Oppositely-verging thrusting structures in the North Argentine Andes compared with the German Variscides

Abstract

The Andes of north Argentina and the german Variscides, in spite of their different ages and geological settings, are considered good examples of conve rgent orogens displaying oppositely ve rgent thrusts. The back-thrusting is related to deep seated blind thrusts forming conjugate fault combinations which are significant in upper and mid-crustal shortening. Deep-seated conjugate fault systems in the Variscides were detected in reflection seismic profiles and in the Andes of north A rgentina by mapping and through electrical conductivity investigations. In the Variscides conjugate fault systems may be recognized at eve ry scale in the upper crust and they form giant combinations in the lower crust. In the Andes the enhanced conductivity zones within the lower crust are arranged in conjugate systems, that could represent crustal detachments associated with Nazca plate subduction. In the Andes crustal conjugate fault structures similar to those described in the Variscides, could be present under the west verging eastern border of the Pampean Ranges and beneath the southern Subandean Ranges (Santa Bárbara System). The other oppositely - vergent belts of the north Argentine Andes show fan arrangements similar to those recognized in other orogenic zones such as the Alps and the Pyrenees. The comparison with other well-studied orogens allows speculation about the deep structure of the north A rgentine Andes, where there is insufficient crustal information. The Variscides and other orogens whose deep structure has been investigated by deep seismic reflection profiles, suggest that in the oppositely ve rgent orogens the back-thrusting fronts are systematically associated with crustal - scale thrusting. In the case of the Andes this could imply that significant lithospheric thrusting may be present below the big back-thrusting fronts of the Eastern Cordillera and Pampean Ranges.