effect of grain size on the distribution and transport of deep seated fluids and melts because permeability increases with the square of the grain size for a given fluid fraction it is commonly assumed that porous flow of fluids and melts is more effective in coarse grained domains of deep seated rocks this relative behavior is accurate however only as a description of two systems operating in isolation we demonstrate that if coarse and fine grained domains are in chemical communication then equilization of pore wall curvature across the system results in concentration of fluid or melt in domains of finer grain size because of this localization of fluid the permeability of the finer grained domains will match or exceed that of domains with coarser grains fluid focussing due to differences in grain size has potentially important implications for grain scale transport of metamorphic fluids including those released from subducting slabs into the sheared mantle wedge and also for partial melts in the mantle and deep crust

magmatic history of red sea rifting perspective from the central saudi arabian coastal plain a newly recognized tertiary dike complex and comagmatic volcanic rocks exposed on the central saudi arabian coastal plain record early stages of magmatism related to red sea rifting intrusive and stratigraphic relationships and new potassium argon dating indicate episodic magmatism from about 30 ma to the present additional stratigraphic and radiometric evidence suggests that limited rift related magmatism began as early as about 50 ma an early phase of crustal extension in the region was accompanied by faulting and graben formation and by dike swarm intrusion the style of extension and intrusion changed at 20 ma localized volcanism and sheeted dike injection ceased and were replaced by the intrusion of thick gabbro dikes this change may mark the onset of sea floor spreading in the central red sea the dikes and volcanic rocks consist of a bimodal mafic felsic suite with transitional subalkaline to alkaline chemistry although no unique petrogenetic model can be developed for the suite the following observations and conclusions have been drawn from the available reconnaissance geochemistry 1 strontium isotopic initial ratios overlap in the range 0 7031 to 0 7047 and are not clearly related to alkalinity or silica content the bimodality and alkalinity of the suite are therefore not products of contamination by the radiogenic precambrian upper crustal granitic rocks exposed in the region 2 several of the evolved rocks rhyolites and comendite show large to extreme negative europium anomalies and heavy rare earth element hree enrichment features that cannot be explained by low pressure crystal fractionation of potential parent magmas an intriguing possibility is that hree enrichments and large negative eu anomalies of some peraluminous rhyolites and granites may result from the complete melting of garnet from middle or lower crustal rocks 3 most of the basalts are light rare earth element lree enriched consistent with derivation from fertile mantle in a continental rift setting however interaction of the mafic melts with lower continental crust cannot be ruled out a model of poly baric mantle melt derivation producing several alkaline subalkaline cycles best explains magmatism in the red sea region differences in the depths and dynamics of mantle melt extraction and transport brought about through changes in crust and mantle structure as the rift and paar developed may account for the transition from mixed alkaline subalkaline bimodal magmatism of the pre 20 ma rift basin to exclusively subalkaline tholeiitic magmatism at the red sea spreading axis and to predominantly alkali basalt volcanism inland within the arabian shield

magma systems related to the red sea opening abstract four distinct magma environments can be recognized in association with the red sea basin 1 1 mid ocean ridge basalts within the present actively spreading axial trough 2 2 basaltic dikes and layered gabbros within the extended precambrian crust on the arabian margin 3 3 granites and rhyolites developed in the extended precambrian crust and 4 4 plateau basalts asymmetrically distributed on the arabian flanks of the red sea these magmas developed as the result of the initial break up of africa and the establishment of an oceanic spreading center within the red sea original asymmetric continental extension occurred on east dipping detachment faults in the oligocene and caused morb like magmas to invade an extending precambrian crust contemporaneous small scale mantle convection initiated limited partial melting under the arabian plate to form the undersaturated alkali olivine plateau basalts slow extension through the miocene allowed an evaporite basin to develop in the red sea while small volumes of rift and plateau basalts formed on the arabian plate a small change in plate motion in the pliocene initiated the seafloor spreading in the axial trough where morb magmas formed oceanic crust rise of the asthenosphere produced small scale thermal perturbations under the arabian plate controlled by an east dipping master detachment fault small scale mantle convection extended partial melting 2 5 eastward under the arabian plate producing more undersaturated diverse alkali olivine plateau basalts distinct chemical and isotopic signatures of these four magma types lead to obvious actualistic correlations with the geodynamic setting

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

upwelling asthenosphere beneath western arabia and its regional implications two distinct phases of continental magmatism are evident in western arabia the first from about 30 to 20 ma produced tholeiitic to transitional lavas emplaced along nw trends the second from about 12 ma to recent produced transitional to strongly alkalic lavas emplaced along n s trends the older phase is attributed to passive mantle upwelling during extension of the red sea basin whereas the younger phase is attributed to active mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision the younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the west arabian swell after about 14 ma a variety of evidence suggests that the west arabian swell is thermally supported by hot upwelling asthenosphere in contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the red sea basin these processes were symmetric across a n s line marking the central axis of the west arabian swell this axis coincides with two fundamental features the ha il rutbah arch in the north and the makkah madinah nafud mmn volcanic line in the south the symmetry of magmatism is demonstrated by petrochemical evidence that the mmn harrats were derived by greater degrees of partial melting at shallower depths than those harrats lying to the west and east of the mmn line the potential temperature of the asthenosphere is estimated to be about 1436 c beneath the mmn line and about 1354 c beneath the flanking more undersaturated harrats the source of upwelling is either a mantle plume centrally located beneath the west arabian swell or an elongated and extended lobe of hot asthenosphere emanating from the ethiopian mantle plume convective flow may have been channelled along a preexisting regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the afroarabian dome this crest of mantle upwelling underlies the mmn line in the north and the danakil depression and ethiopian rift system in the south it also passes through the red sea basin at the midpoint of an unusual doubly propagating rift system where axial seafloor spreading began 4 5 m y ago the nw trend of the red sea basin was well established by crustal attenuation during the older magmatic phase the pliocene invasion of this basin by a n s zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions

aquifer recharge depletion and connectivity inferences from grace land surface models and geochemical and geophysical data data from the gravity recovery and climate experiment grace and outputs of the clm4 5 model were used to estimate recharge and depletion rates for large aquifers investigate the connectivity of an aquifer s subbasins and identify barriers and preferred pathways for groundwater flow within an aquifer system the nubian sandstone aquifer system and its subbasins dakhla northern sudan platform and kufra in northeast africa were used for demonstration purposes and findings were tested and verified against geological geophysical remote sensing geochronologic and geochemical data there are four major findings 1 the average annual precipitation data over recharge areas in the southern kufra section and the northern sudan platform subbasin were estimated at 54 8 km 3 and 32 8 km 3 respectively and knowing the annual extraction rates over these two areas 0 40 0 20 km 3 recharge rates were estimated at 0 78 0 49 km 3 yr and 1 44 0 42 km 3 yr respectively 2 grace derived groundwater depletion rates over the dakhla subbasin and the northern kufra section were estimated at 4 44 0 42 km 3 yr and 0 48 0 32 km 3 yr respectively 3 the observed depletion in the southern parts of the dakhla subbasin is apparently caused by the presence of the east west trending uweinat aswan basement uplift which impedes the south to north groundwater flow and hence reduces replenishment from recharge areas in the south 4 a major northeast southwest trending shear zone pelusium shear system is apparently providing a preferred groundwater flow pathway from the kufra to the dakhla subbasin our integrated approach provides a replicable and cost effective model for better understanding of the hydrogeologic setting of large aquifers worldwide and for optimum management of these groundwater resources

the arabian continental alkali basalt province part i evolution of harrat rahat kingdom of saudi arabia the harrat rahat lava field forms a major component of an extensive but poorly known continental alkali basalt province extending from yemen in the south through saudi arabia and jordan to syria in the north the continental intraplate volcanism which produced the arabian lava fields harrats was contemporaneous with opening of the red sea collision of the arabian and eurasian plates and uplift of the afro arabian dome harrat rabat has evolved over the past 10 m y it contains two prominent lateritic disconformities which separate three stratigraphic units the shawahit 10 2 5 ma hammah 2 5 1 7 ma and madinah 1 7 recent basalts comprising 68 19 and 13 of the total harrat volume respectively the shawahit basalt is composed of coarse grained dictytaxitic olivine transitional basalt otb and minor 10 alkali olivine basalt aob which erupted through scoria cones and shield volcanoes in contrast the hammah basalt is dominated by equal volumes of fine grained intergranular to intersertal aob and hawaiite which erupted mainly through scoria cones and relatively few shield volcanoes sparse domes and flows of mugearite and benmoreite first appear in the hammah basalt the madinah basalt spans the full compositional range of harrat rahat 103 analyzed samples comprise otb 8 aob 47 hawaiite 32 mugearite 4 benmoreite 8 and trachyte 2 here scoria cones dominate whereas shield volcanoes are sparse and occur only in the lower madinah basalt domes and associated pyroclastic deposits are common the final events of harrat volcanism include 11 post neolithic eruptions 4 500 yr old and two historical eruptions in 641 and 1256 a d arabian harrat magmatism may have been initiated by partial melting of garnet perido tite in the asthenosphere at a depth 100 km accumulation of this partial melt at the crust mantle boundary 37 44 km may have resulted in miocene uplift of the afro arabian dome harrat rahat volcanism began shortly after significant initial uplift with the voluminous extrusion of shawahit otb lava chemical and petrographic data suggest that these otb magmas ascended rapidly with little crustal residence time the data support a model of open system fractionation for aob and hawaiite lavas of the younger hammah and madinah basalts in this model some of the potential shawahit otb was trapped in crustal chambers during magma ascent crystal fractionation in these chambers was accompanied by their periodic replenishment by rising otb magma this resulted in the mixing of magma types and the development of aob and hawaiite magmas which are anomalously enriched in highly incompatible elements the mugearite benmoreite and trachyte lavas are the products of advanced fractionation in these crustal chambers with little or no magma replenishment

upper mantle seismic shear discontinuities of the pacific 1 shear wave impedance discontinuities are inventoried for eight paths connecting circum pacific earthquakes with seismic stations on hawaii and oahu in addition to the transition zone discontinuities we observe a consistent impedance decrease at a depth of 80 km that marks the transition from the fast seismic lid to the low velocity zone the interval over which this transition occurs is less than 30 km the requisite impedance decrease ascribed almost entirely to diminished velocities in the low velocity zone exceeds predictions for peridotite chemistries of appropriate lithospheric age in all cases the transition is better matched by lithosphere of roughly half of the true age four paths show clear evidence of the x discontinuity an impedance increase of 3 8 near 300 km depth the only viable explanations would require extensive eclogite or basalt depleted mantle in the upper mantle of the western pacific the impedance contrast of the 410 km discontinuity which depends on the modal fraction of mantle olivine does not signal the presence of either although a mixture of the two remains a possibility we find no evidence of a low velocity layer atop of the 410 km discontinuity if present it is either consistently thin 15 km highly variable in thickness topography in excess of 60 km or has little impedance contrast 2 3 with overlying mantle the apparent absence of melt atop the 410 km discontinuity an intermittent and weak 520 km discontinuity and a thinned transition zone are consistent with relatively dry conditions in the deep upper mantle of the central pacific

high temperature elasticity of iron bearing olivines the first high temperature data on the nine adiabatic elastic moduli for iron bearing olivine are reported these measurements are on two single crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 k the two specimens contain 8 and 9 modal percent fayalite which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability the rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of co and co2 gas a small increase 1 2 gpa in the adiabatic bulk modulus of each specimen over that of end member forsterite was found the data are high quality to extreme temperatures with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli which is in contrast to published forsterite data the temperature derivatives of the isotropic bulk modulus ks are 1 69 1 80 10 2 gpa k 1 for the two olivine specimens and the shear modulus g derivatives are 1 38 1 36 10 2 gpa k 1 these derivatives are only slightly larger in magnitude than ks t p 1 56 10 2 and g t p 1 30 10 2 gpa k 1 found previously for iron bearing olivine over a very small temperature range there are also no significant differences between the temperature derivatives found here and the average derivatives of end member forsterite from data retrieved over a slightly larger temperature range several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high pressure phases in earth s transition zone one result from these systematics related to the seismic velocities in the earth and especially the shear wave velocities is that an olivine content of less than 50 is implied at the 400 km discontinuity if earth s upper mantle is isochemical furthermore the substitution of almost 10 iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature

the timing of uplift volcanism and rifting peripheral to the red sea a case for passive rifting prior to the formation of the red sea the northeastern afro arabian continent had low relief and was largely below sea level from the late cretaceous to the early oligocene the events leading to the formation of the red sea followed the sequence 1 alkaline volcanism and rifting beginning about 30 32 ma affecting a narrow linear zone in the continent 2 rotational block faulting and detachment faulting well underway by 25 ma 3 gabbro and diorite magmatism andesite to rhyolite volcanism and fine grained nonmarine sedimentation in the rift between 20 and 25 ma 4 fine grained marine sedimentation in the rift as the early shelves started to subside in the middle miocene and 5 uplift of the adjacent continents about 3 km and subsidence of the shelves about 4 km between 13 8 and 5 ma the youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the proterozoic arabian shield that range in age from 13 8 to 568 ma the youngest of these ages coupled with the present high relief along the arabian escarpment and published heat flow measurements indicate that 2 5 4 km uplift has occurred in the last 13 8 m y the sequence volcanism rifting followed by uplift leads to our adoption of a passive mantle model for rift origin models that require uplift to create the rift are rejected because of the late uplift we advocate a model of lithospheric extension caused by two dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere it is acknowledged that traction models could explain the observed data but they imply a rigid static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions neither requirement is necessary in the extension model the rift starts with mechanical extension in a narrow zone of lithosphere between 25 32 ma in our model the thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis once partially melted rocks formerly part of the continental lithosphere join the upwelling asthenosphere resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge the resulting density decrease explains the uplift we think that the red sea began as a consequence of changing plate geometries resulting from the collision of india and eurasia after the collision the segment of the owens fracture zone north of the carlsberg ridge became locked forcing the northeast corner of afro arabia to rotate with the indian plate away from the rest of africa

new k ar ages of basalts from the harrat ash shaam volcanic field in jordan implications for the span and duration of the upper mantle upwelling beneath the western arabian plate the volcanism in the western arabian plate extends from the red sea through the harrat ash shaam system to western syria as far north as the bitlis suture in the taurides the harrat ash shaam volcanic system in jordan consists of northwest trending dikes and a volcanic field that together encompass a width of 220 km in terms of width direction and age of the main volcanic phases the system is similar to the red sea dike belt about 130 new k ar age determinations show that the ages of the harrat ash shaam system dikes and flows range from oligocene to quaternary however there is a distinct gap in the ages between 22 and 13 ma this gap coincides with an apparent decrease in volcanism in the red sea region from around 20 to 12 ma we interpret this 9 m y gap as a quiescent period interrupting the volcanic activity in the region and suggest that from 22 to 13 ma tectonic activity in the arabian plate was mainly restricted to the red sea region a renewal of volcanism along the western margins of the arabian plate at 13 ma was very likely associated with the sinistral movement along the north trending dead sea transform this renewal of volcanism and tectonic activity may reflect the emergence of upper mantle upwelling beneath the western arabian plate at that time

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

the effect of an equilibrated melt phase on the shear creep and attenuation behavior of polycrystalline olivine the impact of a chemically and texturally equilibrated melt phase on the shear creep and attenuation behaviors of polycrystalline olivine has been measured experimentally at seismic to subseismic frequencies the experiments were performed on aggregates that had a particularly uniform and fine grain size 3 m the effect of the melt phase 5 vol is to decrease the newtonian viscosity by a factor of 6 there is no dramatic disaggregation effect or melt induced plummeting of the shear modulus both the melt free and melt bearing aggregates display an attenuation band of the form qg 1 f 1 4 this response cannot be attributed to a variation in microstructure it is intrinsic to the diffusional creep behavior there is no unique effect of the melt phase on the attenuation response the slight increase in absorption of partial melt specimens is explained fully by the effect of the texturally equilibrated melt on aggregate viscosity

timing of mantle upwelling evidence for a passive origin for the red sea rift mantle xenoliths from western saudi arabia some 220 km east of the red sea margin indicate temperatures as high as 900 c in the mantle at about 40 km depth beneath an area of the arabian shield where surface heat flow is lower than the world average this suggests that there has been a large increase in the mantle temperature that has not had time to equilibrate at the surface the amount of uplift and the difference between the observed xenolith temperature and that obtained by projecting the surface heat flow to depth suggests that the lithosphere has been thinned by 30 to 100 km to its present calculated thickness of about 80 km calculations of the time necessary for thermal reequilibration indicate that the lithospheric thinning probably occurred less than 20 m y ago so it postdates rifting which started about 30 ma if there was a prolonged upwelling of deep mantle material resulting in a thinner lithosphere prior to rifting there should have been enough time for some reequilibration at the surface and the observed heat flow should be higher over the broad area of uplift adjacent to the red sea our calculations are consistent with a postrift age of uplift adjacent to the red sea as determined from regional geologic evidence and fission track ages on apatites from shield rocks the red sea appears to have begun with alkaline volcanism on the arabian shield followed by rifting uplift and lithospheric thinning adjacent to the red sea rift began 5 10 m y after rifting this suggests that the red sea is a passive rift that formed because of two dimensional plate motions rather than as a consequence of mantle upwelling a possible cause for the stress in the plates might have been the change of plate motions in the indian ocean that resulted from the collision of india with eurasia

influence of dihedral angle on the seismic velocities in partially molten rocks abstract this article reports the influence of dihedral angle in a partially molten aggregate on its effective elastic properties using theoretical techniques for a given mineral assemblage dihedral angles can vary widely depending on the composition of the melt our results indicate that wetting melts with low dihedral angles have a lower fraction of intergranular contact area contiguity reducing the effective elastic moduli an important consequence of this effect is that the seismic signature of an aggregate with a small volume fraction of a wetting melt will be similar to that of an aggregate containing a larger volume fraction of less wetting melt inferring the extent of melting in seismic ultralow velocity zones therefore needs to incorporate the influence of melt composition via the influence of dihedral angle

tertiary anorogenic granites of the western border of the yemen plateau north yemen y a r was affected from 30 to 20 ma ago by intense magmatic activity trap series contemporary with the main tensional tectonic phases of the afro arabian plate in the trap activity two main phases have been recognized the former mainly basaltic between 30 and 26 ma and the second mainly acidic between 23 and 20 ma during the latter period of activity a basaltic dyke swarm and stock like bodies and dykes granophyres were emplaced along the escarpment and coastal plain some granitic bodies intruded the trap series towards the end of the first phase and during the second one on the basis of geochemical and petrological data the tertiary granites of yemen belong to alkaline or peralkaline associations of a type the textural characteristics indicate that the granites are either hypersolvus or subsolvus the mafic mineral assemblages allow three main types of granites to be distinguished by their different environments of crystallization as regards the origin of these kinds of granites several hypotheses account for the peculiarities observed the lack of crustal xenoliths the preliminary data on the sr isotopic ratio the data on major and trace elements point to a direct origin by fractional crystallization without substantial crustal interaction this agrees well with the strong tensile tectonics linked with the uplift of the afro arabian dome and with the lithosphere thinning which provide a minimal opportunity for interaction of the magmas with the crust

