crustal thickness and vp vs ratio in nw namibia from receiver functions evidence for magmatic underplating due to mantle plume crust interaction a seismological network was operated at the junction of the aseismic walvis ridge with the northwestern namibian coast we mapped crustal thickness and bulk vp vs ratio by the h k analysis of receiver functions in the damara belt the crustal thickness is 35 km with a vp vs ratio of 1 75 the crust is 30 km thick at the coast in the kaoko belt strong variations in crustal thickness and vp vs ratios are found at the landfall of the walvis ridge here and at 150 km northeast of the coast the crustal thickness increases dramatically reaching 44 km and the vp vs ratios are extremely high 1 89 these anomalies are interpreted as magmatic underplating produced by the mantle plume during the breakup of gondwana the area affected by the plume is smaller than 300 km in diameter possibly ruling out the existence of a large plume head under the continent during the breakup

a late neoproterozoic paleomagnetic pole for the congo craton tectonic setting paleomagnetism and geochronology of the nola dike swarm central african republic abstract new structural geochronological and paleomagnetic data were obtained on dolerite dikes of the nola region central african republic at the northern border of the congo craton in this region metavolcanic successions were thrust southward onto the craton during the panafrican orogenic events our structural data reveal at least two structural klippes south of the present day limits of the panafrican nappe suggesting that it has once covered the whole nola region promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes paleomagnetic measurements revealed a stable dual polarity low inclination magnetization component in nine dikes 47 samples carried by pyrrhotite and magnetite this component corresponds to a paleopole at 304 8 e and 61 8 s d p 5 4 d m 10 7 graded at q 6 both metamorphism and magnetic resetting were dated by the 40 ar 39 ar method on amphibole grains separated from the dikes at 571 6 ma the nola pole is the first well dated paleomagnetic pole for the congo craton between 580 and 550 ma it marks a sudden change in direction of the congo craton apparent polar wander path at the waning stages of the panafrican orogenic events

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

composition and tectonic evolution of the chinese continental crust constrained by poisson s ratio article i nfo we have measured both p and s wave velocities vp and vs and poisson s ratios of 60 typical ultrahigh pressure uhp metamorphic rock samples from the chinese continental scientific drilling ccsd main and pre pilot holes and surface outcrops in the sulu dabie orogenic belt at hydrostatic confining pressures up to 850 mpa the experimental results together with those compiled in handbook of seismic properties of minerals rocks and ores ji s c wang q xia b 2002 handbook of seismic properties of minerals rocks and ores polytechnic international press montreal 630 pp reveal that except for monomineralic rocks such as quartzite serpentinite anorthosite limestone and marble the rest of the rock types have poisson s ratios falling along an upward convex curve determined from the correlations between elastic moduli and density poisson s ratios increase with density as the lithology changes from granite felsic gneiss and schist through diorite syenite intermediate gneiss and metasediment to gabbro diabase amphibolite and mafic gneiss and then decrease as the rocks become ultramafic in composition eclogite has a higher density but a lower poisson s ratio than peridotite the results were applied to constrain the crustal composition and tectonic evolution of the chinese continental crust based on crustal thickness h and poisson s ratio from 248 broadband seismic stations measured using teleseismic receiver function techniques the north china yangtze south china and northeast china blocks and songpan ganzi terrane are dominated by low b0 26 and moderate 0 26 b0 28 values n70 suggesting the dominance of felsic composition in the crust the lhasa terrane qiangtang terrane and indochina block are characterized by high proportions 33 42 of measurements with very high values 0 30 and h is found for the south china block northeast block lhasa block qiangtang terrane and indochina block indicating either tectonic thickening of the felsic upper and middle crust by folding and thrusting or the removal of mafic layers from the lower crust into the upper mantle by delamination

major and trace element geochemistry of the archaean ntem plutonic complex south cameroon petrogenesis and crustal evolution the ntem complex crops out in south cameroon and belongs to the congo craton it corresponds to a 2 8 ga basement which in the northern part is made up of plutonic rocks 1 charnockites and 2 soo tonalites both have similar compositions and define a typical tonalite trondhjemite and granodiorite ttg suite the study of isotopes isr major elements and ree allows us to propose a petrogenetic model that may summarized as follows 1 1 partial melting of the mantle gave rise to tholeiites 2 2 tholeiites transformed into garnet bearing amphibolites underwent partial fusion thus generating the parental magma of the ttg suite 3 3 fractional crystallization mainly of hornblende and plagioclase produced the differentiated ttg suite this scheme is typical of archaean ttg suites and is supposed to have occurred in a subduction like geodynamic environment where because of the high archaean geothermal gradients the subducted oceanic crust underwent fusion before dehydration operated it is also possible to show that the ttg suite was not emplaced during the granulite facies metamorphism but that this occurred later affecting only some parts of the basement consequently the ntem complex appears to be an archaean high grade terrane where the granulitic assemblages may be early proterozoic in age characteristic that seems to be more widespread among archaean high grade terranes

tomographic images and 3d earthquake locations of the seismic swarm preceding the 2001 mt etna eruption evidence for a dyke intrusion 1 on july 12 2001 mt etna experienced a sudden increase of seismic activity heralding one of the most intense eruptions of the past 30 years between july 12 and july 18 when the eruption started thousands of small magnitude earthquakes occurred and were recorded by a dense seismic network run by the istituto nazionale di geofisica e vulcanologia sezione di catania ingv ct hypocentral depths of earthquakes were very shallow mostly located above 3 km b s l and clustered near the summit area the high quality seismic dataset gives us the unique opportunity to study the process of magma migration before the eruption in this study we present the three dimensional earthquake locations and the velocity structure obtained by a tomographic inversion the shallowness of seismicity allowed us to enhance the details of the structure beneath the summit craters in a volume poorly defined by previous tomographic studies the presence of a high vp body previously observed at mt etna is confirmed at shallow depth beneath the southeastern part of the summit area the earthquakes preceding the eruption onset concentrated at its western border a low vp vs anomaly is found at 0 1 km depth just at the top of the volume where the magma intruded before the eruption this anomalous zone can be considered as molten material wealthy in gas the relocated seismicity occurs in a cylinder below the vents activated along the fracture system and exhibited an upward migration until the eruption all these results show evidence for the emplacement of a near vertical dyke striking about n s and a few kilometres south of the summit craters

