evidence for microbes in early neoproterozoic stromatolites abstract the biogenesis of stromatolites has long been discussed since many lack convincing evidence of their biological origin this has particularly been the case with older precambrian examples where widespread diagenetic and metamorphic processes have commonly destroyed microbial relics in some cases however evidence has been preserved through very early mineralisation precambrian stromatolites are relatively abundant but it is extremely rare that they contain calcified or dolomitized cyanobacteria and biofilms this study provides evidence from early neoproterozoic columnar stromatolites in the jiuliqiao formation from the southeastern margin of the north china platform for fossilized extracellular polymeric substances eps and cyanobacteria along with nanospheres which may represent permineralised viruses or viral like particles a similar chemical composition and colour along with their embedded and cross cutting relationships with the matrix suggest they are syn depositional products and not modern artefacts or the result of sample preparation the interweaving of cyanobacterial filaments with detrital grains such as quartz and intraclasts indicates trapping baffling and binding processes the presence of virus like particles and mineralized eps with anhedral fine grained calcite suggests they may have provided initial nucleation sites for subsequent carbonate precipitation this study demonstrates the use of a high definition scanning electron microscope to detect fossilized calcified filamentous and spheroidal bacteriomorphs within ancient stromatolites in order to confirm their biogenesis

microstructural changes accompanying the opal a to opal ct transition new evidence from the siliceous sinters of geysir haukadalur iceland opaline silica opal a has formed in marine lacustrine and geothermal environments throughout geological time during diagenesis opal a normally changes to opal ct then opal c and finally to quartz such changes commonly destroy the original fabrics and any fossils that opal a contained the physical changes that accompany the opal a to opal ct transition however are known poorly x ray diffraction analyses electron microprobe analyses and high resolution high magnification scanning electron microscope imagery of siliceous sinters from the geysir geothermal area in iceland show that opal a is formed of heterometric arrays of randomly packed microspheres up to 5 m diameter with neighbouring spheres commonly being joined by small connection pads in contrast enlarged spheres lepispheres inverse opal two types and spindle frameworks with hexagonal motifs characterize opal ct the textures in opal ct which vary on a microscale reflect the complex interplay between dissolution e g inverse opal and precipitation e g enlarged spheres spindle frameworks that probably was mediated by groundwater in a near surface environment the processes deciphered from these young rocks should however be applicable to sedimentary opal a and opal ct of all ages irrespective of their origin

the mar vema fracture zone intersection surveyed by deep submersible nautile one of the two objectives of the vemanaute cruise of the french deep submersible nautile was the geological study of the eastern intersection area between the mid atlantic ridge mar and the vema fracture zone in the equatorial atlantic fourteen dives were conducted that allowed detailed geological survey and sampling of the main morphostructural units of this area the northern and southern walls of the fracture zone the median ridge the northern and southern troughs and the nodal basin in situ observations of recent tectonic features such as furrows ridges and circular depressions concentrated within the southern trough allowed us to establish the location and the size of the present day displacement zone geological investigations have shown that the nodal basin is entirely floored by basalts thus contrasting with other equivalent areas such as the kane and oceanographer fracture zone mar eastern intersections finally this study stresses the great opposition between the relatively old and tectonically inactive northern part of the fracture and the southern part which shows active tectonics and recent volcanic activity

morphology and tectonics of the kane transform from sea beam bathymetry data high resolution sea beam data have been used to construct an accurate regional scale bathymetric map of the kane transform a large offset slowly slipping 30 mm yr central north atlantic transform the kane transform trends about 100 nearly orthogonal to the regional trend of the adjacent mid atlantic ridge mar rift valley the transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the mar the ridges narrow and deepen toward the transform valley over distances of 10 20 km and the intervening troughs widen into distinct closed contour depressions that appear to be relict nodal basins the morphologic elements found on either side of the transform valley exhibit along strike continuity and represent undisrupted displacement markers the swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins and ridges that integrate to create a distinctive transform parallel grain this interval the transform tectonized zone ttz marks the time averaged location of strike slip tectonism as parcels of crust move from each intersection along the transform interface the ttz is widest 6 km near the eastern ridge transform intersection rti and narrows to a strand less than 3 km wide along the central and western portions of the transform along the eastern portion of the transform valley a narrow 3 km median ridge several hundred meters high can be traced for more than 40 km along the valley floor and a similar but shorter feature is also present near the western rti at both rtis the neovolcanic zone from the adjacent rift valley can be traced as a series of constructional edifices across the entire intersection area to the opposing transform valley wall at the eastern rti the neovolcanic zone wraps around the eastern edge of the nodal basin while at the western rti it appears to bisect a paired nodal basin located in the intersection area a line drawn connecting the ends of each neovolcanic zone lies within the narrow ttz and has a strike of 098 using the bathymetric map of the kane transform as a record of the time integrated tectonic evolution of this large slowly slipping ridge transform ridge plate boundary we have constructed a simple model for the evolution of rtis our model emphasizes the interplay between temporal variations in crustal production rates i e variations in the volume of basaltic melt production along oceanic spreading centers and extensional tectonism this model can explain the existence of nodal basins at rtis and relict nodal basins along transforms and fracture zones as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks

do lower crustal rocks record reversals of the earth s magnetic field magnetic petrology of oceanic gabbros from ocean drilling program hole 735b ocean drilling program odp leg 118 at the southwest indian ridge recovered the first vertically oriented continuously cored gabbros from in situ ocean crust magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the earth s magnetic field primary igneous magnetite is common only in evolved fe ti oxide rich gabbros and averages by volume 2 of the rock although substantially higher contents are common secondary magnetite produced by high temperature exsolution and hydrous alteration of olivine and pyroxene is observed in all gabbros studied and is the most important magnetic phase in the majority of recovered gabbros cumulate olivine gabbros because of the high temperatures associated with this alteration it is likely that the primary magnetization of this crustal section was a thermal remanent magnetization or a very high temperature chemical remanent magnetization acquired very near the ridge crest hysteresis loop parameters indicate that all of the studied samples are capable of carrying significant stable remanent magnetization finally previously published drilling results and field studies from odp site 735b indicate that although this crustal section has been tectonically uplifted it formed at a mid ocean ridge spreading center similar to much of the oceanic crust therefore the results of this magnetic study can be extrapolated to include the oceanic gabbro section in general and argue strongly that layer 3 crustal rocks are capable of recording reversals of the earth s magnetic field

horizontal extent of anomalously thin crust near the vema fracture zone from the three dimensional analysis of gravity anomalies recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust with total crustal thickness less than half that of normal oceanic crust the horizontal extent of this anomalous crust is not well known to map the lateral distribution of thin crust in a major fracture zone we have constructed free air anomaly bathymetry and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the vema fracture zone with the mid allantic ridge we isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water crust interface the sediment layer and the crust mantle interface assuming constant crustal thickness and density these calculations are performed using a fully three dimensional model the remaining mantle bouguer anomaly is dominated by the effects of the cooling lithosphere variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three dimensional conduction of heat the attraction of this three dimensional density structure is subtracted from the mantle bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density the residual gravity anomaly map indicates that there is thin crust along much of the length of the vema fracture zone thin crust is concentrated primarily beneath the walls of the fracture zone rather than beneath the center of the fracture zone valley an interpretation confirmed by seismic refraction data the degree of thinning along the length of the fracture zone is variable ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km the excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust other major topographic features such as the median valley and the elevated transverse ridge along the south wall of the fracture zone are also not locally compensated by changes in crustal thickness

migrating transform zone and lithospheric transfer at the galapagos 95 5 w propagator at the 95 5 w propagator system along the cocos nazca plate boundary the spreading axis is offset left laterally about 35 km 1 m y this offset migrates westward 52 km m y relative to points along the nearly east west trending ridge axis transferring lithosphere from the cocos to the nazca plate as it does so the required 58 km m y of right lateral motion across this migrating transform is apparently not accommodated by a throughgoing north south transform fault but rather by an approximately 15 to 20 km wide zone of shear deformation dominated by eastward curving of east west structures to a northwest southeast trend this pattern of curvature the coinciding zone of high seismicity and the abundance of talus in the area suggest that shear is distributed over this entire zone the deformation is inferred to be taken up by a bookshelf mechanism whereby faults oriented at high angles to the shear couple have left lateral slip antithetic to the right lateral shear couple rotating individual blocks clockwise these are interpreted as reactivated abyssal hill bounding normal faults probably dipping 45 60 northeast southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of de steiguer ridge in the zone of transferred lithosphere these faults have apparently had a very complex history initially normal faults formed as abyssal hills were created at the spreading axis and subsequently they had left lateral strike slip motion in the transform zone and reverse motion uplifting de steiguer ridge strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires 0 4 m y

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

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

unusual sea floor fabric near the bullard fracture zone imaged by gloria sidescan sonar the tectonic fabric of the sea floor created at spreading ridges generally curves in the direction of ridge offset near transform faults owing to the influence of shearing stress on transform faults here we present new gloria sidescan sonar images from the south american antarctic ridge showing fabric to the north of the bullard transform with a reversed sense of curvature this curvature and that observed in an area of older sea floor near the eastern ridge transform intersection appears to result from post emplacement deformation caused by the transmission of shear stresses across transforms the deformation was probably related to a major early miocene change in spreading direction which would have imposed a component of compression on the transforms such a sense of curvature has not to our knowledge been observed elsewhere on the deep ocean floor although apparently similar patterns have been reported in ophiolites

