Late Pleistocene flank collapse of Zempoala volcano (Central Mexico) and the role of fault...

Journal of Volcanology and Geothermal Research 177 (2008) 944–958

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Journal of Volcanology and Geothermal Research

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Late Pleistocene flank collapse of Zempoala volcano (Central Mexico) and the role offault reactivation

José Luis Arce a,⁎, Rodolfo Macías a, Armando García Palomo a, Lucia Capra b,José Luis Macías c, Paul Layer d, Hernando Rueda e

a Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, Coyoacán, México, D.F. 04510, Mexicob Centro de Geociencias, Universidad Nacional Autónoma de México, Juriquilla, Qro., Mexicoc Departamento de Vulcanología, Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán, México, D.F. 04510, Mexicod Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, AK 99775-7320, USAe Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Coyoacán, México, D.F. 04510, Mexico

⁎ Corresponding author. Tel.: +52 55 56224288x108;E-mail address: [email protected] (J.L. Arce).

0377-0273/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2008.07.015

a b s t r a c t
a r t i c l e i n f o
Article history:
Zempoala is an extinct Ple Received 4 September 2007Accepted 21 July 2008Available online 5 August 2008
Keywords:debris avalancheZempoala volcanofault reactivationSierra de las CrucesCentral Mexico

istocene (∼0.7–0.8 Ma) stratovolcano that together with La Corona volcano(∼0.9 Ma) forms the southern end of the Sierra de las Cruces volcanic range, Central Mexico. The volcanoconsists of andesitic and dacitic lava flows and domes, as well as pyroclastic and epiclastic sequences, and hashad a complex history with several flank collapses. One of these collapses occurred during the latePleistocene on the S–SE flank of the volcano and produced the Zempoala debris avalanche deposit. Thiscollapse could have been triggered by the reactivation of two normal fault systems (E–W and NE–SW),although magmatic activity cannot be absolutely excluded. The debris avalanche traveled 60 km to the south,covers an area of 600 km2 and has a total volume of 6 km3, with a calculated Heim coefficient (H/L) of 0.03.Based on the textural characteristics of the deposit we recognized three zones: proximal, axial, and lateraldistal zone. The proximal zone consists of debris avalanche blocks that develop a hummocky topography; theaxial zone corresponds with the main debris avalanche deposit made of large clasts set in a sandy matrix,which transformed to a debris flow in the lateral distal portion. The deposit is heterolithologic incomposition, with dacitic and andesitic fragments from the old edifice that decrease in volume as bulking ofexotic clasts from the substratum increase. Several cities (Cuernavaca, Jojutla de Juárez, Alpuyeca) withassociated industrial, agricultural, and tourism activities have been built on the deposit, which pose inevidence the possible impact in case of a new event with such characteristics, since the area is stilltectonically active.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Zempoala volcano (19°03′N, 99°20′W, 3690 masl) is 20 km SW ofMexico City. It is an extinct Pleistocene stratovolcano located at thesouthern end of the Sierra de las Cruces volcanic range, which is partof the Trans-Mexican Volcanic Belt (TMVB) (Fig. 1a). The TMVB is a∼1200 km-long, active volcanic province, related to the subduction ofthe Cocos plate beneath the North America plate (Fig.1a). The Sierra delas Cruces (SC) is an 11 km long volcanic range with a general NNW–

SSE orientation. It consists from north to south of La Catedral, La Bufa,Iturbide, Chimalpa, Salazar, San Miguel, Ajusco, La Corona, andZempoala volcanoes (García-Palomo et al., 2008) (Fig. 1b), as well asother smaller structures. Although it is close toMexico City, the SC hasnot been the subject of intense geological and geophysical studies.There are no detailed stratigraphic studies and the available radio-

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metric data are insufficient to understand the evolution of the volcanicactivity, which includes several lava flow units, pyroclastic deposits(flows and falls), debris avalanche, and lahar deposits (García-Palomoet al., 2002). AlthoughMooser et al. (1974) concluded that volcanism inthe SC migrated from north to south, K–Ar and Ar–Ar dates associatedwith palaeomagnetic studies (Mooser et al., 1974; Mora-Alvarez et al.,1991; Osete et al., 2000; Romero-Terán, 2001; Mejia et al., 2005) showno such migration with volcanism ranging from 3.7 to 0.39 Ma(Pliocene–Pleistocene). In this study we report new 40Ar/39Ar ages forLa Corona and Zempoala volcanoes, the southernmost volcanic struc-tures of Sierra de las Cruces. In addition, Mooser et al. (1974) noticedthat the SC was transected by a series of buried basement fracturesoriented NE–SW.

Furthermore, there are few published studies of the Zempoalavolcano. The volcano was defined as the Zempoala andesite by Fries(1960) and De Cserna and Fries (1981), based on its andesitic anddacitic lava flows. The volcano lacks a central crater due to intenseerosion and faulting. Its summit has an irregular shape, made of highlydissected N–S escarpments dipping to the east (Fig. 2), and E and S–SE

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Fig. 1. a) Map showing the Trans-Mexican Volcanic Belt and the location of Sierra de las Cruces volcanic range. The square in the TMVB represents the area of b). b) Satellite imageshowing the location of Sierra de las Cruces, cities of Toluca and Mexico, as well as several volcanic structures. Radiometric ages were taken from several authors (Mora-Alvarez et al.,1991; Osete et al., 2000; Romero-Terán, 2001; Mejia et al., 2005) and this study (asterisks). Abbreviations are: NT = Nevado de Toluca, Po = Popocatépetl, Ze = Zempoala, LC = LaCorona, Aj = Ajusco, SM = San Miguel, Sa = Salazar, Ch = Chimalpa, It = Iturbide, Ca = La Catedral, LB = La Bufa volcanoes, and Te = Tepic, Gdl = Guadalajara, Co = Colima, Mor = Morelia,Qro = Querétaro, Mx = Mexico City, Pue = Puebla, and V = Veracruz cities.

945J.L. Arce et al. / Journal of Volcanology and Geothermal Research 177 (2008) 944–958

scarps dipping to the south. The summit area is transected and dividedin two parts by a fault, here named the Zempoala fault (describedbelow). Two debris avalanche deposits have been recognized in proxi-mity to the volcano: (i) on the eastern sector of the summit area, ahummocky morphology with up to 30 m high hills that form inter-mountain lakes in the area known as “Lagunas de Zempoala NationalPark”; and (ii) up to 80 km S–SE of the volcano is the Zempoala DebrisAvalanche (ZDA) (Capra et al., 2002). The latter forms the topic of thisstudy because it represents an excellent example of a gravitationalsector collapse triggered by the reactivation of normal faults in theregion.

