Tectonics, v. 25, n. 3, p. 22 pp, 2006. http://dx.doi.org/10.1029/2004TC001723International audienc

The western part of Ecuador is made from several oceanic terranes, which comprise two oceanic plateaus, of Early (c. 120 Ma) and Late Cretaceous age (c. 90 Ma), respectively. The older oceanic plateau was accreted to the Andean margin in the Late Campanian (c. 75 Ma). Fragments of the Turonian–Coniacian plateau were accreted to the Ecuadorian margin in the Late Maastrichtian (c. 68 Ma, Guaranda terrane) and Late Paleocene (c. 58 Ma, Piñón–Naranjal terrane). The Guaranda terrane received either fine-grained oceanic sediments of Coniacian–Maastrichtian age, or island arc/back-arc volcanic suites of Middle Campanian–Middle Maastrichtian age. The Piñón–Naranjal terrane recorded a comparable history, completed in the Maastrichtian–Paleocene, either by pelagic cherts, or by island arc products (Macuchi arc). The Late Cretaceous plateau of Ecuador is interpreted as part of the Caribbean oceanic plateau (COP), because their evolutions are comparable. If so, the COP was not formed by the Galápagos hotspot, but on the Farallón oceanic plate, south of Ecuador and close to the South American margin. The COP belonged to the Farallón plate, until a subduction zone separated both plates in the Middle Campanian, giving way to a well-developed Mid Campanian–Mid Maastrichtian island arc. Accretion in the Late Maastrichtian triggered a change in the subduction system, and the development of a new arc system of Late Maastrichtian–Late Paleocene age, which crosscut the South America–COP plate boundary. The last accretion occurred in the Late Paleocene.

Two accreted oceanic terranes are classically recognized in the Cordillera Occidental of Central Ecuador, the Macuchi island arc to the West, and the Pallatanga oceanic terrane to the East. Detailed stratigraphic studies of the sedimentary cover of the "Pallatanga terrane" show that it actually comprises two terranes. During the late Campanian-early Maastrichtian, the eastern terrane received partially continent-derived turbidites, demonstrating that it was accreted to the Andean margin before mid Campanian times, i.e. 85-80 Ma ago. Meanwhile, the western terrane received fine-grained, pelagic siliceous black cherts indicating that it still belonged to the oceanic realm during mid Campanian-Maastrichtian times. Both series are unconformably overlain by a thick, coarsening upward siliciclastic series of Paleocene age, demonstrating that the western terrane accreted to the eastern one during the late Maastrichtian (≈ 69-65 Ma). The thick Paleocene clastic series recorded the uplift of the Eastern Cordillera, which was triggered by the latter accretion, and enhanced by the Late Paleocene accretion (≈ 58 Ma) of the Piñón oceanic terrane of southern coastal Ecuador.

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