surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

hydroacoustic detection of volcanic activity on the gorda ridge february march 1996 abstract beginning at 0700 gmt on 28 february 1996 intense seismicity was detected in the northeast pacific ocean using the t phase monitoring system developed by noaa pmel to access the u s navy s sound surveillance system sosus in the north pacific the event was preliminarily located on the northernmost segment of the gorda ridge near 42 67 n and 126 8 w in the vicinity of the ridge segment high narrowgate the nature of the seismicity was similar to that observed in june 1993 at the coaxial segment of the juan de fuca ridge which was later documented to be a lateral magma injection with subsequent eruption due to several gaps in the data the detection information was not as comprehensive as during the coaxial event but an initial migration of epicenters from the narrowgate area down rift is inferred based on arrival bearings from a single array there is evidence for an additional diking event on the second and third day of activity there is also indication of a concentration of epicenters located near 42 6 n as occurred during the coaxial episode at what was later determined to be an eruption site examination of t wave rise times generally supports this interpretation based on the nature and duration of the activity a response effort was initiated which later confirmed hot water plumes and fresh lava flows at the site based on both hydroacoustic information and field observations it is proposed that the episode began with a lateral dike injection possibly with eruptive activity in the summit region followed by multiple magma pulses and eventual focusing of the seismic activity and extrusion near 42 6 n

field characteristics of laterally emplaced dikes anatomy of an exhumed miocene dike swarm in reydarfjordur eastern iceland abstract many dikes of the tertiary breiddalur dike swarm in reydarfjordur have a great lateral extent but are vertically upward and or downward discontinuous field relationships indicate that they are not the feeders of the lava pile they intrude their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading krafla volcanic center of the neovolcanic zone average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other tertiary dike swarms in eastern iceland it is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures

fluid flow patterns in fast spreading east pacific rise crust exposed at hess deep tectonic exposures of a volcanic sequence and sheeted dike complex over a 4 km wide region at hess deep equatorial pacific reveal significant spatial heterogeneity 10 103 m in the extent and nature of hydrothermal alteration in young fast spreading east pacific rise crust the volcanic sequence is fairly uniformly altered with only minor oxidation and alteration to clay minerals sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60 their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole clinopyroxene replacement increases 40 whole rock 18o values decreases 4 4 5 5 to 3 5 4 5 and calculated peak metamorphic temperatures increase 250 c to 450 700 c within the deepest dikes localized zones up to 400 m wide are chlorite rich and have low 18o 2 9 4 1 and low peak metamorphic temperatures 345 c these alteration patterns likely formed within broad recharge zones whereby the low 18o zones developed in the regions with the highest fluid flux in the west massive slightly rotated sheeted dikes near the volcanic sheeted dike transition are 18o and cu depleted and display higher peak temperatures 345 c than elsewhere in the shallow dikes these characteristics are consistent with formation within a high temperature hydrothermal discharge zone we propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes similar to those preserved in many ophiolites

seismic images of active magma systems beneath the east pacific rise between 17 05 and 17 35 s seismic reflection data from the east pacific rise between 17 059 and 17 359s image a magma lens that varies regularly in depth and width as ridge morphology changes confirming the notion that axial morphology can be used to infer ridge magmatic state however at 17 269s where the ridge is locally shallow and broad the magma lens is markedly shallower and wider than predicted from regional trends in this area submersible dives reveal recent volcanic eruptions these observations indicate that it is where the width and depth of the magma chamber differ from regional trends indicating an enhanced magmatic budget that is diagnostic of current magmatism

