Variation of extension and volcanism across the southern Sierra Madre Occidental volcanic province,

Ángel F. Nieto-Samaniego* Unidad de Ciencias de la Tierra, Universidad Nacional Autónoma de México, Luca Ferrari Campus Juriquilla, Apdo. Postal 1–742, Querétaro, Qro., 76001, México Susana A. Alaniz-Alvarez } Guillermo Labarthe-Hernández Instituto de Geología, Universidad Autónoma de San Luis Potosí, Dr. Manuel Nava 5, Zona Universitaria, San Luis Potosí, S. L. P., 78240, México José Rosas-Elguera Centro de Ciencias de la Tierra, Universidad de Guadalajara, Blvd. M. García Barragán y Calz. Olímpica, 44840, Guadalajara, , México

ABSTRACT INTRODUCTION

The middle to late Cenozoic tectonic-magmatic evolution of the Three major tectonic events occurred in the northern half of the Mexican Sierra Madre Occidental volcanic province south of the Tropic of Can- Pacific coast during Tertiary time: (1) an episode of subduction that lasted cer is summarized and analyzed for the first time, based on new geo- until middle time (Atwater, 1989); (2) a progressive waning of sub- logic and structural work and published information. In the eastern duction through various episodes of microplate capture induced by the ap- part of the study (Mesa central physiographic province) silicic proach of the East Pacific Rise to the trench (Lonsdale, 1991); and (3) the volcanism occurred in a short-lived episode culminating at ca. 30 Ma opening of the (e.g., Stock and Hodges 1989; Lonsdale, and was followed by crustal-scale extension between 30 and 27 Ma. In 1989). These events shaped the volcanic and structural configuration of the the western part of the study area (Sierra Madre Occidental physio- central and western parts of , forming the Sierra Madre Oc- graphic province) a voluminous episode of volcanism at cidental volcanic province and a wide extensional province, referred to as the 24–21 Ma was succeeded by east-west extension that produced regu- “real southern Basin and Range” (Henry and Aranda-Gómez, 1992) (Fig. 1). larly spaced affecting only the upper crust. In the westernmost In the past two decades there have been many contributions to the recon- part of the study region, an andesitic to rhyolitic arc, formed between struction of the volcanic evolution of the northern and central part of the 17 and 12 Ma, was affected by crustal-scale, north-northwest–trending, Sierra Madre Occidental volcanic province (McDowell and Keizer, 1977; extensional faulting, leading to the formation of the Gulf of California. McDowell and Clabaugh, 1979; Henry and Fredrikson, 1987; Wark et al., In the Mesa central the maximum extension was oriented approxi- 1990; Aguirre and McDowell, 1991). Structural studies in (Henry, mately east-west and amounted to ~20%. In the eastern Sierra Madre 1989), (Aguirre-Diaz and McDowell, 1993; Aranda-Gómez et al., Occidental physiographic province extension was only 8% and ori- 1997), and (Gans, 1997; McDowell et al., 1997) (Fig. 2) provide in- ented approximately east-west. We observe that trenchward shifting of formation on the timing and style of the extensional . In general, the climax of subduction volcanism and extension occurred during late these works report a westward migration of extension and volcanism dur- , early Miocene, and late Miocene time. Comparison with the ing post- time, as well as an inception of extension well before the offshore tectonics indicates that the first two tectonic-magmatic pulses first direct interaction between the Pacific and the North American plates. coincide with periods of fast spreading at the Pacific-Farallon bound- The Sierra Madre Occidental volcanic province south of the Tropic of ary, south of the Shirley zone. We propose that increases in the Cancer (hereafter referred to as the southern Sierra Madre Occidental vol- spreading rate are related to periods of high subduction rate, which in canic province) is less well-known, at least in the international geologic lit- turn correspond to episodes of retreating subduction. A retreating slab erature. This region can be divided into the Sierra Madre Occidental and the may have generated a flux of hotter asthenospheric material into the Mesa central physiographic provinces (Fig. 1). Geologic mapping and struc- mantle wedge, producing widespread melting at the base of the crust as tural studies carried out in the past 15 yr have documented the volcanic and well as intraarc extension in the overriding plate. Boundary conditions tectonic evolution of the Mesa central in good detail (Labarthe-Hernández (i.e., plate tectonics) ultimately determined timing, magnitude, and ori- et al., 1982; Aranda-Gómez et al., 1989; Henry and Aranda-Gómez, 1992; entation of extension, whereas volcanic and tectonic styles are con- Nieto-Samaniego, 1990; Martínez-Reyes, 1992; Quintero-Legorreta, 1992; trolled by the internal structure of crustal blocks and by the gravita- Nieto-Samaniego and Alaniz-Alvarez, 1994; Nieto-Samaniego et al., 1996, tional and thermal effects of . 1997), but most of this information is published in university reports or na- tional journals, with limited international distribution. *E-mail: [email protected]. The timing of the volcanic and extensional events in the Sierra Madre

Data Repository item 9919 contains additional material related to this article.

GSA Bulletin; March 1999; v. 111; no. 3; p. 347–363; 10 figures; 2 tables.

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Figure 1. Geodynamic setting, physiography (A), and Cenozoic volcanic arcs (B) of Mexico. The study region (boxed) comprises the southern Sierra Madre Occidental volcanic province (SMO VP) and extends through the physiographic provinces of the SMOc (Sierra Madre Occidental), Mesa central (MC), (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. Oligocene–Miocene volcanic rocks south of the MVB are diagonally ruled.

Occidental is less well-known than for the Mesa central. Good strati- graphic and geochronologic information is available only for a few places 110˚N 105˚N (Lyons, 1988; Scheubel et al., 1988; Nieto-Obregón et al., 1985), whereas 30˚N there are only reconnaissance geologic and structural works (Gastil et al., SON 1978; Damon et al., 1979; Webber et al., 1994; Ferrari, 1995; Ferrari et al., N 1999) and several scattered ages of volcanic rocks for the remaining region CHIH (Clark et al., 1981; Nieto-Obregón 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- SIN 25˚N DGO naissance studies along roads. In this paper we integrate and summarize our regional stratigraphic and ZAC structural information with published geological and geophysical data to Tropic of Cancer provide the first synthesis of the middle to late Cenozoic volcanic and tec- SLP A tonic evolution of the southern Sierra Madre Occidental volcanic province. NAY Our data document an Oligocene three-dimensional strain in the southern 0 km 500 GTO JAL Mesa central, an early Miocene two-dimensional strain in the eastern part 20˚N 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. Figure 2. Boundaries of Mexican states cited in the text. SON— We propose that variations in deformation and volcanism observed since Sonora, SIN—Sinaloa, CHIH—, NAY—, DGO— Oligocene time in the southern volcanic province are due to the difference Durango, ZAC—, A—, SLP—San Luis Po- in thickness and composition of the major crustal blocks that form the Sierra tosí, JAL—Jalisco, GTO—. 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 TOPOGRAPHY AND CRUSTAL THICKNESS a general phenomenon in Oligocene–Miocene time, although in the south- ern volcanic province geologically significant extension affected a much The spatial variations in the Sierra Madre Occidental volcanic province wider region than to the north. The westward migration is tentatively related are illustrated in an east-west topographic profile in Figure 3. This profile to the along-trench variation of the Farallon– subduction rate distinguishes three physiographic provinces with different elevations: an during Oligocene and Miocene time. eastern province with mean elevation of 1400 m corresponding to the Sierra

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TABLE 1. K/AR AGES FOR THE SOUTHERN SIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE Sample Site Rock type Material Long Lat K 40Ar* 40Ar* Age ±1σ dated (W) (N) (wt%) (mol/gr) (%) (Ma) (Ma) SLP 7* Cerro El Büey, SLP Hb 100.68 22.44 0.53 3.50E-11 52.0 37.6 1.9 GL 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 Montaña, Gto. Ignimbrite WR 101.28 21.18 4.63 2.73E-10 93.1 29.3 1.5 MP-B1† Campuzano, Gto. Ignimbrite WR 101.14 20.84 6.43 3.02E-10 98.2 28.6 1.4 GL 406† E of Calvillo, Gto. Ignimbrite WR 101.04 21.10 4.56 2.39E-10 95.8 26.9 1.3 CTO-02§ Sierra Nochistlán Ignimbrite San 102.84 21.59 7.89 3.67E-10 76.2 26.6 0.7 GL 137† Cerro Cano, Gto. WR 101.07 21.09 1.59 8.05E-11 67.7 25.9 1.3 GL 241† E of Calvillo, Gto. Ignimbrite WR 101.08 21.12 3.71 1.83E-10 96.8 25.2 1.3 SLP 19* La Salitrera, SLP Bas.- Andesite Pl 100.52 22.93 0.57 1.31E-11 35.0 13.2 0.6 Notes: Hb—hornblende, San—sanidine, Pl—plagioclase, gms—groundmass, WR—whole 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.

