aeolian dune migration along the ceara coast north eastern brazil aeolian dune dimensions and migration rates are analysed along the ceara coast north east brazil dunes that are currently mobile along the ceara coast are composed of barchans and sand sheets the results show that barchans maintain an equilibrium form which can be characterized by values of dimensionless shape parameters h w and w l where h is the dune height w is the wing to wing width and l is the dune length dunes are highly mobile with average migration rates of 17 5 m year 1 for barchans and 10 m year 1 for sand sheets the calculated migration rates were found to depend strongly on dune dimensions for both barchans and sand sheets i e the larger the dune is the lower the migration rate will be this size dependence was associated with the existence of a representative common transport rate along the dune fields which induces a different dune migration rate dependent on dune size finally from the observed dune evolution an aggregated scale aeolian sediment transport was inferred this bulk transport rate of the order of 90 100 m3 m 1 year 1 is only valid for a timescale of years to decades which is the timescale used in dune evolution analysis

natural electromagnetism in the rhine graben summary together with experiments by differential geomagnetic soundings magnetotelluric soundings have been performed in the rhine graben the results of the former have been presented and discussed before we try to show that it is not necessary to call upon the notion of induction in surrounding areas to justify the characteristics of the observed anomalous magnetic fields theoretical calculations of these fields on structures defined by the interpretation of magnetotelluric sounds or by geological and other geophysical data agree very well with experimental data we conclude that magnetotelluric experiments and interpretations are valid as the rhine graben is a well known region allowing us to verify our interpretation

fluid evolution and graphite genesis in the deep continental crust metamorphism in deep continental crust is often accompanied by carbon rich h2o poor fluids1 5 and is characterized by the development of granulite facies mineral assemblages3 5 graphite a minor phase in many high grade metamorphic rocks6 17 has also been recognized in recent xenoliths of deep crustal material18 demonstrating its presence in modern deep continental crust although graphite is most common in biotite schists it has also been recorded in pyroxene gneisses two feldspar quartz pegmatites marbles and quartzo feldspathic gneisses11 15 19 the presence of supracrustal sequences in granulite facies regions6 19 suggests that the latter developed through progressive metamorphism and dehydration of shallow level crustal material much of this material must have experienced a metamorphic recrystallization in the amphibolite facies accompanied by the development of hornblende although graphite in the high grade rock may develop from oxidation of organic material this may not be the most common means for graphite genesis in much of west greenland14 and in southern norway12 the low grade hornblende bearing material does not contain significant quantities of organic compounds although graphite is found in higher grade metamorphic equivalents this strongly suggests that introducing co2 into deep crustal material leads to fluid evolution and graphite formation i outline here the evolution of fluid composition in the c o h system which is in equilibrium with common crustal mineral assemblages thermodynamic data for co2 h2o mixtures20 and mineral components21 are used to model the chemical changes in fluid composition which accompany introduction of co2 into various types of rock the results demonstrate that graphite genesis is a direct consequence of deep crustal metamorphism in the presence of a co2 rich fluid phase in addition the calculations demonstrate that the deep continental crust will not be a reservoir for large volumes of methane rich gas

residual colluvio aeolian aprons in the negev highlands israel as palaeo climatic indicator abstract slope features treated previously mainly pedogenetically and described as alluvial terraces or as a catenary compound of stony loessial sierozem are shown to be remains of former colluvial loessial aprons the loess was deposited in the central negev during the uppermost pleistocene by dust laden rainstorms which triggered debris flows under conditions twice as humid as today during the holocene the aprons were selectively eroded by slope runoff resulting in a wide exposure of the basal slope truncation of the wadi fill and the final formation of the present day patchy slope morphology the dust laden rainy regime fits the last more humid period i e 70 000 10 000 yr b p

