a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

soil seal formation and its effect on infiltration uniform versus nonuniform seal approximation two different approaches to the solution of the problem of flow through the dynamic stage of seal formation as well as through soil with a fully developed seal are studied the first approach considers the disturbed seal layer and the undisturbed soil underneath as a continuous nonuniform soil profile the second replaces the nonuniform seal by a uniform equivalent layer thereby generating a homogeneous two layer flow system the depth dependent properties of the nonuniform seal are expressed in terms of the exponential model of mualem and assouline 1989 the dynamics of seal formation are modeled according to assouline and mualem 1997 during the first rainfall on an undisturbed soil profile when the seal layer is formed the application of the first or the second approach has only a minor effect on the calculated infiltration curves however there is a significant difference between the two solutions regarding the dynamic changes of the water content in the soil surface and consequently within the seal layer during subsequent rainfalls on a sealed soil profile when the seal layer is completely developed the differences between the two ways of accounting for the seal layer become evident and their effects on the infiltration curve are much more significant representing the seal as an equivalent uniform layer increases the ponding time and the infiltration rates at the early stage of the process the amplitude of these effects is increased when the rainfall rate is higher and the seal layer is thicker an important result is that the relationship between infiltration rate and cumulative infiltration is unique in the case of a completely developed seal and when the seal is considered as a nonuniform layer however this relationship is not unique during seal formation independent of the approach applied to represent the seal layer

nontransform offsets of the pacific cocos plate boundary and their traces on the rise flank a multi beam bathymetric reconnaissance of the fast spreading crest of the east pacific rise between 7 n and 2 6 n found the axial ridge to be interrupted by four 4 13 km non transform offsets they have probably persisted since initiation by a small change in spreading direction 3 5 ma at each offset the volcanic rift zones veer 15 toward each other and overlap for 5 25 km around deep basins intervening lateral displacements of the rift zones are too short 2 5 km to break the linear axial volcanoes but cause local bends in their plan shapes some examples have a morphology that mimics the larger intervolcano offsets interpretation of a thorough bathymetric and magnetic survey around the 9 km 5 5 n offset indicates that the mainly one sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small scale jumps of the eruptive zone the northern rift zone is elongating propagating at the expense of the southern one the resulting along axis migration of this and other examples at speeds comparable to spreading rates causes the rise flank scars of the nontransform offsets equivalent to the fracture zones of transform faults to be oblique to the spreading direction these traces are recognizable within the highly lineated fault block terrain of the rise flanks because of the distinctive morphology of the 5 25 km wide bands that accreted at overlapped sections of the rift zones here lineation is less marked and oblique to the spreading direction and volcanic relief is more important than relief caused by faulting the west flank trace of the 5 5 n offset is occupied by a line of volcanoes and shallow volcanic ridges several other volcanic chains previously identified as hot spot traces seem to have grown in similar tectonic settings

cluster analysis of eastern north pacific tropical cyclogenesis precursors tropical cyclogenesis in the eastern north pacific is investigated by applying an objective cluster analysis technique to wind and geopotential height data from the 1985 to 1989 hurricane seasons the first 20 principal components derived from the 850 hpa european centre for medium range weather forecasting global analysis data are used to separate tropical cyclogenesis events into three relatively homogeneous groups composites of the height and wind fields for the days prior to cyclogenesis reveal distinct differences in the environment for the three groups two of the groups are associated with large scale height and wind anomalies which lie upstream of the genesis region on the days before tropical cyclones are formed the remaining group is associated with anomalies to the west of mexico which provide a favorable environment for tropical cyclogenesis an examination of the initial locations of the circulations which grew into named storms reveals that the groups associated with the upstream anomalies are characterized by tropical cyclones which formed over a broad range of longitudes while the group associated with the anomalies in the pacific contains storms which formed very near the coast of mexico

hydrothermal activity on near arc sections of back arc ridges results from the mariana trough and lau basin 1 the spatial density of hydrothermal venting is strongly correlated with spreading rate on mid ocean ridges with the interesting exception of hot spot affected ridges evidently because spreading rate is a reliable proxy for the magma budget this correlation remains untested on spreading ridges in back arc basins where the magma budget may be complicated by subduction induced variations of the melt supply to address this uncertainty we conducted hydrothermal plume surveys along slow spreading 40 60 mm yr and arc proximal 10 60 km distant sections of the southern mariana trough and the valu fa ridge lau basin on both sections we found multiple plumes overlying 15 20 of the total length of each section a coverage comparable to mid ocean ridges spreading at similar rates these conditions contrast with earlier reported results from the two nearest arc segments of a faster spreading 60 70 mm yr back arc ridge the east scotia ridge which approaches no closer than 100 km to its arc there hydrothermal venting is relatively scarce 5 plume coverage and the ridge characteristics are distinctly slow spreading small central volcanic highs bookended by deep median valleys and axial melt lenses restricted to the volcanic highs two factors may contribute to an unexpectedly low hydrothermal budget on these east scotia ridge segments they may lie too far from the adjacent arc to benefit from near arc sources of melt supply and subduction aided migration of mantle from the bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening analogous to the reykjanes ridge thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid ocean ridges a well defined trend of spreading rate versus hydrothermal activity on most ridge sections plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity

effects of rainfall intensity and slope gradient on the dynamics of interrill erosion during soil surface sealing abstract soil erosion during rainfall is strongly affected by runoff and slope steepness runoff production is drastically increased when a seal is formed at the soil surface during rainfall therefore a complex interaction exists between soil erosion and surface sealing in this study the dynamics of interrill erosion during seal formation is studied under different simulated rainfall and slope conditions a sandy soil was exposed to 70 mm of rainfall at two intensities 24 mm h 1 and 60 mm h 1 and five slope gradients from 5 to 25 infiltration runoff and soil loss rates were monitored during rainfall final infiltration rates increased with slope gradient at both rainfall intensities this effect being stronger for the higher intensity cumulative runoff at the end of the rainfall event was lower as slopes were steeper while an opposite trend was obtained for soil loss for the 5 and the 9 slopes the sediment concentration in runoff reached quickly a stable value during the whole rainfall event while it reached a peak value before declining for the higher slopes the peak value and its timing were rainfall intensity dependent soil erodibility during seal formation was evaluated using two empirical multiplication of factors type models it seems that slope and rainfall erosivity are accounted for only partly in these models for mild slope gradients below 9 the value of k i estimated by means of the two expressions becomes practically constant shortly after runoff apparition consequently the estimates resulting from this type of expressions remain valuable from the practical point of view

