Paleosecular Variation Models Research Articles

Two thousand years of archeointensity from West Africa

This study presents 17 archeointensity estimates from Senegal and Mali, two neighboring countries in West Africa, for the period 1000 BCE to 1000 CE. The archeological artifacts used in this study were collected during the course of two separate projects, together spanning 22 years and across 8 separate excavations. A primary objective of this study was to get accurate dates, hence, only samples with independent age constraints from pottery style, detailed stratigraphy and 14C dates were used. A total of 236 specimens from 63 samples were subjected to a double heating paleointensity experiment (IZZI method) from which 95 specimens were selected using a set of very strict selection criteria. The paleointensity results were corrected for differential cooling rate effects and remanence anisotropy. Additionally, we demonstrate the equivalence of using tensors derived from anhysteretic and thermal remanences for correcting remanent anisotropy of the specimens and use the former for the anisotropy correction. Our data show good agreement with the most recent paleosecular variation model but are lower than the pre-existing data, which are mostly from Egypt and Morocco. The presence of substantial non-axial-dipolar contributions in the region is evident when virtual axial dipole moments (VADMs) from the published literature are calculated for 20° latitudinal bands and compared with our data—the average VADM values show a distinct latitudinal gradient. A prominent feature of this dataset is an intensity high observed prior to 700 CE in both Senegal and Mali. Comparing this peak with existing records from regions further to the north suggests a small but significant temporal offset and is interpreted to be additional evidence for a geomagnetic field with a significant and rapidly changing non-axial-dipolar contribution.

Equatorial paleosecular variation of the geomagnetic field from 0 to 3 Ma lavas from the Galapagos Islands

Complete progressive thermal demagnetization of nearly 400 oriented samples from 58 sites (lava flows) from the Galapagos Islands of Santa Cruz, San Cristobal and Floreana provide data for the statistical characterization of the time-averaged geomagnetic field near the Equator for the past few million years. Estimates of VGP dispersion due to paleosecular variation range from 9.2° to 11.8° depending on site selection criteria; our preferred estimate based on 64 site VGPs (51 accepted from this study and 13 from the 1971 study by Cox) is 11.4° (95% confidence interval 10.2–13.0°), consistent with previous estimates from the Galapagos Islands as well as paleosecular variation Model G, and confirming that angular dispersion of VGPs near the Equator is relatively low. The mean direction is not significantly different from a geocentric axial dipole field when account is taken of southward plate motion over the Galapagos hotspot. Preliminary paleointensity results from a comparison of the natural remanence with a total thermal remanence produced in a lab field of 15 μT on a subset of 321 samples from 48 sites that had relatively small changes in magnetic susceptibility after laboratory heating suggest that the time-averaged field was about 21 μT, or only two-thirds the present strength, in agreement with some other recent estimates.

Paleosecular variation models for ancient times: Clues from Keweenawan lava flows

Statistical paleosecular variation models predict distributions of paleomagnetic vectors as a function of geographic position. Such models have been used in a variety of applications that test whether a given data set fairly represents the variability and average properties of the geomagnetic field. The simple relationship between inclination of the geomagnetic field and latitude predicted by geocentric axial dipole (GAD) models has been a cornerstone for plate reconstructions for decades, yet many data sets exhibit a tendency to be shallower than expected for a dominantly axial geocentric magnetic field. Too shallow inclinations have variously been interpreted as plate motion, permanent non-dipole field components or bias in inclination from sedimentary processes. Statistical PSV models could in principle be used to resolve the cause of inclination anomalies because there is a simple relationship between the elongation of the distribution of directions in the vertical plane and the average inclination. Shallowing of inclinations from sedimentary processes results in a progressive transformation of the elongation direction in the vertical plane containing the average direction into a pronounced elongation in the plane perpendicular to that. However, the applicability of statistical models based on the last 5 million years for more ancient times is an open question. Here we present new data from the Keeweenawan North Shore Volcanics ( ∼ 1.1 Ga). These data are consistent with statistical PSV model predictions and are less well fit by models that include a 20% axial octupole component. We also find evidence for a pervasive overprinting by hematite in a shallower direction and find support for the contention that the asymmetric reversal(s) observed in Keweenawan aged rocks along the North shore of Lake Superior can be explained as an age progression, with the reverse directions being older than the normal directions. Finally, we re-consider implications from an analysis of inclinations from the Global Paleomagnetic Database for the Paleozoic and Pre-Cambrian. We find that the data are inconsistent with a random sampling of any simple geomagnetic field model and conclude that the data set under-samples the field in a spatial sense.