dislocation damping and anisotropic seismic wave attenuation in earth s upper mantle crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves although these defects known as dislocations potentially contribute to the attenuation of seismic waves in earth s upper mantle evidence for dislocation damping from laboratory studies has been circumstantial we experimentally determined the shear modulus and associated strain energy dissipation in pre deformed synthetic olivine aggregates under high pressures and temperatures enhanced high temperature background dissipation occurred in specimens pre deformed by dislocation creep in either compression or torsion the enhancement being greater for prior deformation in torsion these observations suggest the possibility of anisotropic attenuation in relatively coarse grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in earth s upper mantle

diffusion creep of dry melt free olivine 1 deformation experiments were conducted on fine grained 3 6 m fully synthetic fo90 olivine aggregates in a gas medium apparatus at 300 mpa confining pressure and temperatures of 1150 1360 c the strain rates of the solution gelation derived and therefore genuinely melt free dry samples are about two orders of magnitude lower than the strain rates for nominally melt free aggregates at the same pressure and temperature conditions and grain size benchmark deformation tests with anita bay dunite and mild steel reproduce published data the creep strength of melt added sol gel olivine is similar to the published creep strength of dry melt bearing olivine derived from natural rocks nonlinear least squares fits to the melt free deformation data give an activation energy of 484 kj mol a stress exponent of 1 4 and a grain size exponent of 3 over a range of stresses from 15 to 210 mpa these results suggest that small amounts of melt may be similarly effective in reducing the creep strength of upper mantle rocks as small amounts of water however a possible contribution of grain boundary composition to the observed differences in rheology in the absence of melt cannot be conclusively ruled out by the current experiments

comment on topographic and volcanic asymmetry around the red sea constraints on rift models by t h dixon e r ivins and j f brenda

a model for intergranular pressure solution in open systems abstract intergranular pressure solution ips represents a well known mechanism of bulk deformation and pore volume change especially in sedimentary rocks which has been documented in the field and reproduced in laboratory experiments theoretical descriptions of the process in terms of simple pressure solution creep laws can explain features such as grain size dependence of creep rates but have typically remained restricted to closed system conditions with negligible long range transport in the fluid phase and somewhat biased toward grain boundary diffusion controlled creep the recasting of the classical weyl rutter model of ips considered in this paper emphasizes open system conditions and the competition between diffusion and dissolution reaction rate processes the thermodynamic force which drives ips is expressed in terms of an effective grain boundary normal traction and a solute supersaturation this suggests the occurrence of a transition from ips to grain boundary healing at a certain level of supersaturation of the pore fluid although the process of healing is not studied here the explicit appearance of the solute concentration in the driving force highlights the coupling of the ips deformation process to the long range transport of dissolved material and the need to model the diagenesis of compacting sedimentary sequences accordingly the conceptual grain boundary model used in this paper is that of a fluid invaded grain to grain contact zone which is visualized to remain in a roughened state during continued ips the use of simple linear phenomenological rate laws for dissolution and grain boundary diffusion allows the ranges of rate control by dissolution or diffusion to be succinctly characterized in terms of the ratio of corresponding time constants the associated compaction behaviour is explored quantitatively in this paper by presenting solutions for the uniaxial and isotropic compaction creep strain vs time for a simple cubic packing of spheres

utilizing the geophysical and hydrogeological data for the assessment of the groundwater occurrences in gallaba plain western desert egypt the investigated region is located in the western desert fringes of the nile valley which requires studies of groundwater related to the many projects of land reclamation the key objective of this paper is to estimate the qualitative and geometrical features of the investigated aquifer using 60 vertical electrical sounding and time domain electromagnetic soundings allows us to suggest one possible model of the geometrical features of the local aquifer a hydrogeological monitoring has been undertaken to investigate the current groundwater situation at the gallaba plain such hydrological monitoring has not been undertaken before in detail the results show that the investigated region has high groundwater potentialities in two main aquifers which belong to pleistocene shallow fresh water and deep brackish water the lithological and structural elements contribute mainly to recharge and store the groundwater in the western part of the river nile in kom ombo graben the geochemical properties of the groundwater of the studied aquifers reflect meteoric water which is a fresh to slightly brackish water the small amount of groundwater salinity arises from silicate weathering and evaporation processes occurring in the aquifer matrix moreover most of the studied groundwater samples are unfit for human consumption such samples are very satisfactory for livestock and poultry purposes and they can be used for irrigation using modern and improved irrigation methods e g sprinkler and drip methods furthermore the hydrogeological monitoring of the concerned area indicates that it has high groundwater potentialities which will support its sustainable development

numerical modeling of groundwater resource management options in the east oweinat area sw egypt southwest egypt is an arid area with no surface water and limited resources of useable groundwater in the well known nubian sandstone aquifer system these groundwater reserves have been heavily exploited since the 1960s which has led to substantial decline in the potentiometric surface of the aquifer a calibrated regional numerical model with refined grids on the pumping centers has been used to investigate the hydrodynamic impacts of different groundwater management options on the potentiometry of the aquifers the results indicate that there is a real danger of either dewatering the shallow aquifer in some areas e g kharga oasis or increasing the water depth to uneconomic lifting depth they also indicate that although the planned extraction rates in dakhla farafra and bahariya oases are feasible for at least the coming 100 years the present rate for kharga oasis and the planned rate for the east oweinat area have to be reduced substantially

groundwater flow modeling and particle tracking for chemical transport in the southern voltaian aquifers groundwater is the major source of water supply for most uses in the rural settlements in ghana a groundwater flow model was calibrated for some aquifers of the southern voltaian sedimentary system under steady state conditions the objective was to determine estimates of the hydraulic conductivities of the different hydrostratigraphic units of the southern voltaian and the distribution of recharge from precipitation data on the stable isotopes of oxygen and hydrogen from the study area suggest that groundwater recharge in the area is from rainfall the calibrated steady state model suggests that aquifer hydraulic conductivities in the study area range from 1 19 to 6 3 m day hydrostratigraphic unit specific hydraulic conductivities are discussed the calibrated recharge ranges from 3 81e 05 m day to 6 0e 04 m day which represents 0 9 6 of the precipitation in the form of rainfall six distinct flowpaths have been defined using particle tracking the particle tracking simulation suggests travel times in the range of 380 to 5 199 years from recharge areas to discharge areas along the flowpaths identified a contaminant dropped at the recharge areas in the central sections of the model area would travel at these rates along the flowpaths assuming that advection is the dominant transport process inverse geochemical modeling indicates the dissolution of albite k feldspars and anorthite respectively along flowpaths i and iv the inverse modeling along flowpaths i and iv suggest the dissolution of albite k feldspar and anorthite respectively at 1 085e 06 3 16e 08 and 3 168e 07 mmol year

