anelasticity and thermal structure of the oceanic upper mantle temperature calibration with heat flow data the thermal structure of the oceanic upper mantle is examined on the basis of seismic attenuation surface heat flow and mid ocean ridge basalt morb composition the comparison of seismic q with laboratory q determined at sufficiently high pressure and temperature allows us to constrain temperature in the earth since the inferred absolute temperature relies on knowledge of the frequency dependence of q if any we calibrate an unknown frequency effect on q by comparing the derived temperatures with geotherms calculated from surface heat flow and morb composition this comparison yields a correction factor found to be small for the geotherm derived from laboratory ultrasonic q examples of thermal structure are presented for the iceland plateau and the pacific ocean under the iceland plateau we expect partial melt for the asthenosphere only in the vicinity of the spreading ridge in asthenosphere older than 5 m y of the iceland plateau and older than 40 m y of the pacific ocean temperatures are below the dry peridotite solidus and no partial melt is expected these results are consistent with the thermal structure derived from a comparison of seismic and laboratory velocities it is shown that there is a good agreement between temperatures determined from seismic attenuation heat flow and morb chemistry since the laboratory measurements were made on a dry peridotite and we employ a dry peridotite solidus for determining the geotherm the consistency in temperatures derived from three independent data strongly suggests that the upper mantle remote from subduction zones is essentially dry it appears that the spreading process in mid ocean ridge causes asthenosphere material to ascend from greater depths where it was at higher temperature this gives rise to pressure release melting

formation and mechanics of magma reservoirs in iceland summary provided magma flow in a partially molten layer follows darcy s law of flow in porous media it can be demonstrated that magma in the proposed magma layer beneath iceland flows towards regions of reduced crustal thickness in these regions magma accumulates to form partially molten magma reservoirs which are thus located in the magma layer these reservoirs either give rise to surface eruptions or feed totally molten shallow magma chambers located in the crust that may erupt it is argued that the melt fraction varies throughout a reservoir being highest near the top where the reservoir may be totally molten but that the average is 0 25 it is concluded that for individual eruptions the reservoirs behave on the whole as a poroelastic material it is suggested that formation of crustal magma chambers is facilitated by the occurrence of stress barriers that lead to the formation of thick sills while liquid such sills absorb the magma of all dykes that enter them and may evolve into magma chambers ideal sites for stress barriers and thus for magma chambers are formations where individual layers have different elastic properties i e where the crust behaves as a multilayer denoting the bulk volume of the reservoirs by vb the volume of the crustal chambers by v and the volume of lava and dyke material in individual eruptions by ve it is found that vb 5365ve and v 1850ve using the estimated volumes of exposed plutons as a basis it is found that provided the feeder reservoir does not participate in the eruption the volume of individual eruptions from elastic crustal magma chambers in iceland is less than 0 25 km3 usually less than 0 1 km3 and that typical central volcano eruptions should be about 0 02 km3 for eruptions of the order of 1 km3 or larger the reservoirs must supply magma during the eruption it is concluded that from the point of view of volume historical eruptions of typical size could have come from elastic crustal magma chambers

numerical models on the influence of partial melt on elastic anelastic and electrical properties of rocks part ii electrical conductivity abstract partial melting influences both the elastic and electrical properties of mantle rocks significantly but in a different manner a set of theoretical models of the electrical conductivity of partially molten rocks is developed for different geometries of the melt distribution the models correspond to a similar set on elasticity and anelasticity presented in a preceding paper part i both model sets allow combined interpretations of seismic and geoelectric observations from anomalous hot mantle regions by a systematic variation of the melt geometry the following melt geometries are assumed in the conductivity models melt films tubes isolated ellipsoidal inclusions films and ellipsoidal inclusions with a variable degree of interconnection distributed geometries and superpositions of spheres with films and with tubes unexplored conduction mechanisms near the melting temperature film thicknesses of molecular sizes or effects of volatiles are not considered resistor network theory is utilized to account for a variable degree of interconnection and distributed geometries thereby a new possible interpretation of archie s law is found in terms of varying interconnection furthermore it is found that a reduced degree of interconnection may have an important influence on the conductivity at melt fractions characteristic for a weak to moderate decrease in seismic velocity the model results are compiled together with the elastic models of part i in a set of diagrams which may be applied systematically to observed electrical and seismic data to find constraints on the state of melting

