Melange Units Research Articles

Radar İnterferometri Teknikleri Kullanarak Derin Kompleks Heyelanların İzlenmesi: Karaoğlan Havzası (Mersin-Türkiye)

Landslide inventory mapping studies around Mersin and Erdemli regions of southern Turkey revealed that numerous old large-scale and recent small-scale landslides were presumably aroused in different time intervals. While the recent active landslides have been mostly triggered by excessive rainfall, old landslides are considered to be occurred by geomorphologic valley incision processes besides the other preparatory environmental conditions. The evaluation of the spatial distribution of old landslides present retrogressive, deep-seated with complex and rotational slides in the ophiolites and ophiolitic melange units comprising also overlying reefal limestones. In this study, surface deformations caused by landslides were evaluated using radar interferometry techniques for a specified period in Karaoglan catchment. Landslide related deformations were detected over radar images of L-band ALOS-PALSAR sensor for the years between 2007 and 2011. Landslides which are active are investigated in detail cross-sections. The mean displacement from differential interferogram cross-sections was measured 3.5 cm in LOS direction. Differential SAR interferometry for studied period depicts that the average rate of movement for the entire area is extremely slow with a rate of 10 mm/yr.

The upper Cretaceous Ermioni VMS deposit, Argolis Peninsula, Peloponnese, Greece: Type, genesis, and geotectonic setting

The age, type and geotectonic setting of the Ermioni VMS are explored. Although obscured by post-ore deformation and metamorphism, the Ermioni VMS is genetically and spatially related to hydrothermally altered volcaniclastic rocks and arkoses at the footwall, and turbidites at the hangingwall. The Ermioni VMS deposits occur at Karakasi, Roro and Cambrorosso mine sites, and in close proximity to each other. At Karakasi, the identified hydrothermal alteration zones include a lower and innermost grunerite zone, succeded by an intermediate silica-chlorite zone which shows the largest spatial development and surrounded by a silica-epidote-chlorite zone at the flanks, whereas an albite-prehnite zone is developed at the top of the VMS system. The Ermioni VMS comprises nearly monomineralic pyrite with elevated Cu and Co content, and Cu/(Cu + Zn) and Co/Ni ratios typical for “Mafic-Pelitic”-type VMS deposit. Two ore stages are recognized, Stage I euhedral and coarse-grained pyrite (major Co carrier) with minor chalcopyrite, magnetite and pyrrhotite was followed by Stage II pyrite and minor chalcopyrite and sphalerite. Rubidium-Sr and Os-Re radiogenic pairs on pyrite from the Roro massive and Karakasi stringer ores define an upper Cretaceous age of VMS formation (65.58 ± 0.9 to 66.02 ± 0.1 Ma and 64.96 ± 0.9 to 65.12 ± 0.1 Ma, respectively). This age debunks the previously proposed classification as Cyprus-type. The stable isotope compositions of the hydrothermal fluids, including δ18OΗ2Ο, δDΗ2Ο, δ13CCO2, δ30SiNBS, point to mixing between subduction related hydrothermal fluids, seawater and seafloor turbidites, which accords well with the proposed classification. The δ34SH2S and δ57Fe values confirm a magmatic origin of sulfur and leaching of the footwall volcaniclastic rocks with contributions from seawater and turbidites. Radiogenic isotope and trace element geochemistry indicate that the Ermioni “Mafic-Pelitic” VMS was formed at a partly sediment-covered, upper Cretaceous back-arc spreading center above a retreating subduction zone setting within the “Adheres Melange unit”. The Ermioni VMS is the only known upper Cretaceous sulfide ore identified in Greece. The findings of this work may be used in expanding future research on the metallogenic potential of Greece.

Late Palaeozoic extensional volcanism along the northern margin of Gondwana in southern Turkey: implications for Palaeotethyan development

