tertiary arc-magmatism of the sierra madre del sur, …rmolina/documents/moran-etal-1999.pdf · and...

Tertiary arc-magmatism of the Sierra Madre del Sur, Mexico,and its transition to the volcanic activity of the Trans-Mexican

Volcanic Belt

D.J. MoraÂ n-Zentenoa,*, G. Tolsona, R.G. MartõÂ nez-Serranob, B. Martinya, P. Schaafb,G. Silva-Romoc, C. MacõÂ as-Romoa, L. Alba-Aldavea, M.S. HernaÂ ndez-Bernala,

G.N. SolõÂ s-Pichardoa

aInstituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, Cd. Universitaria, Mexico, D.F., 04510bInstituto de GeofõÂsica, Universidad Nacional AutoÂnoma de MeÂxico, Cd. Universitaria, Mexico, D.F., 04510

cFacultad de IngenierõÂa, DivisioÂn de Ciencias de la Tierra, Universidad Nacional AutoÂnoma de MeÂxico, Cd. Universitaria, Mexico, D.F., 04510

Abstract

The Tertiary magmatic rocks of the Sierra Madre del Sur (SMS) are broadly distributed south of the Trans-Mexican VolcanicBelt (TMVB) and extend to the southern continental margin of Mexico. They represent magmatic activity that originated at atime characterized by signi®cant changes in the plate interactions in this region as a result of the formation of the Caribbean

plate and the southeastward displacement of the Chortis block along the continental margin of southwestern Mexico. Thechange from SMS magmatism to an E±W trending TMVB volcanism in Miocene time re¯ects the tectonic evolution ofsouthwestern Mexico during these episodes of plate tectonic rearrangement.

The distribution and petrographic characteristics of the magmatic rocks of the SMS de®ne two belts of NW orientation. The®rst is represented by the nearly continuous coastal plutonic belt (CPB), which consists of batholiths and stocks ofpredominantly felsic composition. The second belt is inland of the ®rst and consists of discontinuously distributed volcanic ®elds

with piles of andesitic to rhyolitic ¯ows, as well as epiclastic and pyroclastic materials. These two belts were emplaced along acontinental crust segment constituted by a mosaic of basements with recognizable petrologic and isotopic di�erences. Thesebasements originated during di�erent tectono-thermal events developed from the Proterozoic to the Mesozoic.

Major and trace element data of the SMS magmatic rocks de®ne a clear sub-alkaline tendency. Variations in the generalgeochemical behavior and in the Sr and Nd isotopic ratios indicate di�erent degrees of magmatic di�erentiation and/or crustalcontamination. These variations, specially in the inland Oligocene volcanic regions of Guerrero and Oaxaca states, seem to havebeen controlled by the particular tectonic setting at the time of magmatism. In northwestern Oaxaca greater extension related to

transtensional tectonics produced less di�erentiated volcanic rocks with an apparently lower degree of crustal contaminationthan those of northeastern Guerrero.The geochronologic data produced by us up to now, in addition to those previously reported, indicate that the Tertiary

magmatic rocks of the SMS range in age from Paleocene to Miocene. The general geochronologic patterns indicate asoutheastward decrease in the age of igneous activity, rather than a gradual northeastward migration of the locus of magmatismtoward the present-day TMVB. SMS magmatic rocks exposed to the west of the 1008W meridian are dominantly Late

Cretaceous to Eocene, while those to the east range from Oligocene to Miocene, also following a southeastward age-decreasingtrend. Paleocene and Eocene magmatic rocks of the western region of the SMS seem to keep a general NNW trend similar tothat of the Tertiary magmatic rocks of the Sierra Madre Occidental (SMO). In the eastern region of the SMS the Oligocene

magmatic rocks show a trend that roughly de®nes an ESE orientation. The change in the trend of arc magmatism may be thee�ect of the landward migration of the trench, for a given longitude, as a result of the displacement of the Chortis block. The

Journal of South American Earth Sciences 12 (1999) 513±535

0895-9811/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved.

PII: S0895-9811(99 )00036 -X

* Corresponding author. Tel: +5-616-0557; fax: +5-550-6644.

E-mail address: [email protected] (D.J. MoraÂ n-Zenteno).

transtensional tectonic regime developed in Oligocene time in NW Oaxaca probably accentuated this trend by facilitating

magma generation and ascent in these northerly regions.The geochronologic data of the SMS, in conjunction with those of the TMVB, suggest that there is a spatial and temporal

magmatic gap in south central Mexico between 97 and 1008W longitude during late Oligocene and middle Miocene time (24±16

Ma). This magmatic gap is interpreted in terms of a combination of the relatively rapid change in the subducted slab geometryafter the passage of the Chortis block from a moderate to a shallow angle and the time needed for the mantle wedge to maturesu�ciently to produce magmas. # 1999 Published by Elsevier Science Ltd. All rights reserved.

Resumen

Las rocas magmaÂ ticas terciarias de la Sierra Madre del Sur (SMS) se encuentran ampliamente distribuidas al sur de la Faja

VolcaÂ nica Transmexicana (FVTM) y se extienden hasta la margen continental del sur de MeÂ xico. La actividad magmaÂ tica quedio origen a estas rocas se desarrolloÂ en un tiempo caracterizado por cambios signi®cativos en las interacciones de las placastectoÂ nicas en esta regioÂ n, lo que resultoÂ de la formacioÂ n de la placa del Caribe y el desplazamiento al sureste del bloque de

Chortis a lo largo de la margen suroccidental de MeÂ xico. El cambio del magmatismo de la SMS al volcanismo con orientacioÂ nE±W de la FVTM, en el Mioceno, re¯eja la evolucioÂ n tectoÂ nica del suroeste de MeÂ xico durante estos episodios detransformacioÂ n en la con®guracioÂ n de las placas tectoÂ nicas.La distribucioÂ n y las caracterõÂ sticas petrograÂ ®cas de las rocas magmaÂ ticas de la SMS de®nen dos cinturones con orientacioÂ n al

NW. El primero de ellos estaÂ representado por un cinturoÂ n casi continuo de plutones ubicados a lo largo de la costa (CPB), elcual consiste principalmente de batolitos y troncos de composicioÂ n dominantemente feÂ lsica. El segundo cinturoÂ n se encuentrahacia el interior del continente y consiste de campos volcaÂ nicos discontinuamente distribuidos que incluyen secuencias con

derrames de andesita a riolita, asõÂ como materiales epiclaÂ sticos y piroclaÂ sticos. Estos dos cinturones fueron emplazados en unsegmento de corteza continental constituido por un mosaico de basamentos con diferencias petroloÂ gicas e isotoÂ picasreconocibles. Estos basamentos se originaron en diferentes eventos tectono-teÂ rmicos desarrollados entre el Proterozoico y el

Mesozoico.Los datos de elementos mayores y traza de las rocas magmaÂ ticas terciarias de la SMS de®nen una tendencia claramente

subalcalina. Las variaciones en el comportamiento geoquõÂmico general, asõÂ como en las razones isotoÂ picas de Sr y Nd, indican

diferentes grados de diferenciacioÂ n magmaÂ tica y/o contaminacioÂ n cortical. Estas variaciones, especialmente en las regionesvolcaÂ nicas del Oligoceno del interior de los estados de Guerrero y Oaxaca, parecen haber sido controladas por el ambientetectoÂ nico particular en el tiempo del magmatismo. En el noroeste de Oaxaca, la mayor extensioÂ n asociada a la tectoÂ nicatranstensional produjo rocas volcaÂ nicas menos diferenciadas con un aparente menor grado de contaminacioÂ n cortical que

aquellas del noreste de Guerrero.Los datos geocronoloÂ gicos producidos por nosotros hasta ahora, asõÂ como a aquellos previamente reportados por otros

autores, indican que las rocas magmaÂ ticas de la SMS varõÂ an en edad del Paleoceno al Mioceno. Los patrones geocronoloÂ gicos

generales indican una edad decreciente del magmatismo hacia el sureste, maÂ s que una migracioÂ n gradual del eje del magmatismohacia el norte, hasta la posicioÂ n de la FVTM. Las rocas magmaÂ ticas de la SMS expuestas al oeste del meridiano 1008W, tienendominantemente edades entre el CretaÂ cico TardõÂ o y el Eoceno, mientras que aqueÂ llas ubicadas al este varõÂ an entre el Oligoceno

y el Eoceno, mostrando tambieÂ n un patroÂ n decreciente hacia el sureste en sus edades. Las rocas magmaÂ ticas del Paleoceno y elEoceno de la regioÂ n occidental de la SMS parecen guardar la tendencia general hacia el NNW que tienen las rocas magmaÂ ticasde la Sierra Madre Occidental (SMO). En la regioÂ n oriental de la SMS las rocas magmaÂ ticas del Oligoceno muestran unatendencia en su distribucioÂ n con una orientacioÂ n cercana al ESE. El cambio en la orientacioÂ n del magmatismo puede ser el

efecto de la migracioÂ n de la trinchera hacia el interior del continente, para una longitud dada, como resultado deldesplazamiento del bloque de Chortis. El desarrollo de la tectoÂ nica extensional en el Oligoceno en el NW de Oaxacaprobablemente acentuoÂ esta orientacioÂ n, al facilitar la generacioÂ n y acenso de magma en estas regiones septentrionales.

