change in fabric of melange in the shimanto belt japan change in relative convergence systematic changes in fabrics of melanges have been detected in rocks of the shimanto belt japan several shear indicators including s c structures riedel shears and folds in the melanges indicate consistent sense of shear both in outcrop and microscopic scales the deformation mechanisms independent particulate flow and pressure solution suggest that the deformation started under unlithified conditions and progressed through lithification due to strain hardening after lithification cataclastic deformation occurred locally but most of the earlier deformation features are preserved this process appears to have taken place along the decollement beneath the off scraped accretionary prism sinistral reverse sense of shear is observed in the campanian melange whereas a dextral reverse sense of shear is preserved in the early eocene melange in the shimanto belt this change in fabric is interpreted to be linked to a change in relative convergence between the subducting and overriding plates three different plate models have been proposed for the late cretaceous to early tertiary western pacific margin 1 subduction of the kula pacific ridge along the continental margin 2 subduction of the pacific plate since 85 ma and 3 subduction of the aborted kula pacific ridge after 43 ma the observed change in fabric of melange is consistent only with the third model which predicts a change in relative convergence from sinistral to dextral at the appropriate time

a mid ordovician temperate carbonate shelf the black river and trenton limestone groups of southern ontario canada abstract mid ordovician limestones of southern ontario are usually considered to be tropical shelf limestones and interpreted using facies models derived from recent tropical carbonate environments nevertheless depositional rates grain types faunas erosion surfaces mineralogy and geochemistry are more compatible with a temperate or even cold shelf environment in view of the low latitudes 20 s indicated by paleomagnetism this suggests deposition in relatively cold seas deposition below a tropical thermocline or that the paleomagnetic data are wrong similar features in other caradocian limestones of eastern north america show that these inferred cooler conditions were widespread if deposition occurred below a tropical thermocline most shallow water caradocian limestones must be re interpreted as deeper water deposits more likely in view of the evidence for temperate conditions in shallow water is that the whole eastern seaboard of north america was affected by cold currents arising from southern polar ice caps and thus was a temperate shelf environment

quantitative estimation of fluid pressure ratio of shear vein in an on land accretionary complex the yokonami melange the cretaceous shimanto belt kochi southwest japan abstract the fluid pressure ratio and effective friction coefficient of shear veins in an exhumed accretionary complex in the cretaceous shimanto belt southwest japan were constrained by fluid inclusion and stress inversion analyses the shear veins are considered to be developed along the decollement temperature and pressure conditions for shear veins were found to be in the range of 175 225 c and 143 215 mpa respectively fluid pressure ranged between the lines of 70 100 lithostatic pressure when the geothermal gradient was assumed on the basis of the slab age at the trench the effective friction coefficient of shear veins was estimated to be 0 1 0 2 by stress inversion analysis which corresponds to a fluid pressure ratio of 0 67 0 83 both pressure temperature and friction coefficient results indicate that the fluid pressure ratio is higher than about 0 7 in this paper we propose the procedure to constrain paleo fluid pressure ratio from natural materials wedge states were examined using the weak decollement friction coefficient with shallow and steep taper angles indicating that the wedge state is stable or compressively critical and is controlled by the fluid pressure ratio within the wedge

detrital zircon reference for the north china block abstract u pb analyses of 250 single detrital zircons from upper proterozoic to ordovician strata collected from the zhuozi shan in north central china provide a detrital zircon reference for the north china block a major crustal entity in the asian tectonic collage the results which range in age from 1 72 to 2 97 ga shed new light on the age of the crystalline basement in north china much of which is covered by younger sedimentary units in addition this detrital zircon reference can be used to help determine the provenance of clastic sedimentary units and for assessing validity of paleogeographic and regional tectonic models that include the complex history of asian continental amalgamation terrane accretion and subsequent translation that is ongoing today

strontium isotopic evidence for the origin of siderite ankerite and magnesite mineralizations in the eastern alps metasomatic magnesite siderite and ankerite mineralizations within paleozoic sediments of the northern greywacke zone and its metamorphic counterpart in the crystalline basement upper austroalpine unit were investigated with respect to their strontium isotopic composition the results support a sedimentary s l model for the magnesites within visean platform carbonates of the veitsch nappe and an epigenetic hydrothermal model for the siderites and ankerites in devonian platform carbonates of the noric nappe and from huttenberg the fe transporting fluids can be characterized either as magmatogene or more probably as metamorphogene by increasing the stage of recrystallization and or remobilization due to at least two later metamorphic overprints sr in the various ore minerals beame enriched in radiogenic 87sr

paleoceanographic modeling of temperature salinity profiles from stable isotopic data temperature salinity t s profiles in ancient seawater can be estimated from oxygen isotopic compositions of well preserved calcitic fossils sampled from known paleodepth gradients the proposed model uses isotopic compositions of fossils to determine possible combinations of temperature and salinity during deposition given estimates of average salinity isotopic composition and net global hydrologic fractionation for ancient oceans increasing density with depth constrains those combinations to narrow ranges this graphic method superposes curves for density and isotopic composition in t s space to reveal possible ancient t s profiles application of this method to oxygen isotope data from ordovician brachiopods deposited on the eastern continental slope of north america demonstrates its usefulness and yields a stable seawater column in which brackish intermediate waters overlie warmer more saline deep waters

a contribution to the ore genesis of the magnesite deposit of eugui navarra spain in the area of the magnesite deposits of eugui navarra spain studies on illite crystallinity the degree of graphitization of carbonaceous material measurements of vitrinite reflectivity and fluid inclusions have been carried out on dolomites magnesites schists and carbonaceous matter these rocks have suffered metamorphism of very low to low grade the magnesite appears generally concordant with the namurian dolomitic rocks showing a typically banded structure the genetic model proposed involves an early mg concentration during sedimentation syndiagenetic dolomitization lateral circulation of saline solutions and formation of diagenetically crystallized rhythmites dcr and final compaction the formation of magnesite took place under the conditions of low pressure and temperatures close to 150 c very similar to all strata bound ore deposits deformation and regional metamorphism only caused minor removal recrystallization and transformation of the clay minerals and carbonaceous matter

