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

coastwise transport reconsidered lateral displacements in oblique subduction zones and tectonic consequences coastwise transport margin parallel displacement of arc and forearc slivers is a common feature of zones of oblique subduction today but at rates that rarely exceed a few tens of kilometers per million year paleomagnetic studies strongly indicate that coastwise transport in the north american cordillera went on during late cretaceous early tertiary time at rates several times higher than this other studies have shown that obliquity is commonly partitioned between oblique subduction and coastwise transport of a detached sliver of over riding crust the analysis developed here indicates that in the absence of a buttress a physical impediment to sliver motion at its leading edge sliver motion should obey the relationship tan p r tan 1 1 r2 sin2 12 where is the angle of obliquity is the angle of subduction and r rfrs is the ratio of resistance to slip on strike slip faults cutting the over riding plate to resistance to slip on the subduction zone in this equation p is a partitioning factor 0 p 1 where p 1 implies oblique subduction and no sliver motion and p 0 is the case in which the sliver moves at velocity v sin equivalent to the transverse component of relative motion rates of coastwise transport calculated from this relationship are systematically too high presumably indicating that most modern subduction zones are buttressed key factors in the buttress effect are 1 the greater strength of oceanic vs continental lithosphere and 2 the energy required to thicken continental crust large scale tectonic transport in the north american cordillera probably was the result of a combination of favorable factors long continued highly oblique convergence and a plate margin geometry that avoided the buttress problem

igneous microstructures in migmatites clear differences between the microstructures of leucosomes and mesosomes occur in some nebulitic migmatites in the proterozoic arunta block central australia the leucosomes show crystal faces of k feldspar microcline microperthite cordierite andalusite and plagio clase against quartz indicating crystallization of a melt in contrast the mesosomes show predominantly polygonal grain shapes modified by 001 crystal faces of biotite and rare crystal faces in some porphyroblasts of garnet cordierite and orthopyroxene indicating re crystallization in the solid state the occurrence of abundant crystal faces in minerals such as feldspar cordierite and andalusite which typically grow crystal faces in magmas but generally not in metamorphic rocks is evidence of former melt therefore igneous microstructures can be preserved in leucosomes that have not undergone penetrative subsolidus deformation and recrystallization the cores of many of the crystals of cordierite and k feldspar have small locally oriented inclusions similar to those in the same minerals in the mesosomes these indicate that the cores remained solid during crystallization of the leucosome which produced inclusion free euhedral rims these crystals provide criteria for recognizing possible restite in granitoid magmas

thermal consequences of mantled gneiss dome emplacement abstract bodies of rock within the crust that have higher thermal conductivities than surrounding rocks serve as conduits for the diffusive flow of heat the resultant focusing of heat creates an apparent contact metamorphic aureole in the surrounding rocks without the high conductivity rocks being a heat source themselves two dimensional thermal models of mantled gneiss domes are used to consider these effects and their role in the thermal histories of metamorphic belts the complex metamorphic histories and patterns observed around gneiss domes may be the result of the emplacement of relatively high thermal conductivity rocks dome cores into lower thermal conductivity metasediments mantling strata the degree of the thermal anomaly is primarily affected by the conductivity contrast but is also dependent upon the velocity of emplacement crustal level and heat flux many of the metamorphic isograd features and pressure temperature time paths that are observed around mantled gneiss domes can be explained merely by the emplacement of high conductivity rocks into low conductivity rocks without invoking additional heat sources or significant post metamorphic deformation

estimates of modern arc parallel strain rates in fore arcs deflections of slip vectors of interplate thrust earthquakes from expected directions are used to estimate arc parallel strain rates within the overriding plates at the world9s major convergent plate margins arc parallel extension strain rates between 10 8 yr and 10 7 yr and significant at 2 standard deviations are observed in the fore arcs of the aegean aleutian mariana sumatran southern kuriles new hebrides scotia and southern central american lat 8 to 12 n subduction zones and the himalayas northern chile lat 17 to 31 s and central america lat 11 to 18 n show arc parallel compression available geologic and geodetic estimates of fore arc slip and strain rates agree within a factor of two with slip vector estimates arc parallel strain in fore arcs is rapid enough to produce geologically significant effects such as unroofing of high grade metamorphic rocks and disruption of transported fore arc terranes fore arcs deform even where convergence is perpendicular to curved margins demonstrating that head on subduction can produce a three dimensional strain field