volatiles in a peralkaline system abiogenic hydrocarbons and f cl br systematics in the naujaite of the ilimaussaq intrusion south greenland agpaitic rocks comprise most of the exposed part of the 1 16 ga old 8 17 km large and about 1700 m thick ilimaussaq intrusion in south greenland within these more than 600 m thick sequence of sodalite rich naujaites mainly sodalite arfvedsonite alkali feldspar nepheline eudialyte aenigmatite are interpreted as a sodalite flotation cumulate sodalites show two to three different zones in cathodoluminescence cl and at least two zones in thin sections the cl zones can be related to chemical differences detectable by electron microprobe whereas relations with optical zonations are less obvious compositional trends in sodalite reflect trends in the evolution of volatile contents in the melt the sodalite at ilimaussaq is almost free of ca and closely corresponds to the pure na cl sodalite endmember with about 7 wt of cl s contents reach up to 0 9 wt cl br ratios range from 500 to 1700 raman spectroscopy shows that s is present as so4 2 in sodalite although sphalerite zns is a stable phase in naujaites peralkalinity and fo2 conditions allow s2 and so4 2 to be present contemporaneously the whole naujaite sequence is divided into two parts an upper part with low homogeneous s contents and cl br ratios in the sodalite cores and a lower part with strongly variable and higher s contents and with cl br ratios which are decreasing downwards the details of the s content and the cl br ratio evolution show that sodalite strongly influences the halogen contents of the melt by scavenging cl and br the naujaites were formed from a highly reduced halogen rich magma in equilibrium with magmatic methane at about 800 c which upon ascent cooling and fractionation exsolved an aqueous fluid phase both fluids were trapped in separate inclusions indicating their immiscibility micrometer sized aegirine crystals and primary hydrocarbon bearing inclusions are abundant in the crystal cores the inclusions were trapped at pressures up to 4 kbar although the emplacement pressure of the intrusion is about 1 kbar this indicates growth of the sodalite during melt ascent and a very effective mechanism of trace element scavenging during sodalite growth sodalite rims are devoid of aegirine or primary hydrocarbon inclusions and probably reflect the emplacement stage

origin of gassmann s equations gassmann s relations are receiving more attention as seismic data are increasingly used for reservoir monitoring correct interpretation of underground fluid migration from seismic data requires a quantitative understanding of the relationships among the velocity data and fluid properties in the form of fluid substitution formulas and these formulas are very commonly based on gassmann s equations nevertheless confusion persists about the basic assumptions and the derivation of gassmann s 1951 well known equation in poroelasticity relating dry or drained bulk elastic constants to those for fluid saturated and undrained conditions it is frequently stated for example but quite incorrect to say that gassmann assumes the shear modulus is constant i e mechanically independent of the presence of the saturating fluid this note clarifies the situation by presenting an unusually brief derivation of gassmann s relations that emphasizes the true origin of the constant shear modulus result while also cla

s wave velocities and anisotropy of typical rocks from yunkai metamorphic complex and constraints on the composition of the crust beneath southern china abstract in order to constrain the interpretation of seismic data from receiver functions and deep profiles of the crust beneath southern china cathaysia and yangtze blocks we have measured s wave velocities v s and splitting as a function of hydrostatic confining pressure up to 650 mpa for 22 representative samples i e granite diorite felsic gneiss and mylonite amphibolite schist and marble from the yunkai metamorphic complex china that represent the crystalline basement beneath the region the experimental data were combined with electron backscattering diffraction ebsd analysis of rock forming minerals to constrain variations of v p v s ratios and understand the origin of seismic anisotropy the crusts beneath the yangtze and cathaysia blocks have different average thicknesses h 35 4 6 3 km and 29 8 1 8 km respectively but display almost the same v p v s values 1 73 0 08 and 1 74 0 04 respectively these ratios correspond to an average of bimodally distributed granitic and gabbroic lithologies which are dominant respectively in the upper and lower crusts instead a homogeneous andesitic composition of the overall crust positive and negative correlations between h and v p v s occur in west and east parts of southern china respectively the negative correlation indicates basaltic underplating from a partially molten mantle wedge above the subducting pacific plate into the southern china crust whereas the positive correlation implies that much larger thinning strain has taken place in the high temperature mafic lower crust high temperature than in the low temperature felsic upper crust during mesozoic cenozoic tectonic extension

the concentration and isotopic composition of carbon in basaltic glasses from the juan de fuca ridge pacific ocean the abundance and 13c12c ratios of carbon were analyzed in basaltic glass from twenty locations along the juan de fuca ridge using a 3 step combustion extraction technique carbon released during the first two combustion steps at 400 500 c and 600 650 c is interpreted to be secondary and only the carbon recovered during a final combustion step at 1200 c is thought to be indigenous to the samples for carbon released at 1200 c glasses analyzed as 1 2 mm chips contained 23 146 ppm c with 13c values of 4 8 to 9 3 whereas samples crushed to 38 63 m or 63 90 m yielded 56 103 ppm c with 13c values of 6 1 to 9 2 the concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean ridge basalts the juan de fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor evidence of disequilibrium includes 1 1 a large range of carbon contents among glasses collected at similar depths 2 2 a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample and 3 3 a lack of correlation between vesicle abundance carbon concentration and depth of eruption variations in carbon concentration and 13c ratios along the ridge do not correlate with major element chemistry the observed relationship between carbon concentrations and 13c values may be explained by late stage variable degrees of open system rayleigh like degassing

seismic velocities poisson s ratios and potential auxetic behavior of volcanic rocks abstract the present compilation of p and s wave velocities vp and vs for 395 dry and 664 wet basalt 231 dry and 63 wet rhyolite samples provide a new database for evaluating the porosity dependence of vp vs and poisson s ratio and the potential auxetic behavior i e negative poisson s ratio of volcanic rocks the correlation is always positive for the water saturated rocks but can be either positive rhyolite or negative basalt for dry rocks depending on the poisson s ratio of the non porous solid matrix 0 auxetic behavior cannot occur in wet volcanic rocks but may appear in dry basalts with extremely high porosity 70 for basalt such that a re entrant foam structure forms the auxeticity of highly porous volcanic rocks such as pumice and reticulite will be suppressed by infiltration of water into the rocks hence volcanic rocks cannot be auxetic in nature if their vesicles are filled with water under the laboratory conditions however a transition from conventional to auxetic behaviors in pumice and reticulite is expected to occur by changing the degree of water saturation these results provide a set of basic parameters for the modeling of ground deformation and temporal and spatial variations in vp vs vp vs ratio as precursors of volcanic eruption or volcanic edifice collapse