1.1. Terminology

A debris avalanche is a rapid movement of an incoherent, unsortedmass of rock and soil mobilized by gravity (Schuster and Crandell,1984). The resulted deposit is generally described based on two tex-tural facies (Glicken, 1998, 1991; Palmer et al., 1991: block and matrix(or mixed) facies. Block facies consists of debris avalanche blocks,unconsolidated or poorly consolidated pieces of the old mountain(Glicken, 1998). This facies is exposed in the proximal area with atypical hummocky morphology. The matrix (or mixed) facies consistsof megaclasts (N1 m) and clasts (b1 m) embedded in abundant matrix(b2 mm). This facies characterizes the main body of the deposit and itcan distally transform to a debris flow (Ui et al., 1986; Palmer et al.,1991; Takarada et al., 1999).

Lahar is a general term for rapidly flowingwater-saturatedmixturesof rock debris and water from a volcano. Debris flow and hyperconcen-trated flow types are defined based on their water-sediment concentra-tion (∼60% by volume) (Costa and Schuster, 1988).

2. Local geology

The local basement underneath Zempoala volcano is representedby several rock types. The oldest lithologies consist of Mesozoic sedi-mentary rocks (Xochicalco, Morelos, Cuautla, andMexcala formations)(Fig. 3) (Fries, 1956, 1960; De Cserna et al., 1974; De Cserna and Fries,1981). These rocks are covered by the Cenozoic Tilzapotla Formation,which consists of rhyolitic ignimbrites and lava flows dated by K–Ar at26 Ma (Fries, 1960), 31.9 Ma (Morán-Zenteno et al., 1998), and 38.3±1.0 Ma (García-Palomo et al., 2000). The Tepoztlán Formation,consisting of volcaniclastic deposits (Fries, 1960; De Cserna et al.,1988) varying in age between 21.6 and 7.5 Ma (García-Palomo et al.,2000), rests unconformably above the Tilzapotla Formation.

The Cuernavaca Formation (Fries,1956,1960) lies along the SE flankof the volcano and in the outskirts of Cuenavaca. It is a thick sequence(up to 300 m) of hyperconcentrated flow and debris flow depositsthat are heterolithologic, poorly sorted, and contain subrounded-to-rounded blocks, set in a compact, brown, sandymatrix. These depositsare intercalated with fluvial beds displaying sedimentary structuressuch as cross-bedding and channel-infilling. This unit represents animportant gap in the volcanic activity in the area. The youngest rocks in

Fig. 2. Perspective view of a digital elevation model of Zempoala volcano. Note the complexmorphology of the volcano, strongly dissected by faults, as well as the two scars left by theE and S–SE sector collapses. ZF = Zempoala fault.

946 J.L. Arce et al. / Journal of Volcanology and Geothermal Research 177 (2008) 944–958

the area are represented by the Chichinautzin Volcanic Field (CVF), amonogenetic volcanic field with several scoria cones and andesitic tobasaltic lava flows, ranging in age from 1.7 to 0.0016 Ma (Siebe et al.,2004a,b, 2005).

2.1. Structural framework

Based on previous work, aerial photographs, satellite imagery,topographic maps, and fieldwork we recognized two fault systemswith E–Wand NE–SWorientations (Fig. 4). The E–W system is relatedto the Tula–Chapala Fault Zone (Johnson and Harrison, 1990), whereasthe NE–SW system is related to the Tenochtitlán Shear Zone (DeCserna et al., 1988). Both fault systems have been reactivated severaltimes since the Miocene.

2.1.1. E–W fault systemOn the western portion of Zempoala volcano a series of E–W

trending, N-dipping faults, named the Tenango Fault System (TFS), ischaracterized by a set of curved and discontinued faults and fractures,with a right-stepping en-echelon arrangement (Mooser and Maldo-nado-Koerdell, 1961; Bloomfield, 1973; García-Palomo et al., 2002). TheTFS has undergone at least two episodes of movement, first strike-slipmotion during the Miocene, followed by normal faulting during thePleistocene–Holocene (García-Palomo et al., 2000). The latter episodewas accompanied by volcanic activity and sector collapse of Nevado deToluca volcano, the emplacement of the 8500 yr BP Tenango lava flow,and recent seismic swarms (Bloomfield, 1973; Yamamoto and Mota,1988; García-Palomo et al., 2000; Norini et al., 2006).

In the eastern portion of Zempoala volcano there is a topographichorst, called Chichinautzin Horst (ChH) (Siebe et al., 2004a,b; Colín-Rodríguez, 2006), with a substrate made up of Oligocene (XochitepecFormation) and middle Miocene (Tepoztlán Formation) units, com-posed of volcanic and volcaniclastic rocks, respectively (Mooser, 1986;García-Palomo et al., 2000). Crowning the highest portion of the horst

there are about 220 overlapping monogenetic volcanoes of theChichinautzin Volcanic Field (Bloomfield, 1975; Lugo-Hubp, 1984;Siebe et al., 2004a,b, 2005).

The ChH is bounded to the north by the Tláhuac–Tulyehualco half-graben (Montiel-Rosado,1990; Campos-Enríquez et al., 1997; Magaña,2003; Colín-Rodríguez, 2006) and to the south by the La Peradetachment normal fault (Delgado-Granados et al., 1995; Siebe et al.,2004a) marked by the alignment of scoria cones, topographiclineaments, and geophysical anomalies (Márquez and De Ignacio,2002; Ferrari and Capra, 2001). The ∼1200 m throw of the MorelosFormation on the La Pera fault probably originated by several episodesof normal movement.

2.1.2. NE–SW fault systemDe Cserna et al. (1988) proposed the NE-trending Tenochtitlán

Shear Zone as a series of faults extending between Petatlán, Guerreroand theNE edge of the studyarea. Twoepisodes ofmovement occurredon these faults and similar faults: sinistral strike-slip motion duringtheMiocene and normalmotion during the Pliocene to Recent (García-Palomo et al., 2002). In the Apan region, northeast of theMexico Basin,there are several extensional structures that control the monogeneticvolcanism and the seismicity (García-Palomo et al., 2002). At the northend of the study area, in the Guadalupe and SC volcanic ranges ofMiocene and Pliocene–Pleistocene age, respectively, these fault sys-tems form horst and graben structures (García-Palomo et al., 2008).West of the Nevado de Toluca volcano they produced the San Miguelgraben and control the Pleistocene activity of the Valle de BravoVolcanic Field (Blatter and Carmichael,1998; Aguirre-Díaz et al., 2006).

The NE–SW fault system in the study region outlines a 45 km long,25 km wide graben structure here named the Cuernavaca Graben(Fig. 4). In the south it is bounded by two horsts, the Malinalco on thewest and Cañon de Lobos on the east (Fig. 4).

TheMalinalco horst ismade up of Cretaceous limestones, and Tertiary(Miocene and Pleistocene) volcanic rocks. The horst (5-km wide and

Fig. 3. Composite stratigraphic column of the Zempoala area. Description and radiometric data were taken from different authors: (⁎1) Siebe et al. (2004a,b), (⁎2) Osete et al. (2000),(⁎3) García-Palomo et al. (2000), (⁎4) Alba-Aldave et al. (1996), (⁎5) Morán-Zenteno et al. (1998), (⁎6) Fries (1956, 1960).


