Discussion about the emplacement of some of the Southern Pyrenees nappes (Spain)

The Solithern Pyrenees or the Prepyrenees as the Spa-nish geologists use to refer them are one of the main tec-tonic units of the Pyrennes and extend for more than 600 km from east t o west. They separate the cristalli-ne Paleozoic axis to the north of the wide flat expanse of de Ebro basin to the south (fig. 1). Seguret (1972) and Garrido and Rios (1972) proved that the Pyrenees are formed by superimposed nappes and slices of Mesozoic and Paleogene rocks that glided southward some tens of kilometres. Althoung the ailochthonous origin of some of these unit had already been suggested by the earliest geologist working in the Pyrenees (Dalloni, 1913, 1931; Jacob et al., 1927) their point of view was not widely accepted until the middle of the seventies. Nappe emplacement spans from the Early Eocene to the Oligocene and its mechanism and style of deforma-tion changes throughout the orogenic process (Solé-Su-grañes, 1978 a; 1979). The time of the emplacement and the paleogeographic conditions mark the difference among the nappes and smaller ailochthons that forms the Prepy renees. Former geologists suggested that the nappes were formed by gravity gliding alone (Van Lith, 1968, Van de Velde, 1967) or by gravity instabilities produced by crustal shortening (Seguret, 1972). Solé-Sugrañes (1979) poited out that some of the earliest small nappes may be formed by gravity gliding of carbonate rocks into a turbidic basin, but the Latte Eocene nappes and dé-collements involving persasive folding and thrusting of Paleozoic rocks must have been strongly pushed form the back due t o crustal shortening. Structures due t o nappe movement and their mecha-nics are discussed. Development of cleavage near para-ilel to the sole thrusts beneath and above them sug-gest a ductile behaviour more likely than a pure brittle, behaviour with drastic decrease of sliding friction ailowed by high geostatic ratio (i. e. ratio of fluid be-D e t a c h e d m a s s e s Figu.re 1.- Main structural units of the Pyrenees. Detache