palaeoseismology review of the state of the art abstract historical seismicity provides only a partial indication of the seismotectonic character i e earthquake potential of a given region even in italy where a relatively long historical record exists available catalogues are generally unsatisfactory for this purpose fortunately strong earthquakes often leave geological evidence herein referred to as seismites that are mainly related to surface faulting folding or other deformation and occasionally to secondary ground failures e g landslides liquefaction features etc these features may be preserved and therefore palaeoseismological studies may detect and date them thereby permitting extension of knowledge beyond the local historical threshold up to 2000 years in the most fortunate cases this paper provides a summary of criteria which may aid in the identification of areas suitable for palaeo earthquake investigation historical seismicity and the presence of well developed tectonic geomorphic features are recognized as the most reliable criteria for identifying active faulting however lack of these characteristics cannot be considered conclusive evidence for lack of activity palaeoseismological techniques aim at identifying not only the obvious effects of surface faulting such as offset of young geologic units but also the more subtle tectonic or non tectonic features including liquefaction structures gravity features and coseismic uplift and or subsidence finally we discuss some questions that pertain to palaeoseismicity evaluation techniques among these are the reliability of the quaternary dating techniques the evaluation of the seismic energy released minimum magnitude and principally the unequivocal identification of a seismic origin for the considered geological structures

tracking slabs beneath northwestern pacific subduction zones article i nfo article history this study uses the amplitudes of bottom side reflected shear waves to constrain the morphology and dynam ics of subducted oceanic lithosphere beneath northwestern pacific subduction zones across honshu arc the 410 and 660 km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the wadati benioff zone their topographies are negatively correlated along slab dip showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone the base of the upper mantle shows broad depressions as well as localized zones of shallow average depths be neath korea and northeast china the 15 km peak to peak topography west of the wadati benioff zones suggests that the stagnant part of the subducted pacific plate is not as flat as previously suggested eastward slab pile up is also possible at the base of the upper mantle across southern kuril arc the shear wave reflec tion coefficients of major olivine phase boundaries fall below 5 within the wadati benioff zone the appar ent reflection gaps and the spatial connection between a strong reflector at 900 km depth may imply 1 possible compositional variations at the top and bottom of the transition zone and 2 substantial mass heat flux across the 660 km seismic discontinuity we also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths the depths and strengths of these reflectors are highly variable be tween honshu and southern kuril islands

petrogenesis of permian triassic felsic igneous rocks along the truong son zone in northern laos and their paleotethyan assembly abstract felsic igneous rocks in northern laos are key elements of the truong son and song ma tectonic zones and are important for constraining the paleotethyan evolution in southeastern asia this paper presents new geochronological geochemical and sr nd hf o isotopic data from granitoids and rhyolites in northern laos geochronological data show that these samples have zircon u pb ages ranging from ca 281 ma to 221 ma these samples can be subdivided into four groups based on their lithology and geochemical composition group 1 samples are granodiorite and granite with i type geochemical characteristics they have lower 87sr 86sr i ratios 0 7036 0 7059 and higher end t values 0 7 0 8 than group 2 biotite granite samples 87sr 86sr i 0 7087 0 7129 and end t 5 3 to 9 2 and group 3 rhyolite samples 87sr 86sr i 0 7092 0 7103 and end t 5 9 to 8 9 zircon grains from group 1 samples yield positive ehf t values of 5 1 13 8 tdm ages of 0 37 0 71 ga and 18o values of 5 6 6 3 in contrast zircons from group 2 and group 3 samples have mainly negative ehf t values 9 6 to 1 6 and 4 7 to 14 7 older tdm ages 1 08 1 70 ga and 1 42 1 97 ga and higher 18o values 5 7 10 3 and 6 7 8 0 respectively our geochemical data suggest that the group 1 granite samples are the products of the partial melting of newly underplated mafic rocks whereas the group 2 and 3 samples originated from a mixed source of ancient metaigneous and metagreywacke components group 4 samples include late triassic i type granodiorite and granite and have end t 1 9 to 3 9 87sr 86sr i 0 7079 0 7084 zircon ehf t 0 0 7 5 and 18o 5 1 6 9 indicating a derivation from a metagreywacke source with a component of juvenile mafic crust our data suggest that the permian felsic igneous rocks in northern laos formed in an arc setting related to the song ma paleotethyan branch bab subduction whereas the late triassic i type granitoids formed in a post collisional setting in response to the gravitational collapse of thickened crust during the collision of the south china and indochina blocks the temporal spatial pattern of igneous rocks along the truong son zone is related to the paleotethyan evolution and resembles that along the ailaoshan song ma zone and its eastward extension in northern hainan

a geophysical study of the taupo volcanic zone new zealand the taupo volcanic zone is a region of intense volcanic activity from which over 16 000 km3 of lavas and pyroclastics have been erupted since the late pliocene analysis of gravity and magnetic anomalies over the zone has revealed the presence of five major centers each associated with an approximately circular negative gravity anomaly and a positive pseudogravity anomaly these centers are interpreted as collapse features resulting from large scale withdrawal of magma from the upper crust four of the depressions occur beneath recognized volcanic centers or calderas the fifth kapenga is inferred to underlie an extinct hidden volcanic center three dimensional models of the thicknesses of low density and magnetized rocks in the taupo volcanic zone have been determined by inversion of gravity and magnetic anomalies beneath three of the centers the thicknesses of low density and magnetized rocks are approximately equal 2 3 km that is the collapse structures are infilled by magnetized volcanics however there is a significant discrepancy between the thicknesses of the gravity 5 km and magnetic models 1 5 km in the region of the okataina volcanic centre it is therefore suggested that a significant volume of low density nonmagnetic rock possibly acidic magma underlies okataina

chronology and dynamics of a large silicic magmatic system central taupo volcanic zone new zealand the central taupo volcanic zone in new zealand is a region of intense quaternary silicic volcanism accompanying rapid extension of continental crust at least 34 caldera forming ignimbrite eruptions have produced a complex sequence of relatively short lived nested and or overlapping volcanic centers over 1 6 m y silicic volcanism at taupo is similar to the yellowstone system in size longevity thermal flux and magma output rate however taupo contrasts with yellowstone in the exceptionally high frequency but small size of caldera forming eruptions this contrast reflects the thin rifted nature of the crust which precludes the development of long term magmatic cycles at taupo 11 refs 4 figs 1 tab

effect of stress fields on magma chamber stability and the formation of collapse calderas 1 the summits of many of the earth s and other planets larger volcanoes are occupied by calderas that formed by collapse into an evacuating underlying magma chamber these collapse calderas are typically several tens of square kilometers in area and are commonly elliptical in shape we show that the long axes of late quaternary collapse calderas in the kenya rift valley the western basin and range province the snake river yellowstone plateau and the iceland rift zone are parallel to the upper crustal minimum horizontal stress direction sh as determined by independent criteria we suggest that circular magma chambers beneath these volcanoes became elliptical by stress induced spalling of their chamber walls by a mechanism that is analogous to the formation of breakouts in boreholes and tunnels in breakouts the hole becomes elongate parallel to the far field minimum stress in the kenya rift late pleistocene caldera collapse was accompanied by a 45 rotation of sh and an increase in the magnitude of the maximum horizontal stress sh the breakout model predicts increasingly unstable caldera walls under these conditions a possible explanation for the sudden appearance of so many collapse events in a volcanic setting that had never experienced them before this mechanism of stress change induced collapse may have played a role in other caldera settings

