mch089.qxd

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Geological Society of America

Map and Chart Series MCH089

2002

Geology of Nevado de Toluca Volcano and

surrounding areas, central Mexico

*Armando García-Palomo, José Luis Macías, José Luis Arce

Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán 04510, México D.F., México

Lucia Capra

Instituto de Geografía, Universidad Nacional Autónoma de México, Coyoacán 04510, México D.F., México

Victor Hugo Garduño

Departamento de Geología y Mineralogía, Instituto de Investigaciones Metalúrgicas,

Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán, México

Juan Manuel Espíndola

Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán 04510, México D.F., México

ABSTRACT

Nevado de Toluca is an andesitic-dacitic stratovolcano of Pliocene-Holocene age

located in central Mexico. The volcano is built on a complex sequence of metamorphic

and sedimentary formations of Jurassic-Cretaceous age, rhyolitic ignimbrites of late

Eocene age, and massive andesitic lava flows of late Miocene. In the northwest corner

of the map area, on top of this basement sequence, a complex andesitic-dacitic strato-

volcano, San Antonio, and a series of andesitic-dacitic domes and cones of Pliocene–

early Pleistocene age were also built. The first andesitic-dacitic emissions of Nevado de

Toluca occurred 2.6 Ma and continued during late Pleistocene–Holocene time contem-

porarily with basaltic to dacitic emissions of the Chichinautzin Volcanic Field in the

eastern parts of the map area.

Volcanism in the area has been controlled by the interplay of three fault systems

active since late Miocene. These systems, from older to younger, are the Taxco-Querétaro

Fault System (NNW–SSE), the San Antonio Fault System (NE–SW), and the Tenango

Fault System (E–W). Nevado de Toluca was built at the intersection of these three fault

systems, which have influenced its volcanic history as evidenced by at least three sector

collapses and several large explosive eruptions. The Pliocene to Holocene volcanism at

Nevado de Toluca and the late Pleistocene–Holocene activity at Chichinautzin Volcanic

Field, together with the regional tectonic activity and recent seismic swarms, suggest that

the Tenango Fault System represents an active segment within the Trans-Mexican Vol-

canic Belt. The potential reactivation of either this fault system or Nevado de Toluca Vol-

cano would pose earthquake and/or volcanic hazards to more than 25 million inhabitants

in the vicinity, including large cities such as Toluca and Mexico.

García-Palomo, A., Macías, J.L., Arce, J.L., Capra, L., Garduño, V.H., and Espíndola, J.M., 2002, Geology of Nevado de Toluca Volcano and surrounding areas,

central Mexico: Boulder, Colorado, Geological Society of America Map and Chart Series MCH089, 26 p.

*Present address: Instituto de Geologia, Universidad Nacional Autónoma de

México, Coyoacán 04510, México D.F.; E-mail: [email protected]

INTRODUCTION

The Trans-Mexican Volcanic Belt (TMVB) is a 1200-km-

long continental arc, predominantly of andesitic-dacitic composi-

tion, extending westward across central Mexico and ending

nearly at the Gulf of Mexico (Fig. 1). Volcanoes of the Trans-

Mexican Volcanic Belt are generally viewed as resulting from the

subduction of the Cocos and Rivera plates beneath the North

American plate along the Middle American Trench (MAT)

(Ponce et al., 1992; Singh and Pardo, 1993; Pardo and Suarez,

1993, 1995). Other authors suggest that the Trans-Mexican Vol-

canic Belt is related to a crustal fracture zone or to a mega-shear

(Cebull and Shurbet, 1987). More recently Márquez et al. (1999)

have proposed the existence of a mantle plume and the subduc-

tion of the Cocos and Rivera plates as coexisting mechanisms to

explain several geochemical features of the belt. Sheth et al.

(2000) argue against this hypothesis, and proposed rather that the

Trans-Mexican Volcanic Belt represents a rift-like structure that is

experiencing active extension.

Recent petrological studies, particularly Osmium isotopic

analyses, reveal that magmatism of the Trans-Mexican Volcanic

Belt shows assimilation of the lower crust (Chesley et al., 2000).

These results correlate with the study of the Chichinanautzin

Volcanic Field (CVF) by Verma (2000), who concluded that

andesites and dacites were derived from partial melting of an het-

erogeneous mafic granulite from the lower crust, while alkaline

magmas are mantle derived mafic-magmas.

The beginning of volcanic activity in the Trans-Mexican Vol-

canic Belt has been controversial. In the western part of the

Trans-Mexican Volcanic Belt, Gastil et al. (1979) placed the

beginning circa 4.5 Ma, while Allan (1986) placed the oldest

known calc-alkaline volcanism at 10 Ma. According to Mooser et

al. (1974), volcanism in the central Trans-Mexican Volcanic Belt

started 30 Ma ago, whereas in the eastern Trans-Mexican Vol-

canic Belt, Cantagrel and Robin (1979) suggested a beginning

some 20 Ma ago. From stratigraphic studies in western and cen-

tral Trans-Mexican Volcanic Belt, Pasquaré et al. (1991) found

that the oldest units consisted of massive fissure-vent lava flows

(basaltic andesites to rhyolites in composition) to which they

gave the general name of “Basal Sequence.” Two samples from

this sequence yielded K-Ar ages of 8.1 Ma (sample from south

of Querétaro) and 7.8 Ma (sample from 15 km north of Morelia).

The Trans-Mexican Volcanic Belt is dominated by huge,

geologically young andesitic stratovolcanoes, some of which

form short north-south volcanic chains younger toward the south,

i. e., toward the trench. Examples are Cantaro-Nevado de Col-

ima-Colima, Telapón-Iztaccíhuatl-Popocatépetl, and Cofre de

Perote-Las Cumbres-Pico de Orizaba (see Fig. 1). K-Ar data for

the andesitic calc-alkaline volcanoes Sanganguey and San Juan

in the western Trans-Mexican Volcanic Belt indicate that cone

construction began circa 0.6 and 0.2 Ma, whereas in central

Trans-Mexican Volcanic Belt (Iztaccíhuatl-Nevado de Toluca)

cone growth began 1.7 and 2.6 Ma ago, respectively. This age

progression in the onset of growth of the andesitic volcanoes

along the Trans-Mexican Volcanic Belt and the trenchward

migration of volcanism suggest different subduction rates

between the Cocos and Rivera plates (Nixon et al., 1987; Pardo

and Suarez, 1995; Ferrari et. al., 1999).

In this work, we present the stratigraphy, distribution, and

structural setting of rock units around Nevado de Toluca Volcano

and surrounding areas. We summarize the petrological affinities

of the stratigraphic units as a basis for elucidation of the volcanic

evolution of the region, which in turn bears on the problem of the

beginning of volcanic activity in the Trans-Mexican Volcanic

Belt. Particular emphasis is given to the complex fault systems in

the region, which have affected the basement complex upon

which the andesitic-dacitic stratovolcanoes have been built since

the late Pliocene. For our study we reviewed several regional geo-

logical maps (De Cserna and Fries, 1981; Sánchez-Rubio, 1978;

Bloomfield,1975; Elías-Herrera,1993; Ramírez-Espinosa,1977).

The study area covers about 4000 km

, encompassing six quad-

rangle maps (scale 1:50 000) published by Instituto Nacional de

Estadística Geografía e Informática (INEGI). Geological data

were collected and compiled with the aid of these topographic

maps. The topographic contour lines, geological contacts and

faults, and cultural features were then combined into a single digi-

tal product at a 1:100 000 scale. For the description of the map

units, we follow customary usage in published works as employed

by García-Palomo (1998) and García-Palomo et al. (2000).

Tectonic setting of central Mexico

The Trans-Mexican Volcanic Belt consists of three distinc-

tive segments, each with its own tectonic, volcanological, and

petrological characteristics (Pasquaré et al.,1987) (Fig.1). These

segments are the western Trans-Mexican Volcanic Belt, domi-

nated by the Colima-Chapala-Tepic triple junction where both

alkaline and calc-alkaline volcanic products are present; the cen-

tral Trans-Mexican Volcanic Belt, characterized by disperse

andesitic-dacitic stratovolcanoes, silicic calderas, and mono-

genetic cone fields where calc-alkaline volcanism is dominant;

and the eastern Trans-Mexican Volcanic Belt, characterized by

andesitic-dacitic calc-alkaline stratovolcanoes and some alkaline

products. Under this scheme, the Nevado de Toluca is located

along the boundary between the central and eastern segments.

According to the tectonic model of Mexico proposed by

Johnson and Harrison (1990), Nevado de Toluca Volcano is

located within the Guerrero Block (Fig. 2). This block is confined

to the north by the Chapala-Tula fault system and to the south by

the Chapala-Oaxaca fault system. In this model, Nevado de

Toluca astrides a series of NNW–SSE trending normal faults,

forming a Basin-and-Range like pattern (Demant, 1978; Suter et

al., 1992), and a series of E-W trending normal faults (Fig. 2).

These two fault systems were originally identified as major frac-

tures in the Toluca–Mexico City area (Mooser, 1969; Mooser and

Maldonado-Koerdell, 1961). Based on satellite imagery, Flores

(1978) concluded that Nevado de Toluca Volcano lies at the inter-

section of NW-SE and NE-SW fault systems. However, in these

2 A. García-Palomo et al.

mch089 2 of 26

general studies no data are available on the kinematics of the fault

systems. Recently, García-Palomo et al. (2000) concluded that

Nevado de Toluca Volcano was built upon the intersection of

three fault systems named Taxco-Querétaro (NNW-SSE), San

Antonio (NE-SW), and Tenango (E-W), respectively.

Nevado de Toluca Volcano

The Nevado de Toluca Volcano (19° 09′N; 99° 45′W,

4 680 m.a.s.l.), the fourth highest peak in Mexico, is located

about 23 km southwest from the city of Toluca, capital of the

State of Mexico. Nevado de Toluca is also known as “Xinantécatl,”

a Náhuatl term meaning “Nude Man.” Romero-Quiroz (1959)

argued that this word does not have solid Náhuatl roots and pro-

posed instead the word Tzinacantépetl, or Bat Mountain. How-

ever, García-Martínez (2000), through comprehensive research

of the historical archives, recently concluded that the most appro-

priate Náhuatl name for Nevado de Toluca is “Chicnauhtécatl,”

meaning Nine Hills.

Nevado de Toluca is a large andesitic-dacitic stratovolcano

of Late Pliocene–Holocene age (Bloomfield and Valastro, 1974;

Cantagrel et al., 1981; this work). The northern flanks of the

mch089 3 of 26

Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico 3

Figure 1. Location of Nevado de Toluca

Volcano and other volcanic features of the

Trans-Mexican Volcanic Belt (TMVB).