Los datos geocronoloÂ gicos de la SMS, en combinacioÂ n con aquellos de la FVTM, sugieren que existioÂ un `gap' temporal en elmagmatismo para un intervalo entre el Oligoceno tardõÂ o y el Mioceno medio (24±16 Ma), entre los 978 y 1008W. Este `gap'magmaÂ tico es interpretado en teÂ rminos de una combinacioÂ n entre el cambio relativamente raÂ pido en la geometrõÂ a de la placa

subducida de un aÂ ngulo moderado a un aÂ ngulo bajo, despueÂ s del paso del bloque de Chortis, y el tiempo necesario para que lacunÄ a del manto madurara su®cientemente para producir magma. # 1999 Published by Elsevier Science Ltd. All rights reserved.

1. Introduction

The oblique orientation of the Trans-MexicanVolcanic Belt (TMVB) with respect to the Acapulco(Middle America) Trench is a relatively unusual fea-ture of the circum-Paci®c that has been related to the

regional variations in the kinematic interactionsamong the North America, Cocos and Rivera plates(Fig. 1, inset) (Urrutia-Fucugauchi and Del Castillo-GarcõÂ a, 1977; SuaÂ rez et al., 1990; Pardo and SuaÂ rez,1995). Geochronological evidence indicates thatTMVB activity started at about 16 Ma before the pre-

D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535514

sent and evolved to E±W trending ma®c to intermedi-ate volcanism that is active to this day (Ferrari et al.,1994).

Northwest of the TMVB, the volcanic arc productsrelated to the convergent limit between the Farallonplate and the North America plate form the NNWtrending Sierra Madre Occidental (SMO) provinceconstituted by andesitic to rhyolitic sequences rangingin age from Paleocene to middle Miocene. The tran-sition of volcanic activity from the SMO to the TMVBin western Mexico occurred without interruption,whereas in some areas of the northern part of the cen-tral and eastern sectors of the TMVB, a volcanic hia-tus has been recognized between 26 and 16 Ma(Ferrari et al., 1994).

The aerial distribution, as well as the structural andgeochronologic characteristics of the Tertiary arc-mag-matic rocks of the Sierra Madre del Sur (SMS), suggesta di�erent tectonic scenario for pre-middle Miocene

time with respect to the present-day arc-trench con®gur-ation. The Tertiary magmatic rocks of the SMS consti-tute a broad province of intrusive and extrusive rocksthat extends from the State of MichoacaÂ n to the Isthmusof Tehuantepec (Fig. 1). The southernmost outcrops ofthe younger volcanic rocks of the TMVB de®ne thenorthern limit of the SMS. To the south, the igneousproducts of the SMS extend to the southwestern conti-nental margin of Mexico (Bellon et al., 1982). The distri-bution and petrologic characteristics of the Tertiarymagmatic rocks of the SMS de®ne two di�erent NWtrending belts (Fig. 1). The ®rst, the CPB, is representedby a conspicuous chain of batholiths along with smallerintrusives that outcrop along the southwestern continen-tal margin of Mexico and indicate relatively rapid upliftsince the Oligocene (MoraÂ n-Zenteno et al., 1996). Theother is represented by a series of relatively isolated vol-canic ®elds that are distributed between the CPB and theTMVB. This paired con®guration of the Tertiary mag-

Fig. 1. Geologic map of southern Mexico showing the distribution of Cenozoic magmatic rocks with selected dates or age ranges, as well as the

location of transects shown in Fig. 6. The methods and uncertainties associated with these dates are presented in Table 2. The inset shows the

distribution of Tertiary magmatic provinces in Mexico and the present-day plate tectonic features (SMO=Sierra Madre Occidental, IVF=Inland

volcanic ®elds of the SMS, CPB=Coastal plutonic belt of the SMS, TMVB=Trans-Mexican Volcanic Belt) (States: J=Jalisco, M=MichoacaÂ n,

G=Guerrero, O=Oaxaca, C=Chiapas; MC=Mexico City; IT=Isthmus of Tehuantepec).

D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 515

matic rocks of the SMS portrays not only the exposureof di�erent crustal levels but also some di�erences inpetrological characteristics and tectonic relationships.

The SMS magmatic rocks play a crucial role towardthe understanding of the regional geologic frameworkof south central Mexico. They originated at a timecharacterized by signi®cant changes in the plate tec-tonic interactions around the Paci®c margin ofsouthern Mexico (Ross and Scotese, 1988;Mammerickx and Klitgord, 1982). The stratigraphicdetails, geochemical characteristics and particular tec-tonic relationships of the SMS Tertiary volcanicsequences are essential to an understanding of thetransition to the present day trench-arc con®guration,because they are the result of the changes whichbrought about this transition. In spite of this, studieson regional geochronological or geochemical patternsare relatively scarce (Guerrero-GarcõÂ a, 1975; Schaaf;1990; MoraÂ n-Zenteno, 1992; FerrusquõÂ a-Villafrancaand McDowell, 1991; Herrmann et al., 1994; Schaaf etal., 1995; MartõÂ nez-Serrano et al., 1997; Martiny et al.,1997; MoraÂ n-Zenteno et al., in press).

In this paper we discuss our geochronological andgeochemical database, as well as data previouslyreported by other authors in order to evaluate theirsigni®cance in the interpretation of the regionalchanges in the magmatic patterns and their possible re-lationships with the plate tectonic interactions and thedeformation of the continental crust.

2. The basement of the SMS Tertiary magmatic rocks

The Tertiary plutonic and volcanic rocks of theSMS are found in di�erent basement terranes, each ofwhich display contrasting stratigraphic characteristics

as well as tectonic a�nities (Ortega-GutieÂ rrez, 1981;

Campa and Coney, 1983). The main characteristics of

the basement units and their isotopic compositions are

summarized in Table 1. The inset of Fig. 2 shows the

tectonostratigraphic division of Campa and Coney

(1983) for southern Mexico. According to this division,

the SMS is distributed along the Guerrero, Mixteca,

Oaxaca, Xolapa and JuaÂ rez terranes.

The Guerrero terrane is represented by island-arc

sequences, mostly Early Cretaceous in age, although

some Late Jurassic units have also been reported

(Centeno-GarcõÂ a et al., 1993a, and references therein).

The stratigraphic relationships of these arc sequences

as well as the nature and age of their basement have

been the matter of debate (Campa, 1978; De Cserna et

al., 1978; Campa and RamõÂ rez-Espinosa, 1979; Campa

and Coney, 1983; Lang et al., 1996). There is evidence

indicating that the Guerrero terrane sequences north-

west of Acapulco unconformably overlie a pre-

Cretaceous assemblage of oceanic a�nity (Centeno-

GarcõÂ a et al., 1993b), while the easternmost sector

thrusts over continental crust probably related to the

Mixteca terrane basement (Campa, 1978; Campa and

Coney, 1983; ElõÂ as-Herrera and SaÂ nchez-Zavala, 1990).

The AcatlaÂ n Complex, which constitutes the base-

ment of the Mixteca terrane, is represented by a tecto-

nically heterogeneous assemblage of mostly oceanic

basin a�nity, including metamorphic units ranging

from greenschist to eclogite facies (Ortega-GutieÂ rrez,

1978, 1993). The metamorphism and intense defor-

mation of the AcatlaÂ n Complex have been interpreted

as the result of its accretion to the continental nucleus

represented by the Oaxaca Complex (Ortega-GutieÂ rrez,

1993). The AcatlaÂ n Complex is covered by igneous

and sedimentary sequences, including Late Paleozoic

and Middle Jurassic to Cretaceous rocks (FerrusquõÂ a-

Table 1

Summary of the main petrologic characteristics and typical isotopic ranges of the tectono-stratigraphic terrane basements of southern Mexico.

Isotopic ranges are based on recalculated data (30 Ma) from Ruiz et al. (1988a,b), YanÄ ez et al. (1991), MoraÂ n-Zenteno (1992), Centeno-GarcõÂ a

et al. (1993b), Talavera-Mendoza et al. (1995)

Guerrero Terrane Mixteca Terrane Oaxaca Terrane Xolapa Terrane

Basement or

characteristic

sequences

Mesozoic volcano-sedimentary

sequences

AcatlaÂ n Complex Oaxaca Complex Xolapa Complex

Age Jurassic to Lower Cretaceous Lower Paleozoic

metamorphism

Precambrian (Grenville)

metamorphism

Mesozoic metamorphism and

anatexis of older metamorphic

and plutonic units

Petrologic

features

Volcano-sedimentary arc

sequences unconformably

underlain by oceanic-like

sequences. Post tectonic

plutons

Greenschist facies metapelites

and greenstones. Eclogite

facies metapelites, ultrama®c

rocks and granitoids

Granulite facies para- and

orthogneisses, meta-

anorthosites, metagabbros

and charnockites

Igneous and sedimentary

migmatites including

metagraywackes, quartz-

amphibolites, quartz-

feldspathic gneisses andmarble

lenses87Sr/86Sr 0.7035±0.7060 0.705±0.750 0.704±0.716 0.704±0.715

eNd from +1 to +7 from +3 to ÿ12 from ÿ9 to ÿ13 from +3 to ÿ12


Villafranca, 1976; MoraÂ n-Zenteno et al., 1993 andreferences therein).

The metamorphic basement of the Oaxaca terrane isrepresented by a Grenvillian age (900±1110 Ma) gran-ulite facies complex with ma®c and felsic ortho-gneisses, paragneisses as well as meta-anorthosites andcharnockites (Ortega-GutieÂ rrez, 1981, 1993). The post-metamorphic cover includes sedimentary sequences ofthe Tremadocian, Mississippian±Permian andJurassic±Cretaceous time intervals (Pantoja-Alor,1970; Schlaepfer, 1970).

