crustal lineaments distribution of lower crustal intrusives and structural evolution of the voring margin ne atlantic new insight from wide angle seismic models abstract five lineaments on the volcanic voring margin ne atlantic have been identified in crustal scale models derived from ocean bottom seismograph obs data it is suggested that the voring basin can be divided in four compartments bounded by the jan mayen fracture zone lineament a new lineament defined from this study the gleipne lineament the surt lineament and the bivrost lineament the nw se trending jan mayen gleipne and bivrost lineaments probably represent old zones of weakness controlling the onset of the early eocene seafloor spreading whereas the surt and new lineaments rotated ca 30 symmetrically from the azimuth of the gleipne lineament may represent adjustment features related to the early cretaceous early tertiary rifting the longest landward extent of a lower crustal high velocity body assumed to represent intrusions related to the last phase of rifting is found between the new lineament and the gleipne lineament where the body extends across the helland hansen arch northeastwards in the voring basin the landward limit of the body steps gradually seawards closely related to the interpreted lineaments northeast of the gleipne lineament the body terminates close to the fles fault complex north of the surt lineament it extends across the nyk high and northeast of the bivrost lineament the intrusions terminate around the voring escarpment evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin the active component is evident through the decrease in magmatism with increased distance from the icelandic plume and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement the increased thickness of the continental crust on the seaward side of the voring escarpment the upwarping of moho and thinning of the lower crustal high velocity layer in the western part of the voring basin as well as a strong shallowing of the moho observed in parts of the area between the jan mayen fracture zone lineament and the new lineament can be explained by lithospheric delamination models

a high resolution p wave model for the top 1200 km of the mantle this work attempts to map aspherical velocities in the top 1200 km of the mantle that are illuminated by the international seismological centre p wave data the processing includes summary ray sorting and hypocentral redetermination to cope with the extreme variation of ray coverage a multicell inversion is used to simultaneously constrain anomalies in overlapping cells the multicell solutions are summed after inversion to form the final model named p1200 at cell size of 1 1 and about 50 km in thickness the lateral resolution of the model is about 1 2 in major subduction zones and about 5 15 in most places major features in the model withstood tests on signal coherency and different reference models the long wavelength parts of the p1200 model correlate well with previous global models especially the s12 model of shear wave among the large features in the transition zone and below there are two fast bands one from canada to south america and another from siberia through east asia and the philippine sea to western australia there are also slow patches under the central pacific and from india to eastern africa though most subduction zones have slab like high velocity anomalies they are sandwiched by broad slow velocities at shallow depths some of the slabs appear to penetrate the 660 km discontinuity and stagnant slabs are also seen in the transition zone plume like slow anomalies exist below some prominent hot spots like hawaii iceland yellowstone and french polynesia

a moderate melting model for the voring margin norway based on structural observations and a thermo kinematical modelling implication for the meaning of the lower crustal bodies high p wave velocities 7 1 7 8 km s lower crustal bodies lcbs imaged along volcanic margins are commonly interpreted as plume and breakup related thick mafic underplating this interpretation is partly challenged in this paper based on new seismic observations and modelling of the outer voring basin norway an exceptional strong amplitude reflection the t reflection is particularly well defined below the north gjallar ridge ngr between 7and 8 s twt the t reflection is located near the volcanic lava flows emplaced during the ne atlantic breakup 55 54 ma ago and coincides with the top of the lcb forming a mid crustal dome based on structural and temporal relationships we show that the dome clearly influences the structural development of the ngr and predates the continental breakup at least by 10 15 ma using a thermo kinematical model we tried also to investigate and quantify the relationships between the extension lcb and the magmatic production modelling suggests that significant paleocene early eocene magmatism can be produced without any temperature anomaly in the mantle if differential stretching occurs during the breakup initiation the conclusion of 2d thermo kinematical parametric analysis is that the magmatic model predicts either little extension 5 with significant melting along the outer voring basin we suggest that the continental part of the lcb could not necessarily be breakup related and so magmatic as has often been stated previously it is concluded here that the continental part of the lcb observed beneath the outer voring basin may be partly or fully attributed to inherited high pressure granulite eclogite lower crustal rocks the real amount of mafic material emplaced along the outer voring basin could be 20 40 less than thought

experimental grain size sensitive flow of hot pressed brazilian quartz aggregates abstract grain size sensitive flow may be of particular importance in the flow of rocks in natural high strain zones hot pressed aggregates of synthetic ultrafine grained brazilian quartz of nominal grain sizes 0 4 1 3 and 4 5 m were deformed at 300 mpa confining pressure and temperatures of 1273 1473 k fully ductile flow at differential stresses below 200 mpa followed 0 4 exp 220000 r t d 2 where is strain rate s 1 is stress mpa d is grain size microns and r is the gas constant grains remained equant and mostly dislocation free without significant crystallographic fabric water pressure 25 50 mpa was estimated from water adsorbed onto grain boundaries during specimen fabrication flow was inferred to be controlled by silicon volume diffusion and to be sensitive to water fugacity at differential stresses 200 mpa non linear flow involved dislocation motion extrapolation to natural strain rates and temperatures that predict grain size sensitive flow in quartz should only be important at 800 k and very small grain sizes 1 m flow law parameters may change over the extrapolation interval e g through a transition to control by silicon grain boundary diffusion but geological evidence suggests that flow of even very fine grained quartzites commonly occurs by recrystallization accommodated dislocation creep rather than by diffusional accommodation

formation and maintenance of shear zones the processes that initiate and maintain prograde ductile shear zones are not well understood we have performed shear experiments 1 to 3 on a fine grained 100 150 m gneiss 59 quartz 28 plagioclase 13 aligned but not interconnected biotite to determine the evolution of deformation mechanisms that produce dramatic strain weakening and localization at conditions 1 5 gpa 800 c dot gamma 2 10 5 s where pure quartz aggregates deform homogeneously by dislocation creep initial yield occurs where stress concentrations at the tips of weak biotite grains produce semibrittle deformation in intervening quartz or plagioclase allowing local biotite interconnection by slip on 001 and initiating strain weakening after yield the interconnected biotites kink and thus strengthen but the highly strained parts of grains react to a fine grained mixed phase assemblage which deforms by grain size sensitive creep allowing further strain weakening and localization at 3 3 the strain and strain rate in the 100 m thick shear zone are 100 times that of the enclosing host rock similar to the localization observed in natural shear zones thus the processes that initiate strain localization are not necessarily the same as those that preserve the weak shear zone and once formed a shear zone may be permanently weakened

frictional viscous flow of phyllosilicate bearing fault rock microphysical model and implications for crustal strength profiles it is widely believed that around the brittle ductile transition crustal faults can be significantly weaker than predicted by conventional two mechanism brittle ductile strength envelopes factors contributing to this weakness include the polyphase nature of natural rocks foliation development and the action of fluid assisted processes such as pressure solution recently ring shear experiments using halite kaolinite mixtures as an analogue for phyllosilicaterich rocks for the first time showed frictional viscous behavior i e both normal stress and strain rate sensitive behavior involving the combined effects of pressure solution and phyllosilicates this behavior was accompanied by the development of a mylonitic microstructure a quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model in this paper a microphysical model for shear deformation of foliated phyllosilicate bearing fault rock by pressure solution accommodated sliding along phyllosilicate foliae is developed the model predicts purely frictional behavior at low and high shear strain rates and frictional viscous behavior at intermediate shear strain rates the mechanical data on wet halite kaolinite gouge compare favorably with the model when applied to crustal materials the model predicts major weakening with respect to conventional brittle ductile strength envelopes in particular around the brittle ductile transition the predicted strength profiles suggest that in numerical models of crustal deformation the strength of high strain regions could be approximated by an apparent friction coefficient of 0 25 0 35 down to depths of 15 20 km

p wave propagation heterogeneity and earthquake location in the mediterranean region summary we analyse p wave traveltimes for the mediterranean area using both teleseismic and regional arrivals for shallow earthquakes reported in the bulletins of the international seismological centre we model delays between pairs of 0 5 0 0 5 0 cells obtaining a detailed representation of the p traveltime heterogeneities examination of these anomalies shows the clear presence of geographically coherent patternsconsistent with known geological featuresdue to signi cant structure in the upper mantle we present a scheme based on an empirical heterogeneity correction ehc to p wave traveltimes to improve earthquake location this method provides similar bene ts to those of a location procedure based on ray tracing in a 3 d model but it is simpler and computationally more e cient the de nition of the traveltime heterogeneity model being based on a statistical procedure bypasses most of the critical points and possible instabilities involved in model inversion ehc relocation applied to mediterranean earthquakes allows one to predict about 70 per cent of the estimated signal due to heterogeneity and produces epicentral and origin time shifts of respectively 4 22 km and 0 35 s rms from a synthetic experiment in which we use the proposed algorithm to retrieve known source locations we estimate that the rms improvement achieved by the ehc relocation over a simpler standard 1 d location is more than 20 per cent for both epicentral mislocation and origin time shifts

a possible station bias in travel time measurements reported to isc a station bias in the travel times of p waves may exist in the international seismological centre isc data base it appears that low gain seismographs report systematically late arrival times average station delays confirm this with low gain stations generally slower than high gain stations

