MOREIRAS Stella Maris
congresos y reuniones científicas
Palaeo-landslide damming in the Western Precordillera, Argentina (31°S).
Congreso; Annual Meeting. British Society of Geomorphology (virtual); 2021
Institución organizadora:
British Society of Geomorphology
Slope instabilities are one of the Environmental Effects of Earthquakes (EEE) introduced in the assessment of seismic intensity and hazard. Their preservation allows to extend the seismic activity of a region or a certain structure if the seismic event can be associated with an active fault. Numerous prehistoric landslides associated with earthquakes have been identified in the seismically active scenario of the Argentine Precordillera. Specifically, in the Quebrada de Los Ratones (northern area of the Sierra del Tontal-30ºS), two rock avalanches (RA-1 and RA-2) as well as lacustrine sediments were observed. The source areas of the mobilised material were located at the headwaters of two small branches of the main gully. In the upper part of both headwaters, slope deformation structures and landslide scarps are identified and show evidence of continuous displacements, probably up to the present day. In both cases, the onset of movement points to a collapse of a huge rock mass from the headwaters. Due to the steep slope and relative fall height (≈850 m), the velocity reached in both cases is estimated to exceed 120 km/h. The sequence of lake sediments is arranged partially covering the RA-1 deposit and laterally covering the RA-2 deposit. Therefore, it is interpreted that the deposit of the older rock avalanche (RA-1) was sectioned and eroded by the later rock avalanche (RA-2), which naturally dammed a small part of the stream catchment with the consequent generation of a paleolake. In order to constrain the age of deposition of the lake sediments and estimate the relative age of the most recent rock avalanche, a sample of fine sediments from the lake was extracted for OSL (Optically Stimulated Luminescence) dating to 11.2 ± 0.6 ka cal BP.Regarding the triggering mechanism, rainfall could be considered unlikely, as several analyses of slope stability in rock slides along the Sierra del Tontal have shown them to be stable under saturated conditions. Furthermore, rainfall seems to be more likely to generate debris flows that correlate with changes in drainage basin dynamics. If a seismic trigger is postulated for RA-1 and RA-2, with volumes of 28.4 and 21.6 million m3, respectively, an earthquake of M 6.9 and M 6.8 would be required, respectively, in each case. A priori, according to the expected maximum seismic magnitudes and proximity, seismic events of these magnitudes could be generated exclusively by the El Tigre fault. However, exceptional cases of earthquakes whose magnitude far exceeds the maximum magnitude expected for the active fault are not uncommon. Therefore, the El Carrizal and La Cantera faults should not be discarded as possible seismogenic sources also because of their proximity.Based on the regional neotectonics, the high seismic activity, previous studies and using the ESI scale and the EEE catalogue, a destructive earthquake of intensity ≥ X-MMI (equivalent to a M 7.0 earthquake) is inferred as the trigger for the rock avalanches analysed. Therefore, the OSL age obtained in the basal sequence of the lacustrine sediments would allow extending the seismicity in the Precordillera to the Late Pleistocene/Early Holocene.