Magmatism and crustal evolution in Costa Rica (Central America)

New major element and trace element data of Lower Tertiary volcanics and of rocks of the Aguacate volcanic series permit a better understanding of the petrochemical evolution in the Costa Rica area. This evolution of the magmatic and tectonic processes shows clearly the transformation of oceanic crust with basic magmatism into a continental crust with intermediate to acidic magmatic activity. The main trend since the upper Cretaceous is characterized by a continual increase of the silica level of the magmatic products which build up the present tectonitic crust of Costa Rica.Only the Aguacate volcanic series is contrasting from all other magmatic series by predominance of alkaline rocks which are petrochemically quite similar to the volcanics of the East-Pacific Islands. It is considered, therefore, that the Aguacate volcanics represent remnants of Upper Tertiary island volcanoes.

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Zusammenfassung Neue chemische Gesteinsanalysen und Spurenelement-Daten von tertiären Vulkaniten Costa Ricas erlauben eine bessere Interpretation der petrochemischen Entwicklung im südlichen Mittelamerika. Sie ist gekennzeichnet durch einen kontinuierlichen Anstieg des SiO₂-Gehaltes der magmatischen Förderprodukte von der Oberkreide bis zur Gegenwart. Hierin kommt die Umwandlung ozeanischer in kontinentale (tectonitic) Kruste zum Ausdruck.Abweichend von allen anderen magmatischen Serien Costa Ricas weist die Aguacate-Serie eine Vorherrschaft von alkalinen Gesteinen auf, die petrochemisch denen der ost-pazifischen Inselvulkane ähnlich sind. Es wird daher die Möglichkeit erörtert, da\ die Aguacate-Serie stark abgetragene Reste tertiärer Inselvulkane darstellt.

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Resumen Nuevos análisis químicos y datos de elementos trazas en los volcanitos terciarios de Costa Rica permiten una mejor interpretación de la evolución quimica del magmatismo en la zona de América Central. Dicha evolución está caracterizada por un continuo aumento en el contenido de SiO₂ de las rocas volcánicas desde el Cretácico superior hasta el Reciente. El aumento de silice manifesta la transformación de la corteza oceánica en una corteza continental (tectonítica).La Serie Aguacate se diferencia de todas las otras series magmáticas de Costa Rica, mostrando una predominación de las rocas alcalinas, que químicamente se parecen a las de las islas volcánicas en el Pacífico oriental. Por esta razón se discute la posibilidad de que la Serie Aguacate representa los restos fuertemente erosionados de islas volcánicas terciarias.

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The origin of oceanic plagiogranites from the karmoy ophiolite,western Norway

Both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the Karmoy ophiolite of western Norway. The plutonic zone of this ophiolite can be subdivided into three complexes; the East-Karmoy Igneous Complex, the Visnes High Level Complex and the Veavagen Igneous Complex and plagiogranitic rocks are well developed in the first two of these.Within the East-Karmoy Igneous Complex, plagiogranites are associated with high temperature, pre-basic dyke, shear zones. Rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite (hydrated diabase) assuming a starting material consisting of 40% hornblende and 60% plagioclase and that batch melting occurred within the stability field of hornblende.In comparison, plagiogranite occurs in a number of bodies in the upper part of the Visnes High Level Complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes, microgabbros and magnetite gabbros, and a floor assemblage of layered and non-layered gabbros. The R.E.E. modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari-textured gabbros, although the distribution of highly hygromagmatophile elements such as K, Rb, Ba, etc. cannot be explained satisfactorily by this model alone. Depletion in these elements appears to be an autometasomatic effect.     

