Mean Direction Of Magnetization Research Articles

Plio‐Pleistocene paleomagnetic secular variation and time‐averaged field: Ruiz‐Tolima volcanic chain, Colombia

AbstractPaleomagnetic results obtained from 47 Plio‐Pleistocene volcanic flows from the Ruiz‐Tolima Volcanic Chain (Colombia) are presented. The mean direction of magnetization among these flows, which comprise normal (n = 43) and reversed (n = 4) polarities, is Dec = 1.8°, Inc = 3.2°, α95 = 5.0°, and κ = 18.4. This direction of magnetization coincides with GAD plus a small persistent axial quadrupolar component (around 5%) at the site‐average latitude (4.93°). This agreement is robust after applying several selection criteria (α95 < 10º; α95 < 5.5º; polarities: normal, reversed, and tentatively transitional). The data are in agreement with Model G proposed by McElhinny and McFadden (1997) and the fit is improved when sites tentatively identified as transitional (two that otherwise have normal polarity) are excluded from the calculations. Compliance observed with the above mentioned time‐averaged field and paleosecular variation models, is also observed for many recent similar studies from low latitudes, with the exception of results from Galapagos Islands that coincide with GAD and tend to be near sided.

Magnetic microstructure and magnetotransport in Co2FeAl Heusler compound thin films

We correlate simultaneously recorded magnetotransport and spatially resolved magneto-optical Kerr effect (MOKE) data in Co2FeAl Heusler compound thin films micropatterned into Hall bars. Room temperature MOKE images reveal the nucleation and propagation of domains in an externally applied magnetic field and are used to extract a macrospin corresponding to the mean magnetization direction in the Hall bar. The anisotropic magnetoresistance calculated using this macrospin is in excellent agreement with magnetoresistance measurements. This suggests that the magnetotransport in Heusler compounds can be adequately simulated using simple macrospin models, while the magnetoresistance contribution due to domain walls is of negligible importance.

Magnetization dynamics in polycrystalline Permalloy and epitaxial Co platelets observed by time‐resolved photoemission electron microscopy

AbstractWe studied the dynamic magnetization response in rectangular polycrystalline Permalloy and also epitaxial Co structures (lateral sizes comprised tens of microns at a thickness of tens of nanometers) during the action of a magnetic field pulse, using time‐resolved X‐ray photoemission electron microscopy with a time resolution of 10 ps. In the case of Permalloy platelets the restoring torque that is necessary for the stroboscopic image acquisition is provided by the Landau flux closure structure representing a minimum of the free energy. We investigated the dynamic response of 90° Néel domain walls. The main results are: the maximum velocity of the domain wall is 1.5 × 104 m/s, the intrinsic frequency of the magnetization change in these structures is estimated to be several Gigahertz. For the case of epitaxial Co platelets grown on Mo(110) the magnetic uniaxial anisotropy with an easy axis along Mo[1$ \bar 1 $0] restores the homogeneous magnetization structure after each field pulse. We observed a rotation of the mean magnetization direction within the first 100 ps of the field pulse. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarized Neutron Reflectometry for the Analysis of Nanomagnetic Systems

In recent years, polarized neutron reflectometry (PNR) has played an essential role for the exploration of magneto- and spintronic structures. Well known systems extensively studied include exchange coupled magnetic superlattices, exchange bias systems between ferromagnetic and antiferromagnetic films, exchange spring valves between soft and hard magnetic films, and more recently magnetic semiconductors and ferromagnetic Heusler alloy films and superlattices. In addition to studies of laterally extended layered systems, neutron scattering has now been applied to the exploration of periodic magnetic arrays, such as stripes and islands on the submicrometer scale. Although the competition with magneto-optics and X-ray resonant magnetic scattering (XRMS) has increased in recent years, there are some advantages PNR offers that are hard to challenge. One of those is the quantitative analysis of the data via fits to theoretical models based on the distorted wave Born approximation (DWBA), which accounts for both specular and off-specular scattering. The second one is spin flip (SF) scattering, which has no counterpart in XRMS. SF scattering probes magnetization fluctuations transverse to the mean magnetization direction and gives access to magnetic roughness and magnetic domain states. Specular and off-specular PNR work is exemplified by most recent work on spintronic materials and on patterned and functionalized magnetic layers.