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

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

effect of rainfall induced soil seals on soil water regime wetting processes the effects of a rainfall induced soil seal on wetting processes are studied in the cases of two different soils hamra sandy loam and loess loam the soil seal is identified as the disturbed upper layer where the average hydraulic properties have been changed during rainfall from the initial properties that were identical to those of the undisturbed soil bed these properties are derived by applying the seal model of mualem and assouline 1989 calibrated for the two soils determination of the seal thickness bulk density retention curve and hydraulic conductivity function made it possible to solve systematically the flow equation for different boundary conditions allowing the simulation of the wetting processes within the seal layer as well as the undisturbed profile underneath the infiltration curves and the water content profiles during and after rainfall are derived for three rainfall intensities the water content and the capillary head profiles as well as the variation of these variables with time at different depths under the soil surface were also calculated smaller amounts of the rainfall infiltrate as the rainfall intensity increases the infiltration rate diminishes faster as the rainfall intensity increases the idea of a single infiltration curve as a function of the cumulative rainfall seems to be invalid each rainfall intensity yields a different infiltration curve a rough validation test of the model yielded predicted infiltration curves closely bounded by the experimental results the final infiltration rate after 50 mm of rainfall also seems to be a function of the rainfall intensity with the higher rate obtained for the higher rainfall intensity as experimentally observed in other studies unlike the explanations found in literature this study suggests that a shallower saturated zone within the seal layer is the reason for this phenomenon this study points out that the disturbed layer which forms the surface seal is not saturated from the very beginning of rainfall it has a significant water holding capacity and thus takes time to reach saturation furthermore under rainfall of high intensity the seal layer reaches saturation only at its upper zone close to the soil surface the lower part may remain unsaturated even after 50 mm rainfall and is larger for the soil of coarser texture

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

the valu fa ridge the pattern of volcanic activity at a back arc spreading centre abstract the central valu fa ridge an active back arc spreading segment bounded by small overlapping spreading centres can be divided into three regions each 8 12 km long with distinct bathymetric morphological and magnetic characteristics these observations suggest that each region has a different volcanic history driven by variations in melt supply along axis two of the three regions are centred on small deviations of the ridge axis which appear to be the local foci of extrusion there appears to have been a recent re organisation of the magmatic supply to the ridge in the past the highest magmatic budget was at the southern end of the ridge but it has now switched to the northern end where it coincides with a small overlapping spreading centre this re organization may be associated with island are activity beneath the ridge a bright reflector which is interpreted as being from the roof of a magma chamber is laterally more continuous that the inferred volcanic segmentation this implies that mixing of magma within the chamber must be limited along strike the magmatic budget at various parts of the ridge inferred from axial morphology can be correlated with variations in the width and brightness of the reflector this correlation suggests that the size of the chamber is dynamic and changes in response to variations in supply from below or delivery upwards

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

morphology of the valu fa spreading ridge in the southern lau basin multibeam echosounder surveys and seismic reflection profiling were conducted in the southern lau back arc basin sw pacific the bathymetric survey covered a 130 km long section of the active spreading ridge 3 cm yr half spreading rate the n s trending valu fa ridge vfr while seismic profiles concentrated on a 35 km long ridge section analysis of these data shows a morphologically segmented ridge which is underlain by a continuous magma chamber the ridge shows similarities but also differences to the segmentation model developed for mid ocean ridges analogous to this model the section of the vfr studied displays third and fourth order segmentation the amplitudes of the along axis undulations however are a factor of 5 to 10 greater than at mid ocean ridges additionally the vfr has numerous small volcanic cones along the axis which have previously only been observed at much slower spreading ridges in cross section the vfr has a very steep narrow triangular shape which is thought to result from the high viscosity of its andesitic lavas the axial magma chamber appears to vary in width 1 4 km and location relative to the axis beneath different morphological segments perhaps the most significant deduction from our combined data is that the magma chamber is widest beneath the most prominent axial discontinuity a small overlapping spreading center osc this is counter to observations from mid ocean ridges off axis traces of ridge segments allow us to reconstruct the evolution of this osc for the past 220 kyr we conclude that this back arc spreading axis shows a segmentation pattern which is similar to mid ocean ridges but it is influenced and modified by the geochemistry of its lavas and its proximity to the island arc

petrology of mariana trough and lau basin basalts the crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid ocean ridge basalts n morb but some subtle but distinctive differences exist these are largely due to differences in volatiles fe oxidation ratio abundances of large ion lithophile elements lil and some high field strength hfs elements isotope data for sr pb nd he and o are variable but broadly are morb like we present data contrasting mariana trough mtb and lau basin basalts lbb showing how they differ in certain trace element and isotopic signatures and compare both to n morb relative to n morb and lbb the mtb are enriched in alkalis alkaline earths light rare earth elements ree h2o and fe fe even for primitive samples while having morb type he 3 4 7 8 to 8 4 ra sr 87 86 mean 0 70293 and nd e 10 2 the lau basin is zoned with a central area having n morb characteristics for h2o lil ree nd e 8 1 to 9 0 and o but a depletion in hfs elements sr is more radiogenic mean 87 86 0 70347 and 3he 4he 3 4 7 2 to 11 0 ra is high relative to n morb the lau basin has a zone bordering the adjacent island arcs which is like mtb in both back arc basins relatively primitive unfractionated basalts with mg 100 mg fe 62 are found which resemble postulated primitive n morb calculated parental magmas obtained by adding xenocrystic olivine plot near or on 3 and 4 phase boundaries at 1 1 5 gpa for the ol di si system low p fractional crystallization and magma mixing explain most of the observed rock types possible explanations for differences between mtb and lbb include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere the long and complex history of mariana trough mantle sources led to a metasomatized mantle which gives rise to the distinctive mtb in contrast the lau basin is dominated by basalts derived from a source depleted in volatiles lil and hfs elements a previous melting history is implied mixing with a mantle component enriched in 87sr and 3he may be due to mantle counterflow driven by the subduction process

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

an observational study of easterly waves over the eastern pacific in the northern summer using fgge data abstract fgge revel iii b data provided by the european centre for medium range weather forecasts and the outgoing longwave radiation data measured by satellites are used to investigate observationally relationships between deep cloud activity and large scale meteorological fields during the northern hemisphere summer may september of 1979 over the eastern pacific aside from the summer monsoon area over southern and eastern asia the eastern pacific is the area where strong deep convection frequently develops in the northern summer this is also one of the tropical ocean areas that have been least explored meteorologically a power spectral analysis using the maximum entropy method is made for the meridional wind component at 850 mb at various grid points in the analysis domain for each mouth from may through september the waves with a period of 4 6 days are not only stronger in july and august than those of other months but active in the regions of 100 130 w in the eastern pacific and 130 160 e i

multiple hydrothermal sources along the south tonga arc and valu fa ridge 1 quantifying hydrothermal venting at the boundaries of tectonic plates is an outstanding geoscience problem considerable progress has been made by detailed surveys along mid ocean ridges mors but until recently little was known about fluid venting along volcanic arcs we present the first systematic survey for hydrothermal venting along the 425 km long south tonga arc and new chemistry data for particle and thermal plumes previously reported along an adjacent 88 km long section of the back arc valu fa ridge vfr eleven hydrothermal plumes recognized by their anomalous light backscattering eh temperature ph dissolved he 3 ch4 and total dissolvable fe and mn were identified arising from seven volcanic centers along the arc five plumes on the vfr were characterized chemically vent field density for the south tonga arc was 2 6 sites 100 km of arc front comparable to that found by surveys of the kermadec arc 1 9 to 3 8 sites 100 km and to mors in the eastern pacific average value for 2280 km of surveyed ridgecrest 3 2 sites 100 km a vent gap occurs along a 190 km section of the arc closest to the vfr and a site density twice the average for mors on the eastern edge of the pacific plate was found on this part of the vfr 6 6 sites 100 km we suggest magmas ascending under the adjacent south tonga arc have been captured by the vfr while chemical enrichments of plumes on the south tonga arc were in general slightly less than those on the kermadec arc several instances of excessive anomalies in ph suggest a similar presence of fluids enriched in magmatic volatiles co2 so2 h2s locally venting on the vfr has contributed to accumulations of 3 he fe and mn within the southern lau basin on a broader scale our results provide considerable support for the notion that venting from intraoceanic arcs on the convergent margin of the pacific plate adds significantly to the total hydrothermal input into the pacific ocean