history of metamorphic fluids along outcrop scale faults in a paleogene accretionary prism sw japan implications for prism scale hydrology 1 synkinematic quartz crystals collected from late stage syn to post cleavage faults in the paleogene shimanto accretionary complex contain fluids that provide constraints on fluid flow within the accretionary wedge in particular the trapped fluids provide a glimpse of intraprism flow during a phase of widespread prism thickening carbonic ch4 co2 rich and aqueous h2o rich inclusions occur intermixed within single crystals that nucleated primarily in releasing stepovers along faults that deform well indurated and or cleaved wallrock invariably the carbonic inclusions contain an aqueous liquid as a perimeter phase and the aqueous inclusions contain a carbonic vapor phase suggesting coeval trapping of unmixed phases from a miscible parent fluid these observations are interpreted to reflect crystal growth following deformation driven dilitation and consequent unmixing of the parent fluid at one location cape oyama the occurrence of ch4 and h2o in the absence of co2 enables explicit constraints to be placed on pressure and temperature conditions during crystal growth these conditions suggest geothermal gradients of 50 c km during late stage faulting our results suggest local intraprism flow that was driven by deformation the scale of flow that is suggested is quite restricted and this leads us to hypothesize that the paleogene shimanto accretionary prism acted largely as a barrier to fluid flow during late stage deformation as a consequence the paleogene rocks probably served to focus fluid escape elsewhere most likely trenchward and arcward the combination of the elevated geothermal gradient which would have contributed to increasing the relative strength of this package of rocks and the lack of fluid pathways within it would have favored the development of new out of sequence thrust faults outboard of the paleogene prism or the reactivation of older more landward structures

magnesite deposits and occurrences in the west carpathians of czecho slovakia abstract magnesite occurrences in the west carpathians a part of the alpine orogenic system have been known since the second half of the 19th century they are hosted by cambrian silurian devonian carboniferous permian lower and upper triassic sedimentary units economically most important are crystalline magnesite veitsch type and locally associated talc in the devonian sequence of the veporides and in the caboniferous of the gemerides yearly production 2 3 million tonnes traditionally the slovak magnesite deposits had been interpreted as hydrothermal metasomatic related to magmatically derived fluids more recently a sedimentary diagenetic origin of the magnesite with mg derived in full or in part from the contemporaneous submarine basaltic volcanism is considered more probable after deposition and diagenesis the magnesites and their host rocks were metamorphosed in greenschist carboniferous of the gemerides to amphibolite devonian of the veporides facies

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

a new genetic model for the north african metasomatic siderite deposits based on published accounts of the diagenetic evolution of evaporite diapirs and geological preliminary geochemical and fluid inclusion data a new genetic model is presented for the north african metasomatic siderite deposits

magnetic and structural fabrics of the melange in the shimanto accretionary complex okinawa island implication for strain history during decollement related deformation the magnetic fabrics of the melange in the shimanto accretionary complex of okinawa island were obtained from the anisotropy of magnetic susceptibility ams data the magnetic fabrics show a marked contrast between the structurally lower and upper parts of the melange the lower part records deformation in the footwall of the decollement d1 whereas the upper part documents that in the hanging wall of the decollement d2 in the lower part of the melange the magnetic fabrics show good correlations with s1 layer parallel foliation reflecting strain geometries under layer perpendicular flattening as s c fabrics were locally developed by heterogeneous shear during d1 some of the previous magnetic fabrics were rotated with s1 in this case the resultant magnetic fabrics reflect s surfaces and maximum ams axes are reoriented parallel to shear directions without clear change in shape and anisotropy degree of the ams ellipsoids thus s surfaces did not act as flattening planes but may represent the deformation through independent particulate flow and mechanical rotation the lower part of the melange was pervasively sheared during d2 reflecting that it laid immediately above the decollement however d1 magnetic fabrics were not modified because shear strain was accomplished by brittle microfaults without development of penetrative strain in contrast penetrative strain was significant in the upper part of the melange resulting in the magnetic fabrics recording incremental strain associated with d2 therefore deformation mechanism and proximity to a decollement play an important role on development of magnetic fabrics in a melange

metallogenetic evolution of the east alpine paleozoic basement paleo metallogenetic sketch maps of the east alpine area are presented for the ordovician silurian the devonian lower carboniferous and the upper carboniferous permian mineralization and tectonic evolution are then discussed in the frame of a plate tectonic model compatible with recent results in western europe

gangue mineral 87 sr 86 sr ratios and the origin of mississippi valley type mineralization the 87 sr 86 sr ratio measurements in mississippi valley type gangue minerals and host marine carbonates ranging in age from upper cambrian to mississippian show that in northwestern ohio where the mineralization is sparse the minerals precipitated from fluids in local isotopic equilibrium with the host carbonates in which the minerals were emplaced in contrast most gangue minerals from districts of major mineralization in illinois kentucky virginia and tennessee contain strontium with higher 87 sr 86 sr ratios than the carbonate hosts and appear to have crystallized from fluids containing strontium from silicate minerals in the sedimentary succession no evidence has been found for a magmatie component in the ore fluid strontium data from elmwood tennessee show that the early ore fluids like those of northwestern ohio were in isotopic equilibrium with the host carbonates but that they were mixed with and displaced by introduced fluids containing more radiogenic strontium these findings are consistent with the two fluid mixing model for the mississippi valley mineralization where a fluid indigenous to the host carbonates and containing reduced sulfur was mixed with introduced fluids analogous to metal bearing and low sulfide oil field brines with an extensive history of chemical exchange with clays and other silicate minerals in the sedimentary succession

