UPLIFT AND EXHUMATION OF THE SOUTHERNMOST SEGMENT OF THE WESTERN CORDILLERA (COLOMBIA) AND DEVELOPMENT OF THE NEIGHBORING TUMACO BASIN

Colombia’s Western Cordillera is characterized as geologically complex mountainous terrain developed on a series of discrete Cretaceous oceanic crustal blocks accreted along the northern South America margin. Uplift and exhumation processes are assessed for the southern Western Cordillera using geochronology, low-temperature thermochronology and paleothermometry data. Sampled sites include the Tumaco Basin (Remolinogrande-1 and Majagua-1 wells), the Gorgona Island, the Patía Sub-basin, and mountain massifs of the Western Cordillera. Time temperature (t-T) solutions for the data were generated using inverse-forward modeling. Best-fit solutions reveal an isothermal period from 58 to 40 Ma and five events of rapid cooling: (i) ~70?-58 Ma (average cooling rates (CR) ~23°C/Ma), (ii) ~40-22 Ma (CR ~12.5 – 50 °C/Ma), (iii) ~20-16 Ma (average CR ~22.8 °C/Ma, peak CR ˃ 150 °C/Ma at ~16 Ma), (iv) ~14-10 Ma (CR ~40 °C/Ma, peak CR ˃ 65 °C/Ma at ~12 Ma), and (envy) ~6-4 Ma (CR ~35°C/Ma). A comparison between episodes of rapid exhumational cooling and changes in the geometry and/or rates of convergence between Farallon (Nazca) and South American plates since ~70 Ma suggest that the first three events were possibly driven by: i) Caribbean Large Igneous Province accretion from ~75-58 Ma, ii) Farrallon plate break-up during ~35-28 Ma, and iii) Panamá-Chocó block accretion at ~23-26 Ma. However, the last two events (~14-10 Ma and ~6-4 Ma) cannot be explained by increased regional convergence rates. Alternatively, these two periods could be associated with subduction of anomalously thickened local crust and/or seamounts. The isothermal period is not consistent with high convergence rates. We attribute this period to sustained oblique convergence (transpression) which leads to crustal extension thereby promoting the formation and development of the Tumaco Basin. After extension, the region began to record changes in sedimentary dynamics controlled by the four mentioned tectonic events accompanied by intense volcanism. These processes have promoted the present configuration of the south Colombia relief.