a submarine hydrothermal manganese deposit from the south west pacific island arc hydrothermal deposits of a wide variety of types are being found with increasing frequency on or near actively spreading mid ocean ridges1 4 however they also have a potential to occur in other submarine volcanic settings including island arcs5 to follow up indications of mineralization associated with submarine hydrothermal activity in the south west pacific island arc6 a joint new zealand oceanographic institute imperial college research cruise was mounted in may 1981 aboard the rv tangaroa during this cruise over 130 sampling stations were occupied at one of which were dredged manganese deposits with strong hydrothermal affinities this is the first report of such deposits from an island arc setting

observed evolution of linear and nonlinear effects at the dahan downhole array taiwan analysis of the september 21 1999 m 7 3 chi chi earthquake sequence in this paper the site characteristics of the dahan downhole array are studied by analyzing the september 21 1999 m 7 3 chi chi earthquake sequence including the main shock and some aftershocks the four level array 0 m 50 m 100 m and 200 m is located to the north of hualien city in eastern taiwan polarization analysis is used to check the orientation errors of the seismometers at different levels of depth if the surface instrument is chosen as reference the angle between the major polarization axes of the surface and any downhole records is the orientation error that must be corrected for the downhole accelerographs the orientation errors at depths of 50 m 100 m and 200 m are 32 120 and 84 after the corrections the coherency between the surface and downhole records is substantially improved spectral ratio analysis shows that the predominant frequency of the chi chi main shock shifts to a lower frequency we also simulate ground motions at different depths by using the haskell method with a linear velocity structure model the record at surface is chosen as the input motion compared with the observed data ground acceleration can be well reproduced for the aftershocks weak motion events of the september 21 1999 m 7 3 chi chi earthquake however for the chi chi main shock the synthetic waveform cannot match well with the observation neither in amplitude nor in phase this indicates that large ground shaking probably induced the nonlinear site effect at that time and the model used cannot support it

zabyat formation semail nappe oman sedimentation on to an emplacing ophiolite the pelagic sediments of the semail ophiolite are depositionally overlain by ophiolite derived sediments of the zabyat formation dominated by matrix supported rudites with subordinate arenites and lutites the detritus was locally derived from the ophiolite in a deep marine environment indicated by both underlying and overlying pelagic carbonates the zabyat rocks are restricted to fault zones in the ophiolite which were active during emplacement they are interpreted as debris shed from the sub vertical fault zones during the initial disruption of the semail ocean floor prior to the final emplacement on to the arabian continental margin this interpretation places constraints on possible mechanisms of ophiolite emplacement

east pacific rise near 13 n geology of new hydrothermal fields abundant massive sulfide deposits are present at the crest of the east pacific rise near 13 north where the opening rate is about 12 centimeters per year large manganese and helium 3 anomalies in seawater samples evidence of intense present day activity of hydrothermal springs indicate that sulfides are still being produced along this segment of the rise massive sulfides also occur on adjacent off axis seamounts

el nino southern oscillation signal associated with middle holocene climate change in intercorrelated terrestrial and marine sediment cores north island new zealand a synchronous textural variation in intercorrelated high resolution sediment records from floodplain continental shelf and continental slope settings of the eastern north island new zealand provides evidence of increased storminess after ca 4 ka an upcore change in sediment texture reflects the transition to landsliding which supplanted fluvial incision as the dominant mode of sediment production in the middle holocene this signal which appears in all three records indicates a regional response to external forcing and records the impact of an intensified atmospheric circulation marking the establishment of the contemporary climate that is strongly influenced by the el nino southern oscillation the change in climate was a hemispheric event and in the southern hemisphere its timing is confirmed by independent proxy records from elsewhere in new zealand and the circum south pacific region

