Bornova Flysch Zone Research Articles

P–T–tevolution of the Cycladic Blueschist Unit in Western Anatolia/Turkey: Geodynamic implications for the Aegean region

AbstractEclogite and blueschist facies rocks occurring as a tectonic unit between the underlying Menderes Massif (MM) and the overlying Afyon Zone/Lycian Nappes and the Bornova Flysch Zone in western Anatolia represent the eastward continuation of the Cycladic Blueschist Unit (CBU) in Turkey. This high‐Punit is attributed to the closure of the Pindos Ocean and consists of (a) a Triassic to Upper Cretaceous coherent series derived from passive continental margin sediments and (b) the tectonically overlying Upper Cretaceous Selçuk mélange with eclogite blocks embedded in a pelitic epidote‐blueschist matrix. The coherent series has experienced epidote‐blueschist facies metamorphism (490 ± 25°C/11.5 ± 1.5 kbar; 38 km depth).40Ar/39Ar white mica and206Pb/238U monazite dating of quartz metaconglomerate from coherent series yielded middle Eocene ages of 44 ± 0.3 and 40.1 ± 3.1 Ma for epidote‐blueschist facies metamorphism, respectively. The epidote‐blueschist facies metamorphism of the matrix of the Selçuk mélange culminates at 520 ± 15°C/13 ± 1.5 kbar, 43 km depth, and is dated at 57.5 ± 0.3–54.5 ± 0.1 Ma (40Ar/39Ar phengite). Eclogite facies metamorphism of the blocks (570 ± 30°C/18 ± 2 kbar, 60 km depth) is early Eocene and dated at 56.2 ± 1.5 Ma by206Pb/238U zircon. Eclogites experienced a nearly isothermal retrogression (490 ± 40°C/~6 to 7 kbar) during their incorporation into the Selçuk mélange. The retrograde overprints of the coherent series (410 ± 15°C/7 ± 1.5 kbar from Dilek Peninsula and 485 ± 33°C/~6 to 7 kbar from Selçuk–Tire area) and the Selçuk mélange (510 ± 15°C/6 ± 1 kbar) are dated at 35.8 ± 0.5–34.3 ± 0.1 Ma by40Ar/39Ar white mica and 31.6 ± 6.6 Ma by206Pb/238U allanite dating methods, respectively. Regional geological constrains reveal that the contact between the MM and the CBU originally formed a lithosphere‐scale transform fault zone.40Ar/39Ar white mica age from the contact indicates that the CBU and the MM were tectonically juxtaposed under greenschist facies conditions during late Eocene, 35.1 ± 0.3 Ma.

Conceptual model of the Gülbahçe geothermal system, Western Anatolia, Turkey: Based on structural and hydrogeochemical data

The Gülbahçe Geothermal Field is located on the eastern margin of the Karaburun Peninsula, about 45km from the city of İzmir, western Anatolia, Turkey. The stratigraphy of the study area is represented by a Miocene volcano-sedimentary succession, including several sedimentary and volcanic units. These units overlie the basement rocks of the Karaburun Platform and Bornova Flysch Zone which consist of sandstones, shales and carbonate blocks. These rock units are cut and deformed by a series of NW-SE- to NE-SW-trending faults, extending from Sığacık Bay to Gülbahçe Bay. Structural studies suggest that while most of the geothermal systems in western Anatolia are controlled by normal faults, the geothermal system at Gülbahçe is controlled by a strike-slip dominated shear zone, previously named the İzmir-Balıkesir Transfer Zone. Along the fault zone, associations of active fault segments accommodate deep circulation of hydrothermally modified sea water, and thus the resulting negative flower structure is the primary control mechanism for the geothermal system.Hydrogeochemical properties of the field show that surface temperature of fluid ranges from 30 to 34°C. Geothermal fluids in Gülbahçe have high salinity (EC>34 mS/cm) and low enthalpy. Piper and Schoeller diagrams indicate that geothermal fluid is in the NaCl facies. Chemical geothermometers suggest that the reservoir temperature is around 53–136°C. The isotopic data (oxygen-18, deuterium and tritium) suggest that geothermal fluids are formed by local recharge and deep circulation of sea water.

