lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

instability of a lithospheric step beneath western north island new zealand lithospheres of different thicknesses are often juxtaposed by movement on a continental transform boundary such a boundary with a step change in densities may trigger a gravitational instability as lateral pressure gradients are created where normal mantle lithosphere terminates against less dense asthenospheric mantle here we show for plausible values of the lithospheric viscosity a mechanism by which the thicker mantle lithosphere will drip off into the lower density asthenosphere as the mantle deforms it also progressively thickens and then thins the overlying crust creating a topographic wave that migrates in concert with the removal of mantle lithosphere within western north island new zealand geophysical data define a sharp lithospheric step across the taranaki ruapehu line and geological observations provide a history of uplift and subsidence that has propagated southward in the past 12 m y the rate of observed north to south migration of the wave 30 mm yr its wavelength 250 km and amplitude 1 km are compatible with it being caused by progressive removal of mantle lithosphere if the viscosity of the uppermost lithospheric mantle is 5 10 20 pa s providing one of the clearest examples yet of this fundamental geological process

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

rapid crustal accretion and magma assimilation in the oman u a e ophiolite high precision u pb zircon geochronology of the gabbroic crust 1 new high precision u pb zircon geochronology from the oman united arab emirates u a e ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust the new data come from a well preserved and exposed crustal section in the center of the wadi tayin massif single grain and grain fragment 206pb 238u dates from upper level gabbros tonalites trondhjemites and gabbroic pegmatites corrected for initial th exclusion range from 112 55 0 21 to 95 50 0 17 ma with most data clustered between 96 40 0 17 to 95 50 0 17 ma zircon dates from upper level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half rates of 50 100 km ma dates from tonalites trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper level gabbros suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates three of the dated upper level gabbros and a single gabbroic pegmatite from the base of the crust have 1 ma intrasample variability in single grain dates suggesting assimilation of older crust during the formation or crystallization of the magmas whole rock end t of seven samples including the upper level gabbros with variable zircon dates have tightly clustered initial values ranging from end 96 ma 7 59 0 23 to 8 28 0 31 the end values are similar to those from other gabbros within the ophiolite suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas the new dates suggest that the studied section formed at a fast spreading mid ocean ridge between 96 4 95 5 ma the large intrasample variability in zircon dates in some samples is unexpected in this setting and may be related to propagation of a younger ridge into older oceanic lithosphere

seaward dipping reflectors and the continent ocean boundary at passive continental margins abstract the definition of the continent ocean boundary at passive continental margins has proved to be an elusive task even the relatively direct method of seismic refraction experiments has yielded results that cannot always be interpreted unequivocally multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors dipping seaward to form a wedge shaped structure their general characteristics and velocity structure suggest that they may be largely volcanic in nature but there is no agreed upon model for their origin nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean and as such they are extremely important structural markers that may yield important evidence concerning the structure and evolution of passive margins

two forms of volcanism implications for mantle flow and off axis crustal production on the west flank of the southern east pacific rise seamarcii side scan sonar and bathymetric data on the west flank of the east pacific rise 16 18 s reveal a large number of seamounts organized primarily in chains the easternmost ends of the chains near the ridge axis appear to be active as indicated by fresh lava flows in addition areas of unusually high reflectivity representing recent lava flows were found as far as 60 to 80 km from the spreading axis the flows are sometimes dammed by abyssal hill scarps formed near the ridge axis that have maximum relief of the order of 100 m in other cases the scarps appear to be buried by flows the disappearance of scarps can be used to estimate the extent of old lava flows or volcanic debris that are not detected acoustically in the sonar image under this criterion lava flows and volcanic debris surrounding the seamounts cover 40 50 of the survey area of about 18 000 km2 with an estimated average thickness of about 100 m the volume of lava flows and volcanic debris is about 0 8 0 3 of that of the total crust the volume of the seamounts is estimated by isolating the seamounts from the background topography with an anisotropic median filtering technique the volume of seamount edifices is 1 05 0 05 of the volume of the crust thus the total volume of volcanic material extruded off axis is about 1 5 to 2 2 of the volume of the crust the decreasing volcanic activity away from the ridge axis suggests that near axis seamounts may be associated with the upwelling system beneath the ridge perhaps by preferential melting of embedded passive heterogeneities however the decrease in volcanism with increasing distance from the axis also could be caused by a decrease in lithospheric vulnerability to penetration by magma since no apparent east west faults and no simultaneous volcanic activity along the length of a seamount chain are observed north south extension and ridge perpendicular convection are unlikely to be major causes of seamount formation in the study area the existence of closely spaced linear chains and the tendency for fresh flows to be found at near ridge ends of chains suggest that there are discrete sources in the upper mantle active for extended periods over 1 8 m y passive heterogeneities embedded in the upwelling mantle would have to be highly elongated along streamlines to produce persistent nearly stationary melting anomalies a variety of mechanisms may be responsible for the off axis volcanism but the single hypothesis most consistent with all observations is a mini hotspot origin although fresh lavas 80 km off axis are consistent with broad mantle upwelling beneath the ridge the possibility of narrow upwelling cannot be ruled out because the seamounts could be caused by independent buoyant upwelling of mini plumes that can penetrate thin lithosphere more easily near the axis