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

populations of faults and fault displacements and their effects on estimates of fault related regional extension abstract measured populations of fault displacements derived from regional seismic oilfield seismic coalmine plans and outcrop data show a power law distribution with exponents s of 0 45 to 0 95 for singleline samples across an array of faults the more negative values indicate relatively larger numbers of smaller faults an expression for a population of active faults derived from the gutenberg richter magnitude frequency relation for earthquakes is log n a b d log d where d maximum displacement of a fault n number of faults of maximum displacement d or greater and b d 1 0 and has the same value as b for the corresponding earthquake population populations of dead faults existing at the end of a tectonic episode have been numerically derived using a fault growth model and satisfy the relation log n a e log d where e has a value between 1 6 and 2 0 numerically derived populations of fault displacements in a dead fault population have slopes of s where s e 1 the contribution of an individual fault to the regional strain varies with the lifetime seismic moment of the fault and is proportional to d 2 estimation of fault related extension by summing heaves on faults of a limited size range is valid only if the measured size range of faults accommodates most of the extension correction can be made if the s value of the fault displacement population is known

characteristics of three recent earthquake sequences in the taupo volcanic zone new zealand abstract we have analysed three recent earthquake sequences in the northern part of the taupo volcanic zone a 1998 sequence at haroharo with a largest event of ml 4 8 and a 2004 sequence near lake rotoehu largest event ml 5 4 had normal b values and displayed an aftershock decay pattern with most of the activity within the first few days in contrast a 2005 sequence a few tens of kilometres away at matata largest event ml 4 1 had very different characteristics with a slow development and decay no tendency for enhanced seismicity after the larger events and a very high b value the focal mechanisms of the rotoehu and matata events are normal and have stress patterns consistent with the geodetically observed extension of the taupo volcanic zone in a northwest southeast direction the extensive recent volcanism in the okataina volcanic centre does not seem to have affected the stress pattern in this area the rotoehu sequence showed a strong resemblance particularly in the time distribution of events to the well known swarm activity in the vogtland region on the german czech border in which larger events were followed by a burst of seismicity as in a normal aftershock sequence some of the arguments that have been advanced to explain the vogtland swarm as seismicity induced by fluid injection apply to rotoehu but there is no direct evidence of fluid involvement the matata sequence appears to have a continuing trigger mechanism either a slow injection of fluid or a slow slip event in an environment in which opening pore spaces prevent high overpressures developing the matata sequence occurred close to the area of the 1987 ml 6 3 edgecumbe earthquake so exhibiting two extremes of seismic temporal pattern namely mainshock aftershock and a swarm with many events of similar magnitude within a small area

quaternary temporal stability of a regional strike slip and rift fault intersection abstract intersecting strike slip and normal fault systems form either two or three plate configurations in circumstances where they intersect to form a triple junction internal block deformation produces a quasi stable 3 d configuration permitting maintenance of both the regional geometry and kinematics of the intersection this paper examines the temporal stability of such a triple junction in new zealand over a range of timescales from individual earthquakes to millions of years and discusses the factors that impact its stability using seismic reflection gravity drill hole and outcrop data the accumulation of vertical displacements is examined both on individual faults and across the intersection zone between the strike slip north island fault system nifs and the taupo rift fault throws and throw rates for the nifs and taupo rift suggest that despite a three fold increase in throw rates in both fault systems at about 0 3 ma which may have modified the orientation of the slip vectors and therefore the contemporary kinematics of the triple junction the intersection of the two fault systems has functioned coherently for the last 0 6 1 5 myr accumulating vertical displacements interdependently since the early stages of rifting the apparent arrest of a large magnitude earthquake on the rift bounding fault adjacent to the intersection suggests that the short term dynamic behaviour of the system is relatively complex compared to the more coherent long term kinematics

late permian to middle triassic sedimentary records in shiwandashan basin implication for the indosinian yunkai orogenic belt south china abstract the shiwandashan basin is an elongate region of the late permian middle triassic potential sediment accommodation that formed on continental crust between the yunkai orogen and the adjacent yangtze craton block south china several terms such as remanent marine trough structural window rift and pull apart basin have been proposed studies of the late permian middle triassic sedimentary sequences and the tectonics lithofacies and paleogeography of the shiwandashan basin indicate that the late permian middle triassic succession is composed of thick coarse molasse deposits and fine clastics organized into coarsening upward and fining upward stratigraphic sequences the shiwandashan basin went through the process of growth and disappearance from molasse to turbidite deposition and finally to volcanic eruption during late permian middle triassic to some extent the facies evolution reflects the tectonic controls on the sedimentation of the foreland basin the coarsening upward sequences hundreds to thousands of metres scale occurring immediately adjacent to the thrust front demonstrate that important episodic thrusting orogenesis and piedmont flexuring subsidence with low velocity took place in the northwestern part of the yunkai orogen whereas fining upward sequences are the indicator of continued deformation with high thrust velocity the following four facts have been demonstrated 1 a foreland basin existed in late permian middle triassic 2 the development of the foreland basin was controlled by the evolution the yunkai orogen 3 the basin and mountains continuously changed and migrated 4 the shiwandashan basin was a key demarcation between an active region in southeastern south china cathaysian block and a relatively stable region in northwestern yangtze block it was a convergent boundary in late permian middle triassic and its tectonic nature changed in the wake of the tectonic evolution of the surrounding area

structural control of volcanism and caldera development in the transtensional taupo volcanic zone new zealand abstract the taupo volcanic zone tvz new zealand comprises several segments with variable components of extension and is characterised by longitudinal segmentation into a central zone dominated by rhyolitic calderas and extremities with only andesitic composite volcanoes modern

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

40ar 39ar geochronology of the charnockites and granulites of the kan nack complex kon tum massif vietnam abstract the truong son belt forms the eastern rim of the indochina block in southeast asia the age of the metamorphism mainly along nw se mylonitic shear zones that affects this belt has been formerly determined at about 240 250 ma this age corresponds to the indosinian tectonometamorphic episode the kon tum massif situated to the south of this belt comprises high temperature rocks the kan nack complex including charnockites and granulites the main charnockitic outcrops restricted to the song ba valley establish the intrusive nature of these magmatic rocks within granulite facies material basic charnockitic rocks are mainly quartz enderbites to norites and hornblende pyroxene granulite facies rocks the 40 ar 39 ar age of intrusion cooling of charnockitic magmas is determined from primary magmatic biotites at about 245 ma in the east of the kan nack complex some granulite facies rocks exhibit relicts of primary granulite facies parageneses whereas others show evidence of overprinting by a retrogressive low grade metamorphism ar ar dating confirm this evolution giving ages of 400 ma for primary relict granulite facies phases and 260 270 ma from the most retrogressed samples establishing the youngest limit for the granulite facies metamorphism granulites intruded by charnockites in the song ba valley yield ages of about 250 ma equivalent to the ages of the charnockites and have evidently been completely reset by these high temperature intrusions therefore the kan nack complex of the kon tum massif is not an independent unit with respect to the indosinian orogen but represents the deep crustal part of this belt

geochronological elemental and sr nd hf o isotopic constraints on the petrogenesis of the triassic post collisional granitic rocks in nw thailand and its paleotethyan implications abstract new u pb geochronological petrologic elemental and sr nd hf o isotopic data for the granites from the inthanon and sukhothai zones in nw thailand in conjunction with correlations with sw china are presented to constrain the age and position of the paleotethys ocean in this region and the associated assembly of southeast asia the geochronological data show that the granitic rocks in the inthanon and sukhothai zones herein named group 1 and group 2 granites respectively yield similar crystallization ages of 230 200 ma group 1 samples are characterized by monzogranite and granite with i and s type geochemical affinity and group 2 samples by i type monzogranite and granodiorite they have generally similar chondrite normalized ree and pm normalized multi element patterns but distinct sr nd hf o isotopic compositions group 1 samples have slightly higher initial 87sr 86sr ratios 0 7111 0 7293 but lower end t values 11 1 to 14 1 than those of group 2 samples 87sr 86sr i 0 7073 0 7278 and end t 8 3 to 11 0 group 1 samples show the lower ehf t values 5 4 to 18 2 older tdm 1 62 2 40 ga and higher 18o values 7 95 to 9 94 than those of group 2 samples ehf t of 11 1 to 4 80 tdm of 0 96 1 95 ga and 18o of 4 95 to 7 98 for the triassic crystallization zircons these geochemical signatures are similar to the kwangsian and indosinian granites in the south china and indochina blocks but distinct from those of the gangdese i type granite and sibumasu paleozoic granite our data suggest that group 1 samples mainly originated from the early paleozoic supracrustal rocks containing metapelite and metavolcanic components which had previously experienced the surface weathering group 2 samples were derived from a hybridized source of an old metamorphic and a newly underplated mafic component synthesis of our data with available regional observations indicates that the inthanon zone represents the main suture zone of the eastern paleotethyan ocean in nw thailand and links with the changning menglian suture zone in sw yunnan sw china in nw thailand a switch from the eastward subduction of the paleotethyan oceanic plate to the collision of the sibumasu with indochina blocks occurred at 237 ma and syn and post collisional time being at 237 230 ma and 200 230 ma respectively the late triassic granites in the inthanon and sukhothai zones are representative of the post collisional magmatic products

