ABSTRACT The marine phytoplankton community responds to latitudinal, longitudinal (coast to ocean), and vertical environmental gradients, an important subject in Oxygen minimum zones (OMZ). The phytoplankton structure and the effect of environmental gradients along the central Mexican Pacific, an area within an OMZ, were studied, especially the importance of size fractions and taxonomic groups. A combination of various methods and protocols, such as microscopic analysis, flow cytometer, and pigment analysis, were followed in this study. Oceanographic conditions included thermal gradients along the study area and unreported evidence of a weak upwelling in the southern zone (Acapulco). Vertical distribution of chlorophyll a showed subsurface maxima (SCM, 15-45 m depth) in all stations, and deeper chlorophyll-a maxima (DCM, 85-95 m depth) in more oceanic stations. Chain-forming diatoms dominated in the northern zone stations. Prochlorococcus, Synechococcus, and picoeukaryotes abundances ranged between 0.01 to 21.7 cells×104 mL−1, although most samples showed the highest contribution to biomass (47.95 µg C L−1) by picoeukaryotes. Expected tendencies of Prochlorococcus distribution were observed: highest densities coincided with the DCM and divinyl chlorophyll-a distribution at oceanic stations. Fucoxanthin had the highest concentrations, whereas fucoxanthin and zeaxanthin concentrations were higher at SCM depths. We documented the co-dominance of the pico- and microplankton: picoplankton was important at the DCM, related to oligotrophic and more stratified water column, whereas microplankton prevailed in coastal stations, with mixed water column, high nutrient concentrations, and diatoms as the dominant group. Picoeukaryotes abundances were related to the concentration of prasinoxanthin, which suggests an important mamiellophyte component not previously revealed.

ABSTRACT. Blooms of stinging pelagic cnidarians have been causing increasing health problems for humans worldwide. Physalia physalis is among the worst stinging species in the Atlantic and Pacific oceans along the North and South American coasts. This siphonophore species has a large gas float and lives at the ocean surface, where its distribution is affected mainly by winds. P. physalis's strandings were observed in the southeastern Pacific Ocean for three consecutive years (2014 to 2016). Data of stranded P. physalis were gathered daily through a collaborative effort between the Chilean Navy and the Chilean Ministry of Health. The association between oceanographic variables and the stranded P. physalis was assessed using a generalized additive mixed model (GAMM) and cross-correlation analysis. The largest stranding occurred along more than 2600 km of coastline and totaled 44,683 colonies. Most of the strandings were along the central and south-central coast of Chile, where almost 200 people were stung, and more than 120 beaches were closed in summer 2016. The GAMM model showed that the oceanographic conditions of warmer temperatures during the winter periods and the weakening of the westerly winds associated with the Niño Southern Oscillation (ENSO) were the most likely causes for the arrival of transport from the colonies to the coast. Also, our analysis illustrates the utility of collaborations among scientists and public organizations to promote large-scale and long-term understanding and provide a model for predicting this dangerous species' arrival and warning for beaches and swimmers, which is of global importance for human health.

The development and initial results from a reanalysis, or best estimate, of the historical state of central Chile's coastal ocean are presented. The Chilean Ocean State Estimate (COSE) covers the western coast of South America southwards of 20°S and for the period 1993 to 2012. The reanalysis is being performed using the 4DVAR driver of the Regional Ocean Modeling System (ROMS). The ROMS solution is constrained to reconcile satellite sea surface temperature and altimetry data with prior estimates of the initial state and boundary conditions derived from the HYCOM (Hybrid Coordinate Ocean Model) global reanalysis. Here we present preliminary results from COSE for central Chile and the year 2008. The regional reanalysis reduces the model-observations mismatch of the global HYCOM solution by 38 and 27%, for sea surface temperature and height respectively, and corrects a spurious on-shore surface current present in the HYCOM reanalysis. Inspection of the assimilation increments suggests that ROMS simulations forced by satellite winds overestimate coastal upwelling.

