Rhyolitic Volcanism in Extension Zone Associated with Strike-Slip Tectonics in the Taxco Region, Southern Mexico

Journal of Volcanology and Geothermal Research 118 (2002) 1^14 www.elsevier.com/locate/jvolgeores

Rhyolitic volcanism in extension zone associated with strike-slip tectonics in the Taxco region, southern Mexico

S.A. Alaniz-AŁ lvarez a;Ã, A.F. Nieto-Samaniego a, D.J. Mora¤n-Zenteno b, L. Alba-Aldave b

a Universidad Nacional Auto¤noma de Me¤xico, Instituto de Geolog|¤a, Unidad de Investigacio¤n en Ciencias de la Tierra, Campus Juriquilla, Quere¤taro, Qro.76230, Mexico b Universidad Nacional Auto¤noma de Me¤xico, Instituto de Geolog|¤a, Ciudad Universitaria, Me¤xico D.F. 04510, Mexico

Received 15 January 2001; accepted 8 January 2002

Abstract

The Taxco Volcanic Field (TVF) is part of a broad magmatic province in southern Mexico. It constitutes an isolated zone of deeply dissected volcanic rocks encircled by outcrops of Mesozoic sedimentary and volcano- sedimentary units. A thick unit of rhyolitic lava flows associated with domes and at least two ignimbrite units forms the TVF. This volcanic sequence is distributed within a well defined zone, it overlies and is in part contemporaneous with continental sedimentary beds limited by major faults. Geochronologic data indicate that most rhyolitic volcanism in the area is Oligocene in age and synchronous with episodes of strike-slip faulting. We document two successive phases of strike-slip faulting for the late Eocene^early Oligocene interval, the first with NNW extension and the second with NE extension. In both cases pre-existing structures were reactivated and sedimentary basins were developed in response to displacement along major faults. The stratigraphic sequence gives evidence that the TVFis located in an extensional basin associated to strike-slip faults. The evolution of the basin underwent a change from sedimentary deposition with subsidence to piling up by volcanism. The result of this change was the development of a volcanic pile with elevations higher than the surrounding Mesozoic rocks. According to the fault kinematics, stratigraphy and the volume of volcanic rocks, the rhyolitic volcanism was emplaced in the area of maximum extension, showing that magma flowed into low pressure zones. The small number of faults within the Oligocene volcanic sequence suggests that volcanism inhibited normal faulting and that magma partially filled the space generated in the extended zone produced by the strike-slip faulting. ß 2002 Elsevier Science B.V. All rights reserved.

Keywords: magma emplacement; rhyolite; strike-slip faulting; Mexico

1. Introduction and stress ¢eld have been widely discussed (e.g. Nakamura, 1977; Takada, 1989; Clemens and Relationships between magma emplacement Mawer, 1992). Kinematic and dynamic analyses of deformation provide valuable information in addressing the problem of the mechanics of mag- * Corresponding author. Tel.: +52-442-238-1116 (ext. 128); Fax: +52-442-238-1100. ma ascent and distribution patterns. Analysis of E-mail address: [email protected] the extensional deformation should consider not (S.A. Alaniz-AŁ lvarez). only the amount of extension calculated from the

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fault displacement but also the increase in crustal volume produced by magma emplacement. There is a considerable volume of magma that remains under the surface during a volcanism pulse (e.g. Crisp, 1984). Parsons and Thompson (1991) and Parsons et al. (1998) proposed that this magma could inhibit faulting due to the changes in the stress ¢eld. It is known that silicic magmatism and strike-slip tectonics are spatially related in many regions (e.g. Petford et al., 1993), but there are few studies documenting how the magmatism a¡ects the fault pattern in strike-slip regime. The distribution of volcanic rocks in southern Mexico (Fig. 1) has been widely studied in the last 5 yr (e.g. Mora¤n-Zenteno et al., 1999; Martiny et al., 2000). In regional maps, the Taxco volcanic Fig. 1. Location map of the Taxco region in relation to the ¢eld (TVF) appears as an isolated outcrop en- main magmatic provinces of south-central Mexico. Areas covered by Figs. 2 and 3 are indicated. TMVB: Trans-Mexi- closed by Mesozoic sedimentary rocks (e.g. Riv- can Volcanic Belt. SMS: Sierra Madre del Sur. era et al., 1998; Fig. 2). There are a small number of studies about Cenozoic faulting. Structural works mainly focus on normal faulting and do

Fig. 2. Regional tectonic map of the Taxco region showing the main Mesozoic and Tertiary structures. This map was modi¢ed from Rivera et al. (1998).