vp vs ratio along the voring margin ne atlantic derived from obs data implications on lithology and stress field a total of 13 regional ocean bottom seismograph obs profiles with an accumulated length of 2207 km acquired on the voring margin ne atlantic have been travel time modelled with regards to s waves the vp vs ratios are found to decrease with depth through the tertiary layers which is attributed to increased compaction and consolidation of the rocks the vp vs ratio in the intra campanian to mid campanian layer 1 75 1 8 in the central voring basin is significantly lower than for the layers above and beneath suggesting higher sand shale ratio this layer was confirmed by drilling to represent a layer of sandstone this mid cretaceous anomaly is also present in the northern voring basin as well as on the southern lofoten margin further north the vp vs ratio in the extrusive rocks on the voring plateau is estimated to be 1 85 conformable with mafic basaltic rocks landward of the continent ocean transition cot the vp vs ratio in the layer beneath the volcanics is estimated to be 1 67 1 75 these low values suggest that this layer represents sedimentary rocks and that the sand shale ratio might be relatively high here the vp vs ratio in the crystalline basement is estimated to be 1 67 1 75 in the basin and on the landward part of the voring plateau indicating the presence of granitic granodioritic continental crust in the lower crust the vp vs ratio in the basin decreases uniformly from southwest to northeast from 1 85 1 9 to 1 68 1 73 suggesting a gradual change from mafic gabbroic to felsic granodioritic lower crust significant 3 5 azimuthal s wave anisotropy is observed for several sedimentary layers as well as in the lower crust all these observations can be explained by invoking the presence of liquid filled microcracks aligned vertically along the direction of the present day maximum compressive stress nw se

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

basal slip and mechanical anisotropy of biotite the basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on 001 samples loaded at 45 to 001 exhibit gentle external rotations associated with dislocation glide high angle kink bands in these samples unlike those developed in micas loaded parallel to 001 are limited to sample corners samples shortened perpendicular to 001 show no evidence of nonbasal slip and fail by fracture over all conditions tested the mechanical response of biotite shortened at 45 to 001 is nearly perfectly elastic plastic stress strain curves are characterized by a steep elastic slope a sharply defined yield point and continued deformation at low mostly 1 stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 mpa and exhibit weak dependencies upon strain rate and temperature assuming an exponential relationship between differential stress d and strain rate e of the form e cexp d exp q rt the data collected over strain rates and temperatures of 10 7 to 10 4 s 1 and 20 to 400 c respectively are best fit by an exponential constant of 0 41 0 08 mpa 1 and an activation energy q of 82 13 kj mol a power law fits the data equally well with n 18 4 and q 51 9 kj mol samples oriented favorably for slip in directions 100 and 110 are measurably weaker than those shortened at 45 to 010 and 310 consistent with the reported burgers vectors 100 1 2 110 and 1 2 110 the anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to 001 previous studies have shown that biotite loaded in the 001 plane is strong prior to the nucleation of kink bands the strength of biotite shortened perpendicular to 001 exceeds that measured parallel to 001 and is pressure dependent application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions its presence within foliated rocks and shear zones may limit locally the stresses that can be supported

fault zone structure and weakening processes in basin scale reverse faults the moonlight fault zone south island new zealand abstract the 200 km long moonlight fault zone mfz in southern new zealand was an oligocene basin bounding normal fault zone that reactivated in the miocene as a high angle reverse fault present dip angle 65 75 regional exhumation in the last c 5 ma has resulted in deep exposures of the mfz that present an opportunity to study the structure and deformation processes that were active in a basin scale reverse fault at basement depths syn rift sediments are preserved only as thin fault bound slivers the hanging wall and footwall of the mfz are mainly greenschist facies quartzofeldspathic schists that have a steeply dipping 55 75 foliation subparallel to the main fault trace in more fissile lithologies e g greyschists hanging wall deformation occurred by the development of foliation parallel breccia layers up to a few centimetres thick greyschists in the footwall deformed mainly by folding and formation of tabular foliation parallel breccias up to 1 m wide where the hanging wall contains more competent lithologies e g greenschist facies metabasite it is laced with networks of pseudotachylyte that formed parallel to the host rock foliation in a damage zone extending up to 500 m from the main fault trace the fault core contains an up to 20 m thick sequence of breccias cataclasites and foliated cataclasites preserving evidence for the progressive development of interconnected networks of partly authigenic chlorite and muscovite deformation in the fault core occurred by cataclasis of quartz and albite frictional sliding of chlorite and muscovite grains and dissolution precipitation combined with published friction and permeability data our observations suggest that 1 host rock lithology and anisotropy were the primary controls on the structure of the mfz at basement depths and 2 high angle reverse slip was facilitated by the low frictional strength of fault core materials restriction of pseudotachylyte networks to the hanging wall of the mfz further suggests that the wide phyllosilicate rich fault core acted as an efficient hydrological barrier resulting in a relatively hydrous footwall and fault core but a relatively dry hanging wall

imaging beneath an opaque basaltic layer using densely sampled wide angle obs data a combined reflection refraction wide angle seismic survey was conducted on the continental shelf north west of britain using a conventional streamer with an air gun source and static ocean bottom seismometers obs to record wide angle energy the shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section the basalts however proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons where an important velocity inversion was anticipated this paper reports on the processing modeling and interpretation of the densely sampled wide angle obs data that were coincident with the reflection profile eleven obs instruments were deployed along a 75 km line and recorded signal from a powerful 149 liter 9100 in sup 3 air gun array fired every 50 m data processing was performed using a standard industrial reflection seismic software package prior to first arrival picking processing steps included geometry definition trace summation and display of the data using various scaling algorithms an initial model was constructed from 1d velocity time profiles digitized every 4 km along the stacked section first arrival travel time modeling rapidly converged to a detailed model of the structure of the top 5 km of the crust modeling revealedmore the existence of a buried low velocity mesozoic sedimentary basin of a prominent basement horst and of a normal fault penetrating to the basement less

shear wave velocity structure of reed bank southern continental margin of the south china sea abstract the shear wave velocity structure of a wide angle seismic profile obs973 2 across reed bank in the southern continental margin of the south china sea scs is simulated by 2 d ray tracing method based on its previous p wave model this profile is 369 km long and consists of fifteen three component ocean bottom seismometers obs the main results are as follows 1 the model consists of seven layers and the shear wave velocity increases from 0 7 km s at the top of sediment layer to 4 0 km s in the lower crust 2 the moho depth decreases from 20 22 km at the reed bank to 9 11 km at the deep oceanic basin with the shear wave velocity of 4 2 km s below the moho 3 the v p v s ratio decreases with depth through the sedimentary layers attributed to increased compaction and consolidation of the rocks 4 in the continental upper crust at model distance 90 170 km s wave velocity 2 5 3 2 km s is relatively low and v p v s ratio 1 75 1 82 is relatively high compared with the other parts of the crust corresponding to the lower p wave velocity in the previous p wave model and normal faults revealed by mcs data indicating that a strong regional extensional movement had occurred during the formation process of the scs at the reed bank area 5 the s wave structures indicate that reed bank crust has different rock compositions from that in the east section of the northern margin denying the presence of conjugate relationship of reed bank with dongsha islands according to p wave models and other data we inferred that reed bank and macclesfield were separated from the same continental crust during the rifting and break up process

modelled palaeo temperature on voring offshore mid norway the effect of the lower crustal body abstract the voring area offshore mid norway has a complex geological history has experienced several extensional phases and was significantly influenced by the break up of the north atlantic we have modelled a cross section over the voring basin aiming to 1 reconstruct the basin evolution in a realistic way and 2 to investigate the heat flow and temperature history in the basin for the modelling we used the following tectonic events the opening of the basin during the permo triassic an event during the middle to late cretaceous and an event in the late cretaceous paleocene the theoretical effects of the lithospheric stretching are depending on the palaeo water depths of the area we present a prediction of the palaeo water depth and a sensitivity analysis of the influence of the palaeo water depth on the estimated beta factors in the area a lower crustal high velocity body found in the area is often interpreted as magmatic underplating related to the break up of the norwegian sea we show the temperature history calculated by models that were run with and without assumption of underplating by a magmatic body emplaced during the early tertiary the observed vitrinite reflectance in the voring area is best explained by numerical calculations of the vitrinite reflectance without a magmatic underplating our conclusion is that the lower crustal body is not related to magmatic underplating or to significant sill intrusions the body may consist of older mafic rocks or a mixture of old continental crust and mafic intrusions

friction on subduction megathrust faults beyond the illite muscovite transition abstract previous experimental studies addressing subduction megathrust friction have focused on shallow fault gouges dominated by smectite illite and quartz here we aim to determine the effect of the transition to muscovite rich fault rock at depths beyond the illite dominated region of the seismogenic zone to achieve this rotary friction experiments were performed on simulated muscovite quartz 65 35 gouge at 100 600 c an effective normal stress of 170 mpa pore pressures of 100 and 200 mpa and sliding velocities of 1 100 m s we isolated the effects of muscovite by shearing pure muscovite gouge at 200 600 c under otherwise identical conditions at low shear strains 1 muscovite quartz gouge showed a friction coefficient of 0 4 0 5 while pure muscovite gouge exhibited 0 3 both gouges showed an increase in with and temperature the muscovite quartz gouge was characterised by velocity strengthening or neutral behaviour at 100 350 c velocity weakening at 350 500 c and velocity strengthening at 500 600 c in contrast pure muscovite gouge showed predominantly velocity strengthening or neutral behaviour demonstrating a key role of quartz in producing velocity weakening the velocity weakening regime in the mixed gouge extends that of 250 400 c recently reported for illite quartz gouge up to 500 c taking into account the observed effect of sliding velocity on a b the data for illite quartz and muscovite quartz together help explain the depth extent of seismogenesis on subduction megathrusts interestingly comparison with previous microphysical models suggests that the mechanism causing velocity weakening in phyllosilicate quartz gouges is one of granular flow involving competition between shear induced dilation and compaction through thermally activated quartz deformation

