Tectonics and volcanism of Sierra Chichinautzin: extension at the front of the Central Trans-Mexican Volcanic belt

Journal of Volcanology and Geothermal Research 93 Ž1999. 125–150 www.elsevier.comrlocaterjvolgeores

Tectonics and volcanism of Sierra Chichinautzin: extension at the front of the Central Trans-Mexican Volcanic belt a,) Alvaro Marquez , Surendra P. Verma b,1, Francisco Anguita ´ Roberto Oyarzun c,3, Jose´ L. Brandle d,4 b

a,2

,

a Departamento de Petrologıa Facultad de Ciencias Geologicas, UniÕersidad Complutense, 28040 Madrid, Spain ´ y Geoquımica, ´ ´ Centro de InÕestigacion de Mexico, Apartado Postal 34, Temixco, Mor. 62580, Mexico ´ en Energıa, ´ UniÕersidad Nacional Autonoma ´ ´ c Departamento de Cristalografıa UniÕersidad Complutense, 28040 Madrid, Spain ´ y Mineralogıa, ´ Facultad Ciencias Geologicas, ´ d Instituto de Geologıa, 28040 Madrid, Spain ´ Consejo Superior de InÕestigaciones Cientıficas, ´

Received 20 January 1998; accepted 3 May 1999

Abstract Because of its recent activity and position at the southern magmatic front of the Trans-Mexican Volcanic Belt ŽTMVB., the Sierra Chichinautzin volcanic field ŽSCN. is a key area for the understanding of this controversial volcanic province. Volcanic activity has built more than 220 monogenetic volcanoes Žshields, scoria cones, thick lava flows, and hydromagmatic structures. during the last 40,000 years, for a total volume of about 470 km3. The SCN basalts are geochemically similar to OIBs, while the intermediate and felsic volcanic rocks show a calc-alkaline trend and abundant evidence for magma mixing. The structural analysis of this volcanic field and surrounding areas has been based on field data, satellite images, and a method for detecting volcanic center alignments. The tectonic data, together with geophysical evidence, confirm active general N–S extensional conditions with a strike–slip component for the SCN area, the same structural setting that prevails in the rest of the Central TMVB. Extensional tectonics, a negative regional Bouger gravity anomaly, a low-velocity mantle, high heat flow, and shallow seismicity suggest a rift-type setting involving the upwelling of anomalous mantle beneath the Central TMVB. The combined petrological, structural and geophysical arguments support that the SCN volcanism is rift-related, and rule out processes involving the subduction of the Cocos plate, which casts further doubts on the standard subduction model for the TMVB volcanism. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Trans-Mexican Volcanic Belt; Chichinautzin; volcanology; tectonics; petrology; OIB; calc-alkaline

1. Introduction ) Corresponding author. Tel.: q34-913944906; fax: q34915442535; E-mail: [email protected] 1 Tel.: q 52-73250044; fax: 52-73250018. E-mail: [email protected]. 2 Tel.: q34-913944906; fax: q34-915442535; E-mail: [email protected]. 3 Tel.: q34-913944878; fax: q34-913944872; E-mail: [email protected]. 4 Tel.: q34-913944903; fax: q34-915442535; E-mail: [email protected].

The Trans-Mexican Volcanic Belt ŽTMVB. is a linear, 1000-km-long, 20 to 200-km-wide east–weststriking volcanic province that crosses Mexico from Puerto Vallarta on the Pacific coast to Veracruz on the Gulf of Mexico ŽFig. 1a. ŽRobin, 1982a; Verma, 1987.. The volcanic activity spans at least from the Miocene up to present, with six of the volcanoes ŽPico de Orizaba, Popocatepetl, Jorullo, Paricutın ´ ´

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 8 5 - 2

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A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

Colima, and Ceboruco. ŽFig. 1a. showing historical activity. The belt has more than 8000 volcanic structures ŽRobin, 1982a. with various morphologies, including stratovolcanoes, calderas, domes and monogenetic cone fields. The TMVB is not a typical magmatic arc: Ž1. it is orientated at a 15–208 angle to the Middle America trench ŽMolnar and Sykes, 1969., so that the trench is some 400 km from the eastern end of the TMVB, but only 150 km from the western one; Ž2. the

Wadati–Benioff zone is poorly defined, and is absent below the volcanic central belt ŽNixon, 1982; Singh and Pardo, 1993; Pardo and Suarez, 1995. ´ ŽFig. 1b.; Ž3. there is a deep gravity low Ž- y200 mGal. that suggests the existence of low-density mantle layer ŽVp s 7.6 kmrs, densitys 3.29 grcm3 . beneath the lower crust ŽMolina-Garza and UrrutiaFucugauchi, 1993. ŽFig. 1c.; and Ž4. geochemically distinct types Ždominant calc-alkaline and minor OIB-type. of volcanic rocks occur in close associa-

Fig. 1. Geologic and geophysical features of the TMVB. Ža. TMVB Žshaded area., active volcanic centers Žsolid triangles., and major cities Žsolid squares.. Žb. cross-section ŽA–AX . of seismicity Žsee a for location. and the volcanic front Žsolid triangles. Žsimplified after Pardo and X Suarez, 1995.. Žc. crustal model for gravity profile B–B Žsee a for location.; values indicate densities expressed in grcm3 Žsimplified after ´ Molina-Garza and Urrutia-Fucugauchi, 1993..

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

tion ŽAllan et al., 1991; Righter et al., 1995; Luhr, 1997.. Two contrasting hypotheses has been suggested to explain the origin of the TMVB: Ž1. a standard subduction model Že.g., Pardo and Suarez, 1995, and ´ references therein.; and Ž2. a less conventional model involving the accumulation of volcanic products over an active fracture zone Že.g., Shurbet and Cebull, 1984.. Additionally, Moore et al. Ž1994. suggested the presence of a mantle plume to explain the OIB volcanism in the western TMVB. The subduction model is complicated by the peculiar orientation of the TMVB with respect to the trench, the low-density mantle layer and seismic-gap, and the pervading OIB volcanism. The main difficulty encountered by the fracture model is the dominantly calc-alkaline nature of the volcanic rocks. Some authors have published papers defending aspects of both viewpoints, and proposed hybrid models Že.g., Allan, 1986; Urrutia-Fucugauchi and Bohnel, 1988.. The ¨ latter suggest that the subducted plate is responsible for the magmatism, and the regional fractures determine the geometry and, to a certain extent, the chemistry of magmatism. It seems clear that new data on the relationship between tectonics and magmatism in the TMVB are needed.

2. Regional geology of Sierra Chichinautzin Volcanic Field (SCN) area A key zone within the TMVB is the SCN ŽFig. 1a and Fig. 2., located on its southern boundary. The SCN comprises over 220 Quaternary monogenetic volcanic centers. Volcanic materials cover approximately 2400 km 2 Ž98840 X –99840 X W, 18830 X – 19830X N. of the Distrito Federal ŽDF., and Mexico and Morelos states. With a general E–W direction, Chichinautzin also marks the southern limit of the Mexico Basin, and is situated between two QuaterŽSiebe et al., nary stratovolcanoes: Popocatepetl ´ 1996. and Nevado de Toluca ŽMacıas ´ et al., 1997.. The Popocatepetl–Iztaccıhuatl, the Sierra de Las ´ ´ Cruces, and Nevado de Toluca volcanoes occur on an important NNW structural trend. The relationships between volcanism and this structural trend have been recently stressed by Alaniz-Alvarez et al. Ž1998., who relate the polygenetic volcanoes to the

