LONG-TERM EROSION RATES DERIVED FROM RECONSTRUCTION OF A 10 MA DEPOSITIONAL SURFACE, NORTHERN CHILE

The relationship between erosion rates, slopes and climate in arid mountain front settings remains poorly understood. We determine erosion rates for two separate regions of the western Andean mountain front by reconstruction of an ~ 10 Ma regionally extensive depositional surface that occurs in northern Chile between 18°30' and 29°30' south latitude. The depositional surface remnants, spanning elevations of 1000 to 4000 m, were isolated and mapped utilizing geologic maps, slope maps derived from 90 m topography and Thematic Mapper images. The original extent of the depositional surface was reconstructed from the mapped remnants. Using the amount of material removed between the the reconstructed surface and the modern surface topography, eroded volumes are calculated for 22 catchments that drain the western Andean mountain front. Eroded volumes are converted to erosion rates based on the chronologic constraints on the youngest strata. These erosion rates are compared to the mean annual precipitation and average slope of the areas eroded below the surface within each catchment. Our results show that a significant correlation (r-squared > 0.5) exists between erosion rates and the average slope of the eroding areas. The six southernmost basins that we consider span the transition between Westerlies moisture and the hyper-arid core of the Atacama Desert. These southern basins show a strong correlation between precipitation, slopes and erosion. Our results suggest a strong linkage between erosion rates and precipitation, at least in the range of 0-150 mm/yr of mean annual precipitation.