UPLIFT AND EROSION HISTORY OF THE EASTERN CORDILLERA IN NW ARGENTINA FROM DETRITAL GEOCHRONOLOGY OF THE FORELAND BASIN SEDIMENTS OF THE NEOGENE ORAN GROUP EXPOSED ALONG THE RÍO IRUYA

Foreland basin sediments in northwestern Argentina preserve a record of the eastward extension, uplift and erosion of the eastern margin of the high topography of the Puna plateau and Eastern Cordillera. We describe new sedimentologic and geochronologic data from the Neogene Oran Group at 23°S exposed along the Río Iruya to reveal the timing and effects of growth of the modern plateau margin. Traditional sedimentologic approaches, including conglomeratic clast counts, sandstone point-counts, and X-ray diffraction of clay minerals are unable to document provenance changes over the past 12 Ma, but we show that detrital zircon U-Pb age distributions preserve statistically-robust signatures that track changes in sediment source. We argue that significant growth of the modern plateau edge commenced between 12 and 7 Ma and erected a topographic barrier that cut off the supply of sediment from sources west of the modern Eastern Cordillera. Erosion of this rising block has progressively dissected a cover of early Paleozoic rocks, leading to deep incision into the Precambrian metasediments of the Puncoviscana Formation that continues today. We also present apatite and zircon fission-track data from both the Oran Group and modern streams that document erosion of the frontal block. Analytical modeling of these data that accounts for both a transient thermal field and the cooling-rate dependence on closure temperature suggests an average erosion rate of this block of about 0.6 mm/a over at least the past 6 Ma.