ocean plate stratigraphy in east and southeast asia abstract ancient accretionary wedges have been recognised by the presence of glaucophane schist radiolarian chert and melange recent techniques for the reconstruction of disrupted fragments of such wedges by means of radiolarian biostratigraphy provide a more comprehensive history of ocean plate subduction and successive accretion of ocean floor materials from the oceanic plate through offscraping and underplating reconstructed ocean floor sequences found in ancient accretionary complexes in japan comprise from oldest to youngest pillow basalt limestone radiolarian chert siliceous shale and shale and sandstone similar lithologies also occur in the melange complexes of the philippines indonesia thailand and other regions this succession is called ocean plate stratigraphy ops and it represents the following sequence of processes birth of the oceanic plate at the oceanic ridge formation of volcanic islands near the ridge covered by calcareous reefs sedimentation of calcilutite on the flanks of the volcanic islands where radiolarian chert is also deposited deposition of radiolarian skeletons on the oceanic plate in a pelagic setting and sedimentary mixing of radiolarian remains and detrital grains to form siliceous shale in a hemipelagic setting and sedimentation of coarse grained sandstone and shale at or near the trench of the convergent margin radiolarian biostratigraphy of detrital sedimentary rocks provides information on the time and duration of ocean plate subduction the ages of detrital sediments becomes younger oceanward as younger packages of ops are scraped off the downgoing plate ops reconstructed from ancient accretionary complexes give us the age of subduction and accretion direction of subduction and ancient tectonic environments and is an important key to understanding the paleoenvironment and history of the paleo oceans now represented only in suture zones and orogenic belts

recognition of the shimanto hp metamorphic belt within the traditional sanbagawa hp metamorphic belt new perspectives of the cretaceous paleogene tectonics in japan abstract the sanbagawa metamorphic belt in sw japan has been traditionally referred as a typical subduction related high pressure hp metamorphic belt this belt extends e w direction for more than 800 km from the kanto mountains to kyushu island in part protoliths of many hp metamorphic rocks formed as an accretionary complex after ca 90 80 ma and suffered a blueschist facies metamorphism at 66 61 ma this metamorphic event clearly postdates the sanbagawa metamorphism of 120 110 ma this newly recognized hp rocks occur extensively in the traditional sanbagawa hp metamorphic belt throughout shikoku kii peninsula and kanto mountains thus it was named as the shimanto hp metamorphic rocks thus the traditional sanbagawa metamorphic belt comprises two distinct hp metamorphic belts and the sanbagawa metamorphic rocks cover less than half of the areas for the traditional sanbagawa metamorphic belt importantly the metamorphic grade of the shimanto metamorphic rocks ranges from the pumpellyite actinolite to epidote amphibolite facies just like many sanbagawa metamorphic rocks the similarity between metamorphic rocks of these two belts in spatial distribution metamorphic grades and metamorphic facies series was the main reason why these two belts had not previously been differentiated a pacific type orogenic belt in general comprises a belt of accretionary complex and hp metamorphic belt on the ocean side plus a fore arc basin and a granite batholith belt on the continent side in sw japan the sanbagawa metamorphic belt is chronologically accompanied with the accretionary complex southern chichibu belt lower cretaceous fore arc sediments and the ca 120 70 ma sanyo batholith belt with ryoke low pressure metamorphic rocks likewise the shimanto hp metamorphic belt is accompanied with an upper cretaceous accretionary complex northern shimanto belt campanian to maastrichtian fore arc sediments and the san in batholith belt the opening of a back arc basin japan sea at ca 20 ma extensively modified the primary spatial arrangements of the two cretaceous paleogene orogenic belts in particular the southward thrusting of the ryoke sanyo granitic batholith belt over the sanbagawa metamorphic belt formed the present structural superposition of the ryoke above the sanbagawa and shimanto metamorphic belts

paleothermal structure of the shimanto accretionary prism shikoku japan role of an out of sequence thrust vitrinite reflectance study on the cretaceous shimanto accretionary prism of southwest japan shows that the vitrinite reflectance varies from 0 6 to 3 0 corresponding to the estimated maximum paleotemperature variation of 90 210 c the thermal structure shows a sharp discontinuity across an out of sequence thrust separating it into two thermal blocks in each block vitrinite reflectance values increase systematically toward the prism younging direction and the thermal structure is clearly overprinted on the tectonic imbrication of turbidite and melange units this provided us with a set of data to calculate the geometry and displacement of the out of sequence thrust the fission track zircon partial annealing ages indicate that this thrust was active about 43 ma probably in response to the accretion of eocene rocks this study shows that initial thickening of the accretionary prism in an order of several kilometres was mainly controlled by out of sequence thrust

palaeontological and isotope evidence for warm saline deep waters in ordovician oceans today s oceans are characterized by cold bottom waters produced at high latitudes here we present morphological and oxygen isotope data for brachiopods from the caradocian upper ordovician trenton group in new york which indicate that both temperature and salinity increased with depth in ordovician oceans production of warm saline deep waters in low to mid latitude evaporative seas and reduced production of cold deep water may have been favoured by high sea level warm global climate and fortuitous positioning of continents1 5 the resulting ocean circulation system was markedly different from that of today lower oxygen contents of warm saline deep waters may have contributed to the deposition of organic rich sediments which are abundant in caradocian strata6 7but rare in modern ocean sediments

