Outline
Comptes Rendus

Internal Geophysics
Absolute paleointensity of the Earth's magnetic field during Jurassic: case study of La Negra Formation (northern Chile)
Comptes Rendus. Géoscience, Volume 335 (2003) no. 8, pp. 661-670.

Abstracts

We carried out a detailed rock-magnetic and paleointensity study of the ∼187-Ma volcanic succession from northern Chile. A total of 32 consecutive lava flows (about 280 oriented standard paleomagnetic cores) were collected at the Tocopilla locality. Only 26 samples with apparently preserved primary magnetic mineralogy and without secondary magnetization components were pre-selected for Thellier paleointensity determination. Eleven samples coming from four lava flows yielded reliable paleointensity estimates. The flow-mean virtual dipole moments range from 3.7±0.9 to 7.1±0.5(1022 A m2). This corresponds to a mean value of (5.0±1.8)×1022 A m2, which is in reasonably good agreement with other comparable quality paleointensity determinations from the Middle Jurassic. Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field.

Une étude détaillée du magnétisme des roches et de la paléointensité a été menée sur la succession volcanique, datée d'environ 187 Ma, du Nord-Chili. Un total de 32 coulées de lave consécutives (environ 280 carottes paléomagnétiques standard orientées) ont été échantillonnées à Tocopilla. Seuls 36 échantillons ayant apparemment conservé une minéralogie magnétique primaire et ne comportant pas de constituants à aimantation secondaire ont été présélectionnés pour la détermination de la paléointensité de Thellier. Onze échantillons, provenant de quatre coulées, ont fourni des estimations de paléointensité fiables. Les moments dipolaires virtuels de la coulée moyenne s'échelonnent entre 3,7±0,9 et 7,1±0,5(1022 A m2). Ceci correspond à une valeur moyenne de (5,0±1,8)×1022 A m2, ce qui est en bon accord avec les autres déterminations de paléointensité, comparables en qualité, du Jurassique moyen. Étant donné la grande dispersion et la médiocre répartition de données d'intensité absolue fiables, il est difficile de tirer des conclusions solides à propos de l'évolution du champ géomagnétique dans le temps.

Metadata
Received:
Accepted:
Published online:
DOI: 10.1016/S1631-0713(03)00116-0
Keywords: paleointensity, paleomagnetism, Jurassic, northern Chile
Keywords: paléointensité, paléomagnétisme, Jurassique, Nord-Chili
Juan Morales 1; Avto Goguitchaichvili 1; Luis M. Alva-Valdivia 1; Jaime Urrutia-Fucugauchi 1

1 Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofisica, UNAM, Ciudad Universitaria, 04510 México DF, Mexico
@article{CRGEOS_2003__335_8_661_0,
     author = {Juan Morales and Avto Goguitchaichvili and Luis M. Alva-Valdivia and Jaime Urrutia-Fucugauchi},
     title = {Absolute paleointensity of the {Earth's} magnetic field during {Jurassic:} case study of {La} {Negra} {Formation} (northern {Chile)}},
     journal = {Comptes Rendus. G\'eoscience},
     pages = {661--670},
     publisher = {Elsevier},
     volume = {335},
     number = {8},
     year = {2003},
     doi = {10.1016/S1631-0713(03)00116-0},
     language = {en},
}
TY  - JOUR
AU  - Juan Morales
AU  - Avto Goguitchaichvili
AU  - Luis M. Alva-Valdivia
AU  - Jaime Urrutia-Fucugauchi
TI  - Absolute paleointensity of the Earth's magnetic field during Jurassic: case study of La Negra Formation (northern Chile)
JO  - Comptes Rendus. Géoscience
PY  - 2003
SP  - 661
EP  - 670
VL  - 335
IS  - 8
PB  - Elsevier
DO  - 10.1016/S1631-0713(03)00116-0
LA  - en
ID  - CRGEOS_2003__335_8_661_0
ER  - 
%0 Journal Article
%A Juan Morales
%A Avto Goguitchaichvili
%A Luis M. Alva-Valdivia
%A Jaime Urrutia-Fucugauchi
%T Absolute paleointensity of the Earth's magnetic field during Jurassic: case study of La Negra Formation (northern Chile)
%J Comptes Rendus. Géoscience
%D 2003
%P 661-670
%V 335
%N 8
%I Elsevier
%R 10.1016/S1631-0713(03)00116-0
%G en
%F CRGEOS_2003__335_8_661_0
Juan Morales; Avto Goguitchaichvili; Luis M. Alva-Valdivia; Jaime Urrutia-Fucugauchi. Absolute paleointensity of the Earth's magnetic field during Jurassic: case study of La Negra Formation (northern Chile). Comptes Rendus. Géoscience, Volume 335 (2003) no. 8, pp. 661-670. doi : 10.1016/S1631-0713(03)00116-0. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/S1631-0713(03)00116-0/

