observations of first order mantle reverberations we have observed first order mantle reverberations specifically sh polarized scsn and sscsn phases reflected at near normal incidence from upper mantle discontinuities as discrete phases on long period digital seismograms of the hglp and sro networks such arrivals correspond to an n or n 1 member dynamic ray family denoted by scsn sd s where n is the multiple number of the parent phase d signifies a reflection from the top of an internal discontinuity at depth d and d signifies a reflection from the bottom the travel times and attenuation of these phases place important constraints on the nature of the transition zone we have employed the phase equalization and stacking algorithm of jordan and sipkin 1977 to obtain the differential attenuation operators of sscsn s650 s sscsn and scsn s650 s scsn phase pairs from deep focus tonga events recorded by the hglp station kip on the island of oahu the apparent q s of the zeroth order and first order reverberations over the frequency band of 10 to 30 mhz have been inverted for the average quality factors of the upper mantle qum and lower mantle qlm and the reflection coefficient of the 650 km discontinuity r 650 at 20 mhz the results are qum 82 18 qlm 231 60 and r 650 0 080 0 004 at normal incidence the qlm estimate is significantly less than recent normal mode solutions suggesting that lower mantle attenuation structure is frequency dependent at very low frequencies we have measured the differential travel times between the first order reverberations sscsn s400 s and sscsn s650 s which yields a direct estimate of the vertical shear wave travel time between the discontinuities along the tonga to kip path the observed two way time 92 2 1 5 sec is less than predicted by some recent oceanic upper mantle models indicating either a smaller separation of the discontinuities and or greater shear velocity in the transition zone measurements of differential travel times between first order reverberations and multiple scs phases suggest that the former is more likely and are consistent with depths of 400 and 650 km for the transition zone discontinuities observations of first order mantle reverberations can potentially provide unique constraints on lateral heterogeneity of the earth s mantle especially within the transition zone

kinematics of the north american caribbean cocos plates in central america from new gps measurements across the polochic motagua fault system 1 the polochic motagua strike slip fault system in guatemala marks the on land plate boundary between the north american na and the caribbean ca plates gps observations in 1999 and 2003 show that the far field velocity across the system na ca relative velocity is 20 mm yr this is significantly higher than the nuvel 1a velocity but is consistent with the gps based ca na velocity proposed by demets et al 2000 the observations are modeled by a fault centered on the motagua fault locked at a depth of 20 km with a slip rate decreasing from eastern to central guatemala from 20 to 12 mm yr towards the na ca cocos triple junction this decrease is accommodated by 8 mm yr of e w extension in the westernmost part of ca south of the motagua fault about 10 mm yr of dextral slip is observed across the mid american volcanic arc the na ca cocos triple junction is thus a complex 400 km wide wedge shaped area

constraints on the central indian basin thermal structure from heat flow seismicity and bathymetry abstract the lithosphere beneath the central indian basin is characterized by high heat flow widespread deformation of sediment and acoustic basement and unusually high seismicity the high heat flow suggests that temperatures in the lithosphere may be higher than expected for its age we explore the constraints on the temperature distribution with depth by combining heat flow bathymetry and seismicity data the high heat flow 30 mw m 2 greater than expected constraints the near surface temperature distribution the bathymetric depths on average those expected for these lithospheric ages after corrections for sediment loads constrain the integral of the temperature with depth since the depths of oceanic intraplate earthquakes appear to be limited by an isotherm 750 c the maximum earthquake depth 40 km constrains the minimum depth of deformation and the maximum temperature there the present thermal structure is investigated by examining different models matching these three observational constraints first the possibility is examined that the high heat flow results from anomalous lithosphere with a basal temperature or thickness different from typical oceanic lithosphere in this case the temperature constraints from the earthquake depths restrict the additional heat flux to less than 20 mw m 2 however since the heat flux anomaly requires plate thicknesses substantially thinner than usually assumed it appears that at most 10 mw m 2 can be added by this mechanism second the possibility is examined that the extra flux results from reheating the bottom portion of the lithosphere to asthenospheric temperatures substantial reheating 30 40 km below the surface is required to match the high heat flow such reheating should produce 1 km of uplift whereas the average basement depth is no shallower than expected for its age thus significant deep lithospheric reheating cannot be widespread over the deformed region third the effects of a temperature perturbation within the lithospheric column and the resulting change in surface heat flux with time are examined a temperature perturbation at shallow depths could produce the present heat flow anomaly but no evidence for shallow intrusion or other mechanism for such an effect exists it is concluded that despite the heat flow anomaly the lack of an average bathymetric anomaly and the observation of seismicity to a depth of 40 km indicate that lithospheric temperatures in the central indian basin are not significantly different from those expected for its age

lateral heterogeneity and azimuthal anisotropy in the north atlantic determined from ss s differential travel times to explore upper mantle heterogeneity beneath the north atlantic we have measured 70 ss s differential travel times using the waveform cross correlation method both the two way preliminary reference earth model prem and j b jeffreys bullen residuals exhibit a maximum variation of 17 s ranging from 10 to 7 and 9 to 8 s respectively the lack of correlation between ss s and s residuals along with the strong correlation between ss s and ss residuals suggests that ss s differential travel times eliminate source and receiver effects and sample the upper mantle anomalies near the ss bounce points without being strongly affected by lower mantle heterogeneities we find evidence for a strong local anomaly in the north atlantic residuals for bounce points immediately to the north of the azores gibraltar plate boundary average about 4 s faster than for the bounce points to the south of the boundary we infer that this velocity contrast exists in the lithosphere and continues below a depth of 200 km a series of linear multiple regression experiments demonstrate that variation in velocity with increasing age of the seafloor and azimuthal anisotropy are both significant contributors to the prem and j b residuals assuming that travel time delays are a linear function of the square root of the seafloor age we find the coefficient for age to be about 1 0 s m y this is smaller than but not significantly different from the corresponding slope for the s delays of intraplate earthquakes in the atlantic reported by j d duschenes and s c solomon 1977 and favors the presence of small amounts of water e g 0 1 in the upper mantle in modeling anisotropy we carry out an experiment using synthetic seismograms to examine the shear wave splitting phenomena in the anisotropic medium and investigate three possible forms for sh wave delay as a function of azimuth sinusoidal variations as a function of 2 4 and a combination of 4 predominant and 2 that mimics the delay expected in olivine crystals when the axis is aligned horizontally our regression models strongly prefer the predominance of a 4 to a 2 variation and the last type of anisotropy explains our data best from the preferred anisotropy model the olivine axis or the upper mantle shearing flow is inferred to be oriented n s the magnitude of the azimuthal variation suggests that anisotropy extends over a depth range of several hundred kilometers in the upper mantle