new insight into a subduction related orogen a reappraisal of the geotectonic framework and evolution of the japanese islands abstract the geotectonic framework and the evolutionary history of the japanese islands need revision in accordance with the various geophysical geological evidence gathered by new methodologies in the recent years including seismic tomography vibroseis ground breaking seismic experiments and detrital zircon chronology these investigations have addressed various themes such as 1 seismic profile of the crust and mantle beneath the japanese islands 2 high precision ages of the protoliths of high p t metamorphic rocks and 3 provenance of terrigenous clastics the results have led to a number of important findings including 1 detection of a large mass of slab around the mantle boundary layer suggesting the long term oceanic subduction beneath japan 2 confirmation of the subhorizontal piled nappe structure for the entire crust of japan 3 finding a new high p t metamorphosed accretionary complex unit that represents the youngest blueschist in japan 4 finding of the oldest early cambrian arc batholith and cover sediments and 5 the identification of plural arc batholiths which have already been erased from the surface based on a synthesis of these new data this article presents a re evaluation of the conventional geotectonic subdivision of the subduction related orogen in japan re definition of the elements and their mutual boundaries and re consideration of the geotectonic evolution of the japanese islands in particular the historical change in provenance suggests that proto japan has experienced large scale tectonic erosion in multiple stages and the corresponding large amounts of continental crust materials were subducted for understanding the orogenic growth of japan during the last ca 500 million years the significance of tectonic erosion coupled with continental contraction as well as the oceanward accretionary growth requires further attention

asymmetrical melange fabrics as possible indicators of the convergent direction of plates a case study from the shimanto belt of the akaishi mountains central japan abstract scaly foliated melanges in the upper maastrichtian lowest paleogene strata occurring in a zone several kilometers wide in the shimanto belt of the akaishi mountains central japan often exhibit composite planar fabrics on a mesoscopic scale with distinct monoclinic symmetry they are characterized by block in matrix fabrics including scaly foliations in mudstone matrices and asymmetric lensoidal and or ellipsoidal clasts of sandstone which were generated by extension parallel to the layers these fabrics are geometrically very similar to those of foliated fault related rocks another remarkable fabric is the imbricated stacks of lensoidal sandstone clasts forming mesoscopic duplexes by layer parallel contraction the co existence of these extensional and contraction planar fabrics suggests that non coaxial deformation had considerably progressed and that strain distribution was heterogeneous during formation of the melange systematic analysis of attitudes of these melange fabrics in many outcrops indicates that the fabrics have a remarkably consistent preferred orientation suggesting that a south trending overthrust shear about 40 70 counterclockwise with respect to the trend of strata prevailed during melange formation this direction is fairly consistent with the relative plate motion deduced from the regional tectonic framework of this age which indicates the oblique subduction of the pacific plate relative to the eastern margin of the eurasian continent thus the melange fabrics with monoclinic symmetry probably reflect the relative motion of consuming plates and may become excellent geological indicators for determining the direction of convergence of plates during melange formation since data on melange fabrics are directly and easily available in the field

consolidation patterns during initiation and evolution of a plate boundary decollement zone northern barbados accretionary prism borehole logs from the northern barbados accretionary prism show that the plate boundary decollement initiates in a low density radiolarian claystone with continued thrusting the decollement zone consolidates but in a patchy manner the logs calibrate a three dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid and which portions have consolidated the seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid rich conduit extending down the decollement surface fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch

the late paleoproterozoic orogeny in the north china craton abstract granulite ttg and khondalite formations are the two typical types of precambrian metamorphic basement in the north china craton the former belongs to the basement of the archean craton while the latter is a paleoproterozoic metasedimentary rock series the formation of the ttg rock series took place during the period of 2500 to 2450 ma near the end of the archean and it has undergone multiple times of metamorphism the cooling rate of the ttg lower crust is quite low less than 0 3 ma developed during slow crustal cooling in a rather stable tectonic environment the khondalite rock series underwent amphibolite to granulite facies metamorphism that is isotopically constrained to 2000 to 1800 ma followed by fast crustal cooling from 2 4 to 3 4 ma based on the thermal and dynamic features of different types of metamorphic rocks the tectonic state of the crust and the obvious change of crustal cooling rate from neo archean to paleoproterozoic it is clear that continental orogeny existed in the north china craton between 2000 ma and 1800 ma

isotopic fronts in hydrothermally mineralized carbonate rocks advective dispersive fluid flow through permeable and porous rock causes systematic alteration of the infiltrated rock o and sr isotopes can be used as tracers to monitor exchange processes between mineralizing hydrothermal solutions and carbonate host rocks fluid infiltration into rock of initially uniform isotopic composition leads to characteristic changes in 18o and 87sr in the rock that depend on infiltration distance fluid velocity diffusivity and reaction kinetics for fast fluid solid equilibrium during advective flow the shape of such alteration profiles can be described by the dimensionless peclet number p e which expresses the fluid flux infiltration distance and diffusivity in the case of mineralization from low to moderate temperature solutions isotope exchange is not instantaneous but kinetically controlled and a similar parameter the damkohler number n d expressing fluid flux isotope exchange rate and infiltration distance becomes more useful n d and p e values have been estimated for various examples of carbonatehosted hydrothermal mineralization in the eastern alps austria and the benue trough nigeria by modelling o and sr isotope data from ore or gangue minerals and host rock the values obtained enable a rough differentiation between pervasive and channelled fluid flow as the prevailing transport mechanism to be made results indicate that both highly channelled fluid flow through fractures without significant interaction with the wall rock and pervasive fluid flow with infiltration distances into the percolated host rock of the order of 100 m occur near mineral deposits in the latter case the extent of o and sr isotopic alteration of the host rock can be used as an effective exploration tool

