APATITE FISSION TRACK CONSTRAINTS ON THE LONG-TERM EROSIONAL EXHUMATION AND LANDSCAPE EVOLUTION OF THE “ALTIPLANO ANTIOQUEÑO”, NORTHERN ANDES COLOMBIA

Apatite fission track (AFT) results from twenty-two samples collected from ~2 km of exposed paleocrustal depths, along two profiles, constrain Cenozoic erosional exhumation of the Antioqueño Plateau (AP), Cordillera Central (Colombia), the largest elevated plateau in the Northern Andes. AFT ages, generated by the LA-ICP-MS method, from both profiles vary between ca. 30-49 Ma and display almost invariant ages (at ±2σ) between 1500 to 2400 m elevations. Confined track length distributions from samples within the same elevation interval display uni-modal distributions with an average mean track length of ~ 14.2 um, indicating relatively rapid ecooling. Most AFT ages are older than previously reported AHe ages from the same profiles and are virtually identical to AFT ages produced previously using the EDM and by a non-vertical profile approach. Modeling of the AFT age and track length data suggests significant cooling at the same time as the Pre-Andina orogenetic phase (ca. 42-50 Ma). Assuming a paleogeothermal gradient of 25°C, this cooling episode corresponds to exhumation rates in the order of 0.64 km/Ma. Sediment produced from erosional exhumation along with surface uplift of the Central Cordillera are recorded by strata of the same age in the Eastern Cordillera. The Matasanos-Porce AFT profile resembles earlier AHe transects and gives an inflection point at ~1400 m, which defines the upper boundary of an apatite partial annealing zone exhumed during the 45-50 Ma cooling event. The elevation of the inflection point indicates that about 1.3 km of crust have been removed since Eocene time, which gives an average denudation rate of ~ 0.028 km/Ma. The Magdalena basin, which is the major depression that separates the Central and Eastern Cordilleras, and the Magdalena delta were receptors of large amounts of Cenozoic sediments sourced from the northern Andes. Long-term average erosion rates from these data are similar to previously reported erosion rates for the region and are two to three orders of magnitude less than modern, anthropogenically enhanced erosion rates in the AP. This finding corroborates that the elevated erosional surface is not at equilibrium and that humans have played an important role in exacerbating erosion well above background rates.