PLIO-PLEISTOCENE PALEOMAGNETIC RECORD FROM THE MICHOACÁN-GUANAJUATO MONOGENETIC VOLCANIC FIELD (WESTERN MEXICO)
RAFAEL MACIEL PEÑA1, AVTO GOGUITCHAICHVILI1*, BERNARD HENRY2, LEDA SÁNCHEZ-BETTUCCI3, JUAN MORALES1, BERTHA AGUILAR REYES1, ANA MARÍA SOLER-ARECHALDE4 AND MANUEL CALVO-RATHERT5
1 Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede
2 Paléomagnétisme, Institut de Physique du Globe de Paris and Centre National de la Recherche Scientifique, 94107 Saint Maur, France
3 Departamento de Geología, Area Geofísica-Geotectónica, Facultad de Ciencias, Universidad de la República, 11200 Montevideo, Uruguay
4 Laboratorio de Paleomagnétismo, Instituto de Geofísica, Universidad Nacional Autónoma de México, 04510 México DF., México
5 Laboratorio de Paleomagnetismo, Departamento de Física, Escuela Politécnica Superior, Universidad de Burgos, C/Francisco de Vitoria, s/n, 09006, Burgos, Spain
* Corresponding author
Received: February 22, 2010; Revised: June 22, 2010; Accepted: January 15, 2011
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, 95 = 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
Continuation of LATINMAG Special Issue #4, Stud. Geophys. Geod., 54 (2010)
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312 Stud. Geophys. Geod., 55 (2011)
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.
Ke y wo rd s : paleosecular variation, reversals, Western Mexico, time-averaged field,
geocentric axial dipole, Trans Mexican Volcanic Belt
1. INTRODUCTION
Revealing the variability of the geomagnetic field with time is essential for understanding the conditions in the Earth’s liquid core and at the core-mantle boundary. Detailed paleomagnetic records allow remote sensing of the activity in the Earth’s deep interior at very different time scales. Apart from the Paleosecular Variation (PSV) studies, the Time Averaged Field (TAF) is being considered as the decisive factor to precisely describe the fluctuations of the Earth magnetic field. The TAF may indicate long-term departures from the Geocentric Axial Dipole (GAD), the most basic field model used throughout paleomagnetism (Lawrence et al., 2006, 2009). The TAF initiative has begun to update the database of geomagnetic observations over the last five million years (Tauxe et al., 2004; Lawrence et al., 2006, 2009; Johnson et al., 2008). The analysis of these new generation paleomagnetic results shows that the data at low latitudes seems to be more scattered than those at high latitude. This point depends critically on a set of data or latitude about 20°N. Many paleomagnetic records from the Trans Mexican Volcanic Belt (TMVB) (see compilation in Ruiz-Martinez et al., 2000 and Mejia et al., 2005) show a relatively high scatter and evidence of tectonic rotations (mostly anticlockwise, of 10° to 20°) which makes it difficult to make any firm conclusions about the variability of the Earth’s magnetic field. Steele (1985) and Herrero-Bervera et al. (1986) investigated the paleosecular variation in a recent section of Brunhes Chron and concluded than the angular standard deviation was in agreement with the value predicted from the PSV models. Bohnel et al. (1990) revised this conclusion and found the PSV for central Mexico was lower than the PSV models, similar to the value for Hawaii and that the difference with the study of Herrero-Bervera et al. (1986) was due to the selection criteria applied to the data set (Urrutia-Fucugauchi, 1997; Conte-Fasano et al., 2006).
This investigation is aimed to contribute to the time averaged field global database and to the paleosecular investigations at low latitudes, based on a detailed rock-magnetic and paleomagnetic study of seventeen lava flows (about 140 standard paleomagnetic cores) associated with the Michoacan Guanajuato Volcanic Field (MGVF). The available ages range from 2.78 Ma to 0.56 Ma. The MGVF with about 2000 volcanic build-ups seems to
Plio-Pleistocene Paleomagnetic Record from the Michoacán-Guanajuato Volcanic Field
Stud. Geophys. Geod., 55 (2011) 313
be most adequate unit for paleomagnetic targets. There is probably no other Plio-Quaternary volcanic field in the world displaying such a large number of monogenetic volcanoes and it can be said that in this respect it is a unique feature.