Stratigraphy and depositional history of the Cretaceous carbonate successions in the Spil Mountain (Manisa, W Turkey)

The Bornova Flysch Zone (western Turkey) consists of huge Mesozoic limestone and ophiolite blocks embedded into sheared siliciclastic sedimentary rocks of Maastrichtian−Paleocene age. The limestone blocks, which range in age from Late Triassic to Cretaceous, are considered to be olistoliths or deformed and sliced platform parts. In the Spil Mountain, two successions of Cretaceous carbonates are tectonostratigraphically differentiated: (1) a Lower Cretaceous and Campanian(?)−Maastrichtian relatively autochthonous succession showing sedimentary transition to the Bornova Flysch, and (2) a Cenomanian(?)−lower Campanian allochthonous succession overthrusted to the flysch. These successions represent separate parts of the same platform. The autochthonous succession bears Lower Cretaceous peritidal carbonates at its base and is named Unit 1. The succession is often composed of fenestral mudstone and algal wackestone microfacies. Unit 2 disconformably overlies Unit 1 and consists of platform-derived litho and bioclastic packstones of Campanian(?)−Maastrichtian age. This unit reveals a typical thinning and fining upward sequence, finally passing into pelagic wackestones of Unit 3. The two aforementioned units record a platform drowning event, which occurred rapidly based on the presence of planktonic foraminifera within matrix of basal breccia. Carbonate deposition ceased due to the input of siliciclastic sediments during the late Maastrichtian−Paleocene. The allochthonous succession consists of two vertically superimposed units: (1) Cenomanian(?)−Santonian rudistid limestones (Unit 4) deposited in restricted platform environments and (2) Santonian−lower Campanian pelagic limestones (Unit 5) indicating open platform to slope conditions. The Spil Mountain Cretaceous carbonate sequences are correlated with those in peri-Mediterranean platforms. They show close similarities to the Bey Dağları (western Taurides) carbonate sequences in stratigraphy and facies. Paleontological and sedimentological analyses and the microfacies enable us to reconstruct a paleoenvironment evolution and a facies model for the Spil Mountain carbonate deposits during the Cretaceous period.

Jurassic and Cretaceous radiolarian assemblages from the Bornova mélange in northern Karaburun Peninsula (western Turkey) and its connection to the İzmir–Ankara mélanges

Two large blocks of red bedded chert identified within the Late Cretaceous–Paleocene Bornova mélange in northern Karaburun Peninsula yielded Jurassic (late Bathonian–early Oxfordian) and Cretaceous (middle–late Albian) radiolarian assemblages. These new data confirm the correlation of the Bornova mélange with the Bornova Flysch Zone (BFZ) and the İzmir–Ankara mélanges. A review of all previously obtained ages in chert blocks of the BFZ and the İzmir–Ankara mélanges is provided in order to strengthen this correlation.

Late Maastrichtian-Late Palaeocene planktic foraminiferal biostratigraphy of the matrix of the Bornova Flysch Zone around Bornova (İzmir, Western Anatolia, Turkey)