The Late Palaeozoic–Early Mesozoic Tethyan development of the Eastern Mediterranean region remains debatable, especially in Turkey, where alternative northward and southward subduction hypotheses are proposed. Relevant to this debate, new whole-rock geochemical data are provided here for early Carboniferous (Late Tournaisian-Late Visean; c. 340–350 Ma) tuffaceous sedimentary rocks within the Çataloturan thrust sheet (Aladağ nappe), eastern Taurides. The tuffs accumulated from evolved alkaline volcanism, variably mixed with terrigenous and radiolarian-rich sediments. In addition, Late Palaeozoic meta-volcanic rocks, c. 150 km farther NE, within the Binboğa (= Malatya) metamorphics (a low-grade high-pressure unit), are indicative of a within-plate setting. An impersistent geochemical subduction signature in these volcanics may represent an inherited, rather than contemporaneous, subduction influence, mainly because of the absence of a continental margin arc or of arc-derived tuff. Both the Binboğa metamorphics and the Çataloturan thrust sheet (Aladağ nappe) restore generally to the north of the relatively autochthonous Tauride carbonate platform (Geyik Dağ), within the carbonate platform bordering north-Gondwana. The Çataloturan thrust sheet is interpreted, specifically, as a c. E–W, deep-water, volcanically active rift that progressively infilled. Regional geological evidence suggests that melange units (Konya Complex, Afyon zone), Teke Dere unit, Lycian nappes), and Chios–Karaburun melange, E Aegean) accreted to the north-Gondwana continental margin during the late Carboniferous; this was coupled with localised calc-alkaline granitic magmatism (Afyon zone of Anatolide crustal block). We propose an interpretation in which Late Devonian–Carboniferous alkaline intra-plate volcanism relates to extension/rifting along the north-Gondwana margin. In contrast, the melange accretion and granitic magmatism could relate to short-lived late Carboniferous southward subduction that accompanied the diachronous closure of Palaeotethys.

Origin and geochemical evolution of groundwaters at the northeastern extend of the active Fethiye-Burdur fault zone within the ophiolitic Teke nappes, SW Turkey

Thermal and nonthermal, mineral and non-mineral spring waters in different physical and chemical characters occur along the active Fethiye-Burdur fault zone (FBFZ) in the southwestern Turkey and discharge mainly within the supra-subduction-type ophiolitic units of the Teke nappes. This research aims to determine the physical and hydrogeochemical characteristics, origin, and geochemical evolution of the groundwaters discharging from the same lithology and tectonic zone within the northeastern extend of the active Fethiye-Burdur fault zone. A total of representative nine groundwater samples were collected from the study area. Of which, one of the samples is thermal, and others are nonthermal groundwaters. One of the nonthermal groundwaters is mineral water type, and the other one is hyperalkaline (pH > 10). Based on the analytical results, the waters in the study area are divided into six different groundwater facies: (i) Mg-Ca-HCO3-SO4, (ii) Mg-HCO3-CO3, (iii) Ca-HCO3, (iv) Ca-Mg-HCO3, (v) Mg-HCO3, and (vi) Ca-Na-Cl-CO3 facies. The chemical characteristics of the groundwaters are mainly related to mineralogic and hence chemical composition of sub-surface rocks, residence time of the groundwater, and water-rock intereactions, whether or not the conditions closed to CO2. Neutral to moderate alkaline (pH 10) Ca-Na-Cl-CO3-type waters within deeper ultramafic rock aquifers under the conditions closed to CO2. Mainly altered mafic-ultramafic silicates and carbonate rocks are the major sources of mineralization in the study area. On the other hand, H2S (most probably magmatic origin) reacted with carbonate rocks underlying the ophiolitic melange units produced CO2(g). Dissolution of CO2 within groundwaters is thought to cause arising of acidic conditions, higher degree dissolution of mafic-ultramafic silicates, and mineral water formation.

Preliminary results of the radiolarian-chert hosted manganese deposit within the Vezirler ophiolitic mélange (Kula-Manisa, western Turkey): constraints on the origin, paleo-redox conditions, and depositional environments

In Turkey, Late Cretaceous ophiolitic complexes and melange assemblages of the Izmir-Ankara-Erzincan Suture Zone, part of the northern branch of the Neotethys Ocean are rich in Mn-oxide deposits. These deposits are related to the red-violet radiolarian-chert blocks within the melange units. Of these, the Vezirler Melange, consisting of pelagic limestone, pillow basalt, radiolarite blocks, and highly altered serpentinite are distributed in the northern part of the Kula (Manisa) region, western Turkey. Here, the Mn-oxide deposit occurs as interlayers and, to a lesser degree in sharp contact with the radiolarian-chert-basalt-mudstone associations, and are highly folded and fractured. Based on XRD and petrographic studies, mineral paragenesis of the Mn-ores consists predominantly of braunite, pyrolusite, pysilomelane, quartz, and calcite. Geochemically, Mn-ores have high Ba (mean 10,854 ppm) and Sr (mean 4491 ppm) and low contents of Co ( 7) and low Co/Ni (< 1), Co/Zn (< 2), Y/Ho (< 41) ratios and low ΣREE (< 100) indicate that Kula Mn-ores formed in a submarine hydrothermal system. Redox-sensitive element ratios [V/(V + Ni) = 0.13–0.46, Ni/Co = 1.74–4.59)] and positive Mn*sn (0.22–0.92) indicate that the Kula Mn-ores formed in an oxic depositional environment. Furthermore, a low Lan/Ndn (1.19–3.04), Lasn/Ndsn (< 1.36); moderate Lasn/Cesn and Al/(Al + Fe) ratios; weak negative Ce/Ce* anomaly (0.58); low Ceanom values (< − 0.32); and high Sr concentrations (up to 20,178 ppm) indicate that the Kula manganese deposit formed in a high-salinity and oxygenated marine sedimentary environment with as part of a distal hydrothermal system. Radiolarians from different blocks within the Late Cretaceous melange complexes hosting the Mn-oxide deposits along Izmir-Ankara-Erzincan Suture Zone in northern Turkey yielded a wide range of ages, Middle Triassic to Late Cretaceous. Hence, the Mn-oxides could have formed at any time during that interval, when oceanic crust was generated in different tectonic settings of the Izmir-Ankara Ocean. The ophiolite was then emplaced southward onto the Menderes Platform prior to the middle Eocene.

