Mean Pole Position Research Articles

A revised magnetostratigraphy for the Mid-Proterozoic Gardar lava succession, South Greenland

New palaeomagnetic results from the ca. 1300 Ma Gardar lava succession within the Eriksfjord Formation of South Greenland are presented. This study incorporates stepwise thermal demagnetisation of 256 samples from 66 lava flows from all three lava groups in this formation, accompanied by principal component analysis. The results identify two complete reversals (N → R → N) in the upper lava group and two (R → N and N → R) in the lower group, with a possible palaeoexcursion in the lower group. A minimum of two further reversals are predicted in the time interval represented by the sandstone units intercalated with the lava flows. Both complete reversals in the upper group exhibit inclination asymmetries of 30°. This degree of asymmetry is satisfied by a geomagnetic field model equivalent to a dipole axially offset by 570 km. However, a more complex source is not precluded by the present evidence. Four new pole positions are calculated for the Gardar lavas, two for the upper lava group and one each for the middle and lower groups. The middle lava group shows an anomalous mean direction and pole position, which is unrepresentative of the remainder of the lava succession and is thought to record a virtual geomagnetic pole representative of a transitional field direction. The presence of a non-axial geocentric dipole field means that the use of the Laurentian apparent polar wander path for dating the new poles is not valid.

Phanerozoic reconstructions: What and how do we know it

Paleobiogeography will play an important role in furthering our understanding of global change, particularly with respect to providing constraints on past climates. As a result of this there is increasing emphasis within the paleontological community on the past spatial distributions of faunas, extinctions, and originations. Thus there is an increasing interest in reconstructions of the past distributions of continents, so that maps and paleo-latitudes of faunas, extinctions or originations can be determined. This results in an increasing need to understand the state of the art of global plate reconstructions through the Mesozoic and Cenozoic, and global continental reconstructions through the Paleozoic, i.e. what we know and how well do we know it. The talk will summarize our present understanding of Mesozoic and Cenozoic plate motions using computer animations followed by a brief summary of the present understanding of individual continental apparent polar wander paths (APWP's) as well as motions in the global frame. Global and continental mean poles at approximately 20 Ma intervals have been computed for all of the major continents of the Phanerozoic using data from Van der Voo (1992) and plate motions derived from a detailed analysis of Mesozoic and Cenozoic seafloor spreading data. Comparison of mean continental poles reconstructed into the same reference frame shows that although there is general agreement, statistically different pole positions are quite commonly observed. This is particularly true for the Late Cretaceous and pre-Late Jurassic of the northern Hemisphere, and has resulted in some controversy concerning appropriate Atlantic fits. The differences in predicted (or reconstructed) paleo-latitudes are not trivial, particularly for areas of esatern Gondwanaland such as Australia and New Zealand, and are not easily visualized from a simple comparison of mean pole positions. A final focus will be with regards to reconstructions of Asia. Comparison of recently published Permian global reconstructions in the McKerrow and Scotese (1989) volume demonstrates that vast differences of interpretation persist and that paleontologists among others cannot accept exisiting reconstructions uncritically.

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.

An Upper Carboniferous-Lower Permian (280 Ma) palaeomagnetic pole from the western margin of the Bohemian Massif

In the area of the drill site of the German Continental Deep Drilling Project KTB on the western margin of the Bohemian Massif 15 sites of Uppermost Carboniferous to Lowermost Permian rocks (quartz porphyries, granites and a siltstone, geographic mean coordinates: 12°E, 49.8°N) have been studied in order to obtain a reliable pole position for this time interval. The investigated rocks are syngenetic or only slightly younger than the youngest granites of the Fichtelgebirge for which a K/Ar and Rb/Sr age of 280 Ma has been determined by Besang et al. (1976). While four sites (quartz porphyries) showed too weak intensities of the natural remanent magnetization NRM, the results of two sites with otherwise good rock magnetic properties had to be discarded because of uncertain tectonic position in a shear zone. The remaining nine sites have all reversed polarity as expected for a remanence age within the reversed Kiaman interval and a mean pole position at 165.9°E, 42.3°N with N = 9, R = 8.8776, A95 = 6.4°. This is in good agreement with the mean Upper Carboniferous-Lower Permian European pole position of Stable Europe according to Irving & Irving (1982) and of the Bohemian Massif (Krs 1968). Rock magnetic investigations revealed that the ChRM of the siltstone is due to primary magnetite, while the mostly single-component characteristic remanent magnetization ChRM of the magmatic rocks is carried by primary magnetite and by primary haematite produced by syngenetic high-temperature oxidation of primary magnetite.

