basement drilling in the western atlantic ocean 2 a synthesis of construction processes at the cretaceous ridge axis petrologic geochemical and geophysical studies of cores from dsdp sites 417 and 418 show that the cretaceous eruptive crust was formed at a stable slow spreading axis similar to the present mid atlantic ridge probably not close to major transform fractures interpretations of whole rock and glass chemical variation suggest that magma fractionation occurred in temporally and spatially transient storage reservoirs at various depths between the zone of magma segregation and the ridge crest we suggest that melt generation uprise and eruption are part of a related episode of activity involving rapid upward propagation of fractures eruptive episodes are recorded by stratigraphic sequences of contiguous mostly cogenetic lavas separated by breccias sediment and or extreme alteration zones suggestive of lengthly quiescent intervals the section drilled at 417d comprises three such episodes in 365 m of basement whereas seven are recorded in 418a in 540 m patterns of downhole changes in magnetic inclinations and polarities strongly support the suggestion that iterative burial of successive lava episodes occurs with crustal spreading from the axis this model predicts that the eruptive layer is imbricated and that it shows increasing complexity of lithology chemical compositions and magnetic character with depth studies of crack distribution and laboratory measurements of core physical properties indicate preferential zones of water circulation which decrease with depth permeability of the lava pile is largely controlled by the imbricate eruptive structure

basement drilling in the western atlantic ocean 1 magma fractionation and its relation to eruptive chronology petrologic geochemical studies of basalts drilled in cretaceous oceanic crust dsdp legs 51 53 in the western atlantic ocean are synthesized in quantitative magma fractionation models from least squares analysis of whole rock glass and phenocryst phase compositions it is concluded that fractionation by crystallization of olivine plagioclase and clinopyroxene involved gravitative separation of mafic phases from plagioclase flotation of plagioclase together with disequilibrium reaction morphologies suggest that crystallization began shortly after the release of primitive liquids from the mantle polybaric fractionation and mixing of derivative magma batches are indicated by evidence for solid liquid reaction and low pressure reequilibration distinct eruptive series bounded by magnetic and other stratigraphic discontinuities correspond to chemically defined magma fractionation series the latter are probably differentiated from one another through small differences in degree of partial melting their lithophile element character shows evidence for homogeneous depleted source material persisting during crustal construction at the atlantic ridge axis the whole rock and glass selvedge compositional averages for eruptive units reflect the importance of posteruptive phenocryst redistribution and syn eruptive changes in liquid fraction compositions

an experimental investigation of the influence of water and oxygen fugacity on differentiation of morb at 200 mpa crystallization experiments were performed at 200mpa in the temperature range 1150 950 c at oxygen fugacities corresponding to the quartz fayalite magnetite qfm and mno mn3o4 buffers to assess the role of water and fo2 on phase relations and differentiation trends in mid ocean ridge basalt morb systems starting from a primitive mgo 9 8 wt and an evolved morb mgo 6 49 wt crystallization paths with four different water contents 0 35 4 7 wt h2o have been investigated in primitive morb olivine is the liquidus phase followed by plagioclase clinopyroxene amphibole is present only at water saturated conditions below 1000 c but not all fluid saturated runs contain amphibole magnetite and orthopyroxene are not stable at low fo2 qfm buffer residual liquids obtained at low fo2 show a tholeiitic differentiation trend the crystallization of magnetite at high fo2 mno mn3o4 buffer results in a decrease of melt feo mgo ratio causing a calc alkaline differentiation trend because the magnetite crystallization temperature is nearly independent of the h2o content in contrast to silicate minerals the calc alkaline differentiation trend is more pronounced at high water contents residual melts at 950 c in a primitive morb system have compositions approaching those of oceanic plagiogranites in terms of sio2 and k2o but have ca na ratios and feo contents that are too high compared with the natural rocks implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites

comments on composition and depth of origin of primary mid ocean ridge basalts by d c presnall and j d hoover presnall and hoover 1984 in their evaluation of the nature of primary mid ocean ridge basalts morbs concluded that earlier data suggesting many of the most primitive morbs are derived from high mgo picritic magmas are not valid in reaching this conclusion presnall and hoover 1984 ignore several important considerations regarding 1 analytical uncertainties 2 tio2abundances in morbs 3 least squares calculations 4 complex models of magmatic evolution and 5 enstatite saturation i briefly discuss these points below