Version originale du texte intégral

1 Introduction

Geomagnetic paleointensity data are critical for understanding the workings of the Earth's dynamo. Coe et al. [7] recently suggested that absolute intensity should be a fundamental constraint in numerical models that promise to provide unprecedented insight into the operation of the geodynamo. However, absolute paleointensity data are still scarce and cannot be yet used to document the general characteristics of the Earth's magnetic field [36]. Reliable paleointensity values are generally much more difficult to obtain than reliable directional data, because only volcanic rocks which satisfy some very specific magnetic criteria [4,20] can be used for absolute paleointensity determination.

Judging from the existing paleointensity database (ftp://ftp.dstu.univ-montp2.fr/pub/paleointb, see also [26]) only large-scale features can be resolved, such as the pronounced lowering of the Earth's Virtual Dipole Moment (VDM) in the whole Mesozoic and part of Paleozoic [29], during which the dipole structure of the field was preserved [25]. However, high geomagnetic intensities have been recently reported [14,38] for the Middle Cretaceous and thus the reliability of Mesozoic Dipole Low (MDL) may be questioned. To try to obtain more constraints for the Mesozoic geomagnetic field strength, we carried out Thellier paleointensity experiments on ∼187 Ma old La Negra volcanic succession in northern Chile. Obtaining of new paleointensity and paleodirection determination from the southern America volcanics is critical, because the geographic distribution of paleointensity data is still very uneven (e.g., [37]). In particular, very few data are available from the southern hemisphere, which impedes an accurate analysis of the fine-scale changes in the statistical characteristics of paleosecular variations [16].

2 Sampling details

The La Negra Formation (Figs. 1 A and B) comprises a thick pile of lava flows, largely of high-K basaltic andesites, with thin intercalated sediments. It seems that they erupted in an extensional environment [34] and that, together with their chemical similarity to the Puente Piedra volcanics in Peru [3] and the lavas at Bustamante Hill in Central Chile [22], suggests they were erupted in an ensialic back-arc basin rather than in an actual volcanic arc.

Fig. 1

(A) Sketch map of northern Chile illustrating the locality of paleomagnetic sampling (modified from [34]). (1) La Negra Formation, (2) and (3) are Gatico (∼158 Ma) and Tocopilla (∼155 Ma) intrusions, respectively. (B) Simplified stratigraphy of Antofagasta region, northern Chile, with the principal names of geological formations (redrawn from [2]).

(A) Carte schématique du Nord-Chili, montrant l'emplacement des lieux d'échantillonnage paléomagnétique (modifié d'après [34]). (1) Formation de La Negra ; (2) et (3), intrusions de Gatico (∼158 Ma) et Tocopilla (∼155 Ma). (B) Stratigraphie simplifiée de la région d'Antofagasta, Nord-Chili, avec les principales dénominations des formations géologiques (redessiné à partir de [2]).

Paleomagnetic sampling was done near of town of Tocopilla (Fig. 1). Rogers et al. [34] proposed a mean age of 187 Ma as the best estimate of the time of emplacement of the lower La Negra volcanic sequence. It is only regrettable that these authors do not report the error on their K–Ar age. We note, however, that the sample dated by Rogers and Hawksworth [34] most probably belongs to site NE06 of our collection (lower part of La Negra sequence [1]). In total, about 280 oriented samples belonging to 32 consecutive lava flows were collected. Commonly, the outcrops extend laterally over a few tens of meters and, in these cases, we drilled typically 6–11 standard paleomagnetic cores per flow. The samples were distributed throughout each flow both horizontally and vertically in order to minimize effects of block tilting and lightning. Cores were obtained with a gasoline-powered portable drill, and then oriented with both magnetic and sun compasses.