underplated units in an accretionary complex melange of the shimanto belt of eastern shikoku southwest japan imbricated thrust stacks composed of melange and oceanic slabs occur in the shimanto belt eastern shikoku sw japan the melanges are divided into two types melange i consists of lenticular sandstone blocks surrounded by scaly shale matrix and melange ii is composed of blocks of oceanic material such as basalts cherts and red shale in a scaly matrix of shale melange ii is observed only near the boundaries between melange i and oceanic slabs the fabric of melange i indicates progressive deformation during underthrusting of originally coherent trench fill turbidites layer parallel extension characterizes early deformation and is divided into two stages an early stage formed during normal faulting and a subseguent stage formed perpendicular to the early extension the early extension is interpreted in terms of riedel shear associated with layer parallel shearing and tectonically induced loading normal to the layering the second extension is due to sticking thrust movement late deformation of melange i is represented by folding in association with layer parallel shearing this appears to be related to compression just before or during underplating of melange i beneath the accretionary prism layer parallel shear extends downward due to strain hardening of sheared sediments and then reaches to the base of the sediments subsequently the shear zone penetrates pelagic sediments and finally oceanic basement at this time melange ii is formed by the shearing an imbricated stack of melanges i ii and oceanic slabs forms duplexes during the underplating process the weak metamorphism of the melange complex in the study area suggests that underplating occurs at shallow depths where trench fill is thin

high field strength element ratios in archean basalts a window to evolving sources of mantle plumes abstract in terms of high field strength element ratios nb th zr nb nb y and zr y most basalts from non arc type archean greenstones are similar to oceanic plateau basalts suggestive of mantle plume sources a large number of these basalts have ratios similar to primitive mantle composition perhaps the archean mantle was less fractionated than at present and primitive mantle comprised much of the deep mantle and made a significant contribution to mantle plumes the near absence of archean greenstone basalts similar to nmorb in composition is also consistent with a relatively unfractionated mantle in which a shallow depleted source dm was volumetrically insignificant the element ratios in basalts also indicate the existence of recycled components himu em1 em2 in the mantle by the late archean this suggests that oceanic lithosphere was recycled into the deep mantle and became incorporated in some mantle plumes by the late archean high field strength element ratios also indicate an important contribution of continental crust or and subcontinental lithosphere to some non arc archean greenstone basalts this implies that at least thin continental lithosphere was relatively widespread in the archean

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

well preserved underplating structure of the jadeitized franciscan complex pacheco pass california duplex structures several hundred metres thick bounded by marker beds exhibiting basalt chert association have been found in the franciscan complex near pacheco pass central diablo range california the terrane was regionally metamorphosed and deformed under blueschist facies conditions at 150 c and 7 8 kbar detailed mapping of the basalt chert sequence has clarified the change of environment from mid oceanic ridge to subduction zone eastward convergence of duplexing indicates eastward subduction of the oceanic plate during early cretaceous time the presence of duplexing and a reconstruction of oceanic plate stratigraphy recorded within the underthrust unit demonstrate the accretionary origin of protoliths of the franciscan coherent unit which has long been believed to have an in situ origin interpreted as either near trench volcanism or accreted terranes in which geosynclinal type volcanism occurred

provenance diversification within an arc trench system induced by batholith development the cretaceous japan case by comparing detrital zircon u pb age spectra of coeval fore arc and back intra arc basin sandstones we identified the overall distributary pattern of terrigenous clastic material within the cretaceous arc system of sw japan abundant proterozoic c 1500 2500 ma detrital grains from the interior of east asia are present in the cretaceous intra arc basin however after a barrier mountain range formed during batholith emplacement proterozoic clastics were rarely transported into the fore arc domain episodic batholith formation in pacific type orogens likely played a major role in controlling terrigenous supply routes between coeval back arc and fore arc domains the cretaceous orogen in japan thus provides a good template for analysing the tectono sedimentary development of other arc related basins

peeling oceanic crust in subduction zones thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes estimates of the average thicknesses of scraped off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m with a mean thickness of 80 100 m in the complexes studied the permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust this mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones

origin of siderite mineralization in western guizhou sw china constrains from rees c o sr and s isotopes abstract numerous siderite deposits in western guizhou of sw china are mainly hosted in the middle devonian dolostones the larger caiyuanzi cyz and xiongxiongjia xxj deposits have an ore reserve in excess of 300 mt with an average ore grade of 35 total iron there are banded and massive siderite ores and sulfide siderite ores the major ore mineral is magnesian siderite and gangue minerals are ankerite dolomite kaolinite quartz and carbon there are also metallic minerals including pyrite chalcopyrite and minor galena tetrahedrite and enargite rare earth elements rees and carbon oxygen and strontium isotope data indicate a marine origin of the host dolostones and a hydrothermal metasomatic origin of the siderite mineralization the variations of rees and carbon and oxygen isotopic compositions of siderite from the cyz and xxj deposits reflect the variable mineralization processes and different fluid rock ratios the lree depletion of siderite from the cyz deposit could be due to crystallographic control whereas the lree depletion along with mree enrichment and positive eu anomalies of siderite from the xxj deposit was possibly controlled by both crystallography and fluid composition siderite and calcite have 13 c pdb and 18 o smow values ranging from 8 to 4 01 and from 13 42 to 20 07 respectively the variations in c and o isotopic compositions of siderite and calcite may reflect metasomatic replacement of marine carbonates by hydrothermal fluids at different fluid rock ratios dolostones have 87 sr 86 sr initial values of 0 70844 0 70920 whereas siderite and calcite have values ranging from 0 71396 to 0 71708 negative correlations of initial sr isotope ratios versus carbon and oxygen isotopes of the host dolostones siderite and calcite indicated that carbonate minerals were metasomatic products of marine carbonates by hydrothermal fluids leaching from the proterozoic basement rocks iron was possibly derived from the basement leaching rather than from the continental weathering the 34 s cdt values 19 9 to 13 6 of sulfides associated with siderite indicated that the sulfur was probably derived from the marine sulfates through either bacterial sulfate reduction or organic mediated thermochemical reduction

