prograde and retrograde metamorphism of hematitebearing basic schists in the sanbagawa belt in central shikoku the mineral assemblages of hematite bearing basic schists in intermediate high pressure metamorphism are temperature dependent for assemblages with excess hematite albite muscovite and quartz the paragenetic relations can be dealt with in terms of a four component system without omitting or grouping major components in the sanbagawa belt in central shikoku the dominant amphibole in the hematite bearing basic schists changes from winchite via crossite and barroisite to hornblende the stability of amphibole is described chemographically within a pseudoternary system with another excess phase epidote many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low t p amphibole around the prograde amphibole the examination of amphibole zoning makes it possible to draw a retrograde p t trajectory which passes on the lower pressure side of the prograde one

implications of mylonitic microstructures for the geotectonic evolution of the median tectonic line central japan the median tectonic line mtl the most prominent onshore fault in japan demarcates the cretaceous hiji quartz dioritic gneiss of the ryoke belt on the west from the high p t type sambagawa metamorphic rocks on the east in the takato area toward the mtl the hiji gneiss grades into strongly mylonitized rocks characterized by grain size reduction of quartz in the mylonitic rocks the development of fluxion banding sm is remarkably influenced by the existence of k feldspar forming a myrmekitic intergrowth brittle microstructures indicative of truly cataclastic deformation are observed only in mylonitic rocks close to the mtl early deep level ductile deformation apparently gave way to shallower brittle deformation at a later stage the attitude of stretching lineations lm and asymmetric microstructures observed in the mylonites suggest that sinistral strike slip shearing with a subordinate component of vertical slip took place during mylonitization in mid cretaceous time the mylonitic rocks and their protolith supposed to have constituted the eastern limb of the shear zone were probably eroded out and lost by upheaval of the sambagawa belt relative to the ryoke belt

the origin of strain patterns resulting from contemporaneous deformation and metamorphism in the sambagawa metamorphic belt the magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the sambagawa high p type metamorphic belt of western shikoku japan the strain magnitude increases with increasing metamorphic temperature from several per cent to 250 the a2 a3 ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a1 a2 ratio increases rapidly the long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt a map of strain zones within the sambagawa high p type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high grade zones represent a deep seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate

double saloon door tectonics in the japan sea fossa magna and the japanese island arc abstract a number of criteria considered diagnostic of double saloon door rifting and seafloor spreading are matched by data from the japanese arc these include i a pair of terranes sw and ne honshu which rotated in opposite directions from 22 21 ma to 14 11 ma ii rotated terranes which comprise a retro arc fold thrust belt attached to an accretionary wedge intruded by a magmatic arc iii contemporaneous backarc extension from 24 to 21 ma which is brought to a halt by progressive collision of the izu bonin and japan arcs from 15 to 5 ma iv isolation of blocks of thicker continental crust by areas of thin continental or oceanic crust during backarc rifting v such isolation may be due to simultaneous rifting or to progressively seaward rifts associated with ridge jumps towards the subduction zone vi opposite rotations are accommodated by subduction rollback demonstrated by seaward migration of the volcanic front from 30 26 ma to 16 15 ma vii concurrent development of a major arc orthogonal rift the fossa magna from 23 18 ma to 14 ma which was thereafter inverted from 15 ma to the recent viii a northeast propagating rift in the northern japan basin demonstrated by the relationship of linear magnetic anomalies to the mapped continent ocean boundary driving mechanisms for double saloon door tectonics are discussed in relation to various reconstructions of the northwest pacific opposite rotational torques leading to opposite terrane rotations may be caused by rollback of a curved trench hingeline or by the divergent slab sinking forces of the pacific and philippine sea plates

pressure temperature trajectory of the sanbagawa metamorphism deduced from garnet zoning abstract most garnets in a small volume of the pelitic sanbagawa schists nucleated simultaneously grew and then ceased with their growth maintaining near equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism chemical trends of zoned garnets systematically shift from the pyrope poor to pyrope rich portions in a ternary mn fe mg system in order of ascending metamorphic grade a ternary mn fe mg model system suggests that the higher grade garnets started to grow at lower pressures and higher temperatures than the lower grade ones during prograde metamorphism it follows that high grade metamorphism took place at lower pressures in the garnet to oligoclase biotite zones of the sanbagawa metamorphic terrain in central shikoku

exhumation of the sanbagawa blueschist belt sw japan by lateral flow and extrusion evidence from structural kinematics and retrograde p t t paths

