electrical conductivity of orthopyroxene and its high pressure phases electrical conductivity of orthopyroxene clinopyroxene and ilmenite garnet with the same starting material san carlos orthopyroxene mg 0 92 fe 0 08 sio 3 containing 2 89 al 2 o 3 by weight were measured within their respective stability fields in a multi anvil press in the formula for conductivity 0 exp h kt pre exponential factors 0 of orthopyroxene clinopyroxene and ilmenite garnet are 5248 1778 2239 s m respectively and the activation enthalpies are 1 80 1 87 and 1 66 ev respectively these results are required for defining a laboratory based conductivity depth profile of the upper mantle

landform regolith mapping in the west african context abstract in this paper we present an integrated landform regolith mapping protocol chart for west africa developed using a case from central burkina faso the chart accounts for i the specificities of the ubiquitous duricrust capped remnants of the regional paleolandsurfaces particularly the residual or detrital character of the fe duricrusts and ii the diversity of the remaining non duricrusted portions of the landscape with an emphasis on the pitfalls encountered in determining the in situ or transported character of the regolith the paper also provides field identification criteria and illustrations of the typical west african landform regolith associations the landform regolith mapping chart is integrated into a prospectivity chart which is used to define exploration guides i defining sampling strategies ii interpreting potential geochemical anomalies and iii targeting lateral extensions of known bedrock mineralizations the mapping protocol further allows for the definition of a class of landform regolith association propitious to supergene mineral concentrations the mapping protocol is shown to be transposable to wetter contexts and more generally to other tropical shield provinces

common conversion point stacking of receiver functions for estimating the geometry of dipping interfaces summary we develop a new method to estimate geometries of dipping seismic velocity discontinuities from common conversion point stacking of receiver functions the multistage fast marching method is applied to the ray tracing with refraction at dipping interfaces although geometry of interfaces dipping at 30 70 cannot be correctly estimated with a 1 d velocity model it can be correctly estimated when refraction at dipping interfaces is taken into account we apply this method to synthetic and observed data and confirm that geometries of dipping interfaces are successfully estimated this method will contribute to revealing the structures beneath subduction zones where dipping interfaces exist

behavior of gold in the lateritic equatorial environment weathering and surface dispersion of residual gold particles at dondo mobi gabon abstract a supergene gold dispersion halo has developed at dondo mobi southern gabon from the weathering of an auriferous lisvenite under equatorial humid rain forest conditions morphological and chemical studies were performed on gold particles extracted from both fresh and weathered rocks from different parts of the dispersion halo the results demonstrate that the gold particles are residual and are subjected to increasing weathering upward through the profile from the protore to the surface in the central part of the halo and at the surface both laterally upslope and downslope weathering continues downward into the formerly barren saprolites which were derived from proterozoic schist and archean gneiss rim of the halo thus the au content progressively decreases as does the ag content of gold particles with distance from the center of the ore body gold particles subjected to increased weathering are smaller with more rounded shapes and surfaces pitted by dissolution the dispersion pattern is controlled by au dissolution and translocation of residual gold particles these processes involve an autochthonous evolution in the central part of the halo and a para autochthonous evolution at the rim of the halo at dondo mobi chloride and humic substances are the only ligands present in significant concentrations that are capable of complexing au thermodynamic calculations demonstrate that under such acidic conditions ph ranges from 4 to 6 gold stability increases with increasing silver content the dominant au complexes formed have been calculated to be an aurous hydroxichloride complex and an organometallic complex both of which are stable at surficial conditions thus both chemical and translocation processes control the mobility of au and gold particles in equatorial lateritic weathering profiles

including anisotropy in 3 d velocity inversion and application to marlborough new zealand summary two types of seismic velocity heterogeneity are often observed in a given region pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes we seek to combine these types of observations by including anisotropy in 3 d velocity inversion the fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model the ray paths are estimated for each of the shear wave splitting observations and the 3 d initial model is determined via an averaging process and a coherence analysis the initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements the traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node the ray paths are defined with approximate 3 d ray tracing and for each point along a ray path the velocity is defined as a function of the ray path azimuth at that point a blended approach is used to damp the perturbations to the magnitude of anisotropy and or perturbations to the fast direction tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable application to the marlborough region shows variations in anisotropy in the brittle crust ductile lower crust mantle and subducted slab the patterns are consistent with the shear wave splitting observations but show some additional features the maximum anisotropy is approximately 12 per cent oriented northeast between the awatere and wairau faults this high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain in the region of interaction of the ductile lower crust with the shallow subducting slab moderate anisotropy with east west orientation is imaged this is consistent with ductile deformation roughly parallel to the plate velocity

tararua broadband array north island new zealand abstract the tararua broadband array consisting of nine three component seismometers was deployed in southern north island new zealand from february 1991 to september 1992 this l shaped array had an approximately 40 km n s arm and 30 km e w arm its primary purpose was to record waveforms from tonga kermadec earthquakes whose propagation paths have spent the majority of their time in the pacific plate that has been subducted beneath the australasian plate along the tonga kermadec hikurangi margin the continuous recording allowed simultaneous acquisition of an excellent teleseismic and regional broadband seismogram dataset for the main experiment broadband three component waveforms with p wave precursive slab phases up to 15 sec early relative to the jeffreys bullen travel times have been recorded for 71 events multiple frequency analysis of the p waveform shows that the high frequencies 5 hz arrive first with coherent dispersion below about 4 5 hz this behavior can be attributed to a high velocity layer 8 to 10 km thick lying above the cold fast lithosphere of the subducted pacific plate polarization analysis of the p wave precursors show them to arrive at steep incidence angles of 20 to 30 suggesting the phase refracts off the subducted plate9s surface to the array we observe exceptionally large pl leaking modes from shallow events in the tonga kermadec region to the north

plate coupling in the northern south island and southernmost north island new zealand as illuminated by earthquake focal mechanisms subduction of the pacific plate in the northern south island and southernmost north island of new zealand is transitional insofar as the crustal thickness of the pacific plate increases significantly along strike in the northern south island focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes the stress regime in the subducted plate appears to be dominated by slab pull t axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone and the average azimuth of these t axes is rotated some 25 clockwise out of the direction of dip of the subducted plate this can be related to the asymmetrical shape of the subducted slab in contrast the stress regime in the overlying plate appears to be dominated by subhorizontal compression low angle thrust events near the plate interface in cook strait and the southernmost north island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from global positioning system gps observations an absence of low angle thrust events near the plate interface in the northern south island and the tendency of p axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost north island differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern south island

morphopedologie et anomalie geochimique l intense alteration geochimique qui a affecte l ensemble de la zone transcontinentale des boucliers pendant plusieurs millions d annees a dissous partiellement la partie superieure de la croute

intermediate depth earthquakes in a region of continental convergence south island new zealand it is rare to find earthquakes with depths greater than 30 km in continent continent collision zones because the mantle lithosphere is usually too hot to enable brittle failure however a handful of small intermediate depth earthquakes 30 97 km have been recorded in the continental collision region in central south island new zealand the earthquakes are not associated with subduction but all lie within or on the margins of thickened crust or uppermost mantle seismic high velocity anomalies the largest of the earthquakes has m l 4 0 corresponding to a rupture radius of between 100 and 800 m providing bounds on the upper limit to the rupture length over which brittle failure is taking place in the deep brittle plastic transition zone the earthquake sources may be controlled by large shear strain gradients associated with viscous deformation processes in addition to depressed geotherms

exploration of mid ocean ridges with a frequency domain electromagnetic system summary electromagnetic survey methods using controlled sources have been applied to ocean floor and mid ocean ridge surveys and have shed new light on the distribution of fluids within these structures the present state of knowledge of the electrical nature of oceanic crust is reviewed and some alternative mid ocean ridge electromagnetic surveys are simulated using a numerical modelling algorithm to see what further information they could provide this simulation considers the response of both large scale crustal targets such as magma bodies and zones of hydrothermal circulation and also the effect of near surface heterogeneity that may spatially alias the data the magnitude of this heterogeneity is estimated and it is shown that it could be large enough to prevent useful structural constraints being obtained if the effect spatial aliasing was negligible or could be removed a trans ridge experiment could delinate zones of partial melting and hydrothermal circulation within the oceanic crust

electromagnetic active source sounding near the east pacific rise an active source electrical conductivity sounding was carried out near the east pacific rise at 21 n the average electrical conductivity from 2 km beneath the seafloor into the upper mantle was found to be about 0 004 s m there is no evidence of fully molten magma at a distance of 7 km from the spreading axis the inferred conductivity with depth of the crustal rock is consistent with brace s model of connected seawater filled pores in rocks heated from beneath

connectivity of molten fe alloy in peridotite based on in situ electrical conductivity measurements implications for core formation in terrestrial planets abstract the connectivity of molten fe s in peridotite has been experimentally investigated by means of in situ electrical conductivity measurements at high temperatures and 1 gpa starting materials were powdered mixtures of peridotite klb 1 with various amounts 0 3 6 13 19 24 vol of the 1 gpa eutectic composition in the fe fes binary system at temperatures above the eutectic point in the fe fes system 980 c and below the solidus of klb1 1200 c molten fe s in a solid silicate matrix interconnects when the volume fraction is over 5 conductivity temperature paths indicate that in the presence of partial silicate melting the connectivity of molten fe s in a peridotite matrix is inhibited based on observations of retrieved samples the percolation threshold of fe s melts in the presence of low to moderate degrees of silicate melt is estimated at 13 2 vol these results indicate that if the volume fraction of fe alloy in a planetesimal was initially greater than 5 and if early heating by decay of radionuclides raised the temperature of the interior above the fe alloy melting point initial metal segregation was controlled by permeable flow of molten iron alloy in a solid silicate matrix these conditions were likely met by many terrestrial objects in the early solar nebula efficient removal of residual fe alloy 5 vol from silicate requires high degree melting of silicate so that metal can segregate as droplets giant impacts during the final stage of accretion of large planetary objects could supply the energy required for high degrees of melting alternatively if initial metal segregation were delayed until a planetary object grew to large size 1000 km in diameter release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean

subduction zone anisotropy beneath corvallis oregon a serpentinite skid mark of trench parallel terrane migration 1 we studied the back azimuth dependence of ps converted phases at gsn station cor corvallis oregon using broadband p receiver functions computed from 602 teleseismic earthquakes the amplitudes and polarities of the transverse ps phases are largely two lobed which indicates anisotropy with a tilted symmetry axis a double peaked ps conversion at 4 5 6 5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the ps converted phase by the dipping interface a polarity flip on the transverse receiver function near north south back azimuth indicates an anisotropic symmetry axis aligned north south far from the convergence direction n68 e the ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed vp 6 0 6 5 km s near 40 km depth at the slab interface with the overriding north american plate one dimensional modeling suggests 10 anisotropy in the supraslab layer with a slow symmetry axis oriented n5 w at a 60 tilt from the vertical adjustments for the effect of slab interface dip on ps amplitude suggest a somewhat lower 7 anisotropy we infer a thin 7 km anisotropic detachment zone for the northward slippage of the siletz forearc terrane along the top of the descending slab a motion consistent with gps measurements and models of regional lithospheric dynamics serpentinite is a likely constituent for the deep anisotropic layer owing to its reduced vp and high poisson ratio the ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone

velocity and anisotropy structure at the hikurangi subduction margin new zealand from receiver functions summary we examine seismic velocity and anisotropy at the interface between the australian and the subducting pacific plate in southern north island new zealand using receiver functions from teleseismic earthquakes recorded on two l shaped seismic arrays iris station snzo makes up the corner of the southern array velocity models are tested with a ray theoretical method that allows both dipping boundaries and anisotropy assuming a 2 d structure beneath each array the strongest arrival on the northern array consistently appears at 2 s the arrival is modelled by a jump in velocity at a flat boundary at 12 km which may be caused by a change in metamorphic grade this prominent arrival correlates with a weaker arrival at 1 5 s 7 km at station snzo isotropic layers dipping with the slab can explain many features in the first 20 s of the records but anisotropy is needed to explain both the transverse receiver functions and a change in the polarity of a prominent ps arrival at 3 s delay relative to p o nthe radial component this arrival is consistent with a change between an upper layer that is either isotropic or anisotropic with a horizontal symmetry axis and a supraslab metasediment layer having anisotropy with av ertical slow axis of symmetry there is tradeoff between boundary dip the plunge of the axis of symmetry layer thickness and the amount of anisotropy a dip of 20 shallowing to 15 under the tararua array thickness of 4 km and a vertical symmetry axis fits the backazimuth variation of the amplitudes and polarities of the arrivals with anisotropy of 4 6 per cent observed layer multiples are smaller than synthetic predictions to explain this discrepancy either layer boundaries are rough attenuation is strong or the layer boundaries are more gradational at the periods 1 s used in this study compared to previous studies 5 s

the lithosphere asthenosphere boundary beneath the south island of new zealand abstract lithosphere asthenosphere boundary lab properties beneath the south island of new zealand have been imaged by sp receiver function common conversion point stacking in this transpressional boundary between the australian and pacific plates dextral offset on the alpine fault and convergence have occurred for the past 20 my with the alpine fault now bounded by australian plate subduction to the south and pacific plate subduction to the north using data from onland seismometers especially the 29 broadband stations of the new zealand permanent seismic network geonet we obtained 24 971 individual receiver functions by extended time multi taper deconvolution and mapped them to three dimensional space using a fresnel zone approximation pervasive strong positive sp phases are observed in the lab depth range indicated by surface wave tomography these phases are interpreted as conversions from a velocity decrease across the lab in the central south island the lab is observed to be deeper and broader to the northwest of the alpine fault the deeper lab to the northwest of the alpine fault is consistent with models in which oceanic lithosphere attached to the australian plate was partially subducted or models in which the pacific lithosphere has been underthrust northwest past the alpine fault further north a zone of thin lithosphere with a strong and vertically localized lab velocity gradient occurs to the northwest of the fault juxtaposed against a region of anomalously weak lab conversions to the southeast of the fault this structure could be explained by lithospheric blocks with contrasting lab properties that meet beneath the alpine fault or by the effects of pacific plate subduction the observed variations in lab properties indicate strong modification of the lab by the interplay of convergence and strike slip deformation along and across this transpressional plate boundary

hydrothermal circulation at the cleft vance overlapping spreading center results of a magnetometric resistivity survey we report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the cleft and vance segments of the juan de fuca ridge the data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers areas of reduced crustal electrical resistivities with values approaching that of seawater are seen beneath the neovolcanic zones of both active spreading centers we interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot young oceanic crust

three dimensional lithospheric structure below the new zealand southern alps uppermost mantle seismic structure below the southern alps in south island new zealand is investigated by teleseismic p wave travel time residual inversion the three dimensional tomographic images show a near vertical high velocity 2 4 structure in the uppermost mantle that directly underlies thickened crust along the nnessw axis of the southern alps the center of the high velocity anomaly lies to the east of the alpine fault which bounds pacific and australian plate rocks the oblique collision of these plates resulted in the uplift of the southern alps during the past 5 7 m y also a high velocity anomaly 3 5 corresponding to the hikurangi subduction zone lies to the northeast of the southern alps anomaly and low velocity anomalies 3 underlying parts of northwestern and southern south island may be signatures of late tertiary extension and volcanism the data consist of teleseismic arrival times from the new zealand national seismograph network and arrival times recorded during the 1995 1996 southern alps passive seismic experiment crustal heterogeneity was accounted for by back projecting the rays through an independently obtained three dimensional crustal velocity and moho depth model the southern alps uppermost mantle velocity anomalies are most simply explained by lithospheric thickening below the center of convergence accompanied by thinning and asthenospheric upwelling adjacent to the region of convergence

geoelectric structure of the proterozoic wopmay orogen and adjacent terranes northwest territories canada magnetotelluric mt soundings were made along a transect in northern canada crossing the proterozoic wopmay orogen fort simpson basin and adjacent parts of the slave craton and the nahanni terrane the results are used to define the geoelectric structure and constrain the crustal and lithospheric structure and evolution across the wopmay orogen geoelectric strikes at crustal depths average n34 e and are interpreted to be related to transcurrent faulting that occurred during late distal collisions at the western margin of the orogen weak two dimensionality in the fort simpson basin is interpreted to be due to the sedimentary rocks in the basin at longer periods geoelectric strikes across the wopmay orogen rotate from n43 e at uppermost mantle penetration to n62 e at a depth of 100 km the uppermost mantle strikes are interpreted to be due to ductile shearing linked to the transcurrent faulting in the overlying crust the deeper strikes may be caused by shearing at the base of the present day lith

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

seismic migration processing of p sv converted phases for mantle discontinuity structure beneath the snake river plain western united states we experiment with backprojection migration processing of teleseismic receiver functions from the snake river plain srp broadband seismic experiment previous analyses of data from this experiment have used a common midpoint cmp stacking approach a method widely applied for analysis of p sv converted phases receiver functions to obtain high resolution imaging of upper mantle discontinuities the cmp technique assumes that all p sv conversions are produced by flat lying structures and may not properly image dipping curved or laterally discontinuous interfaces in this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions we first perform synthetic experiments that illustrate the potential improvement of migration processing over cmp stacks application of the migration processing to the srp data set shows most of the major features as in the original cmp work but with a weaker 410 km discontinuity and a more intermittent discontinuity at 250 km apparent depth random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images these tests show that a 20 km elevation of the 660 km discontinuity directly beneath the snake river plain is robust but that the variations in 410 km discontinuity topography that we observe are not stable upon resampling bright spots near 250 km apparent depth are robust upon resampling but interpretation of these features is complicated by possible sidelobe artifacts from topside moho reverberations

electric lithosphere of the slave craton the archean slave craton in northwestern canada is an ideal natural laboratory for investigating lithosphere formation and evolution and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes three deep probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures the magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity 30 m at depths of 80 100 km and is interpreted to be due to dissolved hydrogen or carbon in graphite form this geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer the tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630 2620 ma

the moho in subduction zones abstract the moho in subduction zones exists in two distinct forms one associated with the subducting oceanic plate and second with the overriding plate the seismic expression of both forms is linked to the nature of a landward dipping low velocity zone lvz that has been detected in a majority of subduction zones about the globe and that approximately coincides with wadati benioff seismicity we review seismic studies that constrain the properties of the lvz in cascadia where it has been extensively studied for over a quarter century a model in which the lvz is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal layer 2 is consistent with available geological and geophysical data and reconciles previously conflicting interpretations in this model the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic layer 3 below the resulting seismic velocity contrast approaching 50 for s waves significantly overwhelms that of a weaker underlying oceanic moho at greater depths oceanic crust undergoes eclogitization in a top down sense leading to gradual disappearance of the lvz the large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled forearc mantle wedge pervasive serpentinization and free fluids reduce velocities within the wedge thereby diminishing erasing or even inverting the seismic contrast associated with the moho of the overriding plate this model is tested against observations of lvzs and forearc mantle structure worldwide

