how paleosols influence groundwater flow and arsenic pollution a model from the bengal basin and its worldwide implication in the bengal basin the land surface exposed during the last lowstand of sea level around 20 ka and now buried by holocene sediment is capped by an effectively impermeable clay paleosol that we term the last glacial maximum paleosol lgmp the paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern west bengal and by implication in shallow aquifers across the bengal basin and as polluted deltaic aquifers worldwide the presence of the lgmp defines paleointerfluvial areas it is absent from paleochannel areas a paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic while that beneath paleointerfluvial areas is not paleointerfluvial aquifers are unpolluted because they are protected by the lgmp from downward migration of arsenic and from downward migration of organic matter that drives as pollution via reductive dissolution of as bearing iron oxyhydroxides horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers in which sorption of arsenic minimizes the risk of pollution

on the separation of crustal component from subducted oceanic lithosphere near the 660 km discontinuity the recent mineral physics observations suggest that the crustal and mantle components of subducted oceanic lithosphere have distinct densities and viscosities the deformation of such chemically stratified oceanic lithosphere as it collides with the 660 km discontinuity is examined using a simple analytical model it is shown that the separation of the crustal component can occur through a small scale folding 10 20 km if the viscosity of the crustal garnetite layer is significantly higher than the surrounding mantle 1021 pa s but is less than 1023 pa s leading to a garnet enriched transition zone behind subduction zones

elastic wave velocities of a pyrope majorite garnet to 3 gpa abstract the pressure dependence of the elastic wave velocities for a polycrystalline specimen 100 theoretical density of majorite pyrope garnet py 62 mj 38 has been determined at room temperature to 3 gpa by ultrasonic phase comparison interferometry pressure derivatives of the adiabatic bulk modulus dk s dp 5 3 and shear modulus dgdp 2 0 are derived from the travel times of the compressional p and shear s waves these derivatives clearly demonstrate that pressure has a more pronounced effect on the elasticity of the majorite rich garnet than on the pure pyrope end member presumably in response to the coupled substitution of si and mg for al on the octahedral site in the garnet structure when combined with plausible values for the as yet unmeasured temperature derivatives these new data can be used to calculate velocity depth profiles along a 1400 c adiabat for the range of depths in which majorite is expected to be stable in the transition zone of the earth s mantle 450 600 km for shear waves these majorite gradients match those from regional seismic models much better than those for a pure olivine mantle more reliable calculations of elastic wave velocities at mantle temperatures and pressures await measurement of the temperature derivatives of elastic modulus

effect of water on rheological properties of garnet at high temperatures and pressures abstract the effect of water on the rheological properties of garnet was investigated with triaxial compressive creep experiments at high temperatures 1223 t 1423 k and high pressures 1 6 p 5 6 gpa using a deformation dia d dia apparatus samples fabricated from a natural polycrystalline garnet pyr 23 alm 20 gro 57 together with samples prepared from san carlos olivine were deformed under water saturated conditions with water supplied by the dehydration of talc during the experiments experiments were carried out on a synchrotron x ray beamline in each experiment differential stress and sample displacement were determined using synchrotron x ray diffraction and time resolved radiography techniques respectively water fugacity was calculated based on equation of state for water at experimental p t conditions a fit of our experimental results on the creep of garnet samples to a power law demonstrates strong dependencies of strain rate on water fugacity with a water fugacity exponent of r 1 and on pressure with an activation volume of v 28 10 6 m 3 mol importantly under water saturated conditions the water content of deformed garnet samples measured by infrared spectroscopy increases with increasing pressure i e increasing water fugacity up to an average value of 21 000 h 10 6 si at 3 0 gpa and then decreases with increasing pressure down to 3000 h 10 6 si at 5 6 gpa the flow law for garnet quantified in this study provides an important constraint on the strength of the subducted oceanic crust for a water enriched environment the viscosity of a garnet rich layer will be low above the shallow upper mantle because the water content in garnet increases with increasing pressure at relatively low pressures however at greater depth the viscosity will be high because the direct effect of pressure on the creep through thermally activated processes outweighs the indirect effect on the creep through water solubility

