Sakarya Zone Research Articles

Late Cretaceous-Eocene Geological Evolution of the Pontides Based on New Stratigraphic and Palaeontologic Data Between the Black Sea Coast and Bursa (NW Turkey)

The Late Cretaceous-Eocene geological evolution of northwest Turkey between the Black Sea and Bursa was studied through detailed biostratigraphic characterization of eleven stratigraphic sections. The Upper Cretaceous sequence in the region starts with a major marine transgression and lies unconformably on a basement of Palaeozoic and Triassic rocks in the north (İstanbul-type basement) and on metamorphic rocks and Jurassic sedimentary rocks in the south (Sakarya-type basement). Four megasequences have been differentiated in the Late Cretaceous-Eocene interval. The first one, of Turonian to Late Campanian age, is represented by volcanic and volcanoclastic rocks in the north along the Black Sea coast, and by siliciclastic turbidites and intercalated calcarenites in the south, corresponding to magmatic arc basin and fore-arc basin, respectively. A major ridge along the present southern margin of the Kocaeli Peninsula separated these two realms. In the Late Campanian, volcanism and clastic sedimentation gave way to the widespread deposition of the pelagic limestone and marl of the Akveren Formation; only in the extreme south near Bursa are the pelagic micrites of the Akveren Formation replaced by calciturbidites and siliciclastic turbidites. The age of the Akveren Formation ranges from Late Campanian to Late Palaeocene. The third megasequence is a thick flysch wedge of Early Eocene age, which extends from north of Bursa to the Black Sea coast. The base of the Lower Eocene flysch is marked by a major unconformity. The flysch wedge marks the collision between the Pontides and the Anatolide-Tauride Block. The fourth megasequence is a thick volcanic and volcaniclastic series of late Early to Middle Eocene age, which extends from north of Bursa to the northern margin of the Armutlu Peninsula. The coherent Upper Cretaceous-Eocene stratigraphy, the laterally traceable facies belts, absence of ophiolitic slices and high pressure metamorphic rocks in the Upper Cretaceous-Tertiary series in the region between the Black Sea and Bursa indicate pre-Santonian juxtaposition of the İstanbul and Sakarya zones.

Origin of the Early-Middle Devonian magmatism in the Sakarya Zone, NW Turkey: Geochronology, geochemistry and isotope systematics

A unique example of isotopically-dated Devonian metagranitoid (the Çamlık metagranite; Okay et al., 1996) crops out in the Biga Peninsula, NW Turkey, although its contact relationships with the country rocks and geodynamic setting have remained to be enigmatic so far. Our field work, however, has shown that a number of metagranitoid bodies similar to the Çamlık metagranite intruded the country rocks and developed contact metamorphic zones, consisting of andalusite and calcsilicate hornfelses, garnet–epidote and diopsite–wollastonite skarns. The country rocks of these metagranitoids are made up of regionally metamorphosed metaclastic successions with subordinate metacarbonate–metachert–metabasites (the Kalabak formation), intercalated with tectonic slices of meta-serpentinites. The metagranitoids and the Kalabak formation are collectively termed here the Havran Unit which forms a NE–SW trending, 20 km wide and 80 km long belt in the Biga Peninsula. The Havran Unit is unconformably overlain by the Late Triassic shallow marine sediments and is in tectonic contact with the Permo-Triassic Karakaya Complex, interpreted as the Palaeotethyan subduction–accretion complex. U–Pb SHRIMP-II and LA–ICP–MS dating of the zircons from four individual plutons yielded crystallization ages ranging from 389.1 ± 2.6 to 401.5 ± 4.8 Ma (i.e. Early to Middle Devonian). Based on their geochemical characteristics, the metagranitoids are divided into two groups named the Çamlık and Yolindi metagranitoids. Both granitoids display distinct subduction signature and plot in the volcanic arc granite and post-collisional granite fields on tectonic discrimination diagrams. 87Sr/ 86Sr ( T ) values of these intrusions vary between 0.707367 and 0.715588 while their 143Nd/ 144Nd ( T ) values range from 0.51166 to 0.51187. Very low values of εNd ( T ) (from −5.3 to −9.1) imply that the Lower to Mid-Devonian metagranitoids in the Havran Unit might have been generated by partial melting of lower continental crust, probably driven by a major magma underplating event. Delamination and/or slab-breakoff models are considered to be the most plausible mechanisms for the genesis of these metagranitoids. The data obtained in this study has shown for the first time the widespread existence of Devonian and older basement in NW Turkey. Regional geological correlations imply that this basement may be an exotic terrane in the Sakarya Zone, emplaced during the Late Triassic, prior to its amalgamation with the Palaeotethyan Karakaya Complex.

