Galapagos Rift Research Articles

87Sr/ 86Sr and 18O/ 16O ratios of clays from a hydrothermal area near the Galapagos rift as records of origin, crystallization temperature and fluid composition

Strontium and O isotope compositions of green clay minerals from sediment cores of three boreholes drilled into (sites 424A and 509B) and close to a hydrothermal mound (site 424B) near the Galapagos Spreading Center (DSDP Legs 54 and 70) were determined. The green clays consist mostly of a transition from Fe-smectite (nontronite) to glauconite. 87Sr/ 86Sr ratios were measured on clay size-fractions after gentle acid leaching and on the recovered leachates from different samples. The 87Sr/ 86Sr ratios of the clay residues from both the 424A and B sites are well below the modern seawater value, which points consistently to precipitation from hydrothermal fluids that contained variable amounts of seawater, even away from mound. However, most of the clay residues from mound site 509B have 87Sr/ 86Sr ratios significantly above the seawater value, suggesting the occurrence of a detrital component together with the new authigenic particles. The clay minerals of the hydrothermal mound are mixed with detrital components, and that of the sample taken outside but near the mound as a reference for the surrounding oceanic environment, yields a hydrothermal signature. Crystallization temperatures of the clays range from 32 to 63 °C assuming a δ 18O value of + 2.2‰ for the mineralizing fluids. Hydrothermal fluids generated in the underlying oceanic crust, mixed in varied proportions with ambient seawater and migrated into beds of the mound in a sequence of recurrent processes that ultimately resulted in the formation of the observed clay minerals. No significant temperature differences were detected for crystallization of the K-rich glauconite and K-depleted nontronite. The 87Sr/ 86Sr ratios of the Sr leached off the clay particles are near the value of modern seawater, inferring a progressive replacement of the hydrothermal fluids by seawater in the pore space of the mound sediments.

Desarrollo del centro de dispersión entre las placas de Coco y Nazca y los trazos de los puntos calientes

Se presenta un modelo del desarrollo de las placas tectónicas para el centro de dispersión entrelas placas Coco y Nazca. Nuevos datos magnéticos marinos y datos existentes de la magnetometría, batimetríay gravimetría; junto con nuevas determinaciones de edades, llevan a una nueva interpretación de la complicadasituación del este de la Cuenca de Panamá. En este modelo se interpreta la formación de la parte sur de la placa de Coco como un resultado de un centro de dispersión en tres ejes diferentes, con alternancias entre dorsalesde dispersión, lo que produce un trazo complicado de las anomalías magnéticas. En el área de las dorsales deCoco y Malpelo y en el norte del escarpe de Grijalva se han identificado dos precursores (CNS-1 y CNS-2) delcentro de dispersión entre las placas Coco y Nazca (CNS-3), actualmente activo. El CNS-1 estuvo activo desde22,8 hasta 19,5 Ma, el CNS-2 desde 19,5 hasta 14,7 Ma. La dirección de dispersión del CNS-1 cambió de NW-SE hacia el ENE-WSW (CNS-2) hace 19,5 Ma y hacia el E-W (CNS-3) hace 14,7 Ma. La corteza oceánica delCNS fue subsecuentemente engrosada y sobrepuesta por el volcanismo de un punto caliente. La dorsal de Carnegie en el norte de la placa de Nazca es un producto del punto caliente de Galápagos. Un segundo centro devolcanismo de un punto caliente, probablemente relacionado al punto caliente de Galápagos, está localizado másde 500 km al norte de éste y formó las dorsales de Coco y Malpelo.

Structure and serpentinization of the subducting Cocos plate offshore Nicaragua and Costa Rica

