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

interactions between ridges and plumes in the last 30 years geologists have made much progress on understanding slabs lithosphere and mid ocean ridges but mantle plumes remain enigmatic in their perspective montagner and ritsema 1 chart recent progress largely based on seismic anisotropy toward understanding how such plumes interact with mid ocean ridges existing data are insufficient however to determine whether mantle plumes act independently from plate tectonics the authors call for ocean bottom seismic stations to settle this question 1 http www sciencemag org cgi content full 294 5546 1472

developments in oil shale in 1982 oil shale development activities continue at an impressive level both in the united states and elsewhere but the activity is somewhat reduced compared to prior years the current soft economic environment is probably the cause for the less aggressive development posture of many project developers

effect of subducting sea floor roughness on fore arc kinematics pacific coast costa rica fault kinematics and uplift in the costa rican fore arc of the middle america convergent margin are controlled to a large extent by roughness on the subducting cocos plate along the northwest flank of the incoming cocos ridge seafloor is characterized by short wavelength roughness related to northeast trending seamount chains onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore arc thrust belt and separate blocks with contrasting uplift rates trunk segments of pacific slope fluvial systems typically follow these margin perpendicular faults regionally developed marine and fluvial terraces are correlated between drainages and across faults along the costa rican pacific coast terrace separations across block bounding faults reveal a pattern of fore arc uplift that coincides roughly with the distribution of incoming seamounts magnitude and distribution of quaternary uplift along the costa rican pacific coast suggests that despite a thin incoming sediment pile the inner fore arc shows an accumulation of mass a characteristic that may be due to underplating of seamounts beneath the fore arc high

underthrust sediments fluid migration paths and mud volcanoes associated with the accretionary wedge off costa rica middle america trench styles of deformation and tectonic responses resulting from the convergence of oceanic and continental plates are strongly dependent on fluids in the sediments we estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the middle america trench near costa rica reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion within 4 km of the deformation front where overburden is 800 m more than half of the total water in the pore spaces may have been expelled the rate of fluid transfer from the underthrust section is highest in the zone 0 4 km arcward of the deformation front the clarity of the reflection associated with the decollement is exceptional and shows a phase reversal relative to the seafloor this high amplitude phase reversed reflection ends about 4 km arcward of the deformation front suggesting a rapid reduction in the density and or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50 m and extent of perhaps 500 m 1000 m a reflection traced beneath a seafloor mud volcano crosses through the accretionary wedge slope cover boundary and may mark a fluid conduit locally high amplitude reflections associated with the base of slope cover are coincident with closure of structure and indicate local traps to upward organized or diffusive fluid flow some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section near the trench where fluid expulsion rates are highest water may exit to the seafloor seaward of the downslope pinch out of the slope section

introduction to the special section on fluid flow in the costa rica accretionary prism previous 2d and 3d seismic reflection studies off the nicoya peninsula on the pacific margin of costa rica indicated a reversed polarity decollement beneath the toe of the prism shipley et al 1990 consistent with elevated fluid pressures in addition shipley and moore 1986 reported a decrease in thickness of underthrust sediment beneath the toe of the wedge of up to 50 over the outer 3 5 km implying massive dewatering through the toe because such dewatering must pass through the toe of the wedge we suspected the presence of significant fluid venting during february 1994 we carried out 20 alvin dives in search of indicators of fluid venting on the costa rica prism as the trench off costa rica reaches depths of 4500m the alvin went through a process of refit and navy clearance to dive to depths of up to 4500m and our deepest dive reached that depth in addition to the dives we mapped a broad area of the slope with seabeam obtained a series of pogo heat flow transects and recovered a series of piston cores and giant gravity cores for the study of pore water geochemistry here we report on the structures observed in the dive program mcadoo et al the nature and locations of fluid vents kahn et al the heat flow results langseth and silver the pore water studies zuleger et al and a reinterpretation of surface amplitude data from 3d seismic records finding a remarkable association between amplitude anomalies and regions of fluid vents mclntosh and silver

