influence of offset weak zones on the development of rift basins activation and abandonment during continental extension and breakup 1 we use numerical modelling to investigate reactivation of inherited offset weak zones owz in continental crust and mantle weak zones mwz to form offset rift basins during continental rifting and breakup offset rift basins are basins that are set off offset from the main rift locus of breakup weak zones embedded in a stiff layer are preferentially and rapidly reactivated whereas the same zones are either ignored or slowly reactivated when embedded in pliable layers here stiff implies a nonlinear flow law with a high stress exponent n 10 000 a plastic material and pliable means a low stress exponent n 2 5 as in ductile power law creep of rocks whether offset rift basins form during rifting of a composite lithosphere i e comprising stiff and pliable layers depends on the competition between necking instabilities that develop at the weak zones in the stiff layers and the coupling between the stiff and pliable layers stiff cratonic lithosphere results in early localization of the deformation at the mwz rapid necking and breakup without developing offset rift basins in contrast warm pliable lithosphere develops significant offset basins and has protracted rifting because the mwz is now embedded in a pliable layer we also investigate the influence of owz dip sedimentation and the sensitivity of reactivation to the distance from owz to the mwz and to the size of the mwz a tectonic rifting styles diagram is used to show that the model results agree with natural examples

geochemistry and geodynamic origin of the mesoarchean ujarassuit and ivisaartoq greenstone belts sw greenland abstract the ivisaartoq ca 3075 ma and ujarassuit ca 3070 ma greenstone belts are the largest mesoarchean supracrustal lithotectonic assemblages in the nuuk region sw greenland both greenstone belts underwent polyphase deformation and amphibolite facies metamorphism and were in due course variously dismembered pillow lavas pillow breccia magmatic layering and relic sedimentary structures are well preserved in the ivisaartoq belt volcanic rocks include basalts and minor andesites and picrites boninite like rocks occur in the ujarassuit belt metasedimentary rocks in the ujarassuit belt occur as thin layers 0 5 1 0 m of biotite schists and in the ivisaartoq belt as 500 m thick biotite schists intercalated with minor quartzitic gneisses there is no field evidence indicating that the ivisaartoq and ujarassuit supracrustal rocks were deposited on older continental basement and their volcanic rocks do not exhibit any geochemical trends indicating contamination by archean upper continental crust four groups of amphibolites metavolcanic rocks were recognized in the ujarassuit greenstone belt on the basis of their ree and hfse characteristics 1 group 1 is characterized by near flat ree patterns la smcn 0 77 1 14 la ybcn 0 84 1 24 and moderate negative nb anomalies nb nb 0 60 0 79 2 group 2 displays lree depleted patterns la smcn 0 53 1 02 la ybcn 0 32 0 61 and pronounced negative nb anomalies nb nb 0 32 0 67 3 group 3 consists of lree depleted patterns la smcn 0 69 0 84 la ybcn 0 55 0 91 and absence of significant nb anomalies nb nb 0 92 1 15 and 4 group 4 has concave upward ree patterns la smcn 1 64 2 42 gd ybcn 0 57 1 01 and large negative nb anomalies nb nb 0 28 0 42 the depletion of lree in group 2 amphibolites appears to have resulted from mobility of these elements during metamorphism group 1 3 and 4 amphibolites retain their near primary geochemical signatures and their trace element patterns are comparable to those of phanerozoic oceanic island arc tholeiites iat normal mid ocean ridge basalts n morb and boninites respectively the trace element patterns of the least altered meta ultramafic rocks la ybcn 2 48 1 35 nb nb 0 31 0 60 are comparable to those of modern subduction related picrites amphibolites with an andesitic composition show large negative nb anomalies nb nb 0 21 0 55 enriched lree patterns la ybcn 6 29 15 64 and fractionated hree gd ybcn 2 61 3 12 implying deep melting in equilibrium with residual garnet in the source biotite schists and quartzitic gneisses have low chemical indexes of alteration values cia 46 to 62 and trace element characteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to mafic source rocks collectively the geochemical features of metavolcanic and metasedimentary rocks suggest that the ivisaartoq and ujarassuit greenstone belts represent dismembered fragments of mesoarchean supra subduction zone oceanic crust formed in an arc forearc backarc tectonic setting

arc generated metavolcanic rocks in the anshan benxi greenstone belt north china craton constraints from geochemistry and zircon u pb hf isotopic systematics abstract the anshan benxi granite greenstone belt abgb in the eastern block of the north china craton mainly comprises supracrustal rocks of amphibole plagioclase gneisses plagioclase amphibolites chlorite schists biotite schists quartzites and banded iron formations bifs and intrusive monzogranitic to syenogranitic gneisses based on geology spatial distributions lithologies and zircon u pb geochronology the metavolcanic rocks of the southern abgb can be divided into the 2527 ma southern plagioclase gneiss group 2536 ma southern amphibolite group 2574 to 2560 ma northern chlorite schist group and northern amphibolite group from south to north these rocks outcrop as stratified and stratoid beds whereas some occur as enclaves within later intrusive k rich granitic gneisses the southern plagioclase gneiss group exhibits calc alkaline andesite compositions with high mgo cr and ni fractionated ree patterns and depletion in nb ta which resemble high magnesian andesite hma and are derived from the partial melting of sub arc mantle peridotite that was previously metasomatized by slab melts the southern amphibolite group has tholeiitic basalt compositions with slight enrichment in lrees and high nb ti concentrations but low nb la pm ratios which coincide with nb enriched basalt and are generated by the remelting of residues of sub arc mantle after the extraction of hma melts the northern schist group shows high k calc alkaline andesite dacite compositions and have highest silica and potassium and obvious enrichment in lrees and liles and depletion in nb ta which are comparable to the sanukitoid series rocks these rocks probably formed by low degree partial melting of a sub arc mantle wedge that was metasomatized by slab derived fluids the northern amphibolite group exhibits tholeiite compositions with unfractionated ree patterns and low nb la pm ratios which are consistent with back arc basin basalts babb and are produced by partial melting of depleted mantle combined with lithological associations of the hma neb sanukitoid like rocks and babb like rocks the anshan benxi granite greenstone belt was produced under a convergent plate environment both the zircon ehf values and petrogenesis indicate that the four groups of the metamorphic volcanic rocks directly contributed to continental crustal growth integrating with the widespread 2 5 ga syenogranites and subordinate k rich granitoids that were derived from reworking of ancient crust we propose that the abgb experienced a significant episode of continental growth dominated by a subduction related lateral accretion accompanied with contemporaneously strong reworking of ancient crustal materials

late holocene hydrous mafic magmatism at the paint pot crater and callahan flows medicine lake volcano n california and the influence of h2o in the generation of silicic magmas this paper characterizes late holocene basalts and basaltic andesites at medicine lake volcano that contain high pre eruptive h2o contents inherited from a subduction related hydrous component in the mantle the basaltic andesite of paint pot crater and the compositionally zoned basaltic to andesitic lavas of the callahan flow erupted approximately 1000 14c years before present 14c years b p petrologic geochemical and isotopic evidence indicates that this late holocene mafic magmatism was characterized by h2o contents of 3 to 6 wt h2o and elevated abundances of large ion lithophile elements lile these hydrous mafic inputs contrast with the preceding episodes of mafic magmatism from 10 600 to 3000 14c years b p that was characterized by the eruption of primitive high alumina olivine tholeiite haot with low h2o 0 2 wt lower lile abundance and different isotopic characteristics thus the mantle derived inputs into the medicine lake system have not always been low h2o primitive haot but have alternated between haot and hydrous subduction related calc alkaline basalt this influx of hydrous mafic magma coincides temporally and spatially with rhyolite eruption at glass mountain and little glass mountain the rhyolites contain quenched magmatic inclusions similar in character to the mafic lavas at callahan and paint pot crater the influence of h2o on fractional crystallization of hydrous mafic magma and melting of pre existing granite crust beneath the volcano combined to produce the rhyolite fractionation under hydrous conditions at upper crustal pressures leads to the early crystallization of fe mg silicates and the suppression of plagioclase as an early crystallizing phase in addition h2o lowers the saturation temperature of fe and mg silicates and brings the temperature of oxide crystallization closer to the liquidus these combined effects generate sio2 enrichment that leads to rhyodacitic differentiated lavas in contrast low h2o haot magmas at medicine lake differentiate to iron rich basaltic liquids when these fe enriched basalts mix with melted granitic crust the result is an andesitic magma since mid holocene time mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at medicine lake volcano however during the late holocene h2o poor mafic magmas continued to be erupted along with hydrous mafic magmas although in significantly smaller volumes