The Cocos plate experiences extensional faulting as it bends into the Middle American Trench (MAT) west of Nicaragua, which may lead to hydration of the subducting mantle. To estimate the along strike variations of volatile input from the Cocos plate into the subduction zone, we gathered marine seismic refraction data with the R/V Marcus Langseth along a 396 km long trench parallel transect offshore of Nicaragua and Costa Rica. Our inversion of crustal and mantle seismic phases shows two notable features in the deep structure of the Cocos plate: (1) Normal oceanic crust of 6 km thickness from the East Pacific Rise (EPR) lies offshore Nicaragua, but offshore central Costa Rica we find oceanic crust from the northern flank of the Cocos Nazca (CN) spreading center with more complex seismic velocity structure and a thickness of 10 km. We attribute the unusual seismic structure offshore Costa Rica to the midplate volcanism in the vicinity of the Galapagos hot spot. (2) A decrease in Cocos plate mantle seismic velocities from ∼7.9 km/s offshore Nicoya Peninsula to ∼6.9 km/s offshore central Nicaragua correlates well with the northward increase in the degree of crustal faulting outboard of the MAT. The negative seismic velocity anomaly reaches a depth of ∼12 km beneath the Moho offshore Nicaragua, which suggests that larger amounts of water are stored deep in the subducting mantle lithosphere than previously thought. If most of the mantle low velocity zone can be interpreted as serpentinization, the amount of water stored in the Cocos plate offshore central Nicaragua may be about 2.5 times larger than offshore Nicoya Peninsula. Hydration of oceanic lithosphere at deep sea trenches may be the most important mechanism for the transfer of aqueous fluids to volcanic arcs and the deeper mantle.

El sistema de falla Atirro-Río Sucio y la cuenca de tracción de Turrialba-Irazú: indentación tectónica relacionada con la colisión del levantamiento del Coco

El sistema de falla Atirro-Rio Sucio consiste de una serie de fallas neotectonicas de rumbo NW,predominantemente dextrales, entre las cuales destacan las fallas Atirro, Turrialba, Tucurrique, Lara y Rio Sucio.Rios y quebradas han sido desplazadas dextralmente por las diversas fallas, entre unos pocos metros y 3,5 km.Relevos izquierdos a lo largo de la falla Atirro y entre esta y la Tucurrique han originado las zonas transpresivasde las filas Omega, Rincon de la Esperanza y del Cerro Mirador. Una componente de levantamiento del ladonoreste de la falla Tucurrique provoco represamientos intermitentes del drenaje de los rios Pejibaye, Gato y Tepemechin permitiendo el desarrollo de la cuenca aluvial de Pejibaye. Un salto derecho entre las fallas Atirro-Tucurrique y la falla Rio Sucio ha originado transtension y el desarrollo de la cuenca de traccion (pull-apart) de Turrialba-Irazu. En el sector sureste del pull-apart se han formado cuencas limitadas por las fallas normalesAzul, Chiz y Campano, donde se han acumulado potentes depositos cuaternarios aluviales y laharicos,transportados por los rios Reventazon, Turrialba, Aquiares, Tuis, Pejibaye y Atirro.El volcan Irazu se instalo durante el Cuaternario dentro del sector centro-occidental del pull-apart. Elambiente tensional ha favorecido un mas facil acceso de los magmas provenientes del manto y el desarrollo deuna volumetrica y compleja estructura volcanica. La ubicacion de los crateres de los volcanes Irazu y Turrialbay de los conos monogeneticos que se ubican en el macizo volcanico han sido controlados estructuralmente porfallas normales o fracturas tensionales relacionadas con el desarrollo del pull-apart. Los potentes depositosvolcanicos asociados con la anterior actividad, ha obscurecido la expresion geomorfica del pull-apart en elsector centro-occidental.La sismicidad historica y reciente en el sistema de falla muestra la ocurrencia de varios enjambres yalgunos temblores de M ≥ 5,0, siendo el principal el terremoto de Patillos del 30 de diciembre de 1952 (Ms 5,7).El sistema de falla se asocia con la indentacion tectonica causada por la colision del levantamiento del Coco conel sureste de Costa Rica, evento que se inicio hace unos 5 Ma.

Fallamiento neotectónico de la península de Nicoya y su relación con el escape tectónico del antearco centroamericano

Se ha determinado un arreglo de fallas neotectonicas dentro de la peninsula de Nicoya. En la parte central y NW de la peninsula predominan bloques limitados por fallas de desplazamiento de rumbo dextral con rumbo N-S, dentro de los cuales se reconocen fallas sinestrales de rumbo ENE, oblicuas sinestrales-inversas con rumbo E-W e in- versas de rumbo NW. Las anteriores relaciones espaciales entre los diversos tipos de fallas se explican por traslacion al N de este sector de la peninsula y la transrotacion que ocurre dentro de los bloques. Las fallas cartografiadas en la zona S de la peninsula se han interpretado como sinestrales y dextrales con una relevancia aparentemente similar, ademas se han definido fallas inversas subordinadas. Movimientos translacionales al N y menos probable al E se infieren en la misma. El arreglo de fallas neotectonicas de la peninsula de Nicoya es asociado con la colision de la cordillera del Coco bajo el margen convergente del centro y S de Costa Rica, el cual origina el escape tectonico del bloque del Antearco Centroamericano hacia el NW, paralelo a la fosa Mesoamericana.

