al substituted goethite an indicator of pedogenic and other weathering environments in south africa abstract over 200 goethite containing samples from widely diverse environments along the eastern seaboard of south africa were examined and described mainly by x ray powder diffraction xrd aluminium substituted goethites ranging from 0 to about 32 mole alooh were identified mainly by xrd line shift and chemical extraction they occur in every soil zone and major soil type in one or other form from sources such as 1 non hydromorphic highly weathered strongly acid soils ferruginous bauxites saprolites and moderately weathered moderately acid soils environments 2 hydromorphic environments mottles concentrations pipe stems crusts ferricretes geodes and 3 crusts in calcareous environments these goethites formed in different climatic and weathering environments were grouped according to their amount of al substitution and crystallinity as determined by xrd line broadening al substitution in goethite formed in hydromorphic moderately acid soils and calcareous environments was generally low 0 15 mole goethite formed in non hydromorphic highly weathered environments was generally high in al substitution 15 32 mole and increased in the following order saprolites and strongly acid freely drained soils this study showed that the degree of al substitution and crystallinity of goethites may reflect the environment in which they have formed and serve as an indicator of soilforming processes

geochemistry of leptyno amphibolite complex from haut allier french massif central abstract mafic metaigneous rocks of the leptyno amphibolite group of the french massif central in the haut allier region include amphibolites granulites and other high grade metamorphic rocks such as eclogites many granulites and eclogites were retromorphosed into amphibolite facies most rocks have basaltic compositions some amphibolites have a cumulate character and resemble oceanic gabbros amphibolites without high grade relics retain most of their original compositions typical of oceanic tholeiites including flat ree patterns zr nb 10 17 la nb 0 6 0 9 ti v 18 38 and ti zr 76 97 and were probably formed in the back arc environments the other rocks including amphibolites with high grade relics are enriched in incompatible elements such as light ree rb and ba at least partially due to secondary processes e g retrograde metamorphism effects of neighbouring granite intrusion

pre alpine variscan inheritance a key for the location of the future valaisan basin western alps the boundary between the helvetic and the penninic brianconnais zones has long been recognized as a major fault penninic front in the western alps a narrow oceanic domain has been postulated at least along part of this boundary the valaisan ocean however the information provided by the pre triassic basement has not been fully exploited and will be discussed here in detail the igneous and metamorphic history of the pre triassic basement shows significant differences between the external massifs from the helvetic zone with abundant late carboniferous granites and the basement of the brianconnais zone including the internal massifs dora maira gran paradiso monte rosa devoid of carboniferous granites a major coal bearing basin the zone houillere opened along this boundary this limnic intramontane basin has never been properly investigated the zone houillere is not comparable with the external paralic flexural basins on both sides of the variscan belt but shows similarities with the saar saale basin like the latter we interpret the zone houillere as a transtensional basin opened along a major crustal scale fault zone namely the east variscan shear zone the permian magmatism and sedimentation displays contrasting distributions being absent or very localized in the helvetic zone and widespread in the penninic zone the above data indicate that the structural inheritance from the variscan belt plays a major role in defining the future location of the valaisan basin i e the boundary between the european palaeomargin and the brianconnais microcontinent

u pb zircon and monazite dating of a mafic ultramafic complex and its country rocks u pb data on zircons from the largest mafic ultramafic body 6 2 km of the french central massif sauviat sur vige yield the following age results primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the cambro ordovician 496 25 17 m y variable transformation under eclogite facies conditions was hercynian 320 29 36 m y the same age pattern derived by u pb monazite analyses was found also for the immediate country rocks i e kyanite bearing coarse grained metagranites occurring to the w and n of the sauviat massif due to the fact that there is no regional hercynian high grade metamorphism in this part of the french central massif e g duthou 1977 bernard griffiths 1975 both mafic ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre hercynian acadian caledonian crustal rocks the cause for such a hercynian tectonism is thought to be due to continent continent collision of the spanish with the armorican plate

precambrian u pb zircon ages in eclogites and garnet pyroxenites from south brittany france an old oceanic crust in the west european hercynian belt abstract u pb zircon ages have been determined for two eclogites from the vendee and for two garnet pyroxenites from the baie d audierne in an episodic pb loss model the two discordia would give upper intercept ages around 1300 1250 ma and lower intercepts ages of 436 384 ma two interpretations are proposed 1 the 1250 1300 ma ages may reflect an unspecified upper mantle event or process the paleozoic ages correspond to the tectonic emplacement of an eclogitic mantle fragment into the continental crust 2 the protolith may have been extracted from the upper mantle 1250 1300 ma ago and stored in a crustal environment until it was metamorphosed under high pressure conditions around 400 ma ago this latter model is favoured by available geologic and isotopic data consequently we propose that a 1300 ma old oceanic crust was tectonicly incorporated into a sialic basement during the proterozoic this mixture was subsequently subducted during the paleozoic

mineralogy and stratigraphy of three deep lateritic profiles of the jos plateau central nigeria abstract the fluviovolcanic series fvs of the jos plateau central nigeria are an upper eocene to middle miocene time equivalent to the continental terminal of the surrounding basins it can be divided into a lower part with mafic to intermediate and even felsic volcanic rocks according to the zr tio2 fluvial or limnic sediments and an upper part of mafic volcanic rocks with intercalated transported pisoids and quartz grains palynomorphs of a limnic clay reflect a subtropical to montane tropical vegetation rich in plant species the lower part of the fvs often shows distinct tilting as a result of neotectonic movement along a reactivated conservative plate boundary in continuation of the transatlantic romanche fracture zone the various rocks have been completely converted into deep often 80 m polygenetic soils which consist of repetitive sequences of saprolite mottled horizon and ferricrete the ferricrete represents a paleosurface and protects the profiles against erosion the mineralogy of 3 profiles consists essentially of kaolinite with various amounts of goethite and hematite gibbsite appears in one confined horizon whereas anatase and residual weathered ilmenite and quartz occur as accessories the presence or absence of ilmenite quartz and anatase in the various sections of the profiles were taken as an indicator for different parent rocks the crystallinity of kaolinite hinckley index the crystal size and al substitution of goethite and hematite varied sectionwise indicating different weathering environments for example the goethite in laminar massive ferricretes appearing at the base of all 3 profiles had lower al substitution and somewhat larger crystals than that of the pisolitic vermiform ferricretes and was associated with redoximorphic conditions on the basis of mineralogical paleontological sedimentological and absolute k ar age of basalts the profiles were placed into the transition from paleogene to neogene and stratigraphic correlation was suggested

