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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.

Los Humeros Volcanic Field (LHVF) represents one of the key volcanic calderas in Mexico. Nowadays, LHVF is the third largest geothermal field in Mexico in terms of energy output, with an installed capacity of 94 MWe. The caldera is about 21 by 15 km wide and is located in the Serdán Oriental basin, east of the Trans- Mexican Volcanic Belt in the central-eastern part of the country, roughly 440 km away from the Middle American Trench. In this study we show results of numerical simulations of magma intrusion in order to better understand the deep origin of the caldera. For this purpose, we used high-resolution two-dimensional coupled petrological-thermomechanical numerical simulations of magma intrusion where an initial thermal anomaly was placed in the asthenosphere just below the lithospheric mantle. We performed a parametric study where we investigated the influence of several parameters such as the diameter of the thermal anomaly, the excess temperature and the regional tectonic regime. These 2D simulations were carried out using the finite difference method coupled with the cell marker technique and employing the multigrid method. The physical parameters used for the Earth’s layers (asthenosphere, lithospheric mantle, lower crust and upper crust) and for the composition of the magmatic intrusion were taken from literature and previously established models. In addition, we considered a viscoelastoplastic rheology and the simulations included erosion and surface sediment transport. Modeling results showed that only under certain conditions of temperature excess, initial diameter of the deep thermal anomalies that come in a specific chain-type sequence, it is possible to form a volcanic caldera similar with the dimensions of the LHVF. The temperature excess (ΔT = ~150K) suggested a deep origin for the thermal anomaly with an approximate depth of ~380 km, where currently the surface of the Cocos slab is located below the North American Plate. Additionally, we found that several magmatic pulses can reach the surface only if we consider in our models a small horizontal extension rate consistent with the extensional tectonic regime in the region.

Although the thermal regime of the interface between two overlapping subducting plates, such as those beneath Kanto, Japan, is thought to play an important role in affecting the distribution of interplate and intraslab earthquakes, the estimation of the thermal regime remains challenging to date. We constructed a three-dimensional (3-D) thermal convection model to simulate the subduction of the Pacific plate along the Japan Trench and Izu-Bonin Trench, including the subduction of the Philippine Sea beneath Kanto and investigated the slab thermal regime and slab water contents in this complex tectonic setting. Based on the subduction parameters tested in generic models with two flat oceanic plates, a faster or thicker plate subducting in a more trench-normal direction produces a colder slab thermal regime. The interplate temperature of the cold anomaly beneath offshore Kanto was approximately 300°C colder than that beneath offshore Tohoku at a same depth of 40 km and approximately 600°C colder at a depth of 70 km. The convergence between the two subducting plates produces an asymmetric thermal structure in the slab contact zone beneath Kanto, which is characterized by clustered seismicity in the colder southwestern half. The thermo-dehydration state of the mid-ocean ridge basalt near the upper surface of the subducted Pacific plate controls the interplate seismicity beneath the Kanto-Tohoku region according to the spatial concurrence of the thermo-dehydration and seismicity along the megathrust fault zone of the subducted Pacific plate.

Subducting plates around the globe display a large variability in terms of slab geometry, including regions where smooth and little variation in subduction parameters is observed. While the vast majority of subduction slabs plunge into the mantle at different, but positive dip angles, the end-member case of flat-slab subduction seems to strongly defy this rule and move horizontally several hundreds of kilometers before diving into the surrounding hotter mantle. By employing a comparative assessment for the Mexican, Peruvian and Chilean flat-slab subduction zones we find a series of parameters that apparently facilitate slab flattening. Among them, trench roll-back, as well as strong variations and discontinuities in the structure of oceanic and overriding plates seem to be the most important. However, we were not able to find the necessary and sufficient conditions that provide an explanation for the formation of flat slabs in all three subduction zones. In order to unravel the origin of flat-slab subduction, it is probably necessary a numerical approach that considers also the influence of surrounding plates, and their corresponding geometries, on 3D subduction dynamics.