geophysical characteristics of the southern mariana trough 11 50 n 13 40 n despite slow opening rates generally inferred for the mariana trough the southernmost part of the basin has fast spreading geophysical and morphologic characteristics that are unlike the features of the basin to the north a side scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11 50 n and 13 40 n and identifies the following characteristics the ridge across axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross sectional area variations of 1 to 7 km2 the along axis mantle bouguer gravity gradient is 0 2 mgal km axial segmentation occurs as overlapping axes and small deviation in trend no transform fault offsets exist despite significant changes in the trend of the spreading center characteristics of the surrounding basin include shallower overall depth than in the north no well developed frontal arc high in the southernmost trough the close proximity of submarine arc type volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank these characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin we propose that these effects are related to 1 the geometry of trench rollback in the southern trough leading to trench parallel extension generating inward radiating extensional faults 2 decoupling of the trench parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin and 3 augmentation of the spreading center s magmatic budget by arc magmatic sources contributing to its fast spreading character although these effects may be accentuated in the southern mariana trough by the geometry of trench rollback and position of the slab which here underlies the spreading center they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid ocean ridges

hundreds of small volcanoes on the median valley floor of the mid atlantic ridge at 24 30 n we have identified 481 seamounts from 6 000 km2 of sea beam swaths of the median valley floor of the mid atlantic ridge mar between 24 and 30 n we find that seamount abundances are an order of magnitude larger on average 80 per 1 000 km2 with relief above 50 m than estimated in previous studies along mid ocean ridges in the pacific1 and south atlantic2 suggesting that at this section of the mar seamount volcanism plays an important part in the formation of new oceanic lithosphere the neovolcanic ridge often observed on the inner valley floor3 appears to be constructed totally of individual coalesced seamounts so that our counts represent a minimum estimate the frequency distribution of summit heights of the mar seamounts is consistent with the exponential model describing the heights of off axis seamounts in the eastern pacific4 6 but with different parameters such a distribution of heights may be a fundamental aspect of seamount volcanism related to lithospheric properties and or magmatic plumbing seamount dominated axial volcanism must create a crustal structure very different from classic fissure fed models

the axial summit graben and cross sectional shape of the east pacific rise as indicators of axial magma chambers and recent volcanic eruptions abstract the axis of the east pacific rise epr undulates up and down hundreds of meters over distances of 30 200 km along strike the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers oscs we have suggested that systematic variations in depth and cross sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment 1 a comparison of recently collected multichannel seismic mcs data 2 with our sea beam and seamarc ii data have allowed us to test and advance this hypothesis along the epr from 9 to 13 n there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment the cross sectional shape of the rise the presence or absence of an axial summit graben and the presence or absence of a shallow axial magma chamber as interpreted from mcs data where the axial magma chamber is present the cross sectional shape of the ridge is broad and an axial summit graben is recognized along the axis in contrast where the cross sectional shape of the rise is narrow and triangular an axial magma chamber is not detected and an axial summit graben is absent these ridge axis characteristics tend to occur along deeper portions of a given ridge segment often near ridge axis discontinuities we suggest that these systematic variations in ridge axis morphology cross sectional shape and structure presence or absence of an axial graben reflect spatial and temporal variations in the magmatic budget of the ridge axis where the magmatic budget is waxing shallow level magma reservoirs in the crust and underlying upper mantle swell creating a broad axial bulge with a summit graben where the magmatic budget is diminished the crustal magma chamber is small this proposed correlation of shape structure and magmatic parameters fails along only two short portions of the ridge in these areas there is evidence for an axial magma chamber and the rise has a broad cross sectional shape but there is no summit graben bottom photographs and submersible results however show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting suggesting that the summit graben has been recently filled in by lava flows and the development of a summit graben or a linear caldera by volcano tectonic collapse has not yet occurred

a conceptual model for infiltration and redistribution in crusted soils treating the crust as a single layer a simplified but accurate infiltration and redistribution model for a crust topped soil profile is developed the method is an extension of an earlier infiltration and redistribution model for homogeneous soil profiles the crust layer is conceptually subdivided into regions associated with surface water content and interface water content respectively the extension of the wetting front into the subsoil is described by the homogeneous soil method described earlier two ordinary differential equations are written which match pressure heads and fluxes at the soil interface and are solved by a runge kutta technique the model is tested against the results of the richards equation with variations in the key variables of capillary length scale and saturated hydraulic conductivity for both soil and crust and for cases where ponding occurs both in the crust region and after wetting has entered the subsoil region in all cases the profile water contents and infiltration rates are very reasonably simulated

the interaction of planetary scale tropical easterly waves with topography a mechanism for the initiation of tropical cyclones abstract the interaction of a basic state flow consisting of a planetary scale easterly wave superimposed on a uniform easterly zonal wind with an isolated topographic feature is examined by numerical integration of the shallow water equations in an equatorial beta plane channel the topographic parameters are chosen to represent the sierra madre mountains of mexico the zonal wavelength and phase speeds of the basic state easterly waves are chosen to correspond with previously published observations the topographic modification of a slow rossby mode in which the basic state vorticity maximum is located near 15 latitude is characterized by the generation of a cyclonic vorticity maximum in the lee of the mountain which extends the entire length of the ridge and a secondary maximum which propagates downstream a fast mixed rossby gravity mode in which the vorticity maximum is located at the equator interacts less strongly with the mountain with a lee vorticity maximum that is confined near the southern

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

magma distribution across ridge axis discontinuities on the east pacific rise from multichannel seismic images detailed studies of the morphology of the east pacific rise axis have shown that its linearity is disrupted by many small but distinct non transform offsets including overlapping spreading centres oscs and deviations from axial linearity devals which display variable geochemical signals1 9 using multichannel seismic reflection profiling we have mapped the distribution of a bright shallow reflector that detrick et al 10 have associated with an axial magma chamber we have found that it is neither continuous across the 9 03 osc6 nor separated into two parallel bodies2 and that its lateral offset does not conform to that of the topographic offset associated with the 9 17 deval these observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the east pacific rise and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery

modeling infiltration into a sealing soil rain infiltration is often controlled by a less permeable layer known as a seal infiltration through this layer was modeled numerically by continuously updating the seal properties as a function of rainfall characteristics the seal remains unchanged until the surface has reached incipient ponding subsequently a rapid change in the hydraulic properties of the seal is imposed to reflect the effect of raindrop impact finally an equilibrium state develops between seal formation due to raindrop impact and seal erosion due to the rainstorm intensity effects model predicted infiltration was similar to that observed during simulated rainstorms having various combinations of intensities and kinetic energy rates per millimeter of rain a sensitivity analysis was performed to determine the effects of changes in seal and bulk soil properties as well as simulation parameters on infiltration seal formation was dependent on cumulative rainfall energy and the rainstorm intensity