early to late pleistocene human settlements and the evolution of lithic technology in the nihewan basin north china a macroscopic perspective the paleolithic of the nihewan basin is characterized by large numbers of sites a long term scale definite strata and rich cultural remains recently obtained evidence of human activities and dates indicate that the early to late pleistocene paleolithic sites in the nihewan basin form a concentrated time and cultural sequence lithic technology developed according to a continuous evolution model the development of the small tool industry is generalized as the donggutuo zhiyu also called shiyu series and the origin of microlithic technology could be traced back to the early pleistocene which supports the theory of local origin the complexity of lithic technology development is also discussed

late quaternary evolution of the burullus lagoon region north central nile delta egypt abstract the evolution of late quaternary stratigraphic sequences in the burullus lagoon region of the north central nile delta egypt is determined by petrological analysis of radiocarbon dated sediment borings shallow marine lagoonal and fluvial deposits accumulated during the late pleistocene and delta plain to coastal facies during the holocene since about 7500 yrs b p the latter sequence is characterized primarily by an irregular succession of delta plain deposits i e distributary channel fill marsh and lagoon complete coarsening upward sequences formed by prodelta to coastal units and previously defined in the northeastern nile delta are not observed in this region long term sediment accumulation rates for holocene facies during the build up of the delta range from about 100 to 300 cm 1000 yrs the location and ages of three nile distributary channel systems are recognized by means of cores from east to west sebennitic saitic and bolbitine rosetta the mouth of the sebennitic which is oldest of the three was located on what is now the inner to middle shelf this would explain the absence of prodelta and delta front sequences and a presence of a belt of medium to coarse grained sands extending seaward of the coast the distribution in time and space of the holocene facies is interpreted in terms of the interplay of factors controlling sedimentation i e eustatic sea level oscillations climatic changes and land subsidence minor in this region 0 04 cm yr of the three sea level changes dominate paleogeographic maps show through a series of time slices the changing configuration of this region during the past 30 000 years maps indicate the shifting channel systems and the major displacement of associated marshes and lagoons this study suggests that without the emplacement of coastal protection barriers and containment structures an expansion of burullus lagoon may be expected in the near future

major element chemistry of galapagos rift zone magmas and their phenocrysts abstract basalts dredged from the galapagos rift zone between 85 w and 100 w were analyzed by electron microprobe to determine the chemistry of the glass exteriors and included phenocrysts microphenocrysts and quench minerals the basalts come from both normal mid ocean ridge segments and from ridge segments that cross the galapagos platform the basalts fall into two chemical and geographical groups group a basalts come from outside the central region of the galapagos platform i e outside 89 92 5 w and are chemically similar to basalts from normal ocean ridge segments group b basalts come from the center of the galapagos platform 89 92 5 w and are enriched in incompatible elements like plume influenced basalts from the mid atlantic ridge the spinel olivine plagioclase and clinopyroxene phenocrysts in both groups of basalts are low pressure equilibrium phases but the chemical difference among basalts from within each group indicates high pressure fractional crystallization is also responsible for the chemical evolution of some of these basalts presently no crystallization or partial melting model can relate the chemistry of the two groups of basalts and the compositional influence of a large ion lithophile elements and water rich mantle beneath the galapagos platform is a viable alternate hypothesis the eruption temperature of magmas from the normal ridge segments as determined by olivine liquid thermometry is 1217 10 c suggesting steady state conditions but on the galapagos platform the eruption temperatures are lower and more variable than on the normal ridge 1186 c 30 c suggesting a more complex plumbing system and the absence of a steady state magma chamber beneath the platform