Paleomagnetic results from Mesozoic strata in the Mérida Andes, Venezuela

We report paleomagnetic results from Jurassic through Cretaceous sedimentary rocks (nine sites) in the Mérida Andes, Venezuela. Detailed thermal and alternating field demagnetization yielded well‐grouped directions of both polarities. The mean of the normal directions is statistically identical to the mean of the reversed directions and the site means clusters better after structural correction, both at the 95% significance level, indicating that the primary magnetization has been isolated. The mean pole position lies at 86.5°N, 151.1°E, A95 = 5.6°. Combining this pole position with 10 other poles from the southern part of cratonic South America establishes a mean pole for the Late Jurassic through Cretaceous at 87.6°N, 211.0°E, A95 = 3.3°. These results imply, within the error limit of the paleomagnetic data, that South America is indeed one rigid plate and that the motion of this plate was restricted to east‐west translation since the opening of the South Atlantic Ocean.

A palaeomagnetic study of the Basal Gneiss Region and Jotun Nappe Complex, central-southern Scandinavian Caledonides

SUMMARY In the southern sector of the Scandinavian Caledonides a metamorphic gneiss complex (the Basal Gneiss Region or BGR) underthrusts a nappe complex of Precambrian granulites (the Jotun Nappe) in an assemblage collectively recording orogenic events culminating in the Scandian (Upper Silurian and Lower Devonian) episode. Palaeomagnetic study of a large regional sample spanning the combined outcrop identifies properties strongly controlled by metamorphic retrogression to amphibolite and greenschist facies. Low blocking temperature (Ibt) components in the BGR have E+ directions with a mean of D = 69° 1 = 60° (N = 19 sites,α95= 7.0°); sporadic higher blocking temperature components define a dual polarity northeast negative/southwest positive (NE - /SW+) axis with a mean of D = 42°, 1 = -24° (20 sites, α95= 8.1°). Granulite facies rocks in the Jotun Nappe yield a high-quality palaeomagnetic record reflected in high $LQn values and dominated by medium and high blocking temperature (hbt) components. A dual polarity (‘A’) axis defines a palaeofield migration from NE+ to E+ during uplift and cooling of this terrane with a predominance of normal polarities. A smaller population of Ibt (‘B’) components show E+ directions distributed along the Mesozoic palaeofield migration path for Eurasia, and probably acquired during brittle tectonic events of this age. ‘A’ magnetizations from 86 sites define mean south pole positions between 301°E, 2°N and 325°E, 10°S linked to uplift-related cooling following the climactic Scandian orogenic episode and dated at 420–400 Ma from the collective geological and radiometric evidence. Both the outward and return segments of an APW loop may be represented. The polar swathe correlates with (i) a higher blocking temperature component from the BGR, (ii) the uplift magnetization record in the western orthotectonic Caledonides of Scotland, and (iii) primary magnetizations from the Siluro-Devonian molasse facies in Britain. This study shows that the westerly extension of the APW path identified by British Siluro-Devonian data is not peculiar to that crustal segment but applies to continental Europe, and presumably to Laurentia, as well. The pattern of remanence acquisition within the Caledonides suggests contrasting zones of thermal and thermochemical remanence, with the latter probably linked to fluid migration consequent upon orogenic loading.

No relative rotation detected between Corsica and Sardinia

Forty dikes of Upper Paleozoic age have been investigated paleomagnetically on both sides of the Straits of Bonifacio to detect a possible rotation of Sardinia relative to Corsica. A primary component of magnetization has been isolated after thermal and alternating field cleaning. Eleven sites in Corsica yield a paleomagnetic mean of D = 134.7, I = −11.3 ( α 95 = 6.8), while 11 sites from Sardinia showed a paleomagnetic mean of D = 132.7, I = −1.6 ( α 95 = 7.3). The two directions are not significantly different at the 95% confidence level. No relative rotation between Corsica and Sardinia occurred across the Bonifacio Straits and the two islands maintained the present spatial relationship since the Upper Paleozoic. A mean paleomagnetic direction of D = 133.7, I = −6.5 ( α 95 = 5.1) for the Sardo-Corso Massif yield a corresponding virtual mean pole position at 249.4°E, 33.7°N coincident with Upper Paleozoic data of the Mediterranean region. This data implies about 70° of counterclockwise rotation of the block respect stable Europe.

