Konu "Craton" ile AYBE- Katı Yer Bilimleri Lisansüstü Programı - Yüksek Lisans'a göz atma
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ÖgeEclogite İnduced Deformation of the Siberian Craton(Eurasia Institute of Earth Sciences, 2019-05-03) Ballı, Açelya ; Göğüş, Oğuz Hakan ; 602171001 ; Solid Earth Sciences ; Katı Yer Bilimleri Anabilim DalıThe deformation of the cratons, whose roots are approximately 250 km deep is very difficult. The removal of the mantle lithosphere, which is one of the proposed mechanisms for the deformation of the craton that is stable for long periods, is carried out by many different processes. Deformation of the craton as a result of a gravitational instability is one of the most likely mechanisms. According to isopycnic hypothesis, lithospheric mantle of cratons thought to be buoyant due to their depleted composition, even though most of them Archean in age and cold. Since the mantle lithosphere of the craton is lighter in density than asthenosphere, an additional force is required for a gravitational instability to occur. This thermo - mechanical force causes deformation of the roots of the craton by creating an instability between the mantle lithosphere and the asthenosphere. The Siberian craton is one of the world's largest Archean - Proterozoic cratons. The Siberian craton has approximately 100 - 1300 m surface topography, 35 - 53 km MOHO thickness, and a maximum depth of 350 km LAB which are acquired from petrological studies, seismic tomography and gravity anomalies. Specifically, the LAB varies among 170-350 km and such depth change is not well understood. Until the formation of the Siberian craton is completed, it hosts many tectonic and magmatic events. These include active margin zones, continent collisions, and rift zones. As a result of pressure change in the active boundary regions, the transformation of basalt to eclogite takes place. Therefore, it creates a gravitational instability in the environment. Gravity anomalies observed near kimberlite fields, reflect the possibility of denser eclogitic bodies under the crust of Siberian craton. Our study focuses on testing potential deformation of the Siberian continental lithosphere with the presence of these eclogitic bodies. We performed 2D numerical experiments to investigate the effects of eclogite blocks that are varying in size and density. Crust rheology was prepared in accordance with Siberian craton. The density of the mantle lithosphere (3330 kg / m3 - 3410 kg / m3 +20 kg / m3) is changed to observe its effect on the system, and eclogite blocks of different size (5 km x 500 km, 10 km x 250 km, 25 km x 100 km) are added to the lower crust base to start a gravitational instability. According to model results, depending on the deformation of the mantle lithosphere, eclogite block can either stay attached to the lower crust, or it can be detached from it. In the case where the eclogite block attached to the lower crust, two different conditions: localized deformation (do not occur the drip mechanism) and non-localized deformation occurs due to the small-scale convection movement. Also, two different removal mechanism for the case where eclogite becomes detached are also observed: high degree deformation of mantle lithosphere, and the eclogite block pierce through the mantle lithosphere. Comparison of experimental results with geophysical data for MOHO and LAB depths showed that, the most convenient models for Siberian craton are the models where the dripping were not observed. Mantle lithosphere densities of 3350 kg / m3 or less yields the most consistent results. While the width of the eclogite block causes high-degree deformation, it is observed that with increasing thickness it leads to formation of viscous drips. Taking MOHO and LAB depths into account obtained from the model results, it has been observed that the model #A1, #A2 and #A3 agrees well with the BB' cross-section at 20.92 Ma, 25.36 Ma and 20.92 Ma, respectively. Experimental results indicate that, eclogite block(s) under the Siberian craton may still be there and craton itself does not undergo any significant deformation.
ÖgeMobile Cratons, Subcretion Tectonics and Formation of Ttgs(Eurasia Institute of Earth Sciences, 2019-05-03) Çetiner, Uğurcan ; Göğüş, Oğuz Hakan ; 602171003 ; Solid Earth Sciences ; Katı Yer Bilimleri Anabilim DalıThe 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.