Late Cretaceous-Paleogene orogenic build-up of the Ecuadorian Andes: Review and discussion
2022
Jaillard, Etienne
A review of the tectonic events recorded by sediments on the Andean continental margin of northern Peru and Ecuador and on the accreted oceanic terranes of Ecuador, allows to identify three tectonic events of late Campanian to late Paleocene age (~75 to ~55 Ma). Each tectonic event resulted in the arrival of quartz-rich deposits on the oceanic units and in a sedimentary hiatus in the backarc zone (Oriente basin), which both indicate an uplift of the South-American continental margin. This is interpreted as the underplating of oceanic terranes beneath the Andean continental margin. After the Incaic contractional phase (~40 Ma), the Oriente Basin recorded a sharp increase in the subsidence rate, the westward migration of the depocenters and a drastic change in the detrital source around 25 Ma, which indicate the onset of its foreland basin evolution, driven by flexural subsidence. It is therefore proposed that the Ecuadorian Andes resulted first from the evolution of a western, west-verging orogenic wedge, made of accreted oceanic material (~75–40 Ma). Once this western wedge overthickened and submitted to strong vertical body forces, the strain related to ocean-continent convergence was transmitted to the continental plate, where a second, eastern, east-verging orogenic wedge formed about 25 Ma ago. This scenario accounts for the tectonic and sedimentary evolution of the Ecuadorian-north Peruvian margin, and explains why, in spite of a moderate elevation, the partly oceanic crustal root of the Ecuadorian Andes, is almost as thick as beneath the high-altitude Altiplano of southern Peru and Bolivia, where the crustal root is mainly of continental nature.
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