venezuela basin crustal structure velocity depth profiles derived from six two ship expanding spread experiments in combination with other geophysical data define the characteristics of two distinct types of venezuela basin crust and the boundary between them each two ship common midpoint reflection refraction profile is automatically transformed into the p plane picked and interpreted to provide v z functions with appropriate confidence bounds the results together with gravity magnetic and near vertical incidence reflection data reveal a 50 000 km2 triangle of venezuela basin crust which resembles normal oceanic crust in a magnetic quiet zone north and west of this triangle lies the previously defined thick caribbean crust having two distinct layers above the m discontinuity acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at deep sea drilling project sites 146 149 sills and 150 flows also depths to mantle are greater than normal interpretations of near vertical and wide angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal the boundary between the two crustal areas has a ne sw trend which parallels the dominant structural and magnetic lineations this boundary coincides in position though not in trend with the previously defined central venezuela basin fault zone further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production

oceanic crustal thickness and seismic character along a central pacific transect seismic reflection data along a flow line of crustal generation in the central pacific that spans ages of zero to 85 ma and spreading half rates of 30 to 100 km m y 1 shows a nearly constant travel time of 2 s through igneous crust to reflection moho the highest amplitude and most laterally continuous moho reflections were recorded over 20 30 ma crust that was emplaced at superfast 75 95 km m y 1 spreading rates the superfast spread portion also records the lowest scatter about the 2 s average travel time to reflection moho seismic images show that lower crustal reflectors dip consistently eastward toward the ridge crest these dipping reflectors are truncated by the reflection moho

plastic deformation and magmatic impregnation in serpentinized ultramafic rocks from the garrett transform fault east pacific rise the microstructural study of a set of serpentinized ultramafic samples dredged in the garrett transform fault shows that these rocks have been deformed plastically at temperatures probably in excess of 1000 c and under moderate deviatoric stresses below 1 kbar such temperature and stress conditions probably prevail in the asthenosphere or in the deepest and hottest part of the lithosphere below the ridge the deformed paragenesis of the studied samples belong to two groups harzburgites with petrological characteristics typical of residual mantle and plagioclase dunites a melt circulated in these two groups of deformed ultramafic rocks at the end of their deep seated plastic deformation this melt crystallized plagioclase clinopyroxene and also possibly orthopyroxene in one sample in dykelets and in interstices between the minerals of the deformed host rock the deformation and magmatic impregnation history of the studied ultramafic samples recall the results of on land studies of the crust mantle transition zone of ophiolitic massifs this suggests that our set of samples may represent fragments of the east pacific rise crust mantle transition zone the deformed ultramafic paragenesis of these samples was chemically modified by interactions with the impregnating melt the deformed spinel was enriched in tio2 and cr2o3 the deformed clinopyroxene and orthopyroxene of the harzburgitic samples were also enriched in tio2 and the orthopyroxene was significantly depleted in al2o3 the high mobility of titanium during the impregnation process suggests that the ti content of a melt often used as an indicator of its degree of evolution or of the degree of partial melting undergone by its source rock may also be modified by melt rock interactions during the circulation of the magma through the crust and upper mantle

reconstruction of the geometry of accretion during formation of the bay of islands ophiolite complex two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the bay of islands region of southwestern newfoundland canada the first called the coastal complex cc is exposed as a long linear belt along the west coast it consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history the second called the bay of islands complex boic crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history an autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs indicating that each evolved along a single ridge transform system the internal structure of both the boic and the cc is consistent with the above interpretations the ne sw structural trends in the cc transform are oriented approximately parallel to the sea floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the boic one way chilling statistics from the sheeted diabase dike unit growth directions of dendritic crystals along inclined comb layers in the plutonic section and the sense of shear along the cc transform all indicate that the boic was accreted on the ne side of a nw se trending spreading center present geographic reference frame juxtaposition of the boic with the ne sw trending cc occurred at a ridge transform intersection the transform exhibited a left lateral offset magma chamber profiles have been identified in the layered gabbroic sections of the boic by using the patterns outlined by the attitudes of fine scale igneous layering these patterns depict arcuate structures which appear to define roughly a cross sectional view of a systematically oriented half cylinder the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts thus we believe that a large cylindrical shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial plane at the time of formation of the bolc the cc marks the abrupt truncation of this long linear magma chamber petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30 km depth beneath the crustal level magma chamber after formation these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted a geometric and tectonic model is presented for the accretion process responsible for formation of the bay of islands ophiolite

the variability in seismic structure of layer 2 near the east pacific rise at 12 n seismic refraction data collected by using a large air gun source and oceanbottom hydrophone receivers show that within two areas on the flanks of the east pacific rise the structure of the uppermost 2 km of the crust is laterally homogeneous the two areas are 20 and 40 km in extent on 0 5 and 4 m y old crust respectively and are separated laterally by 110 km a small but significant difference in structure exists between the two areas the younger crust exhibiting the higher velocities ray tracing through laterally heterogenous models of the shallow crust the careful calculation of corrections for seafloor topography and the objective comparison of data sets by using the slant stacking approach suggest that the established apparent variability in shallow crustal structure may in part be an artifact of experimental and interpretative techniques we propose a reasonable model for shallow crustal structure in which the isovelocity contours parallel the seafloor topography this permits determination of lateral homogeneity of the velocity gradient in the shallow crust over the subdued topography of the east pacific rise or in the deeper crust in general where vertical velocity gradients are sufficiently small that topography has negligible effect on the velocity field

s local wave seismic anisotropy in the forearc above the subducted nazca plate between 33 s and 34 5 s s wave splitting from local earthquakes within the nazca plate that are deeper than the interplate seismogenic zone enabled the determination of the fast velocity direction and the lag time t in the forearc of the overriding plate data were collected from 20 seismic stations most of which were temporary deployed between 33 5 s and 34 5 s and included part of the normal subduction section to the south and part of the transitional section to flat subduction to the north the fast velocity direction has a complex pattern with three predominant directions northwest southeast north south and northeast southwest and relatively high t a quality evaluation of the highest measurements enabled us to identify possible cycle skipping in some of the measurements which could be responsible for the large observed lag time we consider that most of the anisotropy that was observed in the forearc is probably located in the mantle wedge and a minor part is located in the crust the complex pattern of splitting parameters when the anisotropy is associated at the mantle wedge could be the result of three dimensional variations in the subducting nazca plate at these latitudes also similarities between the splitting parameters and the principal compressional stress direction from pliocene and quaternary rocks suggest that the anisotropy in the crust could originate by tectonic local stress

mantle crust transition zone and origin of wehrlitic magmas evidence from the oman ophiolite abstract abundant exposure and well preserved outcrops in the oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment the mantle crust transition zone shows large lateral variations in thickness and composition but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations and gabbro lenses compositionally the transition zone marks a gradational passage from mantle to crust the moho may be placed at the base of the continuous gabbroic crustal section structures both within the transition zone and near the base of the crust are nearly parallel to the moho however the dominant flow mechanisms differ from the upper mantle to the lower crust plastic flow dominates in the upper mantle while viscous flow resulting in strong magmatic fabrics dominates at crustal levels wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted the parent magma of the wehrlitic instrusions is in fact a crystal melt mixture composed of olivine and chromite probably including a component of mantle xenocrysts and xenoliths and of reequilibrated basaltic melt