Other Quaternary volcanoes are: 1—

Ceboruco, 2—La Primavera, 3—Colima,

4—Parícutin, 5—Los Azufres, 6—

Amealco, 7—San Antonio, 8—Jocotitlán,

9—Popocatépetl,10—Iztaccihuati. MAT—

Middle American Trench.

Figure 2. Map showing prominent NE–SW and NW–SE

trending fault systems associated with the Trans-Mexican

Volcanic Belt (after De Cserna et al., 1988; Johnson and

Harrison, 1990; Nieto-Samaniego et al.,1995). The heavy-

line box shows the map area in central Mexico. Volcanoes

shown are: PO—Pico de Orizaba, CP—Cofre de Perote,

NT—Nevado de Toluca, C—Colima, Ma—La Malinche,

TE—Telapon, TL—Tlaloc, I—Iztaccíhuatl, J—Jocotitlán,

and P—Popocatépetl. Cities shown are MC—Mexico

City, T—Toluca, PU—Puebla, Q—Querétaro, O—Oax-

aca, CHI—Chilpancingo, MO—Morelia, and G—Guadala-

jara. Fault systems shown include the Oaxaca fault (OF),

the Tenochtitlán fault system (TFS), the Chapala-Oaxaca

fault system (CHOFS), and the Chapala-Tula fault sys-

tem (CHTFS). Other structures are the Tepic Graben

(TG), the Colima Graben (CG), and the Penjamillo

Graben (PG) (after Siebe et al., 1999).

volcano are made of coalescent fans of pyroclastic materials

spreading toward the Lerma Basin. This basin encloses the City

of Toluca de Lerdo and the lakes of Chiconahuapan and Lerma,

which are drained by the Lerma River toward the Pacific Coast.

The relief of Nevado de Toluca with respect to the Lerma River

basin is 2100 m. The southern flanks of Nevado de Toluca consist

of incised valleys that drain to the Chontalcoatlán River and

thereafter to the Pacific Ocean. The volcano rises circa 2700 m

with respect to the towns of Ixtapan de la Sal and Tonatico

located on its southern flanks (Fig. 3).

The summit crater of Nevado de Toluca is roughly elliptical

(approximately 2 ×1.5 km) with its major axis trending E-W; it has

a horseshoe shape opened to the east (Fig. 4). The present crater

rim is composed of two or more remnants of older crater structures,

appearing in air photographs and satellite imagery as straight walls

intercepting at high angles. The western crater wall contains rem-

nants of lava flows and domes that are well observed toward the

uppermost part of the wall (Fig. 5). The crater contains two lakes

(Lake of the Sun and Lake of the Moon) separated by a dacitic

dome intrusion known as El Ombligo (Spanish word meaning

navel). The surface of floor of these two alkaline lakes (5.6 pH;

Armienta et al., 2000) is about 4,200 m.a.s.l., and both contain

diatom flora (Caballero-Miranda,1996). Pre-Hispanic pottery and

obsidian blades are commonly found on the surface of the central

dome and in the deep waters of the lakes; apparently these pottery

shards are the remains of offerings made by the Aztecs or by earlier

dwellers such as the Matlazincas (Quezada,1972).

The volcano’s morphology shows the effect of glacial

advances during the Holocene (Heine, 1988). Rock glaciers and

debris flows derived from glacial activity are well exposed on

both flanks of the present crater. Despite some hydrothermally

altered areas within the crater, there are no signs of modern

hydrothermal activity on the volcano.

STRATIGRAPHY

In our study area we have recognized and mapped seventeen

stratigraphic units, as shown in the geologic map and the compos-

ite stratigraphic column diagram on the map sheet (located above

the legend). A description of each unit follows.

Mesozoic

Guerrero Terrane. The Guerrero Terrane forms most of the

western part of Mexico, is one of the largest terranes of the North

American Cordillera, and is characterized by an Upper Jurassic-

Lower Cretaceous volcanic-sedimentary sequence of arc affinity

(Centeno-García et al., 1993). In the study area, the Guerrero

Terrane consists of three formations: the Ixtapan-Teloloapan

sequence, the Acuitlapan Formation, and the Amatepec Forma-

tion. These formations are described below.

The volcano-sedimentary metamorphic Ixtapan-Teloloapan

sequence consists of conglomerates, limestones, sandstones, and

lava flows affected by greenschist facies metamorphism (Campa

et al., 1974). Based on the presence of ammonites and other

invertebrates, Campa et al. assigned this unit to late Jurassic-early

Cretaceous. This sequence is exposed in the southern portion of

the area close to Ixtapan de la Sal, Zumpahuacán, and Coatepec

de Harinas, where it consists of long narrow outcrops oriented

NW–SE. Its base is not exposed in the area, and it is overlain by

Tertiary and Quaternary volcanic rocks.

In the Taxco region, the Acuitlapan Formation consists of

interbedded gray to green shales and poorly sorted graywackes

formed by feldspar and quartz grains derived from metamor-

phic and volcanic rocks (Fries, 1966). In the Santa Rosa area,

this formation consists of meter-thick interlayering of meta-

morphosed carbonates, arenites, and lutites. These rocks are

well bedded, foliated, rather friable, and weather to a buff or tan

color. The calcareous portion is gray and more resistant to

weathering than the other lithologies. The main outcrops of this

formation are located to the southwest of the study area. Accord-

ing to its ammonite assemblage, the age of this unit is early

Jurassic (Tolson, 1993).

The Amatepec Formation (De Cserna, 1983), which is

exposed in the southwestern portion of Nevado de Toluca Vol-

cano, consists of dark, bluish-gray to black limestones with cen-

timeter-thick bands of white calcite and quartz. Locally, thin beds

of black silicic and slate rocks are intercalated with the lime-

stones, suggesting that the probable protolith of these rocks was

pelagic micritic limestone, with thin cherts and shales. The lime-

stone is dark gray, with thin and laminated beds with dark-gray

shale intervals. The age of the Amatepec Formation is Cretaceous

(Albian-Cenomanian) (De Cserna, 1983).

Guerrero Morelos Platform. The Guerrero Morelos Plat-

form includes the Morelos and Mexcala formations of Tithon-

ian-Coniacian age (De Cserna et al., 1974a). This map unit

consists of massive to thickly bedded limestones and dolomitic

limestones associated with calcareous terrigenous rocks toward

the upper portion of the sequence. In the study area it is exposed

around Zumpahuacán, where it forms NNW–SSE trending

open anticline-syncline folds with the same alignment. Here,

the Morelos Formation is exposed, and consists of light-gray

dolomitic limestones with chert bands of massive to metric bed-

ding stratification. The thickness of the unit varies from place to

place from few meters up to 1000 m (De Cserna and Fries,

1981). This unit is in normal-fault contact, with the volcanic

rocks of the Tepoztlán Formation of middle Miocene age.

Cenozoic: Eocene

Intrusive Felsic Igneous rocks. This unit includes a small

stock of quartz-rich porphyritic dacite dated at 55 ± 6 Ma by

De Cserna et al. (1974b) (Table1). This intrusive body is located

in the southwestern part of the mapped area close to the towns of

Almoloya de Alquisiras and Malinaltenango. They form an elon-

gate dome-like structure with northwesterly trend, the maximum

elevation of which (~2,000 m.a.s.l.) rises to about 300 m above

the depressions filled with volcaniclastic sediments to the east.

4 A. García-Palomo et al.

mch089 4 of 26

mch089 5 of 26

Figure 3. Landsat image of the map area showing NT—Nevado de Toluca, SA—San Antonio Volcano, TA—Tenango Andesite, T—City of Toluca,

IX—Ixtapan de la Sal, To—Tonatico, VG—Villa Guerrero, C—Coatepec, and M—Malinalco.

Cenozoic: Eocene-Oligocene

The Balsas Formation. The Balsas Formation consists of

calcareous conglomerates, lava flows, sandstones, volcanic silt-

stones, and lacustrine deposits (Fries, 1956; 1960), with a maxi-

mum thickness of 500 m (De Cserna and Fries, 1981). This

formation ranges in age from Upper Eocene to Early Oligocene

and probably represents a late stage of Laramide tectonism that

affected the pre-Cretaceous rocks of this region. Its distribution

within the map area is limited to scarce faulted and tilted outcrops

southeast of the town of Malinalco. Here, the Balsas Formation is

represented by red polymictic conglomerates that consist of

rounded to subrounded clasts (1-5 cm in diameter) set in a red-

dish matrix of silty sand. The formation overlies the limestones of

the Morelos Formation with an angular unconformity.

The Tilzapotla Formation. The Tilzapotla Formation was

originally described as a sequence of rhyolites, rhyodacites,

dacitic lava flows, and pyroclastic flow deposits of early Oligo-

cene age as its lower age limit (Fries, 1960). Recently, Morán-

Zenteno et al. (1999), in their study of the Tertiary Volcanic

Province of Southern Mexico (TVPSM), recognized three series

of rocks distributed south of the Trans-Mexican Volcanic Belt:

the Taxco volcanic sequence of rhyolitic composition (31-38 Ma,

Table1); the Buenavista-Quetzalapa volcanic sequence of mainly

andesitic-dacitic composition (24-31 Ma, Table 1) but including

the Tilzapotla Rhyolite; and the Huautla volcanic sequence of

dominantly andesitic composition as yet not dated.

Outcrops of this formation are best exposed in the southern

part of the map area near the towns of Porfirio Díaz and Chiltepec,

where they form large and extensive plateaus, locally intruded by

felsic dikes. In these localities, the Tilzapotla Formation is com-

posed of several units of pink ignimbrite, as thick as 300 m, with

white pumices and crystals of quartz, plagioclase, and biotite

imbedded in a vitric matrix.

Based on its stratigraphic position, Fries (1960) proposed the

age of the Tilzapotla Formation as early Oligocene. Rocks from

this Formation yielded K-Ar ages of 26 Ma (Fries, 1960), 31.9

± 0.9 Ma (Morán-Zenteno et al., 1999), and 49 ± 3 Ma (Linares

and Urrutia-Fucugauchi, 1981) (Table 1). We obtained a K-Ar

date of 38.3 ± 1.0 Ma for a sample from this sequence. This age,

together with the areal distribution of these rocks in the southern

portion of the study area, just north of the City of Taxco, suggest

that this unit correlates with the lower unit (Acamixtla Forma-

tion) of the Taxco volcanic sequence dated at 38.2 ± 1 Ma (K-Ar

method) (Morán-Zenteno et al., 1999).