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

stress field associated with elliptical inclusions in a deforming matrix mathematical model and implications for tectonic overpressure in the lithosphere abstract shear zones and competent layers and boudins represent viscosity heterogeneities within the rock mass differences in viscosity impel differences in strain rates between the background material and the heterogeneities in this work we represent the viscosity heterogeneities as elliptical inclusions we use the kolosov muskhelishvili equations for the incompressible viscous flow problem in and around elliptical inclusions systematic investigation of the stress equilibrium at the matrix inclusion interface shows that the mean stress equivalent to the total pressure is not continuous across viscosity boundaries the results predict that pressure and stress perturbations depend strongly on the orientation of the elliptical heterogeneity with respect to the far field stresses a viscosity ratio of 10 between the inclusion and the surrounding material is sufficient to produce pressure discontinuities approximately equal to the magnitude of the effective shear stress of the strongest rock under the considered physical conditions comparison of the analytical solutions with thermo mechanical models confirms pressure incongruity and suggests that dynamic parameters such as pressure and effective shear stress vary spatially and temporally within deforming two viscosity rock systems as a corollary the dependence of metamorphic phase equilibria on thermodynamic pressure implies that shear zones taken as weak inclusions and boudins taken as hard inclusions may record non lithostatic pressure during deformation

comments on the interpretation of deformation textures in rocks abstract in rocks that undergo ductile deformation preferred orientation develops as a result of intracrystalline slip and mechanical twinning the orientation distribution is a consequence of the microscopic mechanisms and of the strain path it can be used to get some insight into the deformation history however it is never unique the interpretation relies largely on polycrystal plasticity theory the concepts of stress equilibrium and strain compatibility which are two extreme assumptions made to model deformation are discussed new approaches such as the viscoplastic self consistent theory are a compromise and may be applicable to mineral systems which display a high degree of plastic anisotropy important extensions allow for heterogeneous deformation in the polycrystal from grain to grain and even within grains in correspondence with microstructural observations all these theories defy the popular notion which is becoming entrenched in the geological literature that the microscopic slip plane normal aligns with the axis of maximum principal compressive stress and that in simple shear the crystallographic slip plane rotates into the macroscopic shear plane and the slip direction into the macroscopic shear direction an orientation referred to by geologists as easy glide it is emphasized that future work on texture development of rocks should be based on rigorous physics rather than ingenious intuition in accordance with an old recommendation of walter schmidt

