Post-emplacement fluid-driven intrusion fracturing and quartz-sand injections at the basal shear zone of the Esla Nappe (Cantabrian Zone, NW Iberia)

The Esla Nappe is located in the external foreland and thrust belt of the Variscan Orogen in the NW Iberian Massif (Cantabrian Zone, NW Iberia). It is formed by a near-complete Palaeozoic sedimentary succession. With a displacement of around 19 km, the nappe was emplaced along a thin (<2–3 m) basal shear zone (ENBSZ) located at an estimated minimum depth of 4 km. Emplacement took place during the Moscovian (ca. 312 Ma). Fault-rock assemblages record a variety of alternating deformation mechanisms and processes, including cataclastic flow,  pressure solution and hydrofracturing and vein precipitation. All these processes are considered evidence of an aseismic stable behaviour of the ENBSZ, where deformation was influenced by secular variations in the fluid pore pressure.

Following emplacement, the ENBSZ was breached by clastic dykes and sills which were intruded following re-opened previous anisotropies, including bedding planes, thrust surfaces, joints and stylolites. Together, they constitute an interconnected network of quartz sand-rich lithosomes reaching structural heights occasionally exceeding 20 m above the ENBSZ. The orientation of the dykes suggests that the injection process took place under low differential stress conditions in the hangingwall, and near-lithostatic fluid pore overpressure conditions in the footwall. The injected slurry consisted of overpressured pore fluid, quartz-sand grains derived from the footwall and entrained host-derived fragments. Depending on fracture aperture and slurry composition, a variety of fluid velocities can be inferred in the order of 15–30 cm/s. Thin pure injections of quartz grains (ca. <1 cm) were characterised by a laminar flow (Re<2100), whereas the thickest quartz and host-derived mixed injections (~1 m) displayed a fully turbulent flow (Re~2 x 10⁴).

The causes for the fluids to reach near-lithostatic fluid overpressures within the uppermost footwall remain unknown. It is not possible to rule out a seismic trigger, but the absence of extreme shear localization structures typical of seismic slip suggests that the injection process was driven by fluid progressive accumulation, possibly related with clay dehydration reactions, tectonic loading, pore compaction or fluid migration from underlying formations. Actual breaching and injection may have been allowed by a decrease in bedding-parallel compressive stresses in the Esla Nappe associated with the subsequent evolution of the thrust-wedge.