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Fig. 3. Faults and major volcanoes along the Taxco^San Miguel de Allende fault system. Ages were obtained from: (1) Nieto-Sa- maniego and Alaniz-Alvarez (1994), (2) Pe¤rez-Venzor et al. (1996), (3) Reyes-Zaragoza (2001), (4) Zu¤n‹iga et al. (1998), (5) Aguirre-D|¤az (1996), (6) Garc|¤a-Palomo et al. (2000), (7) this work, (8) Valdez-Moreno et al. (1998), (9) Aguirre-D|¤az, written communication, (10) Ferrari, written communication. not document the relationships between volcan- emplaced in an extensional zone during strike-slip ism and faulting or volcanism and tectonics (Nie- tectonics. to-Samaniego et al., 1995; Jansma and Lang, 1997). In this paper, we present evidence of two phases of Cenozoic strike-slip faulting in the Tax- 2. Geological setting co region. The sedimentary record provides evidence of a basin formed by NW extension in the The Sierra Madre del Sur is a physiographic late Eocene, whereas Oligocene NE extension is province characterized by a widespread arc-mag- deduced from the kinematics of faults and the matic record that includes plutonic and volcanic distribution of synchronous rhyolitic domes. We rocks ranging in age from Paleocene to Miocene propose that the TVFis isolated because it was (Fig. 1). Granodioritic to tonalitic batholiths form

VOLGEO 2483 21-10-02 4 S.A. Alaniz-AŁ lvarez et al./ Journal of Volcanology and Geothermal Research 118 (2002) 1^14 an almost continuous WNW-trending belt along metamorphic units (Taxco Schist and Taxco Viejo the present-day coast of southern Mexico. Vol- Formation) that crop out near Taxco City. The canic rocks are discontinuously exposed inland complete age range of these sequences has not up to the northern part of the Sierra Madre del been well constrained, but it includes Jurassic to Sur, displaying compositional variations from an- Lower Cretaceous units (Campa and Ram|¤rez, desite to rhyolite (Mora¤n-Zenteno et al., 1999). 1979). Rocks of both domains are involved in a A mosaic of metamorphic segments, with well series of N^S-trending regional folds and over- de¢ned di¡erences in age and petrotectonic fea- thrusts originated during Laramide Orogeny of tures, constitutes the basement in the region. Late Cretaceous and early Tertiary times. The origin of these segments has been interpreted in terms of accretionary and deformation episodes 3.1. Red conglomerate that occurred from the Late Proterozoic to Meso- zoic time (e.g. Ortega-Gutie¤rrez, 1981; Campa The basal unit of the Tertiary sequence is com- and Coney, 1983). Pre-Mesozoic terranes are cov- posed of conglomerates and sandstones that crop ered by continental and marine sequences de- out in the Acamixtla area, east of Taxco City, and formed by E^W shortening during the Laramide in the southern part of the TVF, near Atzala (Fig. Orogeny, probably at the end of the Cretaceous. 4). At a regional scale, similar sequences of early The TVFis located at the southern termination Tertiary age have been considered under the geof a long NNW^SSE fault system (Fig. 3), known neric denomination of Balsas Group (Fries, 1960; as the Taxco^San Miguel de Allende lineament De Cserna and Fries, 1981). In the study area, the (Demant, 1978), which crosses three di¡erent Ce- conglomerates contain pebbles of limestone, silt- nozoic geological provinces of central Mexico. stone and shale with minor quantities of andesite, The lineament is about 500 km long and is con- polycrystalline quartz and calcite. The composi- stituted by a zone of subparallel faults that, in tion of the clasts corresponds to the rocks under- some places, reaches 50 km wide. There is evi- lying the conglomerate, although there are no dence of a complex kinematic history, character- clasts of the Taxco Viejo schist. According to Ed- ized by reactivation of distinct segments of the wards (1955) ma¢c lavas are interbedded with the lineament at di¡erent times. Timing of volcanic lower conglomerate beds and a rhyolite tu¡ tops activity along the structure shows that emplace- the unit. This author reported 425 m of red con- ment occurred synchronously with fault activity glomerates for the Acamixtla outcrop; however, (Fig. 3). in the Atzala outcrop the conglomerate has a maximum thickness of 30 m. This unit unconformably overlies the Mesozoic rocks and we 3. Tertiary stratigraphy did not observe any features that indicate deformation by shortening. The age of this unit is con- The Tertiary sequence of the Taxco region cov- strained by its stratigraphic relationship between ers folded Mesozoic rocks of marine sedimentary the Upper Cretaceous Mexcala Formation and and volcano-sedimentary units. Lateral variations the overlying Eocene Acamixtla Ignimbrite. of the Cretaceous sequences indicate a transition from a volcanic arc domain in the west to a shal- 3.2. Acamixtla Ignimbrite low marine platform in the east (Campa, 1978; Campa and Ram|¤rez, 1979). The platform marine The name Acamixtla Ignimbrite is applied to a beds are represented by mid-Cretaceous lime- sequence of silicic volcanic layers that crop out in stones of the Morelos Formation that are overlain the eastern part of the Taxco region and cover the by terrigenous beds of the Late Cretaceous Mex- red conglomerate (Fig. 4). It constitutes up to 100 cala Formation (Fries, 1960; De Cserna and m of crystal-rich ignimbrites, welded breccias and Fries, 1981). The volcano-sedimentary sequences layers of vitrophyre. The crystal-rich layer is char- of the western domain include some low-grade acterized by abundant phenocrysts of sanidine

