Natural Remanent Magnetization Directions Research Articles

A Pennsylvanian paleomagnetic pole from the mineralized Late Cambrian Bonneterre Formation, southeast Missouri

One hundred eighty‐eight oriented samples of the Late Cambrian Bonneterre carbonates of southeastern Missouri were taken from Viburnum Trend mines and local surface outcrops. Initial natural remanent magnetization directions have south‐southeasterly declinations and downward inclinations. Progressive demagnetizations using thermal and AF techniques give two directions: one is a viscous present‐day field direction, and the other is of reversed polarity, with south–southeasterly declinations and very shallow inclinations. The resulting paleopole, at 43°N, 126°E, is located near the Pennsylvanian to Early Permian segment of the North American apparent polar wander path. A negative conglomerate test supports this assignment for the magnetization age. Magnetic extracts from the mineralized as well as unmineralized zones of the Bonneterre contain spherical crystal aggregates of pure magnetite of late diagenetic origin. The secondary magnetization resides in the magnetite spheroids, and we infer that the spheroids formed prior to sulfide mineralization.

Palaeomagnetic investigations in the Hercynian mountain belt of central Europe

The palaeomagnetism of sediments and volcanics of Middle Devonian to Early Carboniferous age has been studied in the Hercynian realm of the Vosges, Harz and Franconian Forest (France and Germany). After appropriate cleaning, characteristic directions of natural remanent magnetization (NRM) have been determined, which are considered to be of pretectonic origin according to rock magnetic and structural geological evidence. A mean palaeopole for the Devonian rocks in the areas investigated was determined at — 29.7°N and 9.0°E. The site latitude derived from the Devonian rocks agrees very well with that observed in Great Britain and Britanny. The significant difference in longitude between our Devonian poles and the other European data may be due to an anticlockwise rotation of central Europe with respect to the northern Hercynian Foreland. The Carboniferous rocks yield a mean palaeopole at —41.7°N and 340.4°E, matching the Carboniferous part of the European polar wander path. A fold test within Carboniferous greywackes from the Harz mountains proves a pre-folding origin of the NRM of these rocks. However, this stable magnetization direction indicates local or regional tectonic displacement. Our results do not support any major latitudinal motions of continental segments in central Europe during the Hercynian orogeny. The Palaeozoic palaeomagnetic data from the Vosges, Franconian Forest, Harz, Britanny and Central Massif can be used to interpret the Hercynides in terms of a large scale shear belt.

Paleomagnetism of Late Jurassic rocks in the Northern Canelo Hills, southeastern Arizona

The Canelo Hills volcanics are exposed in the Canelo Hills, a northwest trending range in Santa Cruz County, southeast Arizona. The formation is composed of silicic tuffs and flows as well as volcaniclastic conglomerates and sandstones. Strikes of the rocks are generally to the northwest with moderate dips to the southwest and northeast. Apparent age results from the sequence studied paleomagnetically include two published isotopic dates of 147±6 Ma (K‐Ar, biotite) and 149±11 Ma (whole rock, Rb‐Sr) and a Rb/Sr isochron age, reported here, which indicates an age of 151±2 Ma. Paleomagnetic samples were collected from 17 cooling units in the northern Canelo Hills. Samples from most of these units responded to alternating field (af) demagnetization, and secondary components were generally erased by peak af between 10 and 50 mT. Samples from five sites showed no response to af demagnetization. Thermal demagnetization of samples from these units produced no significant changes in direction of natural remanent magnetization (NRM), although within‐site clustering of NRM directions was improved. Data from two sites were rejected because of failure to isolate a well‐determined characteristic NRM. Of the remaining 15 sites, 10 sites were of normal polarity, while five sites showed reversed polarity. Intensities of the characteristic NRM ranged from 4×10−3 to 3×10−1 A/m. The data from these 15 cooling units yield a formation mean direction of I = 29.9°, D = 334.9° with k = 33.4 and α95 = 6.7°. The resulting paleomagnetic pole is at 62.2°N, 130.3° (dp = 4.1°, dm = 7.4°). This pole is between poles obtained from the Summerville and lower Morrison formations. The Canelo Hills pole is thus consistent both in position and age with the Late Jurassic episode of rapid apparent polar wander originally defined by paleomagnetic data from the Summerville and Morrison formations.

