ultramafic exposures and the gravity signature of the lithosphere near the fifteen twenty fracture zone mid atlantic ridge 14 16 5 n abstract we analyze geological and geophysical data near the fifteen twenty fracture zone to constrain the processes of magmatic accretion and tectonic extension in the area characterized by extensive outcrops of ultramafic rocks at the seafloor close to the fracture zone 60 km the crust is thicker associated with circular or elongated gravity lows bull s eyes axis parallel faults and abyssal hills and no peridotite outcrops consistent with a more homogeneous and magmatic crust simple melting models predict magmatic crustal thicknesses that differ substantially from the gravity estimates and the outcrop of peridotites seems inconsistent with the high degrees of partial melting in the area inferred from basalt and peridotite geochemistry we explain these discrepancies primarily a by the entrapment of melt in the lithosphere at deep levels particularly near the transform b by along axis melt migration away from the transform and c by variations in the degree and depth of serpentinization of a heterogeneous crust composed of gabbro and peridotite the along axis variations in melt supply lithospheric thickness and alteration of the lithosphere can result in a change of rheological structure and therefore in fault pattern as observed along the ridge axis identified detachment faults in the area provide a mechanism for the uplift of ultramafic rocks to the seafloor these detachments are not systematically associated with or formed at the inside corner of discontinuities as described elsewhere the origin and evolution of these structures is yet unconstrained but we suggest that they can be initiated at mechanical heterogeneities in the deep lithosphere and that once formed they localize strain due to the presence of weak serpentinites along the fault planes

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

the frictional behavior of lizardite and antigorite serpentinites experiments constitutive models and implications for natural faults laboratory studies of the frictional behavior of rocks can provide important information about the strength and sliding stability of natural faults we have conducted friction experiments on antigorite and lizardite serpentinites rocks common to both continental and oceanic crustal faults we conducted both velocity step tests and timed hold tests on bare surfaces and gouge layers of serpentinite at room temperature we find that the coefficient of friction of lizardite serpentinite is quite low 0 15 0 35 and could explain the apparent low stresses observed on crustal transform faults while that of antigorite serpentinite is comparable to other crustal rocks 0 50 0 85 the frictional behavior of both types of serpentinite is well described by a two mechanism model combining state variable dominated behavior at high slip velocities and flow dominated behavior at low velocities the two mechanism model is supported by data from velocity step tests and timed hold tests the low velocity behavior of serpentinite is strongly rate strengthening and should result in stable fault creep on natural faults containing either antigorite or lizardite serpentinite

the brittle plastic transition in experimentally deformed quartz aggregates deformation experiments have been conducted to provide constraints on the processes responsible for the brittle plastic transition in quartz aggregates a correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and or pressure first there is a transition from cataclastic faulting to semibrittle faulting microstructural observations indicate that this transition occurs due to the activation of dislocations in addition faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide second there is a transition from semibrittle faulting to semibrittle flow this transition corresponds to a change from localized to distributed deformation microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime we conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries third there is a transition from semibrittle flow to dislocation creep microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature in addition microstructural observations indicate that a transition from dominantly mode i axial to mode ii shear microcracking occurs with an increase in confining pressure from 0 4 to 0 8 gpa regardless of temperature the differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions however a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the earth the results of this study provide further evidence to indicate that the brittle plastic transition in the continental crust occurs over a relatively wide range of conditions

the effects of pressure and porosity on the micromechanics of the brittle ductile transition in quartzite room temperature experiments on a quartzite with 7 porosity show that with increasing pressure there is a transition from brittle faulting to ductile cataclastic flow at about 600 mpa in contrast experiments on nonporous quartzite show only faulting up to 1000 mpa the transition to cataclastic flow in the porous quartzite is due to the changing influence of the pores microstructural observations and elastic theory suggest that the transition occurs at the pressure where the differential stress producing a compressive stress concentration sufficient to initiate pore collapse becomes less than that producing a tensile stress concentration sufficient to nucleate axial microcracks at the tops and bottoms of pores cataclastic flow of the porous quartzite is only transient deformation remains distributed and accompanied by work hardening only as long as pore collapse continues to 16 strain at higher strains microcracking produces a net dilatancy which causes strain weakening that leads to faulting at 26 strain at a confining pressure of 750 mpa the faults are stable whereas at 1000 mpa they are unstable the results indicate that porous rocks may undergo limited ductile deformation at shallow levels in the crust but higher strains will cause deformation to localize