from rifting to active spreading in the lau basin havre trough backarc system sw pacific locking unlocking induced by seamount chain subduction associated with pacific australia plate convergence the lau basin havre trough is an active back arc basin that has been opened since 5 5 ma by rifting and southward propagating oceanic spreading current back arc opening rates decrease from 159 mm yr 1 in the northern lau basin to 15 mm y 1 in the southern havre trough major tectonic changes occur at the transition between havre trough rifting and full oceanic spreading of the eastern lau spreading center elsc where the oblique to trench westward subducting louisville seamount chain lsc sweeps southwards along the tonga trench new swath bathymetry seismic reflection data and limited rock sampling in this area constrain a tectonic and kinematic back arc model that incorporates the effects of lsc subduction the elsc which extends southward to 24 55 s forms a deep rift valley propagating southward through older rifted arc basement present day seismicity and fresh and fractured pillow lavas at 23 42 s are consistent with rift valley neovolcanism conversely the northern havre trough has low seismicity and rifted volcanic basement ridges trending 25 45 oblique to the basin axis consistent with low levels of extensional tectonism and volcanism this latter structural fabric is interpreted as an early stage of rifting that is now locked due to compression on the arc exerted by lsc subduction while in the lau basin such effects have passed as the lsc swept along the tonga trench it is proposed that the lau havre back arc opening is controlled by tectonic constraints exerted at the limits of the system by the lsc subduction which determines the southward migration of the tonga arc pole of rotation and associated lau basin opening a discrete three stage back arc opening evolution is proposed comprising 1 an initial phase of back arc rifting along the whole length of the plate boundary beginning at 6 5 ma 2 a subsequent phase mostly present in the southern part of the back arc domain and still active in the havre trough of transpression and transtension starting at 4 ma in the north as the lsc starts to subduct and sweeps southward along the tonga trench and 3 a renewed opening phase in the northern segment of the back arc domain with rifting and spreading starting at 3 5 ma as subduction of the lsc along the northern tonga trench is progressively completed

crustal structure and evolution of the mariana intra oceanic island arc a new high resolution velocity model of the mariana arc backarc system obtained from active source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions the characteristics of the oceanic island arc crust are a middle crust with velocity of 6 km s laterally heterogeneous lower crust with velocities of 7 km s and unusually low mantle velocities petrologic modeling suggests that the volume of the lower crust composed of restites and olivine cumulates after the extraction of the middle crust should be significantly larger than is observed suggesting that a part of the lower crust especially the cumulates is seismically a part of the mantle

transformation of juvenile izu bonin mariana oceanic arc into mature continental crust an example from the neogene izu collision zone granitoid plutons central japan abstract granitic rocks sensu lato are major constituents of upper continental crust recent reviews reveal that the average composition of phanerozoic upper continental crust is granodioritic although oceanic arcs are regarded as a site producing continental crust material in an oceanic setting intermediate to felsic igneous rocks occurring in modern oceanic arcs are dominantly tonalitic to trondhjemitic in composition and have lower incompatible element contents than the average upper continental crust therefore juvenile oceanic arcs require additional processes in order to get transformed into mature continental crust enriched in incompatible elements neogene granitoid plutons are widely exposed in the izu collision zone in central japan where the northern end of the izu bonin mariana ibm arc juvenile oceanic arc has been colliding with the honshu arc mature island arc since middle miocene the plutons in this area are composed of various types of granitoids ranging from tonalite to trondhjemite granodiorite monzogranite and granite three main granitoid plutons are distributed in this area tanzawa plutonic complex kofu granitic complex and kaikomagatake granitoid pluton tanzawa plutonic complex is dominantly composed of tonalite and trondhjemite and characterized by low concentration of incompatible elements and shows geochemical similarity with modern juvenile oceanic arcs in contrast kofu granitic complex and kaikomagatake granitoid pluton consists mainly of granodiorite monzogranite and granite and their incompatible element abundances are comparable to the average upper continental crust previous petrogenetic studies on these plutons suggested that 1 the tanzawa plutonic complex formed by lower crustal anatexis of juvenile basaltic rocks occurring in the ibm arc 2 the kofu granitic complex formed by anatexis of hybrid lower crust comprising of both basaltic rocks of the ibm arc and metasedimentary rocks of the honshu arc and 3 the kaikomagatake granitoid pluton formed by anatexis of hybrid lower crust consisting of k rich rear arc crust of the ibm arc and metasedimentary rocks of the honshu arc these studies collectively suggest that the chemical diversity within the izu collision zone granitoid plutons reflects the chemical variation of basaltic sources i e across arc chemical variation in the ibm arc as well as variable contribution of the metasedimentary component in the source region the petrogenetic models of the izu collision zone granitoid plutons suggest that collision with another mature arc continent hybrid lower crust formation and subsequent hybrid source anatexis are required for juvenile oceanic arcs to produce granitoid magmas with enriched compositions the izu collision zone granitoid plutons provide an exceptional example of the collision induced transformation from a juvenile oceanic arc to the mature continental crust

crustal xenoliths from calbuco volcano andean southern volcanic zone implications for crustal composition and magma crust interaction crustal xenoliths in the 1961 andesite flow of calbuco volcano in the southern southern volcanic zone ssvz of the andes consist predominantly of pyroxene granulites and hornblende gabbronorites the granulites contain plagioclase pyroxene magnetite amphibole and have pronounced granoblastic textures small amounts of relict amphibole surrounded by pyroxene plagioclase magnetite glass symplectites are found in some specimens these and similar textures in the gabbronorites are interpreted as evidence of dehydration melting mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism a moderate pressure high temperature stage accompanied by melting and melt extraction from some samples followed by thermal metamorphism after entrainment in the calbuco andesite lavas high nd t values 4 0 to 8 6 nd isotope model ages of 1 7 2 0 ga and trace element characteristics like chondrite normalized la yb and la nb 1 indicate that the protoliths were oceanic basalts similar oceanic metabasalts of greenschist to amphibolite facies are found in the paleozoic metamorphic belt that underlies the chilean coastal ranges mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate crystallizing from the basaltic andesite magma at calbuco crystallization pressures for the gabbros based on total al contents in amphibole are 6 8 kbar these pressures point to middle to lower crustal storage of the calbuco magma neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of calbuco andesites although an ancient sedimentary contaminant is indicated by the lava compositions the presence of oceanic metabasaltic xenoliths together with the sedimentary isotopic imprint suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt which is an accretionary complex of intercalated basalts and sediments that formed along the paleozoic gondwanan margin if this is the case the geochemical composition of the lower and middle crust beneath the ssvz is significantly different from that of most recent ssvz volcanic rocks

early carboniferous subduction zone metamorphism preserved within the palaeo tethyan rasht ophiolites western alborz iran a polyphase history of oceanic construction and consumption is documented by the distribution of the tethyan ophiolitic suture zones in iran despite the geodynamic significance of these suture zones few modern petrological and geochronological data are available from these ophiolitic domains hampering a full assessment of the timing thermobaric regimes and palaeotectonic scenarios leading to oceanic suturing and continental assembly in iran and more in general along the southern margin of eurasia in this paper we describe a newly discovered hp ophiolite melange within the rasht ophiolites at the nw termination of the palaeo tethyan suture in iran petrological investigations on the metamorphic units embedded within the ophiolite melange are integrated with whole rock geochemistry and ar ar phengite geochronology to constrain the geotectonic setting of formation and to define the associated p t t paths we provide evidence that the rasht ophiolite domain corresponds to an exhumed subduction complex formed during early carboniferous c 350 ma subduction of a branch of the palaeo tethyan oceanic realm with peak metamorphism equilibrated under blueschist to eclogite facies metamorphic conditions implications of these data for the palaeotectonic reconstruction during closure of the palaeo tethyan ocean and the terminal collisional assembly along the southern margin of the eurasia plate are also discussed supplementary material analytical methods and protocols are available at https doi org 10 6084 m9 figshare c 3706852

