Volumetric strains in neutral surface folding

Neutral surface folding is a significant contributor to fold development. This mechanism produces contrasting strains in the inner and outer arcs of the folded layer that arise from principal stress orientations that are approximately parallel and perpendicular to the layer. We demonstrate that such stress patterns imply significant gradients of mean stress across the folding layer, being more tensional on the outer arc and more compressive in the inner arc. This could pump fluids towards the outer arc during folding and result in heterogeneous volume changes. We conclude that the neutral surface folding model should be adapted to accommodate volumetric strains, in order to explain dilatational structures (e.g. open fractures, veining) on the extrados and volume-loss structures (e.g. pressure solution seams, stylolitic cleavages) on the intrados. This dilatation has economic implications as it allows prediction of sites of mineralization and zones of secondary permeability in fold-related hydrocarbon traps.