versão impressa ISSN 1026-8774
Rev. mex. cienc. geol v.25 n.2 México ago. 2008
On the nature and role of the lower crust in the volcanic front of the TransMexican Volcanic Belt and its forearc region, southern and central Mexico
Sobre la naturaleza y el papel de la corteza inferior en el frente volcánico de la Faja Volcánica Transmexicana y su región antearco, sur y centro de México
Fernando OrtegaGutiérrez*, Mariano ElíasHerrera, María Guadalupe DávalosElizondo
Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Del. Coyoacán, 04510 México, D.F., México.
Manuscript received: November 12, 2007
Corrected manuscript received: June 6, 2008
Manuscript accepted: June 6, 2008
With the main objective of further constraining models that debate how and where magmas are generated in the Mexican subduction factory, how they ascend through the overlying viscous layers, and how and where they interact with the traversed crust to produce the diversity of the magmas that compose the system, the nature of the lower crust and its immediate surroundings under the volcanic front of the TransMexican Volcanic Belt (TMVB), as well as its forearc in southern Mexico, are analyzed and integrally characterized in this work. The study is mainly based on the analysis of geophysical and geological existing and new data, as well as on our new data obtained from deepseated xenoliths. Taken as that part of the crust located from the Moho to a depth of 2025 km below the surface, it is concluded from our analysis that all of the lower crust in the study area should be in the granulite fades, if geophysical modeling correctly predicted temperatures of 700800°C for its base, and a crustal thickness varying between 40 and 45 km. Xenoliths and surface geology information, when integrated to tectonic modeling, support the notion that most of the lower crust under the eastern and central sectors of the TMVB should be of Mesoproterozoic age, and tectonically overlapped by Paleozoic andMesozoic juvenile crust in the central sector and its corresponding forearc region. It is also concluded in this work, in agreement with recent seismological high resolution studies, that the apparent differences existing between geophysical modeling and the PT conditions required to generate the primary andesitic, dacitic and adakitic magmas that characterize the TMVB in the study area, may be resolved if the angle of the slab that extends northward and beyond the flat segment is increased substantially, thus creating optimal thermal and rheological conditions in a mantle wedge under the volcanic front much thicker than currently accepted. These conditions would increase the temperature of the entire subduction system to values that would permit the generation of such primary magmas by partial melting of the basaltic part of the subducting slab, the mantle wedge, and the mafic lower crust.
Key words: subduction factory, lower crust, xenoliths, TransMexican Volcanic Belt, southern Mexico.
Con el objetivo principal de contribuir a los modelos que debaten cómo y dónde se generan los magmas de la Faja Volcánica Transmexicana (FVTM), como ascienden atravesando la corteza viscosa, y cómo y dónde interaccionan con la corteza para producir la diversidad de magmas que integran ese sistema, en este trabajo se analizan e interpretan integralmente la naturaleza de la corteza inferior y su entorno inmediato debajo de la FVTM, así como de su región de antearco en el sur de México con base en la información geológica, geofísica y de xenolitos existente. Definida la corteza inferior como la parte de la corteza continental localizada entre el Moho y hasta un profundidad de 2025 km bajo la superficie, se concluye de este análisis que toda la corteza inferior en el área de estudio debería estar en la facies de granulita, si los modelos geofísicos predicen correctamente una temperatura mínima de 700800°C para su base y un espesor cortical entre 40 y 45 km. La información procedente de xenolltos y de la geología superficial integrada a modelos tectónicos, a su vez, apoya la idea de que la mayor parte de la corteza inferior debajo de los sectores oriental y central de la FVTM es del Mesoproterozoico, y está tectónicamente sobrepuesta en su sector central y reglón de antearco correspondiente por corteza juvenil del Paleozoico y Mesozoico. También se considera en este trabajo, en armonía con estudios sismológicos recientes de resolución alta, que las diferencias aparentes que existen entre los modelos geofísicos y las condiciones PT necesarias para generar los magmas andesíticos, dacíticos y adakíticos que caracterizan a la FVTM en el área de estudio, podrían resolverse si el ángulo de subducclón de la placa que se extiende más allá de su segmento subhorizontal se incrementa sustancialmente, creando asi las condiciones térmicas y reológicas óptimas en una columna de manto continental más amplia que la que se ha considerado debajo del frente volcánico del arco. Estas condiciones incrementarían las temperaturas a valores que permitirían la generación de esos magmas primarios por fusión parcial de la cubierta basáltica de la placa en subducclón, la cuña del manto, o la corteza inferior máfica.
Palabras clave: corteza inferior, xenolitos, proceso de subducción, Faja Volcánica Transmexicana, sur de México.
Active tectonics and arc volcanism in southern and central México have been a direct consequence of plate convergence of the Cocos and Rivera plates under the continental crust of Mexico (North American plate) since the early Miocene (20 Ma). The complex nature of this margin stems mainly from the inherent complexity of its preMiocene tectonothermal evolution, the geologic makeup of which in great measure should influence the nature and evolution of magmas generated in the subduction factory, as they migrate from their birthplace at mantle depths to the surface. Indeed, the interaction of these magmas with the traversed crust should be facilitated by the thermal regime and reactivity of the rocks composing that crust, as it is a well known fact (e.g., Sachs and Hansteen, 2000; Dufek and Bergantz, 2005; Gerbi et al, 2006) that a hot crust is essential to drive crust melting and assimilation followed by magma mixing. These processes would certainly affect the ascending magmas changing their primary composition and modifying their original routes of petrologic differentiation.
It is clear that the crust and probably the mantle subtending the Mexican Subduction Zone (MSZ) are anomalous in many aspects compared to many other arc regions of the world. For example, a very important feature of the MSZ is the diversity of crustal blocks or terranes that form the basement there (Figure 1), and it seems to be the only arc in the world that is substantially oblique (16°) to the corresponding trench. Also, the crust under the arc is one of the thickest found among all arcs of the world (Wallace and Carmichael, 1999). The TransMexican Volcanic Belt (TMVB) extends about 1000 km across central México from the Atlantic to the Pacific margins astride at least five tectonostratigraphic terranes, which from east to west are dubbed (Campa and Coney, 1983; Sedlock et al, 1993): Maya, Cuicateco, Zapoteco, Mixteco and Guerrero. In addition, the Chatino terrane, composed of highgrade metamorphic rocks and granites, integrates the southernmost continental crust of the TMVB forearc in southern México. The crystalline basement of these six provinces differ in age as well as in lithology and structure, rising the possibility that magmas ascending from the mantle may interact in different ways according to the geological constitution of the traversed crust. In fact, the extraordinary magmatic diversity of the TMVB (e.g., GómezTuena et al, 2005, and references therein) must be substantially controlled by the structure and composition of the underlying crust, but specific details on the magnitude and nature of these interactions have been limited by the scarcity of deepseated xenoliths in rocks of the TMVB and its forearc region.
In recent years, the multiplicity of igneous rocks and their geographic distribution in the TMVB have prompted a wealth of models to explain its complexity (Luhr, 1997; Wallace and Carmichael. 1999; Márquez et al, 1999; Sheth et al, 2000; Verma, 2002; Ferrari, 2004; GómezTuena et al, 2005, 2007), and yet essential aspects of the problem remain unsolved or under debate, such as the relative petrogenetic roles played by the mantle wedge, the subducted slab, the temporal evolution of the MSZ, and the structure and composition of the deep crust under the arc. However, in contrast with northern Mexico, where lower crustal xenoliths have been found in many places within Neogene basaltic fields (e.g., Luhr and ArandaGómez, 1997, and references therein), only two places with proved lower crust xenoliths and two from the mantle have been reported from the TMVB, but none with mantle xenoliths from the forearc region of southern Mexico. For this reason, modeling the origin and evolution of magmas in the TMVB has been severely limited by the absence or paucity of crucial information about the nature of the lower crust, which constitutes one of the most important potential sources for generation of primary andesiticrhyolitic magmas and an important contaminant for basaltic melts ascending from the mantle.
This paper goes beyond a simple compilation of existing data, as we address the problem by focusing on the integration, analysis, and interpretation of the direct geological evidence (xenoliths and outcrops), and indirect data (geophysical modeling) known and provided in this work about the structure and composition of the lower crust that underlies the frontal region of the TMVB and its forearc in southern Mexico. However, this approach does not attempt to model the whole complexity of the region, but should be useful to constrain future and ongoing multidisciplinary studies that require some factual and integral knowledge about the geology of the local lower crust for the better understanding of how the MSZ has worked in the past 20 Ma.
Definition and PT conditions of the lower crust in the study area
The lower continental crust traditionally had been considered that part of the earth's crust below the Conrad discontinuity, a rather sharp seismic discontinuity present mainly in cratonic areas, and probably separating the granitic from the mafic components of the crust. However, deep continental drilling and detailed geophysical studies more recently have undermined this notion, and presently, the lower crust is usually placed at the transition from amphibolite to granulite facies rocks, a phase boundary that commonly occurs at minimum pressures around 56 kbar or about 1822 km deep in extensional settings. Rudnick and Fountain (1995), for example, place the lower continental crust below 25 km, except in rifted margins, where the upper limit rises to 1718 km (5 kbar) below the surface. Characteristic seismic wave velocities (VP) of 6.77.5 km/s for the lower crust, or the brittleductile transition have been also used to separate the upper from the lower crust. The lower crust in this study is taken as that part of the continental crust extending from the local Moho to an uppermost depth of about 2025 km (ca. 67 kbar). At the highest geothermal crustal gradient of 27.5°C/km modeled for the Mexican region (e.g., Manea et al., 2005,2006) the temperature at the top of this layer would be 550687°C, corresponding to the amphibolite metamorphic facies, where quartz, feldspar and hydrous phases still predominate. The underlying 2025 kmthick lower crust of the study area, with temperatures of 8001000°C and pressures between 7 and 12 kbar, would lay mostly in the high T/P granulite facies, where probably garnet is not stable. On the other hand, for temperatures of 700800°C at the base of the crust previously modeled for the underlying volcanic front of the central TMVB (Currie et al., 2002, Manea et al., 2004), the entire lower crust would barely be in the granulite facies, but with garnet present at the deepest levels. A temperature of 800°C predicted at the base of the crust for the GuerreroOaxaca forearc region (Currie et al., 2002), seems to be more appropriate considering the hightemperature nature of the xenoliths found there (see data and discussions below). The depthpressure relationships used in this work based on the mean density estimated for the crustal layers are shown in Table 1. The depth/pressure equivalence for the whole crust would be approximately 3660 m/kbar, (0.037 km/MPa) and about 3000 m/kbar (0.03 km/MPa) for the underlying mantle wedge.