effects of inelasticity on the apparent depth and detectability of seismic discontinuities in the mantle 1 accurate determination of parameters about seismic discontinuities in the earth such as their depth existence and strength provides critical information about the physical and chemical states and dynamics of the earth s interior unfortunately those parameters can be strongly affected by factors that are not related to the discontinuities we use synthetic seismograms to estimate the influence of one of those factors inelasticity on the apparent depth and detectability of the 410 and 660 kilometer discontinuities obtained from stacking p to s converted phases pds our results show that when pds travels through a zone with lower than normal q values the observed depth of discontinuities could be tens of kilometers deeper than the real value and the stacking amplitudes and consequently the detectability and apparent strength are greatly reduced this study demonstrates the importance of taking q structure into account when seismologically detected discontinuity parameters are interpreted

active seismic deformation in the grabens of northern central america and its relationship to the relative motion of the north america caribbean plate boundary abstract the active seismic deformation along the grabens of northern central america is calculated using a method which relates relative plate velocity with the seismic strain rate tensor the latter is in turn obtained from seismic moment tensors we calculate the typical or average moment tensor from available centroid moment tensors and fault plane solutions this average tensor is then used to obtain the sum of moment tensors of historical and modern earthquakes reported in the literature both historical and modern earthquakes yield an average extension rate of 8 mm yr along the grabens of northern central america while the relative motion between the north america and caribbean plates expressed along the motagua polochic fault system is 20 mm yr this result suggests that part of the seismic deformation related to the plate boundary is taken up as extension along the grabens which play the role of fault termination structures for the motagua polochic system

an extensive region of off ridge normal faulting earthquakes in the southern indian ocean an unusually large number of off ridge earthquakes have occurred within a broad region astride the southeast indian ridge the source mechanisms of nine of these earthquakes in lithosphere from 4 to 27 m y old have been determined with a formal inversion technique based on matching observed and synthetic p and sh waves all events occur in the oceanic mantle from 8 to 22 km below the seafloor the seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events all of the off ridge earthquakes are characterized by normal faulting mechanisms with the t axes nearly parallel to the nearby ridge axis in sharp contrast to typical mid ocean ridge or intraplate earthquakes a variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes including bending stresses thermal stresses of various origins differential subsidence at a fracture zone off ridge volcanism latitudinal plate motions on a nonspherical earth basal shear stress from small scale convection and plate wide perturbations of the stress field due to the continental collision between india and asia no single mechanism is adequate to explain all the observed off ridge normal faulting earthquakes an apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the antarctic and indian plates are caused by different processes the antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the kerguelen plateau and the southeast indian ridge a region which has been suggested as the site of a horizontal pipe for flow in the asthenosphere from the kerguelen hot spot to the vicinity of amsterdam and st paul islands on the southeast indian ridge the off ridge earthquakes in the indian plate are speculatively attributed to part of the plate wide response of the lithosphere to the ongoing continental collision

rotation of the chortis block causes dextral slip on the guayape fault the chortis block is undergoing deformation as a direct result of large magnitude sinistral strike slip faulting on the north america caribbean plate boundary we have studied the chortis block using geologic mapping remote imagery and earthquake seismology to evaluate the effect that large magnitude strike slip faulting has on an adjacent microcontinental block in the eastern part of the chortis block recent geologic mapping has shown that the northeast striking guayape fault is dextral instead of sinistral as previously assumed because this is a major fault that cuts most of the central america isthmus the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the chortis block the presence of several north trending rifts in the western part of the chortis block has led previous workers to propose that the chortis block is divided into smaller independently rotating blocks we use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the chortis block however we have modified their model to account for dextral slip on the guayape fault we believe that rotation of the chortis block causes dextral slip on the guayape fault even though central america is undergoing large sinistral displacement with respect to north america

light elements and li isotopes across the northern portion of the central american subduction zone 1 quaternary mafic rocks have been erupted from volcanoes both on and behind the volcanic front in southeastern guatemala and western el salvador behind the front volcanism extends almost continuously some 110 km from the front b and li contents as well as b nb and li yb ratios are generally higher in the mafic volcanic rocks of the volcanic front and lower in those behind the front beryllium concentrations show the opposite relationships with generally higher values behind the front in addition many behind the front samples have lower 7li 2 3 versus 4 6 for the volcanic front guatemalan volcanic rocks in general have lower 7li than those found along the remainder of the central american margin the light element variations across northern central america clearly indicate that the volcanic front receives a much more pronounced fluid contribution from dehydrating subducting cocos lithosphere than the behind the front region this dehydration pulse occurs at slab depths between 85 and 105 km and selective enrichments of b cs and cl in volcanic rocks of the volcanic front suggest that the source of this pulse is dehydrating serpentinite in addition the dehydration pulse from subducted serpentinite is likely superimposed on a continuous dehydration signal provided by subducting oceanic crust which continues behind the volcanic front to slab depths 200 km the enhanced fluid contributions at the front produce higher degrees of wedge melting thus explaining the lower be contents of front samples there are two possible explanations of the lower 7li of many behind the front volcanic rocks from southeastern guatemala western el salvador selective loss of 7li during progressive dehydration of subducting cocos lithosphere or diffusive fractionation accompanying late stage selective crustal contamination since the slightly lower 7li of guatemalan volcanic rocks in general is consistent with a minor crustal overprint the second explanation is favored and could be relevant to the few subduction zones where lower li isotopic values characterize back arc regions light element variation across southeastern guatemala and western el salvador is consistent with the operation of decompression melting behind the volcanic front be zr ratios across the northern portion of the central american subduction zone may indicate some mobility of be

explosive eruptions of the ayarza calderas southeastern guatemala abstract the ayarza calderas southeastern guatemala form a figure 8 shaped depression that is now occupied by laguna de ayarza the smaller eastern caldera is also the older it formed upon eruption of a mixed plinian air fall that is locally overlain by a comagmatic ash flow deposit called the mixta unit the air fall portion contains rhyolitic 75 2 wt sio 2 pumice basaltic 52 wt sio 2 blobs and compositional hybrids that are either spectacularly banded or homogeneous in color although of limited aerial extent and volume 0 1 km 3 dense rock equivalent the unit is of considerable petrologic interest because the basalt contains pristine phenocrysts and quench crystallites of calcic hornblende virtually identical 87 sr 86 sr ratios 0 70373 of both rhyolite and basalt end members suggest a common magmatic source a log from the mixta ash flow deposit has been c 14 dated at 27 000 1 600 y b p eruption of the younger pyroclastic sequence at ayarza culminated in formation of the western caldera this rhyolitic sequence also consists of a plinian air fall overlain by a comagmatic ash flow deposit the ash flow unit is quite fine grained and probably phreatomagmatic in origin a thin horizon of air fall material is locally interbedded within the ash unit volume dre of the younger sequence has been estimated at a minimum 2 km 3 a log from the younger ash flow unit has been c 14 dated at 23 100 500 y b p the age of formation of the western caldera and the last recognized activity at ayarza although major element geochemistry of the ayarzan rhyolites is very similar to that of other silicic units of northern central america minor and trace element contents permit all the rhyolites to be distinguished from each other sr isotopic ratios of the mixta rhyolite reaffirm that older radiogenic crustal rocks are not involved in the petrogenesis of rhyolites in northern central america