The Xolapa terrane is composed of a lithologicallyheterogeneous mid-crustal amphibolite-facies assem-blage consisting of metagraywackes, quartz-amphibo-lites, quartz-feldspathic gneisses and marble lenses(Ortega-GutieÂ rrez, 1981; Corona-ChaÂ vez, 1997;Tolson, 1998). A characteristic feature of this assem-blage is the occurrence of migmatites indicating di�er-ent degrees of anatexis and subsolidus di�erentiation.The timing of metamorphism of the Xolapa terrane isnot yet clear, since ages between Lower Cretaceousand Eocene have been reported (MoraÂ n-Zenteno,1992; Herrmann, 1994).

The JuaÂ rez terrane consists of Mesozoic sequencesof marine and continental sedimentary and volcano-

genic strata intensively deformed and thrusted with ageneral vergence to the east (Carfantan, 1986;Barbosa, 1994). The western margin of the JuaÂ rez ter-rane north of the city of Oaxaca is de®ned by a west-dipping, NNW trending polygenic mylonitic shearzone which was reactivated in Middle Jurassic andTertiary times (Alaniz-AÂ lvarez et al., 1996).

Information about the deep structure and lateralvariations of the lower crust in southern Mexico is lim-ited, but the general arrangement and tectonic historyportrayed by the exposed basement units suggest thatgradually younger fragments of continental crust wereaggregated around the Oaxaca Complex duringPaleozoic and Mesozoic times (Ortega-GutieÂ rrez,1993).

From an isotopic point of view, the basement rocksof the Guerrero terrane are quite primitive comparedto those of the AcatlaÂ n, Oaxaca and Xolapa complexes(Table 1). Typical 87Sr/86Sr and eNd values for theigneous rocks of the Guerrero terrane sequences (recal-culated to 30 Ma, i.e. the time of magmatism) rangefrom 0.7035 to 0.7060 and +1.0 to +7.0, respectively(Centeno-GarcõÂ a et al., 1993b; Talavera-Mendoza etal., 1995). The other terranes exhibit di�erent ten-dencies toward more evolved 87Sr/86Sr isotopic ratios

Fig. 2. Map of southwestern Mexico showing di�erent deformation domains and indicating their age. Where the age is not constrained it is indi-

cated by a question mark. Active features are indicated with an age of 0 Ma. For references to the timing of deformation see text. The inset

shows the distribution of tectonostratigraphic terranes after Campa and Coney (1983) as well as the Sierra Madre Oriental, the Sierra Madre

Occidental and the Trans-Mexican Volcanic Belt. Abbreviations used in the inset are, G=Guerrero terrane, J=JuaÂ rez terrane, Ma=Maya ter-

rane, Mi=Mixteca terrane, O=Oaxaca terrane, SM=Sierra Madre terrane, SMO=Sierra Madre Occidental, TMVB=Trans-Mexican Volcanic

Belt, X=Xolapa terrane.


and negative eNd values (YanÄ ez et al., 1991; RuõÂ z etal., 1988a,b), probably as a result of the in¯uence ofthe Oaxacan Grenville basement nucleus.

3. Cenozoic structural and tectonic setting of the SMS

The plate tectonic setting and evolution of southernMexico during the Tertiary is complex. From the earlyTertiary and until 29 Ma the Farallon plate was con-sumed beneath southern Mexico. Subduction of theGuadalupe plate, formed by fragmentation of theFarallon plate, occurred during the period between 29and 12.5 Ma. Since 12.5 Ma the Cocos and Riveraplates have been overridden by North America alongthe Acapulco Trench (Mammerickx and Klitgord,1982). It is uncertain whether or not these majorchanges in plate architecture and kinematics played arole in the determination of the patterns of magmatismassociated with subduction in southern Mexico and inthe development of the complex geometry of the defor-mation domains. In Oligocene to Miocene time the re-arrangement of the oceanic plates did not producesigni®cant variations in the age of oceanic crust beingsubducted, but rather slight variations in the conver-gence vectors.

At present, the Rivera and Cocos plates are beingsubducted along the Paci®c margin of Mexico and thesouthern limit of the North America plate is the left-lateral Polochic-Motagua transform fault zone innorthern Guatemala and Belize which extends towardthe east into the Caribbean Sea (Fig. 1 inset). TheCocos±Caribbean±North America triple junction is abroad zone of subsidence o� the coast of Salina Cruzde®ned by shallow normal fault seismicity (GuzmaÂ n-Speziale et al., 1989).

The conspicuous obliquity of the present-day volca-nic arc to the Acapulco Trench has been explained bya lateral decrease in the dip angle of the subductedCocos plate in such a way that the 100 km depth con-tour line is roughly parallel to the southern margin ofthe TMVB (Pardo and SuaÂ rez, 1995). The angle ofsubduction of the Rivera plate at the western end ofthe TMVB and of the Cocos plate to the east of theIsthmus of Tehuantepec is about 458 (Pardo andSuaÂ rez, 1995; Ponce et al., 1992). The notably shallowCocos plate trajectory beneath southern Mexico isthus responsible for the obliquity of the TMVB withrespect to the trench. Since the angle of subductiona�ects the spatial distribution of magmatism withrespect to the trench (e.g. Urrutia-Fucugauchi andMorton-Bermea, 1997), it seems logical to try to usethe timing and distribution of magmatism in thisregion to trace the evolution of the subduction geome-try of the Cocos plate with time.

The truncated Paci®c continental margin of south-

west Mexico in combination with the presence ofmylonitic shear zones with a component of left-lateralmotion along the coast (Fig. 2) have been attributedby some workers to the eastward motion of theChortis block toward its present position in CentralAmerica during Neogene time (Ratschbacher et al.,1991; Herrmann et al., 1994; Schaaf et al., 1995).Comparative stratigraphy and the absence of similarmylonite zones to the northwest of Zihuatanejosuggest that the initial position of the Chortis blockwas no further west than Zihuatanejo (Schaaf et al.,1995). The amount of displacement predicted by thispaleoposition (600 km) is less than that calculated byRosencrantz and Sclater (1986) using ocean ¯oorbathymetry and magnetic anomaly data (1000 km).This discrepancy can be explained by E±W extensionof the Chortis block, as suggested by prominent N±Strending grabens (Schaaf et al., 1995) in Guatemalaand Belize. The displacement of Chortis along thecoast of Mexico brought rocks previously fairly distantfrom the continental margin to lie directly on thenewly formed trench. MoraÂ n-Zenteno et al. (1996) andTolson (1998) have discussed in detail some of the geo-metric consequences of the eastward passage ofChortis, among which are the progressive exhumationof mid-crustal rocks and subduction erosion of uppermantle lithosphere along the newly formed continentalmargin. This o�shore passage of the Chortis blockduring the Oligocene is thus also a distinctly character-istic tectonic event that a�ected the magmatic and de-formation patterns of southern Mexico as well as theirtiming.

The timing and kinematics of Cenozoic regional de-formation in southern Mexico are still far from clear.We have summarized the available data in Fig. 2,including the ages of the structures where these areconstrained. There is general agreement that at leastthe central TMVB is undergoing present day N±S di-rected extension along E±W trending faults as a pos-sible result of gravitational collapse (Suter et al., 1992;Ferrari et al., 1994; Suter et al., 1995a,b). The avail-able data suggest that this extensional regime has beenactive since the late Miocene (Suter et al., 1995a).There is also a general consensus regarding the de®-nition of the fault-bounded Jalisco block at the west-ern end of the TMVB (Fig. 2) during the Miocene butdetails of its past and present kinematics and its inter-action with the Rivera plate are still debated (Barrieret al., 1990; Bandy et al., 1995; Quintero-Legorreta,1995; Ferrari, 1995; Rosas-Elguera et al., 1996).

Studies focusing on Cenozoic deformation inlandfrom the Paci®c and south of the TMVB are stillscarce but several are underway. Jansma and Lang(1997) have reported the presence of a NNW±SSEtrending graben structure in the Arcelia region ofMichoacaÂ n and Guerrero states (Fig. 2) which they as-


sociate with Basin and Range extension, citing evi-dence for its activity prior to 32 Ma. Studies byMoraÂ n-Zenteno et al. (in press) in the Taxco regionhave recently indicated the presence of a NNE trend-ing system of sub-vertical right-lateral faults which cutignimbrites of 38±36 Ma but whose upper age limit isstill uncertain. Further to the east, in the southern partof the State of Puebla and the northwest part of theState of Oaxaca, we have identi®ed a region in whichN±S trending en echelon left-lateral transfer faultsbound down-thrown blocks with lacustrine sediment®lls interbedded with volcanic rocks of Oligocene age.The NW-trending Oaxaca fault (Fig. 2) exhibits acomplex history of displacement beginning in thePaleozoic followed by left-lateral motion in theJurassic (Nieto-Samaniego et al., 1995). Its most recentactivity has been described as normal fault motionwith the down-thrown block to the west and the strati-graphic and geochronologic studies carried out byFerrusquõÂ a-Villafranca et al. (1988) have left implicitthat the latest activity occurred during Miocene time.However, research centered directly on the fault, itskinematics, and timing has failed to provide conclusiveevidence regarding the age of its most recent normalmovement (Centeno-GarcõÂ a, 1988; Alaniz-AÂ lvarez etal., 1996). To the south of the Oaxaca fault, we haverecognized E±W oriented graben structures for whichan early to middle Miocene age can be inferred on thebasis of K±Ar dates of pyroclastic material from thebasin ®ll (FerrusquõÂ a-Villafranca, 1992).