127

NNW tectonic direction. Our catalogue ŽTable 1. shows that volcanic activity of the SCN has built 221 Quaternary volcanoes located in the Sierra Chichinautzin, and in the valleys of Mexico, Toluca and Cuernavaca. Although the latter is outside of the volcanic plateau, it still shows a few scattered cones that can be ascribed to the SCN. General studies on the Sierra Chichinautzin units have been offered by Schlaepfer Ž1968., Fries Ž1960; 1966., Negendank Ž1972a; b; 1973., Mooser et al. Ž1974., Demant Ž1981., De Cserna et al. Ž1988., and Vazquez-Sanchez and Jaimes-Palomera Ž1989.. ´ ´ Verma and Armienta Ž1985. studied the geochemistry of volcanics from the central SCN. Bloomfield Ž1975., Martin del Pozzo Ž1982; 1989. and Swinamer Ž1989. reported more complete studies about the volcanology and petrology of, respectively, the western, central and central-eastern areas of the SCN. However, no comprehensive studies of the whole volcanic field, including volcanological, geochemical and structural data, have been published. The SCN volcanic rocks overlie sediments of the Mexico, Toluca and Cuernavaca basins, other TMVB volcanic materials, and Sierra Madre Oriental sedimentary rocks. The latter crop out in the Cuernavaca valley, and are mainly Cretaceous marine limestones and Paleogene detrital continental rocks that were folded and faulted during Eocene–Oligocene ŽFries, 1960, 1966.. The oldest possible TMVB unit ŽTepoztlan ´ Formation. in this area is an 800 m thick pyroclastic and volcaniclastic flow deposit of probable Middle to Upper Miocene age ŽVazquez-Sanchez ´ ´ and Jaimes-Palomera, 1989.. The Tepoztlan ´ Formation is fractured and tilted to the north, and makes most of the topographic steps between the Cuernavaca valley and the TMVB plateau. In the Sierra de las Cruces–Zempoala area, the TMVB Pliocene volcanics Ždacitic and andesitic domes, lavas, and pyroclastic flow deposits. are found along a NNW– SSE axis, with ages generally decreasing towards the South ŽMora-Alvarez et al., 1991.. The SCN vol´ canic rocks also overlie lavas and pyroclastic flow deposits from the Popocatepetl volcano in the east ´ ŽFries, 1966., and Nevado de Toluca pyroclastic deposits in the west ŽBloomfield, 1975.. The Mexico, Toluca and Cuernavaca basins are filled with Pliocene and Quaternary lacustrine and alluvial sediments ŽFries, 1960, 1966; Schlaepfer, 1968..

128 A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´ Fig. 2. The SCN Žoutlined; modified after Bloomfield, 1975. on a Landsat TM image, displaying location of monogenetic volcanoes Žlava-domes, shield volcanoes, and scoria cones.. Dos: Dos Cerros, Chi: Chichinautzin, Glo: Mesa La Gloria, Lam: Lama, Pel: Pelado, Tab: Tabaquillo, Teh: Tehutli, Tet: Tetepetl, Tex: Texontepec, Tez: Tezoyuca, Tla: Tlaloc, Top: Topilejo, Tres: Tres Cruces, Xic: Xicomulco, Xit: Xitle.

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The volcanism of the SCN is also important from a social point of view because of its very young age, and its close proximity to the densely populated cities of Mexico DF and Cuernavaca, with over 22,000,000 people. Although there have been no historical eruptions in the SCN, the area must be considered as an active one. Xitle lavas have flowed into the Mexico basin where the National University of Mexico ŽUNAM. is now built. These lava flows Ž14 C-dated as 2422 B.P. by Libby, 1955; 2030 and 2060 B.P. by Martin del Pozzo et al., 1997. damaged the Cuicuilco pyramids. Moreover, Chichinautzin means ‘‘Burning Lord’’ in Nauhatl, which probably means that the eruption of Chichinautzin volcano was witnessed by the early inhabitants of the area ŽMartin del Pozzo, 1982.. All the volcanic activity in the SCN is young; paleomagnetic measurements ŽMooser et al., 1974; Herrero and Pal, 1978. show normal polarities, thus, ages should be younger than 700,000 years. 14 C age determinations of paleosoils and organic matter interbedded between SCN volcanics have always given ages younger than 40,000 years ŽBloomfield, 1975; Martin del Pozzo, 1989.. The geomorphology of many volcanoes Že.g., Chichinautzin, Pelado, Tlaloc, Teuhtli, Dos Cerros, Texontepec, Tres Cruces, Tetepetl., indicate Holocene activity Žyounger than 10,000 years. in an area larger than 600 km2 . The SCN volcanism has been coeval with that of Nevado de Toluca and Popocatepetl. Bloomfield and ´ Valastro Ž1974. gave 14 C dates of 25,000 and 11,600 B.P. for two prominent plinian pumice deposits from Nevado de Toluca, and Macıas ´ et al. Ž1997. dates ash flow deposits and surge deposits at 3330 years B.P. 14 C age determinations for Popocatepetl indi´ cate that although the activity of the ‘‘modern volcano’’ began between 50,000 and 30,000 years B.P. ŽBoudal and Robin, 1988., most 14 C age measurements are younger Že.g., Boudal and Robin, 1988; Siebe et al., 1996.. Age determinations for volcanic rocks from the neighbouring Sierra de las Cruces– Zempoala area Ž2.87 to 0.39 Ma; Mora-Alvarez ´ et al., 1991. and Iztaccıhuatl volcano Ž0.9 to 0.08 Ma; ´ Nixon et al., 1987; Nixon, 1988a. show that their activity preceded that of the SCN. From the volcano-tectonic point of view, the SCN marks the front of the alleged magmatic arc resulting from subduction at the Middle America trench, 300

129

km to the southwest. If the volcanism and tectonics of the SCN are related to subduction, the magmatic and tectonic activity should display clear relationships with the subducted Cocos plate. If not, then further doubts may be put upon the classic subduction hypothesis for the TMVB.

3. Volcanism of Sierra Chichinautzin 3.1. Characteristics of Õolcanic actiÕity The volcanism of the SCN is monogenetic. Previous authors ŽBloomfield, 1975; Martin del Pozzo, 1982. have defined three different types of structures ŽFig. 2.: Ža. scoria cones with associated lava flows Že.g., Texontepec, Xitle.; Žb. shield volcanoes, crowned with a cone Že.g., Pelado, Teuhtli.; Žc. lava-domes, with heights of up to 200 m Že.g., Mesa la Gloria, Tabaquillo.. Recent field research has revealed that hydromagmatic activity is also important in the SCN. These deposits are of the base surge type, composed of centimetric layers of ash, with well developed crossbedding structures ŽFig. 3.. These hydromagmatic deposits belong to the sandwave and planar facies ŽWohletz and Sheridan, 1979.. Their abundance with respect to the other facies indicate that the water– magma ratio in these eruptions was between 0.1 and 0.5 ŽWohletz and Heiken, 1991.. The SCN surge deposits are associated with several shield volcanoes Že.g., Pelado, Tres Cruces. and cones, some of the latter showing morphologies similar to tuff cones and tuff rings. It is unknown how many cones have a hydromagmatic component, and how many of them are scoria cones. Further field research is needed, because the possibility of a hydromagmatic eruption in the SCN would notably increase the potential volcanic risk of the area. Our volcanic catalogue includes 221 structures ŽFig. 2., among which cones Žscoria and hydromagmatic structures. are the most frequent type Ž201.. Other structures include 10 shield volcanoes and 10 lava-domes. In some cases, several cones were built during a single eruption process, so the number of eruptions must be less than the number of volcanic

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A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

Table 1 Catalogue of volcanic cones of the SCN Name

Lat ŽN.

Long ŽW.

Map a

State

HASLb

Height

Base Dc

Cone Dd

HrD e

Volume f

Xictontle Xitle Cuatzontle Panza Jumento Malinal La Magdalena Yololica Zompole Ahua Pole Salva Teuhtli Mezontepec Judio S Malacatepe Texoxocol Pelado Toxtepec Cuatzin S Cuatzin T Cuatzin San Miguel Xistune Tune Ocusacayo S Ocusacayo Tlaloc S Tlaloc T Tlaloc C del Agua Tuxtepec El Capulin Raices S Cajete Cajete Tepeyahualco Los Cardos S Los Cardos Tesoyo Cima T Cima Acopiaxco S Acopiaxco El Guarda Tetzacoatl Tumiac Piripitillo Chiguiriteri Tuxtepec San Bartolo San Bartolito

19:14:48 19:14:28 19:14:21 19:13:11 19:12:29 19:13:19 19:13:40 19:13:15 19:12:11 19:12:37 19:12:08 19:12:19 19:13:23 19:11:18 19:10:49 19:09:45 19:09:08 19:09:00 19:09:37 19:09:23 19:09:15 19:09:21 19:09:41 19:08:37 19:08:28 19:08:28 19:07:51 19:06:33 19:06:28 19:06:33 19:05:49 19:07:50 19:07:43 19:06:15 19:06:16 19:06:11 19:06:50 19:05:35 19:05:45 19:05:43 19:06:34 19:06:36 19:07:05 19:07:01 19:06:31 19:08:18 19:08:02 19:07:04 19:06:55 19:07:15 19:06:39 19:06:37

99:13:36 99:13:17 99:12:42 99:17:26 99:18:46 99:13:03 99:11:09 99:10:45 99:06:51 99:06:42 99:06:15 99:05:12 99:01:46 99:13:45 99:16:12 99:15:33 99:13:48 99:13:01 99:08:59 99:06:32 99:06:10 99:05:34 99:00:14 99:00:21 99:01:07 99:04:05 99:04:04 99:02:00 99:01:49 99:01:12 99:00:10 99:16:42 99:17:23 99:15:24 99:14:57 99:14:45 99:14:30 99:16:00 99:15:35 99:13:29 99:11:41 99:11:15 99:09:56 99:09:38 99:09:39 99:07:39 99:06:55 99:07:06 99:06:41 99:06:27 99:05:31 99:05:03

E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49

DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. MEX. MEX. MOR. MOR. MOR. DF. MOR. MOR. MOR. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF. DF.