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

ridge subduction kinematics and implications for the nature of mantle upwelling discussion the authors e farrar and j m dixon in a recent paper on ridge subduction discuss the magmatic and tectonic effects of overridden sea floor spreading centres their paper provides a useful compilation of information on several ridge trench encounters and elucidates a certain type of plate interaction called eduction however much of their analysis is flawed because they 1 do not consider the geometry of subducted slabs and intervening slab windows 2 misrepresent the importance of slab windows to magmatism and tectonism in the overriding plate and to mantle currents and 3 incorrectly link the concept of slab window formation to a model of passive mantle upflow consequently their paper overlooks the controlling influence that slab windows have on asthenosphere lithosphere interactions when diverging plates are subducted since the authors present their material in the style of a comprehensive review their omissions and misrepresentations warrant critical response numerous authors have established that ridge trench encounters tend to affect the overriding plate in three main ways 1 tectonic erosion of forearc and related uplift of forearc and arc 2 change in magmatic regime from arc type volcanism to riftor plume type volcanism extending from forearc to retroarc and 3 broad thermally supported uplift high heat flow and extension stacey 1974 fox et al 1985 merritts and vincent 1989 thorkelson and taylor 1989 hole 1990 hole et al 1991 babcock et al 1992 hole and larter 1993 references cited in the authors paper the authors concur and appeal to active asthenospheric upwelling e g hess 1962 gough 1984 wilson 1988 beneath the overriding plate as a principal cause although the authors depiction of active mantle dynamics is plausible the geological context of their arguments is unacceptable specifically they appeal to ridge subduction and mantle upwelling without providing a framework of subducted slab geometry without full appreciation of slab geometry and the constraints imposed by slab windows their arguments are limited to a vague precept of mantle convection

double ridge subduction recorded in the shimanto accretionary complex japan and plate reconstruction combining a quantitative model relating ridge subduction and radiolarian biostratigraphical data from the shimanto belt including accreted midocean ridge basalt the plate configuration for the western north pacific since 83 ma can be reconstructed the kula north new guinea and north new guinea pacific ridges passed along the japan are assuming a constant plate motion the half spreading rates angles at which ridges entered the trench convergent rates and angles and migration rates of triple junctions can be calculated

geologic consequences of plate reorganization an example from the eocene southern alaska fore arc observation of relative timing of deformation metamorphism and plutonism in a high temperature low pressure metamorphic belt in the eastern chugach mountains of alaska leads to a model of ridge subduction followed by plate reorganization to account for the abnormally high geothermal gradients in the fore arc between 56 and 53 ma a change in the direction of the kula farallon spreading halted the previous southward migration of the kula farallon north american triple junction this forced the triple junction to migrate back north along the plate margin enlarging the slab window beneath the accretionary margin the expanded slab window produced a large scale thermal manifestation now recognized as the chugach metamorphic complex the accretionary complex responded to plate reorganization by orogen parallel extension associated with oblique subduction of the kula plate contraction began again following passage of the triple junction and subduction of the farallon plate