The British Tertiary Igneous Province: Palaeomagnetism and Ages of Dykes, Lundy Island, Bristol Channel

Summary Of 66 dykes collected 52 gave stable directions: 44 reversed, 4 normal and 4 intermediate. The mean direction of magnetization was N3 - 6 E, + 62 7 (down,) equivalent to a pole position of 82.6 ON, 155.0 E. This is significantly different from the present pole and displays right-handed farsidedness. The bulk, at least, of the dykes fall within the apparent span 45-54 My. Yet they display approximately the same small proportion of normal dykes as do the older dyke swarms of the province which were formed during a different polarity period or periods.

On Detecting Changes in the Earth's Radius

Summary Estimates of the Earth’s radius in the geological past can be made from paleomagnetic evidence. A method appropriate to the spherical environment of the data for dealing with this problem is given, which is applied to Devonian, Permian and Triassic data from Europe and Siberia, yielding estimated radii for these periods of 1.12, 0.94 and 0.99 times the present radius respectively. These estimates are not considered to be significantly different from the present radius. Egyed (1960) has suggested that paleomagnetic data could be used to find probable ancient radii for the Earth; the idea is that the mean direction of magnetization of rocks of the same geological age from widely separated collecting sites should be consistent with a geocentric dipole field. The problem is of interest since it has been suggested on geological and cosmological grounds that the Earth’s radius has changed greatly in the geological past. For example, Egyed (1956, 1957) requires a change of z per cent since the Paleozoic, and Carey (1958) a change of 45 per cent in the same time. Cox and Doell (1961) have applied Egyed’s method of calculation to Permian data from Europe and Siberia and gave an estimated Permian radius of 0.991 times the present radius. In this article a generalization of Egyed’s method of calculation is given, which is applied to Devonian and Triassic data as well as Permian data from this region into which some recent values not available to Cox and Doell have been incorporated, yielding estimates of the ancient radii for these three geological periods not significantly different from the present radius. Triassic and Carboniferous data from the North American continent have also been studied, but as yet there are not enough data from this continent to yield a meaningful result. The method given here, like that of Egyed, is based on the assumptions that during any change in the Earth’s radius, the size of each continent remains constant, and that during the particular geological age being studied, the position of the pole has not changed markedly, together with the usual assumptions involved in paleomagnetic studies; that the Earth’s field at the time in question may be represented by a geocentric dipole, and that it is possible by a study of rocks from a given rock unit to determine the approximate direction of the Earth’s field at that place at a known time. The set of relevant observations from a rock unit consists of four quantities, which are average values for the rock unit as a whole: the mean longitude +$ and latitude A$ of the rock unit, and the declination Di and inclination It of the estimated

Correlated magnetic reversal in periodic stripe patterns

The magnetization reversal in a periodic magnetic stripe array has been studied with a combination of direct and reciprocal space methods: Kerr microscopy and polarized neutron scattering. Kerr images show that during magnetization reversal over a considerable magnetic-field range a ripple domain state occurs in the stripes with magnetization components perpendicular to the stripes. Quantitative analysis of polarized neutron specular reflection, Bragg diffraction, and off-specular diffuse scattering provides a detailed picture of the mean magnetization direction in the ripple domains as well as longitudinal and transverse fluctuations, and reveals a strong correlation of those components over a number of stripes.

