SURFACE UPLIFT OF THE FRONTAL CORDILLERA AT 30.5°-31°S CONSTRAINED THROUGH RIVER PROFILE ANALYSIS OF THE CASTAÑO AND CALINGASTA CATCHMENTS, SOUTH-CENTRAL ARGENTINA

At the latitudes of the Pampean flat-slab subduction zone, a recent study points to a north to south gradient in surface uplift between ~30°S to 34°S. However, since the study was limited to exposed Neogene basin deposits, surface uplift of the Frontal and Principal Cordilleras is resolved in bins of one degree of latitude. Nonetheless, a southward increase in surface uplift from 0 (30°S) to 800 ± 400 m (32.5°S) is evident. This study seeks independent evidence of surface uplift between 30°S and 32°S by exploiting well-preserved surface uplift signals in river profiles. In the Calingasta Valley, both the Castaño (~30.5°S) and Calingasta catchments (~31°S) contain knickpoints anywhere between 3100 and 3700 m in the lower reaches of tributary channels regardless of bedrock lithology, which indicates incision of the trunk stream. These knickpoints consistently mark the upstream edge of oversteepened reaches that separate two concave segments of every river channel we've assessed. Regressed slope-area data extracted from SRTM digital elevation models were used to derive stream concavity and steepness indices. This analysis shows that steepness indices consistently double downstream of the convexities. Assuming the relict areas upstream of prominent knickpoints are in equilibrium with their pre-perturbation state, we project the relict portions of the trunk river profiles and use their paleo-outlet elevations to determine total surface uplift relative to the base-level of the Calingasta Valley immediately W of the Precordillera range. Total surface uplift is between ~1,700 m (Castaño catchment) and ~1,200 m (Calingasta catchment). Since the Calingasta does not cut into the core of the Frontal Cordillera, surface uplift must have rotated its channel heads, thus leading to an apparent smaller amount of surface uplift when compared to the Castaño catchment. Within respective uncertainties we find surface uplift to be comparable to that constrained for the Manantiales basin (32°S). These results support the previously proposed notion of along-strike variability in surface uplift.