structural control on sea floor hydrothermal activity at the tag active mound the tag active mound1 13 on the mid atlantic ridge near 26 n is one of the largest known actively forming volcanogenic massive sulphide deposits construction of such a deposit requires that the upflow of hydrothermal fluids be localized at this site over tens of thousands of years but the cause of this localization has been controversial two popular hypotheses propose that it results from young volcanism immediately adjacent to the mound6 or from the intersection of a transfer fault orthogonally crossing the rift valley floor with ridge parallel faults8 here we present high resolution sonar and photographic data which provide no evidence for young eruptions nearby or for transverse through going faults intersecting the tag mound instead our data suggest that the localization of hydrothermal activity at the tag mound is strongly controlled by permeable conduits at the intersection of an actively developing ridge parallel fissure fault complex with a newly recognized but older oblique fault system these observations may aid the interpretation of similar structures in ancient sea floor sulphide deposits where the tectonic disruption caused by emplacement on land has obscured the original geometry

detailed morphology of the tag active hydrothermal mound insights into its formation and growth the detailed structure and morphology of the tag active hydrothermal mound mid atlantic ridge are investigated using near bottom high resolution bathymetry data and digital photographic imagery the mound comprises two distinct circular platforms with one superposed asymmetrically on the other areas of hydrothermal discharge and a surface depression are identified that have not been previously described indicating that the hydrogeology of the system is modified on very short time scales years we propose a model for the growth of the tag active mound in which changes in the near surface permeability of the mound caused by both fissuring and mineral precipitation result in the development of a structure with a surface morphology consistent with that observed

faulting and fault scaling on the median valley floor of the trans atlantic geotraverse tag segment 26 n on the mid atlantic ridge a quantitative assessment of faulting on the median valley floor of a slow spreading ridge is accomplished through the analysis of high resolution dsl 120 sidescan sonar and coregistered bathymetric data from the tag segment near 26 n on the mid atlantic ridge at this location faulting is exposed within a 3 5 km wide ridge parallel zone lying asymmetrically on the eastern half of the median valley floor mapped faults have a normal sense of displacement are 2 km in length and accommodate 1 5 brittle extension evidence of fault linkage within the fault population includes kinked and bent fault traces in map view the development of overlapping fault segments or relay ramps and the presence of multiple local maxima in the displacement distance profiles of some faults faults have a slight tendency to dip to the east or outward relative to the valley axis and exhibit little symmetry of fault dip about the axis of the faulted zone or any other ridge parallel line faults exhibit a roughly linear relationship between maximum fault throw and fault length with a mean ratio of 0 030 for the population regression of length frequency data indicates a power law distribution with an exponent of 1 64 1 96 demonstrating that fractal populations can exist in the mid ocean ridge environment the fractal nature of this length frequency distribution and the ratio of maximum fault throw to fault length differ significantly from those described previously for populations of larger abyssal hill faults in the fast spreading environment where the distribution is exponential and the throw to length ratio is 5 times lower these results suggest that the scaling of fault populations in the midocean ridge setting may vary as a function of spreading rate and or fault size

simulation of nonlinear wave run up with a high order boussinesq model abstract this paper considers the numerical simulation of nonlinear wave run up within a highly accurate boussinesq type model moving wet dry boundary algorithms based on so called extrapolating boundary techniques are utilized and a new variant of this approach is proposed in two horizontal dimensions as validation computed results involving the nonlinear run up of periodic as well as transient waves on a sloping beach are considered in a single horizontal dimension demonstrating excellent agreement with analytical solutions for both the free surface and horizontal velocity in two horizontal dimensions cases involving long wave resonance in a parabolic basin solitary wave evolution in a triangular channel and solitary wave run up on a circular conical island are considered in each case the computed results compare well against available analytical solutions or experimental measurements the ability to accurately simulate a moving wet dry boundary is of considerable practical importance within coastal engineering and the extension described in this work significantly improves the nearshore versatility of the present high order boussinesq approach

hydrothermal activity at the trans atlantic geotraverse hydrothermal field mid atlantic ridge crest at 26 n the first submersible observations of the only known active submarine hydrothermal field on a slow spreading oceanic ridge are incorporated with results of 10 prior years of investigation to derive an understanding of periodicity duration and varying intensity of hydrothermal activity at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge crest near latitude 26 n hydrothermal activity has persisted at this location for at least 1 106 years based on the distribution of hydrothermal and hydrogenous mineralization with respect to crustal age the hydrothermal activity has been cyclic multistage and episodic prior high temperature hydrothermal venting stages with a periodicity of the order of 1 104 years and duration of the order of 101 years are deduced from the estimated ages of discrete sedimentary layers anomalously enriched in cu fe and zn and correspond with the independently determined periodicity of volcanic eruptive cycles on the mid atlantic ridge the most recent episode of high temperature venting is inferred to have ceased in the recent past based on metal enrichment cu fe zn in the surficial sediment layer low temperature hydrothermal venting stages with a duration of the order of 1 104 years intervene between the short high temperature stages and produce stratiform deposits of layered and earthy manganese oxide iron oxide hydroxide and silicate bivalve like forms with the characteristics of vent clams in various stages of dissolution are identified on bottom photographs the fresh appearance of intact tubules composed of iron hydroxide that acted as vents on relict deposits conductive heat flow anomalies in the sediment column and the record of temperature anomalies and excess 3he in the near bottom water column suggest that the low temperature hydrothermal discharge is intermittent at individual vents on a time scale of years

three dimensional seismic structure of a mid atlantic ridge segment characterized by active detachment faulting trans atlantic geotraverse 25 55 n 26 20 n we use air gun shots recorded by ocean bottom seismometers obss to generate a three dimensional 3d p wave tomographic velocity model of the trans atlantic geotraverse tag segment of the mid atlantic ridge and to search for evidence of reflections from a shallow crustal fault interface near vertical reflections were observed in some of the seismic records from obss deployed within the active seismicity zone defined by microearthquake hypocenters forward modeling of synthetic seismograms indicates that these reflections are consistent with a fault interface dipping at a low angle toward the ridge axis our observations suggest that the fault zone may extend beneath the volcanic blocks forming the eastern valley wall our 3d tomographic results show that the across axis structural asymmetry associated with detachment faulting extends at least 15 km to the east of the ridge axis indicating that detachment faulting and uplifting of deep lithologies has been occurring at the tag segment for at least the last similar to 1 35 myr the velocity model contains a 5 km by 8 km velocity anomaly within the detachment footwall this anomaly which is present beneath the active tag hydrothermal mound is characterized by a velocity inversion at 1 5 2 0 km below seafloor underlain by reduced p wave velocities similar to 6 2 6 5 km s compared to surrounding areas similar to 7 0 7 2 km s extending down to 3 5 km below seafloor the velocity anomaly likely results from some combination of thermal and or hydrothermal processes and in either case our results suggest that hydrothermal fluids circulate within the upper section of the detachment footwall beneath the active mound

constraints on the energy and chemical balances of the modern tag and ancient cyprus seafloor sulfide deposits the size chemical composition energy flux and fluid composition of the tag hydrothermal sulfide deposit at the mid atlantic ridge and the size chemical composition and reaction zone characteristics of the skouriotissa volcanogenic massive sulfide deposit of the troodos ophiolite in cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit we conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years with episodes of activity as short as a few tens of years separated by thousands of years of inactivity about 2 1019 j of energy supplied at high temperature is required to form a deposit the size of the tag mound metals up to 4 times the mass of any element present in the sulfide deposit are leached out of relatively small reaction zones most likely at the base of the sheeted dikes chemical balance can be struck for all elements except sulfur with a reaction zone 1 2 km3 in volume from which a small proportion of iron and large proportions of copper zinc and manganese are removed a sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate as suggested by stable isotope analyses we argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section furthermore we speculate that activity of the tag hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in hawaii and iceland

geological context and vents morphology of the ultramafic hosted ashadze hydrothermal areas mid atlantic ridge 13 n 1 recent rov dives and high resolution bathymetric data acquired over the ashadze fields on the mid atlantic ridge 13 n allow us to derive constraints on the regional and local geological setting of ultramafic hosted hydrothermal fields the active vent fields of ashadze hydrothermal fields are located in the western axial valley wall downslope from the termination of a prominent corrugated surface and in a transitional domain with respect to ridge segmentation the study of the shipboard and rov bathymetry shows that decameter 100 m by 60 m to kilometer scaled rockslides shape the axial valley wall slopes in this region the ashadze 1 vent field occurs on a coherent granular landslide rock mass that is elongated in an e w direction the ashadze 1 vent field comprises hundreds of active and inactive sulfide chimneys the ashadze 2 vent field is located in a nne trending linear depression which separates outcrops of gabbros and serpentinized peridotites active black smokers in the ashadze 2 field are located on ultramafic substratum in a 40 m diameter crater 5 m deep this crater recalls similar structures described at some vents of the logatchev hydrothermal field mid atlantic ridge 15 n we discuss the mode of formation for these craters as well as that for a breadcrust like array of radial fissures identified at ashadze 1 we propose that hydrothermalism at ashadze can be an explosive phenomena associated with geyser like explosions our study also constrains the geological and geophysical context of the ultramafic hosted ashadze hydrothermal system that may use the oceanic detachment fault as a preferred permeability conduit

role of melt supply in oceanic detachment faulting and formation of megamullions normal faults are ubiquitous on mid ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases numerous long lived detachment faults that form megamullions with large scale corrugations have been identifi ed on magma poor mid ocean ridges but recent studies suggest counterintuitively that they may be associated with elevated magmatism we present numerical models and geological data to show that these detachments occur when 30 50 of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls under these low melt conditions magmatism may focus unevenly along the spreading axis to create an irregular brittle plastic transition where detachments root thus explaining the origin of the enigmatic corrugations morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid ocean ridges

