Relationship between elastic thickness and the tectonic segmentation of the Andean margin (15-47°S)

Abstract

Using a flexural analysis of the correlation between relief and gravity, the factors that control the Andean segmentation (15-47°S) and the orogenic process are investigated. As a framework for this analysis, the following segmentation is proposed; the first order units are the central Andes (15-33.5°S) and the Southern Andes (33.5-47°S). Along both units, segments of second order are distinguished; Altiplano (15-23°S), Puna (23-28°S) and Frontal Cordillera (28-33.5°S) within the central Andes, Principal Cordillera (33.5-39°S) and Patagonian Cordillera (39-47°S) in the southern Andes. The results of the flexural analysis in this tectonic segmentation framework is expressed in terms of spatial variations of the elastic thickness Te (which defines the lithospheric rigidity), crustal thickness EC, and horizontal compressive force F. Te is high in the forearc and foreland regions, and low under the orogenic axis, showing minimum values (<10 km) and a strong traverse gradient in the central Andes. In contrast, the minimum Te value in the Patagonic Cordillera segment is 35 km. Te has an inverse correlation with EC, parameter that shows its maximum in the western side of the central Andes (65-70 km). F presents maximum values at the north and south edges of the central Andes (reaching 4 x 1012 N/m), which is consistent with the high interplate coupling there. From a rheologic viewpoint, Te is directly proportional to the plagioclase-quartz ratio in the crust as well as strain rate-to-heat flow ratio imposed by the convergence. This qualitative relationship, complemented by geological and geophysical constraints, suggests that the first order segmentation is caused by a contrasting composition of the crust; a more felsic one in the central Andes against a more mafic crust in the southern Andes. The second order segmentation is associated with variations of the thermomechanic regime linked to the convergence process.