Central American Subduction Zone Research Articles

Variation in the thermal and dehydration regime below Central America: Insights for the seismogenic plate interface

Slow earthquakes predominant in Costa Rica indicate unstable faulting of segmented Central American megathrusts, but the recurrence of episodic tremors and slips reported to precede a giant earthquake remains still enigmatic. The underlying mechanism is related to the variation in the coupling along the heterogeneous subduction interface which is poorly understood. In this study, we used up-to-date 3D thermal modeling to provide insights into the along-strike variation in the thermal state and hydraulic distribution beneath the Central American subduction zone. Our results show that the subducted Cocos Plate is much warmer than previously estimated, and the slab geometry exhibits remarkable perturbations along the trench. We found that the regions of large dehydration rate along the slab are consistent with the seismicity occurrence depth beneath the Moho. Below the Nicoya Peninsula and the Guatemala-Nicaragua segment of megathrusts, fluids derived from subducted slab result in increased pore fluid pressures and subsequent recurrence of slow slip events and regular earthquakes.

Chemical composition of sediments from the subducting Cocos Ridge segment at the Southern Central American subduction zone

Chemical composition of sediments from the subducting Cocos Ridge segment at the Southern Central American subduction zone

Shallow Slow Earthquake Episodes Near the Trench Axis off Costa Rica

AbstractSlow earthquakes are mainly distributed in regions surrounding seismogenic zones along the plate boundaries of subduction zones. In the Central American subduction zone, large regular interplate earthquakes with magnitudes of 7–8 occur repeatedly around the Nicoya Peninsula, in Costa Rica, and a tsunami earthquake occurred off Nicaragua, just north of Costa Rica, in 1992. To clarify the spatial distribution of various slip behaviors at the plate boundary, we detected and located very low frequency earthquakes (VLFEs) around the Nicoya Peninsula using a grid‐search matched‐filter technique with synthetic templates based on a regional three‐dimensional model. VLFEs were active in September 2004 and August 2005, mainly near the trench axis, updip of the seismogenic zone. The distribution of VLFEs overlapped with large slip areas of slow slip events. Low frequency tremor signals were also found in high‐frequency seismogram envelopes within the same time windows as detected VLFEs; thus, we also investigated the energy rates of tremors accompanied by VLFEs. The range of scaled energy, which is the ratio of the seismic energy rate of a tremor to the seismic moment rate of accompanying VLFE and related to the rupture process of seismic phenomena, was 10−9–10−8. The along‐dip separation of shallow slow and large earthquakes and the range of the scaled energy off Costa Rica are similar to those in shallow slow earthquakes in Nankai, which shares a similar thermal structure along the shallow plate boundary.

Recycling of nitrogen and light noble gases in the Central American subduction zone: Constraints from 15N15N

How much nitrogen and light noble gases are recycled in modern subduction zones is unclear. Fumaroles act as a means for passive degassing in arcs. They receive variable contributions of volatiles from arc magmas, themselves sourced from the mantle wedge. The gas compositions reflect the extent of volatile enrichment in sub-arc mantle sources and constrain slab dehydration. However, contributions from atmospheric components in fumaroles are unavoidable. For N2, neon and argon, the atmospheric components are challenging to discern from slab-derived components. Here, we report 15N15N measurements from eight fumaroles and seven bubbling springs, along the Central American arc. Our new 15N15N data are coupled with noble gases measurements and show that air-derived components in volcanic gas discharges can easily be underestimated, in both fumaroles and springs, using conventional stable isotope or noble gases methods. We show that, in the absence of 15N15N data, previously used tracers for air (e.g., δ15N, N2/Ar, N2/He, among others) may lead to erroneous conclusions regarding the origin of volatiles in mixed gases. In contrast, 15N15N data provide quantitative constraints on the nature and contributions of both atmospheric and magmatic components. Most springs are heavily dominated by air-derived N2, while fumaroles show substantial contributions of volcanic endmembers. Based on the fumarole data, we show that magma sources beneath the central American arc are enriched in all volatiles relative to 3He, by two to three orders of magnitude compared to the MORB source. We use new 15N15N data to obtain source N2/3He, 3He/36Ar and 3He/22Ne ratios which we then use to compute volcanic N2, Ar and Ne degassing fluxes. Using this approach, we show that outgassing fluxes appear to match subduction fluxes in the Central America subduction zone. We determine an N2 outgassing flux of between 4.0×108 and 1.0×109 mol N2/y, comparable to the subduction flux of 5.7×108 mol N2/yr determined previously. We obtain a similar conclusion for 22Ne and 36Ar. Overall, the volatile fluxes in the central American subduction zone no longer seem to require net transfer of N2, Ar, and Ne, to the deep mantle.