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

patterns of stress at midocean ridges and their offsets due to seafloor subsidence the effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two dimensional 2d horizontal stresses arising from isostatically compensated vertical loads the topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge parallel tensile stresses these may predominate over the ridge perpendicular stresses and explain observations at midocean ridge offsets such as 1 oblique normal faulting at ridge transform intersections trending up to 60 relative to the ridge axis and 2 nontransform offsets consisting of structures oriented at 45 relative to the ridge trend at midocean ridge overlaps rotation of the ridge parallel tensile stresses favours rift propagation at more than 45 relative to the ridge trend it is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets and as a result large overlaps may eventually start to rotate and evolve into a microplate

3 43 billion year old stromatolite reef from the pilbara craton of western australia ecosystem scale insights to early life on earth abstract the 3 43 billion year old strelley pool chert pilbara craton western australia contains compelling evidence of early archaean life in the form of kilometre sized remnants of an ancient stromatolitic carbonate platform reviewing and building on earlier studies we examine the fossilized remains of the platform to seek ecosystem scale insights to earth s early biosphere examining the evidence for biosedimentation and the importance and effect of different environmental processes on biological activity both vertical and lateral trends show that stromatolite abundance and diversity are greatest in the area interpreted as an isolated partially restricted peritidal marine carbonate platform or reef where there is virtually no trace of hydrothermal or terrigenous clastic input in contrast stromatolites are poorly developed or absent among hydrothermal volcaniclastic or terrigenous clastic sedimentary facies and are absent in deeper marine settings that are laterally equivalent to shallow marine stromatolitic facies hydrothermal veins some of which were previously interpreted as vents that exhaled fluids from which the stromatolitic structures precipitated are shown to postdate the stromatolites on the platform stromatolite facies associations varied between different palaeoenvironments but some stromatolite types occurred across different palaeoenvironments highlighting the combined influence of biological and environmental processes on stromatolite formation the regional distribution of stromatolites in the palaeoenvironment suggests a biological response to variations in water depth sediment influx and hydrothermal activity with stromatolite formation favoured by relatively normal shallow marine environments with low clastic chemical sedimentation rates and no direct input from high temperature hydrothermal systems the lithology structure and fabrics of the stromatolites and their close association with abundant evaporite crystal pseudomorphs indicate that evaporitic precipitation was probably the dominant non biological process that contributed to stromatolite formation the study supports a biological interpretation for the origin of the stromatolites and reveals compelling evidence for the conditions that favoured biological activity on the early earth and formation of macroscopic biosignatures that could be preserved for most of earth s history

multibeam bathymetry and sidescan imaging of the rivera transform moctezuma spreading segment junction northern east pacific rise new constraints on rivera pacific relative plate motion to better understand the recent motion of the pacific plate relative to the rivera plate and to better define the limitations of the existing rivera pacific plate motion models for accurately predicting this motion total field magnetic data multibeam bathymetric data and sidescan sonar images were collected during the bart and famex campaigns of the n o l atalante conducted in april and may 2002 in the area surrounding the moctezuma spreading segment of the east pacific rise located offshore of manzanillo mexico at 106 16 w between 17 8 n and 18 5 n among the main results are 1 the principle transform displacement zone of the rivera transform is narrow and well defined east of 107o15 w and these azimuths should be used preferentially when deriving new plate motion models and 2 spreading rates along the moctezuma spreading segment should not be used in plate motion studies as either seafloor spreading has been accommodated at more than one location since the initiation of seafloor spreading in the area of the moctezuma spreading segment or this spreading center is not a rivera pacific plate boundary as has been previously assumed comparison of observed transform azimuths with those predicted by the best fit poles of six previous models of rivera pacific relative motion indicate that in the study area a significant systematic bias is present in the predictions of rivera pacific motion although the exact source of this bias remains unclear this bias indicates the need to derive a new rivera pacific relative plate motion model

regional linear cross strike discontinuities in western aphrodite terra venus western aphrodite terra venus is characterized by a series of parallel linear structural discontinuities 2000 4000 km in length and 100 200 km wide which strike at high angles to the general topographic trend of the highlands the detailed characteristics of these features are similar to terrestrial oceanic fracture zones and strike slip fault zones either interpretation implies the presence of large scale horizontal tectonic movement on venus

structural and geophysical expression of the solea graben troodos ophiolite cyprus field studies show that extensional structures in the north central part of the troodos ophiolite including steep normal faults grabens dikes and low angle detachment faults are related to e w present coordinates spreading the solea graben directly north of the mafic ultramafic rock outcrops around mount olympus consists of a 10 to 15 km wide western portion that is underlain by a low angle detachment fault between the sheeted dike complex and the plutonic section structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40 or more about subhorizontal axes the eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross cutting ophiolitic dikes and hydrothermal mineralization timing of the uplift of the central part of the troodos ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low angle normal faulting the solea graben spreading center provides a model for the structure of modern mid ocean ridges that are spreading amagmatically

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

lithospheric stress near a ridge transform intersection oceanic lithosphere at a ridge transform intersection has distinct structural characteristics the spreading axis and normal fault scarps which parallel the ridge axis away from the intersection locally curve towards the transform fault this implies that the least compressive stress direction becomes oblique to the spreading direction near a ridge transform intersection this stress field may be the result of a tensile ridge axis stress resisting plate separation and a transform shear stress resisting relative plate motion a plane stress plate model with prescribed plate boundary stresses is formulated to explore this hypothesis the orientation of the least compressive horizontal stress near an intersection depends strongly on the ratio of the applied boundary stresses thus ridge axis curvature may place an important constraint on plate boundary forces

investigation of spreading center evolution by joint inversion of seafloor magnetic anomaly and tectonic fabric data spreading center segments that have experienced a complex tectonic history including rift propagation may have a complicated signature in bathymetric and magnetic anomaly data to gain insight into the history of such regions we have developed techniques in which both the magnetic anomaly patterns and seafloor fabric trends are predicted theoretically and the combined predictions are compared numerically with the data to estimate best fitting parameters for the propagation history fitting functions are constructed to help determine which model best matches the digitized fabric and magnetic anomaly data such functions offer statistical criteria for choosing the best fit model we use this approach to resolve the propagation history of the cobb offset along the juan de fuca ridge in this example the magnetic anomaly data prove more useful in defining the geometry of the propagation events while the fabric with its greater temporal resolution is more useful for constraining the rate of propagation it thus appears that joint inversion of magnetic and seafloor fabric data can be valuable in tectonic analyses

a sea beam investigation of the clipperton transform fault the morphotectonic expression of a fast slipping transform boundary a regional scale bathymetric map of the fast slipping 11 0 cm yr clipperton transform boundary and adjoining east pacific rise segments has been constructed using the high resolution capabilities of sea beam the transform domain is 85 km long varies in width from 10 to 20 km and is defined by an assemblage of transform parallel ridges and troughs the largest of which is a 40 km long median ridge that stands up to 700 m above the surrounding seafloor these major topographic elements of the transform domain are bisected by a very narrow 1 km swath of strongly lineated terrain composed of elongate depressions narrow lozenge shaped ridges and gullies that integrate spatially to create a narrow zone of disturbed topography this zone trends 082 and cuts like a knife across the grain of the clipperton transform domain linking one truncated ridge tip with the other we interpret this swath of disturbed terrain to be the product of strike slip tectonism thereby fixing the location of the present day plate boundary the present day structural geometry of the clipperton transform appears relatively simple and characterized by a narrow strike slip plate boundary linking two offset ridge axes

in situ study of the eastern ridge transform intersection of the vema fracture zone abstract fourteen dives of the submersible nautile have been carried out at the eastern intersection of the vema fracture zone with the mid atlantic ridge direct observations allow us to locate precisely the surficial expression of the transform fault motion in the southern trough of the fracture zone valley the present day displacement zone corresponding to an extremely narrow less than 300 m transform fault zone is confined to a belt of disturbed terrain situated at the foot of the southern wall and characterized by transform parallel 30 100 m high ridges of basaltic fragments and pelagic sediments active splays of the transform fault zone lie generally between these ridges creating steep v shaped furrows despite a rather sinuous trend one of the fault strands appears to be laterally continuous and other minor active splays branch out from it or run parallel to it the surface manifestations of the tectonic activity along the present day displacement zone include r riedel shears low angle shears p shears en echelon structures open fissures small pull apart structures and funnel alignments in the sedimentary cover the transform fault motion reactivates large normal faults on the adjacent flanks of the southern wall and the median ridge the transform parallel topography defines a 10 km wide zone with dip slip motion the ridge transform intersection area is entirely floored by fresh basalts contrasting with other intersections of major fracture zones such as the kane and oceanographer fracture zones recent volcanic activity is not restricted to the central volcanic ridge but is widespread across the whole nodal basin area the present day displacement zone cross cuts the neovolcanic zone and the disconnected northernmost extension of the volcanic ridge is attached to the median ridge and so belongs to the african plate the extensive volcanism is suggestive of a magmatic production phase in the intersection area extremely fresh pillowed lava forms have been observed at the rti inside corner high indicating possible off axis volcanism

