Timing and structural evolution of an accretionary orogen, northern Andes

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Vinasco, Cesar۱; Weber, Marion۱; Cordani, Umberto۲; Giraldo, Maria۱; Garcia, Daniel۱
Holding Date 09 September 2008

The geological evolution of the North-western South America is marked by the accretion of oceanic fragments into the former continental margin. This margin for Colombia is represented by part of the Central Cordillera, which comprises an upper Palaeozoic basement mainly composed of low to medium grade metamorphic rocks. The basement was mainly formed during the amalgamation of North America and Gondwana, in the final assembly of Pangea. The Permian collision is believed to be recorded in the northern part of the Central Cordillera by U-Pb SHRIMP ages in inherited metamorphic zircons around 280 Ma and magmatic U-Pb SHRIMP ages in neoformed zircons around 250 Ma within syntectonic crustal granitic rocks. Magmatic U-Pb SHRIMP and Ar-Ar Triassic ages around 228 Ma in undeformed granitic stocks indicate the presence of a late-tectonic magmatism related to the beginning of the break up of the supercontinent. Installation of Central Cordillera in its present position should be in upper Cretaceous times based on tectonic differences with Grenvillian terrains to the east.
Part of the westernmost Central Cordillera, aim of this study, and the Western Cordillera, comprises allochtonous oceanic sequences of basic volcanic rocks and marine sediments of Upper Cretaceous and Cenozoic age related to the evolution of the Caribbean plate, considered by most authors as being of "Pacific" origin. The "Pacific" models propose a Late Cretaceous origin of the Caribbean oceanic crust in the Pacific region about 90 Ma and a known second magmatic activity at about 70 Ma recorded in this work in magmatic zircons in a metavolcanic-sedimentary sequence. The drift of the Caribbean crust into its present position leaves oceanic fragments on its way on the western margin of north-western South America.
Geochemical, isotopic and geochronological results allow identify different crustal blocks in the north-western part of the Central Cordillera. Permian amphibolites probably related to the Alleghenian orogeny shows MORB affinities with some contribution of arc magmatism. Further west, Triassic basic intrusives present MORB characteristics with contribution of immature island arc magmatism likely associated to Triassic north-eastern subduction zones of the Pacific Plate. These Triassic rocks were probably accreted to the continental margin in Eocene times together with the upper cretaceous metavolcanic-sedimentary sequences of MORB affinity.
Structural evidence shows at least 3 deformational episodes related to accretional dynamics during the entire Cenozoic. Regional east verging asymmetrical folds and associated tight isoclinal folds in metavolcanic-sedimentary sequence together with east verging thrust and west verging back-thrusts were likely formed during accretion in the Eocene. Regional strike slip dynamics, associated with the Romeral fault system, active at least since the upper Cretaceous, generates a tectonics of rotated blocks yielding an S-C like fashion.