formation conditions of plagioclase bearing type i chondrules in co chondrites a study of natural samples and experimental analogs abstract about ten percent of type i feo poor chondrules in unequilibrated co chondrites contain plagioclase that appears to be igneous in origin crystallizing at a late stage of solidification of host chondrule melts we have studied plagioclase bearing chondrules in detail and compared them with plagioclase free chondrules in order to determine the formation conditions of plagioclase and the constraints that the presence of plagioclase places on the conditions of chondrule formation plagioclase bearing chondrules have similar textures mineral compositions and bulk compositions to plagioclase free chondrules the only possible chemical difference that might control the presence or absence of plagioclase in a given chondrule is that most plagioclase bearing chondrules have a slightly higher bulk al ca ratio than plagioclase free chondrules we carried out dynamic cooling experiments on a type iab chondrule analog in order to investigate chondrule formation conditions our experiments at slow cooling rates

middle miocene to present plate tectonic history of the southern central american volcanic arc abstract new mid miocene to present plate tectonic reconstructions of the southern central american volcanic arc cava reveal that the inception of cocos ridge subduction began no earlier than 3 ma and possibly as late as 2 ma the cocos ridge has been displaced from the malpelo ridge to the southeast since 9 ma along the panama fracture zone pfz system ambiguous pfz and coiba fracture zone cfz interaction since 9 ma precludes conclusively establishing the age of initial cocos ridge subduction detailed reconstructions based on magnetic anomalies offshore reveal several other variations in subduction parameters beneath southern central america that preceded subduction of the cocos ridge including southeastward migration of the nazca cocos caribbean triple junction along the middle america trench mat from 12 ma to present and subduction of 2 km high scarps both parallel and perpendicular to the trench from 6 to 1 ma the timing of changes in subduction processes has commonly been determined by and correlated with geologic changes in the upper plate however reliable 40 ar 39 ar dating of these events has become available only recently abstr programs geol soc am 2002 these new dates better constrain the magmatic and structural history of southern costa rica observations from this data set include a gap in the volcanic record from 11 to 6 ma which coincides temporally with emplacement of most plutons in southern costa rica normal arc volcanism ceased after 3 5 ma in southern costa rica and pliocene mostly 1 5 ma adakite volcanism was widely distributed from central panama to southern costa rica though volumetrically insignificant this new data reveals that many geologic phenomena commonly attributed to subduction and underplating of the buoyant cocos ridge in fact precede inception of cocos ridge subduction and seem to correlate more favorably in time with earlier tectonic events adakite volcanic activity corresponds in space and time with the subduction of a large scarp associated with a tectonic boundary off southern panama regional unconformities and an 11 6 ma gap in arc volcanism match temporally with oblique subduction of the nazca plate beneath central and southern costa rica cessation of volcanic activity low temperature cooling of plutons in the cordillera de talamanca ct and rapid increases in sedimentation in the fore arc and back arc basins coincide with passage of the nazca cocos caribbean triple junction and initiation of subduction of rough crust associated with cocos nazca rifting 3 5 ma closely followed by initial subduction of the cocos ridge 2 3 ma none of the aforementioned geologic events occurred at a time that would allow for underplating by the cocos ridge rather they are probably related to complex interactions with subduction of complicated plates offshore all of the aforementioned events indicate that the southern central american subduction system has been in flux since at least 12 ma

local earthquake tomography of central costa rica transition from seamount to ridge subduction summary 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 v p v s ratio using about 2000 highquality events the seismicity and slab geometry as well as v p and v p v s 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 v p and v p v s 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

geodynamic evolution of southern costa rica related to low angle subduction of the cocos ridge constraints from thermochronology abstract the late tertiary shallow subduction of the cocos ridge under the caribbean plate controlled the evolution of the cordillera de talamanca in southeast costa rica which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting fission track thermochronology using zircon and apatite as well as 40ar 39ar and rb sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the cordillera de talamanca and the age of onset of subduction of the cocos ridge shallow intrusion of granitoid melts resulted in fast and isobaric cooling a weighted mean zircon fission track age 13 analyses and rb sr biotite ages of about 10 ma suggest rapid cooling and give minimum ages for granitoid emplacement in some cases 40ar 39ar and rb sr apparent ages of amphibole and biotite are younger than the zircon fission track ages which can be attributed to partial resetting by hydrothermal alteration apatite fission track ages range from 4 8 to 1 7 ma but show no correlation with the 3090 m elevation span over which they were sampled the apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes the combination of the apatite fission track ages with subduction parameters of the cocos plate such as subduction angle plate convergence rate and distance of the cordillera de talamanca to the trench implies that the cocos ridge entered the middle america trench between 5 5 and 3 5 ma