Seasonal abundance of brachyuran crab larvae in a tropical estuary: Gulf of Nicoya,Costa Rica,Central America

Brachyuran crab larvae were sampled approximately monthly at four stations in the Gulf of Nicoya, a tropical estuary on the Pacific coast of Costa Rica, Central America. The gulf was divided into two regions based on physical and chemical characteristics. Larvae were identified to five families: Ocypodidae, Pinnotheridae, Portunidae, Majidae, Xanthidae. The abundance and distribution of these taxa were similar to that found in temperature estuaries of North America. Ocypodids, pinnixids, and xanthids were most common in the estuarine upper-gulf region while portunids were more common in the lower gulf. Majids were found in small numbers in the lower gulf. Only the ocypodids showed a consistent pattern in vertical distribution; larvae of this taxon were always more common in surface water. There was no consistent pattern in the seasonal abundance of the various taxa when compared among stations.     

新疆北天山奎屯河蛇绿岩斜长花岗岩锆石SIMS U-Pb年龄及其构造意义

作为中亚造山带南部的最终缝合界线,天山—索伦缝合带记录了古亚洲洋晚期的扩张消减历史。本文对新疆北天山混杂带中奎屯河蛇绿岩的斜长花岗岩进行了SIMS锆石年代学研究及地球化学研究。斜长花岗岩富Na2O,贫K2O(2O3(<15%),微量元素含量整体低于标准N-MORB,稀土配分曲线平坦,无明显Eu异常,是典型的洋中脊斜长花岗岩,其结晶年龄代表准噶尔洋盆扩张时代。24组离子探针测年数据显示,斜长花岗岩加权平均年龄为343.1±2.7 Ma。结合区域地质研究,本文认为在晚古生代石炭纪,伴随着哈萨克斯坦山弯构造的逐渐形成,准噶尔洋盆持续扩张并向外俯冲于周缘地体之下,形成了东、西准噶尔洋内弧,以及天山和阿尔泰安第斯型陆缘弧。     

北天山巴音沟蛇绿岩斜长花岗岩SHRIMP锆石U-Pb年龄及蛇绿岩成因研究

巴音沟蛇绿岩侵位于辉长岩中的斜长花岗岩亏损LREE和不相容元素,(Ce/Yb)_N=0．67～0．88．(Gd/Yb)_N= 0．63～0．73,ε_Nd(t)=8．36～8．52,~87Sr/~86Sr(t)=0．70365～0．70408,为亏损地幔来源花岗岩,是蛇绿岩的组成部分。斜长花岗岩的精细SHRIMP锆石U-Pb年龄为324．8±7．1Ma,表明巴音沟蛇绿岩形成于早石炭世晚期。巴音沟蛇绿岩中辉长岩、辉绿宕、基性熔岩REE球粒陨石标准化和微量元素原始地幔标准化分配型式呈平坦型分布,(Ce/Yb)_N=0．63～1．36,(Gd/ Yb)_N=1．02～1．41,近似于N-MORB分配型式。~87Sr/~86Sr比值变化于0．70306～0．70439,ε_Nd(t)=4．63～8．69,表明其岩浆主体来源于N-MORB,但在形成过程中有OIB组分的加入。结合天山石炭纪区域火山岩的研究,认为巴音沟蛇绿岩形成于大陆裂谷向大洋裂谷转化的构造环境,它是天山造山带晚古生代“红海型”洋盆的地质记录。     

Hydrothermal processes in partially serpentinized peridotites from Costa Rica: evidence from native copper and complex sulfide assemblages

