Rosas-Elguera et al..fm

International Geology Review, Vol. 45, 2003, p. 814–826.
Copyright © 2003 by V. H. Winston & Son, Inc. All rights reserved.
Counterclockwise Rotation of the Michoacan Block:
Implications for the Tectonics of Western Mexico
JOSÉ ROSAS-ELGUERA,1
Centro de Ciencias de la Tierra, Universidad de Guadalajara, Av. Revolución No. 1500, 44840 Guadalajara, Jalisco, Mexico
LUIS M. ALVA-VALDIVIA, AVTO GOGUITCHAICHVILI, JAIME URRUTIA-FUCUGAUCHI,
Laboratorio de Paleomagnetismo Geofisica Nuclear, Universidad Nacional Autónoma de México (UNAM), 04510 D.F., Mexico
MARIA AMABEL ORTEGA-RIVERA,
Centro de Geociencias, Km. 15 Carretera Querétaro–San Luis Potosí, 76230 Juriquilla, Querétaro, Mexico
JUAN CARLOS SALINAS PRIETO,
Consejo de Recursos Minerales, Blvd. Felipe Angeles S/N Carretera Mexico—Pachuca Km. 93.5 42080 Pachuca Hidalgo, Mexico
AND JAMES
K. W. LEE
Department of Geological Sciences and Geological Engineering, Miller Hall, Queen’s University,
Kingston, Ontario, Canada K7L 3N6
Abstract
Subduction of the Farallon plate beneath North America resulted in formation of the Rivera and
Cocos oceanic plates, the extensive magmatic arcs of the Sierra Madre Occidental (SMO), and the
Trans-Mexican Volcanic Belt (TMVB). Southern Mexico consists of crustal blocks separated by a
regional extensional structural system; the latter, called the Guadalajara triple junction, is defined
by the Tepic-Zacoalco (TZR), Colima (CR), and Chapala (CHR) rifts. TZR and CHR separate the
SMO from the Jalisco and Michoacan blocks, whereas CR is the boundary between the Jalisco and
Michoacan blocks.
In this study, we carried out combined radiometric and paleomagnetic analyses in the Michoacan
block. Radiometric dates of 31.60 to 8.39 Ma confirm both the southern extension of the Sierra
Madre Occidental and the early mafic TMVB succession into the Michoacan block. The Oligocene
age agrees well with the radiometric dating reported for the southern SMO and the Tertiary volcanic
fields of the Sierra Madre del Sur. Paleomagnetic data indicate a counterclockwise rotation of ~24°
about a vertical axis for the Michoacan block.
Several plate models suggest either dextral or sinistral oblique convergence of the Cocos plate
relative to North America. Our new results help to constrain these different models. These data
demostrate that deformation in the Michoacan block is as old as late Miocene, and is related to
sinistral oblique convergence of the Cocos plate relative to North America—inducing the southeast
relative motion of the Michoacan block. The structural trends along both CHR and CR are thereby
explained. On the other hand, right-lateral transtension along the TZR is related to the westward
motion of the Jalisco block because of oblique convergence of the Rivera plate.
Introduction
SUBDUCTION OF SEGMENTS of the East Pacific Rise
under Western North America has resulted in fragmentation of the Farallon plate into smaller plates
(e.g. Gorda, Juan de Fuca, Rivera, and Cocos plates;
Atwater, 1970). From Tertiary time to the present
1Corresponding
author; email: [email protected]
0020-6814/03/686/814-13 $25.00
day, subduction of the Farallon plate along the
Pacific margin of Mexico formed the Sierra Madre
Occidental (SMO) and the Trans-Mexican Volcanic
Belt (TMVB) (Fig. 1, inset). Moreover, two important
late Miocene tectonic and volcanic events that have
been documented for western Mexico are: (1) the
opening of the Gulf of California; and (2) the transition of the SMO to the TMVB (Stock and Hodges,
1989; Moore et al., 1994; Ferrari et al., 1999).
