TIMING OF NEOTECTONIC DEFORMATION IN THE WESTERN PRECORDILLERA, CENTRAL ANDES OF ARGENTINA, INFERRED FROM 10BE SURFACE EXPOSURE DATING.

TERRIZZANO, CARLA; ZECH, ROLAND; GARCÍA MORABITO, EZEQUIEL; YAMIN, MARCELA; HAGHIPOUR, NEGAR; WÜTHRICH, LORENZ; CHRISTL, MARCUS; WILLETT, SEAN; CORTES, JOSÉ MARIA; RAPALINI, AUGUSTO E; RAMOS, VICTOR A.

Innsbruck, Alemania.

Simposio; Deutsche Quartärvereinigung (DEUQUA) Tagung 2014; 2014

The active Andean deformation front in the Argentinian forelands between 28° and 33°S is currently concentrated at the boundaries of a main morphotectonic unit: The Precordillera, a north-south trending mountain range tens of kilometers east of the High Andean Cordillera. The neotectonic activity results in widespread deformation of Quaternary deposits and recent devastating earthquakes (e.g. Costa et al., 1999, Siame et al., 2002; Verges et al., 2007; Schmidt et al., 2011, Cortés et al. 2014, Terrizzano et al. 2014). Most studies so far have focused on estimating slip and uplift rates and assessing seismic hazards in the eastern Precordillera, one of the most active zones of thrust tectonics in South America. Rates of uplift and shortening on the reverse faults of the western flank of the Precordillera remain largely unknown. For this study, we combined structural and geomorphic investigations with 10Be surface exposure dating in order to establish a numeric chronology for four deformed alluvial fan surfaces and to estimate uplift rates of the Barreal block (31°30´-31°53´ SL/69°20´ WL) in the western Precordillera. Surface exposure ages were determined for a few large boulders, amalgamated pebbles, and via depth profiles on sand samples. Boulder ages range from 145 to 212 ka for the oldest well-preserved fan remnants (Q1a, n=3), from 63 to 108 ka (Q2, n=3) and 21-28 ka (Q3, n=2, amalgamated pebbles yield ages range from 106 to 127 ka for the oldest fan surface (Q1b, n=79), all calculations assuming no erosion and using the scaling scheme for spallation based on Lal 1991, Stone 2000). Boulders from current channels have 10Be concentration equivalent to 12-31 ka of exposure. The depth profiles yield minimum ages (assuming negligible erosion) of 120 ka (Q1a as well as Q1b) and 79 ka (Q2) and are thus mostly in good agreement with boulder ages. Uplift rates of 0.32 mm/yr and 0.83 mm/yr can be estimated for the eastern and western front of the Barreal block respectively using minimum ages from the depth profiles. This provides the first numerical constraints for the neotectonic activity of the Barreal block. Moreover, the exposure ages may tentatively be interpreted to document fan formation at times of globally low temperatures and glacial maxima (Marine Isotope stages 2, 4 and 6, respectively).