seamount abundances and distributions in the southeast pacific abstract sea beam bathymetry was recorded for 17 277 km of ship track in the southeast pacific and has been analyzed for seamount population characteristics all the ship tracks are over the pacific plate and most fall along 7 lines parallel to the east pacific rise between 7 s and 22 s the lines fall into three age categories one line is over 0 5 2 ma crust three are over 5 10 ma crust and three are over 30 40 ma crust seamount locations were recorded diameters were manually estimated and heights were measured if the swath crossed the seamount center over 382 features were counted along the entire ship track with heights ranging from 50 m to over 2500 m with sampling most consistent at heights between 100 m and 1000 m height to radius ratios vary considerably suggesting that seamount shapes do not scale to a single parameter the observed variety of morphological forms demonstrates that there is a wide range of seamount shapes for small features the distribution of sizes for the population was approximated by exponential dependence and by power law dependence using methods developed by smith and jordan 17 the power law distribution overestimates abundances at the smallest size ranges and both distributions fail to predict abundances of large seamounts determined from wide beam data 17 size distributions were also determined for seamounts in each seafloor age category almost all small seamounts appear to be produced on crust younger than 0 5 2 ma while the number of larger seamounts increases to 5 10 ma for all size ranges observed here the number of seamounts at 5 10 ma and at 30 40 ma is roughly identical these observations suggest that most small seamounts are formed on very young thin lithosphere that permits the passage of small volumes of magma small volumes would cool in older thicker lithosphere before reaching the surface although larger magma bodies might not numerous small sources of melt must exist near the ridge crest to supply the small seamounts probably trapped remnants of the large scale upwelling

the source of a type magmas in two contrasting settings u pb lu hf and re os isotopic constraints the sources of post orogenic a type magmas from two distinct geodynamic settings are compared the end of the ca 514 480 ma delamerian orogeny southeastern south australia was marked by 10 myr of bimodal a type magmatism driven by convective removal of thickened lithosphere initial os and hf isotope ratios record a heterogeneous lithospheric mantle source with some input from aesthenospheric mantle mafic parental melts fractionated to produce the granites in contrast initial os isotope ratios of the a type magmas that comprise the ca 1598 1583 ma mesoproterozoic gawler felsic large igneous province central south australia record a dominant evolved lower crust component however initial hf isotope ratios from these samples are depleted indicating a mantle source for lithophile elements this voluminous bimodal magmatism lasted for 15 myr and ended the wartakan orogeny in both cases the homogenisation of chemical rheological heterogeneities inherited from terrain amalgamation and orogenic thickening strengthened the lithosphere the contemporaneous fusion of heterogeneous mantle crust may represent a common stabilising influence on the lithospheric column regardless of tectono magmatic setting

investigation of the strength contrast at the moho a case study from the oman ophiolite we investigate the viscosity contrast between the crust and mantle by analysis of deformed gabbronorite dikes in the mantle section of the oman ophiolite our study provides a rare opportunity to directly constrain the rheological contrast between a plagioclase rich lithology typical of the lower crust and an olivine dominated harzburgite we constrain deformation conditions using field relationships geothermometry grain size piezometry and electron backscatter diffraction the viscosity of gabbronorite was at least two orders of magnitude lower than the viscosity of the harzburgite consistent with the rheological contrast between plagioclase and olivine calculated via experimental flow laws these flow laws predict that a significant viscosity contrast exists at the crust mantle boundary where the crustal lithology is dominated by plagioclase or where deformation localizes within plagioclase rich layers

large shear zones with no relative displacement in the oman ophiolite the large scale makhibiyah shear zone in wadi tayin massif was generated with no or little relative motion between the two adjacent blocks in contrast with what is reported from otherwise similar shear zones in deep crust and upper mantle this shear zone is asymmetrical with along one margin an asthenospheric mantle similar to 1200 degrees c and along the adjacent margin a lithospheric mantle similar to 1000 degrees c within the hotter side and with increasing shear strain horizontal flow lines smoothly swing towards the shear zone direction before abutting against the wall of the lithosphere side profuse mafic melts issued from the hotter mantle are frozen in the shear zone by cooling along this lithospheric wall tectonic and magmatic activities are entirely localized within the asthenospheric compartment mantle flow lines were rotated during their channelling along this nw se shear zone in the nw and se opposite directions depending on whether the flow lines are deviated nw or se dextral or sinistral shear sense is recorded in the shear band mylonitic peridotites this demonstrates that the shear zone was not generated by strike slip motion a conclusion supported by regional observations