underplating process from melange formation to duplexing example from the cretaceous shimanto belt kii peninsula southwest japan accretionary complexes are considered to form through off scraping and underplating which results in the lateral and vertical growth of an accretionary prism accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process although the origin of melange is considered controversial in order to better understand the origin of melange and its role in accretion in particular during underplating we have studied the relationship between the formation of melange and duplexing in the miyama assemblage here the melange is composed dominantly of shale basalt chert and sandstone this study stresses the following features 1 the structure of the miyama assemblage appears to be a composite suite of landward and seaward dipping duplexes 2 the melange fabric displays a systematic asymmetry which may have resulted through shearing along the decollement and 3 facing of horses within duplexes changes from north to south coinciding with a change in the sense of shear inferred from the melange fabric which suggests that it formed prior to duplexing

strontium isotopes in magnesites from permian and triassic strata eastern alps abstract the sr isotopic composition of sediment hosted magnesites in permian and scythian series of upper austroalpine units eastern alps has been determined the results suggest diagenetic metasomatic magnesite formation by mg rich pore solutions the depositional environment of the magnesite bearing rocks is reflected by different isotopic compositions with initial 87 sr 86 sr ratios close to contemporaneous sea water in marine sediments 0 7071 0 7083 and higher ratios being typical for lacustrine and coastal environments influenced from the hinterland 0 7133 0 7139 coarser grained recrystallized magnesite and magnesite veins show a distinct increase in their sr isotopic ratios 0 7202 0 7220 which can be attributed to metamorphic fluids of eoalpine age the findings of these magnesite occurrences which have been affected only by very low grade metamorphism are compared with similar results from spar magnesites associated with metamorphosed paleozoic sedimentary rocks a similar genetic model is proposed for the first magnesite mineralization in those paleozoic strata but with variably intense later recrystallisation during metamorphism

thermal maturity in the shimanto accretionary prism southwest japan with the thermal change of the subducting slab fluid inclusion and vitrinite reflectance study abstract the p t path of the cretaceous shimanto accretionary complex western shikoku southwest japan was determined by a combination of vitrinite reflectance data and homogenization temperatures of fluid inclusion analysis within the complex the p t paths of the upper cretaceous yokonami melange and surrounding coherent strata were compared no p t difference was found between the yokonami melange and surrounding strata at least two types of fluids methane rich and water rich fluids migrated at different stages during metamorphism within the complex the methane rich fluid was initially trapped in vein quartz under pressures of 260 mpa and a geothermal gradient of 24 c km the water rich fluid was included in calcite at 95 125 mpa and a geothermal gradient of 50 c km the complex sequence of occurrence of the vein minerals and radiometric age data suggest that the water rich fluid was trapped during the kula pacific ridge subduction into the shimanto accretionary complex the paleoheat flow values for the two stages of subduction are consistent with the changes in the inferred slab age the metamorphism of the shimanto accretionary complex was predominantly controlled by the thermal condition of the subducting plate

siderite mineralization of the gemericum superunit western carpathians slovakia review and a revised genetic model ore geology reviews 24 267 298 a reply

organic and inorganic geochemistry of ljubija siderite deposits nw bosnia and herzegovina the ljubija siderite deposits hosted by a carboniferous sedimentary complex within the inner dinarides occur as stratabound replacement type ore bodies in limestone blocks and as siderite sulfides veins in shale three principal types of ore textures have been recognized including massive dark siderite and ankerite siderite with zebra texture and siderite veins the ore and host rocks have been investigated by a combination of inorganic major trace and rare earth element concen trations organic characterization of hydrocarbons includ ing biomarkers and stable isotope geochemical methods isotope ratios of carbonates sulfides sulfates kerogen and individual hydrocarbons new results indicate a marine origin of the host carbonates and a hydrothermal metasomatic origin of the fe mineralization the differ ences in ore textures e g massive siderite zebra siderite are attributed to physicochemical variations e g changes in acidity temperature and or salinity of the mineralizing fluids and to the succession and intensity of replacement of host limestone vein siderite was formed by precipitation from hydrothermal fluids in the late stage of mineralization the equilibrium fractionation of stable isotopes reveals higher formation temperatures for zebra siderites around 245 c then for siderite vein around 185 c sulfur isotope ratios suggest permian seawater or permian evaporites as the main sulfur source fluid inclusion composition confirms a contribution of the permian seawater to the mineralizing fluids and accord with a permian mineralization age organic geochemistry data reflect mixing of hydrocarbons at the ore site and support the hydrothermal metasomatic origin of the ljubija iron deposits

warm saline bottom water in the ancient ocean considerable isotopic evidence indicates that oceanic bottom water was much warmer in the geological past a simple convection model driven by multiple turbulent buoyant plumes and observations of present deep water formation have led to the development of a theory for the formation of warm saline bottom water it is suggested that changes in the size and configuration of marginal seas in net evaporation zones due to lithospheric plate motions and eustatic sea level change caused these seas to become sources of warm saline bottom water evidence from the palaeotemperature record and palaeogeography during the late cretaceous is used to reinforce the hypothesis climatic and chemical consequences of the formation of warm saline bottom water are discussed

late cretaceous genesis of the kula plate abstract we propose that the pacific kula ridge began spreading approximately 85 m y b p during late cretaceous time extrapolation of the great magnetic bight backwards in time results in an implausible ridge configuration this implies that plate velocity vectors for the pacific kula and farallon plates were not constant during this interval evidence for splitting of the kula plate from the pacific plate along the chinook trough is the relationship between the north striking amlia and adak fracture zones the chinook and emperor troughs and the magnetic lineations south of the aleutian trench if this hypothesis is correct it will require that mesozoic reconstructions of the pacific basin and their relation to cenozoic reconstructions be re examined a previously unrecognized mesozoic plate may be required we propose calling this the izanagi plate