conductivity and fluids in the oceanic upper mantle abstract three deep electromagnetic soundings in the north pacific ocean were re examined our inversion of existing data showed the presence of conductors below the three sites in very good agreement with previous results we showed that h 2 o rich fluids as well as partial melting account for the conductivities observed this led us to determine temperatures ranging firstly from 1050 to 1150 c when only h 2 o rich fluids were present and secondly from 1300 to 1350 c for partial melting only we suggest that the upper mantle below the two oldest sites 30 and 72 ma is characterized by about 1 of partial melting in the presence of about 1 of h 2 o rich fluids below the third site that nearest to the ridge zone 1 ma the degree of partial melting might reach 10 the presence of 1 of h 2 o rich fluids would reduce the effect of the asthenospheric outflows on the lithospheric magmatism this agrees with the principle of thermodynamic decoupling of the asthenosphere and lithosphere the weakening effect of fluids particularly water on upper mantle rheology is also discussed we suggest that the asthenosphere and lithosphere might be decoupled mechanically which if true would affect small scale convection below the oceanic lithosphere

aeolian sand drift from the intertidal zone on a temperate beach potential and actual rates this paper compares the amount of sand that is potentially moved into a coastal dune system from a beach using estimates based solely on the wind force with actual measured amounts the actual sand inputs are determined by measuring sand level changes each month over a period of eighteen months along a 1760 m section of foredune on braunton burrows southwest england the calculation procedure used to estimate the potential rates of sand drift is based on the capacity of the wind force to transport sand using a published transport equation white 1979 the results show that the actual movement rates are markedly smaller than the potential rates for all months except in the summer meteorological variables and the watertable level in the adjacent dune system are used in an empirically derived regression equation in an attempt to explain the large discrepancy between the potential and actual movement rates the resulting corrected potential movement rates are a closer estimate to the actual rates but there remains much unexplained variance

magnetovariational study over a seismically active area in the deccan trap province of western india this paper presents the results of a magnetovariational study undertaken over a basalt covered region of western india which has been experiencing a swarm of seismic activity since early 1986 the most striking feature of the transient variations is the anomalous enhancement of the east west y component at stations bounding the seismically active belt from the nature of the horizontal transfer functions and the anomalous induction arrows it is inferred that this behaviour is related to the presence of a resistive body embedded in a north south oriented conductive belt consistent with an evolutionary model of deccan volcanism and other geophysical data it is suggested that the central resistive block coincident with the concentrated belt of earthquake epicentres represents either a dormant volcanic plug or a plutonic body rooted in upwarped conducting mantle to account for the phase relationship between the anomalous y variations and the normal north south x variations it is proposed that the upwarped asthenosphere with its associated high geothermal anomaly acts as a conducting path to channel induced current from the region of the cambay triple junction located towards the northwest of the array the existence of the strong induced currents in the region of the triple junction conforms with the induction arrow pattern observed at the array stations

magnetotelluric study of the teide tenerife and timanfaya tanzarote volcanic areas the application of a magnetotelluric surveying method developed specifically for detection of strong conductivity contrasts has permitted interpretations of the deep structure of two important volcanic formations in the canary archipelago the timanfaya lanzarote and teide tenerife volcanoes beneath the timanfaya volcano a highly conductive body p 1 m has been detected which is interpreted as a shallow magma chamber associated with the eruption which took place between the years 1730 and 1736 the core of this intrusion still largely molten would be located at approximately 4 km depth and would still maintain temperatures in excess of 900 c in the telde caldera the asymmetry of the apparent resistivity components indicates a double depression furthermore a highly conductive level encountered in the centre of the eastern part of the volcanic complex has been attributed to the persistance of relatively high temperatures in the root of the last salic dome extruded inside the caldera in both zones the deep limiting horizon is a highly conductive layer corresponding to the mantle 15 km in lanzarote and 13 km in tenerife whereas other more superficial discontinuities have been related to local structures

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

lithospheric structure of an archean craton and adjacent mobile belt revealed from 2 d and 3 d inversion of magnetotelluric data example from southern congo craton in northern namibia 1 archean cratons and the stitching proterozoic orogenic belts on their flanks form an integral part of the southern africa tectonic landscape of these virtually nothing is known of the position and thickness of the southern boundary of the composite congo craton and the neoproterozoic pan african orogenic belt due to thick sedimentary cover we present the first lithospheric scale geophysical study of that cryptic boundary and define its geometry at depth our results are derived from two dimensional 2 d and three dimensional 3 d inversion of magnetotelluric data acquired along four semiparallel profiles crossing the kalahari craton across the damara ghanzi chobe belts dgc and extending into the congo craton two dimensional and three dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger dgc orogen to the older adjacent cratons we find damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent congo craton the congo craton is characterized by very thick to depths of 250 km and resistive i e cold lithosphere resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during pan african magmatism graphite bearing calcite marbles and sulfides are widespread in the damara belt and account for the high crustal conductivity in the central zone the resistivity models provide new constraints on the southern extent of the greater congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south

