Resumen
The Melipilla Anomaly, a first order geological transition along the Maipo river valley, has been geologically interpreted through an intergration of surface geology, regional magnetic and gravimetric information, and 42 magnetotelluric soundings organizad in two north-south profiles. Magnetically, the Melipilla Anomaly represents the response of a tabular, non magnetic flat-lying body at middle upper crustal depth (~4-6 km), surrounded by a strongly magnetic (k~0.01 cgs), presumably Jurassic batholith. The joint interpretation of surface geology and geoelectrical images with depth would indicate the presence of a compressional horst to the south of the anomalous zone, characterized by resistivities in the range of 1,000-10,000 ohm-m, and a consistent gravimetric response with a positive contrast of ~0.07 gr/cc with respect to the country rock. This compressional horst or rigid body acted as a buttress opposing the tectonic deformation of the jurassic granitoids to the north of the Maipo river. Within this tectonic setting, the non-magnetic tabular body has been interpreted as the detachment zone that allowed the accomodation of the deformation exerted by interaction with the rigid body to the south. The demagnetization of the detachment zone would be associated with fluid migration and alteration in the free surfaces, interpretation which is consistent with its moderate geoelectrical response. From a regional perspective, the development of a rigid block to the south of 33.5ºS, probably represents a feature that conditioned the later segmented evolution of the continental margin. Additionally, the inferred tectonic model suggests an oblique convergence with sinistral sense during the Mesozoic, in agreement with the tectonic evolution of other segments of the Andean margin during the Jurassic and Lower Cretaceous.