|
|
Characteristics and evolution of the Paleozoic accretionary system in southern Mongolia
Sukhbaatar, Turbold ; Lexa, Ondrej (advisor) ; Sun, Min (referee) ; Yuan, Chao (referee)
The microcontinental fragments in central Mongolia are represented by the series of Precambrian blocks consisting of the Baidrag, Zavkhan, Tuva-Mongol, and Tarvagatay blocks forming the so-called Central Mongolian Massif. The Baidrag Block (BB), the southernmost part of the Central Mongolian Massif, records long-lasted evolutionary events of the Central Asian Orogenic Belt. During the Neoproterozoic, the BB had been rifted apart from the Rodinia taking part of the ribbon-like microcontinental block surrounded by the oceanic basins and facing the external Palaeo-Pacific oceanic domain. Our findings suggest the existing oceanic domain north of the BB was subducted and closed in Ediacaran times. For instance, the northern part (in today's coordinate) of the BB is linked to the Cryogenian Bayankhongor oceanic basin (ca. 636--655 Ma) which was considered to be an exclusively reflecting sea- floor section. However, our data support a new interpretation that the Bayankhongor Zone is not entirely consisting of seafloor sequences. According to these findings, southwest-directed Ediacaran (ca. 564--598) subduction of the oceanic plate beneath the Baidrag Block is proposed. In southwestern Mongolia, the important events of the late Neoproterozoic to upper Paleozoic continental growth are recorded. The...
|
|
|
Utilising satellite imagery for geological mapping and structural analysis
Sokolová, Anna ; Lexa, Ondrej (advisor) ; Martínek, Karel (referee)
The use of remote sensing data is becoming very important in Earth sciences. Satellite images are invaluable for studying hardly reachable places on Earth and they can significantly help increase the efficiency of work in areas with existing field data . In arid areas where rocks are exposed and directly visible at the surface, a lithology contacts mapping can be done based on satellite images. An overview of the possibilities of processing multispectral images is included in the theoretical part of this thesis, as well as a description of previously known methods for performing structural analysis based on remote sensing data. As the main part of this thesis, a new method for finding orientations of planar geological structures including a quality control of the input data was developed and described. This method was then applied in the Bodonchin area of the Mongolian Altai where the metamorphic foliation and pegmatite veins were analyzed. The method showed good applicability in the given area despite its limitations which are also discussed in the thesis.
|
|
|
Deformation and petrological record of polyphase quartzo-feldspathic rocks in Erzgebirge, Bohemian Massif
Kryl, Jakub ; Lexa, Ondrej (advisor) ; Závada, Prokop (referee) ; Hasalová, Pavlína (referee)
The Erzgebirge dome, exposing Proterozoic - Early Palaeozoic metagranitoids and metasedimentary rocks of the Saxothuringian domain in the Bohemian Massif, is formed by a tectonic stack of several UHP-HP and HT units exhumed during the Variscan continental subduction-collision process. A systematic study of orthogneiss deformation microstructures carried out on a regional scale has been done to evaluate these units' extent and relative structural position in the subduction complex. Five different microstructural types have been distinguished, revealing two overprinting sequences with 1) coarse-grained microstructures in the parautochthonous Kateřina-Reitzenhain Dome and 2) fine- grained microstructures in the allochthonous HP-HT Gneiss Eclogite Unit I. P-T conditions of the individual overprinting microstructures calculated by conventional thermobarometry suggest continuous deformation during exhumation along two contrasting P-T paths. The warmer one in the parautochthonous unit showed exhumation from ~13.5 kbar and ~820 ◦C to ~3 kbar and ~500 ◦C, while the colder one in the allochthonous unit showed exhumation from ~16 kbar and ~740 ◦C to ~3 kbar and ~400 ◦C. The overprinting microstructures are associated with vertical shortening interpreted as ductile thinning. This deformation started at depths...
