a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

vermeij crushing analysis a new old technique for estimating crushing predation in gastropod assemblages abstract vermeij crushing analysis allows researchers to estimate frequencies of fragmentation due to crushing predation by establishing a taphonomic fragmentation baseline using shells bearing predatory drillholes we calculated the frequencies of several types of shell damage on groups of drilled and undrilled shells shells of the modern species olivella biplicata o baetica nassarius perpinguis and euspira lewisii were collected at torrey pines state beach del mar california the shells were divided into drilled and undrilled groups and examined for multiple types of damage the frequencies of damage in the drilled groups represent taphonomic damage as the cause of death of these shells is known to be naticid gastropod predation in cases where the damage frequencies were higher for the undrilled shells cause of death unknown possibly crushing the differences in fragmentation between the two categories is attributed to crushing predation the frequencies of damage due to crushing predation are used to estimate a minimum frequency of mortality due to crushing predation in the shell assemblage potential confounding factors were also examined such as potential differences in taphonomic susceptibility and hermit crab habitation between drilled and undrilled shells this technique bypasses the logistical difficulties of field predation studies and the problems of interpretation of shell repair frequencies two common methods of assessing crushing predation in shelled prey most significantly vermeij crushing analysis can be performed on both modern and fossil shell assemblages allowing the study of trends in crushing predation over both space and time

three dimensional p and s velocity structure in the coalinga region california the coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three dimensional velocity and hypocenter inversion in an area of complex three dimensional structure dominated by folding and blind thrusts additionally since other varied geological and geophysical studies have been completed in this area the three dimensional inversion solution could be compared to other interpretations inversion of 7696 p and 1511 s first arrivals from earthquakes and 696 p first arrivals from refraction shots produced a three dimensional velocity model with grid spacing of 1 2 km in the hypocentral area the overall shape and location of velocity features correspond well to the mapped surface geology the three dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower the amounts of structural relief inferred for the local folds are similar to values inferred from geologic data the three dimensional velocity solution has several distinctive features there is a linear high velocity body 6 1 6 5 km s about 25 km long from 6 to 8 km depth that may represent a fragment of coast range ophiolite a shallow low velocity zone lvz which extends for 20 km along the fold axis at about 6 km depth and correlates with lvzs observed in both the refraction and the reflection data may indicate high pore pressure caused by lateral compressive strain deeper lvzs occur within inferred franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness 4 8 km wide and 5 10 km long these lvzs may represent multiple thrust faults their locations and geometry are consistent with thrust faults inferred with seismic reflection data the three dimensional velocity solution compares well to prior two dimensional seismic reflection and refraction models and observed gravity the shape of the inferred sedimentary section agrees well with the reflections from the cenozoic strata compared to the refraction model the three dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data the gravity computed from the three dimensional velocities is similar to the observed gravity in both shape and amplitude both a simple one dimensional initial model and a complex initial model derived from the refraction interpretation were tried a simple starting model gave the best results the s velocity solution has different resolution than the p velocity solution because it uses a different set of stations and it has lower resolution because it uses fewer arrival times while the general patterns of velocity variation are similar for both vp and vs the vs solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features

mystery of naticid predation history solved evidence from a living fossil species observations of living cernina fluctuata sowerby the sole extant species of the mesozoic and cenozoic prosobranch gastropod family ampullospiridae show that this species is an algal grazer and is not a shell drilling predatory naticid gastropod all jurassic and early cretaceous gastropods previously assigned to naticidae belong to ampullo spiridae and are now inferred to be herbivores not shell drillers this finding refutes the triassic origin and favors a cretaceous origin for naticid drilling predation a preliminary revision of cenozoic and cretaceous species previously assigned to naticidae suggests that naticids were absent prior to the cretaceous campanian and diversified at a modern level in eocene time this pattern of naticid diversification from the late cretaceous onward coincides roughly with the evidence of naticid predation the fossil record of naticid drill holes once of anomalously short duration now accurately reflects the geologic history of naticid predation activity

the nicoya convergent margin a region of exceptionally low heat flow over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the nicoya peninsula is characterized by remarkably low heat flow values over the accretionary prism average 28 mw m and values in the trench and the ocean crust seaward of the trench average 14 mw m we attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits

three dimensional structure of vp vs and vp vs beneath northeastern japan implications for arc magmatism and fluids we estimated both p v p and s wave velocity v s structures beneath northeastern japan by applying a tomographic method to 169 712 p and 103 993 s wave arrival time data from 4338 local events the average value of v p v s ratio is 1 69 in the upper crust 1 75 in the lower crust and 1 77 in the uppermost mantle these differences in v p v s ratio may mainly reflect the lithological variations with depth low v p low v s and high v p v s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge here the low v p low v s and high v p v s values are interpreted because of partial melting materials which imply the presence of a vast amount of melt in the uppermost mantle in the lower crust low v p low v s and high v p v s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas which suggests that melting of lower crustal materials occurs just beneath each active volcano in contrast the upper crust beneath active volcanoes exhibits low v p low v s and low v p v s these results suggest that the partial melting zones under northeastern japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes the low v p low v s and low v p v s features in the upper crust suggest the existence of h 2 o rather than melt right beneath active volcanoes