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

low pressure facies series metamorphism in an accretionary sedimentary prism southern alaska the low pressure high temperature metamorphism of the chugach metamorphic complex alaska occurred in an ocean continent convergence zone to achieve the high temperatures at a relatively shallow depth in an accretionary prism we propose the large scale transport of heat by fluids which preheated the metamorphic belt by tectonic focusing of fluids followed by injection of melts both of which were generated downdip in a shallow subduction zone

ridge subduction kinematics and implications for the nature of mantle upwelling ridge subduction follows the approach of an oceanic spreading centre towards a trench and subduction of the leading oceanic plate beneath the overriding plate there are four possible kinematic scenarios 1 welding of the trailing and overriding plates e g aluk antarctic ridge beneath antarctica 2 slower subduction of the trailing plate e g nazca antarctic ridge beneath chile and pacific izanagi ridge beneath japan 3 transform motion between the trailing and overriding plates e g san andreas transform or 4 divergence between the overriding and trailing plates e g pacific north america in case 4 the divergence may be accommodated in two ways the overriding plate may be stretched e g basin and range province extension which has brought the continental margin into collinearity and therefore transform motion with the pacific north america relative motion or divergence may occur at the continental margin and be manifest as a change in rate and direction of sea floor

p t evolution of abukuma metamorphic rocks in north east japan metamorphic evidence for oceanic crust obduction the abukuma metamorphic terrane of northeast japan is well known for the type terrains of andalusite sillimanite progressive metamorphism of miyashiro s classic studies 1958 1961 later however relict kyanite was found in the high grade pelitic gneisses kano kuroda 1968 and there have been arguments about whether the abukuma metamorphism was plurifacial or polymetamorphic earlier kyanite sillimanite type and later andalusitesiuimanite type since 1984 we have been studying the geology and petrology of the abukuma metamorphic rocks to reveal their p t t paths and the tectonic evolution of the terrane the purpose of this paper is to outline our new insight into the p t t paths of rocks and their correlation with deformation events of the terrane detailed geology mineralogy and petrology will be presented elsewhere in this paper mineral abbreviations of kretz 1983 are used to represent compositions and reactions of minerals geological and petrological setting the abukuma metamorphic terrane is one of the major geological units of north east japan which is bounded from south west japan by the tanakura tectonic line fig 1 the abukuma metamorphic rocks which are extensively intruded by the cretaceous plutonic rocks shibata uchiumi 1983 shibata tanaka 1987 are lithologically divided into two the gosaisyo series to the north east and the takanuki series to the south west fig 1 the gosaisyo series is composed mainly of metabasites and metacherts with subordinate amounts of meta ultrabasites metapelites and limestones having affinities with oceanic crust original structures and textures however are poorly preserved owing to intense deformation hiroi et al 1987 reported

breakup of young oceanic lithosphere in the upper part of a subduction zone implications for the emplacement of ophiolites it is investigated by means of numerical modelling whether the interaction between young oceanic lithosphere and a subduction zone can lead to the breakup of a young and thin oceanic plate first a thermal model of the upper part of a subduction zone the region between the trench and the volcanic arc is presented temperatures are modelled for situations in which the age of the subducting oceanic lithosphere gradually decreases culminating in the arrival of a spreading ridge at the trench temperatures are used to infer the strength of material within the subducting plate from a pressure and temperature dependent rheology next it is investigated by means of finite element modelling whether the forces acting upon young subducting oceanic lithosphere recently created at a spreading center in the vicinity of a subduction zone can lead to its breakup our modelling results show that very young oceanic lithosphere may indeed break up during the early phase of its subduction whether breakup occurs depends upon the spreading velocity and the length of the ridge segment that interacts with the trench if breakup occurs some young oceanic lithosphere situated in the upper part of the subduction zone and in the region between the ridge and the trench will be detached from the other parts of the subducting plate the detached sheet of thin oceanic lithosphere will not be subducted to mantle depths instead subduction of the trailing plate at the opposite side of the ridge can lead to the incorporation of this sheet into the forearc region our modelling provides a mechanism to incorporate thin oceanic lithosphere into an arc trench region and eventually upon closure of the ocean basin into an orogenic belt it is shown that a large subset of ophiolites those with a harzburgite dominated mantle section has properties in good agreement with an emplacement history in which the breakup of young oceanic lithosphere in the upper part of a subduction zone is the first phase

