Geology of the western Mexican Volcanic Belt

Geological Society of America
Special Paper 334
1999
Geology of the western Mexican Volcanic Belt and
adjacent Sierra Madre Occidental and Jalisco block
Luca Ferrari
Instituto de Geología, Universidad Nacional Autónoma de México, Apdo. Postal 70-276,
Ciudad Univesitaria, 04510, México D.F., México. luca@geologia.unam.mx
Giorgio Pasquarè
Dipartimento Scienze della Terra, Universitá degli studi di Milano, Via Mangiagalli 34, I-20133 Milano, Italy
Saul Venegas-Salgado and Francisco Romero-Rios
Comisión Federal de Electricidad, Gerencia Proyectos Geotermoelectricos,
Departamento de Exploración, A. Volta 655, 58290 Morelia, Michoacan, México
ABSTRACT
The geology of a large portion of western Mexico, including the entire northern
boundary of the Jalisco block, has been compiled at a regional scale. Revision of previous works and new geological surveys allowed the construction of a 1:250,000 scale
map with 28 informal geologic units, 25 for the Tertiary and Quaternary volcanic succession of the Sierra Madre Occidental (SMO) and the Mexican Volcanic Belt (MVB).
In this region, the MVB covers the boundary between the SMO volcanic province
and the Cretaceous to Paleocene batholith and volcano-sedimentary sequences of the
Jalisco block (JB). The 1000-m-thick succession of the SMO was emplaced during
three discrete volcanic episodes of Eocene, Oligocene, and Early Miocene, separated by
volcano-sedimentary deposits or unconformities. This succession, dominated by silicic
ash flows, is absent in the deep geothermal wells of La Primavera, San Marcos and
Ceboruco areas, indicating that the southern boundary of the SMO is located north of
these sites.
Within most of the study area, the MVB is clearly separated from the SMO by a
tectonic unconformity produced in Middle Miocene. The first episode belonging to the
MVB is represented by a widespread and volumetrically significant mafic volcanism,
sometimes with an alkaline composition, which occurred at 11–8 Ma in central-eastern
Nayarit and in the Guadalajara region. A period of reduced volcanic activity between
7.2 Ma and 5.5 Ma was followed by the emplacement of large volumes of rhyolites and
minor ignimbrites in Early Pliocene. Intermediate to mafic volcanism, also with alkaline
composition, becomes again dominant between ~4.5 Ma and the Present, although large
rhyolitic and dacitic dome complexes were emplaced between Guadalajara and Tepic.
In Late Pliocene and Quaternary times, large andesitic to dacitic strato-volcanoes were
built in the northern part of the arc, whereas basaltic shield volcanoes and cinder cones
characterize the volcanic front.
Ferrari, L., Pasquarè, G., Venegas-Salgado, S., and Romero-Rios, F., 1999, Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre
Occidental and Jalisco block, in Delgado-Granados, H., Aguirre-Díaz, G., and Stock, J. M., eds., Cenozoic Tectonics and Volcanism of Mexico: Boulder,
Colorado, Geological Society of America Special Paper 334.
65
66
L. Ferrari et al.
INTRODUCTION
Purpose and limitations of this work
The aim of this work was to produce a geologic map of southwestern Mexico between the Pacific coast and the city of Guadalajara, which could constitute a useful framework for studies on the
tectonic evolution of the Gulf of California and the Jalisco block.
This region includes the Tepic-Zacoalco rift (TZR), thought to represent the northern boundary of the Jalisco block incipient
microplate (Luhr et al., 1985; Allan et al., 1991), and the western
sector of the Mexican Volcanic Belt (MVB), where alkaline (ocean
type) and calc-alkaline volcanism have been occurring in close
association since Late Miocene time (Wallace et al., 1992; Moore
et al., 1994) (Fig. 1). Just north of the MVB is also the southern
termination of the Sierra Madre Occidental silicic volcanic province (Fig. 1), which has been affected by various phases of Basin
and Range extension and strike-slip deformation since the Early
Miocene (Henry and Aranda-Gomez, 1992; Ferrari, 1995; Nieto-
Samaniego et al., 1999). The MVB itself is thought to conceal the
tectonic boundary between different tectonostratigraphic terranes
(Sedlock et al., 1993) which underwent hundreds of kilometers of
strike-slip displacement in Mesozoic to earliest Tertiary times
(Gastil and Jensky, 1973; Anderson and Schmidt, 1983).