2. GEOLOGICAL SETTING AND AGES
The Trans-Mexican Volcanic Belt (TMVB) is an E-W trending zone located between 19° and 20°N latitude, extending ca. 1000 km from the Pacific Ocean to the Gulf of Mexico (Fig. 1). Its origin is related to the subduction of the Cocos Plate beneath the North American Plate. Popocatépetl and Colima volcanoes represent the two major features of this volcanic arc. Less known is the fact that most volcanoes comprising the TMVB are monogenetic scoria cones. Their total amount is difficult to estimate, but their number has been estimated to be well above 3000. Two areas that show the highest concentrations of monogenetic volcanoes are: the Sierra Chichinautzin area, south of
Fig. 1. a) Schematic tectonic setting of Central and Western Mexico indicating the area covered by Michoacan Guanajuato Volcanic Field. b) Digital elevation model for MGVF area showing the location of studied sites.
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314 Stud. Geophys. Geod., 55 (2011)
Mexico-City and the Michoacán-Guanajuato volcanic field. The only registered historic scoria cone eruptions on Mexican grounds are at Jorullo in 1759 and at Paricutín in 1943, which occurred within the Michoacan-Guanajuato volcanic field.
The geographic boundaries of the MGVF are delineated by the 1845N and 2015N parallels and the 100°25W and 10245W meridians. The MGVF, with an area of 40000 km2, contains over 2000 small-sized monogenetic volcanoes including: cinder cones (90%), maars, tuff rings, lava domes and lava flows with hidden vents; all their centers have a calc-alkaline predominant character but alkaline and transitional rocks are also found (Hasenaka et al., 1994). In addition to the small-sized centers, there are about 300 medium-sized volcanos that have mainly erupted calc-alkaline andesites in which the majority of lavas fall in the SiO2 range between of the 55% to 61% (Hasenaka and Carmichael, 1985). These medium-sized volcanoes represent principally Icelandic-type shields, with slopes between 5° and 15°, a basal diameter between 3 and 8 km, heights between 300 and 700 m, and volumes between 1 and 10 km3 (Ban et al., 1992; Conte-Fasano et al., 2006). These volcano shields, were built from an essentially continuous discharge from a central vent, and thus are monogenetic volcanoes. Therefore, these volcanoes represent an important part of the total magma erupted into the volcanic field. Geomorphologically the lava flows associated with these medium-sized volcanoes are older than those from cinder cones, and thus they are possible precursors to the smaller-sized volcanoes (Hasenaka, 1994). Moreover, there are two stratovolcanos, the Volcán Grande and the Cerro Tancítaro. The MGVF is different with respect to the other parts of the TMVB, which are characterized mainly by composite volcanoes (e.g. Popocatépetl, Nevado de Colima, Pico de Orizaba etc.), which typically have more acidic lavas.
Our sampling strategy was largely conditioned by radiometric data availability (Murphy and Carmichael, unpublished results; Hasenaka and Carmichael, 1985; Nixon et al., 1987; Ban et al., 1992; Ferrari et al., 1990). We sampled preferentially sites with available isotopic data (Fig. 1 and Table 1), easy access and, apparently unaltered outcrops. In total, 136 oriented samples belonging to 17 individual lava flows were collected. Twelve out of 17 cooling units were age-dated by Ar-Ar and K-Ar methods. These lavas were erupted between 2.78 Ma and 0.56 Ma (Table 1). Sites Val2 and Val3 (Santa Teresa) as well as Sites Val15 and Val16 (Jaripeo) represent consecutive lava flows with the reddish (scoria) contact between them. The same is true for Sites Val4, 5 and 6 (Cerro Grande) which are composed of three consecutive lava flows. The samples were distributed throughout each flow both horizontally and vertically. All of the sampled lavas are horizontal (or almost horizontal with dip less than 3°). In general, the samples were obtained at the bottom of the 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 the magnetic and sun compasses.
3. MAGNETIC MEASUREMENTS
Detailed rock-magnetic investigations were conducted on one representative sample from each lava flow to identify the magnetic carriers and grain size characteristics. All rock-magnetic measurements were carried out at the palaeomagnetic laboratory of Burgos University (Spain).