|
|
|
Assembly of the Saxothuringian orogenic wedge: the Variscan P-T-t record of the metasediments of Erzgebirge, Bohemian Massif
Jouvent, Marine ; Lexa, Ondrej (advisor) ; Tichomirowa, Marion (referee) ; Pitra, Pavel (referee)
The formation of collisional orogenic wedges involves complex polyphase deformation and metamorphism. This study reveals the tectonic evolution, internal architecture and the timing of the Variscan orogenic wedge evolution in the Saxothuringian Domain (Bohemian Massif). The studied area is the Erzgebirge Crystalline Complex, characterized by numerous occurrences of the (U)HP rocks. In contrast, the surrounding metasediments have been scarcely studied, although they provide an important link between deep subduction and mid-crustal processes. Using field structural geology, petrology, thermodynamic modelling and geochronology (monazite U-Pb and mica 40 Ar/39 Ar dating), we constrained the P-T conditions and timing of four deformation events (D1-D4) identified by structural analysis. Several transects from the low-grade hanging wall phyllites to the footwall medium-grade micaschists have been investigated. The first M1-D1 event is characterized by HP-LT minerals (garnet, chloritoid, phengite, paragonite, and rutile) defining the S1 foliation with an M1 peak pressure conditions increasing from 13 kbar and 520řC in phyllites to 25 kbar and 560řC in micaschists. The corresponding geothermal gradient of 6-11řC/km is typical for subduction environments. The M2-D2 event corresponds to the deformation and...
|
|
|
Pre-Variscan tectonometamorphic evolution of Zábřeh crystalline unit and its geotectonic position
Jůzlová, Klára ; Lexa, Ondrej (advisor) ; Buriánek, David (referee)
This thesis is focused on the study of the evolution of the Zábřeh crystalline unit on the eastern margin of the Bohemian Massif. The main focus is U-Pb dating of detrital zircons obtained from metasediments of the zábřeh series and dating of magmatic zircons from metamorphosed rocks in the vicinity of the village Hynčina. The data are supported by structural analysis, petrographic descriptions and geochemical analyses of rocks. The thesis aims to determine the age of sedimentation and protoliths and thus clarify the interrelationship of the subunits forming the Zábřeh crystalline unit. The results of the thesis indicate that the Zábřeh crystalline unit represents a segment of cambro- ordovician rift zone developed along northern margin of Gondwana affected by early Variscan metamorphism likely related to mid-Devonian back-arc extension. Keywords: Zábřeh crystalline unit, Eastern margin of Bohemian Massif, pre-variscan tektonometamorphic evolution, U-Pb zircon dating
|
|
|
Quantitative correlation of textural data obtained with CIP and EBSD method
Slunská, Petra ; Lexa, Ondrej (advisor) ; Machek, Matěj (referee)
Since 2011, the Institute of Petrology and Structural Geology, Charles University in Prague, worked with CIP - Computer Integrated Polarization microscopy as fast, inexpensive measurement of c-axis orientation of uniaxial minerals, mostly quartz in high definition. CIP method is developed from the early nineties in Switzerland (Heilbronner & Pauli 1993) and later in many other workplaces. The aim of this work was testing and calibration of optical and camera equipment to verify the accuracy and reliability of data obtained. Served as an independent measurement of EBSD (Electron Backscatter Diffraction) data obtained from the same part of the studied thinsections. The data obtained were analyzed by quantitative analysis of microstructures (PolyLX - MATLABTM toolbox; Lexa 2003). The samples used for testing the methodology mentioned were taken on the profile of Hvězdná and Zdobnice near Rokytnice in the Eagle Mountains by contact orlica-snieznik complex and its mantle. Field studies showed the existence of west dipping shear zone along the said contact and deformed orthogneiss show a macroscopic superposition of several deformation events. The resulting frequency histograms similarities and differences of angles c-axes and angles misorientace grains have a high consensus in the azimuthal criterion, axes...