The Bornova Flysch Zone (BFZ), located between the Menderes Massif and the İzmir-Ankara Suture in westernmost Anatolia (Turkey), forms the westernmost part of the Anatolide-Taurides. The BFZ comprises intensely sheared Upper Cretaceous-Palaeocene matrix and blocks of various origins. The matrix of the BFZ is mainly made up of unfossiliferous flysch-type sediments (alternations of sandstones and shales). In Bornova (İzmir, western Turkey) and its surroundings, these clastics locally include planktic foraminifera-bearing pelagic micritic limestone and calcareous shale lenses and interbeds (Beytitepe Limestone). As a result of studies focusing on the planktic foraminifera-bearing pelagic interlayers in three areas (Gökdere, Işıklar and Kocaçay areas) around Bornova, a detailed planktic foraminiferal biostratigraphy of the rocks is documented for the first time. The thickness of the laminated micritic limestones attains 360 m in the Gökdere area. Occurrences of late Maastrichtian species such as Abathomphalus mayaroensis (Bolli), Contusotruncana contusa (Cushman), Globotruncanita conica (White) and Racemiguembelina fructicosa (Egger) within planktic foraminifera assemblages obtained from the laminated micritic limestones and red calcareous shales in the three areas suggest a late Maastrichtian age for these rocks. The occurrence of various species of Globanomalina, Morozovella, Igorina and Parasubbotina within the calcareous shales in the Işıklar area suggests a late Palaeocene age. The Kocaçay area has well-preserved outcrops showing the stratigraphy of the matrix, despite the more complex geology. The upper Maastrichtian laminated micritic limestones are gradationally overlain by upper Maastrichtian calcareous shales. The calcareous shales are represented by rich planktic foraminiferal assemblages and include blocks derived from laminated micritic limestones. The upper Mastrichtian calcareous shales are overlain by Palaeocene red calcareous shales. Poor assemblages including Parasubbotina varianta (Subbotina), Subbotina triangularis (White), Subbotina cf. velascoensis (Cushman), Globanomalina compressa (Plummer) and Globanomalina planoconica (Subbotina) indicate a late Palaeocene age for the lower part and a latest Palaeocene age for the upper part of the calcareous shale sequence. Therefore, the age of conglomerates and flysch overlying the calcareous shales should be latest Palaeocene or younger in the Kocaçay area.

Radiolarian assemblages of Middle and Late Jurassic to early Late Cretaceous (Cenomanian) ages from an olistolith record pelagic deposition within the Bornova Flysch Zone in western Turkey

Abstract The Bornova Flysch Zone (BFZ) in NW Anatolia comprises several olistoliths or tectonic slivers, representing various parts of the Izmir-Ankara ocean. Radiolarian assemblages extracted from one of the olistoliths of the BFZ, cropping out along the Sögütlü section, to the NE Manisa city, were studied in detail. The lowermost part of the section contains latest Bajocian – early Callovian radiolarian taxa, followed by radiolarian assemblages indicating Late Jurassic to early Late Cretaceous (Cenomanian) ages. Previous studies reveal that the Izmir-Ankara oceanic basin was initially opened during late Ladinian – early Carnian. The new radiolarian data obtained from this olistolith reveals that relatively condensed, and possibly more or less continuous, pelagic sedimentation took place during the late Middle Jurassic to early Late Cretaceous in a non-volcanic oceanic basin closer to the Tauride-Anatolide platform margin.

An olistostrome–mélange belt formed along a suture: Bornova Flysch zone, western Turkey

The Bornova Flysch Zone (BFZ) in western Turkey is a regional olistostrome–mélange belt located between the İzmir–Ankara Tethyan suture in northwest and the Menderes Massif in the southeast. The BFZ consists mostly of tectonized gravity mass flows. The blocks are Mesozoic limestone and ophiolite, which are enclosed in Cretaceous–Paleocene sheared sandstone and shale. The limestone blocks are of two types. The first type consists of Late Triassic to Cretaceous shallow marine carbonates. The second type has an Upper Triassic shallow marine section overlain by Jurassic to Cretaceous pelagic limestones. A semi-intact part of the platform occurs in the Karaburun peninsula and on the island of Chios. The ophiolitic blocks in the BFZ include ultramafic rock, gabbro, basalt and radiolarian chert of Middle Triassic to Cretaceous in age. The formation of the BFZ overlaps with the Cretaceous subduction and HP/LT metamorphism of the northern passive continental margin of the Anatolide–Tauride Block. This subduction zone was bounded in the west by a strike–slip tear fault. The BFZ formed in a narrow basin between this tear fault and the Tethyan ocean. The mass flows came from the southeast from the overriding ophiolite and accretionary complex, and from the northwest from the uplifted segments of the platform margin. This model provides an explanation as why the BFZ is unmetamorphosed, whereas the equivalent strata in the Menderes Massif were metamorphosed at depths of over 20km. It also explains the prominence of gravity flows and the southward younging in the BFZ, and for the apparently anomalous observation that, although the Tethyan ocean lay to the northwest, the ophiolitic blocks are more common on the southeastern part of the BFZ. Regions away from the tear fault, such as the Karaburun peninsula, were least affected by subsidence and deformation during the Cretaceous and Paleocene.

