AYBE- Katı Yer Bilimleri Lisansüstü Programı - Yüksek Lisans
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ÖgeMetamorphic Evolution of the Elekdag Eclogites (Central Pontides)(Avrasya Yer Bilimleri Enstitüsü, 2019-01-13)The topic of the thesis is focused on the metamorphic and tectonic evolution of high-pressure metamorphic eclogites and by extension, the surrounding meta-lherzolite and serpentinite of the Elekdag Ophiolite within the Central Pontide Mountains of Turkey. The goal of this study is to continue the work of previous authors in better understanding the timing and conditions of metamorphism and their associated tectonic events through field mapping, detailed petrographic and microstructural analysis, investigation of stable equilibrium assemblages with wavelength-dispersive x-ray, energy-dispersive, angle-selective backscatter spectroscopies coupled with isochemical phase equilibria diagrams and conventional cation exchange geothermobarometry analysis. The Elekdag Ophiolite and other related ophiolites in the Central Pontide Mountains of Northern Anatolia are interpreted as being mantle and crust accretionary prisms which formed along the southern edge of the Laurasian Supercontinent prior to the closure of the Paleotethys Ocean and the Formation of Pangea together with the Gondwana Supercontinent. Elekdag is closely associated with the larger Cangaldag Island Arc and the larger Domuzdag HP mélange. During subduction in the Cretaceous the Elekdag Complex experienced up to eclogite-facies metamorphic conditions. Metamorphic rocks present in the study area include typical ophiolite assemblages with the addition of HP-LT blueschists, greenschist facies mineral assemblages, serpentinite, mica schists and HP-MT eclogites. The HP-LT units are contained within lenses along the boundary of the serpentinite body and are also in contact with the neighboring Domuzdag Complex. Typical mineral assemblages of the eclogites are garnet + omphacite + glaucophane + clinozoisite + white mica ± lawsonite ± tourmaline ± rutile. Blueschist mineral assemblages consist of garnet + glaucophane + clinozoisite + white mica + chlorite ± quartz ± lawsonite ± rutile. Most primary mineral assemblages have been heavily overprinted with lower grade hydrous mineral assemblages. Retrogression occurred due to lower pressures and temperatures during exhumation and the extensive infiltration of hydrothermal fluids within the rocks characterized by chlorite. Typical retrograde phases are glaucophane (for eclogites), as well as the common greenschist facies assemblage of chlorite + albite + clinozoisite ± stilpnomelane. The metabasite samples investigated show mid-ocean ridge basalt affinities (MORB) with at least one protolith being a cumulate. Constraints on maximum pressures and temperatures of metamorphism, and by extent depth of subduction and subsequent exhumation were inferred based upon geochemical and petrographic analysis. Two distinct samples were modeled in isochemical phase diagrams. One, a cumulate metabasite yielded maximum PT conditions of 21±2 kbar at 360 ± 50 °C based upon garnet and phengite compositional isopleths as well as a maximum temperature of metamorphism of 400 °C according to Fe/Mg cation exchange thermometry between garnet and clinopyroxene. A second modeled sample yielded lower pressure ranges of 10±2 kbar at a higher temperature of 500 °C which is in contrast to similarly thermometry results which give maximum temperatures of ~450 °C.
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ÖgeUpper Cretaceous Stratigraphy and Volcanism in İğneada Region, Pontides, NW Turkey(Avrasya Yer Bilimleri Enstitüsü, 2020-07-24)The Pontide Upper Cretaceous magmatic arc can be traced for over 1000 km along the southern Black Sea coast from Georgia to Bulgaria. The arc extrusive sequence is wellexposed in İğneada region in Thrace close to the Bulgarian border. The Upper Cretaceous sequence in İğneada region overlies the schists and phyllites of Strandja Massif with an unconformity. The sequence consists at the base of Cenomanian shallow marine limestone, which pass up into carbonate-rich sandstone, marl and calcareous siltstone indicating deepening upwards. The sedimentary rocks pass up into a volcanic-volcaniclastic sequence of andesitic tuff, lapilli-tuff, lapillistone, agglomerate, andesitic and basaltic-andesitic lava flows. The volcanic-volcaniclastic sequence is divided into three domains for a detailed investigation and they are indicated on the prepared geological map as D1, D2 and D3. The volcaniclastic rocks are intercalated with lava flows, rare pelagic limestone and shale beds. The sequence starts with andesitic volcaniclastic rocks and lava flows, and changes to basaltic-andesitic and then, to andesitic and dacitic rocks. The calc-alkaline characteristic of volcanic rocks and negative Nb-Ta anomaly indicate a volcanic arc setting. Although it is disrupted by several normal faults, the volcanic sequence can be traced from older to younger along the coast of İğneada. The sea floor alteration, which is found in all volcaniclastic and volcanic rocks, the intercalated pelagic limestones, and turbiditic sedimentary structures show that the rocks were deposited in deep submarine conditions in an intra-arc to fore-arc environment.