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

deep structure of the southern kerguelen plateau southern indian ocean from ocean bottom seismometer wide angle seismic data wide angle seismic data collected during the kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern kerguelen plateau and support a new interpretation of the origin of the plateau velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22 km thick crust composed of 1 6 km of sedimentary cover 5 3 km of upper crust 11 0 km of lower crust and a 4 to 6 km thick reflective zone immediately above moho velocities in the upper crust from 3 8 4 5 km s at top to 6 0 6 5 km s at bottom are consistent with the basaltic nature of this layer the top of which was sampled during the ocean drilling program velocities in the lower crust increase continuously from 6 60 km s at the top to 6 90 km s at 19 5 km depth the reflective zone at the base of the crust identified by wide angle reflections is observed only along the nnw sse direction it consists of alternating high and low velocity layers with an average velocity of 6 70 km s in the nnw sse direction and 6 90 km s in the perpendicular direction strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8 60 and 8 00 km s in the nnw sse and e w directions respectively the absence of high velocities at the base of the crust that characterizes many large volume mafic provinces the reflective lower crust and anisotropy in upper mantle suggest that the southern kerguelen plateau represents a stretched continental fragment overlain by basaltic flows isolated from the antarctic margin during the early opening of the indian ocean

generation and obduction of ophiolites constraints from the bay of islands complex western newfoundland constraints provided by the bay of islands ophiolite complex boic western newfoundland suggest that its generation as well as its obduction is related to collision of the dunnage zone island arc system with the irregular continental margin of eastern north america the constraints include the supra subduction zone setting of the boic its extensional environment of generation the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the north american continental margin and the restriction of the boic as well as other appalachian ophiolites and associated taconian allochthons to reentrants in the north american margin during the early phases of collision of the dunnage arc with the north american margin subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation the first develops through trench rollback leading to ophiolite generation in the overriding arc plate outboard of the reentrant whereas the second develops in a transtensional environment with spreading centers offset by strike slip faults which propagate through the overriding plate away from the indenter promontory the configuration of the boic spreading ridge normal to the continental margin and the direction of initial obduction parallel to the margin favor generation of the ophiolite in an overall strike slip regime with complete subduction of the oceanic remnant extension in the overriding plate ceases and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the dunnage zone and for termination of arc volcanism along the north american margin

high seismic velocities associated with reflections from within the lower oceanic crust near the continental margin of eastern canada abstract a deep seismic reflection profile on the continental rise off eastern canada shows strong dipping reflections apparently from within lower oceanic crust over a region of limited lateral extent the origin of these and similar reflections described elsewhere is uncertain this paper reports new seismic refraction results from the region of these reflections they appear to be associated with a layer of p wave velocity of about 7 4 km s 1 or higher while the surrounding crust has a velocity 7 km s 1 the moho with velocity 8 1 km s 1 lies just below the reflectors and the total crustal thickness is around 8 km the crust is interpreted to be oceanic rather than continental and the high velocities of the dipping reflectors are comparable to those of the mafic ultramafic crust mantle transition zone in the bay of islands ophiolite complex

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

lower crustal variability and the crust mantle transition at the atlantis massif oceanic core complex 1 seismic refraction data provide new constraints on the structure of the lower oceanic crust and its variability across the atlantis massif oceanic core complex 30 n on the mid atlantic ridge a 40 km long spreading parallel profile constrains p wave velocities to depths of up to 7 km beneath the seafloor two shorter spreading perpendicular lines provide coverage to 2 km depth the anomalous character of the massif s central dome crust is clear compared to the neighboring rift valley and similar age crust on the opposite ridge flank the domal core of the massif unroofed via detachment faulting has velocities 7 0 km s at depths below 2 5 km sub seafloor increasing to 7 5 7 8 km s over the depth range 4 8 6 8 km within the core complex the moho does not appear to be sharp as no pmp arrivals are observed within the axial valley velocities do not reach mantle transition zone values in the uppermost 6 km we infer that crust there is of normal thickness but that a thinner than average mafic section is present in the central massif near iodp hole u1309d located on the central dome there is a low velocity gradient interval at 1 3 km depth with velocities of 6 6 6 8 km s that coincides with a 3 5 km wide region where shallower velocities are highest given the predominantly gabbroic section recovered from the 1 4 km deep drillhole this seismic structure suggests that the mafic body extends a few km both laterally and vertically

accumulations of melt at the base of young oceanic crust as hot solid rock ascends through the mantle the pressure around it decreases while its temperature decreases approximately adiabatically during the ascent the melting temperature decreases faster than the adiabat temperature and if the initial temperature of the rock is high enough the rock will begin to melt at the depth where the adiabat crosses the melting temperature curve models of upwelling beneath mid ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge axis region suggesting that some of the ascending melt must arrive away from the ridge axis here i report ocean bottom seismograph observations of an uncommon and previously unreported marine seismic phase which provide strong evidence that melt accumulates in sills at the base of young oceanic crust the phase results from the conversion of compressional p waves to shear s waves on interaction with the crust mantle boundary the moho this phase is rare and very difficult to model but when visible it is strikingly prominent and usually appears at unexpectedly great source to receiver ranges the observations imply the presence of bodies of extremely low rigidity presumably melt sills at the moho close to the receiver melt may be fairly mobile at the base of the crust allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle escape of this melt to the sea floor may be responsible for the formation of off axis seamounts

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

structures within the oceanic crust of the central south china sea basin and their implications for oceanic accretionary processes internal structures in mature oceanic crust can elucidate understanding of the processes and mechanism of crustal accretion in this study we present two multi channel seismic mcs transects across the northern flank of the south china sea basin to reveal the internal structures related to cenozoic tectono magmatic processes during seafloor spreading bright reflectors within the oceanic crust including the moho upper crustal reflectors and lower crustal reflectors are clearly imaged in these two transects the moho reflection displays varied character in continuity shape and amplitude from the continental slope area to the abyssal basin and becomes absent in the central part of the basin where abundant seamounts and seamount chains formed after the cessation of seafloor spreading dipping reflectors are distinct in most parts of the mcs data but generally confined to the lower crust above the moho reflection these lower crustal reflectors merge downward into the moho without offsetting it probably arising from shear zones between the crust and mantle characterized by interstitial melt although we cannot exclude other possibilities such as brittle faulting or magmatic layering in the local area a notable feature of these lower crustal reflector events is their opposite inclinations we suggest the two groups of conjugate lower crustal reflector events observed between magnetic anomalies c11 and c8 were associated with two unusual accretionary processes arising from plate reorganizations with southward ridge jumps c 2018 the author s published by elsevier b v