Several studies have recognized the presence ofmylonitic shear-zones along the Paci®c coast of south-western Mexico (Fig. 2) (Ratschbacher et al., 1991;MoraÂ n-Zenteno, 1992; Meschede et al., 1997; Corona-ChaÂ vez, 1997; Tolson, 1998). Geochronological datahave constrained in detail the timing of crystal-plasticdeformation of two of these shear-zones, suggesting ayounging trend toward the southeast along the coast.The mylonite zones in the Zihuatanejo and Juchatengoregions have been studied from a structural point ofview by Meschede et al. (1997), but details of the tim-ing of deformation are lacking. The Tierra Coloradaintrusive, which clearly truncates the Tierra Coloradamylonite zone (Riller et al., 1992), has yielded concor-dant U±Pb zircon ages between 35.2 and 34.3 Ma(Herrmann et al., 1994), setting a minimum age for theshear-zone in this region, at least for crystal-plastic de-formation. In the vicinity of Huatulco, Oaxaca, Tolsonet al. (1993) and Tolson (1998) have shown that the 29Ma (Rb±Sr whole-rock and U±Pb zircon ages ofHerrmann et al., 1994) Huatulco granodiorite isa�ected by crystal-plastic mylonitic processes along theChacalapa shear-zone and that these mylonites aretruncated by granodioritic dikes which yielded a23.721.2 Ma K±Ar age using hornblende separates.It is important to emphasize that not only is there a

di�erence in the ages of the two shear-zones, but alsoa di�erence in the kinematic details. As shown in Fig.2, the Tierra Colorada shear-zone is a normal-left-lat-eral oblique shear zone, with a moderate dip to thenortheast (MoraÂ n-Zenteno, 1992), while the Chacalapashear-zone is a sub-vertical anastomosing shear-zonewith strictly left-lateral kinematic indicators (Tolson,1998). In addition, in the region between Puerto AÂ ngeland Salina Cruz, Delgado-Argote and Carballido-SaÂ nchez (1990) and Tolson (1998) have documentedthe presence of active left-lateral faults with E±W andNE±SW trends.

Meschede et al. (1997) published fault-slip analysisdata from southern Mexico to which they appliedinversion techniques in order to determine the stress®eld. They used crosscutting and overprinting relation-ships to determine relative ages for their data andfurther subdivided them using kinematic compatibilityarguments. They compared the stress tensor orien-tations resulting from the stress inversion to the avail-able data for Cenozoic stage poles for the Farallonplate (and its derivatives) with respect to NorthAmerica. On the basis of this comparison, Meschedeet al. (1997) concluded that there was an e�ectivestress transmission across the North American plateboundary and that the stress ®eld in southern Mexicoprior to 25 Ma had s2 in a sub-vertical orientationand s1 with a northeasterly trend roughly parallel tothe motion vector of the oceanic plate (with respect toNorth America). Their data indicate that, since 25 Ma,the stress ®eld has a vertical maximum stress orien-tation and that s3 is perpendicular to the regionaltrend of topography throughout their study area insouthern part of the State of Oaxaca.

The distribution and characteristics of the major tec-tonic features described above are not necessarily fullycompatible with the stress-®eld evolution described byMeschede et al. (1997), but the discrepancies may beexplained by shear-zone reactivation and mechanicalheterogeneity of the continental crust in southernMexico.

4. Geochronological, petrographic and geochemicalfeatures of Tertiary igneous rocks of the SMS

Although the geochronology and geochemistry ofthe plutonic rocks of the SMS have been studied withsome detail in certain parts of southwestern Mexico(Guerrero-GarcõÂ a, 1975; Pantoja-Alor, 1983, 1992;Schaaf, 1990; Delgado-Argote et al., 1992; MoraÂ n-Zenteno, 1992; Herrmann et al., 1994; Schaaf et al.,1995; HernaÂ ndez-Bernal and MoraÂ n-Zenteno, 1996;Calva-Guerra, 1996; Correa-Mora, 1997), the dataavailable for the volcanic rocks are more limited.However, recent work on the Tertiary volcanic


Table

2a

Geochronologic

data

forTertiary

igneousrocksoftheSMS,mainly

oftheinlandregion,complementary

tothose

presentedbySchaafet

al.(1995)forthecoastalplutonic

belt.(a)Table

includ-

ingdatesobtained

byusandreported

inpublished

papersorin

theses

forwhichcomplete

inform

ationisavailable

Locality

Longitude(W

)Latitude(N

)Sample

no.

Rock

typeorform

ation

AgeMa

Materialdated

Method

Reference

f

MichoacaÂ n

Tzitzio

100856'

19836'

Mx-88-17

Ignim

brite

33.4

21.7

Sanidine

K±Ar

7

MeÂ xico

MataRedonda

100809'240a

19808'120a

AM-11

Rhyolitictu�

33.6

20.9

K-feldspara

K±Ar

13

Cerro

ElPenÄon

100807'070a

19803'380a

AT-2

Rhyolite

31.6

20.8

K-feldspara

K±Ar

13

ArroyoElCastillo

100806'000a

19801'100a

M-01

Lamprophyre

46.6

21.2

Biotite

aK±Ar

13

Guerrero

Teteladel

RõÂ o

100805'a

17858'a

CFE-2

Basalt

42.3

21.4

WR

eK±Ar

2

Xaltianguis

99850.43'

17809.95'

FC19a,b,c

Granodiorite

todiorite

282

1.5

3pt.WR

isochron

Rb±Sr

12

Taxco

99838'550

18837'500

TX-21

Ignim

brite

32.4

20.8

Biotite

K±Ar

15

Taxco

99838'150

18834'300

TX-10

Rhyolite

31.6

21.2

Plagioclase

K±Ar

15

Taxco

99837.4'

18833.4'

M48-51

Rhyolitictu�

492

3WR

K±Ar

5

Taxco

99837'170

18833'500

TX-25

Vitrophyre

31.9

20.8

WR

K±Ar

15

Taxco

99836'b

18834'b

n.r.d

Rhyolite

36.9,35.5

21.3

K-feldspar,WR

K±Ar

3

Taxco

99836'150

18835'500

TX-16

Vitrophyre

32.4

20.9

WR

K±Ar

15

Taxco

99832'450

18834'150

TX-4

Vitrophyre

38.2

21.0

WR

K±Ar

15

SanJuanTetelcingoarea

99831'a

17856'a

n.r.

Tu�

662

2.3

Biotite

K±Ar

4

SanJuanTetelcingoarea

99831'a

17856'a

CFE-1

Basalt

52.4

21.8

WR

K±Ar

2

SierradeAlquitraÂ n

99829'230a

17826'250a

ALV96

Ignim

brite,AlquitraÂ n

Fm.

24.0

20.8

Volcanic

glass

K±Ar

11

SierradeAlquitraÂ n

99828'400a

17819'540a

AOV96

Ignim

brite,AlquitraÂ n

Fm.

22.5

20.6

Volcanic

glass

K±Ar

11

CoxcaclaÂn

99827'300a

18829'450a

BV-21

Granodiorite

32.2

20.7

Biotite-W

RRb±Sr

14

SanMarcos

99824'410a

16846'440a

DM941b

Granodiorite

33.5

20.7

Biotite-W

RRb±Sr

14

Buenavista

99824'050

18817'000

BV-17

Dacititc

lava

30.5

21.1

Plagioclase

K±Ar

15

Buenavista

99823'350

18817'000

BV-12

Daciticlava

24.8

21.3

Hornblende

K±Ar

15

Quetzalapa

99811'150

18821'000

SOL-5

Daciticignim

brite

31.9

20.8

Biotite

K±Ar

15

CruzGrande

99807'300

16843'130

DM94b

Granodiorite

35.7

20.7

Biotite-W

RRb±Sr

14

DSDPLeg

66site

493

98855.53'

16822.87'

493-59-1

Diorite

36.4,34.5

22

WR

K±Ar

6

DSDPLeg

66site

493

98855.53'

16822.87'

493-59-2

Diorite

352

2WR

K±Ar

6

Oaxaca

Zacatepec

97859.06'b

16846.16'b

JG73-16A

Granodiorite

252

1Biotite-W

RRb±Sr

8

Sta.Ma.Zacatepec

97858'250

16853'270

CON-53

Granite

25.5

20.7

Biotite

K±Ar

16

Chila

97852'080b

17857'330b

SIC

HIL

A-1

Andesitic

laccolith

302

1WR

K±Ar

10

LagunadeGuadalupe

97851'230

17811'190

CON-59B

Silicic

tu�

34.8

21.4

Hornblende

K±Ar

16

ZapotitlaÂnPalm

as

97848'290b

17853'230b

SVZA-3

Basalt

322

1WR

K±Ar

10

ZapotitlaÂnPalm

as

97848'290b

17853'230b

SVZA-1

Basalt

312

1WR

K±Ar

10

ZapotitlaÂnPalm

as

97848'290b

17853'230b

SVZA-4

Basaltic

andesite

302

1WR

K±Ar

10

ZapotitlaÂnPalm

as

97848'290b

17853'230b

SVZA-2

Basaltic

andesite

292

1WR

K±Ar

10

Huajuapan

97847'170

17849'470

CON-8A

Andesitic

stock

33.6

21.4

Hornblende

K±Ar

16

Huajuapan

97847'150b

17849'340b

SIL

C-1

Basaltic

andesiteintrusion

312

1WR

K±Ar

10

NorthofHuajuapan

97841'460

18804'510

CON-75

Rhyodacitictu�

31.4

20.8

Biotite

K±Ar

16

NorthofHuajuapan

97840'520

18802'360

CON-91

Andesitic

dike

34.2

21.4

Hornblende

K±Ar

16

Tlaxiaco

97836'440

17821'400

CON-101

Rhyolitictu�

32.9

20.9

Biotite

K±Ar

16

Tamazulapan

97834.08'b

17842.82'b

FV69-180

Tu�,LlanodeLobosFm.