A Late Cretaceous ensimatic arc developed during closure of the northern branch of Neo-Tethys (central-northern Turkey)

Studies on differentiation of ensimatic arc units from melange units in the Northern Anatolian Ophiolitic Belt (NAOB) are very rare. The study area represents the southern edge of the central part of the NAOB. The aim of the study is to distinguish the main units of NAOB and to define the geotectonic setting of the Darmik Ensimatic Arc Association (DEAA). To study the ensimatic arc units as an independent tectonic unit is important and facilitates understanding of the geological evolution of the NAOB. During field studies, the contact relationships within the Darmik Ensimatic Arc Association (DEAA) units and also with other tectonic units were reviewed. Then, paleontological, geochronological, petrological and also geochemical properties of the DEAA have been defined. In the present study, ensimatic arc units have been distinguished from ophiolitic association for the first time, in the region. As a result of the study, the DEAA is divided into two levels and lower level named as the Kartal unit and upper level named as the İzibüyük unit. In the Kartal unit, basaltic and andesitic lavas with their equivalent pyroclastic rock units are dominant. Radiolarite and mudstones interbedded within the Kartal unit of DEAA have Turonian and Santonian age. In addition, the age of basalt samples of DEAA are 98.7 ± 2.4 Ma, defined using the 40Ar/39Ar method. The İzibüyük unit of DEAA comprises conglomerate, sandstone, claystone, mudstone, clayey limestone, micritic limestone with volcanic interbeds and also calciturbidites. Santonian–Maastrichtian ages were obtained from this unit of the DEAA. In conclusion, the age of DEAA is widely accepted as Late Cretaceous. After the evaluation of the analysis, the volcanics of the DEAA originated from calcalkaline-basalts, which reflect ensimatic arc magmatism.

On the nature of the Teisseyre-Tornquist Zone

The authors interpret the structure of the Central Carpathian-North European plates suture zone in Poland, where three main Carpathian tectonic units: the Central Carpathian, Pieniny Klippen Belt (PKB) and Outer Carpathian are present. In general, the PKB follows this zone. Several deep bore-holes were drilled in this region and the seismic lines were tied to bore-hole data and geological maps. The Polish PKB belongs to the complex geological structure stretching from Vienna in Austria to Romania. The rocks included in the PKB tectonic components were deposited within the paleogeographic realm known as the Alpine Tethys, mainly during the Jurassic-Early Cretaceous times. Both strike-slip and thrust components occur within the Polish section of the PKB. The strongly tectonized, few kilometer wide PKB zone is limited by a flower structure marked by two major faults, linked to the strike-slip zone. These faults reach the North European Platform (part of the North European Plate). The flysch sequences, arranged into a series of north-vergent thrust-sheets, constitute the main component of the PKB in the survey zone. They contain olistoliths, which are mainly Jurassic-Early Cretaceous in age. The PKB tectonic components of different age, strike-slip, thrust as well as toe-thrusts and olistostromes are mixed together, giving the present-day melange character of this belt, where individual units are hard to distinguish. Two olistostrome belts (melange units) exist within the PKB structure. The seismic lines show the Central Carpathian Paleogene rocks covering the Paleozoic Central Carpathian Basement south of the PKB. The Subtatric covers the High-Tatric autochthonic and allochthone rocks. The Central Carpathian Plate is thrust over the North European Platform in the Podhale region. The allochthonous Outer Carpathians consist of several nappes (thrust-sheets) verging northward. They are thrust over each other and over the North European Platform which dips gently southward.

