Dispersion Of Virtual Geomagnetic Poles Research Articles

Paleosecular variation and the average geomagnetic field at ±20° latitude

We assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on ±20° latitude: Hawaii, Mexico, the South Pacific, and Reunion. We investigate geomagnetic field behavior over the past 5 Myr and address whether geographical differences are recorded by our data set. We include inclination data from other globally distributed sites with the ±20° data to determine the best fitting time‐averaged field (TAF) for a two‐parameter longitudinally symmetric (zonal) model. Values for our model parameters, the axial quadrupole and octupole terms, are 4% and 6% of the axial dipole, respectively. Our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole (GAD) field. Our estimated octupole term is larger than that from normal polarity continental and igneous rocks, and oceanic sediments, but consistent with that from reversed polarity continental and igneous rocks. The variance reduction compared with a GAD field is ∼12%, and the remaining signal is attributed to paleosecular variation (PSV). We examine PSV at ±20° using virtual geomagnetic pole (VGP) dispersion and comparisons of directional distributions with simulations from two statistical models. Regionally, the Hawaii and Reunion data sets lack transitional magnetic directions and have similar inclination anomalies and VGP dispersion. In the Pacific hemisphere, Hawaii has a large inclination anomaly, and the South Pacific exhibits high PSV. The deviation of the TAF from a GAD contradicts earlier ideas of a “Pacific dipole window,” and the strong regional PSV in the South Pacific contrasts with the generally low secular variation found on short timescales. The TAF and PSV at Hawaii and Reunion are distinct from values for the South Pacific and Mexico, demonstrating the need for time‐averaged and paleosecular variation models that can describe nonzonal field structures. Investigations of zonal statistical PSV models reveal that recent models are incompatible with the empirical ±20° directional distributions and cannot fit the data by simply adjusting relative variance contributions to the PSV. The ±20° latitude data set also suggests less PSV and smaller persistent deviations from a geocentric axial dipole field during the Brunhes.

Paleomagnetic results from the Snake River Plain: Contribution to the time‐averaged field global database

This study presents paleomagnetic results from the Snake River Plain (SRP) in southern Idaho as a contribution to the time‐averaged field global database. Paleomagnetic samples were measured from 26 sites, 23 of which (13 normal, 10 reverse) yielded site mean directions meeting our criteria for acceptable paleomagnetic data. Flow ages (on 21 sites) range from 5 ka to 5.6 Ma on the basis of 40Ar/39Ar dating methods. The age and polarity for the 21 dated sites are consistent with the Geomagnetic Reversal Time Scale except for a single reversely magnetized site dated at 0.39 Ma. This is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records. Combining the new data from the SRP with data published from the northwest United States between the latitudes of 40° and 50°N, there are 183 sites in all that meet minimum acceptability criteria for legacy and new data. The overall mean direction of 173 normally magnetized sites has a declination of 2.3°, inclination of 61.4°, a Fisher concentration parameter (κ) of 58, and a radius of 95% confidence (α95) of 1.4°. Reverse sites have a mean direction of 182.4° declination, −58.6° inclination, κ of 50, and α95 of 6.9°. Normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95% confidence level. Virtual geomagnetic pole dispersion was found to be circularly symmetric, while the directional data were elongate north‐south. An updated and corrected database for the northwestern U.S. region has been contributed to the Magnetics Information Consortium (MagIC) database at http://earthref.org.

Paleomagnetism and 40Ar/39Ar Chronology of Lavas from Meseta del Lago Buenos Aires, Patagonia

