Shallow Inclinations Research Articles

Paleomagnetism of 2.44 Ga mafic dykes in Russian Karelia, eastern Fennoscandian Shield — implications for continental reconstructions

Results of a paleomagnetic study of Paleoproterozoic dykes and Archean host rocks in the eastern Fennoscandian shield are reported here. Most of the dykes are related to ca 2440 Ma layered intrusions and have isotopic ages between 2349 and 2476 Ma. The dykes form an extensive NW–SE and E–W trending swarm. The majority are unaltered gabbronorites which carry stable remanent magnetizations with multicomponent nature. The high temperature remanence component (D), which is thought to represent the primary 2.44 Ga magnetization of the dykes, has a paleomagnetic pole at 20°S, 278.8°E (A95=6.0°, N=11 sites). Another D-like component (D′) with shallower inclination (VGP 9.6°N, 256.2°E, eight samples), at one site from baked Archean host rock may also represent the primary remanence of the dykes. The commonest remanence component (A) records an extensive Svecofennian partial overprint and is interpreted to be ca 1.84 Ga old. It yields a paleomagnetic pole at 52.6°N, 226.8°E (A95=2.7°, N=23). The fourth component, B, has a paleopole at 53.9°N, 169.6°E (A95=6.3°, N=16) and is inferred to be ca 1.75 Ga on the basis of isotopic dating, although ages of 2.1, 1.3 and 0.3 Ga are also possible. Baked contact tests for the 2.44 Ga dykes are negative, as shown by the occurrence of component D in Archean host rocks beyond one dyke width. It is proposed that the negative baked contact test indicates regional reheating and remagnetization of the Archean basement at ca 2.44 Ga ago due to the spatially vast occurrence of coeval 2.44 Ga mafic dykes, layered intrusions, volcanics and granites in the area. Based on this hypothesis, it is suggested that component D in the dykes may be primary and acquired at ca 2.44 Ga ago. The similarity of pole D with poles from coeval dykes and layered intrusions elsewhere in the Karelian Province and Kola Peninsula suggest that, within the Archean domain of the Fennoscandian shield, no large scale block movements have taken place since 2.44 Ga. At 2.44 Ga the Fennoscandian Shield was located at the latitude of ca 30°S which is different to that at 1.84 and 1.75 Ga, when Fennoscandia was located at the latitudes of ca 30°N. A continental reconstruction based on pole D and the pole from the 2.45 Ga Matachewan dykes of the Superior craton in Laurentia, places the Kola Peninsula adjacent to southern Greenland. Reconstructions based on recent geological interpretations and a reconstruction yielding pole D′, are also discussed.

Accelerated tests for the prediction of cut-edge corrosion of coil-coated architectural cladding: Part I: cyclic cabinet salt spray

Various accelerated cabinet tests have been used for the evaluation of the cut-edge corrosion of coil-coated architectural cladding. These include the conventional ASTM B-117 method (5% continuous NaCl spray), the standard Prohesion test (0.35% (NH 4) 2SO 4 + 0.05% NaCl wet/dry spray) as well as modified wet/dry spray tests using a relatively dilute artificial acid rain solution, shallow specimen incline angles with variations in the ratio of the wet and dry periods. Comparisons with outdoor exposure samples reveal that the B-117 test shows unrealistic corrosion morphology with the most realistic cut-edge corrosion given by the modified tests. Furthermore, with all types of wet/dry test specimen-to-specimen variation is considerably reduced compared with the continuous salt spray test. In the novel wet-dry tests the acceleration factor appears consistent with the number of wet and dry cycles however, the overall acceleration factor is limited and, hence, test times are extended. It is concluded that, over a 1000 h test time, the standard Prohesion test appears to show the best combination of realism and acceleration.

