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

mechanics of oblique convergence the distribution and magnitude of the strike parallel component of velocity in an obliquely converging thrust wedge or accretionary prism are determined by the geometry and mechanical properties of the wedge a mechanical analysis based on the assumption of a critical or stable geometry of the wedge for which the rate of cross strike deformation is zero leads to the following conclusions for different bulk rheologies 1 in a linear viscous wedge the strike parallel motion relative to the underthrust slab decreases exponentially away from the rear and is effectively concentrated in a shear zone with a width comparable to the thickness of the wedge at the rear the wedge also deforms by corner flow producing a circulation in the cross strike plane the strike parallel and corner flow velocities depend on the thickness and viscosity of the wedge and on the shear stresses applied to its lower and rear boundaries convergence at the wedge front is normal to strike 2 a critically tapered perfect plastic wedge moves coherently without internal deformation for low and moderate obliquities of the convergence vector the wedge moves at the same velocity as the backstop upper plate for high angles of obliquity the wedge moves laterally relative to the underthrust slab at a maximum velocity dependent on its dimensions and the stress conditions on its boundaries so that it is separated from the upper plate by a strike slip fault defining a forearc sliver no geometrical configuration exists that allows the strike parallel motion to be distributed through the wedge 3 a noncohesive coulomb wedge behaves in much the same way as a plastic wedge but the geometry and velocity depend only on its mechanical properties and the shear stresses on its boundaries and they are independent of scale

viscous flow and deformation of regional metamorphic belts at convergent plate boundaries 1 viscous flow and deformation of a newtonian fluid between rigid boundaries have been modeled for investigating deformation of a forearc wedge and a regional metamorphic belt as a weak deforming zone between the plates at convergent boundaries three types of analytic formulations together with numerical formulations for a newtonian fluid with a constant viscosity are used for investigating various flows and deformation 1 flows induced by squeezing between two parallel boundaries 2 flows induced by squeezing and heterogeneous mass influx in the wedge between two oblique boundaries e g accretionary wedge over the subducting plate with the continental plate as a backstop and 3 flows induced by dragging along the boundaries and by homogeneous mass influx in the wedge as in type 2 all cases include strike slip movements of the boundaries which allows us to investigate three dimensional 3 d deformation e g 3 d corner flow the corresponding flow deformation of infinitesimal and finite elements and the timescale of flow are calculated for each configuration with various mechanical boundary conditions the model results show that configuration and mechanism of the flow can be inferred from the spatial variation of finite deformation e g deformed radiolarians as a strain marker and geometry of folding as finite deformation of a large block in particular prolate strain nearly parallel to the strike of subduction zone together with a large scale folding can be a good indicator for deformation in the forearc wedge associated with 3 d corner flow induced by oblique subduction deformation observed in the cretaceous regional metamorphic belts in southwest japan can be explained by this mechanism

on the role of the upper plate in great subduction zone earthquakes at subduction zones where convergence is not perpendicular to the trench i e oblique slip vectors of interplate thrust earthquakes are often systematically deflected away from the expected plate vector probably by deformation of the leading edge of the upper plate near the epicenters of great thrust earthquakes of mw 8 0 in this century that occurred at trenches systematic deflections of slip vectors for recent 1977 1992 5 interplate thrust earthquakes are significantly less than global averages statistical tests show that great earthquakes nucleate where deviations of slip vectors residuals are smaller and plate convergence is faster but not preferentially where the angle of obliquity is smaller than global averages the small slip vector residuals suggest that great earthquakes nucleate where forces in the forearc due to plate convergence are sustained elastically rather than anelastically and that shear stress on subduction thrust faults with great earthquakes is not necessarily higher than on faults without them shear stress magnitude is less important than rheology in determining the seismic behavior of subduction zones an observed decrease in the slip vector residuals with increasing rates of trench normal convergence is interpreted as evidence that the forearc is increasingly cooled and strengthened by faster subduction for the forearc to cool with increasing convergence rate the shear stress on the subduction thrust fault must be less than about 40 mpa lower temperatures on the fault surface and the possibility that thrust faults below undeforming forearcs are smoother than those below deforming forearcs both act to increase the frictional instability of the fault zone great thrust earthquakes at subduction zones seem to occur where convergence is fast and the forearc is cool and predominantly elastic the ability of the forearc of the upper plate to store elastic strain energy appears to be a major factor controlling where great subduction thrust earthquakes can and cannot occur