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

west african lateritic pediments landform regolith evolution processes and mineral exploration pitfalls this paper is a contribution to the understanding of surface dynamics of tropical shields over geological timescales emphasis is put on the fundamental and applied implications of regolith production and dispersion processes through the formation dissection and preservation of landforms it is based on the case study of sub saharan west africa which recorded neogene 24 3 ma stepwise dissection of its topography through the emplacement of three lateritic pediment systems which still occupy most of its surface pediments are erosional transportation slopes having been weathered and duricrusted pediment regolith associations therefore depend on the parent rock transport dynamics and preservation of the material having transited on their surface as well as on the intensity of their weathering duricrusting iron oxy hydroxide cemented clastic sediments detrital ferricretes and unconsolidated clastic sediments are the dominant outcropping material and as such represent a challenge for mineral exploration that relies on surface geochemical sampling to detect metal concentration in the bedrock landform regolith mapping beyond the scale of modern interfluves combined with paleolandscape reconstitution are relevant to provide exploration guides for i interpreting geochemical anomalies on pediments ii tracing their potential source when they have been transported on pediments and iii targeting suspected ore bodies concealed beneath pediment s past and present latitudinal climatic zonation of pedimentation and weathering patterns suggests a gradation of pedimentation process across the intertropical zone and explains why pediments may have been overlooked in equatorial environments with implications for mineral exploration successive pediment systems adapted to uneven knickzone bearing river networks producing a spatially consistent and reduced 80 m stepping pattern of pediments independent from elevation or distance to base level pediments pediplains are therefore not proxies of uplift and their preservation over geological timescales typifies regions submitted to 10 m my erosion rates the identification and study of lateritic pediments bear important implications on shield sediment routing systems and a better access to the bedrock and its resources which may still be underestimated in the tropics

dissolution features of gold particles in a lateritic profile at dondo mobi gabon abstract this paper describes the morphological evolution of gold particles in a lateritic weathering profile under rain forest inherited from primary quartz veins associated with amphibolite the gold particles can be recognized from the weathering front up through the saprolite and overlying regolith to the land surface in the central part of the supergene gold dispersion halo gold particles undergo some changes in the uppermost 10 m voids and corrosion pits appear on particle surfaces their numbers and sizes increase toward the land surface edges of particles become progressively more blunt from the bottom to the top of the section the morphological changes are attributed to dissolution of gold over time possible causes for such dissolution are discussed occurrence of gold particles in the soil could be helpful for gold explorations in the humid tropics

evolution of regoliths and landscapes in deeply weathered terrain implications for geochemical exploration abstract thick commonly lateritic regoliths are widespread in inter tropical regions of the world and present particular challenges in exploration these are best tackled through a sound understanding of the evolution of the landscapes in which they occur the regoliths formed under humid warm to tropical conditions and although they may have been modified by later climatic changes i e to more humid or more arid conditions many chemical and mineralogical characteristics are retained these include the geochemical expressions of concealed mineralization erosional and depositional processes control the preservation and occurrence of specific regolith units that may be used as sample media and in turn target size element associations and contrast thereby influencing sampling procedures analysis and data interpretation these parameters are best summarized in terms geochemical dispersion models based on the degree of preservation of the pre existing lateritic regolith regolith landform mapping permits an assessment of the terrain in terms of such models in relict regolith landform regimes in which the lateritic regolith is largely preserved broad multi element anomalies in the upper ferruginous horizons lateritic residuum can be detected using sample intervals of 1 km or more in contrast in erosional regimes where this material is absent anomalies in upper saprolite and the soil and lag derived from it are more restricted in area and closer sampling intervals 200 40 m or less may be necessary lag and soil are generally ineffective in depositional areas except where the sediments are very thin e g

variations in the electrical conductivity of the upper mantle beneath north america and the pacific ocean variations in the electrical conductivity of the mantle beneath carty lake in the canadian shield tucson in the southwest united states and honolulu and midway in the north central pacific were determined through the inversion of long period magnetotelluric and geomagnetic depth sounding data inversion of computed response functions is carried out using a minimum structure regularized approach the upper mantle beneath carty lake is approximately an order of magnitude more resistive than the upper mantle beneath tucson and nearly 1 5 orders of magnitude more resistive than honolulu and midway island inversions were also carried out where the minimum structure constraint was removed at known upper mantle discontinuities these models show a jump in conductivity of 1 5 orders of magnitude across the 660 km discontinuity a result that is consistent with laboratory experiments on realistic mantle assemblages mantle conductivity profiles at carty lake are significantly more resistive than those at tucson honolulu and midway to depths of 300 400 km these observations likely reflect differing thermal states the presence or absence of partial melt and volatiles and may also be related to chemical differences between depleted and undepleted upper mantle the observed conductivity variations may be interpreted as lateral variations in temperature partial melt and or dissolved hydrogen in olivine

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

lithospheric geometry of the wopmay orogen from a slave craton to bear province magnetotelluric transect 1 two dimensional inversions of lithospheric probing magnetotelluric mt data at a total of 20 sites acquired along an approximately east west 300 km long profile across the wopmay orogen in the northwest territories canada provide electrical resistivity models of the boundary between the archean slave craton and the adjacent proterozoic bear province an analysis of distortion effects and structural dimensionality indicates that the mt responses are primarily one dimensional or only weakly two dimensional with a depth independent geoelectric strike angle of n32 e consistent with regional structural geology the regional scale model generated from the longer period responses from all of the sites along the profile reveals significant lateral variations in the lithospheric mantle resistive cratonic roots are imaged to depths of 200 km beneath both the slave craton and the hottah terrane of the bear province these are separated by a less resistive region beneath the great bear magmatic zone which is speculatively interpreted as a consequence of a decrease in the grain size of olivine in the wopmay mantle caused by localized shearing compared to its neighboring cratonic roots focused two dimensional models from higher frequency responses at sites on specific sections of the profile reveal the resistivity structure at crustal depths beneath the region these suggest that the root of the slave craton crosses beneath the wopmay orogen and that the wopmay fault zone does not penetrate into the lower crust a comparison of these results with those obtained during the lithoprobe project farther south shows striking along strike variations in the conductivity structure associated with the wopmay orogen

teleseismic p wave delays and modes of shortening the mantle lithosphere beneath south island new zealand a high speed zone in the mantle directly beneath the southern alps of new zealand is required by the recorded pattern of teleseismic p waves two parallel lines of 80 seismographs spaced at 2 km intervals recorded three earthquakes from the western pacific with epicentral distances of 52 53 and 78 azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines differences between measured delays and those predicted from the crustal structure reach 0 8 s along one line and 1 0 s along the other with the rays for the earliest arriving signals passing the depth of 120 km beneath the center of the island assuming these early arrivals are due to structure within the mantle shallower than 200 km they imply that the core of the high speed zone lies beneath the thickest crust which has been shortened by 100 km of convergence during the past 6 7 myr although the shape and position of the high speed body cannot be fixed uniquely a roughly symmetric body centered about a depth of 120 km 80 100 km wide with a depth extent of 100 km and with a maximum speed advance of 7 satisfies the observations the pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other rather the residuals favor thickening of mantle lithosphere by a more homogenous straining of it as if mantle lithosphere beneath continental crust behaved as a continuum an excess mass in the mantle is also required by the observed gravity anomalies once allowance is made for the seismically determined crustal thickness this high density mantle anomaly provides sufficient force per unit length to maintain the crustal root which is approximately twice as thick as that necessary to support the topography

the detection of lateral anisotropy of the ocean floor by electromagnetic methods summary in a typical marine controlled source electromagnetic csem experiment one or more self contained receivers capable of measuring small variations in the electromagnetic field are located in an array on the sea floor a remote horizontal electric dipole is towed close to the sea floor by a surface vessel variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers the variations contain information about the distribution of electrical conductivity of the subsurface rocks the electrical conductivity is related to such critical physical parameters as porosity temperature and fluid content published studies of the response of the sea floor to electromagnetic excitation have not included any systematic variation in the sea floor conductivity with horizontal direction a horizontally isotropic oceanic lithosphere is not an appropriate assumption particularly in the vicinity of the ridge here for example in the near surface layers basalts are extruded onto the sea floor within the very narrow fissured neo volcanic zone the fissures may be filled with sea water and sediment which are better conductors than typical basalt alternatively as time progresses they may become sealed with hydrothermal mineral deposits which can be less conductive than the host rocks in both instances the electrical conductivity of the composite material in the direction oriented along the fisures parallel to the ridge will be greater than that in the transverse direction measures of anisotropy on sea floor of varying age are an indication of the history of the tectonic activity at the ridge the electromagnetic effects of lateral anisotropy are examined through the study of the response of a simple double half space model an upper half space representing sea water is connected to a lower half space representing anisotropic crustal material a general theory describing the excitation of this model by any transient electromagnetic source is developed and the step on responses for systems composed of a horizontal electric dipole transmitter and horizontal magnetic and electric field receivers are computed the results of previous work indicate that following a change in current in the transmitter two distinct changes in horizontal electric field strength separated in time are observed at the receiver the first is caused by the diffusion of the electromagnetic field through the relatively resistive sea floor the second is caused by diffusion through the relatively conductive sea water the times at which the two events occur are robust measures of the isotropic conductivity of the sea floor and the sea water respectively the results of this study show that shape and time of the initial change in the electric field varies systematically with both the average conductivity and the anisotropy of the sea floor for some orientations of the transmitter and the receiver the effect of increasing the anisotropy is similar to the effect of increasing the average conductivity the initial event is delayed however there are orientations for which increasing the anisotropy actually results in an earlier arrival natural science and engineering research council in canada