3050 3150 2960 3600 3000 3150 2940 3040 2610 2540 2560 2650 2710 3470 3420 3410 3350 3620 2920 3420 3450 3330 2840 3100 3150 3170 3090 3690 3640 3500 3580 3150 3160 3160 3120 3140 3060 3170 3160 3180 3050 3080 3300 3310 3170 3310 3330 3140 3190 3129 3220 3160

80 200 110 100 150 170 140 150 110 140 30 50 110 100 100 200 50 200 50 60 100 70 50 110 70 110 50 180 100 20 100 40 140 130 60 200 40 210 130 180 40 30 180 200 100 160 170 30 70 100 140 50

450 750 500 1000 750 750 875 750 675 950 375 475 750 800 750 600 375 1100 300 725 1250 450 425 600 450 500 400 1000 750 200 750 450 600 1000 250 1000 650 400 650 1000 600 400 800 500 675 700 1050 500 500 550 750 1000

175 250 250 400 300 270 400 270 200 425 100 200 250 100 50 200 175 420 200 300 300 150 175 250 170 120 120 350 250 150 300 300 175 300 125 350 450 150 150 200 250 150 250 150 75 250 350 300 120 150 200 250

0.178 0.267 0.22 0.1 0.2 0.227 0.16 0.2 0.163 0.147 0.08 0.105 0.147 0.125 0.133 0.333 0.133 0.182 0.167 0.083 0.08 0.156 0.118 0.183 0.156 0.22 0.125 0.18 0.133 0.1 0.133 0.089 0.233 0.13 0.24 0.2 0.062 0.525 0.2 0.18 0.067 0.075 0.225 0.4 0.148 0.229 0.162 0.06 0.14 0.182 0.187 0.05

0.0066 0.0429 0.0127 0.0412 0.0348 0.0376 0.0472 0.0332 0.0183 0.055 0.0015 0.0048 0.0236 0.0193 0.0159 0.0275 0.0031 0.0977 0.0025 0.0132 0.0536 0.0054 0.0038 0.0166 0.0057 0.0094 0.0029 0.0701 0.0215 0.0005 0.0232 0.0045 0.0183 0.0478 0.0017 0.0778 0.0097 0.0135 0.0186 0.059 0.0061 0.0019 0.0429 0.0184 0.0135 0.0308 0.0716 0.0039 0.006 0.0108 0.0278 0.0173

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

131

Table 1 Žcontinued. Name

Lat ŽN.

Long ŽW.

Map a

State

HASLb

Height

Base Dc

Cone Dd

HrD e

Volume f

Chichinautzin El Hoyo Manteca El Caballito El Palomito Tres Cumbres Tezontle Micro Mixte Cahu Yecahuazac Quimixtecpec Los Otates S Los Otates Suchiooc Spajona Soco Ocotecatl Pajonal Ololica Solo Texontepec Atlas Xalatla pehualtepec Conto Teconto Las Ratas Tilapa El Calpo Cuates S Cuates S Calpul Calpulu Atizapan La Silva Negro Tomasquillo Quatl Santa Fe Coatepec Boludo Coyolte Cinco Tuxtep Tres Cruces Holotepec Tezontle S Tezontle Tepecingo S Cuate T Cuate Cuate

19:05:19 19:05:18 19:05:18 19:05:15 19:05:24 19:03:34 19:03:06 19:04:13 19:04:21 19:04:23 19:04:32 19:03:36 19:03:48 19:03:37 19:03:42 19:05:49 19:05:18 19:05:05 19:04:30 19:03:37 19:03:23 19:14:33 19:13:52 19:12:31 19:12:57 19:11:42 19:11:41 19:11:41 19:11:33 19:11:44 19:11:34 19:11:37 19:11:33 19:11:34 19:10:22 19:10:47 19:09:52 19:10:09 19:09:53 19:09:53 19:08:02 19:04:58 19:04:51 19:05:36 19:05:52 19:05:33 19:05:08 19:01:52 19:02:07 19:00:05 19:03:26 19:03:04 19:02:57

99:08:00 99:10:00 99:10:36 99:10:53 99:10:54 99:13:06 99:15:30 99:08:21 99:07:43 99:06:22 99:05:48 99:07:24 99:07:06 99:06:51 99:06:18 99:03:10 99:00:51 99:01:42 99:02:30 99:02:01 99:01:21 99:24:50 99:23:41 99:20:48 99:24:02 99:21:15 99:22:15 99:22:32 99:24:55 99:27:16 99:26:06 99:26:19 99:26:35 99:26:48 99:29:47 99:22:30 99:22:47 99:23:19 99:25:15 99:26:46 99:26:00 99:23:19 99:23:52 99:26:07 99:27:33 99:29:03 99:28:53 99:27:42 99:28:19 99:24:37 99:24:52 99:24:37 99:25:12

E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A49 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48 E14A48

MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. DF. MOR. MOR. MOR. MOR. DF. MOR. MOR. DF. MOR. MOR. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX.

3480 3240 3290 3240 3250 3210 3020 3190 3210 3140 3160 3170 3150 3180 3040 3300 3370 3490 3240 3280 3110 2960 2890 3510 2980 3390 3160 3210 2920 2700 2933 2900 2880 2860 2510 3200 3190 3040 2990 2700 2720 3020 2920 2800 2750 2910 3050 2900 2730 2640 3000 2940 2990

150 100 100 50 150 60 180 50 100 40 100 120 50 150 150 50 40 190 80 150 70 120 30 50 170 50 100 120 100 90 150 150 170 120 30 160 70 70 160 60 120 200 90 30 50 150 90 100 30 120 80 40 40

750 500 500 325 700 450 1300 450 575 500 800 600 600 600 750 300 250 1000 500 1000 375 500 425 550 750 325 350 500 350 850 550 400 600 300 500 850 750 450 725 500 600 750 800 250 350 750 410 750 275 550 325 350 875

250 300 150 75 350 300 200 150 200 250 350 270 250 270 450 200 100 350 270 350 100 250 300 250 350 250 220 350 170 150 270 270 270 250 200 200 270 200 250 300 450 200 500 200 200 420 200 150 100 200 250 220 225

0.2 0.2 0.2 0.154 0.214 0.133 0.138 0.111 0.174 0.08 0.125 0.2 0.083 0.25 0.2 0.167 0.16 0.19 0.16 0.15 0.187 0.24 0.071 0.091 0.227 0.154 0.286 0.24 0.286 0.106 0.273 0.375 0.283 0.4 0.06 0.188 0.093 0.156 0.221 0.12 0.2 0.267 0.113 0.12 0.143 0.2 0.22 0.133 0.109 0.218 0.246 0.114 0.046

0.0322 0.013 0.0092 0.0018 0.034 0.0068 0.0947 0.0039 0.0128 0.0046 0.0276 0.0189 0.0076 0.0236 0.0437 0.0025 0.001 0.0739 0.0097 0.0584 0.0035 0.0139 0.0032 0.0066 0.0426 0.0033 0.0066 0.0174 0.0056 0.0208 0.0208 0.0135 0.0267 0.0072 0.0031 0.0394 0.0155 0.0062 0.0325 0.0078 0.0264 0.0398 0.0307 0.0012 0.0031 0.0418 0.0069 0.0184 0.0009 0.0144 0.0053 0.0026 0.0107

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

132 Table 1 Žcontinued. Name

Lat ŽN.

Long ŽW.