Early Proterozoic geomagnetic field in western Laurentia: implications for paleolatitudes, local rotations and stratigraphy

Abstract The east-west trending, post-orogenic subvertical Cleaver Dykes from the Great Bear Magmatic Arc are dated by the U–Pb (baddeleyite) method to be 1740 +5/−4 Ma. They have a mean direction of magnetization of D, I=136.5 and 57.4°, k=64, α95=4.5°, based on samples from 17 diabase dykes, a mean location 65.7°N, 118.03°W, and a corresponding paleopole at 19.4°N, 83.3°W, A95=6.1°. Studies of baked contacts, country rocks distant from dykes, and later intrusions that cut them, show that the dykes became magnetized soon after intrusion. The paleopole is not significantly different from that obtained from the 1759±1 Ma post-orogenic Jan Lake Granite from the Hudsonian Orogen in southeast Saskatchewan. These paleopoles fall among a cluster previously determined from post-orogenic overprints in a wide variety of rocks and geological settings across Laurentia. Their mean is 20.8°N, 94.5°W, A95=5.2°. Collectively they record a widespread remagnetization episode following the Hudsonian Orogeny, during which the polarity of the field remained constant. This single polarity interval (the Cleaver Superchron) has a mean age of approximately 1747 Ma, and its duration is at least 13 million years but is probably much longer. With further refinement, it could become a global statigraphical marker. There are 18 paleopoles from red beds and igneous rocks which record the paleofield in western Laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 Ma during the Hudsonian Orogeny. Over this interval, western Laurentia remained at a low and essentially constant latitude. Reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency; there are numerous magnetochrons which could, in future, assist in regional stratigraphic correlations. On the basis of six studies, we estimate that the reference paleopole for this interval is at 1.1°S, 71.7°W, A95=12.1° (N=6). Paleopoles from the other 12 studies are spread over a 100° arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering Archean Slave and Superior cratons during the Hudsonian Orogeny. These reference paleopoles show that during the Hudsonian Orogeny, western Laurentia was in paleolatitudes of 15–30°. During post-Hudsonian time (ca. 1750 Ma) paleolatitudes were about 20° higher. Paleowind directions inferred by others, indicate that Laurentia during these intervals was in the northern hemisphere.

Archeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits

Archeomagnetic dating developed at St. Maur laboratory has been applied to the Mediterranean volcanoes Etna, Vesuvius and Ischia. The method involves samples from lava flows or high temperature emplaced pyroclasts (welded scoriae, pumice, etc.) weighing 0.5–1 kg each, that allows reaching a precision of a few tenths of a degree on the direction of their thermoremanent magnetization, and hence a semi-angle of the Fisher 95% confidence cone between 0.6 and 1.8° for every volcanic unit. Among the factors reducing precision on the mean magnetic direction, the most important appears to be a distortion of the ambient field induced by magnetization of the cooling lava, which means that a number of samples should be collected over a large area. Age determination is based upon similarity between variation curves of the Direction of Earth’s Magnetic Field (DEMF) reconstructed in France from 120 well-dated archeological sites, and on Italian volcanoes from historically dated eruptions. A total of 63 lava flows and pyroclastic units, such as cinder cones or nuée ardente deposits, are shown to be dated with an overall precision of ±40 years for the last 1500 years, and ±50 to ±100 years from AD 500 to 150 BC, this lesser precision resulting from both an increasing uncertainty on the shape of the DEMF curve and a smaller variation of the DEMF itself. This irregularity of the DEMF path plus an increasing number of ambiguities, related to similarity of the DEMF at different times further into the past, are the most serious limitations of the method. Though well-dated eruptions are known for the last two millennia, retrieval of their products is often misleading and about 50% of volcanics presumed of known date prior to the 17th century are in fact of older age, discrepancies usually reaching several hundreds of years. Owing to good agreement between the DEMF curves of France and southern Italy, the method may confidently be extended to volcanic materials from the whole of Mediterranean Europe, provided there are firm constraints that they were erupted within the last 2100 years.

