the significance of phenocryst diversity in tephra from recent eruptions at popocatepetl volcano central mexico tephra lapilli from six explosive eruptions between april 1996 and february 1998 at popocatepetl volcano popo in central mexico have been studied to investigate the causes of magma diversification in thick crusted volcanic arcs the tephra particles are sparsely porphyritic 5 vol magnesian andesites sio2 58 65 wt mgo 2 6 5 9 wt that contain phenocrysts of nio rich up to 0 67 wt nio magnesian olivine fo89 91 cores with inclusions of cr spinel cr 59 70 orthopyroxene mg 63 76 clinopyroxene mg 68 86 intermediate to sodic plagioclase an33 66 and traces of amphibole major and trace element systematics indicate magma mixing the liquid mg melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic mg melt 72 76 and an evolved component magma mg melt 35 40 these component magmas form a hybrid magnesian andesite with an intermediate range of mg melt 50 72 the mafic end member mg melt 72 75 is saturated with olivine and spinel and crystallizes at temperatures 1170 1085 c with oxygen fugacities close to the fayalite magnetite quartz buffer and elevated water contents of several wt h2o a likely location of crystallization is at lower crustal levels possibly at the moho olivine is followed by high mg clinopyroxene which could start to crystallize during magma ascent at depths of 4 to 13 km the mafic magma mixes with an evolved composition containing low mg clino and orthopyroxene and plagioclase at a temperature of 950 c the repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing the tephra is similar to the popo magnesian andesites suggesting similar generic processes for the common lavas of the volcano the advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma building on the elemental systematics of the tephra and a comparison to the near primary basalts from the surrounding monogenetic fields we infer that the popo mafic end member is a magnesian andesite with variable but high sio2 contents of 55 62 wt and near primary characteristics such as high mg melt of 72 75 feo mgo ratios 1 if extrapolated to an mg melt of 72 75 and high ni contents 200 ppm ni this model implies that the typical elemental signature of the popo andesites such as the low cao al2o3 feo high na2o contents and the depletion in high field strength elements e g p zr ti are mantle source phenomena thus determining the elemental budget of the magnesian andesite as it is prior to the modifications by crustal differentiation is central to quantifying the subcrustal mass fluxes beneath popo

geodetic observations during the 2009 eruption of redoubt volcano alaska abstract in march 2009 redoubt volcano about 160 km to the sw of anchorage alaska began its most recent explosive eruption deformation induced by this event was recorded by a gps campaign network consisting of 14 benchmarks which had been established in 1991 after the previous eruption the network was partially reoccupied in 2001 and 2008 and no volcanic deformation was detected during that period in response to precursory unrest starting in january 2009 the alaska volcano observatory temporarily deployed continuously recording gps instruments at four of the campaign benchmarks only days before the onset of explosive activity in march 2009 the only gps instrument recording continuously during the months prior to the eruption was the plate boundary observatory pbo station ac17 about 28 km northeast of the volcano s summit data from this station reveals subtle motion radially outward from the volcano beginning as early as may 2008 which reversed with the onset of explosive activity using simple analytical models we link the precursory activity to a point source intrusion of 0 0194 0 03400 0092 km3 in volume at 13 50 17 3310 17 km below sea level bsl superscripts and subscripts refer to upper and lower ends of confidence intervals at the 95 level during the explosive phase about 0 05 0 10 028 km3 of magma was evacuated from a prolate spheroid with its centroid at 9 17 15 176 92 km bsl a semimajor axis of 4 50 10 001 25 km length and a semi minor axis of 0 475 4 00 3 km the effusive activity is inferred to come from the same source decreasing in volume by 0 0167 0 02280 0106 km3 including observations from seismology and petrology we hypothesize a mid crustal two reservoir system with material sourced from 20 km flowing in at about 13 5 km depth and reheating residual material in the proposed spheroid the mixture migrated to shallower depth 2 4 5 km bsl and reheated material there as this residual magma erupted it was replaced by the material from the spheroidal reservoir at 7 11 5 km depth which renders the shallow source undetectable for geodetic instruments in addition to long term displacements we investigate sub daily kinematic positioning solutions and find that large short term positioning offsets correlate with large explosive events spikes in phase residuals plotted along the sky tracks of individual satellites can be related to individual plumes given favorable satellite station geometry which may be of use in volcano monitoring

