Paleomagnetic and rock-magnetic survey of Brunhes lava flows from Tancitaro volcano, Mexico
ABSTRACT This study presents paleomagnetic results from Tancitaro volcanism in the Michoacan Guanajuato Volcanic Field, western Mexico, as a contribution to the time averaged field global database. Detailed paleomagnetic and rock-magnetic studies were carried out on eleven independent lava flows; 120 oriented, standard paleomagnetic cores were collected from Tancitaro volcano and surrounding areas. All sites were dated by means of 40Ar-39Ar (Ownby et al., 2007) as originating from 793 ka to present. Rock-magnetic experiments included continuous susceptibility and hysteresis measurements. Remanence is carried mostly by Ti-poor titanomagnetite of pseudosingle-domain magnetic structure. Eight out of eleven flows yield normal magnetic polarities while three sites yielded inconsistent paleodirections most probably due to lightning. Mean paleodirection from eight flows is Inc=39.5°, Dec=356.4°, k=29, α95=9.1° which corresponds to a pole position with Plat=84.4°, Plong=219.9°, K=33 and A95=8.5°, practically undistinguishable from expected Plio-Quaternary paleodirections, for the North American Craton. Paleosecular variation is compatible with other studies at the same latitude bands and with recent statistical models. The mean inclination falls within the uncertainties of the Geomagnetic Axial Dipole plus 5% quadrupolar contributions.
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ABSTRACT: The paleosecular variation (PSV) and polarity transitions are two major features of the Earth’s magnetic field. Both PSV and reversal studies are limited when age of studied units is poorly constrained. This is a case of Central and western Mexico volcanics. Although many studies have been devoted to these crucial problems and more than 200 paleomagnetic directions are available for the last 5 Ma, only few sites were dated directly. This paper presents new paleomagnetic results from seventeen independent cooling units in the Michoacán-Guanajuato Volcanic Field (MGVF) in western Mexico. Twelve sites are directly dated by 40Ar/39Ar or K-Ar methods and span from 2.78 to 0.56 Ma. The characteristic paleodirections are successfully isolated for 15 lava flows. The mean paleodirection (inclination I and declination D) obtained in this study is I = 28.8°, D = 354.9°, and Fisherian statistical parameters are k = 28, α95 = 7.3°, N=15, which corresponds to the mean paleomagnetic pole position Plat = 83.9°, Plong = 321.6°, K = 34, A95 = 6.6°. The paleodirections obtained in present study compiled with those, previously reported from the MGVF, are practically undistinguishable from the expected Plio-Quaternary paleodirections. The paleosecular variation is estimated through the study of the scatter of the virtual geomagnetic poles giving SF = 15.9 with SU =21.0 and SL = 12.7 (upper and lower limits respectively). These values agree reasonably well with the recent statistical Models. The oldest sites analyzed (the Santa Teresa and Cerro Alto) yield normal polarity magnetizations as expected for the cooling units belonging to the Gauss geomagnetic Chron. The interesting feature of the record comes from lava flows dated at about 2.35 Ma with clearly defined normal directions. This may point out the possible existence of a normal polarity magnetization in the Matuyama reversed Chron older than the Reunion and may be correlated to Halawa event interpreted as the Cryptochron C2r.2r-1. Another important feature of the geomagnetic record obtained from the MGVF is the evidence of fully reversed geomagnetic field within Bruhnes Chron, at about 0.56 Ma corresponding to the relative paleointensity minimum of global extent found in marine sediments at about 590 ka. Keywordspaleosecular variation–reversals–Western Mexico–time-averaged field–geocentric axial dipole–Trans Mexican Volcanic BeltStudia Geophysica et Geodaetica 04/2011; 55(2):311-328. · 0.98 Impact Factor
