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 ~ 3 m 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 7 months 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 2 cal kyr 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.