ground deformation caused by magma ascent in an open conduit this study examines magma ascent processes of three basic types prior to intermittent explosions such as vulcanian or strombolian types specifically their relations to volcano ground deformation such repetitive explosions eject magma from an open conduit at short time intervals consequently magma pressure is expected to decrease at the conduit and or reservoir and thereby deflate the volcano then magma within the conduit rises exerting normal and shear stresses on the conduit wall thereby inflating the volcano diffusive mass transfer of water molecules from the melt to bubbles might occur subject to a sudden depressurization by eruption when magma includes numerous small gas bubbles such gas bubble expansion lifts the magma in the conduit calculations of ground deformation on a semi infinite medium for such rising magma show that vertical and radial displacements and tilt motions recorded in the far fields are proportional to the 1 5 power of time for low viscosity magma gas bubbles might rise in the melt because of buoyancy force the gas bubbles expand rapidly with time because of decreased ambient pressure pushing the magma upward in the conduit consequently the volcano slowly inflates initially then the rate of deformation increases over time eventually engendering a rapid inflation immediately before eruption these temporal changes in ground deformations contrast against cases in which magma does not include gas bubbles in such cases amplitudes of ground deformation increase almost linearly or even decrease with time at far fields these differences which are recognized as basic characteristics of temporal changes of ground deformation enable us to know the driving forces of magma in an open conduit and to evaluate the gas bubble behavior quantitatively in magma before eruption

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

magma supply dynamics at westdahl volcano alaska modeled from satellite radar interferometry 1 a group of satellite radar interferograms that span the time period from 1991 to 2000 shows that westdahl volcano alaska deflated during its 1991 1992 eruption and is reinflating at a rate that could produce another eruption within the next several years the rates of inflation and deflation are approximated by exponential decay functions having time constants of about 6 years and a few days respectively this behavior is consistent with a deep constant pressure magma source connected to a shallow reservoir by a magma filled conduit an elastic deformation model indicates that the reservoir is located about 6 km below sea level and beneath westdahl peak we propose that the magma flow rate through the conduit is governed by the pressure gradient between the deep source and the reservoir the pressure gradient and hence the flow rate are greatest immediately after eruptions pressurization of the reservoir decreases both the pressure gradient and the flow rate but eventually the reservoir ruptures and an eruption or intrusion ensues the eruption rate is controlled partly by the pressure gradient between the reservoir and surface and therefore it too decreases with time when the supply of eruptible magma is exhausted the eruption stops the reservoir begins to repressurize at a high rate and the cycle repeats this model might also be appropriate for other frequently active volcanoes with stable magma sources and relatively simple magma storage systems

the challenging retrieval of the displacement field from insar data for andesitic stratovolcanoes case study of popocatepetl and colima volcano mexico abstract despite the ability of synthetic aperture radar sar interferometry to measure ground motion with high resolution application of this remote sensing technique to monitor andesitic stratovolcanoes remains limited specific acquisition conditions characterizing andesitic stratovolcanoes mainly vegetated areas with large elevation ranges induce low signal coherence as well as strong tropospheric artefacts that result in small signal to noise ratio we propose here a way to mitigate these difficulties and improve the sar measurements we derive ground motions for two of the most active mexican stratovolcanoes popocatepetl and colima volcano from the time series of sar data acquired from december 2002 to august 2006 the sar data are processed using a method that combines both persistent scatterers and small baseline approaches stratified tropospheric delays are estimated for each interferogram using inputs from the global atmospheric model narr up to a maximum of 10 rad km these delays are validated using spectrometer data as well as the correlation between the wrapped phase and the elevation the tropospheric effect is removed from the wrapped phase in order to improve the unwrapping process on popocatepetl we observe no significant deformation the colima summit area exhibits a constant subsidence rate of more than 1 cm year centered on the summit but enhanced reaching more than 2 cm year around the 1998 lava flow we model this subsidence considering both a deflating magma source at depth and the effect of the eruptive deposits load

magma supply path beneath mt asama volcano japan 1 obtaining a sharp image of magma supply path through dense geophysical observations is important for forecasting time and magnitude of hazardous future eruptions here we reveal a clear magma plumbing system using dense seismic and geodetic networks around mt asama central japan magma intrusions occurred several times beneath the western flank of mt asama forming a wnw ese directed zone with 1 km below sea level the eastern end of this zone connects a narrow vertical pathway extending right under the summit crater which erupted in 2004 monitoring magmatic activity with a well designed observational network is vital to mitigate future volcano hazards