primitive melt replenishment and crystal mush disaggregation in the weeks preceding the 2005 2006 eruption 9 50 n epr abstract the 2005 2006 eruption at 9 50 n east pacific rise provides an exceptional opportunity to investigate the magma plumbing system beneath a well studied ridge system the eruption was preceded by two years of increasingly intense seismicity and occurred from the same location as a previous and well characterized eruption in 1991 1992 here we use the crystal cargo of samples from this eruption to investigate magma reservoir processes in the lead up to the eruption as well as their temporal relationship to the seismicity that preceded it compositional zoning in some plagioclase crystals indicates primitive melt replenishment occurred roughly six weeks or less before the eruption this replenishing event is seen only in the crystals from the central region of the eruption 9 50 9 52 n this is also the area where the most primitive lava compositions are observed and together these observations support models of replenishment being spatially focused the short time between the input of a more primitive melt and eruption onto the seafloor suggests replenishment likely contributed to triggering the eruption rare resorbed plagioclase crystals and glomerocrysts of plagioclase and olivine some of which have rims far from equilibrium with their host melt suggest that disaggregation of a crystal mush occurred within a few days prior to eruption interstitial melt from within this mush zone must have been mixed back into the erupted lava a form of in situ crystallization thus the erupted magmas evolved in a replenished tapped magma reservoir in which at least a part of the crystallization occurred in situ

linked halokinesis and mud volcanism at the mercator mud volcano gulf of cadiz mud volcanoes are seafloor expressions of focused fluid flow that are common in compressional tectonic settings new high resolution 3 d seismic data from the mercator mud volcano mmv and an adjacent buried mud volcano bmv image the internal structure of the top 800 m of sediment at both mud volcanoes revealing that both are linked and have been active episodically the total volumes of extruded mud range between 0 15 and 0 35 km3 and 0 02 0 05 km3 for the mmv and the bmv respectively the pore water composition of surface sediment samples suggests that halokinesis has played an important role in the evolution of the mud volcanoes we propose that erosion of the top of the vernadsky ridge that underlies the mud volcanoes activated salt movement triggering deep migration of fluids dissolution of salt and sediment liquefaction and mobilization since the end of the pliocene since beginning of mud volcanism in this area the mud volcanoes erupted four times while there was only one reactivation of salt tectonics this implies that there are other mechanisms that trigger mud eruptions the stratigraphic relationship of mudflows from the mmv and bmv indicates that the bmv was triggered by the mmv eruptions this may either be caused by loading induced hydrofracturing within the bmv or due to a common feeder system for both mud volcanoes this study shows that the mud volcanoes in the el arraiche mud volcano field are long lived features that erupt with intervals of several tens of thousands of years

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

galapagos hot spot spreading center system 2 87sr 86 sr and large ion lithophile element variations 85 w 101 w thirty eight samples of basalts dredged from the galapagos spreading center gsc between 85 w and 101 5 w longitudes have been analyzed for k rb cs ba and sr contents as well as for 87sr 86sr ratios basalts between 85 w and 89 w segment of the gsc range from 0 7025 to 0 7028 in 87sr 86sr and those between 95 5 w and 101 5 w range from 0 7026 to 0 7027 these basalts are characterized by lree depleted patterns basalts from 89 w to 95 5 w segment having slightly lree depleted to lree enriched patterns la sm e f between 0 8 and 2 4 range from 0 7027 to 0 7031 in 87sr 86sr the longitudinal 87sr 86sr variation shows a maximum at about 92 w where the gsc is nearest to darwin island the alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles but these occur at about 91 w the apparent shift of 87sr 86sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting the highest 87sr 86sr from the gsc is only slightly lower than the highest 87sr 86sr for tholeiitic basalts from the galapagos archipelago several possible hypotheses have been examined to account for these geochemical and sr isotopic variations in the light of the presently available data the most promising explanation seems to be a binary mixing of hot spot or plume type material with the source of normal mid ocean ridge basalts and resulting rift propagation both end members seem to be heterogeneous and therefore it is at present difficult to put severe constraints on this binary mixing nevertheless the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91 w and 92 5 w fracture zones toward the rift