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ÖgeMobile Cratons, Subcretion Tectonics and Formation of Ttgs(Eurasia Institute of Earth Sciences, 2019-05-03)The formation of Archean cratonic lithosphere and TTG (Tonalite-Trondjemite-Granodiorite) suites is not well understood, in part because the style of global tectonics active at that time is uncertain. The non-plate tectonic hypothesis for formation and evolution of continents we test in this study involves: intense magmatism above mantle upwellings in an unstable single plate regime to form cratonic nucleii; imbrication and anatexis of crust-dominated oceanic lithosphere at convergent margins driven by mantle flow, with build-up and thickening of cratonic keels by collisions. We use 2D numerical geodynamic models to investigate whether differential motion between the convecting mantle and cratonic keels can induce horizontal motion of a craton to form an accretionary orogen. Using the convection code StagYY, we attempt to model a self-consistent subcretion of oceanic lithosphere pushed by a pre-imposed craton. Initially, 40 km thick basaltic crust, accompanied by 20 km thick sub-oceanic lithosphere, is introduced on both sides of the 230 km thick cratonic lithosphere, with an initial potential mantle temperature of 1750 K. The domain is divided by 64 vertical cells and 512 lateral cells corresponding to 660 km depth and 2000 km length. Both for upper and lower boundary, free-slip surface conditions are used. Left and right boundaries are periodic. Velocities are forced to be zero until a critical depth of 60 km, after that, a sub-lithospheric mantle flow of 4 cm/yr imposed into the model. Diffusion creep has chosen to be the main deformation mechanism for computational reasons. Our study involves investigating the effects of different parameters on the evolution of the experiments, such as; reference mantle viscosity, eclogite phase transition depth, yield stress of the oceanic lithopshere, and a change in the deformation mechanism. Our experimental results indicate that, cratonic keels can be mobilized by the sub-lithospheric mantle winds. We chose a reference model with typical yield stress (20 MPa), mantle viscosity (1020 Pa s), and eclogite transition depth (40 km) values, where craton becomes mobilized after ~160 Myr from model initiation, and oceanic lithosphere becomes subcreted at the cratonic margin. It has been found that, reference mantle viscosity has a significant impact on the exact time that the craton has become mobilized. Experiments with a 1021 Pa s reference mantle viscosity yielded in faster mobilizaiton times by a factor 22 – 23 times. In these models, subcretion of oceanic lithosphere at continental margins did not occur, but thickened oceanic lithosphere parts created downwellings resembling to subducting oceanic slabs. Lower mantle viscosities (1019 Pa s), however, could not generate sufficient stress to drift the craton away, but they led to a more vigorous convection and thermally eroded the cratonic roots. Increasing yield stresses from 20 MPa to 25 MPa and 30 MPa, made the oceanic lithosphere stronger and elongated the time needed for cratonic mobilization. Increasing it to 40 MPa led to a stable tectonic state, where craton did not become mobilized. Experiments with increased surface yield stresses did not provide an environment for subcretion tectonics, instead, lithospheric removal was due to eclogitic dripping where oceanic lithosphere became thick enough. Removal of the oceanic lithosphere changes velocity and orientation of the flows within the asthenosphere. In relation to that, evolution of some experiments contained convection cells generated within the mantle that ceased the motion of the craton, and even pushed it backwards for brief amount of time in some cases. Experiment performed to investigate the effect of deformation mechanism reflected the best example for this. In this case, rigthward moving craton traveled backwards at some point, created a subcretion on the left margin, and then, it started to move forward again to create a secondary subcretion, which has been classified as asynchronous double-sided subcretion. Our results indicate that, lithospheric removal mechanisms and craton mobilization times can vary with different parameters, but a displacement of 1350 km takes place in 30 to 40 Myr in all experiments, when the craton becomes mobile. Subcretion tectonics can only start in a narrow window, where surface yield stress is 20 MPa and reference mantle viscosity is 1020 Pa s, with the exception of eclogite transition depth being 60 km. Results indicate that subcretion mechanism can be achieved under given conditions, and TTG genesis via this mechanism can be valid when certain P-T conditions are met.