structural variation of the oceanic moho in the pacific plate revealed by active source seismic data abstract the characteristics of the oceanic moho are known to depend on various factors such as seafloor spreading rate crustal age and accretionary processes at a ridge however the effect of local magmatic activities on the seismic signature of the moho is poorly understood here an active source reflection and refraction survey is used to investigate crustal structure and moho characteristics along a 1000 km long profile southeast of the shatsky rise in a pacific ocean basin formed from the late jurassic to early cretaceous and spanning the onset of shatsky rise volcanism although the seismic velocity structure estimated from the refraction data showed typical characteristics of the oceanic crust of the old pacific plate the appearance of the moho reflections was spatially variable we observed clear moho reflections such as those to be expected where the spreading rate is fast to intermediate only at the southwestern end of the profile whereas moho reflections were diffuse weak or absent along other parts of the profile the poor moho reflections can be explained by the presence of a thick crust mantle transition layer which is temporally coincident with the formation of the shatsky rise we inferred that the crust mantle transition layer was formed by changes in on axis accretion process or modification of the primary moho by off axis magmatism induced by magmatic activity of the shatsky rise

the kerguelen isles southern indian ocean new results on deep structure from refraction profiles abstract additional seismic refraction profiles shot across the kerguelen archipelago terres australes et antarctiques francaises known as taaf located in the southern indian ocean substantiate the oceanic type crust of the archipelago and the kerguelen plateau as well the crust mantle boundary is better modelled by a transition than by a discrete interface seismic velocities of 6 60 6 80 km s are in the range of those within oceanic layer 3 velocities of 7 2 7 5 km s observed at long ranges may be interpreted according to at least two distinct hypotheses 1 1 notwithstanding their rather low values they represent upper mantle material and might be assigned to peridotites serpentinised by hydrothermal circulations as reported for various oceanic and continental features 2 2 these velocities are assigned to the lower crust intruded by melt produced by adiabatic decompression of upper mantle material during rifting underplating a small increase in mantle temperature caused by the presence of heat sources i e the southeast indian ridge and the kerguelen mantle plume nearby may have produced huge quantities of melt facilited by the slow movement of the antarctic plate then building up the aseismic kerguelen plateau

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

seismic anisotropy indicators in western tibet shear wave splitting and receiver function analysis abstract using recently collected data from western tibet we find significant variation in the strength vertical distribution and attributes of seismic wave speed anisotropy constrained through a joint application of teleseismic shear wave splitting techniques and a study of p s mode converted waves receiver functions we find that the crust of tibet is characterized by anisotropy on the order of 5 15 concentrated in layers 10 20 km in thickness and with relatively steep 30 45 from the vertical slow symmetry axes of anisotropy these layers contribute no more than 0 3 s to the birefringence in teleseismic shear waves significantly smaller than splitting in many of the observations and much smaller than birefringence predicted by models developed through group inversions of shear wave recordings consequently we interpret models constrained with shear wave observations in terms of structures in the upper mantle near the altyn tagh fault our data favor a two layer model with the upper layer fast polarization approximately aligned with the strike of the fault near the karakorum fault our data are well fit with a single layer of relatively modest 0 5 s delay anisotropy fast polarization in this layer is 60 ne similar to that of the lower layer in the model for the altyn tagh fault site assuming that layers of similar anisotropic properties at these two sites reflect a common cause our finding favors a scenario where indian lithosphere under thrusts a significant fraction of the plateau data from a site at the southern edge of the tarim basin appear to be inconsistent with a common model of seismic anisotropy distribution we suspect that thick sediments underlying the site significantly distort observed waveforms our ability to resolve features of anisotropic structure in the crust and the upper mantle of western tibet is limited by the small amount of data collected in a 6 month observing period we stress the importance of future teleseismic data collection in tibet over extended periods to insure better directional distribution of observed seismic sources

crustal thickness and moho character of the fast spreading east pacific rise from 9 42 n to 9 57 n from poststack migrated 3 d mcs data we computed crustal thickness 5740 270 m and mapped moho reflection character using 3 d seismic data covering 658 km2 of the fast spreading east pacific rise epr from 9 42 n to 9 57 n moho reflections are imaged within 87 of the study area average crustal thickness varies little between large sections of the study area suggesting regionally uniform crustal production in the last 180 ka however individual crustal thickness measurements differ by as much as 1 75 km indicating that the mantle melt delivery has not been uniform third order but not fourth order ridge discontinuities are associated with changes in the moho reflection character and or near axis crustal thickness this suggests that the third order segmentation is governed by melt distribution processes within the uppermost mantle while the fourth order ridge segmentation arises from midcrustal to upper crustal processes in this light we assign fourth order ridge discontinuity status to the debated ridge segment boundary at 9 45 n and third order status at 9 51 5 n to the ridge segment boundary previously interpreted as a fourth order discontinuity our seismic results also suggest that the mechanism of lower crustal accretion varies along the investigated section of the epr but that the volume of melt delivered to the crust is mostly uniform more efficient mantle melt extraction is inferred within the southern half of our survey area with greater proportion of the lower crust accreted from the axial magma lens than that for the northern half this south to north variation in the crustal accretion style may be caused by interaction between the melt sources for the ridge and the lamont seamounts

reconstructed seismic velocity structure of the lewis hills massif and implications for oceanic fracture zones the lewis hills massif of the bay of islands ophiolite complex provides a lower crustal to upper mantle cross section of an oceanic fracture zone segment and adjacent oceanic lithosphere the structural and petrologic character of the fracture zone region is distinctly different from the comparatively simple layered structure of the rest of the bay of islands complex results of detailed field mapping petrography and laboratory seismic velocity measurements have been combined in order to reconstruct the seismic velocity structure of this slice of oceanic lithosphere velocities and velocity anisotropies of each of the major rock units in the massif are restored to their preobduction geometries relative to different stratigraphic levels in the complex and proximity to the fracture zone region although all oceanic fracture zones probably have unique structural and petrologic characteristics in detail from the perspective of the present study it is suggested that in general seismic properties differing from that of normal oceanic crust and upper mantle may be expected within approximately 10 km of the axis of fracture zones in these regions lower crustal as well as upper mantle anisotropy is expected the moho might be expected to shallow due to the presence of abundant ultramafic cumulates resulting in crustal thinning and significant velocity inversions may occur with depth where large cyclic mafic layers occur within cumulate ultramafics in oceanic lithosphere which has passed or is passing through a transform fault domain enhanced mantle anisotropy might occur the middle to lower crust in these regions should be composed of highly anisotropic metamorphic rocks which grade upward into serpentinites and highly fractured gabbros diabases and basalts with relatively low velocities it is suggested that velocities as well as velocity anisotropies produced by these types of structures might be determined in situ by seismic refraction experiments and may be used to map the extent of deformation in the oceanic lithosphere near fracture zones