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

paleogene metamorphism of an accretionary flysch terrane eastern gulf of alaska reconnaissance field studies have identified a regional metamorphic complex at least 200 km long and 25 to 50 km wide in the eastern chugach mountains southern alaska the metamorphic complex was developed within the cretaceous valdez group a flysch sequence that was accreted to the continental margin during the late cretaceous or early tertiary the metamorphic complex shows the progressive development from subgreenschist and green schist facies slate phyllite and semischist that is regionally characteristic of the valdez group to biotite plagioclase quartz schist muscovite cordierite andalusite staurolite garnet and sillimanite in an intermediate zone and amphibolite facies muscovite biotite plagioclase quartz schist and gneiss sillimanite and garnet in a higher grade central zone k feldspar is characteristically absent in the amphibolite facies schists but it is present in migmatitic rocks that are common to the central zone the mineral assemblages and petrographic relations indicate that metamorphism took place under low pressure intermediate facies series conditions partial melting took place at the highest grade of metamorphism melting produced granodioritic magmas in migma titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex the timing of metamorphism as indicated by field relations and k ar data is about 50 to 65 m y ago and overlaps or is coeval with the emplacement throughout the gulf of alaska region of early tertiary plutons that are thought to be anatectic in origin the distribution of these plutons suggests that a high temperature low pressure metamorphic belt about 1 800 km long developed along the leading edge of the continental margin in late paleocene to early eocene time

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

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

monazite xenotime miscibility gap thermometry i an empirical calibration rare earth element ree and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the variscan fold belt in ne bavaria germany the metapelites include a continuous prograde mainly low p 3 5 kbar metamorphic profile from greenschist c 400 c to lower granulite facies conditions c 700 c the lree la sm are incorporated preferentially in monoclinic monazite reo9 polyhedron whereas the hree plus y are concentrated in tetragonal xenotime reo8 polyhedron the major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade monazite consists mainly of la ce and nd la0 20 0 23 ce0 41 0 45 nd0 15 0 18 po4 all other elements are below 6 mol likewise xenotime consists mainly of ypo4 with some dy and gd solid solutions y0 76 0 80 dy0 05 0 07 gd0 04 0 06 in contrast the minor hree concentrations in monazite increase strongly with increasing metamorphic grade y dy and gd increase by a factor of 3 5 from greenschist to granulite facies rocks monazite crystals often show zonation with cores low in hree and rims high in hree that is interpreted as growth zonation attained during prograde metamorphism similarly sm and nd in xenotimes increase by a factor of 3 4 with increasing metamorphic grade prograde zonation in single crystals of xenotime was not observed the xhree y in monazite and xlree in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c 400 c to 700 c the empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry due to its contents of u and th monazite has often been used for u pb age determination the combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature time 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

aeromagnetic evidence for a buried early cretaceous magmatic arc northeast japan positive aeromagnetic anomalies recent drilling and models constructed from these data delineate the plutonic roots of the early cretaceous kitakami magmatic arc in northeast japan buried plutons mostly offshore produce belts of positive magnetic anomalies these anomalies and magnetotelluric data suggest that the plutons form a batholith 70 120 km wide nearly 800 km long and 10 15 km thick the batholith may mark the location of the main kitakami arc most of the exposed kitakami plutons are 2 20 km in diameter some are 3 km thick the small plutons line up along nw trending faults some may have been satellite vents that tapped into the magma supply of the main arc the batholithic roots of the main arc now compose almost half of the modern japan forearc basement steep magnetic gradients offset anomalies and basin stratigraphy portray extensive faulting of the kitakami batholith during oblique subduction in the late cretaceous and rifting in the miocene the eastern boundary of the kitakami batholith lies between 90 and 140 km west of the modern trench much closer than the 300 km distance between the active arc and trench the early cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of the batholith but clear evidence is lacking much of the early cretaceous margin including most of its forearc therefore is missing how the material was removed is unknown it could have been strike slip faulted eroded by subduction related processes or both

