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The Western Cordillera and coastal forearc along the western margin of Ecuador share an allochthonous origin involving the accretion of mafic rocks of an oceanic plateau and intraoceanic volcanic arc. Oceanic plateau mafic rocks of the Pallatanga Formation (Western Cordillera) and Pinon Formation (coastal forearc) form the basement of the Pallatanga and Pinon blocks (terranes), respectively. The basement rocks of both regions are likely derived from the Caribbean Plateau that formed from ~ 100 to 87 Ma. Provenance analyses indicate that detrital material derived from the South American continental margin first reached the Pinon and Pallatanga blocks during Paleocene sedimentation. Specifically, Paleocene-Eocene successions in the coastal forearc and Western Cordillera received detrital input from continental basement rocks of the Eastern Cordillera, and input of igneous material likely derived from the Tandapi volcanic arc and its continuation, the Macuchi submarine arc. In addition, detrital zircon U-Pb ages for uppermost Cretaceous to Miocene clastic deposits of the coastal forearc (including the broader Progreso basin region) and Western Cordillera recorded continuous input from syndepositional volcanic sources, consistent with sustained arc magmatism and close proximity to the Andean magmatic arc. This suggests that the continental magmatic arc in Ecuador has been well established since at least Maastrichtian time. The provenance analyses together with thermochronological, paleomagnetic, and stratigraphic data suggest that the Pinon and Pallatanga blocks were accreted to the South American continental margin at ~ 73 Ma. Therefore, a previously proposed late Eocene age of Pinon accretion is unlikely. Further, a proposed allochthonous origin for the Macuchi arc cannot be reconciled with the fact that this arc supplied detrital material to both the Pallatanga and Pinon blocks. These points lead to a broader recognition that deformation episodes in Ecuador, such as a proposed Eocene increase in shortening, need not be associated with collisional events.

The volcanic basement of the Ecuadorian Western Cordillera (Pallatanga Formation and San Juan unit) is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau. Radiometric ages from these rocks overlap with a hornblende 40 Ar/ 39 Ar plateau age of 88 ± 1.6 Ma age obtained for oceanic plateau basement rocks of the Pinon Formation in coastal Ecuador, and with ca. 92–88 Ma ages reported for oceanic plateau sequences in the Caribbean and western Colombia. These results suggest that the oceanic plateau rocks of the Western Cordillera and fl at forearc in Ecuador are derived from the Late Cretaceous Caribbean-Colombia oceanic plateau. Intraoceanic island-arc sequences (Rio Cala Group) overlie the plateau in the Western Cordillera and yield crystallization ages that range between ca. 85 and 72 Ma. The geochemistry and radiometric ages of island-arc lavas from the Rio Cala Group, combined with the age range and geochemistry of their turbiditic, volcaniclastic products, indicate that the arc was initiated by westward subduction beneath the Caribbean Plateau. They are coeval with island-arc rocks of coastal Ecuador (Las Orquideas, San Lorenzo, and Cayo Formations) and Colombia (Ricaurte Arc). These island-arc

The presence of at least one Block (Pallatanga Block) derived from the Caribbean–Colombian Oceanic Plateau has been proposed in the Western Cordillera of Ecuador. New 40Ar/39Ar dating of the basement of the coastal blocks (90–87 Ma) and fossil ages in the overlying Calentura Fm. (89–84 Ma) argue for late Turonian–Coniacian extrusion of the basement of the Pinon block. Paleomagnetic inclinations indicate that the mafic basement of the Pinon and San Lorenzo blocks extruded at equatorial, low southern latitudes. No subsequent significant latitudinal drift can be observed during the Cretaceous. Rapid changes in paleomagnetic declination between 73 and 70 Ma in both the Pinon and the San Lorenzo blocks indicate that the large igneous province, which originally hosted the coastal blocks experienced a major tectonic event during Late Campanian times. Similar Late Turonian–Coniacian basement ages and Late Campanian tectonic events are reported in the Pallatanga Block of the Western Cordillera. We propose that the Pinon, San Lorenzo and Pallatanga blocks all derive from a common large igneous province, the Caribbean–Colombian Oceanic Plateau, and were incorporated into the South American margin by collision during the Late Campanian (73–70 Ma).