relict hydrothermal zones in the tag hydrothermal field mid atlantic ridge 26 n 45 w two relict hydrothermal zones were delineated between water depths of 3400 and 3500 m at the lower part of the east wall of the rift valley of the mid atlantic ridge in the tag hydrothermal field using a deep towed side scan sonar tow and a camera temperature tow along the northern 3 km of the wall and a submersible transect named the north and mir relict zones they are located about 4 km and 2 km northeast respectively of the known active high temperature sulfide mound between water depths of 3625 and 3670 m on the rift valley floor near the base of the east wall the north zone extends about 2 km along the northern end of the lower east wall the zone includes two moundlike features up to 30 m high by 200 m in diameter imaged by side scan sonar within a 2 km long line of discontinuous hydrothermal deposits comprising inactive toppled and standing chimneys layered material and patchy dark stains on sediment photographed by the camera temperature tow several other moundlike features were imaged with the side scan sonar outside of the photographic coverage the mir relict hydrothermal zone 2 km south of the north zone named after the mir submersible used to investigate it consists of three subzones 1 a 200 m wide area of diverse types of hydrothermal materials exposed by normal faulting at its western margin 2 a 400 m wide by 700 m long central area of discrete groups of toppled and standing inactive sulfide chimneys up to 25 m high on a substrate of red metalliferous sediment and carbonate lutite spires sampled on the highest chimneys are composed of coarse grained recrystallized sulfides dominated by pyrite and chalcopyrite which contain the first primary free gold grains 2 3 m diameter found at a hydrothermal site on a mid ocean ridge and 3 a 150 m wide hummocky area of layered hydrothermal material with the appearance of low temperature precipitates and carbonate lutite with patchy dark stains at its eastern margin the active sulfide mound the north zone and the mir zone are each located on the fractured western margins of gentle dome shaped areas of pillow flows typically 500 m in diameter interpreted as summits of volcanic centers that may have supplied heat to drive adjacent hydrothermal activity the distribution and size of the active and inactive hydrothermal zones of the tag field the chronology and the characteristics of relict samples recovered indicate a history of at least 100 103 years of high temperature hydrothermal episodes with multiple overprinting stages of mineralization accompanied by alteration

geological fate of seafloor massive sulphides at the tag hydrothermal field mid atlantic ridge deep sea mineral deposits potentially represent vast metal resources that could make a major contribution to future global raw material supply increasing demand for these metals many of which are

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

the geological setting of the ultramafic hosted logatchev hydrothermal field 14 45 n mid atlantic ridge and its influence on massive sulfide formation the logatchev hydrothermal field 14 degrees 45 n on the mar is one of a few submarine hydrothermal systems associated with ultramafic rocks it is situated on the eastern inner flank of the rift valley wall 7 km away from the spreading axis and its formation has previously been linked to detachment faulting and core complex formation geological mapping during various rov dives geological sampling and shallow drilling reveal a structural control of hydrothermal activity as well as its location in a debris flow consisting of heterogeneous ultramafic and mafic intrusive rocks the mixed mafic ultramafic host rock lithology is in agreement with published vent fluid and gas chemical data showing characteristics for interaction with mafic as well as with ultramafic rocks massive sulfide formation is more focused than previously thought and likely limited to a thin veneer at the seafloor the logatchev hydrothermal field shows a number of peculiarities that are unusual for most other hydrothermal systems one of these are so called smoking craters seafloor depressions that are several meters wide characterized by an elevated crater rim made up partly of sulfide talus but also of abundant wall rock material at these smoking craters hydrothermal venting occurs directly from holes within the craters and from small cm to dm high cu rich chimneys occurring at the crater rim based on geological mapping and sampling we suggest that these smoking craters are the product of processes related to the regional and local geological setting in an ultramafic hosted off axis location with abundant landslides as well as off axis gabbroic intrusions providing the heat for the hydrothermal convection cell c 2009 elsevier b v all rights reserved

chronology of selected hydrothermal mn oxide deposits from the transatlantic geotraverse tag area mid atlantic ridge 26 n abstract radiometric ages on discrete layers in some hydrothermally deposited mn oxide birnessite samples from the tag hydrothermal field on the mid atlantic ridge range from 16 000 to 4 000 years with marked increased activity noted about 15 000 and 7 000 years ago these observations along with previously published data and inferences on different kinds of hydrothermal activity in the tag field suggest episodic multistage hydrothermal events ranging from high temperature black smokers precipitating sulfides to low temperature events precipitating fe and mn oxides over this entire time range black smokers are presently active the location of the various deposits on the eastern wall of the median valley at this latitude suggests that the high temperature events occur at greater depths

morphology mineralogy and chemistry of hydrothermal deposits from the tag area 26 n mid atlantic ridge abstract metalliferous hydrothermal deposits have been observed and sampled by submersible from the tag hydrothermal field on the mid atlantic ridge at 26 n the deposits include surficial metal rich staining on the surface of carbonate ooze deposits over considerable areas of the sea floor as well as discrete massive layered deposits the latter include hydrothermal precipitates of fe silicate nontronite mn oxide birnessite and fe oxide amorphous the deposits range in size from less than 1 m to 15 m 20 m the composition and morphology of individual deposits varies from thick laminated crystalline birnessite precipitates through fe rich tubular vents to loose earthy birnessite nontronite or amorphous fe oxide interlayered deposits fractionation and crystallinity are presumably controlled by local eh conditions and the hydrodynamics of circulation in the upper crust they are presumed to represent deposition from low temperature hydrothermal solutions which had originally reacted with basaltic oceanic crust at depth at high temperature and had precipitated metal rich sulfides within the crust occasional short duration high temperature black smoker type venting is inferred to have occurred based upon cu fe zn rich layers in the sediments and observations of clam shells in bottom photographs

geochemical time series of hydrothermal fluids from the slow spreading mid atlantic ridge implications of medium term stability abstract time series investigations of hydrothermal fluids from different geological settings at the slow spreading mid atlantic ridge mar indicate a surprising medium term stability of their chemical composition over up to 20 years this does not only apply to tectonically controlled systems with exposure of mantle rocks where magmatic activity is limited such as the logatchev 1 hydrothermal field at 14 45 n we also found the 407 c hot and extremely metal rich fluids at the 5 s site turtle pits 2986 m water depth to discharge at constant composition indicating supercritical phase separation for at least 4 years which contrasts with rather dynamic volcanically driven hydrothermal systems at the fast spreading east pacific rise epr possibly fluid pathways and pressure temperature conditions are more stable in tectonically controlled settings at mar than at the epr where frequent volcanic activity affects permeability within the hydrothermal systems despite some spatial variability hydrothermal fluids discharging at the shallower ca 810 870 m menez gwen hydrothermal field at 37 50 n seem to represent a stable discharge of phase separated vapor type fluids over 16 years this demonstrates that medium term stability of hydrothermal fluid composition seems to be a general feature at slow spreading ridges as a consequence hydrothermal vent organisms at the mar may live in a more stable chemical environment compared to vent systems at fast spreading ridges considering the fact that slow spreading ridges account for more than half of the submarine ridge system such a constant emanation of hydrothermal fluids makes hydrothermal vent systems on slow spreading ridges a medium term stable source of fe and other micronutrients into the ocean

a seafloor gravity profile across the tag hydrothermal mound a series of on bottom gravity stations were completed across the tag hydrothermal mound during a dive program with the japanese shinkai 6500 submersible the mound appears as a gravity high 2 mgal with respect to the surrounding seafloor reflecting the high density sulfides of which it is comprised three dimensional modeling of the data shows no evidence of a dense sulfide root extending deep into the seafloor below the mound which would be associated with a broad highly altered stockwork zone sulfide thickness appears to be greatest 50m to the north of the mound the thickness decreases sharply to the south reaching a steady value of about 10m just to the south of the kremlin area the mass of sulfides within the mound is estimated at 3 million tonnes the surrounding basaltic seafloor appears to have a density of 2400 100 kgm 3 indicating a surficial porosity of between 20 30

serpentinized troctolites exposed near the kairei hydrothermal field central indian ridge insights into the origin of the kairei hydrothermal fluid supporting a unique microbial ecosystem the vent fluids of the kairei hydrothermal field khf on the central indian ridge near the rodriguez triple junction are atypical they have a very high h2 concentration a relatively high si concentration and a remarkably low ch4 h2 ratio recently particular attention has been paid to the khf because the hydrothermal fluids are suggested to support a hydrogen based hyperthermophilic subsurface lithoautotrophic microbial ecosystem hyperslime which is considered to be a likely modern analogue for the early earth ecosystems prior to photosynthesis despite the increasing interest in the fluid chemistry and associated biota the origin of the unusual chemistry of the hydrothermal fluids is still uncertain here we suggest interaction of seawater with troctolite recently discovered from small hills near the khf provide a possible explanation for the composition of the khf fluids dives with the manned submersible shinkai 6500 recovered plagioclase dunite troctolites and olivine gabbros which generally constitute the lower oceanic crust microscopic observations revealed that olivine crystals in the samples were partly to completely replaced by serpentine and magnetite indicating the generation of h2 by serpentinization reactions between olivine and hydrothermal fluids theoretical model calculations predict that the high h2 and high si concentrations of the hydrothermal fluids can be attributed to serpentinization of the troctolites and subsequent hydrothermal reactions with basaltic wall rocks under the khf the unique geological setting of the kairei hydrothermal system where deep crustal rocks are emplaced in the shallower part of oceanic crust is considered to be responsible for the unusual chemistry of the kairei hydrothermal fluids driving the occurrence of the hyperslime

numerical simulation of nonlinear long waves in the presence of discontinuous coastal vegetation abstract we presented numerical simulations of long waves interacting with arrays of emergent cylinders inside regularly spaced patches representing tsunamis interacting with discontinuous patchy coastal vegetation we employed the fully nonlinear and weakly dispersive serre green naghdi equations sgn until the breaking process starts while we changed the governing equations to nonlinear shallow water equations nsw at the vicinity of the breaking wave peak and during the run up stage we modeled the cylinders as physical boundaries rather than approximating them as macro roughness friction our results show that the cylinders provide protection for the areas behind them however they might also cause amplification in local water depth in those areas the presented results are extensively validated against the existing numerical and experimental data our results demonstrate the capability and reliability of our model in simulating wave interaction with emergent cylinders