Chemical composition of sediments from the upper plate at the southern Central American subduction zone and its geological significance

Chemical composition of sediments from the upper plate at the southern Central American subduction zone and its geological significance

A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: Implications for subduction budget in Central America

The geochemical behavior of N during seawater-oceanic crust alteration remains poorly constrained. Yet, it is a central parameter to assess the flux of N to subduction zones. Most studies proposed that hydrothermally altered basaltic rocks are enriched in N relative to fresh basalts. However, published data from DSDP/ODP Hole 504B, a reference site for the composition of the oceanic crust, suggest that seawater alteration leads to the N depletion of the upper ocean crust. To better address this issue, we analyzed N concentration and isotope composition of 21 altered basalts from the lavas and sheeted dikes sampled by Hole 504B. These new analyses show significant N enrichment (up to 14.1 ppm) relative to fresh degassed MORB (∼1 ppm). The differences observed between earlier and modern data are interpreted as resulting from analytical artifact due to the earlier use of a molybdenum crucible for N extraction. Furthermore, our new data show a progressive decrease of N concentration with depth, from 14.1 to 1.4 ppm. Nitrogen isotope compositions display a large range, with δ15N values from −0.9 to +7.3‰, and most likely reflect multiple stages of alteration with fluids of various compositions. In contrast to N concentration, δ15N values do not show a global depth trend but oscillate around a mean value of 3.0 ± 2.2‰ (1SD). The N concentration shows a positive correlation with bulk rock δ18O values, suggesting that N behavior during alteration process is mainly controlled by temperature. We propose that N speciation in the hydrothermal fluid is dominated by NH3/NH4 at low temperature (<200°C) but is transformed to N2, associated with H2, at higher temperature (>200°C). These new data are used to re-evaluate the global flux of N input into Central American subduction zone, showing that the upper basaltic crust represent about 20% of the total N buried in subduction zone. A comparison with previous results obtained on N degassed in volcanic arc illustrates that, in “warm” subduction zone like Central America, up to 50% of the subducted N may be transferred to the deep mantle. This contrasts with “cold” subduction environments, where >80% of the N inputs escape sub-arc slab devolatilization and supports that the geothermal gradient plays a major role in determining the N fate in subduction zones.

Hydrothermal versus active margin sediment supply to the eastern equatorial Pacific over the past 23 million years traced by radiogenic Pb isotopes: Paleoceanographic and paleoclimatic implications