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

intermediate depth earthquakes beneath nelson new zealand and the southwestern termination of the subducted pacific plate 1 there has been debate on how convergence of mantle lithosphere is accommodated during continental collision in the central south island of new zealand whether by intracontinental subduction or continuous deformation of mantle lithosphere intermediate depth earthquakes are sparse beneath the central south island and abundant seismicity 130 230 km deep within the hikurangi subduction zone to the northeast ends abruptly beneath the nelson region of the northern south island this termination of seismicity has previously been interpreted as marking the southwestern edge of the subducted slab here we investigate the stress and strain regime at this termination of activity and find that it is very similar to that within the subducted plate both updip and along strike towards the northeast our results are consistent with subduction continuing further southwest albeit largely aseismically as suggested by shallow earthquakes in the northeastern south island

pn anisotropy beneath the south island of new zealand and implications for distributed deformation in continental lithosphere pn travel times from regional earthquakes recorded both by stations on new zealand and by ocean bottom seismographs deployed offshore indicate anisotropy in the uppermost mantle beneath the region the largest anisotropy of 8 2 1 lies beneath the deforming part of the south island to just off its west coast a zone roughly 100 200 km wide the fastest propagation is aligned n60 e 3 essentially parallel to the largely strike slip relative plate motion since 20 ma also n60 e the magnitude of anisotropy decreases abruptly northwest and southeast of this zone and on the southeast side of the island the orientation of fastest propagation is between n32 w and n s the n60 e orientation of fast propagation is consistent with finite strain within the uppermost part of the mantle lithosphere if the measured 850 km of displacement of the pacific plate past the australia plate is spread over a region with a width of 100 200 km the agreement of this orientation of fast propagation with the orientation or relative plate motion suggests the possibility of but does not require some dynamic recrystallization in rock as cold as 500 800 c where peierls creep seems to be the likely deformation mechanism such a strain distribution matches deformation of a thin viscous sheet that obeys a constitutive relationship of the form e n where e is the average strain rate and is the operative deviatoric stress with an average value of n 3 10 presumably the nw se fast propagation in the region southeast of the island results from strain that precedes the cenozoic deformation that has shaped the island

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

seismicity structure and tectonics of the wellington region new zealand summary the wellington region lies on the border between the pacific and australian plates the former being subducted the plate interface lies at shallow depth under the region 10 60 km so that detailed and long term monitoring of the seismicity of the zone where large thrust earthquakes are expected to occur is possible hypocentres for events recorded by a 12 station seismic network for the period 1978 1982 7231 events have been refined by the use of a threedimensional velocity model derived from a simultaneous inversion of arrival time data from 129 local events for both hypocentres and velocity the hypocentres show that the subducting pacific plate is defined by a nw dipping zone of relatively intense activity the plate interface as given by the upper envelope of this activity is offset vertically by 7 km on average along a nw se line through cook strait parallel to the direction of absolute motion of the pacific plate rather than to the direction of convergence the strike of the interface at deeper levels also changes across this line this distortion may partly reflect effects of the transition to continent continent type convergence further south within the subducting pacific plate the seismic activity defines two parallel zones separated by about 20 km the lower zone being much less active and perhaps merging with the upper zone at a depth of 80 90 km composite focal mechanisms of events in the upper zone imply predominantly down dip tension with a compression axis perpendicular to the plate interface the mechanism in the lower zone also implies down dip tension but the compression axis has rotated to be horizontal and along strike indicating a component of lateral shearing within the plate these two types of mechanism correlate well with those found previously for events at depths of 150 km or more thus it seems that the large negative buoyancy of the subducted lithosphere dominates the internal stress field throughout the length of the seismically active region the lithosphere acting as a stress guide there is no evidence for a significant number of events with mechanisms indicating thrusting along the plate interface the seismic velocity in the region corresponding to the upper seismic band is close to that typical for oceanic crust which here is probably considerably thicker than normal there is no evidence for a substantial increase in velocity down

transient electrical response of san quintin dunite as a function of oxygen fugacity changes information about charge carriers the electrical conductivity of san quintin dunite sqd measured between 950 and 1150 c at controlled oxygen fugacity fo2 within the olivine stability field shows transients in response to changes in fo2 such behavior has not been reported previously for measurements made under similar conditions on either olivine single crystals or polycrystalline samples dunites and lherzolites in which olivine is the major phase in general for olivine an increase in fo2 results in an increase in the transient is manifested as a change in from a stable equilibrium value at a specific fo2 to a quickly established subsequent value the direction of which is opposite that of the final value that will be attained for the fo2 change this transient may be caused by a changing population of electrons produced by oxygen vacancies the grain boundaries or a short lived defect we postulate that the transient is observed in this particular dunite because of its large surface area to volume ratio the transient is more pronounced at relatively high fo23 and temperatures between 950 and 1100 c at relatively low fo2s and temperatures higher than 1100 c the effect is diminished the seebeck coefficient s at 1200 c is slightly smaller than that reported for single crystal olivine and at 1100 c is similar to that reported for single crystal olivine

electrical properties of fine grained olivine evidence for grain boundary transport 1 to investigate the mechanisms responsible for electrical conduction and deduce the relation between grain size and conductivity the electrical conductivity of synthetic polycrystalline forsterite with grain sizes between 1 1 0 4 and 4 7 2 4 m was measured at temperatures up to 1470 c and at 0 1 mpa pressure the complex impedance plots display one clear arc indicating a single dominant conduction mechanism bulk conductivity is inversely proportional to the grain size of the different samples this relation suggests that the electrical conductivity of the samples is controlled by grain boundary diffusion of the charge carriers the apparent activation energy q for diffusion of the charge carriers between 1180 and 1470 c lies between 315 39 and 323 15 kj mol this resembles previous data on grain boundary diffusion of mg in forsterite a geometrical model of less conducting cubic grains and more conducting grain boundaries agrees well with the experimental data this model is applied to predict the conductivity contrast between fine grained shear zones and less deformed regions in the lithosphere upper mantle shear zones are predicted to have 1 5 2 orders of magnitude higher conductivity than less deformed regions in the lithosphere this means grain boundary transport probably has an important role in the existence of anomalously high conductivity zones in the upper mantle and that fine grained shear zones might be detected using magnetotelluric methods

electrical conductivity and partial melting of mafic rocks under pressure we demonstrate the importance of electric conductivity measurements of partially molten mafic rocks by examining of oman gabbro karelia olivinite ronda and spitzbergen peridotites the electrical conductivities of these rocks were estimated using the impedance spectroscopy at temperatures between 800 c and 1450 c and at pressures between 0 3 and 2 gpa in experiments performed in a piston cylinder apparatus at temperatures below and above melting samples were equilibrated during durations on the order of 200 h our results show that a jump in electrical conductivity can be correlated with the temperature range slightly above the solidus due to the delayed formation of an interconnected melt phase thin sections of quenched samples were used to estimate volume fractions and chemical compositions of the partial melts the increase of the electrical conductivity compares well with the connectivity of melt in partially molten samples above the solidus the electrical conductivity increases by 1 to 2 orders of magnitude in comparison with the conductivity of non melted rock below solidus when a complete melt connectivity is established the charge transport follows the network of the formed melt films at grain boundaries durations of up to 200 h are required in order to reach a steady state electrical resistance in a partially molten rock sample the experimental results were compared with the conductivity data obtained from magnetotelluric mt and electromagnetic em measurements in the northern part of the mid atlantic ridge where a series of axial magma chambers amc are presumably located there is good agreement between the measured electric conductivity of gabbroic samples with a melt fraction of 10 to 13 vol and the conductivity estimated at amc beneath the central part of reykjanes ridge as well as between the conductivity of partially molten peridotites and the source zone beneath the mid atlantic ridge at 60 km

sahke geophysical transect reveals crustal and subduction zone structure at the southern hikurangi margin new zealand 1 the seismic array hikurangi experiment sahke investigated the structure of the forearc and subduction plate boundary beneath the southern north island along a 350 km transect tomographic inversion of first arrival travel times was used to derive a well resolved 15 20 km deep p wave image of the crust the refracted phases and migrated reflection events image subducting slab geometry and crustal structure in the west australian plate moho depth decreases westward across the taranaki fault system from 35 to 28 30 km in the east subducted pacific plate oceanic crust is recognized to have a positive velocity gradient but becomes less distinct beneath the tararua ranges where the interface increases in dip at about 15 km depth from 15 this bend in the subducted plate is associated with vertical clusters in seismicity splay fault branching and low velocity high attenuation material that we interpret to be an underplated subduction sedimentary channel we infer that a step down in the decollement transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the tararua ranges reflections from the wairarapa fault show that it is listric and soles into the top of underplated sediments which in turn abut the moho of the overriding plate at 32 km depth near the downdip end of the strongly locked zone the change in dip of the hikurangi subduction interface is spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

