The seismic history of extra-Andean South America is poorly documented. Between lat 26°S and 33°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 Argentina reveal extensive Quaternary reverse faulting. 14C dates of 1080 ± 70 and 1310 ± 40 yr B.P. have been determined on organic material from colluvium on the footwall of the east-dipping Los Molinos branch of the Comechingones fault. They indicate at least 2.1 m of slip, probably coseismic, during the past 1300 yr in an area for which no significant deformation has been reported in association with historical earthquakes.