sliding behavior and deformation textures of heated illite gouge abstract the run products of a series of triaxial friction experiments on an illite rich gouge have been examined petrographically to study the relationship between textural development and sliding mode the samples show a complete range of textures from ones in which the entire gouge layer is deformed to ones in which the deformation is concentrated along narrow subsidiary shears and the rest of the gouge layer is massive the samples with a pervasively developed deformation fabric slide stably whereas the samples containing shear bands show stick slip motion if the intersection angles between boundary parallel and cross cutting riedel shears are also relatively high these textural differences suggest that localization of shear combined with higher angle riedel shears are somehow involved in stick slip motion the orientation of riedel type shears in natural fault zones may also have potential as a paleoseismological tool

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

modelling shear waves in obs data from the voring basin northern norway by 2 d ray tracing three component recordings from an array of five ocean bottom seismographs in the northwestern part of the voring basin have been used to obtain a 2 d shear wave s wave velocity depth model the shear waves are identified by means of travel time differences compared to the compressional p waves and by analyzing their particle motions the model has been obtained by kinematic travel time ray tracing modelling of the obs horizontal components

moho and basement depth in the ne atlantic ocean based on seismic refraction data and receiver functions seismic refraction data and results from receiver functions were used to compile the depth to the basement and moho in the ne atlantic ocean for interpolation between the unevenly spaced data points the kriging technique was used free air gravity data were used as constraints in the kriging process for the basement that way structures with little or no seismic coverage are still presented on the basement map in particular the basins off east greenland the rift basins off nw europe are mapped as a continuous zone with basement depths of between 5 and 15 km maximum basement depths off ne greenland are 8 km but these are probably underestimated plate reconstructions for chron c24 c 54 ma suggest that the poorly known ammassalik basin off se greenland may correlate with the northern termination of the hatton basin at the conjugate margin the most prominent feature on the moho map is the greenland iceland faroe ridge with moho depths 28 km crustal thickness is compiled from the moho and basement depths the oceanic crust displays an increased thickness close to the volcanic margins affected by the iceland plume

new perspectives on mantle dynamics from high resolution seismic tomographic model p1200 recently a high resolution tomographic model the p1200 based on p wave travel times was developed which allowed for detailed imaging of the top 1200 km of the mantle this model was used in diverse ways to study mantle viscosity structure and geodynamical processes in the spatial domain there are lateral variations in the transition zone suggesting interaction between the lower mantle plumes and the region from 600 km to 1000 km some examples shown here include the continental region underneath manchuria ukraine and south africa where horizontal structures lie above or below the 660 km discontinuity the blockage of upwelling is observed under central africa and the interaction between the upwelling and the transition zone under the slow icelandic region appears to be complex an expansion of the aspherical seismic velocities has been taken out to spherical harmonics of degree 60 for degrees exceeding around 10 the spectra at various depths decay with a power law like dependence on the degree with the logarithmic slopes in the asymptotic portion of the spectra containing values between 2 and 2 6 these spectral results may suggest the time dependent nature of mantle convection details of the viscosity structure in the top 1200 km of the mantle have been inferred both from global and regional geoid data and from the high resolution tomographic model we have considered only the intermediate degrees l 12 25 in the nonlinear inversion with a genetic algorithm approach several families of acceptable viscosity profiles are found for both oceanic and global data the families of solutions for the two data sets have different characteristics most of the solutions asociated with the global geoid data show the presence of asthenosphere below the lithosphere in other families a low viscosity zone between 400 and 600 km depth is found to lie atop a viscosity jump other families evidence a viscosity decrease across the 660 km discontinuity solutions from oceanic geoid show basically two low viscosity zones one lying right below the lithosphere the other right under 660 km depth all of these results bespeak clearly the plausible existence of strong vertical viscosity stratification in the top 1000 km of the mantle the presence of the second asthenosphere may have important dynamical ramifications on issues pertaining to layered mantle convection numerical modelling of mantle convection with two phase transitions and a realistic temperature and pressure dependent viscosity demonstrates that a low viscosity region under the endothermic phase transition can indeed be generated self consistently in time dependent situations involving a partially layered configuration in an axisymmetric spherical shell model

rock lithology and porosity determination from shear and compressional wave velocity data examined in this study are previously published laboratory shear s and compressional p wave velocity measurements on water saturated sandstone calcareous sandstone dolomite and limestone cores as well as laboratory porosity measurements on the sandstone and limestone cores sandstone and limestone porosities range from 092 to 299 and from 006 to 229 respectively differential pressure was varied from 500 to 6000 psi corresponding to approximate burial depths from 290 to 3460 m respectively sandstone limestone and dolomite are effectively separated by poisson s ratio or equivalently by the ratio of p to s wave velocity separation of sandstone and limestone appears to result from the difference in of the matrix material namely quartz 056 and calcite 316 respectively an empirical function 1 v a b was fit by regression analysis to sandstone and limestone velocity vs and vp versus porosity values at each differential pressure in this equation a and b are const

frictional behavior of simulated biotite fault gouge under hydrothermal conditions abstract we investigated frictional properties of biotite under hydrothermal conditions by shearing simulated biotite gouge sandwiched between saw cut driving blocks using a testing system with argon as confining medium experiments were conducted under an effective normal stress of 200 mpa with pore pressure of 30 mpa temperatures were varied from room temperature to 600 c with shear displacement rate stepped at standard velocities 1 22 0 122 m s and slow velocities 0 244 0 0488 m s to acquire rate dependence of friction our results show that the friction coefficient of biotite ranges from 0 25 to 0 44 significantly lower than that of muscovite documented in previous studies the steady state rate dependence shows velocity strengthening at temperatures of 25 c to 200 c and transitions to velocity weakening at temperatures above 300 c the velocity weakening is quite minor with a b ranging from 4 10 4 to 9 10 4 and stick slip motions at temperatures from 400 c to 600 c are found to be related to submicron dc values corresponding to critical stiffness higher than the actual stiffness surrounding the fault thus leading to unstable slips microstructures of deformed samples show three types corresponding respectively to three different temperature ranges while brittle structures are dominantly associated with deformation at 100 c significant plastic deformation occurred at higher temperatures deformation at temperatures of 200 c to 400 c are characterized with shear fracture zones accompanied with plastic deformation mostly associated with basal glide intensively deformed zones are the characteristic structures for samples deformed at 500 c to 600 c accompanied with shear fracture zones and moderately deformed zones it is evident that biotite alone is not weak enough to explain a weak fault like the san andreas fault thus overpressured pore fluid or another weaker mineral is needed to produce a weak fault with apparent friction coefficient of 0 1 0 2 the around neutral velocity weakening found for biotite may be appropriate in modeling fault creep transients in the mid crust

crustal structure and evolution of the southern voring basin and voring transform margin ne atlantic abstract the dominantly passive volcanic voring and more margins ne atlantic are separated by the 200 km long voring transform margin vtm the southern voring basin and the vtm have been studied by use of four regional ocean bottom seismograph obs profiles combined by gravity modelling the models demonstrate a complex pattern of magmatism along the transform margin the distribution of magmatism seems to be related to the existence and trend of a lower crustal 8 km s body interpreted as eclogitized rocks present in the southern voring basin early tertiary breakup related magmatic leakage across the continent ocean transition cot appears to be facilitated where this layer is absent these results support earlier workers who have concluded that the jan mayen fracture zone originated from a caledonian zone of weakness we propose that partly eclogitized rocks were uplifted into the lower crust close to this zone during the caledonian orogeny and that this body acted as a barrier to magma emplacement during the late cretaceous early eocene phase of rifting breakup the eclogitized terrain also appears to have caused northeastward channeling of the late cretaceous early tertiary intrusions within the voring basin an up to 10 km thick pre cretaceous sedimentary basin in the southern voring basin may be genetically related to the ns trending late paleozoic and mesozoic rift basins in north east greenland

geology of the earthquake source an introduction earthquakes arise from frictional stick slip instabilities as elastic strain is released by shear failure almost always on a pre existing fault how the faulted rock responds to applied shear stress depends on its composition environmental conditions such as temperature and pressure fluid presence and strain rate these geological and physical variables determine the shear strength and frictional stability of a fault and the dominant mineral deformation mechanism to differing degrees these effects ultimately control the partitioning between seismic and aseismic deformation and are recorded by fault rock textures the scale invariance of earthquake slip allows for extrapolation of geological and geophysical observations of earthquake related deformation here we emphasize that the seismological character of a fault is highly dependent on fault geology and that the high frequency of earthquakes observed by geophysical monitoring demands consideration of seismic slip as a major mechanism of finite fault displacement in the geological record