Simulating particle suspensions by extended point-force distributions in a Navier-Stokes scheme

The Ediacaran–Early Ordovician interval is of great interest to paleogeographer's due to the vast evolutionary changes that occurred during this interval as well as other global changes in the marine, atmospheric and terrestrial systems. It is; however, precisely this time period where there are often wildly contradictory paleomagnetic results from similar-age rocks. These contradictions are often explained with a variety of innovative (and non-uniformitarian) scenarios such as intertial interchange true polar wander, true polar wander and/or non-dipolar magnetic fields. While these novel explanations may be the cause of the seemingly contradictory data, it is important to examine the paleomagnetic database for other potential issues.This review takes a careful and critical look at the paleomagnetic database from Baltica. Based on some new data and a re-evaluation of older data, the relationships between Baltica and Laurentia are examined for ~ 600–500 Ma interval. The new data from the Hedmark Group (Norway) confirms suspicions about possible remagnetization of the Fen Complex pole. For other Baltica results, data from sedimentary units were evaluated for the effects of inclination shallowing. In this review, a small correction was applied to sedimentary paleomagnetic data from Baltica. The filtered dataset does not demand extreme rates of latitudinal drift or apparent polar wander, but it does require complex gyrations of Baltica over the pole. In particular, average rates of APW range from 1.5° to 2.0°/Myr. This range of APW rates is consistent with ‘normal’ plate motion although the total path length (and its oscillatory nature) may indicate a component of true polar wander. In the TPW scenario, the motion of Baltica results in a back and forth path over the south pole between 600 and 550 Ma and again between 550 and 500 Ma. The rapid motion of Baltica over the pole is consistent with the extant database, but other explanations are possible given the relative paucity of high-quality paleomagnetic data during the Ediacaran–Cambrian interval from Baltica and other continental blocks.A sequence of three paleogeographic maps for Laurentia and Baltica is presented. Given the caveats involved in these reconstructions (polarity ambiguity, longitudinal uncertainty and errors), the data are consistent with geological models that posit the opening of the Iapetus Ocean around 600 Ma and subsequent evolution of the Baltica–Laurentia margin in the Late Ediacaran to Early Ordovician, but the complexity of the motion implied by the APWP remains enigmatic.

Early Paleozoic paleomagnetic poles from the western part of the North China Block and their implications

We collected 917 oriented drill-core samples (124 sites) from 22 units spanning the Early Cambrian to Middle Ordovician formations of the North China Block (NCB) to construct an apparent polar wander path (APWP) of the early Paleozoic for the NCB. Sampling areas are located in the Helanshan, Tongchuan-Hancheng and Yuncheng-Luliang regions along the margin of the Ordos Basin. Characteristic remanent magnetizations for five periods between the Cambrian and Ordovician are isolated from 281 samples by careful demagnetization. Directions cluster around northwesterly direction with upward shallow inclination and its antipode after tilt correction. Reliability of these five directions is ascertained through positive fold and reversal tests. The five Cambrian and Ordovician paleopoles plot in the present Atlantic Ocean, which is consistent with previously reported paleomagnetic poles. A newly constructed APWP between the Cambrian and Ordovician indicates that the NCB was located in the Southern Hemisphere around 15°S during Cambro–Ordovician time. Motion of the NCB between the Early and Middle Cambrian comprised: a counterclockwise rotation of 22.3° ± 7.0° and a northward latitudinal displacement of 7.2° ± 6.8°. Combining the paleobiogeographic and paleoclimatic data with the paleomagnetic implies that the NCB was probably located in East Gondwana in the Early Cambrian, and may have separated from Gondwana in post-Early Cambrian time. The NCB is also inferred to have been located close to Siberia and North America during the Middle Ordovician.

Using magnetic anisotropy to correct for paleomagnetic inclination shallowing in some magnetite-bearing deep-sea turbidites and limestones