The Tilzapotla Formation unconformably overlies the Bal-

sas Formation and is overlain with parallel unconformity by the

lava flows of the Basal Sequence.

Cenozoic: Miocene

The San Nicolás Basaltic Andesite. The San Nicolás Basaltic

Andesite is composed of a series of dark-gray, vesicular lava

flows with thickness as great as 100 m (García–Palomo et al.,

2000). They are of basaltic-andesitic composition, aphanitic tex-

ture, and contain zeolite-filled vesicles. The rock is hypocrystalline

and is characterized by an intersertal texture with scarce pheno-

crysts of orthopyroxene, clynopyroxene, olivine, plagioclase set

in a groundmass rich in microlithes of the same minerals, oxides,

and glass. The olivine and pyroxenes have reaction borders to

6 A. García-Palomo et al.

mch089 6 of 26

Figure 4. Aerial view to the west of Nevado

de Toluca with the crater open to the east.

The dacitic central dome “ombligo”

appears in the central part of the crater as

well as the Moon lake, whose surface

stands at ca. 4200 m.a.s.l. (photo by A.

Herrera).

iddingsite. The primary oxide minerals are titanomagnetite and

chromite, and spinel occurs as inclusions in phenocrysts.

These andesites crop out in the vicinity of San Nicolás, a

town located SE from Nevado de Toluca, where they overlie the

Tilzapotla Rhyolite in sharp erosional contact, which may repre-

sent a gap of approximately 10 Ma. García-Palomo (1998)

obtained a K-Ar age of 21.6 ± 1.0 Ma for this unit, corresponding

to early Miocene (Table1).

The Tepoztlán Formation. The Tepoztlán Formation con-

sists of massive lahars rich in subrounded porphyritic andesite

clasts intercalated with fluvial deposits (Fries, 1960; De

Cserna and Fries, 1981). This formation is widely exposed

south of the Trans-Mexican Volcanic Belt in the proximities

of the towns of Tepoztlán, Cocoyoc, and Oaxtepec. The flu-

vial deposits show a wide diversity of stratification types

(cross-bedded, normal grading, etc.), syn-sedimentary faults,

and occasionally, clastic dikes and cut and fill structures as

observed at the entrance of the Malinalco pre-Hispanic ruins.

This map unit is well exposed in the vicinity of the village of

Malinalco and in scattered sites southeast of Nevado de

Toluca (Fig. 6). The total thickness of the Tepoztlán Forma-

tion varies from 200 to 700 m, although close to Malinalco it

pinches out between the San Nicolás Basaltic Andesite and the

overlying Basal Sequence. The exact age of this formation is

mch089 7 of 26

Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico 7

Figure 5. View of the northwest wall of the

Nevado de Toluca crater interior showing

dark-gray lava flows and domes exposed

toward the uppermost portion of the wall.

The Sun Lake appears in the foreground of

the photograph (photo by J.L. Macías).

Figure 6. View of the upper part of the

Tepoztlán Formation at the pre-Hispanic

ruins of Malinalco. Around AD 1400, the

archaeological site of Malinalco was used

by the Aztecs as a training site for their

warriors. This site was carved in lacus-

trine and hyperconcentrated flow deposits

of the Tepoztlán Formation (photo by J.L.

Macías).

still uncertain, but Fries (1960) assigned it to Early Miocene.

The K-Ar dates of the San Nicolás Basaltic Andesite (21.6

± 1.0 Ma, Table 1) and the Basal Sequence above it brackets

this unit within the middle Miocene (García-Palomo, 1998).

The Basal Sequence. The Basal Sequence consists of

interbedded dark-gray lava flows and breccias of andesitic com-

position (García-Palomo et al., 2000). These rocks commonly

have reddish weathered surfaces, show cooling fissures, and

reach a total thickness of 400 m. This sequence is extensively

exposed in the western and eastern parts of Nevado de Toluca

and forms the Tenancingo, Desierto del Carmen, and Chiltepec

ranges. Fries (1960) and Elías-Herrera (1993) grouped this

sequence within the Andesita Zempoala, a series of undifferen-

tiated rocks of late Miocene-Pliocene age. Our field observa-

tions suggest that this sequence is not related to the Andesita

Zempoala, because its age and lithologic characteristics are dif-

ferent. Mineralogically, rocks of the Basal Sequence typically

have microlithic to ophitic texture; and are composed essen-

tially of plagioclase (andesine-labradorite) and minor amounts

of hypersthene and hematite set in a microlithic groundmass

composed of the same constituents.

A whole-rock K-Ar date of this sequence yielded an age of

7.5 ± 0.4 Ma which places this unit in late Miocene (García-

Palomo, 1998; Table 1). According to its age, and because it

unconformably overlies the Tepoztlán Formation (García-

Palomo, 1998), the Basal Sequence can be correlated with rock

of the basaltic sequence of the Querétaro, Morelia, and Río

Grande regions (6-8 Ma) which are inferred to have formed during

the early episodes of formation of the Trans-Mexican Volcanic

Belt (Pasquaré et al., 1991; Ferrari et al., 1994).

The San Antonio Volcanic Sequence. We group under the

name of San Antonio Volcanic Sequence all rocks forming the

San Antonio Volcano (Sánchez-Rubio, 1978). The summit of the

volcano (19º10’ N, 99º52’ W; 3,680 m.a.s.l.) has a truncated

crater opened to the NE; the volcano is cut by NE-SW trending

faults and deep incised valleys (Fig. 3).

The main edifice of San Antonio is composed of a thick

sequence of lava flows, overlain by pyroclastic materials consist-

ing of thick pumice-rich plinian fall layers that are overlain by

pumiceous pyroclastic flow deposits and surges, such as those

observed at Loma de San Francisco (Fig. 7). Sequences of thick

block-and-ash flow deposits are also exposed on the northern

flanks of the volcano close to the town of Santa María del Monte.

A sample from the upper part of the San Antonio sequence

yielded an age 3.0 ± 0.2 Ma (Table 1). On the basis of its strati-

graphic position, the morphology of the edifice, and the perva-

sive fracturing and weathering of the rocks, we consider the early

stages of formation in middle Miocene.

The San Antonio lavas are porphyritic with as much as 40%

phenocrysts, mainly of plagioclase and hornblende, together with

lesser amounts of biotite, augite, olivine, or enstatite. The ground-

mass is hyalopilitic with small crystals of plagioclase, clinopy-

roxene, and Fe-oxides (Sánchez-Rubio, 1984).

Sierra de las Cruces Sequence. The Sierra de las Cruces

Sequence (SCS) is located in the eastern part of the study area

and separates the basins of Mexico and Lerma. The Sierra de las

8 A. García-Palomo et al.

mch089 8 of 26

TABLE 1. K/AR AND OTHER DATES FOR SELECTED ROCKS FROM THE MAP AREA

Map Unit Sample Rock Type Material dated 40*Ar (ppm) 40K (ppm) 40*Ar/K Age Ref

12b NT01 D WR 0.000148 1.965 0.000075 1.3 ± 0.1 Ma 7

12b Me19 A WR 1.3 ± 0.3 Ma 2

11 SJ B-A WR 0.000146 1.796 0.000081 1.4 ± 0.1 Ma 1

12b VnE5 A WR 1.44 ± 0.07 Ma 2

12b Ne20 A WR 1.6 ± 0.15 Ma 2

12a NT05 A WR 0.000301 2.017 0.000149 2.6 ± 0.2 Ma 7

8 SN1 A WR 0.000306 1.754 0.000174 3.0 ± 0.2 Ma 1

9 BMS3 B-A WR 0.000799 1.84 0.000434 7.5 ± 0.4 Ma 1

5BIBWR 0.000971 0.769 0.001261 21.6 ± 1.0 Ma 1

4BV-12 I Hb 0.000831 0.573 0.001450262 24.8 ±1.3 Ma 6

4BV-17 R Pl 0.00155 0.869 0.001783659 30.5 ± 1.1 Ma 6

4 TX-10 R Pl 0.001259 0.68 0.001851471 31.6 ± 1.2 Ma 6

4 TX-25 G WR 0.008059 4.315 0.001867671 31.9 ± 0.8 Ma 6

4SOL-5 D Bt 0.01753 9.372 0.001870465 31.9 ± 0.8 Ma 6

4 TX-21 R Bt 0.01588 8.364 0.001898613 32.4 ± 0.8 Ma 6

4 TX-16 G WR 0.01081 5.686 0.001901161 32.4 ± 0.9 Ma 6

4 En19 R WR 36.9 ± 1.3 Ma 4

4 CH1 R WR 0.00911 4.045 0.002252 38.3 ± 1.0 Ma 1

4 TX-4 G WR 0.009415 4.201 0.002241133 38.2 ± 1.0 Ma 6

M48-51 R WR 49 ± 3 Ma 3

Ig LG-22 D Pb/alpha /mg-hr=2522 Pb=55 (ppm) 55 ± 6 Ma 5

B—Basalt, A—Andesite, D—Dacite, R—rhyolite, G—Glass, I—Ignimbrite, WR—Whole Rock, Hb—Hornblende, Pl—Plagioclase, and Bt—Biotite.

García-

Palomo et al. (2000),

Bloomfield and Valastro (1987),

Linares and Urrutia-Fucugauchi (1981),

De Cserna and Fries (1981),

De Cserna et al. (1974b),

Alva-Aldave et al. (1996), and

This work.

Cruces Sequence was initially described by Schlaepfer (1968)

and Sánchez-Rubio (1984). This range has a general NNW–SSE

orientation. It is formed by several andesitic stratovolcanoes such

as San Miguel, La Corona, Picacho, Zempoala, and Ajusco. In

addition, there are other volcanic structures, mostly domes,

calderas, and scoria cones. Every volcano comprised within the

SCS is made up of pyroclastic flow and fall deposits, lava flows,

and thick sequences of lahar and avalanche deposits associated in

occasions with collapse structures. This sequence pinches out

against the lacustrine, fluvial, and alluvial deposits of the Lerma

Basin. Las Cruces sequence is capped by the younger rocks of

the Chichinautzin Volcanic field and rests discordantly on top of

the Tepoztlán Formation. The age of the Sierra de las Cruces

sequence is considerably variable, going from 13 to 6 Ma in the

northern part (Aguirre-Díaz and Carrasco-Hernández, 1999), 7 to

5 Ma at the Catedral caldera (Aguirre-Díaz et al., 1999), 2.87 to

0.39 Ma in the south (Mora-Alvarez et al., 1991), and to 3.4 to

0.6 Ma at Ajusco Volcano (Romero-Teran, 1998).

Pliocene-Pleistocene

Cones and domes complex. A single map unit, designated

as monogenetic lava cones and domes,

comprises all rocks from

a series of monogenetic volcanic structures distributed around

Nevado de Toluca and San Antonio volcanoes. The lava cones

characteristically have a basaltic andesitic composition and are

exposed in the San Miguel region, where they are crudely aligned

in a NE-SW direction (García-Palomo et al., 2000). Some of

these volcanic centers have open craters with collapse structures

toward the southwest (e.g., Las Palomas). The domes are

andesitic to dacitic, strongly affected by erosion and fractures,

and dispersed throughout the area. The dacites (e.g., San Jose) are

holocrystalline with trachytic texture composed of abundant

plagioclase microlithes that surround the scarce phenocrysts of

plagioclase and pyroxenes, quartz, amphibole, and olivine. Fe-

oxides are present as inclusions in phenocrysts and dispersed

throughout the sample.

