Cycles of Andean Mountain Building Archived in the Amazon Fan

Abstract Detrital zircons (DZs) in terrigenous sediment recovered from the late Pleistocene Amazon Fan have distinct modes of U-Pb crystallization ages and U-Th/He (ZHe) cooling ages that correlate to known South American magmatic and tectonic events. Young ZHe age modes record two phases of Andean exhumation, a late Cretaceous – Paleogene phase, and a late Miocene phase containing the highest density of ZHe age data. Nearly 40% of DZs were found to have Cenozoic ZHe ages, suggesting that the Andes are the main source of clastic detritus in the Amazon Fan. However, Andean upland sediment sources evidently contain a deep-time thermal record of mountain building. Neoproterozoic and early Paleozoic ZHe ages broadly delineate the Cambro-Ordovician Pampean and Famatanian orogenies between ~ 600–400 Ma, while a late Carboniferous to early Permian ZHe age mode (~ 275–300 Ma) likely represents a Paleozoic phase of the Gondwanan orogeny or magmatic arc activity and exhumation along the proto-Andean margin. An ~ 175–225 Ma ZHe mode records Late Triassic – Early Jurassic subduction related magmatism and deformation along the western margin of South America. We also find evidence for recycling and recent exhumation of craton aged DZs, possibly related to the broad drainage reversals experienced by the Amazon River. The first frequency analysis of detrital ZHe age data from the Amazon Fan results in two periods of crustal cooling, one at 57 Ma and another at 92 Ma (90% and 85% confidence, respectively). We attribute this periodicity in ZHe cooling age data to upper and lower plate coupling along the subduction dominated western margin of South America—and more specifically to cycles of orogenesis that include phases of magmatism, orogenic relief construction, and later destruction.