Remanence Vectors Research Articles

Experimental strain of isothermal remanent magnetization in ductile sandstone

Abstract A macroscopically ductile sandstone, to which a homogeneous isothermal remanent magnetization (IRM) was applied, has been deformed at 150 MPa confining pressure and constant strain rate of 10−5 s−1. Hydrostatic compaction does not produce a deflection of the IRM vector although it is reduced in intensity. Pure shear producing shortening in the range 2 to 35% steadily reduces the intensity of magnetization but also homogeneously rotates the remanence vector toward the plane of flattening. The amount of rotation is slightly less than that expected for a non-material line undergoing homogeneous strain. Deformation selectively removes weakly coercive components of remanence, as revealed by alternating field (AF) demagnetization. During deformation a weak deformational viscous remanent magnetization (DVRM) is acquired from the pressure vessel. This is different from a conventional viscous remanent magnetization (VRM) in that it is not acquired when the specimen is subject to hydrostatic confining pressure alone, even for periods three times longer than the longest deformation test.

The palaeomagnetic record of the Santiago volcanics (Republic of Cape Verde); multiphase magnetization and age consideration

Abstract The palaeomagnetism of the older volcanic complex of Santiago constitutes a multicomponent structure, and magnetomineralogical evidence suggests that the various remanence vectors are carried principally by titanomaghaemites. When viewed in the context of the African palaeomagnetic reference frame the predominant magnetization is consistent with an Upper Tertiary age, but two older magnetizations of inferred Lower Tertiary age are also in evidence. The demonstrable secondary nature of all three palaeomagnetic axes (each of them represented by both polarities) makes it likely that the actual emplacement of the older complex is of pre-Tertiary age, being probably an integral component of the pronounced tectonomagmatic uplift phase that affected the entire northwest African margin in Upper Cretaceous time. The older complex of Santiago can probably be correlated with the igneous core of the neighbouring island of Maio, which apparently formed during Turonian time (∼ 90 Ma ago). Some younger lavas of Upper Miocene age (the Flamengos Formation) show a much simpler palaeomagnetic history than rocks of the older complex, yielding only the Upper Tertiary palaeomagnetic field axis.

Estimation of depth and magnetization of magnetic sources from magnetic data: The linearized continuous inverse problem for 21/2D structures

The estimation of the depth to the top and bottom of a magnetic source from magnetic data defines a nonlinear inverse problem, while the evaluation of the distribution of magnetization determines a linear inverse problem. In this paper, these interpretation problems are resolved in the continuous case of 21/2D magnetized bodies with lateral magnetization variations. A formulation of the magnetic problem accounting for different directions of remanent and total magnetization vectors and including a more general definition of apparent susceptibility is presented. Differences between 2D and 21/2D formulations are stressed, as regards the anomaly amplitude, shape and zero-level.

A method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating

SUMMARY Spatial variations in the geomagnetic field must be taken into account if secular variation master curves and directional magnetic dates are to be optimized. Two methods for relocating remanence vectors have been proposed and in this paper their relative accuracies are compared using a numerical model based on the present-day field. A method which converts archaeomagnetic directions via a virtual geomagnetic pole is shown to be the more efficient transformation. For an‘archaeomagnetic region’the size of the British Isles, (900 km radius), the maximum error in relocating vectors to a central location is predicted to be of the order of 1.2°.

Oxide Mineralogy and Magnetic Properties of the Koidu Kimberlite Complex, Sierra Leone, West Africa

SUMMARY A good correlation between opaque mineralogy and rock magnetic properties has been established in a study of a small number of oriented samples from four kimberlite dykes and one pipe in the Koidu complex of Sierra Leone. The pipe sample is mineralogically distinctive, in that it alone contains deuteric titanomaghaemite after titanomagnetite, and haematite after chlorite. All of the kimberlites contain xenocrystic, upper-mantle derived ilmenite that has undergone sub-solidus reduction to form Mg-Ti magnetite. Three of the dykes contain serpentinized olivine with microscopic to sub-microscopic magnetite and identifiable metal Ni-Fe alloys, and finely textured Mg titanomagnetite and magnetite as atolls surrounding complexly zoned Cr-Mg- A1 spinels. Consistent with these observations, Curie and blocking temperatures range from 520 to 590°C and high alternating field (AF) coercivities imply single domain or pseudo-single domain particles as the bulk carrier of natural remanent magnetization (NRM). In the pipe sample, however, the remanence persisting above 600°C is comparatively strong. Preliminary remanent vectors, isolated by either thermal or AF demagnetization, from four of the five samples are moderately consistent, and yield a mean (n = 4) of D = 323, Z = -17, k = 18, and ag5 = 22.