Oxygen and carbon isotope compositions of carbonates in a prominent lithologically mixed unit in the central South Norwegian Caledonides

A prominent pre-Scandian lithologically mixed unit in the central South Norwegian Caledonides contains more than 100 partly carbonated and hydrated metaperidotite bodies and locally fossiliferous detrital serpentinites. The lateral consistency of this mixed unit was not fully appreciated in the past. Therefore, parts of the mixed unit along strike were interpreted to belong to several different tectonostratigraphic levels. Here, we present new carbonate stable isotope data that suggest that the carbonates of the mixed unit between Bergen and Otta (re-)equilibrated at unit-wide similar peak metamorphic conditions. The isotope compositions are characteristic for this unit and indicate that it represented one single tectonic unit during the Scandian Orogeny. The carbonates in the melange are characterized by a narrow range of δ18O (SMOW) values between + 11 and + 15.5‰ and three groups of δ13C (PDB) values: (I) + 1.6 to + 0.3‰, (II) − 1.8 to − 3.9‰, and (III) − 6 to − 8.6‰. Carbonates of group III probably were affected by decarbonation or by a fluid containing organic carbon, whereas carbonates of group I and II overlap with δ13C values typical for Ediacaran–Silurian marine carbonates and may have retained their initial δ13C imprint. We suggest that the δ18O values (re-)equilibrated with unit-wide released metamorphic fluids during Scandian metamorphism. An outcrop-scale homogenisation of the δ13C values reflects the local carbon isotope signature of the released metamorphic fluids that circulated channelized through the melange unit.

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge, SW Japan

ABSTRACTTo understand tectono‐metamorphic processes within or close to the brittle–ductile transition of quartz‐rich crustal rocks in an accretionary wedge, an integrated field, petrological, geochronological and Raman spectroscopic study was conducted on the Mikabu‐Northern Chichibu belt in SW Japan. Field mapping in central Shikoku reveals that the Northern Chichibu belt is comprised of a pile of four tectono‐stratigraphic units, referred to as A, B, C and D units. The A unit (dominated by pelagic sedimentary rocks) represents the structurally lowest and youngest accretionary complex that forms a composite unit with the Mikabu ophiolitic suite. The B unit (consisting of chert‐clastic rock sequences) overlies the A unit and is overlain by the C and D units (mudstone‐matrix mélange units). Raman spectroscopy of carbonaceous material constrains the peak temperature of each unit to be ~290°C for the A unit, 270–290°C for the B unit, 230–250°C for the C unit and ~220°C for the D unit. Ductile deformation and pervasive metamorphism are limited to rocks in the Mikabu, A and B units. Alkali pyroxene and sodic amphibole occur in metabasite from the Mikabu, A and B units, and the widespread occurrence of prograde veins containing lawsonite+quartz pseudomorphs after laumontite was newly recognized from the C unit. Phase petrological data constrain the peak pressure of each unit to be ~0.65 GPa for the Mikabu‐A unit (aragonite stable), ~0.45–0.6 GPa for the B unit (jadeite+albite stable in the structurally lower part), and ~0.35 GPa for the C unit (prehnite+lawsonite stable). The peak metamorphic pressure increases towards structurally lower and younger accretionary complexes, but the thickness of the preserved strata is insufficient to account for the inferred pressure range. The structural–metamorphic relations imply thickening of the accretionary wedge by underplating was followed by a significant phase of thinning by both ductile and brittle processes.

Determination of the Dam Axis Permeability for the Design and the Optimization of Grout Curtain: An Example from Orhanlar Dam (Kütahya-Pazarlar)

Rural projects become an important issue considering the rapid increase of population in Turkey. Alongside the contribution to the national economy, dams serve as an environmental structure, which are utilized in flood prevention, sustainable energy, fighting with forest fire and recreation. However, dam construction must be well planned and projected to minimize the unexpected events such as water leakage. This study comprise the geotechnical studies and the design of the planned grout curtain in Orhanlar Dam (Kütahya/Pazarlar). In this context, field and laboratory studies was realized in Orhanlar Dam axis and reservoir area. Within the scope of field studies, engineering geology map was generated, a suitable axis location was specified for the dam and drilling and in-situ testing was realized. Within the field studies, the joint conditions of the geological units under the dam axis and its effect on permeability was observed. For the geotechnical purposes drilling works performed during the planning stage, 5 boreholes total 166 m was realized on dam axis, 1 borehole total 10 m was realized on cofferdam, 1 borehole total 10 m was realized on diversion tunnel and 1 borehole total 10 m was realized on spillway. To determine the permeability profile on dam axis and design the grout curtain, Lugeon tests in Dağardı ophioloitic melange units observed in dam axis, falling head permeability tests in alluviums observed in thalveg and slope debris observed in right abutment were performed. As a result of these studies geotechnical information about the permeability of Orhanlar Dam was collected and the grout curtain hole was designed.

