Remanence Directions Research Articles

Directional properties of alteration CRM in basic igneous rocks

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

The coast-parallel dolerite dykes of east Madagascar; age of intrusion, remagnetization and tectonic aspects

Palaeomagnetic and whole rock K-Ar data are reported from dolerite dykes from the east coast of Madagascar. The radiometric dates indicate that the dykes were injected 94.5 ± 1.2 Ma ago, during the earliest phase of Cretaceous magmatism on the island. The remanence structure is principally single-component having been acquired in a normal polarity field. The magnetization carrier is inferred to be titanomaghaemite, a product of extensive low-temperature oxidation of original Fe-Ti oxides. When viewed in the context of the African palaeomagnetic reference frame, the characteristic magnetization is consistent with a late Cretaceous-Early Tertiary age, i.e., some 40 million years younger than the emplacement age of the dykes. This low-temperature magnetic resetting is associated with reactivation of the fault/dyke system, this being the consequence of an early Tertiary shear régime provided by 25° anticlockwise rotation of an African/eastern Indian Ocean lithospheric block system. The overall remanence direction yields a pole at 65.5° S, 38° E ( K = 73, α 95 = 1.8), but this pole is regarded as providing only crude paleomagnetic information since the total palaeomagnetic record was most likely acquired at various times during the assumed lithospheric rotation.

Early Silurian paleolatitude for central Newfoundland from paleomagnetism of the Wigwam Formation

Redbeds and minor volcanic rocks of the Early Silurian Wigwam Formation of the Botwood Group were sampled in the Exploits Subzone of the Dunnage Zone of central Newfoundland. At 18 sedimentary and 3 volcanic sites thermal demagnetization isolated a shallow northward (or southward) magnetization (D = 356.0°, I = −16.8°; k = 12.9; α95 = 9.2°; n = 21 site means) after correction for the tilt of bedding. A positive fold test demonstrates that this remanence predates regional Siluro–Devonian folding. A primary age for the remanence is indicated by positive conglomerate tests on volcanic clasts, and reversals correlated with sedimentary stratigraphy. Antiparallel remanence directions at sites of opposite polarity indicate that the primary remanence is not partially overprinted by a secondary component. The Exploits Subzone of the Dunnage Zone is interpreted to have been at low paleolatitudes (8.5°S ± 5°) during the Early Silurian. We reached a similar conclusion for the Notre Dame Subzone of the Dunnage Zone in a previous paleomagnetic study of Early Silurian redbeds and volcanic rocks of the King George IV Lake area. The low paleolatitudes obtained for the Wigwam and King George IV Lake rocks do not differ significantly and, hence, provide no evidence for a large ocean between the two subzones in the Early Silurian. However, the azimuthal orientation of the subzones at that time is still uncertain, and an ocean with its axis oriented in a north–south direction would not be detected by the paleomagnetic method.

Self‐reversal of thermoremanent magnetization in the ilmenite‐hematite System: Order‐disorder, symmetry, and spin alignment

A model of the process by which certain members of the ilmenite‐hematite solid solution series acquire reverse thermoremanent magnetization (reverse TRM) is proposed. It is based on the observation that the cation order‐disorder transformation at work in this mineral system produces cation‐ordered regions, or domains, antiphase with respect to cation positions. Given the fundamental assumption that the so‐called “x phase”, the weakly magnetic phase known to control self‐reversal, is in fact Fe‐rich cation‐disordered boundary material between the two types of cation‐ordered domains, symmetry considerations help eliminate the possibility that the direction of magnetic remanence associated with the boundaries lies parallel to the line of Fe spin. Rather, the net magnetization of the boundary material need be essentially at right angles to the orientation of spin, as is the case for pure hematite. Results of thermomagnetic experiments further suggest that superexchange with the Fe‐rich boundary material produces a femmagnetic structure within each cation‐ordered domain such that its net moment lies along the direction of spin, that is, perpendicular to the net boundary moment. At this stage the moments of each pair of cation‐ordered domains (antiphase in composition) cancel. We propose that reverse TRM arises during cooling in an applied field through rotation of particular Fe spins within each cation‐ordered domain, causing the spin arrangement to become progressively noncollinear. Competing superexchange interactions within the cation‐ordered regions may be responsible for such a spin rotation. According to this model, each cation domain provides a component of magnetization opposite to the net magnetization associated with the boundary material. Provided that there exists in a given grain equal fractions of each type of cation domain, the grain will fully self‐reverse. Synthesized polycrystalline samples as well as rocks containing ferrian ilmenites having compositions in the self‐reversing range will acquire reverse TRM.