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

large and primarily updip afterslip following the 2012 mw 7 6 nicoya costa rica earthquake we present detailed surface measurements of the first 3 5 years of postseismic deformation following the 5 september 2012 moment magnitude mw 7 6 nicoya costa rica earthquake the dominant signal in the first 2 5 years is uniform horizontal trenchward motion totaling 7 26 cm across 40 stations trenchward velocity is strongly diminished by mid 2014 and appears by 2016 to have begun reversing we invert the first 2 5 years to determine the corresponding afterslip on a detailed 3d interface results show significant afterslip both up and down dip of the main coseismic rupture zone with as much as 1 7 m of offset in two patches at 15 20 km depth and immediately up dip of the maximum coseismic slip this updip slip represents an important mechanism to address unrelieved interseismic locking although sufficient strain energy remains to generate up to a mw 7 1 event near the coastline the afterslip patches are anticorrelated with strongly clustered aftershocks at the same depth which is indicative of varying frictional behavior along strike an additional patch of slip is colocated with reoccurring slow slip events beneath the gulf of nicoya the magnitude of the observed slip however cannot be sufficiently explained by the known slow slip events ongoing measurements will be crucial to understanding the relocking process in nicoya

three dimensional seismic imaging of the costa rica accretionary prism field program and migration examples seismic reflection techniques are the primary geophysical tool used to examine the structure of continental margins on convergent plate margins widely separated seismic reflection profiles often do not image complex structural features because out of plane reflections and diffractions obscure the seismic images yet the structural features are often critical to understanding the tectonic processes thus the resulting geologic interpretations are based on images that may not accurately represent the true subsurface structure to image the complex geologic structures of an active continental margin a three dimensional 3 d seismic survey was conducted off of the nicoya peninsula of costa rica in april 1987 over 60 000 shot points of 96 trace multichannel data were collected in a 9 22 km grid this detailed survey was located over the accretionary prism and designed to investigate the active tectonic processes and evolution of this continental margin in this paper we outline the data acquisition program including the navigation processing critical to successful imaging and the seismic processing methodology and then show several examples of images improved by 3 d surveying the resulting 3 d seismic data were migrated both as independent lines and as a 3 d volume seismic images were produced as conventional depth and time sections and as cross sectional slices in depth and time we present comparisons of the migration results that show significant improvements of the subsurface image using 3 d techniques to illustrate these improvements we present images of two complex geologic areas a mud volcano and reflections near the top of the accretionary prism and the top of the subducting oceanic basement beneath the prism comparisons between the original stacked data the data of the two dimensional migration and the results of the 3 d migration illustrate the value of 3 d techniques in studies of active margins in addition to the advantages of the 3 d imaging which include better structural delineation and improved signal to noise ratio in the final image we show that the ability to view the 3 d data volume in both section and plan view offers significant interpretational advantages

deformation structures and implications for fluid flow at the costa rica convergent margin odp sites 1040 and 1043 leg 170 abstract during odp leg 170 five sites were drilled and sampled off the costa rica pacific margin two of them site 1040 and 1043 yielded material from a wedge of deformed sediments the main decollement zone and the underthrusting sedimentary sequence of the subducting cocos plate detailed mesoscopic and microscopic analyses of the deformation features characteristic of each domain help to define four different structural hydrologic regimes unexpectedly the wedge of deformed hemipelagic sediments does not represent an accretionary wedge because little or no transfer of material from the subducting plate has occurred the deformed sedimentary wedge records periods of tectonic bulk strain in which fluid pressure intermittently rises and induces fracturing alternating with periods of gravitational bulk strain the last stage of this alternating strain regime is represented by the growth of thick sub horizontal shear zones the sediments caught up in the decollement zone show distinctly different deformation features reflecting different mechanical behaviour and a different response to fluid flow but again transient pulses of increased pore pressure are interpreted as the driving mechanism of dewatering the underthrust pelagic and hemipelagic section is also characterised by dewatering features but only in the uppermost part the basal sediments of this section in contact with underlying gabbro intrusions have a completely different structural history and hydrologic regime structures here are interpreted as related to near ridge processes and the hydrologic system is not linked to the upper sediment dewatering regime but rather seems to have been fed by a seawater source

three dimensional seismic imaging of the costa rica accretionary prism structural diversity in a small volume of the lower slope conventional two dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms we undertook a three dimensional seismic reflection survey of a small part of a prism arcward of the middle america trench off costa rica to more correctly image structure and to use the improved structural information to examine the processes of accretion this survey reveals small features with dimensions of hundreds of meters while also defining features thousands of meters in lateral extent both of which were underappreciated in conventional two dimensional data from the same area we have imaged active off scraping at the trench and both duplexing and out of sequence faulting a few kilometers arcward of the trench fault spacing and reflector geometry vary dramatically over a space of several hundred meters some of these variations are related to visible changes in morphology of the underlying oceanic basement but others are not so easily documented fault surface reflections define an architecture which may control gross fluid motion through the prism this architecture is apparently formed by duplexing and out of sequence faulting and has been maintained by periodic motion on some of the out of sequence faults the slope sediment apron records multiple phases of deformation abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism a primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism

determining deviatoric stress tensors from calcite twins applications to monophased synthetic and natural polycrystals theoretically calcite twins analysis makes possible the full determination of the deviatoric stress tensor responsible for the formation of the twin lamellae that is the orientations of the principal stress axes and the three differential stress values i j may be calculated assuming conditions of constant resolved shear stress for twinning inverse method using nonlinear regression and robust estimation is proposed applications to monophased synthetic polycrystals that are generated numerically by solving the direct problem and to a natural sample from the quercy southwest of france have given good results comparison with the first method proposed by turner 1953 modified by nissen 1964 for principal stress orientations and by jamison and spang 1976 for differential stress values clearly emphasizes the considerable interest of this new method

a summary of cenozoic tectonic history along the ipod japan trench transect along the international program of ocean drilling ipod japan trench transect a mid cenozoic period of tectonism without arc volcanism separated an earlier cretaceous to paleogene and a later neogene period of plate convergence and subduction the cretaceous to early paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline probably a forearc basin the seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex the arc the syncline and the thickened sediment sequence resemble an arc trench system of the size and structure of the great valley forearc basin and franciscan accretionary complex of california and they were probably continuations of the yezo geosyncline and associated sequences exposed to the north on hokkaido the arc trench system is contemporaneous with the adjacent shimanto arc trench system southwest of tokyo bay in the early paleogene volcanism ceased and part of the thickened sediment sequence of the cretaceous paleogene margin was emergent as a 160 km wide landmass which is recognized in seismic records as a later paleogene sediment source at the end of the paleogene this landmass began to subside and volcanism began again but it was located first briefly on the east and then on the west of the cretaceous arc along the present arc of northern honshu the land mass erosion surface is clearly visible as an angular unconformity in multichannel seismic reflection records the sequence of neogene sediment explosive volcanism and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in deep sea drilling project dsdp cores cretaceous subduction appears to have resulted in a very extensive accretionary complex whereas the neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin

tectonic structure of the convergent pacific margin offshore costa rica from multichannel seismic reflection data the middle america trench between the cocos ridge and a well studied corridor off the nicoya peninsula has a more varied morphology and structure than previously reported the morphological positive features on the lower plate significantly affect the upper plate structure the cocos ridge has uplifted the margin opposite the osa peninsula northwest of cocos ridge numerous seamounts on the oceanic crust sculptured the margin as they subducted a seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted the greater portion of the trench sediment is subducted beneath a lower slope accretionary mass beneath the middle and upper slope is a margin wedge consisting of a high velocity rock with few internal reflections its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area overlying this surface is a low velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred eocene section beneath the shelf the shelf basement is composed of nicoya complex ophiolite with the same acoustic velocity similar structure and no apparent dividing geologic boundary with the margin wedge we favor a seaward continuation of the nicoya complex to the middle slope and emphasize the evidence for a non steady state tertiary tectonic history

detrital record of the early central american magmatic arc petrography of intraoceanic forearc sandstones nicoya peninsula costa rica the framework compositions of sand stones from the costa rican forearc provide a compositional norm for detrital modes from basins associated with intraoceanic arcs a poorly documented tectonic setting nicoya peninsula sandstones are made up of mainly volcanic lithic fragments plagioclase sedimentary lithic fragments and mafic accessory minerals modal point counts of 70 samples average q 3 f 32 l 65 qm 2 f 32 lt 66 qp 1 lvm 72 lsm 27 and qm 6 p 94 k 0 defining these sand stones as less quartzose than sandstones from most other tectonic settings and less lithic than most quartz poor sandstones from intraplate oceanic islands four newly defined modal parameters quantify temporal changes in composition these parameters are 1 vi v the ratio of volcanic fragments with microlitic or felsitic textures to those with lathwork microlitic or felsitic textures 2 mh m the ratio of hydrous to total mafic silicate mineral grains 3 po p the ratio of oscillatory zoned to total plagioclase grains and 4 sc s the ratio of reworked carbonate detritus to total sedimentary lithic fragments all four parameters increase from negligible values in the basement unit to moderate or high values in cover units these increases are followed by generally low values in the youngest miocene sandstones significant temporal changes are not revealed by previously defined modal parameters that are commonly used to analyze sandstone composition petrography of these sandstones provides strong evidence for the initial construction of an andesitic arc edifice by campanian time late cretaceous increases in vi v mh m and po p ratios through most of the section record increased detrital input from evolved volcanic sources a parallel increase in sc s ratios reflects the progressive construction and erosion of a carbonate rich sedimentary are apron the miocene reversal of these trends reflects subaerial erosion of up lifted basement rocks at the forearc structural high miocene sandstones also contain minor plutonic detritus that includes potassium feldspar signaling incipient unroofing of the arc sandstones of the nicoya peninsula thus preserve a relatively complete record of the evolution and erosion of the upper cretaceous through miocene intraoceanic arc of southern central america

