paragenesis of sodic pyroxene bearing quartz schists implications for the p t history of the sanbagawa belt sodic pyroxene jadeite content x jd 0 1 0 3 occurs locally as small inclusions within albite porphyroblasts and in the matrix of hematite bearing quartz schists in the sanbagawa sambagawa metamorphic belt central shikoku japan the sodic pyroxene bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene subcalcic or sodic amphibole phengitic mica albite quartz hematite titanite epidote spessartine rich garnet occurs in mn rich samples sodic pyroxene in epidote bearing samples tends to be poorer in acmite content average x acm 0 26 0 50 than that in the epidote free samples x acm 0 45 0 47 x jd shows no systematic relationship to metamorphic grade and is different among the three sampling regions saruta gawa asemi gawa and bessi besshi the average x jd of the saruta gawa samples 0 21 0 29 is higher than that of the asemi gawa 0 13 0 17 and bessi 0 14 0 23 the p t conditions of the asemi gawa and bessi regions are estimated at 5 5 6 5 kbar 360 c in the chlorite zone 7 8 5 kbar 440 15 c in the garnet zone and 8 9 5 kbar 520 25 c in the albite biotite zone metamorphic pressure of the saruta gawa region is systematically 1 1 5 kbar higher than that of the asemi gawa and bessi regions and materials of the saruta gawa region have been subducted to a level 3 5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the asemi gawa and bessi regions pressure slightly increases toward the north structurally high levels through the sanbagawa belt of central shikoku two types of zonal structure were observed in relatively coarse grained sodic pyroxenes in the matrix one type is characterized by increasing x jd from core to rim the other type by decreasing x jd from core to rim both types of zoned pyroxenes show an increase in x fe 2 fe2 fe2 mg from core to rim the first type of zoning was observed in a sample from the chlorite zone of lowest grade whereas the latter occurs in the garnet and albite biotite zones of higher grade the contrast in zonal structure implies that dp dt during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher grade zones the estimated dp dt of the prograde stage of the chlorite zone is 3 2 kbar 100 c and that of the garnet and albite biotite zones is 1 8 to 0 9 kbar 100 c the variation of dp dt at shallow and deep levels of a subduction system probably reflects the difference of heating duration and or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle

brittle normal faulting in the highest grade sambagawa metamorphic rocks of central shikoku southwest japan indication of the exhumation into the upper crustal level abstract analysis of fault system in the high p t type sambagawa metamorphic rocks of central shikoku southwest japan shows that conjugate normal faults pervasively developed in the highest grade biotite zone upper structural level in three study areas asemi river oriu and niihama areas these conjugate normal faults consist of ne sw to e w striking and moderately north dipping set a and nnw sse striking and moderately east dipping set b faults the fault set a is dominant compared to the fault set b and hence most of deformation is accommodated by the fault set a leading to non coaxial deformation the sense of shear is inferred to be a top to the wnw to nnw based on the orientations of striation or quartz slickenfibre and dominant north side down normal displacement these transport direction by normal faulting is significantly different from that at d1 penetrative ductile flow i e top to the w to wnw it has also been found that these conjugate normal faults are openly folded during the d3 phase about the axes trending nw se to e w and plunging west at low angles or horizontally indicating that normal faulting occurred at the d2 phase d2 normal faults along which actinolite breccia derived from serpentinite by metasomatism sometimes occurs perhaps formed under subgreenschist conditions ca 250 c in relation to the final exhumation of sambagawa metamorphic rocks into the upper crustal level the pervasive development of d2 normal faults in the upper structural level suggests that the final exhumation of sambagawa metamorphic rocks could be caused by distributed extension and normal faulting removal of overburden in the upper crust