Native metals and metal alloys are common in serpentinized ultramafic rocks, generally representing the redox and sulfur conditions during serpentinization. Variably serpentinized peridotites from the Santa Elena Ophiolite in Costa Rica contain an unusual assemblage of Cu-bearing sulfides and native copper. The opaque mineral assemblage consists of pentlandite, magnetite, awaruite, pyrrhotite, heazlewoodite, violarite, smythite and copper-bearing sulfides (Cu-pentlandite, sugakiite [Cu(Fe,Ni)₈S₈], samaniite [Cu₂(Fe,Ni)₇S₈], chalcopyrite, chalcocite, bornite and cubanite), native copper and copper–iron–nickel alloys. Using detailed mineralogical examination, electron microprobe analyses, bulk rock major and trace element geochemistry, and thermodynamic calculations, we discuss two models to explain the formation of the Cu-bearing mineral assemblages: (1) they formed through desulfurization of primary sulfides due to highly reducing and sulfur-depleted conditions during serpentinization or (2) they formed through interaction with a Cu-bearing, higher temperature fluid (350–400 °C) postdating serpentinization, similar to processes in active high-temperature peridotite-hosted hydrothermal systems such as Rainbow and Logatchev. As mass balance calculations cannot entirely explain the extent of the native copper by desulfurization of primary sulfides, we propose that the native copper and Cu sulfides formed by local addition of a hydrothermal fluid that likely interacted with adjacent mafic sequences. We suggest that the peridotites today exposed on Santa Elena preserve the lower section of an ancient hydrothermal system, where conditions were highly reducing and water–rock ratios very low. Thus, the preserved mineral textures and assemblages give a unique insight into hydrothermal processes occurring at depth in peridotite-hosted hydrothermal systems.     

Modeling a tsunami from the Nicoya,Costa Rica,seismic gap and its potential impact in Puntarenas

Although subduction zones around the world are known to be the source of earthquakes and/or tsunamis, not all segments of these plate boundaries generate destructive earthquakes and catastrophic tsunamis. Costa Rica, in Central America, has subduction zones on both the Pacific and the Caribbean coasts and, even though large earthquakes (Mw = 7.4–7.8) occur in these convergent margins, they do not produce destructive tsunamis. The reason for this is that the seismogenic zones of the segments of the subduction zones that produce large earthquakes in Costa Rica are located beneath land (Nicoya peninsula, Osa peninsula and south of Limón) and not off shore as in most subduction zones around the world. To illustrate this particularity of Costa Rican subduction zones, we show in this work the case for the largest rupture area in Costa Rica (under the Nicoya peninsula), capable of producing Mw ～ 7.8 earthquakes, but the tsunamis it triggers are small and present little potential for damage even to the largest port city in Costa Rica.The Nicoya seismic gap, in NW Costa Rica, has passed its ～50-year interseismic period and therefore a large earthquake will have to occur there in the near future. The last large earthquake, in 1950 generated a tsunami which slightly affected the southwest coast of the Nicoya Peninsula. We present here a simulation to study the possible consequences that a tsunami generated by the next Nicoya earthquake could have for the city of Puntarenas. Puntarenas has a population of approximately eleven thousand people and is located on a 7.5 km long sand bar with a maximum height of 2 m above the mean sea level. This condition makes Puntarenas vulnerable to tsunamis.     

藏北羌塘中部存在志留纪洋盆——来自桃形湖蛇绿岩中斜长花岗岩的锆石U-Pb年龄证据

报道了羌塘西部桃形湖蛇绿岩中斜长花岗岩的锆石U-Pb定年结果和岩石地球化学资料及锆石Hf同位素成分。斜长花岗岩中的锆石具有典型的岩浆生长环带,结合其Th/U值(0.16~0.44),表明为典型的岩浆成因。锆石LA-ICP-MS和Cameca U-Pb定年结果分别为441Ma±2Ma和442Ma±3Ma,相当于早志留世,略年轻于伴生的堆晶辉长岩。锆石的εHf(t)值为+8.13~+11.23,平均值为+9.50,具有明显的幔源成因特征。全岩地球化学分析结果表明,斜长花岗岩是洋壳运移过程辉长岩部分熔融形成的。斜长花岗岩的年龄数据表明,羌塘中部地区存在志留纪洋盆。     

Seasonal and diel patterns of total gaseous mercury concentration in the atmosphere of the Central Valley of Costa Rica

