Temperature and flow conditions of quartz-sand injections at the base of the Esla Nappe (Cantabrian Zone, NW Iberia)

The Esla Nappe is located in the foreland and thrust belt of the Variscan Orogen (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 (ENSZ) at a minimum depth of 4 km 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.

Following emplacement, the ENSZ was breached by clastic dykes and sills injected within re-opened previous anisotropies such as bedding planes, thrust surfaces, joints and stylolites. Together, they constitute an interconnected network of quartz sand-rich lithosomes that reach structural heights in excess of 20 m above the ENSZ. The orientation of the dykes suggests that the injection process took place under low differential stress conditions in the hangingwall and near-lithostatic fluid pressure conditions in the footwall. The injected slurry consisted of overpressured pore fluid, quartz-sand grains derived from the footwall and entrained host-rock fragments. The temperature of the fluids estimated from the clumped isotope composition of calcite cements is 71–86 °C, with an average of 80 ± 4 °C. The calcite isotopic composition (δ¹³C = -0.15, δ¹⁸O = -9.53, both VPDB) is well within the typical values of the host Láncara Fm., which suggests that the fluids achieved equilibrium with the host prior to calcite precipitation. Using this calculated temperature and depth estimates for the base of the Esla Nappe, the geothermal gradient during deformation is estimated to be in the order of 16–24 °C/km, a relatively low value.

Flow conditions within the injections have been inferred from properties such as the particle drag coefficient, morphology, diameter and concentration, and the fluid density and viscosity, necessary for the calculation of the terminal fall velocity of the particle array. Thin injections formed of pure quartz, with a thickness <1 cm, are consistent with flow velocities of 0.01–0.35 m/s and a laminar flow (Reynolds number (Re) <800). Thicker pure quartz injections (<10 cm), on the other hand, required faster flow velocities (0.35 m/s) and transitional to turbulent flows (800 < Re < 8000). The thicker injections (≈1 m) that entrained larger host-derived fragments would require transitional to turbulent flows (1200 < Re < 1.2×10⁴) at fast velocities (0.35 m/s).

The estimated geothermal gradient is consistent with the lower estimations for current foreland basins, and very similar to ocean trenches. The velocities and Reynolds numbers derived for the Esla Nappe are larger than usually estimated for deep seated injections without hydraulic connection with the surface, where the vertical pressure gradient driving them is limited. In those cases, laminar flow conditions are usually invoked, but our results suggest that turbulent flow is possible in the thicker injections. Nonetheless, the values are lower than those reported for shallow injections in connection with the surface.