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

counterclockwise p t t paths from amphibolites franciscan complex california relics from the early stages of subduction zone metamorphism overprinting mineral relationships in amphibolites from the franciscan complex california suggest metamorphic evolution from high temperature amphibolite facies to the blueschist facies with increasing p t ratio a counterclockwise p t t path early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are in turn cut or rimmed by sodic amphiboles some amphibolites are overprinted successively by eclogite and blueschist facies assemblages clinopyroxenes are zoned with increasing jadeite component from core to rim multiple generations of phengites are present in the same sample and are more si rich with each successive generation of growth garnets in eclogite overprinted amphibolites are zoned with an initial mgo increase away from the core to a maximum mgo zone followed by mgo decrease toward the rim garnets in other amphibolites show continually increasing mgo from core to rim several samples representative of the variety of franciscan amphibolites are describ

thermal evolution and exhumation in obliquely convergent transpressive orogens most p t t path models to date have considered a linear erosion rate for exhumation from burial depth related to isostatic readjustment of crustal thickness a few have discussed extension enhanced exhumation erosional exhumation can only restore lower crustal rocks from the thickened mountain root to their previous original depth in the pre collisional crust one major assumption of all models to date is that the compressive forces responsible for crustal thickening cease before elevation and erosion begins however compression is often still active even if the crustal thickening has stopped further compression of the thickened and strongly deformed orogenic root is responsible for forceful exhumation extrusion of softened rocks upwards hence the rate of exhumation is related to the rate of convergence of colliding plates extrusional exhumation can elevate buried rocks to any depth depending on the action of fault shear systems in the extrusional exhumation models examined here the ascending rocks cool faster because they approach the zero temperature surface conditions more rapidly than by isostatic erosion the exhumation rate is also governed by the angle between the plate boundary and the displacement vector implying that the convergent plate boundaries are regarded as complex transpressive systems in which the degree of obliquity can be expressed by the ratio of pure to simple shear components a low ratio of pure simple shear typical for wrench dominated plate boundaries implies long distance horizontal transport from the buried position in the original orogen and longer periods during which metamorphic heating occurs a high ratio of pure simple shear characteristic for frontal like convergence implies a rapid exhumation without significant heating thus granulites barrovian type and blueschist facies metamorphism are characterised by increasing angle of obliquity 10 30 90 respectively the high temperature limit at low ratio of pure simple shear is the geotherm t and lies at temperatures that would commonly be taken to indicate that addition of mantle heat was required to generate hot geotherms and p t t paths wrench faulting over 100 s of kilometres at reasonable slow convergence rates could lead to extensive dehydration melting and granulite formation in large scale collisional orogens

formation burial and exhumation of basement nappes at crustal scale a geometric model based on the western swiss italian alps abstract information from geological and reflection seismic data from the western swiss italian alps and from numerical models is used to build a geometrical model that can explain some of the major tectonic metamorphic features of alpine type basement nappes the model gives a geometrical and mechanical explanation for the initiation burial and subsequent uplift and partial exhumation of basement nappes at a crustal scale three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures the first two stages are single vergent nw and correspond to the subduction of continental margin crust and the formation and uplift of high pressure rocks simple shear flow and superimposed wedge shaped pure shear flow is proposed for the creation and intrusion of high pressure nappes of the monte rosa type the third stage is characterized by a doubly vergent style with both pro and retro movements the former created nw vergent nappes as seen in the external alpine massifs and the latter caused backfolding and thrusting typical of the internal alps this third stage corresponds to the neoalpine movements in the western swiss italian alps and is accompanied by a generalized uplift mountain building and molasse sedimentation

backflow and margin parallel shear within an ancient subduction complex lineated and foliated tectonites of the teslin suture zone yukon represent the deep seated part of a west dipping subduction complex that formed seaward of western north america in permian triassic time the tectonites were progressively underplated onto the hanging wall plate during b type subduction with continued subduction and underplating the boundary between the hanging wall and subduction channel migrated oceanward resulting in growth of the hanging wail plate at depth these tectonites preserve a progressive record of backflow dextral margin parallel shear and downflow within the subduction channel early jurassic a type subduction of the leading edge of western north america resulted in eastward overthrusting of the tectonites as a coherent block onto north american strata these tectonites provide empirical evidence that backflow margin parallel displacement and downflow can all occur within the deeper parts of subduction zones

the importance of blueschist eclogite dehydration reactions in subducting oceanic crust the metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure temperature p t paths with a model of metabasalt phase equilibria in steady state subduction zones with high rates of shear heating the upper parts of the subducting oceanic crust progress through the greenschist amphibolite granulite eclogite facies whereas lower parts of the subducting oceanic crust progress through the blueschist eclogite facies in steady state subduction zones with moderate rates of shear heating most of the subducting oceanic crust passes through the blueschist eclogite transition in steady state subduction zones with low rates of shear heating the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km for oceanic crust containing 1 2 wt h 2 o dehydration will not begin until the onset of eclogite or amphibolite facies metamorphism depending on the p t path for many subduction zones the most important dehydration reactions in the subducting oceanic crust occur at the blueschist eclogite facies transition associated with the breakdown of lawsonite or clinozoisite glaucophane and chlorite large amounts of h 2 o released by blueschist eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas

slump strain indicative of paleoslope in cretaceous izumi sedimentary basin along median tectonic line southwest japan submarine slump folds are common in turbidites of the izumi group upper cretaceous south of osaka in southwest japan structural analysis of the folds indicates predominantly eastward movement of the slump sheet however it is noteworthy that this direction is generally opposite to westward paleocurrents obtained from sole marks but it coincides with the direction of younging of the strata and the plunge direction of macroscopic synclines in the izumi group the paleoslope inferred from the slump folds is believed to have been caused by the eastward migration of the depocenter of the izumi sedimentary basin this slumpi ng is thought to have followed tilting of the beds deposited by the westward current the migration of the depocenters and tilting are associated with left slip faulting on the bounding median tectonic line

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

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

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

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

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

kinematic analysis of the taiwan slate belt the taiwan slate belt can be subdivided into two structural domains based on fold and cleavage geometries and finite and incremental strain histories the two belts correspond generally to two topographic ranges in taiwan the hsuehshan range and the backbone range rocks in the hsuehshan range are characterized by upright symmetric folds and a steeply se dipping axial planar pressure solution cleavage synkinematic fibers in pressure shadows are typically straight parallel to cleavage and plunge downdip indicating that the rocks experienced a coaxial strain history in contrast rocks in the backbone range are characterized by inclined asymmetric folds that verge toward the foreland and display a moderately se dipping axial planar pressure solution cleavage synkinematic fibers are straight and parallel to the downdip stretching lineation in xy sections in xz sections however the fibers typically display a clockwise curvature viewed to the ne indicating a noncoaxial strain history we propose that the different strain histories are due at least in part to differences in the original tectonostratigraphic settings of the strata in the two ranges strata in the hsuehshan range are interpreted to have been deposited in a half graben associated with an east dipping normal fault the strata of the backbone range in contrast are considered to have been deposited on the unfaulted chinese continental margin we suggest that the basement horst on the east side of the half graben functioned as a steep faced rigid backstop that concentrated pure shear deformation throughout most of the hsuehshan range shear strains were apparently limited to zones near the base of the sedimentary sequence and adjacent to the backstop the noncoaxial strains characteristic of the backbone range are interpreted to be the result of a regional scale shear zone that formed as chinese basement and its sedimentary cover were underthrust southeastward beneath the luzon arc

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

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

prograde amphiboles in hematite bearing basic and quartz schists in the sanbagawa belt central shikoku relationship between metamorphic field gradient and p t paths of individual rocks the prograde amphibole that coexists with chlorite epidote muscovite albite quartz and hematite in sanbagawa schists was examined to investigate the relationship between the prograde p t paths of individual rocks and the metamorphic field gradient in the sanbagawa metamorphic belt central shikoku the amphibole changes from actinolite through ferri winchite and crossite to barroisite and hornblende with increasing grade along the metamorphic field gradient however the sequence of prograde amphibole compositions in each sample varies in different mineral zones the general scheme can be summarized as magnesioriebeckite riebeckite crossite in the upper chlorite zone of lower grade rocks crossite or glaucophane barroisite in the garnet zone of medium grade rocks and actinolite or winchite barroisite hornblende in the albite biotite zone of higher grade rocks changes of amphibole composition indicate that the prograde p t path recorded in the higher grade rocks was situated on the higher temperature side of that of the lower grade rocks and on the lower pressure side of the metamorphic field gradient the systematic change of p t paths implies an increasing dp dt during continuous subduction these features can be interpreted as documenting prograde metamorphism within a young subduction zone that has a non steady state geotherm

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

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

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

asymmetrical melange fabrics as possible indicators of the convergent direction of plates a case study from the shimanto belt of the akaishi mountains central japan abstract scaly foliated melanges in the upper maastrichtian lowest paleogene strata occurring in a zone several kilometers wide in the shimanto belt of the akaishi mountains central japan often exhibit composite planar fabrics on a mesoscopic scale with distinct monoclinic symmetry they are characterized by block in matrix fabrics including scaly foliations in mudstone matrices and asymmetric lensoidal and or ellipsoidal clasts of sandstone which were generated by extension parallel to the layers these fabrics are geometrically very similar to those of foliated fault related rocks another remarkable fabric is the imbricated stacks of lensoidal sandstone clasts forming mesoscopic duplexes by layer parallel contraction the co existence of these extensional and contraction planar fabrics suggests that non coaxial deformation had considerably progressed and that strain distribution was heterogeneous during formation of the melange systematic analysis of attitudes of these melange fabrics in many outcrops indicates that the fabrics have a remarkably consistent preferred orientation suggesting that a south trending overthrust shear about 40 70 counterclockwise with respect to the trend of strata prevailed during melange formation this direction is fairly consistent with the relative plate motion deduced from the regional tectonic framework of this age which indicates the oblique subduction of the pacific plate relative to the eastern margin of the eurasian continent thus the melange fabrics with monoclinic symmetry probably reflect the relative motion of consuming plates and may become excellent geological indicators for determining the direction of convergence of plates during melange formation since data on melange fabrics are directly and easily available in the field