the role of regional scale faults in controlling a trapdoor caldera coromandel peninsula new zealand abstract regional gravity data from an eroded miocene to pliocene volcanic arc exposed in the coromandel peninsula new zealand delineate a circular 26 mgal 15 km diameter gravity anomaly this anomaly which has steep gradients on its northern and western margins but shallow gradients elsewhere correlates with relatively young volcanic and volcaniclastic rocks within a broad topographic depression gravity modelling using an exponentially decreasing density contrast with depth profile requires very low density rocks ca 2280 kg m 3 in the near surface to account for the observed anomaly giving a total depth of ca 2 8 km for these rocks the northern and western margins of this body dip steeply inward at 70 whereas the southern and eastern margins have shallow inward dips 20 30 the western margin coincides with the regional scale mangakino fault but the northern margin recognizable only in the geophysical data and named here the ohinemuri fault is partially buried under younger volcanic rocks we interpret these deep and steeply bounded low density volcanics in terms of a trapdoor caldera faulted on its northern and western margins with its hinge on the southern and eastern margins epithermal deposits are spatially associated with the mangakino and ohinemuri faults suggesting that both structures may have influenced hydrothermal fluid flow these deposits pre date caldera fill indicating that caldera development followed pre existing regional faults these results delineate the subsurface geometry of a trapdoor caldera and highlight the role of pre existing regional scale faults in controlling such caldera location and collapse

oblique back arc rifting of taupo volcanic zone new zealand taupo volcanic zone tvz is a back arc rift innorth island new zealand its geometry andkinematics are investigated using shaded reliefimages field examination of faults and offset streamchannels the results show that tvz trends nne is 250 km long by hikurangi margin in the north island of new zealand 20 km wide and consists of fivesegments extension is principally manifest as steeplydipping 60 90 normal faults parallel to tvz these in the last 300 kyr have experienced acomponent of dextral shear tvz is therefore anoblique back arc rift the dextral shear is 37 of thetotal displacement which for previously estimatedspreadingrates 7mm yr correspondsto 2 6mm yr thisvalueissimilartopreviousestimatesofthedextralshear from the back arc to the forearc domains in thenorthisland distributeddextralshearacrosstvzthussuggests that strain partitioning across the plateboundary at latitudes of tvz is less significant thanpreviously thought

interdependence of fault displacement rates and paleoearthquakes in an active rift paleoearthquakes at earth s surface often generate faults with variable displacement rates over short time intervals e g 18 k y the nature and origin of these variations and the extent to which they result from systematic and therefore predictable earthquake processes is unresolved we examine the processes underlying fluctuations in displacement rates by charting the accumulation of displacement over the last 60 k y on 25 normal fault traces distributed across the taupo rift new zealand displacement rates become more stable with increasing fault size and are uniform when aggregated across the entire rift the increased stability of fault displacement rates at greater spatial scales suggests that each fault is a component of a kinematically coherent system in which all faults interact and their earthquake histories are interdependent fault interdependencies generate short term complex 18 k y fluctuations in the timing and magnitude of earthquakes but also ultimately result in the stability of displacement rates on million year time scales

seismic velocity structure of the central taupo volcanic zone new zealand from local earthquake tomography abstract the 3 d distribution of p wave velocity vp and the p wave s wave velocity ratio vp vs are derived for the crust in the central taupo volcanic zone tvz new zealand by tomographic inversion of p and s wave arrival time data from local earthquakes resolution in the seismogenic mid crust 4 6 km is good but poorer above and below these depths the 3 d velocity model has several vp anomalies as large as 5 in the mid lower crust 4 10 km and more than 10 in the upper crust 0 4 km the model achieves a 55 reduction in data variance from an initial 1 d model young caldera structures okataina rotorua and reporoa are characterised by low vp anomalies at a depth of about 4 km and these coincide with large negative residual gravity anomalies we attribute these anomalies to large volumes of low vp low density volcaniclastic sediments that have filled these caldera collapse structures although there are no vp anomalies which suggest the presence of molten or semi molten magma beneath the tvz a large high vp anomaly of more than 15 and a high vp vs anomaly are observed coincident with a diorite pluton beneath the ngatamariki geothermal field however vp anomalies cannot be seen beneath the largest geothermal fields waimangu waiotapu and reporoa and consequently if such anomalies exist they must be below the resolution of our data a prominent vp contrast of 5 10 occurs at a depth of about 6 km beneath the boundary between the taupo reporoa depression and the taupo fault belt tfb coincident with the eastern limit of the seismic activity beneath the tfb we interpret this velocity contrast as being caused by the presence of extensive non molten intrusives beneath the taupo reporoa depression we suggest that the high velocity material beneath the taupo reporoa depression is isolated from regional extension in the tvz and from the resulting faulting and seismicity which occurs preferentially within the weaker material of the tfb we are unable to determine whether greywacke which forms the basement beneath the eastern most part of the tvz continues further west or is replaced by a volcanic rock such as andesite which has similar vp and density vp vs anomalies are much smaller than vp anomalies and generally have little spatial relationship to the vp pattern there is a widespread decrease in vp vs from 1 76 to 1 70 1 73 between 4 and 6 km depth over much of the study area and vp vs is high southwest of the okataina caldera where vp is low hypocentres calculated using the 3 d velocity model differ little from those obtained using a 1 d model with station terms however some groups of earthquakes are more tightly clustered following relocation there is a slight decrease in the estimated thickness of the seismogenic zone with 73 of hypocentres between 4 and 7 km depth and a slight increase in the depth of the brittle ductile transition from 6 to 6 5 km

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

fault controls on the geometry and location of the okataina caldera taupo volcanic zone new zealand abstract okataina caldera is located within the taupo rift and formed due to collapse following eruptions at 325 and 61 ka gravity seismic reflection topographic and geological data indicate that active rift faults pass into the caldera and have influenced its location and geometry the caldera has a minimum gravity anomaly of 50 mgal is elongate north south with an inferred minimum depth to caldera floor of 3 0 5 km at the rift axis and occupies a 10 km hard linked left step in the rift the principal rift faults 55 75 dip define the location and geometry of the northwest and southeast caldera margins and locally accommodate piecemeal collapse segments of the east and west margins of the caldera margin are near vertical 70 90 dip trend north south and are inferred to be faults formed by the reactivation of a pervasive mesozoic basement fabric i e faults and or lithological contacts the fault sets which define the caldera geometry predate it while the step in the rift across the okataina volcanic centre ovc is at least as old as the caldera within the ovc displacement on rift faults induced by gravitational caldera collapse at 61 ka exceeds tectonic displacement since this time by at least a factor of two collapse along pre existing rift faults and in particular mesozoic basement fabric are important for caldera formation elsewhere in the taupo volcanic zone

mantle layering from scs reverberations 1 waveform inversion of zeroth order reverberations this is the first paper in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms here we present path averaged estimates of the two way travel times through the crust mantle scs and crust alone m the whole mantle quality factor qscs and the reflection coefficient at the m discontinuity r0 zm obtained by iterative waveform inversion of scs reverberations we show that qscs estimates derived from direct waveform inversion can be biased significantly if reverberations originating within the crust are ignored especially in continental regions where the crust is thick applying our technique which explicitly models crustal reverberations we substantially expand the geographical coverage of scs and qscs measurements available from previous studies the scs data correlate well with whole mantle travel times predicted by the harvard tomographic models both the scs data and the qscs data support the hypothesis that variations in velocity and attenuation structure between old continents and oceans persist to depths of several hundred kilometers in particular the value of qscs derived for paths crossing stable continent is very high 280 30 at 30 mhz compared with that for mature ocean basin 189 16 consistent with the thick plate model of cratons requiring that differences in thermal structure extend to several hundred kilometers the crustal thicknesses implied by the observations of path averaged m are consistent with those inferred from short range refraction data gravity measurements and surface topography the path averaged reflectivity of the m discontinuity is also tectonically correlated becoming brighter as the percentage of continental crust encountered along the path increases this result implies that on average the impedance at the base of the continental crust is significantly lower than at the base of the oceanic crust

