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The nature and provenance of accreted oceanic terranes in western Ecuador: Geochemical and tectonic constraints

Abstract:

Western Ecuador consists of a complex tectonic mélange of oceanic terranes accreted to the continental margin from Late Cretaceous to Eocene time. New geochemical data from these accreted terranes (arising from a 5 year British Geological Survey mapping programme) indicate that they comprise rocks from a variety of oceanic tectonic settings: from thickened (and relatively unsubductable) oceanic plateau basalts, through island-arc tholeiites, with occasional more calc-alkaline lavas, to back-arc basin basalt sequences. This study has enabled us to construct a new geodynamic model for the Cretaceous-Tertiary evolution of the Northern Andes, and has placed important new constraints on the extent of oceanic plateau sequences in Colombia and around the Caribbean. The age and nature of sediments, combined with evidence for the age of peak metamorphism, suggests that a prolonged (15-20 Ma) accretionary event occurred in Late Cretaceous time and involved the collision of an oceanic plateau (represented by the Pallatanga Unit) with the continental margin. This accreted unit can be correlated with similar oceanic plateau sequences from the Western Cordillera of Colombia and those within and around the Caribbean region. The Naranjal and Macuchi island arcs and the associated La Portada back-arc basin developed along the accreted margin from Late Campanian to Eocene time, and these arcs accreted to the continental margin along with oceanic plateau material (represented by the Piñon Unit and Pedernales-Esmeraldas sequences) during Eocene time. The development of island arcs, which separate the two accretionary events, implies that the most westerly (coastal) oceanic plateau sequences, both in Ecuador (Piñon and Pedernales-Esmeraldas) and in Colombia (Gorgona and Serrania de Baudó), cannot belong to the Caribbean-Colombian Oceanic Plateau (CCOP). It therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the Northern Andes. It is possible that the CCOP formed over the Galápagos hotspot, as previously proposed, but the more westerly Coastal plateau was derived from a more southerly hotspot source region, such as Sala y Gomez, in the SE Pacific.