Tuxtlas Volcanic Field Research Articles

The crustal structure on the southern continental margin of the Gulf of Mexico, as inferred by inverse modeling of gravity data

In order to infer the crustal structure in the southern continental margin of the Gulf of Mexico, we performed inverse modeling of isostatic residual gravity data, to obtain a three-dimensional model of density contrast in a region that includes the Los Tuxtlas Volcanic Field, the Veracruz basin and partially the Salina del Istmo basin. As a result of the modeling, the calculated gravity anomaly fits well with the observed data. The model suggests that the Los Tuxtlas Volcanic Field evolved on the verge of a dense body, whose roof is 15 km deep and has a thickness of 10 km, it extends towards the north into the Gulf of Mexico, and it is interpreted as oceanic crust. The southern rim of this body possibly represents the location of the Western Main Transformation Fault, related to the opening of the Gulf of Mexico during the Jurassic, although this inference is speculative. On the inferred basement, possibly constituted by Cretaceous carbonates, the model shows a large sedimentary basin that accommodates Cenozoic siliciclastic sediments with a thickness of up to 10 km, has geometric characteristics related to some known tectonic features, such as the Buried Tectonic Front, in addition to a syncline-type structure, with basal surface at 6 km depth, which accommodates the Western Homocline and the eastern edge of the Tlacotalpan Syncline. The model shows deformed strata that thin towards the tectonic front of the Sierra Zongolica, surrounding the Alto de Anegada and the Los Tuxtlas Volcanic Field, as well as some features related to the Coatzacoalcos Reentrant.

Geochemistry of marine sediments adjacent to the Los Tuxtlas Volcanic Complex, Gulf of Mexico: Constraints on weathering and provenance

The provenance of GoM45 and GoM63 core sediments recovered in the deep-sea area (∼1666 m–1672 m water depth, respectively), in front of the Los Tuxtlas volcanic field, Gulf of Mexico was analyzed based on the textural, mineralogical, and geochemical variations. The radiocarbon age data of Globorotalia menardii group foraminiferal species varied from 17,709 to 20,711 calibrated years B.P for the sediment core GoM45, which represent Pleistocene/Holocene boundary. The core sediments were characterized by detrital components with enriched SiO2, Al2O3, and Fe2O3 concentrations. The weathering indices like Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA) revealed a low intensity of weathering for the core sediments.The V/Cr, Ni/Co, Cu/Zn, and V/Cr elemental ratios indicated an oxic depositional environment for the Gulf of Mexico deep sea area. In addition, the geochemistry data indicated a passive margin setting, which is consistent with the geology of the Gulf of Mexico. The trace and rare earth element concentrations, and Eu anomaly indicated that the sediments were derived from the natural erosion of andesitic and basaltic rocks in the Los Tuxtlas region, SW Gulf of Mexico. Los Tuxtlas region is partly influenced by the Papaloapan and Coatzacoalcos Rivers, which discharges into the SW Gulf of Mexico.

Evidence of hydration of the peridotite mantle wedge recorded in low-CaO olivines from Los Tuxtlas Volcanic Field, Veracruz, Mexico

Los Tuxtlas Volcanic Field (LTVF) is an isolated volcanic center located between the Trans-Mexican Volcanic Belt and the Central America Volcanic Arc. Its volcanic products include alkaline and sub-alkaline rocks with variable subduction signatures, some of them considered as high-MgO rocks and some also include ultramafic xenoliths. Most suites show MgO contents ranging from 11 to 16 wt%, reflecting peridotite-derived melts with a primitive character (Mg# ≥ 66) that suffered crystal settling in magma batches during its ascent from the mantle to surface. Parental magma compositions were reconstructed using the significant forsterite compositions (Fo88 to Fo83) to estimate magmatic temperatures that range from 1095 to 1372 °C. Reconstructed parental magmas and low-CaO magmatic olivines allowed us to determine two main groups with water content between 4 and 6% and 8–9%. These magmatic parameters are consistent with those inferred by thermal models, supporting that magmatic products are derived from the melting of a mantle-wedge peridotite at relatively high temperature and wet conditions. Isotopic compositions point to the existence of a high-μ signature (HIMU; μ = 238U/204Pb) in the mantle source similar to that identified at a regional scale from northern Mexico to South America. Low-CaO olivines and their use as a proxy for magmatic H2O content provide evidence of hydration of the mantle wedge by fluids derived from the slab. Hydrous slab-derived contribution induces a higher extent of melting and formation of wet magmas with high LILE/HFSE ratios. Relatively high-NiO olivine compositions record slight metasomatism of the mantle wedge by a small amount of silicic-rich slab components and the contribution of larger proportions of peridotite mantle to the melt over the slightly metasomatized mantle domains. In general, most of the magmatic products record stages of melting and metasomatism triggered and induced by fluids and melts of the slab into an enriched mantle peridotite wedge.