ridge collision at convergent margins implications for archean and post archean crustal growth abstract growth of continental crust today occurs dominantly at convergent plate margins through magmatic addition and tectonic accretion lateral and vertical related to 1 1 steady state subduction 2 2 non steady state episodic processes ridge forearc collision is one episodic process that can cause significant highly diachronous thermal and tectonic effects when a buoyant bathymetrically high and thermally active oceanic spreading ridge approaches collides with and potentially subducts below a continental margin near field effects within 100 km of collision site in the forearc may include anomalous heat anomalous magmatism ophiolite obduction forearc deformation and tectonic erosion of the base of the forearc far field effects generally postdate collision and may include changes in the thermal structure of the continent changes in the composition of arc magmas a reduction or hiatus in arc magmatism uplift and or subsidence of the continental margin marginal basin formation underplating of subducted slabs and intraplate deformation effects of ridge collision can vary depending on the triple junction geometry the chile margin triple junction cmtj where the actively spreading chile ridge is colliding with the south american forearc is the best modern example of ridge collision near field effects of pliocene recent ridge collisions near the cmtj include 1 1 emplacement of the pliocene taitao ophiolite less than 15 km from the trench 2 2 intrusion of pliocene granitic plutons and 3 3 forearc deformation formation of a pull apart basin on the continental shelf and late tertiary recent movement on a major fault along the arc forearc boundary far field effects include reduction in subduction related seismicity and volcanism chemical changes in arc magmatism and possibly uplift and or subsidence ridge collison features in southern chile are similar to features of archean greenstone belts suggesting that some greenstone sequences may have formed in a ridge collision setting and that ridge collision may have been an important process in archean crustal growth these features include granitic rocks closely associated with ophiolitic rocks formed near a continental margin mafic ultramafic section overlain by basaltic to andesitic volcanic rocks of mixed calc alkaline and morb character intercalated with volcaniclastic sedimentary rocks differences between archean and phanerozoic tectonic processes mainly related to higher heat production in the past probably have affected the nature and importance of ridge collision in archean crustal growth during the archean greater ridge length and smaller plates probably resulted in more ridge collisions and related processes contributing to a higher continental growth rate in particular underplating of subducted slabs and direct slab melting may have been important processes in archean continental growth but are of relatively minor importance today

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

the interpretation of inverted metamorphic isograds using simple physical calculations several zones of major thrust faulting exhibit a juxtaposition of rocks of higher metamorphic grade over rocks of lower grade this configuration may indicate but does not require that temperature gradients were temporarily inverted near the fault we examine the physical conditions under which such a temperature inversion could occur the overthrusting of hotter rock on colder rock can cause a temporarily inverted gradient both above and below the fault only if the time taken to underthrust rock from the land surface to a given depth z on the fault is less than times the characteristic time for diffusion of heat through the block above the fault where is thermal diffusivity an inverted gradient cannot form without heat generation in the fault zone unless v z sin 100 where v is the rate of underthrusting in millimeters per year z is in kilometers and is the dip of the fault this simple criterion is sufficient to demonstrate that several examples of inverted metamorphic gradients cannot be explained simply by the thrusting of hot on cold rock without heat sources in the fault zone dissipative heating accompanying deformation can cause an inverted temperature gradient within and beneath the thrust zone but whereas the overthrusting of hot upon cold rocks cools the fault zone dissipation heats it thus the overthrusting of hot on cold rock and dissipative heating affect the temperature gradient and the maximum temperatures differently we show that the magnitudes of the inverted temperature gradient and of the maximum temperature above the inverted gradient yield independent estimates of the rate of dissipative heating discrepancy between these estimates implies that some additional process must have occurred such as the post thrusting disruption of the isograds if there is such a discrepancy the maximum temperature probably provides the more reliable estimate of the rate of heating at the fault we illustrate this analysis by applying it to reported inverted metamorphic zonation in the pelona schist the st anthony complex the mt everest region of the main central thrust and the olympos thrust petrological inferences of maximum temperatures depths of metamorphism and magnitudes of apparent inverted gradients imply that shear stresses of about 100 mpa accompanied thrust faulting in some of these regions and that some zones of inverted metamorphism have been tectonically thinned after the metamorphism occured

