Nieto Etal 1999

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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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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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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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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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Pacific

Ocean

Jalisco

block

Aguamilpa

Acaponeta

Tepic

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Sta.

Maria

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Sta.Rosa

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Escuinapa

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esa

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Camotln

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Tlaltenango Ja

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Juch

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Zacatecas

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Santia

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Rio

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22N

21N

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0

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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

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SierradeSantaBarbara;

PR9deepgeothermalwellofLaPrimaveracaldera;CB1deepgeothermalwellofCe-

borucovolcano.Stra

tigraphiccolumnsnottoscale.

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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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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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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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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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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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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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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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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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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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