AYBE- Katı Yer Bilimleri Lisansüstü Programı - Yüksek Lisans

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http://hdl.handle.net/11527/268

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Konu "Dinamik modelleme" ile AYBE- Katı Yer Bilimleri Lisansüstü Programı - Yüksek Lisans'a göz atma
  • Öge
    Dynamic Modelling Of Back-arc Extension İs The Aegean Sea And Western Anatolia
    (Eurasia Institute of Earth Sciences, 2016-05-02) Mazlum, Ziya ; Göğüş, Oğuz Hakan ; 602141002 ; Solid Earth Sciences ; Katı Yer Bilimleri Anabilim Dalı
    The Aegean Sea/Western Anatolia back-arc has predominantly been extending due to the southward retreat of the Hellenic subduction zone. This extension has been inferrred by the widespread magmatism, detachment faulting and the exhumation of metamorphic core complexes. While there is an agreement that the active slab retreat has been producing the extension in this back-arc (since the late Oligoce-early Miocene), the real nature of this extension may also be due to the other geodynamic mechanisms (e.g post-orogenic thinning). The major objective of this thesis is to test the geodynamic evolution of the back-arc extension by using numerical modelling and reconcile the model results with the observations from the Aegean Sea and Western Anatolia. Aegean Sea and Western Anatolia were under influence of collision between Sakarya continent and Menderes Taurides until Paleocene. The terminal closure of Northern branch of Neotethys compressed the area and it is thought that crust and lithosphere should have thickened. When the compression is worn of, the whole Aegean region started to extend. The large scale extension has been inferred by the exhumation of metamorphic core complexes (e.g., Kazdağ and Menderes massifs) and detachment faulting since late Oligocene. Interpretations of petrological data from the volcanic units show that the first arc volcanism, associated with the Hellenic subduction zone begun at Rhodope massif during the late Oligocene, and migrated towards SW. The problem is, while the Aegean Sea has possibly extended more than western Anatolia and lowered the topography < 0, the Western Anatolia has an average 1 km elevation above sea level It is possible that the various geodynamic reconfigurations may have been effective in differing the geological evolution of these two regions. For instance, it has been suggested that the slab tear/break-off affect the Western Anatolia inferred by the seismic tomography images. For modelling work, a geodynamic code named "SOPALE" that solves creeping flow for viscoplastic environment was used. A starting model was determined and some parameters were changed in order to understand their effects. Starting model was a simple subduction model with a thick continental lithosphere (40 km crust, 110 km mantle lithsophere) and a thinner oceanic lithosphere (100 km). The oceanic lithosphere was pushed with 1cm/year velocity in order to create a subduction. According to model results, trench was migrated 220 km to the south and crust was thinned down to 28 km. The back-arc topography was subsided 1.5 km. In order to understand the effects of both trench retreat and breakoff, tests for continous slab retreat for Aegean Sea, and discontinous subducting slab or "tear" for Western Anatolia has been conducted. For continous slab retreat, the oceanic lithosphere thickness, density, continental lithosphere thickness and moho temperatures of the back-arc have been changed. Models with different oceanic lithosphere thickness revealed that thicker oceanic lithosphere produced more extension at the back-arc. While 70 km thick oceanic lithosphere is used, slab was retreated 180 km and crust was thinned down to 32 km. But if the lithosphere thickness is increased to 110 km, the total amount of retreat was calculated 260 km and crust was thinned down to 26.5 km. Thicker lithosphere is heavier and produces more slab pull force that required for slab retreat. Likewise models with different oceanic lithosphere densities shown that denser material increases extension at back- arc. If density of oceanic material is selected 3290 kg/m3, the slab migrates 30 km and crustal thickness was calculated 37 km. In spite of that, if the material density is selected 3340 kg/m3, slab retreated 200 km and crustal thickness is decreased to 30 km. The important thing here is the density difference between lithosphere and asthenosphere. Asthenosphere density was selected as 3280 kg/m3. Bigger density difference produces more slab pull force and accordingly more extension. One other important factor is the thickness of the back-arc lithsophere. According to models, thinner continental lithosphere supports back-arc extension. If 90 km thick continent (40 crust, 50 mantle lithosphere) is selected, slab retreat increased to 350 km and crust of the back-arc thinned down to 24 km. On the other hand, model with 130 km back-arc lithosphere (40 km crust, 90 km mantle lithosphere) indicate that amount of slab retreat decreased to 280 km and crustal thickness to 27 km. That means that the thinner back-arc lithosphere may deform easily and contributes slab retreat related extension. Previous studies indicate that lithosphere of the region may have thinned down via convective removal or delamination. Different from Aegean Sea, experiments with discontious slabs or "tears" were conducted for Western Anatolia. To demonstrate the slab break-off, weak and dense material was used for the edge of the slab. According to tomography images, the slab is still under SW Anatolia, so break-off event has to be recent. Our models show that slab break-off is not so significant in terms of extension. Continental lithosphere thickness seems more important to understand the geodynamic properties of the region. If Paleocene compression affected the lithosphere of Western Anatolia more than than the lithsophere of Aegean Sea, there should be an thickness difference between these two. Continental lithsophere thickness models indicate that thicker overriding plate is less likely to extend, so there should be a shear zone within the subducting slab. The tear within the slab evolved at this counterclockwise shear zone. According to this interpretation, slab tear is not a cause, its an effect. Volcanism data also show that Isparta volcanics (where the tear is found) has age of 6-4 million yeas, corresponds to tear event. Slab break-off models show the remnant of the slab still sinking beneath asthenosphere just like the tomography images.