Abstract
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 in basalts on erosion in a bedrock river reach located in the Continental Volcanic Province of the Paraná Basin, southern Brazil. The 120-m-long reach is influenced somewhat by a possible fault that crosses it near one end. The fracture density and fracture direction were evaluated through field photogrammetry in seven sample areas distributed along the reach. The fracture direction and main erosion axes were also surveyed by remote piloted aircraft (RPA) aerial imaging. Tectonic fractures were identified in the field; they do not always appear in the survey of the sample areas but are evident in the RPA survey. 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 more abundant and multidirectional cooling joints act to control the plucking process and not to determine the erosion direction. The fracture density decreases with increasing distance from the fault crossing zone (from 9.62 to 3.73 m/m²), although the lower value is influenced by the presence of an amygdaloidal basalt zone. The higher fracture density favors more intense plucking.
Highlights
Complexity of fracture systems may vary broadly in basalts
To analyze the relationship between fracture density and erosional characteristics, we qualitatively evaluated the spatial distribution of the type and intensity of erosion processes based on field observations and remote piloted aircraft (RPA) imaging
The presence of tectonic fractures in the study area is directly confirmed by some field observations (Figures 3 and 7); considering that the joint directions are distributed among several classes, fractures of tectonic origin may overprint other types of fractures because pre-existing joints may accommodate subsequent tectonic stress (Patrick Muffler et al, 1994; Grant & Kattenhorn, 2004; Moir et al, 2010)
Summary
Complexity of fracture systems may vary broadly in basalts. These rocks have different fracture patterns depending on the morphology and thickness of the lava flow (DeGraff & Aydin, 1993; Thordarson & Self, 1998; Bondre et al, 2004; Vye-Brown et al, 2013). Abrasion is produced by the removal of particles from the rock surface by the impact of sediment carried by the flow, while plucking is a process of extracting blocks bounded by joints, only by hydraulic forces. Cooling joints in basaltic riverbeds typically provide an erosive response involving plucking (Whipple et al, 2000b; Lima & Binda, 2013; Lima & Flores, 2017). Differences in fracture style are not significant for long-term river erosion through plucking, these differences can influence bed morphology on the scale of reaches
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