evolution of the sanbagawa and shimanto high pressure belts in sw japan insights from k ar ar ar geochronology abstract the sanbagawa belt sensu lato in japan is one of the world s classical high pressure hp metamorphic belts and has long been considered to be a coherent hp belt of the cretaceous age however recent progress in detrital zircon geochronology together with the accumulation of abundant phengites k ar ar ar ages revealed that the belt is comprised of two petrotectonic belts sanbagawa sensu stricto hp belt and shimanto hp belt these two belts have distinctly different ages of peak metamorphism and different p t conditions of metamorphism the sanbagawa schists sensu stricto were metamorphosed in the conditions of the pumpellyite actinolite facies through the epidote blueschist to epidote amphibolite facies and up to the eclogite facies during a prograde stage in a subduction zone at 120 115 ma phengite k ar geochronology revealed that the subsequent exhumation after the peak metamorphism took placed in a manner that the higher grade rocks exhumed more rapidly in comparison with the lower grade rocks this is supported by a positive correlation between age and apparent metamorphic gradient that have formed during the post metamorphic peak hydration recrystallization moreover the schists have experienced intense ductile deformation and long term exhumation it took longer than 31 m y to reset the phengite k ar system in contrast the shimanto hp schists were metamorphosed in the epidote blueschist greenschist transitional facies to the epidote amphibolite facies and have experienced a short term deformation for less than 13 m y this short deformation formed a negative correlation between k ar age and apparent metamorphic gradient these two contrasting age temperature relationships suggest different exhumation processes between the two hp belts most likely due to a change in subducting oceanic plates in the cretaceous along the paleo japanese convergent margin

interseismic strain accumulation along the western boundary of the indian subcontinent 1 despite an overall sinistral slip rate of 3 cm yr few major earthquakes have occurred in the past 200 years along the chaman fault system the western boundary of the india plate with the eurasia plate gps and insar data reported here indicate sinistral shear velocities of 8 17 mm yr across the westernmost branches of the fault system suggesting that a significant fraction of the plate boundary slip is distributed in the fold and fault belt to the east at its southernmost on land segment 26 n near the triple junction between the arabia eurasia and india plates we find the velocity across the ornach nal fault is 15 1 13 4 16 9 mm yr with a locking depth probably less than 3 km at latitude 30 n near the town of chaman pakistan where a m6 5 earthquake occurred in 1892 the velocity is 8 5 6 8 10 3mm yr and the fault is locked at approximately 3 4 km depth at latitude 33 n and further north insar data indicate a velocity across the chaman fault of 16 8 2 7 mm yr the width of the plate boundary varies from several km in the south where we observe 2 mm yr of convergence near the westernmost strike slip faults to a few hundreds of km in the north where we observe 6 9 mm yr of convergence and where the faulting becomes distinctly transpressional the shallow locking depth along much of the transform system suggests that earthquakes larger than those that have occurred in the historical record would be unexpected and that the recurrence interval of those earthquakes that have occurred is of the order of one or two centuries similar in length to the known historical record

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

slip partitioning by elastoplastic propagation of oblique slip at depth oblique motion along tectonic boundaries is commonly partitioned into slip on faults with different senses of motion the origin of slip partitioning is important to structural geology tectonophysics and earthquake mechanics partitioning can be explained by the upward elastoplastic propagation of oblique slip from a fault or shear zone at depth the strain field ahead of the propagating fault separates into zones of predominantly normal reverse and strike slip faulting the model successfully predicts the distribution of fault types along parts of the san andreas and haiyuan faults

internal structure of the median tectonic line fault zone sw japan revealed by borehole analysis abstract the internal structure of the median tectonic line mtl fault zone and the processes that prevailed at depths are described based on an analysis of a borehole the fault plane which defines the boundary between the ryoke and sanbagawa derived rocks dips at 56 to the north immediately beneath the boundary approximately 40 m thick fractured rocks form the major strand of the mtl fault zone the hanging wall above the boundary comprises variably deformed ryoke granitoids including several mylonite zones and cataclasite zones the fault zone has evolved through a series of faulting events under temperatures ranging from 400 to 200 c the mineral assemblages of the mylonites and cataclasites immediately above the boundary indicate that these fault rocks were formed at temperatures of about 300 c these mylonites and cataclasites represent therefore fault rocks that formed immediately below and above the brittle plastic transition respectively development of dissolution seams in these cataclasites suggests that the cataclasite has low strength the presence of pseudotachylytes in the cataclasite indicates the occurrence of seismicity immediately above the brittle plastic transition on the other hand the very fine grain size of recrystallised quartz in the mylonites indicates high differential stress immediately below the brittle plastic transition it is therefore likely that the differential stresses immediately below the brittle plastic transition are much higher than those immediately above the transition formation of laumontite in the major strand of the mtl fault zone occurred at temperatures of around 200 c the central slip zone of the major strand is about 30 cm thick and is surrounded by thick gouge zones this situation is favourable for thermal pressurisation during earthquake slips

