The role of tectonics in the late Quaternary evolution of Brazil's Amazonian landscape

Tectonic reactivation has long been suggested to have occurred over several areas of the Amazonian lowland in the Neogene-Holocene. Numerous landscape changes documented in this region, particularly variations in fluvial dynamics and morphologies, have nevertheless been exclusively attributed to climatic fluctuations. Minimizing the effect of tectonics over the Neogene-Holocene evolution of the Amazon basin can result in a distorted interpretation of the sedimentary record and, as a consequence, equivocal paleoclimatic reconstructions. Climate has often prevailed as a hypothesis to explain changes in fluvial dynamics possibly due to the still scarce synthesizing publications focusing exclusively on the neotectonic influence over this region. The present work provides a review of existing data focusing on Quaternary tectonics over a large area of the Amazonian lowland. The existing information is complemented by original morphostructural data on Quaternary terrains from this region, which were acquired through remote sensing. The goals were to analyze the geographic extent of late Quaternary deposits, and to determine the impact of tectonics on their development and on the latest evolution of the Amazon drainage basin. The analysis shows that: 1. the Amazonian lowland experienced geographically widespread sedimentation in the Late Pleistocene-Holocene; 2. sedimentation was promoted by the creation of new depositional sites mostly likely resulting from tectonic activity; and 3. tectonic reactivation impacted the latest evolution of this entire region, with particular influence on drainage basins, as revealed by river courses with an abundance of morphostructural lineaments evidencing fault control. Hence, the NW-, NE-, nearly N-S and, to a lesser extent, W-E morphostructural trends recorded in all areas investigated here are generally associated with strike-slip deformation. In addition, they are conformable with the main orientation of tectonic structures from adjacent crystalline rocks of the Precambrian basement. They also match tectonic trends documented in Neogene and younger sedimentary deposits from this and several other areas of northeastern Brazil. Furthermore, some are detected locally in the subsurface by geophysical data. Furthermore, a profusion of morphostructural anomalies is imprinted on the surface of Neogene-Holocene deposits of the Amazonian lowland, the main ones consisting of: 1. highly asymmetric modern drainage basin still undergoing organization, with anomalous patterns (i.e., trellis, sub-dendritic, sub-parallel and rectangular) that often vary from one another, frequent river captures and rivers flowing in contrary directions along the time, and straight channels that form triangular shapes or triple junctions or with local meandering and subtle enlargements; 2. an abundance of channels, floodplains, lakes and entire valleys with straight and orthogonal margins that locally form rhombic shapes or which may be laterally displaced; 3. fluvial rias (i.e., rivers that became enlarged as their mouth were barred) bounded by sharp and orthogonal lineaments which result in rectangular shapes; 4. termination of megafans into rhombic-shaped depressions bounded by straight lineaments or their lateral displacement several kilometers apart; and 5. rivers with asymmetric terraces laterally displaced by faults. These observations together are consistent with a landscape controlled by neotectonic activity. The modern seismogenic record, which reveals even high-magnitude earthquakes, indicates active deformation over the Amazonian lowland at the Present time. The neotectonic history of the Amazonian lowland is explained within the context of an overarching mechanism most likely related to intraplate stresses. This was probably caused by reactivation of pre-existing structures since the Neogene that was most likely promoted by activity along oceanic fracture zones and transform faults of the Brazilian Equatorial margin, as well as tectonism in the Andean region.