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

alteration of basalt hyaloclastite at the off axis sea cliff hydrothermal field gorda ridge abstract the sea cliff hydrothermal field on the northern segment of the gorda ridge is situated along a rift bounding normal fault about 2 6 km east of the neovolcanic zone and approximately 300 m above the spreading axis the structural setting of this hydrothermal field differs from that of most other active seafloor hydrothermal sites investigated to date which are typically situated in the neovolcanic zone mineralization occurs in basaltic talus covering a large normal fault scarp hydrothermal crusts cover much of the seafloor in the area of the active hydrothermal field these crusts form by extensive alteration of basaltic hyaloclastite in a zone of mixing between ascending hydrothermal fluid and entrained seawater the initial stage of alteration is magnesian metasomatism of both crystalline basalt and basaltic glass converting the rock to mg rich smectite and smectite chlorite further alteration removes nearly all cations and ultimately leads to silicification preservation of basaltic texture in the silicified rocks provides evidence that even such sparingly soluble elements as a1 and ti have been removed oxygen isotopic ratios of the altered rocks constrain initial alteration to temperatures near 220 c close to the maximum measured vent temperatures of 247 c silicification proceeded to much lower temperatures and most amorphous silica deposition occurred at temperatures below 100 c sulfur strontium and lead isotopic data all indicate a predominantly basaltic source with important contributions from seawater but no significant contribution from sedimentary sources comparison with ophiolite hosted massive sulfide deposits shows that the structural setting alteration sequence and depositional environment are all similar and suggests that mineralization and replacement of basaltic breccia in the sea cliff hydrothermal field likely occur in the subsurface beneath a capping layer of silicified hyaloclastite

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

continuous sampling of hydrothermal fluids from loihi seamount after the 1996 event for at least 9 years prior to july 1996 hydrothermal fluids flowed from pele s vents on loihi seamount hawaii in july august 1996 a tectonic volcanic event occurred that destroyed pele s vents creating a pit crater pele s pit and several sites with hydrothermal venting in october 1996 we deployed two new continuous water samplers osmosamplers at two of these hydrothermal sites and collected fluids using traditional sampling techniques to monitor the evolution of crustal and hydrothermal conditions after the event the samplers were recovered in september 1997 and additional discrete vent fluid samples were collected the osmosampler located along the south rift at naha vents captured a change in composition from a low chlorinity high k fluid relative to bottom seawater to a high chlorinity low k fluid these changes are consistent with the fluid cooling during ascent and being derived from several different sources which include high 330 c and low 330 c into which magmatic volatiles were added during the deployment thermal and fluid fluxes decreased at naha the transport of heat and chemicals was decoupled the chemical and thermal evolution of hydrothermal fluids after the event on loihi is consistent with previous models based on events that have occurred along mid ocean ridges the event at loihi clearly had an effect on the local hydrography however the integrated effect of chemical fluxes to global budgets from similar events is uncertain chemical fluxes from similar events may have a global impact if ratios of chemical e g co2 fe mn mg sulfate and k to thermal anomalies greatly exceed or are in the opposite direction to fluxes from mid ocean ridge hydrothermal systems

the magnetic structure of axial seamount juan de fuca ridge axial seamount is a large volcanically active seamount located in the eastern pacific on the central juan de fuca jdf ridge at 46 n 130 w sea surface magnetic anomaly data show that axial lies completely within crust formed during the brunhes normal polarity epoch the edifice however does not produce a simple positive magnetic anomaly related to the excess mass above mean seafloor several pronounced negative magnetic anomalies are associated with the seamount including both a circular shaped low located west of the summit and an elongate low trending nw on the southeast flank of the volcano these two anomalies form part of a nw se magnetic trough which extends across the seamount edifice oblique to the general jdf ridge trend of n20 e a third negative anomaly is observed on the northern flank of the seamount associated with north helium basin a deep elongate basin which indents the northeast flank of the seamount both inversion of sea surface magnetics data and forward modelling of topography constrained by magnetization values of dredged samples show that bathymetric effects can explain the helium basin anomaly but topography alone cannot produce the low magnetization values which extend across the seamount summit magnetization of crustal rocks does not vary spatially over the seamount so that negative anomalies at the summit can be explained by variations in the effective thickness of the magnetic source layer a simple forward model of uniform magnetization shows that the anomaly source layer would have to thin by 50 to less than 700 m thick in order to generate the observed magnetic response a thinned source layer is consistent with the presence of either a substantial magma chamber at depth beneath the summit of axial or a zone of substantial crustal alteration the shape of the anomaly implies the trend of either of these sources must be oriented obliquely to the general ridge trend but parallel to the direction of the absolute plate motion