nature and development of late mesozoic and early cenozoic sedimentary basins in southwest japan abstract the southwestern part of the japanese islands is a typical example of an active continental margin characterized by backarc intra arc and forearc sedimentary basins the back arc basins provide evidence of tectonic continuity with the asian continent the basins are genetically classified as half graben ridge and forearc basins the half graben and ridge basins are controlled by strike slip faulting although the half graben basins are pull apart basins on a much larger scale than the ridge basins and are in many instances closely associated with igneous activity forearc basins occupy the area from the deep sea terrace to the trench floor forearc basin fill sediments form accretionary prisms as a result of subduction processes good examples of these basins are well developed in the cretaceous and paleogene sediments of southwest japan

middle pleistocene age of the nome river glaciation northwestern alaska abstract during the middle pleistocene nome river glaciation of northwestern alaska glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals the age of the nome river glaciation is constrained by laser fusion 40 ar 39 ar analyses of basaltic lava that overlies nome river drift at minnie creek central seward peninsula that average 470 000 190 000 yr 1 milligram size subsamples of the lava were dated to identify and eliminate extraneous 40ar enrichments that rendered the mean of conventional k ar dates on larger bulk samples of the same flow too old 700 000 570 000 yr while the 40 ar 39 ar analyses provide a minimum limiting age for the nome river glaciation maximum ages are provided by a provisional k ar date on a basaltic lava flow that underlies the nome river drift at nearby lave creek by paleomagnetic determinations of the drift itself at and near the type locality and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the anvilian marine transgression that underlie nome river drift on the coastal plain at nome taken together the new age data indicate that the glaciation took place between 580 000 and 280 000 yr ago the altitude of the anvilian deposits suggests that eustatic sea level during the anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression by correlation with the marine oxygen isotope record the transgression probably dates to stage 11 at 410 000 yr and the nome river glaciation is younger still analyses of floor altitudes of presumed nome river cirques indicate that the nome river regional snowline depression was at least twice that of the maximum late wisconsin the cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern alaska during the middle pleistocene

geomorphic response to an active transpressive regime a case study along the chaman strike slip fault western pakistan the chaman left lateral strike slip fault bounds the rigid indian plate boundary at the western end of the himalayan tibetan orogen and is marked by contrasting topographic relief deformed landforms along the fault provide an excellent record for understanding this actively evolving intra continental strike slip fault the geomorphic response of an active transpessional stretch of the chaman fault was studied using digital elevation model dem data integrated with advanced spaceborne thermal emission and reflection radiometer aster visible and near infrared short wave infrared vnir swir and images from geoeye 1 geologic and geomorphic mapping helped in reconstructing the late quaternary landscape history of this transpessional strand of the chaman strike slip fault and the associated spinatizha thrust fault in western pakistan topographic analysis of a part of the transpression the thrust bounded roghani ridge revealed northward growth of the spinatizha fault with the presence of three water gaps and two corresponding wind gaps geomorphic indices including stream length gradient index mountain front sinuosity valley floor width to valley height ratios and entrenchment of recent alluvial fan deposits were used to define the lateral growth and direction of propagation of the spinatizha fault left lateral displacement along chaman fault and uplift along the spinatizha fault was defined using topographic analysis of the roghani ridge and geomorphic mapping of an impressive alluvial fan the bostankaul fan the landforms and structures record slip partitioning along the indian plate boundary and account for the convergence resulting from the difference in the chaman fault azimuth and orientation of the velocity vector of the indian plate copyright 2012 john wiley sons ltd