Intensity and direction of the geomagnetic field on 24 August 1179 measured at Vadum Iacob (Ateret) Crusader fortress, northern Israel

Segal, Y., Marco, S., Ellenblum, R. 2003. Intensity and direction of the geomagnetic field on 24 August 1179 measured at Vadum Iacob (Ateret) Crusader fortress, northern Israel. Isr. J. Earth. Sci. 52: 203–208. Paleomagnetic tests on two archaeological structures from the Crusader fortress Vadum Iacob (Ateret) reveal the use of these structures as an oven and a lime kiln. These fireplaces enable determination of the local direction and intensity of the geomagnetic field on 24 August 1179, when its construction was terminated by the Arab conquest. Stable field directions of the natural remanent magnetization were obtained in 31 specimens from a total of 16 building stones taken from the two archeological structures in the site. We consider the results as a negative stability test, indicating that a temperature above the Curie temperature erased the original in situ magnetization of each building stone when the stones were part of the fireplaces, and a new magnetization was acquired on the day the fire was extinguished. The mean magnetic directions of the two installations are statistically indistinguishable 013o/48o; α 95 = 6.5° and 017o/58o ; α 95 = 9.1o. The paleointensity of the Earth’s magnetic field obtained by the original Thellier double-heating method from 8 basalt specimens from the lime oven is 72 ± 7 μT, about 1.8 times the current field. We demonstrate how the geophysical study of paleomagnetism helped resolve an archaeological question and how the archaeological and historical study helped resolve the dating of a geophysical feature.

Paleomagnetic and geological investigation into southern Sinai volcanic rocks and the rifting of the Gulf of Suez

Two volcanic occurrences from southern Sinai were sampled for paleomagnetic, petrographic and geochemical investigations. 29 oriented samples were taken from six sites of the Middle Jurassic basaltic intrusion at Wadi Budra, southwest Sinai (28°55′N, 33°20′E). After alternating field (AF) demagnetization and tilt correction, all samples exhibit normal magnetization with a mean paleomagnetic direction D=63.3°, I=8.0° and α 95=10.2°. This yields a paleopole position at lat. 25°N, long. 133°E and A 95=8.6°. Also, six oriented blocks (34 core samples) were taken from the Sant Catherine doleritic dike at Wadi Sahl el Raha, south Sinai (28°33′N, 33°55′E). After AF demagnetization all sites showed normal polarity with a mean direction of magnetization D=3°, I=39° and α 95=15.3° and a paleopole position at lat. 83°E, long. 179°E and A 95=16.5°. The paleomagnetic pole of W. Budra is found to be incompatible with poles of similar age from Africa. The cause of this cannot be uniquely determined, but most probably could be referred to local tectonic rotations and/or geomagnetic secular variations affecting the paleomagnetic directions of these samples. In contrast, the Tertiary pole of Wadi Sahl el Raha is consistent with the part of the apparent polar wander path (APWP) of Africa for the time period 20 Ma (Oligo-Miocene age). Petrographical and geochemical investigations revealed that the volcanic rocks of both localities are continental within-plate, rift-related alkali olivine basalt (W. Budra) and olivine dolerite (Sahl el Raha). The primary magnetite is the main opaque mineral present in the samples, which represents the main carrier of magnetization in these rocks.

Magnetic domain structure of micron-size Co line arrays

The magnetic domain structure of lithographic Co line (1μm×280μm×70 nm) arrays was investigated in terms of a shape effect. The strong shape anisotropy along the Co line plays an important role in the formation of the magnetic domain structure. The magnetic stray field distribution above the Co line array was measured by using magnetic force microscope (MFM). As the magnetization of the Co line was saturated along the line, the stripe domain structure was observed. The direction of the magnetic moment was periodically modulated along the mean magnetization direction. The periodicity (λ= 0.8μm) is much longer than that of the magnetic ripple structure (∼10 nm) which is generally observed in the polycrystalline ferromagnetic Co thin film. The stripe domain structure is caused by the competition between the crystalline anisotropy perpendicular to the film plane and the shape anisotropy of the line.

