tectonic erosion and accretion in steady state trenches abstract the geometry of subduction in a steady state trench is analysed to derive simple equations linking the rate of plate convergence across the trench with the outer slope and trough axial sedimentation rates the dips of the outer slope and the slope trough sequence boundary and the angle of decollement below the inner slope it is concluded that tectonic accretion will generally occur where the decollement is steeper than the outer trench slope and conversely that tectonic erosion will occur if the decollement is shallower than the dip of the outer slope the analysis is applied to the andean trench to explain tectonic erosion in the arica bight and to the timor trough to demonstrate that subduction has ceased

evidence of early cretaceous collisional style orogenesis in northern fiordland new zealand and its effects on the evolution of the lower crust structural metamorphic and kinematic data from a well exposed section of lower crustal rocks in northern fiordland new zealand reveal a history of intense contractional deformation and high p metamorphism at the roots of a convergent orogen high p 14 kbars granulite facies garnet clinopyroxene bearing reaction zones occur adjacent to anorthositic veins within gabbroic and dioritic gneiss these veins and reaction zones were variably deformed by two phases of high p granulite facies deformation quantitative kinematic analyses conducted using systems of rotated veins and reaction zones indicate that the first phase produced steeply dipping shear zones within a sinistral pure shear dominated flow regime wk 0 69 this deformation occurred at conditions of p 14 0 1 3 kbars and t 676 34 c and resulted in subhorizontal arc parallel ne sw stretching and up to 60 subhorizontal shortening in high strain zones of the lower crust at depths 45 km the second phase of deformation occurred at p 14 1 1 2 kbars and t 674 36 c and produced vertically stacked gently dipping ductile thrust faults that accommodated arc normal nw directed displacement these features reflect major tectonic thickening of the crust oblique convergence and high p metamorphism during the collision of the roots of a convergent orogen represented by plutons of the median tectonic zone in eastern fiordland with the paleo pacific margin of gondwana represented by western fiordland distinctive kinematic styles suggest that this collision resulted in a partitioning of the arc parallel ne sw and arc normal nw se components of oblique convergence onto sinistral strike slip and ductile thrust faults respectively at lower crustal levels

numerical simulation of floodplain hydrology despite the extensive research into hillslope and channel interactions in headwater catchments surprisingly little attention has been paid to such processes in lowland rivers in particular previous studies have not addressed the influence of hillslope contributions and have concentrated solely on in bank floods rather than more complex out of bank cases accordingly we combine field monitoring and numerical modeling to study hillslope floodplain and channel interactions for a lowland river piezometric precipitation and river stage data were used to parameterize and test a new two dimensional finite element model of saturated unsaturated flow applied to two vertically aligned cross sections through a lowland floodplain data for two major out of bank flood events were simulated which appeared to show the presence of a significant unsaturated zone extending up to 5 m below the surface the model simulated reasonably well the pressure head field that was recorded at a number of piezometers located internal to the computational domain on each transect and we conclude that floodplain hydrology is predominately a two dimensional lateral process three dimensional down reach flow effects would seem to become more significant at the beginning and end of each event the simulations also showed that the unsaturated zone remained close to saturation at all times and that it was not significant in terms of the floodplain hydrology examination of velocity vector patterns showed the formation of a strong groundwater ridge within the floodplain this led to the development of strong velocities directed toward hillslope areas as the inundation front approached the hillslope floodplain junction this suggests that surface water may move into hillslope areas adjoining the floodplain during major floods thus the extent of the hyporheic zone may be larger than previously thought

accelerated hydration of the earth s deep crust induced by stress perturbations the metamorphic cycle associated with the formation of mountain belts produces a lower crust containing little or no free fluid1 2 the introduction of external fluids to dry and impermeable volumes of the earth s crust is thus a prerequisite for the retrogressive metamorphism later observed in such regimes such metamorphism can cause significant changes in the crust s physical properties including its density rheology and elastic properties3 4 on a large scale the introduction of fluids requires the presence of high permeability channels such as faults or fractures which are the result of external tectonic stresses but extensive interaction between externally derived fluids and the fractured rock requires efficient mass transport away from the initial fractures into the rock itself and this transport often occurs over distances much longer than expected from grain boundary diffusion here we present both field observations and a simple network model that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reactions with fluids we also show that the morphology of reaction fronts separating dry from wet rocks depends on the anisotropy of the external stress field

quaternary sedimentation in the japan arc trench system quaternary sedimentation around the japanese islands took place in a dynamic morphotectonic setting that included seismically active trenches a large back arc basin and a tectonically complex slope environment characterized by en echelon folds and fault bounded ridges with small intervening basins these structural ridges created barriers to sediment transport by bottom currents and acted as dams on the slope behind which quaternary sediments ponded sea levels higher than at present throughout most of the pleistocene impeded escape of coarse sediments from the shelf except through a few major submarine canyons that extended completely across the shelf and slope thus deeper water sediments in both the japan fore arc and back arc are dominantly hemipelagic muds and subordinate mud turbidites a drop in sea level to about 130 m below the present level at approximately 18 000 yr b p resulted in deposition of widespread fluviomarine sands and gravels on the continental shelf and an increase in sand silt turbidites in the latest pleistocene record in both the fore arc and the back arc sandy turbidites interbedded with hemipelagic mud or mud turbidites occur particularly in the trench fill of nankai trough southwest japan trench in some fore arc basins on the south west japan slope and in the western part of the japan sea back arc basin they are sparsely represented in the japan trench and the northeast japan slope apparently owing to the poor development of throughgoing submarine canyons on the northeast slope diatomaceous and calcareous muds and oozes occur in some parts of both the fore arc and the back arc and volcanic ash and pumice are virtually ubiquitous throughout the quaternary sediment record in nearly all parts of the japan arc trench system quaternary sedimentation rates are less than about 30 m m y in the open pacific deep sea basins seaward of the trenches but they increase toward the fore arc and reach values as high as 230 m m y in the japan trench inner slope and 300 900 m m y in the trench fill of nankai trough back arc sedimentation rates for the total quaternary range from 20 140 m m y and latest pleistocene holocene rates range from 65 230 mm 1 000 yr

provenance and petrology of deep sea drilling project sands and sandstones from the japan and mariana forearc and backarc regions abstract one hundred and twenty point counts of oligocene to recent sands and sandstones from dsdp sites in the japan and mariana intraoceanic forearc and backarc basins demonstrate that there is a clear compositional difference between the continentally influenced japan forearc and backarc sediments and the totally oceanic mariana forearc and backarc sediments japan forearc sediments average 10 qfl q 0 82 p f 2 framework mica 74 lmlvlst lv and 19 lmlvlst lst in contrast the mariana forearc and backarc sediments average 0 qfl q 1 00 p f 0 framework mica 98 lmlvlst lv and 1 lmlvlst lst sediment compositions in the japan region are variable the honshu forearc sediments average 5 qfl q 0 94 p f 1 framework mica 82 lmlvlst lv and 15 lmlvlst lst the yamato basin sediments dsdp site 299 average 13 qfl q 0 70 p f 3 framework mica 78 lmlvlst lv and 14 lmlvlst lst the japan basin sediments dsdp site 301 average 24 qfl q 0 54 p f 9 framework mica 58 lmlvlst lv and 21 lmlvlst lst p f and framework mica are higher in the yamato basin sediments than in the forearc sediments due to an increase in modal potassium content of volcanic rocks from east to west on the island of honshu site 301 possesses a higher qfl q and lmlvlst lst and lower lmlvlst lv than site 299 because it receives sediment from the asian mainland as well as the island of honshu dsdp site 293 sediments in the mariana region average 0 97 p f 1 framework mica 13 lmlvlst lm and 83 lmlvlst lv due to their proximity to the island of luzon the remaining mariana forearc and backarc sediments show a uniform composition

runaway slip to the trench due to rupture of highly pressurized megathrust beneath the middle trench slope the tsunamigenesis of the 2011 tohoku earthquake off the east coast of northern japan abstract the gigantic 2011 march 11 mw 9 tohoku earthquake is examined from the viewpoint of the pre seismic forearc structure the seismic reflection properties of a megathrust around the usual up dip limit of the seismogenic zone the thermal state of a shallow subduction zone and the dehydration of underthrust sediments at the japan trench the pacific plate is subducting westward beneath northeast japan at a dip angle of 4 6 the middle and lower slopes of the landward side dip eastward at angles of 2 5 and 8 0 respectively the forearc prism beneath the middle and lower slopes is inferred to be in extensionally and compressively critical states respectively based on the presence of clear internal deformation features and on the occurrence of aftershock earthquakes the rapid uplift of the forearc that caused the 2011 tohoku tsunami may have been associated with this internal deformation of the prism the critical state of the prism indicates that the effective basal friction b of the plate boundary megathrust is 0 08 for the lower prism the megathrust especially under the middle slope is characterized by a prominent reflector with negative polarity one of multiple possible explanations is that the megathrust hosts highly pressurized fluids underthrust sediments in this part of the japan trench are dominated by pelagic and siliceous vitric diatomaceous silt with clay the dehydration kinetics of opal a to quartz the clay transformation of smectite illite and the thermal structure of the japan trench suggest that maximum dehydration of the sediments would take place at 50 60 km horizontally from the deformation front where the temperature along the megathrust is 100 120 c the zone of maximum dehydration coincides with the prominent seismic reflector that has negative polarity we hypothesize a possible free slip along this portion of the megathrust during the 2011 tohoku earthquake caused by anomalously high fluid pressure resulting from fluid accumulation over centuries

arc continent collisions sediment recycling and the maintenance of the continental crust author posting geological society of london 2009 this is the author s version of the work it is posted here by permission of geological society of london for personal use not for redistribution the definitive version was published in geological society london special publications 318 2009 75 103 doi 10 1144 sp318 3

crustal structure and seismicity beneath the forearc off northeastern japan marine seismological and other data in the japan trench area north of 38 5 n were used to infer the relation of subducting plate structure seismicity and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates previous crustal models from ocean bottom seismographic refraction obs surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data a wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting pacific plate crust its dip angle increases to about 7 110 km landward of the trench axis a large increase beneath the deep sea terrace is required to reach the well defined angle of 25 beneath the tohoku east coast a large wave speed gradient within layer 2 commonly observed under normal oceans seems to vanish beneath the inner trench slope at about 10 km depth within the overriding plate apparently brittle material with a p wave speed of 6 km s can be found as near as 35 km from the trench axis the upper limit of the seismogenic zone of interplate low angle thrust events is about 15 km depth from obs seismicity and large event analyses both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates

evolution of clastic piles in an arc arc collision zone late cenozoic depositional history around the tanzawa mountains central honshu japan abstract late cenozoic clastic piles around the tanzawa mountains represent trough trench landward and seaward basin fills developed in an arc arc collision zone in central honshu japan the landward basin fills are clastic wedges prograded from the northern orogenic highlands and are characterized by fan deltaic successions succeeding submarine volcano submarine fan slope successions on basin plain deposits the landward basin fills were generally developed intermittently from north to south since the middle miocene evolution of the landward basin fills depended on the interplay of the northward buoyant subduction and suturing of the izu bonin arc in central honshu honshu arc together with submarine magmatism on the landward basin slope emplaced by the subducting pacific plate beneath the izu bonin arc to the west the seaward basin fills however generally lack terrigenous detritus derived from the northern highlands except for a trough fill which received some detritus shed from the rising highs just to the north and are characterized by pyroclastic flow deposits prograded from the southern submarine volcanoes or a volcanic island on the ocean basin the northward progradation of the pyroclastic flow deposits may have been emplaced by the northward drifting of the ocean basin the tanzawa mountains on the other hand consist mainly of early to middle miocene subaqueous volcaniclastic rocks intercalated with some volcanic conglomerates and shallow marine coquinas and can be interpreted as a miocene volcanic island punctuated initiation and propagation of the late cenozoic basin fills diachronously from the north to the south of the tanzawa mountains can be explained by northward drifting and collision of the tanzawa block a paleo volcanic island with central honshu during middle to late miocene and the consequent jump of the subduction zone to the south of it since late miocene preceding the collision of the izu peninsula izu block with the south of the youngest basin fills developed in the southern front of the tanzawa mountains since the middle pleistocene after collision of the izu block with the southernmost basin fills the late cenozoic clastics in and around the tanzawa mountains were uplifted and deformed into northward imbricated structures by rapid slip on the faults framing this area

petrographic classification of sand and sandstone abstract petrographic classifications of sand and sandstone proposed more than half a century ago are still in use although they were formulated at a time when depositional and post depositional sedimentary processes were poorly understood and before the relationships between tectonics and sedimentation could be interpreted in modern plate tectonic terms as a consequence too many scientific articles and technical reports are still encumbered with obsolete concepts graphical tools and ambiguous terminology that make sediment descriptions awkward and misleading a renovation that treasures the legacy of the pioneers is required the descriptive petrographic classification of sand and sandstone proposed in this paper is based on the quasi universally used gazzi dickinson point counting method and simply translates into words ternary compositions of quartz feldspar and lithic fragments without introducing any new names the classic qfl plot is subdivided into 15 fields labelled by adjectives introduced long ago by k a w crook and endorsed by w r dickinson and more recently by g j weltje which reflect relative abundances of the three main framework components provided they exceed 10 qfl according to standard use the less abundant component goes first the more abundant last e g litho feldspatho quartzose composition translates into q f l 10 qfl for lithic rich sand and sandstone information on the prevailing rock fragment type can be added by an additional free adjective e g metamorphiclastic carbonaticlastic as proposed long ago by r v ingersoll for lithic poor feldspatho quartzose and quartzose sand and sandstone further formal subdivisions are proposed based on the q f ratio thus reaching a total of 18 compositional fields overall modern sand known to be derived from different source rocks and found in major world s rivers deserts and deep sea fans fits in the pigeonholes defined by the relative abundance of quartz feldspar and lithic fragments the aim of this classification is to restore directness in sandstone petrology and to avoid ambiguities generated in the past by making reference to badly defined archetypes such as greywacke or arkose thus confusing petrographic composition with subjective considerations about plate tectonic setting texture hydraulic behaviour mechanical durability or chemical durability in the illusion that a classification could be genetic at the same time as descriptive

processes controlling vertical coupling and decoupling between the upper and lower crust of orogens results from fiordland new zealand the pre cenozoic configuration of western new zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a cretaceous orogen from upper to lower crustal levels 10 50 km beginning at 126 ma a composite batholith dominated by diorite was emplaced into the lower crust during emplacement deformation was partitioned into zones weakened by magma and heat leading to the development of two layer parallel shear zones at the upper and lower contacts of the batholith transient vertical decoupling of the crust above and below the batholith occurred from 126 ma until 120 ma as magma was emplaced into and moved through a weak thick lower crust by 116 ma however much of the batholith had crystallized and the lowermost crust had cooled from 750 c t 850 c to t 650 700 c cooling was aided by the juxtaposition of pre existing crust against hot new crust and by the efficient extraction of partial melts out of the lower crust cooling together with dehydration of the lower crust and mafic compositions led to the development of a strong dry lower crustal root by 116 ma a strong lower crust resulted in high degrees of vertical coupling between the upper and lower crust during contraction from 116 to 105 ma even as magma continued to be emplaced into the mid upper crust a narrow focused orogenic style in the upper crust at this time reflected a highly viscous lower crust through which compressional stresses were transferred vertically the results imply that changes in plate boundary dynamics rather than the thermal weakening of thick lower crust during convergence controlled the onset of regional extension at 108 105 ma

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

continental and marine influences expressed by deep sea sedimentation off japan kaiko project abstract a total of about 100 sediments and sedimentary rocks composed of various clays to muddy silstones of late cenozoic age and limestones and sandstones of early cretaceous age were sampled by the submarine nautile during the kaiko project at 3000 to 6000 m water depths in the trench areas south and east of japan a total of 70 samples were selected and studied mainly by optical and electronic microscopy x ray diffraction atomic absorption spectrometry and microprobe analysis the dominant terrigenous influences allow us to identify two main mineralogical provinces to recognize the sedimentary expression of an early pleistocene tectonic event southeast of japan and to propose an indirect stratigraphic use of clay assemblages in poorly fossiliferous deep sea sediments the cretaceous sedimentary environments in kashima seamount area were submitted to various terrigenous and volcanogenic influences with possible early to late diagenetic modifications only very minor post sedimentary changes oxidations metallic coating barite and zeolite formation opal dissolution affect most of the late cenozoic deposits cropping out on the sea floor included those charaterized by the development of deep sea benthic communities e g clams important early diagenetic changes occur only in volcanic rich sediments of south zenisu ridge and south kuril trench in those peculiar deposits very abundant smectites develop with various shapes and chemical compositions pointing to the local peculiarities of volcanic glass morphology chemistry and alteration of permeability and of fluid migration processes

the possible role of fluids for the structuring of the continental crust abstract some well known seismic reflectivity patterns of continental crusts characterize specific tectonic terranes the role of fluids in enhancing or destroying such structures is studied seismicity is strongly dependent on the different pore pressure of fluids in intraplate settings in strike slip or extensional areas hydrostatic pore pressure seems to be approached in accordance with standard rheological models and standard seismicity depth distributions in suture zones or zones of developing loads possibly also along zones of near horizontal normal faults extremely high pore pressure develops which approaches lithostatic values they are accompanied by a number of weakening processes with enhanced shear and reduced viscosity these values correlate with low seismic velocities v p in accordance with an empirical v p relationship

petrofacies and provenance of late mesozoic forearc basin northern and central california data from the great valley group sequence represent the most complete information regarding sandstone petrology of sediment derived from a magmatic arc this information is useful in documenting tectonic and magmatic events within the arc and related terranes and forms the basis for the establishment of petrostratigraphic units for mapping and correlation sandstone and conglomerate compositions are controlled by changes in provenance many of which were basinwide and synchronous clay mineral composition is controlled primarily by burial metamorphism careful attention to sample collection sample preparation and petrographic techniques is essential for uniform results seven petrographic parameters p f lv l m qp q q f and l listed in decreasing importance to petrofacies discrimination define eight petrofacies stony creek platina lodoga grabast boxer cortina los gatos and rumsey listed in approximate order of decreasing age the upper jurassic lower cretaceous petrofacies stony creek platina and lodoga contain higher lithic contents supracrustal sources whereas the upper cretaceous petrofacies especially the rumsey contain higher proportions of plutoniclastic components quartz feldspar and micas the proportion of potassium feldspar increases from near zero in the upper jurassic to nearly 50 of all feldspars in the uppermost cretaceous the lower part of the great valley group upper jurassic and lower cretaceous contains significant quantities of sedimentaclastic and metamorphiclastic material eroded from accreted and deformed terranes tectonic highlands formed by the arc arc collision nevadan orogeny that occurred prior to initiation of the franciscan great valley sierra nevada arc trench system the klamath mountains area provided a major proportion of this detritus ophiolite and serpentinite detritus was deposited locally near the base of the great valley group as a result of deformation along the east side of the growing franciscan subduction complex volcaniclastic detritus was fed into the entire forearc basin as magmatism increased in the sierra nevada area during the cretaceous as the volcanic cover was stripped off plutoniclastic and metamorphiclastic detritus from the underlying batholithic terranes was provided in abundance to the forearc basin crustal components were more continental in the southern sierra nevada and more oceanic in the northern sierra nevada as demonstrated by the higher proportions of metamorphiclastic detritus and by the more felsic nature of volcaniclastic detritus to the south by the middle of the late cretaceous extensive batholithic terranes provided potassium feldspar rich arkosic detritus to the entire forearc basin by the paleogene arc magmatism had migrated eastward sufficiently that deeper levels of the california part of the arc were exposed by erosion tectonic activity decreased in the forearc basin and the basin was filled to sea leve in most parts

small scale deformation structures and physical properties related to convergence in japan trench slope sediments diatomaceous mudstones cored on dsdp legs 56 57 off japan contain numerous postdepositional veins healed fractures and microfaults beneath the forearc basin these secondary structures occur only at depths greater than 620 m miocene in a zone of normal faulting beneath the landward slope of the trench however they occur at depths as shallow as 250 m pliocene in hemipelagic sediments which are inferred to blanket tectonically accreted deposits the intensity of deformation in the trench slope deposits increases in a seaward direction bulk densities indicate that the trench slope deposits have not dewatered normally but are above 400 m more consolidated than sediment deposited on the deep sea terrace a decline in sediment bulk density with increasing depth suggests that open fractures may exist in situ perhaps induced and or maintained by excess pore pressure below about 550 m a normal pattern of increasing density with increasing depth may reflect fracture closure and consolidation under lithostatic load we suggest that tectonic stress related to plate convergence has been communicated to the sediment on the landward japan trench slope causing tectonic dewatering within the subduction complex and inducing pervasive but small scale veining fracturing and faulting in near surface 700 m hemipelagic deposits this deformation occurs in sediment which has not been subject to large tectonic movement and does not eradicate larger 50 100 m thick structural depositional units which are apparent on seismic reflection records the brecciation induced by deformation may reflect natural hydrofracturing under abnormally high pore pressures or opening of fractures formed earlier at shallower depths during the dewatering process the overpressures may in turn locally reduce the sediment shear strength at shallow levels in the sediment column and induce downslope mass sediment movement

incipient subduction and deduction along the eastern margin of the japan sea the eastern margin of the japan sea has been under convergent tectonics since sometime after the pliocene the suture line is on the ocean continent boundary between the japan yamato basins and the tohoku northeast japan arc incipient subduction and obduction of the oceanic and sub oceanic crust are observed along the suture line with the occurrence of numerous n s trending fault bounded ridges and troughs focal mechanism solutions of several compressional type earthquakes with magnitudes of m 6 9 to 7 7 which have occurred along the zone are consistent with the observation of geological thrust faults the convergent stresses in the japan sea may be due to india eurasia collision and its associated intra plate or inter microplate movements in east asia the eastward movement of the amurian plate causes baikal extension along its western margin and the japan sea convergence along its eastern margin

structure and kinematics of oblique continental convergence in northern fiordland new zealand abstract southeast of the alpine fault in fiordland new zealand late tertiary faults envelope exposures of early cretaceous high p 12 16 kbar granulite facies orthogneisses these exposures allowed us to examine how pre cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence fault slip data and kinematic modelling show that oblique slip and reverse displacements preferentially were partitioned east of the alpine fault onto curved ne striking surfaces and moderately dipping 30 52 n and nw striking surfaces respectively approximately 3 0 5 5 km of exhumation and 6 5 7 0 km of oblique dextral displacement occurred on faults that display palm tree style geometries and subhorizontal detachments in profile this heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the alpine fault north of milford sound the principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7 9 ma the effects of these pre existing structures and a focusing of contractional deformation along the northeast margin of fiordland resulted in an unusually high degree of strike slip partitioning across the southernmost onshore segment of the alpine fault

evidence for fluid pathways through archaean crust and the generation of the closepet granite karnataka south india abstract in the archaean amphibolite granulite facies transition zone of southern karnataka anatexis of amphibolite facies peninsular gneiss occurred based on cross cutting relationships this melting event which produced the closepet granite was contemporaneous with the formation of charnockitic rocks this formation of charnockite took place initially due to channelized co 2 rich volatiles moving along an anastamosing network of shears and micro fissures these fluid pathways are now represented by brown veins of charnockite later the fluid became more pervasive and resulted in areas of total inversion to charnockite a lateral change along the fluid pathways from charnockite through incipient charnockite to either a thin granitic vein or a vein of reddening can be traced through the country rocks these different effects along the same laterally continuous vein are interpreted to represent the lateral change of volatile composition which in front of the co 2 rich fluids were increasingly h 2 o dominated

superposition of replacements in the mafic granulites of the jijal complex of the kohistan arc northern pakistan dehydration and rehydration within deep arc crust abstract a deep level crustal section of the cretaceous kohistan arc is exposed in the northern part of the jijal complex the occurrence of mafic to ultramafic granulite facies rocks exhibits the nature and metamorphic evolution of the lower crust mafic granulites are divided into two rock types two pyroxene granulite orthopyroxene clinopyroxene plagioclase quartz 1 and garnet clinopyroxene granulite garnet clinopyroxene plagioclase quartz 2 two pyroxene granulite occurs in the northeastern part of the jijal complex as a relict host rock of garnet clinopyroxene granulite where the orthopyroxene rich host is transected by elongated patches and bands of garnet clinopyroxene granulite garnet clinopyroxene granulite together with two pyroxene granulite has been partly replaced by amphibolite hornblende garnet plagioclase quartz 3 the garnet bearing assemblage 2 is expressed by a compression dehydration reaction hornblende orthopyroxene plagioclase garnet clinopyroxene quartz h 2 o subsequent amphibolitization to form the assemblage 3 is expressed by two hydration reactions garnet clinopyroxene plagioclase h 2 o hornblende quartz and plagioclase hornblende h 2 o zoisite chlorite quartz the mafic granulites include pod and lens shaped bodies of ultramafic granulites which consist of garnet hornblendite garnet hornblende clinopyroxene 4 associated with garnet clinopyroxenite garnetite and hornblendite field relation and comparisons in modal chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet clinopyroxene granulite the results combined with isotopic ages reported elsewhere give the following tectonic constraints 1 crustal thickening through the development of the kohistan arc and the subsequent kohistan asia collision caused the high pressure granulite facies metamorphism in the jijal complex 2 local amphibolitization of the mafic granulites occurred after the collision

reconstruction of subduction zone paleogeometries and quantification of upper plate material losses caused by tectonic erosion we propose in this paper an original method for quantifying the material losses at subduction zones mainly based on vertical movements of the active margins we first test the method using isostatic and elastic models applying it to 15 different well constrained convergent margins the main conclusion is that most of them are presently close to the isostatic equilibrium consequently we consider as a first hypothesis that such was also the case in the past and that erosion or accretion are natural mechanisms restoring the equilibrium in response to a perturbation then we apply this model to compute the geometry of the palcoslab knowing the palcotopography of the margin the comparison between present and past ocean margin lithospheric wedge geometries provides estimates of the removed material that constituted the initial wedge two cases were studied for palcoreconstructions japan and peru the results show that a considerable amount of material was removed during the neogene along both margins a detailed interpretation of the neogene history of the japan margin is proposed on the basis of our results

subduction erosion and accretion in the solomon sea region abstract the solomon sea region is an area of intense tectonic activity characterized by structural complexity a high level of seismicity and volcanism and rapid evolution of plate boundaries there is little accretion in the eastern new britain trench accretion gradually increases westward with thick accretion in the western new britain trench and in the trobriand subduction system the thick accretion in the western part of the new britain trench may be a result of collision from the north of finisterre huon block with new guinea mainland the present boundary of the collision is along the ram markham fault deformation structures and present day seismicity suggest that the northern block is under compression accretion has occurred in the sediment filled trenches in the solomon sea the scale of the accretionary wedge depends on the amount of trench fill sediment available it is unlikely that there is no sediment supply to the eastern part of the new britain trench where no accretion is observed and subduction erosion may be occurring there are two possible mechanisms for subduction erosion of sediment either a rapid rate of subduction relative to the supply of sediment inhibiting sediment accumulation in the trench or horizontal tensional force superimposed on both the forearc and backarc regions of the arc seafloor spreading in both the manus and woodlark basins is fan like with nearby poles in the western margins of the basins this may be a reflection of a horizontally compressional field in the western part and a tensional field in the eastern part of the solomon sea therefore it is possible to conclude that the consumption of sediment in the eastern new britain trench is related to the horizontal tensional field superimposed on both the forearc and backarc regions of the subduction system imbricated thrust and overthrust faults in the western new britain trench and trobriand trough are not linear over long distance but form wavy patterns in blocks with unit distance of approximately 10 km

modeling metamorphic fluid flow with reaction compaction permeability feedbacks existing models of metasomatic flow do not allow for the effect that reaction has on the flow patterns instead it is assumed that the volatiles produced are negligible in volume compared to those infiltrated and that reaction does not modify permeability this is clearly unlikely to be true for infiltration driven decarbonation reactions the rates of porosity creation by reaction and porosity loss by creep have been calculated for a representative volume of calcite quartz wollastonite marble and it is found that even for a weak calcite matrix the rate of porosity generation by reaction is likely to outstrip the collapse of porosity as long as the system is out of equilibrium we have applied a self consistent 1d finite difference model to the reaction of calcite quartz to wollastonite in a 10m thick marble in response to influx of h2o rich fluid with fixed boundary conditions the model allows us to evaluate the effect of reaction on the porosity structure and fluid pressure variation across the layer from which local darcy fluxes can be evaluated the progress of reaction that we model is constrained by hydrological considerations with the requisite parameters recalculated as reaction progresses assuming creep compaction of rock under the stress difference between lithostatic and fluid pressures we fnd that the volume of fluid realised by decarbonation driven by influx of h20 is sufficient to create a back flow so that further advancement of the reaction front is only possible as a result of diffusion of water against the darcy flux the effect of creep driven by differences between fluid pressure and lithostatic pressure is to reduce the permeability of the layer and especially reduce the secondary porosity developed in the zone at and behind the advancing reaction front we predict that in a 3d situation the porous zone of reacted marble becomes a conduit for layer parallel flow and the secondary porosity is infilled by calc silicate minerals due to silica metasomatism

continental and marine paleoenvironments reflected by west pacific clay sedimentation various stratigraphic and paleoenvironmental results arise from mineralogical geochemical and micromorpho logical investigations on late cretaceous and cenozoic sediments recovered by the deep sea drilling project in the west pacific stratigraphic correlations the deep pelagic red clays deposited on either side of the shatsky rise contain a variety of clay minerals which point to the importance of eolian detrital supply and allow rough stratigraphic correlations climate and paleocirculation neogene sediments of the lord howe rise area reflect closely the relationship between the evolution of the antarctic ice sheet and related aridification stages and the atmospheric and oceanic circulation pattern since 14 my regional and global tectonics the post eocene deposits around new zealand express a sequence of tectonic events and of relaxation stages associated with changes in the circulation regime along the inner wall of the japan trench clay mineral successions document the existence of an outer continental landmass until the late oligocene its collapse associated with volcanism close to the paleogene neogene boundary and an increased detrital influence from japan since the early middle miocene volcanism diagenesis and detrital supply the aptian to maastrichtian sediments in the mariana basin document a complex sequence of geochemical and geodynamical events proximal volcanism and local supply from subaerial archipelagoes early marine diagenesis during lithospheric subsidence and plate migration and finally the occurrence and evolution of terrigenous input from distant sources distributed by marine currents

evaluating the influence of aseismic ridge subduction and accretion on detrital modes of forearc sandstone an example from the kronotsky peninsula in the kamchatka forearc abstract the kronotsky peninsula in the forearc region of the kamchatka magmatic arc lies on trend with the emperor seamount chain situated on the currently subducting pacific tectonic plate detrital modes of volcaniclastic sandstone interbedded with mafic eocene basement rocks and within the overlying sedimentary sequence provide insight into the late cenozoic geologic history of this area eocene and basal miocene sandstones are primarily composed of variably altered mafic volcanic debris their detrital modes are similar to those of emperor seamount sandstones and hawaiian beach sands although aspects of the stratigraphy and volcaniclastic sand composition are consistent with a seamount setting there is no physical evidence for an accretion event and the suggested eocene age for this unit makes an emperor seamount origin unlikely a seamount origin cannot be ruled out for older kronotsky basement complexes however a miocene lull in kronotsky volcanism was followed by rapid basin subsidence and influx of arc derived turbidites from the west detrital modes of these sandstones are typical of a moderately evolved continental or micro continental arc an anomalously high proportion of sedimentary lithic fragments is the only possible compositional fingerprint attributable to seamount or ridge subduction

rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal bearing sediments offshore shimokita peninsula japan iodp site c0020 sediments recovered at integrated ocean drilling program iodp site c0020 in a forearc basin offshore shimokita peninsula japan include numerous coal beds 0 3 7 m thick that are associated with a transition from a terrestrial to marine depositional environment within the primary coal bearing unit 2 km depth below seafloor there are sharp increases in magnetic susceptibility in close proximity to the coal beds superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence we investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record using isothermal remanent magnetization irm thermal demagnetization anhysteretic remanent magnetization arm iron speciation and iron isotopes magnetic mineral assemblages in all samples are dominated by very low coercivity minerals with unblocking temperatures between 350 580 c that are interpreted to be magnetite samples with lower unblocking temperatures 300 400 c higher arm higher frequency dependence and isotopically heavy 56fe across a range of lithologies in the coal bearing unit between 1925 1995 mbsf indicate the presence of fine grained authigenic magnetite we suggest that iron reducing bacteria facilitated the production of fine grained magnetite within the coal bearing unit during burial and interaction with pore waters the coal peat acted as a source of electron donors during burial mediated by humic acids to supply iron reducing bacteria in the surrounding siliciclastic sediments these results indicate that coal bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time within the subsequent deep biosphere

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

tectonic regime of the southern kurile trench as revealed by multichannel seismic lines the analysis of the two multichannel seismic reflection records p 855 and p 856 reveals the complexity of the tectonic structure of the kurile margin the absence of trench fill and the relatively constant thickness of the subducted sediments on top of the oceanic basement shows that little or no accretion is occurring at present the lower slope is characterized by active thrusting within a small accretionary prism the toe of the middle slope is a more consolidated unit that constitutes an efficient buttress a piggy back basin possibly filled by sediments that have drifted down slope has formed on top of this unit a narrow high marks the transition to the edge of the continental terrace furthermore the upper slope reveals an unconformity marking the top of the basement the similarity of this unconformity and its sedimentary cover with the well documented data acquired around dsdp legs 56 57 suggests that northern japan and the southern kurile forearcs may have experienced a similar cenozoic vertical history 1 regional setting the southwestern part of the kurile arc in the vicinity of the so called hokkaido corner underwent a polyphase tectonic evolution during the tertiary kurile subduction is believed to have begun in the early jolivet 1986 or late kimura and tamaki 1985 cretaceous after the accretion of the okhotsk terrane to the continent of siberia both the nemuro peninsula and the is

oblique and near collision subduction sagami and suruga troughs preliminary results of the french japanese 1984 kaiko cruise leg 2 abstract leg 2 of the french japanese 1984 kaiko cruise has surveyed the suruga and the sagami troughs which lie on both sides of the northwestward moving and colliding izu bonin ridge the northernmost part of the philippine sea plate the transition from the nankai trough to the suruga trough is characterized by northward decrease in width of the accretionary prism in good agreement with the increasing obliquity between the through axis and the direction of the convergence as the strike of the convergent boundary changes from ene nne to south north south of the area the southern margin of the zenisu ridge shows contractional deformations this supports the interpretation made by the team of leg 1 who studied the western extension of the area we studied that it is an intra oceanic thrusting of the ridge over the shikoku basin in the sagami trough where the relative motion is highly oblique to the plate boundary active subduction is mostly confined in the east west trending portions of the trough located south of the boso peninsula and along the lower boso canyon near the ttt triple junction in between the present motion is mainly right lateral along the northwest trending boso escarpment however an inactive but recent pliocene to lower pleistocene accretionary prism exists south of the boso escarpment which suggests that the relative motion was more northerly than at present before about 1 ma ago

strong b enrichment and anomalous δ11b in pore fluids from the japan trench forearc abstract interstitial pore waters from ocean drilling program odp sites 1150 and 1151 both penetrating 1200 m of sub seafloor sediment from the upper japan trench forearc were analyzed for b concentrations and 11 b isotopes b concentrations cover a wide range from 329 to 3920 m 0 8 9 3 seawater and vary isotopically from 20 to 46 relative to srm 951 in general b concentrations increase more or less steadily with depth at both sites the b enrichment and isotopic changes seem to be related to a deep fluid influence possibly coupled with alteration of volcanic products silica diagenesis does not affect the b geochemistry of the waters as is evident from intact diatom tests and high opal a contents throughout the holes the b enrichment is more pronounced at site 1150 in the seismic portion of the forearc here a large positive anomaly in 11 b values is observed which corresponds with abundant fracturing and two prominent shear zones at about the same depth range at site 1151 the 11 b values show a negative shift attesting to an enrichment in 10 b in the fluid as a result of b desorption from clay due to burial and tectonic stress we conclude that b release from clay minerals is the major contributor to pore fluid anomalies and that fluid flow through permeable out of sequence thrust like the shear zones at site 1150 may be an efficient mechanism for b backflux from the forearc into the hydrosphere

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

compositions of sandstones in circum pacific subduction complexes and fore arc basins arc trench systems of the circum pacific orogenic belt contain voluminous sandstones within fore arc terranes that include complexly deformed subduction complexes and relatively undeformed fore arc basins of several types turbidites incorporated within subduction complexes include not only axially transported trench fill but also abyssal plain sediments deposited on the sea floor beyond the trench and slope basin deposits perched on the accretionary trench slope mean framework modes of circum pacific sandstone suites can be expressed adequately by the use of qfl and qmflt plots supplementary qmpk and qplvls plots depict partial modes of mineral grains alone and polycrystalline lithic fragments respectively source rocks were mainly volcanic and plutonic terranes of he circum pacific magmatic arcs although some recycled sedimentary and metasedimentary debris is also present analysis of point counts for 2 587 rocks grouped into 104 suites from california the pacific northwest southern alaska southwest japan and new zealand indicates that the characteristic fore arc sandstones display similar compositional ranges everywhere and can be termed the circum pacific volcano plutonic graywacke suite knowledge of their petrology may aid the recognition of analogous tectonic settings in the geologic past frameworks vary across a compositional spectrum that ranges chiefly from feldspatholithic sands derived mainly from volcanic cover in undissected magmatic arcs to lithofeldspathic sands derived largely from plutonic batholiths in dissected magmatic arcs quartz contents are typically intermediate and the feldspar to quartz ratio is semiconstant plagioclas is the dominant feldspar and volcanic rock fragments are commonly the most abundant lithic fragments interpretations of compositional trends within subduction complexes and adjacent fore arc basins imply that simple transverse delivery of sediment from the arc orogene to the fore arc region is not the rule instead sand evidently is distributed along subduction zones by longitudinal turbidity currents initiated at widely spaced sediment delivery points where sediment derived from tectonic culminations along the arc trench system can bypass the sediment traps of fore arc basins moreover turbidites shed from dormant or transform segments of arc orogenes are locally spread far into the ocean basin as abyssal plains which are later rafted bodily into the subduction zone finally no sedimentologic explanations of compositional contrasts across fore arc graywacke terranes can be acce ted without considering the alternative of postdepositional tectonic transport by strike slip

the effect of pre tectonic reaction and annealing extent on behaviour during subsequent deformation insights from paired shear zones in the lower crust of fiordland new zealand granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow 20 mm felsic dykes in fiordland new zealand forming 10 80 mm wide garnet reaction zones the metamorphic reaction changed the abundance of minerals and their shape and grain size distribution the extent of reaction and annealing temperature related coarsening and nucleation is greatest close to the dykes whereas further away the reaction is incomplete as a consequence grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres the aspect ratios of clusters of s1 pyroxene and pargasite in the orthogneiss which are variably reacted to post s1 garnet decrease from high in the host to near equidimensional close to the dyke post reaction deformation localized in the fine grained partially reacted areas this produced a pattern of paired shear zones located at the outer parts of the garnet reaction zone our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction the weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage

a summary of cenozoic tectonic history along the ipod japan trench transect along the international program of ocean drilling ipod japan trench transect a mid cenozoic period of tectonism without arc volcanism separated an earlier cretaceous to paleogene and a later neogene period of plate convergence and subduction the cretaceous to early paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline probably a forearc basin the seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex the arc the syncline and the thickened sediment sequence resemble an arc trench system of the size and structure of the great valley forearc basin and franciscan accretionary complex of california and they were probably continuations of the yezo geosyncline and associated sequences exposed to the north on hokkaido the arc trench system is contemporaneous with the adjacent shimanto arc trench system southwest of tokyo bay in the early paleogene volcanism ceased and part of the thickened sediment sequence of the cretaceous paleogene margin was emergent as a 160 km wide landmass which is recognized in seismic records as a later paleogene sediment source at the end of the paleogene this landmass began to subside and volcanism began again but it was located first briefly on the east and then on the west of the cretaceous arc along the present arc of northern honshu the land mass erosion surface is clearly visible as an angular unconformity in multichannel seismic reflection records the sequence of neogene sediment explosive volcanism and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in deep sea drilling project dsdp cores cretaceous subduction appears to have resulted in a very extensive accretionary complex whereas the neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin

detrital modes of the ionian forearc basin fill oligocene quaternary reflect the tectonic evolution of the calabria peloritani terrane southern italy abstract during oligocene neogene time the calabria peloritani terrane cpt a small block of continental lithosphere rifted from the southern margin of the eurasian continent and drifted southeastward to collide with the african continent along the leading edge of the cpt a late oligocene to quaternary mostly siliciclastic succession 2 000 meters thick was deposited in the ionian forearc basin the detritus forming the succession was derived directly from the erosion of the nearby cpt composed of hercynian plutons intruding metasedimentary host rocks contrary to most modern and ancient forearc basins penecontemporaneous volcanic detritus is virtually absent this results from rift related topographic barriers which separated the forearc basin from the associated volcanic arc in spite of an overall quartzofeldspathic continental block provenance the analysis of a large sandstone and conglomerate petrological dataset 228 samples points to three discrete detrital inputs of aphanitic lithic fragments in i late oligocene early miocene ii late messinian and iii quaternary times these petrological variations can be linked to independently determined tectonic events in the western mediterranean which likely modified the lithologic composition of the sediment source terranes input i documents the progressive widespread unroofing of the continental basement of the southern european margin when northwestward subduction of the remnants of the ligurian ocean began input ii is related to widespread intra messinian tectonism possibly due to out of sequence thrusting induced by isostatic readjustments of the perimediterranean orogenic wedges following the messinian drawdown input iii reflects the rapid vertical uplift experienced by the cpt since mid pleistocene time possibly the result of slab detachment along the ionian subduction zone this study indicates that detailed statistical analysis of the detrital modes of ancient terrigenous deposits can be used to identify the tectonic events affecting somewhat petrologically monotonous sediment source areas within small lithospheric terranes

cretaceous high p granulites at milford sound new zealand metamorphic history and emplacement in a convergent margin setting granulite facies orthogneiss of the arthur river complex arc at milford sound western fiordland records a complex early cretaceous magmatic and orogenic history for the pacific gondwana margin that culminated in the emplacement and burial of a dioritic batholith the western fiordland orthogneiss wfo enstatite bearing mafic to intermediate protoliths of the arc and wfo intruded the middle to upper crust the early deformation history of the arc is preserved in the pembroke granulite where two pyroxene s1 assemblages that reflect p 750 c were only patchily recrystallized during later deformation s1 is cut by garnet bearing leucogabbroic to dioritic veins which are cut by distinctive d2 fractures involving anorthositic veins and garnet diopside plagioclase bearing reaction zones these zones are widespread in the arc and wfo and record conditions of p 14 kbar and t 750 c garnet clinopyroxene bearing corona reaction textures that mantle enstatite in both the arc and wfo reflect early cretaceous burial by approximately 25 km of continental crust most of the arc is formed from the milford and harrison gneisses which contain steeply dipping s4 assemblages that envelop the pembroke granulite and involve garnet hornblende diopside clinozoisite rutile and plagio clase with or without kyanite the p t history of rocks in western fiordland reflects pronounced early cretaceous convergence related tectonism and burial possibly related to the collision and accretion of island arc material onto the pacific gondwana margin

large scale sedimentary recycling of tectonic melange in a forearc setting the ionian basin oligocene quaternary southern italy distinctive horizons of melange ranging in maximum thickness from a few tens to several hundreds of meters are conformably interbedded with the regularly stratified oligocene to quaternary predominantly marine deposits of the ionian forearc basin along the leading edge of the calabria peloritani microplate of southern italy these melange horizons were emplaced from the late early to the late miocene during collision of the southern and northern margins of the microplate with the passive margins of northern africa and the apulian platform respectively collisional deformation induced uplift of parts of the accretionary prism and ensuing sedimentary recycling of melange into the adjacent ionian forearc basin the thickest melange horizon can be traced for a distance 80 km with a consistent stratigraphic position within the forearc basin fill and ranges in thickness from 1 km near the collision zone to zero away from it it was emplaced in the late burdigalian and records oblique collision of the calabria peloritani microplate with the northern continental margin of africa in this context portions of the melange of the accretionary prism were extruded tectonically and gravitationally mobilized from southwest to northeast in the remaining undeformed parts of the forearc basin this interpretation is supported by 1 the overall geometry of the melange horizon pinching out away from the collision zone 2 its conformable contacts and chronostratigraphic relationships with the underlying and overlying stratigraphic units 3 its composition and 4 the absence of tectonic deformation at base and top of the horizon younger and thinner melange horizons are interbedded within the latest tortonian messinian succession and resulted from the erosion of thrust tip areas within the inner portion of the accretionary prism following incipient collision of the northern end of the microplate with the apulian platform integration of the results of this and other studies indicate that sedimentary recycling of large volumes of melange from accretionary prisms into forearc basins is an important process along both collisional and noncollisional active plate margins

tectonic evolution of the japanese islands as reflected in modal compositions of cenozoic forearc and backarc sand and sandstone cenozoic tectonic evolution of the japanese rifted continental arc trench system is reflected in the detrital modes of sand and sandstone deposited in forearc and backarc basins sampled by the deep sea drilling project at present the japan arc is divided into two segments along a complex plate boundary where the izu bonin ridge intersects the japan arc creating two triple junctions in front of and behind the ridge southwest of the izu bonin ridge quartzofeldspathic cretaceous forearc sediments were uplifted and recycled into tertiary forearc deposits in response to strike slip movement associated with backarc spreading in the shikoku basin quaternary forearc sections record the first major influx of volcanic detritus to southwest japan sites triple junction related deformation in central honshu has produced sand of mixed volcanic sedimentary provenance which is funneled by the suruga canyon into the nankai trough along the southwest japan margin north of the triple junction oligo miocene forearc sand compositions indicate oligocene forearc uplift and volcanism possibly related to initial backarc rifting and formation of the japan sea and subsequent miocene exposure of arc basement in contrast pliocene and quaternary forearc sand from northeast japan is primarily volcaniclastic and documents uninterrupted arc volcanism within the backarc region sand compositions vary from east to west across the japan sea the asian rifted continental margin and submerged remnants shed quartzofeldspathic sand into the western side of the basin whereas the japan arc sheds volcaniclastic sand into the eastern side of the basin

structure of the frontal part of the andean convergent margin a multichannel seismic reflection record across the central peru margin and trench was improved by processing 24 rather than the 12 channels previously processed and by thorough migrating to reject the strong diffractions that obscured weaker primary reflections the increased resolution clarifies the structure of the 15 km wide frontal accretionary complex and the adjacent truncated continental framework against which the trench sediment was imbricated resolved are individual thrust slices and packets adjacent to the trench axis subducting sediment filled graben in the ocean crust beneath the lower slope and a tertiary stratigraphic section of the upper slope yaquina basin which is cut by normal faults the resolution in this multichannel record provides criteria for reinterpreting single channel data off chile with increased confidence the previously proposed truncation of the south american continent along much of the peru chile trench is confirmed and the development of an accretionary complex in front of the truncated continental crust appears to vary with the amount of sediment seen in the trench axis the andean margin frontal structure is similar to that off central america the aleutian trench and the japan trench which suggests common truncation as well as accretion at the front of convergent margins

cenozoic evolution of the savu basin indonesia forearc basin response to arc continent collision abstract a seismo stratigraphic analysis of the savu basin relates the evolution of a complexly evolved forearc basin to the late miocene recent collision with australia initial early miocene subsidence of the outer forearc basin and the subsequent development of a middle late miocene volcanic proto arc located in the south of the basin both resulted from the late oligocene early miocene eastward propagation of the java timor trench block faulting of the relatively thick continental forearc basement probably determined the rather irregular and scattered pattern of middle late miocene volcanic centres just north of the accretionary prism the forearc basement is flexed down and reshaped into a trenchward dipping backstop that facilitates backthrusting of the accretionary prism the southern forearc basement probably acted as a barrier against compression the thickness of this continental basement is suggested to be a critical factor in the response of the forearc to continent collision owing to its strength the savu basin did not react to collision by the development of northward verging inter arc thrusts in contrast it responded to the underthrusting of continental crust by reactivation of the basement ridges this resulted in the differentiation of the forearc basin into a presently extinct and uplifted miocene south savu basin and a pliocene recent active north savu basin the simultaneous late miocene recent uplift of large segments of the outer forearc and subsidence of the north savu basin is interpreted to result from tectonic underplating of continental material and uplift caused by the buoyancy of the subducted continental crust the relatively recent decoupling of this crust from the oceanic slab and the subsequent isostatic rebound may superimpose the initial effects of post collision extension on arc continent collision

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

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

south kitakami region an allochthonous terrane in japan the pre cretaceous geology of the japanese islands may be interpreted as an accretion complex in which many terranes of several origins are juxtaposed and a number of allochthonous blocks are involved the south kitakami and abukuma regions of northeast japan and probably also silurian and devonian blocks in the kurosegawa fault zone and the border fault zone of the hida metamorphic terrane are the allochthonous blocks the stratigraphic sequence and paleobiofacies of the regions are similar to each other and their lithologic characters are of continental affinity but however are quite different from those of the other terranes having oceanic characters the south kitakami and abukuma regions have been fragments of a lost continent less remotely situated to gondwana during the middle paleozoic and may have drifted and incorporated with the other part of the japanese islands before late mesozoic to early tertiary igneous activity

seaward jump of the subduction zone associated with ridge collision an example from the suruga trough abstract the suruga trough has evolved from a trench to the slope basin environment of sedimentation since the collision between the izu peninsula and honshu island in the middle pleistocene in the future about 0 5 ma after present this slope basin will be filled up by sediments and it is expected that the suruga trough will eventually become a fore arc basin the same systematic change of sedimentary basins cyclically occurred at several times since the late miocene five cycles including an anticipated one are recognized in the suruga trough region sedimentary basins adjacent to the collisional plate boundary show a change from trench to slope basin in one cycle and develop to forearc basin in the next cycle these cycles have gradationally advanced from the landward to the seaward side in that region with the westward motion of the philippine sea plate up to the late stage of izu cycle during this plate motion the dimensions of accreted mass in each cycle have been diminished in size associated with the shortening of time required in each cycle the philippine sea plate motion has changed to the northwest since 0 5 ma b p and the following zenisu cycle the boundary of which strikes at a ne sw trend has been developed in the same plate motion the shichito cycle has already begun at present the nankai trough might have been a dextral slip transform fault between the subduction zone in the suruga trough region and in the hyuga region before 2 ma b p and has changed to an active plate subduction boundary with the northwestward motion of the philippine sea plate during the izu cycle

constraints on element mobility associated with the conversion of granulite to eclogite along fractures in an anorthositic complex on holsnoy norway on holsnoy an island off the coast of western norway an anorthositic complex metamorphosed to granulite facies was partially overprinted by a later eclogite facies metamorphism eclogite facies rocks containing omphacite garnet kyanite and hydrous phases such as mica and zoisite occur in shear zones of various scales and adjacent to veins previous studies of shear zones on holsnoy reported evidence for substantial element mobility jamtveit et al 1990 mattey et al 1994 in this work we compare chemical compositions of granulite and its undeformed eclogitized equivalent adjacent to veins in locations where a single band of granulite can be traced and sampled as it approaches the vein this tracing is crucial because the pre granulite rocks cover a substantial compositional range indicative of a petrologically variable protolith consisting of anorthosite gabbro and jotunite we analysed multiple core samples collected across nine separate granulite eclogite transition zones located at veins in anorthositic jotunitic and gabbroic protoliths for major and trace elements for each transition no compositional difference between the average granulite and average eclogite composition was found at the 90 confidence level except for loi loss on ignition which was consistently significantly higher in the eclogite samples although not significant at the 90 confidence level for any single traverse the average eclogite concentrations of sio2 na2o cs as and br exceed the average granulite concentrations for eight or all nine of the traverses for most traverses statistical analysis of the data limits any gain of sio2 in the eclogites to no more than a few relative per cent other than the introduction of volatile substances presumably an h2o rich fluid eclogitization associated with vein formation was essentially isochemical

continental margin tectonics submarine accretionary prisms submarine accretionary prisms are sites of incipient mountain building where sediments and rocks of the deep ocean are uplifted deformed and ultimately transformed to continental crust accretionary prisms develop at sites of lithospheric convergence that is subduction zones commonly accretionary prisms form along continental margins although accretionary prisms may also occur adjacent to island arcs which are ultimately coalesced into continental landmasses here we review the studies of submarine accretionary prisms during the past four years with primary focus on the contributions of geoscientists from the united states

flooding dynamics on the lower amazon floodplain 1 hydraulic controls on water elevation inundation extent and river floodplain discharge 1 modeling the routing of flood waters across large floodplains is challenging because flows respond to dynamic hydraulic controls from complex geomorphology vegetation and multiple water sources in this study we analyzed the topographic and hydrologic controls of inundation dynamics of a large floodplain unit 2440 km2 along the lower amazon river we combined land topography derived from the shuttle radar topography mission srtm with underwater topography derived from an extensive echo sounding survey to generate a seamless digital elevation model dem floodplain inundation was simulated using lisflood fp which combines one dimensional river routing with two dimensional overland flow and a local hydrological model for the first time accurate simulation of filling and drainage of an amazon floodplain was achieved with quantification of changes in water elevation flooding extent and river floodplain exchange we examined the role of diffuse overbank versus channelized flows on river floodplain exchange diffuse overbank flows represent 93 of total river to floodplain discharge and 54 of floodplain to river discharge floodplain discharge during high water was four times higher than field observation values when the srtm v 4 dem with no correction was used for simulation because of a 4 4 m elevation bias originating from residual motion errors of the srtm interferometric baseline

tectonic history of ulleung basin margin east sea sea of japan a geologic history analysis of an exploratory well drilled on the southern margin of the ulleung back arc basin east sea western part of the sea of japan reveals that tectonic force and sedimentary loading played a major role in the subsidence of the paleo ulleung basin where a thick clastic sequence was deposited during the neogene initial subsidence was most likely caused by a tectonic force related to the back arc rifting and spreading associated with convergent processes the southern margin underwent regional deformation uplift faulting and folding toward the late miocene this event was probably due to back arc deformation caused by changes both in plate motions and in subduction mode at the convergent zones

mid quaternary decoupling of sediment routing in the nankai forearc revealed by provenance analysis of turbiditic sands coring during integrated ocean drilling program expeditions 315 316 and 333 recovered turbiditic sands from the forearc kumano basin site c0002 a quaternary slope basin site c0018 and uplifted trench wedge site c0006 along the kumano transect of the nankai trough accretionary wedge offshore of southwest japan the compositions of the submarine turbiditic sands here are investigated in terms of bulk and heavy mineral modal compositions to identify their provenance and dispersal mechanisms as they may reflect changes in regional tectonics during the past ca 1 5 myrs the results show a marked change in the detrital signature and heavy mineral composition in the forearc and slope basin facies around 1 ma this sudden change is interpreted to reflect a major change in the sand provenance rather than heavy mineral dissolution and or diagenetic effects in response to changing tectonics and sedimentation patterns in the trench slope basin the sands older than 1 ma were probably eroded from the exposed cretaceous tertiary accretionary complex of the shimanto belt and transported via the former course of the tenryu submarine canyon system which today enters the nankai trough northeast of the study area in contrast the high abundance of volcanic lithics and volcanic heavy mineral suites of the sands younger than 1 ma points to a strong volcanic component of sediment derived from the izu honshu collision zones and probably funnelled to this site through the suruga canyon however sands in the forearc basin show persistent presence of blue sodic amphiboles across the 1 ma boundary indicating continuous flux of sediments from the kumano kinokawa river this implies that the sands in the older turbidites were transported by transverse flow down the slope the slope basin facies then switched to reflect longitudinal flow around 1 ma when the turbiditic sand tapped a volcanic provenance in the izu honshu collision zone while the sediments transported transversely became confined in the kumano basin therefore the change in the depositional systems around 1 ma is a manifestation of the decoupling of the sediment routing pattern from transverse to long distance axial flow in response to forearc high uplift along the megasplay fault

along arc structural variation of the plate boundary at the japan trench margin implication of interplate coupling 1 at the japan trench convergent margin many large interplate earthquakes of greater than m7 5 frequently occur their epicenters have uneven distribution mostly located in the northern area to investigate the relationship between this distribution and tectonic structures we have conducted multichannel seismic surveys since 1996 our data show two kinds of interplate sedimentary units a wedge shaped unit and a channel like unit both units have a lower p wave velocity than the basal part of the overriding island arc crust the wedge shaped unit having a velocity of 2 3 km s is widely distributed over the forearc region in the northern area its thickness decreases with depth becoming several hundred meters at a depth of 12 km the channel like unit having a velocity of 3 4 km s is observed in the southern area extending in the downdip direction its thickness reaches 2 km at a depth of 12 km if the low velocity of these units results from the existence of fluid as many authors assume the units being thick implies higher fluid content assuming constant porosity considering that fluid reduces basal friction and with an assumption that fluid available at a specific interface is proportional to the total fluid content in the sediment the thickness variation of the units would cause different degrees of coupling at the plate boundary along the arc this may provide one explanation for the regional disparity in the interplate earthquake occurrence in the margin furthermore we attempt to call attention to the possibility that the channel like sediment works as a shear stress releaser

nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks a subduction shear zone can be modeled as a long narrow channel with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential which combines the effect of both pressure and buoyancy if the channel walls are effectively rigid very slight narrowing or broadening of the channel convergence angles 2 gpa in the channel at only 40 km depth the model is consistent with a horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment 650 m the practical limit for overpressures attainable in subduction zones is determined by the strength and permeability of the channel walls at 40 km depth the channel is effectively confined on both sides by cold lithospheric mantle which should be strong enough to support a significant tectonic overpressure episodic failure of the upper plate to produce great earthquakes at 30 40 km focal depth could vent overpressured fluid from the channel allowing a cyclical buildup and release of both rock and fluid pressure topography on the subducting plate e g seamounts and thinned continental crust may lead to an anvil like jamming of the channel and local high overpressures tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks a decrease in the convergence rate or cessation of subduction with a consequent general warming within the channel and associated viscosity decrease promotes exhumation by buoyant reverse flow the most rapid reverse flow occurs in the region of previously greatest overpressure since the exhumation distance is shorter than for a simple lithostatic pressure distribution and any increase in temperature is coupled with a strong increase in the rate of exhumation preservation of high pressure assemblages at the surface in fossil subduction zones is promoted for such a model

the late jurassic oblique collisional orogen of sw japan new structural data and synthesis the structural configuration of sw japan mainly reflects a late jurassic early cretaceous orogeny the region is divided into an inner belt and an outer belt on the japan sea and pacific ocean sides respectively by a strike slip fault the median tectonic line mtl both consist of a series of stacked nappes the inner belt is divided into a jurassic olistostrome known as the tanba zone and a hinterland area comprising continental triassic jurassic sediments the tanba zone is sliced into two units a lower one with late jurassic matrix and triassic early jurassic radiolarite olistoliths tectonically overthrust by an upper unit comprising an olistostrome with middle jurassic matrix and blocks which include late paleozoic limestone basic lava and radiolarite the tanba zone is overthrust by a paleozoic nappe complex derived from the hinterland the basal sole of the nappe corresponds to a peculiar unit called the ultra tanba zone in the chugoku area the hinterland is divided into an upper nappe the oga nappe formed by permo carboniferous limestone and permian clastic rocks and a lower one the sangun maizuru nappe formed by paleozoic high pressure hp metamorphics the sangun schists the permian maizuru olistostrome and the dismembered paleozoic yakuno ophiolites in the northern part of sw japan the tanba zone is in faulted contact with the circum hida and the hida zones the former is interpreted as the equivalent of the oga and sangun maizuru nappes of the chugoku domain crushed by post cretaceous tectonics the latter consists of paleozoic high temperature ht metamorphic rocks and late triassic early jurassic granite locally mylonitized and covered by early jurassic sandstone the outer belt is formed by a superficial nappe similar to the tanba zone thrust upon a deep domain characterized by a synmetamorphic ductile deformation the deep domain is divided into a lower unit the oboke unit formed by continental derived arenites and a green schist nappe consisting of oceanic sediments and resedimented ophiolites the green schist nappe overthrusts the oboke unit under synmetamorphic conditions with an eastward displacement the two belts are separated by the ryoke zone which corresponds to the southern part of the tanba zone affected by a cretaceous ht metamorphism and sharply cut by the mtl a geodynamic model is proposed for the jurassic orogeny of sw japan assuming that the evolution of the inner and outer belts are linked in the late triassic early jurassic sw japan is an active plate margin the upper plate or south china block consisted of the hinterland and the tanba belt a forearc basin the lower plate consisted of an oceanic area and the south japan continent the basic mechanism of the orogeny is ascribed to the oblique subduction and collision of the south japan continent

implications of measured thermal gradients for water movement through the northeast japan accretionary prism thermal gradients of 24 36 c km have been found in the slope cover sediments of the accretionary prism off northeast japan deep sea drilling project dsdp sites 438a 439 and 440 these gradients lead to the prediction of temperatures in excess of 400 c at depths of 15 km using an increase in thermal conductivity with depth due to the decrease in porosity such temperatures conflict with the predictions of temperatures of 150 c at even greater depths from conductive thermal models and petrologic studies of low temperature blueschists at similar ancient margins one dimensional steady state conduction and advection via darcian water flow of heat up through the prism was simulated numerically the measured thermal gradients can be reconciled with temperatures less than 125 c at the prism base by water flowing upward at rates of the order of 10 10 m s such velocities imply that the total volume of water available from compaction of sediments subducted beneath the northeast japan prism would flow up through a 10 km width of the prism perpendicular to the trench and would require the dsdp sites to have been located fortuitously over the localized zone s of water flow smaller velocities will reproduce the measured gradients and thus greater widths through which water flow occurs are predicted if the temperature at the prism base is higher and or if the prism is thinner and a subduction channel several kilometers high separates the accreted sediments from the descending plate where water flow is significant as likely wherever abundant water rich sediments are subducted thermal gradients below a few kilometers in accretionary prisms may be much lower than near surface values this has profound implications for the thermal structure of the prism off northeast japan and elsewhere

stratigraphic correlation of upper cretaceous to paleocene forearc basin sediments in northeast japan cyclic sedimentation and basin evolution abstract cretaceous to paleocene sediments are distributed across a 200 km wide and 1400 km long belt in northeast japan to sakhalin twenty six stratigraphic sections including land based and offshore drill holes and surface sections are correlated to delineate the sedimentary history and basin evolution of the yezo forearc basin two second order shallowing upward cycles are recognized in hokkaido but are obscure in north honshu shallow marine to paralic sediments of the first cycle early albian to turonian are represented by the mikasa formation late albian to turonian as the western marginal facies while the eustatic curves e g haq curves reach the highstand maximum at the late cenomanian the campanian to maastrichtian shallow and non marine facies of the second cycle coniacian to maastrichtian are characterized by the hakobuchi group in hokkaido and its correlatives in north honshu this large scale shallowing may have reflected directional changes of the izanagi kula plate motion near the early campanian the uppermost maastrichtian to upper paleocene is possibly absent everywhere in the basin the shallow marine to non marine facies often show several third order depositional sequences with upward coarsening facies successions they may have been controlled by global eustasy even in an active margin setting since the stacking patterns of sequences and the timing of sequence boundaries appear considerably concordant with the oscillation patterns of the haq curves

melting of albite and dehydration of brucite in h 2 o nacl fluids to 9 kbars and 700 900 c implications for partial melting and water activities during high pressure metamorphism the melting reaction albite solid h2o fluid albite h2o melt has been determined in the presence of h2o nacl fluids at 5 and 9 2 kbar and results compared with those obtained in presence of h2o co2 fluids to a good approximation albite melts congruently at 9 kbar indicating that the melting temperature at constant pressure is principally determined by water activity at 5 kbar the temperature t mole fraction x h2o melting relations in the two systems are almost coincident by contrast h2o nacl mixing at 9 kbar is quite non ideal albite melts 70 c higher in h2o nacl brines than in h2o co2 fluids for x h2o 0 8 and 100 c higher for x h2o 0 5 the melting temperature of albite in h2o nacl fluids of x h2o 0 8 is 100 c higher than in pure water the p t curves for albite melting at constant h2o nacl show a temperature minimum at about 5 kbar water activities in h2o nacl fluids calculated from these results from new experimental data on the dehydration of brucite in presence of h2o nacl fluid at 9 kbar and from previously published experimental data indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities buffering and limiting crustal melting and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction related metamorphism low water activity in high pressure temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting but may be due to the presence of low a h2o brines

isotope tracking of pore water freshening in the fore arc basin of the japan trench abstract pore waters of sediments in convergent zones typically show a decrease in chloride cl concentrations with depth that suggests the advection of chloride depleted fluids studies from odp leg 186 indicate that sediments accumulated in the fore arc of the japan trench also exhibit a freshening of pore waters with depth here the oxygen 18 o and hydrogen d isotopic composition of pore waters from site 1150 were determined to investigate the source of cl depleted fluids whereas the upper 100 m of sediments have pore waters exhibiting an increase in both 18 o and d values with depth the remaining 1000 m of studied sediments have pore waters exhibiting an overall decreasing trend in isotope values with depth dissociation of gas hydrates is a likely explanation for the elevated isotope values for pore waters in the upper 100 m in contrast the overall decreasing trend in isotope values probably resulted from the advection of cl depleted fluids exhibiting low 18 o and d values depletion of cl 18 o and d likely resulted from the diagenetic transformation of hydrous minerals and the subsequent precipitation of carbonates or other 18 o enriched minerals generation of cl depleted fluids did not occur within the studied depth interval but rather at deeper possible over pressured sites thereby pointing to the significance of deep seated fluid flow in fore arc basins

trace element partitioning during high p partial melting and melt rock interaction an example from northern fiordland new zealand pods of granulite facies dioritic gneiss in the pembroke valley milford sound new zealand preserve peritectic garnet surrounded by trondhjemitic leucosome and vein networks that are evidence of high p partial melting garnet bearing trondhjemitic veins extend into host gabbroic gneiss where they are spatially linked with the recrystallization of comparatively low p two pyroxene hornblende granulite to fine grained high p garnet granulite assemblages in garnet reaction zones new data acquired using a laser ablation inductively coupled plasma mass spectrometer la icpms for minerals in various textural settings indicate differences in the partitioning of trace elements in the transition of the two rock types to garnet granulite mostly due to the presence or absence of clinozoisite garnet in the garnet reaction zone gabbroic gneiss has a distinct trace element pattern inherited from reactant gabbroic gneiss hornblende peritectic garnet in the dioritic gneiss and garnet in trondhjemitic veins from the pembroke granulite have trace element patterns inherited from the melt producing reaction in the dioritic gneiss the distinct trace element patterns of garnet link the trondhjemitic veins geochemically to sites of partial melting in the dioritic gneiss

petrography and geochemistry of holocene sands in the western gulf of mexico implications for provenance and tectonic setting abstract petrographic and geochemical analysis of recent fluvial beach and dune sediments derived from the same source terrane was carried out in the western gulf of mexico to show the usefulness of the these three environments in determining the tectonic setting of the source petrographic analysis showed that dunes concentrate the less heavy minerals and quartz grains by means of the selectiveness of the wind as a transport agent in contrast the heavier grains such as the rock fragments remain in the beach and fluvial environments because of the high transport energy beach and dune sands do not define a particular tectonic setting because of the selective wind action of the backshore of the beach and the dune environments and the less intense chemical weathering the fluvial sands are the most representative in terms of interpretation of tectonic setting because they reflect the volcanic domain of the source rocks of the trans mexican volcanic belt the geochemical results of all three sedimentary environments reflect the calcalkaline character of the igneous source rocks of the trans mexican volcanic belt the samples fall in a continental island arc margin field with basaltic and andesitic source rocks a correlation and factor analysis indicates an alkali basalt source that contributes to the composition of the beach dune and fluvial sediments the geochemical analysis of the sediments more accurately reflects the tectonic setting regardless of the depositional setting of the sediments

transformation of two pyroxene hornblende granulite to garnet granulite involving simultaneous melting and fracturing of the lower crust fiordland new zealand granulite facies gabbroic and dioritic gneisses in the pembroke valley milford sound new zealand are cut by vertical and planar garnet reaction zones in rectilinear patterns in gabbroic gneiss narrow dykes of anorthositic leucosome are surrounded by fine grained garnet granulite that replaced the host two pyroxene hornblende granulite at conditions of 750 uc and 14 kbar major and trace element whole rock geochemical data indicate that recrystallization was mostly isochemical the anorthositic veins cut contacts between gabbroic gneiss and dioritic gneiss but change in morphology at the contacts from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss to zones with a septum of coarse grained garnet surrounded by anorthositic leucosome in the dioritic gneiss the dioritic gneiss also contains isolated garnet grains enclosed by leucosome and short planar trains of garnet grains linked by leucosome partial melting of the dioritic gneiss mostly controlled by hornblende breakdown at water undersaturated conditions is inferred to have generated the leucosomes the form of the leucosomes is consistent with melt segregation and transport aided by fracture propagation limited retrogression suggests considerable melt escape dyking and melt escape from the dioritic gneiss are inferred to have propagated fractures into the gabbroic gneiss the migrating melt scavenged water from the surrounding gabbroic gneiss and induced the limited replacement by garnet granulite

amount of clockwise rotation of southwest japan fan shape opening of the southwestern part of the japan sea abstract paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 ma and 112 ma from yamaguchi and go river areas in the central part of southwest japan the new data together with those of younger igneous rocks published previously define the change of paleomagnetic field direction during the late mesozoic cenozoic period for southwest japan the paleomagnetic direction from this area has pointed 56 3 clockwise from the expected field direction estimated from apwp apparent polar wandering path of the whole of eurasia during the period between 100 and 20 ma in comparison with the expected one from the eastern margin of eurasia korea china siberia the cretaceous field direction of southwest japan shows the clockwise deflection by 44 49 these results establish that while the eastern margin of eurasia including southwest japan was rotated more or less with respect to the main part of eurasia during last 100 ma southwest japan was rotated clockwise through more than 40 with respect to the eastern margin of eurasia since 20 ma the large amount of rotation for southwest japan implies that it is rotated by an opening of the southwestern part of the japan sea which widens northeastward fan shape opening the tectonic feature of southwest japan and the japan sea is analogous to that of corso sardinia and the ligurian sea in the mediterranean indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim

reaction fronts permeability and fluid pressure development during dehydration reactions abstract fluids released by prograde metamorphism are often invoked to explain a range of crustal processes from earthquake triggering to metasomatism these fluids can be either trapped and overpressured or released and channelized depending on the interplay between permeability reaction rate and compaction experimental data are presented measuring permeability porosity and microstructural evolution throughout the dehydration of gypsum to form bassanite reaction fronts regions over which the reaction largely occurs are used as a framework to explain the results experiments were conducted under hydrostatic conditions at a constant temperature of 115 c at two effective pressures of 60 mpa and 110 mpa and three pore fluid pressures of 20 40 and 60 mpa at high effective pressure creep of the gypsum solid framework results in low porosity and permeability producing high pore fluid pressure build up that slows the reaction rate a clearly defined narrow reaction front migrates along the sample and the average permeability remains low until the front sweeps across the entire sample conversely at low effective pressure the reaction front is wide producing a permeable drained network average permeability is enhanced significantly after only a small fraction of the reaction has completed by the interconnection of open pores this study shows that the width of reaction fronts and hence the permeability development is strongly controlled by compaction the reaction front velocity is broadly dependent on permeability and the reaction driving force a simple quantitative model for these relationships is developed

depositional architecture provenance and tectonic eustatic modulation of miocene submarine fans in the shikoku basin results from nankai trough seismogenic zone experiment seismostratigraphy coring and logging while drilling during integrated ocean drilling program expeditions 319 322 and 333 sites c0011 c0012 show three miocene submarine fans in the ne shikoku basin with broadly coeval deposits at ocean drilling program site 1177 and deep sea drilling project site 297 nw shikoku basin the sediment dispersal patterns have major implications for paleogeographies at that time the oldest finer grained kyushu fan has sheet like geometry quartz rich flows were fed mostly from an ancestral landmass in the east china sea during prolonged hemipelagic mud deposition at c0011 c0012 similar to 12 2 to 9 1 ma sand supply continued at sites 1177 and 297 sand delivery to much of the shikoku basin halted during a phase of sinistral strike slip to oblique plate motion after which the daiichi zenisu fan similar to 9 1 to 8 0 ma was fed by submarine channels the youngest fan daini zenisu similar to 8 0 to 7 6 ma has sheet like geometry with thick bedded coarse grained pumiceous sandstones the pumice fragments were fed from a mixed provenance that included the collision zone of the izu bonin and honshu arcs the shift from channelized to sheet like flows was favored by renewal of relatively rapid northward subduction which accentuated the trench as a bathymetric depression increased sand supply appears to correlate with long term eustatic lowstands of sea level the stratigraphic position and 3 d geometry of the sandbodies have important implications for subduction related processes including the potential for focused fluid flow and fluid overpressures above and below the plate boundary fault in sheet like sands pathways for fluid flow have greater horizontal permeability compared with those in channel sands

transport of low ah2o dehydration products to melt sites via reaction zone networks milford sound new zealand the gabbroic dioritic pembroke hornblende granulite phg of milford sound displays a geometrically simple mesoscopic network of sub planar garnet reaction zones grz in which the meta igneous hornblende granulite has been depleted of na si and h2o and c 25 vol almandine rich garnet has formed some studies postulate the initial presence of melt along the centres of all grz explaining the frequent absence of feldspathic veins by selective melt loss a more parsimonious model is necessitated by structural evidence and together with chemical data suggests a relationship between mid range metasomatic transport and anatexis the pembroke outcrops show a process of incipient melting of gabbro diorite in an environment of relatively low ah2o in lithologies that have limited free quartz a non equilibrium steady state is proposed in which a sodic dehydration fluid moves some distance via the grz network towards areas of partial melting only in these areas are na and si reconstituted as albite with more garnet as byproduct having avoided the need for melt percolation the combined structural and chemical evidence directs a focus on mass transport in low ah2o gabbroic environments in subsequent events of shearing and complete transposition both sets of garnet the atypical grz residue and partial melt melanosomes were inherited by the milford gneiss facies of the phg

roles for fluid and or melt advection in forming high p mafic migmatites fiordland new zealand a series of striking migmatitic structures occur in rectilinear networks through western fiordland new zealand involving for the most part narrow anorthositic dykes that cut hornblende bearing orthogneiss adjacent to the dykes host rocks show patchy spatially restricted recrystallization and dehydration on a decimetre scale to garnet granulite although there is general agreement that the migration of silicate melt has formed at least parts of the structures there is disagreement on the role of silicate melt in dehydrating the host rock a variety of causal processes have been inferred including metasomatism due to the ingress of a carbonic mantle derived fluid hornblende breakdown leading to water release and limited partial melting of host rocks and dehydration induced by volatile scavenging by a migrating silicate melt variability in dyke assemblage together with the correlation between dehydration structures and host rock silica content are inconsistent with macroscopic metasomatism and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid

fluvial architecture and sequence stratigraphy of the eocene to oligocene iwaki formation northeast japan channel fills related to the sea level change abstract the iwaki formation the lowermost part of the eocene to oligocene shiramizu group northeast japan consists of conglomeratic braided drainage sediments the formation displays fining upward succession that reflects an overall transgressive sequence with two other overlying formations of the group although the clast supported conglomerates are by far the major components of the formation they also intercalate with many concave up channel sediments filled up with sandy deposits the lower part of the formation mainly consists of multistorey large scale channel sandstones with minor amount of small scale single storey channel sandstones which are related to low rates of base level rise in contrast the middle to upper part of the formation comprises single storey strata with relatively small scale channels which were probably associated with more rapid rates of base level rise the field evidence shows that the change of scale and internal structures of channel sandstones within the conglomerates most likely reflect the rate of changes of sea level rise

do trench sediments affect great earthquake occurrence in subduction zones seismic energy release is dominated by the underthrusting earthquakes in subduction zones and this energy release is further concentrated in a few subduction zones while some subduction zones are characterized by the occurrence of great earthquakes others are relatively aseismic this variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties age of the subducting lithosphere and convergence rate these two properties do not explain all the variance in maximum earthquake size i propose that a third subduction zone property trench sediments explains part of the remaining variance in maximum earthquake size subduction zones are divided into two groups 1 those with excess trench sediments and 2 those with horst and graben structure at the trench thirteen of the 19 largest subduction zone events including the three largest occur in zones with excess trench sediments about half the zones with excess trench sediments are characterized by great earthquake occurrence most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age rate correlation two notable exceptions are the oregon washington and middle america zones overall the presence of excess trench sediments appears to enhance great earthquake occurrence one speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer which at elevated temperature and pressure forms a homogeneous and strong contact zone between the plates

extensional tectonics in the honshu fore arc japan integrated results of dsdp legs 57 87 and reprocessed multichannel seismic reflection profiles

characteristics and causal factors of hysteresis in the hydrodynamics of a large floodplain system poyang lake china abstract a previous modeling study of the lake floodplain system of poyang lake china revealed complex hysteretic relationships between stage storage volume and surface area however only hypothetical causal factors were presented and the reasons for the occurrence of both clockwise and counterclockwise hysteretic functions were unclear the current study aims to address this by exploring further poyang lake s hysteretic behavior including consideration of stage flow relationships remotely sensed imagery is used to validate the water surface areas produced by hydrodynamic modeling stage area relationships obtained using the two methods are in strong agreement the new results reveal a three phase hydrological regime in stage flow relationships which assists in developing improved physical interpretation of hysteretic stage area relationships for the lake floodplain system for stage area relationships clockwise hysteresis is the result of classic floodplain hysteretic processes e g restricted drainage of the floodplain during recession whereas counterclockwise hysteresis derives from the river hysteresis effect i e caused by backwater effects the river hysteresis effect is enhanced by the time lag between the peaks of catchment inflow and yangtze discharge i e the so called yangtze river blocking effect the time lag also leads to clockwise hysteresis in the relationship between yangtze river discharge and lake stage thus factors leading to hysteresis in other rivers lakes and floodplains act in combination within poyang lake to create spatial variability in hydrological hysteresis these effects dominate at different times in different parts of the lake and during different phases of the lake s water level fluctuations creating the unique hysteretic hydrological behavior of poyang lake

sediment flow routing during formation of forearc basins constraints from integrated analysis of detrital pyroxenes and stratigraphy in the kumano basin japan the evolution of sediment flow routing during complete evolution of the kumano forearc basin is determined through integration of stratigraphic and sediment provenance analyses in the upper nankai forearc a new approach uses the compositional variability of detrital clinopyroxenes and orthopyroxenes collected at eight major rivers in japan and three drill sites in the basin and nearby slope environment including the first drill cuttings retrieved by the integrated ocean drilling program iodp joint interpretation of these datasets reveals that the sedimentation history of the basin is characterised by three main phases separated by newly recognised time transgressive boundaries we show that the kumano basin initiated as a trench slope basin in the early quaternary 1 93 ma and that it progressively evolved towards an upper slope environment with increased turbidite confinement and influence from climatic forcing basin initiation was broadly synchronous with development of the nankai megasplay fault suggesting a causal relationship with construction of the nankai accretionary prism unlike preceding studies documenting long distance longitudinal transport of clastic material along the lower nankai forearc only limited longitudinal transport is documented by detrital pyroxenes in the upper forearc these results suggest that transverse canyons are a major control on the sediment flow routing during maturation of forearc basins and that long distance longitudinal flows along convergent margins are principally restricted to near trench environments even in the presence of large forearc basins

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

petrogenesis of migmatites in maine usa possible source of peraluminous leucogranite in plutons high t low p facies series conditions during deformation within a devonian crustal scale shear zone system defined by kilometer scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain at upper amphibolite facies grade metapelites are partially melted the onset of which is introduction