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

rheological properties of the detachment shear zone of an oceanic core complex inferred by plagioclase flow law godzilla megamullion parece vela back arc basin philippine sea abstract we tested plagioclase flow laws for rheological properties of the detachment shear zone developed in an oceanic core complex gabbroic mylonites occur extensively over the 125 km length of the godzilla megamullion an enormous oceanic core complex situated in an extinct philippine sea back arc basin the mylonites were produced in the detachment shear zone at temperatures of 650 850 c in the lower crust over a period of approximately 4 million years corresponding to a slow spreading rate of 2 54 0 21 cm yr i e 8 05 10 10 m s applying the rheological parameters for plagioclase flow laws combined with the geochronological spreading rate we calculated deformation mechanism maps of plagioclase as shear strain rates of 10 12 to 10 8 s 1 corresponding to shear zones in thickness of 0 1 to 1000 m our results show that assuming a constant stress condition defined by a shear strain rate of 10 12 s 1 for grain size of 1000 m in a temperature range between 650 and 850 c petrofabric parameters such as crystal preferred orientations and dynamically recrystallized grain sizes along with estimated equilibrium temperatures can be possibly explained by the deformation mechanism maps of plagioclase for shear strain rates mostly of 10 11 to 10 9 s 1 it suggests that even if the entire thickness of the detachment shear zone may lie several hundred meters below the spreading center the shear zone could be stratified comprising many anastomosing narrow zones it implies that during the development of the detachment fault strain localization would occur in the lower crust over a broad 1000 m zone at high temperatures

strain localization and fluid infiltration in the mantle wedge during subduction initiation evidence from the base of the new caledonia ophiolite despite decades of petrological and geochemical studies the nature and setting of obducted ophiolites remain controversial the influence of supra subduction zone environments on pre existing oceanic lithosphere is yet to assess and the processes leading to subduction obduction initiation are still poorly constrained our study documents successive influx of slab derived fluids and progressive strain localization within the upper mantle in a supra subduction environment during the first few my of the subduction history we focus on strongly sheared mafic amphibolites intruding peridotites near the mantle crust transition of the new caledonia obducted ophiolite and 50 to 100 m above the basal thrust contact of the ophiolite these m to hm long and several m thick shear bands are interpreted as inherited small scale intrusions of mafic melts probably dikes or sills which were derived from a moderately refractory mantle source refertilized by supra subduction zone fluids 40ar 39ar age constraints on pargasite at ca 90 ma suggest that they could be inherited from the former pacific west dipping subduction secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration 1 the first stage of deformation and metasomatism is marked by syn kinematic growth of ca amphibole at 700 800 c and 3 5 kbar with a distinctive supra subduction zone signature and controlled later channelization of aqueous fluids 40ar 39ar dating on magnesio hornblende indicates that this deformation episode occurred at ca 55 ma coincident with east dipping subduction initiation 2 the main metasomatic stage characterized by the development of a phlogopite rich matrix wrapping peridotites and amphibolite boudins points to the percolation of alkali rich aqueous fluids at still high temperature 650 750 c 3 the last low temperature 600 c metasomatic stage results in the formation of deformed veinlets containing talc chlorite and serpentine

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

shape analysis of pacific seamounts abstract shape statistics have been compiled from 85 profiles of well surveyed pacific seamounts in the height range 140 3800 m a flat topped cone was fit to each seamount s cross sectional profile maintaining the slopes of the sides as closely as possible on each profile a basal width d b a summit width d t and a maximum height h were measured the height to basal radius ratio is h is estimated by the ratio 2hd b and flatness f by the ratio d t d b slope angle arctan e is estimated from e 2h d b d t summit height and basal radius are found to be highly correlated r 0 93 the 85 point sample mean of the height to basal radius ratio is h 0 21 0 08 implying that a seamount s summit height is typically one fifth its basal radius despite the high correlation individual points show some scatter and there may be groupings into different morphological types for example all but one of the seamounts with summit heights above 1000 m have values of h that are larger than the sample mean the 85 point sample mean of flatness is f 0 31 0 18 data points show a large scatter with values of f varying between 0 a pointy cone and 0 69 a flat topped cone a histogram representation of flatness however indicates that certain values of f may be more common than others the histogram shows a bimodal distribution with maxima occurring at values of f in the ranges 0 10 0 20 and 0 35 0 50 moreover there is some evidence that the mean flatness decreases with summit height so that the preferred shape of a large sized seamount may be a pointy cone slope angle has an 85 point sample mean of 18 6 individual values of vary between 5 and 36 in addition to having a lower than average mean flatness seamounts with heights above 2600 m also have a lower than average mean slope angle 15 to determine which variables account for most of the observed variation in the seamount shapes a multivariate principal component analysis was performed on the data using five shape variables summit height basal radius summit radius flatness and slope the analysis indicates that most of the variation is described by two variables flatness and summit height