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

volcanic history and magmatic evolution of seguam island aleutian island arc alaska new 40 ar 39 ar dating coupled with detailed field mapping stratigraphy and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of seguam island in the aleutian island arc alaska sixty new 40 ar 39 ar ages from lavas domes and pyroclastic deposits were obtained using furnace incremental heating techniques on replicate samples of whole rock and ground mass separates and they constrain the duration of pleistocene to holocene subaerial volcanism to 318 k y the 40 ar 39 ar plateau ages indicate that over 85 of the complex 68 km 3 of material was erupted almost continuously between 318 ka and 9 ka at ca 9 ka a stratocone on the eastern half of the island partially collapsed producing a 4 km wide crater rhyolitic dome forming eruptions followed from vents in the newly created crater and were likely contemporaneous with 8 0 km 3 of basaltic and basaltic andesitic effusions from pyre peak and a 1 4 km 3 basaltic eruption from a monogenetic cone on the far eastern end of the island geochemical changes over the last 318 k y are subtle most notably the earliest eruptions from 318 to 142 ka produced no andesite and basalt from this period has larger ranges in zr rb and la yb than younger basalts small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal liquid fractionation process that varied only slightly in successive eruptive phases over 318 k y we identified minor geochemical changes in magma composition during each of the three main stages of volcanism but overall the monotonic variations in major and trace element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed system fractional crystallization of basalt using the 40 ar 39 ar geochronology and estimates of individual flow volumes we calculated a time averaged eruptive rate at seguam that is similar to growth rates of other well dated arc volcanoes in the cascades and chilean southern volcanic zone but less than that of mount katmai and mount mageik which are located on the alaska peninsula the eruptive flux at seguam has been highly variable fluctuating more than an order of magnitude from 0 07 km 3 k y during the early history of bimodal volcanism to 1 18 km 3 k y over the past 9 k y

determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches abstract the earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate routinely determined depths of shallow sub oceanic earthquakes published in bulletins are unreliable the depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied in some cases the reflected phases pp and pwp can be clearly identified there exists the possibility that the wave reflected at the water air interface pwp may be misidentified as pp leading to erroneous estimates of depth additional methods of analysis such as surface wave radiation patterns or the apparent frequency dependence of reflection at the crust water interface can remove this possible source of confusion one of the most powerful techniques for depth analysis is the modelling of long period waveforms the pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal deviatoric tension down to a depth of about 25 km with horizontal compression at greater depths

magma rates in feeding conduits of different volcanic centres abstract a quasi stationary magma flow rate in asthenospheric and crustal conduits of central type volcanoes and volcanic centres was studied analytically under the following conditions magma rises through cylindrical channels in which the magma temperature does not change with time but the wall rocks are gradually heated the magma rates were calculated for basaltic andesitic and dacitic volcanoes using the continental and oceanic geotherms it follows from these calculations that the magma supply rate may determine the kind of activity of a volcanic centre being constant for large and very active volcanoes intermittent for usual volcanic centres of island arcs or sporadic for volcamic fields clusters of cinder cones and areal volcanism theoretical conclusions are consistent with observational data

computer simulation models of scoria cone degradation abstract long term erosional modifications of the relatively simple morphology of scoria cinder cones are ideally suited for study by field and computer simulation methods a series of temporally distinct cones in the san francisco and springerville volcanic fields of arizona provides the foundation for documenting the degradational evolution of scoria cones in a semi arid climate progressive changes due to erosion are illustrated by the systematic decrease with increasing age of various morphometric parameters including scoria cone height cone height width ratio hco wco crater depth width ratio and slope angle for example holocene latest pleistocene cones in the san francisco field have a mean hco wco value of 0 178 0 041 a mean maximum slope angle of 29 7 4 2 and a mean average slope angle of 26 4 7 3 whereas the group of pliocene cones have values of 0 077 0 024 20 5 5 8 and 8 7 2 7 respectively comparative morphology of scoria cones is a potentially useful dating tool for mapping volcanic fields in order to better understand the degradational modifications of these volcanic landforms we have developed a numerical approach to simulate the surficial processes responsible for the erosion of a typical scoria cone the simulation algorithm can apply either a linear diffusion equation model or a model with a nonlinear transport law using a finite difference formulation the simulation operates upon a three dimensional scoria cone input as a matrix of elevation values utilizing both field and model results the correlation between changing hco wco value cone age and computer time step was expressed graphically to derive comprehensive values of the transport or diffusion coefficient df for both volcanic fields for the san francisco volcanic field df had a calculated value of 21 4 m2 kyr for the linear model and 5 3 m kyr for the nonlinear model while for the springerville volcanic field df had a calculated value of 24 4 m2 kyr for the linear model and 6 3 m kyr for the nonlinear model

steady state secondary convection beneath lithospheric plates with temperature and pressure dependent viscosity we have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self consistently by the heat transported by convection in this study of the interaction between the lithosphere and upper mantle strongly temperature and pressure dependent rheologies for both newtonian and non newtonian creep mechanisms are employed for a strictly temperature dependent rheology an insignificant amount of heat less than 12 5 mw m2 can be transported convectively for an interior viscosity 0 1021 pas compatible with postglacial rebound on the other hand for similar values of the interior viscosity steady heat fluxes between 20 and 40 mw m2 are produced by introducing pressure dependence into the rheology for the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated once all of the rheological parameters are fixed this finding may have important ramifications for parameterized convection we employ both the single mode mean field approximation and the complete two dimensional equations using finite elements to obtain solutions for the various types of rheologies from evaluating the geophysically relevant observables such as topography free air gravity anomalies surface heat flow and stress fields in the lithosphere we find that the lateral variations of these quantities predicted by a non newtonian rheology are much smaller than those derived from a linear rheology these results suggest that surface variations of geophysical observables are more compatible with a non newtonian rheology in the upper mantle

extension and rotation of crustal blocks in northern central america and effect on the volcanic arc sinistral displacements across the north american caribbean plate boundary in northern central america are distributed among major arcuate faults that have been active in neogene time south of the jocotan and motagua faults in guatemala extensional tectonics has accompanied rotation of the trailing edge of the caribbean plate around these faults segmentation of the volcanic arc in northern central america hitherto attributed to transverse breaks in the subducting cocos plate may instead be a result of this block rotation accompanying changes along the arc are observed in seismicity gravity anomaly patterns volume and composition of volcanic products and topography therefore the complex volcano tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding caribbean plate above the magma production zone along the downgoing cocos slab