TransMexican Volcanic Belt
The TMVB is a continental volcanic arc of Miocene to Recent age located within the CircumPacific belt. It is oriented EW extending from Palma Sola and Los Tuxtlas in the Gulf of Mexico Coastal Plain, to San Bias, Nayarit, and Bahía de Banderas, Jalisco, in the Pacific coast (Figure 1). The province is about 1000 km long and between 80 and 230 km wide, and is forming an angle of about 16° with the WNW trend of the Acapulco trench (GómezTuena et al., 2005) where the MSZ starts. Three plates are involved in the evolution of the TMVB (North America, Cocos, and Rivera) for which a vast literature exists about the nature of the structure and its temporal evolution that cannot be considered in this work. The latter plate moves slowly and subducts the North American plate with a steep angle of about 50° (Pardo and Suárez, 1993; DeMets and Wilson, 1997; Bandy et al, 1999), whereas the Cocos plate in southern Mexico subducts at angles of 0 to 13° (Suárez, et al, 1990; Pardo and Suárez, 1995), with velocities of 5 to 8 cm/year (e.g., DeMets et al., 1994).
The TMVB includes a great variety and large number of volcanoes (over 8000) suchas stratovolcanoes, calderas, cinder cones, lava fields, shield volcanoes, and maars, the rocks of which are mainly of calcalkalic composition, although minor volumes of coexisting alkalic and tholeiitic rocks are also found along the arc. The magmatic province has been subdivided in three sectors (Figure 1) approximately 330 km long each, which in this work correspond to the eastern (Long. W 9699°), central (Long. W 99102°) and western (Long. W 102105°). These segments show distinct structural, compositional and volcanic features that have been genetically related to the nature of the underlying continental crust (Wallace and Carmichael, 1999), as well as to the geodynamic and age properties of the subducting plates. However, for the purposes of this work, the western segment, which is mostly associated with subduction of the Rivera oceanic plate, will not be considered any further.
The region that forms the forearc of the TMVB may be generally referred to as southern Mexico, although because of the obliquity of the arc relative to the trench, this region has a variable width from barely 60 km in the western sector to over 400 km in the central and eastern sectors. The geology of the area is extremely complex and rather poorly understood along the vertical dimension. The main geologic features of the six tectonostratigraphic terranes that compose this region are briefly summarized in Table 2, with some important references included in which more details may be found. Note that the TMVB intersects only four of the southern terranes (Maya, Zapoteco, Mixteco, and Guerrero) depicted in Figure 1, the former two underlain by Precambrian crust, and the Mixteco and Guerrero terranes composed of juvenile (Paleozoic and Mesozoic, respectively) and reworked (preMesozoic) crust. On the other hand, the Cuicateco terrane, composed of juvenile (Mesozoic) and reworked (Precambrian and Paleozoic) crust, wedges out just before reaching the TMVB at the Pico de Orizaba volcano, and therefore it should not be represented in the crust underlying the volcanic province. The Morelos Platform, although is not considered a terrane by itself, the lack of exposed basement rocks in the region, and the major faults that delimit the province, make the boundary between the Mixteco and Guerrero terranes ambiguous (e.g., Campa and Coney, 1983; Sedlock et al, 1993). Nevertheless, the region intersects a large part of the central TMVB, and although its surface geology is well known (Fries, 1960; HernándezRomano et al., 1997; MolinaGarza and OrtegaRivera, 2006) the nature of the buried crust under the platform supracrustal strata is only poorly characterized as probably Precambrian by the presence of highgrade metapelitic xenoliths in Neogene volcanic rocks at Chalcatzingo, Morelos (GómezTuena et al., 2008) and by isotopic geochemical inference (e.g., Levresse et al, 2007).
REGIONAL LOWER CRUST EXPOSED BY XENOLITHS AND OUTCROPS
Xenoliths in the TMVB are common (Figure 1 and Table 3) but apparently most are of shallow origin except in five localities including three places with lower crustal mafic xenoliths (Valle de Santiago maars, Pico de Orizaba stratovolcano, and Amealco caldera), and two with mantle xenoliths (El Peñón, State of Mexico, and Alto Lucero, Veracruz). A summary of most important xenolith localities (18) in the TMVB is given in Table 3. On the other hand, deepseated crustal xenoliths in the forearc of southern Mexico are only known in three places, but none contain mantle xenoliths: Pepechuca, State of México (EliasHerrera and OrtegaGutiérrez, 1997; EliasHerrera, 2004), Chalcatzingo, Morelos (OrtegaGutiérrez et al, 2000) and Puente Negro, Puebla (OrtegaGutiérrez et al, 2004) (Figures 1 and 2). These forearc xenoliths differ from those in the TMVB because they consist mainly of granulite facies metapelites with common pyroperich garnet, sillimanite, rutile, spinel, corundum, cordierite, orthopyroxene, and FeTi minerals, whereas quartz and feldspars are less common, thus probably imparting a relatively high density (>2.8 g/cm3) to the buried forearc crust from which they were derived. Although many xenoliths in these places also originated in the deep middle crust under upper amphibolite facies, they consist of mafic, intermediate, and felsic gneisses and schists for which no exposures in the surrounding terranes are known. Granulite facies xenoliths can be distinguished by the presence of orthopyroxene in both mafic and metapelitic lithologies, rutilebearing quartz and the apparent coexistence of ultrahigh temperature assemblages such as spinelquartz and hypersthenesillimanite (Figure 2). This type of metamorphism is unknown elsewhere in Mexico, and could represent a rather unique Proterozoic crust in the TMVB forearc of southern Mexico that was sampled by the Cenozoic magmas piercing the Morelos Platform.
Exposures of preCenozoic lower crust and estimated densities
Figure 3 illustrates the lithologic columns and densities for the main exposed and buried inferred stratigraphic units for each of the terranes that compose the forearc of the Mexican Subduction Zone. Depth to the Moho is estimated from seismic and gravimetric data as discussed below, and reasonable densities are estimated on the basis of mineralogical extrapolations at depth from exposed formations, although considerations based on more complex petrophysical models for changes in porosity and hence density that take place at depth (e.g., Tassara, 2006), and possible structural stacking of the crust are beyond the scope of this work.
Outcrops of old lower crust in southern Mexico extensively occur in the Mesoproterozoic Oaxacan Complex (OrtegaGutiérrez, 1981, 1984; Solan et al, 2003; Keppie et al, 2003) and Guichicovi Complex (Weber and Kohler. 1999; Weber et al, 2006) of the Zapoteco and Maya terranes, respectively, but may be also present in small portions of the Mesozoic Xolapa Complex in the Chatino terrane (CoronaChávez et al, 2006a). Lower crustal rocks of the Oaxacan and Guichicovi complexes consist of banded, granulite facies orthogneisses and paragneisses, the composition of which vary from quartzfeldspathic gneisses, calcsilicates and aluminous paragranulites, to mafic and intermediate orthogneisses with abundant anorthosite and rare ultramafic rocks. Highdensity phases such as garnet (3.584.32 g/cm3) and pyroxenes (3.23.5 g/cm3) are rather common in all mafic, felsic, and metapelitic lithologies, and compete in abundance with quartz (2.65 g/cm3) and feldspar (2.622.76 g/cm3). Biotite (2.83.2 g/cm3) and hornblende (3.03.4 g/cm3) are also abundant but moderatedensity phases, except where retrogression has replaced them by chlorite group minerals (commonly <2.8 g/cm3), muscovite (2.76 g/cm3), and calcite (2.71 g/cm3). This retrogression is local and represents lowtemperature hydration of the granulites causing the density to decrease substantially. However, at certain depths under the present exposures, this lowgrade retrogression may disappear. Thus, considering the mineralogy of granulites in the Zapoteco terrane of southern Mexico, a mean density for this crustal region may be closely estimated by taking the average of mafic and quartzofeldspathic phases assuming that they constitute respectively approximately 50 % by volume. In this case, the average density of the lower crust (uncorrected for pore fluid contents) could be close to 3.0 g/cm3 in this region. On the other hand, because abundant granitoids together with Precambrian granulites form outcrops (Guichicovi Complex) and subcrops in the buried basement of the Maya terrane, the mean density of the crust in this terrane may be estimated by considering equal parts of garnetiferous granulites (3.0 g/cm3) and granite (2.65 g/cm3), yielding a density of 2.82 g/cm3. The Cuicateco terrane is not considered any further in this context because it wedges out before reaching the underpins of the TMVB (Figure 1). Nevertheless, its general geologic and density features are given in Figure 3.
The Mixteco terrane in outcrop includes eclogite and garnet amphibolite (>3.4 g/cm3), but their added volume may represent only up to 10 % or less of the entire formation, which for the most part shows intense retrogression to garnetfree amphibolite or greenschist with lower densities. The structural depth to which the present Acatlán Complex extends is about 20 km, indicating that this lower limit marks the base of the middle crust. The underlying lower crust, about 20 km thick, probably is composed of granulite facies rocks, as evidenced by xenocrysts of metamorphic orthopyroxene contained in Jurassic granitoids emplaced in the middle crustal migmatites of the Acatlán Complex (OrtegaGutiérrez, 1975). The main part of the exposed Acatlán Complex to structural depths of about 10 km is in fact formed by quartzite and quartzose phyllite, the mean (uncorrected) density of which would be about2.65 g/cm3. Because this sort of Hthology continues down to the base of the Acatlán Complex, albeit changing to denser metamorphic phases such as biotite and some garnet, it is possible to predict the nature of rocks and minerals that compose the lower crust, provided that the Acatlán Complex continues down to this depth. At granulite facies, these rocks, because of the abundance of quartz and feldspar, would still preserve a mean density of about 2.752.8 g/cm3, thus implying a total mean density for the crust underlying the Mixteco terrane of 2.73 g/cm3.