Because of this volcanic and structural complexity, this region
has probably received as much attention as the rest of Mexico in the
last ten years, yet the map of west-central Nayarit by Gastil et al.
(1978) provides the only complete geologic study of at least half of
the region. Other geologic works are concerned with smaller areas
(Demant, 1979; Nieto-Obregon et al., 1985; Gilbert et al., 1985;
Allan, 1986; Nixon et al., 1987; Wallace and Carmichael, 1989;
Lange and Carmichael, 1991; Delgado-Granados, 1992; Quintero
et al., 1992; Righter and Carmichael, 1992; Michaud et al., 1993;
Ferrari et al., 1994a, 1997; Moore et al., 1994; Righter et al., 1995;
Rosas-Elguera et al., 1997), or deal only with late Pliocene-Quaternary volcanic centers (Luhr and Carmichael, 1980, 1981, 1982;
Mahood, 1980; Nelson, 1980; Luhr and Lazaar, 1985; Nelson and
Livieres, 1986; Nelson and Hegre, 1990).
Figure 1. Geodynamic sketch of Mexico depicting the four main plates and the Eocene to Present subduction-related volcanic arcs.
JB = Jalisco block. Boundaries of Mexican Volcanic Belt and Sierra Madre Occidental in central Mexico from Ferrari et al. (1994a).
Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block
A geologic and stratigraphic framework for this region was
particularly needed in order to reconstruct properly its tectonic
evolution, as there was no agreement about the recent and active
structures forming the TZR (Nieto-Obregon et al., 1985; Barrier
et al., 1990; Michaud et al., 1991; Allan et al., 1991; RosasElguera et al., 1993; Ferrari et al., 1994a). On the other hand the
Geothermal Division of Comisiòn Federal de Electricidad (CFE)
has been exploring several areas in the region since the late seventies in cooperation with the second author and also felt the
necessity of organizing its unpublished data into a wider geologic
framework. The project of the map was thus initiated in 1992 as a
cooperative effort between the first author and CFE to integrate
their unpublished data with new geologic survey and to elaborate
them into unique regional stratigraphy.
Previous published and unpublished data were revised and
compiled at a scale of 1:100,000. Areas without geologic information were mapped at a scale of 1:50,000 scale during a total of five
months of field work between 1992 and 1993. The final map
(Plate 1, see inside back cover) was then compiled at a scale of
1:250,000 and stored in an electronic format. The extremely poor
access prevented a detailed survey of the northeastern and southwestern parts of the studied region. These areas were examined at
a reconnaissance level along existing roads and through two helicopter flights and then mapped mostly by aerial photographs. Figure 2 shows the degree of detail of the geologic information
provided by the map.
All geological units we have established are informal; when
already present in the geological literature, we try to maintain
formal or informal names. We group rocks according to their age
and to the presence of a first order bounding unconformity. Volcanic rocks of the same age were subdivided also according to
their lithology, chemical composition and type of volcanic center.
As the map is mainly addressed to the Late Tertiary and Quaternary geologic evolution the stratigraphy and the tectonics of preLate Tertiary rocks is greatly simplified. In some cases, the
Plio-Quaternary stratigraphy may be perhaps oversimplified as
well, but this is due to the uneven detail of the geologic knowledge over the region.
The regional stratigraphy elaborated in this work was supported by a total of 260 radiometric ages compiled from the literature (Table 1) or published here for the first time (Table 2).
Although relevant in number, these ages are often concentrated
in areas of economic or scientific interest, so that the stratigraphy of some parts of the studied region still lack good geochronologic control. The ages reported in Table 1 and 2 are
generally consistent with the stratigraphic setting of the relative samples.
We have tried to incorporate all existing data in the map; in
addition, the use of an electronic storage system will permit us to
modify and integrate the map easily in the future. We consider
the map to be a first attempt to provide a regional geologic frame.
We hope to stimulate more detailed mapping as well as new
stratigraphic and geochronologic studies, which surely will
improve our work.
67
REGIONAL STRATIGRAPHY
The region presents a stratigraphic succession which includes
the Cordilleran basement, the Oligocene to middle Miocene SMO
volcanics and the Late Miocene to Quaternary MVB sequence. In
the following section, we briefly review the geologic units used in
the map but we refer to the existing literature for a more comprehensive description. Some details are given only for units not described
previously. The Neogene tectonics of the studied area is analyzed in
a companion paper (Ferrari and Rosas, this volume), in Ferrari
(1995), Ferrari et al. (1997) and in Rosas-Elguera et al. (1996).
