crustal velocity structure of the omineca belt southeastern canadian cordillera travel time inversion and amplitude modeling of a 350 km lithoprobe seismic refraction wide angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the omineca belt of the canadian cordillera the upper crust to 12 18 km depth has velocities from 5 6 to 6 2 km s 1 and two shear zones the monashee decollement and gwillim creek shear zone are imaged by the wide angle reflections and velocity trends minor velocity differences on either side of the monashee decollement may be related to separate rock origins prominent reflections define the boundaries of a low velocity midcrustal layer from 10 15 km to 20 25 km depth with velocities less than 6 1 km s 1 the low velocities of the midcrust associated with high electrical conductivities and high heat flow may be considered as support for the hypothesis of fluids in the cordilleran crust though other possibilities such as the effect of high temperatures on rock velocities are possible in the lower crust velocities range from 6 4 6 5 km s 1 at the top of the lower crust to 6 6 6 8 km s 1 at its base the moho is very clearly defined by the refraction wide angle reflection data and has a gentle southerly dip crustal thicknesses are 35 37 km a thin crust mantle transition zone of 1 2 km thickness in which velocities vary between 7 6 and 7 7 km s 1 is consistent with coincident reflection data upper mantle velocities range from 7 9 to 8 1 km s 1 with indications from the data of upper mantle layering in comparison with neighboring regions the omineca belt has an anomalously thin crust low crustal velocities and a low velocity upper mantle similar only to the basin and range province the velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method the characteristics of a thin crust and lithosphere along with low velocities from midcrust to mantle suggests that both the basin and range and the southern canadian cordillera are currently being heated from a source within the mantle

new moho map for onshore southern norway a recent seismic refraction study across southern norway has revealed that the up to 2469 m high southern scandes mountains are not isostatically compensated by a thick crust rather the moho depths are close to average for continental crust with elevations of similar to 1 km evidence from new seismic data indicate that beneath the highest topography moho depths are around 38 40 km these measurements are similar to 2 km deeper than early estimates interpolated from coarsely spaced refraction profiles but up to 3 km shallower than receiver function estimates for the area moho depth variation beneath the mountains roughly correlates with changes in surface topography indicating that topography is at least to the first order controlled by crustal thickness however the highest mountains do not overlie the thickest crust and additional support for topography for example from flexural strength in the lithosphere low densities in the upper mantle or mantle dynamics is likely the relationship between topography and moho depth breaks down for the oslo graben and the fennoscandian shield to the east and north high density lower crustal rocks below oslo graben and increasing crust and lithospheric thicknesses below the fennoscandian shield may produce a negative correlation between topography and moho depth

evolution of southern caribbean plate boundary vicinity of trinidad and tobago two processes oblique collision of an island arc and strike slip motion parallel to the south american coast have been suggested as dominant in the tectonic evolution of the southeastern corner of the caribbean field work in the el pilar fault zone of trinidad and interpretation of offshore seismic data lead us to conclude that the dominant process was strike slip motion on five or more major fault systems within a 250 km wide east west trending plate boundary zone extending north to south from grenada to the mouth of the orinoco river during the last 30 m y movements in this fault zone 1 diverted the orinoco river eastward 2 juxtaposed blocks with diverse stratigraphic characteristics and 3 generated secondary extensional compressional and strike slip str ctures including anticlines of northeast to north northeast trend of which the tobago arch the central range of trinidad and the deformed mud diapirs off the southeastern coast of trinidad are the most prominent

seismic refraction measurements within the peninsular terrane south central alaska we present an interpretation of crustal seismic refraction data from the peninsular terrane one of the many exotic terranes that have been accreted to the continental margin of southern alaska in the past 200 m y a seismic refraction line was collected along the glenn highway in the copper river basin of south central alaska in 1984 and 1985 as part of the u s geological survey trans alaska crustal transect tact program p wave velocities of 2 7 3 5 km s and thicknesses of 1 2 km characterize post lower jurassic sedimentary rocks that underlie most of the seismic refraction line an average crustal velocity structure includes the following five velocity divisions beneath the sedimentary rocks lie 1 2 km of 4 0 4 6 km s materials correlating with andesitic volcaniclastic sedimentary rocks and lava flows of the lower jurassic talkeetna formation below these rocks seismic velocity increases rapidly from 5 0 to 6 1 km s in 2 3 km at 7 8 km depth velocity jumps to 6 3 km s and increments to 6 6 km s by 10 12 km depth velocities increase from 6 8 to 7 0 km s between 12 to 20 km depth at about 22 km depth a jump in velocity from 7 0 to 7 4 km s is inferred but is poorly resolved depth to the moho discontinuity could not be determined from our data the absence of clear pmp reflections may indicate that moho is deeper than 40 km data from two offset shotpoints northeast of the line and within the wrangellia terrane constrain the deep structure transition between peninsular and wrangellia terranes the 6 3 6 6 km s material thickens to the northeast toward the suture between peninsular and wrangellia terranes but southwest of its mapped trace at the west fork fault peninsular terrane crustal structure appears dissimilar to that of continental interiors it is similar to velocity structures determined for accreted island arc fragments in california such as the basement of the great valley and the klamath mountains

the siquisique ophiolites northern lara state venezuela a discussion on their middle jurassic ammonites and tectonic implications ammonites from the pillow basalt associations of the siquisique ophiolites located in west central venezuela indicate a minimum bajocian to early bathonian age for emplacement of the complex the ammonites have been identified as stephanoceras skirroceras cf macrum quenstedt emileia ex gr multiformis gottsche et quenstedti westermann and parkinsonia sp this association of cosmopolitan pacific and tethyan elements aids in the location of the western end of the middle jurassic seaway set between the tethys and the pacific oceans hispanic corridor the apparent age of the ammonites indicates that some of the siquisique sediments pillow basalts and serpentines are the oceanic remnants of an arrested early phase of rifting between north and south america

lateral velocity variations in southern california i results for the upper crust from pg waves the plate boundary and major crustal blocks in southern california are imaged by a tomographic backprojection of the pg first arrivals recorded by the southern california array the method formulated specifically for local earthquake arrival times is a fast iterative alternative to direct least squares techniques with it we solve the combined problem of determining refractor velocity perturbations and source and station delays resolution and variance are found empirically by using synthetic examples a map showing lateral velocity variations at a depth of approximately 10 km is presented the results show a strong correlation with surface tectonic features clear velocity contrasts exist across the san andreas the san jacinto and the garlock faults the mojave region has the slowest velocities while the peninsula ranges have the highest the san jacinto block has velocities intermediate between mojave and peninsula range velocities and also has early station delays this may indicate that the san jacinto block has overridden mojave material on a shallow detachment surface no velocity variations are found associated with the transverse ranges which we interpret to mean that the surface batholithic rocks in this area do not extend to pg depths

seismic evidence for widespread serpentinized forearc upper mantle along the cascadia margin petrologic models suggest that dehydration and metamorphism of subducting slabs release water that serpentinizes the overlying forearc mantle to test these models we use the results of controlled source seismic surveys and earthquake tomography to map the upper mantle along the cascadia margin forearc we find anomalously low upper mantle velocities and or weak wide angle reflections from the top of the upper mantle in a narrow region along the margin compatible with recent teleseismic studies and indicative of a serpentinized upper mantle the existence of a hydrated forearc upper mantle wedge in cascadia has important geological and geophysical implications for example shearing within the upper mantle inferred from seismic reflectivity and consistent with its serpentinite rheology may occur during aseismic slow slip events on the megathrust in addition progressive dehydration of the hydrated mantle wedge south of the mendocino triple junction may enhance the effects of a slab gap during the evolution of the california margin