pyroxene orientation within the upper mantle petrofabric analyses of orthopyroxene and olivine for a suite of rocks from the bay of islands ophiolite newfoundland show systematic relationships between the minerals9 fabrics the plane of the mohorovicic discontinuity determined from the contact of the gabbro and ultramafic sections and the spreading direction inferred from the attitude of the sheeted dikes the orthopyroxene c crystallographic axes maxima which are aligned subparallel to the spreading direction coincide with olivine a axes maxima the orthopyroxene b axes and olivine c axes also lie approximately within the plane of the mohorovicic discontinuity but parallel the ridge whereas the orthopyroxene a axes maxima and olivine b axes maxima are approximately perpendicular to the mohorovicic discontinuity using olivine and orthopyroxene elastic constants and their pressure derivatives velocities have been calculated from composite petrofabric data and contoured for varying proportions of olivine and pyroxene the results of these calculations show that within a vertical direction compressional wave velocities change only slightly with increasing pyroxene content whereas parallel to the mohorovicic discontinuity velocities decrease with increasing pyroxene content anisotropy decreases from 4 for a rock with 100 olivine to 2 6 for a rock with 50 olivine and 50 orthopyroxene also within the plane of mohorovicic discontinuity the maximum and minumum velocities occur at approximately the same azimuth for differing percentages of orthopyroxene a comparison of the calculated anisotropy and the observed seismic anisotropy off the coast of california shows a striking agreement from this comparison it is estimated that the percentage of orthopyroxene within the upper mantle of this region varies between 0 and 30

a possible explanation of gravity anomalies over mid ocean ridges gravity fields over the crests of slow spreading mid ocean ridges are characterized by small amplitude free air gravity anomaly lows 30 70 mgal with wavelengths of 40 60 km flanked by smaller gravity highs in general the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until at fast spreading ridges 5 cm yr free air gravity highs 10 20 mgal are observed previous explanations of these anomalies which involve uncompensated ridges thermally expanded ridges and elastic plate compensation models are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest in particular the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid ocean ridge basalts which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more in addition similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers geometrical models of mid ocean ridges derived from detailed structural and petrologic studies of the bay of islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly the ridge topography the crustal level magma chamber the configuration of the isotherms near the ridge crest and a low density partially gabbroic cumulate root extending vertically into the mantle beneath the ridge systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers major contributions are found to come from the topography and from the lowdensity root which broadens with slower spreading rates resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate at extinct spreading centers contributions from the magma chamber isotherms and topography gradually diminish with time once the ridge ceases to spread whereas the low density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the bay of islands ophiolite and the gravity analysis presented here

magma chamber profiles from the bay of islands ophiolite complex models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle evidence from the bay of islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir coherent well defined and systematically oriented large scale arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the bay of islands complex these layering patterns seem to outline the shape of a large continuously evolving steady state magma chamber that extended along a ridge axis and ended at a transform fault intersection

origin of dipping structures in fast spreading oceanic lower crust offshore alaska imaged by multichannel seismic data abstract multi channel seismic mcs reflection profiles across the pacific plate south of the alaska peninsula reveal the internal structure of mature oceanic crust 48 56 ma formed at fast to intermediate spreading rates during and after a major plate re organization oceanic crust formed at fast spreading rates half spreading rate 74 mm yr has smoother basement topography thinner sediment cover with less faulting and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates half spreading rate 28 34 mm yr mcs data across fast spreading oceanic crust formed during plate re organization contain abundant bright reflections mostly confined to the lower crust above a highly reflective moho transition zone which has a reflection coefficient rc of 0 1 the lower crustal events dip predominantly toward the paleo ridge axis at 10 30 reflections are also imaged in the uppermost mantle which primarily dip away from the ridge at 10 25 the opposite direction to those observed in the lower crust dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re organization where a moho reflection is weak or absent our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt the abundance and reflection amplitude strength of these events rc 0 15 can be explained by a combination of solidified melt that was segregated within the shear structures mylonitization of the shear zones and crystal alignment all of which can result in anisotropy and constructive signal interference formation of shear zones with this geometry requires differential motion between the crust and upper mantle where the upper mantle moves away from the ridge faster than the crust active asthenospheric upwelling is one possible explanation for these conditions the other possible interpretation is that lower crustal reflections are caused by magmatic mafic ultramafic layering associated with accretion from a central mid crustal magma chamber considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re organizations associated with ridge jumps or rift propagation we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle and therefore to shearing in the ductile lower crust or alternatively that plate reorganization could produce magmatic pulses which may lead to mafic ultramafic banding

ophiolites synthetic seismograms and oceanic crustal structure 2 a comparison of synthetic seismograms of the samail ophiolite oman and the rose refraction data from the east pacific rise synthetic seismograms for the samail ophiolite computed using the full reflectivity technique are used to test the hypothesis that this ophiolite complex can serve as a prototype for young oceanic crust if so then this ophiolite complex can be compared with the older 45 m y bay of islands boi ophiolite to develop an aging model for oceanic crust synthetics for this young emplacement age 5 8 m y ophiolite are compared with ocean bottom hydrophone data from 0 5 and 4 5 m y age crust obtained during the rivera ocean seismic experiment second arrival phases from reflected refractions from one ophiolite velocity depth model and data from 4 5 m y age crust are in excellent agreement this agreement suggests that the samail ophiolite is a good working model for young oceanic crust earlier work has shown that the boi ophiolite is a good prototype for mature oceanic crust the boi ophiolite data also suggest that alteration of the lower oceanic crust and upper mantle is limited we use the petrologic and geochemical properties of the samail and boi ophiolites to develop a two stage model for crustal aging that directly limits alteration of the lower crust and upper mantle materials

structural history of the metamorphic sole of the bay of islands complex western newfoundland displacement of the early ordovician ophiolitic bay of islands complex was accompanied by the development of a metamorphic sole in underlying accreted gabbro mafic pillow lava pelite and psammite the metamorphic sole contains up to 130 m of amphibolite underlain by up to 80 m of mafic and pelitic psammitic schist which in turn overlies chlorite bearing rocks that overlie mudstone matrix melange a major foliation locally mylonitic and or isoclinally folded is pervasively developed in the greenschists and amphibolites an extension lineation is widely developed and plunges north in amphibolite and trends east west in greenschist consistent with a changing direction of thrust transport from northerly to westerly as the mantle wedge cooled over time shear sense criteria in greenschist facies pelitic psammitic schist indicate thrusting towards the west in the southwestern blow me down massif two sets of folds postdate development of accretion related s l fabrics and resulted in a substantial w

seismological evidence of mantle flow driving plate motions at a palaeo spreading centre at oceanic spreading centres it is unclear whether plate motions drag the underlying mantle or mantle flow pulls the overlying plates seismic imaging of a former speading centre in the pacific plate reveals strong seismic anisotropy that was generated by mantle flowing at speeds greater than plate motions implying that mantle flow pulled this part of the plate

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

seismic velocity structure at deep sea drilling project site 504b panama basin evidence for thin oceanic crust we present an analysis of wide angle reflection refraction data collected in the immediate vicinity of deep sea drilling project hole 504b in the panama basin currently the deepest drill hole 1 288 km into oceanic crust the data were acquired with a 1785 inch3 air gun array and fixed gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths near normal incidence multichannel seismic mcs reflection data were acquired simultaneously observed p and s wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness comparison of the travel times and amplitudes of the p and s wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method forward modeling shows that in contrast to standard oceanic velocity models a velocity depth profile that better explains the observed data is characterized by high velocity gradients up to 0 6 km s km in the middle crust a 1 8 km thick low velocity zone vp 7 1 6 7 km s immediately above moho and a total crustal thickness of only 5 km interpretation of the high velocity gradients in the middle crust is constrained by the observation of p wave amplitude focusing at ranges of 16 19 km although not as well developed in comparison to the p wave arrivals s wave arrivals show similar focusing total crustal thickness is constrained by the combined interpretation of a p wave wide angle reflection event observed at a range of 16 28 km and an mcs reflection event with a crustal travel time of 1 4 1 5 s although these events cannot be directly correlated their travel times are consistent with the assumption that both have a common origin amplitude modeling of the wide angle event demonstrates that these events are generated at the moho

