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

 E., Ozsayın, S., Üner, B., Kahraman, 2021 CC BY-SA E . Ö z s a y ı n e t a l . G e o l o g i c a A c t a , 1 9 . 6 , 1 1 4 ( 2 0 2 1 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 1 . 1 9 . 6 Quaternary subsidence records from Datça graben and Cnidus 2 must be properly separated in order to extract the effect of regional tectonics. The Mediterranean region is recording both global and relative sea-level oscillations (e.g. Haq, 1991; Hsü et al., 1973; Krijgsman et al., 1999). Several historical ruins located in the eastern Mediterranean coasts preserve records of sea-level changes, as well as sedimentological and structural records (e.g. Altunel et al., 2003; Anzidei et al., 2011). Western Anatolia, located in the eastern part of the Mediterranean region, is known to be one of the most seismically active areas in the world. This seismicity is caused by the interaction between African and Anatolian plates, which has given rise to continental extension, and basin formation (e.g. McKenzie, 1978; Şengör and Yılmaz, 1981). The Datça Peninsula, located in the southern part of Western Anatolia, is an area subjected to active tectonics and sea-level changes. It seems that several E–W trending active faults controlled the formation of the Datça graben. The Cnidus ancient cities (old and modern) were settled in the Datça Peninsula between the 7th and 3rd centuries B.C. (Bean and Cook, 1952; Grant, 1986). The Modern Cnidus city is located in the westernmost part of the Datça Peninsula at a distance of around 30km from the old city. Both cities were important coastal mercantile centres in areas of active tectonics. The most remarkable indicators are the harbours of these ancient cities, which are now situated below sea level. This study aims to investigate why the old and modern Cnidus harbours are nowadays located below sea level and to determine the geological features and rates of the driving forces causing it.

Fluctuations incompatible with global sea-level oscillations are defined as relative sea-level changes (Vail et al., 1977). Global and relative sea-level fluctuations must be properly separated in order to extract the effect of regional tectonics.
The Mediterranean region is recording both global and relative sea-level oscillations (e.g. Haq, 1991;Hsü et al., 1973;Krijgsman et al., 1999). Several historical ruins located in the eastern Mediterranean coasts preserve records of sea-level changes, as well as sedimentological and structural records (e.g. Altunel et al., 2003;Anzidei et al., 2011).
Western Anatolia, located in the eastern part of the Mediterranean region, is known to be one of the most seismically active areas in the world. This seismicity is caused by the interaction between African and Anatolian plates, which has given rise to continental extension, and basin formation (e.g. McKenzie, 1978;Şengör and Yılmaz, 1981).
The Datça Peninsula, located in the southern part of Western Anatolia, is an area subjected to active tectonics and sea-level changes. It seems that several E-W trending active faults controlled the formation of the Datça graben.
The Cnidus ancient cities (old and modern) were settled in the Datça Peninsula between the 7 th and 3 rd centuries B.C. (Bean and Cook, 1952;Grant, 1986). The Modern Cnidus city is located in the westernmost part of the Datça Peninsula at a distance of around 30km from the old city. Both cities were important coastal mercantile centres in areas of active tectonics. The most remarkable indicators are the harbours of these ancient cities, which are now situated below sea level.
This study aims to investigate why the old and modern Cnidus harbours are nowadays located below sea level and to determine the geological features and rates of the driving forces causing it.

METHODS
The ancient ruins located in the Datça Peninsula might be affected by: i) active tectonics leading to relative sealevel changes and ii) global/local sea-level fluctuations due to climatic changes. Slip-data of outcropping faults were collected for kinematic and stress inversion analysis. The relative time of fault activity has been inferred from cut-off relationship between the stratigraphic sequence and faults. Angelier's Direct Inversion method (ver. 5.42) has been used to analyse the fault-slip data (Angelier, 1991). For the characterization of the palaeostress field, the vertical/ subvertical stress axis and the R= [(σ 2 -σ 3 ) / (σ 1 -σ 3 )] ratio was taken into account (Delvaux and Sperner, 2003). Stress fields may vary according to R-ratio (Delvaux et al., 1997). Then, palaeostress fields were compared with focal mechanism solutions of the recent earthquakes occurred around the Datça Peninsula in order to check the relationship between the active faults mapped and the present-day stress field. The record of sea-level changes around the Mediterranean and Aegean regions (Vacchi et al., 2014) has been used to extract tectonic subsidence. The two ancient harbour remains have been geo-refereed using Google Earth images and bathymetry maps.

