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

shear zone thickness and the seismicity of chilean and marianas type subduction zones chilean type convergent margins have many large m 7 6 earthquakes whereas marianas type ones do not this dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement whereas it becomes understandable if it is viewed as a relatively thick sediment filled shear zone which thins or thickens arcward depending on subduction speed and sediment supply chilean type margins have thick trench fills and their shear zones generally thin arcward from inlets as much as several thousand metres high the most pronounced thinning being located near backstops tall up to several kilometres seamounts are subducted essentially intact to relatively great depths and confining pressures before jamming into the roof of the channel and becoming seismogenic asperities their near basal ruptures can generate large thrust type earthquakes mainly concentrated in seismic fronts near backstops marianas type margins in contrast have thin trench fills and their shear zones generally thicken arcward from inlets that can be as little as 300 m high seamounts are truncated near the inlet at low confining pressures and generate only small earthquakes after passing the inlet they do not touch the roof and therefore cannot generate large earthquakes a similar mechanism may explain seismic gaps at sediment poor regions of subduction zones

the least radiogenic terrestrial leads implications for the early archean crustal evolution and hydrothermal metasomatic processes in the isua supracrustal belt west greenland abstract vein type galenas from within metasomatized tonalitic gneiss sheets in a metamorphosed oceanic crustal sequence of the arc outbound isua supracrustal belt isb west greenland reveal the hitherto least radiogenic terrestrial lead isotope compositions these leads deviate from commonly applied average continental crust evolution curves for lead and can be modeled by a single stage growth with starting parameters 238u 204pb 7 70 and 232th 238u 4 37 inferring model ages of 3 81 and 3 76 ga for these galenas the initial constraints are indicative of higher u pb and th u in the source of the hydrothermal fluids and arguably in the magmatic precursors of the gneisses they support contentions regarding a time integrated terrestrial pre history with an elevated th u in the early earth s mantle necessary to reconcile the difference between the observed th u and the time integrated th u recorded by pb isotope ratios of present depleted morb mantle derived rocks in addition they are in line with the requirement of preferential enrichment of u relative to pb in a proto continental crust relative to depleted upper mantle in the early earth s history the age equivalency between the galena generations and abundant tonalitic gneiss sheets and their mutual spatial relationship within the isb favors an interpretation of the least radiogenic terrestrial leads to closely approximate the initial pb of 3 75 ga and pre 3 8 ga old tonalitic protoliths and therefore prompts a close genetic relationship between early archean fluid flow and the magmatic events the inferred 3 81 ga model age of the least radiogenic galena pb particularly agrees with previous u pb zircon geochronological results on tonalite sheets within the supracrustals which constrain a ca 3 79 3 81 ga minimum age for the formation to the earth s oldest oceanic crustal sequence in combination with 3 74 ga pb pb ages of post deformational hydrothermal metasomatic minerals our results imply that early archean metasomatic activity is indistinguishable in time from the earliest metamorphic overprint and from the ca 3 75 ga major pulse of tonalite intrusions in the isua area

thermal regime of the costa rican convergent margin 2 thermal models of the shallow middle america subduction zone offshore costa rica at the costa rica margin along the middle america trench along strike variations in heat flow are well mapped these variations can be understood in terms of either ventilated fluid flow where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer we model fluid flow within the subducting aquifer using nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust the sensitivity of these models to the initial thermal state of the plate and styles of fluid flow either ventilated or insulated is explored heat flow measurements on cool crust accreted at the east pacific rise are consistent with ventilated hydrothermal cooling that continues with subduction these models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation heat flow transects on warm crust accreted at the cocos nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100 c and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust

hypocenter determination in strongly heterogeneous earth models using the shortest path method the shortest path method moser 1991 for the calculation of seismic ray paths and travel times along them can be applied directly in the hypocenter location method proposed by tarantola and valette 1982 it uses the analogy between seismic rays in the earth and shortest paths in networks to construct first arrival times from one point to all other points of a three dimensional grid simultaneously in a fast robust way in earth models of arbitrary complexity doing this for all stations of a seismic array one can find the hypocenter location by minimizing the difference between the observed and the calculated travel times at the stations over the three dimensional grid the concept of probability density functions allows then for a fully nonlinear examination of the uncertainties in the hypocenter location due to uncertainties in the travel time data numerical errors in the calculated travel times and to a limited extent incomplete knowledge about the earth model the result is a three dimensional contour map of regions of equal confidence for the earthquake location the method becomes especially attractive when more than one event recorded by the same array is studied because the calculation of the travel times which is relatively the most time consuming operation has to be done only once the method is applied on the location of an event that occurred on january 18 1989 in israel