In sediments, remanence inclination I may be shallower than the inclination I H of the Earth's field in which they were deposited. Use of magnetic anisotropy to correct for this paleomagnetic inclination shallowing is tested for Quaternary turbidite muds from the Scotian Rise and for Cretaceous pelagic limestones from the Pacific Plate. Both are dominated by pseudo-single-domain magnetite carrying a stable remanence whose inclination I (determined by stepwise alternating field demagnetization) is shallower than I H, by an average of 12° for the turbidites and 17° for the limestones. Anhysteretic remanence (ARM), applied identically to various axes, is weakest (ARM min) perpendicular to bedding and strongest (ARM max) parallel to bedding, with little anisotropy in the bedding plane and ARM min ARM max averaging 0.84 for the turbidites and 0.87 for the limestones. Magnetite also dominates susceptibility with minimum ( K min) and maximum ( K max) susceptibility axes perpendicular and parallel to bedding, respectively, and with K min K max averaging 0.91 for the turbidites and 0.95 for the limestones. In theory, we expect to find a correlation between tan I and ARM min ARM max for a suite of specimens with the correlation line's prediction of tan I when ARM min ARM max = 1 yielding an I H estimate corrected for inclination shallowing. This correction method is also expected to succeed if ARM min ARM max is replaced by K min K max , provided susceptibility is dominated by multidomain or pseudo-single-domain magnetite. In practice, both anisotropies do succeed in correcting for inclination shallowing in three of the four turbidite cores. Both anisotropies would also likely succeed for the limestones (if there were enough samples at any one site) since correlating tanI tanI H in place of tan I for the five sites combined predicts tanI tanI H ∼ 1 when ARM min ARM max = 1 or K min K max = 1 . Hence, we recommend routinely measuring anisotropy in paleomagnetic studies of magnetite-bearing sediments and looking for a correlation between tan I and ARM min ARM max or K min K max . Remanence anisotropy is more laborious to measure but provides the more reliable estimate of inclination shallowing. In cases where no significant correlation is found, remanence anisotropy of the magnetite particles in the sediment may also have to be measured to correct for inclination shallowing.

Paleointensity variations across the last geomagnetic reversal at La Palma, Canary Islands, Spain

A detailed volcanic record of absolute paleointensity has been obtained for the period encompassing the last geomagnetic reversal, from a succession of 69 distinct lava flows in two adjacent sections on the island of La Palma. The correlation between the two sections relies on the positions of the Brunhes‐Matuyama boundary and on the flow chronology obtained by a refined potassium/argon (K/Ar) dating technique. Several pretransitional flows exhibit very shallow paleomagetic inclinations, but they cannot be considered as really transitional. Paleointensity experiments conducted on 288 specimens provided 168 successful determinations (14 were obtained after correction) for 50 lava flows that were considered to represent 45 distinct units. The evolution of the paleofield intensity can be constructed by assuming uniform extrusion rates between three dated lava flows. The period preceding the transition is characterized by large fluctuations with typical peak to peak amplitudes between 20 and 40 μT which decrease down to 7 μT prior to the transition. The 100 kyr‐long interval following the reversal is associated with field intensities as high as 60 μT and depict a strong and rapid restoration of the dipole field. The virtual dipole moments (VDMs) have been compared with sedimentary VDMs generated from the record of relative paleointensity from Ocean Drilling Program site 851, to which have been added non dipole components similar to the present geomagnetic field. A reasonable fit can be made between the two curves which implies variations in the eruption rates but not incompatible with the uncertainties in the K‐Ar age determinations. Thus these results can support but do not fully demonstrate the existence of asymmerical saw‐toothed variations in field intensity.

Paleomagnetic data from the Caborca terrane, Mexico: Implications for Cordilleran tectonics and the Mojave‐Sonora megashear hypothesis