identification of structural controls in an active lava dome with high resolution dems volcan de colima mexico monitoring the topography of active lava domes is critical for detecting changes that may trigger or influence collapse or explosive activity internal dome structure and conditions are more difficult to elucidate but also play vital roles here we describe the exposure following an explosion of significant scarps in the active dome at volcan de colima mexico that are interpreted as evidence of brittle failure planes and a complex internal dome morphology in the first use of automated 3d computer vision reconstruction techniques structure from motion and multi view stereo sfm mvs on an active volcanic dome we derive high resolution surface models from oblique and archive photographs taken with a consumer camera the resulting 3d models were geo referenced using features identified in a web sourced orthoimage no ground based measurements were required in december 2010 the dome 2 14 106 m3 had a flat upper surface reflecting an overall ductile emplacement regime between then and may 2011 a period of low explosivity was accompanied by a small volume loss 0 4 105 m3 and arcuate steps appeared in the dome surface suggesting the presence of localized planes of weakness the complex array of summit scarps was exposed following a significant explosion in june 2011 and is interpreted to be the surface expression of fault planes in the dome the 1 m resolution dems indicated that the region of greatest volume loss was not coincident with the assumed location of the conduit and that heterogeneity within the dome may have been important during the june explosion

intra oceanic island arc origin for iratsu eclogites of the sanbagawa belt central shikoku southwest japan abstract new geochemical and sr nd isotopic data for the iratsu eclogite and surrounding metamorphic rocks of the sanbagawa belt japan show that while the protoliths of the metamorphic rocks formed in a variety of tectonic settings the iratsu body represents a deeply subducted and accreted island arc the igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab released fluid as seen in a negative nb anomaly and an enriched mantle similar to that of ocean island basalts oib fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the iratsu body are cumulates from a basaltic magma that crystallized under high o 2 and h 2 o fugacities in the middle to lower crust the source characteristics and crystallization conditions suggest that the protoliths of the iratsu rocks formed in an oceanic island arc quartz eclogites from the marginal zone of the iratsu body have geochemical signatures similar to turbidites from the izu bonin island arc as seen in a negative nb anomaly and a concave ree pattern the protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc geochemical and isotopic signatures of the surrounding mafic schists are similar to normal n and enriched e mid ocean ridge basalt morb and distinct from the rocks from the iratsu body the protoliths of the mafic schists likely formed in a plume influenced mid ocean ridge or back arc basin pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the iratsu body have evolved continental geochemical signatures as seen in a negative e nd t value 5 consistent with their origin as continent derived trench fill turbidites

the 1963 1964 eruption of agung volcano bali indonesia the february 1963 to january 1964 eruption of gunung agung indonesia s largest and most devastating eruption of the twentieth century was a multi phase explosive and effusive event that produced both basaltic andesite tephra and andesite lava a rather unusual eruption sequence with an early lava flow followed by two explosive phases and the presence of two related but distinctly different magma types is best explained by successive magma injections and mixing in the conduit or high level magma chamber the 7 5 km long blocky surfaced andesite lava flow of 0 1 km3 volume was emplaced in the first 26 days of activity beginning on 19 february on 17 march 1963 a major moderate intensity 4 107 kg s 1 explosive phase occurred with an 3 5 h long climax this phase produced an eruption column estimated to have reached heights of 19 to 26 km above sea level and deposited a scoria lapilli to fine ash fall unit up to 0 2 km3 dense rock equivalent dre in volume with plinian dispersal characteristics and small but devastating scoria and ash flow deposits on 16 may a second intense 4 h long explosive phase 2 3 107 kg s 1 occurred that produced an 20 km high eruption column and deposited up to 0 1 km3 dre volume of similar ash fall and pyroclastic flow deposits the latter of which were more widespread than in the march phase the two magma types porphyritic basaltic andesite and andesite are found as distinct juvenile scoria populations this indicates magma mixing prior to the onset of the 1963 eruption and successive injections of the more mafic magma may have modulated the pulsatory style of the eruption sequence even though a total of only 0 4 km3 dre volume of lava scoria and ash fall and scoria and ash pyroclastic flow deposits were produced by the 1963 eruption there was considerable local damage caused mainly by a combination of pyroclastic flows and lahars that formed from the flow deposits in the saturated drainages around agung minor explosive activity and lahar generation by rainfall persisted into early 1964 the climactic events of 17 march and 16 may 1963 managed to inject ash and sulfur rich gases into the tropical stratosphere

probing the accumulation history of the voluminous toba magma the age and compositional zonation in crystals from the youngest toba tuff record the prelude to earth9s largest quaternary eruption we used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150 000 years of magma storage and evolution the dominant subvolcanic magma was relatively homogeneous and thermally stagnant for 110 000 years in the 35 000 years before eruption the diversity of melts increased substantially as the system grew in size before erupting 75 000 years ago