26.2

20.5

Biotite

K±Ar

1


sequences in northwestern Oaxaca and northeasternGuerrero by some of the authors of this paper and byFerrusquõÂ a-Villafranca (1992) has provided infor-mation on the nature of these sequences and regionalpetrological and geochemical variations have beennoted.

The methods used and the materials dated in geo-chronologic studies of the magmatic rocks of the SMSare diverse. Our analysis was based on our own geo-chronologic data as well as on dates published in theliterature or presented in theses that we considered re-liable. In the evaluation of dates published by otherauthors, we took into consideration their general com-patibility with stratigraphic observations and the suit-ability of the method applied with respect the materialdated. The dating methods used for plutonic rocks areindicative of di�erent stages in the crystallization-cool-ing process. For intrusive units dated with di�erentmethods we gave preference in our regional analysis tothose data that better approximate the crystallizationage. Nonetheless, when only one method was applied,we assumed in most cases that cooling occurred in atime interval short enough that it did not signi®cantlydistort the regional geochronological patterns. Thisassumption was based on the fact that most Tertiaryplutons of coastal Oaxaca and Guerrero on whichdi�erent geochronologic methods were applied dis-played U±Pb, K±Ar and Rb±Sr mineral ages that indi-cate a relatively high cooling rate related to a rapiduplift of the continental margin (MoraÂ n-Zenteno et al.,1996). A ®rst compilation of isotopic ages of thesouthwestern continental margin of Mexico hasalready been presented by Schaaf et al. (1995). In thispaper we present a complementary list of dates com-piled for the volcanic rocks and related intrusive unitsof the inland areas (Tables 2a and b). Table 2a liststhose dates for which complete information has beengiven, including the locality, analytical uncertainties, aswell as material and rock unit dated. Table 2b includesthose dates for which complete information has notbeen published. Major and trace element compositionsas well as isotopic data discussed in this paper are pre-sented in more detail elsewhere (Schaaf, 1990;HernaÂ ndez-Bernal and MoraÂ n-Zenteno, 1996; MoraÂ n-Zenteno et al., in press; Martiny et al., in press). Theanalytical details and related uncertainties of the dataare discussed in the cited papers.

A description of the petrology, geochronology andgeochemistry of the coastal plutonic belt as well as thevolcanic rocks in four selected areas is summarizedbelow.

4.1. Coastal plutonic belt

The composition of the plutons along the coastalmargin generally varies from dioritic to graniticE

.ofTamazulapan

97825'b

17834.78'b

FV60-182

YucudaacAndesite

28.9

20.6

WR

K±Ar

1

YanhuitlaÂ n

97823'360

17834'050

CON-7

Daciticlaccolith

40.5

21.7

Hornblende

K±Ar

16

Huitzo-Etla

96851.72'b

17815.49b

FV69-185

Tu�,SuchilquitongoFm.

17.4,16.5

20.3

Biotite

K±Ar

1

Huatulco

96828'a

15850'a

H230

Granite

25.2

20.5

cBiotite-W

RRb±Sr

9

Huatulco

96809'a

15847'a

H113

Granite

25.1

20.5

cBiotite-W

RRb±Sr

9

Huatulco

96807'a

15846'a

H247

Granite

26.7

20.5

cBiotite-W

RRb±Sr

9

Xadani

96802'a

15856'a

H250

Tonalite

20.4

20.4

cBiotite-W

RRb±Sr

9

Ayuta

95852'

15853'

H239

Granite

20.9

20.4

Biotite-W

RRb±Sr

17

JuaÂrez

95827'

16825'

H234

Tonalite

13.6

20.3

Biotite-W

RRb±Sr

17

JuaÂrez

95826'

16820'

H235

Quartzdiorite

14.9

20.3

Biotite-W

RRb±Sr

17

Bomba

95824'

16802'

H029

Granite

16.6

20.3

Biottie-W

RRb±Sr

17

aPersonalcommunicationbyauthor(s).

bCoordinatestaken

from

maporfrom

inform

ationin

originalreference.

cAgereported

previouslyin

Schaafet

al.(1995)andcorrectedin

thepresentstudy.

dn.r.=

notreported.

eWR=whole

rock.

f1.FerrusquõÂ a-V

illafranca

(1976);

2.DeCserna(1981);

3.DeCsernaand

Fries

(1981);

4.Ortega-G

utieÂ rrez(1980);

5.Linaresand

Urrutia-Fucugauchi(1981);

6.Bellon

etal.

(1982);

7.

PasquareÁet

al.

(1991);

8.Guerrero-G

arcõÂ a

and

Herrero-Bervera(1993);

9.Schaafet

al.(1995);

10.Galina-H

idalgo

(1996);

11.HernaÂndez-TrevinÄo

etal.

(1996);

12.Correa-M

ora

(1997);13.

ChaÂvez-A

guirre

andMendoza-Flores(1998);14.Alba-A

ldaveet

al.(1998);15.MoraÂ n-Zentenoet

al.(inpress);16.Martinyet

al.(inpress);17.SolõÂs-Pichardo,PhD

thesis(inprep.).


although more ma®c units have been observed. TheirSiO2 content typically varies from 56 to 74 wt% andin a TAS diagram (Na2O+K2O vs SiO2), these plu-tons show a sub-alkalic trend (Fig. 3a). They are light-REE enriched (LaN/LuN between 4 and 20) and ingeneral display a ¯at pattern for heavy-REE (Fig. 4b).These geochemical features are typical of fractionalcrystallization processes in mantle derived magmas.The plutonic rocks from the Acapulco and Xaltianguisintrusives show a negative Eu anomaly (Fig. 4a) thatmay be explained by plagioclase fractionation(Negendank et al., 1987).

The isotopic age data of the SMS (Table 2 of thispaper and Table 1 of Schaaf et al., 1995) show thatthe Tertiary intrusive rocks of the CPB in the regionbetween the states of MichoacaÂ n and Oaxaca show alarge variation in age, ranging from early Paleocene(6321 Ma hornblende K±Ar; Pantoja-Alor, 1983) inAquila, MichoacaÂ n, near Punta San Telmo, to middleMiocene (13.620.3 Ma whole-rock-biotite Rb±Sr) in

the Isthmus of Tehuantepec area (Table 2a). To thenorthwest, the older Puerto Vallarta and Manzanillobatholiths were emplaced in the Cretaceous.Crystallization ages for the Puerto Vallarta region aregiven by U±Pb zircon dating in granodiorites(10326.5 and 10123.4 Ma) and two Rb±Sr whole-rock isochrons in granitoids (9924 and 9123 Ma).For granitoids in the Manzanillo area, a Rb±Sr whole-rock isochron yielded 6923 Ma (Schaaf et al., 1995).

The ages of the MichoacÂ n and Guerrero coastal plu-tons are similar to those obtained for inland intrusiveslocated in south central MichoacaÂ n. The Santa Elenaand La Huacana plutons yielded ages of 66.121.5 Ma(K±Ar) and 4224 Ma (Rb±Sr isochron), respectively(Pantoja-Alor, 1986; Schaaf et al., 1995). Other plu-tons (La Verde, InguaraÂ n, San Isidro) give biotite,hornblende and sericite K±Ar age determinations of35.620.8 to 31.820.7 Ma for granodiorites and re-lated mineralization (Damon et al., 1983). These agesare somewhat anomalous in the regional context,

Table 2b

(b) List of dates for which complete information is not available

Locality Rock type

or formation

Age Ma Material

dated

Method Referenced

Jalisco

Techalutla Granodioritea 6922 Hornblendea K±Ar 7

Tamazula Batholith 5321 K-feldspara K±Ar 7

MichoacaÂn

Punta San Telmo Granodioritea 5526 Hornblendea K±Ar 7

ZiquiraÂ n Porphyritic diorite 34.328 Hornblendea K±Ar 3

Guerrero

Highway between Cd. Altamirano and Zihuatanejo,

km 30

Andesite 61.221.3 Plagioclasea K±Ar 3

Between Cd. Altamirano and Zihuatanejo Dike 46.525% n.r.b K±Ar 5


km 70

Porphyritic diorite 46.221.1 Plagioclasea K±Ar 3

Between Cd. Altamirano-Zihuatanejo Andesites 43-4622 n.r. K±Ar 6


km 67

Andesitic dike 42.921 Plagioclasea K±Ar 3

10 km W of Cd. Altamirano Diorite 36.628 WRa,c K±Ar 3

Between Cd. Altamirano and Zihuatanejo Dikes 33-4522 n.r. K±Ar 6

San Juan Tetelcingo area Basalt, Tetelcingo Formation 68.822.4 WR K±Ar 2

7 km E of Balsas railroad station Granodiorite 66.121.5 Hornblendea K±Ar 3

Vallecitos de Zaragoza granitoid Diorite to quartz monzonite 3625 n.r. Rb±Sr 6

Poliutla Rhyolitic ignimbrite 42.321 Plagioclasea K±Ar 3

El Ocotito, sample JG74-01 Quartz monzodiorite 3822 Biotite-WR Rb±Sr 1

Acapulco Granite 5020.5 Biotite-WR Rb±Sr 8

Km 91.9 Highway Chilpancingo to Acapulco,

sample JG73-24

Quartz monzonite 32.723 Biotite-WR Rb±Sr 1

Oaxaca

Suchilquitongo-Etla Tu�, Suchilquitongo Formation 20.6, 19.320.3 n.r. K±Ar 4

Tlacolutla-Mitla-MatatlaÂ n area Tu� 16.0ÿ15.3220.8 n.r. K±Ar 4

Nejapa area Tu� 17.4ÿ15.020.8 n.r. K±Ar 4

a Personal communication by author(s).b n.r.=not reported.c WR=whole rock.d 1. Guerrero-GarcõÂ a (1975); 2. Ortega-GutieÂ rrez (1980); 3. Pantoja-Alor (1986); 4. FerrusquõÂ a-Villafranca and McDowell (1991); 5. Kratzeisen

et al. (1991); 6. Frank et al. (1992); 7. Pantoja-Alor (1992); 8. Guerrero-GarcõÂ a and Herrero-Bervera (1993).


which may suggest a local, post-intrusive thermalevent.