garnet yield strength at high pressures and implications for upper mantle and transition zone rheology 1 garnet helps control the mechanical behavior of the earth s crust mantle and transition zone here measurements are presented suggesting that garnet long considered to be a high viscosity phase is actually weaker than the other dominant components in the transition zone the mechanical behavior of garnet at high pressures was examined using radial diffraction techniques in the diamond anvil cell the yield strength of grossular garnet was inferred from synchrotron x ray measurements of differential lattice strains the differential stress was found to increase from 1 3 0 6 gpa at a hydrostatic pressure 5 8 1 1 gpa to 4 1 0 4 gpa at 15 7 1 0 gpa where it was level to 19 gpa the strength results are consistent with inferred strength values for majorite garnet from measurements in the diamond cell normal geometry bolstering the idea that garnet structured materials may all have similar strengths in this low temperature high differential stress regime garnet is shown to be significantly weaker than anhydrous ringwoodite and to have a strength similar to hydrous ringwoodite this result suggests that the presence of water in the transition zone may not be required to explain a weak rheology and therefore models of transition zone behavior built assuming that garnet is the high strength phase may need to be revised

effects of pressure and temperature on the silicon diffusivity of pyrope rich garnet abstract we have determined the pressure and temperature dependencies of si volume diffusion rates in a single crystal of pyp75alm15gr10 garnet at 6 25 gpa and 1400 1800 c by the 29si tracer diffusion method high pressure experiments were conducted using the kawai type multi anvil high pressure apparatus diffusion profiles were obtained using secondary ion mass spectrometry in the depth profiling mode the si diffusion coefficient in garnet dgt is expressed by the arrhenius equation dgt d0 exp e pv rt with log10d0 7 1 1 7 m2 s e 345 56 kj mol and v 4 7 0 5 cm3 mol si diffusion seems to be the slowest in the major constituent elements in a pyrope rich garnet and control rates of plastic deformation the comparison between si diffusion rates in garnet and wadsleyite ringwoodite suggests that garnet has similar or slightly lower strength at most 2 times compared with wadsleyite and has similar or slightly higher strength at most 3 times compared with ringwoodite at temperatures ranging from 900 to 1300 c thus strength contrast between subducted oceanic crust and underlying peridotite layer may be small at the mantle transition zone conditions this result suggests that the separation of subducted oceanic crust from an underlying peridotite layer may not occur

experimental investigation of eclogite rheology and its fabrics at high temperature and pressure eclogite plays an important role in mantle convection and geodynamics in subduction zones an improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology however the deformation processes and associated fabrics in eclogite are not well understood incompatible views of deformation mechanism have been proposed for both garnet and omphacite we present here deformation behaviour of eclogite at temperatures of 1027 1427 c confining pressures of 2 5 3 5 gpa and strain rates of 1 10 5 s 1 to 5 10 4 s 1 we obtained a power law creep for the high temperature and pressure deformation of a dry eclogite 50 vol garnet 40 omphacite and 10 quartz with a 103 3 1 0 n 3 5 0 4 e 403 30 kj mol 1 and v 27 2 cm3 mol 1 the two principal minerals of eclogite have greatly different strengths progressive increase of garnet results in a smooth increase in strength analysis by electron back scattered diffraction shows that 1 garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions 2 omphacite shows pronounced lattice preferred orientations lpos suggesting a dominant dislocation creep mechanism further investigation into the water effects on eclogite show 3 water content does not influence the style of omphacite fabric but increases slightly the fabric strength 4 grain boundary processes dominate the deformation of garnet under high water fugacity or high shear strain conditions yielding a random lpo similar to that of non deforming garnet despite the strong shape preferred orientation spo observed 110 001 slip may dominate the deformation of rutile quartz displays complicated and inconsistent lpos in eclogite these results are remarkably similar to observations from deformed eclogites in nature

melting of pyrope mg3al2si3o12 up to 10 gpa possibility of a pressure induced structural change in pyrope melt the melting of pyrope mg3al2si3o12 has been determined experimentally at pressures up to 10 gpa pyrope melts congruently in the pressure range 3 5 10 gpa at pressures below 7 gpa aluminous enstatite of pyrope composition crystallizes during quenching whereas quench pyrope appears at higher pressures moreover the slope of the melting curve of pyrope decreases significantly at pressures above 7 gpa these changes in both the nature of the quench phase and the slope of the melting curve suggest a pressure induced coordination change for al3 in pyrope melt at around 7 gpa this pressure induced coordination change of al3 probably occurs even in magmatic liquids generated in the deep upper mantle and should affect some of their physical properties