40ar 39ar mineral ages within metamorphic clasts from the kuma group eocene central shikoku japan implications for tectonic development of the sambagawa accretionary prism abstract the sambagawa metamorphic belt exposed in central shikoku records a high p t metamorphism it is comprised by the oboke nappe and the structurally overlying internally imbricated besshi nappe complex the besshi nappe complex locally is unconformably overlain by the kuma group eocene which consists of conglomerates and subordinate sandstones and mudstones clasts within kuma conglomerates exposed in the kamegamori district include low grade schists derived from the unconformably underlying sambagawa complex and high grade metamorphic rocks of uncertain provenance including garnet amphibolite and oligoclase bearing pelitic schist 36 ar 40 ar vs 39 ar 40 ar isotope correlation ages recorded by hornblende from two amphibolite clasts are 131 1 4 9 ma and 156 8 4 3 ma these are interpreted to date post metamorphic cooling through temperatures required for intracrystalline retention of argon and are older than cooling ages previously reported from any presently exposed segments of the sambagawa terrane plateau ages of 108 8 0 7 ma and 115 7 0 6 ma are recorded by muscovite from two clasts of schist muscovite within a proximal basement exposure of unconformably underlying sambagawa pelitic schist records a plateau age of 78 7 0 5 ma combined with isotopic ages previously reported for the sambagawa terrane 40 ar 39 ar data from clasts in the kuma group suggest that metamorphic culmination within a high grade source terrane occured at c 185 145 ma sectors of this complex cooled through c 500 c at c 150 ma other portions cooled through c 500 c at c 130 ma contrasts in the cooling ages likely reflect internal imbrication within the source terrane the besshi nappe complex reached peak metamorphic conditions at c 100 90 ma and experienced relatively rapid uplift and cooling at c 85 75 ma the besshi nappe complex was structurally emplaced onto the oboke nappe which attained peak metamorphic conditions at c 75 ma subsequently the entire sambagawa section was exhumed and most high grade metamorphic rocks were removed the eocene kuma group was unconformably deposited over the sambagawa terrane at c 50 ma

nappe structure of the sambagawa belt the thermal structure of the sambagawa belt which is reflected by minerals produced during the highest temperature of metamorphism generally is characterized by the occurrence of the highest grade schists in the middle of the structural pile the origin of this structure has been analysed in the upper horizon of the sambagawa schist sequence in central shikoku as an example for the structure of the whole belt the upper horizon of the sambagawa schist sequence in central shikoku consists of three nappes the saruta nappe ii the saruta nappe i and the fuyunose nappe in descending order of structural level the saruta nappe ii shows a downward increase of metamorphic grade from the garnet zone through the albite biotite zone to the oligoclase biotite zone the saruta nappe i consists mainly of rocks in the albite biotite zone and partly in the oligoclase biotite zone but the direction of the increase of metamorphic grade is not clear the fuyunose nappe shows an upward increase in metamorphic grade which changes from the glaucophane zone to the barroisite zone it is concluded that the saruta nappe ii and saruta nappe i were overturned and then mechanically coupled with the fuyunose nappe the sambagawa metamorphic field which is of the highest temperature phase of metamorphism appears to have had an inverted thermal gradient and a thermal structure comparable with that expected in the deeper parts of a subduction complex

mass wasting processes in the western wilkes land margin possible implications for east antarctic glacial history abstract multichannel seismic data collected off western wilkes land east antarctica reveal widespread sediment mass movement events mainly in the form of mega debris flows deposited on the continental rise in places they have a seaward extent of more than 200 km and a thickness of up to 400 m debris flow units are commonly separated by thin deposits of well stratified facies interpreted as glaciomarine mixed contouritic and distal turbidite deposits possibly with some hemipelagic sediments these units due to a dominant contouritic component represent weakened layers and could then play a major role in the slope instability based on stratigraphic correlation with dsdp site 268 it is suggested that slope failures in the western wilkes land occurred mostly during the miocene inferred high sedimentation rates due to large amounts of sediment delivered from a temperate wet based ice sheet would have constituted a key factor in the sediment failures the distribution of mass wasting related features appears also strongly related to the amount of meltwater discharge the main trigger mechanism would probably have been earthquakes enhanced by isostatic rebounds following major ice sheet retreats