Two ancient magnetizations have been isolated in rocks of the Caborca terrane, northwest Mexico. The characteristic magnetizations of Neoproterozoic and Paleozoic miogeoclinal shelf‐strata, arc‐derived Lower Jurassic marine strata, and Jurassic volcanic and volcaniclastic rocks are of dual polarity and east‐northeast declination (or south‐south‐west) and shallow inclination. Magnetizations in Neoproterozoic and Paleozoic miogeoclinal strata are interpreted as secondary (J*) and to be of similar age to those observed in Lower and Middle Jurassic rocks. Remanence acquisition is bracketed between about 190 and 160 Ma. The overall mean (D=15.0°, I=8.5°; n=38 sites; six localities; k=19.1, α95=5.5°) suggests a moderate to large clockwise rotation of 12 to 50° (depending on reference direction assumed) of the Caborca terrane, and rocks of the Sonoran segment of the Cordilleran volcanic arc, with respect to the North America craton. When compared with expected inclinations, observed values are not anomalously steep, arguing against statistically significant southward latitudinal displacement of the Caborca block after remanence acquisition. Late Cretaceous intrusions yield primary, dual‐polarity steep inclination “K” magnetizations (D=341.4°, I=52.3°; n=10 sites; five localities; k=38.3, α95=7.9°) and have locally remagnetized Neoproterozoic and Jurassic strata. When present, secondary (K*) magnetizations in Neoproterozoic strata are of higher coercivity and higher unblocking temperature than the characteristic (J*) magnetization. Importantly, the regional internal consistency of data for Late Cretaceous intrusions suggests that effects of Tertiary tilt or rotation about a vertical axis over the broad region sampled (∼5000 km2) are not substantial. Late Cretaceous primary (K) magnetizations and secondary (K*) magnetizations yield a combined mean of D=348.1°, I=50.7° (N=10 localities; 47 sites; k=53.5, α95=6.7°), indicating at most small (<∼10°) clockwise rotation of the Caborca region with respect to the craton. Permissible post‐Late Cretaceous latitudinal displacement is near or below the detection limit of paleomagnetism (<∼300 km). Limited data from Lower Cretaceous strata of the Bisbee Group (D=339.9°, I=47.9°; n=4 sites) suggest that the modest clockwise rotations inferred on the basis of J* magnetizations in Jurassic and older strata occurred in Jurassic time. Together, the lack of evidence for southward displacement, yet evidence for statistically significant clockwise rotation, and the overall similarity of Jurassic magnetizations in the Cordilleran arc with those of the Caborca block, despite the fact that some of them are clearly secondary, are not consistent with the Mojave‐Sonora megashear hypothesis of Late Jurassic left‐lateral strike‐slip motion of the crust of northern Mexico.

Magnetic stratigraphy of the Villafranchian type-section:(Villafranca d'Asti, Italy)

A detailed magnetostratigraphic investigation has been carried out along the section of the Fornace RDB quarry (Villafranca d'Asti, NW Italy), which is the type-section of the Villafranchian and has provided many of the land mammal remains used by Azzaroli (1977) to define the Val Triversa faunal unit (zone MN16a). Poorly consolidated clay and clayey silt are the prevailing lithologies and samples were collected with plastic boxes. Isothermal remanent magnetization measurements showed that haematite is the main ferromagnetic mineral and occurs through the section, whereas iron sulphide is subordinate and only occurs in the lower part. Alternating field demagnetization usually succeeded in isolating a stable component and was used to derive the characteristic remanence by demagnetizing the specimens at 4 to 7 steps in the range 15 to 80 mT. The magnetic fabric was investigated by measuring the anisotropy of the magnetic susceptibility. It was always well defined and characterized by a horizontal foliation matching the bedding plane. The anisotropy of isothermal remanent magnetization was measured on some specimens and yielded fully comparable results. This consistency shows that detrital haematite carries the primary magnetization in these sediments and explains the 20° inclination shallowing of the site mean palaeomagnetic direction. Only one reverse to normal polarity transition has been detected and a direct correlation with the GPTS reference scale of Cande and Kent (1992, 1995) is thus not possible. The age of the Triversa fauna has been much debated in recent literature. Some authors have recently suggested that it is transitional between Ruscinian and Villafranchian, i.e. a little older than previously assumed. According to this hypothesis, the lower part of the RDB section would correlate to the chron C2Ar (upper Gilbert) and the upper part to the chron C2An.3n (lower Gauss), whereas according to the traditional interpretation correlation is to one of the polarity inversions within the middle Gauss (Kaena and Mammoth). An independent age constraint based on magnetostratigraphy will only be possible when other Ruscinian and Villafranchian sections are investigated in the same detail and a correlation between palaeosecular variation is established.

Incursion of sea water into Gucheng Lake detected by magnetic, biologic and chemical data

Magnetic, biological and chemical studies were performed on lacustrine deposits from Gucheng Lake, Jiangsu Province, China. Magnetite and haematite are identified along the entire profile. It is not clear whether magnetite or/and ferrimagnetic iron sulfide (likely greigite) predominates above a striking boundary at 12.0 m, expressed by higher susceptibility (χ) and remanence values. Low NRM intensities above 12.0 m correlate to lower P factors (χmaxχmin) and shallow inclinations of χmin, possibly indicating some disturbing remanence acquisition processes.Pollen, diatom, pigment and TOC analyses reveal that the early Holocene warming (started at the 13.7 m) led to a pulsative sea water incursion at 12.0 m. This incursion resulted in a period of brackish water, which favours the authigenesis of magnetite or ferrimagnetic iron sulphide, resulting in higher magnetic susceptibility and remanences values. Progressive freshing water occurred between 6.0 and 4.0 m. NRM changes correlate with these environmental trends. Magnetic changes above 2.5 m seems to be a result of human-driven deforestation and cultivation, as evidenced from pollen studies and lithological observations. Our results show that pollen and pigment assemblages directly reflect palaeoclimatic changes, whereas magnetic properties and diatoms respond primarily to the changes of hydrology and the limnology of Gucheng Lake.