dust production and deposition in asia and the north pacific ocean over the past 12 myr the silicate fractions of recent pelagic sediments in the central north pacific ocean are dominated by eolian dust derived from central asia an 11 myr sedimentary record at odp sites 885 886 at 44 7 n 168 3 w allows the evaluation of how such dust and its sources have changed in response to late cenozoic climate and tectonics the extracted eolian fraction contains variable amounts 70 of clay minerals with subordinate quartz and plagioclase uniform nd isotopic compositions nd 8 6 to 10 5 and sm nd ratios 0 170 0 192 for most of the 11 myr record demonstrate a well mixed provenance in the basins north of the tibetan plateau and the gobi desert that was a source of dust long before the oldest preserved asian loess formed nd values of up to 6 5 for samples 2 9 ma indicate 35 wt admixture of a young kamchatka like volcanic arc component the coherence of pb and nd in the erosional cycle allows us to constrain the pb isotopic composition of asian loess devoid of anthropogenic contamination to 206pb 204pb 18 97 0 06 207pb 204pb 15 67 0 02 208pb 204pb 39 19 0 11 87sr 86sr 0 711 0 721 and rb sr ratios 0 39 1 1 vary with dust mineralogy and provide an age indication of 250 ma 40ar 39ar ages of six dust samples are uniform around 200 ma and match the k ar ages of modern dust deposited on hawaii these data reflect the weighted age average of illite formation changes from illite smectite with significant kaolinite to illite and chlorite rich kaolinite free assemblages since the late pliocene document changes in the intensity of chemical weathering in the source region such weathering evidently did not disturb the k ar systematics and only induced scatter in the rb sr data we propose that when smectite forms at the expense of illite k and ar are quantitatively lost from what becomes smectite but are quantitatively retained in adjacent illite layers 40ar 39ar age data therefore are insensitive to smectite formation during chemical weathering but date the diagenetic growth of illite the major k bearing phase in the dust over the past 12 myr the dust flux to the north pacific increased by more than an order of magnitude documenting a substantial drying of central asia this climatic change however did not alter the ultimate source of the dust and neoformational products of chemical weathering always remained subordinate to assemblages reworked by mechanical erosion in dust deposited in eastern asia and the pacific ocean

faulting and extension rate over the last 20 000 years in the offshore whakatane graben new zealand continental shelf oblique rifting in the offshore taupo volcanic zone new zealand is expressed in widely distributed active normal faulting in the 20 km wide whakatane graben active faults are identified along seafloor scarps and displacements of the post last glacial transgressive ravinement surface 400 faults we derive an extension rate at seismogenic depths 6 10 km across the graben of 13 6 mm yr by summing surface measurements assuming an average crustal fault dip of 45 15o and correcting for the discrepancies between surface and deep crustal extension estimates structural and kinematic data implies an extension direction 20o oblique to the rift axis resulting in up to 4 6 2 1 mm yr of dextral motion parallel to the rift axis the strike slip motion is accommodated by dip slip displacements on oblique faults in the centre of the graben and oblique slip faulting along the rift margins pure dip slip in the graben centre represents 50 of the total slip with the rangitaiki fault accommodating 25 of the total extension in the graben

the age of lathrop wells volcanic center an 40ar 39ar dating investigation the sup 40 ar sup 39 ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the lathrop wells volcanic center nevada yield eruption ages of 77 3 plus minus 6 0 and 76 6 plus minus 4 9 ka 2 sigma for the ql sub 1 and ql sub 2 flows respectively basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the ql sub 2 the sanidines from the ql sub 2 xenoliths yielded apparent plateau ages n 10 ranging from 69 plus minus 17 to 309 plus minus 11 ka with a strong cluster n 6 at approximately 77 ka because the diffusion dimension of sanidine is approximated by the physical crystal size cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and hf acid etching lead to anomalously old apparent ages these new ages for the ql sub 1 and ql sub 2 flows are analytically indistinguishable and agree well with published eruption ages determined by sup 3 he and sup 36 cl cosmogenic surface exposure dating methods we conclude that the ql submore 1 and ql sub 2 flows are temporally equivalent additionally the agreement between our eruption ages for these units and the published 76 plus minus 20 ka ital zreda ital et al 1993 sup 36 cl exposure age for the lathrop wells scoria cone qs sub 3 suggests that the cone may be temporally equivalent to the ql sub 1 and ql sub 2 flows we believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic copyright 1999 american geophysical union less

the interpretation of inverse isochron diagrams in 40 ar 39 ar geochronology abstract the concepts involved in the construction and interpretation of inverse isochron diagrams used in 40 ar 39 ar geochronology are reviewed the diagrams can be useful as a means of recognising atmospheric argon and excess 40 ar incorporated in the mineral lattice which cannot be recognised from 40 ar 39 ar spectra the age established using an inverse isochron plot unlike that yielded by a spectrum is not affected by trapped argon 40 ar 36 ar ratios that are different from the atmospheric argon ratio e g due to excess 40 ar and may contribute to a better age interpretation however a heterogeneous distribution of excess 40 ar or heterogeneous argon loss can cause false isochrons with axial intercepts indicating an incorrect age and an incorrect trapped argon composition inconsistency between the ages from a spectrum and from the associated inverse isochron plot may indicate the degree of inaccuracy of isochrons however it is possible that both the spectrum and inverse isochron yield the same incorrect age the importance of considering all possible interpretations before assigning an age to a specimen is stressed

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

petrogenesis of volcanic rocks from saipan and rota mariana islands and implications for the evolution of nascent island arcs an 40 ar 39 ar age of 45 1 ma determined for lavas from northern saipan confirms that these high silica rhyolites erupted during the proto arc stage of volcanism in the izu bonin mariana system which is characterized elsewhere by eruption of boninitic lavas incompatible trace element concentrations and sr hf nd and pb isotope ratios for these rhyolites are transitional between those of c 48 ma boninitic lavas and post 38 ma first arc andesites and dacites from saipan and rota that have typical subduction related compositions these transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust a second stage of rayleigh fractionation in the upper crust is required by sio 2 concentrations that exceed 77 wt and near zero compatible element concentrations first arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first arc magmas themselves the mantle sources of the proto arc and first arc lavas from saipan and rota are similar to those of philippine back arc basin basalts based on nd and hf isotopic compositions the pb isotope compositions of these lavas are between those of pacific sea floor basalts and jurassic and younger cherty and clay rich sediments this contrasts with the boninitic proto arc volcanic rocks from guam and deep sea drilling project sites 458 and 459 that have pb isotope compositions similar to pacific basin basalts and volcaniclastic sediments the preferred explanation for the difference in the nature of proto arc volcanism between saipan and other fore arc locations is that the crust ceased extending 3 4 myr earlier beneath saipan this was caused by a change from mantle upwelling fore arc extension and shallow melting to an environment dominated by more normal mantle wedge convection stable crust and deeper melting

continental crust crustal underplating and low q upper mantle beneath an oceanic island arc a detailed structural model of the crust subducting slab and underlying upper mantle across the northern izu ogasawara bonin island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion the model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid crust a highly attenuative mantle that is one with low quality q seems to be confined mainly beneath the presently active rift zone in contrast high p wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin suggesting a large scale magma input responsible for the arc formation

