NRM Directions Research Articles

Low-latitude Oxfordian position of the Oravic crustal segment (Pieniny Klippen Belt, Western Carpathians): Palaeogeographic implications

The Bajocian and the Oxfordian–Kimmeridgian crinoidal limestones of the Pieniny Klippen Belt (PKB), Western Carpathians and the Bathonian–Callovian neptunian dykes which cut them were sampled for rock magnetic and palaeomagnetic study. The sampled sections were: Babiná, Mestečko, Vršatec, Bolešovská dolina and Szaflary. SIRM(T) curves show that the limestone and the dykes from Mestečko contain hematite with blocking temperature T b about 650–670 °C. Babiná limestone and dykes contain magnetite with T b between 470 and 570 °C, usually accompanied by small hematite tail. Sometimes goethite with T b about 80–100 °C is seen. Dykes and limestone from Vršatec contain magnetite with T b of 570 °C or hematite with unblocking temperature of 670 °C. After cleaning of recent soft component by a temperature of 200 °C and a field of 10–20 mT, the hard component was revealed. After tilt correction the locality-mean directions found in limestones have scattered declination but similar reversed inclination between −35° and –40° for the Bajocian and between −35° and −49° for the Oxfordian–Kimmeridgian. An exception was the mean direction for the Bajocian limestone from Bolešovská dolina which has normal polarity. Fold test for inclination only showed pre-folding inclinations for the Bajocian and Oxfordian–Kimmeridgian limestones and scattered inclinations for the in situ and tilt corrected directions for the dykes. The mean palaeoinclination calculated for Bajocian and Oxfordian–Kimmeridgian by means of “inclination-only” test are −38.5° +/−3° and −42.5° +/−11.2°, respectively, which corresponds to 21.7 +/−1.5°N and 24.6 +/−5.6°N of palaeolatitude. This is about 10°–15° lower than the value of inclination expected for that age for the Pieniny Klippen Belt from the Jurassic poles for Stable Europe. As positive inclination only fold test proves pre-folding age of isolating directions other possibility — flattening of inclination should be considered. Anisotropy of magnetic susceptibility (AMS) and magnetic unhysteretic remanent magnetisation (AARM) can be used for the evaluation of origin and for the deviation of natural remanent magnetisation NRM. The axes of AMS are scattered for all localities. The degree of anisotropy is low. The shape parameter T shows equal distribution of oblate and prolate fabric. On the contrary, the ARM maximum axes are well clustered, especially for Mestečko and Vršatec. The maximum axes are distributed in a vertical plane for Mestečko and in horizontal plane for Babiná and Vršatec. This evidences that the AARM fabric is not related to sedimentation and compaction. The anisotropy axes are not close to the NRM directions. For Mestečko they are steeper. This excludes an effect of shallowing of NRM. On this basis we can assume that more southern palaeolatitude of the Pieniny Klippen Belt than expected is a demonstration of palaeogeography and tectonics and points to a close vicinity of the PKB to the African plate in the Jurassic. The results from the Polish part of PKB which emerged in the latest years showed the same palaeolatitude for the Callovian–Kimmeridgian limestones. The low palaeomagnetic inclinations measured in the Bajocian crinoidal limestones show that the southward drift may have started earlier than previously expected.

Magnetic polarity stratigraphy of the Bhuban succession, Surma Group, Tripura-Mizoram accretionary belt

A 1355 m thick Bhuban sequence exposed along Tuirial section in the Aizawl district, Mizoram has been studied for magnetostratigraphic and rock magnetic attributes. The study reveals mono-mineral nature of the rocks with magnetite in the range of Stable Single Domain (SSD) showing the most favourable mineralogy to produce authentic (Natural Remanent Magnetization) NRM directions. Using routine demagnetization methods we reveal a total of 7 normal magneto-zones in the studied part of the Tuirial section. Correlation of the observed polarity with GPTS gives ages ∼12.5 Ma to ∼8 Ma. A notable increase in the rate of sedimentation at ∼9.5 Ma (∼750 m level in the section) indicates facies change from turbidite like sequence to pro-delta system. Detailed sedimentologic work is under progress to relate the magnetostratigraphy to understand some of the basin evolutionary processes.