segmentation and rift propagation at the valu fa rridge lau basin evidence from gravity data gravity and bathymetry data from a detailed ship based survey of a 40 km length of the valu fa ridge allow an analysis of the fine scale segmentation of this young back arc spreading center where the primary morphological segmentation takes the form of small overlapping spreading centers osc separating ridge segments typically 30 km long the gravity survey covers one complete segment and the overlap at its northern end and part of the overlap at its southern end the mantle bouguer anomaly shows a small 10 mgal subcircular negative anomaly the pattern of anomalies implies that the northern osc is the site of particularly vigorous magmatism with either increased crustal thickness or density reduction in the mantle or both accounting for the observed gravity low this interpretation confirms the results of seismic analyses of this spreading center which also imply that the osc is the site of enhanced magmatism the observations appear to contrast with widely accepted models of segmentation at mature ocean basin spreading centers where morphological offsets are believed to coincide with a relatively weak supply of magma on a longer 130 km gravity profile crossing the ridge the mantle bouguer anomaly shows a large 50 mgal longer wavelength 100 km positive anomaly centered on the spreading axis this can be modeled by an increase in crustal density and or a decrease in crustal thickness over a 40 to 50 km wide area centered on the axis this is consistent with recent models for the evolution of the basin involving recent southward propagation of seafloor spreading through preexisting but stretched thinned and intruded island arc lithosphere

the lau havre taupo back arc basin a southward propagating multi stage evolution from rifting to spreading abstract the lau havre taupo lht system is an active 2000 km long back arc basin associated with pacific australian plate convergence in the sw pacific data from the lht back arc system include odp drillhole swath bathymetric and side scan imagery seismic reflection rock geochemistry magnetic anomaly heat flow shallow seismicity gravity and seismic refraction data the lht forms a longitudinal transect from southward propagating true oceanic spreading in the lau basin near tonga similarly southward propagating arc rifting in the havre trough and southward propagating continental rifting in the taupo volcanic zone tvz in northern new zealand four transitional tectonic settings are identified along the lht north to south between the continuum of spreading to rifting and comprise 1 oceanic spreading of the central lau spreading centre clsc propagating into oceanic crust formed by the eastern lau spreading centre elsc 2 oceanic spreading of the elsc propagating into rifted lau havre arc crust 3 rifting within havre arc crust propagating into rifting new zealand continental crust and 4 rifting of continental crust propagating into as yet non rifted but flexurally subsiding continental crust the enhanced spatial and temporal resolution of observed tectonic and magmatic processes along the strike of the lht is used to constrain a model for the evolution of back arc rifting to back arc spreading a sequence of five stages is identified and comprise 1 a combination of incipient simple and pure shear 2 early distributed and spatially heterogeneous half graben rifting with low angle synthetic and antithetic structures associated with rapid graben sediment infilling ephemeral early morb like rift magmatism and the initial trenchward migration of the arc front 3 a latter phase of half graben rifting mostly confined to a contiguous axial zone with continued ephemeral morb like rift magmatism and further trenchward migration of the arc front 4 initial oceanic spreading with evolving steady state morb magmatism along the axial rift graben system and 5 the reconfiguration of ridge geometry in response to changes in opening orientation leading to the development of ridge propagation into newly created oceanic crust rifting stages 2 and 3 can produce at least 120 km wide back arc crust by a balance of crustal extension with an estimated factor of 3 with rift and constructional arc magmatism in the lht system depending on latitude rates of widening during stages 2 and 3 vary between 15 and 45 mm yr 1 whilst stages 4 and 5 have full spreading rates of 90 mm yr 1

tectonic evolution of ridge axis discontinuities by the meeting linking or self decapitation of neighboring ridge segments recent deep tow sea beam and seamarc ii studies indicate that ridge axis discontinuities along the east pacific rise evolve in several different ways where magmatic pulses along the spreading center happen to meet head on a low point or saddle point occurs along the axial depth profile of the spreading center where the magmatic pulses misalign overlapping spreading centers develop and two distinct evolutionary paths are possible one spreading center tip may cut through to and link with the adjacent en echelon ridge chopping off the opposing ridge tip as proposed by macdonald and fox 1983 in addition either spreading center may cut inside or outside of itself repeatedly decapitating its own ridge tip crack propagation studies show that the crack propagation force g drops significantly when the ratio of crack overlap to crack offset l w exceeds 3 applying this relation to spreading centers we suggest that propagation of the individual spreading center tips may stall when l w is greater than 3 if linkage has not yet occurred then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip decapitating the ridge tip in a process we call self decapitation subsequent magmatic pulses may be deflected or derailed from the existing path of the ridge because freezing of the axial magma chamber near ridge axis discontinuities creates a core of coherent unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber in addition the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge for propagating rifts which represent first order changes in plate boundary geometry l w remains 1 5 so that g is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced

infiltration during soil sealing the effect of areal heterogeneity of soil hydraulic properties 1 the combined effects of soil surface sealing and areal heterogeneity of the soil hydraulic properties on the mean infiltration curve are studied here for the first time seal formation during rainfall is simulated according to the dynamic model of assouline and mualem 1997 areal heterogeneity is represented by lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil and by related distributions of the other soil parameters the mean infiltration curve of the heterogeneous field is evaluated presuming that horizontal fluxes are negligible it is found that when soil surface sealing is taken into account in a heterogeneous field the ponding time is only slightly affected however significantly more runoff is produced compared to nonsealing field mulched with the relative area contributing to surface runoff being increased from 65 to 95 the relative effect of field heterogeneity on the infiltration curve increases for higher rainfall intensity the formation of soil surface seal apparently reduces the effect of the field areal variability on the steady infiltration rate the infiltration rate after 60 min of rainfall distributes lognormally under sealing conditions while it distributes normally in the case of the unsealed soil the final infiltration rate resulting from the assumption of a sealed uniform field is underestimated compared to sealed heterogeneous field with the relative effect being larger for a loam soil 51 than for a sandy loam soil 26 also the approximation that the infiltration curve is a unique function of cumulative rainfall independent of rainfall intensity is not valid in a sealing heterogeneous field and its application in such a case would be more erroneous than in uniform fields on the other hand one may disregard the variability of all soil parameters except the hydraulic conductivity when calculating infiltration