physical changes in the lithosphere associated with thermal relaxation after rifting we define the terms modem rift and paleorift with respect to the thermal state of the lithosphere the former being rifted lithosphere not in thermal equilibrium and the latter being thermally stabilized rifted lithosphere a comparison of the physical characteristics of modern rifts with paleorifts shows that they share many of the same geomorphic structural and magmatic features but have important differences in their geophysical signatures using a simple model for the thermal relaxation of the lithosphere in the transition from modern rift to paleorift we show that differences in upper mantle seismic velocities and density surface heat flow thermal structure and curie point depth and magnetotelluric properties of the lithosphere between modern and paleorifts can be explained by lithospheric cooling cooling alone cannot explain the complex subsidence histories associated with and following rifting however nor the presence of high velocity crustal layers and sometimes thickened crust in paleorifts these features can be explained by consideration of lithospheric flexural strength and magmatic thickening of the crust associated with rifting we speculate that the anomalous upper mantle beneath many modern rifts may be associated with the process of magmatic thickening of the rifted crust

magnetotelluric results opposing magma origin of crustal conductors in the rio grande rift although conductive intracrustal layers appear to be characteristic of continental rift zones they also have been detected in stable crustal environments the low resistivity layers are often explained by free water or crustal magma however alternative explanations are possible purely geometrical effects can be mistaken as conductive layers at depth in the interpretation of magnetotelluric or geomagnetic depth soundings certain minerals such as graphite and hydrated minerals e g serpentine may be important thermally activated processes associated with ductile flow mechanisms may also enhance conductivity two dimensional analysis of a portion of 25 new magnetotelluric stations in the central rio grande rift supports the presence of a conductive zone at 10 km depth or less however very significant differences are apparent in the crustal sections at two close localities mid crustal magma lenses well defined by seismic data cannot explain these differences in fact the crust is modeled to be more conductive by at least an order of magnitude where magma is not detected compared to where it occurs at shallow and intermediate levels only the possible effects of regional three dimensional current channeling could conceivably alter this conclusion to explain the unexpected result it is suggested that a conductive horizon occurs where an impermeable ductile cap traps pore fluids beneath the cap may be nearly an order of magnitude more conductive 100 s m than the dry brittle crust above the zone of trapped pore fluids much more conductive by more than an order of magnitude 10 m where active magma injection destroys the integrity of the ductile cap trapped fluids may escape and produce an overall decrease in conductivity the final electrical signature with this dynamic concept depends on the thermal gradient the relative impermeability of the cap the extent of pore fluids beneath and the amount and frequency of magma intrusion the temporal and spatial distribution of earthquake foci in our study area supports the existence of a ductile layer at about 10 km depth and the hypothesis of magma injection

why is the electrical resistivity around the ktb hole so low the low resistivity anomaly close to the ktb borehole coincides with both a self potential and a static magnetic anomaly if this coincidence is not accidental it may yield information about the conditions required for the existence of low resistivity anomalies in the deeper crust a possible answer to the question in the title of this paper is that the oxygen fugacity in the crust around the ktb hole must be low enough to stabilize graphite on a grain boundary scale this could partially explain the extremely low resistivities of the graphitized cataclastic zones the formation of magnetic pyrrhotite from 250 to 4000 m deep and the large self potential anomaly the latter requires a large vertical gradient of the oxygen fugacity and a continuous graphitic conductor through this gradient zone

variations of sea surface temperature wind stress and rainfall over the tropical atlantic and south america abstract empirical orthogonal functions e0fs and composite analyses are used to investigate the development of sea surface temperature sst anomaly patterns over the tropical atlantic the evolution of large scale rainfall anomaly patterns over the equatorial atlantic and south america are also investigated the eof analyses revealed that a pattern of anomalous sst and wind stress asymmetric relative to the equator is the dominant mode of interannual and longer variability over the tropical atlantic the most important findings of this study are as follows atmospheric circulation anomalies precede the development of basinwide anomalous sst patterns over the tropical atlantic anomalous sst originate off the african coast simultaneously with atmospheric circulation anomalies and expand westward afterward the time lag between wind stress relaxation strengthening and maximum sst warming cooling is about two months anomalous atmospheric circulation patterns over northern tropical atlantic are phase