As part of a larger project investigating paleosecular variation in South America, 36 paleomagnetic and geochronology sites were sampled on Meseta del Lago Buenos Aires (47°S, 289°E), 200 km east of the active plate boundary in southern Patagonia. Basaltic lavas sampled range from late Miocene to late Pleistocene yet most of the individual lavas are younger than 3.3 Ma. Thirty‐eight isochron ages determined via 40Ar/39Ar incremental‐heating experiments frame a detailed stratigraphy. The isochron ages range from 66.9 ± 4.1 ka for scoria cones and a youthful lava flow in the Rio Pinturas valley to 10.12 ± 1.35 Ma for thick plateau‐forming lavas exposed along the SE edge of the Meseta. The last 3.3 myr is characterized by seven episodes of volcanism at ca. 3.2–3.0 Ma, 2.4 Ma, 1.7 Ma, 1.35 Ma, 1.0 Ma, 750 ka, 430–330 ka, and <110 ka. The bulk of lavas forming the surface of the Meseta erupted in the last 1.7 my. All sites have stable magnetization, and after step demagnetization using either thermal or alternating field techniques, yield characteristic directions held by magnetite and/or titanomagnetite. Ten sites have distinct transitional directions (defined by pole latitudes <55°), and associated ages indicate possible connections to known reversals within the Matuyama Chron, including the onset of the Jaramillo subchron (1.016 ± 0.01 Ma), the Cobb Mountain subchron (1.25 ± 0.03 Ma), the Ontong‐Java 1 event (1.37 ± 0.03 Ma), and the termination of the Olduvai subchron (1.72 ± 0.02 Ma). Remaining sites are divided into normal (14 sites) and reversed polarity (12 sites). At 95% confidence mean normal and reversed directions overlap. The mean direction for 26 sites is I = −63.0°, D = 3.4°, α95 = 5.4°, which is indistinguishable from the expected geocentric axial dipole. Paleosecular variation, measured by the dispersion of virtual geomagnetic poles about the rotation axis, is higher than expected at 20.0°. Model G, using data for the past 5 Myr, predicts a dispersion of 17°. This discrepancy may be due to true dispersion of the field in southern South America or it may be an artifact of inadequate sampling.

Paleosecular variation from Easter Island revisited: modern demagnetization of a 1970s data set

Sixty-two Brunhes age lava flows from Easter Island in the Eastern Pacific Ocean, collected and processed for paleomagnetic and secular variation studies in the 1970s, have been reprocessed to meet current standards of demagnetization. Alternating field demagnetization in 7–10 steps was performed on all samples previously undemagnetized and characteristic directions determined from principal component analysis. The final data set gives a mean direction of I =−46.1° and D =357.4° with α 95 =3.9° Eliminating sites with 95% confidence circles greater than 10.0° gave an angular dispersion of virtual geomagnetic poles (VGPs) of 14.1° with respect to the rotation axis. This value is slightly larger than that determined prior to detailed demagnetization, and agrees with values predicted for this latitude by Model G. There is no compelling evidence from this study supporting the existence of a secular variation low in the southeastern Pacific region.

Noise in the quiet zone

We have carried out a detailed paleomagnetic investigation of two stratigraphically overlapping sections from the Scaglia Bianca Formation (∼85–89.5 Ma) in the Umbria–Marche area in central Italy. Sampling was conducted over 32 m and 7 m intervals at La Roccaccia and Furlo respectively. After AF cleaning the majority of specimens show the expected normal magnetic field orientation, however a number of specimens are directionally anomalous. Some of these deviant specimens are accompanied by apparent spikes or dips in normalized intensity. A detailed investigation of rock magnetics shows that most of these deviations are not a sign of excursionary geomagnetic field behavior, but rather correspond to specimens with distinct rock magnetic characteristics and are therefore rock magnetic ‘noise’. Such specimens should not be interpreted as records of the geomagnetic field. Our experience suggests that detailed rock magnetic and magnetic fabric analysis should be done on all anomalous directions prior to interpreting them as geomagnetic field behavior. After elimination of rock magnetic noise in the Scaglia Bianca data sets, there is a high degree of agreement in direction and to a lesser extent relative intensity between correlative portions of the two sections. We therefore offer this data set as a robust record of geomagnetic field behavior during the 4.5 Myr interval represented by the La Roccaccia section. A statistical analysis of the relative intensity observations suggests that this period of the Cretaceous Normal Superchron is characterized by a normalized variability in paleointensity (standard deviation about 28% of the mean value) that is significantly lower than seen during the Oligocene over intervals in which reversals or tiny wiggles occur (typically about 50%). The directional stability results in virtual geomagnetic pole dispersion compatible with that found in volcanic rocks from around the same latitude and ranging in age from 80 to 110 Ma.