We investigated the evolution of the Pb isotopic composition of bulk sediments on the Cocos Plate in sedimentary successions of Deep Sea Drilling Project (DSDP) Site 495 and Ocean Drilling Program/Integrated Ocean Drilling Program (ODP/IODP) Site 1256 over the past 23million years of depositional history. Our study addresses the relationship of the sediment Pb isotope record to plate tectonics, weathering inputs, and paleoceanography. It is the first effort to characterize the Pb isotopic evolution of eastern equatorial Pacific sedimentation covering the entire tectonic pathway of the Cocos Plate from its formation at the East Pacific Rise to its arrival at the Central American subduction zone. The Sites 495 and 1256 bulk sediment Pb isotope records are fully consistent over time despite distinct differences between the type of sediment deposited at both locations. A systematic and continuous trend from ∼23 to ∼6–4Ma toward more radiogenic Pb isotopic compositions, e.g., 206Pb/204Pb ratios increase from 18.29 to 18.81, reflects a decrease in the contribution of hydrothermal particles from the East Pacific Rise and an increase in the predominantly eolian contribution of mixed weathering products from the continental arcs of the Northern and south Central Andes as well as from southern Mexico. Surprisingly, both the Pb isotopic composition of the detrital fraction and that of past seawater indicate that inputs from nearby Central America and the Galápagos Archipelago did not significantly contribute to the sediments of our core locations but were overwhelmed by other sediment sources. A systematic change to less radiogenic Pb isotope ratios in sediments younger than ∼4–3Ma, reaching present-day 206Pb/204Pb values near 18.70, reflects a reduction of the continental input from the South Central Volcanic Zone of the Andean Arc and increased contributions from southern Mexican igneous complexes. This isotopic trend reversal took place as a consequence of changes in atmospheric circulation, when the studied sites crossed the Intertropical Convergence Zone due to tectonic drift and concurrent climate cooling. Eolian transport has played a major role in the supply of detrital material over the entire Neogene and Quaternary. The delivery of hydrothermal Pb originating from the East Pacific Rise to the easternmost tropical Pacific has been a persistent feature that is attributed to a remarkably stable central and eastern Pacific deep-water flow pattern over millions of years. Thus, deep ocean circulation did not change significantly either (1) as a consequence of an Early Miocene closure of the deep gateway between the Caribbean and eastern Central Pacific or because (2) a Late Miocene to Pliocene closure of the Central American Seaway had no impact at all.

Estado de esfuerzos estáticos en la interface de placas Coco-Caribe en Costa Rica: Interpretación mecánica para el origen de asperezas en la zona sismogénica

La región del antearco a lo largo de la zona de subducción centroamericana muestra una serie de anomalías positivas de gravedad paralelas a la trinchera con anomalías negativas correspondientes, a lo largo de la fosa y hacia la costa. Estas anomalías se extienden desde Guatemala hasta el norte de Nicaragua. Sin embargo, el segmento costarricense del antearco no sigue este patrón debido a la segmentación de la anomalía negativa de la fosa, la ausencia de la anomalía negativa costera y la presencia de masa continental emergida en el lugar de la anomalía positiva del antearco en la Península de Nicoya. Estudios geodésicos y sismológicos a lo largo de la zona de subducción costarricense sugieren la presencia de áreas de acoplamiento en la zona sismogénica bajo la península de Nicoya previo al sismo de magnitud 7,6 Mw del 5 de setiembre de 2012. Estas áreas han sido asociadas anteriormente con asperezas. Publicaciones previas han propuesto un modelo mecánico para el origen de asperezas a lo largo del margen convergente chileno, basado en la estructura de la placa cabalgante. Basado en la estructura de la placa cabalgante, publicaciones previas han propuesto un modelo mecánico para el origen de asperezas a lo largo del margen convergente chileno, en el cual el material ígneo denso en el antearco de la placa cabalgante sobre la zona sismogénica, modifica el estado de esfuerzos estáticos en la interface de placas e influye en los procesos sismogénicos. En Costa Rica, la geología de superficie y los datos de gravedad indican la presencia de corteza densa compuesta por basaltos y gabros que sobreyace la zona sismogénica en donde el acoplamiento está presente. Los valores de anomalía de Bouguer en esta región alcanzan los 120×10-5 m/s2 y son los más altos medidos para Costa Rica. En este trabajo, el estado de esfuerzos estáticos en la interface de placas Cocos-Caribe se calcula basado en la geometría y en la distribución de densidad de un modelo tridimensional de la litósfera de la zona de subducción interpretado a partir de datos de gravedad de modelos geopotenciales combinados. Los resultados muestran una correlación espacial entre las áreas acopladas en la Península de Nicoya y la presencia de anomalías en el estado de esfuerzos estáticos en la interface de placas. Las anomalías de esfuerzo se calcularon para el componente normal del esfuerzo vertical en la zona sismogénica y se interpretaron como originadas por el material denso que compone el antearco en el área. El material denso de las rocas máficas del Complejo de Nicoya y la carga topográfica de la península sobre la zona sismogénica influyen en la distribución de zonas acopladas y el comportamiento sísmico de la región. Esto se debe a que las anomalías en el esfuerzo normal a la interface de placas incrementan el umbral de esfuerzo cortante para generar la ruptura.

