Central Trans-Mexican Volcanic Belt Research Articles

New constraints on the seismotectonics and crustal structure of the Eastern Trans-Mexican Volcanic Belt based on local seismicity, focal mechanisms and a preliminary state of stress evaluation

After the occurrence of unusual upper crustal seismic activity within the eastern sector of the Trans-Mexican Volcanic Belt (TMVB) in 2019, an area of generally low seismicity, an effort was made to interpret the origin. Concerns leading to the careful analysis presented here, were manifold: Shallow clustered seismicity can indicate active volcanism or related processes, as well as hydrothermalism. Additionally, the area was struck by a major earthquake (Mw 6.4) on January 4, 1920, which had devastating effects on the area's complex geological features and caused hundreds of deaths and significant material damage. As a result, several events of the 2019 sequence of unusual seismic activity have been relocated to precisely know the location. The 34 earthquakes of the 2019 seismic sequence occur in a relatively small area of less than 1600 km2. The seismic sequence can be further subdivided into 2 clusters, separated by the Pico de Orizaba – Cofre de Perote volcanic chain, which itself separates two tectonostratigraphic terranes, is a topographic divide and represents the limit to a considerable topographic drop. We also used the seismic sequence to constrain further the state of local stress and henceforth observe the tectonic regime of the area. For this purpose, focal mechanisms of the events with Mc > 4 (this sequence and previous data) were determined and the inversion of the stress tensor of this group was carried out. Similarly, the state of stress for the central zone of the TMVB was recalculated to establish its possible variations towards the eastern part of the TMVB. We show that the state of stress of the Eastern/Easternmost TMVB presents a transtensional regime, similar to the central TMVB, which has been suggested is mainly caused and controlled by the geometry of the subduction of the Cocos plate beneath the North American plate. A review of Rayleigh wave dispersion curves, calculated through interferometry of ambient seismic noise, a change is observed that suggests some variation in the structure of the eastern and easternmost areas. This contribution is relevant because little is known about the seismotectonics of the TMVB's eastern sector, in part because of the low cortical seismicity reported along the TMVB, the reduced coverage of seismic stations in this region and the distance from the trench Middle American Trench (MAT), where most of the important tectonic processes in Mexico take place. Recently, with the incorporation of a regional seismic network, the detection of small-magnitude events in the study area has significantly been improved.

Late Miocene NE-SW strike-slip fault system of the Mexico basin, central Trans-Mexican Volcanic Belt: Origin, deformation, and reactivation

A NE-SW fault system, observed in the northeast portion of the eastern sector of the Trans-Mexican Volcanic Belt, has been active since the Pliocene-Holocene time with dip-slip kinematics. These NE-SW faults belong to the Tenochtitlan fault system that extends from the southwest coast of Mexico to the NE zone of the Mexico basin. The length of this fault system suggests a complex deformation history, which is unknown before the Pliocene. In addition, the kinematics of the NE-SW faults and the tectonic conditions are unknown before their current activity. In this study, we investigate the geometry and kinematics of the NE-SW faults in the Miocene rocks in the central part of the Mexico basin (Guadalupe Range) for to know the kinematics of these faults during the Late Miocene time. Our results suggest that the Miocene rocks record two deformation events, one event is related to crustal shortening which produced a strike-slip activity in the NE-SW faults during the Late Miocene. The second deformation event is associated with crustal extension and the activity of the NE-SW faults with dip-slip kinematics. This extensional event was active during the Pliocene-Holocene. The re-activation analysis and our observations in the Guadalupe Range suggests that the NE-SW normal faults in the Tizayuca-Chignahuapan region are the product of the re-activation of previous NE-SW strike-slip faults. The change in the kinematics of the NE-SW faults explains the complex geometry of the damage zone and the high amount of the geological fractures in the Miocene rocks of the Mexico Basin. The activity type of the NE-SW faults is probably related to the dynamic of the subduction process in the southwest of Mexico, associated with the change in the inclination (decrease) and the convergence velocity of the Cocos Plate.

