PRELIMINARY RESULTS FROM ROCK MAGNETIC ANALYSES OF QUATERNARY AND TERTIARY BASALTS FROM THE GULF COAST OF MEXICO
Abstract: As the foundation for a palaeomagnetic study of the Gulf Coast of Mexico, rock magnetic measurements were carried out on basaltic samples from 40 sites in the Gulf Coast region. Strong-field thermomagnetic and hysteresis techniques as well as room and low temperature susceptibility analyses were employed. These measurements show that the samples contain titanomagnetites with domain states from single- to multi-domain, with few superparamagnetic grains. Most samples have undergone at least partial deuteric oxidation though a significant quantity have not. The titanomagnetites have a range of compositions from Ti-poor to Ti-rich, where the latter appear to have endured varying degrees of maghematization.
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY OF THE EARLY CRETACEOUS LIMESTONE BEDS FROM THE RIO ARGOS, (CARAVACA, PROVINCE OF MURCIA, SE SPAIN)
Abstract: The Early Cretaceous limestone strata around the Rio Argos (Caravaca Region, Province Murcia, SE Spain) were selected for magnetostratigraphic investigations. This section was chosen due to its importance, detailed geological and paleontological documentation and good outcrops of individual strata. Altogether 361 oriented hand samples were collected covering the Berriasian, Valanginian, Hauterivian, Barremian and the Early Aptian sequence strata. The laboratory specimens were subjected to both the alternating-field and thermal demagnetization procedures, while the thermal treatment by means of the MAVACS apparatus was carried out at relatively dense temperature steps up to 590 °C, in many cases up to 690 °C. Twenty pilot samples were tested for the anisotropy of magnetic susceptibility. Fifteen samples were selected for detailed analysis with the aim of determining the unblocking temperatures with higher precision and verifying the possible occurrence of self-reversal phenomena of remanence during laboratory thermal treatments. All the 361 collected samples were subjected to systematic thermal or combined demagnetization procedures. Multi-component analysis was applied to separation of respective remanent magnetization components, R. Fisher,s (1953) statistics were used for the calculation of the separated remanence components combined with fold tests. - Few samples were found totally weathered, these are characterized by low unblocking temperatures (below 100 °C), and their magnetic susceptibility is markedly lower. The vast majority of samples showed three components of remanence, A-, B- and C-components. It was clearly proved that the studied un-weathered limestones can be divided into two groups of rocks, the first group with syn-tectonic magnetization, and the second group of limestones totally remagnetized in the Neogene. This way, the Early Cretaceous limestones from the Rio Argos were found unsuitable for derivation of a magneto-stratigraphic scale. Apart from totally weathered limestones, magnetite with a well defined unblocking temperature (around 540 °C) was found as the carrier of remanent magnetization in the majority of massive and fresh-looking limestone samples. From the study of the anisotropy of magnetic susceptibility it could be concluded that the fabric of the limestones in both the groups of totally and partially remagnetized samples showed the same features. It is of interest that the limestones under study display no signs of thermal, hydrothermal, chemical, dynamometamorphic or other alterations. The principal aim of the paper is to demonstrate typical case histories aimed at methodological problems since similar rocks may be selected for magneto-stratigraphic studies in other regions of the Tethyan realm and similar remagnetization phenomena may be encountered as already described in the papers by Villalain et al. (1996), Pares & Roca (1996).
PALEOMAGNETIC STUDY OF TRIASSIC SEDIMENTS FROM THE SILICA NAPPE IN THE SLOVAK KARST, A NEW APPROACH
Abstract: Intensive paleomagnetic and rock magnetic study were performed for Triassic limestones from the Silica Nappe in the Slovak Karst. Five exposures situated on the eastern and western side of the Stitnik - Plesivec fault were sampled for this study. In all exposures a secondary component of remanence of normal polarity (N), carried by secondary PSD magnetite was found. In the Silicka Brezova exposure (SB) apart from the N component, another secondary component of reversed polarity (R), carried by hematite, was isolated. Both components were acquired after folding. The R component was acquired during the Odra reversal event in the Oligocene (Birkenmajer et al. 1977). Comparison of its direction with the reference data let us conclude that the area belonged during this time to the African affinity. The declination of the R component suggests that after this magnetization period the studied region rotated anticlockwise by about 90° around an intraplate vertical axis together with the whole Pelso megaunit. According to Marton et al. (1995) and Marton & Fodor (1995) the rotation took place in two phases, the first one by about 50° took place in the Early Miocene, the second one, by about 30° - in the Late Miocene. The N component, isolated by us, seems to have been acquired during the Middle Miocene after the first and before the second. rotational phases: its declination agrees with a counterclockwise rotation of the Nappe by about 30–40° during the Late Miocene, as postulated by the cited authors. The inclination of the N component is lower, than the expected for Miocene, but agrees with the Miocene results for the Bukk region also belonging to the Pelso block, confirming the idea about the Miocene „southern escape“ of the Pelso bloc (Marton 1993). The final tectonic activity in the study area was connected with formation of the Stitnik-Plesivec fault (Late Tertiary - Quaternary). Our results suggest, that the fault is of rotational type and resulted in different tilting of beds situated on its eastern and western sides.