Hyperspectral mapping of Iranian east ophiolite mélanges using neutral network classification method

Mapping in the areas, which are near the collision zone, needs extensive and exact studies and tools because of the variety of rocks and complex structures. Hyperspectral sensor is one of the most advanced tools that can do imaging in hundreds of delicate and continuous bands in visible and infrared spectrums. Therefore, it can identify many lithologies despite spectral similarities and complications. In the present study, as one of the first hyperspectral data survey for separating ophiolite melange units in Iran, we applied the spectral-based method of neutral network classification method on hyperion images of Tabas in southeast of Iran. Based on various laboratory–field studies, the lithology of the study area can be separated into five general groups: ophiolite series, metamorphic units, Oligocene–Miocene to Quaternary volcanic units, and lime and flysch units. In order to perform an accuracy assessment of the image processing results, some locations and points were sampled. These samples were analyzed under the microscope and by electron microprobe system. Based on field–laboratory works, the accuracy of the resulting image was validated. Consequently, the neutral network method as advanced hyperspectral image processing method produced an average user accuracy coefficient of 62 % in some parts of colored melanges. Therefore, the method based on neutral network theory for spectral classification has acceptable ability to discriminate various lithologies in the complex geological scenarios.

Early exhumation of underthrust units near the toe of an ancient erosive subduction zone: A case study from the Northern Apennines of Italy

Apatite fission-track (AFT) analyses were performed on 16 sandstone samples from a tectonic melange unit exposed in three tectonic windows located near the inferred front of the early Miocene subduction system of the Northern Apennines of Italy. The tectonic windows display a block-on-block tectonic melange present under the Ligurian Units. The melange is formed by portions of the upper plate incorporated in the plate boundary shear zone as a consequence of a mechanism of frontal tectonic erosion during the Aquitanian (early Miocene). AFT and structural data, together with stratigraphic constraints, allow the reconstruction of a complete deformation cycle with a phase of underthrusting followed by underplating and early exhumation of the tectonic melange. The exhumation, in particular, took place at ∼10–20 km from the original subduction front. Moreover, the analysis suggests that multiple faults were active at the same time in the frontal part of the subduction zone.

Structural anatomy of the Ligurian accretionary wedge (Monferrato, NW Italy), and evolution of superposed melanges

We document in this study the internal structure of the Late Cretaceous–late Oligocene Ligurian accretionary wedge in northwestern Italy, and the occurrence in this exhumed wedge of broken formation and three different types of melanges that formed sequentially through time. The broken formation is the oldest unit in the accretionary wedge and shows bedding-parallel boudinage structures, which developed as a result of layer-parallel extension at the toe of the internal part of the Alpine wedge front during the Late Cretaceous–middle Eocene. This broken formation experienced an overprint of tectonic, diapiric, and sedimentary processes as a result of continental collision in the late Oligocene. The NE-vergent thrusting and associated shortening produced a structurally ordered block-in-matrix fabric through mixing of both native and exotic blocks, forming the tectonic melange. The concentration of overpressurized fluids along the thrust fault planes triggered the upward rise of shaly material, producing the diapiric melange, which in turn provided the source material for the downslope emplacement of the youngest, late Oligocene sedimentary melange. The sedimentary melange units unconformably cover the collisional thrust faults, constraining the timing of both this episode of contractional deformation related to continental collision and the combination and overlap of tectonic, diapiric, and sedimentary processes. Our multiscale structural analysis of the Ligurian accretionary wedge shows that tectonic, diapiric, and sedimentary processes played a significant role in its evolution, and that the interplay between and the superposition of these different processes strongly controlled the dynamic equilibrium of the accretionary wedge in the NW Apennines–western Alps. This kind of polygenetic melange development may be common in many modern and ancient accretionary complexes, and the processes involved in their formation are likely to be responsible for major tsunamic events in convergent margins.

