Patagonian Andean evolution from Miocene foreland basin detrital zircon geochronology (Invited)

Sedimentary basins record the erosional history of their sediment sources, and in particular, the unroofing evolution of nearby mountain belts. As such, we use the unique U-Pb crystallization age distribution of zircon crystals separated from basin sediments to assess the spatial distribution of mountain belt exhumation. The Magallanes retroarc foreland basin in southernmost Chile and Argentina developed in response to crustal loading of the Patagonian fold-thrust belt during orogenesis. As such, this basin filled with sediment during the active formation of the Patagonian fold-thrust belt. Previous work suggests that the Cretaceous basin fill was derived from the magmatic arc and fold-thrust belt, and that the growth of the thrust belt progressively isolated the basin from detrital input from the magmatic arc. Our project goal is to evaluate the rates and timing of thrust belt growth by comparing the sediment source age and exhumation record in the Paleogene-Neogene basin fill. We separated zircons from paired sandstone and volcanic ash samples collected from the syntectonic Paleogene-Neogene section of the Magallanes Basin in Chile and Argentina, Patagonia (51 30’S). Our analytical strategy involves three steps: 1) refine stratigraphic age of sandstones by U-Pb dating of nearby volcanic ashes, 2) characterize sediment source age by detrital zircon U-Pb dating. These sandstone and ash samples were prepared for zircon U-Pb geochronologic analysis using a variety of mineral separation methods. These included crushing, grinding of bulk samples, Gemini table separation of dense and light grains, and magnetic frantz removal of iron-rich minerals. A heavy liquid, lithium meta-tungstate (LMT) was used to further separate minerals, and finally, samples were handpicked under a binocular microscope. New zircon U-Pb SHRIMP age results from the volcanic ashes indicate the syntectonic sedimentary succession was deposited between ca. 21-18 Ma. These ages refine the existing stratigraphic framework for the Tertiary Magallanes Basin and suggest that significant deformation occurred in the thrust belt at that time. The detrital zircon age distributions from the sandstone samples will provide a sediment source ‘fingerprint’ ca. 21-18 Ma. Foreland basin models predict that growth of the thrust belt during crustal shortening would isolate the foreland basin from the magmatic arc. In this light, we will address whether or not the early Miocene Magallanes Basin maintained a sedimentary connection to the Miocene magmatic arc.