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

mantle flow at a slab edge seismic anisotropy in the kamchatka region the junction of the aleutian island and the kamchatka peninsula defines a sharp turn in the boundary of the pacific and north american plates terminating the subduction zones of the northwest pacific the regional pattern of shear wave birefringence near the junction indicates that trench parallel strain follows the seismogenic benioff zone but rotates to trench normal beyond the slab edge asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge and may influence the shallowing dip of the benioff zone at the aleutian junction

magnetic signature of upper plate structures and subducting seamounts at the convergent margin off costa rica offshore the pacific margin of costa rica the caribbean plate converges with the subducting cocos plate along the middle america trench the tectonics of both plates have been studied on the basis of reflection seismic gravimetric and magnetic data and swath mapping the magnetic data were acquired by geomar kiel germany during cruise so 76 of r v sonne off costa rica in 1992 two different magnetic anomaly patterns are observed off nicoya peninsula the linear magnetic anomalies trend about n50 e and can be traced from the ocean basin to the landward end of the profiles to the southeast of a major structural break trending parallel to fisher seamount and the associated ridge the strike of the magnetic anomalies is n70 e in the area southeast of fisher seamount the magnetic signatures of numerous seamounts are superimposed upon the linear anomalies we interpret the magnetic lineations as chrons 5c to 6b 16 22 5 ma magnetic modeling demonstrates a significant magnetization 1 a m of the margin wedge in the continental slope a structural unit known from seismic studies this is not compatible with the previously discussed origin of the continental slope as an accretionary wedge a composition of the margin wedge similar to the ophiolitic rocks found onshore on nicoya peninsula appears likely a number of morphological furrows observed in the landward trench slope opposite the seamount covered domain of the oceanic crust coincide with magnetic anomalies three dimensional magnetic modeling demonstrates that these anomalies can be explained well by subducting seamounts this implies that the seamounts remain intact at least magnetically for a long time interval during the subduction process thus they may indeed be important seismogenic asperities

surficial evidence of fluid expulsion from the costa rica accretionary prism the nature and distribution of authigenic carbonates chemosynthetic bacterial mats and unique macrobenthic chemosynthetic communities of bivalves and tube worms are important for evaluating and reconstructing present and past fluid venting of accretionary complexes this paper describes the authigenic carbonates chemosynthetic fauna and fluid venting observed at the four tectonic regions of the costa rica accretionary wedge in february 1994 during an alvin diving program of 20 submersible dives we found no surficial evidence of highly focused fluid venting at the toe of the prism outermost 3 km as implied by the absence of authigenic carbonates and chemosynthetic fauna the absence of vent communities on the lower 3 km of the prism and the relatively elevated heat flow with respect to the adjacent incoming cocos plate langseth and silver this issue suggests diffuse rather than focused flow through the toe of the prism twelve active and relict vent sites marked by small clusters of live vesicomyid clams are localized at the bases and tops of out of sequence thrusts implicating fracture permeability as the fluid conduit in the lower slope region but upslope from the toe vast authigenic carbonates and seven active and relict vent sites marked by large dense clusters of chemosynthetic organisms predominate the largest mud diapir in the mid slope region fluid expulsion appears to be more restricted on the upper slope with only 2 small but dense vents marked by chemosynthetic fauna observed at one wall of one canyon

a three dimensional image of shallow subduction crustal structure of the raukumara peninsula new zealand summary earthquake arrival time data from a 36 station deployment of portable seismographs on the raukumara peninsula have been used to determine the 3 d vp and vp vs structure of this region of shallow subduction a series of inversions have been performed starting with an inversion for 1 d structure then 2 d and finally 3 d this procedure ensures a smooth regional model in places of low resolution the subducted plate is imaged as a northwest dipping feature with vp consistently greater than 8 5 km s 1 in the uppermost mantle of the plate structure in the overlying plate changes significantly along strike in the northeast there is an extensive low velocity zone in the lower crust underlying the most rapidly rising part of the raukumara range it is bounded on its arcward side by an upwarp of high velocity a viable explanation for the low velocity zone is that it represents an accumulation of underplated subducted sediment while serpentinization of the uppermost mantle may be responsible for the adjacent high velocity region the low velocity zone decreases and the adjacent high velocity region is less extensive in the southwest this change is interpreted to be related to a change in the thickness of the crust of the overlying plate in the northeast the crust is thinner and subducted sediment ponds against relatively strong uppermost mantle while in the southwest the crust is thicker and the relatively weak lower crust allows sediment subduction to greater depths a narrow zone of high vp vs parallels the shallow part of the plate interface this suggests elevated fluid pressures with the distribution of earthquakes about this zone further suggesting that these pressures may be close to lithostatic the plate interface at 20 km depth beneath the raukumara peninsula may thus be a closed system for fluid flow similar to that

