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ABSTRACT
The middle to late Cenozoic tectonic-magmatic evolution of theSierra Madre Occidental volcanic province south of the Tropic of Can-cer is summarized and analyzed for the first time, based on new geo-logic and structural work and published information. In the easternpart of the study region (Mesa central physiographic province) silicicvolcanism occurred in a short-lived episode culminating at ca. 30 Maand was followed by crustal-scale extension between 30 and 27 Ma. Inthe western part of the study area (Sierra Madre Occidental physio-graphic province) a voluminous episode of ignimbrite volcanism at2421 Ma was succeeded by east-west extension that produced regu-larly spaced grabens affecting only the upper crust. In the westernmostpart of the study region, an andesitic to rhyolitic arc, formed between
17 and 12 Ma, was affected by crustal-scale,north-northwesttrending,extensional faulting, leading to the formation of the Gulf of California.In the Mesa central the maximum extension was oriented approxi-mately east-west and amounted to ~20%. In the eastern Sierra MadreOccidental physiographic province extension was only 8% and ori-ented approximately east-west. We observe that trenchward shifting ofthe climax of subduction volcanism and extension occurred during lateOligocene,early Miocene, and late Miocene time. Comparison with theoffshore tectonics indicates that the first two tectonic-magmatic pulsescoincide with periods of fast spreading at the Pacific-Farallon bound-ary, south of the Shirley fracture zone. We propose that increases in thespreading rate are related to periods of high subduction rate, which inturn correspond to episodes of retreating subduction. A retreating slabmay have generated a flux of hotter asthenospheric material into the
mantle wedge,producing widespread melting at the base of the crust aswell as intraarc extension in the overriding plate. Boundary conditions(i.e., plate tectonics) ultimately determined timing, magnitude,and ori-entation of extension, whereas volcanic and tectonic styles are con-trolled by the internal structure of crustal blocks and by the gravita-tional and thermal effects of magmatism.
INTRODUCTION
Three major tectonic events occurred in the northern half of the Mexican
Pacific coast during Tertiary time: (1) an episode of subduction that lasted
until middle Miocene time (Atwater,1989); (2) a progressive waning of sub
duction through various episodes of microplate capture induced by the ap-
proach of the East Pacific Rise to the trench (Lonsdale, 1991); and (3) th
opening of the Gulf of California (e.g., Stock and Hodges 1989; Lonsdale
1989). These events shaped the volcanic and structural configuration of the
central and western parts of northern Mexico, forming the Sierra Madre Oc
cidental volcanic province and a wide extensional province, referred to as the
real southern Basin and Range(Henry and Aranda-Gmez, 1992) (Fig. 1)
In the past two decades there have been many contributions to the recon
struction of the volcanic evolution of the northern and central part of the
Sierra Madre Occidental volcanic province (McDowell and Keizer, 1977McDowell and Clabaugh, 1979; Henry and Fredrikson, 1987; Wark et al.
1990; Aguirre and McDowell, 1991). Structural studies in Sinaloa (Henry
1989),Durango (Aguirre-Diaz and McDowell,1993; Aranda-Gmez et al.
1997),and Sonora (Gans, 1997; McDowell et al., 1997) (Fig. 2) provide in
formation on the timing and style of the extensional tectonics. In general
these works report a westward migration of extension and volcanism dur
ing post-Eocene time, as well as an inception of extension well before th
first direct interaction between the Pacific and the North American plates
The Sierra Madre Occidental volcanic province south of the Tropic o
Cancer (hereafter referred to as the southern Sierra Madre Occidental vol
canic province) is less well-known, at least in the international geologic lit
erature. This region can be divided into the Sierra Madre Occidental and the
Mesa central physiographic provinces (Fig. 1). Geologic mapping and struc
tural studies carried out in the past 15 yr have documented the volcanic andtectonic evolution of the Mesa central in good detail (Labarthe-Hernndez
et al., 1982; Aranda-Gmez et al., 1989; Henry and Aranda-Gmez, 1992
Nieto-Samaniego, 1990; Martnez-Reyes, 1992; Quintero-Legorreta, 1992
Nieto-Samaniego and Alaniz-Alvarez, 1994; Nieto-Samaniego et al., 1996
1997), but most of this information is published in university reports or na
tional journals, with limited international distribution.
The timing of the volcanic and extensional events in the Sierra Madre
347
Variation of Cenozoic extension and volcanism
across the southern Sierra Madre Occidental volcanic province, Mexico
ngel F. Nieto-Samaniego*Luca Ferrari Unidad de Ciencias de la Tierra, Universidad Nacional Autnoma de Mxico,
Susana A. Alaniz-Alvarez Campus Juriquilla, Apdo. Postal 1742, Quertaro, Qro., 76001, Mxico
Guillermo Labarthe-Hernndez Instituto de Geologa, Universidad Autnoma de San Luis Potos,Dr. Manuel Nava 5, Zona Universitaria, San Luis Potos, S. L. P., 78240, Mxico
Jos Rosas-Elguera Centro de Ciencias de la Tierra, Universidad de Guadalajara,Blvd. M. Garca Barragn y Calz. Olmpica, 44840, Guadalajara, Jalisco, Mxico
GSA Bulletin; March 1999; v. 111; no. 3; p. 347363; 10 figures; 2 tables.
}
*E-mail: [email protected].
Data Repository item 9919 contains additional material related to this article.
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NIETO-SAMANIEGO ET AL.
348 Geological Society of America Bulletin, March 1999
Occidental is less well-known than for the Mesa central. Good strati-
graphic and geochronologic information is available only for a few places
(Lyons, 1988; Scheubel et al., 1988; Nieto-Obregn et al., 1985), whereas
there are only reconnaissance geologic and structural works (Gastil et al.,
1978; Damon et al., 1979; Webber et al., 1994; Ferrari, 1995; Ferrari et al.,
1999) and several scattered ages of volcanic rocks for the remaining region
(Clark et al., 1981; Nieto-Obregn et al., 1981; Moore et al., 1994). Nev-
ertheless, these data permit reconstructing the spatial evolution of the vol-
canism. In addition, we have studied the western Sierra Madre Occidental
at a regional scale through satellite images, aerial photographs, and recon-
naissance studies along roads.
In this paper we integrate and summarize our regional stratigraphic and
structural information with published geological and geophysical data to
provide the first synthesis of the middle to late Cenozoic volcanic and tec-
tonic evolution of the southern Sierra Madre Occidental volcanic province.
Our data document an Oligocene three-dimensional strain in the southern
Mesa central, an early Miocene two-dimensional strain in the eastern part
of the southern Sierra Madre Occidental , and a major middle to late
Miocene extensional phase of deformation in the western part of the south-
ern Sierra Madre Occidental.We propose that variations in deformation and volcanism observed since
Oligocene time in the southern volcanic province are due to the difference
in thickness and composition of the major crustal blocks that form the Sierra
Madre Occidental and the Mesa central. When compared to the rest of the
Sierra Madre Occidental volcanic province, our results indicate that the
westward shift of the zone of maximum volcanic and tectonic activity was
a general phenomenon in OligoceneMiocene time, although in the south-
ern volcanic province geologically significant extension affected a much
wider region than to the north. The westward migration is tentatively related
to the along-trench variation of the FarallonNorth America subduction rate
during Oligocene and Miocene time.
TOPOGRAPHY AND CRUSTAL THICKNESS
The spatial variations in the Sierra Madre Occidental volcanic province
are illustrated in an east-west topographic profile in Figure 3. This profile
distinguishes three physiographic provinces with different elevations: an
eastern province with mean elevation of 1400 m corresponding to the Sierra
Figure 1. Geodynamic setting, physiography (A), and Cenozoic volcanic arcs (B) of Mexico. The study region (boxed) comprises the southernSierra Madre Occidental volcanic province (SMO VP) and extends through the physiographic provinces of the SMOc (Sierra Madre Occidental),Mesa central (MC), Sierra Madre Oriental (SMOr),and Mexican volcanic belt (MVB). Tertiary extension is based on Henry and Aranda-Gomez(1992) and this work. Extension south of the MVB is not shown. OligoceneMiocene volcanic rocks south of the MVB are diagonally ruled.
CHIH
SON
SINDGO
NAY
ZAC
SLP
GTO
JAL
A
105N110 N
30N
25N
20N
0 km 500
Tropic of Cancer
N
Figure 2. Boundaries of Mexican states cited in the text. SONSonora, SINSinaloa, CHIHChihuahua, NAYNayarit, DGODurango, ZACZacatecas, AAguascalientes, SLPSan Luis Po-tos, JALJalisco, GTOGuanajuato.
