SEPARATION OF RIFTING AND LITHOSPHERIC FOLDING SIGNATURES IN THE NW-ALPINE FORELAND

Abstract

The development of the Alpine mountain belt has been governed by the convergence of the African and European plates since the Late Cretaceous. During the Cenozoic, this orogeny was accompanied with two major kinds of intraplate deformation in the NW-European foreland: (1) the European Cenozoic Rift System (ECRIS), a left-lateral transtensional wrench zone striking NNE-SSW between the western Mediterranean Sea and the Bohemian Massif; (2) long-wavelength lithospheric folds striking NE and located between the Alpine front and the North Sea. The present-day geometry of the European crust comprises the signatures of these two events superimposed on all preceding ones. In order to better define the processes and causes of each event, we identify and separate their respective geometrical signatures on depth maps of the pre-Mesozoic basement and of the Moho. We derive the respective timing of rifting and folding from sedimentary accumulation curves computed for selected locations of the Upper Rhine Graben. From this geometrical and chronological separation, we infer that the ECRIS developed mostly from 37 to 17 Ma, in response to north-directed impingement of Adria into the European plate. Lithospheric folds developed between 17 and 0 Ma, after the azimuth of relative displacement between Adria and Europe turned counter-clockwise to NW-SE. The geometry of these folds (wavelength = 270 km; amplitude = 1,500 m) is consistent with the geometry, as predicted by analogue and numerical models, of buckle folds produced by horizontal shortening of the whole lithosphere. The development of the folds resulted in ca. 1,000 m of rock uplift along the hinge lines of the anticlines (Burgundy-Swabian Jura and Normandy-Vogelsberg) and ca. 500 m of rock subsidence along the hinge line of the intervening syncline (Sologne-Franconian Basin). The grabens of the ECRIS were tilted by the development of the folds, and their rift-related sedimentary infill was reduced on anticlines, while sedimentary accumulation was enhanced in synclines. We interpret the occurrence of Miocene volcanic activity and of topographic highs, and the basement and Moho configurations in the Vosges-Black Forest area and in the Rhenish Massif as interference patterns between linear lithospheric anticlines and linear grabens, rather than as signatures of asthenospheric plumes.