Neotectonic influence on tropical rivers of southwestern Amazon during thelate quaternary: the Moa and Ipixuna river basins, Brazil

Neotectonic influence has conditioned the development of the Moa and Ipixun a fluvial belts in the southwestern Brazilian Amazon region. The area under went a compressive tectonics with reverse N-S fault dipping to the west tha t formed an "en echelon system" system from west to east, from the Serras d o Divisor ranges to the Jurua river. As a response to stress, subsiding blo cks with NW-SE trends were developed. The E-W to N55 degrees W topographic trends could represent transcurrent dextral transtensional faults. Width of alluvial belts increases along the sequence of "en echelon system" blocks. Correlated with the increase of width, the quantity of channels formed by avulsion increases as well. The Moa and Ipixuna alluvial belts, respectivel y reaching ca. 9 and 7 km maximum width, are "abnormally" wide because they developed in subsiding blocks stretched in a NW-SE trend. Fluvial dynamics was dominated by avulsion in the subsiding blocks during the late Pleistoc ene and, probably, during part of the Holocene. (C) 2000 Elsevier Science L td and INQUA. All rights reserved.