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SOUTHERN SIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE, MEXICO
Geological Society of America Bulletin, March 1999 34
Madre Oriental; a central province with a mean elevation of 2100 m corre-
sponding to the Mesa central; and a western province with a mean elevation
of 1800 m corresponding to the Sierra Madre Occidental. The difference in
uplift between the Mesa central and the Sierra Madre Occidental is evenmore pronounced if one looks at the geologic units exposed in the two
provinces. In the Mesa central the Mesozoic basement is commonly ex-
posed from 2000 to 2700 m (Fig. 3), whereas in the Sierra Madre Occiden-
tal rocks of the same age are not exposed, even at 800 m inside the major
depression of the area.
The crustal thickness of the physiographic provinces also shows notable
differences: based on a gravimetric profile along the Tropic of Cancer,
Campos-Enriquez et al. (1994) proposed crustal thickness of 37 km for the
Sierra Madre Oriental, 33 km for the Mesa central and 41 km for the Sierra
Madre Occidental. Molina-Garza and Urrutia-Fucugauchi (1993) esti-
mated a similar value for the Sierra Madre Occidental from interpretation
of Bouguer anomaly along a north-south profile at long 103W. Using
group velocity of surface waves, Fix (1975) estimated a crustal thickness
of 30 km for central Mexico, whereas Rivera and Ponce (1986) proposed a
value of 42 km for the Sierra Madre Occidental. The latter value was also
obtained by Meyer et al. (1958) using seismic refraction data. By model-
ing gravity and seismic refraction data, Couch et al. (1991) calculated a
crustal thickness ranging from 40 km at the center of the Sierra Madre Oc-
cidental to 25 km along the coast of the Gulf of California.
These data indicate that the thinner and topographically elevated Mesa
central is bounded by thicker and depressed crustal blocks of the Sierra
Madre Oriental and Sierra Madre Occidental (Fig. 3). The exposure of the
Mesozoic basement in the Mesa central indicates a larger uplift of this block
with respect to the adjacent ones. In the following sections we will show that
the Mesa central and the Sierra Madre Occidental also have important dif-
ferences in the age and style of volcanism and extension.
REGIONAL STRATIGRAPHY
In this section we present a generalized stratigraphy of the Mesa central
and Sierra Madre Occidental that integrates the results of previous works
with our reconnaissance investigations of Tertiary rocks in the region. This
synthesis is supported by 91 published ages (Table DR1, in GSA Data
Repository1) and nine new dates reported in Table 1.
Mesozoic Basement
The oldest rocks of the region are exposed in the Mesa central and belon
to two major paleogeographic domains: the Guerrero terrane island-arc assemblage (Centeno-Garcia et al., 1993) and the Sierra Madre Oriental ma
rine sedimentary sequence. The Guerrero terrane, exposed in the wester
part of the southern Mesa central, is a volcanic-sedimentary sequence mad
of flysch-like sediments interbedded with pillow basalt, which shows a low
metamorphic grade and severe contractile deformation. The succession is ex
posed in the Guanajuato and the Aguascalientes areas (Fig. 4).On the basi
of paleontologic evidence (Chiodi et al., 1988) and K-Ar ages (Monod et al
1990), the succession has been interpreted as Early Cretaceous in age. To th
north and east of the southern Mesa central, a Cretaceous marine carbonat
and clastic sequence (e.g., Carrillo-Bravo, 1971; Labarthe-Hernndez et al
1982) underwent thin-skinned contraction during the Laramide orogeny
In the western Sierra Madre Occidental the existence of a prevolcani
Mesozoic basement is substantiated by localized outcrops of argillite and
limestone exposed along the lower part of the Rio Grande de Santiago
These rocks occur in small and isolated outcrops on top of late Oligocene t
early Miocene intrusive stocks, suggesting that they were roof pendants up
lifted by the plutons. Extensive exposures of Mesozoic rocks are not known
in the Sierra Madre Occidental south of lat 2230N and west of the longi
tude of Zacatecas (Fig. 3).
Paleogene Continental Sedimentary Rocks
This group includes two continental postorogenic units: the Guanajuat
conglomerate (Edwards, 1955) exposed in Guanajuato, and the Cenicer
Formation in San Luis Potos (Labarthe-Hernndez et al., 1982). They con
sist of polymictic conglomerate and sandstone deposited in coalescing al
luvial fans after the Laramide orogeny. According to paleontologic determinations (Edwards 1955) and a K-Ar date from an interbedded andesit
(Aranda-Gmez and McDowell, 1998), the Guanajuato conglomerate i
Eocene in age. Palynologic determinations indicate a Paleocene-Eocen
age for the Cenicera Formation (Labarthe-Hernndez et al., 1982). Th
overlying Casita Blanca Andesite, dated as 44.1 2.2 Ma (Labarthe
Hernndez et al., 1982), gives a minimum age for the Cenicera Formation
Eocene Volcanic Group
Eocene volcanic rocks are widespread in the Sierra Madre Occidental vol
canic province (Aguirre-Daz and McDowell, 1991, and references therein
TABLE 1. K/AR AGES FOR THE SOUTHERN SIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE
Sample Site Rock type Material Long Lat K 40Ar* 40Ar* Age 1dated (W) (N) (wt%) (mol/gr) (%) (Ma) (Ma)
SLP 7* Cerro El Bey, SLP Andesite Hb 100.68 22.44 0.53 3.50E-11 52.0 37.6 1.9GL 409 N of Comanjilla, Gto. Andesite WR 101.45 21.08 2.23 1.43E-10 64.3 32.9 1.6
MCU-1
Cerro La Montaa, Gto. Ignimbrite WR 101.28 21.18 4.63 2.73E-10 93.1 29.3 1.5MP-B1 Campuzano, Gto. Ignimbrite WR 101.14 20.84 6.43 3.02E-10 98.2 28.6 1.4GL 406 E of Calvillo, Gto. Ignimbrite WR 101.04 21.10 4.56 2.39E-10 95.8 26.9 1.3CTO-02 Sierra Nochistln Ignimbrite San 102.84 21.59 7.89 3.67E-10 76.2 26.6 0.7GL 137 Cerro Cano, Gto. Basalt WR 101.07 21.09 1.59 8.05E-11 67.7 25.9 1.3GL 241 E of Calvillo, Gto. Ignimbrite WR 101.08 21.12 3.71 1.83E-10 96.8 25.2 1.3SLP 19* La Salitrera, SLP Bas.- Andesite Pl 100.52 22.93 0.57 1.31E-11 35.0 13.2 0.6
Notes:Hbhornblende, Sansanidine, Plplagioclase, gmsgroundmass, WRwhole rock.*Analyses performed at Instituto Mexicano del Petroleo, Mexico City, in 1988.Analyses performed at Teledyne Isotopes, in 1995.Analyses performed at Geochron Laboratories, Cambridge, Massachusetts, in 1994.
1GSA Data Repository item 9919, data table, is available on request fromDocuments Secretary, GSA, P.O. Box 9140, Boulder, CO 80301. E-mail:[email protected].
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350 Geological Society of America Bulletin, March 1999
22N
21N
22N
21N
104W 102W 100W
100W102W104W
100 km
Figure 3. (A) Generalized topographic map of the southern Sierra Madre Occidental volcanic province. Thick lines bound physiographicprovinces and correspond to major tectonic structures discussed in the text. (I) Sierra Madre Occidental, (II) Mesa central, and (III) Sierra MadreOriental physiographic provinces. Stars represent extensive outcrops of the Mesozoic basement. Thick dashed line indicates the trace of topo-graphic profile AA. SMASan Miguel de Allende; NVMNuevo Valle de Moreno. (B) Topographic profile based on 1:1000000 scale topo-graphic map filtered for the high frequencies. The mean elevation of the three physiographic provinces has been calculated without consideringthe coastal plain. (C) Interpretation of the crustal structure along the profile based on surface geology and geophysical modeling (see text for ref-erences). Structure of middle and lower crust is not to scale and is based on geologic considerations discussed in the text.
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SOUTHERN SIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE, MEXICO
Geological Society of America Bulletin, March 1999 35
although they are commonly obscured by the extensive Oligocene rhyoliticcover. In the southern Mesa central, Eocene rhyolitic ignimbrites and domes
and andesite lava flows crop out locally in Zacatecas (Ponce and Clark, 1988;
Lang et al., 1988),Guanajuato (Bufa Rhyolite, Gross,1975),San Luis Potos
(Labarthe-Hernndez et al., 1982), and Aguascalientes (Nieto-Samaniego
et al., 1996). A sequence of andesitic flows on the Mesozoic basement 80 km
northwest of San Luis Potos yields a K-Ar age of 37.6 1.9 Ma (Table 1).