Time-stratigraphic framework for the Eocene-Oligocene Mogollon-Datil volcanic field, southwest New Mexico

A time-stratigraphic framework for discontinuously exposed regional ignimbrites in the Eocene-Oligocene Mogollon-Datil volcanic field has been established using correlations aided by 40Ar/39Ar age determinations and paleomagnetic analyses. 40Ar/39Ar age spectra from sanidine separates (25 regional ignimbrites, 85 samples, 97 spectra) yield well-defined plateau ages that are precise (within-sample and within-unit 1σ < ± 0.5%) and agree closely with independently established stratigraphic order. Paleomagnetic remanence directions (404 sites) from individual ignimbrite outflow sheets are generally vertically and horizontally uniform throughout facies ranging from thick (100-500 m), densely welded, proximal ignimbrites to thin (1.5-30 m), unwelded, distal fringes. Between-unit differences in paleomagnetic directions provide useful correlation criteria, particularly for units having ages too close to be resolved using 40Ar/39Ar dating. The Mogollon-Datil time-stratigraphic framework clarifies ignimbrite history and provides improved age control for intercalated lavas and sedimentary rocks. Ignimbrite activity was strongly episodic; outflow sheets were primarily erupted in four discrete pulses representing synchronized activity of two separate cauldron complexes. Activity in the southern complex began at 36.2 Ma near Las Cruces, New Mexico, and subsequently migrated 220 km northwest, culminating in the 28.0 Ma Bursum cauldron. Activity in the northern complex, located west of Socorro, New Mexico, underwent a less defined and more modest 40-km westward migration over its 32.0 to 24.3 Ma life span. The four pulses of ignimbrite activity were (1) 36.2-24.3 Ma, 12 major units, >1,500 km3 total volume; (2) 32.0-31.4 Ma, three major units, >1,500 km3 volume; (3) 29.1-27.4 Ma, nine major units, >6,000 km3; and (4) 24.3 Ma, one major unit. The third and largest ignimbrite pulse was accompanied by extensive rhyolitic dome and flow eruptions in the area between the two main cauldron complexes.

Multicomponent magnetization in paleomagnetic records of reversals from continental sediments in Bolivia

A paleomagnetic study of geomagnetic reversals has been attempted from three continental sections of the altiplano in Bolivia. The first section (Viscachani) shows a unique polarity interval with a magnetization carried by magnetite. In the two other sections (Achocalla and Irpavi) the magnetization is characterized by a complex magnetic mineralogy dominated by magnetic sulphides (mostly pyrrhotite) and magnetite. Thermal demagnetization performed under vacuum was found to be much more efficient than in air in isolating the directions of remanence carried by a low temperature (LT) component due to sulphides and a high temperature (HT) component due to magnetite. The directions of magnetization carried by the two minerals have been compared, with particular attention being paid to records of geomagnetic reversals. At Achocalla, the same polarity pattern is revealed by both components. However, the transitional directions of the LT component are located about 50 cm below the abrupt polarity change recorded by magnetite. Such observations are very similar to those from a recent study of Pliocene marine marls in Sicily [1]. These characteristics are compatible with the results of a simulation which assumed that a very early acquisition of the remanence was carried by the LT component and that a post-depositional reorientation of the magnetite occurred. In the last section (Irpavi), the observed LT and HT components have opposite polarities. Although the magnetic parameters indicate the presence of sulphides, we cannot rule out the hypothesis that the LT component has a secondary origin due to coarse magnetite grains. If we assume that the hypothesis of sulphides is correct, this observation emphasizes the difficulties in establishing the origin of sulphide minerals.

