Remanence Inclination Research Articles

Paleomagnetism and Rock Magnetism of Permian sedimentary rocks and Early Jurassic Karoo (Large Igneous Province) sills intersected in the KWV-1 borecore, Eastern Cape Province, South Africa

The nearly 100 Ma record of almost continuous shallow marine to non-marine sedimentation from the Late Carboniferous to the Early Jurassic recorded in the Karoo Basin, South Africa, includes coal deposits and potentially significant shale gas occurrences. As such, numerous studies have addressed the thermal history of the Karoo Basin, using a range of approaches. We use paleomagnetic and rock magnetic data to evaluate the paleotemperature history of the host Permian strata (Ecca Group and lowermost Beaufort Group) intersected in the ∼2320 m deep KARIN KWV -1 drilling project core, obtained in 2015. Where cored, the Karoo sedimentary sequence has been intruded by several Early Jurassic mafic sills, belonging to the Karoo Large Igneous Province, of thickness ranging from less than 10 cm to ∼150 m. The continuous core provides a unique natural laboratory for studying the effects of Karoo magmatism on the basin's sedimentary sequence. The KWV-1 drilling project was spudded ∼10 km east of Willowvale town, Eastern Cape Province.The spatial variation in thermal effects in host sedimentary strata, relative to proximity to Karoo sill contacts, is well-defined on the basis of anisotropy of magnetic susceptibility (AMS) data, paleomagnetic demagnetization response, and thermal alteration index (A40) values based on temperature vs. magnetic susceptibility tests. A zone of pervasive remagnetization of host sedimentary strata characterizes the contact aureoles, the spatial extents of which are to a first order dictated by the thickness of each Karoo sill. A40 data are interpreted to show that ambient paleotemperature estimates increase from ∼150 °C in lower Beaufort/upper Ecca strata to ∼285° in Lower Ecca strata that are sufficiently removed from Karoo sills. However, lower Ecca Group rocks in close proximity to Karoo sills were likely heated to ∼500 °C. Natural remanent magnetization (NRM) intensities of rocks in proximity to sills range from ∼17 to ∼ 438 mA/m, and susceptibilities range from 1.3 to 3.2 × 10−3 SI volume. In contrast, for strata sampled at distances greater than the width of the nearest sill, NRM intensities range from 0.056 to ∼4 mA/m, and susceptibility values range between 193 and 488 X 10−6 SI volume. For host sedimentary rocks, remanence is completely unblocked at temperatures ranging from ∼350 °C to ∼630 °C and acquisition and backfield DC demagnetization of isothermal remanent magnetization (IRM) indicate that specimens are fully saturated in fields of ∼200–600 mT, with coercivity of remanence of ∼30–100 mT. All of the eleven Karoo sills examined in this study yield remanence inclinations that are moderate to steep negative, and are inferred to have recorded a normal polarity field, possibly during a single chron. Our results are interpreted to suggest that substantial thermal effects of Karoo sills on the host sedimentary strata are largely confined to the contact aureoles, with a width that is dependent on sill thickness. We hypothesize that parts of the Ecca sedimentary sequence have remained at ambient, basinal burial temperatures below about 150 °C, and thus that an early-acquired Permian remanence may possibly be preserved in these sedimentary rocks at sufficient distance from thermal contact aureoles. Alternatively, much of the sedimentary section may have been subject to a combination of thermoviscous/chemical remagnetization.

Application of an Anisotropy‐Based Correction to Relative Paleointensity Estimates of Experimentally Deposited Sediments

AbstractThe magnetization of a sediment is acquired when ferromagnetic detrital particles align with the Earth's magnetic field as they settle through the water column. The relative continuity and ubiquity of the global sedimentary record makes sediments and sedimentary rocks an ideal target for studies of relative paleointensity (RPI) and the evolution of the Earth's magnetic field. However, processes including flocculation, contact with the substrate, bioturbation, and compaction disrupt this alignment, leading to shallowed remanent inclinations and biased intensities. The inclination of the depositional field also exerts an influence on the remanent intensity, thus affecting RPI estimates and leading to complications in interpretations of the global record. We present a series of 12 deposition experiments conducted in varying field strengths and inclinations, from which we determine RPI estimates. We then correct these estimates using a modified anisotropy‐based inclination correction technique. We find experimentally that our correction reduces the inclination dependence of relative paleointensity while preserving the RPI's dependence on the depositional intensity. Our results suggest that the anisotropy‐based correction should improve RPI estimates obtained from natural sediments and sedimentary rocks.