Tectonic strain and paleomagnetism: experimental investigation

A porous, clay-rich sandstone with a natural remanent magnetisation shows complex changes in the orientation of the remanence vector when the sandstone is subjected to experimental deformation, at 100 MPa, room temperature and constant natural strain rates of 1 × 10−5 s−1. The paleomagnetic vector does not simply rotate away from the axis of compression. When the rock is shortened perpendicular to bedding the remanence vector rotates toward the bedding, as expected, but the remanence also rotates toward bedding when the rock is shortened parallel to bedding. Moreover, in some cases the remanence vector changes azimuth within the specimen during experimental deformation. Thus bedding anisotropy and the accompanying mineral-fabric anisotropy are more influential than strain in controlling deflections of the paleomagnetic vector in this study. This anisotropy also influences the post-deformational behaviour of remanence: 4 months after deformation, some cores shortened perpendicular to bedding relaxed their remanence vectors back towards the pre-deformational attitude whereas some other cores continued to show progressive changes. Intragranular, recoverable elastic strains in the magnetic grains may be partly responsible for the experimentally induced deflections of remanence.

Ilmenite lamellae and stability of magnetization

I have analyzed magnetic data from four deep‐sea basalts which have undergone the “exsolution” stage of high‐temperature oxidation, but have not begun the “replacement” stage. Even when ilmenite lamellae development was extensive, coercivities were low with a coercive force (Hc) of 11.7 mT (1 mT=10 Oe) and medium destructive field (MDF) of 9.1 mT. The remanence vector exhibited a directional swing of 30° upon alternating field (AF) demagnetization. Thus, few of the titanomagnetite intergrowths behaved like single‐domain grains. This tentative conclusion is supported by those previous studies of subaerial basalts and synthetic materials which optically identified the degree of oxidation and in which no titanomagnetite granulation was observed. In these studies, MDF's generally did not increase above 15 mT during ilmenite lamellae formation. Those natural and synthetic samples which exhibit features of the “replacement” stage have large natural remanent magnetization (NRM) intensities and high coercivities suggesting that despite intense hematization, single‐domain magnetite may still be present.

Project GAMBLE: Ground‐based and airborne magnetic boundary layer experiments

One of the main goals of paleomagnetic studies is to produce evidence for ancient continent displacements and thus produce paleogeographical reconstructions of the earth in the geological past. To achieve this goal, paleomagnetists use the direction of remanent vectors as measured in rocks to derive latitudinal and azimuthal data, according to the model of the Earth magnetic field (EMF). One of the many difficulties faced by these researchers is the determination of whether the rock deformation may have altered the original paleomagnetic direction. If so, the paleomagnetists must correct the effect of such a deformation. Obviously, this aspect is of secondary importance in cratonic areas where beds lie flat or are gently folded, and in earlier times, most paleomagnetic studies were conducted in such areas. However, it now appears that we cannot avoid work within orogenic belts if we want to resolve geodynamic problems [Van der Voo and Channel, 1980]. It therefore became necessary to examine more carefully the interaction between magnetizations and deformation. During the last decade, several teams have conducted research efforts in this direction.

Unstraining paleomagnetic vectors: The current state of debate

One of the main goals of paleomagnetic studies is to produce evidence for ancient continent displacements and thus produce paleogeographical reconstructions of the earth in the geological past. To achieve this goal, paleomagnetists use the direction of remanent vectors as measured in rocks to derive latitudinal and azimuthal data, according to the model of the Earth magnetic field (EMF). One of the many difficulties faced by these researchers is the determination of whether the rock deformation may have altered the original paleomagnetic direction. If so, the paleomagnetists must correct the effect of such a deformation. Obviously, this aspect is of secondary importance in cratonic areas where beds lie flat or are gently folded, and in earlier times, most paleomagnetic studies were conducted in such areas. However, it now appears that we cannot avoid work within orogenic belts if we want to resolve geodynamic problems [Van der Voo and Channel, 1980]. It therefore became necessary to examine more carefully the interaction between magnetizations and deformation. During the last decade, several teams have conducted research efforts in this direction.