possible shallow crustal shear wave anisotropy off lofoten norway inferred from three component ocean bottom seismographs summary p and s wave velocity models along two perpendicular profiles 145 and 175 km long on the undrilled continental shelf off lofoten northern norway have been obtained from the study of nine three component ocean bottom seismographs a vp vs ratio of 1 96 2 15 is found in the sedimentary layers along a ne sw profile these high values are consistent with shaly sediments which are identified in dredged bedrock samples along a nw se profile the s wave velocities in the sedimentary layers are 5 10 per cent higher than along a ne sw profile the most likely explanation for this velocity difference is seismic anisotropy which might be caused by liquid filled microfractures alligned along the direction of the present day maximum horizontal compressive stress the vp vs ratio in the upper crystalline crust is found to be 1 75 along both profiles if the inferred anisotropy described above is caused by aligned microcracks the lack of anisotropy in the crystalline basement indicates that these cracks are restricted to the sedimentary layers it must be emphasized however that the observed azimuthal difference in s wave velocities could be caused by lateral velocity variations in the sediments although the anisotropy hypothesis is considered more likely

lower crustal composition and crustal lineaments on the voring margin ne atlantic a review abstract a prominent lower crustal high velocity layer observed in the voring basin ne atlantic has been investigated with regard to composition and spatial link to crustal scale lineaments in the southernmost part of the basin the lower crustal layer is interpreted as eclogites based on anomalously high p wave velocities 8 4 km s in the remainder of the basin the layer is preferably interpreted as mafic intrusions emplaced during the last phase of rifting but it cannot be excluded that the body consists of older caledonian mafic rocks direct link with high grade caledonian rocks to the east and westwards to east greenland cannot be made since these rocks are mainly of felsic composition the crustal scale lineaments inferred from wide angle seismic data appear to represent the oceanward extension of post caledonian detachments the spatial variation in the thickness of the layer is within the late rifting magmatic hypothesis partly interpreted in terms of pre magmatic variations in thickness of the lithosphere in addition lateral variations in extension mantle upwelling and decompressional melting during the last phase of rifting may also have contributed

crustal structure of the central part of the voring basin mid norway margin from ocean bottom seismographs abstract regional ocean bottom seismograph obs data acquired in the central and northern part of the voring basin mid norway margin have been modelled by use of two dimensional 2 d ray tracing the regional dataset comprises thirty obss deployed along seven 100 170 km long profiles the deeper part of the voring basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels 2 5 km below sea floor but the modelling of the obs data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea floor to the upper mantle the shallow and intermediate sediments to 5 km below sea floor are characterized by a vertical increase in velocity due to increased confining pressure there is also considerable lateral variation in the velocities within sedimentary layers at all levels the obs data confirm that intrusions of sills are important at intermediate and deep sedimentary levels 2 10 km below sea floor in most parts of the area the sills seem to vary in lateral extent from about 20 km to more than 100 km and their thickness is generally inferred to be about 200 m the low velocity in the upper crystalline crust 6 2 km s confirms that the crust in the voring basin is of continental origin in most parts of the area the velocity of the lower crust is as high as 7 3 7 6 km s this high velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness the base of the 7 3 km s layer is interpreted as the moho and the upper mantle velocity is estimated to 8 2 km s

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

correlation between velocity dependence of friction and strain localization in large displacement experiments on feldspar muscovite and biotite gouge abstract room temperature rotary shear experiments on rock powders of albite feldspar and muscovite and biotite micas show that the degree of localization of shear strain in these materials correlates with the frictional behavior in both feldspar and mica experiments increasing localization correlates with a lower coefficient of friction and a more negative velocity dependence of friction feldspar gouges tend to have localized deformation along one or more boundary parallel shears and a negative velocity dependence more localized deformation correlates with a lower coefficient of friction and a more negative velocity dependence in general micas deform throughout a broader zone than feldspar and have a lower coefficient of friction compared to muscovite biotite is weaker has a negative rather than a positive velocity dependence and shows more localized deformation for materials as diverse as feldspars and micas localization along boundary parallel shears correlates with negative velocity dependence along compliant natural faults a negative velocity dependence would result in a greater tendency for unstable sliding thus localized boundary parallel shears may indicate negative velocity dependence and slip by earthquakes on natural faults

effects of simulated clay gouges on the sliding behavior of tennessee sandston abstract the effects of simulated fault gouge on the sliding behavior of tennessee sandstone are studied experimentally with special reference to the stabilizing effect of clay minerals mixed into the gouge about 30 specimens with gouge composed of pure clays of homogeneously mixed clay and anhydrite or of layered clay and anhydrite along a 35 precut are deformed dry in a triaxial apparatus at a confining pressure of 100 mpa with a shortening rate of about 5 10 4 sec and at room temperature pure clay gouges exhibit only stable sliding and the ultimate frictional strength is very low for bentonite mont morillonite intermediate for chlorite and illite and considerably higher for kaolinite anhydrite gouge shows violent stick slip at 100 mpa confining pressure when this mineral is mixed homogeneously with clays the frictional coefficient of the mixed gouge determined at its ultimate frictional strength decreases monotonically with an increase in the clay content the sliding mode changes from stick slip to stable sliding when the frictional coefficient of the mixed clay anhydrite gouge is lowered down below 90 95 of the coefficient of anhydrite gouge the stabilizing effect of clay in mixed gouge is closely related to the ultimate frictional strength of pure clays that is the effect is conspicuous only for a mineral with low frictional strength only 15 20 of bentonite suppresses the violent stick slip of anhydrite gouge in contrast violent stick slip occurs even if the gouge contains as much as 75 of kaolinite the behavior of illite and chlorite is intermediate between that of kaolinite and bentonite bentonite anhydrite two layer gouge exhibits stable sliding even when the bentonite content is only 5 thus the presence of a thin clay rich layer in a fault zone stabilizes the behavior much more effectively than do the clay minerals mixed homogeneously with the gouge this result brings out the mechanical significance of internal structures of a fault zone in understanding the effects of intrafault materials on the fault motion based on the present experimental results incorporated with some other experimental data it is argued that although the stabilizing effect of montmorillonite and vermiculite is indeed remarkable at room temperature the effect should be much less pronounced at elevated temperatures due perhaps to the dewatering of the clays in most geological environments where shallow earthquakes occur the stabilizing effect of clays is probably not so conspicuous as to completely suppress the unstable motion of a fault

vp vs ratio structure of the lofoten continental margin northern norway and its geological implications summary in 1988 an extensive seismic refraction and wide angle reflection study was performed at the lofoten passive continental margin off northern norway by using three component ocean bottom seismographs the lofoten margin was formed by continental rifting and sea floor spreading with extensive volcanism which is responsible for landward flood basalts and seaward dipping reflectors sdr the p wave structure beneath the seaward side of the voting plateau escarpment at the lofoten margin is characterized by a thick oceanic lower crust and the existence of an additional high velocity layer at the bottom of the crust in this study the distribution of the p wave velocity vp to s wave velocity vs ratio down to the uppermost mantle is obtained by studying p to s converted shear wave data vp vs ratios of 3 0 5 5 are obtained in the sedimentary layers these high ratios are attributed to unconsolidated sediments in the upper crust a significant difference in vp vs ratios is found between oceanic side profiles and land side profiles i e 1 86 oceanward of the sdrs and 1 76 landward of the sdrs this variation may have been caused by a difference in porosity due to the difference in extrusive processes the upper crust of the oceanic side of the sdrs was created by submarine eruptions while the land side of the sdrs was formed by submarian eruptions the vp vs ratios in the middle crust and lower crust are estimated as 1 76 1 78 and 1 80 respectively comparing vp and vsv data from the lower crust to those in previous studies clearly indicates the presence of mafic gneisses this result represents new seismic evidence for the hotspot hypothesis concerning the formation of passive volcanic continental margins clear s wave upper mantle refractions can be observed at offsets of up to 100 km even where p wave arrivals cannot be identified in order to model these s wave arrivals a vs 4 8 km s 1 layer is necessary 2 km below the moho

crustal structure of the northern part of the voring basin mid norway margin from wide angle seismic and gravity data abstract three regional ocean bottom seismograph obs profiles with a total of fifteen recovered obss were acquired in the northern part of the voring basin mid norway margin in 1996 the data have been modelled by inversion and forward modelling of the obs vertical components p waves the velocity is found to increase downwards within the sedimentary layers due to increasing depth of burial confining pressure within the two deepest sedimentary layers there seems to be an increase of 0 5 1 km s in velocity northwestwards this increase is most likely caused by high velocity sill intrusions in the sedimentary rocks emplaced during the rift episode leading to early eocene opening of the ne atlantic the presence of significant amounts of sills seems to terminate close to the nyk high along profile 5 dip profile and close to a dome northwest of the shelf edge along profile 6 dip profile the velocity in the top of this dome is anomalously high 3 4 km s at only 1 km depth beneath the seafloor suggesting that the high consists of volcanic rocks or heavily intruded sedimentary rocks the relatively low velocities derived from within the upper crystalline crust 5 9 6 2 km s confirms that the crust is of continental origin within the lower crust there seems to be a clear increase in velocity northwestwards from about 7 0 km s to about 7 4 km s suggesting that the amount of high velocity intrusions underplating in the lower crust decreases landwards the modelling of the strike profile profile 7 suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile the model for profile 7 ties closely southwestwards to the models derived from obs data acquired in 1992 and northeastwards to lofoten obs data acquired in 1988 the models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey and that the transition zone is related to the surt lineament the distribution of magmatic rocks thus seems to correlate strongly with pre breakup structures in addition to the p wave modelling gravity modelling has been performed along all profiles the gravity modelling confirmed all main aspects of the obs models and provided constraints on the crustal structures towards the ends of the profiles