New Cambrian paleomagnetic pole for Yangtze Block

A total of 120 samples from 12 sites were collected from two flanks of a fold. Stepwise thermal demagnetization has successfully revealed characteristic magnetization components from the rocks in each case. A well-defined component determined from red fine-grained sandstone is clustered in the northeasterly direction with shallow upward inclination (D = 29.3°,I= -19.2°,k = 283.7, α95 = 7.3°. tilt-corrected). The pole position (39.5°N, 247.3°E,dp = 4.0°,dm = 7.6°) derived from this component is close to the Permian pole for the Yangtze Block, indicating that the red fine-grained sandstone has been overprinted. The red mudstone reveals two characteristic components Component A with lower unblocking temperature, characterized by northerly declination and moderate to steep inclination corresponds to a pole position overlay with the present North Pole. Component B (D = 129.1°,I=-23.6°,k = 44.6, α95 = 7.8°, tilt-corrected) with higher unblocking temperature, passes fold test, and yields a pole position (39.5°S, 185.l°E,dp = 4.4°,dm = 8.3°) different from the other poles for the Yangtze Block. It is therefore suggested that component B was probably a primary magnetization and the Yangtze Block was situated at low latitudes in the Southern Hemisphere in the Middle Cambrian.

Remagnetization of Cretaceous forearc strata on Santa Margarita and Magdalena Islands, Baja California Sur: Implications for northward transport along the California margin

Paleomagnetic data for two sections of Cretaceous forearc strata with different structural attitudes on Santa Margarita and Magdalena Islands in Baja California Sur, Mexico, indicate that these rocks have been remagnetized, probably during the late Cenozoic. The in situ paleomagnetic directions, however, are similar to data from other Cretaceous rocks on peninsular California with unexpectedly shallow inclinations and easterly declinations. These data have been interpreted as indicating either northward tectonic transport (10°–15° of latitude) and clockwise rotation (>20°) or compaction shallowing of magnetic inclinations in sedimentary rocks combined with southwestward tilting of plutonic rocks. The available paleomagnetic data for Cretaceous forearc strata in southern and Baja California can be divided into three groups: (1) sections with normal‐polarity magnetizations that fail fold tests and are remagnetized, (2) sections with normal‐polarity magnetizations with no or inconclusive fold tests that may or may not be remagnetized, and (3) sections with both normal‐and reversed‐polarity intervals where pervasive remagnetization has not occurred. Other rocks of the Mesozoic Great Valley Group, Coast Range ophiolite, and Franciscan Complex in California also have secondary magnetizations with directions similar to younger geomagnetic field directions. Although these widespread remagnetizations could have variable local causes, we propose regional burial and uplift, related to changes in subduction parameters, as a possible explanation. Two episodes of remagnetization are apparent: one in the Late Cretaceous and a second in the late Cenozoic. On the other hand, the unremagnetized and apparently reliable data from sedimentary and plutonic rocks on the Baja Peninsula consistently indicate northward translation (14° ± 3°) and clockwise rotation (29° ± 8°) with respect to North America since the Late Cretaceous.