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

southward extrusion of eclogite bearing mafic ultramafic bodies in the sanbagawa belt central shikoku japan abstract several mafic rock masses which have experienced eclogite facies metamorphism are distributed in flat lying non eclogitic schists in an intermediate structural level thermal core of the sanbagawa belt the largest iratsu mass and an associated peridotite the higashi akaishi mass extend e w for about 8 km and n s for about 3 km and are surrounded by pelitic basic and quartz schists the iratsu mass consists of metabasites of gabbroic and basaltic origin with intercalations of ultramafic rocks felsic gneiss quartz schist and metacarbonate the iratsu mass can be divided into two layers along a wnw trending metacarbonate layer the higashi akaishi mass consists of peridotite with intercalations of garnet clinopyroxenite it is situated beneath the western half of the iratsu mass and their mutual boundary dips gently or steeply to the n or ne these masses underwent eclogite and subsequent epidote amphibolite facies metamorphism as has been reported elsewhere the iratsu higashi akaishi masses and the surrounding rocks underwent ductile deformation under epidote amphibolite facies or lower p t metamorphic conditions their foliation generally trends wnw and dips moderately to the nne and the mineral lineation mostly plunges to the n and ne in non eclogitic schists surrounding the iratsu higashi akaishi masses the foliation generally trends wnw and dips gently or steeply to the n or s and the mineral lineation mostly plunges to the nw n and ne kinematic analysis of deformation structures in outcrops and oriented samples has been performed to determine shear senses consistent top to the north normal fault displacements are observed in peridotite layers of the higashi akaishi mass and eclogite bearing epidote amphibolite layers of the iratsu mass top to the northeast or top to the northwest displacements also occur in non eclogitic pelitic quartz schists on the northern side of the iratsu mass in the structural bottom of the iratsu higashi akaishi masses and to the south reverse fault top to the south movements are recognized in serpentinized peridotite and non eclogitic schists these observations provide the following constraints on the kinematics of the rock masses 1 northward normal displacement of iratsu relative to higashi akaishi 2 northward normal displacement of non eclogitic schists on the north of the iratsu mass and 3 southward thrusting of the iratsu higashi akaishi masses upon non eclogitic schists in the south the exhumation process of the iratsu higashi akaishi masses can be explained by their southward extrusion

structural and microstructural constraints on the mechanism of eclogite formation in the sambagawa belt sw japan abstract eclogites in the seba basic schist of the sambagawa belt show contrasting textures in one type of eclogite coarse grained omphacite is randomly oriented and cross cuts the schistosity suggesting post tectonic growth in contrast in the second type fine grained omphacite is preferentially aligned parallel to the schistosity suggesting pre to syn tectonic growth the former type of eclogite has been regarded as a product of solid state contact metamorphism after the formation of the schistosity while the latter type has been related to the sambagawa regional metamorphism the idea that eclogite can form by contact metamorphism is unconventional and if true could have major implications for the interpretation of eclogitic rocks in general in the seba basic schist it has generally been assumed that the dominant schistosity formed during a single phase of deformation however field and microstructural studies show i there are two different schistosities one formed before and the other during eclogite formation and ii the differences in eclogite texture reflect the strength of deformation during the second stage therefore the various textural types of eclogite in the seba basic schist all formed during a single phase of regional metamorphism and there is no need to invoke contact metamorphism

coupled micro faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the sambagawa metamorphic rocks southwest japan abstract in the sambagawa schist southwest japan while ductile deformation pervasively occurred at d 1 phase during exhumation low angle normal faulting was locally intensive at d 2 phase under the conditions of frictional viscous transition of quartz c 300 c during further exhumation into the upper crustal level accordingly the formation of d 2 shear bands was overprinted on type i crossed girdle quartz c axis fabrics and microstructures formed by intracrystalline plasticity at d 1 phase in some quartz schists the quartz c axis fabrics became weak and finally random with increasing shear accompanied by the decreasing degree of undulation of recrystallized quartz grain boundaries which resulted from the increasing portion of straight grain boundaries coinciding with the interfaces between newly precipitated quartz and mica we interpreted these facts as caused by increasing activity of pressure solution the quartz grains were dissolved mostly at platy quartz mica interface and precipitated with random orientation and pinned by mica thus having led to the obliteration of existing quartz c axis fabrics in the sheared quartz schist the strength became reduced by the enhanced pressure solution creep not only due to the reduction of diffusion path length caused by increasing number of shear bands but also to enhanced dissolution at the interphase boundaries

