implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

refractory magmas in back arc basin settings experimental constraints on the petrogenesis of a lau basin example we present the results of an experimental study on a refractory back arc basin glass composition 123 95 1 recovered close to the intersection of the north western lau spreading centre and the peggy ridge in the central lau basin we used both the inverse and forward experimental approaches to determine that a picrite composition of 16 wt mgo was parental to the refractory back arc basalt composition and that this picrite was in equilibrium with a residual lherzolite assemblage at 2 1 gpa 1460 c our experimental results suggest that this primary picrite represents neither an aggregate of small melt fractions collected over a depth interval in a melting column nor a small melt fraction instead the primary picrite composition represents a significant melt fraction 15 melting of a lherzolite source and its composition is closely modelled by equilibrium batch melting our preferred model of magma genesis involves three dimensional diapiric flow during which significant solid melt re equilibrium occurs melts are trapped within an ascending diapir until melting reaches a moderate to high fraction 15 25 at which point the primary picrite magmas segregate from the diapir

forty million years of magmatic evolution in the mariana arc the tephra glass record tephra glasses retrieved from 10 deep sea drilling project dsdp cores around the mariana arc system make up a remarkable record of explosive volcanism in the marianas over the past 40 m y major element compositions for approximately 1800 tephra glasses from basalt through rhyolite are reported and used to examine the nature and history of this activity three maxima of volcanic explosivity presumably related to especially vigorous volcanism in the mariana arc are identified with the biggest maximum at around 18 11 ma and two other maxima at 35 24 and 6 0 ma the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western pacific arcs explosive arc volcanism has been predominantly tholeiitic since shortly after arc inception no boninitic glasses were found the tephra glasses belong to the low to medium k suite except during an enigmatic phase of medium to high k explosive volcanism during the late miocene 11 7 ma even though mariana tephra glasses are largely similar in composition to mariana arc lavas the tephra show a much larger compositional range than that found for subaerial mariana arc lavas which totally lack dacitic to rhyolitic volcanic products the tephra glasses define a bimodal population in terms of silica content with a pronounced minimum or daly gap around 65 66 sio2 mariana tephra glasses are fractionated with the least evolved glass being fe rich and having a magnesium number mg 100mg mg fe 2 of 55 basaltic tephra glasses contain 16 a1203 average 14 3 a1203 and are not high alumina basalts this contrasts with the observation that modern mariana arc lavas contain 15 21 average 17 4 a1203 the high alumina basalt lavas of the mariana arc probably reflect plagioclase accumulation and not liquid compositions all tephra glasses plot near low pressure cotectics and reaction curves on the subprojection olivine clinopyroxene quartz mafic and felsic samples define distinct trends the mafic trend reflects fractional crystallization of mantle derived basaltic magma whereas the felsic trend may be due either to anatexis of mariana arc crust or to fractionation of mafic melts the tephra glass data reinforce the model that the magmatic evolution of the mariana arc has been dominated by low pressure fractionation perhaps accompanied by anatexis episodic changes in melting regime to generate miocene potassic tephra may be related to changing mantle sources and processes related to episodes of back arc basin spreading these episodic changes are superimposed on a long term increase in potassium that reflects progressive metasomatism of the mantle source long term increases in k20 contents for mariana arc magmas inferred from the tephra glass record are 0 004 wt m y mafic 0 011 wt m y intermediate and 0 023 wt m y felsic

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

clinoenstatite bearing lava from nepoui new caledonia the lava consists mainly of clinoenstatite phenocrysts 29 sometimes enclosing euhedral chromite a lesser amount of microphenocryst bronzite 20 and a glassy mesostasis clinoenstatite phenocrysts are compositionally homogeneous en 89 fs sub 10 5 wo sub 0 5 with polysynthetic twins on 100 a feature which is believed to indicate inversion from protoenstatite bronzite is variable in composition with en sub 74 84 fs sub 14 22 wo sub 1 5 4 hypersthene en sub 43 61 fs sub 36 52 wo sub 3 4 calcic and subcalcic augite and iron rich hornblende occur as overgrowths on phenocrysts and as quench microlites the new caledonian clinoenstatite boninite is similar chemically mineralogically texturally and in terms of age eocene oligocene and tectonic setting with ophiolite complexes to the three other known clinoenstatite boninite localities cape vogel papua new guinea bonin islands and mariana trench clinoenstatite boninite magmas are believed to result from extensive partial melting of depleted mantle peridotite under hydrous conditions and an abnormally high geothermal gradient modified journal abstract