1 ma east pacific rise oceanic crust and uppermost mantle exposed by rifting in hess deep equatorial pacific ocean abstract a series of dives with the french submersible nautile has been conducted in the region of the galapagos triple junction at the tip of the cocos nazca propagator most of the dives were devoted to the study of hess deep a rift valley produced by rifting of oceanic crust created at the axis of the very fast spreading east pacific rise the dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra rift ridge located to the north of the axis of hess deep this shows that this situation is not specific not slow spreading ridges a nearly complete and unique log of young 1 ma crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered we propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust one model stresses symmetry and vertical tectonics through isostasy diapirism of serpentinized mantle another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low angle detachment faulting in both models which differ mainly in the structure at depth amagmatic extension of east pacific rise crust by the cocos nazca propagator tip results in uplift and denudation of lower crust and upper mantle

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

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

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

modes of extension at oceanic spreading centers evidence from the solea graben troodos ophiolite cyprus abstract the solea graben in the troodos ophiolite cyprus represents a preserved cretaceous age axial valley formed during a brief period of amagmatic seafloor spreading structural analysis of all crustal levels of the graben reveals two periods of deformation an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the tertiary recent amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike plutonic contact and oriented subparallel to ophiolite pseudostratigraphy individual fault segments are essentially planar but link to approximate listric geometries locally bulk structural rotation of fault blocks bounding the graben occurred about a nnw trending axis parallel to the present graben trend average dips in the sheeted complex within the central portions of the graben suggest 40 50 of structural rotation although this is a minimum figure because present dike dips are not a simple function of rotation magnitude tensor analysis of fault populations associated with extensional deformation shows that 3 was oriented approximately perpendicular to the graben axis late near axis extensional deformation segmented the detachment fault along nnw trending normal faults obduction related uplift and doming of the troodos ophiolite is expressed in the solea graben by minor thrust reverse and strike slip faults principally developed during reactivation of pre existing low angle fault zones formed during seafloor deformation tensor analysis of this later phase of faulting shows that 1 was oriented approximately n s and that 2 and 3 alternated position during the deformation the style of deformation documented in the solea graben supports recent speculation for basin and range style detachment faulting at slow spreading ridge crests with low magma budgets such as parts of the mid atlantic ridge near large offset transform faults natural cross sectional exposure of the solea graben shows how extensional structures might link at depth beneath such slow spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large scale ore forming hydrothermal circulation cells

detrital zircon fission track ages from the lower cenozoic sediments nw himalayan foreland basin clues for exhumation and denudation of the himalaya during the india asia collision detrital zircon fission track ft ages from the lower cenozoic sub himalayan foreland basin reflect the progressive effects of crustal thickening and exhumation on the himalayan source rocks as a consequence of the india asia collision the oldest stratum the transgressive marine paleocene eocene subathu formation 57 41 5 ma contains ca 50 ma detrital zircon p1 peak which was derived from the indus tsangpo suture zone and the ladakh batholith of the asian plate a dominant 302 4 21 9 ma peak with a few 520 ma grains in this formation has been derived by erosion of the zircon partial annealing zone zpaz of 240 180 c as the first imprint of the collision this zone affected the himalayan proterozoic basement and its tethyan sedimentary cover since the detritus in the subathu has been derived both from the indian and asian plates the possible suturing of these plates took place during the subathu sedimentation a sudden change in the provenance is recorded in the detrital zircon ft cooling ages in the oligo miocene dagshai and kasauli formations which have dominant 30 and 25 ma p1 peaks respectively we interpret a distinct unconformity spanning 10 m y between the subathu and dagshai formations since ca 30 ma molassic sedimentation coincides with shifting of the source rocks to the himalayan metamorphic belt this belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh pressure high pressure uhp hp metamorphism 53 50 ma in the extreme north and two widespread m1 and m2 metamorphisms 40 30 and 25 15 ma in the middle parts these events appear to be largely responsible for the deposition of the ca 30 ma zircon himalayan peak and ca 25 and 15 ma young himalayan peaks respectively the latter appears within the lower siwalik subgroup 13 11 ma during the lower siwalik deposition pre himalayan peaks gradually decrease with the intensification of the himalayan events in source rocks in spite of uninterrupted fluvial sedimentation in the dagshai kasauli lower siwalik sequences since 30 ma breaks of 5 7 m y in the zircon ft ages reveal pulsative cooling and exhumation in the well identified source areas although cooling and exhumation of the himalayan source rocks remained almost uniform during the eocene source heterogeneity is reflected in fluvial sedimentation since 37 ma from pakistan to nepal in response to the india asia collision