Jalisco block
The JB is a distinctive tectono-stratigraphic assemblage consisting of Late Cretaceous to Early Tertiary volcanic and volcaniclastic
deposits and marine sedimentary sequences (unit 1 and 3) intruded
by granitoid plutons (unit 2). These rocks are often involved in lowto medium-grade metamorphism increasing from the southeast to
the northwest and are affected by folding and faulting, likely related
to the Laramide orogenesis. Turbiditic sequences are often intercalated with the volcanic rocks but are dominant in the western JB
(Sierra Vallejo and Sierra de Zapotan). Volcanic rocks are rhyolitic
ash flow tuff and subordinate andesites, mainly exposed in the central part of the area (Sierra Guamuchil and Sierra Jolapa). Limestone
and minor sandstone of probable Mesozoic age are confined to the
southeastern part of the area, just west of Atemajac.
The ages of the volcanic rocks exposed in the JB range from
114 Ma to 52 Ma (Gastil et al., 1978; Wallace and Carmichael,
1989; Lange and Carmichael, 1991; Righter et al., 1995; RosasElguera et al., 1997) with a cluster of 39Ar/40Ar ages between
60 Ma and 80 Ma (Righter et al., 1995). Plutonic rocks consists of
granite, granodiorite, and tonalite which form a large single
batholith south of Puerto Vallarta and probably represent the basement of the whole JB. Their apparent ages range between 108 Ma
and 45 Ma (Gastil et al., 1978; Allan, 1986; Köhler et al., 1988;
Zimmermann et al., 1988). According to the ages obtained with
different methods and/or on various minerals Zimmermann et al.
(1988) suggest two main periods of emplacements at ~105 Ma
and ~65 Ma with cooling ages up to 20 Ma younger. Zircon U-Pb
dates and Rb-Sr isochrons, however, indicate ages of emplacement ranging between ~100 and 90 Ma (Schaaf et al., 1993).
Based on close similarity in ages and chemical and isotope composition, Böhnel et al. (1992) demonstrated that the Puerto
Vallarta batholith is equivalent to the Los Cabos batholith (Baja
California Sur). In summary, the plutonic and volcanic rocks of
the JB are part of the same Late Cretaceous–Early Tertiary magmatic arc which have been recognized southeastward along the
Guerrero terrane (Campa and Coney, 1983; Sedlock et al., 1993).
Sierra Madre Occidental volcanic province.
The SMO is a huge, middle Tertiary, volcanic province which
extends from the southwestern United States to central Mexico
Figure 2. a) Credits of
the geologic survey.
b) Level of detail of
the geologic map.
Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block
(Fig. 1). In its northern part the SMO is composed of silicic ash flow
tuffs and rhyolitic lavas with minor amounts of andesitic lavas. Their
average thickness exceed 1 km (McDowell and Clabaugh, 1979) and
the ages cluster in two discrete periods of Eocene and Oligocene age
(Montigny et al. 1987 and references therein; Aguirre-Díaz and
McDowell, 1991 and reference therein). This silicic volcanism is
largely the result of fractional crystallization of mantle-derived
basalts produced by the subduction of the Farallon plate (Johnson,
1991; Wark, 1991) and the volcanic activity appears contemporary to
the waning of compression which followed the Laramide orogenesis
and with the first part of Basin and Range extension (Wark et al.,
1990; Aguirre-Diaz and McDowell, 1991, 1993).
The study area encompasses the southernmost part of SMO
which exhibits a stratigraphy mostly similar to the northern part
of the province (Nieto-Samaniego et al., 1999). The existence of
a pre-volcanic (Cretaceous?) basement is substantiated by localized outcrops of argillite and limestone (not mapable at the scale
of Plate 1) which were exposed along the lower part of the Rio
Grande de Santiago canyon before the construction of the
Aguamilpa reservoir. The spatial association of these rocks with
Late Oligocene to Early Miocene granite to granodiorite stocks
(Gastil et al., 1978; Nieto-Obregon et al., 1985)(unit 6) suggest
that they were roof pendants uplifted by the plutons. Although
Eocene rocks are reported by Webber et al. (1994) about 50 km
northeast of San Cristobal and in the central SMO in Durango
(Aguirre-Díaz and McDowell, 1991), no pre-Oligocene volcanic
rocks were found in the study region.