boron geochemistry of the central american volcanic arc constraints on the genesis of subduction related magmas boron contents were measured in representative quaternary lavas from the central american volcanic arc to evaluate along strike variations in subduction processes despite the significant range in b concentrations 2 37 ppm in the mafic lavas b la ratios vary in a systematic fashion along the arc higher values 1 are typical between guatemala and northern costa rica whereas low values most 0 5 typify central costa rica and western panama b la is highly correlated with 10be 9be r2 0 94 excluding one sample and appears to be a useful indicator of subduction contributions to the magma sources if enrichments of both b and 10be are proportional to the flux of subducted sediment along strike variations in b la suggest at least a twofold variation in this flux with maximum values below western nicaragua and minimum values below costa rica and western panama where the cocos ridge is being subducted these data may also reflect significant differences in thermal state of the descending slab which in turn differentially affects release patterns of fluids and fluid mobile trace elements and possibly melting processes beneath different parts of the arc the following scenario is suggested to explain the geochemical results beneath the northwestern part of the arc steep subduction of older relatively cold slab favors more efficient subduction of fluid components to depths beneath the volcanic front the released fluids carry fluid mobile elements to the overlying mantle which upon melting produces calcalkalic magmas shallow subduction of warmer slab beneath the southeastern part of the arc favors shallow release of fluids and limits fluid related metasomatism of sub arc mantle beneath the volcanic front under such conditions b la and ba la ratios in the sub arc mantle vary little from values seen in oceanic island basalts magmas in this part of the arc nevertheless display the highest la yb and lowest ba la and b la ratios which are consistent with prior light rare earth element enrichment in the source significantly lower degrees of melting or a combination thereof because some of the largest volcanoes occur in costa rica and magma flux there is nearly an order of magnitude higher than elsewhere in the arc source enrichment is considered to be the more plausible explanation it is proposed that quaternary magma production below costa rica involved lithospheric sources containing trapped or stored melt components but this enrichment process is unlikely to have involved typical arc magmas or subduction fluids because we see no b enrichment

a gps and modelling study of deformation in northern central america summary we use gps measurements at 37 stations in honduras and el salvador to describe active deformation of the western end of the caribbean plate between the motagua fault and central american volcanic arc all gps sites located in eastern honduras move with the caribbean plate in accord with geologic evidence for an absence of neotectonic deformation in this region relative to the caribbean plate the other stations in the study area move west to west northwest at rates that increase gradually from 3 3 0 6 mm yr 1 in central honduras to 4 1 0 6 mm yr 1 in western honduras to as high as 11 12 mm yr 1 in southern guatemala the site motions are consistent with slow westward extension that has been inferred by previous authors from the north striking grabens and earthquake focal mechanisms in this region we examine the factors that influence the regional deformation by comparing the new gps velocity field to velocity fields predicted by finite element models fems that incorporate the regional plate boundary faults and known plate motions our modelling suggests that the obliquely convergent 20 direction of caribbean north american plate motion relative to the motagua fault west of 90 w impedes the ene directed motion of the caribbean plate in southern guatemala giving rise to extension in southern guatemala and western honduras the fem predictions agree even better with the measured velocities if the plate motion west of the central american volcanic arc is forced to occur over a broad zone rather than along a single throughgoing plate boundary fault our analysis confirms key predictions of a previous numerical model for deformation in this region and also indicates that the curvature of the motagua fault causes significant along strike changes in the orientations of the principal strainrate axes in the fault borderlands in accord with earthquake focal mechanisms and conclusions reached in a recent synthesis of the structural and morphologic data from honduras poor fits of our preferred models to the velocities of gps sites near the gulf of fonseca may be an artefact of the still short gps time series in this region or the simplifying assumptions of our fems

neogene north american caribbean plate boundary across northern central america offset along the polochic fault abstract the polochic fault was a segment of the north american caribbean plate boundary across central america in the neogene its 130 km of left slip was previously determined by matching structures and stratigraphie outcrop patterns of northwest and central guatemala across the fault additional support for the model and the youthfulness of the recorded offset comes from an essentially perfect match of major geomorphic features across the fault a reconstruction process which eliminates 123 km of left slip brings together rivers and drainage divides that existed before the polochic became active with the reconstruction carried across the isthmus on an east west fault the regional structural geology assumes the coherent pattern of a continuous orogenic belt whose geometry is compatible with the model of collisional tectonics centered on the motagua suture zone confined within this belt narrowed to some 60 km by the reconstruction lie the major laramide thrusts folds and tectonically emplaced serpentinites of guatemala crystalline rocks of guatemala re join the chiapas massif and paleozoic sedimentary rocks exposed in the core of an almost continuous anticlinorium extend from southern chiapas to lake izabal the polochic does not bend in eastern guatemala but continues eastward to the motagua fault where it dies westward drift of the northern block resulted in rifting which extended from eastern guatemala into the caribbean along the cayman trough the honduras depression may represent an element of a triple junction along with the polochic and izabal cayman rift the polochic continues westward into the pacific ocean and offsets the middle america trench the polochic has offset the miocene volcanic belt of northern central america confirming the previous estimate of a neogene time of movement about 300 km of relative east west neogene displacement has been recorded on the mid cayman rise only 130 km of which can be accounted for across the polochic it is suggested that cumulative extension on north south faults south of the motagua fault zone between the trench and the honduras depression might make up that difference

aeolian influences on the soils and landforms of the southern yilgarn craton of semi arid southwestern australia abstract within a landscape developed on the deeply weathered dominantly granitic rocks of the yilgarn craton in southwestern australia there is strong evidence of prior aeolian activity multiple arrays of clayey saltation deposits occur as clay dunes or lunettes extending up to 5 km to the southeast of playas and quartzose sand dunes and sheets occur in a 2 km wide band that extends 10 km southeast of an ephemeral creek line parabolic dunes occur within some clayey lunettes the parallel orientation of these diverse features and the elliptical shape of the playas suggest that the winds that have created these geomorphic entities have been from the northwest multiple lunette arrays with up to seven members have not been previously reported in this region there is evidence for a more widespread but more subtle aeolian influence on the soils dust deposits appear to mantle parts of the landscape and are inferred to be coeval with the clayey saltation deposits although these do not occur as a discrete layer evidence includes a plume of calcareous and illitic soils southeast of the major playa in a landscape otherwise characterised by acidic kaolinitic soils similarly the occurrence of deep sandy soils on many slopes with a southeasterly aspect may indicate the interference of topography on the transport of saltating sands during an arid climatic phase the presence of such aeolian deposits has implications for agricultural management mineral exploration and the interpretation of ecological gradients in similar undisturbed landscapes current desertification in the form of salinization and wind erosion has precedence in this landscape the aeolian deposits provide a means of interpreting landscape scale responses to previous climate change and a key to predicting possible outcomes of both desertification and future climate change assuming that bowler s theory that clay dunes require an adjacent salinized playa bed to form is correct the arrays of clayey lunettes together with vegetated former playas indicate that the landscape has undergone multiple cycles of salinization and recovery the lunettes may thus provide a means of predicting landscape hydrological responses to the widespread removal of natural vegetation for farming dating of the various sequences is required to provide a time scale for these landscape processes