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Tab
le 1
. Fl
ow-m
ean
pale
odire
ctio
ns o
f cha
ract
eris
tic re
man
ence
, loc
atio
n an
d av
aila
ble
isot
opic
age
det
erm
inat
ions
for M
GV
F vo
lcan
ics.
n -
num
ber
of s
peci
men
s us
ed f
or c
alcu
latio
n, N
- n
umbe
r of
tre
ated
sam
ples
, I
- in
clin
atio
n, D
- d
eclin
atio
n,
95 a
nd k
- r
adiu
s of
95%
conf
iden
ce c
one
and
prec
isio
n pa
ram
eter
of
Fish
er s
tatis
tics,
resp
ectiv
ely,
Lat
., Lo
ng. -
geo
grap
hic
latit
ude
and
long
itude
of
stud
ied
site
s;VG
P lat
, VG
P lon
g - v
irtua
l geo
mag
netic
pol
e po
sitio
ns fo
r eac
h la
va fl
ow.
Ref
.
Ban
et a
l. (1
992)
H
asen
aka
and
Car
mic
hael
(198
5)
Has
enak
a an
d C
arm
icha
el (1
985)
Ba
n et
al.
(199
2)
Ban
et a
l. (1
992)
Ba
n et
al.
(199
2)
Mur
phy
and
Car
mic
hael
(u
npub
l. re
sults
) Ba
n et
al.
(199
2)
Nix
on e
t al.
(198
7)
Ban
et a
l. (1
992)
Fe
rrar
i et a
l. (1
990)
Fe
rrar
i et a
l. (1
990)
VGP l
ong
[°]
228.
0068
294.
4861
301.
2733
345.
0987
36
6.45
26
342.
6257
22
5.84
85
308.
0294
17.6
851
21
3.68
81
334.
4765
33
2.11
30
91
.298
6 36
1.87
70
307.
7276
VGP l
at [°
]
82.5
495
65.9
269
65.2
045
82.1
308
74.3
213
76.8
594
86.7
639
79.9
482
82.0
161
67
.099
4 80
.840
7 87
.131
0
65.5
789
79.0
194
87.2
164
Age
[Ma]
0.56
0
.07
2.78
0
.07
2.78
0
.08
2.35
0
.25
2.35
0
.25
2.35
0
.25
n.a.
1.2
n.a.
n.
a.
n.a.
0.58
0
.09
2.60
0
.10
1.97
0
.23
0.75
0
.15
0.75
0
.15
n.a.
Long
. W
1013
9.9
1005
2.2
1005
2.2
1004
5.2
10
04
5.2
10
03
1.9
10
04
5.4
1012
6.8
1015
6.7
10
15
4.1
10
15
3.6
10
25
4.9
10
20
4.7
10
10
1.5
10
05
0.4
10
05
0.4
10
11
2.0
Lat.
N
193
547
.6
10.7
20
10.7
20
20.7
20
20.7
20
20.7
20
40.1
20
36.3
20
33.1
20
32.6
19
55.6
19
31.0
19
23.5
19
46.5
19
41.9
19
41.9
19
11.3
k
190
382
137
582
112
275
191 28
62
824 66
236
34
47
15.7
9
95
[]
4.8
3.4
5.7
2.8
8.6
5.5
4.1
11.7
9.9 2.3
8.3
4.4
12.6
9.
8 7.
8
D [
]
173.
8
354.
5
351.
6
352.
6 34
3.5
348.
1 17
7.2
355.
4
351.
5 21
.4
352.
6 35
7.7
354.
6 34
8.6
358.
7
I []
28.
3
5.9
5.7
31.2
33
.7
26.2
33.
9
21.9
37.5
19.3
25
.4
33.2
62.5
32
.7
31.8
n/N
8/8
8/8
7/8
8/8
6/8
5/8
8/8
6/8
5/7
0/8
8/8
7/8
8/8
0/8
3/8
5/8
8/8
R. Maciel Peña et al.
316 Stud. Geophys. Geod., 55 (2011)
3 . 1 . T h e r m o m a g n e t i c C u r v e s
Thermomagnetic curves measured in a high field of 800 mT using a Variable Field Translation Balance (VFTB). A continuous thermomagnetic curve indicates that most of samples contain titanium-poor titanomagnetites, with a Curie temperature between 510°C and 575°C. The significant irreversibility of the heating and cooling in curves (Fig. 2, Samples 98V366A and 98V344A) suggests that the titanomagnetite may have been transformed during heating into hematite or even maghemite during heating in those samples, whose cooling curves are much lower than the heating curves. A few samples have yield apparently two different thermomagnetic phases during heating. The lower Curie point ranges between 180230°C, while the higher one is about 520°C. Both Ti-rich and Ti-poor titanomagnetites seem to co-exist in few lava flows (e.g. Sample 98V396A).