In the Sierra Madre Occidental the oldest volcanic rocks are highly al-
tered and pervasively fractured microcrystalline andesitic lavas, which are
exposed north of Guadalajara and yield a middle Eocene age (Webber et al.,
1994; Ferrari et al., 1999). Their base is not exposed and the top is eroded
and locally covered unconformably by red sandstone and conglomerate
containing andesitic fragments. The minimum thickness of this succession
is 400 m. Based on the geographic distribution of the outcrops mentioned,
the Eocene volcanic rocks, although scattered and of lesser volume, were
probably more extensive than the Oligocene-Miocene rocks.
Oligocene Volcanic Group
Rhyolite domes and lava flows with minor intercalated pyroclastic de-
posits are volumetrically the most important part of the Tertiary volcanism
in the Mesa central. These rocks are exposed all over the southern Mesa
central, often forming the core of major ranges (Fig. 4). Domes are typically
about 2 km in size. They are associated with ore deposits of tin, and with
rhyolite lava that commonly contains topaz. The age is well constrained to
be Oligocene by five K-Ar dates in San Luis Potos and Guanajuato (30.8
0.8 to 29.2 0.8 Ma, Table DR1, see footnote 1). This group is absent west
of Aguascalientes in the Sierra Madre Occidental, as well as to the east inthe Sierra Madre Oriental (Fig. 4).
Oligocene rhyolitic ignimbrites with subordinate andesitic lava flows are
exposed in the southern Mesa central and the southeastern Sierra Madre Oc-
cidental (Fig. 4). The aggregate thickness of these rocks is usually less than
200 m. No clear caldera structures have been genetically or spatially associ-
ated with these ignimbrites. In San Luis Potos and northern Guanajuato
(Fig. 2) the ignimbrites have K-Ar ages ranging from 29.0 to 26.8 Ma (Table
DR1, see footnote 1). This time span is confirmed by three new whole-rock
K-Ar dates ranging between 29.3 and 26.9 Ma from the Sierra de Guanajuato
(Table 1). A second succession of ignimbritic sheets with latest Oligocene
ages (Table DR1, see footnote 1) is exposed in the southernmost part of the
Mesa central. The northernmost exposed ignimbrite and one interbeddebasaltic flow of this succession give ages of 25.2 1.3 Ma and 25.9 1.3 Ma
respectively (Table 1).
In the Sierra Madre Occidental, Oligocene ignimbrites are only reporte
in the Santa Maria del Oro and Juchipila areas (Fig. 5) (Table DR1, see foot
note 1). In addition we obtained a K-Ar age of 26.6 0.7 Ma (Table 1) from
one of the uppermost ignimbrites capping the Sierra de Nochistln (Fig. 5)
These old ignimbrites are commonly interbedded with and capped by vol
caniclastic sequences made of clay, sandstone, and conglomerate, and ca
be interpreted as the westernmost extension of the Mesa central Oligocen
volcanic sequence.
Miocene Volcanic Group
The Miocene volcanic group consists of three main components: earl
Miocene silicic volcanic rocks, early Miocene basaltic andesites, and lat
Miocene basalts. A significant part of the southern Sierra Madre Occidenta
is covered by rhyolitic ignimbrites and minor domes emplaced in a short tim
span (24 to 21 Ma, Table DR1, see footnote 1). More than 1200 m of ash
flow tuffs with ages between 23.7 and 20.1 Ma are exposed in the Bolao
graben, and the upper part of this succession can be followed up to about 10
km southward (Scheubel et al., 1988; Lyons, 1988; Moore et al., 1994). Ig
nimbrites with an aggregate thickness of about 1000 m are exposed in th
Sierra de Santa Lucia, in the core of the Sierra Madre Occidental (Fig. 5)
The uppermost tuffs of this succession have been dated at 23.5 Ma (Clark
et al., 1981), whereas the base yielded an age of about 25 Ma (Ferrari et al
1997a). These silicic ignimbrites resemble the 1000-m-thick and ca. 23-M
El SaltoEspinazo del Diablo sequence, exposed ~100 km to the north isouthwestern Durango (McDowell and Keizer, 1977). Early Miocene as
flows are not found in the Mesa central, which suggests a westward young
ing of the ignimbrite flare up.Ash flows and rhyolites with similar ages ar
exposed in southernmost Baja California (Hausback,1984) and could repre
sent distal facies of the early Miocene Sierra Madre Occidental arc.
North of Guadalajara the Sierra Madre Occidental succession is dissecte
by north- to north-northeasttrending grabens (Fig. 5). Basaltic andesite
are found as lavas at the bases of these grabens and as dikes intruded paral
lel to the normal faults. The age of these rocks is 2119 Ma in the Bolao
graben (Nieto-Obregn et al., 1981) and 21.8 Ma in the Tlaltenango grabe
(Moore et al., 1994).
TABLE 2. COMPUTED PALEOSTRESS TENSORS FORTHE SIERRA MADRE OCCIDENTAL
Eastern Sierra Madre Occidental Western Sierra Madre Occidental
(Calvillo, Juchipila, Tlaltenengo and (Aguamilpa Dam, Jess Maria, La
Bolaos graben) Cofrada, Paso de Lozada,
Presa Baadero, and Sierra Pajaritos)Trend/Plunge Magnitudes relative to Trend/Plunge Magnitudes relative to
1 1
1
225/75 100 222/81 1002 007/11 44 333/03 323 099/08 39 066/07 27
coefficient of friction = = 0.4; n = 61 coefficient of friction = = 0.6;
n = 58Note:The method of Reches (1987) assumes that slip obeys the Coulomb criterion.
Coefficient of friction, cohesion, the misfit angle between observed and calculated slipaxis (V), and the mean misfit angle between general and ideal principal stresses axes ()are used as criteria to separate the fault groups.These analyses simultaneously satisfy thefollowing constraints:0.6 > > 0.4; cohesion near zero; < 25;. < 40.
= = =
2 3
1 3
0 07
.stress ratiostress ratio = = =
2 3
1 3
0 08
.
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NIETO-SAMANIEGO ET AL.
352 Geological Society of America Bulletin, March 1999
The southernmost Mesa central is covered by an extensive basaltic
plateau and by andesitic stratovolcanoes (Ferrari et al., 1994a). Stratigraphic
evidence indicates that the stratovolcanoes are older than the basaltic
plateau extending to the south (Valdez-Moreno and Aguirre-Diaz, 1997;
Prez-Venzor et al., 1997). Ages range from about12 Ma for one of the stra-
tovolcanoes (Prez-Venzor et al., 1997) to 10.58.1 Ma for the basaltic
plateau (Table DR1, see footnote 1).
Basaltic to andesitic lava flows also cap the silicic succession in the Mesa
central to the north. The only age for these rocks is 13.2 0.6 Ma (Table 1)
from one flow of an extensive basaltic-andesitic succession overlying the
Oligocene ignimbrites about 80 km east-southeast of San Luis Potos. This
age is considered representative of the postignimbrite mafic volcanism of the
region, because it coincides with the age of other basalts in the southernmost
Mesa central (Table DR1, see footnote 1), and with the Metates (McDowell
and Keizer, 1977) and Los Encinos basalts (Luhr et al., 1995), which are ex-
posed in eastern Durango and northern San Luis Potos, respectively.
Along the southern limit of the Sierra Madre Occidental, the early Miocene
ignimbrites are covered by 10.28.5 Ma (Table DR1, see footnote 1) basaltic
lava flows, well exposed along the Rio Santiago canyon north of Guadalajara
(Fig. 5), where they have an aggregate thickness exceeding 600 m. This suc-
cession also extends east of Guadalajara for more than 130 km in the Los Al-
tos de Jalisco (Fig. 5), with an average thickness of 200 m and ages of 11 to 8
Figure 4. Geologic map of the Mesa central showing the main stratigraphic units discussed in the text. The figure is based on geologic sheets atscales 1:100 000, 1:50 000, and 1:20 000 published by the Geologic Institute of the San Luis Potos University between 1977 and 1995 (see Nieto-Samaniego et al., 1996; Labarthe-Hernndez et al., 1982, for the complete list of references) and Nieto-Samaniego (unpublished data). PHPaloHurfano Volcano; SCSierra de Codornices; SGSierra de Guanajuato; NVMNuevo Valle de Moreno; SSMSierra de San Miguelito. Theregion shown in Figure 6 is outlined.
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Geological Society of America Bulletin, March 1999 35
Pacific
Ocean
Jalisco
block
Aguamilpa
Acaponeta
Tepic
Pochotitn
Sta.