the paleozoic metamorphic history of the central orogenic belt of china from 40ar 39ar geochronology of eclogite garnet fluid inclusions abstract the pressure temperature time evolution of the uhp eclogites of dabie sulu in the eastern sector of the central orogenic belt of china shows a complex pattern of predominantly triassic and to a lesser extent early paleozoic ages 40 ar 39 ar stepwise crushing analysis of eclogitic garnets yielded consistently paleozoic ages because our initial results from the bixiling eclogite were of limited areal extent their significance required further testing data for garnet bearing gneiss and eclogites from new locations in northern and southern dabieshan shuanghe ganghe and hongmiao confirm the paleozoic garnet results obtained from bixiling eclogite we also tested the technique on a garnet sample of known paleozoic age from altyn tagh in the western central orogenic belt these new results indicate that light colored garnets from dabieshan and altyn tagh yield concordant 40 ar 39 ar ages ranging from 445 465 ma by in vacuo crushing and confirm that a paleozoic u hp metamorphic event took place in the central orogenic belt from altyn tagh in western china to dabieshan in eastern china in contrast dark colored garnets from mafic metavolcanics do not yield 40 ar 39 ar plateaux or isochrons ages

geochemical and nd pb isotopic characteristics of the tethyan asthenosphere implications for the origin of the indian ocean mantle domain abstract it is unclear why the pb nd and sr isotopic composition of the modern mid ocean ridge basalts morb from the indian ocean is different from that of the north atlantic and pacific oceans a possible explanation for this is that the indian morb type isotopic signature is a long lived regional feature of the mantle as evidently shown by the isotopic composition of the 350 ma morb like mian lue northern ophiolite which was formed in the same region presently occupied by the indian ocean however this hypothesis is in conflict with the lack of indian morb type isotopic signature in a number of 150 ma tethyan and indian ocean crusts to further constrain the origin of the indian morb type isotopic signature we analyze the geochemical and pb nd and sr isotopic composition of representative mafic rocks from four tethyan ophiolites ranging in age from 90 to 360 ma the sr isotopic composition of the samples is unreliable due to alteration but the age corrected nd and pb isotopic ratios and geochemical data indicate that these tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear indian morb type isotopic signature we therefore conclude that the bulk of the indian suboceanic mantle was most probably inherited from the tethyan asthenosphere a few regions in both the tethyan and indian oceans however are most probably underlain by pacific and north atlantic morb type mantle and vice versa because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins the indian morb type isotopic signature of the western pacific marginal basin crusts could be due to either flow of the indian ocean mantle into the western pacific or to endogenous production of such an isotopic signature from delaminated east asian sublithospheric materials during closure of the tethys ocean

gravity magnetic and seismic surveys of the caldera complex lake taupo north island new zealand abstract the taupo volcanic centre lies at the southern end of the taupo volcanic zone in the central north island of new zealand major rhyolitic eruptions of 400 and 35 km 3 dense rock equivalent occurred from this centre 26 5 and 1 8 thousand years ago respectively as a result of these eruptions much of the volcanic centre has collapsed and now lies beneath the waters of lake taupo gravity data measured on lake taupo show that the most intense negative bouguer anomaly to occur anywhere within the taupo volcanic zone lies over the northern part of the lake this anomaly is interpreted to outline the area of caldera collapse caused by the 26 5 ka eruption which has been subsequently modified by the 1 8 ka eruption and filled by approximately 3 km of low density uncompacted volcaniclastic material a thick laminated sequence of seismic reflectors forming a distinct unit in the seismic sections can be recognized over most of the lake area and lies beneath material interpreted to have been erupted in the 1 8 ka eruption this sequence of reflectors is interpreted to be the result of the lacustrine deposition of unconsolidated material immediately following the 26 5 ka eruption a prominent magnetic anomaly within the western part of the collapse structure is interpreted to be caused by a body of more highly magnetised material lying at or near the base of the low density caldera fill the laminated sequence of reflectors tilts and thickens towards this magnetic anomaly suggesting that the magnetised material occurs in the deepest part of the 26 5 ka collapse structure the southern margin of the intense low gravity anomaly is marked by a zone of faulting observed in the seismic reflection data magnetic and seismic data in the southern part of the lake are interpreted as a welded ignimbrite sheet overlain in places by rhyolite lavas similar to the pre 26 5 ka surface exposed west of the lake this surface appears to have been down faulted by approximately 500 m and is covered by more recent low density volcaniclastic material the thickness of the volcaniclastic material increases northward as the pre 26 5 ka surface is tilted downwards towards the area of major collapse the northward tilt of this surface suggests that subsidence in the southern part of the lake may also be a response to the 26 5 ka eruption

a geophysical geological transect of the silent canyon caldera complex pahute mesa nevada revision of lithological logs for boreholes penetrating the volcanic center at pahute mesa nevada has led to a thorough review of the volcanic stratigraphy and geologic structure we have combined this review with a compilation of old and newly acquired gravity and seismic travel time data producing a unified interpretation along a northwest to southeast profile the analysis supports a new interpretation of the silent canyon caldera complex the caldera is found to be more asymmetric than previously suggested with the southeastern boundary formed by linear high angle normal faults and a more gently sloping northwestern boundary the total thickness of volcanic units within the caldera complex does not appear to exceed 5 km the shallow structure at pahute mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site the silent canyon caldera complex is actually a set of nested calderas first filled by thick 1 km postcaldera lavas and subsequently buried by outflow sheets of the timber mountain caldera to the south thick postcaldera lavas filled a half graben structure formed west of the west greeley fault dropping the tops of the youngest caldera forming units to depths in excess of 2 km therefore the western boundary of the caldera complex is poorly defined east of the west greeley fault two overlapping calderas are defined and stratigraphic data suggest the presence of even older calderas the youngest caldera the calc alkaline area 20 caldera is well defined from drill hole data the area 20 caldera overlaps the 13 6 ma peralkaline grouse canyon caldera which is less well defined but apparently collapsed in trap door style along the almendro fault for both these calderas collapse continued after the main caldera forming eruption concurrent with the accumulation of thick 1 km lavas within the peripheral collapse zones the geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the nne trending scrugham peak and almendro faults which offset the pre tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 ma age drill hole data show that offsets along these faults increase systematically within older up to 15 ma units which are commonly rotated eastward in a style similar to units at the surface abrupt changes in the subsurface thickness of the caldera forming units occur across the faults indicating that these linear features served as caldera boundaries

geophysical imaging of buried volcanic structures within a continental back arc basin the central volcanic region north island new zealand abstract hidden beneath the 2 km thick low velocity volcaniclastics on the western margin of the central volcanic region north island new zealand are two structures that represent the early history of volcanic activity in a continental back arc these 20 20 km structures at tokoroa and mangakino form an adjacent gravity high and low respectively interpretations from seismic refraction arrivals and gravity modelling indicate the 65 mgal mangakino residual gravity anomaly can be modelled in part by two low density bodies that reach depths of 6 5 km whereas the tokoroa gravity anomaly is due to a higher density rock coming at most to within 650 m of the surface the mangakino anomaly is interpreted to be due to the remnants of magma chambers that fed large ignimbrite eruptions from about 1 2 ma an andesite volcano or complex volcanic structure is the preferred interpretation for the tokoroa gravity high the size of the putative volcanic structure is comparable to the presently active tongariro volcanic complex in the centre of north island