Cuencas sedimentarias de Costa Rica: evolución geodinámica y potencial de hidrocarburos

The Upper Cretaceous/Cenozoic basins of Costa Ricaare underlain by oceanic crustand located along the western margin of the Caribeean Plate. Since Upper Cretaceous time, an intraoceanic arc was formed along the western margin of the Caribeean Plate. This arc marked the beggining of the subdution of the Farallon Plate in the region and also the creationof the first land bridge joining North and South America. Continued subdution of the Farallon Plate from Campanian until the present, created a convergent marginal tectonic setting; with, from SW to NE: a trench, an outer arc, a forearc region, a volcanic arc and a backarcr egion. Localized platforms of shallow-water carbonates, overlain by extensive cyclical turbidites system, characterized the sediments of most of the basis until the Eocene, when a major tectonic compressive event affected the area. As well, about this time, a major sinistral east-west fault (COSTA RICA TRANSCURRENT FAULT SYSTEM) divided the region into north and south segments and transtensional or pull apart basins developed along this major fault system. In the Oligocene time, the north segment was emerged and subject to erosion, whereas in the south segment the sedimentation continued. From Miocene on, shallow marine deposition was widespread in both sements. In the north segment, and extensional stage in the backarc region, created a structural depression filled with Neogene sediments. About the same time, the south segment experienced compressional tectonicis due to noth-directed movementof the Panama block. Later in the Pleistocene, northeasterly-directed shallow subdution of the thick Cocos Ridge, resulted in adiotional thrusting and the developmentof a foreland basin, both, in the back arc and fore arc regions. Organic-rich, deep water, marginally to termally mature source rock, from Cretaceous age, occur in several basins. Also, source rocks of moderate to high TOC values are present in all basins, in the Tertiary. Potential carbonate reservoirs in some of these basins include shallow water carbonates, rudistic reef buildups and grainstone accumulations. Potential clastic reservoirs include turbidite sandstones, regressive deltaic deposits and near-shore porous sandstones. The presence of potential structural and stratigrafic traps in all basins, confer an important as far as the pretoleum potential is concern.

Sismicidad y marco neotectónico de Costa Rica y Panamá

We define the different active tectonic boundaries of the Costa Rica - Panama region. Specially we refine the different seismogenic zones and the structural characteristics of the Panama microplate, a small tectonic block enclose between the Cocos, Caribbean, Nazca and South American plates. The tectonic setting of the Limon earthquake was the North Panama Deformed Belt, the northern limit of the Panama microplate. The deformation along this region is proposed to be tectonically controlled by the collision of the Cocos ridge along the southeastern Costa Rican border.

Neotectónica de la región central de Costa Rica: frontera oeste de la microplaca de Panamá

Datos neotectonicos, de sismicidad y de sismotectonica son usados para definir el Cinturon Deformado delCentro de Costa Rica (CDCCR) como un sistema de fallas reciente, difuso y ancho localizado entre la placa Caribe y lamicroplaca de Panama, el cual es transversal a la region central de Costa Rica y localizado entre la fosa Mesoamericana(lado pacifico) y el Cinturon Deformado del Norte de Panama (sector caribe). En el antearco pacifico central, el sistemade fallas combina desplazamientos de rumbo siniestral y normal. En el arco interno existe: 1) Un sistema externo de fallas inversas, que tiene rumbo predominante WNW en la region norte del pais y 2) Un sistema interno de fallas que estaconstituido principalmente por fallas de desplazamiento de rumbo de tipos dextrales de direccion NW y siniestrales conrumbo entre ENE y NE. En la region trasarco, el sistema de fallas del CDCCR se une con el Cinturon Deformado delNorte de Panama, el cual incluye un sistema externo de fallamiento inverso y plegamiento de rumbo noroeste y otro in-terno de desplazamiento de rumbo dextral combinado con desplazamiento inverso de rumbo noroeste.Las fallas neotectonicas que constituyen el CDCCR son originadas por la colision del levantamiento del Coco con el surde Costa Rica. La indentacion tectonica generada por esta colision provoca el desplazamiento tectonico de la microplacade Panama con respecto a la placa Caribe, tanto hacia el sector este del Caribe como hacia el norte del pais. Este procesoha generado fallamiento extensional combinado con fallamiento transcurrente siniestral en la parte trasera oeste pacificadel limite difuso, lo que ha favorecido el desarrollo de cuencas marinas transversales al antearco. Por otro lado, el movi-miento delantero de la microplaca de Panama con respecto a la placa Caribe, ha originado un cinturon externo de fallasinversas y pliegues en la zona entre el arco interno y el trasarco. El sistema de fallas interno de desplazamiento de rumboque esta localizado dentro del arco interno permite movimientos traslacionales y rotacionales entre los diversos bloquestectonicos que constituyen la zona limite interplaca.