A K-Ar date of the San Jose lava flow was obtained from a

whole-rock concentrated yielding an age 1.4 ± 0.1 Ma (Table1).

From this date and its stratigraphic position, we assigned a

Pliocene-Pleistocene age to this unit.

Pleistocene-Holocene

Nevado de Toluca Volcano. The first geological studies of

Nevado de Toluca Volcano were carried out during the first dec-

ade of the twentieth century (Ordoñez, 1902; Otis, 1902; Flores,

1906; Waitz, 1909). These authors described the general fea-

tures of the volcano, including its morphology, structure, and

petrology. Bloomfield and Valastro (1974, 1977) and Bloom-

field et al. (1977) were the first authors to define the individual

eruptions of the volcano on the basis of radiocarbon chronol-

ogy. They identified several events: a vulcanian eruption about

28 000 yr. BP that produced extensive blue-gray lahars; a plin-

ian eruption that deposited the Lower Toluca Pumice fall ca.

24 000 yr. ago; and another plinian eruption that generated the

Upper Toluca Pumice ca. 11600 yr. BP.

Cantagrel et al. (1981) divided the rocks forming Nevado

de Toluca into two main units. The oldest unit corresponds to a

series of light-gray porphyritic andesitic lava flows that consti-

tute the main edifice of the volcano. These rocks are common

accessory components in younger volcaniclastic and pyroclastic

units and are easy to identify because of their pink-to-red alter-

ation surfaces. The lavas are composed of plagioclase, clinopy-

roxene, and orthopyroxene set in a glass groundmass (Cantagrel

et al., 1981). These deposits crop out near the town of Raíces

mch089 9 of 26

Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico 9

Figure 7. Sequence of pumice fall (black

arrows) and pyroclastic flows exposed on

the northern flanks of San Antonio Volcano

(photo by J.L. Macías).

along the road connecting Coatepec de Harinas and the crater of

the Nevado de Toluca. The K-Ar age of these lavas falls

between 1.60 ± 0.12 and 1.23 ± 0.15 Ma (Pleistocene) (Canta-

grel et al. 1981; Samples Ne20, VnE5, and Me19, Table 1).

However, we recently collected a sample from a sequence of

lava flows exposed on the southern flank of Nevado de Toluca,

which yielded a K-Ar age of 2.6 Ma (sample NT05, Table 1),

being in consequence the oldest products of Nevado de Toluca

Volcano. These lavas were fed by a N-S dike system now highly

dissected and exposed by erosion at several outcrops. These

lavas are porphyritic andesites, with plagioclase, clinopyroxene,

and resorbed hornblende phenocrysts set in a groundmass com-

posed of glass and plagioclase microphenocrysts. The dikes

also cut through an older ignimbrite deposit, unfortunately so

altered that further study on its origin is impossible. We also

dated a dacitic clast found within a debris avalanche deposit

exposed in the southern flanks of the volcano; this clast is very

probably a fragment of the younger edifice and yielded an age

of 1.3 Ma (sample NT01, Table 1) in agreement with the ages

obtained by the above mentioned authors.

The younger unit of Cantagrel et al. (1981) is made up of a

complex sequence of pyroclastic deposits that mantle the flanks

of Nevado de Toluca Volcano. In the northern part, they form

coalescent fans, whereas in the south they were channeled into

NW-SE-trending tectonic basins. This unit is composed of volca-

niclastic deposits (debris flows and fluvial deposits), a debris

avalanche, a debris flow, pyroclastic flow, and surge and fall

deposits, all of which were emplaced during the last 100,000

years (Bloomfield and Valastro, 1974, 1977; Bloomfield, 1975;

Macías et al., 1997; Capra and Macías, 2000). The activity of

Nevado de Toluca between these two periods (~1.5 Ma and

<0.1 Ma) was presumably volcaniclastic, according to Cantagrel

et al. (1981). Numerous volcaniclastic, laharic, and minor lacus-

trine deposits are exposed in the southern flanks of Nevado de

Toluca, where deep canyons have been cut by erosion. In con-

trast, the northern flanks of the volcano have a rather youthful

morphology covered by younger deposits that bury older rocks.

We next summarize the Quaternary stratigraphic record of

Nevado de Toluca (Fig. 8).

Older debris avalanche (DAD-1) and cohesive debris flows

(PDF and MDF) deposits. Macías et al. (1997) first recognized

two debris avalanche deposits emplaced during the Pleistocene

toward the S-SE of the volcano (out to a maximum distance of

55 km). The older deposit (DAD-1, Fig. 9) is constituted by a sin-

gle unit, 10-m thick, massive, matrix supported, and monolitho-

logic with dacitic clasts that vary in diameter from centimeters to

1.5 m. Unfortunately, because outcrops of this older unit are

scarce, no additional information is available. In contrast, detailed

work by Capra and Macías (2000) showed that the younger sec-

tor collapse produced two large cohesive debris flows: the Pil-

caya (PDF) and the El Mogote (MDF) deposits. These covered

an area of 220 km

and involved a total volume of 2.8 km

(Fig. 10a). The PDF has a sharp basal contact, and a flat surface.

Only in the medial area is hummocky morphology present, with

mounds up to 20 m high. Its thickness varies from 6 m in the

proximal section to 40 m in the intermediate zone, where thick-

ening is abrupt (Fig. 10b); assuming an average thickness of

20 m, we obtained a total volume of 2 km

The PDF unit consists of a massive, matrix-supported

deposit containing megaclasts up to 15 m in diameter with jig-

saw puzzle structures. It is heterolithologic in composition and

contains fragments of the andesite and dacite of the older edifice

as well as rocks from the local bedrock, such as andesite and

basalt of the Miocene Basal Sequence, limestone of the Balsas

Formation, schist of the Ixtapan-Teloloapan Formation, rhyolite

from the Tizapotla Formation, conglomerate and lacustrine frag-

ments of the Tepoztlán Formation. The matrix is sandy (as much

as 80% sand fraction) and contains up to 16% clay (olinite-smec-

tite) in the proximal areas. Sorting improves and mean grain size

decreases downstream.

The MDF deposit rests directly on top of the Pilcaya unit

(PDF), cropping out at 40 km from the edifice and extending out

75 km with an average thickness of 6 m (Fig. 10B). It covers an

area of 120 km

for a total volume of 0.8 km

. It consists of a

massive, matrix-supported deposit, with clasts as large as 1 m in

diameter of the same composition of the Pilcaya deposit. The

matrix is sandy (as much as 78% sand) and contains as much as

6% in clay fraction.

From the textural and sedimentological characteristics of

these deposits, we infer that the flank failure occurred because of

intense hydrothermal alteration and tectonic dissection of the old

edifice of the Nevado de Toluca Volcano. Increase of pore pres-

sure, perhaps combined with seismic activity associated with

movements of the Tenango normal fault, could have liquified a

portion of the southern part of the edifice, resulting in a debris

avalanche that transformed directly into a cohesive debris flow.

Subsequent heavy rains then caused the remobilization of the

upper part of the PDF, triggering secondary lahars from which

the MDF deposited.

Pink Pumice Flow (42 ka). A thick, pink pumice-rich pyro-

clastic flow deposit is exposed around Nevado de Toluca Vol-

cano. The deposit consists of at least four units composed of

rounded to semirounded vesiculated pumice fragments set in a

sandy matrix, in which tree trunks have been found (Fig. 11, site

95). This unit represents the oldest radiocarbon-dated material at

42,030 +3530/

–

2445 yr. BP (Macías et al., 1997). Because no

other deposits indicative of dome-destruction events have been

found associated with the Pink Pumice Flow, we assume that the

eruption that produced it took place under open-vent conditions,

most likely after the collapse of the volcano that produced the Pil-

caya debris flow.

Block-and-ash flow deposits (37, 32, 28, 26, 14 ka). The

volcanic activity of Nevado de Toluca during late Pleistocene has

been characterized by cataclysmic eruptions, some of which are

related to the partial or total destruction of central dacitic domes.

Because these deposits are similar in appearance and compo-

nents, previous authors considered them as a single unit, which

they referred to as either “older lahar assemblages” (Bloomfield

10 A. García-Palomo et al.

mch089 10 of 26

and Valastro, 1974, 1977) or “Nueés Ardentes” (Cantagrel et al.,

1981). Bloomfield and Valastro (1977) estimated the age of the

deposits at ca. 28,000 yr. BP, based on a single

C date from a

paleosoil that covers the deposit. However, Macías et al. (1997)

identified two block-and-ash flows, the older of which was dated

as 37 000 ± 1125 yr. BP, which they correlated with the date of

35600 +2600/

–

1800 yr. BP for the “gray lahar” of Heine (1988)

and an underlying paleosoil dated at 38,000 yr. BP by Cantagrel

et al. (1981). The younger block-and-ash flow deposit yielded

ages of 28140 + 865/

–

780, and 28925 + 625/

–

580 yr. BP. These

ages correlate with the upper age limit proposed by Bloomfield

and Valastro (1977), who reported an age of 27580 ± 650 yr. BP

for a fluvial gravel on top of the flow deposit (Table 2).

The block-and-ash flows have entirely blanketed and modi-

fied the morphology of the volcano’s flanks and apron, and at

present most of the localities of these deposits are pits used for

the extraction of construction materials. The northern flanks of

the volcano consist of coalescent tongues of block-and-ash flow

deposits that produce a subdued morphology. All other flanks of

the volcano are highly dissected and show the action of tectonism

and strong erosion, with exposure of the block-and-ash flow

deposits cropping out as isolated tongues along major ravines.