Early abyssal- and late SSZ-type vestiges of the Rheic oceanic mantle in the Variscan basement of the Sakarya Zone, NE Turkey: Implications for the sense of subduction and opening of the Paleotethys

The boundary between the Gondwana and Laurussia, or a terrane separated from it, is traced by a suture recording the closure of the Rheic ocean in the Pulur, Beyçam and Kurtoğlu areas of the eastern Sakarya Zone of Turkey. Pulur lherzolite has a lower Cr# [100Cr/(Cr+Al)=5−37] of the spinel and slightly enriched flat medium-heavy rare earth element (M-HREE) whole-rock pattern typical for those of the fertile abyssal-type lherzolite. By contrast, the Beyçam harzburgite displays the mineralogical and geochemical features of a moderately depleted residue at a supra subduction zone (SSZ). Some of spinels from the Beyçam peridotites show TiO2 contents (0.15–0.48wt.%) identical to a later interaction with a melt of island-arc tholeiite (IAT) composition. Meta-basalt from the Beyçam area is the representative of such a melt and displays the whole-rock M-HREE pattern of a complementary product of the Beyçam harzburgite.Data presented in this study support the primarily northward directed closure of the Rheic ocean, but suggest that the Variscan basement in the eastern part of the Sakarya Zone consists of mantle peridotites formed in a south-vergent SSZ. This dual polarity of subduction in the Rheic ocean, at least in the second half of its closure, is also favored by the geological structure and lithological diversity of the Variscan rocks. These are: (1) spatial distribution of the peridotites, e.g., SSZ-type harzburgites to the north and abyssal-type lherzolites to the south, (2) lack of high-pressure metamorphic rocks, (3) decrease in the degree of Variscan metamorphism toward the north, and (4) emplacement of Variscan syn-collisional magmatism into the low grade metamorphic rocks to the north.Paleotethys was the other ocean of the region created in the Late Paleozoic. Northward subduction of the Rheic ocean from Early Devonian to Early Carboniferous appears to be the most plausible mechanism responsible for the opening of Paleotethys. In this scenario the Paleotethys is envisaged as a back-arc basin opened to the north in the wake of separated terranes from Laurussia. Onset of trench-Rheic mid-ocean ridge collision in the Early Devonian along the northern margin of Laurussia has been proposed to account for the initiation of SSZ-type subduction to the south. The short duration of this subduction appears insufficient to open a fully developed back-arc basin to the south because the elimination of Rheic mid-ocean ridge to the north accelerated the oceanic closure.

Middle Eocene paleomagnetic data from the eastern Sakarya Zone and the central Pontides: Implications for the tectonic evolution of north central Anatolia

A paleomagnetic investigation has been carried out on Middle Eocene volcanic rocks at 23 sites in the eastern Sakarya Zone and the central Pontides of north central Anatolia in order to better understand the regional tectonic evolution of the main zones of northeastern Anatolia in the Middle Eocene. The results constrain the tectonic evolution of the area when evaluated in conjunction with earlier published paleomagnetic directions from 92 localities. Small counterclockwise rotations in the range of R ± ΔR = −1.3 ± 1.4° to R ± ΔR = −17.2 ± 16.8°, with respect to Eurasia, are observed in the central Pontides on the northern side of the North Anatolian Fault (NAF). Clockwise rotations ranging between R ± ΔR = 8.5 ± 15.1° and R ± ΔR = 29.7 ± 12.0° are observed between the NAF and the Sungurlu Fault (SF), in the eastern Sakarya zone. Counterclockwise rotations of R ± ΔR = −18.9 ± 12.4° to R ± ΔR = −42.2 ± 6.9° are observed to the south of the SF. Our findings, combined with previous data, support the indentation model of Kaymakcı et al. (2000, 2003a, 2003b), which postulates the collision and northeast directed indentation of the Kırşehir Block into the Sakarya Zone. This model, which was developed to explain the evolution of the Çankırı Basin, can also explain the variable magnitudes of paleomagnetically determined rotations, thrust directions, and the curvature of the North Anatolian ophiolite belt.