high resolution variation in ostracod assemblages from microbialites near the permian triassic boundary at zuodeng guangxi region south china abstract after the end permian mass extinction epme the marine environment was considered extremely toxic mainly due to anoxic and high temperature conditions and ocean acidification consequently the ecosystem contained few organisms this paper describes a new ostracod fauna from the microbialite bearing permian triassic p tr strata at zuodeng guangxi region china one thousand and seventy ostracod specimens were extracted from forty eight samples fifty three species belonging to fourteen genera were identified ostracods primarily from the family bairdiidae were extremely abundant in the microbialites which suggests that ostracods were opportunists that could survive the epme within this special microbial ecosystem with sufficient food and scarce competitors and predators rather than undergoing a rapid and early recovery after the epme event ostracods having simultaneous paleozoic and meso cenozoic affinities coexisted in this fauna the similarities and differences among the ostracod faunas in the microbialite sections of the p tr boundary around the paleo tethys indicate that there was a long distance dispersal of ostracods but the faunas maintained endemism at the species level previous studies have regarded microbialites as whole units and it is difficult to detect environmental changes within a microbialite interval based on paleoecological groups of super families in this study high resolution sampling was applied to identify changes in the ostracod abundance diversity assemblages and proportion of five dominant species in the section which exhibited an evolutionary trend from the bairdia group to the liuzhinia antalyaensis bairdiacypris ottomanensis group furthermore the transition of the ostracod fauna could be divided into six stages according to changes in the dominant species groups suggesting that the microbialite environment was not entirely monotonous but fluctuated during the postextinction interval

crustal structure of southwest japan revealed by the integrated seismic experiment southwest japan 2002 abstract a multi purpose seismic experiment named the 2002 integrated seismic experiment southwest japan was conducted in 2002 along a more than 240 km long line across southwest japan from the pacific coast to the japan sea coast its profile provides the first crustal scale cross section across the japanese island arc which highlights a number of significant points related to the structural development of the arc major outstanding points are that the japanese island arc is composed of two completely different crusts juxtaposed by the median tectonic line mtl and that the mtl started its activity associated with lower crustal thinning and formation of an upper crustal scale half graben in late cretaceous

mechanics of fore arc slivers insights from simple analog models 1 convergent margins with oblique subduction commonly include a fore arc sliver a portion of the overlying plate bounded by the trench and a major strike slip fault system it has long been noted that relative plate motion near the deformation front as indicated by earthquake focal mechanisms is generally closer to margin normal than would be expected from overall relative plate motion this results from sliver motion as well as from shear within the fore arc we carry out some simple calculations to determine how the rate of sliver motion should vary with plate convergence obliquity we find that the results are related to rheology but that the measurement of sliver rates at natural fore arcs is unlikely to yield real insight into physical properties even for very simple models the system is too complicated and its relation to rheology is not unique the proportion of oblique convergence taken up by sliver motion in simple tabletop experiments depends on the rate of slip and the smoothing of asperities similarities in taper and style of strain between frontal wedges forming with and without slivers suggest that structural observations of exhumed accretionary wedges are unlikely to allow geologists to draw definitive conclusions about the degree of obliquity of relative plate motion at the time when the wedges were formed and in some cases not even whether or not a sliver plate was present at the time of deformation

striations duration migration and tidal response in deep tremor deep tremor in subduction zones is thought to be caused by small repeating shear slip events on the plate interface with significant slow components it occurs at a depth of about 30 kilometres and provides valuable information on deep plate motion and shallow stress accumulation on the fault plane of megathrust earthquakes tremor has been suggested to repeat at a regular interval migrate at various velocities and be modulated by tidal stress here i show that some time invariant interface property controls tremor behaviour using precise location of tremor sources with event duration in western shikoku in the nankai subduction zone japan in areas where tremor duration is short tremor is more strongly affected by tidal stress and migration is inhibited where tremor lasts longer diffusive migration occurs with a constant diffusivity of 10 4 m 2 s 1 the control property may be the ratio of brittle to ductile areas perhaps determined by the influence of mantle wedge serpentinization on the plate interface the spatial variation of the controlling property seems to be characterized by striations in tremor source distribution which follows either the current or previous plate subduction directions this suggests that the striations and corresponding interface properties are formed through the subduction of inhomogeneous structure such as seamounts for periods as long as ten million years

gently north dipping median tectonic line mtl revealed by recent seismic reflection studies southwest japan abstract the median tectonic line mtl with a length of more 1000 km is the most significant fault in japan it juxtaposes the high p t sambagawa metamorphic rocks against the low p t metamorphic rocks of the ryoke belt the mtl was probably formed in the cretaceous with many subsequent reactivations the western segment of the mtl is still active with an almost pure right lateral sense of motion although a great amount of geological information on the mtl has been accumulated information about the subsurface especially the deep seated structure of the mtl is still insufficient it has been generally assumed that the mtl is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion recently new geophysical data have suggested that the mtl dips gently northward at depth we have acquired a complementary set of geophysical profiles seismic reflection and refraction gravity and mt across the mtl in east shikoku our results confirm that the mtl dips northward at about 30 to 40 degrees from the surface to about 5 km depth where it becomes listric this fault geometry more reasonably explains the reactivation history of the mtl the motion has occurred on a listric type fault in so called oblique or lateral ramp manner

