igneous petrogenesis and tectonic setting of plutonic and volcanic rocks of the cordillera de talamanca costa rica panama central american arc

rapid forearc uplift and subsidence caused by impinging bathymetric features examples from the new hebrides and solomon arcs 1 isotopically dated corals from the central new hebrides and new georgia island group solomon islands indicate that both forearcs underwent rapid late quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to 8 mm yr while total plate convergence was only a few kilometers two mechanisms that might account for these rapid reversals in vertical motion include 1 a displacement mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate as the object moves deeper and vacates the base of the forearc the forearc subsides to near its original position and 2 a crustal shortening mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone rapid subsidence follows when the impinging object is broken or otherwise decoupled shallow interplate coupling becomes weak and the uplifted forearc extends and subsides the displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers but it fails to explain the geographic pattern short time frame and abruptness of the change from subsidence to uplift that we observe the crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

inner forearc response to subduction of the panama fracture zone southern central america abstract subduction of the right lateral panama fracture zone along the convergent margin of central america creates abrupt lateral variations in convergence rate obliquity and subducting crustal thickness at its intersection with the middle america trench this intersection known as the panama co nz ca triple junction is migrating to the southeast at a rate of 55 mm yr and currently coincides with the lateral termination of the fila costena thrust belt in the inner forearc of the overriding plate mapping in the inner forearc in the area that straddles the subducting panama fracture zone reveals that cocos caribbean convergence west of the triple junction leads to the development of an inner forearc thrust belt inboard of the colliding cocos ridge while little deformation is evident inboard of nazca caribbean convergence east of the triple junction this results in the lateral termination of the fila costena thrust belt in the region of the forearc that projects over the panama fracture zone where four out of five mapped thrust faults tip out and are buried by lahars three new balanced cross sections indicate a steep gradient in shortening from the center of the thrust belt to its southeastern termination the short term history of the inner forearc recorded in the landscape and topography of the fila costena is consistent with the southeastward migration of the thrust belt and the panama triple junction throughout the past 3 ma with evidence for the growth of a new topographic divide and reorganization of stream channel networks

evidence for active subduction below western panama widespread arc tholeiitic and calc alkaline volcanism characterized the geologic evolution of western panama and southeastern costa rica in middle to late miocene and quaternary time volcanism was and is clearly associated with subduction of nazca lithosphere geochemical data suggest that formation of mantle wedge derived melts and their differentiated products predominated during miocene time whereas slab derived melts were generated in the quaternary this difference in magma chemistry is attributed to a change in the thermal regime of the nazca lithosphere which may be related to emplacement of the galapagos plume

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

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

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

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

geodetic and seismic constraints on some seismogenic zone processes in costa rica new seismic and geodetic data from costa rica provide insight into seismogenic zone processes in central america where the cocos and caribbean plates converge seismic data are from combined land and ocean bottom deployments in the nicoya peninsula in northern costa rica and near the osa peninsula in southern costa rica in nicoya inversion of gps data suggests two locked patches centered at 14 2 and 39 6 km depth interplate microseismicity is concentrated in the more freely slipping intermediate zone suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone the rupture zone for future large earthquakes at least over the short 1 year observation period we also estimate northwest motion of a coastal sliver block at 8 3 mm yr probably related to oblique convergence in the osa region to the south convergence is orthogonal to the trench cocos caribbean relative motion is partitioned here with 8 cm yr on the cocos panama block boundary including a component of permanent shortening across the fila costena fold and thrust belt and 1 cm yr on the panama block caribbean boundary the gps data suggest that the cocos plate panama block boundary is completely locked from 10 50 km depth this large locked zone as well as associated forearc and back arc deformation may be related to subduction of the shallow cocos ridge and or younger lithosphere compared to nicoya with consequent higher coupling and compressive stress in the direction of plate convergence