an electrical resistivity crustal section from the alps to the baltic sea central segment of the egt abstract electrical resistivity models derived from magnetotelluric and magnetovariational measurements in the past 10 years have been compiled along the central segment of the european geotraverse along the profile low resistivity structures exist particularly in the middle and upper crust and probably mask deeper low resistivity zones these structures can be related to different tectonic units along the profile the prominent features are the two dimensional low resistivity structures at the transition from the moldanubian to the saxothuringian and from the saxothuringian to the rhenohercynian unit of the variscan belt the depth correlation of the low resistivity horizon in the rhenish massif with a seismic low reflectivity band the very high conductance of an upper to middle crustal layer in the north german lowlands and its cessation at the river elbe towards the baltic sea the reduction in the resistivity of these zones is very likely due to graphite at mid crustal depths it is obvious that there exists a general increase of electrical conductance in the upper half of the crust from south to north of the central segment which may be due to increasing accumulation of graphite and successive tectonically controlled ordering processes this phenomenon may by relating it to the growing crustal age from south to north be the indicator of a maturing continental crust

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

abstract contribution of physical modeling to understanding salt tectonics in the eastern nile deep sea fan recent multibeam bathymetry backscattering imagery and seismic reflection data indicate that the structural pattern of the east part of the nile deep sea fan differs drastically from that of its central and west parts such differences may result from combined thickskinned crustal scale tectonics and thin skinned gravity driven spreading of the messinian evaporites and their plio pleistocene overburden the eastern deep sea fan comprises a long 200 km nw se deformation corridor that was first interpreted as the possible northern prolongation of the rift of suez along dip the corridor exhibits a structural progression typical of salt bearing passive margins undergoing gravity spreading including small distal buckle folds midslope minibasins surrounded by salt ridges and proximal normal growth faults less typical is the corridor s being bounded by narrow nwse fault zones underlain by narrow salt ridges we used physical models to test whether such pattern was caused by the presence of nw se dormant or active subsalt relief or of a bathymetric high the eratosthenes seamount acting as a buttress during spreading model results clearly indicate that the presence of a passive subsalt relief and or of a buttress rather than that of an active subsalt relief has caused this peculiar structural pattern early gravity spreading caused radial thin skinned extension and the formation of minibasins and nw se and ene wsw salt ridges a pattern also enhanced if basement steps are present later buttressing by the seamount opposed further northeastward extension the salt and overburden spread northwestward reactivating the nw se salt ridges as strike slip zones bounding the corridor

distribution and size of lava shields on the al haruj al aswad and the al haruj al abyad volcanic systems central libya abstract the al haruj volcanic province ahvp consists of two distinct volcanic systems in the north is the system of al haruj al aswad covering an area of 34 200 km2 while in the south the system of al haruj al abyad covering an area of 7 850 km2 the systems have produced some 432 monogenetic volcanoes primarily scoria cinder cones lava shields and maars the density distribution of the volcanoes in each system plotted as eruption points or sites has a roughly elliptical surface expression suggesting similar plan view geometry of the magma sources here suggested as deep seated reservoirs more specifically the al haruj al aswad magma reservoir has major and minor axes of 210 km and 119 km respectively and an area of 19 176 km2 the corresponding figures for the haruj al abyad reservoir being 108 km and 74 km for the axes and 6209 km2 for the area we measured 55 lava shields on the ahvp they are mostly restricted to the northern and southern parts of ahvp and date from late miocene to at least the end of pleistocene while some may have been active into holocene in fact although primarily monogenetic some of the lava shields show evidence of possibly holocene fissure eruptions in the summit parts the early lava shields tend to be located at the edges of volcanic systems and with greater volumes than later more central shields the average lava shield basal diameter is 4 5 km and height 63 m there is strong linear correlation between lava shield volume and basal area the coefficient of determination r2 being about 0 75 when 22 holocene icelandic lava shields are added to the dataset for comparison the correlation between volume and basal area becomes r2 0 95 numerical models suggest that the local stress fields favoured rupture and dyke injection at the margins of the source reservoirs during late miocene early pliocene in agreement with the distribution of the early large volume shields