3 Rock-magnetic characteristics of selected samples

Magnetic characteristics of typical samples selected for Thellier paleointensity measurements are summarized in Fig. 2 (upper part) and could be described as follows.

Fig. 2

Summary of the magnetic characteristics of the typical samples, selected (class 1) and unselected (classes 2 and 3; see also text) for Thellier paleointensity experiments. (A) Orthogonal vector plots of stepwise alternating field demagnetization (stratigraphic coordinates). The numbers refer to peak alternating fields in mT. : projections into the horizontal plane, ×: projections into the vertical plane. (B) Susceptibility versus temperature curves. The arrows indicate the heating and cooling curves.

Résumé des caractéristiques magnétiques des échantillons typiques, sélectionnés (classe 1) et non sélectionnés (classes 2 et 3; voir aussi le texte) pour les expériences de paléointensité de Thellier. (A) Représentation par rapport à des vecteurs orthogonaux de la désaimantation par palier du champ alternatif (coordonnées stratigraphiques). Les nombres se rapportent au pic des champs alternés, en mT. : Projections dans le plan horizontal, × projections dans le plan vertical. (B) Courbes de susceptibilité en fonction de la température. Les flèches indiquent les courbes d'augmentation et de diminution de la température.

1. Selected samples carry essentially a single and stable component of magnetization, observed upon alternating field (Fig. 2A, sample NE2–011C) treatment. A generally minor secondary component, probably of viscous origin, was present but was easily removed. The median destructive fields (MDF) range mostly in the 40–60-mT interval, suggesting the existence of small pseudo-single to single magnetic domain grains as remanence carriers, which are suitable material for the Thellier paleointensity study [8].

2. Low-field continuous susceptibility measurements with temperature show the presence of a single ferrimagnetic phase with Curie point compatible with Ti-poor titanomagnetite. However, the cooling and heating curves are sometimes not perfectly reversible (Fig. 2B, sample NE2–011D) probably due to the heating in air. In addition, hysteresis measurements were carried out on all samples (not shown). However, we believe that the magnetic domain state estimation using room temperature hysteresis parameters in terms of the plot of magnetization ratio vs. coercivity ratio has no resolution for most of natural rocks [12]. Thus, these data were not used to select the most promising samples for Thellier paleointensity experiments.

In all, we selected 26 samples for the paleointensity experiments having the above-described magnetic characteristics. Remaining samples may be divided in two groups (Fig. 2, middle and lower parts) after their magnetic properties: (i) the samples yield reasonably reversible continuous susceptibility curves (Fig. 2B, sample NE24–168A) associated with the remanence dominated by strong secondary magnetization (Fig. 2A, sample NE24–171A); (ii) the kT curves of these samples yield the evidence of two ferrimagnetic phases during heating (Fig. 2B, sample NE9–055). The lower Curie point ranges between 350–400 °C, and the highest one is about 580 °C. The cooling curve shows only a single phase, with a Curie temperature close to that of magnetite. Such irreversible kT curves can be explained by titanomaghemite, which probably transformed into magnetite [24,30] during heating. It is possible that these samples carry chemical remanent magnetization. In addition, this behavior is related by very unstable multivectorial magnetization (new version of Fig. 2A, sample NE09–055B). It is obvious that these samples cannot be used to determine absolute geomagnetic paleointensity.

4 Paleodirections

As part of their efforts to study the details of the volcanic stratigraphy and paleotectonics on Antofagasta region (northern Chile), Alva-Valdivia et al. [1] recently carried out a detailed paleomagnetic study of La Negra Formation. The reliable paleodirections were obtained for 25 sites among 32 sampled (Fig. 3). These directions are considered to be of primary origin because of the occurrence of almost antipodal normal and reversed polarities (Fig. 3). In addition, unblocking temperature spectra and relatively high coercivities point to ‘small’ pseudo-single domain magnetic structure grains as responsible for remanent magnetization. The secondary magnetizations were successfully removed in most cases using the alternating-field demagnetization technique.