extinct spreading on the cocos ridge using recently acquired marine magnetic data and existing magnetic and bathymetric data sets together with odp leg 170 age determinations we present a revised plate tectonic model for the southern cocos and northern nazca plate area according to this model the formation of the southern cocos plate was governed by spreading at different ridge axes with alternations between spreading ridges producing a complex magnetic anomaly pattern in the cocos and malpelo ridge area we have identified two precursors of the recently active cocos nacza spreading system which were active from 22 8 to 14 7 ma with a change in spreading direction from nw se to ene wsw at 19 5 ma the oceanic crust of these abandoned spreading systems was subsequently thickened and overprinted by hotspot volcanism that formed the cocos and malpelo ridges the centre of this hotspot volcanism is about 500 km away from but most probably related to the galapagos hotspot

a plate tectonic model for the mesozoic and early cenozoic history of the caribbean plate abstract we present a model in which the caribbean plate is an intra american feature formed along the caribbean spreading center as opposed to the current model that considers the caribbean plate as a far travelled crustal segment that formed in the pacific region paleomagnetic data which cover an age range from jurassic through paleocene indicate the ophiolite complexes in costa rica and panama formed in an equatorial position the ophiolites did not fundamentally change their position relative to south america since their origin basaltic rocks of the lower part of the ophiolites are of mid ocean ridge type suggesting formation at a spreading center they are interpreted to have formed as part of the proto caribbean crust at a spreading axis in an intra american position during jurassic and early cretaceous times the upper part of the ophiolites is mainly built up by island arc and intraplate basalts the island arc basalts evolved at the central american landbridge which started in the middle cretaceous the intraplate basalts are related to the caribbean plateau basalt which thickened and stiffened the caribbean crust in middle cretaceous to probably campanian times changes in relative plate motions along the middle american convergent plate margin are reflected by orientations and time sequences of paleostress tensors calculated from fault slip data in southern mexico and costa rica kinematic indicators from mylonite zones display sinistral movement along secondary shear zones within the southern mexican crustal complexes and along the northern boundary of the chortis block in the motagua polochic fault system this reflects the eastward drift of the chortis block since the beginning of the cenozoic at this time the chortis block became part of the caribbean plate which moved more than 1000 km towards the east relative to the north and south american plates during the cenozoic

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

slow rates of subduction erosion estimated from subsidence and tilting of the tonga forearc the convergent margins of the western pacific are marked by widespread tectonic erosion of the overriding plate by the subducting oceanic lithosphere at ocean drilling program site 841 on the mid trench slope of the tonga forearc rates of tectonic trenchward tilting and subsidence of the basement appear to be linked supporting models of basal erosion focused near the trench rates of subsidence significantly exceed those predicted from plate cooling models and require crustal thinning to account for the difference in this extensional environment estimated extension at site 841 is 280 tilting and subsidence rates peaked ca 32 34 ma just after spreading in the south fiji basin at 35 ma however the trenchward direction of tilt and higher extension on the trench slope than on the tonga platform suggest a link to tectonic erosion near the trench rather than extension focused in the arc edifice since this time ca 135 km has been eroded from the edge of the plate of which 80 km is related to the ongoing collision of the louisville ridge a probable paleo collision event at 16 ma caused the forearc at site 841 to be uplifted and tilted arcward forming a major unconformity due to mass wasting steady state erosion averages 1 5 km m y there is no compelling evidence to suggest loss of several hundred kilometers of arc crust since the eocene and thus no reason to suspect that subduction initiation and related magmatism may have predated the ca 45 ma boninitic magmatic activity