deformation mechanism maps for feldspar rocks abstract deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates the maps display constant temperature contours in stress grain size space for strain rates ranging from 10 16 to 10 12 s 1 two fields of dominance of grain boundary diffusion controlled creep and dislocation creep are separated by a strongly grain size sensitive transition zone for wet rocks diffusion controlled creep dominates below a grain size of about 0 1 1 mm depending on temperature stress strain rate and feldspar composition plagioclase aggregates containing up to 0 3 wt water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks the strength of water bearing feldspar rocks is moderately dependent on composition and water fugacity for a grain size range of about 10 50 m commonly observed in natural ultramylonites the deformation maps predict that diffusion controlled creep is dominant at greenschist to granulite facies conditions low viscosity estimates of 10 18 10 19 pa s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures 900 c or at lower temperatures diffusion creep of fine grained rocks possibly localized in abundant high strain shear zones for similar thermodynamic conditions and grain size lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks

rheology of synthetic anorthite diopside aggregates implications for ductile shear zones 1 we investigated the high temperature creep strength of fine grained anorthite diopside rocks at temperatures ranging from 1323 k to 1523 k and at 300 mpa confining pressure in a paterson type gas medium deformation apparatus flow stress varied between 20 and 450 mpa resulting in strain rates between 6 1 10 7 s 1 and 7 5 10 4 s 1 pure diopside and anorthite samples were hot pressed from crushed natural single crystals and glass powders respectively two phase samples were produced by hot isostatic pressing of mechanically mixed powders of anorthite glass with 25 50 and 75 vol diopside particles arithmetic mean grain size of the anorthite matrix is dan 3 5 m three different ranges of diopside particle size were used ddi 25 m 35 m and 45 m water content of as is samples was about 0 05 0 02 wt h2o and predried samples contain about 0 004 0 001 wt h2o at experimental conditions as is samples are assumed to be water saturated water content of predried samples is about 3 times less than that of starting diopside single crystals the specimens contain about 1 vol glass located at fluid inclusions and some multiple grain junctions two grain boundaries examined by high resolution transmission electron microscopy did not show amorphous layers to a resolution of 1 nm at experimental conditions pure diopside aggregates are about 2 3 orders of magnitude stronger than pure anorthite samples for as is and predried specimens respectively in general strength of the two phase aggregates increases with increasing diopside content but remains between isostress and isostrain rate bounds aggregate strengths predicted from continuum mechanics models are in good agreement with the experimental data for dilute diopside particle mixtures and high volume fractions when diopside particles form a load bearing framework at low stresses 100 200 mpa the stress exponent is n 1 suggesting diffusion controlled creep at higher stresses mechanical data and microstructures suggest that samples deformed in the transition region between diffusion controlled creep and dislocation creep for pure anorthite and diopside aggregates deforming in dislocation creep we estimated stress exponents of n 3 and n 5 5 respectively for the two phase aggregates n is between n 3 and n 5 depending on diopside content at low stresses deformation microstructures indicate load transfer from a weak anorthite matrix to stronger diopside particles creep activation energies for pure diopside and anorthite mixtures range from 286 kj mol 1 for wet anorthite deformed at low stresses to 691 kj mol 1 for dry diopside deformed at high stresses activation energies of two phase mixtures are between or close to those of the end members as is samples have significantly lower activation energies than predried samples

