pattern of opening rates along the axis of the mid atlantic ridge vine and wilson1 using the vine and matthews2 hypothesis were the first to try to relate the magnetic pattern over the crests of the mid ocean ridges to the known geomagnetic time scale to determine the spreading rate over the past few million years many determinations of spreading rate at the axis of the ridge during plio pleistocene times have been published le pichon3 and morgan4 for example used spreading rates computed for the axial magnetic pattern 10 myr anomaly 5 of heirtzler et al 5 or for the past 5 myr respectively even when trying to assess the properties of instantaneous plate kinematics likewise in the systematic inversion of minster et al 6 anomalies 3 and 5 were generally used thus averaging plate speeds over the past 5 10 myr minster and jordan7 sought to improve this and redetermined the spreading rates using magnetic anomalies 2 and 2 yielding a mean averaging interval of 3 myr here we examine plate behaviour over a shorter time scale in the reasonably well surveyed axial region of the mid atlantic ridge south of the azores the average opening rates we obtained for this region for magnetic anomalies 1 brunhes 2 and 2 using two different geomagnetic time scales indicate that 1 there was a relative slowing down of the opening between anomalies 1 and 2 which may be of global significance 2 the opening is compatible with a rigid plate model when it is averaged over 2 5 myr although for shorter averaging times 1 8 and 0 7 myr there is some evidence of non rigidity of the plates thus different segments of the mid atlantic ridge may display an independent behaviour and with respect to relatively short time intervals may show departures from characteristics typically associated with slow accretion

distributions of hydrothermal activity along the mid atlantic ridge interplay of magmatic and tectonic controls abstract hydrothermal activity has been investigated along three different sections of the slow spreading mid atlantic ridge mar 11 20 30 n 36 38 n and the reykjanes ridge 57 45 63 06 n when considered in total the incidence of venting along these three sections of the mar compares well with the predictions of a model in which frequency of venting is linearly related to ridge crest spreading rate at the scale of individual study areas however departure from the model is observed by up to an order of magnitude venting is anomalously rare along the reykjanes ridge but anomalously abundant along the mar 36 38 n whilst such variability may be within the error of the linear spreading rate model we note that the interplay between magmatic and tectonic processes also differs between the three study areas in the case of the reykjanes ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling investigative strategy because the style of venting which predominates may not give rise to conventional black smoker hydrothermal plumes along the oblique and broadly segmented mar 36 38 n we propose that vigorous hydrothermal venting in broad segment end non transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites

second order segmentation the relationship between volcanism and tectonism at the mar 38 n 35 40 n deep tow sidescan sonar data acquired along 240 km of the mid atlantic ridge mar between 35 40 n and 38 n have been combined with new bathymetric compilations and used to establish the recent tectonic and volcanic history of six second order segments and their bounding non transform offsets ntos the segments show a range of volcanic and tectonic types but in general the northernmost segments i e those with greater influence from the azorean hotspot are shallower and more volcanically robust than those to the south with hydrothermal activity in segment centres for example menez gwen and lucky strike nonetheless this generalisation requires some modification due to temporal variations in the balance between magmatic supply and tectonic dismemberment the ntos are broad right stepping discontinuities locally up to 25 km wide and accommodate offsets between 10 and 50 km the discontinuities are mostly sediment floored and link the spreading segment tips with a range of structures these include locally dense arrays of en echelon extensional normal faults short lengths of linear strike slip fault strands occasionally cutting basement blocks apparently stranded within the offsets basement blocks within the offsets are cross cut by complexes of intersecting faults suggesting that deformation is distributed across the zone the pervasive faulting taking place at the ntos favours fluid circulation and associated hydrothermal activity as at the rainbow site at 36 17 n

non transform offsets along the mid atlantic ridge south of the azores 38 n 34 n ultramafic exposures and hosting of hydrothermal vents ten contiguous non transform offsets ntos along the mid atlantic ridge mar south of the azores between 38 n and 35 40 n have been studied in detail using swath bathymetric acoustic backscatter and deep tow high resolution sidescan sonar tobi data in contrast with discontinuities studied elsewhere at slow spreading ridges these left lateral ntos are consistently broader and larger with complex structural fabrics accommodating the offset they are characterized by a range of elevated and faulted massifs detached from their segment flanks with an irregular acoustic backscatter pattern some of these massifs have been explored and sampled recently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks and sometimes associated with high temperature hydrothermal venting water column surveys adjacent to these massifs show high ch4 and low tdm total dissolvable manganese concentrations possibly resulting from the process of serpentinization of ultramafic rocks the correlation between the shallow dome like shaped massifs and the high concentrations of ch4 associated with low levels of mn is of particular interest to predict the outcrop of ultramafic rocks within the ntos where no geological data are available the exposure of the ultramafic massifs within the ntos is favored by low magmatic supply and low angle detachment faulting occurring at segment ends the pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurrence of high temperature vent sites