Fig. 3

(A) Equal area projections of the flow mean characteristic paleodirections for La Negra Formation. Circles/Crosses denote to the negative/positive inclination. (B) Equal area projection of the mean directions for Antofagasta–Tocopilla volcanic units [1].

(A) Projections en surface des paléodirections caractéristiques moyennes de coulée dans le cas de la Formation de La Negra. Les cercles/croix indiquent l'inclinaison négative/positive. (B) Projections en surface des directions moyennes pour les unités volcaniques d'Antofagasta –Tocopilla [1].

The mean paleodirection obtained in this study is I=-30.7,D=11.3°,k=18,α95=6.8°,N=25 (Fig. 3). The previous paleomagnetic study provide basically similar directions with I=-39.2°,D=9.1°,k=11,α95=12°,N=15 [2]. Combining both data, we obtained a well-defined mean paleomagnetic direction with I=-33.9°,D=10.5°,k=15,α95=6.1°,N=40. These directions are in good agreement with the expected paleodirections at about 180 Ma [35] (Fig. 3B). This indicates that studied units do not show evidence for significant tectonic rotation. Fifteen sites yield reverse polarity magnetization and 11 flows are normally magnetized. The tentative direct magnetostratigraphic correlation suggests that La Negra volcanics have been emplaced during a relatively large time span of about 5 Myr [1].

5 Paleointensity determination

Paleointensity experiments were performed using the Thellier method in its classic form [41]. Heating was made in a vacuum better than 10−2 mbar. All remanences were measured using both JR5A and JR6 spinner magnetometers. Paleointensity data are reported on the Arai-Nagata [23] plot in Fig. 4 and results are given in Table 1. We accepted only determinations: (1) obtained from at least 5 NRM-TRM points corresponding to a NRM fraction larger than about 1/3 (Table 1), (2) yielding quality factor [6] generally above 5, (3) with positive ‘pTRM’ checks i.e. the deviation of ‘pTRM’ checks were less than 15% and (4) with reasonably linear Zijderveld diagrams obtained from the paleointensity experiments. No significant deviation of NRM remaining directions towards the direction of applied laboratory field was observed. To better illustrate this point, we calculated the ratio of potential CRM(T) to the magnitude of NRM(T) for each double heating step in the direction of the laboratory field during heating at T [9]. The values of angle γ – the angle between the direction on characteristic remanent magnetization (ChRM) obtained during the demagnetization in zero field and that of composite magnetization (equal to NRM(T) if CRM(T) is zero) obtained from the orthogonal plots derived from the Thellier paleointensity experiments – are reported in Table 1. For accepted determinations γ values are all <10.6°, which attests that no significant CRM was acquitted during laboratory heating [9]. This approach is probably more restrictive than simple calculation of angle between the characteristic directions determined by Thellier experiments and by thermal demagnetization in zero field.

Fig. 4

The representative NRM–TRM plots and associated orthogonal diagrams for La Negra samples. In the orthogonal diagrams we used same notations as in Fig. 2A.

Courbes représentatives NRM–TRM et diagrammes orthogonaux associés pour les échantillons de La Negra. Dans les diagrammes orthogonaux sont utilisées les mêmes notations que sur la Fig. 2A.

Table 1

Paleodirectional and paleointensity results from La Negra volcanic lava flows: N, number of treated samples; n, number of specimens used for calculation, Dec: declination, Inc: Inclination, k and α95: precision parameter and radius of confidence cone, Inc′ is the individual inclination derived from Thellier paleointensity experiments, N′ is number of NRM–TRM points used for paleointensity determination, TminTmax is the temperature interval used, f,g and q are the fraction of extrapolated NRM used, the gap factor and quality factor [6] respectively. γ is the angle between the direction on characteristic remanent magnetization (ChRM) obtained during the demagnetization in zero field and that of composite magnetization (equal to NRM(T) if CRM(T) is zero) (see text and [9]), FE is the individual paleointensity estimate with standard deviation, VDM and VDMe are individual and average virtual dipole moments

