DETRITAL ZIRCON U-PB AND FISSION TRACK DOUBLE DATING FROM THE AMAZON FAN RECORDS EARLY PHANEROZOIC TO CENOZOIC TECTONIC EVENTS AND RECENT EXHUMATION RATES OF THE ANDES

Detrital geo-thermochronology is a useful tool to understand patterns of erosion, sediment transport, and tectonic-sedimentary system interactions through time. Despite the ubiquity of the technique, it is unclear to what extent detrital geochronologic data sets from large river to submarine fan systems capture geologically recent to long term regimes of erosion and tectonic history of their catchments. Here, we present double dating (U-Pb and zircon fission track (ZFT)) of detrital zircons (DZ) from four samples (n = 400 individual double dates) of turbidite sands recovered from cores from the late Pleistocene Amazon fan (ODP Leg 155). These new DZ U-Pb ages are similar to those previously published, and correspond to recognized tectonic and magmatic events 6565 of South America. New ZFT age data reveal prominent modes at ~525 Ma, at ~260 Ma, and at ~40 Ma. The distribution of ZFT ages likely corresponds to major tectonic cycles affecting South America since the Proterozoic to late Cenozoic time, and thus we interpret these patterns in ZFT ages as the result of increased cooling related to tectonic exhumation. The oldest mode suggests increased exhumation during the early Phanerozoic (~525 Ma) related to the late Pampean and Famatinian orogenies. A late Permian through Triassic (~260 Ma) mode likely records the Gondwanide orogeny, and a late Cenozoic mode (~40 Ma) is likely the result of the construction of the modern Andes. In addition to linking ZFT ages to known orogenic events, we also can interpret exhumation rates in the Andes over late Cenozoic timescales using the youngest, non first-cycle volcanic zircons, which yield nearly identical exhumation rates to those derived from 10Be concentrations in modern river sediments. The results of this study agree with previous U-Pb and U-Th/He double dating from the Amazon fan, and suggest that the Amazon fan records geologically recent and long term tectonic events of South America.