gas rich submarine exhalations during the 1989 eruption of macdonald seamount in january 1989 we observed submarine eruptions on the summit of macdonald volcano during a french german diving programme with the ifremer submersible cyana gas streaming of large amounts of ch4 co2 and so2 from summit vents inferred from water column anomalies and observed by submersible was accompanied on the sea surface by steam bursts turbulence red glowing gases and black bubbles comprising volcanic ash sulphur and sulphides chloride depletion of water sampled on the floor of an actively degassing summit crater suggests either boiling and phase separation or additions of magmatic water vapour submersible observations in situ sampling and shipboard geophysical and hydrographic measurements show that the hydrothermal system of this hotspot volcano is distinguished by the influence of magmatic gases released from its shallow summit

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

axial seamount an active ridge axis volcano on the central juan de fuca ridge axial seamount some of the manuscripts in this special section refer to the edifice with the more precise name of axial volcano a large ridge axis volcano is located on the central segment of the juan de fuca ridge approximately 250 nautical miles west of the washington oregon british columbia coast currently both volcanically and hydrothermally active axial lies directly at the intersection of the cobb eickelberg seamount chain and the juan de fuca ridge figure 1 the volcanic activity associated with the seamount formation strongly interacts with and is affected by the normal seafloor spreading processes at the intersection because of this unique geologic setting its proximity to west coast ports and oceanographie institutions and its shallow depth axial has become the focus of a large number of scientific investigations over the past decade

extensive volcaniclastic deposits at the mid atlantic ridge axis results of deep water basaltic explosive volcanic activity a recent mapping survey followed by submersible diving investigations revealed the existence of extensive volcaniclastic deposits on three adjoining progressively deeper segments of the mid atlantic ridge south west of the azores the maximum water depth at which subaqueous magmatic and hydro magmatic explosive processes can occur is controversial this discovery provides fundamental constraints on the dynamics of deep subaqueous volcanic eruptions using multibeam bathymetry sonar image interpretations submersible observations volcaniclastic sample descriptions and chemical analysis evidence of extensive deep water up to 1700 m magmatic explosive activity of basalt at the axis of a spreading mid ocean ridge is documented for the first time

1998 eruption of axial volcano multibeam anomalies and sea floor observations lava flows erupted during the january february 1998 seismic swarm at axial volcano on the juan de fuca ridge have been identified by differencing of pre and post event multibeam bathymetric surveys and by seafloor observations a sheet flow more than 3 km in length and 500 800 m wide erupted from the uppermost south rift is the site of a robust hydrothermal system 1998 lavas occur over about 9 km of the upper south rift zone or about 20 of the along axis length of the seismicity event 50 km the estimated volume of lava erupted is 18 76 106 m for the extrusion and 100 106 m for the intrusion the total volume is consistent with the volume from modeling of seafloor strain measurements recorded during the event

submarine thermal springs associated with young volcanoes the teahitia vents society islands pacific ocean abstract hydrothermal solutions were sampled close to two summits of teahitia a submarine volcano from the southern extension of the society volcanic chain the temperatures measured within two vents were 30 c the vent solutions have a ph value around 5 3 and are enriched in fe mn si ba li and rees relative to ambiant seawater but show only a small decrease in mg and so4 concentrations the lowest measured 87sr 86sr is 0 70867 while the end is identical to those of the surrounding alkali basalt 2 5 the vent solutions from teahitia have chemical characteristics similar to pele s vents loihi seamount another mid plate volcano the exact temperature of seawater basalt interaction is difficult to constrain however the absence of positive eu anomaly in the fluids the particles and in the hydrothermal deposits from teahitia seems to exclude the participation of a high temperature component comparable as those involved in mid ocean ridge axis vents

rapid emplacement of a mid ocean ridge lava flow on the east pacific rise at 9 46 51 n in april 1991 during a submersible diving expedition to the east pacific rise epr crest at 9 46 51 n a new volcanic eruption on the sea floor was discovered here we report results from numerical modeling of that eruption which indicate that 4 106 6 106 m3 of lava was emplaced in 1 2 h with an average eruption rate of 103 106 m3 s 1 comparable to rates observed in hawaii at kilauea s east rift zone if the rapid emplacement of the 1991 epr lava and its short eruption duration are typical of volcanic events at fast spreading mid ocean ridge crests these characteristics have broad implications for our ability to detect monitor and understand the evolution of magmatic and volcanic processes within the axial zone