microboudin structures of piemontite along the sambagawa metamorphic belt japan implications for lateral variation of differential stress during regional metamorphism evidence is presented of a lateral variation in differential stress during metamorphism along a regional metamorphic belt on the basis of the proportion of microboudinaged piemontite grains p in a quartz matrix in metacherts it is proposed that p is a practical indicator of relative differential stress analysis of 123 metacherts from the 800 km long sambagawa metamorphic belt japan reveals that p values range from 0 01 to 0 7 in this region most samples from wakayama in the mid belt area have p values of 0 4 0 6 whereas those from western shikoku have p values of 0 1 this difference cannot be explained by variations in metamorphic temperature and is instead attributed to a regional lateral variation in differential stress during metamorphism

the higher grade metamorphic zonation of the sambagawa metamorphic belt in central shikoku japan the higher grade metamorphic zonation of the sambagawa sanbagawa belt is established for the first time for the whole area of central shikoku as discontinuous reactions to define the isograd are absent the metamorphic grade is primarily determined by the mg fe partitioning between garnet and chlorite along representative traverses however for regional mapping mineralogical features of the pelitic schists such as using mineral assemblages of more than divariant equilibrium the modal garnet to chlorite ratio and the optical properties of chlorite are employed as auxiliary criteria the presence of the highest grade mineral zone in the middle of the structural level is confirmed but its spatial distribution is far more complex than hitherto accepted thermal axes are now confirmed at three different structural levels a model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress thermal readjustment brought a continuous metamorphic temperature gradient across and within the thrust sheets tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re equilibrated local anomalies of metamorphic grade represented by mixing of schists of different metamorphic grade exist but they are due to a later stage event

vorticity analysis in a metachert from the sanbagawa belt sw japan abstract common tectonic features of deformed metachert include both sets of extended and shortened veins and quartz c axis preferred orientation patterns in such rocks information is also commonly available on the orientation and magnitude of the finite strain these features can be used to derive two independent estimates of the degree of non coaxiality during deformation in an example from the sanbagawa belt sw japan the two methods give consistent results and show that deformation was intermediate between simple and pure shear such types of progressive deformation distributed over a broad region could cause substantial subvertical thinning and be a significant factor in the exhumation of high p t rocks in the region

muscovite k ar ages of the sanbagawa schists japan and argon depletion during cooling and deformation seventy muscovites from schists in the sanbagawa terrain in central shikoku were dated by the k ar method the muscovite ages are consistently older with increasing metamorphic grade within the same zone the ages are significantly younger in schists which have been more severely deformed these k ar age variations could be due to systematic argon depletion during deformation i e to the dynamic recrystallization of muscovites during ductile deformation that formed a large scale recumbent fold during the uplift and cooling argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low temperature deformation than the higher grade zone the relationships between age and grain size in a pelitic schist suggest that coarse grained muscovites lost more argon than the finegrained ones there was no significant resetting of ages in the vicinity of major strike slip faults such as the median tecotonic line or near thrust faults the combination of geochronological and geological data constrains the cooling rate of the sanbagawa schists to 9 12 c ma in the oligoclase biotite zone in central shikoku japan

evolution of the sambagawa metamorphic belt japan abstract the high pressure sambagawa metamorphic belt evolved within a mesozoic accretionary prism that developed in the convergent margin along the east of the eurasian continent although extensively researched recent studies in this region have revealed a number of important features that allow more complete reconstruction of the tectonic history i the grouping of radiometric ages suggests the existence of three major structural units with distinct metamorphic histories one of these has since been completely eroded away ii the difference of ages recorded by different minerals is in many cases small suggesting rapid exhumation after reaching peak metamorphic conditions iii the dominant ductile deformation of the sanbagawa belt representing major orogen parallel flow is commonly related to retrograde metamorphic reactions this shows that the associated fabric developed during exhumation not subduction iv kinematic studies suggest that the main ductile deformation caused major ductile thinning of the region which was probably a significant factor in causing exhumation of the region

early mesozoic subduction in the eastern mediterranean evidence from triassic eclogite in northwest turkey an eclogite forms a 40 m long tectonic lens enveloped in serpentinite within a metabasite phyllite marble unit of permian triassic age in northwest turkey the eclogite consists mainly of garnet omphacite glaucophane and epidote it is associated with other lenses of serpentinite and metagabbro single grain laser probe 40ar 39ar dating of phengites from the eclogite gives ages of 203 to 208 ma corresponding to the triassic jurassic boundary regional geologic considerations also suggest a latest triassic to earliest jurassic metamorphic age for the eclogite and the enclosing metabasite marble phyllite unit the eclogite along with serpentinite and gabbro lenses were probably emplaced as diapirs into the metabasite phyllite marble unit during late triassic subduction the eclogite provides evidence for the often elusive triassic subduction history of the paleo tethys in the eastern mediterranean area

clasts of high grade sanbagawa schist in middle eocene conglomerates from the kuma group central shikoku south west japan middle eocene conglomerates which overlie the sanbagawa metamorphic rocks contain clasts of metamorphic rock with isotope ages of 120 85 ma which fall within the age range reported from the sanbagawa metamorphic rocks they were derived from the chlorite to oligoclase zones of the sanbagawa metamorphic belt clasts of garnet amphibolite and oligoclase biotite schist show a mineral assemblage similar to the highest grade sanbagawa schists however the metamorphic temperatures estimated by various mineralogical thermometers show that some of the clasts were formed at higher temperatures than the in situ sanbagawa metamorphic rocks such higher grade rocks were at the surface by the middle eocene and for the most part they have been eroded away cretaceous and post cretaceous sediments overlie or are in fault contact with the sanbagawa metamorphic rocks which suggests that rocks in the belt were uplifted and eroded from the latest cretaceous to middle eocene time after strike slip movement along the median tectonic line since the middle eocene the belt has experienced relatively slow uplift which was locally around 2 km in central shikoku

40ar 39ar mineral age constraints for the tectonothermal evolution of the sambagawa metamorphic belt central shikoku japan a cretaceous accretionary prism abstract the sambagawa metamorphic belt exposed in central shikoku japan is tectonostratigraphically represented by the oboke and structurally overlying besshi nappe complexes oboke units consist of metamorphic rocks largely derived from clastic protoliths sandstone and conglomerate whereas besshi units are dominated by rocks with oceanic protolith affinities shale chert and greenstone whole rock schist samples from low grade oboke units record intermediate temperature 40 ar 39 ar plateau ages of 70 77 ma which are interpreted to closely date attainment of maximum metamorphic conditions whole rock schist samples from besshi units chlorite zone record plateau ages of 85 94 ma suggesting an earlier metamorphic climax within this structural level amphibole within the albite biotite zone in besshi records 36 ar 40 ar vs 39 ar 40 ar plateau isotope correlation ages of 84 87 ma which are interpreted as dating the post metamorphic cooling through temperatures required for intracrystalline retention of argon a slightly older isotope correlation age 94 ma is recorded by amphibole from the higher grade oligioclase biotite zone contrasts in amphibole cooling ages are interpreted as reflecting thrust related metamorphic inversion and resultant earlier cooling of higher grade structural units muscovite from all structural levels of the besshi nappe complex records 40 ar 39 ar plateau ages of 76 89 ma these are interpreted to date post metamorphic cooling through appropriate closure temperatures following structural assembly of the besshi nappe complex protoliths of the besshi units originated at approx 130 ma and attained maximum metamorphic conditions during entrainment in an accretionary complex at approx 90 100 ma higher grade sequences developed at deeper levels along the hanging wall of the accretionary complex during subsequent structural uplift to shallower crustal levels the besshi nappe complex was tectonically emplaced over oboke units

exhuming the sanbagawa metamorphic belt the importance of tectonic discontinuities tectonic processes that have been proposed to explain the transport to the surface of regional metamorphic belts can be broadly divided into two types i corner flow within a convergent margin bounded by two essentially rigid plates associated with extension at shallow levels this type of model assumes deformation is distributed throughout the margin and that any discontinuities are of secondary importance ii expulsion or extrusion of coherent metamorphic nappes in this second idea tectonic discontinuities are fundamental in the transport to the surface of metamorphic rocks the wealth of geological data available from a variety of studies in the sanbagawa metamorphic belt southwest japan makes it well suited for studying the relative importance of continuous vs discontinuous deformation in the process of exhumation in the sanbagawa belt a sudden decrease in metamorphic pressure going down section of several kilobars suggests the presence of a major tectonic contact separating two major regional nappes an overlying higher pressure besshi nappe and an underlying lower pressure oboke nappe major tectonic discontinuities have also been proposed within the besshi nappe however indicators of metamorphic temperature the results of radiometric age dating and microstructural studies all suggest that post metamorphic discontinuities are minor and that this nappe formed and remained as an essentially coherent unit lithological associations and petrological studies suggest the following positions for the two nappes the besshi nappe formed deep within the former accretionary wedge adjacent to the overlying mantle wedge and with a dip of roughly 30 c in contrast the oboke nappe formed at moderate depths within the accretionary wedge was distant from the mantle wedge and was roughly horizontal penetrative deformation that post dates the peak of metamorphism has affected nearly all of the sanbagawa belt and has played an important role in its exhumation however the presence of a broad coherent besshi nappe overlying the lower pressure oboke nappe suggests that some process such as buoyancy driven extrusion was also important in the exhumation process and in forming the structure of the sanbagawa metamorphic belt

plate movements ductile deformation and geochronology of the sanbagawa belt sw japan tectonic significance of 89 88 ma lu hf eclogite ages prograde p t paths and thermal modelling suggest metamorphism in the sanbagawa belt represents unusually warm conditions for subduction type metamorphic belts and these likely reflect conditions of a convergent margin a few million years before the arrival of an active spreading ridge radiometric age data and kinematic indicators of ductile deformation suggest the sanbagawa belt formed in a cretaceous convergent margin associated with a plate movement vector that had a large sinistral oblique component with respect to the belt the east asian margin plate reconstructions for the cretaceous to tertiary for this region show that the only plausible plate compatible with such motion at this time is the izanagi plate these reconstructions also show that progressively younger sections of the izanagi plate were subducted beneath eastern asia i e a spreading ridge approached until 85 83 ma when the izanagi plate ceased to exist as an independent plate the major reorganization of plates and associated movements around this time is likely to be the age of major interaction between the ridge and convergent margin the ridge approach model for the sanbagawa metamorphism therefore predicts that peak metamorphism is a few million years older than this age range new lu hf dating of eclogite in the sanbagawa belt gives ages of 89 88 ma in excellent agreement with the prediction combining this estimate for the peak age of metamorphism with published p t t results implies vertical exhumation rates of greater than 2 5 cm yr 1 this high rate of exhumation can explain the lack of a significant thermal overprint in the sanbagawa belt during subduction of the ridge

microtectonic evidence for eastward ductile shear in the jurassic orogen of sw japan abstract two jurassic deformation phases are responsible for the thrust and nappe structures in the infrastructure of the outer belt of the mesozoic chain of sw japan the first phase was synmetamorphic and took place under hp lt conditions microtectonic analysis shows that the penetrative deformation corresponded to a ductile shear directed from w to e parallel to the stretching and mineral lineation at the regional scale the first phase corresponded to the obduction of the greenschist nappes upon the kurosegawa continent during the second phase the greenschist nappes were sliced leading to an apparent reverse metamorphic zoning the first phase structures were then locally reworked by folds giving rise to an apparent westwards sense of shear

origin of rutile needles in star garnet and implications for interpretation of inclusion textures in ultrahigh pressure metamorphic rocks the asterism effect of star garnet has been attributed to the oriented distribution of needle like rutile inclusions rutile needles occur in garnet from a wide range of metamorphic settings and rock bulk compositions and their origin has been ascribed to different mechanisms such as exsolution and used to interpret petrological and tectonic processes results from an optical and transmission electron microscopy of idaho star garnet indicate a co precipitation origin it was found that rutile needles are predominantly oriented along the rt grt and rt grt directions following multiple crystallographic orientation relationships cors i e cor 1 2 2 3 4 and 5 in 6 ray star garnet and are oriented solely along the rt grt directions following exclusively cor 2 in 4 ray star garnet the sole presence of cor 2 grt needles in the common 4 ray star garnet in contrast to the presence of both grt and grt needles with multiple cors in the rare 6 ray star garnet suggests that the cor 2 grt needle probably is the energetically most favoured variant as is also supported by the coincidence site lattice considerations the unique crystallography controlled microstructures of 4 ray star garnet including the cloudy domains behind the 111 grt or 100 grt fronts with abundant inclusions of rutile needle rutile compound needle and multiple phase inclusion as well as the clear domains behind the 110 grt fronts with only a few above inclusions concentrated exclusively within the linear grt oriented continuous tube like domains further suggest that the cor 2 grt needles in 4 ray star garnet most likely have a growth in origin co precipitating with garnet at its growth fronts close to thermodynamic equilibrium conditions the 6 ray star garnet on the other hand most likely formed under far from equilibrium conditions thereby yielding a maximum of 99 crystallographic variants of rutile needles with multiple cors in a single crystal in the light of these findings along with the common occurrences of the sole cor in many inclusion host systems owing to the requirement to minimize the energy barrier in an exsolution process the presence of both rt grt and rt grt needles with multiple cors in garnet of sulu eclogite and erzegebirge quartzofeldspathic rock would therefore cast doubt on the assertion of an exsolution origin of rutile needles in garnet from these ultrahigh pressure rocks