displacement transfer between intersecting regional strike slip and extensional fault systems interaction and displacement transfer between active intersecting strike slip or transform and extensional fault systems are examined outcrop data from a well preserved strike slip fault and rift intersection in new zealand are compared to a global data set of 13 such intersections in both continental and oceanic crust displacement transfer between strike slip and normal faults is typically accomplished by gradual changes of fault orientations and slip vectors close to the intersection zone for two and three plate configurations these changes result in sub parallelism of the slip vectors of the component faults with their line of intersection the dimensions of the area over which fault strike and slip vectors change are principally controlled by the extent to which displacements on the dominant of the two intersecting fault systems are confined to a single slip surface or distributed across a zone where slip is spatially distributed the region in which the two displacement fields are superimposed produces transtension and associated oblique slip this distributed off fault deformation facilitates the development of a quasi stable configuration of the fault intersection region maintaining both the regional geometry and kinematics of the intersection zone which in many cases would not be possible for rigid block translations the dimensions of the transition zone are larger for continental crust than for oceanic crust because oceanic crust is thinner fault geometries in oceanic crust are simpler two plate configurations and the slip vectors of the component intersecting fault systems are sub parallel

evidence for weak oceanic transform faults 1 we present the results of a series of 3 d boundary element calculations to investigate the effects of oceanic transform faults on stress state and fault development at adjacent mid ocean ridge spreading centers we find that the time averaged strength of transform faults is low and that on time scales longer than a typical earthquake cycle transform faults behave as zones of significant weakness specifically mechanical coupling of only 5 best explains the observed patterns of strike slip and oblique normal faulting near a ridge transform intersection on time scales shorter than a typical earthquake cycle transient locked periods can produce anomalous reverse faulting similar to that observed at the inside corner ic of several slow spreading ridge segments furthermore we predict that extensional stresses will be suppressed at the ic due to the shear along the transform resisting ridge normal extension this implies that an alternative mechanism is necessary to explain the preferential normal fault growth and enhanced microseismicity observed at many ics index terms 3035 marine geology and geophysics midocean ridge processes citation behn m d j lin and m t zuber evidence for weak oceanic transform faults geophys res lett 29 24 2207 doi 10 1029 2002gl015612 2002

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

structural and volcanic expression of a fast slipping ridge transform ridge plate boundary sea marc i and photographic surveys at the clipperton transform fault a high resolution near bottom survey has been conducted of the clipperton transform fault and adjoining segments of the east pacific rise epr using the sea marc i side looking sonar system and the lamont doherty geological observatory olympus based camera system the transform fault zone tfz is a narrow well defined belt of transform parallel lineaments which varies along strike from a single sharp edged notch to a complex band of subparallel lineaments up to 1 km wide the tfz is set within a 5 km wide band of unusually fine grained side scan texture which could indicate nonbasaltic seafloor and or pervasively sheared and mass wasted basaltic crust the fine grained swath is surrounded by constructional volcanic terrain with no hint of strike slip motion this observation puts an upper limit of 5 km on the extent of lateral migration of the tfz in the last 1 5 m y both ridge transform intersections rtis are dominated by bathymetric highs located on the old plate opposite the spreading center a mantling of fresh looking constructional volcanic terrain on side scan images suggests that the highs are built in part by recent extrusive and intrusive volcanism thermal expansion may also play a part the epr south of clipperton has recently experienced extrusion of high effusion rate basalts burial of faults and fissures by lava flows and development of vigorous hydrothermal circulation on the epr north of clipperton the axial zone of faults and fissures tapers toward the transform fault this may reflect a change in the shape or size of the underlying shallow level magma feeders as a function of distance from the site of magma upwelling or distance toward the transform fault

transform fault earthquakes in the north atlantic source mechanisms and depth of faulting we have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern mid atlantic ridge from an inversion of long period body waveforms the earthquakes occurred on the gibbs oceanographer hayes kane 15 20 and vema transforms we have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width for five of the transforms earthquake centroid depths lie in the range 7 10 km beneath the seafloor and the maximum depth of seismic faulting is 14 20 km on the basis of a comparison with a simple thermal model for transform faults this maximum depth of seismic behavior corresponds to a nominal temperature of 900 100 c in contrast the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike slip mechanisms is 700 100 c the difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments three large earthquakes on the 15 20 transform have shallower centroid depths of 4 5 km and a maximum depth of seismic faulting of 10 km corresponding to a limiting temperature of 600 c the shallower extent of seismic behavior along the 15 20 transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among north american south american and african plates to its present position near the transform the source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes slip vector azimuths agree to within 2 3 of the local strike of the zone of active faulting the only anomalies in mechanism were for two earthquakes near the western end of the vema transform which occurred on significantly nonvertical fault planes secondary faulting occurring either precursory to or near the end of the main episode of strike slip rupture was observed for five of the 12 earthquakes for three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform in all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone we find no evidence to support the conclusions of engeln wiens and stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere

temporal evolution of overlapping spreading centers at 16 20 n on the east pacific rise abstract a compilation of sea beam data on the right stepping overlapping spreading center osc at 16 20 n on the east pacific rise epr and in the surrounding region is presented sea beam data from the rise flanks at 16 20 n reveal three deep basins at least one of which is flanked by a curved ridge whose morphology is similar to that at the offset the widths of these offaxis basins range from 8 to 13 km and allow an estimate to be made of the sizes of the offsets at which they were formed it is proposed that they represent traces on the rise flank which provide a record of the development of the offset and a basis for prediction of its future evolution the repeated pattern of abandoned overlap basins and cuspate ridges supports the notion that oscs are episodic on a time scale of approximately 200 000 years and are cyclic in nature it also reveals an alternation of dominance between the southern and northern segments which have shifted the location of the discontinuity along the axis so as to create zig zag traces of basins on the ridge flanks

stromatolite reef from the early archaean era of australia the 3 430 million year old strelley pool chert spc pilbara craton australia is a sedimentary rock formation containing laminated structures of probable biological origin stromatolites determining the biogenicity of such ancient fossils is the subject of ongoing debate however many obstacles to interpretation of the fossils are overcome in the spc because of the broad extent excellent preservation and morphological variety of its stromatolitic outcrops which provide comprehensive palaeontological information on a scale exceeding other rocks of such age here we present a multi kilometre scale palaeontological and palaeoenvironmental study of the spc in which we identify seven stromatolite morphotypes many previously undiscovered in different parts of a peritidal carbonate platform we undertake the first morphotype specific analysis of the structures within their palaeoenvironment and refute contemporary abiogenic hypotheses for their formation finally we argue that the diversity complexity and environmental associations of the stromatolites describe patterns that in similar settings throughout earth s history reflect the presence of organisms

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

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

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

multibeam bathymetric gravity and magnetic studies over 79 e fracture zone central indian basin a regional scale bathymetric map has been constructed for the 79 e fracture zone fz in the central indian basin between 10 15 s and 14 45 s latitudes and 78 55 e and 79 20 e longitudes using the high resolution capabilities of the hydrosweep multibeam swath bathymetric survey system the bathymetric map clearly reveals the 79 e fz depicting a ridge and trough topography with an elevation difference of over 300 m prominent e w bathymetric lineations and several new features such as seamounts are conspicuous on the map the e w bathymetric lineations bend at the fracture zone toward south resulting in oblique extensional features these features are more prominent in the middle and northern portion of the study area and are less significant in the southern portion the prominence of these features in the north and their subtle presence in the south are probably related to the strong and weak shear coupling at the rti respectively during the various stages of fz evolution gravity and magnetic data were analyzed together with the bathymetric data the observed free air gravity across the 79 e fz is modeled in terms of the lithospheric thickness taking into consideration the thermal evolution of the oceanic lithosphere a lithosphere thickness of about 100 km is inferred from these studies

deep tow studies of the vema fracture zone 1 tectonics of a major slow slipping transform fault and its intersection with the mid atlantic ridge the vema transform fault which slips at a rate of 24 mm yr displaces the mid atlantic ridge mar 320 km in a left lateral sense high resolution deep tow studies of the vema ridge transform intersection rti and the eastern 130 km of the active transform fault reveal a complex pattern of dip slip and strike slip faults which evolve in time and space at the intersection both the neovolcanic zone and the west wall of the mar rift valley curve counterclockwise toward the transform fault along trends approximately 30 oblique to the regional north south trend of the spreading axis the curving of extensional structures in the rift valley such as normal faults and the axial zone of dike injection appears to be related to transmission of transform related shear stresses into the spreading center domain intermittent locking of the american and african lithospheric plates across the rti causes shear stresses to penetrate up to 4 km into the mar axial neovolcanic zone where the lithosphere is relatively thin and up to 12 km into the block faulted west wall of the rift valley where the lithosphere is thicker the degree of shear coupling across the rti may vary with time due to changes in the thickness of the lithosphere along the axis 0 10 km the strength of a mantle weld at depth and the presence or absence of an axial magma chamber so that extensional structures at the rti may be either spreading center parallel when coupling is weak or oblique when coupling is strong oblique extension across the rti in addition to other factors may account for some of the down dropping of lithosphere within the deep nodal basin the easternmost 20 km of the active transform fault zone near the rti displays a braided network of three to nine tectonically active grabens and v shaped furrows in a zone 2 4 km wide interpreted to consist of interwoven riedel shears p shears and oblique normal faults clay cake deformation experiments and deep tow observations suggest that p shears and r shears which are 10 20 oblique to the transform slip direction develop during the initial stages of transform faulting near the rti as the newly accreted lithosphere accelerates to full plate velocity some of the r shears propagate along strike and intercept the oblique normal faults resulting in sharply curving scarps at the rti subsequent to this merging of the two fault types some of the r shears develop a significant component of dip slip while other r shears merge with p shears creating a complex anastomosing fault pattern up to 4 km wide a continuous strand within this braided pattern of faults is interpreted to be the principal transform displacement zone near the rti twenty kilometers west of the rti the active transform fault zone narrows to a furrow generally less than 100 m wide with only a few short discontinuous splays this narrow groove cuts through thinly sedimented basalt 20 40 km west of the rti and continues as a narrow furrow less than 100 m wide through up to 1 5 km of layered turbidite fill most of the way to the western rti such a narrow zone of deformation typifies the mature stages of transform faulting where the lithosphere on both sides of the transform fault is relatively old thick and rigid and has completed its acceleration to full plate velocity the transform fault zone is closely associated with a partially buried median ridge and widens to 1 2 km where it transects exposed portions of the ridge the transform parallel median and transverse ridges create the highest topography associated with the transform fault and may be serpentinized ultramafic intrusions capped by displaced crustal blocks of gabbro metagabbro and basalt