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

deep earthquakes beneath the fiji basin sw pacific earth s most intense deep seismicity in stagnant slabs abstract previous work has suggested that many of the deep earthquakes beneath the fiji basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position since nowhere else in the earth do so many earthquakes occur in slabs stagnated in the transition zone these earthquakes merit closer study accordingly we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well located deep events in the geographic area between the bottoms of the main vanuatu and tonga wadati benioff zones two regions of deep seismogenesis are recognized there i 163 deep shocks have occurred north of 15 s in the vityaz group from 1949 to 1996 these seismological observations and the absence of other features characteristic of active subduction suggest that the vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone ii another group of nearly 50 outboard deep shocks occur between about 450 and 660 km depth west of the complexly buckled and offset western edge of the tonga wadati benioff zone their geometry is in the form of two or possibly three small circle arcs that roughly parallel the inferred motion of tonga trench migration earthquakes in the southernmost of these arcs occur in a recumbent high seismic wavespeed slab anomaly that connects both to the main inclined tonga anomaly to the east and a lower mantle anomaly to the west van der hilst r 1995 complex morphology of subducted lithosphere in the mantle beneath the tonga trench nature vol 374 pp 154 157 both groups show complexity in their focal mechanisms the major question raised by these observations is the cause of this apparent temporary arrest in the descent of the tonga slab into the lower mantle we approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered or was buckled segmented and laid out in the transition zone

mechanical decoupling and thermal structure at the east pacific rise axis 9 n constraints from axial magma chamber geometry and seafloor structures abstract we study the relationships between the seafloor structures and the axial magma chamber geometry in the 9 n overlapping spreading center osc area on the fast spreading east pacific rise epr our observations are based on a new high resolution bathymetric map of the 9 n osc area derived from picks of the seafloor arrival on 3d seismic data and on previously published data that constrain the presence and distribution of melt below the 9 n osc differences in the orientation of structures between the seafloor and the magma chamber indicate a sharp change in principal stress directions with depth suggesting that the brittle crust above the melt sill is decoupled from the melt sill itself and the ductile crust underlying it the stress field within the brittle upper crust results from a local interaction of the two overlapping spreading centers whereas the stress field in the crust below the melt sill corresponds to the regional stress field imposed by plate separation given this mechanical structure of the crust the melt sill shape and location appear to be controlled by the following factors the location of the deep melt source below the melt sill the ambient stress field at the depth of the melt sill and the stress field in the brittle upper crust above the melt sill which thermally shapes the roof of the melt sill through repeated eruptions

controls on segmentation and morphology along the back arc eastern lau spreading center and valu fa ridge back arc spreading centers increasingly depart from mid ocean ridge mor characteristics with proximity to the arc volcanic front the close association of these departures with slab derived materials in erupted lavas suggests that subduction related chemical effects are their primary cause the eastern lau spreading center elsc and valu fa ridge vfr are type examples of this process together they constitute a first order spreading center in the lau back arc basin that progressively converges on the tofua arc volcanic front from north to south here we use ship multibeam and deep towed side scan sonar data to examine variations in axial morphology and volcanism at the second and third order segment scale along these ridges and develop a model for the processes that control them closest to the arc vfr and the southern segment of the elsc shoal toward second order segment ends in contrast to mors northward and beyond 70 km from the arc the axis becomes abruptly deeper and flatter and no longer shoals toward second order segment ends at vfr along axis topographic highs correlate with the location of arc volcanoes along slab flow lines these correlations are weaker along the southernmost elsc segment and absent along elsc segments farther north the observations show a modulation of back arc segmentation with arc proximity that rapidly diminishes with distance they support a model of the mantle wedge with a strongly hydrous domain within 70 km of the arc within which the arc and ridge interact and a much less hydrous domain farther from the arc without evident arc ridge interactions

crustal structure accretionary processes and rift propagation a gravity study of the intermediate spreading valu fa ridge lau basin summary the valu fa ridge is an intermediate spreading full rate of 60 mm yr 1 ridge located in the lau basin in 1995 this ridge was surveyed using a multidisciplinary geophysical approach to image crust and upper mantle structure with the aim of investigating the processes of oceanic crustal accretion in a back arc tectonic environment as part of this experiment a network of gravity profiles was acquired together with seismic magnetic swath bathymetry and controlled source electromagnetic data presented in this paper are the results of forward modelling of a subset of the acquired gravity profiles two oriented ridge perpendicular and one ridge parallel using the preferred seismic models of turner et al 1999 as a basis of initial model construction in addition the gravity data set in its entirety has been used to calculate the mantle and residual mantle bouguer anomalies with the aim of investigating variability in crustal structure both density and layer thickness and the nature of the underlying upper mantle of particular interest are the overlapping spreading centre between the central and northern valu fa ridges where seismic modelling implies a generally thickened crust and a magma chamber located beneath the overlap basin rather than separate chambers supplying each ridge and the propagating rift tip and associated basin bounding pseudo fault modelling results suggest that the pre and post rift crusts have different compositional origins with lower densities required 12 km off axis to fit the observed free air gravity anomaly the locations of the transitions into regions of lower density correspond with those of turner et al 1999 derived from seismic modelling which in turn correspond in location to the rift related pseudo fault identified by wiedicke collier 1993 calculation and interpretation of the mantle and residual mantle bouguer anomalies also confirms the lower off axis densities and indicates a general increase in crustal thickness northwards towards the overlapping spreading centre between the central and northern valu fa ridge segments the presence of thicker crust corresponding to a region of negative mantle and residual mantle bouguer anomaly implies that the region surrounding the overlapping spreading centre has been in the recent geological past or is presently the site of an increased magma supply the residual mantle bouguer anomaly also reveals features related directly to off axis lateral variation in density and layer thickness associated with the southward propagation of the valu fa ridge into the havre trough modelling of the north south variation in crustal thickness along axis shows that the anomaly trend can be explained simply by thicker crust beneath each of the valu fa ridge segments and the overlapping spreading centre between the northern and central valu fa ridges the observation of segment centred crustal thickening is consistent with models of ridge segmentation that suggest that melt influx is located towards segment centres and that segment length is controlled by the maximum lateral extent to which this melt can flow macdonald et al 1988 tolstoy et al 1993 however the overlapping spreading centre remains anomalous from these models in that it also appears to be associated with thicker crust and an increased melt supply

geology and kinematics of the niuafo ou microplate in the northern lau basin we present a revised three plate kinematic model for the lau basin focusing on new interpretations of the geologic characteristics of the niuafo ou microplate boundaries it builds upon the three plate model by zellmer and taylor 2001 while resolving some geologic discrepancies in that model along the southern branch of the mangatolu triple junction and the fonualei rift and spreading center which together form the niuafo ou tonga plate boundary our model predicts slow to ultraslow opening rates 32 8 mm yr southward consistent with axial morphology and magnetic profile modeling our interpretation of the seafloor magnetization pattern and flank morphology along the central lau spreading center clsc indicates that opening rates along the australia niuafo ou boundary are higher than predicted in the previous model thus compensating for slower rates along the niuafo ou tonga boundary lastly we show geologic evidence that peggy ridge pr interpreted previously as an active transform fault and therefore a strong azimuthal constraint on niuafo ou plate motion is inactive and that the active plate boundary is the adjacent lau extensional transform zone extending from the clsc to its intersection with the northwest lau spreading center the removal of the pr azimuthal constraint allows us to determine an euler pole for australia niuafo ou opening that satisfies three plate closure criterion and develop a geologically consistent model of lau basin kinematics while the final model focuses on constraining current plate velocities assuming rigid plate eulerian rotations the geologic analyses presented herein also provide insight into long term basin evolution which appears to involve significant nonrigid plate kinematics