Late Miocene to recent plate tectonic history of the southern Central America convergent margin

AbstractNew plate reconstructions constrain the tectonic evolution of the subducting Cocos and Nazca plates across the southern Central American subduction zone from late Miocene to recent. Because of the strong relationships between lower and upper (Caribbean) plate dynamics along this margin, these constraints have wide‐ranging implications for the timing and growth of upper plate deformation and volcanism in southern Central America. The reconstructions outline three important events in the Neogene history of this margin: (1) the coeval development of the Panama Triple Junction with the initiation of oblique subduction of the Nazca plate at ∼8.5 Ma; (2) the initiation of seamount and rough crust subduction beginning at ∼3–4 Ma; and (3) Cocos Ridge subduction from ∼2 to 3 Ma. A comparison of these events with independent geologic, geomorphic, volcanic, and stratigraphic data sets reveals that the timing, rates, and origin of subducting crust directly impacted the Neogene growth of upper plate deformation and volcanism in southern Central America. These analyses constrain the timing, geometry, and causes of a number of significant tectonic and volcanic processes, including rapid Plio‐Quaternary arc‐fore arc contraction due to Cocos Ridge subduction, the detachment of the Panama microplate at ∼1–3 Ma, and the late Miocene cessation of mantle‐wedge‐derived volcanism across ∼300 km of the subduction zone.

Rock uplift and exhumation of continental margins by the collision, accretion, and subduction of buoyant and topographically prominent oceanic crust

AbstractUnderstanding the causes of rock and surface uplift is important because they control the location of mountain building, depocenters, and drainage characteristics and can influence climate. Here we combine previous thermochronological data with field observations to determine the amount of exhumation, rock, and surface uplift that occurs in the upper plate of Central and South American subduction zones during the collision, accretion, and subduction of oceanic plateaus and aseismic ridges. The collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 Ma. Uplift appears to be an immediate response to collision and is generally independent of the slab dip. The amount of rock uplift is controlled mainly by excess topography associated with the ridge (ultimately linked to buoyancy) and erosion, while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel. The quantity of exhumation is strongly dependant on climate‐induced erosion and the lifespan over which the topography is uplifted and supported. Sediment draining into the trench may leave the elevated ridge axis sediment starved, increasing the shear stresses at the ridge subduction interface, leading to positive feedback between ridge subduction, rock uplift, and exhumation. Trench‐parallel variations in exhumation have a direct impact on exploration paradigms for porphyry‐related metalliferous deposits, and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates.

