Mechanical constraints on the chronology of fracture activation in folded Devonian sandstone of the western Moroccan Anti-Atlas

Abstract

The three-dimensional meter-scale fracture networks, observed on exposed folds between the towns of Tata and Akka, western Moroccan Anti-Atlas, consist mostly of planar discontinuities, which are sub-perpendicular to the bedding and partitioned in three main sets. The chronology of their activation is proposed in five stages since the Hercynian orogeny. Stage 1 predates folding and involves the horizontal compression of the Emsian sandstone. It involves fracture set I, composed of systematic joints parallel to the direction of compression. Stages 2-4 correspond to the folding and are marked in the outer-arc by the activation of fracture set II, composed mainly of joints parallel to the fold axial plane. Stage 5 is a regional shear event during which sets I and III, separated by an angle close to 608, are activated in a conjugate manner. To throw light on the recurrent difficulty in discriminating between activation of inherited and new fractures, an elasto-plastic model is used to construct a stress path in the pervasively fractured medium idealized as a continuum. Each fracture set obeys the Mohr-Coulomb criterion truncated in tension to describe both sliding and opening activations. Finite-element simulations of a simple buckling event accounting for the field fracture sets are presented. It is shown that set I cannot be generated by folding and thus does belong to stage 1. Set II is activated at a later stage of folding than expected from the field interpretation. Set III cannot be activated during stage 2, confirming its role in stage 5. The advantages and limitations of the proposed modeling are finally discussed.

Incidence and Importance of Tectonics and Natural Fluid Migration on Reservoir Evolution in Foreland Fold-And-Thrust Belts

“…In the future, further coupling of forward kinematic tools with rock mechanics are likely to provide a more efficient prognosis of tectonic compaction (LPS) and fracture development (Guiton, 2001;Guiton et al, 2003aGuiton et al, , 2003bOrlov et al, 2004), and to propose more realistic scenarios for the lateral and vertical evolution of permeabilities through times, to be applied during subsequent fluid flow modeling.…”

“…-their initial cementation and secondary porosity characteristics; -their current position in the folded structures (i.e., in the outer arc or instead in an inner arc position, in the forelimb or in the backlimb of the fold; Sassi and Faure, 1996;Mercier et al, 1997;Frizon de Lamotte et al, 1997Saint-Bezar et al, 1998;Grelaud et al, 2000); -their orientation and spacing relative to the modern principal stresses (Guiton, 2001;Guiton et al, 2003a and; -the amount of tectonic uplift and coeval erosion, resulting in a cooling of formation waters and local development of secondary porosity (i.e., synkinematic development of vuggy porosity, ; see also Section 6 of this paper). Among a specific family of microstructures, characterized by a given strike and dipping atitude and spacing, only a fraction of the pre-existing structures are commonly reactivated, indicating that the density and spacing of reactivated structures are more diffuse than those inherited from the foreland autochthon.…”

The impact of different aperture distribution models and critical stress criteria on equivalent permeability in fractured rocks

“…The percolation probability is defined using normalized intersections as a function of density as defined by [Robinson, 1983] and by using the network saturation. [Guiton et al, 2003;.…”

“…For the purpose of this study, we rotated back the data to its prefolding horizontal position, as the fractures were created during burial, prior to folding. The last data set was obtained from Devonian sandstones outcropping in a gently dipping fold in the Anti-Atlas of Morocco [Guiton et al, 2003]. This pavement contains two long orthogonal sets and an oblique set that abuts against the larger systems and is comparable to high-intensity systems such as the Bristol Channel [Belayneh et al, 2006].…”

Folding and inversion tectonics in the Anti‐Atlas of Morocco

“…Special attention has been given to the Proterozoic-Cambrian boundary [Boudda et al, 1979;Bertrand-Sarfati, 1981;Buggisch, 1988b;Latham, 1988;Geyer and Landing, 1995;Benssaou and Hamoumi, 2003]. In contrast to this wealth of stratigraphical and paleontological literature, structural publications dealing with the tectonics of the western Anti-Atlas remain scarce [Leblanc, 1972;Michard, 1976;Soulaimani, 1998;Guiton et al, 2003].…”