upper ocean currents around taiwan abstract current velocity measured by shipboard acoustic doppler current profiler sb adcp during 1991 2000 was used to study the upper ocean the composite and moored currents revealed a branch of the kuroshio that intruded steadily and persistently into the south china sea part of the intruded kuroshio flowed out of the south china sea through the northern luzon strait and re united with the main stream kuroshio the kuroshio had two velocity maximum cores southeast of taiwan but gradually combined into one as the kuroshio flowed north the kuroshio was deflected by the i lan ridge east of taiwan and the zonal running shelf break northeast of taiwan at the shelf break the kuroshio split with one branch intruding onto the shelf west of the luzon strait the kuroshio intruded into the south china sea some water flowed northward into the taiwan strait and re joined the kuroshio currents in the taiwan strait flowed primarily in a northward direction except for the southward current near the coast of mainland china north of the taiwan strait a branch of the northward flow followed the northern coast of taiwan to join the kuroshio the composite current varied consistently from season to season there was generally poor correlation between currents and local winds especially in the deep water regime remote forces were important in the currents around taiwan

late paleozoic mesozoic tectonic evolution of sw japan a review reappraisal of the accretionary orogeny and revalidation of the collisional model abstract this paper makes a review of the interpretations of the tectonic evolution of sw japan during the last three decades in the late 1970s the dominant model was the so called pacific type orogeny emphasizing the purported absence of nappes and the contrast with the alpine chains and interpreting the evolution as due to a steady oceanic subduction since the paleozoic time in the 80s the discovery of the actual structure made of a pile of large thrust sheets led authors to propose collisional models involving the intermittent underthrusting of buoyant blocks like micro continents at the same time the use of high resolution biostratigraphy allowed several authors to recognize ancient accretionary wedges with a reconstructed ocean plate stratigraphy of individual accreted units especially in the tanba and shimanto zones also precise radiometric dating permitted the distinction of metamorphosed units especially in sanbagawa and shimanto belts as a result of these new data since the 1990s the plate tectonic interpretation of the history of the japanese islands was revised by japanese scientists and presented again in terms of accretionary processes linked to a steadily oceanic subduction with an episodic ridge subduction the so called miyashiro type orogeny the review of different data leads to the following conclusions the structure of sw japan is made of a pile of sub horizontal nappes polydeformed with a geometry similar to the one encountered in collisional orogens the geodynamic mechanisms advocated for the tectonic building within the accretionary orogeny concept miyashiro type orogeny are inappropriate a permanent oceanic subduction with the intermittent collision actually subduction of an active ridge or seamount chain is unable to build such structures as this process induces in fact an acceleration of the tectonic erosion and collapse of the upper plate the underthrusting of a micro continent or mature arc is likely needed the exhumation story of sanbagawa hp schists suggests the setting of a continental subduction the petrological and new geochemical data from the literature strongly support the existence beneath the nappes of accretionary complexes of continental bodies showing affinities with south china from which they were once separated the episodic collision underthrusting of such blocks was responsible for the tectonic piling tectonic erosion plaid likely a major role in removing material during the intervening subduction stages a revised geodynamic model implying the collision of the honshu south kitakami kurosegawa and shimanto blocks is proposed for explaining the three orogenic crises which took place respectively at around 240 130 and 80 60 ma ago in sw japan the paleogeographic position and affinity of the hida block with surrounding units in the hinterland are still unclear more work is needed to solve this question

palaeogeographical reconstructions of the pan african brasiliano orogen closure of an oceanic domain or intracontinental convergence between major blocks abstract a reconstruction of the pan african brasiliano orogen has become possible thanks to new data from the southern pan african belt and by using published data geological mapping which included structural thermobarometric and geochemical studies in togo and neighbouring countries have enabled to define the existence of a preorogenic continental rift whose closure followed soon after its opening continental crust convergence induced an oblique collision that caused nappe stacking this involved external nappes thrust over the west african craton intermediate nappes including metabasic rocks with eclogitic assemblages and internal nappes composed of highgrade anatectic gneiss and intrusive granite and charnockite comparison with data from other regions hoggar adrar des iforas northeastern brazil nigeria central africa has led to the recognition of a pre orogenic continental rift that bordered the west africa sao francisco congo cratons in the southern part of the chain farther north in the hoggar and adrar des iforas this continental rift opened on a wider oceanic domain which involved a passive margin along the west african craton and an active margin to the east after these rifting and subduction stages continental collision followed that was dominated by strike slip movement oblique to the plate boundaries the general organization of the pan african brasiliano belt can be considered as the result of a confrontation between three major continental domains these were the west african and sao francisco congo cratons and a reworked shield composed in its inner part of polycyclic basement rocks and bounded by a fringe of monocyclic metasedimentary thrust belts that faced the cratons

