Neotectonics and tree mortality in a forest ecosystem of the Negro basin: Geomorphic evidence of contemporary seismicity in the intracratonic Brazilian Amazonia

Abstract

Neotectonic activity was increasingly recorded over the Amazonian lowlands, even during historical times of a few tens of centuries. However, detailed analyses linking structures and styles of deformation are still few considering the large dimension of the region. Such events in a relatively recent time are expected to have caused conspicuous impacts on Amazonian ecosystems and forest disturbance. Changes in topography and hydrology by fracturing, faulting and folding during either subsidence or uplifting are direct effects on landscape that leave marks on the structure of the current forest, such as tree mortality and community succession. Two rectangular elbow-like shaped wetlands from the left margin of the middle Negro River valley in northwestern Amazonia were attributed to neotectonics. Remote sensing imagery was applied to test this hypothesis and also provide a tectonic model that can explain deformation dynamics in this area. The studied wetlands and adjacent river systems display various morphostructural anomalies compatible with a tectonic control, as well as lineaments paralleling main NW and NE-trending regional structures. The geomorphic analysis led to suggest that the wetlands are depressions formed by NW and NE-trending master boundary faults of horizontal displacements intercepted by various subsidiary faults. Transtensional strike slip regime is recorded by both left and right-lateral faults, with the wetlands corresponding to subsiding areas owned to conjugate strike slip faults. Habitat fragmentation and other ecological processes promoted by tectonic deformation would have impacted the overlying forest canopy by changing its structure due to tree mortality. The complex compartmentalization imposed by active tectonics would have exposed the ground to contrasting hydrological conditions, which controlled the rate of tree mortality within the wetlands. We pose that the tectonic disturbance and associated tree mortality documented in the middle Negro River evidence contemporary seismicity within the intracratonic Brazilian Amazonia.