LONG-TERM EROSION RATES AND EXHUMATION HISTORY OF THE “ALTIPLANO ANTIOQUEÑO”, NORTHERN ANDES (COLOMBIA) FROM APATITE (URANIUM-THORIUM)/HELIUM THERMOCHRONOLOGY

Apatite (U-Th)/He (AHe) data from nineteen samples collected from ~ 2 km of exposed paleocrustal depths along two vertical profiles within the Antioqueño Plateau (AP), Cordillera Central (Colombia), permit reconstruction of long-term erosional exhumation of the largest elevated plateau in the Northern Andes. The two well-defined profiles exhibit excellent reproducibility of AHe ages that increase systematically with elevation from ca. 22 to 49 Ma. Such behavior is in agreement with theoretically predicted curves (e.g.,Wolf et al., 1998) and apparent age versus elevation diagrams obtained for other vertical profile studies (e.g., Stockli et al. 2000). The similarity in the age versus elevation plots for both profiles is a sign of internal consistency of the data sets and can be interpreted as the exhumation of the entire plateau as a discrete unit. A marked inflection point in the age versus elevation plots at 25 Ma defines the bottom of the post-Oligocene He partial retention zone (PRZ). Virtually invariant ages at ca. 25 Ma record the onset of a rapid exhumation pulse for the AP. A more subtle change in slope within the PRZ at ca. 43 Ma is interpreted as a minor exhumation pulse. These two pulses correspond to the Proto-Andina (24 Ma) and Pre-Andina (54-43 Ma) orogenetic phases proposed for the Colombian Andes based on stratigraphic and paleontologic evidence (Van der Hammen, 1960). Such pulses are also synchronous with similar events reported for the Peruvian and Bolivian Andes (Gregory-Wodzicki, 2000; Benjamin, 1997). Our data suggest that that only about 900 meters of AP have been removed via erosion in the last 25 Ma, which constitutes a very low erosion rate for an active orogen (~ 0.036 mm/y). This finding corroborate that the elevated erosional surface is not at equilibrium.

Since the evolution of the AP is marked by the development of a relict, erosional surface, AHe ages also yield spatially averaged, long-term erosion rates. AHe-derived natural erosion rates for geologic timescales (10E5-10E7 years) provide a benchmark against which anthropogenic erosion rates can be referenced. Our results suggest that integrated geologic erosion rates (~ 0.036 mm/y) are well below modern estimates of erosion for the studied area (~ 15 mm/y).