subsidence and extension at a convergent plate margin evidence for subduction erosion off costa rica subduction erosion rather than subduction accretion is proposed as the dominant process currently occurring at the nicoya segment of the convergent plate margin off costa rica based on new results from odp drilling cores and our interpretations of published seismic data we present a tectonic model of subsidence and extension due to tectonic erosion of the forearc wedge and landward migration of the nicoya coastline high seismic velocities in the outermost part of the forearc wedge off the nicoya peninsula below the boss bottom of slope sediment reflector indicate the seaward continuation of the nicoya ophiolite complex into the basement of the forearc wedge odp site 1042 revealed neritic sediment that points to strong subsidence of the forearc basement progradation of the sedimentary succession and landward migration of the coastline tilted block structures are explained by substantial fore arc extension in our interpretation the removal of material from the base of the forearc wedge by the process of basal subduction erosion leads to progressive subsidence and thus to landward migration of the coastline landward shift of the active volcano chain in the same order of magnitude as the coastline migration is consistent with this interpretation

the geologic history of the caribbean cocos plate boundary with special reference to the nicoya ophiolite complex costa rica and d s d p results legs 67 and 84 off guatemala a synthesis abstract the pre upper senonian basement of costa rica crops out in the santa elena and nicoya peninsulas from south to north and from base to top the basement includes the esperanza matapalo and santa elena units the esperanza unit is albian santonian in age and consists mainly of pillow basalt and massive basalt flows the matapalo unit includes callovian to cenomanian radiolarite and includes massive basalt flows basalt and dolerite basement the santa elena unit contains ultramafic and mafic rocks in which harzburgite is the major component the most important tectonic features of the nicoya complex are the large santa elena and matapalo nappes nappe emplacement was from north to south during upper santonian time the sedimentary cover of the nicoya complex comprises 1 1 the campanian el viejo formation that consists of shallow water sediments in the north santa elena peninsula and the campanian maastrichtian sabana grande formation of deep water origin in the south nicoya peninsula 2 2 paleocene strata indicating deposition in a deep water environment comprises the rivas las palmas and samara formations 3 3 a post upper eocene sequence that consists of the shallow water barra honda and montezuma formations two unconformities are significant geological features of the upper senonian to tertiary history of costa rica the lower one is at the base of the sabana grande formation and marks a major change in the geologic conditions basalt is scarce in the campanian tertiary series the upper unconformity at the base of the barra honda and montezuma formations is not as major as the lower one during post campanian time normal faulting occurred in two stages separated by a strong erosional phase the geology of the landward slope of the adjacent middle america trench is outlined by interpreting multifold seismic reflection records off the west coast of costa rica and the dsdp legs 67 and 84 transects off guatemala the western caribbean plate boundary may have been under extensional stress for the last 75 m y the strong landward dipping reflectors of the middle america trench landward slope off guatemala could be equivalent to the on land pre campanian overthrusts of costa rica the available data are consistent with the convergent extensional margin concept

evidence and mechanisms for forearc extension at the accretionary costa rica convergent margin seismic reflection data across the upper trench slope off the nicoya peninsula costa rica reveal a wide zone of nearly trench parallel normal faults although work in the last decade has shown that normal faults are present at many convergent margins most examples e g japan peru chile and guatemala have been associated with margins experiencing subduction erosion or non accretion in contrast extension in the costa rica study area apparently is coeval with frontal accretion and underplating the normal faults across the costa rica forearc are striking in seismic section due to the well layered 2 km thick upper slope apron fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism indicating a deep seated deformation process the zone of extension is from the midslope area to within 10 km of the shelf edge a minimum width of about 20 km the estimated extension across the zone is at least 1 5 to 3 km within the apron section spacing between the faults is generally 200 500 m and nominal fault dip is 20 40 and predominantly landward activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault controlled sedimentary thickening at least some of the faults may be presently active shallow reflectors and possibly the seafloor are displaced by faulting contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out of sequence thrusting primarily below the midslope area the consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes with the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension however the three most likely causes are underplating changes in basal shear stress or a brief episode of subduction erosion