upb and smn edvidence for eburnian and pan african high grade metamorphism in cratonic rocks of southern cameroon abstract u pb analyses of zircons and sm nd mineral and whole rock analyses of samples from southern cameroon document three distinct rock forming or high grade metamorphic events 2900 ma 2050 ma and 600 ma zircons from the ebolowa charnockite in southern cameroon yield an age of 2896 7 ma consistent with published rb sr total rock data for the region sm nd crustal formation ages for these rocks are 3000 ma indicating that the granitoids and ntem series gneisses in the north end of the congo craton formed 2900 ma ago and do not contain major contributions from older archean crust a second metamorphic event of eburnian age is identified from re equilibration of sm and nd in gneisses and metadolerites of the ntem complex at 2050 ma zircons from the younger nyong series gneisses are a complex mixture of cores and overgrowths the overgrowths in most cases formed during a high grade metamorphic event between 2100 and 2000 ma and clearly postdate the ebolowa charnockite the ages of the cores are not precisely known but we believe most represent detrital zircons from 2900 ma protoliths sm nd crustal formation ages for samples of the nyong series gneisses are generally 2500 to 2900 ma indicating that their precursors were apparently derived mainly from the archean craton with variable amounts of younger component sm nd garnet total rock isochrons for metamorphosed mafic rocks in the nyong series yield ages of 2050 ma and zircons from one of them yield an upper intercept age of 2037 10 ma further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 ma we interpret the nyong series metasedimentary gneisses as post 2900 ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 ma eburnian orogeny we cannot define precise limits on the ages of sedimentation preliminary data on metamorphosed intrusive rocks associated with the yaounde series yield u pb concordant ages around 620 ma consistent with sm nd garnet whole rock ages from the same series this dates the pan african tectono metamorphic activity in the southern end mobile belt the pan african lower intercepts of zircons and sm nd garnet whole rock ages from thnyong series are the consequences of this event which is further represented by a 590 ma zircon age from a late syenitic pluton results from cameroon are consistent with recent results from the western part of the congo craton farther south and from the northeastern part of the sao francisco craton in brazil one interpretation of these results is that the congo and sao francisco cratons were joined during the ebumian trans amazonian orogeny 2100 ma and are bordered to the north by a late mesoproterozoic rift basin that was closed during the pan african brasiliano orogeny 600 ma

effects of mechanical layering on volcano deformation summary the migration and accumulation of magma beneath volcanoes often causes surface displacements that can be measured by geodetic techniques usually deformation signals are explained using models with uniform mechanical properties in this paper we study surface displacements due to magma chamber inflation using heterogeneous finite element models we first present a systematic analysis of the influence of mechanical layering showing that the stiffness contrast significantly affects the entity and the pattern of vertical and radial displacements second as an example we apply the models to interpret ground displacements at darwin volcano

seismic heterogeneity of mt etna structure and activity a dense seismic array maintained temporarily for 7 weeks provides an approach to deducing the 3 d heterogeneity of the structure of the upper crust beneath mt etna volcano from the inversion of p and s wave arrivals of local earthquakes independent data from a different source array geometry using waves generated by distant shots at sea confirm a main lateral variation a high velocity body underlies part of the volcano its contrast with the surrounding thick sedimentary pile and its location extending from the present summit to the southeast suggests that it may have been built by the development and migration of past eruptive centres followed by caldera collapse local low velocity anomalies near its sharp western edge may be associated with presently active features

the effects of alteration and porosity on seismic velocities in oceanic basalts and diabases seismic velocities in the lavas that cap normal oceanic crust are affected by both crack porosity and alteration of the primary mineral phases chiefly to clays porosity accounts for 75 80 of the velocity variation in sonic log velocities in the lava sections of holes 504b and 1256d but the effect of alteration on the properties of the basalts has not been assessed in this analysis the grain velocities in basalt and diabase samples are estimated from an empirical linear relationship between grain density and the p wave modulus the theoretical velocity in fresh zero porosity basalt or diabase is 6 96 0 07 km s grain velocities in the diabase samples are statistically indistinguishable from the theoretical velocity and show no variation with depth alteration does not significantly affect the velocities in the diabase samples from hole 504b this result is consistent with previous analyses which demonstrated that velocities in the dikes are controlled by crack porosity in basalt lab samples alteration reduces the average sample grain velocity to 6 74 0 02 km s cracks at the sample scale further reduce the velocity to 5 86 0 03 km s and large scale cracks in the lavas reduce the average in situ velocity to 5 2 0 3 km s cracks account for nearly 90 of the difference between seismic in situ velocities and the theoretical velocity in the unaltered solid material basalt grain velocities show a small but significant systematic increase with depth the influence of alteration decreases with depth in the lavas reaching near zero at the base of the lavas in holes 504b and 1256d

location and size of the shallow magma reservoir beneath kīlauea caldera constraints from near source vp vs ratios we present high resolution compressional wave to shear wave velocity ratios vp vs beneath k lauea s summit caldera by applying an in situ estimation method using waveform cross correlation data for three similar earthquake clusters we observe high vp vs ratios 1 832 and 1 852 for two event clusters surrounded by the low background vp vs value of 1 412 at 2 1 km depth below the surface these high and low vp vs ratios can be explained by melt and co2 filled cracks respectively based on a theoretical crack model the event cluster with the highest vp vs ratio consists of long period events that followed the 1997 east rift zone eruption indicating their association with fluid and magma movement the depths of the two clusters with high vp vs ratios are consistent with the magma reservoir location inferred from geodetic observations their locations east and north of halema uma u crater suggest a horizontal extent of a few kilometers for the reservoir

volatile solubilities in magmas transport of volatiles from mantles to planet surfaces experimental measurements of the solubility of co2 co ch4 h2 and h2o in silicate melt were made at pressures from 10 to 20 kb and temperatures of 1200 c the c o h fluid phase was buffered by graphite iron and wustite detailed measurements were made on melts of naalsi3o8 composition and the results confirmed for alkalic and tholeiitic basalt liquids the melt volatiles have dramatically lower h2 h2o and higher co co2 ratios than the fluid the result is large differences between the c o h compositions of the melt and the fluid fluids about 16 atm c 6 o and 78 h melts about 4 c 32 o and 64 h the measured solubilities allow calculation of activity coefficients for volatile species in the melt they are roughly co 0 1 h2o 0 6 co2 1 h2 3 and ch4 60 this two order of magnitude variation in activity coefficients causes the significant volatile fractionation observed between fluid and melt we argue that the same fractionation will occur in the more usual case of fluid absent partial melting of planetary mantles the fractionation is such that reducing mantles will form magmas that yield volcanic gases about 1 5 orders of magnitude more oxidizing than the mantle source while oxidizing mantles will form magmas with more reduced volcanic gases we conclude that a volcanic gas composition cannot be directly used to estimate either the oxygen fugacity or volatile composition of its magma source region the results suggest that volcanic gases will normally lie in the neutral range of oxygen fugacity near that of the quartz fayalite magnetite buffer these gases are predominately h2o with minor co2 co h2 and ch4