strain localization on an oceanic detachment fault system atlantis massif 30 n mid atlantic ridge 1 microstructural observations mineral chemistry and the spatial distribution of deformation fabrics recorded in outcrop samples collected from atlantis massif the active inside corner high at 30 n mid atlantic ridge suggest that strain is localized near the subhorizontal domal surface hypothesized to be an exposed detachment fault deformation textures in peridotite and gabbro indicate that high temperature 500 c strain occurred via crystal plastic flow and diffusive mass transfer low temperature 400 c shear zones containing brittle and semibrittle microboudinage textures in which tremolite chlorite and or talc replace fractured serpentine or hornblende cut earlier formed high temperature deformation fabrics in peridotite textures indicate strain was localized by cataclasis and reaction softening into zones of intense greenschist and subgreenschist grade metamorphism gabbro is only weakly deformed below amphibolite facies 500 c indicating that strain was partitioned into altered peridotite at low temperature there is a clear relationship between deformation intensity and structural depth beneath the subhorizontal surface of the massif discontinuous high intensity crystal plastic deformation fabrics are found at all structural depths 0 520 m beneath the surface indicating that high temperature granulite and amphibolite grade deformation was not localized in a single shear zone in contrast semibrittle and brittle low temperature shear zones are concentrated less than 90 m structurally beneath the surface and the most intensely brittlely deformed samples concentrated in the upper 10 m localization of brittle deformation fabrics near the upper surface of the massif supports the hypothesis that it is the exposed footwall of a detachment fault the structural evolution of atlantis massif is therefore analogous to a continental metamorphic core complex strain was localized onto the fault by reaction softening and fluid assisted fracturing during greenschist and subgreenschist grade hydrothermal alteration of olivine clinopyroxene serpentine and hornblende to tremolite chlorite and or talc

a view of the lower crustal component of hydrothermal systems at the mid atlantic ridge an extensive suite of hydrothermally altered gabbros was recovered by alvin and dredging from the mid atlantic ridge in the vicinity of the eastern ridge transform intersection rti of the kane fracture zone mark where tectonic extension has provided a window into the lower crustal component of hydrothermal cells four alteration types are distinguished on the basis of metamorphic assemblage mineral composition and deformation textures a conceptual model is presented that places temperature fluid rock ratio relationships in conjunction with styles of deformation into a spatial and temporal framework exsolution of late stage magmatic fluids in the vicinity of the magma hydrothermal interface at temperatures 700 c marks the onset of fluid rock interaction in the mark gabbros kelley et al this issue with subsequent alteration involving seawater derived fluids being strongly influenced by deformation mechanisms in zones of ductile shear hydration was initiated at temperatures between 550 and 700 c and moderate fluid rock ratios elsewhere in the lower crust the onset of seawater penetration occurred at temperatures of 400 550 c and was facilitated by brittle fracturing early vein networks indicate very low initial fluid rock ratios and are interpreted as being related to the solidification of plutons later fractures indicate higher water rock ratios and may be related to the downward propagation of shallow fault systems throughout the crust fluid rock interaction ceased at temperatures 180 300 c subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the rti massif produced moderate fluid rock ratios within localized zones hydrothermal alteration in the crustal section exposed at mark was initiated at lower temperatures and water rock interaction proceeded to lower temperatures than many other plutonic suites these differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust

seismic velocities anisotropy and shear wave splitting of antigorite serpentinites and tectonic implications for subduction zones 1 antigorite the high temperature ht form of serpentinite is believed to play a critical role in various geological processes of subduction zones we have measured p and s wave velocities vp and vs anisotropy and shear wave splitting of 17 serpentinite samples containing 90 antigorite at pressures up to 650 mpa the new results combined with data for low temperature lt lizardite and or chrysolite reveal distinct effects of lt and ht serpentinization on the seismic properties of mantle rocks at 600 mpa vp 5 10 and 6 68 km s vs 2 32 and 3 67 km s and vp vs 2 15 and 1 81 for pure lt and ht serpentinites respectively above the crack closure pressure 150 mpa the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature serpentine contents within subduction zones and forearc mantle wedges where temperature is 300 c should be at least twice that of previous estimates based on lt serpentinization the presence of seismic anisotropy high pressure fluids or partial melt is also needed to interpret ht serpentinized mantle with vp 1 81 the intrinsic anisotropy of the serpentinites 3 8 16 9 with an average value of 10 5 for vp and 3 6 18 3 with an average value of 10 4 for vs is caused by dislocation creep induced lattice preferred orientation of antigorite three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics s l and ls tectonites formed by three categories of strain geometry i e coaxial flattening coaxial constriction and simple shear respectively our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide

constructing constitutive relationships for seismic and aseismic fault slip for the purpose of modeling natural fault slip a useful result from an experimental fault mechanics study would be a physically based constitutive relation that well characterizes all the relevant observations this report describes an approach for constructing such equations where possible the construction intends to identify or at least attribute physical processes and contact scale physics to the observations such that the resulting relations can be extrapolated in conditions and scale between the laboratory and the earth the approach is developed as an alternative but is based on ruina 1983 and is illustrated initially by constructing a couple of relations from that study in addition two example constitutive relationships are constructed these describe laboratory observations not well modeled by ruina s equations the unexpected shear induced weakening of silica rich rocks at high slip speed goldsby and tullis 2002 and fault strength in the brittle ductile transition zone shimamoto 1986 the examples provided as illustration may also be useful for quantitative modeling

el nino like events observed in the tropical pacific abstract the data provided by ships of opportunity operated by surtropac group centre orstom de noumea are used to study surface temperature and salinity in 1979 and 1982 negative anomalies of salinity occurred in the equatorial western pacific these are usually induced by el nino events although no el nino was noticed in the eastern pacific during the beginning of the year however along the tahiti panama shipping route the warm season in 1979 and 1982 lasted longer than usual empirical orthogonal analysis applied to the surface temperature along shipping routes in the eastern and central pacific reveal other events with the same signature as the 1976 el nino a weak event occurred in 1979 and we can also see the beginning of the 1982 major event eastern central and western anomalies are significant correlated

implications of the mechanical and frictional behavior of serpentinite to seismogenic faulting the widespread occurrence of serpentinite along major strike slip seismogenic faults warrants systematic investigation to determine how its frictional characteristics may affect slip along the fault different locations along the motagua fault zone in guatemala were sampled to investigate the sliding mode as a function of composition and texture confining pressure and displacement rate air dried right circular cylinders 7 1 cm in length and 3 3 cm in diameter with a precut 35 to the cylinder and load axes were deformed at confining pressures up to 200 mpa room temperature and displacement rates of 10 4 and 10 7 cm s compositional analyses of specimens cored from five blocks of serpentinite that were tested show that the serpentinite can be divided into two groups one is a mesh textured serpentinite containing up to 70 serpentine mostly lizardite 11 enstatite phenocrysts 19 oxides and minor amounts of olivine and carbonate undifferentiated the other is a flare textured serpentinite containing up to 84 serpentinte mostly antigorite 10 oxides 6 magnesite and dolomite and almost no enstatite or olivine the flare serpentinite undergoes the transition from stable sliding to stick slip sliding at confining pressures as low as 10 mpa the mesh serpentinite results only in stable sliding up to confining pressures of 200 mpa and a displacement rate of 10 4 cm s at a displacement rate of 10 7 cm s only one of the three mesh blocks exhibits stick slip the shear stress required to initiate sliding for the flare textured serpentinites is given by 0 77 n for the mesh textured serpentinites at a displacement rate of 10 4 cm s it is given by 0 56 n and at a displacement rate of 10 7 cm s by 0 50 n differences in fracture strength between the five specimen blocks correlate with differences in frictional strength differences in the sliding behavior and in the deformation observed along the sliding surfaces can be related to the mineralogical components of each serpentinite type extensive deformation occurs in the mesh serpentinites as a result of hard oxide and enstatite indenters ploughing through a soft lizardite substrate lizardite lizardite contacts prevail and these deform by plastic flow the stick slip behavior in the flare textured serpentinites is explained by brittle failure of antigorite antigorite contacts the smaller grain size of the flare serpentinites relative to the mesh serpentinites may also contribute to the higher frictional strength of the flare serpentinites we discuss the implications of the antigorite rich flare textured serpentinite to seismogenic faulting in particular we consider the case of antigorite forming by shear heating induced progressive metamorphism of lizardite chrysotile serpentinites if serpentine dehydration temperatures are approached talc may form having a lower coefficient of friction than serpentine talc may act as a lubricant along the fault plane