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

arc parallel flow within the mantle wedge evidence from the accreted talkeetna arc south central alaska 1 residual mantle exposures in the accreted talkeetna arc alaska provide the first rock analog for the arc parallel flow that is inferred from seismic anisotropy at several modern arcs the peridotites exposed at the base of the jurassic talkeetna arc have a moho parallel foliation and indicate dislocation creep of olivine at temperatures of 1000 to 1100 c slip occurred chiefly on the 001 100 slip system which has only rarely been observed to be the dominant slip system in olivine stretching lineations and olivine 100 slip directions are subparallel to the long axis of the talkeetna arc for over 200 km indicating that mantle flow was parallel to the arc axis the alignment of the olivine 100 axes yields a calculated s wave anisotropy with the fast polarization direction parallel to the arc thus 1 the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog 2 arc parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones fluid filled pockets or glide on the 010 001 h2o induced slip system 3 seismic anisotropy beneath modern arcs may be due to slip on 001 100 with a horizontal foliation rather than slip on 010 100 with a vertical foliation and 4 the observed dominance of the 001 100 slip system may be due to high h2o concentrations suggesting that strain in the oceanic upper mantle may be accommodated dominantly by 001 100 olivine slip

modeling anisotropy and plate driven flow in the tonga subduction zone back arc the goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions the tonga subduction zone in particular can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate we calculated two dimensional mantle flow models using known tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures shear wave splitting was predicted for the anisotropy due to lattice preferred orientation lpo of olivine and orthopyroxene in the flow model finite strain fields the predicted shear wave splitting provides a good match to the fast directions parallel to the azimuth of subducting plate motion and splitting times 0 5 1 5 s observed in tonga both for models where lpo anisotropy develops everywhere above 410 km and for models where lpo anisotropy is confined to regions of relatively high stress if lpo anisotropy does develop over the entire upper 410 km of the mantle the strength of anistropy induced by a given amount of shear strain must be relatively weak 4 for shear strains of 1 5 with a maximum value of 6 for very large strains the splitting observations are comparably fit by a wide range of different viscosity models anisotropy due to melt filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting

depth of the decoupling plate interface and thermal structure under arcs subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate at depth below this mechanically decoupling plate interface the subducting plate couples with the overlying mantle and flow is induced in the mantle wedge this induced flow is the main factor that controls thermal structure under arcs depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern japan arc using a two dimensional numerical simulation of thermal structure the estimated depth is about 70 km the termination depth of thrust earthquakes agrees well with the depth of the coupling transition which indicates that these earthquakes occur at the decoupling plate interface the mechanism of this decoupling plate interface is discussed in this paper it is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2 0 gpa and have little temperature dependence termination depths of thrust earthquakes estimated from seismic studies are nearly the same 60 80 km for various arcs this indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust across arc heat flow profiles observed in various subduction zones show similar features heat flow is very low in the forearc region and increases steeply at the volcanic front considering that the depth of the slab is about 100 km at the volcanic front for various arcs this heat flow feature is closely related to the depth of the slab thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust

ultramafic intrusions in the lewis hills massif bay of islands ophiolite complex newfoundland implications for igneous processes at oceanic fracture zones the coastal complex of western newfoundland has been interpreted as oceanic crust and upper mantle which formed in an oceanic fracture zone within the coastal complex a suite of peridotite bodies and associated basaltic dikes occur the least deformed peridotites in these bodies have cumulate textures and have apparently been separated from any gabbroic cumulates which formed in association with them intrusive relationships with surrounding metamorphic and cumulate rocks are well preserved and suggest that the peridotite bodies are crystal mush intrusions derived from mobilized ultramafic cumulates which were originally deposited within relatively small isolated magma chambers these processes are believed to be characteristic of ridge transform intersections or leaky transform regions two petrologically distinct types of peridotite have been identified one type has lithologies and mineral chemistry similar to those of many oceanic plutonic rocks and these lithologies may be interpreted as differentiates produced in the evolution of typical mid ocean ridge basalts morbs the second type of peridotite although lithologically similar to the first has relatively high cao al 2 o 3 ratios and relatively low tio 2 and na 2 o this second type of peridotite cannot be simply related to the genesis of typical morbs chilled margins and fine grained dikes along the margins of some of these peridotite bodies are high magnesium basalts

two dimensional seismic reflection modeling of the inferred fossil oceanic crust mantle transition in the bay of islands ophiolite we investigate the origin and character of oceanic moho reflections by computing two dimensional synthetic seismogram profiles of the inferred fossil oceanic crust mantle transition observed in the bay of islands ophiolite to simulate a seismic reflection experiment we calculated near vertical incidence seismograms at a horizontal spacing of 500 m for three separate sections of the ophiolite totaling 64 km in length in the synthetic profiles the moho reflection event varies from a single phase to two or more phases of up to 1 s two way travel time total duration individual phases show lateral variation in amplitude and their two way travel times vary by as much as 0 25 s over horizontal distances as short as 10 km lateral discontinuity of phases results in abrupt variations in the travel times of first arriving high amplitude moho phases the geological structures generating the highest amplitude moho reflections vary from high and low velocity lenses of mafic and ultramafic material in the lower crust and upper mantle to interlayered mafic and ultramafic lithologies in the moho transition zone reflection amplitudes from the residual upper mantle are insignificant and our modeling suggests that using the first arriving high amplitude moho phase to estimate thickness of magmatic material might result in errors of up to 1 s two way travel time 3 4 km multichannel seismic data from both the western pacific and western north atlantic show moho travel time variations similar to those observed in the synthetic profiles the western north atlantic data also show multiphase moho reflection events that are laterally discontinuous on a scale similar to that observed in the synthetic data suggesting that the structures observed in the inferred fossil crust mantle transition of the ophiolite are characteristic of oceanic lithosphere

the structure of cretaceous oceanic crust of the nw pacific constraints on processes at fast spreading centers a 725 km long transect along a flow line in the nw pacific provides new images of the internal structure of cretaceous oceanic crust formed at fast spreading rates reflections from the moho transition zone document changes in crustal travel time along the profile variations with a wavelength of 100 150 km may representing fluctuations in the magma supply with a 2 3 m y periodicity to the south the crust may be several kilometers thicker than normal in places possibly resulting from increased magma supply due to the distal effects of the shatsky hotspot at about the depth expected for the boundary between seismic layers 2 and 3 sub horizontal reflections are imaged over much of the profile the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of 0 1 implying a sharp velocity and density discontinuity northward ridgeward dipping reflections in the lower crust are imaged over much of the profile particularly toward the north away from the region possibly affected by the hotspot these reflections stop at the moho reflection but die out perhaps 1 s beneath top basement they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center the discrete character and in places regular spacing every 8 km of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0 1 0 2 m y