evolution of the seismicity in the eastern marmara sea a decade before and after the 17 august 1999 izmit earthquake we review the long term evolution of seismicity in the eastern marmara sea over a decade before and after the 1999 mw 7 6 izmit earthquake we analyze large scale space time variations of seismicity in the region and illustrate the impact of the recent large strike slip earthquakes on the background activity composed of distinct pre existing seismic clusters two types of aftershocks activity are observed the first type of enhancement is on strike slip fault segments izmit fault princes island section of the main marmara fault gemlik fault immediately following the main shock and related to coulomb stress transfer the second type of enhancement is attached to extensional clusters yalova tuzla with a few days delay in the onset of strong activation probably related to pore pressure increase we observe a fast decay of the activity on strike slip segments and slower evolution of seismic clusters with extensional features two years after the izmit earthquake seismic activity returned to the pre earthquake pattern with most of the activity occurring within extensional clusters it appears that the influence of the last large strike slip event on the spatial seismicity distribution in the eastern marmara sea is less significant than the effect of the long term regional extension

free gas and gas hydrates from the sea of marmara turkey chemical and structural characterization gas hydrates and gas bubbles were collected during the marnaut cruise may june 2007 in the sea of marmara along the north anatolian fault system turkey gas hydrates were sampled in the western part of the sea of marmara on the western high and three gas bubble samples were recovered on the western high the central high center part of the sea of marmara and in the cinarcik basin eastern part of the sea of marmara methane is the major component of hydrates 66 1 but heavier gases such as c2 c3 and i c4 are also present in relatively high concentration the methane contained within gas hydrate is clearly thermogenic as evidenced by a low c1 c2 c3 ratio of 3 3 and carbon and hydrogen isotopic data 13cch4 of 44 1 pdb and dch4 of 219 smow a similar signature is found for the associated gas bubbles c1 c2 c3 ratio of 24 4 13cch4 of 44 4 pdb which have the same composition as natural gas from k marmara af field gas bubbles from central high show also a thermogenic origin as evidenced by a c1 c2 c3 ratio of 137 and carbon and hydrogen isotopic data 13cch4 of 44 4 pdb and dch4 of 210 smow whereas those from the cinarcik basin have a primarily microbial origin c1 c2 c3 ratio of 16 600 13cch4 of 64 1 pdb uv raman spectroscopy reveals structure ii for gas hydrates with ch4 trapped in the small 512 and large 51264 cages and with c2h6 c3h8 and i c4h10 trapped in the large cages hydrate composition is in good agreement with equilibrium calculations which confirm the genetic link between the gas hydrate and gas bubbles at western high and the k marmara af offshore gas field located north of the western high we calculate the characteristics of the hydrate stability zone at western high and in the cinarcik basin using the csm gem computer program the base of the structure ii hydrate stability field is at about 100 m depth below the seafloor at the western high site whereas in the cinarcik basin p t conditions at the seafloor correspond to the uppermost range for structure i hydrate formation from microbial gas

the surface rupture and slip distribution of the 17 august 1999 izmit earthquake m 7 4 north anatolian fault the 17 august 1999 izmit earthquake occurred on the northern strand of the north anatolian fault zone the earthquake is associated with a 145 km long surface rupture that extends from southwest of duzce in the east to west of hersek delta in the west detailed mapping of the surface rupture shows that it consists of five segments separated by releasing step overs herein named the hersek karamursel golcuk izmit sapanca lake sapanca akyazi and karadere segments from west to east respectively the hersek segment which cuts the tip of a large delta plain in the western end of the rupture zone has an orientation of n80 the n70 80 e trending karamursel golcuk segment extends along the linear southern coasts of the izmit gulf between karamursel and golcuk and produced the 470 cm maximum displacement in golcuk the northwest southeast striking golcuk normal fault between the karamursel golcuk and izmit sapanca segments has 2 3 m maximum vertical displacement the maximum dextral offset along the izmit sapanca lake segment was measured to be about 3 5 m and its trend varies between n80 e and east west the sapanca akyazi segment trends n75 85 w and expresses a maximum displacement of 5 2 m the karadere segment trends n65 e and produced up to 1 5 m maximum displacement the karadere and sapanca akyazi segments form fan shape or splaying ruptures near their eastern ends where the displacement also diminished

mineralogical and sub microscopic aspects of iron oxides in polygenetic oxisols from minas gerais brazil iron oxides are known to be useful paleo environmental indicators and their properties may give information about past and present circumstances of weathering and soil formation iron oxides from polygenetic oxisols developed on saprolites of metamorphic rocks soil saprolite sequences and on sediments soil sediment sequences from minas gerais brazil were studied by x ray diffraction mossbauer spectroscopy micromorphology and electron microscopy to establish the connection between their properties and circumstances of formation hematite and goethite are the only iron oxides found in the clay and silt fractions micromorphology and mineralogy indicated the presence of three genetically different forms of hematite and four forms of goethite xrd and mossbauer spectroscopy indicate that these forms have differences in mean coherence length mcl and al substitution in soil saprolite sequences hematite is mainly formed during saprolite formation while goethite formed mainly during soil formation microscopic hematite formed in the saprolite has low al substitution and uniform mcl in response to soil formation it diminishes to submicroscopic size and goethite is formed towards toposoils goethite formed in yellow surface soils reflects recent soil formation and has high al substitution hematites and goethites related to gley phenomena have low al substitution and large mcls different coexisting forms of either goethite or hematite point to varying pedo environments either in time corroborating the polygenetic nature of the studied soils or at micron scale

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

crystallization and age of zircon from corsican ophiolitic albitites consequences for oceanic expansion in jurassic times abstract in corsica albitites and albite granites are considered to be the latest products of an ophiolitic magma two albitites have been selected for a study of the morphology typology and age of their zircons applying for the first time the u pb method to western tethyan ophiolites both samples come from the rospigliani series possibly formed in a transform fault zone the first one shows a cumulate texture whereas the second has features indicative of more rapid cooling zircons have precipitated at the end of the fractional crystallization process after a strong enrichment in alkali and incompatible elements in a typological diagram they plot at high temperature and high agpaicity indexes these abundant large euhedral to subhedral zircons sometimes with fine zoning may also contain inclusions of granitic composition which testifies to k fractionation in ophiolitic magmas the u pb age of 161 3myr is in the range of the ages obtained for alpine and apennines ophiolites all these ophiolitic remnants belonging to the liguro piedmont basin may represent a portion of the central atlantic ocean created during middle and upper jurassic times

mineralogical and physico chemical properties of ferralic arenosols derived from unconsolidated plio pleistocenic deposits in the coastal plains of congo abstract the main characteristics of soils under commercial eucalyptus plantations on the atlantic coast of the congo were studied down to a depth of 5 m the objectives were to investigate the mineralogical assemblage and distinguish between current versus old pedogenetic processes and to find evidence of relations between minerals and cation retention in highly weathered sandy soil these soils developed from plio pleistocene sandy deposits were characterized by mineralogical uniformity over depth with quartz contents ranging from 850 to 930 g kg 1 a clay fraction dominated by kaolinite and a cation exchange capacity cec pedogenetic differentiations were weak in these soils because the elapsed time since deposition was too short to differentiate mineralogical horizons from weathered materials our study showed that beyond the properties that minerals confer to soil partially controlling the bioavailability of nutrients their mineralogical and chemical characteristics infer current and former pedogenetic processes in this highly weathered tropical soil