Map a

State

HASLb

Height

Base Dc

Cone Dd

HrD e

Volume f

Tepezingo S Tepezingo Cas de a Tepetzingo Pueblo Quetzal Santa Cruz Cerr Cruz Tezontlala Santa Barbara Citlatepec Xexquixtle Partido Jumiltepec Tlecuilco S Yoteco Yoteco Herradura Tetillas Tezoyuca S Tezoyuca Subes T Tezoyuca S Dos Cerros Dos Cerros Sochol El Guarda Cilcuayo S Cilcuayo Del Agua Zoceyuca Atlalcorra Amoloc Cohuazalo Ahuazatepel S Tlalpexcua Moyocalco Huehuelcon Huehuel Pelagatos Ocoxusco Zoyazal Huipilo Del Aire Teziolo El Tezoyo Loreto La Mesa Escobeta Huiztomayo La Mesa Canta

19:03:25 19:03:16 18:51:28 18:56:11 18:56:29 18:56:19 18:55:58 18:53:39 18:54:23 18:58:25 18:58:45 18:58:18 18:59:10 18:58:54 18:54:31 18:55:58 18:55:19 18:55:19 18:58:47 18:52:43 18:48:39 18:48:20 18:52:49 18:48:27 19:09:11 19:09:10 19:07:19 19:07:15 19:06:06 19:06:03 19:05:19 19:08:10 19:06:26 19:06:26 19:06:16 19:05:52 19:05:54 19:05:06 19:05:00 19:04:54 19:05:32 19:04:15 19:03:27 19:02:57 19:01:19 19:02:32 19:01:49 19:01:36 19:01:42 19:01:35 19:00:34 19:00:26 19:00:21

99:24:24 99:24:29 99:25:50 99:36:20 99:35:07 99:35:31 99:35:33 99:38:39 99:24:14 98:59:56 98:55:20 98:56:04 98:54:09 98:53:34 98:46:56 98:49:00 98:49:26 98:49:39 99:11:29 99:06:43 99:12:22 99:12:14 99:11:19 99:12:22 98:55:56 98:56:25 98:57:09 98:57:33 98:59:13 98:59:31 98:59:25 98:53:54 98:53:41 98:52:31 98:54:57 98:55:06 98:54:29 98:55:50 98:56:15 98:56:51 98:57:43 98:54:27 98:53:21 98:53:04 98:54:49 98:55:33 98:55:38 98:52:44 98:52:11 98:51:51 98:52:26 98:51:53 98:47:26

E14A48 E14A48 E14A58 E14A58 E14A58 E14A58 E14A58 E14A58 E14A58 E14B51 E14B51 E14B51 E14B51 E14B51 E14B51 E14B51 E14B51 E14B51 E14A59 E14A59 E14A59 E14A59 E14A59 E14A59 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41

MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MOR. MEX. MEX. MEX. DF. DF. DF. DF. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MOR. MEX. MEX. MOR. MOR. MOR. MOR. MOR. MEX. MOR. MEX. MEX.

2890 2900 1680 2180 2200 2180 2420 1960 2420 1820 1980 2000 2040 2060 2160 2020 2040 2100 1900 1740 1200 1200 1010 1200 3000 3080 3000 2940 3560 3540 3360 2800 2720 2680 2800 3020 2780 2860 2980 3020 2980 2820 2780 2800 2440 2660 2580 2700 2680 2760 2560 2540 2330

180 20 20 80 60 40 100 40 20 60 80 100 40 80 40 20 20 40 250 100 125 150 30 75 100 100 80 40 160 60 100 180 140 100 120 180 20 60 60 160 60 140 100 260 80 100 80 120 40 100 120 40 20

875 250 250 700 1200 450 700 450 375 450 750 725 400 475 450 450 400 600 650 500 300 300 375 200 675 925 475 500 750 550 850 850 600 700 500 700 250 375 575 950 250 775 600 1050 500 575 625 500 575 625 525 500 375

300 200 175 250 450 100 200 200 150 300 250 225 200 125 200 350 200 250 250 100 125 150 100 75 150 125 150 100 200 200 250 150 200 350 100 225 150 100 150 375 125 250 300 225 225 150 200 100 200 100 175 400 200

0.206 0.08 0.08 0.114 0.05 0.089 0.143 0.089 0.053 0.133 0.107 0.138 0.1 0.168 0.089 0.044 0.05 0.067 0.385 0.2 0.417 0.5 0.08 0.375 0.148 0.108 0.168 0.08 0.213 0.109 0.118 0.212 0.233 0.143 0.24 0.257 0.08 0.16 0.104 0.168 0.24 0.181 0.167 0.248 0.16 0.174 0.128 0.24 0.07 0.16 0.229 0.08 0.053

0.0532 0.0008 0.0007 0.0154 0.0346 0.0027 0.0177 0.0035 0.0012 0.0068 0.0172 0.0195 0.003 0.0064 0.0035 0.0026 0.0015 0.0061 0.0428 0.0082 0.0047 0.0062 0.0015 0.0012 0.0153 0.0261 0.0068 0.0033 0.0318 0.0072 0.0264 0.0415 0.0192 0.0227 0.0098 0.0332 0.0006 0.003 0.007 0.0592 0.0017 0.0317 0.0167 0.0955 0.0087 0.0116 0.0117 0.0098 0.0051 0.0122 0.0126 0.0064 0.0014

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

133

Table 1 Žcontinued. Name

Lat ŽN.

Long ŽW.

Map a

State

HASLb

Height

Base Dc

Cone Dd

HrD e

Volume f

Xoyacan Quatepel Aholo Tlacuayol Sacromonte Tepenacasco Tapeixte La Joya Chiconquiat Xaxa Tenayo

19:05:17 19:05:16 19:05:28 19:05:31 19:07:37 19:07:30 19:10:17 19:10:32 19:10:08 19:10:03 19:11:11 19:09:52 19:09:29 19:10:02 19:14:06 19:14:17 19:05:19 19:05:10 19:05:36 19:02:11 19:03:24 19:05:11 19:02:54

98:48:29 98:51:35 98:51:16 98:50:55 98:46:32 98:48:57 98:48:01 98:47:39 98:49:05 98:48:27 98:48:57 98:51:22 98:52:39 98:57:12 98:51:57 98:51:55 98:51:39 98:48:23 98:55:41 98:56:56 98:59:25 99:31:40 99:07:12

E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14B41 E14A48 E14A49

MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. MEX. DF. MEX. MEX. MEX. MEX. MEX. MOR. MOR. MEX. MOR.

2720 2740 2820 2700 2580 2460 2720 2660 2900 2700 2740 2500 2480 2940 2240 2300 2680 2700 2880 2580 3000 2770 3070

80 200 80 60 80 20 100 60 160 60 140 40 60 140 20 100 140 40 60 80 200 45 100

925 925 475 350 625 350 925 750 1125 475 700 300 375 550 400 575 800 500 300 375 750 350 500

100 250 225 200 300 75 250 225 275 200 250 150 150 100 200 150 125 200 150 150 150 50 125

0.086 0.216 0.168 0.171 0.128 0.057 0.108 0.08 0.142 0.126 0.2 0.133 0.16 0.255 0.05 0.174 0.175 0.08 0.2 0.213 0.267 0.129 0.2

0.0203 0.0608 0.0081 0.0037 0.0141 0.0008 0.0304 0.0124 0.0698 0.0057 0.0269 0.0017 0.0035 0.0136 0.0015 0.0116 0.028 0.0041 0.0025 0.0046 0.0369 0.0017 0.0087

Cuajoma Ayaqueme S Cocotitlan Cocotitlan S Cuatepetl Sxoyacan Huihuilanco Cuatepe Chalchu Fuego Cuiloyo a