Origin of apparent magnetic excursions in deep‐sea sediments from Mendeleev‐Alpha Ridge, Arctic Ocean

Arctic deep‐sea sediments often record intervals of negative inclination of natural remanence that are tens of centimeters thick, implying magnetic excursions with durations of tens of thousand years that far exceed excursion durations estimated elsewhere, and the lack of tight age control usually provides excessive freedom in the labeling of Arctic excursions. Fortuitous variations in sedimentation rate have been invoked to explain the “amplified” excursions. Alternating field demagnetization of natural remanent magnetization (NRM) of sediment cores 08JPC, 10JPC, 11JPC, and 13JPC recovered by the Healy Oden Trans‐Arctic Expedition in August 2005 (HOTRAX05) to the Mendeleev‐Alpha Ridge yields apparent magnetic “excursions” in sediments deposited in the Brunhes Chron. Thermal demagnetization of the NRM, however, implies multiple magnetization components with negative inclination components usually “unblocked” below ∼350°C. Analysis of isothermal remanent magnetization acquisition curves from magnetic extracts indicates two magnetic coercivity components superimposed on one another. Magnetic experiments conducted under high and low temperatures show features that are characteristic for (titano)magnetite and titanomaghemite. Presence of the two magnetic phases is further confirmed by elemental mapping on a scanning electron microscope equipped for X‐ray energy dispersive spectroscopy (EDS) and by high‐resolution X‐ray diffraction (XRD). It is unlikely that anomalously thick intervals of negative inclination in these Brunhes‐aged sediments are caused by unusual behavior of the magnetic field in the Arctic area. We therefore attribute low and negative NRM inclinations in these cores to partially self‐reversed chemical remanent magnetizations, apparently carried by titanomaghemite and acquired during the oxidation of detrital (titano)magnetite grains. The high Ti contents and high oxidation states indicated by EDS and XRD data provide the conditions required for partial self‐reversal by ionic reordering during diagenetic maghemitization, and this process appears to have affected all HOTRAX05 cores collected from the Mendeleev‐Alpha Ridge.

Correcting distorted paleosecular variation in late glacial lacustrine clay

An undisturbed, horizontal chronostratigraphic marker horizon of laminated red glacio-lacustrine clay crops out over ∼25,000 km 2 in northern Ontario, Canada. The primary, clastic hematite laminae possess two stable vector components of magnetization. We sampled a 1.6 m vertical section at overlapping 2 cm intervals in cubic specimens (8 cm 3, n = 106), precisely oriented in geographic coordinates which permitted measurement of inclination and declination. Alternating field demagnetization (12–17 steps per specimen) isolated a characteristic (ChRM), primary component (coercivity ≥40 mT) approximately 30° shallower than the mean geomagnetic field inclination at this latitude, with distorted paleosecular secular variation (PSV). A lower coercivity overprint (20–40 mT) is 6° shallower than the present geomagnetic field and similarly inclined to the depositional-remanence deflection when the clay was re-sedimented in the laboratory. We believe this angle to be representative of the inclination of the deflected primary remanence inclination during deposition, caused by the magnetic anisotropy of the clay. Using this as a proxy for the initial inclination, the ChRM (hard-component) inclinations were restored to their original values assuming vertical compaction, which averages to 51%. The restored inclinations are compatible with site-latitude and the restored secular-variation loops centre reasonably on the geographic North Pole. The structural correction based on homogeneous vertical shortening over-simplifies the reality of heterogeneous particulate flow in which grain rotations depend on shape, size and packing and effectively combines influences of compaction and depositional settlement. Nevertheless, this correction makes the PSV data more useful and interpretable in terms of paleopole migration. The more logical correction technique using anisotropy of anhysteretic remanence (AARM; Jackson, M.J., Banerjee, S.K., Marvin, J.A., Lu, R., Gruber, W., 1991. Detrital remanence, inclination errors, and anhysteretic remanence anisotropy: quantitative model and experimental results. Geophys. J. Int. 104: 95–103) is foiled here due to the high-coercivity of the remanence-bearing grains and their unknown orientation-distribution and shape-distribution.