Tectonic setting of the Jurassic bimodal magmatism in the Sakarya Zone (Central and Western Pontides), Northern Turkey: A geochemical and isotopic approach

The Lower to Middle Jurassic Mudurnu formation of the Sakarya Zone (Northern Turkey) was deposited in an extensional basin. This unit crops out along the southern Pontide range and consists of marine sedimentary rocks including debris flows, lignite-bearing clastic rocks and Ammonitico Rosso horizons alternating with mafic and felsic volcanic and volcaniclastic rocks. Magmatic rocks of the Mudurnu formation comprise two compositionally different groups; 1) a mafic group including diabase–microgabbro–basaltic lavas and their pyroclastic equivalents, and 2) a felsic group including granite porphyries and felsic pyroclastic rocks. All the magmatic members of the Mudurnu formation are subalkaline and display a calc-alkaline affinity. They are bimodal, with a significant silica gap between the mafic and felsic members with the exception of a few samples. These magmatic rocks display enrichment in LILE and depletion in Nb, Ta, P and Ti, implying a subduction-related magmatic signature. Melting modelling for the mafic rocks indicates that they originated possibly from subcontinental lithospheric mantle (SCLM) composed of spinel lherzolite. εNd(i) values (+ 1.5 to + 4.3) imply that the mafic volcanic and hypabyssal rocks were possibly derived from a time-integrated LREE-depleted mantle source. The initial Sr and Nd isotope values, and εNd(i) of the felsic hypabyssal rocks are comparable to the mafic ones. The isotope data point to a genetic relationship between the felsic and mafic members. Results obtained from the geochemical modelling of incompatible versus compatible trace elements show that the felsic rocks were derived from the mafic melts by fractional crystallization (FC) process. In the light of their regional geological setting and these geochemical characteristics, we propose that the magmatic rocks of the Mudurnu formation formed in an extensional basin situated on an active and/or just ended subduction zone during the Jurassic period. The Mudurnu formation is a good example of extensional magmatism derived from a depleted mantle source that displayed subduction signatures and that was modified by fluids from a long-lived subduction event.

Geochemical constraints on the genesis of the Sarıcakaya intrusive rocks, Turkey: Late Paleozoic crustal melting in the central Sakarya Zone

The Sakarya Zone is intruded by several Late Paleozoic granitoids, of which the Sarıcakaya intrusive rocks in the central Sakarya (Eskişehir) region, is the least-studied. The Sarıcakaya intrusive rocks consist mainly of quartz diorite-granodiorite, granodiorite and granite. They are, geochemically, divided into two groups: diorites and granites. The former is medium-K and calc-alkaline (mainly I-type), whereas the latter is high-K to shoshonite and calcic (I-type). Typical minerals for both rock types are plagioclase, K-feldspar, quartz, biotite, hornblende and Fe–Ti oxides. Chondrite-normalized REE patterns for the Sarıcakaya intrusive rocks are moderately fractionated and have small negative Eu anomalies. They are enriched in LILE and LREE relative to HFSE showing characteristics of arc-related granitoids. Geochemical characteristics of the Sarıcakaya intrusive rocks indicate a hybrid origin through partial melting of lower crustal source rocks.

Cenomanian – Turonian Oceanic Anoxic Event (OAE2) in the Sakarya Zone, northwestern Turkey: Sedimentological, cyclostratigraphic, and geochemical records

The Cenomanian/Turonian oceanic anoxic event as recorded in many parts of the world is characterized by presence of black shales and global carbon isotope peaks. Cyclic variations within the anoxic event and association with pelagic red beds are indicated by many studies. Global tectonic/volcanic controls on the origin and the chronostratigraphic position of the anoxic event within the global sea-level changes are important for understanding the paleoceanographic and paleoclimatic conditions during the deposition of Cenomanian/Turonian sediments. Three measured stratigraphic sections in the Central Pontides and the Central Sakarya region of Turkey on the Sakarya Zone of the Pontides have been studied in detail. Cyclic facies alternations have been analysed and Ficher plot curves have been established for the measured sections. According to the cyclostratigraphic analysis, position of the C/T boundary with black shales/mudstones coincides with the global sea-level rise. This indicates that global sea-level change is one of the controlling parameters of black shale generation at the boundary and the long distance correlation between pelagic/hemipelagic successions of different parts of the Sakarya Zone and with global counterparts displays the effect of a large-scale event rather than local. A positive shift in the δ 13C curve has been obtained just below the C/T boundary similar to many curves obtained around the world. The depletion of Mn indicates oxygen-depleted bottom conditions. Parallel increase of Cd, P, Ba, and Fe indicates increased productivity. Increase in Zn, Cu, Pb and the presence of volcanoclastic sands just below the boundary indicate the fact that volcanism was present during the deposition of anoxic sediments and affected the productivity. TOC values of black shales reach up to 1, 95% at the boundary but drop suddenly after the boundary in parallel with other proxies. The Milankovitch-scale frequent rise and fall of the sea-level superimposed on larger cycles also triggered the nutrient enrichment and the ventilation of the basin resulting in the repetitive alternation of black shales and red marls or cherts. Therefore, this study indicates that the OAE2 including cyclic black shale and red marl/chert alternations on the Sakarya Zone was developed by the cooperation of volcanism, productivity and small and large-scale sea level changes.