Uplift and lateritization history of the Çaldağ ophiolite in the context of Neo-Tethyan ophiolite obduction and uplift: implications for the Cenozoic weathering history of western Anatolia

Abstract: The Çaldağ ophiolite, in the Bornova Flysch Zone, is a fragment of Neo-Tethys in the Aegean province, western Turkey. The Bornova Flysch Zone is probably of Cretaceous age, and is locally covered by Early Eocene sediments of the Başlamış Formation that show the first evidence of exposure of ultramafic rocks. Field mapping confirms eight tectonic events correlated with the published regional history. The Çaldağ ophiolite would have been exposed to weathering only from the Mid-Palaeocene to the Late Miocene and from the Mid-Pliocene until the present. This weathering has resulted in the formation of an average c . 69 m thick oxide-dominated Ni-laterite. Biological, palaeoecological and isotopic records indicate favourable conditions for weathering, comparable with those of regions in which laterites form today. These data suggest that tropical to subtropical climatic conditions dominated through most of the Cenozoic; the Palaeocene–Eocene was significantly warmer than the Oligo-Miocene. Combining the exhumation history with the available climate data better constrains the periods suitable for laterite formation with implications for laterite formation regionally. The data indicate a possible total period of 50 Ma when laterite formation was possible, yet the profile measured at Çaldağ is indicative of either a shorter period of actual weathering or partial erosion of the formed profile.

Late Middle Jurassic (Late Bathonian-early Callovian) radiolarian cherts from the Neotethyan Bornova flysch zone, Spil Mountains, Western Turkey

Alternating radiolarian cherts and mudstones associated with basaltic lavas occur in a olistolith within the Late Cretaceous Bornova Flysch in W Anatolia. Cherts yielded diverse and moderately preserved, Nassellaria-dominated radiolarian faunas of late Bathonian-early Callovian age. Associated volcanic rocks are geochemically classified as remnants of oceanic seamounts. This is so far the first late Middle Jurassic age from the crust of the Neotethyan Izmir-Ankara Ocean in W Anatolia, and suggests that its spreading started earlier. Similar ages were reported from the Vardar and Meliata-Hallstadt Tethyan oceanic branches in Greece and Serbia, which also opened in Late Triassic but closed earlier.

DISCOVERY OF THE OLDEST (UPPER LADINIAN TO MIDDLE CARNIAN) RADIOLARIAN ASSEMBLAGES FROM THE BORNOVA FLYSCH ZONE IN WESTERN TURKEY: IMPLICATIONS FOR THE EVOLUTION OF THE NEOTETHYAN IZMIR-ANKARA OCEAN

Diverse upper Ladinian to middle Carnian radiolarian faunas have been obtained from different ribbon cherts blocks and tectonic slices of the Izmir- Ankara Suture Complex and the marginal sediments of the Tauride-Anatolide platform within the Bornova Flysch Zone, western Turkey. Four isolated samples with late Ladinian, early Carnian, late early Carnian, early middle Carnian ages and one continuous section with late Ladinian to late early Carnian age were considered. As a result of taxonomic studies, sixty-six taxa from different families were defined from these time intervals, definition of sixteen taxa remained in open nomenclature were given and one new species (Pseudostylosphaera mostleri) was determined. The radiolarian ages obtained indicate an upper Ladinian to upper lower Carnian deepening of the Middle Triassic Tauride-Anatolide carbonate platform. Combined with regional geological data this event is interpreted to reflect the initial opening of the Neotethyan Izmir-Ankara oceanic seaway.

Evidence for two episodes of volcanism in the Bigadiç borate basin and tectonic implications for western Turkey

