relic magma chamber structures preserved within the mesozoic north atlantic crust the north atlantic transect seismic reflection data collected southwest of bermuda have been reinterpreted following post stack migration and reveal two major intracrustal reflections the shallower of these two events located 1 s below the igneous basement is a subhorizontal undulating surface that in some places is continuous for as much as 10 km on the basis of its position within the section and its laterally discontinuous nature we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro a second set of lower crustal reflections dipping 20 40 eastward is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection moho several lines of evidence argue against these features being either artifacts or out of the plane events instead their presence may be ascribed either to crustal penetrating fault zones or to mafic ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo spreading center because of the laminated character of these events and their typical occurrence within 1 0 to 1 5 s of the reflection moho we prefer a compositional versus a structural interpretation for their origin the gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities significant departures are recognized elsewhere similarly the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the blake spur fracture zone an interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal thinning model of mutter and others

cryptic variation within the cumulate sequence of the oman ophiolite magma chamber depth and petrological implications the cumulate sequence of the oman ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic long lived crustal magma chamber that underlay a late cretaceous spreading ridge regional variations in cumulate sequence stratigraphy reflect not only contrasting degrees of mantle fractionation but also contrasting primary magma compositions microprobe data are presented from a 600 m section of modally layered olivine gabbros coherent cryptic cyclic variation is shown with both normal and reversed geochemical gradients present these are explained in terms of fractionation mixing and eruption in an open system magma chamber the height of the liquid column required to precipitate the observed cyclic units is calculated to be about 100 m the implications of this result are discussed a contrast between the fo an co variation between olivine and plagioclase in the oman cumulate sequence and that observed for morb votcanics is noted this may reflect compositional stratification in spreading ridge magma chambers or signifcant differences between the petrogenesis of the oman ophiolite and morb

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

inverse square root dependence of mid ocean ridge flank roughness on spreading rate the topographic roughness of mid ocean ridge flanks is known to increase with decreasing spreading rate1 4 but the exact form of this relationship has not been established as the topographic features that make up ridge flank roughness abyssal hills are created near the axes of mid ocean ridges by faulting and volcan ism5 7 the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres here i present measurements of ridge flank roughness on profiles crossing the world mid ocean ridge system which show that roughness is proportional to the inverse square root of the spreading rate this result is consistent with some very simple inferences of how the topographic roughness of mid ocean ridge flanks scales with the lithospheric thickness near ridge axes as determined by thermal models

a low velocity zone within the layer 3 region of 118 myr old oceanic crust in the western north atlantic summary during the north atlantic transect nat experiment 11 expanding spread profiles esps were acquired using a 3300 cubic inch 30 element tuned airgun array s v prospekta and a 48 channel seismic streamer r v moore extremely high density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter tau p with high precision velocity structures have been obtained by tau sum inversion ray tracing and synthetic wkbj and reflectivity seismogram methods both in tau p and x t domains a detailed analysis of esp 5 suggests the presence of a low velocity zone within the lower oceanic crust at the location of magnetic anomaly mo 118 ma it is apparently 2 86km thick has a p wave velocity of 6 5 km s 1 s wave velocity of 3 6 km s 1 and occupies much of the depth interval normally associated with oceanic layer 3 converted shear wave arrivals have been used to obtain the shear velocity structure we find that the vp vs ratio in the uppermost part of the crust is about 1 9 and much of the crustal section has a vp vsratio greater than 3 attenuation of p and s waves has been studied by amplitude analysis and we find that in most of the lower crust qs is equal to qp comparison with solutions from esps in younger and older parts of the basin suggests that esp 5 is unique in displaying a low velocity zone despite this the total thickness at esp 5 is not significantly different than at other locations we suggest that this provides strong evidence that the low velocity zone occurs within the lower crust and does not result from serpentinization of the upper mantle beneath thin oceanic crust

complex structure along a mesozoic sea floor spreading ridge birps deep seismic reflection cape verde abyssal plain summary as part of an intensive study of a small area of oceanic lithosphere the british institutions reflection profiling syndicate birps acquired closely spaced deepseismic reflection profiles over the early cretaceous crust of the cape verde abyssal plain off west africa the survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea floor spreading axis isochron profiles and orthogonal dip lines oriented in the original direction of spreading flow profiles a large capacity well tuned airgun source and very quiet shooting conditions ensured a high signal to noise ratio for deep reflection devising a strategy for mitigating contamination from wrap around multiples arriving from previous shots enabled us to use the minimum possible shot point interval 50 m allowed for collecting long 18 s records data processing was oriented towards a medium with low root mean square velocity steeply dipping structure and pervasive low apparent velocity noise from diffraction at the top of the igneous crust the contrast between the isochron and flow profiles is striking isochron profiles are typically highly reflective throughout the igneous crust consisting of bright bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted moho reflection these reflections do not offset intracrustal or top basement structure and thus are not interpreted as faults an igneous intrusive origin seems more likely flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust continuing down in places to the moho dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust the dominant dip of these reflections is to the west towards the spreading ridge axis reflections from the vicinity of the moho while well developed in some places are not particularly prominent across the survey area moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles on isochron profiles dipping reflections occasionally flatten out into and may merge with the moho reflection on flow profiles as dipping crustal reflections approach the moho reflection they are usually abruptly cut off by it without extending deeper this survey shows how oceanic crustal structure can vary rapidly over relatively small areas provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust and gives a conclusive observation of faults that penetrate the entire oceanic crust

seismic reflection imaging of the juan de fuca plate from ridge to trench new constraints on the distribution of faulting and evolution of the crust prior to subduction we present prestack time migrated multichannel seismic images along two cross plate transects from the juan de fuca jdf ridge to the cascadia deformation front df offshore oregon and washington from which we characterize crustal structure distribution and extent of faults across the plate interior as the crust ages and near the df in response to subduction bending within the plate interior we observe numerous small offset faults in the sediment section beginning 50 70 km from the ridge axis with sparse fault plane reflections confined to the upper crust plate bending due to sediment loading and subduction initiates at 120 150 km and 65 80 km seaward of the df respectively and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity most bend faulting deformation occurs within 40 km from the df on the oregon transect bright fault plane reflections that extend through the crust and 6 7 km into the mantle are observed if attributed to serpentinization 0 12 0 92 wt water within the uppermost 6 km of the mantle is estimated on the washington transect bending faults are confined to the sediment section and upper middle crust the regional difference in subduction bend faulting and potential hydration of the jdf plate is inconsistent with the spatial distribution of intermediate depth intraslab seismicity at cascadia a series of distinctive ridgeward dipping 20 40 lower crustal reflections are imaged in 6 8 ma crust along both transects and are interpreted as ductile shear zones formed within the ridge s accretionary zone in response to temporal variations in mantle upwelling possibly associated with previously recognized plate reorganizations at 8 5 ma and 5 9 ma