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ABSTRACT: A significant number of new palaeomagnetic poles have become available since the last time a compilation was made (assembled in 2005, published in 2008) to indicate to us that a new and significantly expanded set of tables with palaeomagnetic results would be valuable, with results coming from the Gondwana craton-ic elements, Laurentia, Baltica/Europe, and Siberia. Following the Silurian Caledonian Orogeny, Laurentia's and Baltica's Apparent Polar Wander Paths (APWPs) can be merged into a Laurussia path, followed in turn by a merger of the Laurussia and Siberia data from latest Permian time onward into a Laurasian combined path. Meanwhile, after about 320 Ma, Gondwana's and Laurussia/Laurasia's path can be combined into what comes steadily closer to the ideal of a Global Apparent Polar Wander Path (GAPWaP) for late Palaeozoic and younger times. Tests for True Polar Wander (TPW) episodes are now feasible since Pangaea fusion and we identify four important episodes of Mesozoic TPW between 250 and 100 Ma. TPW rates are in the order of 0.45–0.8°/M.y. but cumulative TPW is nearly zero since the Late Carboniferous. With the exception of a few intervals where data are truly scarce (e.g., 390–340 Ma), the palaeomagnetic database is robust and allows us to make a series of new palaeogeographic reconstructions from the Late Cambrian to the Palaeogene.Earth-Science Reviews 08/2012; · 7.34 Impact Factor
Geofísica Internacional 48 (4), 375-384 (2009)
Paleomagnetic and rock-magnetic survey of Brunhes lava flows
from Tancitaro volcano, Mexico
R. Maciel Peña1*, A. Goguitchaichvili1, V. H. Garduño Monroy2, V. C. Ruiz Martinez3, B. Aguilar
Reyes1, J. Morales1 L. Alva-Valdivia4, C. Caballero Miranda4 and J. Urrutia-Fucugauchi4
1Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica – Sede Michoacán, Universidad Nacional
Autónoma de México, Campus Morelia, Mexico.
2Departamento de Geología y Mineralogía, Instituto de Investigaciones Metalúrgicas, Universidad Michoacana, San
Nicolás de Hidalgo, Morelia, Michyaoacán, Mexico.
3Departamento de Física de la Tierra, Astronomía y Astrofísica I, Universidad Complutense de Madrid, Madrid, Spain.
4Laboratorio de Paleomagnétismo y Geofísica Nuclear, Instituto de Geofísica, Universidad Nacional Autónoma de
México, Mexico City, Mexico.
Received: March 31, 2009; accepted: May 13, 2009
Este estudio presenta los resultados paleomagnéticos del Volcán Tancitaro, (perteneciente al Campo
Volcánico Michoacán Guanajuato), en el oeste de México, como una contribución a la base de datos de
promediados en el tiempo del campo global. Se realizó el estudio paleomagnético y de magnetismo de roca en
once flujos de lava. Se colectaron 120 núcleos orientados en el volcán Tancitaro y sus áreas aledañas. Todos los
sitios fueron previamente fechados por 40Ar -39Ar (Ownby et al., 2007,) y van desde 793 ka hasta el presente. Se
realizaron experimentos de magnetismo de roca, curvas de susceptibilidad e histéresis magnética y en la mayoría
de los casos la remanencia magnética fue por la presencia de titanomagnetita pobre en Titanio con estructura
magnética de pseudodominio simple. Ocho de los once flujos de lava dieron polaridad normal, mientras que los
tres restantes produjeron paleodirecciones inconsistentes y no se tomaron en cuenta para el análisis y se debió
probablemente por efectos de rayos. La paleodirección principal obtenida de los ocho flujos es Inc=39.5º, Dec=
356.4º, k=29, α95=9.1º lo cual corresponde a la posición del polo con Plat=84.4º, Plong=219.9º, K=33 y α95=8.5º.
Esta dirección es prácticamente consistente con la paleodirección esperada del Plio Cuaternario, derivada del
polo de referencia para el Craton Norteamericano. La variación paleosecular coincide con otros estudios de la
misma latitud y con modelos estadísticos recientes. La inclinación media cae dentro de la incertidumbre del Eje
Dipolar Geomagnético más 5% de contribuciones cuadrupolares.