laser ablation inductively coupled plasma mass spectrometry and tephras a new approach to understanding arc magma genesis the development of laser ablation inductively coupled plasma mass spectrometry has revolutionized the analysis of tephras by providing 1 an efficient and precise method for determining abundances of a wide variety of trace elements at low concentrations in individual glass shards and 2 assessment of geochemical heterogeneities within individual ash horizons this development is important for petrogenetic studies of intraoceanic arc systems where tephras provide the most complete temporal record of magmatism results from the izu bonin and mariana arc systems indicate that despite close geographical proximity and similar tectonic evolution they contrast strongly in terms of geochemical evolution since 35 ma whereas the mariana tephras have exceptional compositional diversity ranging from low k oligocene to high k miocene and subsequently medium k compositions pliocene quaternary the izu bonin arc has been dominated by low k compositions throughout the mariana increases in k are paralleled by increases in abundances of incompatible trace elements and by increased values of diagnostic ratios e g nb yb and th yb regarded as monitors of potential mantle source fertility the relative uniformity of nb yb and nb zr ratios in izu bonin tephras indicates that cyclic processes of backarc basin development and mantle depletion do not necessarily induce large scale temporal geochemical variations in the associated arc temporal variability within the mariana arc and its divergence from the izu bonin arc ca 13 ma can be traced to a major injection of subducted sediment in the mariana system at this time

reconstructing the geological and structural history of an active geothermal field a case study from new zealand abstract the utilisation of geothermal systems benefits from an understanding of the host rock geology locations and controls of permeability pathways and the nature and timing of magmatic sources providing thermal energy kawerau geothermal field in the central taupo volcanic zone tvz of new zealand is currently developed for electricity generation and direct uses of high temperature steam to 200 mw electrical output the kawerau geothermal system is hosted in a sequence of volcanic lithologies tuffs lavas and intrusive bodies and sediments that overlie faulted mesozoic metasedimentary greywacke basement identification of lithologies in the volcanic sedimentary sequence is challenging due to the levels of hydrothermal alteration and lithological similarities a combination of detailed petrological investigations consideration of the emplacement processes and greater certainty of crystallisation or eruption ages through u pb age determinations on zircons is used to reconstruct the depositional and faulting evolution of the rocks hosting the currently active hydrothermal system the oldest event inferred is faulting of the greywacke along northwest southeast orientated dominantly strike slip structures to generate half grabens that were filled with sediments incorporating two dated ignimbrites 2 38 0 05 and 2 17 0 05 ma a 1 46 0 01 ma ignimbrite was deposited relatively evenly across the field implying that any topographic relief was subdued at that time subsequent deposition of ignimbrites occurred in episodes around 1 0 0 55 0 6 and 0 32 ma interspersed with thin sedimentary sequences that accumulated at average rates of 0 06 mm yr 1 andesite lavas from a buried composite cone occur as a conformable package between units dated at 1 0 and 0 6 ma bodies of coherent rhyolite occur at multiple stratigraphic levels two magma types with associated tuffs were emplaced as domes and sills at 0 36 0 03 ma and a third type at 0 138 0 007 ma as dikes and domes that are exposed at surface the andesitic putauaki composite cone southwest of the field first erupted around 8 ka but earlier hydrothermal eruption breccias imply that magma was intruded to shallow depths as early as 16 ka age data and associated correlations show that post 1 5 ma normal faulting has accompanied episodic subsidence of the kawerau area with fault movement focused between northeast southwest structures associated with the geometry of the modern tvz and the reactivated northwest southeast structures associated with most displacement in the area prior to 1 5 ma contrasts between emplacement of coherent rhyolite as sills at 0 36 ma and a dike at 0 138 ma reflect a shift in orientation of the principal stress axes in response to initiation of the modern tvz rifting regime most volcanic rocks at kawerau are distally sourced from elsewhere in the tvz but form local marker horizons that delineate topographic relief within the field and additionally constrain past subsidence rates current rates of subsidence and thermal output at kawerau are geologically recent features associated with latest quaternary rifting processes

origin of enclaves in s type granites of the lachlan fold belt abstract the more mafic s type granites of the lachlan fold belt contain a distinctive assemblage of lithic inclusions of deep crustal origin two types predominate the schistose enclaves and the microgranular enclaves there is also a small proportion of fragments of local country rock all the features of the schistose enclaves are consistent with an origin as lithic restite fragments from the source their ubiquity in the mafic s type granites and absence from both felsic s type granites and i type granites conform with such an origin the argument that these enclaves are not in chemical equilibrium with the host granite because they do not complement its composition is not valid since they represent less fertile parts of the source that had different compositions that as a result melted to a lesser extent models that derived these enclaves from regions of the crust above the zones of partial melting so that they are not restites are complex unnecessary do not conform with their ubiquity in the more mafic s type granites and are not consistent with their chemical compositions the origin of the microgranular enclaves is controversial in most s type granites these enclaves have been recrystallized to a similar mineral assemblage to the host granite with a higher proportion of biotite in a few places such material is seen to be forming by recrystallization of material in the core of the enclave which has the assemblage quartz calcic plagioclase orthopyroxene cordierite biotite with accessory ilmenite sulfide and apatite all of the microgranular enclave cores show pseudo doleritic texture in which calcic plagioclase crystals an60 and orthopyroxene crystals project into or are enclosed by quartz large plagioclase crystals are zoned with some core compositions as calcic as an94 the low na contents of the microgranular enclaves are not consistent with an igneous origin we consider that these enclaves were derived by metamorphism of calcareous mudstones or argillaceous limestones the pseudo igneous textures result from the presence of a partial melt during metamorphism an analysis of a calc silicate lens of upper amphibolite grade from the wilsons group of victoria land antarctica is remarkably similar in composition to the microgranular enclaves from lachlan fold belt s type granites we suggest that rocks equivalent to that group which is of appropriate age and general composition were the source rocks for the s type granites of the lachlan fold belt

partial melting at the earth s surface implications for assimilation rates and mechanisms in subvolcanic intrusions abstract the relatively recent routine availability of isotopic and elemental analytical results from large sample suites of igneous rocks has revolutionized the study of magmatic processes large uncertainties remain however with regard to the highly variable rates of magma differentiation and ascent through the crust although the repose interval prior to an eruption may be equivalent to magma residence time at a certain level in the crust this cannot be assumed a priori even where this happens to be the case uncertainties in determining such intervals for prehistoric eruptive sequences may be large with respect to process rates consequently independent insights from geologically well constrained situations must be sought this paper first presents documentation of an example wherein fragments of dacite incorporated by a mafic lava flow on the surface were melted to the point that they viscously deformed in response to stresses in this flow the transformation from the solid state to a largely molten state is inferred to have occurred within a few weeks these results are then discussed in the context of examples from the literature that support rapid melting and digestion of xenoliths in magma it is suggested that the rate of crustal assimilation may be sufficiently rapid that it can operate within the durations of extended eruptive episodes years decades which are generally much shorter than typical repose periods at arc volcanoes decades millennia

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

construction of an island arc and back arc basin system in eastern central asian orogenic belt insights from contrasting late carboniferous intermediate intrusions in central inner mongolia north china abstract characterizing island arc and back arc basin system through detailed magmatic profile remains a pivotal task in any regional paleotectonic and supercontinental reconstructions along ancient convergent plate margins as is of paramount importance for the northern china southern mongolia ncsm tract along the eastern central asian orogenic belt caob here we document episodic late carboniferous intermediate to felsic intrusive suites in the west ujimqin region of central inner mongolia spanning from ca 315 ma granodiorites through ca 310 ma hornblende diorites to ca 305 301 ma diorites the earlier granodiorites exhibit dominant magnesian tholeiitic affinity enrichment in large ion lithophile elements liles and depletion in high field strength elements hfses and possess whole rock isr t ratios of 0 707387 0 708799 negative nd t of 2 3 to 2 4 and zircon hf t values of 0 69 to 4 87 and 18o of 6 33 to 8 40 these features are consistent with the magmatic derivation from dehydration melting of tholeiitic mafic to intermediate lower crustal underplates with obvious high 18o supracrustal inputs the ca 310 ma pod like hornblende diorites display high ca boninite like elemental traits and morb like isotopic compositions isr t 0 704190 to 0 704203 nd t 7 9 to 8 3 zircon hf t 14 7 to 17 5 and 18o 4 88 to 6 16 indicative of a prior melt depleted then fluid enriched mantle source by contrast subsequent 301 305 ma diorites show arc like elemental hallmarks with notable enriched liles and depleted hfses as well as variable isotopic values isr t 0 703690 0 704065 nd t 3 5 to 8 4 zircon hf t 7 10 to 16 4 and 18o 3 75 to 6 83 they are interpreted to represent the evolved products of hornblende dominated fractional crystallization from depleted basaltic parental magmas given their coincidence with regional back arc basin type ophiolitic stratigraphy and detrital zircon age spectra these intermediate to felsic intrusive suites tend to document the timeline of a back arc basin formation from initial rifting to maturation in conjunction with early carboniferous forearc type ophiolitic complexes and forearc magmatic records at west ujimqin and the adjacent region a carboniferous island arc and back arc system can be reconstructed from xilinhot to west ujimqin the characterization of this northward present day coordinates subduction system in terms of its temporal evolution not only provides overwhelming evidence that modern pacific like archipelago type arc configuration had prevailed in central inner mongolia during the carboniferous but also offers vivid insights into the diversity of crustal generation mechanism in a subduction factory that ushered in seminal continental crustal construction in the eastern caob