Quaternary outer fore-arc deformation and uplift inboard of the Panama Triple Junction, Burica Peninsula

[1] The Panama Triple Junction (Cocos-Nazca-Caribbean) represents the point that abruptly separates the thick and rapidly subducting Cocos plate to the northwest from the thin and obliquely subducting Nazca plate to the southeast along the Central American convergent margin. New structural and geomorphic analyses on the Burica Peninsula, an outer fore-arc peninsula located only ∼100 km inboard the Panama Triple Junction, reveal that the peninsula is dominated primarily by contractional deformation along three listric thrust faults that root in the underlying plate boundary. The geometry and spatial distribution of these thrusts indicate that this deformation occurs primarily in response to the change in crustal thickness occurring as a result of eastern migration of the flank of the Cocos Ridge coeval with migration of the Panama Triple Junction at a rate of ∼55 mm/yr to the southeast. Mapping and detailed elevation surveys reveal eight marine terraces on the peninsula with a distribution of inner edge elevations indicating that uplift is spatially uniform from north to south along strike in this area. However, terraces along the northwest part of the peninsula are offset across major thrust faults. Age control provided by 14C, OSL and soil chronosequences indicate that the terraces within the easternmost portion of the peninsula range in age from Marine Isotope Stage (MIS) 3 to Holocene, a result that indicates that this portion of the peninsula is younger than ∼60 ka. Time-averaged uplift rates calculated from marine terraces and other Quaternary marine deposits yield consistent uplift rates that range between 2.1 ± 0.1 and 7.7 ± 0.5 mm/yr for samples older than 1 ka and between 6.9 ± 1.0 and 19.3 ± 8.0 mm/yr for samples younger than 1 ka. We interpret this temporal distribution in uplift rates to suggest that the eight terraces preserved on the peninsula are produced coseismically wherein the anomalously high uplift rates calculated from the youngest samples (<1 ka) are not yet averaged over a complete seismic cycle. These observations, combined with (1) shortening estimates from balanced cross sections indicating that minimum shortening decreases from northwest to southeast and (2) the observation of growth strata within the youngest marine units, are consistent with a space-for-time model for triple junction migration. These results indicate that both triple junction migration and the change in bathymetry occurring at the triple junction boundary are far more dominant factors in outer fore-arc deformation than the change in rate and obliquity of subduction and basal tractions that also occur on either side of the triple junction.

Arc‐parallel shear deformation and escape flow in the mantle wedge of the Central America subduction zone: Evidence from P wave anisotropy

The P wave anisotropy of the crust and uppermost mantle has been determined in two separate areas of the continental margin of Central America covering offshore and onshore parts of southern Nicaragua and northern and central Costa Rica. Our study is based on traveltime data from local earthquakes recorded with three combined onshore and offshore seismic networks that had been deployed in three half‐year intervals between 2002 and 2006. The P wave traveltime data have been tomographically inverted in terms of an elliptical anisotropic P wave velocity distribution. In the upper crust beneath Costa Rica, the seismic anisotropy is of the order of 2%–5% and locally strongly variable in orientation corresponding to the complicated tectonic structure. In contrast, the upper mantle beneath both study regions shows stronger anisotropy of the order of 5%–10% and a more coherent orientation of the axis of fast P wave velocity that varies systematically on regional scales: The upper mantle of the incoming oceanic plate is characterized by a trench‐normal orientation of the fast P wave velocity axis presumably corresponding to mineral alignment (lattice preferred orientation LPO) in transport direction. This pattern is corroded in the uppermost part of the subduction zone, possibly by the influence of bending‐related trench‐parallel faults and serpentinization that can overprint or annihilate the LPO anisotropy. The upper mantle of the overriding plate and mantle wedge shows a clear trench‐parallel orientation of the fast P wave velocity axis. The anisotropy is stronger in Nicaragua than in southern central Costa Rica (factor of 2 orders of magnitude). The development of a stronger anisotropy in the Nicaragua could be driven by a change in the stress regime from compressional in southern Costa Rica to transpressional in Nicaragua corresponding to a change from near‐orthogonal subduction in the SE to oblique subduction and slab retreat in the NW. From a comparison with S/SKS wave, GPS, and geochemical observations follows that the most likely explanation for the observed pattern of P wave anisotropy in the mantle wedge is LPO caused by a trench‐parallel shear deformation and/or NW oriented escape flow originating in the compressional zone near the Cocos Ridge collision area.

