Detrital provenance of modern rivers and stable isotope paleoaltimetry of quaternary volcanic glasses in the northern Andes of Ecuador and Peru

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2019-05

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Advances in paleoaltimetry and detrital provenance techniques provide opportunities to generate important new constraints on past elevation, climate, and erosion that enhance reconstructions of mountain building and landscape evolution. Stable isotope paleoaltimetry uses the ratio of hydrogen or oxygen isotopes (δD or δ¹⁸O values, respectively) of paleometeoric water as an indicator of paleoelevation. Deuterium (δD) values in Ecuadorian Quaternary volcanic glasses hydrated by paleometeoric water provide validation of the volcanic glass stable isotope proxy. Deuterium values in late Pleistocene- Holocene (<200 ka) volcanic glasses deposited over a 4 km elevation range spanning the Pacific coastal forearc and Andean magmatic arc of Ecuador (0°–1.5°S) record systematic isotope-elevation relationships. Glasses deposited under different climate states (i.e., cool glacial vs. warm interglacial) exhibit values that are offset from modern water values, suggesting that they recorded different isotopic lapse rates. An elevation reconstruction with volcanic glass δD values incorporating estimated changes in surface air temperatures between glacial and interglacial climates yields accurate elevations. Along the Andean convergent margin of Ecuador, distinct tectonic provinces with contrasting bedrock signatures were surveyed by U-Pb geochronological analyses of zircons in sand-size sediment from modern rivers. Sand was collected from 12 rivers across the forearc, magmatic arc, internal (hinterland) and external (foreland) segments of the fold-thrust belt. U-Pb geochronological results show diverse ages spanning the Precambrian through the Cenozoic and reveal that distinctive age distributions with characteristic age populations clearly define unique provenance signatures for the various tectonic provinces. For one large catchment, variations in U-Pb age distributions highlight a complex downstream propagation of provenance signal. The Marañón River in northern Peru is a major tributary to the Amazon River that runs axially through the mountainous Andean region between the Western and Eastern Cordilleras. New zircon U-Pb ages from unconsolidated river sands from the Marañón River and several of its tributaries characterize modern provenance signatures of the northern Peruvian Andes. When compared to existing ages from consolidated Phanerozoic sedimentary and igneous rocks in the catchment, river sand age spectra reveal a representative picture of the geology in the upstream drainage area.

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