the seismic source parameters of the 1991 costa rica aftershock sequence evidence for a transcurrent plate boundary the april 22 1991 valle de la estrella costa rica earthquake ms 7 6 was a back arc thrusting event associated with the underthrusting of the caribbean plate beneath central america a network of three passcal type portable instruments was deployed to monitor the aftershock activity in southern costa rica 2 to 6 weeks after the main shock the waveforms recorded on three component midperiod seismometers were used to recover source information for 15 small aftershocks magnitudes between 3 2 and 4 4 with a linear moment tensor inversion method we conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery the longer period waveforms in general are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data the earlier part of the seismic wave forms has proven to be the most important carrier of the source information a gross crustal model can be used to describe the structure for the source study the small changes in the waveform character resulting from the mislocation of the events or inexact green s functions generated from the oversimplified crustal model do not prohibit us from the recovery of the source orientation at local distances in contrast the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information our focal mechanisms are generally in good agreement with p wave first motion fault plane solutions determined from a local short period network the aftershocks show a clear spatial segmentation based on focal mechanism type most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock in contrast a cluster of aftershocks northwest of the main shock showed dominantly left lateral strike slip motion on a northeasterly striking nodal plane this suggests that a diffuse deformation zone exists in central costa rica and is characterized by left lateral strike slip motion this diffuse transcurrent deformation zone coincides with several geologic and geophysical features and perhaps is a result of the slower subduction rate of the buoyant cocos ridge than its adjacent segments along the middle america trench mat the diffuse transcurrent boundary may intersect with the north panama deformed belt npdb near limon costa rica and is very likely a plate boundary for the proposed panama block

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake

tomographic evidence for a subducted seamount beneath the gulf of nicoya costa rica the cause of the 1990 mw 7 0 gulf of nicoya earthquake 1 tomographic images constrain the existence of a subducted seamount beneath the gulf of nicoya costa rica the subducted seamount is found at a depth of 30 km within the rupture area of the march 25 1990 mw 7 0 gulf of nicoya earthquake the gulf of nicoya earthquake was a typical thrust type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting cocos plate and the overriding plate precise relocation of the mainshock and its aftershocks in a 3 d p wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate we interpret the subducted seamount as an asperity whose rupture caused the 1990 gulf of nicoya earthquake

melting and seismic structure beneath the northeast japan arc transportation of h2o associated with subduction of the pacific plate beneath the northeast japan arc is modeled to predict distribution of aqueous solution and melt and the consequent p wave velocity structure the observed velocity structure coincides well with the model for equilibrium transport of h2o most of h2o subducted is brought down to 150 200 km depth along the slab then the aqueous fluid generated ascends to initiate melting which explains the low velocity regions 6 reduction observed beneath the backarc rather than beneath the volcanic front if equilibrium transport of h2o occurs initiation of melting beneath the backarc with deep subduction of h2o is likely to be the case also for other subduction zones with slabs older than several tens of m y cold enough to stabilize serpentine to a great depth

drill hole analysis reveals evidence of targeted predation on modern brachiopods abstract if drill holes are made in prey either by mistaken or opportunistic predators or parasites rather than those which do so on a more regular basis we might predict that such drill holes will commonly be incomplete or repaired and show no evidence of behavioural familiarity such as size selection or constancy of attack site these predictions were used to examine the patterns of drill holes found in bulk samples of death assemblages of modern rhynchonelliform brachiopods from around new zealand four samples yielding data for five different taxa four terebratulides and one rhynchonellide were examined in each the proportion of incomplete or repaired holes was low samples of terebratella sanguinea fiordland and gyrothyris mawsoni antipodes islands show statistically significant evidence of both size selection and preferential drill hole siting the sample of notosaria nigricans waikawa shows only evidence of size selection examination of drill hole morphology and the co occurring gastropod fauna suggests that the distinctive countersunk holes in t sanguinea and g mawsoni were likely made by the trophonid gastropod xymene ambiguus aquarium experiments demonstrate that the ranellid gastropod argobuccinum tumidum is also capable of feeding on brachiopods but that the resulting hole is not likely to be readily recognisable in death assemblages

simultaneous inversion for earthquake location and velocity structure beneath central costa rica we have imaged the complex crustal and upper mantle structure beneath central costa rica using p wave arrival times from locally recorded earthquakes thurber s 1983 iterative inversion method is used to simultaneously estimate velocities along a three dimensional grid and hypocentral parameters of local earthquakes our data consist of over 12 000 arrival times from more than 1300 earthquakes recorded by stations of a permanent seismographic network in costa rica our resulting velocity model correlates well with mapped geologic units at very shallow depth and with tectonic features at greater depth we find low velocities 4 0 to 4 8 km sec in the shallow crust above 10 km near the active volcanoes and associated with a nw se trending late cretaceous to late tertiary sedimentary basin southeast of herradura peninsula high velocities 5 4 to 5 7 km sec in the shallow crust correlate with outcrops of late jurassic to early tertiary ultramafic ophiolitic units and with basic tertiary volcanic units at depths between 20 and 30 km high velocities 6 8 to 7 2 km sec are associated with the subducting cocos plate under costa rica and two prominent low velocity bodies 6 3 to 6 5 km sec are present about 30 km trenchward of the volcanic arc and along the projection of the aseismic cocos ridge as it subducts beneath costa rica the thickened oceanic crust of the cocos ridge is most likely responsible for its low velocities the deep low velocity anomaly located trenchward of the axis of the volcanoes may indicate the presence of a low density intrusive resulting from an earlier phase of magmatism possibly the late miocene episode that produced the talamanca intrusive complex

