70-6 Selecting appropriate zircon for crustal thickness estimates: Implications for delayed faunal migration in western Anatolia

Abstract:

Recent paleoaltimetry estimates of early Miocene western Anatolia suggest high (>3.5 km) topography, which has been interpreted to have acted as a significant barrier to faunal migration. Assuming Airy isostatic balance, this elevation predicts thickened (>50 km) crust. Although thermobarometry estimates support the notion of a thickened crust at this time, no quantitative estimates of crustal thickness exist. We conducted laser ablation ICP-MS for U-Pb geochronology and trace element geochemistry on >2500 detrital zircon grains from eight samples collected from measured stratigraphic sections across western Türkiye. We applied these data to a new multivariate classification routine to determine potential rock types of detrital zircon, trained on a global data set of >20,000 published analyses. Measured stratigraphy and detrital zircon maximum depositional ages from the Gördes, Selendi, and Gediz basins are consistent with mixed fluvial-lacustrine deposition between 18 and 14 Ma. Detrital zircon U-Pb age distributions record a broad mix of 600-350 Ma and 250-200 Ma ages related to Gondwanan and Pangean orogenic episodes, and prominent early Miocene age modes interpreted to be related to synextensional magmatism during orogenic collapse. Classified Miocene zircons are consistent with the local geology and potential sediment sources, as they are inferred to be dominantly sourced from granites, felsic volcanic rocks, and lesser intermediate and mafic igneous rocks. Estimates of early Miocene crustal thickness vary based on rock type. Specifically, zircon inferred to be formed in felsic rocks yield median crustal thickness estimates of ~30 km, with corresponding elevation estimates below sea level. On the other hand, zircon inferred to be from intermediate to mafic igneous rocks give crustal thickness estimates >50 km, consistent with paleoaltimetry and thermobarometry, collectively supporting the interpretation that high topography delayed early Miocene faunal migration across western Anatolia. This study demonstrates that proper data filtering and geochemical classification of zircon are critical to obtaining realistic estimates of paleocrustal thickness.

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