Relation between asymmetry of drainage basins, discrete Alpine faults and crustal structure in the Cantabrian Mountains (NW Iberia)

Crustal deformation producing the tilting of the topographic surface has an effect on the arrangement of drainage networks. We study some of the effects of tilting in a 300 km long segment of the Cantabrian Mountains (CM), an alpine mountain belt parallel to the northern coast of Spain. The eastern part of the CM studied preserves an orogenic root with crustal thickness in excess of 45 km. At the surface, there are discrete alpine faults that control the boundaries of Cenozoic basins and current Quaternary drainage within the northern slope of the main orogenic water divide. While the convergence originating the CM was active during the Paleogene, it remains tectonically inactive since middle Miocene. Thus, in this contribution, we examine the effects of tectonic tilting on drainage at two different spatio-temporal scales in the CM: in relation to discrete alpine faults operating at hectometric to kilometric scale several million years ago (short wavelength), and in response to a lateral gradient in crustal thickness, acting currently at kilometric length scales (long wavelength). From the range of geomorphic indices defined for drainage basins, we selected two that quantify drainage asymmetry, which we relate to surface tilting at both local and regional scale. The geomorphic analysis strengthens the identification of the main alpine structures by their protracted effects on the current landscape, but also the regional impact that current larger wavelength deformation has on the drainage of part of the mountain belt.