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

chronology of sanbagawa metamorphism by collating age data based on the fossil age of the protoliths radiometric dating of the metamorphic minerals and sedimentary records of erosion at the earth s surface the history of the sanbagawa metamorphism can be summarized as follows 1 the pre metamorphic sedimentary rocks carboniferous jurassic early cretaceous became mixed and formed a thickened packet in the vicinity of an ancient trench through a variety of subduction related tectono sedimentary processes probably in early cretaceous time c 130 120 ma 2 the subducted protoliths underwent progressive metamorphism reaching a maximum depth of c 30 km in late early cretaceous time c 116 10 ma 3 the high p t metamorphic rocks began to rise toward the surface during the interval 110 50 ma with minimum estimates for the average cooling rate around 9 12 c ma and an average uplift rate around 0 4 0 5 mm year 4 finally at some stage after reaching the erosional surface the high p t metamorphic rocks were covered unconformably by the middle eocene c 50 42 ma kuma group on the basis of the present chronological summary of the sanbagawa metamorphism the areal extent of the sanbagawa metamorphism is also discussed with respect to the weakly metamorphosed subduction accretion complex of the next tectonic belt to the south the northern chichibu belt

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

trench proximal volcanism following ridge crest trench collision along the antarctic peninsula on the basis of multichannel seismic reflection magnetics and bathymetry data a pleistocene holocene 0 1ma volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the antarctic peninsula the seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3 1 and 6 0 ma ago new trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt the basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts e g th ta 1 0 1 5 rb nb 0 5 87sr 86sr 0 70265 0 7028 143nd 144nd approximately 0 5129 and are totally lacking in any evidence for interaction with subduction enriched mantle geochemical similarities with other more trench distal postsubduction alkalic basalts along the antarctic peninsula are striking strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region the basalts were most likely to have been generated as a result of the formation of a slab window beneath the antarctic peninsula following ridge crest trench collision subduction component free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision and decompressional melting resulted this type of trench proximal volcanism following ridge crest trench collision differs from that in other locations where calc alkaline volcanism persisted or ophiolite obduction occurred

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

cenozoic volcanism in the california coast ranges numerical solutions we present the results of numerical simulations of cenozoic volcanism in the coast ranges of california associated with the northward migration of the mendocino triple junction mtj three aspects of the thermal evolution and magmatism in the wake of the mtj are investigated 1 mantle flow and thermal perturbation in the slabless window 2 pressure release partial melting in the upwelling asthenosphere and 3 thermally induced crustal anatexis numerical results show that upwelling of asthenosphere in the slabless window causes a drastic change in the thermal structure of the lithosphere and results in significant pressure release partial melting in the upwelling asthenosphere up to 4 5 km of basaltic magma may be produced most of this mafic magma is likely stored near the base of the crust thermally induced crustal anatexis mainly occurs in the deep crust below 20 km where a high degree 50 of partial melting may be predicted most of the crustal melt is probably generated within 1 m y after the passage of the mtj this work also suggests that the spatial and temporal distribution of cenozoic volcanism in the california coastal ranges is directly related to the variation of the relative velocity between the north america and pacific plates in the last 30 m y the voluminous volcanic rocks in the northern california coast ranges correlate to the relatively high velocity between these two plates in the last 10 m y the reduced volcanism in the central coast ranges may be related to the slow migration of the mtj between 10 and 20 ma