Formation and evolution of the Masirah ophiolite constrained by paleomagnetic study of volcanic rocks

The extrusive rocks of the lower ophiolitic nappe of Masirah Island (Oman) were paleomagnetically studied. Four mean directions of magnetization were isolated, one at low temperature, the others at high temperature. The low-temperature component, found in most samples, corresponds to a recent viscous and / or chemical remagnetization. The high-temperature component, found at three sites in the Centre area, is interpreted as a possible primary magnetization acquired during extrusion or, more probably, as an early remagnetization related to subsequent hydrothermal alteration and indicates formation of the Masirah ophiolite around a paleolatitude of 40°S, close to the present position of the West Somali basin. A second high-temperature component, found in three other sample sites in the Hakl and in the Thumi areas, is probably a chemical remagnetization related to the emplacement of the upper ophiolite nappe in late Maastrichtian to Paleocene times. Finally a last high-temperature component, found only in the two sites from Naft area, could be a post-tectonic remagnetization acquired in Oligocene or Miocene times or represents differential tilting of pillows and sedimentary blocks. The high-temperature components are usually fairly scattered due to the very complex tectonic history of the area. Nevertheless, the relation between the defined paleopoles and the Indian polar wander is unambiguous and the evolution of the Masirah ophiolite from formation up to the emplacement of the second ophiolitic nappe is clearly related to the northward movement of the Indian plate. This new paleomagnetic study rules out the possibility of a common or related origin for the Masirah and the Semail ophiolites.

Correlation of widespread tephra deposits based on paleomagnetic directions: Link between a volcanic field and sedimentary sequences in Japan

Abstract Magnetic measurements were made of the Yabakei and the Imaichi pyroclastic-flow deposits of the late Matuyama Chron in central Kyushu and their correlative co-ignimbrite ashes: the Pink and the Azuki tephra deposits in the Kinki district, and the Ku6C and the O7 tephra deposits in Boso Peninsula. Site mean magnetic directions of the Imaichi deposit are all of reversed polarity with declinations deflected slightly to the east. These directions are consistent with those of the Azuki and the Ku6C ashes. Magnetizations of the Yabakei, the Pink, and the O7 ashes are also consistent with one another. All are characterized by shallow inclinations (about 30°) of normal polarity with slightly westerly declinations. The identical paleomagnetic directions of these widespread tephra confirm correlations that have been proposed mainly on the basis of glass and mineral chemistry, and provide a tephrochronological linkage between the source volcanic area and the lacustrine or marine sequences over distances of at least 1000 km. This linkage allows us to correlate the oxygen isotope record of the Kazusa Group in Boso Peninsula with the alternating marine/non-marine lithology of the Osaka Group in the Kinki district. Radiometric dates from the Yabakei and the Imaichi pyroclastic-flow deposits are concordant with the astronomically-calibrated geomagnetic polarity time scale.

Large rotation of the Easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor

One of the goals of the Pito93 cruise along the perimeter of the Easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study. Using a new orienting device, the Geocompass, 14 oriented blocks were obtained during 6 dives conducted using the Ifremer DSRV Nautile. Paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and AF demagnetizations, and reveals a mean Characteristic Remanent Magnetization (ChRM) direction within each block, with both normal and reverse polarities. After elimination of data from 5 blocks situated outside the microplate, and one block which bears an anomalous direction, we have computed a mean ChRM direction at D m = 49.0°, I m = −39.0° (k = 10.2, α 95 = 19.8 °, n = 7). Although the mean inclination is in agreement with the inclination of the recent dipole magnetic field (D = 0°, I = −43°), the average declination is significantly different from this direction, which we interpret as due to a bulk clockwise rotation of 48.5° ± 11° (after the bivariate average) of the Easter microplate over the past 2–3 Ma. This value is in excellent agreement with previous estimates based on remote geophysical measurements and models.