Orientations of palaeotectonic features as a key to understanding the neotectonic block rotation of the Kocaeli peninsula, NW Turkey

Due to northward subduction of Neotethys, the İstanbul zone collided with the Sakarya zone in northwest Turkey during the early Eocene. Subsequently, this region was subjected to compressional forces during the late Eocene–early Miocene period. Folds, thrusts and reverse faults developed approximately parallel to long axes of the İstanbul zone. NNW–SSE oriented conjugate strike‐slip faults developed with continued contraction. In addition to the orientations of palaeotectonic features, the morphotectonic, stratigraphic and seismic characteristics expose differences between the northeastern Marmara peneplain and the southern Black Sea highland. This study reports causes of this diversity reflecting the neotectonic evolution of the İstanbul zone. The diversity is related to the clockwise rotation of the Kocaeli peninsula between two dextral zone‐bounding faults and two sinistral block‐bounding faults. The principle factors of this process were the development of the North Anatolian fault zone (NAFZ) and the related evolution of the Adapazarı–Karasu fault zone (AKFZ), the Bosphorus fault zone (BFZ) and the Northern Boundary fault (NBF).

Variscan amphibolite-facies rocks from the Kurtoğlu metamorphic complex (Gümüşhane area, Eastern Pontides, Turkey)

The Kurtoglu metamorphic complex, that forms part of the pre-Liassic basement of the Sakarya zone in northern Turkey, consists of at least two tectonic units. Blueschist-facies rocks of unknown metamorphic age in the southern part of the complex are tectonically overlain by Variscan low-pressure high-temperature metamorphic rocks. The latter comprise mica schists and fine-grained gneisses, cut by metaleucogranitic dikes, as well as migmatitic biotite gneisses and subordinate amphibolite intercalations. Structural data indicate that metamorphism and penetrative deformation occurred after dyke intrusion. Peak metamorphic conditions of the mica schists, fine-grained gneisses and metaleucogranites are estimated to ∼650°C and ∼0.4 GPa, based on phase relationships in the system NCKFMASH, Fe–Mg partitioning between garnet and biotite as well as garnet-aluminosilicate-quartz-plagioclase (GASP) and garnet-plagioclase-biotite-quartz (GBPQ) barometry. Peak temperatures of the migmatitic biotite gneisses and amphibolite intercalations are not well constrained but might have been significantly higher (690–740°C), as suggested from hornblende-plagioclase thermometry. 40Ar–39Ar incremental dating on muscovite and biotite fractions from the mica schists and fine-grained gneisses yielded plateau ages of ∼323 Ma. Significantly older model ages of ∼329 and ∼337 Ma were obtained on muscovite fractions from two metaleucogranite samples. These fractions contain both relict igneous and newly formed metamorphic muscovite.

2–2: Ada Tepe sedimentary-hosted, low-sulphidation epithermal Au deposit, SE Bulgaria: District: Lat. 41°26′ N, Long. 25°39′ E