From stratigraphic and radiometric age evidence, five

block-and-ash flow (BAF) deposits have been identified at

Nevado de Toluca with dates of about 37, 32, 28, 26, and 14 ka

(see Fig. 8 and Table 2). Eruptions at 37, 28, and 14 ka were

produced by the destruction of large central dacitic domes that

occupied the volcano’s central crater. This interpretation is sug-

gested by the roughly radial pattern of exposures of pyroclastic

deposits, although the oldest two also show important ponded

deposits. The aspect, texture, composition, and chemistry of

juvenile products are very similar for all deposits; therefore,

paleosoils and/or charcoal fragments were very useful in identi-

fying individual units.

The 37 and 28 ka deposits consist of massive block-and-ash

flow deposits (commonly more than 10 m thick) composed of

mch089 11 of 26

Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico 11

Figure 8. Composite stratigraphic column of Nevado de Toluca Volcano (after Macías et al., 1997).

more than three flow units of which the uppermost has a pink-red

color, probably suggesting oxidation due to contact with the

ambient air. These flow deposits contain charred logs and

fumarolic pipes, and are associated with minor pyroclastic surge

deposits. Both flow deposits are composed of gray porphyritic

juvenile lithics with minor amounts of pumice, glassy lithic

clasts, and red-oxidized dacite clasts from the volcanic edifice.

The juvenile lithic clasts have millimeter-sized phenocrysts of

plagioclase, hornblende, augite, and minor hypersthene, quartz,

and biotite embedded in an aphanitic groundmass of the same

constituents. Pumice and dense juvenile lithic clasts have a uni-

form chemical composition in the range of 65 to 67% in SiO

(Table 3 and Macías et al., 1997).

The 14 ka block-and-ash flow deposit differs strikingly from

the other two because it has a more radial distribution around the

volcano. A typical proximal section of this unit consists, from the

base upward, of a gray cross-bedded surge deposit (15 cm thick)

overlain by two massive gray block-and-ash flow units made of

gravel to boulder size lithics set in a coarse sand-size matrix with

a total thickness of 10 m (Fig. 12, Section 268). In other localities

(e.g. Section 35), the basal surge is rather thick (30-40 cm) with

well-developed cross bedding and accretionary lapilli as large as

1cm in diameter, capped by a massive pink ash flow deposit with

few gravel size lithics embedded in a fine sand matrix (Fig. 11).

In distal areas (20 km northeast from the vent), this unit has been

found in subaerial exposures (Newton and Metcalfe, 1999) and in

drill holes at the margins of Lake Chiconahuapan (Caballero et

al., 2001) (Fig. 11). At this distance, the 14 ka event is repre-

sented by a gray, massive ash flow about 90-cm thick composed

of fine sand- to silt-size particles. Organic-rich material collected

some 50 cm above this layer at pit La Isla II yielded an age of

13 870 +445/

–

420 yr. BP; for which Caballero et al. (2001)

assigned an approximate age for this unit of >14000 yr. BP. How-

ever, at Site 256, a paleosoil sample lying below the block-and-

ash flow deposit yielded an age of 13160 ± 89 yr. BP. Therefore

the age of this event remains uncertain.

12 A. García-Palomo et al.

mch089 12 of 26

Figure 9. Stratigraphic correlation of the Late Pleistocene deposits around Nevado de Toluca Volcano. Abbreviations of deposits are UTP—Upper

Toluca Pumice, LTP—Lower Toluca Pumice, BAF—Block-and-ash flow, PDF—Pilcaya Debris Flow, and DAD1—Debris Avalanche Deposit 1.

mch089 13 of 26

Figure 10. A: Digital elevation model showing the distribution of the Pilcaya (PDF) and El Mogote (MDF) cohesive debris-flow deposits on the southern flanks of Nevado de Toluca Volcano. TE—

Tenago, CH—Coatepec Harinas, IxS—Ixtapan de la Sal, PI—Pilcaya, and MO—El Mogote. B: Stratigraphic correlation of selected sections of the Pilcaya and El Mogote deposits (for locations see

Figure 10A).

Massive block-and-ash flows deposits represent the other

two events dated ca. 32 and 26 ka; however, they have a poor dis-

tribution around Nevado de Toluca. The scattered nature of out-

crops of these deposits probably reflects direct explosions or

partial collapse of the central dacitic domes or to burial by subse-

quent pyroclastic deposits.

Pumice Fall Deposits

Previous work described two plinian eruptions at the Nevado

de Toluca volcano. They were dated in ~24 500 and ~11 600 yr.

BP and named Lower Toluca Pumice and Upper Toluca Pumice,

respectively (Bloomfield and Valastro, 1974, 1977). Radiocarbon

ages were determined for paleosoil and carbonized material col-

lected below the deposits; however, no charcoal material was

collected in the deposits themselves. In this work, we recognize

four plinian eruptions (older to younger): Ochre Pumice Fall

(~36 000–37 000 yr. BP), Lower Toluca Pumice (~24 500 yr.

BP), White Pumice Flow (12 100 yr. BP), and Upper Toluca

Pumice (10 500 yr. BP) (see Table 1). Of these four units, only

the Ochre Pumice Fall was not previously reported (Macías et

al., 1997).

Ochre Pumice Fall (OPF). The Ochre Pumice Fall occurs

on the northern slope of Nevado de Toluca Volcano ~ 5 km from

the summit (Fig. 9, Section 268). From bottom to top, it consists

of an alternating sequence of pumice fall layers and surge and

flow deposits as follows: pumice fall layer (22 cm), surge deposit

(10 cm), thin pumice fallout (7 cm), surge and ash flow deposits

(62 cm total thickness), pumice fall (25 cm), and surge horizon

(10 cm). The fall layers (Fig. 12) consist of ochre vesiculated

pumice fragments and rare accidental lithic clasts. The sequence

is crowned by a pink pyroclastic flow deposit rich in gravel-size

pumice fragments set in a sand to silt size matrix (3–5 m thick).

Two charcoal samples from the second surge horizon and from

the base of the next fall layer yielded approximate ages of 39355

+1385/

–

1180 yr. BP, and 36780 +3325/

–

2345 yr. BP. These dates

bracket the age of the eruption as between 36 and 39 ka ago.

Lower Toluca Pumice (LTP). The Lower Toluca Pumice is

composed of an ochre fallout deposit with reverse grading and

an average thickness of 55 cm. It is separated from the 28 ka

BAF by an ash flow deposit and a dark-brown paleosoil, dated

at 24260 ± 670 yr. BP by Bloomfield and Valastro (1977). The

Lower Toluca Pumice is clast-supported and rich in ochre pum-

ice, with lesser amounts of clasts of gray dense juvenile dacite,

hydrothermally altered lithic clasts, and schist fragments from

the local basement. According to Bloomfield et al. (1977), the

Lower Toluca Pumice fallout contains approximately 62%

pumice, 27% lithic clasts, and 11% crystals. It covers an

approximate area of 400 km

and has a dispersal axis trending

northeast from the crater. The above authors calculated its vol-

ume at 0.33 km

(0.16 km

D.R.E. = dense rock equivalent).

However, the most complete exposures of this deposit

(Fig. 11, Section 62) show that the Lower Toluca Pumice is more

complex than previously described. The base of the unit always

consists of the pumice fall layer proper (LTP), which is overlain

by pale-brown cross-stratified surge deposits as thick as 35 cm,

rich in ochre rounded to subrounded pumice and lithic clasts. The

unit is then covered by a pale-brown ash flow deposit 42 cm

thick, which commonly shows clear signs of reworking toward

the top that ends with a poorly developed paleosoil.

White Pumice Flow (WPF). The White Pumice Flow con-

sists, from bottom to top, of three fallout members, and at least

two pyroclastic flow units (Fig. 9, Section 161). The lower fall-

out member (55 cm thick) has asymmetric grading (reverse to

normal) and poor sorting; it is rich in white pumice fragments,

lesser lithics, and abundant crystals of biotite, hornblende, and

plagioclase. The middle fallout member (7 cm thick) is com-

posed of fine sand grains (lithics, pumice, and crystals of horn-

blende, biotite, and plagioclase), and it shows high-angle

cross-lamination (laminae dark and white). The upper fallout

member (12 cm thick) has asymmetric grading and is composed

of coarse gravel of pumice, lithics (more than the basal mem-

bers), and crystals. At Section 161 it developed high-angle

cross-stratification, suggesting wind reworking during deposi-

tion (Fig. 13). Thin cross-stratified pyroclastic surges are

interbedded with the fallout layers. On top of the fallout

sequence lays the white pumice flow deposit, which consists of

two massive units; both have a minimum thickness of 15 m and

contain pumice and lithic fragments as large as 35 and 20 cm in

diameter, respectively. The pumice is dacitic (65.67% in SiO

)

and rich in phenocrysts of plagioclase, hornblende, biotite, and

quartz. The White Pumice Flow has been found mainly on the

southeast slopes of the volcano, with very thick exposures close

to Villa Guerrero and Tenancingo. This unit correlates with the

Lower Almoloya Tephra found in a pit drilled in the northwest-

ern edge of the Chiconahuapan Lake (Newton and Metcalfe,

1999); these authors dated an organic rich horizon below this

layer at 12400 ± 60 yr. BP. The White Pumice Flow is underlain

by a paleosoil dated at 26275 +1210/

–

150 yr. BP (Macías et al.,

1997); however, recent findings of charred logs within the pum-

ice flows at different outcrops (Fig. 11, Sections 161 and 200),

yielded ages of 12 414 +290/

–

280, and 12 040 ± 92 yr. BP (see

Table 2). Because the lower limit of the Lower Almoloya

Tephra has been dated at 12 400 ± 60 yr. BP, we consider the

approximate age of the White Pumice Flow eruption to be

closer to the 12040 ±92 yr. BP date, although additional dating

needs to be carried out to establish a more definite age.

Upper Toluca Pumice (UTP). The Upper Toluca Pumice

represents the largest plinian eruption at Nevado de Toluca Vol-

cano. Bloomfield and Valastro (1974, 1977) subdivided the

Upper Toluca Pumice into two members (Lower and Upper);

however, recent detailed studies of this deposit demonstrate that

the eruption was rather complex (Macías et al., 1997; Arce,

1999). Extensive field data attest that the Upper Toluca Pumice is

composed of four fallout horizons (PC0, PC1, PC2, and PC3),

that correspond to four plinian columns ~ 25, 37, 42, and 28 km

in height (Arce, 1999). Four pyroclastic flows (F0, F1, F2, and

F3) and two pyroclastic surges (S1, and S2) are included in this

14 A. García-Palomo et al.

mch089 14 of 26

mch089 15 of 26

Figure 11. Distribution of block-and-ash flow deposits around Nevado de Toluca Volcano. The locations of some key stratigraphic sec-

tions discussed in the text are indicated by a numbered open circle. Contour intervals every 100 m.

mch089 16 of 26

TABLE 2: RADIOCARBON AGES OF PYROCLASTIC DEPOSITS OF NEVADO DE TOLUCA VOLCANO.