upper mantle seismic wave attenuation effects of realistic partial melt distribution frequency dependence of seismic velocity and attenuation resulting from viscoelastic relaxation of partially molten mantle is estimated we consider the contribution of the melt squirt mechanism through which pressure differences between disk shaped inclusions are equalized by melt passing through connecting tubes the pressure differences arise as a result of shear strain compressing disk shaped pores differently on the basis of disk orientation with respect to the applied shear the frequencies over which the transition from the unrelaxed to the relaxed states occurs are determined by representing the melt as a network of tubes connecting oblate ellipsoidal pores the pressure equalization process is modeled by a system of first order linear differential equations whose eigenvalues are the characteristic frequencies for melt squirt relaxation it is shown that in this framework the set of frequencies is invariant to the absolute scale of the system but is sensitive to melt bulk modulus and viscosity as well as distribution of melt inside pores and conduits use of realistic solid and melt physical properties and pore and conduit geometries demonstrates that it is the relaxed modulus that is most likely excited in the seismic band and that melt mobility has little effect on seismic attenuation some conceivable melt distributions however would result in detectable attenuation in the seismic band in all cases investigated attenuation increases with frequency indicating that melt squirt is not responsible for global upper mantle q observations

the yemen trap series genesis and evolution of a continental flood basalt province abstract the yemen basaltic plateau trap series was built up by fissure eruptions related to tensional tectonics in the afro arabian plate two main phases of activity have been identified and dated between 30 and 26 and 23 and 19 ma respectively the first phase was dominated by eruptions of alkaline to transitional basalts with a few rhyolitic ignimbrites during the second phase rhyolites with minor transitional basalts were erupted in the southern and southwestern part of the area geochemical and isotopic data indicate that the basalts are the result of combined processes of crystal fractionation and crustal contamination on mantle derived parental liquids the degree of crustal contamination generally decreases from south to north

upper mantle structure beneath the arabian peninsula and northern red sea from teleseismic body wave tomography implications for the origin of cenozoic uplift and volcanism in the arabian shield upper mantle structure between 150 and 400 km depth is imaged beneath the arabian shield and northern red sea by modeling p and s traveltime residuals from teleseismic events recorded on the saudi arabia national digital seismic network the 1995 1997 saudi arabian passcal experiment and three permanent stations rayn eil and mrni relative traveltime residuals were obtained using a multichannel cross correlation method and inverted for upper mantle structure using vandecar s inversion method the resulting images reveal a low velocity region 1 5 for the p model and 3 for the s model trending nw se along the western side of the arabian shield and broadening to the northeast beneath the makkah madinah nafud volcanic line we attribute the low velocities to a mantle thermal anomaly that could be as large as 330 k and that is associated with the cenozoic uplift of and volcanic centers on the shield our tomographic images are not consistent with models invoking separate mantle upwellings beneath the northern and southern regions of the shield and instead favor single plume or superplume models we also find little evidence for low velocities beneath the northern red sea suggesting that there might not be a geodynamic link between rifting in the red sea and plateau uplift and volcanism in the shield

geological evolution of the afro arabian dome abstract the afro arabian dome includes the elevated continental regions enclosing the red sea gulf of aden and the ethiopian rift system and extends northwards as far as jordan it is more than an order of magnitude larger than other african uplifts both the structures and the igneous rocks of the dome appear to be products of the superimposition of two perhaps three semi independent generating systems initiated at different times but all still active a strain pattern dominated by nw trending basins and rifts first became established early in the cretaceous by the end of the oligocene much of the extensional strain had been taken up along the red sea and gulf of aden axes which subsequently developed into an ocean palaeogene trap volcanism of mildly alkaline to transitional character was related to this horizontal extension rather than to doming further west the east sahara swell has a history of intermittent alkaline volcanicity which began in the mesozoic and was independent of magmatism in the afro arabian dome volcanicity specifically related to doming began in the miocene along a n s zone of uplift extending from ethiopia to syria this elongated swell forms the northern termination of the east african system of domes and rifts characterized by episodic vertical uplift but very little extension superimposition of epeirogenic uplift upon structures formed by horizontal extension took place in the neogene volcanicity related to vertical tectonics is mildly alkaline in character whereas transitional and tholeiitic magmas are found along the spreading axes

the arabian continental alkali basalt province part ii evolution of harrats khaybar ithnayn and kura kingdom of saudi arabia three coalesced basaltic lava fields harrats khaybar ithnayn and kura constitute the largest contiguous area of cenozoic basalt in saudi arabia similar in extent 20 564 km2 and volume 1 850 km3 to harrat rahat which is situated only 25 km to the south harrat kura with an age range from about 11 to 5 ma is the oldest of these harrats it contains a single stratigraphic unit the kura basalt primarily composed of alkali olivine basalt aob with subordinant basanite and hawaiite and a single cluster of four phonolite domes in contrast harrats khaybar 5 ma to present and ithnayn 3 ma to present are composed of more mildly alkaline basalt types the two oldest units on harrat khaybar the jarad and mukrash basalts are dominated by dictytaxitic to ophitic olivine transitional basalt otb flows which were extruded primarily from major arterial lava tubes to form unusual morphological features here designated whaleback lava flows the lava tubes emanate from a 50 km long north trending fault scarp along the western edge of the central vent zone which marks the site of voluminous outwelling of otb lava the fault scarp ceased to be a major site of extrusion at the end of mukrash time immediately to the east of the fault scarp a thin 0 7 to 1 0 km uptilted slab of crust formed the roof of a high level magma chamber 60 x 25 km that dominated the eruptive history of the abyad basalt the youngest stratigraphic unit on harrat khaybar the abyad basalt occurs as seven stratigraphic subunits the two youngest of which qb6 and qb7 are post neolithic and historical in age in contrast to the jarad and mukrash basalts the abyad basalt is dominated by aob and hawaiite with only minor otb no abyad differentiated rocks occur on harrat ithnayn however fractional crystallization in the high level magma chamber beneath harrat khaybar has resulted in an extreme compositional range of rock types from mugearite to benmoreite trachyte and comendite the vent distribution at harrat khaybar shows a distinct zonation indicative of a high level compositionally stratified magma chamber whole rock chemistry suggests that even the most primitive basalts are not primary magmas but rather fractionated melts derived from primary magmas of variable melt fractions generated at differing mantle depths the more undersaturated harrat kura basalts were derived from primary magmas representing the smallest degrees of partial melting at the greatest depth crystal fractionation of most primary magmas appears to have taken place in reservoirs at the crust mantle boundary chemical and petrographic data suggest that open system magma recharge was an important process in the high level magma chamber beneath harrat khaybar the chemical and mineralogical heterogeneity of the harrat khaybar comendites their sr isotope values and their zr nb ratios all indicate that they were derived by varying degrees of crystal fractionation 95 crustal melting and halogen complexing the north trending volcanic axes of harrats rahat khaybar and ithnayn form a single 600 km long linear vent system designated the makkah madinah nafud mmn volcanic line the three transitional to mildly alkaline harrats extruded from this volcanic line differ from most arabian harrats which like harrat kura to the west are significantly more undersaturated the relatively large degrees of partial melting evident in harrats rahat khaybar and ithnayn suggest that the mmn volcanic line may overlie a north south thermal lineation in the mantle the petrochemical stratigraphic sequence in these three harrats is nearly identical early voluminous extrusions of otb were followed by later less extensive extrusions of minor otb and abundant aob hawaiite and differentiated lavas as this petrochemical stratigraphy evolved it progressed northward along the mmn trend with time beginning on harrat rahat about 10 ma on harrat khaybar about 5 ma and on harrat ithnayn about 3 ma the interplay of open system magmatic processes beneath this migrating volcanic line has resulted in a classic example of a strongly fractionated mildly alkaline basalt series