EPISÓDIOS INTRUSIVOS NO ARCO DE PONTA GROSSA, DETERMINADOS ATRAVÉS DE UM ESTUDO PALEOMAGNÉTICO

This work presents paleomagnetic data on the mafic dyke swarm associated with the tectonic structure known as the Ponta Grossa Arch. It is generally accepted that the dykes were emplaced when this structure were already well developed, during the Lower Cretaceous. The existing faults and fractures were then filled with magmatic material, mainly of basic composition giving rise to the so called Ponta Grossa Swarm. For the paleomagnetic analyses, 127 dykes were sampled which are widely distributed in the whole area of the Ponta Grossa Arch. These dykes cut the Parana Basin sediments, mainly Paleozoic in age, as well as the crystalline basement. About 400 hand-samples and cylinders were collected and oriented by both magnetic and sun compasses. The magnetic stability was investigated by both thermal and alternate magnetic field demagnetization methods. The characteristic remanent magnetization of each sample was identified through Zijderveld diagrams. Normally 3 specimens from each sample were submitted to the proper fields or temperatures. The mean magnetization direction of each dyke was calculated by averaging the data from the corresponding specimens. Statistic parameters associated to the means were calculated by Fisher's statistics (1953). Paleomagnetic directions recorded within each of the areas where the dykes are concentrated: Fartura, Sapopema, Telemaco Borba, Curitiba and Guapiara, show significant differences suggesting that the dykes were emplaced in different intrusive phases. A statistical comparison of the magmatic phases from different areas indicate that frequently not all the areas were active at the same time. It is suggested that the Ponta Grossa Arch was affected by nine main intrusive episodes, from which five took place during a normal polarity geomagnetic interval, three took place a during reversed interval and one occurred when the geomagnetic field presented anomalous directions. This latter episode was only recorded in Guapiara and Fartura. In general each episode affected two or more areas. However, one of the normal field episodes was identified only in Fartura and one of the reversed field episodes affected only Curitiba.

Palaeomagnetic data from the Borrowdale Volcanic Group: volcano-tectonics and Late Ordovician palaeolatitudes

Nineteen palaeomagnetic sampling sites from the Borrowdale Volcanic Group between Borrowdale and Langdale (English Lake District) yield high unblocking temperature magnetization components which postdate the structural tilt, and therefore postdate the formation of the broad block-faulted syncline which characterizes this part of the central Lake District. The mean magnetization direction before tilt correction (Dec: 347.2°, Inc:–61.9°, α 95 : 6.9°) corresponds to a palaeolatitude of 43°S (+ 5°/–7°). The magnetization is associated with pervasive Late Ordovician hydrothermal alteration, and is certainly pre-Devonian based on comparison with available palaeomagnetic data from southern Britain. This implies that the block-faulted synclinal structure of the Borrowdale/Langdale region is pre-Acadian in age, consistent with recent interpretations of Late Ordovician volcano-tectonics based on mapping and volcanological studies.

Late Devonian paleomagnetic age for the Polaris Mississippi Valley-type Zn–Pb deposit, Canadian Arctic Archipelago

The Polaris Mississippi Valley-type (MVT) zinc–lead deposit is located in the Middle to Upper Ordovician Thumb Mountain Formation of the Cornwallis Fold Belt in the Canadian Arctic Archipelago. Paleomagnetic analysis was done on samples from 29 sites in the sphalerite–galena ore, the surrounding dolomitic envelope, and the adjacent host-rock limestones, using alternating-field and thermal step demagnetization, scanning electron microscopy with energy-dispersive analysis, and saturation isothermal remanente analyses. Excluding the modern viscous remanence component, the ore, dolomite alteration, and host limestones carry only a prefolding A remanent magnetization component that resides in single- and pseudosingle-domain magnetite, both as discrete grains and as inclusions in the MVT ore minerals. All evidence indicates that the mineralization and magnetization events were coeval. The mean magnetization direction of D = 145.8°, I = −31.1° (α95 = 5.9°, k = 26.2, N = 24) after tilt and a minor plunge correction gives a Late Devonian pole position of 28.6°N, 121.2°E (dp = 3.7°, dm = 6.6°, N = 24). The results indicate that the ore deposit was formed during the onset of the Ellesmerian Orogeny and tilted later in the orogeny.