Paleomagnetic field properties at high southern latitude

Statistical analyses of paleomagnetic data from lava flows are used to study geomagnetic field behavior on million year timescales. Previous paleomagnetic studies have lacked high‐latitude measurements necessary to investigate the persistence of geomagnetic anomalies observed in the recent and historical field and replicated in some numerical geodynamo simulations. These simulations suggest that reduced convective flow inside the tangent cylinder may affect the magnetic field at high latitude, whereas lower‐latitude observations are expressions of columnar/helical flow outside the tangent cylinder. This paper presents new paleointensity and paleodirectional data from 100 volcanic sites in the Erebus Volcanic Province (EVP), Antarctica, and 21 new age determinations by the 40Ar/39Ar incremental heating method. The new EVP data are combined with previously published paleomagnetic and geochronological results, providing 133 sites, 91 having radioisotopic dates. Modified Thellier‐Thellier paleointensity estimates are reported for 47 sites (37 have dates). Ages for the combined data set span 0.03 to 13.42 Ma. The 125 high‐quality EVP directional data selected from the merged data set have a non‐Fisherian distribution and a mean direction with an inclination anomaly of ∼3°, but 95% confidence limits include the prediction from a geocentric axial dipole. Virtual geomagnetic pole (VGP) dispersions for Brunhes, Matuyama, and the combined 0–5 Ma data set are consistently high compared with values from middle‐ to low‐latitude regions regardless of the criterion used to determine transitional fields. With VGP latitude cut off at 45°, the dispersion (23.9 ± 2.1°) for the combined 0–5 Ma EVP data set is consistent with earlier high‐latitude data and paleosecular variation (PSV) in Model G but not with some more recent statistical PSV models. Mean EVP paleointensity of 31.5 ± 2.4 μT, derived from 41 high‐quality sites, is about half the current value at McMurdo (∼63 μT). The result is essentially independent of data selection criteria. High VGP dispersion and low‐intensity values support the global observation of anticorrelation between directional variability and field strength. Simulations of time‐varying dipole strength show that uneven temporal sampling may bias the mean EVP intensity estimate, but the possibility of persistently anomalous field behavior at high latitude cannot be excluded.

Testing corrections for paleomagnetic inclination error in sedimentary rocks: A comparative approach

Paleomagnetic inclinations in sedimentary formations are frequently suspected of being too shallow. Recognition and correction of shallow bias is therefore critical for paleogeographical reconstructions. This paper tests the reliability of the elongation/inclination ( E/ I) correction method in several ways. First we consider the E/ I trends predicted by various PSV models. We explored the role of sample size on the reliability of the E/ I estimates and found that for data sets smaller than ∼ 100–150, the results were less reliable. The Giant Gaussian Process-type paleosecular variation models were all constrained by paleomagnetic data from lava flows of the last five million years. Therefore, to test whether the method can be used in more ancient times, we compare model predictions of E/ I trends with observations from five Large Igneous Provinces since the early Cretaceous (Yemen, Kerguelen, Faroe Islands, Deccan and Paraná basalts). All data are consistent at the 95% level of confidence with the E/ I trends predicted by the paleosecular variation models. The Paraná data set also illustrated the effect of unrecognized tilting and combining data over a large latitudinal spread on the E/ I estimates underscoring the necessity of adhering to the two principle assumptions of the method. Then we discuss the geological implications of various applications of the E/ I method. In general the E/ I corrected data are more consistent with data from contemporaneous lavas, with predictions from the well constrained synthetic apparent polar wander paths, and other geological constraints. Finally, we compare the E/ I corrections with corrections from an entirely different method of inclination correction: the anisotropy of remanence method of Jackson et al. [Jackson, M.J., Banerjee, S.K., Marvin, J.A., Lu, R., Gruber, W., 1991. Detrital remanence, inclination errors and anhysteretic remanence anisotropy: quantitative model and experimental results. Geophys. J. Int. 104, 95–103] which relies on measurement of remanence and particle anisotropies of the sediments. In the two cases where a direct comparison can be made, the two methods give corrections that are consistent within error. In summary, it appears that the E/ I method for recognizing and corrected the effects of sedimentary flattening is reasonably robust for at least the Mesozoic and Cenozoic when the source of scatter is geomagnetic and sedimentary flattening in origin.