Geophysical investigation of the Honningsvåg igneous complex, Scandinavian Caledonides

The Honningsvåg igneous complex within the Mageroy nappe, northern Norway, has marked magnetic and gravimetric signatures. Palaeomagnetic studies reveal a Siluro-Devonian (Scandian) dual-polarity remanence (NE and up—SW and down; mean declination = 045°, inclination = -38° and a 95 = 8.4°). Rock-magnetic and petrological studies indicate a pure magnetite host for the remanent magnetization component. Magnetic-fabric ellipsoids delineate Dl and D2 Scandian structures affecting the area, and demonstrate an internal reclined fold-structure of the Honningsvåg igneous complex. Forward-modelling demonstrates that a short-wavelength, poúitive aeromagnetic anomaly (c. 700 nT) over the northwestern part of the complex is associated with an area of reverse remanent polarity (SW and down). Conversely, areas of normal remanence polarity (NE and up most common) are associated with magnetic ‘lows’ or ‘quiet’ zones. The gravity field in the vicinity of the complex displays a symmetrical positive anomaly of approximately 200 g.u. (wavelength 15 km). This can be attributed to a density contrast of approximately 0.20 Mg/m 3 between the gabbros of the Honningsvåg complex (2.93) and surrounding metasediments (2.73). An optimal geophysical and geological model demonstrates that the complex has been tilted through 90°, and now has the form of a steeply dipping reclined fold-structure extending to a depth of at least 6 km. Palaeomagnetic remanence acquisition post-dates emplacement of the Magerøy nappe (late Dl), and probably originated during post-Dl uplift. Brittle, upper-crust, D2 deformation which continued into Devonian times, folded the Honningsvåg igneous complex and introduced a small but systematic dispersion in the palaeomagnetic remanence directions. The magnetization is, therefore, post-Dl but pre-D2 in age, and can be termed ‘syn-Scandian’. Palaeomagnetic and isotopic evidence suggests a syn-tectonic magnetization age of c. 410 Ma. The palaeomagnetic pole (N7° and E344°) resembles that from the underlying Kalak nappe complex. It is therefore inferred that the Scandian emplacement of the Magerøy nappe was synchronous with a thermal event which affected the older and underlying Finnmarkian-defor-med Kalak nappe complex.

The Dala dolerites, central Sweden, and their palaeomagnetic signature

Abstract New palaeomagnetic results are presented from nine dolerites from central Sweden. Data are also presented from four porphyry sites and one granite site. The data obtained are compared with previously published data from the area. Based on the directions of the characteristic magnetizations isolated in the various rock units a division into five groups, A-E, has been made. Group A consists of dolerites of late Sveconorwegian (c. 1050–850 Ma) age and group B comprises dolerites belonging to the Central Scandinavian Dolerite Group (c. 1250–1150 Ma). Groups C and D are less clearly defined. ‘Group C, with a shallow negative northerly remanence inclination represents the Tuna dykes (c. 1370 Ma). The D direction, with a shallow positive northerly inclination, may represent an older generation of dolerites. The border between groups C and D is diffuse and may be the result of a prolonged mid-Proterozoic period with dolerite intrusions in Dalarna during which significant apparent polar wander occurred. Group E comprises dolerites with a steep positive inclination close to the direction of the Earth's present field. The porphyries and the granite studied carry group D and E remanence directions. This strengthens the evidence for a D-group generation of dolerites contemporary with the porphyries and the granite, c. 1650 Ma ago.

Lava Block Rotation around Vertical Axis Inferred from Remanence Directions of Miocene Yoka Formation, Southwest Japan.

Paleomagnetic study was made on the lava flows of the Miocene Yoka Formation in the Hokutan Group, Southwest Japan. Although the stable remanence directions are consistent within the same sampling site, the site-mean directions are widely scattered and are distributed along a small circle on an equal area projection. The radius of the small circle is 45.4° and its center locates at the point of D=187.5°, I=81.0°. The shifting of the center by 9.0 degrees is concordant with northward tilting of the Yoka Formation. Average inclination value 44.6° of the Yoka Formation derived from the radius of the small circle is consistent with inclination values in Southwest Japan during 20-30Ma. These results lead to the conclusion that the distribution of remanence directions along a small circle is caused by rotations of lava blocks around a vertical axis after cooling below blocking temperature.