shear modulus and q of forsterite and dunite near partial melting from forced oscillation experiments an apparatus designed to determine the complex shear modulus of rock samples by forced torsion oscillations at high temperature and in the seismic frequency band 0 003 30 hz is briefly described measurements were performed on natural dunite from aheim norway up to 1400 c and on polycrystalline forsterite up to 1500 c at 1 atm pressure the two materials were chosen to study by comparison the effect of melt on the elasticity and anelasticity of mantle rocks between 1000 and 1200 c the absolute values of the shear modulus g are almost equal for both materials above 1200 c g for natural dunite decreases progressively with temperature and at 1400 c and 1 hz reaches 13 of its value at 1100 c in contrast g of pure forsterite depends little on temperature for petrological reasons supported by simultaneous measurements of the electric resistivity there is strong evidence that the decrease of g in dunite above 1200 c is due to melt from the lower melting components of the dunite based on different models estimates of the melt fraction are made at high temperature in both materials q 1 is characterized by a monotonic decrease with frequency according to with 0 25 an apparent activation energy of 38 5 kcal mol 1 for forsterite and 48 8 kcal mol 1 for dunite was found with no significant change in the regime of partial melting from this it is concluded that q 1 even at partial melting is dominated by solid state high temperature background absorption there is no indication from these experiments for a constant q band at low seismic frequencies or an increase of q proportional to frequency as suggested by some seismologists the present results are in good qualitative agreement with those for young s modulus obtained previously by strain retardation experiments

variable viscosity convection and partial melting in the continental asthenosphere convection beneath continents may be associated with the generation of partially molten zones at depth experimentally determined non newtonian temperature and pressure dependent creep laws for olivine were used to model upper mantle convection in the presence and absence of a melt phase in our models convection is chaotic and exhibits a strong time dependence using model dry and water undersaturated solidus relations partially molten regions develop at asthenospheric depths forming low viscosity regions the existence of these regions is linked to convection and consequently varies in space and time with an episodicity of 5 to 10 m y in good agreement with observations based on seismic tomography and the episodicity and volumetric abundance of surface volcanics for an average continental heat flux of about 53 3 mw m2 maximum melt fractions range between 0 and 2 dry solidus and 2 and 4 water undersaturated solidus on the basis of experimental deformation studies we employed an empirical relationship between stress and grain size to assess the grain size variations possible in the upper mantle the grain sizes obtained millimeters to centimeters are in good agreement with grain sizes observed in natural examples the smallest grain sizes occur along narrow zones of high stress within the deepest part of the lithosphere the largest grain sizes are found in the overlying lithosphere and in partially molten asthenospheric regions as our models scale well to natural observations these results appear to preclude diffusion creep n 1 as a viable upper mantle steady state deformation mechanism

the crustal magma chamber of the katla volcano in south iceland revealed by 2 d seismic undershooting summary results of a 2 d seismic undershooting experiment on the katla central volcano in south iceland are reported large localized traveltime anomalies 0 4 s are observed on an array within the katla caldera the traveltimes are forward modelled using a wavefront tracker developed in appendix a thus non linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity an extreme variation in compressional velocity is required to extend over a significant volume in order to model the data the resulting model is not unique but constraints on the allowable range of velocities 2 5 6 0 km s render the basic features well constrained a clear s wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly low amplitude p waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly the model is interpreted in terms of a magma chamber containing extensively molten rock the magma chamber is shallow with a bottom at a depth of about 1 5 km below sea level 3 0 km below surface and measures about 5 km across the depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved on the other hand the bottom of the chamber is well resolved the chamber is underlain by rocks of average or high velocity for that depth the magma chamber is a persistent feature big enough 10 km3 to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels the chamber is fed by magma fracturing from below the model agrees with phenomenological models of magma chambers in iceland based on geological observations and provides a quantification of those models in terms of depth and size on the other hand it is fundamentally different from recent models of magma chambers at mid ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath iceland this underlines the important effect of the icelandic hotspot on tectonics and volcanism in iceland and implies a substantially different crustal and thermal structure in iceland from that of normal mid ocean ridges