Morphology and distribution of monogenetic volcanoes in Los Tuxtlas Volcanic Field, Veracruz, Mexico: implications for hazard assessment

Morphology and distribution of monogenetic volcanoes in Los Tuxtlas Volcanic Field, Veracruz, Mexico: implications for hazard assessment

Crustal and Upper‐Mantle Structure Below Central and Southern Mexico

AbstractThe Mexican subduction zone is a complex convergent margin characterized by a slab with an unusual morphology, an abnormal location of the volcanic arc, and the existence of slow earthquakes (slow slip events and tectonic tremors). The number of seismic imaging studies of the Mexican subduction zone has increased over the past decade. These studies have revealed the seismic wave velocity structure beneath Central and Southern Mexico. However, the existing tomographic models are either confined to specific areas or have coarse resolution. As a consequence, they do not capture the lateral and finer‐scale variations in the mantle and the crust structure. Here, we fit frequency‐dependent traveltime differences between observed and synthetic seismograms in a three‐dimensional model of Central and Southern Mexico to constrain crustal and upper mantle seismic velocity structures jointly. We use ∼3,300 seismic records, filtered between 5 and 50 s, from 74 regional earthquakes. We perform point spread function tests to assess the resolution and trade‐offs between model parameters. Our tomographic model reveals several seismic wave velocity features. These features include the geometry of the Cocos slab, the partial melting zone beneath the Trans‐Mexican Volcanic Belt, and the Yucatan slab diving below Los Tuxtlas Volcanic Field that are consistent with previous studies. We also identify horizontal structures in the lower crust and the shallow upper mantle that were not previously identified. These structures include slow seismic wave velocity anomalies that may be correlated with ultraslow seismic velocity zones and high conductivity regions, where slow earthquakes have been identified.

Provenance and palaeoenvironmental significance of microtextures in quartz and zircon grains from the Paseo del Mar and Bosque beaches, Gulf of Mexico

Microtextures of quartz and zircon grains from the Bosque and Paseo del Mar beaches in the Gulf of Mexico were examined using a Scanning Electron Microscopy (SEM), to infer their provenance and palaeoenvironment. A total of 16 microtextures from 200 grains were identified and were grouped into mechanical, chemical, and mechanical/chemical origin. Microtextures of mechanical origin represent parallel striations (ps), cracks due to collision (ck), conchoidal fractures (cf), v-shaped marks (vs), abrasion fatigue (af), meandering ridges (mr), chattermarks (ch), curved (cg) and straight grooves (sg), which are attributed to a high-energy fluvial and sub-aqueous marine environment. Solution pits (sp), crystal overgrowth (cro), and silica globules (sgl) of chemical origin indicated precipitation and dissolution in a silica saturated marine environment. Adhered particles (ap), elongated depressions (dp), and relief are grouped as mechanical/chemical origin, suggested a sub-aqueous nearshore marine environment. Although zircon is considered as a resistant mineral, the frequency of microtextures observed was higher in zircon than in quartz. The results of this study revealed that the Los Tuxtlas volcanic field, Chiapas Massif and Oaxacan Complexes in the SW Gulf of Mexico are the potential source areas, which supplied sediments to the Paseo del Mar and Bosque beach areas.