Compaction‐corrected inclinations from southern California Cretaceous marine sedimentary rocks indicate no paleolatitudinal offset for the Peninsular Ranges terrane

Paleomagnetic data have been used extensively to delineate the terrane displacement and accretion history of the western margin of North America. However, the anomalously shallow paleomagnetic inclinations used to indicate large‐scale northward translation might be alternatively interpreted as due to postmagnetization tilting of batholithic rocks and compaction of marine sediments. To understand the magnitude of burial compaction effects on the post‐Cretaceous motion history of the Peninsular Ranges‐Baja Borderlands terrane, a rock magnetic, compaction, and paleomagnetic study of the Ladd Formation and the Point Loma Formation from southern California was conducted. The anhysteretic anisotropy of remanence of the characteristic remanence‐carrying grains and individual magnetic grain anisotropy were used to correct the inclinations of each formation. Individual magnetic grain anisotropy was determined by both compaction experiments and redeposition of a magnetic separate in a DC magnetic field. Standard paleomagnetic studies of the units indicated that previous Ladd Formation results could be reproduced, and a correction was made at the sample level. We were unable to adequately reproduce earlier results of the Point Loma Formation, so the average remanence anisotropy was used to correct the previously reported mean direction. The mean inclination of the Ladd Formation was conected from 46° (α95 = 8°) to 58° (α95 = 4°), and the mean inclination of the Point Loma Formation was conected from 39.5°±5.4° (normal) and −36.4° ± 16.6° (reversed) to 56.0°±5.1° (normal) and −53.0°±16.7° (reversed). These results suggest that the Peninsular Ranges‐Baja Borderland terrane has been part of the western North America since the Late Cretaceous and that clay‐containing sedimentary rocks may typically experience from 10° to 15° of inclination shallowing due to burial compaction.

Palaeomagnetism of Neoproterozoic glacial rocks of the Huabei Shield: the North China Block in Gondwana : Piper J.D.A. & Zhang Qi Rui, Tectonophysics, 1997, 283/1–4 (145–171)

We collected 917 oriented drill-core samples (124 sites) from 22 units spanning the Early Cambrian to Middle Ordovician formations of the North China Block (NCB) to construct an apparent polar wander path (APWP) of the early Paleozoic for the NCB. Sampling areas are located in the Helanshan, Tongchuan-Hancheng and Yuncheng-Luliang regions along the margin of the Ordos Basin. Characteristic remanent magnetizations for five periods between the Cambrian and Ordovician are isolated from 281 samples by careful demagnetization. Directions cluster around northwesterly direction with upward shallow inclination and its antipode after tilt correction. Reliability of these five directions is ascertained through positive fold and reversal tests. The five Cambrian and Ordovician paleopoles plot in the present Atlantic Ocean, which is consistent with previously reported paleomagnetic poles. A newly constructed APWP between the Cambrian and Ordovician indicates that the NCB was located in the Southern Hemisphere around 15°S during Cambro–Ordovician time. Motion of the NCB between the Early and Middle Cambrian comprised: a counterclockwise rotation of 22.3° ± 7.0° and a northward latitudinal displacement of 7.2° ± 6.8°. Combining the paleobiogeographic and paleoclimatic data with the paleomagnetic implies that the NCB was probably located in East Gondwana in the Early Cambrian, and may have separated from Gondwana in post-Early Cambrian time. The NCB is also inferred to have been located close to Siberia and North America during the Middle Ordovician.

High-resolution record of geomagnetic secular variation from Late Pleistocene Lake Lisan sediments (paleo Dead Sea)

We measured geomagnetic secular variation in Lake Lisan sediments (paleo Dead Sea). More than 1500 oriented samples were collected from a 27.3-m section of alternating aragonite and detritus laminae in the Dead Sea basin ranging in age from 67 to 32 ka. The natural remanent magnetization (NRM) is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic. The NRM is very stable and was acquired several hundred years after deposition. The mean direction of 878 horizons is D = 005°, I = 45° ( α 95=1°; κ=22). We observed three modes of directional geomagnetic variation as a function of (and by inference, time): very rapid inter-sample changes, slow variation in mean direction, and inclination shallowing of about 1°/m. The overall rate of change in direction is 0.57±0.57°/year, not significantly different from zero. For about 83% of the record the rate of change is less than 1°/year and comparable to historical values. High rates of change are observed more frequently in the Lisan than in historical records, and peak rates are up to ten times faster. A smoothed curve resulting in a maximum rate of change of 0.66°/year and a mean 0.10±0.10°/year may be a more realistic representation of the field behavior. No reverse NRMs were observed, but geomagnetic field excursions may be present where the VGPs deviate by more than 40° from the geographic north at about 52 and 41 ka; the latter may represent the Laschamp event.