814
TECTONICS OF WESTERN MEXICO
815
FIG. 1. Structural systems of central-western Mexico (TZR = Tepic-Zacoalco rift; CHR = Chapala rift; CR = Colima
rift) bounding crustal blocks (JB = Jalisco block; MB = Michoacan block). Inset shows the Sierra Madre Occidental
(SMO) and Trans-Mexican Volcanic Belt (TMVB) volcanic arcs. Abbreviations: G = Guadalajara; T = Tepic; MGVF =
Michoacan-Guanajuato volcanic field.
The west-central part of Mexico is segmented by
several structural systems bounding crustal blocks.
The Tepic-Zacoalco, Colima, and Chapala rifts form
a rift-rift-rift triple junction south of the city of
Guadalajara, forming the so called rift-rift-rift
Guadalajara triple junction (Fig. 1). The Colima and
Chapala rift system confines the Michoacan block,
whereas the Colima and Tepic-Zacoalco rift structures bound the Jalisco block. In the early phases of
formation, the rift systems experienced different
kinematics. Ferrari (1995) suggested that right-lateral transtension was dominant along the TepicZacoalco rift during late Miocene time; the Chapala
rift, however, was affected by left-lateral transtension (e.g., Garduño et al., 1993), and finally in the
Colima rift, ESE extension has been documented
(Barrier et al., 1990; Rosas-Elguera et al., 1996).
In an oblique convergent margin system, the
deformation induced is partitioned into an arc-normal component, accommodated at the trench, and
an arc-parallel component, accommodated at the
magmatic arc (Jarrard, 1986). In late Miocene time,
both a dextral-lateral (Schilt et al., 1982) and leftlateral motion (Wilson, 1997) at the northwest end of
the Cocos plate relative to the North America plate
had been proposed. Any model requires tectonic
rotation around a vertical axis for the involved
crustal blocks of the overriding plate.
Thus knowledge of the relative motions between
these plates involved with the southwest tectonics of
Mexico is important because it allows us to propose
a link between these relative motions and the onland
structural systems. Evaluation of these contrasting
plate models may be constrained by the onland geological record in the continental margin of southern
Mexico. The purpose of this study was to determine:
(1) if some movement had occurred in the Michoacan block; and (2) the age of deformation. These
points are important because our results can be
related to the convergence of the Cocos plate relative to North America and its implications for the
Guadalajara triple junction.
We performed an integrated paleomagnetic and
radiometric dating study to document the regional
stratigraphy and tectonics of the the Cotija half-graben, located in the Michoacan block. Radiometric
dating suggests a southern extension to this area of
the SMO and the early mafic basal TMVB sequence.
Paleomagnetic data indicate a counterclockwise
rotation of the Michoacan block. These results are
interpreted in support of a sinistral convergence
component of the Cocos plate relative to North
816
ROSAS-ELGUERA ET AL.
America, suggesting passive formation of the Colima
and Chapala rifts.
The Michoacan Block
According to Mosser (1972), southern Mexico
consists of several crustal blocks (Fig. 1). The
Michoacan block is bounded by the NNE segment of
the Rio Balsas in the east, the Colima tectonic
depression in the west, the Chapala rift and the
Michoacan-Guanajuato volcanic field to the north,
and the Middle America Trench to the south. The
Michoacan block consists of a late Cretaceous–early
Tertiary batholith (Schaaf et al., 1995), which
intruded Cret aceous vo lcano-sedi me nt ary
sequences of the Alberca and Tepalcatepec formations (Pimentel, 1980). Intercalated with the volcano-sedimentary sequences is the Tecalitlan
Formation, which consists of ash-flow tuffs and
minor andesitic lava flows (Rodriguez, 1980). A
youngest Cretaceous limestone rests on the above
sequence. According to subsurface geology by
PEMEX (Mexican petroleum agency), total thickness of the Mesozoic rocks is between 2500 and
6000 m.