volcanic reconstruction of paleoproterozoic arc volcanoes the hidden and louis formations flin flon manitoba canadathis is a companion paper to dewolfe y m gibson h l and piercey s j 2009 petrogenesis of the 1 9 ga mafic hanging wall sequence to the flin flon callinan and triple 7 massive sulphide deposits flin flon manitoba canada canadian journal of earth sciences 46 this issue the hanging wall to the flin flon callinan and triple 7 volcanogenic massive sulphide deposits of the flin flon district is composed of the hidden and louis formations the contact between these formations is marked by mafic tuff that represents a hiatus in effusive volcanism the formations form a composite volcanic edifice that was erupted and grew within a large volcanic tectonic subsidence structure hosting the deposits that developed within a rifted arc environment the formations are evidence of resurgent effusive volcanism and subsidence following a hiatus in volcanism marked by ore formation since they consist of dominantly basaltic flows sills and volcaniclastic rocks with subordinate basaltic andesite and rhyodacitic flows and volcaniclastic rocks the hidden formation is interpreted to represent a small shield volcano and the louis formation a separate shield volcano that developed on its flank both the hidden and louis volcanic edifices were constructed by continuous low volume eruptions of pillow lava a gradual change from a dominantly extensional environment during the formation of the footwall flin flon formation to a progressively more dominant convergent environment during the emplacement of the hanging wall suggests that the hidden and louis formations are unlikely to host significant volcanogenic massive sulphide type mineralization however synvolcanic structures in the formations define structural corridors that project downwards into the footwall where they encompass massive sulphide mineralization indicating their control on ore formation longevity and reactivation as magma and fluid pathways during the growth of the hidden and louis volcanoes

seismic images of a magma chamber beneath the lau basin back arc spreading centre we present normal incidence seismic reflection profiles from a back arc spreading centre in the southern lau basin which show a bright mid crustal reflector beneath the ridge axis analysis of wide angle seismic data collected simultaneously with the reflection profiles shows that the reflector coincides with the top of a low seismic velocity zone we interpret the reflector as the top surface of a magma chamber the lau basin is one of only two places where a magma body has been clearly imaged beneath a spreading centre the other being the east pacific rise between 9 and 13 n ref 1 the lau basin reflector is imaged beneath an entire 35 km long ridge segment it is also present beneath both ridges and the overlap basin of a small overlapping spreading centre this is the first observation of a widespread magma body beneath an overlapping spreading centre and indicates that the offset here is not related to a break in continuity of the magma chamber

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

seismic structure and indicators of magma budget along the southern east pacific rise in this paper we re examine the relationship between seismically constrained variations in crustal structure along the southern east pacific rise sepr and the segment scale variations in axial depth morphology basalt geochemistry and hydrothermal activity that have often been attributed to along axis differences in the supply of magma to the mid ocean ridge along 800 km of the fast spreading sepr good correlations exist between axial depth ridge cross sectional area mantle bouguer anomaly and the mgo weight percent of basalts recovered from the rise axis these correlations indicate along axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of 100 000 years in contrast we show that the depth and width of the midcrustal magma sill the thickness of seismic layer 2a and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross sectional area we suggest that the emplacement geometry width of the intrusion zone and flow lengths not magma supply controls extrusive layer seismic layer 2a thickness we hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events dike intrusion eruptions faulting which occur on much shorter timescales 10 100 years than the longer term variations in magma supply reflected in along axis changes in the shape and depth of the ridge axis

segmentation of the mid atlantic ridge between 24 n and 30 40 n the mid atlantic ridge mar extends for over 12 000 km from iceland to the bouvet triple junction in the south atlantic most of our knowledge of the geology of this slow spreading centre comes either from widely spaced low resolution echo sounding lines or from detailed studies of small areas using deep towed instruments or submarines high resolution multibeam bathy metry1 2 has been insufficient to define the variability of accretionary processes along this plate boundary here we present the results of a recent investigation of a 800 km long section of the mar carried out using the multibeam echo sounder seabeam3 analysis of these data reveals the scale and nature of the segmentation of the mar between the kane fracture zone 24 n and latitude 30 40 n

opposing trends in crustal thickness and spreading rate along the back arc eastern lau spreading center implications for controls on ridge morphology faulting and hydrothermal activity abstract along back arc ridges located near the arc volcanic front crustal thickness is observed to vary systematically with arc proximity independent of spreading rate this effect is especially well expressed by the 400 km long eastern lau spreading center elsc where approaching the arc volcanic front from 100 to 40 km crustal thickness nearly doubles 5 5 9 km as spreading rates decrease by more than half 97 39 mm yr the crustal thickness variations at the elsc appear to result primarily from changes in mantle wedge composition caused by subduction we investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology faulting and hydrothermal activity as part of the first phase of ridge2000 integrated studies in the lau back arc basin we used deep towed side scan sonar instruments dsl120a and imi30 to continuously map the near axis region within broader coverage ship multibeam bathymetry and acoustic imagery swaths an array of optical sensors maprs concurrently surveyed the near bottom water for hydrothermal plume anomalies hydrocasts made at identified plume sites confirmed their hydrothermal origin the data show that as spreading rates increase and crustal thicknesses decrease along the elsc 1 morphology transitions from shallow peaked volcanic highs to a deeper flat axis 2 faults become larger and more widely spaced 3 hydrothermal activity as measured by plume incidence increases and appears to reach levels higher than the global mid ocean ridge trend with spreading rate the observations indicate that crustal thickness magma supply has a greater control on ridge morphology and faulting than spreading rate even at fast rates where the thermal lithosphere should be thin however heat input from mantle advection proportional to spreading rate perhaps in combination with increased fault permeability appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the elsc the observed effects may have generic implications for controls of seafloor spreading characteristics at mid ocean ridges

hydrothermal sedimentation adjacent to the propagating valu fa ridge lau basin sw pacific abstract the partition geochemistry and hydrothermal metal accumulation rates ar during the late quaternary 3 km from hydrothermally active areas still shows an enrichment of mn ag ba and zn when compared to regional 500 m depth lau basin sea water the elements ni zn cu co and mo are associated with mn oxides in the sediment and are thought to have been scavenged by hydrothermal oxides during the transport of hydrothermal oxides away from the spreading center only a low proportion of fe is bound to the hydrothermal oxide phase in the sediments and this element is mainly associated with silicate phases principal components analysis pca of geochemical data in the detrital free caco 3 free fraction of the sediments supports a geochemical hydrothermal oxide association between mn ni mo cu zn pb p be ba and la with fe associated with the silicate weathered detrital fraction iron manganese ratios in the oxide phase range between 1 2 and 2 8 these values are below those found in sediments at mid ocean ridges peaks in the downcore distribution in mn appear to be contemporaneous in the cores studied and are probably the result of hydrothermal discharges on the vfr hemipelagic sedimentation rates throughout the area average 2 3 cm ka 1 hydrothermal mn ar range between 3 and 82 mg cm 2 ka 1 during the 0 30 ka b p period and between 5 and 65 mg cm 2 ka 1 during the 30 120 ka b p period maximum mn ar adjacent to the vfr are comparable to the highest values previously reported for recent sediments proximal to the east pacific rise epr during the entire period 0 120 ka b p mn ar are highest in core so48 101 located about 90 km behind the propagating tip of the vfr recent hydrothermal input has also occurred about 33 km behind the propagator this is in accord with the suggested location of fully evolved steady state magma chambers occurring some distance tens of kilometers behind propagating rift tips

small non overlapping offsets of the east pacific rise high resolution bathymetric surveys1 2 of the east pacific rise epr reveal that in addition to overlapping spreading centres oscs the crest of the epr is offset tiny amounts by smaller features recently langmuir et al have shown that despite their subtle bathymetric expression these small offsets bends and kinks of the epr collectively termed deviations from axial linearity devals represent important petrological and tectonic boundaries some devals are small non overlapping offsets snoos that offset the epr axis by up to a few hundred metres bathymetric contours near snoos are usually disturbed from their axis parallel trend in ways that we interpret as evidence for strike slip faulting and volcanism at high angles to the trend of epr axis we offer a model for the formation of snoos and their associated faults that invokes independent magma supply stochastic active spreading and limited non rigid behaviour of the neovolcanic zone

