Transient fluvial incision and spatial distribution of active rock uplift in the Uspallata-Calingasta-Iglesia Valley, Central Argentina.

Abstract:

In the southern portion of the Pampean flat-slab subduction zone, a narrow, elongate intermontane depression separates the Precordillera fold-and-thrust belt from the Frontal and Principal Cordilleras of the high Andes. This depression, the Uspallata-Callingasta-Iglesia Valley, is divided into three catchments based on drainage divides between three large, transverse rivers. These catchments have their headwaters in the western Precordillera, Frontal Cordillera and Principal Cordillera ranges. Deformation is thought to have ceased near the core of the range (Frontal and Principal Cordilleras) and propagated to the east during the Miocene to present. In this study, river profiles and knickpoint topographic data are assessed to determine the distribution of active rock uplift and amount of fluvial incision in the Calingasta and Iglesia valleys. Slope-area data extracted from a digital elevation model consistently show slope-break knickpoints separating distinct sections of river profiles, which suggests relative bedrock uplift. Catchment-wide, normalized steepness indices (k_sn) - using a concavity index of 0.45 (common in steady state profiles in the study area) - for both basins revealed consistently high k_sn signals (>300) in the main stems and tributaries draining the Frontal Cordillera and the western Precordillera. The highest k_sn values are situated in an elevation range of 2500 to 4000 m, west of the fold and thrust belt in the Principal and Frontal Cordilleras. Slope-break knickpoints are not clustered in elevation and there is no obvious correlation between upstream drainage area at knickpoints and respective distance from the tributaries' mouths, which suggests that they are not part of an upstream migrating wave of knickpoints. The data presented here indicate active rock uplift in the eastern edge of the Frontal Cordillera, which was previously considered inactive.