Western Trans-Mexican Volcanic Belt Research Articles

A new species of Caecidotea (Crustacea: Isopoda: Asellidae) from a well in the western slope of Mexico, with occurrence records of C. alvarezi.

Caecidotea isabelae sp. nov., a new freshwater asellid isopod species from Jalisco, Mexico is described. During fieldwork in 2021, we discovered eight new populations of freshwater isopods belonging to the genus Caecidotea. One population corresponded to a new species, Caecidotea isabelae sp. nov., while the other seven populations were identified as Caecidotea alvarezi. These populations are reported from freshwater bodies on the western Trans-Mexican Volcanic Belt, expanding the distributional range of C. alvarezi through the state of Jalisco. The new species can be distinguished of their congeners mainly by the protopod of pleopod I triangular with 3 retinacula, exopod of pleopod III with 6 presutural setae, pleopod IV with 3 spines in the proximal external margin. The present study increases the number of known epigean asellid species in Mexico to 12, and the number of currently described Caecidotea species in North America to 102 with three subspecies.

Petrogenesis of mafic-intermediate magmatism of the Michoacán–Guanajuato volcanic field in Western Mexico. A geochemical review

The Michoacán–Guanajuato volcanic field (MGVF) in the western Trans-Mexican Volcanic Belt is one of the largest and most diverse monogenetic volcanic fields in the world holding more than 1200 volcanic vents. Its eruptive activity goes back to 7 Ma, it is considered an active volcanic field, and the composition of its rocks varies from mafic to silicic. It is essential to understand the geochemical evolution of its products, the complex petrogenetic processes, and the origin of magmas in central Mexico. Although these processes are linked to the subduction of the Cocos plate beneath the North American plate, the magmatic plumbing system of the MGVF remains not completely understood. The MGVF has been studied for decades, focusing in its dominant intermediate magmas. Nevertheless, the origin and evolution of the mafic components and their relation with the intermediate rocks have been poorly discussed. Here, we compile geological and geochemical data of the MGVF to discuss the petrogenesis of mafic magmas along the volcanic field and the role they play in the generation of intermediate melts. We used data published for 429 samples of mafic and intermediate volcanic rocks. Conventional procedures and statistical techniques were used to process the dataset. We propose that MGVF mafic magmas are derived from low degrees (∼1–15%) of partial melting of a spinel-bearing lherzolite source/mantle related to the rapid ascent of the asthenosphere caused by an extensional regime that is present in the area. In contrast, intermediate magmas where divided into two main groups based on the Mg content: high-Mg intermediate rocks, which seems to be derived from different rates of assimilation and fractional crystallization process, and low-Mg intermediate rocks, which can be related to fractional crystallization of mafic magmas. In addition, mafic and intermediate magmas display a chemical diversity which is related to mantle heterogeneity domains in the mantle wedge.

Seed integrity, effect of temperature and storage time on germination of <em>Populus luziarum</em> and <em>P. primaveralepensis</em>, endangered subtropical species from Mexico

Background: Populus luziarum and P. primaveralepensis are endemic species of western Mexico; growing in riparian forests they are critically endangered. The best way to conserve their seeds is unknown, which could be limiting for their conservation. Hypothesis: The germinability of both subtropical species is like that of boreal and template Salicaceae species that disperse seeds in spring and early summer, as they germinate quickly with high percentages, and rapidly lose their viability when stored at ambient temperature. Studied species: Populus luziarum and P. primaveralepensis. Study site and dates: Western Trans-Mexican Volcanic Belt. Jalisco, Mexico. October 2019. Methods: The physical integrity of the seeds was assessed by X-ray imaging and compared with germinability. In addition, the effect of storage time (nine weeks) under two temperatures (4 and 21 °C) on the percentage and mean germination rate was evaluated. Results: No significant differences were found between physical integrity and germination in freshly collected seeds for both species. Germination in the first 24 hrs was 91 and 95 % for Populus luziarum and P. primaveralepensis, respectively (week 0). Germination percentages were lower when stored at 21 °C, but P. primaveralepensis was decreased more slowly. Conclusions: Seeds of subtropical Populus respond similarly to those of species from temperate and boreal climates with early seed dispersal, a crucial condition for establishing ex situ reforestation and conservation programs.