Paleomagnetic study of late Miocene through Pleistocene igneous rocks from the southwestern USA: Results from the historic collections of the U.S. Geological Survey Menlo Park laboratory

Seventy sites from the southwestern United States provide paleomagnetic results that meet certain minimum criteria and can be considered for the Time‐Averaged Field Initiative (TAFI). The virtual geomagnetic poles for these 70 units are circularly distributed, and their mean is nearly coincident with the rotational axis. When other published data for the southwestern United States are included (N = 146), the virtual geomagnetic poles are again circularly distributed, but their mean is significantly displaced from the rotational axis. Whichever of these data sets is used, the mean poles for normal‐ and reversed‐polarity data differ by ∼170° and are not antipodal at greater than 95% confidence. When the data are separated into specific age groups, the 95% confidence limits about the mean poles for the Brunhes, Matuyama, combined Gauss/Gilbert, and late Miocene intervals all include the rotational axis. Angular dispersion about these four mean poles increases systematically with increasing age and is consistent with paleosecular variation Model “G.”

Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows

We present a synthesis of 0–5 Ma paleomagnetic directional data collected from 17 different locations under the collaborative Time Averaged geomagnetic Field Initiative (TAFI). When combined with regional compilations from the northwest United States, the southwest United States, Japan, New Zealand, Hawaii, Mexico, South Pacific, and the Indian Ocean, a data set of over 2000 sites with high quality, stable polarity, and declination and inclination measurements is obtained. This is a more than sevenfold increase over similar quality data in the existing Paleosecular Variation of Recent Lavas (PSVRL) data set, and has greatly improved spatial sampling. The new data set spans 78°S to 53°N, and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time‐averaged field (TAF) and paleosecular variation (PSV) for the Brunhes and Matuyama chrons, and for the 0–5 Ma interval combined. The Brunhes and Matuyama chrons exhibit different TAF geometries, notably smaller departures from a geocentric axial dipole field during the Brunhes, consistent with higher dipole strength observed from paleointensity data. Geographical variations in PSV are also different for the Brunhes and Matuyama. Given the high quality of our data set, polarity asymmetries in PSV and the TAF cannot be attributed to viscous overprints, but suggest different underlying field behavior, perhaps related to the influence of long‐lived core‐mantle boundary conditions on core flow. PSV, as measured by dispersion of virtual geomagnetic poles, shows less latitudinal variation than predicted by current statistical PSV models, or by previous data sets. In particular, the Brunhes data reported here are compatible with a wide range of models, from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude. Discriminating among such models could be helped by increased numbers of low‐latitude data and new high northern latitude sites. Tests with other data sets, and with simulations, indicate that some of the latitudinal signature previously observed in VGP dispersion can be attributed to the inclusion of low‐quality, insufficiently cleaned data with too few samples per site. Our Matuyama data show a stronger dependence of dispersion on latitude than the Brunhes data. The TAF is examined using the variation of inclination anomaly with latitude. Best fit two‐parameter models have axial quadrupole contributions of 2–4% of the axial dipole term, and axial octupole contributions of 1–5%. Approximately 2% of the octupole signature is likely the result of bias incurred by averaging unit vectors.