Valeurs de paléodirections et paléointensité obtenues sur les coulées volcaniques de La Negra. N, nombre d'échantillons traités; n, nombre d'échantillons utilisés pour le calcul ; Dec: déclinaison ; Inc: inclinaison ; k et α95: paramètre de précision et rayon du cône de confiance ; Inc ': inclinaison individuelle dérivée des expériences de paléointensité de Thellier. N′: nombre de points NRM–TRM utilisés pour la détermination de la paléointensité ; TminTmax: intervalle de température utilisé ; f,g,q représentent respectivement la fraction des NRM extrapolés utilisée, le facteur de lacune, le facteur de qualité [6]; γ est l'angle existant entre la direction d'aimantation rémanente caractéristique (ChRM) obtenue pendant la désaimantation dans le champ zéro et celle de l'aimantation composée (égale à NRM(T) si CRM(T) est égal à zéro [9]); FE est la paléointensité individuelle estimée par déviation standard ; VDM et VDMe sont les moments dipolaires virtuels individuels et moyens

Paleodirections Paleointensity
Site n/N Dec (°) Inc (°) α 95 ( ° ) k Specimen Inc ' ( ° ) TminTmax N f g q γ FE±σ(FE) VDM VDMe
NE02 6/6 14.8 −49.2 5.5 148 01N007C −48.4 350−520 6 0.54 0.78 5.9 3.3 18.2±1.7 3.59 4 . 2 ± 0 . 5
01N008B −49.6 400−520 5 0.55 0.78 8.4 4.4 21.5±1.3 4.18
01N011C −56.2 400−540 6 0.58 0.81 9.8 1.5 26.1±2.6 4.69
01N012C −54.3 450−560 6 0.75 0.79 6.5 2.9 24.0±2.1 4.41
NE11 6/6 203.4 57.3 4.3 159 01N062C 51.4 250−500 7 0.42 0.82 4.5 10.6 35.1±3.1 6.68 7 . 1 ± 0 . 5
01N063B 54.3 250−500 7 0.45 0.85 6.6 6.3 41.7±3.0 7.67
01N066C 60.5 250−520 8 0.51 0.87 10.7 2.1 40.4±1.5 6.87
NE19 7/7 11.3 −36.2 6.9 76 01N142B −38.6 300−520 7 0.45 0.86 9.4 0.8 13.8±0.7 3.0 3 . 7 ± 0 . 9
01N143C −39.3 300−500 6 0.30 0.83 4.4 8.8 21.6±1.7 4.67
01N145B −44.2 300−500 6 0.35 0.87 6.3 5.3 16.3±1.1 3.36
NE20 6/6 5.4 −18.8 5.4 128 01N146B −19.6 250−500 7 0.31 0.82 5.5 7.7 14.5±0.7 3.59

6 Main results and discussion

Only 11 samples, coming from four individual lava flows, yielded acceptable paleointensity estimates. The almost 95% failure rate that we find in our study is not exceptional for a Thellier paleointensity study of old volcanic rocks, if correct pre-selection of suitable samples and strict analysis of the obtained data are made. The Thellier and Thellier [41] method of geomagnetic absolute intensity determination, which is considered the most reliable one, imposes many restrictions on the choice of samples that can be used for a successful determination [4,5,19,21,27,28].

Although our results are not numerous, some credit should be given because of good technical quality determination, attested by the reasonably high Coe et al.'s quality factors [6]. For the accepted determinations, the NRM fraction f used for determination ranges between 0.30 to 0.75 and the quality factor q varies from 4.4 to 10.7 (generally more than 5). The mean virtual dipole moments (VDMs) are ranging from 3.7±0.9 to 7.1±0.5(1022 A m2). This corresponds to a mean value of (5.0±1.8)×1022 A m2, which is in agreement with other comparable quality paleointensity determination for Middle Jurassic [20,36]. The site NE20, which is represented only by single determination, was discarded to calculate mean VDM.

