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Joaquina Alvarez-Marron, CSIC (Spain)
Rosana Menendez-Duarte, University of Oviedo (Spain)
Ulrich Anton Glasmacher, Ruprecht-Karls Universität (Germany)
Finlay Stuart, SUERC (United Kingdom)
Rene Grobe, Ruprecht-Karls Universität (Germany)
Susana Fernández, University of Oviedo (Spain)
Dennis Brown, CSIC (Spain)
Pere Mas, CSIC (Spain)
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Alpine convergence between Iberia and Eurasia resulted in formation of the Pyrenean orogen that extends westward in a mountain chain developed along the northern Spanish coast. The present topography of the Cantabrian Mountains reaches maximum elevations of 2700 m less than 50 km from the coastline. It lowers towards the west, and is correlative with a progressive reduction in N-S shortening. In this area, the different components of the landscape have developed over deformed Variscan bedrock, and include landforms that seem to have a long-term history. These include; 1) elevated surfaces of low relief at mountain summits, 2) a raised marine wave-cut platform along the coast, and 3) a deeply incised fluvial system with steep valley slopes developed over the previous landforms. Our data indicates that the landscape of the area may register an evolution spanning several tens of million of years, with long-term preservation of topographic components. The area may be a good example of landscape evolution corresponding to the initial stages of mountain growth next to a margin that underwent incipient subduction related to slow convergence.
A regional thermochronological study combining apatite fission track (AFT) and (U-Th)/He dating provided initial results with AFT ages that range form 270 (36) Ma (Permian) to 78 (4) Ma (Upper Cretaceous) that are recording a post-Variscan exhumation history and are concordant with very low values of denudation since iniciation of Alpine shortening. The regional AFT study focuses in unravelling rates of landform evolution, in terms of an increase or decrease of relief, and also exhumation rates. This is possible because of the presence of two areas with differences in topography from E to W. A short wavelength topography and abrupt relief, due to deeply incised river valleys in the E (Asturian area), and a long wave length topography and low amplitudes in the W (Lugo area, regional low relief surface at 500 m elevation).
Digital elevation models in a 150-km-long section of the mountains show the elevated surfaces of low relief between elevations of 500 to 800 m, lowering towards the west. These are interpreted to correspond to relicts of a single, ancient peneplain displaced by faults that reactivated existing structures within the bedrock. The raised marine platform that emerged more than 1 Ma ago as indicated by surface-exposure dating that combined three cosmogenic nuclides, reaches more than 3 km in width. The detailed DEM shows the shoreline angle lowering westward along the coastline from 220 to 100 m. This change in elevation occurs in several discrete steps interpreted to correspond to traverse faults with tens of meters of vertical displacement. A morphometric analysis of some fluvial drainage basins indicates at least two phases of incision evidenced by the steepening downwards of the valley's slope morfology.
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