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

the interrelationship between magmatic and ore forming hydrothermal processes in the oman ophiolite the semail ophiolite of oman contains numerous gossans minor sulfide showings and ferromanganoan deposits as well as several massive sulfide deposits the largest of which 8 x 10 6 metric tons is the deposit of lasail detailed field mapping of the lavas and shallow intrusions around this deposit in conjunction with petrographic studies of their metamorphic assemblages enables the ore forming hydrothermal activity to be related to the location and evolution of an underlying magma chamber it can be demonstrated that the deposit formed in the short time interval between the eruption of a lava unit the geotimes unit at a marginal ocean basin spreading axis and the eruption of a second lava unit the lasail unit at an off axis or arc seamount moreover it was the seamount not the spreading axis magma chamber which drove the ore forming hydrothermal convection providing both a near surface heat source and by virtue of its intrusive pressure a localized fracture system on its flanks subsequent eruptions from this same magma chamber then provided a protective capping for the deposit the orientations and distributions of andesitic cone sheets show that the magma chamber lay some 1 5 km from the deposit itself and was in fact a magma cupola developed on the flanks of a much larger magma chamber similar though less detailed studies of other deposits in oman indicate that only very small mound deposits lie within the spreading axis lavas themselves the majority of deposits lie at the boundary between the spreading axis and seamount lavas and another group of deposits is related to the magma chambers that fed a third submarine lava unit the alley unit also associated with the construction of the seamounts these results may provide useful constraints for future mineral exploration programs in the oman and perhaps also in other ophiolite complexes and on the ocean floor

obduction processes in ancient modern and future ophiolites geologic and tectonic processes of ophiolite obduction deduced from structural mapping timing of deformation and metamorphism and palinspastic reconstruction of ancient ophiolite complexes compare closely with modern processes operative in the western pacific many ongoing arguments about the origin of ophiolite complexes oceanic island arc or marginal basin and their emplacement subduction related or not are largely semantic and depend on exactly what individuals regard as island arcs marginal basins or subduction zones a review of tectonic processes currently active in the western pacific region shows a wide range in size shape and duration of arcbasin complexes and some of these show uniformitarian models similar to those proposed for emplacement of ancient ophiolites the tethyan ophiolites of the semail oman mountains and spontang western himalaya of ladakh zanskar and the ordovician western newfoundland ophiolites are compared with recent examples of the mariana basin yap trench banda arc luzon arc and papua new guinea it is concluded that they are all connected in some way with marginal basins and island arcs in varying stages of development and maturity emplacement of these ophiolites is related to continental underthrusting of a trapped marginal basin and accretionary prism comparable to present day examples of the n australian margin underthrusting timor and the banda sea and the se china margin underthrusting the luzon arc

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

nonlinear soil response a reality geotechnical models consistently indicate that the stress strain relationship of soils is nonlinear and hysteretic especially at shear strains larger than 10 5 to 10 4 nonlinear effects such as an increase in damping and reduction in shear wave velocity as excitation strength increases are commonly recognized in the dynamic loading of soils on the other hand these effects are usually ignored in seismological models of ground motion prediction because of the lack of compelling corroborative evidence from strong motion observations the situation is being changed by recently obtained data explicit evidence of strong motion deamplification accompanied by changes in resonant frequencies are found in the data from the 1985 michoacan mexico and the 1989 loma prieta california earthquakes the events recorded by the vertical and surface accelerograph arrays in taiwan as well as a number of other events throughout the world evidence of nonlinear behavior becomes apparent beyond a threshold acceleration of 100 to 200 gal nonlinearity is considerable in cohesionless soil but may be negligible in stiff soils the findings of recent years indicate that nonlinear site effects are more common than previously recognized in strong motion seismology

active hydrothermal vents and sulfide deposits on the southern juan de fuca ridge massive sulfide deposits rich in zinc and silver were recovered from the juan de fuca ridge 500 km west of oregon in september 1981 the samples recovered are composed largely of zinc sulfide with lesser amounts of iron lead and copper sulfide most of the deposits occur at a series of hydrothermal vents within a relatively continuous depression in the center of a smooth 1 km wide valley along the ridge axis the depression appears to be formed by collapse of a lava lake possibly modified by extensional faulting the axial valley floor outside the depression is underlain by fresh glassy ferrobasalt sheet and lobate flows two types of sulfide mineral deposits were dredged from one of the hydrothermal vents 1 angular slabs of dark gray zinc sulfide and 2 subrounded fragments of porous light gray zinc sulfide the samples contain fresh sphalerite zoned wurtzite pyrite and minor amounts of marcasite galena and chalcopyrite cubanite the spreading rate of the juan de fuca ridge and the composition of the sulfide samples are generally similar to the east pacific rise lat 21 n site however the texture and geologic setting of the sulfide deposits are significantly different

shear modulus degradation curves of gravelly and clayey soils based on kik net in situ seismic observations abstractin this paper the nonlinearity of the gravelly and clayey soils in japan is investigated based on the kiban kyoshin network kik net downhole array seismic observations shear modulus red