Neoglacial activity in Finland: dating the evidence from glaciolacustrine sediment sequences in the Halti-Ridnitšohkka region

Palaeomagnetic dating of sediment sequences from lakes Pihtsusjärvi and Haltijärvi in Enontekiö, northwestern Finnish Lapland, provided a chronology for a study of the palaeohydrological and glacier history of the Halti-Ridnitšohkka fells area. Both sequences exhibited a stable primary component of the NRM direction, allowing a comparison of the secular variation records with the chronology of Lake Nautajärvi reference curves (NRM) to provide a chronological framework for the present study. The basins of Lakes Pihtsusjärvi and Haltijärvi were formed 10 500 and 5500 cal. BP, respectively. Minerogenic layers found from studied Lake Pihtsusjärvi sequences provided evidence of postglacial activity of the Halti fell valley glacier. The results indicated that the Halti glacier was active soon after the continental ice sheet had retreated from the area at around 10 000 9000 cal. BP, during which time the Kovdajohka valley terminal and recessional moraines were formed. This was followed by a Holocene thermal maximum (HTM) in northern Fennoscandia, which was interrupted by climate cooling at around 6000 cal. BP. The basal age of the Lake Haltijärvi sediment core and Lake Pihtsusjärvi sediment stratigraphy indicated minor, short-term neoglacial activity in the Halti fell vicinity between c. 6000 and 5500 cal. BP.

Archaeomagnetic investigation of oriented pre-Columbian lime-plasters from Teotihuacan, Mesoamerica

Abstract Results of an archaeomagnetic study of two excavation field seasons at the Xalla and Teopancazco residential areas of the ancient Prehispanic city of Teotihuacan, Central Mexico are reported. One-hundred and fifty three oriented samples of lime-plasters from the two archaeological sites were collected for the study. NRM directions are reasonably well grouped. Alternating field demagnetization shows single or two-component magnetizations. Rock magnetic measurements point to fine-grained titanomagnetites with pseudo-single domain behaviour. Characteristic site mean directions from both sites are correlated to the available palaeosecular variation curve for Mesoamerica. The mean directions obtained from Xalla site point to average dates of 550 AD±25 years that matches with the documented ‘Big Fire’ of Teotihuacan (AD 575). Two consecutive construction levels at Teopancazco were estimated as AD 250–350 and AD 350–425 respectively.

Rock magnetism, geologic history and aeromagnetic anomalies: A case study of the Ulupa Siltstone, Adelaide Geosyncline

Marine siltstones and fine sandstones of the Brachina Formation and olive-green siltstones of its more distal equivalent, the Ulupa Siltstone, were deposited during the Marinoan in the Adelaide Geosyncline. These sediments have since been deformed and metamorphosed during the Ordovician Delamerian Orogeny. In the Flinders Ranges, the relatively unmetamorphosed sediments, though iron-rich, are only weakly magnetic. In the Mount Lofty Ranges, where peak metamorphic grades were reached, the Brachina Formation and its equivalent, the Ulupa Siltstone, are extremely magnetic. Magnetic markers within these formations have been used to trace the macrostructure of the Southern Adelaide Foldbelt and have been especially useful where outcrop is limited. While the magnetic markers are consistent with respect to their continuity and presence, the amplitude and character of the anomaly changes significantly along strike. This is basically a function of metamorphic grade as anomalies at lower grades can be successfully interpreted (since both depth and dip are often known) assuming induced magnetisation only. At the highest grade, where the rocks are andalusite schists, the magnetic anomalies are inconsistent with the direction of the present magnetic field, and remanence must represent the greater component of the anomaly. The results of the rock magnetism study show a close correlation between metamorphic grade and the variation in magnetic mineralogy and natural remanent magnetisation. The rocks display multiple components of NRM including a weak primary detrital component acquired during deposition and a much stronger thermal or chemical component acquired during the Delamerian Orogeny. The secondary component has been related to the NRM direction indicated by the nearby Black Hill Norite which is believed to have intruded during the waning phases of the Delamerian Orogeny. The results of the rock magnetism study have been interpreted in terms of the geologic history of the Ulupa Siltstone and have been used in the interpretation of the aeromagnetic anomalies it causes.