Current-scattered (?) detrital remanence directions in the Zn-rich Pennsylvanian Stark black shale, USA

SUMMARY Palaeomagnetic results are reported from the metalliferous Stark black shale in the Upper Pennsylvanian (Missourian/Kasimovian) Kansas City Group. Palaeomagnetic analysis of 400 specimens from 28 sites gives a characteristic remanent magnetization in 17 sites of the shale that yields a Late Mississippian to Middle Pennsylvanian palaeopole at 32.2°N 128.5°E (dp = 4.7° and dm = 8.8°). The observed palaeomagnetic age is slightly older than the host rock, indicating that the mineralization of the Stark Shale has, excluding recent alteration, a primary sedimentary or syngenetic origin. The reason for the slightly older age is likely due to trace modern hematite that slightly steepens the remanence inclination. The large oval of 95 per cent confidence is interpreted to be caused by clay–magnetite aggregates that formed during sediment transport and the biasing effect of the gentle palaeocurrent at each site acting on the large aggregates. Therefore, the scattered distribution of the site mean remanence declinations found for the Stark Shale is evidence of a detrital remanent magnetization that is formed by primary sedimentary processes with an enriched metallic content and not remagnetization with mineralization by secondary hydrothermal processes.

New late Proterozoic paleomagnetic pole for the Rio de la Plata craton: Implications for Gondwana

Major uncertainties remain on the precise kinematic and temporal aspects of Gondwana assembly. In order to provide new constraints on them, a paleomagnetic study was carried out on the Ediacaran succession of red claystones exposed in Sierra de los Barrientos (37.8°S, 59.0°W), Buenos Aires province, Argentina. This succesion belongs to the late Proterozoic–early Paleozoic sedimentary cover of the Rio de la Plata craton. Sixty-two oriented samples from 13 sites were submitted to standard stepwise demagnetization procedures yielding a characteristic remanence carried by hematite. A primary origin for the remanence is suggested by the apparent recording of secular variation along the stratigraphic column including a reversal of the earth magnetic field and an improvement in clustering of mean site directions after application of bedding corrections, although the latter is not statistically significant. Since the study samples show a significant anisotropy of magnetic susceptibility (AMS) degree ( P up to 1.2) and an ellipsoid shape compatible with significant compaction during diagenesis, remanence inclination shallowing was investigated through oriented isothermal remanent magnetization (IRM) acquisition and demagnetization experiments. These permitted to confirm significant inclination shallowing and to compute a correction factor that was applied to the mean site directions producing a significantly further improvement in the statistical parameters. A paleomagnetic pole was computed for the Los Barrientos claystones at 15.1°S, 252.6°E, dp: 10.9°, dm: 14.2°. When rotated into a Gondwana framework, this new pole agrees with previous poles of ca. 550 Ma from several Gondwanan cratons, allowing the construction of a single APWP for the whole of Gondwana since that time, and suggesting that by ca. 550 Ma Gondwana was already assembled or in the final stages of that process.