Regional Stratigraphy and Geochronology
South of the Chapala rift, regional geological
mapping shows an area (Cotija sector) where the
SMO and the TMVB overlap (e.g., Ortega et al.,
1992; Consejo de Recursos Minerales, 1999), but no
radiometric ages were reported. Furthermore, radiometric databases (Ferrari et al., 1999) do not report
any Tertiary absolute ages in this part of the Michoacan block. West of the study area, however, a flatlying ignimbrite was dated as 23.5 ± 0.9 Ma (Ferrari
et al., 2002, Table 1). We completed our geologic
mapping and collected four samples for geochronology. Two basalts from the Cotija half-graben area
were collected for 40Ar/39Ar laser step-heating dating as part of a larger geological study of the structures in the Michoacan block. Standard whole-rock
sample preparation was done at the mineral separation laboratory at Unidad de Ciencias de la Tierra,
Juriquilla, Mexico. 40Ar/39Ar analyses were performed at the Geochronology Research Laboratory
of Queen’s University, Kingston, Ontario, Canada. A
third sample, an andesite, was dated at Mass Spec
Services, Geonuclear Divison by K-Ar method.
Finally a biotite concentrate and 40Ar/39Ar analysis
were done for an ash-flow tuff at CICESE, Mexico.
Analytical methods are detailed in Appendix 1.
The Cotija half-graben in the Michoacan block is
~25 km long and 10 km wide and shows a N60°W
trend. Geologic field work along this structure, supported by results of our geochronologic study, allow
us recognize three Tertiary to Quaternary magmatic
provinces (Fig. 2): (1) a granitic intrusive that is part
of a plutonic belt; (2) an Oligocene–Miocene pyroclastic succession (the SMO); and (3) late Miocene–
Quaternary volcanism of the TMVB.
Coastal plutonic belt. The granitic rocks south of
Cotija belong to the so-called coastal plutonic belt
studied by Schaaf et al. (1995). Radiometric ages for
this unit are between 53 and 68 Ma, and represent
the roots of a Tertiary magmatic arc parallel to the
volcanic fields of Sierra Madre del Sur developed
during the earliest Tertiary (Moran-Zanteno et al.,
1999).
Sierra Madre Occidental. Spatial-temporal evolution of Cenozoic volcanism in Mexico was used to
propose a transition from the SMO to the TMVB
(Ferrari et al., 1999). The SMO is defined as a NNW
volcanic arc of dominantly silicic composition, with
ages ranging from Paleocene to middle Miocene
(McDowell and Clabaugh, 1979). South of the
TMVB is the NNW-trending Tertiary arc magmatism
of the Sierra Madre del Sur, which may represent a
southeastern extension of the SMO with middle
Miocene ages (Fig. 1, Moran-Zenteno et al., 1999).
Most of the SMO was built through two episodes of
silicic volcanism in ~31.5–28 Ma and 23.5–20 Ma
related to the occurrence of two slab detachment
events (Ferrari et al., 2002).
The Oligocene–Miocene volcanic succession of
the SMO is exposed in the southwestern part of the
Cotija area. Although previously it was mapped as
SMO, no radiometric or lithologic descriptions were
made. It is deeply eroded, showing a well-developed
drainage network compared with that of the TMVB
(Fig. 3). The Oligocene rocks are a ~300 m thick,
northward-tilted volcaniclastic succession, reddish
to brownish in color. Within this unit, large andesitic
fragments (<0.5 m in diameter) are ash-matrix supported and are intercalated andesitic lavas. We
determined a K-Ar age of 29.3 ± 1.5 Ma (Table 1) for
sample MMD-3 that belongs to an andesitic lava
flow exposed in the valley of the Rio Huertas (Figs.