Maria
delOro
Sta.Rosa
Dam
SanCristobal
Guadalajara
Escuinapa
M
esa
delNayarJe
ss
Maria
SanPedro
ElZopilote
PuntaMita
Pte.de
Camotln
Bolaos
Sierrad
eMoro
nes
Sierra
deNo
chist
ln
Tlaltenango Ja
lpa
Calvillo
Rio
Santiago
Rio
San
Pe
dro
RioAte
ngo
SierraSta.Lucia
Juch
ipila
Zacatecas
Rio
Santia
go
Rio
Verde
22N
21N
Aguascalientes
0
50
100km
Celaya
Quer
taro
Len
Atoton
ilco
Penjam
illo
grab
enG
uanajuato
San
Miguel
de
Allen
de
Do
lores
Hidalgo
San
Lu
is
de
laPaz
San
Lu
is
Po
tos
Pinos
Sal
inas
de
Hidalgo
Ojuelo
s
S.S.
B.
S.C.
Lagos
deMoreno
102W
104W
106W
II
Ia
Ib
III
VilladeR
eyesgrab
en
ElBajogr
aben
Altos de J al is c o
S.S.M.
CB1
SanP
edro-C
eboruc
ograb
en
PR9
Pla
nde
Barra
ncas-Santa
Rosagrab
en
SierraAlica
S.Pajaritos
Figure5.Structuralmapandge
neralizedstratigraphyofthesouthernSierraMadreOc-
cidentalvolcanicprovince,basedo
nsourcescitedinthetextfortheMesacentral(MC)and
oninterpretationofsatelliteimage,digitalterrainmodels,aerialphotogr
aphsandrecon-
naissancefieldwork(FerrariandR
osas-Elguera,1998,personalcommun.).T
hickgraylines
areboundariesofstructuraldomainsdescribedinthetext.I
IISierraMad
reOriental,II
Mesacentral,Iaea
sternSierraMadreOccidental,Ibwes
ternSierraMadreOccidental.
S.C.
SierradelCubo;S.S.M.
SierradeSanMiguelito;S.S
.B.
SierradeSantaBarbara;
PR9deepgeothermalwellofLaPrimaveracaldera;CB1deepgeothermalwellofCe-
borucovolcano.Stra
tigraphiccolumnsnottoscale.
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NIETO-SAMANIEGO ET AL.
354 Geological Society of America Bulletin, March 1999
Ma (Ferrari et al., 1994a, 1996); these ages allow them to be correlated with
the late Miocene basalts and andesites described from the Mesa central.
Volcanic Rocks Related to the Gulf of California
In the southwestern part of the Sierra Madre Occidental, north of Santa
Maria del Oro, eroded andesitic volcanoes commonly cover the early
Miocene ash flows. One of these centers has been dated as 14.7 Ma (Table
DR1, see footnote 1). Gastil et al. (1979) also reported a succession of an-
desitic and basaltic flows with subordinate rhyolites with ages between 20.4
and 13.8 Ma south of Tepic (Table DR1, see footnote 1). These rocks sug-
gest the existence of a middle Miocene intermediate-composition volcanic
arc in the western part of the Sierra Madre Occidental. This volcanism rep-
resents the southern occurrence of the early to middle Miocene andesitic
arc, recognized by Gastil et al. (1979) and Fensby and Gastil (1991) around
the Gulf of California, that was subsequently separated along its axis by the
rifting process.
A plateau of alkali basalts with ages of 11 to 8.9 Ma (Table DR1, see
footnote 1) is exposed between the Sierra Madre Occidental and the Pacific
coast north of Tepic (Fig. 5). These rocks unconformably overlie upon tilted
ignimbrites of the Sierra Madre Occidental. Several mafic dikes with ages
of 1211 Ma (Damon et al., 1979; Clark et al., 1981; Table DR1, see foot-note 1) and with north-south to northwest trends are found in the western-
most part of the Sierra Madre Occidental. This mafic, commonly alkalic
volcanism marks the initiation of the rifting process at the mouth of the Gulf
of California.
Quaternary Volcanic Rocks
The youngest volcanic event consists of scattered cinder cones, maars,
and lava flows emplaced in the northern part of the Mesa central. These vol-
canic rocks include nephelinite, basanite, alkaline basalt, and basalt. Ac-
cording to 8 K-Ar age determinations, they are early to middle Pleistocene
in age (Aranda-Gomez and Luhr, 1996).
Continental Sedimentary Deposits
Poorly to mildly consolidated alluvial and lacustrine deposits fill numer-
ous basins cut in the Tertiary succession. They consist of conglomerate,
gravel, sand, sandstone, and subordinate marl, limestone, and chert. These
deposits probably span the entire late Tertiary time and possibly Quaternary
time, as indicated by intercalation of Miocene ignimbrites and basalts and
by Pleistocene vertebrate fossils discovered in the upper part of the se-
quence near San Miguel de Allende (Carranza-Castaeda et al., 1994) and
Aguascalientes (Reynoso-Rosales and Montellano-Ballesteros, 1994). The
Juchipila graben is floored by a stratified volcanic-sedimentary succession
consisting of 10 to 20 m of a coarse volcanic conglomerate, followed by as
much as 250 m of lacustrine clays, silts, and limestone. Alluvial deposits
(gravel and sandstone) of unknown age also fill the southernmost part of theTlaltenango graben.
Summary of the Volcanic Evolution
The variation in the stratigraphic record across the southern parts of the
Mesa central and Sierra Madre Occidental is summarized in the stratigraphic
columns of Figure 5. Eocene, mostly andesitic, volcanism appears to have
occurred across the entire Mesa central and in the eastern part of the Sierra
Madre Occidental. In early Miocene time voluminous rhyolitic ignimbrites
were emplaced in the Sierra Madre Occidental while erosion and continen-
tal sedimentation were underway in the Mesa central. In late Miocene time
basaltic to andesitic volcanism related to the Mexican volcanic belt and to the
opening of the Gulf of California occurred at the southern boundaries of the
Mesa central and the Sierra Madre Occidental. A few basalts and andesites
were also emplaced within the Mesa central but are not known in the Sierra
Madre Occidental. In Quaternary time,minor alkaline basalts were emplaced
in the Mesa central only.
The most striking difference between the Mesa central and the Sierra
Madre Occidental concern the age and the style of the silicic volcanism: in
the former, silicic volcanism consists largely of rhyolitic domes and oc-
curred mostly in Oligocene time; in the latter, it was essentially represented
by caldera-related ignimbrites of early Miocene age. We note a westward
(or west-southwestward) shift of the focus of silicic volcanism in Oligocene
to Miocene time, similar to that observed in the northern and central Sierra
Madre Occidental (Gans, 1997; Aranda-Gomez et al., 1997).
MIDDLE TO LATE TERTIARY FAULTING IN THE SOUTHERNSIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE
Geometry and Kinematics
The southern volcanic province is bounded to the south and also divided
longitudinally by major crustal structures (Figs. 3 and 5). Although the ge-omorphic expression of these structures is locally modest, their tectonic sig-
nificance is given by the fact that they bound different types of basement
and/or control the style of Cenozoic extension and volcanism. Faulting also
affected the interior of these blocks at different times. In this section we de-
scribe the geometry, kinematics, and age of the boundary faults and of the
other faulting inside the Mesa central and Sierra Madre Occidental blocks.
Boundary Fault Systems. The boundaries between the Mesa central andSierra Madre Occidental and Oriental are major north-south fault systems,
along which Cenozoic normal movement formed half grabens. The bound-
ary between Sierra Madre Oriental and the Mesa central is a complex system
of north-northwest to north-trending and west-dipping normal faults, here
named the San Miguel de AllendeCatorce fault system (Fig. 5). The fault
system separates the intensely and complexly faulted Mesa central to the
west from the Sierra Madre Oriental, which shows no Cenozoic extension.
The southern part of this structure is the TaxcoSan Miguel de Allende fault,
first identified by Demant (1978) as a major tectonic boundary. Minimum
offsets of the fault system in the San Miguel de Allende area is 450 m (Nieto-
Samaniego and Alaniz-Alvarez, 1994). North of San Miguel de Allende the
fault system cuts Quaternary sediments, forming a 50-m-high scarp. The
fault trace is interrupted within the Sierra del Cubo and appears again as the
northern extension of the Villa de Reyes graben (Fig. 5). Because there are
no western bounding faults to form a complete graben, we deduce that the
fault system is a half graben, the master fault dipping to the west.
We chose the Aguascalientes fault as the western boundary of the Mesa
central because it is a large-displacement fault that marks the western limit
of Mesozoic basement and separates regions of different structural style. The
north-striking, east-dipping fault has a 250-m-high scarp into a basin filledwith at least 500 m of sedimentary rocks and intercalated ignimbrites. Water
wells indicate a total displacement of 1000 m (Jimnez-Nava, 1993). Sub-
sidiary high-angle, normal faults parallel the main scarp. A west-dipping, an-
tithetic fault that creates a full graben is found only near Aguascalientes, so
the basin is mostly a half graben with an east-dipping master fault (Fig. 5).