ABSTRACT Seagrasses are aquatic angiosperms that grow submerged in shallow marine and estuarine environments worldwide. Halodule wrightii is a circumtropically distributed seagrass species found primarily in the Atlantic Ocean, but also in parts of the Indian Ocean and eastern Pacific Ocean. Along the Pacific coast of Central America, large swaths of the reported distribution of H. wrightii are assumptions based on interpolation between known seagrass point locations, and despite confirmation of H. wrightii occurrence in Bahía de Jiquilisco in El Salvador, little is known about its distribution and associated species, which can hamper efforts to conserve this seagrass ecosystem. To address these gaps in data, we provide the first assessment of H. wrightii distribution and associated macroinvertebrate species in Bahía de Jiquilisco. We identified six areas where seagrasses occurred and two of these were selected for monitoring. At the two sampled areas, heterogeneous patches of H. wrightii covered 27.1 km2 with 22 associated macroinvertebrate species from three taxonomic groups in three phyla. We conclude by discussing local threats to H. wrightii, including implications for endangered hawksbill (Eretmochelys imbricata and green (Chelonia mydas agassizii) turtles that depend on these ecosystems, and describe opportunities for conservation in Bahía de Jiquilisco.

Prior to 2007, efforts to monitor and conserve hawksbill turtles (Eretmochelys imbricata) in the eastern Pacific Ocean were opportunistic and records were virtually non-existent. The first abundance estimates were published in 2010, but contained limited data on the species. Ongoing research since that time has led to the identification of several rookeries, including sites containing large proportions of the overall hawksbill nesting currently known to occur in the region. Monitoring projects were established at several sites and have since provided substantial nesting data on the species. Here we summarize data collected between 1983 and March 2016 from all sites (n = 9) confirmed to host >10 nests in any given season to provide an update on hawksbill nesting in the eastern Pacific. We documented a total of 3,508 hawksbill nests, 265,024 hatchlings and 528 individual nesting females in the region. The vast majority of these records (99.4%, 99.9% and 99.6%, respectively) were generated subsequent to 2007, coinciding with the discovery of eight of the nine rookeries included in this study and the organization of monitoring efforts at those sites, which led to the increased documentation conferred here. Our findings should not be misconstrued as increases in actual nesting or signs of recovery, which could diminish the ongoing need for conservation actions, but rather as optimism, that there is still an opportunity to restore the species in the eastern Pacific. The top three sites in terms of average annual number of nests were Estero Padre Ramos (Nicaragua; 213.2 ± 47.6 nests), Bahia de Jiquilisco (El Salvador; 168.5 ± 46.7 nests) and Aserradores (Nicaragua; 100.0 ± 24.0 nests), and all three sites are located in mangrove estuaries in Central America, highlighting the importance of these rookeries/habitats for the survival and recovery of hawksbills in the region. The remaining six sites received between 6.9 ± 7.3 nests (Costa Careyes, Mexico) and 59.3 ± 17.7 nests (Los Cobanos, El Salvador) annually. By integrating data collected on nesting hawksbills with local conservation realities at the most important known hawksbill rookeries in the eastern Pacific, we provide a more holistic view of the conservation status and management needs of the species in this ocean region.

ABSTRACT Janthina typica is an extinct, rare, floating species of gastropod from the early Pliocene whose fossils have an unusual geographic distribution, appearing in the eastern North Atlantic archipelagos (Canary, Azores, Madeira and Selvagen Islands), Morocco, and Pacific (New Zealand, Australia and Japan). This study examines the origin of this biogeography and how the species may have dispersed via sea surface currents. We have considered the published ecological aspects of the genus Janthina, the Janthina typica fossil localities, and ocean palaeocurrents. Abundant specimens of J. typica are found in marine deposits on Gran Canaria island (northeast Atlantic), 40Ar/39Ar dated at 4.2 Ma. These deposits therefore accumulated just before the end of the early Pliocene warm climate and closely predate the start of global changes that gave rise to the worlds present climate. In the early Pliocene, the cold Canary Current did not yet exist. The subtropical northeastern Atlantic Ocean was warmer than today and its waters would have met the Circumtropical Current that crossed the Central American Seaway from the Caribbean to the Pacific. From there, the South Equatorial Current flowed towards the eastern coast of Indonesia before splitting north towards Japan and south as the East Australian Current. The latter must also have extended along the southern coast of Australia, crossing the Bass Strait before reaching the area of modern-day Perth in southwestern Australia. The reverse journey (from Australia to the eastern Atlantic Ocean) would have posed far more obstacles, and is considered improbable. J. typica therefore likely originated in the East Atlantic. The main causes for its extraordinary geographic distribution are its ecology as a floating animal in warm water, tectonic plate movements that permitted an open Central American Seaway and a restricted Indonesian Seaway, and Earths rotation and its influence on marine currents.

