Neotectonics of the subandes-Brazilian craton boundary using geomorphological data : the Maranon and Beni basins

Active and abandoned fluvial traces are used together with neotectonic, seismotectonic and subsurface structural data to study the neotectonic evolution of the Peruvian and Bolivian foreland basins. The Maranon Basin to the north and the Beni Basin to the south are located near the ends of the Peru-Bolivia Andean segment. This segment lies above the flat-slab subduction of the Nazca plate beneath the Andes. The surface of these basins shows a complex network of present-day rivers and fossil river traces. A relative chronology of the river traces deduced from morphological criteria allows the identification of successive shifting of fluvial belts up to the present position of the main rivers. As it enters the Ucamara Depression, the Ucayali River is deflected to the northeast. Successive shifts of the deflection point are directed upstream along the upper reaches of the river along the foothills. Simultaneously the Maranon River is deflected to the north, lining up with the straight, NE-trending lower reaches of the Huallaga River. In the central part of the depression the rivers trend northeast-southwest, parallel to the main strike of the basement faults. This regional trend is also parallel to the greatest principal stress observed both to the east and the west of the depression, and to the neotectonic faults on the margin of the Brazilian craton. The Beni Basin is characterized by a counterclockwise shift of the Beni River. Successive shifts of the river involve the northward migration of the deflection point of the Beni River as it enters the basin. A fault connected to the foothill margin controls this downstream movement. Recent erosion of the flood plain surface to the east suggests a tendency of the craton margin to ascend. No coaxial stress resulting from either plate motion or from collapse of the Andean topography seems to control the directional shift of rivers in the basins. (Résumé d'auteur