Paleomagnetic evidence for hot pyroclastic debris flow in the shallow submarine Shirahama Group (Upper Miocene‐Pliocene), Japan

The cooling history of coarse andesite blocks erupted and deposited proximally in the shallow marine environment has been investigated by paleomagnetic studies and heat conductivity calculations. Six to 13 samples were cored and measured from six essential andesite blocks 1–5 m in diameter. Thermal demagnetization techniques were used to detect the directional change of remanence through demagnetization steps, each representing different blocking temperatures (Tb). Each sample has three components of remanent magnetization (C1, C2, and C3). C1 is characterized by a Tb of 250°C and an orientation coincident with the present geomagnetic field. C2 has a Tb of 250°–500°C and shows a consistent direction with reversed polarity. C3 has a Tb of 500°–600°C, and its orientation is nearly consistent from samples collected from nearby parts of the same clast and are random from samples from different clasts. On the basis of the remanence direction of each component, the differing Tb, and the petrography of the samples, C1 is interpreted as secondary viscous remanence, whereas C2 and C3 are the partial thermoremanent magnetizations obtained during and immediately after emplacement. C3 was probably acquired while the blocks were rolling or sliding downslope, and C2 formed during continued cooling of the blocks after they came to rest. Heat conductivity calculations suggest that the blocks were emplaced less than an hour after being erupted. If the eruptive vent was only 2–3 km distant, as suggested by field relations, a downslope transport rate of less than 36 km/h is implied.

Flexural flow folding and the paleomagnetic fold test: An example of strain reorientation of remanence in the Mauch Chunk Formation

The relationship between the remanent magnetization and the detailed strain geometry around a first‐order fold in the Appalachian Valley and Ridge Province was investigated to examine whether penetrative strains associated with folding can generate a apparent synfolding geometry from a prefolding magnetization. Paleomagnetic results from the Mississippian Mauch Chunk Formation on both limbs of the Frackville Anticline near Lavelle, Pennsylvania, yield two magnetic components, an intermediate unblocking temperature (300°C–600°C) Kiaman remagnetization and a two‐polarity high unblocking temperature (650°C–680°C) characteristic magnetization. When the magnetic directions are incrementally corrected for bedding tilt, the intermediate‐temperature component is most tightly clustered at 85% unfolding (D=176°, I=3°) and the high‐temperature component is most tightly clustered at 75% unfolding (D=184°, I=27°). Mesoscopic and microscopic structural fabric analyses suggest a strain history that includes a significant component of flexural slip/flow folding. In the coarser‐grained sandstone units, folding has largely been accommodated by slip on bedding, while in the finer‐grained beds, folding has been accommodated by grain‐scale deformation mechanisms such as pressure solution and low‐temperature plasticity. Finite strain measurements, determined from center‐to‐center distances between quartz grains, yield strain ellipsoids consistent with this folding model. Inclination of the characteristic component varies as a function of the magnitude of the finite strain. This variation suggests that the characteristic magnetization has been systematically reoriented with respect to bedding during folding. Remanence directions on the south dipping limb have been rotated to shallower inclinations, while those on the north dipping limb have been rotated to steeper directions causing the prefolding magnetization to appear synfolding. These rotations are in agreement with models of rigid particle rotation in deforming viscous media. Unlike the characteristic magnetization, the secondary component appears to be unaffected by the deformation, and its synfolding behavior is interpreted as the acquisition of a secondary magnetization during Alleghenian folding. These results show that it is important to consider penetrative strains when evaluating the significance of apparent synfolding magnetizations.