conductive heat flow at the tag active hydrothermal mound results from 1993 1995 submersible surveys we report 70 measurements of conductive heat flow at the 50 m high 200 m diameter tag active hydrothermal mound made during submersible surveys with alvin in 1993 and 1995 and shinkai 6500 in 1994 the stations were all measured with 5 thermistor 0 6 or 1 m long alvin heat flow probes which are capable of determining both gradient and thermal conductivity and were transponder navigated to an estimated accuracy of 5 10 m relative to the 10 m diameter central complex of black smokers within 20 m of this complex conductive heat flow values are extremely variable 0 1 100 w m which can only be due to local spatial and possible temporal variability in the immediate vicinity of the vigorous discharge sites a similar local variability is suggested in the kremlin area of white smokers to the southeast of the black smoker complex on the south and southeast side of the mound there is very high heat flow 3 7 25 w m on the sedimented terraces that slope down from the kremlin area heat flow is also high 0 3 3 w m in the pelagic carbonate sediments on the surrounding seafloor within a few tens of meters of the southwest northwest and northeast sides of the mound on the west side of the sulfide rubble plateau that surrounds the central black smoker peak there is a coherent belt of very low heat flow 20 mw m 20 50 m west of the smokers suggestive of local shallow recharge of bottom water the three submersible surveys spanned nearly two years but showed no indication of any temporal variability in conductive heat flow over this time scale whether natural or induced by odp drilling in 1994

chemical characteristics of hydrothermal fluids from the tag mound of the mid atlantic ridge in august 1994 implications for spatial and temporal variability of hydrothermal activity the tag hydrothermal mound on the mid atlantic ridge 26 08 n 44 50 w was revisited in august 1994 with the submersible shinkai 6500 in order to characterize time series fluid chemistry prior to the odp drilling fluid samples were taken from both black smokers and white smokers si ph alkalinity h 2 s major cations na k ca 2 mg 2 major anions cl so 4 2 and minor elements li sr b fe mn cu zn br as well as sr isotope ratios were measured we report the first br ci ratios for the tag hydrothermal fluids showing no fractionation between br and cl during the fluid rock interaction this study shows small changes in composition of the black smoker fluids from the 1990 data edmond et al 1995 changes of ph alkalinity fe k and 87 sr 86 sr values are suggestive of subsurface fes precipitation and a decrease of water rock ratio at a deeper reaction zone differences in chemical characteristics between the black and white smoker fluids were similarly observed as in 1990

underwater hyperspectral imaging using a stationary platform in the trans atlantic geotraverse hydrothermal field underwater hyperspectral imaging is a relatively new method for characterizing seafloor composition to date it has been deployed from moving underwater vehicles such as remotely operated vehicles and autonomous underwater vehicles while moving vehicles allow relatively rapid surveying of several 10 1000 m2 they are subjected to short term variations in vehicle attitude that often compromise image acquisition and quality in this study we tested a stationary platform that was landed on the seabed and used an underwater hyperspectral imager uhi on a vertical swinging bracket the imaged seafloor areas have dimensions of 2 3 m times 1 m and are characterized by very stable uhi data of high spatial resolution the study area was the trans atlantic geotraverse hydrothermal field at the mid atlantic ridge 26 n in water depths of 3530 3660 m uhi data were acquired a 12 stations on an active and an inactive hydrothermal sulfide mound based on supervised classification 24 spectrally different seafloor materials were detected including hydrothermal and non hydrothermal materials and benthic fauna the results show that the uhi data are able to spectrally distinguish different types of surface materials and benthic fauna in hydrothermal areas and may therefore represent a promising tool for high resolution seafloor exploration in potential future deep sea mining areas

reduced crustal magnetization beneath the active sulfide mound tag hydrothermal field mid atlantic ridge at 26 n abstract a detailed near bottom magnetic field survey was carried out by the submersiblealvin over the actively venting mound located within the tag trans atlantic geotraverse hydrothermal field on the mid atlantic ridge at 26 08 n 44 49 w three dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound this magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure in contrast the sea surface magnetic anomaly is associated with a broad 2 8 km magnetization low elongated along the axis that includes both the active and inactive mounds the short wavelength 250 m near bottom magnetic anomaly over the active mound is far too small to produce the long wavelength 8 km sea surface magnetic anomaly at tag however and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly other hypotheses including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely the existence of vigorous hydrothermal activity at tag and the lack of microseismic activity in the tag area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly the thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long wavelength sea surface anomaly pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long wavelength magnetic signal could be preserved in the crust the overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites these studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late stage intrusive bodies 1 while detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones only drilling will ultimately test these hypotheses

heat flow and mineralogy of tag relict high temperature hydrothermal zones mid atlantic ridge 26 n 45 w submersible investigations employing heat flow measurements 12 stations sampling and imagery of the two relict high temperature hydrothermal zones of the tag field the alvin and mir sulfide zones elucidate relations between heat sources and mineralization including an active sulfide mound that has been the focus of prior studies values of heat flow in the mir zone and at the margin of the active mound are inversely proportional to distance from adjacent volcanic centers this observation supports the hypothesis that intrusions at volcanic centers adjacent to the high temperature hydrothermal zones supply the heat to drive hydrothermal activity the chronology of hydrothermal deposits in the different zones indicates that the intrusions are episodic with field wide high temperature hydrothermal events recurring at intervals of tens of thousands of years while activity at individual zones may recur at intervals of hundreds to thousands of years a sequence of hydrothermal deposits ranges to at least 140 000 years ago from massive sulfides forming at the active mound to recrystallization of sulfides in the active and relict zones to pyritization of an inactive mound in the alvin zone low temperature mineral phases precipitate before during and after the sulfides

the internal structure of an active sea floor massive sulphide deposit the hydrothermal circulation of sea water through permeable ocean crust results in rock water interactions that lead to the formation of massive sulphide deposits these are the modern analogues of many ancient ophiolite hosted deposits1 4 such as those exposed in cyprus here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the mid atlantic ridge a complex assemblage of sulphide anhydrite silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor indeed the deposit grows largely as an in situ breccia pile as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits during inactive periods the collapse of sulphide chimneys dissolution of anhydrite and disruption by faulting cause brecciation of the deposit the abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures 150 c currently maintained within the mound and implies substantial entrainment of cold sea water into the interior of the deposit these observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits despite its absence in those preserved on land

a near bottom magnetic survey of the mid atlantic ridge axis at 26 n implications for the tectonic evolution of the tag segment 1 an extensive deep tow magnetic survey of the tag ridge segment on the mid atlantic ridge reveals new information about the relationship between the magnetic anomaly field and the tag hydrothermal deposits results show the strongest magnetization is located over the neovolcanic axis and asymmetrically toward the western side of the central brunhes anomaly a well defined linear magnetization low is located over the eastern rift valley wall of the tag segment the near bottom data show no direct correlation between this crustal magnetization low and the hydrothermal deposits the magnetization low is explained by crustal thinning caused by 4 km of horizontal extension along a normal fault previous observations and sampling indicate exposures of gabbros and dikes in the eastern rift valley wall suggesting slip along a normal fault has revealed this crust modeling suggests the fault has been active since 0 35 0 1 ma at a horizontal slip rate of roughly half the spreading rate of 22 km myr the tag hydrothermal system is located on the hanging wall of this fault within 3 km of its termination over the past several hundred thousand years movement on the detachment fault may have episodically increased the permeability of the hanging wall reactivating the overlying hydrothermal systems significant vents like tag may be typically associated with hanging walls of long term detachment faults near seafloor spreading centers this would imply that it is the reactivation of permeability in the hanging wall related to repeated fault movement that controls the longevity of these hydrothermal systems

fault rotation and core complex formation significant processes in seafloor formation at slow spreading mid ocean ridges mid atlantic ridge 13 15 n 1 the region of the mid atlantic ridge mar between the fifteen twenty and marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension normal faults show large amounts of rotation dome shaped corrugated detachment surfaces core complexes intersect the seafloor at the edge of the inner valley floor and extinct core complexes cover the seafloor off axis we have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments 13 and 15 n segments steep outward facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30 the rotation occurs very close to the ridge axis as much as 20 within 5 km of the volcanic axis and is complete by 1 my producing distinctive linear ridges with roughly symmetrical slopes this morphology is very different from linear abyssal hill faults formed at the 14 n magmatic segment which display a smaller amount of rotation typically 15 we suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults if faults are long lived a dome shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex a single ridge segment can have several active core complexes some less than 25 km apart that are separated by swales we present two models for multiple core complex formation a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales either model can explain the observed morphology