tectonic erosion and consequent collapse of the pacific margin of costa rica combined implications from odp leg 170 seismic offshore data and regional geology of the nicoya peninsula the convergent margin off the pacific coast of the nicoya peninsula of costa rica exhibits evidence for subduction erosion caused by the underthrusting cocos plate critical evidence for efficacy of this process was recovered at the ocean drilling program odp drilling site 1042 leg 170 positioned 7 km landward of the middle america trench axis off the nicoya peninsula the primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity the so called base of slope sediment boss horizon or surface the boss horizon which can be traced landward from near the trench to the nicoya coastal area and parallel to it for hundreds of kilometers separates a low velocity 2 0 2 5 km s 1 sequence of slope sediment from an underlying sequence of much higher velocity 4 4 5 km s 1 rock site 1042 reached the acoustically defined boss horizon at a below sea level depth of 3900 m and yielded a carbonate cemented calcarenitic breccia of early middle miocene age sedimentological geochemical paleontological and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment on the basis of coastal exposures the boss horizon as a margin wide geologic interface can be temporally and lithostratigraphically correlated to a regional angular unconformity this unconformity known as the mal pais unconformity separates neogene and younger shelf to littoral beds from the underlying mafic units of the mesozoic nicoya complex and cretaceous and early tertiary sedimentary sequences at site 1042 it is inferred that tectonism caused the vertical subsidence of the early neogene breccia from a shallow to a deep water setting the mal pais unconformity of the boss horizon thus connects the rock fabric of the outermost part of margin to that of coastal nicoya and implies that in the early neogene the nicoya shelf extended seaward to near the present trench axis this circumstance requires that the early neogene trench axis was at least 50 km seaward of where it is now located the long term effects of subduction erosion similar to those described for the scientifically drilled japan tonga and peru margins best account for offshore and onshore evidence for a post paleogene history of crustal thinning and landward trench migration of costa rica s pacific margin during the past 16 17 myr the calculated mass removal and landward migration rates are 34 36 km myr 1 km 1 of margin and 3 km myr 1 respectively these values are similar to those found for other pacific margins dominated by nonaccretionary subduction zone processes

slab dip vs lithosphere age no direct function one paradigm of subduction relates the dip of the slab to the buoyancy of the downgoing lithosphere along subduction zones with the negative buoyancy proportional to the age of the oceanic lithosphere we measured the dip of the slab down to depths of 250 km along 164 sections crossing 13 subduction zones and compared it with the age of the subducting oceanic lithosphere both at the trench and at depth we show here that this relationship is far more irregular than previously suggested and that it is not possible to simply correlate the increase of the slab dip to the increasing age of the downgoing cooler lithosphere younger oceanic lithosphere may show steeper dip than older segments of slabs e g central america vs south america in contrast with predictions of models considering only slab pull the combination of slab age and subduction rate better accounts for slab dip however the correlation is not satisfactory correlation coefficient equal to 0 450 these results suggest that supplemental forces or constraints have to be accounted for such as thickness and shape of the hangingwall plate absolute plate velocity presence of lateral density variations in the hosting upper mantle effects of accretion erosion subduction of oceanic plateaus and slab deformation due to the motion of the mantle relative to the subducting plate d 2005 elsevier b v all rights reserved

pacific offshore record of plinian arc volcanism in central america 3 application to forearc geology 1 sediment gravity cores collected on the pacific slope and incoming plate offshore central america reach up to 400 ka back in time and contain numerous ash layers from plinian eruptions at the central american volcanic arc the compositionally distinct widespread ash layers form a framework of marker horizons that allow us to stratigraphically correlate the sediment successions along and across the middle america trench moreover ash layers correlated with 26 known eruptions on land provide absolute time lines through these successions having demonstrated the correlations in part 1 we here investigate implications for submarine sedimentary processes average accumulation rates of pelagic sediment packages constrained by bracketing tephras of known age range from 1 6 cm ka on the incoming plate to 30 40 cm ka on the continental slope there are time intervals in which the apparent pelagic sedimentation rates significantly vary laterally both on the forearc and on the incoming plate where steady conditions are usually expected a period of unsteadiness at 17 25 ka on the forearc coincides with a period of intense erosion on land probably triggered by tectonic processes unsteady conditions on the incoming plate are attributed to bend faulting across the outer rise triggering erosion and resedimentation extremely low apparent sedimentation rates at time intervals 50 80 ka suggest stronger tectonic activity than during younger times and indicate bend faulting is unsteady on a longer timescale submarine landslides are often associated with ash layers forming structurally weak zones used for detachment ash beds constrain ages of 60 ka 19 ka and 6 ka for three landslides offshore nicaragua phases of intense fluid venting at mud mounds produce typical sediments around the mound that become covered by normal pelagic sediment during phases of weak or no activity using intercalated ash layers we determine for the first time the durations several hundred to 9000 years of highly active periods in the multistage growth history of mud mounds offshore central america which is essential to understand general mud mound dynamics

quaternary volcanic stratigraphy of managua nicaragua correlation and source assignment for multiple overlapping plinian deposits a tephrochronological study that began as an active fault mapping program in managua nicaragua following the 1972 earthquake has yielded information with broader geologic relevance seven principal widespread quaternary air fall deposits the managua sequence have been defined the deposits are basaltic or rhyodacitic span probably 11 km 3 dense rock equivalent and are classified as plinian or phreatoplinian regardless of chemical composition two of the units were radiocarbon dated at 6 600 yr b p and 21 000 yr b p as a result of this study ages of the other layers are not as well constrained but are estimated on the basis of soil development and position in the section correlation and source assignment were particularly difficult because of incomplete exposure and similarity of physical and chemical features but they were accomplished with a combination of detailed measurement and sampling at managua reconnaissance mapping and sampling at potential vent areas and application of petrographic mineralogic and chemical correlation methods the close similarity among the rhyodacite pumices posed the greatest difficulty for correlation cluster analysis applied to multielement xrf chemical analyses of pure glass separates gave the best results the clarification of the managua sequence has revealed the local geologic history of managua has characterized the average largest recent nicaraguan eruptions and has defined time stratigraphic markers used for archaeologic and engineering purposes additionally it has provided a regional framework within which to interpret and correlate more detailed studies of individual nicaraguan volcanoes