The Tethyan Metallogenic Belt (TMB), extending from Europe through Anatolia to Iran, is one of the world's major metal producing belts, and consists of many sectors. Mineral deposits of the Biga Peninsula in northwestern Turkey exhibit, in many ways, the characteristics of mineral deposits found throughout the belt. Biga Peninsula tectonically forms the westernmost part of the Sakarya Zone and easternmost part of the Rhodope Zone at the intersection of Gondwana and Laurasia.The Biga Peninsula metallogeny research and exploration project created a GIS inventory of mineral deposits and prospects, and classified them genetically to evaluate the mineral deposit potential using genetic models based on descriptive data. The GIS database, consisting of 128 deposits or prospects, helped to generate new prospects and potential prospects. This field-based study indicated that the Biga Peninsula forms a prime target for gold–copper exploration not only in Turkey but in the world.The current economically significant mineral deposits of the Biga Peninsula were shaped by Cenozoic calc-alkaline magmatism, ranging between 52 and 18 Ma, and related to mainly collisional and post-collisional tectonic regime. Epithermal Au–Ag deposits including high-(HS), low-(LS) and intermediate-sulfidation (IS) styles, porphyry Au–Cu–Mo and base-metal skarn systems are economically the most important. Though there are no currently economic examples of some of them in the Biga Peninsula, other deposit types include Carlin-like distal disseminated Au–Ag, orogenic Au, especially listwanite hosted, volcanogenic Mn and U, lateritic (ferricrete) Fe deposits, carbonate replacement (CR) and placers.Several active metal mines, such as Balya, Arapucandere and Koru, are operating in the Biga Peninsula. Kucukdere Au–Ag deposit in Balikesir is the only gold mine in the Biga Peninsula, except for by-product gold produced from base-metal deposits. Results of the study show current total gold endowment of the Biga Peninsula including reserves and/or resources is 9.18 Moz gold [284.2 t] contained in twelve different deposits. Of these only 6 contain significant gold [> 0.3 Moz or 10 t]. Halilaga porphyry and Agi Dagi and Kirazli HS epithermal systems have an ongoing resource estimate, and Halilaga is a candidate to be one of the largest Cu–Au deposits not only in the Biga Peninsula, but in Turkey. Currently newly discovered Tepeoba is the largest porphyry Cu–Mo–Au deposit with known resources in the Biga Peninsula.Here, the first40Ar/39Ar step-heating age data conducted on some of the major HS epithermal gold deposits and causative intrusives in the Biga Peninsula are reported. Geochronological results from this project, evaluated with previous studies, indicate at least 3 phases of porphyry and 2 phases of high-sulfidation epithermal gold mineralization in the Biga Peninsula. The most important mineralizing phases and related host rocks for gold mineralization range from 38 to 22 Ma. The Oligocene is especially important for economic epithermal and porphyry systems in the Biga Peninsula, which is comparable to deposits in the Oligo-Miocene Serbomacedonian-Rhodope metallogenic belt of the Balkan Peninsula in SE Europe.

Alpine high-P/low-T metamorphism of the Afyon Zone and implications for the metamorphic evolution of Western Anatolia, Turkey

Carpholite + chloritoid + pyrophyllite association occurs widely in the Triassic metaclastic rocks of the Afyon Zone in west-central Turkey. Fe-Mg-carpholite is associated with rare aragonite pseudomorphs and glaucophane in marbles and metabasites, respectively. The Afyon Zone consists stratigraphically of a Pan-African basement and an overlying Mesozoic cover sequence. The Pan-African basement, which shows Barrovian-type amphibolite-facies metamorphism, comprises garnet–mica schists, intruded by sodic amphibole-bearing metagabbros and leucocratic metagranites. It is unconformably overlain by a continuous metasedimentary sequence extending from Triassic to early Palaeocene. This cover sequence begins with metaconglomerates, which pass upwards into phyllites. Fe-Mg-carpholite occurs within this metaclastic sequence as rosette-like crystals in metapelites and fibres in quartz segregations. The metaclastic rocks are succeeded by metamorphosed platform carbonates, grading into Latest Mesozoic metamorphosed pelagic limestones, which in turn progress up to a Late Mesozoic–Early Tertiary olistostrome. This sequence is tectonically overlain by the blueschists of the Tavşanlı Zone. Fe-Mg-carpholite-bearing assemblages imply temperatures of about 350 °C and minimum pressures of 6–9 kbar, corresponding to burial depths of about 30 km for the Mesozoic passive continental margin sediments and the underlying Pan-African supracrustal metasediments and metaintrusives. The metamorphic rocks of the Afyon Zone are unconformably overlain by Upper Palaeocene–Lower Eocene sedimentary rocks, indicating a Paleocene age for the regional HP/LT metamorphism. This implies continuous younging of HP/LT metamorphism in the Anatolides related to northward subduction of the Anatolide–Tauride platform beneath the Sakarya Zone. From north to south this involved the Tavşanlı Zone (Campanian, 80 ± 5 Ma), the Afyon Zone (Palaeocene?), the Menderes Massif (Middle Eocene) and the Lycian Nappes (Late Cretaceous–Eocene?), all of which were probably derived from the frontal part of the Anatolide–Tauride platform.