siderite mineralization of the gemericum superunit western carpathians slovakia review and a revised genetic model ore geology reviews 24 267 298 a discussion radvanec et al 2004 provided a revised version of their metamorphic hydrothermal model for siderite and associated mineral deposits of the western carpathians compared to the previous version grecula 1995 grecula et al 1996 the upgraded model emphasizes the dominant role of retrograde variscan metamorphic processes and admits remobilization of the variscan siderite and other ore components during the alpine orogenic cycle similar to the previous versions the variscan ore forming processes genetically related to metamorphism are believed to dominate not only in the siderite vein and metasomatic deposits but also in the superimposed vein sulphides p 292 the revised model is unsustainable in light of recent knowledge the major pitfalls are summarized and commented as follows

early tertiary fluid flow and pressure temperature conditions in the shimanto accretionary complex of south west japan constraints from fluid inclusions the pressure and temperature history of the tertiary shimanto belt of south west japan has been elucidated by analysing fluids trapped in quartz crystals which grew syn kinematically along late stage brittle faults the samples come from three areas that span the paleogene exposures on the muroto peninsula of shikoku island applying microthermometric and laser raman microsampling techniques to coeval water rich and carbonic fluid inclusions we have constrained the pressure and temperature conditions that accompanied a widespread and kinematically distinct phase of deformation the results suggest elevated geothermal gradients during late stage deformation conditions that are in disaccord with previous plate reconstructions that have depicted old thermally mature pacific crust subducting beneath eurasia during the early to middle tertiary these conditions can most easily be accounted for by including an additional plate boundary in the western pacific during paleogene time plate reconstructions that include the kula plate in this region are therefore consistent with our findings in addition our results provide clues to the conditions that likely accompany seismogenic deformation at active convergent plate boundaries

siderite deposits in northern italy early permian to early triassic hydrothermalism in the southern alps abstract we present a minero petrographic geochemical and geochronological study of siderite orebodies from different localities of the southern alps northern italy siderite occurs as veins cutting the variscan basement and the overlying lower permian volcano sedimentary cover collio fm and as both veins and conformable stratabound orebodies in the upper permian verrucano lombardo and bellerophon fms and lower triassic servino and werfen fms sedimentary sequences of the lombardian and the venetian alps all types of deposits show similar major and rare earth ree element patterns suggesting a common iron mineralizing event the compositions of coexisting siderite fe rich dolomite and calcite suggest formation from hydrothermal fluids at relatively high temperature conditions 250 c geochemical modelling supported by ree analyses and by literature and new 1 3 c and 18 o isotopic data suggests that fluids responsible for the formation of siderite in the variscan basement and in the overlying lower permian cover were derived from dominant fresh water which leached fe and c from volcanic rocks mainly rhyolites rhyodacites and organic carbon bearing continental sediments on the basis of u th pb microchemical dating of uraninite associated with siderite in the val vedello and novazza deposits lombardian alps the onset of hydrothermalism is constrained to 275 13 ma early mid permian i e it was virtually contemporaneous to the plutonism and the volcanic sedimentary cycle reported in the same area orobic basin the youngest iron mineralizing event is represented by siderite veins and conformable orebodies hosted in lower triassic shallow marine carbonatic successions in this case the siderite forming fluids contained a seawater component interacted with the underlying permian successions and eventually replaced the marine carbonates at temperatures of 250 c the absence of siderite in younger rocks suggests an early triassic upper limit for the iron pulse in the southern alps which would thus predate the middle triassic magmatism based on the overlap between hydrothermalism extensional tectonics and in part magmatism the genesis of siderite in the southern alps may be related to plutonic activity and or magmatic underplating occurring since the permian in a geodynamic scenario preluding the opening of the neo tethys

recognizing oib and morb in accretionary complexes a new approach based on ocean plate stratigraphy petrology and geochemistry abstract we present a new approach for recognizing the origin of accreted basaltic rocks based on ocean plate stratigraphy ops and on the petrology and geochemistry of basalts from mid oceanic ridges morb and oceanic islands oib using examples from four accretionary complexes ac in sw japan akiyoshi mino tamba chichibu and shimanto the key to the problem is the model of ops which includes an association of igneous and sedimentary rocks that form on an oceanic plate during its travel from a mid oceanic ridge to a subduction zone we propose the reconstruction of the tectonic settings of basalts according to their relationships with associated ops sediments their petrogenesis and their geochemical features five types of ops are recognized in the accretionary complexes of sw japan 1 sandstone shale 2 sandstone shale and chert 3 sandstone shale chert and morb 4 sandstone shale chert morb and gabbro peridotite 5 seamount ops including oib cap carbonates slope clastics and basal shale chert the alkaline tholeiitic or calc alkaline composition of basaltic melts which are typical of oceanic islands mid oceanic ridges and island arcs respectively can be identified by the sequence in crystallization of their major phenocrysts i e olivine ol clinopyroxene cpx and plagioclase pl and by their compositions alkaline and calc alkaline mafic lavas are characterized by an ol cpx pl succession whereas tholeiitic melts by their ol pl cpx succession titanium rich minerals e g ti augite kaersutite ti biotite are typical of alkaline lavas the application of geochemistry based tectonic discrimination diagrams is also a powerful tool if not supported by geological and petrological data may result in confusion due to magma contamination post magmatic alteration and secular change of mantle thermal conditions we propose that a direct comparison of normalized multi element patterns and key binary plots from older volcanic rocks with their modern analogues provides a more viable and reliable method of basalt discrimination our ops petrology geochemistry method allows us to confirm the above conclusions that the lavas of the akiyoshi mino tamba and southern chichibu ac formed in oceanic islands because they are associated with seamount ops sediments crystallized from ol to cpx and pl contain ti augite and kaersutite and are enriched in tio2 lree and nb in this paper we present geochemical data from the inuyama basalts of the mino tamba ac and from the toba complex in the huge mikabu greenstone belt of the chichibu ac the inuyama basalts are in contact with jurassic pelagic cherts but their geochemical features are confusing they contain phenocrysts of ol ti augite and kaersutite and therefore probably formed in seamounts the toba volcanic rocks are a part of the huge ophiolite belt they have flat to slightly lree enriched ree patterns are characterized by an ol cpx succession of phenocrysts and they plot in the oib field in binary plots suggesting they formed in an oceanic plateau