role of viscous plate coupling in the late tertiary andean tectonics 1 plate coupling between oceanic and continental plates in convergent margins of andean type is analyzed from the continuum mechanics approach we postulate a simple mechanism that accounts for the compressive regime in andean type environments in this mechanism deformation in the continental lithosphere is split into two distinctive domains the forearc domain and the arc foreland domain the forearc deformation is controlled by the balance between buoyancy forces associated with the trench and continental slope relief and the stress transferred from the convergence velocity through the age and velocity dependent slip zone the arc foreland deformation is controlled by the absolute plate velocity of the continental plate and the resistance at the slip zone strength of the coupling zone is determined by analyzing the dynamic trench topography along the active margin of south america between 0 and 55 s using this approach we found strength values in the range of 20 95 mpa in strong direct correlation with the age of the subducting plate the slip layer strength observation has been successfully tested against a thermal and strain rate dependent rheological model from this theoretical result we define an empirical relationship between strength of the slip zone and the age and convergence velocity applying this plate coupling model we reproduce shortening rates in the order of 1 10 km myr in agreement with those reported for the late tertiary evolution of the andes model results reproduce some first order features of the geological evolution of the margin such as the shape of the trench the overall andes relief the altiplano buildup and block rotation patterns in addition the model provides a mechanism to explain the evolution of the central depression the inversion of tertiary basins under slow convergence rates during the miocene and the segmentation of the margins tectonic erosion

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

resistivity identification of shallow mining cavities in real del monte mexico abstract a geophysical study was carried out in the real del monte silver mining centre in central mexico this mining district has been worked since the 12th century a complex network of tunnels was the result of intense exploitation and the irregular mineralization the urban zone is situated over the main mineralized veins this investigation attempted to apply geoelectrical prospecting methods to determine areas of risk related to the potential collapse of shallow tunnels geoelectrical data were reinterpreted using a modification of militzer s method which integrated a 1 versuslgraphics over 4 ab 2 different distances high resistivity anomalies were found to be associated with the presence of shallow cavities

the crustal tongue melting model and the origin of massive anorthosites recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid crust levels recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10 13 kbar indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40 50 km depths in the sveconorwegian province terne boundaries have been traced in deep seismic profiles to moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km in southern norway we suggest that a lithospheric scale weakness zone the feda transition zone has channelled the rogaland anorthosites through linear delamination asthenospheric uprise and melting of a mafic lower crustal tongue

active thrusting landscape evolution and late pleistocene sector collapse of baru volcano above the cocos nazca slab tear southern central america in this paper stratigraphy geochronology and geologic mapping are used to characterize a sequence of quaternary deposits associated with edifice failure of baru volcano which together with balanced cross sections illustrate the upper plate s response to along strike variations in subduction properties that occur across the tear in the subducting slab located at the panama fracture zone the subducting panama fracture zone is an active transform that separates disparate styles of subduction between the thick rapid and flat subduction of the cocos plate to the west and the thinner more oblique and steeper subduction of the nazca plate to the east we focus on the arc forearc region above the cocos nazca slab tear where both the fila costena inner forearc fold and thrust belt and the exhumed cordillera de talamanca terminate along strike to the southeast the fila costena thrust belt imbricates an eocene late miocene forearc basin sequence with up to 40 km of shortening inboard of cocos plate subduction and crosscuts a sequence of pleistocene and younger volcano sedimentary units on the southwestern flanks of the active baru volcano at its southeastern termination these units constrained in age by radiocarbon dating n 11 soil color and 40 ar 39 ar n 5 data include a sequence of late pleistocene to holocene debris avalanche and lahar deposits landscape morphology the areal distribution of units and new shortening estimates from balanced cross sections collectively suggest that the southeastern termination of the fila costena thrust belt actively propagates to the southeast coeval with migration of the panama triple junction the prevailing map pattern suggests that the forearc s response to southeastward migration of the slab tear varies with distance to the trench in contrast to deformation patterns in the outermost forearc plate boundary tractions associated with shallow subduction and cocos caribbean convergence are more influential in the development of arc forearc deformation than either oblique subduction of the 2 km bathymetric scarp or right lateral shear which both occur astride the subducting panama fracture zone