k metasomatism and detachment related mineralization harcuvar mountains arizona the bullard detachment fault a regional low angle normal fault exposed in the harcuvar mountains of west central arizona separates lower plate mylonitic rocks and chloritic breccia from upper plate volcanic and sedimentary rocks areally extensive k metasomatism has converted upper plate mafic flows and felsic ash flow tuffs into rocks with 8 to 12 wt k 2 o 2 o and a simple k feldspar hematite quartz mineralogy the secondary k feldspar is very pure or 95 to 99 5 monoclinic and structurally similar to orthoclase differences in 18 o values between secondary k feldspar replacing sanidine phenocrysts 9 to 11 and k feldspar replacing groundmass 11 to 14 in the tuffs imply differential o exchange with migrating fluids whole rock 18 o values for tuffs 10 to 14 and mafic flows 6 to 9 do not therefore represent primary igneous values the rocks apparently became k metasomatized and 18 o enriched while interacting with low to moderate temperature neutral to alkaline oxidizing brines that accumulated in an extensional basin above the detachment fault cu au ag mineralization is concentrated in faults and fissures in metasomatized mafic flows above the detachment fault fluid inclusion studies show that mineralizing fluids had minimum temperatures of 290 to 330 c along the detachment fault 240 to 290 c in mafic flows above the detachment fault and 100 to 130 c in barite calcite mn oxide veins farther from the detachment fault the predominant mineralizing fluids near the detachment fault and in the mafic flows were saline brines with 13 to 17 equivalent wt naci more dilute brines with 6 to 12 equivalent wt naci formed the barite calcite mn oxide veins inferred 18 o values of the mineralizing fluids range from 3 for high temperature quartz sulfide veins to 5 for lower temperature barite calcite mn oxide veins the high salinities oxygen isotope compositions and geologic setting indicate that the mineralizing fluids were basinal brines the mineralizing fluids apparently evolved from early deep level reducing basin brines to a later stage marked by the influx of higher level oxidizing basin brines relatively minor amounts of less evolved lower 18 o meteoric water entered the system during the very late stages of mineralization paragenetic relations and geochemical and isotopic data indicate that mineralization was superimposed on previously k metasomatized rocks mineralization and k metasomatism may be indirectly linked however because both occurred during detachment faulting and both involved basinal brines specifically k metasomatism of upper plate units liberated elements such as cu pb zn mn sr and na that were incorporated into the mineralizing basin brines multiple fluid regimes during detachment faulting are indicated because basin brine dominated mineralization overprinted lower plate mylonitic rocks and breccia that had probably previously equilibrated with igneous or metamorphic fluids at deeper levels of the detachment system

gases and helium isotopes in high temperature solutions sampled before and after odp leg 158 drilling at tag hydrothermal field 26 n mar hydrothermal fluids were sampled for dissolved gases at tag 26 n mar during two alvin dive series in april may 1993 and march 1995 respectively 17 months before and 4 months after ocean drilling program leg 158 total gas volumes extracted from the 1993 and 1995 samples are of the same order of magnitude even if some increase in h2s c02 and ch4 was noticeable in the 1995 samples no significant difference was observed in helium concentration and helium isotopic ratio 3 he 4 he 8 2 0 1 ra the ch4 3 he ratio found in black smokers sampled in 1993 is around 9 x 10 6 close to ratio found in mar basalts in 1995 this ratio is around 4 times higher due to the ch4 increase the 13 c in c02 measured on two black smokers sampled in 1993 is uniform at 8 4 to 8 8 versus pdb while a 13 c value of 13 is measured at a new site sampled in 1995 located 40 meters east of the main black smokers complex bsc ch4 3 he and 13 c values both point to a mainly magmatic abiogenic origin of the carbon species in the system overall comparison of the 1993 and 1995 data suggests that the hydrothermal circulation is continuing through the basaltic layer carrying helium c02 and ch4 of magmatic origin even if some thermogenic ch4 contribution seems likely in the 1995 samples collected in the new active area we can speculate that the 1994 drilling may have modified the previously steady state hydrothermal circulation in the tag mound and temporarily enhanced the hydrothermal circulation by opening new pathways favouring transient input of ch4 rich fluids before coming back to a new steady state

reduced crustal magnetization beneath relict hydrothermal mounds tag hydrothermal field mid atlantic ridge 26 n submersible magnetic profiles over the mir and alvin relict hydrothermal zones of the tag hydrothermal field on the mid atlantic ridge at 26 n show magnetic anomalies that are consistent with the presence of reduced crustal magnetization these magnetic anomalies are similar to the anomaly found associated with the tag active sulfide mound and suggests that hydrothermal alteration rather than thermal demagnetization is responsible for these reduced magnetization zones magnetic field mapping thus can be useful in determining the subsurface extent of upflow zones and alteration pipes of midocean ridge hydrothermal systems

structural control evolution and accumulation rates of massive sulfides in the tag hydrothermal field the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge is one of the best studied hydrothermal systems to date however high resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle auv reveal new information about the distribution of active and inactive hydrothermal deposits and their relation to structural features the discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow spreading ridges the interpretation of ship based and high resolution auv based data sets allowed for the determination of the main tectonic stress regimes that have a first order control on the location and distribution of past and present hydrothermal activity the data reveal the importance of cross cutting lineament populations and temporal variations in the prevalent stress regime a dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material 29 mt the accumulation rate of 1 500 t yr is comparable to those of other modern seafloor vent fields however our observations suggest that the tag segment is different from many other slow spreading ridge segments in its tectonic complexity which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation the inactive and weakly active mounds contain almost 10 times the amount of material as the active high temperature mound providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits

on the sr isotope and ree compositions of anhydrites from the tag seafloor hydrothermal system abstract sr isotope analyses have been conducted on anhydrite samples from the tag trans atlantic geotraverse active hydrothermal mound 26 08 n mid atlantic ridge that have previously been shown to exhibit two distinct patterns of ree behavior when normalized to tag end member hydrothermal fluid despite differences in ree patterns the sr isotope data indicate that all the anhydrites precipitated from fluids with a similar range of hydrothermal fluid and seawater components and all but one were seawater dominated 52 75 speciation calculations using the eq3 6 software package for geochemical modeling of aqueous systems suggest that the ree complexation behavior in different fluid mixing scenarios can explain the variations in the ree patterns anhydrites that exhibit relatively flat ree patterns labs ybbs 0 8 2 0 subscript bs indicates normalization to end member black smoker hydrothermal fluid and a small or no eu anomaly eubs eu bs 0 8 2 0 are inferred to have precipitated from mixes of end member hydrothermal fluid and cold seawater ree complexes with hard ligands e g fluoride and chloride are less stable at low temperatures and trivalent eu has an ionic radius similar to that of ca2 and the other ree and so they behave coherently in contrast anhydrites that exhibit slight lree depletion labs ybbs 0 4 1 4 and a distinct negative anomaly eubs eu bs 0 2 0 8 are inferred to have precipitated from mixes of end member hydrothermal fluid and conductively heated seawater the lree depletion results from the presence of very stable lree chloro complexes that effectively limit the availability of the lree for partitioning into anhydrite above 250 c eu is present only in divalent form as chloride complexes and discrimination against eu2 is likely due to both the mismatch in ionic radii between eu2 and ca2 and the strong chloro complexation of divalent eu which promotes stability in the fluid and inhibits partitioning of eu2 into precipitating anhydrite these variations in ree behavior attest to rapid fluctuations in thermal regime fluid flow and mixing in the subsurface of the tag mound that give rise to heterogeneity in the formation conditions of individual anhydrite crystals

run up of tsunamis and long waves in terms of surf similarity abstract in this paper we review and re examine the classical analytical solutions for run up of periodic long waves on an infinitely long slope as well as on a finite slope attached to a flat bottom both cases provide simple expressions for the maximum run up and the associated flow velocity in terms of the surf similarity parameter and the amplitude to depth ratio determined at some offshore location we use the analytical expressions to analyze the impact of tsunamis on beaches and relate the discussion to the recent indian ocean tsunami from december 26 2004 an important conclusion is that extreme run up combined with extreme flow velocities occurs for surf similarity parameters of the order 3 6 and for typical tsunami wave periods this requires relatively mild beach slopes next we compare the theoretical solutions to measured run up of breaking and non breaking irregular waves on steep impermeable slopes for the non breaking waves the theoretical curves turn out to be superior to state of the art empirical estimates finally we compare the theoretical solutions with numerical results obtained with a high order boussinesq type method and generally obtain an excellent agreement

hydrothermal sulfide accumulation along the endeavour segment juan de fuca ridge highlights high resolution auv bathymetry is used to identify hydrothermal sulfides on the seafloor 1 2 mt of massive sulfide is identified along 24 km of mid ocean ridge length massive sulfide accumulated at a rate of 400 t yr efficiency of sulfide deposition is 5 of total mobilized metals and reduced sulfur current global seafloor sulfide estimates likely underestimate amount present by about a factor of four abstract hydrothermal sulfide deposits that form on the seafloor are often located by the detection of hydrothermal plumes in the water column followed by exploration with deep towed cameras side scan sonar imaging and finally by visual surveys using remotely operated vehicle or occupied submersible hydrothermal plume detection however is ineffective for finding hydrothermally inactive sulfide deposits which may represent a significant amount of the total sulfide accumulation on the seafloor even in hydrothermally active settings here we present results from recent high resolution autonomous underwater vehicle based mapping of the hydrothermally active endeavour segment of the juan de fuca ridge in the northeast pacific ocean analysis of the ridge bathymetry resulted in the location of 581 individual sulfide deposits along 24 km of ridge length hydrothermal deposits were distinguished from volcanic and tectonic features based on the characteristics of their surface morphology such as shape and slope angles volume calculations for each deposit results in a total volume of 372 500 m3 of hydrothermal sulfide sulfate silica material for an equivalent mass of 1 2 mt of hydrothermal material on the seafloor within the ridge s axial valley assuming a density of 3 1 g cm3 much of this total volume is from previously undocumented inactive deposits outside the main active vent fields based on minimum ages of sulfide deposition the deposits accumulated at a maximum rate of 400 t yr with a depositional efficiency proportion of hydrothermal material that accumulates on the seafloor to the total amount hydrothermally mobilized and transported to the seafloor of 5 the calculated sulfide tonnage represents a four fold increase over previous sulfide estimates for the endeavour segment that were based largely on accumulations from within the active fields these results suggest that recent global seafloor sulfide resource estimates which were based mostly on the sizes and distribution of hydrothermally active deposits may be similarly underestimating the amount of sulfide along the global submarine neovolcanic zones