RESUMEN Janthina typica es un molusco gasterópodo flotante, extinguido y raro del Plioceno. Sus fósiles presentan una distribución geográfica inusual apareciendo en los archipiélagos del este del Atlántico norte (Islas Canarias, Azores, Madeira γ Salvajes), en Marruecos γ en el oeste del Pacífico (Nueva Zelanda, Australia y Japón). En el presente trabajo se estudia la causa de esta biogeografia y de su dispersión mediante corrientes marinas de superficie. Para ello se han considerado los aspectos ecológicos del género Janthina, las localidades fósiles de J. typica y las ρ aleo corrientes oceánicas. Abundantes ejemplares de J. typica se han encontrado en depósitos marinos de la Isla de Gran Canaria que han sido datados en 4.2 Ma. Esta época es justamente anterior a la terminación del clima cálido del Plioceno inferior y se aproxima al comienzo de los cambios globales que van a dar lugar al modelo climático actual En el Plioceno inferior la corriente fría de Canarias no existía. El Atlántico norte subtropical era más cálido que en la actualidad y sus aguas se encontrarían con la Corriente Circumtropical que cruzaba el Paso de América Central desde el Mar Caribe al Océano Pacífico. Desde ahí, la Corriente Ecuatorial del Sur fluye hacia las costas de Indonesia en donde se divide, al norte hacia Japón y al sur hacia las costas de Australia llegando a alcanzar una posición frente a la actual ciudad de Perth tras atravesar el Estrecho de Bass. El trayecto inverso (de Australia al este del océano Atlántico) encontraría más obstáculos y puede considerarse altamente improbable. De ello se deduce que J. typica se originó en el Atlántico europeo. Las principales causas de su extraordinaria distribución geográfica son su ecología, por ser un animal flotante en aguas cálidas, los movimientos de las placas tectónicas, que permitieron el paso de América Central y un estrechamiento del paso de Indonesia, y el movimiento de rotación de la Tierra y su influencia sobre las corrientes marinas.

Regional Ocean Modeling System (ROMS) results, combined with chlorophyll-α (Chl-α) and satellite altimetry information as well as information from oceanographic cruises were analyzed to identify interactions between intrathermocline eddies (ITEs) and the Juan Fernández Archipelago (JFA), and discuss their potential impact on surface Chl-α concentrations. The JFA is located off the coast of central Chile (33°S), and is composed of three main islands: Robinson Crusoe (RC), Alejandro Selkirk (AS) and Santa Clara (SC). Results indicate that the surface and subsurface anticyclonic eddies that interact with the JFA are formed primarily within the coastal transition zone between 33° and 39°S. ITEs are present within the JFA region with a semiannual frequency, mainly during the austral autumn, and have a weak surface expression in relation to the adjacent surface eddies, with a slow displacement (1.16 to 1.4 km d-1) in a northwest direction and a coherent structure for periods of ≥ 1 year. During the ITEs' interaction with RC-SC islands and an adjacent seamount, a slight (prominent) thermocline deflection of the upper limit (lower) was observed. The horizontal extent (~70-100 km) was greater than the internal Rossby deformation radius and the average vertical extent was ~400 m. The interaction between the weak surface expression of ITEs, identified with satellite altimetry, and the JFA persisted during autumn for nine weeks until reaching the winter period. Approximately one month after the beginning of the interaction between ITEs and the islands, increases in surface Chl-α associated with the eddy were observed, with values up to three times higher than adjacent oceanic waters.

Se analizaron los resultados de un modelo oceánico regional (ROMS), combinado con información de clorofila-α (Clo-α) y altimetría satelital, además de información proveniente de cruceros oceanográficos, para identificar la interacción entre los remolinos intratermoclina (ITEs) y el Archipiélago Juan Fernández (AJF), y discutir su potencial impacto sobre las concentraciones de Clo-α superficial. El AJF se encuentra ubicado frente a la costa central de Chile (33°S), y está conformado por las islas Robinson Crusoe, Santa Clara y Alejandro Selkirk. Los resultados indican que los remolinos anticiclónicos superficiales y subsuperficiales que interactúan con el AJF se forman principalmente en la zona de transición costera entre 33° y 39°S. Los ITEs se presentan en la región del AJF con una frecuencia semianual, principalmente durante el período de otoño austral, y poseen una débil expresión superficial respecto de los remolinos superficiales adyacentes, un lento desplazamiento (1,16-1,4 km d-1) con dirección noroeste y una estructura coherente por períodos ≥ 1 año. Durante la interacción de los ITEs con las islas RC-SC y el monte submarino adyacente, se observó una leve (prominente) deflexión del límite superior (inferior) de la termoclina. La escala horizontal (~70-100 km) fue mayor que el radio interno de deformación de Rossby y la escala vertical promedio fue de ~400 m. La interacción entre el ITE de débil expresión superficial identificada con altimetría satelital durante el período de otoño y el AJF perduró durante nueve semanas alcanzando el período invierno. Aproximadamente un mes después del comienzo de la interacción entre el ITE y las islas, se observaron incrementos de Clo-α superficial asociados al remolino con valores hasta tres veces mayores, respecto de las aguas oceánicas adyacentes.