plastic deformation of garnets systematics and implications for the rheology of the mantle transition zone abstract plastic properties of materials with garnet structure have been studied under wide temperature conditions ranging from room temperature to 95 of the melting temperatures using uniaxial compression and hot microhardness tests garnets studied include single crystals of oxide garnets y 3 al 5 o 12 gd 3 ga 5 o 12 and y 3 fe 5 o 12 and silicate garnets various solid solutions including grossular almandine andradite pyrope spessartine and uvarovite both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions t t m and and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the earth s mantle based on both stress dip tests and microstructural observations it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes this conclusion is consistent with the high peierls stress inferred from the hot hardness tests the high peierls stress in garnets is presumably due to the large unit cell i e long burgers vectors and or the bcc packing which are common to all garnets we postulate therefore that the present results can be applied to the strength of high pressure garnet majorite and suggest that garnet rich layers in the earth such as subducted oceanic crust in the transition zone or a possibly garnet rich bottom part of the transition zone will be considerably stronger than surrounding regions

rheological control of oceanic crust separation in the transition zone mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle we have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle confirming previous results the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling however a soft layer may exist between the strong garnet crust and cold slab interior models employing a simplified newtonian sandwich model show that this thin weak layer can effectively decouple the crust and slab once entrained into the lower mantle the then lighter crust can rise sufficiently fast as a rayleigh taylor instability to avoid further entrainment these results suggest that the crustal component of slabs may be trapped at 670 km depth leading to a garnet enriched transition zone

rock deformation ductile and brittle rock deformation studies are conducted to constrain the non elastic behavior of rocks subjected to non hydrostatic stresses in various geological and geophysical situations including long term creep responsible for large scale tectonics such as mantle convection and brittle failure related to earthquakes significant advancements made in these areas during the last quadra annual period are reviewed in ductile deformation important progress has been made toward understanding rheological properties under deep mantle conditions these studies provide important insights into the dynamics of the earth s deep interior in addition significant progress has been made toward understanding the role of chemical environment on rheology and the influence of microstructures such as grain size in the brittle field important advances were made in the observation of the spatio temporal evolution of faulting the complex interplay of microcracking crystal plasticity diffusive mass transfer and pore collapse in controlling the brittle plastic transition under crustal conditions have been elucidated recent investigations demonstrate the importance of fluid flow on frictional strength new measurements have clarified the influence of normal stress temperature and mineralogy on friction constitutive behavior some of these experimental observations are being incorporated into crustal faulting models that provide physical insights into the dynamics of the earthquake cycle

the strength of mantle silicates at high pressures and room temperature implications for the viscosity of the mantle the rheological properties of the upper mantle important in convective and tectonic processes have long been studied through low pressure creep experiments on olivine1 5 in comparison our understanding of deformation in the deep earth is incomplete because there are few experimental constraints on the rheology of the transition zone 400 670 km depth and the lower mantle estimates of the viscosity at these depths have had to rely on geophysical models of for example post glacial uplift the geoid and changes in the earth s rate of rotation6 13 recently we developed techniques for measuring solid state creep at mantle pressures14 and here we use these methods to make direct laboratory measurements of the strength of key mantle minerals we study mg fe 2sio4 olivine representative of about 60 of the upper mantle mg fe 2sio4 spinel which constitutes a large fraction of the transition zone and mg fe sio3 perovskite and perovskite mg fe o magnesiowiistite assemblages which comprise most of the lower mantle all at room temperature and pressures of up to 60 gpa our results suggest that the transition zone may form a layer of relatively high viscosity or strength between the upper and lower mantle a simple theological model shows that this behaviour may be compatible with current geophysical observations

layered rheological structure of subducting oceanic lithosphere abstract the subducting oceanic lithosphere which was formed by petrological differentiation of pyrolite at mid ocean spreading centers is in this paper no longer considered as a compositionally and mechanically homogeneous rigid plate or viscous slab as it was in earlier papers but as a sandwich made up of different lithological layers the effect of phase transformation as a function of depth with respect to changes in rheology of the downgoing slab is also considered it is shown that the rheological behavior of the descending slab is strongly controlled by its lithological and thermal structure our results provide alternative and plausible interpretations for the distribution of seismicity within the wadati benioff zone and for the fate of subducted oceanic lithosphere at the 670 km discontinuity these results although they are limited by the availability and quality of thermodynamical and rheological parameters for different rock types will advance our understanding of rheological properties of the subducting oceanic lithosphere