Palaeomagnetic Reversals in Mississippian Sediments of Missouri

—Normal to reversed polarity transitions have been found in Mississippian limestones of Missouri at longitude 269.7 and latitude 38.6 in an area of approximately 2,000 sq.km centered on the city of St. Louis. The transitions suggest a reversal of the geomagnetic field during the time these limestones were deposited. The mean normal NRM direction of declination 323.2, inclination 14.8, alpha-95 10.0 and corresponding reversed direction declination 144.8, inclination of 11.7 with alpha-95 of 5.2, are each tightly grouped, while in between these two groups the circles of confidence reveal a large scatter. Six normally magnetized groups of sites and corresponding VGP positions were found while seven groups are reversed. The mean normal VGP was at longitude 146.2 and latitude 44.6 and the reversed one at longitude 313.9 and latitude −34.8. The large scatter in between these two groups is interpreted as being due to rapid changes in the direction of the geomagnetic field when compared with the rate of deposition of the sediments. Alternating field (AF) and thermal demagnetization (TH) techniques were used to remove secondary components of the NRM.

A preliminary magnetic study on lacustrine sediments from Zoigê Basin, eastern Tibetan Plateau, China: Magnetostratigraphy and environmental implications

A preliminary magnetic study was conducted on Core RM (310 m long), drilled in the Zoigê Basin (33 °57′N, 102 °21′E), on the eastern Tibetan Plateau. Greigite in SD state exists as a main magnetic contributor along the whole core. In addition, magnetite and haematite are detected between 160-45 m. NRM directions show a complex record with many apparent changes in inclination, for which greigite or the remanence acquisition process may be responsible. Nevertheless, the Matuyama/Brunhes boundary and, possibly, the Blake Event are identified. Between 310 and 160 m (as well as between 45 and 4.5 m), predominant anoxic conditions favour the formation of greigite, leading to higher susceptibility (χ) and remanence peaks. However, higher carbonate content during dry periods in these zones, may dilute the concentration of magnetic minerals, resulting in lower values of magnetic parameters. Between 160 and 45 m, weak oxic conditions preserve detrital magnetic minerals. The increasing χ background in this zone may indicate changes in the concentration of magnetite and haematite related to the uplift of the Tibetan Plateau, while higher χ peaks may indicate fluctuations to anoxic conditions, which enhance greigite concentration.

Paleointensity of the Geomagnetic Field in Egypt from 4000 BC to 150 AD Using the Thellier Method.

A total of 170 ceramic specimens from 16 archaeological sites representing 16 well determined ages from 4000 BC to 150 AD were investigated using the Thellier method. These paleointensities were compared with data from countries near Egypt after reduction to a common latitude to determine a secular variation curve of the geomagnetic field for the time period from 4000 BC to 150 AD. The field intensity increases from about 30 μT around 3500 BC to a maximum of 70 μT at about 400 BC when it starts to decrease until 150 AD. The Thellier double heating method was used together with the following additional test measurements : a) during the Thellier experiments the laboratory field F Lab . pointed in the direction of stable NRM, b) magnetic susceptibility was measured after each double heating step to check for chemical alteration, c) a TRM check was made, by repeating the pTRM induction at a lower temperature, d) recent samples which were fired in a known field were used to test the reliability of our experimental setup and method. Based on rock magnetic measurements magnetite was identified as the predominant carrier of magnetization in the ceramics and bricks.

