The origin of mesetas in the Iberian Chain (Spain)

Abstract

Topography results from the interaction of tectonics that moves rock masses and surface processes that shape and lower them. In this study we investigated the recent evolution of the Iberian Chain landscape, an intraplate orogen located in the centraleastern Spain. It originated during the alpine orogeny in Upper Cretaceous-Middle Miocene. In the whole Iberia, the Iberian Chain represents a unique case of domeshaped topography. Its central sector is dominated by an extensive planation surface which lies at an altitude of 1300 m. This surface records a period of tectonic quiescence (Upper Neogene?) during which most of previous compressive structures were almost completely eroded creating a wide peneplain. In Late Pliocene (?)-Quaternary, this denudation episode was interrupted by the onset of an uplift that controlled the organization of the present fluvial network. The origin and evolution of the highstanding plateau in a relatively tectonically stable region is still a controversial issue. To quantitatively characterize the influence of surface and tectonic processes, we studied the morphometry of the Iberian Chain. In detail, we analyzed the topography (map of local relief, swath profiles) and the hydrography (basin hypsometric curve and integral, basin asymmetry factor, river longitudinal profiles and relative geomorphic indices) of the Iberian Chain using the SRTM DEM as a main data source. Our results have been coupled with the incision rates of the High Tagus R. and Martin R. areas, calculated using fill terraces dated by the Uranium-series (230Th/U) method carried out on calcareous tufa deposits. Our morphometric results indicate that rock-type erodibility is the main factors ruling landscape (topography and drainage pattern) evolution of the study area as well as tectonic uplift. Its dominance is confirmed by the values of incision rate that are very similar throughout the central sector of the range. Nevertheless, the accentuated low values of local relief and intrinsic concavity of the stream longitudinal profiles as well as hypsometric convex curves and relatively high integrals indicate the Iberian Chain landscape is in a transient state in response to a recent uplift. Indeed, the fluvial processes that so weakly incised this landscape are still far from counterbalancing the tectonics input. These results depict the Iberian Chain as a well distinct topographic unit with respect to the rest of Iberia. To simulate the evolution of a landscape characterized by the same tectonic and erosion inputs of the Iberian Chain area, we performed 3-D numerical experiments. We tested the models combining process laws (hillslope diffusion, fluvial incision, sedimentation) and physical parameters calibrated on the field, radiometric, and morphometric data. For our purposes, we used SIGNUM (Simple Integrated Geomorphological Numerical Model), a Matlab TIN-based landscape evolution model able to simulate tectonic uplift, hillslope diffusion and river incision. A validation of the parameters obtained from 3-D numerical simulations was performed by the inversion of synthetic 2-D longitudinal profiles. The comparison between real and simulated 3-D landscapes provide us a quantitative view on the origin of present topographic features of the Iberian Chain. In detail, 3-D models show that the onset of the tectonic uplift occurs at ~3.2 Myr with non-uniform rates that range between 0.5 mm/yr in the interior sector to 0.25 mm/yr. The inversion of the drainage network, starting at around 2 Ma, occurs after the onset of the uplift, through the topographic barriers that preserve the landscape in the inner Iberian Chain at high elevation.