36 km long) is bounded by the NE–SWChalmaNormal Fault, which has acurvilinear shape dipping to the SE (Fig. 4). The Chalma Fault affectslimestones of the Morelos Formation, volcaniclastic deposits of theTepoztlán Formation, as well as lava flows of the Chichinautzin VolcanicField (Fig. 4). TheNE–SWZempoalanormal fault dips SE70° and isparallelto the Chalma Fault (Fig. 4). It affectsMiocene and Pliocene volcanic rocks.

The NE-trending Cañón de Lobos horst is ∼1500 masl, and is madeup of Cretaceous limestones of the Morelos Formation. The horst (25-km long and 6-kmwide) is bounded to the north by the NE–SWCañónde Lobos normal fault, which dips 50° to the NW (Fig. 4).

From these data, it is clear that tectonic movements have played animportant role to produce fractures, faults, earthquakes, and volcan-

ism in the area. This is attested by the occurrence of late Holocenevolcanism (e.g. the ca. 1670 yr BP eruption of Xitle volcano; Siebe,2000), and recent earthquakes whose focal mechanisms indicate anorth–south extension along the E–Wtrending normal fault (Márquezet al., 1999; Delgadillo, 2000; Chavacán, 2003; Quintanar et al., 2005;Colín-Rodríguez, 2006).

3. The Zempoala debris avalanche deposit

The Zempoala debris avalanche deposit (ZDA) was first consideredas part of the Cuernavaca Formation (Fries, 1960). Subsequently, DeCserna and Fries (1981) described a series of coalescent alluvial fans

Fig. 4. a) Simplified map of the structural scenario of the Zempoala region, showing the Cuernavaca Graben, Cañón de Lobos, La Pera, and Chalma faults; b) Schematic representationof La Pera and Chalma normal faults intersection. The two normal fault systems show a 130° angle and the S–SE sector collapse is in an apical direction. Abbreviations: ZDA =Zempoala debris avalanche deposit, CF = Cuernavaca Formation.


fromZempoala volcano, and recently, Capra et al. (2002) proposed thatPliocene flank collapse of thewestern portion of the volcano producedthe 80-km long debris avalanche deposit south of the volcano.However, extensive field work carried out during this study suggeststhat the collapse originated from the S–SE sector of the volcanoproducing a debris avalanche emplaced towards the S–SE lowlands.

The deposit is divided based on its spatial distribution and mor-phological features into three main zones: 1) proximal, 2) axial, and3) lateral distal zone (Fig. 5).

3.1. Proximal zone

This zone is located close to the southern part of the volcano, up toa distance of 2 km, and is characterized by an abrupt morphology thatconsists of block facies hummocks (type A, Glicken, 1998), up to 100 mhigh, of highly fractured andesitic and dacitic lava fragments that arechemically and mineralogically similar to the rocks of the mainvolcano (Fig. 6). The dacitic lavas (67.66 wt.% SiO2) are porphyritic,

with large phenocrysts (up to 3 mm) of plagioclase, pyroxene, am-phibole, and rare biotite, set in a glassy matrix (Table 1). Andesiticlavas (61.59 wt.% SiO2) are porphyritic containing phenocrysts ofplagioclase, amphibole, and pyroxene, in a glassy matrix.

3.2. Axial zone

The axial zone extends from 20 up to 80 km from the vent (fromTemixco up to Pueblo Viejo, Morelos), after a gap of approximately25 km where the deposit is not exposed (Fig. 5). In this zone thedeposit shows a uniform morphology with small mounds probablyresulted from a selective erosion process. The deposit is dark-pale grayin color, with variable thickness (15–20 m) and consists of matrixfacies, with subangular to subrounded clasts andmegaclasts up to 3 min diameter, many of themwith jigsaw-fit puzzle structures (Fig. 7) setin a fine sandy matrix. The fragments consist of angular gray andesite,red dacite (similar to those observed in the hummocky facies, withphenocrysts of plagioclase, amphibole, and Fe–Ti oxides), and

Fig. 5. a) Satellite image showing the three main zones of the Zempoala debris avalanche deposit (ZDA), and the path of the flow. The discontinuous line circles the area where thedeposit was completely remobilized. b) Vertical section A–B showing the different zones of the ZDA deposit.


subrounded exotic fragments such as basaltic lavas, red-altered olderdacitic lavas and limestones. In particular, rounded exotic clasts aremore abundant toward the base of the deposit. Also in this zone arecommon red-brown 1 m-size elongated blocks composed of clay andblocks from older pyroclastic deposits (Fig. 7). Granulometric analysesof 35 samples of this zone (performed using one-phi spacing sieves,from −6ϕ to 4ϕ) indicate two modes in the histogram, at −5ϕ and 1ϕ,and low sorting (σϕ 1.8 to 3.2) (Fig. 8).

3.3. Lateral distal zone

This zone crops out around the towns of PasoMorelos, Nexpa, and LaMesquitera, near the Amacuzac River (Fig. 5) where it forms a planarmorphology with terraces up to 4 m. It is gray, massive, matrix-sup-

ported, with heterolithologic fragments (same components as thosedescribed for the axial matrix facies) set in an abundant matrix thatrepresents up to 60 vol.% (Fig. 9). Fragments are from angular (stillshowing jigsaw texture) to rounded in shape, mostly decimetric in sizebut with few megaclasts up to 2 m. Granulometry of seven analyzedsamples, showpolimodal curveswith themost importantmodebetween−3 and −2, and moderate sorting (σϕ2) (Fig. 8). By comparing both theaxial and lateral zones, the latter is finer and enriched in sand particles.

3.4. Secondary lahar deposits

The lateral distal zone of the ZDA deposit is covered by debris flowsinterbeddedwith fluviatile and lacustrine deposits that form 20m-thickflat fans. Thedebrisflowdeposits are brown, commonlyheterolithologic,

Fig. 6. Total alkalis versus silica diagram for samples from Zempoala, Ajusco, and SanMiguel volcanoes. For comparison a sample from the Chichinautzin Volcanic Field,collected from a lava flow near Pueblo Viejo village is also plotted. Note that Zempoalasamples show a major variation in silica content. The Ajusco and San Miguel data weretaken from Romero-Terán (2001).


massive, indurated, and moderately sorted. In places these depositsdevelop a crude normal grading of 20 cm-size, rounded clasts of dacite,basalt and limestone, set in a coarse sand matrix.

Fluvial deposits are pale brown and commonly heterolithologic,developing laminations, cross-bedding, and filled-channel structures.Main constituents are rounded dacite, basalt, and limestone clasts of5 cm size. Granulometric results of this zone display two modes, at−4ϕ and 1ϕ and sometimes one mode at −1ϕ, according to its type ofdeposit (i.e.: debris flow, hyperconcentrated flow, fluviatile deposits).