pacific offshore record of plinian arc volcanism in central america 2 tephra volumes and erupted masses 1 sediment gravity cores collected from the pacific seafloor offshore central america contain numerous distal ash layers from plinian type eruptions at the central american volcanic arc dating back to more than 200 ka in part 1 of this contribution we have correlated many of those ash layers between cores and with 26 tephras on land the marine ash layers cover areas of up to 106 km2 in the pacific ocean and represent a major fraction 60 90 of the erupted tephra volumes because the pacific coast lies within a few tens of kilometers downwind from the volcanic arc combining our own mapping efforts on land and published mapping results with our marine data yields erupted volumes of all major tephras along the arc that range from 1 to 420 km3 recalculated to erupted magma mass the widespread tephras account for 65 of the total magma output at the arc complementing our tephra data with published volumes of the arc volcanic edifices and volcano ages we calculate the long term average magma eruption rates for each volcano moreover we use incompatible element variations to calculate the cumulate masses that were fractionated during variable degrees of differentiation this yields a minimum estimate of long term average magma production rate at each volcano because intrusives without surface expression and losses by erosion are not accounted for peak magma production rates increase from costa rica to guatemala but there is considerable scatter within each region and large differences even between neighboring volcanoes

the platanar aguas zarcas volcanic centers costa rica spatial temporal association of quaternary calc alkaline and alkaline volcanism the platanar volcanic center is dominated by a calc alkaline basalt andesite dacite rhyolite magma series with unusual lree enrichment adjacent and overlapping the calc alkaline rocks are the most alkaline basalts found along the volcanic front of central america these basalts are mafic lil and lree enriched transitional to alkaline basalts several are found on the north flank of platanar in the aguas zarcas region where there are nine cinder cones and a few isolated flows however they are also found in isolated lava outcrops at least as far south as porvenir volcano along the volcanic front the addition of mafic alkaline magmas with high la yb and low ba la into the platanar magma chamber or chambers may contribute to the lree enriched character of the platanar basaltic andesites and andesites at platanar the field and geochemical evidence suggest mixing between calc alkaline and alkaline magmas a process that has probably occurred throughout the development of the cordillera central of costa rica the presence of negative ce anomalies in several of the calc alkaline lavas also make the platanar complex very unusual compared to the rest of the central american volcanic front in the center of the platanar complex is the chocosuela caldera an apparent remnant of an avalanche caldera created by the collapse in the middle pleistocene of an ancestral stratovolcano toward the nnw in a directed blast type eruption rhyolite is present as pumice lapilli in pyroclastic flow deposits outside the caldera rim whole lapilli analyses span the daciterhyolite range the previous eruption of high silica tephra as pyroclastic flows the current long dormant period and the repeated occurrence of earthquake swarms on the flanks of the platanar complex make it a candidate for volcanic hazard mapping detailed geological mapping and emergency planning

the influence of crustal structure on compositions of subduction related magmas abstract a survey of sr isotopic ratios and other compositional features of subduction related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust i e thickness composition and age these observations lead to the conclusion that crust and or mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas typically basalt or basaltic andesite it is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks the extent to which such processes manifest themselves will depend upon details of the local crustal structure in particular the composition and age of the crust will strongly influence such parameters as sr nd and pb isotopic compositions such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust e g central andes depending upon the level of magma stagnation and evolution within the crust different trends in isotopic composition may result these isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas and locally they may reflect subduction of continental sediments interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust cascades e eleutians and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes consequently most island arc magmas and especially those more evolved than basalt are probably not primary in the sense that they do not represent direct melts of the upper mantle studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered it appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle derived magmatic components are more mafic in composition than average continental crust

minimum volume of a tephra fallout deposit estimated from a single isopach a simple method is presented to calculate the minimum volume of an exponentially thinning fall deposit for which only one isopach is available this method exploits the fact that the product of the area enclosed within an isopach a and its thickness t reaches a maximum value ta max for a certain isopach it is shown that the minimum volume is vmin 3 69ta which is equal to the actual volume when ta ta max calculation of vmin for 327 isopachs from 74 fallout deposits shows that it is often just a bit lower than the volume calculated from all the available isopachs it is therefore proposed that vmin provides a useful minimum volume estimate for fallout deposits for which only one isopach is available

interplate seismicity at the crisp drilling site the 2002 mw 6 4 osa earthquake at the southeastern end of the middle america trench we investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity westward from osa peninsula in costa rica a major change in the character of the incoming cocos plate is displayed by abrupt lateral variations in seafloor depth and thermal structure here a mw 6 4 thrust earthquake was followed by three aftershock clusters in june 2002 initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the middle america trench off central costa rica the earthquake sequence occurred while a temporary network of obh and land stations 80 km to the northwest were deployed by adding readings from permanent local stations we obtain uncommon p wave coverage of a large subduction zone earthquake we relocate this catalog using a nonlinear probabilistic approach within both a 1 d and a 3 d p wave velocity models the main shock occurred 25 km from the trench and probably along the plate interface at 5 10 km depth we analyze teleseismic data to further constrain the rupture process of the main shock the best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope supporting the nucleation of the osa earthquake at 6 km depth the location and depth coincide with the plate boundary imaged in prestack depth migrated reflection lines shot near the nucleation area aftershocks propagated downdip to the area of a 1999 mw 6 9 sequence and partially overlapped it the results indicate that underthrusting of the young and buoyant cocos ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central costa rica margin