The most widespread unit in the southern SMO is a succession
of welded ash flow tuffs and minor andesitic lava flows (unit 4)
which attains an aggregate thickness of 800 m in the southeastern
part of the area (Fig. 3). Oligocene ash flows (34–28 Ma; Damon
et al., 1979; Nieto-Obregon et al., 1981) are found in the Santa
Maria del Oro and Juchipila areas (Fig. 3 and 4). In the western part
of the area these rocks are interbedded with and capped by volcanoclastic sequences made of clay, sandstone and conglomerate.
Although this sequence was not encountered in the northeastern part
of the study area, Spinnler et al. (this volume) recognized an angular unconformity within the ignimbritic succession. Most of the
SMO, however, is covered by rhyolitic ash flow and pumice-flow
and minor domes with ages of 26–17 Ma (Damon et al., 1979;
Gastil et al., 1978; Nieto-Obregon et al., 1981, 1985; Scheubel et al.,
1988; Moore et al., 1994; Webber et al., 1994; Nieto-Samaniego
et al., 1999) which are only found in the southern SMO. Ash flows
and rhyolites with similar ages exposed in southernmost Baja California (Hausback, 1984) and as far as 230 km east of Guadalajara
(Pasquarè et al., 1991; Tuta et al., 1988) may represent respectively
the front and the back arc of the Early Miocene SMO.
Between San Cristobal and García de la Cadena a succession
of basaltic-andesitic lava flows is found capping the Early
Miocene ash flows (Unit 5). This succession yielded ages comprised between of 21.8 and 19 Ma (Nieto-Obregon et al., 1981;
Moore et al., 1994; Ferrari and Lopez-Martinez unpublished data)
and has geochemical characteristics (Moore et al., 1994) correlative with the Southern Cordillera Basaltic Andesite (SCORBA)
73
first studied in Chihuahua by Cameron et al. (1989) and also
reported in central Durango (Aguirre-Díaz and McDowell, 1993).
North of Guadalajara the SMO succession is dissected by
north-south to north-northeast–southsouth-west trending, ~15 km
wide grabens (Nieto-Samaniego et al., 1999). One of this grabens,
the Juchipila graben, is floored by a stratified volcano-sedimentary succession (unit 8). This consists of some meter of a coarse
volcanic conglomerate, followed by up to 250 m of lacustrine
clays, silts and limestones.
In the eastern part of the study area, north of Santa Maria del
Oro, the SMO ash flows are sometimes covered by eroded
andesitic volcanoes which in one case has been dated 14.7 Ma
(Rodriguez-Castaneda and Rodriguez-Torres, 1992). South of
Tepic, a succession of andesitic and basaltic flows with subordinate rhyolites gave ages comprised between 20.4 and 13.8
(± 3.1) Ma (Gastil et al., 1979a). These rocks (unit 7) suggest the
existence of a middle Miocene volcanic arc shifted in the western
part of the SMO province and characterized by products of intermediate composition. We believe that this volcanism represents
the southern occurrence of the Early to Middle Miocene andesitic
arc recognized by Gastil et al. (1979b) and Fenby and Gastil
(1991) around the Gulf of California which was subsequently
separated along its axis by the opening of the gulf itself.
Mexican Volcanic Belt
The Mexican Volcanic Belt (MVB) is a 1000-km-long volcanic arc formed in response to the subduction along the Acapulco trench since middle Miocene (Ferrari et al., 1994b; 1999)
(Fig. 1). The western MVB has been often regarded as mainly
Plio-Quaternary in age (Nelson and Sanchez-Rubio, 1986; Nixon
et al., 1987) but a large amount of mafic lavas were also
emplaced in Miocene time. The older rocks are exposed along the
northwestern shores of the Chapala lake, in the Sierra Las Vigas
(unit 9). They are volcanic breccias made of decimetric to metric
clasts of andesitic to dacitic composition sometimes supported by
a sandy matrix (Garduño et al., 1993). These rocks are often
highly altered by hydrothermal processes and fractured. There
are no ages available for these rocks but they must be pre-Late
Miocene because of their stratigraphic position.
In the Guadalajara region, a monotonous sequence of thin
basaltic lava flows is exposed all along the Rio Santiago east of
Tequila (unit 10). This succession yielded ages of 10.2–8.5 Ma
(Table 1) (Fig. 3) and the composition tend to evolve upsection
from alkali-basalts to basaltic andesites (Moore et al., 1994).