Magnetostratigraphic (re)calibration of the Paleocene/Eocene boundary interval in Holes 550 and 549, Goban Spur, eastern North Atlantic

A magnetostratigraphic reinvestigation of the Paleocene/Eocene boundary intervals in DSDP Holes 550 and 549, Goban Spur, has removed uncertainties associated with the previously published magnetostratigraphic information for these chronostratigraphically important sections. In Hole 550, levels assigned to nannoplankton zones NP4 and 5, previously considered to be of normal polarity, carry a reverse polarity characteristic magnetisation which is correlated with geomagnetic Chron C26r. Unconformably overlying these levels is a thin interval of reversely magnetised sediment which records the latter part of Chron C25r. Chron C25n is redefined and its lower/upper boundaries are placed respectively at 425.08 and 422.12 mbsf in Hole 550. Chron C24r extends upwards ∼62.4 m. Respectively, the start and end of Chron C24n.3n are placed at 359.69 and 349.41 mbsf in this hole. Within Core 28, just above C24n.3n, is a record of Chron C24n.2n as well as interval of indeterminate polarity within C24n.2r. The problematic issue of assigning numerical ages to the various events associated with the Paleocene/Eocene transition in Hole 550 is then discussed. The remanence in Hole 549 is difficult to interpret because magnetic inclinations in the interval previously assigned to Chrons C25n and C24r are typically <20° compared with the ∼60° expected for the Paleogene sediments at this site. Acknowledging the complexities of this record, we have constructed the magnetostratigraphy for this hole, based on results from the present study and previous work [H.A. Townsend, The paleomagnetism of sediments acquired from the Goban Spur on the Deep Sea Drilling Project Leg 80, in: P.C De Graciansky, C.W. Poag et al. (Eds.), Initial Reports DSDP 80 (1985) 389–410]. Contrary to the previous study, we are not able to fix the base of Chron C25n; it is located either in the lower part of Core 18, where the core pieces are too small to be used reliably for paleomagnetic studies, or in the gap between Cores 18 and 19. The Chron C25n–24r boundary is blurred (∼351 to 353 mbsf) because of the shallow inclination problem. Chron C24n.3n, which is characterised by inclinations of ∼35°, has its lower/upper boundaries at ∼318 and 307.62 mbsf respectively. An apparent record of Chron C24n.2n, based on two samples, is present between 305.53 and 304.65 mbsf.

Shallow bias of paleomagnetic inclinations in the Paleozoic and Precambrian

An updated analysis of the global paleomagnetic database shows that the frequency distributions of paleomagnetic inclinations for the Cenozoic and Mesozoic eras (younger than 250 Ma) are compatible with a random geographical sampling of a time-averaged geomagnetic field that closely resembles that of a geocentric axial dipole. In contrast, the frequency distributions of paleomagnetic inclinations for the Paleozoic and Precambrian eras (prior to 250 Ma) are over-represented by shallow inclinations. After discounting obvious secondary causes for the bias, such as from data averaging, sedimentary inclination error, inhomogeneous lithological distributions, and tropical remagnetization, we show that the anomalous inclination distributions for the Paleozoic and Precambrian can be explained by a geomagnetic field source model which includes a relatively modest (∼25%) contribution to the axial dipole from a zonal octupole field and an arbitrary zonal quadrupolar contribution. The apparent change by around 250 Ma to a much more axial dipolar field geometry might be due to the stabilization of the geodynamo from growth of the inner core to some critical threshold size, a gross speculation which would imply that either the threshold size was rather large or the inner core nucleated rather late in Earth history. Alternatively, if a geocentric axial dipole model is assumed or can eventually be demonstrated independently, the anomalous inclination distributions for the Paleozoic and Precambrian may reflect a tendency of continental lithosphere to be cycled into the equatorial belt, perhaps because geoid highs associated with long-term continental aggregates influence true polar wander.