Con el fin de evaluar el efecto de la Subducción de la Dorsal Asísmica de Cocos (DAC) durante las etapas finales de la formación del Istmo. Realizamos muestreos con bultos en afloramientos fosilíferos en las penínsulas de Burica y Nicoya. Las condiciones paleoecológicas y paleoambientales fueron reconstruidas a partir de la comparación entre la estructura de las comunidades fósiles, con las comunidades modernas dragadas de los mares de Panamá, usando análisis de componentes principales. Los resultados indican que antes del cierre del Istmo, existieron islas oceánicas y un afloramiento moderado en Burica. Posterior al cierre, el choque de la DAC provocó la elevación del fondo marino y las aguas que se encontraban a 2 300m pasaron a 40m. El afloramiento se intensificaba en mar abierto pero la dorsal había formado islas en Burica que limitaban el efecto del afloramiento en la costa. La subducción de la DAC continuó y las islas se unieron gradualmente a tierra firme y desaparecieron, permitiendo el afloramiento. Durante el Pleistoceno medio un segundo proceso de levantamiento acelerado continuó elevando el fondo marino y formó la Cordillera de Talamanca. La cordillera creó una barrera que bloqueó el paso de los vientos Alisios y originó condiciones ecológicas y optimas que permiten el crecimiento de los mejores arrecifes de coral costeros del Pacífico oriental tropical (POT) entre Panamá y Costa Rica.

The evolution of the marine communities along the Pacific coast of Central America, may have changed in response to the formation of the Isthmus of Panama. To evaluate the effect of the Aseismic Cocos Ridge (DAC) subduction on the marine benthic communities, we reconstructed benthic assemblages from Neogene fossiliferous formations in Burica and Nicoya peninsulas of Panama and Costa Rica. Paleoecological and paleoenvironmental conditions were reconstructed by comparing community structure from bulk fossil samples with dredge collections from modern Tropical American seas, using principal component analysis. Our results indicate that during the early Pliocene, before the closing of the Isthmus, some oceanic islands existed with moderate upwelling in the Burica region. After the closure, during the late Pliocene and early Pleistocene the collision of the DAC caused an uplift of the seafloor, where water depth of 2 300m became shallow waters of less than 40m depth. Meanwhile, upwelling intensified in the open ocean the uplift that had formed small islands in coastal areas of Burica, creating protected areas and limiting the upwelling effect that was given in open ocean. The subduction of the DAC continued until the islands were joined to the mainland and gradually disappeared, allowing the return of the upwelling. During the middle Pleistocene a second process of accelerated uplift ith speeds of 8m/1 000 years provoked again the elevation of the seafloor and later the elevation of the Talamanca Range. The new range formed a barrier that blocked the passage of the Trade winds, created new ecological conditions and optimized and allowed the growth of the best coral reefs in the coasts of the tropical Eastern Pacific (POT) between Panama and Costa Rica. Rev. Biol. Trop. 60 (2): 893-908. Epub 2012 June 01.

A total of 126 sword-fish (Xiphias gladius) stomach were analyzed. The stomachs were obtained from specimens captured within the 25° to 34°S and 78° to 90°W área, by industrial longline vessels. Only 72 (57.1%) of the total stomach had food ítems in their stomachs, whit a sex proportion of 32 males (44.4%o), 37 females (51.4%) and 3 indeterminate (4.2%). Traditional methods for the stomach contents analysis such as the numeric, frequency of occurrence, gravimetric and índex of relative importance (IRI) were used. Cephalopods were the main food ítems (%IIR > 90) followed by the fishes (%IIR < 4). The squid (Dosidicus gigas) was the principal food in this study (%IIR = 85.1). No significant differences were found between the feeding of males and females of swordfishes (U = 81; p = 0.4862, p < 0.05). A cluster analysis showed higher stomach contents similarity between fall and winter than those from spring, within the size groups the I group was different than others and finally, the feeding from zone 3 were different from others zones.

