A DEEP-SEA RECORD OF SOUTH AMERICAN TECTONICS AND SURFACE PROCESSES: INSIGHTS FROM DETRITAL ZIRCON DOUBLE DATING FROM THE AMAZON SUBMARINE FAN

Deep-sea fans have been shown to record the physical and chemical signatures of their linked continent-scale river catchments. Quantitative assessments of sediment provenance, sediment recycling, and rock exhumation can be acquired from terrigenous sediment in deep-sea fans using paired detrital zircon (DZ) U-Pb and U-Th/He (ZHe) double dating techniques. Here, we target the late Pleistocene (marine isotope stage 2 – 6) Amazon fan (ODP Leg 155), offshore northeast South America, to garner new insights into current and ancient continental tectonics and surface processes in the Amazon basin, and the potential implications for the formation of the South American passive margin.

U-Pb DZ ages, and thus their associated ZHe cooling ages, reflect the distribution of sand-sized material transferred from the Amazon river to the deep-sea fan. Measured ZHe ages have distinct populations that broadly correlate to known South American orogenic events since the Phanerozoic. Small populations of early Paleozoic ZHe ages may broadly delineate the Pampean and Famatanian orogenies between ~550 – 400 Ma, while a larger population of Permian ZHe ages (~275 – 300 Ma) delineates a temporal relationship to the Gondwanan orogeny. The largest population of ZHe ages represents Andean exhumation beginning between ca. 70 – 50 Ma with a major increase in rate between 10 – 20 Ma. Neogene ZHe ages measured in old U-Pb age populations (Mesozoic, Paleozoic, and Precambrian) not only delineate the Andes as a major source of Amazon fan sediment, but also illustrate the common occurrence and importance of sediment burial and exhumation (recycling) within the overall Andean orogenic system. The presence of significant Andean-derived zircons, and by proxy sand sized terrigenous sediment, in the deep-sea fan suggests that sediment supply to the Amazon margin via exhumation of the eastern cordillera and Andean foreland since the Neogene, has been an important control on resultant stratigraphic architecture and evolution of the South American Atlantic margin. These new interpretations based on DZ double dating from the Amazon fan are consistent with recently published interpretations of efficient sediment throughput from Andean source to Atlantic sink during the last two glacioeustatic lowstands.