magnetotelluric imaging of lower crustal melt and lithospheric hydration in the rocky mountain front transition zone colorado usa we present an electrical resistivity model of the crust and upper mantle from two dimensional 2d anisotropic inversion of magnetotelluric data collected along a 450 km transect of the rio grande rift southern rocky mountains and high plains in colorado usa our model provides a window into the modern day lithosphere beneath the rocky mountain front to depths in excess of 150 km two key features of the 2d resistivity model are 1 a broad zone 200 km wide of enhanced electrical conductivity 20 m in the mid to lower crust that is centered beneath the highest elevations of the southern rocky mountains and 2 hydrated lithospheric mantle beneath the great plains with water content in excess of 100 ppm we interpret the high conductivity region of the lower crust as a zone of partially molten basalt and associated deep crustal fluids that is the result of recent less than 10 ma tectonic activity in the region the recent supply of volatiles and or heat to the base of the crust in the late cenozoic implies that modern day tectonic activity in the western united states extends to at least the western margin of the great plains the transition from conductive to resistive upper mantle is caused by a gradient in lithospheric modification likely including hydration of nominally anhydrous minerals with maximum hydration occurring beneath the rocky mountain front this lithospheric hydration front has implications for the tectonic evolution of the continental interior and the mechanisms by which water infiltrates the lithosphere

identification of a magma chamber in the ghoubbet asal rift djibouti from a magnetotelluric experiment abstract a very broad band 10 3 to 10 4 hz magnetotelluric investigation of the axial zone of the ghoubbet asal rift djibouti has revealed a shallow 2 4 km magma chamber which can be mapped in some detail the suggested roof of the chamber is shallowest very close to the ardoukoba volcano which was built during the november 1978 rifting episode

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

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

the late pleistocene and holocene stratigraphy and paleolimnology of lakes kivu and tanganyika abstract evidence from diatoms chemistry and sedimentology shows that lakes kivu and tanganyika experienced late pleistocene low stands 86 to 300 m for kivu and 250 to 350 m for tanganyika both lakes had higher 2 10 fold sedimentation rates prior to 11 500 yr b p about 9900 yr b p upwelling became important in southern lake tanganyika which had been deepening for at least several millenia by 9400 yr b p kivu had become deep and dilute with overflow via the ruzizi river to lake tanganyika southern lake tanganyika became more stably stratified about 5200 yr b p volcanism and hydrothermal activity in kivu began about 5000 yr b p and the salinity reached 3 ppt and the lake became strongly stratified the ruzizi river outflow was interrupted from 3500 yr b p until recently the sedimentary record implies that sudden hydrothermal activity and sublacustrine volcanism in kivu has probably been the cause of its impoverished fauna these results show that high lake levels in the eastern rift of africa from 10 000 to 7000 yr b p were paralleled in the western rift given the great volumes of these lakes climate has not caused great variations in salinity the diatom floras however have been remarkably sensitive to changes in stratification and nutrient supply volcanism has been a major force in the limnological and biological evolution of lake kivu while both kivu and tanganyika have responded to changes in precipitation and wind stress

the significance of terrestrial electrical conductivity variations the degree to which different materials of the earth can conduct electricity varies enormously from the insulating qualities of cold quartz rich crys talline rock to the metallic behavior of the liquid core a well recognized physical property the specific resistance or resistivity expresses quanti tatively the resistance to current flow and the fact that it can be sensed through measurements on the earth s surface makes it an important geophysical parameter the history of the investigation of resistivity is similar to that of other geophysical quantities such as seismic velocity when it was recognized that the earth as a whole was in some degree a conductor the initial aim was to determine a global average value of resistivity this was followed by the search for a function describing the radial variation while the latter effort is by no means completed a great deal of recent work has been concerned with departures from the average behavior and that aspect forms the basis of this paper lateral variations in electrical resistivity may provide information on the composition and water content of the crust and on the temperature and degree of partial melting in zones of the mantle the success of any investigation depends on the adequacy of electromagnetic measurements at the earth s surface the ability to mathematically infer a distribution of resistivities on the basis of induction theory and by no means least the knowledge through laboratory mea surements of electrical properties of earth materials under different phys ical conditions recent progress in each of these aspects is considered first a word as to units resistivity is conveniently expressed in ohm meters qm the resistance of a meter cube of the material as an indication

