Seepage forces, important factors in the formation of horizontal hydraulic fractures and bedding-parallel fibrous veins ('beef' and 'cone-in-cone')

International audienceBedding-parallel fibrous veins ('beef' and 'cone-in-cone') are common to a number of sedimentary basins, especially those containing black shale. The type locality is SW England. The commonest mineral in the fibres is calcite. The fibres indicate vertical opening, against the force of gravity. In the past, this has been attributed to fluid overpressure. However, a simple analysis, based on Von Terzaghi's concepts, leads to the conclusion that, for the fractures to be horizontal, either the rock must be anisotropic, or it must be subject to horizontal compression. By means of a more complete analysis, supported by physical models, we show that horizontal fractures are to be expected, even if the rock is isotropic and there are no tectonic stresses. Upward fluid flow, in response to an overpressure gradient, imparts seepage forces to all elements of the solid framework. The seepage forces counteract the weight of the rock, and even surpass it, generating a tensile effective stress. The process may lead, either to tensile hydraulic fracturing, or to dilatant shear failure. We suggest that these two failure modes, and the availability of suitable solutes, explain the frequent occurrence of 'beef' and 'cone-in-cone' respectively

Understanding biomolecular motion, recognition, and allostery by use of conformational ensembles

We review the role conformational ensembles can play in the analysis of biomolecular dynamics, molecular recognition, and allostery. We introduce currently available methods for generating ensembles of biomolecules and illustrate their application with relevant examples from the literature. We show how, for binding, conformational ensembles provide a way of distinguishing the competing models of induced fit and conformational selection. For allostery we review the classic models and show how conformational ensembles can play a role in unravelling the intricate pathways of communication that enable allostery to occur. Finally, we discuss the limitations of conformational ensembles and highlight some potential applications for the future