Madre Oriental; a central province with a mean elevation of 2100 m corre- Mesozoic Basement 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 The oldest rocks of the region are exposed in the Mesa central and belong uplift between the Mesa central and the Sierra Madre Occidental is even to two major paleogeographic domains: the Guerrero island-arc as- more pronounced if one looks at the geologic units exposed in the two semblage (Centeno-Garcia et al., 1993) and the Sierra Madre Oriental ma- provinces. In the Mesa central the Mesozoic basement is commonly ex- rine sedimentary sequence. The Guerrero terrane, exposed in the western posed from 2000 to 2700 m (Fig. 3), whereas in the Sierra Madre Occiden- part of the southern Mesa central, is a volcanic-sedimentary sequence made tal rocks of the same age are not exposed, even at 800 m inside the major of flysch-like sediments interbedded with pillow basalt, which shows a low of the area. metamorphic grade and severe contractile deformation. The succession is ex- The crustal thickness of the physiographic provinces also shows notable posed in the Guanajuato and the Aguascalientes areas (Fig. 4).On the basis differences: based on a gravimetric profile along the Tropic of Cancer, of paleontologic evidence (Chiodi et al., 1988) and K-Ar ages (Monod et al., Campos-Enriquez et al. (1994) proposed crustal thickness of 37 km for the 1990), the succession has been interpreted as Early in age. To the Sierra Madre Oriental, 33 km for the Mesa central and 41 km for the Sierra north and east of the southern Mesa central, a Cretaceous marine carbonate Madre Occidental. Molina-Garza and Urrutia-Fucugauchi (1993) esti- and clastic sequence (e.g., Carrillo-Bravo, 1971; Labarthe-Hernández et al., mated a similar value for the Sierra Madre Occidental from interpretation 1982) underwent thin-skinned contraction during the . of Bouguer anomaly along a north-south profile at long 103°W. Using In the western Sierra Madre Occidental the existence of a prevolcanic group velocity of surface waves, Fix (1975) estimated a crustal thickness Mesozoic basement is substantiated by localized outcrops of and of 30 km for central Mexico, whereas Rivera and Ponce (1986) proposed a exposed along the lower part of the de Santiago. value of 42 km for the Sierra Madre Occidental. The latter value was also These rocks occur in small and isolated outcrops on top of late Oligocene to obtained by Meyer et al. (1958) using seismic refraction data. By model- early Miocene intrusive stocks, suggesting that they were roof pendants up- ing gravity and seismic refraction data, Couch et al. (1991) calculated a lifted by the plutons. Extensive exposures of Mesozoic rocks are not known crustal thickness ranging from 40 km at the center of the Sierra Madre Oc- in the Sierra Madre Occidental south of lat 22°30’ N and west of the longi- cidental to 25 km along the coast of the Gulf of California. tude of Zacatecas (Fig. 3). These data indicate that the thinner and topographically elevated Mesa central is bounded by thicker and depressed crustal blocks of the Sierra Paleogene Continental Sedimentary Rocks 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 This group includes two continental postorogenic units: the Guanajuato with respect to the adjacent ones. In the following sections we will show that conglomerate (Edwards, 1955) exposed in Guanajuato, and the Cenicera the Mesa central and the Sierra Madre Occidental also have important dif- Formation in San Luis Potosí (Labarthe-Hernández et al., 1982). They con- ferences in the age and style of volcanism and extension. sist of polymictic conglomerate and sandstone deposited in coalescing al- luvial fans after the Laramide orogeny. According to paleontologic deter- REGIONAL STRATIGRAPHY minations (Edwards 1955) and a K-Ar date from an interbedded andesite (Aranda-Gómez and McDowell, 1998), the Guanajuato conglomerate is In this section we present a generalized stratigraphy of the Mesa central Eocene in age. Palynologic determinations indicate a -Eocene and Sierra Madre Occidental that integrates the results of previous works age for the Cenicera Formation (Labarthe-Hernández et al., 1982). The with our reconnaissance investigations of Tertiary rocks in the region. This overlying Casita Blanca Andesite, dated as 44.1 ± 2.2 Ma (Labarthe- synthesis is supported by 91 published ages (Table DR1, in GSA Data Hernández et al., 1982), gives a minimum age for the Cenicera Formation. Repository1) and nine new dates reported in Table 1. Eocene Volcanic Group 1GSA Data Repository item 9919, data table, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301. E-mail: Eocene volcanic rocks are widespread in the Sierra Madre Occidental vol- [email protected]. canic province (Aguirre-Díaz and McDowell, 1991, and references therein),

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22˚N 22˚N

21˚N 21˚N

104˚W 102˚W 100˚W

Figure 3. (A) Generalized topographic map of the southern Sierra Madre Occidental volcanic province. Thick lines bound physiographic provinces and correspond to major tectonic structures discussed in the text. (I) Sierra Madre Occidental, (II) Mesa central, and (III) Sierra Madre Oriental physiographic provinces. Stars represent extensive outcrops of the Mesozoic basement. Thick dashed line indicates the trace of topo- graphic profile A–A′. SMA—San Miguel de Allende; NVM—Nuevo Valle de Moreno. (B) Topographic profile based on 1:1 000 000 scale topo- graphic map filtered for the high frequencies. The mean elevation of the three physiographic provinces has been calculated without considering the coastal plain. (C) Interpretation of the crustal structure along the profile based on surface 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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TABLE 2. COMPUTED PALEOSTRESS TENSORS FOR THE SIERRA MADRE OCCIDENTAL Eastern Sierra Madre Occidental Western Sierra Madre Occidental (Calvillo, Juchipila, Tlaltenengo and (Aguamilpa , Jesús Maria, La Bolaños ) Cofradía, Paso de Lozada, Presa Bañadero, and Sierra Pajaritos) Trend/Plunge Magnitudes relative to Trend/Plunge Magnitudes relative to σ σ 1 1 σ 1 225/75° 100 222°/81° 100 σ 2 007°/11° 44 333°/03° 32 σ 3 099°/08° 39 066°/07° 27

σσ– σσ– stress ratio ==φ 23 =008. stress ratio ==φ 23 =007. σσ σσ 13– 13–

coefficient of friction = µ = 0.4; n = 61 coefficient of friction = µ = 0.6; n = 58 Note: The method of Reches (1987) assumes that slip obeys the Coulomb criterion. Coefficient of friction, cohesion, the misfit angle between observed and calculated slip axis (V), and the mean misfit angle between general and ideal principal stresses axes (ξ) are used as criteria to separate the groups. These analyses simultaneously satisfy the following constraints: 0.6 > µ > 0.4; cohesion near zero; ς < 25°;. ξ < 40°.