Palaeosecular variation over the past 5 Myr based on a new generalized database

A generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0–5 Ma has been constructed for use with a relational database management system. The database includes all data whose virtual geomagnetic poles (VGP) lie within 45° of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling. Because many of these data were collected and processed more than 15–20 years ago, each result has been characterized according to the demagnetization procedures carried out. Analysis of these data in terms of the latitude variation of the angular dispersion of VGPs (palaeosecular variation from lavas) strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods. Differences between the angular dispersions for separate normal- and reverse-polarity data sets confirm that many older studies have not been adequately cleaned magnetically. Therefore, the use of the database for geomagnetic field modelling should be carried out with some caution. Using a VGP cut-off angle that varies with latitude, the best data set consists of 2636 results that show a smooth increase of VGP angular dispersion with latitude. Model G for palaeosecular variation, which is based on modelling of the antisymmetric (dipole) and symmetric (quadrupole) dynamo families, provides a good fit to these results.

The myth of the Pacific dipole window

The angular dispersion of virtual geomagnetic poles (VGP) from lava flows is often cited as having been anomalously low in the Pacific during the Brunhes epoch because the dispersion from Hawaiian lavas is said to be much lower than measured elsewhere. This led to the concept of the Pacific dipole window or Pacific non-dipole low. Because lavas tend to be erupted in bursts of activity, many of the Hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations. Geochronological controls, as are available for Hawaii, have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field. Unfortunately, in that analysis the angular dispersion of VGPs was calculated about the mean VGP instead of about the spin axis, and thus underestimated the dispersion. We have therefore reanalyzed the relevant data using the following criteria. (1) Only those lavas with α 95 < 10° have been considered. (2) A latitude-dependent cut-off angle is used to eliminate those vectors that are not part of the normal secular variation. (3) We have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector, because suitable geochronological controls are rarely available. Application of this new method to the Hawaiian data shows excellent agreement with the results from using geochronological controls. Where possible we have therefore included this method in our overall analysis. As expected, the resulting global data are compatible with a Fisher distribution about the spin axis. Brunhes age data from Hawaii ( N = 96 independent measurements) give a VGP angular dispersion about the spin axis of S F = 12.4°, and for the Pacific region as a whole S F = 12.5° ( N = 190) between latitudes 15° and 30° (north or south). These values are the same as S F = 12.4° ( N = 160) calculated for the rest of the world in the same latitude range. This clearly demonstrates that the hypothesis of the Pacific dipole window may confidently be rejected.

New palaeomagnetic results from Icelandic Neogene lavas

SUMMARY In this paper, some statistical properties of new collections of palaeomagnetic direction and intensity data are presented and compared with results from older collections. The new data come mostly from three composite surveys in the basalt lava pile of Iceland, totalling about 1000 lavas of ages 7-14 Myr. The palaeomagnetism of all these lavas converges on a reproducible long-term pattern in several important features of the Neogene field behaviour. Previous conclusions which are confirmed in this paper include: (1) the distribution of intermediate virtual geomagnetic poles (VGPs) is not confined to preferred longitude intervals; (2) normal- and reverse-polarity states of the geomagnetic field are essentially equivalent as regards the intensity of the field, mean field directions, and secular variation; (3) the virtual geomagnetic dipole moment fdls off with decreasing VGP latitude, by a factor of 3.5-4 between polar and equatorial latitudes: (4) a significant reduction in the between-flow angular dispersion of VGPs may have occurrcd during the last 14 Myr; and (5) palaeomagnetic directions from Icelandic basalt lavas are stable and reliable.

