A comparison of exfoliation joint formation mechanisms: what is the role of surface processes?

The formation of granitic domes via exfoliation jointing produces some of the most celebrated and hazardous landforms on Earth. In 1904, G.K. Gilbert outlined three mechanisms to explain exfoliation jointing as: (1) related to the original cooling of the rock body, (2) related to decompression of the rock body as it is exhumed to the surface of the Earth, or (3) related to processes at Earth’s surface - a hypothesis recently supported by observations of thermal cycling in crack initiation and propagation. Despite more than a century of study, our understanding of the mechanisms driving exfoliation jointing remains incomplete. This research seeks to address the question: is the formation of exfoliation joints more sensitive to surface processes (e.g., biotite weathering, thermal cycling), topographic, or regional (i.e. tectonic) stresses? To test this hypothesis, we predicted the orientation of fractures subject to variable geologic conditions with a multi-scale weathering model of damage and fracture propagation implemented in the finite element method. We present predictions resulting from thermal contraction during cooling of the rock body, depressurization during rock exhumation, and regional tectonic compression. We then compare fractures generated under variable topographic stresses, surface weathering processes, and rock geochemistry (i.e., biotite fraction and orientation). By improving our understanding of how significantly pre-existing geologic conditions and rock fabrics influence fracturing, we can work towards disentangling this effect on observed fracture orientations and better interpret paleo-stresses for major tectonic events or potentially paleo-topography. Additionally, enhancing models for weathering mechanics and fracturing in granitic bodies may reveal sensitivities to changes in climate and critical zone evolution, with implications for the forecasting of rockfall hazards in relation to projected temperature and climatic changes.