fore arc motion and cocos ridge collision in central america 1 we present the first regional surface velocity field for central america showing crustal response to interaction of the cocos and caribbean plates elastic half space models for interseismic strain accumulation on the dipping subduction plate boundary fit the gps data well and show strain accumulation offshore and beneath the nicoya and osa peninsulas in costa rica but not in nicaragua since large subduction zone earthquakes occur in nicaragua we suggest that interseismic locking in nicaragua and some other parts of central america occurs but is mainly shallow 20 km depth too far offshore to be detected by our on land gps measurements our data also show significant trench parallel motion for most of the region generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates however trench parallel motion is also observed in central costa rica where plate convergence is normal to the trench and in the nicaraguan fore arc where trench parallel motion is fast up to 9 mm a 1 but mechanical coupling is low a finite element model of collision as opposed to subduction involving the aseismic cocos ridge also fits the gps surface velocity field most significantly reproducing the pattern of trench parallel motion we infer that buoyant thickened cns 2 cocos ridge crust resists normal subduction and instead acts as an indenter to the caribbean plate driving crustal shortening in southern costa rica and contributing to trench parallel fore arc motion in costa rica and perhaps nicaragua as a type of tectonic escape

forearc response to subduction of the cocos ridge panama costa rica stratigraphic paleontologic and structural data from two panama costa rica forearc peninsulas located landward of the aseismic cocos ridge along the southernmost middle america trench document rapid pliocene subsidence and basin infilling followed by quaternary deformation and uplift on the burica peninsula an approximately 3 000 m thick pliocene pleistocene clastic sedimentary sequence characterized by fine grained turbidite deposits and volumetrically minor coarser grained channel fill slump and debris flow deposits bears depth diagnostic foraminifera that document rapid pliocene subsidence and deposition on a south dipping paleoslope new paleobathymetric and age estimates from foraminifera in these rocks indicate shallowing of depositional paleodepths from 2 000 to 1 200 m during the late pliocene present day exposure of these rocks indicates an average quaternary uplift rate of 1 mm yr the turbidite section on the burica peninsula is interpreted to represent a trench slope deposit and is correlated to a similar marine sedimentary section on the adjacent osa peninsula of costa rica structures developed within pliocene pleistocene strata of the outer forearc burica and osa peninsulas record minor 5 subhorizontal margin perpendicular shortening and margin perpendicular extension bedding orientations on the osa and burica peninsulas generally record arcward tilting by 10 30 of these outer forearc blocks on the burica peninsula the medial fault zone a north striking high angle right lateral strike slip fault has truncated and reoriented the pliocene pleistocene sedimentary sequence by fault bend forced folding a linear margin parallel fault herein named the ballena celmira fault zone bounds the arcward side of the osa and burica peninsulas and has accommodated vertical displacements between the outer forearc burica osa area and inner forearc terraba area throughout the tertiary structural and seismicity observations from the forearc region suggest that neogene deformation of the leading edge of the arc in this area reflects the effects of isostatic uplift and possibly increased interplate shear stresses as a result of subduction of the cocos ridge beneath this part of the arc since 1 m y ago

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

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

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

seismic rupture associated with subduction of the cocos ridge the april 3 1983 costa rica earthquake mw 7 4 ms 7 3 occurred where the aseismic cocos ridge has been partially subducting beneath the caribbean plate the epicenter is at the eastern edge of the 70 110 km initial aftershock area which later shows considerable expansion p wave directivity analysis indicates that the rupture front propagated westward 63 km from the epicenter to where the major moment release was truncated the source time history of the 1941 event ms 7 5 which occurred in the same epicentral area appears to be similar to the 1983 event but with more complexity and a slower initiation of moment release the low stress drop of the 1983 event 2 3 bar and low ratio of seismic slip to tectonic slip 0 15 for the 1983 event suggest that the coupling between the descending slab and the overlying plate is weak yet a recurrence time for large events mag 7 0 in this zone of about 40 years is implied by the 1904 1941 and 1983 events suggesting that the cocos ridge causes increased partial coupling compared with that in the adjacent subduction zone the slow source rupture process may reflect the nature of the contact area between the costa rica arc and the subducting plate in the vicinity of the bouyant cocos ridge the normal faulting event of july 1 1979 mb 5 3 is consistent with present day uplift of the burica peninsula which previous studies suggest is related to the subduction of the cocos ridge the burica and osa peninsulas may behave as detached blocks which influence the distribution of partial coupling reflected in the aftershock expansion pattern