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Fig. 4. Geologic map and stratigraphic column of the Taxco area. Solid lines enhance major fault structures. Localities of dated samples are indicated with white stars. Outline of the Mesozoic unit is based on the map by Rivera et al., (1998). Unit leveled Qlh represents lahars derived from the Plio^Quaternary volcanic activity of the Trans-Mexican Volcanic Belt. and quartz and the breccia is composed of frag- of this unit are distributed in restricted areas ments of rhyolite altered to zeolite (Alba-Aldave along the Chontalcuatla¤n and San Jose¤ rivers to et al., 1996; AŁ ngeles-Garc|¤a et al., 1997). The unit the north of the TVFand in the southern area, is well exposed along highway 95 at the eastern near Atzala. In the northern outcrops there are access to Taxco City where it is cut by pyroclastic remnants of volcanic necks. In the Atzala locality and vitrophyre dikes, as well as subvolcanic rhyo- the unit unconformably overlies the red conglom- lites. K^Ar determinations of whole rock and erate. Andesites underlie sedimentary deposits of mineral concentrates carried out in one vitrophyre the Atzala and Chontalcuatla¤n formations. and three samples of crystal-bearing ignimbrites have yielded dates ranging from 36.5 to 38 Ma 3.4. Chontalcuatla¤n Formation (De Cserna and Fries, 1981; Alba-Aldave et al., 1996; Mora¤n-Zenteno et al., 1999)(Fig. 4, Table The Chontalcuatla¤n Formation constitutes a se- 1). quence up to 350 m thick that is broadly exposed to the north of the TVF. Its distribution de¢nes a 3.3. Andesites sedimentary basin 10 km wide and 18 km long that is bounded to the east and to the west by This unit consists of porphyritic andesite lava two major strike-slip faults and to the north by £ows and autobreccias containing phenocrysts of highlands of Mesozoic rocks (Figs. 2 and 4). The plagioclase, hornblende and pyroxene. Outcrops lithology of the sequence comprises beds of sand-

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Table 1 Age determinations for the Tertiary magmatic rocks of Taxco area Sample Long. Lat. Mineral Rock type 40Ar* K 40Ar rad. Age (W) (N) (ppb) (%) (%) (Ma) TX4a 99‡34P36Q 18‡33P28Q WR vitrophyre 9.415 3.52 10.6 38.2 þ 1.0 TX-DM1 99‡35P05Q 18‡34P15Q hbl diorite 0.882 0.34 13.5 36.6 þ 1.9 TX-JL06 99‡37P58Q 18‡41P42Q sanidine ignimbrite 21.310 9.22 81.5 33.1 þ 0.8 TX21a 99‡38P55Q 18‡37P50Q biotite tu¡ 15.880 7.01 40.1 32.4 þ 0.8 TX16a 99‡36P15Q 18‡35P50Q WR vitrophyre 10.810 4.77 39.5 32.4 þ 0.9 TX25!!a 99‡37P17Q 18‡33P50Q WR vitrophyre 8.059 3.62 46.9 31.9 þ 0.8 TX10a 99‡38P50Q 18‡34P58Q plg rhyolite 1.259 0.57 29.9 31.6 þ 1.2 TX3M 99‡36P18Q 18‡32P02Q hbl gabbro 51.8 47.6 þ 3.6b TX10M 99‡36P24Q 18‡32P13Q hbl gabbro 70.5 52.3 þ 1.1b 40Ar* refers to radiogenic 40Ar. a Alba-Aldave et al. (1996). b Total gas ages, considered as minimum ages, obtained by the furnace incremental heating age spectrum 40Ar/39Ar in horn- blendes from a dike with hydrothermal alteration. A minimum age of 54 Ma was estimated from the highest temperature pla- teaus. stone as the major component, as well as subor- 3.6. Atzala Formation dinate interbedded layers of conglomerate and shale. The coarse-grained facies dominate towards The Atzala Formation is a sedimentary unit the margins of the basin, whereas ¢ner sediments interbedded with the lower part of the Oligocene with some lacustrine layers appear in the central rhyolitic sequence of the TVF. The Atzala For- part. The clastic nature of the sequence indicates a mation is constituted by about 150 m of £uvial local sedimentary source with a conspicuous dom- deposits dominated by beds of sandstone and inance of limestone fragments in the conglomer- conglomerates. In contrast with the Chontalcuat- ate. la¤n Formation, these beds include clastic frag- The Eocene age is inferred from the strati- ments of silicic volcanic rocks in addition to the graphic position of this sequence, underlying the limestone fragments. Sediments of this formation San Gregorio Ignimbrite and overlying andesites are exposed at Tetipac and in the southern margin that regionally have yielded ages older than 36.6 of the Oligocene rhyolites. At Tetipac they under- Ma. In the northern part of the basin the Chon- lie the lower member of the Tener|¤a Formation talcuatla¤n Formation is unconformably overlain and overlie the San Gregorio Ignimbrite. This re- by Plio^Quaternary lahar deposits derived from lationship with the Oligocene volcanic rocks sug- the Trans-Mexican Volcanic Belt. gests that it is a younger sedimentary sequence than the Chontalcuatla¤n Formation. 3.5. San Gregorio Ignimbrite As with the Chontalcuatla¤n Formation, the coarse-grained mainly limestone beds are present Near San Gregorio, an ignimbrite is widely dis- at the edge of the basin. The tectonic limits of this tributed and has abundant phenocrysts of sani- depositional basin are inferred from the linear dine, plagioclase, quartz and biotite in a vitric contacts of the Atzala Formation with Mesozoic matrix. It rests over the Chontalcoatla¤nForma- rocks, because these contacts are nearly parallel to tion and is covered by pyroclastic deposits from the fault systems. The SW boundary of the Atzala the Huixteco Formation. This ignimbrite also Formation outcrop area coincides with the NW- crops out near Tetipac, where it is covered by a trending strike-slip Chichila fault, whereas the SE conglomerate of the Atzala Formation described boundary parallels the NE fault system (Figs. 4 below (Fig. 4). A sanidine concentrate obtained and 5). from the base of the sequence yielded a K^Ar In the Atzala area we measure strata attitudes age of 33.1 þ 0.8 (Table 1). from the top to the bottom of the sedimentary