petrology of the axial ridge of the mariana trough backarc spreading center abstract the axial ridge of the mariana trough backarc basin between 17 40 n and 18 30 n rises as much as 1 km above the floor of a 10 15 km wide rift valley physiographic segmentation with minor ridge offsets and overlaps coincides with a petrologic segmentation seen in trace element and isotope chemistry analyses of 239 glass and 40 aphyric basalt samples collected with alvin and by dredging show that the axial ridge is formed largely of olivine hypersthene normative tholeiitic basalt about half of these are enriched in both lil elements and volatiles but are depleted in hfs elements like other rocks found throughout much of the mariana trough the lil enrichments distinguish these rocks from n morb even though nd and sr isotope ratios indicate that much of the crust formed from a source similar to that for n morb in addition to lil enriched basalt there is lil depleted basalts even more closely resembling n morb in major and trace elements as well as sr nd and pb isotopes both basalt varieties have higher al and lower total fe than morb at equivalent mg level mg ranges from relatively primitive e g mg 65 70 to more highly fractionated e g mg 45 50 highest parts of the axial ridge are capped by pinnacles with elongated pillows of basaltic andesite e g 52 56 sio 2 these are due to extreme fractional crystallization of basalts forming the axial ridge active hydrothermal vents with chimneys and mats of opaline silica barite sphalerite and lesser amounts of pyrite chalcopyrite and galena formed near these silicic rocks the vents are surrounded by distinctive vent animals polychaete worms crabs and barnacles isotope data indicate that the mariana trough crust was derived from a heterogeneous source including mantle resembling the morb source and an arc source component the latter was depleted in hfs elements in previous melting events and later modified by addition of h 2 o and lil elements

controls on platinum group elemental distributions of podiform chromitites a case study of high cr and high al chromitites from chinese orogenic belts abstract a study of podiform chromite deposits from the asiatic orogenic belt and the qilian qiangling kunlun himalaya tectonic domain provides new insights into the geochemistry of the pges in podiform chromite deposits and the genesis of the deposits themselves the bulk of deposits which occur in mantle peridotites of ophiolites have typical ophiolitic pge patterns that are depleted in pt and pd relative to the average upper mantle and have negatively sloping distributions on mantle normalized diagrams type i high cr chromitites have higher os ir ru and rh contents than type ii high al chromitites although both have similar pd and pt most of the type i and ii chromite deposits have lower pd and pt contents than the upper mantle peridotites in which they occur podiform chromitites are essentially products of melt rock interaction in the upper mantle their cr and pges were contributed by not only the invading magmas but also by the upper mantle host the chromite deposits are in part metasomatic replacement bodies the type i high cr chromitite pge patterns were produced by interaction between s undersaturated boninitic magmas and depleted harzburgites whereas the type ii high al chromitite pge patterns were formed by interaction between initially s saturated tholeiitic magmas and depleted harzburgites the low to very low pd and pt contents of both type i and type ii chromitites require that the mantle assemblage in which the chromite deposits were formed had lost their sulfides and hence pd and pt prior to formation of the chromite deposits in addition no or little pd and pt were deposited by the invading magma which either remained s undersaturated boninite or became morb s undersaturated due to interaction with the s depleted harzburgitic mantle it is suggested that the very low ir os and ru contents of boninites in general might be due to loss of ir during the formation of podiform chromitites it is suggested that podiform chromitites with type i pge patterns were formed in an island arc environment whereas those with type ii pge patterns were formed in a back arc setting

mesozoic transpression transtension subduction and metallogenesis in northern and central california middle paleozoic to middle jurassic terrane assemblies in the klamaths and sierran foothills consist of mafic ultramafic complexes fine grained terrigenous strata derived from previously accreted continental margin belts sutured oceanic terranes reflect c 230 myr of margin parallel slip involving chiefly transtension and transpression quartzofeldspathic clastic rocks and blueschists eclogites are very rare little devolatilization occurred at magmagenic depths hence coeval hydrothermal ore deposits and granitoids are uncommon in contrast nearly head on cretaceous subduction of the farallon plate generated the massive klamath sierra nevada volcanic plutonic arc reflecting dewatering of the eastward descending oceanic lithosphere in the magmagenic zone immature great valley forearc and franciscan trench deposits shed from the arc record c 70 myr of rapid crustal growth au bearing solutions rising from magmagenic depths exsolved from plutons and expelled from heated wall rocks were mobilized attending arc construction precipitation of gold bearing quartz veins occurred where h2o co2 bearing fluids encountered major geochemical discontinuities in the wall rocks