Igneous Rock Associations 13. Focusing on the Central American Subduction Zone

Central America has recently been an important focus area for investigations into the complex processes occurring in subduction zones. Here we review some of the new findings concerning subduction input, magma production and evolution, and resultant volcanic output. In the Nicaraguan portion of the subduction zone, subduction input is unusually wet, likely caused by extensive serpentinization of the mantle portion of the incoming plate associated with bending-related faulting seaward of the Middle America trench. The atypical influx of water into the Nicaraguan section of the subduction zone ultimately leads to a regional maximum in the degree of mantle melting. In central Costa Rica, subduction input is also unusual in that it includes oceanic crust flavored by the Galapagos plume. Both of these exotic subduction inputs are recognizable in the compositions of magmas erupted along the volcanic front. In addition, Nicaraguan magmas bear a strong chemical imprint from subducting hemipelagic sediments. The high-field-strength-element depletions of magmas from El Salvador through Costa Rica are related to local variations in the depth to the subducting Cocos plate, and, therefore, to segmentation of the volcanic front. Minor phases, probably amphibole or rutile, control these variable depletions. Silicic magmas erupted along the volcanic front exhibit the same along-arc geochemical variations as their mafic brethren. This and their mantle-like radiogenic isotopic compositions suggest the production of juvenile continental crust all along the Central American subduction zone. Punctuated times of enhanced magmatic input from the mantle may aid in crustal development.SOMMAIREL’Amérique centrale a récemment été le lieu de recherches sur les processus complexes se produisant dans les zones de subduction. Ici nous passons en revue certaines découvertes sur nature des intrants de subduction, la production et l’évolution des magmas, ainsi que les extrants volcaniques résultants. Dans le segment nicaraguayen de la zone de subduction, les intrants de subduction sont exceptionnellement humides, probablement à cause de la serpentinisation généralisée de la portion mantélique de la plaque en subduction, fissurée par flexure dans partie marine de la fosse océanique de l’Amérique centrale. L'afflux atypique en eau dans le segment nicaraguayen de la zone de subduction induit ultimement un maximum régional de la proportion de fusion du manteau. Dans la portion centrale du Costa Rica l’intrant de subduction est lui aussi atypique en ce qu’il comprend une croûte océanique teintée par le panache des Galápagos. Ces deux intrants de subduction atypiques sont répercutés dans la composition des magmas éjectés le long du front volcanique. En outre, les magmas nicaraguayens affichent une forte empreinte chimique héritée des sédiments hémipélagiques en subduction. Les appauvrissements en éléments à fortes liaisons atomiques des magmas, du El Salvador jusqu’au Costa Rica, sont liés à des variations localisées de la profondeur de la plaque en subduction de Cocos, et donc, à la segmentation du front volcanique. Des phases mineures, probablement amphibole et rutile, déterminent ces appauvrissements variables. Les magmas siliceux éjectés le long du même front volcanique montrent les mêmes variations géochimiques le long de l’arc que leur contrepartie mafique. De plus, les compositions radiogéniques de leurs contreparties mantéliques évoquent la production d’une croûte continentale juvénile le long de la zone de subduction de l’Amérique centrale. Des épisodes d’accroissements ponctuels des intrants magmatiques du manteau peuvent contribuer au développement d’une croûte.

Volatile (H2O, CO2, Cl, S) budget of the Central American subduction zone

After more than a decade of multidisciplinary studies of the Central American subduction zone mainly in the framework of two large research programmes, the US MARGINS program and the German Collaborative Research Center SFB 574, we here review and interpret the data pertinent to quantify the cycling of mineral-bound volatiles (H2O, CO2, Cl, S) through this subduction system. For input-flux calculations, we divide the Middle America Trench into four segments differing in convergence rate and slab lithological profiles, use the latest evidence for mantle serpentinization of the Cocos slab approaching the trench, and for the first time explicitly include subduction erosion of forearc basement. Resulting input fluxes are 40–62 (53) Tg/Ma/m H2O, 7.8–11.4 (9.3) Tg/Ma/m CO2, 1.3–1.9 (1.6) Tg/Ma/m Cl, and 1.3–2.1 (1.6) Tg/Ma/m S (bracketed are mean values for entire trench length). Output by cold seeps on the forearc amounts to 0.625–1.25 Tg/Ma/m H2O partly derived from the slab sediments as determined by geochemical analyses of fluids and carbonates. The major volatile output occurs at the Central American volcanic arc that is divided into ten arc segments by dextral strike-slip tectonics. Based on volcanic edifice and widespread tephra volumes as well as calculated parental magma masses needed to form observed evolved compositions, we determine long-term (105 years) average magma and K2O fluxes for each of the ten segments as 32–242 (106) Tg/Ma/m magma and 0.28–2.91 (1.38) Tg/Ma/m K2O (bracketed are mean values for entire Central American volcanic arc length). Volatile/K2O concentration ratios derived from melt inclusion analyses and petrologic modelling then allow to calculate volatile fluxes as 1.02–14.3 (6.2) Tg/Ma/m H2O, 0.02–0.45 (0.17) Tg/Ma/m CO2, and 0.07–0.34 (0.22) Tg/Ma/m Cl. The same approach yields long-term sulfur fluxes of 0.12–1.08 (0.54) Tg/Ma/m while present-day open-vent SO2-flux monitoring yields 0.06–2.37 (0.83) Tg/Ma/m S. Input–output comparisons show that the arc water fluxes only account for up to 40 % of the input even if we include an “invisible” plutonic component constrained by crustal growth. With 20–30 % of the H2O input transferred into the deeper mantle as suggested by petrologic modeling, there remains a deficiency of, say, 30–40 % in the water budget. At least some of this water is transferred into two upper-plate regions of low seismic velocity and electrical resistivity whose sizes vary along arc: one region widely envelopes the melt ascent paths from slab top to arc and the other extends obliquely from the slab below the forearc to below the arc. Whether these reservoirs are transient or steady remains unknown.