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

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

constraints on inner forearc deformation from balanced cross sections fila costena thrust belt costa rica 1 the fila costena thrust belt in the forearc of costa rica is accommodating a significant portion of the convergence of the cocos plate and panama microplate geologic mapping of the thrust belt depicts a duplex with three horses that incorporate eocene limestones and oligocene to early miocene clastics inboard of the subducting cocos ridge axis by constructing a cross section at this location along a ne sw trending transect perpendicular to the thrust belt we constrain a shortening rate of approximately 40 mm a and propose that as much as 50 of the total plate convergence rate is taken up in the inner forearc the eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the cocos ridge axis owing to decrease in slip on faults and a lateral ramp in the basal decollement the thrust belt terminates near the panama border at the onland projection of the subducting panama fracture zone these observations suggest that shortening is propagating to the east with the migration of the panama triple junction and the onset of shallow subduction of the thickened edge of the cocos plate the absence of similar features in the nicaraguan forearc where the subducting crust is older subducts more steeply and lacks incoming ridges and seamounts indicates that deformation of the forearc basin in costa rica reflects greater coupling between the converging plates inboard of the cocos ridge

melange formation by subduction erosion the case of the osa melange in southern costa rica the convergent plate margin off the osa peninsula in southern costa rica is characterized by the indentation of the cocos ridge at 4 5 ma the indentation causes the uplift of the osa melange which we interpret to represent an exhumed major channel for the transport of tectonically eroded material down into the subduction zone we present evidence that similar to the nicoya segment of the costa rica convergent margin subduction erosion rather than accretion has been the dominant process along the plate boundary the composition of the osa melange is dominated by tectonized material of the upper plate nicoya ophiolite complex basalt radiolarite limestone strong deformation is concentrated in numerous discrete shear zones and produced the layered fabric of large rock volumes which partly experienced temperatures 200 c we thus interpret the osa melange to be a product of subduction erosion at the base of the outer arc wedge structure

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

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

benthic foraminiferal distribution and sedimentary structures suggest tectonic erosion at the costa rica convergent plate margin distribution patterns of benthic foraminiferal faunas from odp leg 170 sites 1041 and 1042 show that the costa rican convergent margin subsided from coastal to abyssal depth from middle miocene to present this favours the model of a margin undergoing active subduction erosion we propose that subduction erosion leads to the removal of material from the base of the forearc wedge and as a consequence to progressive subsidence of the forearc a mean subsidence rate is estimated to be approximately 0 4 mm yr 1

accelerating late quaternary uplift of the new georgia island group solomon island arc in response to subduction of the recently active woodlark spreading center and coleman seamount abstract the new georgia island group of the solomon islands is one of four places where an active or recently active spreading ridge has subducted beneath an island arc we have used coral reef terraces paleobathymetry of neogene sedimentary rocks and existing marine geophysical data to constrain patterns of regional quaternary deformation related to subduction of the recently active woodlark spreading center and its overlying coleman seamount these combined data indicate the following vertical tectonic history for the central part of the new georgia island group 1 subsidence of the forearc region tetepare and rendova islands to water depths of 1500 m and deposition of marine turbidites until after 270 ka 2 late quaternary uplift of the forearc to sea level and erosion of an unconformity 3 subsidence of the forearc to 500 m bsl and deposition of bathyal sediments and 4 uplift of the forearc above sea level with holocene uplift rates up to at least 7 5 mm yr on tetepare and 5 mm yr on rendova in the northeastern part of the new georgia island group our combined data indicate a slightly different tectonic history characterized by lower amplitude vertical motions and a more recent change from subsidence to uplift barrier reefs formed around new georgia and vangunu islands as they subsided 300 m by 50 100 ka subsidence was replaced by uplift that accelerated to holocene rates of 1 mm yr on the volcanic arc compared with rates up to 7 5 mm yr in the forearc area of tetepare and rendova uplift mechanisms such as thermal effects due to subduction of spreading ridges tectonic erosion or underplating of deeply subducted bathymetric features are not likely to function on the 270 ka period that these uplift events have occurred in the new georgia island group a more likely uplift mechanism for the post 270 ka accelerating uplift of the forearc and volcanic arc of the new georgia island group is progressive impingement of the coleman seamount or other topographically prominent features on the subducting plate regional effects we relate to this ongoing subduction related process include 1 late quaternary post 270 ka accelerating uplift of the rendova tetepare forearc area in response to initial impingement of the coleman seamount followed by exponentially increasing collisional contact between the forearc and seamount 2 later quaternary propagation of uplift arcward to include the volcanic arc as the area of collisional contact between the forearc and seamount increased and 3 large wavelength folding that has produced regional variations in late holocene uplift rates observed in both forearc southern rendova tetepare and volcanic arc new georgia island areas we propose that the dominant tectonic effect of coleman seamount impingement is horizontal shortening of the forearc and arc crust that is produced by strong coupling between the subducting seamount and the unsedimented crystalline forearc of the new georgia island group the horizontal forces due to mechanical resistance to subducting rugged ridge and seamount topography may have terminated spreading of the woodlark spreading center entering the trench ghizo ridge and converted it to a presently active strike slip fault zone

