Péter Márton

Crystal-size distributions and possible biogenic origin of Fe sulfides

Sedimentary greigite (Fe3S4) can form either by “biologically controlled” or by “biologically induced mineralization” (BCM and BIM, respectively). In order to identify the origin of magnetic Fe sulfides, we studied and compared the sizes and morphologies of greigite crystals produced by a magnetotactic microorganism (previously described and referred to as the “many-celled magnetotactic prokaryote”, MMP) and Fe sulfides from two specimens of Miocene sedimentary rocks (from Laka, in the foredeep of the Western Carpathians and from Michalovce, in the Transcarpathian Depression). Greigite grains from the MMP and the Laka rock show nearly Gaussian crystal-size distributions (CSDs), whereas the CSD is lognormal for Fe sulfides from the Michalovce rock. We simulated various crystal-growth mechanisms and matched the calculated and observed CSDs; crystals from the MMP and the Laka rock have CSDs that are consistent with random growth of crystal nuclei in an open system, whereas the CSD of the Michalovce Fe sulfides is consistent with surface-controlled growth followed by supply-controlled growth in an open system. On the basis of CSDs and characteristic contrast features in the transmission electron microscope, greigite in the Laka rock is likely of BCM origin, whereas the Fe sulfide crystals in the other rock sample were produced by BIM processes. Our results indicate that the methods we applied in this study may contribute to the identification of the origin of magnetic Fe sulfide minerals in sedimentary rocks. This paper was presented at the “Biogenic Iron Minerals” symposium held in Tihany, Hungary (May 2000)

Implications of a combined biostratigraphic and palaeomagnetic study of the Umbrian Maiolica Formation

Abstract The study of a 275.5 m thick section of white, pelagic limestones occupying the valley of the Fonte del Giordano river on the southern slope of Mt. Montagnola has yielded a biostratigraphically controlled clear magnetic reversal pattern after thermal cleaning. The magnetic stratigraphy of the lower 131 m of the section (Calpionellid zones) is correlatable with the M-sequence of oceanic magnetic anomalies between M-19 and M-14. The reversal stratigraphy of the upper 81.5 m of the section (Radiolaria zone) has also been tied to the oceanic polarity time scale by making linear interpolation for a missing 63 m thickness underneath it. Besides the Fonte del Giordano section two Berriasian outcrops each with a different bedding attitude were studied at Gubbio and near Cagli for tectonic tilt test giving positive results. The mean palaeomagnetic pole position for the Late Jurassic/Early Cretaceous after bedding correction is: Φ = 19.1°, Λ = 288.2°, k = 148.7, α95 = 10.2° (N = 3), confirming the presence of a large swing in the polar path, a common behaviour of apparent polar wandering for the peri-Adriatic region during this time.

A refined apparent polar wander curve for the transdanubian central mountains and its bearing on the mediterranean tectonic history

Abstract Recently obtained palaeomagnetic data have given a fairly detailed apparent polar wander (A.P.W.) curve for the Mesozoic of the Transdanubian Central Mountains which has been compared with other A.P.W. curves from the Mediterranean area. The similarity of the A.P.W. curves enabled a unified A.P.W. curve to be constructed for the Central Mediterranean region. It has been shown that the new Mediterranean A.P.W. curve is in full agreement, even in details, with the updated A.P.W. curve for Africa. This confirms the existence during the entire Mesozoic of a single Central Mediterranean megatectonic unit rigidly attached to Africa. Its positions with respect to the pole can now be traced back in more detail than before.

Miocene to Quaternary deformation in NE Slovenia: complex paleomagnetic and structural study

Abstract Combined paleomagnetic and structural research was carried out in the Mura-Zala Basin including the western and southern surrounding hills in northeastern Slovenia. The Mura-Zala Basin was formed due to ENE–WSW trending crustal extension in the late Early Miocene (18.3–16.5 Ma). First, marine sedimentation took place in several more or less confined depressions, then in a unified basin. During thermal subsidence in the late Miocene deltaic to fluvial sediments were deposited. After sedimentation, the southernmost, deepest depression was inverted. Map-scale folds, reverse and strike-slip faults were originated by NNW–SSE compression. This deformation occurred in the latest Miocene–Pliocene and is reflected also in the magnetic fabric (low field susceptibility anisotropy). After this folding, the Karpatian sediments of the Haloze acquired magnetization, then suffered 30° counterclockwise rotation relative to the present north (40° counterclockwise with respect to stable Europe). This Pliocene (Quaternary?) rotation affected a wide area around the Mura-Zala Basin. The latest Miocene to Quaternary folding and subsequent rotation may be connected to the counterclockwise rotation of the Adriatic microplate.