Monitoring of Hg in the atmosphere near volcanoes is limited with no previous data for Costa Rica. Seasonal and daily patterns of total gaseous mercury (TGM) were observed at the main sampling location at the Universidad Nacional, Heredia, Costa Rica. The area (lat. 10.000230 long. −84.109499) is located in the Central Valley of Costa Rica and is 27 km SE of the Poas volcano (lat. 10.199486 long. −84.231388). Measurements were made from May 2008 to May 2009 at this location with some additional values obtained at other sites near the Poas volcano including San Luis and Grecia as well as near, Turrialba and Irazu volcanoes. Total gaseous Hg was determined in samples collected at a height of 2 m using the Tekran 2537A (Tekran Inc.) gas-phase Hg vapor analyzer. Meteorological data (temperature, relative humidity, wind speed, wind direction, radiation and precipitation) were obtained from the airport weather station located at Alajuela. Monthly precipitation is typically 85 mm during the dry season (December to April) with winds from the west. The wet season begins in late April and continues to December with monthly rainfall of 328 mm and winds from the NE. The annual mean temperature is 20 °C. With the onset of the wet season TGM increased from typical values near 10 to 905 ng m⁻³. Measurements made within 5 km of the Poas volcano were higher than at Heredia at that time. Diel values measured at the university site increased until midday along with temperature and radiation. Relative humidity showed a reciprocal pattern. It was found that high values of TGM were not related to wind velocity or direction. The strong diel pattern increased with sunrise, peaked at midday and was lowest during the night time. It would seem that Hg⁰ from the volcano is oxidized and is deposited to the soils during the dry season when winds are blowing from the volcano. With the onset of heavy rains in April, Hg in the soil is reduced and re-volatilized resulting in the high levels in the atmosphere. Values at other volcano sites were provided. The role of atmospheric pollutants such as H₂O₂ and O₃ should be included in future studies as they may result in oxidation of reduced Hg. The instability in the air masses may also be a factor and local pollution sources may result in high levels that is emitted from the volcanoes of Hg being circulated to ground level as radiation intensity increases.     

Fluid venting activity on the Costa Rica margin: new results from authigenic carbonates

Carbonate precipitates on mounds and along tectonic scarps off the Costa Rica margin are manifestations of subduction-induced dewatering. The long-term dewatering history is recorded in mineralogical, petrological and isotope signals of carbonates recovered from these sites. The carbonates are strongly depleted in ¹³C (–11 to –53 PDB) and enriched in ¹⁸O (+4 to +8 PDB). Thermogenic methane and biogenic methane were identified as sources of the carbon. Chemoherm carbonates and seepage-associated carbonates formed in a focused flow regime have lighter ¹³C values, while others formed in a more diffusive flow regime have slightly enriched C isotope values. Three fluid components were inferred based on the calculation of equilibrium ¹⁸O: clay dehydration water, gas hydrate water and seawater. Calculated equilibrium ¹⁸O values of carbonates from different down-core depths as well as from different precipitation stages show that the ¹⁸O of the precipitating fluid is progressively depleted with time. Dolostones showing a methane-C source and a well constrained O-isotope signature are thought to have formed at depth in the sediment and subsequently became exhumed. Glauconitic sandstones cemented by methane-derived carbonate provide evidence that fluid and solid material have been expelled by the mud volcano.     

The geologic history of the Nicoya Ophiolite Complex, Costa Rica, and its geotectonic significance

