Hidefumi Tanaka

Palaeomagnetism of Proterozoic mafic intrusions and host rocks of the Mount Isa Inlier, Australia: revisited

A palaeomagnetic study was carried out on Proterozoic dolerite dykes, plugs and some of their host rocks in the intrusion-rich Mount Isa Inlier for which only one other similar study has been reported (Duff and Embleton, 1976). After rejection of intrusions with unstable remanences and internally inconsistent directions, the results from eleven dykes and five plugs remain. These yield two main palaeomagnetic directions: a direction with both polarities and moderate northerly upward (southerly downward) inclination, obtained from metamorphosed dykes and plugs; and a near-vertical downward direction, obtained from unmetamorphosed dykes. The first appears to be associated with a period of regional metamorphism in the Mount Isa Inlier, estimated at ∼ 1620-1500 Ma, and having the last major metamorphic peak near 1554 Ma. A moderate directional dispersion suggests that this magnetization post-dates periods of major deformation of the region, estimated to have ended at around 1550 Ma. Thus the likely age of the magnetization is 1550-1500 Ma or younger, depending on the rate of cooling. Its pole (IM) plots at 110.6°E, 79.0°S (A95=8.4°). The second direction is concordant with the IA group of Duff and Embleton (1976), which includes results from the unmetamorphosed Lakeview Dolerite dyke dated at 1116 Ma. The combined results from previous and present studies on the unmetamorphosed dykes give a pole (IAR) at 311.1°E, 9.5°N (A95=17.4°). Poles IAR and IM suggest very high and middle palaeolatitudes at around 1100 and 1500 Ma, respectively. The new results agree with the most recent version of the Australian pole path by Idnurm and Giddings (1988), and do not challenge the concept of a single path for the continent.

Age and magnetism of lavas in Jökuldalur area, Eastern Iceland: Gilsá event revisited

Abstract We present results of paleomagnetic measurements and K–Ar age determinations of 38 lava flows collected in five separate sections in the Jokuldalur area of Eastern Iceland, including one section previously studied by Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsa geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444]. These sites are close to the type locality of the normal “Gilsa event” in the Matuyama chron first identified by McDougall and Wensink [McDougall, I., Wensink, H., 1966. Paleomagnetism and geochronology of the Pliocene–Pleistocene lavas in Iceland. Earth Planet. Sci. Lett. 1, 232–236]. Using new experimental results as well as field observations, we could correlate the sequence of lava flows in the Jokuldalur area spanning the time interval between 1.8 and 0.5 Ma. The magnetic polarities and ages obtained in this study are quite consistent with the standard time scale for Brunhes–Matuyama ages given by Cande and Kent [Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095] based on marine magnetic anomalies, except that the Gilsa event needs to be added. Existence of Olduvai age lavas under Gilsa was inferred but not certain. Our results are fairly in good agreement with the former studies by Wensink [Wensink, H., 1964a. Secular variation of Earth magnetism in Plio–Pleistocene basalts of eastern Iceland. Geol. Mijnbouw 43, 403–413; Wensink, H., 1964b. Paleomagnetic stratigraphy of younger basalts and intercalated Plio–Pleistocene tillites in Iceland. Geol. Rund. 54, 364–384] and Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsa geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444], but the assignment to the polarity zones is different because of the newly obtained K–Ar ages. Based on the present study, we propose that the Gilsa event is a short normal subchron in the Matuyama chron distinct from and above the more well-established Oldvai subchron.

Paleomagnetism of the late Quaternary Ontake Volcano, Japan: directions, intensities, and excursions

Study of paleosecular variation was carried out to mostly andesite lavas from the late Quaternary Ontake Volcano which have detailed stratigraphy and accurate radiometric ages. Among 42 sites, some of the results from lava successions were combined due to possible equivalent ages, remaining 35 sites. After excluding two low latitude VGPs, angular dispersion of 15.6° was obtained for a period which is accurately confined between 20 ka and 90 ka. The obtained angular dispersion is not different from the typical value for the last 5 my, and this is not surprising if we consider that the global paleointensity was not necessarily low and rather oscillatory during the period of 40–100 ka which precedes the global low in 20–40 ka. Two low latitude VGPs from 48 ka lava (41.9°N, 196.2°E) and 80 ka lava (15.9°N, 183.3°E) indicate existence of excursions in Japan during the latest Pleistocene. Preliminary paleointensity experiments indicate that the 80 ka excursion is accompanied by a low paleointensity of about 5 μT while the 48 ka one is not. These two excursions are probably related to those reported previously not only from the Ontake Volcano and its proximity but also from other several sites in Japan, although they were not conclusive. The 48 ka excursion probably correlates to the Laschamp excursion. This is consistent with the fact that the VGP position from the 48 ka lava comes to the central Pacific region, similar to those from the Laschamp excursion found in New Zealand. Possible correlation of the 80 ka excursion is the one from Norwegian-Greenland Sea and Arctic Ocean, which are stratigraphically between the Laschamp and the Blake excursions.

Circular asymmetry of the paleomagnetic directions observed at low latitude volcanic sites

The shape of the distribution in field directions and VGP positions was studied by Bingham statistics applied to a paleomagnetic dataset from lavas for the last 5 my by McElhinny and McFadden (1997), which includes those with VGP latitude higher than 45°. Data from Hawaii clearly show an oval shaped distribution of field directions, elongated along the meridian plane, while distribution of VGP is almost circular. Analysis on the global data divided by latitudinal bands also indicate more elongation in field directions for low latitude bands. This feature was interpreted as a general indication of dipole nature of the paleomagnetic field. For Hawaii data, however, possibility of the Pacific Nondipole Low is also suggested due to the large elongation parameter and very high significance of Fisher distribution to describe the VGP positions together with the small ASD. Although this elongated shape in the distribution of field directions is not clear for most of the individual site, the directions of the principal axes are sensitive enough to depict this feature at almost all sites.

Paleointensities for 10–22 ka from volcanic rocks in Japan and New Zealand

Abstract Eight volcanic rocks from Japan and New Zealand in the age range 10–50 ka B.P. were studied using the Thellier method paleointensity experiment and yielded five successful results for 10–22 ka. Samples from the Shikotsu, Kuttara and Daisen volcanoes in Japan were taken from pyroclastic flows (both welded and non-welded) and a pumice fall deposit. One of the two New Zealand rocks is a rhyolite lava from the Taupo Volcanic Zone. Four sites out of seven located on rocks that are not usually subjected to Thelliers experiment yielded successful results, which indicates that volcanic products such as pyroclastic flows and rhyolite lavas are as good a material as basalt and andesite lavas for application of the paleointensity experiment. One paleointensity from New Zealand at 9.9 ka is very large, at about 100 μT, and provides support for the paleointensity high found in Europe and Japan [1,2]. The other four paleointensities for 14–22 ka are smaller than the present-day value, and this agrees with the idea of a broad paleointensity minimum for the period 10–50 ka that was suggested by McElhinny and Senanayake [3]. It is also noted that the variation curve of absolute paleointensity from the volcanic rocks is reasonably similar to the relative paleointensity curves from marine and lake sediments which were summarized by Tauxe [4].

Spherical harmonic analysis of paleomagnetic data: The case of linear mapping

We consider the problem of describing the ancient magnetic field as completely as possible using paleomagnetic data. One of the best ways to do this is to represent the field by the use of Gauss coefficients. Unlike ordinary cases, however, spherical harmonic analysis of paleomagnetic data is a complicated problem because (1) the data cannot be considered as contemporaneous and (2) they are mostly available only in the form of directions. If the mapping between the model parameters (Gauss coefficients) and the data is linear (e.g., the three components of the field), there is one-to-one correspondence between the means of the parameters and the means of the data. Conventional least squares techniques can be applied to the means as the relations between the data and parameters are the same as in the instantaneous case. One-to-one correspondence also exists between the variances of the parameters and the variances of the data. However, the functions appearing in these relations are not orthogonal to each other, and the matrix to be inverted is quite ill conditioned. For magnetic field directions that are nonlinearly related to Gauss coefficients, one-to-one correspondence is completely absent between the means (variances) of the parameters and those of the data. This means that inversion becomes quite complicated because the mean of such data contains information on both the mean and fluctuations of the parameters. We applied inversion to the paleointensity data of the past 5 Myr and obtained the means and variances of Gauss coefficients for that period. The means are well determined and show that the time-averaged field is dominated by the axial dipole component. It is more axisymmetric than the recent field, suggesting that equatorial dipole components are largely averaged out as assumed by paleomagnetic dipole hypothesis. Inversion of the variances gave less well determined results. Among the variances of Gauss coefficients that are significantly different from zero that of the axial dipole is much larger than those of other coefficients, indicating that the observed variation in paleointensity is largely due to the fluctuation of the magnitude of the dipole moment.

Paleointensity database provides new resource

A global paleointensity database constructed from all published data based on volcanic rocks in geological time older than 0.03 Ma is now available and ready for use. It can be obtained from the authors as text files on a 3.5″ diskette in MS-DOS or Macintosh format or via e-mail. It includes not only major parameters, but also minor details that can be used to assess data quality. Each datum consists of reference, latitude and longitude of the site, age and its error, experimental method, site identification, site mean paleointensity and its error, number of samples, virtual dipole moment (VDM), polarity, inclination, and declination. The database contains a total of 1123 flow mean data retrieved from 83 original papers, which were published before or during 1991.

Magnetic fabrics study and inferred flow directions of lavas of the Old Pali Road, O′ahu, Hawaii

Abstract A section 600–700 m deep in the Koolau tholeiitic lava-shield volcano consists of mostly thin pahoehoe and aa flows cut by occasional dikes and outward-dipping gravity-driven intrusive sheets. A magnetic fabric study of 250 specimens from 18 sites shows a tight clustering of K1 axes for some of the lava flows and is thought to fairly reliably define the lava flow direction. A systematic plunge of the K1 axes appears to define an imbrication and yields a flow azimuth. The azimuth differs by 60° from that anticipated but is readily explained by a change in shape of the Koolau Volcano during growth. The tightest clustering is in massive aa lava flows 5 m thick. Samples collected from near the flow base give the most consistent orientations. Compound pahoehoe lavas with many small flow units give the least tightly clustered anisotropy of magnetic susceptibility (AMS) axes and, in some flows, the orientation is random. The gravity-driven sheet has an AMS fabric indicating a down-dip magma flow direction. After the Koolau Volcano was deeply eroded, a cinder cone and lava flows of nephelinite were formed in the rejuvenation stage of volcanism. The nephelinite is normally magnetized and the AMS fabric indicates a generally downslope flow.

Palaeosecular variation for 0.1-21 Ka from the Okataina Volcanic Centre, New Zealand

Studies of palaeodirections and palaeointensities were carried out on mainly rhyolitic lavas and pyroclastics from the Okataina Volcanic Centre, New Zealand, which has erupted during the past 32 kyr. Of the 17 sites studied, 14, spanning the period 0.1-21 Ka yielded good mean palaeodirections, while three carried unstable natural remanent magnetizations. Of 49 specimens from 7 sites, on which Thellier palaeointensity experiments were carried out, 21 specimens gave successful results, yielding 3 site mean palaeointensities for 1886 AD, 5 Ka, and 7.5 Ka. When the new palaeodirections, together with previously reported archaeo and volcanic data, were compared with sedimentary records from New Zealand and eastern Australia for the last 10 kyr, good agreement was obtained in inclination but discrepancies were observed in declination. The new 7.5 Ka and 5 Ka palaeointensities are moderately high and relatively low, respectively, and are concordant with the global trend. The mean palaeointensity obtained for 1886 AD is, 11% higher than the IGRF1900. The difference is scarcely significant, but might indicate a small bias toward high values. Although the sedimentary directional curves show excellent agreement with the prediction from CALS7K, the fit of the palaeointensity data to model values was relatively poor over the wider Pacific region. Further reliable palaeointensity data are needed to solve the discrepancy.

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

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 (A95 = 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.