Name of the INEGI 1:50000 map where the cone is located. Meters above sea level. c Basal diameter of the cone. d Diameter of the cone at the top. e Ratio between height and basal diameter of the cone. f Volume of the cone in cubic meters. b

structures. The heights and diameters of 181 cones Žfrom types a and b. have been measured ŽTable 1. using 1:50,000 published topographic maps from INEGI ŽInstituto Nacional de Estadıstica, Geografıa ´ ´ e Informatica; the official mapping institution of ´ Mexico.. The heights of the cones range between 20 and 300 m Žmean s 104 m.. The basal diameters range from 200 to 1600 m Žmean s 644 m., and the cone diameters from 50 to 500 m Žmean s 225 m.. The HrD ratios Žheightrbasal diameter. range from 0.004 up to 0.525. These values for the whole volcanic field are similar to those reported for parts of it by Bloomfield Ž1975., Martin del Pozzo Ž1982., and Swinamer Ž1989., and also to those reported for the Michoacan–Guanajuato Volcanic Field ŽFig. 1a., a ´ monogenetic volcanic field located 300 km west of SCN. The cone mean values for Michoacan–Guana´ juato ŽHasenaka and Carmichael, 1985. are the fol-

lowing: heights 100 m, basal diameters 830 m, and cone diameters 225 m. The hydromagmatic component of several cones in the SCN makes these measurements unreliable in defining a relationship between morphology and age of the cones, because the eruptive mechanisms and the initial morphologies of the cones are different. Most of the lavas are block flows, with minor aa and pahoehoe Že.g., Xitle lavas.. They are typically 5 to 10 m thick, but can reach thicknesses of up to 20 m in the Cuernavaca valley where the lava filled stream channels. Some of the flows are more than 10 km long. The Xitle lava flows are 13 km long and cover an area of 70 km2 ŽTable 2.. The shield volcanoes are 5 to 10 km in diameter and are crowned by a scoria cone. They resemble, both in size and morphology, those of Michoacan–Guanajuato ´ ŽHasenaka, 1994.. Pelado volcano is 8 to 10 km in

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134

Fig. 3. Photographs of hydromagmatic Žbase surge. deposits in SCN, showing cross-stratification. Note hammer for scale.

diameter, 600 m high, and covers 64 km2 ; and Teuhtli volcano is 6 to 7 km in diameter, 500 m

high, and covers 35 km2 ŽTable 2.. These volcanoes are usually circular with gentle slopes of - 108. Dos

Table 2 Main characteristics of some of the most important volcanoes of the SCN Name

Lat ŽN.

Long ŽW.

Type

Area Žkm2 .

Height Žm.

Long Žkm.

Diameter Žkm.

Volume Žkm3 .

Pelado Teuhtli Dos Cerros Chichinautzin Ocusacayo Tlaloc La Gloria Tabaquillo Xicomulco Topilejo Tetepetl Lama Jumento Agua Holot.q 3 cru Texontepec Los Cardos Cuatzin 3 Cumbres Oclayuca Xitle Total

19:09:00 19:13:23 19:09:10 19:05:19 19:08:28 19:06:33 19:05:30 19:07:05 19:12:20 19:12:16 19:05:55 19:05:08 19:12:29 19:05:49 19:05:08 19:14:30 19:05:35 19:09:23 19:03:34 19:02:48 19:14:28

99:13:01 99:01:46 98:56:25 99:08:00 99:04:05 99:02:00 99:18:36 99:17:22 99:03:55 99:08:43 99:32:20 99:33:02 99:18:46 99:00:10 99:28:53 99:24:50 99:16:00 99:06:32 99:13:06 99:03:30 99:13:17

Shield Shield Shield Shield Shield Shield Dome Dome Dome Dome Dome Dome Cone Cone Cone Cone Cone Cone Cone Cone Cone

64 36 10 118 20 85 18 8 9 2.5 16 10 2.1 20 54 20 4.8 40 50 70 75

600 500 700 350 200 200 100 400 250 250 -

6 3.5 6 11 9 8 13 5.5 9 3.5 4 2.5 3 10 9 6 4.5 6.5 10 8 13

10 7 6 7 -

10 5 8 5 8 3.5 0.85 0.8 0.35 0.15

2.16 1.05 3.2 2.5 3.5 3.2 57.26

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

Cerros volcano, is elongated in a N–S direction, because it erupted on a tilted base. Lava-domes are thick lava flows of coulee ´ or of the low-dome type ŽBlake, 1990.. They were formed by more viscous magmas than the scoria cones and shields. In some cases a scoria cone appears at the vent, but usually the location of the vent must be deduced by the orientation of flow ridges. Some lava-domes Že.g., Tabaquillo. have flow fronts more than 200 m high; or can reach 13 km in length ŽMesa La Gloria.. The volume of the SCN volcanic rocks has been estimated from topographic maps and Digital Elevation Models. Using the measurements of the basal and crater diameters, and heights of the cones, we have calculated the volume using a truncated cone geometric model. Typical volumes of scoria cones range from 0.5 = 10y6 to 95 = 10y6 km3 , for a total volume of about 3.5 km3 of scoria. Satellite images and aerial photographs have been used to outline the extent of the volcanic structures, and Digital Elevation Models were used to calculate the volume of some volcanoes ŽTable 2.. The overall volume of volcanics emitted in the SCN is difficult to estimate, because measuring lava flow thickness is difficult at many points. However, we have made a minimum estimate, using three 1:50,000 scale Digital Elevation Models Žmaps E14A48, E14A49, and E14B41. from INEGI, and calculating the volume of material above 2500 m, which is the maximum height of the Tepoztlan ´ Formation. More than 450 km3 of volcanic materials occur just in this zone. The southern zone of the volcanic field Žsouth of Tepoztlan ´ scarp. is not included in this estimate. Since the southern volcanic materials crop out in an area larger than 400 km2 , their volume must be greater than 15 km3. Thus, the overall volume of SCN materials must be about 470 km3. Volume and age data indicate that the magma output rate of SCN is very large Ž470 km3 in 40,000 years s 11.75 km3r1000 years.. At Michoacan– ´ Guanajuato Volcanic Field ŽFig. 1a. about 50 km3 has erupted in the last 40,000 years ŽHasenaka, 1994.; the magma output rate is 1.2 km3r1000 years, 10 times smaller than in the SCN. The volume of the SCN is similar to that of the neighbouring Popocatepetl stratovolcano Ž600 km3 ; Boudal and ´ . Robin, 1988 . However, the SCN activity is only 40,000 years old, and Popocatepetl volcano began its ´

135

activity more than 300,000 years ago. The most recent activity at Popocatepetl began between 50,000 ´ and 30,000 years B.P. ŽBoudal and Robin, 1988., contemporaneously with the beginning of the SCN volcanism. From this time onwards, a cone of 8 km in diameter and 1700 m high has been built ŽRobin and Boudal, 1987., with a volume of 30 km3. Popocatepetl’s recent magma output rate, less than 1 ´ km3r1000 years, is much smaller than that of the SCN. These data highlight the importance of SCN volcanism for the understanding of magma generation and evolution processes in the TMVB. 3.2. Petrography and geochemistry The petrography of the SCN rocks has been described previously by Negendank Ž1972a., Bloomfield Ž1975., Martin del Pozzo Ž1989., and Swinamer Ž1989.. The SCN rocks are mainly aphanitic and porphyritic with few Ž- 10%. phenocrysts; some lava domes have phenocryst content as high as 50%. Rock types include: Ž1. olivine–basalts; Ž2. olivine– andesites; Ž3. pyroxene–andesites and pyroxene– dacites; and Ž4. mixed-lavas Ž sensu Nixon, 1988b.. The textures are porphyritic, with a fine-grained groundmass of plagioclase, pyroxene, and glass. The basalts have olivine Žusually with spinel inclusions., and minor augite and plagioclase phenocrysts, while the olivine–andesites have both olivine and orthopyroxene phenocrysts. In the pyroxene–andesites Žolivine-free. and pyroxene–dacites the phenocrysts are orthopyroxene Žhypersthene., clinopyroxene Žaugite., with subordinate plagioclase. The mixedlavas are characterized by coexisting olivine and quartz, the presence of large phenocrysts of plagioclase with complex zoning and twinning, and the occurrence of biotite and hornblende phenocrysts. Rounded quartz xenocrysts with pyroxene aureoles are common in these intermediate and felsic rocks. The oxide phases are titanomagnetite and minor ilmenite. Xenoliths of volcanic and plutonic rocks of dioritic–tonalitic composition are an outstanding feature of the SCN rocks. They are usually rounded, 5 to 20 cm in diameter, and some of them show glassy reaction rims. They appear in lava domes, lava flows, and scoria cones, but they are especially abundant in some lava domes Že.g., Tabaquillo and Lama volcanoes.. Some of these xenoliths ŽFig. 4a. have ovoid

136

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

Fig. 4. Ža. Enclave in Tabaquillo lava-dome ŽSCN.. Note the rounded shape, the reaction rims, and the fine grained texture. This type of enclave is interpreted as resulting from magma mixing Žsee text.. Handlens for scale. Žb. Microphotograph of a quartz grain Ž0.1 mm. with a pyroxene reaction rim in a lava-dome of SCN, cross polars.