extraterrestrial ni rich spinel in upper eocene sediments from massignano italy evidence of a major cosmic event at the end of the eocene is given by the finding of at least one possibly two or more impact horizons containing microtektites microkrystites shocked quartz and unusually high iridium concentrations we report here the discovery in the global stratotype of the eocene oligocene boundary at massignano in italy of ni rich spinel a mineral that forms by the interaction of meteoritic bodies with the earth s atmosphere a maximum concentration of 800 crystals mg 1 corresponding to a flux of 2 107 crystals cm 2 is observed in a well dated layer 35 7 0 4 ma that also contains shocked quartz and iridium the crystals are found clustered in flattened spheroids the probable relic of their host bodies the chemical and physical characteristics of the spinel crystals indicate that the spheroids formed by interaction of dust particles in the upper part of the atmosphere rather than by ablation of large objects as proposed for the spinel bearing spheroids found at the cretaceous tertiary boundary the most likely explanation is that the dust particles were generated by a large cometary impact whether this impact is the one that produced microtektites and or microkrystites or another one that may have occurred shortly after or before is still questionable additional searching for ni rich spinel at other upper eocene sections may help to answer this question

geochemistry and intrusive directions in sheeted dikes in the troodos ophiolite implications for mid ocean ridge spreading centers 1 sheeted dikes at mid ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity as such they are crucial for the interpretation of many observations at mid ocean ridges or other volcanoes with pronounced rift zones including topography hydrothermal systems petrology and geochemistry we carried out a structural magnetic and chemical investigation of a 4 10 km sheeted dike section in the troodos ophiolite cyprus on the basis of major and trace element geochemistry we distinguish dikes that may be correlated with the basal high ti series hts lavas from those of the overlying low ti series lts lavas all dikes studied are nearly parallel to each other with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center anisotropy of magnetic susceptibility measurements suggests that the hts and lts dikes intrude in fundamentally different ways hts dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber lts dikes show a bimodal orthogonal set of intrusive directions one shallow and one near vertical near lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system and near vertical dike propagation directions are probably associated with feeder dikes to down rift surface flows our study suggests that the types of dike intrusive behavior in the troodos ophiolite may also be typical for normal mid ocean ridges or other major shield volcanoes with well developed rift zones

contribution of volcanism and tectonism to axial and flank morphology of the southern east pacific rise 17 10 17 40 s from a study of layer 2a geometry multichannel seismic images from the east pacific rise 17 10 17 40 s are used to study the geometry of seismic layer 2a in the near axis region wave equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2a event processed stacks are compared with seafloor morphology and tectonic fabric imaged in side scan sonar data assuming layer 2a corresponds with the extrusive crust these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge crest and ridge flank morphology we find variations in axial thickening of layer 2a which imply twofold variation in the width of the extrusive layer accumulation zone as well as systematic changes in the pattern of accumulation of this layer on the ridge flanks the layer 2a horizon mimics abyssal hill relief consistent with a horst and graben origin for this topography short wavelength variations in the thickness of layer 2a are superimposed on this relief which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone the integrated off axis thickness of layer 2a is correlated with ridge cross sectional area and indicates persistent 100 000 years spatial gradients in extrusive layer thickness systematic changes in the cross axis shape of the ridge observed over distances of 5 10 km cannot be attributed to the extrusive layer and it appears that axial structure beneath the volcanics governs the cross axis morphology of the ridge