late pleistocene to holocene temporal succession and magnitudes of highly explosive volcanic eruptions in west central nicaragua the stratigraphic succession of widespread tephra layers in west central nicaragua was emplaced by highly explosive eruptions from mainly three volcanoes the chiltepe volcanic complex and the masaya and apoyo calderas stratigraphic correlations are based on distinct compositions of tephras the total tephras combine to a total on shore volume of about 37 km3 produced during the last 60 ka the total erupted magma mass including also distal volumes of 184 gt dre distributes to 84 into 9 dacitic to rhyolitic eruptions and to 16 into 4 basaltic to basaltic andesitic eruptions the widely dispersed tephra sheets have up to five times the mass of their parental volcanic edifices and thus represent a significant albeit less obvious component of the arc volcanism eruption magnitudes m log10 m 7 with m the mass in kg range from m 4 1 to m 6 3 most of the eruptions were dominantly plinian with eruption columns reaching variably high into the stratosphere but minor phreatomagmatic phases were also involved two phreatomagmatic eruptions one dacitic and one basaltic andesitic produced mostly pyroclastic surges but also fallout from high eruption columns comparison of fallout tephra dispersal patterns with present day seasonally changing height dependant wind directions suggests that 8 eruptions occurred during the rainy season while 5 took place during the dry season the tephra succession documents two major phases of erosion the first phase 17 ka ago appears to be related to tectonic activity whereas the second phase may have been caused by wet climatic conditions between 2 to 6 ka ago the apoyo caldera had two large plinian caldera forming eruptions in rapid succession about 24 ka ago and should be considered a silicic volcano with long repose times three highly explosive basaltic eruptions were generated at the masaya caldera within the last 6 ka since then frequent but small eruptions and lava effusion were largely limited to the caldera interior the dacitic chiltepe volcanic complex experienced six plinian eruptions during the last 17 ka and seems to be an accelerating system in which eruption magnitude increased while the degree of differentiation of erupted magma decreased at the same time we speculate that the chiltepe system might produce the next large magnitude silicic eruption in west central nicaragua

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

new pliocene pleistocene 40ar 39ar ages fill in temporal gaps in the nicaraguan volcanic record abstract the volcanic record of western nicaragua documents a significant lull in volcanic activity that has persisted from the late miocene 7 ma to the formation of the modern volcanic front around 350 ka this study fills this gap for the first time with samples collected in northwest nicaragua between cosiguina and san cristobal volcanoes and with samples collected from the nicaraguan depression we found two previously unknown volcanic units ranging from 3 6 to 1 3 ma and the improved volcanic record allows us to reconstruct the geochemical evolution of the nicaraguan arc u th values increased by nearly threefold since the miocene following the carbonate crash at 10 ma when dominantly carbonate sediment deposition shifted toward hemipelagic sediment deposition this transition was thought to be abrupt however our new data show that it took place gradually over the last 7 ma northwest nicaragua is a particularly interesting case study because it contains middle miocene volcanism on either side of the nicaraguan depression the coyol formation 25 7 ma to the east and the tamarindo formation 14 7 11 7 ma to the west the presence of mid miocene volcanism on either side of the nicaraguan depression has led to the hypothesis that the two coeval units currently separated by 100 km were once connected and have since been separated by extension here we present data suggesting that the tamarindo and coyol are geochemically distinct and therefore cannot be considered part of the same unit

constraining explosive volcanism subjective choices during estimates of eruption magnitude when estimating the magnitude of explosive eruptions from their deposits individuals make three sets of critical choices with respect to input data the spacing of sampling sites the selection of contour intervals to constrain the field measurements and the hand contouring of thickness isomass data respectively volcanologists make subjective calls as there are no accepted published protocols and few accounts of how these choices will impact estimates of eruption magnitude here for the first time we took a set of unpublished thickness measurements from the 1959 k lauea iki pyroclastic fall deposit and asked 101 volcanologists worldwide to hand contour the data first there were surprisingly consistent volume estimates across maps with three different sampling densities second the variability in volume calculations imparted by individuals choices of contours is also surprisingly low and lies between s 5 and 8 third volume estimation is insensitive to the extent to which different individuals smooth the raw data in constructing contour lines finally large uncertainty is associated with the construction of the thinnest isopachs which is likely to underestimate the actual trend of deposit thinning the net result is that researchers can have considerable confidence in using volume or dispersal data from multiple authors and different deposits for comparative studies these insights should help volcanologists around the world to optimize design and execution of field based studies to characterize accurately the volume of pyroclastic deposits

geomorphic and exhumational response of the central american volcanic arc to cocos ridge subduction 1 the timing of collision of the cocos ridge at the middle america trench remains one of the outstanding questions in the tectonic evolution of the central american convergent margin new analyses of the tectonic geomorphology of the cordillera de talamanca the extinct volcanic arc inboard of the cocos ridge coupled with low temperature thermochronometry data provide insight into the cooling and erosional history of the arc from late miocene to present we identify a low relief surface at high elevation along the northeastern flanks of the range which represents a relict erosional landscape cut across shallow plutonic rocks of the arc edifice longitudinal profiles of rivers on this surface are isolated from steep downstream sections by prominent knickzones that are interpreted to reflect a migrating wave of transient incision generated during differential rock uplift of the range reconstruction of pre incision profiles suggests that rock uplift during the growth of the cordillera de talamanca is no greater than 2 km this inference is corroborated by results from our apatite u th he and apatite fission track analyses along an elevation transect on mt chirripo the highest mountain in the cordillera de talamanca low temperature cooling ages overlap significantly with published high temperature40ar 39ar ages the combined results imply that rapid cooling in the late miocene was related to secular cooling of a shallow pluton rather than exhumation our results imply that rapid incision along downstream channel segments differential rock uplift and growth of the talamanca as a bivergent orogenic wedge associated with the onset of cocos ridge subduction are relatively young characteristics of the range a review of previously published radiometric ages and revised plate reconstructions for the late miocene further suggest that the cessation of arc volcanism in both the cordillera de talamanca and the cordillera central of western panama was coeval with the initiation of oblique subduction of the nazca plate during the late miocene thus we conclude that the cessation of arc volcanism in the late miocene 5 8 ma and the onset of cocos ridge collision 3 ma are separate events that reflect recent changes in the configuration of the plate boundary system