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

the origin of low k rhyolites from the mariana frontal arc low k rhyolites and overlying 2 pyroxene andesites both of late eocene age comprise the oldest volcanic units exposed on saipan the mineralogy and geologic setting of these rocks indicate they were erupted in a volcanic arc setting the presence of andesite and lack of basalts of similar age suggest that the rhyolites are not part of a typical bimodal basalt rhyolite suite

formation of arcs and backarc basins inferred from the tectonic evolution of southeast asia since the late cretaceous abstract results of the geological and geophysical surveys in the daito ridges and basin in the northern west philippine basin suggest that the daito ridge was an arc facing toward the south from the late cretaceous to the early tertiary the late cretaceous and tertiary history of southeast asia is evaluated based on these data in the daito ridges and basins and reconstructed based on overall plate kinematics that have operated in this area during the late cretaceous the daito ridge and the east philippine islands were positioned along the boundary between the indian and pacific plates the western half of the philippines setting on the indian plate approached from the south and collided with the east philippine daito arc either during the latest paleocene or the earliest eocene it is inferred that the bulk of the philippine archipelago rotated clockwise and borneo spun counterclockwise during the tertiary from the reconstruction the formation of backarc basins and their spreading direction are assessed as a result some primary causes and significant characteristics are suggested for the opening of backarc basins in southeast asia first opening of some backarc basins commenced with or was triggered by collisions second backarc basins opened approximately parallel to oceanic plate motion third the formation of some backarc basins was triggered by the approach of a hot spreading center fourth the spreading mode or direction of backarc basins was greatly affected by the configuration of the surrounding continent and was also rearranged to spread approximately parallel to oceanic plate motion the formation of backarc basins and their spreading direction can be reasonably explained by plate kinematics however the generative force responsible for their formation is possibly within the subduction system particularly to form horizontal tensional force in backarc side

partial assimilative recycling of the mafic plutonic roots of arc volcanoes an example from the chilean andes at long lived arc volcanic centers open system interactions between new magma and the plutonic roots of the arc will increase with time relative to interactions with older crust assimilative recycling of mafic plutonic arc roots will have different isotopic consequences than incorporation of aged continental crust in the tatara san pedro complex 30 comagmatic basaltic lavas are characterized by xenocrysts and microxenoliths of olivine augite and plagioclase derived from solidified cumulates wide but poorly correlated ranges of incompatible and compatible major and trace elements and limited isotopic variability this mineralogical chemical signature reflects assimilation wherein grain boundary melting and disaggregation of xenoliths led to blending of incompatible element enriched melts derived mainly from hornblende phlogopite and plagioclase in combination with variable retention of xenocrysts hence variable but high compatible element concentrations

the late jurassic panjeh submarine volcano in the northern sanandaj sirjan zone northwest iran mantle plume or active margin abstract the tectonic setting in which jurassic igneous rocks of the sanandaj sirjan zone sasz of iran formed is controversial sasz igneous rocks are mainly intrusive granodiorite to gabbroic bodies which intrude early to middle jurassic metamorphic basement jurassic volcanic rocks are rare here we report the age and petrology of volcanic rocks from the panjeh basaltic andesitic rocks complex in the northern sasz southwest of ghorveh city the panjeh magmatic complex consists of pillowed and massive basalts andesites and microdioritic dykes and is associated with intrusive gabbros the overall sequence and relations with surrounding sediments indicate that this is an unusually well preserved submarine volcanic complex igneous rocks belong to a metaluminous sub alkaline medium k to high k calc alkaline mafic suite characterized by moderate al2o3 13 7 17 6 wt and variable fe2o3 6 0 12 6 wt and mgo 0 9 11 1 wt contents zircon u pb ages 145 149 ma define a late jurassic tithonian age for magma crystallization and emplacement whole rock compositions are enriched in th u and light rare earth elements lrees and are slightly depleted in nb ta and ti the initial ratios of 87sr 86sr 0 7039 0 7076 and end t values 1 8 to 4 3 lie along the mantle array in the field of ocean island basalts and subcontinental metasomatized mantle immobile trace element ti v zr y nb yb th and co behavior suggests that the mantle source was enriched by fluids released from a subducting slab i e deep crustal recycling with some contribution from continental crust for andesitic rocks based the chemical composition of panjeh mafic and intermediate rocks in combination with data for other gabbroic to dioritic bodies in the ghorveh area we offer two interpretations for these and other jurassic igneous rocks of the sasz as reflecting melts from a subduction modified oib type source above a neo tethys subduction zone or b plume or rift tectonics involving upwelling metasomatized mantle mostly reflecting the 550 ma cadomian crust forming event

paleoseismology of the 1912 acambay earthquake and the acambay tixmadeje fault trans mexican volcanic belt the acambay graben is a major intra arc basin of the trans mexican volcanic belt trenching studies at four sites along the acambay tixmadeje fault reveal evidence for the 1912 acambay earthquake and at least four late pleistocene and holocene ground rupturing earthquakes with similar displacements to the 1912 event evidence for multiple events was recognized by a combination of upward terminating faults fissure fills erosionally truncated fissures colluvial wedge fills and downward increasing separation of stratigraphic units sites along the central part of the ese wnw trending rangefront fault indicates large dip slip separations across a multi event scarp in holocene colluvium with a penultimate event that occurred since 5230 calibrated years before present cal yr b p on the huapango plain near the eastern end of the 1912 rupture where slip was oblique the penultimate earthquake occurred after 4700 cal yr b p and there is evidence for at least four more earthquakes in the last 34 000 cal yr b p three dimensional excavation of lacustrine and channel deposits at las lomas yields a left normal slip vector of s40 5 e with 105 10 cm of slip for the last three events there this slip vector is consistent with nne oriented extension across the graben the amount and style of slip documented for prehistoric ruptures are also consistent with the 1912 rupture with a mean of 60 cm event of dip slip on the rangefront and a mean of 35 cm event of oblique slip on the n48 w trending huapango plain assuming these values represent a characteristic behavior yields an average recurrence interval of 3600 years for the last four large earthquakes since 11 570 cal yr b p and a slip rate of 0 17 mm yr across the rangefront empirical earthquake parameters calculated from our paleoseismic data support instrumental measures indicating that the 1912 acambay earthquake was of m 6 8 7 0 the 1912 event is representative of ground rupturing earthquakes on the acambay tixmadeje fault and demonstrates that large seismic events m 7 can be generated by faults in the trans mexican volcanic belt and in other intra arc settings

