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Mode and timing of terrane accretion in the forearc of the Andes in Ecuador

By
Cristian Vallejo
Cristian Vallejo
Geological Institute, Eldgenössische Technische Hochschule Zürich, CH-8092 Zürich, Switzerland
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Wilfried Winkler
Wilfried Winkler
Geological Institute, Eldgenössische Technische Hochschule Zürich, CH-8092 Zürich, Switzerland
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Richard A Spikings
Richard A Spikings
Department of Mineralogy, University of Geneva, CH-1205 Geneva, Switzerland
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Leonard Luzieux
Leonard Luzieux
Geological Institute, Eldgenössische Technische Hochschule Zürich, CH-8092 Zürich, Switzerland
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Friedrich Heller
Friedrich Heller
Geological Institute, Eldgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland
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François Bussy
François Bussy
Institute of Mineralogy and Geochemistry, University of Lausanne, CH-1015 Lausanne, Switzerland
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Published:
June 01, 2009

The volcanic basement of the Ecuadorian Western Cordillera (Pallatanga Formation and San Juan unit) is made up of mafic and ultramafic rocks that once formed an oceanic plateau. Radiometric ages from these rocks overlap with a hornblende 40Ar/39Ar plateau age of 88 ± 1.6 Ma obtained for oceanic plateau basement rocks of the Piñon 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 flat 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 units may be related to the Late Cretaceous Great Arc of the Caribbean. Paleomagnetic analyses of volcanic rocks of the Piñon and San Lorenzo Formations of the southern external forearc show that they erupted at equatorial or low southern latitudes.

The initial collision between South America and the Caribbean-Colombia oceanic plateau caused rock uplift and exhumation (>1 km/m.y.) within the continental margin during the Late Cretaceous (ca. 75–65 Ma). Magmatism associated with the Campanian–early Maastrichtian Rio Cala Arc ceased during the Maastrichtian because the collision event blocked the subduction zone below the oceanic plateau. Paleomagnetic data from basement and sedimentary cover rocks in the coastal forearc reveal 20°–50° of clockwise rotation during the Campanian, which was synchronous with the collision of the oceanic plateau and arc sequence with South America.

East-dipping subduction beneath the accreted oceanic plateau formed the latest Maastrichtian to early Paleogene (ca. 60 Ma) Silante volcanic arc, which was deposited in a terrestrial environment. Subsequently, Paleocene to Eocene volcanic rocks of the Macuchi unit were deposited, and these probably represent a continuation of the Silante arc. This submarine volcanism was coeval with the deposition of siliciclastic rocks of the Angamarca Group, which were mainly derived from the emerging Eastern Cordillera.

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GSA Memoirs

Backbone of the Americas: Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision

Suzanne Mahlburg Kay
Suzanne Mahlburg Kay
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Víctor A. Ramos
Víctor A. Ramos
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William R. Dickinson
William R. Dickinson
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Geological Society of America
Volume
204
ISBN print:
9780813712048
Publication date:
June 01, 2009

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