shapes, doleritic textures with plagioclase and biotite, and chilled margins, indicating magma mixing

Že.g., Bacon, 1986; Davidson et al., 1990; De Silva et al., 1994.. Metamorphic xenoliths are also present,

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137

Fig. 5. TAS diagram ŽLe Bas et al., 1986. of SCN rocks Ždata from: Gunn and Mooser, 1971; Negendank, 1972a; Bloomfield, 1973, 1975; Robin, 1982b; Verma and Armienta, 1985; Nixon, 1988a,b; Martin del Pozzo, 1989; Swinamer, 1989; Verma, 1999-this issue, in press.. Also plotted is the alkaline–subalkaline line of Irvine and Baragar Ž1971.. Note the main calc-alkaline trend, and the transitional and alkaline character of several samples.

but they are less abundant. Quartz xenocrysts indicate that crustal contamination has also occurred in some of the SCN magmas. Disequilibrium textures, typical of the mixed-lavas, include coexisting olivine and quartz, each surrounded by pyroxene aureoles ŽFig. 4b., and plagioclase showing reaction textures. These features are more abundant in lava domes, but some lavas Že.g., Jumento scoria cone., have disequilibrium textures and show mixing of two different glasses. Notwithstanding the suggestion of Martin del Pozzo Ž1989. that these textures are due to crustal contamination, many authors ŽAnderson, 1976; Eichleberger, 1978; Gans et al., 1989. ascribe this type of mineralogical disequilibrium to magma mixing. Furthermore, as discussed below, the chemistry of the SCN volcanic rocks clearly indicates that magma mixing is the main mechanism for the generation of the intermediate and felsic rocks. In a TAS classification diagram the SCN rocks project in the fields of basalt, trachy–basalt, basaltic trachy–andesite, trachy–andesite, andesite and dacite. The vast majority of samples plot along a well defined calc-alkaline trend ŽFig. 5.. In a multi-

element MORB-normalized diagram the SCN mafic rocks lack the typical negative Nb and Ti anomalies of subduction-related volcanic rocks, and are similar to OIBs ŽFigs. 6 and 7.. Additionally, the intermediate and felsic rocks of the SCN have lower contents of highly incompatible elements ŽFig. 7. and REE

Fig. 6. Zr–ZrrY discrimination diagram ŽPearce and Norry, 1979., for SCN basalts Ždata from Verma, in press..

138

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

Fig. 7. Multi-element MORB-normalized ŽPearce, 1983. plot showing the trace-element characteristics of the OIB-type mafic volcanic rocks from the SCN ŽRobin, 1982b; Nixon, 1988b; Swinamer, 1989; Verma, in press.. Shaded field enclose the OIB field ŽFitton et al., 1991.. A dacite from the SCN ŽVerma, 1999-this issue. is also included; note its lower values in incompatible elements ŽNb, Zr..

than the mafic rocks of the same area ŽVerma, 1999., and have a negative Nb anomaly. The Sr and Nd isotope ratios of the SCN mafic rocks display values Ž87 Srr 86 Sr s 0.70353–0.70432, 143 Ndr 144 Nd s 0.51275–0.51293; Nixon, 1988b; Verma, in press. that fall within the mantle array, between the HIMU and BSE fields. Although a detailed geochemical study is beyond the scope of this paper, geochemical and isotopic evidence argue against fractional crystallization of the mafic magmas to explain the more evolved rocks. For instance, negative Nb–Ti anomalies, one of the most important features of volcanic arc basalts, are absent in the SCN basalts, however, the SCN andesites and dacites do show them ŽFig. 7.. The decrease in incompatible elements, as silica increases, argues against a fractional-crystallization origin for the intermediate and felsic calc-alkaline rocks from such basalts. Moreover, incompatible elements data suggest that the evolution of SCN volcanic rocks is related to magma mixing between OIB-type basalts and a crustal component of dacitic composition ŽFig. 8..

4. Tectonics of Sierra Chichinautzin Since several shallow earthquakes Ž- 20 km depth. of small magnitude Ž- 4. have occurred in the SCN in the last few decades, the area can be regarded as tectonically active ŽDe Cserna et al.,

Fig. 8. Rb contents Žppm. plotted against SrrRb for Chichinautzin volcanic rocks: basalts, basaltic trachy–andesites, basaltic andesites, andesites and dacites ŽSwinamer, 1989; Verma, in press,Verma, 1999.; the line shows the mixing curve between a basalt and a dacite from the SCN.

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

1988; Yamamoto and Mota, 1988; Gonzalez´ Pomposo and Valdes-Gonzalez, 1995; UNAM and ´ ´ CENAPRED Seismology Group, 1995.. The analysis of the SCN tectonics is complicated by the recent volcanic activity and covering vegetation, both of which obscure the tectonic structures. The best outcrops are in the Tenango area at the western limit of SCN, where several E–W en-echelon normal faults, with throws to the north of up to 50 m, are clearly visible even in Landsat TM images. The extrusion of the Tetepetl dome at Tenango, 8500 years B.P. ŽBloomfield, 1975., was clearly related to these faults. Macıas ´ et al. Ž1997. found slickensides on these fault planes showing left-lateral and normal types of motion. Several normal faults in the Toluca valley, with a combined throw of 2.5 m to the north, intersect ashes of the Tres Cruces volcano ŽBloomfield, 1975.. This volcano belongs to the SCN and is younger than 8440 years ŽBloomfield, 1975.. In the Cuernavaca valley, normal faults with throws of 1 to 4 m, also cut Quaternary deposits ŽFries, 1960.. Due to limited field exposures, tectonic structures were deduced from satellite images ŽLandsat TM and MSS., aerial photographs, topographic maps, and volcanic alignments. 4.1. Lineaments from satellite images In a previous satellite image analysis, De Cserna et al. Ž1988. proposed the existence of a main N608E lineament direction in the Mexico Basin. Vazquez´ Sanchez and Jaimes-Palomera Ž1989. likewise ´ showed two predominant fracture directions at E–W and N658E. Our analysis found some discrepancies with these earlier results ŽFig. 9.. In order to improve the recognition of lineaments, several digital methods such as edge-enhancement and high pass filters ŽSabins, 1987. have been applied to the Landsat images. Only those lineations that appear to be recent faults, either because of their topographic or geologic expression, have been drawn ŽFig. 9a.. Clear tectonic fractures are not easy to distinguish in the central, densely vegetated part of the SCN, although E–W trending, northward dipping faults can be seen. Some of these faults are associated with lava-domes Že.g., Tetepetl dome and Topilejo– Xicomulco area., which apparently flowed from these fractures. Some NE–SW lineaments have been also observed.

139

The tectonic lineaments are easier to identify in the satellite images of some areas surrounding the central SCN, such as Sierra de las Cruces–Zempoala, where E–W, NW–SE, and NE–SW lineaments can be distinguished, and volcanic centers are aligned in a roughly NNW direction. The Iztaccıhuatl–Popo´ catepetl lineation appears to be controlled by a NNW ´ fracture. Another possible marker of the NNW–SSE structural trend in this area is the southward decrease of volcanic ages in the Sierra de las Cruces– Zempoala area ŽMora-Alvarez ´ et al., 1991., and in the Iztaccıhuatl–Popocatepetl complex ŽBoudal and ´ ´ Robin, 1988; Nixon, 1988a.. In the Cuernavaca valley and Tenancingo area, several trends of lineaments can be distinguished ŽE–W, NNE–SSW, NNW–SSE.. Some of the outcrops in this area are older Cretaceous limestones. This complicates the detection of the lineaments related to recent faults, because some of the lineaments may be related to older deformation phases. Because of this reason, only those that were clearly related to volcanic activity were considered in the study. In short, the analysis of the SCN satellite images indicates a predominant E–W tectonic trend, with subordinate NNE–SSW, NNW–SSE, and NE–SW directions ŽFig. 9b.. 4.2. Volcanic alignments The usefulness of aligned volcanic centers in tectonic regional studies was highlighted by Nakamura Ž1977., who showed that these alignments are perpendicular to the horizontal minimum stress Ž s hmin ., which in an active volcanic area must be equal to either s 3 or s 2 in order for magmas to reach the surface. In the SCN, several authors ŽFries, 1966; Bloomfield, 1975; Martin del Pozzo, 1982. pointed out that volcanic alignments indicate that s hmin is essentially N–S. Demant Ž1981. and De Cserna et al. Ž1988. emphasized the importance of the subsidiary N608E direction marked by cone alignments. Several mathematical methods have been put forward to resolve the problem of visual analysis of volcanic alignments, methods whose aim is to eliminate the subjective component ŽWadge and Cross, 1988; Connor, 1990; Lutz and Gutmann, 1995.. Regional alignments detected using these methods are usually co-linear or parallel to the mapped regional

140 A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

Fig. 9. Ža. Landsat TM image of the SCN volcanoes, and lineaments detected in the Landsat image Žsee text.. Žb. Rose diagram of the lineament directions.