2 and 3). In addition, we have dated a biotite concentrate from the bottom of the rhyolitic ash flow
succession (sample JRE-227) located in the bed of
the Rio Huertas. This sample gave an absolute age
NW of Cotija
Rio Huertas
Cazos River
Gallineros
Location
Rio Huertas
Ixtlan
JIQ-15
JRE-227
CO
248
Sample
MMD-3
ROE142
Basalt
Andesite
Rock Type
Ignimbrite
Ignimbrite
Ash-flow tuff
Basalt
Basalt
Rock type
Whole rock
Whole rock
Material
Feldspar
Feldspar
Biotite
Whole rock
Whole rock
Material
20.025
19.745
Latitude, °N
–
19.895
19.865
19.944
19.878
Latitude, °N
wt%
Age, Ma
102.392
102.855
K/Ar ages
80.4
81.6
0.99
0.99
0.11
0.117
8.8 ± 0.8
29.3 ± 1.5
3
This study
Reference1
40Ar*,
1
This study
This study
This study
Longitude, °W Average K, wt% Average 40Ar
23.5 ± 0.9
31.6 ± 0.3
8.39 ± 0.92
9.21 ± 0.92
Reference1
2
ages
Age, Ma
26
–
103.041
102.883
102.807
102.929
40Ar/39Ar
Longitude, °W
1 = Ferrari et al., 2002; 2 = Rosas-Elguera et al., 2002; 3, Rosas-Elguera et al., 1989.
ENE of Cotija
RF-1
1References:
Location
Sample
TABLE 1. New Isotopic Ages for the Michoacan Block
TECTONICS OF WESTERN MEXICO
817
818
ROSAS-ELGUERA ET AL.
FIG. 2. Simplified geologic map of the Cotija region.
of 31.6 ± 0.3 Ma obtained for 40Ar/39Ar analyses
(Table 1). A well-defined plateau was obtained for
83% of the 39Ar released, as is shown in Figure 4A.
Our new geochronologic data correlate well with
those reported for the SMO and for the Sierra Madre
del Sur (Moran-Zenteno et al., 1999; Ferrari et al.,
2002). For instance, ages of 31 Ma have been
reported for the most northern area of Jalisco (in the
Huejuquilla area) about 330 km NNW of Cotija
(Ferrari et al., 2002). On the other hand, the Tertiary
volcanic rocks of the Sierra Madre del Sur form a
belt parallel to the present-day trench (Moran-Zenteno et al., 1999). The data base of Moran-Zenteno
et al. (1999) report ages between 22.5 and 49 Ma for
ignimbrites and rhyolitic tuffs of the Sierra Madre
del Sur, but ages between 31.6 to 33.6 Ma are
exposed in the state of Michoacan. These results
agree well with our geochronologic data (Table 1).
In the Cotija area, the Oligocene succession is
covered by a younger ignimbrite about 10 km south
of the town of Cotija. In this area, the ignimbrite
forms a unit with ~100 m thick, flat, yellow ash-flow,
partly welded tuff. Because of its stratigraphic position this unit is correlated with a pink ash-flow tuff
dated by Ferrari et al. (2002) at 23.5 Ma located
immediately west of the Cotija area. This age and
the northward tilted nature of the underlying Oligocene succession suggest an extensional tectonic
event between 31.6 Ma and 23 Ma.
Trans-Mexican Volcanic Belt. This E-W magmatic arc is located along 19–20°N Latitude with
ages younger than ~10 Ma (Fig. 1, inset). According
to radiometric ages, geographic distribution, and
chemical characteristics, the TMVB consists of a
basal late Miocene mafic succession and overlying,
thick Plio-Quaternary calc-alkaline sequences, but
a smaller volume of alkaline rocks have been
reported to the north of Guadalajara (Moore et al.,
1994) and at the western shoulder of the Colima graben (Allan, 1986; Righter and Rosas-Elguera 2001).
Although older dates (e.g., 13 Ma) have been
reported for the northern TMVB in the Los Altos
Plateau (Fig. 1; Urrutia-Fucugauchi, 1980; Castillo
and Romero, 1991; Verma et al., 1985), they may in
fact be younger than this, as recently shown by
Alva-Valdivia et al. (2000). In summary, most of the
radiometric dates range between 11 and 8 Ma.