time resolved study of charge generation and propagation in igneous rocks electrical resistivity changes ground potentials electromagnetic em and luminous signals preceding or accompanying earthquakes have been reported many times in addition to ground uplift and tilt and other parameters however no concept exists that would tie these diverse phenomena together into a physically coherent model using low to medium velocity impacts to measure electrical signals with microsecond time resolution it is observed that when gabbro and diorite cores are impacted at relatively low velocities 100 m s highly mobile charge carriers are generated in a small volume near the impact point they spread through the rocks causing electric potentials exceeding 400 mv em and light emission as the charge cloud spreads the rock becomes momentarily conductive when a granite block is impacted at higher velocity 1 5 km s the propagation of the p and s waves is registered through the transient piezoelectric response of quartz after the sound waves have passed the surface of the granite block becomes positively charged suggesting the same charge carriers as observed during the low velocity impact experiments expanding from within the bulk during the next 2 3 ms the surface potential oscillates indicating pulses of electrons injected from ground and contact electrodes the observations are consistent with positive holes e g defect electrons in the o2 sublattice traveling via the o 2p dominated valence band of the silicate minerals the positive holes propagate as charge clouds rather than as classical em waves before activation they lay dormant in form of electrically inactive positive hole pairs php chemically equivalent to peroxy links o3x oo xo3 with x si4 al3 etc phps are introduced into the minerals by way of hydroxyl o3x oh which all nominally anhydrous minerals incorporate when crystallizing in h2o laden environments the fact that positive holes can be activated by low energy impacts and their attendant sound waves suggests that they can also be activated in the crust by microfractures during the dilatancy phase depending on where in the stressed rock volume the charge carriers are activated they will form rapidly moving or fluctuating charge clouds that can account for earthquake related electrical signals and em emission wherever such charge clouds intersect the surface high fields are expected causing electric discharges and earthquake lights

roughness of the mantle transition zone discontinuities revealed by high resolution wavefield imaging we processed two independent data sets 2 458 973 three component seismograms from all earthscope transportable array usarray stations and 141 080 pairs of high quality receiver functions from the earthscope automated receiver survey using the generalized iterative deconvolution method and shaped the output with a 0 1 hz ricker wavelet we used these data as input to our 3 d plane wave migration method to produce an image volume of p to s scattering surfaces under all of the lower 48 states model simulations show that compared to the widely used common conversion point stacking method our method is capable of resolving not only dipping discontinuities but also subtler details of amplitude and topography variations at scales near the diffraction limit maps of attributes derived from these discontinuities reveal several previously unobserved features of the 410 and 660 discontinuities d410 and d660 topography with many tens of kilometers is resolved at a range of scales additionally we observed large variations of amplitude on the radial component and in the radial to transverse amplitude ratio that correlate with inferred variations in discontinuity topography we argue that this combination of observations can be explained by roughness at a range of scales the structural fabric we imaged is preferentially aligned orthogonal to the farallon north american relative motion two regions we infer to have exceptional roughness show spatial correlation with locations where tomography models indicate that the subducted slab is passing through the transition zone we suggest that these rough regions are indicators of vertical mantle flows through the transition zone

1 d electrical conductivity structure beneath the philippine sea results from an ocean bottom magnetotelluric survey abstract eight months of observation using ocean bottom electro magnetometers obems have allowed us to estimate the regional electrical conductivity structure beneath the philippine sea six obems were deployed along a line crossing the philippine sea from nw to se and five of them recorded useful data the raw time series data were cleaned up before we estimated the magnetotelluric mt impedance tensor conductivity structure at five sites is estimated using 1 d occam s inversion to fit the determinant average of each mt impedance tensor after a correction for the effect of topography we examined effect from two dimensionalities on the 1 d conductivity structure and the robustness of solutions the results of the 1 d conductivity structural model are strongly related to tectonic setting and the crustal age beneath each site the structure beneath the spreading axis of the mariana trough shows a distinct low conductivity structure at depths of 50 150 km and it probably reflects the upwelling dynamics operating beneath the spreading axis these low values are comparable with that of olivine with low hydrogen content implying that 1 the melting process extracts water from minerals such as olivine and 2 the melt beginning depth in the mariana trough is deeper than that of the typical morb source region the off axis conductivity profiles infer the existence of a high conductivity peak or a conductivity gradient change at mid depth the depth level of the peak increases with crustal age suggesting that the conductivity structure is related to a geothermal structure and that these conductivity profiles are explained by the temperature gradient change possibly combined with the presence of partial melt our results suggest that further ocean bottom em study has high potential to investigate the temperature gradient change and amount of hydrogen water and melt in the upper mantle

in defence of a resistive oceanic upper mantle reply to a comment by tarits chave and schultz in a previous paper heinson constable 1992 we discussed the effect of coastlines on sea floor magnetotelluric mt data collected deep within the ocean basins in order to quantify the geomagnetic coast effect we developed a model for oceanic upper mantle electrical conductivity independent of previous sea floor mt interpretations based on sea floor controlled source electromagnetic csem soundings laboratory studies of mantle materials and global geomagnetic sounding estimates for lower mantle conductivity we demonstrated that the coast effect of our model was large was not manifest as severe anisotropy in the mt response if the ocean basins were bounded by coastlines on three or more sides and that a qualitative agreement existed between the mt response of our simple coastline model and published sea floor mt data the implication of the latter is that by including the effect of the coastlines additional mechanisms for enhanced upper mantle conductivity such as volatiles carbon melt hydrogen and other factors may not be required by the mt data tarits chave schultz 1993 question the validity of a large number of aspects of our work their principal conclusions are that a horizontal or vertical leakage paths between the ocean and other oceans or the mantle will substantially reduce the magnitude of the ocean wide coast effect b the principle of extrapolating laboratory electrical conductivity measurements to mantle conditions is unreliable from uncertainties in the understanding of pressure temperature and petrological states of the upper mantle c published sea floor mt data sets examined by us were of such vintage that any conclusions drawn were unreliable or incorrect and d the conductivity profile compiled by chave flosadottir cox 1990 based on 1 d interpretations of sea floor mt and csem data that took no account of the coast effect is consequently a more reliable estimator of mantle conductivity structure than our model while all aspects of tarits et d s comments were addressed to some extent in our original paper we did not investigate all details fully either in the interests of conciseness or because the impact on our conclusions was minimal we will take this opportunity to delve further into some of these issues but before considering the complications it will be useful to reiterate the motivation for our work first oldenburg s 1981 and oldenburg whittal parker s 1984 1 d interpretations of three sea floor mt data sets included a high conductivity zone hcz in the oceanic upper mantle that decreased in magnitude and deepened with increasing age the existence of a hcz has become a paradigm not only amongst the electromagnetic community but also with seismologists petrologists and mineral physicists many entire papers are written on the subject attempting to explain the disparities between laboratory measurements of olivine conductivity at upper mantle temperatures and the hcz correlating the hcz with seismic low velocity zones lvz examining the temporal evolution of the hcz etc the impact of oldenburg s work has been so large that it seemed appropriate to re examine the basis of the hcz and question whether it is indeed required by the data note that

upper mantle temperature determined from combining mineral composition electrical conductivity laboratory studies and magnetotelluric field observations application to the intermontane belt northern canadian cordillera abstract using laboratory derived temperature dependences of the electrical conductivity of mantle minerals coupled with appropriate mixing laws we determine the bulk conductivity of mantle mineral assemblages for the ternary olivine orthopyroxene clinopyroxene ol opx cpx system we calculate physical property bounds hashin shtrikman bounds as a function of the fraction of different phases present these limits correspond to the extreme situations where the most conducting phase is either fully interconnected or fully disconnected the relationships we present between temperature mineral composition and bulk electrical conductivity allow constraining one of them given the other two we apply this approach to an area of the north american cordilleran intermontane belt in the yukon territory northern canada where xenolith evidence indicates bimodal upper mantle mineral assemblages harzburgite and lherzolite this locality coincides spatially with an upper mantle region of low electrical conductivity determined by long period magnetotelluric data given the mantle mineral composition and the maximum and minimum bounds on the electrical conductivity deduced by non linear model appraisal we determine the permitted extremal temperature bounds of the intermontane belt mantle rocks directly below the moho to a depth of some 80 km we show that the mantle in this region is at a minimum temperature of 820 c and a maximum temperature of 1020 c the latter is some 200 c colder than that suggested in a recent interpretation of an observed collocated low velocity zone from a teleseismic survey

electrical conductivity of olivine a dunite and the mantle laboratory studies of the electrical conductivity of rocks and minerals are vital to the interpretation of electromagnetic soundings of the earth s mantle to date the most reliable data have been collected from single crystals we have extended these studies with electrical conductivity measurements on a dunite from north carolina in the temperature range of 600 1200 c and under controlled oxygen fugacity observations of conductivity as a function of oxygen fugacity and temperature demonstrate that conduction in the dunite is indistinguishable from conduction in single olivine crystals thus the common practice of exaggerating the single crystal conductivities to account for conduction by grain boundary phases in the mantle is unnecessary because the dunite conductivity is consistent with that published for single crystals under similar conditions we have made a combined analysis of these data conductivity as a function of temperature between 600 and 1450 c displays three conduction mechanisms whose activation energies may be recovered by nonlinear least squares fitting yielding activation energies of 0 21 2 56 10 19 j 0 13 1 60 ev below 720 c 2 56 0 02 10 19 j 1 60 0 01 ev between 720 c and 1500 c and 11 46 0 90 10 19 j 7 16 0 56 ev above 1500 c the behavior of conductivity as a function of oxygen fugacity is well explained by a model in which an o2 independent population of charge carriers is supplemented at high oxygen fugacities with a population that is proportional to o20 3 this parametrization produces a clear correlation of the o2 dependent term with iron content which is otherwise obscured by variations in conductivity among olivines