Western Turkey has been dominated by N–S extension since the Early Miocene. The timing and cause of this N–S extension and related basin formation have been the subject of much debate, but new data from the Bigadiç borate basin provide insights that may solve this controversy. The basin is located in the Bornova Flysch Zone, which is thought to have formed as a major NE-trending transform zone during Late Cretaceous-Palaeocene collisional Tethyan orogenesis and later reactivated as a transfer zone of weakness, and which separates two orogenic domains having different structural evolutions. Volcanism in the Bigadiç area is characterized by two rock units that are separated by an angular unconformity. These are: (1) the Kocaiskan volcanites that gives K/Ar ages of 23 Ma, and (2) the Bigadiç volcano-sedimentary succession that yields ages of 20.6 to 17.8 Ma. Both units are unconformably overlain by Upper Miocene-Pliocene continental deposits. The Kocaiskan volcanites are related to the first episode of volcanic activity and comprise thick volcanogenic sedimentary rocks derived from subaerial andesitic intrusions, domes, lava flows and pyroclastic rocks. The second episode of volcanic activity, represented by basaltic to rhyolitic lavas and pyroclastic rocks, accompanied lacustrine–evaporitic sedimentation. Dacitic to rhyolitic volcanic rocks, called the Sındırgı volcanites, comprise NE-trending intrusions producing lava flows, ignimbrites, ash-fall deposits and associated volcanogenic sedimentary rocks. Other NE-trending olivine basaltic (Gölcük basalt) and trachyandesitic (Kayırlar volcanites) intrusions and lava flows were synchronously emplaced into the lacustrine sediments. The intrusions typically display peperitic rocks along their contacts with the sedimentary rocks. It is important to note that the Gölcük basalt described here is the first recorded Early Miocene alkali basalt in western Turkey. The oldest volcanic episode occurred in the NE-trending zone when the region was still experiencing N–S compression. The angular unconformity between the two volcanic episodes marks an abrupt transition from N–S collision-related convergence to N–S extension related to retreat of the Aegean subduction zone to the south along an extensional detachment. Thrust faults with top-to-the-north sense of shear and a series of anticlines and synclines with subvertical NE-striking axial planes observed in the Bigadiç volcano-sedimentary succession suggest that NW–SE compression was reactivated following sedimentation. Geochemical data from the Bigadiç area also support the validity of the extensional regime, which was characterized by a bimodal volcanism related to extrusion of coeval alkaline and calc-alkaline volcanic rocks during the second volcanic episode. The formation of alkaline volcanic rocks dated as 19.7 ± 0.4 Ma can be related directly to the onset of the N–S extensional regime in western Turkey. Copyright © 2005 John Wiley & Sons, Ltd.

First evidence of high-pressure metamorphism in the “Cover Series” of the southern Menderes Massif. Tectonic and metamorphic implications for the evolution of SW Turkey

The Menderes Massif occupies a large part of western Turkey. It is tectonically overlain by nappes of the Izmir–Ankara Suture Zone (including the Bornova Flysch Zone) on its northern flank, the Afyon Zone on its eastern border, and the Lycian Nappes on its southern flank. The discovery of magnesiocarpholite–kyanite assemblages in the Mesozoic ‘cover’ sequence of the southern Menderes Massif leads to the very first consideration that a major high-pressure (HP) metamorphic event affected the massif during its Alpine evolution. Estimations of minimum pressure–temperature conditions about 10–12 kbar and 440 °C suggest a minimum burial of 30 km for the metasedimentary rocks of the Menderes Massif, similar to that for the Eocene Dilek Blueschists, which belong to the Cycladic complex. This finding of high-pressure rocks within the Menderes ‘cover’ series, as well as the recent description of low-grade high-pressure parageneses in the basal thrust sheets of the overlying Lycian nappe complex, has strong implications for the tectono-metamorphic evolution of these units. The characteristics of this high-pressure metamorphism and the deformation features observed in the southern Menderes Massif allow consideration of a fast burial during Alpine subduction and nappe stacking. Three high-pressure units were piled up during the Eocene collision of the Sakarya continent and the Tauride–Menderes block. The lowermost unit consists of the imbricated ‘core’ and ‘cover’ of the Menderes Massif. The intermediate unit is the Cycladic Blueschist complex (Dilek and Samos regions) and the uppermost unit is made of the Lycian Nappes. The retrogression of high-pressure parageneses from the Menderes Massif follows an isothermal decompression path and the associated deformation is characterized by a severe stretching oriented N–S to NE–SW during the high-pressure event and its exhumation. We propose a tectono-metamorphic model in three main stages: (1) the southward translation of the high-pressure Lycian Nappes and the burial of the Menderes (and Cycladic) metasediments to a depth of at least 30 km to allow their Eocene high-pressure metamorphism; (2) a northward continuum of deformation responsible for top-to-the-north fabrics associated with the Main Menderes Metamorphism peak (Paleocene–Eocene) and its progressive cooling (Late Eocene), north-verging folding, thrusting, imbrication of ‘core’ and ‘cover’ series, and N–S to NE–SW intense stretching associated with the Menderes high-pressure metamorphism and its exhumation. These top-to-the-north to top-to-the-northeast movements are probably related to the northward backthrusting of the Lycian Nappes, reactivating the contact between the Menderes Massif and the Lycian Nappes as a top-to-the-NE shear zone that allowed exhumation of the Lycian high-pressure rocks; (3) Oligo-Miocene N–S extension during which the final exhumation of the metamorphic rocks along kilometre-scale low-angle detachments occurred.