incipient subduction at the contact with stretched continental crust the puysegur trench a seismic benioff zone and plate kinematics show puysegur trench south of new zealand transitioning to subduction because the local structure and its influence on subduction initiation is poorly understood we conducted a seismic survey with ocean bottom seismometers and multichannel seismic profiles our early results show that the overriding pacific plate beneath the solander basin is composed of block faulted and thinned continental crust and the inner trench wall of northern puysegur ridge is composed of folded and faulted sediment the megathrust interface has been imaged and shows 500 m of downgoing undisturbed sediments combining plate kinematic history with seismic velocity inferred density we show that the density difference across the plate boundary changed as oblique strike slip plate motion juxtaposed dense oceanic crust with thinned continental crust the density difference rapidly increased 18 to 15 ma coincident with subduction initiation suggesting that compositional differences have a large influence on subduction initiation

structures in natural serpentinite gouges abstract serpentinite gouges from cataclastic fault zones in the voltri massif nw italy are dominated by few discrete boundary parallel y shears and oblique synthetic riedel r 1 shears separating domains of massive gouge unlike natural clay rich fault gouges the cohesive microbreccia domains only rarely show foliations these structures dominated by discrete shears indicate a significant degree of localization of the deformation at the scale of the gouge the dominant gouge forming mechanisms were fracturing of olivine and pyroxene grains along serpentinized grain boundaries and initiation of transgranular fractures in pyroxene grains in domains of lattice bending and grain indentation syn tectonic serpentinization added to the weakening of fragments textural similarities with experimentally formed gouges suggest that the voltri fault zones were dominated by stick slip sliding hence that they may represent fossil seismogenic faults a notable aspect of the gouge texture in the voltri fault zones is a sigmoidal shape of the riedel shears which bend into parallelism with the y shears it is argued that this geometry was largely controlled by local perturbations of the stress field near heterogeneities on y or p shears the average spacing between the r 1 shears increases linearly with 0 47 times the distance between bounding y shears this proportionality may reflect a balance between the cohesive strength of the gouge material and the shear stresses acting on the bounding slip planes

oceanic versus emplacement age serpentinization in the josephine ophiolite implications for the nature of the moho at intermediate and slow spreading ridges we present field petrographie and geochemical evidence for oceanic serpentinization in the 162 ma josephine ophiolite of nw california and sw oregon undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation the dikes intruding serpentinites are of two types 1 fe ti enriched dikes with normal mid ocean ridge basalt nmorb magmatic affinity which are geochemically linked to the uppermost lavas and a late fe ti dike within the crustal sequence of the ophiolite and 2 hornblende bearing calc alkaline dikes intruded during ophiolite emplacement between 150 and 146 ma based on crosscutting relationships between dikes and serpentinites serpentinization of upper mantle peridotites took place at or near the ridge axis during periods of amagmatic structural extension in the absence of a magma chamber lizardite bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of 350 c within the oceanic upper mantle oceanic lizardite bearing serpentinized shear zones from the ultramafic cumulate section to the basal sole indicate that the entire josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement the ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement and we suggest that the paleo moho in the josephine ophiolite is a serpentinization boundary the basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement oceanic serpentinization in the josephine ophiolite took place beneath a 2 3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid ocean ridges as well as at slower spreading ridges

two types of brittle fracture of silicate rocks under confining pressure and their implications in the earth s crust abstract four silicate rocks granite gabbro dunite and eclogite were triaxially compressed up to a confining pressure of 3 gpa at room temperature and were found to exhibit brittle fracture behaviour from the measurements of the compressive and frictional strengths and the activity of acoustic emission ae and the observations of the microstructure of fractured specimens the pattern of fracture was found to change when the compressive strength became equal to the frictional strength two types of fracture mechanism low pressure and high pressure types were inferred the low pressure type fracture exhibits features of brittle fracture equivalent to those recorded by previous workers the features of high pressure type fracture are similar to those observed between brittle fracture and ductile creep in high temperature triaxial experiments microcracks are not concentrated close to the main fault and the main fault is sharp and oriented at approximately 45 to the stress direction this suggests that the high pressure type fracture might correspond to the transitional fracture type between the brittle and ductile regimes at room temperature for granite and gabbro samples of a few centimeters size high pressure type fracturing begins at 0 8 gpa confining pressure while for dunite and eclogite it begins at 1 0 and 2 0 gpa respectively the size effect on compressive strength under confining pressure was estimated based on the suggestion that the uniaxial strength decreases with increasing specimen size and ceases to decrease for specimens 1 m this estimation suggests that the compressive strength for a granite specimen a few meters in size would become equal to the frictional strength at confining pressures as low as 60 mpa the significance of this is that it may be necessary to model earthquakes using the high pressure type fracturing because in the absence of pore pressure 60 mpa confining pressure corresponds to a depth of 3 km faulting processes in the crust including both the initial faulting and the movement of existing faults were inferred using the experimental results and the estimated size effect on strength