although they are commonly obscured by the extensive Oligocene rhyolitic Mesa central. The northernmost exposed ignimbrite and one interbedded cover. In the southern Mesa central, Eocene rhyolitic and domes basaltic flow of this succession give ages of 25.2 ± 1.3 Ma and 25.9 ± 1.3 Ma, and andesite flows crop out locally in Zacatecas (Ponce and Clark, 1988; respectively (Table 1). Lang et al., 1988), Guanajuato (Bufa , Gross, 1975), San Luis Potosí In the Sierra Madre Occidental, Oligocene ignimbrites are only reported (Labarthe-Hernández et al., 1982), and Aguascalientes (Nieto-Samaniego in the Santa Maria del Oro and Juchipila areas (Fig. 5) (Table DR1, see foot- et al., 1996). A sequence of andesitic flows on the Mesozoic basement 80 km note 1). In addition we obtained a K-Ar age of 26.6 ± 0.7 Ma (Table 1) from northwest of San Luis Potosí yields a K-Ar age of 37.6 ± 1.9 Ma (Table 1). one of the uppermost ignimbrites capping the Sierra de Nochistlán (Fig. 5). In the Sierra Madre Occidental the oldest volcanic rocks are highly al- These old ignimbrites are commonly interbedded with and capped by vol- tered and pervasively fractured microcrystalline andesitic , which are caniclastic sequences made of , sandstone, and conglomerate, and can exposed north of Guadalajara and yield a middle Eocene age (Webber et al., be interpreted as the westernmost extension of the Mesa central Oligocene 1994; Ferrari et al., 1999). Their base is not exposed and the top is eroded volcanic sequence. and locally covered unconformably by red sandstone and conglomerate containing andesitic fragments. The minimum thickness of this succession Miocene Volcanic Group is 400 m. Based on the geographic distribution of the outcrops mentioned, the Eocene volcanic rocks, although scattered and of lesser volume, were The Miocene volcanic group consists of three main components: early probably more extensive than the Oligocene-Miocene rocks. Miocene silicic volcanic rocks, early Miocene basaltic , and late Miocene . A significant part of the southern Sierra Madre Occidental Oligocene Volcanic Group is covered by rhyolitic ignimbrites and minor domes emplaced in a short time span (24 to 21 Ma, Table DR1, see footnote 1). More than 1200 m of ash- Rhyolite domes and lava flows with minor intercalated pyroclastic de- flow tuffs with ages between 23.7 and 20.1 Ma are exposed in the Bolaños posits are volumetrically the most important part of the Tertiary volcanism graben, and the upper part of this succession can be followed up to about 100 in the Mesa central. These rocks are exposed all over the southern Mesa km southward (Scheubel et al., 1988; Lyons, 1988; Moore et al., 1994). Ig- central, often forming the core of major ranges (Fig. 4). Domes are typically nimbrites with an aggregate thickness of about 1000 m are exposed in the about 2 km in size. They are associated with deposits of , and with Sierra de Santa Lucia, in the core of the Sierra Madre Occidental (Fig. 5). rhyolite lava that commonly contains topaz. The age is well constrained to The uppermost tuffs of this succession have been dated at 23.5 Ma (Clark be Oligocene by five K-Ar dates in San Luis Potosí and Guanajuato (30.8 ± et al., 1981), whereas the base yielded an age of about 25 Ma (Ferrari et al., 0.8 to 29.2 ± 0.8 Ma, Table DR1, see footnote 1). This group is absent west 1997a). These silicic ignimbrites resemble the 1000-m-thick and ca. 23-Ma of Aguascalientes in the Sierra Madre Occidental, as well as to the east in El Salto–Espinazo del Diablo sequence, exposed ~100 km to the north in the Sierra Madre Oriental (Fig. 4). southwestern Durango (McDowell and Keizer, 1977). Early Miocene ash Oligocene rhyolitic ignimbrites with subordinate andesitic lava flows are flows are not found in the Mesa central, which suggests a westward young- exposed in the southern Mesa central and the southeastern Sierra Madre Oc- ing of the “ignimbrite flare up.”Ash flows and with similar ages are cidental (Fig. 4). The aggregate thickness of these rocks is usually less than exposed in southernmost Baja California (Hausback, 1984) and could repre- 200 m. No clear structures have been genetically or spatially associ- sent distal facies of the early Miocene Sierra Madre Occidental arc. ated with these ignimbrites. In San Luis Potosí and northern Guanajuato North of Guadalajara the Sierra Madre Occidental succession is dissected (Fig. 2) the ignimbrites have K-Ar ages ranging from 29.0 to 26.8 Ma (Table by north- to north-northeast–trending grabens (Fig. 5). Basaltic andesites DR1, see footnote 1). This time span is confirmed by three new whole-rock are found as lavas at the bases of these grabens and as dikes intruded paral- K-Ar dates ranging between 29.3 and 26.9 Ma from the Sierra de Guanajuato lel to the normal faults. The age of these rocks is 21–19 Ma in the Bolaños (Table 1). A second succession of ignimbritic sheets with latest Oligocene graben (Nieto-Obregón et al., 1981) and 21.8 Ma in the Tlaltenango graben ages (Table DR1, see footnote 1) is exposed in the southernmost part of the (Moore et al., 1994).

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Figure 4. Geologic map of the Mesa central showing the main stratigraphic units discussed in the text. The figure is based on geologic sheets at scales 1:100 000, 1:50 000, and 1:20 000 published by the Geologic Institute of the San Luis Potosí University between 1977 and 1995 (see Nieto- Samaniego et al., 1996; Labarthe-Hernández et al., 1982, for the complete list of references) and Nieto-Samaniego (unpublished data). PH—Palo Huérfano Volcano; SC—Sierra de Codornices; SG—Sierra de Guanajuato; NVM—Nuevo Valle de Moreno; SSM—Sierra de San Miguelito. The region shown in Figure 6 is outlined.

The southernmost Mesa central is covered by an extensive basaltic age is considered representative of the postignimbrite volcanism of the and by andesitic stratovolcanoes (Ferrari et al., 1994a). Stratigraphic region, because it coincides with the age of other basalts in the southernmost evidence indicates that the stratovolcanoes are older than the basaltic Mesa central (Table DR1, see footnote 1), and with the Metates (McDowell plateau extending to the south (Valdez-Moreno and Aguirre-Diaz, 1997; and Keizer, 1977) and Los Encinos basalts (Luhr et al., 1995), which are ex- Pérez-Venzor et al., 1997). Ages range from about12 Ma for one of the stra- posed in eastern Durango and northern San Luis Potosí, respectively. tovolcanoes (Pérez-Venzor et al., 1997) to 10.5–8.1 Ma for the basaltic Along the southern limit of the Sierra Madre Occidental, the early Miocene plateau (Table DR1, see footnote 1). ignimbrites are covered by 10.2–8.5 Ma (Table DR1, see footnote 1) basaltic Basaltic to andesitic lava flows also cap the silicic succession in the Mesa lava flows, well exposed along the Rio Santiago north of Guadalajara central to the north. The only age for these rocks is 13.2 ± 0.6 Ma (Table 1) (Fig. 5), where they have an aggregate thickness exceeding 600 m. This suc- from one flow of an extensive basaltic-andesitic succession overlying the cession also extends east of Guadalajara for more than 130 km in the Los Al- Oligocene ignimbrites about 80 km east-southeast of San Luis Potosí. This tos de Jalisco (Fig. 5), with an average thickness of 200 m and ages of 11 to 8

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Celaya Villa de Reyes graben Reyes de Villa Dolores Hidalgo Potosí SanLuis S.S.M. Guanajuato S.S.B. graben Pinos Ojuelos Penjamillo El Bajío graben León Salinas de Hidalgo Lagos deMoreno II 102˚W Aguascalientes Atotonilco Zacatecas Calvillo

ltos de Jalisco Sierra de Nochistlán de Sierra

A Ia

Jalpa

iVerde RioV

ones Juchipila Sierra de Mor de Sierra Tlaltenango PR9 San Cristobal Guadalajara Bolaños Dam Sta.Rosa Mesa central, Occidental, Sierra Madre Ia—eastern Occidental. Sierra Madre Ib—western S.C.—Sierra del Cubo; S.S.M.—Sierra de San Miguelito; S.S.B.—Sierra Santa Barbara; caldera; CB1—deep geothermal well of Ce- PR9—deep geothermal well of La Primavera Stratigraphic columns not to scale. boruco volcano.

ntiago Sa

Rio 104˚W

Pte.de Camotlàn i Atengo Rio S. Pajaritos Sierra Sta.Lucia Plan de Barrancas-Santa Rosa graben Sierra Alica Sta. Maria Sta. del Oro Jesús Maria 50 100 km CB1 Mesa delNayar

o-Ceboruco graben Ib block Aguamilpa

Tepic SanPedr Pochotitán Jalisco 0

San Pedro El Zopilote

RioSantiago i a Pedro San Rio Acaponeta Punta Mita Escuinapa 22˚N 21˚N 106˚W Figure 5. Structural map and generalized stratigraphy of the southern Sierra Madre Oc- Sierra Madre of the southern 5. Structural map and generalized stratigraphy Figure cidental volcanic province,cidental volcanic the Mesa central (MC) and cited in the text for based on sources of satellite image,on interpretation digital terrain models, aerial photographs and recon- and Rosas-Elguera, (Ferrari work naissance field 1998,Thick gray lines personal commun.). Oriental, boundaries of structural domains described in the text. III—Sierra Madre are II—

