Intrusion fracturing and quartz sand-rich injections in thrust-related fault rocks within the basal shear zone of the Esla Nappe (Cantabrian Zone, NW Iberia)

Abstract

The Esla Nappe, located in the Cantabrian Zone (Variscan Orogen, NW Iberia), has an estimated 19 km displacement to the NE accommodated in a thin (<2–3 m) shear zone (ENSZ) at least 4 km depth. Fault-rock assemblages record different deformation processes operating in the hangingwall and footwall. While the hangingwall was largely deformed through cataclastic flow localised in a <50 cm band, hangingwall-footwall intervening ultracataclasites and footwall lithologies preserve evidence for an interplay of cataclastic flow and pressure solution. Final hydrofracturing and calcite vein precipitation has contributed to the vertical dilation of the fault rock assemblages.In the late stages of emplacement, the ENSZ was breached through intrusion fracturing and associated clastic sills and dykes injected along re-opened previous discontinuities and anisotropies. Together, they conform an interconnected network of quartz sand-rich lithosomes reaching structural heights of 20 m over the ENSZ. Injections are formed by a mixture of injected quartz grains and host-derived fragments. Dyke orientations suggest that the injection process occurred under large fluid overpressure conditions exceeding lithostatic values in the footwall. Fluid overpressure may have been caused by progressive fluid accumulation beneath a low-permeability ultracataclasite layer at the base of the nappe. Breaching through the shear zone was favoured by a change in the stress regime following the Esla Nappe emplacement.