Paleomagnetism of the Patagonian Plateau Basalts, southern Chile and Argentina

A total of 505 paleomagnetic samples were collected from 65 sites (volcanic flows) of the Patagonian plateau basalts at four locations in southern Chile and Argentina. K/Ar analyses indicate that 38 flows from two locations form a Late Cretaceous group (64–79 Ma), while the remaining 27 flows are Eocene in age (42–56 Ma). Progressive demagnetization indicates that a characteristic remanent magnetization (ChRM) has been successfully isolated from 59 flows. Rock‐magnetic properties and analysis of ChRM directions within and between sites allow the secure inference that the ChRM is a thermoremanent magnetization acquired during original cooling. The Eocene flows yield 15 independent samplings of the paleomagnetic field. However, these data fail the reversals test and probably do not adequately average geomagnetic secular variation. The Late Cretaceous flows yield 18 independent samplings of the paleomagnetic field, six of normal polarity and 12 of reversed polarity. These data pass the reversals test and have angular dispersion of virtual geomagnetic poles (VGPs) consistent with adequate sampling of geomagnetic secular variation. The mean of these 18 VGPs yields a Late Cretaceous paleomagnetic pole for South America: latitude = 78.7°S; longitude = 358.4°E; A95 = 6.3° (K = 31.6; S = 14.5°). This pole is consistent with similar age poles from North America and Africa when the Atlantic is reconstructed to 70 Ma. Contrary to previous indications, the South American apparent polar wander path does not reach the present rotation axis until after Late Cretaceous time.

Palaeomagnetic data from central Mexico and their use for palaeosecular variation studies

Abstract Three Brunhes-age palaeomagnetic data sets from central Mexico exhibit different dispersion of virtual geomagnetic poles. Volcanics from the eastern part of the Trans-Mexican Volcanic Belt result in a S F = 14.8 °. This value is interpreted to represent an upper limit, as these rocks probably are affected by localized tectonic rotations. For a stalagmite, S F = 9.5 °, which is interpreted as a lower limit, as the stalagmite is only 1200 years old and the data are strongly biased in time. Seventy-four volcanic units from the Chichinautzin Formation result in S F = 11.7°, with confidence limits of 10.4° and 13.3°. This dispersion is much lower than expected from palaeosecular variation models, but corresponds to data obtained for other areas in or beneath the Pacific Basin, e.g. the Hawaiian islands.

Northward motion of the Whitehorse Trough: paleomagnetic evidence from the Upper Cretaceous Carmacks Group: Discussion

Discordant paleomagnetic directions in mid-Cretaceous plutons of the Coast Plutonic Complex and North Cascades have been interpreted as indicating mid-Cretaceous paleolatitudes 2000-2500 km south relative to North America (Beck et al. 1981; Irving et al. 1985; Umhoefer 1987). But this magnitude of northward transport contradicts interpreted pre-midCretaceous links between the North American miogeocline and westerly parts of the Cordillera (Price and Charmichael 1986). Because paleohorizontal cannot be directly determined in plutonic rocks, the discordant paleomagnetic directions may also be explained by tilting of the plutons about a horizontal axis. Primarily because of the consistent nature of the paleomagnetic discordance over a large region, Beck et al. (1981) and Irving et al. (1985) have preferred the interpretation of northwards transport. However, Butler et al. (1989) recently pointed out geological evidence indicating that widespread northeast-side-up tilting may have affected the western portion of the Coast Plutonic Complex and the North Cascades. Resolution of this debate is of obvious importance to understanding the tectonic evolution of the Canadian Cordillera. Marquis and Globerman reported results of a paleomagnetic study of the Upper Cretaceous Carmacks Group in the Whitehorse Trough. The Carmacks Group is composed of gently dipping basaltic and andesitic lava flows that have been dated at 70.4 -+ 2.4 Ma. These rocks lie west of the Tintina Northern Rocky Mountain Trench fault and the Teslin suture zone. Because paleohorizontal can be unambiguously determined for these volcanic rocks, the paleomagnetic data presented by Marquis and Globerman are potentially very important in resolving the tilt versus translation question. Marquis and Globerman collected paleomagnetic samples in three regions spread over 300 km. Based on detailed laboratory studies, they successfully isolated characteristic remanent magnetization (ChRM) from 18 of 27 paleomagnetic sites (= flows). The acquired paleomagnetic data pass a fold test of stability and contain sites of both normal and reversed polarity; and magnetic properties indicate that the ChRM is a primary thermoremanent magnetization. The observed angular dispersion of virtual geomagnetic poles (VGP's) indicates some degree of required sampling of geomagnetic secular variation. Marquis and Globerman determined a paleomagnetic pole from the Carmacks Group at 82N, 109E, with 95% confidence region (Ag5) of radius 7.8. Using a 70 Ma North American reference paleomagnetic pole of 77.7N, 185.8E (Ag5 = 7.7) and the methods of Beck (1980) as modified by Demarest