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

problems with inferring p t t paths in low p granulite facies rocks microstructural evidence commonly is used to infer metamorphic reactions which are used to infer pressure temperature time p t t paths however this approach in low p high t lpht granulite facies metamorphic terranes has two main problems 1 microstructural evidence may be inconclusive so that reactions cannot be inferred with confidence in particular relative timing of mineral growth inferred from inclusions moulding relationships and foliation porphyroblast relationships is commonly ambiguous or invalid the most reliable indicators of metamorphic reactions are partial pseudomorphs and corona structures especially if symplectic intergrowths indicating simultaneous growth of two or more minerals are involved 2 even reactions that can be inferred with confidence do not indicate unique p t trends owing to p t slopes of reaction curves where successive reactions can be shown to have occurred in the same rock a line or curve joining reaction curve intersections gives an apparent single event path however isotopic evidence is needed to prove that polymetamorphism involving more complex paths making fortuitous intersections with the apparent single event path did not occur although these problems are well known their importance is not always emphasized in metamorphic investigations the difficulties are illustrated by published work on p t t paths for proterozoic lpht granulite facies rocks of central australia and antarctica recent work in antarctica has shown that p t t paths may be episodic and more complex than the simple single event paths commonly inferred from microstructural evidence alone

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

structural patterns and tectonic evolution of supracrustal domains in the archean slave province canada a new plate tectonic model accounts for lithological relations and regional structural patterns in late archean supracrustal domains of the slave province multiple dykes and pillowed mafic flows most common in the western part of the province suggest sea floor spreading the mafic volcanics lying in narrow homoclinal belts stratigraphically below more extensive turbidites are viewed as megaxenolithic remnants of oceanic crust preserved on the periphery of granitoid plutons and blocks of sialic crust closure of an oceanic basin was marked by emplacement of the granitoid plutons and coeval felsic volcanics the latter predominating over mafic volcanics in northeastern domains the felsic calc alkaline magmas may have risen from a shallow dipping subduction zone westerly verging folds westerly convex fold arcs and inclinations of later foliations particularly in lower level rocks of higher metamorphic grade are in accord with underthrusting to the east the zone of underthrusting shifted progressi

earth science trickle down geodynamics analysis of the platinum group elements in a particular type of ancient volcanic rock provides clues about earth s early history as well as a fresh approach to understanding mantle dynamics

two types of ductile deformation regional metamorphic belt abstract the geometry of plastic strains of regional metamorphic rocks is divided into two types one is the uniaxial extension strain prolate and the other the biaxial shortening strain oblate from the plane strain simple shear to the uniaxial shortening geometry the uniaxial extension geometry is found in deformation of metamorphic rocks of the sambagawa ryoke kamuikotan and franciscan belts while the biaxial type is seen in deformation of the shimanto the alpine the appalachian and the caledonian metamorphic rocks the classification of the metamorphic belts by means of strain geometry and strain path is independent of the metamorphic facies series the difference in strain geometry of metamorphic belts is responsible mainly for the tectonic stress regime in metamorphism it is suggested that the biaxial type belongs to the collision type orogeny and the uniaxial type to the cordilleran type orogeny

short paper accretion and uplift in the shimanto belt sw japan k ar cleavage dating and fission track cooling ages have been combined with existing stratigraphic data to constrain the evolution of the shimanto belt a cretaceous neogene accretionary complex cleavage ages record two major phases of accretion during the late cretaceous and late eocene early oligocene the minimum time periods between sedimentation and deformation are constrained to 9 5 9 and 0ma intervals within four tectonic units containing youngest sediments of maastrichtian late eocene late eocene and early oligocene ages respectively one phase of middle to late miocene uplift is indicated for the whole study area by the fission track cooling ages frontal underplating combined with out of sequence thrusting are suggested as mechanisms for this regional uplift the cretaceous neogene shimanto belt fig la forms the outermost onshore exposure of accreted material in sw japan the boundaries of constituent accreted units of the shimanto belt are defined by a detailed stratigraphy taira et al 1980taira et al 1988 providing an opportunity to test the application of cleavage dating and fission track studies to accretionary complex evolution the butsuzo tectonic line fig la juxtaposes the shimanto belt to the jurassic sanbosan belt in the north younger accreted material extends offshore to the south as far as the present day boundary of the eurasian plate the nankai trough fig la continued accretion seaward of the shimanto belt has back rotated older accreted units to their present day steeply dipping orientations the type section of the shimanto belt taira et al 1980 lies in the eastern hata peninsula fig 1b which contains the study