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Ma (Ferrari et al., 1994a, 1996); these ages allow them to be correlated with basaltic to andesitic volcanism related to the Mexican volcanic belt and to the the late Miocene basalts and andesites described from the Mesa central. 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 Volcanic Rocks Related to the Gulf of California were also emplaced within the Mesa central but are not known in the Sierra Madre Occidental. In time, minor alkaline basalts were emplaced In the southwestern part of the Sierra Madre Occidental, north of Santa in the Mesa central only. Maria del Oro, eroded andesitic volcanoes commonly cover the early The most striking difference between the Mesa central and the Sierra Miocene ash flows. One of these centers has been dated as 14.7 Ma (Table Madre Occidental concern the age and the style of the silicic volcanism: in DR1, see footnote 1). Gastil et al. (1979) also reported a succession of an- the former, silicic volcanism consists largely of rhyolitic domes and oc- desitic and basaltic flows with subordinate rhyolites with ages between 20.4 curred mostly in Oligocene time; in the latter, it was essentially represented and 13.8 Ma south of Tepic (Table DR1, see footnote 1). These rocks sug- by caldera-related ignimbrites of early Miocene age. We note a westward gest the existence of a middle Miocene intermediate-composition volcanic (or west-southwestward) shift of the focus of silicic volcanism in Oligocene arc in the western part of the Sierra Madre Occidental. This volcanism rep- to Miocene time, similar to that observed in the northern and central Sierra resents the southern occurrence of the early to middle Miocene andesitic Madre Occidental (Gans, 1997; Aranda-Gomez et al., 1997). 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 MIDDLE TO LATE TERTIARY FAULTING IN THE SOUTHERN rifting process. SIERRA MADRE OCCIDENTAL VOLCANIC PROVINCE 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 Geometry and Kinematics coast north of Tepic (Fig. 5). These rocks unconformably overlie upon tilted ignimbrites of the Sierra Madre Occidental. Several mafic dikes with ages The southern volcanic province is bounded to the south and also divided of 12–11 Ma (Damon et al., 1979; Clark et al., 1981; Table DR1, see foot- longitudinally by major crustal structures (Figs. 3 and 5). Although the ge- note 1) and with north-south to northwest trends are found in the western- omorphic expression of these structures is locally modest, their tectonic sig- most part of the Sierra Madre Occidental. This mafic, commonly alkalic nificance is given by the fact that they bound different types of basement volcanism marks the initiation of the rifting process at the mouth of the Gulf and/or control the style of Cenozoic extension and volcanism. Faulting also of California. 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 Quaternary Volcanic Rocks other faulting inside the Mesa central and Sierra Madre Occidental blocks. Boundary Fault Systems. The boundaries between the Mesa central and The youngest volcanic event consists of scattered cinder cones, maars, Sierra Madre Occidental and Oriental are major north-south fault systems, and lava flows emplaced in the northern part of the Mesa central. These vol- along which Cenozoic normal movement formed half grabens. The bound- canic rocks include nephelinite, basanite, alkaline basalt, and basalt. Ac- ary between Sierra Madre Oriental and the Mesa central is a complex system cording to 8 K-Ar age determinations, they are early to middle Pleistocene of north-northwest– to north-trending and west-dipping normal faults, here in age (Aranda-Gomez and Luhr, 1996). named the San Miguel de Allende–Catorce fault system (Fig. 5). The fault system separates the intensely and complexly faulted Mesa central to the Continental Sedimentary Deposits west from the Sierra Madre Oriental, which shows no Cenozoic extension. The southern part of this structure is the Taxco–San Miguel de Allende fault, Poorly to mildly consolidated alluvial and lacustrine deposits fill numer- first identified by Demant (1978) as a major tectonic boundary. Minimum ous basins cut in the Tertiary succession. They consist of conglomerate, offsets of the fault system in the San Miguel de Allende area is 450 m (Nieto- gravel, sand, sandstone, and subordinate marl, limestone, and chert. These Samaniego and Alaniz-Alvarez, 1994). North of San Miguel de Allende the deposits probably span the entire late Tertiary time and possibly Quaternary fault system cuts Quaternary sediments, forming a 50-m-high scarp. The time, as indicated by intercalation of Miocene ignimbrites and basalts and fault trace is interrupted within the Sierra del Cubo and appears again as the by Pleistocene vertebrate fossils discovered in the upper part of the se- northern extension of the Villa de Reyes graben (Fig. 5). Because there are quence near San Miguel de Allende (Carranza-Castañeda et al., 1994) and no western bounding faults to form a complete graben, we deduce that the Aguascalientes (Reynoso-Rosales and Montellano-Ballesteros, 1994). The fault system is a half graben, the master fault dipping to the west. Juchipila graben is floored by a stratified volcanic-sedimentary succession We chose the Aguascalientes fault as the western boundary of the Mesa consisting of 10 to 20 m of a coarse volcanic conglomerate, followed by as central because it is a large-displacement fault that marks the western limit much as 250 m of lacustrine clays, silts, and limestone. Alluvial deposits of Mesozoic basement and separates of different structural style. The (gravel and sandstone) of unknown age also fill the southernmost part of the north-striking, east-dipping fault has a 250-m-high scarp into a basin filled Tlaltenango graben. with at least 500 m of sedimentary rocks and intercalated ignimbrites. Water wells indicate a total displacement of 1000 m (Jiménez-Nava, 1993). Sub- Summary of the Volcanic Evolution 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 variation in the stratigraphic record across the southern parts of the the basin is mostly a half graben with an east-dipping master fault (Fig. 5). Mesa central and Sierra Madre Occidental is summarized in the stratigraphic The southern edge of the Mesa central is the dominantly normal Bajío fault columns of Figure 5. Eocene, mostly andesitic, volcanism appears to have (Aranda-Gomez et al., 1989). The eastern half of the Bajío fault trends east occurred across the entire Mesa central and in the eastern part of the Sierra and, according to subsurface data, shows an offset about 700 m west of Celaya Madre Occidental. In early Miocene time voluminous rhyolitic ignimbrites (Ramos-Salinas, 1996). The western half strikes N50°W and bounds Meso- were emplaced in the Sierra Madre Occidental while and continen- zoic basement and Paleogene plutons (Fig. 4). Using displacements reported tal sedimentation were underway in the Mesa central. In late Miocene time by Quintero-Legorreta (1992) and Aranda-Gómez et al. (1989) and adding an