Large scale rotations in North Hungary during the Neogene as indicated by palaeomagnetic data

Abstract Tertiary igneous and sedimentary rocks were collected for palaeomagnetic study at about 90 localities from the North Hungarian Central Range. Rock ages range from the late Eocene to the Mid-Miocene but the majority of the samples are Early Miocene in age. After laboratory treatment 66 localities yielded palaeomagnetic results, which fall into two groups. The first group is characterized by a westerly declination rotation of 70–90° accompanied by shallowed inclinations, and the second group is characterized by a declination rotation of only 30° in the same sense. The first group is older and comprises rocks of late Eocene to early Miocene age, whereas the second group comes from rocks of late Ottnangian to Karpatian age. These results are indicative of two successive tectonic rotations, one in the late Ottnangian and another just preceding the Badenian. The first rotation was accompanied by a northward shift of the area.

Remanent magnetization of a Pliensbachian limestone sequence at Bakonycsernye (Hungary)

Abstract Remanent coercivity spectra derived from IRM acquisition curves and thermal demagnetization of the IRM indicate that magnetite, haematite and minor amounts of goethite determine the magnetic properties of the Pliensbachian limestones at Bakonycsernye. These limestones have been sampled at approximately 7-cm intervals along a 10-m stratigraphic section which covers the whole Pliensbachian stage (Lower Jurassic) without any recognizable break in sedimentation. The primary natural remanent magnetization (NRM) is carried by detrital particles of magnetite and haematite, but it is seriously overprinted by a normal magnetization which originates from secondary haematite with a wide range of blocking temperatures. This haematite is believed to have formed diagenetically during one of the Mesozoic periods of normal polarity. However, the reversal pattern obtained after NRM thermal demagnetization at temperatures ≥450°C is thought to be characteristic of the Pliensbachian stage.

Tectonic evolution of the northwestern Internal Dinarides as constrained by structures and rotation of Medvednica Mountains, North Croatia

Abstract This paper attempts to explain the tectonic history and possible reasons for the change of trend of the northwestern part of the Internal Dinarides in a transitional area between the Southeastern Alps, central Dinarides and Tisia, north of Zagreb. Structural and palaeomagnetic data collected in pre-Neogene rocks at Medvednica Mountains, combined with palaeomagnetic data available from Neogene rocks in the surrounding area, point to the following conclusions: (1) The reason for dramatic deflection in structural trend of the Internal Dinarides in the area north of Zagreb is a 130° clockwise rotation and eastward escape of a tectonic block comprising Medvednica Mountains and the surrounding inselbergs, bounded to the north by the easternmost tip of the Periadratic Lineament. In Medvednica Mountains, the main period of tectonic escape and associated clockwise rotation occurred in the Late Palaeogene, possibly in the Oligocene–earliest Miocene. (2) When rotated into the original position, the trend of observed pre-Neogene structures of Medvednica Mountains becomes parallel to the major structural trend of the central Dinarides. In view of their original orientation, these structures are interpreted in the following way: (a) The first D1 deformational event is attributed to the Aptian–Albian nappe stacking in the central–northern Dinarides that was accommodated by a top-to-the-north directed shearing and northward propagation of already obducted ophiolites of the Central Dinaridic ophiolite zone. This nappe stacking, which resulted in a weak regional metamorphism in tectonic units underlying the ophiolites, was orogen-parallel or at a very acute angle to known structural (and possibly palaeogeographic) trends. This implies a major left-lateral shear component along the former Adriatic margin and obducted Dinaridic ophiolite zone. (b) This was followed by Early Albian orogen-perpendicular shortening (D2) that was accommodated by folding and top-to-the-west thrusting. This deformation resulted in gradual cooling of the metamorphic stack and also in uplift and erosion of the higher structural units. (c) The D3 deformational event was driven by renewed E–W shortening that took place after the Paleocene, most probably during the Middle Eocene–Oligocene, i.e. synchronous with the main Dinaridic tectonic phase of the External Dinarides. This shortening was probably triggered by collision and thrusting of Tisia over the northern segment of the Internal Dinarides. (d) This was finally followed by D4 pervasive, right-lateral N–S shearing that is tentatively interpreted as being related to the right-lateral shearing of the Sava zone during the Eocene–Oligocene. (e) Following the main period of tectonic escape and induced clockwise rotation along the Periadriatic fault, possibly in the Oligocene–earliest Miocene, the Medvednica Mountains and the surrounding area were affected by repeated extensions and inversions since the Early Miocene to recent times. Palaeomagnetic data suggest that in the Early Miocene (but probably before the Karpatian) this area was part of a regional block that shifted northwards and rotated in a counter-clockwise sense. A second episode of counter-clockwise rotation occurred at the present latitude in post-Pontian times (since c. 5 Ma), driven by the counter-clockwise rotating Adriatic Plate.