accurate hypocentre determination in the seismogenic zone of the subducting nazca plate in northern chile using a combined on offshore network the coupled plate interface of subduction zones commonly called the seismogenic zone has been recognized as the origin of fatal earthquakes a subset of the after shock series of the great antofagasta thrust type event 1995 july 30 mw 8 0 has been used to study the extent of the seismogenic zone in northern chile to achieve reliable and precise hypocentre locations we applied the concept of the minimum 1 d model which incorporates iterative simultaneous inversion of velocity and hypocentre parameters the minimum 1 d model is complemented by station corrections which are influenced by near surface velocity heterogeneity and by the individual station elevations by relocating mine blasts which were not included in the inversion we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth a study of the resolution parameters ale and dspr documents the importance of offshore stations on location accuracy for offshore events based on precisely determined hypo centres we calculate a depth of 46 km for the lower limit of the seismogenic zone which is in good agreement with previous studies for this area for the upper limit we found a depth of 20 km our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies the results are also in agreement with thermal studies for the antofagasta forearc region

a fossil record full of holes the phanerozoic history of drilling predation the evolutionary history of drilling predation despite a long and rich fossil record precambrian holocene contains a 120 m y gap late triassic early cretaceous drilled bivalve and brachiopod shells from jurassic deposits of hungary india and four localities documented in the literature indicate that drillers may have existed continuously throughout the mesozoic they may have been descendants of paleozoic predators unknown mesozoic carnivores or precursors of modern drillers a literature database suggests three major phases in the phanerozoic history of drilling predators 1 the paleozoic phase latest precambrian carboniferous dominated by rare to moderately frequent drillings in brachiopods and sessile echinoderms 2 the mesozoic phase permian early cretaceous with very rare or even facultative drillers that had little impact on marine benthic communities but nevertheless may have been present continuously and 3 the cenozoic phase late cretaceous holocene dominated by frequent gastropod drillings in mollusks

antitaxial crack seal vein microstructures and their relationship to displacement paths abstract the microstructures developed in an example of a syntectonic crack seal vein indicate that antitaxial fibres do not necessarily track the incremental opening history during crack seal vein growth this observation serves as a caution against the uncritical use of fibrous microstructures to make inferences about displacement histories during fibre formation it is demonstrated that crack seal processes can lead to the development of both irregular and laminated vein microstructures as well as fibrous microstructures the types of microstructures which develop in crack seal veins and the extent to which they reflect the opening paths of veins depend on a range of factors the most important of these include the nucleation and growth kinetics of phases precipitated from fluids in opening fractures and the location of sites of material accretion during successive crack seal increments

hypocenter velocity structure coupling in local earthquake tomography abstract local earthquake tomography involves the use of seismic wave arrival times from earthquake sources with uncertain locations for the inference of three dimensional variations in seismic velocity formally this inverse problem contains explicit coupling between the earthquake hypocenters and the velocity structure however many studies fail to treat the hypocenter velocity structure coupling explicitly an examination of previous studies provides evidence that this failure can lead to significant bias in the resulting velocity model a set of simple hypothetical test cases clearly demonstrates the seriousness of the potential bias when coupling is neglected producing models that underestimate the actual velocity perturbations by about a factor of four under the same conditions strict simultaneous inversion and simultaneous inversion with parameter separation both produce accurate inverted models the use of appropriate station corrections and iteration of the solution can improve the results when the coupling is ignored but velocity anomalies will still be underestimated by about a factor of two at best

drilling predation on gryphus vitreus brachiopoda off the french mediterranean coasts abstract a total of 145 valves of gryphus vitreus with naticid drillholes have been recorded from samples during 21 cruises in the northwestern mediterranean out of about 30 000 empty valves drilling was lower than 1 in all stations except in one station where it reached up to 25 5 selectivity of drillhole site by the naticid euspirella pulchella seems to apply in g vitreus as drillholes are mainly concentrated in the relatively meaty distal half of the shell which is also the thickest part of the valve about 0 6 2 mm attacks occur generally on the ventral valve 86 2 according to the life position of gryphus the predation pressure on g vitreus by drilling appears exceptional in that it has hardly any influence on the evolution of the species this contradicts several previous statements on the fossil record

three dimensional models of p wave velocity and p to s velocity ratio in the southern central andes by simultaneous inversion of local earthquake data the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 seismological network of 31 digital broad band and short period three component seismometers was deployed in northern chile between the coastal cordillera and the western cordillera more than 5300 local seismic events were observed in a 100 day period a subset of high quality p and s arrival time data was used to invert simultaneously for hypocenters and velocity structure additional data from two other networks in the region could be included the velocity models show a number of prominent anomalies outlining an extremely thickened crust about 70 km beneath the forearc region an anomalous crustal structure beneath the recent magmatic arc western cordillera characterized by very low velocities and a high velocity slab a region of an increased vp vs ratio has been found directly above the wadati benioff zone which might be caused by hydration processes a zone of lower than average velocities and a high vp vs ratio might correspond to the asthenospheric wedge the upper edge of the wadati benioff zone is sharply defined by intermediate depth hypocenters while evidence for a double seismic zone can hardly be seen crustal events between the precordillera and the western cordillera have been observed for the first time and are mainly located in the vicinity of the salar de atacama down to depths of about 40 km