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Fig. 5. Structural map of the Taxco area showing the sites of detailed structural analysis and corresponding equal-area nets, lower hemisphere. Dots on girdles show the slickenside lineations. Arrows indicate the sense of movement of the hanging wall. Note that in some cases, two faults with the same strike have opposite senses of movement. See text for the explanation. Stars indicate the points of data acquisition for paleostress analysis. sequence. The dip direction of the strata changes Tertiary unit of the area and is formed by a 300- gradually from NNW in the upper contact to ESE m-thick pile of slightly to densely welded ignim- in the lower contact (Fig. 4). This change indi- brites that top the high ranges formed by the cates an incremental clockwise rotation of the Tener|¤a Formation (Fig. 4). Volcanic layers with- block containing the sedimentary beds. in the sequence display in general a subhorizontal attitude. They include pumice bearing ignimbrites, 3.7. Taxco Volcanic Field (TVF) slightly welded breccias, layers of vitrophyre and some ash fall tu¡s. A K^Ar whole rock age ob- The Tener|¤a Formation is one of the thickest tained from the top of the sequence yielded an age volcanic units in the region (V600 m) and con- of 32.4 þ 0.8 (Mora¤n-Zenteno et al., 1998) which, sists predominantly of rhyolitic lava £ows with within error, is compatible with dates obtained phenocrysts of sanidine and quartz, as well as from the underlying Tener|¤a Formation. some interlayered ignimbrites with phenocrysts of sanidine, quartz, biotite and hornblende. These rhyolites are associated with some small-scale 4. Tertiary structures domes aligned along fractures. Alba-Aldave et al. (1996) obtained K^Ar dates from rhyolites Several Tertiary kilometer-scale strike-slip and intermediate ignimbrites that are compatible faults are recognized in the Taxco region and sur- with the stratigraphic position of the unit. These rounding areas (Fig. 5). The sense of movement dates range from 31 to 32 Ma (Fig. 3, Table 1). was determined by using asymmetric structures The Huixteco Formation is the youngest silicic formed on fault surfaces; these structures are

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Fig. 6. Schematic blocks showing the kinematic conditions of deformation during late Eocene and early Oligocene in the Taxco region. E1 and E3 indicate the principal horizontal stretching and shortening directions. known as kinematic indicators. Three types of ki- The major faults described below, Tetipac, Chi- nematic indicators were used in the studied area: chila, Acamixtla, El Muerto, San Gregorio, Coa- crystal ¢bers, grain grooves and related fractures pango, and Taxco faults, are visible lineaments (Doblas, 1998). The crystal ¢bers are neoformed represented by fault zones wider than 50 m, in- quartz that grew congruously in the shadow pres- cluding associated segments or minor faults. sure zones. The grain grooves are conic-shaped The Acamixtla fault is a 15-km-long, N10‡E- features left by the movement of rigid grains trending, steeply dipping structure located east which ¢nally are indented on the fault surface. of the TVF( Fig. 5). It de¢nes the contact of the We used two types of fractures related to the Acamixtla Ignimbrite or the Chontalcuatla¤nFor- faults: open fractures dipping in opposite direc- mation with the eastern Mesozoic rocks and has a tion of the fault surface known as Tension gashes, well de¢ned linear geomorphic expression (Fig. 4). and synthetic faults with angles between 30 and In the northern segment of the Acamixtla fault, 60‡ with respect to the fault surface. In order to some ENE-trending faults cross the lineament determine the shear sense unambiguously, we zone and there are indications (mentioned below) used at least two types of kinematic indicators that both systems moved synchronously. In the on the same fault plane. southern termination of the Acamixtla fault (lo-