microseismicity of the mid atlantic ridge at 7 s 8 15 s and at the logatchev massif oceanic core complex at 14 40 n 14 50 n lithospheric formation at slow spreading rates is heterogeneous with multiple modalities favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate we report microseismicity from three deployments of seismic networks at the mid atlantic ridge mar two networks surveyed the mar near 7 degrees s in the vicinity of the ascension transform fault three inside corner high settings were investigated however they remained seismically largely inactive and major seismic activity occurred along the center of the median valley in contrast at the logatchev massif core complex at 14 degrees 45n seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted logatchev hydrothermal vent field to the north and south of the massif however seismic activity occurred along the ridge axis emphasizing the asymmetry of seismicity at the logatchev segment focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3 5 km depth which we interpret to reflect volume expansion accompanying serpentinization at shallower depth of 2 4 km some earthquakes in the vicinity of the vent field showed normal faulting behavior suggesting that normal faults facilitates hydrothermal circulation feeding the vent field further a second set of cross cutting faults occurred indicating that the surface location of the field is controlled by local fault systems

drilling constraints on lithospheric accretion and evolution at atlantis massif mid atlantic ridge 30 n expeditions 304 and 305 of the integrated ocean drilling program cored and logged a 1 4 km section of the domal core of atlantis massif postdrilling research results summarized here constrain the structure and lithology of the central dome of this oceanic core complex the dominantly gabbroic sequence recovered contrasts with predrilling predictions application of the ground truth in subsequent geophysical processing has produced self consistent models for the central dome the presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100 220 kyr and not as a single magma pulse isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth below 800 m alteration is restricted to narrow zones surrounding faults veins igneous contacts and to an interval of locally intense serpentinization in olivine rich troctolite hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies but was predominantly in the amphibolite and greenschist range deformation of the sequence was remarkably localized despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation presumably during unroofing via detachment faulting both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent southern ridge of the massif there the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70 of recovered samples we develop a working model of the evolution of atlantis massif over the past 2 myr outlining several stages that could explain the observed similarities and differences between the central dome and the southern ridge

solution to the volume problem in serpentinization kernel pattern consists of a rim of completely serpentinized peridotite surrounding a core of partly serpentinized or unserpentinized peridotite it is found adjacent to shear zones in some serpentinites kernels are commonly rectangular with length width ratios of 1 to 3 the boundary between the completely serpentinized rim and the unserpentinized core is commonly cut at a high angle by cross fractures the cross fractures which contain either picrolite or chrysotile asbestos taper out in the core and widen in the rim they are almost always perpendicular to the core rim boundary and point into the core of the kernel in this sense they are radially distributed in the rim the kernel pattern can be observed in thin section in outcrop and on the megascopic scale of tens of metres cross fractures represent expansion fractures in serpentinite in the rim of the kernel caused by an increase in volume acompanying serpentinization of the peridotite in the core of the kernel as the serpentinization front penetrates into the kernel the serpentinized peridotite expands however the serpentinite in the rim cannot expand and must fracture to accommodate the increase in volume as serpentinization proceeds more expansion occurs extending the fractures farther into the core and requiring the fractures in the existing serpentinite to widen these fractures are subsequently filled by serpentine minerals the resulting pattern consists of a rim of serpentinite cut by serpentine veins that surrounds a core of peridotite measured densities of cross fractures in the rims of kernels are consistent with those expected from the magnitude of the volume change accompanying serpentinization of peridotite