Low-grade metamorphic rocks from the Pulur complex, NE Turkey: implications for the pre-Liassic evolution of the Eastern Pontides

In the Pulur complex (Sakarya Zone, Eastern Pontides, Turkey) a low-grade tectonometamorphic unit (Dogankavak) is exposed in three tectonic windows beneath a complex medium-pressure high-temperature metamorphic unit of late Carboniferous age. The thrust plane between both units is transgressively covered by Liassic conglomerates. The Dogankavak unit comprises a sequence of metabasites with MORB-type chemical compositions and phyllites, with subordinate calcareous phyllites, marbles, quarzofeldspathic schists and metacherts. This sequence is interpreted as a former accretionary complex related to the consumption of the Palaeotethys. Mineral parageneses in the metabasites allow for the distinction of two domains with slightly different peak metamorphic conditions, i.e. 375–425 °C/0.5–0.8 GPa (greenschist facies) and 400–470 °C/0.6–1.1 GPa (albite-epidote amphibolite facies). The age of metamorphism is constrained at ~ 260 Ma (early Late Permian) by two Rb-Sr mineral-whole rock ages (hornblende, phengite) and one 40Ar/39Ar single step total fusion age (phengite). In conjunction with previous data on other accretionary complexes in the Sakarya zone in Northern Turkey, the data presented in this study suggest a continuous subduction of the Palaeotethys at least from Early/Late Permian to Late Triassic and a discontinuous preservation of accretion complexes in both space and time.

A comparison of the geological and geoelectrical structures in the Eastern Marmara region (NW Turkey)

Northwestern Anatolia contains different tectonic units which are presently bounded by the two branches of the North Anatolian Fault. The Pontide Zone in the north consists of the İstanbul–Zonguldak Unit to the west and the Ballıdağ–Küre Unit (a remnant of Paleo-Tethyan ocean) in the east. The Sakarya Zone in the south is juxtaposed against the Pontide Zone due to the closure of Paleo-Tethys. The Armutlu–Ovacık Zone represents a tectonic mixture of both zones in the north and south. Neotectonic units, such as the İzmit–Sapanca Corridor and the Northern Sakarya Low, which developed south of the İstanbul Zone and north of the Sakarya Zone, respectively, represent an extensional tectonic regime. The relationship and continuations of these tectonic entities in the region were examined by a 90 km-long, NW–SE trending magnetotelluric profile. Electrical resistivity is quite low (10 ohm m) beneath the Northern Sakarya Low and the Sakarya Zone at a depth of 26 km, compared to the northern areas. This low-resistivity zone may provide an indication of thinning in the total crustal thickness and a viscoelastic lower crust. Furthermore, in the northern part of the profile, the five-layered conductive sequence obtained by magnetotelluric modelling, may correspond to fragments of the İstanbul Zone and the Ballıdağ–Küre Unit underlying it.

Electrical resistivity structure of the northwestern Anatolia and its tectonic implications for the Sakarya and Bornova zones

The crustal structure of the northwestern Anatolia was studied by magnetotelluric measurements with periods up to 500 s at 20 locations along a line (line KS) crossed Istanbul, Sakarya and Bornova Zones to investigate deep electrical resistivity patterns that contribute to the understanding of tectonic setting of the region. The results obtained from the two-dimensional (2D) inversion of magnetotelluric data for the modes transverse electric (TE) and transverse magnetic (TM) show that the tectonic structure up to 5 km depth was comparatively complex and that the structural pattern was generally disturbed by the grabens and Neogene basins of low resistivity feature. The low resistivity (<80 Ω m) zones stretched from the Istanbul Zone towards the northern part of the Sakarya Zone and from the southern border of the Bornova Zone towards the Gediz Graben in south characterize sedimentary sequences for this depth range. Based on the more detailed near-surface 2D geoelectrical model constructed using dc Schlumberger resistivity data the basin-fill deposits is about 1.5 km and 750 m thick in the Gediz and Görede Grabens, respectively. Other smaller regions of low resistivity were also interpreted as the small-scale sedimentary Neogene basins within the Sakarya and Bornova Zones. Below about 5 km depth geoelectric models except a zone beneath Kazdağ range show more resistive structure underlying these sediments. The resistive structure was correlated with the Precambrien crystalline rocks and gneiss schist basement of Sakarya and Bornova Zones, respectively. A major zone with relatively conductive (∼10 Ω m) occurred within highly resistive mid crust beneath Kazdağ range represents an electrically macro-anisotropic structure which could be followed from magnetotelluric sounding data in both TE and TM modes. It was discussed that the origin of this zone is associated with a coeval plutonism and metamorphism of latest Oligocene age, which yields a considerable low conductivity in the crystalline rocks. Of special interests was the zone of electrical conductivity, as it seems to exist a correlation between conductive structure and geotectonic features, which may support ideas about the high-grade metamorphism and accordingly north–south extending in the northwestern Anatolia.