introduction to the special issue on sfb 574 volatiles and fluids in subduction zones climate feedback and trigger mechanisms for natural disasters combine field work on land with marine cruises probing the seafloor during the course of the sfb 574 many different aspects of subduction zone processes were investigated geophysical investigations identified and quantified the input of water through hydration of the bend faulted subducting plate as far as mantle depths this process and water release from the subducting slab deeper in the subduction zone was investigated by numerical modeling these studies highlighted the important role of hydration and dehydration of mantle rocks for the global water cycle subducted oceanic fracture zones are another conduit for water transported deep into the subduction zone compactional and thermal dehydration of the subducted sediment layer affects the strength of interplate coupling and the depth and lateral extension of the seismogenic zone where disastrous earthquakes are generated the fluids largely generated by clay mineral transformation are expelled through the forearc by splay faults at the seafloor the sites of cold seeps often associated with mud volcanoes are populated by biota that control the carbon transfer to the ocean the most prominent manifestations of cold seeps are authigenic carbonates that form from anaerobic oxidation of methane and which serve as archives of forearc processes the largest cold seep emissions occur at faults generated by the subduction of volcanic seamounts petrological analyses of ancient exhumed subduction zone metamorphic rocks revealed the important role of pervasive typically channelized fluids in high pressure metamorphic reactions of dewatering subducted igneous crust and mantle slab derived fluids hydrate the mantle wedge along the slab wedge interface and form a subduction channel in which mixing of different fluids and fluid rock interaction causes metasomatic overprinting the element redistributions associated with the liberation of fluids from sonderforschungsbereiche sfbs are a successful funding model in use by the german science foundation dfg for over 30 years to strengthen basic research first locally at universities and later also supra regionally by including academic institutions at different cities and states literally translated sfb means special research area that comprises research that complements but does not duplicate research at participating institutions and departments the english terminology used by the dfg is collaborative research centre which better describes the expected approach by emphasizing collaboration and interdisciplinary efforts in such a way that the overall result is better than the sum of individual results the sfb 574 had united more than 70 scientists with expertise in structural geology geophysics sedimentology geochemistry empirical and experimental petrology volcanology and biology for 11 years 2001 2012 the overarching theme addressed the role of volatiles in subduction zone tectonic hydrological metamorphic and magmatic processes and resulting hazards the main areas of research were the subduction zones of central america and southern chile both extend across shorelines from deepsea trenches to arc volcano summits and thus required to

grain size areal thickness distribution and controls on sedimentation of the 1991 mount pinatubo tephra layer in the south china sea the june 15 1991 climactic eruption of mt pinatubo produced an extensive largely co ignimbrite derived airfall ash layer on luzon island and across the central south china sea the layer covers an area of 4 105 km2 with a volume of 5 5 km3 near the coast of luzon the deposit consists of two units a normally graded basal ash bed unimodal in grain size and a finer grained internally structureless upper ash bed showing grain size bimodality with increasing distance from the source the coarse particle populations of the two units merge and migrate towards a near constant fine population 11 m the distal region is covered by a fine mode dominated virtually ungraded single ash layer the reversal of the winds from easterly directions at upper tropospheric and stratospheric levels to westerly directions in the middle and lower troposphere indicates that both the coarse and fine mode components fell out from high altitude eruption clouds the high velocity upper level winds however would have transported fine grained ash particles far beyond the south china sea which suggests that their settling was accelerated by aggregation the boundary between the units thus marks a change from fallout of predominantly discrete pyroclasts to simultaneous fallout of aggregated fines and freely falling coarse grained particles the particle populations composing the upper ash bed were almost completely removed from the proximal areas by the upper level winds at lower elevations the counterclockwise circulation of a typhoon over the coastal area advected the ash south and eastward producing a thickness maximum in the medial region at about 160 km from source the strong displacement of fines possibly aided by wind turbulence led to a break in bulk tephra thinning rates close to the coastline in the distal region outside the influence of the typhoon southwest monsoonal winds caused a distinct lobe axis inflection and thickness asymmetry within this region at about 420 km from source fallout of particle aggregates created a second thickness maximum comparison of the field data with previous experimental observations and tephra flux records in the deep sea wiesner et al 1995 carey 1997 mccool 2002 implies that the transport of ash in the water column was largely determined by vertical density currents differences in the reaction of coarse and fine particles to turbulence in the descending plumes probably suppressed the segregation of fines but allowed the coarser pyroclasts to maintain their initial order of arrival at the sea surface considering typical fall rates of convective plumes modifications of the initial fallout position of the particles by the south china sea current system are on the order of only a few kilometers the results suggest that convective sedimentation processes ensure the preservation of atmospheric particle transport directions distances and fallout modes in the deep sea