Paleomagnetic study from radiometrically dated lavas associated to the Tepic-Zacoalco Rift (western Mexico): Possible evidence of a new geomagnetic excursion within Gilbert chron

This work presents the results of a detailed paleomagnetic and rock-magnetic study carried out in the igneous units associated with the Tepic-Zacoalco Rift (western Trans-Mexican Volcanic Belt). Standard paleomagnetic drilled cores were collected from 21 sites distributed over the Rift extension, all of them with available radiometric ages between 3 and 5 Ma. Rock magnetic experiments showed that, in most cases, the minerals carrying the magnetic remanence are titanomagnetites with variable titanium content. Stepwise magnetic treatment allowed to determine the paleomagnetic directions for 17 sites, of which 4 are of normal polarity, 8 reverse, and 5 sites have apparently intermediate polarities (based on paleolatitude less than 45° as a cut-off angle to separate stable and transitional geomagnetic regimes). The mean paleodirection is Dec. = 347.6°, Inc = 29.4°, N = 12, α95 = 9.5°, corresponding to a paleomagnetic pole position Plat = 77.2° N, Plong = 144.3° E, A95 = 8.8°, N = 12. These results agree, within uncertainties, with the expected directions retrieved from the stable North America reference poles and to the previously reported values for the same region and age. Thus, major tectonic deformation or vertical axis rotation is discarded at least since Early Pliocene. An interesting feature of this study is the occurrence of intermediate paleodirections around 3.86 Ma which probably correspond to post-Cochiti geomagnetic excursion within the Upper Gilbert chron.

Ceboruco hazard map: part I - definition of hazard scenarios based on the eruptive history

Of the 48 volcanoes in Mexico listed as potentially active by the National Center for Disaster Prevention (CENAPRED), Ceboruco, located in the western Trans-Mexican Volcanic Belt, is considered among the 5 most hazardous. Its recent eruptive history includes a large magnitude Plinian (VEI 6) eruption ~ 1000 years ago and the historical 1870–1875 vulcanian (VEI 3) eruption, as well as recent fumarolic and seismic activity.Ceboruco is a relatively young (< 400,000 years) stratovolcano characterized by abrupt changes in eruptive behavior. Individual eruptive episodes have great variations in style (effusive andesitic to highly-explosive rhyodacitic) and duration. These factors complicate hazard assessment.Three main eruptive scenarios of different magnitudes (large, intermediate, small) and eruption characteristics (likelihood of occurrence: high, medium, small) have been identified and will be presented as a background to build the volcanic hazard map for Ceboruco volcano (presented in part II of this work). Here, we report on the detailed eruptive history, with emphasis on the volcanic products of each of the eruptions, in order to identify those deposits that can serve as a reference for calibrating the modeling software (Tephra2 and Hazmap for ash fallout, Eject! code for ballistics, Etna Lava Flow Model for lava flows, Titan2D for pyroclastic density currents, and Flo-2D and LaharZ for lahars) that will be used in further steps to simulate different volcanic phenomena and lead to the construction of the hazard map.

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 (Mw6.9) and the 1920 Jalapa (Mw6.4) 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 (Mw7.2 ± 0.2) and a second large event took place in 1875 (Mw7.0 ± 0.2). Two other earthquakes are reported in the western TMVB in 1611 and 1749 (Mw6.4). The 1858 earthquake (Mw7.6 ± 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 (Mw5.7) occurred east of the city of Puebla. The 1887 (Mw6.2 ± 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.

The role of complex terrain in the generation of tornadoes in the west of Mexico

Tornadoes are extreme manifestations of severe storms that occur around the world. In Mexico, the most affected region by the tornado phenomenon is the Trans-Mexican Volcanic Belt (TMVB), a complex topographic region in the central part of the country with a large population density. This research work aims to investigate the role of the complex topography in the generation of instability conditions that favored the formation of two tornadoes almost in the same place (western TMVB) on August 7, 2012, and September 16, 2014. Numerical experiments with the WRF-ARW model were performed in order to obtain knowledge about several important weather conditions preceding each tornado event and to identify the role of the complex terrain in the generation of instability necessary for their formation. Notwithstanding this real complexity, similar patterns in instability parameters and meteorological variables were found for the two tornadoes. The complex terrain seems to be essential in the generation and increase in instability preceding each tornado event. This work is the first approach to understand the meteorological phenomena, in the complex topography of Mexico, which leads to the formation of tornadoes. Understanding natural hazards such as tornadoes represents a first phase in the process of disaster risk reduction.