tem observations of several spinel garnet assemblies toward the rheology of the transition zone in order to better identify the mineral phase which controls the rheology of the transition zone between 410 and 660 km depth transmission electron microscopy observations were made on several coexisting spinel garnet assemblies alkremite xenolith pyrope rich mgo 1 1al2o3 spinel assembly deformed at 1173k 800 mpa in a griggs apparatus mg fe 3 al mg si 2si3o12 majorite mg fe 2sio4 spinel assembly synthesized in a laser heated diamond anvil cell it was found that garnet crystals systematically remain undeformed while spinel crystals are plastically deformed these results are in accord with the assumption that the rheology of majorite is stronger than the rheology of spinel in the conditions of the transition zone

ultradeep 300 kilometers ultramafic xenoliths petrological evidence from the transition zone the seismologically delineated transition zone at depths between 400 and 670 kilometers is a fundamental discontinuity in the earth that separates the upper mantle from the lower mantle xenoliths from within or close to the transition zone are dominated by pyropic garnet and associated pyroxene or mineralogically heterogeneous garnet lherzolite these xenoliths show evidence for the high pressure 90 to 120 kilobars transformation of pyroxene to a solid solution of pyroxene in garnet majorite and silicon in octahedral coordination low pressure less than 80 kilobars exsolution of clinopyroxene or orthopyroxene from the original majorite is preserved although mineral modes and rock proportions below the transition zone and the relative amount of eclogite present cannot be accurately assessed from the xenoliths it is likely that both majorite and beta spinel help produce the observed seismic gradient of the transition zone

arsenic incorporation into authigenic pyrite bengal basin sediment bangladesh abstract sediment from two deep boreholes 400 m approximately 90 km apart in southern bangladesh was analyzed by x ray absorption spectroscopy xas total chemical analyses chemical extractions and electron probe microanalysis to establish the importance of authigenic pyrite as a sink for arsenic in the bengal basin authigenic framboidal and massive pyrite median values 1500 and 3200 ppm as respectively is the principal arsenic residence in sediment from both boreholes although pyrite is dominant ferric oxyhydroxides and secondary iron phases contain a large fraction of the sediment bound arsenic between approximately 20 and 100 m which is the depth range of wells containing the greatest amount of dissolved arsenic the lack of pyrite in this interval is attributed to rapid sediment deposition and a low sulfur flux from riverine and atmospheric sources the ability of deeper aquifers 150 m to produce ground water with low dissolved arsenic in southern bangladesh reflects adequate sulfur supplies and sufficient time to redistribute the arsenic into pyrite during diagenesis

vulnerability of deep groundwater in the bengal aquifer system to contamination by arsenic arsenic levels in shallow groundwater in the bengal basin exceed thresholds for safe drinking water groundwater modelling indicates that deep wells that reach safe water below 150 m could remain safe for centuries if used for domestic water only whereas the intensive use of deep groundwater for irrigation could contaminate this resource within decades shallow groundwater the primary water source in the bengal basin contains up to 100 times the world health organization who drinking water guideline of 10 g l 1 arsenic as threatening the health of 70 million people groundwater from a depth greater than 150 m which almost uniformly meets the who guideline has become the preferred alternative source the vulnerability of deep wells to contamination by as is governed by the geometry of induced groundwater flow paths and the geochemical conditions encountered between the shallow and deep regions of the aquifer stratification of flow separates deep groundwater from shallow sources of as in some areas oxidized sediments also protect deep groundwater through the ability of ferric oxyhydroxides to adsorb as basin scale groundwater flow modelling suggests that over large regions deep hand pumped wells for domestic supply may be secure against as invasion for hundreds of years by contrast widespread deep irrigation pumping might effectively eliminate deep groundwater as an as free resource within decades finer scale models incorporating spatial heterogeneity are needed to investigate the security of deep municipal abstraction at specific urban locations

volcanic deposits in antarctic snow and ice major volcanic eruptions are able to spread large amounts of sulfuric acid all over the world acid layers of volcanic origin were detected for the first time a few years ago by hammer in greenland ice the present paper deals with volcanic deposits in the antarctic the different methods that can be used to find volcanic acid deposits in snow and ice cores are compared electrical conductivity sulfate and acidity measurements numerous snow and ice samples collected at several antarctic locations were analyzed the results reveal that the two major volcanic events recorded by h2so4 fallout in antarctic ice over the last century are the eruptions of krakatoa 1883 and agung 1963 both located at equatorial latitudes in the southern hemisphere the volcanic signals are found to be particularly well defined at central antarctic locations apparently in relation to the low snow accumulation rates in these areas it is demonstrated that volcanic sulfuric acid in snow is not even partially neutralized by ammonia the possible influence of antarctic volcanic activity on snow chemistry is also discussed using the three recent eruptions of the deception island volcano as examples only one of them seems to have had a significant effect on the chemistry of snow at a location 200 km from this volcano it is concluded that antarctic volcanic ice records are less complicated than greenland records because of the limited number of volcanos in the southern hemisphere and the apparently higher signal to background ratio for acidity in antarctica than in greenland