In the Cotija area, the TMVB consists of
two units: a late Miocene mafic volcanic unit and a
Plio-Quaternary andesitic to basaltic volcanic field
TECTONICS OF WESTERN MEXICO
819
FIG. 3. Age spectrum obtained on samples JRE-227 and RF-1. The arrows indicate the fraction used in the plateau
age (tp) calculation. The width of the boxes represent 2σ errors. Dates and errors were calculated using formulas given
by Dalrymple et al. (1981) and the constants recommended by Steiger and Jäger (1977).
FIG. 4. Digital elevation model for the Cotija half-graben showing the location of paleomagnetic sites. The white line
separates the deeply eroded SMO from the TMVB. Although some volcanoes can still be distinguished in the footwall, in
the hanging wall they are unrecognized.
820
ROSAS-ELGUERA ET AL.
(Fig. 2). Up to now, the mafic succession has only
been documented north of the TMVB (Fig. 1; Gastil
et al., 1978; Moore et al., 1994; Ferrari et al., 2000;
Alva-Valdivia et al., 2000). In the Michoacan block,
the basal mafic succession is formed by basaltic
andesites and andesitic-basalt lava flows with individual thickness <10 m, forming plateau-like structures. Deeply eroded volcanoes are present as well
(Fig. 3). In fact, a thick (~20 m on average) weathered red to white cover is a conspicuous characteristic of this area. Intercalated with the mafic
succession is a ~50 m thick, brownish volcaniclastic
unit that is a stratigraphic marker because of its
broad distribution.
Two volcanic samples from the Cotija halfgraben area were collected for radiometric dating as
part of the geologic study of the Michoacan block.
Two new 40Ar/39Ar laser step-heating dates were
obtained (see details in the Appendix). The sample
RF-1 (basalt) date is 9.21 ± 0.92 Ma whole-rock and
the sample JIQ-15 (basalt) date is 8.39 ± 0.92 Ma
whole-rock (Table 1). Figure 4B shows the age
spectrum for sample RF-1. Our results correlate
well with those reported for the northern TMVB,
suggesting that the late Miocene mafic TMVB succession extends to this sector of the Michoacan
block.
The youngest volcanic rocks related to the
TMVB are Plio-Quaternary shield volcanoes,
andesites, and basalts related to the so-called
Michoacan-Guanajuato volcanic field (Hasenaka
and Carmichael, 1985). Debris flows have been
reported for this volcanic field (Rosas-Elguera et
al., 2002).
Counterclockwise Rotation of Michoacan
Block: Paleomagnetic Evidence
Recent paleomagnetic studies along the TMVB
have shown block rotations around a vertical axis.
The Cotija half-graben main fault cuts the late
Miocene volcanic succession with a minimum vertical offset of 400–700 m. Because of physical properties of the basaltic rocks, the striations along the
fault planes are poorly developed. Thus, we used the
paleomagnetic method to characterize the deformation, and define the potential vertical-axis block
rotations. Location of the studied sites are given in
Figure 3.
The remanent magnetization was measured with
a JR-5A spinner magnetometer (sensitivity ~10–9
Am2). Measurements were recorded after stabiliza-
FIG. 5. Orthogonal alternating-field (A) and stepwise-thermal (B) vector plots of representatives samples from Cotija
(stratigraphic coordinates). The numbers refer to the peak
alternating fields in mT. Symbols: filled circles = projections
into the horizontal plane, crosses = projections into the vertical plane.
tion of the remanence in this magnetometer. Both
alternating field demagnetization (AF), using laboratory made AF-demagnetizer, and stepwise thermal
demagnetization up to 575°–675°C, using a noninductive Schonstedt furnace, were carried out.
Sixty-five samples from 12 sites were progressively
demagnetized. Typically, 5 or 6 samples per
flow were subjected to alternating magnetic field
treatment.
