Wyniki wyszukiwania - BazTech

1

Evolution of Neoarchean-Paleoproterozoic basement in the Brunovistulia terrane, S Poland : geological, P-T and geochemical records

Żelaźniewicz Andrzej Ryszard, Jastrzębski Mirosław

Geological Quarterly

|

2021

|

Vol. 65, No. 2

art. no. 20

EN

Brunovistulia is a composite terrane of Gondwana descent that eventually was accreted to the SW margin of Baltica, central Europe. It is built of metagneous and metasedimentary rocks that originated mainly between 650 and 550 Ma. However in the Upper Silesian part of Brunovistulia, much older fragments have been drilled, which yielded U-Pb zircon ages between 2.75 and 2.0 Ga. They have been interpreted as an “exotic” constituent of the Brunovistulia superterrane, named the Rzeszotary Terrane. Our geological and geochemical studies of the Rzeszotary borehole cores yielded new data on the composition, provenance and evolution of that terrane. Precursors of the Rzeszotary complex were separated from the depleted mantle prior to or around 3.2-3.0 Ga. At 2.75-2.6 Ga, a juvenile magmatic arc edifice formed, beneath which oceanic lithosphere was subducted. Decompression melting of the mantle brought about tholeiite magmas of IAT/MORB composition with LILE additions. Tonalitic and trondhjemitic precursors of gneisses present today were formed at that time, probably due to partial melting of mantle-derived wet basalts at the base of the island arc. Around 2.0 Ga, the arc collided with an unspecified cratonic mass and was subject to orogenic deformation, metamorphism and migmatization. The entire arc edifice was then strongly shortened and forced down to depths equivalent to ~6-12 kbar where the rocks underwent contractional deformation and metamorphism (~500-700°C). Tonalites and trondhjemites were changed to gneisses, and basites to epidote- and garnet amphibolites. These rocks underwent syntectonic migmatization through the mechanism of segregation/differentiation in the presence of fluids and incipient partial melting. Synmetamorphic shortening of the rock pile, which led to folding and heterogeneous development of shear zones with thrust kinematics, terminated with intrusions of K-granites at 2.0 Ga, being followed by some brittle-ductile deformation of unconstrained timing. The 2.0 Ga event may have been connected with the 2.1-1.8 Ga global amalgamation of the Paleoproterozoic supercontinent of Columbia. Later the future Rzeszotary terrane was detached from the Gondwana mainland, reassembled and eventually, in the Neoproterozoic, it became part of the foreland of the Cadomian Orogen in Central Europe.

2

Ekspedycja IODP 360 : pierwszy etap odwiertu do płaszcza Ziemi

Ciążela J., Dick H. J. B., Mac-Leod Ch., Blum P.

Przegląd Geologiczny

|

2016

|

Vol. 64, nr 11

889--895

EN

The aim of this paper is to provide a report on the IODP expedition 360 to the Polish geoscientific community. Expedition 360 to the Atlantis Bank along the Southwest Indian Ridge was Leg 1 of the SloMo Project. The primary objective of the SloMo Project is to test competing hypotheses on the nature of the Moho at the slow-spreading oceanic lithosphere. Based on a seismic survey and geologic mapping, the Moho beneath Atlantis Bank is believed to represent a serpentinization front, and not an igneous boundary between gabbro andperidotite. Expedition 360 started on November 30,2015 in Colombo (Sri Lanka), and ended on January 30,2015 in Port Louis (Mauritius). Hole U1473A was drilled 790 m through massive gabbro. Core recovery ranges from 44 to 96% towards the bottom of the hole, where excellent drilling conditions occurred. This deepest single-leg basement hole drilled into ocean crust is in overall good condition andcan be re-entered at Leg 2. For the first time, a Polish nominee has been selectedfor the scientific party of an oceanic IODP expedition. The mantle drilling project raised much attention in the Polish media. One hundred rock samples have been collected to investigate in Poland.

3

Results of Crust and Mantle Soundings in Central and Northern Europe in the 21st Century

Semenov V. Yu.

