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

Paleogene tectonic and sedimentation history of the Andaman-Nicobar Accretionary Arc, Northeast Indian Ocean

The Andaman-Nicobar archipelago in the northeastern Indian Ocean is a nonvolcanic outer arc island chain developed by tectonic accretion of sediments and ocean crust along the eastern margin of the subducting Indian lithosphere. The Paleogene stratigraphy of the island chain comprises olistostromes, olistoliths and coarse-grained volcaniclastic turbidite facies, and reefal limestones of late Paleocene- Eocene MithakhariMelange and finer-grained siliciclastic turbidites (Andaman Flysch) of Oligocene age. The lower Paleogene sedimentary rocks were deposited in shallow-water basins formed on the upper trench slope and growing accretionary wedge and contain material sourced from a local volcanic arc and eroded ophiolite. By contrast, the Oligocene continent-derived siliciclastic sediments were originally deposited outside of the accretionary wedge as part of a large submarine fan system. Subsequent deformation and thrusting juxtaposed these different formations as trench rollback progressed and the accretionary wedge expanded westwards