ground deformation associated with the precursory unrest and early phases of the january 2006 eruption of augustine volcano alaska 1 on january 11 2006 augustine volcano erupted after nearly 20 years of quiescence global positioning system gps instrumentation at augustine consisting of six continuously recording telemetered receivers measured clear precursory deformation consistent with a source of inflation or pressurization beneath the volcano s summit at a depth of around sea level deformation began in early summer 2005 and was preceded by a subtle but distinct increase in seismicity which began in may 2005 after remaining more or less constant deformation rates accelerated on at least three stations beginning in late november 2005 after this date gps data suggest the upward propagation of a small dike into the edifice which based on the style of deformation and high levels of gas emission appears to have ascended to shallow levels by mid december 2005 about four weeks before the eruption began

petrology of pelitic schists in the oligoclase biotite zone of the sanbagawa metamorphic terrain japan phase equilibria in the highest grade zone of a high pressure intermediate type of metamorphic belt the oligoclase biotite zone of the bessi area central shikoku is characterized by sodic plagioclase xca 0 10 0 28 bearing assemblages in pelitic schists and represents the highest grade zone of the sanbagawa metamorphic terrain mineral assemblages in pelitic schists of this zone all with quartz sodic plagioclase muscovite and clinozoisite or zoisite are garnet biotite chlorite paragonite garnet biotite hornblende chlorite and partial assemblages of these two types correlations between mineral compositions mineral assemblages and mineral stability data assuming ph2o psolid suggests that metamorphic conditions of this zone are about 610 25 c and 10 1 kbar based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase biotite zone of the sanbagawa terrain with those in the new caledonia omphacite zone as an example of a typical high pressure type of metamorphic belt and with those in a generalized upper staurolite zone as an example of a medium pressure type of metamorphic belt progressive assemblages within these three zones can be related by reactions such as

metamorphic evolution of the kamuikotan high pressure and low temperature metamorphic rocks in central hokkaido japan the kamuikotan complex in the sorachi yezo belt located in central hokkaido japan has been extensively subjected to subduction related high pressure and low temperature metamorphism on the basis of geological petrological and geochronological data the coherent metamorphic rocks are divided into six units as follows susunai horokanai harushinai biei pankehoronai and shizunai based on their metamorphic mineral sequences the six units can be classified into the following three baric types high pressure 1 hp1 high pressure 2 hp2 and high pressure intermediate hi the horokanai and shizunai units belong to the hp1 type the harushinai and biei units belong to the hp2 type and the susunai and pankehoronai units belong to the hi type metabasites of the hp1 type are defined by the association of lawsonite na amphibole or epidote na amphibole glaucophane or crossite the hp2 type is characterized by the assemblage of pumpellyite na pyroxene chlorite and the representative low variance asssemblages are lawsonite pumpellyite na pyroxene chlorite and pumpellyite na pyroxene na amphibole chlorite in the hi type the most common low variance assemblage is pumpellyite epidote actinolite chlorite these results together with the previously published k ar and 40ar 39ar ages suggest that the kamuikotan metamorphic rocks gradually decreased in pressure during the cretaceous this tendency indicates an increase in the kamuikotan subduction geotherm during early cretaceous to early eocene time the increase in the paleogeotherm may result from the change of subduction rates and subducting slab ages

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

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

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

precursory inflation of shallow magma reservoirs at west sunda volcanoes detected by insar 1 we use 2006 2009 alos interferometric synthetic aperture radar data over the entire west sunda arc indonesia home of 13 of the world s active volcanoes to derive arc wide time dependent ground deformation data we present unambiguous evidence of inflation at six volcanoes three of which erupted after the observation period we show that these volcanoes have shallow magma reservoirs at 1 3 km depth below the average regional elevation a global comparison of reservoir depths at arc volcanoes suggests that volcanoes in extensional and strike slip settings west sunda can develop shallow reservoirs whereas volcanoes in compressional settings may lack them thus magma ascend through the upper crust could be influenced by intra arc tectonic settings

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

polyphase late paleozoic tectonothermal evolution of the southwestern meguma terrane nova scotia evidence from 40ar 39ar mineral ages 40ar 39ar incremental release ages of hornblende muscovite and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late paleozoic tectonothermal evolution for the southwestern meguma terrane regional d1 folding with cleavage formation under greenschist lower amphibolite facies m1 metamorphic conditions occurred at ca 400 410 ma and was followed by emplacement of a series of granitic stocks ranging in age between ca 375 and 315 ma these were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade these are locally superposed on m1 regional metamorphic assemblages and result in a complex isograd pattern 40ar 39ar mineral ages suggest episodes of contact metamorphism occurred at 1 360 375 ma possibly related to emplacement of the south mountain batholith or temporal equivalents 2 350 356 ma around the port mouton pluton and northeastern shelburne pluton 3 ca 315 325 ma near the wedgeport pluton and in