Paleomagnetism of Unzen volcano: A volcanic record (Senbongi excursion) of the Iceland Basin event and the Brunhes VGP distribution for Japan

Abstract A paleomagnetic study was carried out on volcanic rocks from Unzen volcano: samples were collected from a total of 69 sites with 19 sites in pyroclastic flows and 50 sites in lava flows. Ages for the flows were determined either by K-Ar methods or detailed field surveys, and indicate that all of the flows were deposited during the Brunhes chron. After demagnetization 10 pyroclastic and 48 lava flows had stable site-mean directions. One lava flow in the Senbongi area with a K-Ar age of 197±17 ka had an intermediate virtual geomagnetic pole (VGP) at 8.3°N, 21.6°E. The age and VGP position apparently correlate with the Iceland Basin event, suggesting that it is a record of the event on volcanic rocks. Our study added 53 Brunhes-aged VGPs to the overall collection for Japan, increasing it by 40% to a total of 175 VGPs. After excluding all site-mean directions having α95 > 10° and VGP latitude < 50°, the remaining 148 VGPs have a mean pole at 89.7°N, 40.9°E (A 95 = 2.2°) showing no significant deviation from the geographic pole. The angular standard deviation (ASD) was calculated as 15.2° ± 1.2° (N = 148), which is compatible with paleosecular variation models from the literature. However, the data set was found to deviate from a Fisher distribution. The actual meaning of the ASD value after removing the intermediate VGPs needs to be reconsidered.

Holocene paleosecular variation from dated lava flows on Maui (Hawaii)

Low inclination and low secular variation seem to be present for at least the past 5 Myear in central Pacific. The period covering the past 10 kyear is crucial to the study of the field variability and to fill the gap between historical field measurements and long-term paleomagnetic records. We have conducted a paleomagnetic study of 13 sites of basaltic lava flows from Maui island with 14C ages between 10.3 and 0.2 ka. Two other sites dated at 45 and 730 ka were also sampled. Eight to 10 samples from each site were demagnetized using thermal treatment and companion specimens from the same samples were demagnetized by alternating fields (af). Thermomagnetic and hysteresis measurements indicated that magnetite (575 °C) in fine grains was the dominant magnetic carrier, although in many cases we also observed a low-temperature phase which is likely carried by titanomagnetite with low titanium content. The existence of relatively high coercivities associated with these two mineralogical phases generated overlapping components which could not be properly isolated using af demagnetization. Successful results were obtained after thermal demagnetization for 13 sites with a mean inclination of 36.9 ± 4°. The mean inclination (inc. = 36.3°) of the eleven sites younger than 10.5 ka is also very close to the value (37°) of the geocentric axial dipole (GAD) at the site latitude, but the angular dispersion of the virtual geomagnetic poles (VGPs) about the spin axis (9° with N = 13 and 6.7° with N = 11) is significantly lower than the predictions of the models of paleosecular variation at this latitude. The inclination variations for the past 10 kyear are in excellent agreement with the very detailed dataset which has previously been obtained from the big island of Hawaii. The mean inclination of all volcanic records ( N = 132) is 1.8° lower than expected, but this is likely caused by the lack of records between 5 and 7 ka B.P. We note also that the inclinations from Lake Waiau sediments (big island) are shallower than those of the volcanic records (Mauna Kea, Hawaii) and were thus probably affected by compaction. We infer that there is no striking evidence for an inclination anomaly under Hawaii during this period, being aware of the need for additional records covering at least the 5–7 ka B.P. time interval. The absence of a systematic deviation going beyond the dispersion of the inclinations obtained for the past millions of years neither pleads for a significant long-term persistent anomaly. However, all studies report a low dispersion of the VGPs which must reflect low secular variation.