Late Miocene Ne-Sw Strike-Slip Fault System of the Mexico Basin, Central Trans-Mexican Volcanic Belt: Origin, Deformation, and Reactivation

Late Miocene Ne-Sw Strike-Slip Fault System of the Mexico Basin, Central Trans-Mexican Volcanic Belt: Origin, Deformation, and Reactivation

Active Crustal Deformation in the Trans‐Mexican Volcanic Belt as Evidenced by Historical Earthquakes During the Last 450 Years

AbstractThe Trans‐Mexican Volcanic Belt (TMVB) is oblique to the subduction zone. Background seismicity is very low. Nevertheless, two large instrumental earthquakes were recorded: the 1912 Acambay (Mw 6.9) and the 1920 Jalapa (Mw 6.4) earthquakes. Macroseismic data were systematically searched for crustal earthquakes on the TMVB contained in historical accounts. The macroseismic reports were inverted for the best fitting magnitude and epicenter. The western end of the TMVB shows the highest level of seismic activity. An earthquake is reported in 1568 (Mw 7.2 ± 0.2) and a second large event took place in 1875 (Mw 7.0 ± 0.2). Two other earthquakes are reported in the western TMVB in 1611 and 1749 (Mw 6.4). The 1858 earthquake (Mw 7.6 ± 0.3), in the central TMVB occurred on faults located to the west of the 1912 Acambay event. The eastern TMVB is less active. An earthquake in 1575 (Mw 5.7) occurred east of the city of Puebla. The 1887 (Mw 6.2 ± 0.2) and the 1920 Jalapa earthquakes are the larger events in this region. The seismicity observed from the instrumental and historical seismicity catalogs show that active tectonic deformation occurs throughout the TMVB and that it is not confined to specific regions where instrumental earthquakes have been located and active faults mapped. The number of large earthquakes in the TMVB, suggest unique large‐scale deformation of this complex volcanic belt. The presence of large earthquakes in this tectonic province, where 40% of the population of Mexico live, underlines the importance of reevaluating hazard in this densely populated region.

Geology and stratigraphy of the Mexico Basin (Mexico City), central Trans-Mexican Volcanic Belt

ABSTRACT We present a new geological map of the Mexico Basin (Mexico City) based on field descriptions, a compilation of data from previous publications, and new 40Ar/39Ar geochronology data. The oldest rocks described in the Mexico Basin are Cretaceous limestones, overlaid by Oligocene (26.0 Ma) and Miocene (22.8–5.0 Ma) volcanic successions, followed by Pliocene-Pleistocene (3.7–1.2 Ma) to Recent volcanic rocks around the basin. The Mexico basin is surrounded by volcanic ranges mainly andesitic and dacitic in composition: Sierra de Guadalupe (∼20 to ∼13 Ma), Sierra de las Cruces (3.7–0.03 Ma), Sierra Nevada (1.4 Ma to Recent), and Sierra de Chichinautzin (1.2 Ma to Recent). The basin has formed and evolved through complex tectonic and volcanic events: A NNW and NNE-Cañón de Lobos trending reverse fault affected the Cretaceous basement, the NW-SE Mixhuca normal fault displaced Oligocene-Miocene volcanics, the NE-SW Tenochtitlan fault system displaced Plio-Pleistocene rocks, and finally E-W normal faults affected the most recent volcanic rocks, paleosols, and lacustrine sediments.

Biohermal thrombolites of the crater lake Rincón de Parangeo in Central México

This work describes the distribution, macro and microstructures of modern microbialites deposited in the marginal facies of a Quaternary paleo-lake at Rincon de Parangueo, within the central trans-mexican volcanic belt. This paleo-lake formed after the volcanic eruption and maar formation that provided suitable conditions at its margins for substrate colonization and microbial proliferation. Vesicular thrombolites, grouped into domical and irregular clusters, were mapped, measured and analyzed. These microbialites are characterized by a 2 cm-thick stromatolitic crust and a thrombolitic matrix that accreted by carbonate precipitation. The stromatolitic crusts represent the last accretion stages at intertidal conditions before the complete evaporation of the paleo-lake. The asymmetrical distribution and preservation of these microbial deposits suggest that the rapid subsidence and tectonic deformation began before the lake completely dried out. The maar at Rincon de Parangeo represents not only an important record of recent geological processes in dynamic interaction with microbial processes under increasingly extreme alkaline conditions (pH between 10 and 10.6), but also a possible analogue for some Precambrian sequences. It provides an example of a rapidly transforming environment where macro and micro geobiological interactions directly influence biological succession.