tomographic estimates of sub moho seismic velocities in fennoscandia and structural implications abstract pn and sn velocity estimates are derived for the sub moho structure beneath fennoscandia using a tomographic conjugate gradient scheme observational data stem from local earthquake recordings of crustal phases pg sg lg and the sub crustal phases pn and sn by the fennoscandian seismograph network unknowns are pn and sn velocities and event hypocenter parameters crustal thicknesses are assumed as known and are not estimated whereas velocities are allowed to vary the most prominent p and s velocity anomalies are found in the northwest inland lofoten and southwest more of norway and for shear waves also in central finland there is a good correlation between the velocity anomalies in norway and observed negative bouguer anomalies indicating the presence of low density sub moho material in these areas geologically the pn and sn velocity anomalies appear to be associated with geodynamic processes tied to the opening of the norwegian greenland sea

geophysical constraints on washington convergent margin structure gravity and magnetic maps of western washington reveal the lateral structure and fabric of the washington coast range puget basin and southern washington cascade range the magnetic and gravity maps show large amplitude positive anomalies associated with the shallow but largely buried section of washington coast range mafic rocks which are separated by negative anomalies over deep sedimentary basins the positive anomalies indicate that the coast range mafic basement extends farther east than previously thought at least as far east as the longitude of seattle linear and steep gravity and magnetic gradients indicate many unmapped often buried faults in the washington coast range province magnetic highs are also associated with mapped batholiths in the cascade arc several magnetic highs observed east of the coast range rocks and west of these batholiths may be associated with buried tertiary plutons or ophiolites two dimensional gravity and magnetic modeling constrained with geological and other geophysical data indicate that the coast range province rocks are about 1 km thick at the coast thickening to as much as 30 km near their postulated eastern edge a maximum boundary on the average density of the upper 15 20 km of the rocks that compose the coast range province of 2920 kg m3 was established by the modeling suggesting a composition largely of basalt and gabbro with little interbedded sediments under these rocks may be mantle or a subduction complex composed of dense mafic ultramafic and sedimentary rocks like that proposed to underlie vancouver island previous gravity models of the washington margin include lower densities for the proposed subduction complex than for vancouver island suggesting a lower component of mafic and ultramafic rocks than the rocks underlying vancouver island however my washington model requires that the proposed subduction complex be more dense than the trench sediments and therefore that material denser than sediments be incorporated within it the absence of continental mantle and the modeled wedge shape of the coast range province upper crust suggest that erosion of the bottom of the overriding plate by subduction processes may have occurred

scandinavia s north atlantic passive margin 1 the tectonics of the eastern passive margin of the north atlantic are reexamined the scandinavian north atlantic passive margin includes not only the offshore exploration and basin domain but also large portions of the onshore domains of the scandinavian caledonides combined information from structural geology potential field data regional geology basin development and geomorphology made it possible to propose a new definition of the passive margin the rift shoulder is formed by linked faults defining the present rift flank these faults are the extensional structures located furthest to the east onshore norway sweden that can be linked to the rifting that led to the formation of the north atlantic this innermost boundary fault system ibf is formed by a set of normal west dipping crustal faults it extends over a distance exceeding 2000 km from the north sea across the caledonian mountain belt to the barents sea the passive margin width between the continent ocean boundary and the ibf ranges from 550 km in the south to over 700 km in mid norway to 165 km north of lofoten rifting and faulting on the ibf started in permo carboniferous and a succession of rift phases eventually culminated in continental breakup and the formation of the north atlantic basin development between permian and cretaceous was toward the future breakaway fault but faulting was also active on land from mesozoic to present onshore offshore crustal scale cross sections show the geometry of the passive margin the dip orientation of the major faults and the changes in crustal thickness

a seismic refraction study of the oregon cascades a 275 km long reversed refraction profile in the oregon cascades two shallow earthquakes of magnitude 5 in southern washington a shallow earthquake of magnitude 4 6 in northern california and a previously published analysis of the bouguer gravity field are used to develop a crustal p wave velocity model for the oregon cascades travel time analysis of the refraction profile indicates a crustal structure characterized by surface layers with p wave velocities that vary from 2 9 to 5 2 km s and thicknesses that vary from 2 5 to 5 0 km upper crustal velocities of 6 1 to 6 5 km s between the depths of 3 and 29 km lower crustal velocities near 7 0 km s between the depths of 29 and 44 km and a mantle reflector at a depth of 44 km comparison with synthetic seismograms supports this structure and shows that the lower crustal and mono transitions can be better modeled by continuous velocity gradients than by first order discontinuities arrival times from two shallow earthquakes in southern washington in 1981 across 14 telemetry stations in the oregon cascades show an apparent velocity of 7 62 km s on the basis of a previously published gravity analysis the mean north south component of the dip to the crust mantle boundary is estimated at 1 down dip to the south the apparent velocity from the earthquakes and the dip estimate from gravity indicate that the true pn velocity is 7 70 km s arrival times from a shallow earthquake in northern california in 1978 across telemetry stations in the washington cascades are consistent with this model and indicate that the upper mantle velocity reaches 8 20 km s at a depth of approximately 100 km

crustal structure beneath the high lava plains of eastern oregon and surrounding regions from receiver function analysis 1 we analyze teleseismic p to s receiver functions to image crustal structure beneath the high lava plains hlp of eastern oregon and surrounding regions coverage from 206 broadband seismic stations provides the first opportunity to resolve variations in crustal composition thickness and heterogeneity on scales of a few km in depth and tens of km laterally across the hlp region we utilize both h stacking and a new gaussian weighted common conversion point stacking technique we find crust that is 40 km thick beneath the cascades idaho batholith and owyhee plateau and thinner 31 km crust beneath the hlp and northern great basin low poisson s ratios of 0 240 characterize the granitic crust beneath the idaho batholith while the owyhee plateau exhibits values of 0 270 typical of average continental crust the owyhee plateau is a thick simple crustal block with distinct edges at depth the western hlp exhibits high average values of 0 304 typical for regions of widespread basaltic volcanism combined with other geological and geophysical observations the areas of abnormally high poisson s ratios 0 320 and low velocity zones in the crust beneath north central and southern oregon are consistent with the presence of partial melt on either side of the hlp trend suggesting a central zone where crustal melts have drained to the surface perhaps enabled by the brothers fault zone thicker crust and an anomalous n s band of low poisson s ratios 0 252 skirting the steens mountain escarpment is consistent with residuum from a midcrustal magma source of the massive flood basalts supporting the view of extensive mafic underplating and intraplating of the crust from cenozoic volcanism

the upper mantle beneath the north east pacific rim a comparison with the gulf of california summary seismograms from 22 earthquakes along the north east pacific rim recorded in southern california form the data set for investigation of the upper mantle beneath the cascade range juan de fuca region a transitional area encompassing both very young ocean floor and a continental margin these data consist of 853 seismograms 6 a 42 which produce 1068 travel times and 40 ray parameter estimates we compare these data directly to another large suite of records representative of structure beneath the gulf of california an active spreading centre we use the spreading centre model gca as a starting point in wkbj synthetic seismogram modelling and perturb gca until the ne pacific data are matched application of wavefield continuation to these two groups of data provides checks on the model s con sistency with the data as well as an estimate of the resolvability of differences between the two areas the resulting ne pacific model cjf is very similar to gca disparities in travel times and waveforms from 16 to 22 however are interpretable in ierms of lateral structural variations between the two regions from 200 to 410 km depth cjf features velocities from 200 to 350 km that are inter mediate between the very low values of gca and the more moderate values of t7 a model for the inland western united states models of continental shield regions have even higher velocities in this depth range but all four model types are similar below 450 km this regular pattern observed in the rate of velocity increase with tectonic regime suggests an inverse relationship between velocity gradient and lithospheric age above 400 km depth