The upper and middle crust of the Chatino terrane are dominated by garnetpoor, quartzofeldspathic gneisses and intrusive granitic plutons, the mean density of which would by about 2.7 g/cm3. The lower crust of the Chatino terrane is not exposed, although some granulite facies rocks are apparently cropping out in limited areas of the Xolapa Complex (CoronaChávez et al., 2006a). Thus, it may be safely assumed that most of the crust under the present exposures of the upper amphibolite facies Xolapa Complex is composed of granulitic and metapelitic gneisses with some garnet and pyroxenes, but with quartz, micas and feldspar constituting the great majority of that crust. A mafic lowermost crust may also be present if the source of the heat that generated the Xolapa Complex and its intrusive batholiths originated from underplated basaltic magmas. However, because garnet would not be stable in the relatively thin and hot crust of the continental margin of southern Mexico (1030 km, see below), a garnetfree gabbroic rock in the granulite facies would have a density less than 3.0 g/cm3. Thus, a mean density for the lower and middle crust in the continental margin of southern Mexico can be roughly estimated taking one quarter of this crust to be mafic and the other three quarters quartzofeldspathic, yielding a value of about 2.72 g/cm3.
Outcrops of the Guerrero terrane, considering a maximum exposed thickness of about 6 km in the Tejupiclo area (ElíasHerrera, 2004), consist of island arc and ocean floor volcanic and sedimentary assemblages generally metamorphosed at low grade (greenschist and subgreenschist facies). Protoliths are dominantly slate, graywacke, and andesitic lavas and pyroclastic rocks, for which a mean density of 2.7 g/cm3 may be safely assumed. Deeper structural levels include amphibolite facies schists and granitoids with densities estimated at 2.72.75 g/cm3. The deep buried crust of the Guerrero terrane south of the Balsas River cannot not be much denser because even for crustal thicknesses calculated by seismology at 32 km (Domínguez et al., 2006), the lowgrade facies rocks would be hydrous quartzofeldspathic schists and gneisses with minor amphibolites, where hornblende, some pyroxene, and epidote (3.3 g/cm3) would be present. It is important to mention, however, that recent magnetotelluric experiments across two profiles in southern and centralnortheastern Mexico (Jodicke et al., 2006) concluded that the lower crust in these regions has been hydrated by fluids derived from the top of the subducting slab, and that porosities between 5 to 3 % may exist in this crust where excess water is being stored. Therefore, densities derived above were corrected down by a maximum factor of 5 % (Table 4) to account for the pores occupied by water (ca. 1 g/cm3). Note that hydration of the granulitic lower crust to form amphiboles, biotite and probably epidote with mean densities similar to pyroxene, but with garnet remaining stable at those pressures and temperatures, would not change much an overall density of the lower crust estimated at 2.80 g/cm3.
Modeling of the crust by gravity methods requires some critical assumptions such as isostatic compensation and density profiles for a layered crust, which in turn rest on the adequate conversion of seismic wave velocities to densities. However, it should be borne in mind that the heterogeneous geology, active tectonics, and reduced width (<100 km) of many of the crustal blocks that compose the basement of the TMVB in some cases may not be isostatically compensated, thus somewhat weakening the models. The first comprehensive attempt to gravitationally picture the third dimension of the Mexican crust was made by Woollard et al. (1969) based on a survey of the Bouguer anomaly distribution in the country. The results, expressed in a map, proposed a close isostatic compensation of the Mexican crust given by the relation H = (BA35)/0.079, where BA is the Bouguer anomaly in mgals and H the topographic height in meters (e.g., MolinaGarza and UrrutiaFucugauchi, 1993).
More recent modeling of the Bouguer anomaly in three NS profiles representing the main part of the TMVB (UrrutiaFucugauchi and FloresRuiz, 1996) determined a simple Bouguer anomaly pattern characterized by negative values between 200 and 250 mgals corresponding to a mean topographic height of about 2000 m (Figure 4). Maximum and mean crustal thicknesses of about 50 and 42 km and assuming mean crustal and mantle densities of 2.85 g/cm3 and 3.30 g/cm3, respectively, were pictured along the eastern profile, corresponding to the central region of the TMVB across Mexico City (see Figure 4a). The profile across the western part of the TMVB, on the other hand, showed a maximum crustal thickness of 41 km and an average of 38 km (UrrutiaFucugauchi and FloresRuiz, 1996). Interestingly, the central profile across the GuanajuatoMichoacán volcanic field that contains the only granulitic xenoliths in the TMVB also yielded similar crustal thicknesses between 40 and 47 km. The thickness of the crust underlying the early Mesozoic Arteaga Complex in the TMVB forearc adjacent to the Michoacán monogenetic volcanic front, modeled by gravity and magnetics, was calculated at 2832 km, assigning to the lower crust a density of 2.9 g/cm3 and a thickness between 13 and 17 km (GarcíaPérez and UrrutiaFucugauchi, 1997). However, considering the petrologic composition of the Guerrero terrane in its lower preCretaceous levels, where quartz and feldsparrich metasedimentary rocks predominate (CentenoGarcía et al.,1993,2003; ElíasHerrera et al, 2000; ElíasHerrera, 2004), the real density of the lower crust there is probably much lower. The thinnest crust found along the volcanic front of the TMVB occurs in its western sector beneath the Colima rift, where a maximum thickness of 18 km was measured by the gravity modeling (Figure 4a).
A detailed gravity interpretation of the crustal structure under the State of Oaxaca, southern Mexico (Mena et al., 1995), based on two profiles along and across the continental margin from Ometepec, Guerrero to Salina Cruz, Oaxaca, and from Puerto Ángel to Teotitlán del Camino, Oaxaca, respectively, derived a complex crust consisting of six layers with densities ranging from 2.67 to 2.9 g/cm3, and a constant Moho depth of 2830 km along the Pacific coast of Oaxaca. The presence of this relatively thick continental crust adjacent to the deep oceanic trench is probably due to the Cenozoic truncation of the continental structure in southern Mexico.
Seismicity and magnetotellurics
Several seismic refraction profiles and other seismological local studies, or extending from the Pacific margin into the TMVB (e.g., Meyer, 1961; Fix, 1975; Valdés et al., 1986; Geolimex Working Group, 1994; ValdésGonzález and Meyer, 1996; Campillo, etai, 1996; CamposEnriquez and SánchezZamora, 2000; Iglesias et al, 2001), differ considerably in the crustal thickness estimates (e.g., CruzAtienza, 2000) from the gravity results for the Moho depth in southern Mexico and under the TMVB. A crustal model based on magnetotelluric profiles of the Oaxacan continental margin (Arzate et al., 1993) determined a relatively thin crust of about 35 km under the central part of the profile (Oaxaca city), contrasting with the maximum Moho depth of 44 km found on the basis of gravity modeling. This 10km difference was attributed to the presence of fluids, porosity or fracturing of the lower crust, which shifted the magnetotelluric spectra towards lower values in relation to gravity and seismic modeling.
A more detailed magnetotelluric profile extending from Acapulco, in southern Mexico, to Tampico, in northeastern Mexico, across the TMVB, was designed to better understand the complex terrane structure, geometry of tectonic boundaries and constitution of the crust (Jording et al., 2000). Results showed that some of the terrane boundaries are shallow structures that do not extend at depth, except for the case of the northern limit of the Xolapa Complex (Chatino terrane) along its entire length. It is rather surprising that this study did not consider the Oaxaca fault as a fundamental tectonic boundary, despite its 200 km rectilinear minimum extension separating the Proterozoic Oaxacan Complex from a Mesozoic tectonostratigraphic unit, which is formed by volcanosedimentary units partly deposited on oceanic lithosphere represented by mafic and ultramafic dismembered ophiolitic sequences in the Cuicateco terrane.
Similar magnetotelluric profiles across southern and central Mexico were studied with the main objective of depicting processes associated with fluid and melt release from the subduction zone and the influence of obliquity of the arc in relation to the trench (Jodicke et al., 2006). In the central Mexico profile (Acapulco, GuerreroTampico, Tamaulipas) a high conductivity structure set astride the lower and middle crust boundary and extending over 300 km across the TMVB was interpreted as a partially molten zone produced by hydrous melting of the lower crust, whereas along the southern Mexico profile, which lacks volcanism (Puerto Escondido, OaxacaAlvarado, Veracruz) various high conductivity anomalies were explained only by the presence in the lower crust of fluids released from the shallow slab and trapped under an impermeable middle crust. The absence of melts in this case was ascribed to differences in the geometry and age of the deformed subducting plate, with a steeper angle of subduction in southern Mexico (13°) compared to the profile across central Mexico, where the slab is younger and less inclined (10°). Otherwise, the geothermal gradient along the trajectory of the subduction plane that best fitted the conductivity anomalies in relation to dehydration reactions of the downgoing plate was 8.5°/km for both regions.
Surface heat flow measurements in southern Mexico or the TMVB are so few (e.g., ProlLedezma and Juárez, 1985; Ziagos et al., 1985), with the exception of the Texcoco site in the Valley of Mexico, of very shallow depth (100350 m), that their use for constraining the thermal structure of the crust overlying the MSZ is severely limited. It should be mentioned, however, that the corrected geothermal gradients obtained from 15 sites along an area running from the forearc (8 sites), across the arc (2 sites), and into the backarc (5 sites) regions of the central and western segments of the TMVB (Ziagos et al, 1985) yielded low values in the forearc (8.416.3°C/km), but substantially higher (as expected) in the arc (36.956.5°C/km), and the backarc (17.861°C/km). In fact, these specific gradients and its large variations respond more probably to local and transient geologic conditions rather than to the secular heat field that should affect deeper levels of the underlying crust. On the other hand, the contoured heat flow for southern and central Mexico roughly depicts a forearc with values below 40 mW/m2, which double in the arc and backarc regions to values between 40 and 80 mW/m2. Averages for the forearc, arc, and back arc regions reported by Ziagos et al. (1985) were 26 ± 5 mW/m2, 91 ± 26 mW/m2, and 89 ± 27 mW/m2, respectively, indicating contrasting thermal conditions, most probably related to the presence in the last two regions of stagnant magmas in the crust, or an underlying anomalous hot mantle. The forearc, on the other hand, appears to be refrigerated with respect to normal conditions in the continental crust of Mexico. Corresponding geotherms for the entire forearc crust of southern Mexico is extremely low at 5°C/km, whereas it rises to 22.5°C/km in the backarc region of the TMVB (Ziagos et al, 1985, fig. 9).