Magnetic reversal history of Pleistocene sediments at Old Crow, northwestern Yukon Territory

The banks of the Porcupine River near Old Crow in the unglaciated part of the northern Yukon consist of about 55 m of interbedded fluvial and lacustrine gravels, sands, silts, and clays. This sequence was sampled closely for palaeomagnetic analysis and the sediments were found to possess a natural remanent magnetization (NRM) that is stable in direction with mean destructive fields of 200–300 Oe (16–24 kA/m). Therefore, good palaeomagnetic data can be obtained from sediments that have been subjected to permafrost conditions, although some samples show disturbance of NRM directions by cryoturbation.Magnetic polarity transitions are centred at 17 and 26 m above river level and the 15 samples between them define an interval of reversed polarity of considerable duration. The behaviour of NRM during the transition periods is complex with declination switching abruptly by 180° but with the inclination displaying very erratic variation over a considerable time. This may be caused by mixing in single samples of components of NRM with ages that straddle the transition—a situation that would be possible if NRM were post-depositional in origin and so could be locked in at times long after sedimentation.Sedimentological, palaeobotanical, and palaeomagnetic data all support the presence of a major unconformity in the middle portion of the sedimentary sequence. This unconformity lies at the top of the transition that closes the reversed episode so that the sediments below it are likely older than 700 000 years.These results demonstrate that palaeomagnetic studies on the sedimentary sequences exposed in the many isolated sites within the Old Crow region should greatly facilitate their correlation and indirect dating, effecting thereby an improved understanding of the recent geological history of this anthropologically important area.

Magnetic properties of Cenozoic ophiolites in the Hayama-Mineoka and Setogawa belts, South-Central Honshu, Japan.

Intensity and directions of natural remanent magnetization and their stability against alternating field demagnetization as well as thermomagnetic behavior, opaque mineralogy and chemical composition of constituent ferromagnetic minerals have been extensively investigated with pillow and dyke basalts, dolerites, gabbros and ultramafic rocks composing the Hayama-Mineoka and Setogawa ophiolite complexes.It has been demonstrated by the present work that degree of alteration of most basalts in these ophiolite complexes is low and comparable to that of the oceanic basalts. Titanomagnetite is subjected to low-temperature oxidation mostly owing to bleaching of ferrous ions but directions of the original TRM appear to be unaffected by the oxidation, although its intensity may be reduced and additional CRM may be acquired. Hydrothermal alteration recognized in a restricted area substantially reduces intensity of NRM and causes scatter in directions. Ultramafic rocks have stable and relatively large NRM, which is carried by fine magnetite particles precipitated along margins of olivine crystals.

Archeomagnetism of a 19th century pottery kiln near Jordan, Ontario

The Jordan pottery kiln near St. Catharines, Ontario, was last fired about 1840–1841. Bricks from the kiln floor have intense NRM's (natural remanent magnetizations) in the range 0.2 × 10−2 to 3.7 × 10−2 emu cm−3 which are directionally stable against alternating field (AF) demagnetization to 1000 Oe (7.96 × 104 A/m). Of 31 specimens tested by the modified Thellier double-heating method, 27 yielded reliable paleofield intensities averaging 1.166 ± 0.092 times the present field intensity. Natural remanent magnetization directions are "streaked" in inclination and shallower than expected. Their average, D = 359.4°, I = 71.3 °(α95 = 2.2°, k = 310, N = 15 samples), differs significantly from both 1845 and 1979 fields in the area. Upon thermal demagnetization to 520 or 540 °C, streaking disappears and remanence vectors systematically steepen. The thermally cleaned mean direction, D = 357.8°, I = 73.7 °(α95 = 1.5°, k = 635, N = 15 samples), is indistinguishable at the 95% confidence level from the 1845 field, but differs significantly from the 1979 field.Part of the "inclination error" and streaking of NRM directions could result from fabric anisotropy or tilting of the bricks, but the greater part probably results from shape anisotropy of the strongly magnetic kiln floor, which deflects the internal field away from the external field and into the plane of the floor during cooling. The internal field is also weaker than the external field as a result of this self-demagnetization. The stronger the NRM, the greater the inclination error and the weaker the apparent paleofield intensity recorded. Thus, high-blocking-temperature fractions of NRM should record a less magnetically refracted (i.e., a steeper) paleofield, as observed. Also observed is approximate correlation between the spatial variation of NRM intensity in the kiln floor and variations in inclination and apparent paleointensity. The shape anisotropy of strongly magnetized horizontal sheets is a probable source of shallow inclinations, scattered directions, and weak apparent paleointensities in many submarine lavas sampled by the Deep Sea Drilling Project.