filamentous fossil bacteria from the archean of western australia abstract four morphotypes of structurally preserved filamentous fossil bacteria have been discovered in petrographic thin sections of laminated carbonaceous cherts from the 3500 ma old warrawoona group of northwestern australia these tubular and septate microfossils are interpreted here as being syngenetic with warrawoona sedimentation as such they are apparently the oldest such fossils now known in the geological record the diversity of this assemblage and the evident complexity of its individual components suggest that the beginnings of life on earth may have appreciably pre dated the deposition of the warrawoona sediments

structure and seismotectonics of the vema fracture zone atlantic ocean the structure and seismo tectonics of the vema fracture zone and the adjacent regions of its transverse ridge to the south and the mid atlantic ridge spreading centre to the north is discussed based on an analysis of seismic refraction profiles micro earthquake activity and gravity anomalies crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers micro earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge transform intersection immature crust similar to that reported by fowler 1976 from the famous area lies beneath the spreading centre the lower crust thins and normal mantle appears as the fracture zone is approached the thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection this is also the locus of current microearthquake activity at sub bottom depths of 3 4 km which a composite fault plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley within the central region of the fracture zone valley the crust thickens but still retains an anomalous velocity structure seismic layer 3 is absent and crustal velocities are low this is probably the consequence of attenuated crustal accretion of faulting and cracking and of hydrothermal alteration the pattern of residual gravity anomalies after correction for topography for sediment thickness and for mantle temperature variations is consistent with the crustal structure defined by the seismic experiments the gravity measurements indicate that there is a broad thinning of the crust over 20 40 km in the vicinity of the fracture zone on which are superimposed localized regions of low and high densities to the south of the fracture zone the crust thickens and assumes a normal velocity structure including a mature seismic layer 3 the seismic moho is warped upwards under the transverse ridge suggesting a tectonic origin from buckling rather than uplift by serpentinization

controls on development and diversity of early archean stromatolites the 3 450 million year old strelley pool formation in western australia contains a reef like assembly of laminated sedimentary accretion structures stromatolites that have macroscale characteristics suggestive of biological influence however direct microscale evidence of biology namely organic microbial remains or biosedimentary fabrics has to date eluded discovery in the extensively recrystallized rocks recently identified outcrops with relatively good textural preservation record microscale evidence of primary sedimentary processes including some that indicate probable microbial mat formation furthermore we find relict fabrics and organic layers that covary with stromatolite morphology linking morphologic diversity to changes in sedimentation seafloor mineral precipitation and inferred microbial mat development thus the most direct and compelling signatures of life in the strelley pool formation are those observed at the microscopic scale by examining spatiotemporal changes in microscale characteristics it is possible not only to recognize the presence of probable microbial mats during stromatolite development but also to infer aspects of the biological inputs to stromatolite morphogenesis the persistence of an inferred biological signal through changing environmental circumstances and stromatolite types indicates that benthic microbial populations adapted to shifting environmental conditions in early oceans

closure of the africa eurasia north america plate motion circuit and tectonics of the gloria fault we examine the closure of the current plate motion circuit between the african north american and eurasian plates to test whether these plates are rigid and whether the gloria fault is an active transform fault we also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries and compare the predicted direction of motion along the african eurasian plate boundary in the mediterranean with the direction of slip observed in earthquakes from marine geophysical data we obtain 13 transform fault azimuths and 40 3 m y average spreading rates 34 of which are determined from comparison of synthetic magnetic anomaly profiles to 140 observed profiles slip vectors from 32 earthquake focal mechanisms further describe plate motion detailed magnetic surveys north of iceland provide 11 rates in a region where prior plate motion models had few data magnetic profiles north of the azores triple junction record a rate of 24 mm yr 4 mm yr slower than used by prior models gloria and sea beam surveys accurately measure the azimuths of seven transform faults our plate motion model fits six of the seven within 2 two transform faults surveyed by gloria side scan sonar lie near famous area transform faults a and b and give azimuths 13 clockwise of them because recent studies show that short offset transforms such as transforms a and b are in many places oblique to the direction of plate motion we exclude azimuths from transforms with less than 35 km offset the best fitting and closure enforced vectors fit the data well except for a small systematic misfit to the slip vectors on right lateral slipping transforms slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths whereas on left lateral slipping transforms slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths we search the long eurasia north america boundary for evidence of an additional plate but find no systematic misfits to the data in particular if a spitsbergen plate exists and moves relative to eurasia its motion is less than 3 mm yr an africa eurasia euler vector determined by adding the eurasia north america and africa north america euler vectors is consistent with the gloria fault trend and with slip vectors from eastern azores gibraltar ridge focal mechanisms a small circle centered at the africa eurasia closure enforced pole fits the trace of the gloria fault the model in which closure was enforced predicts 4 mm yr slip across the azores gibraltar ridge and west northwest convergence near gibraltar 45 more oblique than suggested by a recent model based on compressive axes of focal mechanisms moreover our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between gibraltar and sicily because closure enforced vectors fit the data nearly as well as the best fitting vectors we conclude that the data are consistent with a rigid plate model and with the gloria fault being a transform fault

stromatolite provinces of hamelin pool physiographic controls on stromatolites and associated lithofacies recent studies recognized distinct stromatolite provinces in hamelin pool western australia each with morphologically distinct stromatolite structures paired with characteristic shelf physiography in the present paper we couple detailed lithofacies mapping with hamelin pool bathymetry and consider physiography as a control of sedimentation processes including stromatolite development bathymetric transects derived from a high resolution bathymetry map with depths from 0 to 11 meters allow calculation of slope gradients in the provinces as in other settings bathymetry is linked to energy regimes which in turn appear to be coupled with variations in stromatolite morphologies and associated lithofacies as follows 1 low gradient ramps with low energy settings are associated with sheet mats and elongate clustered stromatolites that exhibit regular spatial patterns possibly indicative of self organization 2 low gradients coupled with high energy settings resulting from strong winds result in seif stromatolites with pronounced directional bands 3 medium to steep gradients coupled with medium to high energy are associated with individual and merged stromatolites often with thin basal necks 4 headlands and promontories where the topography deflects currents are associated with elongate nested stromatolites and 5 medium to high energy slopes typically found at promontory edges and shelf margins are dominated by blocky pavement observations linking stromatolite morphology to physiography in a modern microbial system provide insight into the long lived debate about biology versus environment in controlling stromatolite morphology when physiography leads to a high energy regime environmental controls are the main factor determining stromatolite morphology in contrast when physiography promotes a low energy environment the response of biological communities becomes the main driver of macroscale stromatolite morphology

oscillatory spreading explanation of anomalously old uplifted crust near oceanic transforms rocks significantly older than their theoretically predicted age have been recovered in the vicinity of some atlantic transform zones rocks of palaeocene 55 58 myr and possibly even of cretaceous age are found near the vema transform on tectonically uplifted crust which should not be older than 30 myr we present here a model which might explain the presence of uplifted anomalously old crust near the vema transform zone tz the proposed model involves axial rift propagation and reorientation and migration of the transform as a result of a change in the regional stress field during transform migration a crustal wedge formerly part of the african plate is transferred to the south american plate the ensuing oscillatory spreading can explain the anomalous age of the uplifted crustal block compression resulting from rift propagation and transform migration provides a mechanism for crustal uplift