Paleomagnetic evidence for Post‐Jurassic stability of southeastern Mexico: Maya Terrane

The tectonic evolution of southeastern Mexico has been a subject of major controversy, not only in regard to past geometry but also in the timing of proposed geological events as well. For the past 10 years, most, if not all, investigators agree that the Gulf of Mexico Basin was formed by Late Jurassic time and that the Maya Terrane was in its current location prior to the Cretaceous. In order to gain further insight into the drift history of the Maya Terrane we have undertaken a paleomagnetic study of the uppermost Jurassic‐Lower Cretaceous (Tithonian‐lower Neocomian?) San Ricardo Formation in southeastern Mexico, at 93.7°W, 16.8°N. The sampling site is located east of the Isthmus of Tehuantepec, on the southwest side of the Maya block, at the base of the Yucatan Peninsula. A suite of 133 samples was collected in stratigraphic succession from a 114‐m‐thick sequence of red shales and sandstones near Cintalapa, Chiapas, Mexico. After progressive thermal demagnetization of all samples at six steps from 350°C to 630°C, 89 samples were selected for final paleopole analysis on the basis of their magnetic stability. Four different polarity intervals were observed, the sequence being from bottom to top: N, R, N, R which assists in the assessment of the reliability of the observations. The mean pole position obtained, 160.0°E, 69.8°N, agrees with the mean pole position of the upper part of the Morrison Formation of Colorado, a unit of virtually identical age. These results indicate that no discernible rotation or displacement of the Maya block has occurred since at least early Neocomian times.

Reconstruction of the Laurasian and Gondwanan segments of Permian Pangaea

Abstract A reappraisal of the geological and geometrical constraints on the fits of the continents around the Atlantic, Indian and circum-Antarctic oceans is used as a base upon which to re-examine Early and Late Permian palaeomagnetic data. The palaeomagnetic poles from each of the three main Late Palaeozoic palaeo-continents: Eurasia, North America, and Gondwana define statistically well determined means for the Early and Late Permian. When these poles are reconstructed, the means, and their associated A 95 cones of confidence do not superimpose. The lack of superposition of Late Palaeozoic palaeomagnetic poles on Wegener-style Pangaea reconstructions based on Mesozoic and younger seafloor spreading geometries has been noted for some time. Most solutions to this problem have involved alternative 'tight fits' of the Pangaean continents for the Late Palaeozoic. Solutions invoking the non-dipole behaviour of the magnetic field have also been proposed. We examine the geometric consequences of the 'tight fit' Mesozoic-based Pangaean reconstructions that allow better fit of the palaeomagnetic data and conclude that the magnitudes of overlap (650-950 km) required by these modifications are not compatible with the geological and tectonic evidence from these regions. We prefer to use a 'looser' Pangaea fit that does not necessarily result in the superposition of palaeomagnetic poles. However, when the individual poles from Europe, North America, and Gondwana are combined into a global mean pole, it is observed that both the 'tight' and the 'loose' fit yield virtually indistinguishable mean pole positions. Considering the strong geological and geometrical arguments against the palaeomagnetically derived fits and the lack of a clear statistical difference between the two we use our revised Pangaean reconstruction to determine palaeo-latitudinal framework of the Eurasian, North American, and Gondwanan segments of Pangaea for the Early and Late Permian. The validity of these palaeo-latitudinal reconstructions is tested with climatically sensitive floristic and lithological data by Ziegler, and found to be satisfactory.

Paleomagnetism of Upper Carboniferous limestones from the Carnic Alps

Limestones of Stephanian age belonging to the Carboniferous-Permian sedimentary sequence of the Auernig Group at Passo Pramollo (Carnic Alps) were investigated by a paleomagnetic study. These limestones represent the late-tectonic molassic stage of the Hercynian orogeny in the Paleozoic chain of the Carnic Alps. After a preliminary sampling at twenty sites, unstable magnetization was found to affect the remanence at eight sites. Of twelve sites studied in detail (about 180 specimens), 10 sites had a mean paleomagnetic direction of D = 141.8° and I = −8.1°, and α 95 = 17.8° after cleaning and corrections for bedding attitudes tilted by subsequent, mainly Alpine tectonics. The corresponding virtual mean pole position lies at 243.3°E, 36.2°N, coincident with data of comparable age from Morocco, and is located east of the African poles of Late Carboniferous to Permian age. In addition, the pole from the Pramollo limestones is also comparable with the younger Permian poles from the Southern Alps and from most Mediterranean crustal blocks, showing that the area was also rotated counterclockwise with respect to Europe. Paleogeographically, the area of the Carnic Alps already belonged to the Southern Alpine domain by Upper Carboniferous times, and was located in the Northern Hemisphere at that time.