The palaeomagnetism of Jersey volcanics and dykes, and the Lower Palaeozoic apparent polar wander path for Europe

Summary. Stable natural remanent magnetization (NRM) in the Jersey Volcanics and in a single rhyolite dyke was probably acquired during the Cambrian before folding of the volcanics in the Cadomian Orogeny. After dip correction, the volcanics yield a palaeomagnetic pole at 323 E, 52 N (dp = 33, dm=35). In Jersey dolerite dykes three groups of stable NRM directions are recognized, with palaeomagnetic poles at 248' E, 26 N (dp = lo, dm = 20°), 339 E, 1 S (dp = 9, dm = 12), and 336 E, 31 S (dp = 5, dm = 9). Comparison with the European apparent polar wander path implies that stable NRM in these groups was acquired respectively during Late Precambrian or early Cambrian, Siluro-Devonian and middle Carboniferous time. The stable NRM of the Jersey lamprophyre dykes yields a palaeomagnetic pole at 322 E, 16 N (dp = 31°, dm = 38) and is probably of Silurian or Devonian age. These palaeomagnetic poles and other new data determined by the author for the Armorican Massif can be fitted to a common apparent polar wander path for Europe, and this implies that the basement of Lower Palaeozoic Europe extended from the Baltic Shield at least as far south as the Armorican Massif. The Hercynian Orogeny in these parts of Europe was therefore probably intracratonic. This polar wander path implies that in early Cambrian time the pole did not move significantly relative to Europe, but that this was followed by a large middle to late Cambrian polar shift which corresponded to rapid drift of Europe across the South Pole.

Magnetic fields of the solar nebula as recorded in chondrules from the Allende meteorite

The natural remanent magnetization (NRM) in individual chondrules from the Allende meteorite was measured. These had previously been oriented relative to each other. The NRM directions of the chondrules are not initially random, but they become scattered after either alternating field (AF) or thermal demagnetization. The NRM is less stable than anhysteretic remanent magnetization (ARM) against AF-demagnetization. The bulk of the NRM in the matrix is erased by 300°C. For the larger chondrules it is erased by 550°C, but for the smaller chondrules and the white inclusion a substantial decrease in NRM occurs by 350°C leaving about 20% up to 600°C. The behavior of the laboratory-induced ARM and the NRM under alternating field demagnetization suggest that the NRM of the chondrules consists of at least two components of TRM. One is a high-temperature component which was acquired when the individual chondrules were cooled through the Curie temperature and before they were assembled into the Allende meteorite. The other is a low-temperature component which was probably acquired in a field of about 1 Oe when the meteorite experienced thermal metamorphism or during the assembly of the meteorite.

Iterative method of isolating primary and secondary components of remanent magnetization illustrated by using the Upper Devonian Catombal Group of Australia

Directions of natural remanent magnetization from Upper Devonian Catombal Group red beds retain after magnetic cleaning a large secondary component acquired after the rocks were folded. A computer program was developed to eliminate progressively this ‘hard’ secondary component, which was found to lie along the present dipole field direction with an average intensity 1.6 times that of the primary component. It is interpreted to be due to weathering. The best estimate of primary direction was taken as the one in which the site mean directions, after removal of some secondary component and unfolding, gave minimum dispersion. The Upper Devonian field direction at Wellington derived from this procedure is (171°, −14°), with A95 = 8°. The southern hemisphere paleomagnetic pole (49°S, 312°E) obtained is not significantly different from the only published Australian pole position of Upper Devonian age, from the Lochiel Formation, at (58°S, 320°E).