accurate vp vs relationship for dry consolidated sandstones 1 we use a finite element method to study the elastic properties of dry sandstone morphologies we show that the dry poisson s vp vs ratio becomes independent of the poisson s ratio of the solid phase at a critical porosity and that this limiting behaviour holds for any number of solid phases and is insensitive to the manner in which the phases are distributed we represent the behaviour of the poisson s ratio as a simple flow diagram and present an empirical structure property relationship that holds for all sandstone morphologies independent of the number of phases and the poisson s ratio of the solid phase

paleomagnetism of the 765 ma luakela volcanics in northwest zambia and implications for neoproterozoic positions of the congo craton the 765 ma luakela volcanics of northwest zambia are a northeast trending belt of basaltic to andesitic flows within an essentially undeformed and unmetamorphosed siliciclastic succession that unconformably overlies neoarchean and paleoproterozoic rocks of the congo craton margin paleomagnetic analysis of 65 samples from nine sites isolated three magnetization components component a carried mainly by single domain magnetite is directed very shallowly to the southeast and northwest and is interpreted as a primary thermoremanent magnetization component b carried mainly by hematite and oriented mainly shallowly upwards to the southwest is interpreted to be a secondary overprint possibly acquired during latest neoproterozoic pan african deformation in the lufilian arc a low stability component c probably carried by multi domain magnetite or hematite or both is directed very steeply downward and is similar to late paleozoic results from the region paleopoles lva and lvb corresponding to components a and b are similar to previous results from mafic rocks of similar age in tanzania lva 40 2 s 122 0 e coincides with a reliable pole for the 748 ma mbozi syenite gabbro complex confirming the position of the congo craton at 765 to 750 ma the secondary lvb pole 38 3 n 90 9 e of uncertain age but younger than 765 ma coincides with the pole for the ca 795 ma gagwe lavas casting doubt on the assumed primary nature of the gagwe remanence our new data imply that the congo craton rotated ca 90 clockwise between 750 ma and the time of b component acquisition contrary to previous scenarios in which congo rotated counter clockwise between ca 800 and 750 ma

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

modeling of stress dependent static and dynamic moduli of weak sandstones 1 static and dynamic moduli of a sandstone may be different and stress sensitive knowledge about this is important for the interpretation of seismic data and sonic logs we use a numerical model which is based on the discrete element method dem to study the static and the dynamic mechanical behavior of weak sandstones we developed a constitutive contact law and implemented it into the dem model in order to properly simulate the deformation of weak sandstones the contact law includes the effect of nonlinear elasticity and plasticity at grain contacts the dem model can also capture the effect of the closing of pre existing cracks the formation of new cracks and slip at uncemented grain contacts due to stress alteration using such a model we simulated sandstone specimens under laboratory experimental conditions we calculated the static and the dynamic moduli of the dem model at different stress states the modeling results qualitatively agree with some published laboratory observations by applying different stress paths and stress histories we studied how the moduli were influenced by the stress state stress path and stress history in particular the simulations show the growing difference between the static and the dynamic moduli as failure is approached the simulations thus support the assumption that theoretical insight in the grain scale deformation mechanisms helps to understand and explain the difference between static and dynamic moduli as well as for the failure process

differences between static and dynamic elastic moduli of a typical seismogenic rock summary static and dynamic elastic moduli of calcare massiccio mudstone limestone the typical seismogenic rock in the italian apennines are measured using a standard uniaxial static compression test a dual cantilever forced oscillation test and ultrasonic measurement of elastic wave velocities these measurements cover nine decades in frequency including the seismic domain neither a significant frequency dependence nor a pronounced strain amplitude dependence was observed providing a young s modulus of 75 7 gpa and a poisson s ratio of 0 28 0 02 these values are characteristic of calcare massiccio in undamaged condition

poisson s ratio values for rocks abstract compared to other basic mechanical properties of rocks poisson s ratio is an elastic constant of which the significance is generally underrated yet in rock mechanics there is a considerable number of diverse areas which require a prior knowledge or estimation of the value of poisson s ratio this paper examines the values and applications of poisson s ratio in rock mechanics following an historical account of the initial controversy whether it was a material constant or not the effects of poisson s ratio in the elastic deformation of materials intact rocks and rock masses are briefly reviewed also the reported values of poisson s ratio for some elements materials and minerals are compiled while typical ranges of values are presented for some rocks and granular soils finally poisson s ratio classifications are recommended for isotropic intact rocks