Effects of tectonic deformation on the remanent magnetization of rocks

Paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector. Deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis. However, the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known. Anisotropy of magnetic susceptibility (AMS) provides a quantitative basis for strain determination. The principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid. A linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types. If the correlation method used is valid, AMS observations can be used to establish the magnitudes and directions of the principal strains. Strain modifies bedding planes and realigns the grains that carry magnetic remanence. Compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain. The effects of strain on bedding can be corrected using passive marker theory. Attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive. During deformation linear and planar elements deflect away from the axis of maximum shortening. The passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter. Tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions. The passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix.

Drilling-induced remanence in carbonate rocks: occurrence, stability and grain-size dependence

Summary. A strong grouping of the directions of natural remanent magnetization in a collection of Ordovician limestones and dolomites, prior to correction for the in situ orientation of the samples, led us to suspect the presence of a substantial spurious magnetization acquired during sample collection and preparation. A close correspondence between the directions of the remanence vectors and the direction of the ambient magnetic field during sawing and drilling of the samples suggested that the remanence was dominated by a component acquired during cutting of the samples. Ten specimens of the Camp Nelson Limestone and Shakopee Dolomite were demagnetized to 100 mT, given an anhysteretic remanence along their axes, and then sawed again. A substantial magnetization parallel to the ambient field during cutting was acquired by all of the specimens, and the resultant directions deviated by 7-70 from the direction of the anhysteretic magnetization. Stepwise alternating-field and thermal cleaning to 60 mT and 400°C respectively failed to remove preferentially the cutting-induced magnetic contamination. Since the fraction of magnetite grains cut during drilling and sawing must be a linear function of grain size, modified Lowrie-Fuller tests were carried out and the results are interpreted to indicate the presence of a multidomain magnetite fraction in the Shakopee Dolomite, Camp Nelson Limestone and Oregon Dolomite. Ratios of initial to anhysteretic susceptibility (dxARM) correlate well with the angular deviation of magnetic directions produced by sawing. This indicates that acquisition of drilling-induced remanence is a function of magnetite grain size, compatible with the notion that the drillinginduced component resides in large magnetite grains which have been cut.

A palaeomagnetic study of Svecokarelian basic rocks from northern Sweden

Abstract This palaeomagnetic investigation comprises basic rocks from six localities from the Svecokarelian zone in northern Sweden. Most of the pole positions in this study and other reported poles of Svecokarelian and post-Svecokarelian rocks fall within an approximately 12 degrees wide band running from east to west representing ages of magnetizations from 1880-1700 to ∼ 1530 Ma. Thermal demagnetizations of specimens of the probably oldest massifs indicate a possible backward continuation of the polar wandering path. Mineralogical studies of thin sections of the rocks show ore symplectites and myrmekitic textures indicating a slow rate of cooling at least at the end of the rock formation. Signs of metamorphism are demonstrated by the existence of secondary minerals, including magnetite, not related to late magmatic alterations. The distribution of site means as well as the change of directions of the remanence vectors during thermal demagnetization can be explained by a slow rate of cooling and where signs of metamorphism exist by partial remagnetization of the rock. This study has, apart from the palaeomagnetic results, demonstrated the difficulty of correlating radiometric ages with ages of magnetization.

Effect of progressive deformation on remanent magnetization of permian redbeds from the alpes maritimes (France)