Low palaeolatitude of Late Proterozoic glaciation: early timing of remanence in haematite of the Elatina Formation, South Australia

Previous palaeomagnetic study of a ∼ 10 m thick rhythmite member of the Elatina Formation, part of the Late Proterozoic (∼ 650 Ma) Marinoan glacial succession in South Australia, argued strongly for a low palaeolatitude (∼ 5°) of deposition. However, a fold test was thwarted by the approximate alignment of the remanence with the axis of tectonic folding. Here we report results of a fold test on soft-sediment slump folds with a wavelength of 14–22 cm in the Elatina rhythmites, the axes of which are approximately perpendicular to the direction of remanence. Fifty-one standard palaeomagnetic specimens sampled across the slump structure gave α 95 = 3.7° prior to unfolding, α 95 = 1.8° with 67% unfolding, and α 95 = 2.4° with 100% unfolding. The results indicate that the remanence was acquired very soon after deposition; apparently it was acquired prior to soft-sediment slumping and then slightly sheared by the disturbance. The Elatina pole must be considered a virtual geomagnetic pole because the tidal rhythmite member studied represents only 60–70 years of deposition. However, the mean pole position is similar to other Late Proterozoic poles for South Australia, which implies that the very low inclination (< 10°) for the Elatina rhythmites does not record a geomagnetic excursion or reversal but does indeed indicate deposition in low palaeolatitudes. The similarity of many of the Late Proterozoic pole positions is evidence also that inclination error is not significant. The low palaeolatitude of Late Proterozoic glaciation is one of the major enigmas in contemporary Earth science, raising questions concerning the nature of the geomagnetic field, climatic zonation, and the Earth's rotational parameters in Late Proterozoic time.

Reworking characteristics of quaternary pyroclastics, Thera (Greece), determined using magnetic properties

The magnetic properties of unwelded, poorly consolidated Quaternary tephra from Thera, Greece, are examined for the evidence on their emplacement, particularly the extent to which individual clasts have been reworked after initial deposition. Most samples have stable remanences with single component magnetization acquired between 350–450°C (some from 540°C) and 100/150°C, indicating that each of these samples was in a fixed position, relative to the ambient geomagnetic field, when they originally cooled over these temperature ranges. The between-sample directions within each tephra layer are either: (a) mutually consistent, i.e. within measurement and collection error; (b) grouped but more scattered than expected; or (c) randomly scattered. These groupings are interpreted in terms of hot emplacement followed by respectively: (a) no re-working; (b) cold, internal settlement of individual clasts; and (c) extensive cold, reworking. In each case, the percentage of reworked material within any given layer can be estimated from the scatter of the directions of remanence of individual clasts. Such estimates are consistent with some, but not all, volcanological appraisals. This magnetic method provides a simple, inexpensive method for establishing the degree of cold reworking in such unwelded tephra.

A Cretaceous pole from south China, and the Mesozoic hairpin turn of the Eurasian apparent polar wander path

To contribute to the apparent polar wander path (APWP) of the South China Block and Eurasia in general, we collected paleomagnetic samples from Mesozoic red beds around the city of Ya' an (30°N, 103°E) in the western tip of the Sichuan Basin. In this paper we present the results from 373 oriented cores taken from one section representing 3 km of sedimentary rocks. The section is dated with continental ostracods and with a magnetostratigraphic correlation between a densely sampled 272‐m sequence and the polarity time scale, giving an upper Jurassic to Upper Cretaceous or Lower Tertiary age. The remanent direction is remarkably stable throughout the section (D=2.0°, I=34.2°, k=63.1,α95=3.6°, N=26/28 sites). While this fact might suggest that the section has been remagnetized, paleomagnetic and rock magnetic tests indicate that the remanence is primary. The pole position (78.6°N, 273.4°E, dp=2.4°, dm=4.1°) corresponds to a rather low paleolatitude (λ=18.8°±2.4°) but is consistent with other Cretaceous poles from China. If one accepts the Eurasian APWP of Irving and Irving (1982), this result would imply that more than 1000 km of shortening took place between South China and Eurasia, following the acquisition of the remanence. However, there is no geological evidence for this large shortening. We propose that the remanence was acquired within the time corresponding to the tip of the hairpin turn (∼150–50 Ma) in the revised APWP of Besse and Courtillot (this issue). The local geology suggests that the syncline from which the samples were taken has been rotated by 15°±5° counterclockwise, which is reflected in a similar discrepancy between the measured paleodeclination and that predicted by the Besse and Courtillot (this issue) Eurasian APWP. After correcting for this rotation, the pole position is 70.9°N, 225.2°E (dp=2.4°, dm=6.5°). We conclude that Eurasia was fully assembled by the end of the Jurassic and that the Mesozoic Eurasian hairpin turn is a real feature.