Rock-magnetic and paleomagnetic study on a 27-m-long core from Lake Chapala, western Mexico: Paleoenvironmental implications for the last 10 ka

We report detailed rock-magnetic and paleomagnetic results from a 27-m-long sedimentary core collected at depocenter of Lake Chapala, western Trans-Mexican Volcanic Belt. Rock-magnetic parameters point to PSD grain-size titanomagnetites as the dominant magnetic mineralogy. Observed variations, in both concentration and grain-size dependent magnetic parameters, reflect changes in the sediment supply through the studied depth, but mainly during the last half century. Stable characteristic remanent magnetization, representative of secular variation of the geomagnetic field, was retrieved after alternating field demagnetization of the natural remanent magnetization (NRM). The comparison of the obtained magnetic inclination and declination logs against the corresponding curves predicted by the Global Geomagnetic Field Model SHA.DIF.14k enables to establish an alternative sedimentation-rate independent age-depth model for the whole core, in reasonably good agreement with the one calculated using conventional radiocarbon dating. This approach works not only for Lake Chapala, but also for Lake Chalco. The correlation established between rock-magnetic parameters and paleoenvironments, based on diatom identification of representative samples within specific zones, can be considered as a novel proxy methodology. This approach opens a new perspective for new, more detailed rock-magnetic, paleomagnetic and paleoenvironmental investigations for better paleoclimatic reconstructions.

Ceboruco hazard map: part II—modeling volcanic phenomena and construction of the general hazard map

Ceboruco volcano in the western Trans-Mexican Volcanic Belt is one of the eleven most active stratovolcanoes in Mexico. Due to its recent eruptive history including a large Plinian eruption ~ 1000 years ago, the AD 1870 eruption, and recurrent recent seismic activity, it seemed highly appropriate to construct a hazard map in order to be prepared for future eruptions and their associated hazards. Ceboruco volcano eruptions are predominantly effusive; however, it also has been characterized by a great variability of eruptive styles throughout its record of activity. In fact, some eruptions comprise a significant diversity of volcanic processes, including lava flows, tephra fallout, ballistic emission, pyroclastic flows and surges, and lahars. In this work, we present (1) an integrated and simplified hazard map and (2) more detailed scenario-based hazard maps showing the areas affected by the different expected volcanic phenomena attempting to account for this great diversity of eruptive processes. The maps represent the basis to identify the main hazard zones during a future eruption and the related impacts on population and infrastructure within the area of influence of Ceboruco (~ 700 km2), as well as for undertaking subsequent vulnerability and risk analyses. The maps provide a tool to develop measures of prevention and mitigation of volcanic hazards (preparedness of the population, establishment of evacuation routes and refuges, etc.), as well as for decision-making by authorities during territorial planning (urban expansion for example). The integrated simplified hazard map can also be a tool for dissemination purposes, in order to create awareness of associated hazards derived from a possible future activity of the volcano among the public in general. This is important because in the western sector of the Trans-Mexican Volcanic Belt (and specifically in the State of Nayarit) most volcanic edifices (with the exception of Colima volcano) are closed-vent volcanoes (sealed volcanic vent vs. open-vent systems) with long repose periods (up to ~ 16,000 years for example in the case of San Juan volcano 60 km to the W), a situation that consequently and unfortunately has led to a practically nonexistent volcanic risk perception.

Subsurface structural imaging of Ceboruco Volcano area, Nayarit, Mexico using high-resolution aeromagnetic data