minor and trace element geochemistry of volcanic rocks dredged from the galapagos spreading center role of crystal fractionation and mantle heterogeneity a wide range of rock types abyssal tholeiite fe ti rich basalt andesite and rhyodacite were dredged from near 95 w and 85 w on the galapagos spreading center computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization however the p2o5 k2o ratio averages 0 83 at 95 w and 1 66 at 85 w and implies distinct mantle source compositions for the two areas these source regions also have different rare earth element ree abundance patterns with la sm ef 0 67 at 95 w and 0 46 at 85 w the sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95 w the mantle source regions for these two areas are interpreted to be depleted in incompatible and volatile elements although the source region beneath 95 w is less severely depleted in la and k incompatible trace element abundances in 26 samples are used to infer that the range of fe ti rich basalt from 85 w represents 19 to 35 residual liquid following crystal fractionation of a mineral assemblage of plagioclase clinopyroxene and lesser olivine the most highly differentiated samples have also had less than 1 titanomagnetite removed most samples from 85 w can be related to a common parental magma that contained approximately 9 wt feo 1 wt tio2 and had an mg number mg 100 mg mg fe2 of about 65 although the samples from 95 w cannot all be derived from a common parental magma the inferred parental magmas may have been derived by varying degrees of partial melting of a common source the fractionation sequence consists of two parts an initial iron enrichment trend followed by a silica enrichment trend we interpret the trace element data to indicate that the most iron rich lavas represent about 32 residual liquid derived by crystal fractionation of plagioclase clinopyroxene and lesser olivine from a parental magma with an mg number of about 66 the silica enrichment trend results from crystallization of titanomagnetite and some apatite fractionation of pigeonite which is a minor phase in the major element models cannot be distinguished from clinopyroxene fractionation by using trace elements

gravity anomalies over extinct spreading centers a test of gravity models of active centers gravity fields over active mid ocean ridges display long wavelength free air gravity highs with superimposed short wavelength anomalies over the spreading axis these axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30 70 mgal and wavelengths of 40 60 km but decrease in amplitude and wavelength with increasing spreading rate until at fast spreading centers the anomalies are positive 10 20 mgal with wavelengths of about 20 30 km similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo spreading rate dependency suggesting that a large component of the anomaly is not dynamically maintained calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows the amplitudes of the residual lows at slow paleo spreading rates are 25 45 mgal with half widths of about 20 45 km and decrease with increasing spreading rate to about 5 mgal amplitude and 10 15 km half width over fast paleo spreading ridges the amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers most previously proposed models of mid ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid ocean ridges this research provides a further constraint on geometric models of accretion as these models predict vastly different gravity fields results indicate that a frozen in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis this conclusion is consistent with the presence of a low density gabbroic root situated at shallow mantle depths beneath the ridge axis this root tends to narrow with increasing spreading rate producing the required residual gravity field variation

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

low pressure phase equilibria of anhydrous anorthite bearing mafic magmas 1 one of the most persistent questions regarding the phase equilibria of mid ocean ridge basalts morb pertains to the petrogenesis of the anorthitic plagioclase phenocrysts an90 that are characteristic of the more primitive members of such suites anorthitic phenocrysts are present in many if not most morb suites in spite of the fact that no naturally occurring morb glasses have ever been discovered to be in equilibrium with plagioclase more calcic than an85 we have addressed this paradox by attempting to saturate natural basalts with anorthite in a series of 1 atm experiments using three different natural basaltic starting compositions an n morb an e morb and a continental high alumina basalt to ensure duplication of the olivine and anorthite saturation observed in natural anorthite bearing basalt the experiments were run in an93 6 capsules with fo92 olivine added to the starting glass the compositions of experimental liquids are generally colinear with the trends observed in the lava suites used as the source material for the starting glasses significantly aluminous spinel al2o3 contents of 61 68 wt was produced at 1290 c in all compositions and chromites al2o3 contents of 33 42 wt at lower temperatures in n morb derived liquids despite no spinel having been added to the starting mixture in addition the experiments produced basaltic liquid in equilibrium with both fo89 olivine and an85 feldspar at temperatures of 1230 and 1210 these liquids have compositions with mg at mg mg fet 100 that range from 63 to 85 the tio2 mgo correlation indicates large 16 23 amounts of crystallization for each percent decrease in mgo these results suggest the possibility that dry anorthite bearing basaltic magmas are the product of the interaction between primary melt and al spinel bearing upper mantle in addition the results indicate that morb magmas can undergo a large amount 50 of crystallization prior to reaching 8 mgo further although anorthite bearing magmas have characteristics consistent with their being a significant volumetric component of morb parent magmas the reaction mechanism suggested for their petrogenesis indicates that they are not necessarily primary magmas