hydrothermal activity and magma genesis along a propagating back arc basin valu fa ridge southern lau basin valu fa ridge is an intraoceanic back arc spreading center located at the southern prolongation of the lau basin bathymetric observations as well as detailed sampling have been carried out along the spreading axis in order to trace hydrothermal and volcanic activity and to study magma generation processes the survey shows that widespread lava flows from recent volcanic eruptions covered most of the vai lili hydrothermal vent field only diffuse low temperature discharge and the formation of thin layers of siliceous precipitates have been observed evidence of present day hydrothermal activity at the hine hina site is indicated by a thermal anomaly in the overlying water column our studies did not reveal any signs of hydrothermal activity either above the seismically imaged magma chamber at 22 25 s or across the southern rift fault zone 22 51 s lavas recovered along the valu fa ridge range from basaltic andesites to rhyolites with sio2 contents higher than reported from any other intraoceanic back arc basin on the basis of the highly variable degrees of crystal fractionation along axis the development of small disconnected magma bodies is suggested in addition the geochemical character of the volcanic rocks shows that the transition zone from oceanic spreading to propagating rifting is located south of the hine hina vent field in the vicinity of 22 35 s

morphotectonic characteristics of a propagating spreading system in the northern lau basin bathymetric sea beam mapping of a southward propagating rift system in the lau basin sw pacific shows a morphotectonic segmentation of the two competing axes over a distance of 70 km the propagating axis develops from a narrow rift graben at the propagator tip via a leaky graben into a juvenile volcanic ridge and finally into a fully developed spreading ridge the competing dying spreading axis 70 km farther east shows steady deepening towards the north combined with a change from a small volcanic ridge to a narrow depression before it is buried by sediments the relay zone between the two axes contains two deep n s striking grabens the eastern one 3200 m deep may act as a short lived spreading center a ne trending string of slightly offset deep sediment filled basins is interpreted as fossil analoges to these grabens originally generated in the southward migrated relay zone the string of basins suggests an episodic rather than steady state southward migration propagation and opening of grabens led to a rotation of the inherited ridge parallel crustal fabric however the propagating tip and the grabens cut obliquely through the fabric thus generating small scale structural and morphological inhomogeneities of the crust at the propagating tip old and new tectonic lineaments closely interfinger

axial crustal structure of the lau back arc basin from velocity modeling of multichannel seismic data located west of the tonga trench the lau back arc basin is a prime environment for studying the interplay between oceanic spreading systems and an active subduction zone within the basin lie two complementary intermediate rate spreading systems the central lau spreading center clsc and the eastern lau spreading center valu fa ridge elsc vfr that are positioned 170 to 40 km away from the active volcanic arc respectively multichannel seismic mcs images of both systems reveal systematic variations in axial crustal structure primarily related to the proximity of the volcanic arc but also related to spreading rate morphology and petrology upper crustal refraction data selected from the along axis seismic lines collected during the 1999 mcs survey were modeled in both the time versus range t x and intercept time versus slowness p domains to provide validation and detail to the seismic reflection observations the results show that as both the proximity to the arc and the spreading rate decrease southward 1 seismic layer 2a thickens by 0 6 km between the clsc and vfr from 0 4 km to 1 0 km 2 the average depth of the axial magma chamber amc increases from 1 5 km at the clsc to 2 8 km at the southern vfr excluding the northern section of the elsc that shows no continuous amc reflector but does show an isolated melt sill 3 the upper crustal basement velocity decreases from 2 1 km s at the clsc to 1 8 km s at the vfr and 4 the velocities of both layer 2a and 2b decrease between the clsc and northern vfr from 3 2 to 2 5 km s and 5 0 to 3 9 km s respectively along with a broad axial high morphology these features of the clsc thinner layer 2a shallower amc and faster crustal velocities correlate best with the fast spreading east pacific rise and intermediate spreading juan de fuca ridge conversely the structural characteristics of the central elsc vfr have no known counterpart in the global mid ocean ridge system we attribute this primarily to the volatile concentration in the magmas coming from the tonga subduction zone

mechanical and hydraulic resistance relations in crust topped soils abstract the formation of soil surface crusts leads to increased mechanical and hydraulic resistances in this study changes and relationships of both resistances under simulated sprinkle irrigation or rainfall and sprinkle followed by flooding were examined results indicated that a silt loam soil developed a thicker surface crust than a clay soil for any given kinetic energy ke crusts as thick as 3 9 and 2 6 mm formed on the silt loam and clay soils respectively mechanical resistance r m increased with increasing ke where the effect was greater in the silt loam and was attributed to intrinsic resistance and crust thickness steady state infiltration rate i was much lower in crusted clay than crusted silt loam soil changes of both r m and i closely followed changes in crust thickness z c thicker crusts showed more resistance against external force than thinner crusts due to more extended particle interlocking obtained functions indicated that the effect of thickness on strength was more significant in the lower range of crust thickness the effect of z c on i strongly followed a negative power function for both soils with higher i in the silt loam soil mechanical resistance showed high sensitivity to crusting and remained responsive to increasing severity change of r m in the silt loam soil was steeper than that of the clay soil r m of the silt loam soil increased from 0 5 n non crusted to 43 n highest ke with a relative change rel o r m of 86 for clay r m increased from 0 77 n to 22 n producing a rel o r m of 28 6 trends were consistent when evaluated relative to the lowest applied ke rel 1 r m where r m increased under highest ke about 6 9 and 4 7 times for the silt loam and clay soils respectively hydraulic resistance r h increased with increasing treatment intensity and was higher in the clay soil both r m and the work of resistance r w showed highly significant relationships with r h the silt loam soil displayed greater r w for any given r h although r h was markedly higher in the clay soil the rate of change was greater in the silt loam soil infiltration rate appeared highly sensitive to increased crusting particularly in the clay soil the i r h relationship strongly followed the negative power function i a r h b relative changes in i under ponding showed reduction factors of 130 and 180 relative to original soils and of approximately 23 and 28 times relative to the least intense treatment for the silt loam and clay soils respectively relative changes in r m correlated positively while rel i correlated negatively with rel r h in power equations the change of transmission zone suction u with r h fitted the function a r h m where a and m were constants characteristic of the soil using a function of the form k a n the characteristic constants a and n were determined as 0 174 and 3 01 for the silt loam and 1 367 and 1 61 for the clay soil and the characteristic k functions were obtained

seismic mapping of a magma chamber beneath the valu fa ridge lau basin we analyze a dense grid of seismic normal incidence and wide angle data collected over an active back arc spreading center in the lau basin of the southwest pacific our survey area covers the whole of a 35 km long morphological segment and a small overlapping spreading center a bright reflector which is coincident with a velocity inversion at a depth of 3 2 0 2 km below the seafloor is observed on every one of the 40 across axis profiles we interpret this reflector as being the roof of a crustal magma chamber the widest magma chamber reflector occurs beneath the overlapping spreading center where it extends up to 4 km being imaged beneath both ridges and in places beneath the overlap basin elsewhere the width of the reflector varies between 0 6 and 2 3 0 4 km the narrowest reflectors are observed beneath deviations devais of the ridge axis in addition to being observed on each of the kilometer spaced across axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis the reflection coefficient of the event is between 0 34 and 0 65 this requires the melt at the top of the chamber to have a p wave velocity less than 2 7 km s if the interface is planar or less than 3 8 km s if the interface is layered