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´

tectonic structures Že.g., Lutz and Gutmann, 1995.. This correlation between regional structures and volcanic alignments suggest that the vents are located at the top of vertical fractures Žperpendicular to the horizontal minimum stress, s hmin ., along which magma ascends ŽNakamura, 1977.. The mathematical procedures used in this work ŽAncochea et al., 1994. allow the graphic definition of the main cone alignment directions. The process is iterative, and performed sequentially, both along the X and Y directions on a grid previously defined. Lines with increasing slopes Ž18, 28, 38 . . . 1808. are traced from each grid node Ž x i , y i . until a full 1808 arc is completed. The lines that are eventually considered as true cone alignments are selected in terms of the number of associated points; in our case, no less than 10 volcanic centers per line. Associated points are defined as those which fall within a given interval of confidence for the line, i.e., a belt having the line as its axis. In our case, a belt width of 1000 m was chosen. These procedures were refined to allow the integration of data in a geographical information system ŽGIS., which ultimately displays the location of cones and associated alignments on digital maps ŽFigs. 10 and 11.. Since in this type of volcanic activity several cones can be formed during the same eruptive event, it is necessary to decide which points will be used for the calculations. If all the cones are taken into account, the program will compute all these points as belonging to the line. Therefore, the lines traced along centers originating during the same eruptive episode will have more points and the results will be biased. Landsat TM images and topographic maps were used to group the cones clearly formed during the same eruptive episode, reducing the number of points from 221 to 187. In the first case study, of the whole volcanic field, the lines containing more than 15 points were chosen ŽFig. 10a.. When lines with fewer points were chosen, the process yields a large number of lines in all directions that have no tectonic meaning, and are the product of the point cloud. Twenty-two lines with more than 15 points were detected, with lengths from 36 to 88 km. There are two main directions: E–W Žgenerally the longest. and N608E, with subordinate N–S and NW–SE trends ŽFig. 10b.. The results are similar to those from satellite images, but in the

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latter, the NE and NW trends are more important. The explanation probably relates to the age of these fractures, because the NE and NW trends are in the Cuernavaca valley where older formations crop out. Therefore, some of the faults detected by remote sensing may be much older than the SCN volcanic activity. The analysis of the N608E alignments from Fig. 10a shows that many of the associated points lie at the extreme ends of the lines. In order to suppress possible complicating effects of heterogeneity of volcano density in the SCN Že.g., higher density of points at either end of the alignments., a zone in the higher density part of the field was chosen for a second analysis. This area is 77 = 25 km and contains 146 points. In this case, only lines with more than 10 points were chosen. The results are very similar to those for the whole volcanic field, and again E–W and N608E are the most important trends ŽFig. 11., which further supports the existence of the latter ŽFig. 11b.. Some of the lines coincide with the fractures drawn from Landsat images. For example, the NNE trend of fractures situated in the Cuernavaca valley are the same as the alignments associated with the volcanoes of this valley Žsee Figs. 9a and 10a.. Also the E–W fractures related to the domes of Tetepetl or Xicomulco coincide with some longer E–W alignments of volcanoes. The most probable explanation for the greater length of volcanic alignments is that the vegetation of Sierra Chichinautzin is obscuring the fractures. 4.3. Stress Our method shows that the volcanic alignments in the SCN are mainly E–W and N608E. The E–W lines are generally longer and contain more points than the N608E lines. These results, together with seismic ŽUNAM and CENAPRED Seismology Group, 1995. and field data ŽFries, 1960; Bloomfield, 1975; Macıas ´ et al., 1997. suggest a general N–S extensional field in the SCN region. The focal mechanism of the 12-km-deep Milpa Alta earthquake in the SCN indicated E–W normal-faulting with a significant Ž50%. sinistral strike–slip component ŽUNAM and CENAPRED Seismology Group, 1995.. Therefore the area must be also undergoing compression to produce this strike–slip component.

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Fig. 10. Ža. Landsat TM image of the SCN showing volcanic alignments containing more than 15 points Žsee text.. Žb. Rose diagram of the alignments directions.

A. Marquez et al.r Journal of Volcanology and Geothermal Research 93 (1999) 125–150 ´ Fig. 11. Ža. Landsat TM image of the SCN showing volcanic alignments of the central zone containing more than 10 points Žsee text.. Žb. Rose diagram of the alignments directions. 143

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Since a N–S extensional component is obvious, s 1ŽV . may be roughly similar to s 2 . Following the method of Angelier Ž1979. Žin Davis and Reynolds, 1996. this assumption gives an acceptable solution. If s 1 s s 2 then the slip direction is determined in two steps, finding first the great circle that contains s 3 and the pole to the fault, and then finding the intersection between the great circle representing the fault plane and the former one. The result for a typical E–W oriented fault dipping to the north, in a zone subjected to a stress environment marked by s 1ŽV . s s 2ŽN E – ENE. ) s 3ŽNW – NNW. Ž s 2 / E–W to allow strike–slip movement., yields a normal movement with a sinistral component. Slip direction along the plane would be WNW to NW oriented, which matches the field evidence and the results of Suter et al. Ž1992.. 5. Sierra Chichinautzin: an extension-related volcanic field at the front of the TMVB 5.1. Geophysical eÕidence The SCN is located at the front of the TMVB, 300 km northeast of the trench. Many authors Že.g., Pardo and Suarez, 1995; Suter et al., 1995a; and ´ references therein. have suggested that the volcanism of the central TMVB is related to subduction of the Cocos plate under Mexico. If this were the case, the Cocos plate should be located approximately 125 km below SCN Žthe typical setting in subduction zones; Ž1995., in a e.g., Gill, 1981.. Pardo and Suarez ´ comprehensive study of the seismicity of southern Mexico, suggested that the Cocos plate is presently at 80–100 km depth beneath the Central TMVB. However, since this zone lacks deep seismic activity ŽPardo and Suarez, 1995. the 80–100 km must be ´ taken as an assumption. The subduction-related seismicity is always shallower than 60 km, and it disappears more than 50 km before the volcanic front, all along the central TMVB ŽFig. 1b.. If we assume that the slab exists under the Central TMVB, then the

subducting plate would have to change its angle abruptly in order to reach the 100 km depth beneath the volcanic front. Seismicity beneath the SCN is shallow; De Cserna et al. Ž1988. and Gonzalez-Pomposo and Valdes´ ´ Gonzalez ´ Ž1995. show that earthquake focii beneath the SCN and surrounding areas are always shallower than 50 km, and most of them are less than 20 km deep. This is similar in the rest of the Central TMVB, where seismic activity is shallow and related to N–S extensional conditions ŽSuter et al., 1992, 1995b; Singh and Pardo, 1993; Gonzalez´ Pomposo and Valdes-Gonzalez, 1995; UNAM and ´ ´ CENAPRED Seismology Group, 1995; CamposEnrıquez et al., in press.. Molina-Garza and Urrutia´ Fucugauchi Ž1993. presented a N–S gravimetric profile across the SCN with a regional anomaly related to the existence of a low density Ž3.29 grcm3 . and low velocity ŽVp s 7.6 kmrs. mantle layer at the base of the crust Ž40 km depth. beneath the central TMVB ŽFig. 1c.. These values are typical of mantle beneath rifts and continental extensional areas, as in Rio Grande Rift, USA Že.g., Sinno et al., 1986., Gulf of California Že.g., Couch et al., 1991., or East African Rift Že.g., Braile et al., 1995.. Fix Ž1975. also showed a decrease in seismic velocities at the base of the TMVB crust, and suggested the presence of a low-density upper mantle beneath the TMVB. This profile is different from those beneath well-known subduction zones Že.g., Aleutians, Japan; Gill, 1981., showing a greater decrease in S waves velocities extending to 100 km depth. Geophysical data from the SCN and surrounding areas, such as the shallow character of the seismicity, high heat flow ŽZiagos et al., 1985., and the occurrence of a low-density mantle with seismic and gravity values more typical of regions undergoing rifting processes Že.g., Braile et al., 1995., are different from those observed in similar subduction-related settings developed on continental active margins Že.g., the Andean realm, south volcanic zone: 338– 458S; e.g., Wilson, 1989..