u pb ages of detrital zircons in pacheco pass metagraywackes sierran klamath source of mid cretaceous and late cretaceous franciscan deposition and underplating 1 using laser ablation icp ms techniques u pb ages are reported for 338 detrital zircons separated from four franciscan metagraywacke samples from imbricate thrust sheets cropping out in the pacheco pass area east central california coast ranges structurally higher slabs are clastic quartz albite bearing rocks typified by neoblastic lawsonite jadeitic pyroxene the lowest exposed slab lacks jadeitic pyroxene but contains traces of newly grown pumpellyite and lawsonite in addition to abundant quartz albite studied specimens contain moderate amounts of phengite titanite chlorite stilpnomelane carbonate iron oxides rock fragments and carbonaceous matter these diablo range clastic sediments were deposited in mid cretaceous and late cretaceous time the highest allochthon unit i was deposited after 102 ma unit iv was deposited after 90 102 ma and unit v was deposited after 86 ma metagraywacke depositional accretionary ages within slab iv young upward but ages of the slabs decrease progressively downward in the four sample aggregate most zircons have igneous ages falling in the 135 170 ma range with a smaller population at 85 120 ma the rocks contain rare zircons of middle proterozoic late archean ages a minor source in northern california and nw nevada seems likely for the pacheco pass metagraywackes but the sierran klamath calcalkaline arc provided most of the clastic debris judging by the volcanic nature of lithic clasts in these eastern belt metagraywackes and because massive sierran plutonism occurred at 85 120 ma the abundant 135 170 ma zircons probably were derived chiefly from the eroding comagmatic volcanic arc rather than from the less voluminous jurassic arc plutons

plate reconstruction based upon age data of japanese accretionary complexes the ages of the oceanic plates for the japanese accretionary complexes are plotted against time of accretion the plots show that early cretaceous transform faulting modified the arrangement of the pre middle cretaceous accretionary complexes and that three or four mid ocean ridges have collided with the japan arc since permian time the plate configuration for the western north pacific is reconstructed

late stage exhumation of hp metamorphic rocks progressive localization of strain and changes in transport direction sambagawa belt japan abstract new geologic mapping delimits major structures associated with the later stages of exhumation of high pressure hp metamorphic rocks of the sambagawa belt of central shikoku the main sets of structures define an inclined n dipping wedge bounded by s vergent thrust faults below and n vergent normal faults above such faults bound every metamorphic mineral zone of the sambagawa belt faults also occur within metamorphic mineral zones such as the asemigawa detachment which is entirely within the oligoclase biotite zone the highest grade metamorphic zone this fault is the structurally lowest of the n vergent normal faults these d2 structures accommodate the exhumation of the higher grade rocks relative to the lower grade rocks within the sambagawa belt rather than relative to units bounding the belt so they are termed an internal extrusional wedge the extrusional wedge is truncated to the east against footwall chlorite zone rocks by the hamegano out of sequence thrust oost the oost divides the traditional chlorite zone into an upper unit consisting of varicolored schist l tectonite and a lower unit consisting of regionally extensive pelitic schist to the north the metamorphic mineral zones are stacked over the oost forming the tomisato duplex and these structures are further modified by the oboke anticline and tsuneyama syncline the evolution of exhumation structures of the sambagawa suggests progressive localization of strain as exhumation proceeded as well as a change in movement direction from d1 oblique orogen subparallel exhumation to the d2 structures with dip parallel orogen normal transport that we describe in detail the evolution of exhumation structures in the sambagawa belt differs from that of the hp rocks of the franciscan complex of california in that the exhumation of the latter is accommodated entirely by discrete faults and the franciscan lacks internal normal faults the internal extrusional wedge structure of the sambagawa hp rocks resembles that of hp rocks of leti island of eastern indonesia

areal extent of eclogite facies metamorphism in the sanbagawa belt japan new evidence from a raman microprobe study of quartz residual pressure application of raman spectrometry to determine residual pressure retained by quartz grains sealed in garnet reveals significant differences between eclogite facies rocks and lower grade schistose rocks of the sanbagawa metamorphic belt in central shikoku japan garnet in the eclogitic lithologies commonly exhibits two chemically distinct growth stages the inner segment commonly includes quartz grains with higher residual pressures than those in the outer segment suggesting that the former represents prograde eclogite facies and the latter formed during exhumation regional variation of the quartz residual pressure suggests that the eclogite unit has a far greater extent than previously recognized and reveals a large pressure gap between the newly proposed eclogite and noneclogite units