the nature and importance of phyllonite development in crustal scale fault cores an example from the median tectonic line japan abstract like many large crustal scale faults the median tectonic line mtl in sw japan has a long history of movement having been active predominantly as a strike slip fault since the mid cretaceous fault rock exposures in the core of the mtl preserve a history of deformation at a range of mid to shallow crustal depths ryoke mylonites 1 4 km north of the main contact record deeper level cretaceous top to the south sinistral movements the remainder of the fault zone core is surprisingly narrow exhibiting a wide variety of fault rocks that illustrate both the interaction and effects of syn tectonic fluid influx over a range of deformation conditions exposures within 50 m of the central slip zone display a progressive sequence in fault rock evolution from ultramylonite cataclasite foliated cataclasite phyllonite breccia gouge this sequence occurs because cataclasis in the vicinity of the fault core creates permeable pathways for the ingress of chemically active fluids into the fault zone this leads to the replacement of load bearing phases such as feldspar by fine grained foliated aggregates of intrinsically weaker phyllosilicates such as white mica and chlorite the grain size reduction associated with both cataclasis and mineral alteration creates conditions ideal for the operation of fluid assisted stress induced diffusive mass transfer mechanisms comparison with the findings of recent experimental studies suggest that the fault zone processes observed in the core of the mtl will lead to long term weakening provided the network of phyllosilicate rich fault rocks are able to form an interconnected thin layer of weak material on kilometre to tens of kilometre length scales

deep structures and breakup along volcanic rifted margins insights from integrated studies along the outer voring basin norway three dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the norwegian volcanic rifted margin a surprising high velocity lower crustal dome marked by a strong amplitude reflection t reflection is described below the north gjallar ridge ngr near the volcanic plateau formed during the late paleocene early eocene breakup faulting along the ngr occurs during early campanian paleocene and is controlled by the crustal dome the t reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup the influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup based on the structural relationship and the results of the modelling it is therefore concluded that the t reflection does not necessarily originate from an anomalous tertiary magmatic event related to the breakup underplated material but could be also partly attributed to pre breakup mafic material or and inherited high pressure granulite eclogite rocks in the continental domain q 2004 elsevier ltd all rights reserved

experimental deformation of muscovite abstract the strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45 0 and 90 to the basal plane 001 at temperatures t from 20 to 400 c confining pressures pc from 10 to 400 mpa and strain rates e from 2 5 10 7 to 2 3 10 4 s 1 samples deformed as 45 to 001 exhibit smooth undulatory extinction in thin section associated with dislocation glide with few well developed low angle kink boundaries basal shear strengths depend on confining pressure at pc e a exp a exp q rt results from temperature and strain rate stepping experiments performed at pc 200 mpa yield an activation energy q of 47 19 kj mol 1 and an exponential constant a of 0 5 0 2 mpa 1 samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45 to 001 and exhibit significant pressure dependence 0 8 up to confining pressures of 200 mpa kink bands are generated at the ends of samples where contact is made with the pistons samples loaded at 90 to 001 are much stronger than samples of all other orientations and deformation is accomplished by fracture fracture strengths exhibit a strong dependence on confining pressure 0 9 similar in magnitude to other silicates that are brittle at the same conditions by comparison with biotite muscovite is weaker in shear on 001 at the experimental conditions tested with an activation energy for glide q that is significantly smaller than that for glide in biotite

earthquake locations in southern california obtained using source specific station terms we relocate 297 400 events recorded by the southern california seismic network scsn between 1975 and 1998 using spatially varying station terms to improve relative location accuracy our method uses existing scsn p and s picks a smooth one dimensional velocity model and an iterative grid search approach based on the l1 norm we apply empirical corrections for three dimensional structure by computing station timing corrections that continuously vary as a function of source position station terms for each event are obtained by smoothing the residuals from nearby events using a natural neighbor delaunay tessellation of the seismicity and then iterating until a stable set of locations and station terms is achieved our approach achieves relative location accuracy comparable locally to master event methods but can be applied uniformly over large regions median estimated standard errors for our final locations are 328 m in horizontal position and 741 m in depth our locations exhibit much less scatter particularly in depth than those of the scsn catalog and a greater tendency to align into linear and planar features suggestive of fault structures our results appear comparable to and in some cases better than previous scsn relocation studies using joint hypocenter velocity inversion techniques plots of daytime versus nighttime events permit discrimination between clusters of natural and artificial seismicity we observe no simple relationship between the maximum depth of seismicity and surface geology

flow behavior of fine grained synthetic dunite in the presence of 0 5 wt h2o the deformation behavior of synthetic dunite mg forsterite plus 2 5 vol enstatite with a grain size of 1 m has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850 to 1000 c strain rates of 10 7 to 10 5 s 1 and a confining pressure of 600 mpa nominally dry material behaved purely elastically in contrast samples with 0 5 wt added water flowed at stresses between 9 and 81 mpa reaching strains up to 12 fitting a dorn type power law to the wet data yielded stress exponents n of 1 7 0 4 and apparent activation energies in the range 302 22 kj mol 1 the wet deformed samples showed polygonal grains no crystallographic preferred orientation no subgrains low dislocation densities and evidence for grain boundary cavitation grain growth was minor it is concluded that the wet samples probably deformed by a grain size sensitive grain boundary sliding dominated mechanism though the nature of the accommodation process remains unclear and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere

misoriented faults in exhumed metamorphic complexes rule or exception abstract low angle normal faults and other weak faults are common in the metamorphic core of collisional orogens they frequently show a phyllosilicate rich mylonitic foliation that was reactivated under brittle conditions experimental and theoretical works indicate that mechanical anisotropies exert a substantial influence on shear failure and frictional sliding eventually inhibiting the nucleation and propagation of new andersonian shear fractures and favoring the localization of brittle failure along the pre existing foliations metamorphic phyllosilicate rich rocks may show a friction coefficient varying from 0 6 at high angles to the foliation to 0 2 0 4 for shear along the inherited foliation to test the influence of mechanical anisotropies on the development of non andersonian faults we have applied a modified slip tendency analysis to three misoriented phyllosilicate rich faults of the european alps the analysis accounts for anisotropy in friction coefficients and has been named anisotropic slip tendency analysis here we show that brittle deformation along misoriented phyllosilicate rich foliations is more probable than the development of new andersonian faults the presence of a well developed network of weak phyllosilicate rich faults may influence the overall structural style and mechanical properties of the brittle lithosphere in collisional orogens

fault zone weakening and character of slip along low angle normal faults insights from the zuccale fault elba italy a seismically active low angle normal fault is recognized at depth in the northern apennines italy where recent exhumation has also exposed ancient examples at the surface notably the zuccale fault on elba field based and microstructural studies of the zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability promoting influx of co 2 rich hydrous fluids this triggered low grade alteration and the onset of stress induced dissolution precipitation processes e g pressure solution as the dominant grain scale deformation process in the pre existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine grained hydrous mineral phases these rocks exhibit ductile deformation textures very similar to those formed during pressure solution accommodated frictional viscous creep in experimental fault rock analogues the presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core which significantly enhanced the sealing capacity of the fault zone a slip model for low angle normal faults in the apennines is proposed in which aseismic frictional viscous creep occurs on a weak slow moving slip rate 1 fault interspersed with small seismic ruptures caused by cyclic hydrofracturing events our findings are potentially applicable to other examples of low angle normal faults in many tectonic settings

weakening and strain localization produced by syn deformational reaction of plagioclase there are many observations in naturally deformed rocks on the effects of mineral reactions on deformation but few experimental data in order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase h2o more albitic plagioclase zoisite kyanite quartz we utilized fine grained 2 6 m plagioclase aggregates of two compositions an54 and an60 both dried and with 0 1 0 4 wt h2o present in shear deformation experiments at two sets of conditions 900 c 1 0 gpa in the plagioclase stability field and 750 c 1 5 gpa in the zoisite stability field dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep but samples with 0 1 to 0 4 wt water deformed in the zoisite stability field showed extreme strain localization into very narrow 1 3 m shear bands after low shear strain in these samples the microstructures of reaction products in the matrix differ from those in the shear bands in the matrix large up to 400 m zoisite crystals grew in the direction of finite extension and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic in the shear bands the reaction products albitic plagioclase zoisite white mica and traces of kyanite form polyphase aggregates of very fine grained 0 1 m dislocation free grains most of the sample strain after 2 has occurred within the shear bands within which the dominant deformation mechanism is inferred to be diffusion accommodated grain boundary sliding dagbs the switch from dislocation creep in dry samples deformed without reaction to dagbs in reacted samples is associated with a decrease in flow stress from 800 to 200 mpa these experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine grained polyphase assemblage leading to a switch in deformation mechanism and significant weakening thus localization of deformation in polyphase rocks may occur on any pressure p temperature t path where the equilibrium composition of the constituent minerals changes