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Table 2 spaced, in some places with a frequency of about Fault data used in the computed paleostress tensor for the two faults per meter. Most of the structures mea- Oligocene event sured in the ¢eld trend in general N^S and are Fault Dip Dip Trend Plunge Movement high angle left-lateral faults. There are some direction NW-trending faults with dip-slip displacement. Taxco 80 73 350 3 Left The faults cut ignimbrites of the Acamixtla Ignim- San Gregorio 90 90 0 10 Left brite and rhyolite lava £ows of the Tener|¤aFor- San Gregorio 70 85 160 1 Left San Gregorio 272 90 2 5 Left mation. Several faults host pyroclastic dikes that Tetipac 230 80 140 1 Right presumably fed the Huixteco Formation, which is NW 55 90 325 1 Right the only pyroclastic deposit overlying the Tener|¤a NW 37 80 127 2 Right Formation. NW 65 55 90 52 Right-normal The Tetipac fault is located in the northwestern zone of the TVFand extends about 50 km farther cation Cst1, Fig. 4) there is a wide deformation NW (Rivera et al., 1998). It trends VN45‡W and zone that probably resulted from the convergence de¢nes the western boundary of the Chontalcuat- of the Acamixtla fault with the Coapango fault. la¤n Formation. The fault cuts the volcanic rocks The Coapango fault trends N^S and is 9 km of the TVFand the Mesozoic volcano-sedimenta- long, extending from Acamixtla to Coapango. ry unit, which extends to the west of the fault, There are good exposures of the fault zone on forming a highland area. Two phases of faulting highway 95 (Cst1, Fig. 5). There are many asso- can be inferred from the ¢eld relationships. Kine- ciated faults that have a spacing of about 15 m. matic indicators record a phase of faulting with Most of the measured N^S faults are left-lateral left-lateral displacement. This phase included a and the main fault is oriented N13‡W/80E‡, show- normal component, as is indicated by the subsi- ing horizontal slickenside lineations. Within the dence that enabled the accumulation of the Chon- fault zone there are also many right-lateral and talcuatla¤n Formation. The second phase seems to some dip-slip faults, commonly oriented N^NW. be subsequent and was inferred from kinematic The faults cut the Acamixtla Ignimbrite. indicators and local displacements of ignimbrite The San Gregorio fault trends N^S and extends layers. for about 20 km in the eastern zone of the TVF. The El Muerto fault is located at the southeast- It is located to the west of the Coapango and ern limit of the TVFand extends from north of Acamixtla faults. The trace of this structure can Taxco City to San Juan de Dios. It trends WNW be followed nearly to Chontalcuatla¤n, but there and is 10 km long, de¢ning the contact of the are geomorphologic indications of its continua- Taxco Schist with the red conglomerate and the tion to the north. The fault zone is well exposed Acamixtla Ignimbrite (Fig. 4). The main fault on highway 95 and on the road to San Gregorio trends N65‡W/48‡NE and is characterized by 6 m (Fig. 5). In the highway exposures the fault zone of fault breccia. Several conjugated faults dipping is 130 m wide and includes a main fault zone with to the south are observed in the hanging wall 20 m of breccia. This fault is vertical and oriented block. These faults have a moderate dip angle N^S. The slip vector was horizontal with left-lat- and the observed kinematic indicators suggest eral movement. There are some NW-trending oblique movement with normal and right-lateral faults crossing the N^S system that contains kine- components (Fig. 5). matic indicators that show both right-lateral and The Chichila fault trends NW and is located 13 left-lateral movement. Some NNW- and NW- km south of the Tetipac fault. It de¢nes the trending faults are ¢lled with pyroclastic material. southwestern boundary of the Atzala Formation The Taxco fault is 9 km long, trends N10‡W and its contact with the Mesozoic rocks (Fig. 5). and is well exposed along highway 95, 1 km east This structure extends for about 70 km to the of Taxco City. The fault zone is 160 m wide and is southeast. Rivera et al. (1998) mapped this fault formed by several discrete faults that are narrowly as part of a set of regional strike-slip parallel

VOLGEO 2483 21-10-02 10 S.A. Alaniz-AŁ lvarez et al./ Journal of Volcanology and Geothermal Research 118 (2002) 1^14 faults. The right-lateral movement is deduced gion. Late Eocene deformation is inferred mainly from the large-scale displacements observed in from the stratigraphic characteristics of the area the Mesozoic rocks (Fig. 2) and is consistent and the distribution of the Eocene sediments with with the clockwise rotation of the block contain- respect to major lineaments and the surrounding ing the Atzala Formation (Fig. 4). Mesozoic rocks. Oligocene deformation can be Several minor ENE-trending faults cut volcanic documented from the relationship between fault rocks within the TVF. The relative age of the kinematics and distribution of the volcanism. ENE faults with respect to the NW and N^S faults is equivocal. At an outcrop scale we docu- 5.1. Late Eocene NW extension mented ENE faults that moved before, after and synchronously with the N^S and NW faults at The Chontalcuatla¤n Formation, a sedimentary localities P10, P6, and CH1, respectively. Rocks sequence more than 350 m thick, is bounded by of the Huixteco Formation are cut by ENE, NW steeply dipping strike-slip faults and is topograph- and N^S faults (locations P10, P11, P12, S1, S2, ically lower than the surrounding Mesozoic rocks. and S3 in Fig. 5), showing that multiple faults The sediments are restricted to a narrow, deep with multiple striae sets a¡ected the youngest and sharply de¢ned structural basin. Early Oligo- rocks of the TVF. Because there are no strati- cene ignimbrites (sample JL06, Table 1) lie con- graphic indications of any other signi¢cant defor- formably above this sequence. These features in- mation phase postdating the TVF, we inferred dicate that the Chontalcuatla¤n Formation was that these faults moved synchronously. deposited in a basin formed by extension before There are some indications suggesting that N^S 33 Ma and after the extrusion of the andesite that and NW structures predate the documented lies at the bottom of the sequence. phases of Cenozoic deformation in the Taxco re- The extension required to form the basin ¢lled gion. The VN^S Acamixtla, San Gregorio and by the Chontalcuatla¤n Formation is more com- Taxco faults are located at the southern termina- patible with right-lateral movement of the VN^ tion of the Taxco^San Miguel de Allende linea- S-trending Acamixtla, San Gregorio and Taxco ment. This major structure has been interpreted as faults and with a left-lateral displacement of the a boundary of crustal blocks, that separates re- VNW-trending Tetipac fault (Fig. 6). In Tetipac, gional structural domains and, in some areas, ignimbrites of the Tener|¤a Formation are cut by controls the distribution of di¡erent Mesozoic NW faults showing both left- and right-lateral rocks (Nieto-Samaniego et al., 1999). Folds and displacement. Also, the N^S faults that cut the thrusts a¡ecting Mesozoic sequences in the Taxco Acamixtla Ignimbrite display kinematic indicators region are N^S-trending, suggesting that Cenozoic showing both senses. Coherent kinematic indica- faults inherited this structural trend (Fig. 2). tors are present in both systems but were later Indications of the pre-Eocene age of the NW- masked by the Oligocene deformation. trending faults are provided by Ar^Ar ages of Fig. 6 shows that these kinematics produced ma¢c dikes accompanying the Ag^Pb^Zn veins extension that was oriented circa NNW. It is no- hosted in faults with this orientation in the Taxco table that c1 bisects the obtuse angle between the mining district (Fig. 5). The analysis of the Ar^Ar faults instead of the acute angle, as is predicted spectra of two samples obtained from these dikes for Andersonian fault systems. We think that the indicates a minimum age of V54 Ma (Table 1). orientation of the faults in Taxco was imposed by the Mesozoic structural trend. The occurrence of widespread Paleogene defor- 5. Phases of faulting mation associated with NNW extension in southern Mexico is suggested by the kinematics of fault At least two phases of strike-slip faulting that systems documented in di¡erent regions (Gonza¤- are associated with the development of sedimen- lez-Torres, 1987; Tolson, 1998; Silva-Romo et al., tary basins can be documented in the Taxco re- 1999).