new insight into a subduction related orogen a reappraisal of the geotectonic framework and evolution of the japanese islands abstract the geotectonic framework and the evolutionary history of the japanese islands need revision in accordance with the various geophysical geological evidence gathered by new methodologies in the recent years including seismic tomography vibroseis ground breaking seismic experiments and detrital zircon chronology these investigations have addressed various themes such as 1 seismic profile of the crust and mantle beneath the japanese islands 2 high precision ages of the protoliths of high p t metamorphic rocks and 3 provenance of terrigenous clastics the results have led to a number of important findings including 1 detection of a large mass of slab around the mantle boundary layer suggesting the long term oceanic subduction beneath japan 2 confirmation of the subhorizontal piled nappe structure for the entire crust of japan 3 finding a new high p t metamorphosed accretionary complex unit that represents the youngest blueschist in japan 4 finding of the oldest early cambrian arc batholith and cover sediments and 5 the identification of plural arc batholiths which have already been erased from the surface based on a synthesis of these new data this article presents a re evaluation of the conventional geotectonic subdivision of the subduction related orogen in japan re definition of the elements and their mutual boundaries and re consideration of the geotectonic evolution of the japanese islands in particular the historical change in provenance suggests that proto japan has experienced large scale tectonic erosion in multiple stages and the corresponding large amounts of continental crust materials were subducted for understanding the orogenic growth of japan during the last ca 500 million years the significance of tectonic erosion coupled with continental contraction as well as the oceanward accretionary growth requires further attention

reactivation of an old plate interface as a strike slip fault in a slip partitioned system median tectonic line sw japan abstract in models for strain partitioning at obliquely convergent plate boundaries trench parallel slip occurs on a vertical fault trench parallel slip at the nankai subduction zone sw japan is mapped along the median tectonic line mtl which dips approximately 40 n to understand its structural context and how the mtl functions in this slip partitioned system we collected a set of three seismic profiles in the kii peninsula south of osaka using a multi scale acquisition strategy that provides increasingly fine resolution to understand its fault kinematics we analyzed microseismic activity in two locations on the fault using source data from japan s hi net monitoring network structural details suggest that the mtl functioned as a megathrust during subduction of the cretaceous sanbagawa hp metamorphic belt its current pattern of microseismicity shows that it behaves as a strike slip fault with no indication of a vertical fault at or around its surface trace thus trench parallel slip at the nankai is now accommodated on an inclined fault plane in an unusual form of partitioning this system appears to have developed out of a two phase tectonic history in which a thrust structure that formed under initial phase compressive stresses has been reactivated as a strike slip fault under subsequent phase shear stresses its unusual kinematics show that shear failure can occur on an existing non vertical fault plane at a regional scale in preference to the rupture of a new ideal vertical fault plane

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

shrimp u pb zircon dating of quartz bearing eclogite from the sanbagawa belt south west japan implications for metamorphic evolution of subducted protolith in order to decipher the origin of eclogite in the high p t sanbagawa metamorphic belt shrimp u pb ages of zircons from quartz bearing eclogite and associated quartz rich rock metasandstone were determined one zircon core of the quartz rich rock yields an extremely old provenance age of 1899 79 ma suggesting that the core is of detrital origin eight other core ages are in the 148 134 ma range and are older than the estimated age for trench sedimentation as indicated by the youngest radiolarian fossil age of 139 135 ma from the sanbagawa schists ages of metamorphic zircon rims 132 112 ma from the quartz rich rock are consistent with metamorphic zircon ages from the quartz bearing eclogite indicating that eclogite facies metamorphism peaked at 120 110 ma these new data are consistent with both the iratsu eclogite body and surrounding highest grade sanbagawa schists undergoing coeval subduction zone metamorphism and subsequent re equilibration under epidote amphibolite facies conditions during exhumation

