonic and magmatic evolution as well as the recent structure of the crust in the Central Andes, a combined geological-geophysical investigation has been made along a traverse through the Andes between 20° and 24°S (Fig. 1). The following pages present a review of the geological development together with the first results of work from the Chilean section of the traverse.

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