Geochemical and isotopic composition of volcanic rocks of the heterogeneous Miocene (~ 23–19 Ma) Tepoztlán Formation, early Transmexican Volcanic Belt, Mexico

We present the first geochemical data (major and trace elements, as well as Sr, Nd, and Pb isotopes) on volcanic rocks from the Tepoztlán Formation in the central Transmexican Volcanic Belt. The Tepoztlán Formation is up to 800m thick and comprises a wide range of primary volcanic rocks (lavas, pyroclastic density current deposits, pyroclastic fall deposits), and their secondary reworked products due to mass flow (lahars) and fluvial processes. Magnetostratigraphy combined with K/Ar and Ar/Ar geochronology suggests an age of Early Miocene (23–19Ma) for this Formation. Lava flows, pyroclastic rocks, dykes and volcanic clasts range from basaltic andesite to rhyolite, with a clear dominance of andesites and dacites. All samples are subalkaline and hy-normative. These rocks show homogeneous REE patterns with LREE enrichment and higher LILE concentrations with respect to HFSE, notably the typical negative anomaly of Nb, Ta, and Ti, suggesting a subduction-related magma genesis. Major and trace element concentrations show that either assimilation of heterogeneous continental crust or crustal recycling by subduction erosion and fractional crystallization are important processes in the evolution of the Tepoztlán Formation magmas. Isotopic compositions of the Tepoztlán Formation samples range from (87Sr/86Sr)t=0.703693 to 0.704355 and (143Nd/144Nd)t=0.512751 to 0.512882, falling within the mantle array. All geochemical characteristics indicate that these rocks originated from a heterogeneous mantle, modified and evolved through assimilation of country rock and fractional crystallization in the upper crust.

Holocene paleo-earthquakes recorded at the transfer zone of two major faults: The Pastores and Venta de Bravo faults (Trans-Mexican Volcanic Belt)

We present evidence of five late Holocene earthquake ruptures observed at two paleoseismological trenches in the Laguna Bani sag pond (Trans-Mexican Volcanic Belt, central Mexico). The trenches exposed two fault branches of the western termination of the Pastores fault, one of the major fault systems within the central Trans-Mexican Volcanic Belt. The site was studied by combining geomorphological and structural approaches, volcanic mapping, ground-penetrating radar, and paleoseismological analysis. The study revealed that coseismic surface rupture was noncharacteristic, and that the exposed fault branches had not always moved simultaneously. The fault tip has ruptured at least 5 times within the past 4 k.y., and the rupture events followed and preceded the deposition of an ignimbrite. The close temporal relationship of the seismic rupture with the volcanic activity of the area could be the result of volcanism triggered by faulting and its associated seismicity. The relatively high recurrence of seismic events (1.1–2.6 k.y.) and the noncharacteristic fault behavior observed at this tip of the Pastores fault suggest that the fault might have been active as a primary fault rupturing along segments of variable length or depth, and/or that the fault ruptured eventually as a secondary fault. The secondary ruptures would likely be related to earthquakes produced at major neighboring faults such as the Acambay fault, which moved during the 1912 Acambay earthquake, or the Venta de Bravo fault. A relatively large slip rate estimated for this fault branch (0.23–0.37 mm/yr) leads us to contemplate the possible connection at depth between the Pastores and the Venta de Bravo faults, increasing the maximum expected magnitude for central Mexico.

Petrology and geochemistry of the Valle de Santiago lower-crust xenoliths: Young tectonothermal processes beneath the central Trans-Mexican volcanic belt