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

crooked line 2d seismic reflection imaging in crystalline terrains part 2 migration seismic reflection surveys are frequently conducted over very complicated geological structure but surveying often must be confined to existing crooked roads or tracks typically data from such 2d crooked line surveys are processed using standard common midpoint cmp stacking techniques to obtain a 2d time section which is then 2d migrated in part i we show that a reflector dip component across the processing line can cause serious problems for standard cmp stacking we also propose a supplementary processing step in which cross dip is determined locally and cross dip moveout cdmo is removed from data to form an optimum cross dip stack however a crooked line survey is really a swath 3d survey and ideally we would like to obtain a 3d image of reflectivity surrounding the profile here we investigate the potential of 3d prestack kirchhoff migration to directly image all observed reflections i e we attempt to construct a 3d image volume of all reflectors viewed by the survey because reflectors t

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

systematic changes in the incoming plate structure at the kuril trench 1 recent seismic structural studies in trench outer rise regions have shown that vp within the incoming oceanic plate systematically decreases toward the trench probably owing to bending and fracturing of the plate to understand the mechanisms acting to reduce vp vs is critical because the vp vs ratio is a sensitive indicator of lithology porosity and the presence of fluid in the outer rise region of the kuril trench we conducted an extensive seismic refraction and reflection survey that revealed systematic changes in vp vs and vp vs our results suggest that water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending related fractures at the top of the oceanic crust consistent with the seawater percolation our results support the idea that plate bending and fracturing during the bending in the outer rise of the trench play an important role in the water cycle of subduction zones

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

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

madagascar s escape from africa a high resolution plate reconstruction for the western somali basin and implications for supercontinent dispersal accurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models here we provide a new plate tectonic reconstruction of the opening of the western somali basin during the breakup of east and west gondwana the model is constrained by a new comprehensive set of spreading lineaments detected in this heavily sedimented basin using a novel technique based on directional derivatives of free air gravity anomalies vertical gravity gradient and free air gravity anomaly maps also enable the detection of extinct mid ocean ridge segments which can be directly compared to several previous ocean magnetic anomaly interpretations of the western somali basin the best matching interpretations have basin symmetry around the m0 anomaly these are then used to temporally constrain our plate tectonic reconstruction the reconstruction supports a tight fit for gondwana fragments prior to breakup and predicts that the continent ocean transform margin lies along the rovuma basin not along the davie fracture zone dfz as commonly thought according to our reconstruction the dfz represents a major ocean ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as madagascar drifted southwards and offshore tanzania is an obliquely rifted rather than transform margin new seismic reflection evidence for oceanic crust inboard of the dfz strongly supports these conclusions our results provide important new constraints on the still enigmatic driving mechanism of continental rifting the nature of the lithosphere in the western somali basin and its resource potential

polyphase rifting and break up of the central mozambique margin abstract the break up of the gondwana supercontinent resulted in the formation of the central mozambique passive margin as africa and antarctica were separated during the mid jurassic period although plate kinematics during the oceanic spreading phase are well constrained the initial fit of africa and antarctica their earliest relative movements and margin architectures remain active areas of interest this study uses high quality multi channel seismic reflection profiles to identify the major crustal domains in the angoche and beira regions of the central mozambique margin our results show that the central mozambique passive margin is characterised by intense but localised magmatic activity evidenced by the existence of seaward dipping reflectors sdrs in the angoche region and magmatic sills and volcanoclastic material marking the beira high the angoche and beira regions possess faulted upper continental crusts with possible exhumation of lower crustal material forming an extended ocean continent transition oct the beira high segment reveals an offshore continental fragment which is overlain by a faulted pre rift sedimentary unit likely to belong to the karoo group the combination of our seismic interpretation with existing geophysical and geological data has allowed us to propose a break up model which supports the idea that the central mozambique margin was affected by polyphase rifting the beira high basement is formed by a strike slip deformation along a proposed lithospheric weakness the lurio pebane shear zone northwestern southeastern oriented extension follows and results in continental break up and oceanic spreading our results suggest a segmentation of the central mozambique margin with oceanisation first occurring in the angoche segment the formation of the first oceanic crust in the beira segment followed likely delayed by the formation and failure of the northern beira high rift

a major perturbation of the carbon cycle before the ghaub glaciation neoproterozoic in namibia prelude to snowball earth 1 a large 11 15 negative shift in 13c is observed in shallow water carbonates directly beneath neoproterozoic glacial deposits or correlative disconformity in northwest namibia ascribed to a snowball earth reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin and field relations show it predates the fall in sea level associated with the ghaub glaciation we crudely estimate the duration of the isotopic shift as 0 6 106 years from a simple thermal subsidence model similar or larger 13c anomalies are found directly beneath neoproterozoic glacial units in australia canada china scotland and svalbard after considering conventional interpretations for negative 13c anomalies we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale magnitude and geological context of the anomaly in namibia counterintuitively an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation

faulting and hydration of the juan de fuca plate system abstract multichannel seismic observations provide the first direct images of crustal scale normal faults within the juan de fuca plate system and indicate that brittle deformation extends up to 200 km seaward of the cascadia trench within the sedimentary layering steeply dipping faults are identified by stratigraphic offsets with maximum throws of 110 10 m found near the trench fault throws diminish both upsection and seaward from the trench long term throw rates are estimated to be 13 2 mm kyr faulted offsets within the sedimentary layering are typically linked to larger offset scarps in the basement topography suggesting reactivation of the normal fault systems formed at the spreading center imaged reflections within the gabbroic igneous crust indicate swallowing fault dips at depth these reflections require local alteration to produce an impedance contrast indicating that the imaged fault structures provide pathways for fluid transport and hydration as the depth extent of imaged faulting within this young and sediment insulated oceanic plate is primarily limited to approximately moho depths fault controlled hydration appears to be largely restricted to crustal levels if dehydration embrittlement is an important mechanism for triggering intermediate depth earthquakes within the subducting slab then the limited occurrence rate and magnitude of intraslab seismicity at the cascadia margin may in part be explained by the limited amount of water imbedded into the uppermost oceanic mantle prior to subduction the distribution of submarine earthquakes within the juan de fuca plate system indicates that propagator wake areas are likely to be more faulted and therefore more hydrated than other parts of this plate system however being largely restricted to crustal levels this localized increase in hydration generally does not appear to have a measurable effect on the intraslab seismicity along most of the subducted propagator wakes at the cascadia margin

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

network of off axis melt bodies at the east pacific rise at faster spreading mid ocean ridges the creation of new oceanic crust through magmatism usually occurs within a narrow zone on the ridge axis three dimensional seismic images of the fast spreading east pacific rise reveal a network of magmatic bodies 4 8 km away from the ridge axis that seem to be connected to the axial magma chamber

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

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

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

alteration of the subducting oceanic lithosphere at the southern central chile trench outer rise hydrothermal circulation and brittle faulting processes affecting the oceanic lithosphere are usually confined to the upper crust for oceanic lithosphere created at intermediate to fast spreading rates lower crust and mantle rocks are therefore relatively dry and undeformed however recent studies at subduction zones suggest that hydration of the oceanic plate is most vigorous at the trench outer rise where extensional bending related faulting affects the hydrogeology of the oceanic crust and mantle to understand the degree of hydration we studied the seismic velocity structure of the incoming nazca plate offshore of southern central chile 43 s here the deep sea trench is heavily filled with up to 2 km of sediments seismic refraction and wide angle data complemented by seismic reflection imaging of sediments are used to derive a two dimensional velocity model using joint refraction and reflection traveltime tomography the seismic profile runs perpendicular to the spreading ridge and trench axes the velocity model derived from the tomography inversion consists of a 5 3 km thick oceanic crust and shows p wave velocities typical for mature fast spreading crust in the seaward section of the profile with uppermost mantle velocities as fast as 8 3 km s approaching the chile trench seismic velocities are significantly reduced however suggesting that the structures of both the oceanic crust and uppermost mantle have been altered possibly due to a certain degree of fracturing and hydration the decrease of the velocities roughly starts at the outer rise 120 km from the deformation front and continues into the trench even though the trench is filled with sediment basement outcrops in the outer rise frequently pierce the sedimentary blanket anomalously low heat flow values near outcropping basement highs indicate an efficient inflow of cold seawater into the oceanic crust hydration and crustal cracks activated by extensional bending related faulting are suggested to govern the reduced velocities in the vicinity of the trench considering typical flow distances of 50 km water might be redistributed over most of the trench outer rise area where trapped in faults seawater may migrate down to mantle depth causing up to 9 of serpentinization in at least the uppermost 2 km of the mantle between the outer rise and the trench axis

a global scale plate reorganization event at 105 100 ma abstract a major plate reorganization is postulated to have occurred at approximately 100 ma however this reorganization has received limited attention despite being associated with the most prominent suite of fracture zone bends on the planet and many other geological events we investigate tectonic events from the period 110 to 90 ma and show that the reorganization occurred between 105 and 100 ma was global in scale and affected all major plates seafloor evidence for plate motion changes is abundant during this period with either fracture zone bends or terminations preserved in all ocean basins long lived eastern gondwanaland subduction ended along a 7000 km long section of the margin while elsewhere around the proto pacific rim subduction continued and there is evidence that compressional stresses increased in the overriding plates thrusting in western north america transpression and basin inversion in eastern asia and development of the present day andean style margin along western south america occurred contemporaneous with the development of an extensional regime in eastern gondwanaland basin instability in africa and western europe further demonstrates that lithospheric stress regime changes were widespread at this time considering the timing of the reorganization and the nature of associated plate boundary changes we suggest that eastern gondwanaland subduction cessation is the most likely driving mechanism for the reorganization subduction is the dominant driver of plate motion and therefore this event had the potential to strongly modify the balance of driving forces acting on the plates in the southwestern proto pacific and neighboring plates whereby producing widespread changes in plate motion and continental lithospheric stress patterns we propose that major changes in ridge trench interaction triggered the cessation of subduction the progressive subduction of two closely spaced perpendicular mid ocean ridges at the eastern gondwanaland subduction zone to the east of australia and new zealand respectively resulted in very young crust entering the trench and we suggest that by 105 100 ma there was insufficient negative buoyancy to drive subduction finally we propose that the plume push force of the bouvet plume that erupted near the african antarctic south american triple junction contributed to plate motion changes in the southern atlantic region