morphological and structural studies of the society and austral hotspot regions in the south pacific abstract the southeastern extension of the society teahitia hotspot and austral macdonald hotspot volcanic chains were extensively studied 1986 1989 using multi channel echo sounding seabeam bottom imaging systems and submersible dives sea floor within these hotspot regions consists of recently constructed submarine highs built on crust that shows morphological and structural discontinuities with the surrounding ancient abyssal hill regions 4200 m in depth recent volcanic and hydrothermal activity is present on several volcanoes within the society hotspot and on the macdonald volcano previous studies carried out in the society hotspot region showed several geological provinces 1 1 eight major highs 1000 m in height which extend from above sea level down to a depth of 3750 m these features have been formed by lateral eruptive events along rift zones which are aligned along four main structural directions n150 180 n80 n30 40 and n110 which correspond to the orientations of ancient ridge axes transform faults and riedel type shear zones respectively 2 2 the sea floor which slopes away from the base of each major volcano 3750 4200 m depth has an average slope of 0 5 and is characterized by smooth topography and volcanic cones less than 500 m high 3 3 the surrounding abyssal hill region which has an average slope of 0 1 near the society hotspot consists of faulted terrane with a n170 orientation and ancient elongated structural highs from the structural lineations observed in the society hotspot volcanoes a reconstruction of volcanic features has been made it is proposed that the formation of the seamounts occurs in an organized fashion along pre existing preferential structural orientations that are associated with zones of crustal weakness

drilling the snake pit hydrothermal sulfide deposit on the mid atlantic ridge lat 23 sup 0 22 n a major high temperature hydrothermal area has been discovered in the mid atlantic ridge rift valley about 25 km south of the kane fracture zone the vent field consists of a wide area 40 000 m sup 2 of dark hydrothermal deposits numerous sulfide chimneys and mounds some up to 11 m high and high temperature black smoker vents ten shallow holes the first ever drilled in an active submarine hydrothermal area recovered friable unconsolidated fe cu fe and zn sulfides and several large fragments of massive sulfide mainly chalcopyrite from the locally thick 13 m hydrothermal deposits the vents are also associated with an unusual biological community of smaller more mobile organisms than reported from the east pacific rise

en echelon axial volcanic ridges at the reykjanes ridge a life cycle of volcanism and tectonics abstract new deep towed sidescan records and multibeam bathymetric data along the reykjanes ridge allow us to assess the varying thermal effect of the icelandic hotspot the increasing predominance of faulting speculated to be derived from mantle upwelling and outflow over simple extensional neotectonics is traced from northeast to southwest numerous en echelon volcanic ridges built along the plate boundary are seen in various stages of growth and decay and a cycle of alternating construction and dismemberment is proposed unfaulted short narrow lensoid ridges occasionally in small groups are interpreted as immature and developing axial volcanic ridges broad convex flanked ridges appear to be mature sites of volcanic construction where the magma supply has been maintained narrow bow form ridges occasionally with rectilinear flanks represent axial volcanic ridges which have ceased construction and are in initial stages of disaggregation isolated polygonal blocks generally seen off axis represent the most advanced stage of dismantling of the surface crustal accretionary features while this cyclicity is speculated to be related to periodic magmatic and tectonic focusing and defocusing along the axis due to the restricted magma supply at this slow spreading ridge it is recognised that this needs modelling although the oblique spreading of the reykjanes ridge allows one to clearly distinguish between structures attributable to plate spreading from those related to small scale accretionary processes we propose that the major processes identified in this study are directly applicable to all slow spreading sections of mid ocean ridges

factors controlling large wood transport in a mountain river as with bedload transport wood transport in rivers is governed by several factors such as flow regime geomorphic configuration of the channel and floodplain or wood size and shape because large wood tends to be transported during floods safety and logistical constraints make field measurements difficult as a result direct observation and measurements of the conditions of wood transport are scarce this lack of direct observations and the complexity of the processes involved in wood transport may result in an incomplete understanding of wood transport processes numerical modelling provides an alternative approach to addressing some of the unknowns in the dynamics of large wood in rivers the aim of this study is to improve the understanding of controls governing wood transport in mountain rivers combining numerical modelling and direct field observations by defining different scenarios we illustrate relationships between the rate of wood transport and discharge wood size and river morphology we test these relationships for a wide multithread reach and a narrower partially channelized single thread reach of the czarny dunajec river in the polish carpathians results indicate that a wide range of quantitative information about wood transport can be obtained from a combination of numerical modelling and field observations and from document contrasting patterns of wood transport in single and multithread river reaches on the one hand log diameter seems to have a greater importance for wood transport in the multithread channel because of shallower flow lower flow velocity and lower stream power hydrodynamic conditions in the single thread channel allow transport of large wood pieces whereas in the multithread reach logs with diameters similar to water depth are not being moved on the other hand log length also exerts strong control on wood transport more so in the single thread than in the multithread reach in any case wood transport strongly decreases with increasing piece volume although this relation is not linear we also document a nonlinear relationship between wood transport and flood magnitude a threshold discharge was identified below which wood transport is negligible this threshold is higher in the multithread reach while in the single thread reach floods of lower magnitude are able to transport wood downstream wood transport ratio increases with discharge until it reaches an upper threshold or tipping point and then decreases or increases much more slowly this threshold is clearly related to bankfull discharge but it is much higher for the multithread reach than for the single thread one although modelling input and field observations were taken from a specific river our findings and conclusions are likely to be applicable to a much larger suite of mountain rivers

geotectonic setting of hydrothermal activity on the summit of lucky strike seamount 37 17 n mid atlantic ridge we have investigated the relations between volcanic tectonic and hydrothermal activity on lucky strike seamount 37 17 n mid atlantic ridge using a nested survey strategy involving collection of data from different deep sea mapping systems the highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic pillow lava and hydrothermal sulfide rubble and hydrothermally cemented breccias substrates the large volume of sulfide rubble together with the nature of the sulfide structures indicates that hydrothermal activity has been episodic but ongoing for a long period of time hundreds to thousands of years on the basis of the distribution of hydrothermal deposits we propose a model of alternation between tectonic and volcanic control on hydrothermalism at lucky strike seamount midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high temperature hydrothermal activity over long time periods during periods of amagmatic extension active faulting within the summit depression provides multiple near surface fluid flow pathways for discharge of high temperature fluids and widespread deposition of massive sulfides during eruptive events rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits the new sheet flows form a cap on the hydrothermal system and fluid upflow is reorganized discharge of high temperature fluids is restricted to isolated sites with relatively high permeability for example the edges of the lava lake much of the upwelling hydrothermal fluid pools in the subsurface conductively cools and mixes with entrained seawater before discharging as widespread low temperature diffuse flow hyaloclastites become cemented further augmenting the sealing of the system present day activity at lucky strike seamount represents this locally volcanically controlled phase of activity despite the segment as a whole being dominantly amagmatic

the geochemical controls on vent fluids from the lucky strike vent field mid atlantic ridge abstract hydrothermal vent fluids were collected from the lucky strike site at 37 17 n on the mid atlantic ridge in both 1993 and 1996 seven vents were sampled with the dsv alvin in 1993 and six vents were sampled in 1996 using the rov jason during the lustre 96 cruise as three of the vents were sampled in both 1993 and in 1996 a time series of vent fluid chemistry is also reported measured temperatures ranged from 202 to 333 c at the 1618 1726 m depth of the vent field which is located on lucky strike seamount these fluids are either equal to or less than the local bottom seawater in chlorinity while the range in fluid compositions at lucky strike is generally within that observed elsewhere the unusual aspects of the fluid chemistries are the relatively high ph and low fe mn li and zn we attribute this as well as an usually low sr ca ratio to reaction with a highly altered substrate the high si and cu contents suggest a deep as well as hot source for these fluids the fluid compositions therefore suggest formation by super critical phase separation at a depth not less than 1300 m below the seafloor and reaction with a relatively oxic and previously altered substrate there is temporal variability in some of the vent fluid compositions as li k ca and fe concentrations have increased in some of the vents as has the fe mn molar ratio although the chlorinities have remained essentially constant from 1993 to 1996 while there is not a simple relationship between vent fluid compositions or temperatures and distance from the lava lake at the summit of the seamount the vent fluids from many of the vents can be shown to be related to others often at distances 200 m the most southeasterly vents eiffel tower and the marker mounds vents are distinct in chlorinity and other chemical parameters from the rest of the vents although closely related to each other within the southeastern area similarly all of the vents not in this one area appear closely related to each other this suggests one or two source fluids for many of the vents as is also inferred to be the case at tag but which is in contrast to observations on faster spreading ridges this may suggest inherently different plumbing for hydrothermal systems at slower versus faster spreading ridges