Individual flows range between 2 and 10 m in thickness and the
exposed succession is over 600 m thick. A distinctive silicic
welded ash flow (Inversely Welded Tuff of Moore et al., 1994) is
interbedded with the basalts in the lowest part of the exposed succession and a heterogeneous silicic succession made of pumice
flows, welded ash flows, ash falls and reworked pyroclastics (Los
Caballos tuff of Moore et al., 1994) overlies the basalts both to
the north and to the south of San Cristobal. North of Tequila, the
basalts rest in unconformity above folded ash flows of the SMO
74
L. Ferrari et al.
Figure 3. Composite stratigraphic columns for the
northeastern part of the mapped area at the longitude of Guadalajara. The first column refers to the
SMO succession exposed north of Rio Santiago.
The second column refers to the succession exposed
in the Rio Santiago valley between San Cristobal
and the Santa Rosa dam. At San Cristobal, only the
MVB succession is exposed. Volcanic rocks belonging to the MVB filled a basin (symbolized by the
dashed line), cut into the SMO succession, which is
exposed in the Santa Rosa dam area. Number to the
left of columns are unit thickness in m. Absolute
ages from Nieto-Obregon et al. (1981, 1985),
Gilbert et al. (1985), Moore et al. (1994) and this
work are also reported in Table 1 and 2.
(Ferrari et al., 1997). In the San Cristobal area the basal contact is
never exposed at the 800 m elevation of the Rio Santiago base,
but ash flows belonging to the SMO outcrop at an average elevation of 2000 m just 25 km north of San Cristobal. Thus the basalts
probably flowed southward into a preexisting depression cut into
the SMO sequence. Several eroded basaltic shield volcanoes are
actually exposed north of Rio Santiago, resting on Early Miocene
ash flows. One of these centers has been dated 11 Ma (Moore
et al., 1994), and the others probably have similar ages since their
flows join with the San Cristobal plateau.
South of Rio Santiago, the basalts are covered by younger
rocks but, about 35 km south of San Cristobal the geothermal
wells drilled in the La Primavera caldera encountered a monotonous sequence over 800 m thick of basaltic and basalticandesitic flows under about 1100 m of ash flows, rhyolites, and
lacustrine deposits correlative with the Pio-Quaternary succession (Fig. 5). These basalts are both aphyric and porphiritic with
phenocrysts of plagioclase and olivine and, sometimes, a glassy
texture in the groundmass. A glassy rhyolitic ash flow is also
intercalated in the central part of the basaltic succession. This
succession shows a very similar petrography and stratigraphy
with respect to the one exposed in the San Cristobal area and a
whole rock age of 12.5 ± 0.6 Ma has been obtained for a basalticandesite at the base of the succession (Table 2, Fig. 5). The San
Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block
75
Figure 4. Composite stratigraphic columns for the western
part of the mapped area. The first column includes rocks
cropping out in the Rio Santiago valley between Sierra Alica
and Tepic. The second column shows the stratigraphy
exposed in southwestern Nayarit along the western edge of
the Jalisco block. Absolute ages from Gastil et al. (1978,
1979b), Damon et al. (1979), Zimmermann et al. (1988),
Rodriguez-Castaneda and Rodriguez-Torres (1992), Righter
et al. (1995), and this work are also reported in Table 1 and 2.
Cristobal plateau extends also east of Guadalajara for over
130 km with an average thickness of 200 m and ages comprised
between 11 and 8 Ma (Ferrari et al., 1994c; 1999). As a consequence, the Late Miocene basalts of the Guadalajara region
cover an area comparable to that of the Late Pliocene-Quaternary Michoacan-Guanajuato volcanic field (Hasenaka and
Carmichael, 1985), and their volume exceeds the total volcanic
output of the western MVB during the Pliocene and Quaternary
(Ferrari unpublished data). Thus the Late Miocene volcanism
represents a major stage in the building of the MVB.