Sample No. Lab. No. Age (yr. BP) Material dated Location Ref.

NT95140 A-8880 3,140 +195/-190 Charcoal in pumice flow deposit 19°02′19″, 99°54′34″ 1

NT96144B A-9175 3,435±50 Charcoal in ash flow deposit 19°13′26″, 99°47′22″ 1

MX9787C A-9926 4,200±110 Charcoal in dark-organic paleosoil 19°01′55″, 99°30′00″ 2

MX97455 A-9708 9170+160/-155 Charcoal in brown paleosoil 19°05′59″, 99°26′01″ 2

NT9570-A A-9173 10,445±95 Charcoal in pyroclastic flow of UTP 19°17′30″, 99°21′00″ 2

La Isla-2 A-9778 10,820±365 Charcoal in lacustrine sediment, jus below UTP 3

KBC-38 Tx-1658 11,580±70 Charcoal from thin layer at base of UTP; average of 4 samples 19°04′00″, 99°22′00′ 4

NT97200 A-9781 12040±92 Charcoal in White Pumice Flow deposit 2

NT96161 A-9472 12,415+290/-280 Charcoal in White Pumice Flow deposit 19°04′05″, 99°39′20″ 2

KBC-40a Tx-1660 13,620±150 Paleosoil 19°10′00″, 99°39′00″ 4

KBC-40b Tx-1661 13,870±180 Paleosoil 19°10′00″, 99°39′00″ 4

NT97193 A-9776 15,340+155/-150 Lacustrine organic horizon below block-and –ash flow deposit 19°13′37″, 99°28′45″ 3

MX9887D A-9921 16,215+135/-130 Paleosoil 19°01′55″, 99°30′00″ 2

KBC-40c Tx-1662 17,090±220 Humic horizon in Paleosoil just above reworked LTP 19°10′00″, 99°39′00″ 4

KBC-25 Tx-1604 20,100±140 Paleosoil on Pleistocene lavas below KBC-9; contaminated by modern humus 19°06′00″, 99°56′00″ 4

KBC-20 Tx-1599 21,030±430 Paleosoil on valley lahar 19°05′00″, 99°51′00″ 4

KBC-19 Tx-1598 21,170±170. Paleosoil on Tertiary lavas overlain by lahar: ?=KBC-17 and -18,

contamination suspected 19°04′00″, 99°52′00″ 4

KBC-26 Tx-1605 21,790±200 Paleosoil on valley lahar overlain by fluvial sand and gravel 19°11′00″, 99°51′00″ 4

KBC-9 Tx-1725 23,800± 490 Charcoal fragments in lithic ash at base of valley lahar , 2 samples 19°06′00″, 99°56′00″ 4

KBC-7 Tx-1523 23,940± 600 Paleosoil on lahar to E of Nevado: mean of 7 samples 19°12′00′, 99°39′00″ 4

KBC-42 Tx-1664 24,160±420 Thin peaty layer at base of valley lahar, W of Nevado 19°05′00″, 99°51′00″ 4

KBC-8 Tx-1524 24,260± 670 Paleosoil on lahar to E of Nevado: mean of 7 samples 19°10′00′, 99°39′00′ 4

KBC-15 Tx-1594 24,400±430 Paleosoil on Tertiary lavas; Sierra de Las Cruces 19°14′00″, 99°23′00″ 4

KBC-2 Tx-1518 24,410± 590 Paleosoil on lahar to E of Nevado: mean of 7 samples 19°04′00′, 99°39′00″ 4

KBC-18 Tx-1597 24,440± 550 Paleosoil on Tertiary lavas and old lahars to W of Nevado 19°03′30″, 99°52′30″ 4

KBC-17 Tx-1596 24,590± 280 Paleosoil on Tertiary lavas and old lahars to W of Nevado 19°03′00″, 99°52′30″ 4

KBC-4 Tx-1520 24,930± 670 Paleosoil on lahar to E of Nevado: mean of 7 samples 19°09′00″, 99°37′00″ 4

KBC-5 Tx-1521 25,020± 590 Paleosoil on lahar to E of Nevado: mean of 7 samples 19°09′30″, 99°37′30″ 4

KBC-6 Tx-1522 25,250± 760 Paleosoil on lahar to E of Nevado: mean of 7 samples 19°09′30″, 99°39′30″ 4

KBC-3 Tx-1519 25,620± 680 Paleosoil on lahar to E of Nevado: mean of 7 samples 19°12′00″, 99°41′00″ 4

NT9545 A-8485 26,273-1208/-1050 Paleosoil below the white pumice flow deposit 19°02′54″, 99°39′00″ 1

NT97204-B A-9782 26,610±320 Charcoal in block-and-ash flow deposit 19°07′05″, 99°40′50″ 2

KBC-41 Tx-1663 27,590±650 Paleosoil on fluvial gravel derived from lahar 19°05′00″, 99°51′00″ 4

NT9535B A-8244 28,140+865/-780 Charcoal in block and ash flow deposit 19°13′26″, 99°47′09″ 1

NT9521 A-8388 28,925+625/-580 Charcoal in ash flow deposit 19°00′24″, 99°38′56″ 1

MX9787A A-9927 31,450+660/-610 Brown paleosoil 19°01′55″, 99°30′00″ 2

NT9535-1 A-9315 32,730±50 Paleosoil below block-and-ash flow deposit 19°13′26″, 99°47′22″ 2

NT9521-A A-8389 32,860+2025/-1615 Charcoal in block-and-ash flow deposit 2

NT98268-C A-10000 36,780+3325/-2345 Charcoal in ochre pumice fall deposit 19°10′30″, 99°46′20″ 2

NT9550 A-8486 37,000 ± 1125. Charcoal in block and ash flow deposit 19°00′09″, 99°38′56″ 1

NE 12 CGR-4955 38,000 Paleosoil at Jaral gully 5

NT9595 A-8657 42,030+3530/-2445 Charcoal in ash flow deposit 19°12′52″, 99°45′44″ 2

Compiled data from:

Bloomfield and Valastro (1977),

Cantagrel et al. (1981), 1Macías et al., (1997),

Caballero et al., (2000), and

This work.

deposit (Fig. 14). The fallout layers of the Upper Toluca Pumice

covered portions of the Lerma and Mexico City basins with lay-

ers ~1 m and 10 cm thick, respectively; such areas presently con-

tain major population centers of Toluca and Mexico City, which

have a combined population of more than 25 million. These

deposits are composed mostly of pumice with minor amounts of

gray juvenile dacite, red-altered andesitic lithic clasts, and green-

altered dacite clasts. This pyroclastic sequence (fall, flow, and

surges) is well exposed at El Zaguan gully on the eastern flank of

Nevado de Toluca. The pyroclastic flow deposits are probably

those described by Palacio-Prieto (1988) at El Zaguan gully as

pumice-rich lahars, and by Bloomfield and Valastro (1977)

mch089 17 of 26

Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico 17

TABLE 3.

HEMI

AL ANALY

ELE

TED

AMPLE

M THE MAP AREA

Sample CH33 CH46 CH47 CH49 CH84 SJ BS3 BI NT30 NT54 NT84 NT90

SiO2 61.04 60.67 61.25 61.63 56.7 64.46 60.08 52.69 60.89 65.06 63.47 62.73

TiO2 0.75 0.77 0.8 0.8 0.82 0.58 0.78 0.79 0.77 0.58 0.65 0.70

Al2O3 15.92 16.12 15.97 16.01 16.07 15.72 15.5 16.33 16.44 16.21 16.60 16.32

Fe2O3 5.59 5.72 5.43 5.58 6.54 4.09 5.8 7.87 5.72 4.08 4.95 5.13

MnO 0.09 0.09 0.09 0.09 0.1 0.07 0.09 0.13 0.09 0.07 0.08 0.07

MgO 4.91 4.69 4.37 4 6.07 2.2 5.61 6.74 3.51 2.09 2.55 3.56

CaO 5.62 6.08 6.02 5.69 6.9 4.35 5.76 8.04 5.86 4.18 4.73 4.89

Na2O 4.38 4.24 4.25 4.39 3.99 4.18 4.02 3.68 4.22 4.34 4.39 4.32

K2O 1.56 1.62 1.7 1.86 1.41 1.98 1.95 0.83 2.20 2.29 1.98 1.86

P2O5 0.19 0.24 0.26 0.26 0.24 0.17 0.3 0.17 0.28 0.16 0.19 0.20

LOI -0.01 -0.01 -0.01 -0.01 0.12 1 0.08 0.86 0.14 1.43 0.57 0.66

Total 99.97 100.11 99.99 100.29 98.96 98.81 99.97 98.12 100.1 100.49 100.2 100.45