Conductive crust imaged in western Turkey by MT

Wide‐band magnetotelluric (MT) data across the İzmir‐Ankara Suture Zone (İASZ), the Bornova Flysch Zone and the Menderes Massif of western Turkey images shallow and deep crustal structures. Inversions of the MT data reveals two subzones of the crust with varying thickness; the viscoelastic lower crust appears as a conductive zone (<75 ohm m), while the brittle upper crust is shown as a relatively high resistivity zone in the geoelectrical model. We also observe a very interesting twin‐core structure with a very high resistivity (>2000 ohm m) at a depth from 5 km to 25 km under the İASZ. A hypocentral cross‐section of earthquakes along the profile shows more dense seismic activity in and around the twin‐core structure. Due to this correspondence, we interpret that decoupling of the twin‐core structure could be responsible for the denser seismic activity around the İASZ. The depth of the lithosphere/asthenosphere boundary is rather shallow in the geoelectrical model, about 45 km at the SE and NW end of the profile, and 55 km in the middle.

Magnetotelluric data in western Turkey: Dimensionality analysis using Mohr circles

During the summer of 1996, wideband (0.003125–2000 s) magnetotelluric (MT) data were collected from 52 sites across western Turkey, with a site spacing of ∼5 km. The extensional regime is dominant as a result of the convergence of the African and Arabian plates against the Eurasian plate, and western Turkey is characterized with east‐west oriented, “continental escape” tectonics. To contribute to the knowledge of the geological structures along the profile, the dimensionality characteristics of the MT impedance tensors are computed, taking real and imaginary parts of the tensor elements separately. The rotationally invariant parameters of central impedance (d3) and anisotropy angle (λ) are also computed, the two being good dimensionality indicators. Pseudosections of these parameters reveal the presence of major geological structures in western Turkey, such as the zone between the Menderes Massif and the Bornova Flysch zone, the Izmir‐Ankara Suture zone, the western part of the North Anatolian fault zone, and grabens (Demirci, Gördes, and Bigadiç), which are the characteristic of the region. Interpretation of the full set of Mohr circles shows a strong anisotropy and high central impedance anomalies, while changes are shown in geological strike direction at estimated depths of about 7–8 km, 15–20 km, and 35–40 km along the profile from south to north. These anomalies are indicative of changes in the thickness of upper crust.

A Section Across a Tethyan Suture in Northwestern Turkey

A section across a major Tethyan suture in northwestern Turkey is described in detail. The suture of Early Tertiary age juxtaposes two continental blocks with distinct stratigraphic, structural, and metamorphic features. The Sakarya Zone in the north is represented by Permo-Triassic accretion-subduction complexes, which are unconformably overlain by Jurassic to Paleocene sedimentary rocks. The Anatolide-Tauride Block to the south of the suture consists of two tectonic zones. The Tavsanli Zone consists of a coherent blueschist sequence with Late Cretaceous isotopic ages. This blueschist sequence is tectonically overlain by Cretaceous oceanic accretionary complexes and peridotite slabs. The Bornova Flysch Zone consists of Triassic to Cretaceous limestone blocks in an uppermost Cretaceous to Paleocene flysch. The suture is represented by a N-vergent thrust fault separating lithologies from these two continental blocks. The orogenic history of the region can be considered in two stages. In the Late Cretaceous...