VOLGEO 2483 21-10-02 S.A. Alaniz-AŁ lvarez et al./ Journal of Volcanology and Geothermal Research 118 (2002) 1^14 11

5.2. Oligocene NE extension near the tip of several major faults (Acamixtla, Coapango, San Gregorio, and Tetipac). It is Oligocene volcanism in the TVFwas synchro- known that the stress ¢eld is disturbed near the nous with strike-slip faulting as is inferred from tips of faults (e.g. Segall and Pollard, 1980)orby the alignments of rhyolitic domes, the presence of the dynamic interaction between neighboring pyroclastic material ¢lling N^S-trending left-later- faults (e.g. Cashman and Ellis, 1994). Thus, with- al faults, and faults with the same kinematics cut- in a basin associated with strike-slip faults, the ting the ‘pyroclastic dikes’. Although faulting af- movement of minor faults is produced by local fects the Oligocene volcanic rocks, major faults stress tensors, which do not necessarily corre- are more conspicuous outside of the outcrop spond to the regional stress ¢eld. area of volcanic rocks. Nieto-Samaniego and Alaniz-Alvarez (1997) As is indicated in Fig. 5, N^S faults cutting the proposed that two faults interact kinematically if Oligocene Tener|¤a Formation show only left-lat- the slip of one fault depends on the movement of eral kinematic indicators, whereas right-lateral another fault, and therefore both have to move displacement indicators prevail on the NW faults. simultaneously. The resulting movement direction Taking this into account, the zone between the of the block bounded by the faults is parallel to Acamixtla and Tetipac faults is expected to have their intersection. We documented only three been a¡ected by shortening in Oligocene time. cases within the TVFwhere the N^S fault system The same zone was a¡ected by extension during appears to have been reactivated simultaneously the Eocene phase of deformation. with the ENE^WSW system (Fig. 5). The coeval The rhyolitic domes of the Tener|¤a Formation activation of two major faults is not common in are distributed between the Chichila and the Te- the TVFbecause the intersection of faults, and tipac faults within the area de¢ned by the right- thus the movement of the block between them, handed stepover. These NW faults had right-lat- is nearly vertical. Nevertheless, our structural eral movement during the Oligocene. data suggest that the ENE^WSW, N^S and NW The direction of maximum extension associated faults were activated independently during the with the emplacement of the Oligocene volcanism same phase of deformation. can be obtained from the kinematic analysis of We used the method described by Reches fault-slip data. A reasonable assumption is that (1987) to calculate the directions and relative the minimum principal stress (c3) parallels the magnitudes of a stress tensor that could satisfy maximum extension (E1). In order to use slicken- the Oligocene fault-slip data. Reches’ method as- side lineations to obtain the stress tensor it is nec- sumes that the slip occurs in the direction of max- essary to verify that the fault-slip data used satisfy imum shear stress along the fault and considers the assumption of parallelism between the slicken- that slip direction satis¢es the Coulomb yield cri- side lineations and the maximum shear stress on terion. The fault-slip data used were major faults the fault plane. Since there are kinematic incom- with more than 2 m of gouge and breccia (Table patibilities among slip-fault data of the same 2, Fig. 5). We used eight faults for the inversion, phase of deformation that must be eliminated, four correspond to the N^S striking faults and the we removed fault data of the Eocene deformation, other four to the NW striking faults. We obtain minor faults and those slickenside lineations that c1 = 154‡/4‡ (trend/dip), c2 = 353‡/85‡, c3 = 244‡/ seem to have been produced by interaction with 1‡, and stress ratio R =(c23c3/c13c3) = 0.77. other fault planes. Considering the two phases of deformation de- Most of the minor faults located in the central scribed above, we interpret a change in the c3 part of the TVFdisplay indications of movements direction from NNW to N64‡E that took place incompatible with the kinematics of the major about 33 Ma. This age corresponds to that of faults and do not correspond to a unique stress the ignimbrite located between the sediments of tensor. Figs. 2 and 5 show that the TVFis located Chontalcuatla¤n and Atzala formations.