Table 1Whole-rock chemical analysis of Zempoala volcano, Pueblo Viejo lava flow (PV), and the MioInstituto de Geologia at UNAM, Mexico by X-RF method. Total Fe as Fe2O3. Major elements in

Sample ZEM30-DL ZEM32 ZEM3

Latitude N 18° 48′ 39″ 19° 03′ 01″ 19° 02

Longitude W 99° 14′ 02″ 99° 14′ 02″ 99° 19

Altitude (masl) 1167 2832 2815

SiO2 62.90 61.28 65.96TiO2 0.81 0.83 0.62Al2O3 15.85 16.92 15.07Fe2O3t 5.35 5.71 4.29MnO 0.05 0.07 0.08MgO 2.89 3.34 1.56CaO 4.98 5.74 3.50Na2O 3.98 3.88 3.91K2O 1.82 1.55 2.30P2O5 0.21 0.19 0.18LOI 1.02 0.71 3.03Total 99.84 100.2 100.5Rb 37 27 92Sr 709 769 365Ba 505 454 863Y 26 16 22Zr 238 218 193Nb 6 4 34V 141 145 74Cr 138 78 35Co 15 18 7Ni 50 20 6Cu 24 11 3Zn 62 65 70Th 3 3 10Pb 9 b5 11

At Temixco, the ZDA flow probably spread up to the limestone hillsto the east, but it was rapidly eroded by the rivers from the north,which produced an important secondary laharic sequence (Fig. 5), acommon phenomenon associated to hydrological modifications thatfollow debris avalanche emplacement (i.e. Scott, 1988; Palmer andNeall, 1989; Capra and Macías, 2002).

3.5. Distribution and volume

The ZDAdeposit is exposed on the S–SE portions of the volcano from6 km to 80 km. Close to the edifice the deposit is covered by the youngerCuernavaca Formation (Fig. 5). However, fromTemixco,Morelos (locatedat 20 km from the summit) up to Pueblo Viejo, Guerrero (at 80 km fromthe volcano), the deposit crops out continuously.

The thickest outcrops of the deposit (∼20–30 m) appear where thedebris avalanche encountered rough topography of the Morelos For-mation limestones. At a certain point around 5 km to the south ofAlpuyeca, the avalanche was emplaced through the Amacuzac River(Fig. 5). The area covered by the ZDA deposit was delimited with theconstruction of 50 stratigraphic sections and calculated in thetopographic map of INEGI (scale 1:50,000) by using the AUTOCADprogram. The minimum area is 600 km2 that today is occupied bytowns as Alpuyeca, Puente de Ixtla, Jojutla de Juárez, Nexpa, and LaMesquitera, in the State of Morelos; and Paso Morelos, in the State ofGuerrero. Using an average thickness of 10 m, a minimum volume of6 km3 may be considered for the whole deposit, including that mater-ial incorporated by bulking that is difficult to quantify. However acrude estimation made in the two most important zones (axial anddistal lateral) at a distance of 20 and 80 km from the summit, (aroundTemixco and Pueblo Viejo) yielded values of 30 and 60% by volume ofbulking, respectively (Fig. 7). Therefore the final volume ismore than 2fold the initial volume.

cene dacitic lava (DL). Analyses were performed at the Geochemistry Laboratory of thewt.%, and trace elements in ppm. LOI = Loss on ignition; masl = meters above sea level

8 ZEMPV ZEM0601 ZEM0602

′ 52″ 18° 31′ 32″ 19° 02′ 11″ 19° 02′ 20″

′ 13″ 99° 11′ 46″ 99° 20′ 19″ 99° 20′ 11″

823 3200 3100

51.18 69.09 60.491.92 0.63 0.8216.21 16.87 17.199.54 2.06 5.430.13 0.01 0.087.22 0.55 3.187.67 3.42 5.283.99 4.46 3.881.34 2.06 1.580.60 0.11 0.21−0.23 0.98 1.5199.56 100.24 99.6422 57 32538 454 746414 467 41526 23 17247 198 19725 3 4158 91 118298 55 5236 23 18134 25 1935 19 1789 36 74b3 b3 b35 6 9

Fig. 7. a) Photograph of the block-and-matrix facies of the ZDA deposit at a site located 55 km from the summit of Zempoala volcano. The black line circles a big block with jigsaw-fitpuzzle structure; b) another view of the block-and-matrix facies of the ZDA deposit. Notice the thickness and the presence of orange blocks made of shale, embedded in the matrix.For scale the person is 1.75 m in both photographs.


In order to estimate the mobility of the debris avalanche, wecalculated the Heim coefficient (H/L). By assuming that the collapsestarted at an altitude of 3690 masl in the volcano and traveled 80 kmfrom the volcano to an average elevation of 900 masl, a H/L=0.03 isobtained.

3.6. Stratigraphic relations: age of the deposit

In the vicinity of Temixco, the stratigraphy begins with a daciticlava flow (64 wt.% SiO2 (Table 1; Fig. 10) with phenocrysts of plagio-clase, pyroxene, amphibole, and Fe–Ti oxides, set in a microlitic ma-trix. This lava flow contains abundant rounded and, in places, shearedxenoliths that have equigranular textures with phenocrysts of plagio-clase and pyroxene. The rounded xenoliths commonly show reactionrims. The exact age of this unit is unknown but Fries (1960) assigned ita Miocene age. The dacitic lava is covered discordantly by ca.1-m thickdebris flow deposit containing several types of rounded clasts, set in a

coarse sandy matrix. The ZDA deposit overlies this debris flow with asharp contact and underlies the Cuernavaca Formation, although thecontact between themwas not observed. Sometimes, the ZDA depositis directly overlain by products of the Chichinautzin Volcanic Fieldthat represent the youngest products in the region.

3.7. 40Ar/39Ar analyses

Due to the lack of radiometric data of the southern portion of Sierrade las Cruces and in order to constrain the age of the ZDA deposit, wecollected five samples of Zempoala and La Corona volcanoes in order todetermine their 40Ar/39Ar ages (Table 2). The samples were crushed,washed, sieved and hand-picked for small whole-rock chips suitable fordating alongwith standardmineral TCR-2 sanidine (27.87Ma; LanphereandDalrymple, 2000). Analytical procedure details are described inYorket al. (1981), Layer et al. (1987), McDougall and Harrison (1999), andLayer (2000).

Fig. 8. Sedimentological characteristics of the axial and lateral zones of the Zempoala debris avalanche deposit. a) Cumulative weight percent versus grain-size (phi units); andb) triangular classification diagram of gravel, sand, and mud.