electrical conductivity of molten basalt and andesite to 25 kilobars pressure geophysical significance and implications for charge transport and melt structure electrical conductivities of molten hawaiian tholeiite and crater lake andesite were measured between 1200 c and 1400 c at atmospheric pressure and at pressures up to 17and 25 kbar respectively isobaric plots of log versus 1 t is electrical conductivity are linear with the exception of the zero pressure tholeiite melt data conductivities decrease with increasing pressure in both melts with the andesitic melt exhibiting a greater pressure dependence between 5 and 10 kbar abrupt decreases in the slopes of isothermal log versus p plots i e decreases in activation volume are observed for both rock melts this discontinuity probably reflects changes in melt structure as opposed to changes in conduction mechanism in each pressure range the data for each rock melt can be described reasonably well by an equation of the form 0 exp ea p v kt where 0 is a preexponential constant ea is the activation energy and v is the activation volume a qualitative model involving depolymerization of the melt with increased pressure leading to increased efficiency of packing can explain the observed discontinuity in activation volume as well as the observed pressure dependences of other melt physical properties such as viscosity and density conductivity versus melt fraction curves for partially molten peridotite are reevaluated using high pressure tholeiitic melt conductivities and crystalline conductivity values recently determined by other workers minimum melt fraction estimates of 5 10 are required to explain upper mantle regions of anomalously high electrical conductivity in terms of a partial melting hypothesis

on the mechanism of magma injection and plate divergence during the krafla rifting episode in ne iceland geodetic observations of horizontal and vertical point displacements in a 100 km wide network across the krafla fissure swarm of ne iceland are analyzed with simple elastic models the observed displacements accumulated from 1975 to 1980 in several events of magma injection into fissures that were widened the fissure swarm was extended and it subsided while the adjacent lithosphere contracted or was compressed accompanied by surface uplift we applied two models to fit the far field deformation an infinite horizontal pressure line source or a pipe and an infinite vertically limited vertical dyke source loaded by normal stress at the walls both embedded in an elastic half space in the far field inversion the two models cannot be distinguished but geology and the observed axial graben subsidence clearly discriminate against the pipe and for the dyke the inversion suggests that the widened dykes cut through the whole lithosphere which in the fissure swarm is only 4 6 km thick during the rifting episode the general magma level in the fissure swarm rose to near the surface by 1980 but the loading increments appear to have decreased whith time from 1975 to 1980

crustal thickness beneath libya and the origin of partial melt beneath as sawda volcanic province from receiver function constraints this study investigates crustal thickness and properties within the libyan region results obtained from 15 seismic stations belonging to the libyan center for remote sensing and space science are reported in addition to 3 seismic stations publically available using receiver functions the results show crustal thicknesses ranging from 24 km to 36 km with uncertainties ranging between 0 10 km to 0 90 km more specifically crustal thickness ranges from 32 km to 36 km in the southern portion of the libyan territory then becomes thinner between 24 km and 30 km in the coastal areas of libya and thinnest between 24 km and 28 km in the sirt basin the observed high vp vs value of 1 91 at one station located at the as sawda volcanic province in central libya indicates the presence of either partial melt or an abnormally warm area this finding suggests that magma reservoirs beneath the libyan territory may still be partially molten and active thereby posing significant earthquake and volcanic risks the hypothesis of an active magma source is further demonstrated though the presence of asthenospheric upwelling and extension of the sirt basin this study provides a new calculation of unconsolidated sediment layers by using the arrival time of the p to s converted phases the results show sediments thicknesses of 0 4 km to 3 7 km with the vp vs values ranging from 2 2 to 4 8 the variations in crustal thickness throughout the region are correlated with surface elevation and bouguer gravity anomalies which suggest that they are isostatically compensated