three dimensional p velocity structure of the crust below newberry volcano oregon utilizing teleseismic p residuals we have detected a column of high p velocity material extending from within 10 km of the surface below the summit of newberry volcano oregon to midcrustal depths near 25 km we interpret this column to be the expression of a swarm of predominantly subsolidus gabbroic sills and dikes which were intruded as the volcano was built the high p velocities observed below the volcano severely limit the size of magma chambers which could presently exist in the crust below newberry volcano those possible include a few percent of partial melt distributed through large volumes of a mafic intrusion zone in the midcrust a few smaller higher melt fraction zones in the midcrust with dimensions less than 6 km and whose aggregate volume is only a few percent of enclosing volumes of 200 km sup 3 small magma bodies with dimensions of a few kilometers located within the upper 10 km of the crust or a mafic crystal rich magma of arbitrary dimensions located in the upper few km the low p velocities detected in the upper 4 km beneath the center of the summit caldera may be partially caused by a magma chamber in the second ofmore these catagories less

on upper mantle seismic heterogeneities beneath fennoscandia abstract three dimensional seismic mapping of the upper mantle beneath fennoscandia baltic shield using an ach type of inversion technique in combination with p wave travel time residual observations from the local seismograph network gave the following results the central parts of the baltic shield are characterized by relatively high seismic velocities down to approximately 300 km those parts of the shield most affected by the caledonide orogeny exhibit relatively low velocities particularly in the uppermost 100 km depth interval the lower part of the upper mantle 300 600 km does not exhibit pronounced seismic velocity anomalies and in this respect is in contrast to results from similar studies in regions subjected to neotectonic processes like parts of central and southeastern europe the seismic anomaly pattern in the presumed thickened lithosphere is in quantitative agreement with similar ones derived from surface wave dispersion analysis and inversion of electrical measurements the general orientation of these anomalies coincides with that of the glacial uplift

petrochemistry and tectonic significance of cretaceous island arc rocks cordillera oriental dominican republic abstract cretaceous island arc rocks of the caribbean island arc system have been exposed by cenozoic faulting in the cordillera oriental in eastern hispaniola high k 2 o intermediate to felsic volcanic rocks loma la vega volcanics are interbedded with marine epiclastic sedimentary rocks and tuffs las guajabas tuffs and unconformably overlie pre aptian sedimentary rocks low k 2 o volcanic rocks guamira volcanics and a granodioritic to tonalitic intrusion el valle pluton the petrology and geochemistry of these units in conjunction with regional stratigraphic data are used to speculate on the tectonics of the newly developing caribbean island arc system during early and late cretaceous time the loma la vega volcanics are characterized by the presence of large phenocrysts of sanidine and minor amounts of clinopyroxene opaque oxides and rare leucite in a devitrified matrix of chlorite and clay although the volcanic rocks have undergone some low temperature alteration metamorphism which redistributed some major elements and large ion litho phile trace elements the high field strength elements rare earth elements and radiogenic isotopes appear to have been minimally affected based on abundances of the relatively immobile elements trace element enrichment patterns and isotopic compositions the loma la vega volcanics are considered part of the high k calc alkaline ca or shoshonitic island arc volcanic series in contrast pre aptian early cretaceous volcanic and plutonic rocks of the underlying los ranchos formation have chemical characteristics similar to rocks in the island arc tholeiitic or primitive island arc pia series that form coeval and along strike sections of the early cretaceous caribbean island arc in other parts of present day hispaniola cuba puerto rico and the virgin islands an abrupt and regional change in composition from island arc tholeiites to high k calc alkaline rocks is coincident with a hypothesized reversal in subduction polarity in pre aptian time as inferred from previously published tectonic models polarity reversal may have been triggered by attempted subduction of the caribbean oceanic plateau beneath this segment of the caribbean island arc the observed magmatic and tectonic effects of the proposed cretaceous caribbean arc reversal are similar to the better documented neogene subduction reversal event in the solomon islands arc in the southwest pacific

seismic refraction study of upper crustal structure in the lassen peak area northern california in september 1982 the u s geological survey recorded a 125 km long refraction profile through the lassen peak area of the cascade range this profile extended northwestward to the klamath mountains and southeastward to the sierra nevada foothills interpretation of the refraction data determined the upper crustal structure in this volcanic region of northern california near surface velocities 1 5 5 55 km s are consistent with a thickness of 2 4 km for the cascade volcanic rocks between 0 5 km above and 1 8 km below sea level velocities are about 4 7 5 55 km s the highest velocities are modeled below the devils kitchen terminal geyser area possibly indicating hydrothermal alteration of the volcanic rocks the absence of low velocities less than about 3 4 5 km s deeper than 1 km above sea level suggests that the cretaceous sedimentary rocks exposed west of lassen peak do not underlie the volcano southeast of lassen peak at 1 8 km below sea level a 0 3 km thick layer with a velocity of 6 1 km s overlies a lower velocity 5 7 5 8 km s 2 9 km thick layer because these modeled velocities do not change across the boundary between the sierra nevada foothills and the cascade range sierran basement could continue under the cascade volcanic rocks as far northwest as lassen peak the velocity model does not indicate the presence of a magma chamber larger than 3 km in diameter within the top 8 km of the crust

the deep structure of the scandes and its relation to tectonic history and present day topography abstract we review the results of the toposcandiadeep project a component of the topoeurope project in which we have studied the crustal and upper mantle structures of southern norway in relation to its high topography the scandinavian mountain chain the scandes is an intracontinental mountain chain at the western edge of the baltic shield and has its southern part located in southern norway the timing as well as the processes causing the formation of the scandes are disputed we bring new geophysical constraints to this issue by providing crustal and mantle seismic models for the area and by integrated modeling of the lithosphere and its potential deformation new maps of moho depth and crustal seismic velocities have been compiled using data from refraction lines p receiver functions and noise cross correlation these results show a thickening of the crust from southwest to northeast and a small crustal root not directly located below the topographic high p s and surface wave tomography infer seismic mantle velocities lower than in normal shield structure with a possible sharp boundary close to the oslo graben these low velocities are imaged in the lithosphere and in the underlying mantle down to the 410 km discontinuity integrated modeling of seismic models and gravity data shows that the low velocities below southern norway are compatible with a change in lithosphere thickness from c 100 km under southern norway to nearly 200 km under southern sweden with possible additional differences in composition the study also indicates that the topography can be isostatically sustained by the density distribution in the crust and lithospheric mantle we argue that the lithospheric lateral variation has been present for at least 300 my and has had a significant influence on the localization of the topography independently of the mechanism for uplift

