CENOZOIC EVOLUTION OF THE LATACUNGA-AMBATO BASIN DURING ANDEAN SHORTENING AND ARC MAGMATISM WITHIN THE WESTERN CORDILLERA AND INTERANDEAN VALLEY OF ECUADOR

A stratigraphic and structural investigation of the Western Cordillera and Interandean Valley of Ecuador helps clarify the effects of Andean crustal structures and subduction of the Carnegie Ridge on Cenozoic basin evolution. Paleogene and Neogene successions show contrasting depositional systems. The Paleogene Angamarca Group of the Western Cordillera recorded transitions from early-middle Eocene marine turbidite deposition (Apagua unit) to late Eocene-early Oligocene marine and then nonmarine fan delta to alluvial fan conditions (Rumi Cruz unit). Detrital zircon U-Pb geochronological data indicate a mixed or composite provenance consistent with derivation from both a proximal magmatic arc in the Western Cordillera and distal bedrock sources in the Eastern Cordillera. The uppermost Angamarca Group (Rumi Cruz unit) is unconformably overlain by the middle Miocene-Pliocene Zumbagua Group. This >10 Myr hiatus is related to nondeposition and/or erosion in the Western Cordillera, potentially due to uplift associated with increased shortening and magmatism. The Zumbagua Group is affiliated with multiple igneous units, including volcanic flows and intrusions that represent Miocene arc magmatism in the Western Cordillera. Proximal volcanic facies of the Zumbagua Group include lahars and debris flows derived from adjacent volcanoes. Less abundant are fluvial and lacustrine facies developed farther downslope from igneous source areas. The Zumbagua Group represents principally mass-flow processes of an intra-arc basin within a magmatic arc that spanned from at least 15 to 6 Ma based on detrital zircon U-Pb age distributions. Following Zumbagua deposition, development of the Latacunga-Ambato Basin was directly linked to upper crustal shortening in the Interandean Valley. Growth structural relationships involve steep monoclines documented above east- and west-dipping basement structures that compartmentalized the Latacunga-Ambato Basin. This phase of Pliocene-Quaternary shortening was accompanied by a transition from fluvial-lacustrine conditions to widespread emplacement of late Quaternary volcanic ashflow units. Further detrital zircon U-Pb geochronological analyses will improve assessments of sediment provenance in the Western Cordillera and Interandean Valley, with emphasis on the age of key growth strata in the Latacunga-Ambato Basin.