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

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

segmentation and holocene surface faulting on the median tectonic line southwest japan the median tectonic line mtl active fault system is a 300 km long arc parallel right lateral strike slip fault related to oblique subduction of the philippine sea plate beneath the eurasian plate at the nankai trough southwest japan the fault on shikoku island has a slip rate of 5 10 mm yr one of the highest in onshore japan we identified 12 geometric segments along the mtl separated by discontinuities such as en echelon steps bends changes in strike and gaps in the surface trace a chronology of latest holocene surface faulting on the mtl was constructed based on previously published and new work including trenching geomorphic mapping and acoustic profiling and piston coring of the submarine extension of the fault zone the mtl probably does not rupture along its entire length in a single earthquake but instead consists of multiple earthquake segments that rupture independently of one another the recurrence interval and surficial offset for surface rupturing earthquakes at four individual sites on the mtl in shikoku island are 1000 3000 years and 5 8 m respectively the relatively long recurrence interval and large surficial offset per earthquake suggest large seismic moment release during surface rupturing earthquakes on the mtl as commonly observed for japanese moderate to large intraplate earthquakes because the predicted rupture lengths based on empirical relations between surface displacement and rupture length are considerably larger than those of individual segments large earthquakes on the mtl probably rupture multiple geometrically defined segments the fault was previously thought to be unruptured at least in the past 1000 years based on the absence of historical evidence for destructive earthquakes however trench excavations show that part of the mtl ruptured most recently during or after the 16th century a d with at least 6 9 0 7 m of slip along the main fault trace this rupture may be correlated to the 1596 keicho kinki earthquake but further geological and historical investigations are needed to confirm this hypothesis

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

paired metamorphic belts revisited abstract the modern plate tectonics regime is characterized by a duality of thermal environments one representing the subduction zone and the other representing the arc backarc or orogenic hinterland this duality is the hallmark of one sided asymmetric subduction and the characteristic imprint of one sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt one of high dt dp type and the other low dt dp type the broadly contemporaneous occurrence of granulite and ultrahigh temperature metamorphism with eclogite high pressure granulite metamorphism in the geological record since the neoarchean era is evidence of dual thermal environments and indicates that subduction has operated on earth since that time classic paired metamorphic belts in which an inboard high dt dp metamorphic belt is juxtaposed against an outboard low dt dp metamorphic belt along a tectonic contact such as the ryoke and sanbagawa belts in japan are found in phanerozoic accretionary orogens of the circum pacific generally they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far traveled this is a consequence of the difference between globally continuous subduction generating a low to intermediate dt dp environment in the subduction zone and a high dt dp environment in the arc backarc system and metamorphic imprints in the geological record that represent discrete events due to changes in plate kinematics or subduction boundary dynamics or as a result of collision of ridges arcs or continents with the upper plate at the trench the concept of paired metamorphic belts may be generalized and extended more widely than in the original proposition to subduction to collision orogenic systems in addition to accretionary orogenic systems in this wider application the term paired metamorphic belts may be used for penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients one warmer and the other colder juxtaposed by plate tectonics processes brown 2009 this extends the original concept of miyashiro 1961 beyond the simple pairing of high dt dp and low dt dp metamorphic belts in circum pacific accretionary orogens and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings this is particularly useful in subduction to collision orogenic systems where the suture and lower plate materials will register the imprint of low to intermediate dt dp and the upper plate will register penecontemporaneous high dt dp metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record

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

timing and displacement of holocene faulting on the median tectonic line in central shikoku southwest japan abstract the median tectonic line mtl extends about 900 km parallel to the general trend of the southwest japan island arc the central segment of the mtl has been active during the quaternary although the average slip rate along the mtl is up to several mm a 1 there is no historical record of destructive earthquakes along it in order to collect data on the behavior of the mtl during late holocene times especially on the timing of seismic events and the amount of displacement during each event trench studies were conducted on the active segment in central shikoku in 1984 and 1988 the results are summarized as follows 1 the latest two seismic events produced by slip on the mtl in central shikoku occurred during the early 8th century a d and between 3250 and 2820 years b p 2 the amount of displacement during the latest event is inferred to have been about 5 7 m 3 the earthquake magnitude for the latest event is estimated at 7 9 richter scale and a fault segment trace about 70 km long ruptured during that event 4 an average slip rate along the active trace of the mtl in central shikoku during the late quaternary is estimated to be 5 8 mm a 1

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

a 15 year slow slip event on the sunda megathrust offshore sumatra in the banyak islands of sumatra coral microatoll records reveal a 15 year long reversal of interseismic vertical displacement from subsidence to uplift between 1966 and 1981 to explain these coral observations we test four hypotheses including regional sea level changes and various tectonic mechanisms our results show that the coral observations likely reflect a 15 year long slow slip event sse on the sunda megathrust this long duration sse exceeds the duration of previously reported sses and demonstrates the importance of multidecade geodetic records in illuminating the full spectrum of megathrust slip behavior at subduction zones