Fracture variability in basalts and its effect on river erosion: a case study in the Paraná Volcanic Province

How to cite item Lima., A., G., Pelegrina, M. A., & Pontarolo, M. (2021). Fracture variability in basalts and its effect on river erosion: a case study in the Paraná volcanic province. Earth Sciences Research Journal, 25(1), 13-20. DOI: https:// doi.org/10.15446/esrj.v25n1.74167 The variation in the structural characteristics (cooling joints and tectonic fractures) of basaltic flows implies potential variability in the intensity of erosion by plucking. The erosive behavior of the rivers that sculpt these areas depends on their interaction with the diverse fracture systems. In view of this, we analyzed the effect of fracture variability (tipology, density and direction) in basalts on erosion in a bedrock river reach located in the Continental Volcanic Province of the Paraná Basin, southern Brazil. The fracture density and fracture direction were evaluated through field photogrammetry in seven sample areas distributed along a reach of 120 m. The fracture direction and main erosion axes were also surveyed by remote piloted aircraft (RPA) aerial imaging. The main erosion axes coincide with the principal fracture directions (tectonic fractures), which are disposed obliquely to the channel flow direction, making an average angle of 50°. The small, more abundant, and multidirectional cooling joints control the plucking process, but do not determine the erosion direction. The fracture density systematically decreases upstream from 9.62 to 3.73 m/m2, probably related to distance from a structural lineament which river crosses downstream. The higher fracture density favors more intense plucking due to decrease in the size of the rock blocks. The lower fracture density limits the plucking and favors the macroabrasion, mainly if associated with vesicular-amygdaloidal basalt. ABSTRACT Fracture variability in basalts and its effect on river erosion: a case study in the Paraná volcanic province