Anomalously shallow inclination in middle-northern part of the South China block: palaeomagnetic study of Late Cretaceous red beds from Yichang area

SUMMARY We present new palaeomagnetic results from Late Cretaceous red beds of the Paomagang Formation collected at 42 sites in the Yichang area (30.7°N, 111.7°E), middle–northern part of the South China Block. A high unblocking temperature component around 680°C was isolated from 26 sites by stepwise thermal demagnetization. Fold tests are positive at the 95 per cent confidence limit for 14 and 11 sites of the Paomagang and Wangdian areas, respectively. Normal and reversed polarity sequence found in the Wangdian area passed a reversal test at the 95 per cent confidence limit. The tilt-corrected mean direction for Yichang area is D= 3.8°, I= 23.3° (α95= 5.1°, N= 26), corresponding to a palaeopole at 71.5°N, 280.0°E with A95= 4.3°. Comparison with the expected inclination from the 80 Ma Eurasian APWP pole indicates a inclination flattening of 26.3°± 6.4°. Middle Cretaceous to Cenozoic NW–SE extension tectonics within the SCB contributes only 2.0° to the inclination flattening. While the massive samples of the Paomagang Formation show a moderate degree of anisotropy (1.017 < Pj < 1.086) and steeper inclination (I= 29.3°), the laminated samples, which form more than 75 per cent of the studied samples, show a higher degree of anisotropy (1.052 < Pj < 1.227) and shallower inclination value (I= 22.7°). A good correlation between remanence inclination and facies types indicates that depositional processes produced the observed shallow inclination in the Yichang area.

Borehole geophysical techniques to define stratigraphy, alteration and aquifers in basalt

This paper concerns the interpretation of borehole geophysical data from basalt sequences, especially continental basalt sequences that host aquifers. Based on modifications of the rules used for interpreting borehole data from sedimentary rocks, new rules are proposed to identify the internal stratigraphy, aquifer boundaries, and alteration features in continental basalts. The value of several wireline tools is critiqued. Natural gamma logs have limited utility in basalt sequences unless anomalously high-potassium or low-potassium basalt flows and/or sedimentary interbeds exist which can act as marker beds for stratigraphic correlations. Neutron logs can usually discriminate between individual flows, flow breaks and interbeds, even in unsaturated basalts. Neutron logs and temperature logs can also be used to map aquifer thickness in basalt. Gamma–gamma density logs are usually sensitive to the density contrasts between interbeds and basalt flows, and in combination with neutron and natural gamma logs are crucial for the correct interpretation of large void spaces in basalt such as collapsed lava tubes and formerly inflated pahoehoe lobes. Basalt porosity calculated from neutron, resistivity and/or gamma–gamma density logs is commonly overestimated due to the presence of hydrous alteration minerals. Velocity and resistivity logs are best at discriminating between flows in saturated conditions. Magnetic susceptibility logs may capture magnetic mineralogy variations at a finer scale than that of flows and flow breaks and therefore should always be interpreted in combination with other logs. Non-spectral neutron–gamma logs are not useful in basalt, though spectral neutron–gamma logs have been used successfully for stratigraphic correlation and to locate pollutants. Geochemical logs or the inclination of magnetic remanence provide the best data to discriminate individual flows with a basalt sequence, and thus establish an internal stratigraphy. Other tools used alone cannot provide reliable stratigraphic information, but a combination of tools may work. We recommend the combination of natural gamma, neutron, and gamma–gamma density logs in unsaturated rocks, and these logs plus velocity and resistivity logs in saturated rocks.

Syn-folding remagnetization events in the French-Belgium Variscan thrust front as markers of the fold-and-thrust belt kinematics