rain infiltration into loess soils from different geographic regions abstract loess soils are among the most erodible soils therefore evaluating and enhancing infiltration is paramount in controlling soil loss a laboratory study was conducted to evaluate the relative difference in infiltration among selected loess surface and subsurface soils from belgium canada china and the united states representing the major loess belts of the northern hemisphere soils were subjected to simulated rainstorms of constant intensity i 41 1mm h 1 duration 2 hours and energy rate 27 0 j m 2 per mm of rain infiltration runoff and soil water pressure were continuously monitored infiltration was described by a linear relationship for the pre ponding period and by a power series for the post ponding period differences in infiltration response were attributed to differences in soil properties such as differences in organic matter particle size swelling clay content fe oxyhydroxides and carbonates despite the high organic carbon content and coarser texture of the canadian loess surface soil the presence of highly expansive smectitic clay caused a rapid reduction in infiltration rates indicating the importance of soil mineralogical constituents in surface seal development

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

seismic imaging of the axial region of the valu fa ridge lau basin the accretionary processes of an intermediate back arc spreading ridge in 1995 a multidisciplinary geophysical experiment targetted the intermediate spreading valu fa ridge full rate 60 mm yr 1 which is centred on 22 20 s 176 40 w in the lau basin as part of this experiment wide angle and normal incidence seismic profiles were collected both along and across axis to determine the crustal structure of the central valu fa ridge cvfr and its overlap with the northern valu fa ridge nvfr controlled source electromagnetic profiles and underway gravity magnetic and bathymetry data were also collected in this paper we describe the results of forward modelling of the along and across axis wide angle and normal incidence seismic data an axial low velocity block and its underlying slightly broader zone of depressed seismic velocities low velocity zone have been identified and these features are interpreted as corresponding to a melt lens and underlying magma chamber the low velocity block is 1 2 km wide and has a first order upper boundary from which large amplitude reflections are observed amplitude analysis of these indicates an interconnected melt fraction the nature of the lower boundary is more poorly constrained as no reflection event corresponding to the base of the low velocity block is observed modelling indicates that velocities similar to those observed at the base of layer 2 within the axial region 5 5 km s 1 are achieved by 250 m below the upper boundary possibly suggesting a gradational lower boundary with high velocity gradient the low velocity zone lvz is interpreted as an 4 km wide magma chamber delineated by a seismic velocity anomaly of 0 2 km s 1 extending down through layer 3 to within 1 5 2 km of the moho the velocity anomaly and dimensions of the lvz are generally smaller than those observed at the east pacific rise epr and mid atlantic ridge mar the observed along axis continuity of the low velocity block is remarkable extending from the southern tip of the cvfr to the overlapping spreading centre osc with the nvfr a low velocity block is modelled beneath the inside flanks i e the slopes that dip into the overlap basin of both ridges at the osc although the existence of a single low velocity block beneath the overlap basin itself cannot be ruled out the identification of a single lvz centred on the overlap region rather than two merged lvzs beneath each segment implies that the material in each low velocity block originates from the same crustal magma source a reflection event from the moho is observed from directly beneath the axis on both across axis profiles which indicates that a distinct crust mantle boundary may be formed within the axial region many of the observations at the valu fa ridge are consistent with those at the epr and the reykjanes ridge mar which implies that regardless of spreading rate crustal accretionary processes at mid ocean ridges with similar magmatic budgets are also broadly similar

micromorphological and mineralogical components of surface sealing in loess soils from different geographic regions mineralogy of clay sized particles and rainstorm characteristics are among the major factors that determine the nature of soil sealing this research was designed to further our knowledge about the role of clay mineralogy and rainstorm characteristics on crust formation four loess soils from different geographic locations canada usa china and belgium were used in this study preliminary studies were carried out on grenada bt usa lishi china and bierbeek bt belgium horizons detailed infiltration and microscopic studies were performed only on the bm1 horizon of the saskatchewan soil canada and the ap horizon of the grenada soil mississippi usa two levels of rainstorm were applied 40 and 80 mm h 1 while smectite was the major component in the saskatchewan bm 1 horizon vermiculite and illite were found to be the dominant clay minerals in the bt horizon of the grenada soil formation of the washed layer together with thin lamellar crust coincided with the preferential movement of clay sized particles through runoff in both the saskatchewan bm1 and grenada ap horizons the higher initial infiltration rate with higher rainfall intensity in both soils were likely due to higher matric suction but rapid reduction in infiltration in this treatment was due to rapid aggregate destruction and a dense packing of the fundamental soil particles no washed in zone could be observed in the bm1 horizon of the saskatchewan soil in both rainstorm intensities however drastic reduction in infiltration especially with high rainstorm intensity after 7 8 min was attributed mainly to the swelling of smectite in this soil material washed in materials were visible in the ap horizon of grenada under high rainstorm intensity due to the presence of iron oxides and clay coatings aggregates were stable and no sealing could be observed in the grenada bt and bierbeek bt materials the degree of seal development in the lishi soils was low this was due to low clay and high carbonate contents this study shows clearly that higher and continuous rainstorm intensity causes rapid seal development and as a result more erosion would occur especially in the soils with high smectite content

an air sea interaction theory for tropical cyclones part ii evolutionary study using a nonhydrostatic axisymmetric numerical model abstract in part i of this study an analytical model for a steady state tropical cyclone is constructed on the assumption that boundary layer air parcels are conditionally neutral to displacements along the angular momentum surfaces of the hurricane vortex the reversible thermodynamics implied by this assumption allows the mature storm to be thought of as a simple carnot engine acquiring heat at the high temperature ocean surface and losing heat near the low temperature tropopause although the oceanic heat source is universally recognized as the sine qua non for the mature hurricane there is also wide acceptance of conditional instability of the second kind cisk which makes no specific reference to surface heat fluxes as the formative mechanism this ambivalence is seen in that all numerical simulation studies find it essential to have transfer from the ocean surface yet all start from a conditionally unstable atmosphere the hypothesis put forward in part i based on the steady state theory is t

mantle wedge control on back arc crustal accretion at mid ocean ridges plate separation leads to upward advection and pressure release partial melting of fertile mantle material the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away in back arc basins the subducting slab is an important control on the pattern of mantle advection and melt extraction as well as on compositional and fluid gradients modelling studies predict significant mantle wedge effects on back arc spreading processes here we show that various spreading centres in the lau back arc basin exhibit enhanced diminished or normal magma supply which correlates with distance from the arc volcanic front but not with spreading rate to explain this correlation we propose that depleted upper mantle material generated by melt extraction in the mantle wedge is overturned and re introduced beneath the back arc basin by subduction induced corner flow the spreading centres experience enhanced melt delivery near the volcanic front diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away our model explains fundamental differences in crustal accretion variables between back arc and mid ocean settings