crustal resistivity structure from magnetotelluric soundings in the colorado plateau basin and range provinces central and western arizona resistivity structure to about 25 km depth is defined from two dimensional modeling of 29 magnetotelluric mt soundings 0 002 5 hz that traverse 280 km of the southwestern colorado plateau transition zone and basin and range provinces in arizona from the surface to 5 km depth the mt model suggests structural relationships between low resistivity sedimentary and volcanic rocks 50 300 ohm m and high resistivity granitic and gneissic basement 500 9000 ohm m in the basin and range province the mt model and a seismic reflection section show a generally consistent distribution of supracrustal rocks that have relatively low to moderate resistivity mt and relatively strong locally coherent reflectivity the supracrustal zone defined by these physical properties is inferred to be composed of upper plate rocks above a middle tertiary detachment fault system which has been mapped in surrounding ranges some low angle fault zones inferred from seismic reflections to extend into high resistivity basement below the supracrustal rocks are not resolved by the mt model a low resistivity zone with a conductance of 500 s or more is modeled in the crust at an average depth of about 15 km in the basin and range province and transition zone and may deepen below the southwestern part of the colorado plateau in the basin and range province the top of the low resistivity may correspond to a reflective layer with a 6 s two way travel time this deep low resistivity zone might be caused by a small fraction of connected hydrous solutions or silicic melts

integrating magnetotelluric and seismic images of silicic magma systems a case study from the laguna del maule volcanic field central chile

late quaternary climatic reconstruction for the maghreb north africa abstract the paleoclimatic evolution of the maghreb since 40 000 yr b p is not yet known in great detail for that purpose data from more than 35 sites have been collected although only about 11014c dates and a few prehistoric remains can be used to correlate these data however the sites are well distributed over the maghreb and the convergent results from paleosoils paleolakes alluvian or eolian deposits pollen analysis etc induce one to draw up a certain pattern in climatic evolution for the period between 40 000 and 20 000 yr b p the data are still fragmentary but are concordant in indicating wetter environments generalized as far as the northern sahara the effect of dropping temperatures on the increasing water budget and soil moisture is not yet clear but rainfall could have been more regular and more abundant as a consequence of the shifting of middle latitude cyclone paths towards the south in both summer and winter the environmental indicators show that between about 20 and 18 000 yr b p rainfall progressively became more irregular however proofs of aridity in the south maghreb and the northern sahara is evident only for the period between 12 10 000 yr b p at that time the saharan high pressure belt was reinforced and well established at higher latitudes than at present this dry climate lasted more or less until the thermal optimum after 6000 yr b p there is some evidence that the climate became wetter again but the environmental conditions were rather different from those of before 20 000 yr b p for instance the middle holocene dark humic sois are the result of a pedogenesis different from that of the late pleistocene reddish paleosoils for the last two millennia it is not easy to distinguish between a natural trend towards aridification and desertification through man s intervention

climatic changes in the chalbi desert north kenya the chalbi flat located 60 km outside the rift to the east forms a moist salt clay encrusted playa which also functions as an inland sink for alluvial fans residual lacustrine beds including freshwater carbonates and travertines crop out along its higher periphery mammalian bones of lacustrine and terrestrial elements and k ar dating of the capping lava suggest a lake or a swamp environment 2 2 5 million years ago radiocarbon dating of melanoides tuberculata and planorbis and a sediment assemblage composed of clotted micrite calcareous molds root marks and stromatolites point to an additional early holocene chalbi lake or swamps 11000 9500 yr bp the data suggest wet environmental conditions outside the rift which is consistent with the occurrence of a wet period in the early to middle holocene in intertropical africa but out of phase with the climatic conditions in sinai and israel and with the high latitude maximum lake levels and glacier advance these relations have probably been caused by migration of climatic belts