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

improving seismic event location an alternative to three dimensional structural models we devise and apply a method to account for the effect of the aspherical structure of the earth in locating earthquakes this technique relies upon the ability to detect the average structural signal present in the residuals between source and receiver and correct for this signal during location using a phenomenological description that we call empirical heterogeneity corrections ehc ehc are employed in the relocation of a large set of well constrained tcleseismic earthquakes selected among the events reported by the bulletins of the international seismological centre 1964 1995 the rms length of ehc relocation vectors for these events is about 10 km the method is also tested against a selected set of ground truth events both earthquakes and explosions whose locations are independently known by nonseismic means the rms length of the mislocation vectors for the test events compared to their original mislocation in the reference 1 d model sp6 is reduced in the ehc relocation by 17 for explosions and 12 for earthquakes our technique provides a successful alternative to the use of 3 d structural models approximately reaching the same value of effectiveness in improving event location

mapping of p wave slab anomalies beneath the tonga kermadec and new hebrides arcs abstract p wave velocity anomalies of several per cent fast relative to the jeffreys bullen jb model are seen around subduction zones under the tonga kermadec and new hebrides trenches from inversion of international seismological centre isc travel time residuals from intermediate and deep focus earthquakes mantle heterogeneities outside the model are corrected using previous velocity models ray paths inside the model region are traced allowing two dimensional velocity variations the rays of similar source and receiver locations are combined into summary rays and the hypocenters are relocated as part of the inversion iterations the resulting images are interpreted in conjunction with resolution and error estimations the slab like anomalies are generally continuous in places of high seismicity and decrease in amplitude with depth most coherent slab anomalies are surrounded by large slow patches between the surface and 350 km depth the fast slab beneath the tonga arc appears to end at 550 km depth before the seismicity ends but there is a fast band at 750 1000 km depths below the deepest tongan earthquakes the fast slab like anomaly beneath the kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle and it occurs aseismically near new zealand down to 500 550 km depth interpretations in the lower mantle in many places such as kermadec and new hebrides arcs however are hampered by poor resolution

crustal transect from the north atlantic knipovich ridge to the svalbard margin west of hornsund the crustal structure along a 312 km transect stretching from the axial mountains of the north atlantic knipovich ridge to the continental shelf of svalbard has been obtained using seismic reflection data and wide angle obs data the resulting seismic vp and vs models are further constrained by a 2 d gravity model the principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow spreading counterpart of the north atlantic ridge systems vp is estimated to be 3 50 6 05 km s for the upper oceanic crust oceanic layer 2 with a marked increase away from the ridge the measured vp of 6 55 6 95 km s for oceanic layer 3a and 7 10 7 25 km s for layer 3b both with a vp vs ratio of 1 81 except for slightly higher values at the ridge axis does not allow a clear distinction between gabbro and mantle derived peridotite 10 40 serpentized the thickness of the oceanic crust varies a lot along the transect from the minimum of 5 6 km to a maximum of 8 1 km the mean thickness of 6 7 km for the oceanic crust is well above the average thickness for slow spreading ridges 10 mm year half spreading rate the areas of increased thickness could be explained by large magma production rates found in the zones of axial highs at the ridge axis which also have generated the off axial highs adjacent the ridge we suggest that these axial and off axial highs along the ridge control the lithological composition of the oceanic crust this approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments the axial and off axial highs and mantle derived peridotite outside these zone

assessment of earthquake location accuracy and confidence region estimates using known nuclear tests i have used the known epicenters and depths of 307 nuclear tests at the nevada test site nts and 75 tests at the east kazakhstan test site to compare the distances between actual and calculated hypocenters mislocations with calculated location confidence limits errors each test was located using its isc compiled p first arrivals a starting depth of 33 km and the iasp91 velocity model i also applied azimuth and incident angle dependent station corrections to the arrival times the most accurately located tests are those at the kazakhstan test site where all of the mislocations in latitude and longitude are less than 10 km and 90 of the depth mislocations are less than 20 km bayesian confidence limits calculated from the least squares covariance matrix satisfactorily account for these mislocations at the nevada test site 90 of the mislocations are less than 9 km in longitude 15 km in latitude and 33 km in depth the overestimates in depth are compensated for by origin times that are 2 to 5 sec late the larger mislocations at nts occur in spite of the fact that the closest recording sites are less than 200 km away compared to 1800 km away at the kazakhstan test site a significant fraction of the depth mislocations at nts are much larger than their calculated errors i interpret this failure of the calculated errors to adequately account for the nts mislocations as being due to systematically late arrivals at northwestern american stations relative to the predicted iasp91 values by using a locally determined three layer velocity model for the travel times of phases recorded at distances of less than 1500 km i was able to reduce 90 of the depth mislocations to less than 11 km as well as significantly reducing the latitude longitude and origin time mislocations i conclude that differences between the iasp91 model and velocities in the vicinity of the explosions may introduce a significant bias into the determined depths of nuclear tests a similar bias would therefore be expected in the similarly determined depths of shallow earthquakes

a revisit to p wave travel time statics at teleseismic stations to account for local deviations from the one dimensional reference velocity model the station correction formulation of dziewonski and anderson 1983 is extended to include the travel time variation with respect to the ray takeoff angle the station statics are obtained through an iterative retrieval of the lower degree spherical harmonics normalized over the data takeoff angle range on station residual spheres and similar event statics are introduced to decouple the effects of near station and near event heterogeneities except the zero degree constant term all higher degree static terms are obtained on a regional rather than an individual station event location the p wave static corrections up to degree and order 2 are derived for 1343 stations using nearly 3 million international seismological center travel time residuals with respect to the iasp91 model and with event relocation and summary ray sorting the resulting station statics account for nearly 27 of the variance of the p wave travel time residuals patterns of the zero degree statics correlate well with surface tectonic features and with previous results at most stations over subduction zones the slowest direction of the secondorder statics or the second degree in azimuth is perpendicular to the trench

the effect of mineralogy and effective normal stress on frictional strength of sheet silicates abstract phyllosilicates are common constituents of upper crustal faults and subduction forearcs we studied the effect of mineralogy and controlled effective normal stress between 5 mpa and 100 mpa on frictional strength although the effect of mineralogy on frictional strength of single phase phyllosilicate gouges has been previously studied the influence of effective normal stress has not we conducted water saturated and vacuum dry frictional tests on single phase phyllosilicate gouges using a triaxial apparatus minerals included talc pyrophyllite kaolinite lizardite illite montmorillonite chlorite muscovite phlogopite and biotite particle size 20 mpa dry friction coefficients are constant and wet friction coefficients show a small increase this is attributed to loss of water and increased contact area the results indicate that frictional strength of clay rich faults increases at depths less than 1 km under hydrostatic pore fluid pressures

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

towards an improved understanding of the mechanical properties and rheology of the lithosphere an introductory article to rock deformation from field experiments and theory a volume in honour of ernie rutter ernie rutter s influential contribution to our understanding of rock deformation spans the whole continental crust from deformation just under the grass e g rutter green 2011 to melt migration in the lower crust e g rutter neumann 1995 ernie has also worked on many aspects of the deformation of oceanic crust rutter brodie 1988 however here we reflect on the wide range of deformation conditions ernie has studied by presenting key aspects of the strength and rheology of the continental lithosphere as we understand it today with the aim of providing the context for the contributions included within this volume the continental crust forms just over a third of the earth s surface and it is different from the crust of any other planet in our solar system it is rich in those elements that partition in a silicate melt and its formation and evolution determined the composition of the mantle and that of the atmosphere hawkesworth 2006 thanks to its physical and chemical fingerprint the earth s crust has sustained life uninterrupted for 3 8 ga and it is the source of major natural resources such as hydrocarbons and mineral deposits and others such as geothermal energy the continental crust is less dense than its oceanic counterpart and is therefore more buoyant it forms a substantial part of the cold thermal layer of the earth and its rheology from greek r1v rhe flow and logia logia study the study of the flow of matter is a key factor controlling plate tectonic processes at the earth s surface and in its interior davies 2011 beneath the continental crust lies the lithospheric mantle and together they form the continental lithosphere considerable variations in structure thickness and composition of the continental lithosphere mean that despite its importance we currently have limited understanding of the rheology of this complex system thatcher pollitz 2008 significant advances in our knowledge of continental rheology have been possible through

spatial correlation of deformation and mineral reaction in experimentally deformed plagioclase olivine aggregates abstract shear deformation of hot pressed plagioclase olivine aggregates was studied in the presence and absence of mineral reaction experiments were performed at 900 c 1500 mpa and a constant shear strain rate of 5 10 5 s 1 in a solid medium apparatus whether the mineral reaction between plagioclase and olivine takes place or not is controlled by choosing the appropriate plagioclase composition labradorite an 60 does not react anorthite an 92 does labradorite olivine aggregates deformed without reaction are very strong and show strain hardening throughout the experiment syndeformational reaction between olivine and anorthite causes a pronounced strain weakening the reaction produces fine grained opx cpx spinel aggregates which accommodate a large fraction of the finite strain deformation and reaction are localised within a 0 5 mm wide sample three representative samples were analysed for their fabric anisotropy r and shape preferred orientation fabric angle with the shear plane using the autocorrelation function acf fabric anisotropy can be calibrated to quantify strain variations across the sheared samples in the deformed and reacted anorthite olivine aggregate there is a strong correlation between reaction progress and strain regions of large shear strain correspond to regions of maximum reaction progress within the sample the derived strain rate variations range up to almost one order of magnitude