the high p t sambagawa extrusional wedge japan abstract a metamorphic inversion along the asemigawa section of the besshi unit in the sambagawa high p t metamorphic zone has been explained previously by s vergent major recumbent folding or thrusting through careful structural field investigations however we recognize the d2 asemigawa detachment fault at the boundary of the highest grade oligoclase biotite zone thermal culmination in the belt we have also discovered a series of normal faults in the hanging wall of the major detachment in the upper domain and a series of thrust faults bounding every metamorphic mineral zone of the footwall in the lower domain outcrop scale d2 asymmetric folds are closely associated with axial planar pressure solution cleavages and are genetically related to the above d2 normal faulting and thrusting as fault propagation folds northward fold vergence is found only in the upper domain and southward vergence is found only in the lower domain exceptions to this rule occur only for parasitic folds in the overturned limbs of mesoscopic folds which indicates flexural slip as a likely fold mechanism we propose an extrusion wedge model for the inverted metamorphism and accordingly this mechanism may also account for a large part of the exhumation of the sambagawa metamorphic rocks although westward shear at the time of d1 stretching lineation has partly contributed to the exhumation by counter flow in the accretionary wedge we infer that m1 with its retrograde actinolite schists represents a d1 ductile shear zone but it contributes as much as a component of the d2 asemigawa detachment other normal and thrust faults and asymmetric folds as reactivated brittle shear zones thus shear direction had changed orthogonally and drastically between ductile d1 stretching and ductile brittle transition to brittle d2 extrusion we infer that this transition reflects the collision of an actively spreading mid ocean ridge and is a consequence of increased coupling of plates and orogen normal stress followed by oblique subduction d0 and d 1 record eastward down flow during the oblique underthrusting of oceanic plate before peak metamorphism m1

minimal advective travel time along arbitrary streamlines of porous media flows the fermat leibnitz bernoulli problem revisited summary travel time of marked fluid particles along arbitrary streamlines in arbitrary porous streamtubes is estimated from below based on the cauchy bunyakovskii schwartz and jensen inequalities in homogeneous media the estimate is strict and expressed through the length of the streamline hydraulic conductivity porosity and the head fall the minimum is attained at streamlines of unidirectional flow the bounds for heterogeneous soils non darcian flows and unsaturated media are also written if such bounds are attained the corresponding trajectories become brachistochrones for example in a two layered aquifer and seepage perpendicular to the layers there is a unique conductivity porosity ratio which makes a broken streamline brachistocronic similarly if conductivities of two layers are fixed there is a unique incident angle between flow in one medium and the interface which makes a refracted streamline brachistocronic

provenance and thermal history of the franciscan accretionary complex constraints from zircon fission track thermochronology the franciscan accretionary complex of california contains large belts of sediments that were subducted to depths of 10 to 30 km metamorphosed under high pressure and low temperature conditions and later returned to the surface reconnaissance zircon fission track ft data 21 samples place constraints on the sedimentary provenance thermal history and or exhumation history of various franciscan units in a transect across the northern coast ranges through mendocino pass metagraywacke samples from the valantine springs formation of the franciscan eastern belt experienced maximum temperatures tmax ranging from 250 to 310 c zircon ft and previously reported apatite ft data indicate that parts of the western eastern and eastern central belts cooled at about 100 70 ma from 300 c to 100 c considering the low paleogeothermal gradient of roughly 9 c km during franciscan subduction these rocks were apparently uplifted and exhumed from 30 to 10 km depth at this time one sample from the eyler mountain sequence in the northwestern diablo range similarly shows tmax 310 c and major cooling at 100 70 ma this uplift and exhumation may have reflected accelerated accretion at the base as well as the front of the franciscan wedge accompanied by extensional thinning at shallower levels as a result of massive sediment influx into the franciscan trench from the sierra nevada magmatic arc in the northern coast ranges the western part of the central belt and the coastal belt experienced lower tmax probably 200 c and retain source area detrital zircon ft ages burnt hills terrane samples in the diablo range also experienced tmax 230 c and retain detrital ages the detrital single grain age patterns from these areas are compatible with those expected from the sierra nevada magmatic arc and its country rocks this is consistent with derivation of franciscan sediments from arc terranes in california and provides no particular suggestion that these parts of the franciscan are highly allochthonous paleogene coastal belt sandstones also yield detrital ages compatible with derivation from the sierra nevada but also include early tertiary zircon populations compatible with reconstructions suggesting these sandstones were sourced partly from the idaho batholith