Palabras clave: Paleomagnetismo, variación paleosecular, promedio de tiempo de campo, epoca Brunhes, Cinturón Vol-
cánico Trans- Mexicano.
This study presents paleomagnetic results from Tancitaro volcanism in the Michoacan Guanajuato Volcanic
Field, western Mexico, as a contribution to the time averaged field global database. Detailed paleomagnetic and
rock-magnetic studies were carried out on eleven independent lava flows; 120 oriented, standard paleomagnetic
cores were collected from Tancitaro volcano and surrounding areas. All sites were dated by means of 40Ar-39Ar
(Ownby et al., 2007) as originating from 793 ka to present. Rock-magnetic experiments included continuous
susceptibility and hysteresis measurements. Remanence is carried mostly by Ti-poor titanomagnetite of pseudo-
single-domain magnetic structure. Eight out of eleven flows yield normal magnetic polarities while three sites
yielded inconsistent paleodirections most probably due to lightning. Mean paleodirection from eight flows is
Inc=39.5°, Dec=356.4°, k=29, α95=9.1° which corresponds to a pole position with Plat=84.4°, Plong=219.9°,
K=33 and A95=8.5°, practically undistinguishable from expected Plio-Quaternary paleodirections, for the North
American Craton. Paleosecular variation is compatible with other studies at the same latitude bands and with
recent statistical models. The mean inclination falls within the uncertainties of the Geomagnetic Axial Dipole
plus 5% quadrupolar contributions.
Key words: Paleosecular variation, time averaged field, Brunhes chron, Trans-Mexican Volcanic Belt.
Geofis. Int. 48 (4), 2009
Fluctuations of the geomagnetic field with time are
essential for understanding the geodynamo, and conditions
in the Earth’s liquid core and at the core-mantle boundary.
Paleosecular variation may indicate modulation of
geodynamo action in the core by the convective state of
the lower mantle. Thus, determinations of these variations
are decisive for understanding the processes in the core
that give rise to the geomagnetic field and how and why
the Earth’s magnetic field reverses polarity.
The fundamental factors in the variability of the
Earth Magnetic Field are the Time Averaged Field (TAF)
and paleosecular variation (PSV). The TAF initiative
has begun to update the database of geomagnetic
observations over the last five million years (Mejia et al.,
2005; Tauxe et al., 2004, Lawrence et al., 2006; Lawrence
et al., 2009). A simple method to estimate the PSV
consists in observing of the angular standard deviation
(ASD) of virtual geomagnetic poles (VGPs) for a given
locality. Several combinations of dipole and non-dipole
components predict the ASD characteristic of PSV with
latitude (McFadden et al., 1988, 1991; Lawrence et al.,
2006; Johnson et al. 2008)).
Johnson et al. (2008) reported a detailed synthesis of
a new generation of paleomagnetic data compilations.
The latitudinal dependence of VGP (virtual geomagnetic
poles) scatter for these data appears much less important.
The data at low latitudes seems to be more scattered than
those at high latitude. This issue depends critically on a set
of data of latitude 20°N. Published directional data from
Trans Mexican Volcanic Belt (TMVB) meeting some
very basic selection criteria (Herrero-Bervera et al., 1986;
Ruiz-Martinez et al., 2000; Osete et al., 2000, Morales et
al., 2001, Alva-Valdivia et al., 2001; Mejia et al., 2005;
Petronille et al., 2005, Rodriguez-Ceja et al., 2006) show
an unusually high degree of scatter, which might be due to
genuine geomagnetic field behavior.
The present study reports time averaged field global
database and paleosecular investigations at low latitudes,
based on a detailed rock-magnetic and paleomagnetic
study of eleven lava flows associated to Tancitaro Volcano
in the Michoacan Guanajuato Volcanic Field (MGVF). All
sites were recently dated by means of Ar40-Ar39 systematics
(Ownby et al., 2007). The available ages range from 793
Ka to present.
