Abstract

In Ecuador and northwestern Peru the Andes and adjacent country, particularly on the Pacific side, are composed of at least five distinctive geologic terranes. The terranes are distinguished from one another and from cratonic South America to the east by dissimilar basements, cover rocks, intrusive rocks, and Bouguer gravity anomaly fields.The Piñón terrane, occupying most of coastal Ecuador, has a basaltic basement characterized by the largest known on-land positive Bouguer anomalies in the western hemisphere. The Tahuín terrane occupies most of northwestern Peru and the southwestern corner of Ecuador. The terrane has an especially complex basement and is the site of generally positive Bouguer anomalies. The small Birón terrane has an unusual basement composed in part of cordierite gneiss and amphibolite that give consistent Late Cretaceous K – Ar mineral ages. The wedge-shaped Chaucha terrane lies in part on the western Andean slope, between the oceanic Piñón terrane on the north and the continental Birón terrane on the south. The vast Santiago terrane composes the high Andes of southern Ecuador and northwestern Peru. It is the site of the unique Santiago Formation, a thick succession of Lower Jurassic limestones found nowhere else in the region.Geologic and geophysical evidence supports the view that the five terranes are parautochthonous or allochthonous fragments emplaced against cratonic South America from Middle Jurassic to Late Eocene time. Continental-border subduction alone (at the so-called "Andean margin") may have been an inadequate engine for orogeny. Additional allochthonous terranes perhaps await identification at other places along the Andes. Whether the emplacement of allochthonous terranes has been an important process elsewhere in the tectonic development of the Andes remains to be established. Geologic mapping on the oceanward western border of the Andean orogen, studies of basement petrology and chronology, and paleomagnetic studies are particularly needed.The distribution of mineral deposits (including petroleum) in Ecuador and northwestern Peru is not uniform but is instead related spatially to the five terranes and cratonic South America. This relationship can be useful to prospectors.

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