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

the characteristics of seismic signals produced by lahars and pyroclastic flows volcan de colima mexico abstract the general characteristics of seismic signals produced by pyroclastic flows generated by either the collapse of a lava dome or an eruptive column and lahars at volcan de colima mexico are discussed the paper concentrates on the 2004 2006 activity associated with and following the extrusion of andesitic block lava in october november 2004 it is shown that the duration of the broad band seismic records of pyroclastic flows lasts a few minutes while the duration of seismic records of lahars continues for tens of minutes or hours the spectra of seismic records produced by pyroclastic flows are characterized by lower peak frequencies around 3 4 hz than for lahars around 6 8 hz this difference in the frequency content together with the difference in the duration of seismic signals allows early diagnostic of the events in real time

time dependent changes in volcanic inflation rate near three sisters oregon revealed by insar 1 an extensive area near the three sisters volcanic center oregon has been actively uplifting since 1996 in this study we use interferometric synthetic aperture radar insar to assess the three sisters uplift in time and space from 1992 through 2010 we present the first insar line of sight time series of the deformation refine the onset of volcanic intrusive activity assess the changes in deformation through time and determine whether inflation is still occurring we model insar data to determine the source geometry that best describes the uplift and create an inflation time series of the deformation our results reveal an intrusion located at 5 7 km depth with a cumulative volume of magma of 5 7 107 m3 as of fall 2010 the input of magma started gradually around the summer of 1996 increased significantly from 1998 to 2003 and then decreased in rate from 2004 through 2010 we present evidence that a swarm in 2004 was associated with the decrease in the source inflation rate

systematic insar tropospheric phase delay corrections from global meteorological reanalysis data 1 despite remarkable successes achieved by differential insar estimations of low tectonic strain rates remain challenging in areas where deformation and topography are correlated mainly because of the topography related atmospheric phase screen aps in areas of high relief empirical removal of the stratified component of the aps may lead to biased estimations of tectonic deformation rates here we describe a method to correct interferograms from the effects of the spatial and temporal variations in tropospheric stratification by computing tropospheric delay maps coincident with sar acquisitions using the era interim global meteorological model the modeled phase delay is integrated along vertical profiles at the era i grid nodes and interpolated at the spatial sampling of the interferograms above the elevation of each image pixel this approach is validated on unwrapped interferograms we show that the removal of the atmospheric signal before phase unwrapping reduces the risk of unwrapping errors in areas of rough topography

argon retentivity of hornblendes a field experiment in a slowly cooled metamorphic terrane hornblende from samples of amphibolite and granitic gneiss collected within a single outcrop in the central adirondacks yield significantly different sup 40 ar sup 39 ark sub k dates of 948 plus minus 5 and 907 plus minus 5 ma assuming that this terrane cooled slowly following high grade metamorphism and that the samples have experienced the same thermal history the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples the fe fe mg mn of the hornblende samples are 0 8 and 0 6 the higher ratio corresponding to the younger sup 40 ar sup 39 ar sub k date transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution but contain zones of fibrous phyllosilicates approximately 0 1 to 2 mu m wide parallelling 100 and 110 these alteration zones probably formed during post metamorphic cooling as a result of the migration of fluids through the hornblendes and are obvious pathways for argon escape from hornblende as these features are more abundant in the hornblende sample with the younger sup 40 ar sup 39 ar date and higher fe fe mg mn they may influence the argon blocking temperature by effectively partitioning the hornblende grains intomore diffusion domains of varying size biotite from the granitic gneiss yields an sup 40 ar sup 39 ar sub k date of 853 plus minus 2 ma with a mildly discordant step heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite plagioclase from the amphibolite yields a sup 40 ar sup 39 ar sub k integrated date of 734 plus minus 3 ma less

space geodetic imaging of rapid ground subsidence in mexico city since the late 1950s several areas of mexico city have undergone accelerated ground subsidence and have developed associated fracturing and faulting new interferometric synthetic aperture radar insar and global positioning system gps data indicate that rates of current land subsidence in mexico city exceed 350 mm yr these rates are close to historical maximum levels of the mid twentieth century when mitigation efforts were fi rst undertaken to reduce damage to urban infrastructure the locus of maximum subsidence has shifted from its historical location in the old city center to the east correlation of our insar results with seismically mapped stratigraphic units suggests that subsidence is primarily controlled by compaction of quaternary lacustrine clays and silts we also evaluate spatial gradients in subsidence and suggest that this rather than subsidence magnitude is the key factor in risk assessment subsidence represents a major geologic risk for mexico city and imposes serious constraints to any further urban development

