calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

how does anisotropy in bedrock river granitic outcrops influence pothole genesis and development this research was partially funded by geomateriales 2 programme s2013 mit 2914 funded by the regional government of madrid spain

causes consequences and chronology of large magnitude palaeoflows in middle and late pleistocene river systems of northwest europe this study examines the record of high palaeoflow phases in river systems in northwest europe investigating their causes whether due to unique events such as the formation of the dover strait or as characteristic consequences of climate change examining their consequences with regard to landscape evolution and possible effects on climate and determining the chronology of key events large magnitude palaeoflows more than an order of magnitude larger than present day flood peaks are shown to be characteristic of rivers in this region at particular times within the pleistocene these events the most recent of which were during heinrich events 2 and 1 at 25 and 17 ka were evidently caused by the combined effects of glacial outwash rainfall snowmelt and melting of permafrost in some proportion they released such large volumes of water that the thermohaline circulation of the atlantic ocean and thus the climate may well have been affected these large magnitude palaeoflows are thus a significant aspect of the pleistocene earth system that has hitherto gone unquantified

alkali mapping of the japanese quaternary volcanic rocks abstract of ca 200 quaternary volcanic centers in the japanese islands ca 120 are characterized by k 2 o and na 2 o concentrations of their ejecta normalized to sio 2 60 k 2 o values give a geographically well defined trend of regularly increasing away from the volcanic front na 2 o values show no geographical trend relationships between the sio 2 normalized k 2 o and the depth of the deep seismic zone k h relation are different for different arcs the southwest japan arc shows distinctly higher k 2 o and less regular geographic trend than the others probably due to crustal contamination in the normal segments of the northeastern japan belt k 2 o increases ca 2 5 to 3 0 per 100 km depth of the seismic zone groups of volcanic centers near the junction between the northeast honshu and izu mariana arcs give characteristically lower k 2 o while rishiri volcano far behind the arc junction between the kurile and honshu arcs also gives a low k 2 o value

bedform genesis in bedrock substrates insights into formative processes from a new experimental approach and the importance of suspension dominated abrasion bedrock channels are common in the natural environment and bedrock channel erosion sets the pace of denudation in many river catchments however in comparison to the large number of studies concerning the formation of alluvial bedforms relatively few investigations have concerned bedrock bedform genesis field based analysis of sculptured forms within bedrock channels has been restricted notably by the slow rate of bedform development in such environments furthermore only a limited number of flume scale experiments have been conducted that attempt to simulate the genesis of sculpted bedforms in bedrock channels this study demonstrates that optimisation of clay beds through analysis of clay strength enables the development of features analogous to bedrock river channel bedforms even at a scale that is orders of magnitude smaller than some natural examples three sets of suspended sediment laden experiments were carried out using hard medium and soft clay bed substrates a suite of erosive bedforms including potholes flutes and furrows developed on all experimental beds all observed erosional features have clear equivalents to those observed in natural bedrock rivers bed shear strength was found to be a significant factor for the genesis of different types of simulated bedrock bedforms in our experiments with other factors such as flow velocity bed slope and flow depth held approximately constant importantly in a subset of experiments performed with an absence of suspended sediment fluid flow did not result in the erosion and development of bedforms in the clay bed hence this work illustrates that abrasion by suspended sediments is the key process required for the formation of these simulated bedrock bedforms in our experiments in the absence of bedload abrasion other processes such as plucking cavitation and dissolution will have been negligible

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

critical flow in rockbed streams with estimated values for manning s n the writer presented interesting experimental observations the discusser would like to comment on some aspects of the paper regarding critical flow or more accurately near critical flow critical slope calculations and boundary shear stress distributions in undular flows he hopes that the information will assist in refining the author s work

origin of vertical shafts in bedrock along the eramosa river valley near guelph southern ontario numerous vertical to subvertical cylindrical shafts occur in the rugged exposures of the middle silurian amabel formation bedrock along the eramosa river approximately 10 km northeast of guelph these shafts vary from 1 m to 10 m in diameter with depths as great as 12 m they can occur as isolated depressions but more commonly are clustered closely together and many coalesce to form sinuous bedrock walls these depressions may contain soil fine sediment rubble from the surrounding dolomite bedrock and rounded clasts of precambrian basement lithologies till is conspicuously absent these shafts occur in close proximity to cavernous pores that were formed during pre wisconsinan to early wisconsinan time suggesting a possible karstic origin the cylindrical shafts however with their greater midpoint diameters sculpted interiors and inclined axes and their lack of vertical flutes and till are more likely to have originated from glaciofluvial discharge at the end of the late wisconsinan the d

rates and timing of earth surface processes from in situ produced cosmogenic be 10 beryllium 10 is the longest lived of the seven known unstable isotopes of be it results mostly from the interaction of cosmic radiation primarily neutrons with a variety of target atoms by spallation the splitting of nuclei lal 1988 it can also be produced at very low levels by radio disintegration of u and th sharma and middleton 1989 although all radioactive be isotopes are produced in the atmosphere via cosmic ray reactions arnold 1956 lal and peters 1967 morris et al chapter 5 this volume in this paper we are most interested in 10be produced in rock which we refer to as in situ produced such in situ produced 10be is a cosmic ray dosimeter functioning as a quantitative monitor of near surface residence time in some geologic situations 10be can be used as a chronometer allowing one to estimate the duration of surface exposure in other settings 10be can be used as a tracer allowing one to estimate the rate distribution and behavior of earth surface processes rates of 10be production are very low in rocks exposed at earth s surface 102 atoms g 1 y 1 resulting in typical in situ produced 10be concentrations termed activities of 103 to 107 atoms g 1 the development of accelerator mass spectrometry ams in the 1980s allowed routine measurement of 10be at levels typical of rocks exposed near earth s surface elmore and phillips 1987 analytical refinements in the 1990s lowered detection limits by an order of magnitude allowing dating of exposure periods 103 years in favorable geologic and geographic situations simple exposure histories at high altitude and latitude most in situ 10be is measured in quartz mineral separates where the primary spallation target is o lal and arnold 1985

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

imaging of co2 bubble plumes above an erupting submarine volcano nw rota 1 mariana arc nw rota 1 is a submarine volcano in the mariana volcanic arc located 100 km north of guam underwater explosive eruptions driven by magmatic gases were first witnessed there in 2004 and continued until at least 2010 during a march 2010 expedition visual observations documented continuous but variable eruptive activity at multiple vents at 560 m depth some vents released co2 bubbles passively and continuously while others released co2 during stronger but intermittent explosive bursts plumes of co2 bubbles in the water column over the volcano were imaged by an em122 12 khz multibeam sonar system throughout the 2010 expedition numerous passes were made over the eruptive vents with the ship to document the temporal variability of the bubble plumes and relate them to the eruptive activity on the seafloor as recorded by an in situ hydrophone and visual observations analysis of the em122 midwater data set shows 1 bubble plumes were present on every pass over the summit and they rose 200 400 m above the vents but dissolved before they reached the ocean surface 2 bubble plume deflection direction and distance correlate well with ocean current direction and velocity determined from the ship s acoustic doppler current profiler 3 bubble plume heights and volumes were variable over time and correlate with eruptive intensity as measured by the in situ hydrophone this study shows that midwater multibeam sonar data can be used to characterize the level of eruptive activity and its temporal variability at a shallow submarine volcano with robust co2 output

feedbacks between erosion and sediment transport in experimental bedrock channels natural bedrock rivers flow in self formed channels and form diverse erosional morphologies the parameters that collectively define channel morphology e g width slope bed roughness bedrock exposure sediment size distribution all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes to explore the mechanics of river incision we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate sand and cement a single sediment size primarily transported as bedload a single erosion mechanism abrasion and sediment starved transport conditions we find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field sediment transport and bottom roughness abrasion only occurs where sediment impacts the bed and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows however the spatial focusing of erosion results in tortuous flow paths and erosional forms inner channels scoops potholes which dissipate flow energy this energy dissipation is a negative feedback that reduces sediment transport capacity inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load copyright 2007 john wiley sons ltd

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

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

aeromagnetic data a tool for studying the variscan arc of western europe and its correlations with transatlantic structures abstract a new compilation of magnetic data from france with the french continental shelf and portugal allows the delineation of the geodynamic features of the variscan domain of western europe specific techniques such as the use of shadowgrams provide a better definition of the variscan structures in france especially of those below the mesozoic basins and offshore on the western continental shelf in addition hypotheses on the extension of the western part of the variscan arc may be considered comparison of the magnetic trends using a permian palaeogeographic reconstruction suggests new constraints on the interpretation of the ibero armorican arc and hitherto unknown relations hips between the main variscan faults and sutures defined in europe and their possible extensions into north america

controls on p t t deformation path from amphibole zonation during progressive metamorphism of basic rocks estuary of the river vilaine south brittany france edenite tremolite and edenite magnesio hornblende in equilibrium with plagioclase chlorite epidote quartz and vapour involve several types of reactions for which kd can be related to t and p thermodynamic calculation of these equilibria leads to isopleth systems given knowledge of the progressive changes of end member activities in zoned ca mg amphiboles based on microprobe analyses it is possible to construct precise pressure temperature time paths p t t paths which have been followed by metabasites during polyphase metamorphism when applied to basic rocks from the river vilaine area this method allows us to construct a p t t path that can be compared directly to the p t t path constructed from interbedded acid rocks aluminous micaschists in the same structural unit through time both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the l s fabric system of flinn this allows us to construct a p t t deformation path for this structural unit these paths are interpreted in terms of an under overthrusting continental collision belt the hercynian belt and represent an illustration of the time delay caused by stacking of more than two crustal units

tremolite and h 2 o activity attending metamorphism of hornblende plagioclase garnet assemblages estimation of the activity of tremolite component atr in calcic amphiboles is an important problem in igneous and metamorphic petrology because equilibria involving tremolite are used in the estimation of the activity of h2o attending crystallization of igneous and metamorphic rocks estimated values of atr from hornblende analyses using both ionic and coupled substitution crystalline solution models can be compared to values of atr calculated from vapor absent mineral equilibria in addition these values of atr can be used in calculations of ah2o for rocks for which there is an independent estimate of ah2o the values of atr calculated from vapor absent equilibria are generally consistent with those estimated from the different crystalline solution models but uncertainties in the calculations preclude choosing a preferred solution model from computed mineral equilibria it is clear that mineral assemblages with low values of atr can be in equilibrium with high values of ah2o consequently the low values of ah2o estimated from hornblende bearing high grade rocks with low values of atr may be real rocks from mica creek british columbia probably achieved extremely low values of ah2o by vapor absent metamorphism

rb sr evidence for the presence of ordovician gsranites in the deformed basement of the badajoz cordoba belt sw spain a rb sr whole rock investigation of suites of samples from two orthogneiss bodies in the almendralejo area of the oporto portalegre badajoz cordoba belt points to an ordovician age for the granitic magmatism about 470 ma for the almendralejo gneiss peralkaline metagranite and about 425 ma for the ribera del fresno gneiss leucocratic subaluminous metagranite both with high initial87sr 86sr ratios this points to a post ordovician hercynian age for the metamorphism and tectonic deformation that affected the area including the gneisses and rules out the alleged cadomian or even older age

isotopic dating of the mylonitization of the azuaga group in the badajoz cordoba belt sw spain on the basis of rb sr muscovite dating an age of about 355 ma late tournaisian early visean is assigned to the m1 metamorphism and associated mylonitization of the ultramylonites of the las grullas formation in the azuaga group badajoz cordoba belt this age may possibly be related to the accretion process suturing of the ossa morena zone to the central iberian zone thought to be responsible for the metamorphism and mylonitization muscovite from the valencia de las torres formation displays a somewhat lower rb sr age of about 337 ma the corresponding k ar ages indicate that after the termination of the metamorphism it took some 20 ma before the rocks had cooled below the closure temperature of muscovite to k ar whole rock rb sr data indicate that the las grullas ultramylonites were probably derived from ordovician granitoids

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

empirical and theoretical investigations of sculpted forms in buckeye creek cave west virginia abstract sculpted forms play important roles in channel erosion but the controls on their growth or appearance are uncertain empirical evidence from sculpted forms eroded into limestones within buckeye creek cave west virginia shows that types of sculpture morphology are determined by the types of flow or vortex structures present within sculptures during floods small hemispherical sculptures maintain constant form as they enlarge and occupy fixed locations on channel walls because of their association with joints styolites and bedding planes taking advantage of the systematic growth and stability of the sculptures a geometrical model of sculpture growth is presented that relates the effects of sculpture size form and wall retreat to the relative erosion efficiencies and excesses required for sculpture growth numerical results obtained from the theoretical model reveal that small sculptures must erode more efficiently than large sculptures or they will be removed by wall retreat and that erosi

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

hydrothermal fluid flow system around the iheya north knoll in the mid okinawa trough based on seismic reflection data seismic reflection data around the iheya north knoll hydrothermal field provide insights into geological structures that control subseafloor hydrothermal fluid flow in the sediment covered continental backarc basin of the mid okinawa trough we identified the seismic expression of widespread porous volcaniclastic pumiceous deposits and intrusions as a result of silicic arc volcanism the porous and permeable volcanic deposits are distributed in an area extending updip from the thick succession of the deep trough to the seafloor at the hydrothermal field their regional structure focuses the flow of hydrothermal fluids derived from the surrounding trough fill sediments and directs them to the vents of the hydrothermal field the high concentrations of ch4 and nh4 in the fluids of the hydrothermal field are likely derived from the interaction of migrating fluids with trough fill sediments

seismic record of tectonic evolution and backarc rifting in the southern ryukyu island arc system abstract the southern ryukyu island arc system is located at a convergent plate margin where the philippine sea plate is subducting under the eurasia plate we have conducted multi channel seismic reflection surveys to study tectonic evolution and backarc rifting of the southern ryukyu island arc system using the r v tansei maru of the ocean research institute university of tokyo in june 1993 and 1994 we describe systematically a complete cross section from the east china sea continental shelf to the ryukyu trench from the viewpoint of seismic stratigraphy seven major seismic units and three stages in the tectonic evolution of the system are identified 1 during stage 1 from late miocene to earliest pleistocene pre rift deposits of the shimajiri group accumulated over a wide region from the east china sea continental shelf to the forearc region 2 stage 2 is defined by a series of tectonic processes involving crustal doming erosion subsidence and sedimentation in association with initial rifting of the southern okinawa trough during most of early pleistocene time and 3 the backarc rifting is still in progress and syn rift sedimentation has been under way since the late pleistocene stage 3 a new significant observation lies in the fact that the pliocene shimajiri group is not distributed in most of the present day southern okinawa trough this implies that the backarc rifting of the southern okinawa trough was probably initiated after the deposition of the shimajiri group that is during the early pleistocene crustal scale simple shear allowing an asymmetrical half graben structure rather than pure shear governed the initial rifting of the southern okinawa trough the possibility however cannot be excluded that pure shear associated with symmetrical rifting may be predominant in the present day rifting of the southern okinawa trough as a result of northwestward migration of the rifting axis the average extension rate of the southern okinawa trough is estimated to be approximately 1 2 cm year on the basis of fault geometry observed on the acoustic basement

granulite facies metamorphism and retrogressive evolution of the monts du lyonnais metabasites massif central france abstract garnet bearing pyriclasites have been found within the monts du lyonnais formation near st andrela cote in the eastern part of the massif central the rocks appear to have differentiated from a tholeiitic magma of continental affinity and they may be genetically related to quartzofeldspathic granulites considered to be the products of crustal melts by means of electron microprobe analyses the consistent pressure and temperature estimates based essentially on pyroxene garnet equilibria show that the peak of metamorphism was at about 860 c and 10 kbar furthermore this study tends to confirm the affiliation of monts du lyonnais granulites to the early variscan granulite eclogite facies metamorphism which extends from bohemia to spain through central europe this event may be expected in the vicinity of an ancient suture zone where a continental collision followed a subduction

nd and sr isotopic study of volcanic rocks from japan two older granitic rocks and some selected quaternary volcanic rocks from the japanese islands were analyzed in a reconnaissance study for the purpose of examining the relationships between nd and sr isotopic abundances and the megatectonic structure around the japanese islands model ages of 0 9 ae were determined by the nd and sr methods on a paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the japanese islands the nd and sr isotopic data show that the cretaceous granodiorite is the result of partial melting of older crust the nd of tholeiitic rocks from the izu arc gives e nd ranging from 8 3 to 9 3 and with the corresponding e sr from 14 5 to 18 5 these results are identical to those found for the mariana arc these values are distinctly lower than typical morb by around 1 2 eu this difference in e nd between arcs and morb is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust the hakone volcano is clearly confirmed as belonging to an oceanic source by nd isotopic results e sr e nd values of the volcanics from a section along the fossa magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material the crustal component clearly increases in going from south to north volcanics across the northeast japan arc also show a distinct correlation of e sr e nd related to the position relative to the active subduction zone but with the opposite trend these relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the japan sea in general there appear to be regular isotopic relationships between the izu mariana oceanic island arc and the continental island arc of japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas

detailed volcanic morphology of daisan miyako knoll in the southern ryukyu arc abstract high resolution bathymetric mapping using an autonomous underwater vehicle and a survey vessel was conducted at daisan miyako knoll in the southern ryukyu arc this paper presents for the first time the detailed volcanic morphology of a submarine edifice that previously was poorly explored and discusses its formation process and relation to arc volcanism and tectonics bathymetry shows morphological evidence that daisan miyako knoll is a stratovolcano with three caldron like depressions 1 km in diameter a central cone and multiple lava flows at the summit the main edifice is conical with a basal diameter of 8 10 km and relief of 1000 1100 m with a summit depth of 779 m the central cone that stands inside the depressions has a circular crater numerous blocks up to 10 m wide are distributed around the crater indicating a previous explosive eruption lava flows erupted from the central cone have a maximum thickness of 46 m at the flow margins and a maximum flow length of 1 9 km the total flow area is 1 5 km2 and the estimated volume is 0 03 km3 the relatively thick lava flows up to 46 m with steep sided margins 30 40 and curtain folded surface textures suggest that silicic magmas erupted and their fresh appearance indicates that lava flow eruptions the most recent volcanic activity occurred during the quaternary the volcanic features at daisan miyako knoll are aligned mainly in the nw se direction suggesting that volcanism here may be controlled primarily by arc parallel extension no volcanic edifices have been confirmed on the trench side of daisan miyako knoll therefore this edifice defines the location of the arc volcanic front in this region from the larger rhyolitic edifices 10 km wide around daisan miyako knoll along the front it is inferred that silicic arc volcanism is extending from the central to southern submarine ryukyu arc

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

postglacial recession of niagara falls in relation to the great lakes abstract the recessional history of niagara falls in the present niagara gorge during the postglacial period has been a focus of study throughout this century radiocarbon ages of clam shells suggest that niagara falls migrated very slowly around the narrowed gorge section at niagara glen from 10 500 to 5500 yr b p when upper great lakes water bypassed lake erie and flowed to the ottawa river via the outlet at north bay ontario prior to that interval river discharge and recession rates were similar to those at present and similar rates resumed after 5200 yr b p by about 4500 yr b p the present gorge had intersected a buried gorge of the pre late wisconsinan niagara river st davids gorge the sediment derived from the excavated buried valley fill may be present as a distinct marker horizon in the sediments in southwestern lake ontario

hydrothermal plumes imaged by high resolution side scan sonar on a cruising auv urashima an autonomous underwater vehicle auv is an ideal platform that enables a closer approach to the seafloor than other platforms the auv provides remarkable advancement not only in imaging small scale bathymetry with high resolution but also in imaging the spatial distribution of scattering signals within a water column acoustic echoes can be recorded continuously by auvs potentially providing information related to hydrothermal plumes within a water column in may 2007 such an auv urashima of the japan agency for marine earth science and technology captured fantastic acoustic images within the water column above an active hydrothermal field the iheya north field off okinawa numerous filamentous and hyperbolic echoes were recorded on the side scan sonar images according to the temperature and geochemical anomaly the scattering signals within the water column were hydrothermal plumes upwelling from the vent area this proof of concept has implications for long range exploration for vent field with auvs

geochemistry of the quaternary volcanic rocks of the northeast japan arc major and trace element data are presented for a series of lavas from 17 volcanic centres in the ne japan arc these represent a transect of the quaternary arc from its volcanic front type a volcanoes in the east across its central zone type b to its western margin type c rocks range from basalt to dacite in composition and the variation is attributed to fractionation of plagioclase mafic silicates and titanomagnetite for comparative purposes trace element data for each volcano are presented on a 55 wt sio2 normalised basis the volcanoes display the characteristic features of arc volcanism viz enrichment in large ion lithophile elements rb sr ba k and depletion in high field strength elements zr nb hf ta using these data and derived primary magma compositions tatsumi et al 1983 mixing calculations give the percentages of fractionating mineral phases necessary to derive the 55 wt sio2 liquids these data together with published distribution coefficients allow the calculation of trace element abundances in the primary magmas reciprocal trace element plots indicate that irrespective of position with respect to the volcanic front the elements la ba k sr nb y and zr of the primary magmas all lie on a common line strongly suggesting that the melts are the products of varying degrees of melting of a common homogeneous source however those melts close to the volcanic front type a show anomalous rb th and pb values suggesting that their mantle source was enriched in these elements further calculations suggest that the hfs depletion is not due to retention by residual mineral phases but is a feature of the source thus the data suggest a that the source of the arc volcanism was homogeneous and b that the characteristic chemical features were an inherent feature of the source since zr nb ratios in the source melts are close to those found in n type morb it is suggested that a major component of arc volcanic source rocks was a depleted mantle of the type which has given melts of n type morb composition lil elements added to such a depleted source are derived by the loss of silica rich aqueous fluids from the descending subducting slab such losses occur within the first 100 km and are fixed in the overlying mantle wedge this contaminated material is transported down by drag induced convection producing a homogeneous but metasomatised mantle within the melting zone diapiric uprise produced by small scale melting produces further melting with the major control of melt chemistry being the degree of partial melting superimposed on this chemistry is a localised addition of pb rb and th in the source of those volcanoes nearest the volcanic front

the gondou hydrothermal field in the ryukyu arc a huge hydrothermal system on the flank of a caldera volcano high resolution geophysical mapping was conducted from an autonomous underwater vehicle on the flank of daisan kume knoll in the ryukyu arc southwest of japan 1 m resolution bathymetry identified 264 spires 173 large mounds and 268 small mounds within a depression that is up to 1600 m wide and up to 60 m deep at water depths between 1330 and 1470 m hydrothermal venting is strongly inferred from the observation of plumes in sidescan sonar imagery and positive temperature anomalies over the spires and mounds this field named the gondou field has a giant mound g1 with a diameter of 280 m and a height of 80 m mound g1 has distinctive summit ridges comprised of multiple spires where acoustic plumes with temperature anomalies up to 1 12 c are observed indicative of high temperature venting other than mound g1 a number of active large mounds more than 30 m wide and spires over 10 22 m tall are common and they concentrate in the central and southern areas of the field suggesting that these areas are the center of present hydrothermal activity acoustic plumes imaged by side scan sonar at the gondou field are different in character from bubble plumes imaged in other hydrothermal fields in the ryukyu arc the plumes are diffused and deflected as they rise through the water column and have a shape consistent with black smokers

detailed morphology and bubble plumes of daiichi amami knoll in the central ryukyu arc abstract high resolution geophysical mapping operations were conducted from a survey vessel and autonomous underwater vehicle auv in the daiichi amami knoll ryukyu island arc for the first time both the detailed morphology and the extent of the bubble plume in this region are revealed the knoll is a conical edifice rising 500 700 m from the surrounding seafloor with a peak depth at 247 m the summit region of the knoll has a complex structure and is divided into three parts the northeastern rim central caldera and southern cones among the three parts the southern cones has characteristic morphological features such as cones and a depression two cones cone a and cone b rise 30 m above the surrounding basement and have fissure like linear features the depression depression d1 faces the steep slope of cone a and is bounded by cone a and b the horizontal dimensions are 500 m 200 m and it is 10 40 m lower than the surrounding basement water column anomalies detected on side scan sonar images which are thought to be bubble plumes were intensively distributed in and around depression d1 and cone b with horizontal dimensions of 300 m 800 m the volcanic morphology and bubble plumes indicators of hydrothermal activity suggest that the daiichi amami knoll is an active volcanic knoll this result proposes a new insight that dual volcanic chains exist in the north to central ryukyu arc the daiichi amami knoll is located behind the volcanic front not on the front the knoll surrounding volcanoes and volcanic edifice behind the front are aligned with one straight line 20 km behind and parallel to the front making a rear arc volcanic chain the presence of dual volcanic chains in the north to central ryukyu arc revealed by our detailed geophysical survey provides valuable information to consider toward the subduction processes and magma generation in the ryukyu arc

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

tectonic volcanic and hydrothermal features of a nascent rift graben in the southern okinawa trough abstract high resolution geophysical mapping using an autonomous underwater vehicle auv and surface vessel revealed tectonic volcanic and hydrothermal features of a nascent rift graben in the southern okinawa trough ship based bathymetry identified ene wsw trending lineaments at a water depth of 1900 2000 m that form a rift graben the ishigaki rift which is 50 km long 7 km wide and has relief of 100 m shipboard em122 multibeam sonar detected acoustic bubble plumes and high backscatter intensity on the seafloor along the rift the auv high resolution bathymetry 2 m and sidescan imagery revealed the presence of a ropey texture with high acoustic reflectivity interpreted to be relatively young lava flows evidence of hydrothermal venting was observed at two previously unknown sites along the rift steep sided spires up to 23 m in height and a mound discharging smoke like acoustic plumes were identified the auv temperature sensor observed sharp temperature increases of up to 0 13 c around the plumes even at the vehicle s altitude of 100 m above the seafloor the presence of relatively young lava flows and indicators of high temperature hydrothermal venting and gas bubbles suggest the ishigaki rift is magmatically active compared with adjacent active back arc rifts the ishigaki rift has a shorter length narrower width and lower relief and occurs at a shallower water depth suggesting that the rift is relatively young and immature considering that the ishigaki rift is located near the arc volcanic front and that other mature rifts are typically located farther from the front it is suggested that a new axial rift system is evolving within the okinawa trough and that back arc rifting processes may be gradually migrating arcward in the trough during the quaternary

amphibole plagioclase equilibria an empirical model for the relation albite tremolite edenite 4 quartz equilibria between plagioclase calcic amphibole and quartz can be described in part by the relation among mineral components naalsi3o8 ca2mg5si8o22 oh 2 naca2mg5alsi7o22 oh 2 4sio2 this relation governs the partitioning of na between plagioclase and the a site of coexisting amphibole data from natural amphibolites reveal that this partitioning is systematic and sensitive to metamorphic grade the ideal portion of the equilibrium constant kid xna a x a xab derived from natural samples is sensitive to bulk composition inasmuch as both plagioclase and amphibole are highly non ideal samples from a single outcrop have values ranging from 0 5 xab 0 74 to 4 1 xab 0 10 the continuous reaction naalsi3o8 ca2mg5si8o22 oh 2 naca2mg5alsi7o22 oh 2 4sio2 proceeds to the right with increasing grade of metamorphism and for a given bulk composition kid increases with increasing temperature two related discontinuous reactions actinolite albite hornblende oligoclase quartz and actinolite oligoclase hornblende anorthite quartz also proceed to the right with increasing metamorphic grade and result in changes in the topology of a phase diagram that describes the partitioning of na between plagioclase and amphibole a site a schreinemakers net is presented that is consistent with natural occurrences the results of this study should aid in the delineation of metamorphic facies within amphibolites

cadomian subduction collision and variscan transpression in the badajoz cordoba shear belt southwest spain abstract the tectonothermal evolution of the badajoz cordoba shear belt southwest iberian massif spain involved the build up of a cadomian upper proterozoic lower paleozoic subduction collision orogenic wedge complex made up of five stacked tectonic slices and its deformation under transpression during hercynian late paleozoic intracontinental shearing episodes cadomian metamorphism in the high grade gneissic units of the wedge complex is characterized by peak metamorphic conditions of 720 c 8 kbar for the lowermost tectonic slice higuera de llerena mylonite gneiss and 670 c 10 kbar in the overlying gneissic slice blastomylonitic migmatic gneisses the p t conditions recorded by eclogites structurally overlying the previous gneisses enhance a scheme of inverted metamorphism since these eclogites equilibrated at 685 700 c 15 kbar during peak metamorphic conditions after the high pressure event the highest temperatures attained by the eclogites correspond to a period of nearly isothermal decompression with a drastic decrease in pressure later on the retrogressed eclogites experienced a pressure increase before they began to reequilibrate under progressively lower pandt conditions similar to those recorded by the retrogression of gneisses such a p increase in eclogites might be related to a period of underthrusting below continental crust responsible for the amalgamation of the different tectonic units of the badajoz cordoba shear belt the present results unravel that the models concerned with the geotectonic evolution of the ibero armorican arc and the respective correlations among hercynian and pre hercynian cadomian for our aims terranes across the north atlantic must be reconsidered

the iapetus model a plate tectonic concept for the variscan belt of europe abstract a plate tectonic model is presented explaining the evolution of the variscan belt of europe it regards the iapetus ocean and its branch the tornquist sea as the only ocean involved in the caledonian variscan and alleghenian mega orogen the iapetus and its marginal basins disappeared through oblique subduction and collision in the following sequence 1 1 northeastern iapetus including the tornquist sea caledonian orogeny 2 2 central iapetus acadian orogeny 3 3 adjacent marginal basins variscan orogeny and 4 4 southwestern iapetus alleghenian orogeny the area of the variscides was part of the continental margin of gondwana made up of ridges and ensialic basins no ocean lay within the variscan area during the palaeozoic although patches of oceanic crust probably existed in rift basins e g the lizard ophiolite after the silurian early devonian closure of northwestern and central iapetus the variscan area was surrounded by cratons on three sides in a manner similar to that around the modern mediterranean sea the subsequent variscan orogeny was essentially an ensialic thrusting and folding induced by a further approach of gondwana toward laurentia after the variscan collision gondwana rotated clockwise to the west by moving along a strike slip zone this rotation during the carboniferous opened the palaeo tethys in the east and caused the alleghenian collision in the west the middle ordovician to permian palaeogeography is presented on a series of maps modelling tectonic displacement and plate motions avalonia is interpreted as the ordovician volcanic island arc of gondwana separated from mainland gondwana by an oceanic gulf or a back arc basin the iberian arc may have originated by a continental escape of iberia from the center of collision into the oceanic gulf the variscan austro alpine and southern alpine units are today located near their earliest early carboniferous position

stress field along the ryukyu arc and the okinawa trough inferred from moment tensors of shallow earthquakes abstract we investigated the stress field along the ryukyu arc and the okinawa trough using regional nied mt and global harvard cmt moment tensor catalogs using the combined dataset of 4 years of data from the nied mt catalog and 24 years of data from the harvard cmt catalog we found an arc parallel extensional stress province along the forearc of the ryukyu arc and observed a detailed extensional stress field in the okinawa trough this arc parallel extension is observed in the entire region of the ryukyu arc except at its northeastern end this stress field is clearly separated from the backarc extensional stress field in the okinawa trough by the volcanic chain since it exists in a normal subduction region as well as in a region of weakly coupled subduction the effects of oblique subduction might not be enough to explain the formation of the arc parallel extension thus we consider that the backarc opening process should play an important role in this arc parallel extension along the okinawa trough an arc perpendicular extensional stress field is observed in the southeastern and central ryukyu arc while in the northeastern okinawa trough the direction of the extensional axis is oblique to both the arc normal direction and the direction of plate motion two dimensional mechanical interaction models which consider the interactions between the subducting plate forearc and backarc are inadequate because an arc parallel extensional stress field is observed in the entire ryukyu arc and the extension axis in the northeastern okinawa trough is oblique to the arc trend the oblique backarc stress may originate beneath the backarc or on the continental side of the backarc one of the simplest ways to explain this observation might be active rifting in the northeastern okinawa trough

large vertebrate burrow from the upper mississippian mauch chunk formation eastern pennsylvania usa abstract a burrow of probable amphibian origin was discovered in the upper member of the mississippian mauch chunk formation in eastern pennsylvania facies analysis of the upper member indicates that deposition occurred in an ephemeral braided stream setting the burrow is housed in a mudstone and is filled by two graded beds of conglomerate to sandstone it is characterized by a flared opening leading into a narrower slightly helical tunnel that ends in an inflated ovate chamber approximately 51 to 60 cm in diameter with a maximum height of 20 cm the flared opening is stratigraphically higher than the elevation at the base of the chamber the tunnel has a semicircular base the geometries abrupt angle changes inflated termination width to height ratios of the terminal chamber and graded fill indicate that the structure was an open void prior to sedimentation the geometry and size of this structure are incompatible with known invertebrate burrows and erosional features palaeosauropus primaevus an amphibian footprint ichnotaxon recovered from the mauch chunk formation was made by an amphibian of sufficient size to create such a large burrow this type of burrowing was most likely a response to seasonal droughts as local water sources evaporated

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

multiscale structural and lithologic controls in the development of stream potholes on granite bedrock rivers abstract jointing veins dikes and fracture patterns influence the genesis of potholes in bedrock rivers we use measures of surface rock strength schmidt hammer readings joint orientation and spacing and subsurface rock properties ultrasound velocity to analyze the spatial relationship between joints and potholes in three rivers of the spanish central system the tietar manzanares and alberche rivers at each site we measured the dimensions of between 45 and 77 potholes at least 50 randomly located schmidt hammer rebound values as well as schmidt hammer values in a 15 15 cm grid around each pothole and ultrasonic p wave velocities measured in a 15 15 cm grid around a subset of 12 of the potholes results support our hypotheses that most potholes correlate with joints 89 of potholes are related to joint sets potholes exhibit preferred orientations associated with dominant joints and pothole genesis correlates more strongly with variations in substrate resistance than with hydraulics we classify potholes by morphology and present a genetic sequence for specific morphologies in relation to substrate characteristics

strontium isotope composition in volcanic rocks from the northeast japan arc abstract in the northeast japan arc a number of quaternary volcanoes form a long narrow belt parallel to the japan trench 87 sr 86 sr ratios were determined in 52 specimens of volcanic rocks from 27 volcanoes in the northeast japan arc area the results reveal that the ratios change systematically in space decreasing 87 sr 86 sr ratios across the arc were confirmed over a wide area of northeast japan in the same direction increases in both rb and sr contents were also found the regular trends are considered to be a strong constraint for elucidation of subduction originated magma genesis at the eurasia plate vs pacific plate boundary in the northern region of the northeast japan arc 87 sr 86 sr ratios in volcanic rocks along the volcanic front were almost constant 0 7038 0 7045 and slightly higher than those from the izu ogasawara arc 0 7032 0 7038 this suggests that interactions between the eurasia plate and the pacific plate and those between the philippine sea plate and the pacific plate are slightly different the southern region of the northeast japan arc where the direction of the volcanic front bends from southward to westward showed anomalously high 87 sr 86 sr ratios reaching to 0 7077 this region coincides with the triple junction of the eurasia pacific and philippine sea plates suggesting anomalous interaction at the triple junction

detailed volcanic and tectonic morphology of nakadomari hill in the southern okinawa trough abstract nakadomari hill is an unexplored edifice in the southern okinawa trough back arc basin where crustal extension by continental rifting occurs and few submarine eruptions associated with back arc volcanism have been mapped in detail high resolution bathymetric mapping with an autonomous underwater vehicle auv reveals that nakadomari hill is a submarine volcanic complex with recent lava flow eruptions the high resolution 2 m bathymetry shows striking contrast between rugged terrain in the central area and the adjacent smooth terrain the cone shaped features in the rugged terrain are covered by ropey fabric with high acoustic reflectivity and are devoid of tectonic deformation thus they are interpreted to be relatively younger lava domes the cone shaped features in the smooth terrain show low reflectivity and are dissected by ene wsw trending tectonic lineaments suggesting they were formed during older volcanism nakadomari hill developed through multi stage eruptions before and after back arc rifting and faulting the ropey and curtain folded morphology with high acoustic reflectivity are interpreted to be a field of lava flows and they are relatively young the area considered to be lava flows is estimated to be over 13 km2 showing that eruptions in the southern okinawa trough have been extensive an active volcanic system is supported by indicators of high temperature hydrothermal venting hydrothermal plumes with concurrent sharp temperature increases of up to 0 17 c were present even at the vehicle altitude of 100 m above the bottom the alignment of volcanic cone shaped features at nakadomari hill is in the nnw sse direction which is clearly different from the orientation of local rifting ene wsw at a larger scale the volcanic edifices west of kume island from the arc to back arc are also aligned roughly in the nnw sse direction and they are sub parallel to a major tectonic structure in the arc the kerama gap the agreement of the local volcanic and tectonic trends implies that magmatic focusing may have taken place along the tectonic structures of the kerama gap and in turn caused the extensive back arc volcanism in this area

geology and tectonics of the ryukyu islands abstract the ryukyu islands are divided morphologically and geologically into three island groups north central and south ryukyus north central ryukyu represents the geological continuation of the outer belt of southwest japan composed of mesozoic eocene sedimentary sequences whereas south ryukyu is characterized by high pressure metamorphic rocks eocene volcanics and limestone and lower miocene sediments the geological and structural contrasts between north central and south ryukyus are conspicuous before the late miocene transgression covered the whole area the ryukyu islands have been established since then the successive developments of the goto tunghai senkaku basins since the paleogene of the okinawa trough since the miocene and of the grabens near the island group since the pliocene signify a southeastward shift of the basins in relation to the activity in the granite diapir zones accompanied by volcanism

variscan oceans ages origins and geodynamic implications inferred from geochemical and radiometric data abstract a review of the magmatic associations providing evidence for the existence of extensional regimes during the palaeozoic evolution of variscan europe is presented the available geochronological data allow recognition of two major rifting events one in the early ordovician and the other in the middle late devonian scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage it is suggested that the variscan belt resulted from a complex two stage collision process through successive suturing in the silurian and the early carboniferous of these oceanic domains the superposition of two distinct opening closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the variscan orogen

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

deep structure of the northeastern japan arc and its implications for crustal deformation and shallow seismic activity abstract seismic tomography studies in the northeastern japan arc have revealed the existence of an inclined sheet like seismic low velocity and high attenuation zone in the mantle wedge at depths shallower than about 150 km this sheet like low velocity high attenuation zone is oriented sub parallel to the subducted slab and is considered to correspond to the upwelling flow portion of the subduction induced convection the low velocity high attenuation zone reaches the moho immediately beneath the volcanic front or the ou backbone range running through the middle of the arc nearly parallel to the trench axis which suggests that the volcanic front is formed by this hot upwelling flow aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the backbone range where they are expelled from solidified magma and migrate further upward the existence of aqueous fluids may weaken the surrounding crustal rocks resulting in local contractive deformation and uplift along the backbone range under the compressional stress field of the volcanic arc a strain rate distribution map generated from gps data reveals a notable concentration of east west contraction along the backbone range consistent with this interpretation shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone based on these observations a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern japan arc

possible hydraulic significance of two kinds of potholes examples from the paleo potomac river potholes preserved along abandoned sections of bedrock bounded rivers should provide significant paleohydraulic data if the conditions for pothole formation are understood in a prototype study we recognize two kinds of potholes along the paleo potomac river near great falls virginia vertical potholes drilled into the strath floor are circular in plan and are interpreted to form at sites of small stream depths and high flow velocity lateral potholes noncircular with overhanging roofs are found on flanks of rock obstacles they are interpreted to be eroded by sediment laden vertical flow near the air water interface the vortices circulate water in the horizontal plane and begin to form near the free surface in flow separation zones downstream of the obstacles the geometry of flow separation zones and thus the location of the lateral potholes are scaled by flow reynolds number therefore lateral potholes could be used to reconstruct paleoflow depths and velocities if parts of the channel bed adjacent to the flow obstruction are preserved

a rigid block rotation model for the gps derived velocity field along the ryukyu arc abstract using a gps derived velocity field we try to estimate the kinematic features of the ryukyu arc and discuss its implications to the spreading of the okinawa trough the gps derived strain rate field implies that internal deformation within the ryukyu arc is not significant therefore we adopt a rigid block rotation model for the motion of the ryukyu arc the velocity field in the ryukyu arc is best explained by a model with three blocks rather than one with only one block the estimated block rotations predict the spreading along the okinawa trough the predicted spreading is slow 10 mm per year in the north and fast 50 mm per year in the southwest the direction of the predicted spreading is normal to the okinawa trough n170 e n180 e in the southwest while in the north the direction of the spreading is oblique to the okinawa trough n150 e n180 e