Granitic inselberg erosion controlled by dike swarm array in semiarid Brazil

Abstract

Numerous inselbergs rise above the regional erosional surface in semiarid northeastern Brazil. The inselbergs are underlain by the Quixadá Pluton and cut by cm-thick granite dike swarms. Five inselbergs with abundant dikes from two different parts of the pluton were selected to investigate the role of dike patterns in the evolution of residual granite topography, including morphological features of hillslopes. The methods included field mapping of landforms, structural measurements of accessible dike arrays, and rock strength determinations using a Schmidt hammer. Unmanned Aerial Vehicle (UAV) photogrammetry was used to build detailed digital surface models of inselbergs and to recognize geometric patterns of dike arrays. In addition, 34 rock samples were collected to determine rock composition and evaluate their resistance to weathering. The results show that >60 % of dikes display a sub-horizontal array composed of low- and medium-angle dikes (up to 45° dip), which tend to increase rock-mass resistance, providing effective barriers against weathering and vertical fracturing, and are associated with the most prominent, steep-sided residual hills. Besides, dike swarm geometry controls the slope outlines of some inselbergs, which commonly conform to dike attitudes. By contrast, steeply-dipping dikes (>60° dip) promote weathering along the dike-host rock interface and are hypothesized to underlie the plains between the inselbergs. At a small scale, dikes control the occurrence and evolution of minor rock landforms such as weathering pans, solution runnels, overhangs, and split boulders. Our findings show that the resistance of inselbergs to erosion is not homogeneous in the granitic pluton but is controlled by local factors such as dike occurrence and dip.