the external massifs relics of variscan basement in the alps chemical and structural data give new insight into the complex metamorphic evolution of crystalline basement of the helvetic realm central and western alps the relics of pre carboniferous basement the so called external massifs may be subdivided into an older basement unit with a younger cover which together suffered the following metamorphic evolution from late caledonian until variscan times 1 a late caledonian high pressure grade metamorphic event with formation of kyanite bearing mineral assemblages conditions of 8 10 kb 550 c may have been attained it is not excluded that the high pressure evolution has been accompanied by formation of late caledonian nappe structures crustal thickening due to collision or ductile thrusts 2 depending on the maximum temperature attained the later geothermal evolution probably of early variscan age led to at least one of the following stages a mineral assemblages of biotite sillimanite garnet b mineral assemblages of sillimanite cordierite k feldspar c second regional anatexis with formation of cordierite bearing granitoids conditions of about 4 5 kb 600 650 c have been attained 3 during the late cooling history alpine type tension gaps of variscan age with quartzk feldspar andalusite were formed latest joints were covered by muscovite andalusite

pre mesozoic alpine basements their place in the european paleozoic framework prior to their alpine overprinting most of the pre mesozoic basement areas in alpine orogenic structures shared a complex evolution starting with neoproterozoic sediments that are thought to have received detrital input from both west and east gondwanan cratonic sources a subsequent neoproterozoic cambrian active margin setting at the gondwana margin was followed by a cambrian ordovician rifting period including an ordovician cordillera like active margin setting during the late ordovician and silurian periods the future alpine domains recorded crustal extension along the gondwana margin announcing the future opening of the paleotethys oceanic domain most areas then underwent variscan orogenic events including continental subduction and collisions with avalonian type basement areas along laurussia and the juxtaposition and the duplication of terrane assemblages during strike slip accompanied by contemporaneous crustal shortening and the subduction of paleotethys under laurussia thereafter the final pangea assemblage underwent triassic and jurassic extension followed by tertiary shortening and leading to the buildup of the alpine mountain chain recent plate tectonic reconstructions place the alpine domains in their supposed initial cambrian ordovician positions in the eastern part of the gondwana margin where a stronger interference with the chinese blocks is proposed at least from the ordovician onward for the visean time of the variscan continental collision the distinction of the former tectonic lower plate situation is traceable but becomes blurred through the subsequent oblique subduction of paleotethys under laurussia accompanied by large scale strike slip since the pennsylvanian this global collisional scenario has been replaced by subsequent and ongoing shortening and strike slip under rising geothermal conditions and all of this occurred before all these puzzle elements underwent the complex alpine reorganization

pre alpine evolution of the seckau complex austroalpine basement eastern alps constraints from in situ la icp ms upb zircon geochronology abstract the variscan european belt is a complex orogen with its southern margin partly obscured by alpine tectonics and metamorphism we present a study of one of the units the seckau complex that constitute the southern part of the variscan european belt in the eastern alps in order to clarify its origin age and lithostratigraphy the magmatic and geochronological evolution of this complex in the northwestern part of the seckau nappe as part of the austroalpine silvretta seckau nappe system was investigated by zircon u pb dating of paragneisses and metagranitoids coupled with petrological and geochemical data this reveals the distinction of three newly defined lithostratigraphic lithodemic sub units 1 glaneck metamorphic suite 2 hochreichart plutonic suite and 3 hintertal plutonic suite the glaneck metamorphic suite is mainly composed of fine grained paragneisses that yield u pb zircon ages in the range between 2 7 ga and 2 0 ga as well as concordia ages from 572 7 ma to 559 11 ma all of these ages are interpreted as detrital zircon ages originating from an igneous source the paragneisses are the host rock for the large volumes of metagranitoids of the hochreichart plutonic suite and the hintertal plutonic suite the hochreichart plutonic suite comprises highly fractionated melts with mainly s type characteristics and late cambrian to early ordovician u pb zircon ages 508 9 ma to 486 9 ma interpreted as magmatic protolith ages the hintertal plutonic suite is composed of metagranitoids with late devonian to early carboniferous 365 11 ma and 331 10 ma protolith ages that intruded during an early phase of the variscan tectonometamorphic event the metagranitoids of the hintertal plutonic suites define a magmatic fractionation trend seen in variable rb sr ratios on this base they can be further subdivided into a the griessstein pluton characterized by s type metagranitoids and b the pletzen pluton distinguished by intermediate to acidic metagranitoids with i type affinity the detrital zircon age spectra suggest a neoproterozoic ancestry of the glaneck metamorphic suite which was located west of the arabian nubian shield probably next to the trans saharan belt the early paleozoic evolution of the recent seckau complex shows similarities to basement units of the southalpine unit parts of the austroalpine unit and the tatric and veporic units of the central western carpathians

revealing exhumation of the central alps during the early oligocene by detrital zircon u pb age and fission track double dating in the taveyannaz formation

recalculated probability of m 7 earthquakes beneath the sea of marmara turkey 1 new earthquake probability calculations are made for the sea of marmara region and the city of istanbul providing a revised forecast and an evaluation of time dependent interaction techniques calculations incorporate newly obtained bathymetric images of the north anatolian fault beneath the sea of marmara le pichon et al 2001 armijo et al 2002 newly interpreted fault segmentation enables an improved regional a d 1500 2000 earthquake catalog and interevent model which form the basis for time dependent probability estimates calculations presented here also employ detailed models of coseismic and postseismic slip associated with the 17 august 1999 m 7 4 izmit earthquake to investigate effects of stress transfer on seismic hazard probability changes caused by the 1999 shock depend on marmara sea fault stressing rates which are calculated with a new finite element model the combined 2004 2034 regional poisson probability of m 7 earthquakes is 38 the regional time dependent probability is 44 18 and incorporation of stress transfer raises it to 53 18 the most important effect of adding time dependence and stress transfer to the calculations is an increase in the 30 year probability of a m 7 earthquake affecting istanbul the 30 year poisson probability at istanbul is 21 and the addition of time dependence and stress transfer raises it to 41 14 the ranges given on probability values are sensitivities of the calculations to input parameters determined by monte carlo analysis 1000 calculations are made using parameters drawn at random from distributions sensitivities are large relative to mean probability values and enhancements caused by stress transfer reflecting a poor understanding of large earthquake aperiodicity index terms 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics 8150 tectonophysics plate boundary general 3040 keywords earthquake probability sea of marmara seismic hazard turkey stress interaction north anatolian fault