In most of the studied units a stable paleomagnetic component could be recognized (Fig. 5). In a
few cases, a secondary component, probably of viscous origin, was easily removed by applying a field
of 20 mT (Fig. 5A). The greater part of the remanent
magnetization, in most case, was removed at temperatures between 500 and 580°C (Fig. 5B), indicating
low-Ti titanomagnetites as carriers of the magnetization. A characteristic magnetization direction was
821
TECTONICS OF WESTERN MEXICO
and the statistical parameters calculated assuming a
Fisherian distribution. Paleomagnetic site mean
directions of cleaned remanence and the corresponding virtual geomagnetic pole positions for the
Michoacan block (Cotija area) are given in Table 2.
Most sites are characterized by normal polarity
magnetizations. The volcanic rocks of the Cotija
area yield an overall mean paleodirection of I =
33.3°, D = 337.1°, k = 82, α95 = 4.8° (Fig. 6), which
deviates counterclockwise from the expected direction estimated from the North American apparent
polar wander path (Besse and Courtillot, 1991).
Discussion and Conclusions
FIG. 6. Equal area projection of the mean paleodirections
for all flows, as indicated in Table 2. Symbols: circle/crosses
denote negative/positive inclination, respectively.
Implications for the relative motions between
Cocos and North America plates
Between 10 and 7 Ma, oblique convergence
resulted in a dextral-lateral system at the northwestern end of the Cocos plate (Schilt et al., 1982).
Wilson (1997) suggested a moderate oblique convergence between the Cocos plate relative to the
North America plate for the same period but with a
determined by the least squares method (Kirschvink, 1980), four to eight points being taken in the
principal components analysis for this determination. The obtained directions were averaged by unit,
TABLE 2. Paleomagnetic Mean Directions of Cleaned Remanence
and Corresponding VGP Positions for Cotija Samples1
Site
n/N
Dec
Inc
α95
k
Plat
Plong
Pol
RF1
5/5
344.2
42.1
11.6
44.1
65.6
183.6
N
RF2
5/6
345.5
37.2
8.1
93.5
76.3
175.4
N
RF3
6/6
337.8
34.6
7.8
98.5
69.1
169.9
N
RF4
5/6
339.8
28.9
10.5
62.3
70.3
158.8
N
VJ1
1/6
323.9
27.8
–
–
55.2
164.5
N
SF1
6/6
348.8
37.7
14.5
28.4
79.2
179.3
N
SF2
5/5
162.1
-27.6
9.2
70.2
72.2
345.4
R
SF3
5/5
328.6
34.4
10.7
63.8
60.4
171.6
N
SF4
4/5
338.2
32.1
12.5
38.2
69.1
165.3
N
SF5
5/5
347.6
35.7
9.9
60.3
78.3
170.9
N
CO1
5/5
337.9
36.1
8.1
90.2
69.2
172.8
N
CO2
5/5
323.8
22.6
12.1
40.3
54.4
160.4
N
1N = number of treated samples; n = number of specimens used for calculation; Dec = declination; Inc = inclination; k and
α95 = precision parameter and radius of 95% confidence cone of Fisher statistics, respectively; Plat/Plong = latitude/longitude of VGP position; Pol = magnetic polarity.
822
ROSAS-ELGUERA ET AL.
FIG. 7. Tectonic map of west-central Mexico showing the crustal blocks involved in late Miocene kinematics. Late
Miocene relative motion of the Jalisco block after Ferrari et al. (2000) was changed for Plio-Quaternary time according
to Rosas-Elguera et al. (1996). Discontinuous/continuous arrows represent the rotated/expected directions for the late
Miocene, respectively. No rotation is depicted in the Tepic area.
left-lateral component of motion. These models suggest either a sinistral or a dextral oblique convergence component between the Cocos plate and
North America since late Miocene time. If correct,
tectonic rotation around a vertical axis is required
for the involved crustal blocks.
