Inyecciones clásticas en la base del manto del Esla (Zona Cantábrica, NO de Iberia)

Abstract

The Esla Nappe is located in the foreland and thrust belt of the Variscan Orogen in NW Iberia (Cantabrian Zone). It is composed of an almost-complete Palaeozoic sedimentary succession. Its displacement was calculated in the order of 19 km (Alonso, 1987), and the associated strain is recorded in a thin (ca. 2–3 m) shear zone located at its base (Arboleya, 1989). A variety of fault rocks are found in it, including cataclasites and ultracataclasites, formed by cataclasis and pressure solution during the movement of the thrust sheet. The hangingwall flat and ramp were intruded by clastic dykes and sills formed by a mixture of quartz sand grains and limestone fragments, reaching some tens of meters above the thrust surface, with a continuous basal level present at the nap- pe-scale. These injections followed pre-existing discontinuities in the hangingwall, including bedding planes, thrusts, joints and stylolites. The orientation of the dykes is consistent with an injection event taking place in conditions of low differential stress and high fluid pressure. The scarcity of simple shear strain recorded in the injectites suggests that the injection process took place in a late stage of the emplacement history of the nappe. It is interpreted as a result of a change in the stress regime at the base of the nappe following a transfer of displacement into other thrust surfaces underneath ahead of the footwall ramp