Costa Rica forms part of an intra-oceanic arc between the Pacific and Caribbean oceans; the Nicoya Ophiolite Complex is located along its Pacific border. In this study, evidence is given that the Nicoya Complex is composed of ridge-formed oceanic crust that suffered a strong compressional stress during Late Santonian times. As a result of this, isoclinal folding and large-scale nappe emplacement occurred at a shallow crustal depth. The principal component of this compressional stress was E-W-directed. It is also demonstrated that, from this time, the complex was situated between a subducting plate and a volcanic arc. From that Campanian until the Middle Eocene the zone was undulated, and generally at a great depth below sea level. During the Eocene—Oligocene epoch a new tectonic stress affected the area. It produced open folding with upthrusting in the ophiolite complex and overthrust folding of the overlying rock series. As a result of crustal thickening during this tectonic phase, the area was uplifted. From Miocene times, the zone was shaped into a dome and a synform. These undulations are attributed to compression of the subducting Coco Plate, west of the area.The Upper Santonian tectonic phase demonstrates how compressional stress produced the break-up of the Caribo-Pacific plate west of the study area, as a result of which, a Caribbean plate without an associated oceanic ridge and a Pacific plate originated. The compressional stress in question was presumably generated by the opposed spreading directions of the new Mid-Atlantic Ridge and an older ridge to the west of the study area.Furthermore, it is argued that the Cretaceous obduction of the ophiolite belt along the Pacific coast of the American continents, was produced by the directional change of these continents during the birth of the Mid-Atlantic Ridge. This created intra-plate compressional stress and converted originally passive continental margins into active zones, where thrusting of oceanic crust on to a continental margin (obduction) could occur. When the Mid-Atlantic Ridge started spreading, the obduction phase ended due to subduction of the oceanic plate below the leading edge of the continent.     

Geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites, Central Anatolian Crystalline Complex, Turkey

Plagiogranites associated with the Sarikaraman ophiolite of the Central Anatolian Crystalline Complex, Turkey, closely resemble other plagiogranites from supra-subduction zone-type ophiolites of Neotethys. The ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite (ca. 10% by volume) than is usually associated with such bodies. The Sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex. The plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite. Field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber. Both the deep-seated trondhjemites and the volcanic rhyolites constitute the Sarikaraman plagiogranite suite. Geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites, which are characterised by (MORB-normalized) low HFS element contents with small negative Nb---Ta anomalies and variably enhanced LIL element abundances. Unlike other plagiogranites, however, the Sarikaraman suite is not characterized by consistently low K₂O contents; a feature that reflects the variable mobilization of the LIL elements under lower greenschist facies conditions. The REE are uniformly enriched relative to the basic components of the complex, but have similar normalized patterns exhibiting mild light REE depletion. In terms of their origin, the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization (70–85% of clinopyroxene-feldspar ± amphibole) or partial melting (5–15% batch melting) of a gabbroic ‘source material’, although only the first process can produce most of the range of the plagiogranite compositions. As a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole.     

Petrogenetic and geotectonic significance of Neoproterozoic suprasubduction mantle as revealed by the Wizer ophiolite complex,Central Eastern Desert,Egypt

Ophiolite complexes, formed in a suprasubduction zone environment during Neoproterozoic time, are widely distributed in the Eastern Desert of Egypt. Their mantle sections provide important information on the origin and tectonic history of ocean basins these complexes represent. The geochemistry and mineralogy of the mantle section of the Wizer ophiolite complex, represented by serpentinites after harzburgite containing minor dunite bodies, are presented. Presence of antigorite together with the incipient alteration of chromite and absence of chlorite suggests that serpentinization occurred in the mantle wedge above a Neoproterozoic subduction zone. Wizer peridotites have a wide range of spinel compositions. Spinel Cr# [100Cr/(Cr + Al)] decrease gradually from dunite bodies (Cr# = 81–87) and their host highly depleted harzburgites (Cr# = 67–79) to the less depleted harzburgites (Cr# = 57–63). Such decreases in mantle refractory character are accompanied by higher Al and Ti contents in bulk compositions. Estimated parental melt compositions point to an equilibration with melts of boninitic composition for the dunite bodies (TiO₂ = ~<0.07–0.22 wt%; Al₂O₃ = 9.4–10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO₂ = <0.09–0.28 wt%; Al₂O₃ = 11.2–14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO₂ = ~<0.38–0.51 wt%; Al₂O₃ = 14.5–15.3 wt%). Estimated equilibrium melts are found in the overlying volcanic sequence, which shows a transitional MORB–island arc geochemical signature with a few boninitic samples. Enrichment of some chromites in TiO₂ and identification of sulfides in highly depleted peridotites imply interaction with an impregnating melt. A two-stage partial melting/melt–rock reaction model is advocated, whereby, melting of a depleted mantle source by reaction with MORB-like melts is followed by a second stage melting by interaction with melts of IAT–boninitic affinities in a suprasubduction zone environment to generate the highly depleted harzburgites and dunite bodies. The shift from MORB to island arc/boninitic affinities within the mantle lithosphere of the Wizer ophiolite sequence suggests generation in a protoarc-forearc environment. This, together with the systematic latitudinal change in composition of ophiolitic lavas in the Central Eastern Desert (CED) of Egypt from IAT–boninitic affinities to more MORB-like signature, implies that the CED could represent a disrupted forearc-arc-backarc system above a southeast-dipping subduction zone.     

