Late Holocene tectonic influence on hydrology and vegetation patterns in a northern Amazonian megafan

Abstract

The factors that control the heterogeneous canopy of the Amazonian rainforest have long been a topic of interest for research. Among all hypotheses, changes in landscape due to geological processes have been increasingly defended. Large areas of open vegetation in the northern Amazonian lowlands are confined to paleolandforms created by the abandoning of megafan depositional systems. Previous investigations related the megafan sedimentation in this region to Late Pleistocene-Holocene tectonic reactivations. However, the influence of neotectonics on both the Amazonian megafans and associated vegetation cover remains to be further investigated. We studied a depression preliminarily noticed in the south-central sector of one Amazonian megafan (i.e., Viruá megafan) aiming to determine if its genesis was due to recent tectonic subsidence after the megafan abandoning. The investigation combined morphostructural, hydrological and remote sensing reflectance analyses, as well as characterization of floristic communities in the modern and ancient environments based respectively on field inventories and palynological data. The results indicated a nearly 10-km long and rectangular-shaped area having straight margins paralleling NNE/SSW-trending regional structural lineaments. Hydrological analysis suggests that this area experiences the largest floods during wet seasons, a condition compatible with the topographic data that revealed a smoother gradient of ~3m comparing this area with others from the megafan plain. In the studied depression, numerous forest patches are arranged as sets of parallel straight lines trending consistently in the NE/SW direction. The patches have spectral values that differ significantly from other forest patches over the megafan surface, being compared to those of seazonally-flooded forests, such as igapós and várzeas, that surround the megafan paleolandform. In addition, a forest patch within the depression revealed trees physiologically adapted to tolerate submersion 6 to 7months per year, which is also a characteristic of the inundated forests external to the megafan. These data altogether led us to conclude that the studied depression consists of a shallow subsiding basin formed by tectonic reactivations in the mid/late Holocene. Tectonic instability at around 2calkyr BP further disturbed this system by creating NE/SW-trending lakes and leading to the replacement of seazonally-flooded forests by grasslands. Thus, tectonic activity in a relatively recent geological time must be accounted when analyzing plant distribution in the Amazonian wetlands.