fluids p t paths and the fates of anatectic melts in the earth s crust this paper presents a model for the longevity and physical behaviour of anatectic melts in the earth s crust it is assumed that a thermal anomaly has caused partial fusion of biotite plagioclase quartz rocks to produce a granitic melt and restitic solids the focus is on the potential chemical and physical outcomes as a function of the various p t fluid conditions rocks undergoing fluid present melting will sustain an increase in bulk density and will become more gravitationally stable diapiric up welling of such material should not occur in fluid absent melting the bulk density will decrease and foster gravitational instability stresses in the surrounding rocks could however lead to fracture and rapid evacuation of the melt with excess fluid and without melt segregation isobaric cooling will cause immediate freezing if t rises above the biotite quartz melting reaction freezing will still occur at the original reaction t diapiric up welling should not occur biotite bearing diatexites might be formed by large degrees of fusion but magma ascent is unlikely in isothermal decompression immediate freezing will ensue and the migmatite should contain orthopyroxene rather than biotite with excess fluid and melt segregation cooling will induce freezing at the fluid saturated granite solidus magma ascent could however occur even for xflh2o 1 in isothermal decompression melt will persist to the granite solidus for the imposed xflh2o limited magma ascent could occur all granites should contain primary igneous biotite for fluid deficient rocks without melt segregation freezing will occur at the t of the original melting if the reaction is overstepped solidification will occur at the original xflh2o magma formation is unlikely migmatites should contain either pyroxene retrogressed to biotite or biotite alone on decompression immediate freezing is likely if t remains at the biotite quartz reaction for higher t melt will survive decompression and freeze at the fluid saturated solidus for some increased xflh2o magma ascent is possible and the melt will freeze very close to the wet granite solidus in fluid deficient rocks with melt segregation cooling and crystallization will cause fluid saturation in situ segregated melt will freeze at the original xflh2o melt ascent will however cause exsolution of a co2 rich fluid with fluid escape melt will freeze close to the wet granite solidus an increase in melt proportion may accompany magma ascent or source up welling any low p granite intrusion should be charnockitic for fluid absent rocks without melt segregation freezing will occur on cooling to the t of the original reaction only transitory migmatites could be formed but melt will survive decompression almost to the h2o saturated granite solidus the rocks will reveal former melt presence only in the existence of veins of coarse retrograde biotite with isothermal decompression further melting may occur forming an orthopyroxene bearing granulite facies migmatite in fluid absent situations with melt segregation magma could be efficiently transported to higher crustal levels if the melt proportion were insufficient for magma formation granulite facies migmatites would result ascending melt would progressively dissolve any entrained restite the granites produced should be charnockitic to aid in interpretation of the modelling results we suggest that it may be slightly more common for the highest grade metamorphic terranes to exhibit isobaric cooling paths that fluid present anatexis is normally limited to the shallower parts of the crust and that fluid absent reactions are more likely at greater depths we conclude that the best models for natural fluid present anatexis are those dealing with essentially pure h2o fluid and that fluid deficient conditions will dominate over fluid excess we also believe that it is common for melts to be physically segregated on scales larger than the diffusion paths of many elements

core formation in the shergottite parent body and comparison with the earth the mantle of the shergottite parent body spb is depleted relative to the bulk spb in siderophile and chalcophile elements these elements are inferred to reside in the spb s core our chemical model of these depletions rests on a physically plausible process of segregation of partially molten metal from partially molten silicates as the spb grows and is heated above silicate and metallic solidi during accretion metallic and silicate phases equilibrate at low pressures as new material is accreted to the spb surface later movement of the metallic phases to the planet s center is so rapid that high pressure equilibration is insignificant partitioning of siderophile and chalcophile elements among solid and liquid metal and silicate determines their abundances in the spb mantle using partition coefficients and the spb mantle composition determined in earlier studies we model the abundances of ag au co ga mo ni p re s and w with free parameters being oxygen fugacity proportion of solid metal formed proportion of metallic liquid formed and proportion of silicate that is molten abundances of all elements except au mo and re may be reproduced adequately with oxygen fugacity near one log unit below the iron wustite buffer proportion of molten silicate near 0 3 and a core composed of subequal parts of solid and liquid metal constituting approximately 30 of the mass of the spb the abundances of au and re may be reproduced using different but still permissible partition coefficients by invoking a small chondritic veneer mixed into the mantle after core formation or by assuming that small fractions of solid and liquid metal are trapped in the mantle during core formation molybdenum cannot be modeled adequately problems may include inaccurate partition coefficients inaccurate estimation of the abundance of mo in the spb mantle and the possibility that the mantle source of the snc meteorites is not representative of the whole spb mantle in contrast to the spb there is at present no hypothesis capable of quantitatively reproducing elemental abundances in the earth s mantle the contrast suggests that these two terrestrial planets may have accreted or differentiated differently but the cause of such differences is unclear

timing of deformation in the sarandi del yi shear zone uruguay implications for the amalgamation of western gondwana during the neoproterozoic brasiliano pan african orogeny u pb and hf zircon sensitive high resolution ion microprobe shrimp and laser ablation inductively coupled plasma mass spectrometry la icp ms ar ar hornblende and muscovite and rb sr whole rock muscovite isochron data from the mylonites of the sarandi del yi shear zone uruguay were obtained in order to assess the tectonothermal evolution of this crustal scale structure integration of these results with available kinematic structural and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630 625 ma during the collision of the nico perez terrane and the rio de la plata craton the shear zone underwent dextral shearing up to 596 ma under upper to middle amphibolite facies conditions which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 ma after emplacement of the cerro caperuza granite at 570 ma the shear zone underwent only cataclastic deformation between the late ediacaran and the cambrian the sarandi del yi shear zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the rio de la plata craton and the nico perez terrane furthermore the obtained data reveal that strain partitioning and localization with time magmatism emplacement and fluid circulation are key processes affecting the isotopic systems in mylonitic belts revealing the complexity in assessing the age of deformation of long lived shear zones

compressional wave attenuation in oceanic basalts to understand better the seismic attenuation in the upper volcanic regions of the oceanic crust compressional wave attenuations of oceanic basalts have been measured as a function of confining pressure using an ultrasonic pulse echo spectral ratio technique capable of measuring attenuations to pressures of 500 mpa seven basalts five from deep sea drilling project cores and two from dredge samples have wide ranges of densities porosities and alterations making possible an analysis of the parameters influencing basalt attenuation attenuation increases with the volume of secondary minerals present and with increasing porosity thus vesicularity and compositional changes associated with basalt alteration will produce variations in attenuation with the application of hydrostatic pressure cracks close thereby reducing attenuations this pressure dependence should be manifested in oceanic layer 2 by decreasing attenuation with depth an inverse relationship between velocity and attenuation is observed at high hydrostatic pressures water saturation increases attenuation at pressures below 200 mpa and enhances the sensitivity of attenuation to pressure thus making the state of saturation important in the 40 to 100 mpa range generally found in layer 2 these results provide a framework for interpreting marine attenuation data