Ophiolite emplacement by strike-slip tectonics between the Pontide Zone and the Sakarya Zone in northwestern Anatolia, Turkey

Northwestern Anatolia contains three main tectonic units: (a) the Pontide Zone in the north which consists mainly of the Istanbul–Zonguldak Unit in the west and the Ballidag-Kure Unit in the east; (b) the Sakarya Zone (or Continent) in the south, which is juxtaposed against the Pontide Zone due to the closure of Paleo-Tethys prior to Late Jurassic time; and (c) the Armutlu–Ovacik Zone which appears to represent a tectonic mixture of both zones. These three major tectonic zones are presently bounded by the two branches of the North Anatolian Transform Fault. The two tectonic contacts follow older tectonic lineaments (the Western Pontide Fault) which formed during the development of the Armutlu–Ovacik Zone. Since the earliest Cretaceous, an overall extensional regime dominated the region. A transpressional tectonic regime of Coniacian/Santonian to Campanian age caused the welding of the Istanbul–Zonguldak Unit to the Sakarya Zone by an oblique collision. In the Late Campanian, a transtensional tectonic regime developed, forming a new basin within the amalgamated tectonic mosaic. The different tectonic regimes in the region were caused by activity of the Western Pontide Fault. Most of the ophiolites within the Armutlu–Ovacik Zone belong to the Paleo-Tethyan and/or pre-Ordovician ophiolitic core of the Istanbul–Zonguldak Unit. The Late Cretaceous ophiolites in the eastern parts of the Armutlu–Ovacik Zone were transported from Neo-Tethyan ophiolites farther east by left-lateral strike-slip faults along the Western Pontide Fault. There is insufficient evidence to indicate the presence of an ocean (Intra-Pontide Ocean) between the Istanbul–Zonguldak Unit and the Sakarya Zone during Late Cretaceous time.

Palaeomagnetic study of the Galatean Volcanic Province, north‐central Turkey: neogene deformation at the northern border of the Anatolian Block

A regional palaeomagnetic study is reported from 24 lavas of the (Late Miocene) Galatean Volcanic Province bordering the North Anatolian Fault Zone (NAFZ) in northern Anatolia. Rock magnetic studies show that the lavas have a ferromagnetic content dominated by low-Ti magnetite assemblages which have been subjected to variable degrees of high and low temperature oxidation. Although larger fractions of ferromagnetic grains are multidomain, significant fractions of single domain grains are always present and presumed to be responsible for the stable remanence at all sites. Magnetic remanence is interpreted to be essentially primary because it preserves a record of both polarities which has since been tectonically rotated. Fifteen sites define a coherent dipolar axis with a mean direction of D/I=16·8/56·5° (α95=6·9°). This is rotated clockwise by 10·2±7·2° from the Eurasian field at 10 Ma and is interpreted to record block rotation during westward expulsion of lithosphere along the northern margin of the Anatolian region late in the Neotectonic history. Upper Cretaceous rocks from the northern side of this sector of the NAFZ exhibit rotations relative to Eurasia; whilst these may have been imparted partly during Eocene collision of the İstanbul and Sakarya zones, consistent counterclockwise motions show that differential block rotations are occurring here with the Galatean region bounded at its northern margin by the NAFZ. Copyright © 1999 John Wiley & Sons, Ltd.

Pre-Cenozoic tectono-stratigraphic components of the Western Pontides and their geological evolution

The Western Pontides of northern Turkey are a tectonic mosaic formed as a result of progressive welding of continental and oceanic fragments during the Palaeozoic and Mesozoic. In this region, three approximately east–west trending zones can be distinguished: the Pontide Zone representing the Pontides sensu stricto; the Sakarya Zone which is regarded as the southerly continental fragment; and the Armutlu–Ovacik Zone which is viewed as a tectonic mixture of the two zones. The Pontide Zone records development of an ensimatic island arc which are emplaced upon a continental fragment of Laurasian origin prior to the development of the Ordovician sediments. On top of this amalgamated basement, association thick sediments were deposited during the Palaeozoic and Early Mesozoic. During the Dogger, Palaeotethyan ophiolites were obducted onto the Pontides. The thick pile of cover sediments and the ophiolite slab were exhumed along a detachment surface while the underlying rocks were elevated to form a metamorphic core complex during the Early Cretaceous. The metamorphic rocks are exposed along the Ballidag, Sunnice, Almacik and Armutlu mountain ranges. This east–west trending structural high separated two coeval basins, the Ulus Basin and the Boyali Basin, which are located to the north and south respectively. During the Late Cretaceous, collision occurred between the Sakarya continent and the Pontide Zone. The continental convergence affected the region until the late Early Eocene. The present tectonic style of the region was established during this phase. From the Middle Eocene onwards only structural rearrangements have occurred. Copyright © 1999 John Wiley & Sons, Ltd.