Available age and sampling
The Trans-Mexican Volcanic Belt contains
stratovolcanoes, cinder cone fields and silicic caldera
complexes. Volcanism in this region dates from late
Miocene related to the subduction of Cocos and Rivera
plates at the Middle America trench. The western TMVB
is associated with subduction of ~ 9 Ma Rivera plate,
whereas the 12-18 Ma Cocos plate subducts under North
America to the east (Klitgord and Mammerickx, 1982).
The Michoacan-Guanajuato volcanic field is located
in the west-central part of TMVB (Fig. 1), east of
Chapala Lake. The geographic boundaries of the MGVF
are 18°45’ N to 20°15’ N and 100°25’ W to 100°45’ W,
with an area of 40,000 km2, It contains over 2,000 small
monogenetic volcanoes including cinder cones (90%),
maars, tuff rings, lava domes and lava flows with hidden
vents all are predominant calc-alkaline but some alkaline
and transitional rocks are also found Silica content varies
from 47% to 70% for olivine basalt and basalt-andesite
rocks (Hasenaka, 1994; Hasenaka et al., 1994).
Volcán Tancítaro (VT) is a large andesitic, composite
volcano in the Michoacán–Guanajuato Volcanic Field in
west-central Mexico. With a height of 3840 m (Ownby
et al., 2007), VT is the dominant feature in the MGVF.
Twenty-six new Ar40-Ar39 ages indicate that Tancítaro
became active ≥793±22 ka and that the most recent
effusive activity occurred at 237± 4 ka. Two catastrophic
sector-collapse events are identified and dated; the first
one on the west flank between 694 and 571 ka, and the
second on the east flank between 261 and 238 ka (Ownby
et al., 2007).
Our sampling strategy was largely conditioned by
Ownby et al. (2007) who gave 26 new Ar-Ar incremental
heating ages for Tancitaro and adjacent sorounding areas.
We sampled only sites with available radiometric dating
information (Table 1 and Fig. 2 trough 4), of easy access
and yielding fresh, apparently unanltered outcrops. In
total, 120 oriented samples belonging to 11 individual
lava flows were collected. The samples were distributed
throughout each flow both horizontally and vertically.
All lava flows sampled were horizontal (dip less than
4°). In general, samples were obtained at the bottom of
flows with the hope of collecting samples with the finest
grained material. Cores were sampled with a gasoline-
powered portable drill, and oriented in most cases with
both magnetic and sun compasses.
Summary of magnetic experiments
In order to obtain the directions of characteristic
remanent magnetization and to identify the principal
magnetic carriers, following experiments were carried
out: a) Thermal and alternating field demagnetizations, b)
measurements of continuous thermomagnetic curves (low
field susceptibility versus temperature), and c) hysteresis
Geofis. Int. 48 (4), 2009
Fig. 1. Tectonic setting of western Mexico, adopted from Delgado-Granados (1993) and Ownby et al. (2007). Numbered triangles refer
to volcanic centers: (1) V. Tancítaro, (2) V. Colima, (3) Sierra La Primavera, (4) V. Tequila, (5) V. Ceboruco, (6) V. Tepetiltic, (7) V.
Sangangüey, (8) V. Las Navajas, (9) V. San Juan. S.J.B.F. = San Juanico–Bueanavista Fault. The dashed box represents the Michoacán
Guanajuato Volcanic Field (MGVF).
Flow-mean paleodirections of characteristic remanence, location and available isotopic age determinations (Ownby
et al. 2007) for Tancitaro volcanics. N, number of treated samples; n, number of specimens used for calculation; Inc,
Inclination; Dec, Declination; k and α95: precision parameter and radius of 95% confidence cone of Fisher statistics.