Cenozoic northward translation of the Kitakami massif in northeast Japan: paleomagnetic evidence

Paleogene and early Cretaceous welded tuffs have been sampled from eleven sites at the Omoe Peninsula (39.6°N, 142.0°E) in the Kitakami massif of northeast Japan for paleomagnetic study. Characteristic paleomagnetic directions with a high unblocking temperature component above 560°C are isolated: Westerly declinations with shallow inclination are identified after tilt correction in the 62–71 Ma Heizaki volcanics ( D=283.5°, I=9.4°, α 95=8.0°) and in the 114–119 Ma Harachiyama Formation ( D=274.2°, I=−20.4°). The shallow inclination, which was previously reported from early Cretaceous plutonic rocks without tilt correction in the Kitakami massif, has been discovered in the tilt-corrected paleomagnetic directions. The presence of both normal and reversed polarities suggests reliability of this shallow and westerly paleomagnetic direction. The shallow inclination indicates that the Kitakami massif was located at low latitude (5°±4°N) at 71–62 Ma. Taking into account a reference paleomagnetic pole of the Paleocene expected from Sikhote Alin, the Kitakami massif was translated northward over 25° in latitude since 65 Ma. The Kitakami massif can be subjected to latitudinal northward motion more than 24° during the last 70 Ma, when it was transported by push of the moving Pacific plate and reached the Asian continental margin near Sikhote Alin later than 30 Ma. We conclude that the Kitakami massif was incorporated into northeast Japan at some time between 30 and 22 Ma after the Cenozoic northward translation.

A database for the Matuyama-Brunhes magnetic reversal

We present a paleomagnetic database, designated MBD97, comprising sedimentary and lava data for the Matuyama-Brunhes reversal transition field. Using criteria regarding suitability of the rock samples, measurement methods and temporal resolution of the data, we consider 62 different studies of the Matuyama-Brunhes reversal; of these, only 11 satisfy our selection criteria: four lava sites, one loess site, three shallow-marine sedimentary sites, and three deep-ocean sedimentary sites. We discuss our reasons for accepting or disguarding each candidate data set. We investigate the distribution of transitional virtual geomagnetic poles (VGPs); they fall along American and Asian-Australian longitudes. The spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar. The directional transition of the Matuyama-Brunhes reversal took ∼ 2300–5000 years, during which time the surface intensity fell to ∼ 10–20% of its pre- and post-transitional value. We do not find any systematic bias due to site distribution or inclination shallowing, although the errors in the data do vary over the duration of the reversal. MBD97 is a small database, but it has a fairly good geographic distribution and is suitable for studies of the Matuyama-Brunhes transition, nonetheless, it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis. The data in MBD97 are listed.

Measurements of the Cretaceous Paleolatitude of Vancouver Island: Consistent with the Baja-British Columbia Hypothesis

A previously unsampled outcrop of gently dipping or flat-lying Upper Cretaceous sedimentary strata in the Vancouver Island region, which contains unaltered aragonitic mollusk fossils, yielded a stable remanent magnetization that is biostratigraphically consistent with Cretaceous magnetochrons 33R, 33N, and 32R. These results, characterized by shallow inclinations, indicate an Upper Cretaceous paleolatitude of about 25 ± 3 degrees north, which is equivalent to that of modern-day Baja California. These findings are consistent with the Baja–British Columbia hypothesis, which puts the Insular Superterrane well south of the Oregon-California border in the Late Cretaceous.

Synthetic and sedimentary records of geomagnetic excursions

The geomagnetic excursion recorded in the sediments of Lac St.Front (Massif Central, France) is characterized by shallow and negative inclinations followed by a younger steep inclination interval (Vlag et al., 1996). In the corresponding interval of the nearby Lac du Bouchet only steep inclinations are found. Sedimentary records of the Mono Lake excursion show similar inclination patterns; ‘complete’ records of this excursion show a succession of a shallow by a steep inclination interval, while ‘incomplete’ records only show only steep inclinations (Coe and Liddicoat, 1994). Due to a non‐instantaneous acquisition of the remanence, sedimentary records reflect only a smoothed geomagnetic signal. It will be shown that smoothing of a small low‐intensity ‘reversed’ interval embedded in a non‐antipodal normal field interval may result in records of a shallow inclination interval followed by a steep inclination interval, while further smoothing results in only steep inclinations. Realignment of magnetic grains by the stronger normal field can also produce such records and may explain why such an unusual large lock‐in depth is required by the conventional smoothing model. Whatever the mechanism, the similarities between these synthetic records and the excursional records of Lac St. Front‐Lac du Bouchet and Mono Lake suggest that the latter are more or less affected by vector addition of two non‐antipodal directions.