Abstract New paleomagnetic studies have been carried out within the Ardennes segment of the N France - S Belgium Variscan fold-and-thrust belt to set constraints on the fold-thrust belt kinematics and reveal the casual relationships between vertical-axis rotations and major strike deviated zones localised along the general trend of the belt. Magnetite-bearing Devonian and Carboniferous limestones yielded two characteristic, secondary components of the natural remanent magnetization : a low temperature component recorded most probably during the late stages of folding and a high temperature component, acquired during incipient stages of deformation. Both post- and synfolding magnetizations were identified in the Lower Devonian hematite bearing sandstones. Ages of magnetization, inferred from the analysis of characteristic remanence inclinations compared to the reference curves for the stable parts of the Old Red Sandstones Continent (ORC), suggest the previous remagnetization event to be due to the burial of sedimentary rocks under the thick molassic foreland basin of Namurian-Westphalian age and the second to the final out-of-sequence activation of the thrust front in Stephanian times. Irrespective of the age of the magnetizations, orientations of paleomagnetic directions are dominantly governed by second-order structural trends. Clockwise rotations are observed in relatively narrow zones featuring deviated orientations of fold axes, other sites show paleomagnetic directions akin to those known from the ORC. We interpret this feature as a result of local transpressive deformations and related rotations, which occurred at lateral borders of propagating thrust-sheets. The latter deformation zones are suggested to be controlled by deep-seated discontinuities inherited from the Devonian Rheno-hercynian basin development. The Ardennes thrust belt was thus not rotated as a whole unit with respect to the ORC after the Namurian, preserving the initial orientation of the continental margin.

Magnetic anisotropy and paleomagnetic inclination shallowing in red beds: Evidence from the Mississippian Mauch Chunk Formation, Pennsylvania

Red, hematite‐bearing sedimentary rocks are an important source of paleomagnetic data, particularly for continental apparent polar wander paths during the Paleozoic. This study presents magnetic anisotropy data from the Mississippian Mauch Chunk Formation of eastern Pennsylvania, indicating that these red beds have suffered from a significant amount of paleomagnetic inclination shallowing. Fourteen oriented block samples were collected from normal and reversed polarity strata identified in a previous study. Thermal demagnetization isolated the characteristic remanent magnetization at seven normal polarity horizons at unblocking temperatures greater than 670°C (mean direction, D = 354.0°, I = −18.4°, α95 = 10.2°). Anisotropy of remanence measurements (anisotropy of isothermal remanent magnetization and thermal remanent magnetization) indicate a bedding parallel, foliated magnetic fabric with foliations ranging from 1.1 to 1.35. Thermal demagnetization at 670°C of the isothermal remanent magnetization isolates the magnetic fabric of the characteristic remanence‐carrying grains and indicates stronger foliations (1.15–1.48). Anisotropy of magnetic susceptibility (AMS) also indicates bedding parallel magnetic foliations of 1.01–1.04, typical of red beds. Chemical leaching isolates the AMS of the characteristic remanence‐carrying grains to range from 1.02 to 1.07. This AMS fabric was used to correct the characteristic remanence inclination isolated by thermal demagnetization. The critical parameter needed for an accurate inclination correction, the individual particle anisotropy aχ, was determined from the correlation between the normalized principal axes of remanence and susceptibility anisotropy to be 1.06. The corrected Mauch Chunk direction, D = 354.5°, I = −56.4° results in a corrected Mauch Chunk Formation paleopole (12°N, 108°E) that is consistent with a European igneous paleopole.

Magnetic anisotropy of serpentinized peridotites from the MARK area: Implications for the orientation of mesoscopic structures and major fault zones

[1] Mantle-derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments. Here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from Ocean Drilling Program (ODP) Site 920 (Mid-Atlantic Ridge south of Kane, MARK) to provide insights into the tectonic processes responsible for the exposure of these deep-seated rocks at the seafloor. Paleomagnetic data from 214 samples from Site 920 document a remarkably consistent inclination (36.1° +0.8°/-1.4°) that is shallower than either the expected geocentric axial dipole inclination (40.7°) or present-day inclination (41.9°) at the site. We show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence, possibly augmented by viscous processes at moderate temperatures. These properties were acquired during cooling from the relatively high temperatures (>350°C) at which serpentinization occurred. The remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif. However, interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric, which presumably resulted in a deflection of the remanence. We estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy. The corrected remanent inclinations (mean 39.5°) more closely approximates the time-averaged inclination at the site, indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350°C. Accounting for the anisotropy-related deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation. After this reorientation the dominant mesoscopic foliation in these rocks, defined by the preferred orientation of orthopyroxene and subparallel serpentine veins, has an average orientation that closely parallels the regional-scale fault zones on the western median valley wall.