petrological model of the northern izu bonin mariana arc crust constraints from high pressure measurements of elastic wave velocities of the tanzawa plutonic rocks central japan abstract ultrasonic compressional wave velocities vp and shear wave velocities vs were measured with varying pressure up to 1 0 gpa in a temperature range from 25 to 400 c for a suite of tonalitic gabbroic rocks of the miocene tanzawa plutonic complex central japan which has been interpreted as uplifted and exposed deep crust of the northern izu bonin mariana ibm arc the vp values of the tonalitic gabbroic rocks increase rapidly at low pressures from 0 1 to 0 4 gpa and then become nearly constant at higher pressures above 0 4 gpa the vp values at 1 0 gpa and 25 c are 6 3 6 6 km s for tonalites 56 4 71 1 wt sio2 6 8 km s for a quartz gabbro 53 8 wt sio2 and 7 1 7 3 km s for a hornblende gabbro 43 2 47 7 wt sio2 combining the present data with the p wave velocity profile of the northern ibm arc we infer that 6 km thick tonalitic crust exists at mid crustal depth 6 1 6 3 km s vp overlying 2 km thick hornblende gabbroic crust 6 8 km s vp our model shows large differences in acoustic impedance between the tonalite and hornblende gabbro layers being consistent with the strong reflector observed at 12 km depth in the ibm arc the measured vp of tanzawa hornblende bearing gabbroic rocks 7 1 7 3 km s is significantly lower than that vp modeled for the lowermost crustal layer of the northern ibm arc 7 3 7 7 km s at 15 22 km depth we propose that the ibm arc consists of a thick tonalitic middle crust and a mafic lower crust

trace element mobility during spheroidal weathering of basalts and andesites in hawaii and guatemala abstract trace element mobility has been studied in spheroidally weathered corestone shell complexes developed on late tertiary and quaternary volcanic rocks the parent materials are basalts and andesites related to 1 a subduction zone tecuamburro and moyuta volcanoes guatemala and 2 a hot spot hawaiian islands of oahu maui and hawai i the initial major element compositions are largely preserved in the corestones each corestone sample shows major element abundances characteristic of the different magmatic processes through which the lavas were generated however the changes in trace element compositions due to weathering show similar patterns in the corestone shell complexes independent of the tectonic setting and age element mobility within the different corestone shell complex was determined using an isovolumetric reference frame in this method bulk density is chosen as an independent measure of the extent of weathering based on volumetric concentrations the notion of element immobility cannot be applied to all the corestone shell complexes elements commonly assumed to be immobile e g zr ti and al vary in volumetric concentration for some corestone shell complex despite petrographic evidence of only minor weathering two corestones oahu and tecuamburro have become enriched in ree 3 relative to other lava flows from the same volcanoes and display negative ce anomalies the exfoliated shells from these complexes tend to have lower concentrations of ree 3 and have positive ce anomalies the fractionation of ce from the other rare earth elements ree is inferred to be a manifestation of changes in the valence state of ce from trivalent to tetravalent under oxidizing conditions in settings where the development of corestone shell complex is advanced an acidic solution can mobilize trivalent ree leach them out of more extensively weathered materials and transport them to less acidic portions of the regolith undergoing incipient weathering the precipitation of the ree 3 in the rinds and centers of the corestones are likely due to increases of the ph of the weathering solution systematic weathering induced ree behavior enrichment of most ree with lesser ce enrichment resulting in a negative ce anomaly can be discerned even in corestone samples that exhibit major element abundances and petrographic features otherwise similar to unweathered basalts and andesites

missing oligocene crust of the izu bonin arc consumed or rejuvenated during collision the 50 myr old izu bonin mariana ibm arc consists mostly of oligocene middle and lower crust that underlies the upper crust these units are in turn covered by quaternary volcanic rocks seismic imaging forearc geology ocean drilling program drilling and magnetic anomalies suggest that most ibm arc crust was created in eocene oligocene times however remnants of this old crust have never been found at the northern end of the arc where it is colliding with the honshu arc izu collision zone two batholiths in this collision zone thetanzawa tonalites and the kofu granitic complex were emplaced during the miocene 4 17 ma major elements zr y rare earth element ratios and normalized abundance patterns and sr nd isotopic data indicate that these plutonic bodies are compositionally similar to the oligocene ibm volcanic rocks and that they are dissimilar to the miocene pliocene and quaternary ibm lavas and volcaniclastic rocks we suggest that the miocene plutonic rocks in the izu collision zone were derived from partially melted oligocene middle crust a model is proposed in which ibm arc middle crust in the collision zone was partially melted during the collision and then intruded into the overlying upper crust of the honshu and ibm arcs this resulted in the complete loss of chronological information related to their original source

distribution of quaternary deformation in the central main ethiopian rift east africa 1 the main ethiopian rift mer is a narrow continental rift characterized by an along axis variation in rift evolution with early stages in the south evolving to incipient breakup in the north although distribution and style of quaternary volcanotectonic deformation is well known in the northern rift sector knowledge of these characteristics is comparatively less constrained southward in this paper we present the results of a field structural study carried out to better constrain the time space distribution of faulting in the central sector of the mer central mer the new field structural data coupled with new 14c radiometric dating of faulted rocks suggest a localization of faulting at both rift margins of the central mer where radiometric dating of faulted material has allowed establishing a late pleistocene holocene activity of border faults conversely in rift faulting wonji fault belt wfb is subordinate highlighting a major difference with the northern sector of the mer where deformation is essentially accommodated in the axial zone this is consistent with an along axis variation in rift evolution showing the central mer less evolved than the northern rift sector inversion of cumulative fault slip data reveals a variation in the extension direction between the rift margins n105 110 e and the rift floor n90 95 e which accords well with the current nubia somalia plate kinematics the variation in extension direction across the rift could manifest a slip partitioning between the boundary faults and in rift wfb faults

systematic variation in the depths of slabs beneath arc volcanoes summary the depths to the tops of the zones of intermediate depth seismicity beneath arc volcanoes are determined using the hypocentral locations of engdahl et al these depths are constant to within a few kilometres within individual arc segments but differ by tens of kilometres from one arc segment to another the range in depths is from 65 km to 130 km inconsistent with the common belief that the volcanoes directly overlie the places where the slabs reach a critical depth that is roughly constant for all arcs the depth to the top of the intermediate depth seismicity beneath volcanoes correlates neither with age of the descending ocean floor nor with the thermal parameter of the slab this depth does however exhibit an inverse correlation with the descent speed of the subducting plate which is the controlling factor both for the thermal structure of the wedge of mantle above the slab and for the temperature at the top of the slab we interpret this result as indicating that the location of arc volcanoes is controlled by a process that depends critically upon the temperature at the top of the slab or in the wedge of mantle immediately below the volcanic arc

low δ18o magmas in the carboniferous intra oceanic arc central tibet implications for felsic magma generation and oceanic arc accretion abstract low 18o magmas 18ozircon pb dating indicates that they were formed simultaneously at 350 ma slightly younger than the early carboniferous ophiolites 355 ma the forearc ophiolites and volcanic rock assemblages constitute an intra oceanic subduction system evidenced by their similar sr nd isotopes sims zircon o isotope analyses indicate that the felsic rocks have 18ozircon values similar to or lower than that of the mantle the dacite and trondhjemite samples have homogeneous 18ozircon values of 5 1 0 4 2sd and 4 6 0 5 2sd respectively the tonalite samples exhibit a large range of 18ozircon values from 2 7 to 5 0 most of which are lower than the mantle like value of 5 3 0 6 2sd we suggest that the dacites and trondhjemites were produced by partial melting of unaltered lower crust whereas the low 18o tonalites originated by melting of hydrothermally altered lower crust and both were generated in the early stage of this intra oceanic arc this process resulted in a weak middle arc crust of intermediate composition by allowing decoupling of upper and lower arc crust this plays an important role in the accretion of intra oceanic arcs to continents and ultimately in the crustal growth

history of basin formation and tectonic evolution at the termination of a large transcurrent fault system deformation mode of central kyushu japana abstract the central part of the kyushu island is a locus of active arc volcanism associated with subduction of the philippine sea plate being located at a bend in the boundary between the philippine sea and eurasian plates a complicated lateral displacement has generated half grabens a zone of strike slip and rhomboidal basins filled with volcanic material in central kyushu initial northward subduction in latest miocene time activated the n s trending kokura tagawa tectonic line ktl as a left lateral slip fault that bounds the western margin of the area of volcano tectonic depression the hohi volcanic zone which was initially formed about 6 ma ago the depocenter which is now called kuju basin in the hohi volcanic zone was at the corner of the ktl and the e w trending median tectonic line mtl relative convergence direction of the philippine sea plate shifted counterclockwise at about 1 5 ma and active motion on the ktl declined at the same time west northwestward subduction enhanced right slip on the mtl geologic gravimetric and seismic data indicate that the mtl has shifted its active trace northward in central kyushu as far as 10 km as a result the depocenter adjacent to the transcurrent fault migrated northeastward in the hohi volcanic zone specifically from kuju basin pliocene via shonai basin early quaternary to beppu bay basin late quaternary the latest depocenter of the beppu bay is surrounded by active faults that clearly delineate a rhomboidal basin on the mtl central kyushu exemplifies the basin forming history and tectonic evolution at the termination of a large transcurrent fault system