evidence from rayleigh wave data for magma in an upper crustal dike in the albuquerque belen basin of the rio grande rift new mexico we have found evidence for a dikelike structure in the middle of the albuquerque belen basin of the rio grande rift from our interpretation of two station rayleigh wave amplitude and phase velocity data two dimensional finite element modeling suggests that the structure is about 200 m thick extends vertically from a depth of about 2 1 2 km to at least 20 km and contains material characterized by a shear wave velocity of about 1 2 km s simple physical arguments suggest that the length of the structure may be at least 28 km long nearby high heat flow values historical swarmlike seismicity recent 0 14 ma volcanism and an extensive sill like magma body at midcrustal depths all suggest that the structure is a magma filled dike source characteristics and seismometer response limit our reliable data to the frequency band of 0 08 hz to 0 250 hz thus we cannot tell whether the dike is solely an upper crustal feature or whether it extends through the crust and in to the upper mantle copyright american geophysical union 1987

the rio grande rift new electromagnetic constraints on the socorro magma body abstract the socorro magma body in central new mexico is a prime example of an intrusive magma body associated with continental extension in one of the major active rift zones of the world the rio grande rift to study this feature further in particular its relation to large scale features delineated elsewhere beneath the rift in 1988 brown university occupied a sequence of densely spaced 5 10 km wide band 10 4000 s remote referenced three component magnetic variation sites along an east west line which transected the southern margin of the magma body our profile was deployed so as to cross precisely above the center of what has been the most seismically active portion of the structure southwest of socorro complex magnetic transfer coefficients at up to 15 periods from 21 sites along our profile have been inverted to determine two dimensional conductivity models for vertical cross sections transverse to the strike of the rift two independent generalized inverse algorithms are used an overdetermined block inversion and a mixed determined flat continuous resistivity inversion both inversions require the presence of a conducting zone beneath the rift at intermediate depths 15 20 km in the crust this feature has properties which are similar to those of the intracrustal conductor seen elsewhere along the rift in addition to this conducting zone in the deeper crust the data require a more conductive core at shallow depth 10 15 km which appears to be associated with the base of a zone of increased microseismicity described by sanford and his colleagues this latter feature might be a zone of diffuse magmatic intrusions enhanced hydrothermal circulation or a combination of both finally our two inversions suggest that the resistivity structure beneath the western end of our profile which is physiographically adjacent to the colorado plateau has properties which are similar to those reported for interior regions of the plateau

magnetotelluric soundings and the geological structure and tectonics of the senegalo mauritanian basin in northern senegal west africa the results of the magnetotelluric soundings provide new insights on the geological structure and tectonics of the onshore senegalo mauritanian basin in northern senegal west africa electrical resistivities can be correlated with the major geological units in the upper 1000 m resistivity variations indicate a transition from freshwater saturated sediments near the surface to saline water saturated sediments above the basement the resistivity of the sediments is dominated by their porosity and the resistivity of the contained fluids the resistivity of the cretaceous sediments is unexpectedly low especially in the western part of the basin the top of the metamorphic granitic basement cannot be located on the west portion of the profile because of the feeble resistivity contrast between the impervious compacted aptian jurassic limestones and the metamorphic crystalline rocks the magnetotelluric estimates of the depth of the basement range from 600 m in the east to 4000 m in the west based on the magnetotelluric interpretation and other independent geophysical and geological information a cross section through the senegalo mauritanian basin along the magnetotelluric traverse is presented a zone of high electrical conductivity is observed in the crust at depths of 4 6 km on the eastern margin of the deep basin this conductive structure may be the extension of one further south that was revealed by an earlier investigation the entire conductive zone which runs north south between longitudes 15 and 16 w may be a major fracture zone related to the opening of the atlantic ocean however its origin remains to be established