chemical composition of basement fluids within an oceanic ridge flank implications for along strike and across strike hydrothermal circulation compositions of basement fluids are presented for four sites along a 3 5 m y old partly buried basement ridge on the eastern flank of the juan de fuca ridge this ridge is roughly parallel to the active ridge axis of the endeavor segment 100 km to the west from south to north these sites are baby bare outcrop ocean drilling program odp site 1026 and the southern and northern sides of mama bare outcrop the composition of basement fluids is determined or estimated from analyses of pore water samples that were extracted from sediments at each of these sites spring waters from baby bare and basement fluids that vented from the open odp hole 1026b chemical trends in basement fluids along this transect show increasing alteration from south to north a similar trend was observed along an odp transect perpendicular to the ridge axis with increasing fluid alteration from west to east much of the increase in fluid alteration along the odp transect is explained by greater water rock exchange with increasing basement temperature to the east in contrast the trend along the 3 5 m y old ridge is best explained by diffusive exchange with the overlying sediment the rate of this exchange is used to constrain hydrologie properties within basaltic basement flow within the 3 5 m y old ridge is inferred to occur from south to north and lacks significant exchange with basement fluids from the active ridge crest to the west thus the two flow systems are hydrologically distinct and flow paths are likely influenced by the complex distribution of permeability in basement the pattern of seafloor morphology and the type and rate of sedimentation

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

loihi seamount hawaii a mid plate volcano with a distinctive hydrothermal system the initial discovery of deep sea hydrothermal vents at the galapagos rift1 2 has resulted in a decade of exploration and experimentation subsequent discoveries in the pacific3 8 and atlantic9 ocean basins have established the importance of these features as sources of mantle derived gases and solutes10 2 and as loci for heat dissipation13 14 polymetallic sulphide mineral deposition15 16 and geochemical exchange17 19 the presence of dense communities of bacteria and specialized macrofauna20 22 has called into question the absolute role of sunlight as the principal energy source for marine organisms until recently these deep sea vents were known only at or near plate boundaries but recent dredging23 24 near bottom water sampling25 27 and sea floor camera survey28 programmes have indicated that hydrothermal vents are present at the summit of loihi seamount a mid plate hotspot submarine volcano at the southern end of the hawaiian island chain29 here we report results obtained from sampling two active vent fields the fluid chemistry especially high co2 and fe presence of alkalic lavas and iron depositing bacteria and absence of macrofauna suggest that mid plate vents may differ from the hydrothermal vents found at mid ocean ridges

the 2016 south alboran earthquake m w 6 4 a reactivation of the ibero maghrebian region abstract on 25 january 2016 an earthquake of magnitude m w 6 4 occurred at the southern part of the alboran sea between southern spain and northern morocco this shock was preceded by a foreshock m w 5 1 and followed by a long aftershock sequence focal mechanism of main shock has been estimated from slip inversion of body waves at teleseismic distances solution corresponds to left lateral strike slip motion showing a complex bilateral rupture formed by two sub events with most energy propagating along a plane oriented n30 e plane dipping to the nw relocation of larger events of the aftershock series show two alignments of epicentres in ne sw and nne ssw direction that intersect at the epicentre of the main shock we have estimated the focal mechanisms of the largest aftershocks from moment tensor inversion at regional distances we have obtained two families of focal mechanisms corresponding to strike slip for the nne ssw alignment and thrusting motion for the ne sw alignment among the faults present in the area the al idrisi fault or fault zone may be a good candidate for the source of this earthquake the study of coulomb failure stress shows that it is possible that the 2016 earthquake was triggered by the previous nearby earthquakes of 1994 m w 5 8 and 2004 m w 6 3 the possible seismic reactivation of the central part of the ibero maghrebian region is an open question but it is clear that the occurrence of the 2016 earthquake confirms that from 1994 the seismicity of central part of imr is increasing and that focal mechanism of largest earthquakes in this central part correspond to complex ruptures or zone of fault