seismicity distribution in conjunction with spatiotemporal variations of coseismic slip and postseismic creep along the combined 1999 izmit duzce rupture abstract the north anatolian fault zone nafz in nw turkey as one of the most active and best studied strike slip faults provides a unique opportunity to study earthquake related relaxation processes through analyzing co and postseismic deformation we study the spatial and temporal distributions of seismicity related to the two consecutive 1999 m 7 izmit and duzce earthquakes a high resolution aftershock catalogue including 10 000 hypocenters extending along the combined rupture zone and extending from prior to the izmit event to after the duzce event is studied spatial and temporal distributions of events allow to identify distinct seismically active and inactive fault patches their location is related to the co and postseismic deformation within and below the seismogenic layer respectively four seismically inactive patches extending 30 50 km along the rupture zone and down to 10 km depth are identified with a systematic spatial shift between them introduced by the duzce mainshock the cumulative distribution of sub areas hosting coseismic slip aftershock clusters and postseismic creep shows that the entire upper seismogenic and lower ductile portions of the crust along the combined izmit and duzce rupture zone are activated between rupture initiation and a two year postseismic period this observation was only achieved due to the subsequent occurrence of two adjacent m 7 strike slip earthquakes in combination with a distinct local seismic and geodetic monitoring our findings suggest that a coseismically introduced lateral and vertical slip deficit is systematically compensated postseismically in both the brittle and ductile portions of the crust

the petrology and geochemistry of dykes of the lizard ophiolite complex cornwall dykes from the coastal section of the lizard ophiolite sw england have been analysed for both major and trace elements the dykes vary from fresh olivine dolerites to lower amphibolite grade metadolerites all are basaltic and tholeiitic and possess broadly oceanic characteristics three magma groupings can be defined geochemically on the basis of trace element contents particularly rare earth elements and p y ratios these magma groupings are thought to have been generated by differing degrees of partial melting and from differing source rocks each group is thought to have undergone fractional crystallization the spatial and suggested temporal separation of these groups supports a pulsatory magma chamber model for the spreading centre at which the lizard ophiolite developed

obduction processes in ancient modern and future ophiolites geologic and tectonic processes of ophiolite obduction deduced from structural mapping timing of deformation and metamorphism and palinspastic reconstruction of ancient ophiolite complexes compare closely with modern processes operative in the western pacific many ongoing arguments about the origin of ophiolite complexes oceanic island arc or marginal basin and their emplacement subduction related or not are largely semantic and depend on exactly what individuals regard as island arcs marginal basins or subduction zones a review of tectonic processes currently active in the western pacific region shows a wide range in size shape and duration of arcbasin complexes and some of these show uniformitarian models similar to those proposed for emplacement of ancient ophiolites the tethyan ophiolites of the semail oman mountains and spontang western himalaya of ladakh zanskar and the ordovician western newfoundland ophiolites are compared with recent examples of the mariana basin yap trench banda arc luzon arc and papua new guinea it is concluded that they are all connected in some way with marginal basins and island arcs in varying stages of development and maturity emplacement of these ophiolites is related to continental underthrusting of a trapped marginal basin and accretionary prism comparable to present day examples of the n australian margin underthrusting timor and the banda sea and the se china margin underthrusting the luzon arc

weathering trends and parent material characteristics of polygenetic oxisols from minas gerais brazil i mineralogy in geologically stable areas in the tropics climatic changes and geomorphic cycles give origin to polygenetic soils polygenesis involves new soil formation phases taking place on preweathered materials from previous phases resulting in soils with rather similar chemical and mineralogical properties polygenetic soils from minas gerais brazil were investigated with the purpose of distinguishing mineralogical aspects in relation to polygenesis the soil materials were studied by micromorphology and electron microscope and analyzed by xrd all soils are strongly weathered and even show weathered grains of ilmenite and quartz the soils show two phases of kaolinite formation pseudomorphs after mica and precipitation from solution and of accumulation of iron compounds desilication causes gibbsite formation from kaolinite occasionally even in the deep saprolite superposed processes can be due to progressively deeper weathering or to changes of the external environment

gondwana derived microcontinents the constituents of the variscan and alpine collisional orogens abstract the european variscan and alpine mountain chains are collisional orogens and are built up of pre variscan building blocks which in most cases originated at the gondwana margin such pre variscan elements were part of a pre ordovician archipelago like continental ribbon in the former eastern prolongation of avalonia and their present day distribution resulted from juxtaposition through variscan and or alpine tectonic evolution the well known nomenclatures applied to these mountain chains are the mirror of variscan resp alpine organization it is the aim of this paper to present a terminology taking into account their pre variscan evolution at the gondwana margin they may contain relics of volcanic islands with pieces of cadomian crust relics of volcanic arc settings and accretionary wedges which were separated from gondwana by initial stages of rheic ocean opening after a short lived ordovician orogenic event and amalgamation of these elements at the gondwanan margin the still continuing gondwana directed subduction triggered the formation of ordovician al rich granitoids and the latest ordovician opening of palaeo tethys an example from the alps external massifs illustrates the gradual reworking of gondwana derived pre variscan elements during the variscan and alpine tertiary orogenic cycles

evolution of late neoproterozoic to early paleozoic tectonic elements in central and southeast european alpine mountain belts review and synthesis abstract late neoproterozoic to early paleozoic cadomian tectonic elements are widespread in the southeastern alpine mediterranean mountain belts with discontinuous exposure extending from the alps to the menderes massif in turkey the sequences include voluminous plutonic island and continental arc successions some ophiolites of variable late neoproterozoic and cambrian to early ordovician ages high to medium grade metamorphic sequences and subordinate metasediments geological and geochronological data suggest that these cadomian elements partly experienced tectonothermal activity including metamorphism and magmatism between ca 650 and 600 ma followed by further i and s type plutonism in an andean type continental margin setting ca between 570 and 520 ma subsequently a major rift zone developed which resulted in continental stretching and subsequent formation of a back arc basin which is dated as late cambrian rifting is followed by oceanic spreading which commenced ca at the cambrian ordovician boundary possibly in a back arc setting this tectonic scenario suggests that alpine mediterranean cadomian tectonic elements were accreted to gondwana during early cadomian events ca 600 650 ma subsequently they were likely part of a long lasting outer subduction zone of gondwana at the margin of a panthalassa type ocean during late neoproterozoic iii and cambrian due to back arc spreading continental pieces started to split off from gondwana ca at the cambrian ordovician boundary

spatiotemporal earthquake clusters along the north anatolian fault zone offshore istanbul we investigate earthquakes with similar waveforms in order to characterize spatiotemporal microseismicity clusters within the north anatolian fault zone nafz in northwest turkey along the transition between the 1999 izmit rupture zone and the marmara sea seismic gap earthquakes within distinct activity clusters are relocated with cross correlation derived relative travel times using the double difference method the spatiotemporal distribution of microearthquakes within individual clusters is resolved with relative location accuracy comparable to or better than the source size high precision relative hypocenters define the geometry of individual fault patches permitting a better understanding of fault kinematics and their role in local scale seismotectonics along the region of interest temporal seismic sequences observed in the eastern sea of marmara region suggest progressive failure of mostly nonoverlapping areas on adjacent fault patches and systematic migration of microearthquakes within clusters during the progressive failure of neighboring fault patches the temporal distributions of magnitudes as well as the number of events follow swarmlike behavior rather than a mainshock aftershock pattern

