OROGENIC COLLAPSE AND EVOLUTION OF THE WESTERN ANATOLIA MIGRATION CORRIDOR

Fossil records suggest a delay of 2–3 Myr between the establishment of the “Gomphotherium Landbridge” and the dispersal of hominoids to central Anatolia, but only 1-2 Myr between the establishment of the landbridge and the appearance of hominoids at a far more remote location in China. In addition, vast differences existed between the small mammal populations of Europe and Anatolia in the early Miocene. Airy isostacy, applied to crustal thicknesses based on mineral barometry and structural restorations, indicates elevations of 3.5–4.1 km in western Anatolia during the early Miocene. These observations lead to the hypothesis that extreme elevations in western Anatolia presented migration barriers in the early Miocene.

To investigate how high western Anatolia was in the early Miocene, we collected δD data from 14 volcanic glass samples and δ¹⁸O data from 18 carbonate samples. Paleowater isotopic compositions were calculated assuming standard isotopic fractionation during precipitation and a 15°C precipitation temperature for CO₃. δ¹⁸O_{paleowater(pw)} values range from -6.1 to -12.9‰ and δD_pw values of volcanic glass samples range from -113.7 to -68.7‰. A weak negative trend between δD and wt% H₂O reflects hydration by dominantly environmental waters instead of magmatic water. The δ¹⁸O, δ¹³C, and microphotographs of alluvial carbonate samples show that these samples are not diagenetic. Volcanic glass samples with wt% H₂O> 2 and alluvial carbonate samples were used to calculate paleoelevation. We used a 16 Ma paleosol sample formed in a marginal marine environment as a low-elevation baseline for calculating Miocene paleoelevations. The most negative values of the δ¹⁸O_pw and δD_pw are -13.2‰ and -113.7‰, which yield elevations of 3.6 ± 0.7 and 4.3 ± 0.9 km, respectively. These results from independent isotopic systems confirm that early Miocene topography in western Anatolia was ~3 km higher than the modern. Furthermore, the calculated elevations are consistent with independent estimates based on Airy isostactic considerations. All of these are consistent with the hypothesis that elevated topography posed formidable barriers to faunal dispersion in the early Miocene and may explain the delay in faunal dispersion in western Anatolia. If that is the case, a Miocene-Pliocene decrease in elevation which accompanied extension may have produced faunal migration corridors.