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.

Outstanding student paper awards: Geomagnetism and paleomagnetism, tectonophysics

The following members received Outstanding Student Paper Awards at the 2004 Fall Meeting in San Francisco, California.Kristin Lawrence, University of California, San Diego, Evaluation of agreement between paleosecular variation models and empirical distributions at 20° latitude.

地球磁場の形と変動―平均地球磁場と古地磁気永年変化―

Paleomagnetism has been applied to tectonics studies on the basis of the geocentric axial dipole (GAD) field hypothesis. However, the applicability of the GAD hypothesis is still under investigation in the paleomagnetic community. It is directly connected to the precision of the tectonic movements derived from paleomagnetism, so that the the morphology of the geomagnetic field should draw wide attention among Earth scientists. The morphology of the long term geomagnetic features is often represented by the time-averaged field (TAF) and the paleosecular variation (PSV). They are extracted from the paleodirections of lavas erupted during the past 5 My.The recent TAF and PSV models indicate following; (1) the GAD hypothesis is not strictly accurate, (2) the geocentric axial quadupole (g02) field exists in the TAF and it cannot be neglected, and (3) the inclination anomaly calculated from the averaged paleomagnetic data consists also of the apparent effect of the PSV, which additionally cannot be ignored. These components should be paid close attentions to specially in the case of detailed tectonics such as highresolution plate motion. The inclination anomaly due to g02 in the TAF and the effect of the PSV would produce a mistake of the paleolatitude, which becomes totally up to several degrees.

Paleomagnetism of the southwestern U.S.A. recorded by 0–5 Ma igneous rocks

The issue of permanent nondipole contributions to the time‐averaged field lies at the very heart of paleomagnetism and the study of the ancient geomagnetic field. In this paper we focus on paleomagnetic directional results from igneous rocks of the southwestern U.S.A. in the age range 0–5 Ma and investigate both the time‐averaged field and its variability about the mean value. Several decades of work in the southwestern United States have resulted in the publication of paleomagnetic data from over 800 individual paleomagnetic sites. As part of a new investigation of the San Francisco Volcanics, we collected paleomagnetic samples from 47 lava flows, many of which have been previously dated. The new data combined with published data are highly scattered. Contributions to the scatter were considered, and we find that removal of data sets from tectonically active areas and judicious selection according to Fisher's [1953] precision parameter results in an axially symmetric data distribution with normal and reverse modes that are indistinguishable from antipodal. Monte Carlo simulations suggest that a minimum of 5 samples per site are needed to estimate the precision parameter sufficiently accurately to allow its use as a determinant of data quality. Numerical simulations from statistical paleosecular variation models indicate the need for several hundred paleomagnetic sites to get an accurate determination of the average field direction and are also used to investigate the directional bias that results from averaging unit vectors rather than using the full field vector. Average directions for the southwestern U.S.A. show small deviations from a geocentric axial dipole field, but these cannot be considered statistically significant. Virtual geomagnetic pole (VGP) dispersions are consistent with those from globally distributed observations analyzed by McElhinny and McFadden [1997]. However, a systematic investigation of the effect of imposing a cutoff on VGPs with large deviations from the geographic axis indicates that while it may reduce bias in calculating the average direction, such a procedure can result in severe underestimates of the variance in the geomagnetic field. A more satisfactory solution would be to use an unbiased technique for joint estimation of the mean direction and variance of the field distribution.