About 160 km west of Guadalajara, a plateau of alkali-basalts
with ages between 11 and 8.9 Ma (Righter et al., 1995; Table 2) is
exposed between the SMO and the Pacific coast north of Tepic
(unit 11) (Fig. 4). The lavas are nearly aphyric with microphenocryts of olivine and minor plagioclase. These rocks rest in unconformity above tilted blocks of the SMO. Several mafic dikes with
ages of 12–11 Ma (Damon et al., 1979; Clark et al., 1981; Table 2)
and north-south to north-northwest trend are found the western
part of the SMO. Basaltic lavas and north-northeast trending dikes
of the same age are also found along the southern coast of Nayarit,
north of Punta Mita. In this area, many of the basalts were emplaced
as pillow lava. In addition, Gastil et al. (1979b) reported forambearing volcanic sandstones interbedded with basalts dated
10 Ma, which testify that marine ingression related to the “Protogulf opening” was already underway by this time.
A very thick succession of mafic lava flows of Late Miocene
age has been also found in the deep geothermal well drilled 3 km
south of the crater of the Ceboruco volcano by CFE (CB1 well,
Fig. 5a). This succession, composed by microporphyritic to
aphyric basalts and basaltic-andesites with minor intercalations
of pumice and ash flows, rests directly over altered ignimbrites
and andesites correlative with the Jalisco block succession
exposed 10 km to the south.
The volcanic succession in the Chapala lake region is more
complex and has been studied in less detail. Rocks to the south
and the west of the Chapala lake are lava flows and breccias of
Late Miocene to Early Pliocene age (unit 12). In the Sierra de
Tapalpa, most of the succession consists of alkaline basaltic and
andesitic lava flows with ages comprised between 5.4 and 4.4 Ma
(Allan, 1986) although Late Miocene lavas are also reported
(Rosas-Elguera et al., 1997) (Table 2). The succession exposed
to the north and the west of the Chapala lake comprise also silicic
Figure 5. Stratigraphy of deep geothermal well drilled by CFE in the Ceboruco area (a) and in the La Primavera and San Marcos areas (b). Absolute
elevation (asl) is indicated on the left of the column. Thicknesses for the units encountered in the La Primavera caldera are averages of data from 10
wells. All but one well (PR 9) stopped before 450 m bsl. Absolute age for the CB2 well from this work (Table 2).
Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block
77
Figure 6. Composite stratigraphic columns of the
zone of intersection between SMO and MVB in
the central part of the mapped area. Absolute ages
from Nieto-Obregon et al. (1985) and Moore
et al. (1994) are also reported in Table 1 and 2.
lavas and welded ash flows with ages ranging between 6.7 and
4.2 Ma (Delgado-Granados, 1992) (unit 13). Locally, deltaic and
lacustrine deposits including diatomitic facies (unit 14) are
interbedded in the previous successions. These deposits consist
mainly of reworked silicic ash fall and lavas and are related to the
initial formation of the Chapala lake.
In the Guadalajara area, the Late Miocene basalts are covered by the Cerro La Col and Cerro Los Bailadores rhyolitic
dome complexes (5.5–5.2 Ma) and by various silicic ash flows
and air fall deposits among which are the 4.7 Ma San Gaspar
Ignimbrite and the 3.3 Ma Guadalajara Ignimbrite (Gilbert et al.,
1985) (unit 15). As a whole this silicic succession is up to 800 m
thick and correlative rocks have been found in the La Primavera
wells (Fig. 5b). A similar silicic sequence is exposed in the Sierra
de Ahuisculco (unit 16), where an ignimbrite yielded an age of
4.7 Ma (Rosas-Elguera et al., 1997), suggesting a possible correlation of this succession with the Guadalajara group.
Another silicic sequence is exposed continuously between
Plan de Barranca and Compostela (unit 17). It consist of welded
ash flows, a pumice flow and rhyolitic domes and flows (Fig. 6).
Righter et al. (1995) obtained a 4.3 Ma 39Ar/40Ar age from one of
the topmost ash flow whereas Gastil et al. (1979b) report a K/Ar
age of 4.6 Ma for the summit rhyolites. Other undated silicic
complexes are the Tepic caldera and a dome complex east of San
Blas. Based on stratigraphic relations we correlate them with the
Early Pliocene silicic sequence.