Palaeomagnetism of Precambrian rocks from Southeast Norway and South Sweden

Precambrian amphibolite and hyperite rocks from the Bamble and Kongsberg areas in SE Norway, and amphibolite rocks from SW Sweden were investigated for evidence of remagnetization by the Sveconorwegian metamorphic episode. The similarity of the characteristic natural remanent magnetization directions, shown by the various rocks from the Bamble and Kongsberg areas, indeed supports the idea of remagnetization on a regional scale. Therefore the average pole position at 3°S, 153°W, determined from six sites in these areas, is considered to reflect the average virtual pole position for the post-Sveconorwegian period of uplift and cooling (1,120–975) · 10 6 year ago. The pole positions determined from the characteristic natural remanent magnetization directions of amphibolite rocks in SW Sweden are indicative of being somewhat younger. In addition, two hyperite dikes were studied near Karlshamn in SE Sweden. Their characteristic natural remanent magnetization is consistent with that of the hyperite dikes in central south Sweden (Mulder, 1971). The Precambrian apparent polar wandering path for Europe is reconstructed on the basis of twenty-three pole positions from the Baltic Shield and three pole positions from Great Britain. This pole path requires an average angular rate of apparent polar wandering of 0.2–0.3° per 10 6 year.

Natural remanent magnetization studies of a layered Breccia Boulder from the lunar highland region

The average directions of natural remanent magnetization (NRM) of three texturally distinct layers (72215, 72255, and 72275) of a 2 m-sized breccia boulder were found to be the same, while the directions of their stable components of NRM were found to be widely divergent. One clast from 72275 yielded a stable NRM direction which was different from that of the matrix. Approximate paleointensity measurements showed that 72255 and 72275 could have obtained their stable remanence from an ancient magnetic field of the same magnitude. However, 72215 probably was magnetized by a magnetic field of a different intensity. We concluded that the coincident NRM directions owe their origin to a secondary imprint of less stable magnetization imparted during the assembly of the boulder at moderate temperatures (∼ 450°C) on the South Massif. The stable directions, on the other hand, date from the last, higher-temperature (∼ 770°C) magnetizing event experienced by the mineral and lithic components while they were part of the immature pre-Serenitatis regolith.

Application of low-field hysteresis and memory-phenomenon studies in determining the stability of magnetization of Deccan Trap basalts from Mount Girnar and Mount Pavagarh regions, India

Two techniques, namely alternating-field demagnetization and thermal demagnetization, are widely being used for determining the stability of magnetization of a rock specimen. Recently a faster and simpler technique known as low-field hysteresis loop and memory-phenomenon test has been developed for determining the stability of magnetization directions of igneous rocks. In this paper the results of this new technique, after applying it to about 1000 specimens obtained from 250 oriented rock samples collected from 42 sites of Deccan Trap basalts from Mount Girnar (21°30′N; 70°30′E) and Mount Pavagarh (22°30′N; 73°30′E), India, are presented. The agreement between the mean natural remanent magnetization directions determined by this procedure and those computed after alternating-field demagnetization has been found to be very good. All the specimens from Mount Girnar and 94% of Mount Pavagarh specimens showed a stable line without any memory in a field ≈ 10 Oe. This indicates that the rocks from these two localities are highly stable and are most suitable ones for the determination of a precise palaeomagnetic direction.

A magnetic study of titanomagnetite substituted by magnesium and aluminium

Polycrystalline samples of two solid solution series Fe 2.4−δ M δ Ti 0.6 O 4 (where M = Al or Mg) have been prepared in the compositional range found in naturally occurring samples (0 < δ < 0.25). The variation of unit cell size, saturation magnetization and Curie point have been measured as a function of degree of substitution. The cell edge, a, falls with substitution for both series yielding determinative curves: 1. (1)| a = (8.485 ± 0.001) − (0.07 ± 0.02) δ + (0.16 ± 0.09) δ 2 for Mg, and 2. (2)| a = (8.485 ± 0.001) − (0.20 ± 0.02) δ + (0.15 ± 0.05) δ 2 for Al (0 ⩽ δ ⩽ 0.25). The variation in saturation magnetization with substitution may be ascribed to: (1) the location of Al ions on octahedral sites together with the displacement of an equal number of Fe 2+ ions onto tetrahedral sites; (2) an almost random distribution of Mg between the two sublattices. The Curie point falls as δ increases in both series, the effect of Al substitution being the greater. Using the data from the synthetic samples the effect of Al and Mg on the Curie points of titanomagnetites in a number of basalts is estimated. It is concluded that all the samples considered are appreciably non-stoichiometric and that this could lead, in some of the samples, to a reversed direction of natural remanent magnetization.