heat production in the continental crust part i data converted from seismic velocities and their attempted interpretation abstract when using the simple exponential distribution of radiogenic heat production a with depth z a z a o exp z d the statistical analysis of a large number of heat production profiles converted from the observed seismic velocity vs depth profiles and the experimental knowledge of the relationship between seismic velocity and heat production disclosed certain relations between the logarithmic decrement d value and heat flow tectonic age and crustal thickness the d valuein this respect characterizes the depth rate of the differentiation of the heat production in all tectonic units the d parameter decreases with depth as the next step the more complicated distribution law a z a o exp z d a was applied where a characterizes the quantity of removed heat sources by the processes accompanying the crustal evolution such as remelting metamorphosis leaching by underground waters etc all parameters a o a and d are constant within each particular tectonic province and parameter d does not depend on depth contrary to the former law the amount of removed heat sources a reflects the initial concentration of heat sources and is higher in young tectonic terranes in areas of high heat flow and in regions of thin crust

tomography of crustal p and s travel times across the western united states 1 data from the usarray combined with local regional and national networks provide an unprecedented opportunity for imaging the crust of the western united states we employ a simple tomography scheme to invert pg and sg travel times for velocity structure the approach is analogous to pn tomography we assume a great circle arc between source and receiver and solve for station and event terms along with 2 d velocity structure pg velocities are high in the snake river plain western washington and the columbia plateau and in central california the colorado plateau falls midrange while the basin and range is slow as are the geysers region north of the san francisco bay and the salton trough for sg the snake river plain is fast as is the entire cordillera from the southern sierra nevada up through the cascades both the colorado plateau and most of the rocky mountains are also fast at the highest point of the rocky mountains the aspen anomaly is a low velocity region the basin and range is a composite of slower to the west abutting the sierra nevada and faster in eastern nevada coastal california is slow with the exception of the san francisco bay region and far northern california the continental crust of the north american craton appears slightly fast for both pg and sg our observations and their trends compare well with middle to upper crustal velocity estimates from previous studies they also compare reasonably well with average crustal velocities from the same studies

crustal thickness map of the western united states by partitioned waveform inversion the partitioned waveform inversion method nolet 1990 was used to obtain a new crustal thickness map of the western united states we fitted 200 seismograms containing higher and fundamental mode rayleigh waves for events and stations in the western united states which provided average crustal thickness constraints between each source receiver pair all these constraints were combined in a linear inversion to obtain a crustal thickness map local crustal thickness estimates from published seismic reflection refraction work were used as additional constraints to overcome deficiencies of path coverage in some areas the resulting crustal thickness map shows thicker 45 km crust beneath the cascades thick 40 km crust beneath the sierra nevada thin 30 km crust beneath the basin and range and thick greater than 45 km crust beneath the colorado plateau the boundary between the colorado plateau and the southern leg of the basin and range is a prominent feature of the model thick crust is seen at the bend of the san andreas fault near los angeles our model is broadly consistent with the crustal thickness map of mooney and weaver 1989 a quantitative comparison with the crustal thickness model 3smac nataf and ricard 1996 was performed by inverting for a model that deviates least from it results show that we are able to retain most of the features of that model while maintaining a good fit to the surface wave and point constraint data

tobago west indies a fragment of a mesozoic oceanic island arc petrochemical evidence the pre cenozoic rocks of tobago west indies represent an exceptional cross section through a portion of a composite mesozoic arc system other parts of which may be found in the villa de cura klippe the aruba blanquilla island chain and perhaps in the western cordillera of colombia and ecuador the geochemical data from tobago presented in this paper provide a basis from which a regional view of the petrologic character of the accreted mesozoic arc of the southern caribbean mountain system can be deduced the mesozoic rocks of tobago can be divided into four main groups the north coast schist the ultramafic to mafic plutonic complex the tobago volcanic group and late mafic dykes rocks of the north coast schist served as wallrock for the plutonic volcanic dyke complex which evolved during the mid cretaceous geochemical and nd and sr isotopic data presented demonstrate the oceanic island arc character of all the pre cenozoic rocks of the island all suites span the calc alkaline and tholeiitic regions on discrimination diagrams rare earth element ree patterns for the north coast schist tobago volcanic group and plutonic suite are essentially identical and are moderately lree enriched la yb 1 1 2 7 the dykes exhibit the most unevolved ree patterns and are interpreted as most closely representative of parental liquid compositions initial nd 6 6 to 9 4 and 87 sr 86 sr 0 70303 0 70364 for all samples except north coast schist and tobago volcanic group argillites are similar in value and overall range to other oceanic island arcs limited isotopic data from argillite samples from tobago suggest an important change in the continental derived sediment contribution during mesozoic arc growth nd isotopic data from north coast schist siliceous argillite indicate virtually no continental detritus in this neocomian sediment while nd isotopic data from the tobago volcanic group argillite suggest a definite continental contribution during albian arc growth

seismicity of mount hood and structure as determined from teleseismic p wave delay studies a 16 station seismic network was established in the mount hood area of oregon in 1977 as part of a multidisciplinary study to evaluate the geothermal potential of a typical cascade volcano the immediate objective was to monitor local seismicity and to study the p wave velocity structure of the crust and upper mantle during the 13 months the network was in operation 10 local earthquakes were recorded all these events are spatially associated with the volcano and occurred at shallow depths roughly defining a zone striking north northwest the largest earthquake a magnitude 3 4 event was preceded by a single foreshock and followed by three locatable aftershocks these and the five other located earthquakes were of magnitude less than 2 0 fifty five teleseisms were recorded by the network and used in a teleseismic p wave delay study the residuals from the p delay study indicate that there is no significant velocity anomaly associated with the mount hood volcano and only small velocity variations in the mount hood region

upper crustal structure of the mount hood oregon region as revealed by time term analysis seismic refraction data with a dense areal distribution were collected to study the seismic structure of mount hood and the surrounding region this area is typical of cascade volcanoes and is geologically quite complex the prime goals of this project were to search for velocity variations in the upper crustal rocks and to determine if the velocity of these rocks is anisotropic a new system including 100 remote recording units was developed to facilitate the collection of data in this type of survey the data collected in this study reveal a large variation in velocity and thickness of the uppermost crustal rocks that is probably typical of the high cascade province a regional structural pattern surrounding mount hood where there is a marked thinning of low velocity near surface rocks suggests that the present edifice of mount hood lies on top of a much larger structure possibly the roof of a large batholith that was emplaced prior to the eruption of the volcanic rocks that form the modern mountain a straightforward time term analysis of the data reveals this structure when the time terms are used to correct the travel time observations for the variation in structure under each station the remaining set of residuals indicates a variation of travel time with azimuth while a systematic error in the time term reduction cannot be ruled out this variation is probably the result of anisotropic velocity structure in the underlying refractor the alignment of the direction of maximum velocity with the direction of maximum principal stress and the decrease in anisotropy with depth suggest that the anisotropic velocity structure is related to the opening and closing of fractures in the present stress field the direction of maximum velocity is about n25 w and the degree of anisotropy appears to vary with depth from about 3 near the surface to zero at depths below 8 km

