High-elevation western Anatolian topography delayed faunal migration during the early Miocene

Anatolia is a major thoroughfare for faunal migration and its paleogeography impacted faunal dispersals from, and to, Africa, Europe, and Asia. For example, the first appearance of hominoids in central Anatolia was 2-6 Myr after the formation of the "Gomphotherium Landbridge" according to fossil records, yet the arrival of hominoids at a far more distant location in China occurred only 1-2 Myr after the formation of the landbridge. Furthermore, in the early Miocene, the populations of small mammals in Europe and Anatolia differed greatly. Mineral barometry-based crustal thickness calculations and Airy isostatic considerations suggest paleoelevations of 3.5–4.1 km in early Miocene western Anatolia. This presents the possibility that the observed delays in faunal dispersion and differences in faunal populations were the result of topographic barriers in western Anatolia. Nevertheless, the hypothesis that high elevations posed migration barriers in western Anatolia lacks supporting paleoelevation data. To test the hypothesis, we first established a new geochronological model for the Gördes Basin based on U-Pb ages from sandstones and tuffs collected from new stratigraphic sections and then measured hydrogen isotopic ratios of 13 volcanic glass samples and oxygen isotopic ratios of 28 carbonate samples from that basin. The onset of the sedimentation of the Gördes Basin at 18-19 Ma based on both maximum depositional ages (sandstone) and true depositional ages (tuffs) is younger than previously estimated at 21-20 Ma. We calculated the paleowater isotopic compositions with standard isotopic fractionation during precipitation and a 15°C precipitation temperature for CO3. Volcanic glass samples have δDpaleowater(pw) values ranging from -113.7 to -67.5‰ and δ18Opw values ranging from -12.9 to -6.1‰. Hydration by primarily ambient waters rather than magmatic water is indicated by a slight negative trend between δD and weight percentage H2O. The analysis of the δ18O and δ13C of alluvial carbonate samples and microphotographs demonstrate that they are not diagenetic. Paleoelevation was calculated using alluvial carbonate materials and volcanic glass samples with wt% H2O> 2. A 16 Ma paleosol sample in a marginal marine environment was chosen as a low-elevation baseline for determining Miocene paleoelevations. Calculated paleoelevations of 19-16 Ma western Anatolia are 3.6 ± 0.7 and 4.3 ± 0.9 km (1σ), based on the most negative δ values of -12.9‰ and -113.7‰ for δ18Opw and δDpw, respectively. Paleoelevations calculated based on the most negative quartile are 3.2 ± 0.5 km and 3.9 ± 0.6 km (2σ) for δ18Opw and δDpw, respectively.We conclude that the early Miocene topography in western Anatolia was approximately 2-3 km higher than the current topography, based on independent oxygen and hydrogen isotopic compositions of carbonate and volcanic glass paleoelevation proxies. Moreover, independent estimations based on Airy isostacy agree with the calculated paleoelevations. These factors together support the model of extreme early Miocene paleoelevations in western Anatolia and the hypothesis that early Miocene faunal dispersal was hampered by high relief. If that is the case, extensional deformation throughout the Miocene-Pliocene could cause a decrease in paleoelevation and an establishment of faunal migration corridors in the western Anatolia.