VOLGEO 2483 21-10-02 12 S.A. Alaniz-AŁ lvarez et al./ Journal of Volcanology and Geothermal Research 118 (2002) 1^14

6. Relationships between volcanism and strike-slip be expected to occur in normal and strike-slip faulting faulting regimes. It has been established that for each unit of 6.1. Theoretical considerations volcanic rock there could exist an important amount of intrusive rocks as dikes, plutons and Stress ¢eld, magmatic input (Takada, 1989, other intrusions (Takada, 1989; Parsons and 1994), buoyancy and rheology contrast (Wata- Thompson, 1991). The ratios of intrusive to ex- nabe et al., 1999) have been proposed as the fac- trusive volumes of silicic magmas has been esti- tors that determine magma transport and em- mated between one and 10 (Crisp, 1984 and refer- placement. The type of volcanism (monogenetic ences therein). Therefore, by each volumetric unit or polygenetic) can be determined by the magma of volcanic rocks there are between one and 10 trajectory across the lithosphere, induced by the units of plutonic or subvolcanic rocks. In zones tectonic stress ¢eld plus the e¡ect of the magma with a large volume of volcanic rocks this ratio pressure at di¡erent levels of the crust. could imply an important change in crustal vol- It is expected that magma migrates toward the ume and therefore should be considered in strain direction of minimum pressure. According to Se- analysis. Parsons and Thompson (1991) and Par- cor and Pollard (1975), excess magmatic pressure sons et al. (1998) proposed that magmatism inhib- is de¢ned as Pe = P3c3, where P is magma pres- its normal and strike-slip faulting when the vol- sure and c3 the minimum principal compressive ume occupied by dikes accommodates all the stress. The direction of magma migration is con- tectonic extension. trolled by the maximum gradient of excess magmatic pressure, whose value increases as c3 dimin- 6.2. Volcanism and faulting in the TVF ishes. With c3 horizontal and varying greater than P with depth, the maximum gradient of excess In a strike-slip regime the maximum extension magmatic pressure is upward (Secor and Pollard, is located in the pull-apart zones which, at the 1975; Takada, 1989). time of movement, has a di¡erent local stress ¢eld The most e¡ective mechanism of magma migra- with c1 vertical (e.g. Segall and Pollard, 1980; tion is through dikes. The low viscosity of basaltic Bertoluzza and Perotti, 1997). For the late Eocene magma has been considered the cause of melt in- deformation in Taxco, the maximum extension trusion as relatively narrow tabular bodies (Emer- zone should have been near the southwestern tip man and Marrett, 1990). Nevertheless, felsic mag- of the Acamixtla fault (Fig. 6). The source of the ma could migrate by dikes under the appropriate Acamixtla ignimbrites is unknown, but the lack of conditions (e.g. Weinberg, 1996; Clemens and pyroclastic dikes in the Eocene conglomerates Mawer, 1992). The migration of magma as dikes (Edwards, 1955 and this work) suggests that the could occur along pre-existing fractures as well as ignimbrites could have been generated outside of by forming a new fracture (resembling hydraulic the TVFarea. The extensional strain recorded by fractures, e.g. Anderson, 1951). In the ¢rst case the Chontalcuatla¤n basin ¢ll and the lack of vol- dikes grow when magmatic pressure exceeds the canic rocks suggest that subsurface magmatism regional normal stress on the dike. If the magma was scarce at this time. propagates by new fracture, dikes are formed in The Oligocene rhyolitic volcanism is located the plane normal to c3. Delaney et al. (1986) pro- within an extensional basin formed in the right- posed that magma preferentially invaded pre-ex- handed stepover between the Chichila and Tetipac isting fractures oriented at angles 6 40‡ to c3. dextral faults. The Atzala sequence, at the bot- According to those arguments the following tom, indicates that subsidence began prior to the conditions are necessary for magma ascent and volcanism. Subsidence ceased at the time of vol- volcanism: (1) upward orientation of the maxi- canism, nevertheless there is evidence of synchro- mum value of Pe, (2) dPe/dZ 6 0 and (3) c3 hor- nous faulting. The result of these events is that izontal. Given these conditions, volcanism would instead of a basin, a relatively high topographic