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ÖgeSubduction Roll Back and the Generation of Wet and Decompression Melting(Eurasia Institute of Earth Sciences, 2019-05-03)Subduction zones are the major element of active tectonics (55.000 km) of planet Eart (Stern, 2002). Subduction zones are regions of the Earth affected by the sinking of relatively cold and dense oceanic lithospheres into the mantle. Geophysical and geological evidences have led to interpretation of oceanic lithosphere subduction beneath the Sunda and Japan subduction region. Active subduction is taking important role to creation of serial volcanic province. These volcanic areas show variable chemical properties such as alkaline and calc-alkaline compositions. Alkaline composition is related with low pressure conditions and common at ridge regions however they are observed at some subduction zones such as Sunda arc. Calc-alkaline magmatism is related with dehydration reactions at subduction slab. Volatiles inside the top of the subducted oceanic lithosphere are releasing at 80 - 200 km depth condition. Volatiles decrease the melting temperature and cause partial melt of mantle wedge (triangular asthenospheric window beneath the volcanic arc). Thickness of the subducting slab is changing with oceanic lithosphere age. Feature of the subduction is dominated by thickness of the slab which is changing with age. Numerous 2D numerical geodynamic experiments (I2ELVIS) in the context of the tectonic evolution of the region are conducted to test the effects of the oceanic lithosphere age on melt generation. Within the scope of the models, the age of the oceanic lithosphere has been tried by increasing the age from 50 million to 120 million years. The plate convergence rate was defined as 4 cm / year and 8 cm/yr. The model boundaries are 1400 km vertical and 4000 km horizontal. as defined. The geology of the layers used in the models is defined as follows; 10 km atmosphere, 2 km. ocean, 20 km. felsic upper continental crust (wet quartzite), 15 km. felsic lower crust (wet kurtzite), 3 km. upper oceanic crust (basalt), 5 km. lower oceanic crust (gabbro) and 2 km. width is used for the zone of weakness hydrated mantle. Model result for subduction are comparable with observations related to the geodynamic evolution of the Sunda. The mantle structure compared by seismic profiles, considering convergent rate of plate motion. Chemical composition distribution of volcanics are correlating with geochemistry studies.
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ÖgeModeling the Structural Evolution of the Detachment Faults at Western Anatolia Back Arc System(Eurasia Institute of Earth Sciences, 2019-05-03)Extensional tectonics in the western Anatolia-Aegean region feature exhumation of the metamorphic core complexes that is accommodated by low angle normal (detachment) fault systems. Specifically, the central Menderes massif contains two symmetrically developed outward facing (Gediz and Büyük Menderes) detachment faults, which accommodated large scale displacements. Additionally, there are many younger high-angle normal faults in conjunction with the initiation of extension and synextensional magmatism since the Early Miocene. The standard fault mechanical theory does not allow such orientations, the occurrence of these faults at low angle and the seismicity on them are still not well-understood. Here, we investigate the evolution of the normal fault systems on lithospheric scale using thermomechanical forward models. We employ the numerical finite element code ASPECT to compute the visco-plastic deformation within a model domain that is 500 km wide and 165 km deep. The initial condition of our model is designed to reproduce the first-order lithospheric structure at the onset of Western Anatolia extension approximately 20 million years ago and consists of an upper crust (25 km thick) with wet quartzite rheology, a lower crust (25 km thick) with wet anorthite rheology, and a mantle lithosphere (30 km thick) with dry olivine rheology. We conduct two model suits where we investigate the impact of key parameters within a plausible range: (1) we vary the extension velocities imposed on the margins of the model boundary from Vext = 1- 4 cm/year full rate. (2) we vary the friction strain weakening factor of the upper crust (fc = 0.1 to 0.5). Our models show that these two parameters directly control the initial dip angle and development of the normal faults. We find that major faults are formed initially at 50-52° dip but evolve towards shallower dipping angles, 10-15°, because of the isostatic adjustment due to thinning/exhumation of the crust. The sequentially tilted faults on where slip can no longer be accommodated are abandoned and left behind as inactive low angle fault surfaces. Basin ward migration of newer fault is formed in the hanging wall to accommodate strain. The tectonic evolution of the central Menderes region is best reproduced in our reference model with a friction strain weakening factor of 0.2 and an extension rate of Vext = 3 cm/yr. Namely, this model agrees well with the detachment faults shallowing dip angles, outward facing faults and symmetry with respect to the central Menderes massif. In addition, the exhumed massif has a dome shaped structure and the distance to one another (80 km) is comparable to those of Western Anatolia. Also, high angle normal faults are formed above the detachment faults, typical for Gediz and Büyük Menderes grabens. When the friction strain weakening factor of the upper crust and extension rates are changed, differences in these structural elements are observed. We conclude that our reference model supports the two rolling-hinge detachment system separated by elongated metamorphic domes with fold axes perpendicular to the direction of extension.