Pedogenetic Effects in Fault Evolution

Abstract

During the late Oligocene – middle Miocene, an extensive deformation took place in the Western Mediterranean. In this process, main faults of NE-SW striking direction bordering horsts and grabens of the Catalan margin and minor faults perpendicular to the former were formed. We have studied some of these minor faults affecting Miocene conglomerates in the Barcelona Plain. The fault zone is formed by an absent damage zone and a poorly developed fault core. Their fault rocks are obliterated by later pedogenetic processes that have changed fault and fault rock properties and consequently fault rock classification. Nowadays fault rock is a cataclasite but because it is cemented by later pedogenetic processes, in fact, in origin was a gouge. When fault rock was a gouge (stage 1), and considering fault zone architecture, cross-fault flow could exist. During the second stage, after fault rock pedogenesi, new minerals reduced effective porosity and so permeability of the fault rock. The consequence was the seal of faults and compartmentalisation of flow. In conclusion, faults occurring in the upper meteoric environment can have other kind of processes responsible of their impermeabilization than deeper faults.