control of seafloor roughness on earthquake rupture behavior earthquake rupture complexity is described for three recent large underthrusting earthquakes along the costa rican subduction zone the 1983 osa 1990 nicoya gulf and 1999 quepos events these earthquakes occurred in regions characterized by distinctly different morphologic features on the subducting plate the 1990 and 1999 events occurred along linear projections of subducting seamount chains and had fairly simple earthquake rupture histories both events are interpreted as failure of the basal contact of closely spaced isolated seamounts acting as asperities in contrast the 1983 event occurred along the subducting cocos ridge and had a complex rupture history comparison of rupture characteristics of these large underthrusting earthquakes with size and location of subducting features provides evidence that seamounts can be subducted to seismogenic depths and that variations in seafloor bathymetry of the subducting plate strongly influence the earthquake rupture process

seismic and gravity constraints on the nature of the basement in the africa eurasia plate boundary new insights for the geodynamic evolution of the sw iberian margin we present a new classification of geological domains at the africa eurasia plate boundary off sw iberia together with a regional geodynamic reconstruction spanning from the mesozoic extension to the neogene to present day convergence it is based on seismic velocity and density models along a new transect running from the horseshoe to the seine abyssal plains which is combined with previously available geophysical models from the region the basement velocity structure at the seine abyssal plain indicates the presence of a highly heterogeneous thin oceanic crust with local high velocity anomalies possibly representing zones related to the presence of ultramafic rocks the integration of this model with previous ones reveals the presence of three oceanic domains offshore sw iberia 1 the seine abyssal plain domain generated during the first stages of slow seafloor spreading in the ne central atlantic early jurassic 2 the gulf of cadiz domain made of oceanic crust generated in the alpine tethys spreading system between iberia and africa which was coeval with the formation of the seine abyssal plain domain and lasted up to the north atlantic continental breakup late jurassic and 3 the gorringe bank domain made of exhumed mantle rocks which formed during the first stages of north atlantic opening our models suggest that the seine abyssal plain and gulf of cadiz domains are separated by the lineament south strike slip fault whereas the gulf of cadiz and gorringe bank domains appear to be limited by a deep thrust fault located at the center of the horseshoe abyssal plain

crustal deformation dynamics and stress evolution during seamount subduction high resolution 3 d numerical modeling funding was received from the people programme marie curie actions of the european union s seventh framework programme fp7 2007 2013 under rea grant agreement 604713 zip zooming in between plates v s and c r r acknowledge financial support received through the hades project ctm2011 30400 c01 and ctm2011 30400 c02 funded by spanish mineco

margin architecture and seismic attenuation in the central costa rican forearc seismic attenuation across the central costa rican margin wedge is determined fromamplitude analysis ofwideangle seismic data travel time and amplitude modeling are applied to ocean bottom hydrophones along two trench parallel profiles located 30 km p21 and 35 km p18 landward of the deformation front northeast of quepos plateau tomographic inversion images a progressively thinning margin wedge from the coast to the lower slope at the trench a 1 1 5 km thick decollement zone with seismic velocities of 3 5 4 5 km s is sandwiched between the marginwedge and the subducting cocos plate for strike line p21 amplitude modeling indicates a qp value of 50 150 for the upper margin wedge with seismic velocities ranging from 3 9 km s to 4 9 km s along strike line p18 qp values of 50 150 are determined with velocities of 4 3 5 0 km s in the upper margin wedge increasing to 5 1 5 4 km s in the lower margin wedge quantitative amplitude decay curves support the observed upper plate qp values in conjunction with earlier results from offshore nicoya peninsula our study documents landward decreasing attenuation across the margin wedge consistent with a change in lithology from the sediment dominated frontal prism to the igneous composition of the forearc middle prism

fluid accumulation along the costa rica subduction thrust and development of the seismogenic zone in 2011 we acquired an 11 55 km 3 d seismic reflection volume across the costa rica margin nw of the osa peninsula to accurately image the subduction thrust in 3 d to examine fault zone properties and to infer the hydrogeology that controls fluid accumulation along the thrust following processing to remove water column multiples noise and acquisition artifacts we constructed a 3 d seismic velocity model for kirchhoff prestack depth migration imaging images of the plate boundary thrust show high reflection amplitudes underneath the middle to lower slope that we attribute to fluid rich poorly drained portions of the subduction thrust at 5 km subseafloor beneath the upper slope the plate interface abruptly becomes weakly reflective which we interpret as a transition to a well drained subduction thrust mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude there is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a plumbing system faults commonly have fault plane reflections and are presumably fluid charged the faults and layered fabric form three compartmentalized hydrogeologic zones 1 a shallow ne dipping zone beneath the slope 2 a steeply sw dipping zone beneath the shelf slope break and 3 a ne dipping zone beneath the shelf the more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure elevates effective stress and creates greater potential for unstable coseismic slip