High-Pressure - Low-temperature evolution in the Indus-Tsangpo Suture along the Kohistan Arc (Kaghan Valley, NE Pakistan)

North of Naran in the Kaghan Valley (NE Pakistan), the metabasites of the melange units lying below the Kohistan Arc, contain glaucophane. Typically they reflect blueschist-metamorphic conditions (0.7 GPa, 400°C). Associated graphite-rich metapelites with quartz veins underwent upper greenschist to amphibolite conditions. Near Naran we observed in quartz grains of type 3 veins first minute relics of Fe-Mg carpholite indicating earlier blueschist metamorphic conditions. P-T estimates indicate 1.2-1.6 GPa at 380-410 °C, pressure and temperature values typical for blueschist metamorphic conditions. Changes in mineral assemblages and abundant sudoite component in associated chlorite point to a pressure drop after peak I conditions. We assign the observed changes to peak I conditions occurring during a Cretaceous subduction event. Temperatures estimated with Raman graphite-thermometry clearly indicate a significant subsequent rise of post- peak I temperatures up to 500°C. This is compatible with the amphibolite peak II assigned to the Tertiary continental collision that leads to subduction of the Indian Plate and ultra-highpressure metamorphism. During subduction the blueschist metamorphic metapelites underwent dehydration, which caused alteration in the overlying lithospheric mantle. In a hydrated lithospheric mantle density is significantly reduced which enhanced subduction of continental crust in the Higher Himalaya. This P-T evolution is typical for a collision orogen with a high plateau but remarkably contrasting findings from Eastern Anatolia, where plateau building is in statu nascendi (e.g., Oberhansli et al., 2010).

Late Dolomitization in Basinal Limestones of the Southern Apennines Fold and Thrust Belt (Italy)

The Triassic pelagic carbonates of the Lagonegro Units from the southern Apennines fold and thrust belt host discordant bodies of dolostone. These rocks show textural features typical of zebra or saddle dolomites. In this contribution, the results of field observations, petrography and geochemical analyzes performed on samples from three different outcrops are presented.Field data indicate that the peculiar zebra-type rock and brecciated fabrics were controlled by regular bedding of the micritic, pelagic limestones. The dolomitization resulted in the replacement of the host limestones and subsequent void-filling precipitation of dolomite in a dilatational stress field.Homogenization temperatures (Th) are in the range 80-120°C, with a clear mode of 95 ÷ 10°C. When corrected for maximum pressure, they indicate an upper limit for dolomite formation around 110-115°C. Melting temperatures of ice (Tmi ) point to salinities in the range 2-6 wt% NaCl eq, with a mean of 4.2%. δ 13 C values overlap those of the Upper Triassic seawater, whereas the δ 18 O values are significantly depleted compared to the coeval seawater. 87 Sr/86 Sr values are higher with respect to Upper Triassic seawater, and partially overlap Middle-Upper Miocene values.The performed analyzes indicate a dolomitization process driven by warm fluids (110-115°C) with a salinity close to that of seawater and O-isotope ratios comprised between seawater and formation waters. It is suggested that the dolomitization was accomplished by formation waters squeezed out or from surrounding lithologies or from the underlying melange units. Integration with available thermal data on the collisional history of the belt suggests that the fluid-flow took place after maximum burial, in the early stages of exhumation, in the assumption of a fluid in thermal equilibrium with the host rocks.According to literature data, the dolomitization event was quite widespread. It can be argued that it was part of a major fluid-flux associated with fold and thrust belt development, which possibly affected the Apulian Platform carbonates located in the foreland. The latter, now buried below the nappe stack, hosts some of the major oil fields of continental Europe.

Hinterland-directed transtensional faulting at an orogen structural front: The example of the Cap-Chat melange, Quebec Appalachians

The Cap-Chat melange is a regional-scale dismembered unit that occurs close to the Quebec Appalachian front. The melange records early (D 1 , Taconian) and late (D 3 , Acadian) regional deformations. The greater degree of disruption of the melange compared with surrounding units is due to the superimposition of D 2 structures that are rare outside the melange. D 2 structures include both layer-extensional features such as block disruption along shear zones and faults and layer-contractional features such as folds that record the transtensional collapse of the Taconian orogenic wedge along a rheologically weak deformation zone. The Cap-Chat melange is interpreted as the westernmost remnant of a zone of mechanical decoupling in the upper crust, which allows normal or transtensional faulting in the inner parts of the orogen. This paper presents evidence that beyond the general complexity of melange units, geological observations may still contribute to unraveling their tectonic history and that transtensional reactivation of original thrust faults is a viable mechanism in the frontal parts of orogens.