exsolution in hornblende and its consequences for 40ar39ar age spectra and closure temperature abstract step heating studies of hornblende and biotite which coexist in an amphibolite near north walpole vermont reveal dramatically different 40 ar 39 ar age spectra the biotite mg 59 is characterized by a plateau over 98 percent 39ar release at 343 5 ma in strong contrast to the amphibole which exhibits both a complex age spectrum and a k ar age significantly younger than the biotite the amphibole release pattern contains three complete age cycles which range between extremes of 199 ma and 408 ma interestingly the k ca ratio calculated from reactor produced isotopes also varies with ar release but follows a trend which is antipathetic to the 40 ar 39 ar age pattern detailed microstructural examination including transmission electron microscopy tem reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40ar 40ar retentivity in the amphibole this result underscores the variability of closure temperature tc for 40ar in metamorphic amphiboles and should warn against accepting a single universal value of tc

phase relations of pumpellyite actinolite facies metabasites in the sanbagawa metamorphic belt in central shikoku japan abstract sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpellytic actinolite facies were examined in terms of phase equilibria in the five component system al2o3 fe2o3 cao mgo feo the fe3 content of epidote measured as x ep fe fe fe al of epidote in the assemblage epidote chlorite actinolite pumpellyite decreases gradually towards the higher grade pumpellyite free areas the progressive change in xfeep can be detected within one metabasite bed 200 meters thick near the upper limit of the pumpellyite actinolite facies the mg fe2 substitution as expressed by variation of fe fe mg in chlorite 0 40 0 55 has little effect on the fe3 al ratios of epidote and pumpellyite in the above mentioned assemblage the lowet xfeep in the pumpellite bearing assemblage is 0 15 and hence the upper limit of the pumpellyite actinolite facies is defined by the appearance of an epidote chlorite actinolite assemblage with x ep fc c 15

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

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

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

interferometric synthetic aperture radar gps integration interseismic strain accumulation across the hunter mountain fault in the eastern california shear zone 1 the principal limitations of interferometric synthetic aperture radar insar to measure subtle long wavelength deformation are uncertainties associated with the satellite orbits we propose a method to remove orbital phase errors from the insar data by integrating insar and continuous gps time series we model the along track variation of the baseline errors as second order polynomials and estimate the coefficients using the continuous gps measurements we apply this method to a 600 km long region encompassing the basin and range and the eastern california shear zone comparison of the corrected insar velocities with independent gps data shows that this method removes the long wavelength insar errors the insar data reveal a region of sharp variation in the line of sight velocity across the hunter mountain fault we model the deformation as interseismic elastic strain accumulation across a strike slip fault the modeling suggests a fault slip rate of 4 9 0 8 mm yr and a locking depth of 2 0 4 km the shallow locking depth suggests that the hunter mountain fault is a transfer fault between low angle normal faults in the area

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

widespread uplift and trapdoor faulting on galapagos volcanoes observed with radar interferometry volcanic uplift caused by the accumulation of magma in subsurface reservoirs is a common precursor to eruptions1 2 but for some volcanoes uplift of metres or more has not yet led to an eruption3 here we present displacement maps of volcanoes in the galapagos islands constructed using satellite radar interferometry that might help explain this dichotomy we show that all but one of the seven volcanoes on the islands of isabela and fernandina deformed during 1992 99 cerro azul and fernandina erupted4 5 6 during the observation period and show evidence of inflation co eruptive deflation and shallow dyke intrusion in contrast the largest volcano sierra negra has not erupted yet exhibits spatially and temporally variable deformation with a maximum uplift of 2 7 m between 1992 and 1999 which can be modelled by a shallow inflating sill inflation during 1997 98 however was accompanied by trapdoor faulting on a steeply dipping fracture system within the caldera repeated trapdoor faulting over geological time has formed an arcuate intra caldera ridge within sierra negra and may have acted to relax stresses above the magma chamber inhibiting summit eruptions similar processes may help explain large uplift unaccompanied by eruptive activity at other volcanoes

neoproterozoic high pressure low temperature metamorphic rocks in the avalon terrane southern new brunswick canada high p low t metamorphic rocks of the hammondvale metamorphic suite hms are exposed in an area of 10 km2 on the nw margin of the caledonian avalon terrane in southern new brunswick canada the hms is in faulted contact on the se with c 560 550 ma volcanic and sedimentary rocks and co magmatic plutonic units of the caledonian terrane the hms consists of albite and garnet porphyroblastic mica schist with minor marble calc silicate rocks and quartzite pressure and temperature estimates from metamorphic assemblages in the mica schist and calc silicate rocks using twq indicate that peak pressure conditions were 12 4 kbar at 430 c peak temperature conditions were 580 c at 9 0 kbar 40ar 39ar muscovite ages from three samples range up to 618 615 ma a minimum age for high p low t metamorphism in this unit these ages indicate that the hms is related to the c 625 600 ma subduction generated volcanic and plutonic units exposed to the se in the caledonian terrane the ages are also similar to those obtained from detrital muscovite in a neoproterozoic cambrian sedimentary sequence in the caledonian terrane suggesting that the hms was exposed by latest neoproterozoic time and supplied detritus to the sedimentary units the hms is interpreted to represent a fragment of an accretionary complex similar to the sanbagawa belt in japan it confirms the presence of a major cryptic suture between the avalon terrane sensu stricto and the now adjacent brookville terrane