cenozoic plate tectonic history of the northern venezuela trinidad area geological and geophysical data coupled with recent plate tectonic reconstructions suggest that the cenozoic geologic history of the northern venezuela trinidad area has been dominated by strike slip displacement of discrete crustal blocks allochthonous terranes within the area include metavolcanic rocks of the cretaceous villa de cura group and metamorphic rocks of the precambrian to cretaceous cordillera de la costa a relatively competent crustal block margarita block is defined by an outline around the metamorphic basement of margarita island the araya paria peninsula the northern range of trinidad and tobago island reconstruction of the margarita block to its original position requires at least partial closure of the falcon basin closure of the bonaire and cariaco basins and restoration of about 50 km of motion on both the oca and bocono faults post middle eocene eastward translation of the caribbean plate caused eastward motion of the margarita block a minor change in relative plate motion during the late oligocene or early miocene produced a right step in the moron fault forming the cariaco pull apart basin and el pilar fault zone maximum offset on el pilar fault is estimated to be no more than 125 km though displacement along the entire fault zone may have been greater transpressional stresses between the caribbean plate and northern south america caused folding of the serrania del interior of venezuela and the central range of trinidad eastward migration of transpressional stresses at the southeastern corner of the caribbean south american plate boundary is being accommodated by formation of oblique thrusts transpressive anticlines and downwarping of the crust bouguer gravity data suggest that jurassic aged atlantic oceanic crust is being depressed as the caribbean plate expands into the demerara plateau area this study suggests that the faults and transtensional transpressional compressional structures identified in this study are the result of stresses produced during the large eastward translation of the caribbean plate since the paleocene and are not the product of a shear couple

upper mantle heterogeneities beneath central europe summary an inversion of isc travel time data from selected earthquakes in the distance range 30 90 to 53 stations in central europe has been used to model velocity down to 600 km depth the model explains 0 1 0 2s of the residuals as for other array studies leaving 0 5 s unexplained as noise the uppermost 100 km of the mantle and crust contains inhomogeneities that correlate remarkably well with the geology this may be due to deep seated thermal anomalies or in some areas to delays introduced by passage of the rays through sedimentary cover the deeper anomalies are smaller and unrelated to those in the lithosphere which suggests that the asthenosphere is decoupled from the rigid lithosphere the structure at 600 km depth is again quite inhomogeneous and might be due to undulations of the 650 km discontinuity the models show some suggestion of a high velocity slab trending from east to west beneath the alps

event location at any distance using seismic data from a single three component station this paper demonstrates the usefulness of a novel technique for analysis of single site three component seismogram records for reliable epicenter determinations for even small seismic events at any distance parameters that may be extracted by this technique are p phase identifications in terms of x 2 probabilities and the associated slowness vectors using data from short period and broadband seismometers installed in the new noress norway array single site event locations are demonstrated azimuth estimates are found directly from the slowness vector while epicenter distance estimates are derived from differential arrival times between pn pg sn and lg sg simultaneous inversion for crustal model and hypocenter data indicate a crustal thickness beneath noress of 33 5 km if sn and or lg are well recorded focal depth estimates are often accurate to within a few kilometers locating teleseisms is problematic since secondary phases are not fully readily identifiable and hence distance can be estimated only by conversion of the measured apparent phase velocity for distances out to approximately 1000 km event location errors as compared to network solutions seldom exceed 50 km and are due mainly to errors in azimuth estimates with sn phases available focal depth errors appear to be of the order of 4 km for teleseismic p waves short period location errors occasionally exceeded 7 and were due mainly to poor distance estimates using broadband records from relatively strong events location errors were about 1 further refinements seem feasible by using seismicity information and or recognition of stationarity in the p wave decomposition pattern for events within the same region at least in certain cases such approaches reflect genuine location refinements as demonstrated

tomography of the western united states from regional arrival times first arrival times from regional distances 200 1200 km in the western united states extracted from the international seismological centre isc data set were inverted in a tomographic study to map the laterally varying pn velocity structure of the uppermost mantle i e the mantle lid and to estimate the crustal static delay at each seismograph station synthetic data were used to evaluate resolution results correlate well with major tectonic features we find low uppermost mantle velocities v 7 9 km s these low apparent pn velocities are primarily due to the effects of isostasy hotspot volcanism and crustal extension we interpret the low apparent pn velocities beneath the sierra nevada 7 6 7 7 km s to be the result of time delays introduced by ray paths tunneling through the deep crustal root the yellowstone hotspot migrating along the snake river plain has left low velocity 7 6 7 8 km s hot upper mantle in its wake lower pn velocities 7 8 km s beneath the extensional basin and range are also presumably due to hotter uppermost mantle based on station static delays for the basin and range we infer that moho depths there do not vary significantly from 30 km normal continental pn velocities 8 0 km s as well as thicker crust 40 50 km underlying the rocky mountains wyoming basin province colorado plateau and the northern columbia plateau indicate the presence of relatively cool uppermost mantle beneath these regions

an introduction to the european geotraverse project first results and present plans abstract the european geotraverse egt is an international multidisciplinary project focused on a north south orientated lithospheric profile 4000 km long and of varying width extending from northernmost scandinavia to north africa this profile consists of three interlinking segments northern central and southern comprising a continuous succession of tectonic provinces ranging from the oldest precambrian areas of the baltic shield to the currently active area of the western mediterranean the broad aim of the egt project is to obtain a better three dimensional picture of the structure state and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics all of the 12 major projects that constitute the egt joint programme have now been initiated and several of these projects are nearing completion along the northern segment data from the fennoscandian long range project fennolora a 2000 km long seismic profile across the precambrian baltic shield show that except beneath southern sweden the shield is characterized by a high velocity 40 50 km thick crust including a 5 10 km thick crust mantle transition zone an alternating series of 4 6 high and low velocity zones is present in the subcrustal lithosphere the base of which increases in depth from ca 110 km to ca 230 km from south to north beneath the shield the top of the mantle transition zone lies at a depth of about 450 km the second major project along this segment the eugeno s european geotraverse northern segment southern part project is a multidisciplinary study of the fennoscandian border zone and was largely completed in 1984 with the realization of a large scale seismic experiment preliminary interpretation of the excellent data obtained indicate the presence of strong lateral variations in internal crustal structure beneath the danish basin field work for a third major project a multidisciplinary transect of the archaean and early proterozoic terrains in the northernmost part of the shield the polar profile was carried out in 1985 a series of deep seismic reflection lines has so far been realized in the area of the central segment in the context of german national programmes first interpretations of the seismic data from a 260 km long profile across the two main intra variscan hercynian lineaments have shown the presence of numerous horizons making up a highly reflective zone in the lowermost 10 km of the crustal section studied and distinct changes in reflectivity between the main variscan tectonic zones in 1986 the entire segment will be investigated in detail in an ambitious international programme of integrated geological and geophysical studies a series of seismic experiments termed egt s have been carried out across the southern segment in 1982 1983 and 1985 interpretation of data from the 1982 and 1983 experiments have led to several interesting results including 1 1 the suggestion that two crust mantle like interfaces exist beneath the po basin at depths of about 35 and 50 km and adjacent tectonic units these interfaces marking a deep contact zone between the adriatic and european plates 2 2 in the area between genoa and corsica the ligurian sea is underlain by a greatly thinned distinctly layered section of continental crust and 3 3 corsica and sardinia are underlain by bowl shaped typically variscan continental crusts the 1985 phase of seismic surveys focused on crustal structure beneath tunisia and the adjacent seas in addition two off traverse projects are being realized first a wide aperture network of autonomously recording seismic stations nars installed along the line gothenburg malaga between 1982 and 1984 is already yielding high quality data on the upper 600 700 km of the mantle second an investigation of lithospheric seismic anisotropy in the area of the iberian peninsula is being organized for 1987 1988 finally of great importance are the systematic compilation of existing data and where needed in critical regions collection of new geophysical and geological data presently being carried out for the entire area encompassed by the egt it is expected that these compilations will be completed by 1987 at about the same time that full results from the main large scale seismic experiments become available enabling the construction of an integrated lithospheric cross section along the egt requiring a final phase of intensive multidisciplinary collaboration