dynamics of intraoceanic subduction initiation 1 oceanic detachment fault inversion and the formation of supra subduction zone ophiolites subduction initiation is a critical link in the plate tectonic cycle intraoceanic subduction zones can form along transform faults and fracture zones but how subduction nucleates parallel to mid ocean ridges as in e g the neotethys ocean during the jurassic remains a matter of debate in recent years extensional detachment faults have been widely documented adjacent to slow spreading and ultraslow spreading ridges where they cut across the oceanic lithosphere these structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration and can therefore effectively localize deformation here we show geochemical tectonic and paleomagnetic evidence from the jurassic ophiolites of albania and greece for a subduction zone formed in the western neotethys parallel to a spreading ridge along an oceanic detachment fault with 2 d numerical modeling exploring the evolution of a detachment ridge system experiencing compression we show that serpentinized detachments are always weaker than spreading ridges we conclude that owing to their extreme weakness oceanic detachments can effectively localize deformation under perpendicular far field forcing providing ideal conditions to nucleate new subduction zones parallel and close to or at spreading ridges direct implication of this is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context and calls for future research on three dimensional modeling of subduction initiation and how upper plate extension is associated with that

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

the instantaneous rate dependence in low temperature laboratory rock friction and rock deformation experiments 1 earthquake occurrence probabilities that account for stress transfer and time dependent failure depend on the product of the effective normal stress and a lab derived dimensionless coefficient a this coefficient describes the instantaneous dependence of fault strength on deformation rate and determines the duration of precursory slip although an instantaneous rate dependence is observed for fracture friction crack growth and low temperature plasticity in laboratory experiments the physical origin of this effect during earthquake faulting is obscure we examine this rate dependence in laboratory experiments on different rock types using a normalization scheme modified from one proposed by tullis and weeks 1987 we compare the instantaneous rate dependence in rock friction with rate dependence measurements from higher temperature dislocation glide experiments the same normalization scheme is used to compare rate dependence in friction to rock fracture and to low temperature crack growth tests for particular weak phyllosilicate minerals the instantaneous friction rate dependence is consistent with dislocation glide in intact rock failure tests for each rock type considered the instantaneous rate dependence is the same size as for friction suggesting a common physical origin during subcritical crack growth in strong quartzofeldspathic and carbonate rock where glide is not possible the instantaneous rate dependence measured during failure or creep tests at high stress has long been thought to be due to crack growth however direct comparison between crack growth and friction tests shows poor agreement the crack growth rate dependence appears to be higher than the rate dependence of friction and fracture by a factor of two to three for all rock types considered

deformation of antigorite serpentinite at high temperature and pressure abstract deformation experiments were conducted in a griggs type apparatus to constrain the rheology of antigorite serpentinite at high temperatures and pressures samples deformed at temperatures of 400 625 c and pressures of 0 85 1 5 gpa experienced semi brittle deformation resulting in localization on faults at 5 to 25 strain samples deformed at 300 c exhibited transitional behavior the stresses measured for our samples are in good agreement with recent experiments conducted in the deformation dia apparatus however our results indicate that steady state ductile flow is not achieved in contrast with interpretations based on the deformation dia experiments extrapolation of our data to geologic conditions suggests that serpentinites may be weak enough during localized frictional sliding to account for slab wedge decoupling inferred from geologic observations experiments conducted above the thermal stability of antigorite at 700 c and 1 5 gpa show evidence for distributed deformation this observation does not support the hypothesis that intermediate depth earthquakes result directly from dehydration embrittlement in addition samples exhibit velocity strengthening from 300 to 700 c a frictional behavior that inhibits earthquake nucleation