phase relations in the transition zone phase relations in the systems mgo al2o3 sio2 and cao mgo sio2 have been investigated experimentally at 166 226 kbar pressures and 1350 2500 c temperatures with a split sphere anvil apparatus ussa 2000 mgsio3 ilmenite is stable to lower pressures 175 kbar than found previously casio3 perovskite forms at p bar 15t c 151 000 three new high pressure phases are reported a phase with the diopsidic composition the cm phase a high pressure polymorph of al2o3 and superphase b with the tentative composition mg10si3o14 oh f 4 the invariant point for coexisting majorite mg4si4o12 perovskite mgsio3 beta phase mg2sio4 and liquid has been located at 224 kbar 2430 c with the eutectic melt composition on the enstatite forsterite join at 40 44 wt of forsterite the melting temperature of mgsio3 perovskite at 225 kbar is predicted to be 2600 c the new data made possible the calculation of a temperature pressure phase diagram for the system cao mgo al2o3 sio2 cmas in the pressure range 0 280 kbar the mineralogy of the earth s mantle and the possible mechanisms for producing large seismic velocity gradients in the transition zone are discussed on the basis of the derived phase relations

ultradeep ultramafic mantle xenoliths transmission electron microscopy preliminary results abstract the defect microstructure of ultradeep garnets from the jagersfontein kimberlite pipe south africa was investigated by transmission electron microscopy these crystals exhibit a high density of dislocations most of which form well organized and regularly spaced subgrain boundaries free dislocations are also detected most of them have a burgers vector 1 2 111 shortest lattice repeat but a few 100 burgers vectors are also characterized especially in junctions small needle shaped pyroxene exsolutions are also detected they exsolved from the si rich garnets during assent to the earth s surface their nucleation and growth required bulk diffusion and this process may have affected the original dislocation microstructure it is believed however that the observed dislocation pattern still reflects the original deformation regime of garnets at very high pressure and temperature and this dislocation pattern strongly suggests that garnets were highly ductile finally a number of precipitates of a dense amorphous phase of pure carbon are also detected they might result from the amorphization of small diamonds exsolved from the garnet as the pressure was progressively released

the distribution of hydroxyl in garnets from the subcontinental mantle of southern africa 166 garnets of dominantly mantle origin were analyzed for oh content by infrared ir spectroscopy ir spectra in the 3400 3700 cm 1 region display consistent absorption patterns attributable to oh structurally bound within the garnet crystal occasionally contaminated by low intensity oh absorptions from microscopic inclusions the principal structural oh absorption occurs near 3570 cm 1 with the appearance of additional absorptions near 3512 cm 1 and 3650 cm 1 dependent on garnet composition or paragenesis samples derive from a wide variety of rock types occurring as xenoliths in kimberlites of southern africa oh abundances using the best currently available calibration range from less than 1 up to 135 ppm h 2o and increase in the general order as follows on craton eclogites group 1 basaltic kimberlite alnoite alkali basalt the oh contents of common lithospheric granets from coarse peridotites including several phlogopite bearing samples are typically less than 20 ppm h 2o for tectonic settings of kimberlites both on and off the archaean kaapvaal craton ti rich garnets from deformed peridotites are richer in oh supporting previous suggestions of association of these xenoliths with putative megacryst magmas subcalcic cr rich xenocrysts diamond inclusion garnets and garnets from diamondiferous eclogites have very low oh contents similar to eclogites and depleted peridotites without macroscopic diamonds the oh content of southern african peridotite and eclogite garnets are significantly lower on average than those previously examined from the colorado plateau diatremes while details of emplacement related h mobility in garnets remain to be established our results suggest that garnets record useful information on the role of water or other hydrous volatile species in petrological processes at their source regions in the mantle although garnets do not appear to constitute a large reservoir of mantle hydrogen the large stability range of oh bearing garnet in the crust and mantle implies wide applicability as a qualitative hydrobarometer

