PALEOSEISMICITY DEDUCED FROM LAKE SEDIMENT DEFORMATIONS IN THE CALCHAQUÍ VALLEY, NW-ARGENTINA

Vestiges of at least two strong earthquakes occur in lake sediments of a former rock-avalanche dammed lake in the Calchaquí valley at the transition from the Cordillera Oriental to the Sierras Pampeanas tectonic provinces of the NW Argentine Andes. Two reverse-fault offsets with buckle faults in the foot wall and slump faults in the hanging wall indicate that these earthquakes took place while the lake sediments were water saturated and therefore during the lake phase. Within strike of these faults overturned folds occur in a lake sediment segment 1-km-long. Perpendicular to the folds are two deformation horizons with convolute bedding interpreted as seismites. These seismite horizons occur in various sub-basins of this former lake but are restricted to well sorted deposits of silty to sandy composition. No seismites occur within the center of the former lake characterized by clayey to silty composition. The age of the earthquakes causing lake sediment deformation is bracketed by preliminary cosmogenic nuclide ages of surfaces of three rock-avalanche deposits and breakaway scarps, two of them having minimum ages of ~ 13.5 ka and the other one of ~ 4.5 ka. While the older rock-avalanche deposits dammed the valley and formed this lake the young rock avalanche fell in a sub-basin of the lake which was followed by dam erosion and lake-dry out. In addition, one seismite horizon could be AMS carbon-14 dated to 7,500 +/- 70 a `CAL BP´ by organic material. This age coincides with the age of a cluster of 4 simultaneous landslides dated by a combination of cosmogenic nuclide dating and tephrostratigraphy to have occurred at about the same range of time (7 - 7.5 ka ago). These data spanning several thousand years differ from the historic seismic record of the past 200 years and the instrumental record of the last 40 years which indicate, respectively, that no earthquakes of intensity MM > 6 or no earthquake of magnitude M > 4 has occurred in this area. Therefore, paleoseismic investigations indicate that the seismic hazard in this area is stronger than suggested by the historical and instrumental seismic records and that strong earthquakes do occur with a recurrence interval of a few ka.