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

deformation and surface uplift associated with serpentinization at mid ocean ridges and subduction zones 1 we employ the classical problem of an inclusion in an elastic half space to model effects of sub surface serpentinization on crustal deformation change of stress state and surface uplift at the tag hydrothermal field on the mid atlantic ridge the model suggests that an anomalous salient 3 km in diameter and 100 m high that projects 3 5 km westward from the east valley wall may have resulted from a relatively deep seated well serpentinized body exhibiting transformational strain the associated large strains would likely result in sub surface fracturing or faulting but surface uplift may be relatively insensitive to the exact depth and shape of the serpentinized region serpentinization of a region beneath the footwall of the tag detachment fault will tend to promote slip along some overlying normal faults which may then enhance fluid pathways to the deeper crust to continue the serpentinization process our solution for the miyazaki plain above the kyushu palau subduction zone in sw japan explains the observed uplift of 120 m the small transformational strains associated with serpentinization in this region may promote thrust type events in the aseismic slip zone near the upper boundary of the subducting philippine sea plate as well as intraplate earthquakes associated with normal faulting the rate of serpentinization needed to produce the observed uplift at the miyazaki plain is significantly greater than that needed at tag though significantly smaller on per unit volume basis thermal effects of serpentinization in both regions appear to be small but uplift data provide an additional constraint on inferring serpentinization from geological and seismological observations

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

anatomy of an intruded coal i effect of contact metamorphism on whole coal geochemistry springfield no 5 pennsylvanian coal illinois basin abstract if time and heating rate are important agents in coal maturation one might expect to see differences in chemical changes in response to maturation depending on the means of increased rank using a suite of samples obtained from an intruded pennsylvanian age coal in southern illinois we present whole coal chemical data comparing these data to extant geochemical data for coals that have undergone normal burial maturation we evaluated the hypothesis that if coal alteration occurs rapidly due to intrusion rather than gradually burial maturation then different relationships are seen in chemical composition proximate and ultimate analyses and vitrinite reflectance the pennsylvanian age asturian westphalian d springfield no 5 coal is mined at the big ridge mine near eldorado southern illinois this high volatile b bituminous coal was intruded by an ultramafic igneous intrusion during the early permian alteration occurs out to 1 2 dike thickness and includes an increase in random vitrinite reflectance rm from levels 0 7 to over 5 3 loss of liptinites and formation of devolatilization vacuoles and fine mosaic texture decreases in volatile matter vm and increases in fixed carbon fc appear to be less than would be expected for the level of reflectance seen within the alteration halo carbonate minerals have a major influence on proximate analyses but even following the removal of carbonates the decrease in vm is still less than would be seen in coals of similar vitrinite reflectance that were altered by normal burial maturation carbonate mineralization also contributes to variability in ultimate analysis values approaching the intrusion particularly for c and o after carbonate removal data for these coals do not appear to follow the normal burial coalification tracks when plotted on a van krevelen diagram and on a seyler chart these differences suggest that a slightly different maturation pathway may have been followed by these coals and by others that were altered rapidly by intrusion

faulting patterns near 19 30 s on the east pacific rise fault formation and growth at a superfast spreading center 1 structural mapping from high resolution sonar and bathymetric data collected near 19 30 s on the southern east pacific rise epr reveals multiple packets of small closely spaced outward dipping faults that form antithetic to larger inward dipping faults the mean spacing and height of inward dipping scarps are 1 6 and 1 9 times greater than those of outward dipping faults and the median spacing and height of inward dipping scarps are 2 0 and 1 7 times greater respectively accordingly inward dipping faults are less abundant 35 45 but accommodate more 50 55 of the roughly 4 0 strain although previous studies have shown that epr fault populations exhibit exponential size frequency distributions one dimensional throw and spacing data from the 19 30 s region are not described well by this model the first major 40 m scarp height fault is observed within 1 4 km of the ridge crest and invariably dips inward further off axis a transition from greater inward dipping to greater outward dipping fault abundance constrains the minimum half width of the active plate boundary zone to be 5 15 km inspection of additional data sets from 16 and 18 30 s indicates a pattern of large inward dipping faults with associated hanging wall deformation similar to that observed in the 19 30 s region this pattern is distinct from the horst and graben morphology observed along the fast spreading northern epr the pervasive hanging wall deformation observed on the flanks of the southern epr suggests master fault geometries that are more sharply curved or kinked at depth relative to their abyssal hill equivalents on the northern epr boundary element modeling indicates the pattern and geometry of faulting observed along the southern epr is consistent with fault nucleation above a shallow sill like magma chamber with internal overpressure since the influence of the sill on fault growth is strongly dependent on the depth of the sill the different patterns of faulting observed along the northern and southern epr may reflect the small difference in axial magma chamber depth between these two regions with a slightly shallower chamber beneath the southern epr

