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

Abstract

Las Chacras Fault System is a Quaternary structure located in the western piedmont of Sierra de la Huerta in the Sierras Pampeanas Occidentales, in west-central Argentina. This fault system is subsidiary of the Valle Fértil Megafault System, the NW-striking master structure that affects the western flank of the Valle Fértil - La Huerta ranges. The Las Chacras Fault System comprise several sections including the Pan de Azúcar fault, which presents the most conspicuous evidence of tectonic Quaternary activity. The Pan de Azúcar fault affects three Quaternary alluvial surfaces (Q 1 , Q 2 and Q 3 ) along 26 km in the western piedmont of Sierra de La Huerta, where three natural exposures allowed us to observe Triassic rocks thrust over Quaternary deposits. The observation of different kinematic markers and the relative position of the units affected by the fault, allow us to define this fault as a reverse fault. This structure trends 325° and dips 25° NE. With a DGPS we carried out detailed topographic profiles perpendicular to the average strike of the fault, along interfluves and rivers that cross the structure, 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 the piedmont geomorphology, generating west-facing scarps and drainage anomalies such as rivers with convex Hack 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 ± 0.2 m since the Late Pleistocene to Holocene, yielding a shortening rate and a slip rate of ∼0.16 ± 0.1 mm/yr. Thus, the Pan de Azúcar fault is a potential seismogenic source, which could produce a moderate earthquake between Mw = 6.4 and 7.0, with a recurrence interval between 2600 and 6900 years.