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

characterization of open and closed volcanic systems in indonesia and mexico using insar time series 1 we use 2007 2011 advanced land observing satellite alos data to perform an arc wide interferometric synthetic aperture radar insar time series survey of the trans mexican volcanic belt tmvb and to study time dependent ground deformation of four indonesian volcanoes selected following the 2007 2009 study of chaussard and amelung 2012 our objectives are to examine whether arc volcanoes exhibit long term edifice wide cyclic deformation patterns that can be used to characterize open and closed volcanic systems and to better constrain in which cases precursory inflation is expected we reveal deformation cycles at both regularly active and previously inactive indonesian volcanoes but we do not detect deformation in the tmvb reflecting a lower activity level we identify three types of relationships between deformation and activity inflation prior to eruption and associated with or followed by deflation kerinci and sinabung inflation without eruption and followed by slow deflation agung and eruption without precursory deformation merapi colima and popocatepetl at merapi no significant deformation is detected even during eruption the first two cases correspond to closed volcanic systems and suggest that the traditional model of magmatic systems and eruptive cycles do apply to andesitic volcanoes i e inflation and deflation episodes associated with magma accumulation or volatile exsolution in a crustal reservoir followed by eruptions or in situ cooling in contrast the last case corresponds to open volcanic systems where no significant pressurization of the magmatic reservoirs is taking place prior to eruptions and thus no long term edifice wide ground deformation can be detected we discuss these results in terms of insar s potential for forecasting volcanic unrest

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

mesozoic tectonothermal development of the sambagawa mikabu and chichibu belts south west japan evidence from 40ar 39ar whole rock phyllite ages five whole rock 40ar 39ar plateau ages from low grade sectors of the sambagawa belt besshi nappe complex range between 87 and 97 ma two whole rock phyllite samples from the mikabu greenstone belt record well defined 40ar 39ar plateau ages of 96 and 98 ma together these ages suggest that a high pressure metamorphism occurred in both the sambagawa and mikabu belts at c 90 100 ma the northern chichibu sub belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north c 110 ma to south c 215 ma the northern chichibu sub belt is correlated with the kuma nappe complex sambagawa belt two whole rock phyllite samples from the kurosegawa terrane display markedly older metamorphic ages than either the sambagawa or the chichibu belts accretion of sambagawa chichibu protoliths began prior to the middle jurrasic depositional ages decrease from middle jurassic kuma chichibu nappe complex to c 100 ma oboke nappe complex toward lower tectonostratigraphic units the ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units the kurosegawa belt and the geological units to the south belong to distinctly different terrances than the sambagawa chichibu belts these have been juxtaposed as a result of transcurrent faulting during the cretaceous

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

u pb ages of detrital zircons from the sanbagawa belt in western shikoku additional evidence for the prevalence of late cretaceous protoliths of the sanbagawa metamorphics the sanbagawa metamorphic belt in japan is one of the best studied high pressure low temperature metamorphic belts recent work applying new dating techniques has challenged the previously accepted temporal framework for the evolution of the belt as it was shown that large parts of the belt contain detrital zircons of late cretaceous age younger than 100 ma i e they have protolith ages younger than the previously accepted age of metamorphism at ca 110 ma a 2000 m bore hole from north western shikoku provided an excellent opportunity to further evaluate the areal extent of late cretaceous protoliths as the drill hole was drilled in an area considered to be part of the jurassic to early cretaceous part of the sanbagawa belt dating of single zircon grains using the la icp ms u pb dating method shows that all but one sample contain zircons younger than 100 ma and thus the protoliths are younger than the previously accepted age of metamorphism of the sanbagawa belt the single sample that contains only zircons dated at 136 3 ma apparently is of volcanic origin and could be a clast representing the source of 130 140 ma zircons of the sample taken about 120 above this sample in addition three surface samples were analysed two of these also contain zircons younger than 100 ma whereas the third sample contains only zircons older than 159 ma the zircons from this sample also exhibit an age spectrum different from that exhibited by the other samples the exact significance of this sample is not clear as yet