Xenoliths age and pressuretemperature conditions in the local lower crust
The only data on the PT conditions based on deepseated xenoliths from the crust under the TMVB are reported in two studies. AguirreDiaz et al. (2002) reported one granulite xenolith from the Amealco caldera (Figure 1), State of Querétaro, that yielded entrainment pressures of 2.93.2 kbar (1012 km) calculated from trapped fluid inclusions and assuming temperatures of 800 and 900°C forthe generation of the enclosing granulites. However, the original layer that provided the xenoliths, considering its highgrade metamorphism and relatively young Nd model age (683 Ma), more probably represents the actual lower crust buried at least 25 km, and from which the xenolith was carried up by a magma that stagnated in the upper crust (<12 km), where the fluids inclusions might have been formed. The Amealco caldera is located between extensive outcrops of the Mesozoic Guerrero terrane at El OroTlalpujahua region in the south, and Guanajuato area in the north, and thus this terrane should underlie the caldera. Because a minimum structural thickness of about 6 km has been measured in the Guerrero terrane of southern Mexico (ElíasHerrera, 2004), the upper and middle crust underlying the southern edge of the central TMVB should be composed essentially of lowgrade metamorphic rocks typical of that terrane, and the Precambrian granulites proposed by AguirreDiaz et al. (2002) may be present only in the lower crust. It should be pointed out, however, that the Precambrian age obtained by AguirreDiaz et al. (2002) for the xenolith is a Nd model age, but the true age of the buried basement should be younger.
Xenoliths from Valle de Santiago, Guanajuato, although are abundant and have been geochemically well studied (UribeCifuentes, 2006), their age and pressuretemperature conditions have been only poorly determined. Righter and Carmichael (1993) concluded that xenocrysts of olivine, pyroxene and plagioclase in the country lavas and pyroclastic rocks of this locality were taken from underplated bodies of gabbro and pyroxenite buried about 30 km. Thermobarometry on the granulite xenoliths yielded a pressure range of 5 to 10 kbar equivalent to lower middle and lower crust, although the mineral composition of granulites studied by us from Valle de Santiago consist of OpxCpxOlPlgSpl, showing all levels of concentration of the major phases from anorthositic to pyroxenitic in irregular bands or elongate zones, indicating that this part of the lower crust may represent intraplated basic magmas where fractional crystallization of plagioclase and pyroxene played a major role. The problem, however, is that some of these granulites show a foliation across the presumably magmatic banding, representing a tectonic event after crystallization, and therefore it is difficult to consider them equivalent to the magmas of the overlying TMVB, unless the deformation is associated with deep fault movements active in the Neogene. Considering that garnetfree, plagioclaseolivine, and orthopyroxenespinel pairs coexist in textural equilibrium in most of the mafic granulites studied by us from that locality, maximum pressures between 5 kbar (olivineplagioclase) and 8 kbar (orthopyroxenespinel) may be estimated for the crystallization of these xenoliths based on petrogenetic grids for mafic and ultramafic rocks under granulite facies conditions (e.g., Gasparik, 1984; Herzberg and Gasparik, 1991), and yet thermal models for the crust in the Michoacán sector of the Mexican subduction zone (e.g., Currie et al, 2002; Manea et al, 2005) predict a temperature of 800°C at the base of a 40 km thick crust. These conditions require a geothermal gradient of 20°C/km and the stability of garnet for a mafic lower crust. Indeed, partial melting of the mafic lower crust (fluid absent), according to experimental data on MORBtype starting samples (Rapp et al., 1991), at temperatures up to 1075°C and between 8 and 32 kbar produces highly depleted HREE tonalitic to trondhjemitic melts, with garnet remaining in the residue at pressures above 8 kbar, but magmas or xenoliths with this composition are absent from Valle de Santiago volcanic field. Therefore, whether the mafic xenoliths come from underplated Mesozoic or Cenozoic subductionrelated magmas, or were entrained well above the local Moho from an older crust, it is not known. If representing the lowermost crust under Valle de Santiago, a maximum thickness of about 3036 km (810 kbar) would be suggested by the mineral assemblages in the garnetfree granulites, which is much thinner than the 4045 km (1112 kbar) suggested by geophysical methods (UrrutiaFucugauchi and MolinaGarza, 1992; CamposEnríquez and SánchezZamora, 2000). Here we propose that these granulites may in fact represent the upper part (2530 km depth) of probably Precambrian lower crust affected by Cenozoic thermal processes that include the coeval massive access of mafic magmas (underplating), granulite facies metamorphism, partial melting, and some mixing of the magmas evolved from these processes. Unfortunately, the age obtained for these xenoliths (UribeCifiientes, 2006), of 1.6 Ga, and from high grade gneissic xenoliths of Nevado de Toluca at 1.21.3 Ga (MartínezSerrano et al, 2004) all are depleted mantle Nd model ages and therefore do not represent the real age of the correspondent basement, which is also the case for the Amealco granulite single xenolith (AguirreDíaz et al, 2002).
The thermal state of the lower crust plays a crucial role in the generation and modification of magmas by processes such as anatexis and assimilation. Thus, the understanding of the temperature distribution as a function of depth (geothermal gradient) constitutes an important factor for modeling geodynamic process associated with arc volcanism. In the absence of precise heat flow measurements along the TMVB and its forearc (Ziagos et al., 1985), analogs of other similar arc settings in the world may be useful to assess the thermal structure of the lower crust under the TMVB, and compare it with data from recent numerical models mentioned above for the Mexican province. The young and hot Cascadia slab of the northwestern United States (8 Ma) shows a 13° angle of subduction and moderate temperatures of about 800°C for the top of the slab at about 8085 km depth under the volcanic front and maximum temperatures of 1050°C for the overlying mantle (Hacker et al, 2003; Preston et al., 2003), yielding an integrated geothermal gradient for the slab of 10°C/km (hot subduction). In contrast, the Costa Rica subduction zone, where steep slab angle only heats to 500°C the upper plate boundary (Peacock et al, 2005), developed a lower geothermal gradient of 7°C/km (cold subduction).
On the other hand, the geometry of the Cocos plate west of the Tehuantepec ridge (Kostoglodov et al, 1996) includes (Figure 5) a subhorizontal (13°) segment extending from the trench to the Pacific coastline, there it bends over and follows a horizontal trajectory for 150 km, plunging again to acquire its final angle of around 21° under the Popocatepetl volcano situated about 330 km from the trench. Based on this geometry and that of Pardo and Suárez (1995), Currie et al. (2002) carried out the first numerical thermal modeling of the MSZ along four profiles from Jalisco to Oaxaca in southern Mexico set across the forearc and into the TMVB. The most important result of that work was the definition of PT fields for the four regions predicting maximum temperatures at the base of the local crust (mean of 40 km) between 700°C for the JaliscoMichoacán and 800°C for the GuerreroOaxaca forearc sections. These data would yield a rather low geothermal gradient of 17.5 °C/km and pressures of about 11 kbar for the base of the crust underlying the volcanic front at Popocatepetl volcano. Further thermal modeling of this subduction system, which considered the partly coupled nature of the slab interface (Manea et al, 2004,), predicted forearc and volcanic arc surface heat flows of 2835 mW/m2 and 60 mW/m2 respectively, yielding a similar temperature of 700°C for the Moho under Popocatepetl. In this case, the top of the subducting plate would be at 600°C, and about 70 km deep (21 kbar) thus recrystallized in the wet (zoisite) eclogite facies, whereas maximum temperatures modeled for the mantle wedge under Popocatepetl volcano are about 750°C. Clearly, these PT conditions are not sufficient to melt the basaltic crust of the slab nor the mafic lower crust, and also are too low for melting (wet solidus at 980°C at 70 km) of the overlying mantle peridotite (Peacock, 1990; Schmidt and Poli, 1998; Poli and Schmidt, 2002) to account for the primary andesitic and adakitic magmas that characterize the TMVB volcanic front.
However, when shear heating in the slab interface and a stresstemperature dependence of mantle viscosity are considered (e.g., Manea et al, 2005, 2006), temperatures dramatically rise to about 900°C along the thrust plane, to a maximum of 1230°C in the overlying mantle, and to 1100°C at the base of the crust under Popocatepetl or Nevado de Toluca volcanoes, implying a mean crustal geothermal gradient of 27.5°C/km. Under these conditions, hydrated mantle peridotite, eclogitic oceanic crust, and granulitic lower mafic crust would easily yield partial melts of andesitic composition, making it clear that factional heating and a temperaturedependent olivine rheology of the mantle wedge are critical factors for the generation of intermediate and mafic magmas along the TMVB volcanic front. Another way to reconcile the actual presence of andesitic and adakitic volcanism in the frontal zone of the TMVB, would be to increase (beyond the flat segment) the present seismically unconstrained inclination of the slab from its proposed value of 1921° (Manea et al, 2004; Kostoglodov et al, 1996) to much steeper angles (e.g., Ferrari, 2006; Manea and Gurnis, 2007), which would then permit a thicker mantle column than the presently proposed value of only 32 km under Popocatepetl, and consequently also an intense asthenospheric heating of the wedge under the stratovolcano. In fact, an angle greater than 45° for the Cocos plate beyond the inflection point has been recently suggested from MASE (Middle America Subduction Experiments) receiver function data (Xyoli PérezCampos, unpublished data), thus creating optimal conditions within a mantle column 80 km thick (37 kbar at the slab interphase) for increasing the temperature of the entire subduction system to values that permit the generation of andesitic magmas. Note that adakites, if generated at these pressures, would have to be the product of coesite eclogite partial melting, where calcic or sodic amphiboles are no longer stable.
Measured surface and reduced heat flow in the study region, as described above (e.g., Ziagos et al., 1985), calculated a mean geotherm of 22.5°C/km for the arcbackarc regions of the TMVB, which when translated to temperatures at a Moho buried 4050 km, would yield values at the base of the local crust of 9001125°C. These temperatures, if the crust is indeed that thick, would also cause hydrous melting of the lower mafic crust and produce TMVB magmas of intermediate composition albeit without an adakitic signature because garnet or amphibole are not stable at those PT conditions.
Precambrian lower crust under the TMVB?
Although outcrops of Precambrian rocks have not been found forming the basement of the TMVB volcanic rocks, it is quite possible that the eastern sector is underlain by Oaxaquiantype (i.e., Grenvillain) crust, which is widely exposed less than 100 km south of Pico de Orizaba volcano, and at a similar distance north of Los Humeros Caldera (Figure 1). The involvement of this crust is infact shown by the higher radiogenic (Pb, Nd, and Hf) isotopic signatures measured for Pico de Orizaba (Schaaf et al., 2005; Macías, 2005; Cai et al., 2007) compared to the other sectors of the TMVB, and by the presence of granulite facies xenoliths in volcanic rocks at and around the Citlaltepetl (Pico de Orizaba) stratovolcano (CarrascoNuñez et al., 2005).