cathodoluminescence studies and u pb dating of zircons in pre mesozoic gneisses of the tauern window implications for the penninic basement evolution the pre mesozoic basement in the eastern and central part of the tauern window eastern alps consists mainly of anatectic plagioclase gneisses and amphibolites intruded and migmatized by carboniferous granitoids the gneisses are geochemically similar to calcalkaline dacites of modern primitive island arcs cathodoluminescence investigations and u pb data on zircons indicate an uppermost proterozoic or cambrian extrusion age of the dacitic protoliths and a carboniferous syngranitoid anatexis around 320 ma high k calcalkaline meta rhyolites in the north central tauern window form a separate part of the basement not affected by carboniferous anatexis and plutonism their protoliths extruded in the lower carboniferous in the permian high k granitoids intruded the anatectic basement

variations of seismic b value at different stages of the seismic cycle along the north anatolian fault zone in northwestern turkey abstract we studied spatiotemporal b value variations along the north anatolian fault zone nafz in northwestern turkey with a focus on the combined 1999 izmit and duzce rupture and the eastern sea of marmara we used a local seismicity catalog of the izmit duzce region covering a time span from 2 5 years prior to the izmit until 14 months after the duzce mainshock and a four year hypocenter catalog in the eastern sea of marmara we consistently calculated moment magnitudes to ensure a homogeneous dataset and applied strict quality criteria this allows studying variations of b values throughout the region and at different stages of the seismic cycle with a standard gridding technique b value maps depth sections and time series were calculated which reveal a very heterogeneous b value distribution in the study area the variety of b value observations cannot be interpreted unambiguously given that the b value most likely depends on a combination of fault zone characteristics like local stress conditions heterogeneity of the crust and damage distribution by presenting a comprehensive set of possible interpretations we point out that a biased discussion of the results towards stress or another individual parameter may lead to erroneous conclusions furthermore the applied data discretization scheme influences the appearance of the final b value distribution leading to potential misinterpretations

early ordovician continental break up in variscan europe ndsr isotope and trace element evidence from bimodal igneous associations of the southern massif central france abstract the high grade marvejols group and the low grade albigeois cevennes sedimentary sequence contain bimodal igneous rocks of early ordovician age which are representative of a widespread thermal event in the european variscides comparison of their nd sr isotope and trace element characteristics provides additional evidence for their origin in an ensialic extensional setting as an alternative to the back arc model proposed by previous authors we propose that these associations record a continental break up episode unrelated to contemporaneous subduction in this model the widespread early paleozoic bimodal magmatism marks the birth of a mid european oceanic arm we propose that the break up was controlled by both transtensional processes and mantle plume activity high grade associations such as the marvejols group and low grade bimodal associations such as the albigeois cevennes might be regarded as representing opposite rifted passive margins the northern marvejols type margin was involved in a late ordovician silurian subduction like process with hp ht metamorphism in contrast the southern albigeois cevennes margin remained largely unaffected possibly as part of the overriding plate of the subduction zone pseudo calc alkaline signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break up episodes in this case negative nb anomalies are produced by the addition of crustal components enriched in th and lree relative to nb this inferred alternative origin of nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data

distribution and genesis of bauxite on the mambilla plateau se nigeria abstract only small low grade occurrences of bauxite are known in nigeria associated with ferricretes on the jos plateau widespread bauxite has been discovered on the mambilla plateau laterite is developed on gneisses and basalt while on trachytic parent rocks low silica bauxite occurs with a thickness of more than 5 m and covering an area of at least 1 km2 this bauxite consists of gibbsite and goethite in contrast to the ferricretes of the jos plateau weathering on the mambilla plateau led to the formation of stone line profiles ferricretes were developed on the upper jurassic gondwana surface under contrasting climate and are preserved as relicts in the southern part of the mambilla plateau where weathered precambrian basement occurs in a highly uplifted position dismantling of these ferricretes is attributed to a gradual change of paleoclimate towards more humid conditions after the extrusion of volcanic rocks in the northern part of the plateau presumably during lower to mid miocene these volcanics underwent intense chemical weathering bauxite and laterite formed depending on the type of parent rock weathering profiles are mostly truncated and overlain by reworked material allochthonous components occur both within and also below the stone line this is shown by mineralogical indicators such as ilmenite or quartz and by a shift in the zr ti ratio similar profiles are missing further north on the jos plateau where ferricretes formed instead this is attributed to regional differences in paleoclimatic conditions which were more humid at mambilla while jos was under the influence of dryer conditions the widespread formation of laterite on different parent rocks on the mambilla plateau could be related to a global mid miocene climatic optimum which led to a general intensification of chemical weathering processes

downhole geophysical observatories best installation practices and a case history from turkey downhole sensors of different types and in various environments provide substantial benefit to signal quality they also add the depth dimension to measurements performed at the earths surface sensor types that particularly benefit from downhole installation due to the absence of near surface noise include piezometers seismometers strainmeters thermometers and tiltmeters likewise geochemical and environmental measurements in a borehole help eliminate near surface weathering and cultural effects installations from a few hundred meter deep to a few kilometer deep dramatically reduce surface noise levels the latter noticeably also reduces the hypocentral distance for shallow microearthquakes the laying out of a borehole network is always a compromise of local boundary conditions and the involved drilling costs the installation depth and procedure for a long term downhole observatory can range from time limited installations with a retrieval option to permanently cemented sensors permanently cemented sensors have proven to be long term stable with non deteriorating coupling and borehole integrity however each type needs to be carefully selected and planned according to the research aims a convenient case study is provided by a new installation of downhole seismometers along the shoreline of the eastern marmara sea in turkey these stations are being integrated into the regional net for monitoring the north anatolian fault zone here we discuss its design installation and first results we conclude that despite the logistical challenges and installation costs the superior quality of downhole data puts this technique at the forefront of applied and fundamental research

new constraints on the origin and age of variscan eclogitic rocks ligurian alps italy gabbro and eclogite boudins are preserved within the amphibolites of the composite para and ortho gneiss variscan basement of the savona crystalline massif ligurian brianconnais italy whole rock trace element patterns low initial end 5 4 to 8 8 data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts which most likely underwent crustal contamination during emplacement gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation whereas the eclogites represent evolved melts u pb and trace element micro analyses on zircons separated from one amphibolitised gabbro and one eclogite help to constrain coeval ages at 468 ma for their igneous protoliths the occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks in the eclogite concordant zircon ages younger than the protolith age testify to metamorphic re crystallisation or new growth from about 420 to 305 ma zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392 376 ma ago the younger zircon portions yielding a mean concordia age of 333 7 ma are related to equilibration or new growth during the post eclogite amphibolite facies equilibration