on the rheologically critical melt fraction abstract with increasing melt fraction the strength of partially molten granite decreases from a value characteristic of solid competent rock to a value nearly equal to that of the melt previously published mechanical and microstructural data indicate that deformation in partially molten rocks often involves brittle processes thus the pressure in the melt is expected to be important in determining strength when the volume changes during deformation strength and fluid flow will be coupled by such parameters as permeability k storage capacity per unit volume or storativity s melt compressibility f grain size d fluid viscosity and strain rate experiments on brittle rock at low temperatures show that the strain rate at which the internal fluid pressure can be maintained constant is approximately proportional to k s an evaluation of published experimental data suggests that this relation also holds for partially molten granites indicating that the strength of these rocks depends on their transport properties since the permeability is related to below a critical melt fraction rcmf the melt pressure will change if deformation is not iso volumetric by assuming a power law relationship between k and we estimate that rcmf f d m 1 n 1 where m and n relate permeability to grain size and respectively

simulation of impact of present and future groundwater extraction from the non replenished nubian sandstone aquifer in southwest egypt a numerical time dependent groundwater model of the nubian sandstone aquifer in egypt is presented a complete database of the hydrogeological and drilling information of 850 water wells drilled in the period 1960 2000 was evaluated and used for the model parameter input as well as for its calibration a steady state condition is rejected even for the pre development time as the subsurface inflow across the egyptian sudanese borders is lower than discharge and the observed hydraulic gradient is residual gradient and not due to steady state flow the results of the long period simulation 10 000 years indicated that the nubian sandstone aquifer is still under the influence of the past humid period and has been in an unsteady deplenishing process therefore groundwater development plans should be based on this concept the calibrated model was used to investigate the hydrodynamic impacts of the present and planned groundwater extraction on the potentiometry of the aquifer the simulation results indicate that there is a real danger of groundwater depletion particularly in the shallow aquifer in some areas in fact if the planned extraction of 1 200 million m3 year in east oweinat area is imposed drawdown after 100 years could be as much as 200 m relative to1960s levels by this time the cone of depression will extend to dakhla and kharga oases

influence of melt on the creep behavior of olivine basalt aggregates under hydrous conditions the influence of melt on the creep behavior of olivine basalt aggregates under hydrous conditions has been investigated by performing a series of high temperature triaxial compression experiments samples with melt fractions of 0 02 0 12 were deformed under water saturated conditions at temperatures between 1373 and 1473 k and a confining pressure of 300 mpa in a gas medium apparatus at constant differential stress and temperature the rate of deformation increased rapidly but systematically with increasing melt fraction in the diffusion creep regime at a given differential stress samples with melt fractions of 0 02 and 0 12 deformed a factor of 2 and 20 respectively faster than a melt free sample in the dislocation creep regime a sample with a melt fraction of 0 12 deformed a factor of 40 faster than a melt free sample for partially molten olivine basalt aggregates deformed under hydrous conditions the dependence of creep rate on melt fraction can be expressed in the form 0 exp where 26 for diffusion creep and 31 for dislocation creep the results of this study combined with reasonable estimates for the spatial variation in the concentrations of water and melt as well as for the geotherm and the activation volume for creep provide constraints on the viscosity structure of earth s upper mantle as an example we present a viscosity profile for the mantle wedge above a subducting plate demonstrating that the viscosity in that region can vary by 3 orders of magnitude over a depth of 60 km due to the combined effects of water and melt weakening

mitigation of groundwater level deterioration of the nubian sandstone aquifer in farafra oasis western desert egypt el farafra oasis is included in the new valley project which started in the 1960s this project is based exclusively on groundwater of the well known nubian sandstone aquifer nsa the hydraulic head drawdown problem in farafra oasis constitutes a big problem because it threatens the availability of the only water source in the oasis hydrogeologically farafra oasis belongs to the huge artesian unrenewable fossil water nubian from sandstone aquifer the problem began to appear at the end of the 1990s instantaneously with increased drilling of deep water wells huge quantities of water 267 million m3 are derived from the aquifer every year hydraulic head depression cones are formed around the main reclamation areas where the groundwater surface lowered by 35 m below the ground surface numerical modeling was used to investigate the hydrodynamic impacts of the present and planned groundwater extraction on the groundwater levels besides the possible means of mitigating the continuous decrease of the groundwater levels the aquifer simulation indicated that there is a significant head decline of the groundwater particularly in the first productive zone four scenarios were applied to signify the present and future development plans for the next 20 years i e year 2033 applying the present conditions reveal that heads are more declined than now another 30 and 17 8 m in the first and second productive zones respectively are expected to be added to the present groundwater drawdowns groundwater is expected to decline significantly if the pumping rates are to be increased by 50 in response to natural development of the agricultural society and continuous need to more cultivation lands groundwater heads are expected to be lower by about 40 m in the first productive zone and 24 m in the second productive zone the third plan examines the impact of reducing the pumping rate by about 25 on groundwater levels by applying recent irrigation techniques as drop and sprinkler irrigation methods instead of the presently used flood irrigation in this case simulated drawdowns are expected to decline and the maximum drawdowns recorded are 19 and 10 m through the next 20 years in the first and second productive zones respectively to mitigate the groundwater level deterioration the study suggests the reduction of pumping rates by about 20 by applying the recent irrigation techniques besides reclamation of about 11 000 feddan 4620 hectare to accommodate the increasing population and need of cultivation lands in the oasis in the next 20 years according to plan the simulated groundwater levels are expected to decrease by about 20 3 and 12 m at the end of the simulation period year 2033 this means that the rate of head decline is lowered to about 1 m year for the first productive zone and 0 6 m year for the second productive zone with the increase in the cultivation area by about 16 therefore the study recommends the fourth plan to manage the groundwater resource in farafra oasis