structure and tectonics of the erosional convergent margin off antofagasta north chile 23 30 s subduction erosion has dominated the evolution of the north chile convergent continental margin since at least the mesozoic we investigate the structure of the antofagasta 23 s sector of this margin along a transect using coincident wide angle and near vertical seismic profiling and gravity data a 2 d velocity field of the overriding and subducting plates was obtained using joint refraction and reflection travel time tomography a velocity derived density distribution was used to model marine gravity data and substantiate the velocity model the gravity and velocity models imply that the overriding plate is mainly made of arc type igneous basement the upper plate is constructed of two main rock bodies separated by a subhorizontal layer defined by a velocity inversion the top coincident with a reflection in near vertical seismic images the seismic boundary is interpreted as a detachment separating an upper extended domain with large scale normal faulting from a lower domain apparently undergoing a different type of deformation velocity derived porosity indicates that the front of the margin is probably fluid saturated and disaggregated by fracturation as a consequence of frontal subduction erosion fluids carried into the subduction channel within slope debris filling underthrusting grabens reduce basal friction and probably induce hydrofracturing and basal erosion along the underside of the overriding plate at depths greater than 20 km porosity and density values imply that most fluids have been exhausted and the lower part of the upper plate is structurally coherent and little fractured the change in physical properties leads to increased mechanical coupling along the plate boundary and occurs at the updip limit of the distribution of aftershocks of the 1995 antofagasta earthquake m w 8 0 defining the seismogenic zone

moho structure of central america based on three dimensional lithospheric density modelling of satellite derived gravity data the central american isthmus hosts a highly variable moho structure due to the diverse origin and composition of the crustal basement and the influence of large scale neotectonic processes gravity data from the combined geopotential model egm2008 were interpreted via forward modelling to outline the three dimensional lithospheric density structure along the middle american trench as well as the segmentation of the oceanic cocos and nazca plates and the overriding caribbean plate in this work results for the depth of the moho obtained from the density model are presented the quaternary volcanic arc correlates with a maximum moho depth of 44 km in western guatemala to the south east of the continental shelf the caribbean plate shows moho depths between 20 and 12 km whereas to the north values as shallow as 8 km are observed at the cayman trough for the oceanic cocos plate depths between 16 and 21 km are obtained for the moho along the cocos ridge contrasting with values between 15 and 12 km for the seamount segment and 8 and 11 km for the segments of the crust that are not affected by the galapagos hot spot track

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

seismogenic zone structure of the southern middle america trench costa rica 1 the shallow seismogenic portion of subduction zones generates damaging large and great earthquakes this study provides structural constraints on the seismogenic zone of the middle america trench offshore central costa rica and insights into the physical and mechanical characteristics controlling seismogenesis we have located 300 events that occurred following the mw 6 9 20 august 1999 quepos costa rica underthrusting earthquake using a three dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers we use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region these events define a plane dipping at 19 that marks the interface between the cocos plate and the panama block the majority of aftershocks occur below 10 km and above 30 km depth below sea level corresponding to 30 35 km and 95 km from the trench axis respectively relative event relocation produces a seismicity pattern similar to that obtained using absolute locations increasing confidence in the geometry of the seismogenic zone the aftershock locations spatially correlate with the downdip extension of the oceanic quepos plateau and reflect the structure of the main shock rupture asperity this strengthens an earlier argument that the 1999 quepos earthquake ruptured specific bathymetric highs on the downgoing plate we believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level

the early archaean itsaq gneiss complex of southern west greenland the importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution geochemical and isotopic studies of small volumes of variably preserved 3600 ma rocks in gneiss complexes are crucial for documenting early earth history in the itsaq gneiss complex of the nuuk region west greenland there is dispute whether the granitic sensu lato orthogneisses dominating it are mainly products of a single ca 3650 ma crust formation super event or whether they formed in several unrelated events between ca 3850 and 3560 ma which of these interpretations of the dates is correct has major implications regarding what the whole rock radiogenic isotopic record pb pb sm nd rb sr reveals about continental crust formation and early terrestrial differentiation there is also debate whether some west greenland metasedimentary rocks with 12c 13c data interpreted as evidence for life are 3850 ma or only 3650 ma old establishing the correct age for these rocks is important for debates concerning early surficial environments and origin of life controversies have arisen because of different approaches taken by different workers specifically with respect to how much emphasis is placed on field geology in interpreting dates and isotopic data in this paper field observations and sampling from low strain zones where the origin and geological context of the rocks are best preserved and understood are closely integrated with u pb zircon dates and cathodoluminescence cl imagery of the zircons this approach shows that most single phase well preserved meta granitoid samples have simple zircon populations dominated by oscillatory zoned prismatic grains formed when their host magmas crystallized on the other hand migmatites and some strongly deformed banded gneisses have much more complex zircon populations the combined field evidence and zircon geochronology on the itsaq gneiss complex demonstrate that 1 some areas contain exposed orthogneisses formed during multiple magmatic thermal events between ca 3850 and 3560 ma and are not as suggested by kamber and moorbath 1998 dominated by ca 3650 ma granitoids containing abundant 3650 ma zircons inherited from cryptic unexposed older rocks 2 abundant 3750 ma granitoids are present which are locally well preserved 3 some water lain sediments reported as showing c isotope evidence for life were deposited as early as 3850 ma 4 the whole rock sm nd isochron approach fails to distinguish with any confidence 3650 ma from 3800 ma rocks 5 however it reinforces previous indications for markedly depleted 2 5 nd domains in the pre 3750 ma mantle