along arc variation in water distribution in the uppermost mantle beneath kyushu japan as derived from receiver function analyses 1 we investigated the seismic velocity discontinuities in the uppermost mantle of kyushu a subduction zone in japan using receiver function analyses developed especially for discontinuities with high dipping angles we elucidated the regional variation of discontinuities as being the continental moho oceanic moho and upper boundary of the philippine sea slab from the geometry of these discontinuities and contrast in s wave velocities we discovered that the oceanic crust of the philippine sea slab has a low velocity and is possibly hydrated down to 70 km beneath south kyushu 80 90 km beneath central kyushu and less than 50 km beneath north kyushu the fore arc mantle wedge beneath central kyushu has a low velocity zone and possibly contains hydrated materials and free fluid but such a low velocity zone does not exist in the fore arc mantle wedge beneath north kyushu and south kyushu beneath south kyushu water dehydrated from the slab could move to the back arc side and cause arc volcanism beneath central kyushu water dehydrated from the slab could move to the fore arc side and cause a gap in volcanism and hydration of fore arc mantle materials beneath north kyushu the oceanic crust does not appear to convey water abundantly in the mantle wedge the low velocity hydrated fore arc mantle extends landward beneath the northern part of central kyushu and could produce volcanic rocks partially contaminated by slab derived fluid

a wave equation migration method for receiver function imaging 2 application to the japan subduction zone 1 the newly developed wave equation poststack depth migration method for receiver function imaging is applied to study the subsurface structures of the japan subduction zone using the fundamental research on earthquakes and earth s interior anomalies freesia broadband data three profiles are chosen in the subsurface imaging two in northeast ne japan to study the subducting pacific plate and one in southwest sw japan to study the philippine sea plate the descending pacific plate in ne japan is well imaged within a depth range of 50 150 km the slab image exhibits a little more steeply dipping angle 32 in the south than in the north 27 although the general characteristics between the two profiles in ne japan are similar the imaged philippine sea plate in eastern sw japan in contrast exhibits a much shallower subduction angle 19 and is only identifiable at the uppermost depths of no more than 60 km synthetic tests indicate that the top 150 km of the migrated images of the pacific plate is well resolved by our seismic data but the resolution of deep part of the slab images becomes poor due to the limited data coverage synthetic tests also suggest that the breakdown of the philippine sea plate at shallow depths reflects the real structural features of the subduction zone rather than caused by insufficient coverage of data comparative studies on both synthetics and real data images show the possibility of retrieval of fine scale structures from high frequency contributions if high frequency noise can be effectively suppressed and a small bin size can be used in future studies the derived slab geometry and image feature also appear to have relatively weak dependence on overlying velocity structure the observed seismicity in the region confirms the geometries inferred from the migrated images for both subducting plates moreover the deep extent of the pacific plate image and the shallow breakdown of the philippine sea plate image are observed to correlate well with the depth extent of the seismicity beneath ne and sw japan such a correlation supports the inference that the specific appearance of slabs and intermediate depth earthquakes are a consequence of temperature dependent dehydration induced metamorphism occurring in the hydrated descending oceanic crust

low frequency electrical response of polycrystalline olivine compacts grain boundary transport the complex electrical impedance of sintered polycrystalline compacts of san carlos olivine grain size 75 180 m has been determined over the frequency range 104 to 10 4 hz at temperatures between 800 and 1400 c at the fo2 of the wustite magnetite buffer complex plane plots im z vs re z show separate effects of grain interior and grain boundary conductivity grain interior transport controls the response above 102 hz whereas grain boundary transport dominates between 102 and 10 2 hz overall response of the compacts is modeled with an equivalent circuit in which parallel rc circuit elements representing grain interior and grain responses act in series bulk dc conductivity is lower than either grain interior or grain boundary conductivity alone inversion of magnetotelluric profiles that employ lower bulk conductivities of the grain interior grain boundary system yield upper mantle temperature estimates 30 to 50 c higher than those based on single crystal values

electrical conductivity of dry lower crustal rocks this study provides values of electrical conductivity of possible lower crustal materials to assist interpretation of lower crustal magnetotelluric soundings we present mean values of conductivity measurements collected from the literature for dry mafic and silicic rocks in the temperature range of 500 c to 1000 c we observe statistically significant differences between rock types mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group aphanitic fine grained rocks have higher conductivity than phaneritic coarse grained ones best fitting curves of log conductivity versus temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity because the laboratory rocks are dry their conductivities are lower at a given temperature than they would be if fluids or volatiles were present hence any temperatures inferred from

can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts 1 constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late quaternary peridotite xenoliths from the southern sierra nevada allow evaluation of models commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle in these models two conductive melts basalt and sulfide are embedded in a resistive matrix bounds on the amount of sulfide 0 06 0 4 and the bulk conductivity 0 03 0 1 s m place constraints on the degree of interconnection between the melts because the sulfide melt is very conductive even a small fraction of well connected melt results in a bulk conductivity larger than 0 1 s m similarly completely disconnected melts result in bulk conductivities much less than 0 03 s m the only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction 1 interconnected basalt melt with a discontinuous sulfide phase such a texture is observed in laboratory experiments with much larger sulfide melt fractions but has not been reported for small melt fractions a variant of the hashin shtrikman model and a hybrid model consisting of cascaded hashin shtrikman calculations were successful in matching the magnetotelluric and petrologic constraints with a model that appropriately simulates the melt interconnectivity we suggest that electrical conductivity may be used to infer in situ melt properties in the mantle

magnetotelluric image of the fluid cycle in the costa rican subduction zone fluids entering the subduction zone play a key role in the subduction process they cause changes in the dynamics and thermal structure of the subduction zone1 and trigger earthquakes when released from the subducting plate during metamorphism fluids are delivered to the subduction zone by the oceanic crust and also enter as the oceanic plate bends downwards at the plate boundary however the amount of fluids entering subduction zones is not matched by that leaving through volcanic emissions4 or transfer to the deep mantle implying possible storage of fluids in the crust here we use magnetotelluric data to map the entire hydration and dehydration cycle of the costa rican subduction zone to 120 km depth along the incoming plate bend we detect a conductivity anomaly that we interpret as sea water penetrating down extensional faults and cracks into the upper mantle along the subducting plate interface we document the dehydration of sediments the crust and mantle we identify an accumulation of fluids at 20 30 km depth at a distance of 30 km seaward from the volcanic arc comparison with other subduction zones5 14 indicates that such fluid accumulation is a global phenomenon although we are unable to test whether these fluid reservoirs grow with time we suggest that they can account for some of the missing outflow of fluid at subduction zones

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

continental subduction and three dimensional crustal structure the northern south island new zealand the three dimensional vp and vp vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment and 3146 earthquakes have been relocated between 40 km and 100 km depth the subducted plate is imaged as a relatively low velocity feature in the uppermost mantle reflecting the continental nature of the subducted crust in this region an increase in amplitude of this low velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction velocity variations within the subducted and overlying plates show some spatial correlation this suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath marlborough is currently not accommodating much active subduction in the overlying plate the awatere fault is a major structural feature associated with a low velocity zone extending to 23 km depth there is a marked change in structure near this fault with seismic velocities being lower to the southeast a relatively high level of seismicity occurs in this region of lower seismic velocities suggesting a relationship between the two a possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust the low velocity region in the overlying plate coincides with the region of most intense active deformation suggesting it is relatively weak

the effects of crustal heterogeneity on ray based teleseismic imaging summary we evaluate the efficacy of crustal scale receiver function imaging in the presence of crustal scattering first we show that the resolution of an image is not appreciably affected by geologically realistic crustal scattering rather we show that the image contrast is reduced the image of actual targets get buried in the noise created by arrivals arising from crustal scattering then we construct three classes of models in which we simulate a flat moho a moho with topography and a subduction zone for all the models we simulate the moho as a finite thickness velocity transition zone and the crust with a three layer heterogeneous zone that contains binary von karman statistics with varying degrees of rms density and velocity contrasts we use finite difference simulations to generate a suite of varying slowness seismograms which we then image with pre stack 2 d kirchhoff migration and ccp stacking we find that for a small degree of rms density velocity contrasts both imaging methodologies faithfully image the models but for rms density velocity variations greater than approximately 5 per cent per cent we observed distortion and or artefacts to the images for migration the distortion takes the form of amplitude variations for horizontal model features whereas for the ccp imaging the main affect is the enhancement of any horizontal feature in the model

the electrical structure of the slave craton the slave craton in northwestern canada a relatively small archean craton 600400 km is ideal as a natural laboratory for investigating the formation and evolution of mesoarchean and neoarchean sub continental lithospheric mantle sclm excellent outcrop and the discovery of economic diamondiferous kimberlite pipes in the centre of the craton during the early 1990s have led to an unparalleled amount of geoscientific information becoming available over the last 5 years deep probing electromagnetic surveys were conducted on the slave using the natural source magnetotelluric mt technique as part of a variety of programs to study the craton and determine its regional scale electrical structure two of the four types of surveys involved novel mt data acquisition one through frozen lakes along ice roads during winter and the second using ocean bottom mt instrumentation deployed from float planes the primary initial objective of the mt surveys was to determine the geometry of the topography of the lithosphere asthenosphere boundary lab across the slave craton however the mt responses revealed completely serendipitously a remarkable anomaly in electrical conductivity in the sclm of the central slave craton this central slave mantle conductor csmc anomaly is modelled as a localized region of low resistivity 10 15 v m beginning at depths of f80 120 km and striking ne sw where precisely located it is spatially coincident with the eocene aged kimberlite field in the central part of the craton the so called corridor of hope and also with a geochemically defined ultra depleted harzburgitic layer interpreted as oceanic or arc related lithosphere emplaced during early tectonism the csmc lies wholly within the ne sw