the 322 ka tiribi tuff stratigraphy geochronology and mechanisms of deposition of the largest and most recent ignimbrite in the valle central costa rica the tiribi tuff covered much of the valle central of costa rica currently the most densely populated area in the country 2 4 million inhabitants underlying the tuff there is a related well sorted pumice deposit the tibas pumice layer based on macroscopic characteristics of the rocks we distinguish two main facies in the tiribi tuff in correlation to the differences in welding devitrification grain size and abundance of pumice and lithic fragments the valle central facies consists of an ignimbritic plateau of non welded to welded deposits within the valle central basin and the orotina facies is a gray to light bluish gray densely to partially welded rock with yellowish and black pumice fragments cropping out mainly at the grande de tarcoles river gorge and orotina plain this high aspect ratio ignimbrite 1 920 or 1 1 10 3 covered an area of at least 820 km2 with a long runout of 80 km and a minimum volume outflow of 25 km3 15 km3 dre geochemically the tuff shows a wide range of compositions from basaltic andesites to rhyolites but trachyandesites are predominant replicate new 40ar 39ar age determinations indicate that widespread exposures of this tuff represent a single ignimbrite that was erupted 322 2 ka the inferred source is the barva caldera as interpreted from isopach and isopleth maps contours of the ignimbrite top and geochemical correlation 10 km in diameter the tiribi tuff caldera forming eruption is interpreted as having evolved from a plinian eruption during which the widespread basal pumice fall was deposited followed by fountaining pyroclastic flows in the sw part of the valle central the ignimbrite flowed into a narrow canyon which might have acted as a pseudo barrier reflecting the flow back towards the source and thus thickening the deposits that were filling the valle central depression the variable welding patterns are interpreted to be a result of the lithostatic load and the influence of the content and size of lithic fragments

late miocene to recent plate tectonic history of the southern central america convergent margin new plate reconstructions constrain the tectonic evolution of the subducting cocos and nazca plates across the southern central american subduction zone from late miocene to recent because of the strong relationships between lower and upper caribbean plate dynamics along this margin these constraints have wide ranging implications for the timing and growth of upper plate deformation and volcanism in southern central america the reconstructions outline three important events in the neogene history of this margin 1 the coeval development of the panama triple junction with the initiation of oblique subduction of the nazca plate at 8 5 ma 2 the initiation of seamount and rough crust subduction beginning at 3 4 ma and 3 cocos ridge subduction from 2 to 3 ma a comparison of these events with independent geologic geomorphic volcanic and stratigraphic data sets reveals that the timing rates and origin of subducting crust directly impacted the neogene growth of upper plate deformation and volcanism in southern central america these analyses constrain the timing geometry and causes of a number of significant tectonic and volcanic processes including rapid plio quaternary arc fore arc contraction due to cocos ridge subduction the detachment of the panama microplate at 1 3 ma and the late miocene cessation of mantle wedge derived volcanism across 300 km of the subduction zone

late cenozoic tephrostratigraphy offshore the southern central american volcanic arc 2 implications for magma production rates and subduction erosion pacific drill sites offshore central america provide the unique opportunity to study the evolution of large explosive volcanism and the geotectonic evolution of the continental margin back into the neogene the temporal distribution of tephra layers established by tephrochonostratigraphy in part 1 indicates a nearly continuous highly explosive eruption record for the costa rican and the nicaraguan volcanic arc within the last 8 m y the widely distributed marine tephra layers comprise the major fraction of the respective erupted tephra volumes and masses thus providing insights into regional and temporal variations of large magnitude explosive eruptions along the southern central american volcanic arc cava we observe three pulses of enhanced explosive magmatism between 0 1 ma at the cordillera central between 1 2 ma at the guanacaste and at 3 ma at the western nicaragua segments averaged over the long term the minimum erupted magma flux per unit arc length is 0 017 g ms tephra ages constrained by ar ar dating and by correlation with dated terrestrial tephras yield time variable accumulation rates of the intercalated pelagic sediments with four prominent phases of peak sedimentation rates that relate to tectonic processes of subduction erosion the peak rate at 2 3 ma near osa particularly relates to initial cocos ridge subduction which began at 2 91 0 23 ma as inferred by the 1 5 m y delayed appearance of the oib geochemical signal in tephras from barva volcano at 1 42 ma subsequent tectonic re arrangements probably involved crustal extension on the guanacaste segment that favored the 2 1 ma period of unusually massive rhyolite production

variations in lava composition associated with flow of asthenosphere beneath southern central america existing trace element and isotopic data as well as new u series data for nicaragua and costa rica are examined in light of the recent suggestion that asthenospheric mantle is flowing northward behind the subducting nazca plate around the northwest corner of south america lavas from central nicaragua have trace element and isotopic compositions suggesting that they are derived from relatively depleted mantle with a long history of interaction with the subducting cocos plate lavas from northern and central costa rica have compositions indicating a more enriched mantle source that has been influenced little by the subducting slab these compositional traits vary irregularly between the two regions suggesting that mantle compositions and subduction influences intermingle in a complex manner beneath northern costa rica the dip of the subducting slab decreases abruptly southward and accretion of sediment begins in this same vicinity these changes in composition and physical parameters may all be induced or influenced by an influx of asthenospheric material from behind the retreating nazca plate