the seismic structure of 140 myr old crust in the western central atlantic ocean summary a detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone obh receivers was carried out over mesozoic magnetic anomaly m17 about 300 km south west of bermuda amplitude and travel time interpretations show this 140myr old crust to be uniform on the seismic scale over a lateral distance of almost 1ookm the best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7 2 km the principle components beginning with a 5 km s l velocity at the top of layer 2 are a 0 4 km thickness with gradient 1 1 s a 1 9km thickness with gradient 0 64s 1 7 km thickness with gradient 0 1 s a 3 2 km thickness with zero gradient and constant velocity of 7 km s below which lies a 0 5 km thick moho transition zone the uppermost few kilometres of the upper mantle apparently has little or no velocity gradient the normal incidence two way reflection time through this structure agrees to better than 0 1 s with the location of the moho reflection seen on the ipod usgs multichannel reflection profile that passes within 1 km of this experiment the travel times of the converted shear wave arrivals constrain poisson s ratio in the igneous crust to 0 28 k 0 01 a delay time study of a 20km radius circle of shots fired around a small 6 km five element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres the observed differences in shallow crustal delay time between the five obh instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile these experiments were located clear of fracture zones or other structural anomalies in a region of well defined linear constant amplitude magnetic anomalies we propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres we suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies unknown at the time and the

seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern alaska laboratory velocity measurements and structural field relations explain the observation of a significant seismic anisotropy within highly foliated rocks in southern alaska the orientation of the principal compressive stress indicates that this anisotropy is not related to extensive dilatancy but can be satisfactorily explained by the foliation of the metasedimentary rocks laboratory measurements indicate a significant anisotropy within these rocks even at pressures well above those thought to close microcracks these observations document that anisotropy related to preferred mineral orientation may be widespread in continental rocks thus pervasive foliation can play a more important role in causing anisotropy than dilatancy and inferences of principal stress orientations solely from anisotropy may be erroneous

new type of olivine fabric from deformation experiments at modest water content and low stress a new type of olivine fabric was found by high strain simple shear deformation experiments in the presence of trace amounts of water at 0 5 2 2 gpa and 1470 1570 k in this fabric called e type fabric the olivine 100 axis is subparallel to the shear direction and the 001 plane is parallel to the shear plane this geometry suggests that the 100 001 slip system makes the dominant contribution to total strain this fabric is dominant at a modest water content 200 coh 1000 h 10 6 si at low stresses and high temperatures some mylonites from peridotite massifs show this type of olivine fabric which suggests the presence of water during the shear localization the seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric a type but the magnitudes of anisotropy are different between the two types for horizontal flow the amplitude of vsh vsv anisotropy is weaker but the amplitude of shear wave splitting is stronger for the e type fabric than for the a type dry fabric seismic anisotropy in the oceanic upper mantle may be due to the olivine e type fabric

shear wave splitting measurements problems and solutions abstract we compare sensitivity and reliability of three basic techniques of shear wave splitting analysis cross correlation of wave components minimization of covariance matrix eigenvalues and minimizing energy on the transverse component of both synthetic and real data point out some problems and propose solutions we discuss major difficulties of evaluating shear wave splitting parameters which are not always obvious in routine processing and can sometimes lead to false results and wrong interpretations we describe effects of pre processing of digital records of shear waves e g filtering and setting lengths of analyzed signals and discuss reliability of results to minimize outliers in evaluated splitting parameters for reliable evaluation of splitting of medium quality sks wave forms which are valuable mainly in temporary deployments of stations we recommend a careful band pass filtering of the signal using a window length not shorter than the dominant period inspecting the signal noise ratio on t component and the particle motion the transverse minimization method provides usually more robust and stable solutions than the other two methods in the conclusion we suggest how to proceed in evaluation of splitting parameters to ensure reliable solutions even for medium or low quality shear waveforms recorded frequently during temporary field measurements

geologic and seismic velocity structure of the crust mantle transition in the bay of islands ophiolite complex geological investigations of the bay of islands ophiolite complex show that while a typical layered ophiolite suite is present the thickness of major lithologic units is extremely variable from place to place the composition and internal structure of map scale kilometers across lithologic units as well as the contacts that bound them are laterally variable inferred velocity depth functions of the crust mantle moho transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure the crust mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick the mafic ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long these units megalenses may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units solid state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units substantial relief and slopes of up to at least 12 over lateral distances of about 10 km occur on the top of the crust mantle transition zone as well as other geologic unit contacts although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere

mineral chemistry of ultramafic cumulates from the north arm mountain massif of the bay of islands ophiolite evidence for high pressure crystal fractionation of oceanic basalts the mineral chemistry of ultramafic cumulates forming the basal portions of the plutonic section of the north arm mountain massif bay of islands ophiolite complex is not consistent with crystal liquid fractionation of a primitive mid ocean ridge basalt at low pressures the appearance of clinopyroxene and orthopyroxene having very high mg numbers 100 mg mg fe 85 to 92 and the absence of plagioclase as early fractionating phases coprecipitating with forsteritic olivine fo86 92 suggest that moderate to high pressure phase relationships are applicable we propose on the basis of mineral chemistry crystallization sequences and structural relationships that the basal ultramafic cumulates on the north arm massif represent the products of high pressure 10 kbar polybaric crystal fractionation of primary basaltic melts generated within the mantle beneath a mid ocean spreading center these basal ultramafic cumulates were probably precipitated due to in situ crystallization processes along the margin of a vertical conduit extending downward from a crustal level magma chamber to mantle depths that exceed 30 km after their formation the cumulates were transported to the near surface along the axis of upwelling and divergence as the lithosphere was separating beneath a mid ocean ridge

seismic anisotropy above and below the subducting nazca lithosphere in southern south america 1 the goal of this study is to better constrain anisotropy and mantle flow above and below the nazca slab from 28 s to 42 s through modeling of shear wave splitting in local s sks and skks sk k s phases comparisons of local s splitting times and path lengths in the slab mantle wedge and upper plate indicate that splitting times for arc and back arc stations are consistent with anisotropy in the mantle wedge but long slab paths to fore arc stations imply that slab anisotropy is also significant sk k s shear wave splitting observations and models for sub slab anisotropy show that significant anisotropy is present below the slab and that the orientation of sub slab anisotropy sometimes differs from anisotropy above the slab anisotropy both above the slab and below the slab in the south american subduction zone is consistent with mantle flow that is driven by a combination of entrainment with downgoing slab motion and flow complexity related to variations in slab shape and slab rollback