3 . 2 . H y s t e r e s i s P r o p e r t i e s
Magnetic hysteresis loop determinations were also conducted using the VFTB instrument. The saturation remanent magnetization (Jrs), the saturation magnetization (Js)
Fig. 2. Susceptibility versus temperature (in air) curves of representative samples. Arrows indicate the heating and cooling cycles.
Plio-Pleistocene Paleomagnetic Record from the Michoacán-Guanajuato Volcanic Field
Stud. Geophys. Geod., 55 (2011) 317
and coercive force (Hc) were calculated after correction for the paramagnetic contribution. The coercivity of remanence (Hcr) was determined by applying a progressively increasing backfield after saturation. Representative hysteresis plots are reported in Fig. 3 (Samples 98V366A and 98V396A). Judging from the ratios of hysteresis parameters (Fig. 4), it seems that all samples fall into the pseudo-single domain (PSD) grain size region (Day et al., 1977; Dunlop, 2002). This may indicate a mixture of multidomain (MD) and a significant amount of single domain (SD) grains (see the SD-MD trend reported by Dunlop, 2002). Corresponding isothermal remanence (IRM) acquisition curves were found to be very similar for all samples. Saturation is reached in moderate fields in the order of 150200 mT, which points to some spinels (titanomagnetites) as remanence carriers. A well defined wasp-waisted behavior, which reflects the presence of magnetic
Fig. 3. Typical examples of hysteresis loops (uncorrected, left column) and associated isothermal remanence acquisition curves (right column).
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318 Stud. Geophys. Geod., 55 (2011)
phases with different coercivities (Tauxe et al., 1996) was obtained for one case (Fig. 3, Sample 98V344A). Both hysteresis and thermomagnetic curves indicate a very important contribution of Ti-hematite in this sample. At low applied fields, the hysteresis curve is thin and the IRM curve is steep, pointing to the presence of magnetite or Ti-poor titanomagnetite (Cervantes Solano et al., 2010). In contrast, at high applied fields the hysteresis curve is clearly open and IRM acquisition shows no full saturation. This indicates that both (titano)magnetite and (titano)hematite co-exist in studied samples.
3 . 3 . R e m a n e n c e P r o p e r t i e s
The remanent magnetization of the studied samples were measured with a JR6 (AGICO) spinner magnetometer (nominal sensitivity ~ 109 Am2). Both alternating field (AF) demagnetizations using a Molspin AF-demagnetizer and stepwise thermal demagnetizations using an ASC-TD48 furnace were carried out.
Two-component magnetizations were recognized for most of the studied units (e.g. Fig. 5a, Samples 98V320 and 98V372). The secondary components are sometimes much stronger than primary ones (this is the case of Sample 98V320). The characteristic magnetization components are isolated after applying 30 mT peak alternating field. The omnipresence of lightning-produced secondary components is well illustrated in the equal area projection of natural; remanent magnetization (NRM, Fig. 5b), which show extremely scattered directions. It should be noted that the AF treatments proved to be more efficient. Other samples displayed stable, uni-component magnetizations.
Fig. 4. Dunlop plot (modified after Dunlop, 2002) showing the relationship between the hysteresis parameters.
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Stud. Geophys. Geod., 55 (2011) 319
Fig. 5a. 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. - projections into the horizontal plane, - projections into the vertical plane.
Fig. 5b. The equal area projection of the NRM directions before magnetic treatments. - positive inclination, - negative inclination.
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4. MAIN RESULTS AND DISCUSSION
The characteristic magnetization direction was determined by the least squares method (Kirschvink, 1980), using 4 to 9 data points being taken in the principal component analysis for this determination. The obtained directions were averaged by unit and the statistical parameters were calculated assuming a Fisherian distribution. The average unit directions are rather precisely determined for the 13 sites (Table 1, Fig. 6) yielding an 95 of less than 10°. Sites Val8 (San Nicolas) and Val15 (Jaripeo) gave slightly higher values
Fig. 6. a) Equal area projections of the flow-mean characteristic paleodirections for the MGVF volcanics, and b) corresponding virtual geomagnetic pole positions. Thick ellipses represent the mean values.
Fig. 7. The same as in Fig. 6 for all available results.
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Stud. Geophys. Geod., 55 (2011) 321
of 11.7° and 12.6° respectively. We note however, that the paleodirections retrieved from Val15 are based on three samples. Due to a strong magnetic overprints, no paleodirections were obtained from the Sites Val10 (Yuridia) and Val14 (Brinco del Diablo).