Geochemistry, petrogenesis, and tectonic setting of the Los Tuxtlas Volcanic Field, Mexico

We present new major and trace element geochemical data, including the rare‐earth elements (REE), platinum group elements and Au, and combine them with the published information to constrain the highly debated origin of magmas in the Los Tuxtlas Volcanic Field (LTVF) of the Late Miocene to historic age. The LTVF magmas originated in the garnet‐bearing mantle as confirmed from the partial melting models of the REE. The mantle‐derived magmas may have subsequently undergone fractional crystallization of mainly olivine. The controversial tectonic setting of the LTVF was solved by log‐ratio transformed major as well as combined major and trace element data as a continental strike‐slip fault setting during the Miocene–Pliocene. During the Pleistocene to Holocene, the more numerous samples having only major elements alone indicated the continental strike‐slip fault setting, whereas the lesser number of samples having combined major and trace elements were more consistent with the continental extension. Importantly, no geochemical evidence for the involvement of the subducting slab in the magma genesis of the LTVF could be inferred. The inferences presented in this work were consistent with the entirety of geological, geochemical, and geophysical evidence. The multidimensional models also showed that the monogenetic volcanic field of the Xalapa region of the easternmost part of the Mexican Volcanic Belt represents a continental extension setting.

Sediment chemistry and detrital zircon record in the Bosque and Paseo del Mar coastal areas from the southwestern Gulf of Mexico

Abstract In this study, we analyzed the mineralogy and bulk chemical composition of 20 sediment samples, as well as trace, rare earth element (REE) concentrations and U–Pb ages of 400 detrital zircons retrieved from the Bosque (PB) and Paseo del Mar (PM) coastal sediments, southwestern Gulf of Mexico, to investigate the provenance. The XRD study revealed that the PB sediments are associated with quartz, muscovite, cordierite, albite, and anorthite. The PM sediments are associated with calcite, aragonite, and biogenic materials, which derived probably within the coastal environment. The weathering indices like Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA) revealed a moderate intensity of weathering for the PB sediments. The chondrite normalized REE patterns of the PB and PM sediments are with negative and positive europium anomalies (Eu/Eu*), suggesting that the sediments were derived by the combination of felsic and mafic source rocks. The Th/U ratios (>0.3) of the PB and PM zircons indicated an igneous origin. The chondrite normalized REE patterns of the PB and PM zircons are depleted in low REE (LREE) and enriched in heavy REE (HREE) with positive cerium and negative europium anomalies. A wide variation in chondrite normalized REE patterns of zircons with respect to ages was observed. The PB is dominated by Proterozoic (~551–2500 Ma) and Paleozoic zircons (~445–480 Ma; Ordovician), whereas PM is dominated by Cenozoic zircons (~5–21 Ma; Miocene), indicating that the zircons were derived from different source terranes. The Chiapas Massif (CMC) and Chuacus complexes are identified as the potential source areas, which contributed Proterozoic zircons to PB. On the other hand, Cuicateco terrane (CUI) and Tuxtla Volcanic Field (TVF) are considered as the potential source areas, which supplied significant number of Cenozoic zircons to the PM coastal area. The zircon U–Pb ages and the bulk sediment geochemistry together indicate that the source terranes, which supplied sediments to the PB and PM coastal areas, consist of mafic and felsic rocks. The results of this study further illustrated the importance of littoral currents in distributing sediments, although rivers are considered as a carrier and agent of distributing sediments along the coastal areas.

Delamination of sub-crustal lithosphere beneath the Isthmus of Tehuantepec, Mexico: Insights from numeric modelling

The Isthmus of Tehuantepec (IT) is a tectonically active and complex physiographic province in southern Mexico marked by an anomalous volcanism with unusual location and geochemical signature, an extensional stress regime and a well-imaged high seismic-velocity anomaly in form of a slab-like structure whose origin remains still not well understood. Here we use numeric modelling, tectonic information and composition of volcanic rocks from the Los Tuxtlas Volcanic Field (LTVF) to reconstruct the geodynamic evolution of the Isthmus of Tehuantepec from the Late Miocene to the present, and propose a model that reconciles the seismic observations with the tectonic evolution of the region. Using a high-resolution thermomechanical petrological coupled model, we show that rollback of delaminated continental lithosphere and associated asthenospheric upwelling provides a plausible mechanism for generating the observed high seismic-velocity anomalies beneath the Isthmus of Tehuantepec. Our model proposes that delamination of the entire lithospheric mantle initiated some 7 Myr ago by infiltration of hot and buoyant asthenospheric material into the base of the crust, when a slab gap appeared in the subducting Cocos slab. A geodynamic model involving sub-crustal lithosphere delamination reconciles the geological and geophysical observations and explains the complex oceanic-continental tectonics of the Isthmus of Tehuantepec.