Tertiary coastal plutons of the Guerrero terranebetween Zihuatanejo and Acapulco were emplaced inlate Eocene to Oligocene time (Fig. 1). A Rb±Srwhole-rock isochron yielded an age of 36.525 Ma forthe granitoids of the Zihuatanejo area (Schaaf et al.,1995). The granitic batholith exposed in the PetatlaÂ n

area, southeast of Zihuatanejo, yielded hornblende andbiotite K±Ar ages ranging from 40.720.9 to37.421.5 Ma (Delgado-Argote et al., 1992; Schaaf etal., 1995; Stein et al., 1994). In this same general area,the ultrama®c rocks of El Tamarindo and Loma Bayaare related to Cretaceous magmatism and have beenstudied in detail by Delgado-Argote et al. (1992).

Farther to the southeast, a U±Pb zircon crystalliza-

Fig. 3. Chemical classi®cation of Tertiary magmatic rocks of the Sierra Madre del Sur, Mexico. (a) Samples of the coastal plutonic belt (TAS

diagram after Wilson, 1989); data of the Xaltianguis pluton after Correa-Mora (1997). (b) Samples of the inland Tertiary volcanic sequences

(TAS diagram after Le Maitre et al., 1989). Note that in the northwestern Oaxaca region basaltic andesite and andesitic compositions are domi-

nant. The three samples of rhyolitic composition correspond to minor pyroclastic units. Volcanic rocks of Guerrero and Morelos regions range

from andesites to rhyolites but most of them fall in the dacite and rhyolite ®elds. The data plotted are in Martiny et al. (in press) and MoraÂ n-

Zenteno et al. (in press).


tion age of 35 Ma was obtained for a granodiorite SEof Atoyac (Herrmann et al., 1994). The TierraColorada, Xaltianguis, San Marcos and Cruz Grandegranodioritic plutons give biotite Rb±Sr ages rangingfrom 35 to 26 Ma (Schaaf et al., 1995; Alba-Aldave etal., 1998). In addition to these dates, U±Pb zircon agesare reported for the intrusives of Tierra Colorada (34.3Ma), San Marcos (31 Ma) and Cruz Grande (32 Ma)(Herrmann et al., 1994).

The Acapulco granite displays notably older coolingages than other plutonic rocks in the region (43.420.9Rb±Sr biotite-whole-rock; Schaaf, 1990; 5020.5 Ma

(Guerrero-GarcõÂ a and Herrero-Bervera, 1993).Furthermore, the Acapulco intrusive is remarkable inthat it includes a granite with pseudorapakivi textureand a two pyroxene-bearing syenite facies. Fayaliteand ¯uorite have also been reported as accessories andsome samples have trace-element distributions thatplot in the ``within-plate-granites'' ®eld of Pearce et al.(1984).

Farther to the east, between Acapulco andHuatulco, there are younger intrusive rocks. The U±Pb ages obtained by Herrmann et al. (1994) fromPinotepa Nacional to Huatulco range from 30 to 27

Fig. 4. Chondrite-normalized REE patterns of Tertiary magmatic rocks of the Sierra Madre del Sur (chondrite values of Nakamura, 1974). (a)

Coastal plutonic belt; note the Eu anomaly in the intrusives of Xaltianguis and Acapulco. (b) Inland Tertiary volcanic sequences of northeastern

Guerrero and western Oaxaca.


Ma. The K±Ar dates from hornblende concentratesreported by HernaÂ ndez-Bernal and MoraÂ n-Zenteno(1996) for the Rio Verde batholith, east of PinotepaNacional, are 29.9 and 27.7 Ma. K±Ar biotite determi-nations of rocks exposed to the north and east ofPinotepa Nacional yielded 27.7 to 23.5 Ma(HernaÂ ndez-Bernal and MoraÂ n-Zenteno, 1996;Martiny et al., in press). Between Puerto AÂ ngel andSalina Cruz, K±Ar and Rb±Sr age determinationsfrom biotite concentrates obtained by SolõÂ s-Pichardorange from 2720.5 to 13.620.3 Ma (Schaaf et al.,1995; Table 2a, this paper). O�shore of PinotepaNacional the dioritic rocks cut by Leg 66 of the DSDPyielded whole-rock ages of 34.522 to 36.421.8 Ma(Bellon et al., 1982).

The plutons between Punta San Telmo and the areawest of Acapulco, with a few exceptions, typicallyshow initial 87Sr/86Sr ratios between 0.7035 and 0.7049and eNd values from +5.7 to +3.1. Initial 87Sr/86Srratios of the La Verde, InguaraÂ n and San Isidro inlandplutons range from 0.7039 and 0.7055 (Damon et al.,1983). In the Acapulco region, the Oligocene intrusiveshave slightly higher initial 87Sr/86Sr ratios of 0.7041±0.7050, and lower initial eNd of +2.9 and +1.6(Schaaf, 1990; Correa-Mora, 1997). Between Acapulcoand Huatulco the initial 87Sr/86Sr ratios of the plutonicrocks range from 0.7040 to 0.7054; eNd values are gen-erally lower than to the west of Acapulco, and rangefrom +3.5 to ÿ3.0; (MoraÂ n-Zenteno, 1992;Herrmann, 1994; HernaÂ ndez-Bernal and MoraÂ n-Zenteno, 1996; Martiny et al., 1997).

4.2. Inland volcanic ®elds

The volcanic sequences of the Sierra Madre del Surare not as well studied as those of the CPB and thedata presented below were obtained from two transectsperpendicular to the coast. The volcanic units consistof intermediate- to high-K, sub-alkalic rhyolites tobasaltic andesites, with SiO2 content between 53 and76 wt% (Fig. 3b). Chondrite normalized REE patternsdisplay typical magmatic arc behavior, with LREEenrichment relative to HREE (LaN/LuN between 5 and14; Fig. 4b). Eu anomalies in the volcanic rocks of theTaxco area (Eu/Eu�0 0.4) are probably related to crys-tal fractionation of plagioclase. In the area ofBuenavista, Huautla and northwestern Oaxaca, thevolcanic rocks do not display a signi®cant Euanomaly.

Initial 87Sr/86Sr ratios of andesites and basaltic ande-sites from northwestern Oaxaca range from 0.7042 to0.7046, and eNd values, from 0 to +2.6 (Martiny etal., in press). Dacites and rhyolites of similar age fromthe Taxco area show evidence of either a greaterdegree of crustal contamination or more evolvedsource rocks since their initial 87Sr/86Sr ratios range

from 0.7051 to 0.7063 (MoraÂ n-Zenteno et al., inpress). This inference is corroborated by the lead isoto-pic data reported by Martiny et al. (1997). Pb isotoperatios of the Tertiary magmatic rocks from westernOaxaca show a narrow range [(206Pb/204Pb)=18.669±18.749; (207Pb/204Pb)=15.587±15.623; (208Pb/204Pb)=38.442±38.588] whereas the more acidic volcanic rocksfrom northeastern Guerrero are slightly more radio-genic [(206Pb/204Pb)=18.745±19.000; (207Pb/204Pb)=15.599±15.651; (208Pb/204Pb)=38.557±38.818].

4.2.1. Western GuerreroIn the southwestern part of the State of Guerrero,

the Tertiary volcanic rocks of the Sierra Madre delSur have been dated in the Zihuatanejo-Vallecitos area(Pantoja-Alor, 1986; Frank et al., 1992; Kratzeisen etal., 1991). Between Zihuatanejo and Cd. Altamiranothe most widespread unit is a 1500 m thick series ofhigh-K, and pyroxene-bearing porphyritic basalticandesite to dacitic lava ¯ows, and intermediate to felsictu�s and ignimbrites (Kratzeisen et al., 1991). Thissequence is cut by numerous andesitic to dacitic dikesof varying dimensions trending NE±SW and NW±SE.The time range for these igneous sequences is given byK±Ar age determinations and varies from 46 to 43 Mafor the andesites and from 45 to 33 Ma for the dikes(Kratzeisen et al., 1991). Given the scarcity of datafrom this region, and their broad range, it is not poss-ible to con®rm if these constitute a single event of 12million year duration or two separate events. In theCd. Altamirano area, dikes and tu�s were dated at46.2±42.3 using K±Ar (Pantoja-Alor, 1986). The onlyother Oligocene volcanic rocks in the general regionare exposed in the Mil Cumbres, MichoacaÂ n, areawhere a sanidine K±Ar age of 33.421.7 Ma has beenreported (PasquareÁ et al., 1991).