crustal structure along the west flank of the cascades western washington knowledge of the crustal structure of the washington cascades and adjacent puget lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region we present a model for crustal velocity structure derived from analysis of seismic refraction wide angle reflection data collected in 1991 in western washington the 280 km long north south transect skirts the west flank of the cascades as it crosses three tectonic provinces including the northwest cascades thrust system nwcs the puget lowland and the volcanic arc of the southern cascades within the nwcs upper crustal velocities range from 4 2 to 5 7 km s 1 and are consistent with the presence of a diverse suite of mesozoic and paleozoic metasediments and metavolcanics in the upper 2 3 km of the puget lowland velocities drop to 1 7 3 5 km s 1 and reflect the occurrence of oligocene to recent sediments within the basin in the southern washington cascades upper crustal velocities range from 4 0 to 5 5 km s 1 and are consistent with a large volume of tertiary sediments and volcanics a sharp change in velocity gradient at 5 10 km marks the division between the upper and middle crust from approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from 6 0 to 7 2 km s 1 these high velocities do not support the presence of marine sedimentary rocks at depths of 10 20 km beneath the cascades as previously proposed on the basis of magnetotelluric data crustal thickness ranges from 42 to 47 km along the profile the lowermost crust consists of a 2 to 8 km thick transitional layer with velocities of 7 3 7 4 km s 1 the upper mantle velocity appears to be an unusually low 7 6 7 8 km s 1 when compared to velocity models from other regions this model most closely resembles those found in active continental arcs distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect in the nwcs and puget lowland most of the seismicity occurs below the base of the upper crust as defined by a seismic boundary at 5 10 km depth and continues to 20 30 km depth the region of transition between the nwcs and the puget lowland appears as a gap in seismicity with notably less seismic activity north of the boundary between the two earthquakes within the cascades are generally shallower 0 20 km and are dominated by events associated with the rainier seismic zone

ammonite radiolarian assemblage tobago volcanic group tobago west indies implications for the evolution of the great arc of the caribbean an unusually fossiliferous sedimentary sequence is exposed near scarborough tobago west indies it occurs within the tobago volcanic group above an undifferentiated sequence of volcaniclastic breccia and lava and includes volcanogenic sedimentary rocks at the base of the bacolet formation an ammonite radiolarian assemblage has been recovered from this sedimentary interval in the tobago volcanic group ammonites from the spring gardens quarry are moderately evolute and have unbranched ribs they are probably juveniles of mojsisoviczia of middle albian age another ammonite from a stratigraphically higher locality i e volcanogenic sedimentary rocks at the base of the bacolet formation is identified as manuaniceras decsernae young of early late albian age radiolarian assemblages from intercalations of black siliceous argillite of the tobago volcanic group are characterized by abundant a rchaeospongoprunum spp and the common occurrence of pseudodictyomitra pentacolaensis dictyomitra montisserei thanarla brouweri and pantanellium ventosum also present in lesser abundances are other members of the family archaeodictyomitridae xitus napora durhami and sciadiocapsa speciosa this assemblage correlates with the romanus subzone of o dogherty 1994 1 and kozurium zingulai zone of pessagno 1977 2 both of which are dated as early to middle albian these newly reported fossils indicate an albian age for deposition of the tobago volcanic group earlier published 40ar 39ar radiometric ages from the group are consistent with the stratigraphic age assignment of the fossils and thereby suggest that volcanism associated with the tobago volcanic group began ca 105 ma possible correlative accreted fragments of the mesozoic oceanic arc of the southern caribbean include the curacao lava formation the lower part of the washikemba formation bonaire the tiara formation of north central venezuela and possibly the subsurface mejillones complex of the carupano basin the mesozoic rocks of tobago consist of two distinct oceanic arc rock sequences the older north coast schist 120 ma exhibits polyphase deformation and lower greenschist facies metamorphism the structurally overlying tobago volcanic group is not penetratively deformed or folded it has undergone prehnite pumpellyite facies metamorphic conditions and has been tilted to moderate dips by various brittle faults we interpret the hiatus that separates these oceanic arc rock sequences as representing a significant event in the evolution of the great arc of the caribbean consequently we propose that the commonly postulated reversal in subduction zone polarity of the great arc occurred prior to deposition of the tobago volcanic group ca 105 ma and that older more highly deformed and metamorphosed oceanic arc sequences e g north coast schist served as a basement for younger sequences or as wall rock for intrusive elements of younger magmatic suites of the great arc 1 ref 23 2 ref 26

the crustal structure of the wrangellia terrane along the east glenn highway eastern southern alaska recently acquired seismic refraction data from eastern southern alaska provide new information on the structure and composition of the wrangellia and adjacent terranes the data comprise a 160 km long refraction profile along the east glenn tok cutoff highway that was collected as part of the u s geological survey s multidisciplinary trans alaska crustal transect program the upper 3 km of the wrangellia terrane and associated rocks is characterized by low compressional wave velocities vp 1 9 3 3 4 6 5 6 km s 1 and high velocity gradients common to most onshore seismic refraction profiles there is also clear seismic expression of the west fork fault system as a steep down to the southwest fault that separates the peninsular terrane to the southwest and the metamorphic complex of gulkana river to the northeast in contrast no seismic expression occurs for the paxson lake fault which separates the wrangellia terrane from the metamorphic complex of gulkana river adjacent to the denali fault within the wrangellia terrane two high velocity bodies vp 6 6 km s 1 occur in the upper crust one of these extends to 10 km depth and correlates with a late paleozoic dioritic complex suggesting that the wrangellia terrane is at least 10 km thick in this part of alaska from 5 to 23 km depth the crust appears seismically homogeneous with velocity increasing from vp 6 2 to vp 6 6 km s 1 beneath this level the crust is less well resolved although evidence exists for a low velocity zone between 23 and 26 km and a possible southwest dipping interface at 35 km no identifiable mantle refraction or reflection is observed possibly indicating a crust as thick as 55 km the relatively low seismic velocities in the upper 23 km of the crust compare favorably with laboratory measured velocities on pelitic schists and intermediate composition plutonic rocks granites and granodiorites both of which are recognized in wrangellia we interpret the seismic velocities to indicate that silicic to intermediate composition rocks are important constituents of the basement of this part of wrangellia geologic evidence indicates that the alaskan part of the wrangellia terrane is a paleozoic and mesozoic island arc our seismic evidence indicates it may have been built mostly on continental crust as opposed to the fragment of wrangellia from vancouver island which was probably built on oceanic crust

the 14 february 1981 elk lake washington earthquake sequence a magnitude 5 5 ml earthquake occurred on 14 february 1981 near elk lake in the southern washington cascade range it was the largest earthquake to occur in washington since the 1965 seattle earthquake mb 6 5 the epicenters of 1050 located aftershocks mcoda 0 delineate a fault zone 6 km long north south by 3 km wide that extends from 5 to 12 km in depth immediately after the main shock aftershocks occurred at generally deeper depths to the north and south indicating that the break propagated bilaterally and downward first motion patterns for the main shock and 84 aftershocks mcoda 2 0 indicate predominantly right lateral strike slip faulting along a nearly vertical plane striking essentially north south in agreement with the alignment of epicenters earthquake activity at elk lake for the 7 yr period to 1980 was low nine months before the main shock an earthquake swarm began just southeast of what was to become the present aftershock zone the swarm lasted 2 months and was followed by 7 months of quiescence before two immediate foreshocks occurred on 11 february 1981 the overall pattern of seismicity observed throughout the elk lake sequence corresponds well to that predicted by an asperity model examination of the magnitude frequency relation b indicates that there was no significant temporal variation in b throughout the swarm main shock after shock sequence however b is significantly higher on the east side of the fault zone than on the west side for the entire aftershock sequence b 0 77 0 05 this earthquake sequence is important for three reasons 1 the main shock is the largest earthquake to occur in the pacific northwest since 1965 2 the sequence has been long and aftershocks are numerous and 3 its occurrence indicates that the potential seismic hazard in southwest washington must be reevaluated