A geodynamical perspective on the subduction of Cocos and Rivera plates beneath Mexico and Central America

Abstract The Middle America subduction zone (MASZ) is one of the world’ most complex convergent margins as it involves the subduction of the Rivera and Cocos young oceanic plates beneath the North American and Caribbean plates and is bounded by the Gulf of California rift and the Panama slab window. Characterized by contorted and unusual slab geometry, irregularly distributed seismicity and volcanism, exceptionally large slow slip events (SSE) and non-volcanic tremors (NVT), this subduction system represents a great natural laboratory for better understanding geodynamic processes at a fundamental level. Based on a solid observational foundation, and incorporating the latest experimental results into a coherent geodynamical framework, we shed light on the main processes controlling the subduction system evolution in this region. The tectonics, volcanism, slab geometry and segmentation along the margin are reviewed from a geodynamical perspective. We proposed and discussed a series of evolutionary scenarios for the Mexican and Central American subduction zones, providing a coherent starting base for future geodynamical modeling studies tailored to this active margin. We discuss comparatively the recently discovered SSEs and NVTs along the MASZ, and try to differentiate among the proposed mechanisms responsible for these observations. Finally we discuss the recent seismic anisotropy observations in a geodynamic context, offering an integrated view of mantle flow pattern along the entire active margin. Although the MASZ as a whole may be considered a fairly complicated region with many unusual features and sometimes controversial interpretations, its complexity and unusual characteristics can improve our knowledge about the linkage between deep and surface processes associated with subduction zone dynamics.

An Evaluation of the Applicability of Current Ground-Motion Models to the South and Central American Subduction Zones

The applicability of existing ground‐motion prediction equations (GMPEs) for subduction‐zone earthquakes is an important issue to address in the assessment of the seismic hazard affecting the Peru–Chile and Central American regions. Few predictive equations exist that are derived from local data, and these do not generally meet the quality criteria required for use in modern seismic hazard analyses. This paper investigates the applicability of a set of global and regional subduction ground‐motion models to the Peru–Chile and Central American subduction zones, distinguishing between interface and intraslab events, in light of recently compiled ground‐motion data from these regions. Strong‐motion recordings and associated metadata compiled by Arango, Strasser, Bommer, Boroschek, et al. (2011) and Arango, Strasser, Bommer, Hernandez, et al. (2011) have been used to assess the performance of the candidate equations following the maximum‐likelihood approach of Scherbaum et al. (2004) and its extension to normalized intraevent and interevent residual distributions developed by Stafford et al. (2008). The results of this study are discussed in terms of the transportability of GMPEs for subduction‐zone environments from one region to another, with a view to providing guidance for developing ground‐motion logic trees for seismic hazard analysis in these regions. Online Material: Tables summarizing the statistics for the Scherbaum et al. (2004) scoring system.

The seismically active Andean and Central American margins: Can satellite gravity map lithospheric structures?

The spatial resolution and quality of geopotential models (EGM2008, EIGEN-5C, ITG-GRACE03s, and GOCO-01s) have been assessed as applied to lithospheric structure of the Andean and Central American subduction zones. For the validation, we compared the geopotential models with existing terrestrial gravity data and density models as constrained by seismic and geological data. The quality and resolution of the downward continued geopotential models in the Andes and Central America decrease with increasing topography and depend on the availability of terrestrial gravity data. High resolution of downward continued gravity data has been obtained over the Southern Andes where elevations are lower than 3000m and sufficient terrestrial gravity data are available. The resolution decreases with an increase in elevation over the north Chilean Andes and Central America. The low resolution in Central America is mainly attributed to limited surface gravity data coverage of the region.To determine the minimum spatial dimension of a causative body that could be resolved using gravity gradient data, a synthetic gravity gradient response of a spherical anomalous mass has been computed at GOCE orbit height (254.9km). It is shown that the minimum diameter of such a structure with density contrast of 240kgm−3 should be at least ∼45km to generate signal detectable at orbit height. The batholithic structure in Northern Chile, which is assumed to be associated with plate coupling and asperity generation, is about 60–120km wide and could be traceable in GOCE data. Short wavelength anomalous structures are more pronounced in the components of the gravity gradient tensor and invariants than in the gravity field.As the ultimate objective of this study is to understand the state of stress along plate interface, the geometry of the density model, as constrained by combined gravity models and seismic data, has been used to develop dynamic model of the Andean margin. The results show that the stress regime in the fore-arc (high and low) tends to follow the trend of the earthquake distributions.