Block rotations in central and southern Mexico
have been reported (e.g., Urrutia-Fucugauchi,
1981). Just to the north of Cotija, in the Chapala rift,
Urrutia-Fucugauchi and Rosas-Elguera (1994)
found ~15° counterclockwise rotation for the late
Miocene to Pliocene lavas (Fig. 7). Besides this,
paleomagnetic data suggest a counterclockwise
rotation of 24° in the Los Altos region, and tectonic
data suggest that a transtensional deformation dominated during the Late Miocene (Fig. 7; AlvaValdivia et al., 2000; Ferrari et al., 2000).
We have shown, based on new radiometric data,
that volcanic activity of the TMVB in the Cotija area
is as old as late Miocene. Furthermore, our paleomagnetic data show a counterclockwise rotation
around a vertical axis, suggesting deformation
related to left-lateral faulting. This can be explained
if southeast motion of the Michoacan block is
assumed (Fig. 7) and this motion can be induced by
sinistral oblique convergence of the Cocos plate relative to North America.
Implications for the Guadalajara triple junction
Late Miocene right-lateral transtension along the
Tepic-Zacoalco rift has been documented (Ferrari,
1995), but paleomagnetic results suggest that
no major block rotation has occurred since then
(Goguitchaichvili et al., 2002). Reconstruction of
the motion of the Rivera plate relative to North
America for the same period shows that convergence
along the plate boundary was NNE (DeMets and
Traylen, 1999), inducing a right-lateral component
in the Tepic-Zacoalco rift because of westward
motion of the Jalisco block (Fig. 7; Ferrari et al.,
2000).
Structural data along the Chapala rift, and in
eastern areas, have shown that left-lateral transtension played a principal role in the early phases of
formation (Garduño et al., 1993). These results
agree with counterclockwise rotations at the Los
Altos area and in eastern Chapala (UrrutiaFucugauchi and Rosas-Elguera, 1994, AlvaValdivia et al., 2000). In an oblique convergent margin system, the deformation induced because of the
823
TECTONICS OF WESTERN MEXICO
involved plates is partitioned into an arc-normal
component accommodated at the trench, and an arcparallel component accommodated at the magmatic
arc (Jarrad, 1986; McCarey, 1994).
Thus, structural and paleomagnetic observations
in the Chapala rift can be explained if a left-lateral
component, in a transtensional framework, associated with southeastward motion of the Michoacan
block is considered. On the other hand, the Colima
rift (eastern boundary of Michoacan block) has
experienced an ESE extension (Barrier et al., 1990;
Rosas-Elguera et al., 1996) which is in agreement
with late Miocene alkaline volcanism within it
(Allan, 1986). In this case also, the observed extension can be related to the southeast motion of the
Michoacan block, as previously was proposed by
DeMets and Stein (1990).
We propose that sinistral oblique convergence of
the Cocos plate relative to the North America plate
inducing southeastward motion of the Michoacan
block can explain the structural features found
along both the Chapala and Colima rifts; on the
other hand, right-lateral transtension along the
Tepic-Zacoalco rift is related to the westward motion
of the Jalisco block because of the oblique convergence of the Rivera plate (Fig. 7). Thus the Guadalajara triple junction is related to the boundary force
plate regime.
Conclusions
1. New K-Ar and Ar-Ar radiometric data support
extension of the SMO to the Michoacan block in the
Cotija area, which is covered by the late Miocene
mafic TMVB succession. Thus, this succession has
a broader extension in central and southern Mexico
than previously considered. Consequently, further
detailed studies are required to define its regional
extension and geological significance.
2. Paleomagnetic data support the occurrence of
a counterclockwise vertical axis rotation of the
Michoacan block.
3. Our radiometric and paleomagnetic results
support a late Miocene left-oblique convergence of
the Cocos plate relative to North America plate.
4. Early phases of the evolution of Guadalajara
triple junction are related to crustal motions of
southern Mexico induced by oblique subduction of
both the Rivera and Cocos plates relative to the
North America plate.