Thermoviscous remanent magnetism of Columbia River basalt blocks in the cascade landslide

We studied sixteen basalt samples from a landslide in the Columbia River Gorge to determine if they had acquired a thermoviscous remanent magnetism (TVRM) since the slide was emplaced about 800 years ago. All samples were thermally demagnetized at 20 heating steps until 360°C, where a large change in susceptibility was noted. Analysis of the directional changes during demagnetization indicates that the samples contain up to four TVRM components, in addition to an NRM component. The TVRM components with the lowest blocking temperatures are tightly clustered around the present field direction while the NRM directions are consistent with a random distribution, as expected for a landslide deposit. Measurements of hysteresis parameters and thermomagnetic analyses of the samples demonstrate that the dominant magnetic mineral in the basalt is single‐domain magnetite. The temperature at which the first TVRM component was removed ranges from 70°C to 100°C. This result can be compared to nomographs that relate the time and temperature used to demagnetize a TVRM with the time and temperature at which the TVRM was acquired. Our results are more consistent with the nomograph of Pullaiah et al. [1975] than with the nomograph of Middleton & Schmidt [1982].

Magnetic properties of howardite, eucrite and diogenite (HED) meteorites: Ancient magnetizing fields and meteorite evolution

The remanent magnetism and other magnetic properties of four howardites (Kapoeta, Petersburg, Le Teilleul and EET 87503), two eucrites (Sioux County and PCA 82502) and three diogenites (Shalka, Johnstown and EET 79002) have been investigated. The dominant carrier of magnetic properties is low-nickel kamacite, present to the extent of 3–5% by weight in Petersburg and 0.1–1% in the others. A wide range of remanent magnetization (NRM) intensities, characteristics and behaviour during alternating field and thermal demagnetization is observed. Two common features are scattered directions of NRM between meteorite subsamples and anomalous behaviour of NRM direction and intensity during demagnetization. All the meteorites show evidence of a primary NRM acquired during an early evolutionary and some acquired secondary NRM. The heterogeneous NRM, with which anomalous demagnetization appears to be associated, implies magnetization of constituent fragments of the meteorite prior to, and no remagnetization after final meteorite assembly. It is tentatively proposed that the primary magnetizing field was of internal origin in the parent body, and that thermoremanent magnetization occurred when the body cooled after differentiation. The coherent magnetic fabric observed in all but two of the HEDs was most probably imparted at the time of final assembly of the meteorites.

Analysis of NRM directions from the Rossello composite: implications for tectonic rotations of the Caltanissetta basin, Sicily

The paleomagnetic results of the Pliocene marine marls from the Rossello composite section [30] in the Caltanissetta basin of Sicily have been analysed to determine the amount and timing of tectonic rotations. The analysis shows that there is a consistent inclination error for the NRM component, which is removed between 500 and 600°C, which may be caused by compaction of the sediment. Differences in inclination error throughout the composite section are related to differing carbonate content, a higher carbonate content implying less compaction and smaller inclination error. Furthermore, consistent differences are found between directions of normal and reversed polarities as well as between directions of the lower and upper part of the composite section. These differences are to a large extent related to differences in carbonate content, porosity and influence of weathering. The Caltanissetta basin shows a total clockwise rotation of 34° occurring since the early Pliocene. The magnetostratigraphy of the Rossello composite section shows that from the Miocene/Pliocene boundary (5.2 Ma) up to the Gilbert/Gauss boundary (3.6 Ma) there were no tectonic rotations of the Caltanissetta basin. Further, we speculate that 10° of this total clockwise rotation occurred during approximately Kaena times (ca. 3.05 Ma), and that at least 24° of this clockwise rotation is younger than the Gauss/Matuyama boundary (2.6 Ma).

Tectonic implications of Paleomagnetic data from the Northland ophiolite, New Zealand

Paleomagnetic sampling of 12 sites from two separate massifs of the Northland ophiolite, New Zealand, yields consistent results. Two characteristic NRM directions are recognised: one of normal polarity associated with weakly magnetised rocks (particularly intrusive dolerites), and another of reversed polarity associated with strongly magnetised rocks (particularly basaltic pillow lavas). Thermal and AF demagnetisation shows that the normal directions are due to a secondary viscous component which reflects the present-day field direction, whilst the reversed directions are stable (typically to 500°C) reflecting either the primary TRM or a hard secondary component. The declination and inclination of this stable magnetisation differ substantially from that of the present-day field implying significant tectonic movement or deformation of the ophiolite since formation. Anomalous declinations and inclinations are interpreted here in terms of rotations about vertical axes and, tentatively, latitudinal translation since no widespread significant tilting has been recognised within the massifs. Declinations are consistent with clockwise rotations of 50° and 120° for the Cape Reinga and Mangakahia massifs, respectively, probably associated with obduction at a convergent margin with a right-lateral slip component, situated to the northeast of Northland. The anomalously low inclination vectors (30–40°) are consistent with an origin at lower than present latitudes which, if true, implies long-lived N-S translation of oceanic crust in the southwest Pacific region between Cretaceous and Oligocene times.