The basal tectonic mélange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea

Recent field work and mapping in the lower units of the Cabo Ortegal Complex provided new data about the tectonic melange that appears in the lowest structural position: the Somozas Melange. This melange unit with average thickness of 1800 m is restricted to the eastern part of the complex, and is located at the advancing front of the allochthonous complexes of NW Iberia. Three rock units are involved in the melange: 1) an ophiolitic melange consisting of igneous rocks mixed with serpentinites; 2) a metasedimentary unit with phyllites and phyllonites, with scarce conglomerates, marbles and quartzites; 3) high-T metamorphic rocks with varied types of amphibolites and orthogneisses. Two granitic rocks within the ophiolitic melange were dated using U-Pb zircon geochronology at 527 ± 2 Ma and 499 ± 1 Ma. Two different series of igneous rocks can be distinguished in this melange. The first series consists of gabbros, diorites, granitoids and basalts-basaltic andesites with calc-alkaline affinities. The second series contains common basaltic rocks, diabasic dikes and gabbros with chemical compositions typical of island-arc tholeiites. Both igneous series shared a common geographic setting, but the island-arc tholeiites are younger than the calc-alkaline igneous rocks. The two igneous series were probably generated in a mature volcanic arc located along the periphery of Gondwana. In the metasedimentary unit, a conglomerate from a large tectonic block included in serpentinites yielded age populations of detrital zircons suggesting that the sediments were deposited along the periphery of the West-African Craton. This conglomerate contains a large number of zircons (n = 24) with ages ranging 630-464 Ma, probably representing the chronology of the Pan-African event, including the magmatic activity in the volcanic arc where the igneous lithologies involved in the melange were generated. The maximum age of sedimentation for this conglomerate is estimated as latest Cambrian – earliest Ordovician, and constraints the end of the magmatic activity in the volcanic-arc. Within the unit of high-T rocks, an orthogneiss yields a U-Pb protolith age of 485 ± 6 Ma, which is similar to other ages of igneous rocks in the basal allochthonous terrane in NW Iberia. The three rock assemblages forming part of the Somozas Melange may be linked to the evolution of a mature peri-Gondwanan volcanic arc. This volcanic arc was affected by pronounced extension which caused the opening of intra-arc basins, culminating the rifting and subsequent drift of the external parts of the arc during the opening of the Rheic Ocean. This opening started during a time interval constrained by the peak activity in a mature volcanic arc (c. 527-499 Ma) and the generation of intraarc basins around the Cambrian-Ordovician boundary. Tectonic melanges including high-P rocks have been classically related to subduction zone environments. Regional relationships in NW Iberia and the nature of the rock units involved in the Somozas Melange, suggest that two different subduction zones generated during oblique convergence and collision between Gondwana and Laurussia were active during the final stages of the assembly of Pangea. The first related to the underthrusting beneath Laurussia of the most external Gondwana margin (c. 370 Ma). The second subduction zone was a new one which accreted later remnants of a peri-Gondwanan arc and sediments of the continental margin below a layer of exhumed high pressure rocks. This oblique collision finished the closure of the Rheic Ocean and contributes to define the new oceanic domain located to the East of Pangea, the Palaeotethys.

NEW GEOCHEMICAL AND AGE DATA ON THE OPHIOLITES FROM THE OTHRYS AREA (GREECE): IMPLICATION FOR THE TRIASSIC EVOLUTION OF THE VARDAR OCEAN

This paper presents new radiolarian biostratigraphic and volcanic geochemical data for the Agoriani Melange Unit and Fourka Unit, which represent the basal part of the Othrys ophiolitic nappe (central Greece). The Agoriani Melange Unit is the base of the Othrys ophiolitic nappe, consists of a tectono-sedimentary melange that is, in turn, tectonically overthrust by the Fourka Unit, which consists of basalt-radiolarian chert sequences. Geochemical data show that, in the study area, the Agoriani Melange includes alkaline within-plate basaltic (WPB) and normal-type mid-ocean ridge (N-MORB) sequences in the pillowfacies, while the Fourka Unit mainly consists of normal-type mid-ocean ridge basaltic (N-MORB) sequences, as well as low-K tholeiitic (L-KT) sequences, both in occurring in the pillow-facies. Alkaline WPB have a clear enriched nature, with high concentration of high field strength elements (HFSE), relative enrichment in light (L-) rare earth elements (REE) with respect to heavy (H-) REE, and high Nb/Y (> 0.7), Ce/Y (> 1.5) Ta/Hf (> 0.35), Th/Yb (> 0.7) and Ta/Yb (> 0.3) ratios. N-MORB rocks have a more depleted nature with flat N-MORB normalized HFSE patterns, flat or slightly LREE depleted chondrite normalized REE patterns, and low Nb/Y (< 0.3), Ce/Y (< 0.5) Ta/Hf (< 0.1), Th/Yb (< 0.08) and Ta/Yb (< 0.05) ratios. L-KT basalts show an evident subduction- related geochemical signature exemplified by depletion of Ta, Nb, Zr, and Ti relative to other HFSE, slight LREE/HREE enrichment, and relative enrichment of Th with respect to Ta. Biostratigraphic data for the Fourka Unit show that radiolarites associated with N-MORBs were deposited, on the whole, in Middle and Late Triassic time. In particular the late Anisian - early Ladinian age found in one stratigraphic section represents the oldest age ever reported for a MORB sequence in the Hellenides. Radiolarites deposited over the L-KT basaltic sequence are early Carnian - middle Norian/Late Norian in age. The radiolarites associated with alkaline WPB in the Agoriani Melange did not yield determinable radiolarians, while a radiolarite sequence in this melange not associated with basalts is middle Bathonian to late Bathonian/early Callovian in age. The data presented in this paper provide new constraints on the early (Middle- Late Triassic) phase of evolution of the Vardar Ocean in the Hellenides. The MORBs are interpreted to have erupted during Middle-Late Triassic time, related to the initial opening of the Vardar oceanic basin. The L-KT basalts are interpreted to have erupted during Middle-Late Triassic time (i.e., in part contemporaneously with MORBs), in the ocean-continent transition zone adjacent to the rifted continental margin. Moreover, we speculate that the alkaline WPB have formed in oceanic seamounts or in the ocean-continent transition zone adjacent to the rifted continental margin.

