Neotectonic, morphotectonic and paleoseismologic analysis of the Las Chacras Fault System, Sierras Pampeanas Occidentales, San Juan, Argentina

ROTHIS, LUIS MARTIN; PERUCCA, LAURA PATRICIA; SANTI MALNIS, PAULA; ALCACER, JUAN MANUEL; HARO, FEDERICO MIGUEL; VARGAS, HORACIO NICOLAS

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2019 vol. 91 p. 144 - 153

0895-9811

Las Chacras Fault System is a Quaternary structure located in the western piedmont of Sierra de la Huerta in theSierras Pampeanas Occidentales, in west-central Argentina. This fault system is subsidiary of the Valle FértilMegafault System, the NW-striking master structure that affects the western flank of the Valle Fértil - La Huertaranges. The Las Chacras Fault System comprise several sections including the Pan de Azúcar fault, which presentsthe most conspicuous evidence of tectonic Quaternary activity. The Pan de Azúcar fault affects threeQuaternary alluvial surfaces (Q1, Q2 and Q3) along 26 km in the western piedmont of Sierra de La Huerta, wherethree natural exposures allowed us to observe Triassic rocks thrust over Quaternary deposits. The observation ofdifferent kinematic markers and the relative position of the units affected by the fault, allow us to define thisfault as a reverse fault. This structure trends 325° and dips 25° NE. With a DGPS we carried out detailed topographicprofiles perpendicular to the average strike of the fault, along interfluves and rivers that cross thestructure, for the purpose of establishing the relationship between the topography and the slope of the terrain.This analysis indicates that Quaternary activity of the fault has played an essential role in controlling thepiedmont geomorphology, generating west-facing scarps and drainage anomalies such as rivers with convexHack profiles. Paleoseismic analyses indicate that the fault has moved after 18.360 ± 2990 yr BP. Furthermore,we calculated a net displacement for the Pan de Azúcar fault of ∼50 ± 0.2m since the Late Pleistocene toHolocene, yielding a shortening rate and a slip rate of ∼0.16 ± 0.1 mm/yr. Thus, the Pan de Azúcar fault is apotential seismogenic source, which could produce a moderate earthquake between Mw=6.4 and 7.0, with arecurrence interval between 2600 and 6900 years.