Ceboruco volcano is one of the largest volcanoes of the Trans-Mexican Volcanic Belt (TMVB), which extends along central Mexico. Among the western TMVB, Ceboruco Volcano comes in the second place after Colima Volcano in its activity. Also, it is considered as the only one that has historically-documented eruptions. Few geophysical studies have been published with the aim of studying the internal structure of the volcano. In the current work, and throughout the support of the CeMIEGeo-P24 geothermal exploration project, we aim to delineate the possible subsurface structural trends, to determine the approximate depth to the basement surface, and to provide an illustrative 3D model for its subsurface structure.To achieve such goals, a detailed analysis of the aeromagnetic data for the volcano area was performed. 2D interpretation was carried out for the aeromagnetic data. Edge detection filters, such as Tilt Derivative, and Analytic Signal, were used to map the contact/faults within the study area. In addition, 3D inversion was used to image the subsurface structure of the volcano. Aeromagnetic data was inverted using the GMSYS-3D inversion program by applying Parker algorithm. Four subsurface layers at depths of 250, 500, 750 and 1000 m were assumed during the inversion process. Results indicate that the limits “boundary” of the subsurface magma chamber of the volcano can be traced easily from the first subsurface layer. A secondary structure line appears towards the east from the volcano boundary and it can be observed from the inversion of the second and third layers. This indicates that both, the base of the volcano and the structure line, stem from the same source. Finally, they disappear in the inversion results for the last layer, which concludes that we reach the basement above which the volcano is formed. Combination of all obtained results with the geological information, helped to understand the main structure of the Ceboruco volcano.

Populus luzae (Salicaceae), a new species of white poplar endemic to the western Transmexican Volcanic Belt, in Zapopan, Jalisco, Mexico

Populus luzae, a new species of white poplar from the western Transmexican Volcanic Belt, Zapopan, Jalisco, Mexico, is described and illustrated. Populus luzae belongs to section Populus and is morphologically close to P. guzmanantlensis, but it differs from the latter in having smaller size and soboliferous habit, narrower twigs and petioles, ovate to ovate-deltoid and chartaceous leaf blades, adaxial leaf surface with less depressed veins, abaxial surface puberulent, thrice smaller ovate male bracteoles with entire margin, twice longer female bracteoles sparsely denticulate but none lobed, smaller number of pistillate flowers, capsules pubescent, twice longer mature male inflorescence, and a lax and longer infrutescence.

Phylogeography and population differentiation in the Psittacanthus calyculatus (Loranthaceae) mistletoe: a complex scenario of climate–volcanism interaction along the Trans‐Mexican Volcanic Belt

AbstractAimThe formation of the Trans‐Mexican Volcanic Belt (TMVB) played an important role in driving inter‐ and intraspecific diversification at high elevations. However, Pleistocene climate changes and ecological factors might also contribute to plant genetic structuring along the volcanic belt. Here, we analysed phylogeographical patterns of the parrot‐mistletoe Psittacanthus calyculatus to determine the relative contribution of these different factors.LocationTrans‐Mexican Volcanic Belt.MethodsUsing nuclear and chloroplast DNA sequence data for 370 individuals, we investigate the genetic differentiation of 35 populations across the species range. We conducted phylogenetic, population and spatial genetic analyses of P. calyculatus sequences along with ecological niche modelling and Bayesian inference methods to gain insight into the structuring of genetic variation of these populations.ResultsOur analyses revealed population structure with three genetic groups corresponding to individuals from Oaxaca and those from the central‐eastern and western TMVB regions. A significant genetic signal of demographic expansion, an east‐to‐west expansion predicted by species distribution modelling, and approximate Bayesian computation analyses strongly supported a scenario of habitat isolation and invasion of TMVB by P. calyculatus during the late‐Pleistocene.Main conclusionsThe genetic differentiation of P. calyculatus may be explained by the combined effects of (1) geographical isolation linked to the effects of the glacial/interglacial cycles and environmental factors, driving genetic differentiation from congeners into more xeric vegetation and (2) the invasion of TMVB from east to west, suggesting a role for both colonization and glacial/interglacial cycles models.

Toroidal, Counter-Toroidal, and Upwelling Flow in the Mantle Wedge of the Rivera and Cocos Plates: Implications for IOB Geochemistry in the Trans-Mexican Volcanic Belt