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average 500 m of basin-fill deposits estimated from Hernández-Laloth (1991), range from 1500 to 2000 m (Lyons, 1988). The graben is slightly asymmet- the Bajío fault reaches a vertical offset of 1350 m near León and 1100 m near ric; the maximum relief is 1200 m on the western scarp (Scheubel et al., Guanajuato. However, if we consider the vertical offset of the parallel Veta 1988). The three grabens are bounded by high-angle faults with dominant Madre fault in the city of Guanajuato (Gross, 1975), the cumulative offset can dip-slip displacements. Inside the grabens, volcanic or sedimentary beds be as much as 2850 m. The Bajío fault vanishes west of León, where it inter- show tilts of 10° to 20° either to the east and the west. sects the north-trending Penjamillo graben (Fig. 5) (Ferrari et al., 1994b). Be- The region between the Bolaños graben and the coastal plane is affected tween the Penjamillo graben and Guadalajara, the southern boundary of the by four north-south– to north-northwest–trending major structures: the Mesa central and of the Sierra Madre Occidental is poorly defined because Puente de Camotlán, the Sierra de Pajaritos-Atengo, and the Sierra Al- most of the area is covered by late Tertiary continental sediments and by ex- ica–Jesus Maria half grabens and the Pochotitán–San Pedro fault system. tensive late Miocene basalts. These rocks are cut by two approximately east- These structures systematically dip to the west and tilt the hanging blocks west normal fault systems with 200–400 m of vertical offset (Fig. 5). South of as much as 35° to the east and east-northeast. The major structure is the the Los Altos de Jalisco plateau is the east-west–trending Chapala graben Sierra Alica–Jesus Maria half graben, which has a maximum vertical offset (Campos-Enriquez et al., 1990; Urrutia-Fucugauchi and Rosas-Elguera, exceeding 2.2 km. The region west of this structure is interpreted as a series 1994). None of these structures has the large vertical displacement shown by of parallel listric faults, whose style, age, and orientation strongly indicate a the Bajío fault, and this suggests that the Penjamillo-Guadalajara region is a genetic relation with the opening of the Gulf of California (Ferrari et al., transitional zone with a diffuse deformation. 1997b; Ferrari and Rosas-Elguera, 1999). These structures are limited to the West of Guadalajara the Sierra Madre Occidental is separated from the south by a left-lateral normal accommodation zone that separates extension Jalisco block by two N55°W trending en echelon grabens (Ferrari and to the north from transpressional folding at the boundary with the Jalisco Rosas-Elguera, 1999) (Fig. 5). Bounding faults on both grabens display block (Fig. 5) (Ferrari, 1995). mainly dip-slip motions. However, older right-lateral transtensional motions are observed at some places (Ferrari et al., 1997b). In addition a middle Age of Faulting Miocene left-lateral transpression was documented in the southernmost part of the Sierra Madre Occidental and some of this deformation was probably Southern Mesa central. The age of the main depressions in the Mesa accommodated by strike-slip movement along major faults of the graben central is constrained by the volcanic units interbedded in the sedimentary (Ferrari, 1995). filling. The filling of the San Miguel de Allende basin contains a ca. 11 Ma Mesa central. Within the Mesa central there are three major fault sys- basalt the counterpart of which was identified in the range. However, the tems. The N30°E trending Villa de Reyes graben cuts the eastern part of the basalt shows less than 20% of the total displacement of the fault (Nieto- Mesa central. This is a symmetric graben but it is segmented by secondary Samaniego and Alaniz-Alvarez, 1994). Both the Aguascalientes and the El orthogonal graben and faults (Fig. 5). In the city of San Luis Potosí the sed- Bajío basins contain late Miocene basalts and undated ignimbrites. In the imentary deposits that fill the basin are 480 m thick and contain interbedded Villa de Reyes graben, the Panalillo rhyolite (Fig. 6), with age of 26.8 Ma, Oligocene ignimbrite sheets. A vertical offset of ~680 m was calculated us- was identified within the basin fill. According to these data, we consider that ing subsurface stratigraphic sections based on water exploration wells and major faulting in the southern Mesa central began in late Oligocene time. geologic mapping (Tristán-González, 1986; Martínez-Ruiz and Cuellar- The oldest Cenozoic unit exposed in Sierra de Guanajuato, the Eocene González, 1978). Guanajuato conglomerate, presents dramatic changes in thickness, varying The San Luis de la Paz–Salinas de Hidalgo fault system is a well-defined from 1500 m (Gross, 1975) to less than 100 m in adjacent areas. These varia- group of parallel faults that strike N50°W and dip southwest (Fig. 5). These tions, together with the presence of dikes, the intercalation of lava flows, and faults have mainly normal movements (Labarthe-Hernandez and Jiménez- the tilting of sandstone horizons, have been interpreted as an evidence of the López, 1993; Silva-Romo, 1996) with limited offset, and there are no basins inception of extension (Nieto-Samaniego, 1990; Randall-Roberts et al., associated with these faults. 1994). Andesitic dikes (Echegoyén-Sánchez et al., 1970), rhyolitic domes The third major fault system forms the northwest-trending aligned along fractures, and rhyolitic lava flows that cover normal faults Dolores Hidalgo–Ojuelos depression and cuts the Villa de Reyes graben record extension prior to 32 ± 1 Ma (Gross, 1975; Nieto-Samaniego, 1990). near San Felipe (Figs. 4 and 5). The basin is poorly defined and only few The tectonic episode with the higher deformation rate occurred in middle- bounding faults were mapped. Other minor faults in the southern Mesa Oligocene. The Veta Madre fault offsets 30 Ma rhyolite domes about 1500 m central have dominantly normal displacement. (Nieto-Samaniego et al., 1996) and is filled by minerals dated as 29.2 ± Sierra Madre Occidental Physiographic Province. The structural style 2 Ma (Gross, 1975). Post-Miocene extensional phases with lower intensity are of this province is dramatically different from that in the Mesa central. In- recorded by displacements of 200 to 600 m of an ignimbrite dated as 24.8 ± stead of two sets of orthogonal faults, several major parallel faults are found 0.6 Ma (Nieto-Samaniego et al., 1996) and basaltic lavas dated as 13 to 11 Ma (Fig. 5). In the eastern part of the province three nearly symmetric grabens (Table DR1, see footnote 1). strike north-northeast to north-south. In the western part of the province, In the San Luis Potosí region (Fig. 6) the first clear evidence of extension is north-northwest–trending half grabens and major listric faulting are domi- the emplacement of the San Miguelito rhyolitic domes and dikes along faults, nant (Fig. 5). which implies an extensional phase ca. 30 Ma. The main phase of extension The easternmost structure in the province is the Juchipila graben and its is documented by the displacement of ~500 m of the Cantera ignimbrite (29.0 eastward splay, the Calvillo graben. The Juchipila graben is a 80-km-long ± 1.5 Ma) (Tristán González 1986) and the emplacement of the El Zapote rhy- and 15-km-wide depression with a N10°E strike and a relief of as much as olite (27.0 ± 0.7 Ma) and Panalillo rhyolite (26.8 ± 1.3 Ma) through the faults 1000 m. The Calvillo graben, is a narrow, N15°E trending depression, with cutting the Cantera ignimbrite. Minor offsets of the Panalillo rhyolite a maximum relief of 900 m (Fig. 5). Its total length is 40 km and its average (Labarthe-Hernández and Jimenez-López 1994) record a minor phase of ex- width is 12 km. West of Juchipila, the Tlaltenango graben trends N10°E and tension in post-Oligocene time. has a length of 120 km and a width of 15 km, forming a depression of 400 The outlined stratigraphic relationships indicate that extension could m. The largest structure is the north-south Bolaños graben, which is 140 km have begun in Eocene time, at least in Guanajuato and San Luis Potosí, and long and 15–20 km wide. Here offsets of stratigraphic units commonly that three subsequent phases of extension can be identified: (1) a mild ex-

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010km101º 100º50' Continental sedimentary deposits (Oligocene-Holocene) Late Miocene basalt Cabras Basalt Panalillo Rhyolite (upper member) SAN LUIS POTOSÍ Andesite A' Panalillo Rhyolite (lower member) El Zapote Rhyolite

A Ignimbrites and andesites Cantera Ignimbrite

San Miguelito Rhyolite Oligocene volcanic group 22º00' Portezuelo Latite

Ojo Caliente Traquite Rhyolite domes Santa María Ignimbrite Eocene volcanic group Casita Blanca Andesite Paleogene continental sediments Cenicera Formation Mesozoic basement Caracol Formation

SLSH

21º45' 21º45'

Villa de Reyes graben A 101º 100º50' A'

2450 2150

m a.s.l. 1900

1km

Figure 6. Geologic map of Sierra de San Miguelito. Geologic mapping is modified after Tristán-González (1986), and faults are simplified after Labarthe-Hernández and Jiménez-López (1992, 1993, 1994). Formal units defined by Labarthe-Hernández 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 graben and San Luis de la Paz–Salinas de Hidalgo fault system (SLSH). Section is after Labarthe-Hernández and Jiménez-López (1994); the marker is the lower member of the Cantera Ignimbrite.

tensional phase before 30 Ma; (2) a well constrained phase constituting the mineralization event took place prior to 21 Ma and fills N30°E trending faults peak of extension between 30 and 27 Ma; and (3) minor phases after about that cut a 23.7 Ma ignimbritic succession with offsets of 100–200 m (Lyons, 24 and after about 11 Ma. 1988). The peak of deformation is associated with the north-south bounding Sierra Madre Occidental Physiographic Province. The age of deforma- faults of the graben, which displace a 21.3 Ma ignimbrite by more than 1 km tion in this province is less well-constrained than in the Mesa central but can and expose the mineralization (Lyons, 1988; Scheubel et al., 1988). Several be reasonably estimated in some places. In the Bolaños graben an important mafic dikes intrude the N30°E faults, in some cases feeding basaltic flows that