seafloor massive sulphide exploration using deep towed controlled source electromagnetics navigational uncertainties deep towed geophysical surveys require precise knowledge of navigational parameters such as instrument position and orientation because navigational uncertainties reflect in the data and therefore in the inferred geophysical properties of the sub seafloor we address this issue for the case of electrical conductivity inferred from controlled source electromagnetic data we show that the data error is laterally variable due to irregular motion during deep towing but also due to lateral variations in conductivity including those resulting from topography to address this variability and quantify the data error prior to inversion we propose a two dimensional perturbation study our workflow enables stable and geologically reliable results for multi component and multi frequency inversions an error estimation workflow is presented which comprises the assessment of navigational uncertainties perturbation of navigational parameters and forward modelling of electric field amplitudes for a homogeneous and then a heterogeneous sub seafloor conductivity model some navigational uncertainties are estimated from variations of direct measurements other navigational parameters required for inversion are derived from the measured quantities and their error is calculated by means of error propagation some navigational parameters show direct correlation with the measured electric fields for example the antenna dip correlates with the vertical electric field and the depth correlates with the horizontal electric field for the perturbation study each standard deviation is added to the navigational parameters forward models are run for each perturbation amplitude deviations are summed in quadrature with the stacking error for a total laterally varying data error the error estimation is repeated for a heterogeneous sub seafloor model due to the large conductivity range several orders of magnitude which affects the forward model the approach enables us to utilize data from several components multiple electric fields frequencies and receivers in the inversion to constrain the final model and reduce ambiguity the final model is geologically reasonable in this case enabling the identification of conductive metal sulphide deposits on the seafloor

a poroelastic model for the tidal modulation of seafloor hydrothermal systems 1 time series measurements of the temperature and exit velocity of hydrothermal effluent suggest that seafloor hydrothermal systems are modulated by tidal processes here we apply the theory of poroelasticity to predict the magnitude and phase of tidally induced changes in the temperature and flow rate of hydrothermal effluent at the seafloor we construct a model in which the steady state upwelling of buoyant fluid in the crust is modulated by tidal loading of a one dimensional seafloor by the overlying water column the nature of the solution is controlled by the relative magnitudes of three length scales these are 1 the depth h of the heat source below the seafloor 2 the skin depth d over which pore pressure signals can diffuse during one tidal cycle and 3 the advective length scale a over which the upwelling flow advects thermal signals during one tidal cycle we consider the likely magnitude of the parameters in a real system as well as the limitations of a one dimensional representation of that system we then discuss how observational data on the magnitude and phase lag of temperature and flow rate could be used to constrain the subseafloor parameters that govern hydrothermal circulation within the seafloor

constraints on the behavior of trace elements in the actively forming tag deposit mid atlantic ridge based on la icp ms analyses of pyrite the distribution of trace ore elements in different paragenetic stages of pyrite has been documented for the first time in the sub seafloor of the actively forming tag massive sulfide deposit trace element distributions have been determined by in situ laser ablation inductively coupled plasma mass spectrometry la icp ms of pyrite formed at different stages of mineralization and at different temperatures constrained by previously published fluid inclusion analyses the data reveal a strong dependence on paragenetic stage with distinct low and high temperature enrichments porous pyrite and marcasite formed at low temperatures 350 c at the base of the hydrothermal mound and in the stockwork zone is enriched in co se bi cu ni and sn a number of different sub types of pyrite also have characteristic trace element signatures e g the earliest pyrite formed at the highest temperatures is always enriched in co and se compared to later stages ablation profiles for co se and ni are smooth and indicate that these elements are present mainly in lattice substitutions rather than as inclusions of other sulfides profiles for as sb tl and cu can be either irregular or smooth indicating both lattice substitutions and inclusions lead and ag have mostly smooth profiles but because pb cannot substitute directly into the pyrite lattice it is interpreted to be present as homogeneously distributed micro or nano scale particles the behavior of the different trace elements mainly reflects their aqueous speciation in the hydrothermal fluids at different temperatures and for some elements like co and se strong partitioning into the pyrite lattice at elevated temperatures adsorption onto pyrite surfaces controls the distribution of a number of redox sensitive elements i e mo v ni u particularly in the upper part of the mound which is infiltrated by cold seawater where micro or nano scale inclusions of chalcopyrite sphalerite galena or sulfosalts are present there is still a strong temperature dependence on the inclusion population e g more abundant chalcopyrite in the highest temperature pyrite suggesting that the inclusions were co precipitated with pyrite rather than overgrown however at the deposit scale the trace element distributions are also strongly controlled by remobilization and chemical zone refining as previously documented in bulk geochemical profiles the results show that pyrite chemistry is a remarkably good model of the chemistry of the entire hydrothermal system for many trace elements the concentrations in pyrite are highly predictive in terms of the conditions of mineral formation over a wide range of temperatures from the stockwork zone to the cooler outer margins of the deposit calculated minimum concentrations of the trace elements in the fluids needed to account for the observed concentrations in pyrite show good agreement with measured vent fluid concentrations particularly pb as mo ag and tl however significantly higher concentrations are indicated for co and se than have been measured in sampled fluids confirming the strong partitioning of these elements into high temperature pyrite

runup of solitary waves on a circular island this is a study of the interactions of solitary waves climbing up a circular island a series of large scale laboratory experiments with waves of different incident height to depth ratios and different crest lengths is described detailed two dimensional run up height measurements and time histories of surface elevations around the island are presented a numerical model based on the two dimensional shallow water wave equations including runup calculations was developed numerical model predictions agreed very well with the laboratory data and the model was used to study wave trapping and the effect of slope under certain conditions enhanced runup and wave trapping on the lee side of the island were observed suggesting a possible explanation for the devastation reported by field surveys in babi island off flores indonesia and in okushiri island japan

tsunami run up and draw down on a plane beach tsunami run up and draw down motions on a uniformly sloping beach are evaluated based on fully nonlinear shallow water wave theory the nonlinear equations of mass conservation and linear momentum are first transformed to a single linear hyperbolic equation to solve the problem with arbitrary initial conditions we apply the fourier bessel transform and inversion of the transform leads to the green function representation the solutions in the physical time and space domains are then obtained by numerical integration with this semi analytic solution technique several examples of tsunami run up and draw down motions are presented in particular detailed shoreline motion velocity field and inundation depth on the shore are closely examined it was found that the maximum flow velocity occurs at the moving shoreline and the maximum momentum flux occurs in the vicinity of the extreme draw down location the direction of both the maximum flow velocity and the maximum momentum flux depend on the initial waveform it is in the inshore direction when the initial waveform is predominantly depression and in the offshore direction when the initial waves have a dominant elevation characteristic

new age data for mid atlantic ridge hydrothermal sites tag and snakepit chronology revisited the chronologies of tag and snakepit hydrothermal fields have been established using 210pb pb 230th 234u and 14c dating at the tag field a mn oxide record indicative of low temperature events began at least 125 000 years and possibly 140 000 years ago with maximum intensities at 15 000 7000 and 4000 years before present high temperature events giving rise to sulfide deposits began about 100 000 years ago and have been intermittent to the present day a presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20 000 years decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000 7000 and 9000 years ago after a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago the snakepit field is much younger and no sulfides older than 4000 years have been recovered relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at tag reactivation of snakepit took place about 80 years ago and is presently concurrent with that of tag comparison with hydrothermal sites on the east pacific rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits

continuation of the hydrothermal fluid chemistry time series at tag and the effects of odp drilling new data are presented for major dissolved species in vent fluids from the tag hydrothermal mound on the mid atlantic ridge sampled in 1993 1994 and 1995 the latter two cruises bracket the drilling of the mound by the ocean drilling program in september november 1994 changes in venting patterns have been observed over this time period both before and after drilling however the major ion chemistry of venting fluids appears to be stable indicating that the drilling did not penetrate the primary plumbing system of the mound

silver lead zinc veins metamorphic core complexes and hydrologic regimes during crustal extension the kokanee range ag pb zn au vein and replacement deposits formed during eocene crustal extension and unroofing of the valhalla metamorphic core complex in the hanging wall of the transcrustal slocan lake fault the 34ssulfide 18oquartz 18osiderite and galena pb isotopic ratios for vein minerals display regional zonations revealing fluid flow paths of a large fossil hydrothermal system sulfur was derived from local country rocks but carbon was derived from a deep seated reservoir possibly the upper mantle lead is a mixture of local upper crustal pb with lower crustal and depleted upper mantle pb regional isotopic zonations were controlled by deep fracture zones such as the slocan lake fault which channeled lower crustal and mantle pb and mantle co2 to higher crustal levels where mixing occurred with highly evolved meteoric waters that had leached local sulfur and upper crustal ph this is the first ag pb zn vein district for which a genetic link between mineralization and metamorphic core complex unroofing is demonstrated

tectonics of the mid atlantic rift valley between the tag and mark areas 26 24 n evidence for vertical tectonism abstract a segment of the mid atlantic ridge between the tag and mark areas between 26 and 24 n was studied during march april of 1988 from the r v akademik mstislav keldysh using mir submersibles the rift valley is depressed here to depths of 3200 4700 m and includes seven independent magmatic cells which are in different phases of their evolution some of them are now in the constructive phase whereas others are in the destructive phase and are strongly fractured slopes of two types flank the rift valley the normal slopes similar to the famous area are block faulted terrains with small elevations above the neovolcanic ridge crest in the center of the rift valley the abnormal slopes are elevated from 1 to 3 km above the neovolcanic ridge crest an almost undisturbed section of the upper oceanic crust from gabbro through sheeted dikes to pillow lavas outcrops on the abnormal slopes as the dikes are dipping nearly vertically and pillow lavas preserve their initial inclinations no block rotation or back tilting occurred during the development of rift mountains pure vertical uplift was responsible for rift mountain growth speculations are made that serpentinization of the uppermost mantle peridodite can be considered as the main process leading to oceanic crust uplift serpentinization takes place during the nonvolcanic destructive phases when seawater can easily penetrate into the deeper crust and uppermost mantle reaching the peridodites as a result serpentinites are added to the oceanic crust of slow spreading ridges