496 my age of plagiogranites in the chamrousse ophiolite complex external crystalline massifs in the french alps evidence of a lower paleozoic oceanization genuine ophiolitic series are extremely rare in the variscan orogenic belt and the chamrousse formation appears as an exception the main lithological and geochemical characters of the ophiolite are summarized and a special attention is paid to the field relationships and the magmatic filiation of the leucocratic rocks the time evolution of the mafic magmas is considered to express the transition of attenuated continental lithosphere to oceanic lithosphere the silicic rocks represents the final products from a multisequence fractional crystallization of a tholeiitic lil enriched magma and their compositional diversity results from the local increase of the h2o partial pressure during syn accretion tectonics dating the plagiogranites using theup b method on zircons also give the date of the oceanization the resulting age of496 6ma is closely comparable 1 to the accretion age of ophiolites from peri atlantic paleozoic belts and 2 to the protolith crystallization age of the leptyno amphibolitic groups lags from western europe but these relics ophiolites and lags of the cambro ordovician distensional event later undergo very different tectonometamorphic evolution the distinct post magmatic evolutions may be related to their rather distinct original geodynamic significance and to their primitive location with respect to the subsequent zonation of the hercynian belt

u pb zircon and sm nd geochronology of mafic and ultramafic rocks from the central part of the tauern window eastern alps geochronological and geochemical analyses were carried out in order to identify the pre variscan basement of the tauern window eastern alps maficultramafic rocks from the central part of the tauern window have been studied by ree analysis and u pb and sm nd isotopic analyses on whole rock zircons garnets and sphene u pb and sm nd zircon dating define both magmatic pan african and cambro ordovician events from 650 ma to 486 ma within the alpine fold belt this indicates a time span of 150 ma for magmatic activities in the tauern window of the eastern alps the ages of 657 ma u pb zircon and 644 sm nd zireon obtained from an amphibolite are the oldest dates of the eastern alps they may be related to the pan african orogeny and imply an early cycle of magmatic intrusion before major activity started at around 500 ma sm nd whole rock analyses of the precambrian rocks do not define an isochron reflecting heterogenities within the mantle source the initial end values 1 2 to 4 7 are very low implying an enrichment of the magma source the second main phase of magmatic activity 539 486 ma is characterized by the emplacement of mafic ultramafic rock sequences as no ophiolitic relies are observed in these domains the early paleozoic magmatism was likely associated with extensional tectonics obtained ages of 301 3 and 314 4 3 ma point to a variscan metamorphism the first combined u pb zircon sm nd zircon data for an amphibolite from the basal amphibolite formation baf favoured the sm nd zircon isochron age as a magmatic age whereas the low initial end value point to an enriched magma source as well as to heterogenities within the magma source the obtained ages suggest that parts of the pre variscan basement within the alpine fold belt were formed during the pan africa cycle the detection of pre variscan ages within the alpine basement must reffect a complex history involving significant pre variscan activity

seismic wave propagation in shallow layers at the gonaf tuzla site istanbul turkey using the first dataset available from the downhole geophysical observatory of the north anatolian fault we investigated near surface seismic wave propagation on the tuzla peninsula istanbul turkey we selected a dataset of 26 seismograms recorded at tuzla at sensor depths of 0 71 144 215 and 288 m to determine near surface velocities and attenuation structures the waveforms from all sensors were pairwise deconvolved and stacked this produced low noise empirical green s functions for each borehole depth interval from the green s functions we identified reflections from the free surface and a low velocity layer between 90 and 140 m depth the presence of a low velocity zone was also confirmed by a sonic log run in the borehole this structure plus high near surface p and s wave velocities of 3600 4100 and 1800 m s lead to complex interference effects between upgoing and downgoing waves as a result the determination of quality factors q with standard spectral ratio techniques was not possible instead we forward modeled the green s functions in the time domain to determine effective q values and to refine our velocity estimates the effective q p values for the depth intervals of 0 71 0 144 0 215 and 0 288 m were found to be 19 35 39 and 42 respectively for the s waves we obtained an effective q s of 20 in the depth interval of 0 288 m considering the assumptions made in our modeling approach it is evident that these effective quality factors are biased by impedance contrasts between our observation points our results show that even after correcting for a free surface factor of 2 the motion at the surface was found to be 1 7 times greater than that at 71 m depth our efforts also illustrate some of the difficulties of dealing with site effects in a strongly heterogeneous subsurface online material plots of resistivity and caliper logs and the spectra of all 26 events

a unified earthquake catalogue for the sea of marmara region turkey based on automatized phase picking and travel time inversion seismotectonic implications abstract the marmara section of the north anatolian fault zone nafz is late in its seismic cycle and can be expected to produce a magnitude m up to 7 4 earthquake during the next decades in direct vicinity to the 15 million population center istanbul this setting translates the seismic hazard into very high risk and makes a thorough understanding of the current seismotectonic setting of this nafz section a pressing task the absence of near fault stations along the most part of the offshore marmara section limits the reliability of existing seismicity catalogues for this region for the first time we combine the different regional permanent networks thereby optimizing azimuthal coverage and present a refined hypocenter catalogue for the sea of marmara on this basis compared to the original locations adoption of a refined automated technique to determine precise onset times for the different body waves and an iterative travel time inversion scheme lead to substantial improvement of 6812 absolute earthquake locations particularly in the epicentral distribution the automated processing is explained in detail further optimization is achieved through relative relocation of 4407 earthquakes our catalogue covers more than a decade 2006 2016 with a regional moment magnitude of completeness of mc 2 1 the epicentral distribution delineates the marmara section i e the northern nafz branch as the seismically most active fault strand we identify several aseismic fault patches that are interpreted to represent locked parts of the fault seismic activity in the past decade predominantly occurs off the main fault on the edges of the aseismic patches supporting previous studies that the marmara section of the nafz contains both locked and creeping fault portions single event focal mechanisms 2 7 mw 4 5 indicate that currently both strike slip and normal faulting occur confirming the transtensional setting of the region

geochemistry of ophiolites from the chamrousse complex belledonne massif alps the ophiolite complex of chamrousse belledonne massif alps consists of mafic to ultramafic cumulates and non cumulates metamorphosed to amphibolite facies grade the non cumulitic rocks are similar in chemical composition to recent ocean floor olivine tholeiites both n type and enriched p type the distribution of lithophile elements shows that the non cumulitic rocks represent several magmas of different parentage the character of the magmas varies according to the time of emplacement

microearthquake imaging of the parkfield asperity microearthquake data from a downhole seismometer network on the san andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the parkfield characteristic earthquakes the source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most furthermore the magnitude frequency statistics suggest that at local magnitude 0 6 the cumulative number of small events begins to fall off the usual gutenberg richter b 1 relation in which the number of events increases exponentially with decreasing magnitude the downhole seismometer data establish a baseline from which the evolution of the earthquake process at parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical gutenberg richter relation may be operating for the smallest of parkfield microearthquakes