We carried out analog laboratory modeling at a scale 1:4,000,000 and computer rendering of the flow patterns in a simulated western Middle American subduction zone. The scaled model consists of a transparent tank filled with corn syrup and housing two conveyor belts made of polyethylene strips. One of the strips dips 60° and moves at a velocity of 30 mm/min simulating the Rivera plate. The other one dips 45°, moves at 90 mm/min simulating the subduction of the Cocos plate. Our scaled subduction zone also includes a gap between the simulated slabs analogous to a tear recently observed in shear wave tomography studies. An acrylic plate 3 mm thick floats on the syrup in grazing contact with the polyethylene strips and simulates the overriding North America plate. Our experiments reveal a deep toroidal flow of asthenospheric mantle through the Cocos–Rivera separation. The flow is driven by a pressure gradient associated with the down-dip differential-motion of the slabs. Similarly, low pressure generated by the fast-moving Cocos plate creates a shallow counter-toroidal flow in the uppermost 100 km of the mantle wedge. The flow draws mantle beneath the western Trans-Mexican Volcanic Belt to the Jalisco block, then plunges into the deep mantle by the descending poloidal cell of the Cocos slab. Moreover, our model suggests a hydraulic jump causes an ~250 km asthenosphere upwelling around the area where intra-arc extensional systems converge in western Mexico. The upwelling eventually merges with the shallow counter-toroidal flow describing a motion in 3D space similar to an Archimedes’ screw. Our results indicate the differential motion between subducting slabs drives mixing in the mantle wedge of the Rivera plate and allows the slab to steepen and retreat. Model results are in good agreement with seismic anisotropy studies and the geochemistry of lavas erupted in the Jalisco block. The model can explain the eruption of OIB lavas in the vicinity of the City of Guadalajara in western Mexico, and the south shoulder in the central part of the Tepic-Zacoalco fault system.

The 1567 Mw 7.2 Ameca, Jalisco earthquake (western Trans-Mexican Volcanic Belt): Surface rupture parameters, seismogeological effects, and macroseismic intensities from historical sources

The 1567 Mw 7.2 Ameca, Jalisco earthquake (western Trans-Mexican Volcanic Belt): Surface rupture parameters, seismogeological effects, and macroseismic intensities from historical sources

Intracanyon basalt lavas of the Debed River (northern Armenia), part of a Pliocene–Pleistocene continental flood basalt province in the South Caucasus

Late Pliocene to Early Pleistocene (~3.25–2.05Ma), 200–400m thick basalt lavas outcrop in the South Caucasus region, including the Kars–Erzurum Plateau (northeastern Turkey), the Javakheti Plateau (Georgia–Armenia), and the Lori Plateau (northern Armenia). These fissure-fed, rapidly erupted fluid lavas filled pre-existing river valleys over many tens of kilometres. The basalts exposed in the Debed River canyon, northern Armenia, are ~200m thick and of three morphological types: (1) basal pillow basalts and hyaloclastites, overlain by (2) columnar-jointed pahoehoe sheet flows, in turn overlain by (3) slabby pahoehoe and rubbly pahoehoe flows. The lower and middle lavas show evidence for damming of river drainage, like many lavas of the Columbia River flood basalt province, Scotland, Ireland, and Iceland. There is also evidence for syn-volcanic faulting of the early lavas. Related basalts also outcrop in the Gegham Uplands and the Hrazdan River basin in Armenia. This 3.25–2.05Ma South Caucasus basalt province, covering parts of Turkey, Georgia and Armenia, has an estimated areal extent of ~15,000km2 and volume of ~2250km3. Because its main geological features are remarkably like those of many continental flood basalt (CFB) provinces, we consider it a true, albeit small, CFB province. It is the smallest and youngest CFB in the world. An analogue closely similar in major features is the Late Miocene Altos de Jalisco CFB province in the western Trans-Mexican Volcanic Belt. Both provinces formed during lithospheric pull-apart and transtensional faulting. Their broader significance is in showing flood basalt size distribution to be a continuum without natural breaks, with implications for geodynamic models.

Petrogenesis and geodynamic significance of silicic volcanism in the western Trans-Mexican Volcanic Belt: role of gabbroic cumulates