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

holocene forearc block rotation in response to seamount subduction southeastern peninsula de nicoya costa rica the southeastern tip of the peninsula de nicoya costa rica on the caribbean plate margin lies inboard of the rough bathy metric terrain on the subducting cocos plate and along the land ward projection of the convergence vector for the fisher seamount group the southern tip of the peninsula has nearly orthogonal coastlines and extensive well preserved holocene marine terraces

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

stress changes in the costa rica subduction zone due to the 1999 mw 6 9 quepos earthquake abstract stress changes after large earthquakes can trigger substantial seismicity along many fault systems as subduction zones produce the majority of large earthquakes it is essential to understand how significant earthquakes may affect stress conditions and aftershock locations calculations of stress changes due to large underthrusting earthquakes can be used to examine likely zones of triggered seismicity in subduction environments however associations between stress changes and locations of small aftershocks are typically difficult because of large errors on offshore aftershock locations based on land based seismic observations after the 1999 quepos earthquake m w 6 9 in the subduction zone offshore costa rica small magnitude earthquakes were located using a local onshore offshore temporary seismic network providing a data set of high quality aftershock locations in light of these well located aftershocks we compute coulomb stress changes for the 1999 quepos earthquake for comparison our calculations show lobes of increased stress in regions coinciding with most of the small magnitude seismicity following the mainshock as seen in many cases of large strike slip earthquakes few small earthquakes occur in regions of decreased stress within three years a large m w 6 4 earthquake occurred in this region however based on this modeling it does not appear that the 2002 earthquake occurred in a zone of stress increase from the 1999 event

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

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

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to

landscape evolution within a retreating volcanic arc costa rica central america subduction of hotspot thickened seafloor profoundly affects convergent margin tectonics strongly affecting upper plate structure volcanism and landscape evolution in southern central america low angle subduction of the cocos ridge and seamount domain largely controls landscape evolution in the volcanic arc field mapping stratigraphic correlation and 4 0 ar 3 9 ar geochronology for late cenozoic volcanic rocks of central costa rica provide new insights into the geomorphic response of volcanic arc landscapes to changes in subduction parameters slab thickness roughness dip late neogene volcanism was focused primarily along the now extinct cordillera de aguacate quaternary migration of the magmatic front shifted volcanism northeastward to the caribbean slope creating a new topographic divide and forming the valle central basin stream capture across the paleo aguacate divide led to drainage reversal toward the pacific slope and deep incision of reorganized fluvial networks pleistocene caldera activity generated silicic ash flows that buried the valle central and descended the tarcoles gorge to the orotina debris fan at the coast growth of the modern cordillera central accentuated relief along the new divide establishing the valle central as a pacific slope drainage basin arc migration relocation of the pacific caribbean drainage divide and formation of the valle central basin resulted from slab shallowing as irregular hotspot thickened crust entered the subduction zone the geomorphic evolution of volcanic arc landscapes is thus highly sensitive to changes in subducting plate character

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