We present a comprehensive petrologic study of lower-crust mafic and felsic xenoliths hosted by Quaternary alkaline basalts of the Valle de Santiago monogenetic volcanic field. This is the only locality along the entire Trans-Mexican volcanic belt where the abundance and size of xenoliths allow the understanding in great detail of processes associated with interactions of young subduction-related magmatism and the deep continental crust. Mafic xenoliths (two pyroxene ± spinel granulites and metanorthosites), olivine-rich gabbroic xenoliths, and transitional xenoliths compose the bulk of the population, although a few belong to the charnockitic suite (enderbite and faersundite). Thermobarometric calculations (two-pyroxene, ilmenite-magnetite, Ti-in amphibole, amphibole-plagioclase, and phase equilibria in the system NCMAS (Na-Ca-Mg-Si)) result in pressures around 9 kbar and temperatures of 1000–1100 °C for the granulite-facies metamorphism, which would give a very hot lower crust, ∼33 km thick, beneath Valle de Santiago and a mean geothermal gradient of ∼30 °C/km. Igneous zircons (Th/U = 0.03–0.87) extracted from one of the felsic granulites yielded a major peak of latest Cretaceous age (67.1 Ma), interpreted as the crystallization age of the granitic protolith, without inheritance from Precambrian or Paleozoic crust. Minor peaks at 45.1 and 25.5 Ma are interpreted as partial Pb losses from some of the Cretaceous zircons. Trace-element geochemistry, as well as Sr, Nd, and Pb isotopic studies performed on two granulites, is consistent with the juvenile and coeval nature of both the mafic metagabbroic xenoliths and the alkaline basaltic magmas that lifted the xenoliths from the lower crust. Two intermediate stages in the thermal evolution of the sampled xenoliths include the emplacement at different depths of volatile and K-Fe-Ti–rich oxidized melts represented by igneous assemblages with kaersutite, biotite, titanomagnetite, spinel, plagioclase, Fe-rich epidote, clinopyroxene, fayalitic olivine, and glasses that pervasively invaded most granulite xenoliths before being taken to the surface. A preferred plumbing system model is presented depicting a protracted Miocene to Quaternary basaltic intraplate magmatic system that sampled former basaltic batches stationed in the lower crust, together with the Late Cretaceous deep-seated granitoids beneath Valle de Santiago in the backarc of the central Trans-Mexican volcanic belt. Both components were later subjected to granulite-facies conditions in the lower crust, most probably related to the continued heating of the crust by basaltic magmas underplated in the central Trans-Mexican volcanic belt backarc region.

Reply to "Comment on 'Estimation of Ground Motion in Mexico City from a Repeat of the M 7.0 Acambay Earthquake of 1912' by S. K. Singh, A. Iglesias, M. Ordaz, X. Perez-Campos, and L. Quintanar" by M. Suter

Prediction of ground motion in Mexico City from postulated earthquakes in the central trans‐Mexican volcanic belt (CTMVB) is clearly important in the estimation of seismic risk. Unfortunately, moderate seismicity and sparse seismic network in the region makes the task difficult. Few useful recordings of CTMVB earthquakes are available in the near‐source region and in the Valley of Mexico. Deterministic modeling of wavefield, reliable at frequencies of interest, remains a challenge due to the complex and mostly unknown crustal structure of the CTMVB and the Valley of Mexico in which the city is located. Even reliable determination of the magnitude of small and moderate earthquakes in the CTMVB from regional recordings is problematic due to the large site effect. Singh et al. (2011) provide an estimation of ground motion in the city from a postulated M w 7 earthquake in the Actopan region, about 120 km to the north of the city. For reasons mentioned by Singh et al. (2011), this estimation may also be valid for the M ∼7 Acambay earthquake of 1912. The estimation is based on the empirical Green’s function (EGF) technique and uses seismograms of two small earthquakes that occurred in the Actopan area. These two events were recorded at a …

Experimental constraints on the origins of primitive potassic lavas from the Trans-Mexican Volcanic Belt

Primitive magmas in the Trans-Mexican Volcanic Belt (TMVB) span a wide geochemical range that includes calc-alkaline basalt and basaltic andesite, potassic shoshonites, and intraplate alkaline basalts, indicating that the subarc mantle wedge is chemically heterogeneous. The aim of this study is to experimentally constrain the origins of potassic lavas that have erupted along the volcanic front in the TMVB. We used a piston-cylinder apparatus to determine the P–T–H2O near-liquidus phase relations for two primitive potassic lavas: a hornblende trachybasalt (shoshonite) from Cerro La Pilita in the central TMVB and a high-K calc-alkaline basalt from Ayutla in the western TMVB. Experiments were conducted at mantle pressures (0.8–2.5 GPa) and temperatures (1,100–1,400 °C) with 1.5–6 wt% H2O. Results show that both samples were last equilibrated with an olivine + clinopyroxene assemblage at upper mantle pressures. Integrating our results with trace element characteristics, we conclude that the potassic magmas formed by a complex, multistage process in which melts from the hottest part of the mantle wedge either reequilibrated with clinopyroxene-rich veins in the shallow upper mantle or caused melting of such veins by advective heating. We combine our results with previous experiments on TMVB lavas to provide an along-arc perspective of melt equilibration depths in the mantle wedge. The results suggest that although melts may initially form deep in the wedge, they commonly reequilibrate with heterogeneous mantle at shallower depths. Primitive, medium-K basaltic andesites in the TMVB form by reequilibration with harzburgite, which we infer to be a common lithology in the upper mantle, whereas some potassic magmas like the ones studied here form through reequilibration with or melting of veins of olivine + clinopyroxene ± phlogopite. Though somewhat rare at the volcanic front relative to the more abundant medium-K volcanic rocks, the potassic magmas are an important lava type for revealing mantle chemical heterogeneities.