Síntesis geocronológica del magmatismo, metamorfismo y metalogenia de Costa Rica, América Central

Una recopilación completa de 651 determinaciones de edades radiométricas desde 1968 (415 dataciones 40Ar/39Ar, 211 K/Ar, 5 U/Th, 4 Rb/Sr, 2 U/Pb y de fisión termocronología con zircón), tanto aquellas publicadas desde 1968 como muchas nuevas, proporcionan un marco completo de la estratigrafía ígnea de Costa Rica y las inferencias acerca de la edad del metamorfismo y los eventos metalogenéticos. Las rocas ígneas del Jurásico Superior temprano al Eoceno Medio (~ 160 a ~ 41 Ma), corresponden principalmente con acreciones ofiolíticas. Durante el Campaniano (~ 71 Ma) comienza a establecerse la actual zona de subducción, conformada por rocas volcano-sedimentarias, con una composición desde básica a félsica. Sin embargo, las rocas volcánicas primarias, subalcalinas, in situ y abundantes, aparecen hasta después de los ~ 29 Ma. Los intrusivos e hipoabisales de granitoides hasta gabroides (plutones, stocks, diques y sills), están presentes de SE a NW en: la cordillera de Talamanca (~ 12,4 a 7,8 Ma), cerros de Escazú (~ 6,0 a 5,9 Ma), fila Costeña (~ 18,3 a 16,8 y ~ 14,8 a 11,1 Ma), Tapantí-montes del Aguacate-Carpintera (~ 4,2 a 2,2 Ma) y Guacimal (~ 6,4 a 5,2 Ma). Las rocas asociadas al arco con edades entre los 29 y 11 Ma (denominado Frente Proto- Volcánico), están presentes en la llanura de San Carlos y en el sur de Costa Rica. La ubicación y la edad de las rocas ígneas indican que el arco dio un giro de 20° al NW entre 15 y 8 Ma, con un polo de rotación que se centró en el sur de Costa Rica. Esta rotación se atribuye a la deformación en la placa superior (acortamiento en el sur y extensión en el noroeste), acompañado por el retroceso de la trinchera hacia el sur. A los ~ 3,45 Ma, el arco volcánico en el sur cesó su actividad, mientras que el vulcanismo adakítico localizado persistió en la cordillera de Talamanca (4,2 - 0,95 Ma) debido a la subducción de las Placa Cocos. El Frente Paleo-Volcánico está representada por rocas del arco (8 - 3,5 Ma) que se extienden a lo largo de Costa Rica, de forma paralela al arco volcánico moderno y fue seguido por el evento efusivo Monteverde (2,1 - 1,1 Ma), que progresivamente se retrocedió hacia el NE. El Frente Neovolcánico se estableció entre el 2,1 y el presente, con el crecimiento de los actuales volcanes, mayoritariamente hace unos 0,6 Ma y, en general, están representados por tres episodios, no isocrónicos entre volcanes: volcanes ancestrales entre 1,61 y 0,85 Ma (Proto-Cordillera), en parte coetánea con Monteverde; un evento importante cerca de ~ 0,74 - ~ 0,2 Ma (Paleo-Cordillera), y una actividad volcánica relativamente pequeña, pero activa aún en 0,25 - 0 Ma (Neo-Cordillera). El vulcanismo submarino en aguas del Pacífico costarricense, está representado por la serranía Fisher (30,0 Ma y 19,2 Ma), la cordillera del Coco (14,5 - 0,6 Ma) y el vulcanismo subaéreo de la isla del Coco (2,2 - 1,5 Ma). Los principales eventos magmáticos, metamórficos y metalogéneticos están relacionados con los principales eventos geotectónicos, incluyendo discordancias regionales en Cretácico Superior (~ 82 Ma), Eoceno Medio (~ 45 Ma) y el Mioceno Superior (~ 8 Ma).

