Tectonics and drainage development in central Amazonia: The Juruá River

Abstract

Understanding the factors that controlled the development of large rivers lacks information from modern and ancient analogues. The Amazonian basin contains several of the largest rivers on Earth, with many having tectonic control. The study of these rivers can contribute to test the claim that large rivers have a tectonic forcing. Addressing the evolution of the Juruá River is relevant, since this is the longest Amazonian river that parallels the Solimões Megashear Zone, a major NE-striking structure with a long history of reactivation of faults and folds even in the Cenozoic. This work discusses the geological evolution of the Juruá River along the Pleistocene-Holocene based on digital elevation models, integrated with subsurface data (well log and seismic reflection). The goal was to characterize some ancient large river valleys in forested areas of Amazonia and to analyze whether they might be related to previous paths of the Juruá River. A physical connection was found between this river and three elongated belts (i.e., PV1, PV2, and PV3) with low topographic and dissection values and surfaces marked by sinuous to meandering strings or curved lines. These characteristics led to relate the belts to ancient Juruá River valleys, an interpretation also suggested by the correspondence between paleovalley morphologies and subsurface Pleistocene-Holocene deposits with concave-up erosional bases. A tectonic hypothesis could explain the development of these paleovalleys around drainage basins with an annular pattern associated with a large convex-up relief, which we relate to rock upwarping by folding. Reconstructing the geological evolution of the Juruá River can introduce new data in discussions focused on the origin of large tropical rivers and on the neotectonic history of the South American intraplate.