paleozoic to triassic ocean opening and closure preserved in central iran constraints from the geochemistry of meta igneous rocks of the anarak area the anarak area belongs to an ophiolitic belt along the northern border of the central east iranian microcontinent and is thought to contain fragments of the former paleotethys and neotethys oceans a wide range of meta igneous rocks from the late paleozoic to triassic anarak metamorphic complex amc and nearby meraji area have been studied to constrain the origins and modes of emplacement of oceanic remnants in central iran our samples occur as layers and lenses embedded in extensive sequences of deformed meta sediments and smaller bodies of serpentinized ultramafic rocks petrographical and geochemical data combined with field and satellite observations allow recognition of seven types of meta igneous rocks preserved from low grade to blueschist facies conditions their origins based on relative abundances of immobile trace elements include subduction zone mid ocean ridge ocean intraplate and continental rift settings these data and existing geochronological constraints show the amc formed an accretionary complex formed exhumed incrementally during the carboniferous permo triassic and triassic igneous rocks from meraji formed in the early devonian due to opening of the paleotethys and belong to a rift sequence extending over 300 km along the edge of the central east iranian microcontinent the amc and nearby rock associations record the evolution of the paleotethys during a complete wilson cycle between ca 450 and 225 ma with implications for 1 continental rifting 2 ocean opening 3 subduction initiation 4 ocean intraplate and continued mid ocean volcanism 5 ridge subduction and 6 final closure of the ocean during continent continent collision alternate interpretations of the anarak metabasites are possible but require radical departures from the widely accepted model for tectonic evolution of the paleotethys with the existence of paleotethyan backarc basin s and permian or earlier collision of continental blocks in central iran in any case our results show accretionary complexes preserved along suture zones contain an important record of the evolution of oceanic crust from ancient ocean basins

new seismological constraints on growth of continental crust in the izu bonin intra oceanic arc the process by which continental crust has formed is not well understood though such crust mostly forms at convergent plate margins today it is thus imperative to study modern intra oceanic arcs such as those common in the western pacific ocean new seismic studies along the representative izu bonin intra oceanic arc provide unique along strike images of arc crust and uppermost mantle to complement earlier cross arc lithospheric profiles these reveal two scales 1000 10 km scale of variations one at the scale of the izu versus bonin thick versus thin arc crust the other at the intervolcano 50 km scale these images show that 1 the bulk composition of the izu bonin arc crust is more mafic than typical continental crust 2 the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes 3 the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments and 4 a process to return lower crustal components to the mantle such as delamination is required for an arc crust to evolve into continental crust continued thickening of the izu bonin crust accompanied by delamination of lowermost crust can yield velocity structure of typical continental crust

ancient depleted mantle as a source of boninites in the izu bonin mariana arc evidence from os isotopes in cr spinel and magnetite abstract boninite is a volcanic rock derived from shallow melting of highly depleted hydrous mantle fluxed with water from subducted slabs the eruption of boninite early in the history of the izu bonin mariana ibm arc 48 45 ma suggests generation by melting of upper mantle material that was relatively unmodified by subducted components thus the boninite composition should largely reflect that of the sub arc mantle for better understanding of the mantle sources of nascent arc settings and the contributions of different components to arc melts we analyzed os isotope ratios 187os 188os of bulk rocks and mineral separates euhedral cr spinel from boninites and euhedral cr spinel magnetite mixtures from tholeiites younger than 45 ma that erupted after boninites from the bonin islands and guam the age corrected initial os isotope ratios of the whole rock samples 0 1179 0 2050 were more radiogenic and variable than those of the mineral separates possibly because of contamination with crustal materials during magma ascent or alteration after emplacement the age corrected os isotope ratios of euhedral cr spinel in boninite from the bonin islands 0 1187 0 1254 and from guam 0 1220 0 1269 are unradiogenic relative to primitive mantle and those of the cr spinel magnetite mixtures from the tholeiites from the bonin islands are similar to or slightly more radiogenic 0 1224 0 1382 the most depleted os isotope ratio of the cr spinel from boninite yielded a model re depletion trd age of 1 4 ga suggesting that the mantle source of the boninite experienced melt extraction prior to 1 4 ga the source of the boninites is interpreted to be mostly highly depleted mantle with a small contribution of slab flux arising from altered oceanic crust that has radiogenic os components with or without contributions from components with relatively unradiogenic os such as volcaniclastics of oceanic island basalt affiliation or very young mid ocean ridge basalt

subarctic pacific evidence for a glacial deepening of the oceanic respired carbon pool abstract measurements of benthic foraminiferal cadmium calcium cd ca have indicated that the glacial interglacial change in deep north pacific phosphate po4 concentration was minimal which has been taken by some workers as a sign that the biological pump did not store more carbon in the deep glacial ocean here we present sedimentary redox sensitive trace metal records from ocean drilling program odp site 882 nw subarctic pacific water depth 3244 m to make inferences about changes in deep north pacific oxygenation and thus respired carbon storage over the past 150 000 yr these observations are complemented with biogenic barium and opal measurements as indicators for past organic carbon export to separate the influences of deep water oxygen concentration and sedimentary organic carbon respiration on the redox state of the sediment our results suggest that the deep subarctic pacific water mass was depleted in oxygen during glacial maxima though it was not anoxic we reconcile our results with the existing benthic foraminiferal cd ca by invoking a decrease in the fraction of the deep ocean nutrient inventory that was preformed rather than remineralized this change would have corresponded to an increase in the deep pacific storage of respired carbon which would have lowered atmospheric carbon dioxide co2 by sequestering co2 away from the atmosphere and by increasing ocean alkalinity through a transient dissolution event in the deep sea the magnitude of change in preformed nutrients suggested by the north pacific data would have accounted for a majority of the observed decrease in glacial atmospheric pco2

silicic magmas in the izu bonin oceanic arc and implications for crustal evolution abundant rhyolitehaseruptedfromthe izu bonin marianavolca nic arc ibm arc from its earliest stage eocene to the present geochemically three types of quaternary rhyolites exist in the izu bonin arcfront and theyare closely related tovolcanotypeand crus tal structure the dominantly basaltic islands of the volcanic frontproduce small volumes of rhyolites that we call r1 the submarinecalderas of the volcanic front erupt mostly rhyolite that we call r2 seamounts knolls and pillow ridges in thebackarcextensionalzoneare mostly basaltic but also contain rhyolites that we call r3 thethickest total crust and the thickest intermediate compositionmiddle crust occurs below the dominantly basaltic volcanoes whereas the intermediate composition middle crust tends to be thin nerbeneaththesubmarinecalderas r1rhyolitesmaybederivedfromquaternary andesitic sources whereas r2 and r3 rhyolites may bederivedfromoligoceneones thehighercao al

geology and eruptive history of unzen volcano shimabara peninsula kyushu sw japan abstract during the past 500 thousand years unzen volcano an active composite volcano in the southwest japan arc has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben the volcano can be divided into the older and the younger unzen volcanoes the exposed rocks of the older unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200 300 ka drill cores recovered from the basal part of the older unzen volcano are dated at 400 500 ka the volcanic rocks of the older unzen exceed 120 km3 in volume the younger unzen volcano is composed of lava domes and pyroclastic deposits mostly younger than 100 ka this younger volcanic edifice comprises nodake myokendake fugendake and mayuyama volcanoes nodake myokendake and fugendake volcanoes are 100 70 ka 30 20 ka and