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

tomographic imaging of the alaska subduction zone the alaska subduction zone is characterized by the pacific plate descending beneath the north american plate causing abundant seismic activity in the crust and along the wadati benioff zone down to a depth of approximately 200 km we have used 142 908 p wave arrival times from 12 237 shallow and intermediate depth earthquakes recorded by the alaska earthquake information center jointly run by the geophysical institute university of alaska fairbanks and u s geological survey in the period from january 1977 to november 1991 to investigate the three dimensional 3 d p wave velocity structure beneath central and southern alaska travel times and ray paths are accurately calculated by using an efficient 3 d ray tracing technique the nonlinear tomographic problem is solved by iteratively conducting linear inversions and the velocity structure and hypocentral locations are simultaneously determined we conducted two types of inversions one is an inversion with a laterally homogeneous starting model the others are what we call slab inversions in which we introduce into the initial model the high velocity subducting pacific plate as a priori information we found that the slab inversions gave a seismologically more plausible result and a final root mean square travel time residual significantly smaller than that of the inversion with the homogeneous starting model detailed p wave tomographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30 60 km the tomographic image of the upper crust correlates well with the major surface geological features such as slow sedimentary basins and fast ultramafic bodies prominent low velocity anomalies exist in the crust and upper mantle beneath active volcanoes in the mantle wedge the low velocity anomalies dip toward the continental side and extend to a depth of about 150 km which are considered to be associated with the active volcanism in the alaska subduction zone the results suggest that the subducting pacific plate has a thickness of 45 55 km and a p wave velocity 3 6 higher than that of the surrounding mantle

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

structure of the costa rica convergent margin offshore nicoya peninsula we present the results of a seismic refraction survey conducted offshore costa rica near the nicoya peninsula a dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers these data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the middle america trench onto the nicoya peninsula the best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron velocities increase from 3 5 to 4 2 to 4 6 km s at distances of 10 20 and 30 50 km landward of the trench these velocities are higher than observed within margin wedges at other well studied convergent margins but lower than the velocities within the adjacent nicoya complex which are 5 5 km s at similar depths below the surface we interpret the margin wedge velocities as indicating that material similar to the nicoya complex extends seaward to near the lower slope but that fracturing alteration or accretion processes have lowered the velocity of the margin wedge with respect to the nicoya complex the seismic refraction data cannot constrain the exact thickness or velocity of a possible low velocity zone lvz overlying the subducting plate however geologically reasonable structures are only produced with a lvz 400 m thick velocities in the upper part of oceanic layer 2 are 3 5 4 0 km s within the subducting slab these velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust the top of the subducted slab is well resolved and deepens from 5 km depth at the trench to 15 16 km depth at the nicoya peninsula coastline the dip angle of the subducting plate increases from 6 to 13 at a distance of 30 km from the trench interplate seismicity appears to become common 55 km from the trench where the plate boundary is at 14 km depth

boring predation and mesozoic articulate brachiopods abstract a systematic survey of all the mesozoic brachiopods in the large collections of the sedgwick museum cambridge university has revealed more than 213 predatory boreholes from a wide range of largely british localities it seems likely that this type of predation was not infrequent at least locally as evidenced by the proportion of bored valves at some localities and the degree of stereotypy shown in borehole positioning the size of the boreholes and their neat circular cross sections suggest that they were made by large epifaunal predators using chemical attack and those of post albian age may be comfortably if not confidentally assigned to muricid gastropods although it is likely that bivalves with their greater flesh yields were the preferred prey item in most mesozoic shelly benthic communities it is apparent that boring predators may have resulted in a significant predation pressure on articulate brachiopods

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

circum mediterranean oligo miocene biogeographic evolution the gastropods point of view based on studies in iran qom basin esfahan sirjan basin zagros mountains in turkey mut and sivas basins in the mesohellenic basin and in northeastern egypt a new palaeobiogeographic concept for the oligocene and miocene in the circum mediterranean area with special emphasis on the distribution patterns of gastropod faunas is presented a very strict biogeographic terminology is proposed to avoid the common fusion of geographic faunistic and geodynamic terms in the palaeontologists jargon our main interest focuses on the affinities and the faunistic interactions between the early mediterranean and the early indo pacific regions during the oligo miocene especially the role of the early indo pacific faunas as the mythic centre of origination of early neogene european mollusc faunas as often vaguely indicated in the literature is critically evaluated this leads to the establishment of a mediterranean iranian province and a western indian eastern african province as palaeobiogeographic subunits of the western tethys region during the oligocene due to major geodynamic and related biogeographic changes in the early miocene the western tethys region no longer existed and was replaced by the proto mediterranean atlantic region for the area of the paratethys the palaeobiogeographic unit danubian province which includes the protocaspian subprovince is proposed furthermore the underrated transatlantic communication of the oligo miocene nearshore mollusc faunas is demonstrated altogether this paper is a contribution to the discussion on the biogeographic concepts classification and nomenclature in palaeontology as initiated by westermann palaeogeogr palaeoclimatol palaeobiogeogr 158 2000 1 13 163 2000 49 68 2002 elsevier science b v all rights reserved