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

matuyama brunhes reversal and kamikatsura event on maui paleomagnetic directions 40 ar 39 ar ages and implications abstract eighty nine basaltic lava flows from the northwest wall of haleakala caldera preserve a concatenated paleomagnetic record of portions of the matuyama brunhes m b reversal and the preceding kamikatsura event as well as secular variation of the full polarity reversed and normal geomagnetic field they provide the most detailed volcanic record to date of the m b transition the 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles vgps that move from reverse to normal along the americas concluding with an oscillation in the pacific ocean to a cluster of vgps east of new zealand and back finally to stable polarity in the north polar region all but one of the 16 kamikatsura vgps cluster in central south america the full polarity flows with 40ar 39ar ages spanning a total of 680 kyr pass a reversal test and give an average vgp insignificantly different from the rotation axis with standard deviation consistent with that for other 0 5 ma lava flows of similar latitude precise 40ar 39ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775 6 1 9 and 900 3 4 7 ka for the m b and kamikatsura transitional flows respectively this matuyama brunhes age is 16 kyr younger than ages for m b flows from the canary islands tahiti and chile that were dated using exactly the same techniques and standards suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals

the geoid small scale convection and differential travel time anomalies of shear waves in the central indian ocean we have measured 37 differential travel times for the phase pairs ss s sss ss scs2 scs scs2 s sscs2 sscs and sscs2 ss each pair included a surface bounce point in the central indian ocean between 4 n and 21 s and 69 and 85 e residuals were calculated relative to both the j b travel time tables and the pem c earth model and were corrected for the elevation of the bounce points the mean j b residual associated with bounce points in the central indian basin is 2 8 0 7 s fast this contrasts with 4 2 s slow reported for the western pacific by sipkin and jordan and is similar to the 4 0 s found for old continental nucleii after correcting for known differences between continental and oceanic crustal structure and for differences in measurement technique we find that travel times in the mantle beneath the central indian basin are midway in character between those beneath pre cambrian shields and those beneath the western pacific ocean basin the fastest differential travel times occur along a line passing close to the center of the large indian ocean geoidal low there is a clear increase in travel time with distance from the center of the geoid low suggesting that the source of the geoid anomaly may also be largely responsible for the difference between the central indian and pacific basins the chagos laccadive ridge seems to coincide with a transition from the fast differential travel times of the central indian basin to slower differential travel times associated with bounce points beneath young seafloor near active spreading centers there are also variations within the central indian basin that are not related to surface geological features or to the age of the seafloor these variations are periodic with a wavelength of about 640 km and with a nne trend which agrees approximately with the trend that would be expected for linear convective rolls in the upper mantle

topography and geoid undulations caused by small scale convection beneath continental lithosphere of variable elastic thickness summary the effect of subcontinental upper mantle convection on topography gravity and geoid undulation is studied with special regard to the effective elastic thickness of the continental lithosphere hel we designed a numerical model of the upper mantle lithosphere system consisting of a constant viscosity fluid overlayed by a partly elastic plate the boundary tractions stresses at the top of the convecting mantle model were applied to the lithosphere model for which we used the thin plate approximation the total thickness of the lithosphere was kept constant to 150 km while the elastic thickness was varied between 0 and 150 km the aspect ratio of the convecting mantle was varied between 0 7 and 3 and the rayleigh number between 104 and 106 our calculations demonstrate the effect of a thick elastic lithosphere on the surface observables peak to peak averaged amplitude of topography geoid undulation and gravity are strongly reduced and their spectra are low pass filtered for an aspect ratio less than 1 and a very thick elastic lithosphere topography tends to zero and while geoid undulation and gravity may even become negative above the up welling plume the spectral relation of geoid or gravity to topography admittance is nearly unaffected for hie 70km for aie 70km admittance values for all wavelengths generally decrease with increasing elastic thickness of the plate and may become negative geoid admittance for the basic mode increases with larger aspect ratio while gravity admittance for hie 100 km slightly decreases associating higher rayleigh numbers of the convecting fluid with lower viscosity an increase in rayleigh number leads to considerable decrease of the surface observables however admittance values are only slightly affected and decrease approximately linearly with increasing rayleigh number comparison of our results with observations of the topography spectrum and the free air gravity admittance for north america suggests that large aspect ratio cells beneath continents are unlikely however convection cells of aspect ratio 1 or less underlying an elastic lithosphere of at least 100 km thickness is in agreement with data the rayleigh number should be as high as 106

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

mantle layering from scs reverberations 1 waveform inversion of zeroth order reverberations this is the first paper in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms here we present path averaged estimates of the two way travel times through the crust mantle scs and crust alone m the whole mantle quality factor qscs and the reflection coefficient at the m discontinuity r0 zm obtained by iterative waveform inversion of scs reverberations we show that qscs estimates derived from direct waveform inversion can be biased significantly if reverberations originating within the crust are ignored especially in continental regions where the crust is thick applying our technique which explicitly models crustal reverberations we substantially expand the geographical coverage of scs and qscs measurements available from previous studies the scs data correlate well with whole mantle travel times predicted by the harvard tomographic models both the scs data and the qscs data support the hypothesis that variations in velocity and attenuation structure between old continents and oceans persist to depths of several hundred kilometers in particular the value of qscs derived for paths crossing stable continent is very high 280 30 at 30 mhz compared with that for mature ocean basin 189 16 consistent with the thick plate model of cratons requiring that differences in thermal structure extend to several hundred kilometers the crustal thicknesses implied by the observations of path averaged m are consistent with those inferred from short range refraction data gravity measurements and surface topography the path averaged reflectivity of the m discontinuity is also tectonically correlated becoming brighter as the percentage of continental crust encountered along the path increases this result implies that on average the impedance at the base of the continental crust is significantly lower than at the base of the oceanic crust

episodic back arc extension during restricted mantle convection in the central mediterranean abstract in the central mediterranean during the last 80 myr africa has been slowly converging toward eurasia with an average relative velocity of less than 1 cm yr 1 for 50 myr this slow convergence led to the build up of the alpine orogenic belt and to the initiation of consumption of the former tethys ocean in the last 30 myr however the whole central western mediterranean has been swept by rapid episodes of trench migration up to 6 cm yr 1 and back arc opening which consumed the whole former tethys ocean we combine plate tectonic history geological timing and laboratory modelling to reconstruct the subduction history and the evolution of the central mediterranean over the last 80 myr we find that the dynamic evolution of the subducting oceanic lithosphere can reconcile the rapid episodic back arc migration with the slow african convergence but only if the subduction process is restricted to the upper mantle