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

an alternative early opening scenario for the central atlantic ocean abstract the opening of the central atlantic ocean basin that separated north america from northwest africa is well documented and assumed to have started during the late jurassic however the early evolution and the initial breakup history of pangaea are still debated most of the existing models are based on one or multiple ridge jumps at the middle jurassic leaving the oldest crust on the american side between the east coast magnetic anomaly ecma and the blake spur magnetic anomaly bsma according to these hypotheses the bsma represents the limit of the initial basin and the footprint subsequent to the ridge jump consequently the evolution of the northwest african margin is widely different from the northeast american margin however this setting is in contradiction with the existing observations in this paper we propose an alternative scenario for the continental breakup and the mesozoic spreading history of the central atlantic ocean the new model is based on an analysis of geophysical data including new seismic lines an interpretation of the newly compiled magnetic data and satellite derived gravimetry and recently published results which demonstrate that the opening of the central atlantic ocean started already during the late sinemurian 190 ma based on a new identification of the african conjugate to the ecma and on the extent of salt provinces off morocco and nova scotia the identification of an african conjugate magnetic anomaly to bsma the african blake spur magnetic anomaly absma together with the significant change in basement topography are in good agreement with that initial reconstruction the early opening history for the central atlantic ocean is described in four distinct phases during the first 20 myr after the initial breakup 190 170 ma from late sinemurian to early bajocian oceanic accretion was extremely slow 0 8 cm y at the time of blake spur 170 ma early bajocian a drastic change occurred both in the relative plate motion direction from nnw sse to nw se and in the spreading rate an increase to 1 7 cm y after a small increase between chron m25 154 ma kimmeridgian and chron m22 150 ma tithonian the spreading rate slowed down to about 1 3 cm y and remained fairly constant until chron m0 125 ma barremian aptian boundary in addition kinematic reconstructions illustrate a significant spreading asymmetry during the early history of the central atlantic ocean the accretion rates were higher on the american side and led to the formation of more oceanic crust on this plate we infer that this asymmetry could be related to the fact that the thermal anomaly responsible for the significant magmatism of the central atlantic magmatic province camp was preferentially located below the african plate

new seismic images of oceanic crustal structure remarkable new images of the internal structure of oceanic crust have been recorded from a two ship multichannel seismic survey in the western north atlantic results include strong planar dipping reflectors in the lower crust evidence of whole crustal failure generating reflectors from the top of the basement to the moho and banded layering in the lower crust and in some places in the upper mantle anomalous crust in the blake spur fracture zone is documented from both normal incidence and wide angle seismic profiles the internal structure of the crust records the magmatic and tectonic history of the crust as it was generated at and then moved away from the spreading center

genesis of fabric destructive dolostones a case study of the brac formation oligocene cayman brac british west indies abstract the origin of fabric destructive and fabric retentive textures in dolostones of all ages remains open to debate evolution of the fabric destructive textures which effectively destroys all traces of the original limestones remains particularly controversial on cayman brac a small island located in the caribbean sea the brac formation lower oligocene is formed of limestones and fabric destructive dolostones the top of this formation is defined by the brac unconformity which developed as a karst surface during the late oligocene and early miocene the overlying cayman formation lower to middle miocene is pervasively dolomitized and characterized by fabric retentive textures that have preserved the depositional textures of the original limestones the stratigraphic juxtaposition of these two formations provides an ideal opportunity to compare the factors that controlled the development of fabric destructive and fabric retentive fabrics in dolostones the dolostones in the brac formation are formed of dolomite that replaced the limestone and dolomite cements typically zoned that were precipitated in pores throughout the dolostones interpretations based on the composition of the dolomite 13 c values 18 o values and sr fe and mn contents indicate that dolomitization of both the brac formation and the cayman formation was probably controlled by similar slightly modified seawater with the distribution of the dolomite in the limestones and the evolution of dolostone textures being controlled largely by permeability pathways that governed circulation patterns of the dolomitizing fluids the large sucrosic dolomite crystals which are commonly zoned probably developed as a result of repeated cycles of limestone matrix dissolution and dolomite precipitation

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

seismic structure of oceanic crust in the western north atlantic we examine the seismic structure of cretaceous aged ocean crust north of the blake spur fracture zone in the western north atlantic by using a combination of multichannel seismic and wide angle reflection refraction data although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure it displays large variations in crustal reflectivity on both ridge normal and ridge parallel profiles the upper crust is highly reflective and contains both subhorizontal or shallow dipping events and more steeply dipping reflectors subhorizontal reflectors are typically present between 1 5 and 2 km depth and are in some cases laterally continuous for distances of 15 to 20 km generally these events do not correlate with the depth of the seismic layer 2 layer 3 boundary determined from refraction data and their origin is still poorly understood the steeply dipping events are generally confined to the upper 2 5 km of the crust we interpret these reflectors as the subsurface expression of ridge parallel extensional faults commonly mapped at mid ocean ridges the lower crust in this area exhibits alternating regions of high and low reflectivity the highly reflective zones are made up of packages of linear or concave upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust some of these dipping reflectors appear to cut through the whole crustal section although most fade out in the acoustically transparent midcrust on ridge normal profiles the majority of these events dip to the east toward the paleo spreading center whereas on ridge parallel profiles the events typically dip south toward the blake spur fracture zone the base of the crust is generally not associated with a strong discrete moho reflection but is a comparatively indistinct boundary usually associated with a 1 to 2 km thick band of diffuse high reflectivity we interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8 10 km with the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge normal and ridge parallel profiles we may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults alternatively these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere

evidence from the oman ophiolite for active mantle upwelling beneath a fast spreading ridge it is commonly accepted that as oceanic lithosphere cools it slides away from the spreading centre under its own weight1 it has been suggested2 5 that even at the spreading axis this process controls the dynamics causing asthenospheric mantle material to rise passively before being dragged away from the ridge by the overlying lithosphere alternatively the upwelling of the asthenosphere at the ridge might be active caused by the buoyancy of partial melt6 10 or of the less dense peridotite left behind by such melting11 12 at slow spreading ridges bull s eye gravity anomalies13 14 have been interpreted as suggesting focused accre tion and hence active mantle diapirs at fast spreading ridges however these anomalies are weaker and less clearly three dimensional15 which may reflect passive mantle upwelling16 here we show that structural data from the oman ophiolite which is thought to have been created at a fast spreading ridge17 18 are consistent with active mantle upwelling beneath the ridge and not with models of passive flow the absence of well marked bull s eye anomalies at fast spreading ridges may be explained by their relatively constant crustal thickness and near isostatic equilibrium