Magnetic anisotropy of granitic rocks of the Bergell massif (Switzerland)

Magnetite as well as ilmenohematite are contained by the Bergell granitic rocks, but the fabric of low field susceptibility is due to the magnetite only and is equivalent to the macroscopic biotite fabric. Microscopic observations and high field anisotropy measurements show that the ilmenohematite trigonal axes are aligned parallel to the directions of minimum low field susceptibility anisotropy. The deflection of the stable direction of natural remanent magnetization caused by the alignment of ilmenohematite is corrected using the susceptibility anisotropy data. The resulting virtual palaeopole position deviates from other Oligocene to Miocene pole positions of stable Europe suggesting that since the time of intrusion some 25 my ago the Bergell massif has been rotated anti-clockwise by an angle of ∼30° about a vertical axis.

Remagnetization Hypothesis Discounted: A Paleomagnetic Study of the Trenton Limestone, New York State

Twenty-nine samples collected from an 80-m section of the Trenton Limestone give almost vertical directions of natural remanent magnetization (NRM), but these become nearly horizontally directed after magnetic cleaning (D = 173°, I = 18°, α 95 = 5°). The result is confirmed by thermal demagnetization experiments, and the consequent low latitude suggested for the time of deposition is supported by the paleoclimatic evidence. Samples collected from a penecontemporaneous slump bed demonstrate that both the NRM and cleaned directions are postdepositional. Magnetic-anisotropy measurements show good agreement between the mean principal axis of maximum susceptibility and the cleaned remanence direction, suggesting that the alignment of the long axes of magnetic grains was controlled by the ancient magnetic field direction. This can only have arisen through a postdepositional precompaction in situ rotation of the magnetic grains, a process originally suggested by Irving and Major (1964) from laboratory deposition experiments. This study represents the first positive demonstration that such a process occurs naturally. The Ordovician paleomagnetic pole position for North America deduced from these rocks (lat 36° S., long 66° W.) is in complete conflict with the predictions of the remagnetization hypothesis, which requires near-polar conditions at that time. The result, however, is in excellent agreement with that obtained from igneous rocks of similar age from the Front Range of Colorado and shows that the Ordovician pole position lies close to the late Paleozoic position. A consequence of this conclusion is the existence of an Ordovician proto-Atlantic Ocean, as has been suggested independently from geological evidence.

Magnetism and magnetic reversals in ferromanganese nodules

We have studied the strength, stability and direction of natural remanent magnetization of 25 deep-sea, JOIDES, shallow marine and fresh-water ferromanganese nodules. The nodules contain a reasonably stable and isotropic NRM of about the same magnitude as deep-sea sediments. The recording of magnetic reversals in four of the seven large deep-sea nodules supports the slow overall growth rates measured by radioactive techniques on deep-sea nodules. In contrast with the reversed layers found in some of the deep-sea samples, all three shallow marine nodules less than 15 000 yr old have magnetic inclinations which appear parallel to the Earth's present field.

Paleomagnetic and rock-magnetic studies of the Permian Cutler and Elephant Canyon Formations in Utah

Study of the Permian Cutler formation and the upper 15 meters of the Permian Elephant Canyon formation at 0.6-meter stratigraphic intervals southwest of Moab in eastern Utah. The directions of natural remanent magnetization show a pronounced streak distribution, but thermal demagnetization successfully isolates the stable paleomagnetic direction. All directions are reversed, and no significant long-term change in pole position is observed throughout the entire section. The pole calculated from the Elephant Canyon data lies at 43.6 N, 119.6 E; the Cutler pole lies at 44.4 N, 116.2 E. Rock-magnetic analyses suggest that the secondary magnetization results from the iron hydroxides and was acquired after recent surface exposure.

Paleomagnetic Correlations and Potassium-Argon Dating of Middle Tertiary Ash-Flow Sheets in the Eastern Great Basin, Nevada and Utah

Directions of natural remanent magnetization are used to identify and correlate individual cooling units in the middle Tertiary ash-flow province in central and eastern Nevada and western Utah. Potassium-argon dating indicates that the minimum time between eruptions of individual but genetically related ash-flow cooling units is on the order of 0.8 m.y. As this interval is long in comparison with the secular variation of the direction of the geomagnetic field, in a given volcanic province the direction of natural thermoremanent magnetization is a unique characteristic of each cooling unit. Ash-flow sheets investigated include the Stone Cabin Formation, the tuff of Pancake Summit, the Windous Butte Formation, the Needles Range Formation, the Bates Mountain Tuff, and the tuff of Clipper Gap, of Oliogcene to early Miocene age. The original areas of individual ash-flow cooling units are as great as 8,000 km 2 ; the volumes, 1,300 km 3 . The paleomagnetic correlations, made over distances up to 200 km, confirm most of the previously made lithologic correlations and allow more accurate delineation of single cooling units. These correlations are particularly useful in those parts of the Basin and Range province where the Tertiary stratigraphic section consists mainly of ash-flow sheets, and the outcrops are confined to mountain ranges that are separated by alluvium-filled valleys.