the nature and distribution of carbon in submarine basalts and peridotite nodules primary carbonaceous material has been identified in submarine basaltic glasses and mantle derived peridotite nodules from alkali basalts using electron microprobe techniques in the submarine rocks carbon occurs 1 in quench produced microcracks in glasses and phenocrysts 2 in vesicles where it is preferentially concentrated on the sulfide spherules attached to vesicle walls and 3 in microcracks and co2 rich bubbles in inclusions of glass completely enclosed by phenocrysts in peridotite nodules carbon exists in intergrain cracks along grain boundaries and on the walls of fluid inclusions disposed in two dimensional arrays the carbonaceous material is believed to consist of a mixture of graphite other forms of elemental carbon and possibly small amounts of organic matter it is suggested that carbon precipitates by disproportionation of co according to the reaction 2 co c co2 and that this reaction is catalyzed by sulfide oxide surfaces in vesicles once deposition has begun the reaction continues on carbon surfaces as well based on the large amounts of condensed carbon observed in some vapor inclusions and the apparent lack of oxidation features associated with them it is proposed that carbon condensed from a magmatic vapor in which co was a significant constituent this implies that oxygen fugacities of undegassed basaltic melts under confining pressures of the shallow crust are typically lower than those of the qfm buffer at equivalent temperatures this is in agreement with some intrinsic oxygen fugacity measurements on similar undegassed materials regardless of the mechanism of its formation the presence of carbon in co2 rich vesicles and inclusions in basaltic glasses and mantle nodules adds uncertainty to estimates of minimum pressures of entrapment based on measurements of fluid densities condensed carbon also accounts for some of the carbon isotopic characteristics of these rocks

velocity porosity relationships 1 accurate velocity model for clean consolidated sandstones we use numerical simulations to derive the elastic properties of model monomineralic consolidated sandstones the model morphology is based on overlapping spheres of a mineral phase we consider model quartzose and feldspathic sands we generate moduli porosity relationships for both the dry and water saturated states the ability to control pore space structure and mineralogy results in numerical data sets which exhibit much less noise than corresponding experimental data the numerical data allows us to quantitatively analyze the effects of porosity and the properties of the mineral phase on the elastic properties of porous rocks the agreement between the numerical results and available experimental data for clean consolidated sandstones is encouraging we compare our numerical data to commonly used theoretical and empirical moduli porosity relationships the self consistent method gives the best theoretical fit to the numerical data we find that the empirical relationship of krief et al is successful

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

dynamic and static moduli static and dynamic bulk moduli ks and kd are measured as continuous functions of pressure from zero to 2 3 kilobars for two sandstones a tuff limestone granite and oil shale results for the sandstones and granite are in good agreement with previously reported data with ks kd varying from about 0 5 at atmospheric pressure to close to unity at pressures 2 kilobars and above for rocks behaving elastically under static loading the ks kd ratio is inversely related to the microcrack density for the limestone time dependent deformation associated with pore collapse results in ks kd ratios approaching 0 1 at high pressure upon unloading while initially high 1 0 at high pressures ks kd becomes lower than values obtained during loading at low pressures 1 kilobar due to opening of microcracks generated during pore collapse for the oil shale in which few microcracks exist ks kd remains relatively constant with pressure at a value of about 0 7

evidence for a neoproterozoic carbonate ramp on the northern edge of the central african craton relations with late proterozoic intracratonic troughs during late proterozoic times the archaean central african craton was affected by trough faulting which led to the formation of grabens the sangha aulacogen being the main structure of this type in the studied area this transverse basin connects with other basins on the northern and south western borders of the craton during the cryogenian this network of basins was filled with fluvio deltaic and lacustrine periglacial deposits the glacio eustatic transgression in neoproterozoic iii end proterozoic times flooded extensive areas of shelf on the northern edge of the craton leading to the development of carbonate sedimentation in a broad outer shelf environment associated with nearshore barriers and evaporitic lagoons these facies are similar to those developed in the west congolian schisto calcaire shale limestone ramp succession the north central african ramp succession sediment slope contains an example of tidal rhythmites in vertical accretion which occurs beneath the barrier deposits on the subtidal outer shelf mathematical analysis of the bedding pattern yields a period of 29 30 days for the lunar month a result which is in agreement with astrophysical evidence for this epoch i e 650 ma ago major subsidence and seismic activity on this gently sloping platform associated with the proximity of the sangha aulacogen caused the triggering of carbonate turbidites and mass flow deposits the proliferation of microbial mats under euphotic conditions on an extensive shelf led to the build up of a carbonate platform during early neoproterozoic iii times the west congolian and north central african ramps prograded northwards and southwards respectively into the sangha aulacogen the sea at that time was restricted to a long graben like basin while a remaining area of marine sedimentation persisted into the palaeozoic thus the pattern of end proterozoic carbonate sedimentation on the borders of the central african craton can be interpreted in terms of an overall gently sloping ramp model with progradation converging towards the sangha aulacogen

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

high grade metamorphism dehydration and crustal melting a reinvestigation based on new experiments in the silica saturated portion of the system kalo 2 mgo sio 2 h 2 o co 2 at p 1 5 gpa the system kalo2 mgo sio2 h2o co2 has long been used as a model for the processes of granulite facies metamorphism and the development of orthopyroxene bearing mineral assemblages through the breakdown of biotite bearing assemblages there has been considerable controversy regarding the role of carbon dioxide in metamorphism and partial melting we performed new experiments in this system at pressures of 342 to 1500 mpa with t between 710 and 1045 c and xflh2o between 0 05 and 1 00 accurately locating most of the dehydration and melting equilibria in p t xflh2o space the most important primary result is that the univariant reaction phl qtz fl en sa melt must be almost coincident with the fluid absent reaction phl qtz en sa melt in the co2 free subsystem in conjunction with the results of previous measurements of co2 solubility in silicate melts and phase equilibrium experiments our theoretical analysis and experiments suggest that co2 cannot act as a flux for partial melting crustal melting in the presence of h2o co2 mixed fluids will always occur at temperatures higher than with pure h2o fluid present magmas produced by such melting will be granitic s l in composition with relatively high sio2 and low mgo contents irrespective of the h2o co2 ratio in any coexisting fluid phase we find no evidence that lamprophyric magmas could be generated by partial fusion of quartz saturated crustal rocks the granitic melts formed will not contain appreciable dissolved co2 the channelled passage of hot co2 rich fluids can cause local dehydration of the rocks through which they pass in rock dominated as opposed to fluid dominated systems minor partial melting can also occur in veins initially filled with co2 rich fluid as dehydration and local disequilibrium drive the fluid towards h2o rich compositions however co2 is unlikely to be a significant agent in promoting regional granulite grade metamorphism melting magma generation metasomatism or long range silicate mass transfer in earth s crust the most viable model for the development of granulite facies rocks involves the processes of fluid absent partial melting and withdrawal of the melt phase to higher crustal levels