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Figure 7. Schmidt projection, lower hemisphere of faults measured in the southern Sierra Madre Occidental volcanic province. Contouring was performed with the Kamb method.

have been dated as 19.9 Ma at Veta Rica (Table DR1, see footnote 1). Field model proposed by Krantz (1988), in the Sierra de San Miguelito area, lo- evidence suggests that the dikes are synextensional and thus their age proba- cated southwest of San Luis Potosí (Fig. 6). Using these results we calcu- bly represents the peak of extension. The minimum age of faulting is not con- lated the direction and the amount of extension for the entire Mesa central strained by any geologic unit. An early Miocene age is deduced for the Tlal- (see Appendix 1 for a complete description). Principal extension was ori- tenango graben because the bounding faults cut an ignimbritic succession of ented 258°/12° with a magnitude of 20%. An extension of 11% also oc- about 23 Ma but do not affect a 21.8 Ma shield volcano inside the depression curred oriented 162°/02°, showing the three-dimensional character of the (Moore et al., 1994). The uppermost ignimbrite cut by the Sierra Alica–Jesus deformation. Maria half graben, dated as 23.5 Ma (Clark et al., 1981), provides a maximum Sierra Madre Occidental Physiographic Province. In this province age for this structure, but no ages are available for the ending of faulting. it is clear that deformation was two dimensional and that it affected an al- In the westernmost part of the province, the Pochotitán–San Pedro fault most horizontal and undeformed ignimbritic plateau. Thus we consid- system cuts ignimbrites as young as 19 Ma (Table DR1, see footnote 1), and ered it adequate to use fault-slip inversion methods to calculate the paleo- tilted hanging-wall blocks are covered by late Miocene basalts north of stress tensor. In the eastern part of the province we measured a total of Tepic. Several mafic dikes with ages between 11.9 and 10.9 Ma (Table DR1, 137 faults, 108 of which have good kinematic indicators. Fault trends see footnote 1) are intruded parallel with the fault system and are inferred to show a peak near N10°E (Fig. 7), which is also the trend of the faults at mark the major phase of extension (Ferrari and Rosas-Elguera, 1999). larger scale (Fig. 5). There is a minor concentration with an east-west di- In summary, these stratigraphic relations indicate two main phases of de- rection. We interpret these faults as secondary structures that accommo- formation: the first between ca. 21 and 18 Ma in the eastern part of the date local strain incompatibilities because they are nearly vertical and province and the second during middle to late Miocene in the western part show oblique- or strike-slip displacements. Furthermore, there are no of the province, adjacent to the Gulf of California. major faults belonging to this group, and the grabens are not interrupted σ by major transverse depressions. The minimum principal stress ( 3) - Amount of Extension tained with the inversion method proposed by Reches (1987) is oriented 099°/08°, nearly horizontal and orthogonal to the trend of the grabens Southern Mesa central. The Mesa central represents a good example of (Table 2). In the western part of the province we measured a total of 54 triaxial deformation style. Strong evidence of triaxial strain in the southern faults with good kinematic indicators. Using the same method we obtain σ Mesa central includes the following: (1) the rhombohedral arrangement of a N66°E trending 3 (Table 2), which is orthogonal to the dominant fault fault traces both at large scale (Fig. 6), and in the intersection of the Villa de trend (Fig. 5). Reyes graben with the San Luis de la Paz–Salinas de Hidalgo fault system To estimate the amount of extension we use the area-balance method de- (Fig. 6); (2) both fault systems show dip-slip displacement; (3) there is no scribed by Groshong (1994) (Fig. 8). Although there is no good strati- systematic crosscutting relation between the faults, indicating that the ac- graphic control, the method may work because there is a well-defined re- tivity was synchronous (Nieto-Samaniego, 1990; Labarthe-Hernández and gional reference level not disturbed by major faulting within the horsts Jiménez-López, 1992, 1994). (Figs. 5 and 8). Some minor fault zones between the major grabens are neg- The orientations of the principal strains were obtained, using the odd-axis ligible because they do not tilt the sequence and produced displacements

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Figure 8. Estimation of amount of extension along a profile perpendicular to the graben of the eastern Sierra Madre Occidental. The area vertically ruled is equal to the gray one. The figure shows the most consistent solution ac- cording to the geology of the region (see text for details). ε = Vertical exaggeration is 10x. Dashed lines represent in- ferred faults at depth. Lo = ini- tial length; Lf = final length.

∆

less than 100 m. We carried out many experiments varying the depth of de- age is found in the western part of the Sierra Madre Occidental, and exten- tachment and obtained 5 km as a realistic approximation. This corresponds sion appears to slightly postdate it. Extension direction changed orientation to a horizontal extension of 8% (Fig. 8). counterclockwise by ~30°, from NW81°SE to NE66°SW, culminating Extension in the western part of the province is obviously greater than to with the opening of the Gulf of California, which now bounds the western the east because the crust has been thinned into the oceanic rift of the Gulf edge of Sierra Madre Occidental. The trenchward pattern of tectonic- of California. We have not estimated the amount of extension in this region magmatic migration is clearly shown in Figure 9D, where dated volcanic because a reference level is lacking. 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 DISCUSSION AND CONCLUSIONS northern Sierra Madre Occidental volcanic province (e.g., Aranda-Gomez et al., 1997; Gans, 1997). Tectonic-Magmatic Evolution of the Southern Sierra Madre Occidental volcanic province Comparison with the Central and Northern Sierra 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 There are some peculiarities in the geometry and kinematics of extension blocks that behaved as roughly independent units. These blocks correspond in the southern Sierra Madre Occidental volcanic province. The first differ- to the physiographic provinces of the Sierra Madre Oriental, Mesa central ence concerns the size of the extended area. It has been long suggested that and Sierra Madre Occidental. The Sierra Madre Oriental is a relatively sta- the inner core of the Sierra Madre Occidental is a relatively unfaulted ble block, without volcanism and extension during the Cenozoic. The Mesa “horst,” separating the highly extended areas of the southern Basin and central is a thinned and elevated block bounded by the thicker and relatively Range and the Gulf of California (Henry, 1989; Henry and Aranda-Gomez, lower Sierra Madre Oriental and Occidental blocks (Fig. 3). Within these 1992). Although this appears to be true in Durango and southern Chi- blocks volcanism and deformation show marked differences in style and tim- huahua, our data indicate that this is not the case for the southern Sierra ing. Volcanism in the Mesa central occurred between about 30 and 25 Ma, Madre Occidental; here the crust appears pervasively faulted, and distin- and is mainly represented by rhyolitic lava domes capped by a thin ig- guishing Basin and Range from Gulf of California faulting is difficult both nimbritic cover. Deformation was three dimensional, with horizontal, ap- in space and in time (Fig. 1). Furthermore, the area affected by extension proximately east-west extension of as much as ~20%. Extension was in part and volcanism in the southern Sierra Madre Occidental volcanic province is concurrent with volcanism, but the peak of the deformation postdates the nearly twice as wide as in the central volcanic province, directly north of the principal volcanic event. study region. Volcanism and significant Oligocene–Miocene extension in Volcanism and deformation occurred later toward the west. By early the former extend as far as eastern San Luis Potosí and Querétaro, about 500 Miocene time a thick ignimbritic cover was emplaced in the eastern Sierra km from the present coast, whereas in the central Sierra Madre Occidental Madre Occidental, with rare rhyolitic lava flows and domes and subordi- volcanic province they are only observed in Durango and Sinaloa. nate mafic lava. Deformation was two dimensional, and the principal A second difference regards the direction of extension. In the northern stretching of ~8% was oriented approximately east-west. Extension was Sierra Madre Occidental, Gans (1997) reported a minimum extension of partly synchronous with volcanism, but its climax postdates the “ig- 90% in the northeast-southwest direction from 26 to 20 Ma followed by ap- nimbrite flare up.” During middle to late Miocene time, volcanism and ex- proximately east-west extension of 10%–15% between 20 and 17 Ma. In tension continued to shift toward the west-southwest. Arc volcanism of this the central Sierra Madre Occidental (Durango and Sinaloa) dominant ex-