unzipping of the volcano arc japan abstract a working hypothesis for the recent evolution of the southern volcano arc japan is presented which calls upon a northward progressing sundering of the arc in response to a northward propagating back arc basin extensional regime this model appears to explain several localized and recent changes in the tectonic and magrnatic evolution of the volcano arc most important among these changes is the unusual composition of iwo jima volcanic rocks this contrasts with normal arc tholeiites typical of the rest of the izu volcano mariana and other primitive arcs in having alkaline tendencies high concentrations of light ree and other incompatible elements and relatively high silica contents in spite of such fractionated characteristics these lavas appear to be very early manifestations of a new volcanic and tectonic cycle in the southern volcano arc these alkaline characteristics and indications of strong regional uplift are consistent with the recent development of an early stage of inter arc basin rifting in the southern volcano arc new bathymetric data are presented in support of this model which indicate 1 1 structural elements of the mariana trough extend north to the southern volcano arc 2 2 both the mariana trough and frontal arc shoal rapidly northwards as the volcano arc is approached 3 3 rugged bathymetry associated with the rifted mariana trough is replaced just south of iwo jima by the development of a huge dome 50 75 km diameter centered around iwo jima such uplifted domes are the immediate precursors of rifts in other environments and it appears that a similar situation may now exist in the southern volcano arc the present distribution of unrifted volcano arc to the north and rifted mariana arc to the south is interpreted not as a stable tectonic configuration but as representing a tectonic snapshot of an arc in the process of being rifted to form a back arc basin

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

volcanism accompanying back arc basin development in the southwest pacific abstract the age and composition of magmatism in fiji and to a lesser extent vanuatu and tonga are evaluated in the light of the fragmentation which has sundered the island arc system they once collectively constituted any of six magmatic phenomena during the past 10 m y may be consequences of adjacent plate reconfigurations trondhjemitic plutonism about 10 m y change from tholeiitic to more calc alkaline volcanism between 10 and 6 m y eruption near the rifted arc margin of voluminous crustally derived high si low k rhyolites 7 m y ago shoshonite dominated basaltic volcanism 5 5 to 3 0 m y change to ocean island like basaltic volcanism at 3 0 0 2 m y and geochemically peculiar as well as tectonically anomalous acid andesites 0 7 3 0 m y ago in kandavu southwestern fiji crustal derivation of the low k rhyolites may reflect maximal heating during initial rifting shoshonites may accompany extreme regional extension above a still arc like source eruption of ocean island like basalts begins once subducted lithosphere no longer contributes to erupted melts or separates fiji from ocean island sources coeval eruption of hybrid andesites may accompany oblique subduction of south fiji basin crust beneath kandavu along the northeast hunter fracture zone

chemical transects across intra oceanic arcs implications for the tectonic setting of ophiolites systematic spatial variations in ti y and zr are found in volcanic sequences sampled as transects from back arc through arc to trench within the western pacific intra oceanic arc systems these correlations occur regardless of eruption age reflecting the retention of relative positions between individual tectonic units concentrations of these incompatible elements are taken to reflect the nature of the underlying mantle at a particular trench volcano distance during the subduction process volcanics become more depleted trenchwards implying that progressively shallower levels melts were extracted from more refractory or harzburgitic mantle a model is proposed whereby a subduction system is superimposed on a compositionally stratified mantle which is a product of an earlier spreading event on the basis of this evidence the trench distance variation model is applied to the troodos ophiolite complex similar spatial changes in lava chemistry are found across the ophiolite providing evidence for the relative position of the troodos trench and direction of subduction beneath the troodos basin these findings have implications for studies in other orogenic terrains where structure and exposure permit their application