The steeply subducting edge of the Cocos Ridge: Evidence from receiver functions beneath the northern Talamanca Range, south‐central Costa Rica

The deep structure of the south‐central Costa Rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible. We present a receiver function study along a transect of broadband seismometers through the northern flank of the Cordillera de Talamanca (south Costa Rica). Below Moho depths, the receiver functions image a dipping positive conversion signal. This is interpreted as the subducting Cocos Plate slab, compatible with the conversions in the individual receiver functions. In finite difference modeling, a dipping signal such as the one imaged can only be reproduced by a steeply (80°) dipping structure present at least until a depth of about 70–100 km; below this depth, the length of the slab cannot be determined because of possible scattering effects. The proposed position of the slab agrees with previous results from local seismicity, local earthquake tomography, and active seismic studies, while extending the slab location to greater depths and steeper dip angle. Along the trench, no marked change is observed in the receiver functions, suggesting that the steeply dipping slab continues until the northern flank of the Cordillera de Talamanca, in the transition region between the incoming seamount segment and Cocos Ridge. Considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision, the southern Costa Rican subduction zone may at present be undergoing a reconfiguration of subduction style, where the transition to shallow underthrusting may be underway but still incomplete.

Forearc motion and deformation between El Salvador and Nicaragua: GPS, seismic, structural, and paleomagnetic observations

We combine geodetic, structural, and paleomagnetic data from El Salvador with Global Positioning System (GPS) data from southern Honduras and Nicaragua to describe the motions of the Salvadoran and Nicaraguan forearcs and determine the location and style of faulting across the Gulf of Fonseca offset of the volcanic arcs of eastern El Salvador and western Nicaragua. Finite-element modeling of GPS measurements at 35 sites in El Salvador, southern Honduras, and Nicaragua indicates that the Nicaraguan and Salvadoran forearcs both move west to northwest, parallel to their respective trenches, at 15 ± 2 mm yr −1 (95% limit) in a Caribbean plate reference frame. The similar motions of the two forearcs, despite an ∼20°–25° difference in the obliquity of subduction beneath them and absence of any significant convergence obliquity offshore from El Salvador, are consistent with a recent hypothesis that the Nicaraguan forearc pushes the Salvadoran forearc to the northwest, possibly driven by northwestward lateral escape of the Central America forearc from its collision zone with the Cocos Ridge offshore from Costa Rica. The Gulf of Fonseca and adjacent eastern El Salvador form an ∼60-km-wide extensional zone with E-W elongation, determined by diffuse seismicity, GPS velocities, and numerous young, N-S–striking normal faults mapped with a 10 m digital elevation model (DEM), structural measurements, and Lidar (light detection and ranging). Strike-slip earthquakes in the Fonseca pull-apart structure and evidence for modest (∼10°) vertical-axis fault block rotations from paleomagnetic measurements at 33 sites in the Fonseca pull-apart structure both indicate that extension may be accompanied by bookshelf faulting.

Plume–subduction interaction in southern Central America: Mantle upwelling and slab melting

The volcanic front in southern Central America is well known for its Galapagos OIB-like geochemical signature. A comprehensive set of geochemical, isotopic and geochronological data collected on volumetrically minor alkaline basalts and adakites were used to better constrain the mantle and subduction magma components and to test the different models that explain this OIB signature in an arc setting. We report a migration of back-arc alkaline volcanism towards the northwest, consistent with arc-parallel mantle flow models, and a migration towards the southeast in the adakites possibly tracking the eastward movement of the triple junction where the Panama Fracture Zone intersects the Middle America Trench. The adakites major and trace element compositions are consistent with magmas produced by melting a mantle-wedge source metasomatized by slab derived melts. The alkaline magmas are restricted to areas that have no seismic evidence of a subducting slab. The geochemical signature of the alkaline magmas is mostly controlled by upwelling asthenosphere with minor contributions from subduction components. Mantle potential temperatures calculated from the alkaline basalt primary magmas increased from close to ambient mantle (~1380–1410°C) in the Pliocene to ~1450°C in the younger units. The calculated initial melting pressures for these primary magmas are in the garnet stability field (3.0–2.7GPa). The average final melting pressures range between 2.7 and 2.5GPa, which is interpreted as the lithosphere–asthenosphere boundary at ~85–90km. We provide a geotectonic model that integrates the diverse observations presented here. The slab detached after the collision of the Galapagos tracks with the arc (~10–8Ma). The detachment allowed hotter asthenosphere to flow into the mantle wedge. This influx of hotter asthenosphere explains the increase in mantle potential temperatures, the northwest migration in the back-arc alkaline lavas that tracks the passage of the hotter asthenosphere, and the presence of a slab melting signature in the volcanic front caused by recycling of Galapagos Hotspot tracks.