subduction zone structure and magmatic processes beneath costa rica constrained by local earthquake tomography and petrological modelling summary a high quality data set of 3790 earthquakes were simultaneously inverted for hypocentre locations and 3 d p wave velocities in costa rica tests with synthetic data and resolution estimates derived from the resolution matrix indicate that the velocity model is well constrained in central costa rica to a depth of 70 km northwestern and southeastern costa rica are less well resolved owing to a lack of seismic stations and seismicity maximum h2o content and seismic wave speeds of mid ocean ridge basalt and harzburgite were calculated for metamorphic phase transformations relevant to subduction both the 3 d p wave velocity structure and petrological modelling indicate the existence of low velocity hydrous oceanic crust in the subducting cocos plate beneath central costa rica intermediate depth seismicity correlates well with the predicted locations of hydrous metamorphic rocks suggesting that dehydration plays a key role in generating intermediate depth earthquakes beneath costa rica wadati benioff zone seismicity beneath central costa rica shows a remarkable decrease in maximum depth toward southeastern costa rica the presence of asthenosphere beneath southeastern costa rica which entered through a proposed slab window may explain the shallowing of seismicity due to increased temperatures and associated shallowing of dehydration of the slab tomographic images further constrain the existence of deeply subducted seamounts beneath central costa rica large low p wave velocity areas within the lower crust are imaged beneath the southeasternmost volcanoes in central costa rica these low velocities may represent anomalously hot material or even melt associated with active volcanism in central costa rica tomographic images and petrological modelling indicate the existence of a shallow possibly hydrated mantle wedge beneath central costa rica

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

the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake and its tectonic implications a broadband seismic study the rapture process of a large back arc thrusting earthquake the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake is investigated using broadband body waves and long period surface waves we find the source process to be relatively simple with the source models separately obtained from body and surface waves being very consistent the event occurred on a shallow southwest dipping rapture plane on which most energy is released updip of the hypocentral location 10 20 km deep high frequency radiation appears to have been released over a relatively small source area our preferred model has a focal mechanism with strike 102 10 dip 17 14 and rake 63 17 a seismic moment of 3 8 1 5 1020 n m and a total rupture duration of 40 6 s the earthquake appears to be associated with the north panama deformed belt npdb a thrust and fold complex that has accommodated the oroclinal deformation of panama this event along with previous large events north of panama in 1882 and 1916 indicates that there is substantial convergence along the npdb marking the npdb as a probable emerging plate boundary it remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

seismic structure and magmatism of the young kyushu subduction zone seismic tomography shows that slow velocity anomalies exist in the mantle wedge and extend to the forearc region down to the subducting philippine sea slab under northern kyushu we also conducted numerical simulations with petrologic data to estimate the fluid distribution in the mantle wedge the seismic and simulation results indicate that regimes of melting and magmatism in a subduction zone with a young slab are different from those with old slabs dehydration and melting occur beneath the arc and forearc above a young and warm slab such as in northern kyushu 26 ma while fluids aqueous solution and melt occur mainly beneath the back arc above old slabs

attenuation tomography in the western central andes a detailed insight into the structure of a magmatic arc high quality data from 1498 local earthquakes recorded by the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 and ancorp 96 andean continental research project 1996 temporary seismological networks allowed the detailed determination of the three dimensional 3 d attenuation structure qp 1 beneath the recent magmatic arc in the western central andes 20 to 24 s assuming a frequency independent qp 1 in a frequency band between 1 and 30 hz whole path attenuation t was estimated from the amplitude spectra of the p waves using spectral ratios and a spectral inversion technique the damped least squares inversion tomography of the data reveals a complex attenuation structure crust and mantle of the forearc and subducting slab are generally characterized by low attenuation qp 1000 crust and mantle beneath the magmatic arc show elevated attenuation the strongest anomaly of extremely low qp is found in the crust between 22 and 23 s beneath the recent volcanic arc qp 100 n s variations can be observed the western flank of the crustal attenuation anomaly follows the curved course of the volcanic front north of 21 s the attenuation is less developed in the northern part of the study area the low qp zone penetrates in the forearc mantle down to the subducting slab in the south a deeper zone of high attenuation is resolved between 23 and 24 s directly above the subducting slab low qp in the mantle correlates with earthquake clusters the strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts the attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction related hydration processes in the mantle wedge from slab derived fluids

the role of hydraulic fractures and intermediate depth earthquakes in generating subduction zone magmatism the presence of magmatism and intermediate depth 70 300 km deep seismicity at subduction zones is at first sight surprising magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle the current model for subduction zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge leading to a reduction in melting temperature and magmatism1 2 3 4 but there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge the occurrence of intermediate depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths one proposal put forward to explain intermediate depth seismicity is that high pore pressure might facilitate faulting by decreasing the friction5 6 7 the hypothesis presented here is that non percolating water provides the high pore pressure that the consequent faulting temporarily interconnects the water pores and when a sufficient vertical height of water is interconnected a hydrofracture is produced which transports the water out into the mantle wedge thereby generating subduction zone magmatism