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35 30 25 20 15 10 Escuinapa (Ma) Age C stored to their pre 12.5 Ma position for the amount proposed in Lonsdale (1991). Lines per- the amount proposed 12.5 Ma position for to their pre stored examples of computed half rate sea-floor spread- show pendicular to sea-floor isochrons zone rate south of the Shirley fracture half-spreading Note the systematic higher average ing. during Oligocene and early Miocene time (B) the close match be- intervals) (at 1 m.y. episodes pulses and of the main extensional volcanic tween the timing of high spreading on the mainland (B and C). Possible accommodation zone 15 with vergence reversal (Axen, 1995) 16 16 18 14 20 South of Shirley North of Shirley 14 43 mm/yr 47 mm/yr 20 16 22 18 24 25 20 Age (Ma) 26 22

34 mm/yr 30 hre rcueZone Fracture Shirley

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 28 (mm/yr) rate Half-spreading B A 24 Figure 9. Comparison between continental and offshore tectonics. The map in (A) shows tectonics. 9. Comparison between continental and offshore Figure the main continental structure with time of faulting and average direction of extension (from direction with time of faulting and average the main continental structure the magnetic deduced from pattern sea-floor isochron and the corresponding this work) was Atwater (1989) and Lonsdale (1991). Baja California and anomaly map of Severinghaus position considering that no embayment existed at the mouth of to its prerift restored re- and extinct ridges (thick double lines) were Sea-floor age (circled) Gulf of California.

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tension in late Oligocene to middle Miocene and in Pliocene–Quaternary Gulf of California plates configuration by closing the Gulf of California and time was east-northeast trending, with a minor episode of west-northwest restoring the right-lateral motion along the Tosco-Abreojos fault system extension in between (Aranda-Gomez et al., 1997). By contrast, two thirds (Fig. 9).This restoration places the Shirley Fracture Zone at the same latitude of the southern volcanic province (Mesa central and eastern Sierra Madre as the northern limit of the southern Sierra Madre Occidental volcanic Occidental) were affected by approximately east-west extension in province, and the vergence reversal in the Gulf of California area. Oligocene and early Miocene time, and only in middle to late Miocene time In Figure 9B we calculated the western half rate of spreading at the East Pa- did east-northeast extension affect the western Sierra Madre Occidental. cific Rise between 30 and 20 Ma along two flow lines to the north and to the A final difference concerns the tilt vergence in the western Sierra Madre south of the Shirley Fracture Zone. Considering a nearly symmetric spread- Occidental. South of lat 23° N blocks are ubiquitously tilted toward the east- ing at the rise (Lonsdale, 1991), one can take these values as representative of northeast, whereas to the north, in Sinaloa, they are systematically tilted in the eastern (Farallon) half-spreading rate. These rates show two rapid in- the opposite direction (Henry, 1989; Aranda-Gomez et al., 1997). This ver- creases, at 30–26 Ma and 24–20 Ma that match the two main episodes of vol- gence reversal implies that a reverse accommodation zone must exist some- canism and extension in the Sierra Madre Occidental volcanic province where in the Escuinapa area (Fig. 9). Axen (1995) suggested a possible (Fig. 9B and C). Given that the trench has actually moved oceanward since matching structure on the other side of the Gulf of California, separating Mesozoic time (Lightgow-Bertelloni and Richards, 1998), we propose that in- hanging wall and footwall segments along the Main Gulf in creases of the spreading rate at the Eastern Pacific Rise were a consequence of southern Baja California (Fig. 9A). increases in the subduction rate at the trench and, therefore, that the Faral- lon–North America boundary west of the southern volcanic province was a Causes of Extension in the Southern retreating subduction boundary. Spreading south of the Shirley Fracture Zone Sierra Madre Occidental volcanic province 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 In general, extension in continental volcanic arcs is caused by the com- occurred much farther inland than in the central volcanic province. Along this petition among lithospheric body forces, such as those related to the gravi- part of the Farallon–North America plate boundary, the difference between tational and thermal effects of magmatism (e.g., Gans et al., 1989), and plate subduction and convergence rate probably exceeded a critical threshold boundary forces, related to the relative motion vectors between subducting needed to induce extension in the overriding plate. and overriding plates or to direct interaction between diverging oceanic and These observations led us to conclude that, at a continental scale (over continental plates (e.g., Bohannon and Parsons, 1995). 100 km of length), timing, magnitude, and orientation of extension as well Magmatism can induce extension in two ways: 1) injection of mafic mag- as age of volcanic episodes in the Sierra Madre Occidental volcanic mas at the base of the crust may induce a buckling of the lithosphere be- province were ultimately controlled by plate tectonic forces, namely by the cause of the accumulation of low-density hot material; 2) rapid piling up of latitudinal variation of the retreating subduction boundary between the Far- volcanic rocks at the surface can build a topographically elevated region, allon and North America plates. where the vertical lithostatic stress may exceed the regional horizontal On a regional to local scale (less than 100 km of length), we believe that stress. At a regional scale, deformation induced by high topography is likely the variations in the tectonic and volcanic style in the southern Sierra Madre to be bidimensional, with extension orthogonal to the axis of the volcanic Occidental volcanic province were controlled by the difference in the struc- arc, whereas deformation related to magmatic doming may be three dimen- ture of the Mesa central and Sierra Madre Occidental crustal blocks and by sional and close to radial extension. In both cases, however, extension is lithospheric body forces. In the Mesa central, the existence of a prefractured likely to affect only the upper part of the crust. crust and a moderate magmatic rate induced a three-dimensional strain, a However, extension in the overriding plate can be induced by a “retreat- complicated fault geometry, and the emplacement of silicic as rhy- ing subduction boundary,” such as that where the subduction rate exceeds olitic domes. We believe that, in this case, plate boundary forces and body the convergence rate (Jarrad, 1986; Royden, 1993), and this deformation is forces are equally responsible in controlling the dynamics of extension and usually accommodated along the volcanic arc (Hamilton, 1995). Extension magmatism. In the Sierra Madre Occidental, a stronger and relatively ho- driven by a retreating subduction boundary is expected to involve the entire mogeneous crust and a high magmatic rate caused a bidimensional strain, lithosphere, and deformation is expected to be bidimensional if the litho- regularly spaced faults, and the formation of . In this case - sphere is homogeneous. The extension direction, however, will depend tism was probably the dominant cause of extension. upon the obliquity of the convergence and the occurrence or not of strain We consider that the silicic volcanism flare-up was triggered by the com- partitioning. In addition, a retreating slab will generate a flux of hotter bined effect of extensive mafic underplating and the beginning of upper asthenospheric material into the opening mantle wedge, producing wide- plate extension induced by the slab retreat. 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 CONCLUSIONS volcanism in the overriding plate. A detailed discussion of the relative role of body forces and plate boundary Our main conclusions can be summarized as follows: forces in determining the tectonic-magmatic evolution of the southern Sierra 1. In the southern Sierra Madre Occidental volcanic province we observed Madre Occidental volcanic province is beyond the scope of this paper. How- a clear pattern of trenchward shifting of the climax of subduction volcanism ever, the data presented herein permits at least a qualitative analysis of the is- and extension during three distinct episodes in middle Oligocene, early sue. The late Miocene episode of extension (proto-Gulf of California) was Miocene, and late Miocene time. In all the episodes, the peak of extension clearly related to the capture of the Magdalena microplate and Baja California slightly postdates volcanism. by the Pacific plate (e.g., Stock and Hodges, 1989). Thus, plate tectonics was 2. Tectonic and volcanic style, as well as fault pattern, vary strongly from the dominant factor governing the latest period of extension. However, the east to west. first two episodes of extension occurred when remnants were 3. Extension and volcanism in the southern volcanic province took place still subducting, magmatism was still active, and both kinds of forces were ac- much farther inland than in the central volcanic province. tive. To evaluate the role of plate boundary forces, we reconstructed the pre- 4. There is a very good correlation between the timing of extensional and

360 Geological Society of America Bulletin, March 1999

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A Unrotated

SSM faults VRG faults 227º/48º 120º/65º k = -0.52

odd= =071º/77ºe odd, e 3 3 similar=e 1 =258º/12º VRG striae intermediate, e 097º/63º 2 a =36º

SSM striae 216º/47º similar, e 90º 1 a intermediate=e 2 =168º/02º

Corrected for rotation B

SSM faults VRG faults 228º/70º 120º/65º k = -0.49

odd= =064º/78ºe 3 similar=e =259º/11º 1 VRG striae 097º/63º a =35º

SSM striae 213º/69º

90º

intermediate=e 2 =169º/02º

Figure A1. Application of the odd-axis model (Krantz, 1988) to the Sierra de San Miguelito area. The model is based on the Reches’s slip-model and considers four fault planes with orthorhombic symmetry. Simultaneous motion of the faults accommodates a triaxial nonrotational defor- mation. The odd axis is the one with opposite “sign” (shortening of stretching) with respect to the other principal axes, assuming no volume ε ε ε ε ε α 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 an- gle between the fault traces on the intermediate-similar plane. The Krantz model holds for nonrotational faulting. However, we consider that it can be applicable to the San Miguelito area. In any case, correcting the data for the rotation produces nearly identical results in the similar axis orientation (cf. A and B). Data in B were rotated 22° counterclockwise about an axis 311°/00°.