78
L. Ferrari et al.
The Plio-Quaternary MVB consists mostly of basaltic cinder
cones and shield volcanoes, andesitic to dacitic stratovolcanoes
and dacitic to rhyolitic domes (Nixon et al., 1987 and references
therein; Wallace and Carmichael, 1989; Lange and Carmichael,
1991; Wallace and Carmichael, 1994; Righter and Carmichael,
1992; Moore et al., 1994; Righter et al., 1995). Lava types are
both alkaline and calc-alkaline in composition and fall in the
ocean island and subduction related fields (Righter and Carmichael, 1992; Moore et al., 1994; Righter et al., 1995), although
the former are subordinated in volume. Basaltic lavas with oceanic island affinity occur all over the western MVB but seem concentrated in the northern part of the arc (unit 18 and 22). In the
Guadalajara and Hostotipaquillo areas, a peculiar variety of Early
Pliocene porphyritic olivine-basalts with megacrysts of plagioclase are found within this group (Moore et al., 1994) but the rest
of these rocks are microporphyritic to aphyric. Subductionrelated basalts and andesites are found in close association with
the OIB-type volcanics. Calc-alkaline and alkaline basaltic cinder
cones and shield volcanoes of Early Pliocene to Holocene age
unconformably cover the JB and constitute the volcanic front of
the western MVB (Wallace and Carmichael, 1989; Lange and
Carmichael, 1990; Righter and Carmichael, 1992) (unit 19b
and 23). Late Pliocene and Quaternary andesitic to dacitic stratovolcanoes are located mostly at the rear of the arc (Luhr and
Carmichael, 1980, 1981, 1982; Nelson, 1980; Wallace and
Carmichael, 1994; Nelson and Livieres, 1986) (unit 19a and 24).
Late Pliocene to Quaternary silicic volcanism is restricted to the
Magdalena and San Pedro (Castillo and De la Cruz, 1992,
Table 2) rhyolitic and dacitic dome complexes and the Las Navajas (Nelson and Hegre, 1990) and La Primavera (Mahood, 1980,
1981) peralkaline rhyolitic centers (unit 25 and 26). The Quaternary Acatlán ignimbrite, described by Wright and Walker (1981),
is exposed in the southern part of the mapped area (unit 21). It is
covered by a basaltic flow dated at 0.65 Ma (Allan, 1986) and is
probably related to rhyolitic domes located north of Acatlàn
(Ferrari et al., 1997).
Several fluvio-lacustrine deposits crop out at the base of the
tectonic depressions inside the JB (unit 20) and are presently
undergoing erosion. In the Puerto Vallarta and Amatlàn de Cañas
depressions, they are represented by a poorly cemented conglomerate with clasts of granite supported in a sandy matrix. In
the Ameca area, volcano-lacustrine deposits are also present
within this unit. All these deposits are probably related to the
Early Pliocene formation of these tectonic depressions.
DISCUSSION AND CONCLUSIONS
The boundary between the Jalisco block and the
Sierra Madre Occidental
The regional mapping presented in Plate 1 allows us to infer
the location of the northern boundary of the JB and to discuss its
nature. The ages of plutonic and volcanic rocks to the north and
to the south of the western MVB indicate that the boundary
between JB and SMO is concealed beneath Late Miocene to
Quaternary volcanic rocks. The northernmost exposure of Cretaceous-Paleocene ash flow and granitoids are south of Volcán San
Juan, in the Sierra El Guamuchil and Sierra de Ameca horsts, and
just south of Plan de Barranca (Fig. 7). The latter location is also
the only site where the JB is in contact (tectonic) with the SMO
ash flows. In addition, the deep geothermal well drilled south of
San Pedro and Ceboruco volcanoes and in the La Primavera
caldera found rocks correlatives with the JB without encountering the SMO succession (Fig. 5). In order to satisfy these data,
we propose that the JB-SMO boundary follows a zig-zag pattern
under the San Juan, Tepetiltic, Ceboruco and Tequila stratovolcanoes (Fig. 7). This boundary appears to control the composition of the MVB volcanism. Volcanic rocks emplaced within the
JB are dominantly mafic whereas a bimodal volcanism is typical
of the region underlain by the SMO.
Along the northern edge of the JB, Late Cretaceous to Paleocene granitoids and volcanics which often display low-grade
metamorphism are presently exposed at elevations ranging
between 1500 m and 2600 m. By contrast the silicic succession of
the SMO is underlain by granitoids of Oligocene to Early
Miocene age (Fig. 3, 4 and 6), which outcrops along the Rio Santiago at ~500 m at most. This implies that the JB has been uplifted
considerably with respect to the SMO and that a major discontinuity must exist between the two domains. Inside the JB, however,
Plio-Quaternary lavas rest directly over Cretaceous-Paleocene
rocks. Therefore south of the MVB the Oligocene–Early Miocene
SMO succession was either: a) removed prior to the emplacement
of the mafic lavas or b) never deposited. In our view, the best
explanation is that the JB was already uplifted by Oligocene times
and formed a barrier for the SMO ash flows coming from the
north. The other option, in fact, looks very unlikely because this
implies a huge and complete erosion in a short time and because
no subvolcanic or plutonic rocks of middle Tertiary age, representing the roots of SMO ignimbrites, are exposed in the JB. This
hypothesis appears further confirmed by the stratigraphy of the JB
shown at the inner slope of the trench between Puerto Vallarta and
Manzanillo (Mercier de Lepinay et al., 1996), which indicate an
uplift of the block in Eocene time.