influence of flow width on mean velocity of debris flows in wide open channel debris flow in a wide open downstream channel has a significant transverse velocity component that strongly influences its mean longitudinal velocity investigation of observation data of debris flow surges at jiangjia ravine in china shows the dependency of manning resistance of debris flows on the ratio of flow width to depth regression fit reveals a power function relationship between the manning resistance coefficient and the width to depth ratio this derives a new formula of mean velocity incorporating the influence of flow width the result indicates that the width to depth ratio can be viewed as a kind of shape roughness analogous to grain roughness in the darcy weisbach resistance coefficient expression doi 10 1061 asce hy 1943 7900 0000648 c 2013 american society of civil engineers

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

partial melt below iceland a combined interpretation of seismic and conductivity data a combined interpretation of published seismic and electric data is carried out to impose constraints on the state of melting below central iceland these data indicate high electrical conductivity and low vp velocity regionally below 6 km s for the crust mantle transition region while the mantle below shows lower conductivity but still anomalous low velocity the velocity data are treated by utilizing gravity data to quantify the elastic modulus decrease due to partial melt a comprehensive set of theoretical models on elasticity anelasticity and electrical conductivity of partially molten rock is applied to this data set the geometry the fraction and the interconnection of the melt phase are varied systematically the conductivity models are based on published laboratory data on conductivity of solid and melt phases of rocks anisotropy and possible unexplored conductivity mechanisms near the melting temperature are not considered here by means of the melt models the data can be explained most easily as follows in a depth range roughly between 20 and 50 km only a small percentage of melt 2 at the most 5 if melt pockets are also present occurs in thin films possibly with a distribution of aspect ratios and a low degree of interconnection if in the crust mantle transition region zones of low p wave velocity 6 km s are correlated with those of high conductivity 0 08 s m melt fractions between 10 and 20 are likely and the degree of interconnection should be high thin films can be excluded as a dominant melt geometry in that region apart from improved constraints on melt geometry the results are in good agreement with published models based only on conductivity data however they disagree with published models based entirely on seismic data the discrepancy is now resolved by the combined interpretation

magnetotelluric survey across the active spreading zone in southwest iceland abstract the results of 10 magnetotelluric soundings performed along a 110 km long profile crossing the constructive plate boundary in southwest iceland are presented apparent resistivities are interpreted by a horizontally stratified earth model to yield a pseudo cross section along the profile the crust mantle interface contains a well conductive layer the depth to the good conductor increases with age of the crust and the distance from the axial zone this layer is interpreted as partially molten basalt at a temperature about 1100 c and a volume fraction of the melt phase in the range 10 20 the high conductivity layer probably disappears west of the borgarnes anticlinal axis which separates the older to the west and younger to the east flood basalts in western iceland indicating that the temperature below the oldest part of the profile lies below the solidus curve of basalt recent seismic crustal investigations in the same area indicate a state of partial melting or a magma chamber which agrees with the results of the magnetotelluric soundings

three dimensional mapping of magma source and transport regions from seismic data the mantle wedge beneath northeastern japan we investigate the distribution of partial melt in island arc using the seismic velocity structure of the mantle wedge beneath northeastern japan the comparison of the seismic tomography with laboratory velocity data on a partially molten mantle rock yields estimates of melting zones in three dimensions we employ experimental data on the degree of partial melt in hydrous peridotite to give constraints on the melt fraction and temperature melting and magma rich zones derived from the velocity structure coincide with observed low q zones the results of the three dimensional mapping indicate that the source of magma in island arc is diapir like melting patches localized within the low velocity zones of the mantle wedge extensive volcanic activity along the volcanic front is due to the presence of vast magma rich zones just beneath the moho those melting zones in the uppermost mantle may in turn cause melting of lower crustal materials and produce felsic magma melt appears to stay at and beneath the moho where crystallization fractionation may proceed melt exists at greater depths in the back arc region which may correlate with across arc variations of chemical compositions of the volcanic rocks observed in northeastern japan we suggest that magma migration in the ductile lower crust may cause low frequency microearthquakes and magma penetration into the brittle upper crust may produce mid crustal s wave reflectors