Palaeomagnetic correlation and dating of mineralization and intrusive events

In the mining district of Santa Eulalia, Chihuahua, northern Mexico, the mineralization (consisting of simple massive concentrations of lead-zinc-iron sulphides and oxides confined in largely unaltered Cretaceous limestones) has traditionally been related to several intrusive emplacement events that occurred during the Tertiary. However, the relative age relationships of the intrusives, volcanics and mineralization have proved difficult to establish. In this work we report palaeomagnetic results which permit definition of the mean relative age relationships for the igneous units and their correlation with the mineralizing event. The mean directions and pole positions for the intrusives, mineralization and volcano-sedimentary cover (capping series) indicate a Tertiary age (Eocene-Miocene). The intrusives show normal, intermediate and reverse polarities, suggesting several emplacement events or a long period of intrusion. The direction and polarity of the remanent magnetization for the mineralization correlates with those for the felsites, which occur as a complex set of sills and dykes, and they are therefore penecontemporaneous with the mineralization.

Paleomagnetic study of the Cretaceous Morelos Formation, Guerrero State, southern Mexico

Paleomagnetic results are reported for a ~ 30 m thick limestone section of the Albian-Cenomanian Morelos Formation, Guerrero State, in southern Mexico (−17.75 °N, 99.50° W). Seventy-two specimens from 35 samples were subjected to detailed alternating field (AF) or thermal demagnetizations. Most specimens were stable and presented a single magnetization component remaining after removal of low-coercivity, low-blocking temperature components. End-point and vector analyses give a mean direction and pole position after structural correction (dip ~ 14°) of D = 332.7°, I = 46.8°, k = 215, α 95 = 4° and 63° N, 198° E ( dp = 3, dm = 5), respectively. The pole position agrees with mid-Cretaceous poles reported for cratonic North America, suggesting that the area was already keeping a similar latitudinal position relative to the North American craton by mid-Cretaceous time. Most samples present normal polarity, as expected within the Cretaceous normal interval, but four samples from a single bed present intermediate and reverse polarity, suggesting the occurrence of a reverse event during the late Albian-earliest Cenomanian.

Random walk of the Earth's pole related to the Chandler wobble excitation

The equation governing the polar motion shows that the polar secular drift and the Chandler wobble amplitude are related to each other. In particular, a drift of the mean pole position comes out as a consequence of the maintenance of the Chandler wobble by possible step perturbations of the Earth's inertia tensor. The minimum excitation functions necessary to explain the Chandler wobble amplitude variations for the period 1901—84 are derived from the Chandler term, with the hypothesis that the excitations follow a uniform random distribution in time. It is shown that they have the statistical properties of the steps of a two-dimensional random walk. These functions are then used to derive, from a statistical simulation, a lower limit of the secular drift which may result from the excitation of the Chandler wobble. The drift generated by the random walk is of the same order of magnitude as the observed secular drift for the period 1901—84, but their time dependence is different. This indicates that the observed secular drift cannot be explained as the consequence of an excitation of the Chandler wobble by random steps of the Earth's inertia tensor. However, the possible contribution of the Chandler wobble excitation to the polar drift has to be taken into account when other mechanisms, such as lithospheric rebound related to deglaciation, are proposed.

Magnetostratigraphy, paleomagnetism, and remanence acquisition in the Triassic chugwater formation of Wyoming

The magnetic reversal sequence determined at five sections of the Chugwater Formation can be correlated over a distance of more than 200 km. A detailed study of 203 cores constituting a grid of samples taken from a normal to reverse transition zone at the Red Grade section shows that the transition is characterized by a fairly smooth 180° direction change accompanied by an intensity decrease in about 80 cm of section. The transition occurs in the same way in all vertical columns within the grid and in other columns at the same stratigraphic level up to 80 m from the grid and appears identical to the transition reported by Herrero‐Bervera and Helsley at the same lithologic horizon about 1 km from the grid. Thermal and chemical demagnetization of grid samples removes a secondary remanence in the general direction of the earth's present magnetic field and does not reveal the presence of normal and reverse components in the same sample. The transition appears to have been magnetized during a reversal of the earth's field. Samples probably became magnetized in a few hundred years or less while they were close to the deposition surface. Paleomagnetic pole positions for all five sites are very closely clustered, and there appears to be no consistent long‐term polar wander from bottom, to top in the sections. The mean pole position (46.6°N, 113.5°E; α95 = 1.88°) is significantly different from pole positions for the Early Triassic Moenkopi Formation from the Colorado Plateau, suggesting that post‐Triassic rotation has occurred between the plateau and the basement uplifts of Wyoming.