Plastically deforming clay-rich sediment to help measure the average remanence anisotropy of its individual magnetic particles, and correct for paleomagnetic inclination shallowing

The inclination of remanence in fine-grained sediment may be shallower than the inclination of the field in which it was acquired. This paleomagnetic inclination shallowing can lead to an underestimation of paleolatitude. Fortunately, compaction also induces a magnetic anisotropy in the sediment that can help correct for the inclination shallowing. Such correction generally requires measuring the average remanence anisotropy of the individual magnetic particles in the sediment, which can be difficult. For example, various authors have tried diluting the sediment with a glue, placing it in a strong magnetic field to align the long axes of the magnetic particles while the glue hardens, and then measuring the anisotropy of the resulting sample. This often overestimates particle anisotropy, probably because the magnetic particles tend to form chains along the magnetic field lines. We describe a method that avoids this problem, in the case of clay-rich soft sediment; we align the magnetic particles, without using a magnetic field, by plastically deforming the sediment with an axial compression. This method was applied to a suite of clay-rich deep-sea turbidite samples that bear pseudo-single-domain magnetite particles. Measuring the remanence anisotropy induced by compressing a composite sample yielded an estimate of the average remanence anisotropy of the individual magnetite particles in the sediment. Although this proved to be an underestimate, it can be useful in conjunction with the overestimate yielded by the magnetic field alignment method. We also experimented with a modification of our method that involves measuring the change in remanence inclination as well as the remanence anisotropy induced by compressing the sample. This yielded a particle remanence anisotropy estimate that allowed us to successfully correct for the inclination shallowing observed in the turbidites. Our method should be applicable to any clay-rich soft sediment with magnetic particles of any domain state if the particles are dominated by uniaxial shape anisotropy.

Geomagnetic paleointensity for the last 100 kyr from the sub-antarctic South Atlantic: a tool for inter-hemispheric correlation

We report relative paleointensity proxy records from four piston cores collected near the Agulhas Ridge and Meteor Rise (South Atlantic). The mean sedimentation rate of the cores varies from 24 cm/kyr to 11 cm/kyr. The two cores with mean sedimentation rates over 20 cm/kyr record positive remanence inclinations at 40–41 ka coeval with the Laschamp Event. Age models are based on oxygen isotope data from three of the cores, augmented by radiocarbon ages from nearby Core RC11-83, and by correlation of paleointensity records for the one core with no oxygen isotope data. The relative paleointensity proxy records are the first from the South Atlantic and from the high to mid-latitude southern hemisphere. Prominent paleointensity lows at ∼40 ka and ∼65 ka, as well as many other features, can be correlated to paleointensity records of comparable resolution from the northern hemisphere. The records are attributable, in large part, to the global-scale field, and therefore have potential for inter-hemispheric correlation at a resolution difficult to achieve with isotope data alone.

A Maastrichtian palaeomagnetic pole for the Pacific plate from a skewness analysis of marine magnetic anomaly 32

The asymmetry (skewness) of marine magnetic anomaly 32 (72.1–73.3 Ma) on the Pacific plate has been analysed in order to estimate a new palaeomagnetic pole. Apparent effective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire Pacific plate and spanning a great-circle distance of ∼12 000 km. The data were inverted to obtain a palaeomagnetic pole at 72.1°N, 26.8°E with a 95 per cent confidence ellipse having a 4.0° major semi-axis oriented 98° clockwise of north and a 1.8° minor semi-axis; the anomalous skewness is 14.2° ± 3.7°. The possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible effect on our palaeopole analysis over the range of relevant spreading half-rates, ∼25 to ∼90 mm yr−1. The new pole is consistent with the northward motion for the Pacific plate indicated by coeval palaeocolatitude and palaeoequatorial data, but differs significantly from, and lies to the northeast of, coeval seamount poles. We attribute the difference to unmodelled errors in the seamount poles, mainly in the declinations. Comparison with the northward motion inferred from dated volcanoes along the Hawaiian–Emperor seamount chain indicates 13° of southward motion of the Hawaiian hotspot since 73 Ma. When the pole is reconstructed with the Pacific plate relative to the Pacific hotspots, it differs by 14°–18° from the position of the pole relative to the Indo–Atlantic hotspots. This has several possible explanations including bias in one or more of the palaeomagnetic poles, motion between the Pacific and Indo–Atlantic hotspots, and errors in plate reconstructions relative to the hotspots.