New GPS measurements in Chiapas (Mexico), Guatemala and El Salvador are used to constrain the fault kinematics in the North America (NA), Caribbean (CA) and Cocos (CO) plates triple junction area. The regional GPS velocity field is first analysed in terms of strain partitioning across the major volcano-tectonic structures, using elastic half-space modelling, then inverted through a block model. We show the dominant role of the Motagua Fault with respect to the Polochic Fault in the accommodation of the present-day deformation associated with the NA and CA relative motion. The NA/CA motion decreases from 18-22 mm yr−1 in eastern Guatemala to 14-20 mm yr−1 in central Guatemala (assuming a uniform locking depth of 14-28 km), down to a few millimetres per year in western Guatemala. As a consequence, the western tip of the CA Plate deforms internally, with ≃9 mm yr−1 of east-west extension (≃5 mm yr−1 across the Guatemala city graben alone). Up to 15 mm yr−1 of dextral motion can be accommodated across the volcanic arc in El Salvador and southeastern Guatemala. The arc seems to mark the northern boundary of an independent forearc sliver (AR), pinned to the NA plate. The inversion of the velocity field shows that a four-block (NA, CA, CO and AR) model, that combines relative block rotations with elastic deformation at the block boundaries, can account for most of the GPS observations and constrain the overall kinematics of the active structures. This regional modelling also evidences lateral variations of coupling at the CO subduction interface, with a fairly high-coupling (≃0.6) offshore Chiapas and low-coupling (≃0.25) offshore Guatemala and El Salvador.

The Trans-Mexican Volcanic Belt (TMVB) is a 1000 km long Neogene continental arc showing a large variation in composition and volcanic style, and an intra-arc extensional tectonics. It overlies the Rivera and Cocos slabs, which display marked changes in geometry. Geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc, the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc, and the lower crust beneath the arc is partially molten. East of 101°W the TMVB is built on a Precambrian to Paleozoic crust with thickness of 50–55 km. West of 101°W the TMVB is underlain by Jurassic to Cenozoic marine and continental arcs with a 35–40 km thick crust. The evolution of the TMVB occurred in four stages: 1) from ~ 20 to 10 Ma the initial andesitic arc moved inland showing progressively drier melting and, eventually, slab melting, suggesting flattening of the subducted slab; 2) since ~ 11 Ma a pulse of mafic volcanism migrated from west to east reaching the Gulf of Mexico by 7 Ma. This mafic lavas marks the lateral propagation of a slab tear, triggered by cessation of subduction beneath Baja California; 3) thereafter, the volcanic front started moving trenchward, with a marked phase of silicic volcanism between 7.5 and 3 Ma, local emplacement of small volume intraplate-like basalts since 5 Ma, and development of extensional faulting. These features are related to slab rollback, enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust; 4) the modern arc consists of a frontal belt dominated by flux and slab melting, and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint.

How flat slab geometries are generated has been long debated. It has been suggested that trenchward motion of thick cratons in some areas of South America and Cenozoic North America progressively closed the asthenospheric wedge and induced flat subduction. Here we develop time-dependent numerical experiments to explore how trenchward motion of thick cratons may result in flat subduction. We find that as the craton approaches the trench and the wedge closes, two opposite phenomena control slab geometry: the suction between ocean and continent increases, favoring slab flattening, while the mantle confined within the closing wedge dynamically pushes the slab backward and steepens it. When the slab retreats, as in the Peru and Chile flat slabs, the wedge closure rate and dynamic push are small and suction forces generate, in some cases, flat subduction. We model the past 30 m.y. of subduction in the Chilean flat slab area and demonstrate that trenchward motion of thick lithosphere, 200–300 km, currently ∼700–800 km away from the Peru-Chile Trench, reproduces a slab geometry that fits the stress pattern, seismicity distribution, and temporal and spatial evolution of deformation and volcanism in the region. We also suggest that varying trench kinematics may explain some differing slab geometries along South America. When the trench is stationary or advances, the mantle flow within the closing wedge strongly pushes the slab backward and steepens it, possibly explaining the absence of flat subduction in the Bolivian orocline.

The mobility of gravity-driven granular flows such as debris flows or pyroclastic density currents are extremely sensitive to topographic changes, such as break in slopes, obstacles, or ravine deviations. In hazard assessment, computer codes can reproduce past events and evaluate hazard zonation based on inundation limits of simulated flows over a natural terrain. Digital Elevation Model (DEM) is a common input for the simulation algorithm and its accuracy to reproduce past flows is crucial. In this work, we use TITAN2D code to reproduce past block-and-ash flows at Colima volcano (Mexico) over DEMs with different cell size (5, 10, 30, 50, and 90 m) in order to illustrate the influences of the resolution on the numeric simulations. Our results show that topographic resolution significantly affects the flow path and runout. Also, we found that simulations of past flows with the same input parameters (such as the basal friction angle) over topography with different resolutions resulted in different flow paths, areas, and thickness of the simulated flows. In particular, the simulations performed with the 5- and 10-m DEMs produced similar results. Also, we obtained consistent simulation results for the 30- and 50-m DEMs. However, for the coarser 90-m DEM results are largely different and inaccurate. We recommend generating a benchmark table in order to acquire characteristic values for the basal friction angle of studied events. In case of rugged topographies, a DEM with high resolution should be used for more confident results. © 2011 Springer Science+Business Media B.V.

