Gürsel Sunal

Palaeostress analysis of Tertiary post-collisional structures in the Western Pontides, northern Turkey

Fingerprints of the opening of the Western Black Sea Basin and collision of Pontides and Sakarya Continent along the Intra-Pontide suture can be traced in the area between Cide (Kastamonu) and Kurucasile (Bartin) in northern Turkey, along the southern coast of the Black Sea. The Western Black Sea Basin is an oceanic basin opened as a back-arc basin of the northward- subducting Intra-Pontide Ocean. Basement units related to this opening are represented by Lower Cretaceous and older units. The first arc magmatism related to this subduction began during Turonian times. Coeval with this magmatism, back-arc extension affected the region and caused development of horst-graben topography. This extensional period resulted in the break-up of continental crust and the oceanic spreading in the Western Black Sea Basin during Late Santonian times. During the Late Campanian-Early Maastrichtian period, the Sakarya Continent and Pontides collided and arc magmatism on the Pontides ended. After this collision, the Western Pontides thickened, imbricated and developed a mainly N-vergent foreland fold and thrust belt character since Late Eocene- Oligocene times. The palaeostress directions calculated from thrust faults of this foreland fold and thrust belt are 4.6°/156.6° for σ1, 6.4°/66.1° for σ2, and 83.2°/261.9° for σ3. The nature of the imbrica- tion indicates that it was a northward prograding foreland system connected to a floor thrust (detach- ment) fault at the bottom. Field observations on curved slickenfibres support the theory that the thrust faults of this imbricated structure have transformed to oblique thrusts and strike-slip faults over time.

Devonian magmatism in the western Sakarya Zone, Karacabey region, NW Turkey

The Karacabey Pluton is a large magmatic body in the northwestern Turkey overthrust by the probable Triassic metamorphic rocks of the Lower Karakaya Complex. Both the metamorphic rocks and the Karacabey Pluton are unconformably overlain by a Lower Jurassic and younger sedimentary sequence. The Karacabey Pluton was regarded as a Carboniferous intrusion based on the previous K-Ar biotite geochronological data. Here, we provide new geological, geochemical and geochronological data from the Karacabey Pluton. Zircon U-Pb results from two samples yielded ages of 393.8 +/−2.7 to 395.9 +/−4.09 Ma, suggesting that the granitoids intruded in the crust throughout the Early to Middle Devonian. The Karacabey Pluton consists mainly of biotite and locally hornblende bearing granitoid with lesser amounts of S-type leucocratic granodiorite, all of which are cut by pegmatitic bodies. It belongs to the high-K calc-alkaline series with distinct Nb and Ta anomalies in multi-element spider diagram. Sr and Nd isotopes’ initial values are 0.709–0.712 and 0.511–0.512, respectively. εNd(i) values range between −7.8 and −9.4. The isotopic characteristics of the rocks indicate lower crustal sources of both metapelitic and metaigneous origin. Geochemical features of the rocks suggest that they developed in an arc-related environment, along with the other Devonian granitoids described from the Biga Peninsula in northwest Turkey. The granitoid shows a low-temperature alteration/metamorphism marked by recrystallization of quartz, sericitization of the feldspar and formation of late chlorite, epidote and muscovite. Possibly because of these, the Ar–Ar biotite ages are scattered with a possible concentration at around Permo–Carboniferous boundary. Zircon (U-Th)/He geochronology suggests that after the granitoid was reburied during Early Jurassic to Early Cretaceous sedimentation, there was renewed uplift and erosion during the Late Cretaceous (Turonian), which is possibly related to the closure of the Intra-Pontide Ocean in the north.

The Strandja Massif and the İstanbul Zone were once parts of the same palaeotectonic unit: new data from Triassic detrital zircons

ABSTRACT Spatially continuous rock assemblages that share similar environmental evolution or structural features can be classified as a single tectonic unit. This approach enables to link dispersed units or massifs with each other and sometimes can be subjective, depending on the classification criteria. The relationship and the nature of the contact between the Strandja Massif and the İstanbul Zone have been controversial due to the Cainozoic cover. Amalgamation of these units was claimed as early as the Aptian-Albian. Lower Triassic sedimentary rocks, which are overlain by the Carboniferous flysch with a N-verging thrust fault are exposed NW of the İstanbul Zone. This study reveals the spatial relationship between the Strandja Massif and the İstanbul Zone deduced from the U-Pb dating and Lu-Hf isotopes of the detrital zircons from these Lower Triassic clastics. Our results show that the early Triassic basin was fed from a provenance that included arc-related Upper Carboniferous-Lower Permian magmatic rocks which is much more likely to be the Strandja Massif than the İstanbul Zone. The second outcome of this study is that a unit that previously assigned to Palaeozoic turned out to be Triassic, which brings the Strandja Massif farther to the east, into the northern İstanbul Zone.

The Thrace Basin and the Black Sea: the Eocene–Oligocene marine connection

The Late Cretaceous – Recent West Black Sea Basin and the Eocene–Oligocene Thrace Basin are separated by the Strandja arch comprising metamorphic and magmatic rocks. Since Late Cretaceous time the Strandja arch formed a palaeo-high separating the two basins which accumulated clastic sediment of >9 km thickness. During late Eocene – early Oligocene time the marine connection between these basins existed through the Catalca gap west of Istanbul. The Catalca gap lies on the damage zone of a major Cretaceous strike-slip fault; it formed a 15 km wide marine gateway, where carbonate-rich sediments of thickness c. 350 m were deposited. The sequence consists of upper Eocene shallow marine limestones (SBZ18-20) overlain by upper Eocene – lower Oligocene (P16-P19 zones) pelagic marl with a rich fauna of planktonic foraminifera; the marls are intercalated with 31–32 Ma acidic tuff and calc-arenite beds. The Catalca gap is bounded in the west by a major normal fault, which marks the eastern boundary of the Thrace Basin. Seismic reflection profiles, well data and zircon U–Pb ages indicate that the Thrace Basin sequence west of the fault is late Eocene – middle Oligocene (37–27 Ma) in age and that the fault has accommodated 2 km of subsidence. Although there was a marine connection between the West Black Sea and Thrace basins during late Eocene – early Oligocene time, no significant exchange of clastic sediment took place. Sedimentation in the Catalca gap ended abruptly during early Oligocene time by uplift, and this eventually led to the paralic conditions in the Thrace Basin.