Mesozoic palaeomagnetism of the transdanubian central mountains and its tectonic implications

Abstract The Mesozoic apparent polar wandering (APW) of the Transdanubian Central Mountains, determined from thermally isolated natural remanences at 13 localities, shows a remarkable similarity to the Mesozoic APW of Africa in that they both exhibit the same loop-like movement. Moreover, the difference between the two APWs can practically be eliminated by a 35° clockwise rotation of the palaeodeclinations. It is concluded, therefore, that the region of the Transdanubian Central Mountains was part of the African (-Adriatic) plate up to some time in the Cenozoic when it moved to its present position, resulting in a 35° anticlockwise rotation relative to Africa.

Archaeomagnetic directions: The Hungarian calibration curve

Abstract The first archaeomagnetic dating in Hungary was made using a reference inclination curve derived by interpolation between three inclination curves then available from continental Europe, namely from France, Bulgaria and the Ukraine. As the corresponding declination curves could not be interpolated with confidence, all three declination curves were used only for an estimation of the time interval to which the measured declinations might be assigned. As data accumulated it became feasible for dating to be made exclusively on the basis of archaeomagnetic and direct observational results for Hungary. The results now cover the last 2000 years with relatively short gaps (sixth and thirteenth centuries ad), for which data interpolation is plausible. The variation of inclination is fairly sinusoidal and well resolved. It exhibits two maxima (c. 70°) at approximately ad 800 and ad1500 and three minima at about ad300 (c. 58°), ad 1300 (c. 55°) and in the twentieth century (c. 62°). Although the variation of the declination is twice as large as that of the inclination, the declination record has inherently less resolution (dD = dI/cosI > dI). Approximately zero declination values during the first half of the first millennium ad were followed by westerly values during the sixth to eighth centuries (c. 10°). It appears that by ad 900 the declination was already easterly and rapidly increasing until ad 1000 when it peaked with a value over 20°E. A newly discovered feature is a short oscillation beginning with 16° at ad 1300 and ending with 16.5° at ad 1600 during which the declination became westerly with a minimum of −10° in the first half of the fifteenth century. From ad 1600 on the declination rapidly decreased and described a negative half circle between about ad 1620–30 and 1950 with a minimum of −18° at ad 1800. The general pattern of the directional secular variation for Hungary is in agreement with that for France, Sicily, Britain, the Ukraine and the Balkans.

Palaeomagnetic evidence of tectonic rotations in the southern margin of the Inner West Carpathians

Abstract Palaeomagnetic samples were collected at 33 Triassic localities from three nappes and a pseudo-autochthonous unit in the Aggtelek-Rudabanya Mountains in order to establish the rotation pattern of the area as a whole with respect to the enclosing principal continents, and to check whether relative movements in the past between the various units that make up the region are resolvable by the palaeomagnetic data. After detailed thermal demagnetization and careful data selection it has emerged that the entire study area was rotated anticlockwise with respect to both Europe and Africa, i.e., in a similar manner and to a similar degree as other units in the Central Mediterranean region (e.g., the Transdanubian Central Mountains). On the other hand, small-scale relative movements such as those between the Aggtelek and Rudabanya areas are not apparent in the present data.