Internal architecture of the Tuxtla volcanic field, Veracruz, Mexico, inferred from gravity and magnetic data

The Tuxtla Volcanic Field (TVF) is a basaltic volcanic field emerging from the plains of the western margin of the Gulf of Mexico in the Mexican State of Veracruz. Separated by hundreds of kilometers from the Trans-Mexican Volcanic Belt to the NW and the Chiapanecan Volcanic Arc to the SE, it stands detached not only in location but also in the composition of its rocks, which are predominantly alkaline. These characteristics make its origin somewhat puzzling. Furthermore, one of the large volcanoes of the field, San Martin Tuxtla, underwent an eruptive period in historical times (CE 1793). Such volcanic activity conveys particular importance to the study of the TVF from the perspective of volcanology and hazard assessment. Despite the above circumstances, few investigations about its internal structure have been reported. In this work, we present analyses of gravity and aeromagnetic data obtained from different sources. We present the complete Bouguer anomaly of the area and its separation into regional and residual components. The aeromagnetic data were processed to yield the reduction to the pole, the analytic signal, and the upward continuation to complete the interpretation of the gravity analyses. Three-dimensional density models of the regional and residual anomalies were obtained by inversion of the gravity signal adding the response of rectangular prisms at the nodes of a regular grid. We obtained a body with a somewhat flattened top at 16km below sea level from the inversion of the regional. Three separate slender bodies with tops 6km deep were obtained from the inversion of the residual. The gravity and magnetic anomalies, as well as the inferred source bodies that produce those geophysical anomalies, lie between the Sontecomapan and Catemaco faults, which are proposed as flower structures associated with an inferred deep-seated fault termed the Veracruz Fault. These fault systems along with magma intrusion at the lower crust are necessary features to understand the origin and structure of the TVF.

Evidence of pre-Columbian settlements in the forest of the Tuxtla Volcanic Field, Veracruz, Mexico

The basaltic Los Tuxtlas Volcanic Field (LTVF) is located at the western margin of the Gulf of Mexico in the State of Veracruz, Mexico. The field is a massif composed of four large volcanic structures and hundreds of scoria cones, lava domes and maars. This area was in the past covered by a dense forest in whose margins flourished several of the ancient cities of importance in central and southern Veracruz. Within the forest no enduring archeological ruins have been found; but the present inhabitants of the area frequently find fragments of ceramics and stone that attest to the presence of what could have been small settlements with a seminomadic regime. Unfortunately the objects found have been removed from their emplacement and are difficult to date. However in the course of our study of the volcanic deposits in the area we found four mudflow deposits containing pre- Columbian pottery objects and shards, as well as charcoal in some of them. Sections of the deposits were observed in detail and sampled for granulometric analysis. The charcoal samples were dated using standard radiocarbon methods (C-14); where charcoal was absent the pottery shards were dated with thermoluminescence (TL) techniques. The samples from these sites yielded ages of 1176±100 years BP (TL), 1385±70 BP years (C-14), 1157±105 years BP (TL), and 2050+245-235 years BP (C-14). Since in the area there is no clayey and silty material suitable for production of pottery; the closest and largest prehispanic cities, Tres Zapotes or Matacapa, are located to the west of the LTVF.

Hazard assessment at San Martín volcano based on geological record, numerical modeling, and spatial analysis

The San Martin shield volcano, located in the Los Tuxtlas Volcanic Field, has experienced effusive shield-building activity, as well as explosive eruptions, as evidenced by direct observations during the last eruption in 1793. The threat to the surrounding villages consists principally of lahars, especially because of the tropical climate in the region. Ash fallout and lava flows represent additional hazards. In addition, the surrounding Quaternary monogenetic field includes more than 300 scoria cones and about 40 explosion craters (mainly maars) that also represent a hazard source. In the present study we constructed hazard maps using field data, orthophotos, spatial analysis, and specialized software (LAHARZ and HAZMAP) to deliminate lahar inundation zones, areas that could potentially be affected by ash fallout (including the evaluation of houses prone to roof collapse due to ash load), and the most susceptible areas for hosting future monogenetic vent formation.