We investigate the regional thermal structure of the crust in Mexico using Curie Point Depth (CPD) estimates. The top and bottom of the magnetized crust were calculated using the power-density spectra of the total magnetic field from the freely available “Magnetic Anomaly Map of North America”. We applied this method to estimate the regional crustal thermal structure in overlapping square windows of 2° × 2°. The CPD estimates range between 10 and 40 km and show several regions of relatively shallow and deep magnetic sources, with a general inverse correlation with measured heat flow. A deep CPD region (20–30 km) is located in the fore-arc area where the subducting Cocos plate has a flat-slab geometry. This deep region is bound to the NW and SE by shallow CPD areas beneath the states of Michoacan (CPD = 12–16 km) and Oaxaca (CPD = ~16 km), respectively. There is a good spatial correlation between this deep CPD area and two main fracture zones located on the incoming Cocos plate (Orozco and O’Gorman fracture zones), suggesting that subduction plays an important role in setting apart different CPD provinces along the Mexican coast. Another deep CPD (16–32 km) area corresponds to the region where the Rivera plate subducts beneath Jalisco block. The Trans-Mexican Volcanic Belt is characterized by a decrease in Curie depths from west (16–20 km) to east (10–12 km). Finally, several deep CPD areas are situated in the back-arc region where old Mesozoic terrains are present. Our results suggest that the main control on the crust’s regional thermal structure in the fore-arc and volcanic arc regions is due to the subduction of the Cocos and Rivera plates beneath Mexico.

Lahars are hazardous events that can cause serious damage to people who live close to volcanic areas; several were registered at different times in the last century, such as at Mt St Helens (USA) in 1980, Nevado del Ruiz (Colombia) in 1985 and Mt Pinatubo (Philippines) in 1990. Risk maps are currently used by decision-makers to help them plan to mitigate the hazard-risk of lahars. Risk maps are acquired based on a series of tenets that take into account the distribution and chronology of past lahar deposits, and basically two approaches have been used: (1) The use of Flow Simulation Software (FSS), which simulates flows along channels in a Digital Elevation Model and (2) The Geochronological Method (GM), in which the mapping is based on the evaluation of lahar magnitude and frequency. This study addresses the production of a lahar risk map using the two approaches (FSS and GM) for a study area located at Popocatépetl volcano – Central Mexico. Santiago Xalitzintla, a town located on the northern flank of Popocatépetl volcano, where volcanic activity in recent centuries has triggered numerous lahars that have endangered local inhabitants, has been used for the case study. Results from FSS did not provide satisfactory findings because they were not consistent with lahar sediment observations made during fieldwork. By contrast, the GM produced results consistent with these observations, and therefore we use them to assess the hazard and produce the risk map for the study area.

Although commonly attributed to a mantle plume, time-transgressive magmatism of the Snake River Plain–Yellowstone (SRPY) province differs in important ways from that associated with typical oceanic hotspots. A fundamental question concerns the relative contributions of lithosphere vs. upwelling sub-lithospheric mantle to formation of SRPY basaltic magmas. Specifically, association of this province with initially thick and cold Archean lithosphere (Wyoming craton) poses a problem in that this lid will hinder and possibly prevent melting of rising plume material. Assuming an anhydrous peridotite mantle, melting can only occur if (1) the lid can be substantially thinned over geologically reasonable time and/or (2) the upwelling material is exceptionally warm, or (3) the lid was suitably thin to begin with. Petrologic modeling indicates that SRPY primitive basalts last segregated from mantle at conditions (< 1500 °C and ~100 km depth; Leeman, W.P., Schutt, D.L., Hughes, S.S., 2009. Thermal structure beneath the Snake River Plain: implications for the Yellowstone hotspot. J. Volcanol. Geotherm. Res.) only slightly warmer than MORB-source mantle and significantly cooler than sources of many oceanic hotspot magmas. In this study, geodynamic models were developed to evaluate lithospheric thinning processes. The motivation for the modeling is the observation that if the lithosphere is initially more than 200 km thick – typical of many cratons – then thinning by at least a factor of two is required to allow decompression melting of an ascending plume, assuming low volatile content and high excess temperature (potential temperature > 1500 °C). Fully dynamic models were applied to investigate the extent and rate of lithosphere thinning assuming an initial structure representative of the Wyoming craton. We find that thermal erosion by plume impingement alone appears incapable of providing the required lithospheric thinning. Alternative models (e.g., low-angle Laramide subduction, lithospheric delamination) also conflict with geochemical evidence that SRPY basalts contain a dominant contribution of old, isotopically evolved mantle material — presumably derived from subcontinental lithospheric mantle (SCLM). We conclude that SCLM is likely to be preserved, that the thick SCLM lid prevents substantial melting of rising plume material (tomographically imaged), and that SRPY basalts are predominantly derived by melting of lithospheric mantle.