A source study of the October, 2007 earthquake sequence of Morelia, Mexico

En este artículo se analiza una secuencia de siete sismos (2.5<Mw<3.0) ocurridos en la Ciudad de Morelia, México. Esta serie de temblores ocurrieron en un intervalo de 33 horas en el mes de octubre de 2007. Fueron registrados por dos estaciones locales ubicadas en esa Ciudad. Morelia se encuentra en la la parte central de la Faja Volcánica Trans-Mexicana (CTMVB, por sus siglas en inglés). Las formas de onda y los espectros de estos sismos son sorprendentemente similares, sugiriendo que sus localizaciones y mecanismos focales son casi idénticos. La inversión de forma de onda, restringida a partir de fallas descritas anteriormente en el área (rumbo ~E-O, buzando al norte), arroja un mecanismo focal definido por , y , lo cual es consistente con los mecanismos focales reportados previamente en la región. Dado que, para estos pequeños eventos, la señal se confunde con el ruido para frecuencias f<0.2Hz, se estimó el momento sísmico a partir del espectro de las ondas S en una banda de frecuencias definida en el intervalo 0.2≤f≤1Hz. Sin embargo, en esta banda de frecuencias, existe una amplificación significativa de las ondas símicas debida a una capa de baja velocidad provocada por rocas volcánicas superficiales presentes en cualquier sitio localizado en el CTMVB. En la estimación del y en la interpretación de los espectros observados, se aproximó esta amplificación usando el cociente espectral H/Z. Asumiendo un modelo de fuente , los espectros observados pueden ser explicados con ternas (Δσ, t*, ) (5MPa, 0.02s, 20Hz) y (20 MPa, 0.03 s, 20 Hz), donde Δσ es la caída de esfuerzos asumiendo el modelo de Brune y t* y .son los parámetros de atenuación. Con el fin de simular el movimiento fuerte del terreno, para un sismo postulado de , se usaron estas combinaciones de parámetros junto con las técnicas de Empirical Green Function (EGF) y Random Vibration Theory (RVT). Las aceleraciones horizontales PGA y velocidades PGV en los sitios de referencia están en el rango de 23 a 46 cm/ y de 1.5 a 3.52 cm/s para una caída de esfuerzos de Δσ=5Mpa. Los valores pronosticados para una caída de esfuerzos Δσ=20Mpa son casi el doble (44-89 cm/ and 2.5-6.1 cm/s). Las estimaciones obtenidas, especialmente para Δσ=5MPa, son considerablemente más pequeñas que las reportadas a partir de datos globales. Esta comparación sugiere que existe una alta atenuación en la región volcánica o una inadecuada estimación del efecto de t* y .

Experimental insights into the formation of high-Mg basaltic andesites in the trans-Mexican volcanic belt

High-Mg basaltic andesites and andesites occur in the central trans-Mexican volcanic belt, and their primitive geochemical characteristics suggest equilibration with mantle peridotite. These lavas may represent slab melts that reequilibrated with overlying peridotite or hydrous partial melts of a peridotite source. Here, we experimentally map the liquidus mineralogy for a high-Mg basaltic andesite (9.6 wt% MgO, 54.4 wt% SiO2, Mg# = 75.3) as a function of temperature and H2O content over a range of mantle wedge pressures. Our results permit equilibration of this composition with a harzburgite residue at relatively high water contents (>7 wt%) and low temperatures (1,080–1,150°C) at 11–14 kbar. However, in contrast to the high Ni contents characteristic of olivine phenocrysts in many such samples from central Mexico, those of olivine phenocrysts in our sample are more typical of mantle melts that have fractionated a small amount of olivine. To account for this and the possibility that the refractory mantle source may have had olivine more Fo-rich than Fo90, we numerically evaluated alternative equilibration conditions, using our starting bulk composition adjusted to be in equilibrium with Fo92 olivine. This shifts equilibration conditions to higher temperatures (1,180–1,250°C) at mantle wedge pressures (11–15 kbar) for H2O contents (>3 wt%) comparable to those analyzed in olivine-hosted melt inclusions from this region. Comparison with geodynamic models shows that final equilibration occurred shallower than the peak temperature of the mantle wedge, suggesting that basaltic melts from the hottest part of the wedge reequilibrated with shallower mantle as they approached the Moho.

