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Provenance and geochronological insights into Late Cretaceous-Cenozoic foreland basin development in the Subandean Zone and Oriente Basin of Ecuador

Abstract:

The evolution of sediment source regions in the Andes of Ecuador can be linked to Late Cretaceous-Cenozoic development of a subduction-related magmatic arc, retroarc fold-thrust belt, and flexural foreland basin. Andean shortening and crustal thickening in the Eastern Cordillera and Subandean Zone generated not only topographic loading sufficient for flexural subsidence but also surface uplift of diagnostic sources that supplied sediment eastward to the Oriente foreland basin. An assessment of the chronostratigraphic, provenance, and depositional characteristics of a ~4.5-km-thick clastic succession in the Subandean and Oriente foreland basin provides insights into important Cretaceous and Cenozoic shifts in the paleogeographic framework and tectonic evolution of the Ecuadorian Andes. Detrital zircon U-Pb geochronological results for 17 samples (1845 individual ages) provide an integrated provenance framework for sedimentary fill of the Oriente Basin. The U-Pb results characterize six stratigraphic levels (Hollín, Tena, Tiyuyacu, Chalcana, Arajuno, and Mesa/Mera Formations) that range in age from late Early Cretaceous to Pliocene-Quaternary. Contributing source regions can be identified by diagnostic age populations representative of four principal tectonic provinces: (1) Western Cordillera magmatic arc (<100Ma); (2) Eastern Cordillera basement (650-250Ma), Triassic intrusions (250-200Ma), and Early and limited Late Cretaceous igneous rocks (145-66Ma); (3) Subandean Zone fold-thrust belt (200-145Ma ages accompanied by recycled signatures from Cretaceous strata); and (4) eastern cratonic regions of the Amazonian shield (>900Ma). Upsection shifts in representative U-Pb age spectra demonstrate the systematic introduction of new source regions as well as progressive recycling (cannibalization) of older basin fill incorporated into younger foreland deposits during eastward advance of the Andean fold-thrust belt. Detrital zircon geochronologic results for the uppermost Cretaceous Tena Formation, including a young population of 68.6±3.5Ma (n =3 grains), indicate a wholesale reversal from eastern cratonic sediment sources to early Andean sources that can be assigned to a Maastrichtian onset of shortening, flexural subsidence, and shift from marine to nonmarine deposition in the Ecuadorian segment of the northern Andes. A 90-68-Ma age population confirms the potential existence of a Late Cretaceous igneous complex with continental affinities, consistent with records of coeval arc magmatism reported in Colombia and southern Ecuador.