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

the deformation of plagioclase investigated using electron backscatter diffraction crystallographic preferred orientation data abstract extensive plagioclase grain orientation datasets have been collected using the rapid and accurate electron backscatter diffraction sem technique the crystallographic preferred orientation cpo of a porphyroclastic rich plagioclase layer and an equigranular plagioclase layer in an amphibolite facies shear zone from harris scotland were investigated in order to study their cpos and deformation mechanisms the layers have random cpos together with other microstructures and misorientation distributions these suggest dynamic recrystallisation followed by dominant grain boundary diffusion creep this is surprising given the coarse 100 250 m recrystallised grain size of the layers plagioclase within tectonites of these grain sizes in the amphibolite facies are generally inferred to deform by dislocation creep from microstructures and observed strong cpos deformation mechanism maps show that this style of deformation is possible at very slow strain rates

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

texture and elastic anisotropy of a mylonitic anorthosite from the morin shear zone quebec canada a sample of anorthosite from the granulite facies morin shear zone quebec canada was investigated for crystal preferred orientation and elastic anisotropy time of flight neutron diffraction data obtained with the hippo diffractometer at lansce were analyzed with the rietveld method to obtain orientation distribution functions of the principal phases plagioclase clinopyroxene and orthopyroxene texture and microstructures are compatible with the plastic deformation of the aggregate under high t conditions all mineral phases depict a significant preferred orientation that could be related to the general top to the north shearing history of the morin shear zone texture patterns suggest that 010 001 in plagioclase and 110 001 in clinopyroxene are likely dominant slip systems using preferred orientation data p and s waves velocities and elastic anisotropy were calculated and compared with previous studies to explore elastic properties of rocks with different pyroxene plagioclase mixtures p wave velocity s wave splitting and anisotropy increase with clinopyroxene content seismic anisotropy is linked to the texture symmetry which can lead to large deviations between actual anisotropy and that measured along cartesian xyz sample directions lineation foliation reference frame this is significant for the prediction and interpretation of seismic data particularly for monoclinic or triclinic texture symmetries

plagioclase preferred orientation in layered mylonites evaluation of flow laws for the lower crust thesis s m joint program in marine geology and geophysics massachusetts institute of technology dept of earth atmospheric and planetary sciences and the woods hole oceanographic institution 2008

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

new type of olivine fabric from deformation experiments at modest water content and low stress a new type of olivine fabric was found by high strain simple shear deformation experiments in the presence of trace amounts of water at 0 5 2 2 gpa and 1470 1570 k in this fabric called e type fabric the olivine 100 axis is subparallel to the shear direction and the 001 plane is parallel to the shear plane this geometry suggests that the 100 001 slip system makes the dominant contribution to total strain this fabric is dominant at a modest water content 200 coh 1000 h 10 6 si at low stresses and high temperatures some mylonites from peridotite massifs show this type of olivine fabric which suggests the presence of water during the shear localization the seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric a type but the magnitudes of anisotropy are different between the two types for horizontal flow the amplitude of vsh vsv anisotropy is weaker but the amplitude of shear wave splitting is stronger for the e type fabric than for the a type dry fabric seismic anisotropy in the oceanic upper mantle may be due to the olivine e type fabric

sea floor lava fields on the east pacific rise a seabeam and seamarc i survey of a seamount group located several kilometres west of the axis of the east pacific rise epr at lat 9 53 n defines the existence of a large submarine lava field at the base of one of the volcanoes mib the field must be relatively young with respect to the surrounding terrain because lavas lap up against the base of mib seamount obscuring older erosional terrain and extend out over the faulted sedimented terrain of the flank of the epr volcanic cones within the lava field are aligned with the strike of fault scarps that are buried by the lavas mib seamount is characterized by a large well developed caldera and we suggest that the lava field was created during a recent volcanic episode when magmas migrated laterally away from the seamount9s reservoir along preexisting faults parallel to the ridge axis the withdrawal of magmas from the mib reservoir into the flanking parasitic eruptive plumbing system created the lava field and deflated the seamount summit this event contributed to the development of the mib caldera draining of shallow level magma chambers under seamounts by pressure controlled basal flank eruptions that form lava fields is an important mechanism for the development of seamount morphology