Estimation of Ground Motion in Mexico City from a Repeat of the M 7.0 Acambay Earthquake of 1912

Abstract The Acambay earthquake of 1912 ( M ∼7.0), which occurred in the central Trans-Mexican Volcanic Belt (CTMVB) about 100 km west–northwest of Mexico City, has been thought to be a critical scenario event in the estimation of seismic hazard of the city. We use seismograms of two small earthquakes located near Actopan (15 December 2003; 18 May 2010) and recorded at station CUIG, a hill-zone site in Mexico City, as empirical Green’s functions (EGFs) for the Acambay region. Because Actopan, like Acambay, is situated in the CTMVB and both are located at about the same distance from CUIG, the use of the recordings of the Actopan earthquakes as EGFs is partly justified. We first analyze data of the two small earthquakes at a local station, DHIG, to estimate their seismic moment and stress drop. As there is considerable uncertainty in the estimated stress drops of the two events, we choose a reasonable range of values for them and apply a technique of random summation of EGFs to simulate ground motion at CUIG from a postulated M w 7 earthquake. The estimated geometric mean horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) at CUIG range from 2.7 to 9.7 cm/s 2 and from 1.1 to 3.0 cm/s, respectively. Ground-motion maps for the entire city are presented using a simulated trace at CUIG and the known transfer functions of many sites within the city. The results are consistent with reported seismic intensities in Mexico City, and PGA and peak ground displacement (PGD) at a 2-s period at the seismic station of Tacubaya during the Acambay earthquake. Estimated ground motions suggest that a repeat of the event does not present significant hazard to Mexico City.

The southern Mexico block: main boundaries and new estimation for its Quaternary motion

AbstractNumerous studies, mainly based on structural and paleomagnetic data, consider southern Mexico as a crustal block (southern Mexico block, SMB) uncoupled from the North American plate with a southeast motion with respect to North America, accommodated by extension through the central Trans-Mexican volcanic belt (TMVB). On the other hand, the accommodation of this motion on the southeastward boundary, especially at the Cocos–Caribbean–North American triple junction, is still debated.The boundary between the SMB and the North American plate is constituted by three connected zones of deformation: (1) left-lateral transtension across the central TMVB, (2) left-lateral strike-slip faulting along the eastern TMVB and Veracruz area and (3) reverse and left-lateral strike-slip faulting in the Chiapas area. We show that these three active deformation zones accommodate a counterclockwise rotation of the SMB with respect to the North American plate. We specially discuss the Quaternary motion of the SMB with respect to the surrounding plates near the Cocos–Caribbean–North American triple junction. The model we propose predicts a Quaternary counterclockwise rotation of 0.45°/Ma with a pole located at 24.2°N and 91.8°W.Finally we discuss the geodynamic implications of this counterclockwise rotation. The southern Mexico block motion is generally assumed to be the result of slip partitioning at the trench. However the obliquity of the subduction is too small to explain slip partitioning. The motion could be facilitated by the high thermal gradient and gravitational collapse that affects central Mexico and/or by partial coupling with the eastward motion of the Caribbean plate.

