MDF: The structure and evolution of near-surface massively dilatant faults

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Close to the Earth's surface, cohesive rocks fail in extension, and normal faults in these rocks are commonly massively dilatant. Although near surface massively dilatant faults are common, their internal structure is poorly known, which makes structural and fluid flow predictions notoriously difficult. In an integrated project of scaled sandbox experiments, fieldwork and geomechanical modeling, we will study near surface massively dilatant faults (MDF), to better predict their subsurface structure and understand their evolution. In a parameter study using analogue modeling, the main geometric and mechanical controls on MDF will be quantified, and results compared with high-resolution morphometric measurements of the surface expression of MDF; firstly on an exposed mid ocean ridge, and then in sandstones with pre-existing joints. Comparison of our experiments with the field data and calibration against numerical models will allow predicting their inaccessible internal structure at depth. We will also address the influence of deformation rate and its relation to the shape, size, structure and formation of MDF.