Geological and isotopic constraints on the structure of the Bilbao anticlinorium (Basque–Cantabrian basin, North Spain)

The structure of the Bilbao anticlinorium is re-evaluated based upon new geological and isotopic data on Mesozoic evaporites and related groundwaters. A detailed geological survey to guide and monitor the construction of a 3.3 km tunnel provided new data on Early Cretaceous rocks with evaporite intercalations. S and O isotopes from evaporite minerals and from sulfate dissolved in groundwaters permitted us to ascribe the sampled subsurface rocks to the Early Cretaceous, rather than to the Triassic, and trace their distribution. We present a new structural map of the anticlinorium compartmented by four major branching thrusts. New and reinterpreted cross sections resolve its deep structure. Cover rocks extend to depths below 6–8 km. Hangingwall anticlines occur above hangingwall and footwall thrust ramps and the ensemble of slices forms an antiformal stack directly above a south dipping basement ramp. Shortening of ca. 15–20 km was accompanied by alongstrike sinistral displacements. Currently, the Bilbao anticlinorium relates to major geophysical anomalies and is regarded as the surface expression of an indenting wedge of deep crustal rocks at mid-crustal depths.