Magnetic properties of chemical remanent magnetization in synthetic and natural goethite: Prospects for a natural remanent magnetization/thermoremenant magnetization ratio paleomagnetic stability test?

Magnetic properties of chemical remanent magnetization in synthetic and natural goethite: Prospects for a natural remanent magnetization/thermoremenant magnetization ratio paleomagnetic stability test?

Directional properties of alteration CRM in basic igneous rocks

SUMMARY Remagnetization experiments, involving the acquisition of secondary CRM, VRM and PTRM by reheating in controlled laboratory fields of 0-75pT, have been carried out on three different basic igneous rocks. The initial magnetic phases in all three rocks are titanomagnetites modified by varying degrees of deuteric- and low-temperature oxidation. Laboratory reheating results in further modification of the parent magnetic mineral phases, and the formation of new cubic and/or rhombohedra1 phases as daughter products. The induced CRM precludes identification of primary remanence directions, despite maximum reheating temperatures well below the original unblocking temperatures. The breakdown of a cation-deficient SD/PSD cubic parent phase to haematite in a sample from a basaltic lava flow leads to a directionally stable CRM intermediate between the NRM and external field directions, while a similar process in a dolerite leads to CRM directions parallel to the external field. Chemical alterations within the cubic system in an alkaline dyke sample with slightly larger (PSD) magnetic ‘grain sizes’ also lead to CRMs controlled by the external field. The intensities of all secondary remanence components show a linear dependence on laboratory field strength.

Paleomagnetism of the Shakotan Peninsula, West Hokkaido, Japan.

A paleomagnetic study was made for Miocene-Pliocene volcanic and sedimentary rocks from the Shakotan Peninsula, West Hokkaido, Japan (43.2°N, 140.5°E). Out of 20 rock units sampled, 18 sites were successful to reveal stable remanence but three sites were rejected because of their anomalous directions. Although most of the sites showed in situ directions coinciding with the direction of the present earth's field (PEF), we did not adopt the usual rejection criterion; the NRM in which the in situ direction is close to the PEF should be automatically rejected. Our opinion is that as long as the original stable remanence is detected there is no need to, rather better not to, reject such NRM directions. To support this idea we compared decay curves of remanence in AF demagnetization among NRM, IRM, and ARM. The total mean paleomagnetic direction (I=55.6°, D=14.7°, α95=9.5°) is slightly eastward and gives a paleomagnetic pole (77.9°N, 249.5°E, α95=12.1°) which might suggest a clockwise block rotation of the Shakotan Peninsula. However, we concluded that this pole for middle Miocene to Pliocene is not significantly apart from the present geographical pole on 95% confidence level. On the same confidence level, the pole position agrees with some of the contemporary poles from Northeast Japan implying the same block of West Hokkaido and Northeast Japan. This pole position also indicates that the Shakotan Peninsula did not suffered much tectonic deformation from the collision of East Hokkaido and the Kuril arc to the middle part of Hokkaido which is considered to have occurred after the late Miocene (KIMURA and TAMAKI, 1985). The northerly paleodirection revealed by the lowest sedimentary rock (-15Ma) might be an indication of the timing of rotation of Northeast Japan earlier than Southwest Japan.

Rock magnetism of the Early Pliocene Trubi Formation at Eraclea Minoa (Sicily)

The most important magnetic mineral in the finegrained marls of the Early Pliocene Trubi formation near Eraclea Minoa is magnetite. Some goethite is also present. The magnetite is of high purity and of primary origin.Hysteresis and low‐temperature measurements show that most magnetite grains are single‐domain. The presence of superpara‐magnetic grains is suspected. Multi‐domain grains are rare. It is stewn that AF demagnetization does not yield correct NRM directions.