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

old pacific crust near hawaii a seismic view an expanding spread profile esp over 80 ma pacific ocean crust near hawaii was modeled with a one dimensional solution for travel times relative amplitudes and sometimes frequency content the resulting profiles of compressional and shear wave velocities and poisson s ratio are similar to those determined by other ocean crustal studies except that the shear wave q in the sediments is very low 10 and some unusual high frequency high phase velocity arrivals require a laminated structure within the mid crustal section at about 3 km below the base of the sediment the laminations are probably alternating layers of gabbro and ultramafic cumulates the cumulates may be fractionation products from an upper level magma chamber that was present at the time of crustal formation reflections which may correspond to these laminations have been observed in reflection records both near this esp and in the north atlantic comparison of different ophiolite models for ocean crust reveal that the most commonly used velocity lithology model is probably in error for the pillow and dike basalts resulting in an interpretation of overly thick layers in previous studies of refraction lines the preferred interpretation is that the average total thickness of the pillow and dike sequences is between 1 0 and 1 5 km

geological and geophysical investigation of the mid cayman spreading centre seismic velocity measurements and implications for the constitution of layer 3 summary in this paper we present laboratory measurements of compress ional and shear wave velocities of a diverse suite of gabbroic rocks collected from the walls of the mid cayman spreading centre with dsrv alvin the degree of deformation and alteration affecting these gabbros is quite variable and we believe that they are typical of plutonic rocks emplaced at shallow levels upper portion of seismic layer 3 and shallower of the oceanic crust the compositional and textural variations are reflected in the wide range of laboratory velocities which span most of the range of seismic velocities reported for oceanic and ophiolite rock samples including basalts gabbros ultramafics and their altered derivatives based upon the laboratory velocities and the geological setting of the mid cayman gabbros it is argued that no unique lithology except anhydrous peridotite can be unequivocally identified in the oceanic lithosphere from seismic velocity data alone furthermore these data allow for the possibility of considerable lithologic heterogeneity within portions of the oceanic crust at the scale of a few centimetres to a few hundred metres such heterogeneities would go unrecog nized because seismic refraction studies mask these variations resulting in a picture of apparent uniformity

seismic anisotropy in the sumatra subduction zone an important tool for understanding deformation occurring within a subduction zone is the measurement of seismic anisotropy through observations of shear wave splitting sws in sumatra two temporary seismic networks were deployed between december 2007 and february 2009 covering the fore arc between the fore arc islands to the back arc we use sks and local sws measurements to determine the type amount and location of anisotropy local sws measurements from the fore arc islands exhibit trench parallel fast directions which can be attributed to shape preferred orientation of cracks fractures in the overriding sediments in the sumatran fault region the predominant fast direction is fault trench parallel while in the back arc region it is trench perpendicular the trench perpendicular measurements exhibit a positive correlation between delay time and raypath length in the mantle wedge while the fault parallel measurements are similar to the fault parallel fast directions observed for two crustal events at the sumatran fault this suggests that there are two layers of anisotropy one due to entrained flow within the mantle wedge and a second layer within the overriding crust due to the shear strain caused by the sumatran fault sks splitting results show a nnw sse fast direction with delay times of 0 8 3 0 s the fast directions are approximately parallel to the absolute plate motion of the subducting indo australian plate the small delay times exhibited by the local sws 0 05 0 45 s in combination with the large sks delay times suggest that the anisotropy generating the teleseismic sws is dominated by entrained flow in the asthenosphere below the slab

ephemeral magma chambers in the trinity peridotite northern california abstract the trinity massif comprises the major lithologies of an ophiolite as defined at the 1972 penrose conference previous studies have shown however that it differs from the semail oman or table mountain newfoundland ophiolitic massifs particularly because its crustal section is thin and because its mantle section has vertical plastic flow planes these features have led to an interpretation of the trinity massif as a fragment of slow spreading oceanic lithosphere le sueur et al 1984 boudier and nicolas 1985 in this paper we show that the trinity gabbros occur in discontinuous kilometre sized pockets intrusive into the mantle peridotites the internal stratigraphy and the petrological characteristics of these gabbros suggest that they formed in short lived magma chambers these ephemeral magma chambers developed after the end of the plastic deformation in the surrounding mantle when it had cooled down to lithospheric temperatures we discuss the possibility that these small and ephemeral magma chambers formed at a slow spreading oceanic ridge

ophiolites synthetic seismograms and ocean crustal structure 1 comparison of ocean bottom seismometer data and synthetic seismograms for the bay of islands ophiolite a series of synthetic seismograms based on the bay of islands ophiolite data are compared with ocean bottom seismometer seismic refraction observations which sample 60 m y crust in the northeastern pacific the general agreement between the synthetics for the ophiolite and the observations support the idea that some ophiolites are representative of normal ocean crust this support extends to similarities in complex second arrivals in the synthetics and observations which often are not used in the interpretation of marine seismic data the use of the ophiolite model also allows us to show how the interpretation of seismic data is affected by experimental design and interpretation procedures as the sampling density increases and as more phases in the observations are accounted for the velocity depth model more closely approaches that found for the ophiolite the results of this modeling support sharper p velocity transitions within the crust than are found in young 12 m y age oceanic crust the need for a thin approximately 1 km thick p low velocity zone at the base of the crust is interpreted as support for serpentinization at the crust mantle boundary serpentinization of the ultramafic component in the bay of islands ophiolite therefore may include a small preemplacement component

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

crustal structure of the propagating tammar ridge segment on the mid atlantic ridge 21 5 n active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time the mechanisms controlling ridge propagation however are poorly understood at the slow spreading mid atlantic ridge at 21 5 n a seismic refraction and wide angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply thus crust is thickest 8 km at the domed segment center and decreases in thickness toward both segment ends however thicker crust is formed in the direction of ridge propagation suggesting that melt is preferentially transferred toward the propagating ridge tip further while seismic layer 2 remains constant along axis seismic layer 3 shows profound changes in thickness governing variations in total crustal thickness this feature supports mantle upwelling at the segment center thus fluid basaltic melt is redistributed easily laterally while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply the onset of propagation seems to have coincided with the formation of thicker crust suggesting that propagation initiation might be due to changes in the melt supply after a rapid initiation a continuous process of propagation was established the propagation rate seems to be controlled by the amount of magma that reaches the segment ends the strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length

upper mantle peridotites in the bay of islands ophiolite newfoundland formation during the final stages of a spreading centre abstract the structure of upper mantle rocks in ophiolites reflects the high temperature flow beneath an oceanic spreading centre locally overprinted by detachment related strain the structure of the mantle section in the ophiolitic table mountain massif bay of islands complex boic newfoundland was investigated in detail it is correlated with other massifs in the boic and the spreading related mantle flow history is reconstructed six structural units can be recognized on the basis of changes in the attitude of the high temperature mineral stretching lineation the relative age relationships between the structural units can partly be established they support the plate thickening model for oceanic lithosphere formation the structural history of the mantle section suggests that early plate driven flow was followed by forced flow underplating of an off axis diapir to the base of the lithosphere and detachment this complex history is explained in a model of a decreasing spreading rate which tracks the waning stages of spreading before detachment of the ophiolite during this final history the rate of spreading was decoupled from the rate of mantle upwelling so that a large forced flow field could build up in the blow me down bmd massif of the boic all spreading related flow structures are parallel to the ridge suggesting that the structures and the massif were frozen in near the ridge itself the presence of several kilometres of massive dunites in the bmd massif might indicate that in the final stages of spreading melt production was higher than could be accommodated by spreading so that melt ponded at the base of the crust lherzolites occurring at the base of the boic are considered as product of non adiabatic mantle upwelling prevalent in the final stages of spreading

