Abstract

TEJADA, Flavia et al. Neotectonic and paleoseismic analysis of the Loma Negra Oriental fault, orogenic front of the Central Precordillera, Argentina. Bol. Soc. Geol. Mex [online]. 2021, vol.73, n.2, 00008.  Epub Jan 31, 2022. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2021v73n2a080121.

The reverse fault known as Loma Negra Oriental is located on the orogenic front of the Central Precordillera, characterized by N-S trending thrusts with eastern vergence. This fault limits the eastern flank of the Loma Negra brachianticline, a fault propagation fold that deforms Neogene rocks and Quaternary deposits outcropping on the eastern piedmont of the Sierra de Talacasto. The faults related to this fold trend N-S and affect progressively younger deposits to the east, .showing the advance of the Andean deformation in that direction. In the northern section of the Loma Negra Oriental Fault, a natural exposure shows deposits associated with single-event displacements, allowing us to determine its kinematics. The Loma Negra Oriental thrust shows a NNW trend (130°) and dips with low angle (22° SW) affecting recent alluvial deposits of early Holocene age (8,700 ± 1,370years BP). To characterize the morphology of the scarps associated with this fault, we constructed detailed topographic profiles transverse to the analysed structure. Two scarps of 0.9 m and 3.1 m high were recognized, suggesting several seismic events. The fault seems to continue to the north and south as a blind (buried) structure that limits the entire eastern flank of the brachianticline, with a length of at least 16.5 km. Based on the scarps measurement, the surface and subsurface rupture extension, and the paleoseismic analysis of a natural exposure, with a total vertical slip of 0.9 ± 0.1 m (~ 0.4 m for the last event and ~ 0.5 m for the penultimate) and a dip-slip per event of 1.16 ± 0.15 m, we suggest the occurrence of two seismic events during the Late Pleistocene-Holocene. In this way, we consider this structure as a potential seismogenic source which could generate a maximum probable earthquake of magnitude Mw 6.5 or larger, with an estimated recurrence period of ~9,000 years, posing a threat to infrastructure and populated areas of the province of San Juan (>800,000 inhabitants).

Keywords : thrust; active fault; Holocene; seismic hazard; Central Andes.

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