COSMOGENIC EXPOSURE AGES OF LAVA FLOWS THAT TEMPORARILY DAMMED THE RIO GRANDE AND RIO SALADO, MENDOZA PROVINCE, ARGENTINA

The relationship between tectonics and river incision is an area of considerable interest in modern Cordilleran geology. An often-overlooked effect on the fluvial system in tectonically active areas is lava flow damming. Lava flows can easily dam or divert major rivers, typically causing upstream aggradation and a host of possible downstream effects. In order to understand better the effect of lava damming on major Cordilleran river systems, we determined 3He exposure ages of lava flows that temporarily dammed two separate rivers in the Mendoza Province, Argentina. The Rio Grande was likely dammed by a series of basaltic lava flows that erupted from the large volcanic center to the east. Cosmogenic 3He exposure ages of primary lava flow surfaces (at ~36.31°S; 69.66°W) range from 37±3 to 44±2 ka (mean ±1s is 41±1, n=3). The Rio Grande is now incised below the tops of the lava flow surfaces. A small fluvially beveled and polished bedrock strath ~9 m below the top of the lava flow surface records a break in river incision. A water polished bedrock knob on this fluvial strath yielded a 3He exposure age of 16±1 ka. The Rio Salado, located along highway 222 on the road to Las Leñas, was likely temporarily dammed by a lava flow (at ~35.18°S; 69.78°W) that emanated from a vent located to the northeast of the valley. Upstream of the lava flow a package of unconsolidated sediments is preserved and exposed by tributary incision. A dense, impermeable, basal clay layer may be recording lacustrine deposition in a lava-dammed lake. Above the clay layer is 5–8 m of alluvial silts to rounded gravels possibly recording fluvial aggradation behind a breached but still effective lava dam barrier. Three 3He exposure ages of primary lava flow surfaces from the Rio Salado lava dam are forthcoming. These features demonstrate that lava damming can have a significant effect on Cordilleran drainages and may need to be addressed when investigating relationships between river incision and tectonics in volcanically active areas.