4.2.2. Northeastern GuerreroThe Oligocene volcanic rocks in northeastern

Guerrero are distributed in three main areas: Taxco,Buenavista-Quetzalapa and Huautla. Other Tertiaryvolcanic rocks of Oligocene age have been reported inthe Tejupilco area, State of Mexico, and include exten-sive sequences of ignimbrites forming the Sierra de laGoleta, Sierra de Nanchititla and the Mesa delNaranjo. Similar rocks have also been reported in theTenancingo-Ixtapan de la Sal, State of Mexico, area(De Cserna, 1982; GarcõÂ a-Palomo, 1997). The volcanicsection in the Taxco area consists of rhyolitic ignim-brites, vitrophyres, ¯ow and ash fall tu�s, forming asequence up to 800 m thick. K±Ar dates in volcanicglass, plagioclase and biotite show that the age of thelower part of the sequence ranges from 38.2 to 35.5Ma (De Cserna and Fries, 1981; MoraÂ n-Zenteno et al.,in press), whereas that of the upper part, from 32.4 to


31.6 Ma (MoraÂ n-Zenteno et al., in press). These resultssuggest that the Taxco sequence originated from twodistinct volcanic events.

The Buenavista±Quetzalapa range, 30 km southeastof Taxco, is formed by a series of dacitic ignimbritesand lava ¯ows with a maximum total thickness ofabout 900 m. A 31.921.1 Ma biotite K±Ar age wasobtained for the lower part of the sequence, rep-resented by dacitic ignimbrites of the TilzapotlaFormation, whereas the upper part, represented bydacites of the Buenavista Formation, yielded a plagio-clase K±Ar age of 30.521.1 Ma (MoraÂ n-Zenteno etal., in press) (Table 2a). These rocks are related to acaldera about 20 km in diameter, which originatedduring the early stage of volcanic activity and under-went later resurgent activity.

4.2.3. Northwestern OaxacaIn the Huajuapan±Tamazulapan±YanhuitlaÂ n region,

the volcanic sequences are dominated by andesite andbasaltic andesite lavas that overlie a sequence of epi-clastic beds and some silicic ash fall and ash ¯ow tu�s(Fig. 3b). Farther south in the Tlaxiaco region, theTertiary volcanic rocks range from felsic to intermedi-ate pyroclastic and epiclastic deposits. The maximumthickness of the Tertiary volcanic sequences in thisregion is about 500 m. Throughout northwesternOaxaca, numerous dacitic and andesitic hypabyssalintrusives (stocks and dikes) are emplaced at di�erentlevels of the volcanic sequence (MartõÂ nez-Serrano etal., 1996). In some areas the volcanic rocks are inter-layered with lacustrine deposits, which have been inter-preted as accumulated in fault-bounded tectonic basins(Martiny et al., 1995). The time range for the volcan-ism in northwestern Oaxaca is 34.8±31.4 Ma (Table2a) based on biotite and hornblende K±Ar ages of thetu�s and hypabyssal rocks (Martiny et al., 1997).These dates are relatively consistent with the whole-rock K±Ar ages (32±29 Ma) for hypabyssal rocks andlavas in the Huajuapan area (Galina-Hidalgo, 1996).In the Tamazulapan±YanhuitlaÂ n area, FerrusquõÂ a-Villafranca and McDowell (1991) obtained a youngerwhole rock K±Ar age of 28.9 Ma for an andesitic lavaand a biotite age of 26.2 Ma for a tu�. Up to now, inthis region only one Eocene K±Ar hornblende age of40.521.7 Ma (Table 2a) has been obtained for a lac-colith associated with the YanhuitlaÂ n Formation thatseems to represent the earliest manifestations ofTertiary magmatism in this region.

4.2.4. Central and southeastern OaxacaThis region covers a broad area including NW of

the city of Oaxaca, Tlacolula-Mitla and Nejapa withvariations in the age and composition of volcanic pro-ducts. In general, the volcanic sequences are consti-tuted by siliceous tu�s, volcaniclastic deposits,

rhyolitic to andesitic lava ¯ows interbedded with lacus-trine deposits and intruded by rhyolitic to latitic hypa-byssal rocks (FerrusquõÂ a-Villafranca and McDowell,1991). Pyroclastic deposits are more abundant thanlava ¯ows in this region. The K±Ar age determinationsof central and southeastern Oaxaca range from 20.6near the city of Oaxaca to 15.0 Ma in Nejapa(FerrusquõÂ a-Villafranca and McDowell, 1991).

5. Geochronologic patterns

The geochronological database of the SMS mag-matic rocks (Table 2 and Schaaf et al., 1995) indicatesthat arc-magmatism was active, with di�erent spatialdistributions, from Paleocene to middle Miocenetimes. Fig. 5 shows the distribution of arc magmaticrocks for four di�erent times. There is a recognizabletendency of the older magmatic rocks to dominate inthe westernmost region of the SMS. This behavior wasidenti®ed by Schaaf et al. (1995) for the plutonic rocksalong the continental margin between Puerto Vallataand Acapulco. Fig. 6a shows a plot of ages versus dis-tance from Puerto Vallarta. The ages of intrusiverocks along the coastal segment between PuertoVallarta and Zihuatanejo range from 100 to nearly 35Ma. From Zihuatanejo to the Huatulco region there isalso a decreasing age trend in the extinction of mag-matism but with a higher rate of migration. Schaaf etal. (1995) calculated a rate of 1.2 cm/a for the ®rst seg-ment and 7.7 cm/a for the second. Based only on U±Pb zircon ages of di�erent plutons from the Acapulcoregion to Huatulco, Herrmann et al. (1994) calculateda decreasing age rate of 5.59 cm/a.

The Late Cretaceous±Paleocene magmatism is rep-resented by the Puerto Vallarta, Manzanillo and SanTelmo plutons (Fig. 1) (Schaaf, 1990; KoÈ hler et al.,1988; Schaaf et al., 1995). Paleocene to Eocene mag-matic rocks, including the inland volcanic areas, liemainly to the west of 1008W meridian, distributedfrom the coastal margin to a distance up to 300 kmfrom the present-day trench (Fig. 5a). Eocene magma-tism dominates in the Zihuatanejo region and inlandfrom it, including the extensive outcrops of andesitesof the Balsas River basin in the Presa del In®ernilloregion (Kratzeisen et al., 1991; Delgado-Argote et al.,1992; Pantoja-Alor, 1983). In the inland region ofMichoacaÂ n and western Guerrero, there are also sev-eral calc-alkaline plutonic rocks for which Eocene ageshave been reported (Pantoja-Alor, 1983; Alba-Aldaveet al., 1996). The relatively scarce basaltic ¯ows inter-calated in the continental sequences of the BalsasFormation are indicative of isolated Eocene volcanismthat extends toward the east to the eastern part of theState of Guerrero (De Cserna, 1981). In addition tothe basaltic intercalations of the Balsas Formation,


Fig.5.DistributionoftheTertiary

magmaticarc

rocksoftheSierraMadre

del

Suratdi�erenttimes

(a)Paleocene±Eocene;

(b)Oligoceneand(c)middle

Miocene±Plio-Q

uaternary.


east of 1008W longitude, few localities of magmaticrocks have been reported with Eocene ages (MartõÂ nez-Serrano et al., 1997).

The early Oligocene magmatic rocks (36±30 Ma) aredistributed mostly in the eastern half of the SMS andde®ne a broad zone that extends eastward from the

Zihuatanejo and Taxco±Tejupilco regions (Alba-Aldave et al., 1996) to west central Oaxaca, includingsouthern Puebla and the Huajuapan±Tlaxiaco region(Fig. 5b) (Martiny et al., 1996). In the Acapulco±Taxco sector Oligocene magmatism was in general coe-val, indicating the occurrence of an arc of at least

Fig. 6. (a) Graph of pluton ages vs distance from Puerto Vallarta along the coast. Only data from the coastal plutonic belt are plotted. (b)

Graph of age vs distance inland from the Acapulco Trench along section A±A '. (c) Graph of age vs distance inland from the Acapulco Trench

along section B±B '. Section lines for (b) and (c) are shown on Fig. 1. For A±A ' and B±B ' sections age uncertainties are indicated.


200 km wide (Fig. 6b). In northwestern Oaxaca theOligocene rocks tend to be slightly older in the inlandvolcanic zones (Huajuapan±Tamazulapan±YanhuitlaÂ n)with respect the plutonic rocks of the coastal zone(Fig. 6c), however the K±Ar ages from 29.6 to 25.9(Table 2a) reported by FerrusquõÂ a-Villafranca (1976)for the inland sequences give indications of coevalmagmatism in both regions at some time after 30 Ma.Late Oligocene arc-magmatic rocks (30±23 Ma) aremainly distributed along the coastal region fromAcapulco to Huatulco and correspond mostly to felsicbatholiths (Herrmann et al., 1994; HernaÂ ndez-Bernaland MoraÂ n-Zenteno, 1996). Early and middle Miocenerocks are distributed in the region between the city ofOaxaca and the Isthmus of Tehuantepec (Fig. 5c)mainly within elongated, E±W trending tectonic basinslocated inland of the Puerto AÂ ngel-Salina Cruz sector.In this region it is evident that volcanism in the inlandregions was active after the cessation of Oligocene plu-tonism in the Huatulco region.