crustal structure of the trans atlantic geotraverse tag segment mid atlantic ridge 26 10 n implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges new seismic refraction data reveal that hydrothermal circulation at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 10 n is not driven by energy extracted from shallow or mid crustal magmatic intrusions our results show that the tag hydrothermal field is underlain by rocks with high seismic velocities typical of lower crustal gabbros and partially serpentinized peridotites at depth as shallow as 1 km and we find no evidence for low seismic velocities associated with mid crustal magma chambers our tomographic images support the hypothesis of tivey et al 2003 that the tag field is located on the hanging wall of a detachment fault and constrain the complex dome shaped subsurface geometry of the fault system modeling of our seismic velocity profiles indicates that the porosity of the detachment footwall increases after rotation during exhumation which may enhance footwall cooling however heat extracted from the footwall is insufficient for sustaining long term high temperature hydrothermal circulation at tag these constraints indicate that the primary heat source for the tag hydrothermal system must be a deep magma reservoir at or below the base of the crust

propagation and amplification of tsunamis at coastal boundaries when tsunamis hit a coastal area they often cause substantial damage and loss of life at locations otherwise well protected from normal even severe wind generated waves an example of such a catastrophe took place on 12 december 1992 when an earthquake of magnitude ms 7 5 occurred in the eastern region of flores island indonesia1 2 one of the areas hardest hit by the resulting tsunamis was babi a small coastal island two villages on the island were completely destroyed even though both were located in sheltered areas where wave conditions are normally calm here we describe the results of numerical and laboratory experiments that show how unlike wind generated waves tsunamis are capable of penetrating into sheltered coastal areas without significantly losing their energy for the case of babi island the original tsunami wave was split into two with one wave propagating around each side of the island the two waves met in the sheltered region and the subsequent amplification of wave amplitude resulted in the destructive flow onto the beach

time series studies of vent fluids from the tag and mark sites 1986 1990 mid atlantic ridge a new solution chemistry model and a mechanism for cu zn zonation in massive sulphide orebodies

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

the trans atlantic geotraverse hydrothermal field a hydrothermal system on an active detachment fault abstract over the last ten years geophysical studies have revealed that the trans atlantic geotraverse tag hydrothermal field 26 08 n on the mid atlantic ridge is located on the hanging wall of an active detachment fault this is particularly important in light of the recognition that detachment faulting accounts for crustal accretion extension along a significant portion of the mid atlantic ridge and that the majority of confirmed vent sites on this slow spreading ridge are hosted on detachment faults the tag hydrothermal field is one of the largest sites of high temperature hydrothermal activity and mineralization found to date on the seafloor and is comprised of active and relict deposits in different stages of evolution the episodic nature of hydrothermal activity over the last 140 ka provides strong evidence that the complex shape and geological structure of the active detachment fault system exerts first order but poorly understood influences on the hydrothermal circulation patterns fluid chemistry and mineral deposition while hydrothermal circulation extracts heat from a deep source region the location of the source region at tag is unknown hydrothermal upflow is likely focused along the relatively permeable detachment fault interface at depth and then the high temperature fluids leave the low angle portion of the detachment fault and rise vertically through the highly fissured hanging wall to the seafloor the presence of abundant anhydrite in the cone on the summit of the tag active mound and in veins in the crust beneath provides evidence for a fluid circulation system that entrains significant amounts of seawater into the shallow parts of the mound and stockwork given the importance of detachment faulting for crustal extension at slow spreading ridges the fundamental question that still needs to be addressed is how do detachment fault systems and the structure at depth associated with these systems e g presence of plutons and or high permeability zones influence the pattern of hydrothermal circulation mineral deposition and fluid chemistry both in space and time within slowly accreted ocean crust

characterizations of geothermal springs along the moxi deep fault in the western sichuan plateau china abstract abundant geothermal springs occur along the moxi fault located in western sichuan province the eastern edge of the qinghai tibet plateau highlighted by geothermal water outflow with an unusually high temperature of 218 c at 21 5 mpa from a 2010 m borehole in laoyulin kangding earthquake activity occurs relatively more frequently in the region and is considered to be related to the strong hydrothermal activity geothermal waters hosted by a deep fault may provide evidence regarding the deep underground their aqueous chemistry and isotopic information can indicate the mechanism of thermal springs cyclical variations of geothermal water outflows are thought to work under the effect of solid earth tides and can contribute to understanding conditions and processes in underground geo environments this paper studies the origin and variations of the geothermal spring group controlled by the moxi fault and discusses conditions in the deep ground flow variation monitoring of a series of parameters was performed to study the geothermal responses to solid tides geothermal reservoir temperatures are evaluated with na k mg data the abundant sulfite content dissolved oxygen do and oxidation reduction potential orp data are discussed to study the oxidation reduction states strontium isotopes are used to trace the water source the results demonstrate that geothermal water could flow quickly through the moxi fault the depth of the geothermal reservoir influences the thermal reservoir temperature where supercritical hot water is mixed with circulating groundwater and can reach 380 c to the southward along the fault the circulation of geothermal waters becomes shallower and the waters may have reacted with metamorphic rock to some extent our results provide a conceptual deep heat source model for geothermal flow and the reservoir characteristics of the moxi fault and indicate that the faulting may well connect the deep heat source to shallower depths the approach of hot spring variation research also has potential benefits for earthquake monitoring and prediction

hydrothermal systems and tertiary low angle normal faulting in the southwestern united states many tertiary normal faults in the southwestern united states dip gently 30 some of these normal faults apparently were initiated with gentle dips in conflict with the predictions of elementary dynamic analysis close correspondence between low angle normal faulting and intense hydrothermal alteration in the mojave desert western arizona and the rio grande rift is consistent with the hypothesis that hydrothermal systems play an important role in the genesis of these faults we present a mechanical model to demonstrate that low angle normal faults could form in response to stress trajectories that become reoriented in a periodically sealed geothermal system beneath a sloping surface

deep seated fluid involvement in ductile brittle deformation and mineralization south mountains metamorphic core complex arizona in the south mountains metamorphic core complex of central arizona a middle tertiary pluton exhibits the entire spectrum of ductile to brittle structures characteristic of metamorphic core complexes apart from minor cu enrichments in brecciated samples original major and minor element abundances of the deformed igneous rocks are remarkably undisturbed and there is little correspondence between chemical composition and deformational state similarly biotite and feldspar behaved almost isochemically during deformation except in the most severely deformed samples where plagioclase became somewhat more sodic and where biotite lost fe mg relative to al ti and was partly converted to vermiculite the oxygen and hydrogen isotope compositions of the samples are also quite uniform and are typical for continental granitoid rocks the range in whole rock 18 o values is extremely limited for the entire suite 8 3 to 9 9 versus v smow from undeformed granodiorite to mylonite green breccia and microbreccia although 18 o values for quartz 9 4 to 11 2 and feldspar 7 8 to 9 2 are similarly restricted the highest values occur in the most strongly deformed samples variably vermiculitic biotite has mildly and apparently randomly disturbed 18 o values 4 9 to 6 0 but d values 75 4 are remarkably uniform and are typical for unaltered plutonic rocks the 18 o values of barren ductilely deformed quartz veins 9 1 to 9 7 and of late brittlely deformed gangue quartz from the max delta gold mine 12 1 are consistent with the persistent circulation of a single isotopically uniform fluid reservoir at temperatures from 600 c ductile deformation barren quartz veins to 400 c brittle deformation mineralized quartz veins the inferred fluid 18 o 7 and d 35 values imply a deep seated igneous or high t metamorphic origin for the fluids fluids present during the entire history of ductile to brittle deformation were apparently close to chemical and oxygen hydrogen isotope equilibrium with the rocks the temporal association of deformation with tertiary plutonism and the evidence for high temperature fluid rock near equilibrium support the interpretation that the fluids were expelled from differentiated late crystallizing portions of the south mountains plutonic complex

tidally driven effluent detected by long term temperature monitoring at the tag hydrothermal mound mid atlantic ridge abstract during aug 13 21 1994 temperatures and current velocity were simultaneously monitored on the tag hydrothermal mound three giant kelps gks vertical thermistor arrays of 50 m height were moored on the periphery of the central black smoker complex cbc a manatee multi monitoring system including current velocity was deployed 50 m east of cbc four daibutsu geothermal probes penetrated the sediment south to west of cbc compilation of all data revealed semi diurnal variations in water temperatures and current velocity and allowed us to discuss the source of these anomalies temperature anomalies of gks correlate well with current velocity and are interpreted to be caused by the main plume from cbc that was bent over by the tidal current we identified two types of asymmetric periodic temperature variations at daibutsu probes 2 and 8 located 20 m to the south of cbc by comparing temperatures and current velocity they are attributed to non buoyant effluents laterally advected by the tidal current the source of one variation is located east to ese of the probes and the source of the other is located to the north on aug 31 a new periodic anomaly emerged on probe 2 with its amplitude up to 0 8 c the 6 h offset between the new anomaly and the previous one suggests that the source of the new anomaly lies to the west of probe 2 the heat flux of these non buoyant effluents is estimated to range from 30 to 100 kw m 2 which is of the same order as direct estimates of diffuse flow at the tag mound it suggests that a significant amount of diffuse effluent is laterally advected by the prevailing current near the seafloor

tectonic versus magmatic extension in the presence of core complexes at slow spreading ridges from a visualization of faulted seafloor topography we develop a forward model of the generation of faulted seafloor topography visualization to estimate the relative roles of tectonic and magmatic extension in the presence of core complexes at slow spreading ridges the visualization assumes flexural rotation of 60 normal faults a constant effective elastic thickness te of young lithosphere and a continuous infill of the depressed hanging wall by lava flowing from the spreading axis we obtain a new estimate of te 0 5 1 km from the shapes of the toes of 6 well documented oceanic core complexes we model an 80 km long bathymetric profile in the equatorial atlantic across a core complex and the ridge axis at 13 20 n and estimate the variation in tectonic extension which yields the variation in the fraction of upper crust extension m by magmatic diking at the ridge axis core complex formation appears to be stable for all values of m 0 5 the visualization shows how gabbro emplaced at the base of the lithosphere during extension by magmatic diking is partitioned to each side of the spreading axis and predicts a high probability of finding gabbros in the domes of core complexes