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

global upper mantle structure from long period differential travel times we have made over ten thousand measurements of pp p and ss s differential travel times from long period global digital seismograph network recordings of all events with mb 5 5 which occurred during the years 1976 to 1986 we have obtained reasonable global coverage with very good sampling of several regions of the world the residuals range from 5 to 5 s for pp p and 12 to 12 s for ss s and reveal a large scale pattern of heterogeneity when plotted at the reflected phase s bounce point our experiments indicate that lower mantle structure and source receiver structure can each contribute approximately 0 5 s to our measured pp p residuals so there is considerable signal to be explained the pattern observed in the pp p measurements is similar to the pattern observed in the ss s measurements with the ss s residuals 2 to 4 times larger in magnitude comparisons of our measured residuals to those predicted by the upper mantle models of woodhouse and dziewonski show that the overall patterns are quite similar but the amplitude of the model residuals is roughly a factor of 2 too small comparisons with the predictions of a whole mantle model of tanimoto again shows that the predicted pattern of residuals is reasonably consistent with the observations but now the predicted residuals are too large by about a factor of 2 this variation in predicted amplitudes is probably mainly due to differences in the near surface structure of the models we have also binned our measurements according to the tectonic regionalization gtr1 of jordan and find a qualitative correlation of average residual with tectonic region in particular precambrian shields show a strong anomaly and there is a correlation of residual size with the age of oceanic crust at the bounce point for all tectonic regions the ratio of ss s to pp p residuals is approximately 2 this ratio is consistent with a thermal origin for the observed signal finally our measurements show no compelling evidence for azimuthal anisotropy which might be related to fossil spreading direction or the direction of absolute plate motion such a signal is probably overwhelmed by the large signal from three dimensional structure

scales of heterogeneity near the core mantle boundary we have mapped shear velocity variations near the base of the mantle with scs s differential travel times beneath the north atlantic there is a systematic variation in velocity over a distance of 1500 km or more with a pronounced low velocity anomaly centered roughly beneath madeira island beneath the northern indian ocean there is no similar long wavelength anomaly the average scs s residual beneath the indian ocean is 3 6 0 7 s more negative than the atlantic residual indicating faster average velocities near the base of the mantle

tectonic stress field in the indian subcontinent a map of maximum horizontal compressive stress orientation in the indian subcontinent has been prepared using orientations derived from three different stress indicators borehole elongation breakouts in situ hydraulic fracturing measurements and earthquake focal mechanisms most part of the subcontinent appears to be characterized by a compressional stress regime thrust and strikeslip faulting imposed by plate boundary forces although shmax orientations do not in general show clear correlation with the direction of motion of the indian plate four provinces are recognized on the basis of regionally consistent orientations these are the midcontinent stress province the southern shield the bengal basin and the assam wedge their boundaries have been determined taking into consideration regional tectonics and seismicity central and northern india including the shillong plateau stretching up to the great himalaya pakistan and nepal are included in a broad midcontinent stress province characterized by nne ene oriented shmax the mean orientation of shmax in this province is n23 e subparallel to the direction of compression expected to arise from the net resistive forces at the himalayan collision zone suggesting that it is largely determined by the tectonic collision processes much of southern india mysore plateau and the high grade metamorphic terrain south of the plateau appears to be part of a second stress province characterized by nw oriented shmax these appear close to those of the intraplate stress field prevailing in the central indian ocean a third stress province was recognized in the bengal basin including parts of west bengal tripura manipur and mizoram in northeastern india and most of bangladesh this province extends eastward from the marginal fault in the western margin of the bengal basin to the indo burma subduction zone and is bounded on the north by the e w striking dauki fault shmax within the sedimentary pile of the bengal basin is oriented in e w direction while p axes of earthquakes within the basement and the crust beneath the basin and within the subducted slab of the indian plate beneath the indo burman ranges generally trend north n30 e shmax orientations within the sedimentary pile of the basin are parallel to the local approximately e w direction of the convergence of indian and burmese plates suggesting a casual relation to the resistive forces at the subduction zone in the indo burma region interestingly the stress field in the basement and the crust beneath the bengal basin and in the subducted slab is similar to the one prevailing in the midcontinent stress province assam wedge stress region occupies the northeastern corner of the indian plate including upper assam arunachal pradesh and much of nagaland this region subducts beneath a sharply bent continental collision boundary consisting of the northeastern limb of the himalayan and northern limb of the indo burman fold belts as a result the stress field in this province is depth differentiated and most likely responsible for the absence of consistent shmax directions

worldwide distribution of group velocity of mantle rayleigh waves as determined by spherical harmonic inversion we have determined the worldwide distribution of group velocity of mantle rayleigh waves for periods between 100 and 300 sec without assuming any regionalization group slowness 1 u is expressed by spherical harmonics and the coefficients up to angular order 7 have been determined from travel times of rayleigh waves by a least squares method from these u has been synthesized since we cannot obtain information about the odd terms of the expansion from one circuit measurements around the world we have used group velocities of mainly r 2 and r 3 the overall pattern of u for periods between 100 and 200 sec is consistent with results of previous pure path and regional studies group velocities for tectonically active regions are low and those of the shields and the northwestern pacific are high

constraints from finite element modeling on the active tectonics of northern central america and the middle america trench 1 we have developed an elastic finite element model in order to study the role of the different forces acting on the northwestern part of the central american volcanic arc and the chortis block we present synthetic focal mechanisms maps of tectonic regime and strain crosses to analyze the results the models show that to achieve the observed state of stress on the volcanic arc the arc must be modeled as a lithospheric weak zone also the forces related to the eastward drift of the caribbean plate must be higher than those related to the subduction of the cocos plate the coupling on the subduction interface must be low with or without slip partitioning due to the obliquity of the subduction at the trench at guatemala the western edge of the chortis block is pinned against north america even with low trench normal forces making the triple junction between the cocos north american and caribbean plates a zone of diffuse deformation the extension in the western part of the chortis block from guatemala to the honduras depression is explained by the geometry of the north american caribbean plate boundary and the direction of motion of the caribbean plate with respect to north america the direction of extension in the chortis block is always e w regardless of the magnitude of the applied forces and the main part of the deformation is absorbed between the ipala graben and the honduras depression both features being consistent with our models

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

magnetic crust in the bay of bengal abstract four marine magnetic profiles across the bay of bengal are analyzed and the magnetic nature of the upper part of the crust is investigated by inversion in the frequency domain the magnetization distribution at the average depth of the basement below the sediments is computed and shows high magnetization with alternating polarity the spectral decay provides a maximum estimate of the depth to basement of 8 and 10 km along profiles approximately parallel to 10 and 14 n respectively this suggests a thick pile of sediments in this region with thickness increasing towards the north the inversion of the recorded magnetic field along the 10 n profile at an average depth of 8 km provides alternating magnetization of approximately 1 2 10 3 cgs units corresponding to basic rocks this indicates a magnetic basement in the bay of bengal with alternating polarities typical of oceanic crust greater magnetization 3 10 3 cgs units over the 85 east ridge and the pattern of computed magnetization distribution on either side of this ridge indicate that it might have been a spreading ridge at sometime in the geological past which subsequently aborted

the growth and erosion of cinder cones in guatemala and el salvador models and statistics abstract morphologic data for 147 cinder cones in southern guatemala and western el salvador are compared with data from the san francisco volcanic field arizona usa cima volcanic field california usa michoacan guanajuato volcanic field mexico and the lamongan volcanic field east java the guatemala cones have an average height of 110 50 m an average basal diameter of 660 230 m and an average top diameter of 180 150 m the general morphology of these cones can be described by their average cone angle of slope 24 7 average height to radius ratio 0 33 0 09 and their flatness 0 24 0 18 although the mean values for the guatemalan cones are similar to those for other volcanic fields e g san francisco volcanic field arizona cima volcanic field california michoacan guanajuato volcanic field mexico and lamongan volcanic field east java the range of morphologies encompasses almost all of those observed worldwide for cinder cones three new 40ar 39ar age dates are combined with 19 previously published dates for cones in guatemala and el salvador there is no indication that the morphologies of these cones have changed over the last 500 1000 ka furthermore a re analysis of published data for other volcanic fields suggests that only in the cima volcanic field of those studied is there clear evidence of degradation with age preliminary results of a numerical model of cinder cone growth are used to show that the range of morphologies observed in the guatemalan cinder cones could all be primary that is due to processes occurring at the time of eruption