Variable dispersion of Neogene geomagnetic field directions in Iceland

Analysis of the primary paleomagnetic directions in 4000 lava flows in Iceland reveals that the magnitude of total virtual geomagnetic pole dispersion has decreased during the last 15 Ma. This dispersion, which is independent of the polarity of the field, is in part caused by the occurrence of erratically varying paleofield directions during intermittent periods possibly as long as 100 ka.

Geomagnetic paleointensities from radiocarbon‐dated lava flows on Hawaii and the question of the Pacific nondipole low

Radiocarbon ages have been published for nine basaltic lava flows on the island of Hawaii; the ages range from 2600 to somewhat older than 17,900 years B.P. By using the Thelliers' method in vacuum, geomagnetic paleointensity values were obtained from eight of the lavas; the ninth proved unsuitable. The paleointensities for the four youngest flows (2600–4600 years B.P.) yield virtual dipole moments (VDM's) that are 20% greater to more than twice the worldwide values for those times obtained by V. Bucha from archeomagnetic data. The dispersion of virtual geomagnetic poles for the eight lavas is 15.5°, appreciably larger than the average for older lava flows on Hawaii. These results contrast with the historic magnetic field in the region of Hawaii, in which both secular variation and nondipole components are very low. At about 10,000 years B.P. the measured VDM is not very different from the long‐term worldwide average but differs considerably from a smooth extrapolation of Bucha's average curve. At about 18,000 years B.P. the measured VDM is very low and is associated with an unusually shallow paleomagnetic inclination for the latitude of Hawaii. These new paleointensity and paleodirectional data strongly suggest that sizable nondipole geomagnetic fields have existed in the vicinity of Hawaii at various times during the Holocene epoch and perhaps earlier.

Paleomagnetic study of lower Mesozoic diabase dikes and sills of Connecticut and Maryland

Paleomagnetic results from 61 sites on diabase dikes and sills in Connecticut and Maryland yield a mean pole of position of 100.9 °E. 68.6 °N, α95 = 1.6°. These results combined with those from 7S sites on four diabase sills in the Gettysburg Basin of Pennsylvania produce a mean pole position of 101, 6 °E. 65.4 °N, α95 = 1.3 °from 139 VGPs. These rocks arc probably of Early to early Middle Jurassic age by stratigraphic and inferred stratigraphic position. The paleomagnetic results indicate that the Gettysburg and Hartford Basins did not subside simultaneously.The dispersion of virtual geomagnetic poles about the mean pole of the Connecticut, Pennsylvania, and Maryland intrusive rocks is probably the result of 'homogenization' of magnetic directions during low-temperature oxidation of titanomagnetite to titanomaghemite over a period long enough to erase a considerable amount of dispersion due to secular variation.Rotation of Europe against North America in the prerifting configuration causes good convergence of European Jurassic mean poles and the pole of the Connecticut. Maryland, and Pennsylvania intrusive rocks. This is evidence that separation of the two continents had not begun by Early to early Middle Jurassic time, which is in agreement with earlier findings based on sea-floor magnetic anomalies and faunal assemblages.