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

merapi 2010 eruption chronology and extrusion rates monitored with satellite radar and used in eruption forecasting abstract despite dense cloud cover satellite borne commercial synthetic aperture radar sar enabled frequent monitoring of merapi volcano s 2010 eruption near real time interpretation of images derived from the amplitude of the sar signals and timely delivery of these interpretations to those responsible for warnings allowed satellite remote sensing for the first time to play an equal role with in situ seismic geodetic and gas monitoring in guiding life saving decisions during a major volcanic crisis our remotely sensed data provide an observational chronology for the main phase of the 2010 eruption which lasted 12 days 26 october 7 november 2010 unlike the prolonged low rate and relatively low explosivity dome forming and collapse eruptions of recent decades at merapi the eruption began with an explosive eruption that produced a new summit crater on 26 october and was accompanied by an ash column and pyroclastic flows that extended 8 km down the flanks this initial explosive event was followed by smaller explosive eruptions on 29 october 1 november then by a period of rapid dome growth on 1 4 november which produced a summit lava dome with a volume of 5 10 6 m 3 a paroxysmal vei 4 magmatic eruption with ash column to 17 km altitude destroyed this dome greatly enlarged the new summit crater and produced extensive pyroclastic flows to 16 km radial distance in the gendol drainage and surges during the night of 4 5 november the paroxysmal eruption was followed by a period of jetting of gas and tephra and by a second short period 12 h of rapid dome growth on 6 november the eruption ended with low level ash and steam emissions that buried the 6 november dome with tephra and continued at low levels until seismicity decreased to background levels by about 23 november our near real time commercial sar documented the explosive events on 26 october and 4 5 november and high rates of dome growth 25 m 3 s 1 an event tree analysis for the previous 2006 merapi eruption indicated that for lava dome extrusion rates 1 2 m 3 s 1 the probability of a large 1872 scale eruption was 10 consequently the order of magnitude greater rates in 2010 along with the explosive start of the eruption on 26 october the large volume of lava accumulating at the summit by 4 november and the rapid and large increases in seismic energy release deformation and gas emissions were the basis for warnings of an unusually large eruption by the indonesian geological agency s center for volcanology and geologic hazard mitigation cvghm and their volcano research and technology development center bpptk in yogyakarta warnings that saved thousands of lives

role of extensional tectonics in exhumation of eclogites and blueschists in an oblique subduction setting northeastern venezuela eclogites and blueschists in northeastern venezuela belong to a mid to late cretaceous subduction complex fore arc terrane related to right oblique subduction of the proto caribbean plate beneath the farallon plate along the leeward antilles volcanic island arc these rocks were metamorphosed at high pressures and subsequently underwent retrogradation at decreasing pressures but at constant or slightly increasing temperatures they underwent five phases of deformation three synmetamorphic and two postmetamorphic the major principal extensions x axes of each of the three synmetamorphic deformations are subparallel to the volcanic arc the extension parallel to the arc and the associated exhumation and uplift of the subduction complex fore arc terrane may have resulted primarily from an increase of the arc parallel displacement rate of this terrane caused by an increase of obliquity of plate convergence

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

underplated units in an accretionary complex melange of the shimanto belt of eastern shikoku southwest japan imbricated thrust stacks composed of melange and oceanic slabs occur in the shimanto belt eastern shikoku sw japan the melanges are divided into two types melange i consists of lenticular sandstone blocks surrounded by scaly shale matrix and melange ii is composed of blocks of oceanic material such as basalts cherts and red shale in a scaly matrix of shale melange ii is observed only near the boundaries between melange i and oceanic slabs the fabric of melange i indicates progressive deformation during underthrusting of originally coherent trench fill turbidites layer parallel extension characterizes early deformation and is divided into two stages an early stage formed during normal faulting and a subseguent stage formed perpendicular to the early extension the early extension is interpreted in terms of riedel shear associated with layer parallel shearing and tectonically induced loading normal to the layering the second extension is due to sticking thrust movement late deformation of melange i is represented by folding in association with layer parallel shearing this appears to be related to compression just before or during underplating of melange i beneath the accretionary prism layer parallel shear extends downward due to strain hardening of sheared sediments and then reaches to the base of the sediments subsequently the shear zone penetrates pelagic sediments and finally oceanic basement at this time melange ii is formed by the shearing an imbricated stack of melanges i ii and oceanic slabs forms duplexes during the underplating process the weak metamorphism of the melange complex in the study area suggests that underplating occurs at shallow depths where trench fill is thin