ediacaran to cambrian oceanic rocks of the gondwana margin and their tectonic interpretation in tectonic maps of variscan europe allochthonous pieces of cadomian basement clearly stand out with their predominant metabasic to ultrabasic elements the so called exotic terranes with ophiolites most of these domains are observed in basements of the central iberian allochthone the south armorican domain the nappe structures of the french massif central the saxothuringian zone and the bohemian massif similar relics can be recognized in many alpine basement areas and correlations with supposedly more autochthonous basements such as the ossa morena zone and the central iberian basement can be envisaged all of these relics are thought to represent the interrupted trace of a former continuous or discontinuous structure characterized by the presence of ocean derived proto rheic rock suites these can be interpreted as pieces of former magmatic arcs of ediacaran to cambrian age accreted to the gondwana margin which later were scattered as allochthonous units during the variscan plate tectonic processes the presence of similar rock suites of ordovician age in the alpine realm is explained by the accretion of exotic china derived basements and their collision with the gondwana margin during the opening of the rheic ocean

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

the variscan belt in central europe main structures geodynamic implications open questions abstract the variscan belt of europe originated from the confrontation of northern europe and gondwana with intervening pre variscan blocks cadomian and older armorica tepla barrandean moravo silesian though a strike slip component cannot be excluded geological evidence suggests a subduction collision process newly discovered features include basaltic rocks with morb affinities high pressure metamorphic rocks trench deposits and large scale allochthonism which also affects the pre variscan basement and has often led to an inversion of the metamorphic isogrades large scale thrusting is evident at all crustal levels and is also observable by geophysical methods the structure of the variscan crust is best described as horizontal tectonic layering late ordovician through carboniferous convergence closed one rhenohercynian and one saxo thuringian basin to the north and at least one mediterranean basin to the south of the pre variscan blocks subduction occurred from the north as well as from the south down under the dorsal core region the exact timing and the driving mechanism of this bilateral activity remains a problem likewise uncertain are the exact limination of the pre variscan blocks and the existence and width of oceanic areas in the saxothuringian zone and in the south facing part of the moldanubian zone there is good evidence of early palaeozoic rifting and of late ordovician through carboniferous plate convergence palaeomagnetic and palaeoclimatic constraints however suggest only a limited extent of oceanic basins in these areas while they appear to allow a wider ocean at the rhenohercynian saxothuringian boundary which is relatively unobtrusive from the geological point of view research during the past decade has produced a wealth of information on the variscan fold belt in europe the resulting geodynamic models however are extremely diverging evidently it is not the time yet for definitive statements the present paper intends to review the state of knowledge to deduce such basic items of a geodynamic model as appear reasonably well documented and to point out the most urgent open questions stress will be laid on central europe which offers a complete cross section through the northern flank of the variscan belt and where we have carried out most of our studies during the last years

geochronology of ophiolites of the newfoundland appalachians precise u pb zircon ages 2 errors are reported for four ophiolite complexes from the newfoundland appalachians zircons from trondhjemite from blow me down mountain bay of islands complex give an age of five points those from gabbro from blow me down brook give a minimum 207pb 206pb age of 480 ma one point confirming that this is the age of the main plutonic episode these data refute previously determined latest cambrian u pb zircon and sm nd mineral rock isochron ages of 504 10 and 505 ma for the complex gabbro from the betts cove complex yielded zircon that gives an age of four points this refutes the interpretation of an earlier u pb zircon age of 463 6 ma for a trondhjemite dyke as the age of the ophiolite it was inconsistent with the presence of arenigian graptolites in the overlying snooks arm group two zircon ages from a pegmatitic and medium grained trondhjemite of the annieopsquotch complex are five points and two points zircon from trondhjemite of the pipestone pond c

early cambrian oceanic plagiogranite in the silvretta nappe eastern alps geochemical zircon u pb and rb sr data from garnet hornblende plagioclase gneisses garnet hornblende plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the austro alpine silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates it is proposed here that the gneisses were formerly oceanic plagiogranites an end 530 value of 5 6 for the gneisses as well as initial87sr 86sr values of 0 7036 0 7037 for the gabbroic rocks and 0 7026 0 7027 for the ultramafic rocks suggest a mantle source for this rock association the geochemical characteristics of the garnet hornblende plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well the combined u pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532 30 ma similar to previously dated mostly calcalkaline orthogneisses in the same area high quality transparent zircons showed the least degree of discordance but contain extremely low u and pb levels the rock suite including this plagiogranite was emplaced within oceanic crust which formed in the latest precambrian early palaeozoic off the northern continental margin of gondwana

tertiary weathering profiles in central nigeria as indicators of paleoenvironmental conditions abstract in central nigeria the fluviovolcanic series of the jos plateau are composed by deeply weathered volcanic rocks and sediments the sequence is often capped by ferricretes ferricretes remains of former soils are also intercalated in the series profile sections between two ferricretes are deep 10 m the repeated saprolite solum sequence sometimes exceeds 100 m it can be separated into a lower section without bauxitisation but often with total bleaching of the saprolite a middle section with bauxite and hematite in the saprolite and an upper kaolinitic section with hematite and goethite bleaching by total removal of fe 2 and bauxitisation by total desilification are both indicators of wet and hot climates with a high biomass production in the first case the groundwater was permanently high in the latter a well drained fluctuating groundwater can be assumed changing groundwater conditions may be the result of plateau uplift paleoenvironmental changes are thus reflected in relatively datable paleosols undatable paleosoils with comparable characteristics on the planation surfaces of central and northeast nigeria also result from paleoenvironmental conditions consequently concepts of landscape evolution must take into account changing environments

u pb dating of an early paleozoic bimodal magmatism in the french massif central and of its further metamorphic evolution in the marvejols area southern french massif central the gneissic marvejols supergroup is overthrust on the metasedimentary serie du lot deposited in part prior to 540 ma the allochtonous terranes are characterized by the occurrence of a leptyno amphibolitic group a complex association of mafic and felsic rocks of igneous and sedimentary derivation a 480 10 ma age has been obtained by u pb dating of zircons for the crystallization of both mafic and felsic meta igneous rocks these rocks were emplaced during an important extensional tectonics relics of eclogites pyrigarnites coronite gabbros and hp trondhjemites are clear evidence for a further hp ht event dated at 415 6 ma on zircons from a hp trondhjemite subsequently the marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345 10 ma