origin of silicic magmas along the central american volcanic front genetic relationship to mafic melts abstract silicic pyroclastic flows and related deposits are abundant along the central american volcanic front these silicic magmas erupted through both the non continental chorotega block to the southeast and the paleozoic continental chortis block to the northwest the along arc variations of the silicic deposits with respect to diagnostic trace element ratios ba la u th ce pb oxygen isotopes nd and sr isotope ratios mimic the along arc variation in the basaltic and andesitic lavas this variation in the lavas has been interpreted to indicate relative contributions from the slab and asthenosphere to the basaltic magmas carr m j feigenson m d bennett e a 1990 incompatible element and isotopic evidence for tectonic control of source mixing and melt extraction along the central american arc contributions to mineralogy and petrology 105 369 380 patino l c carr m j and feigenson m d 2000 local and regional variations in central american arc lavas controlled by variations in subducted sediment input contributions to mineralogy and petrology 138 3 265 283 with respect to along arc trends in basaltic lavas the largest contribution of slab fluids is in nicaragua and the smallest input from the slab is in central costa rica similar trends are observed in the silicic pyroclastic deposits data from melting experiments of primitive basalts and basaltic andesites demonstrate that it is difficult to produce high k 2 o na 2 o silicic magmas by fractional crystallization or partial melting of low k 2 o na 2 o sources however fractional crystallization or partial melting of medium to high k basalts can produce these silicic magmas we interpret that the high silica magmas associated central america volcanic front are partial melts of penecontemporaneous mantle derived evolved magmas that have ponded and crystallized in the mid crust or are melts extracted from these nearly completely crystallized magmas

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

silicic ignimbrites within the costa rican volcanic front evidence for the formation of continental crust abstract the origin of silicic magmas greater than 65 silica in areas without old evolved continental crust has long been an enigma in oceanic arcs silicic volcanic deposits are not common which has generally been attributed to subduction related magmas rising without stalling or fractionating through a relatively thin mafic oceanic crust the volcanic arc in costa rica is built on a thick 40 km oceanic plateau and extensive silicic magmatism dominated by silicic ignimbrites has occurred in the region in the absence of old evolved continental crust the silicic deposits display the common large ion lithophile element lile enrichment and high field strength element hfse depletion observed in magmas generated by subduction processes which are not characteristic of melts generated from oceanic plateaus key trace element ratios in the ignimbrites vary with position along the volcanic front and this variation is similar to that of the volcanic front lavas ignimbrites contain unique cumulative frequency distributions of incompatible trace element ratios that demonstrate different magma batches for each unit we propose that the caribbean oceanic plateau in costa rica is being converted to continental crust in large part by the emplacement of silicic magmas in this area the origin of these silicic magmas is due to reprocessing juvenile mantle derived subduction related magmas that have ponded in the crust

rapid pulses of uplift subsidence and subduction erosion offshore central america implications for building the rock record of convergent margins integrated ocean drilling program iodp expedition 334 to southern costa rica central america documented unprecedented subduction erosion in an area of active seismic slip widespread subduction erosion of the upper plate initiated when the cocos ridge an overthickened aseismic ridge arrived at the middle america trench subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate the completely recovered shelf sequence constrains a short 2 m y interval of extreme subsidence 1200 m with a rapid pulse occurring during the first 0 3 m y this event removed an estimated 1 2 106 km3 of forearc material at a rate of 1690 km3 m y km of trench during a time of rapid 1035 m m y shelf sediment accumulation at this erosive margin a sediment starved trench persisted in spite of abundant sediment supply because subduction erosion led to the creation of forearc basins similar rapid pulses of subduction erosion may punctuate the evolution of many margins contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts and recycling of continental material into the mantle

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

could mars be dark and altered there is a long known dichotomy in the martian albedo with an associated but mostly assumed mineralogical split as well the bright red regions are inferred to be weathered oxidized dust and the dark grey regions unaltered volcanic material a number of recent analyses suggest this division is unnaturally simplistic and the association of many dark regions with the former presence of water requires a re examination of the spectra in light of potential alteration minerals i present an alternate interpretation of the reflectance spectral characteristics of some dark regions on mars that includes dark layer silicates if their presence is confirmed on mars this will have implications for sequestration of current and past volatile inventories clues to the extent and type of geochemical weathering and potential zones where bacterial life forms may have emerged

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

a new stratigraphy for the tertiary volcanic rocks of the nicaraguan highland three major volcanic events have occurred in nicaragua since middle tertiary time first the oligocene was dominated by rhyolite shield volcanism producing the highland ignimbrite second creation of northeast and later northwest striking dilatational fracture zones allowed the extrusion of basaltic to andesitic magmas along the pacific coast constructing the northwest oriented coyol volcanic arc this miocene arc can be followed all along the pacific coast of the chortis block third a pliocene pleistocene southwestward shift of arc volcanism created the modern volcanic arc this volcanic history is supported by a new stratigraphy based on volcanic centers and related facies the old cyclic layer cake stratigraphy of the coyol group has been abandoned