The origin of El Chichon volcano is poorly understood, and we attempt in this study to demonstrate that the Tehuantepec Ridge (TR), a major tectonic discontinuity on the Cocos plate, plays a key role in determining the location of the volcano by enhancing the slab dehydration budget beneath it. Using marine magnetic anomalies we show that the upper mantle beneath TR undergoes strong serpentinization, carrying significant amounts of water into subduction. Another key aspect of the magnetic anomaly over southern Mexico is a long-wavelength (~150 km) high amplitude (~500 nT) magnetic anomaly located between the trench and the coast. Using a 2D joint magnetic-gravity forward model, constrained by the subduction P–T structure, slab geometry and seismicity, we find a highly magnetic and low-density source located at 40–80 km depth that we interpret as a partially serpentinized mantle wedge formed by fluids expelled from the subducting Cocos plate. Using phase diagrams for sediments, basalt and peridotite, and the thermal structure of the subduction zone beneath El Chichon we find that ~40% of sediments and basalt dehydrate at depths corresponding with the location of the serpentinized mantle wedge, whereas the serpentinized root beneath TR strongly dehydrates (~90%) at depths of 180-200 km comparable with the slab depths beneath El Chichon (200- 220 km). We conclude that this strong deserpentinization pulse of mantle lithosphere beneath TR at great depths is responsible for the unusual location, singularity and, probably, the geochemically distinct signature (adakitic-like) of El Chichon volcano.

During 1997 and 2001 two lahars took place in the Tenenepanco and Huiloac gorges on the northeastern sector of the Popocatepetl volcano in Mexico. These lahars were the result of volcanic activity characterized by a pyroclastic fall and flow. The lahars were triggered by the mass failures produced hours after the eruption. The well-known superelevation technique commonly used in this kind of study was applied to infer the lahar's mean velocity. Estimations show a broad flow velocity range of 1.3– 13.8 m/s depending on the following geomorphological parameters: channel depth and slope, peak discharge and distance from the initial source. These results were compared with lahar velocities from Mount St Helens (1980) and Nevado del Ruiz (1985), which were obtained by the same superelevation technique. According to the data, the lahars from Nevado del Ruiz and Popocatepetl were triggered by similar conditions, while at Mount St Helens they were the result of the conversion of a pyroclastic surge cloud. Actually, the Mount St Helens lahars were almost twice as fast as the ones at Nevado del Ruiz and Popocatepetl. This observation shows the importance of the lahars' genesis in that the pyroclastic surge clouds produce faster lahars than do pyroclastic falls. Such results should be taken into account in lahar risk assessment studies for communities located near active volcanoes. © 2007 Elsevier B.V. All rights reserved.

The origin of the Central Mexican Volcanic Belt (CMVB) and the influence of the subducting Cocos plate on the CMVB volcanism are still controversial. In this study, the temperature and mantle wedge flow models for the Mexican subduction zone are developed using the finite element method to investigate the thermal structure below CMVB. The numerical scheme solves a system of 2D Stokes equations and 2D steady-state heat transfer equation. Two models are considered for the mantle wedge: the first one with an isoviscous mantle wedge and the second one with strong temperature-dependent viscosity. The first model reveals a maximum temperature of ∼830 ◦ C in the mantle wedge, which is not sufficient for melting of wet peridotite. Also, the geotherm of the subducting plate upper surface does not intersect the dehydration-melting solidus for mafic minerals. The second model predicts temperatures of more than 1200 ◦ C beneath the CMVB for a wide range of rheological parameters (reference viscosity and activation energy). Up to 0.6 wt.% H2O can be released down to 60 km depth through metamorphic changes in the oceanic crust of the subducting slab. The melting of this oceanic crust apparently occurs in a narrow depth range of 50–60 km and also melting of the mantle wedge hydrated peridotite is now expected to take place beneath CMVB. Considering that the melting processes on and in the vicinity of the subducting plate surface generate the most of the volcanic material, a dynamic model for the blob tracers is developed using Stokes flow at infinite Prandtl number. The blobs of 0.2–10.0 km in diameter migrate along very different trajectories only at low wrapping viscosities ( ηw =1 0 14 –5 × 10 17 Pa s). The modeling results show that the “fast” trajectories terminate at the same focus location at the base of the continental crust, while the arrival points of “slow” trajectories, which are common for the blobs of smaller size (∼0.4–0.5 km), are scattered away from the average focus location. This observation may give us a hint on a possible mechanism of strato and mono volcanoes genesis. The rise