3.7.1. 40Ar/39Ar resultsAll samples showed well-behaved argon systematics with heating

steps forming isochrons and plateau-like features (Fig. 11) (analyticaldata are presented in Appendix A). For samples ZEM-0601 and ZEM-0602, two runs were done and the results combined to calculatecomposite ages (Table 2). ZEM-0601 shows evidence of excess argon,as seen by the down-stepping ages in the age spectrum (Fig. 11), andhence, the plateau age for this sample is significantly older than the

Fig. 9. Photograph of the axial zone at a site located ∼80 km from the summit of ZempoaFormation. Note the sharp contact between these two units.

isochron age. However we feel that the isochron age (694±19 ka)better accounts for the excess argon and is our preferred age. For ZEM-0602, no significant excess argon is seen and the isochron and plateauages agree (819±13 ka). The three samples from La Corona showedwell-defined plateaus and isochrons and only one run was done oneach sample yielding ages of 924±12 ka, 909±20 ka, and 913±9 ka, forsamples LCR-M02, LCR-M03, and LCR-M04 respectively (Table 2;Fig. 11). LCR-M02 has a slight amount of excess argon while the other

la volcano (close to Pueblo Viejo town). The ZDA deposit is underlain by the Mexcala

Fig. 10. Stratigraphic relations of the ZDA deposit observed around Temixco, Morelos.This field relation places the ZDA in the middle Pliocene.


two do not. Based on this and our experience with ZEM-0601, weinterpret the isochron ages (bold in Table 2) to reflect the age of thesesamples, although the plateau and isochron ages are indistinguish-able. At 2-sigma, the three ages from La Corona are identical andsignificantly older than those from Zempoala.

These new Ar dates of the southern portion of SC provide newinsights in the chronological evolution of the volcanic range. Asmentioned above, Sierra de las Cruces volcanic range is composed ofseveral volcanic structures made of andesitic to dacitic products.Mooser et al. (1974) first proposed a N–S migration of the magmatismof the SC based on a few radiometric data. Subsequent works (Mora-Alvarez et al., 1991; Osete et al., 2000; Romero-Terán, 2001; Mejiaet al., 2005) have contributed with K–Ar and Ar–Ar data at all the

Table 240Ar/39Ar summary of the five samples from Zempoala (ZEM-0601 and ZEM-0602) and La CDeviate

Sample # and location Lithologic description Integrat(ka)

ZEM-0601 19° 02′ 20″N 99° 20′ 11″ W

Porphiritic dacite with abundant plagioclasephenocrysts and some pseudomorphs of amphibole

Run 1: 7

Run 2: 8

2 run co

ZEM-0602 19° 02′ 11″N 99° 20′ 19″ W

Porphiritic andesite, with few plagioclaseand pyroxene phenocrysts.

Run 1: 8

Run 2: 7

2 run co

LCR-M02 19° 06′ 55″N 99° 19′ 59′ W

Porphiricitc dacite with abundant plagioclasephenocrysts and some pseudomorphs of amphibole

961±22

LCR-M03 19° 07′ 02″N 99° 19′ 56″ W

Andesitic lava, with few plagioclase andpyroxene phenocrysts.

884±26

LCR-M04 19° 08′ 30″N 99° 19′ 25″ W

Porphiricitc dacite, similar to LCR-M02 899±11

range, with ages ranging from 3.7 Ma (at La Bufa volcano, in the north)to 0.4 Ma (at Ajusco volcano, in the south). 40Ar/39Ar data reported inthis work represent the first results for La Corona and Zempoalavolcanoes, for which we got younger ages than they were expected,although they could just represent the latest products of thesevolcanoes. However, we think that more radiometric data is needed towell understand the evolution of this volcanic range.

4. Discussion

Collapse of stratovolcano flanks is a common phenomenon in vol-canoes around theworld (Siebert et al., 1987). In addition to magmaticintrusive and eruptive activity, other mechanisms identified or pro-posed as triggers of flank and sector collapses include regional earth-quakes, gravitational spreading, phreatic activity, intense rainfall, andsimple gravitational failure in response to progressive weakening ofthe edifice by hydrothermal alteration (Voight,1981; vanWyk de Vriesand Borgia, 1996; Scott et al., 2001; Kerle and vanWyk de Vries, 2001;Capra et al., 2002; Siebert, 2002). Experimental models have demon-strated that the intersection and reactivation of normal faults beneathvolcanoes can induce collapse in an apical direction between the twofaults (Vidal and Merle, 2000). This appears to be the case of theZempoala volcano, as we discuss below.

4.1. Origin of the instability

Considering the particular geological and structural setting of theZempoala volcano, several factors explain the origin of the S–SE sectorcollapse. The 2100maltitude difference between Zempoala volcano andthe Cuernavaca region makes the volcano unstable towards the south.However, the most important factor favoring the instability of thevolcano is the intersection and reactivation of two normal fault systems(La Pera Fault and Chalma–Zempoala fault systems), which intersect atthe Zempoala volcano. These two faults intersect at an angle of ∼130°,and the direction of the collapse bisects this angle (Fig. 4). Experimen-tal surveys have demonstrated that such a mechanism can promote

orona (LCR-M02, LCR-M03, and LCR-M04) volcanoes. MSWD = Mean Square Weighted

ed age Plateau age(ka)

Plateauinformation

Isochron age(ka)

Isochroninformation

41±56 732±16 7 of 11 fractions 657±30 10 of 11 fractions76% 39Ar release 40Ar/36Ari=307±3MSWD=0.3 MSWD=1.6


mposite 766±14 14 of 22 fractions 694±19 19 of 22 fractions82% 39Ar release 40Ar/36Ari=303±1MSWD=0.7 MSWD=1.1



mposite 815±8 11 of 21 fractions 819±13 13 of 21 fractions82% 39Ar release 40Ar/36Ari=296±5MSWD=1.6 MSWD=1.9

932±13 6 of 8 fractions 924±12 8 of 8 fractions90% 39Ar release 40Ar/36Ari=300±2MSWD=2.0 MSWD=2.0



Fig. 11. 40Ar/39Ar age spectra and isochrons for fivewhole-rock samples from Zempoala (ZEM-0601, ZEM-0602) and La Corona (LCR-M02, LCR-M03, and LCR-M04) volcanoes. MSWD =Mean Square Weighted Deviates. Errors are quoted at ±1 sigma.



voluminous collapsesof stratovolcanoes (Vidal andMerle, 2000). Probablythe E–W fault played the most important role for the collapse, because itshows a higher vertical displacement, and it was active during thePleistocene and Holocene with associated volcanism. Additionally, thedebris avalanche deposit is composed of fractured dacitic and andesiticlava blocks without any other volcanic deposit associated (e.g. pyroclasticdeposits) that could suggest another mechanism for the collapse.

The S–SE sector collapse left a scar structure in the Zempoalavolcano. The flow deposited hummocks in proximal zones, andtraveled ca. 80 km to the south. Much of the material emplaced istoday buried by the deposits of the Cuernavaca Formation. The texturalzonation of the deposit reflects the particular morphology of the area.