a new regional electrical model for the southern section of the rio grande rift and the adjacent basin and range and great plains one of the earliest geophysical studies in the rio grande rift involved a geomagnetic induction profile across the southern rift by schmucker 1964 1970 the interpretation of these data and later studies by reitzel gough and porath clearly established the presence of an electrical conductivity anomaly along the axis of the rift extending from colorado to west texas this anomaly was originally thought to lie well within the mantle the earliest magnetotelluric mt studies in this region generally corroborated the magnetic variation models although the conducting mantle appeared to be somewhat shallower than previous geomagnetic studies had indicated on the other hand more recent mt interpretations differ substantially from the earlier work in that they suggest that the major anomalous features are likely to be in the crust rather than in the upper mantle our purpose here is to reconcile the interpretation of the magnetic variation and mt data bases in order to obtain a new preliminary regional model along a latitude of 32 n that fits both data sets applying constraints from mt interpretations we argue for the presence of an intracrustal layer at a depth of about 20 km having a conductance of about 2000 s allowing for a small portion of the z field to be of normal and external origin the magnetic variation data suggest that the conductor disappears or is greatly attenuated to either side of the rift but the location of its boundaries are not well resolved by the present data neither data set could clarify whether the conductor is underlain by normal mantle on the other hand neither the mt data nor the magnetic variation data strictly requires the presence of abnormal mantle material beneath the southeastern basin and range the southwestern great plains and the southern rio grande rift

on the electrical conductivity of the oceanic lithosphere abstract below the bottom of the sediments the oceanic lithosphere is difficult to sound electromagnetically because the resistivity is high compared to over and under lying structures there are three methods which can contribute information 1 1 magnetotelluric methods provide the impedance as a function of frequency for signals of sufficiently low frequency to penetrate through the ocean in the deep sea the upper frequency limit is a few cph 2 2 a quasi static electric field can be measured at the ocean floor the field is generated by charges built up by the motion of ocean water currents through the geomagnetic field by providing a current path below the ocean the lithospheric conductivity tends to reduce this field in order to interpret the measurements one needs to know the spatial character of the water motions 3 3 an artificial em source which transmits at frequencies near 1 hz can be used at the ocean floor energy propagation through the lithosphere has been detected on the ocean floor to horizontal distances of 18 km detection is aided by the very low noise level at the high frequencies concerned estimates of the typical conductivity distribution throughout the ocean crust are given

width height and slope relationships and aerodynamic maintenance of barchans the allometric relationship between the height of barchan dunes and horn to horn widths is re examined the addition of new data from barchans in the namib desert and elsewhere confirms the relationship that the height of a barchan is approximately one tenth of the width of the dune height and width of the dune are closely linked to the three dimensional morphology of the dune in particular the relatively constant mean angles of around 11 at which the side slopes of the dune are maintained we suggest that the 3 d barchan form is directly controlled by aerodynamic processes constant feedback between three dimensional stoss face windward shape of the dune slipface height and horn to horn width and shear stress produces an equilibrium dune form

aeolian transport of sediment on a beach during and after rainfall wildwood nj usa abstract a field investigation was conducted 9 march 1994 on a dissipative beach to compare wind characteristics and aeolian transport during and after a light rain winds blew alongshore during the monitoring period moisture content of surface sediment on the backbeach was above 7 during rain and 4 four hours after rainfall ceased trapping rates measured using a cylindrical vertical trap were 14 1 kg m 1 h 1 during rainfall and 140 2 kg m 1 h 1 for a period beginning 3 h 33 min after rainfall ceased results indicate that light rain resulting in bulk surface moisture values over 7 is not sufficient to eliminate transport but the rate of transport is lower than can occur on the beach three to four hours following rain the amount of sand trapped at wildwood during rainfall is relatively high when compared to many previous studies on beaches in the absence of rain the great length of the beach as a source area combined with parallel or oblique winds are significant in increasing rates of sediment transport

