Subduction and tectonics on the continental margin off northern Spain: observations with the submersible <i>Cyana</i>

Abstract

Summary Short-term latest Mesozoic-early Cenozoic convergence between the Iberian and European plates transformed the passive north Spanish margin into an active margin with southward subduction of the oceanic crust of the Bay of Biscay. This margin became passive again when subduction ceased in late Eocene or Oligocene time. Nine dives with the submersible Cyana allowed us to sample the rocks outcropping on the Le Danois Bank slope. A Mesozoic sequence includes shelf limestones of the Dogger to the Lower Cretaceous, and Lower Cretaceous detrital sediments (sandstones, conglomerates, pelites); it overlies a typical Palaeozoic Asturian basement. The main objective of the diving campaign was to study the structure of these strata in order to elucidate deformation of the continental wall of an active margin during its early stages of evolution, before the formation of an accretionary prism. The occurrence in Lower Cretaceous pelites of a weak schistosity in a reversed fold limb gives evidence of compressive structures, probably northward-verging overfolds. Additional evidence of compressive tectonics is the dislocation of quartz grains and dolomite crystals in Lower Cretaceous sandstones and Jurassic dolomites. These observations suggest that the continental slope in the Le Danois Bank area has a predominantly imbricate structure. Deformation and uplift of the northern Spanish margin appear to be related to the early stages of subduction of the European plate under Iberia.