Preservation of biosignatures in metaglassy volcanic rocks from the Jormua ophiolite complex,Finland

Evidence of microbially-mediated alteration of basaltic glass is preserved in originally glassy basalts (rims of pillow lavas, hyaloclastite breccias, and chilled margins of dykes) from the well-preserved 1.95 Ga Jormua ophiolite complex (JOC) of Northeastern Finland. Although textural evidence of microbial alteration is commonly observed in relic glass from recent oceanic crust and some ophiolites, these textures have been destroyed during greenschist to lower amphibolite facies regional metamorphism and deformation of the JOC. However, another robust biosignature is found in the generally depleted δ¹³C values of disseminated carbonate extracted from originally glassy basalts, relative to crystalline samples. The same distribution of δ¹³C values is well documented in samples from recent oceanic crust as well as ophiolites of Phanerozoic age. This characteristic contrast in the δ¹³C values of disseminated carbonate is interpreted to result from microbe-induced fractionation during oxidation of organic matter. X-ray mapping of initial alteration zones has identified residual carbon associated with highly-concentrated S that is unrelated to carbonate. We attribute these biosignatures to microbially-mediated alteration of originally glassy material prior to ophiolite emplacement.     

Late Quaternary moisture export across Central America and to Greenland: evidence for tropical rainfall variability from Costa Rican stalagmites

We present a high-resolution terrestrial archive of Central American rainfall over the period 100–24 and 8.1–6.5 ka, based on δ¹⁸O time series from U-series dated stalagmites collected from a cave on the Pacific Coast of Costa Rica. Our results indicate substantial δ¹⁸O variability on millennial to orbital time scales that is interpreted to reflect rainfall variations over the cave site. Correlations with other paleoclimate proxy records suggest that the rainfall variations are forced by sea surface temperatures (SST) in the Atlantic and Pacific Oceans in a fashion analogous to the modern climate cycle. Higher rainfall is associated with periods of a warm tropical North Atlantic Ocean and large SST gradients between the Atlantic and Pacific Oceans. Rainfall variability is likely linked to the intensity and/or latitudinal position of the intertropical convergence zone (ITCZ). Periods of higher rainfall in Costa Rica are also associated with an enhanced sea surface salinity gradient on either side of the isthmus, suggesting greater freshwater export from the Atlantic Basin when the ITCZ is stronger and/or in a more northerly position. Further, wet periods in Central America coincide with high deuterium excess values in Greenland ice, suggesting a direct link between low latitude SSTs, tropical rainfall, and moisture delivery to Greenland. Our results indicate that a stronger tropical hydrological cycle during warm periods and large inter-ocean SST gradients enhanced the delivery of low latitude moisture to Greenland.     