Volcanic CO2output at the Central American subduction zone inferred from melt inclusions in olivine crystals from mafic tephras

[1] The volatile contents of olivine-hosted (Fo89–71) melt inclusion glasses in rapidly quenched mafic tephras from volcanic front volcanoes of the Central American Volcanic Arc (CAVA) in Guatemala, Nicaragua, and Costa Rica, were analyzed by secondary ion mass spectrometry (SIMS) in order to derive the minimum eruptive output of CO2, along with H2O, Cl, and S. Details of the analytical method are provided that establish melt inclusion CO2 analyses with the Cameca ims6f at the Helmholtz Centre Potsdam. The highest CO2 concentrations (up to 1800 μg/g) are observed in Nicaraguan samples, while melt inclusions from Guatemala and Costa Rica have CO2 contents between 50 and 500 μg/g. CO2 does not positively covary with sediment/slab fluid tracers such as Ba/La, Ba/Th, or U/La. Instead, the highest CO2 concentrations occur in the inclusions with the most depleted incompatible element compositions and low H2O, approaching the composition of mid-ocean ridge basalts (MORBs), whereas the most H2O-rich inclusions are relatively CO2-poor (<800 μg/g). This suggests that CO2 degassing was more extensive in the melts with the highest slab contribution. CO2/Nb ratios in the least degassed CAVA melt inclusions are similar to those of primitive MORBs. These are interpreted here as recording a minimum CO2 output rate from the mantle wedge, which amounts to 2.8 × 104 g/s for the ∼1100 km long CAVA. Previously published estimates from quiescent degassing and numerical modeling, which also encompassed the slab contribution, are 3 times higher. This comparison allows us to estimate the proportion of the total CO2 output derived from the mantle wedge.

A strong-motion database from the Central American subduction zone

Subduction earthquakes along the Pacific Coast of Central America generate considerable seismic risk in the region. The quantification of the hazard due to these events requires the development of appropriate ground-motion prediction equations, for which purpose a database of recordings from subduction events in the region is indispensable. This paper describes the compilation of a comprehensive database of strong ground-motion recordings obtained during subduction-zone events in Central America, focusing on the region from 8 to 14° N and 83 to 92° W, including Guatemala, El Salvador, Nicaragua and Costa Rica. More than 400 accelerograms recorded by the networks operating across Central America during the last decades have been added to data collected by NORSAR in two regional projects for the reduction of natural disasters. The final database consists of 554 triaxial ground-motion recordings from events of moment magnitudes between 5.0 and 7.7, including 22 interface and 58 intraslab-type events for the time period 1976–2006. Although the database presented in this study is not sufficiently complete in terms of magnitude–distance distribution to serve as a basis for the derivation of predictive equations for interface and intraslab events in Central America, it considerably expands the Central American subduction data compiled in previous studies and used in early ground-motion modelling studies for subduction events in this region. Additionally, the compiled database will allow the assessment of the existing predictive models for subduction-type events in terms of their applicability for the Central American region, which is essential for an adequate estimation of the hazard due to subduction earthquakes in this region.