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

rheology and deformation mechanisms of an isotropic mica schist we have investigated the transitional semibrittle deformation of a mica schist 75 biotite by shortening cylinders cored at 0 45 and 90 to foliation to varying strains at confining pressures pc to 500 mpa constant strain rates e from 1 5 10 7 to 1 6 10 4 s 1 and temperatures t from 25 to 400 c deformation is concentrated within one or more throughgoing millimeter wide shear zones at all conditions these localize at low strains e 2 through the nucleation and coalescence of dense sets of intragranular microkink bands mkbs despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions the differential stresses required for shear zone development vary little with fabric orientation biotite schist undergoes a transition from strain softening to steady strength mechanical response at confining pressures in the range 75 to 150 mpa the pressure sensitivity of strength characterized by the slope of the mohr envelope decreases from 0 5 at pc 100 mpa to 0 1 at pressures greater than 200 mpa reflecting the increasing contribution of glide and kinking in biotite at higher pressures however dilatancy associated with microcracking and void formation along mkb boundaries persists to at least 500 mpa within the pressure insensitive regime 200 pc 500 mpa temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along t decreasing or e increasing paths path dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments by assuming an exponential relationship between differential stress d and strain rate e of the form e c exp d exp q rt we fit the data with two end member flow laws with a single activation energy q 89 kj mol and exponential constants ss 0 15 0 01 mpa 1 and ws 0 55 0 04 mpa 1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to t and e respectively application of the results to crustal deformation suggests that mica rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones

microstructure of serpentinite mylonites from the josephine ophiolite and serpentinization in retrogressive shear zones california the josephine peridotite has experienced deformational episodes over a wide range of temperatures deformation within the stability field of a serpentine group mineral appears invariably accompanied by retrogressive metamorphism serpentinization two general types of deformed serpentinites have been identified 1 incohesive serpentinites most common which appear to have experienced low temperature predominantly brittle deformation and 2 serpentinite mylonites which have experienced plastic flow near the upper limit of antigorite stability the distinction between these two is based on textural and mineralogical criteria serpentinite mylonites are cohesive homogeneously foliated and typically strongly lineated rocks composed primarily of antigorite and magnetite they occur in planar bounded zones enclosed within the josephine peridotite that were the loci of both noncoaxial deformation and serpentinization we discuss the development of two important shear sense indicators porphyroclast systems and oblique shear band foliations as observed in these rocks porphyroclast systems in serpentinite mylonites are often comprised of cores and mantles of different mineralogy cores are usually composed of primary pyrogenetic minerals whereas mantles including tails are composed of products of the serpentinization reaction these systems are therefore termed porphyroctasts with neocrystallization tails oblique shear bands are common micro structures in serpentinite mylonites shear bands which offset the mylonitic foliation along narrow discrete fractures are termed f type whereas those which offset the mylonitic foliation along narrow ductile micro shear zones are termed d type the latter are considered more reliable for shear sense determinations strains accomplished by movement on shear bands are probably small strain resulting from a volume increase dur ing serpentinization while holding the shear zone width constant is sufficient to produce the shear bands therefore we suggest that oblique shear bands may develop to accom modate volume increases associated with deformation

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

microfracturing damage and failure of brittle granites 1 the evolution of stress induced damage and the eventual brittle failure are experimentally analyzed for mount scott granite of oklahoma we quantify the damage intensity in two methods and directly compare model predictions and actual damage the 14 samples of the medium grain size granite were loaded triaxially at dry conditions room temperature and under 41 mpa confining pressure microfractures were mapped in five samples and the majority of them 80 belong to two groups tensile microfractures trending subparallel to the loading axis and shear microfractures trending 11 40 off the loading axis the tensile microfractures dominate the low stress stage and they remain intragranular with a stress increase the relative density of shear microfractures increases with increasing stresses and they formed elongated intergranular zones of coalescing microfractures we compared two independent values of damage intensity 1 the macroscopic experimentally measured reduction of the deformation modulus and 2 the expected reduction of this modulus calculated with several damage models for the density of the mapped microfractures our fracture density data best fit the model of noninteracting cracks of kachanov 1992

stabilizing effect of high pore fluid pressure on slip behaviors of gouge bearing faults this work is supported by the department of energy office of science the office of basic energy sciences chemical sciences geosciences biosciences division under award no de fg 0207er15916 and by the national science foundation under grant no ear 1761912

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data