Volcanic hazard from San Martin Tuxtla Volcano, Tuxtla Volcanic Field, Veracruz, México: probability maps of ashfall deposition from possible future eruption

San Martin Tuxtla Volcano, the largest and highest edifice in the Tuxtla Volcanic Field, had explosive eruptions in 1664 and 1793. This volcano poses the highest hazard for the growing population centers surrounding it. The impacts and deposits of the 1793 eruption have been studied recently, and some of its characteristics, such as eruptive mass, grain-size distribution of the products and plume height, have been estimated. These data, together with daily data on wind velocities taken in the nearby city of Veracruz, have been used to conduct numerical modeling of ash distribution from an eruptive column comparable to that estimated during the 1793 event. Using wind velocities of randomly chosen days, we constructed probability maps of areas likely to receive, at least 1, 5, or 10 cm of ash. Our results indicate that an area about 1.3 × 104 km2 has a 12% probability of being covered by an ash fall more than 1 cm thick, if a mass of 1014 g was erupted and deposited from a 10-km-high eruptive plume. The results presented in this paper serve as a scientific basis to estimate volcano risk in the area from a possible eruption, consisting of one or several large explosive events, at San Martin Tuxtla volcano.

Imaging the Moho and Subducted Oceanic Crust at the Isthmus of Tehuantepec, Mexico, from Receiver Functions

Using teleseismic data recorded along a transect, which we call VEOX (for Veracruz-Oaxaca seismic line), of 46 broadband stations installed across the Isthmus of Tehuantepec in southern Mexico, we obtained receiver functions and stacked them to study the Moho topography and back projected them to visualize the subducted slab geometry beneath the isthmus. We observed a back-azimuth dependent Moho thickness across the transect, particularly beneath the Los Tuxtlas Volcanic Field. Also, we observed the Cocos plate which subducts with an angle of 26° between 140 and 310 km from the trench. Comparison with regional seismicity indicates that it occurs below the oceanic crust.

Evaluation of Recent Tectonomagmatic Discrimination Diagrams and their Application to the Origin of Basic Magmas in Southern Mexico and Central America

Discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years. Although old diagrams have been extensively used, the recent ones based on discriminant functions of ratio variables, with or without log-transformation, proposed during 2004–2010 for the discrimination of four tectonic settings of island arc, continental rift, ocean-island and mid-ocean ridge, were newly evaluated to show their high success rates of 57.3–100% and 58.5–100% for major-element and immobile-element based diagrams, respectively. For the continental arc of the Andes evaluated for its similarity to island arc, these four sets of diagrams showed success rates of 62.1–83.8%. These four sets of five diagrams per set were therefore used to infer tectonic setting of the Mexican Volcanic Belt (MVB), Los Tuxtlas volcanic field (LTVF), and Central American Volcanic Arc (CAVA). Using this approach, the MVB, especially its western, central and eastern parts, and the LTVF of Southern Mexico show a dominantly continental rift setting and the CAVA shows an arc setting. The west-central part of the MVB is consistent with dual tectonics of arc and rift. These results confirm the application of an unusual mantle upwelling rift-model for the Mexican on-land volcanism, whereas the conventional plate tectonic subduction model seems to be applicable for the CAVA from Guatemala to north-western Costa Rica.

Crustal thickness at the Tuxtla Volcanic Field (Veracruz, Mexico) from receiver functions