Acknowledgments
Research supported by CUCEI 2002-5 (Departamento de Ingeniería Civil y Topografía), Consejo
Nacional de Ciencia y Tecnología (CONACyT), and
Dirección General de Apoyo al Personal Académico
(DGAPA) projects GEO-05, and J32727-T IN100100, IN-116201, and IN-102897. The 40Ar/39Ar
analytical work was supported by the Natural Sciences and Engineering Research Council of Canada
Major Facilities Access grant and Individual
Research grant to JKWL. MAOR is thankful for an
Internal Research grant from the Instituto de Geologia, UNAM, and a CONACyT Research grant. JRE
would like to acknowledge field assistance from M.
Tostado-Plascencia. Also, we thank Donald W. Carr
for revision of the English exposition.
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Appendix 1
Prepared whole-rock samples together with flux
monitors (standards), were wrapped in aluminum
foil. The resulting disks were stacked into a 11.5 cm
long and 2.0 cm diameter container, and then irradiated with fast neutrons in position 5C of the McMaster nuclear reactor (Hamilton, Ontario) in one 8h
irradiation. Groups of flux monitors (typically 12 in
total) were located at ~l cm intervals along the
irradiation container, and J values for individual
samples were determined by second-order polynomial interpolation. Typically, J values are between
~ 0.003 and 0.03, and vary by <10% over the length
of the capsule. No attempt was made to monitor
horizontal flux gradients, inasmuch as these are
considered to be minor in the core of the reactor.
For total fusion of monitors and step-heating
using a laser, the samples are mounted in an alumi-
num sample-holder, beneath the sapphire viewport
of a small, bakeable, stainless steel chamber connected to an ultra-high vacuum purification system.
Following an overnight bakeout at 200°C, an 8W
Lexel 3500 continuous argon-ion laser is used. For
total-fusion dating, the beam is sharply focused; for
step-heating the laser beam is defocused to cover
the entire sample. Heating periods are ~3 minutes at
increasing power settings (0.25 to 7 W). The evolved
gas, after purification using an SAES C50 getter (~5
minutes), is admitted to an on-line, MAP 216 mass
spectrometer, with a Bäur Signer source and an
electron multiplier (set to a gain of 100 over the
Faraday). Blanks, measured routinely, are subtracted from the subsequent sample gas fractions.
The extraction blanks are typically <10 × 10–13,
<0.5 × 10–13, <0.5 × 10–13, and <0.5 × 10–13 cm–3
826
ROSAS-ELGUERA ET AL.
STP for masses 40, 39, 37, and 36, respectively. At
least 24 flux monitors (Mac-83 biotite, few grains
each; Sandeman, et al., 1999) were individually
degassed at 1,200°C. Measured argon isotope peak
heights were extrapolated to zero time, normalized
to the 40Ar/36Ar atmospheric ratio (295.5) using
measured values of atmospheric argon, and corrected for neutron-induced 40Ar from potassium,
39Ar and 36Ar from calcium (using the production
ratios of Onstott and Peacock, 1987), and 36Ar from
chlorine (Roddick, 1983). Dates and errors were
calculated using formulas given by Dalrymple et al.
(1981) and the constants recommended by Steiger
and Jäger (1977). Isotope correlation analysis was
based on the formulas and error propagation of Hall
(1981) and the regression of York (1969).
Errors shown in Table 1 and on the age spectrum
and isotope correlation diagrams represent the ana-
lytical precision at 2σ, assuming that the errors in
the ages of the flux monitors are zero. This is suitable for comparing within-spectrum variation and
for determining which steps constitute a plateau
(McDougall and Harrison, 1988, p. 89). A conservative estimate of the error in the J value is 0.5%, and
this can added for inter-sample comparison. The
dates and J values for the intralaboratory standard
(e.g., MAC-83 biotite at 24.36 Ma) are referenced to
TCR sanidine at 28.0 Ma (Baksi et al., 1996). A plateau age is obtained when the apparent ages of at
least three consecutive steps, comprising a minimum of 70% of the 39Ark released, agree within 2σ
error with the integrated age of the plateau segment.
A so-called pseudoplateau follows the requirement
of a plateau age, but has an 39Ark percentage that
can be lower than 70%.