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

temperature time path for the low pressure ryoke metamorphism japan based on chemical zoning in garnet garnet crystals from low pressure high temperature lpht ryoke metamorphic rocks in the yanai district south western japan show several kinds of chemical zoning patterns that systematically vary with grain radius between c 0 1 and 0 5 mm large grains c 0 4 mm show normal zoning and small grains 600 c shorter than 0 5 myr which was presumably caused by the intrusion of a granodiorite sheet chemical zoning of garnet grains with different radii simulated for the t tpath using a numerical model of continuous nucleation and diffusion controlled growth in combi nation with intracrystalline diffusion compares well with the observed zoning patterns in garnet grains with different radii this is in spite of the fact that the simulated zoning patterns vary greatly depending on subtle differences in the t thistory therefore they suggest that the t tpath gives a good explanation for the lpht ryoke metamorphism although this study only refers to the ryoke metamorphism the technique may be applicable to thermal modelling of other metamorphic terranes

exhumation process of the nago subduction related metamorphic rocks okinawa ryukyu island arc abstract the kunigami zone in okinawa is an extension of the shimanto zone japan the rocks make up the main part of the nago metamorphic rocks and such metamorphic rocks are exceptional in the shimanto zone the anne complex in the older motobu zone is also metamorphosed the reason for why and how this kind of the metamorphism occurred and especially why and how the metamorphic rocks were exhumed is yet uncertain and unresolved to understand the metamorphic and exhumation process in okinawa a structural study is undertaken and its relation to the eocene ridge subduction is discussed we believe exhumation was performed by formation of a d2 extrusion wedge made up of the nago metamorphic rocks the base for this wedge is a subduction thrust and the roof is a detachment fault internally there exists another kijoka detachment fault which is a brittle low angle fault with top to the northwest shear sense and the d2 major recumbent folds and thrusts show top to the southeast opposite shear sense in the kunigami zone this is the first report that finds detachment faults from the typical and ancient accretionary complex m2 is mostly retrograde related to exhumation but its medium p t type prograde metamorphism abnormal at subduction zones represents a high thermal gradient during ridge subduction as a result this ridge subduction is responsible for exhumation at the time of accretion of the kunigami zone d1 ductile contraction and constriction exhibited top to the southeast shear sense but an opposite and extensional shear sense is recognized in the proto wedge during d1 the wedge had already been active and begun to exhume m1 of the miyagi complex is accretion related and also of medium p t type metamorphism and is a consequence of cretaceous ridge subduction without any ability to cause much exhumation

cooling history of the sanbagawa metamorphic belt inferred from fission track zircon ages abstract the sanbagawa metamorphic belt in central shikoku japan is a classic example of a high pressure intermediate facies series metamorphic belt the lowest structural horizon of the sanbagawa belt exposed in central shikoku is the oboke unit and this is overlain by the besshi unit which contains high grade mineral zones seven fission track ft zircon ages were obtained from five fractions of felsic igneous pebbles in conglomerate the matrix schist of conglomerate and an intercalated psammitic schist from the oboke unit the ages are in the range 56 69 ma and are younger than 40ar 39ar ages previously reported from the oboke unit 70 77 ma these concordant grain ages are interpreted as being post metamorphic cooling ages because the samples analyzed are composed of zircon crystals of igneous and various detrital origins and therefore of variable provenance age the cooling rate of the oboke unit is less than 8 c ma which is lower than that obtained for the besshi unit our results of ft zircon dating are close to those of the melange zone of the northern cretaceous shimanto belt ca 65 ma this suggests that the underplating of the high pressure type sanbagawa metamorphic rock occurred contemporaneously within the same subduction system as the growth of the shimanto accretionary complex