Palaeomagnetic Results From the Tibetan Sedimentary Series of the Manang Area (North Central Nepal)

The remanent magnetization of Palaeozoic and Mesozoic carbonates from the Tibetan Sedimentary Series (TSS) of the Manang area north of the Annapurna Range (north central Nepal) has been investigated. the TSS forms part of the tectonic unit between the Indian-Eurasian plate boundary in the north and the Main Central Thrust (MCT) in the south. Apart from a present field direction, three remanence components have been separated: a normal polarity, post-folding remanence (negative fold test) is carried by pyrrhotite (the core corrected mean direction for five sites with 47 specimens from Silurian/Devonian rocks is decl. = 312.9°, incl. = 58.3°, α95 = 17.6°, k = 19.8). the origin of this component is not understood. A more precisely determined, reversed polarity, post-folding remanence (99 per cent negative fold test) is also carried by pyrrhotite (the core corrected mean direction for 12 sites with 229 specimens from Carboniferous/Triassic rocks is decl. = 196.4°, incl. = -65.9°, α95 = 3.2°, k = 183). One site of Lower Carinan age shows—in addition to the pyrrhotite component—a second, stable remanence direction carried by magnetite. the fold test is 99 per cent positive and there is a high probability that the remanence is of detrital origin (the bedding corrected direction for one site with 40 specimens is decl. = 334.3°, incl. = -54.1°, α95 = 4.2°, k = 30.2, palaeolatitude 34.6°S). The direction of the magnetite component coincides with other Triassic palaeodirection data from the literature and the inclination suggests a palaeoposition of the TSS at the northern margin of India at this time. the northern extent of India in the Triassic cannot be reconstructed from the data because of the strong anticlockwise rotation (48°) of stable India since this time. The inclination of the reverse polarity pyrrhotite component is 19° steeper than expected from the maximum northern position of the area. This suggests a regional northward tilt along the MCT which is supported by steep inclinations found in other areas. Declination data for the reverse polarity pyrrhotite component and for the magnetite component deviate clockwise from expected values for the area by 23°-34° (calculated from the Indian APWP). Utilizing Klootwijk, Conaghan & Powell's (1985) rotational underthrusting model, the minimum magnitude of continental underthrusting between the TSS and stable India at the longitude of central Nepal (84°E) has been calculated from the declination of our reverse polarity pyrrhotite component. A possible range of 520–1100 km results from consideration of different tectonic models.

A new Late Cretaceous paleomagnetic pole from the Adel Mountains, west central Montana

North America's apparent polar wander path has been poorly defined between the mid‐Cretaceous and Paleocene reference pole positions. Existing data allowed 13° of apparent polar motion over about 22 m.y. (87–65 Ma) roughly coinciding with the beginning of Laramide deformation (∼80 Ma). We report on a paleomagnetic study of the Adel Mountain Volcanic rocks to refine the North American apparent polar wander path for this interval. The shonkinite rocks of the Adel Mountain Volcanic field are on the eastern edge of the Cretaceous‐Paleocene fold and thrust belt; some of these structures disturb the western edge of the volcanic pile. We obtained two new K‐Ar dates from the Adel rocks. One date, on biotite ( from a shonkinite dike that crosscuts most of the volcanic rocks, is 71.2±2.7 Ma. The other, a whole rock date from a flow deep in the volcanic pile, is 81.1±3.5 Ma. We collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows, dikes, and laccoliths. Positive fold and conglomerate tests, along with alternating field and thermal demagnetization, indicate that our characteristic remanent directions are primary magnetizations acquired before Late Cretaceous to Paleocene thrust belt deformation. Averaging the virtual geomagnetic poles from 26 reliable sites, all of normal polarity, yields a paleopole at 82.2°N, 209.9°E (α95 = 6.80°, k = 18.38). This pole is concordant with the Paleocene reference pole (82.0°N, 170.2°E, α95 = 3.5°, k = 18.6 (Diehl et al., 1983)) and is 11.6° from the Globerman and Irving (1988) mid‐Cretaceous pole at 71°N, 196°E. The youngest information in the Cretaceous stillstand pole is from the Niobrara Formation (Shive and Frerichs, 1974) at about 85–89 Ma. If we take the average age of the Adel Mountain Volcanics to be 76 Ma, then ∼12° of apparent polar motion occurred between 87 Ma and 76 Ma. Thus, rapid apparent polar motion correlates well with the onset of Laramide deformation.

