Veins, such as those contained within mudrock sequences, provide clear evidence of dilational deformation. A common explanation is that the deformation event involves high pore pressures, with low effective stress leading to natural hydraulic fracturing. The analysis here counters that view. The basis of the argument comes from micro-mechanics understanding that reveals the pore-scale process interactions that occur during geomechanics//fluids interactions in constrained situations. The analysis reveals that increasing pore pressure (from external causes) leads to increases in stress components, due to the strain constraints typical of the subsurface. For a pre-existing, closed fracture, an increase of pressure keeps the fracture closed. Fracture opening requires elongational strain in the matrix rocks, reducing the fracture-normal stress, and finally, the movements of the walls of the matrix rocks, due to relaxation and separation, allows fluid to enter the fracture. Once within a fracture, the pore fluid causes the matrix rocks to shorten, and thus does increase the aperture. Applying these findings, it is proposed that veins may occur well up into the caprock sequence, where a stretching deformation allows fracture creation or opening that is exploited by high-volume and high-pressure fluid flows rooted to depth – and which leave behind some of their dissolved solid content to record the multi-process dilational event.
For geomechanical processes that occurred in the geological past, we lack any real-time observations that would help to constrain conditions and processes. But, some rock-masses are subsequently uplifted and eroded to provide outcrops across which a quasi-2D arrangement of consequences can be observed and recorded. Geologists study outcrops and develop process explanations for deformations that leave behind certain features, such as fractures or deformation bands that cut across lithological layering. Similarly, veins – which are mineral-filled discontinuities – form arrays that occur through some rock-mass exposures. The volume occupied by the added mineral filling represents in some way a net dilation of the rock-mass, preserved as hard evidence of this elusive deformation parameter. Veins preserve other useful information – primarily their spatial patterns, as well as the context – that helps to constrain the process model of the operative deformation.