6. Discussion and conclusions

The petrologic and geochemical variations of theTertiary magmatic rocks along the SMS indicate di�er-ent degrees of magmatic di�erentiation and crustalcontamination. The factors that controlled these vari-ations are not completely understood. There is nostraightforward correlation between the degree of con-tamination by older crust, as inferred from the Sr andNd isotopic data, and the age of the oldest crust in thedi�erent terranes of the tectonic mosaic in southernMexico. In spite of this, there is a tendency for theTertiary and Late Cretaceous magmatic units,emplaced in the Guerrero terrane (i.e. Manzanillo,PetatlaÂ n-Papanoa, Punta San Telmo), to display thelowest 87Sr/86Sr ratios and the highest eNd values(Schaaf et al., 1995). Other intrusives in the Guerreroterrane have 87Sr/86Sr and eNd values similar to thoseof the eastern SMS where older crustal componentsseem to have played a greater role in the magmaticevolution.

The SiO2 and alkali content of the SMS magmaticrocks, in conjunction with the rare earth element abun-dances and isotopic data, show some variations thatmay be the result of di�erent degrees of crystal frac-tionation. If we assume that the degree of fractionationis related to the rate of ascent of magmas to the uppercrust (Burkart and Self, 1985; Glazner and Ussler,1989) and that the rate of ascent will be controlled bythe stress regime and the particular deformation con-ditions at the time of magma emplacement (Apperson,1991; Takada, 1994), we can use the geochemical char-acter of the igneous rocks to make a ®rst order guessat the state of stress of the crust during magmatism.

The early Oligocene volcanic rocks of the Taxco±

Tilzapotla area in northeastern Guerrero have in gen-eral a higher SiO2 concentration than coeval volcanic

rocks of northwestern Oaxaca (Fig. 3b). This higher

SiO2 content, particularly for the rhyolitic rocks of theTaxco range, could be caused by a greater degree of

crystal fractionation (plagioclase), as inferred from the

Eu anomaly displayed by the REE chondrite-normal-

ized diagrams. In addition to this, the Sr, Nd and Pbisotopic data (Martiny et al., 1997) indicate that, for

the Taxco±Buenavista volcanic rocks, there is a greater

in¯uence of old crustal components or more evolved

source rocks than for the northwestern Oaxaca volca-nic zone. The tectonic and stratigraphic features in

both areas indicate that western Oaxaca underwent left

lateral transtension that produced pull-apart basins ofN±S orientation at the time of the volcanism. For the

Taxco±Tilzapotla area there are no such indications of

major extensional features contemporary with the

silicic magmatism. The high rate of extension in north-western Oaxaca during early Oligocene time seems to

have favored not only a greater volume of volcanism

but also lower degrees of di�erentiation and crustalcontamination, particularly for the upper andesitic

sequences (Martiny et al., in press).

In the CPB of Oaxaca and eastern Guerrero the in-¯uence of the tectonic setting in the geochemical vari-

ations of the plutonic rocks is less evident. They were

emplaced during a period of transtensional tectonics

related to the Chortis block displacement. The beha-vior of major and trace elements of the plutonic rocks

as well as their isotopic compositions indicate vari-

ations in the degree of magmatic di�erentiation andcrustal contamination. The causes of these variations

for rocks emplaced in the same general tectonic setting

remain elusive.

The geochronological patterns described re¯ect in a

general way the transition from a zone in¯uenced by

the subduction of the Farallon plate in western

Mexico to a more complex plate tectonic scenario insouthern Mexico. The magmatic rocks of the late

Paleocene±Eocene time interval, mainly distributed

from Manzanillo to Zihuatanejo and inland up to the

01008W meridian, seem to de®ne the southern exten-sion of coeval magmatic rocks of the Lower Volcanic

Complex of the SMO (Clark et al., 1982; Montigny et

al., 1987; Aguirre-DõÂ az and McDowell, 1991). Thisfact suggests that the continuation of the magmatic arc

to the south of MichoacaÂ n and western Guerrero was

probably removed laterally with the Chortis block.

The scarcity of geochronological data in the Chortisblock and the extensive cover of Quaternary volcanic

sequences prevent us from con®rming this interpret-

ation at present. The volcanic members of the BalsasFormation in eastern Guerrero and other isolated


Eocene localities in western Oaxaca were distant mani-festations of magmatic arc activity.

The southeastern migration of the trench±trench±transform triple junction that accompanied the displa-cement of the Chortis block produced a gradualextinction of the magmatism since Eocene time as canbe observed in the age decreasing trend along the con-tinental margin southeast of Zihuatanejo (Herrmann etal., 1994; Schaaf et al., 1995). The decreasing age trendin the segment northwest of Zihuatanejo does notseem to be directly related to the displacement of theChortis block, at least not during the latest history of

displacement when the Caribbean plate was formed.The lack of correlative stratigraphic features along thiscontinental margin and the northern limit of theChortis block do not support a paleo-position of theChortis block o� the coast of Jalisco, Colima andnorthwestern MichoacaÂ n (Schaaf et al., 1995). In ad-dition, no evidence has been found indicating Tertiaryleft lateral shear zones that could suggest a continu-ation of the kinematic conditions similar to those ofthe Zihuatanejo±Huatulco sector. Schaaf et al. (1995)interpreted this decreasing trend as the product of aprior eastward migration of magmatism (Late

Fig. 7. Cartoon showing the evolution of the continental margin geometry since the early Oligocene and associated igneous products.

Abbreviations are: Ac=Acapulco, MC=Mexico City, Mz=Manzanillo, PA=Puerto AÂ ngel, Zt=Zihuatanejo.


Cretaceous±Eocene) followed by episodes of tectonicerosion of the continental margin. Lateral removal ofcontinental slices, by oblique subduction, prior to thedisplacement of the Chortis block and possible subduc-tion erosion processes could have played a role in thetectonic truncation of this margin.

The distribution of the Oligocene magmatic rocksindicates not only an eastward migration of the mag-matism, but also a change toward an ESE trend (Fig.5b). There are no indications that the ESE orientationin the distribution of the early and late Oligocenerocks from Guerrero to Oaxaca (Fig. 5b) could be theresponse of a change in the subducted slab geometry.According to the reconstruction of the convergencevectors of the Farallon plate with respect to NorthAmerica (Schaaf et al., 1995), the orientation of thearc at this time corresponds neither with a period ofsigni®cant increase of the convergence rate, nor with amajor change in the convergence direction. The ageconstraints of the shear zones in the continental mar-gin and the paleogeographic reconstructions based onthe ocean ¯oor magnetic data of the Cayman Trough(Rosencrantz and Sclater, 1986; Ross and Scotese,1988; Pindell and Barrett, 1990) indicate that a signi®-cant segment of the Chortis block was still o�shore ofsouthern Oaxaca. Given the geometry of the northwest-ern part of the Chortis block, its displacement towardthe ESE (Fig. 7a) produced a gradual northeastwardmigration of the trench. This migration probablycaused the arc magmatism to reach the inland regionsas far as northeastern Guerrero and northwesternOaxaca. In northwestern Oaxaca (Huajuapan±Tlaxiacoregion), the occurrence of very active N±S transten-sional tectonics could have favored the development ofthe intermediate volcanism that gave the arc its rela-tively broad con®guration. The largest volume of mag-matism in late Oligocene time (<30 Ma) wasconcentrated along the present-day continental marginof eastern Guerrero and Oaxaca (Fig. 7b). This distri-bution seems to have been determined by the NW±SEshear zones associated with the transtensional tectonicsat the boundary between southern Mexico and theChortis block (Herrmann et al., 1994; Tolson, 1998).Herrmann et al. (1994) suggested that the heating andfracturing produced by extension facilitated themagma ascent during the eastward displacement of theChortis block. Magmatism in the inland regionbetween the city of Oaxaca and Nejapa continueduntil early and middle Miocene times, whereas lateOligocene plutonic rocks are predominant along thecoastal area. This suggests a decrease in the displace-ment rate of the Chortis block at this time that lead toa more gradual northward migration of the magma-tism.

The removal of the Chortis block has been largelyinvoked as the cause of the initiation of arc-magma-

tism farther north in the TMVB as a consequence ofthe gradual development of the Acapulco Trench atthe truncated margin of southwestern Mexico (e.g.Demant, 1978; Ratschbacher et al., 1991; Schaaf et al.,1995). Since the earliest manifestations of magmaticactivity of the eastern and central TMVB are as old asmiddle Miocene (016 Ma; Ferrari et al., 1994 andreferences therein), a 010 Ma magmatic gap followingthe extinction of the late Oligocene magmatism can berecognized for a broad region between 100 and 978W(Fig. 7c).

In addition to the emplacement of the arc magma-tism in the present day oblique position of the TMVB,the present-day geometry of the subducted slabbeneath the eastern sector of the SMS (Pardo andSuaÂ rez, 1995) is suggestive of a shallowing of the sub-duction angle between the late Oligocene and themiddle Miocene (Fig. 7c). The relative rapid change ofplate inclination and the time required for the mantlewedge beneath the TMVB to achieve the metasomaticconditions necessary to produce arc-volcanism are themost probable causes of the magmatic gap. The gra-dual transition of SMO magmatism to the volcanic ac-tivity of the western TMVB occurred in a di�erentscenario. The subducted slab of the present-day Riveraplate seems to have remained at its present angle sinceits formation by the fragmentation of the Guadalupeplate at 12.5 Ma.

Acknowledgements

Di�erent phases of this research were funded by theConsejo Nacional de Ciencia y TecnologõÂ a(CONACyT) (project 3361 T9309) and PAPPIT-DGAPA (grant IN102794) at the UniversidadNacional AutoÂ noma de MeÂ xico (UNAM). We thankJuan Julio Morales Contreras and Faustino JuaÂ rez forlaboratory support at UNAM, Teodoro HernaÂ ndez-TrevinÄ o for assistance in the ®eld and Margarita-Reyes-Salas for petrographic analyses.

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