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.

Comparing remanence anisotropy and susceptibility anisotropy as predictors of paleomagnetic inclination shallowing in turbidites from the Scotian rise

We studied 79 turbidite mud samples from 3 piston cores from the continental rise off Nova Scotia, Canada. Natural remanence and susceptibility are mainly due to magnetite. The stable remanence inclination, Iobs, is on average 12 degrees shallower than that predicted from the geocentric axial dipole model. The average percent magnetic anisotropy is 17% for anhysteretic remanent magnetization (ARM) and 10% for susceptibility (χ). Both magnetic anisotropies are foliated in the bedding plane with the hard (minimum) axis perpendicular to bedding (except for 15% of the specimens which are likely disturbed and are excluded from further discussion). As expected theoretically for a suite of samples with inclination shallowing (Hodych and Bijaksana, 1993), a significant correlation was found between tan Iobs and ARMmin/ARMmax (the ratio of intensities of ARM given identically perpendicular and parallel to bedding). A significant correlation was also found using χmin/χmax (the ratio of susceptibilities perpendicular and parallel to bedding) in place of ARMmin/ARMmax. In individual cores, the correlation using ARM anisotropy is generally higher than using susceptibility anisotropy. However, when the results from the three cores are combined, either correlation extrapolated to zero anisotropy succeeds in predicting the observed inclination shallowing. Because susceptibility anisotropy measurement is so fast, we recommend that it become routine in paleomagnetic studies of magnetite-bearing sediments to warn of possible inclination shallowing.

Magnetic properties and emplacement of the Bishop tuff, California

Anisotropy of magnetic susceptibility (AMS) and characteristic remanence were measured for 45 sites in the 0.76 Ma Bishop tuff, eastern California. Thirty-three sites were sampled in three stratigraphic sections, two in Owens gorge south of Long Valley caldera, and the third in the Adobe lobe north of Long Valley. The remaining 12 sites are widely distributed, but of limited stratigraphic extent. Weakly indurated, highly porous to dense, welded ash-flow tuffs were sampled. Saturation magnetization vs temperature experiments indicate two principal iron oxide phases: low Ti magnetites with 525–570 °C Curie temperatures, and maghemite with 610°–640 °C Curie temperatures. AF demagnetization spectra of isothermal remanent magnetizations are indicative of magnetite/maghemite predominantly in the multidomain to pseudo-single domain size ranges. Remeasurement of AMS after application of saturating direct fields indicates that randomly oriented single-domain grains are also present. The degree of anisotropy is only a few percent, typical of tuffs. The AMS ellipsoids are oblate with Kmin axes normal to subhorizontal foliation and Kmax axes regionally aligned with published source vents. For 12 of 16 locality means, Kmax axes plunge sourceward, confirming previous observations regarding flow sense. Topographic control on flow emplacement is indicated by the distribution of tuff deposits and by flow directions inferred from Kmax axes. Deposition east of the Benton range occurred by flow around the south end of the range and through two gaps (Benton notch and Chidago gap). Flow down Mammoth pass of the Sierra Nevada is also evident. At least some of the Adobe lobe in the northeast flowed around the west end of Glass mountain. Eastward flow directions in the upper Owens gorge and southeast directions in the lower Owens gorge are parallel to the present canyon, suggesting that the present drainage has been established along the pre-Bishop paleodrainage. Characteristic remanence directions from 45 sites (267 samples) yield an overall mean of D=348°, I=53° for the Bishop tuff. A correlation is found in two of the three profiles between density and remanence inclination. A mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash-fall tuff at Lake Tecopa is: D=353°, I=54°, k=172, α95=2.9°, N=15.