The southern edge of the Mesa central is the dominantly normal Bajo fault
(Aranda-Gomez et al., 1989). The eastern half of the Bajo fault trends east
and, according to subsurface data,shows an offset about 700 m west of Celaya
(Ramos-Salinas, 1996). The western half strikes N50W and bounds Meso-
zoic basement and Paleogene plutons (Fig. 4). Using displacements reported
by Quintero-Legorreta (1992) and Aranda-Gmez et al. (1989) and adding an
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SOUTHERN SIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE, MEXICO
Geological Society of America Bulletin, March 1999 35
average 500 m of basin-fill deposits estimated from Hernndez-Laloth (1991),
the Bajo fault reaches a vertical offset of 1350 m near Len and 1100 m near
Guanajuato. However, if we consider the vertical offset of the parallel Veta
Madre fault in the city of Guanajuato (Gross, 1975), the cumulative offset can
be as much as 2850 m. The Bajo fault vanishes west of Len, where it inter-
sects the north-trending Penjamillo graben (Fig. 5) (Ferrari et al., 1994b). Be-
tween the Penjamillo graben and Guadalajara, the southern boundary of the
Mesa central and of the Sierra Madre Occidental is poorly defined because
most of the area is covered by late Tertiary continental sediments and by ex-
tensive late Miocene basalts. These rocks are cut by two approximately east-
west normal fault systems with 200400 m of vertical offset (Fig. 5). South of
the Los Altos de Jalisco plateau is the east-westtrending Chapala graben
(Campos-Enriquez et al., 1990; Urrutia-Fucugauchi and Rosas-Elguera,
1994). None of these structures has the large vertical displacement shown by
the Bajo fault, and this suggests that the Penjamillo-Guadalajara region is a
transitional zone with a diffuse deformation.
West of Guadalajara the Sierra Madre Occidental is separated from the
Jalisco block by two N55W trending en echelon grabens (Ferrari and
Rosas-Elguera, 1999) (Fig. 5). Bounding faults on both grabens display
mainly dip-slip motions. However, older right-lateral transtensional motions
are observed at some places (Ferrari et al., 1997b). In addition a middle
Miocene left-lateral transpression was documented in the southernmost partof the Sierra Madre Occidental and some of this deformation was probably
accommodated by strike-slip movement along major faults of the graben
(Ferrari, 1995).
Mesa central. Within the Mesa central there are three major fault sys-tems. The N30E trending Villa de Reyes graben cuts the eastern part of the
Mesa central. This is a symmetric graben but it is segmented by secondary
orthogonal graben and faults (Fig. 5). In the city of San Luis Potos the sed-
imentary deposits that fill the basin are 480 m thick and contain interbedded
Oligocene ignimbrite sheets. A vertical offset of ~680 m was calculated us-
ing subsurface stratigraphic sections based on water exploration wells and
geologic mapping (Tristn-Gonzlez, 1986; Martnez-Ruiz and Cuellar-
Gonzlez, 1978).
The San Luis de la PazSalinas de Hidalgo fault system is a well-defined
group of parallel faults that strike N50W and dip southwest (Fig. 5). These
faults have mainly normal movements (Labarthe-Hernandez and Jimnez-
Lpez, 1993; Silva-Romo, 1996) with limited offset, and there are no basins
associated with these faults.
The third major fault system forms the northwest-trending
Dolores HidalgoOjuelos depression and cuts the Villa de Reyes graben
near San Felipe (Figs. 4 and 5). The basin is poorly defined and only few
bounding faults were mapped. Other minor faults in the southern Mesa
central have dominantly normal displacement.
Sierra Madre Occidental Physiographic Province. The structural styleof this province is dramatically different from that in the Mesa central. In-
stead of two sets of orthogonal faults, several major parallel faults are found
(Fig. 5). In the eastern part of the province three nearly symmetric grabens
strike north-northeast to north-south. In the western part of the province,north-northwesttrending half grabens and major listric faulting are domi-
nant (Fig. 5).
The easternmost structure in the province is the Juchipila graben and its
eastward splay, the Calvillo graben. The Juchipila graben is a 80-km-long
and 15-km-wide depression with a N10E strike and a relief of as much as
1000 m. The Calvillo graben, is a narrow, N15E trending depression, with
a maximum relief of 900 m (Fig. 5). Its total length is 40 km and its average
width is 12 km. West of Juchipila, the Tlaltenango graben trends N10E and
has a length of 120 km and a width of 15 km, forming a depression of 400
m. The largest structure is the north-south Bolaos graben, which is 140 km
long and 1520 km wide. Here offsets of stratigraphic units commonly
range from 1500 to 2000 m (Lyons, 1988). The graben is slightly asymmet
ric; the maximum relief is 1200 m on the western scarp (Scheubel et al.
1988). The three grabens are bounded by high-angle faults with dominan
dip-slip displacements. Inside the grabens, volcanic or sedimentary bed
show tilts of 10 to 20 either to the east and the west.
The region between the Bolaos graben and the coastal plane is affected
by four north-south to north-northwesttrending major structures: the
Puente de Camotln, the Sierra de Pajaritos-Atengo, and the Sierra Al
icaJesus Maria half grabens and the PochotitnSan Pedro fault system
These structures systematically dip to the west and tilt the hanging blocks
as much as 35 to the east and east-northeast. The major structure is the
Sierra AlicaJesus Maria half graben, which has a maximum vertical offse
exceeding 2.2 km. The region west of this structure is interpreted as a serie
of parallel listric faults, whose style, age, and orientation strongly indicate a
genetic relation with the opening of the Gulf of California (Ferrari et al.
1997b; Ferrari and Rosas-Elguera,1999). These structures are limited to the
south by a left-lateral normal accommodation zone that separates extension
to the north from transpressional folding at the boundary with the Jalisco
block (Fig. 5) (Ferrari, 1995).
Age of Faulting
Southern Mesa central. The age of the main depressions in the Mescentral is constrained by the volcanic units interbedded in the sedimentary
filling. The filling of the San Miguel de Allende basin contains a ca. 11 Ma
basalt the counterpart of which was identified in the range. However, the
basalt shows less than 20% of the total displacement of the fault (Nieto
Samaniego and Alaniz-Alvarez, 1994). Both the Aguascalientes and the E
Bajo basins contain late Miocene basalts and undated ignimbrites. In the
Villa de Reyes graben, the Panalillo rhyolite (Fig. 6), with age of 26.8 Ma
was identified within the basin fill. According to these data, we consider tha
major faulting in the southern Mesa central began in late Oligocene time
The oldest Cenozoic unit exposed in Sierra de Guanajuato, the Eocen
Guanajuato conglomerate, presents dramatic changes in thickness, varying
from 1500 m (Gross, 1975) to less than 100 m in adjacent areas. These varia
tions, together with the presence of dikes, the intercalation of lava flows,and
the tilting of sandstone horizons, have been interpreted as an evidence of the
inception of extension (Nieto-Samaniego, 1990; Randall-Roberts et al.
1994). Andesitic dikes (Echegoyn-Snchez et al., 1970), rhyolitic dome
aligned along fractures, and rhyolitic lava flows that cover normal fault
record extension prior to 32 1 Ma (Gross, 1975; Nieto-Samaniego, 1990)
The tectonic episode with the higher deformation rate occurred in middle
Oligocene. The Veta Madre fault offsets 30 Ma rhyolite domes about 1500 m
(Nieto-Samaniego et al., 1996) and is filled by vein minerals dated as 29.2
2 Ma (Gross, 1975). Post-Miocene extensional phases with lower intensity ar
recorded by displacements of 200 to 600 m of an ignimbrite dated as 24.8
0.6 Ma (Nieto-Samaniego et al., 1996) and basaltic lavas dated as 13 to 11 M
(Table DR1, see footnote 1).
In the San Luis Potos region (Fig. 6) the first clear evidence of extension ithe emplacement of the San Miguelito rhyolitic domes and dikes along faults
which implies an extensional phase ca. 30 Ma. The main phase of extension
is documented by the displacement of ~500 m of the Cantera ignimbrite (29.0
1.5 Ma) (Tristn Gonzlez 1986) and the emplacement of the El Zapote rhy
olite (27.0 0.7 Ma) and Panalillo rhyolite (26.8 1.3 Ma) through the faults
cutting the Cantera ignimbrite. Minor offsets of the Panalillo rhyolite
(Labarthe-Hernndez and Jimenez-Lpez 1994) record a minor phase of ex-
tension in post-Oligocene time.