geochemical evidence for the origin of late triassic melange units in the oman mountains as a small ocean basin formed by continental rifting a useful tool to elucidate past tectonic environments is the geochemistry of volcanic and sedimentary rocks when used together the regional structural setting of the oman mountains indicates that deep water sediments and volcanic rocks formed adjacent to the rifted arabian margin in the late triassic near the axis of a narrow ocean basin of red sea type tholeiitic to trachytic extrusives formed seamounts associated with late triassic reefal build ups immobile trace element compositions point to a within plate origin the interbedded and overlying late triassic deep sea sedimentary cover comprises ribbon radiolarites and both distal siliclastic and calcareous turbidites that accumulated on an abyssal plain at least ca 180 km northeast of the arabian continent associated ferromanganiferous oxide sediments are interpreted as chemical precipitates derived from high temperature vents in the spreading axis of the young ocean basin pervasive regional subsidence took place during end triassic early jurassic time later in the cretaceous oceanic crust was consumed in a northeast dipping subduction zone morb type crust was subducted while late triassic volcanic edifices and sedimentary cover were accreted during eventual trench margin collision the semail ophiolite split into blocks allowing sub ophiolite melange rocks to be expelled upwards through corridors creating the batinah melange as the ophiolite nappe ploughed inboard over already thrust assembled abyssal plain sediments hawasina complex some duplexes were uplifted oversteepened overturned and then slid backwards onto the ophiolite to form the batinah sheets

in situ soil specific nonlinear properties back calculated from vertical array records during 1995 kobe earthquake in situ nonlinear soil properties are investigated based on vertical array records obtained during the 1995 hyogoken nambu earthquake kobe earthquake at soft soil sites near the earthquake fault zone spectrum ratios between ground surface and deeper levels are calculated for the main shock and associated small shocks and s wave velocities and damping ratios in surface soil layers to best reproduce them are back calculated by means of an inversion analysis extended bayesian method assuming one dimensional vertical horizontal shear sh wave propagation obvious differences in s wave velocities and damping ratios are found between the main shock and the small shocks clear strain dependent modulus degradations which can be differentiated for different soil types are recognized the degradations are essentially consistent with some of laboratory test results to date for each soil type at least for g g0 0 5 or larger though for gravelly soils back calculated values tend to show milder degradations tha

the lead and strontium isotope geochemistry of metalliferous sediments associated with upper cretaceous ophiolitic rocks in cyprus syria and the sultanate of oman metalliferous sediments consisting dominantly of fine grained iron and manganese oxides and hydroxyoxides have been widely recorded from the crests of sea floor spreading ridges and as a basal faci

geochemistry and palaeo oceanography of metalliferous and pelagic sediments from the late cretaceous oman ophiolite abstract metalliferous and pelagic sediments are exposed within and above the extrusive successions of the upper cretaceous oman ophiolite which on the basis of mostly geochemical evidence is believed to have formed in an incipient marginal basin setting located above a ne dipping subduction zone the ophiolitic extrusives document various volcano tectonic settings which include the axial zones of a spreading ridge fault controlled seamounts and off axis volcanic edifices most of the fe mn and trace metal enriched sediments studied are interpreted as precipitates formed by oxidation of solutions derived from high temperature sulphide precipitating vents the trace element content e g ree and sr was largely scavenged from seawater the sediments are similar to the dispersed metalliferous sediments on the flanks of modern spreading ridges and the basal sediments of dsdp wells and of other ophiolite complexes e g troodos cyprus distinctive mound structures located low in the lavas are attributed to percolation of sulphide rich solutions into already deposited metalliferous oxide sediments the resulting iron silica rock was probably originally precipitated as ferruginous silicates major massive sulphides formed off axis at the base of intermediate basic edifices of volcanic arc affinities fe mn and trace metal enrichment in the sediment cover of a flat topped seamount of axial lavas is interpreted as a dispersion halo around the largest massive sulphide orebody which is situated 5 km away lasail small massive sulphide bodies are common in the axial lavas particularly along major seafloor fault zones the metalliferous sediments locally precipitated near these vents are ferromanganiferous but trace metal depleted the metalliferous and pelagic sediment cover of the extrusive successions generally documents waning hydrothermal input after volcanism ended in the area a model is discussed in which the ophiolite was created at a spreading axis above a subduction zone dipping away from the arabian continental margin with progressive subduction this crust approached the margin initially calcareous sediment accumulated above the calcite compensation depth ccd but then non calcareous radiolarites were deposited as the ophiolitic crust approached the continental margin where the ccd was higher and marginal upwelling possibly enhanced productivity as the edge of the arabian continental margin entered the trench the over riding ophiolite was regionally uplifted allowing short lived chalk accumulation above the ccd this was followed by volcaniclastic deposition related to the tectonic emplacement