gravity anomalies isostasy and mantle flow at the east pacific rise crest bathymetry and gravity data obtained during a detailed hydrosweep survey of the southern east pacific rise from 7 s to 9 s are used to investigate isostasy at the axis of a fast spreading mid ocean ridge in particular we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160 km long 7 12 s 8 38 s ridge segment the crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km the summit is broad and flat and an axial summit caldera is present for the entire length of the ridge segment however the width and cross sectional shape of the crestal high vary systematically along the axis it is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment the ridge crest is marked by a free air gravity anomaly high about 15 20 km wide with an amplitude of 10 15 mgal relative to the ridge flanks mantle bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment the axial mantle bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross sectional area of the crestal bathymetric high the effects of cooling and subsidence away from the axis were removed from the bathymetry and free air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest the residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis the lithosphere was treated as a broken plate either with a constant flexural rigidity or an effective elastic thickness te which grows at a rate proportional to the square root of distance from the axis the best fitting values of te for the constant rigidity case are in the range of 0 3 0 6 km for the growing plate model te increases at a rate of 0 2 0 3 km the gravity constrains the mass deficiency to extend to a depth of 20 30 km for both lithospheric models we interpret this low density material as a region of partial melt feeding magma to the ridge axis the best fitting density anomalies imply that a 4 9 melt fraction is present beneath the crestal high upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis the mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment it thus appears that differences between the along axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three dimensional focused upwelling at slow spreading ridges to two dimensional sheetlike upwelling at fast spreading ridges rather the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along axis magma distribution

mafic granulite rheology implications for a weak continental lower crust abstract the jelly sandwich model of continental rheology long the consensus model has recently been challenged the controversy is based on the rheology of mafic granulite that is thought to make up most of the lower continental crust however because of technical issues in experimental rock deformation there are very few data on this problem here we report the rheology and fabrics of a reconstituted fine grained nominally dry i e no hydrous mineral mafic granulite 57 plagioclase 24 clinopyroxene 14 orthopyroxene 5 opaque minerals 0 16 0 28 wt h 2 o deformed at 1 1 1 2 gpa pressure in a modified griggs type deformation apparatus the rheology of this mafic granulite can be described by the constitutive equation of e a 3 2 0 4 exp 244 35 kj mol r t where e is in s 1 in mpa t in kelvin and a 10 2 0 1 6 mpa 3 2 s 1 our results provide new experimental evidence in support of the jelly sandwich lithosphere strength model and imply that mafic granulite with a moderate amount of water 0 05 0 08 wt h 2 o is likely to be a weak layer in the lithosphere both pyroxenes and plagioclase develop pronounced fabrics in responding to axial deformation the deformation mechanism is dislocation creep with 100 001 and 001 100 010 100 being the dominant slip systems for pyroxenes and plagioclase respectively the low strength of mafic granulite is ascribed largely to the significant weakening effect of dissolved water in pyroxenes and plagioclase a weak continental lower crust has many important implications for geodynamics of crust mantle interactions such as lower crust channel flow and delamination

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

preferred orientation of anorthite deformed experimentally in newtonian creep abstract synthetic anorthite aggregates were deformed in a paterson gas deformation apparatus at confining pressures up to 400 mpa in torsion and axial compression at temperatures between 950 c and 1200 c samples deformed in torsion under newtonian creep display development of texture or crystallographic preferred orientation as documented with synchrotron x ray diffraction measurements complex diffraction patterns were deconvoluted with the rietveld method to obtain quantitative texture information torsion samples deformed up to shear strains of 4 and samples deformed in compression at higher stresses to total strains of 0 3 develop clear textures texture and shape preferred orientation spo of torsion samples display a monoclinic pattern with an asymmetry inclined against the sense of shear consistent with polycrystal plasticity simulations that assume the deformation is accomplished by dislocation glide these results show that a material deforming in linear viscous creep can develop a strong texture in striking contrast to the paradigm that the presence of a texture precludes low stress newtonian behavior our observations show that the presence or absence of crystallographic preferred orientation is not sufficient to uniquely infer the dominant rheological mechanical regime as sometimes applied for interpretation of seismic anisotropy in the earth