Geochemistry of cumulus peridotites and gabbros from the Marum ophiolite complex,northern Papua New Guinea

The layered cumulus rocks of the Marum ophiolite complex in northern Papua New Guinea range from highly magnesian dunite, wehrlite, and lherzolite through pyroxenite to norite-gabbro with minor anorthosite and ferronorite-gabbro near the top of the sequence. Most of the cumulates, particularly the gabbroic rocks, are characterised by recrystallised adcumulus textures and all intercumulus melt (mesostasis) has been expelled. Trends in the cumulate sequence from Mg-rich to more Fe-, Ca- and Al-rich compositions are consistent with the formation of the layered sequence by magmatic accumulation from mafic tholeiitic magmas with varying degrees of differentiation. The cumulates are characterised by extremely low levels of ‘incompatible’ elements (K, Ba, Rb, P, Zr, Nb, Hf, Y and REE) at all levels of differentiation. REE patterns are strongly depleted in LREE; HREE abundances range from ≦0.3 chondrites in peridotite to 3 x chondrites in the norite-gabbros. The Marum cumulates resemble low-Ti peridotites and gabbros found in other orthopyroxene-bearing ophiolite sequences. The parent magmas of the Marum cumulates are inferred to have been strongly depleted in ‘incompatible’ trace elements (～ 2,000 ppm Ti, ～20 ppm Zr, 6–9 x chondrites HREE with La_N/Sm_N～0.5). These abundances are lower than found in typical MORB and back-arc basin basalts or their picritic parents. The dissimilarity of trace element abundances of the inferred Marum parent magmas with MORB-type high-alumina olivine tholeiites supports the conclusion drawn previously from the petrology of the cumulates that the parent magmas to the Marum ophiolite were not of MORB composition but resembled the strongly depleted, Ni-rich magnesian olivine-poor tholeiites and quartz tholeiites of the Upper Pillow Lavas of the Troodos ophiolite. The Marum parent magmas are believed to have been formed by shallow melting of refractory peridotite, and are chemically and genetically distinct from the LREE-enriched high-Ti lavas (Tumu River basalts) which occur in faulted contact. The geochemical data do not permit unequivocal assignment of a tectonic environment for the formation of either the Tumu River basalts or the plutonic suite; their juxtaposition results from thrust emplacement.     

The Aguacaliente Fault,source of the Cartago 1910 destructive earthquake (Costa Rica)

On 4 May 1910, the most destructive earthquake in the history of Costa Rica (Ms 6.4) destroyed the city of Cartago, a major city located in the Valle Central of Costa Rica. Using both palaeo‐seismological and morphotectonic analyses, we have found evidence that points to the Aguacaliente Fault (AF) as the source of this earthquake. This structure is a N100° E trending, strike‐slip fault situated to the south of Cartago and within a wide band of deformation. We excavated two trenches near Bermejo, south of Cartago. We found evidence of three surface ruptures within the last 1000 years on this fault. The age of the most recent rupture is consistent with the Cartago 1910 earthquake. The AF is a seismogenic source capable of producing large earthquakes (Mw 6.5–6.9) with an estimated recurrence interval of about 500 years.     

Seismic swarms,fluid flow and hydraulic conductivity in the forearc offshore North Costa Rica and Nicaragua

At the continental margin of north Costa Rica and Nicaragua, the strongly hydrated Cocos Plate subducts beneath the Caribbean Plate. From the downgoing Cocos plate fluids are released through extensional fractures in the overriding plate. At the seafloor, they form fluid seeps, mounds and other types of fluid expulsion. Using an offshore temporary seismic network, we investigated seismicity possibly related to these processes and observed several swarms of earthquakes located on the continental slope trenchward of the seismogenic zone of S Nicaragua. The seismicity occurred within the downgoing plate, near the plate interface and in the overriding plate. We interpret these swarm events as an expression of pore pressure propagation under critical stress conditions driven by fluid release from the downgoing plate. In order to estimate hydraulic diffusivity and permeability values, we applied a theory developed for injection test interpretation to the spatio-temporal development of the swarms. The resulting diffusivity and permeability values are in the ranges of 28–305 m²/s and 3.2 × 10^?14 m²–35.1 × 10^?14 m², respectively, applying to the continental and oceanic crust near the plate interface. These values are somewhat larger than observed in drill logs on the margin wedge off north Costa Rica, but of comparable magnitude to values estimated for the Antofagasta 1995 earthquake aftershock sequence.