genesis of the batinah melange above the semail ophiolite oman abstract the batinah melange which overlies the late cretaceous semail ophiolite in the northern oman mountains comprises mostly sedimentary rocks of deep water facies alkalic lavas and intrusives all of continental margin affinities together with smaller volumes of semail ophiolitic and metamorphic rocks four intergradational textural types of melange can be recognized sheet melange has large 1 km intact sheets either with little intervening matrix or set in other melange types and with an organised sheet orientation fabric slab melange is finer textured 100 m and more disrupted block melange has smaller m blocks with some matrix and a weak to random block fabric clast melange is matrix supported rudite with a weak depositional clast fabric structural relationships particularly the absence of tectonic fabrics the decreasing strength of fragment fabrics with increasing fragmentation and the abundance of brittle fragmentation suggest that these melange types formed by either gravity driven sedimentary processes or superficial sliding or thrusting of individual rock slabs in the slab melange long sequences can be pieced together passing up from upper triassic mafic sub marine extrusives and sediments into radiolarian cherts hemipelagic and redeposited limestones and terminating in non calcareous radiolarities with mn deposits of early cretaceous age mafic sills are numerous these sequences can be matched with sub ophiolite rocks now exposed in fault corridors through the semail these sequences become progressively disrupted upwards in the corridors and can be traced continuously into overlying melange which then thins away from the corridors we argue that during late cretaceous emplacement over the arabian margin active fault corridors split the semail slab and acted as conduits up which sub ophiolite rocks were supplied to the ophiolite surface there the rocks were redisributed by superficial processes

properties of vertical ground motions a typical engineering approach to developing site specific design vertical ground motions starts with rock outcrop horizontal motions converts them into the vertical component using an empirical vertical to horizontal v h ratio for response spectra and propagates the resulting motion through the soil column as a vertically incident p wave in the absence of data on strain dependent soil properties in compressional deformation strain compatible shear wave properties from the horizontal component analyses are utilized this approach makes two assumptions 1 that the vertical motions are primarily composed of compressional waves and 2 that strain dependent material properties in shear deformation can be extrapolated to compressional deformation our study deals with the empirical validation of both assumptions first we investigated the ratio of sv to p wave spectra of the vertical component of ground motions from significant recent events in california to find which wave type predominantly contributed to vertical motions in the frequency range of 0 5 25 hz the results indicate that shear waves dominate the vertical motions at frequencies up to approximately 10 hz above which the contribution of compressional deformation is about as strong or greater this result holds for both soil and rock sites second using the data from the kik net borehole arrays in japan we estimated the nonlinearity in compressional deformation by studying p wave amplification at variable amplitude levels frequency shifts and in some instances reduced amplification compatible with the hysteretic softening type of nonlinearity known for shear waves is found as the amplitude of compressional strain increases a tentative curve of constrained modulus reduction is also similar to the existing shear modulus reduction curves the results of this study suggest that for most practical applications vertical motions can be modeled as nonvertically propagating sv waves this could be implemented through conventional one dimensional horizontal component modeling using shake and the application of empirical depth dependent v h correction factors to account for the inclined propagation path at high frequencies vertical motions may have to be modeled as near vertically propagating p waves with strain dependent properties specifically developed for compressional deformation however these frequencies may be of lesser importance for design applications manuscript received 11 january 2002

a hydrothermal deposit from explorer ridge in the northeast pacific ocean abstract crusts composed of nontronite and ferromanganese oxides were recovered from explorer ridge a spreading ridge segment in the northeastern pacific ocean located off the west coast of canada the chemical and mineralogical composition of the crusts closely resembles that of the mound like hydrothermal deposits recently discovered at the famous site on the mid atlantic ridge and on the galapagos spreading centre compositional anomalies suggest that the crusts are precipitates of hydrothermal vent solutions which were ejected discontinuously and subsequently mixed with seawater