Geomagnetic field model for the last 5 My: time-averaged field and secular variation

Abstract Structure of the geomagnetic field has bee studied by using the paleomagetic direction data of the last 5 million years obtained from lava flows. The method we used is the nonlinear version, similar to the works of Gubbins and Kelly [Nature 365 (1993) 829], Johnson and Constable [Geophys. J. Int. 122 (1995) 488; Geophys. J. Int. 131 (1997) 643], and Kelly and Gubbins [Geophys. J. Int. 128 (1997) 315], but we determined the time-averaged field (TAF) and the paleosecular variation (PSV) simultaneously. As pointed out in our previous work [Earth Planet. Space 53 (2001) 31], the observed mean field directions are affected by the fluctuation of the field, as described by the PSV model. This effect is not excessively large, but cannot be neglected while considering the mean field. We propose that the new TAF+PSV model is a better representation of the ancient magnetic field, since both the average and fluctuation of the field are consistently explained. In the inversion procedure, we used direction cosines instead of inclinations and declinations, as the latter quantities show singularity or unstable behavior at the high latitudes. The obtained model gives reasonably good fit to the observed means and variances of direction cosines. In the TAF model, the geocentric axial dipole term (g10) is the dominant component; it is much more pronounced than that in the present magnetic field. The equatorial dipole component is quite small, after averaging over time. The model shows a very smooth spatial variation; the nondipole components also seem to be averaged out quite effectively over time. Among the other coefficients, the geocentric axial quadrupole term (g20) is significantly larger than the other components. On the other hand, the axial octupole term (g30) is much smaller than that in a TAF model excluding the PSV effect. It is likely that the effect of PSV is most clearly seen in this term, which is consistent with the conclusion reached in our previous work. The PSV model shows large variance of the (2,1) component, which is in good agreement with the previous PSV models obtained by forward approaches. It is also indicated that the variance of the axial dipole term is very small. This is in conflict with the studies based on paleointensity data, but we show that this conclusion is not inconsistent with the paleointensity data because a substantial part of the apparent scatter in paleointensities may be attributable to effects other than the fluctuations in g10 itself.

A statistical analysis of magnetic fields from some geodynamo simulations

We present a statistical analysis of magnetic fields simulated by the Glatzmaier‐Roberts dynamically consistent dynamo model. For four simulations with distinct boundary conditions, means, standard deviations, and probability functions permit an evaluation based on existing statistical paleosecular variation (PSV) models. Although none closely fits the statistical PSV models in all respects, some simulations display characteristics of the statistical PSV models in individual tests. We also find that nonzonal field statistics do not necessarily reflect heat flow conditions at the core‐mantle boundary. Multitaper estimates of power and coherence spectra allow analysis of time series of single, or groups of, spherical harmonic coefficients representing the magnetic fields of the dynamo simulations outside the core. Sliding window analyses of both power and coherence spectra from two of the simulations show that a 100 kyr averaging time is necessary to realize stationary statistics of their nondipole fields and that a length of 350 kyr is not long enough to fully characterize their dipole fields. Spectral analysis provides new insight into the behavior and interaction of the dominant components of the simulated magnetic fields, the axial dipole and quadrupole. Although we find spectral similarities between several reversals, there is no evidence of signatures that can be conclusively associated with reversals or excursions. We test suggestions that during reversals there is increased coupling between groups of spherical harmonic components. Despite evidence of coupling between antisymmetric and symmetric spherical harmonics in one simulation, we conclude that it is rare and not directly linked to reversals. In contrast to the reversal model of R. T. Merrill and P. L. McFadden, we demonstrate that the geomagnetic power in the dipole part of the dynamo simulations is either relatively constant or fluctuates synchronously with that of the nondipole part and that coupling between antisymmetric and symmetric components occurs when the geomagnetic power is high.

Extensional deformation and paleomagnetism at the western margin of the Gulf extensional province, Puertecitos Volcanic Province, northeastern Baja California, Mexico