Rock magnetic properties and structural development in the core of the Hazara‐Kashmir Syntaxis, NE Pakistan

Paleomagnetic and structural analyses have been conducted on three well‐exposed sections through a 6–8 km thick pile of molassic red beds (Murree formation) in the lowermost tectonic unit of the Hazara‐Kashmir Syntaxis. Micropaleontological age determinations of the lowermost Murree sediments indicate Late Paleocene deposition (55 m.y.). From south to north, the sections are situated in the Jhelum, Neelum (both in Azad Kashmir), and Kaghan (northeastern Pakistan) valleys. Thermal demagnetization experiments suggest that haematite with high unblocking temperatures carries stable characteristic remanence directions. The relationship between finite strain and magnetic fabric was established by mapping deformed reduction spot strain markers and by measuring the anisotropy of magnetic susceptibility (AMS). For the Jhelum valley the weakly tectonized Murree beds are characterized by flattened AMS ellipsoids resulting from diagenetic compaction. Inclination values suggest that the Murree foreland basin started to develop at about 8°N during the early suturing of India and the development of island arcs to the north. India has moved northward by at least 2600 km since collision with Eurasia in the Paleocene. Declination values suggest 45° of clockwise rotation of the block supporting the Murree formation relative to the Indian craton. For the Neelum and Kaghan valleys, quantitative strain mapping shows a progressive increase of deformation northward. NRM directions rotate passively toward the cleavage plane which parallels the foliation of the AMS ellipsoids. NRM directions and AMS ellipsoids are deformed because of superposition of tectonic strain on a primary compactional strain. The AMS pattern is interpreted in light of this superposition, and a regional deformation path from south to north is suggested. A tectonic rotation model is proposed which is consistent with the transport directions around the Hazara‐Kashmir Syntaxis and the rotation of thrust sheets indicated by the NRM data. This model relates the convergent transport directions with the mean indentation direction of India into Asia.

Alteration of magnetic minerals and its effect on the NRM of Deccan trap flows of the Narmada region, India

Magnetic and geochemical investigations have been carried out on suites of Deccan lava flows from both sides of the river Narmada. Multidomain magnetite is more abundant in the flows on the southern side than among the northern ones. Flows from south of the river seem to have absorbed more water than those on the northern side, possibly well after their emplacement. Multidomain magnetite-bearing samples shattered often into pieces when standard cores of these were heated to a temperature beyond 570 K and several such samples lost their primary NRM direction.

MAGNETIC PROPERTIES OF LATE CRETACEOUS-CENOZOIC VOLCANIC ROCKS OF THE SOVIET FAR EAST SOUTH

Magnetic properties of late Cretaceous-Cenozoic basalts are discussed in relation to their composition and conditions of crystallization during the cooling of the melt in subaerial and subaqueous environments. The possible effects of secondary oxidation (maghemitization), at least in certain cases, local tectonic deformations and paleosecular variations of geomagnetic field on the distribution of NRM directions are noted. Paleomagnetic reference data for individual volcano genic complexes of the Sikhote-Alin, Sakhalin and South Kurils regions are given, and a comparison with paleomagnetic data for the same aged rocks of Japan and Korea is made.

Selective destructive demagnetization of breccias from DSDP Leg 83: a microconglomerate test

The magnetization of four breccia samples from the Leg 83 section of DSDP Hole 504B was analyzed by selective destructive demagnetization in order to study the origin and stability of hydrothermally altered basalts. The NRM directions of the clasts for three of the four samples are randomly oriented and much more strongly magnetized than the bulk sample. Clasts which were individually demagnetized show two or more components of magnetization, but neither are coincident with those of the bulk sample, indicating that NRM was probably acquired prior to the consolidation of the breccia and suggesting that any overprint (VRM or otherwise) can be removed by AF demagnetization to at most 50 Oe. Reflected light microscopy and electron microprobe analysis of two samples show that the unexpectedly high NRM of the matrix regions is apparently the result of secondary magnetic phases precipitated from hydrothermal solutions.