The Tuxtla Volcanic Field (TVF) is a structure of basaltic rocks on the western margin of the Gulf of Mexico in the Mexican State of Veracruz. Located some 150 km from the easternmost tip of the Mexican Volcanic Belt, its tectonic relationship is still unclear. The volcanism, mostly alkaline, is younger than 7 Ma and has given origin to hundreds of cinder and scoria cones, maars and four large composite volcanoes, one of which, San Martín Tuxtla, erupted explosively in 1793. Due to its volcanological importance, it has been the subject of several geological studies, none of which focused on its crustal structure. Moreover, because the seismicity level in the area is relatively low, no broadband seismometers of Mexico's National Seismological Service are currently installed in the area. In this paper we present the results of the analyses of 24 teleseismic events occurring between 2004 and 2008 recorded in two broadband stations deployed around San Martín volcano. The aim of this study was to determine the depth to the Moho, any major intracrustal interface in the area, and a velocity model by means of receiver function analysis. The results show that the crustal thickness in the area varies between roughly 28 and 34 km. The receiver functions at one station suggest a second interface at a depth between 10 and 14 km. This interface is probably the contact between an upper sedimentary layer and the transitional crust found elsewhere in the margins of the Gulf of Mexico. The determination of the crustal thickness in the TVF is of importance to characterize the area and as a framework to pursue further studies of this volcanic field.

The Neogene Veracruz fault: evidences for left-lateral slip along the southern Mexico block

AbstractStructural data combined with analysis of satellite images and seismic profiles show that a major left-lateral strike-slip fault affects the Veracruz basin and post-5 Ma volcanic rocks of the Los Tuxtlas volcanic field (LTVF). The main volcanic alignment of the LTVF is located along this fault. Additional structural data collected in the Trans-Mexican volcanic belt (areas of Xalapa, Teziutlán and Huauchinango) show that the shear zone affects Pliocene Trans-Mexican volcanic rocks. Low seismicity associated to faulted Quaternary markers such as alluvial fans, alluvial terraces and volcanoes argue for active faulting in this area. Plio-Quaternary strike-slip faulting in the Veracruz basin and in the eastern Trans-Mexican volcanic belt is important because it connects two important structural provinces: the left-lateral strike-slip faults province to the south and the left-lateral transtensive faulting that affects the central part of the Trans-Mexican volcanic belt. These three active deformation zones constitute the boundary between the southern Mexico block and the North American plate. It is generally assumed that strike-slip faulting along the Trans-Mexican and Central America volcanic arcs is the result of oblique subduction of the Cocos plate under the North American and Caribbean plates. However slip vectors along the Middle America trench are almost perpendicular to the trench. This Neogene sinistral strike-slip motion could be partially driven by the eastward motion of the Caribbean plate rather than by strain partitioning along the oblique Middle America trench subduction zone.

Discriminating four tectonic settings: Five new geochemical diagrams for basic and ultrabasic volcanic rocks based on log — ratio transformation of major-element data

We present five new discriminant function diagrams based on an extensive database representative of basic and ultrabasic rocks from four tectonic settings of island arc, continental rift, ocean-island, and mid-ocean ridge. These diagrams were obtained after loge-transformation of concentration ratios of major-elements — a technique recommended for a correct statistical treatment of compositional data. Higher % success rates (overall values from ∼ 83 to 97%) were obtained for proposing these new diagrams as compared to those (∼82 to 94%) obtained from the discriminant analysis of the raw major-element concentration data (i.e., without the loge-transformation and without taking ratios of the compositional data, but using exactly the same database to provide an unbiased comparison), suggesting that such a data transformation constitutes a statistically correct and recommended technique. The new diagrams also resulted in less mis-classification of basic and ultrabasic rocks from known tectonic settings than the diagrams obtained from the raw data. The use of these highly successful new discriminant function diagrams is illustrated using Miocene to Recent basic and ultrabasic rocks from three areas of Mexico with complex or controversial tectonic settings (Mexican Volcanic Belt, Los Tuxtlas volcanic field, and Eastern Alkaline Province), as well as older rocks from three areas (Deccan, Malani, and Bastar) of India. Additionally, the major-element data from two ‘known’ continental arc settings are used to show that they are similar to those from the island arc setting. Continental rift setting is inferred for all Mexican cases and for one cratonic area of India (Malani) and an IAB setting for the Bastar craton. The Deccan flood basalt province of India is used to warn against an indiscriminate use of those discrimination diagrams that do not explicitly include the likely setting of the area under evaluation. An Excel template is also provided for an easy application of these new diagrams for discriminating the four settings considered in this work.