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

geochemistry of diabase dikes from the lewis hills massif bay of islands ophiolite evidence for partial melting of oceanic crust in transform faults the lewis hills massif of the bay of islands ophiolite is interpreted to have had a transform fault history in an oceanic fracture zone prior to its abduction diabase dikes from the lewis hills massif are divisible into two groups on the basis of their trace element compositions one group is similar to mid ocean ridge basalts and is termed the lh normal suite the other group termed the lh depleted suite is characterized by high cao ti zr and y zr with relatively low sio 2 zr and p 2 o 5 the low sio 2 abundances are suggestive of silica undersaturated rocks of the alkali basalt nephelinite series of magmas but the very low abundances of incompatible trace elements in these magmas contrast with their high abundances in most silica undersaturated rocks we interpret the geochemical characteristics of thesediabase dikes to indicate that the magmas from which they solidified were produced by partial melting of adcumulate gabbroic and troctolitic rocks at low pressures 2 3 kbar within the ophiolitic fracture zone the effects of partial melting of gabbroic and troctolitic rocks have not been documented in rocks described from present day oceanic fracture zones but samoles have not been obtained from those structural levels of fracture zones where such evidence mightbe found

four dimensional upper crustal construction at fast spreading mid ocean ridges a perspective from an upper crustal cross section at the hess deep rift abstract the faulted walls of the hess deep rift in the equatorial pacific provide one of the few significant tectonic windows into the oceanic crust created at a fast spreading mid ocean ridge the 5000 m deep rift graben exposes a slightly dismembered section of oceanic crust created at the east pacific rise 1 ma the upper portion of the north wall of the rift features a laterally extensive exposure of upper crustal rock units similar to those found in many ophiolite complexes for tens of kilometers along the rift wall basaltic lavas with sparse dikes grade down into a spectacular sheeted dike complex that is underlain locally by a heterogeneous assemblage of gabbroic rocks the geometry of lava flows dikes and fault zones that developed during magmatic accretion provide constraints on the cross axis temporal evolution of the upper crust in this area outcrop scale digital imagery from submersible vehicles provides a well constrained geological framework for samples collected from these upper crustal rock units and the geochemistry of lava and dike samples provides additional temporal constraints on crustal accretion and the role of along axis magma transport during accretion integrated results of geological and geochemical investigations suggest a four dimensional model for upper crustal construction whereby tectonic and magmatic processes act in concert to construct the upper oceanic crust within a few kilometers few tens of thousands of years of a fast spreading ridge axis in this model a sub axial melt lens where it exists is recharged from below with distinct parental melts and limited along axis mixing preserves geochemical segmentation of the melt lens dikes emanate from distinct portions of the melt lens and transport magmas vertically as well as significant distances laterally along axis erupting lavas in the special case where magmas ascend through dikes to higher levels in the crust and intersect the seafloor differences in the phenocryst contents and compositions of lavas and dikes suggests that density filtering of buoyant magmas among other factors including magma overpressure and orientations of local stresses plays a role in the eruption of magma on the surface during spreading and magmatic construction subaxial subsidence accommodates thickening of the basaltic lava unit sub axial subsidence faulting and block rotation result in an upper crust with inward dipping toward the axis lava flows and outward dipping away from the axis dikes substantial differences in upper crustal geology may result from segment scale variations in which vertical magma delivery dominates in areas of robust magma supply likely over sites of recently recharged melt lenses at segment centers but lateral magma transport dominates in areas of relatively lower magma supply possibly toward segment ends

