Late Quaternary subsidence records from the Datça graben and Cnidus ancient city (SW Turkey): sea-level changes versus tectonics

The Datca graben in southwestern Anatolia is a WNW-trending seismically active depression, with tectonic activity since Pliocene time. This tectonic activity is controlled by normal faults, which have effected ancient settlements. The Cnidus city (old and modern) –an ancient mercantile centre during the Hellenistic, Roman and Byzantine periods– is one of the places that has recorded this activity. The ancient harbour walls of Cnidus, lying 2.2-4.0m below sea level, contain important traces about sea-level changes and tectonics over the past 2.6kyr. Palaeostress analysis along boundary faults in the Datca graben yields an almost N–S oriented pure tensional regime, compatible with earthquake focal mechanism solutions located around the Datca Peninsula. Additionally, an almost E−W trending surface rupture related to a historical earthquake in modern Cnidus, which shows normal fault characteristics, gives further support to the ongoing extension along the Kizlan, Karakoy and Cnidus fault zones. Previous studies on late Quaternary sea-level changes around the Datca Peninsula suggest that 2.6kyr ago sea level was 1.0-1.25m lower than today. From the present-day depth of the Old Cnidus harbour remains and regional sea-level records, it can be inferred that tectonics has played a significant role. Our calculations show that the Datca graben is subsiding at rates of 0.36-0.46mm/yr in the central part and 1.05-1.15mm/yr in the southern part. These values match those found in other areas around the Datca Peninsula.