Early Triassic magnetic polarity time scale—integration of magnetostratigraphy, ammonite zonation and sequence stratigraphy from stratotype sections (Canadian Arctic Archipelago)

Stratotypes defining the stages of the Early Triassic (Griesbachian, Dienerian, Smithian and Spathian) are located on Ellesmere and Axel Heiberg islands in the northern Canadian Arctic. Ammonite-rich horizons are within a clastic outer shelf-to-slope facies of thick progradational wedges of mudstones and siltstones. Three sections were sampled for magnetostratigraphy and interpreted for transgressive and regressive pulses of sedimentation. Using the ammonite zonation as a guide, the transgressive-regressive cycles and magnetostratigraphies have been correlated among the sections and to the published Triassic sequence stratigraphy time scale, thus enabling definition of the magnetic polarity pattern for the upper Griesbachian to Smithian stages in multiple sections. The magnetic polarity and associated sequence stratigraphy pattern for the lower Griesbachian and for the Spathian were derived from single sections. The Griesbachian and Dienerian stages each have two pairs of normal- and reversed-polarity chrons; the Smithian is predominantly of normal polarity, and the Spathian is predominantly of reversed polarity. This magnetic polarity time scale may help to resolve age correlations of North American redbed facies and to define the Permian-Triassic boundary. After correction for variable structural orientations, the mean directions of magnetization from the three sites converge at 296° declination, 57° inclination ( k = 60, α 95 = 16.5° ; equivalent pole = 41°N, 161°E; paleolatitude = 38°N), which is consistent with the pole derived from nearby Early Permian volcanics and supports a postulated post-Early Triassic, pre-Tertiary counterclockwise rotation of this region with respect to cratonic North America.

High‐latitude paleomagnetic poles from Middle Jurassic Plutons and moat volcanics in New England and the controversy regarding Jurassic Apparent Polar Wander for North America

A paleomagnetic study of Middle Jurassic plutonic and volcanic rocks in New England (White Mountains Magma Series) yields high‐latitude pole positions for North America. High unblocking temperature, moderate to high coercivity magnetizations of normal polarity have been isolated in three plutons (White Mountains batholith, Mount Monadnock, and the Belknap Mountains; mean age ∼169 Ma), but the mean pole (88.4°N, 82.1°E, A95 = 6.1°) is not distinguishable from the geographic axis and therefore the hypothesis that the plutons have been contaminated by recent field overprints can not be rejected. However, a dual polarity, high unblocking temperature, and high coercivity magnetization isolated from the Moat volcanics (169 Ma, Rb‐Sr age) was apparently acquired soon after caldera collapse and tilting, at about the time of intrusion and cooling of the Conway granite (reported ages K‐Ar biotite, 168 Ma; zircon fission track, 163 Ma). The Moat volcanics pole position (78.7°N, 90.3°E, dp = 7.1°, dm = 10.2°) calculated using the mean magnetization direction of reversed polarity (the Cr component) falls at high latitude but is distinguishable from the spin axis. Moreover, published Middle Jurassic paleomagnetic poles from Gondwana (Africa, Australia, and East Antarctica) transferred to the North American reference frame also suggest a high‐latitude Middle Jurassic pole position for North America, in agreement with the Moat volcanics pole. The new evidence for a Middle Jurassic loop to high latitudes in the North American apparent polar wander path conflicts by 15°–20° with some key published Jurassic reference poles (e.g., the Newark Trend N2 and the Corral Canyon poles) used to constrain current paleomagnetic Euler pole (PEP) apparent polar wander paths for the Jurassic. We suggest that a plausible explanation for the discrepancy is that the N2 and Corral Canyon magnetizations are in fact secondary and were acquired after tilting. The hypothesis that the North American apparent polar wander path ventured to high latitude in the Middle Jurassic requires further testing, however the results of this study already suggest that the path may be more complicated than that proposed by recently published PEP studies.