16.5 N 19º 24’ 58.3”
Long (°) Age (Ka)
W 102º 18’ 17.5” 209± 41
Tan2 (TV) 8/8 43.5 353.5 199 3.9 idem idem 209± 41
Tan3 (UR-4) 8/8 60.9 339.9 242 3.6 N 19º 22’ 27.7” W 102º 05’ 03.3” 429 ±64
Tan4 (San Fco) 0/8 - - - - N 19º 22’ 08.8” W 102º 21’ 54.2” 339 ±23
Tan5 (Tan 28) 2/8 41.2 348.5 - - N 19º 23’ 40.2” W 102º 24’ 43.8” 269± 22
Tan6 (Tan 26) 7/8 58.6 17.9 41 9.6 N 19º 25’ 32.6” W 102º 26’ 07.9” 256 ±18
Tan7 (Tan 10) 6/8 28.1 348.6 115 6.3 N 19º 18’ 32.0” W 102º 32’ 22.5” 373 ±61
Tan8 (Tan 43’) 8/8 27.7 2.5 359 2.9 N 19º 15’ 42.7” W 102º 33’ 50.6” 612 ±41
Tan9 (Tan 43) 8/8 31.3 352.7 39 9.2 N 19º 16’ 05.5” W 102º 34’ 08.44” 612 ±41
Tan10 (NI 19) 7/8 32.8 1.2 69 7.3 N 19º 09’ 52.4” W 102º 12’ 56.7” 82 ±24
Tan11 (NI 18) 7/8 43.6 349.3 198 4.3 N 19º 00’ 36.9” W 102º 04’ 03.5” 163± 37
Geofis. Int. 48 (4), 2009
The remanent magnetizations of seven to nine samples
from each lava flow (Table 1) were measured with a JR-6
(AGICO LtD) spinner magnetometer (nominal sensitivity
~ 10-9 Am2) at the Laboratorio Interinstitucional de
Magnetismo Natural (LIMNA) in Morelia (Mexico). Both
alternating field (AF) demagnetization (using a molspin
AF-demagnetizer) and stepwise thermal demagnetization
up to 560 °C (using a ASC TD-48 furnace) were carried
out. During thermal demagnetization, the low-field
susceptibility at room temperature was measured after
each step with a Bartington susceptibility meter.
Fig. 3. Greater details of sampled site location (adopted from Ownby et al. 2007).
Fig. 2. Digital elevation model for Tancitaro area showing the location of sites.
Geofis. Int. 48 (4), 2009
Fig. 4. Orthogonal vector plots of stepwise thermal or alternating field demagnetization of representative samples. The numbers refer
either to the temperatures in °C or to peak alternating fields in mT. o - projections into the horizontal plane, x – projections into the
vertical plane. Also shown (middle) is the equal area projections of NRM directions before magnetic treatments.
Two-component magnetizations were systematically
recognized for most of studied units (Fig. 4). The
secondary components are sometimes much stronger
than primary ones (samples 08T012A, 08T012B and
08T093A). The characteristic magnetizations components
are isolated after applying 40 mT peak alternating field.
It should be noted that AF treatments proved to be more
efficient than thermal demagnetization. This is illustrated
at Fig. 4. Samples 08T012B and 08T012A belong to the
same core. While thermal treatment is unable to isolate
primary remanence, the alternating fields could reveal
the primary, characteristic magnetization at last steps of
demagnetization procedure. We believe that the origin
of this strong secondary overprint is due to the lightning
effect. This is in agreement with the fact that the NRM
(natural remanent magnetization) directions show huge
dispersion on the equal-area projection. We note that no
ChRM directions were obtained from sites TAN1, TAN4
For remaining sites, relatively small, secondary
components, probably of viscous origin were detected
and were easily removed applying 10 mT (Fig. 4, sample
08T107A). The greater part of remanent magnetization,
in most cases was removed at temperatures between 520
and 560°C, which indicate to low-Ti titanomagnetites as
responsible for magnetization. The median destructive
fields (MDF) range mostly from 20 to 40 mT, suggesting
pseudo-single domain grains as remanent magnetization
carriers (Dunlop and Özdemir, 1997).