tectonic evolution of the troodos ophiolite within the tethyan framework a new tectonic model reconciles conflicting structural and geochemical evidence for the origin of the troodos ophiolite a well preserved remnant of neotethyan oceanic crust grabens and normal faults within the sheeted dike complex and the extrusive sequence of the troodos ophiolite resemble those of oceanic spreading centers diverse intrusive and tectonic contact relationships between the sheeted dike complex and the underlying plutonic sequence indicate multiple and episodic intrusion of magma and along and across strike variation in volcanic and tectonic activity during development of oceanic crust coupled with the existence of the arakapas transform fault to the south these structural and intrusive relationships suggest origin at an intersection between a spreading center and a transform fault the arclike chemistry of sheeted dikes and related extrusive rocks and the inferred highly depleted and hydrous nature of the mantle source of the late stage intrusive and extrusive rocks argue however for generation of part of the ophiolite within a subduction zone environment regional reconstructions suggest that the mesozoic neotethys may have evolved as a marginal basin both to the afro arabian continent and the paleotethyan ocean over an active or recently active south dipping subduction zone the troodos ophiolite and other eastern mediterranean ophiolites whose magma compositions were affected by the subducted paleotethyan slab may have formed along east west trending spreading centers separated by north south trending transform faults within this marginal basin a rapid change in relative plate motion in late cretaceous time between eurasia and afro arabia created a regional compressive regime that may have resulted in plate boundary reorganizations within the neotethyan realm and in initiation of north dipping subduction zone s beneath the troodos and other ophiolites in the region the apparent forearc setting of the troodos ophiolite is a consequence of this intraoceanic displacement after its formation and thus is unrelated to its generation

bottom water observations in the vema fracture zone the vema fracture zone trough at 11 n between 41 and 45 e is open to the west at the 5000 m level but is silled at the 4650 m level on the east where it intersects the axis of the mid atlantic ridge the trough is filled with antarctic bottom water aabw with a potential temperature of 1 32 c and salinity of 34 82 ppt the bottom water is thermally well mixed in a nearly homogeneous layer about 700 m thick the great thickness of this bottom layer as compared with the bottom water structure of the western atlantic basin may result from enhanced mixing induced by topographic constriction at the west end of the fracture zone trough a benthic thermocline with potential temperature gradients of about 1 2 mdeg m 1 is associated with an abrupt increase in turbidity with depth at about 1200 m above bottom a transitional layer of more moderate temperature gradients about 0 4 mdeg m 1 lies between the benthic thermocline above and the aabw below the aabw layer whose depth averaged suspended paniculate concentrations range from 8 to 19 g l 1 is consistently higher in turbidity than the overlying waters at the eastern end of the trough 140 m below sill depth very low northeastward current velocities with maximums of 3 cm s 1 were recorded for an 11 day period

constructing the upper crust of the mid atlantic ridge a reinterpretation based on the puna ridge kilauea volcano the volcanic morphology of a number of segments of the slow spreading mid atlantic ridge mar have been reinterpreted based on our understanding of dike emplacement dike propagation and eruption at the east rift zone of kilauea volcano hawaii and its submarine extension the puna ridge the styles of volcanic eruption at the submarine puna ridge are remarkably similar to those of the axial volcanic ridges avrs constructed on the median valley floor of the mar we use this observation to relate volcanic processes occurring at kilauea volcano to the mar we now consider that volcanic features e g seamounts and lava terraces built on the flanks of the avrs are secondary features that are fed from lava tubes or channels not primary features fed directly from an underlying dike we examine simple models of pipe flow and conclude that lava tubes can transport lava down the flanks of submarine rifts to build all of the volcanic features observed there in addition deep water lava tubes are strong enough to withstand the pressures of a few megapascals that the building of a volcanic structure 150 m high at the end of the tube would generate the volumes of individual volcanic terraces and seamounts on the puna ridge and at the mar are large 0 1 1 km3 and similar to the volumes of lava flows that are broadly distributed at the subaerial east rift zone of kilauea this striking difference in the volcanic morphology on a scale of 1 2 km producing terraces and seamounts underwater and low relief flows on land must be related to the enhanced cooling and to the greater mechanical stability of tubes in the submarine environment we suggest that at the mar a crustal magma reservoir most likely located beneath shallow flat sections of the segment provides magma to the rift axis through dikes that propagate laterally tens of kilometers the zone of dike intrusion at least in the neighborhood of the magma body is likely narrower than the width resurfaced by flows yielding a crustal structure that has a rapid vertical transition from lavas to sheeted dikes at segment ends the zone of dike intrusion is likely to be wider giving a resulting structure with a more gradual transition from lavas to dikes

vema fracture zone central atlantic tectonic and magmatic evolution of the median ridge and the eastern ridge transform intersection domain the eastern vema fracture zone intersection with the mid atlantic ridge mar axis was surveyed with the french submersible nautile during the vemanaute cruise in 1989 at this ridge transform intersection rti an elongated e w ridge more than 50 km long is present in the transform valley this median ridge rises up to 1000 m above the surrounding seafloor the crest of the median ridge is lower and presents an arcuate shape at the tip of the mar axis dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation this detrital cover consists of polymictic sedimentary breccias sandstones and siltstones composed of basaltic doleritic and gabbroic clasts with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation disaggregation mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls this event could be related to the uplift of the southern wall of the fracture zone the transverse ridge which took place probably between 10 and 3 ma ago since this uplift episode the transverse ridge which is now undergoing subsidence and the detritus covered transform valley floor separated by the transform fault zone have migrated westward and eastward respectively vertical tectonics of the median ridge at the approach of the rti can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors a possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western rti is more than 1 km out of isostatic equilibrium recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the mar axis have been recorded on the southern flank of the median ridge several stages in the very recent tectonic volcanic history of the eastern rti that is roughly during the last 300 000 years can thus be defined the lower elevation and narrow arcuate shape of the median ridge east of 40 57 w are inferred to have resulted from tectonic extension during the creation of the nodal basin

evolution of migrating transform faults in anisotropic oceanic crust examples from iceland major transform fault zones link extensional segments of the north american eurasian plate boundary as it transects the iceland hotspot changes in plate boundary geometry involving ridge jumps

axial topographic relief associated with ridge transform intersections the axial depths of mid ocean ridges are consistently observed to increase along strike from the central part of a ridge segment to its intersection with a transform fault variations in the magnitude of this axial relief probably reflect variations in the magmatic and tectonic processes that characterize plate accretion and seafloor spreading at each individual segment constrained by observations from 41 ridge transform intersections we investigate the relative effect on axial depths of transform offset segment length regional depth and spreading rate we first solve for a model of ridge segment relief as a function of the age offset of the transform and the segment length both scaled by spreading rate through a series of regression analyses covering a range of spreading rate scalings we determine a model which predicts segment relief at both fast and slow spreading centers to within 240 m standard deviation we find that half segment scale deepening is more than a direct thermal effect of the juxtaposition of a cold lithospheric edge and the end of a ridge at a transform fault a second inversion problem is solved to determine the parameters that contribute to the absolute depth of the ridge axis we can show that the absolute depth of the shallowest section of a ridge segment is strongly dependent on regional depth and that this dependence is more complex at slow spreading ridges reflecting the common but not ubiquitous existence of a median valley there these results in combination with the segment relief analysis clearly indicate that the axial depth at a ridge transform intersection is a function of both long wavelength processes such as may control hot spot distribution and local tectonic factors including but not limited to transform offset

distortions rotations and crustal thinning at ridge transform intersections when an oceanic ridge intersects a transform fault the newly formed oceanic crust may be severely distorted frequently exhibiting major rotations of fault blocks1 4 such effects typically extend over a 10 km wide strip parallel to the transform by contrast studies of contemporary oceanic transforms suggest that deformation is restricted to a single fault trace within the transform domain5 9 this apparent inconsistency can be reconciled if the rotations occurred at the ridge transform intersection during accretion of the crust here i develop a kinematic model that considers the effect of an ephemeral period of stretching on a region that broadens as the transform is approached along the ridge the strains calculated from this model indicate extension at the ridge simple shear at the transform and composite strains in the region of distributed deformation the model predicts very rapid rotation rates 100 in 104 105 yr the implications for the topography and structure of ridge transform intersections are considered and the alternating periods of extension by stretching and by intrusion are illustrated by reference to contemporary transforms on the mid atlantic ridge the model may also explain the common occurrence of cross cutting dykes near transform faults in ophiolites

lithosphere age depth and structural complications resulting from migrating transform faults if oceanic transform faults migrate back and forth across a finite width of the lithosphere as a consequence of minor ridge propagations and recessions many structural complications can arise patterns of lithosphere age and depth as well as internal structure may be determined by the frequency and magnitude of such migrations such processes are expected to be most likely at transforms bounded by relatively young thin weak lithosphere due to either short ridge offsets or fast spreading rates

gloria investigations of oceanic fracture zones comparative study of the transform fault zone the tectonic patterns of some twenty spreading centre offsets have been determined using the long range sidescan sonar gloria and are reviewed in this paper offsets of less than about 20 km fail to produce transform faults but are accommodated by short sections of oblique spreading with overlapping spreading centres in the fast spreading case at slow slip rates such offsets can be associated with topographic fracture zones that are indistinguishable from those produced at true transform faults such features may account for a significant proportion of spreading centre offsets on slowly spreading mid ocean ridges offsets larger than about 30 km produce a true 9transform fault9 this comprises a 1 to 5 km wide band of individual faults that are parallel or subparallel to the spreading direction the band corresponds to the 9transform fault zone9 tfz of continental wrench faulting it includes the 9principal transform displacement zone9 ptdz and secondary structures such as riedel shears a narrow tfz is normally associated with a prominent ptdz which appears to be either a narrow furrow or a single scarp on the other hand wider zones often fail to show a clear ptdz on gloria both types are common on the slowly spreading mid atlantic ridge and may even occur at different points along the same large offset transform such as romanche although the narrow type appears to occur preferentially where the local ridge axis is oblique to the spreading direction narrow tfzs and prominent ptdzs appear to be more common on the fast spreading east pacific rise it is suggested that broader tfzs and less prominent ptdzs may form in response to the complicating effects of small components of compression across the transform fault perhaps as a result of departure of the transform fault trace from a true small circle such compression would be more easily supported in the thicker stronger lithosphere of slowly spreading ridges but could be relieved by tension due to the ridge push force where the ridge is oblique to the spreading direction