Ba 389 529 587 528 495 390 466 197 605 448 455 463

Rb 0.1 35 51 44 34 27 22 18 47 50 48 44

Sr 465 573 661 571 545 729 822 381 720 493 514 500

Y1717181818161617 20151820

Zr 137 169 176 174 149 135 161 94 175 149 149 152

Th 2.7 3.6 4.3 4 3.3 3.1 3.4 0.8 5.9 4.6 4.1 4

Pb 12 10 14 12 9 8 10 5 0 0 0 5

Zn 63 66 67 71 69 75 70 76 57 47 52 62

Cu 21 22 21 19 35 24 25 49 12 6 12 17

Ni 106 87 76 67 126 38 148 161 13 17 20 49

V 101 102 99 105 119 73 104 167 123 74 94 96

Cr 193 200 167 145 262 71.7 240 384 57.3 38.8 62.9 113

Hf 3.4 4.2 5.2 4.4 4 3.5 4 2.5 5.1 2.6 2.7 2.7

Cs 1.4 1.5 1.1 1.3 0.8 0.7 1 0.4 1.2 2.7 1.6 1.8

Sc 13.5 14.4 14.1 14.1 17.5 9.1 13.3 20.8 15.1 9 13.1 14.1

Ta 0.5 0.9 1.2 0.6 0.4 0.3 0.3 0.3 0 0 0 0

Co 48.4 54.9 51.5 56.7 43.9 16.6 22.6 33.8 33.9 16.9 19.9 20.5

Be 1 1111121 2112

U 1.1 1.4 4.3 1.7 1.1 0.9 1 0.1 2.2 1.5 2.2 1.5

W 260 271 283 322 139 23 1 14 124 34 39 37

Mo 1 1131222 3224

Au 1 111222344000

La 14.9 24.6 28.1 25.1 21.6 18.5 22.3 7.3 28.3 15.7 19.7 21.8

Ce 37 58 61 60 50 30 46 18 57 31 36 37

Nd 18 28 31 27 24 17 22 11 27 15 18 21

Sm 3.85 4.88 5.66 5.14 4.69 3.19 4.53 2.86 5.98 3.28 4.34 4.6

Eu 1.21 1.57 1.49 1.65 1.53 1.02 1.45 1 1.63 1.04 1.29 1.28

Tb 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.8 0.4 0.5 0.7

Yb 1.74 1.82 1.76 1.98 1.83 1.13 1.41 1.5 1.9 1.43 1.88 1.95

Lu 0.26 0.27 0.26 0.28 0.23 0.17 0.22 0.24 0.28 0.22 0.28 0.29

N19°13′ 19° 04′ 19° 07′ 19° 06′ 19° 01′ 19° 09′ 19° 05′ 18° 58′ 19° 03′ 19° 01′ 19° 02′ 19° 03′

W99°26′ 99° 25′ 99° 28′ 99° 31′ 99° 27′ 99° 58′ 99° 37′ 99° 30′ 99°47′ 99° 47′ 99° 45′ 99° 43′

Note: Major elements reported in weight percentages, minor and trace elements in ppm.

Samples are: CH—Holocene scoria cones and lava flows of the Chichinautzin Volcanic Field, SJ—San José, cones and domes complex, BS3—Basal Sequence,

and BI—San Nicolás Basaltic-Andesite.

All samples were analyzed for major elements by Inductively coupled plasma-direct current (ICP-DC), and for trace elements by Instrumental neutron activation

analysis (INAA), at Activation Laboratories, Ancaster, Canada.

southeast of Tlanisco as “pink valley-fill lahars.” Cantagrel et al.

(1981) also saw similar deposits in some cuts of the Rio Grande

and correctly catalogued them as “coulées pyroclastiques pon-

ceuses” although they considered them an independent event

apart from the Upper Toluca Pumice eruption.

The chemical composition of juvenile products is dacitic,

ranging from 61 to 65% silica (Macías et al., 1997; Arce, 1999).

The pumice and juvenile lithic clasts consist of crystals of plagi-

oclase, orthopyroxene, and hornblende with minor biotite and

oxides set in a glassy vesiculated groundmass.

Bloomfield and Valastro (1974, 1977) obtained a minimum

age of 11 600 yr. BP on the basis of

C dates obtained from

samples collected at the base of the unit (Table 2). New radio-

carbon dates of a sample found within the basal pyroclastic

flow deposit yielded an age of 10 445 ± 95 yr. BP (Macías and

Arce, 1997; Arce, 1999). These data, along with a radiocarbon

date of 10 820 ± 365 yr. BP for a sample from the La Isla II drill

hole taken just below of the Upper Toluca Pumice layer

(Caballero et al., 2001) and the revised data available in the lit-

erature, suggest that the Upper Toluca Pumice eruption

occurred some 10 500 years ago.

The Chichinautzin Volcanic Field

The Chichinautzin Volcanic Field (CVF) represents the

youngest volcanic activity in the area and consists of a series of

predominantly basaltic-andesitic scoria cones (e.g., Tres Cruces

Volcano, Holotepec) and fissure-fed lava flows (e.g., Tenango).

These volcanic edifices, which constitute only the western part

of the Chichinautzin Volcanic Field are located in the eastern-

most portion of the study area. The age of the volcanic centers

of the Chichinautzin Volcanic Field varies from 38000 to 1500

years BP (Bloomfield 1974, 1975; Martin del Pozzo, 1982;

Velasco-Tapia and Verma, 2001; Siebe, 2001). The Upper

Toluca Pumice deposit, which was produced by a large plinian

eruption of Nevado de Toluca some 10 500 yr. BP, represents

an excellent marker to distinguish between late Pleistocene and

Holocene rocks within this western edge of the The Chichin-

autzin Volcanic Field (Fig. 15A). Based on stratigraphic rela-

tions, it is obvious that fissure-vent lava flows and cinder cones

related to the E–W Tenango fault system are mainly Holocene.

Alternatively, scattered scoria cones in the Lerma Basin are pre-

Upper Toluca Pumice and are bracketed between 38 000 and

10500 yr. BP.

The late Pleistocene stratigraphic sequence of this region of

the Chichinautzin Volcanic Field can be further subdivided into

two groups based on the stratigraphic position of the scoria fallout

layers derived from the Negro Cone that has a main dispersion

axis to the NW. A paleosoil below this fallout horizon yielded an

age of 19 530 ± 160 yr. BP (Bloomfield, 1975); therefore, the old-

est group of cones includes Quilotzi and Pehualtepec, while the

younger group, with ages between 19 530 and 10 500 yr. BP,

includes Capulhuac, Cuautl, and others (Fig. 15B).

Geochemistry

Twelve new chemical analyses of major, trace, and rare-earth

elements were performed on the volcanic rocks in the study area.

These results, combined with those of previous work, represent a

total of 53 calc-alkaline and 18 alkaline rocks plotted on the clas-

sification diagram of Figure 16 (Bloomfield and Valastro, 1974;

Macías et al., 1997, Morán-Zenteno et al., 1999; Marquez et al.,

1999; Wallace and Carmichael, 1999; Verma, 1999). Young prod-

ucts belonging to Nevado de Toluca Volcano and older units have

calc-alkaline affinity (Fig. 16). However, a few samples from the

central and eastern parts of the Chichinautzin Volcanic Field

18 A. García-Palomo et al.

mch089 18 of 26

Figure 12. Panoramic view of Site 268

where the Ochre Pumice Fall deposit is

well exposed on the northern flanks of

Nevado de Toluca Volcano.

overlap the alkaline field (Bloomfield, 1975; Márquez et al.,

1999; Wallace and Carmichael, 1999).

The Nevado de Toluca samples plot in the andesite and

dacite fields, ranging from 55 to 67% SiO

. Rocks of the western

Chichinautzin Volcanic Field have mainly andesitic composition,

in particular the Holocene scoria cones and fissure-fed lava flows.

The rocks of the Tilzapotla Formation are mainly rhyolites and

represent the most differentiated eruptive products in the area

(Fig. 16). The San Nicolas Basaltic Andesite plots in the left part

of the basaltic andesite field, while the Basal Sequence falls in

the andesitic field.

The andesites and dacites of Nevado de Toluca Volcano have

MgO values (1.5–3.5 wt%) and slightly differ from the andesites

and dacites of the Chichinautzin Volcanic Field (4–7.2 wt%

MgO), as was previously recognized by Wallace and Carmichael

(1999). Alkaline products of the Chichinautzin Volcanic Field

have MgO values between 6 and 10 wt% that plot in the upper left

corner of Figure 17A. Three clusters of samples grouped accord-

ing to their MgO concentrations are evident in Figure 17A:Toluca

andesites, Toluca dacites, and Chichinautzin calc-alkaline, alkaline

and the older Basal Sequence, and San Nicolás Basaltic Andesite

rocks. Other Harker diagrams, however, show that Nevado de

Toluca samples plot separately from the Chichinautzin calc-alka-

line and alkaline rocks and the Basal Sequence and San Nicolás

Basaltic Andesite samples (Fig. 17B–D). A similar behavior is

observed in the abundances of Ni, V, and Cr among the trace ele-

ments, with the San Nicolás Basaltic Andesite being the more

mafic sample (Fig. 17C). The silica versus alkalis diagrams clearly

show the alkaline rocks of the Chichinautzin Volcanic Field to be

distinct from the rest of the samples (Fig. 17E–F).

Figure 18 shows the spider diagram for ten of our twelve

chemical analyzed rocks, in which the Nevado de Toluca vol-

canics define a gray-shaded pattern. The Nevado de Toluca rocks

fit well in respect to the Trans-Mexican Volcanic Belt with LREE

(La, Ce, and Nd) enrichments between 30 to 60 times and deple-

tions of HREE (Tb, Yb, and Lu). The San Jose Dacite (1.3 Ma)

follows the same pattern as the Nevado de Toluca rocks. The

youngest calc-alkaline Chichinautzin samples are slightly

enriched in light REE with respect to the Toluca samples; sur-

prisingly, the Basal Sequence (7.5 Ma) shows a similar pattern.

The only sample that diverges from this general trend is the

San Nicolas Basaltic Andesite (21 Ma), which shows smaller

mch089 19 of 26

Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico 19

Figure 14. Composite stratigraphic section of the Upper Toluca Pumice

(UTP), which consists of a complex set of pyroclastic fall, surge, and

flow deposits. The upper and lower members of the UTP described by

Bloomfield and Valastro (1974) are shown.

Figure 13. Detailed view of the base of the White Pumice Flow (WPF)

sequence consisting of alternate layers of poorly sorted pumice fall lay-

ers bracketing a high-angle cross-stratified fall horizon that we attribute

to reworking by strong winds during deposition (see coin for scale).

mch089 20 of 26

Figure 15. A. Simplified geological map of the Chichinautzin Volcanic Field (CVF) in the Lerma basin zone, east of the Nevado de

Toluca Volcano. B. Stratigraphic relations of the cinder cones and lava flows are based on their ages (Bloomfield, 1975; this work) and

their relative positions with respect to the Upper Toluca Pumice fall deposit.

enrichments of light REE in regard to the mantle and a similar

pattern of the heavy REE with respect to the Basal Sequence

falling within the Nevado de Toluca trend.