Regional geology and tectonics
The Anatolia region is affected by the northward motion of the African and Arabian plates against the relatively stable margin of the Eurasian plate (Şengör and Yılmaz, 1981;Şengör et al., 1985). This convergence is accommodated by the North and East Anatolian Fault Systems that displaced westwards the Anatolian plate (Barka and Reilinger, 1997;Şengör, 1979;Şengör and Yılmaz, 1981). The southwestern movement of the Anatolian plate towards the Aegean arc causes N-S oriented continental extension (McKenzie, 1978;Meulenkamp et al., 1988;Şengör and Yılmaz, 1981) (Fig. 1A). Different models have been proposed for the origin of this extension such as tectonic escape, back-arc spreading, orogenic collapse, and episodic two-stage graben (Bozkurt, 2000;Meulenkamp et al., 1988;McKenzie 1978;Şengör and Yılmaz, 1981;Şengör et al., 1985;Seyitoğlu and Scott, 1991). GPS measurements indicate counter-clockwise rotation of the western Anatolia region (McClusky et al., 2000;Morris and Robertson, 1993).

Records of sea-level changes in the Mediterranean region
Sea-level change is the sum of eustatic and tectonic factors. A study, performed in Italy, indicated +4m eustatic sea-level change during the Holocene for the Mediterranean region (Lambeck et al., 2004a). The Holocene relative sealevel fluctuation shows regional differences that result from N o r t h A n a t o l i a n F a u l t S y s t e m E a s t A n a to li a n F . S y s t.
Aegean-Cyprean arc 0 20 km N K ız la n Fa ul t Z on e C ni du s F. Z .
K a r a k ö y F a u l t Z o n e A C FIGURE 1. A) Simplified map showing the major plates and bounding structures around the Anatolian plate. B) Major neotectonic structures in the western Anatolian province (Şenel, 2002;Tur et al., 2015;simplified from Özsayın, 2016). C) Geological map of the Datça Peninsula showing location of the old and modern Cnidus (Dirik, 2007;Ersoy 1991).
According to the location and position of the ruins, several sea-level measurements have been carried out from neighbouring regions such as Greece, Cyprus, Israel or Lebanon as well as Turkey for the same period (e.g. Lambeck et al., 2004b;Lambeck and Purcell, 2005;Lykousis, 1991;Morhange et al., 2006;Pavlopoulos et al., 2013).

Geology of the Datça Peninsula
The units exposed in the Datça Peninsula are divided into two main groups: basement rocks of Pre-Pliocene age, and Plio-Quaternary deposits that constitute the basin-infill (Fig. 1C).
Quaternary subsidence records from Datça graben and Cnidus 5 fault zone in the north and the Karaköy fault zone in the south (Kahraman et al., 2013). Furthermore, two major faults are located close to Modern Cnidus, the Cnidus fault in the south and the Damlaca fault in the north (Fig. 3).
The N100º-trending Kızlan fault zone is about 10kmlong and formed by S-dipping normal faults (Fig. 4A). This trend can be followed for approximately 2km from the northeast of the Körmen harbour to Kızlan. At Kızlan, the fault changes its trend to the E-W and continues for about 2km eastwards, and finally it changes the strike to N122º. Fault scarps, triangular facets, alluvial fans and the lineation of the vegetation along with the fault trend evidence the recent activity of these faults (Fig. 5A). The Kızlan fault zone juxtaposes basement units and Pliocene clastics with the Pliocene marine sequence and younger units of the basin infill. Colluvium and fault breccia can be observed in the fault planes (Fig. 4B). Slickenlines are well-preserved along the basement (footwall), which have been used for paleostress analysis yielding N−S-oriented extension ( Fig. 2; Table 1).
The NW-trending Karaköy fault zone is approximately 9km-long and composed of two segments (Fig. 2). The western segment extends 3km from the Körmen harbour to Hızırşah with a trend of N097º. The eastern segment trends N117º and has a trace ca. 2km-long. At Hızırşah, the fault steps over 1 km northwards and continues 3km to the Datça harbour with a trend of N120º. The Karaköy fault zone marks a slope break and a vegetation lineament (Fig. 5B). This fault zone juxtaposes basement units with Pliocene clastic and marine rocks. A previous study indicates a NNE-SSW to NE-SW oriented extensional regime for this fault (İnce, 2019) (Fig. 2). Modern Cnidus is cross-cut by NE-and NW-trending normal faults (Fig. 3). The trace of the N050º-trending Damlaca fault has approximately 3km-long and is NWdip-slipping, with a similar trend with that the Datça fault, which is bounding the Gökova bay (Fig. 1B). At surface, fault scarps and topographic slope breaks are highlighting the fault trace. This fault juxtaposes basement units with Quaternary rocks. There are three normal faults cutting the marbles of the basement with N075º, N080º and N081º trends. These fault traces are 300 to 500m-long and also dip to the NW.
The Cnidus fault is composed of several segments with lengths ranging between 200m and 2km. These segments have N105º trend and dip steeply to the SW, juxtaposing basement units with Quaternary deposits. Fault scarps and topographic slope breaks are the expression of the fault trace (Fig. 5C). The stress inversion for this fault indicates a stress regime with maxima horizontal extension oriented NNE-SSW (Altunel et al., 2003).