quaternary uplift astride the aseismic cocos ridge pacific coast costa rica the pacific coast of costa rica lies within the central american forearc and magmatic arc region that was created by northeastward subduction of the cocos plate beneath the caribbean plate at the middle america trench from the peninsula de nicoya south eastward toward the peninsula de osa and the peninsula de burica on the panamanian border the middle america trench loses its physiographic expression where it intersects the aseismic cocos ridge interaction between subduction of the buoyant aseismic cocos ridge and the overriding caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from nicoya to burica the pliocene and pleistocene stratigraphic record and holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the peninsula de osa immediately landward of the aseismic cocos ridge crustal uplift rates decrease northwest toward the peninsula de nicoya and to a lesser extent southwest toward the peninsula de burica the late quaternary stratigraphy on the peninsula de osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates crustal uplift rates calculated from these sequences systematically decrease from 6 5 to 2 1 m ka north east across the peninsula deformation of the peninsula is modeled as uplifted and down to the northeast tilted fault blocks with an angular rotation rate of 0 03 to 0 06 per thousand years although less well constrained crustal uplift rates on the peninsula de nicoya 200 km to the northwest of the peninsula de osa vary from 1 m ka for pliocene and pleistocene sediments to 2 5 m ka for holocene marine terraces in the quepos region 100 km to the northwest of the peninsula de osa calculated uplift rates derived from incision of late quaternary fluvial terraces range from 0 5 to 3 0 m ka on the peninsula de burica only 60 km to the southwest of the peninsula de osa calculated uplift rates range from 4 7 m ka for a late holocene marine terrace to 1 2 m ka for post late pliocene deep sea sediments the variations in calculated uplift rates on the peninsula de osa constrain a dynamic model for subduction of the cocos ridge and the resulting uplift of the overriding caribbean plate deflection of the caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted cocos ridge because the shape of the subducted end of the cocos ridge is unknown two scenarios are evaluated 1 a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and 2 a ridge where the subducted end was truncated by the panama fracture zone the best fit model utilizes a truncated ridge that has been subducted during the past 0 5 m y 50 km beneath the overriding caribbean plate which has an effective elastic thickness of 5 km the model predicts that the highest uplift rate should be 3 7 m ka and occur on the southwest coast of the peninsula de osa the rate of uplift slows considerably to the northeast and indicates that the peninsula de osa is tilting to the northeast which agrees with observations in that region the predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the peninsula de osa resulting in little uplift that can be attributed to cocos ridge subduction in the northwestern portions of the peninsula de nicoya

the crustal structure of the cocos ridge off costa rica 1 the submarine cocos ridge in the northwestern panama basin a bathymetric feature more than 1000 km long and 250 500 km broad is about 2 km shallower than the adjacent basin it is generally interpreted as the trace of the galapagos hot spot two 127 and 260 km long seismic wide angle sections were recorded along and across this ridge offshore the osa peninsula costa rica crustal thickening is seen everywhere along the sections on the northwestern outer ridge flank increased thickness is exclusively attributed to the upper crust and expressed by 2 km thick flow basalts the quepos plateau caps the upper crust in this area toward the center of the cocos ridge the moho deepens from 11 12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80 of the crust and is three times the thickness of normal oceanic lower crust it is homogeneously structured and the velocities which range from 6 5 km s at the top to 7 35 km s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition similarities to the crustal velocity structure of iceland central kerguelen plateau and broken ridge are consistent with a formation of this 13 15 ma old cocos ridge segment by excessive magmatism in a near plate boundary setting

lead isotope composition of central american volcanoes influence of the galapagos plume 1 lead isotopic analyses of lavas from central america both along and behind the volcanic arc help to clarify source components in the mantle wedge analysis of previous pb isotopic data had implied that little or no marine sediment lead was added to the central american source region as all samples fell within the morb field in contrast to other information e g ba la 10be 87sr 86sr that indicated a high subduction component the data presented here include several analyses of local marine sediment showing it to be exceptionally unradiogenic in pb and thus permitting high sediment contributions to the mantle source region without significant changes in pb isotopes combined pb nd and pb sr isotopic diagrams clearly illustrate the influence of crustal contamination for samples from guatemala and honduras and of subducted sediment for all lavas of the volcanic front samples collected behind the volcanic front are derived from mixing between enriched and depleted mantle sources and in central costa rica extending to the back arc overlap pb sr and nd isotope values for both cocos island and some components of the galapagos hot spot the restricted geographical occurrence of the enriched mantle signature in central america coupled with the persistence of the signal well into the back arc region imply that these lavas are sampling galapagos plume influenced mantle the presence of this plume component beneath southern central america and extending to the northeast beneath the caribbean confirms a galapagos hot spot origin for this part of caribbean plate

symmetrical and segmented variation of physical and geochemical characteristics of the central american volcanic front abstract the regional variation of physical and geochemical characteristics of central american volcanoes occurs in two fundamentally different patterns the first pattern is symmetrical about nicaragua crustal thickness silica contents of mafic lavas and volcanic edifice heights are lowest in nicaragua and increase smoothly toward costa rica to the south and guatemala to the north magma density is maximum in nicaragua and decreases smoothly outward the regional variation in crustal thickness is just enough so that magma densities calculated at appropriate moho pressures are the same at the base of the crust throughout the region this is consistent with magma ponding at the base of the crust the bulk compositions of central american basalts show the same symmetrical variation suites of nicaraguan basalts plotted in pseudo ternary cmas projections indicate large olivine and plagioclase primary phase volumes toward costa rica and guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller which is consistent with fractionation at increasing depth the second pattern is the segmentation of the volcanic front and the plate margin in general the segmentation strongly affects the spacing and size of volcanic centers at segment boundaries volcanic centers are generally small and unusually widely spaced toward segment interiors volcano spacing and size increase systematically the lil element contents of lavas strongly reflect this pattern for lavas with similar silica contents the larger the volcano the higher the lil element contents the relationships between segmentation volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab the relationship between volcano volume and lil element content is qualitatively in agreement with an open system fractionation model

mass and energy budgets of explosive volcanic eruptions analysis of the frequency size relationship of explosive volcanic eruptions for the past 2000 years reveals that the time averaged eruptive mass flux of tephra from subaerial explosive eruptions is 3 104 kg s 1 this mass flux corresponds to a thermal energy release of 4 1010 w half of the long term 10 year mass and thermal energy release from explosive volcanic eruptions is accounted for by eruptions of 10 km of tephra