evolution of backarc rifting mariana trough 20 24 n the mariana trough is an actively opening backarc basin in the western pacific the trough formed by extension which longitudinally split an earlier arc massif creating a crescent shaped basin between the remnant west mariana ridge and eastern active volcanoes of the mariana arc opening increases southward from the volcano islands near 24 n where the two arcs join at 18 n in the central mariana trough the basin is widest and may be opening primarily by seafloor spreading we present a synthesis of closely spaced shipboard gravity magnetic and bathymetry measurements from the northern basin 20 24 n an area that undergoes a significant progressive southward increase in extension we have identified three stages in the evolution of rifting in this area 1 asymmetric rifting from 24 n to 22 15 n where faulting and magmatism have migrated laterally to remain near the active volcanic arc side of the basin 2 a localization of rifting from 22 15 n to 21 n where the primary zone of rifting separates from the active volcanic arc and 3 a further concentration of rifting from 21 n to 20 n leading to the formation of deep tectonic grabens near the center of the basin this last stage may be a precursor to or incipient seafloor spreading we describe a new type of magnetic lineation resolved in a three dimensional seafloor magnetization inversion of the trough between 20 and 24 n the detailed character of these lineations their association with tectonic structures and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting less magnetic rifted arc crust the development of these well defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough although tectonic deformation appears to be more widely distributed in addition the geometry of the magnetization bands with respect to the remnant arc border faults indicates that rifting propagated rapidly northward

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

the lau havre taupo back arc basin a southward propagating multi stage evolution from rifting to spreading abstract the lau havre taupo lht system is an active 2000 km long back arc basin associated with pacific australian plate convergence in the sw pacific data from the lht back arc system include odp drillhole swath bathymetric and side scan imagery seismic reflection rock geochemistry magnetic anomaly heat flow shallow seismicity gravity and seismic refraction data the lht forms a longitudinal transect from southward propagating true oceanic spreading in the lau basin near tonga similarly southward propagating arc rifting in the havre trough and southward propagating continental rifting in the taupo volcanic zone tvz in northern new zealand four transitional tectonic settings are identified along the lht north to south between the continuum of spreading to rifting and comprise 1 oceanic spreading of the central lau spreading centre clsc propagating into oceanic crust formed by the eastern lau spreading centre elsc 2 oceanic spreading of the elsc propagating into rifted lau havre arc crust 3 rifting within havre arc crust propagating into rifting new zealand continental crust and 4 rifting of continental crust propagating into as yet non rifted but flexurally subsiding continental crust the enhanced spatial and temporal resolution of observed tectonic and magmatic processes along the strike of the lht is used to constrain a model for the evolution of back arc rifting to back arc spreading a sequence of five stages is identified and comprise 1 a combination of incipient simple and pure shear 2 early distributed and spatially heterogeneous half graben rifting with low angle synthetic and antithetic structures associated with rapid graben sediment infilling ephemeral early morb like rift magmatism and the initial trenchward migration of the arc front 3 a latter phase of half graben rifting mostly confined to a contiguous axial zone with continued ephemeral morb like rift magmatism and further trenchward migration of the arc front 4 initial oceanic spreading with evolving steady state morb magmatism along the axial rift graben system and 5 the reconfiguration of ridge geometry in response to changes in opening orientation leading to the development of ridge propagation into newly created oceanic crust rifting stages 2 and 3 can produce at least 120 km wide back arc crust by a balance of crustal extension with an estimated factor of 3 with rift and constructional arc magmatism in the lht system depending on latitude rates of widening during stages 2 and 3 vary between 15 and 45 mm yr 1 whilst stages 4 and 5 have full spreading rates of 90 mm yr 1

distribution and origin of igneous rocks from the landward slopes of the mariana trench implications for its structure and evolution the landward slope of the mariana trench is composed largely of igneous rocks serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m seamountlike features on the trench slope break are the surface expression of serpentinite diapirs cumulate and massive gabbros are found several varieties of volcanic rocks are common including boninites altered and metamorphosed basalts andesites and dacites the chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism together with the gabbros these volcanic rocks represent what is probably a late eocene arc complex these rocks were probably the first volcanic products to result from the subduction of the pacific plate beneath the phillippine sea plate their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since eocene time most of this removal of material appears to have occurred during the early stages of subduction there are isolated occurrences on the landward slope of rock assemblages including alkalic basalts chert hyaloclastites upper cretaceous siliceous sediments and shallow water limestones these assemblages are very similar to rocks dredged from seamounts on the offshore flankmore of the trench and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion there has been episodic accretion of seamount fragments to the landward slope less