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

exhumation rates and age of metamorphism in the sanbagawa belt new constraints from zircon fission track analysis zircon fission track dating and track length analysis in the high grade part of the asemigawa region of the sanbagawa belt demonstrates a simple cooling history passing through the partial annealing zone at 63 2 5 8 2 ma combining this age with previous results of phengite and amphibole k ar and 40ar 39ar dating gives a cooling rate of between 6 and 13 c myr 1 which can be converted to a maximum exhumation rate of 0 7 mm year 1 using the known shape of the p t path this is an order of magnitude lower than the early part of the exhumation history in contrast zircon fission track analyses in the low grade oboke region show that this area has undergone a complex thermal history probably related to post orogenic secondary reheating younger than c 30 ma this event may correlate with the widespread igneous activity in south west japan around 15 ma the age of subduction related metamorphism in the oboke area is probably considerably older than the generally accepted range of 77 70 ma

world s youngest blueschist belt from leti island in the non volcanic banda outer arc of eastern indonesia abstract the timor tanimbar region constitutes part of the non volcanic outer banda arc of eastern indonesia here the world s youngest a type high pressure metamorphic belt crops out with different stages of evolution whereas an advanced domal uplift stage is seen in timor island the high pressure hp metamorphic belt is still in the first stage of tectonic extrusion on the eastern small islands of kisar leti moa sermata and laibobar the metamorphic rocks on leti are among the best exposed in the islands they are tectonically juxtaposed against overlying ultramafic rocks and underlying unmetamorphosed continental shelf sediments bound by normal and reverse faults respectively the leti metapelites display four progressive metamorphic zones chlorite biotite garnet chloritoid staurolite and kyanite zones with increasing grade zonation in leti metabasite unit is comparable and progressively changes from blueschist greenschist transition bs gs through epidote amphibolite ea to amphibolite am facies with increasing grade the highest grade metapelites and metabasites occupy structurally intermediate levels overprinting mineral relationships in bs gs transition schists indicate that decompression occurred from 5 5 7 kbar to the protoliths of leti metamorphic rocks are originally permo triassic the younger sediments and igneous rocks at the margin of the northward advancing australian continent entered the subduction zone immediately prior to the commencement of the banda arc australia collision in the pliocene burial reached a maximum depth of 30 35 km at the main stage of peak p t conditions and was related to the main stage of the collisional event slab break off at depth in the collision zone facilitated rapid uplift by wedge extrusion and active erosion during the exhumation stage the tectonic juxtaposition of the schist units at mid crustal levels and hydration may correspond to the final stage of barrovian overprinting which continued through transportation from depth the leti island belongs to the eastern portion of the world s youngest blueshist belt which has not yet entered the second stage mountain building and is slightly younger than the western timor which evolved to the extensive mountain building stage by doming

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

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

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

coupling of early tertiary extension in the great valley forearc basin with blueschist exhumation in the underlying franciscan accretionary wedge at mount diablo california in the california coast ranges east of san francisco post 3 5 ma folding and erosion of the mount diablo anticline have created unusual three dimensional exposures of late mesozoic cenozoic stratigraphic and structural relations among rocks of the franciscan subduction complex coast range ophiolite and great valley forearc basin these relations offer new insights into the kinematics of late cretaceous early tertiary synsubduction extension attenuation and blueschist exhumation within the ancestral california forearc region map relations and subsurface data reveal that marine strata of the great valley forearc basin northeast of mount diablo accumulated in an 100 km long north south trending extensional graben system normal faults in the graben system cut steeply downsection through late cretaceous and early tertiary growth strata and terminate against the clayton marsh creek fault which is at a low angle to bedding and is interpreted to be the basal detachment for part of the graben system the clayton marsh creek fault in turn is linked to a system of faults that juxtapose blueschist facies rocks of the underlying ancestral franciscan accretionary prism against attenuated remnants of forearc crust that were never deeply buried including the coast range ophiolite and basal great valley strata these faults are the local expression of the coast range fault in this area franciscan rocks exposed beneath the faults were metamorphosed at 20 km depths after 108 ma based on jadeitic pyroxene and 108 ma detrital zircons u pb in metagraywackes apatite fission track data indicate that these blueschist facies rocks were subsequently uplifted and exhumed from 9 km to 3 km depths in latest cretaceous early tertiary time coeval with the subsidence and extension in the structurally overlying forearc graben system much of their earlier rise from 20 km to 9 km depth presumably also occurred coeval with graben development although our data cannot determine this directly these relations demonstrate that the california forearc around mount diablo was in an extensional regime undergoing active sedimentation during major franciscan exhumation and thus support models for exhumation of franciscan blueschist facies rocks via synsubduction extensional processes rather than via erosional processes