Origin and Modification of Magnetic Fabric in Fine-Grained Detrital Sediment by Depositional and Postdepositional Processes

Glaciolacustrine varved clay of late Wisconsinan age in western New York has stable remanent magnetization and anisotropic magnetic susceptibility (AMS). The results of rock magnetic tests demonstrate that remanence is carried by interacting single domain grains of magnetite, but coarse multidomain grains of magnetite are also present. As in many fine-grained detrital sediments, remanent inclination is anomalously shallow, given the latitude of the area of deposition and the existence of a geocentric dipole field at the time of deposition. The AMS consists of a foliation that is gently inclined to bedding and a weaker lineation in the plane of foliation. Independence of magnetic fabric and direction of remanence can be demonstrated by comparing remanence and AMS at closely spaced sites in the same beds and also between widely spaced sites of different age. The magnetic fabric suggests that deposition is accompanied by transient density currents which align large multidomain grains; magnetic alignment of single domain grains occurs later in a dilute slurry at rest on the surface of deposition, rather than during settling in still water. Compaction after deposition results in decrease of remanent inclination and increase in magnetic foliation. Remanent declination and magnetic lineation are unaffected by compaction.

Magnetic Mineralogy and Diagenesis of the Middle Ordovician Everton Formation, Northern Arkansas

Three remanent directions can be isolated in rocks from the Everton Formation: a relatively low-intensity, primary remanence, carried by apparently detrital magnetite, a higher intensity, secondary remanence of Late Pennsylvanian age, carried by apparently authigenic magnetite; and a recent direction carried as a CRM (chemical remanent magnetization) by authigenic goethite, and as a VRM (viscous remanent magnetization) by magnetite. Hematite is present in some samples, but does not appear to carry a significant remanent signal. The primary component is mostly recoverable from sandy dolomite and dolomitic sandstone that contain a relatively high concentration of accessory detrital minerals. The Late Pennsylvanian overprint is dominant in the lower half of the Everton Formation. Intensity of overprinting is laterally consistent and peaks in intensity correlate approximately with bounding silstone/shale that may have acted to restrict the upward migration of parent fluids responsible for authigenic magnetite. Correlation between lithologic character and magnetic mineralogy permits segregation of the remnant components independent of magnetic directions.

Quaternary Paleomagnetic Studies in Japan.

Various geomagnetic phenomena have been revealed by paleomagnetic and archeomagnetic studies on the Japanese Quaternary. Archeo-secular variations of the geomagnetic field in Japan are clarified for the past 2, 000 years. Fairly big local gradients in the geomagnetic direction are found within Japan.Studies on tephra layers and marine and lake sediments, revealed that several geomagnetic events or excursions occurred during the Brunhes normal polarity epoch. These events and excursions will provide very good time markers to correlate with the middle and late Quaternary strata.Age of paleo-earthquakes can be determined by using remanent direction of sediments squeezed out by ancient earthquakes, because their magnetization recorded the geomagnetic field at the time when the earthquakes occurred.

Possible high-frequency geomagnetic polarity changes in Lower-Middle Jurassic carbonates in Iraq

Palaeomagnetic results from Iraqi standard 2.5-cm Jurassic limestone specimens show weak intensities and directions of remanence that are initially randomly scattered, other than an absence of high inclinations. When these are sliced parallel to the bedding in ∼ 1-cm discs, their intensity increases and their directions show greater stratigraphic consistency. The age of the remanence appears to be primary (early diagenetic) on the basis of their sedimentological properties. The magnetization is unrelated to mineralogy and is interpreted as normal and reversely magnetized components within standard sized samples. This suggests that, at this time, frequent polarity changes took place resulting in similar polarities within stratigraphic zones < 2–3 cm thick, corresponding to at least five polarity transitions per million years.