The origin of a primitive trondhjemite from the Trans-Mexican Volcanic Belt and its implications for the construction of a modern continental arc

A remarkable suite of Miocene high-silica trondhjemites discovered in the central Trans-Mexican Volcanic Belt indicates that slab melts can ascend through the mantle and crust while suffering only minor compositional modifications. Despite carrying an assortment of deep crustal xenoliths, the trondhjemites preserve the most depleted isotopic compositions ever measured in the Mexican arc, with values that are nearly identical to those of the Pacific mid-oceanic-ridge basalts. These rocks also have high Sr/Y ratios, and extremely fractionated heavy rare earth element patterns at relatively high Mg number (Mg#), features that are all consistent with melts from the subducted oceanic crust that had only limited interaction with mantle peridotite during ascent. Nonetheless, modeling results indicate that these unusual geochemical features can be modified by more extensive mantle assimilation, resulting in compositions that could match those of more typical intermediate rocks from Mexico. The data thus indicate that the slab melt component uniquely recorded by the Miocene trondhjemites represents a likely constituent for most volcanic sequences of the Mexican arc, and suggest that a modern andesitic continental crust can be constructed directly from mantle-modified slab melts without a basaltic precursor.

Effects of prolonged flat subduction on the Miocene magmatic record of the central Trans-Mexican Volcanic Belt

Temporal changes in the chemical compositions of middle to late Miocene rocks from the central Trans-Mexican Volcanic Belt elucidate how prolonged flat subduction influences arc magmatism. These are recorded in the Palo Huérfano–La Joya–Zamorano Volcanic Complex (PH–LJ–Z; 12–10 Ma), a group of andesitic to dacitic stratovolcanoes located at ∼ 500 km from the trench; and in the Querétaro Volcanic Succession (QVS; 9–6 Ma), a basaltic to basaltic–andesitic plateau that stratigraphically overlies the stratovolcanoes. The two rock-groups display trace element patterns that are typical of arc magmas, but the PH–LJ–Z suite shows higher Sr/Y and Gd/Yb ratios than the QVS, and a few PH–LJ–Z samples also display Sr isotopic ratios that are similar to those of the East Pacific Rise mid-ocean ridge basalts. These features are typical of adakitic magmas. In contrast, QVS rocks display an overall weaker subduction signature (e.g. lower Ba/Nb ratios), do not show adakite features, and have lower SiO 2/(MgO + Fe 2O 3 tot) ratios at equivalent Mg# (∼ 40–70), MgO and Na 2O contents than the PH–LJ–Z suite. Since Fe in arc magmas is a sensitive proxy of water content or melting pressure, and Na may reflect the thickness of the mantle column that controls the extent of melting, the overall chemical differences of both suites can only be reconciled if they equilibrated with the mantle wedge at roughly the same pressures but with different amounts of dissolved water. Once formed, both magmatic suites also suffered contamination at different crustal levels during ascent, but their primary compositional differences are mainly related to different mechanisms of element recycling that occurred without major changes in the local tectonic configuration. The adakite features of PH–LJ–Z rocks, and their emplacement at a large distance from the trench, are consistent with a sub-horizontal subduction geometry that favored slab melting at relatively low pressures. On the other hand, a protracted flat subduction will most likely produce a gradually cooler thermal structure in the subduction zone, hindering slab melting, and instead favoring slab dehydration and fluid fluxing of the mantle wedge as a mechanism to form the more typical arc volcanics of the QVS.

A Shallow Crustal Earthquake Doublet from the Trans-Mexican Volcanic Belt (Central Mexico)

The Trans-Mexican volcanic belt is an active volcanic arc related to subduction along the Middle America trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc-parallel faulting. The Mezquital graben is a major intra-arc basin of the central Trans-Mexican volcanic belt. A doublet of moderate shallow shocks occurred in March and October 1976 in the region of this graben. These earthquakes were recorded by the Mexican National Seismological network, in particular, by the Bosch-Omori seismograph ( T = 18 sec) at the Tacubaya Observatory in Mexico City. We have carefully relocated the two main shocks and their major aftershocks by reading the original records and using a modified crustal velocity model for this region. A difference of ∼50 km is observed between the locations reported by the Mexican Seismological Service and those obtained in this study, which are additionally supported by the damage distribution of these earthquakes. A first-motion analysis, based on regional and teleseismic records, defines for the March and October shocks normal fault mechanisms, characterized by east–west-striking fault planes, which coincides with the orientation of the master faults of the Mezquital graben. After calculating the instrumental response, the source parameters were obtained from the Bosch-Omori seismograph records by body-wave modeling. For the March earthquake, we estimate a seismic moment of 4.5 × 1023 dyne cm (equivalent to M w 5.0) and a stress drop of 0.7 MPa, assuming a circular rupture model (radius, 3 km). Given the poor quality of the Bosch-Omori record for the October earthquake, we used comparison with the March event of long-period ( T = 20 sec) teleseismic records at two stations to obtain corresponding source parameters for the October event. By assuming a similar stress drop as for the March event, we obtain a M of 5.6 × 1023 dyne cm and M w 5.1 with a rupture length of 6.5 km. According to gravity data, the regional east–west faults are longer than reported. In particular, our detailed measurements indicate that the master fault of the Mezquital graben continue westward in the subsurface below the basin fill. In a more regional context, additional magnetic data suggest that the El Mezquital graben and Aljibes half-graben might be connected at their southern limits by part of the same fault system. This is the first earthquake doublet reported from central Mexico and the biggest seismic event in that zone during the past 50 years. From a physical point of view, it is possible that the seismic energy released by the March event was partially transferred toward the east, triggering the October shock further east on the same fault system. The presence of moderate seismicity in this zone should therefore be taken into account when assessing its seismic hazard.