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

microstructures and flow mechanisms in regional metamorphic rocks of japan a number of microstructural features indicate a difference in the dominant deformation mechanism between the higher temperature ryoke and the lower temperature sambagawa and shimanto metamorphic belts of japan the microstructures of metacherts containing deformed radiolaria are divided into two types in both the sambagawa and shimanto belts the quartz grains are tabular while in the ryoke belt they are equiaxed tem studies of these metacherts revealed that the tabular grains contain abundant subboundaries consisting of large numbers of network dislocations and bowe out dislocations while the equiaxed grains contain no subboundaries and have low densities of dislocations which are not bowed out there is a corresponding difference in the textures lattice preferred orientation of quartz the ryoke metacherts display randomly distributed c axes of quartz while the sambagawa and shimanto metacherts show conspicuous crossed girdle patterns with some asymmetry there is a third difference between these regions in the metacherts of the ryoke metamorphic belt the strain magnitudes determined from deformed radiolaria increase with increasing volume fraction of mica in the same metamorphic p and t conditions while in the sambagawa and the shimanto metamorphic cherts the strain magnitudes decrease with increasing the mica fraction

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

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

middle paleocene terrane juxtaposition along the median tectonic line southwest japan evidence from 40ar 39ar mineral ages abstract the median tectonic line mtl is a regional transcurrent fault separating anadalusite sillimanite type metamorphic rocks and granitic rocks within the ryoke terrane from glaucophanic type metamorphic rocks within the sambagawa terrane in southwest japan the late cretaceous izumi group unconformably overlies ryoke metamorphic rocks and is tectonically separated from the sambagawa terrane by the mtl recent 40 ar 39 ar mineral ages reported for the sambagawa terrane in this area suggest extensive late cretaceous uplift however no sambagawa erosional debris has been described from the izumi group 40 ar 39 ar ages have been determined for hornblende and biotite separated from samples systematically collected along a traverse from the ryoke terrane into the mtl kashio area chubu district in addition whole rock samples of protomylonite within the mtl have been analyzed similar 68 70 ma isotope correlation ages are recorded by hornblende within massive ikuta and foliated minakata granite and protomylonite within the mtl biotite from these samples records ages ranging between 66 and 68 ma together the mineral ages suggest relatively rapid post magmatic cooling through appropriate argon closure temperatures these also indicate that temperatures maintained during protomylonite formation within the mtl were less than those required to effect even partial rejuvenation of intracrystalline argon systems within hornblende protomylonite samples yielded well defined whole rock plateau ages of 62 63 ma which are interpreted to closely date the last phase of ductile flow within the mtl these results suggest that a significant ductile phase of movement occurred within the mtl in the middle paleocene this may have been associated with significant sinistral displacement and resultant tectonic juxtaposition of previously separated portions of the sambagawa and ryoke belts with unconformably overlying izumi group

progressive refolding in ductile shear zones abstract the foliation formed in a ductile shear zone can become folded by continuing shear in the zone without the foliation having to enter the shortening sector of the flow field if the foliation lies parallel to the shear plane infinitesimal variations in the rate of shear strain cause compensating rotations that may amplify into folds small increases in the rate cause backward rotations counter to the sense of shear these do not amplify small decreases cause forward rotations that do amplify

tectonic history of subduction zones inferred from retrograde blueschist p t paths many phanerozoic convergent plate junctions are marked by discontinuous blueschist belts reflecting relatively high pressure p prograde trajectories common blueschist paragneisses such as those of the western alps exhibit widespread overprinting by greenschist and or epidote amphibolite facies assemblages for this type of high p belt retrograde metamorphism involved fairly rapid nearly isothermal decompression some terranes underwent continued heating during early stages of pressure release uplift probably occurred as a consequence of the entrance of an island arc oceanic plateau or segment of continental crust into the subduction zone collision resulting in marked deceleration or cessation of underflow and buoyant approximately adiabatic rise of the stranded recrystallized subduction complex other high p belts such as the franciscan of western california preserve metamorphic aragonite and lack a low p overprint retrogression approximately retraced the prograde p t temperature path or for early formed high grade blocks occurred at even higher p t ratios parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration upward motion took place as tectonically imbricated slices as laminar return flow in melange zones and perhaps partly a lateral spreading extension of the underplated accretionary prism retrograde p t trajectories of high pmore belts therefore provide important constraints on the tectonic evolution of convergent plate junctions less