lateral heterogeneity in the seismic structure of the oceanic crust inferred from velocity studies in the bay of islands ophiolite newfoundland received 1981 june 26 in original form 1980 october 17 summary in the bay of islands ophiolite complex of western newfoundland lower ordovician crust and upper mantle are exposed in four massifs two of these the blow me down and north arm massifs contain complete or nearly complete ophiolite successions ranging from clastic sediments at the top through pillow basalts sheeted dykes gabbros and peridotites at the base the detailed velocity structure of the north arm massif has been reconstructed from compressional and shear wave velocity measurements to confining pressures of 6 kbar and low pore pressures for 125 cores collected from 3 sites of known stratigraphic level and is compared with the previously determined seismic structure of the blow me down massif within the upper 2km of north arm compressional wave velocities increase from 5 0 to 7 0 km s and shear wave velocities increase from 2 8 to 3 8 km s velocities at this level in the blow me down massif are higher due to the presence of pumpellyite in both massifs the top of layer 3 occurs within the sheeted dyke complex at a depth of approximately 1 5 km and marks the transition from greenschist to amphibolite facies metadolerites high level gabbros and plagiogranites at depths of between 2 and 4 km are responsible for velocity inversions in north arm poisson s ratios at this depth are relatively high 0 3 1 0 34 whereas in blow me down abundant quartz rich plagiogranites give rise to low poisson s ratios 0 25 0 27 the lowest crustal unit in north arm which consists primarily of gabbro and amphibolite has average compressional and shear wave velocities of 7 0 and 3 8 km s respectively within this section 4 6 km the velocities decrease slightly with increasing depth in north arm but increase abruptly at 5 5 km depth in blow me down in the north arm massif the transition from crust to mantle is rather gradual due to the presence of a relatively thick mixed zone of gabbro and ultramafics

gabbro layering induced by simple shear in the oman ophiolite moho transition zone we investigate the origin of modal layering in gabbro lenses of the moho transition zone in the oman ophiolite with a microstructural study gabbro lenses exhibit a shape preferred orientation of plagioclase crystals that are euhedral and devoid of any intracrystalline deformation this texture and field kinematic indicators show a strong simple shear deformation in magmatic conditions the parallelism of the lineation in gabbros and the plastic lineation of the host dunite indicate that their development is contemporaneous and that the magmatic features are passively coupled to the solid plastic flow of the host mantle we also found undeformed isotropic and weakly deformed roughly layered gabbros the host rock is plagioclase and clinopyroxene impregnated dunite it contains euhedral plagioclase suggesting that some grains crystallized in suspension there is no clear boundary between the gabbro and the host rock on the contrary the limit is diffuse with increasing plagioclase and clinopyroxene content from the dunite to the gabbro over tens of meters this suggests that the gabbro corresponds to melt impregnated dunite we defined 4 gabbro types from unlayered type 1 to well layered type 4 with progressively more continuous and distinct layers we characterized deformation with crystal shape and crystallographic preferred orientation cpo measurements unlayered samples have a random fabric type 2 have a very weak shape fabric and planar cpo defined by olivine and plagioclase 010 planes type 3 have a weak fabric with a lineation defined by the 001 olivine axis and the 100 plagioclase axis which reflects the shape fabric type 4 have a strong shape fabric and cpo with a lineation defined by the 100 olivine and plagioclase axis we interpret the progression from isotropic to clear s l fabrics as a result of increasing deformation imposed by the flowing host mantle the type 2 gabbros must result from compaction with increasing simple shear the lineation is developed in type 3 samples finally with lower melt content suspension flow switches to plastic deformation in the olivine rich layers leading to plastic olivine fabrics in type 4 samples we deduce that layering originates from simple shear which other studies show induces strong segregations in suspended particles and plastically deformed rocks to build the isotropic stage melt must accumulate faster than the deformation suggesting that melt is extracted by dikes

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

anisotropic gradients in the upper mantle pn amplitudes in some widely spaced sets of orthogonal marine refraction lines on young oceanic crust are greater in the fast direction than in the slow direction this is inconsistent with the predicted amplitude behavior for simple head waves but can be explained by an increase in anisotropy with depth it appears that these gradients are due to increasing olivine crystal orientation although changes in the relative abundance of two anisotropic minerals without variable tectonization could also account for the observations depth variation of tectonization most probably indicates very high temperature gradients at the moho this would imply a substantial amount of convective heat transport in the whole oceanic crust near mid ocean rises

quantifying uncertainties in attenuation estimation at methane hydrate bearing zones using sonic waveform logs abstractapplication of seismic attenuation estimation using sonic waveform data is limited because the estimation methods have not yet been fully developed although the median frequency shift method is considered to be effective and robust compared to the conventional spectral ratio method we demonstrated that the median frequency shift methods strongly depend on reference data under lower signal to noise ratios we modified an existing median frequency shift method not to depend on arbitrarily choosing a reference value and to quantify the uncertainties in attenuation estimation furthermore we implied the optimum selection of receiver pairs used for more stable attenuation analysis our numerical experiments supported the advantage of the proposed method although our main findings by applying the proposed methods in methane hydrate bearing sediments are almost consistent with past field sonic logging measurements we find some differences in the magnitude of attenuation values compared to existing s

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization

crustal structure of the eastern algerian continental margin and adjacent deep basin implications for late cenozoic geodynamic evolution of the western mediterranean the karukera spur is an easternmost submerged rise of the lesser antilles fore arc in the guadeloupe archipelago culminating about 4000 m above the fore arc basin 150 km west of the deformation front the analysis of 3500 km 2d high resolution multichannel seismic reflection and multibeam bathymetric data and the study of 14 core samples allow for the first time to reconstruct its sedimentary anatomy and tectonic evolution seven seismic units us1 to us7 are evidenced organised into four major depositional sequences separated by erosional surfaces units us1 to us4 thicken to the south southeast on the spur the sedimentary deposits rest upon a deeply eroded metamorphic basement of the caribbean plate as in la desirade island sequence 1 units us1 and us2 remains unknown in its lower part because it does not outcrop its upper part comprises early miocene shallow water carbonates sequence s2 unit us3 only comprises an upper miocene transgressive systems tract deposited into basinal environment it is bounded by major erosional surfaces sb2 and sb3 sequence 3 us4 to us6 is zanclean to pleistocene it is dominantly composed of shallow water reefal deposits sequence 4 corresponds to recent deposits the karukera spur experienced three main extensional episodes with alternations between uplift and subsidence periods uplifts are responsible for emersions and erosions subsidence severely drowned shallow water deposits the first extensional episode led to the formation of horsts and grabens in the basement prior to early miocene deposits the second extensional episode occurred during upper miocene in accordance with data in surrounding islands the third tectonic episode developed since calabrian noticeable is the constant reactivation of n130 n150 n150 n180 and n40 trending faults since early miocene and the neoformation of e w trending faults since calabrian our results suggest that at the latitude of guadeloupean archipelago the lesser antilles subduction zone is an erosional margin because of the successive sweepings of the santa lucia then tiburon ridges

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma