along arc lateral variation of rb sr and k ar ages of cretaceous granitic rocks in southwest japan cretaceous granitic rocks were emplaced over a distance of 700 km along arc in southwest japan rb sr and k ar ages of a major group of these granitic rocks with ilmenite series ore mineralogy were examined rb sr whole rock ages of 92 8 4 0 ma and rb sr and k ar biotite ages of 80 88 ma were obtained on one group of these granitic rocks from kamo sera area of central hiroshima prefecture the k ar ages of various minerals combined with the rb sr whole rock age give a smooth cooling curve which suggests a 5 to 10 ma time lag between intrusion and cooling at 300 c for the cretaceous granitic rocks the rb sr whole rock and rb sr k ar biotite ages of these granitic rocks become younger eastward along the southwest japan arc and the time lag between the two systems remains constant at 5 to 10 ma over the entire area the along arc age variation does not support the genetical relationship of the cretaceous granitoids with steady state subduction the cretaceous granitic province at the eastern margin of eurasian continent was at least partly formed by an episodic event such as ridge subduction

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

an experimental study of rill erosion and morphology abstract rill erosion is recognized as an important process of water erosion on agricultural land the objectives of this study are to examine the effects of rainfall intensity on rill network development and to present some indicators for a quantitative description of rill morphology a soil pan 10 m long 3 m wide and 0 5 m deep and with an adjustable slope gradient from 0 to 30 was subjected to three successive rains under rainfall intensities of 50 and 100 mm h 1 the results showed that rainfall intensity significantly affected rill erosion especially in the active period of rill network development the magnitude of rill erosion was 28 5 and 33 1 kg m 2 and contributed 78 6 and 76 2 to the soil loss under rainfall intensities of 50 and 100 mm h 1 respectively the formation of rill network under the 50 mm h 1 intensity was more complex than that under the 100 mm h 1 intensity for the latter rill networks developed fast and then varied slightly the mean rill inclination angle mean rill density degree of rill dissection and mean rill tortuosity complexity cmean increased with the increase of rains under the same rainfall intensity the value was the optimal derivative morphological indicator to estimate rill erosion and morphology which was followed in descending order by mean cmean and

k ar age and chemistry of phengite from the sanbagawa schists in the kanto mountains central japan and their implication for exhumation tectonics abstract epma analyses and k ar age determinations were carried out on phengite in pelitic schist from the sanbagawa metamorphic belt of the kanto mountains central japan phengite from the sanbagawa pelitic schist in the kanto mountains generally occurs as aggregates of fine grained crystals it is extremely fine grained in domains adjacent to relatively rigid garnet and albite porphyroblasts this suggests that deformation induced grain size reduction took place in phengite during the ductile deformation accompanying the exhumation of the host schists epma analysis shows that phengite is chemically heterogeneous at the thin section scale suggesting that it formed during retrograde metamorphism in restricted equilibrium domains the retrograde chemical reaction was promoted by the ductile deformation k ar ages of phengite get younger from the southern unit 82 ma to the northern unit 58 ma in the kanto mountains the age range is similar to that in central shikoku the older schists occur in the higher metamorphic grade zone in central shikoku and in the lower grade zone in the kanto mountains the thermal structures in central shikoku are inverted so that the highest grade zone occurs in the upper or middle parts of the apparent stratigraphic succession in contrast the kanto mountains have a normal thermal structure the higher grade zone is in the lower part of the apparent stratigraphic succession the different tectonic features in exhumation produced the two contrasting age temperature structure relations at the western side of sanbagawa belt in central shikoku and the eastern end of the sanbagawa belt in the kanto mountains that are 800 km distant from each other namely the western sanbagawa belt in central shikoku underwent longer ductile deformation during the exhumation than the eastern sanbagawa belt in the kanto mountains

age k ar phengite temperature structure relations a case study from the ishigaki high pressure schist belt southern ryukyu arc japan the ishigaki high pressure schist belt in the southern ryukyu arc is correlated with the suo high pressure schist belt in southwest japan the former metamorphic sequence is composed mainly of basic and pelitic schists and is subdivided into three zones the lower grade zone a the medium grade zone b and the high grade zone c based on the mineral assemblages of the basic schists the k ar phengite age gives 188 205 ma for zone a 196 206 ma for zone b and 208 220 ma for zone c while the apparent d 002 spacing of carbonaceous materials is 3 590 3 437 a 3 415 3 390 a and 3 387 3 364 a respectively the age d 002 relationships suggest that the ages become older with increasing metamorphic temperature this positive age temperature relationship in the ishigaki area contrasts with a negative relationship in the nishiki area in the suo belt the two areas also display a contrasting thermal structure with the former area having an inverted metamorphic gradient and the latter displaying a normal thermal structure these contrasting age temperature structure relationships in the metamorphic belt could be due to different tectonic styles relating to the exhumation of the metamorphic sequences we suggest that the ages obtained are related directly to the ductile deformation history of the matrix phengite below the closure temperature 500 c during exhumation of the host rocks the duration from the beginning of exhumation to the apparent resetting of the phengite k ar system was different between the two metamorphic sequences and significantly longer in the ishigaki than the nishiki

unpairing metamorphic belts p t paths and a tectonic model for the ryoke belt southwest japan southwest japan is divided into outer and inner zones by the median tectonic line mtl a major transcurrent fault the outer zone is composed of the sambagawa high pressure intermediate or high p t type metamorphism chichibu and shimanto belts in the inner zone the ryoke belt andalusite sillimanite or low p t type metamorphism was developed mainly within a jurassic accretionary complex this spatial relationship between high p t type and low p t type metamorphic belts led miyashiro to the idea that metamorphic belts were developed as paired systems textural relationships and petrogenetically significant mineral assemblages in pelites from the ryoke belt imply peak p t conditions of 5 kbar and up to 850 c in migmatitic garnet cordierite rocks from the highest grade metamorphic zone it is likely that the thermal anomaly responsible for metamorphism of the ryoke belt was related to a segment of the farallon izanagi ridge as it subducted under the eastern margin of the asian continent during the cretaceous the sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dp dt slope inferred to have been generated by a nested set of hairpin like clockwise p t paths these p t paths are characterized by limited prograde thickening minor decompression at peak t and near isobaric cooling features that may be typical of p t paths in low p t type metamorphic belts caused by ridge subduction a ridge subduction model for the ryoke belt implies that juxtaposition of the high p t metamorphic rocks of the sambagawa belt against it was a result of terrane amalgamation belt parallel ductile stretching recorded as syn metamorphic predominantly constrictional strain in both ryoke and sambagawa belt rocks and substantial sinistral displacement on the mtl are consistent with left lateral oblique convergence diachroneity in fast cooling of the ryoke belt is implied by extant thermochronological data and is inferred to relate to progressive sw to ne docking of the sambagawa belt thus an alternative interpretation of paired metamorphic belts in japan is that they represent laterally contemporaneous terranes rather than outboard and inboard components of a trench arc paired system amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of paired metamorphic belts in the geological record

post magmatic cooling of the elberton granite bearing on the late paleozoic tectonothermal history of the georgia inner piedmont the elberton granite is an undeformed pluton which intrudes polydeformed gneisses along the eastern margin of the georgia inner piedmont magmatic biotite and muscovite display internally concordant 40 ar 39 ar release spectra which record plateau dates of 235 241 m y biotite from adjacent gneisses and from recrystallized xenoliths yield similar plateau ages hornblende from country rock amphibolite and from recrystallized xenoliths record well defined 40 ar 39 ar plateau ages of 267 276 m y the 40 ar 39 ar ages are markedly younger than the 320 20 m y crystallization age of the pluton u pb zircon ross and bickford 1980 and are interpreted to date times of post magmatic cooling through temperatures required for argon retention in the various mineral species examined because similar ages are recorded by equivalent minerals from the country rock terrane post magmatic cooling must have closely followed post metamorphic cooling of the inner piedmont 40 ar 39 ar date

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

formation of distinct granitic magma batches by partial melting of hybrid lower crust in the izu arc collision zone central japan the miocene kofu granitic complex kgc occurs in the izu collision zone where the izu bonin mariana ibm arc has been colliding with the honshu arc since the middle miocene the kgc includes rocks ranging in compositions from biotite bearing granite the shosenkyo and mizugaki plutons and hornblende biotitebearing granodiorite tonalite quartz diorite and granite the shiodaira sanpo hirose and sasago plutons to hornblende bearing tonalite and trondhjemite the ashigawa tonogi pluton indicating that it was constructed from multiple intrusions of magma with different bulk chemistry the sr isotopic compositions corrected to sensitive high resolution ion microprobe shrimp zircon ages sri suggest that the primary magmas of each pluton were formed by anatexis of mixed lower crustal sources involving both juvenile basalt of the ibm arc and shimanto sedimentary rocks of the honshu arc after the primary magmas had formed the individual plutons evolved by crystal fractionation processes without significant crustal assimilation or additional mantle contribution shrimp zircon u pb ages in the kgc range from 16 8 to 10 6ma and overlap the resumption of magmatic activity in the ibmand honshu arcs at c 17ma and the onset of ibm arc honshu arc collision at c 15ma the age of the granite plutons is closely related to the episodic activity of arc magmatism and distinct granitic magma batches could be formed by lower crustal anatexis induced by intrusion of underplated mantle derived arc magmas based on pressures determined with the al in hornblende geobarometer the kgc magmas intruded into the middle crust thus the kgc could represent an example of the middle crust layer indicated throughout the ibm arc by 6 0 6 5 km s seismic velocities this granitic middle crust layer acted buoyantly during the ibm arc honshu arc collision leading to accretion of buoyant ibmarc middle crust to the honshu arc

discrete plumbing systems and heterogeneous magma sources of a 24 km3 off axis lava field on the western flank of east pacific rise 14 s the largest known mid ocean ridge off axis lava flow field occurs slightly off axis near 14 s along the east pacific rise epr it comprises at least four volcanologically discrete units we collected lava samples from 24 sites within the off axis lava field and 7 sites on the adjacent ridge crest using the shinkai 6500 submersible the lava field comprises at least three distinct morb compositions all of which are different from the lavas collected from the nearby ridge axis the east and west cones and the northern lobe of the lava field consist of normal morb n morb lavas with a low concentration of incompatible elements and low lile hfse and lree hree ratios by contrast the samples from the west plain of the field have a higher concentration of incompatible elements and higher lile hfse and lree hree ratios indicating t morb character the lava samples collected from the summit of the east cone show the highest concentration of lile elements and lrees among the lava field emorb lava the n morb of the off axis lava field are more depleted in incompatible elements than the adjacent epr axis lavas possibly reflecting the re melting of the residual mantle in the off axis region the e morb lava was probably derived from fertile mantle that did not undergo melting beneath the spreading center t morb which occupies the main part of the 14 s lava field is the product of magma mixing between n morb and e morb magmas 2007 elsevier b v all rights reserved

trace element composition and degree of partial melting of pelitic migmatites from the aoyama area ryoke metamorphic belt sw japan implications for the source region of tourmaline leucogranites abstract degree of partial melting of pelitic migmatites from the aoyama area ryoke metamorphic belt sw japan is determined utilizing whole rock trace element compositions the key samples used in this study were taken from the migmatite front of this area and have interboudin partitions filled with tourmaline bearing leucosome these samples are almost perfectly separated into leucosome melt and surrounding matrix solid this textural feature enables an estimate of the melting degree by a simple mass balance calculation giving the result of 5 11 wt of partial melting similar calculations applied to the migmatite samples which assume average migmatite compositions to be the residue solid fraction give degree of melt extraction of 12 14 wt from the migmatite zone the similarity of the estimated melting degree of 5 11 wt with that in other tourmaline leucogranites such as harney peak leucogranite and himalayan leucogranites in spite of differences in formation process implies that the production of tourmaline leucogranites is limited to low degrees of partial melting around 10 wt probably controlled by the breakdown of sink minerals for boron such as muscovite and tourmaline at a relatively early stage of partial melting because the amount of boron originally available in the pelitic source rock is limited on average 100 ppm 10 wt of melting locally requires almost complete breakdown of boron sink mineral s in the source rock in order to provide sufficient boron into the melt to saturate it in tourmaline this in turn means that boron depleted metapelite regions are important candidates for the source regions of tourmaline leucogranites

two dimensional modelling of the within field variation in rill and gully geometry and location related to topography abstract the topography rill pattern and geometry were surveyed within a small agricultural field the rill network and dimensions were converted to a grid based system and used to calculate the slope gradient and contributing area for each grid cell a strong correlation was found between the rill cross sections and a power function of slope gradient and contributing area the slope and area exponents were 1 28 and 0 89 respectively for all rill points and 1 02 and 0 87 respectively for the rill points not lying in the depositional area these exponents are within the range reported for field studies in a one dimensional context our data confirmed the potential of an inverse slope area relationship with an area exponent of 0 5 to predict areas prone to rilling and the location of ephemeral gullies however the prediction of individual rill starting points was not possible because of the major influence of small local variations in topography on overland flow discharge both the prediction of the rill cross sections and the prediction of the location of the two ephemeral gullies in the field were significantly improved when the tillage direction was taken into account in calculating the contributing area therefore tillage patterns should be given more attention when developing routing procedures for hydrological and erosion models

a 1 d morphodynamic model for rill erosion 1 the key objective of this paper is to develop a 1 d hydrodynamic and sediment transport model namely rill1d that handles transcritical flows over abrupt changes e g formation of pool crests and width change in a single rill and also predicts changes in rill bed elevation replication of rill conditions in terms of flow and bed evolution changes necessitated the use of an enhanced tvd maccormack scheme with implementation of tseng s surface gradient method to provide an oscillation free solution over formed pool crests the model at the end of each time step calculates the flow depth velocity and bed shear stress and provides changes in bed elevation and grain size distribution within a cell the hydrodynamic performance of rill1d is evaluated by comparing its results with either analytical solutions or experimental observations from various benchmark tests with rigid bed conditions that include 1 formation of a hydraulic jump in a rectangular channel 2 steady subcritical and transcritical flow without a shock over a hump 3 formation of a hydraulic jump in a converging diverging frictionless channel and 4 flows over step pools it is shown that the enhanced tvd maccormack scheme adequately simulates transcritical flows by preserving the mass conservation and reducing the artificial numerical error the scheme also approximates satisfactorily depth and velocity for a single rill with a fixed bed consisting of steps and pools except near the pool headwalls where a backroller forms with negative velocities the sediment component of rill1d is evaluated by two field investigations of single rills with mobile beds with the field data the code is tested for its ability to reproduce measured values of sediment transport rates a sensitivity analysis is performed to assess the effects of cell size and critical erosional strength in the predictive ability of the model rill1d performs reasonably well in these simulations in terms of sediment prediction rates and fared adequately in terms of replicating rill bed morphology

miocene clockwise rotation of southwest japan and formation of curvature of the median tectonic line paleomagnetic implications the origin of the curvature of the median tectonic line mtl in southwest japan is still controversial in order to investigate its formation we carried out a paleomagnetic study in the nohi area north of the curved part of the mtl in the eastern part of southwest japan more than 420 paleomagnetic samples were collected at 35 sites from cretaceous to paleogene welded tuffs in the kasagatake rhyolites oamamiyama group and nohi rhyolite characteristic directions with high unblocking temperature component above 560 c were isolated from 32 sites paleomagnetic directions of the kasagatake rhyolites and oamamiyama group 55 66 ma have a clockwise deflected declination d 51 7 i 52 6 95 8 6 which is consistent with the characteristic paleogene paleomagnetic direction of the central part of southwest japan a similarly deflected declination is also observed in the peripheral part of the nohi rhyolite region far away from the atera fault system paleomagnetic data indicate that the nohi area experienced a clockwise rotation through more than 45 with respect to the asian continent as a part of southwest japan but that the amount of rotation of the nohi area is 23 smaller than that of the central part of southwest japan a new model is developed to explain the tectonic rotation at about 15 ma during the latest stage of the clockwise rotation of southwest japan the eastern end moved southward by more than 60 km along a left lateral strike slip fault which comprised the present north south segment of the mtl and the akaishi tectonic line we conclude that the curvature of the mtl is a composite of the 15 ma strike slip fault system and a small bend of the mtl associated with the small differential rotation of the nohi area with respect to the central part of southwest japan

morphology mineralogy and chemistry of hydrothermal deposits from the tag area 26 n mid atlantic ridge abstract metalliferous hydrothermal deposits have been observed and sampled by submersible from the tag hydrothermal field on the mid atlantic ridge at 26 n the deposits include surficial metal rich staining on the surface of carbonate ooze deposits over considerable areas of the sea floor as well as discrete massive layered deposits the latter include hydrothermal precipitates of fe silicate nontronite mn oxide birnessite and fe oxide amorphous the deposits range in size from less than 1 m to 15 m 20 m the composition and morphology of individual deposits varies from thick laminated crystalline birnessite precipitates through fe rich tubular vents to loose earthy birnessite nontronite or amorphous fe oxide interlayered deposits fractionation and crystallinity are presumably controlled by local eh conditions and the hydrodynamics of circulation in the upper crust they are presumed to represent deposition from low temperature hydrothermal solutions which had originally reacted with basaltic oceanic crust at depth at high temperature and had precipitated metal rich sulfides within the crust occasional short duration high temperature black smoker type venting is inferred to have occurred based upon cu fe zn rich layers in the sediments and observations of clam shells in bottom photographs

an adakitic pluton on kyushu island southwest japan arc abstract an adakitic pluton is designated the shiraishino granodiorite ca 15 km2 in outcrop area on kyushu island southwestern japanese arc sr y and rare earth element concentrations and crystallization characteristics of the pluton are similar to those of adakites geochemical and tectonic considerations show that the pluton was derived from partial melting of a subducted slab with minor contamination of high 87sr 86sr material and subsequent fractional crystallization of plagioclase hornblende compiled sr and y data indicate that most cretaceous granitic rocks of southwestern japan generated by partial melting of mafic lower crust adakitic plutons derived by slab melting occur rarely and have older ages it is concluded that plutonism in the southwestern japanese arc during cretaceous time was began with the emplacement of small adakitic intrusions and progressed to more abandant granitic rocks of the type derived from the mafic lower crust

timing of rotational motion of southwest japan inferred from paleomagnetism abstract paleomagnetic field directions for the period younger than 35 ma are obtained from igneous rocks distributed in the san in district inner zone of southwest japan the remanent magnetization of samples between 30 and 35 ma old are fairly well grouped with a mean direction of d 65 9 i 48 6 and 95 6 5 this result establishes that southwest japan rotated clockwise 56 12 during the past 30 m y a declination value of about 60 is observed in the rocks of 28 ma d 52 2 i 33 5 and 21 ma d 69 9 i 49 5 comparing this with results from dacitic rocks with an age of 15 ma in other areas of southwest japan suggests that rotational motion did not occur possibly until 15 ma these results require that the rotation of southwest japan occurred just after the shikoku basin had been created

detrital zircon u pb geochronology and stratigraphy of the cretaceous sanjiang basin in ne china provenance record of an abrupt tectonic switch in the mode and nature of the ne asian continental margin evolution abstract the age spectra obtained from 505 spots of detrital zircon u pb ages of five representative sandstone samples from the sanjiang basin in ne china point to a significant change in its provenance during the coniacian santonian the predominant detrital source for the sanjiang basin during the early cretaceous was the zhangguangcai range magmatic belt and jiamusi block along its western and southern periphery whereas it changed in the late cretaceous to its eastern periphery the timing of these inferred changes in the detrital source regions and drainage patterns nearly coincide with the age of a regional unconformity in and across the basin the time interval of non deposition and unconformity development was coeval with a transitional period between an extensional tectonic regime in the early cretaceous and a contractional deformation episode in the late cretaceous the sanjiang basin evolved during this time window from a backarc to a foreland basin the migration of the coastal orogenic belt and the fold and thrust belt development farther inland during the late cretaceous marked the onset of regional scale shortening and surface uplift in the upper plate of a flat or very shallow dipping subduction zone the stratigraphic record the detrital source and geochronology of the basinal strata and the internal structure of the sanjiang basin present therefore an important record of a tectonic switch in the nature of continental margin evolution of northeast asia during the late mesozoic

thermal effects of ridge subduction and its implications for the origin of granitic batholith and paired metamorphic belts distribution of temperature and h2o in subduction zones during the course of ridge subduction are investigated numerically in order to constrain the timing and extent of metamorphism and melting within the subducting oceanic crust and the overlying arc crust the model assumes that after ridge subduction the two plates move together without creating a slab window the results suggest that before ridge subduction temperature along the subducting slab starts to rise when the slab is younger than 10 myr a drastic thermal impact occurs associated with subduction of the ridge crest followed by a diffusion stage of the impact the temperature elevation is limited within a few tens of kilometers from the slab crust interface and within 30 myr after ridge subduction although it will last for a longer period if a slab window is formed during the peak stage slab melting occurs and generates a significant amount of granitic melt mainly at depths shallower than 40 km at p t conditions corresponding to low p t metamorphism high p t metamorphism with a significant water flux is also achieved near the interface when a subducting slab is less than 10 myr old before ridge subduction or 20 myr old after ridge subduction consequently the low and high p t metamorphism accompanied by a significant amount of granitic melt is associated closely in time and space with the ridge subduction this model can explain the observed characteristics of the granitic batholith and the paired metamorphic belts through these processes arcs and continents can grow within the short period of time associated with the episodic subduction of ridges

prehistoric and roman gullying in the european oess belt a case study from central belgium in contrast with the understanding of present day soil erosion processes knowledge on past soil erosion phenomena is still rather limited although some studies report on severe gully erosion phases during the fourteenth and eighteenth centuries almost no evidence is available that documents earlier gully erosion phases this study investigates the development and age of two old permanent gullies that are conserved in the ancient meerdaal forest in central belgium the development history of both gullies is very similar in the first gully archaeological evidence was found indicating an erosion phase during roman times followed by a partial infilling of the gully in the second gully radiocarbon dating provided evidence of the same roman activity phase cal yr 46 bc ad 78 but also of an earlier incision phase during the middle bronze age cal yr 1743 1602 1568 1533 bc also here the erosion phase was followed by a partial infilling this limited infilling indicates that the catchment of the gullies was reforested quite rapidly hereby cutting off all runoff and sediment production this has led to a unique situation in the meerdaal forest with the conservation of about 43 similar large gullies in an area of about 17 km 2 this area has a high geovalue as the studied gullies are among the oldest and best conserved gullies in northwestern europe

hybridization of a shallow i type granitoid pluton and its host migmatite by magma chamber wall collapse the tokuwa pluton central japan the miocene tokuwa pluton of i type granitoid affinity was emplaced discordantly into a cretaceous to paleogene accretionary complex and induced a contact aureole in which various thermally metamorphosed rocks were developed including hornfels metatexite diatexite and cordierite bearing tonalite crd tonalite of s type granite affinity the thermally metamorphosed rocks show low pressure reaction textures culminating in partial melting peak p t conditions of 3 kbar at 7808c are estimated on the basis of the twq thermobarometer for the garnet bearing rocks the rocks in the contact aureole exhibit a gradual transition from hornfels through metatexite and diatexite to crd tonalite the sr isotopic composition at the time of tokuwa pluton emplacement at 12 ma decreases systematically from metatexite 0 7100 0 7112 through diatexite 0 7078 0 7094 to crd tonalite 0 7067 0 7068 this trend is interpreted in terms of mixing between the tokuwa magma and the aureole migmatites the field relationships geochemical data and isotopic data collectively suggest that the emplacement of the tokuwa pluton triggered partial melting of the surrounding metasedimentary rocks subsequent hybridization of the tokuwa magma with the metatexite in variable proportions produced the crd tonalite and diatexite the hybridization was caused by invasion of the tokuwa magma into the migmatite zone accompanied by gravitational collapse of the previously crystallized wall of the magma chamber the data presented demonstrate that even a relatively low temperature shallow i type granitoid pluton can induce contact anatexis and hybrid s type granitoid formation at the intrusive contact

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

high pmetamorphism pattern of induced flow in the mantle wedge and the link with plutonism in paired metamorphic belts the convex form of subduction stage pressure temperature p t paths up to c 2 0 gpa implies the sambagawa high p metamorphic belt japan formed a few million years before ridge subduction additional compilation of p t conditions for higher p sambagawa rocks c 2 0 2 5 gpa reveals that the thermal profile along the slab surface shows a remarkable high t ward warping at c 2 0 gpa c 65 km previous thermal models indicate that this warping corresponds to the onset of induced mantle flow towards the subducting slab if a normal thickness continental crust of c 30 km was present this implies the hangingwall region between 30 and 65 km depth was occupied by serpentinized wedge mantle isolated from large scale mantle flow subsequent arrival of the spreading ridge reheating and dehydration of the serpentinized wedge probably supplied the water necessary for causing granitic magmatism in the ryoke high t metamorphic belt which is paired with the sambagawa belt

paleomagnetism of triassic and jurassic red bedded chert of the inuyama area central japan a paleomagnetic study was conducted on triassic and jurassic red bedded chert from the inuyama area central japan paleomagnetic specimens were collected from single chert beds at nine sites within mesoscopic 1 5 m folds whose ages were determined by radiolarian fossils progressive thermal demagnetization experiments were conducted and three of the four magnetization components provided tectonic implications component d unblocking temperature of 560 680 c from six sites passes the fold test with the mesoscopic folds paleolatitude was estimated as 12 3 15 6 n or s 19 0 10 5 n or s and 25 7 16 7 n or s for 237 ma 223 5 ma and 210 ma respectively in the case of northern latitude a northward migration of 5 6 9 cm yr is estimated between 237 ma and 210 ma with no significant latitudinal migration from 210 ma to 160 ma component c unblocking temperature of 350 530 c has normal polarity and is thought to have been acquired as a chemical remanent magnetization carried by magnetite which was acquired as synfolding magnetization with the sakahogi synform the magnetization is considered to have been acquired through heating in the accretionary wedge during the cretaceous normal superchron 120 5 83 5 ma component b unblocking temperature of 200 350 c has reversed polarity and is thought to have been acquired as a thermoviscous remanent magnetization after the bending of the sakahogi synform the most probable period of the magnetization acquisition is between c27r and c24r 58 63 ma after the emplacement of late cretaceous granitic batholiths

the chemistry and mineralogy of ferromanganese encrustations on rocks from the sierra leone rise equatorial mid atlantic ridge and new england seamount chain abstract chemical and mineralogical compositions of ferromanganese oxide coatings on rocks dredged from the new england seamounts the sierra leone rise and the mid atlantic ridge near the equator have been determined in an investigation of regional differences in atlantic ferromanganese deposits most encrustations are clearly of hydrogenous origin consisting mainly of todorokite and mno 2 but several recovered from the equatorial fracture zones may be hydrothermal accumulations mn fe ratios are similar to those observed in nodules and encrustations elsewhere in the atlantic generally falling below unity except for samples from the new england seamounts cu 0 02 0 2 is lower than in other atlantic deposits ni contents vary from 0 064 to 0 478 the latter value being found near the st paul s fracture zone a large difference in co is observed between the northwest and equatorial atlantic 0 048 0 135 new england seamounts 0 355 1 010 near the equator ferromanganese coatings on phosphatized limestones from the sierra leone rise being richest in co differences in the chemistry of the water column and in growth rates of the ferromanganese coatings may be important in producing this regional contrast in composition fine scale changes in element abundances within the encrustations indicate that the nature of the substrate has little influence on compositional variations

progressive sequence of reactions of the ryoke metamorphism in the yanai district southwest japan the formation of cordierite the compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low to medium grade pelitic rocks of the ryoke metamorphism in the yanai district southwest japan the biotite and muscovite that coexist with k feldspar have a higher k component in an a kf diagram than those in rocks lacking k feldspar this fact reflects an increase in the k2o content in muscovite but in biotite it reflects an increase of not only the k2o content but also of the octahedral vacancy at higher metamorphic grade beyond the cordierite isograd where cordierite coexists with neither chlorite nor k feldspar the biotite shows an increase in illite k aliv xii 1 si 1 and tschermak components alvi aliv r 1 si 1 where xii and r denote the interlayer vacancy and fe mg mn respectively a reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite i e k al poor biotite phengitic muscovite k al rich biotite cordierite quartz water by treating this as a key reaction in medium grade metamorphism a set of reaction in a progressive metamorphism is established for the ryoke metamorphism a typical low pressure type metamorphism some textures in one of the high grade areas the k feldspar cordierite zone suggest that a further two prograde reactions have taken place i e andalusite biotite quartz cordierite k feldspar water and andalusite sillimanite quartz cordierite k feldspar water this implies that this zone probably has a p t path involving isobaric heating

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

emplacement ages geochemical and sr nd hf isotopic characterization of mesozoic to early cenozoic granitoids of the sikhote alin orogenic belt russian far east crustal growth and regional tectonic evolution abstract the sikhote alin range of the russian far east is an important accretionary orogen of the western pacific orogenic belt in order to study the formation and tectonic evolution of the orogen we performed zircon u pb dating as well as geochemical and sr nd hf isotopic analyses on 24 granitoid samples from various massifs in the primorye and khabarovsk regions the zircon dating revealed that the granitoids were emplaced from 131 to 56 ma cretaceous to paleogene in the primorye region granitoids in the coastal sikhote alin intruded the cretaceous taukha accretionary terrane from ca 90 to 56 ma whereas those along the central sikhote alin fault zone intruded the jurassic samarka accretionary terrane during ca 110 75 ma the oldest monzogranite 131 ma was emplaced in the lermontovka area of the nw primorye region granitoid massifs along the central sikhote alin fault zone in the khabarovsk region formed from 109 to 58 ma thus the most important tectonothermal events in the sikhote alin orogen took place in the cretaceous geochemical analysis indicates that most samples are i type granitoids they have initial 87sr 86sr ratios ranging from 0 7040 to 0 7083 and initial nd isotopic ratios expressed as end t values from 3 0 to 5 0 mostly 0 to 5 the data suggest that the granitoid magmas were generated by partial melting of sources with mixed lithologies including the subducted accretionary complex hidden paleozoic proterozoic basement rocks based on whole rock nd isotopic data we estimated variable proportions 36 77 of juvenile component mantle derived basaltic rocks in the generation of the granitic magmas furthermore zircon hf isotopic data ehf t 0 to 15 indicate that the zircon grains crystallized from melts of mixed sources and that crustal assimilation occurred during magmatic differentiation the quasi continuous magmatism in the sikhote alin orogen suggests that the paleo pacific plate subduction was very active in the late cretaceous the apparently regular progression of granitic intrusion ages from 80 to 56 ma in the taukha terrane may reflect oblique underflow of the paleo pacific plate beneath the eurasian continental margin subduction was not only manifested by granitic intrusion but also by abundant silicic volcanism the late cretaceous paleo pacific plate motion probably changed from parallel or sub parallel to oblique relative to the continental margin of the sikhote alin leading to the change of magmatic source region and geochemical characteristics of the derived igneous rocks late cretaceous rapid sea floor spreading at ca 100 ma induced highly active subduction and led to voluminous magmatism in the entire circum pacific realm finally the present age and isotopic study lends support to the hypothesis of geologic and tectonic correlation between sikhote alin and sw japan

the thermal significance of potassium feldspar k ar ages inferred from age spectrum results 40ar39ar age spectrum analyses of three microcline separates from the separation point batholith northwest nelson new zealand which cooled slowly 5 c ma 1 through the temperature zone of partial radiogenic 40ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages 80 ma corresponding to the time that the samples cooled below about 100 c the last 35 percent of 39ar released from the microclines yields plateau ages 103 99 and 93 ma which reflect the different bulk mineral ages and correspond to cooling temperatures between about 130 to 160 c theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates 5 c ma 1 diffusion parameters calculated from the 39ar release yield an activation energy e 28 8 1 9 kcal mol 1 and a frequency factor grain size parameter d0l2 5 6 3 9 14sec 1 this arrhenius relationship corresponds to a closure temperature of 132 13 c which is very similar to the independently estimated temperature from the observed diffusion compensation correlation this d0l2 implies an average diffusion half width of about 3 m similar to the half width of the perthite lamellae in the feldspars the range in microcline k ar ages from the separation point batholith is the result of relatively small temperature differences within the pluton during cooling comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous k feldspars e 40 to 52 kcal mol 1 reveals that ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state previously published u shaped age spectra are probably the result of the superimposition of excess 40ar upon diffusion profiles of the kind described here

the latest cretaceous early paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern pacific margin episodic growth of accretionary complexes along the northwestern pacific margin is well documented from detailed biostratigraphy and radiometric dating of metamorphic rocks the latest cretaceous to early paleogene rapid growth of the accretionary complexes observed in hokkaido and sakhalin islands was caused by the supply of a large amount of terrigenous sediments from an intracontinent collision zone the mongolia okhotsk zone in the northern asian continent rapid growth of the accretionary prism resulted in episodic tectonic events in the forearc and magmatic arc rapid underplating caused uplift and exhumation of previously accreted complexes along with high pressure series metamorphic rocks now exposed as the kamuikotan complex in hokkaido and the susunai complex in sakhalin the forearc basin shallowed as a result of the uplift the uplift collapsed the old accretionary complex and supplied olistoliths onto the forearc region magmatic activity along the continental margin was also active at the same time subduction of a large amount of terrigenous sediments into the mantle may have contributed to active magmatism at that time a large supply of water trapped in the subducted sediments entered the mantle and increased partial melting of the mantle wedge

geothermometry from 40ar39ar dating experiments abstract we have selected a variety of minerals from three basic intrusions within the high grade metamorphic terrane near haliburton ontario for k ar dating by application of 40 ar 39 ar incremental heating experiments we conclude that the discordant k ar mineral dates can best be explained by a slow cooling history from the stepwise heating experiments we calculated isotopic closure temperatures using dodson s theory for slow cooling and from these constructed a cooling curve this curve was then compared to independent constraints to test the overall validity of our procedures and assumptions the results are geologically reasonable insofar as 1 a the maximum calculated temperatures from hornblendes of 700 c are consistent with independent approximate metamorphic grid arguments 2 b the ar closure temperatures for biotites 350 400 c are consistent with independent evidence and 3 c minerals with older k ar dates have higher calculated closure temperatures the resultant geological cooling curve implies that a maximum temperature of 700 c was reached before 1000 ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200 c by 700 ma ago we have discovered that plutonic plagioclases are very sensitive indicators comparable to k felds pars of ar loss and therefore can usefully extend the range of application of the 40 ar 39 ar stepwise heating method we also describe experiments in which we have tested the effects on the ar systematics of acid leaching of plagioclases

ephemeral stream processes implications for studies of quaternary valley fills abstract three unstable ephemeral stream channels arroyos which drain source areas that have high sediment yields ranging from predominantly sand arroyo calabasas to a mixture of sand silt and clay sand creek to largely silt and clay sage creek were resurveyed to provide data on the rates and mechanics of erosion and sedimentation processes during periods ranging from 14 to 22 yr channel morphology changed significantly erosion occurred through nickpoint recession and bank collapse but erosional reaches are separated by aggrading or stable channel reaches in general sediment that is eroded as the nickpoint recedes upstream is trapped in the widened channel downstream in this manner sediment is transported episodically out of these basins during a series of cut and fill cycles the manner by which the channels aggrade and the morphology of the aggraded stable channels are controlled by the sediment type the wide and shallow channel of arroyo calabasas is filled by vertical accretion of sand size sediment the narrow and deep channels of sage creek and sand creek are created by the lateral accretion of cohesive fine grained sediment the channel modification and the cut and fill episodes are dependent on high sediment yields and therefore they are independent of subtle climatic shifts cut and fill deposits that have been created in this manner should not be equivalent in age from basin to basin and therefore channel trenching and filling in the semiarid western united states during the holocene need not be synchronous

soil erosion in the swiss midlands results of a 10 year field survey abstract long term field monitoring of soil erosion by water was conducted on arable land in the swiss midlands all visible erosion features in 203 fields were continuously mapped and quantified over 10 years the eroded soil volume associated with linear erosion features was calculated by measuring the length and cross sectional area in rills at representative positions and the extent of interrill erosion was estimated averaged across the 10 study years just under one third 32 2 of the fields exhibited erosion with 0 75 t ha 1 yr 1 mean and 0 56 t ha 1 yr 1 median the average annual soil loss of the region was relatively small the year to year variation in soil loss of the region was great and ranged from 0 16 to 1 83 t ha 1 yr 1 the maximum annual soil erosion in a single field was 96 t yr 1 or 58 t ha 1 yr 1 thus demonstrating that only a few erosion events on a few fields may decisively contribute to the total extent of soil erosion in a region linear and interrill erosion accounted for 75 and 25 of total soil loss respectively wheel tracks furrows headlands and slope depressions were important on site accelerators of erosion run on from adjacent upslope areas was an important trigger of erosion of the soil moved by erosion 52 was deposited within the field of origin a high proportion 72 of the linear erosion features caused off site damage part of the total eroded soil 20 was transported into water thereby contributing to their contamination the long term field assessment of soil erosion helps to fill existing knowledge gaps concerning temporal and spatial variability of soil erosion on arable land the extent and severity of soil erosion and its sources and causes as well as subsequent off site damage

meltwater erosion process of frozen soil as affected by thawed depth under concentrated flow in high altitude and cold regions changes in thawed depth of frozen soil caused by diurnal and seasonal temperature fluctuations are commonly found in high altitude and latitude regions of the world this is of great importance in terms of its impact on hydrologic and erosion processes there is a need for experimental data to improve our knowledge and modeling of the phenomenon laboratory experiments were conducted in beijing to assess the impacts of thawed depth slope gradient and flow rate on soil erosion by concentrated meltwater flow over underlying frozen soil layer flumes were filled with soil collected from watershed deposited sediments before being saturated and frozen after the soil was completely frozen flumes were taken out of storage to thaw the frozen soil from top to designed depths meltwater was simulated with a tank filled with ice cubes to supply water flow at approximately 0 c the erosion experiments involved four thawed depths of 1 2 5 and 10 cm three slope gradients of 5 10 and 15 and three flow rates of 1 2 and 4 l min with seven rill lengths of 0 5 1 2 3 4 5 and 6 m sediment laden water samples were collected at the lower end of the flume for determination of sediment concentration which showed that it increased exponentially with rill length to approach a maximum value the sediment concentrations were closely correlated with thawed soil depth flow rate and slope gradient shallower thawed depths produced more sediments than thick thawed depths however the effect of flow rate on sediment concentration was not as great as slope gradient which declined with increases in slope gradient results from these experiments are useful for understanding the effect of thawed depth on erosion process in thawed soils subject to freezing and for estimating erosion model parameters

argon retentivity of hornblendes a field experiment in a slowly cooled metamorphic terrane hornblende from samples of amphibolite and granitic gneiss collected within a single outcrop in the central adirondacks yield significantly different sup 40 ar sup 39 ark sub k dates of 948 plus minus 5 and 907 plus minus 5 ma assuming that this terrane cooled slowly following high grade metamorphism and that the samples have experienced the same thermal history the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples the fe fe mg mn of the hornblende samples are 0 8 and 0 6 the higher ratio corresponding to the younger sup 40 ar sup 39 ar sub k date transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution but contain zones of fibrous phyllosilicates approximately 0 1 to 2 mu m wide parallelling 100 and 110 these alteration zones probably formed during post metamorphic cooling as a result of the migration of fluids through the hornblendes and are obvious pathways for argon escape from hornblende as these features are more abundant in the hornblende sample with the younger sup 40 ar sup 39 ar date and higher fe fe mg mn they may influence the argon blocking temperature by effectively partitioning the hornblende grains intomore diffusion domains of varying size biotite from the granitic gneiss yields an sup 40 ar sup 39 ar sub k date of 853 plus minus 2 ma with a mildly discordant step heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite plagioclase from the amphibolite yields a sup 40 ar sup 39 ar sub k integrated date of 734 plus minus 3 ma less

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

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

boron in metapelites controlled by the breakdown of tourmaline and retrograde formation of borosilicates in the yanai area ryoke metamorphic belt sw japan tourmaline out isograd formed by the breakdown of tourmaline is defined in the upper amphibolite facies metapelites in the yanai area ryoke metamorphic belt sw japan the rim composition of tourmaline progressively becomes aluminous with ascending metamorphic grade and the chemical zoning of tourmaline is controlled by x alna 1mg 1 and mgtiyal 2 vectors in low to medium grade zones where muscovite is stable whereas it is controlled by mg oh yal 1o 1 camgox 1 yal 1 oh 1 and mgtiyal 2 vectors in further higher grade muscovite unstable zones the size of tourmaline increases drastically where breakdown of muscovite quartz takes place probably due to the growth of tourmaline during breakdown of muscovite on the high temperature side of the tourmaline out isograd depletion of whole rock boron is observed escape of boron bearing melt or the fluid evolved from the melt during its crystallization probably caused this depletion although locally trapped boron bearing melt or fluid formed irregularly shaped tourmaline and dumortierite during retrograde metamorphism

the ryoke plutonometamorphic belt crustal section of the cretaceous eurasian continental margin abstract the ryoke belt comprising low pressure regional metamorphic rocks and low level granitoids expresses a deep in the crust of the cretaceous eurasian continental margin before the opening of japan sea the petrological p t estimation of the metamorphic rocks indicates the condition of upper to middle crust level the zonally arranged areal distribution of the san yo shallow facies to ryoke deep facies granitic provinces from back arc side to fore arc side is concordant with the occurrence of the associated low grade to high grade regional metamorphic rocks they are regarded as an upper to middle crustal section of the cretaceous eurasian continental margin these metamorphic rocks are derived mainly from the jurassic accretionary complex in the migmatitic high grade part the metasedimentary rocks were just being transformed into continental crust isotopic ages of these metamorphic and granitic rocks show an along arc variation of the systematic eastward younging from 100 ma to 70 ma for 800 km it cannot be interpreted with the steady state subduction model and indicates that the orogeny was caused by an episodic event presumably the subduction of kula pacific ridge beneath the eurasian continent during cretaceous the present juxtaposition of the ryoke and the sanbagawa paired metamorphic belts inevitably needs a secondary displacement from their birthplace two possibilities should be examined along arc transcurrent movement or across arc overthrusting

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

ferromanganese crusts as indicators for paleoceanographic events in the ne atlantic hydrogenetic ferromanganese crusts reflect the chemical conditions of the sewater from which they formed fine scale geochemical analysis of crust layers in combination with age determinations can therefore be used to investigate paleoceanographic changes which are recorded in geochemical gradients in the crusts at tropic seamount off northwest africa uniform crust growth influenced by terrigenous input from the african continent occurred during approximately the past 12 ma phosphatization of these crusts is minor in contrast crusts from lion seamount located between madeira and the portuguese coast display a much more variable growth history a pronounced increase in ni cu and zn is observed in some intervals of the crusts which probably reflects increased surface productivity a thick older phosphatized generation occurs in many samples hydrographic profiles indicate that mediterranean outflow water mow may play an important role in the composition of these crusts 10be dating of one sample confirms that the interruption of the mow during the messinian salinity crisis 6 2 5 ma ago resulted in changes in element composition sr isotope dating of the apatite phase of the old crust generation has been carried out to obtain a minimum age for the older generation of atlantic crusts and to determine whether crust phosphatization in the atlantic can be related to phosphatization episodes recorded in pacific crusts the preliminary data show that the old phosphatized crust generation might be as old as approximately 30 40 ma

tourmaline breakdown in the migmatite zone of the ryoke metamorphic belt sw japan a sharp line delimitating the distribution of tourmaline termed as a tourmaline out isograd is defined in the migmatite zone of the ryoke metamorphic belt japan the trend of the tourmaline out isograd closely matches that of the isograds formed through the regional metamorphism suggesting that it represents the breakdown front of tourmaline during regional metamorphism this is confirmed by the presence of the reaction textures of tourmaline to sillimanite and cordierite near the tourmaline out isograd the breakdown of tourmaline would release boron into associated melts or fluids and be an important factor in controlling the behaviour of boron in tourmaline bearing high temperature metamorphic rocks near the tourmaline out isograd large tourmaline crystals occur in the centre of interboudin partitions containing leucosome in the melanosome of the intervening matrix reaction textures involving tourmaline are locally observed these observations imply that tourmaline breakdown is related to a melting reaction and that the boron in the leucosome is derived from the breakdown of tourmaline in the melanosome during prograde metamorphism boron released by tourmaline breakdown lowers both the solidus temperature of the rock and the viscosity of any associated melt considering that the tourmaline out isograd lies close to the schist migmatite boundary these effects might have enhanced melt generation and segregation in the migmatite zone of the ryoke belt the evidence for the breakdown of tourmaline and the almost complete absence of any borosilicates throughout the migmatite zone suggest that boron was effectively removed from this region by the movement of melt and or fluid this implies that the tourmaline out isograd can reflect a significant amount of mass transfer in the anatectic zones

rbsr dating of acidic rocks from the middle part of the inner zone of southwest japan tectonic implications for the migration of the cretaceous to paleogene igneous activity abstract rb sr age determinations by mineral and whole rock isochron methods have been performed on cretaceous to paleogene granitic and rhyolitic rocks from the middle part of the inner zone of southwest japan in order to examine some chronological problems and tectonic models for the migration of igneous activity our chronological results include 1 isotopic homogenization of granitic and rhyolitic rocks in the miyazu izushi area was disturbed by mixing of acidic and basic magmas 2 the rokko granite was emplaced at 77 8 ma and reheated at 72 ma 3 mineral isochron ages obtained for the acidic volcanic rocks range from 62 6 to 85 8 ma and are considered to date the time of eruption igneous activity migrated eastward from 95 ma to 70 ma which may be related to a similar eastward migration of the development of sedimentary basins in fore arc regions the observed eastward migration has been discussed with relevance to a ridge subduction model in which the heated position by subducted ridge moved from west to east although the cause for the migration of the igneous activity is poorly understood several difficulties with the simple ridge subduction model are pointed out

tethyan ferromanganese oxide deposits from jurassic rocks in hungary fossil manganese nodules and encrustations from seamount and basin localities in the transdanubian central mountains of hungary are lithologically mineralogically and chemically similar to some modern marine ferromanganese oxide deposits and show no evidence of postdepositional changes other than cementation five groups of deposits were encountered fe mn nodules encrusted shells pavements stains and fe oxide encrusted intraclasts the first three of which are specific to the 9seamount9 environment and the last to the basins optical and electron microprobe investigation of the samples shows them to exhibit many similarities with modern ferromanganese oxide deposits and that many of the nodules are surrounded by a halo of dispersed ferromanganese oxides strongly suggesting that they continued to accrete metals through the pore waters of unlithified sediments for a period after burial by contrast pavements which appear to have grown on hardgrounds at the sea floor show little or no evidence of derivation of metals from underlying sediments geochemical investigations on the deposits show the seamount varieties to be closer in composition to most modern nodules and crusts than the basin varieties and that the latter are essentially manganese and trace element poor ferruginous deposits nevertheless all can be more or less compositionally equated with modern ferromanganese oxide deposits forming in marginal atlantic environments which would be in accord with the proposed depositional environment of the transdanubian central mountains based on other evidence

the success of recent land management efforts to reduce soil erosion in northern france abstract soil erosion is an important problem in open field agricultural landscapes with almost no permanent vegetation in small headwater catchments and with few physical obstacles to reduce runoff velocities runoff concentration along linear landscape elements plot boundaries or thalwegs frequently causes ephemeral gullies to form the latter reflecting the poor hydrogeomorphic condition of the land and soilscape to address this problem and to remediate negative on and off site effects land management efforts have multiplied over the past decades in many regions this includes amongst other measures the implementation of vegetation barriers called fascines in the loess dominated aa river basin of northern france where cropland accounts for 67 of the cover we investigated the effect of fascines on ephemeral gully erosion dynamics together with rainfall characteristics and cropland management this was accomplished through a spatially explicit study of 269 sites prone to ephemeral gullying using a diachronic analysis of historical aerial photographs between 1947 and 2012 ephemeral gully densities at the scale of the aa river basin 643 km2 varied between 0 39 and 1 31 m ha 1 long term average of 0 68 m ha 1 with local maxima up to 9 35 m ha 1 densities are however much higher when only considering the most erosion vulnerable municipalities long term average of 2 23 4 30 m ha 1 those values should be used when comparing results from this study to other reports of ephemeral gully erosion fascines were introduced in 2001 and were present in 30 of the gully erosion sites by 2012 although the presence of fascines has an effect on gully length reduction spatial and temporal variations in gully length were mainly driven by cumulative precipitation measurement of sediment deposition at 29 fascines in 2016 showed that only 47 of the fascines functioned as sediment sinks they stored on average 1 7 mg of sediment per winter half year corresponding to 0 009 mg ha 1 the results suggest that fascines positively impact the landscape s resilience and reduce ephemeral gully erosion rates the use of vegetation barriers such as fascines are increasingly implemented for erosion control in western europe but pose problems for the management of open field landscapes

migration of igneous activities related to ridge subduction in southwest japan and the east asian continental margin from the mesozoic to the paleogene abstract in the inner zone of southwest japan the northern part of the median tectonic line mtl granites and their volcanic equivalents were formed in large quantities during the late cretaceous to early tertiary migration of magmatic activity related to ridge subduction is proposed to explain the widely distributed magmatism and its tectonics for this purpose the ages of granitic rocks were carefully inspected according to the criteria of the dating methods and procedures concerned and compiled with the sampling localities they clearly show a trend in rock ages along the mtl in agreement with the general direction of strikes in southwest japan from the trend of the ages and the cooling histories of the bodies it was concluded that the magmatism migrated eastward along the mtl with a rate of 30 km m y this migration concept is extended to the mesozoic igneous rocks of the east asian continental margin there two magmatic belts ranging in age from mesozoic to paleogene are found to become younger northeastward along the margins which is ascribed to the subduction of the farallon izanagi and kula pacific ridges

high mg diorites derived from sanukitic hma magmas kyushu island southwest japan arc evidence from clinopyroxene and whole rock compositions abstract high mg diorites that have similar whole rock composition to high mg andesites hmas should not be simply interpreted as rocks solidified from the hma magmas because the high mg diorites may be mafic cumulates derived from a different magma from the hmas the hmas contain unique clinopyroxenes with higher mg and si than those of other sub alkaline series igneous rocks the mg and si are controlled by the source magma composition rather than its crystallized conditions such as pressure and temperature the chemical composition of clinopyroxenes would present important information for the investigation of the source of high mg diorites we considered the source of early cretaceous high mg diorites on kyushu island southwest japan arc based on their clinopyroxene and whole rock compositions the clinopyroxenes have similar chemical characteristics to those in hmas rather than those in other sub alkaline rocks moreover the whole rock compositions are equivalent to the sanukitic hma and do not show features of mafic cumulates this indicates that the high mg diorites solidified from sanukitic hma magmas it is generally believed that the sanukitic hma magmas involve the subduction of a young and or hot oceanic slab was situated in their genesis therefore the occurrence of the high mg diorites suggests that kyushu was situated in the tectonic setting of young and or hot slab subduction in the early cretaceous

rill and gully development during an extreme winter runoff event in norway erosion on agricultural fields in norway has earlier been reported to be mainly sheet and minor rill erosion exceptional rainfall events during january and february 230 of 30 years monthly mean in 1990 led to extreme runoff and widespread erosion also on areas normally not considered to have a high erosion risk this study presents the results of a field survey in three counties where rills and ephemeral gullies were measured after this event shifting weather conditions with freezing and thawing had given frozen subsoil topsoil conditions varied from ice and snow covered surface to thawed surface with frozen subsoil in one of the areas with a high clay content only the upper topsoil layer was unfrozen during this event extended sheet erosion and smaller rills occurred in the southernmost locations more of the topsoil profile was unfrozen combined with a high silt sand content more severe rilling and ephemeral gullies developed gullies developed down to the depth of the drainpipes in all locations this erosion equals soil losses of more than 100 tons ha 1 or 8 9 mm soil lost from the entire field area different mechanisms for gully development were registered gullies developed as enlarged rills with headward migration and sidewall sloughing knickpoints in the slope could also lead to gully development concentrated water flow entering the field from farmyards roads neighbouring fields and woods could be the starting point for uncontrolled rilling and ephemeral gullying the combination of frozen subsoil saturated soil with low strength and intense rainfall led to gully development also on areas with gentle slopes especially sandy soils management practices like crop cover tillage and lack of surface water control highly influenced the development of gullies

radiolarian contourites record eocene aabw circulation in the equatorial east atlantic abstract 50 m of middle eocene pure radiolarian ooze were drilled at odp site 660 in the equatorial east atlantic 80 km northeast of the kane gap the oozes comprise a 10 m high and 2 km broad mound of seismic reverberations covered by manganese rich sediment and contain trace amounts of sponge spicules and diatoms negligible organic carbon 0 15 clay and variable amounts of pyrite the yellow to pale brown silty sediments are relatively coarse grained 30 45 coarser than 6 m little bioturbated and commonly massive or laminated on a cm scale the unlithified radiolarian ooze may indicate an interval of high oceanic productivity probably linked to a palaeoposition of site 660 close to the equatorial upwelling belt during middle eocene time the absence of organic matter however and both the laminated bedding and the mound like structure of the deposit on the lower slope of a continental rise indicate deposition by relatively intense contour currents of oxygen rich deep water which passed through the kane gap winnowed the fine clay fraction and prevented the preservation of organic carbon the ooze may be either a contourite lag deposit or a contourite accumulation of displaced radiolarians originating south of the kane gap and being deposited in its northern lee thus documenting the passage of a strong cross equatorial bottom water current formed near antarctica these eocene contourites may be an analogue for ancient radiolarites in the tethyan ocean

laboratory investigations of rill dynamics on soils of the loess plateau of china abstract we conducted a laboratory study on rill network evolution with detailed measurements under controlled experimental conditions the concepts of fractal geometry entropy and topology were used to quantify rill erosion intensity and morphology the objectives of this study are to reveal rill erosion processes and rill network development under different rainfall intensities and to present some indicators to provide a quantitative description of rill morphology a soil pan 5 m long 1 m wide and 0 6 m deep with slope gradient of 20 was subjected to three successive rainfall events under rainfall intensities of 66 94 and 127 mm h 1 the results showed that rainfall intensity played a major role in rill erosion the development of a rill network is most rapid under medium intensity rainfall rainfall intensity that is too weak or too strong is not conducive to rill development as rainfall duration increased under the same rainfall intensity the values of soil loss rill density geomorphologic comentropy and nodes increased rill network development was slow at the early stage but became more rapid at a later stage the results also indicated that the geomorphologic comentropy was the best basic morphological indicator of rill erosion and the number of rill nodes was the best derived morphological indicator of rill morphology which was followed in descending order by the fractal dimension rill density and bifurcation ratio

u pb ages of zircons from mesozoic intrusive rocks in the yanbian area jilin province ne china transition of the paleo asian oceanic regime to the circum pacific tectonic regime abstract geology in the yanbian area ne china is a key to understanding the relationship between the paleo pacific tectonic regime and the older paleo asian oceanic regime especially the transition between the regimes during the mesozoic a compilation of 131 u pb zircon ages including 18 obtained in the present study shows that the late paleozoic to mesozoic magmatism in the yanbian area took place from 285 to 105 ma mainly 205 160 ma and that there was a peak of activity at 175 ma and a hiatus from the late jurassic to early cretaceous 150 130 ma provisionally we subdivide the magmatic activity into four dominant stages middle permian to middle triassic 280 240 ma late triassic 230 205 ma early to middle jurassic 205 160 ma and early cretaceous 130 105 ma middle permian intrusive rocks including pyroxenite gabbro diorite granodiorite tonalite and monzogranite are interpreted to have been emplaced along an andean style continental margin in association with the subduction of the paleo asian oceanic plate the late permian middle triassic intrusive rocks are mainly quartz monzonite monzogranite and syenogranite and are considered to have been the result of syn orogenic post collisional activities the late triassic magmatism involved not only the emplacement of mafic ultramafic intrusive rocks but also coeval i and a type granitoids indicating an extensional environment after the final closure of the paleo asian ocean since the jurassic the tectonic setting has been dominated by subduction of the paleo pacific oceanic plate previously published data for the yanbian area and age data from surrounding regions show that the entire region from the yanbian area to the korean peninsula and the japanese islands experienced peaks in igneous activities that ranged from 200 to 165 ma during the jurassic and 120 to 110 ma and 90 to 65 ma during the cretaceous the jurassic intrusive rocks were emplaced mainly in the west and cretaceous intrusive rocks in the east in other words there is a southeastwards younging of mesozoic intrusive rocks that appears to have been closely associated with the evolution of the paleo pacific ocean taking all this into account we conclude that the transition from the paleo asian oceanic regime to the circum pacific tectonic regime took place during the late triassic to early jurassic

cobalt in ferromanganese crusts as a monitor of hydrothermal discharge on the pacific sea floor ferromanganese oxide crusts which accumulate on unsedimented surfaces in the open ocean1 6 derive most of their metal content from dissolved and particulate matter in ambient bottom water7 8 in proportions modified by the variable scavenging efficiency of the oxide phase for susceptible ions9 they differ in this respect from abyssal nodules much of whose metals are remobilized from host sediments here we present maps of cobalt concentration and inferred accumulation rate of ferromanganese crusts from the pacific ocean we propose that depletion of cobalt in pacific crusts measures the location and intensity of submarine hydrothermal discharge use of the cobalt chronometer an algorithm inversely relating cobalt content and crust growth rate permits mapping of the accumulation rate of ferromanganese crusts with only indirect recourse to radioactivity based dating methods these maps show that crusts in hydrothermal areas grow from two to more than four orders of magnitude faster than in the central pacific ocean cobalt enriched crusts are found where water masses are most isolated from continental coastal and hydrothermal sources of metals now and in the past this relationship can resolve the problem of cobalt enrichment in crusts without recourse to hypotheses invoking special cobalt sources or enrichment mechanisms

uplift patterns and orogenic evolution in the scottish dalradian rb sr and k ar mineral ages from the dalradian rocks of angus and perthshire indicate considerable diachronism of cooling following the caledonian regional metamorphism the oldest mineral ages 515 ma are similar to the independent estimates of peak metamorphic age and hence indicate very rapid cooling they occur in the highest grade samples of the angus region and are associated with localized uplift regional rapid uplift events at 460 440 ma and 410 390 ma may also be recognized and correlated with structural and igneous activity and metamorphic age dates within the scottish highlands thermal models show the importance of localized and discontinuous uplift in determining both the timing of metamorphism and the shape of the p t paths they suggest that the major diachronous metamorphism and granite production in the scottish highlands is depth controlled rather than representing major lateral diachronism of thickening or heat sources localized uplift may exercise considerable control on mineral facies variation within orogenic belts the local uplift in angus introduces hot high pressure tarfside nappe rocks into higher level tay nappe rocks thus the uniform block uplift models frequently used in the interpretation of metamorphic and structural phenomena in orogenic belts are an oversimplification of the real events

roles of mantle diapir and ductile lower crust on island arc tectonics abstract in order to examine the relationships between surface geological phenomena in island arcs and lower crustal and upper mantle dynamics crustal and upper mantle responses induced by upper mantle density heterogeneities with magnitudes expected from the seismic tomography beneath the northeastern japan arc have been evaluated according to the rheological model with a low viscosity lower crust and low viscosity uppermost mantle low density lower crustal material above mantle diapiric upwelling is mechanically dragged along by asthenospheric shear stresses and then lower crustal material flows from the back arc corner to the fore arc corner the surface subsidence for this process is about 300 m on a geological time scale of 5 50 my and the formation of a sedimentary basin with about the same lateral scale as the mantle diapir can be expected in the area above the mantle diapir stage s the subducted slab in island arc will operate as a barrier to the lateral movement of lower crustal material accumulation of low density lower crustal material may occur at the edge of the overriding plate during this process at a critical stage of the pressure in the mantle wedge of fore arc region high temperature lower crustal material may then be exposed along the tectonic line stage e as the result of discharging lower crustal material along the tectonic line the pressure in the lower crust will be decreased and accelerated subsidence accompanied by volcanism is expected in the sedimentary basin cretaceous geological phenomena in southwest japan may be explained by this physical mechanism its linked surface crustal movement and the flow of lower crustal material a relatively slow deposition in the period of about 140 120 ma observed in the kanmon sedimentary basin may be interpreted as the geological representation of stage s from about 110 ma the kanmon sedimentary basin was supplied with a large quantity of volcanic sediments and simultaneously expanded and suddenly subsided at about the same time low pressure high temperature metamorphism and granitic activity ryoke belt occurred along the areas to the south of the kanmon group it may be possible to interpret these geological phenomena along the median tectonic line mtl as the surface geological events corresponding to stage e it is however stressed that the time and spatial scale for this coupling is determined by the rheological structure of the lower crust and upper mantle and by the scale of mantle diapir

significance of iron and cobalt partitioning between plagioclase and biotite for problems concerning the eu2 eu3 ratio europium anomaly and magnetite ilmenite series designation for granitic rocks from the inner zone of southwestern japan abundances of rees and other trace elements were determined for granitic rocks and their mineral separates from the sanin sanyo and ryoke belts in the inner zone of southwest japan among those data obtained the fe and co partition coefficients between plagioclase and biotite were found to be specially important the partition coefficients for fe are negatively correlated with those for co and values for the magnetite series granites from sanin belt are clearly separated from those for the ilmenite series granites of ryoke belt the sanyo belt samples which include both types of granites fall within the range of the ryoke ilmenite series granites the variation of fe and co partition coefficients can be interpreted by the difference in redox states crystal chemistry of these minerals and kinetic disequilibrium effect the sanyo belt data suggest that the ilmenite series granites were converted from a magma with magnetite series affinity during later solidification stages moreover the eu2 eu3 ratios in minerals were calculated by the philpotts method in which the equivalence of eu2 and sr2 partitioning is assumed the eu2 eu3 ratio in plagioclase from the sanyo and ryoke belt granites seems to be correlated with the whole rock fe2 fe3 though the sanin belt granites tend to show high plagioclase eu2 eu3 ratios probably due to a disequilibration caused by rapid solidification furthermore effect of redox states on the size of the eu anomaly in plagioclase melt partition coefficient was examined from the plagioclase eu2 eu3 data of the granites