The relationships among the anisotropy of magnetic susceptibility, the finite strain and the characteristic remanent magnetization (ChRM) have been investigated in deformed red sediments from the Alpes Maritimes in southeast France. The magnetic properties of these rocks are due to hematite. The finite strain is known from field studies of the shapes and orientations of green reduction spots and from optical microscopy and X-ray texture goniometry studies of chlorite and illite. The strain data permit a classification of the magnetic results according to four main stages of progressive deformation. In the compaction stage magnetic anisotropy is flattened in the bedding due to compactional loading; the ChRM directions approximate to a Fisherian distribution about a mean value close to the known Permian direction for the region. As deformation proceeds, pencil structure first develops and the anisotropy is characterized by a prolate pattern with maximum axis parallel to the subhorizontal azimuth of the pencil structure and fold axes. With increasing deformation the magnetic susceptibility ellipsoid becomes oblate and finally triaxial. At these stages of high deformation the anisotropy patterns show that the hematite grain orientations are strongly influenced by the cleavage. The remanence directions in deformed rocks do not correspond to a Fisher distribution and eventually deviate strongly from Permian directions. These results show that paleomagnetic vectors are affected by the distortional strains which accomodate shape change in the rock. Simple tilt correction without compensation for the strain would yield incorrect paleopole positions. An attempt has been made to compensate for the effects of deformation by regarding the remanent vector as a passive marker (March, 1932). Equations for strain removal were applied to the remanence in two ways; 1. (a) using strain data directly obtained in the field for each site and 2. (b) by converting the magnetic susceptibility anisotropy of each sample to a corresponding finite strain value. Correcting for the finite strain by both methods improves the within-site grouping of paleomagnetic vectors. In strongly deformed rocks strain removal assuming the March model results in overcorrection of the rémanent magnetization data. In low-strain regions distortion may produce mechanical rotation of the hematite particles, but recrystallization may alter the deformation mechanism in regions of high strain. The limits of applicability of the passive marker treatment could not be tested adequately in the deformed red sediments of the Alpes Maritimes.

Changes in direction of the remanence of rocks produced by stationary alternating field demagnetization

A theoretical investigation of the way in which an isotropic rock containing single-domain particles acquires both IRM and ARM (or TRM) has indicated that stationary single-axis alternating field (af) demagnetization with the af axis at an angle to the remanence vector should produce progressive angular changes in a single-component remanence as demagnetization proceeds. Just before the remanence is completely removed it should lie at 90° to the af axis irrespective of the original orientation of the remanence (apart from 0°). Experimental observations on a rock sample support these deductions. This analysis has been extended to investigate the way in which ARM (or TRM) and IRM are demagnetized by static three-axis demagnetization methods which are used by some workers in palaeomagnetism. Theory, in conjunction with the use of a numerical model, predicts that an ARM or TRM should not undergo significant direction changes when these methods are applied but an IRM should undergo progressive direction changes as demagnetization proceeds, usually moving until it makes an angle of cos−1 (1/√3) with each of the three af axes just before it is removed. Confirmation that such changes do occur have been obtained by experiments on a rock sample. The relative merits of static and tumbling af demagnetization methods are also discussed.

Paleogene magnetic stratigraphy in Umbrian pelagic carbonate rocks: The Contessa sections, Gubbio

The Umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between Paleogene biostratigraphy and geomagnetic polarity history. The red-to-pink, Paleocene to middle Eocene Scaglia Rossa limestone, the varicolored upper Eocene Scaglia Variegata limestone, and the gray-green Oligocene Scaglia Cinerea marlstone form a 250-m-thick, continuous exposure in the Contessa Valley near Gubbio, Italy. Magnetostratigraphic, lithostratigraphic, and biostratigraphic investigations in three sections covering the entire Paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period. The ferromagnetic mineral in the Scaglia Cinerea is magnetite and AF demagnetization to peak fields of 20 mT is sufficient to define the characteristic remanent magnetization. The Scaglia Variegata and pink Scaglia Rossa samples contain an additional hematite component which is very pronounced in dark red Paleocene Scaglia Rossa samples. Changes of magnetic mineralogy take place in these limestones during heating, especially above 500 °C. However, thermal demagnetization is effective in isolating the characteristic remanence vectors, which form almost antipodal clusters of directions representing normal and reversed polarities. The directions are rotated counterclockwise, partly due to post-Oligocene tectonism. Paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation, but the major epoch and stage 29, as in the Gubbio Bottaccione section; Paleocene-Eocene, just above anomaly 25; Eocene-Oligocene, between anomalies 13 and 15; Oligocene-Miocene, just below anomaly 6C. These correlations require slight modifications to previous conclusions on Paleogene sea-floor spreading rates.

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.