circulation mechanisms related to northeast brazil rainfall anomalies this study explores the role of the upper ocean for circulation anomalies in the overlying atmosphere with focus on the late boreal winter and extreme climatic events in northeast brazil data sources comprise rainfall records in the nordeste surface ship observations of the sea surface temperature sst pressure and wind fields coads period 1948 1990 satellite observations of tropical convection hrc 1971 1988 and upper air analyses of the european centre for medium range weather forecasts ecmwf 1980 1991 all for the tropical atlantic sector interhemispheric sst gradients are most strongly associated with north south contrasts in pressure and the meridional wind component whereas the inverse sst pressure relations in situ are less close enhanced northward temperature increase in the tropical atlantic is accompanied by steeper meridional pressure gradient and accelerated southerly wind component which is representative of a northward displaced intertropical convergence zone itcz with the latter in turn leading to drought in northeast brazil interhemispheric contrasts in the 850 1000 mbar layer mean temperature are similar to those of sst and account for most of the meridional surface pressure gradient during wet as compared to dry years in northeast brazil the tropical north atlantic is cooler and the south atlantic warmer and accordingly the lower tropospheric thickness is reduced to the north but inflated to the south of the equator resulting in increased reduced surface pressure over the north south atlantic as well as reduced southerly surface wind stronger subsidence over the outer tropics of the northern hemisphere and intensified ascending motion and convective activity over the nordeste related to a southward displaced near equatorial convection belt the surface and upper air evidence thus indicates the ways in which the interhemispheric sst gradients exert a hydrostatic control on the lower tropospheric thickness pattern and thus force the south north surface pressure gradients and the surface meridional wind component and hence modulate the latitude position of the itcz and nordeste rainfall

implications of magnetotelluric modeling for the deep crustal environment in the rio grande rift abstract block forward modeling and smooth backus gilbert linearized inversion have independently been used to two dimensionally model two east west magnetotelluric profiles in the central rio grande rift a crustal conductive zone of at least 1500 s has been detected at about 10 km depth along the northern line however along a line 40 km to the south where shallow active magma is inferred and microseismicity is highest no high conductivity zone is found the conductive zone beneath the northern line is hypothesized to occur where water is trapped beneath an undisturbed ductile cap under the southern line magma injection through the cap may release the water thereby resulting in a relatively more resistive crust with no conductive midcrustal layer the resulting paradox is that where there is active magma injection into the upper crust of the rio grande rift the crust is more resistive rather than more conductive

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

wind blown sand on beaches an evaluation of models five models for predicting rates of aeolian sand transport were evaluated using empirical data obtained from field experiments conducted in april 1994 at a beach on inch spit co kerry republic of ireland measurements were made of vertical wind profiles to derive shear velocity estimates beach slope and rates of sand transport sediment samples were taken to assess characteristics of grain size and surface moisture content estimates of threshold shear velocity were derived using grain size data after parsing the field data on the basis of the quality of shear velocity estimation and the occurrence of blowing sand 51 data sets describing rates of sand transport and environmental conditions were retained mean grain diameter was 0 17 mm surface slopes ranged from 0 02 on the foreshore to about 0 11 near the dune toe mean shear velocities ranged from 0 23 m s 1 just above the observed transport threshold to 0 65 m s 1 rates of transport ranged from 0 02 kg m 1 h 1 to more than 80 kg m 1 h 1 these data were used as input to the models of bagnold bagnold r a 1936 the movement of desert sand proc r soc london a157 594 620 kawamura kawamura r 1951 study of sand movement by wind translated 1965 as university of california hydraulics engineering laboratory report hel 2 8 berkeley zingg zingg a w 1953 wind tunnel studies of the movement of sedimentary material proc 5th hydraulics conf bull 34 iowa city inst of hydraulics pp 111 135 kadib kadib a a 1965 a function for sand movement by wind university of california hydraulics engineering laboratory report hel 2 8 berkeley and lettau and lettau lettau k and lettau h 1977 experimental and micrometeorological field studies of dune migration in k lettau and h lettau eds exploring the world s driest climate university of wisconsin madison ies report 101 pp 110 147 correction factors to adjust predictions of the rate of transport to account for the effects of slope and moisture content were calculated using the models of bagnold bagnold r a 1973 the nature of saltation and bed load transport in water proc r soc london ser a 332 473 504 and belly belly p y 1964 sand movement by wind u s army corps eng cerc tech mem 1 washington d c 38 pp respectively none of the models was able to produce a strong correspondence between measured and predicted rates of transport best results were obtained using the bagnold and zingg models and the kadib model was the least viable of this group the influence of sediment moisture content appeared to be the critical factor in degrading model viability overall none of the models is adequate for general applications to coastal aeolian environments where moisture content complications tend to override the predictive competence of the simple transport formulations c 1998 elsevier science b v