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

spatial distribution and temporal evolution of crustal melt distribution beneath the east pacific rise at 9 10 n inferred from 3 d seafloor compliance modeling determining the melt distribution in oceanic crust at mid ocean ridges is critical to understanding how magma is transported and emplaced in the crust seafloor compliance deformation under ocean wave forcing is primarily sensitive to regions of low shear velocity in the crust making it a useful tool to probe melt distribution analysis of compliance data collected at east pacific rise between 9 and 10 n through 3 d numerical modeling reveals strong along axis variations in the lower crustal shear velocities as well as temporal variation of crustal shear velocity near 9 48 n between measurements spanning 8 years compliance measured across the rise axis at 9 48 n and 9 33 n suggest a deep crustal low velocity zone beneath the ridge axis with a low vs vp ratio consistent with melt in low aspect ratio cracks or sills changes in compliance measured at 9 48 n between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval perhaps following the 2005 2006 seafloor eruption this temporal variability provides direct evidence for short term variations of the magmatic system at a fast spreading ridge

seismic structure of 15 ma oceanic crust formed at an ultrafast spreading east pacific rise evidence for kilometer scale fracturing from dipping reflectors 1 a grid of seismic reflection data from 15 ma cocos plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading 200 mm yr full rate east pacific rise epr the data contain an unexpected quantity of bright reflectors throughout the crust generally 5 km in length significant reflection dip angles 10 30 are seen only on the isochron profiles and several reflectors have a confirmed isochron dip orientation certain upper crustal reflectors ucrs appear to form a single network with isochron dip the most prominent example apparent reflection coefficient 0 2 projects toward a 70 m disruption in the igneous seafloor and waveform analysis demonstrates that the reflector is likely the product of a low velocity zone on the order of tens of meters thick our observations suggest that the isochron dipping ucrs represent small slip kilometer scale faults fractures and none are primarily related to the seismic 2 3 boundary or to the sheeted dike gabbro transition lower crustal reflectors lcrs have no preferred orientation or regular pattern on the 30 km scale of this survey certain lcrs may be similar to the ucrs in origin while others may originate from near axis ductile flow lithologic banding off axis magmatism or some combination thereof overall reflectors in this area are abundant and complex consistent with previous off axis studies but at odds with observations of young crust 5 ma near the epr this dichotomy may be resolved by a delay in reflector formation through some combination of postaccretionary tectonics magmatism and hydrothermal alteration

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

laterally confined volcanic successions lcvs recording rift jumps during the formation of magma rich margins abstract seaward dipping reflectors sdrs are a characteristic feature of magma rich margins and represent the generation of large volumes of flood basalts at the point of continental breakup a number of recent studies provide new insights into the emplacement and tilting of sdrs and conclude that the majority of sdrs are contained within new magmatic crust that has a close affinity to oceanic crust however the process by which these initial magmatic systems evolve into a fully established spreading centre remains poorly understood several characteristic features of magma rich margins may be explained by the occurrence of rift jumps during sdr emplacement yet the cause and prevalence of such rift jumps remain unknown here we constrain the 3d geometry of the continent ocean transition in the orange basin offshore south africa this allows us to test if where and why such rift jumps occur our results demonstrate an order of along strike segmentation previously unobserved in these settings we demonstrate that the sdr belt is disrupted by the occurrence of a volcanic stratigraphic package defined as the laterally confined volcanic succession lcvs not previously identified on a rifted margin we interpret this as a magmatic spreading centre that was abandoned by a subsequent rift jump identification of lcvss is important for two reasons first we argue that the lcvs formed via the same process as sdrs and hence provides a unique example of sdr geometry prior to their separation onto conjugate plates second as we can map out the 3d geometry of the lcvs and sdrs we propose that rift jumps during magma rich margin formation may be fundamental to the establishment of a laterally continuous incipient spreading centre

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

extinct spreading on the cocos ridge using recently acquired marine magnetic data and existing magnetic and bathymetric data sets together with odp leg 170 age determinations we present a revised plate tectonic model for the southern cocos and northern nazca plate area according to this model the formation of the southern cocos plate was governed by spreading at different ridge axes with alternations between spreading ridges producing a complex magnetic anomaly pattern in the cocos and malpelo ridge area we have identified two precursors of the recently active cocos nacza spreading system which were active from 22 8 to 14 7 ma with a change in spreading direction from nw se to ene wsw at 19 5 ma the oceanic crust of these abandoned spreading systems was subsequently thickened and overprinted by hotspot volcanism that formed the cocos and malpelo ridges the centre of this hotspot volcanism is about 500 km away from but most probably related to the galapagos hotspot

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

birth of an intraoceanic spreading center the cocos nazca spreading center is one of the few examples of the formation of a spreading center by splitting of oceanic lithosphere it was created when the farallon plate broke up in the early miocene following the collision of the pacific farallon spreading center with the north american continent much of the ancient farallon plate corresponding to the area of opening is lost to subduction beneath central america and south america but new data from the conjugate area on the pacific plate allow the first detailed reconstruction of the break up process the opening began after chron 7 25 ma at a location of focused crustal extension caused by overlapping spreading centers that had evolved in response to a slight reorientation of a pacific farallon ridge segment beginning at chron 6b 22 7 ma eastward progressing seafloor spreading started along an axis that most likely migrated toward the region of weak lithosphere created by the galapagos hotspot by chron 6 19 5 ma plate splitting from the spreading center to the trench was complete allowing the fully detached cocos and nazca plates to move independently this kinematic change resulted in a significant ridge jump of the newly established pacific nazca spreading center a change in plate motion direction of the nazca plate by 20 clockwise and a large increase in pacific cocos plate velocity in the middle miocene