age of izu bonin mariana arc basement documenting the early tectonic and magmatic evolution of the izu bonin mariana ibm arc system in the western pacific is critical for understanding the process and cause of subduction initiation along the current convergent margin between the pacific and philippine sea plates forearc igneous sections provide firm evidence for seafloor spreading at the time of subduction initiation 52 ma and production of forearc basalt ocean floor drilling international ocean discovery program expedition 351 recovered basement forming low ti tholeiitic basalt crust formed shortly after subduction initiation but distal from the convergent margin nominally reararc of the future ibm arc amami sankaku basin asb radiometric dating of this basement gives an age range 49 3 46 8 ma with a weighted average of 48 7 ma that overlaps that of basalt in the present day ibm forearc but up to 3 3 m y younger than the onset of forearc basalt activity similarity in age range and geochemical character between the reararc and forearc basalts implies that the ocean crust newly formed by seafloor spreading during subduction initiation extends from fore to reararc of the present day ibm arc given the age difference between the oldest forearc basalt and the asb crust asymmetric spreading caused by ridge migration might have taken place this scenario for the formation of the asb implies that the mesozoic remnant arc terrane of the daito ridges comprised the overriding plate at subduction initiation the juxtaposition of a relatively buoyant remnant arc terrane adjacent to an oceanic plate was more favourable for subduction initiation than would have been the case if both downgoing and overriding plates had been oceanic

remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs an example from the izu bonin arc

rapid evolution of subduction related continental intraarc rifts the taupo rift new zealand the evolution of the continental intra arc taupo rift in the north island new zealand is rapid significantly faster than comparative intra continental rifts such as the african rifts based on our faulting data and published geological geophysical and borehole data we show that activity in the 2 ma taupo rift has rapidly and asymmetrically narrowed via inward and eastward migration of faulting at rates of ca 30 km my 1 and 15 km my 1 respectively and has propagated southwards along its axis 70 km in 350 kyr the loci of voluminous volcanic eruptions and active faulting are correlated in time and space suggesting that a controlling factor in the rapid rift narrowing is the presence of large shallow heterogeneities in the crust such as large rhyolitic magma bodies generated by subduction processes which weaken the crust and localize deformation eastward migration of faulting also follows the eastward migration of the volcanic arc which may be related to rollback of the pacific crust slab at the hikurangi subduction zone southward propagation of the rift is linked with southward migration of the hikurangi plateau chatham rise subduction point and occurs episodically aided by stress changes associated with voluminous local volcanism the large magma supply during early continental intra arc rift stages explains faster evolution from tectonic to magmatic than intracontinental rifts however the fast changes in magma supply from the subduction zone can also lead to evolution reversals more evolved magmatic stages reverting to less evolved tectonic stages rift cessation and thus failed continental break up

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

rare earth element tetrad effect and negative ce anomalies of the granite porphyries in southern qiangtang terrane central tibet new insights into the genesis of highly evolved granites abstract highly evolved granites are geochemically similar to the mature continental crust and are sometimes associated with w sn nb ta and rare metal mineralizations these rocks have important implications for tracing the compositional evolution of continental crust and magma metallogenesis they were commonly considered to have been produced by partial melting of crustal materials or by high degree fractional crystallization in this study we report on high sio2 granite porphyries in the xiabie co area of southern qiangtang terrane central tibet which provide new insights into the petrogenesis of the highly evolved granites zircon dating results indicate that they were generated in the late early cretaceous 107 1 ma they consist mainly of orthoclase albite and quartz with accessory minerals including zircon fluorite and fe ti oxides geochemically they have high sio2 but low mgo fe2o3t cao and p2o5 and are enriched in rb but depleted in ba sr eu and ti similar to highly evolved granitic rocks they are characterized by distinct rare earth element ree tetrad effects and negative ce anomalies they also have slightly higher end t values 0 99 to 0 36 than those of the adjacent early granites 2 05 to 3 01 and relatively high and variable zircon ehf t values ranging from 0 3 to 5 4 compared to typical highly evolved granites and petrological experiment data we suggest that the xiabie co granite porphyries represent highly evolved melts produced by fractional crystallization the interaction between fractionated melts and f rich aqueous fluids played an important role in the genesis of their ree tetrads effect involvement of meta sedimentary rocks that went through surficial chemical weathering into their source region is the predominant reason for the generation of negative ce anomalies this indicates that in addition to fractional crystallization the melt fluid interactions and partial melting of source materials also play important roles in the generation of highly evolved granites

origin of silicic magmas and the compositional gap at sumisu submarine caldera izu bonin arc japan abstract sumisu caldera volcano izu bonin arc japan is dominated by the bimodal assemblage of basalt basaltic andesite 66 wt sio2 andesites 56 61 wt sio2 are minor in volume and a compositional gap of 6 wt sio2 exists between the andesites and the dacites most andesites contain sieve textured plagioclase and some dacites have resorbed plagioclase containing many glass inclusions some orthopyroxene phenocrysts in andesites and dacites have overgrowth rims with a high ca content a feature consistent with crystallization from a higher temperature magma than their cores dacites rhyolites and glasses in the matrix and plagioclases possess compositions similar to those of partial melts produced by dehydration melting of calc alkaline andesite tanzawa tonalite at low pressure 3 kbar phenocryst assemblages of these andesites dacites and rhyolites resemble restite phase assemblages of the dehydration melting prolonged fractionation or partial melting of basalts in contrast is untenable based on petrography and fractionation calculations we suggest that the silicic magmas at sumisu caldera volcano have been produced by dehydration melting of solidified calc alkaline andesite protolith in the upper to middle crust which segregated efficiently from restite crystals in rare situations reheating and en mass remobilization permitted the same andesites to erupt as andesites 56 61 wt sio2 thus sumisu has a relatively narrow temperature difference 900 1100 c associated with a large compositional range 56 74 wt sio2

geology and geochemistry of early arc volcanic rocks from guam the island of guam is located at the southern end of the mariana fore arc and was the site of early arc volcanism between 43 and 32 m y ago two volcanic units were erupted during the early evolution of guam the late middle eocene facpi formation and the late eocene to early oligocene alutom formation the facpi formation is composed largely of interbedded boninite series pillow lavas pillow breccias and dikes although arc tholeiite series rocks cap the formation in some areas the alutom formation is composed of interbedded volcanic breccias tuffaceous sandstones lava flows and sills calc alkaline arc tholeiite and boninite series compositions are all found in the alutom formation primitive boninite series rocks are relatively high in sio 2 mgo and ni and low in a1 2 o 3 and tio 2 contents compared to basaltic rocks from the arc tholeiite series additionally they have low ti zr ratios and high k zr rb zr and ba zr ratios although no calc alkaline basalts are found in the early arc formations of guam basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc alkaline series among the analyzed samples here are a few with anomalously high y and rare earth element ree contents chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase clinopyroxene and orthopyroxene the arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase clinopyroxene orthopyroxene and magnetite the composition of the least silicic calc alkaline series andesite can be modeled by fractional crystallization of olivine clinopyroxene plagioclase and magnetite from a transitional basalt derivation of more silicic calc alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende all basalts from the early volcanic series of guam were erupted in an arc setting the boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels whereas the arc tholeiite and calc alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels we suggest that because the calc alkaline series parent basalts most likely were richer in incompatible high field strength hfs elements and sio 2 than were the tholeiitic parents they were generated by lower degrees of melting

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

extension and rifting the zeit region gulf of suez abstract a field analysis of faults and fractures in the ras gharib ras gemsa region of the gulf of suez shows that the main late cenozoic extension occurred perpendicular to the rift axis three main types of dip slip normal faults successively developed as the tilt of blocks bounded by antithetic normal faults increased determinations of the amount of extension from structural data are compatible with estimates made using subsidence data through a simplified model of lithospheric stretching the uplift of rift shoulders is related in chronology and volume to the subsidence of the rift the geometry of fault patterns and directions of extension suggests that the late cenozoic total movement corresponds to a counterclockwise rotation of 4 5 of sinai relative to africa with a pole close to cairo