SUMMARY Temperature is one of the most important factors that controls the extent and location of the seismogenic coupled and transition, partially coupled segments of the subduction interplate fault. The width of the coupled fault inferred from the continuous GPS observations for the steady interseismic period and the transient width of the last slow aseismic slip event (M w ∼ 7.5) that occurred in the Guerrero subduction zone in 2001‐2002 extends up to 180‐220 km from the trench. Previous thermal models do not consider this extremely wide coupled interface in Guerrero subduction zone that is characterized by shallow subhorizontal plate contact. In this study, a finite element model is applied to examine the temperature constraints on the width of the coupled area. The numerical scheme solves a system of 2-D Stokes equation and 2-D steady-state heat transfer equations. The updip limit of the coupling zone is taken between 100 and 150 ◦ C, while the downdip limit is accepted at 450 ◦ C as the transition from partial coupling to stable sliding. From the entire coupled zone, the seismogenic zone extends only up to ∼82 km from the trench (inferred from the rupture width of large subduction thrust earthquakes), corresponding to the 250 ◦ C isotherm. Only a small amount of frictional heating is needed to fit the intersection of the 450 ◦ C isotherm and the subducting plate surface at 180‐205 km from the trench. The calculated geotherms in the subducting slab and the phase diagram for MORB are used to estimate the metamorphic sequences within the oceanic subducting crust. A certain correlation exists between the metamorphic sequences and the variation of the coupling along the interplate fault.

For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes. Subduction of fracture zones is predicted to have local geochemical and physical manifestations in volcanic arcs. Here, the authors show boron enrichment near fracture zones in some arcs and infer the processes occurring there using detailed geodynamic modelling.

Lahars are a type of catastrophic event that is generated in volcanic environments. Due to the recent technological advances computer simulated models have become widely used by the scientific community to predict the distribution of catastrophic events, such as floods, debris avalanches, earth landslides or lahars, and to mitigate possible damage to populations located in a risk area. Our study risk area is located in the Popocatepetl volcano, a large and active stratovolcano in the Trans Mexican Volcanic Belt. Here, lahars have been occurring during the last century, and some of them have seriously affected nearby towns. Since one of the main input parameters into a lahar simulation is the Digital Elevation Model (DEM), this paper focuses on studying lahar simulations on Popocatepetl using DEMs obtained from various sources with the aid of a popular package known as LAHARZ. The first simulations were performed using a 1:50,000 topographic map with 20-m contours interval. The second set of tests used an enhanced DEM with profiles measured during fieldwork in the lahar channel. In the last set of tests, we added lateral artificial barriers to the channel profiles used in the previous enhanced DEM. Our results show that the use of high resolution and detailed DEMs does not necessarily guarantee a realistic lahar simulation as was previously thought, and a detailed explanation as to why the LAHARZ code computation creates ragged edges is offered. Our results also offer useful information on the level of detail necessary for an input DEM into LAHARZ in order to obtain lahar simulations closer to real lahars. The minimum resolution for the DEM used in simulations must be less than the width of the narrowest cross-section of the channel to realistically reproduce the lahars. For DEMs with lower resolution, sometimes the modeled lahars are shorter in length, and therefore underestimate real hazards to populations. Also, the results show that lateral barriers are sometimes needed to obtain more realistic inundation areas.

