GEOMORPHIC CHANGE AND SEDIMENT PRODUCTION IN RESPONSE TO UPLIFT OF THE ANDEAN EASTERN CORDILLERA, NW ARGENTINA

The geomorphology of the Eastern Cordillera (EC) of Northwest Argentina reveals a complex history of uplift and erosion that provides a rapidly growing sediment load to the foreland. Drainage cuts low-slope, high elevation surfaces to create valleys with up to 2.5 km relief. This topography contains evidence for divide migration, knickpoint retreat, capture, and drainage reversal on the former Puna-level surface. Foreland basin data suggests that substantial erosion of the EC basement began ~ 4 Ma, and a cosmogenic burial date from the base of a valley fill implies that a deeply incised, reversed drainage in the headwaters of the Río Iruya (Río Colanzuli) was established by 1.3 Ma. Thus relief grew at nearly 1 m/a. In this reversed drainage, extreme incision combined with headward erosion and capture yielded enough sediment to cause complex response leading to a valley filling episode lasting through 200 ka. Removal of the valley fill and subsequent bedrock incision occurs today at rates exceeding 1 mm/a, although complex response continues in the form of cut/fill cycles during removal of the valley fill and deepening of the valleys. Incision of epigenetic gorges and bedrock reaches demonstrates that the stream power is sufficient to promote valley deepening, but is locally insufficient to remove debris from oversteepened tributaries. Likewise, propagation of > 500 m knickpoints into the headwaters of the Río Nazareno induces valley aggradation downstream of gorges cut into low-relief uplands. Former drainage parallel to the strike of the Eastern Cordillera is being cut by deep transverse drainage through headward erosion and capture. Sediment transport by the modern Río Iruya is the highest in the region, even greater than geologic rates determined from production and removal of the valley fill. The geomorphic evidence and sediment flux data support the notion that positive feedback by the geomorphic system in response to uplift produces increasingly coarse and voluminous sediment for a clastic wedge in the piggyback basins and megafans of the foreland to the east.