The outlined stratigraphic relationships indicate that extension could
have begun in Eocene time, at least in Guanajuato and San Luis Potos, and
that three subsequent phases of extension can be identified: (1) a mild ex
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NIETO-SAMANIEGO ET AL.
356 Geological Society of America Bulletin, March 1999
tensional phase before 30 Ma; (2) a well constrained phase constituting the
peak of extension between 30 and 27 Ma; and (3) minor phases after about
24 and after about 11 Ma.
Sierra Madre Occidental Physiographic Province.The age of deforma-tion in this province is less well-constrained than in the Mesa central but can
be reasonably estimated in some places. In the Bolaos graben an important
mineralization event took place prior to 21 Ma and fills N30E trending faults
that cut a 23.7 Ma ignimbritic succession with offsets of 100200 m (Lyons,
1988). The peak of deformation is associated with the north-south bounding
faults of the graben,which displace a 21.3 Ma ignimbrite by more than 1 km
and expose the mineralization (Lyons, 1988; Scheubel et al., 1988). Several
mafic dikes intrude the N30E faults, in some cases feeding basaltic flows that
Caracol Formation
Cenicera Formation
Casita Blanca Andesite
Santa Mara Ignimbrite
Ojo Caliente Traquite
Portezuelo Latite
San Miguelito Rhyolite
Cantera Ignimbrite
El Zapote Rhyolite
Panalillo Rhyolite(lower member)
Andesite
Late Miocenebasalt
Panalillo Rhyolite(upper member)
Continental sedimentary deposits(Oligocene-Holocene)
SAN LUIS POTOS
101
101
10050'
10050'
2200'
A
A'
2145'2145'
Rhyolitedomes
Eocene volcanicgroup
Oligocenevolcanicgroup
Ignimbritesanda
ndesites
Cabras Basalt
0 10 km
PaleogenecontinentalsedimentsMesozoicbasement
Vill
ade R
eye
sg
rab
en
SLSH
A A'
1900
2150
2450
1 km
m
a.s.l.
Figure 6. Geologic map of Sierra de San Miguelito. Geologic mapping is modified after Tristn-Gonzlez (1986), and faults are simplified after
Labarthe-Hernndez and Jimnez-Lpez (1992, 1993, 1994). Formal units defined by Labarthe-Hernndez et al. (1982) are in italics. Faults in-side the boxed area are reproduced in the lower right corner to show the rhombohedral array at the intersection between Villa de Reyes grabenand San Luis de la PazSalinas de Hidalgo fault system (SLSH). Section is after Labarthe-Hernndez and Jimnez-Lpez (1994); the marker isthe lower member of the Cantera Ignimbrite.
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SOUTHERN SIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE, MEXICO
Geological Society of America Bulletin, March 1999 35
have been dated as 19.9 Ma at Veta Rica (Table DR1, see footnote 1). Field
evidence suggests that the dikes are synextensional and thus their age proba-
bly represents the peak of extension. The minimum age of faulting is not con-
strained by any geologic unit. An early Miocene age is deduced for the Tlal-
tenango graben because the bounding faults cut an ignimbritic succession of
about 23 Ma but do not affect a 21.8 Ma shield volcano inside the depression
(Moore et al., 1994). The uppermost ignimbrite cut by the Sierra AlicaJesus
Maria half graben,dated as 23.5 Ma (Clark et al.,1981),provides a maximum
age for this structure, but no ages are available for the ending of faulting.
In the westernmost part of the province, the PochotitnSan Pedro fault
system cuts ignimbrites as young as 19 Ma (Table DR1, see footnote 1), and
tilted hanging-wall blocks are covered by late Miocene basalts north of
Tepic. Several mafic dikes with ages between 11.9 and 10.9 Ma (Table DR1,
see footnote 1) are intruded parallel with the fault system and are inferred to
mark the major phase of extension (Ferrari and Rosas-Elguera, 1999).
In summary, these stratigraphic relations indicate two main phases of de-
formation: the first between ca. 21 and 18 Ma in the eastern part of the
province and the second during middle to late Miocene in the western part
of the province, adjacent to the Gulf of California.
Amount of Extension
Southern Mesa central. The Mesa central represents a good example oftriaxial deformation style. Strong evidence of triaxial strain in the southern
Mesa central includes the following: (1) the rhombohedral arrangement of
fault traces both at large scale (Fig. 6), and in the intersection of the Villa de
Reyes graben with the San Luis de la PazSalinas de Hidalgo fault system
(Fig. 6); (2) both fault systems show dip-slip displacement; (3) there is no
systematic crosscutting relation between the faults, indicating that the ac-
tivity was synchronous (Nieto-Samaniego, 1990; Labarthe-Hernndez and
Jimnez-Lpez, 1992, 1994).
The orientations of the principal strains were obtained,using the odd-axis
model proposed by Krantz (1988), in the Sierra de San Miguelito area, lo
cated southwest of San Luis Potos (Fig. 6). Using these results we calcu
lated the direction and the amount of extension for the entire Mesa centra
(see Appendix 1 for a complete description). Principal extension was ori
ented 258/12 with a magnitude of 20%. An extension of 11% also oc
curred oriented 162/02, showing the three-dimensional character of th
deformation.
Sierra Madre Occidental Physiographic Province. In this provincit is clear that deformation was two dimensional and that it affected an al
most horizontal and undeformed ignimbritic plateau. Thus we consid
ered it adequate to use fault-slip inversion methods to calculate the paleo
stress tensor. In the eastern part of the province we measured a total o
137 faults, 108 of which have good kinematic indicators. Fault trend
show a peak near N10E (Fig. 7), which is also the trend of the faults a
larger scale (Fig. 5). There is a minor concentration with an east-west di
rection. We interpret these faults as secondary structures that accommo
date local strain incompatibilities because they are nearly vertical an
show oblique- or strike-slip displacements. Furthermore, there are n
major faults belonging to this group, and the grabens are not interrupte
by major transverse depressions. The minimum principal stress (3) ob
tained with the inversion method proposed by Reches (1987) is oriented099/08, nearly horizontal and orthogonal to the trend of the graben
(Table 2). In the western part of the province we measured a total of 54
faults with good kinematic indicators. Using the same method we obtai
a N66E trending 3 (Table 2), which is orthogonal to the dominant fautrend (Fig. 5).
To estimate the amount of extension we use the area-balance method de
scribed by Groshong (1994) (Fig. 8). Although there is no good strati
graphic control, the method may work because there is a well-defined re
gional reference level not disturbed by major faulting within the horst
(Figs. 5 and 8). Some minor fault zones between the major grabens are neg
ligible because they do not tilt the sequence and produced displacement
Figure 7. Schmidt projection, lower hemisphere of faults measured in the southern Sierra Madre Occidental volcanic province. Contourinwas performed with the Kamb method.
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NIETO-SAMANIEGO ET AL.
358 Geological Society of America Bulletin, March 1999
less than 100 m. We carried out many experiments varying the depth of de-
tachment and obtained 5 km as a realistic approximation. This corresponds
to a horizontal extension of 8% (Fig. 8).
Extension in the western part of the province is obviously greater than to
the east because the crust has been thinned into the oceanic rift of the Gulf
of California. We have not estimated the amount of extension in this region
because a reference level is lacking.
DISCUSSION AND CONCLUSIONS
Tectonic-Magmatic Evolution of the SouthernSierra Madre Occidental volcanic province
On the basis of stratigraphy, structure, topography, and crustal thickness,
we propose that the southern volcanic province contains three major crustal
blocks that behaved as roughly independent units. These blocks correspond
to the physiographic provinces of the Sierra Madre Oriental, Mesa central
and Sierra Madre Occidental. The Sierra Madre Oriental is a relatively sta-
ble block, without volcanism and extension during the Cenozoic. The Mesa
central is a thinned and elevated block bounded by the thicker and relatively
lower Sierra Madre Oriental and Occidental blocks (Fig. 3). Within these
blocks volcanism and deformation show marked differences in style and tim-
ing. Volcanism in the Mesa central occurred between about 30 and 25 Ma,
and is mainly represented by rhyolitic lava domes capped by a thin ig-
nimbritic cover. Deformation was three dimensional, with horizontal, ap-proximately east-west extension of as much as ~20%. Extension was in part
concurrent with volcanism, but the peak of the deformation postdates the
principal volcanic event.