what triggered the early to mid pleistocene tectonic transition across the entire eastern mediterranean abstract subduction plays a fundamental role in plate tectonics and when interrupted it may trigger a series of geodynamic and sedimentary responses synchronous structural modifications recorded across the entire eastern mediterranean region are dated to a relatively short period early to mid pleistocene these deformations are documented within plates e g arabian sinai and african plates along plate boundaries e g dead sea and north anatolian faults and cyprus arc and in the mediterranean basin during the same period the northward subduction of the sinai plate was interrupted when the eratosthenes seamount cyprus arc collision initiated subduction collision processes of the eastern mediterranean serve as a unique modern analogue for similar settings worldwide understanding their association with accompanying neo tectonic processes is therefore predominantly important by fostering a detailed and comprehensive approach this research provides a coherent tectonic picture for the eastern mediterranean early to mid pleistocene tectonic transition in order to explore its triggering mechanisms since the neogene convergence across the eastern mediterranean was accompanied by eurasian indentation by arabia northward motion westwards anatolia escape and southwards aegean propagation this semi counterclockwise plate motion was temporarily interrupted by the incipient seamount arc collision which is suggested here as a trigger of the early to mid pleistocene tectonic transition

petrology of a leucogabbroic interval within basal layered gabbros at north arm mountain bay of islands ophiolite data from detailed sample traverses in the layered gabbro unit of the north arm mauntain massif bay of islands ophiolite allow meter scale resolution of magmatic processes in spreading ridge magma chambers one suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase an73 7 87 5 average modal abundance 75 clinopyroxene mg 80 3 86 0 18 and olivine fo76 6 82 1 6 with intergranular orthopyroxene mg 78 0 83 3 1 and accessory cr al spinel cr 32 3 41 4 ilmenite rims spinel in one sample whole rock zr contents range from 6 to 15 ppm plots of stratigraphic height in the traverse versus petrogenetic indicators e g mg s of mafic phases and an in plagioclase reveal both normal and reverse cryptic variation patterns the patterns for all indices are generally correlated the normal portions of the patterns formed during fractional crystallization of basalt batches ranges of mineral compositions in the normal trends suggest that 29 38 crystallization of each batch of basalt occurred before magmatic replenishment the reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends experiments and observations of natural assemblages indicate that 55 modal plagioclase crystallizes from basalts at the olivine plagioclase clinopyroxene liquid piercing point the average plagioclase content of this suite of leucogabbros from north arm mountain is too high to have formed from simple crystallization at the piercing point petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount 10 of plagioclase was resorbed during mixing the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume other suites of layered gabbros from north arm mountain are not so plagioclase rich as the leucogabbros described above crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers or may occur episodically in the overall lifetimes of the magma chambers

the petrogenesis of plagioclase phyric basalts at mid ocean ridges 1 plagioclase ultraphyric basalts pubs have been sampled along most mid ocean ridges with ultraslow to intermediate spreading rates over the past 40 years the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation however when the global population of pub lavas is examined this hypothesis becomes less compelling pub host lavas demonstrate a large range of compositions and densities similar to aphyric glasses from the same ridge segments most importantly the majority of pub host liquids are less dense than their phenocryst cargo meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment furthermore pub lavas have never been sampled on axis at fast spreading centers or from locations with noted contemporaneous axial magma chambers where pubs should be abundant if plagioclase is buoyant in mid ocean ridge basalt morb instead we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust possibly followed by loss of olivine during magma ascent pubs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts which precludes long crustal residence times for these magmas in addition to being a proxy for lower spreading rates our findings also suggest that pub eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber

mio pleistocene zanda basin biostratigraphy and geochronology pre ice age fauna and mammalian evolution in western himalaya article i nfo the pliocene 5 3 2 6 ma of tibet witnessed the drying of the northern tibetan plateau and the approach to the pleistocene ice age within the background of intensifying indian and east asian monsoons yet little is known about pliocene mammals living on the high tibetan plateau despite the fact that fossil mammals elsewhere consti tute an important knowledge base for terrestrial environments the late miocene to pleistocene zanda basin at the northern foothills of the himalayas affords a welcome opportunity to evaluate the biological response to environ mental change at high elevations abundant well preserved fossil mammals and fish from an 800 m continuous section of fine to coarse grained sediments thus open a rare window into a past biological world for example the discovery of an ancestral wooly rhino from zanda basin that was the precursor of its late pleistocene megafaunal descendants leads to our out of tibet hypothesis suggesting that the high tibetan plateau was a pliocene cradle for ice age cold adaptations in this paper we document in detail the mammalian biostratigraphy chronology and paleozoogeography based on zanda basin fossil mammals our high resolution biostratigraphy and biochronology offer for the first time independent constraints that both support and modify recent magnetostratigraphic correlations using charac teristic pliocene and pleistocene mammals particularly the small mammal assemblages in the lower part of the section and monodactylid equus from the upper section we propose a correlation to c1n to c3an 1r with an age range of 400 ka to 6 4 ma within the 800 m zanda section the lower 0 150 m is of latest miocene age spanning 6 4 5 3 ma sparsely fos siliferous the lower section has produced five taxa so far ochotona panthera qurliqnoria palaeotragus a nd hipparion all are consistent with a late miocene age the middle 150 620 m section spans the entire pliocene this section is by far the most fossiliferous including such typical pliocene small mammals as prosiphneus mimomys apodemus a ndtrischizolagus as well as large mammals such as coelodonta thibetana hipparion zandaense chasmaporthetes nyctereutes meles antilospira and others in the upper 620 800 m section the fos sils are rare but do include characteristic pleistocene taxa such as equus zoogeographically zanda basin mammals are a mixture from two major sources taxa such as mimomys prosiphneus trischizolagus chasmaporthetes nyctereutes meles a ndxenocyon are commonly found in north china or east asia in contrast several forms such as unique species of pikas ochotona squirrels aepyosciurus and ancestral tibetan antelope qurliqnoria seem to belong to an indigenous tibetan fauna evolved within the plateau a lack of shared taxa with the oriental realm suggests a formidable barrierby the himalayas despite a short distance 100 km between zanda basin and the indian subcontinent

late cretaceous crustal growth in the gangdese area southern tibet petrological and sr nd hf o isotopic evidence from zhengga diorite gabbro abstract recent studies of gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern tibet during the jurassic early eocene however the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes here we report on the zhengga diorite gabbro suite from the gangdese area they consist of plagioclase labradorite and anorthite amphibole clinopyroxene biotite and minor magnetite epidote zircon and apatite la icp ms zircon u pb dating for two samples gives a ca 94 ma age for the zhengga intrusive rocks i e the late cretaceous apart from one high sio 2 52 2 wt diorite sample with slightly high k 2 o 1 72 wt and initial 87 sr 86 sr 0 7068 and low e nd t 5 6 values the gabbro samples are geochemically characterized by low sio 2 39 8 50 1 wt and k 2 o 0 3 1 1 wt strongly negative nb ta and positive sr anomalies and uniform initial 87 sr 86 sr 0 7043 0 7048 the gabbros can be divided into two groups group i gabbros with relatively low total rare earth element ree rb and sio 2 contents and positive eu anomalies and group ii gabbros with slightly higher total ree rb and sio 2 contents and negative eu anomalies the group i gabbros have e nd t 1 7 to 4 1 and zircon e hf t 6 5 to 11 1 and 18 o 5 89 to 7 24 values which are slightly different to those of the group ii gabbros 0 2 to 2 0 2 9 to 6 5 and 6 24 to 7 05 trace element compositions of amphibole and clinopyroxene grains suggest that the zhengga mafic magmas contained a significant fluid transported component probably released from subducted oceanic lithosphere we suggest that the parental magmas of the zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts fluids the group i gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas with insignificant crustal contamination whereas the group ii gabbros were probably produced by assimilation and fractional crystallization afc processes from mafic magmas that were geochemically similar to the group i gabbros pre collisional underplating of mantle derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late late cretaceous cenozoic granitoids of the gangdese batholiths this study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust

subsidence in the northeastern nile delta rapid rates possible causes and consequences holocene fluvial and marine deposits have accumulated in a graben like structure on the northeastern margin of the nile delta this part of the delta which includes lake manzala port said and the northern suez canal has subsided rapidly at rates of up to 0 5 centimeter per year since about 7500 years ago this subsidence has diverted at last four major distributaries of the nile river into this region the combined effects of continued subsidence and sea level rise may flood a large part of the northern delta plain by as much as 1 meter by the year 2100 the impact of continued subsidence now occurring when sediment input along the coast has been sharply reduced because of the aswan high dam is likely to be substantial particularly in the port said area and as far inland as south of lake manzala