4.2. Chronology of the debris avalanche

Theeventbeganwith the collapseof theS–SEpart of thevolcano. Thehummocky of the proximal zone probably consist of large debris ava-lanche blocks (pieces of the old mountain) that simply slide and stopclose to the edifice. The rest of the sliding mass, probably because of itsfragmentation, alteration and water content, rapidly transformed to aflow that continued to move down the slope. During flow, it progres-sively entrained exotic clasts eroded from the basement (limestones andbasalts) and from the substratum or a river bed where it was flowing.This is also evidenced by the presence of important amounts of wellrounded clasts concentrated at its base, and generally attributed tomaterial eroded from river sediments, process that also contribute inincreasing the flow's water content (Vallance and Scott, 1997) (Fig. 8).This flowdeposited themain volume of the debris avalanche in the axialzone. At peripheral areas, the flow encountered an unconfinedtopography that caused the flow to spread out (Fig. 5) and rapidlydepositing coarse material. This process caused a dilution of the flowwith the relative increase of the proportion water/sediment. In thesemarginal areas, the floodwas probably behaving as a debris flow. In fact,despite a few jigsaw fragments (that can be still preservedwithin debrisflows; Scott et al., 2001; Capra and Macías, 2002) the lateral zone ismuch more similar to a debris flow deposit, both for its texturalcharacteristics and planar morphology forming small terraces.

This particular zonation of the ZDA and its transformation to adebris flow is similar to that described by Palmer et al. (1991) for theunconfined wet debris avalanche deposit in Ruapehu volcano, NewZealand. In the ZDA case (H/L 0.03), the water should have played animportant role, not only on the mobility of the flow, but also on thetriggering mechanism. During transport, as the primary debrisavalanche dilutes due to selective deposition, the proportion ofwater increases to produce a lateral transformation in debris flow(Palmer et al., 1991).

As stated earlier, the Zempoala volcano was probably highlyunstable due to the morphology of the volcano and the tectonicsetting. As noted above, the triggering mechanismwas not necessarilyvolcanic activity. The other triggers noted can act in conjunction withan edifice destabilized by tectonic activity, possibly in the case of the

Appendix A

40Ar/39Ar analytical data for five samples from Zempoala and La Corona volcanoes

19° 02′ 11″N; 99° 20′ 19″ W; 3200 masl

ZEM-0601 Whole rock run #1

Laser(mW)

Cum.39Ar

40Ar/39Armeasured

+/− 37Ar/39Armeasured


+/−

500 0.0152 1017.360 16.104 0.4358 0.0131 3.4112 0.06981000 0.0808 218.014 4.049 0.4471 0.0137 0.7362 0.01991200 0.1619 76.437 0.503 0.4038 0.0035 0.2339 0.0018

ZDA and in possible future collapses, because the La Pera Fault is stillactive. The distal transformation of the ZDA to a debrisflow, ascribed toan originally high water content, suggests the possibility of triggeringby intense rainfall.

4.2.1. Hazard ImplicationsAlthough Zempoala is an extinct volcano ∼0.8 million years old,

there are several factors that make it dangerous. The volcanic edificehas steep walls, the rocks are highly fractured, there is activetectonism, and the area is subjected to high rainfall. Another importantfactor is the high topographic gradient to the south, which also makesthe volcano unstable. In addition, the collapse of extinct volcanoes canbe unexpected, and people living nearby do not know the devastatingeffect of such a collapse (i.e. Casita volcano; Sheridan et al., 1999; Scottet al., 2005). Around Zempoala, a future avalanchewould damage citiessuch as Cuenavaca and associated agricultural and touristic activities aswell as the Mexico City to Acapulco highway.

5. Concluding remarks

The distribution of the ZDA deposit is reported, including the threezones (proximal, axial and lateral distal zones), in addition of newdata ofthe areal distribution (600 km2), volume (6 km3), trajectory, the locationof the scar related to the collapse, a run out of 80 km, and a frictionalcoefficient of 0.03. The reactivation of two fault systems (the active E–Wand the NE–SW faults) caused the instability of Zempoala volcano thatcombined with another non-volcanic cause (an earthquake or intenserainfall) provoked the collapse of the S–SE flank of the volcano.

We established the age and the composition of the Zempoalavolcano rocks varying from 0.7–0.8 Ma and from andesite to dacite(61.59–67.66 wt.% SiO2), with phenocrysts of plagioclase, pyroxene,amphibole, and Fe–Ti oxides, very similar to those rocks found inthe ZDA deposit. Based on the 40Ar/39Ar dates reported in this work(Table 2) the ZDAD must be younger than 0.7 Ma.

Finally, this work will be useful for future investigations aboutgeologic hazards in the studied area, considering a potential sectorcollapse of Zempoala volcano.

Acknowledgments

This work was supported by CONACYT (45843) and PAPIIT(IN103205) grants (J.L. Arce), and a scholarship provided by theInstituto de Geologia to the second author to conclude his thesis. Wethank D. Aparicio from the Instituto de Geología, UNAM, for his helpduring laboratory work and sample processing. We are indebted to R.Lozano who performed chemical analyses at the X-ray fluorescencelaboratory. The Servicio Geológico Metropolitano (SEGEOMET) pro-vided digital topography of the area. J.D. Keppie made grammaticalimprovements to the first version of the manuscript. We are indebtedto C. Siebe and K.M. Scott for their constructive reviews, thatsubstantially improved the ideas stated in the paper.

Weighted average of J from standards=8.542e−05+/−2.803e−07

% Atm.40Ar

Ca/K +/− Cl/K +/− 40Ar⁎/39ArK

+/− Age(ka)

+/−(ka)

99.1 0.7999 0.0241 0.00826 0.00334 9.375 13.444 1444.2 2070.399.8 0.8206 0.0252 0.00161 0.00085 0.466 4.316 71.9 665.290.4 0.7411 0.0065 0.00136 0.00027 7.329 0.380 1129.1 58.5

(continued on next page)

Appendix A19° 02′ 11″N; 99° 20′ 19″ W; 3200 masl

ZEM-0601 Whole rock run #1Weighted average of J from standards=8.542e−05+/−2.803e−07

Laser(mW)

Cum.39Ar

40Ar/39Armeasured



+/− % Atm.40Ar

Ca/K +/− Cl/K +/− 40Ar⁎/39ArK

+/− Age(ka)

+/−(ka)

1400 0.2368 40.802 0.313 0.3784 0.0040 0.1192 0.0016 86.3 0.6945 0.0073 0.00087 0.00016 5.568 0.463 858.0 71.31600 0.3121 34.024 0.225 0.3599 0.0027 0.0993 0.0016 86.3 0.6606 0.0050 0.00049 0.00020 4.675 0.448 720.4 69.11800 0.3825 21.939 0.201 0.3661 0.0039 0.0588 0.0011 79.1 0.6719 0.0071 0.00038 0.00013 4.570 0.353 704.2 54.42000 0.4446 15.745 0.171 0.3687 0.0041 0.0366 0.0008 68.6 0.6766 0.0076 0.00045 0.00020 4.941 0.268 761.3 41.32500 0.5320 17.586 0.196 0.3759 0.0044 0.0437 0.0008 73.3 0.6900 0.0081 0.00053 0.00009 4.685 0.269 721.9 41.53000 0.6306 16.475 0.181 0.3920 0.0059 0.0394 0.0008 70.5 0.7194 0.0108 0.00061 0.00014 4.850 0.265 747.2 40.85000 0.8533 16.817 0.101 0.4630 0.0028 0.0412 0.0005 72.2 0.8499 0.0052 0.00092 0.00008 4.665 0.172 718.8 26.58000 1.0000 22.615 0.175 0.5920 0.0041 0.0601 0.0008 78.4 1.0867 0.0075 0.00142 0.00006 4.889 0.246 753.3 38.0Integrated 54.327 0.133 0.4343 0.0016 0.1676 0.0013 91.1 0.7971 0.0029 0.00102 0.00009 4.809 0.361 740.9 55.6