conductivity structure of oceanic upper mantle beneath the pacific plate summary ocean bottom magnetotelluric responses from three ages on the pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age the conductivity achieves its maximum value 0 1 sm 1 at depths near 70 120 and 180 km for lithospheric ages of 1 30 and 72 myr respectively the existence of this conductive structure has firmly been established by constructing a number of different acceptable models and also by obtaining unique averages over depth of the conductivity an explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the high conductivity beneath jdf t 1 myr the lower bound lies in the range 0 04 0 12 beneath cal t 30 myr it lies in the range 0 02 0 10 and beneath ncp t 72 myr it lies in the range 0 008 0 02 the distributions of partial melt show that the major portion of melt beneath jdf lies between 40 and 90 km and that melting has ceased by depths of about 170 km at all three sites geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value and then possibly decreases with depth the depth at which the maximum temperature is reached is about 125 km for cal and about 175 km for ncp and the maximum temperatures are estimated to be 1400 1500 c lastly as an independent check upon the partial melt explanation the location and magnitude of the seismic low velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites

mechanical interaction between volcanic systems in libya abstract the spatial distributions of monogenetic volcanoes primarily volcanic craters within the four principal volcanic provinces of libya are examined and presented on a volcano density map six main volcanic clusters have been identified referred to as volcanic systems remarkably the al haruj ahvp and nuqay nvp volcanic provinces have double peak volcano density distributions while the gharyan gvp and as sawda svp volcanic provinces have single peak volcano density distributions we interpret each volcano density peak as corresponding to a separate volcanic system so that there is a total of six systems in these four provinces there was an overlap in volcanic activity in these provinces with at least three simultaneously active we propose that each of the 6 volcanic systems was is supplied with magma from a large sill like reservoir similar in lateral dimensions to the systems clusters themselves numerical results show zones of high tensile and shear stresses between the reservoirs that coincide roughly with the main swarms of extension dykes and volcanic fissures and shear faults fractures in the areas the most recent volcanic eruptions in libya fall within the modelled high stress concentration zones primarily eruptions in the volcano waw an namus and the holocene al mashaqaq lava flow there are no known eruptions in libya in historical time but some or all the volcanic systems may have had one or more arrested historical dyke injections in particular part of the recurrent seismic events in the hun graben in the northwest libya may be related to dyke propagation and arrest if some of the inferred magma reservoirs are still fluid as is likely they pose earthquake and volcanic hazards to parts of libya particularly to the city of gharyan and zallah town as well as to many oil field operations

temperature and melting of a ridge centred plume with application to iceland part ii predictions for electromagnetic and seismic observables summary the dynamic and melting processes of a ridge centred plume have been investigated in a companion paper by ruedas et al hereafter referred to as paper i in a set of 3 d numerical fluid dynamic models with varying plume excess temperatures and melt extraction thresholds in paper i the modelled thickness of the generated crust has been compared to observations of the icelandic crust using the results of those plume models magnetotelluric mt transfer functions and seismic velocity anomalies are predicted in this paper together with paper i a dynamically consistent set of geophysical observables of a ridge centred plume is presented and applied to iceland temperature partial melting and the connectivity of the melt phase influence the electrical conductivity of crust and mantle rocks the temperature and melt fraction of our plume models are used to calculate 3 d conductivity models for mt modelling for the melt geometry ellipsoidal inclusions with appropriate aspect ratios were assumed to control melt connectivity the resulting transfer functions are compared to each other and to models not including a plume to separate signals from the ridge and the plume they may be applied to observed mt measurements if the plume head contains only 1 per cent of melt the plume signal cannot be distinguished from the ridge signal at least 3 per cent melt is needed for such distinction the other predicted observables calculated from the different numerical models are seismic velocity anomalies the temperature induced vp and vs anomalies were estimated including anharmonic and anelastic effects as well as the water induced increase of dislocation mobility that lowers seismic velocities realistic melt geometries as observed in laboratory experiments were used to calculate the effect of partial melts on the seismic velocities vs anomaly distributions are synthesized from the different plume models and compared to seismic observations to reconcile seismic anomalies of the plume head and plume stem a wet plume stem overlain by a partially molten dehydrated plume head is favoured the combined interpretation of available observations crustal thicknesses paper i and seismic results with our dynamic plume models paper i leads to a favoured plume model with 135 k excess temperature and a vertical velocity of approximately 13 cm yr 1 at 200 km depth with 1 per cent melt extraction threshold and a melting zone of approximately 500 km width and 100 km depth extent