La Negra mean VDM is shown in Fig. 5 together with 16 other selected mean VDMs and VADMs (virtual axial dipole moment) for the period 5–200 Ma. We selected data using quite modest criteria [33] demanding (a) the mean based on more than nine successful determinations from at least three cooling units, (b) no transitional data and (c) paleointensity estimates obtained with Thellier method with pTRM checks. The perhaps most striking observation from inspecting Fig. 5 is that, in spite of the many published paleointensity studies, there exists only 17 paleomagnetic dipole moment estimates in the 5–200-Ma period that fulfill the above mentioned modest criteria. Among them, six are from submarine basaltic glass, one from single plagioclase crystal and remaining VDMs come from whole-rock samples. The excellent technical quality of paleointensity data obtained from basaltic glasses, first underlined by Tauxe and Pick [39,40] and later by Juarez et al. [17,18] is not by itself a proof of the geomagnetic validity of the paleostrength found. Goguitchaichvili et al. [10] showed that the magnetite found in this kind of material may be not of magmatic origin, but crystallizes at rather low temperatures as a result of either glass demixtion or alteration. In such a case, the Thellier paleointensity results would underestimate the actual paleofield strength. More data are needed before the intriguing differences between different materials can be put into a geomagnetic context.

Fig. 5

Evolution of mean virtual dipole moments (VDMs) and virtual axial dipole moments (VADMs) for 5 to 200 Ma. Open squares are mean VDM obtained from continental lava flows [11,13,14,28,31–33,42] (this study – big symbols). Diamonds are mean VADMs obtained from submarine basaltic glass [36]. The closed circles are mean VDM-obtained single plagioclase crystals [38].

Évolution des moments dipolaires virtuels moyens (VDMs) et des moments dipolaires axiaux virtuels (VADMs) entre 5 et 200 Ma. Les carrés représentent les VDM moyens obtenus à partir de coulées de laves continentales [11,13,14,28,31–33,42] (cette étude – grands symboles). Les losanges représentent les VADMs moyens obtenus à partir de verre basaltique sous-marin [36]. Le cercle noir représente le VDM moyen obtenu à partir de monocristaux de plagioclase [38].

Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. As showed by Guyodo and Valet [15], a very large number of records of absolute intensity, reinforced by numerous and precise ages, are necessary to provide a reasonable picture of the field variations trough time.

Acknowledgements

The comments of Mireille Perrin and one anonymous referee on an early version of this manuscript lead to significant improvements of the scientific content and English style of this paper. Economic support for this project has been provided by CONACYT project J32727-T. LA and AG are grateful to financial support given by DGAPA-UNAM grant No. IN100100.

The authors are grateful to Dr Fernando Henríquez for the invaluable help during the field trip.


References

[1] L. Alva-Valdivia, A. Goguitchaichvili, J. Urrutia-Fucugauchi, Paleomagnetism and rock-magnetism of the Jurassic La Negra Formation, Northern Chile: implications for tectonics and volcanic stratigraphy, Int. Geol. Rev., in press

[2] C. Arriagada, P. Roperch, C. Mpodozis, G. Dupont-Nivet, P. Cobbold, A. Chauvin, J. Cortes, Paleogene clockwise tectonic rotations in the fore-arc of Central Andes, Antofagasta region, northern Chile, J. Geophys. Res. – Solid Earth, in press

[3] M.P. Atherton; W.S. Pitcher; V. Warden The Mesozoic marginal basin of central Peru, Nature, Volume 305 (1983), pp. 303-306

[4] M. Calvo; M. Prévot; M. Perrin; J. Riisager Investigating the reasons for the failure of paleointensity experiments: A study on historical lava flows from Mt. Etna (Italy), Geophys. J. Inter., Volume 149 (2002), pp. 44-63

[5] R. Coe Paleointensity of the Earth's magnetic field determined from Tertiary and Quaternary rocks, J. Geophys. Res., Volume 72 (1967) no. 12, pp. 3247-3262

[6] R. Coe; S. Grommé; E.A. Mankinen Geomagnetic paleointensity from radiocarbon-dated flows on Hawaii and the question of the Pacific nondipole low, J. Geophys. Res., Volume 83 (1978), pp. 1740-1756

[7] R. Coe; L. Hongre; G. Glatzmaier An examination of simulated geomagnetic reversals from a paleomagnetic perspective, Philos. Trans. Roy. Soc. London Ser. A, Volume 357 (2000), pp. 1787-1813

[8] D. Dunlop Ö. Özdemir, Rock-Magnetism, Fundamentals and Frontiers, Cambridge University Press, Cambridge, UK, 1997