500 0.0091 1262.248 21.042 0.5073 0.0184 4.0956 0.0716 95.9 0.9312 0.0337 0.01247 0.00348 52.022 7.238 7999.4 1110.61000 0.0427 399.366 4.704 0.4756 0.0114 1.3270 0.0197 98.2 0.8730 0.0210 0.00236 0.00104 7.255 3.700 1117.7 569.81200 0.0875 163.798 1.863 0.4512 0.0092 0.5287 0.0065 95.4 0.8282 0.0168 0.00221 0.00048 7.567 0.864 1165.8 133.11400 0.1376 89.495 0.630 0.4275 0.0039 0.2821 0.0038 93.1 0.7846 0.0071 0.00106 0.00040 6.144 1.051 946.6 161.91600 0.1988 64.867 0.425 0.4042 0.0045 0.2005 0.0028 91.3 0.7418 0.0083 0.00083 0.00041 5.633 0.790 867.9 121.81800 0.2640 47.702 0.309 0.3882 0.0032 0.1429 0.0020 88.5 0.7124 0.0058 0.00075 0.00020 5.483 0.599 844.8 92.22000 0.3197 33.108 0.389 0.3855 0.0055 0.0943 0.0013 84.1 0.7076 0.0101 0.00035 0.00018 5.256 0.413 809.8 63.72500 0.4066 30.099 0.258 0.3789 0.0030 0.0839 0.0013 82.3 0.6955 0.0056 0.00023 0.00008 5.318 0.413 819.3 63.73000 0.5089 24.671 0.215 0.3798 0.0033 0.0669 0.0010 80.1 0.6971 0.0060 0.00052 0.00012 4.895 0.312 754.3 48.15000 0.7501 22.356 0.148 0.4411 0.0041 0.0585 0.0009 77.2 0.8096 0.0075 0.00080 0.00008 5.094 0.269 784.9 41.58000 1.0000 24.807 0.104 0.5694 0.0028 0.0671 0.0009 79.8 1.0452 0.0051 0.00124 0.00007 4.996 0.264 769.7 40.6Integrated 62.351 0.135 0.4542 0.0015 0.1914 0.0007 90.7 0.8337 0.0028 0.00104 0.00007 5.797 0.198 893.2 30.7

19° 02′ 20″ N; 99° 20′ 11″ W; 3100 masl


Laser(mW)

Cum.39Ar

40Ar/39Armeasured



+/− % Atm.40Ar

Ca/K +/− Cl/K +/− 40Ar⁎/39ArK

+/− Age(ka)

+/−(ka)

500 0.0227 156.367 1.241 0.5970 0.0078 0.5224 0.0067 98.7 1.0959 0.0142 0.00717 0.00050 2.010 1.804 309.8 277.91000 0.0800 78.751 0.700 0.7932 0.0097 0.2554 0.0033 95.8 1.4563 0.0178 0.00385 0.00031 3.309 0.924 510.0 142.31250 0.3488 14.612 0.141 0.7154 0.0069 0.0315 0.0004 63.4 1.3133 0.0128 0.00128 0.00006 5.343 0.153 823.2 23.51500 0.6865 7.870 0.046 0.6838 0.0041 0.0086 0.0002 31.5 1.2553 0.0076 0.00125 0.00005 5.373 0.075 827.9 11.61700 0.8413 7.365 0.065 0.8201 0.0068 0.0081 0.0005 31.6 1.5056 0.0124 0.00271 0.00011 5.021 0.149 773.6 22.91900 0.9032 9.143 0.119 1.1581 0.0127 0.0113 0.0010 35.5 2.1268 0.0233 0.00624 0.00018 5.879 0.314 905.8 48.32200 0.9396 12.583 0.169 1.6062 0.0225 0.0194 0.0022 44.6 2.9506 0.0413 0.01396 0.00041 6.967 0.670 1073.3 103.33000 0.9763 16.765 0.209 2.4301 0.0278 0.0277 0.0022 47.7 4.4667 0.0512 0.02800 0.00035 8.776 0.679 1352.0 104.65000 0.9947 44.926 0.662 6.5558 0.0928 0.0954 0.0050 61.6 12.0860 0.1719 0.03011 0.00075 17.327 1.523 2668.3 234.48000 1.0000 278.219 5.359 9.5307 0.1920 0.7773 0.0202 82.3 17.6082 0.3571 0.03113 0.00187 49.626 5.217 7631.8 800.6Integrated 19.733 0.069 1.0005 0.0037 0.0474 0.0003 70.7 1.8371 0.0068 0.00421 0.00005 5.784 0.102 891.2 16.0


500 0.0306 134.135 1.651 0.6368 0.0083 0.4505 0.0064 99.2 1.1690 0.0152 0.00513 0.00028 1.044 1.696 160.9 261.31000 0.1202 55.159 0.776 0.8970 0.0130 0.1725 0.0029 92.3 1.6470 0.0240 0.00265 0.00017 4.236 0.856 652.7 131.81200 0.2688 13.803 0.078 0.8298 0.0039 0.0281 0.0004 59.7 1.5234 0.0072 0.00130 0.00008 5.548 0.125 854.8 19.21400 0.4442 7.931 0.060 0.7272 0.0043 0.0084 0.0003 30.8 1.3350 0.0079 0.00103 0.00006 5.470 0.095 842.7 14.61600 0.5972 7.395 0.067 0.7272 0.0053 0.0078 0.0003 30.3 1.3351 0.0097 0.00110 0.00004 5.138 0.111 791.7 17.01800 0.7155 7.845 0.085 0.7964 0.0085 0.0093 0.0004 34.3 1.4621 0.0157 0.00184 0.00010 5.139 0.145 791.8 22.32000 0.7939 7.993 0.101 0.9237 0.0113 0.0098 0.0007 35.5 1.6960 0.0207 0.00292 0.00017 5.144 0.221 792.5 34.12500 0.8799 8.999 0.099 1.1337 0.0107 0.0130 0.0007 41.6 2.0819 0.0196 0.00564 0.00017 5.241 0.212 807.5 32.73000 0.9404 11.260 0.150 1.5016 0.0184 0.0210 0.0011 54.2 2.7583 0.0338 0.01093 0.00025 5.150 0.338 793.4 52.15000 0.9895 24.187 0.319 3.3856 0.0417 0.0655 0.0019 79.0 6.2272 0.0770 0.02283 0.00035 5.083 0.567 783.2 87.38000 1.0000 458.005 3.026 10.2331 0.0664 1.5162 0.0115 97.6 18.9157 0.1237 0.03233 0.00126 10.857 2.844 1672.4 437.9Integrated 22.675 0.080 1.0918 0.0033 0.0596 0.0003 77.4 2.0049 0.0060 0.00399 0.00004 5.119 0.112 788.7 17.4