Sites VAL2 and VAL3 (Santa Teresa) are consecutive lavas with a clearly defined contact zone between them. Thus, they represent two distinct geomagnetic field features. The same is true for the Sites VAL4 through VAL6 (C. Grande) and VAL15 and VAL16 (Jaripeo). In case, of C. Grande and Jaripeo these consecutive lavas yielded statistically distinguishable paleodirections while the Santa. Teresa lavas provided almost identical results. Thus, it is quite possible that these lavas were formed during a relatively short time.
Table 2. Flow-mean paleodirections of characteristic remanence, location and available isotopic age determinations (1 - Gonzalez et al., 1997, 2 - Bohnel and Molina, 2002, 3 - Panfil, 1999, 4 - Michalk et al., 2010, 5 - Maciel Peña et al., 2009) for TMVB volcanics. For the meaning of the symbols, see Table 1.
The mean paleodirection obtained in this study is inclination I = 28.8°, declination D = 354.9°, Fisherian parameters k = 28, 95 = 7.3°, N = 15, which corresponds (Fig. 6) to the mean paleomagnetic pole position Plat = 83.9°, Plong = 321.6°, K = 34, A95 = 6.6°. These directions, are practically indistinguishable from the expected Plio-Quaternary paleodirections, as derived from the reference poles for the North American polar wander curve (Besse and Courtillot, 2002) and is in agreement with previous studies and compiled data from the MGVF (Tables 2 and 3, Fig. 7) considering all previously reported studies (Gonzalez at al., 1997; Bohnel and Molina, 2002; Panfil, 1999; Michalk et al., 2010; Maciel Peña et al., 2009). This suggests that no major tectonic deformation occurred in the studied area since the Late Pliocene to the present. Thus, these data may be used for the paleosecular variation (PSV) estimation.
The formula 2 2 2F T WS S S n was used for estimating paleosecular variation in this
study, where SF is the angular dispersion, SW dispersion within the site (following McEllhinny and McFadden, 1997), n the average number of samples per site, and
1 2211 1 N
T iiS N is the total angular dispersion (Cox, 1969). Here, N is the
number of sites used in the calculation, i the angular distance of the i-th virtual geomagnetic pole (VGP) from the axial dipole. All VGPs obtained in this study yield lesser colatitudes than generally adopted 45 cut-off angles (Johnson et al., 2008). We obtained SF = 15.9 with SU = 21.0 and SL = 12.7 (upper and lower limits respectively) which reasonably agrees with the Model G of the McFadden et al. (1988, 1991) fit to the Johnson et al. (2008) databases for the last 5 Ma (Fig. 8). Considering all previously reported paleodirections from the MGVF (Table 3 and Fig. 9) we have obtained values (SF = 13.9 with SU = 16.9 and SL = 11.9) which are almost identical to those reported by Herrero-Bervera et al. (1986).
Table 3. Summary of paleomagnetic data and angular dispersion of the VGPs from volcanic rocks of Central Mexico and MGVF.
This Study (MGVF) Brunhes/Matuy. 15 84.0 321.6 7.3 28 15.9 21.0 12.7 All MGVF Selected Brunhes/Matuy. 45 88.7 99.1 5.3 17 13.9 16.9 11.9
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In order to obtain a relatively detailed paleomagnetic record for the last 3 Ma, we have combined our data with that of the nearby, Ar-Ar dated lava flows associated with the Tancitaro Volcano (Maciel Peña et al., 2009). The paleomagnetic declination, inclination and paleolatitude of the VGP (virtual geomagnetic poles) are shown in Fig. 10 against their stratigraphic position. The oldest sites analyzed are the Santa Teresa locality Site
Fig. 8. Paleosecular variation of lavas (PSVL) for the last 5 Ma (data from this study). (Adopted from McFadden et al., 1988 and 1991 - black line, and Johnson et al., 2008 - grey line.)
Fig. 9. The same as in Fig. 8 using all available MGVF data.
