Different Wilson Cycle styles in Western Europe: the key role of inheritance

In the classical Wilson Cycle concept, orogeny is assumed to follow protracted subduction of a wide oceanic domain. Such subduction systems form magmatic arcs associated with high-temperature and low-pressure metamorphism in the subduction upper plate, and depletion of the mantle wedge above the slab. Since its emergence, the Wilson Cycle concept has been largely used to study collisional orogens in general, and yet, in Western Europe, the Pyrenees and the Alps are both devoid of magmatic arc remnants. Recent field studies and paleogeographic restorations suggest that both the Pyrenees and the Alps resulted from the closure of narrow proto-oceanic domains that may have never reached the stage of steady-state seafloor spreading. This would imply that rift systems may be inverted at any stage of their development, including prior to the onset of steady-state seafloor spreading. Inversion of such a rift system would not form a magmatic arc due to the limited length of the slab, and hence orogeny would essentially be a mechanical process mainly controlled by the inherited rift architecture. In this presentation, we first describe the paleogeographic settings of the Alpine TethysPyrenean rift systems. We show the results of an innovative kinematic reconstruction approach that integrates field observations, realistic margin widths and pre-rift tight full fit restorations. Second, we discuss how the margins along-dip architecture has controlled the two-dimensional architecture of the Pyrenean and Alpine orogens. We show that the major escarpments inherited from rifting and separating the thick-crusted and buoyant proximal domain from the thin-crusted and denser distal domain have become first-order ramp structures that today separate the external- from the internal part of both orogens. Finally, we explore how the along-strike segmentation of the Pyrenean and Alpine rift systems have controlled the three-dimensional architecture of the subsequent orogens. We show that the segmentation of the Pyrenean and Alpine rift systems, which both used to display ribbons of thick continental crust between overstepping rift basins, can explain most of the non-cylindricity observed today in both the Pyrenean and Alpine orogens.