A characteristic magnetization direction was deter-
mined by the least squares method (Kirschvink, 1980),
4 to 10 points being taken in the principal component
analysis for this determination. The obtained directions
were averaged by unit and the statistical parameters
calculated assuming a Fisherian distribution.
Susceptibility vs. Temperature
Low-field susceptibility measurements (k-T curves)
under air were carried out using Agico-Kapabrdige
susceptibility meter equipped with furnace in Saint Maur
(France) IPGP laboratory. One sample from each site
were heated up to about 630°C at a heating rate 20°C/
min and then cooled at the same rate. Curie temperature
was determined by the Prévot et al’s (1983) method.
Alternatively, low-temperature (from about –185°C to
room temperature) susceptibility was recorded using the
Geofis. Int. 48 (4), 2009
Low-T susceptibility experiments (Fig. 5) show a
rather monotonic increase from about -185°C to room
temperature with no indication of Verwey transition.
Some titanium-poor titanomagnetite may be responsible
for remanent magnetization. As showed by Özdemir
et al. (1993), the Verwey transition may be largely
suppressed for the titanomagnetites with variable
titanium content. Alternatively, similar behavior may
also belong to non-stoichiometric (partially oxidized)
magnetite. Corresponding high-T susceptibility experi-
ments (Fig. 6) also indicate the presence of Ti-poor
titanomagnetites. However, the cooling and heating
curves are not perfectly reversible, probably because of
low initial value of magnetic susceptibility. This may also
be due to some moderate mineralogical alteration at highmoderate mineralogical alteration at high
temperatures. Both Ti-rich and Ti-poor titanomagnetites
seem to co-exist in few lava flows (samples 08T107 and
08T004A). These curve yields apparently two different
thermomagnetic phases during heating. The lower Curie
point ranges between 300-400°C, while the highest one is
Magnetic hysteresis measurements were performed at
room temperature on a specimen from all sampled sites
at IPGP (Saint Maur) laboratory apparatus in fields up
to 0.8 Tesla. The histeresis parameters were calculated
after correction for the paramagnetic contribution. The
coercivity of remanence (Hcr) was determined by applying
progressively increasing backfield after saturation. Typical
hysteresis plot are reported in Fig. 7a. The representative
curve is simple, symmetrical and reflects very restricted
ranges of the coercivities (Tauxe et al., 1996). Judging
from the ratios of hysteresis parameters (Fig. 7b), it seems
Fig. 5. representative curve of susceptibility vs. temperature curve recorded from -185°C to room temperature.
that all samples fall in the pseudo-single domain (PSD)
grain size region (Day et al., 1977). This may also indicate
a mixture of multidomain (MD) and a significant amount
of single domain (SD) grains (Dunlop and Özdemir, 1997;
Results and discussion
Beside strong lightning effect, the average unit
directions are rather precisely determined for 8
independent lava flows out of 11 collected (Table 1, Fig.
8a). All α95 are less than 10°. All flows yielded normal
polarity magnetization as may be expected for the Bruhnes
age rocks. We consider the paleodirections determined
in this study to be of primary origin. Thermomagnetic
curves show that the remanence is carried in most cases
by Ti-poor titanomagnetite, resulting of oxy-exsolution of
original titanomagnetite during the initial flow cooling,
which indicates that the primary magnetization is a TRM
(thermoremanent magnetization). Moreover, unblocking
temperature spectra and relatively high coercivities
point to pseudo-single domain magnetic structure grains
as responsible for remanent magnetization. The mean
paleodirection obtained from eight flows is Inc=39.5°,
Dec=356.4°, k=29, α95=9.1° which corresponds to the pole
position Plat=84.4°, Plong=219.9°, K=33 and A95=8.5°.
These directions are practically undistinguishable (Fig.
8a and b) from both the spin axis and the expected Plio-
Quaternary paleodirections, as derived from reference
poles for the North American craton (Besse and Courtillot,
2002). This may indicates that no major regional tectonic
rotation occurred in the area since about 1 My. The mean
inclination overlaps within the uncertainties to those
derived from the GAD (Geomagnetic Axial Dipole) and
GAD plus a 5% quadrupolar contributions.