Palaeomagnetic study of the Galatean Volcanic Province, north-central Turkey: neogene deformation at the northern border of the Anatolian Block

A regional palaeomagnetic study is reported from 24 lavas of the (Late Miocene) Galatean Volcanic Province bordering the North Anatolian Fault Zone (NAFZ) in northern Anatolia. Rock magnetic studies show that the lavas have a ferromagnetic content dominated by low-Ti magnetite assemblages which have been subjected to variable degrees of high and low temperature oxidation. Although larger fractions of ferromagnetic grains are multidomain, significant fractions of single domain grains are always present and presumed to be responsible for the stable remanence at all sites. Magnetic remanence is interpreted to be essentially primary because it preserves a record of both polarities which has since been tectonically rotated. Fifteen sites define a coherent dipolar axis with a mean direction of D/I=16·8/56·5° (α95=6·9°). This is rotated clockwise by 10·2±7·2° from the Eurasian field at 10 Ma and is interpreted to record block rotation during westward expulsion of lithosphere along the northern margin of the Anatolian region late in the Neotectonic history. Upper Cretaceous rocks from the northern side of this sector of the NAFZ exhibit rotations relative to Eurasia; whilst these may have been imparted partly during Eocene collision of the Istanbul and Sakarya zones, consistent counterclockwise motions show that differential block rotations are occurring here with the Galatean region bounded at its northern margin by the NAFZ. Copyright © 1999 John Wiley & Sons, Ltd.

Geological Units and Structures in Suture Zone Around Balıkesir NW Anatolia

The region norheast of Balıkesir comprises different group of rocks reflecting different tectonic settings. Sakarya zone and Tavşanlı Zone are separated by a Mesozoic Tethyan suture zone called İzmir-Ankara suture. Sakarya zone, represented by Karakaya Complex in the study area, is located to the north of a major fault, which marks the northern boundary of Neo-Tethyan suture zone. The Karakaya complex reflects active continental margin units that were formed by southward subduction of Paleo-Tethyan oceanic crust during Permo- Triassic. At this perod, Karakaya complex units were internally imbricated. The closure of the Neo-Tethys ocean, leading HP/LT metamorphizm of northward subducting platform sediments (Orhaneli unit), resulted in the formation of two distinct tectonic units during late Mesozoic to Paleocene; an ophiolotic melange (Ovacık melange) and fault controlled wild-flysch (Bornova flysch). Ovacık melange, which was originally tectonically underlein by Orhaneli unit, detached during uplift of blueschist. Bornova flysch has back-thrushed over Sakarya zone during late Paleocene-?Eocene. A large garnitic pluton, Çatldağ granodiorite, which is a pruduct of extensive calc-alkaline magmatism in NW Anatolia, emplaced into the older units during Early Miocene, and caused titing of older structures around pluton.

Deep Conductivity Structure of the North Anatolian Fault Zone and the Istanbul and Sakarya Zones Along the Gölpazari-Akcaova Profile, Northwest Anatolia

Magnetotelluric (MT) measurements at six locations along a 90-km profile across the area between Golpazari and Akcaova have been modeled in two dimensions to increase understanding of the deep conductivity structure of the western part of the North Anatolian fault zone (NAFZ) and the Istanbul and Sakarya zones. It is well known from surface geology that the branch of the NAFZ that passes through the Pamukova Valley with an E-W strike separates the region into two sub-areas, containing contrasting sets of geological features. These two areas also exhibit significant differences in terms of their deep conductivity structure. Electrical resistivity is quite low (10 ohmm) south of the fault at an approximate depth of 26 km, compared to the area north of the fault zone. This low-resistivity zone may provide an indication of partial melting at this depth. In the northern part of the profile beneath Ucgaziler (DUC) and Akcaova (DAK), a five-layered conductivity sequence obtained by magnetotelluric modeling and t...