J. Kruczyk

Paleomagnetism of middle-late Jurassic sediments from Poland and implications for the polarity of the geomagnetic field

Abstract Paleomagnetic data for the middle-late Jurassic sediments from nine localities of Krakow-Czestochowa Upland belonging to stable Europe and from three valleys of the Lower Subtatric Nappe belonging to mobile Europe are presented. The majority of the rocks investigated comprising limestones, stromatolites and radiolarites were found to be normally magnetized. Natural remanent magnetizations (NRM) of reversed polarity were found in limestones and stromatolites of middle Callovian age from the stable region and in one exposure of Oxfordian radiolarites from the mobile region. Along one of the middle Callovian sections studied the NRM direction of the specimens changed from R to N. The mean paleomagnetic pole positions obtained for the Krakow-Czestochowa Upland (lat. 72.3° N, long. 150.4°E, α 95 = 7.3° and for the Lower Subtatric Nappe (lat. 71.7 N, long. 132.2° E, α 95 = 4.3° ) are consistent with the data for stable Europe (lat. 74.7 °N, long. 147.1 °E, α 95 = 5.0° ). The paleomagnetic results presented do not reveal any major relative rotations either within the Lower Subtatric Nappe or between this area and the Krakow-Czestochowa Upland. The reversed directions of NRM observed indicate the existence of a period of reversed field direction in the Middle Callovian and perhaps also in the Oxfordian.

Paleomagnetism of Jurassic sediments as evidence for oroclinal bending of the Inner West Carpathians

Abstract The Jurassic carbonates from the Krizna Nappe contain a pre-tectonic characteristic component of remanence. The declinations for individual nappe fragments differ, and form a fan-like pattern. They are close to the tectonic vergencies measured in the field, which suggests that the arcuate structure of the Inner Carpathians results from an oroclinal bending of an originally straight belt.

Origin of magnetic minerals and natural remanent magnetization of basaltic rocks from Lower Silesia, Poland

SummaryTwo groups of basaltic rocks with normal and reversed NRM were examined. It was found that in both groups the primary NRM, synchronous with the period of rock formation, is preserved. Relations of the primary NRM to the origin and the degree of oxidation of magnetic minerals were considered in detail.

Thermomagnetic properties of some Late Mesozoic diabase dikes of South Spitsbergen

SummaryLaboratory investigations of thermomagnetic properties of specimens from seven diabase dikes of South Spitsbergen have been performed. Studies of temperature variation of isothermal saturation remanence combined with Hopkinson effect measurements and investigations of low temperature remanence give information on the mineral composition of the magnetic phases present in the magnetic fraction of the diabases. The experiments show that the ‘clean’ natural remanence of these dikes has been acquired at the time of their, consolidation in the Late Mesozoic, though three of them may have been partially remagnetized at the time of Tertiary orogeny. Since the dikes reveal several normal and reversed directions of ‘clean’ natural remanence, it is concluded that they were formed at different times in the Late Mesozoic, even though some of them may have been subjected to secondary influences at a later date.

Relationships between magnetic and structural fabrics revealed by Variscan basement rocks subjected to heterogeneous deformation—a case study from the Kłodzko Metamorphic Complex, Central Sudetes, Poland

Abstract The Kłodzko Metamorphic Complex comprises a number of thrust units, consisting of meta-igneous and metasedimentary rocks of the Variscan basement of the Sudetes, NE Bohemian Massif. The thrust sheet stack rests upon unmetamorphosed Nowa Ruda ophiolite and is unconformably overlain by Frasnian-Fammenian sediments. The studied rocks underwent six deformation events, scattered in time between the Middle Devonian and Late Carboniferous. The multiphase deformation produced a steep foliation that consistently trends WNW-ESE and a mineral lineation plunging to the ESE at a shallow to moderate angle. The results of the AMS study show that, despite the complex structural evolution, all the studied rocks bear a similar magnetic fabric mirroring the principal structural directions. This relationship between magnetic and structural fabrics is apparent in all tectonic units irrespective of the considerable variations in strain rate and metamorphic grade. This suggests a rather low dependence of magnetic anisotropy on the changing conditions and intensity of deformation.

Fifty Years of Palaeomagnetic Studies in the Institute of Geophysics, Polish Academy of Sciences

Palaeo- and archaeomagnetic investigations in the Institute of Geophysics were initiated at the beginning of 1960s. The archaeomagnetic study was performed in the Geomagnetic Observatory in Hel till the mid-1980s and was resumed lately in the Warsaw laboratory. Palaeomagnetic study begun in 1963 but the palaeomagnetic group obtained necessary place for the laboratory in the Central Geophysical Observatory in the newly finished building in Belsk later in 1966. Formally, the Paleomagnetic Laboratory was established in 1972. From the very beginning, palaeomagnetic studies were accompanied by studies of magnetic properties of rocks and minerals-carriers of natural remanence and by measurements of magnetic susceptibility and its anisotropy. Therefore, the laboratory became successively equipped with the apparatus for strictly palaeomagnetic study (astatic and spinner magnetometers and demagnetizing devices) as well as with devices for rock-magnetic investigations (apparatus for thermoanalysis, system for measuring of hysteresis parameters and for study of magnetic remanence at low temperatures, torsion balance, device for study of magnetic properties under mechanical stress and temperature, device for study of alterations of magnetic minerals based on thermally stimulated emission of exo-electrons (TSEE). The last three devices have not been in use for several years. At the end of 1980s, the Institute moved to a new building and the whole laboratory was moved to Warsaw. Moving palaeomagnetic measurements to town was possible due to the replacement of spinner magnetometer by the cryogenic magnetometer SQUID, which is much more sensitive and less affected by the urban disturbances. Computer software packages that are written in the Institute or obtained from their inventors became a great help in the interpretation of experimental results. Palaeomagnetic investigations have been performed in our laboratory on rocks of various origins and age, coming from various regions of Poland and other countries often in cooperation with scientists from foreign laboratories. Numerous palaeomagnetic results that are obtained in our laboratory are in the world data palaeomagnetic basis and on Apparent Polar Wander Paths constructed for various regions. Study of anisotropy of magnetic susceptibility brought interesting results concerning problems of tectonics; study of rock-magnetic properties helped to identify carriers of primary and secondary components of natural magnetic remanence in many cases. In the last 10 years, a group in our laboratory has been studying very important problems connected with environmental magnetism by applying of magnetometric methods for investigations of soil and air pollution.