Tectonic setting of the Nevado de Toluca region

The Nevado de Toluca Volcano is localized by the intersec-

tion of three complex fault systems of different age, orientation,

and kinematics (García-Palomo et al., 2000). These systems,

from older to younger, are the Taxco-Querétaro Fault System

(trending NNW) located south of the volcano; the San Antonio

Fault System (NE–SW) that runs between the San Antonio and

Nevado de Toluca Volcanoes; and the Tenango Fault System

(E–W) located to the east of the volcano. Our field data, consist-

ent with previous work, suggest that these systems have co-

existed since the late Miocene. In addition, the stratigraphy,

chronology, and kinematics of fault planes indicate the occur-

rence of at least three main deformation events that have affected

central Mexico since late Miocene. During early Miocene, an

extensional phase with the same deformation style as the Basin

and Range tectonics of northern Mexico caused the formation of

horsts and grabens south of Nevado de Toluca and the intrusion

of subvertical dikes oriented NW–SE and NNW–SSE. During

middle Miocene, a dominantly transcurrent event generated

NE–SW faults with two main components of motion. The first

movement corresponds to a left-lateral strike-slip fault (σ

ori-

ented NW–SE) that then turned into a normal fault through a

counterclockwise rotation of σ

close to a N–S position. The lat-

est deformation phase started during late Pliocene, involving an

oblique extension (σ

oriented NE–SW) accommodated by E–W

right-lateral faults that changed to normal faults by the shifting of

to a N–S orientation. Late Pleistocene-Holocene monogenetic

volcanism, explosive central eruptions of Nevado de Toluca Vol-

cano, and present-day seismicity in the region and in central

Mexico are all related to these faults.

Volcanic history of the Trans-Mexican Volcanic Belt in the

map area

The oldest rocks in the area, while of similar age, belong to

two different tectonic environments. During Late Jurassic–Early

Cretaceous, Mexico was bounded on the west by an island arc

trench system that resulted from subduction of the Farallon and

Kula plates beneath Mexico (Coney, 1982). The eruptive output

from these volcanic arcs are represented in our map area by the

Ixtapan-Teloloapan sequence and the Acuitlapan and Amatepec

Formations that belong to the Guerrero Terrain (Campa and

Coney, 1983; De Cserna and Fries, 1981; De Cserna, 1983). Geo-

logical conditions in the east, however, were different; there the

development of a calcareous reef platform (Guerrero-Morelos

Platform) and sedimentation of the Morelos Formation occurred.

These deposits represent a marine transgression associated with

the opening of the Gulf of Mexico. Soon afterwards, the inshore

part of the platform was exposed to clastic flysch sedimentation

(Mexcala Formation) from the continental emergence in the region.

During late Mesozoic-early Tertiary, the continental land-

mass underwent compression from the southwest as a conse-

quence of accretion of the insular arcs to Mexico during the

Laramide orogeny (Coney, 1978). This compressional phase

caused the insular rocks composed of the Ixtapan-Telolopan

mch089 21 of 26

Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico 21

Figure 16. Silica versus alkali diagram of

Le Bas et al. (1986) andesitic and dacitic

rocks from Nevado de Toluca are shown by

the small black (Cantagrel et al., 1981) and

empty gray circles (Macías et al., 1997;

this work). Other symbols plotted are open

triangles, the alkaline rocks of Chichin-

autzin Volcanic Field (CVF) (Wallace and

Carmichael, 1999); black triangles, Holo-

cene lava flows, and cinder cones from the

CVF (this work); solid diamond, the San

Nicolás Basaltic Andesite (SNBA); black

square, the Basal Sequence (BS); open dia-

mond, San José dacite; and open squares,

the Tilzapotla rhyolite (Morán-Zenteno et

al., 1999).

sequence to override the calcareous formations of the Guerrero-

Morelos platform through a N–S thrust fault. The Laramide orog-

eny caused substantial continental emergence, and subsequent

marine regression continued through the Eocene with the end of

the marine sedimentation and the beginning of continental depo-

sition of the red beds of molasse deposits (Balsas Formation)

(Fries, 1956, 1960). During this time, the southwestern part of the

map area was subjected to the intrusion of a felsic igneous body.

During the Oligocene, the Farallon plate continued to be sub-

ducted beneath the continent, with the ensuing generation of a wide

continental volcanic arc along western Mexico. In the northwestern

part of Mexico, it formed the Sierra Madre Occidental (McDowell

22 A. García-Palomo et al.

mch089 22 of 26

Figure 17. Binary diagrams of major and trace elements, symbols as in Figure 16. Alkaline rocks from the Chichinautzin Volcanic Field plot

in the left side of the diagrams. The Nevado de Toluca volcanics from the San Nicolás Basaltic Andesite to the younger products follow a lin-

ear trend in MgO, CaO, and K

O toward higher silica concentrations, and the Tilzapotla Rhyolite that are the most silic rocks.

and Clabaugh, 1981); in the southwestern part, it formed the Terti-

ary Volcanic Province of Southern Mexico (Alba-Aldave et al.,

1996; Martiny et al., 1996). The last sequence involved the vol-

canic rocks of the Taxco-Huautla region (Morán-Zenteno et al.,

1999), which includes the rhyolitic volcanics of the Tilzapotla

Formation exposed south of Nevado de Toluca Volcano.

During the early Miocene, the tectonic conditions in western

Mexico apparently changed and resulted in a hiatus of volcanic

activity (Morán-Zenteno et al., 1999). However, in our map area,

about 21 Ma, the apparently effusive volcanism that produced the

San Nicolas Basaltic-Andesite took place, suggesting a volcanic

phase during this time. Nevertheless, the scarcity of outcrops and

data for this episode precludes a good understanding of its role in

the volcanic-tectonic evolution of the region.

During middle Miocene, volcanism occurred in scattered

areas of central Mexico along the present Trans-Mexican Vol-

canic Belt. In the map area, intense volcaniclastic sedimentation

during this time deposited the Tepoztlán Formation; its outcrops,

hundreds of meters thick, are exposed in the southeastern part of

Nevado de Toluca and Zempola volcanoes. Primary structures

and the distribution of the Tepoztlán Formation suggest that this

unit was emplaced by an older volcano located somewhere north

of the deposits.

Late Miocene was characterized in the area by an episode

of erosion without deposition followed by the formation of

large andesitic-dacitic stratovolcanoes forming the Las Cruces

Volcanic Range, located just to the east of Nevado de Toluca.

During this time, the Chortis Block was moving along a left-lat-

eral fault from its former position south of Mexico to its present

position in Central America, initiating an important geodynamic

reorganization of the Pacific plates (De Cserna, 1969). Evidence

of this movement are the mylonitic bands observed along the

coast of Guerrero (Tolson, 1998), the brecciated calcareous rocks

of early Cretaceous age (Mills, 1998), and the diminishing age

of the magmatic rocks of the Sierra Madre del Sur (Morán-Zen-

teno et al., 1999). From this process of plate reorganization

spawned the Acapulco trench on the Guerrero coast and a com-

pressive deformation in central Mexico, as suggested by strike-

slip faults such as those found in the regions of Morelia and

Guadalajara (Ferrari et al., 1994), Hidalgo (Suter et al., 1991),

and in the San Antonio Fault System in the study region (García-

Palomo et al., 2000).

The construction of extensive lava-flow plateaus in several

sectors of the Trans-Mexican Volcanic Belt marks a striking epi-

sode of widespread volcanism during Late Miocene; these

plateau-forming rocks have been recognized around the cities of

Guadalajara, Querétaro, and Toluca (Ferrari et al., 1994; García-

Palomo et al., 2000). This widespread effusive activity, which

produced mainly calc-alkaline products, might represent the

beginning of the volcanic activity that resulted in the Trans-Mex-

ican Volcanic Belt, as suggested by Pasquaré et al. (1991).

During the Pliocene, the subduction of the Cocos and Rivera

plates beneath Mexico produced the volcanic activity of the Trans-

Mexican Volcanic Belt across central Mexico; it probably also is

associated with a distensive tectonic regime as previously dis-

cussed. During Late Pliocene, sporadic activity mostly resulted in

cinder cones and domes, and a few fissure-fed lava flows. San

Antonio Volcano is the oldest stratovolcano in the map area, with

its activity beginning approximately in Middle Miocene and

involving effusive and explosive events that deposited voluminous

block-and-ash flow deposits and plinian fall units. During the

Pliocene, numerous monogenetic cones and domes erupted within

the vicinity of the San Antonio Volcano. Nevado de Toluca Vol-

cano began to develop during this time (ª2.6 Ma), with the extru-

sion of thick andesite lava flows that constructed the central

edifice. Between 1.2 Ma and 42000 yr. BP, the activity was domi-

nated by the emplacement of thick laharic sequences, some of

which were associated with major collapses of the volcanic edi-

fice. While the specific triggering mechanisms for collapse can-

not be deduced from field evidence, the abundance of highly

altered materials in the deposits suggests that intensive hydrother-

mal alteration significantly weakened the edifice, thereby con-

tributing to periodic structural failure. The collapse-generated

volcaniclastic deposits were emplaced within the southern sectors

of the volcano and channeled into the grabens of the Taxco-Queré-

taro fault system. Approximately 42000 years ago resumption of

explosive activity produced a thick sequence of pumice and block-

and-ash flow deposits, that formed extensive fans around the vol-

cano. According to our revised stratigraphy, at least five

dome-destruction events and four plinian eruptions have occurred

since. The Lower Toluca Pumice and the Upper Toluca Pumice

tephra fall from plinian eruptive columns blanketed extensive

areas; isopachs of these deposits show dispersal axes toward the

population centers of Toluca and Mexico City. While the explosive

mch089 23 of 26

Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico 23

Figure 18. Chondritic REE patterns of selected samples using the normal-

ization values of the Haskin et al. (1966) symbols as in Figure 16. The gray

pattern represents the field of juvenile products of dacitic composition of

Nevado de Toluca Volcano (Macías et al., 1997). The hachured pattern rep-

resents the field of the Tilzapotla Rhyolite (Moran-Zenteno et al., 1999)

activity of Nevado de Toluca (< 42 000 yr.) was developing,

numerous monogenetic cinder cones also erupted in the Lerma

and Toluca basins (Chichinautzin Volcanic Field). Holocene fis-

sure-vent lava flows, such as the Tenango Andesite, were also

deposited in this period. The most recent volcanic activity recog-

nized in the area (3300 yr. BP) was a minor magmatic eruption at

Nevado de Toluca Volcano, which emplaced a surge and a pyro-

clastic flow deposit on its northeast flank.

ACKNOWLEDGMENTS

M. Abrams from the Jet Propulsion Laboratory, NASA,

provided useful satellite imagery, K. Cervantes drafted the geo-

logical map, and J.C. Mora helped with petrographic descrip-

tions of some units. This study was financed by the CONACYT

(CONACYT 0179P; 27993-T) and Dirección General de Asun-

tos del Personal Académico (DGAPA), Universidad Nacional

Autónoma de México (UNAM) (IN107196 and IN102297)

grants. R.I. Tilling, S.P. Verma, Ch. Fridrich, R.A. Thompson,

S. Hughes, and D. Blatter provided useful and constructive

reviews of the paper.

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