Further evidence for inclination shallowing in red beds

Red beds and related shallow marine siliciclastic sediments within the extensional Catalan Neogene basins offer a unique opportunity to study the relationship between sediment anisotropy, remanence acquisition and inclination shallowing. On the basis of the dominant fabric features, sedimentary facies are grouped into laminated, massive and bioturbated. The bulk susceptibility appears to be dominated by paramagnetic phyllosilicates and clay minerals. The magnetic susceptibility fabric is dominantly foliated while the degree of anisotropy is very variable and is dependent on the facies type. Strong foliation dominates in the laminated sediments whereas weak anisotropy characterizes the bioturbated and massive mass‐flow facies. Moreover, we observe a correlation between facies type and remanence inclination. The undisrupted thinly laminated facies provides the most biased inclination values: 20° as opposed to 60° of the expected geomagnetic field. Steeper inclinations are reported from bioturbated muds, while the best record of the geomagnetic inclination is found in the weakly anisotropic mass flow deposits.

Inclination shallowing and preferred transitional VGP paths

Transitional VGP paths recorded in sediments cluster into two antipodal preferred longitude bands that tend to lie 90° away from their site longitudes, the latter also being clustered. VGP paths obtained from lava flow sequences, though much fewer, appear not to show these biases, suggesting a rock-magnetic influence on VGP paths recorded in sediments. Inclination shallowing of detrital magnetic remanence, enhanced under low transitional field strengths, is the most likely candidate.We illustrate the effects of inclination shallowing by applying a simple shallowing model (tan IR = f tan IA, where IA is the inclination of the magnetic remanence and IA is the inclination of the ambient field) with field variation to hypothetical data sets. Shallowing-induced clustering increases as f decreases and becomes extreme as f approaches 0.1.We have used the model to ‘de-shallow’ the available set of transitional VGP sediment records for various values of f. The probability that the observations arise from inclination shallowing of a uniform random distribution of paths increases as f decreases. When f drops to 0.13 there is a 50% chance of getting at least as much grouping as observed. To decide if inclination shallowing is a dominant factor in the clustering, we need to know whether such extreme shallowing is widespread in sedimentary records under transitional field conditions. Field and laboratory redeposition data are not yet adequate to resolve this question.

Inclination error linked to sedimentary facies in Miocene detrital sequences from the Vallès-Penedès Basin (NE Spain)

Abstract Palaeogeographic reconstructions based on kinematic and other integrated models are rarely in perfect agreement with palaeomagnetic data. Furthermore, the reliability of rocks that record the geomagnetic field is often questioned, particularly for detrital remanent magnetization (DRM). Among the diverse types of magnetization bias, inclination shallowing caused by several mechanisms such as gravitational torques upon deposition and post-depositional compaction can be a major source of error in palaeolatitudinal estimations. Inclination shallowing for a variety of fine-grained alluvial sediments from the Neogene Vallès-Penedès Basin (NE Spain) has been observed to be dependent on sedimentary facies. The correlation of inclination shallowing to sediment fabric is investigated in samples from 11 stratigraphic profiles. Each profile has a dominant lithology, from thinly laminated to edafized silts and massive muds and breccias. A measure of particle alignment in sediment can be determined by means of the degree of anisotropy (P) and shape symmetry (T) AMS parameters. Since each sedimentary facies shows a particular range of P and T , a representative mean AMS ellipsoid can be estimated from each lithologically homogeneous profile. Estimated mean remanence inclination ( I NRM ) has a strongly consistent relationship to the mean AMS ellipsoid and a positive linear correlation is inferred between ln ( P ) and ln tan ( I NRM ). Given the extreme case of no sediment anisotropy, i.e. P = 1, inclination error would be cancelled and I NRM would record the true magnetic field inclination of the sampling site. The regression function predicts a corrected magnetic inclination of 60°, consistent with that of the expected geomagnetic dipole for the studied site and age.