the fina nagu volcanic complex unusual submarine arc volcanism in the rapidly deforming southern mariana margin in the mariana convergent margin large arc volcanoes disappear south of guam even though the pacific plate continues to subduct and instead small cones scatter on the seafloor these small cones could form either due to decompression melting accompanying back arc extension or flux melting as expected for arc volcanoes or as a result of both processes here we report the major trace and volatile element compositions as well as the oxidation state of fe in recently dredged fresh pillow lavas from the fina nagu volcanic chain an unusual alignment of small closely spaced submarine calderas and cones southwest of guam we show that fina nagu magmas are the consequence of mantle melting due to infiltrating aqueous fluids and sediment melts sourced from the subducting pacific plate into a depleted mantle wedge similar in extent of melting to accepted models for arc melts fina nagu magmas are not as oxidized as magmas elsewhere along the mariana arc suggesting that the subduction component responsible for producing arc magmas is either different or not present in the zone of melt generation for fina nagu and that amphibole or serpentine mineral destabilization reactions are key in producing oxidized arc magmas individual fina nagu volcanic structures are smaller in volume than mariana arc volcanoes although the estimated cumulative volume of the volcanic chain is similar to nearby submarine arc volcanoes we conclude that melt generation under the fina nagu chain occurs by similar mechanisms as under mariana arc volcanoes but that complex lithospheric deformation in the region distributes the melts among several small edifices that get younger to the northeast

paleomagnetism and geochronology of mafic dykes from the southern granulite terrane india expanding the dharwar craton southward abstract the southern granulite terrane sgt of india is a collage of archean to neoproterozoic crustal blocks hosting several generations of mafic dykes we report new paleomagnetic and geochronologic results from mafic dykes in the salem block of the sgt the 2370 ma bangalore dyke swarm is confirmed in this area with a u pb zircon age of 2363 6 6 ma on dykes with steep dual polarity paleomagnetic directions yielding a pole at 2 n and 73 e a95 14 these results precisely place the dharwar craton and northern sgt at high latitudes in the early paleoproterozoic the episodic paleoproterozoic magmatic record of india is also recovered from the sgt several groupings of sgt paleomagnetic directions compare favorably with the 2 21 ga 2 18 ga and 1 88 ga dharwar craton dykes we also report a grand mean pole of 81 n and 259 e a95 11 that we attribute to a widespread ediacaran age remagnetization associated with the final stages of gondwana assembly these paleomagnetic and geochronologic data permit a better resolved reconstruction of indian crustal elements from the paleoproterozoic onward

relating seismic velocities thermal cracking and permeability in mt etna and iceland basalts we report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from etna italy and columnar basalt from seljadur iceland measurements were made in a servo controlled steady state flow permeameter at effective pressures from 5 80 mpa during both increasing and decreasing pressure cycles selected samples were thermally stressed at temperatures up to 900 degrees c to induce thermal crack damage acoustic emission output was recorded throughout each thermal stressing experiment at low pressure 0 10 mpa the p wave velocity of the columnar seljadur basalt was 5 4 km s while for the etnean lava flow basalt it was only 3 0 3 5 km s on increasing the pressure to 80 mpa the velocity of etnean basalt increased by 45 60 whereas that of seljadur basalt increased by less than 2 furthermore the velocity of seljadur basalt thermally stressed to 900 degrees c fell by about 2 0 km s whereas the decrease for etnean basalt was negligible a similar pattern was observed in the permeability data permeability of etnean basalt fell from about 7 5 x 10 16 m 2 to about 1 5 x 10 16 m 2 over the pressure range 5 80 mpa while that for seljadur basalt varied little from its initial low value of 9 x 10 21 m 2 again thermal stressing significantly increased the permeability of seljadur basalt whilst having a negligible effect on the etnean basalt these results clearly indicate that the etnean basalt contains a much higher level of crack damage than the seljadur basalt and hence can explain the low velocities 3 4 km s generally inferred from seismic tomography for the mt etna volcanic edifice c 2005 elsevier ltd all rights reserved

ab initio elastic properties of talc from 0 to 12 gpa interpretation of seismic velocities at mantle pressures and prediction of auxetic behaviour at low pressure talc is a hydrous magnesium rich layered silicate that is widely disseminated in the earth from the seafloor to over 100 km depth in ultra high pressure metamorphism of oceanic crust in this paper we determine the single crystal elastic constants at pressures from 0 to 12 gpa of talc triclinic c 1 and monoclinic c2 c polytypes using ab initio methods we find that talc has an extraordinarily high elastic anisotropy at zero pressure that reduces with increasing pressure the exceptional anisotropy is complemented by a negative poisson s ratio for many directions in crystal space calculations show that talc is not only one of very few common minerals to exhibit auxetic behaviour but the magnitude of this effect may be the largest reported so far for a mineral the compression vp and shear vs wave velocity anisotropy is 80 and 85 for the triclinic polytype at pressures where talc is known be stable in the earth up to 5 gpa the vp and vs anisotropy is reduced to about 40 for both velocities which is still a very high value vp is slow parallel to the c axis and fast perpendicular to it this remains unchanged with increasing pressure and is observed in both polytypes the shear wave splitting difference between fast and slow s wave velocities at low pressure has high values in the plane normal to the c axis with a maximum near the a axis in the triclinic and the b axis in the monoclinic polytype the c axis is the direction of minimum splitting the pattern of shear wave splitting does not change significantly with pressure the volume fraction of talc varies between 11 and 41 for hydrated mantle rocks but the lack of data on the crystallographic preferred orientation cpo precludes a detailed analysis of the impact of talc on seismic anisotropy in subduction zones however it is highly likely that cpo can easily develop in zones of deformation due to the platy habit of talc crystals for random aggregates of talc the isotropic vp vs and vp vs ratio have significantly lower values than those of antigorite and may explain low velocity regions in the mantle wedge vp vs ratios are more complex in anisotropic media because there are fast and slow s waves resulting in vp vs1 and vp vs2 ratios for every propagation direction making interpretation difficult in deformed polycrystalline talc with a cpo talc on the subduction plate boundary can strongly influence guided wave velocity as cpo would develop in this region of intense shearing the very low coefficient of friction 0 1 of talc above 100 c could also explain silent earthquakes at shallow depths ca 30 km along the subduction plate boundaries frequently responsible for tsunami