gas emissions and active tectonics within the submerged section of the north anatolian fault zone in the sea of marmara the submerged section of the north anatolian fault within the marmara sea was investigated using acoustic techniques and submersible dives most gas emissions in the water column were found near the surface expression of known active faults gas emissions are unevenly distributed the linear fault segment crossing the central high and forming a seismic gap as it has not ruptured since 1766 based on historical seismicity exhibits relatively less gas emissions than the adjacent segments in the eastern sea of marmara active gas emissions are also found above a buried transtensional fault zone which displayed micro seismic activity after the 1999 events remarkably this zone of gas emission extends westward all along the southern edge of cinarcik basin well beyond the zone where 1999 aftershocks were observed the long term monitoring of gas seeps could hence be highly valuable for the understanding of the evolution of the fluid fault coupling processes during the earthquake cycle within the marmara sea

a sm nd isotopic study of 500 ma old oceanic crust in the variscan belt of western europe the chamrousse ophiolite complex western alps france we report the results of a sm nd isotopic study of whole rock samples from the chamrousse ophiolite complex in the variscan basement of the western alps france our data point to a 497 24 2 ma age in excellent agreement with u pb results menot et al 1984 on subordinate plagiogranites strong isotopic heterogeneity and time evolution of magmas from relatively less depleted nd 5 to very depleted nd 9 sources are also implied available data favour a mixing model between n morb and e morb type end members but a third component with distinct supra subduction zone affinity is also required these results preclude any tectonic setting such as intracontinental rift ensialic marginal basin or major ocean ridge but substantiate an oceanic back arc environment they also demonstrate that the well documented upper cambrian lower ordovician extensional phase here reached a true oceanic stage

neoproterozoic to early palaeozoic magmatic evolution in the gondwana derived austroalpine basement to the south of the tauern window eastern alps in the austroalpine basement to the south of the tauern window distinct suites of metabasites occur with orthogneisses in pre early ordovician units tholeiitic and alkaline within plate basalt type metabasites are associated with acid meta porphyroids in the post early ordovician thurntaler phyllite group according to their correlated trace element abundances metabasite zircons crystallized with their host rocks protolith pb pb zircon ages whole rock ta yb th yb and oxygen sr nd isotope data define two principal evolution lines an older evolution at elevated th yb typical of subduction related magmatism started by 590 ma n morb type and 550 530 ma volcanic arc basalt type basic suites which mainly involved depleted mantle sources it finished with mainly crustal source 470 450 ma acid magmatites an other evolution line by tholeiitic and 430 ma alkaline within plate basalt type suites in both pre and post early ordovician units is characterized by an intraplate mantle metasomatism and enrichment trend along multicomponent sources these magmatic evolution lines can be related to a plate tectonic scenario that involved terranes in a progressively mature neoproterozoic to ordovician active margin and a subsequent palaeo tethys passive margin along the north gondwanan periphery

the epigenetic replacement of kaolinite by hematite in laterite petrographic evidence and the mechanisms involved abstract this paper describes the petrographical and mineralogical transformations accompanying the accumulation of iron oxides within the deep mottled layer of a laterite from burkina faso in this layer the proportions of purple red hematitic spots or nodules which form at the expense of volumes occupied by a soft microporous kaolinite plasma increase as the surface of the soil is approached the antagonism between hematite and kaolinite was observed at different levels in all cases from the border to the core of the hematitic domains kaolinite experiences corrosion soon followed by complete destruction at this later stage hemalite crystals are organized into thin lamellar structures showing clearly the shapes of former kaolinite platelets a mechanism explaining coincidence of the accumulation of hematite with the destruction of kaolinite is proposed we postulate that in the deep layers of laterites fed by fe 2 solutions the protons necessary to dissolve kaolinite are generated by a reaction similar to the oxidation hydrolysis step of ferrolysis in the mottled layer of this laterite therefore the progressive building up of a high concentration of iron appears to be the result of two closely related processes i e the precipitation of iron oxides from solution and the concommitant dissolution of kaolinite

u pb and pb pb zircon dating of the older orthogneiss suite in the silvretta nappe eastern alps cadomian magmatism in the upper austro alpine realm two orthogneiss suites dominate the silvretta nappe primary crystallization of the larger suite younger orthogneisses is assumed to be ordovician in age the second adjacent magmatic suite consists of older alkaline to calc alkaline ultrabasic basic to intermediate and granitic rocks known as older orthogneisses u pb data of multigrain zircon fractions as well as single zircon stepwise evaporation 207pb 206pb results suggest a latest proterozoic to early cambrian intrusion age for the protoliths of the older orthogneisses as both dating methods yield early cambrian crystallization ages of 526 7 and 519 7 ma for an alkaline granite gneiss similar results were obtained for two neighbouring calc alkaline orthogneisses 207pb 206pb ages of 533 4 and 568 6 ma respectively the crystal habitus corresponds to p5 s19 and s9 zircons of magmatic origin whole rock initial sr isotope ratios indicate a primitive source the igneous protoliths of these older orthogneisses represent a fragment of a cadomian pan african crust found in places within the basement of the european hercynides

microseismicity at the north anatolian fault in the sea of marmara offshore istanbul nw turkey 1 the north anatolian fault zone nafz below the sea of marmara forms a seismic gap where a major earthquake is expected to occur in the near future this segment of the fault lies between the 1912 ganos and 1999 izmit ruptures and is the only nafz segment that has not ruptured since 1766 to monitor the microseismic activity at the main fault branch offshore of istanbul below the cinarcik basin a permanent seismic array pires was installed on the two outermost prince islands yassiada and sivriada at a few kilometers distance to the fault in addition a temporary network of ocean bottom seismometers was deployed throughout the cinarcik basin slowness vectors are determined combining waveform cross correlation and p wave polarization we jointly invert azimuth and traveltime observations for hypocenter determination and apply a bootstrap resampling technique to quantify the location precision we observe seismicity rates of 20 events per month for m 2 5 along the basin the spatial distribution of hypocenters suggests that the two major fault branches bounding the depocenter below the cinarcik basin merge to one single master fault below 17 km depth on the basis of a cross correlation technique we group closely spaced earthquakes and determine composite focal mechanisms implementing recordings of surrounding permanent land stations fault plane solutions have a predominant right lateral strike slip mechanism indicating that normal faulting along this part of the nafz plays a minor role toward the west we observe increasing components of thrust faulting this supports the model of nw trending dextral strike slip motion along the northern and main branch of the nafz below the eastern sea of marmara

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake

late proterozoic zircon ages from a basic ultrabasic complex a possible cadomian orogenic complex in the hercynian belt of western europe the basic ultrabasic complex of belle isie en terre northwestern part of the armorican massif france has been dated at 602 4 1 m y by the u pb zircon method the calc alkaline trend of the metabasic rocks and the geologic context suggest that this ophiolite like complex is related to a back arc orogenic setting similar to the geodynamic environment that has been proposed for pan african calc alkaline ophiolite like complexes of late proterozoic age around the west african craton our results are compatible with the closing in late proterozoic time of a channel ocean north of the basement and formed during the cadomian orogeny rather than with possible caledonian or hercynian subduction