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Val2 and Val3 and the Cerro Alto locality Site Val13. All yielded normal polarity magnetizations as expected for the units belonging the Gauss geomagnetic Chron. An interesting feature of the record comes from the lava flows Val4 through Val16 (Cerro Grande) dated at about 2.35 Ma these yielded clearly defined normal directions. The possible existence of a normal polarity magnetization in the Matuyama reversed (Fig. 10) epoch older than the Reunion events, identified as anomaly X in the magnetic anomaly profiles by Heirtzler et al. (1968) and Emilia and Heinrichs (1969), has not been confirmed by subaerial volcanic rocks with the exception of a 2.37 Ma normal polarity lava from Argentina (Valencio et al., 1970). More recently however, Herrero-Bervera et al. (2007) reported transitional paleodirecions at Halawa Valley, Koolau Volcano, Oahu, Hawaii (Fig. 10). The corresponding virtual geomagnetic pole positions are located close to Madagascar and 40Ar/39Ar incremental heating experiments yielded isochrons between 2.64 ± 0.23 to 2.37 ± 0.17 Ma with a weighted mean age of 2.514 ± 0.039 Ma, which, combined with the overall reversed polarity of whole section and the absence of polarity reversals, strongly suggests that the excursion corresponds to Cryptochron C2r.2r-1 (Cande and Kent, 1995). The normal polarity was also detected for the Site Val8 (San Nicolas) dated at about 1.2 Ma. It may be speculated that this lava flow was erupted during the worldwide observed Reunion geomagnetic event.
Another important feature of the geomagnetic record obtained from the MGVF field is that lava flow VAL-1 dated radiometrically at about 0.56 Ma yield fully reversed paleodirections. Quidelleur and Valet (1996), on the basis of transitionally magnetized
Fig. 10. Flow-mean magnetic inclination, declination and paleolatitude of virtual geomagnetic poles against age based on data from Central and Western Mexico (see text for more details).
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Stud. Geophys. Geod., 55 (2011) 325
lavas (Canary Islands) that gave an unspiked K-Ar age of 602 ± 24 ka, interpreted the event as a new excursion and proposed the name ‘La Palma’. In addition, a relative paleointensity minimum of global extent was found in marine sediments at about 590 ka (Guyodo and Valet, 1999) supporting the findings of Quidelleur and Valet (1996). However, Singer et al., (2002) based on new high quality Ar-Ar data, proposed that the lavas at Barranco de los Tilos recorded the Big Lost event (incremental heating 40Ar/39Ar age of 580.2 ± 7.8 ka) (Figs. 7 and 8), rather than ‘La Palma’ excursion. The Big Lost event was first described by Champion et al. (1981, 1988) in lava flows in Idaho. There are some marine evidences of the Big Lost event as well: Lund et al. (1998) identified in ODP leg 172 three cases of anomalous directions at 610 ka; Langereis et al. (1997) found additional evidences for the Big Lost between 560570 ka recorded in a piston core from the Ionian Sea. More recently, Carcaillet et al. (2004) found three VDM (virtual dipole moment) minima between 750 and 500 ka, the oldest (about 700 ka) being identified as the Delta excursion. The only evidence of a fully reversed short event at 630640 ka comes from the studies of Liu et al. (1985, 1988) who investigated in detail the Lishi Loess Series in Xifeng area in China. Additional evidence that support these anomalous paleodirections comes from the Ceburuco lavas studied by Pétronille et al. (2005).
5. CONCLUSIONS
We have carried out a rock-magnetic and paleomagnetic survey on seventeen independent cooling units from the Michoacan Guanajuato Volcanic Field. A continuous thermomagnetic curve indicates that most of the samples contain titanium-poor titanomagnetites occasionally associated with titanohematites. Judging from the ratios of hysteresis parameters, it seems that all samples fall into the pseudo-single domain grain size region indicating a mixture of multidomain (MD) and a significant amount of single domain (SD) grains. The characteristic paleodirections are successfully isolated for 15 lava flows. The mean paleodirection obtained in this study is I = 28.8°, D = 354.9°, k = 28, 95 = 7.3°, N = 15, which corresponds to the mean paleomagnetic pole position Plat = 83.9°, Plong = 321.6°, K = 34, A95 = 6.6°. The compilation of previously reported paleomagnetic data yielded similar results which are practically indistinguishable from the expected Plio-Quaternary paleodirections, as derived from the reference poles for the North American polar wander curve. The calculation of angular standard deviation of the virtual geomagnetic poles using all currently available data belonging to the MGVF yielded SF = 13.9 with SU = 16.9 and SL = 11.9 (upper and lower limits, respectively) which reasonably agrees with available statistical models for the last 5 Ma. The interesting feature of the record comes from the 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 events 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 a 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.
R. Maciel Peña et al.
326 Stud. Geophys. Geod., 55 (2011)
Acknowledgments: The authors are grateful for the financial support given by CONACYT Project No.54957. MCR acknowledges partial support of Junta de Castilla y Leon Project No.BU004A0S.
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