sn velocities in western and eastern north america we have investigated the use of the new delay time data set of engdahl et al 1997 for the construction of tomographic images of sn velocity in north america the delays show a significant difference in average delay between paths located west and east of 105 w this difference is larger when paths longer than 10 are included indicating a positive gradient for the s velocity in eastern north america discarding outliers two separate regions in the western and eastern united states and canada where imaged low velocities were found under the colorado plateau extending westwards towards the gulf of california in the eastern u s the michigan basin stands out with low sn velocity and the grenville province is characterized by a positive anomaly

are the norwegian mountains compensated by a mantle thermal anomaly at depth article i nfo article history the norwegian mountains or scandes represent a long mountain range stretching from southern norway to the arctic and is characterised by a rugged topography and peaks up to 2 5 km high the origin of this mountain chain far away from any plate boundary remains a matter of passionate debates inside the geoscientific community hot mantle fingers originating from the iceland plume impacting the base of the scandinavian lithosphere and creating asthenospheric diapirs is one of the most accepted hypotheses for explaining cenozoic uplift in norway in order to test this hypothesis we conducted integrated gravity and thermal modelling we used the dense ngu gravity grid i e one measurement every 3 km and modelled the depth extent and the mass deficit associated to the compensating loads located below the southern scandes assuming that the density deficit below the scandes is purely thermal in origin thermal modelling allowed for testing the magnitude of the potentially associated temperature anomaly and its impact on surface heat flow recently acquired heat flow data were used in order to constrain the results from the thermal modelling the results of our integrated geophysical modelling rule out the possibility that the present day topography of the southern scandes is compensated by a deep seated asthenospheric diapir however our modelling does not exclude that thermal processes in the deep mantle could have initiated or assisted recent uplift of the southern scandes

the crustal structure of central east greenland ii from the precambrian shield to the recent mid oceanic ridges we present a 3 d crustal model of the east greenland fjord region between 69 n and 74 n the model covers the precambrian shield and the caledonian orogenic belt the adjoining devonian and mesozoic basins the continent ocean transition and the cenozoic oceanic areas as far as the kolbeinsey and the mohns mid oceanic ridges existing seismic models of the crustal structure are extrapolated into adjacent areas using 3 d gravity modelling for this purpose we compile a new regional scale bouguer anomaly map the precambrian shield west of the caledonian orogen approximately west of 32 w shows a mean thickness of 35 km with only small scale undulations this thickness is at the lower limit of the global range in shield thickness the caledonian orogen exhibits a pronounced mountain root with overall crustal thicknesses up to 51 km beside the urals the east greenland caledonides are one of the two palaeozoic mountain belts where a crustal root has preserved to the present day continuation of the crustal model to the east beyond the continent ocean transition yielded thicknesses of the crystalline oceanic crust from 9 km near the kolbeinsey ridge to 3 km west of the mohns ridge differences in the thermal structures of the old continental and the young oceanic lithosphere are responsible for the low density mantle beneath the oceanic crust which is also demonstrated by 3 d gravity modelling

p wave detection thresholds pn velocity estimates and t wave location uncertainty from oceanic hydrophones p wave arrivals recorded by the u s navy s sound surveillance system sosus hydrophone arrays were used to estimate earthquake detection thresholds and pn velocities in the northeast pacific ocean the navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary t waves however use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment the p wave detection threshold of the sosus hydrophones was quantified using the epicentral distance and magnitude of 250 northeast pacific ocean earthquakes earthquakes with body wave magnitudes as low as 2 have detectable p wave arrivals at epicentral distances of 500 km earthquakes with mb between 3 5 and 5 were detected 50 of the time at distances of 100 1500 km while events with mb 5 were all detected even out to distances of 1000 1500 km both p and t wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes the peak amplitude of the t wave coda and the onset of the p wave were used as the earthquake arrival times to estimate event locations t wave arrival time residuals have a gaussian distribution with zero mean which implies that using t wave peak amplitude is consistent with using the p wave onset as the arrival time there are typically 6 stations used to derive a t wave based location hence location error ellipses are not well constrained a monte carlo technique was employed to estimate t wave event location uncertainty t wave locations have error bars of 1 km in latitude and longitude when 3 hydrophones are used for a location estimate the detected p wave arrivals and earthquake locations were used to measure pn velocities pn velocity values of 7 9 0 1 and 8 0 0 1 km s were found for the pacific and juan de fuca plates respectively a pn velocity of 7 5 0 1 km s was measured for rays traveling northward from the mendocino triple junction along the cascadia subduction zone a pn velocity of 7 7 0 3 km s was estimated for ray paths originating onshore western north america and traveling to the offshore hydrophones

paleogeography of jurassic fragments in the caribbean jurassic rocks of the caribbean are a sampling of 100 million years of farallon plate history with fragments originating at diverse paleolatitudes and from varied tectonic settings fragments with clear paleogeographic signatures are components of the basement complexes of duarte in hispaniola bermeja in puerto rico and la desirade off guadeloupe paleolatitudinally sensitive radiolarian faunas document origination of duarte as equatorial la desirade as higher latitude and various bermeja cherts as both equatorial and higher latitude red ribbon chert of duarte and bermeja of the same age physical appearance and lithological association are probably dismembered components of the same slab of pacific crust la desirade red ribbon chert is slightly younger than the duarte and bermeja red ribbon chert and was deposited at higher latitude bermeja tuffaceous chert is also of higher latitude and probably had an arc proximal origin on the basis of modeled plate trajectories in the pacific the origin of various cherts from different paleolatitudes that end up in the same location requires different arrival times at the trench between north and south america based on radiolarian paleobiogeography plus indications of origin at a spreading ridge and ignoring the poorly constrained modeled trajectories for the late jurassic at least one of the higher latitude fragments may have originated in the southern hemisphere the accumulation of multifarious chert greenstone and other ocean floor components was accomplished by offscraping strata transported to the subduction zone along the eastern pacific margin and warehousing this material in an accretionary complex prior to entry of the caribbean plate into the gap between north and south america

thin superficial layer and lateral heterogeneities in southern sweden using short period rayleigh wave dispersion summary using short period rayleigh wave dispersion data recorded along refraction lines the shear velocity background structure of the uppermost part of the crystalline in the southern baltic shield was found on a regional scale the dispersion was grouped into dispersion regions and inverted to shear velocity models down to about 2 3 km depth the inferred p velocity models are given the models separated naturally into categories in close agreement with the large scale geology of the area investigated the regional shear velocity structure varied strongly over the area and the uppermost few hundred metres of the crust at deeper levels the structure showed a normal variation the variation in these uppermost layers could be explained by the variation of seismic velocities with pressure in pre stressed rock due to partial closure of crack porosity as there was no significant sedimentary cover in the area this superficial layer could be attributed to the weathering layer of crystalline rocks on a local characteristic horizontal scale of about 6 km the lateral heterogeneities were studied using the dispersion in the 1 5 2 5 hz band with a perturbation method the local variations were mapped into the background shear velocity structure along the eugeno s profile iv array the local shearvelocity models were constrained to be increasing functions of depth and to satisfy the values and slopes of the known regional dispersion most of the 48 models could be mapped using a reference model constructed from the profile iv average model the models defined a strongly heterogeneous weathering layer with a thickness of 0 4 0 5 km the uppermost crustal shear background velocities correlated in detail with the local surface geology the velocities formed two distinct populations composed essentially of granites population i and granitic gneisses population 11 with pi 3 19 f 0 08 and p 2 84 i 0 15 km s respectively these velocities correspond to an in situ crack porosity density of 0 1 0 5