VOLGEO 2483 21-10-02 S.A. Alaniz-AŁ lvarez et al./ Journal of Volcanology and Geothermal Research 118 (2002) 1^14 13 feature consisting of rhyolitic domes was formed Alba-Aldave, L., Reyes-Salas, A.M., Mora¤n-Zenteno, D.J., Ł (Fig. 4). The absence of a related caldera indicates Angeles-Garc|¤a, S., Corona-Esquivel, R., 1996. Geoqu|¤mica de las rocas volca¤nicas terciarias de la regio¤n de Taxco- that the volume expulsed was emitted without Huautla. Memoria del VII Congreso Nacional de Geoqu|¤- forming an empty space near the surface that mica, San Luis Potos|¤,. Actas INAGEQ 2, 39^44. could collapse. These features suggest that the Anderson, E.M., 1951. The Dynamics of Faulting and Dyke volume occupied by magma inhibited normal Formation with Application to Britain. 2nd edn., Oliver and faulting during extension. Boyd, Edinburgh. AŁ ngeles-Garc|¤a, S., Reyes-Salas, M., Cruz-Sa¤nchez, E., 1997. Caracterizacio¤n de la zeolita de la brecha de Taxco. Actas INAGEQ 3, 341. 7. Conclusions Bertoluzza, L., Perotti, C.R., 1997. A ¢nite-element model of the stress ¢eld in strike-slip basins: implications for the Per- From the analysis of the Tertiary stratigraphy, mian tectonics of the Southern Alps (Italy). Tectonophysics 280, 185^197. the distribution of the rock units and the charac- Campa, M.F., 1978. La evolucio¤n tecto¤nica de Tierra Caliente, teristics of the faults, we documented two exten- Guerrero. Bol. Soc. Geol. Mex. 39, 52^64. sional basins, associated with strike-slip faults, Campa, M.F., Ram|¤rez, J., 1979. La Evolucio¤n geolo¤gica y la formed successively during the Eocene^Oligocene Metaloge¤nesis del Noroccidente de Guerrero. Universidad interval. The oldest phase of faulting was related Auto¤noma de Guerrero, Serie Te¤cnico Cient|¤¢ca, 1. Campa, M.F., Coney, P.J., 1983. Tectono-stratigraphic ter- to NNW extension and occurred in the late Eo- ranes and mineral resource distributions of Me¤xico. Can. cene. The younger phase took place in the early J. Earth Sci. 20, 1040^1051. Oligocene and was related to NE extension. The Cashman, P.H., Ellis, M.A., 1994. Fault interaction may gen- transition between both regimes occurred about erate multiple slip vectors on a single fault surface. Geology 33 Ma. 22, 1123^1126. Clemens, J.D., Mawer, C.K., 1992. Granitic magma transport The voluminous Oligocene volcanism of the by fracture propagation. Tectonophysics 204, 339^360. Taxco region occurred synchronous with strike- Crisp, J.A., 1984. Rates of magma emplacement and volcanic slip faulting within zones under extension. The output. J. Volcanol. Geotherm. Res. 20, 177^211. volcanic rocks overlie the sediments that ¢lled De Cserna, Z., Fries, C. Jr., 1981. Hoja Taxco 14Q-h(7) escala the younger basin. Extension allowed rhyolitic 1:100 000, Geolog|¤a de los Estados de Guerrero, Me¤xico y Morelos: Universidad Nacional Auto¤noma de Me¤xico, In- magma to rise to the surface. The positive relief stituto de Geolog|¤a, Me¤xico. formed by the volcanic rocks indicates that the Delaney, P.T., Pollard, D.D., Ziony, J.I., McKee, E.H., 1986. volume of magma exceeded the stretching within Field relations between dikes and joints: emplacement pro- the extended zone and prevented subsidence. cesses and paleostress analysis. J. Geophys. Res. 91, 4920^ 4938. Demant, A., 1978. Caracter|¤sticas del Eje Neovolca¤nico Trans- Acknowledgements mexicano y sus problemas de interpretacio¤n. Universidad Nacional Auto¤noma de Me¤xico, Instituto de Geolog|¤a, Re- vista, 2, pp. 172^187. We thank to Barbara Martiny for useful dis- Doblas, M., 1998. Slickenside kinematic indicators. Tectono- cussions and revision of the manuscript. We also physics 295, 187^197. thank Juan Va¤zquez and Crescencio Gardun‹o for Edwards, J.D., 1955. Studies of some early Tertiary red con- technical assistance. The comments and sugges- glomerates of central Mexico. Geol. Surv. Prof. Pap. 264-H. Emerman, S.H., Marrett, R., 1990. Why dikes? Geology 18, tions of Mike Hall and Ray Cas greatly improved 231^233. the manuscript. This research was supported by Fries, C. Jr., 1960. Geolog|¤a del Estado de Morelos y de partes UNAM, PAPIIT-IN113198. adyacentes de Me¤xico y Guerrero, regio¤n central-meridional de Me¤xico. Universidad Nacional Auto¤noma de Me¤xico, Instituto de Geolog|¤a, Bolet|¤n 60, 236 pp. References Garc|¤a-Palomo, A., Mac|¤as, J.L., Gardun‹o, V.H., 2000. Mio- cene to Recent structural evolution of the Nevado de Toluca Aguirre-D|¤az, G., 1996. Volcanic stratigraphy of the Amealco volcano region, Central Me¤xico. Tectonophysics 318, 281^ caldera and vicinity, central Mexican Volcanic Belt. Rev. 302. Mex. Cienc. Geol. 13, 10^51. Gonza¤lez-Torres, E., 1987. Geolog|¤a y paleomagnetismo del

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