on the rifting dynamics of plate divergence and magma accumulation at oceanic ridge axes rifting dynamics at spreading axes is governed by two processes the large scale plate divergence and the local magma accumulation in the crust mantle transition layer both evolve simultaneously a model is developed particularly for the situation in iceland where a well studied rifting episode occurred in the krafla volcanic system 1975 1984 both the divergence and the buoyant rise of magma create tensile deviatoric stress in the axial region but while divergence generates an altogether extensional stress field uprising of buoyant melt produces tension only near the axis but compression of the sides the buoyant rise is driven by the differential pressure gradient in rock and melt the processes are studied with a two dimensional finite element routine presently thermal effects are neglected the model parameters are density difference size of the buoyant body externally applied stress field mechanical properties of rock and melt relatively small amounts of divergence and small increases of buoyancy are shown to generate axial tension which can overcome the tensile strength axial tension produced by buoyant bodies can even overcome lateral compression observed long intervals of quiescence require either large rock strength quasi continuous stress relaxation small buoyant bodies and or a compressive deviatoric stress normal to the axis during much of the time between rifting episodes buoyant rise and injection of melt must be important in generating compression

peridotite sills and metasomatic gabbros in the eastern layered series of the rhum complex mapping of the eastern layered series els of the rhum ultrabasic complex on the northern flank of hallival shows that peridotite and allivalite troctolite or gabbro layers are laterally discontinuous and vary both in thickness and lithology peridotite generally has sharp upper and lower contacts against the allivalites which sometimes cut across the layering in the allivalite reaction dissolution and hybridization effects between peridotite and allivalite are developed locally some troctolite layers terminate as isolated fingered blocks in peridotite there are many small peridotite bodies which are clearly intrusive into allivalite and have previously been identified as distinct peridotite sheets and plugs they are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them we propose that many of the peridotite layers in the els formed as thick sills of picritic magma emplaced into a partly solidified layered troctolite complex the stratiform gabbros of the els are heterogeneous layered rocks that commonly contain relicts of troctolite and anorthosite wavy metre scale contacts between gabbro and troctolite cut across pre existing grain size modal and rhythmic layering with little disruption these metasomatic gabbros mimic the textures grain size and rhythmic layering of their troctolitic protoliths we propose that many of the els gabbros formed as a result of interaction between porous troctolites and a low temperature basaltic melt residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism

geophysical constraints on partial melt in the upper mantle this paper addresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical anomaly typical observed anomalies are high electrical conductivity of the order of 0 1 s m or greater velocity decreases of 7 10 seismic q values less than 100 and a frequency band for seismic effects in the region near 1 hz existing theories of electrical conduction in partial melts and of frequency dependent seismic properties together with recent measurements of melt electrical conductivity viscosity and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods from electrical anomalies mainly sensitive to melt volume and its interconnection one can require a minimum melt fraction of several percent at temperatures close to the solidus 1150 1300 c however seismic models demand only a small volume in very flattened shapes aspect ratio 0 001 melt fraction 0 1 further if melt configuration permits seismic dissipation in bulk that is there exist flattened voids intersecting more or less equant voids then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies observed equilibrium textures of partly melted peridotite together with inferred melt solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern thus if partial melt causes mantle geophysical anomalies it should exist in a variety of void shapes and probably of sizes while the association of electrical and elastic anomalies with indications of reduced density volcanism and high heat flow makes the hypothesis of partial melting an attractive explanation the minimum physical requirement is for existence of relatively high temperature