κ≈ µ magmatic pulses and periods of rapid spreading at the East Pacific Rise in –0.4 and a coefficient of friction = 0.8. In order to estimate the post-Oligocene strain we need to assume the principal strain the offshore region. orientations obtained for Sierra de San Miguelito as valid for all the Mesa central, based 5. The good match among continental and oceanic tectonic events sug- on the similarity in strike of the principal fault systems (cf. Figs. 5 and 7). The crust of gests that boundary conditions (i.e., plate tectonics) ultimately determine the southern Mesa central is thinner and more elevated relative to the adjacent Sierra the timing, magnitude, and orientation of extension. Madre Oriental and Occidental crustal blocks. A reasonable assumption is to consider that Mesa central had a thickness similar to that of the adjacent crustal blocks prior to 6. The style of tectonics and volcanism and the distribution of faulting are the deformation. The symmetry of crustal blocks shown in Figure 3 suggests that de- controlled by the internal structure of the crustal blocks and by the gravita- formation within the Mesa central was produced by the relative movement of the Sierra tional and thermal effects of magmatism. Madre Oriental and Occidental crustal blocks, as supported by the orientation of the bounding structures (Aguascalientes graben and San Miguel de Allende–Catorce fault APPENDIX I system), approximately north-south (Fig. 5), and by the approximately east-west orien- ε tation of the maximum principal strain ( 1) obtained in the Sierra de San Miguelito. The crust in the Sierra Madre Occidental is 40 km thick, and we estimated 8% stretching during Cenozoic time. Considering the crustal thickness before deformation of the ON THE ESTIMATION OF THE AMOUNT Mesa central to be equal to the original thickness of the Sierra Madre Occidental, OF EXTENSION IN THE MESA CENTRAL ε l0 = 43 km, and a final thickness lf = 32 km, the vertical principal elongation ( 3) is ε = (l –l )/l = –0.25, where the negative sign indicates that shortening occurred in the The method of Krantz (1988) was used to determine the principal directions and the 3 f 0 0 vertical direction. To calculate the horizontal principal elongations we need to assume strain ratio κ = ε /ε = tan2α, where ε are the principal elongations, and ε > ε > ε 2 3 i 1 2 3 no volume change. Considering an undeformed cube with unit edge and applying to it (Fig. A1). We obtained a value of κ = ––0.52. (the negative sign indicates that ε is a 3 a strain tensor with principal strains without volume change, we obtain shortening 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- + εεε+ + = (1) soid, the principal plane that contains the maximum and intermediate strain axes is (111123)( )( ) 1. nearly horizontal. We also used the graphics proposed by Reches (1983), obtaining

Geological Society of America Bulletin, March 1999 361

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Using the mean value for strain ratio κ = –0.45 in equation 1 and introducing the Carrillo-Bravo, J., 1971, La Plataforma Valles–San Luis Potosí: Boletín de la Asociación Mexi- ε ε ε ε cana Geólogos Petroleros, v. 23, 102 p. calculated value 3 = –0.25, we obtained 1 = 0.2, 2 = 0.11, and 3 = –0.25. These values represent the bulk deformation of southern Mesa central, without considering Centeno-Garcia, E., Ruiz, J., Coney, P. J., Patchett, P. J., and Ortega-Gutiérrez, F., 1993, Guerrero the material transfer due to the magmatism. terrane of Mexico: Its role in the Southern Cordillera from new geochemical data: Geology, v. 21, p. 419–422. Some comments are needed regarding the use of the odd-axis model for Sierra de Chiodi, M., Monod, O., Busnardo, R., Gaspar, D., Sanchez, A., and Yta, M., 1988, Une discor- San Miguelito. The San Luis de la Paz–Salinas de Hidalgo fault system shows a dance ante Albienne datée par une faune d’ámmonites et de braquiopodes de type Téthysien domino style with beds dipping 20° to the northeast (Labarthe-Hernández and au Mexique Central: Geobios, v. 21, p. 125–135. Jiménez-López, 1992, 1994). This bed inclination implies a rotational component in Clark, K. F., Damon, P. E., Shafiquillah, M., Ponce, B. F., and Cardenas, D., 1981, Sección ge- the deformation tensor, whereas the odd-axis model supposes an irrotational strain. ológica-estructural a través de la parte sur de la Sierra Madre Occidental, entre Fresnillo y Nevertheless, we use the odd-axis model for the following reasons: la costa de Nayarit: Asociación Ingenieros Mineros, Metalurgicos y Geólogos de México, 1. We try to estimate the strain for all the southern Mesa central, whereas tilting is Memoria Técnica, v. XIV, p. 69–99. local. The rotation of 20° observed in the beds of Sierra de San Miguelito is not wide- Couch, R. W., Ness, G. E., Sánchez-Zamora, O., Calderon-Riveroll, O., Doguin, P., Plawman, T., Coperude, S., Huehn, B., and Gumma, P., 1991, Gravity anomalies and crustal structure of spread. The bulk of Sierra de San Miguelito is only tilted ~5°. Adjacent to the Sierra the Gulf and Peninsular Province of the Californias, in Dauphin, J. P., and Simoneit, B. R. T., de San Miguelito there are other faults and grabens parallel to the San Luis de la eds., The Gulf and the Peninsular Province of the Californias: American Association of Pe- Paz–Salinas de Hidalgo fault system (Bledos and Enramadas grabens, Fig. 7), that do troleum Geologists Memoir 47, p. 47–70. not produce significant tilting. At a regional scale tilting produced by the major Damon, P. E., Nieto-Obregon, J., and Delgado-Argote, L., 1979, Un plegamiento neogénico grabens is <10°. en Nayarit y Jalisco y evolución geomórfica del Rio Grande de Santiago: Asociación In- 2. Rotation introduced error in the odd-axis model only in the orientation of the genieros Mineros, Metalurgicos y Geólogos de México, Memoria Técnica, v. XIII, slip vector of the San Luis de la Paz–Salinas de Hidalgo fault system. It is possible to p. 156–191. correct this error (Fig. A1), but the variation produced by rotation does not affect the Demant, A., 1978, Características del Eje Neovolcánico Transmexicano y sus problemas de in- terpretación: Universidad Nacional Autónoma de México, Instituto de Geología, Revista, result significantly because it is less than the variation due to the quality of the geo- v. 2, p. 172–187. logic data. Echegoyén-Sánchez, J., Romero-Martínez S., and Velázquez-Silva, S., 1970, Geología y We conclude that the orientations of the principal strains estimated using the odd- yacimientos minerales de la parte central del Distrito Minero de Guanajuato: Consejo de axis model are sufficiently good, in relation to the quality of our geologic data. Recursos Naturales No Renovables (México), Boletin, v. 75, 36 p. Edwards, J. D., 1955, Studies of some early Tertiary red conglomerates of central Mexico: U.S. Geological Survey Professional Paper 264-H, 183 p. ACKNOWLEDGMENTS Fensby, S., and Gastil, G., 1991, Geologic-tectonic map of the Gulf of California and surround- ing areas, in: Dauphin, J. P., and Simoneit, B. R. T., eds., The Gulf and the Peninsular Province of the Californias: American Association of Petroleum Geologists Memoir 47, This work was supported by grants CONACYT P152T (to Ferrari) and p. 79–83. 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