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

crustal structure across the pacific margin of nicaragua evidence for ophiolitic basement and a shallow mantle sliver seismic wide angle measurements across the pacific margin of nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots the structure and the p wave velocity of the subducting cocos and overriding caribbean plates were determined by modelling wide angle data and further constrained by coincident seismic reflection borehole and gravity data the oceanic crust of the cocos plate is 5 5 km thick with a thin pelagic sediment cover the plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network a major feature of the upper plate is an 80 km wide high velocity high density rock unit reaching from the front of the margin to about the middle of the shelf this wedge shaped unit is 15 km thick beneath the shelf edge and is composed of a 5 km thick upper part with velocities increasing from 3 5 km s 1 near the trench to 5 2 km s 1 at the shelf break overlying a 10 km thick lower part with velocities of 6 0 km s 1 analysis of the gravity field requires densities of 2 6 2 7 g cm 3 for the upper part and 2 9 g cm 3 for the lower part of this unit these velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore guatemala and exposed on the nicoya peninsula costa rica the velocity structure of this ophiolitic unit ends about 50 km offshore landward the basement underneath the sandino basin is formed by older igneous rock drilled beneath upper cretaceous sedimentary rock onshore nicaragua beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12 18 km and coincides with the down dip limit of the seismogenic zone mantle densities are required for the sliver to match the gravity data in a tectonic reconstruction the suture of an oceanic plateau on the farallon plate against the chortis block in upper cretaceous time is suggested suturing left the former trench and margin in deep water consistent with the late cretaceous to palaeocene deep water sediment of the rivas formation at the base of the sandino basin suture of the farallon plate and chortis block might have initiated the strike slip movement along the motagua polochic fault system the development of a new subduction zone in eocene oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the chortis block and shifted the volcanic arc about 70 km southwestwards close to its present position

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

earliest part of earth s stratigraphic record a reappraisal of the 3 7 ga isua greenland supracrustal sequence investigations of the isua supracrustal rocks in west greenland allow us to identify the protoliths and alteration history for most isua rocks the protoliths consisted of alternating basalt and banded iron formation this sequence was invaded by dunitic sills metamorphosed and later intruded by felsic gneisses pervasive carbonation and k metasomatism produced a sequence of lithologies mimicking those found in modern platform deposits however the protoliths could have originated in a purely oceanic environment with no sialic detrital components the isua sequence probably consists of several tectonic panels some of which are cut by 3810 ma felsic intrusive rocks

sources of iron formations in the archean isua and malene supracrustals west greenland evidence from la ce and sm nd isotopic data and ree abundances abstract isotopic ratios of ce and nd and abundances of rare earth elements rees and ba are reported for the iron formations of the archean isua and malene supracrustals west greenland it is suggested that the sm nd system of the isua iron formations was disturbed by metamorphisms at 1680 ma oxide facies iron formation and 2740 ma carbonate facies iron formation however the malene iron formation samples show similar nd isotopic ratios and ree patterns indicating that they suffered little disturbance after deposition at 2800 ma initial ce and nd values estimated at 2800 ma for the malene iron formation are close to chur chondritic uniform reservoir of 3 samples two show negative ce and positive nd and the other gives chondritic ce and negative nd these initial values suggest that the sources for the malene iron formation had a chiefly chondritic or rather mantle like depleted nature without a notable long term negative ce anomaly similar ree features are indicated for the sources of the isua iron formation by the estimated initial ce and nd values at 3760 ma of these samples