Fig. 12. Sketch displaying the main tecno-volcanic features of the central TMVB and SCN. Ža.: main structural trends Žheavy lines. in the central TMVB and location of the SCN Žbackground: shaded relief image., arrows indicate decreasing ages of volcanic activity along well defined NNW structural trends; Žb. volcanic alignments containing more than 15 points Žheavy lines. in the SCN Žoutlined on a Landsat TM image.; Žc.: main tectonic elements in the SCN, showing relationships between regional stresses Ž s 1ŽV . , s 2 , s 3 ., combined normal–strike– slip faulting ŽE–W., extensional fractures ŽN608E. and volcanism Žsolid triangles..

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5.2. Tectonic eÕidence In a regional context, the SCN can be interpreted as the southernmost structural domain of a group of six parallel E–W oriented tectonic structures. From south to north the structures are ŽFig. 12a.: Ž1. Chichinautzin ŽSCN.; Ž2. Perales Semi-graben; Ž3. Acambay Graben; Ž4. Nopala Graben; Ž5. Mezquital Graben; and Ž6. Aljibes Semi-graben ŽMartınez-Re´ yes and Nieto-Samaniego, 1990; Suter et al., 1992, 1995a,b; Macıas et ´ et al., 1997; Campos-Enrıquez ´ al., in press.. Altogether, these structures form an extensional N–S active zone of more than 150 km wide, across the TMVB. Most of these structural features and dynamic conditions can be observed throughout the central TMVB Žfrom the Lake Chapala to the Mezquital Graben, a distance of more than 500 km., where earthquake and structural data show active N–S extensional conditions with a strike–slip component from Miocene to present times ŽPasquare` et al., 1988; Johnson and Harrison, 1990; Martınez-Reyes and Nieto-Samaniego, 1990; Suter et ´ al., 1992; Ferrari et al., 1994; Suter et al., 1995a,b; UNAM and CENAPRED Seismology Group, 1995.. Moreover, it has been suggested that the extensional faulting has guided the emplacement of the TMVB volcanism from Late Miocene to present ŽFerrari et al., 1994; Alaniz-Alvarez et al., 1998.. Suter et al. Ž1992. proposed that the N–S extension in the TMVB is related to isostatically compensated surface loads, produced by the high elevation of the TMVB. This causes horizontal shallow tension and normal faulting in the TMVB, parallel to the axis of the volcanic belt. However, extensional tectonics and related volcanism of the SCN also occur in the Cuernavaca valley, outside of the TMVB plateau. This dynamic scheme is similar to that of the southernmost volcanoes of the Michoacan– ´ Guanajuato Volcanic Field. Normal faulting in the central TMVB has a strike–slip component reflecting a stress environment marked by s 1ŽV . s s 2ŽNE – ENE. ) s 3ŽNW – NNW. . Note that different from other extensional settings developed on continental active margins Že.g., the Andes., s 3 is oblique to the trench ŽWNW.. Since the main direction of extension Ž s 3 . is NW–NNW oriented, the volcanic activity in the SCN can be explained in terms of a combination of extensional

processes. The main directions of cone alignments in the SCN are E–W and N608E ŽFig. 12b.. The first one corresponds to the orientation of normal faults with a sinistral strike–slip component, while the N608E trend can be interpreted as related to tensional fractures opening perpendicular to s 3 ŽFig. 12c.. Thus, in both cases extension would be the most important mechanism for the ascent of magmas in the SCN. Another problem arises from the probable counter-clockwise rotation of the southern border of the TMVB ŽUrrutia-Fucugauchi and Bohnel, 1988., ¨ which may be suggesting that the whole structural belt is opening from west to east. This is further supported by the work of Suter et al. Ž1995a. who indicate a rotational deformation component with a pole of rotation located eastward of the main zone of deformation. For example, extension directions in the large Acambay graben vary from NNE Žnorthern sector. to NNW Žsouthern sector. ŽSuter et al., 1995a., thus suggesting a scissors-like opening. The whole tectonic scenario is further complicated by a series of oblique northwest- to northtrending normal faults related to the southern continuation of the US Basin and Range ŽHenry and Aranda-Gomez, 1992; Jansma and Lang, 1997.. Whether this older extensional structural trend interplays with the mainly E–W extensional faults of the TMVB might be regarded as conjectural, however, it would nicely explain some very important volcanic alignments such as those of the Popocatepetl– ´ Iztaccıhuatl and Sierra de Las Cruces ŽAlaniz-Al´ varez et al., 1998. ŽFig. 12a.. Finally, extensional processes are not restricted to the TMVB, but extend to the whole domain stretching from the TMVB to the southern coast, where extensional conditions have been dominant from Miocene onwards ŽMeschede et al., 1997.. 5.3. Geochemical eÕidence As discussed previously the geochemical and isotopic data for the SCN basalts indicate that they are of the OIB-type ŽFigs. 6 and 7., and probably originated by partial melting of a mantle that was not affected by subduction. Petrographic and geochemical evidence ŽFig. 4a,b and Fig. 8. indicate that magma mixing processes are

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abundant in the SCN felsic and intermediate rocks. The interaction of mantle-derived basalts with crustal-derived felsic magmas would result in a calc-alkaline trend similar to those observed in typical subduction-related calc-alkaline series, but with a very different origin Že.g., Gans et al., 1989.. Examples of the latter are provided by the U.S. Basin and Range ŽGans et al., 1989., Gulf of California ŽMartın-Barajas et al., 1995., and Rio Grande Rift ´ ŽMcMillan and Dungan, 1986.. Magma mixing between basaltic and dacitic magmas also seems to be the dominant magmatic process in the neighbouring Popocatepetl and Iztaccıhuatl ´ ´ stratovolcanoes Že.g., Cantagrel et al., 1984; Robin and Boudal, 1987; Nixon, 1988a,b.. Similar processes are common in several other areas all along the TMVB, such as Tequila volcano ŽWallace and Carmichael, 1994., Ceboruco ŽNelson, 1980., Colima ŽLuhr and Carmichael, 1982; Robin et al., 1991; Robin and Potrel, 1993., Paricutın ´ ŽMcBirney et al., 1987., Amealco caldera ŽVerma et al., 1991., Pico de Orizaba ŽCantagrel et al., 1984., or the easternmost TMVB ŽBesch et al., 1995..

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SCN. The long-lived ŽMiocene to present. OIB volcanism along the entire TMVB, the position of the SCN at the front of the TMVB, and the similarities of geophysical and tectonic features in the SCN to the rest of the Central TMVB, cast reasonable doubts on the standard subduction-zone model for the origin of the TMVB volcanism.

Acknowledgements This work was supported by CONACYT project 0196P-T and DGAPA project IN-100596. A. Marquez and F. Anguita thank the Universidad Com´ plutense ŽUCM. –Universidad Nacional Autonoma de ´ ŽUNAM. bilateral program for partial supMexico ´ port, enabling them to visit the UNAM, and to conduct part of the field work for this project. Remote-sensing studies were performed at the R q D department of Infocarto ŽMadrid, Spain.. The first manuscript was completed during the stay of the three first authors in CICESE ŽEnsenada, Mexico.. This paper benefitted from constructive comments from J.F. Luhr, B.D. Marsh, and two anonymous referees.

6. Conclusions Geophysical, tectonic, and geochemical data suggest an extensional origin for the monogenetic SCN magmatism at the front of the TMVB. Satellite images, field data, and volcanic alignments studied in this work show that volcanic activity of the SCN is related to N–S and NNW–SSE extension, the same stress environment displayed by the rest of the Central TMVB. The negative regional Bouguer gravity anomaly, a low-velocity mantle, high heat flow, and shallow seismicity, point to a rift-type environment, involving the upwelling of low-density mantle beneath the central TMVB. This is reflected in the geochemistry of the SCN basalts, which are similar to OIBs and different from those of continental volcanic arcs related to subduction. Petrographic and geochemical data highlight the importance of magma mixing in the intermediate and felsic rocks of the SCN. The mixing of OIB magmas produced by partial melting of an enriched mantle, with more silicic magmas Žprobably related to crustal partial melting. can explain the calc-alkaline rocks of the

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