10be imaging of sediment accretion and subduction along the northeast japan and costa rica convergent margins sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity the seismogenic zone and the subduction recycling of volatiles and continentally derived materials to the earth s mantle identifying and quantifying sediment accretion essential for a complete mass balance across the margin can be difficult seismic images do not define the processes by which a prism was built and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy this contribution reports the first use of cosmogenic 10be depth profiles to define the origin and structural evolution of forearc sedimentary prisms biostratigraphy and 10be model ages generally are in good agreement for sediments drilled at deep sea drilling project site 434 in the japan forearc and support an origin by imbricate thrusting for the upper section forearc sediments from ocean drilling program site 1040 in costa rica lack good fossil or paleomagnetic age control above the decollement low and homogeneous 10be concentrations show that the prism sediments are older than 3 4 ma and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments low 10be in costa rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion

intra arc extension in central america links between plate motions tectonics volcanism and geochemistry this study revisits the kinematics and tectonics of central america subduction synthesizing observations of marine bathymetry high resolution land topography current plate motions and the recent seismotectonic and magmatic history in this region the inferred tectonic history implies that the guatemala el salvador and nicaraguan segments of this volcanic arc have been a region of significant arc tectonic extension extension arising from the interplay between subduction roll back of the cocos plate and the 10 15mm yr slower westward drift of the caribbean plate relative to the north american plate the ages of belts of magmatic rocks paralleling both sides of the current nicaraguan arc are consistent with long term arc normal extension in nicaragua at the rate of 5 10mm yr in agreement with rates predicted by plate kinematics significant arc normal extension can hide a very large intrusive arc magma flux we suggest that nicaragua is in fact the most magmatically robust section of the central american arc and that the volume of intrusive volcanism here has been previously greatly underestimated yet this flux is hidden by the persistent extension and sediment infill of the rifting basin in which the current arc sits observed geochemical differences between the nicaraguan arc and its neighbors which suggest that nicaragua has a higher rate of arc magmatism are consistent with this interpretation smaller amplitude but similar systematic geochemical correlations between arc chemistry and arc extension in guatemala show the same pattern as the even larger variations between the nicaragua arc and its neighbors

brittle ductile deformation and tensile rupture of dome lava during inflation at santiaguito guatemala gas and ash explosions at the santiaguito dome complex guatemala commonly occur through arcuate fractures following a 5 to 6 min period of inflation observed in long period seismic signals observation of active faults across the dome suggests a strong shear component but as fault propagation generally proceeds through the coalescence of tensile fractures we surmise that explosive eruptions require tensile rupture here we assess the effects of temperature and strain rate on fracture propagation and the tensile strength of santiaguito dome lavas indirect tensile tests were conducted on samples with a porosity range of 3 30 and over diametral displacement rates of 0 04 0 004 and 0 0004 mm s at room temperature the tensile strength of dome rock is rate independent within the range tested and inversely proportional to the porosity of the material at eruptive temperatures we observe an increasingly ductile response at either higher temperature or lower displacement rate where ductile deformation is manifest by a reduction in loading rate during constant deformation rate tests resulting in slow tearing viscous flow and pervasive damage we propose a method to conduct indirect tensile tests under volcanic conditions using a modification of the brazilian disc testing protocol and use brittleness indices to classify deformation modes across the brittle ductile transition we show that a degree of ductile damage is inevitable in the lava core during explosions at the santiaguito dome complex and discuss how strain leading to rupture controls fracture geometry which would impact gas pressure release or buildup and regulate explosive activity

heat transfer and intraplate deformation in the central indian ocean nineteen new heat flow measurements made across deformed oceanic lithosphere in the central indian ocean and previously published data show that heat flow is significantly higher than predicted by models for cooling oceanic lithosphere over much of the region many of the temperature depth profiles are nonlinear upward convection of water is the most likely explanation for the curvature of the temperature profiles since other possible causes including variations in bottom water temperatures conductivity changes with depth in the sediments and experimental error can be eliminated this interpretation requires water velocities of the order of 7 x 10 sup 8 m s which is unusual because the lithosphere is relatively old 72 82 m y and a thick sedimentary cover 1 2 5 km is present these observations suggest that the processes causing deformation of the plate have increased the heat flux through the sediment water interface we infer that extra heat is being generated at shallow depths perhaps less than 35 km in the plate although the specific mechanism by which deformational energy is converted into heat is difficult to determine

source composition and distribution of the fluid in the kurile mantle wedge constraints from across arc variations of b nb and b isotopes b and nb concentrations and b isotope composition were determined for lavas covering the entire length of the kurile arc b nb and 11b of these lavas show clear across arc variations in which both are highest at the trench side and continuously decrease as the slab depth increases b nb in the volcanic front lavas 8 25 is significantly higher than those of morb and oib indicating that the mantle wedge is extensively metasomatized by b rich fluid derived from the subducting slab the frontal 11b values are also out of the range of morb and oib and systematic difference exists between northern kurile nk 5 4 0 5 and central and southern kurile csk 4 3 0 3 however these two parameters approach or coincide with mantle values at the most back arc side the 11b and 87sr 86sr linearly correlate with nb b suggesting control by simple mixing of two isotopically homogeneous components that is slab derived fluid and mantle wedge the estimated 11b and 87sr 86sr of the fluid are 7 0 0 6 and 0 70326 0 00005 for nk and 6 3 0 3 and 0 7033 for csk respectively these fluid compositions are consistent with a source constituted by 95 altered oceanic crust and 5 oceanic sediment with the slight difference between nk and csk resulting from the different lithologies of subducting sediments the mantle wedge beneath the kurile arc has b sr and nd isotope compositions identical with morb but shows significantly higher b nb the above trends bear a close resemblance to those observed in the izu arc strongly suggesting their universality in the arc system the across arc decline of b nb observed both in kurile and izu implies that the fluid flux from the slab continuously decreases as the slab depth increases however this relates only to a final stage of slab dehydration since most of the volatile components are lost at the fore arc region in contrast to b k is uniformly enriched across the arc this requires control by a k bearing mineral phase either in the slab phengite or the mantle wedge phlogopite although such a mineral is responsible for only a small part of the whole fluid transport