nuclear explosion locations at the balapan kazakhstan nuclear test site the effects of high precision arrival times and three dimensional structure abstract we have investigated the potential contributions of improved arrival times using waveform cross correlation and the use of three dimensional 3 d velocity models for seismic event location capability our analyses are applied to a dataset of nuclear explosions at balapan kazakhstan for which ground truth locations and some absolute origin times are available this ground truth information allows us to determine excellent origin time estimates for the remaining explosions the combination of excellent ground truth location information and high quality origin time estimates permits us to 1 carry out a detailed examination of the quality of isc picks 2 identify probable timing errors in the digital data 3 evaluate relative and absolute location capability using data from a sparse network 4 assess the influence of event signal to noise ratio snr on relative location accuracy 5 utilize the balapan events as a source array for 3 d tomography beneath the test site and 6 test the influence of 3 d structure local and global on relative location accuracy and precision in a controlled situation our principal finding is that improved arrival times are the primary contributor to improved locations joint and individual relocations of balapan events using the full digital dataset result in average mislocations of less than 1 km and 95 confidence regions of a compatible size to mimic a ctbt scenario more realistically we also carry out relocations using very few stations 4 10 observations location accuracy degrades somewhat but the high quality picks generally result in mislocations less than 10 km even for events with very large azimuthal gaps uncertainty is generally underestimated in these cases tests with artificially degraded snr show that mislocation increases slowly as snr decreases 3 d velocity structure makes a smaller contribution to relative location accuracy than accurate time picks travel time variations due to global 3 d structure vary little across the source region so that location scatter is not reduced when travel time corrections for global 3 d structure are applied travel time variations due to the local 3 d structure estimated using source region tomography are also modest applying travel time corrections that account for the local structure does yield slight location improvement

comparison of p and s station corrections and their relationship to upper mantle structure we use international seismological centre data between the years 1964 and 1994 to derive a data set of p and s station corrections by restricting the raw p and s data to those which share the same source to receiver paths we minimize any bias to the station corrections from differential p and s data distributions we find that both static and azimuth dependent terms are highly correlated in addition we perform synthetic tests to determine the sensitivity of our data to mantle structure and show that static terms are sensitive to structure at least throughout the upper mantle in determining the regression slope of the static terms we allow for regional offsets and find a best fit global slope of s to p correction a 2 85 0 19 this is lower than that obtained in previous station correction studies but is in agreement with results in the upper mantle using differential travel times and with values derived for the upper parts of the lower mantle regionalizing the data by tectonics does not give statistically distinct ratios of s to p station correction the result for shield areas however is significantly smaller than previously believed probably because of our ability to derive station corrections from similar data sets variations in the second azimuthal terms are similar in both fast direction and relative magnitude in another synthetic experiment we find that their general trends can be predicted well by isotropic models of heterogeneous velocity structure from this result and also because they do not correlate with sks splitting data we conclude that station correction second azimuthal terms should not be interpreted in terms of anisotropy

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

modelling of porphyroclasts in simple shear and the role of stress variations at grain boundaries abstract grain scale numerical experiments involving simple shear of a two phase non linear viscous material are described and compared with mineral fish or lozenge shaped porphyroclasts such as muscovite two types of 2d models are considered either a single elongate grain or two parallel elongate grains in both cases supported by a weaker polygonal grain matrix the relative viscosities of the contrasting grain structures were systematically varied allowing us to observe the effects of non linear viscous rheology on the resulting microstructure and flow patterns the results show that the finite rotation of the hard elongate grain was similar within any one experiment but was largely influenced by viscosity contrast the geometry of the model and the imposed shear strain models involving single elongate hard grains show increasing instability at their ends and less strain compatibility with the deforming matrix grains as the viscosity contrast is increased in the paired grain models the greatest variation in the matrix grain microstructure is seen in the region where the two hard grains are oriented at a high angle to the direction of shear finally we consider the changes in intragranular stress by comparing microstructural observations using different viscosities with the distribution of stress in space and during progressive shear in the plane approximately parallel to the maximum principal stress direction 1 a localised change of stress occurs across and along the interface between the hard and soft grains variations in the mean stress at these boundaries are directly attributable to changes in the minimum principal stress we propose that with shear strains greater than 2 it is the minimum principal stress that can control diffusion processes at the grain boundary rather than mean stress in conclusion we suggest that our models have the potential for providing useful insights into why metamorphic reactions can occur at the interface between a porphyroclast and matrix at high shear strains and how stress distribution can control the initiations of such reactions

shear waves from three component ocean bottom seismographs off lofoten norway indicative of anisotropy in the lower crust summary data from four three component ocean bottom seismographs obs situated on a 145 km long profile on the rost high on the continental shelf off lofoten northern norway have been modelled using 2 d seismic ray tracing and a p and s wave velocity model along the profile is presented the s wave model has been obtained from a study of the high quality horizontal component data a vp vs ratio of 1 95 2 15 is found in the sedimentary layers on the rost high these high values are consistent with shaly sediments which are identified in dredged bedrock samples south of the rost high a considerably lower vp vs ratio ca 1 6 is estimated for the sediments which can be attributed in part to a higher degree of compaction in the deepest sediments and in part to a more sandy lithology in the upper and middle crystalline crust a vp vs ratio of 1 75 is obtained strong s reflections and p to s conversions at the moho are observed on all four instruments and the modelling of these arrivals indicates that the s wave velocity in the lower crust varies with angle of incidence from 3 5 km s 1 vp vs 1 95 at vertical incidence to 4 0 km s 1 vp vs 1 7 at 60 70d this angle of incidence dependent s wave anisotropy the order of 10 per cent is consistent with some horizontal or subhorizontal layering in the lower crust

the role of tectonic grain size reduction in the rheological stratification of the lithosphere with the aid of published constitutive flow laws for common crustal and upper mantle rock types it is possible to construct curves showing variation of rock strength with depth in the continental lithosphere assuming a particular crustal stratification of rock types and a particular geothermal gradient such exercises lead to the conclusion that marked changes in strength can occur at major petrological interfaces such as the moho discontinuity which are therefore likely to be associated with a tendency for major sub horizontal detachments to form constitutive flow laws used for such purposes are usually assumed to represent steady state flow at constant rock microstructure

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

v p v s and lithology published laboratory investigations suggest an association exists between the ratio of seismic compressional and shear wave velocities vp vs and sedimentary rock lithology comparisons of some theoretical models with these laboratory studies suggest that crack or pore geometry has a stronger effect on observed vp vs values than elastic constants of the minerals comprising the matrix further it can be inferred that the observed association between lithology and vp vs is a result of an association between lithology and distribution of pore and crack shapes direct observation of crack shapes for a variety of lithologies is a next step in strengthening these inferences the present study reviews the empirical relations between vp vs and lithology and examines two published theoretical models of cracked elastic media the models suggest that seismic velocities of sandstones may be controlled by cracks and pores with aspect ratios in the range of 10 1 to 1 dolomite in the range of 10 2 to 10 1 and dens

a regional shear wave velocity model in the central voring basin n norway using three component ocean bottom seismographs abstract a regional ocean bottom seismometer obs dataset was acquired in the central and northern part of the voring basin n norway in 1992 thirty three component obss were deployed along seven regional profiles and the shear waves recorded on the horizontal components have been modelled by use of 2 d ray tracing utilizing an established p wave model in the shallow sediments 1 4 km depth mean vp vs ratios of 2 1 2 2 and 2 3 2 5 are inferred along the nw se and ne sw trending profiles respectively the relatively high mean vp vs ratios in this interval are probably related to unconsolidated sediments in the uppermost layers the azimuthal variation in the vp vs ratios with the fast direction nw se indicates a shear wave anisotropy of 5 10 in this interval consistent with observations further north in the lofoten area in the 5 10 km depth interval high velocity sill intrusions in the sediments are mapped by the p wave modelling these high velocity intrusions act as efficient conversion interfaces confirmed by the shear wave modelling a regional decrease in the vp vs ratios to 1 7 1 8 are observed at 4 6 km depth possibly related to sand rich sediments in the middle cretaceous the upper crust is clearly mapped by the shear waves and the inferred vp vs ratio of 1 75 confirms that the crust in the voring basin is of continental origin an increase of the vp vs ratio 1 8 1 85 in the lower crust indicates a change in its composition possibly explained by a mixing of continental crust and magmatic underplating

