THE CRETACEOUS AND TERTIARY TECTONIC HISTORY OF WESTERN COLOMBIA

The Cretaceous and Tertiary subduction history of the Northern Andes is unique within the Andean chain because it was interrupted by the collision and accretion of arc and oceanic plateau rocks. This work aims to quantify i) the timing of accretion of arc and plateau rocks and ii) the thermal and exhumational response of the buttressing continental and indenting oceanic rocks to collision, accretion and post-accretion subduction in the Colombian Andes.

We have obtained more than 120 new thermochronological ages from the Western (WC) and Central Cordilleras (CC) in Colombia. Our data record a complex cooling history from the Early Cretaceous to late Miocene.

Alkali-feldspar ⁴⁰Ar/³⁹Ar cooling ages obtained from crystalline rocks located in the CC yielded ages of ~138–130 Ma and are contemporaneous with the cessation of Jurassic arc-magmatism and a major unconformity within the retro-forearc region of the Northern Andes. We interpret these ages as cooling driven by exhumation in response to fragmentation of the Jurassic slab. Oceanward back-stepping of the slab during the earliest Cretaceous gave rise to the Lower Cretaceous Quebradagrande oceanic arc sequence.

Medium-temperature thermochronometers in Jurassic rocks from a particular structural block in the CC reveal the presence of a previously unidentified cooling event at 107–117 Ma. This event has been attributed to exhumation driven by the collision and accretion of the Quebradagrande arc against the continental margin, and the obduction of the subduction channel onto the forearc.

Thermal modelling of the low-temperature thermochronometric data acquired from rocks sampled throughout the CC and WC reveals three periods of rapid cooling since the Late Cretaceous. The earliest phase is recorded by granitoids emplaced in the CC and cooling rapidly from ~550˚C to ~60˚C during 75–65 Ma. We attribute cooling to exhumation of the continental margin due by the collision and accretion of the Caribbean Large Igneous Province in the Campanian. The CC exhumed at moderate rates during the Eocene, and was probably caused by an increase in plate convergence rates. Elevated exhumation rates in the middle - late Miocene have been identified from the apatite (U-Th)/He data and are probably related to the collision and subduction of the buoyant Carnegie Ridge.