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.