Metamorphic sole rocks and their mafic dykes in the eastern Tauride belt ophiolites (southern Turkey): implications for OIB-type magma generation following slab break-off

The metamorphic sole rocks observed between the Pozantı–Karsantı ophiolite and the melange unit are located on the eastern part of the Tauride carbonate platform. They consist of ortho-amphibolites at the top and metasedimentary lithologies at the base. Amphibolites from the metamorphic sole rocks are represented by OIB, MORB and IAT based on their major, trace and REE compositions. The isolated dolerite dykes intrude both the metamorphic sole rocks and the ophiolitic unit at different structural levels. The dolerite dykes cutting the metamorphic sole rocks are enriched in LILE and depleted in HFSE. Enrichment in LILE such as Th, relative to HFSE, is indicative of the presence of a subduction component. Flat-lying REE patterns of the dolerite dykes also confirm an IAT source. Pyroxenite and albitite dykes also cut the metamorphic sole rocks. REE patterns of pyroxenite dykes show prominent LREE enrichment, similar to that observed in within-plate alkaline basalts. The alkaline isolated pyroxenite dykes were probably the result of late-stage magmatism fed by melts that originated from an OIB source, shortly before the emplacement of the Pozantı–Karsantı ophiolite onto the Tauride carbonate platform. A hydrothermal alteration stage is characterized by albitite formation in the joints of the metamorphic sole rocks and by secondary mineralization along the contact zones of dolerite dykes. Mineral parageneses in the metamorphic sole rocks exhibit amphibolite and greenschist-facies assemblages. Geothermobarometric studies based on a newly recognized mineral assemblage (e.g. kyanite) and chemical compositions of minerals in the metamorphic sole rocks indicate that the metamorphic temperature during the metamorphism was 570–580°C and the pressure was around 5–6 kbar.

Stratigraphy and geochemistry of pillow basalts within the ophiolitic mélange of the Izmir–Ankara–Erzincan suture zone: implications for the geotectonic character of the northern branch of Neotethys

The Izmir-Ankara-Erzincan suture zone of Turkey is a broad zone of ophiolitic melange containing numerous fragmented blocks ranging in age from Triassic to Cretaceous. Stratigraphic sequences for various melange units are compared, together with the geochemistry of associated basaltic pillow lavas of Cretaceous age. A review of geochemical data for the pillow lavas demonstrate: (a) a dominant group of alkalic basalts with enriched incompatible elements, variable Zr/Y and Zr/Nb ratios, and (b) a range of tholeiitic basalts with slightly depleted to mildly enriched (normalized) rare earth patterns, (La/Yb)N 0.4-3.0, and generally low Zr/Y ratios. The alkalic basalts can be chemically matched to Pacific Ocean seamounts, although the close association of red radiolarites and cherts suggests that many basalts represent the margins of such structures, rather than the main seamount edifice. Nd-Sr isotope data are typical for ocean island basalts and represent an admixture of a dominant EM-1 source and a depleted MORB-like source. Enhanced δ18O compositions are a consequence of submarine alteration and not crustal contamination. Tholeiitic compositions have affinities with both N- and E-type MORB, although most are probably representative of tholeiitic ocean islands. Overall the basalts are mainly representative of structures built on the ocean floor, rather than the oceanic crust itself, being scraped off the subducting crust and preserved in the melange of the accretionary wedge.