Paleomagnetism and potassium-argon age of St. Paul Island, southeastern Indian Ocean: Contrasts in geomagnetic secular variation during the Brunhes epoch

A total of 135 oriented cores have been collected from 18 separate basaltic bodies on St. Paul Island on the axis of the mid-Indian ridge, and subjected to paleomagnetic and K/Ar analyses. Like Amsterdam Island, 70 km to the north, St Paul is restricted to the Brunhes epoch ( t = 0−0.69 m.y.) in outcrop. An average volcanic production rate of 10 4 m 3 per year is suggested for that part of St. Paul above sea level. Of the two islands, St. Paul has a much higher angular dispersion of virtual geomagnetic poles ( S F). Because of this, it is concluded that despite their restricted age ranges, St. Paul and Amsterdam are likely to have minimal overlap in age. As support for this interpretation, data from a nearby deep-sea core, which has a Brunhes epoch record spanning 17 m of sediment, is consistent with a highly variable geomagnetic secular variation during the Brunhes epoch.

Brunhes epoch geomagnetic secular variation on Terceira Island, central North Atlantic

A paleomagnetic survey has been made of Brunhes epoch lavas and ignimbrites on Terceira in the Azores. At least six widely spaced samples were taken from each of 33 separate bodies. The corrected dispersion of virtual geomagnetic poles (SF) is 13.9 (with upper and lower 95% confidence limits of 16.3 and 12.2, respectively), consistent with the present geomagnetic field behavior in the same latitudes. Analysis of the data as nine independent surveys involving from two to seven separate specimens per body provides values of SF that are not significantly different. Thus an equally accurate value of SF could have been obtained with less than half the samples collected. Analysis of the effect of Kw (the within-body precision parameter) and other factors involved in the computation of SF suggests that the largest number of separate cores per separate body required for defining SF is four. Two separate lavas recorded a possible single departure of the geomagnetic field, and thus evidence of instability of the geocentric dipole during the Brunhes epoch is provided.

Potassium-Argon Ages and Paleomagnetism of the Waianae and Koolau Volcanic Series, Oahu, Hawaii

Paleomagnetic and potassium-argon measurements on 786 oriented cores from 99 volcanic units at 18 sites in the Waianae and Koolau Ranges, Oahu, when combined with data from previous studies, show that the sub-aerial Waianae Volcano was active only from about 3.6 to 2.4 m.y. ago and the subaerial Koolau Volcano from about 2.6 to 1.8 m.y. ago. There is some evidence that Waianae Volcano was still active when Koolau Volcano emerged from the sea. The predominantly tholeiitic lower and middle members of the Waianae Volcanic Series are approximately contemporaneous and were extruded during the late Gilbert and early Gauss geomagnetic polarity epochs. They were followed within less than 0.2 m.y. by the alkalic lavas of the upper member, which were probably extruded largely during the later part of the Gauss normal polarity epoch. The Koolau Volcanic Series was extruded entirely during the early Matuyama reversed polarity epoch. Data from three thick stratigraphic sections in the Waianae and Koolau Volcanic Series indicate that stacks of lava flows as much as 470 m thick can be formed in less than 0.25 m.y. and that the maximum average period between superimposed lava flows is on the order of 103 yrs. Additional data on the hawaiite flow that led to the discovery of the Kaena reversed event indicate that this reversed flow is 2.85 ± 0.05 m.y. old. Angular dispersion of virtual geomagnetic poles (VGP) in the Hawaiian Islands appears to have decreased during the past 5 m.y. This may be caused by a decrease in dipole wobble, a decrease in the nondipole component of the Earth9 magnetic field, or the accumulated effects of weathering, tectonism, and geomorphic processes in older rocks. The mean Waianae and Koolau VGPs are slightly on the side of the Earth9s rotation axis away from Oahu. This supports, but does not prove, the hypothesis that the axial dipole is displaced slightly northward from the Earth9s center. Three VGP “excursions” were recorded in sections of lava in the Waianae and Koolau ranges. During these excursions, the VGPs appear to have traveled away from or toward the geographic axis along great circle paths, suggesting they may be related to the dipole rather than the nondipole field and may record aborted reversals in polarity or rapid and infrequent dipole tilts.