Volcanism and deformation occurred later toward the west. By early
Miocene time a thick ignimbritic cover was emplaced in the eastern Sierra
Madre Occidental, with rare rhyolitic lava flows and domes and subordi-
nate mafic lava. Deformation was two dimensional, and the principal
stretching of ~8% was oriented approximately east-west. Extension was
partly synchronous with volcanism, but its climax postdates the ig-
nimbrite flare up. During middle to late Miocene time,volcanism and ex-
tension continued to shift toward the west-southwest. Arc volcanism of this
age is found in the western part of the Sierra Madre Occidental, and exten-
sion appears to slightly postdate it. Extension direction changed orientation
counterclockwise by ~30, from NW81SE to NE66SW, culminating
with the opening of the Gulf of California, which now bounds the western
edge of Sierra Madre Occidental. The trenchward pattern of tectonic-
magmatic migration is clearly shown in Figure 9D, where dated volcanic
rocks and extensional events are plotted against their distance from the pa-
leotrench. A similar pattern of migration has been observed in the central and
northern Sierra Madre Occidental volcanic province (e.g., Aranda-Gomez
et al., 1997; Gans, 1997).
Comparison with the Central and NorthernSierra Madre Occidental volcanic province
There are some peculiarities in the geometry and kinematics of extension
in the southern Sierra Madre Occidental volcanic province. The first differ-
ence concerns the size of the extended area. It has been long suggested that
the inner core of the Sierra Madre Occidental is a relatively unfaulted
horst, separating the highly extended areas of the southern Basin and
Range and the Gulf of California (Henry,1989; Henry and Aranda-Gomez,
1992). Although this appears to be true in Durango and southern Chi-
huahua, our data indicate that this is not the case for the southern Sierra
Madre Occidental; here the crust appears pervasively faulted, and distin-
guishing Basin and Range from Gulf of California faulting is difficult both
in space and in time (Fig. 1). Furthermore, the area affected by extensionand volcanism in the southern Sierra Madre Occidental volcanic province is
nearly twice as wide as in the central volcanic province,directly north of the
study region. Volcanism and significant OligoceneMiocene extension in
the former extend as far as eastern San Luis Potos and Quertaro,about 500
km from the present coast, whereas in the central Sierra Madre Occidental
volcanic province they are only observed in Durango and Sinaloa.
A second difference regards the direction of extension. In the northern
Sierra Madre Occidental, Gans (1997) reported a minimum extension of
90% in the northeast-southwest direction from 26 to 20 Ma followed by ap-
proximately east-west extension of 10%15% between 20 and 17 Ma. In
the central Sierra Madre Occidental (Durango and Sinaloa) dominant ex-
=
=
Figure 8. Estimation ofamount of extension along a
profile perpendicular to thegraben of the eastern SierraMadre Occidental. The areavertically ruled is equal to thegray one. The figure shows themost consistent solution ac-cording to the geology of theregion (see text for details).Vertical exaggeration is 10x.Dashed lines represent in-ferred faults at depth. Lo = ini-tial length; Lf = final length.
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Geological Society of America Bulletin, March 1999 35
Jalisco
block
Tepic
Guadalajara
ShirleyFractureZon
e
Aguas-
calientes
0
100
200km
LenS
anLuis
Potos
S.M.
de
Allende
22
20
18
18
16
16
16
24
15P
ossibleaccommodationzone
withvergencereversal(Axen,
1995)
23-19
17-9
29-27
20
22
24
14
14
Possibleaccommodationzone
withvergencereversal(thiswork)
North
ernlimitofOligo-Mioceneextensional
faultingandvolcanismintheMC
43mm/yr
47mm/yr
34mm/yr
26
28
B
C
A
Escu
inapa
00.2
0.4
0.6
0.8
11.2
1.4
1.6
20
25
30
Age(Ma)
Half-spreadingrate(mm/yr)
SouthofShirley
NorthofShirley
Zacatecas
400
300
500
6
00
700
510
15
20
25
30
35
Mainextensional
episode(thiswork)
Datedvolcanicrock
witherrorbar(Table1&2)
Age(Ma)
Distancefrom
thepaleotrenc
h(km)
N
Figure9.Comparisonbetween
continentalandoffshoretectonics.Themapin(A)shows
themaincontinentalstructurewithtimeoffaultingandaveragedirectiono
fextension(from
thiswork)andthecorrespondingsea-floorisochronpatterndeducedfromthemagnetic
anomalymapofSeveringhausand
Atwater(1989)andLonsdale(1991).Ba
jaCaliforniawas
restoredtoitspreriftpositioncon
sideringthatnoembaymentexistedat
themouthofthe
GulfofCalifornia.Sea-floorage
(circled)andextinctridges(thickdoub
lelines)werere-
storedtotheirpre12.5Mapositionfortheamountproposed
inLonsdale(1991).L
inesper-
pendiculartosea-floorisochronsshowexamplesofcompute
dhalfrateofsea-floorspread-
ing.Notethesystematichigheraveragehalf-spreadingrateso
uthoftheShirleyfracturezone
(at1m.y.intervals)
duringOligoceneandearlyMiocenetime(B)andtheclosematchbe-
tweenthetimingofhighspreadingpulsesandofthemainextensionalandvolcanicepisodes
onthemainland(BandC).
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NIETO-SAMANIEGO ET AL.
360 Geological Society of America Bulletin, March 1999
tension in late Oligocene to middle Miocene and in PlioceneQuaternary
time was east-northeast trending, with a minor episode of west-northwest
extension in between (Aranda-Gomez et al., 1997). By contrast, two thirds
of the southern volcanic province (Mesa central and eastern Sierra Madre
Occidental) were affected by approximately east-west extension in
Oligocene and early Miocene time, and only in middle to late Miocene time
did east-northeast extension affect the western Sierra Madre Occidental.
A final difference concerns the tilt vergence in the western Sierra Madre
Occidental. South of lat 23 N blocks are ubiquitously tilted toward the east-
northeast, whereas to the north, in Sinaloa, they are systematically tilted in
the opposite direction (Henry, 1989; Aranda-Gomez et al., 1997). This ver-
gence reversal implies that a reverse accommodation zone must exist some-
where in the Escuinapa area (Fig. 9). Axen (1995) suggested a possible
matching structure on the other side of the Gulf of California, separating
hanging wall and footwall segments along the Main Gulf escarpment in
southern Baja California (Fig. 9A).
Causes of Extension in the SouthernSierra Madre Occidental volcanic province
In general, extension in continental volcanic arcs is caused by the com-
petition among lithospheric body forces, such as those related to the gravi-tational and thermal effects of magmatism (e.g.,Gans et al., 1989), and plate
boundary forces, related to the relative motion vectors between subducting
and overriding plates or to direct interaction between diverging oceanic and
continental plates (e.g., Bohannon and Parsons, 1995).
Magmatism can induce extension in two ways:1) injection of mafic mag-
mas at the base of the crust may induce a buckling of the lithosphere be-
cause of the accumulation of low-density hot material; 2) rapid piling up of
volcanic rocks at the surface can build a topographically elevated region,
where the vertical lithostatic stress may exceed the regional horizontal
stress. At a regional scale, deformation induced by high topography is likely
to be bidimensional, with extension orthogonal to the axis of the volcanic
arc, whereas deformation related to magmatic doming may be three dimen-
sional and close to radial extension. In both cases, however, extension is
likely to affect only the upper part of the crust.
However, extension in the overriding plate can be induced by a retreat-
ing subduction boundary, such as that where the subduction rate exceeds
the convergence rate (Jarrad, 1986; Royden, 1993), and this deformation is
usually accommodated along the volcanic arc (Hamilton, 1995). Extension
driven by a retreating subduction boundary is expected to involve the entire
lithosphere, and deformation is expected to be bidimensional if the litho-
sphere is homogeneous. The extension direction, however, will depend
upon the obliquity of the convergence and the occurrence or not of strain
partitioning. In addition, a retreating slab will generate a flux of hotter
asthenospheric material into the opening mantle wedge, producing wide-
spread melting and magma injection at the base of the crust. Thus, episodes
of retreating should match pulses of magmatism and control the location of
volcanism in the overriding plate.A detailed discussion of the relative role of body forces and plate boundary
forces in determining the tectonic-magmatic evolution of the southern Sierra
Madre Occidental volcanic province is beyond the scope of this paper. How-
ever, the data presented herein permits at least a qualitative analysis of the is-
sue. The late Miocene episode of extension (proto-Gulf of California) was
clearly related to the capture of the Magdalena microplate and Baja California
by the Pacific plate (e.g., Stock and Hodges, 1989). Thus, plate tectonics was
the dominant factor governing the latest period of extension. However, the
first two episodes of extension occurred when Farallon plate remnants were
still subducting,magmatism was still active,and both kinds of forces were ac-
tive. To evaluate the role of plate boundary forces, we reconstructed the pre-
Gulf of California plates configuration by closing the Gulf of California and
restoring the right-lateral motion along the Tosco-Abreojos fault system
(Fig. 9).This restoration places the Shirley Fracture Zone at the same latitude
as the northern limit of the southern Sierra Madre Occidental volcanic
province, and the vergence reversal in the Gulf of California area.