accreted kula plate fragment at 94ma in the yokonami melange shimanto belt shikoku japan abstract the cretaceous shimanto belt that is distributed along the pacific side of southwest japan contains one of the best records of accretionary processes and history of oceanic plate subduction in the circum pacific region yokonami melange shimanto belt shikoku sw japan is composed of pillowed basalt that erupted in the equatorial region about 200 m thick bedded radiolarian chert sequence that contains radiolarians with age determinations spanning ca 135 ma upper valanginian to 93 9 ma cenomanian younging southward with repetition and alternated sandstone and mudstone although the chronology of the bedded chert sequence is well studied using the radiolarian the chronology of the clastic units was not reported sufficiently we performed a detailed geologic mapping investigation of rock outcrops along the goshiki no hama coast the yokonami melange we have determined u pb ages of zircon grains extracted from tuffs and sandstones to determine the depositional age of the turbidite and hemipelagic units the youngest depositional age of these clastic sediments is interpreted to be the arrival time at the trench we reconstructed the ocean plate stratigraphy with our new chronological data which includes mid oceanic basalt bedded chert hemipelagic sediments and trench turbidite deposits we recognize that the turbidite sequence has been repeated by layer parallel thrust in the study area we have delineated the travel trajectory of the oceanic lithosphere based on 1 the relative plate motion model of the paleo pacific ocean 2 the arrival time of the subducted lithosphere at the trench 94 32 ma and 3 paleomagnetic measurement of the basalt and turbidite the trajectory suggests that the morb in the goshiki no hama coast yokonami melange erupted at the kula pacific ridge in the paleo pacific ocean traveled northward and eventually arrived at the continental margin of east asia at 94 32 ma

off scraping accretionary process under the subduction of young oceanic crust the shimanto belt of okinawa island ryukyu arc the eocene kayo formation exposed at the southwestern end of the shimanto belt in the ryukyu arc records the progressive development of off scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint the formation consists almost wholly of coherent trench turbidites supplied by axial currents detailed structural analysis has revealed that two successive deformation stages are recognized in the formation the early deformation stage 1 occurred by horizontal shortening related to the initial stage of off scraping accretion in this stage sands were water rich and nearly unlithified and the competence contrast between sandstone and shale was low the late deformation stage 2 is marked by the formation of seaward verging imbricate fold and thrust system within the accretionary wedge compared with stage 1 sands in stage 2 were more dewatered and lithified and the competence contrast between sandstone and shale increased in accordance with this deformation history structures caused by high pore fluid pressure indicate increased cohesion of sands during off scraping accretion studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the formation became buried and suffered thermal imprint pressure solution cleavage developed as an axial planar to the stage 2 folds thermal imprint on the formation is characterized by high paleotemperatures at approximately 250 c 300 c in a region of the prehnite pumpellyite facies metamorphism this thermal event took place during and slightly after the stage 2 probably associated with subduction of a young and hot oceanic plate

high paleogeothermal gradient with ridge subduction beneath the cretaceous shimanto accretionary prism southwest japan paleotemperature in the jurassic chichibu and the cretaceous shimanto accretionary complexes southwest japan increases southward generally without regard to the early phase geologic structure there is an exceptional peak in the okitsu melange and the thermal structure probably formed after accretion of the complex the estimated geothermal gradient is more than 90 c km in the okitsu melange and more than 50 c km elsewhere in the complex the ophiolitic okitsu melange formed concurrently with eocene subduction of the kula pacific ridge under the cretaceous accretionary prism the previous thermal structure was overprinted by this event

accreted oceanic materials in japan abstract the phanerozoic circum pacific orogenic belts contain numerous ocean derived materials accreted through plate converging processes japanese islands in particular display various kinds of oceanic materials of different origins including fragments of seamounts oceanic reef limestone morb like rocks and oceanic mantle and pelagic sediments the compilation of these rocks in many subduction complexes of late permian to the present led to following conclusions accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust i e layer 1 and layer 2 many fragments of seamount with alkali basalt 600 hot spot seamount 26 oceanic reef limestone 291 morb like basalt 200 and numerous cherts more than 1000 are recognized as ancient oceanic materials accreted to the japanese islands however gabbros and mantle materials of layer 3 and lower parts of the oceanic lithosphere scarcely occur in subduction accretion complexes except for a few examples of back arc basin or fore arc origin accretion occurs episodically in southwest japan oceanic materials were accreted intermittently in 1 a end permian 2 b middle late jurassic 3 c late cretaceous times 4 d at ca 50 ma and 5 e in miocene times while in northeast japan and hokkaido this occurred in b middle late jurassic c late cretaceous and f early cretaceous times in contrast to the general belief on accretion of younger oceanic plates the majority of japanese subduction accretion complexes were formed during the subduction of plates up to 160 ma old the accretionary events in end permian and middle late jurassic times coincide with northward collision of ancient island arcs oceanic rises or seamount chains of hot spot origin with the asian continent accretion relevant to subduction of older plates may be controlled by the collision subduction process of these topographic reliefs on an oceanic plate in addition the chronological coincidence with the continent collision amalgamation between the sino korean and siberian platforms and between the sino korean and yangtze blocks also implies collision induced voluminous supply of elastics from back arc regions and its contribution to the formation of huge accretionary complexes accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts this implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels with respect to the metamorphic grades for japanese subduction complexes oceanic materials have less than 1 vol in the zeolite facies 15 20 in the prehnite pumpellyite metagraywacke facies and ca 30 in the greenschist glaucophane schist facies and albite epidote amphibolite fades this relationship indicates that the major process for landward accretion of oceanic materials is not off scraping or sedimentary mixing at the trench but underplating subcretion at much deeper levels of a subduction zone