Moreover, Precambrian Nd model ages (1.21.6 Ga) similar to those in Oaxaquia Mesoproterozoic crust have been documented from xenoliths, both in the front of the TMVB at Valle de Santiago (UrrutiaFucugauchi and UribeCifuentes, 1999) and Nevado de Toluca volcano (MartinezSerrano et al., 2004), as well as in the forearc region at Pepechuca (ElíasHerrera and OrtegaGutiérrez, 1997; ElíasHerrera, 2004), which correspond to the central sector of the Mexican arc. Inherited zircons in latest Cretaceous granitoids of the Morelos Platform have been interpreted to come directly from underlying Grenvillian crust (Levresse et al., 2007). On the other hand, the granulitic xenolith reported at Amealco (AguirreDíaz et al., 2002) yielded a Nd model age of 683 Ma, probably indicating substantially less Precambrian crust contamination along the central sector of the TMVB. Although the thick crust (4050 km) found by geophysical methods along the internal regions of the TMVB volcanic front, as presented above, would suggest the presence of lower crustal roots related to Paleozoic and Precambrian orogens, it is clear from Figure 1 that the distribution of terranes in the forearc of southern Mexico favors the existence of a younger middle crust (Paleozoic and Mesozoic) buried under the volcanic front along the central sector of the TMVB. The Bouguer gravity anomaly map and section across the tectonic boundary between the Guerrero (Mesozoic) and Mixteco (Paleozoic) terranes (GarcíaPérez, 1995) between west longitudes 98 and 101° (Figure 4b and c) show a marked gradient trending NS along 100.3° Meridian evidently dividing a thicker crust (Figure 4d) to the east (probably Mesoproterozoic lower crust) from a thinner crust to the west (essentially Mesozoic crust). Major geologic structures aligned with this trend are the Arcelia graben in southern Mexico and the Querétaro fault in the TMVB (AlanizÁlvarez and NietoSamaniego, 2005). The tectonic significance of this feature constitutes a central issue because the contact between the Mesozoic Guerrero and the Paleozoic Mixteco terranes would lie 40 km (Campa and Coney, 1983) or 200 km (Sedlock et al, 1993) east of the gravity gradient trend. The thickening of the crust across this boundary may indicate thrusting of the Mesozoic rocks over the Paleozoic terrane and its possible Precambrian substrate (Figure 4d), and therefore the extension of preMesozoic crust and Mesozoic thrusting for more than 240 km west of its outcropping limit in the Papalutla fault. In conclusion, however, the lack of Precambrian outcrops or properly dated xenoliths showing that age in the crustal structure of the western and central sectors of the TMVB requires extreme caution and further studies are needed before the presence of Precambrian crust under the arc is considered a valid assumption.
Role of the lower crust as a contaminant and potential source of andesitic and adakitic magmas in the TMVB
The chemical influence of the crust in the petrogenesis of the TMVB has been a constant subject of discussion, and many authors (e.g., Verma, 1999, 2000; Chesley et al, 2002; Verma and CarrascoNúñez, 2003) have shown that crustal assimilation in some places of the TMVB had a central role in the process, whereas many others (Wallace and Carmichael, 1999; GómezTuena et al., 2007; Blatter et al., 2007) tend to minimize this role in the studied areas (Chichinautzin, Zitácuaro), placing the main controls in processes affecting the mantle wedge and subducted slab. However, given the continental setting of the TMVB and the thickness of the underlying crust as discussed above, crustal assimilation should be an important process in the petrogenesis of its magmas, which in many places is clearly indicated at least for shallow levels by the thermal and chemical interaction of upper crustal xenoliths with the magma (e.g. Wilcox, 1954; McBirney et al, 1987; Márquez et al, 1999; Luhr, 2001; Schaaf et al, 2005). And yet, with the exception of some volcanoes in the eastern sector, the great majority of the andesitic and basaltic magmas so far studied show little if any isotopic evidence of contamination by an evolved crust.
On the other hand, adakites which may be generated by partial melting of a thickened or delaminated lower crust (Chung et al, 2003; Kay et al, 2005), have been identified in the TMVB in several places, varying in age from Miocene to the present (Luhr, 2000; GómezTuena et al, 2003, 2006, 2008; MartínezSerrano et al, 2004). However, these rocks and following Defant and Drumond (1990), have been geochemically explained by melting the mafic crusts of the young Rivera plate (Luhr, 2000) or due to prolonged flat subduction episodes of the Cocos plate (e.g., Mori et al., 2007). For crustal thickness of 4550 km (1214 kbar) as modeled by gravity and seismology in the central and eastern sectors of the TMVB, and temperatures of 700°C at the base of this crust (calculated by isoviscous modeling of the thermal structure of the MSZ, as discussed above), garnet granulites of gabbroic composition and Mg# <60 should compose most of the lower crust underlying the Mexican stratovolcanoes in those sectors. This sort of crust, even if wet, cannot melt at 700°C to produce andesitic magmas, but it certainly would do so for temperatures above 1000°C as modeled using temperaturestress dependent mantle rheologies. Abundant garnet in this case would be absent in the residue because pressures above 15 kbar are required to generate it at those temperatures, thus yielding ordinary andesiticdacitic magmas. Nevertheless, Nevado de Toluca stratovolcano shows adakitic signature (MartínezSerrano et al., 2004), whereas it seems to be absent in the Popocatepetl and Pico de Orizaba volcanoes despite the fact that the crust there is thicker and probably cooler. If the adakitic signature of Nevado de Toluca was acquired by partial melting of the wet eclogitic top of the Cocos plate at pressures around 20 kbar (2 GPa, or 70 km deep), minimum temperatures should be of the order of 700750°C, which may occur if the slab and mantle wedge were sufficiently coupled to produce frictional heating. Indeed, higher temperatures in the mantle wedge and steeper plate geometry models (e.g., Ferrari et al., 2004) would have a maj or impact on the thermal conditions of the lower crust to make it a potential contaminant or a source for some magmas of the TMVB.
It should be noted that melting of eclogites, where garnet and rutile remain in the source, not always generates the adakitic signature, as melting of zoisite and apatiterich eclogites at high and ultrahigh pressures in the slab (e.g., Nagasaki and Enami, 1998; de Hoog et al, 2007) will not produce a high Sr/Y ratio because zoisite and apatite retain some Sr and Y leaving behind a refractory kyanitebearing eclogite at temperatures that must exceed 800°C. Therefore, melting of the oceanic subducted crust recrystallized to zoisite eclogite may commonly occur in arc regions without imprinting a clear adakite signature. Popocatepetl and Pico de Orizaba could represent such case.
An additional model recently proposed for the origin of the highmagnesium andesites with an adakitic signature at Mount Shasta of the Cascades arc (Streck et al, 2007) is the mixing of dacitic (>50%) and basaltic (4030%) magmas contaminated by ultramafic rocks (57%) from preexisting ophiolites. The adakitic signature of the highMg andesites was considered to be inherited from the more strongly adakitic dacites presumably produced by melting of the lower crust. A similar scenario may be entertained for some of the TMVB lavas with a weak adakitic signature, because the traversed thick crust underlain by the Mixteco and Guerrero terranes flooring the arc front contains ultramafic bodies and the pressuretemperature conditions at the base of that crust may in some cases permit the stability of garnet, rutile, and hornblende in the granulite facies.
The main conclusion that may be derived from this work is that much more geologybased information is required to really understand the nature and evolution of the lower crust under the TMVB and the forearc region before its role in the evolution of the magmas composing the province is properly assessed. In particular, the prospecting and petrologic study of a lot more deepseated xenolith localities should be undertaken, as these objects can provide first hand information about the age and many physical and chemical parameters of the crust that are required for modeling the origin, ascent, modification and emplacement histories of the TMVB magmas. Also, the differences among geophysical (thermal, seismologic, magnetotelluric, and gravimetric) and petrologic models regarding the true geometry, structure and composition of the lower crust in the study area, will be better resolved once interdisciplinary research is designed more closely and considering the geologic data provided by xenoliths and basement outcrops. Ongoing research such as MASE (e.g., Ferrari, 2006) will undoubtedly provide a highresolution picture of the subducted Cocos plate, as well as the topography of the Moho, which will determine the thickness of the entire crust. Together, these data will certainly influence and constrain models that debate how and where magmas are generated in the subduction factory, how they ascend through the overlying viscous and fractured layers, and how and where they interact with the crust and mantle to produce the diversity that characterizes the magmas that compose the TMVB. In particular, the role of melts provided by the subducting slab (adakitic) and their interaction with peridotites of the metasomatized mantle wedge, should be examined more carefully in order to assess in great detail the ultimate origin of primitive magmas in the TMVB and to distinguish them from the more evolved products of fractionation, magma mixing, and of meltingassimilation processes in the lower and upper crust.
This work was funded by DGAPAPAPIITIN104706 project grant to F. OrtegaGutiérrez. Fruitful discussions with Gerardo Suárez and Xyoli PérezCampos, both researchers at the Instituto de Geofísica at UNAM, were very stimulating in regards to the geometry of the subducted plate. Finally, the insights and suggestions provided by Kevin Righter in his thoughtful review of this paper were very helpful and inspiring, which we very gratefully acknowledge.
