Relationships between pore space anisotropy and anisotropy of physical properties of silicoclastic rocks from the Corbières–Minervois fold-and-thrust-belt (north-east Pyrenees, France)

Abstract

We present a study integrating petrophysical measurements along a general N–S section of the Corbières–Minervois fold and thrust belt (NE Pyrenees, France) in order to study the petrofabric of weakly deformed sediments. We focus on the relationship between the evolution of deformation measured with the anisotropy of various physical properties (anisotropy of magnetic susceptibility (AMS), anisotropy of P-waves velocity (APV) and anisotropy of electrical conductivity (AEC)) and the distribution of the porous space. The origin of AMS fabric is interpreted as recording a layer parallel shortening oriented N145, steadily throughout the studied section with local gradient associated with folding as sites close to the forelimbs. AEC is in good agreement with AMS results and records the same shortening direction. The most probable source of anisotropy for AMS and AEC is the preferred orientation of phyllosilicate minerals. APV, which reflects the distribution of porous space, is more complex and is controlled by the tectonic style of the magnetic fabric. The porosity is a flat-lying-bedding porosity, where the magnetic fabric is poorly evolved and it becomes a flat-lying-pressure-solution-cleavage porosity where the magnetic fabric is tectonic. At regional scale, we show that changes in anisotropy of petrophysical properties are controlled by individual structures, like regional fold or thrust fault and occurred mainly before folding as a consequence of a well-expressed NNE–SSW layer parallel shortening. The folds and thrust faults are developed subsequently from inherited E–W basement faults. This explains why the structural grain of the sedimentary cover, expressed notably by the magnetic lineation, is oblique to the major structures of the cross-section: the Alaric Anticline and the North-Mouthoumet Fault.