During the late Miocene east-northeast–directed extension in the Gulf of California extensional province, the western rift margin in northeastern Baja California, Mexico, was segmented at the northwest-striking Matomi accommodation zone. The accommodation zone passed through the northern Puertecitos Volcanic Province and separated pre–6 Ma extension to the north from the unextended region to the south. Pre–6 Ma northeast-side-down displacement is documented across the accommodation zone, which may have undergone dextral oblique-slip motion. The rift margin migrated westward during the latest Miocene or Pliocene, bypassing accommodation-zone structures and incorporating the Puertecitos Volcanic Province into the region of Gulf extensional province deformation. East-northeast– to east-directed extensional deformation is at least post–6 Ma in the northern Puertecitos Volcanic Province and deformation currently continues. Pliocene changes in the Gulf of California spreading center system may have triggered incorporation of the Puertecitos Volcanic Province into the Gulf extensional province as recently as 2–3 Ma. Paleomagnetic analyses of 6.3–6.6 Ma pyroclastic flow deposits show no consistent evidence for rotational deformation, although minor (∼10°–15°) clockwise rotation is possible given anomalous declination directions recorded at some sites. Comparisons with paleosecular variation models and the late Miocene paleopole for stable North America imply no statistically significant rotation relative to geomagnetic north, although minor (∼10°) clockwise rotation is permissible given uncertainties. Geologic relationships show that pre–6 Ma accommodation-zone structures identified in this study did not mark the southern boundary of later rotational deformation documented to the north. The boundary of Pliocene to Holocene rotations may be a broad, diffuse zone of extensional shear encompassing the northeastern Puertecitos Volcanic Province, which accommodated small rotations.

Paleomagnetism of Pliocene to Pleistocene lava flows in the northern part of Hyogo prefecture, Southwest Japan and Brunhes Chron paleosecular variation in Japan

Paleomagnetic investigation was performed on 26 Potassium-Argon dated Pliocene to Pleistocene lava flows, collected at 34 sites from northern Hyogo, Southwest Japan. Most magnetic polarities were found to be coincident with the geomagnetic polarity timescale (GPTS), although two lava flows do not agree with the timescale. One of 2 lava flows incompatible with the GPTS, the Oliveine Andesite lava flow from the Oginosen volcano group dated at 0.92 + / − 0.05Ma, shows normal polarity. This finding is a possible evidence confirming the existence of the Santa Rosa Event proposed recently. Twenty-two paleomagnetic data sets were used to investigate paleosecular variation (PSV) having passed the following criteria: 1) characteristic components are obtained from four specimens or more for a lava flow, 2) ケ95 of the lava flow mean direction is 50° or < −50°. The mean VGP was calculated to be latitude = 85.5°N and longitude = 285.3°E (k = 34.8 and ケ95 = 5.3°). The resultant angular standard deviation (ASD) with respect to the geographic pole was calculated to be 14.5° with the upper limit = 18.3° and the lower limit = 12.1°. The ASD value is almost equal to most existing results from the Japan Islands. Brunhes Chron PSV in Japan was estimated using the identical criteria for selection; 66 of 87 available data sets passed. The mean VGP was calculated to be latitude = 89.3°N and longitude = 14.4°E (k = 33.1 and ケ95 = 3.1°). The resultant ASD with respect to the geographic pole was calculated to be 14.2° with the upper limit = 16.1° and the lower limit = 12.7°. The ASD values for the past 3 million years in northern Hyogo and for the Brunhes Chron in Japan are compatible with the global trend for the past 5 million years fitted by physical PSV Model F and phenomenological PSV Model G. This suggests that the PSV in Japan is not low compared with anomalously low PSV area in the Pacific Region.

Anisotropic paleosecular variation models: implications for geomagnetic field observables