high paleogeothermal gradient with ridge subduction beneath the cretaceous shimanto accretionary prism southwest japan paleotemperature in the jurassic chichibu and the cretaceous shimanto accretionary complexes southwest japan increases southward generally without regard to the early phase geologic structure there is an exceptional peak in the okitsu melange and the thermal structure probably formed after accretion of the complex the estimated geothermal gradient is more than 90 c km in the okitsu melange and more than 50 c km elsewhere in the complex the ophiolitic okitsu melange formed concurrently with eocene subduction of the kula pacific ridge under the cretaceous accretionary prism the previous thermal structure was overprinted by this event

carbon isotopic study on coexisting calcite and graphite in the ryoke metamorphic rocks northern kiso district central japan carbon isotope fractionation between coexisting calcite and grpahite 13ccc gr has been determined in metamorphosed limestones and calc silicate rocks from the ryoke metamorphic belt in the northern kiso district in this district the ryoke metamorphic rocks ranging from the lower greenschist facies to the upper amphibolite facies are widely distributed the fractionation of 13c 12c between calcite and graphite decreases regularly with increasing metamorphic grade and is independent of absolute 13c values of calcite this evidence suggests that carbon isotopic exchange equilibrium has been attained during metamorphism even in the greenschist facies and isotopic modification possibly caused by retrogressive metamorphism is not distinguished for t 270 650 c the fractionation is expressed by the following equation 13ccc gr 8 9 106t 2 7 1 t in k

structural metamorphic evolution of the tia complex new england fold belt thermal overprint of an accretion subduction complex in a compressional back arc setting rocks in the tia complex underwent a simple clockwise p t t deformation path moving from mid crustal levels at blueschist facies conditions d1 d2 6 kbar 200 c to upper crustal levels at high t amphibolite facies conditions d4 d5 2 5 kbar 600 c in a relatively short time 15 ma the rocks remained under these conditions for approximately 40 ma d6 before they cooled and were brought to the surface d7 d8 during d6 permian rift basins were opening as thermal gradients in the tia complex reached a maximum d1 2 deformation was associated with accretion subduction processes and was followed by progressively less intense mainly e w oriented compression and uplift during d3 5 bringing the rocks to within 2 kbars of the surface d6 is characterized by limited extensional deformation and final uplift took place during d7 8 along shear zones that have orientations similar to d3 5 thrusts the shift from subduction dominated processes to thermally dominated processes occurred during d3 4 uplift of 4 kbar around 310 300 ma this coincided with a change in tectonic setting of the tia complex from an accretionary prism to a back arc position as the subduction zone stepped eastward an increased heat flow associated with back arc processes gradually caused thermal weakening of the lithosphere which due to external subduction related compressive forces resulted in uplift d3 with continued heating of the crust thermal doming resulted and uplift stopped d4 5 whilst thermal weakening led to limited crustal collapse as permian basins opened d6 the extremely high heat flow that was maintained for 40 ma suggests a heat source external to the crust possibly associated with thermal relaxation dehydration and detachment of the remnant stab left beneath the new england orogen after the subduction zone shifted east a stable crustal thermal structure was re established by 260 ma as the crust strengthened and could further support subduction induced compressional structures d7 8