Late Quaternary Activity of the La Rinconada Fault Zone, San Juan, Argentina

Abstract

Most of the permanent deformation in the Pampean Flat slab segment of the central Andes is taken up at the Andean Orogenic Front in Argentina, a narrow zone between the Eastern Precordillera and Sierras Pampeanas that comprises one of the world's most seismically active thrust zones. Active faults and folds in the region have been extensively mapped but still largely lack information on style and rates of deformation, which is essential for understanding the distribution of regional strain and estimating the seismic potential of individual faults. Structural, geomorphic, and 36Cl cosmogenic radionuclide surface exposure age methods are used to focus on key sites along the 30-km-long La Rinconada Fault Zone in this region of west-central Argentina, which is ~15 km away from the highly populated (~500,000) city of San Juan, to define a late Quaternary average shortening rate of 0.41 ± 0.01 mm/year. This slip rate is the same order of magnitude, but slightly lower than nearby similar east dipping Eastern Precordillera faults including the La Laja and Las Tapias Faults. Relatively low slip rates are interpreted as being a consequence of distributed deformation between the latitude of the La Rinconada Fault Zone (31 and 32°S), as compared to between latitudes 32 to 33°S where deformation appears to be focused on fewer structures, including the Las Peñas and La Cal Thrust Faults. The La Rinconada Fault Zone is capable of generating earthquakes of Mw 6.6–7.2, but further investigations are required to determine timing and recurrence intervals of discrete events.