closure of the east paleotethyan ocean and amalgamation of the eastern cimmerian and southeast asia continental fragments abstract the phanerozoic record of southeast asia preserves the history of opening and subsequent consumption of the paleotethyan ocean and the assembly of continental fragments into asia as part of the broader scale reconstruction of pangea however uncertainty remains as to which of the many suture zones in southeast asia represents the relict of the main ocean when final ocean closure occurred and the assembly history of the eastern cimmerian and southeast asia continental fragments our overview of the geological features of the suture zones the bounding continental fragments and their magmatic metamorphic and sedimentary records resolves many of these key issues the sedimentary biogeographical structural lithological geochemical and geochronological data from the changning menglian inthanon and bentong raub suture zones argue for their linkage with the longmu co shuanghu suture zone in central tibet and together constitute the main east paleotethyan ocean relict the eastward subduction of the ocean resulted in the development of a series of magmatic arc back arc basin and continental fragments in southeast asia including from west to east the lincang sukhothai east malaya arc the jinghong nan sa kaeo back arc basin the simao west indochina fragment the luang prabang loei back arc basin the south indochina fragment the wusu and truong son back arc basins the north indochina fragment the jinshajiang ailaoshan song ma branch back arc basin and the south china block assembly of these fragments resulted in indosinian high temperature and high pressure metamorphism and related tectonothermal event available data indicate a switch from subduction of the main east paleotethyan ocean to the collision of the sibumasu with simao indochina blocks at 237 ma with subsequent syn and post collisional events at 237 230 ma and 230 200 ma respectively along the changning menglian inthanon and bentong raub suture zones the timing of initial syn and post collision events along the jinshajiang ailaoshan song ma suture zone with its record of back arc basin closure is at 247 ma 247 237 ma and 237 200 ma generally 10 ma older than that along the changning menglian inthanon and bentong raub suture zones our synthesis of all available data enables establishment of a comprehensive geodynamic model for the east paleotethyan evolution this model links the spatial temporal pattern across southeast asia into a series of tectonic events including ocean back arc basin opening subduction closure subsequent assemblage and orogenic collapse along with associated igneous metamorphic and sedimentary activities

the early triassic indosinian orogeny in vietnam truong son belt and kontum massif implications for the geodynamic evolution of indochina new structural field data at various scale and 40ar 39ar geochronological results from the basement rocks in the truong son belt and kontum massif of vietnam confirm that ductile deformation and high temperature metamorphism were caused by the early triassic event of the indosinian orogeny in the range of 250 240 ma a compilation of isotopic data obtained in other countries along the sibumasu indochina boundary broadly indicates same interval of ages this tectonothermal event is interpreted as the result of a synchronous oblique collision of indochina with both sibumasu and south china inducing dextral and sinistral shearing along e w to nw se and n s fault zones respectively the collision along song ma follows the northwards subduction of indochina beneath south china and the subsequent development of the song da zone which in turn was affected by the late triassic indosinian phase of shortening within the indochina plate internal collisions occurred coevally in the early triassic as along the poko suture at the western border of the kontum massif

asymmetric multiple block collapse at rotorua caldera taupo volcanic zone new zealand calderas worldwide have been classified according to their dominant collapse styles although there is a good deal of speculation about the processes involved recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials however many other factors also contribute to final caldera morphology rotorua caldera formed during the eruption of the mamaku ignimbrite collapse structure and evolution of rotorua caldera is interpreted based its geophysical response geology and geomorphology and the stratigraphy of the mamaku ignimbrite rotorua caldera is situated at the edge of the extensional taupo volcanic zone in which major faults strike ne sw a second less dominant fault set strikes nw se these two fault sets have a strong influence on the morphology of rotorua caldera no one style of collapse can be applied to rotorua caldera it was formed during a single eruption but subsided as many blocks and shows features of trapdoor piecemeal and downsag types of collapse here rotorua caldera is described according to its composition activity and geometry as a rhyolitic single event asymmetric multiple block single locus collapse structure the mamaku ignimbrite is the only ignimbrite to have erupted from rotorua caldera extracaldera thickness of the mamaku ignimbrite is up to 145 m whereas inside the caldera it may be greater than 1 km thick the mamaku ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence the main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower middle and upper parts lmi mmi umi respectively based on crystal content welding jointing devitrification and vapour phase alteration juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments only the most evolved pumice type occurs in the basal tephra sequence all three pumice types occur together throughout the main ignimbrite sequence whereas the andesitic fragments are only present in umi lithic lag breccias in umi indicate a late stage of caldera collapse concentration of lithic fragments increases towards the middle of the ignimbrite and may also reflect increased subsidence rate during an earlier stage collapse of rotorua caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the mamaku ignimbrite allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence

basalt triggering of the c ad 1305 kaharoa rhyolite eruption tarawera volcanic complex new zealand abstract the ad 1305 kaharoa eruption episode of tarawera volcanic complex was the largest 4 km3 magma eruption in new zealand in the last 1000 years high silica 76 sio2 rhyolite was erupted from seven vents along an 8 km linear zone to form pumiceous pyroclastic deposits and lava domes initial vent clearing explosions were followed by a series of plinian pumice eruptions that spread tephra downwind and pyroclastic density current deposits over the volcano slopes the early pyroclastic eruptions were followed by extrusion of a dome in the summit vent and the migration of further plinian activity to adjacent vents the episode finished with the extrusion of three summit lava domes accompanied by block and ash flows two main types of rhyolite were erupted 1 a low zr low sr high rb type comprising the early plinian pyroclastics and 2 a high zr high sr low rb type comprising the late eruptives including the summit domes basaltic material basalt and basaltic andesite is commonly present as free clasts within the kaharoa pyroclastic deposits along with rare clasts of granodiorite diorite gabbro and olivine clinopyroxenite the basaltic clasts are often veneered by kaharoa rhyolite basaltic material is also common as inclusions within rhyolite pumices along with olivine and augite xenocrysts derived from the basalt the basaltic clasts and inclusions contain complexly zoned plagioclase and corroded olivine commonly with a reaction rim consistent with disequilibrium caused by magma mixing some basaltic clasts and inclusions are hornblende free others contain groundmass hornblende plumes of brown glass often extend from hornblende bearing basaltic inclusions into the surrounding rhyolite these plumes demonstrate magma mixing as does the presence of rhyolitic glass and quartz xenocrysts in some basaltic inclusions most basaltic inclusions have crenulate boundaries suggesting they were fluidal when coming into contact with the rhyolite magma diorite gabbro and olivine clinopyroxenite clasts have mineralogy similar to the basalts commonly contain patches of fine grained basaltic material and are considered co magmatic with the basalts intrusion of basalt magma into the base of the rhyolite magma chamber appears to have occurred for some time before the kaharoa eruptions began allowing considerable mixing with the rhyolite magma transfer of water to some basalt inclusions and formation of hornblende in their groundmass conversely the hornblende free basalt inclusions did not have time to react with the rhyolite magma and must have been intruded shortly before the start of the eruption intrusion of this hornblende free basalt appears to have triggered the kaharoa eruption after the magma chamber had been primed by the earlier intrusions

fingerprints of the paleotethyan back arc basin in central hainan south china geochronological and geochemical constraints on the carboniferous metabasites hainan of southeast asia has been regarded as a key area for understanding the late paleozoic tectonic regime and amalgamation process of the indochina with south china blocks that are not well constrained this paper presents a set of new geochronological elemental and sr nd isotopic data for the paleozoic bangxi and chenxing metabasites in central hainan the geochronological data show that the representative samples yield the 40ar 39ar plateau age of 328 1 2 6 ma and zircon u pb age of 330 7 4 4 ma respectively they are sio2 and tio2 poor al2o3 rich mafic rocks the chenxing samples are characterized by left sloping chondrite normalized ree and n morb like multi elemental patterns the bangxi samples have the e morb like geochemical affinity all samples show high e nd t values ranging from 5 61 to 9 85 such signatures suggest their origination of a morb like source with the input of subduction derived components our investigation has verified the presence of the carboniferous metabasites with both morb and arc like geochemical affinities at the bangxi chenxing area in central hainan in combination with the available data from the jinshajiang ailaoshan and song ma suture zones it is proposed for the development of a carboniferous back arc basin along the ailaoshan song ma and central hainan suture zones in response to the subduction of the paleotethyan main ocean

magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics the volcanic rift zone in iceland is characterized by elongate volcanic systems consisting of tension fractures normal faults and volcanic fissures where most of the volcanotectonic activity takes place most volcanic systems develop central volcanoes the formation of which is still poorly understood there is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes in the central volcanoes eruptions are frequent typically one every several hundred years of small volume normally less than 0 1 km3 and fed mainly by thin average thickness of 0 5 m inclined sheets outside central volcanoes the eruptions are rare typically one every several thousand years but of large volume typically more than 1 km3 and fed mainly by thick average thickness of 4 5 m subvertical dikes using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi infinite plate these empirical relations as well as the formation of specific central volcanoes in the volcanic systems can be explained once a cavity like magma chamber has formed its existence in the rift zone concentrates tensile stress this stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system which partly explains the formation of central volcanoes and their high eruption frequencies this concentration also gives rise to a local stress field that encourages injection of small volume sheets in all directions and at various dips from their source magma chamber the magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma through inclined sheets toward a limited area at the surface where the central volcano gradually forms the inverse relationship between eruption frequency and eruption volume when the central volcano is compared with other parts of the volcanic system is also partly due to the trap like nature of the magma chamber the chamber is normally much smaller than its source mantle reservoir so that a single magma flow through a dike from the reservoir lasting perhaps many years may trigger tens of magma flows through sheets from the chamber many of which would reach the surface in the central volcano

geochemical ar 40 ar 39 geochronological and sr nd isotopic constraints on the origin of paleoproterozoic mafic dikes from the southern taihang mountains and implications for the ca 1800 ma event of the north china craton abstract the wide occurrence of paleoproterozoic mafic dikes in the basement of the trans north china orogen provides important constraints on the major 1800 ma tectonothermal event of the north china craton these mafic dikes form a tholeiitic series dominated by basalts and basaltic andesites and can be geochemically classified into three groups the groups 1 and 2 rocks are characterized by high total feo contents 12 69 15 61 of which the group 1 rocks are more enriched in lile and lree contents and depleted in hfse th nb n 2 5 3 8 la yb cn 3 0 5 8 nb la n 0 27 0 38 and have more radiogenic nd isotopic ratios nd t 3 40 to 5 14 than the group 2 rocks th nb n 0 8 1 2 la yb cn 1 5 2 1 nb la n 0 65 0 87 nd t 0 60 to 1 67 in contrast the group 3 rocks are typified by low feot 7 86 11 04 and high mgo 5 62 9 56 contents higher la yb cn 4 22 7 13 ratios less radiogenic nd isotopic ratios nd t 2 75 to 5 52 and more significant th u and nb ta depletion are also apparent features 40ar 39ar geochronology of three representative samples from each group yielded plateau ages of 1780 7 0 5 ma 1765 3 1 1 ma and 1774 7 0 7 ma respectively these elemental and isotopic data suggest that the geochemical variations of these rocks cannot be simply explained by crystallization from a common parental magma involving crustal contamination during emplacement instead they most likely originated from variable sources under different degrees of partial melting the group 1 rocks were derived from relatively low degrees of partial melting of a refractory lithospheric mantle previously metasomatised by subduction related fluids whereas the group 2 rocks originated from a hybridized source involving ca 40 subduction modified lithospheric mantle similar to the group 1 and ca 60 n morb component in contrast the group 3 rocks show significant th u depletion that may mark the involvement of portions of gabbroic lower crust trapped in the source region during subduction based on all the available data we propose a tectonic model of a full subduction syncollision postcollision rifting cycle for the evolution of the ncc between 1870 and 1765 ma a variety of source regions beneath the southern taihang mountains initially developed during the period of subduction collision and subsequent extension rifting resulted in the melting of the subduction modified lithospheric mantle which produced widespread paleoproterozoic mafic dikes on the trans north china orogen in response to upwelling of convective mantle

three dimensional distribution of seismic anisotropy in the hikurangi subduction zone beneath the central north island new zealand 1 we use earthquake arrival time data in the central north island new zealand in inversion for 3 d vp azimuthal anisotropy it is parameterized with an isotropic component and two azimuthal anisotropy parameters for each node and local earthquake shear wave splitting observations define the initial anisotropy model our results suggest that mantle flow either within the mantle wedge or below the subducted slab is not the primary source of anisotropy in the hikurangi subduction zone the largest region of anisotropy that we image is within the subducted slab where anisotropy is consistently trench parallel and 5 9 magnitude from 32 to 185 km depth bending induced yielding in the slab offshore and as it passes through the trench provides an explanation for this slab anisotropy beneath the taupo volcanic zone tvz there is 1 4 trench normal anisotropy within 30 km of the surface of the slab within a region entrained with motion of the slab in contrast only very weak 0 2 anisotropy is imaged in the region of high temperature partial melt arising from active corner flow in the mantle wedge in the crust geologic structure is important with the largest crustal anisotropy 14 related to schist we image significant anisotropy aligned with the margins of the tvz related to extensive fracturing on the margins as the tvz actively extends within the heavily intruded and underplated tvz lower crust the orientation of anisotropy switches to align with the extension direction this orientation is consistent with other geophysical data suggesting the presence of connected melt in this region

triassic high strain shear zones in hainan island south china and their implications on the amalgamation of the indochina and south china blocks kinematic and 40ar 39ar geochronological constraints abstract a kinematic and geochronological study has been carried out on the triassic high strain shear zones in hainan island the southern south china block there are wnw and ne trending high strain shear zones with greenschist to amphibolite facies metamorphism in this island kinematic indicators suggest a dextral top to the nne thrust shearing for the wnw trending high strain shear zones and a sinistral top to the se thrust shearing for the ne trending shear zones the quartz c axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system the timing of shearing for these shear zones has been constrained by the 40ar 39ar dating analyses of synkinematic minerals middle triassic 242 250 ma and late triassic early jurassic 190 230 ma have been identified for the wnw and ne trending shear zones respectively a synthesis of these kinematic and thermogeochronological data points to a two stage tectonic model for hainan island that is top to the nne oblique thrusting at 240 250 ma followed by top to the se oblique thrusting at 190 230 ma in combination with the available data from the southern south china and indochina blocks it is inferred that south hainan and north hainan have affinity to the indochina and south china blocks respectively the tectonic boundary between south hainan and north hainan lies roughly along the wnw trending changjiang qionghai tectonic zone probably linking to the song ma and ailaoshan zones the middle triassic structural pattern of hainan island is spatially and temporally compatible with those of the south china and indochina blocks and thus might be a derivation from the amalgamation of the indochina with south china blocks in response to the closure of the paleotethys ocean and subsequent subduction collision

p t t paths and post metamorphic exhumation of the day nui con voi shear zone in vietnam abstract the day nui con voi belt in vietnam is the southeasternmost part of the red river shear zone in asia it is a narrow high grade metamorphic core complex consisting of garnet sillimanite biotite gneisses mylonite bands amphibolite layers and migmatites geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite facies conditions of 690 60 30 c and 0 65 0 15 gpa and the subsequent mylonitization occurred under greenschist facies conditions of 480 c and under 0 3 gpa fifteen synkinematic hornblende and biotite separates from gneisses amphibolites and mylonites were dated with the k ar method hornblende separates from the day nui con voi give k ar ages of 26 4 28 5 ma and the biotite separates do give 24 5 24 7 ma combination of thermobarometric and geochronological data yields the cooling history of 500 c at 28 ma and 300 c at 24 ma with a cooling rate of 70 110 c ma 1 and 23 km post metamorphic exhumation of the core complex the first 16 km exhumation from the peak of metamorphism at probably 31 ma to 28 ma was triggered by the left lateral strike slip displacement of the red river shear zone