some additional observations on inclusions in the granitic rocks of the sierra nevada microgranular quartz diorite and diorite inclusions are widespread in central sierra nevada granitoid rocks and are almost exclusively restricted to hornblende bearing rocks most commonly felsic tonalites and mafic granodiorites strontium 87 strontium 86 values of the inclusions and host granitoids from individual plutons generally plot on single isochrons that agree closely with previously determined ages neodymium 143 neodymium 144 values of five inclusions ranged from 0 51229 to 0 51248 and yield end values that correlate closely with values of their respective hosts the nd sm and rb sr systematics indicate that most inclusions were in isotopic equilibrium with enclosing materials at the time of formation silica contents of inclusions and granitoids are contiguous but inclusions generally contain less than and granitoids more than 60 sio2 linear continuity of the femic oxide and h2o trends relative to sio2 suggests many inclusions formed as concentrations of hydrous mafic minerals variation of other major element oxides and trace elements support this inference the aforementioned data are generally inconsistent with a direct genetic link between the mafic inclusions and the mantle derived basalt however many initial sierra nevada magmas were produced by mixing of mafic mantle derived melts and felsic crustal materials resulting in isotopically varied tonalitic and granodioritic magmas most inclusions thus represent fragmented crystal accumulations of early formed near liquidus minerals generated from these previously mixed magmas

the mid rivera transform discordance morphology and tectonic development to better define the morphotectonic elements and tectonic development of the mid rivera transform discordance multibeam bathymetric seafloor backscatter multichannel seismic reflection and total field marine magnetic data were collected along the entire rivera transform west of 107 w during the bart and famex campaigns of the n o l atalante conducted in 2002 these data show that although the transform tectonized zone of the rivera transform west of 107 30 w is a single continuous morphologic basin this basin consists of two distinct morphotectonic domains an eastern domain which morphologically is a deep rhombochasm within which organized seafloor spreading has occurred and a western leaky transform domain these new data in conjunction with the results of previous studies support the idea that the rivera pacific euler pole is migrating southward towards the eastern half of the rivera transform and further indicate a recent 0 14 ma and most likely ongoing clockwise reorganization of the principle transform displacement zones of the rivera transform west of 108 w we propose that the mid rivera transform discordance owes its origin to this eastward progressing clockwise reorganization of the transform segments that is occurring in response to recent changes in rivera pacific relative plate motion

deep tow studies of the overlapping spreading centers at 9 03 n on the east pacific rise the deeply towed instrument package of the scripps institution of oceanography was used to study for the first time the fine scale structure of an overlapping spreading center osc system the 9 03 n osc on the east pacific rise epr the eastern and western limbs of this osc system which are 8 km apart and overlap by 27 km show marked differences away from the tips the spreading centers are highly tectonized and differ greatly from the ridge tips the western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area in both cases the ridge tip fissures are 10 15 oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively the eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through going trace of the east pacific rise in contrast the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis some of the very few tectonic features observed in the overlap basin may reflect the existence of a short lived shear couple due to the interaction of the two overlapping spreading centers sediment cover over the whole area and especially in the overlap basin is relatively thick supporting the idea of a time averaged deficit in the magmatic budget at this osc we suggest that the 9 03 n osc developed where two magmatic pulses independent in space and time and propagating along the strike of the east pacific rise away from the loci of melt emplacement failed to meet misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing osc geometry independent lines of evidence derived from the three dimensional inversion of the magnetic field at the 9 03 n osc sempere et al 1984 from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field sempere and macdonald 1986 and from geochemical data langmuir et al 1986 support our interpretation of the deep tow data we suggest that deviations from axial linearity of the spreading centers devals langmuir et al 1986 and small nonoverlapping offsets snoos batiza and margolis 1986 are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on oscs and some saddle points i e devals snoos are all small rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells beneath the east pacific rise

microearthquake evidence for extension across the kane transform fault we report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the kane fracture zone the kane is a slow slipping 25 mm yr large offset 150 km transform delineated by a pronounced transform valley the experiment site lies in the eastern half of the transform 40 km west of the eastern ridge transform intersection in a region of marked transform parallel topography the network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width spanning the transform valley and a narrow median ridge to the north hypocentral parameters were determined for 86 earthquakes epicenters within the network define a narrow transform parallel trend along the steep southern wall of the transform valley maximum focal depths increase from 6 to 9 km from east to west across the network significant activity was also detected outside the transform tectonized zone to the southeast this activity may be associated with the inside corner of the ridge transform intersection while to the north about 20 events occurred in 7 m y old crust in a region of ridge parallel bathymetry six focal mechanisms were obtained from p wave first motions for events within the transform valley the best constrained solutions for four earthquakes within the network show normal faulting on fault planes subparallel to the trend of the transform all the mechanisms indicate a tension axis perpendicular to the trend of the transform these results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere the observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere potential sources of extension across the transform include thermal stress in the young oceanic lithosphere topographic loading a small component of plate divergence normal to the transform and northward motion of the asthenosphere relative to the surface plate

photosynthetic microbial mats in the 3 416 myr old ocean recent re evaluations of the geological record of the earliest life on earth have led to the suggestion that some of the oldest putative microfossils1 and carbonaceous matter were formed through abiotic hydrothermal processes2 3 similarly many early archaean more than 3 400 myr old cherts have been reinterpreted as hydrothermal deposits rather than products of normal marine sedimentary processes2 4 5 here we present the results of a field petrographic and geochemical study testing these hypotheses for the 3 416 myr old buck reef chert south africa from sedimentary structures and distributions of sand and mud we infer that deposition occurred in normal open shallow to deep marine environments the siderite enrichment that we observe in deep water sediments is consistent with a stratified early ocean6 7 we show that most carbonaceous matter was formed by photosynthetic mats within the euphotic zone and distributed as detrital matter by waves and currents to surrounding environments we find no evidence that hydrothermal processes had any direct role in the deposition of either the carbonaceous matter or the enclosing sediments instead we conclude that photosynthetic organisms had evolved and were living in a stratified ocean supersaturated in dissolved silica8 9 3 416 myr ago

a compendium of causes and effects of processes at transform faults and fracture zones a flowchart has been constructed which consolidates into a single conceptual framework a large number of the phenomena and processes which have been observed or postulated at transform faults tfs and fracture zones fzs oceanic ridge ridge transform faults and their aseismic extensions are emphasized the upstream end of the flowchart contains large scale boundary conditions imposed by the position of a particular transform fault in space and time for example slip rate the downstream end of the flowchart contains small scale features characteristic of individual tfs or fzs for example the existence of a fz parallel ridge features of both the upstream and the downstream ends of the flowchart can be mapped measured or otherwise derived by direct observation the middle of the flow chart includes processes and phenomena which are inferred to result from the upstream boundary conditions and to cause the downstream small scale features for example thermal conduction across the plate boundary phenomena in this black box in the center of the flowchart cannot be directly observed with present technology but must be inferred the advantages of ordering knowledge into such a flowchart are that inconsistancies gaps in understanding self limiting phenomena feedback loops and previously unrecognized relationships stand out and that the boundary between inference and observation is clearly drawn

increased mud deposition reduces stromatolite complexity microbial mat growth can produce diagnostic delicate structures in low energy environments but mat morphology and thus preservation of delicate structures as morphological biosignatures is affected by mud accumulation in lake joyce antarctica varying rates of siliciclastic mud deposition affected the morphology of partially calcified microbial mats these mats were characterized along a shore parallel transect from higher to lower sedimentation regions of the lake mats with high mud accumulation rates were predominantly flat with sparse pinnacles at lower sedimentation rates mats developed more pinnacles and more delicate features including vertical webs of biofilm suspended between pinnacles morphological transitions suggest that mud deposition disrupts growth of delicate microbial structures like biofilm webs and inhibits pinnacle growth at higher accumulation rates the simplification of mat morphology with increasing mud accumulation has important implications for the distribution of stromatolite types in the rock record stromatolites with intricate biofilm structures are rare possibly due to the paucity of appropriate environments rather than the intrinsic properties of microbial communities communities capable of forming delicate microbialites likely produced morphologically simpler pinnacles and flat mat in environments with significant siliciclastic mud or micrite influx thus delicate mats have narrow environmental ranges for growth and temporal or spatial variations in siliciclastic mud or micrite sedimentation rates bias the record of microbial activity preserved in stromatolites

stress fields associated with oceanic transform faults abstract although their formation is commonly attributed to shear stresses oceanic transform faults contain many structural elements indicating extension across them examples include transform parallel tension fractures normal faults grabens dykes and small scale spreading centres at the junctions between transform faults and adjacent ridge segments there are commonly sets of oblique fractures a detailed field study of oblique fractures in the vicinity of a junction between a mid ocean ridge and an oceanic fracture zone in north iceland shows that they are mostly tension fractures and normal faults this paper presents the results of a boundary element study of the stress field associated with the junction between an oceanic transform fault and its ridge segments the results show that uniaxial tensile loading plate pull parallel with the transform fault tends to lock it but also gives rise to shear stress concentration in a zone that coincides with the fault this shear stress is primarily responsible for the strike slip faulting and associated earthquakes in the transform fault to generate the transform parallel fracture zone graben normal faults tensile fractures dykes and spreading centres however requires tensile loading parallel with the ridge axis the results suggest that biaxial ridge parallel and ridge perpendicular tensile loading generates a stress field that 1 unlocks and makes slip easier on the transform faults and 2 explains the oblique fractures at the ridge transform junctions