the relation of mesozoic cainozoic volcanism to tectonics in the afro arabian dome abstract the mesozoic cainozoic volcanism of ne africa and arabia is described in terms of four major magmatic provinces these are dominantly basaltic and vary in general composition from strongly alkaline to tholeiitic with some overlap between provinces most but not all of this volcanism has taken place within the confines of the afro arabian dome and its attendant rifts but the magmatism is not explicable in terms of a mantle plume beneath afar because such a plume explains neither the spatial nor temporal distribution of volcanicity instead i propose a multi stage model in which basaltic magmas of transitional composition were generated in paleogene time as a response to regional extension with subsequent evolution of the red sea and gulf of aden into proto oceans these magmas took on the character of oceanic tholeiites in the neogene alkalic activity related to epeirogenic doming was superimposed on this extension related volcanism a third independent magma generating mechanism appears to have operated nw of the afro arabian dome where small volumes of alkaline basalt magma have been erupted intermittently since early in the cretaceous this activity may result from periodic tapping of deep asthenosphere by tensile fracturing and upwarping of the northward drifting african plate

pathline calibrated groundwater flow models of nile valley aquifers esna upper egypt abstract strongly concentrated agriculture along the river nile in egypt combined with hydrologic changes related to the construction of the aswan high dam in the 1970 s has led to increasing salinization and waterlogging of agricultural areas successful control and remediation of these problems requires accurate understanding of the shallow quaternary aquifers within the nile valley while extensive conceptual models have been developed by the egyptian rigw published numerical models have yet to incorporate all features of the conceptual model in particular marine affinity of some shallow groundwaters within the valley cl as the predominant anion indicates significant leakage from deeper cretaceous aquifers into the shallow quaternary aquifers a feature that is not present in current models in this study groundwater profile modeling incorporating the bedrock leakage demonstrates that its shallow appearance requires hydraulic separation of surficial from deep recharged zones of the quaternary aquifer this separation occurs near the boundary between reclaimed and traditional agricultural lands which is also the primary site of waterlogging apparently excessive recharge presumed to occur beneath the reclaimed lands does not penetrate deeply and therefore might be easily remediated with shallow drains profound similarities exist between the nile valley salinization cases and the occurrence of shallow nuisance water in desert southwestern u s cities e g las vegas the u s experience with this problem may provide useful guidance in addressing nile valley salinization and waterlogging issues in the future in general irrigation related recharge from the reclaimed lands in the nile valley may have a much more localized impact on traditional lands than previously thought

groundwater potential for irrigation in the east oweinat area western desert egypt the nubia sandstone aquifer system is one of the most extensive groundwater systems in north africa covering an area of about 2 000 000 km2 including parts of egypt libya sudan and chad in the western desert of egypt the nubian formation has a thermal gradient of 1 1 5 c 100 m 1 with the exception of the east oweinat area located in the southern part of the western desert this is the only part of this huge system where ground water occurs under unconfmed conditions in an area where the nubian sandstone crops out and is underlain by shallow basement rocks in this area groundwater has no thermal characteristics the aquifer system in the east oweinat area attains a relatively high hydraulic conductivity the direction of groundwater flow is generally northeastwards but is distorted at faults and fracture zones chemical analyses of groundwater in the area indicate a low salt content and suitability for irrigation purposes as the estimated recharge to the area is low compared with the foreseen irrigation water requirement the development of groundwater in the east oweinat should be based on groundwater mining although the evaluation of the groundwater resources in east oweinat has indicated that groundwater can be extracted at a rate of 4 7 106 m3 d 1 the long term economics of extraction that can sustain large scale development projects has to be assessed

shear wave attenuation and dispersion in melt bearing olivine polycrystals 2 microstructural interpretation and seismological implications 1 the torsional forced oscillation tests of melt bearing olivine aggregates reported by jackson et al 2004 consistently show a peak in attenuation that is absent from melt free aggregates tested under similar conditions and grain sizes characterization by sem shows that the melt resides in triple junction tubules and larger pockets as previously described tem imaging and eds analysis reveals that olivine olivine grain boundaries are characterized by a region 1 nm wide which is structurally and chemically distinct from olivine grain interiors from the possible mechanisms that can produce an anelastic attenuation peak melt squirt can be eliminated for our samples and experimental conditions we attribute the observed attenuation peak to elastically accommodated grain boundary sliding requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt while the nanometer scale grain boundary structure in the melt bearing aggregates is essentially the same as for melt free aggregates studied previously elastically accommodated sliding in the latter is apparently inhibited by tight three grain edge intersections the exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle

thermal history of the eastern gulf of suez ii reconstruction from apatite fission track and k feldspar measurements abstract apatite fission track aft measurements from miocene graben fill sediments of the eastern gulf of suez in three deep boreholes belayim 113 m 2 6 ras garra m 1 and alma 2 yield a wide range of ages from 125 to 320 ma and mean track lengths 10 1 12 7 m this range is similar to that recorded from precambrian crystalline basement flanking the graben which is the major source for the rift fill since the aft ages exceed the age of the host strata which are presently at their highest post depositional temperatures the rift related thermal regime is one of only moderate heating downhole aft data in the belayim 113 m 2 6 borehole attest to a higher rift temperature than in the ras garra m 1 borehole some 85 km to the south where little or no thermal overprinting is evident these findings are consistent with previously reported bottom hole temperatures and vitrinite reflectance data in the study area despite the higher syn rift thermal regime indicated for belayim 113 m 2 6 apatite provenance ages in ras garra m 1 are considerably younger thus the aft data in the ras garra m 1 area do not record significant rift related thermal effects but rather they mainly retain a pre rift provenance signature which reflects the order and depth of erosion at the uplifted flanks the younger aft ages in ras garra m 1 despite the weaker rift related thermal effect suggest a deeper level proportional to an additional 5 10 c of exhumation of the uplifted crystalline basement southwards along the eastern rift flank by mid miocene time this result is consistent with earlier findings which indicate both increased extension and heat flow southwards in the gulf of suez and earlier exhumation of the rift flank immediately preceding extension and opening of the gulf of suez the most deeply exhumed basement rocks presently exposed on the rift flanks were heated to temperatures 110 c total annealing of apatite but 40 ar 39 ar data and non resetting of zircon ft clocks in sinai 40 ar 39 ar data from granite penetrated in graben drilling at 3 89 km further corroborates pre rift palaeotemperatures