mantle heterogeneity beneath eastern california from magnetotelluric measurements 1 a 300 km long magnetotelluric profile across the great valley sierra nevada and california basin and range reveals minimum resistivity differences of 1 order of magnitude at depths from moho at 30 35 km to depths greater than 200 km horizontal wavelengths of 25 km are seen for these changes at depths shallower than 100 km and the isabella high seismic wave speed anomaly is present in the section between depths of 100 and 200 km as a minimum tenfold resistivity increase a transformation from electrical resistivity to temperature and melt fraction using a petrological model and laboratory measurements reveals that partial melt is relatively uncommon and confined primarily to depths between 30 and 100 km the remainder of the observed resistivity variation is attributed to lateral variations in temperature predicted melt fractions match those inferred from miocene quaternary xenoliths but modeled temperatures are 200 c higher than inferred from thermobarometric analyses of the xenoliths possible causes for this discrepancy include an incorrect starting material for the petrological model exotic conductive phases higher conductivity due to oxidation state and or iron content and diffusion of ions through the solid matrix i show that hydrogen diffusion or higher oxygen concentrations are plausible explanation for the higher model temperatures less than 200 ppm h 106 si in olivine would increase the conductivity of the matrix sufficiently to allow temperatures inferred from the resistivity section to match those from the xenoliths a possible source of the hydrogen would be fragments of sinking metasomatized mantle lithosphere that melted during cenozoic delamination

illuminating the plate interface structure beneath cook strait new zealand with receiver functions 1 using teleseismic receiver functions derived from broadband seismic arrays on the north end of the south island and south end of the north island of new zealand we image seismic impedance discontinuities in the upper mantle beneath cook strait using common conversion point ccp and kirchhoff migration methods our primary findings are observations of discontinuities associated with the top of the subducting pacific plate our results suggest that contrary to recent suggestions the plate is continuous under the northern south island through the region of seismicity deeper than 50 km west of cook strait the slab dips steeply to the northwest we find evidence for a low velocity layer at the top of the slab near which much of the seismicity is concentrated we see hints of two crustal discontinuities consistent with observations from previous studies we also see substantial and continuous energy on the transverse receiver function stacks above the mantle wedge extending to the northwestern edge of our stacks which may indicate seismic anisotropy above 50 km depth

the electrical conductivity of the oceanic upper mantle summary previous inversions of sea floor magnetotelluric mt sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest and controlled source field electromagnetic csem soundings require a lithospheric mantle conductivity of 3 x lop5 sm which is so low that the electromagnetic em coast effect would produce more anisotropy in mt soundings than is observed we address these issues by constructing an olivine mantle model for conductivity and examining the inversion of mt data from three sea floor sites and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered our mantle model is based on recent measurements of olivine conductivity the conductivity of tholeiite melt a thermal model for the upper mantle based on lithospheric cooling and the temperature of a a 3 olivine transition the pyrolite model of mantle petrology and conductivities derived from csem sounding we propose archie s law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity and archie s law with exponent 1 5 as the preferred estimate the 1 d forward response of this model is not compatible with observed sea floor mt data three data sets presented by oldenburg 1981 pass a test for one dimensionality based on the size of the residuals when fit with parker s d algorithm but two of the three soundings fail a test for independence of residuals we also find that the presence of the upper mantle high conductivity zone previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount re inversions of the mt data produce models which are incompatible with our petrological model of mantle conductivity however by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3 d problem using thin sheet analysis we predict mt responses which are distorted in a manner that is remarkably similar to the observed data

sensitivity study of marine csem data for reservoir production monitoring using numerical and analytical modeling we assess the feasibility of marine controlled source electromagnetic csem data for monitoring the flooding front during water flooding of an oil reservoir we discuss the ability of time lapse csem data to resolve changes in the electric conductivity small enough to be of interest for monitoring purposes measurement and modeling errors are discussed briefly and analytical calculations comparing time lapse signals with a certain type of time lapse modeling errors are performed numerical calculations performed with a volume integral equation method are used to study the effect on the electromagnetic em fields of relevant conductivity changes the numerical investigation includes robustness with respect to survey parameter values discrimination between different front shapes and ability to overcome time lapse error simulated signals are found to be strong enough to overtake typical measurement errors and are fairly robust toward perturbations of survey parameters it is found analytically and numerically that certain modeling errors experience a high degree of time lapse cancellation

the electrical conductivity of lherzolite between 700oc and 1200oc and time over a period of 1700 hours are reported for a lherzolite nodule from mount porndon australia analysis of complex impedance collected at 100 hz 1 khz and 10 khz indicates that there is significant frequency dispersion in this rock at temperatures below 900oc 1000oc by choosing the resistance of an equivalent parallel rc network at the frequency having minimum phase the effect of dispersion is largely avoided conductivity as a function of time collected following changes in fo2 at 1010 and 1200oc has been analyzed to determine the diffusivity of defects relating fo2 to the electrical conduction mechanism in olivine the diffusivities so obtained are in remarkably good agreement with those determined from strain measurements during creep tests which implies that magnesium vacancies are the rate limiting step for conductivity reequilibration after fo2 changes a longer term process is observed in this rock in which the conductivity drifts upward or downward with a time constant of hundreds of hours after initial reequilibration to gas mix fo2 changes we speculate that this long term drift could be related to equilibration of the iron distribution between coexisting olivine and pyroxene as a function of fo2 the sense is an increase in conductivity olivine gains iron at low fo2 and vice versa the lherzolite conductivity data are not significantly different from measurements made on olivine single crystals and polycrystals even though the rock contains about 34 modal pyroxene this consistency of laboratory measurements of electrical conductivity of olivines from many localities and geological settings supports the use of recent models relating mantle temperature with electrical conductivity in the interpretation of mantle geo electromagnetic soundings

scsp observed on north island new zealand implications for subducting plate structure seismic phase conversions provide important constraints on the layered nature of subduction zone structures recordings from digital stations in north island new zealand have been examined for converted scs to p scsp arrivals from deep 150 km tonga kermadec earthquakes to image layering in the underlying hikurangi subduction zone consistent p wave energy prior to scs has been identified from stations in eastern and southern north island where the subducted plate interface is at a depth of between 15 and 30 km two scs precursors are observed ray tracing indicates that the initial precursor scsp1 corresponds to conversion from the base of an 11 14 km thick subducting pacific crust the second precursor is interpreted as a conversion from the top of the subducting plate the amplitude ratio scsp1 scs increases from 0 10 to 0 19 from northern to southern north island this is within the range expected from a simple first order velocity discontinuity at an oceanic moho a 1 2 km thick layer of low velocity sediment at the top of the subducting plate is required to explain the remaining scsp waveform our results imply that the abnormally thick hikurangi chatham plateau has been subducting beneath new zealand for at least 2 9 myr thus explaining the high uplift rates observed across eastern north island

distributed deformation in the lower crust and upper mantle beneath a continental strike slip fault zone marlborough fault system south island new zealand converted phases from teleseisms recorded by a seismic array spanning the northern half of the marlborough fault system south island new zealand show a continuous unbroken moho underlying a seismically anisotropic lower crust beneath the two north ernmost faults of the fault system these observations suggest that distributed deforma tion not slip on a narrow vertical fault accommodates displacement in the lower crust below the 120 480 km of right lateral slip across the wairau fault one splay of the marl borough fault system and the northward continuation of the alpine fault beneath the wairau fault the moho dips 258 308 southeast from a depth of 26 km northwest of the fault to a depth of 34 km southeast of the fault farther to the southeast ps conversions from the moho continue under the awatere fault 34 6 10 km of slip with a constant amplitude and depth of 34 km across the two faults converted energy from 16 20 km depth varies with back azimuth in a manner suggesting the presence of anisotropy in the lower crust these observations imply that one of the tenets of plate tectonics that faults defining plate boundaries pass through both crust and upper mantle does not apply to new zealand or to continents in general

on the electrical crust mantle structure in fennoscandia no moho and the asthenosphere revealed summary an interpretation of the geomagnetic inductive response function c 0 observed at kiruna in northern sweden is herein undertaken the bounds of acceptable solutions are initially discovered by a monte carlo random search procedure and the best fitting solutions are examined by the application of linear theory to the problem the data are shown to have a higher degree of internal consistency than that described by the estimated variances of each datum a further monte carlo inversion of the variance reduced data set gives solutions with well defined model parameters the two major features of the models are 1 a small or non existent electrical conductivity variation across the seismic moho boundary and 2 the unequivocal existence of an electrical asthenosphere under the fenno scandian shield beginning at a depth of between 155 185 km and of 60km minimum thickness both of these observations have seismic counterparts finally possible mantle temperature profiles are deduced which depend on the assumptions and laboratory data employed