Se analizaron 126 estómagos de pez espada (Xiphias gladius), capturados entre los 25° y 34°S y 78° y 90°W, por barcos palangreros de la pesquería industrial frente a Chile centro-norte. Del total de estómagos analizados, 54 se encontraron vacíos (42,9%) y 72 (57,1%) presentaron sus estómagos con contenidos, de estos últimos la proporción sexual correspondió a 32 machos (44,4%), 37 hembras (51,4%o) y 3 indeterminados (4,2%o). Para el análisis de contenidos estomacales se utilizaron los métodos numéricos, de frecuencia, gravimétricos e índice de importancia relativa (IIR). Los cefalópodos fueron el alimento principal (%IIR > 90), mientras que los peces aparecen como alimento secundario (%IIR < 4). El cefalópodo Dosidicus gigas fue la presa principal (%IIR = 85,1). No se encontró diferencia significativa entre la alimentación por sexos ({/= 81; p = 0,4862, p < 0,05). Los análisis de conglomerados revelaron que los contenidos estomacales son más similares en otoño e invierno que en primavera; mientras que entre los distintos grupos de tallas el grupo I aparece como distinto del resto de las tallas, y los estómagos analizados provenientes de la zona 3 serían más diferentes que el resto de las áreas estudiadas.

The species composition, geographical distribution and abundance of the euphausiids, captured in 64 sampling stations between Valparaíso (33°0'S, 71°40'W), Juan Fernández Archipelago (33°37'S, 68°49'W) and Desventuradas Islands (26°20'S, 80°00W) between September and October 2000, were de-scribed. Between euphausiids adults, a total of 22 species belonging to six genera were identified {Euphausia, Nematobrachion, Nematoscelis, Stylocheiron, Thysanoessa and Thysanopoda). The higher abundance was found near Valparaíso (6,238 ind 1000 m-3), only integrated by Euphausia mucronata (62,3%). The captures were influenced by the diel vertical migration, which generated a horizontal discontinuity in the distribution of most of the euphausiids, due to the sampling depth (0-200 m). An inverse correlation was estabhshed between the abundance of E. mucronata and Nematoscelis megalops and the dissolved oxygen concentration.

Se describe la composición, distribución y abundancia de los eufáusidos capturados entre Valparaíso (33°0'S, 71°40'W), el archipiélago de Juan Fernández (33°37'S, 68°49'W) y las islas Desventuradas (26°20'S, 80°00'W), entre septiembre y octubre de 2000. Se identificaron 22 especies pertenecientes a seis géneros {Euphausia, Nematobrachion, Nematoscelis, Stylocheiron, Thysanoessa y Thysanopoda). La mayor abundancia se encontró próxima a la costa de Valparaíso (6.238 ind 1000 m-3 ), sólo constituida por Euphausia mucronata (62,3%). Las capturas estuvieron influenciadas por la migración vertical diaria, lo que generó una discontinuidad en la distribución horizontal de la mayoría de los eufáusidos, dada la profundidad del muestreo (0-200 m). Se estableció una relación inversa entre la abundancia de E. mucronata y Nematoscelis megalops y la concentración de oxígeno disuelto.