widespread active detachment faulting and core complex formation near 13 n on the mid atlantic ridge oceanic core complexes are massifs in which lower crustal and upper mantle rocks are exposed at the sea floor1 2 3 they form at mid ocean ridges through slip on detachment faults rooted below the spreading axis2 3 4 5 6 to date most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes here we present a survey of the mid atlantic ridge near 13 n containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis the detachment faults are apparently all currently active and at various stages of development a field of extinct core complexes extends away from the axis for at least 100 km our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity preliminary examination of seismicity and seafloor morphology farther north along the mid atlantic ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow spreading ridge than previously suspected

marine mineral exploration with controlled source electromagnetics at the tag hydrothermal field 26 n mid atlantic ridge seafloor massive sulphide sms deposits are of increasing economic interest in order to satisfy the relentless growth in worldwide metal demand the trans atlantic geotraverse tag hydrothermal field at 26 n on the mid atlantic ridge hosts several such deposits this study presents new controlled source electromagnetic csem bathymetric and magnetic results from the tag field potential sms targets were selected based on their surface expressions in high resolution bathymetric data high resolution reduced to the pole magnetic data show negative anomalies beneath and surrounding the sms deposits revealing large areas of hydrothermal alteration csem data sensitive to the electrical conductivity of sms mineralization further reveal a maximum thickness of up to 80 m and conductivities of up to 5 s m sms samples have conductivities of up to a few thousand s m suggesting that remotely inferred conductivities represent an average of metal sulphide ores combined with silicified and altered host basalt that likely dominates at greater depths

laboratory experiments of tsunami runup on a circular island laboratory experiments of a 7 2 m diameter conical island were conducted to study three dimensional tsunami runup the 62 5 cm tall island had 1 on 4 side slopes and was positioned in the center of a 30 m wide by 25 m long flat bottom basin solitary waves with height to depth ratios ranging from 0 05 to 0 20 and source lengths ranging from 0 30 to 7 14 island diameters were tested in water depths of 32 and 42 cm twenty seven capacitance wave gages were used to measure surface wave elevations at incident and four radial transects on the island slope maximum vertical runup measurements were made at 20 locations around the perimeter of the island using rod and transit a new runup gage was located on the back or lee side of the island to record runup time series

possible controls of base and precious metal mineralization associated with tertiary detachment faults in the lower colorado river trough arizona and california base and precious metal mineral deposits are located along and adjacent to tertiary detachment faults in western arizona and southeastern california mineralization is characterized by early copper and iron sulfides followed by massive specular hematite in turn followed by fracture filling chrysocolla and malachite mineralization occurred during faulting and was associated with unusually high geothermal gradients that were at least partially due to detachment fault movement that brought hot lower plate rocks up from great depth and juxtaposed them with cool near surface upper plate rocks mixing of ascending reduced metal bearing aqueous solutions with higher level oxidized fluids is proposed as an important process leading to oxide mineralization movement of the redox interface down normal fault ramps at the same rate as displacement of hanging wall rocks resulted in continuous mineralization at some upper plate sites over significant periods of geologic time this produced the largest orebodies aqueous solutions that precipitated iron oxides near the detachment fault carried manganese oxides to higher levels where vein and strata bound sedimentary manganese oxides were deposited

estimate of heat flux and its temporal variation at the tag hydrothermal mound mid atlantic ridge 26 n 1 from august 1994 to march 1995 three 50 m high vertical thermistor arrays designated giant kelps gks were deployed around the central black smoker complex cbc at the tag hydrothermal mound mid atlantic ridge 26 08 n 44 49 w these were designed to monitor the temporal variability of the vertical temperature distribution in the hydrothermal plume one small high temperature probe hobo was also deployed in one of the black smoker vents of cbc over the observation period two typical characteristics are recognized in plume temperatures measured with gks 1 the amplitudes of temperature anomalies decrease with increasing height above the top of cbc 2 maximum temperature anomalies on the upper thermistors occurred periodically and nearly simultaneously across the array about every 6 hours conversely maximum temperature anomalies on the lower thermistors occurred periodically every 12 hours indicating that the location of the plume discharged from cbc was forcibly moved by the change in direction of tidally modulated current flow the heat flux from cbc was estimated from temperatures measured by gks based on a model of buoyant hydrothermal fluid rising in a stable stratified density environment the estimated heat flux from cbc gradually decreases from about 86 to 55 mw over the 7 months of measurement with a mean rate of decrease of 0 17 mw d 1 since the black smoker effluent temperature measured with hobo was almost stable over the measurement period a plausible cause of the decrease is a reduction in the volume of hydrothermal fluid provided to the cbc in which case the estimated mean rate of decrease in volume flux of cbc is 8 9 m3 d 1 estimated heat flux temperature anomalies observed by hobo and diffuse flow and subbottom temperature anomalies recorded by other long term monitoring instruments before during and after odp leg 158 indicate that the drilling probably affected the fluid flow pattern within the mound but had little effect on the total heat flux from cbc

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

black smokers massive sulphides and vent biota at the mid atlantic ridge the discovery of black smokers massive sulphides and vent biota in the rift valley of the mid atlantic ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate to fast spreading oceanic ridges hydrothermal exchange processes may thus be important at the ridges which extend through the atlantic ocean and western indian ocean comprising more than half of the 55 000 km global length of seafloor spreading centres

modeling the effects of tidal loading on mid ocean ridge hydrothermal systems 1 tidal signals are observed in numerous time series measurements obtained from mid ocean ridge hydrothermal systems in some instances these tidal signals are clearly the result of ocean currents but in other instances it appears that the signals may originate in the subseafloor formation in order to explore the effect of ocean tidal loading on mid ocean ridge hydrothermal systems we apply a one dimensional analytical model of tidal loading on a poroelastic half space and develop a two dimensional numerical model of tidal loading on a poroelastic convection cell the one dimensional models show that for a reasonable range of fluid elastic and hydrological properties the loading efficiency may vary from near zero to near unity and the diffusive penetration depth for tidal pressure signals may vary from tens of meters to kilometers the two dimensional models demonstrate that tides may generate significant vertical and horizontal pressure gradients in mid ocean ridge hydrothermal systems as a result of spatial variations in fluid temperatures and the elastic and hydrological properties of the crust these continuum models predict that outflow temperature perturbations will be very small 10 4 c but in real systems where the continuum hypothesis does not always apply the perturbations may be on the order of 0 1 c the models predict relatively large perturbations to fluid velocity at the seafloor for high temperature vents the outflow perturbations normalized to the mean flow velocity increase as the permeability decreases flow reversals at the seafloor are predicted in some regions of net low temperature outflow and net inflow during the tidal cycle in the subseafloor tidally induced flow perturbations are likely to significantly enhance mixing and fluid exchange below the seafloor in regions of slow flow and in regions where there are strong gradients in temperature or in the mechanical and hydrological properties of the crust tidally enhanced mixing and fluid exchange may significantly influence the extent and character of microbial production in the subseafloor

geochronology of tag and snakepit hydrothermal fields mid atlantic ridge witness to a long and complex hydrothermal history geochronological studies of a large number of precipitates from the tag hydrothermal field and of few samples from snakepit hydrothermal field of the mid atlantic ridge show intermittent repeated hydrothermal events at both sites 210pb pb and 230th 234u measurements of sulfides iron and manganese oxides and 14c measurements of carbonates combined with observations of hydrothermal events recorded as discrete layers in sediment cores provide the basis for unravelling the temporal history of the fields the tag field shows intermittent activity over the past 120 000 years as evidenced by ages of low temperature mn oxides the presently active black smoker mound first formed about 40 000 50 000 years ago with precipitation of massive sulfides it has had intermittent pulsed high temperature activity every 5000 6000 years over the past 20 000 years which may reflect renewed magmatic activity at the ridge axis fluid flow is focussed at the mound site by structural and tectonic control suggested by the intersection of n s ridge parallel lystric normal faults and an e w transform fault periods of inactivity are marked by covering of the mound with pelagic carbonate ooze which is probably partially dissolved and reprecipitated as aragonite at the end of each high temperature event the snakepit field had an initial event about 4000 years ago probably shortly after the eruption of the volcanic ridge on which it sits a recent renewal still presently active was probably initiated by recent fissuring of the volcanic pile

history and geochemistry of a metalliferous sediment core from the mid atlantic ridge at 26 n fourteen thousand years of hydrothermal deposition are recorded in a metalliferous sediment core recovered from the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 n an estimated 26 to 96 of the sediment at this core site is hydrothermally derived as determined from the caco3 al and fe data layers of essentially pure vent precipitates contain 3 caco3 and 0 5 al with high concentrations of fe 43 cu 4 1 zn 1 2 mn 1 1 v 480 ppm pb 175 ppm cd 32 ppm and hg 3 8 ppm sediment accumulation rates vary from 1 to 30 g cm2 1000 y throughout the core a function of the intensity of hydrothermal inputs distinct hydrothermal events are recorded at 6000 and 8500 y b p in layers containing 90 vent derived material vertical metal profiles and interelement relationships in the core result from variable deposition of oxides and sulfides oxidation and dissolution of sulfide phases and scavenging of metals from seawater

the runup of solitary waves this is a study of the runup of solitary waves on plane beaches an approximate theory is presented for non breaking waves and an asymptotic result is derived for the maximum runup of solitary waves a series of laboratory experiments is described to support the theory it is shown that the linear theory predicts the maximum runup satisfactorily and that the nonlinear theory describes the climb of solitary waves equally well different runup regimes are found to exist for the runup of breaking and non breaking waves a breaking criterion is derived for determining whether a solitary wave will break as it climbs up a sloping beach and a different criterion is shown to apply for determining whether a wave will break during rundown these results are used to explain some of the existing empirical runup relationships