AguirreDiaz, G., Dubois, M., Laureyns, J., Schaaf, P., 2002, Nature and PT conditions of the crust beneath the central Mexican Volcanic Belt based on a Precambrian crustal xenolith: International Geology Review, 44, 222242. [ Links ]
AlanizÁlvarez. S.A., NietoSamaniego, A.F., 2005, El sistema de fallas TaxcoSan Miguel de Allende y la Faja Volcánica Transmexicana, dos fronteras tectónicas del centro de México activas durante el Cenozoico: Boletín de la Sociedad Geológica Mexicana. Volumen Conmemorativo del Centenario, 57(3), 6582. [ Links ]
ÁngelesMoreno, E., 2006, Petrografía, geología estructural y geocronología del borde noroccidental del terreno Cuicateco, sierra Mazateca, estado de Oaxaca, México: México, D.F., Universidad Nacional Autónoma de México, tesis de maestría, 194 p. [ Links ]
Arzate, J.A., Mareschal, M., UrrutiaFucugauchi, J., 1993, A preliminary crustal model of the Oaxaca continental margin and subduction zone from magnetotelluric and gravity measurements: Geofísica Internacional, 32, 441452. [ Links ]
Bandy, W., Kostoglodov, Y., HurtadoDíaz, A., Mena, M., 1999, Structure of the southern Jalisco subduction zone, Mexico, as inferred from gravity and seismicity: Geofísica Internacional, 38(3), 127136. [ Links ]
Blatter, D.L., Carmichael, I.S.E., 1998, Hornblende peridotite xenoliths from central México reveal the highly oxidized nature of subarc upper mantle: Geology, 26, 10351038. [ Links ]
Blatter, D.L., Farmer, G.L., Carmichael, I.S.E., 2007, A North South transect across the Central Mexican Volcanic Belt at ~100°W: Spatial distribution, petrological, geochemical and isotopic characteristics of Quaternary volcanism: Journal of Petrology, 48(5), 901950. [ Links ]
Cai, Y., Goldstein, S, L., Langmuir, C.H., GómezTuena, A., LaGatta, A., Straub, S. M., MartinDel Pozzo, A. L., 2007, Implications from hafnium isotopes on subduction contributions and mantle heterogeneity in the eastern TransMexican Volcanic Belt (abstract): International Conference on Evolution, Transfer and Release of Magmas and Volcanic Gases, Taipei, 2227 April 2007, MAG 2007, p. 5. [ Links ]
Campa, M.F., Coney, P.J., 1983, Tectonostratigraphic terranes and mineral resource distributions in Mexico: Canadian Journal of Earth Sciences, 20, 10401051. [ Links ]
Campillo, M., Singh, S. K., Shapiro, N., Pacheco, J., Herrmann, R.B., 1996, Crustal structure south of the Mexican volcanic belt, based on group velocity dispersion: Geofísica Internacional, 35, 361370. [ Links ]
CamposEnríquez, J.O., SánchezZamora, O., 2000, Crustal structure across southern Mexico inferred from gravity data: Journal of South American Earth Sciences, 13(6), 47989. [ Links ]
CarrascoNuñez, G., Righter, K., Chesley, J., Siebert L., ArandaGómez, J.J., 2005, Contemporaneous eruption of calcalkaline and alkaline lavas in the continental arc (Eastern Mexican Volcanic Belt): chemically heterogeneous but isotopically homogeneous source: Contribution to Mineralogy and Petrology, 150, 423440. [ Links ]
CentenoGarcía, E., Ruiz, J., Coney, P.J., Patchett, P.J., OrtegaGutiérrez, F., 1993, Guerrero terrane of Mexico: Its role in the Southern Cordillera from new geochemical data: Geology, 21, 419422. [ Links ]
CentenoGarcía, E., CoronaChavez, P., TalaveraMedoza, O., Iriondo, A., 2003, Geology and tectonic evolution of the western Guerrero terrane a transect from Puerto Vallartato Zihuatenajo, Mexico, in Geologic transects across Cordilleran Mexico, Guidebook for field trips of the 99th Annual Meeting of the Cordilleran Section of the Geological Society of America, México, D.F., April 46, 2003: Universidad Nacional Autónoma de México, Instituto de Geología, Publicación Especial 1, Field trip 9, 201228. [ Links ]
Chesley, J.T., Ruiz, J., Righter, K., Ferrari L., GómezTuena, A., 2002, Source contamination versus assimilation: an example from the TransMexican Volcanic Arc: Earth and Planetary Science Letters, 195, 211221. [ Links ]
Chung, S.L., Liu, D., Chu, M.F., Lee, H.Y., Wen, D.J., Lo, C.H., Lee, T.Y., Quain, Q., Zhang, Q., 2003, Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet: Geology, 31, 10211024. [ Links ]
CoronaChávez, P., Poli, S., Bigioggero, B., 2006a, Syndeformational migmatites and magmaticarc metamorphism in the Xolapa Complex, southern Mexico: Journal of Metamorphic Geology, 24(3), 169191. [ Links ]
CoronaChávez, P., ReyesSalas, M., GarduñoMonroy, V.H., IsradeAlcántara, I., LozanoSanta Cruz, R., MortonBermea, O., HernándezAlvarez, E., 2006b, Assimilation of granitic xenoliths in the MichoacánGuanajuato volcanic field: the case of Arócutin, Michoacán, Mexico: Revista Mexicana de Ciencias Geológicas, 23, 233245. [ Links ]
CruzAtienza, V.M., 2000, Inversión global con algoritmos genéticos y cristalización simulada, aplicada a funciones de receptor: modelos estructurales de velocidades para la corteza en la República Mexicana: México, D.F., Universidad Nacional Autónoma de México, tesis de licenciatura, 236 p. [ Links ]
Currie, C.A., Hyndman, R.D. Wang, K. 2002, Thermal models of the México subduction zone: Implications for the megathrust seismogenic zone: Journal of Geophysical Research, 107 (B12), 2370, doi:10.1029/2001JB000886. [ Links ]
Defant, M.J., Drumond, M.S., 1990, Derivation of some modern arc magmas by partial melting of young subducted lithosphere: Nature, 347, 662665. [ Links ]
DeMets, C., Wilson, D.S., 1997, Relative motions of the Pacific, Rivera, North American, and Cocos plates since 0.78 Ma: Journal of Geophysical Research, 102(B2), 27892806. [ Links ]
DeMets, C., Gordon, R., Argus, D., Stein, S., 1994, Effect of recent revisions of the geomagnetic reversal time scale on estimates of current plate motions: Geophysical Research Letters, 21, 21912194. [ Links ]
Domínguez, J., Suárez, G., Comte, D., Quintanar, L., 2006, Seismic velocity structure of the Guerrero gap, México: Geofísica Internacional, 45(2), 129139. [ Links ]
Ducea, M.N., Gehrels, G.E., Shoemaker, S., Ruiz, J., Valencia, V.A., 2004, Geological Evolution of the Xolapa Complex, Southern Mexico. Evidence from UPb Zircon geochronology: Geological Society of America Bulletin, 116, 10161025. [ Links ]
Dufek, J., Bergantz, G. W., 2005, Lower crustal magma genesis and preservation: a stochastic framework for the evaluation of basaltcrust interaction: Journal of Petrology, 46(11), 2167 2195. [ Links ]
ElíasHerrera, M., 2004, Geología precenozoica de la región de Tejupilco, estado de México, y sus implicaciones tectónicas: México, D.F., Universidad Nacional Autónoma de México, tesis doctoral, 201 p. [ Links ]
ElíasHerrera, M., OrtegaGuitérrez, F., 1997, Petrology of highgrade metapelitic xenoliths in an Oligocene rhyodacite plugPrecambrian crust beneath the southern Guerrero terrane, Mexico: Revista Mexicana de Ciencias Geológicas, 14(1), 101109. [ Links ]
ElíasHerrera, M., SánchezZavala, J.L., MacíasRomo, C., 2000, Geological and geochronological data of Guerrero terrane in the Tejupilco area, southern Mexico: new constrains on its tectonic interpretation: Journal of South American Earth Sciences, 13, 355375. [ Links ]
Ferrari, L., 2004, Slab detachment control on mafic volcanic pulse and mantle heterogeneity in central México: Geology, 32(1), 7780. [ Links ]
Ferrari, L., 2006, Cenozoic evolution of the central part of the Mexican subduction zone from geologic and geophysical data: in the eve of the result from the "MASE" experiment: Geos, 26(1), 177. [ Links ]
Ferrari, L., Orozco, M.T., Petrone, CM., 2004, Mantle wedge perturbation induced by slab detachment and the MioPliocene bimodal volcanism in the TransMexican Volcanic Belt (abstract): Eos Trans. AGU, 85 (47), Fall Meeting Supplement, V51B0578. [ Links ]
Fix, J.E., 1975, The crust and upper mantle of central Mexico: Geophysical Journal of the Royal Astronomical Society, 43, 453499. [ Links ]
Fries, Carl, Jr., 1960, Geología del estado de Morelos y de partes adyacentes de México y Guerrero, región central meridional de México: Universidad Nacional Autónoma de México, Instituto de Geología, Boletín 60, 236 p. [ Links ]
GarcíaPérez, F., 1995, Caracterización geofísica de la región de Tierra Caliente y áreas colindantes, estados de Guerrero y Morelos: México, D.F., Universidad Nacional Autónoma de México, tesis de maestría, 55 p. [ Links ]
GarcíaPérez, F., UrrutiaFucugauchi, J., 1997, Crustal structure of the Arteaga complex, Michoacán, southern Mexico, from gravity and magnetics: Geofísica Internacional, 36(4), 235244. [ Links ]
Gasparik, T., 1984, Twopyroxene thermobarometry with new experimental data in the system CaOMgOAl2O3SiO2: Contributions to Mineralogy and Petrology, 87, 8797. [ Links ]
Geolimex Working Group, 1994, Reflection of the subducting plate? First results of a Mexican geotraverse: Zentralblatt für Geologie und Paläontologie, 1, 541553. [ Links ]
Gerbi, C.C., Johnson, S.E., Koons, P.O., 2006, Controls on lowpressure anatexis: Journal of Metamorphic Geology, 24, 107118. [ Links ]
Giosa, T.A., Nelson, S.A., 1985, Gabbroic xenoliths in alkaline lavas in the region of Sanganguey volcano, Nayarit, Mexico (abstract), in The Geological Society of America, 98th Annual Meeting, Orlando, Florida, USA: The Geological Society of America, Abstracts with Programs, 17(7), 593. [ Links ]