We present a family of statistical models for paleosecular variation (PSV) of the geomagnetic field that are compatible with paleodirectional and paleointensity variations in lava flows sampling the last 5 Ma, and explore what paleomagnetic observables might be used to discriminate among the various family members. We distinguish statistical models with axial anisotropy, which provide a suitable description for an earth with homogeneous boundary conditions at the core–mantle interface from those with more general anisotropy corresponding to geographically heterogeneous boundary conditions. The models revise and extend earlier ones, which are themselves descendants of CP88, devised by Constable and Parker [Constable, C.G., Parker, R.L., 1988. Statistics of the geomagnetic secular variation for the past 5 m.y. J. Geophys. Res. 93, 11569–11581]. In CP88, secular variation is described by statistical variability of each Gauss coefficient in a spherical harmonic description of the geomagnetic field, with each coefficient treated as a normally distributed random variable: the Gauss coefficients of the non-dipole part of the field exhibit isotropic variability, and the variances are derived from the present field spatial power spectrum. The dipole terms have a special status in CP88, with a non-zero mean for the axial-dipole, and lower variance than predicted from the spatial power spectrum. All non-dipole terms have zero mean except the axial-quadrupole. CP88 is untenable for two reasons: it fails to predict the observed geographic dependence of directional variability in the magnetic field, and it grossly underpredicts the variance in paleointensity data. The new models incorporate large variance in the axial-dipole, and in the non-axial-quadrupole Gauss coefficients, g 2 1 and h 2 1. The resulting variance in paleomagnetic observables depends only on latitude (zonal models), unless the variance in h 2 1 is different from that in g 2 1 (non-zonal models). Non-zonal (longitudinal) variations in PSV, such as the flux lobes seen in the historical magnetic field, are simulated using the non-zonal models. Both the zonal and non-zonal models fit summary statistics of the present dataset. We investigate the influence of persistent non-zonal influences in PSV on various paleomagnetic observables. It is shown that virtual geomagnetic pole (VGP) dispersion is rather insensitive to longitudinal variations in structure of PSV, and that inclination dispersion has the potential to be more informative given the right site distribution. There is also the possibility of using paleointensity and geographic variations in the frequency of occurrence of excursional directions to identify appropriate PSV models.

Geomagnetic paleosecular variation at Hawaii around 3 Ma from a sequence of 107 lava flows at Kaena Point (Oahu)

We have sampled a sequence of 107 lava flows in the Waianae series on Oahu, Hawaii, in two separate stretches. The first (51 flows) at Kaena Point extends from sea level to an altitude of 190 m, and the second (56 flows) follows the Satellite Tracking Station road from 218 m to 360 m above sea level. Thermal and af demagnetization yield very similar results, and reveal only normal polarities. K/Ar dating at five different horizons combined with the paleomagnetic results indicate that the sequence was emplaced entirely in the normal polarity interval between the Upper Mammoth and the Lower Kaena polarity transitions (3.22–3.11 Ma). Some of the flows are serially correlated. Filtering does not, however, drastically affect the final results. Inclinations are shallower than those expected from a centered dipole field. Although large (∼13°), the inclination anomaly is consistent with results from other sites at the same latitude. The scatter of the VGPs about the geographic pole is consistent with available data from other regions at similar latitudes. The scatter of the directions is also consistent with the predictions of recent statistical models of paleosecular variation. Therefore, these results do not support the hypothesis of a Pacific dipole window.

Geomagnetic paleosecular variation in the Brunhes period, from the island of El Hierro (Canary Islands)

We have sampled a sequence of 69 flows in the island of El Hierro (Canary Islands). 46 of them belong to the El Golfo formation and 23 to the most recent rift formation. Radiometric dating brackets this section between 442 ka and 134 ka with six intermediate datings. All the samples were stepwise demagnetized, partly with alternative field, partly thermally with very similar results. The large majority of the samples are characterized by a single stable component of magnetization. ChRM isolated after the first steps of demagnetization are all of normal polarity. Inclinations are on average shallower than expected from a geocentered dipole field, but consistent with results from other areas at the same latitude. Mean declinations of the rift and El Golfo sequences are both easterly deviated (about 5° and 15° respectively) and significantly different suggesting that the section has undergone a progressive clockwise rotation. VGP scatter and directional scatter calculated for the rift sequence are consistent with available data from other geographical areas, and also with recent models of paleosecular variation. On the other hand, a significantly smaller directional scatter is obtained for the lower El Golfo sequence, suggesting that a period of low secular variation occurred in the Canary Islands area between approximately 440 and 260 ka.