Magmatic history and evolution of the Central American Land Bridge in Panama since Cretaceous times

Chemical compositions for 310 igneous rocks from the Cordillera de Panama and the Sona and Azuero peninsulas were supplemented by 40 Ar/ 39 Ar dating and Sr-, Nd-, Pb-, and O-isotope analysis to determine the magmatic evolution and oceanic plate interactions over the past 100 Ma in western Panama. An initial phase of intraplate magmatism, having geochemical characteristics of the Galapagos hotspot, formed the oceanic basement of the Caribbean large igneous province from 139 to 69 Ma. Younger accreted terranes with enriched trace element patterns (accreted ocean island basalt [OIB]) were amalgamated between 70 and 20 Ma. A second magmatic phase in the Azuero and Sona peninsulas has trace element patterns (Sona-Azuero arc) suggesting the initiation of subduction at 71–69 Ma. Arc magmatism continued in the Chagres basin region (Chagres-Bayano arc) from 68 to 40 Ma. A third phase formed discrete volcanic centers across the Cordillera de Panama (Cordilleran arc) from 19 to 5 Ma. The youngest phase consists of isolated volcanic centers of adakitic composition (Adakite suite) in the Cordillera de Panama that developed over the past 2 million years. Initiation of arc magmatism at 71 Ma coincides with the cessation of Galapagos plateau formation, suggesting a causal link. The transition from intraplate to arc magmatism occurred relatively quickly and introduced a new enriched mantle source. The arc magmatism involved progressive transition to more homogeneous intermediate mantle wedge compositions through mixing and homogenization of subarc magma sources through time and/or the replacement of the mantle wedge by a homogeneous, relatively undeleted asthenospheric mantle. Adakite volcanism started after a magmatic gap, enabled by the formation of a slab window.

Equatorial paleosecular variation of the geomagnetic field from 0 to 3 Ma lavas from the Galapagos Islands

Complete progressive thermal demagnetization of nearly 400 oriented samples from 58 sites (lava flows) from the Galapagos Islands of Santa Cruz, San Cristobal and Floreana provide data for the statistical characterization of the time-averaged geomagnetic field near the Equator for the past few million years. Estimates of VGP dispersion due to paleosecular variation range from 9.2° to 11.8° depending on site selection criteria; our preferred estimate based on 64 site VGPs (51 accepted from this study and 13 from the 1971 study by Cox) is 11.4° (95% confidence interval 10.2–13.0°), consistent with previous estimates from the Galapagos Islands as well as paleosecular variation Model G, and confirming that angular dispersion of VGPs near the Equator is relatively low. The mean direction is not significantly different from a geocentric axial dipole field when account is taken of southward plate motion over the Galapagos hotspot. Preliminary paleointensity results from a comparison of the natural remanence with a total thermal remanence produced in a lab field of 15 μT on a subset of 321 samples from 48 sites that had relatively small changes in magnetic susceptibility after laboratory heating suggest that the time-averaged field was about 21 μT, or only two-thirds the present strength, in agreement with some other recent estimates.

Local earthquake tomography of central Costa Rica: transition from seamount to ridge subduction

The structure and seismicity of the subduction zone of central Costa Rica have been investigated with local earthquake tomography down to ca. 50 km depth. Seismic traveltime data sets of three on- and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations, 3-D structure of P-wave velocity and Vp/Vs ratio using about 2000 high-quality events. The seismicity and slab geometry as well as Vp and Vp/Vs show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest, a seamount province in the centre and the subducting Cocos Ridge in the southeast of the investigation area. Three prominent features can be identified in the Vp and Vp/Vs tomograms: a high-velocity zone with a perturbation of 4–10 per cent representing the subducting slab, a low-velocity zone (10–20 per cent) in the forearc crust probably caused by deformation, fluid release and hydration and a low-velocity zone below the volcanic arc related to upwelling fluids and magma. Unlike previously suggested, the dip of the subducting slab does not decrease to the south. Instead, an average steepening of the plate interface from 30° to 45° is observed from north to south and a transition from a plane to a step-shaped plate interface. This is connected with a change in the deformation style of the overriding plate where roughly planar, partly conjugated, clusters of seismicity of regionally varying dip are observed. It can be shown that the central Costa Rica Deformation Belt represents a deep crustal transition zone extending from the surface down to 40 km depth. This transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well.