Paleomagnetism and magnetic fabric of the Gallen massive sulfide deposit, Rouyn-Noranda, Canada

Twenty oriented drill cores were collected from the outcrop of the Gallen massive sulfide deposit in the Rouyn-Noranda area in order to investigate remanent magnetization and anisotropy of magnetic susceptibility in the deposit. The purpose was to obtain inforrnation on the emplacement of the deposit. Electrical resistivity is high enough to allow 60-Hz alternating fields (AF) to be used for stability testing of the remanent magnetization. For thermal demagnetization, the heating chamber was first evacuated and then filled with argon. Alternating field demagnetization in a peak field of 30 mT yielded well-grouped stable directions carried by relatively high coercivity magnetite. The mean direction and pole position are: 137, +50, and 43 degrees W, 2 degrees S, dm=9 degrees , dp=6 degrees , respectively. The pole position falls on the 1,750- to 1,700-m.y. part of the apparent polar wander curve for North America, suggesting that the magnetization was acquired after the probable Archean formation of the deposit. The anisotropy of susceptibility (generally

Apparent polar wander paths carboniferous through cenozoic and the assembly of Gondwana

Directions of magnetization observed from rock-units from a given continental block are conveniently summarized by a path of apparent polar wander (apw) relative to that block. This article documents apw paths for the major continental blocks spanning the interval from Late Carboniferous (approximately −300 Ma) to the present. It is shown that by passing moving windows of 30 or 40 Ma duration over the data an apw path can be obtained with little or no loss of features through smoothing. It also is shown that mean pole positions obtained using Fisher's statistics or by calculating modes do not differ significantly. In the analysis ‘poor’ results are screened out by applying certain minimum reliability criteria. Unit weight is then given to each of the remaining results. No attempt has been made to weight the results differentially because of the risk of added subjectivity.

Geomagnetic excursion in late‐glacial basalt outcrops in south‐western Iceland

Three hills in the active volcanic zone of the Reykjanes peninsula, SW‐Iceland, are largely composed of volcanic tuffs and thin lava flows having stable, shallow reverse directions of magnetization. These hills are most probably contemporaneous and of late Wisconsinan age. Their mean virtual pole position is at 254°E, 12°S. Along with a geomagnetic event of similar age recently discovered elsewhere in SW‐Iceland, this excursion may define a useful stratigraphic horizon in studies of the Mid‐Atlantic ridge crest in Iceland.

Analysis of star pair latitudes

Star pair latitude observations form the basis for the pole positions reported by the International Polar Motion Service (IPMS). The IPMS processes these observations to produce a mean pole position. However, the time series of observations contains high‐frequency information which is lost in the calculation of the mean pole. In this study, 2931 star pair observations are analyzed. A possible large excitation at one cycle per solar day is observed. The average power level in the frequency band of the tesseral tides is seen to be high, although the peaks cannot be conclusively identified as tidal phenomena.

Polar Motion through 1977 from Doppler Satellite Observations

Doppler observations of Navy Navigation Satellites have been used to compute pole positions on a daily basis since 1969. Limited results exist for the period 1964 to 1969. Based on Doppler observations from four or five satellites, the standard error for a five-day mean pole position is less than 20 cm. Comparisons are made between BIH, IPMS and ILS results and those obtained from Doppler. It is shown that the six years of reliable Doppler data since 1972 contribute little in finding the Chandler period. Using observations from the three astronomical sources over 12 years yields a Chandler period of 432.0 ± 0.2 days.

Palaeomagnetism of the Cretaceous Pirgua Subgroup (Argentina) and the age of the opening of the South Atlantic

Palaeomagnetic data for the Cretaceous Pirgua Subgroup from 14 different time units of basalts and red beds exposed in the north-western part of Argentina (25° 45′ S 65° 50′ W) are given. After cleaning all the units show normally polarized magnetic remanence and yield a palaeomagnetic pole at 222° E 85° S (dΦ= 7°, dχ= 10°). The palaeomagnetic poles for the Pirgua Subgroup (Early to Late Cretaceous, 114–77 Myr), for the Vulcanitas Cerro Rumipalla Formation (Early Cretaceous,<118 Myr, Valencio & Vilas) and for the Pocos de Caldas Alkaline Complex (Late Cretaceous, 75 Myr, Opdyke & McDonald) form a ‘time-group’ reflecting a quasi-static interval (mean pole position, 220° E 85° S, α95= 6°) and define a westward polar wander in Early Cretaceous time for South America. Comparison of the positions of the Cretaceous palaeomagnetic poles for South America with those for Africa suggests that the separation of South America and Africa occurred in late Early Cretaceous time, after the effusion of the Serra Geral basalts. The K-Ar ages of basalts of the Pirgua Subgroup (114 ± 5; 98 ± 1 and 77 ± 1 Myr) fix points of reference for three periods of normal polarity within the Cretaceous palaeomagnetic polarity column.