origin of petroleum in the neoproterozoic cambrian south oman salt basin abstract the south oman salt basin sosb is host to the world s oldest known commercial deposits most of the south oman oils have been proven to be associated with the source rocks of the neoproterozoic to cambrian huqf supergroup but the assignment of oils to specific huqf intervals or facies has been hampered by the geochemical similarity of the organic matter across the entire huqf sequence possibly as a consequence of limited change in the local palaeoenvironment and biota over the time of its deposition this study was conducted to establish improved correlations between organic rich rock units and reservoir fluids in the sosb through detailed molecular and isotopic analysis of the huqf supergroup with special emphasis directed towards understanding the ara carbonate stringer play unusual biomarkers tentatively identified as a norsteranes show distinctive patterns among carbonate stringer oils and rocks different from those observed in nafun sediments and ara rocks from the athel basin these putative a norsteranes form the basis for new oil source correlations in the sosb and provide for the first time geochemical evidence of a self charging mechanism for the carbonate stringer play the paucity of markers specific to the nafun group shuram buah and masirah bay formations confounds attempts to quantify their respective contributions to huqf oil accumulations nafun inputs can only be determined on the basis of subtle differences between nafun and ara biomarker ratios the most useful geochemical characteristics delineating nafun group organic matter from ara group intra salt source rocks included low relative abundance of mid chain monomethyl alkanes x compounds low relative abundance of gammacerane 28 30 dinorhopane 25 28 30 trinorhopane and 2 methylhopanes low c 22 t c 21 t and high c 23 t c 24 t cheilanthanes ratio values based on these parameters molecular evidence for major contributions of liquid hydrocarbons from nafun group sediments shuram buah and masirah bay formations is lacking our results suggest that the majority of sosb hydrocarbon accumulations originate from within the ara group either from the carbonate stringers or from the package of sediments that comprises the thuleilat athel silicilyte and u shale formations subtle aspects of the composition of some carbonate stringer and post salt huqf oils could suggest some degree of sourcing from the nafun rocks but stronger evidence is needed to confirm this

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

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

link between plate fabric hydration and subduction zone seismicity in alaska subduction carries water into the earth where it can influence seismicity analysis of the structure of the alaskan subduction zone suggests fluid delivery is influenced by faults in the oceanic plate that formed at the mid ocean ridge

multichannel seismic images of the oceanic crust s internal structure evidence for a magma chamber beneath the mesozoic mid atlantic ridge near the base of the mesozoic oceanic crust of the western north atlantic we recognize a discontinuous diffractive layer horizon r moho is a distinct uniform and more continuous horizon beneath variations in seismic thickness of the crust occur primarily through changes in the r to moho layer the maximum thickness occurring midway between fracture zones we propose that horizon r may mark the top of a layered cumulate sequence that formed on the floor of a relatively persistent crustal magma chamber the crust having minimum thickness is spatially restricted to a small strip near fracture zones and comprises only the super r unit we further propose that this implies the sustained accretion of normal oceanic crust at the mesozoic mid atlantic ridge required the presence of a magma chamber

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

seismic anisotropy in the upper mantle 2 predictions for current plate boundary flow models 1 the anisotropic seismic structure due to flow induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary the orientation distributions of olivine enstatite aggregates evolve along streamlines of each flow model based on each grains plastic response to the local stress strain field the effective elastic tensor for these textured aggregates provides predictions of p wave anisotropy and shear wave splitting throughout the model space p and s travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model comparison of predicted fast p direction for our method versus a finite strain based estimate shows that agreement is acceptable for much of the model space but notable differences occur in regions up to several tens of kilometers in size two dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle the effect of flow in the third dimension is addressed in a few calculations for one comparison of flow in the mantle wedge at a subduction zone the introduction of trench parallel flow causes significant changes in the predicted patterns of p wave anisotropy magnitude more than orientation and sks splitting

deep and high temperature hydrothermal circulation in the oman ophiolite petrological and isotopic evidence a systematic search for evidence of high temperature hydrous alteration within the gabbros of the samail ophiolite oman shows that most of the gabbros have been affected by successive stages of alteration starting above 975 c and ending below 500 c sr and o isotopic analyses provide constraints on the nature and origin of the fluids associated with these alteration events massive gabbros dykes and veins and their associated minerals depart from mid ocean ridge basalt morb source magma signatures 87sr 86sr 0 7032 and depleted 18o 10 a second group of samples including massive gabbros and high temperature dykes and veins is characterized by lower water rock ratios in the range of 3 5 these samples display a high temperature hydrothermal alteration related to seawater ingression the main fluid channels may be sub millimetre sized microcracks with a dominantly vertical attitude which constitute the recharge hydrothermal system whereas the dykes and veins represent the discharge part this model requires that these dykes have been generated by hydration of the crystallizing gabbros via seawater penetration near the internal wall of the magma chamber

a 2 d tomographic model of the juan de fuca plate from accretion at axial seamount to subduction at the cascadia margin from an active source ocean bottom seismometer survey we report results from a wide angle controlled source seismic experiment across the juan de fuca plate designed to investigate the evolution of the plate from accretion at the juan de fuca ridge to subduction at the cascadia margin a two dimensional velocity model of the crust and upper mantle is derived from a joint reflection refraction travel time inversion to interpret our tomography results we first generate a plausible baseline velocity model assuming a plate cooling model and realistic oceanic lithologies we then use an effective medium theory to infer from our tomography results the extent of porosity alteration and water content that would be required to explain the departure from the baseline model in crust of ages 1 ma and away from propagator wakes and regions of faulting due to plate bending we obtain estimates of upper crustal hydration of 0 5 2 1 wt and find mostly dry lower crust and upper mantle in sections of the crust affected by propagator wakes we find upper estimates of upper crustal lower crustal and upper mantle hydration of 3 1 0 8 and 1 8 wt respectively at the cascadia deformation front we find that the amount of water stored at uppermost mantle levels in the downgoing jdf plate is very limited 0 3 wt with most of the water carried into the subduction zone being stored in the oceanic crust

a numerical model of hydrothermal cooling and crustal accretion at a fast spreading mid ocean ridge 1 there are two competing models for lower crustal accretion at fast spreading ridges in the gabbro glacier model all the lower crust solidifies in a magma lens at the dike gabbro boundary and then flows to its final level in the sheeted sill model the lower crust is formed by sills emplaced at a variety of depths we present a steady state numerical representation of the sill model that explicitly includes hydrothermal circulation the crust is accreted uniformly at the ridge axis at all depths and hydrothermal circulation occurs in crustal rocks that have cooled below a threshold temperature the results show that when the permeability exceeds a threshold value 4 10 14 m2 for a cracking temperature of 800 c and a uniform permeability hydrothermal circulation can cool the entire crust near the ridge axis producing vertical isotherms in the lower crust at lower permeabilities the properties of seawater lead to bimodal cooling with the lowermost crust cooling more slowly than shallower depths these results are reasonably consistent with studies of the seismic structure of the east pacific rise and of plagioclase crystal size distributions in gabbros from oman we infer that the sheeted sill model cannot be discarded on thermal grounds