[9] A. Goguitchaichvili; M. Prévot; N. Roberts; J. Thompson An attempt to determine the absolute geomagnetic field intensity in Southwestern Iceland during the Gauss–Matyama reversal, Phys. Earth Planet. Lett., Volume 115 (1999), pp. 53-66

[10] A. Goguitchaichvili; M. Prévot; J.-M. Dautria; M. Bacia Thermo-detrital and crystalline magnetizations in an Icelandic hyaloclastite, J. Geophys. Res., Volume 104 (1999), pp. 29219-29239

[11] A. Goguitchaichvili; L. Alva-Valdivia; J. Urrutia-Fucugauchi; J. Morales; L. Ferrari Absolute paleointensity results from the Trans Mexican Volcanic Belt: implication for the Late Miocene Geomagnetic field strength, Gephys. J. Int., Volume 143 (2000), pp. 977-985

[12] A. Goguitchaichvili; J. Morales; J. Urrutia-Fucugauchi On the use of thermomagnetic curves in paleomagnetism, C. R. Acad. Sci. Paris, Ser. IIa, Volume 333 (2001), pp. 699-704

[13] A. Goguitchaichvili; L. Alva-Valdivia; J. Urrutia-Fucugauchi; C. Zesati; C. Caballero Paleomagnetic and paleointensity study of Oligocene volcanic rocks from Chihuahua (northern Mexico), Phys. Earth Planet. Int., Volume 124 (2001), pp. 223-236

[14] A. Goguitchaichvili; L. Alva-Valdivia; J. Urrutia-Fucugauchi; J. Morales; O. Ferreira-Lopes On the reliability of Mesozoic dipole low: new absolute paleointensity results from parana flood basalts (Brazil), Geophys. Res. Lett., Volume 29 (2002) no. 13, p. 1655

[15] Y. Guyodo; J.-P. Valet Integration of volcanic and sedimentary records of paleointensity: constraints imposed by irregular eruption rates, Geophys. Res. Lett., Volume 26 (1999), pp. 3669-3672

[16] J.A. Jacobs Reversals of the Earth Magnetic Field, Cambridge, New York, 1994

[17] M.T. Juàrez; L. Tauxe The intensity of time-averaged geomagnetic field: the last 5 Myr, Earth Planet. Sci. Lett., Volume 175 (2000), pp. 169-180

[18] M.T. Juàrez; L. Tauxe; J.S. Gee; T. Pick The intensity of the Earth's magnetic field over the past 160 million years, Nature, Volume 394 (1998), pp. 878-881

[19] A. Kosterov; M. Prévot Possible mechanism causing failure of Thellier paleointensity experiments in some basalts, Geophys. J. Inter., Volume 134 (1998), pp. 554-572

[20] A.A. Kosterov; M. Prévot; M. Perrin; V. Shashkanov Paleointensity of the Earth's magnetic field in the Jurassic: new results from a Thellier study of the Lesotho Basalt, southern Africa, J. Geophys. Res., Volume 102 (1997) no. B11, pp. 24859-24872

[21] S. Levi The effect of magnetite particle size in paleointensity determination of the geomagnetic field, Phys. Earth Planet. Inter., Volume 13 (1977), pp. 245-259

[22] B. Levi; L. Aguirre; J.O. Nystrom Metamorphic gradients in burial metamorphosed vesicular lavas: comparison of basalt and spilite in Cretaceous basic flows from central Chile, Contrib. Mineral. Petrol., Volume 80 (1982), pp. 49-58

[23] T. Nagata; R.M. Fisher; K. Momose Secular variation of the geomagnetic total force during the last 5000 years, J. Geophys. Res., Volume 68 (1963), pp. 5277-5281

[24] Ö. Özdemir Inversion of titanomaghemites, Phys. Earth Planet. Inter., Volume 65 (1987), pp. 125-136

[25] M. Perrin; V.P. Shcherbakov Paleointensity of the earth magnetic field for the past 400 My: evidence for a dipole structure during the Mesozoic low, J. Geomagn. Geolectr., Volume 49 (1997), pp. 601-614

[26] M. Perrin; E. Schnepp; V. Shcherbakov Paleointensity database updated, EOS, Trans. Am. Geophys. Union, Volume 79 (1998), p. 198