palaeomagnetism of the itajai castro and bom jardim groups from southern brazil summary palaeomagnetic results corresponding to geological units associated with the brasiliano pan african cycle late pre cambrian early palaeozoic are presented a total of 160 hand samples from the itajai castro and bom jardim groups from southern brazil were submitted to laboratory treatment 46 acid volcanic samples from the campo alegre formation itajai group yielded two magnetic components labelled a1 dm 36d im 40d a95 10d and a2 dm 44d im 8d a95 4d with respective palaeomagnetic poles ca 223de 57ds a95 9d and sv 200de 43ds 20 acid volcanic samples from the castro group yielded a single stable direction of magnetization dm 347d im 52d a95 12d with palaeopole ct 4de 76ds a95 15d 12 acid samples from the acampamento velho formation bom jardim group yielded two magnetic components labelled b1 dm 156d im 69d a95 17d and b2 dm 207d im 15d a95 42d with respective palaeopoles av1 338de 61ds a95 28d and av2 345de 44dn a95 37d seven sedimentary samples from the vargas formation bom jardim group yielded a magnetic component dm 110d im 81d a95 17d with palaeopole va 322de 34ds a95 30d finally 11 andesitic volcanic samples yielded a magnetic component dm 108d im 4d a95 12d with palaeopole hi 228de 14dn a95 9d these results are compared with apwp s constructed for the gondwana continents

ultrasonic velocity measurements in volcanic rocks correlation with microtexture summary this paper summarizes results of a study of porosity permeability microstructure and acoustic properties of volcanic and pyroclastic rocks from campi flegrei cf and mt etna et italy we have measured the hydraulic transport and acoustics properties of 28 room dry samples at ambient conditions 25 room dry samples under confining pressure up to 60 mpa and 5 brine saturated samples under pressure up to 45 mpa effective pressure we established the following range of porosity permeability and vp and vs variations as functions of mineralogy and differential pressure in cf and et lithologies 1 porosity cf pyroclastic rocks 30 60 per cent cf and et lava rocks 2 20 per cent 2 permeability cf pyroclastic rocks 10 1000 md cf and et lava rocks 0 01 100 md 3 velocity and quality factor cf pyroclastic rocks vp 2 3 km s x1 vs 1 2 km s x1 qp 5 80 cf and et lava rocks vp 3 5 5 5 km s x1 vs 2 3 km s x1 qp 10 115 mineralogy and microstructure govern the acoustic and petrophysical properties of these rocks under pressure in pyroclastic rocks changes in acoustic response are directly related to presence of zeolites and pumice and their reactions with the pore fluid in dry conditions collapse of the internal texture of pumice leads to decreasing velocity with pressure in saturated conditions the water zeolite interactions compete with effects due to collapsing internal textures and so velocity change with pressure is not as pronounced the microstructural changes were confirmed by analyses of optical hydraulic and transport properties after pressure ct scans show a macroscopically more compact structure under optical microscopy pumice and zeolitized pumice appear shredded a significant reduction in porosity and permeability is measured after pressurization in lava rock samples acoustic velocities increase in function of pressure and the velocity pressure relationships are characteristic of samples with rounded pores our results emphasize the importance of conducting velocity measurements at simulated in situ conditions by constraining the computations using site and depth specific rock physics properties differences between ground deformation models in volcanic areas can be assessed and predicted more reliably thus reducing volcanic hazard

emplacement and deformation of the fomopea pluton implication for the pan african history of western cameroon abstract the fomopea pluton 622 613 ma is located in the western part of the pan african belt in cameroon it comprises of 3 units biotite hornblende granitoids bhg biotite monzogranites bmg and edenite syenogranites esg the bhg unit displays magnetic fabrics characterized by foliations gently dipping towards the ese and ene trending lineations d1 event microstructures are magmatic to submagmatic in discrete n s deformation bands d2 lineations are rotated towards the north and microstructures indicate solid state deformation at mid to low t conditions with kinematic indicators pointing to a sinistral motion the second unit bmg displays lineation trajectories suggesting emplacement during the d2 event the esg unit displays fabrics consistent with a later emplacement with no superposed deformation the last event d3 corresponds to a dextral shear zone that runs along the southeastern border of the fomopea pluton it was responsible for protomylonitic deformation of the granitic rocks in greenschist facies conditions whereas the core of the shear zone registered higher temperatures and strain this shear zone induced a rejuvenation of the rb sr isotopic system in the pluton at ca 572 ma it belongs to the central cameroon shear zone system regarded as the prolongation of the dextral patos shear zone system in brazil

physical processes in volcanic eruptions mechanisms are modeled in terms of dynamic physico chemical processes in this respect volcanology shares with much of earth science in general the benefits that accrue from the application of the methods and concepts of other disciplines to its problems basic concepts from the fields of thermo dynamics fluid mechanics and rheology have been particularly impor tant in fostering new insights recent developments have been so numerous as to make it necessary to limit the scope of this review and thus we concentrate here on the basic physical processes involved in the rise of magma to the surface and the distribution of cruption products on thc ground or in the atmosphere the perspective of planetary volcanism wilson hcad 1983 underlines the fact that volcanoes can be regarded as probes of the composition and structure of a planetary interior as a function of position depth and time

origin and evolution of the late precambrian high grade yaounde gneisses cameroon abstract the yaounde series is composed of low to high grade garnet bearing schists and gneisses belonging to the pan african north equatorial fold belt the high grade rocks consist of kyanite garnet gneisses and garnet plagioclase gneisses containing layers of calcsilicate rocks marbles quartzites and magnetite rich orthopyroxenites the chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite rocks dolomitic marls evaporitic beds quartzites and iron rich sediments some volcanogenic greywackes may have been of alkaline affinity this sequence was probably deposited in shallow water near shore semi barred conditions which can be related either to an intracontinental distensive environment or to a passive margin at the northern edge of the congo craton the yaounde gneisses were deformed during two main tectonic phases the first phase d1 is marked by a layering s0 s1 resulting from tectonic transposition the second one d2 corresponds to tangential tectonics with isoclinal folds and flat lying shear zones associated with a s2 schistosity and a l2 lineation a third phase late d2 is marked by the development of low temperature mylonitic shears scarce wrench faults indicate later e w compression the d1 d2 transition is characterized by the emplacement of mafic and ultramafic rocks and by the development of high pressure granulite facies conditions t 750 800 c p 10 12 kbar these peak metamorphic conditions triggered a widespread migmatization dated at 565 22 ma and coeval with the d2 event the gradual decrease of the metamorphic conditions toward the south and the existence of a syn d2 retrograde inverse metamorphic gradient suggest that the overall structure of the yaounde series is a large tectonic nappe thrust onto the congo craton a geotectonic scheme for the north equatorial fold belt is discussed