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

oceanic inside corner detachments of the limassol forest area troodos ophiolite cyprus abstract flat lying extensional detachment faults have been imaged in the inside corner regions of ridge transform intersections on the mid atlantic ridge exposed detachment surfaces are 10 km or more across and are corrugated in the direction of spreading as are continental detachments beneath the detachments lie core complexes of peridotite and gabbro these are overlain by blocks of crustal material we argue here that similar detachments are an essential component of the limassol forest area of the troodos ophiolite in cyprus which lies south of the arakapas fault zone previously recognized as a palaeo transform fault and here interpreted as a transform fault that evolved into a fracture zone in the limassol forest core complexes of mantle peridotite can be shown to have been exposed at the sea floor or to have been covered by overlapping crustal blocks separated from the peridotite core and from each other by low angle extensional faults the extension can be shown to have occurred shortly after crustal construction and the already extended terrain was then intruded by swarms of dykes and plutons we interpret these relations as arising when crust is constructed in an inside corner area extended by detachment faulting deformed further during slip along the transform and then intruded by new magma as it passes the second spreading centre the structurally deeper parts of the crustal blocks that overlie the detachment lie broadly towards the west indicating that the spreading axis lay in that direction the ophiolite north of the transform is much less extended and we interpret this as a section of outside corner crust in this interpretation the troodos ophiolite formed to the east in its current orientation of a ridge transform ridge intersection in which the transform had a dextral offset and sinistral slip the part of the ophiolite that forms the limassol forest was produced at the western inside corner and spread eastwards until it passed the second spreading axis at which point the ophiolite north of the arakapas fault was created and welded to the limassol forest when the transform became a fracture zone

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

global positioning system measurements of deformations associated with the 1987 superstition hills earthquake evidence for conjugate faulting large station displacements observed from imperial valley global positioning system gps campaigns are attributed to the november 24 1987 superstition hills earthquake sequence thirty sites from a 42 station gps network established in 1986 have been reoccupied during 1988 and or 1990 displacements at three sites within 3 km of the surface rupture approach 0 5 m eight additional stations within 20 km of the seismic zone are displaced at least 10 cm this is the first occurrence of a large earthquake ms 6 6 within a preexisting gps network best fitting uniform slip models of rectangular dislocations in an elastic half space indicate 130 8 cm right lateral displacement along the northwest trending superstition hills fault and 30 10 cm left lateral displacement along the conjugate northeast trending elmore ranch fault the geodetic moments are 9 4 1025 dyn cm and 2 3 1025 dyn cm for the superstition hills and elmore ranch faults respectively consistent with teleseismic source parameters the data also suggest that postseismic slip along the superstition hills fault is concentrated at shallow depths distributed slip solutions using singular value decomposition indicate near uniform displacement along the elmore ranch fault and concentrated slip to the northwest and southeast along the superstition hills fault a significant component of nonseismic secular displacement is observed across the imperial valley which is attributed to interseismic plate boundary deformation

deep tow seismic profiles from the vema transform and ridge transform intersection a recently developed near bottom reflection profiling system has been used to obtain high resolution seismic sections across the vema transform fault at the western ridge transform intersection deep towed 3 5 khz echo sounder data show evidence of fresh lava flows at the seabed within the nodal deep over one kilometre of sediment fills the bottom of the ridge transform intersection deep many strong discontinuous reflectors within the sedimentary section caused by lava layers indicate that the intersection deep has been the site of sustained igneous activity on the eastern side of the deep is a large uplifted fault block created as the newly formed crust moved out of the spreading centre dipping strata within the fault block document back tilting as the new crust is extended by normal faulting profiles across the active transform fault reveal that the zone of deformation throughout the one kilometre thick sediment column is narrow suggesting that the location of the fault has remained stable throughout at least the last million years

the mechanics of the south iceland seismic zone the south iceland seismic zone appears to act as a transform fault between offset sections of the mid atlantic ridge however there is no morphological expression of a throughgoing transform between the ridges the anticipated throughgoing east west transform is represented by a series of north south strike slip faults we study the mechanics of the south iceland seismic zone using boundary element methods first we examine the implications of the differences between the geometry of an orthogonal ridge transform ridge system and the geometry of the structures observed in iceland then we consider the role of the north south faults in accommodating transform motion in an east west direction we conclude that the north south faults can accommodate deformation if they are longer or more spatially frequent than mapped the south iceland seismic zone is subject to sequences of large earthquakes every 45 112 years in comparing the seismic moment release derived from earthquake magnitudes with that predicted by our models we confirm that the system of north south faults can act as a transform fault the seismic moment release derived from the earthquake magnitudes is 1 4 times greater than that predicted by our models if our application of the moment magnitude relation is correct the above could imply that the brittle layer extends to 14 km rather than the 10 km assumed in our models other possibilities are that the magnitudes of the earthquakes have been slightly overestimated or that the historical seismic record documents a period of seismicity greater than that expected from rates of plate motion inferred from geological and very long baseline interferometry vlbi

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

vertical crustal movements at the vema fracture zone in the atlantic evidence from dredged limestones abstract a crustal block shallower by over 3 km than the level predicted by the thermal contraction model for the ocean crust is located on the southern side of the vema transform in the atlantic lithology magnetic and gravity data suggest the anomalous block is an uplifted piece of crust and upper mantle and not a constructional volcanic feature the crest and upper slopes of the anomalous block are capped by shallow water reef limestones determination of paleofacies and ages of samples of these limestones suggest that 1 a the crest of the anomalous crustal block reached sea level and was exposed subaerially in middle pliocene time about 3 m y ago and subsequently sank to its present depth at an average rate of about 0 3 mm year that is a rate one order of magnitude faster than that predicted by the thermal contraction model for crust of the same age 2 b the anomalous block reached at or above sea level either continuously or intermittently throughout the late miocene from roughly 10 m y ago 3 c paleocene sediments dated at about 55 58 m y have been recovered from the uplifted block where assuming a spreading rate of 1 1 cm year the crust should not be more than about 35 m y old vertical crustal movements at the vema transform provide an example of vertical tectonism in the large oceanic transform fracture zones

thermal contraction and the state of stress in the oceanic lithosphere nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three dimensional state of deviatoric stress which can be separated into two parts a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature we examine thermal contraction stresses due to temperature changes in a thin semi infinite rectangular plate bounded by the spreading center axis two stress free boundaries representing transforms or fracture zones define the plate width or ridge segment length l with the bottom of the elastic plate defined by a prescribed temperature the plate thickens as the square root of age as expected for a thermal boundary layer due to vertical conductive cooling above this prescribed temperature elastic stresses are assumed to relax quickly stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress free material is added to bottom of the cooling thickening plate the state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior at the ridge axis ridge parallel tensile stresses are about 300 mpa the same as if the plate were not allowed to contract in this direction along the transform boundary the maximum transform parallel tensile stress occurs at a distance l 2 from the ridge transform intersection where its magnitude is comparable to the stress at the ridge axis the tangential stresses at the plate boundary decrease rapidly with distance from the boundary at a distance of l 4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude the stress magnitudes are independent of both spreading rate and ridge segment length a large transform parallel tensile stress may control the length of transform offsets thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis however a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate thus providing a natural length scale controlling the spacing of transforms

nonlinear viscoelasticity and the formation of transverse ridges transverse ridges are regions of anomalously high uplift that run parallel to many of the transform fattits at mid ocean ridges previous models of the formation of these ridges generally fail to explain the magnitude of uphft between 1 and 8 km or account for geophysical observations which indicate that the ridges are dynamically uphfted by neutral or heavy but not buoyant material however the potentially large maxwell relaxation time of the lithosphere and the high strain rates and finite deformation of transform faults imply that nonlinear viscoelasticity may play an important role in the formation of these ridges we present simple theoretical and experimental models of viscoelastic flow beneath a transform fattit to show that the purely horizontal motion of the fattit can generate vertical uplift typical of transverse ridges the theoretical model is of an infinite half space of fluid driven from above by two plates moving parallel and in opposite directions to one another the constitutive relation for the viscoelastic rheology is for a second order fluid i e nonlinear viscoelastic effects are treated as perturbations to newtonian purely viscous flow the theoretical model not only predicts the two transverse ridges that are often observed on either side of the transform fault but the central transform valley that separates them as well for typical hthospheric flexural rigidity and deformation associated with transverse ridges the viscoelastic effect provides sufficient vertical stress to produce the observed uplift further the dynmnic uplift is a purely mechanical effect i e it does not involve buoyant materim which corresponds to geophysical observations the experimental model is comprised of a viscoelastic fluid 2 aqueous solution of high molecular weight carboxymethylcellulose driven by a rotating plate this is analogous to a t ransk rm fattit because the boundary of a spiroting circular tectonic plate surrounded by stationary plates is one continuous transform fattit the experiment produces a large ridge of fluid along the edge of the plate with a shght trough outside of the ridge this result is qualitatively described by a cyhnch ical version of of the above theoretical model in conclusion both experimental and theoretical models show that when nonlinear