seismic structure in the northeastern south china sea s wave velocity and vp vs ratios derived from three component obs data a nearly 500 km long seismic profile with reflective and refractive wide angle ocean bottom seismometer obs data and multi channel seismic mcs data was acquired across the northeastern continental margin of the south china sea scs the s wave crustal structure and vp vs ratios have been obtained based on a previously published p wave model using the software raylnvr modeling of vertical and horizontal component obs data yields information on the seismic crustal velocities lithology and geophysical properties along the obs 2001 seismic profile s wave velocities in the model increase generally with depth but exhibit high spatial variability particularly from the shelf to the upper slope of the northeastern scs margin vp vs ratios also reveal significant lithological heterogeneity dongsha penghu uplift dpu is a tectonic zone with a thicker crust than adjacent areas and a high magnetic anomaly with a vp vs of 1 74 and a p wave velocity of 5 0 5 5 km s the dpu primarily consists of felsic volcanic rocks in the upper crust and is similar to the petrology of zhejiang fujian volcanic provinces which perhaps is associated with a mesozoic volcanic arc the ocean continent transition oct in the northeastern scs is characterized by a thinning continental crust volcanoes in the upper crust and a high velocity layer hvl in the lower crust the s wave velocity and vp vs ratio suggest that the hvl has a mafic composition that may originate from underplating of the igneous rocks beneath the passive rifted crust after the cessation of seafloor spreading c 2009 elsevier b v all rights reserved

modeling of residual spheres for subduction zone earthquakes 1 apparent slab penetration signatures in the nw pacific caused by deep diffuse mantle anomalies we have computed focal residual spheres for 145 subduction zone earthquakes along the northwest edge of the pacific using regional and global mantle velocity models from tomographic inversions the mantle models explain much of the observed residual sphere data and to a certain extent suggest the location of mantle velocity heterogeneities which are responsible for various residual sphere patterns for most deep events considered the fast slablike residual sphere anomalies are caused by diffuse heterogeneities mainly of deep lower mantle and receiver mantle origin rather than by an extension of the slab the region immediately below the deepest earthquakes depths of 650 1500 km has an effect usually smaller than or comparable to the effect of other regions of the mantle without a proper account of the teleseismic effect attributing the long wavelength anomalies of the residual sphere to near source slab effects alone or even primarily is not valid the fast bands in many observed residual spheres agree with seismicity trends once the deep mantle and receiver mantle effects are removed these may give the approximate orientation but not the depth extent of near source fast velocities for most deep earthquakes under japan the predominant fast band is subhorizontal rather than near vertical this type feature would be overlooked in conventional residual sphere studies using only steeply diving rays and cosine weighting of the data

on the mechanical properties of synthetic kaolinite quartz fault gouge summary the mechanical properties and microstructures of mixed kaolinite quartz fault gouge have been studied by means of triaxial tests wherein a 1 mm thickness of fault gouge was sheared between rigid sintered alumina sliders test conditions ranged up to 200 mpa confining pressure 175 mpa pore water pressure and temperatures to 600 c constant displacement rate tests were carried out at 5 10 4 mm s 1 and the stress relaxation technique was used to access a wide range of lower displacement rates down to 10 10 mm s 1 the effects of different ratios of clay quartz were studied and some experiments were cafried out using crushed tennessee sandstone gouge and pre faulted cylinders of tennessee sandstone deformation induced microstructures were studied by optical scanning and transmission electron microscopy and mineral chemistry changes were identified using eds analysis in the stem the most important factor determining the strength of these gouges is effective confining pressure and the effective stress law was obeyed at all test conditions increasing the proportion of quartz in the gouge slightly increases the strength as does increase of temperature under all test conditions the gouges deformed by strain hardening stable sliding at 2 mm displacement the friction coefficient is similar to that for rock and rock sliding 0 85 over wide ranges of strain rate and temperature the strength of these gouges is little affected if the effective pressure is high but strength drops rapidly with decreasing strain rate if effective pressure is low or if pore water pressure is high reloading of the gouge after stress relaxation at high temperatures demonstrated hardening arising from hydrothermal cementation during stress relaxation which led to sudden rupture of the cemented gouge this phenomenon may be responsible for cyclic rupture of natural faults new mineral phases were produced in these experiments only at 600 c at 400 c anticipated production of pyrophyllite from kaolinite plus quartz was not found but recrystallization of kaolinite is believed to have occurred and evidence of healing of cracks in quartz was seen microstructural studies revealed the ubiquitous development of p and r1 foliations and other features which are identical to those found in natural clay bearing fault zones microstructural evolution is associated with the strain hardening and apparent hardening is strongly associated with the stress path during loading

comparison between a regional and semi regional crustal obs model in the voring basin mid norway margin semi regional ocean bottom seismograph obs data acquired in the central and northern part of the voring basin mid norway margin have been modeled by use of 2 d ray tracing the semi regional model derived from the study of twenty five obss deployed along a 120 km long profile is compared with a regional model consisting of five obss from the same profile the semi regional model is somewhat more detailed than the regional model due to the considerably closer receiver spacing the overall geometry and velocity distribution of the two models are remarkably similar however proving that the regional procedure with large obs spacing provides a reliable regional model intrusions of sills related to early tertiary continental rifting and break up are important at intermediate and deep sedimentary levels 2 10 km below sea floor in most parts of the area the semi regional modeling suggests that one of the deepest sills extends much further east and is substantially thicker locally more than 500 m than indicated in the regional model another important difference is a high velocity body within the upper crystalline crust at 11 12 km depth in the nw part of the area indicating that the closer obs spacing in the semi regional modeling allows detection of local intra crustal intrusions local differences are also inferred in the lower crust at about 20 km depth a structure is inferred within the lower crust from wide angle reflections this might suggest that the high velocity lower crustal layer interpreted as magmatic underplating consists of a mixture of underplated intruded magmatic material and blocks of continental lower crust

crustal seismic velocity structure in the epicentral region of the latur earthquake september 29 1993 southern india inferences from modelling of the aftershock seismograms abstract we present 1 d models of crustal p and s wave velocity structure in the epicentral region of the devastating 1993 latur earthquake in southern india travel time and relative amplitude modelling of a seismogram section of well located aftershocks out to 80 km offset by reflectivity synthetic seismograms reveal alternating low velocity layers lvls for p and s waves in the upper crust at depths of 6 5 9 0 km and 12 3 14 5 km with about 7 velocity reduction a lower crustal lvl at 24 26 km depth is also inferred by modelling the seismic activity in the latur region is essentially confined to shallow upper crustal depths of less than 6 km above the lvls the upper crustal velocity models further reveal a relatively low vp vs ratio of 1 65 while synthetic seismogram models consistent with the upper crustal p wave reflection phases reveal a qp qs ratio certainly less than 1 a high conductivity zone at a shallow depth of 6 10 km brought out by broad band mt soundings in this region is found to be coincident with the upper crustal lvls these coincident anomalous properties of the upper crust suggest the presence of fluids probably in a partially or slightly under saturated state below the hypocentral region of the latur earthquake and its aftershocks

a reappraisal of the sibson scholz fault zone model the nature of the frictional to viscous brittle ductile transition along a long lived crustal scale fault outer hebrides scotland the widely cited sibson scholz conceptual fault zone model suggests that seismically active upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10 15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep the crustal scale outer hebrides fault zone ohfz in nw scotland has been described as the type example of such a continental fault zone it cuts precambrian basement gneisses and is deeply exhumed allowing direct study of the deformation products and processes that occur across a wide range of crustal depths a number of fault rock assemblages are recognized to have formed during a long lived displacement history lasting in excess of 1000 myr during caledonian movements that are recognized along much of the 190 km onshore fault trace brittle cataclasite bearing faults in the west of the ohfz are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust related crustal thickening or shear heating microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids this led to retrogression with the widespread development of new finegrained phyllosilicate bearing fault rocks phyllonites and the onset of fluid assisted grain size sensitive diffusional creep in the most highly deformed and altered parts of the fault zone phyllonitic fault rocks also occur in older more deeply exhumed parts of the fault zone implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature depth range within crustal scale faults our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal scale faults containing interconnected networks of phyllosilicate bearing fault rocks will be characterized by long term relative weakness and shallow 5 km frictional viscous transition zones similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the san andreas fault

experimental deformation of muscovite shear zones at high temperatures under hydrothermal conditions and the strength of phyllosilicate bearing faults in nature abstract layers of fine grained muscovite were hot pressed then sheared between alumina sliders to shear strains up to 2 at temperatures between 300 and 700 c confining pressures of 206 mpa and various pore water pressures high pore water pressures helped suppress dehydroxylation of the mica and permitted testing at temperatures higher than previously used shear strain rates between 10 3 s 1 and 10 7 s 1 were accessed using constant shear strain rate and stress relaxation testing except for strain rates 5 s 1 at 700 c deformation was strain rate and temperature insensitive but effective normal stress sensitive with a friction coefficient at yield of 0 3 rising with strain to 0 5 steady state sliding was not attained from the mechanical data and microstructural study deformation was inferred to have occurred by a mixture of brittle frictional and crystal plastic processes at 700 c and low strain rates the shear strength falls rapidly with a linear viscous characteristic in a way not previously reported this is tentatively attributed to rate control by viscous glide of basal dislocations extrapolating these results to geological strain rates we expect mica rich fault zones will exhibit frictional behaviour with a low friction coefficient between 0 25 and 0 5 giving way at mid crustal conditions to a rapid strength drop as viscous creep supervenes thus mid to lower crustal mica dominated faults of any orientation and terrains of schistose metapelitic rocks in the cores of orogeneic belts are expected to be very weak supporting shear stresses in the range 1 10 mpa the frictional behaviour of mica rich faults in upper crustal regions is however expected to be too strong to account alone for the proposed weakness of some major fault zones