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

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

along strike variability in the seismogenic zone below nicoya peninsula costa rica 1 at the subduction zone in northwestern costa rica the seismogenic zone lies directly beneath the nicoya peninsula allowing for near source seismic studies of earthquake activity we located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the nicoya peninsula using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth 10 km in the south to 20 km in the north that occurs where the subducting oceanic crust changes from warmer cocos nazca spreading center cns origin to colder east pacific rise epr origin we argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath nicoya costa rica subducting colder oceanic crust deepens the seismogenic updip limit

effects of subducting seafloor roughness on upper plate vertical tectonism osa peninsula costa rica 1 subduction of seamounts and ridges along thinly sedimented convergent margins results in deformation of the overriding forearc exposures of newly recognized late pleistocene shallow water deposits i e marenco formation record intervals of rapid subsidence and uplift across the costa rican forearc inboard of the subducting cocos ridge in general the marenco formation is a fining upward fossiliferious late pleistocene marine sand disconformably overlying beveled surfaces cut across the competent osa melange basement the 50 to 27 ka age of the marenco formation is constrained by 12 accelerator mass spectrometry and two conventional 14c dates obtained on marine macrofossils the deposition of this sequence coincident with a general fall in sea level during oxygen isotope stage 3 requires 6 mm yr 1 subsidence inboard of the northwest flank of the subducting cocos ridge presently exposures of the marenco formation are found at 75 m above sea level requiring uplift rates in excess of 6 mm yr 1 we interpret the down and up history of vertical tectonism recorded by the marenco formation as the response of the upper plate to variations in the elevation of the subducting cocos ridge on the basis of a model where the upper plate deforms through bends because of roughness on a rigid downgoing plate the rate duration and spatial distribution of vertical tectonism across the forearc are determined by the magnitude of the orthogonal component of the relative convergence vector and the bathymetry of the underthrusting plate application of this model to bathymetric data for the cocos plate offshore yields a broad agreement between predicted future rates of subsidence and uplift and rates over the last 50 kyr recorded by the marenco formation furthermore analysis suggests that the arrival of the blunt tipped leading edge of the cocos ridge 0 5 3 ma resulted in an initial period of very rapid 30 mm yr 1 uplift

influence of incoming plate relief on overriding plate deformation and earthquake nucleation cocos ridge subduction costa rica 18 pages 7 figures supporting information https doi org 10 1029 2019tc005586 data are available at the pangaea world data center https doi org 10 1594 pangaea 909047

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

the seismogenic zone in the central costa rican pacific margin high quality hypocentres from an amphibious network transition from subduction of normal to thickened oceanic crust occurs in the central portion of the costa rican margin where large interplate earthquakes m 7 and abundant interseismic seismicity have been associated with subduction of bathymetric highs we relocated 1 300 earthquakes recorded for 6 months by a combined on and offshore seismological network using probabilistic earthquake relocation in a 3d p wave velocity model most of the seismicity originated at the seismogenic zone of the plate boundary appearing as an 18 dipping planar cluster from 15 to 25 30 km depth beneath the continental shelf several reverse focal mechanisms were resolved within the cluster the upper limit of this interseismic interplate seismicity seems to be controlled primarily by the overlying plate thickness and coherency which in turn is governed by the erosional processes and fluid release and escape at temperatures lower than 100 to 120 c along the plate boundary the downdip limit of the stick slip behaviour collocates with relative low temperatures of 150 to 200 c suggesting that it is controlled by serpentinization of the mantle wedge the distribution of the interseismic interplate seismicity is locally modified by the presence of subducted seamounts at different depths unlike in northern costa rica rupture of large earthquakes in the last two decades seems to coincide with the area defined by the interseismic interplate seismicity

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

model studies of near inertial waves in flow over the oregon continental shelf abstract two dimensional primitive equation model studies of wind forced flow over a continental shelf show that under upwelling conditions high levels of near inertial wave energy are found in the interior over the shelf the regions of elevated wave energy with maximum wave amplitudes of around 0 2 m s 1 persist for up to two weeks and have spatial scales of 20 40 m vertically and 5 20 km horizontally relatively high dissipation levels are associated with these concentrations of wave energy when forced with downwelling favorable winds model results show very little subsurface inertial energy on the shelf a comparison of inertial wave propagation using the primitive equation model and a linearized version of the model demonstrates strong dependence on the background flow field the behavior of inertial waves using the linearized model is completely different very little subsurface inertial energy is seen on the shelf in either upwelling or downwelling conditions except where the bottom slope