Seismicity of the region
The western Anatolia province is one of the most seismically active regions in the world. Due to the subduction along the Aegean arc and related magmatism and back-arc extension processes, several destructive D a m l a c a f a u l t

C n id u s f a u lt Modern Cnidus
Bozdağ hill MEDITERRANEAN FIGURE 3. Google Earth satellite image, showing NE-and NW-trending normal faults located at the Modern Cnidus city (modified from Dirik et al., 2003;Dirik, 2007). o l o g i c a A c t a , 1 9 . 6 , 1 -1 4 ( 2 0 2 1  Quaternary subsidence records from Datça graben and Cnidus 6 earthquakes occurred which were recorded in both historical and instrumental times. Archaeological studies indicate that two major earthquakes hit Modern Cnidus in the 2 nd century B.C. and in 459 A.D. (Altunel et al., 2003;Dirik et al., 2003;Guidoboni et al., 1994). A total of 149 earthquakes around the Datça Peninsula were recorded between 1917 and 2020 with magnitudes ≥4 (Fig. 6). The focal mechanism solutions of these earthquakes fall into two main groups. In the first group the earthquakes were generated by NE-and NW-trending strike-slip faults with a subordinate normal component, whereas in the second group the earthquakes were compatible with E-W trending normal faults. The second group is mostly located in the Gökova graben bounded by the Gökova fault to the north and the Datça fault to the south (see Fig. 1B for locations). These are mostly shallow earthquakes. The most recent activity around the Gökova graben was the 21 st July 2017 Bodrum-Kos earthquake with a magnitude of Mw: 6.6.

