Rasgos morfoestructurales de la llanura amázonica del Perú: efecto de la neotéctonica sobre los cambios fluviales y la delimitación de las provincias morfológicas

Abstract

Morphostructural trends of the amazonian regions of Peru: the effect of neotectonics on fluvial changes and upland wetland boundaries Abstracts The Amazonian regions of Peru are discussed within the context of current knowledge on recent Andean tectonics and their impact on river drainage. The main morphostructural provinces are the foothills or Subandean Zone which are bordered on the east by large depressions in the Marañón Basin of northeastern Peru, the Madre de Dios region of southern Peru and further to the east the Brazilian craton (Iquitos Geanticline). The structural framework of the Subandean Zone consists of the Subandean Thrust and Fold Belt (STFB) West which crops out mostly in the Upper Foothills, and the Subandean Tilted Block Zone (STBZ) East which is concentrated mainly in the Lower Foothills. The main rivers which cross the STFB are antecedent or superimposed, but the secondary drainage channels conform to these structures and are subsequent. The STBZ river basins run parallel to the structural contour. The Marañón basin comprises a northern depression along the Pastaza River, and to the south is the large Ucamara depression with its extended floodplains and structurally controlled lakes. To the east, the Iquitos geanticline represents a positively epeirogenic structure on the western border of the South American craton. The rivers showing active meander belts (Marañón, Ucayali and Tapiche) are drained by white (silty) water from the Andes. Watersheds are drained by stable black water (from organic acids) streams with underfit patterns like the Samiria and Pacaya rivers. The underfit pattern is interpreted as the abandonment of formerly active large streams of white water. The position in the depression of the black water streams suggests successive shiftings of the Marañón and Ucayali rivers respectively northward (50 km) and southeastward (100 km) during the recent time, probably the Holocene. The geodynamical environment of the Ucamara depression suggests a close relation between the recent tectonic activity and the successive shifting of the Marañón and Ucayali rivers. The three large areas of swamp and lakes are presented: the Concordia, Puinahua and the Punga lakes. Their features are discussed in relation to deep faults and fold belts in the basement. Neotectonic data as well as data from earthquake focal mechanisms are compared to the present morphostructural scheme of the area, and discussed in term of quaternary tectonic evolution.