magmatic processes and segmentation at a fast spreading mid ocean ridge detailed investigation of an axial discontinuity on the east pacific rise crest at 9 37 n 1 geophysical and petrological boundaries on mid ocean ridges provide ideal locations to study the relationships between magmatic tectonic and hydrothermal processes alvin based observational data and geochemical data for basalts and hydrothermal fluids are used to investigate these relationships at an axial discontinuity on the east pacific rise epr crest between 9 36 n and 9 38 n this ridge crest discontinuity is morphologically expressed by the overlap of an eastern and western axial summit collapse trough asct that delimits the primary volcanic and hydrothermal loci along the ridge crest in this area the ascts overlap by 3 km and are offset in a right lateral sense by 0 45 km near bottom imaging of this area in 1989 and 1991 shows changes in volcanic morphology and increases in hydrothermal and biological activity consistent with the occurrence of a magmatic event during that time interval when combined with the inferred age and structure of the seafloor basalt geochemistry and hydrothermal fluid chemistry these temporal changes suggest active southward propagation of the eastern asct and show that the western asct was unaffected by the recent magmatic event numerous extinct hydrothermal vents and older looking lava flow surfaces suggest waning of magmatic activity in the western asct young looking lava flows within or proximal to the eastern asct have anomalously high mg numbers relative to the regional trend and are chemically similar to lava erupted in 1991 along the 9 46 52 n epr region we propose that the young looking lava flows in the eastern asct are related to the 1991 eruption data show that the 9 37 n axial discontinuity marks a magmatic and hydrothermal boundary along the epr ridge crest and we argue that it be classified as a third order discontinuity this result is consistent with geophysical evidence suggesting fundamental differences in the crust and upper mantle north and south of the 9 37 n discontinuity

the rrr triple junction at the southern end of the pacific cocos east pacific rise abstract a sea beam survey of the pacific cocos galapagos triple junction at 2 40 n on the east pacific rise epr establishes that it is a ridge ridge ridge junction with the epr axis making a small but sharp bend where it is intersected by a slow spreading 15 mm yr fast propagating rift at the tip of the narrow triangle of crust accreted at this slow spreading axis is volcanic ridge built on epr crust tholeiitic lavas erupted along the slow spreading axis here and at the rather similar pacific nazca galapagos junction differ in minor element and rare earth element geochemistry from lavas on adjacent segments of the epr axis being slightly enriched in incompatible elements

axial and off axial heterogeneity of basaltic rocks from the east pacific rise at 12 35 n 12 51 n and 11 26 n 11 30 n surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the epr at 12 35 n 12 51 n and at 11 26 n 11 30 n and from four adjacent seamounts centered at less than 18 km from the rise axis the basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition there is no correlation between lava flow morphology geological settings and compositions based on mineral and compatible element composition these basalts were classified into three types the least evolved olivine basalts and highly phyric plagioclase basalts type a have high fo87 89 high mg 66 70 and high ni 100 contents the most evolved plagioclase olivine basalts with or without pyroxene type b and c have relatively low fo78 86 low mg 55 65 and low ni 60 100 ppm contents simple crystal fractionation 3 18 accounts for some of the observed compositional range however based on their incompatible element contents the above basalts are divided into three groups the various types a to c are found in each group depleted basalts with low k ti 0 04 0 15 ratios low zr 100 ppm and low nb 0 4 ppm contents undepleted basalts with the highest k ti 0 25 0 46 zr 150 170 ppm and nb 8 16 ppm contents and the transitional basalts having compositions between these two extremes these three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3 4 km in width near 12 43 n 12 51 n on the epr similar volcanic diversity in a more limited area 3 km2 is observed on off axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle the transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting we suggest that a lherzolitic mantle with minerals having a variable behavior under partial melting constitutes this type of composite mantle indeed clinopyroxene enriched in incompatible elements will be consumed at an earlier stage of melting than the olivine plus orthopyroxene eruption of compositionally distinct magma batches in close proximity on axial and off axial structures implies periods of quiescence between successive magmatic stages and cycles

continuous mantle melt supply beneath an overlapping spreading center on the east pacific rise tomographic images of upper mantle velocity structure beneath an overlapping spreading center osc on the east pacific rise indicate that this ridge axis discontinuity is underlain by a continuous region of low p wave velocities the anomalous structure can be explained by an approximately 16 kilometer wide region of high temperatures and melt fractions of a few percent by volume our results show that oscs are not necessarily associated with a discontinuity in melt supply and that both osc limbs are supplied with melt from a mantle source located beneath the osc we conclude that tectonic segmentation of the ridge by oscs is not the direct result of magmatic segmentation at mantle depths

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b