In Figure 9B we calculated the western half rate of spreading at the East Pa-
cific Rise between 30 and 20 Ma along two flow lines to the north and to the
south of the Shirley Fracture Zone. Considering a nearly symmetric spread-
ing at the rise (Lonsdale,1991), one can take these values as representative of
the eastern (Farallon) half-spreading rate. These rates show two rapid in-
creases,at 3026 Ma and 2420 Ma that match the two main episodes of vol-
canism and extension in the Sierra Madre Occidental volcanic province
(Fig. 9B and C). Given that the trench has actually moved oceanward since
Mesozoic time (Lightgow-Bertelloni and Richards, 1998), we propose that in-
creases of the spreading rate at the Eastern Pacific Rise were a consequence of
increases in the subduction rate at the trench and, therefore, that the Faral-
lonNorth America boundary west of the southern volcanic province was a
retreating subduction boundary. Spreading south of the Shirley Fracture Zone
was always higher than to the north by as much as 20% during the peaks of
spreading rate. This explains why extension in the southern volcanic province
occurred much farther inland than in the central volcanic province. Along this
part of the FarallonNorth America plate boundary, the difference betweensubduction and convergence rate probably exceeded a critical threshold
needed to induce extension in the overriding plate.
These observations led us to conclude that, at a continental scale (over
100 km of length), timing, magnitude, and orientation of extension as well
as age of volcanic episodes in the Sierra Madre Occidental volcanic
province were ultimately controlled by plate tectonic forces, namely by the
latitudinal variation of the retreating subduction boundary between the Far-
allon and North America plates.
On a regional to local scale (less than 100 km of length), we believe that
the variations in the tectonic and volcanic style in the southern Sierra Madre
Occidental volcanic province were controlled by the difference in the struc-
ture of the Mesa central and Sierra Madre Occidental crustal blocks and by
lithospheric body forces. In the Mesa central, the existence of a prefractured
crust and a moderate magmatic rate induced a three-dimensional strain, a
complicated fault geometry,and the emplacement of silicic magmas as rhy-
olitic domes. We believe that, in this case, plate boundary forces and body
forces are equally responsible in controlling the dynamics of extension and
magmatism. In the Sierra Madre Occidental, a stronger and relatively ho-
mogeneous crust and a high magmatic rate caused a bidimensional strain,
regularly spaced faults, and the formation of calderas. In this case magma-
tism was probably the dominant cause of extension.
We consider that the silicic volcanism flare-up was triggered by the com-
bined effect of extensive mafic underplating and the beginning of upper
plate extension induced by the slab retreat.
CONCLUSIONS
Our main conclusions can be summarized as follows:
1. In the southern Sierra Madre Occidental volcanic province we observed
a clear pattern of trenchward shifting of the climax of subduction volcanism
and extension during three distinct episodes in middle Oligocene, early
Miocene, and late Miocene time. In all the episodes, the peak of extension
slightly postdates volcanism.
2. Tectonic and volcanic style, as well as fault pattern, vary strongly from
east to west.
3. Extension and volcanism in the southern volcanic province took place
much farther inland than in the central volcanic province.
4. There is a very good correlation between the timing of extensional and
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SOUTHERN SIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE, MEXICO
Geological Society of America Bulletin, March 1999 36
magmatic pulses and periods of rapid spreading at the East Pacific Rise in
the offshore region.
5. The good match among continental and oceanic tectonic events sug-
gests that boundary conditions (i.e., plate tectonics) ultimately determine
the timing, magnitude, and orientation of extension.
6. The style of tectonics and volcanism and the distribution of faulting are
controlled by the internal structure of the crustal blocks and by the gravita-
tional and thermal effects of magmatism.
APPENDIX I
ON THE ESTIMATION OF THE AMOUNTOF EXTENSION IN THE MESA CENTRAL
The method of Krantz (1988) was used to determine the principal directions and thestrain ratio = 2/3 = tan
2, where i are the principal elongations, and 1 > 2 > 3(Fig. A1). We obtained a value of= 0.52. (the negative sign indicates that 3 is ashortening axis), an odd-axis (shortening axis) oriented 071/77, and a similar axis(maximum stretching) oriented 248/12 (Fig. A1). Thus, considering the strain ellip-soid, the principal plane that contains the maximum and intermediate strain axes isnearly horizontal. We also used the graphics proposed by Reches (1983), obtaining
0.4 and a coefficient of friction = 0.8.In order to estimate the post-Oligocene strain we need to assume the principal strain
orientations obtained for Sierra de San Miguelito as valid for all the Mesa central, basedon the similarity in strike of the principal fault systems (cf. Figs. 5 and 7). The crust ofthe southern Mesa central is thinner and more elevated relative to the adjacent SierrMadre Oriental and Occidental crustal blocks. A reasonable assumption is to considethat Mesa central had a thickness similar to that of the adjacent crustal blocks prior tothe deformation. The symmetry of crustal blocks shown in Figure 3 suggests that de-formation within the Mesa central was produced by the relative movement of the SierrMadre Oriental and Occidental crustal blocks, as supported by the orientation of th
bounding structures (Aguascalientes graben and San Miguel de AllendeCatorce faulsystem), approximately north-south (Fig. 5), and by the approximately east-west orientation of the maximum principal strain (1) obtained in the Sierra de San Miguelito. Thcrust in the Sierra Madre Occidental is 40 km thick, and we estimated 8% stretchingduring Cenozoic time. Considering the crustal thickness before deformation of theMesa central to be equal to the original thickness of the Sierra Madre Occidentall0
= 43 km, and a final thickness lf= 32 km, the vertical principal elongation (
3) i
3
= (lfl
0)/l
0= 0.25, where the negative sign indicates that shortening occurred in th
vertical direction. To calculate the horizontal principal elongations we need to assumeno volume change. Considering an undeformed cube with unit edge and applying to ia strain tensor with principal strains without volume change, we obtain
. (1)1 1 1 11 2 3
+( ) +( ) +( ) =
odd, e 3intermediate, e 2
similar, e1 a
odd= =064/783similar= =259/111
intermediate= =169/022
SSM faults
228/70
SSM
striae
213/69
VRGstriae
097/63
VRG faults
120/65
a =35
90
B
odd= =071/773similar= =258/121
intermediate= =168/022
SSM faults
227/48
SSM
stria
e
216/47
VRGstriae
097/63
VRG faults
120/65
a =36
90
A
k = -0.52
k = -0.49
Unrotated
Corrected for rotation
Figure A1. Application of the odd-axis model (Krantz, 1988) to the Sierra de San Miguelito area. The model is based on the Rechess slip-modeand considers four fault planes with orthorhombic symmetry. Simultaneous motion of the faults accommodates a triaxial nonrotational deformation. The odd axis is the one with opposite sign (shortening of stretching) with respect to the other principal axes, assuming no volume
change. When the odd-axis is 1 then the similar-axis is 3 and vice-versa. We assumed the stretching to be positive and 1 > 2 > 3. 2 is the angle between the fault traces on the intermediate-similar plane. The Krantz model holds for nonrotational faulting. However, we consider that ican be applicable to the San Miguelito area. In any case, correcting the data for the rotation produces nearly identical results in the similar axiorientation (cf. A and B). Data in B were rotated 22 counterclockwise about an axis 311/00.
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NIETO-SAMANIEGO ET AL.
362 Geological Society of America Bulletin, March 1999
Using the mean value for strain ratio = 0.45 in equation 1 and introducing thecalculated value 3 = 0.25, we obtained 1 = 0.2, 2 = 0.11, and 3 = 0.25. Thesevalues represent the bulk deformation of southern Mesa central,without consideringthe material transfer due to the magmatism.
Some comments are needed regarding the use of the odd-axis model for Sierra deSan Miguelito. The San Luis de la PazSalinas de Hidalgo fault system shows adomino style with beds dipping 20 to the northeast (Labarthe-Hernndez andJimnez-Lpez, 1992, 1994). This bed inclination implies a rotational component inthe deformation tensor, whereas the odd-axis model supposes an irrotational strain.Nevertheless, we use the odd-axis model for the following reasons:
1. We try to estimate the strain for all the southern Mesa central, whereas tilting islocal. The rotation of 20 observed in the beds of Sierra de San Miguelito is not wide-spread. The bulk of Sierra de San Miguelito is only tilted ~5. Adjacent to the Sierrade San Miguelito there are other faults and grabens parallel to the San Luis de laPazSalinas de Hidalgo fault system (Bledos and Enramadas grabens, Fig. 7), that donot produce significant tilting. At a regional scale tilting produced by the majorgrabens is
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