In the western Trans-Mexican Volcanic Belt voluminous silicic volcanism has been associated with the rifting of the Jalisco block from mainland Mexico. Rhyolitic volcanism started at 7.5 Ma after a major pulse of basaltic volcanism aged 11–8.5 Ma associated with slab detachment. This was followed by a second period, between 4.9 and 2.9 Ma, associated with rhyolitic domes and ignimbrite coexisting with basaltic volcanism. The similarity in rare earth element contents between basalts and rhyolites excludes a simple liquid line of descent. The low Ba and Sr contents and the ferroan character of the rhyolites suggest extensive fractional crystallization. Late Miocene–early Pliocene rhyolite Sr isotope values are only slightly more radiogenic than the basalts, whereas Nd isotope ratios are indistinguishable. We successfully modelled the 7.5–3 Ma silicic magmatism as a result of partial melting of crustal gabbroic complexes that we infer to have formed in the mid-lower crust due to the high-density Fe-enriched composition of the late Miocene basaltic volcanism. Slab rollback since ~7.5 Ma favoured decompression melting and arrival of additional mafic magmas that intruded in the lower crust. These basalts heated and melted the gabbroic complexes forming the silicic magmas, which subsequently underwent assimilation and fractional crystallization processes. The first silicic pulse was emplaced during a period of low tectonic activity. Extensional faulting since the Pliocene favours the eruption of both silicic magma and lesser amount of mafic lavas.

The origin of intraplate magmatism in the western Trans-Mexican Volcanic Belt

Alkaline basalts with geochemical features similar to those of intraplate ocean islands have been emplaced along the main trace of the Tepic-Zacoalco rift (TZR), a unique tectonic structure of the western Trans-Mexican Volcanic Belt in which extension is superimposed to a convergent margin. New geochemical and petrologic data on mafic volcanic rocks along the rift indicate the existence of a highly heterogeneous pre-subduction mantle wedge that has been slightly overprinted by slab-derived chemical agents. Most mafic volcanic rocks display geochemical and isotopic compositions that are indistinguishable from those of the Pacific islands Socorro and Isabel, and confirm the existence of an ancient, recycled, high-μ component (HIMU; μ = 238 U/ 204 Pb) in their mantle source. Olivines separated from samples carrying the HIMU signature have NiO and CaO contents similar to olivines from mid-ocean ridge basalt (MORB), indicating that the source of enrichment must be entirely hosted in peridotite. In contrast, more evolved rocks within the TZR have stronger subduction signatures and water contents, and display a distinctive isotopic array that points to slab-derived contributions. Olivines from these rocks are slightly less forsteritic but also extend to higher NiO and lower CaO contents than those from more mafic magmas, suggesting provenance from a secondary pyroxenite source. The overall geochemical evidence thus indicates that the pre-subduction background mantle wedge in western Mexico must be identical, and just as diverse, as that below the Pacific basin. Extension-driven mantle upwelling in a continental setting can only melt a dry peridotitic mantle to its lowest extents, and therefore preferentially sample its most enriched and easily fusible components. Yet the addition of even a small amount of slab-derived silica promotes a secondary petrologic transformation to pyroxene-rich lithologies that upon melting create magmas with compositions that are more akin to a volcanic arc setting.

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.

Rock-magnetic and paleomagnetic results from the Tepic-Zacoalco rift region (western Mexico)

A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40 Ar/ 39 Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude  = 81.1, longitude  = 94.3) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed

A model for the state of brittle failure of the western Trans-Mexican Volcanic Belt

The Trans-Mexican Volcanic Belt (TMVB) is an igneous arc built above the Middle America subduction zone. Its western section is being extended orthogonally to its axis by several arrays of active normal faults with a combined length of 450 km and including up to 1.5 km of throw. Until now, intra-arc extension in the TMVB has been considered the result of either rifting or retreat of the Rivera and Cocos plates. Observations worldwide and numerical models, however, appear to contradict these ideas. Continental extension in convergent margins takes place where the upper plate moves away from the trench, and the subduction zone is only weakly coupled with the overlying plate. In western Mexico, neither of these relationships applies. A new numerical model presented here is able to explain satisfactorily the state of brittle failure of the TMVB. The model embodies the first-order physics of the northern Middle America subduction zone, and its boundary conditions are consistent with the convergence history of the Rivera and North America plates. Modelling results show that periods of accelerated subduction between the Rivera and North America plates give rise to an increase in suction force under the fore arc. The over-riding plate then bends downwards, building up tensional stress inside the volcanic arc. Failure of the arc follows within 1 million years of pulse initiation. Analysis of the results shows that the steep subduction angle of the Rivera slab, the relief of the volcanic plateau, and the thermal weakening of the lower crust facilitated the failure of the arc. The model demonstrates that a highly coupled subduction zone can cause extension, albeit limited, in the over-riding plate.