Quaternary alluvial terraces in an active tectonic region: the San Juan River Valley, Andean Ranges, San Juan Province, Argentina

The San Juan River, located in San Juan Province (Argentina). crosses the P recordillera and other geologic units including the Ullum tectonic valley a nd the La Laja Zone between latitudes 31 degreesS and 32 degreesS. The San Juan River is antecedent as is suggested by its two perpendicular segments linked by an almost parallel segment to the main structural trend. Along th e Precordillera, the San Juan River valley has many different alluvial fans at the river junctions with its tributaries. The Quaternary alluvial fans display surfaces cut in a series of steps which we consider to be alluvial terraces generated by aggradation and repeated incision episodes. The studi ed sector includes one area with recent major seismic activity (La Laja Zon e), another without major seismic activity (Precordillera area). and a subs ident area (Ullum area) where a large lake was formed 6500 yr BP. The old S an Juan River was captured by the Quebrada de Ullum valley by means of a 25 -m incision. which resulted in river-gradient headward erosion. The San Jua n River gradient shows some irregularities that, although unrelated to the main structures, are associated with river dynamics, which emphasizes litho logic differences. The main river valley width, the geometry and gradient o f each tributary, together with the basement rock lithologies and the size of each local source area are the major factors which control the alluvial terrace generation processes. In the La Laja Zone. where the uppermost terr ace is capped by travertine, dating of travertine deposits suggests that th e maximum incision rate is 0.9-1 mm/yr related to episodic activity on the La Laja Fault. (C) 2000 Published by Elsevier Science Ltd.