Acta Geophysica

|

2015

|

Vol. 63, no. 1

103--124

EN

The first decade of 21st century is characterized by the appearance of new approaches to deep induction soundings. The theory of magnetovariation and magnetotelluric soundings was generalised or corrected. Spatial derivatives of response functions (induction arrows) were obtained for the ultra-long periods. New phenomena have been detected by this method: secular variations of the Earth’s apparent resistivity and the rapid changes of induction arrows over the last 50 years. The first one can be correlated with the number of earthquakes, and the second one – with geomagnetic jerks in Central Europe. The extensive studies of geoelectrical structure of the crust and mantle were realized in the frame of a series of international projects. New information about geoelectrical structures of the crust in Northern Europe and Ukraine was obtained by deep electromagnetic soundings involving controlled powerful sources. An influence of the crust magnetic permeability on the deep sounding results was confirmed.

4

Induction Sounding of the Earth's Mantle at a New Russian Geophysical Observatory

Moilanen J., Pushkarev P.Yu.

Acta Geophysica

|

2015

|

Vol. 63, no. 2

385--397

EN

Deep magnetotelluric (MT) sounding data were collected and processed in the western part of the East European Craton (EEC). The MT sounding results correspond well with impedances obtained by magnetovariation (MV) sounding on the new geophysical observatory situated not far from the western border of Russia. Inversion based on combined data of both induction soundings let us evaluate geoelectrical structure of the Earth’s crust and upper and mid-mantle at depths up to 2000 km, taking into account the harmonics of 11-year variations. Results obtained by different authors and methods are compared with similar investigations on the EEC such as international projects CEMES in Central Europe and BEAR in Fennoscandia.

5

Are the crustal and mantle conductive zones isotropic or anisotropic?

Berdichevsky M., Pushkarev P.

Acta Geophysica

|

2006

|

Vol. 54, no. 4

333-342

EN

One of the significant problems of modern deep magnetotellurics is the recognition of anisotropy in the crustal and mantle conductive zones. In the paper we perform numerical experiment comparing several 2D models of crustal and mantle isotropic and anisotropic prismatic conductors. Anisotropy is modeled by alternat-ing horizontal or vertical thin layers of different resistivities (the vertical layers are parallel to the prism strike). Using these models, we examine conditions under which the magnetotelluric and magnetovariational response functions distinguish between isotropy and anisotropy. The resolution of MT and MV studies depends on the sediments conductance, lithosphere resistance and deep conductor width. Calcu-lations show that the most favorable conditions for anisotropy studies are observed in the active regions characterized by small sediments conductance (10-20 S) and moderate lithosphere resistance (108 Ohmźm2). However, in the stable regions, where sediments conductance exceeds 50-100 S and the lithosphere resistance comes up to 109 Ohmźm2, the crustal and mantle anisotropic and isotropic conduc-tors manifest themselves in the equivalent magnetotelluric and magnetovariational functions, which cannot distinguish between anisotropy and isotropy and admit both the interpretations.

6

Mantle plumes and dynamics of the Earth interior : towards a new model

Cwojdziński S.

Przegląd Geologiczny

|

2004

|

Vol. 52, nr 8/2

817-826

EN

Seismic tomography provides reconstructions of thermal-density structure of the Earth's mantle as deep as the mantle/core boundary (CMB). For the first time, a direct image of dynamic processes, occurring inside the globe, was obtained. Existing plate-tectonic models of modern geodynamics lead to a number of discrepancies. Most important are: stationary position of mantle plumes as the assumption of the convection process in the Earth's mantle, mantle convection versus data on both its viscosity and the existence of global seismic discontinuities, possibility of horizontal displacements of lithospheric plates above the discontinuous LVZ zone which disappears under deep-seated continental "roots", the model of radially growing distance between mid-oceanic ridges and Africa (also Antarctica), the growing separation between hot spots occur in neighbouring plates with time, geophysical data indicative of considerable input of energy and material from the Earth's core into the mantle, uncompensated by any exchange between the lower and upper mantle. New models (multi-layered convection or a plate-tectonic hybrid convection model) intend to explain tomographic image with taking into consideration geochemical data but with miserable results. The nature of mantle convection still remains controversial. The phenomenon of stationarity of hot spots relative to the accepted plate movements and the absence of evidence indicating deformations of mantle plumes by the convection system are also unclear and controversial. The presented model of the expanding Earth's offers a reasonable solution to these discrepancies and paradoxes.