GEOMORPHIC EVIDENCE OF TRANSIENT FLUVIAL LANDSCAPE RESPONSE IN AN ACTIVE OROGEN, CENTRAL BETIC CORDILLERA, SPAIN

The landscape of the Betic Cordillera forearc, southern coastal Spain is being shaped by a poorly understood interaction among subduction rollback-related regional rock uplift, drainage integration, and erosional/tectonic exhumation of Sierra Nevada. New optically-stimulated luminescence (OSL) ages from alluvial fills in the south-flowing Andarax, Adra, and Guadalfeo rivers indicate recent, rapid (0.1 - ~1.0 m/yr) and probably unsteady rates of fluvial incision that we here use to explore these contrasting models of crustal deformation and rock uplift. Short term incision rates are an order of magnitude larger than predicted by low temperature thermochronolgy or perserved uplifted near-shore landforms. These three rivers were chosen because of their similar underlying bedrock geology, the presence of marine and geomorphic stratigraphic markers at the coast and in the forearc interior, and good regional coverage of seismic and GPS-geodetic data of crustal deformation. GIS based topometric characterization of the Central Betics was conducted using 5m resolution DEM from the Spanish Geologic Survey (IGMES). Channel long profiles, and maps of normalized steepness indices (k_sn), chi (χ), and knickpoints provide a quantitative, regional context for the field terrace stratigraphic data. A terrace stratigraphic model extracted from the Andarax river indicates that there are fill terraces with ages ranging from 15 ka to 111 ka. Using our average stream concavity and steepness values, we are able to project the age of terrace Qt₁ from our OSL dated site where it is 111 ± 45 ka, down to the coast where it intersects a marine terrace known to be > 300 ka. This simple projection implies little, if any internal deformation for the Betic forearc block, but is consistent with regional uplift and upstream migration of knickpoints from the coast to their present mapped locations in the watersheds. This style of crustal deformation is consistent with thick skinned or block uplift type of tectonic-surficial process feedbacks, whereas thin skinned normal faulting models for crustal deformation are largely excluded.