[1] Peneplanation of mountain ranges is generally considered the result of long-term erosional processes that smooth relief and lower elevation near sea level. Therefore peneplain remnants at high elevation in mountain ranges are used to infer posttectonic surface uplift. Such an interpretation has been proposed for the Pyrenees where high-elevation, low-relief erosional surfaces rose up to more than 2000 m. Because the Pyrenean foreland basins are filled with very thick continental deposits, which have buried the early jagged landscape, we challenge this hypothesis by pointing out that relief applanation does not necessarily require elevation lowering. We propose an alternative interpretation in which piedmont aggradation of detrital sediment that comes from erosion of the high chain induces the rise of the base level of the range, therefore reducing strongly the erosive efficiency of the drainage system and resulting in the progressive smoothing of the relief. Such a process allows a high-elevation, lowrelief erosional surface to develop at the scale of the range. In the Pyrenees, occurrence of high-elevation, low-relief erosional surface remnants does not imply a posttectonic uplift, but is instead due to the dissection of the initial Miocene high-elevation, low-relief surface by the recent drainage system, the erosive activity of which has been enhanced by global climate change from the late Pliocene onward.

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[1] The High Atlas of Morocco is a still-active, linear intracontinental mountain chain in the NW African plate, which results from weak crustal thickening associated with rift inversion during the Cenozoic and from uplift related to mantle thermal doming. A striking morphological feature of the High Atlas is the occurrence of both transverse and longitudinal (i.e., strike-parallel) drainage characterized by deep fluvial incision of more than 1000 m in low-relief topography of the axial zone of the chain. Most of the transverse component of the drainage appears to postdate the longitudinal component as indicated by recent or incipient captures and wind gaps. The longitudinal drainage is inherited from an early stage of fluvial organization controlled by the tectonic structures developed during upper crustal folding and thrusting in the post-Paleozoic cover. Amplification of N-S regional slope in the western High Atlas by continued crustal shortening and thickening triggered: (i) higher erosion rates in transverse than in longitudinal catchments and (ii) captures of longitudinal streams by transverse ones, creating a new organization of the drainage system toward the regional slope. Such evolution from a longitudinal to a transverse-dominated drainage may represent a common mechanism of fluvial network development in mountain belts where the amplification of the regional slope results from long-lived lithospheric convergence.