Paleomagnetism and K-Ar Age of Mafic Dikes from the Wind River Range, Wyoming

Research Article| September 01, 1971 Paleomagnetism and K-Ar Age of Mafic Dikes from the Wind River Range, Wyoming HENRY SPALL HENRY SPALL Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80302 AUTHOR'S PRESENT ADDRESS: GEOMAGNETISM LABORATORIES (ERL), NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION, BOULDER, COLORADO 80302 Search for other works by this author on: GSW Google Scholar Author and Article Information HENRY SPALL AUTHOR'S PRESENT ADDRESS: GEOMAGNETISM LABORATORIES (ERL), NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION, BOULDER, COLORADO 80302 Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80302 Publisher: Geological Society of America Received: 15 Jul 1970 Revision Received: 05 Apr 1971 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1971, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1971) 82 (9): 2457–2472. https://doi.org/10.1130/0016-7606(1971)82[2457:PAKAOM]2.0.CO;2 Article history Received: 15 Jul 1970 Revision Received: 05 Apr 1971 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation HENRY SPALL; Paleomagnetism and K-Ar Age of Mafic Dikes from the Wind River Range, Wyoming. GSA Bulletin 1971;; 82 (9): 2457–2472. doi: https://doi.org/10.1130/0016-7606(1971)82[2457:PAKAOM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Potassium-argon ages have been determined on fresh pyroxenes from three of five dikes in the Louis Lake quadrangle of the Wind River Range, Wyoming. They indicate a minimum apparent age of intrusion at 1880 m.y. This is older than published whole-rock ages from the same dikes, but is consistent with whole-rock ages of up to 2060 m.y. from other dikes in this area. The within-dike dispersion of the directions of natural remanent magnetization decreases during alternating field demagnetization up to 1400 oe. Thermal demagnetization, however, is not effective in removing secondary magnetizations. After structural correction according to overlying Cambrian sediments, an over-all mean paleomagnetic pole from four dikes is obtained at 121° W., 43° N., which is in agreement with poles from Canadian Shield rocks of similar age. There is evidence in two of the dikes for a remagnetization episode, and the equivalent pole positions suggest that this may have occurred at the time of deposition of the Cambrian Flathead Sandstone. Alteration has greatly influenced the magnetic and isotopic properties of the dikes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Polarity Transitions of the Geomagnetic Field Deduced From the Natural Remanent Magnetization of Tertiary and Quaternary Rocks in Southwest Japan

Miocene, Pliocene and Pleistocene volcanic and plutonic rocks were collected from the southwestern part of Japan, and directions of natural remanent magnetization were measured. Rocks with stable magnetization from various tests in the laboratory are classified into four large groups according to geological sequence or radiometric datings; the groups being of upper Miocene, Mio-Pliocene, middle Pliocene and Plio-Pleistocene. It was observed that the rocks belonging to Mio-Pliocene, middle Pliocene and Plio-Pleistocene have both the normal and reversed magnetizations almost in the same numbers, but the reversed polarity was not found in the rocks belonging to the upper Miocene time.Results of paleomagnetic observations obtained from the rocks in southwest Japan show that the geomagnetic field during the Miocene and Pliocene ages had close to the field of the present axial dipole with exception of duration of polarity changes. However, the field has often changed its polarity since the Miocene time. A transition which occurred in Mio-Pliocene is unusual as compared with that in the other ages, and the paleomagnetic poles during the transition are situated only in a definite zone along a great circle around the earth (about 10°E and 170°W in longitude). On the other hand, transitions in middle Pliocene and Plio-Pleistocene appear to have been abrupt, because no intermediate direction of magnetization, suggesting that the geomagnetic field changed its polarity gradually, was found in every section of rocks.