the reporoa caldera taupo volcanic zone source of the kaingaroa ignimbrites the reporoa caldera occupies the northern end of the reporoa depression previously described as a tectonic fault angle depression earlier confirmation of the topographic basin as a caldera had been hindered by the lack of an associated young pyroclastic flow deposit of large enough volume to have caused caldera collapse new exposures on the eastern margin of the reporoa basin reveal thick lithic lag breccias 30 m interbedded within the 0 24 ma kaingaroa ignimbrites these ignimbrites were previously attributed to the adjacent okataina volcanic centre lag breccia thicknesses and maximum clast sizes decrease rapidly outward from the caldera rim and discrete breccias are absent from ignimbrite sections more than 3 km from the rim the lithic lag breccias together with structural and geophysical evidence confirm reporoa caldera as the source of the c 100 km3 kaingaroa ignimbrites adding another major rhyolitic volcanic centre to the seven previously recognized in the taupo volcanic zone other older calderas may also be present in the reporoa depression

volcanic and structural evolution of the okataina volcanic centre dominantly silicic volcanism associated with the taupo rift new zealand article the okataina volcanic centre ovc contains the northeasternmost caldera complex in onshore taupo volcanic zone tvz new zealand sited largely within the taupo rift pyroclastic fall deposits and ignimbrites deposited in the bay of plenty coast area between 420 and 625 ka were probably erupted from ovc and these provide a maximum age for the centre the earliest ignimbrite for which there is good evidence for eruption from ovc is the c 550 ka quartz biotite ignimbrite exposed to the west of lake okataina and to se of ovc this ignimbrite probably correlates with one of the early ignimbrites found in the kawerau geothermal wells and is large enough to have been accompanied by caldera collapse it was followed by extensive rhyolitic explosive eruptions of the murupara subgroup culminating with eruption of the matahina ignimbrite at c 325 ka this c 160 km 3 magma volume eruption was accompanied by caldera collapse to form the southern part of the present day okataina caldera complex a long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of ovc before eruption of the 100 km

tectonics of south china key to understanding west pacific geology abstract south china is not a post caledonian platform but a composite of orogenic belts this late proterozoic mesozoic orogen is a collage of three continental fragments the three blocks are yangzi huanan and dongnanya and they are separated by the banxi nanpanjiang formerly xianggangzhe and gunanhai suture zones yangzi was separated from gondwanaland during late precambrian when an open ocean called banxi was present between the two continents tectonic processes at an active margin during sinian and early paleozoic time led to the genesis of an accretionary wedge complex the banxi melange and the huanan flysch nappes on the northern margin of huanan which was then the northern margin of the gondwana continent reorganization of plates during the devonian suspended subduction at the huanan active margin huanan was uplifted and unconformably overlain by transgressive deposits of devonian and or carboniferous age a remnant ocean the nanpanjiang sea still existed between yangzi and huanan and deep water sedimentation continued both at the southern margin of yangzi and at the northwestern margin of huanan the latter again became an active margin during late paleozoic when permian and triassic flysch sediments were deposited in foredeeps ahead of advancing nappes huanan and yangzi collided during the triassic and resulted in the deformation of the passive margin of the latter to form the yangzi deformed belt huanan was separated from a more southerly continent dongnanya in the devonian by seafloor spreading which created a late paleozoic and early mesozoic ocean gunanhai ocean a continuously deposited sequence ranging in age from devonian to triassic was laid down on the southern passive margin of huanan on the other side of the ocean the dongnanya permian strata include glacial marine deposits of gondwanaland affinity dongnanya became separated from gondwanaland during the late permian when it marched northward to be reunited with huanan the huanan passive margin sequence was deformed by folding and overthrusting after the late mesozoic collision of huanan and dongnanya scattered outcrops of the suture melange resulting from this collision occur at a few localities in coastal fujian dongnanya in our model was the mesozoic continent of southeast asia peripheral to mainland asia this may or may not have been the eastern continuation of the microcontinent sibumasu siam thailand burma malaysia sumatra the collision of dongnanya and asia gave rise not only to the gunanhai melange of coastal fujian but also to the ophiolite melanges parts of which are present in taiwan tailuko the philippines and western malaysia

first shrimp u pb zircon dating of granulites from the kontum massif vietnam and tectonothermal implications abstract the kontum massif in central vietnam represents the largest continuous exposure of crystalline basement of the indochina craton the central kontum massif is chiefly made of orthopyroxene granulites enderbite charnockite and associated rocks of the kannack complex mineral assemblages and geothermobarometric studies have shown that the kannack complex has severely metamorphosed under granulite facies corresponding to p t conditions of 800 850 c and 8 1 kbars twenty three shrimp ii u pb analyses of eighteen zircon grains separated from a granulite sample of the kannack complex yield ca 254 ma and one analysis gives ca 1400 ma concordant age for a zoned zircon core this result shows that granulites of the kannack complex in the kontum massif have formed from a high grade granulite facies tectonothermal event of indosinian age triassic the cooling history and subsequent exhumation of the kannack complex during indosinian times ranged from 850 c at ca 254 ma to 300 c at 242 ma with an average cooling rate of 45 c ma

a back arc basin formed within continental lithosphere the central volcanic region of new zealand abstract the central volcanic region is an area of quaternary volcanism and tensional tectonics within the north island of new zealand that appears to be a direct continuation of a young oceanic back arc basin the havre trough into the continental structure of new zealand volcanism is predominantly rhyolitic yet the region possesses many of the geophysical characteristics often associated with oceanic back arc basins upper mantle seismic velocities are low pn and sn velocities of 7 4 and 3 95 km s respectively the crust is anomalously thin for its continental setting and the heat flow almost all of which is expressed in the discharge of hot water through geothermal systems is about 700 mw m2 i e some twelve times greater than normal continental heat flow in order to account for such a high heat flow a large scale mass transfer of heat is required it is proposed therefore that the central volcanic region represents a site of active back arc spreading within a continental lithosphere

crust and upper mantle structure of a continental backarc central north island new zealand summary near vertical and wide angle seismic data provide evidence for a gradational crust mantle boundary in a depth range of 15 20 km beneath the central volcanic region cvr new zealand this volcanic area includes the taupo volcanic zone and is a direct extension of tonga kermadec oceanic backarc spreading into continental lithosphere long range seismic refraction data show velocities of 6 km s 1 and less within the top 15 km of the crust of the cvr at a depth of 15 km compressional seismic velocities increase to 6 8 km s 1 and then to 7 4 0 2 km s 1 at 20 km depth these 7 4 km s 1 seismic wave speeds are interpreted as anomalous upper mantle as beneath this level passive seismic studies show similar pn wave speeds that increase slowly to 7 8 km s 1 at about 80 km deep we interpret rocks between 15 and 20 km to be a layer of new crust formed by underplating the strongest reflection observed and what might also be interpreted as a reflection moho is from the top of the proposed underplated layer at 15 km depth at 20 km depth no such distinct reflection is observed rather wide angle reflection data show a continuum of low level reflectivity between 15 and at least 35 km depth indicating some heterogeneity and or structure within the lower crust and upper mantle thus the transition from lower crust to upper mantle is broad and a conventional reflection moho does not exist beneath the cvr buoyancy force calculations based on rock uplift for the central north island indicate that the subjacent mantle to a depth of 80 100 km is 70 kg m 3 or 2 per cent less dense than normal mantle best estimates attribute half of this density anomaly to the effects of increased temperature with additional contributions from partial melt 1 2 per cent and melt residuum

tectonic framework and phanerozoic evolution of sundaland abstract sundaland comprises a heterogeneous collage of continental blocks derived from the india australian margin of eastern gondwana and assembled by the closure of multiple tethyan and back arc ocean basins now represented by suture zones the continental core of sundaland comprises a western sibumasu block and an eastern indochina east malaya block with an island arc terrane the sukhothai island arc system comprising the linchang sukhothai and chanthaburi blocks sandwiched between this island arc formed on the margin of indochina east malaya and then separated by back arc spreading in the permian the jinghong nan uttaradit and sra kaeo sutures represent this closed back arc basin the palaeo tethys is represented to the west by the changning menglian chiang mai inthanon and bentong raub suture zones the west sumatra block and possibly the west burma block rifted and separated from gondwana along with indochina and east malaya in the devonian and were accreted to the sundaland core in the triassic west burma is now considered to be probably cathaysian in nature and similar to west sumatra from which it was separated by opening of the andaman sea basin south west borneo and or east java west sulawesi are now tentatively identified as the missing argoland which must have separated from nw australia in the jurassic and these were accreted to se sundaland in the cretaceous revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of sundaland and adjacent regions are presented