hydrothermal activity on a 105 year scale at a slow spreading ridge tag hydrothermal field mid atlantic ridge 26 n new radiometric dates are presented for the hydrothermal deposits in the alvin and mir relict hydrothermal zones located on the lower east wall of the rift valley within the tag hydrothermal field and placed in context of the evolution of the tag field as a whole over the past 105 years the field includes an active high temperature sulfide mound situated near the base of the east wall and a low temperature zone at middepth on the east wall all within a 5 by 5 kilometer area the oldest hydrothermal deposits sampled in the mir relict zone the low temperature zone and an inactive sulfide mound in the alvin relict zone are low temperature manganese crusts dated at 140 000 125 000 and 74 000 years respectively high temperature activity began about 100 000 years ago in the mir zone a 50 000 year high temperature event took place at the active sulfide mound in the mir zone and at the inactive sulfide mound in the alvin zone this event is followed by sporadic activity at the mir zone and the active sulfide mound until 10 000 and 5000 years ago respectively a reactivation of the system affected the mir zone from about 2000 to 600 years ago the presently active sulfide mound was reactivated only 60 years ago after a quiescence of about 4000 years the radiochronologic record reveals that the hydrothermal activity has been episodic with a major high temperature event throughout the tag field at 50 000 years and other more localized events

analytical solutions for tsunami runup on a plane beach single waves n waves and transient waves in the literature it has so far been common practice to consider solitary waves and n waves composed of solitary waves as the appropriate model of tsunamis approaching the shoreline unfortunately this approach is based on a tie between the nonlinearity and the horizontal length scale or duration of the wave which is not realistic for geophysical tsunamis to resolve this problem we first derive analytical solutions to the nonlinear shallow water nsw equations for the runup rundown of single waves where the duration and the wave height can be specified separately the formulation is then extended to cover leading depression n waves composed of a superposition of positive and negative single waves as a result the temporal variations of the runup elevation the associated velocity and breaking criteria are specified in terms of polylogarithmic functions finally we consider incoming transient wavetrains generated by monopole and dipole disturbances in the deep ocean the evolution of these wavetrains while travelling a considerable distance over a constant depth is influenced by weak dispersion and is governed by the linear korteweg de vries kdv equation this process is described by a convolution integral involving the airy function the runup on the plane sloping beach is then determined by another convolution integral involving the incoming time series at the foot of the slope a good agreement with numerical model results is demonstrated

structural aspects of fluid rock interactions in detachment zones metamorphic core complexes and associated detachment faults of the north american cordillera represent gently dipping normal displacement shear zones detachment zones along which hot deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate structures that formed during the ductile to brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones mylonitization which occurred along deeper segments of detachment zones below the brittle ductile transition was locally accompanied by tensile failure indicative of low deviatoric stress and high fluid pressure this condition persisted into the earliest phases of brittle deformation after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress brittle deformation of upper plate rocks occurred largely under conditions of low fluid pressure structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems 1 an upper plate system driven by convection and dominated by meteoric and connate fluids at near hydrostatic pressures and 2 a system within deeper levels of the shear zone where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping the late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear zone rocks to be overprinted by mineralization related to the upper plate fluid system

an experimental study of wave run up at a steep beach this paper presents experiments on run up of strongly nonlinear waves on a beach of 10 54 inclination velocity fields are obtained by the piv particle image velocimetry technique acceleration measurements are also attempted but it is difficult to obtain useful results in every case in addition free surface profiles are extracted from digital images and wave resistance probes the investigation focuses on the dynamics of the early stages of the run up when steep fronts evolve in the vicinity of the equilibrium shoreline but maximum run up heights are also reported measurements on moderately nonlinear waves are compared to results from long wave theories including a numerical boussinesq model and analytic shallow water results from the literature in particular the applicability of the long wave theories is addressed however most attention is given to run up of high incident solitary waves that are on the brink of breaking at the shoreline in one case a temporarily slightly overturning wave front is found that neither develops into a plunger or displays appreciable spilling this feature is discussed in view of measured velocity and acceleration patterns and with reference to the dam break problem effects of scaling as well as viscous damping are also briefly discussed

the equatorial neutral thermospheric response to geomagnetic thermospheric temperature and neutral density measurements from dynamics explorer 2 de 2 have been used to study the time dependent response of the equatorial thermosphere to geomagnetic forcing data taken following the onsets of several different isolated geomagnetic storms have been used in a superposed epoch scheme to investigate the characteristic response of the density and temperature structure of the equatorial thermosphere at 350 lcm altitude in the rooming and evening local time sectors the principal post storm onset features that have been observed are as follows 1 two travelling waves one generated at northern high latitudes and one at southern high latitudes pass through the equatorial thermosphere with wave crests crossing the equator at a time lag of 3 6 hours after the storm onset the maximum neutral density perturbations associated with these waves are 15 25 for atomic oxygen and 30 40 for molecular nitrogen 2 following the passage of these travelling waves the densities and temperatures in the evening sector return relatively rapidly to their undisturbed state whereas those in the morning sector have significant variations at longer lag times in particular significant increases in neutral density and temperature occur in the morning sector about 12 hours after the onset of the storm the n2 density enhancements are seen earliest in the summer hemisphere while the o density enhancements are seen earliest in the winter hemisphere

evolution of an active sea floor massive sulphide deposit hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to 400 c before venting at spectacular black smokers these hydrothermal systems exert a strong influence on ocean chemistry1 yet their structure longevity and magnitude remain largely unresolved2 the active transatlantic geotraverse tag deposit at 26 n on the mid atlantic ridge is one of the largest oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black smoker fields here we report ages of sulphides and anhydrites from the recently drilled3 tag substrate structures determined from 234u 230th systematics analysed by thermal ionization mass spectrometry the new precise ages combined with existing data4 5 show that the oldest material 11 000 to 37 000 years old forms a layer across the centre of the deposit with younger material 2 300 7 800 years old both above and below this stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid6 the age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2 000 years

mineralogy chemical composition and structure of the mir mound tag hydrothermal field the study of samples collected from the surface of the mir mound tag hydrothermal field by video controlled hydraulic grab allowed identification of a number of mineralogical types these include pyrite chalcopyrite py cp bornite chalcopyrite opaline bn cp op and sphalerite opaline sp op sulfide chimneys massive sulfides composed of pyrite py chalcopyrite pyrite cp py marcasite pyrite opaline mc py op sphalerite pyrite opaline sp py op and sphalerite chalcopyrite pyrite opaline sp cp py op as well as siliceous and fe mn oxide hydrothermal deposits most of the minor elements ag au cd ga hg sb and pb are associated with zn rich massive sulfides co bi pb and as with ferich ones while cu rich sulfides are depleted of trace metals cu enriched assemblages are concentrated in the northern part zn enriched in the center and siliceous rocks in the south of the mir mound according to paragenetic relations the development of the mound started with the formation of quartz originally opaline rocks and dendritic assemblages of melnikovite pyrite followed by deposition of chalcopyrite and recrystallization of primary pyrite subsequent generation of sphalerite rich assemblages and final deposition of opaline rocks the late renewal of hydrothermal activity led to local formation of cu rich chimneys enriched in au ag hg and pb probably due to their remobilization from inner parts of the deposit

stochastic analysis of exit fluid temperature records from the active tag hydrothermal mound mid atlantic ridge 26 n 2 hidden markov models of flow episodes 1 i develop a stochastic signal model for episodic modes of variability in hydrothermal flow records using probabilistic functions of markov processes i e hidden markov models hmms and fit the model to exit fluid temperature time series data from diffuse flow sites on the active tag hydrothermal mound the flow states are modeled using gamma densities to provide flexibility for application to a range of signal types between three and five flow states are needed to fit the diffuse flow temperature records from tag which correspond to models with between 10 and 28 degrees of freedom the number of flow states required to fit a given record is related to the signal variance with more variable records requiring a larger state space hmms thus provide an efficient signal model for episodic variability in hydrothermal flow records suggesting that markov processes may provide a means to generate stochastic subsurface flow models for deep sea hydrothermal fields if the spatial flow correlations can be incorporated into a statistical framework i also use the viterbi algorithm to decode the time series data into best fitting state sequences which can be used to classify the records into discrete flow episodes this may provide an objective means to identify discrete events in a flow record if misclassification issues arising from nonepisodic variability e g tidal forcing can be addressed

oceanic detachment faults focus very large volumes of black smoker fluids it is generally assumed that the seawater derived fluids that feed black smoker vent fields on the seafloor are discharged vertically from depths of 1 3 km we present new oxygen and strontium isotope data that show that fluids at black smoker temperatures of 300 400 c were focused along a low angle detachment fault at 15 45 n near the mid atlantic ridge isotopic alteration is the most extreme ever reported from oceanic rocks altered at similar temperatures indicating intensely focused fluid flow both in recharge and discharge parts of the hydrothermal system rare earth element mobility in the fault rocks demonstrates isotopic alteration by evolved hydrothermal fluids not conductively heated seawater the fault zone protolith was predominantly ultramafic but also included mafic rocks with metasomatic alteration to talc tremolite chlorite schists resulting mainly from chemical exchange between these lithologies during fluid flow fluids in equilibrium with this assemblage would be similar to ultramafic hosted black smoker fluids we present a new model in which hydrothermal circulation around detachment faults evolves from basalt hosted tag type to footwall ultramafic hosted rainbow type to low temperature ultramafic hosted lost city type key features of our model are the intrusion of gabbro bodies immediately below the detachment to provide a heat source for circulation and focusing of fluid flow into the detachment fault to allow venting away from the neovolcanic axis