Observations in the ruins of old and modern Cnidus
The Modern Cnidus, located at the western end of the Datça Peninsula, was one of the most important mercantile centres during the Hellenistic, Roman, and Byzantium periods in the eastern Mediterranean region (Altunel et al., 2003;Anzidei et al., 2011).  l o g i c a A c t a , 1 9 . 6 , 1 -1 4 ( 2 0 2 1  Quaternary subsidence records from Datça graben and Cnidus 7 Cnidus city (old Cnidus) was established in the early 7 th century B.C. in the centre of the Datça Peninsula, near the actual town of Datça (Bean and Cook, 1952;Grant, 1986) ( Figs. 1; 2). The city was moved to the westernmost Datça Peninsula in the 3 rd century B.C. (Bean and Cook, 1952) (Figs. 1; 4).
Two harbours in old Cnidus were used during the Hellenistic period before the 3 rd century B.C. (Altunel et al., 2003). They have important structures that directly mark the sea level at the time of construction. Harbour ruins at the first inhabited area provide an exact measure of sealevel change since 2.6kyr and 2.2kyr. The southern ruin is  l o g i c a A c t a , 1 9 . 6 , 1 -1 4 ( 2 0 2 1  Quaternary subsidence records from Datça graben and Cnidus 8 400m-long, and the northern one is 250m-long (Fig. 7). Both harbour walls which were constructed in the coastline and prolonged into the sea at that time to serve as barriers for ships, are under sea level today (Kayan, 1988a) (Fig. 8).
The base levels of the walls, originally constructed to the coastline, are standing below 2.2m for the deepest part for the northern harbour and 4.0m for the southern harbour of current sea level.
Modern Cnidus was built between the 3 rd -2 nd centuries B.C. (Bean and Cook, 1952). Two harbours were constructed, namely the northern (military) and southern (commercial). The southern harbour walls were constructed onto sandy sea floor, and nowadays are located 26-27m below sea level (Büyüközer, 2012). Although the southern wall remains in its original position, 140m long the northern wall is located at depths between 1.0-5.0m below current sea level (Büyüközer, 2012) (Figs. 9; 10).     l o g i c a A c t a , 1 9 . 6 , 1 -1 4 ( 2 0 2 1  Quaternary subsidence records from Datça graben and Cnidus 9 harbours that can be useful to study sea-level fluctuations (Anzidei et al., 2011;Flemming et al., 1973). In this case, regional sea-level changes come into prominence to distinguish the effect of active tectonics.
Taking into account the sea-level record and our observations in the old Cnidus harbours (2.2m depth-1m/1.25m below current sea level for the northern one, 4.0m depth-1m/1.25m for southern one), 0.95-1.20m of tectonic subsidence can be calculated for the northern harbour and 2.75-3.00m for the southern harbour. With these data, the calculated subsidence rates for the northern harbour are 0.36-0.46mm/yr and for the southern harbour 1.05-1.15mm/yr. The rates obtained for the northern harbour perfectly match those calculated for the Gökova graben (0.3-0.4mm/yr) (Uluğ et al., 2005). Yıldırım et al. (2016) suggested a rate of 1.93±0.3mm/yr, which is higher than our estimate for the southern harbour. Unfortunately, it was not possible to calculate the subsidence rates for the modern Cnidus harbours as the walls were constructed with rock fill technique and the depth of the sea floor at that time is not known. Nevertheless, it could be discussed the situation of the northern wall, which is below present-day sea level (unlike the southern wall), and its possible relation to the activity of the Cnidus fault (Figs. 9; 10). Both, northern and southern harbour walls are located on the hanging wall of this fault. Previous studies indicate 11.4m of vertical displacement on the Cnidus fault for the last 5.9±0.5kyr, with at least 7 earthquakes, having a recurrence period of 350yrs (Yıldırım et al., 2016(Yıldırım et al., , 2019. The fact that the northern harbour wall is below presentday sea level whereas 120m of the southern wall remain over the sea might be explain by northward tilting of the footwall along the Cnidus fault plane.
The location and seismic activity of the Kızlan, Karaköy, and Cnidus fault zones and the subsidence rates obtained from the current location of the ancient city and harbour ruins are compatible with the tectonic setting of the region.

CONCLUSIONS
The structural data of the faults that border the Datça graben compared with regional sea-level fluctuations allow the quantification of regional tectonics.
The Datça graben is an active depression bounded by the Kızlan and Karaköy fault zones.
The palaeostress analysis of the Kızlan fault zone indicates an almost N-S oriented extension, which matches with the focal mechanism solutions of the earthquakes recorded in the Gökova bay.
From the depth of the ancient harbour remnants in the old and modern Cnidus cities it can be inferred that, in addition to global sea-level hanges in the late Holocene, regional tectonics have played a major role in the local sea-level changes.
Subsidence rates of 0.36-0.46mm/yr and 1.05-1.15mm/yr are estimated for the old Cnidus harbours. Although the rates obtained from the northern harbour match those calculated in the Gökova bay, the rates determined from the southern harbour are higher. The increase in the rates from north to south might be related to the seismic/aseismic activity along the Karaköy fault zone controlling the southern margin of the graben.
Overall, our investigation highlights that ancient coastal settlements can preserve clues about sea-level changes and tectonic activity. Nevertheless, these clues should be carefully investigated in order to obtain meaningful results.