heterogeneity in the lower mantle beneath africa as revealed from s and scs phases abstract s and scs phases propagated beneath africa and its adjacent regions were analyzed using their differential travel times t and the differences of anelastic parameter t between the two phases the residuals of the scs s differential travel times had strongly positive correlation with those of the scs travel times the largest residual of 7 9 s was in central africa 10 5 n 22 6 n the residuals decreased with increasing distance northwards from central africa the scs bounce points having large positive values greater than 4 s were concentrated in a region with a horizontal length of about 500 km at the core mantle boundary beneath central africa this region was characterized by an s wave velocity around 3 5 slower than prem in the d region the anelastic parameter t of the s phases propagated beneath africa was larger than that of the scs phases suggesting that the apparent q structure in the mid mantle 1000 2000 km deep was slightly lower than that at the base of the mantle these results probably imply a mantle plume beneath africa

speculations on the thermal and tectonic history of the earth summary the connection between the earth s thermal history and convection in the mantle is exploited to elucidate the early evolution of the earth it appears probable that convection extending over almost all of the mantle has dominated vertical heat transport throughout the whole of the earth s history only in boundary layers at the surface and at a depth of 650 700 km is conduction likely to be important the resulting evolution appears to be consistent with geological observations on early precambrian rocks various arguments are put forward in favour of two horizontal scales of convective flow in the mantle at depths less than 650 km the large scale flow is related to the motion of major plates and must be ordered over distances of more than 5000 km its evolution and energetics are discussed and there are no obvious problems in maintaining the proposed convective motions small scale flow with an extent of the order of 500 km appears necessary both to explain the heat flow through older parts of the earth s surface and to reconcile the geophysical observations with the results of numerical experiments though the existence of the small scale flow is at present speculative various tests of its presence are proposed

monogenetic behind the front volcanism in southeastern guatemala and western el salvador 40ar 39ar ages and tectonic implications abstract eighteen lavas erupted from cinder cones behind the volcanic front in southeastern guatemala and western el salvador have been dated using the 40 ar 39 ar method on groundmass separates plateau ages range between 1069 5 22 1and 37 12 0 ka the majority of dated lavas are younger than 600 ka importantly cinder cone lavas become progressively younger toward the volcanic front trench in addition the lavas become systematically depleted in cs ba rare earth elements cs zr la zr and 18 o with time the baseline values of these same geochemical parameters increase with distance from the volcanic front the cinder cone lavas are closely associated with rifting south of the north american caribbean plate boundary the trenchward migration of behind the front volcanism may be linked to a westward migration of this rifting existing evidence suggests there was an important change in the face of volcanism behind the volcanic front in southeastern guatemala from a small number of large polygenetic volcanoes to abundant smaller monogenetic cones the temporal spatial changes in the geochemistry of cinder cone lavas is attributed to decreasing contamination by paleozoic basement rocks both with time and with diminishing distances to the volcanic front

petrogenesis of lavas from cinder cone fields behind the volcanic front of central america small monogenetic cinder cones and shield volcanoes occur in platform cone fields behind the volcanic front of central america their lavas bvf suite are distinctly different from typical volcanic front lavas vf suite most vf lavas are nearly aphyric basalts containing only olivine and plagioclase phenocrysts they have high al 2 o 3 na 2 o k 2 o ba and sr contents and moderate ni cr and rb contents in contrast to petrographic observations major and trace element modeling and the decrease in the cao al 2 o 3 ratio with fractionation require substantial clinopyroxene fractionation and moderate degrees of fractionation for the bvf lavas the contradiction between petrography and compositional trends can be resolved by the following model 1 crystallization and separation of plagioclase clinopyroxene and olivine at approximately 5 10 kb and 2 rapid magma ascent and surface eruption of nearly aphyric lavas near surface crystallization involves only plagioclase and olivine du

preservation of contrasting geothermal gradients across the caribbean north america plate boundary motagua fault guatemala 1 strike slip plate boundaries juxtapose crustal blocks that may have different geodynamic origins and therefore different thermal structures thermo kinematic modeling of this type of strike slip plate boundary predicts an asymmetric signature in the low temperature thermochronologic record across the fault age elevation profiles of zircon u th he ages across the motagua fault a 500 km long segment of the transform boundary between the north american and caribbean plates document a sharp cooling age discontinuity across the fault this discontinuity could be interpreted as a difference in denudation history on each side of the fault however a low relief miocene erosional surface extends across the fault this surface has been uplifted and incised and provides a geomorphic argument against differential denudation across the fault by integrating magmatic volcanic and heat flow data age elevation profiles and thermo kinematic modeling we propose that large horizontal displacement along the motagua fault has offset a persistent geothermal asymmetry across the fault and explains both the age discontinuities and the age elevation patterns this study illustrates how thermochronology can be used to detect large strike slip displacements and more generally opens new perspectives in understanding the impact of nonuniform thermal structures on thermochronologic results

structure of the earth mantle and core the deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years most studies have emphasized quantitative three dimensional mapping of the lateral velocity heterogeneity of the mantle and core these aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes a first generation of global models for the lateral velocity variations in the deep earth has been produced providing tantalizing images of large scale structures suggestive of a non steady state thermal convection system the upper 400 km of the mantle has the strongest lateral velocity variations of up to 10 for shear velocity surface wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations thus the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics the upper mantle models support the contention that continents have deep roots with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km very long wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km as well as throughout the lower mantle these deep seated variations have little correspondence to surface tectonics and efforts to interpret their nature are just beginning the lowermost 200 km of the mantle d region has lateral velocity fluctuations comparable to those in the upper mantle and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the d layer a combined thermal and compositional boundary layer roughly mirroring the lithosphere is a likely explanation for this anomalous zone the core mantle boundary appears to have significant 10 km long wavelength topography presumably sustained by dynamic stresses from deep mantle convection the inner core may have strong lateral heterogeneity or axially symmetric anisotropy suggesting a complex thermal and compositional state significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short period body wave band models for teleseismic p wave attenuation operators have converged with t values of 0 7 1 0 s appropriate at 1 hz and t values of 0 4 0 6 s appropriate at 4 hz regional variations of attenuation are slowly being mapped out as well

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

multiple scs travel times and attenuation beneath mexico and central america multiple scs phases recorded for the june 19 1982 e1 salvador earthquake m s 7 0 are analyzed to determine whole mantle attenuation and upper mantle travel time anomalies beneath mexico central america and northern south america recordings from the ultra long period seismographs at pasadena and berkeley and the sro station anmo provide eight scs n arrivals with 15 bouncepoints concentrated within mexico the whole mantle qscs for these paths is estimated as 145 n 27 in the frequency range 8 to 40 mhz and the average scsn scs n jb residual atsc s at a period of 50 sec is 5 sec eight scs n phases of similar periods recorded at gdsn stations in south america yield a qscs estimate of 232 n 89 and atsc s 3 9 sec for paths beneath central america and cs 93 n 17 and atsc s 0 3 sec for paths beneath northern south america these results indicate that the variations in travel times and attenuation beneath continental regions are as pronounced as differences reported between old oceanic and stable shield regimes