|
|
|
Mechanisms of fracturing and origin of veins on Mokrsko deposit
Švagera, Ondřej ; Lexa, Ondrej (advisor) ; Franěk, Jan (referee)
Locality Mokrsko-west is located inside the Josef mining gallery and belongs to the ore district Psí Hory Mountains, known for its high content of micro-granular gold (~ 100 g/t). It is situated within the apophysis of Sázava tonalite, which is a part of Central Bohemian Plutonic Complex, located southerly from Prague. Besides other structural elements, the whole locality contains the network of sheeted quartz veinlets, which is the aim of this thesis. Data from vectorization and image analysis of the photographic documentation from the gallery and electron microscope has been used. The statistical approach has been used to quantify proportions of mineral phases within the quartz veinlets, their cumulative spacing and fractal distribution. Image analysis confirmed the presence of K-feldspathic metasomatism which affects the plagioclase grains. It forms rims on the edges of the quartz veinlets. The proportional relationship of K-feldspar and quartz within the veinlets wasn't confirmed. It's therefore possible that they were two separate processes of the uncertain time relation. Transfer from the lognormal distribution in histograms of vein spacing in drill-cores to the more normal distribution of the veinlets in the gallery walls was observed. Results from the cumulative spacing analysis confirmed...
|
|
|
Structural evolution of infra- and superstructure boundary in orogens
Peřestý, Vít ; Lexa, Ondrej (advisor) ; Franěk, Jan (referee)
ENGLISH ABSTRACT This work focuses on structural and deformational aspects of the crust development during orogeny, based on historical suprastructure/infrastructure concept. The uppermost suprastructure is dominated by steep fabrics, which evolved in a brittle regime within early stages of orogeny, and which sustain almost unchanged for the whole period of orogenic evolution. On the other hand, the infrastructure is located in the lower crust, where ductile processes dominate. Because the ductility is a thermally activated process, sufficiently long time is needed to heat the lower crust, and therefore the infrastructure is younger than suprastructure. Gently to shallowly dipping structures develop as a consequence of ductile flow. The aim of this study is to describe processes on the suprastructure/infrastructure boundary. Position of this boundary is closely related to brittle/ductile transition, and so that a brief introduction to rheology is included. Rheological boundaries are often places of high strain and high displacement gradients, which must be somehow accomodated. One possibility is to lose communication between adjacent levels and create a detachment plane, which is commonly supposed in the conventional thrust tectonics model. The other possibility is, that layers are at least partially...
|
|
|
Numerical simulations of geodynamic processes
Krýza, Ondřej ; Lexa, Ondrej (advisor) ; Ježek, Josef (referee)
ENGLISH ABSTRACT The aim of this work is to provide a basic overview in numerical modeling of geodynamic processes. Therefore, work is divided into two main parts that almost independently describe its main topic. Both parts are designed to allow reader easily locate sources for more detailed and deeper description of the problem. The first part deals with the mathematical and physical apparatus, which is used to describe the geodynamic processes and their formalization due to the application for the construction of mathematical models. This section is mainly based on the description of conservation laws and related equations (continuity equation, Navier-Stokes equations, heat transfer equation) and also gives an overview of relations describing the force interactions in solids and rheology. The second aspect of this work is explanation of the principle of numerical methods (finite differences, finite volumes, finite elements and spectral methods) and their brief overview. In this part, the work is focused on finite differences, a description of which is the main topic of this part. In last chapter of this work, an example of the implementation of finite difference method for modeling of the thermal evolution of fold structures is given. We elaborate simple kinematic and thermal model to simulate time and...
|
|
|
Mechanisms of granite fracturing and their implications for origin of gold-bearing veins in Mokrsko deposit
Švagera, Ondřej ; Lexa, Ondrej (advisor) ; Franěk, Jan (referee)
The stress concept in geology is widely used for analysis and description of tectonic events. These can be documented directly in the field or by interperetation of results gained by laboratory analysis. The expressions of brittle tectonics and geological events linked with them are often important clues leading to understanding processes giving unique forms to many localities. The origin of gold-bearing veins in Mokrsko deposit is result of long-lasting tectonic processes. During these were created at least 4 generations of brittle structures. The proximity of Variscides is the source of this brittle fracturing. Sázava pluton intruded during the syn- orogenic phase and in the Slapy apophysis of Sázava pluton started the circulation of fluids because of the contact metamorphism. These fluids were using pre-existing EW joint systems. After that, the precipitation of minerals rich in Bi-Te-Cu gave origin to the local gold-bearing mineralization. This mineralisation is partly bound to the quartz-veins localized on Mokrsko- Západ. Their thickness is often in sub-milimeter scale with very regular spacing.
|
guest ::