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

timing and mechanisms of north atlantic cenozoic uplift evidence for mantle upwelling postrift domal uplift patterns are a distinct feauture of northern north atlantic margins on the basis of apatite fission track data offshore seismic stratigraphy geomorphology gravity and seismic tomography we argue that southern norway is characterized by predominantly neogene domal uplift currently available data suggest similar processes for other uplifted regions such as spitsbergen northern norway the british isles aad parts of east greenland

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

the lithosphere asthenosphere system in fennoscandia abstract we present a summary of the available information on rayleigh wave dispersion data for the fennoscandian region the observations have been combined to produce regional dispersion relations which have then been subjected to the hedgehog inversion procedure the results are presented on a map outlining the thickness of the lid and the shear velocities in both the lid and the asthenosphere channel lid thickness up to around 135 km is found in the bothnia north central finland area with if any weak shear velocity contrast to the underlying layer the surrounding areas are characterized by lid thickness up to around 75 km a stronger low velocity zone to lid contrast may be found in the caledonian and baltic sea area 0 25 0 45 km s taking into account moho depth data and the aforementioned results a map of the lithosphere asthenosphere system was derived

receiver function analysis of the crust and upper mantle in fennoscandia isostatic implications abstract the mountains across southern norway and other margins of the north atlantic ocean appear conspicuously high in the absence of recent convergent tectonics we investigate this phenomenon with receiver functions calculated for seismometers deployed across southern fennoscandia these are used to constrain the structure and seismic properties of the lithosphere and primarily to measure the thickness and infer the bulk composition of the crust such parameters are key to understanding crustal isostasy and assessing its role or lack thereof in supporting the observed elevations our study focuses on the southern scandes mountain range that has an average elevation 1 0 km above mean sea level the crust mantle boundary moho is ubiquitously imaged and we occasionally observe structures that may represent the base of the continental lithosphere or other thermal chemical or viscous boundaries in the upper mantle the moho resides at 25 30 km depth below mean sea level in southeastern coastal norway and parts of denmark 35 45 km across the southern scandes and 50 60 km near the norwegian swedish border that section of thickest crust coincides with much of the transscandinavian igneous belt and often exhibits a diffuse conversion at the moho which probably results from the presence of a high wave speed mafic lower crust across inner fennoscandia a zone of thinned crust

thrust emplacement of the hispaniola peridotite belt orogenic expression of the mid cretaceous caribbean arc polarity reversal articulo sobre el emplazamiento fallero en la zona de peridotitas de la hispaniola republica dominicana estudiando si es la expresion orogenica de la polaridad reversa en la zona del caribe en el cretacico medio

eruptive history and geochronology of mount mazama and the crater lake region oregon geologic mapping k ar and 40 ar 39 ar age determinations supplemented by paleomagnetic measurements and geochemical data are used to quantify the quaternary volcanic history of the crater lake region in order to define processes and conditions that led to voluminous explosive eruptions the cascade arc volcano known as mount mazama collapsed during its climactic eruption of 50 km 3 of mainly rhyodacitic magma 7700 yr ago to form crater lake caldera the mazama edifice was constructed on a pleistocene silicic lava field amidst monogenetic and shield volcanoes ranging from basalt to andesite similar to parental magmas for mount mazama between 420 ka and 35 ka mazama produced medium k andesite and dacite in 2 1 proportion the edifice was built in many episodes some of the more voluminous occurred approximately coeval with volcanic pulses in the surrounding region and some were possibly related to deglaciation following marine oxygen isotope stages mis 12 10 8 6 5 2 and 2 magmas as evolved as dacite erupted many times commonly associated with or following voluminous andesite effusion establishment of the climactic magma chamber was under way when the first preclimactic rhyodacites vented ca 27 ka the silicic melt volume then grew incrementally at an average rate of 2 5 km 3 k y 1 for nearly 20 k y the climactic eruption exhausted the rhyodacitic magma and brought up crystal rich andesitic magma mafic cumulate mush and wall rock granodiorite postcaldera volcanism produced 4 km 3 of andesite during the first 200 500 yr after collapse followed at ca 4800 yr b p by 0 07 km 3 of rhyodacite the average eruption rate for all mazama products was 0 4 km 3 k y 1 but major edifice construction episodes had rates of 0 8 km 3 k y 1 the long term eruption rate for regional monogenetic and shield volcanoes was d 0 07 km 3 k y 1 but only 0 02 km 3 k y 1 when the two major shields are excluded plutonic xenoliths and evidence for crystallization differentiation imply that the amount of magma intruded beneath mount mazama is several times that which has been erupted the eruptive and intrusive history reflects competition between 1 crystallization driven by degassing and hydrothermal cooling and 2 thermal input from a regional magma flux focused at mazama before ca 30 ka relatively small volumes of nonerupted derivative magma crystallized to form a composite pluton because the upper crust had not been heated sufficiently to sustain voluminous convecting crystal poor melt subsequently and perhaps not coincidentally during mis 2 a large volume of eruptible silicic magma accumulated in the climactic chamber probably because of heating associated with mantle input to the roots of the system as suggested by eruption of unusually primitive magnesian basaltic andesite and tholeiite west of mazama

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin

tomographical mapping of the lithosphere and asthenosphere beneath southern scandinavia and adjacent areas abstract in this paper we report on a seismic tomographical survey of the lithosphere asthenosphere beneath southern scandinavia and adjacent areas the ach inversion technique has been used and the observational data in the form of p wave travel time residuals have been obtained from seismogram records from networks in denmark southern norway and southern sweden the volume under investigation subdivided into four levels layers of 7 7 knots blocks each for estimation of velocity perturbations covers the area 54 62 n and 7 20 e and depth range 0 600 km major findings are as follows level 1 0 100 km pronounced velocity lows beneath southern jutland zealand the variscan foredeep and the sveco norwegian province of southeastern norway 1500 ma old the svecofennides of southeastern sweden exhibit relatively high velocities while the sveco norwegian province of southwestern sweden 1700 ma old is characterized by moderately high velocities the transition between velocity highs and lows coincides with the tornquist teisseyre line ttl or fennoscundian border zone level 2 100 300 km velocity anomalies are less pronounced at this level although velocity lows are found beneath the northern jutland zealand area and slightly northwestwards of the oslo rift the only velocity high is found beneath the southeasternmost part of sweden the ttl does not appear to have a clear counterpart in the velocity anomaly pattern in this depth range although a clearly unambiguous answer here would require far more extensive observational data a research goal of the future level 3 300 500 km and level 4 500 600 km velocity anomalies are weak which is not unexpected for the upper mantle beneath shield areas