LATE HOLOCENE FAULTING IN THE SOUTHEAST SIERRAS-PAMPEANAS OF ARGENTINA

The seismic history of extra-Andean South America is poorly documented . Between lat 26 degrees S and 33 degrees S, about 375 km east of the fold-and-thrust belt that marks the present orogenic front, the block- faulted crystalline basement uplifts of the Sierras Pampeanas in Argen tina reveal extensive Quaternary reverse faulting. C-14 dates of 1080 +/- 70 and 1310 +/- 40 yr B.P. have been determined on organic materia l from colluvium on the footwalI of the east-dipping Los Molinos branc h of the Comechingones fault. They indicate at least 2.1 m of slip, pr obably coseismic, during the past 1300 yr in an area for which no sign ificant deformation has been reported in association with historical e arthquakes.