Volcanic evolution and plate tectonics
The Oligocene to Quaternary volcanic evolution within the
study area appears to be marked by episodes of vigorous volcanism separated by periods of low volcanic activity. This is displayed in Fig. 8, which shows the distribution of radiometric ages
subdivided according to the petrography of the dated samples. In
Fig. 8, the Plio-Quaternary volcanism is certainly overrepresented but we believe that the figure is representative of the main
variations of the volcanism. It can be seen that a change in the
dominant composition of volcanic rocks took place after a period
of reduced volcanic activity in Middle Miocene. In fact, in the
Oligocene and the Early Miocene the composition of the dated
rocks is largely silicic whereas since middle Miocene time, mafic
Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block
79
Figure 7. Tectonic sketch map showing the Late Miocene to Quaternary structures and volcanic centers. The inferred boundary between JB and SMO
is depicted with a bold dashed line.
and intermediate products become dominant. This change has
been observed in the volcanism of the whole of central Mexico
and has been interpreted as the limit between the SMO and the
MVB (Ferrari et al., 1994a; 1999).
The silicic SMO volcanism shows a peak at the beginning
of Early Miocene and then gradually stops in the middle
Miocene (Fig. 8). The time of termination of the SMO volcanism fits well with the waning of the subduction off Baja California, which, at this latitude, took place between 14 and 12 Ma
(Lonsdale, 1991). During the Middle Miocene, volcanic activity
is sporadic in the west and totally absent in the central and eastern part of the study area. During this period, the boundary
between SMO and JB underwent contractile and transpressive
deformations (Ferrari, 1995).
The volcanism related to the MVB can be divided into three
episodes: a period of dominantly mafic, sometimes alkaline, vol-
canism between 11 and 8 Ma, an interval of reduced and chemically evolved volcanism between 8 and 5 Ma, and a new episode
of mafic and alkaline volcanism since 5 Ma to the Present. The
first period corresponded to the main phase of opening of the
Gulf of California. This event may have produced a tear in the
subducting slab, allowing the upwelling of asthenospheric material and the emplacement of alkaline basalts along the preexisting discontinuity of the JB-SMO boundary. The 8 to 5 Ma
decrease of the volcanic activity corresponds to the plate reorganization which preceded the birth of the Rivera plate and the
existence of the Mathematician microplate (Mammerickx and
Klitgord, 1982; Lonsdale 1991; Lonsdale, 1995). A reduced volcanic rate for this period may indicate a decrease in the subduction velocity and thus in the magma production rate at depth. In
this case, however, we do not see a clear relation between volcanism and plate tectonics and we think that more details on
80
L. Ferrari et al.
Figure 8. Frequency and composition of Oligocene to Present ages of volcanic rocks in the map area (Table 1 and 2).
Limit between SMO and MVB according to Ferrari et al. (1994a) and corresponds to a change in the dominant composition of dated rocks. Silicic = rhyolites to dacites, intermediate = andesites, mafic = basalts.
both subjects is needed to solve the problem. The new occurrence of mafic and alkaline volcanism since 5 Ma appears
related to the extensional tectonics which affected the boundaries of the JB in this period, which, in turn, could be associated
with a retreating subduction of the Rivera plate (Rosas-Elguera
et al., 1996).
ACKNOWLEDGMENTS
We thanks Dr. G. Hiriart-Le Bert, Gerencia de Proyectos
Geotermoelectricos of C.F.E., for his effective and continuous
support to the project. J. Rosas-Elguera provided much useful
information on the geology of Zacoalco and Chapala region.
K. Righter, I. S. E. Carmichael, and H. Delgado-Granados kindly
made available radiometric dates in advance of publications.
Revisions by G. Moore, J. Allan, and C. Henry contributed to
improve the manuscript. L.F. was supported by a post-doctoral
grant of University of Milan, and by grants 205.05.15 of C.N.R.
and M.U.R.S.T. 40% (1991, 1992, 1993), Italy. Map updating
between 1995 and 1999 was accomplished thanks to grant
UNAM PAPIIT IN 108196 to L.F.
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