GómezTuena, A., LaGatta, A., Langmuir, C, Goldstein, S., OrtegaGutiérrez, F., CarrascoNúñez, G., 2003, Temporal control of subduction magmatism in the Eastern TransMexican Volcanic Belt: mantle sources, slab contributions and crustal contamination: Geochemistry, Geophysics, Geosystems, 4, doi:10.1029/2003GC000524. [ Links ]
GómezTuena, A., OrozcoEsquivel, M.T., Ferrari, L., 2005, Petrogénesis ígnea de la Faja Volcánica Transmexicana: Boletín del la Sociedad Geológica Mexicana. Volumen Conmemorativo del Centenario, 57(3), 227283. [ Links ]
GómezTuena, A., Capra, L., Cai, Y., Goldstein, S.L., Langmuir, C.H., 2006, Geochemical evidence for melting of a complete section of the subducted slab at Nevado de Toluca Volcano, Central Mexico (abstract): Geophysical Research Abstracts, European Geosciences Union 2006, 8, SrefID: 16077962/gra/EGU06A03051. [ Links ]
GómezTuena, A., Langmuir, Ch.H., Goldstein, S.L., Straub, S.M., OrtegaGutiérrez, F., 2007, Geochemical evidence for slab melting in the TransMexican Volcanic Belt: Journal of Petrology, 48(3), 537562. [ Links ]
GómezTuena, A., Mori, L., RincónHerrera, N.E., OrtegaGutiérrez, F., Solé, J., Iriondo, A., 2008, The origin of a primitive trondhjemite from the TransMexican Volcanic Belt and its implications for the construction of a modern continental arc: Geology, 36(6), 471474. [ Links ]
Hacker, B.R., Peacock, S.M., Abers, G.A., Holloway, S.D., 2003, Subduction factory 2. Are intermediatedepth earthquakes in subducting slabs linked to metamorphic dehydration reactions?: Journal of Geophysical Research, 108(B1), 116. [ Links ]
Herzberg, C., Gasparik, T., 1991, Garnet and pyroxenes in the mantle: a test of the majorite fractionation: Journal of Geophysical Research, 96, 1626316274. [ Links ]
HernándezRomano, U., AguileraFranco, N., MartínezMedrano, M., BarcelóDuarte, J., 1997, GuerreroMorelos platform drowning at the CenomanianTuronian boundary, Huitziltepec area, Guerrero state, southern Mexico: Cretaceous Research, 18, 661686. [ Links ]
Hoog de, J.C.M., Janák, M., Vrabec. M., 2007, Partial melting of zoisite eclogite and implications for traceelement transport in subduction zones (abstract): International Eclogite Field Symposium, Scotland, U.K, 2021. [ Links ]
Iglesias, A., CruzAtienza, V.M., Shapiro, N.M., Singh S.K., Pacheco J.F., 2001, Crustal structure of southcentral Mexico estimated from the inversion of surfacewave dispersion curves using genetic and simulated annealing algorithms: Geofísica Internacional, 40(3), 181190. [ Links ]
Jödicke, H., Jording, A., Ferrari, L., Arzate, J., Mezger, K., Rupke, L., 2006, Fluid release from the subducted Cocos plate and partial melting of the crust deduced from magnetotelluric studies in southern México: Implications for the generation of volcanism and subduction dynamics: Journal of Geophysical Research, 111(B08102), 122. [ Links ]
Jording, A., Ferrari, L., Arzate, J., Jödicke, H., 2000, Crustal variations and terrane boundaries in southern Mexico as imaged by magnetotelluric transfer functions: Tectonophysics, 327(1), 113. [ Links ]
Kay, S.M., Godoy, E., Kurtz, A., 2005, Episodic arc migration, crustal thickening, subduction erosion, and magmatism in the southcentral Andes: Geological Society of America Bulletin, 117(12), 6788. [ Links ]
Keppie, J.D., Dostal, J., Cameron, K.L., Solari, L.A., OrtegaGutiérrez, F., López, R., 2003, Geochronology and geochemistry of Grenvillian igneous suites in the northern Oaxacan Complex, southern Mexico: tectonic implications: Precambrian Research, 120, 365389. [ Links ]
Kostoglodov, V., Bandy, W., Domínguez, J., Mena, M., 1996, Gravity and seismicity over the Guerrero seismic gap, México: Geophysical Research Letters, 23, 33853388. [ Links ]
Levresse, G., Tritlla, J., Deloule, E., PintoLinares, P., 2007, Is there a Grenvillian basement in the GuerrerroMorelos Platform?: Geológica Acta, 5(2), 167175. [ Links ]
Luhr, J.F., 1997, Extensional tectonics and the diverse primitive volcanic rocks in the western Mexican Volcanic Belt: Canadian Mineralogist, 35, 473500. [ Links ]
Luhr, J.F., 2000, The geology and petrology of Volcan San Juan Nayarit, Mexico and the compositionally zoned Tepic Pumice: Journal of Volcanology and Geothermal Research, 95, 109156. [ Links ]
Luhr, J.F., 2001, Glass inclusions and melt volatile contents at Paricutin Volcano, Mexico: Contributions to Mineralogy and Petrology, 142, 261283. [ Links ]
Luhr, J.F., ArandaGómez, J.J., 1997, Mexican peridotite xenoliths and tectonic terranes: Correlations among vent location, texture, temperature, pressure, and oxygen fugacity: Journal of Petrology, 38(8), 10751112. [ Links ]
Macías, J.L., 2005, Geología e historia eruptiva de algunos de los grandes volcanes activos de México: Boletín de la Sociedad Geológica Mexicana, Volumen Conmemorativo del Centenario, Temas Selectos de la Geología Mexicana, 57(3), 379424. [ Links ]
Manea, V., Gurnis, M., 2007, Subduction zone evolution and low viscosity wedges and channels: Earth and Planetary Science Letters, 264, 2245. [ Links ]
Manea, V.C., Manea, M., Kostoglodov, V., Currie, C., Sewell, G., 2004, Thermal structure, coupling and metamorphism in the Mexican subduction zone beneath Guerrero: Geophysical Journal International, 158(2), 775784. [ Links ]
Manea, V.C., Manea, M., Kostoglodov, V., Sewell, G., 2005, Thermomechanical model of the mantle wedge in Central Mexican subduction zone and a blob tracing approach for the magma transport: Physics of the Earth and Planetary Interiors, 149(12), 165186. [ Links ]
Manea, V.C., Manea, M., Kostoglodov, V., Sewell, G., 2006, Intraslab seismicity and thermal stress in the subducted Cocos plate beneath central Mexico: Tectonophysics, 420(34), 389408. [ Links ]
Márquez, A., Oyarzún, R., Doblas, M., Verma, S., 1999, Alkalic (oceanicisland basalt type) and calcalkalic volcanism in the Mexican volcanic belt: A case for plumerelated magmatism and propagating rifting at an active margin?: Geology, 27(1), 5154. [ Links ]
MartínDel Pozzo, A.L., 1990, Geoquímica y paleomagnetismo de la Sierra Chichinautzin: México, D.F., Universidad Nacional Autónoma de México, tesis doctoral, 235 p. [ Links ]
MartínezSerrano, R., Schaaf, P., SolísPichardo, G., HernándezBernal, M., HernándezTreviño, T., MoralesContreras, J., Macías, J., 2004, Sr, Nd and Pb isotope and geochemical data from the Quaternary Nevado de Toluca volcano, a source of recent adakitic magmatism, and the Tenango Volcanic Field, México: Journal of Volcanology and Geothermal Research, 138(12), 77110. [ Links ]
McBirney, A.R., Taylor, H.P., Armstrong, R.L., 1987, Paricutin reexamined: a classic example of crustal assimilation in calcalkaline magma: Contibution to Mineralogy and Petrology, 95, 420. [ Links ]
Mena, M., de la Fuente, M., MoránZenteno, D., Espíndola, J.M., NúñezCornú, F., Medina, F., 1995, Anomalías gravimétricas y espesor de corteza en la región de Oaxaca, México: Geofísica Internacional, 34, 7991. [ Links ]
Mendoza, O.T., Suastegui, M.G., 2000, Geochemistry and isotopic composition of the Guerrero terrane (western Mexico): implications for the tectonomagmatic evolution of southwestern North America during the late Mesozoic: Journal of South American Earth Sciences, 13, 297324. [ Links ]
Meyer, R.P., Steinhart, J.S., Woollard, G.P., 1961, Central Plateau, 1957. Explosion studies of continental structure, in Steinhart, J.S., Meyer, R.P., (eds.): Carnegie Institution of Washington, Publication, 622, 199225. [ Links ]
MolinaGarza, R.S., OrtegaRivera, A., 2006, Chronostratigraphy and paleomagnetism of the Balsas Group of the TuzantlánCopalillo basin, northern Guerrero state, Mexico: Revista Mexicana de Ciencias Geológicas, 23(2), 215232. [ Links ]
MolinaGarza, R.S., UrrutiaFucugauchi, J.H., 1993, Deep crustal structure of central Mexico derived from interpretation of Bouguer gravity anomaly data: Journal of Geodynamics, 17(4), 181201. [ Links ]
Mori, L., GómezTuena, A., Cai, Y., Goldstein, S.L., 2007, Effects of prolonged flat subduction on the Miocene magmatic record of the central TransMexican Volcanic Belt: Chemical Geology, 244, 452473. [ Links ]
Nagasaki. A., Enami, M., 1998, Srbearing zoisite and epidote in ultrahigh pressure (UHP) metamorphic rocks from the SuLu province, eastern China: An important Sr reservoir under UHP conditions: American Mineralogist, 83, 240247. [ Links ]
Nance, R.D., Miller, B.V., Keppie, J.D., Murphy, J.B., Dostal, I., 2006, Acatlán Complex, southern Mexico: Record spanning the assembly and breakup of Pangea: Geology, 34, 857860. [ Links ]
Nelson, S.A., GonzálezCaver, E., Kyser, K.T., 1995, Constraints on the origin of alkaline and calcalkaline magmas from Los Tuxtlas volcanic field, Veracruz, México: Contribution to Mineralogy and Petrology, 122, 191211. [ Links ]
Nixon, T.G., 1989, The geology of Iztaccihuatl volcano and adjacent areas of the Sierra Nevada and the Valley of México: Geological Society of America, Special Paper, 219, 58 p. [ Links ]
OrtegaGutiérrez, F., 1975, The preMesozoic geology of the Acatlán area, south México: Leeds, University of Leeds, Ph.D. thesis, 166 p. [ Links ]
OrtegaGutiérrez, F., 1981, Metamorphic belts of southern Mexico and their tectonic significance: Geofísica Internacional, 20(3), 177202. [ Links ]
OrtegaGutiérrez, F., 1984, Evidence of Precambrian evaporites in the Oaxacan granulite complex of southern México: Precambrian Research, 23(34), 377393. [ Links ]