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

behavior of zircon in the upper amphibolite to granulite facies schist migmatite transition ryoke metamorphic belt sw japan constraints from the melt inclusions in zircon behavior of zircon at the schist migmatite transition is investigated syn metamorphic overgrowth is rare in zircon in schists whereas zircon in migmatites has rims with low th u that give 90 3 2 2 ma u pb concordia age between inherited core and the metamorphic rim a thin dark cl annulus containing melt inclusion is commonly developed suggesting that it formed contemporaneous with the rim in the presence of melt in diatexites the annulus is further truncated by the brighter cl overgrowth suggesting the resorption and regrowth of the zircon after near peak metamorphism part of the zircon rim crystallized during the solidification of the melt in migmatites preservation of angular shaped inherited core of 5 10 m in zircon included in garnet suggests that zircon of this size did not experience resorption but developed overgrowths during near peak metamorphism the ostwald ripening process consuming zircon less than 5 10 m is required to form new overgrowths curved crystal size distribution pattern for fine grained zircons in a diatexite sample may indicate the contribution of this process zircon less than 20 m is confirmed to be an important sink of zr in metatexites and ca 35 m zircon without detrital core are common in diatexites supporting new nucleation of zircon in migmatites in the ryoke metamorphic belt at the aoyama area monazite from migmatites records the prograde growth age of 96 5 1 9 ma using the difference of growth timing of monazite and zircon the duration of metamorphism higher than the amphibolite facies grade is estimated to be ca 6 myr

exhumation of triassic hp lt rocks by upright extrusional domes and overlying detachment faults ishigaki jima ryukyu islands abstract the tomuru formation of ishigaki jima in the southernmost part of the ryukyu arc comprises blueschist facies subduction complex rocks metamorphosed in triassic time d1 structures related to subduction blueschist facies mineral growth and possibly early stages of exhumation are deformed by d2 structures that appear to reflect the last stage of exhumation d2 structures define several anticlines with parasitic overturned folds verging away from anticlinal axes the shortening recorded by this deformation appears to reflect upward extrusion relative to flanking material the anticlines are flanked by detachment faults with normal sense of shear parallel to d2 vergence hanging wall rocks that include the fusaki formation an accretionary prism with early cretaceous metamorphic ages and late eocene limestone conglomerate and andesitic volcanics the eocene strata contain metamorphic detritus derived from the tomuru and fusaki formations indicating pre late eocene surface exposure of these units ultramafic rocks and gabbro blocks of the tomuru formation were incorporated by sedimentary sliding into the trench prior to subduction and high pressure metamorphism rather than being emplaced as diapirs along a post metamorphic fault as previously proposed geochronologic metamorphic and thermal considerations suggest exhumation of the tomuru formation to relatively shallow crustal depths prior to or concurrent with early cretaceous metamorphism of the fusaki formation arcward vergent thrusting may have placed the younger and formerly structurally lower subduction complex fusaki formation over the older one tomuru formation d2 extrusional doming began after the emplacement of the fusaki formation at high structural levels the d2 transport directions are subparallel to the strike of the orogen suggesting that the upright extrusion may have occurred along a forearc strike slip fault system this final stage of exhumation concluded in the late eocene with extensional collapse and the development of detachment faults the progression from initial exposure of the tomuru and fusaki formations deposition of late eocene strata extrusional doming and late detachment faulting may have been associated with migrating step overs rather than changes in regional tectonics

thermobaric structure of the franciscan complex in the pacheco pass region diablo range california examination of the thermobaric structure of jadeite glaucophane bearing franciscan rocks in the pacheco pass region of the diablo range show that they are subhorizontal and the structurally intermediate horizon has the highest metamorphic grade mineral parageneses of more than 700 metagraywackes plus 38 metagraywacke samples from up to 1200 vertical feet of drill core were examined modal compositions of about 100 metagraywackes were determined and more than 2600 microprobe analyses were completed to quantify compositions of jadeitic pyroxene from about 80 metagraywackes the jadeite jd zone is defined by the assemblage jadeitic clinopyroxene quartz pm albite lawsonite pm sodic amphibole pm aragonite this zone occurs at an intermediate structural position sandwiched between subhorizontal jadeitic clinopyroxene free albite ab zones transition zones occur both above and below the jd zone the jd zone covers an area 20 km wide and 400 m thick the nearly flat lying boundaries betwee