UPLIFT OF THE SOUTHERN CENTRAL ANATOLIAN PLATEAU FROM 87Sr/86Sr STRATIGRAPHY AND CRN DATING OF RIVER TERRACES

The Central Anatolian Plateau is at the crux of one of the most complicated tectonic regions on Earth, bounded by the Aegean extensional province to the west, enigmatic subduction to the south, the Bitlis-Zagros collisional zone to the east, and the continental-scale North Anatolian Fault Zone (NAFZ) to the north. Although several mechanisms have been proposed to explain the high elevation of the Eastern Anatolian Plateau, little attention has been given to the Central Anatolian Plateau. Critical for understanding the mechanism of surface uplift and how it fits within the regional tectonic setting is determining the temporal framework and pattern of surface uplift. The southern margin of the plateau, which is extensively overlain by Miocene marine sediments uplifted to ~2 km elevation and contains well-preserved fluvial terraces within the Mut Basin, offers excellent potential to explore the history of surface uplift.

Our data provide constraints on uplift that followed initial deformation and uplift of the Tauride belt along the southern plateau margin. We present ⁸⁷Sr/⁸⁶Sr data on the highest and youngest marine sediments that presently cap the margin, and onlap older Tauride units at the southeastern and southwestern edges of the margin. Analyzed samples, including oysters and foraminifera, cover a region of > 150 km along the southern margin, from the Antalya Basin to the Mut/Ermenek Basin. Measured ratios average 0.708888 in the west (1450 m elevation), 0.708832 at the middle site (1885 m elevation, ~200 m below the highest marine sediments), and 0.708813 at our eastern-most site (1845 m elevation). Comparing these values to the global sea water ⁸⁷Sr/⁸⁶Sr curve (LOWESS IV) suggests ages of ~10 Ma in the west, ~12.5 Ma at the middle site, and ~13 Ma in the east. However, a potential complication concerns the Miocene Mediterranean sea water ⁸⁷Sr/⁸⁶Sr curve, which shows a major departure from the global ⁸⁷Sr/⁸⁶Sr curve when the Mediterranean became restricted from global ocean circulation (e.g., McCulloch and De Deckker, 1989). This interpretation of the Mediterranean Sr isotope curve would imply ages of ≤ 10 Ma at the western site, ≤~8 Ma at the middle site, and ≤ ~7 to 8 Ma at the eastern site. If the latter interpretation of our Sr measurements is correct, which is suggested by biostratigraphic constraints from the same sections, this indicates a post-middle Tortonian phase of uplift that was nearly simultaneous along the southern margin of the plateau, or slightly earlier in the west.

We suggest a regional tectonic framework that can explain this uplift pattern. Initial westward extrusion of the Anatolian microplate likely led to localized uplift along restraining regions of the bounding faults, such as the nearly N-S striking Kirkkavak fault near Antalya. Later uplift across the southern margin may have resulted from a change in the deformation pattern, with Central Anatolia undergoing counter-clockwise rotation. Such a pattern of modern deformation is supported by paleomagnetic rotation data as well as GPS velocity data. This deformation pattern should have produced a compressional or transpressional regime throughout the southern margin.

To determine the most recent uplift history, we analyzed cosmogenic exposure ages of fluvial clasts preserved on river terraces of the Göksu River (Mut Basin). Sampled terraces range from ~30 to 140 m above the present river. Model exposure ages of chert clasts from the terraces are derived from cosmogenic ²¹Ne and ¹⁰Be measurements, while exposure ages of carbonate clasts are calculated from ³⁶Cl measurements. Initial ²¹Ne results suggest a model exposure age of ~160 ka for the upper terrace level, however, complications in the interpretation of the age with respect to initial inherited cosmogenic nuclides and surface erosion will be tested as additional analyses are completed. Nonetheless, if our initial model ages are even approximately correct, this implies that the latest Quaternary was characterized by a relatively rapid phase of uplift. This more rapid uplift was probably driven by different mechanisms, which may have included deeper crustal or lithospheric processes.