Extension-related origin of magmas from a garnet-bearing source in the Los Tuxtlas volcanic field, Mexico

The Los Tuxtlas volcanic field (LTVF) of late Miocene to Recent age is a key area to understand the consequences of the current subduction of the Cocos plate beneath the North American plate, as well as the competing effects of the ongoing extension along the Gulf of Mexico coast. Geochemical and radiogenic (Sr, Nd, and Pb) isotope data are used to constrain the origin of these 7 Ma to Recent magmas in this area. The basanitic and alkaline basaltic rocks show highly steep light rare-earth element-enriched patterns implying residual garnet in their mantle source, whereas the evolved alkaline and sub-alkaline rocks have less steep rare-earth element patterns consistent with a contribution from the continental crust. Geochemical and isotope data from the LTVF are compared with those from continental rifts, extension-related areas, continental break-up regions, and island and continental arcs, including the Central American volcanic arc related to the subduction of the same oceanic plate (Cocos plate), as well as with those from the two nearby Mexican provinces [the Eastern Alkaline Province (EAP) and the eastern part of the Mexican Volcanic Belt (E-MVB)]. These data for the LTVF primitive rocks are similar to rifts, extension-related areas and continental break-up regions, including the two Mexican provinces, but different from island and continental arcs, including the northern part of the Central American Volcanic Arc (CAVA). The LTVF rocks show an unusual Th and U enrichment with respect to Ba and Rb, which also renders a distinct negative Nb anomaly (with respect to Th and K) in them. These rocks also show a negative Nb anomaly (with respect to Ba and La) that is similar to numerous rift, extension-related areas, and continental break-up regions, but distinct from all arcs around the world, indicating that the magma genesis processes in the LTVF are similar to those in rifts. The “Sr-shift”, shown to be a typical feature of most, if not all, island and continental arcs including the CAVA, is not present in the LTVF rocks. Numerous discrimination diagrams, including the new discriminant function diagrams, suggest a rift setting for the LTVF. An essentially extension-related origin of the LTVF is, therefore, inferred in this study. Furthermore, in the light of major and trace element data for LTVF primitive rocks and their modelling an incompatible element-enriched garnet-bearing source seems plausible. The LTVF source is likely to reside in the lithosphere rather than the asthenosphere although the asthenospheric contribution cannot be completely ruled out. The evolved alkaline and sub-alkaline rocks might have a lower crustal component. Finally, it appears that the LTVF shows more affinity to the EAP rather than to the Mexican Volcanic Belt (MVB), implying that the LTVF should probably be considered as a part of the EAP.

Further constraints for the Plio-Pleistocene geomagnetic field strength: New results from the Los Tuxtlas volcanic field (Mexico)

A rock-magnetic, paleomagnetic and paleointensity study was carried out on 13 Plio-Pleistocene volcanic flows from the Los Tuxtlas volcanic field (Trans Mexican Volcanic Belt) in order to obtain some decisive constraints for the geomagnetic field strength during the Plio-Pleistocene time. The age of the volcanic units, which yielded reliable paleointensity estimates, lies between 2.2 and 0.8 Ma according to the available K/Ar radiometric data. Thermomagnetic investigations reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution that probably occurred during the initial flow cooling. Unblocking temperature spectra and relatively high coercivity point to ‘small’ pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Six flows yield reverse polarity magnetization, five flows are normally magnetized, and one flow shows intermediate polarity magnetization. Evidence of a strong lightning-produced magnetization overprint was detected for one site. The mean pole position obtained in this study is Plat = 83.7°, Plong = 178.1°, K = 36, A95 = 8.1°, N = 10 and the corresponding mean paleodirection is I = 31.3°, D = 352°, k = 37, α95 = 8.2°, which is not significantly different from the expected direction estimated from the North American apparent polar wander path. Thirty-nine samples were pre-selected for Thellier palaeointensity experiments because of their stable remanent magnetization and relatively weak-within-site dispersion. Only 21 samples, coming from four individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 6.4 to 9.1 × 1022 Am2. Combining the coeval Mexican data with the available comparable quality Pliocene paleointensity results yield a mean VDM of 6.4 × 1022 Am2, which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and are of dissimilar quality. Additional high-quality absolute intensity determinations are needed to better constraint the geomagnetic field strength during the Plio-Pleistocene time.