19° 06′ 55″ N; 99° 19′ 59″ W; 3775 masl

LCR-M02 Whole rockWeighted average of J from standards=8.430e−05+/−2.136e−07

Laser(mW)

Cum.39Ar

40Ar/39Armeasured



+/− % Atm.40Ar

Ca/K +/− Cl/K +/− 40Ar⁎/39ArK

+/− Age(ka)

+/−(ka)

300 0.0245 754.396 13.204 0.3883 0.0114 2.5174 0.0473 98.6 0.7127 0.0209 0.01438 0.00104 10.494 5.182 1595.3 787.4600 0.0976 64.220 0.219 0.4996 0.0040 0.1927 0.0013 88.6 0.9170 0.0074 0.00726 0.00025 7.297 0.377 1109.4 57.21000 0.2866 12.092 0.044 0.5248 0.0056 0.0195 0.0005 47.5 0.9633 0.0102 0.00305 0.00008 6.337 0.141 963.5 21.51500 0.5252 9.192 0.055 0.4803 0.0038 0.0099 0.0003 31.6 0.8816 0.0070 0.00170 0.00007 6.271 0.097 953.5 14.72000 0.7363 8.639 0.036 0.4942 0.0042 0.0091 0.0003 30.7 0.9071 0.0078 0.00151 0.00007 5.973 0.087 908.1 13.33000 0.9225 9.464 0.054 0.5332 0.0056 0.0116 0.0004 36.0 0.9787 0.0103 0.00167 0.00010 6.038 0.119 918.1 18.1

Appendix A (continued)


Appendix A19° 02′ 11″N; 99° 20′ 19″ W; 3200 masl


Laser(mW)

Cum.39Ar

40Ar/39Armeasured



+/− % Atm.40Ar

Ca/K +/− Cl/K +/− 40Ar⁎/39ArK

+/− Age(ka)

+/−(ka)

5000 0.9632 13.964 0.096 1.0990 0.0136 0.0291 0.0017 61.0 2.0179 0.0251 0.00275 0.00028 5.436 0.509 826.5 77.49000 1.0000 17.969 0.108 1.3059 0.0111 0.0399 0.0019 65.1 2.3982 0.0204 0.00297 0.00061 6.269 0.546 953.2 83.0Integrated 32.468 0.070 0.5562 0.0022 0.0885 0.0005 80.5 1.0210 0.0039 0.00272 0.00005 6.321 0.141 961.1 21.5

19° 07′ 02″ N; 99° 19′ 56″ W; 3750 masl


Laser(mW)

Cum.39Ar

40Ar/39Armeasured



+/− % Atm.40Ar

Ca/K +/− Cl/K +/− 40Ar⁎/39ArK

+/− Age(ka)

+/−(ka)

300 0.0069 1446.373 49.735 1.6774 0.0812 4.9669 0.1734 101.5 3.0812 0.1494 0.00498 0.00632 −21.258 11.290 −3236.0 1720.1600 0.0597 214.440 1.207 1.8785 0.0210 0.7045 0.0063 97.0 3.4509 0.0386 0.00174 0.00052 6.389 1.828 971.5 277.91000 0.1932 37.132 0.173 1.5402 0.0110 0.1058 0.0017 84.0 2.8289 0.0202 0.00105 0.00015 5.955 0.496 905.5 75.31500 0.3925 21.714 0.125 1.3766 0.0109 0.0531 0.0010 71.9 2.5282 0.0201 0.00089 0.00014 6.099 0.293 927.3 44.62000 0.5855 11.870 0.065 1.3754 0.0109 0.0207 0.0005 50.9 2.5260 0.0201 0.00131 0.00012 5.825 0.153 885.7 23.23000 0.8515 11.447 0.049 1.1760 0.0067 0.0193 0.0007 49.2 2.1594 0.0123 0.00262 0.00010 5.810 0.206 883.3 31.45000 0.9724 16.696 0.121 2.2440 0.0213 0.0358 0.0012 62.4 4.1235 0.0391 0.00349 0.00017 6.278 0.358 954.5 54.49000 1.0000 36.537 0.424 4.0761 0.0598 0.1018 0.0036 81.6 7.4990 0.1103 0.00283 0.00076 6.744 1.045 1025.4 158.8Integrated 38.949 0.093 1.5530 0.0050 0.1124 0.0006 85.1 2.8524 0.0091 0.00190 0.00008 5.816 0.172 884.3 26.3

19° 08′ 30″ N; 99° 19′ 25″ W; 3540 masl


Laser(mW)

Cum.39Ar

40Ar/39Armeasured



+/− % Atm.40Ar

Ca/K +/− Cl/K +/− 40Ar⁎/39ArK

+/− Age(ka)

+/−(ka)

300 0.0218 278.995 5.267 0.3608 0.0092 0.9303 0.0181 98.5 0.6622 0.0168 0.00602 0.00076 4.080 2.434 620.5 370.1600 0.1021 31.734 0.084 0.4955 0.0045 0.0876 0.0010 81.5 0.9094 0.0082 0.00187 0.00008 5.865 0.278 891.7 42.31000 0.3218 7.926 0.049 0.4742 0.0037 0.0059 0.0002 21.6 0.8704 0.0068 0.00080 0.00006 6.193 0.071 941.7 10.81500 0.6174 6.820 0.040 0.4600 0.0039 0.0029 0.0002 12.2 0.8443 0.0071 0.00076 0.00005 5.963 0.059 906.7 9.02000 0.8505 6.707 0.034 0.5379 0.0044 0.0027 0.0002 11.2 0.9872 0.0080 0.00131 0.00005 5.932 0.068 902.0 10.43000 0.9711 7.364 0.044 0.8806 0.0069 0.0055 0.0004 21.2 1.6167 0.0126 0.00392 0.00009 5.780 0.114 878.8 17.35000 0.9918 13.812 0.178 2.5184 0.0391 0.0279 0.0021 58.5 4.6286 0.0720 0.01490 0.00039 5.726 0.608 870.6 92.49000 1.0000 49.217 0.609 4.3143 0.0675 0.1538 0.0063 91.8 7.9384 0.1245 0.01791 0.00135 4.069 1.810 618.7 275.3Integrated 15.532 0.048 0.6070 0.0022 0.0326 0.0002 61.9 1.1141 0.0040 0.00192 0.00003 5.915 0.068 899.4 10.7

Appendix A (continued)


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Late Pleistocene flank collapse of Zempoala volcano (Central Mexico) and the role of fault...

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