GPS-derived coupling estimates for the Central America subduction zone and volcanic arc faults: El Salvador, Honduras and Nicaragua

We invert GPS velocities from 32 sites in El Salvador, Honduras and Nicaragua to estimate the rate of long-term forearc motion and distributions of interseismic coupling across the Middle America subduction zone offshore from these countries and faults in the Salvadoran and Nicaraguan volcanic arcs. A 3-D finite element model is used to approximate the geometries of the subduction interface and strike-slip faults in the volcanic arc and determine the elastic response to coupling across these faults. The GPS velocities are best fit by a model in which the forearc moves 14–16 mm yr−1 and has coupling of 85–100 per cent across faults in the volcanic arc, in agreement with the high level of historic and recent earthquake activity in the volcanic arc. Our velocity inversion indicates that coupling across the potentially seismogenic areas of the subduction interface is remarkably weak, averaging no more than 3 per cent of the plate convergence rate and with only two poorly resolved patches where coupling might be higher along the 550-km-long segment we modelled. Our geodetic evidence for weak subduction coupling disagrees with a seismically derived coupling estimate of 60 ± 10 per cent from a published analysis of earthquake damage back to 1690, but agrees with three other seismologic studies that infer weak subduction coupling from 20th century earthquakes. Most large historical earthquakes offshore from El Salvador and western Nicaragua may therefore have been intraslab normal faulting events similar to the Mw 7.3 1982 and Mw 7.7 2001 earthquakes offshore from El Salvador. Alternatively, the degree of coupling might vary with time. The evidence for weak coupling indirectly supports a recently published hypothesis that much of the Middle American forearc is escaping to the west or northwest away from the Cocos Ridge collision zone in Costa Rica. Such a hypothesis is particularly attractive for El Salvador, where there is little or no convergence obliquity to drive the observed trench-parallel forearc motion.

Tectonics and magmatism of the Hess deep region of the Galapagos rift

The joint analysis of data on the anomalous maganetic field, seismicity, and structures of the Hess deep basalts have allowed us to specify the elongation of zone of spreading and to correct the spatial distribution of the neovolcanic zone. The precise petrogeochemical analysis of various types of basalts composing the uneven-aged oceanic crust of the basin showed that the neovolcanic zone magmatics are related to the primitive type in contrast to rift boards of differential basalts. A model of the Galapagos rift’s deep structure in the area of the western Hess deep has been suggested.

Rough crust subduction, forearc kinematics, and Quaternary uplift rates, Costa Rican segment of the Middle American Trench

Orthogonal subduction of bathymetrically rough oceanic lithosphere along the northwestern flank of the Cocos Ridge imprints a distinctive style of deformation on the overriding Costa Rican forearc. We divide the Costa Rican forearc into three 100–160-km-long deformational domains based on the bathymetric roughness and thickness of the Cocos plate entering the Middle American Trench, the dip of the subducting plate, the variation in surface uplift rates of late Quaternary coastal deposits, and the orientations and types of faults deforming Paleogene and Neogene sedimentary rocks. In the ~100-km-long Nicoya domain, coastal deposits show localized surface uplift and arcward tilting above the downdip projections of the fossil trace of the Cocos-Nazca-Panama (CO-NZ-PA) triple junction and the Fisher seamount and ridge. In the ~120-km-long central Pacific forearc domain between the Nicoya Peninsula and Quepos, shallower (~60°) subduction of seamounts and plateaus is accompanied by trench-perpendicular late Quaternary normal faults. Steeply dipping, northeast-striking, margin-perpendicular faults accommodate differential uplift associated with seamount subduction. Uplift and faulting differ between the segments of the forearc facing subducting seamounts and ridges. Inner forearc uplift along the seamount-dominated segment is greatest inboard of the largest furrows across the lower slope. Localized uplift and arcward tilting of coastal deposits is present adjacent to subducting seamounts. In contrast, inboard of the underthrusting aseismic Cocos Ridge, along the ~160-km-long Fila Costena domain between Quepos and the Burica Peninsula, mesoscale fault populations record active shortening related to the ~100-km-long Fila Costena fold-and-thrust belt. The observed patterns of faulting and permanent uplift are best explained by crustal thickening. The uplifted terraces provide a first-order estimate of permanent strain along the forearc in Costa Rica. The permanent strain recorded by uplift of these Quaternary surfaces exceeds the predicted rebound of stored elastic strain released during subduction-zone earthquakes.