Active fault systems and tectono-topographic configuration of the central Trans-Mexican Volcanic Belt

The central Trans-Mexican Volcanic Belt (TMVB) reflects the interplay between three regional fault systems: the NNW–SSE to NW–SE striking Taxco–Querétaro fault system, the NE–SW striking system, and the E–W striking Morelia–Acambay fault system. The latter is the youngest and consists of fault scarps up to 500 m high, whose formation caused structural and morphological reorganization of the region. In this paper, we investigate possible activity of the three systems within the central TMVB, and assess the role that they play in controlling the tectono-topographic configuration of the area. Our study is based on DEM-derived morphometric maps, longitudinal river profiles, geomorphologic mapping, and structural field data concerning recent faulting. We find that all three regional fault systems are active within the central TMVB, possibly with different displacement rates and/or type of motion; and that NNW–SSE and NE–SW striking faults control the major tectono-topographic elements that build up the region, which are being re-shaped by E–W striking faults. We also find that tectonic information can be deciphered from the topography of the youthful volcanic arc in question, regardless its complexity.

Magma degassing and basaltic eruption styles: a case study of ∼2000 year BP Xitle volcano in central Mexico

To investigate the relationship between volatile abundances and eruption style, we have analyzed major element and volatile (H 2O, CO 2, S) concentrations in olivine-hosted melt inclusions in tephra from the ∼2000 yr BP eruption of Xitle volcano in the central Trans-Mexican Volcanic Belt. The Xitle eruption was dominantly effusive, with fluid lava flows accounting for ∼95% of the total dense rock erupted material (1.1 km 3). However, in addition to the initial, Strombolian, cinder cone-building phase, there was a later explosive phase that interrupted effusive activity and deposited three widespread ash fall layers. Major element compositions of olivine-hosted melt inclusions from these ash layers range from 52 to 58 wt.% SiO 2, and olivine host compositions are Fo 84–86. Water concentrations in the melt inclusions are variable (0.2–1.3 wt.% H 2O), with an average of 0.45±0.3 (1σ) wt.% H 2O. Sulfur concentrations vary from below detection (∼50 ppm) to 1000 ppm but are mostly ≤200 ppm and show little correlation with H 2O. Only the two inclusions with the highest H 2O have detectable CO 2 (310–340 ppm), indicating inclusion entrapment at higher pressures (700–900 bars) than for the other inclusions (≤80 bars). The low and variable H 2O and S contents of melt inclusions combined with the absence of less soluble CO 2 indicates shallow-level degassing before olivine crystallization and melt inclusion formation. Olivine morphologies are consistent with the interpretation that most crystallization occurred rapidly during near-surface H 2O loss. During cinder cone eruptions, the switch from initial explosive activity to effusive eruption probably occurs when the ascent velocity of magma becomes slow enough to allow near-complete degassing of magma at shallow depths within the cone as a result of buoyantly rising gas bubbles. This allows degassed lavas to flow laterally and exit near the base of the cone while gas escapes through bubbly magma in the uppermost part of the conduit just below the crater. The major element compositions of melt inclusions at Xitle show that the short-lived phase of renewed explosive activity was triggered by a magma recharge event, which could have increased overpressure in the storage reservoir beneath Xitle, leading to increased ascent velocities and decreased time available for degassing during ascent.