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

breakup of pangaea and plate kinematics of the central atlantic and atlas regions summary a new central pangaea fit type a is proposed for the late ladinian 230 ma together with a plate motions model for the subsequent phases of rifting continental breakup and initial spreading in the central atlantic this model is based on 1 a reinterpretation of the process of formation of the east coast magnetic anomaly along the eastern margin of north america and the corresponding magnetic anomalies at the conjugate margins of northwest africa and the moroccan meseta 2 an analysis of major rifting events in the central atlantic atlas and central mediterranean and 3 a crustal balancing of the stretched margins of north america moroccan meseta and northwest africa the process of fragmentation of central pangaea can be described by three major phases spanning the time interval from the late ladinian 230 ma to the tithonian 147 7 ma during the first phase from the late ladinian 230 ma to the latest rhaetian 200 ma rifting proceeded along the eastern margin of north america the northwest african margin and the high saharan and tunisian atlas determining the formation of a separate moroccan microplate at the interface between gondwana and laurasia during the second phase from the latest rhaetian 200 ma to the late pliensbachian 185 ma oceanic crust started forming between the east coast and blake spur magnetic anomalies whereas the morrocan meseta simply continued to rift away from north america during this time interval the atlas rift reached its maximum extent finally the third phase encompassing the time interval from the late pliensbachian 185 ma to chron m21 147 7 ma was triggered by the northward jump of the main plate boundary connecting the central atlantic with the tethys area therefore as soon as rifting in the atlas zone ceased plate motion started along complex fault systems between morocco and iberia whereas a rift drift transition occurred in the northern segment of the central atlantic between morocco and the conjugate margin of nova scotia the inversion of the atlas rift and the subsequent formation of the atlas mountain belt occurred during the oligocene early miocene time interval in the central atlantic this event was associated with higher spreading rates of the ridge segments north of the atlantis fz an estimate of 170 km of dextral offset of morocco relative to northwest africa in the central atlantic is required by an analysis of marine magnetic anomalies five plate tectonic reconstructions and a computer animation are proposed to illustrate the late triassic and jurassic process of breakup of pangaea in the central atlantic and atlas regions

architecture of on and off axis magma bodies at epr 9 37 40 n and implications for oceanic crustal accretion abstract crustal accretion at fast spreading mid ocean ridges is believed to be concentrated in a narrow zone up to a few kilometers wide centered beneath the ridge axis however there is increasing evidence for off axis magmatism occurring beyond this narrow zone here we present 3d multichannel seismic mcs images from the east pacific rise 9 37 40 n extending to 11 km on the ridge flanks in the axial region two offset axial magma bodies underlie a small ridge axis discontinuity at 9 37 n displaying an overlapping geometry similar to that of the seafloor structures above on the ridge flanks a series of off axis magma lenses oaml are imaged they are located 2 10 km from the ridge axis at 700 to 1520 ms two way travel time below seafloor bsf 1 6 to 4 5 km bsf with variable areas ranging from 0 5 km2 to 5 2 km2 the largest body is centered 4 km east of the ridge axis and is composed of a large continuous flat topped lens and a series of small discontinuous westward dipping bodies along its western edge the flat crest of the oaml lies at approximately the same depth beneath layer 2a as the axial magma lens and we infer that this oaml has formed by aggregation of ascending melts that accumulate at the base of the sheeted dike section a cluster of reflections underlying the oaml at 1260 1510 ms bsf are observed that may be deeper lenses feeding melts to the upper lens this largest oaml is associated with moho travel time anomalies of 120 260 ms within a zone that extends up to 2 km from the edge of the oaml suggesting a lower crust that is partially molten with lower crustal velocities reduced by 8 18 and or thicker than normal by up to 1 km local volcanic edifices are found above two of the three oamls imaged in our study area and are inferred to be the eruptive products of the oamls from the volume of these edifices and the moho travel time anomalies we estimate the potential contribution of off axis magmatism to the total volume of the crust to be 0 01 3 the oamls imaged in our study area are present over roughly the same distance range as the zone of formation of near axis seamounts we speculate that oamls and the volcanic edifices found above them are small scale manifestations of the off axis magmatism that gives rise to near axis seamounts

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

reflective oceanic crust formed at a fast spreading center in the pacific we present new seismic images of cretaceous crust formed at a fast spreading center in the pacific the high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast spreading centers are not seismically detectable subhorizontal reflections can be traced at 600 800 ms two way time below the top of basement for tens of kilometres suggesting the presence of a widespread seismic boundary possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes lower crustal reflections dipping dominantly toward the paleo spreading center may represent mafic ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections

shallow structure of oceanic crust in the western north atlantic from seismic waveform inversion and modeling seismic reflection data from mesozoic oceanic crust in the western north atlantic in the vicinity of the blake spur fracture zone have imaged a number of short subhorizontal events in the upper 1 km of the crystalline crust wide angle expanding spread profile esp data from the same region indicate the presence of first order velocity discontinuities in this depth interval using esp data reprocessed to enhance precritical reflection events we investigate the relationship between the precritical and postcritical reflections with three different approaches amplitude analysis forward modeling and waveform inversion the precritical reflections have high apparent reflection coefficients 0 1 and may be divided into two types those which can be traced out to critical ranges 5 6 km representing velocity steps and those which fade out at ranges of 3 4 km representing shorter wavelength velocity variations synthetic seismogram modeling indicates that the velocity steps occur within a 50 m depth interval while the shorter wavelength features may correspond to high or low velocity zones with a thickness of 20 m 10 of the dominant seismic wavelength or less we investigate the velocity structure in detail by full waveform inversion of a selected portion of esp data transformed into the intercept time slowness domain the inversion method minimizes the sample by sample misfit between the data and reflectivity synthetics in a least squares sense a conjugate gradient algorithm was used starting from a velocity structure derived from forward modeling of wide angle esp data the inversion tightly constrains short wavelength components of the velocity structure and indicates that intracrustal reflections result both from major velocity steps and from zones of alternating high and low velocity synthetic seismograms generated from sonic log data from deep sea drilling project hole 418a show reflections of similar amplitude and character to those seen in the esp data these reflections probably originate from rather subtle features of the velocity depth profile related primarily to changes in porosity in the pillow lava sequence a deeper reflector 800 ms below oceanic basement in the blake spur fracture zone may correspond to the top of a zone of serpentinized upper mantle

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

on the roughness of mesozoic oceanic crust in the western north atlantic summary seismic reflection profiles from mesozoic oceanic crust around the blake spur fracture zone bsfz in the western north atlantic have been widely used in constraining tectonic models of slow spreading mid ocean ridges these profiles have anomalously low basement relief compared to crust formed more recently at the mid atlantic ridge at the same spreading rate profiles from other regions of mesozoic oceanic crust also have greater relief the anomalous basement relief and slightly increased crustal thickness in the bsfz survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation if so the intracrustal structures observed may be representative of an atypical tectonic regime

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

crooked line 2d seismic reflection imaging in crystalline terrains part 1 data processing for cost and access reasons most of the seismic reflection data collected in crystalline terrains have been acquired by 2d crooked line profiling when the survey geometry is significantly irregular and the geologic structures have cross profile dip several standard 2d imaging procedures severely underperform as a result reflection signal is poorly aligned across individual common midpoint cmp gathers and much is lost during the cmp stack to improve imaging either the methods used to align signal before stack need to be modified or more tolerant methods of combining trace signals than the standard cmp stack need to be applied because a high fold 2d crooked line profile is really a 3d survey of a swath of terrain around the processing line better signal alignment before cmp stacking may be achieved by revisiting the traveltime equation and including the cross dip terms into the moveout calculations therefore in addition to the correction of nmo and in line dip moveout dmo we also locally com

downdip variations in seismic reflection character implications for fault structure and seismogenic behavior in the alaska subduction zone seismic reflection data collected offshore of alaska peninsula across the western edge of the semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth suggesting changes in structure that may relate to seismic behavior from the trench to 40 km landward two parallel reflections are observed which we interpret as the top and bottom of the subducted sediment section from 50 95 km from the trench the plate interface appears as a thin 400 ms reflection band deeper and farther landward the plate interface transitions to a thicker 1 1 5 s package of reflections where it appears to intersect the forearc mantle wedge based on our preferred interpretation of the continental moho synthetic waveform modeling suggests that the thin reflection band is best explained by a single 100 to 250 m thick low velocity zone whereas the thick reflection band requires a 3 to 5 km thick zone of thin layers the thin reflection band is located at the center of the 1938 mw 8 2 semidi earthquake rupture zone that now experiences little interplate seismicity the thick reflection band starts at the downdip edge of the rupture zone and correlates with a dipping band of seismicity and projects to the location of tremor at greater depth we interpret the thin reflection band as a compacted sediment layer and or localized shear zone the thick reflection band could be caused by a wide deformation zone with branching faults and or fluid rich layers representing a broad transition from stick slip sliding to slow slip and tremor

spreading rate dependence of three dimensional structure in oceanic spreading centres segmentation by transform faults and other types of along axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid ocean ridge buoyant upwelling driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust exhibits a spreading rate dependent transition between two dimensional and three dimensional upwelling structures for low spreading rates and mantle viscosities an initial two dimensional structure transforms into a three dimensional one at high spreading rates an initially two dimensional structure remains two dimensional these results suggest that the origin of spreading centre segmentation may be different at fast and slow spreading rates

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

variability in oceanic crustal thickness and structure multichannel seismic reflection results from the northern east pacific rise multichannel seismic reflection data acquired between 8 50 and 9 50 n and between 12 30 and 13 30 n along the east pacific rise provide a three dimensional view of the young oceanic crust seafloor to moho reflection travel times vary by up to 0 9 s within our study areas the total range of crustal travel times in the 9 n area is 1 55 to 2 45 s the total range in the 13 n area is 1 60 to 2 05 s the variation is systematic indicating thinner crust locally associated with overlapping spreading centers oscs and in the 9 n area segment scale variation along crustal isochrons crustal travel time is found to be a valid proxy for oceanic crustal thickness outside of the axial low velocity volume thickness can be calculated from time to 500 m even in the axial region thickness can be calculated to 1 km if low velocity zone position is known crustal thicknesses calculated from travel times vary by 2 6 km in the 9 n area and by 1 5 km in the 13 n area the majority of this variation is attributed to seismic layer 3 the lower crust segment scale variation of 1 8 km 5 5 to 7 3 km thickness is observed in the 9 n area with thinnest crust formed between 9 40 and 9 50 n and thickest formed between 9 15 and 9 20 n results imply a three dimensional pattern of magma supply to the 9 n segment the osc at 9 03 n is associated with major disruptions of the segment scale pattern in the form of local thin areas within the discordant zone the smaller osc at 12 54 n is not associated with dramatic changes in thickness of the surrounding crust in the absence of oscs the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment thicker crust does not always correlate with shallower ridge bathymetry broader axial cross section or more negative mantle bouguer or subcrustal gravity anomaly variable thickness of the crust mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result we hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick moho transition zone and a subcrustal gravity low such a diapiric upwelling center appears to be now located beneath the east pacific rise near 9 40 to 9 50 n