lithospheric stresses associated with nontransform offsets of the mid atlantic ridge implications from a finite element analysis nontransform offsets are a fundamental aspect of the offset geometry exhibited along the mid oceanic ridge system independent of spreading rate along the slow intermediate opening 40 mm y full rate mid atlantic ridge these offsets of the ridge axis range in length from less than 10 km to approximately 30 km and vary in age offset from 0 5 to 2 0 m y the variable morphotectonic geometries associated with these discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike slip tectonism and that the geometries are unstable in time in many cases there appears to be an evolutionary relationship between transform fault boundaries and nontransform offsets as the result of prolonged differential asymmetric spreading between adjoining ridge segments the finite element method is used to study the complex stress field associated with these small offset discontinuities of ridges with slow 30 mm y and fast 100 mm y total opening rates a plane stress plate model examines the variation in the horizontal tectonic stress field produced by offsets with different lengths and changes in the ratio of a ridge normal tensile stress resisting plate separation to a shear stress resisting relative plate motion along the discontinuity the predicted fault patterns based on the calculated stress field are compared with seafloor observations in terms of the morphotectonic patterns and evolution of nontransform offsets for a slow spreading rate the analysis shows that all structural geometries observed can be modeled by a range of offset lengths 5 10 20 30 and 40 km and by a ridge normal stress 3 to 5 times greater than the discontinuity shear stress these findings suggest that nontransform offsets are zones of mechanical weakness relative to the surrounding lithosphere an offset length between 10 and 20 km is predicted to be the threshold length for maintaining a transform fault geometry as inferred from ridge axis morphology there seems to be a strong link between the magnitude of the stress ratio and the time varying magmatic activity along and between ridge segments while our models are consistent with a weak discontinuity shear stress relative to the ridge normal stress to explain the geometries of nontransform offsets of slow spreading centers a weaker ridge normal stress to discontinuity shear stress most closely models the development of an overlapping spreading center geometry the distinctive geometry of nontransform offsets of spreading centers opening at fast rates this difference is attributed to magma supply along axis relatively continuous for fast spreading centers and intermittent for slow spreading centers and a preexisting zone of mechanical weakness linked to the evolution of nontransform offsets from transform faults on slow spreading centers

the role of density in the accumulation of basaltic melts at mid ocean ridges it is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust at the east pacific rise this neutral buoyancy level lies only 100 400 m below the sea floor significantly shallower than the depths 1 2 km of the magma bodies imaged in multichannel reflection data suggesting that other factors must control the collection of melt in these reservoirs the apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis not the buoyancy of melt is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid ocean ridges

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

kinematics of the north american african plate boundary between 28 and 29 n during the last 10 ma evolution of the axial geometry and spreading rate and direction abstract a high resolution survey sara segmentation ancienne de la ride atlantique collected sea beam bathymetry magnetic gravity and seismic reflection data on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to complement a portion of an on axis study carried out between the atlantis and kane transform faults which revealed the configuration of the spreading axis the sara survey extends this coverage to an age of approximately 10 ma on either flank a sinuous isochron pattern was established from anomaly identification made using a two dimensional model the plate tectonic reconstruction based solely on this isochron pattern provides an accurate kinematic description of the relative motion between the african and north american plates within the zone our results confirmed by data from the famous area and a flowline comparison with the trend of the kane fracture zone show that a change in axial orientation spreading direction and spreading rate occurred at approximately anomaly 4 time 7 01 ma this was coincident with the appearance of a new axial segment all the axial segments observed have changed length continously during the last 10 ma by advancing or retreating at one or both ends in any combination so as to leave non parallel trails on the flanks that do not align with either the spreading direction or with the absolute plate motion factors contributing to the sinuous magnetic anomaly pattern are discussed

a method for determining significant structures in a cloud of earthquakes the interpretation of a cloud of earthquake hypocenters in terms of causative structures is not a simple task locations are subject to uncertainties which will not be the same for every earthquake the data should therefore not be interpreted simply by inspection which is difficult in the case of three dimensional data anyway instead we propose using the location uncertainties as a guide in processing the data earthquake locations are moved inside their uncertainty or confidence ellipsoids until a simplified picture of the earthquake cloud is obtained which can then be interpreted in terms of some simplified structure such as faults the aim of the approach is to give the simplest possible structure that is consistent with all the location and confidence ellipsoid data the method is applied to three synthetic sets of data these illustrate the potential and limitations of the method application to a real earthquake data set from rabaul caldera in papua new guinea gives an image of the caldera ring fault that suggests departures from the simple ring fault structure previously assumed sensitivity analysis on the rabaul data shows that the method is not unduly sensitive to the assumptions that have to be made in applying it

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

small scale earthquake mechanisms induced by fluid injection at the enhanced geothermal system reservoir soultz alsace in 2003 using alternative source models the geothermal reservoir at soultz sous forets is a valuable natural laboratory for understanding the mechanisms of microearthquakes generated during stimulations and circulation tests an ongoing effort currently exists regarding the retrieval of mechanisms aimed to indicate the type of fracturing of the rock massif as a default a moment tensor description has been applied nevertheless the retrieval of the mode of fracturing still remains ambiguous recent studies indicate a prevailing shear slip but rarely a non shear pattern has also been observed the moment tensor used today as a universal tool for descriptions of the mechanism captures general balanced dipole sources however in the case of small scale earthquakes the moment tensor need not always be reliably determined in an effort to fit the data there may be notable non shear components caused by the low quality of input data constraining the source model to directly determine a simpler one is convenient for describing the physical phenomena expected for a particular focus an opening of new fractures can be described to a first approximation by a tensile crack optionally combined with a shear slip such an alternative model is called a shear tensile crack stc source model the combination is practical and can be used to both identify events that reflect purely mode i tensile failure and to determine the dilation angle of the fracture undergoing shear the latter is particularly important in enhanced geothermal system reservoirs such as soultz where shear related dilation is believed to be the primary mechanism underpinning permeability creation during stimulation injections we performed a synthetic case study by simulating seismic data as recorded by the actual seismic array installed at soultz sous forets synthetic p and s amplitudes for several shear tensile source models were inverted for several types of station coverage the analysis explored how results were influenced by mislocation mismodeling and noise contamination of data in most cases the orientation of the mechanism was well resolved determination of shear vs non shear content within the mechanism was more difficult from all of the factors influencing resolution that we explored the quality of the monitoring system the number of stations and their distribution with respect to the focus and noise contamination were of the highest impact the stc source model yielded considerably less spurious non shear fracture components than the moment tensor from the bulk of the seismicity recorded during the stimulation in 2003 we concentrated on the first phase of the injection when only a single borehole at the site was stimulated we processed thirteen small earthquakes with magnitudes larger than 1 4 that were not treated in previous studies we determined that their source mechanisms were dominantly pure shear slips on pre existing faults just as the earthquakes investigated earlier were the results were also in agreement with the stress pattern from in situ measurements

heterogeneous and asymmetric crustal accretion new constraints from multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 36 15 n at slow spreading mid ocean ridges crustal accretion style can vary significantly along and across ridge segments in magma poor regions seafloor spreading can be accommodated largely by tectonic processes however the internal structure and formation mechanism of such highly tectonized crust are not fully understood we analyze multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 35 40 n 36 40 n a section of the ridge that shows diverse accretion styles we identify volcanic tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain t and the fraction of magmatic accretion m estimated t values range from 0 2 0 4 on ridge segments to 0 6 0 8 at the rainbow nontransform discontinuity ntd at segment ends t is asymmetric reflecting asymmetries in accretion rate topography and faulting between inside and outside offset corners detachment faults have formed preferentially at inside corners where tectonic strain is higher we identify at least two oceanic core complexes on the fossil trace of the ntd in addition to the rainbow massif which occupies the offset today a gravity high and low magnetization suggest that the rainbow massif which hosts a high temperature hydrothermal system was uplifted by a west dipping detachment fault asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis which may have caused emplacement of magma into the footwall hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep penetrating second generation faults

