How does deep granite weather? Fractures matter but fracture size counts

How the weathering zone propagates into un-weathered rock at depth is subject of increasing research. Some models suggest that the depth of the ground water table controls weathering zone thickness. Others predict that regolith thickness is pre-conditioned by fracture density. Still others invoke weathering-induced fracturing by iron oxidation.

To explore the influence of climate versus tectonic processes on weathering intensity and regolith thickness we drilled 5 cores in granitic bedrock along a climatic gradient within the EarthShape project (from N to S dry dessert, semi-arid, Mediterranean, humid) along the Chilean Coastal Cordillera. We combined borehole logging with geochemical and geophysical analyses of core and soil samples to evaluate the controls on weathering. Based on this geochemical data set we are able to show that the studied weathering profiles consist of three main weathering parts characterized by distinct geochemical gradients. The first part comprises the soil layer and is characterized by high weathering intensities, the second part comprises the saprolite down to the bedrock and includes the weathering front, the third part is found below the weathering front and is associated with fractures. Thus multiple weathering zones developed at major fractures. We found that the depth of the weathering front is mainly controlled by the characteristics of the fracture network: fracture aperture and dip, whereas weathering intensity is controlled by a combination of fracture network and the influence of vegetation through soil CO2 and organic acids. Generally, our data show that for a given fracture density fracture apertures control regolith thickness.