northeastern pacific mantle conductivity profile from long period magnetotelluric sounding using hawaii to california submarine cable data we present results of a long period magnetotelluric mt investigation of the electrical structure beneath the eastern north pacific the electric field data consist of 2 years of continuously recorded voltages across an unpowered 4000 km long submarine telephone cable haw 1 extending from point arena california to oahu hawaii the electric field measurements are coherent to some degree with magnetic field measurements from honolulu observatory at periods of 0 1 to 45 days this coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length significantly diminishing their effect robust controlled leverage mt response estimates and their jacknife confidence limits are computed for the haw 1 to honolulu data an equivalent scalar mt response obtained from honolulu magnetic variations data is used to correct the haw 1 mt response for static shift and to extend the mt response estimate to periods of 100 days the composite response function satisfies necessary and sufficient conditions for consistency with a one dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km inversion of the mt response reveals a conductive zone 0 05 0 1 s m between 150 and 400 km depth and a positive gradient below 500 km these observations are consistent with previous mt studies in the north pacific this upper mantle conductivity is too high to be explained by solid state conduction in dry olivine using reasonable mantle geotherms calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that h dissolved in olivine possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy provide sufficient conductivity enhancement to explain the inversion results the high conductivity may also be explained by the presence of gravitationally stable partial melt comparison of the haw 1 results with long period mt studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes in particular the upper mantle beneath the pacific ocean is considerably more conductive than that beneath the canadian shield and similar in conductivity to that beneath the basin and range

chromite from the blue ridge province of north carolina accessory chromite in dunite shows a variety of textures that indicate alteration one group type a consists of four types of chromite clean chromite lattice chromite in which the invading chlorite occurs along three directions in the 100 plane optically zoned chromite and poikiloblastic chromite most of type a chromites are surrounded by chromian clinochlore the other group type b consists of euhedral to subhedral chromite grains which are included in olivine or pyroxene the accessory chromites define a trend exhibited by chromite from other areas that have undergone metamorphism olivine spinel geothermometry indicates equilibration temperatures near 700 degrees c roughly consistent with mineral assemblages in the host gneisses thus the blue ridge dunites are metamorphic rocks and not primary mantle peridotites modified journal abstract

the transition from subduction to continental collision crustal structure in the north canterbury region new zealand summary the north canterbury region marks the transition from pacific plate subduction to continental collision in the south island of new zealand details of the seismicity structure and tectonics of this region have been revealed by an 11 week microearthquake survey using 24 portable digital seismographs arrival time data from a well recorded subset of microearthquakes have been combined with those from three explosions at the corners of the microearthquake network in a simultaneous inversion for both hypocentres and velocity structure the velocity structure is consistent with the crust in north canterbury being an extension of the converging chatham rise the crust is about 27 km thick and consists of an 11 km thick seismic upper crust and 7 km thick seismic lower crust with the middle part of the crust being relatively aseismic seismic velocities are consistent with the upper and middle crust being composed of greywacke and schist respectively while several lines of evidence suggest that the lower crust is the lower part of the old oceanic crust on which the overlying rocks were originally deposited the distribution of relocated earthquakes deeper than 15 km indicates that the seismic lower crust changes dip markedly near 43 s to the south west it is subhorizontal while to the north east it dips north west at about lo fault plane solutions for these earthquakes also change near 43 s for events to the south p axes trend approximately normal to the plate boundary reflecting continental collision while for events to the north t axes are aligned down the dip of the subducted plate reflecting slab pull while lithospheric subduction is continuous across the transition it is not clear whether the lower crust near 43 s is flexed or

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

seismic anisotropy from local earthquakes in the transition region from a subduction to a strike slip plate boundary new zealand shear wave splitting is used to investigate anisotropy in the crust and upper mantle in a subduction zone lower half of the north island of new zealand and its transition to oblique transform faulting marlborough area northern south island in marlborough delay times show almost no increase with depth and it is most likely that the higher frequency phases used in this study respond mainly to lithospheric anisotropy in the central marlborough fault system mfs fast polarizations are subparallel to the faults anisotropy is attributed to the presence of metamorphosed schist eclogite of 30 10 km thickness and located 50 80 km beneath the mfs on the edges of the mfs fast polarizations are parallel to the maximum compressive stress direction consistent with crack induced anisotropy in the crust the shear zone which is as wide as the island in the mantle as inferred from sks phases seems to occur in a narrower zone in the crust in the lower half of the north island fast polarizations from events at all depths are oriented parallel to the strike of the hikurangi subduction zone as well as to the faults polarisations are similar to those of sks phases which mainly sample the mantle this suggests that the lithosphere and the upper mantle asthenosphere deform in a coherent strike slip shear we calculate 1 2 0 3 velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth in order to match the sks delay times this result requires the presence of anisotropic material down to 580 100 km depth or a change in anisotropy with depth or frequency dependent splitting

a marine electromagnetic survey to detect gas hydrate at hydrate ridge oregon gas hydrates are a potential energy resource and hazard for drilling and infrastructure yet estimates of global volume vary by over three orders of magnitude hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations a marine electromagnetic survey was carried out at hydrate ridge oregon usa with the aim of testing the use of controlled source electromagnetic csem and magnetotelluric mt methods to map gas hydrate and free gas below the gas hydrate stability zone a 2 d csem inversion supports the scenario deduced from previous seismic and drilling results which indicate two mechanisms of hydrate emplacement a transport dominated and reaction dominated regime a prominent resistive region of 2 5 4 m at a depth of about 130 mbsf near the seismic bottom simulating reflector bsr suggests that 27 to 46 per cent of the bulk volume is filled with hydrate depending on whether archie s law or the hashin strikman bounds are used this is representative of a reaction dominated regime for hydrate emplacement and where a significant low velocity zone exists based on a seismic tomography inversion suggests large quantities of free gas below the bsr electrical resistivity logging while drilling lwd data show general agreement with the csem inversion model except for a csem derived resistive region at seismic horizon a known to transport free gas into the gas hydrate stability zone inversion of mt data collected simultaneously during the csem survey provides a complimentary low resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model

electrical conductivity variations around the palmdale section of the san andreas fault zone our analysis of a decade of observations from the palmdale california telluric array is presented the array is a system of large scale telluric field measurements used for the passive monitoring of electrical conductivity variations over a large region centered over the san andreas fault zone northeast of los angeles since 1985 systematic changes in the relationships between the telluric field measurements around the array have been observed these changes amounted to 1 to 2 by the time the array was closed down in 1989 with the fields measured along the fault and northeast of the fault showing signal decreases relative to the fields southwest of the fault because of the possible implications of these changes for conductivity variations related to tectonic activity on the san andreas we have attempted to sort out the geography of the zones that could cause the changes the simplest though nonunique interpretation of the observed variations in the telluric relationships is a steady increase in the conductivity of the lower crustal region of the fault zone from two dimensional models of the telluric response around the array we infer that an increase of about 60 milli siemens in the conductance of the lower crustal fault zone could account for the changes seen over the period 1985 to 1989 assuming changes were in a kilometer wide zone and that the lower crustal fluids were very conductive this could represent a strain of about 0 4 10 6 the surface strain would be much smaller and would be lost in surface strain noise because of the possibility that fluid pressure changes are involved these small strains could still be significant for earthquake cycle phenomena alternatively the observed changes could have been produced by increases in the conductivity of the upper crust of the mojave desert northeast of the fault or by decreases in the conductivity of the upper crust in the san gabriel mountains southwest of the fault to produce the observed telluric changes however the additional conductivity and the associated strain changes would have to be much larger the strain needed to accommodate the increased conductivity on the mojave side would have to be of the order of 6 10 4 which seems unrealistically high smaller strains could account for conductivity changes in the san gabriel mountains but these strains would have to be compressive water level changes are not considered to be a possible cause of the conductivity changes because the measurement scale is much larger than the typical scale of water level changes and no seasonally correlated changes in the telluric fields were observed

electrical conductivity of san carlos olivine along 100 under oxygen and pyroxene buffered conditions and implications for defect equilibria the electrical conductivity along 100 of single crystal san carlos olivine was measured as a function of temperature between 1100 and 1200 c and oxygen fugacity between 10 6 and 10 0 5 pa at 1200 c and either with pyroxene buffered or without self buffered an added natural pyroxene buffer from a san carlos lherzolite under these temperature and o2 conditions electrical conduction in the self buffered sample is attributed to polarons fe and electrons e and in the pyroxene buffered sample is attributed to polarons fe and magnesium vacancies v mg over the range of temperature and o2 investigated the electrical conductivity of the self buffered sample is given by sb 100 2 27 s m e 0 55 ev kt o20 18 306 3 s m e 2 25 ev kt o2 0 18 and for the pyroxene buffered sample by pb 100 0 18 s m e 0 34 ev kt o20 17 15 2 s m e 1 3 ev kt where k is boltzmann s constant t is in kelvin and o2 is in atmospheres the conductivity of the pyroxene buffered sample is lower than that of the self buffered sample primarily as a result of a decrease in the polaron concentration the electrical conductivity of both samples was found to decrease irreversibly once the samples experienced an oxygen fugacity more reducing than approximately the wustite magnetite buffer electron microprobe analyses indicate that this effect results from loss of iron from the olivine samples to the iridium electrodes a series conduction model based on the observed compositional gradient adequately accounts for the magnitude of the irreversible conductivity decrease and limits the thickness of any surficial pyroxene phase to 0 1 m mantle temperature profiles based on laboratory measurements of self buffered samples predict temperatures of the order of between 25 and 150 c colder depending on the ambient oxygen fugacity than those based on measurements of pyroxene buffered samples