hydrography and high temperature heat flux of the rainbow hydrothermal site 36 14 n mid atlantic ridge on the basis of an extensive set of conductivity temperature depth lowered acoustic doppler current profiler ladcp and nephelometry profiles and tow yos the hydrography flow field and particle plume of the rainbow hydrothermal site on the mid atlantic ridge are analyzed in the rift valley the water column is less dense and stratified than both eastern and western off ridge water with t s characteristics consistent with inflow across a sill from the east the buoyant hydrothermal plumes rise into a strong boundary current flowing along the slope of a topographic high which partially blocks the rift valley below 1950 m the bulk of the neutrally buoyant plume is advected across a sill which forms both the narrowest and the shallowest part of the valley and acts as a hydraulic control point for the flow below 2000 m large amplitude internal waves consistent with tidal forcing are observed near the sill but ladcp measurements suggest that the tidal signal is not strong enough to lead to flow reversal at plume depth above 2000 m the mean current across the topography generates lee waves which radiate energy upward and downstream density averaged light scattering profiles show the hydrothermal particle plume to be gaussian in depth even in the near field where many of the individual profiles are characterized by multiple peaks and the horizontal variability is highest the temperature anomalies associated with the mean near source particle plume are of order 5 10 3 c that is the plume is cold fresh as expected from the background hydrography of the deep atlantic using the flow field light scattering and hydrographic anomaly observations the heat flux associated with the hydrothermal particle plume at rainbow is estimated to lie between 1 and 5 gw

black smokers fuelled by freezing magma if ocean floor hot spring activity is to continue long enough to produce a large massive sulphide deposit heat must be stripped by the circulating water from a store within the ocean crust we argue here that this heat store can only be a magma chamber and that production of a large sulphide deposit requires very rapid crystallization of large volumes of magma 1 km3 per 100 yr for several hundred years the effects of such rapid solidification should be apparent in the gabbroic components of ophiolite complexes as well as in plutons in other environments where large high temperature massive sulphides are formed

seismicity of the atlantis massif detachment fault 30 n at the mid atlantic ridge 1 at the oceanic core complex that forms the atlantis massif at 30 n on the mid atlantic ridge slip along the detachment fault for the last 1 5 2 ma has brought lower crust and mantle rocks to the seafloor hydroacoustic data collected between 1999 and 2003 suggest that seismicity occurred near the top of the massif mostly on the southeastern section while detected seismicity along the adjacent ridge axis was sparse in 2005 five short period ocean bottom seismographs obs were deployed on and around the massif as a pilot experiment to help constrain the distribution of seismicity in this region analysis of six months of obs data indicates that in contrast to the results of the earlier hydroacoustic study the vast majority of the seismicity is located within the axial valley during the obs deployment and within the array seismicity was primarily composed of a relatively constant background rate and two large aftershock sequences that included 5 teleseismic events with magnitudes between 4 0 and 4 5 the aftershock sequences were located on the western side of the axial valley adjacent to the atlantis massif and close to the ridge transform intersection they follow omori s law and constitute more than half of the detected earthquakes the obs data also indicate a low but persistent level of seismicity associated with active faulting within the atlantis massif in the same region as the hydroacoustically detected seismicity within the massif the data indicate a north south striking normal fault and a left lateral strike slip fault near a prominent transform parallel north facing scarp both features could be explained by changes in the stress field at the inside corner associated with weak coupling on the atlantis transform alternatively the normal faulting within the massif might indicate deformation of the detachment surface as it rolls over to near horizontal from an initial dip of about 60 beneath the axis and the strike slip events may indicate transform parallel movement on adjacent detachment surfaces

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

seismic strain rates in regions of distributed continental deformation we compare the rates and configurations of seismic deformation determined from moment tensor mechanisms of earthquakes in five continental regions with the rates and configurations expected if the seismicity reflected the relative plate motions in the regions the five regions the aegean iran california western new guinea and central asia are characterized by high levels of seismicity distributed over hundreds to thousands of kilometers from the accepted plate boundaries the earthquake mechanisms are compiled primarily from the catalog of centroid moment tensor solutions which is nearly complete over the interval 1977 1987 for earthquakes greater than ms 5 the thickness of the seismogenic layer in each region is constrained by the results of modeling broadband body wave phases and of previous earthquake studies in the aegean california and western new guinea the observed configurations of deformation agree with a simple model of distributed accommodation of relative plate motion in iran and central asia the moment tensors indicate that there is appreciable strain parallel to the strikes of the belts we augment our 10 year estimates of seismic deformation with the available historical moment and magnitude estimates for the five regions and find that moderate and large earthquakes account for between 25 and 70 of the deformation of the seismogenic layer

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