Axial-current-mode shock depoling of PZT 56/44 ferroelectric ceramic disks

Gas-gun impact techniques have been used to investigate the electrical response of shock-depoled PZT 56/44 ferroelectric ceramic disks in the stress range rom 0.7 to 8.8 GPa. External short-circuit current pulses were measured as the shock wave propagated axially through the disk in a direction either parallel or antiparallel to the remanent polarization vector. Complete charge release was not observed in any of the shots. About 82% of the available charge was released for both orientations in the stress range from about 1.5 to 5.6 GPa. At stress levels of 6.8 and 8.8 GPa the charge release for the antiparallel orientation shots was less than for the parallel orientation shots. A series of electrical polarization reversal measurements was performed under nonshock conditions on PZT disks to estimate the electrical properties in the region ahead of the shock front. Maximum electric fields for the regions ahead of and behind the shock front were estimated to be 1–6 and 4–24 kV/mm, respectively, as the shock stress was increased.

Emplacement temperatures of unsorted and unstratified deposits of volcanic rock debris as determined by paleomagnetic techniques

Unsorted and unstratified deposits of volcanic rock debris typically flank recently active stratovolcanoes. It is often difficult, using standard geologic procedures, to establish whether a particular deposit was emplaced by a pyroclastic flow, lahar, rock avalanche, or glacier. Determination of the emplacement temperatures of clasts contained in the deposit aids in discriminating among some of these possibilities. The emplacement temperature of a clast can be estimated by analyzing its thermoremanent magnetization. To do this, oriented samples of the clasts are submitted to progressive thermal demagnetization; the directions and magnitudes of the resulting residual remanent vectors provide the information necessary to estimate the temperatures at which the clasts were emplaced. Studies of samples that were given artificial emplacement temperatures reveal that estimates are within ±25 °C of the actual values. The temperature range within which estimates are possible depends on the thermoremanent magnetization acquisition curve of a clast. Data obtained from deposits of volcanic rock debris from Mount St. Helens, Washington, indicate that for andesitic and dacitic rocks, the range is roughly 100 to 550 °C. The procedure probably can be applied to other geologic problems that require temperature information.

The effect of stress cycling and inelastic volumetric strain on remanent magnetization

The relation between remanent magnetization (natural and thermal), volumetric strain, and stress was determined for five rock types. Twenty different samples (fine‐grained intrusive, gabbro, basalt, tuff, and andesite) were repeatedly cycled to 95% of their fracture stresses at confining pressures of up to 1250 bars. At the completion of the first cycle there was always a permanent demagnetization of the rock. Subsequent stress cycles show a progressively smaller demagnetization, and the changes of remanence with loading become nearly reproducible by the fourth cycle. The onset of dilatancy produced changes in the rate of magnetization change. For two rock types the magnitude of the remanence increased up to the onset of dilatancy and then decreased as the stress was augmented. For these two rock types the remanent vector first rotated toward the axis of applied stress up to the onset of dilatancy; continued loading caused a rotation away from the applied stress axis. Typically, the changes in remanent intensity were of the order of 5%, which is about 3 times smaller than the changes in the individual components parallel and perpendicular to the axis of the greatest compressive stress. In fact, it was not uncommon for one component to increase while the other decreased. As a consequence, large rotations, up to 10°, of the remanent moment were observed, and at differential stresses in excess of half the compressive strength the remanent vector always rotated away from the axis of greatest compression. In addition, time dependent changes in remanent magnetization of several percent accompanied time dependent increases in crack porosity during constant stress creep tests at high differential stresses.Table 2 is available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 2006. Document J78‐003; $1.00. Payment must accompany order.

Spectral analysis of Quaternary palaeomagnetic data from British Columbia and its bearing on geomagnetic secular variation

Palaeomagnetic results obtained from a 7-m sedimentary sequence in southern British Columbia spanning approximately 9000 yr (˜ 22 000 to ˜31 000 yr BP based on radiocarbon dates) are reported and analysed. Remanence directions from 37 horizons spread throughout the section have been determined, and no evidence of the geomagnetic excursion observed at Lake Mungo, Australia (˜ 31 000–28 000 yr BP), or that observed at Mono Lake, California (25 000–24 000 yr BP) is present. However, regular oscillations in the remanence vectors are observed, and maximum entropy spectral analysis, after mapping the remanence directions on to the complex plane, reveals peaks at periods of approximately 2000 and 5000 yr. The 2000-yr peak is associated with clockwise looping of the geomagnetic vector, and therefore most likely represents the time associated with one full cycle of the westward drift of the non-dipole field. The 5000-yr peak is associated with elliptical counterclockwise looping of the local geomagnetic vector and may be indicative of counterclockwise motion of the geomagnetic dipole axis.