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

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

a thermal tectonic model for high pressure metamorphism and deformation in the sesia zone western alps the sesia zone western alps is a slice of austroalpine continental crust that was once metamorphosed at t 500 560 c and p 14 16 kbar at 130 100 ma b p pervasive blueschist eclogite facies assemblages record this event and a later greenschist facies overprint is only well developed in the nw part of the zone on the basis of thermal calculations it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift erosion an alternative model is proposed in which the sesia zone was emplaced by underthrusting of the austroalpine continental margin prior to subduction of oceanic lithosphere of the piemonte basin subduction of relatively cool oceanic lithosphere below the underthrust sesia zone then continued for a period of time e g 20 ma thus suppressing thermal recovery of this latter unit exhumation of sesia zone rocks began during this subduction episode by uplift on the insubric line erosion and by underplating of oceanic

the transition from brittle to ductile deformation in the sambagawa metamorphic belt japan the dominant deformation mechanism during the sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature the magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote pumpellyite actinolite zone and the epidote actinolite zone the synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks and it decreases sharply as the grade reaches the epidote actinolite zone hence the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote actinolite zone of the sambagawa metamorphic belt the sharp change of the ductile strain magnitude also takes place at the epidote actinolite grade in the shimanto metamorphic belt of japan an example of the intermediate pressure facies series of metamorphism it is concluded that the transition from brittle to ductile deformation takes place at about 300 400 c and is independent of pressure of metamorphism

geodetic measurement of horizontal crustal deformation in eastern taiwan abstract four medium aperture trilateration networks in eastern taiwan have been surveyed three to four times since 1981 one is the ilan network crossing an elongated active seismic zone the others are the hualien yuli and taitung networks located at the northernmost middle and southernmost portions of the longitudinal valley respectively based on changes of the observed line length the three components of the surface strain rate tensor for each of the networks are obtained by a least squares adjustment technique then the principal strain rates are calculated the ilan network gives a principal strain rate of uniaxial extension at 2 3 strain yr in the direction of n45 w the hualien network has a principal strain rate of 1 2 strain yr extension in n62 e and 1 2 strain yr contraction in n152 e the yuli network yields essentially a principal strain rate of uniaxial contraction at 8 4 strain yr in n117 e whereas the taitung network has a principal strain rate of 1 5 strain yr extension in n24 e and 3 9 strain yr contraction in n114 e the directions of contractionof both the yuli and taitung networks are consistent with the direction of the maximum compressive stress of this area furthermore average velocities of the relative motion between geodetic stations in the central range and the coastal range are estimated from the average rates of changes in line length stations in the hualien network show a left lateral relative motion in a direction more or less parallel to the strike of the longitudinal valley while stations in the yuli and taitung networks move toward each other in the direction approximately perpendicular to the trend of the longitudinal valley

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

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

uplift and exhumation of high pressure metamorphic terrains the example of the cycladic blueschist belt aegean sea the cycladic blueschists emerged from a depth of more than 50 km during the tertiary these rocks are now covered only by a thin fraction of the synmetamorphic overburden upper plate remnants of the upper plate include low p crystalline rocks and sediments which have been juxtaposed onto the lower plate blueschists in several stages since the early miocene the low angle contacts operated as normal faults which removed the overburden from above the cycladic blueschists detailed investigations on tinos island indicate that the fractured blocks of the upper plate lying above a low angle normal fault did not rotate this shows that the faults initiated as rather gently dipping fractures similar structures were also reported from other blueschist terrains kinematic analysis of such flat lying faults implies that the synmetamorphic overburden should be mostly located several hundred kilometers away from the present exposure of the blueschists considering that the cycladic blueschist belt exposes successively shallower units in the west and that rocks of the upper plate have an anatolian nature we suggest that these blueschists were dragged from beneath turkey we recognize that the blueschist unit has undergone further thrusting in the final stage of the exhumation as have its counterparts elsewhere in the hellenides the emergence of the blueschists in the tertiary was concomitant with continental collision and thrusting in the hellenides the operation of low angle normal faults with large extensional displacements occurred within this context of collision tectonics this extension occurred parallel to the strike of the convergent orogen

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

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

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

the na ca amphibole albite chlorite epidote quartz geothermobarometer in the system s a f m c n h2o 1 an empirical calibration the proposed geothermobarometer is based on an empirical calibration which takes account of two equilibria involving the tremolite edenite pargasite and hastingsite components in amphiboles it has applications to assemblages found in metabasic rocks of widely different chemical compositions magnesian to fe rich metabasalts and for metamorphism ranging from lower greenschist to highest amphibolite facies knowing the si t1 aliv alvi fe3 fe2 mg ca nam4 naa and a vacancy in an amphibole and the al3 and xmg in coexisting epidote and chlorite it is possible to calculate two values of in kd for this assemblage these equilibria involve edenite tremolite and pargasite hastingsite tremolite end members in amphibole the calculation program is given for these equilibria the isopleths iso values of kd have been calculated for 0 27 xmg 0 75 and 0 xfe3 fe3 fe3 alvi 0 8 it is then possible to determine pressure and temperature directly when xmg xfe3 in kd for tremoliteedenite and in kd for pargasite hastingsite tremolite are known application of this geothermobarometer is limited to ca free plagioclase assemblages and complete p t paths can be drawn only if all the minerals are considered together phase relations at successive stages of crystallization can be constrained by studying the relationships between the coexisting minerals their zoning and the metamorphic fabrics

syn convergent exhumation implied by progressive deformation and metamorphism in the valle dell orco transect nw italian alps structural metamorphic and geochronological data from the valle del9orco are best explained by an orogenic wedge model in which crustal slices from continent and ocean acted as passive blocks at different times in the alpine orogeny deformation generally migrated westwards in this section and metamorphic recrystallization at any given point was largely controlled by the pervasiveness of strain the data suggest a dynamic crustal stacking system lasting from 70 to 35 ma rather than continental collision following long lived b type subduction and offer a relevant and simple model for eclogite exhumation that requires exhumation of ductile shear bounded slices within the orogenic wedge the easternmost internal unit of the sesia lanzo zone the eclogitic micaschist complex was subducted and pervasively recrystallized under eclogite facies conditions during the early stages of convergence in the late cretaceous during the development of the major preserved structures the eastern eclogitic micaschist complex was undergoing general non coaxial shear with an overall top down to the east vorticity suggesting a potential mechanism of exhuming an eclogitic 9pip9 slices of pre alpine basement the vasario sparone subunit were incorporated into the lower western parts of this unit during the latest part of this phase but do not show penetrative effects of the early alpine deformation they were passively folded during the early decompressional part of the p t history prior to the unit being juxtaposed against the underlying gneiss minuti complex under blueschist greenschist facies conditions the gneiss minuti complex itself was then recrystallized at high strain in the greenschist facies in the latest eocene fabrics developed at this stage indicate top down to east extension which may have accommodated the exhumation of eclogitized piemont oceanic material and the european continental material of the gran paradiso massif evidence of northwest directed thrusting within the gran paradiso unit at the same time suggests again that the crustal block was transported on a deeper reverse sense shear zone that presumably links to the more external thrusts now exposed to the west high pressure mineral lineations within the piemont zone cannot be unequivocally assigned to either the burial or exhumation paths but are consistent with the generally east west transport directions observed in the structurally higher units

melange forming processes in the development of an accretionary prism evidence from fission track thermochronology to give thermotectonic constraints on melange forming processes and to understand the significance of melange in the evolution of accretionary complexes fission track analysis was carried out on samples collected from sandstone blocks in melange matrix and coherent sandstone sequences adjacent to melange units in the cretaceous to miocene shimanto accretionary complex southwest japan which is one of the typical accretionary complexes with an accumulation of geological information nineteen zircon ages and 15 track length distributions from melanges indicate with some local variation that materials with higher maximum paleotemperatures are exposed at the rear part of the prism as is the case of the coherent units two apatite ages are contemporaneous with previously reported apatite ages of 10 ma obtained from coherent units in the region emphasizing the regional cooling through the apatite closure temperature of 100 c at that time there is no evidence for substantial relative motion of the melanges with respect to the surrounding coherent units melange formation is explained by processes that exclude large vertical transport i e 5 km of melange forming materials these thermotectonic characteristics observed in the shimanto accretionary complex are maintained during subsequent local internal deformation caused by the successive growth of the wedge

tectonic setting of western pacific marginal basins abstract global kinematics as well as magnetic anomalies of marginal basins with continental geology and paleomagnetic data as additional constraints are used to present a set of reconstructions of the western pacific marginal basins between 56 ma and the present at key periods 56 43 32 20 12 and 3 ma our model accounts for the rapid motion of exotic terranes along the whole of the western pacific convergent zone marginal basins appear to open in a great variety of tectonic settings the two extreme examples being the mariana trough where trench suction may be the predominant driving force and the south china sea where intracontinental deformation appears to be the major mechanism the study of marginal basins is a 3 d problem which must take into account the whole tectonic context subduction related tectonics in cross section and upper plate deformation in map view and not only a 2 d problem the classical trench arc back arc problem

pressure temperature path of sanbagawa prograde metamorphism deduced from grossular zoning of garnet the zonal structure of prograde garnet in pelitic schists from the medium grade garnet zone and the higher grade albite biotite zone was examined to investigate the evolution of prograde p t paths of the sanbagawa metamorphism the garnet studied shows a bell shaped chemical zoning of the spessartine component which decreases in abundance from the core towards the rim almandine and pyrope contents and xmg mg mg fe2 increase monotonously outwards the general scheme of the zonal structure for grossular content xgrs ca fe2 mn mg ca can be summarized as 1 xgrs increases outwards inner segment and reaches a maximum at an intermediate position between the crystal core and the rim then decreases towards the outermost rim outer segment 2 the inner segment of garnet in the garnet zone samples tends to have a higher xgrs xsps values for a given xsps than those in the albite biotite zone samples 3 average xsps at the maximum xgrs position in the albite biotite zone samples ranges from 0 02 to 0 12 and is lower than that in the garnet zone samples 0 13 0 32 4 the maximum xgrs in the albite biotite zone samples 0 34 0 39 on average tends to be higher than that in the garnet zone samples 0 26 0 36 and 5 differences of xgrs between the maximum and rim in the albite biotite zone samples are between 0 10 and 0 14 and higher than those in the garnet zone samples 0 11 these facts imply that albite biotite zone materials a were recrystallized under lower dp dt conditions at an early stage of the prograde metamorphism b began their exhumation under higher p t conditions and c have been continuously heated during exhumation for a longer duration than the garnet zone materials the systematic changes of prograde p t paths can be interpreted as documenting the evolution of the sanbagawa subduction zone