three dimensional velocity structure of the upper mantle beneath costa rica from a teleseismic tomography study summary a tomographic study has been carried out in costa rica utilizing teleseismic events recorded by the national seismic network ovsicori una the data consist of 130 earthquakes which occurred at epicentral distances of 23d 100d p phases and 110d 180d pkp phases 1378 iasp91 traveltime residuals were computed and inverted using an iterative linearized inversion procedure which incorporates 3 d minimum traveltime ray tracing the 3 d velocity structure of the upper mantle as derived from teleseismic tomography shows a northeastward dipping high velocity body beneath northern costa rica interpreted as the subducting cocos plate although less evident a similar pattern of lateral heterogeneities characterizes the deep structure of central costa rica there is no evidence for deep high velocity bodies beneath southern costa rica the 3 d reconstruction of the subducting cocos plate shows a steeply dipping slab subvertical beneath northern costa rica here the oceanic lithosphere is older and denser than in central costa rica where the slab dips at about 60d southern costa rica does not show a dipping slab this is in accordance with the subduction collision of the thick and buoyant cocos ridge the geometry of the slab the maximum velocity anomalies and the sudden change in the local seismicity depth pattern suggest the existence of a tear in the subducted plate located between the northern and central costa rican subduction zones the strain field in the overriding plate indicates a strong structural control induced by the along trench variations in subduction northern costa rica shows slight tensile deformation while southern costa rica displays moderate to strong compressive deformation the maximum horizontal gradient of the strain field is located in central costa rica where a sinistral transcurrent fault system crosses the country from the pacific to the atlantic coast this system is likely to be generated by the along trench variations in subduction

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

local earthquake tomography of shallow subduction in north chile a combined onshore and offshore study selected travel time data from the aftershock series of the great antofagasta earthquake m w 8 0 have been inverted simultaneously for both hypocenter locations and three dimensional v p and v p v s struc ture the data were collected with a dense 44 station seismic network including ocean bottom hydrophones we performed a series of inversions with increasing complexity 1 d 2 d and 3 d this approach was cho sen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution special efforts were made to assess the solution quality including standard resolution esti mates and tests with synthetic travel times the subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high v p feature high v p v s ratios within the oceanic crust possibly indicate elevated fluid content underplating of material eroded close to the trench is found beneath the mejillones peninsula the lower crust of the overlying plate is characterized by an average v p of 6 8 6 9 km s and an average to low v p v s ratio large areas of anomalously high v p are found in the lower crust south of the city of antofagasta they are interpreted as remants of magmatic intrusions a zone of high v p v s ratios is found within the rup ture area of the antofagasta main shock just above the subducted slab its location within the region of high est stress release from the main shock suggests that the main shock rupture causes the high v p v s ratio the high v p v s ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overly ing lower crust

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

the geometry of the wadati benioff zone under southern central america and its tectonic significance results from a high resolution local seismographic network abstract we present here a detailed geometry of the wadati benioff zone under costa rica obtained from seismicity recorded by a dense local seismographic network jointly operated by the costa rica volcanological and seismological observatory national university and the charles f richter seismological laboratory university of california santa cruz underneath the nicaragua costa rica border the wadati benioff zone smoothly contorts from steep to shallow dip angles nw to se but does not show evidence of a brittle tear as postulated by others however further to the se ne of puerto caldera costa rica the wadati benioff zone does show a segmentation the quesada sharp contortion at intermediate depths h 70km nw of this sharp contortion the deepest portion of the seismically active slab dips at about 80 and reaches maximum depths ranging from 200 km near the nicaragua costa rica border to 135 km under ciudad quesada to the se the deeper portion of the wadati benioff zone dips at about 60 and the seismicity does not extend below depths ranging from 125 km behind the volcanic arc to 50 km east of quepos in southern costa rica east of 83 55 w we find no evidence of the wadati benioff zone deeper than 50 km the obtained geometry and other known tectonic features related to the subduction of the cocos plate under the caribbean plate along the southern terminus of the middle america trench nicaragua and costa rica correlate well with along trench variations in age of the subducted cocos plate some of these tectonic features are 1 the shallowing of middle america trench bathymetry from nw to se 2 variations in the energy release within the subducted slab 3 differences in coupling between cocos and caribbean plates 4 the termination of the central america volcanic chain in central costa rica 5 distinct stress field variations on the overriding caribbean plate the subduction of the cocos ridge under southern costa rica is partially responsible for some of these features shallowing of the middle america trench in southern costa rica and for the high uplift rates of the outer arc however as the presence of the panama fracture zone limits the subducted extension of the cocos ridge to less than 100 km from the trench we propose that the overall geometry of the wadati benioff zone is controlled by abrupt along trench changes in the age of the subducted cocos plate