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

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

a recent phase of accretion along the southern costa rican subduction zone abstract in 2011 we acquired a 3d seismic reflection volume across the costa rica margin nw of the osa peninsula to investigate the complex structure and the development of the seismogenic zone within the costa rican subduction zone in the vicinity of recent international ocean drilling program iodp drilling in contrast to previous interpretations these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments similar to materials recovered from iodp drilling that have been thrust and thickened into thrust bounded folded sequences these structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model we interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the cocos nazca caribbean triple junction and accreted during recent margin subduction that also accelerated with passage of the triple junction the margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last 5 ma that was probably preceded throughout the neogene by periods of non accretion or erosion

upper plate rigidity determines depth varying rupture behaviour of megathrust earthquakes seismological data provide evidence of a depth dependent rupture behaviour of earthquakes occurring at the megathrust fault of subduction zones also known as megathrust earthquakes1 relative to deeper events of similar magnitude shallow earthquake ruptures have larger slip and longer duration radiate energy that is depleted in high frequencies and have a larger discrepancy between their surface wave and moment magnitudes1 3 these source properties make them prone to generating devastating tsunamis without clear warning signs the depth dependent rupture behaviour is usually attributed to variations in fault mechanics4 7 conceptual models however have so far failed to identify the fundamental physical causes of the contrasting observations and do not provide a quantitative framework with which to predict and link them here we demonstrate that the observed differences do not require changes in fault mechanics we use compressional wave velocity models from worldwide subduction zones to show that their common underlying cause is a systematic depth variation of the rigidity at the lower part of the upper plate the rock body overriding the megathrust fault which deforms by dynamic stress transfer during co seismic slip combining realistic elastic properties with accurate estimates of earthquake focal depth enables us to predict the amount of co seismic slip the fault motion at the instant of the earthquake provides unambiguous estimations of magnitude and offers the potential for early tsunami warnings models compiled from subduction zones worldwide show that the elastic properties of the rock overlying shallow subduction megathrusts can be used to estimate potential slip possibly enabling early tsunami warnings

early archaean amitsoq tonalites and granites of the isukasia area southern west greenland development of the oldest known sial amphibolite facies early archaean amitsoq gneisses envelop and intrude the c 3 800 ma isua supracrustal belt isukasia area southern west greenland most of these gneisses are strongly deformed but in a c 75 km2 augen of lower deformation the amitsoq gneisses are seen to comprise predominantly 3 750 3 700 ma tonalitic grey gneisses that were intruded first by thin bodies of mafic to dioritic composition known collectively as the inaluk dykes and then by c 3 600 ma white gneisses and finally by sporadic c 3 400 ma pegmatitic gneiss sheets the grey gneisses could have formed by partial melting of crust consisting predominantly of basic rocks the inaluk dykes are interpreted as strongly fractionated basic melts of mantle origin contaminated by crustal material the white gneisses consist mostly of medium grained granite and occur as lenses and anastomosing sheets throughout their host of grey gneisses with subordinate inclusions of supracrustal rocks the white gneisses have chemistry compatible with formation by partial melting at depth of a source dominated by grey gneisses the igneous chemistry including ree abundances of the grey gneisses and white gneisses has been modified to varying degrees by metasomatism and assimilation reactions during the crystallisation of the white gneisses and also during subsequent tectonometamorphic events the white gneisses are evidence for considerable reworking by anatexis of sialic crust in the early archaean 150 to 100 ma after its formation the white gneisses and the pegmatitic gneisses show that granitic rocks s s were important in the earliest archaean and are further evidence of the diversity of the oldest known sial

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

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

recognizable primary volcanic and sedimentary features in a low strain domain of the highly deformed oldest known 3 7 3 8 gyr greenstone belt isua west greenland a low strain domain has been identified in the metamorphosed mostly highly deformed volcanic and sedimentary rocks of the early archaean isua supracrustal belt this domain contains well preserved volcanic and sedimentary features including basaltic pillow lavas pillow breccia heterogeneous volcanic breccia amygdules in metabasalt and polymict conglomerate dominated by recrystallized chert and volcanic clasts the low strain domain is bounded by highly deformed rocks mostly derived from basalt chert and banded iron formation these discoveries demonstrate that some primary features have escaped the pervasive metasomatism dominant in other parts of the belt and furthermore strengthen the characterization of the isua supracrustals as a typical greenstone belt

do subducting seamounts generate or stop large earthquakes seamount subduction is a common process in subduction zone tectonics contradicting a widely held expectation that subducting seamounts generate large earthquakes seamounts subduct largely aseismically producing numerous small earthquakes on rare occasions when they do produce relatively large events the ruptures tend to be complex suggesting multiple rupture patches or faults we explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault the complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures

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

seismogenic zone structure beneath the nicoya peninsula costa rica from three dimensional local earthquake p and s wave tomography the subduction plate interface along the nicoya peninsula costa rica generates damaging large mw 7 5 earthquakes we present hypocenters and 3 d seismic velocity models vp and vp vs calculated using simultaneous inversion of p and s wave arrival time data recorded from small magnitude local earthquakes to elucidate seismogenic zone structure in this region interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern nicoya peninsula respectively microseismicity offset across the plate suture of east pacific rise derived and cocos nazca spreading center derived oceanic lithosphere is 5 km revising earlier estimates suggesting 10 km of offset interplate seismicity begins downdip of increased locking along the plate interface imaged using gps and a region of low vp along the plate interface the downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental moho intersection possibly due to the onset of ductile behaviour slow forearc mantle wedge p wave velocities suggest 20 30 per cent serpentinization across the nicoya peninsula region while calculated vp vs values suggest 0 10 per cent serpentinization interpretation of vp vs resolution at depth is complicated however due to ray path distribution we posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes

overriding plate structure of the nicaragua convergent margin relationship to the seismogenic zone of the 1992 tsunami earthquake we present 2 d seismic velocity models and coincident multichannel seismic reflection images of the overriding plate and the inter plate boundary of the nicaragua convergent margin along two wide angle seismic profiles parallel and normal to the trench acquired in the rupture area of the 1992 tsunami earthquake the trench perpendicular profile runs over a seamount subducting under the margin slope at the location where seismological observations predict large coseismic slip along this profile the igneous basement shows increasing velocity both with depth and away from the trench reflecting a progressive decrease in upper plate rock degree of fracturing upper mantle like velocities are obtained at approximate to 10 km depth beneath the fore arc sandino basin indicating a shallow mantle wedge a mismatch of the inter plate reflector in the velocity models and along coincident multichannel seismic profiles under the slope is best explained by approximate to 15 velocity anisotropy probably caused by subvertical open fractures that may be related to fluid paths feeding known seafloor seepage sites the presence of a shallow partially serpentinized mantle wedge and the fracture related anisotropy are supported by gravity analysis of velocity derived density models the downdip limit of inter plate seismicity occurs near the tip of the inferred mantle wedge suggesting that seismicity could be controlled by the presence of serpentinite group minerals at the fault gouge near the trench the inferred local increase of normal stress produced by the subducting seamount in the plate boundary may have made this fault segment unstable during earthquake rupture which could explain its tsunamigenic character

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

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

the steeply subducting edge of the cocos ridge evidence from receiver functions beneath the northern talamanca range south central costa rica the deep structure of the south central costa rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible we present a receiver function study along a transect of broadband seismometers through the northern flank of the cordillera de talamanca south costa rica below moho depths the receiver functions image a dipping positive conversion signal this is interpreted as the subducting cocos plate slab compatible with the conversions in the individual receiver functions in finite difference modeling a dipping signal such as the one imaged can only be reproduced by a steeply 80 dipping structure present at least until a depth of about 70 100 km below this depth the length of the slab cannot be determined because of possible scattering effects the proposed position of the slab agrees with previous results from local seismicity local earthquake tomography and active seismic studies while extending the slab location to greater depths and steeper dip angle along the trench no marked change is observed in the receiver functions suggesting that the steeply dipping slab continues until the northern flank of the cordillera de talamanca in the transition region between the incoming seamount segment and cocos ridge considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision the southern costa rican subduction zone may at present be undergoing a reconfiguration of subduction style where the transition to shallow underthrusting may be underway but still incomplete

protoliths of the 3 8 3 7 ga isua greenstone belt west greenland abstract the isua greenstone belt fig 1 contains the oldest known relatively well preserved metavolcanic and metasedimentary rocks on earth the rocks are all deformed and many were substantially altered by metasomatism but both the deformation and metasomatism were heterogeneous transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated likewise different kinds and different episodes of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths new geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents by this means the primary nature of the schists that make up most of the isua greenstone belt was reinterpreted and a new map that better portrays the primary nature of the rocks has been produced the previously mapped stratigraphy was found to be of little value in understanding the geology stratigraphic units were defined by different and diverse criteria such as current composition structure metamorphic texture and inferred protoliths much of this stratigraphy represents a misinterpretation of the primary nature of the rocks the new work indicates that most of the isua greenstone belt consists of fault bounded rock packages mainly derived from basaltic and high mg basaltic pillow lava and pillow lava breccia chert bif and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks a previously mapped extensive unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite

seismic tomography with local earthquakes in costa rica abstract the costarican isthmus is located at the western limit of the caribbean oceanic plateau where the cocos plate subducts along the middle american trench this plate shows strong lateral variations in its morphology and structure the main objective of this study is to investigate the effects of subduction related magmatism as a function of morphology and structure of the subducting plate by performing a simultaneous inversion of the 3 d crustal velocity field and hypocenter locations from local earthquakes for that we used the traveltimes of p waves first arrivals from more than 5000 events recorded at the costarican seismic networks between 1991 and 1998 in order to prevent data and modeling inaccuracies we followed an inversion scheme consisting of four steps 1 selection of an adequate data subset for tomographic inversion 2 estimation of the best reference 1 d model 3 determination of the finest parameterization and 4 evaluation of resolution the results show that the uppermost levels of the crust 0 6 km are consistent with geology since the velocity anomalies reflect the most meaningful geological features observed in the surface below these levels 6 20 km deep we found two different zones separated by a sw ne seismic alignment the northern part is characterized by a highly heterogeneous velocity field and is seismically active while the southern part is much more homogeneous and practically inactive moreover the northern part shows a certain accumulation of low velocity material within the upper mantle we suggest and illustrate that the structural differences between the northern and southern zones can be a consequence of the differences on the geometry and structure of the subducting slab

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

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

weak interplate coupling by seamounts and repeating m 7 earthquakes subducting seamounts are thought to increase the normal stress between subducting and overriding plates however recent seismic surveys and laboratory experiments suggest that interplate coupling is weak a seismic survey in the japan trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude m 7 both observed seismicity and the pattern of rupture propagation during the 1982 m 7 0 event imply that interplate coupling was weak over the seamount a large rupture area with small slip occurred in front of the seamount its northern bound could be determined by a trace of multiple subducted seamounts whereas a subducted seamount itself may not define the rupture area its width may be influenced by that of the seamount

crustal structure of the cocos ridge northeast of cocos island panama basin 1 the submarine cocos ridge in the northwestern panama basin pacific ocean is generally interpreted as the trace of the galapagos hotspot a 278 km long seismic wide angle transect was carried out across a comparatively narrow ridge segment 150 km northeast of cocos island the results indicate a huge thickened crust where crustal thickening is mainly achieved by the lower crust which makes up 75 of the crustal thickness and is tripled compared to normal oceanic lower crust the velocities are comparable to normal lower crust and suggest no differences to a gabbroic composition the moho deepens from 10 to 18 km depth below the ridge similarities to other hotspot related oceanic ridges and plateaus with massive lower crusts and velocities below 7 4 km s suggest the formation of this cocos ridge segment near or at the plate boundary

from seamount accretion to tectonic erosion formation of osa melange and the effects of cocos ridge subduction in southern costa rica 1 the costa rica portion of the middle america trench mat is characterized by active tectonic erosion a process that causes the removal of material from the base of the upper plate as the plate boundary migrates upward offshore studies demonstrate accelerated subduction erosion starting at the miocene pliocene boundary as the result of subduction of thickened galapagos related crust as presently represented by the cocos ridge the subduction of the cocos ridge also caused uplift and exposure of the outer forearc on the osa peninsula which offers a window to explore the tectonic evolution of the area the rocks outcropping on osa peninsula are a middle eocene middle miocene melange dominated by basalt chert and limestone resulting from accretion of seamounts the accretion dominated period of the mat evolution ended at the miocene pliocene boundary when thickened crust a paleo cocos ridge produced at the galapagos hot spot arrived at the trench the thick crust caused uplift and severe tectonic erosion of the accretionary edifice allowing exhumation of the osa melange the change from accretion to erosion caused the outer forearc to be offset along subvertical faults that define small kilometer to tens of kilometers size blocks that are going through differential vertical movements in response to the morphology of the subducting ridge subduction accretion and erosion are two processes that can alternate in time and space or coexist along the same margin so that mass removal can develop on a previously growing margin and completely remove an accretionary prism

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

corrugated megathrust revealed offshore from costa rica exhumed faults are rough often exhibiting topographic corrugations oriented in the direction of slip such features are fundamental to mechanical processes that drive earthquakes and fault evolution however our understanding of corrugation genesis remains limited due to a lack of in situ observations at depth especially at subducting plate boundaries here we present three dimensional seismic reflection data of the costa rica subduction zone that image a shallow megathrust fault characterized by corrugated and chaotic and weakly corrugated topographies the corrugated surfaces extend from near the trench to several kilometres down dip exhibit high reflection amplitudes consistent with high fluid content pressure and trend 11 18 oblique to subduction suggesting 15 to 25 mm yr 1 of trench parallel slip partitioning across the plate boundary the corrugations form along portions of the megathrust with greater cumulative slip and may act as fluid conduits in contrast weakly corrugated areas occur adjacent to active plate bending faults where the megathrust has migrated up section forming a nascent fault surface the variations in megathrust roughness imaged here suggest that abandonment and then reestablishment of the megathrust up section transiently increases fault roughness analogous corrugations may exist along significant portions of subduction megathrusts globally mature parts of the shallow megathrust beneath costa rica are characterized by striking corrugations that may channel fluids according to seismic images nascent sections of the subduction zone plate boundary appear only weakly corrugated