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

systematic distribution of incompatible elements in mantle peridotite importance of intra and inter granular melt like components in this paper we examine the distribution of incompatible elements in earth s mantle based on data reported for 20 mantle xenoliths collected from 5 localities worldwide a structural model combined with an element partitioning model forms the basis for our analyses the former separates a bulk peridotite into mineral crystal lattices interfaces grain and interphase boundaries and intra and inter granular inclusions as sites for incompatible elements the latter relates the distribution of elements among these sites based on lattice strain theory by treating both intra and inter granular inclusions as a melt like phase the combined models successfully reproduce the relative concentrations of incompatible elements among minerals clean rock reconstituted from mineral compositions and mineral mode and whole rock the analyses reveal common signatures in the rocks 1 incompatible elements in the crystal lattices of olivine orthopyroxene and clinopyroxene achieved chemical equilibrium 2 olivine orthopyroxene and clinopyroxene grains contain similar amounts of an intra granular melt like component possibly in the form of sub micron inclusions with weight volume fractions between 5 10 5 and 1 10 2 3 all rocks contain an inter granular melt like component with a fraction between 10 4 and 10 2 well above the amount expected to be stored along interfaces 4 fractions of the inter and intra granular components are positively correlated indicating that they were originated from the same process 5 the inter and intra granular melt like phases are chemically equilibrated with other structural components based on plausible upwelling rates for mantle xenoliths it is unlikely that the melt like component formed during ascent instead its ubiquitous appearance its invisibility to optical microscopy and its absorption of the incompatible elements in a manner similar to a melt phase even at sub solidus condition all might be explained by the presence of amorphous silica precipitates such as those observed previously in naturally occurring and experimentally annealed mantle composites from the mineral mode grain size and mineral plus whole rock concentrations of incompatible elements we can ascribe the chemical signatures of xenoliths to achievement of chemical equilibrium at mantle conditions rather than to a consequence of some disequilibrium metasomatic effect as has been done previously although it should be tested by additional analytical studies our model will make it possible to determine whether or not a rock is chemically equilibrated in terms of the distribution of incompatible elements or if a metasomatic disequilibrium event is required

an integrated approach for identification of potential aquifer zones in structurally controlled terrain wadi qena basin egypt abstract wadi qena basin represents one of the most promising regions for future development in egypt fresh water supplies are crucial for such plans we provide an integrated remote sensing landsat aster dem geoeye 1 geophysical aeromagnetic isotopic 18 o 2 h field stratigraphic and structural interpretation and geochemical major dissolved ions approach to delineate zones of potential groundwater resources in wadi qena basin four water bearing horizons were sampled fractured crystalline aquifer nubian aquifer system nas post nubian aquifer system pnas and the quaternary aquifer findings include 1 spatial analysis of remote sensing data in a gis environment indicates extensive structural deformation by dextral faults trending ne sw i e qena safaga shear zone qs and sinistral faults trending nw se i e najd shear zone and sufficient surface water supply from the east through wadi fattera sub basin 2 analysis of geophysical data indicates that these faults control the water bearing horizons in the subsurface 3 isotopic analysis reveals four isotopic groups including two end members one mixed group and one mixed and evaporated group group i highly depleted fossil nubian waters range 18 o from 6 39 to 6 74 and 2 h from 48 21 to 52 46 group ii modern waters in fractured basement range 18 o from 1 41 to 1 51 and 2 h from 5 46 to 6 04 group iii a mixed cluster between nas and modern waters range 18 o from 4 82 to 5 05 and 2 h from 33 28 to 38 54 and group iv samples which have both mixing between the nubian and meteoric waters and also have a considerable deviation from the global meteoric water line gmwl range 18 o from 0 58 to 4 69 and 2 h from 19 59 to 38 68 4 samples with a mixed isotopic signature in group iii which tap the nas and are located along the main channel of wadi fattera area 3600 km 2 provide evidence for modern recharge along surface exposures of the nas and enhanced infiltration along deep seated faults 5 samples with a mixed isotopic signature in group iv which tap the quaternary and pnas aquifers and are located along deep seated faults provide evidence of artesian upward leakage from the deep nas into the shallower quaternary and pnas aquifers the present study improves our understanding of the role of structural control and modern recharge in exploration for aquifer potential in arid environments

conceptualization of the hydrogeological system of some sedimentary aquifers in savelugu nanton and surrounding areas northern ghana groundwater resources play a pivotal role in the rural water delivery system in ghana the hydrogeological system of middle voltaian terrain was simulated using available data on hydraulic heads and boundary conditions the objective was to characterize the general groundwater flow pattern and provide local estimates of the distribution of hydraulic conductivity and recharge fields the results suggest a predominant ne sw flow direction which ties in with the general regional structural trend and indicates that the hydrogeological conditions of the rocks are controlled by structural entities created in the wake of fracturing and or weathering of the rocks whose primary permeabilities are considerably reduced because of high compaction and low grade metamorphism calibrated hydraulic conductivities range between 1 90 and 10 81 m d the spatial distribution appears to reflect the intensity of fracturing and or weathering of the rock and the proportion of the clay fraction of the weathered zone vertical groundwater recharge has been estimated to range between 0 3 and 4 1 of the annual rainfall this recharge rate is quite low and reflects the imperviousness of the thick overburden because of high clay content in some places and high compaction in others despite this apparently low recharge rate groundwater resources potential in the area appear to be high and increased abstraction from existing abstraction wells by up to 50 does not appear to register significant effects on groundwater budgets at the simulated recharge rates this suggests that the well yields are much lower than the potential of the aquifer system the apparently low yields might be associated with poor well development and the choice of inappropriate well completion materials this study recommends a monitoring system to be developed for a much more regional groundwater flow simulation under transient conditions copyright 2012 john wiley sons ltd

the gutenberg discontinuity melt at the lithosphere asthenosphere boundary the lithosphere asthenosphere boundary lab beneath ocean basins separates the upper thermal boundary layer of rigid conductively cooling plates from the underlying ductile convecting mantle the origin of a seismic discontinuity associated with this interface known as the gutenberg discontinuity g remains enigmatic high frequency ss precursors sampling below the pacific plate intermittently detect the g as a sharp negative velocity contrast at 40 to 75 kilometer depth these observations lie near the depth of the lab in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base i propose that the g reflectivity is regionally enhanced by dynamical processes that produce melt including hot mantle upwellings small scale convection and fluid release during subduction