provenance of eclogitic metasediments in the north qilian hp lt metamorphic terrane western china geodynamic implications for early paleozoic subduction erosion abstract in this contribution we present new petrological geochemical zircon u pb and hf isotopic data for eclogitic metasediments from the north qilian orogenic belt in which early paleozoic ophiolitic sequences and hp lt metamorphic rocks have been previously recognized the studied metasediments contain eclogite facies assemblages reflecting p t conditions of 450 560 c and 1 9 2 4 gpa consistent with those of adjacent eclogites ree compositions of the eclogitic metasediments overlap those of average upper continental crust the shrimp and la mc icpms u pb data of zircons from metasediments demonstrate input from sources with major age components about 500 ma and 1600 1900 ma peak at 1800 ma with a smaller population at 660 800 ma peak at 750 ma and minor 1 9 ga zircon ages the youngest detrital zircons suggest a maximum depositional age of ca 475 ma and combined with p t conditions and previously determined metamorphic age suggest a burial rate of 0 6 0 7 cm yr zircon hf isotopic analyses show that 500 ma zircons have positive ehf t mainly between 8 and 16 by contrast meso to paleoproterozoic detrital zircons show a broad spectrum of ehf t mainly between 5 and 10 with t dm hf of 1800 2500 ma these data suggest that eclogitic metasediments are derived from a mixture of proterozoic continent crust and juvenile early paleozoic oceanic and or island arc crust and their protoliths likely were deposited in a terrigenous dominated continental margin basin rather than a pelagic oceanic crust environment the new results are consistent with subduction erosion of the active continental margin during south dipping subduction as these sediments formed in a fore arc environment close to the qilian block to the south were transported in the subduction zone to 60 70 km depth prior to their exhumation

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

quaternary uplift and seismic cycle deformation peninsula de nicoya costa rica differing rates and styles of quaternary deformation along the costa rican fore arc reflect segmentation of the trench corresponding with three contrasting domains of subducting sea floor offshore rapid upward flexure of the southern fore arc segment results from the subduction of the buoyant cocos ridge whereas moderate deformation along the central fore arc segment reflects the subduction of buoyant seamounts in contrast the northern costa rican fore arc deforms in response to the subduction of relatively dense sea floor devoid of major bathymetric anomalies quaternary geomorphic evidence and earthquake oral histories from the peninsula de nicoya within the northern costa rican fore arc demonstrate arcward tilting of the fore arc crust with discrete uplift events occurring during large subduction earthquakes uplift rates calculated from the late holocene cabuya terrace along the peninsula s trench perpendicular southeastern coast decrease systematically toward the arc from 4 5 m k y at cabuya to 1 7 m k y at montezuma 8 km to the northeast an uplifted carbonate beachrock horizon radiocarbon age 4500 5200 yr b p correlated between cabuya and montezuma is tilted 0 1 downward toward the arc although quaternary uplift is evident as far arcward as tambor 10 km northeast of montezuma uplifted terraces are absent between tambor and the golfo de nicoya a submerged archaeological site radiocarbon age 2500 yr b p located along the golfo de nicoya coast 30 km northeast of montezuma demonstrates late holocene subsidence of 0 5 m k y these data indicate net late holocene arcward tilting of the peninsula at an angular rotation rate of between 0 01 and 0 02 k y oral histories describing the m 7 7 nicoya subduction earthquake of 5 october 1950 provide evidence of seismic cycle deformation along the peninsula s southwestern coast between puerto carrillo and nosara interviews with 48 residents show that coseismic uplift of at least 1 m affected this coastline and that a significant fraction of this uplift has subsequently been reversed during four decades of gradual subsidence the coseismic deformation pattern estimated from a uniform slip dislocation model for the 1950 earthquake is consistent with both geomorphic and oral history evidence these observations suggest that seismic cycle deformation functions as an important mechanism of vertical tectonism within the costa rican fore arc arcward rotation and doming of the peninsula de nicoya during the quaternary may reflect repeated cycles of sudden coseismic deformation followed by gradual postseismic and or interseismic crustal movement

crustal structure of the middle american trench off costa rica from wide angle seismic data off costa rica seismic wide angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones obh and land stations for recording three profiles perpendicular to the middle american trench mat and one parallel to it are presented here the coverage from closely spaced obh and structural constraints from the coincident near vertical reflection seismic survey provided detailed velocity depth distributions over 4 0 km s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the mat axis the interpretation of the wide angle data required a low velocity zone lvz between the upper and lower plate indicating its nature as subducted and underplated sediment the velocity depth distributions support the hypothesis that the continental margin wedge off costa rica is most likely an offshore extension of the nicoya complex ophiolitic rocks which are exposed along the coast the subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope

origin of the central american ophiolites evidence from paleomagnetic results the ophiolite complexes exposed along the pacific coast of costa rica and western panama evolved from jurassic ocean floor to the island arc of the tertiary to the present we attribute late cretaceous volcanism to the caribbean sill event that thickened the oceanic crust to form an oceanic plateau tectonostratigraphic units formed by late cretaceous nappe thrusting and block rotation rotated blocks as revealed by paleomagnetic data are covered by neoautochthonous sedimentary and volcanic rocks collision with the cocos ridge in the late tertiary fragmented and rotated the ophiolite complexes south of the costa rica fracture zone the oceanic basement formed in the jurassic period in an equatorial position its latitudinal drift path as deduced from paleomagnetic inclination data matches that of the south american continent and differs from that of the farallon phoenix plates we favor formation of the ophiolite complexes in a caribbean inter american position as opposed to a distant origin in the pacific region

petrogenesis of a voluminous quaternary adakitic volcano the case of baru volcano the origin of adakite magmas remains controversial because initially the term adakite had petrogenetic significance implying an origin by direct melting of the eclogitized subducting oceanic crust many models have been produced for their origin and until now there has not been a straightforward method to discriminate between these models in a given adakite suite here we use detailed chronological and geochemical studies of selected adakitic edifices that allows for the determination of the magmatic output rate parameter qe which has been correlated with the rates of magma generation deep within subduction zones by providing temporal and eruption rate estimates we provide constraints on the possible petrogenetic processes involved in the generation of adakite like signatures adakite magmas derived from the melting of the subducting slab should be volumetrically insignificant when compared to the adakite like magmas produced by typical arc magma generation processes in this study we use this observation and the extraordinary stratigraphic exposure from miocene to present in an adakitic volcano in panama and to study the temporal and chemical variation in erupted magmas to estimate rates of magma generation detailed chemical and geochronological analyses of baru volcano indicate that the volcanic edifice was constructed in its entirety during the quaternary and magmas display adakite like features such as steep rare earth elements patterns pronounced depletions in the heavy rare earth elements low y high sr and high sr y the magmatic output rates qe that we have calculated show that compared to other typical adakitic volcanoes most of the volcanic edifice of baru volcano was constructed extremely rapidly 213 k a and in time frames that are similar to typical arc volcanoes the observed chemical and mineralogical variation coupled with the high magma production rates indicate that baru volcano is more representative of a typical arc volcano than a small volume melt of the subducting oceanic crust the technique we outline may have broader application in determining the petrogenetic conditions of other adakite suites

cold seep driven carbonate deposits at the central american forearc contrasting evolution and timing in escarpment and mound settings continuous surface cores of cold seep carbonates were recovered offshore pacific nicaragua and costa rica from 800 to 1 500 m water depths meteor 66 3 in order to decipher their evolution and methane enriched fluid emanation in contrasting geological settings cores from the mounds iguana perezoso baula v and from the jaco scarp escarpment were used for a multi method approach for both settings aragonite was revealed as dominant authigenic carbonate phase in vein fillings and matrix cementation followed by mg calcite as second most abundant this common precipitation process of caco3 polymorphs could be ascribed as indirectly driven by chemical changes of the advecting pore water due to anaerobic oxidation of methane a more direct influence of seep related microbial activity on the authigenic mineral assemblage in both settings is probably reflected by the observed minor amounts of dolomite and a dolomite like camg carbonate mgco3 42 13c data of jaco scarp samples are significantly lower 43 to 56 pdb than for mound samples 22 to 36 pdb indicating differences in fluid composition and origin noteworthy 18o values of scarp samples correlate most closely with the ocean signature at their time of formation documenting the archive potential a high resolution case study of a mound core implies at least 40 changes in fluid supply within a time interval of approximately 14 ky as most striking difference the age data indicate a late stage downward progressing cementation front for all three mound cap structures approx 2 5 cm ky but a significantly faster upward carbonate buildup in the bulging sediments on top of the scarp environment approx 120 cm ky the latter data set leads to the hypothesis of chemoherm carbonate emplacement in accord with reported sedimentation rates until decompression of the advective fluid system probably caused by the jaco scarp landslide and dating this to approximately 13 000 years ago