The mid-Tertiary Coastal Magmatic Belt in south-central Chile, which crops out both in the Central Valley and, south of 41°S, in the Coastal Cordillera as far west as the Pacific coast, formed when the locus of Andean magmatic activity expanded, both to the west and to the east relative to its previous and current location in the Main Cordillera. This expansion of the magmatic arc occurred in conjunction with a regionally widespread episode of late Oligocene to Miocene extension which thinned the crust below the proto-Central Valley in south-central Chile and generated sedimentary basins west of, within, and east of the Main Cordillera. The extrusive rocks of the mid-Tertiary Coastal Magmatic Belt are interbedded with the late Oligocene to Miocene continental and marine sediments deposited in these basins, and forty-seven of the fifty new and previously published age determinations for these rocks are within the time period 29 Ma (late Oligocene) to 18.8 Ma (early Miocene). The initiation of extension, basin formation and the westward migration of magmatic activity coincides closely to the beginning, in the late Oligocene, of the current period of both high convergence rate (>10 cm/yr) and less oblique convergence, which together resulted in an approximately three-fold increase in trench-normal convergence rate between the Nazca and South American plates. Extension continued, along with a transient steepening of subduction angle as indicated by the westward migration of the volcanic front during the formation of the mid-Tertiary Coastal Magmatic Belt, during an approximately 10 million year period after the trench-normal convergence rate tripled across the Nazca and South American plate boundary. The mid-Tertiary Coastal Magmatic Belt includes igneous rocks chemically similar to modern Andean arc magmas, as well as rocks with ocean island basalt chemical affinities characterised by lower Ba/La (<19), La/Nb (<1.6) and initial 86Sr/87Sr ratios (<0.7035), and higher epsilonNd(T) values (>+5). The latter formed by melting of mantle uncontaminated by components derived from the dehydration of subducted oceanic lithosphere. This suggests the formation of the mid-Tertiary Coastal Magmatic Belt may have involved upwelling of asthenospheric mantle, possibly through a slab-window, due to the transient episode of invigorated asthenospheric wedge circulation caused by the three-fold increase in late Oligocene trench-normal convergence rates between the Nazca and South Amercan plates. The change in subduction geometry and the transient period of invigorated asthenospheric circulation caused by this increase in convergence rate may have combined to produced moderate extension across the southern South American continental margin by inducing an episode of slab rollback of the subducting Nazca plate

La relación del Cinturón Magmático del Terciario-medio del centro-sur de Chile con el incremento de la razón de convergencia de placas en el Oligoceno superior. El Cinturón Magmático del Terciario-medio de la Costa en el centro-sur de Chile, que aflora en el Valle Central y, al sur de los 41°S, en la Cordillera de la Costa y a lo largo de la costa del Oceáno Pacífico, se formó cuando el foco de la actividad magmática se expandió tanto al oeste como al este de su posición previa y actual en la Cordillera Principal. Esta expansión del arco magmático ocurrió durante el Oligoceno superior-Mioceno en conjunción con un episodio regional de expansión cortical, que adelgazó la corteza bajo el Valle Central embrionario en el centro-sur de Chile y generó cuencas sedimentarias en, al oeste y al este de la Cordillera Principal. Las rocas extrusivas de este cinturón están asociadas con rocas sedimentarias, continentales y marinas, del Oligoceno superior-Mioceno, depositadas en estas cuencas. De un total de cincuenta determinaciones de edades para estas rocas, nuevas y previamente publicadas, cuarenta y siete se ubican entre 29 Ma (Oligoceno superior) y 18,8 Ma (Mioceno inferior). El inicio de la extensión, de la formación de las cuencas y la migración hacia el oeste de la actividad magmática coincide con el comienzo, en el Oligoceno superior, del actual periodo caracterizado por una alta razón de convergencia (>10 cm/año) y de convergencia menos oblicua, lo cual, en conjunto, generó un aumento de tres veces en la velocidad de convergencia de las placas de Nazca y Sudamericana. La extensión continuó por aproximadamente 10 millones de años después que la razón de convergencia, entre las placas de Nazca y Sudamericana, se triplico y la migración hacia el oeste del frente volcánico, durante ese lapso, sugiere un aumento transitorio del ángulo de subducción. El Cinturón Magmático del Terciario-medio de la Costa incluye rocas ígneas con composiciones químicas similares a las rocas del arco volcánico moderno y, también, rocas con afinidades oceánicas, caracterizadas por bajas razones Ba/La (<19) y La/Nb (<1,6), bajas razones 86Sr/87Sr (<0,7035) y altos valores de épsilonNd(T) (>+5). Estas últimas se formaron por fusión parcial de manto astenosférico no contaminado con componentes provenientes desde la deshidratación de litosfera oceánica subductada. Esto sugiere que la formación del Cinturón Magmático del Terciario-medio de la Costa puede haber involucrado alzamiento de manto astenosférico, posiblemente a través de una ventana en la placa subductada, debido a un periodo transitorio de circulación vigorosa en la astenosfera, causado por el incremento en tres veces de la razón de convergencia entre las placas de Nazca y Sudaméricana, ocurrido durante el Oligoceno superior. El cambio en la geometría de subducción y el período transitorio de circulación astenosférica vigorosa, generados por el incremento en las razones de convergencia, se pueden haber combinado para producir una extensión moderada en el margen continental del sector sur de Sudamérica, al inducir un episodio de 'rollback' de la Placa de Nazca subductada