u pb dating and tectonic implication of ophiolite and metabasite from the song ma suture zone northern vietnam the song ma suture zone including the classical song ma suture and the nam co antiform between indochina and south china blocks in northern vietnam is a crucial area for understanding the tectonic evolution of southeast asia to explore their natures and evolutionary history this paper reports new petrological studies and sims zircon u pb age data for the song ma ophiolites metabasites and fine grained quartzite the song ma ophiolites consist mainly of peridotite fo90 91 en92 basalt and gabbro and experienced greenschist to lower amphibolite facies ocean floor metamorphism lenses of rodingite and altered pyroxenite in serpentinites formed during alternation of peridotite we identified two types of zircon in the altered pyroxenite i uniform metamorphic 70 vol and ii zircon having inherited magmatic cores and metamorphic rims 30 vol all u pb analyses of both zircon types yield an upper intercept age of 340 29 ma that we interpret as the protolith age type i zircon and type ii metamorphic rims together yield a mean 206pb 238u age of 280 2 ma with rodingite forming broadly coevally at 283 to 282 ma metabasalt plagioclase amphibole schist has a morb type geochemical affinity magmatic zircon from a plagioclase amphibole schist yields a protolith age of 315 4 ma high pressure eclogite garnet phengite quartz schist and garnet hornblende rock are distributed in the northwestern nam co antiform zircon crystals from both eclogite and garnet hornblende rock have low th u ratios of 0 01 to 0 07 and 0 02 to 0 06 and yield metamorphic ages of 231 8 ma and 228 3 ma respectively the ages of detrital zircon grains from fine grained banded quartzite from the nam co antiform range from paleozoic to proterozoic with a peak at 1 7 to 1 8 ga all available data indicate a three stage tectonic evolution 1 340 to 280 ma the paleotethys that separated the indochina and south china blocks was characterized by oceanic crust formation at 340 to 315 ma and ocean floor metamorphism at 283 to 280 ma 2

lithospheric structure of an active backarc basin the taupo volcanic zone new zealand seismic data from both explosive and earthquake sources have been used to model the crustal and upper mantle velocity structure beneath the taupo volcanic zone tvz an active backarc basin in central north island new zealand volcanic sediments with p wave velocities of 2 0 3 5 km s 1 reach a maximum thickness of 3 km beneath the central tvz underlying these sediments to 16 km depth is material with velocities of 5 0 6 5 km s 1 interpreted as quartzo feldspathic crust east and west of the tvz crust with similar velocities is found to depths of 30 and 25 km respectively beneath the tvz material with p wave velocities of 6 9 7 3 km s 1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust the base of the crust at 30 km depth under the tvz is marked by a strong seismic reflector interpreted as the moho modelling of arrivals from deep 40 km earthquakes near the top of the underlying subducting pacific plate reveals a region with low mantle velocities of 7 4 7 8 km s 1 beneath the crust of the tvz this region of low mantle velocities is best explained by the presence of partially hydrated upper mantle resulting from dehydration of hydrous minerals e g serpentinite carried down by the underlying subducting plate within the lower crust beneath the tvz a region of high 0 34 poisson s ratio is observed indicating the presence of at least 1 per cent partial melt this melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust where it provides a source for the voluminous rhyolitic magmas of the tvz

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

the central ailaoshan ophiolite and modern analogs abstract the central ailaoshan cal ophiolite represents an important tectonic component of the jinshajiang ailaoshan song ma suture zone separating the south china and indochina blocks in the mainland se asia the cal ophiolite occurs as a complex tectonic melange and preserves the history of the opening and closure of the once vast jinshajiang ailaoshan song ma branch of the paleotethys new and existing geological data indicate that the cal ophiolite contains magmatic rocks generated by 1 l devonian e carboniferous ca 380 330 ma volcanic passive margin breakup development in the nw gondwana margin 2 l permian ca 258 ma emeishan large igneous province related continental rift magmatism together with and intruded by 3 earliest m triassic ca 244 ma continent continent syn collisional s type granitoids the devono carboniferous suites of the cal ophiolite are highly comparable with many continental margin type alpine tethyan ophiolites in addition the various cal magmatic suites have strong south china block affinities with coeval magmatism particularly in the western south china block jinshajiang song ma and song da terranes

record of tethyan ocean closure and indosinian collision along the ailaoshan suture zone sw china abstract zircon u pb and lu hf isotopic data along with whole rock elemental and sr nd isotopic analytical results for the xin anzhai and tongtiange granitic plutons in the ailaoshan suture zone record the transition from subduction to collision associated with the accretion of indochina to yangtze blocks the xin anzhai monzogranite yields zircon u pb age of 251 6 2 0 ma and e hf t values of 6 2 9 8 the tongtiange leucogranite gives zircon u pb age of 247 5 2 2 ma and e hf t values ranging from 3 1 to 11 1 the tongtiange leucogranites have lower mgo na 2 o cao feot and tio 2 contents but higher a cnk values than those of the xin anzhai monzogranites the e nd t values for xin anzhai and tongtiange plutons are in the range of 8 5 to 8 8 and 10 6 to 11 4 respectively similar to those of the ailaoshan metamorphic basement the tongtiange leucogranites are the product of dehydration melting of mica rich metasedimentary rocks whereas the primary source of the xin anzhai monzogranites is probably proterozoic gneiss with an addition of 35 45 proterozoic amphibolite our geochronological results together with other published data indicate the presence of permo triassic magmatism associated with the indosinian orogeny along the ailaoshan suture zone this zone links with the jinshajiang suture toward the northwest and the song ma hainan suture to the southeast it is herein proposed that latest permian convergent margin magmatism represented by the xin anzhai granitoid pluton 252 ma terminated through the accretion of the simao indochina to the south china blocks which marks the commencement of the indosinian orogeny resulting in the generation of the 247 ma tongtiange s type leucogranite

a multiple approach radiometric age estimate for the rotoiti and earthquake flat eruptions new zealand with implications for the mis 4 3 boundary abstract pyroclastic fall deposits of the paired rotoiti and earthquake flat eruptions from the taupo volcanic zone new zealand combine to form a widespread isochronous horizon over much of northern new zealand and the southwest pacific this horizon is important for correlating climatic and environmental changes during the last glacial period but has been the subject of numerous disparate age estimates between 35 1 2 8 and 71 6 ka all errors are 1 s d obtained by a variety of techniques a potassium argon k ar age of 64 4 ka was previously determined on bracketing lavas at mayor island volcano offshore from the taupo volcanic zone we present a new more precise 40ar 39ar age determination on a lava flow on mayor island that shortly post dates the rotoiti earthquake flat fall deposits of 58 5 1 1 ka this value coupled with existing ages from underlying lavas yield a new estimate for the age of the combined eruptions of 61 0 1 4 ka which is consistent with u th disequilibrium model age data for zircons from the rotoiti deposits direct 40ar 39ar age determinations of plagioclase and biotite from the rotoiti and earthquake flat eruption products yield variable values between 49 6 2 8 and 125 3 10 0 ka with the scatter attributed to low radiogenic ar yields and or alteration and or inheritance of xenocrystic material with inherited ar rotoiti earthquake flat fall deposits occur in new zealand in association with palynological indicators of mild climate attributed to marine isotope stage mis 3 and thus used to suggest an age that is post 59 ka the natures of the criteria used to define the mis 4 3 boundary in the northern and southern hemispheres however imply that the new 61 ka age for the rotoiti earthquake flat eruption deposits will provide the inverse namely a more accurate isochronous marker for correlating diverse changes across the mis 4 3 boundary in the southwest pacific