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| Subandean Belt | |
|---|---|
| Name | Subandean Belt |
| Type | Fold-and-thrust belt |
| Location | South America |
| Region | Andes |
| Orogeny | Andean orogeny |
| Age | Cretaceous–Cenozoic |
Subandean Belt The Subandean Belt is a major fold-and-thrust province on the eastern flank of the Andes extending along the western edge of the Amazon Basin and adjacent foreland. It developed during the Andean orogeny as a consequence of interaction between the Nazca Plate, the South American Plate, and pre-existing continental structures, and records Cretaceous to Cenozoic shortening, sedimentation, and basin evolution. The region has been the focus of studies by institutions such as the U.S. Geological Survey, IGME (Spain), Servicio Geológico Colombiano, Instituto Geográfico Militar (Bolivia), and universities including Universidad Nacional de San Marcos, Universidad de Buenos Aires, and Universidad de São Paulo.
The belt lies between the eastern cordillera of the Andes and the Amazonian Craton margin, interacting with structures like the Eastern Cordillera (Colombia), the Cordillera Oriental (Ecuador), and the Altiplano-Puna Plateau. Its tectonic framework reflects convergence related to the Nazca Plate subduction beneath the South American Plate, with contributions from microplates such as the Caribbean Plate and the Scotia Plate in the southern sector. Regional shortening produced foreland basins including the Marañón Basin, the Subandean Foreland Basin, and the Beni Basin, influenced by inherited structures of the Precambrian Shield, the Guiana Shield, and the Brazilian Shield. Key tectonic events link to global episodes such as the Cretaceous Normal Superchron, the Paleogene warming, and the Neogene uplift recorded across the Andean orogeny.
Stratigraphic sequences include synorogenic clastic wedges, continental red beds, fluvial megasequences, and marine incursions tied to formations like the Chonta Formation, Puyo Formation, Tambillo Formation, and equivalents correlated with the Maranon Group and Cupiagua Group. Sediment provenance analyses tie sources to the Eastern Cordillera (Bolivia), the Cordillera Central (Peru), and uplifted terrains such as the Cordillera de Mérida, the Yungas Range, and the Cerro de Pasco region. Lithofacies range from conglomerates to fine-grained shales with paleosols recording events synchronous with the Eocene–Oligocene transition, the Miocene climate optimum, and regional eustatic changes associated with the Paraná–Etendeka volcanism and the Caribbean Large Igneous Province.
Structure includes thin-skinned and thick-skinned styles with major thrust sheets, ramps, flats, duplexes, and basement-involved thrusts related to thrust systems such as the Vilcabamba Thrust, the Putumayo Fault System, the Santa Cruz Thrust, and the Huallaga Fault. Folding produces anticlines and synclines that trap hydrocarbons analogous to traps in the Orinoco Belt and the Neuquén Basin. Kinematic indicators record west-directed shortening, reverse faulting, and strike-slip partitioning with links to regional faults like the Ballenita Fault, the Iquique-Ofqui Fault Zone, and the Pasto Fault. Geophysical surveys by groups including Schlumberger, Petrobras, ExxonMobil, and BP have imaged décollement levels and basal shear horizons.
Fossil assemblages include continental vertebrates, freshwater mollusks, palynofloras, and marine invertebrates from intervals correlated with the Paleocene–Eocene Thermal Maximum, the Oligocene, and the Miocene. Notable faunal links connect to taxa described from the Madre de Dios Formation, the Fitzcarrald Arch region, and the Marañón Basin, with affinities to fossils from the Granja Formation, the Petaca Formation, and the Pebas Formation. Paleobotanical records include remains comparable to assemblages from the Amazonian rainforest paleoenvironments, while vertebrate records echo clades reported from the Bolivian Altiplano and Patagonia.
The belt hosts prolific hydrocarbon provinces with producing fields analogous to plays in the Putumayo Basin, Llanos Basin, and the Orinoco Belt, and has been targeted by companies including Repsol, Chevron Corporation, TotalEnergies, and ChevronTexaco. Reservoirs occur in fluvial sandstone bodies, carbonate buildups, and structural traps above regional décollements; source rocks correlate with intervals in the Cretaceous and Paleogene strata. Mineral occurrences include placer gold in rivers draining the belt near Tapajós River, polymetallic veins associated with the Cordillera de la Costa (Venezuela), and industrial minerals exploited by firms such as Barrick Gold Corporation and Glencore. Exploration strategies integrate 2D and 3D seismic, well logs, and basin modeling practiced by laboratories at Imperial College London and the University of Texas at Austin.
Active shortening produces seismicity and surface deformation linked to large events recorded in the historical catalogs of Instituto Geofísico del Perú, Servicio Sismológico Nacional (Ecuador), and the INGV protocols; notable earthquakes in adjacent Andean zones such as the 1868 Arica earthquake and the 1746 Lima earthquake exemplify regional hazard potential. Thrust earthquakes, spontaneous slope failures, and induced seismicity from hydrocarbon activities are monitored by agencies including the International Seismological Centre and the Global Seismographic Network. Mass wasting and riverine flooding along tributaries like the Marañón River and the Madre de Dios River pose secondary hazards to infrastructure and communities governed by regional authorities such as the Municipality of Iquitos.
The belt extends through segments in southern Colombia, northern Ecuador, northeastern Peru, Bolivia, and parts of northwestern Argentina, subdivided into zones often labeled by adjacent basins: the Putumayo Sub-basin, the Maranón Sub-basin, the Huallaga Sub-basin, the Beni Sub-basin, and the Santiago del Estero Sub-basin analogs. Lithotectonic partitioning aligns with physiographic provinces like the Yungas, the Oriente, and the Bolivian Amazonia, and interfaces with structural highs such as the Fitzcarrald Arch and the Marañón Fold Belt. International cooperation on research and resource management involves entities including the World Bank, the Inter-American Development Bank, and regional universities like Universidad Mayor de San Andrés.