Geofis. Int. 48 (4), 2009
Fig. 6. Susceptibility versus temperature (in air) curves of representative samples at high temperatures.
Fig. 7. Typical examples of hysteresis loops (uncorrected) and Day (Day et al. 1977) plot.
Geofis. Int. 48 (4), 2009
The formula SF = ST - SW /n was used for estimating
paleosecular variation in this study where, ST is the total
angular dispersion ST = [(1/N-1)S
the number of sites used in the calculation, di the angular
distance of the ith virtual geomagnetic pole (VGP) from
the axial dipole, SW the within site dispersion (following
McEllhinny and McFadden, 1997) and, n the average
number of sample per site. All new VGPs obtained on this
study yield lesser colatitudes (maximum value is 28.3º)
than generally adopted 45º cut-off angle (Johnson et al.,
2008). We obtained SF=16.0 with SU=25.5 and SL=8.1
(upper and lower limits respectively) which reasonably
agree to the model G of McFadden et al. (1988, 1991) fit
to the McElhinny and McFadden (1997) and Johnson et
al. (2008) databases for the last 5 Myr (Fig. 8b and 9).
N di21/2] [Cox, 1969], N
The combination of our data with previously published
results from Central and Western Mexico (Conte, 2004)
yields that the amplitude of the secular variation is
consistent with values obtained from other worldwide
scattered sites. Thus, the hypothesis of the Pacific dipole
window (Doell and Cox, 1971) may be rejected. This
supports the findings of McElhinny et al. (1996) and
Ruiz-Martínez, (2004). Within the uncertainties, the . Within the uncertainties, the
results obtained in this study agree with the PSV values
for Hawaii (+20° latitude) and Reunion (-20° latitude).
However, the amplitude of dispersion found is much
higher in Mexico with respect to other places at the same
latitude bands in agreement with findings of Lawrence et
al., 2006. More high quality studies and reevaluation of
old sites are needed in order to estimate whether this is a
genuine characteristic of geomagnetic field.
Fig. 8. A) Equal area projections of the flow-mean characteristic paleodirections for the Tancitaro volcanics and B) corresponding
virtual geomagnetic pole positions.
Fig. 9. A) Flow-mean magnetic declination, inclination and paleolatitude of virtual geomagnetic poles against age, B) Paleosecular
variation of lavas (PSVL) for the last 5 Ma. (Adopted from McFadden et al., 1988 and 1991 and Johnson et al. 2008).
Geofis. Int. 48 (4), 2009
This study was supported by UNAM-DGAPA grant
n° 102007 CONACYT grant n° 54957. We thank Bernard
Henry and Maxim le Goff for help during rock-magnetic
measurements in Saint Maur. VCRM is grateful to the
financial support given by MEC (Spain) “José Castillejo
Program” ref. JC2007-00314.
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R. Maciel Peña1*, A. Goguitchaichvili1, V.
Hugo Garduño Monroy2, V. Carlos Ruiz
Martinez3, B. Aguilar Reyes1, J. Morales1, L.
Alva-Valdivia4, C. Caballero Miranda4 and J.
1Laboratorio Interinstitucional de Magnetismo Natural,
Instituto de Geofísica, Sede Michoacán, Universidad
Nacional Autónoma de México, Campus Morelia,
2Departamento de Geología y Mineralogía, Instituto
de Investigaciones Metalúrgicas,
Michoacana, San Nicolás de Hidalgo, Morelia,
3Departamento de Física de la Tierra, Astronomía y
Astrofísica I, Universidad Complutense de Madrid,
28040, Madrid, Spain
4Laboratorio de Paleomagnétismo y Geofísica Nuclear,
Instituto de Geofísica, Universidad Nacional Autónoma
de México, Ciudad Universitaria, Del Coyoacán 04510
México City, Mexico
*Corresponding author: rafaelmacielmx�yahoo.com.rafaelmacielmx�yahoo.com.