COSMOGENIC DATING RANGING FROM 20 TO 700 KA OF A SERIES OF ALLUVIAL-FAN SURFACES AFFECTED BY THE EL-TIGRE FAULT, ARGENTINA

It is crucial to date continental landforms to quantify processes invo lved in terrestrial surface evolution, especially in regions affected by active tectonics, Andean Quaternary alluvial fan surfaces affected by the Fl Tigre strike-slip fault have been studied using combined geo morphic and Be-10 exposure age approaches. Field observations and SPOT (French acronym for ''Satellite for Observation of the Earth'') image analysis enable the identification of six alluvial fan units. Measure ments of in situ-produced cosmogenic Be-10 concentrations in quartzite boulders exposed on the top of fan surfaces show that the depositiona l periods ended during successive major interglacial stages, The calcu lated minimum exposure ages date the abandonments of the alluvial fan surface from 41 000 +/- 8500 yr for the youngest to 670 000 +/- 140 00 0 yr for the oldest unit, When linked to the measured maximum cumulati ve right-lateral displacement of stream channels, the exposure ages yi eld a horizontal slip rate of about 1 mm/yr on the Fl Tigre fault, Thi s study shows that for arid regions, where fan surface erosion is mini mal, in situ-produced Be-10 can be used to constrain the age of strati graphically separate alluvial fan surfaces, These fan surface exposure ages can be further used to calculate slip rates on active faults and infer depositional periods correlative with climatic events.