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

multiple closely spaced transform faults in fast slipping fracture zones the 400 km long offset of the east pacific rise spreading axis between the quebrada fracture zone lat 3 5 s and the gofar fracture zone lat 4 6 s has been surveyed using the long range sidescan sonar gloria the offset is accomplished in three distinct fracture zones each of which contains up to four actively slipping transform faults separated by short spreading centers transform spacings range from 5 to 16 km each transform is associated with its own transverse valley but at quebrada fracture zone and to some extent at the other two these valleys merge into one large slightly oblique fracture zone valley the morphological trends of fracture zone valleys are therefore not necessarily reliable for estimating directions of relative plate movement multitransform fracture zones appear to be much commoner on the fast spreading east pacific rise than on slower spreading ridges the tectonic spreading fabric which comprises closely spaced normal faults formed near the spreading axes and striking subparallel to them is modified to a sigmoidal pattern between the closely spaced transforms even midway between transforms these faults remain oblique to the spreading direction the degree of obliquity increases as the transform separation decreases showing progressive rotation of the stress ellipse as transforms are approached

late amagmatic extension along the central and eastern segments of the west philippine basin fossil spreading axis the spreading processes within the west philippine basin wpb remain partly unknown this study presents an analysis of the tectono magmatic processes that happened along its spreading axis during the conclusion of the last spreading phase at 33 30 ma we demonstrate that the late episode of n s opening from an e w trending spreading system has been followed by a late tectonic event occurring in the central and eastern parts of the basin this event was responsible for transtensional strain accommodated along the nw se faults cutting through the former e w rift valley in the central part of the basin in its eastern part the same event occurred at a larger extent and led to the creation of a new nw se axis obliquely cutting the older e w spreading segments and their associated spreading fabrics at this location several tens of kilometers of slightly oblique amagmatic extension occurred following a v60 direction we propose that this late event is associated with the onset of e w opening of the parece vela basin located along the eastern border of the wpb at 30 ma extensive stresses within this basin were probably transmitted to the hot and easily deformable rift zone of the wpb the newly created nw se axis most likely propagated from east to west being responsible for scissors opening within the wpb ne sw extension ceased when well organized spreading started at 26 ma in parece vela basin accommodating entirely the global extensive stress pattern 2002 elsevier science b v all rights reserved

tectonic processes in the jan mayen fracture zone based on earthquake occurrence and bathymetry jan mayen is an active volcanic island situated along the mid atlantic ridge north of iceland it is closely connected with the geodynamic processes associated with the interaction between the jan mayen fracture zone jmfz and the slowly spreading kolbeinsey and mohns ridges despite the significant tectonic activity expressed by the frequent occurrence of medium to large earthquakes detailed correlation between individual events and the causative faults along the jmfz has been lacking recently acquired detailed bathymetric data in the vicinity of jan mayen has allowed us to document such correlation for the first time the earthquake of 14 april 2004 m w 6 which occurred along the jmfz was studied in detail and correlated with the bathymetry locations of aftershocks within the first 12 hours after the mainshock outline a 10 km long fault plane interactions between various fault systems are demonstrated through locations of later aftershocks which indicate that supposedly normal fault structures to the north of the ruptured fault in the jan mayen platform have been reactivated correlation of the waveforms shows that events located on these structures are significantly different from activity at neighboring structures coulomb stress modeling gives an explanation to the locations of the aftershocks but cannot reveal any information about their mechanisms

distributed deformation in an oceanic transform system applying statistical tools to structural and paleomagnetic data near the husavik flatey fault northern iceland

seismic constraints on the crustal structure within the vema fracture zone results from recent studies of the in well surveyed areas such as the famous area in kane and oceanographer fracture zones suggest that the central north atlantic ramberg et al 1977 significant variations in crustal thickness are transform faults are found every 50 to 100 km associated with parts of these fracture zones making them one of the most common tectonic with total crustal thicknesses in places less than features in the ocean basins morphologically half the thickness of normal oceanic crust the fracture zones on slowly spreading ridges display extent of this anomalous crust and its origin are many similar characteristics deep closed con still very poorly constrained by available geo tour depressions are usually present at ridge physical data here we report results from a 75 axis transform intersections sleep and biehler km long refraction line shot in late 1977 over 1970 the axis of the transform is typically three hawaii institute of geophysics ocean bottom defined by a deep linear trough bounded by escarpments rising eimom uuyuu withe ve valley near its western intersection with the mid transform valley floor a diverse suite of atlantic ridge 10o53 n 43o39 w we show that the apparent crustal thickness beneath this por tion of the vema transform valley is about 5 km not substantially different from crustal thick nesses typically associated with oceanic crust however the vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the kane and oceanographer fracture zones it is characterized by low compressional wave velocities a relatively uniform velocity gradient throughout most of the crust and the absence of a typical layer 3 refractor this anomalous crust is interpreted as intensely fractured hydrothermally altered and serpenti nized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls the large variation in moho depths found in these large offset slowly slipping transforms is attributed to differences in the depth and extent of serpen tnnization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer one important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor

recent faulting and microearthquakes at the intersection of the vema fracture zone and the mid atlantic ridge new air gun reflection profiles 3 5 khz reflection profiles and microearthquake data recorded by an array of ocean bottom seismographs form the basis for this study of the transition from a spreading center to a major transform fault disturbances of the thick normally flat lying turbidite deposits provide indications of recent vertical motions at the western intersection of the fracture zone with the median valley there is a depression in the sediments that represents the southerly extension of the median valley into the fracture zone valley the depression is terminated abruptly on the south by the active transform fault which acts as a locus for vertical as well as horizontal displacement flat lying undisturbed sediments terminate abruptly at the fault the western boundary of the depression is much broader and is characterized by a series of slumplike steps to the west there is little or no evidence for uplift or tilting of sediments which might indicate vertical recovery of the crust as it spreads away from the depression this suggests that uplift and recovery out of the depression is episodic in nature and has been inactive over the last million years along the western boundary to the east there is clear evidence of uplift and tilting of sedimentary layers a basement ridge emerging from the sediments is currently being uplifted and rotated in a manner analogous to processes responsible for the creation and cancellation of median valley relief the transition between the spreading center and the transform fault appears to take place within 1 2 km the width of the transform fault just east of the depression is less than a kilometer microearthquakes were located and displayed by new methods that directly account for nonlinearities associated with small arrays microearthquakes located by three or more ocean bottom seismometers show that the greatest seismic activity occurs along the eastern walls of the median valley at the basement ridge in the eastern portion of the depression and in the crestal mountains very little activity is associated with the western edge of the transform depression and the trace of the transform fault

deep tow studies of the vema fracture zone 2 evidence for tectonism and bottom currents in the sediments of the transform valley floor the deep tow geophysical instrument package has been used to survey a 65 km segment along the principal transform displacement zone ptdz of the vema fracture zone away from either ridge transform intersection the surface trace of the ptdz is a small groove 5 80 m deep and 20 200 m wide incised in the otherwise flat lying turbidites of the transform valley floor the fault trace cuts across the transform valley diagonally from ridge crest to ridge crest hugging the southern flank of the median ridge the groove is straight or gently sinuous relatively free of the splays and bifurcations which characterize the transform fault zone near the ridge transform intersection macdonald et al this issue a pull apart basin was observed at the location of a slight clockwise bend in the fault trace vertical offsets of turbidites suggest that the west end of the median ridge and the east end of the south wall are rising relative to the transform valley floor at rates of the order of millimeters per year sediment waves were observed buried under 10 20 m of sediment suggesting that antarctic bottom water may have flowed through the transform valley more vigorously several tens of thousands years ago than at present

small scale models of oceanic transform zones previous studies about oceanic transform zones reveal major morphological and structural differences that cannot be explained by simple factors such as spreading rates or offsets in order to determine how kinematic and mechanic parameters control the deformation pattern we perform small scale modeling using sand and silicone putty as analog material of brittle and ductile layers of oceanic lithosphere respectively the three dimensional depth geometry of these layers follows the thermal structure of the ridge transform system the structure of the deformed transform zone is very sensitive to input parameters specially to the offset the obliquity of the deformed zone with the extension direction i e spreading direction in a natural system depends on the offset and on the brittle layer thickness for greater thickness or a shorter offset the transform domain widens and the components of extension become significant for smaller lithosphere thickness or larger offset the transform domain becomes narrow with steeper walls and mainly undergoes strike slip deformation such variability in possible controls on the offset length may help explain the structure complexity observed within the oceanic transform zones a comparison with 28 natural systems indicates the same relationship of the offset with the angle between the principal transform displacement zone ptdz and spreading direction a relationship between offset and width is clearly established for mid atlantic ridge mar transforms as on analog models a similar relationship is observed for the east pacific rise epr transforms but with greater variabilty this difference is interpreted as resulting either from greater complexity of transform zones on epr than on mar making it difficult to estimate some parameters from different mechanical behaviors between slow and fast spreading systems or from higher rheological complexity of natural systems than out model