density and size distribution of seamounts in the eastern pacific inferred from wide beam sounding data theoretical expressions relating the distribution of apparent seamount heights per unit length of ship track to the distribution of actual seamount heights per unit area of seafloor are derived for both narrow beam and wide beam echo sounders the shapes of seamounts are approximated as truncated right circular cones of constant slope e height to radius ratio and flatness f i e a seamount of basal radius r has a height h r a flat top of radius fr and a slope angle of arctan e the set of all seamounts in a given physiographic province is assumed to be a poissonian ensemble specified by a function v r the average number of seamounts per unit area for which r r a simple choice for the size distribution of seamounts and one shown to be consistent with data in the eastern pacific is to specify that it decays exponentially with r v r vo exp r in the case of a profile of length l taken in water of depth d with a wide beam echo sounder half beam width the shallow slope deep water approximation e 1 e d yields that the expected number of seamounts with apparent heights h h is n h l 2 1 ed 2 fh lv0e h we apply this sampling theory to apparent height data measured from 44 020 km of wide beam profiles on pacific crust of post mesozoic age although the spatial distribution of seamounts in the eastern pacific is certainly not poissonian the abundance statistics do not involve spatial correlations and we show they are robust with respect to deviations from the poisson model provided the statistics are interpreted as averages over the area actually sampled by the track lines the observed falloff of apparent heights in the eastern pacific is very nearly exponential justifying our use of an exponential model of seamount size distribution the shape parameters are estimated from multibeam surveys of pacific seamounts an analysis of 30 features in the height range 200 4000 m yields 0 21 0 011 and f 0 21 0 028 using these parameters we obtain vo 4 0 1 1 10 9 m 2 and 6 3 0 8 10 4 m 1 these values imply that per 106 km2 in the eastern pacific there are 1600 400 seamounts with summit heights greater than 300 m and 200 60 greater than 1000 m integrated over the entire size distribution seamounts occupy about 6 of the seafloor area and constitute about 0 4 of the oceanic crustal volume our estimates of seamount abundance are appreciably greater than those of most other authors a discrepancy we attribute primarily to the sampling bias of previous studies which generally used uncorrected map counts this hypothesis is corroborated by a comparison of our statistics with map count data for seamounts of heights exceeding 2000 m compiled for the eastern central pacific from the scripps charts above 3000 m the map counts are consistent with the predictions of the exponential model but below this height they are biased to low values

rapid emplacement of young oceanic lithosphere argon geochronology of the oman ophiolite 40 ar 39 ar dates of emplacement related metamorphic rocks beneath the samail ophiolite in oman show that cooling to

lithospheric age dependence of off ridge volcano production in the north pacific data for numbers of seamounts on north pacific ocean crust of different age indicate that the production rate of new off ridge seamounts volcanoes is proportional to the inverse of the square root of the age of the lithosphere this observation is consistent with several hypotheses which have been offered to explain the origin of off ridge oceanic volcanism and in combination with petrologic and paleomagnetic evidence for small oceanic volcanoes leads to a new self consistent model for off ridge volcanism in this model the production rate of off ridge volcanoes is controlled primarily by the availability of fracture zone conduit systems in the thickening lithosphere in contrast mantle plume or hotspot volcanoes may punch through the oceanic lithosphere decreasing production rate of off ridge volcanoes on old oceanic lithosphere may be associated with decreasing extent of partial melting of chemically and isotopically heterogeneous mantle material

amphibole fabric formation during diffusion creep and the rheology of shear zones we conducted experiments to examine the effect of amphibole forming hydration reactions on mineral fabric development and the strength of mafic rocks both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions 800 c 1 gpa amphibole that formed under hydrostatic conditions exhibits a random lattice preferred orientation lpo in contrast amphibole formed during deformation exhibits both a strong shape preferred orientation spo and lpo with a 001 maximum aligned subparallel to the shear direction plagioclase in both hydrostatic and deformed samples shows a very weak to random lpo at low effective strain rates 10 5 s 1 to 10 6 s 1 the stress exponent is 1 1 5 consistent with deformation accommodated by diffusion creep the correlation of the spo and lpo coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation the experimentally produced fabrics are strikingly similar to those observed in amphibolite grade natural shear zones supporting interpretations that such rocks deform by diffusion creep in addition the rheology of the fine grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite thus both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b