tectonometamorphic evolution of the sebadani eclogitic metagabbro and the sambagawa schists central shikoku japan 40ar 39ar mineral age constraints the sambagawa metamorphic belt exposed in central shikoku records a high p t metamorphic event it is represented by the oboke nappe and structurally overlying internally imbricated besshi nappe complex these major structural units are in ductile thrust contact a melange is developed along a ductile internal tectonic contact within the besshi nappe complex tectonic emplacement of a high t enclave sebadani eclogite in the melange zone resulted in the development of a contact metamorphic aureole within the host sambagawa rocks 36ar 40ar versus 39ar 40ar isotope correlation ages recorded by hornblende from the sambagawa basic schists which surround the sebadani enclave are 83 4 0 3 ma within contact aureole and 83 6 0 5 ma outside aureole 40ar 39ar plateau ages recorded by muscovite from the same samples are 87 9 0 3 and 89 3 0 4 ma amphibole from the amphibolite within the sebadani enclave records isotope correlation ages of 93 7 1 1 and 96 5 0 7 ma massive interior and 84 6 1 2 ma marginal shear zone amphibole within the massive amphibolite is significantly higher in xmg than that within the host sambagawa basic schists the older ages recorded by amphibole within the sebadani enclave are interpreted to date cooling through somewhat higher closure temperatures than which characterize the more fe rich amphibole in surrounding schists the younger amphibole age recorded within the marginal shear zone probably indicates that crystallization of amphibole continued until cooling through the relatively lower amphibole closure temperatures these results together with the previously published 40ar 39ar ages of the sambagawa schists suggest i metamorphic culmination occurred in the besshi nappe complex at c 100 90 ma ii at c 95 ma the besshi nappe complex was internally imbricated and tectonic enclaves were emplaced iii at c 85 ma the composite besshi nappe was rapidly exhumed and tectonically emplaced over the oboke nappe which attained peak metamorphic conditions at c 75 ma iv the besshi and oboke nappe complexes were further exhumed as a coherent tectonic unit and unconformably overlain by the eocene kuma group at c 50 ma

a strain reversal method for estimating extension from fragmented rigid inclusions abstract in the standard method of strain analysis using fragmented rigid inclusions such as stretched belemnites extension is estimated by comparing the initial length of the inclusion with its final stretched length this approach fails to recognise that not all of the total extension is recorded by separation of the fragments even when cross fractures are present at the onset of deformation this paper presents an alternative approach in which the extensional strain is reversed the inter fragment gaps being incrementally closed until the inclusion is restored to its original state this allows an estimate of the unrecorded extension accommodated by inhomogeneous matrix flow near the inclusion as well as that recorded by fragment separation model experiments in which the fragmented inclusion is represented by wooden blocks set in a ductile matrix allow the strain reversal method to be compared with the standard method the mean percentage relative error is much smaller in the former 10 2 than in the latter 37 2 the main disadvantage of the strain reversal method is that it requires the collection of more data fragment lengths and gap lengths in correct sequence which must be processed by computer a short basic program is provided although there is no one to one correspondence between strain estimates derived from the two methods an approximate numerical transformation from one to the other may be possible one such transformation based on a study of natural boudinage has been applied to the experimental data and reduces the mean percentage relative error in the standard method estimates to 11 5

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

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

on the mechanism of tectonic transport in zones of oblique subduction abstract many cases of oblique subduction are known throughout the world today and many more must have existed in the past in at least one modern case sumatra the over riding plate has responded to oblique subduction by forming a strike slip fault that traverses the magmatic arc and displaces its leading edge in the direction of the transverse component of oblique convergence conditions favoring this sort of tectonic response are shown to be 1 1 a high angle of oblique convergence 2 2 a low angle of subduction and 3 3 relative thermal softening of the magmatic arc this mechanism may have been responsible in part for northward transport of western cordilleran magmatic arc and subduction zone rocks in the late cretaceous and earliest tertiary it also may help to explain evidence of southward transport and left lateral shear in northern mexico and the caribbean region in pre jurassic time