DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF MODERN RIVER SANDS IN THE ECUADORIAN ANDES: IMPLICATIONS FOR TECTONIC HISTORY AND SEDIMENT RECYCLING

Proposed reconstructions of the Ecuadorian Andes involve contrasting estimates of the timing and style of key orogenic events. U-Pb detrital zircon geochronology of unconsolidated sands from modern rivers with catchment areas spanning different morphotectonic regions yield diagnostic U-Pb age spectra for those regions. Thus, detrital zircon age spectra provide insight into tectonic history by means of elucidating sediment provenance and recycling. We present 1371 new detrital zircon U-Pb ages from 13 samples of unconsolidated sands from 10 rivers in Ecuador that show distinct U-Pb age spectra for various morphotectonic regions. (1) The Oriente Basin is a Cretaceous–Quaternary retroarc foreland basin that is flat-lying in the east (Oriente) with localized uplifts in the west (Subandean Zone). Jurassic arc rocks are restricted to the Subandean Zone, and together with Cretaceous-Neogene sediments make up the dominant surface exposures. River sands in the Oriente-Subandean Zone show a distinctive Jurassic U-Pb age peak, as well as Triassic and Cretaceous age peaks. (2) The Eastern Cordillera contains metamorphosed Paleozoic-Mesozoic rocks and non-metamorphosed Triassic to Jurassic igneous intrusions. Sands from rivers in the Eastern Cordillera have substantial U-Pb age populations of Proterozoic and Paleozoic ages, representing recycled cratonic material. Paleogene-Neogene populations also appear in Eastern Cordillera samples. (3) The Western Cordillera contains Late Cretaceous basement rocks of oceanic plateau origin and intraoceanic island arc sequences, together with Upper Cretaceous-Paleogene sedimentary rocks and Neogene-Quaternary volcanic cover. River sands from the Western Cordillera contain Neogene-Quaternary age populations with minor Triassic and Cretaceous ages. River samples draining Paleogene units in the easternmost Western Cordillera show Proterozoic and Paleozoic ages indicative of exhumation, erosion, and westward transport of detritus from the Eastern Cordillera. Furthermore, the dominant Proterozoic and Paleozoic ages that characterize the Eastern Cordillera rivers provide support for multiple recycling events prior to the current tectonic configuration of the Ecuadorian Andes.