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

wallrock alteration associated with turbidite hosted gold deposits examples from the palaeozoic lachlan fold belt in central victoria australia abstract the study of wallrock alteration phenomena around slate belt hosted gold deposits can provide valuable information regarding the physico chemical conditions of the ore bearing fluids and the timing relationships between deformation peak metamorphism granitoid emplacement and mineralisation thus studying the processes involved in the interaction of the hydrothermal fluids with the host rock is critical in order to constrain genetic models and ultimately as a tool for exploration largely due to the characteristics of the host rocks turbidite hosted mesothermal gold deposits were historically believed to lack such diagnostic alteration haloes and their study has long been neglected however recent studies in the palaeozoic lachlan fold belt in southeastern australia and in similar tectonic settings elsewhere have demonstrated that alteration around many of these deposits is more widespread and pervasive than previously thought a review of existing information and new data strongly support the viability of studying hydrothermal alteration as a guide to turbidite hosted mineralisation and as an indicator of the character of the solutions associated with ore deposition characteristically the auriferous reef structures are surrounded by broad bleached zones while disseminations of carbonate spots pyrite and arsenopyrite porphyroblasts are the most obvious features of alteration depending on parameters such as fault geometry fluid pressure and reactivity and permeability of the host rocks these disseminations can occur up to a kilometre away from the auriferous structures sericitisation and carbonatisation are commonly discernible within 3 to 10 m wide haloes wallrock alteration chemistry of turbidite hosted gold deposits in the lachlan fold belt shows consistent enrichments of k2o co2 s and as and the anomalous values of these elements also provide the most useful dispersion haloes for turbidite hosted gold exploration due to both their regularity and extent

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

central costa rica deformed belt kinematics of diffuse faulting across the western panama block fault kinematics seismicity and geodetic data across central costa rica reveal a diffuse fault zone here named the central costa rica deformed belt ccrdb the ccrdb defines the western margin of the panama block and links the north panama deformed belt npdb along the caribbean coast with the middle america trench mat along the pacific coast the junction of the ccrdb and the mat coincides with an abrupt transition from smooth to rough crust on the subducting cocos plate rough smooth boundary shallow subduction of rough thickened oceanic crust associated with the cocos ridge shifts active shortening into the volcanic arc along faults of the ccrdb variable fault kinematics along this zone may reflect three combined deformation mechanisms horizontal shortening and shear from oceanic ridge indentation basal traction from shallow subduction and localized block uplift from subducting seamount roughness within the forearc domain 1 mesoscale faults express transtension where steep ne striking regional scale faults intersect the pacific coast across the volcanic arc domain 2 mesoscale faults exhibit mostly sinistral and dextral slip on ne and nw striking conjugate faults respectively approaching the npdb in the back arc domain 3 transcurrent faulting is modified by transpression and crustal thickening fault kinematics are consistent with earthquake focal mechanisms and global positioning system gps measurements radiometric age constraints confirm that faulting postdates the late neogene onset of shallow subduction the ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active ccrdb within the forearc the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts

lithospheric structure of the costa rican isthmus effects of subduction zone magmatism on an oceanic plateau we present the results of a multidisciplinary geophysical study conducted to investigate the lithospheric structure of the costa rican isthmus the physical properties of the lithosphere are resolved by three dimensional 3 d simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2 d forward modeling of onshore and offshore seismic refraction and gravity data according to our results the northern half of the costa rican isthmus is constituted by a 40 km thick crust with a 6 to 7 km thick oceanic crust subducting under it the uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities in good agreement with those described for flood basalts the midlevel shows velocities and densities representative of oceanic crust the bottommost level 20 40 km shows high velocities and densities typical of mafic rocks and the upper mantle displays anomalously low densities and velocities intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies these results are consistent with a basement beneath the costa rican isthmus being part of the caribbean plateau originated at 85 90 ma with the onset of the galapagos hotspot the upper level corresponds to the flood basalts extruded during this phase and it includes most of the marginal wedge the second level represents the preexisting oceanic crust the mafic lower crust intracrustal heterogeneities and anomalous upper mantle are interpreted to be built up by underplating intrusion and crystallization of basaltic melts formed under the influence of subducting lithosphere dehydration

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

3d crustal structure from local earthquake tomography around the gulf of arta ionian region nw greece during summer of 1995 local seismicity was recorded in the area around the gulf of arta in northwestern greece by a dense temporary seismic network of the 441 local events observed at 37 stations 232 well locatable events with a total of 2776 p phase readings were selected applying the criteria of a minimum of 6 p observations and an azimuthal gap less than 180 this data set is used to compute a minimum 1d velocity model for the region several tests are conducted to estimate model stability and hypocenter uncertainties leading to the conclusion that relative hypocenter location accuracy is about 500 m in latitude and longitude and 1 km in depth the minimum 1d velocity model serves as initial model in the non linear inversion for three dimensional p velocity crustal structure by iteratively solving the coupled hypocenter velocity problem in a least squares sense careful analysis of the resolution capability of our data set outlines the well resolved features for interpretation the resulting 3d velocity model shows generally higher average crustal velocities in the east and the well resolved area of the eastern gulf of arta exhibits a homogeneous velocity around 6 km s for the whole upper crust a pronounced north south trending zone of low velocities in the upper 5 10 km is observed in the area of the katouna fault zone kfz at greater depths below 10 km the kfz is underlain by high velocity material e w profiles suggest a horst graben structure associated with the kfz