Constraints on upper mantle anisotropy surrounding the Cocos slab from SK(K)S splitting

SKS and SKKS splitting observations are used to constrain the pattern of mantle flow in the Central American subduction zone beneath Costa Rica and Nicaragua. After removing the effects of shallow wedge anisotropy on SK(K)S waveforms, a best fitting model of anisotropy beneath the Cocos Plate and in the deeper mantle wedge is determined. Fast polarization directions and model symmetry axis orientations in both regions (as well as the shallow wedge) are dominated by roughly arc‐parallel azimuths and, therefore, are not consistent with sublithospheric mantle flow entrained by the subducting Cocos Plate or simple two‐dimensional corner flow in the wedge. In conjunction with geochemical data and local S splitting tomography, the SK(K)S splitting observations and anisotropy models are consistent with flow to the WNW within the mantle wedge on the Caribbean side of the Cocos Plate, possibly drawn through a slab window beneath Panama and southern Costa Rica. Anisotropy in the Pacific mantle beneath the Cocos Plate is also best explained by flow with a component that is roughly parallel to the strike of the slab, although the absolute direction of this flow is not uniquely constrained.

Constraints on upper plate deformation in the Nicaraguan subduction zone from earthquake relocation and directivity analysis

In the Nicaraguan segment of the Central American subduction zone, bookshelf faulting has been proposed as the dominant style of Caribbean plate deformation in response to oblique subduction of the Cocos plate. A key element of this model is left‐lateral motion on arc‐normal strike‐slip faults. On 3 August 2005, a Mw 6.3 earthquake and its extensive foreshock and aftershock sequence occurred near Ometepe Island in Lake Nicaragua. To determine the fault plane that ruptured in the main shock, we relocated main shock, foreshock, and aftershock hypocenters and analyzed main shock source directivity using waveforms from the TUCAN Broadband Seismic Experiment. The relocation analysis was carried out by applying the hypoDD double‐difference method to P and S onset times and differential traveltimes for event pairs determined by waveform cross correlation. The relocated hypocenters define a roughly vertical plane of seismicity with an N60°E strike. This plane aligns with one of the two nodal planes of the main shock source mechanism. The directivity analysis was based on waveforms from 16 TUCAN stations and indicates that rupture on the N60°E striking main shock nodal plane provides the best fit to the data. The relocation and directivity analyses identify the N60°E vertical nodal plane as the main shock fault plane, consistent with the style of faulting required by the bookshelf model. Relocated hypocenters also define a second fault plane that lies to the south of the main shock fault plane with a strike of N350°E–N355°E. This fault plane became seismically active 5 h after the main shock, suggesting the influence of stresses transferred from the main shock fault plane. The August 2005 earthquake sequence was preceded by a small eruption of a nearby volcano, Concepción, on 28 July 2005. However, the local seismicity does not provide evidence for earthquake triggering of the eruption or eruption triggering of the main shock through crustal stress transfer.

Nature of crustal terranes and the Moho in northern Costa Rica from receiver function analysis

The Central American subduction zone in northern Costa Rica shows along‐strike variations in both the incoming and overriding plates. By analyzing the subducting oceanic Moho (M1) and the upper plate Moho (M2) with receiver functions, we investigate the variability in the hydration state of the subducting Cocos Plate and the nature of crustal terranes within the overriding Caribbean Plate. We calculate high‐quality P and PP wave receiver functions using broadband data of the Global Seismology Network; Geoscope Project; and the CRSEIZE, Pocosol, and Corisubmod experiments. In addition, we estimate the depth (H) and vertically averaged Vp/Vs (κ) to Moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the H and κ estimates. Our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the Nicoya Peninsula, as compared to a serpentinized oceanic mantle subducting beneath the northern part. In the Caribbean Plate, we describe the nature of the Mesquito, Nicoya, and Chorotega terranes by integrating new and published Vp/Vs estimates. Both the Nicoya and Chorotega terranes display high Vp/Vs (1.80–1.92) consistent with their oceanic character. In contrast, the oceanic Mesquito Terrane mostly displays lower Vp/Vs (1.62–1.80) more compatible with continental crust, which may indicate that subduction zone magmatism is modifying the crust to display continental character. Our estimates show that the deepest M2 (∼42 km) is observed in the volcanic arc region whereas the shallowest M2 (∼27–33 km) is observed in parts of the fore‐arc and back‐arc regions.