THE DEVONIAN PALEOMAGNETIC POLE FOR THE SOUTHERN PART OF THE RUSSIAN PLATFORM (DONETS BASIN) AND ITS GEODYNAMIC IMPLICATION
Abstract: Detailed paleomagnetic studies of Middle-Upper Devonian sedimentary and volcanic rocks from the zone of the junction of the Donbass with the Near-Azov block of the Ukrainian Shield have detected the following main components of natural remanent magnetization: B-metachronic component (R-polarity) whose nature is secondary on the basis of the data of statistical tests (before and after tectonic correction) and magnetic-mineralogical studies: A1 and A2 - synchronous (N and R-polarity) is interpreted as having been acquired during deposition. The southern paleomagnetic pole calculated from mean directions the B-component (Lat.= -36° Long.= 338°, d=6.6°, dm =3.4°) agrees well with the paleomagnetic poles of Permian sediments of the Donets Basin. The paleopoles calculated from A1 (Middle Devonian) and A2 (Upper Devonian) components have the following coordinates: A1 (Lat. = 13°, Long. = 289°, dp = 9.8°, dm = 4.9°) and A2 (Lat. = -3.7°, Long. = 359°, dp = 4.7°, dm = 2.3°). The significant deviation of paleopoles A1 and A2 from the Eifel-Famenian segment of APWP for Baltica may reflect the effect of the local tectonic rotation of the block enclosing the rocks studied.
ANALYSIS OF SELF- AND HETERODIFFUSION IN FERROMAGNETIC AND PARAMAGNETIC á-IRON
Abstract: Self- and heterodiffusion data in á-Fe, in ferromagnetic and paramagnetic state are analysed taking into account the anharmonic properties of the bulk material. The values obtained for the activation enthalpy, entropy and Gibbs energy are in agreement with the experimental values, given by Ceise & Herzing (1987) and Cermak et al. (1989). Using the elastic data of á-Fe, we calculate the ë parameter which in its definition contains the exchange integral for an atom at the saddle point and the equilibrium position. Furthermore, the influence of spin ordering in the activation Gibbs energy, in the Curie temperature zone, is investigated.
ELECTROMAGNETIC VARIATIONS ASSOCIATED WITH THE SEISMICITY OF THE FRONTAL HELLENIC ARC
Abstract: In the present paper we report the results of almost three years of measurements of the electromagnetic variations in 3 and 10 kHz, 41 and 53 MHz. The data are collected using a telemetric network installed along the island of Crete, i.e. in the central segment of the South Hellenic Arc. The experiment indicates that electromagnetic perturbations occur prior to seismic activity and supports previous observations that the time sequence of the variations appears in an invariant pattern.
IRON AGE SLAGS AT SNORUP (DENMARK): MAGNETIC PROSPECTING, MODELLING, RECONSTRUCTION AND DATING
Abstract: A description of the archaeological reconstruction of iron production (100 to 700 AD) based upon bog-iron ore and charcoal in Denmark during the Iron Age is given, the total number of furnaces being of the order of 100,000. Examples of magnetic prospection for slags in general and in more detail, including simple interactive interpretation by inclined magnetic dipoles (spheres) are illustrated. Finally the mean magnetic remanent direction determined by a paleomagnetic study of oriented cores from three slag pits, as well as by magnetic inversion of the surface magnetic field from the same slag pits, is used to obtain a magnetic dating of the slag pits by comparison with the geomagnetic secular variation, suggesting that the mean age of the slag pits is between the 2nd and 5th centuries AD.