Date of Completion
geochronology, thermochronology, detrital, Patagonian Andes, basin, geochemistry, Magallanes
Julie C. Fosdick
Field of Study
Doctor of Philosophy
Retroarc foreland basins are important archives of continental arc magmatism, upper plate deformation, and sedimentation processes that control the evolution of continental lithosphere. Synorogenic foreland basin deposits record orogenic growth, magmatism, paleofluid flow, and sediment provenance histories that link the orogen-to-basin routing system. This dissertation contributes new understanding of these processes in the Magallanes-Austral Basin (MAB) during Mesozoic to Cenozoic sedimentation in southern South America.
In Chapter 1, I investigate the mafic igneous sources to the anomalously mafic-rich lower Miocene Río Guillermo Formation and identify Neogene changes in arc magmatism during multiple ridge subduction events in the Patagonian Andes. Whole rock 40Ar/39Ar geochronology and major and trace element geochemistry data from volcanic clasts reveals latest Oligocene to early Miocene eruptive ages (∼25-22 Ma) of mafic arc-derived volcanics, indicating syndepositional eruption during the ancestral Río Guillermo fluvial sedimentation.
In Chapter 2, I model 160 m.y. of tectonic basin subsidence and sedimentation rates through the Mesozoic to Cenozoic MAB based on a new compilation of stratigraphic data spanning 48-54 °S. This work quantifies along-strike changes in the backstripped tectonic subsidence versus sedimentary loading in relationship to key geologic events in the Patagonian Andes. Subsidence modeling corroborates Late Jurassic – Cretaceous spatially diachronous changes in tectonic regime and elucidates a phase of Late Cretaceous tectonically driven basin uplift that may explain major changes in foredeep sedimentation style.
Finally, in Chapter 3, I explore mechanisms of basin heating and sediment-source exhumation to resolve the influence of heating in relation to the Paleogene Unconformity, an important basin- wide feature within the MAB. Using detrital (U-Th)/He thermochronology, vitrinite reflectance analysis (%Ro), and carbonate clumped isotope paleothermometry (Δ47), I model post- depositional heating and ‘reheating corrected’ sediment-source thermal histories. Independent thermal indices correlate well and indicate maximum temperatures that depart from a conventional down-section temperature increase within sedimentary basins. Temperature estimates from %Ro and Δ47 data suggest an influx of stratigraphically controlled hydrothermal fluid flow that post-dates late Eocene deposition and may account for differences in predicted reheating temperatures from sedimentary overburden alone. Modeling results indicate Paleocene to Eocene and latest Oligocene to early Miocene reheating associated with greater overburden, hydrothermal fluid flow, and/or slab window passage.
VanderLeest, Rebecca, "Detrital Geothermochronology, Geochemistry, and Subsidence Analysis of Tectonics and Sedimentation in the Magallanes-Austral Basin, Patagonian Andes" (2020). Doctoral Dissertations. 2591.
Available for download on Thursday, July 31, 2025