crystal preferred orientations of garnet comparison between numerical simulations and electron back scattered diffraction ebsd measurements in naturally deformed eclogites observations of dislocations sub grains and elongated crystal shapes support plastic deformation of garnet in laboratory experiments and naturally deformed eclogites to evaluate the crystal preferred orientations cpo of garnet formed in axial shortening pure shear and simple shear we performed numerical simulations of cpo development during plastic flow using the visco plastic self consistent model as input for the models we use the slip systems determined by transmission electron microscopy using experimentally deformed specimens although in garnet 66 slip systems are available slip on the k111l 110 system provides over 86 of the total strain in the simulations characteristic cpo distributions are produced for the three deformation paths with the cpo being strongest for axial shortening and weakest for simple shear compared with low symmetry minerals the pole figure densities of garnet which has cubic symmetry are weak k100l axes tend to align with the shortening direction in all three deformation modes the simulations are compared with cpo of naturally deformed garnet from nine eclogite samples from the alps norway and mali which contain 20 40 garnet all samples show weak garnet cpo only two samples have a cpo pattern similar to the simulations for simple shear no samples are similar to simulations for axial strain or pure shear the presence of other weaker minerals such as omphacite and quartz with volume fractions higher than garnet probably prevented garnet from becoming highly strained and developing characteristic cpos in these eclogites higher volume fractions of garnet and higher temperature conditions may however allow the development of garnet cpo in the mantle transition zone particularly within subducted oceanic material morb q 2004 published by elsevier ltd

the significance of large sediment supply active tectonism and eustasy on margin sequence development late quaternary stratigraphy and evolution of the ganges brahmaputra delta abstract borehole data from the ganges brahmaputra delta system unveil an intriguing late quaternary history controlled by immense sediment discharge tectonics and eustasy sea level first intersected a major portion of the lowstand surface 10 000 11 000 cal yr bp marking the onset of sediment trapping and delta growth despite rapid sea level rise 1 cm yr sediment load was sufficient to maintain relative shoreline stability during this time by 7000 cal yr bp accommodation was quickly infilled with slowed sea level rise leading to upstream channel migration and widespread dispersal of sands this forced coastal progradation along the western delta front where the ganges was situated and much of the river s fine grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf concurrently brahmaputra sediments were sequestered to an inland tectonic basin thus starving the adjacent shoreline and leading to transgression along the eastern delta by 5000 cal yr bp the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river mouth estuary present delta configuration appears to have developed by 3000 cal yr bp evolution of the tectonically active ganges brahmaputra g b system reveals important similarities and differences with other deltas overall facies succession follows that of basic models progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain however the timing thickness and controls of these deposits differ immense sediment discharge from the ganges and brahmaputra rivers allowed initial delta growth 2000 3000 yr earlier than the global average despite debouching onto a canyon incised high energy margin subsequently a thick 50 m transgressive systems tract was formed during the early holocene phase of delta aggradation contrasting with extensive shoreline retreat documented along most margins highstand progradation of coastal and shelf sequences ensued by the middle holocene but subsiding inland basins also favored accumulation of a thick to 40 m highstand sequence in the lower floodplains unlike many other delta systems subsidence in the subaerial g b delta is not dominated by compaction but rather by plate driven tectonic processes that generate rates up to 4 mm year overall the huge sediment load tectonic subsidence major seismic events and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta shelf and deep sea bengal fan throughout the late quaternary

melting experiments on the enstatite pyrope join at 80 152 kbar melting temperatures of the assemblage pyroxene garnet on the enstatite pyrope join were experimentally determined at 11 different pressures between 80 and 152 kbar with a split sphere anvil apparatus ussa 2000 the compositions of pyroxene garnet and melt coexisting along the solidus were also determined comparison with previous data at lower temperatures revealed a large temperature dependence of the compositions of garnet coexisting with pyroxene it is proposed that the observed variation in the garnet composition is caused by disorder in garnet the obtained alumina contents of orthopyroxene coexisting with garnet on the solidus at 80 110 kbar contributed to an improvement of the garnet peridotite thermobarometry an internally consistent set of thermodynamic parameters was derived which allows calculation of the temperature pressure phase diagram for the enstatite pyrope join at 20 260 kbar

a griggs deformation apparatus set up at lille the classical solid medium apparatus griggs apparatus is generally viewed as a machine of poor accuracy with a large amount of uncertainty however a new generation of equipment with new assemblies and experimental procedures has greatly enhanced the capability of the apparatus experiments performed on the apparatus set up in our laboratory on t oriented yule marble samples show that the yield strength of material can be measured with reasonable precision within 10 mpa and a yield strength as low as 50 mpa can be detected the apparatus is particularly adapted to work on small specimens e g single crystals a few millimetres in size