Evolution of cataclastic faulting in high-porosity sandstone, Bassin du Sud-Est, Provence, France

Abstract

Cataclastic deformation structures in Cretaceous high-porosity sands in the Bassin du Sud-Est, SE France were surveyed by scan-lines to examine: (i) the role of tectonic loading path on cataclastic deformation band (CDB) network development, and (ii) the development of larger ultracataclastic faults as strain increases. Deformation during Pyrenean–Provençal shortening resulted in a persistent high density (∼10/m 2) of conjugate reverse-sense CDB zones (displacements up to ∼30 cm), with no generation of larger faults. High–low-density undulations occur for each pair of the conjugate set in an alternating manner, suggestive of network hardening, with a wavelength of several tens of metres being in the order of mechanical bed thickness. For two study areas which experienced significant Oligocene–Miocene extension, a moderate, undulating background density (∼4/m 2) of normal-offset CDBs was recorded, which became focussed in places into clusters (∼50/m 2) a few metres wide. Thus tectonic loading path may strongly influence strain distribution. CDB zones develop by the addition of successive bands at the edges until, at a thickness of around 5 cm, new bands tend to stray further away from the zone edges. Coarser sands have thicker CDB zones, suggesting that host grain size, along with mechanical bed thickness, could be an important contributor to the scale limit in CDB zone growth. Larger ultracataclastic faults and discrete slip zones localised within or at the edges of some clusters of CDB zones, post-date cluster development rather than inducing it. This stage of deformation evolution is only reached in extension, not in shortening, suggesting the infeasibility of achieving the critical state line during horizontal compression.