[27] T. Pick; L. Tauxe Geomagnetic palaeointensities during the Cretaceous normal superchron measured using submarine basaltic glass, Nature, Volume 366 (1993), pp. 238-242

[28] M. Prévot; R.S. Mainkinen; R. Coe; S. Grommé The Steens Mountain (Oregon) geomagnetic polarity transition. 2. Field intensity variations and discussion of reversal models, J. Geophys. Res., Volume 90 (1985), pp. 10417-10448

[29] M. Prévot; M. Derder; M.M. McWilliams; J. Thompson Intensity of the Earth's magnetic filed: evidence for a Mesozoic dipole low, Earth Planet. Sci. Lett., Volume 97 (1990), pp. 129-139

[30] P.W. Readman; W. O'Reilly Magnetic properties of oxidized (cation-deficient) titanomagnetites, (Fe, Ti)O4, J. Geomagn. Geoelectr., Volume 24 (1972), pp. 69-90

[31] J. Riisager; M. Perrin; P. Rochette First paleointensity results from Ethiopian flood basalts: implication for the Oligocene geomagnetic field strength, Geophys. J. Inter., Volume 138 (1999), pp. 590-596

[32] J. Riisager; M. Perrin; P. Riisager; G. Ruffet Paleomagnetism, paleointensity and geochronology of Miocene basalts and baked sediments from Valey Oriental, French Massif Central, J. Geophys. Res., Volume 105 (2000), pp. 883-896

[33] P. Riisager; J. Riisager; N. Abrahamsen; R. Waagstein Thellier paleointensity experiments on Faroes flood basalts: technical aspects and geomagnetic implications, Phys. Earth Planet. Inter., Volume 131 (2002), pp. 91-100

[34] G. Rogers; C.J. Hawkesworth A geochemical traverse across the North Andes: evidence for crust generation from the mantle wedge, Earth Planet. Sci. Lett., Volume 91 (1989), pp. 271-285

[35] P. Roperch; G. Carlier Paleomagnetism of Mesozoic rocks from the Central Andes of Southern Peru: importance of rotations in the development of the Bolivian Orocline, J. Geophys. Res., Volume 97 (1992), pp. 8312-8333

[36] P.A. Selkin; L. Tauxe Long-term variations in palaeointensity, Philos. Trans. Roy. Soc. London, Volume 358 (2000), pp. 1065-1088

[37] H. Tanaka; M. Kono; H. Ushimura Some global features of paleointensity in geological time, Geophys. J. Inter., Volume 120 (1995), pp. 97-102

[38] J.A. Tarduno; R.D. Cottrell; A.V. Smirnov High geomagnetic intensity during the Mid-Cretaceous from Thellier analyses of single plagioclase crystals, Science, Volume 291 (2001), pp. 1779-1783

[39] T. Pick; L. Tauxe Geomagnetic palaeointensities during the Cretaceous normal superchron measured using submarine basaltic glass, Nature, Volume 366 (1993), pp. 238-242

[40] T. Pick; L. Tauxe Holocene palaeointensities: Thellier experiments on submarine basaltic glass from the East Pacific Rise, J. Geophys. Res., Volume 98 (1993), pp. 17949-17964

[41] E. Thellier; O. Thellier Sur l'intensité du champ magnétique terrestre dans le passé historique et géologique, Ann. Géophys., Volume 15 (1959), pp. 285-376

[42] R. Zhu; Y. Pan; J. Shaw; D. Li; Q. Li Geomagnetic paleointensity just prior to the Cretaceous normal superchron, Phys. Earth Planet. Inter., Volume 128 (2001), pp. 207-222


Comments - Policy


Articles of potential interest

Further absolute geomagnetic paleointensities from Baja California: evaluation of Pliocene and Early/Middle Pleistocene data

Juan Morales; Avto Goguitchaichvili; Edgardo Cañon-Tapia; ...

C. R. Géos (2003)


Further details on the applicability of Thellier paleointensity method: The effect of magnitude of laboratory field

Juan Morales; Avto Goguitchaichvili; Luis M. Alva-Valdivia; ...

C. R. Géos (2006)


Some characteristics of geomagnetic reversals inferred from detailed volcanic records

Jean-Pierre Valet; Emilio Herrero-Bervera

C. R. Géos (2003)