OrtegaGutiérrez, F., MitreSalazar, L. M., RoldanQuintana, J., SánchezRubio, G., De la Fuente, M., 1990, H3: Middle America TrenchOaxacaGulf of México, Geological Society of America, Centennial Continent/Ocean Transect, 14 p. [ Links ]
OrtegaGutiérrez, F., Ruiz, J., CentenoGarcía, E., 1995, Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic: Geology, 23, 11271130. [ Links ]
OrtegaGutiérrez, F., ElíasHerrera, M., ReyesSalas, M., MacíasRomo, C., López, R., 1999, Late OrdovicianEarly Silurian continental collisional orogeny in southern Mexico and its bearing on GondwanaLaurentia connections: Geology, 27, 719722. [ Links ]
OrtegaGutiérrez, F., ElíasHerrera, M., GómezTuena, AM., ReyesSalas, M., MacíasRomo, C., 2000, High grade gneisses xenoliths in the Chalcatzingo plug, State of Morelos: Shared Precambrian basement across the Guerrero and Mixteco terranes?: Actas INAGEQ, 6 (1), p. 150. [ Links ]
OrtegaGutiérrez, F., Martiny, B., SoléViñas, J., ReyesSalas, A. M., MoránZenteno, D.J., 2004, Discovery of xenocrystic sapphire and probable sapphirine pseudomorphs in the Puente Negro subvolcanic intrusion, Puebla, México (resumen): IV Reunión Nacional de Ciencias de la Tierra, Juriquilla, Qro., p. 49. [ Links ]
PantojaAlor, J., 1994, Formación Las Americas, Tlalpujahua, Michoacán, Méxicouna reinterpretación de los datos petrográficos, petroquímicos, tectónicos y de los mecanismos de erupción: Revista Mexicana de Ciencias Geológicas, 11, 168181. [ Links ]
Pardo, M., Suárez, G., 1993, Steep subduction geometry of the Rivera plate beneath the Jalisco block in western Mexico: Geophysical Research Letters, 20, 23912394. [ Links ]
Pardo, M., Suárez, G., 1995, Shape of the subducted Rivera and Cocos plates in southern México: seismic and tectonic implications: Journal of Geophysical Research, 100(B7), 1235712373. [ Links ]
Peacock, S.M., 1990, Fluid processes in subduction zones: Science, 248, 330337. [ Links ]
Peacock, S.M., van Keken, P.E., Holloway, S.D., Hacker, B.R., Abers, G.A., Fergason, R.L., 2005, Thermal structure of the Costa RicaNicaragua subduction zone: Physics of Earth and Planetary Interiors, 149(12), 187200. [ Links ]
Poli, S., Schmidt, M.W., 2002, Petrology of subducted slabs: Annual Review of Earth and Planetary Sciences, 30, 207235. [ Links ]
ProlLedezma, R., Juárez, G., 1985, Silica geotemperature mapping and thermal regime in the Mexican Volcanic Belt: Geofísica Internacional, 24, 609622. [ Links ]
Preston L.A., Creager K.C., Crosson R.S., Brocher, T.M., Trehu, A.M., 2003, Intraslab Earthquakes: Dehydration of the Cascadia Slab: Science, 302, 11971200. [ Links ]
Rapp, R.P., Watson, E.B., Miller, C.F., 1991, Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites: Precambrian Research, 51, 125. [ Links ]
Righter, K., Carmichael, I.S.E., 1993, Megaxenocrysts in alkali olivine basalts: Fragments of disrupted mantle assemblages: American Mineralogist, 78, 12301245. [ Links ]
Righter, K., RosasElguera, J., 2001, Alkaline lavas in the volcanic front of the western Mexican Volcanic Belt: Geology and petrology of the Ayutla and Tapalpa volcanic fields: Journal of Petrology, 42, 23332361. [ Links ]
Rudnick, R.L., Fountain, D.M., 1995, Nature and composition of the continental crust a lower crustal perspective: Reviews in Geophysics, 33(3), 267309. [ Links ]
Sachs, P.M., Hansteen, T.H., 2000, Pleistocene underplating and metasomatism of the lower continental crust: a xenolith study: Journal of Petrology, 41(3), 331336. [ Links ]
Schaaf, P., Stimac, J., Siebe, C., Macías, J.L., 2005, Geochemical evidence for mantle origin and crustal processes in volcanic rocks from Popocatepetl and surrounding monogenetic volcanoes, central Mexico: Journal of Petrology, 46(6), 12431282. [ Links ]
Schmidt, M.W., Poli, S., 1998, Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation: Earth and Planetary Science Letters, 163(14), 361379. [ Links ]
Sedlock, R.L., OrtegaGutiérrez, F, Speed, R.C., 1993, Tectonostratigraphic terranes and tectonic evolution of Mexico: Geological Society of America, Special Paper, 278, 153 p. [ Links ]
Sheth, H.C., TorresAlvarado, I.S., Verma, S.P., 2000, Beyond subduction and plume: A unified tectonicpetrogenetic model for the Mexican Volcanic Belt: International Geology Review, 42, 11161132. [ Links ]
Solari, L.A, Keppie, J.D., OrtegaGutiérrez, F., Cameron, K.L., Lopez, R., Hames, W.E., 2003, 990 Ma and 1,100 Ma grenvillian tectonothermal events in the northern Oaxacan Complex, southern México: Tectonophysics, 365, 257282. [ Links ]
Solari, L.A., Torres de León, R., HernándezPineda, G., Solé J., SolísPichardo, G., HernándezTreviño, T., 2007, Tectonic significance of CretaceousTertiary magmatic and structural evolution of the northern margin of the Xolapa Complex, Tierra Colorada area, southern México: Geological Society of America Bulletin, 119, 12651279. [ Links ]
Streck, M.J., Leeman, W.P., Chesley, J., 2007, Highmagnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt: Geology, 35, 351354; doi: 10.1130/G23286A.1. [ Links ]
Suárez G., Monfret, T., Wittlinger, G., David, C., 1990, Geometry of subduction and depth of the seismogenic zone in the Guerrero gap, Mexico: Nature, 345, 336338. [ Links ]
TalaveraMendoza, O., Ruiz, J., Gehrels, G.E., MezaFigueroa, D.M., VegaGranillo, R., CampaUranga, M.F., 2005. UPb geochronolgy of the Acatlán Complex and implications for the Paleozoic paleogeography and tectonic evolution of southern Mexico: Earth and Planetary Science Letters, 235, 682699. [ Links ]
Tassara, A., 2006, Factors controlling the crustal density structure underneath active continental margins with implications for their evolution: Geochemistry Geophysics Geosystems, 7, doi: 10.1029/2005GC001040. [ Links ]
UribeCifuentes, R.M., 2006, Xenolitos y volcanismo en la región de Valle de Santiago, Guanajuato Estudios paleomagnéticos y de magnetismo de rocas: México, D.F., Universidad Nacional Autónoma de México, tesis doctoral, 197 p. [ Links ]
UribeCifuentes, R.M. UrrutiaFucugauchi, J., 1995, Lowercrustal xenoliths from the Valle de Santiago maar field (abstracts): Crustal structure and tectonic implications: Geological Society of America Abstract and Programs, A392. [ Links ]
UrrutiaFucugauchi, J., FloresRuiz, J. H., 1996, Bouguer gravity anomalies and regional crustal structure in central México: International Geology Review, 38, 176194. [ Links ]
UrrutiaFucugauchi, J., UribeCifuentes, R.S., 1999, Lowercrustal xenoliths from the Valle de Santiago maar field, MichoacánGuanajuato volcanic field, central Mexico: International Geology Review, 41, 10671081. [ Links ]
UrrutiaFucugauchi, J., FloresRuiz, J.H., Bandy, C.A., MorteraGutiérrez, C.A., 1999, Crustal structure of the Colima rift, western México: gravity models revisited: Geofísica Internacional, 38, 205216. [ Links ]
Valdés, CM., Mooney, W., Singh, K., Meyer, S., Lomnitz, C., Luetgert, J., Hesley, C., Lewis, B., Mena, M., 1986, Crustal structure of Oaxaca, México, from seismic refraction measurements: Bulletin of Seismological Society of America, 76, 547563. [ Links ]
ValdésGonzález, C., and Meyer, R.P., 1996, Seismic structure between the Pacific coast and México City from the Petatlán earthquake (Ms=7.6) aftershocks: Geofísica Internacional, 35, 377 401. [ Links ]
Verma, S.P., 1999, Geochemistry of evolved magmas and their relationship to subductionunrelated mafic volcanism at the volcanic front of the central Mexican volcanic belt: Journal of Volcanology and Geothermal Research, 93, 151171. [ Links ]
Verma, S.P., 2000, Geochemistry of subducting Cocos plate and the origin of subductionunrelated mafic volcanism and the volcanic front of Central Mexican volcanic belt: Geological Society of America, Special Paper, 334, 195222. [ Links ]
Verma, S.P., 2002, Absence of Cocos plate subductionrelated basic volcanism in southern México: A unique case on Earth? Geology, 30(12), 10951098. [ Links ]
Verma, S.P., CarrascoNúñez, G., 2003, Reappraisal of the geology and geochemistry of Volcan Zamorano, Central Mexico: implications for discriminating the Sierra Madre Occidental and Mexican Volcanic Belt Provinces: International Geology Review, 45, 724752. [ Links ]
Wallace, P.J., Carmichael, I.S.E., 1999, Quaternary volcanism near the Valley of Mexico: Implications for subduction zone magmatism and the effect of crustal thickness variations on primitive magma compositions: Contributions to Mineralogy and Petrology, 135(4), 291314. [ Links ]
Weber, B., Köhler, H., 1999, SmNd, RbSr and UPb geochronology of a Grenville terrane in southern Mexico: origin and geological history of the Guichicovi Complex: Precambrian Research, 96, 245262. [ Links ]
Weber, B., Schaaf, P., Valencia, VA., Iriondo, A., OrtegaGutiérrez, F., 2006, Provenance ages of late Paleozoic sandstones (Santa Rosa Formation) from the Maya block, SE Mexico. Implications on the tectonic evolution of western Pangea: Revista Mexicana de Ciencias Geológicas, 23(3), 262276. [ Links ]
Wilcox, R., 1954, Petrology of Paricutin Volcano, Mexico: United States Geological Survey Bulletin, 965C, 281354. [ Links ]
Woollard, G.P., Caldera, J.M., Machesky, L., 1969, Gravity relations in México, in Contributions of the Hawaii Institute of Geophysics, University of Hawaii, technical report, 415422. [ Links ]
Ziagos, J.P., Blackwell, D.D., Mooser, E., 1985, Heat flow in southern Mexico and the thermal effects of subduction: Journal of Geophysical Research, 90, 54105420. [ Links ]