LEE- Jeofizik Mühendisliği Lisansüstü Programı

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http://hdl.handle.net/11527/19323

Gözat

Konu "Litosfer" ile LEE- Jeofizik Mühendisliği Lisansüstü Programı'a göz atma
  • Öge
    Investigation of mantle kinematics beneath Turkey and adjacent regions based on seismological and numerical modelling
    (Fen Bilimleri Enstitüsü, 2020) Confal, Judith Maria ; Taymaz, Tuncay ; 652799 ; Jeofizik Mühendisliği Ana Bilim Dalı
    Upper mantle dynamics (e.g. subduction processes, slab roll-back, slab tearing, and mantle upwelling) influence the tectonics of the Eastern Mediterranean region; however, a detailed understanding of the acting forces has remained elusive. Further progress requires more accurate measurements, not just of the surface kinematics (e.g. GPS measurements, Global Positioning System), but also of indirect indicators of kinematics throughout the lithosphere and convecting upper mantle from seismology. Robust quantification of the magnitude, location, and direction of seismic anisotropy can provide an orientation of present and prior extensional fields, as well as mantle flow patterns. In this PhD Thesis, three novel methods were used to investigate seismic anisotropy and P-wave velocity anomalies in the upper mantle. Separate and combined interpretations of the results and extensive discussions with available literature reveal new findings of mantle dynamics and tectonics in the Eastern Mediterranean region. For the first project, shear-wave splitting (SWS) measurements in the central and southern Aegean were performed. In addition to SKS-waves, direct S-waves (Reference Station Technique) were included in this research, which made it possible to use more observations and therefore get better and more reliable averages than some prior studies. This study was able to use on average 12 events per station, while common local SKS studies calculated splitting parameters with only one to five events. Splitting measurements exhibit mostly NNE-SSW oriented fast polarization directions (FPDs) and large time delays (TDs) at stations in the back-arc region of the Hellenic subduction system. At three stations close to trench FPDs are N-S directed. With the results from the splitting analysis at 35 stations and 81 events, directions of mantle flow, strain, and their strengths are interpreted. The study concluded that FPDs in the back-arc are mostly perpendicular to the trench and parallel to supra-slab mantle flow induced by the roll-back movement of the slab and its extensional regime in the Aegean with station-averaged TDs between 1.15-1.62 s. Due to the curvature of the trench, FPDs in the eastern Aegean and southwestern Turkey seem to be trench-parallel aligned. This could be related to sub-slab mantle flow or it might be influenced by the tearing of the slab in the upper mantle beneath this region. In the second project of this thesis, a numerical approach was used to model the mantle flow and anisotropy of the Eastern Mediterranean and Anatolian region. With complex non-steady-state 3D geodynamic modelling, the plate movement, mantle flow, transverse isotropy, and SKS splitting parameters for the regional subduction system were calculated. The modelled plate movements represent approximately the past 22 million years of the African, Arabian, and the oceanic plate subducting underneath a westward-moving Anatolian and an extending Aegean microplate. Implemented weak zones helped to initiate the detachments of the slab and allowed the slab to roll back. The model shows that tearing beneath southwestern Turkey, a break-off in the collisional regime of eastern Anatolia, as well as the retreat of the slab in the Aegean influence the strength and direction of the mantle flow and anisotropy. Similarities of measured and modelled SWS measurements made it possible to interpret and explain anisotropy data more in detail. SWS splitting parameters are mostly N-S oriented, perpendicular to the movement of the trench, similar to seismological observations, with the highest TDs in the back-arc region due to strong roll-back induced mantle flow. Close to the trench, the transverse isotropy pattern is complex, with vertical and horizontal components parallel and perpendicular to the trench. The influence on mantle flow and FPDs, related to the development of a tear and a break-off in the slab, is significant. The mantle flow appears to be faster through the slab windows and around the edges of the detached parts of the slab, where a circular pattern of strong mantle flow and anisotropy can be observed. At last, a P-wave tomography study of the Eastern Mediterranean region, focusing on the upper mantle, with a large data set was performed. Since strong anisotropy with complicated direction pattern is present in the region, especially due to the active subduction system, a method, first tested on synthetic models, was used to correct the models for anisotropy. The anisotropy of 3D mantle convection simulations and SWS measurements from the literature are taken as a priori constraints to correct P-wave arrival times. Isotropic inversions, as well as the models corrected for anisotropy, show the half-arc shape of the subducting slab in the Aegean, the detached slab in eastern Anatolia, and the slab appears to be fragmented in a few locations. A horizontal tear in western Greece, a deep and pronounced vertical tear in western Anatolia, and a sub-horizontal tear between the eastern and western Cyprian slab can be identified. The steeply dipping Aegean slab flattens around 410 km till the model depth boundary at 750 km and is connected to the Cyprian slab, which does not reach lower than 500 km. Beneath eastern Anatolia, high-velocity anomalies indicate the existence of some Bitlis slab fragments, while a shallow high-velocity anomaly in northern Anatolia (<150 km) might represent a remnant slab from a Neotethyan subduction process. Slow-velocity perturbations beneath volcanic region in central and eastern Anatolia are most probably related to upwelling mantle material. Spatially large first-order velocity perturbations are stable and similar in all three models, but small differences in geometry or strength can be detected when comparing the isotropic model with the anisotropy corrected models. Discrepancies in velocity perturbations reach up to 2% locally; however, the variance reduction only increases minimal, when correcting for anisotropy. It appears that the mostly homogeneous and horizontal anisotropy does not affect the inversion immensely, except in the active slab region, where anisotropy with a dipping and vertical axis of symmetry is present. Adding anisotropy from simple SWS measurements might not influence tomography results immensely due to the horizontal alignments of the splitting parameters. While the anisotropy retrieved from the numerical model allows vertical, multiple layers, and complex anisotropy patterns, the model might not represent the regional settings completely correct. Nevertheless, adding anisotropy enhances P-wave inversions and should be considered when interpreting velocity perturbations in regions with active or fossil subduction slabs.