In southern Mexico, the subducting Cocos slab drastically changes its geometry: from a flat slab in central Mexico to a ∼45° dip angle beneath Chiapas. Also, the currently active volcanic arc, the modern Chiapanecan volcanic arc, is oblique and situated far inland from the Middle America trench, where the slab depth is ∼200 km. In contrast, the Central America volcanic arc is parallel to the Middle America trench, and the slab depth is ∼100 km. A two-dimensional steady-state thermomechanical model explains the calc-alkaline volcanism by high temperature (∼1300 °C) in the mantle wedge just beneath the Central America volcanic arc and the strong dehydration (∼5 wt%) of the Cocos slab. In contrast, the thermal model for the modern Chiapanecan volcanic arc shows high P-T conditions beneath the coast where the extinct Miocene Chiapanecan arc is present, and is therefore unable to offer a reasonable explanation for the origin of the modern Chiapanecan volcanic arc. We propose a model in which the origin of the modern Chiapanecan volcanic arc is related to the space-time evolution of the Cocos slab in central Mexico. The initiation of flat subduction in central Mexico in the middle Miocene would have generated a hot mantle wedge inflow from NW to SE, generating the new modern Chiapanecan volcanic arc. Because of the contact between the hot mantle wedge beneath Chiapas and the proximity of a newly formed cold, flat slab, the previous hot mantle wedge in Chiapas became colder in time, finally leading to the extinction of the Miocene Chiapanecan volcanic arc. The position and the distinct K-alkaline volcanism at El Chichon volcano are proposed to be related to the arrival of the highly serpentinized Tehuantepec Ridge beneath the modern Chiapanecan volcanic arc. The deserpentinization of Tehuantepec Ridge would have released significant amounts of water into the overlying mantle, therefore favoring vigorous melting of the mantle wedge and probably of the slab.

The Tehuantepec Ridge is one of the most prominent lithospheric structures of the Cocos Plate, yet its tectonic evolution has remained poorly constrained until now. Calculated ocean floor ages and spreading rates, morphostructural analysis of the ridge and the surrounding ocean floor are used to infer the tectonic evolution and pattern of the Tehuantepec Ridge and associated structures. The ocean floor age estimates south of the Tehuantepec Ridge suggest an age of ∼26 Ma at the Middle America trench. A mean age difference across the Tehuantepec Ridge of ∼7 Ma suggests that the Tehuantepec Ridge would be formed as a long transform fault on the Guadalupe plate, prior to 15 Ma. The full-spreading rate estimates show that there might be a differential full-spreading rate across the transform fault between 15 and 5 Ma, when the oceanic plate just south of Clipperton Fracture Zone was decelerating between 15 and 10 Ma, then accelerating for the next 5 Ma. We propose a model in which between 13 and 8 Ma temporarily transpressional intratransform-spreading centers would have been existing as a consequence of the East Pacific Rise onset with an angle of ∼10° clockwise with respect to the ceased Mathematician ridge. Our model estimates indicate that between 21 and 15 Ma a prominent pseudofault was formed south of the Tehuantepec Ridge due to an unstable offset on the Mathematician ridge that migrated northward. The Tehuantepec Ridge and the pseudofault trace bound a deeper oceanic basin than the surrounding area, with no corresponding anomalous depth on the western side of the East Pacific Rise. The complete absence of a pseudofault trace and a deeper oceanic basin on the western side of the East Pacific Rise, as well as the spreading rate changes, suggest the existence of a microplate embedded into the Cocos Plate just south of the Tehuantepec Ridge. Also, the asymmetric and sharp morphology of the Tehuantepec Ridge suggests that it may be the expression of a major transpressional structure along the former transform fault on the Guadalupe plate 15–20 Ma ago.

Una secuencia de perfiles de anomalía de gravedad de aire libre a través de la Trinchera de Mesoamérica es usada para modelar el relleno sedimentario de los facies sedimentarios no consolidados pelágicos y hemipelágicos y del material parcialmente alterado del basamento. La diferencia entre los mínimos gravimétricos y batimétricos se utiliza en la estimación de la cantidad de los sedimentos de densidad baja. El efecto de gravedad de relleno es relativamente pequeño, sugiriendo que el proceso mayor en la Trinchera de Mesoamérica es la subducción de sedimentos y el raspar de los sedimentos pelágicos desde la cima de la placa oceánica subducida. El volumen de los sedimentos en la trinchera tiende a incrementarse hacia el sur desde Jalisco hasta Oaxaca. Esta tendencia está menos clara en la Cuenca de Guatemala. Hay una cierta correlación entre el monto de relleno sedimentario fresco y la velocidad de convergencia a la trinchera excepto los perfiles con una contribución terrígena de sedimentos o las áreas de subducción de las entidades batimétricas importantes.