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

dynamics of intraoceanic subduction initiation 2 suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle here we focus on suprasubduction zone ssz ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame ssz ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation structural petrological geochemical and plate kinematic constraints on their kinematic evolution show that ssz crust forms at fore arc spreading centers at the expense of a mantle wedge thereby flattening the nascent slab this leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during ssz crust crystallization former spreading centers are preserved in forearcs when subduction initiates along transform faults or off ridge oceanic detachments we show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone upon inception of slab pull due to e g eclogitization the sole is separated from the slab remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge we propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust our study highlights how far field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle

evidence for forearc seafloor spreading from the betts cove ophiolite newfoundland oceanic crust of boninitic affinity abstract the ordovician betts cove ophiolite of newfoundland has a well developed cumulate sequence in which is rooted a sheeted dyke complex that grades up into pillow lavas dominant chromite olivine orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas dykes and lavas have phenocrysts of olivine high cr al chromite orthopyroxene low tio2 clinopyroxene they have high sio2 and mgo contents and depleted u shaped trace element profiles indistinguishable from those of bonin islands boninites field data imply that cumulates dykes and lavas all are comagmatic while geochemical and mineralogical data indicate that all are of boninitic affinity since boninites are only found in forearcs this implies that the betts cove ophiolitic crust formed in a forearc since the entire oceanic crustal section at betts cove is of boninitic affinity then this implies that true seafloor spreading can initiate in forearc

temporal change in chemistry of magma source under central kyushu southwest japan progressive contamination of mantle wedge volcanism related to subduction of the philippine sea phs plate began in central kyushu at 5 ma after a pause of igneous activity lasting about 10 m y it formed a large volcano tectonic depression the hohi volcanic zone hvz and has continued to the present at a decreasing eruption rate the products are largely andesite and dacite which became enriched in k with time the proportion of tholeiitic to calc alkalic rocks also increases with time calc alkalic high mg basaltic andesites ybbs were erupted in the early stage of the hvz activity 5 3 ma and high alumina basalts kjbs were erupted in the later stage 2 0 ma in contrast to the basalts in the hvz northwest kyushu basalts nwkbs have been erupted on the backarc side of the hvz since 11 ma and hence are not related to the phs plate subduction they are mainly high alkali tholeiitic to alkali basalt that shows no notable chemical change with time nwkb ybb and kjb have morb normalized incompatible element spectra that differ from each other as is well expressed in both nb and sr anomalies the patterns of kjb and nwkb are typical of those for island arc basalt iab and ocean island basalt oib respectively ybb shows a pattern intermediate between the two we suggest that the magma source beneath the hvz changed in composition from an oib type mantle to an iab type mantle as the subduction of phs plate advanced however the magma source remained fertile under northwest kyushu in order to explain the temporal change of source mantle beneath the hvz we propose a model for progressive contamination of the mantle wedge in which three processes contamination by a slab derived component subtraction of magma from the mantle and mixing of the mantle residue and slab derived component are repeated as subduction continues as long as the progressive contamination of mantle wedge proceeds its trace element composition converges at a steady state value for a short period this value does not depend on the initial composition of the mantle wedge but instead on the composition of the slab derived component the trace element composition of the magma produced in such a mantle wedge approaches that of the slab derived component with time but the major element composition is determined by the phase relations of mantle peridotite the slab derived component may be basaltic liquid that is partially melted from rutile bearing eclogite

devonian intra oceanic arc rift sedimentation facies development in the gamilaroi terrane new england orogen eastern australia abstract silurian devonian rocks within the gamilaroi terrane part of the new england orogen of eastern australia provide a well preserved on land example of the development of an intra oceanic arc rift ten sedimentary facies represent different phases in the evolution of the gamilaroi rift and are typified by predominantly volcaniclastic conglomerates and mudstones with minor sandstones all of which were deposited by debris flows and turbidity currents subordinate facies include limestones crystal rich volcaniclastic sandstones volcanic breccias and olistostromes the facies are distributed throughout a succession of six lithostratigraphic units each of which represents a distinct phase in the history of the arc rift felsic volcanic and volcaniclastic rocks at the base of the section represent part of the original arc and are overlain by volcaniclastic sandstones and mudstones deposited within an arc basin lower devonian emsian limestones underwent rapid subsidence concurrent with an influx of volcaniclastic sediments indicating initiation of rift related faulting a thick conglomerate succession was deposited rapidly on a debris apron system adjacent to the steep sided rift walls overlying tuffaceous mudstones indicate a decrease in clastic input later in the history of the rift basin thick intervals of massive and pillowed basalts at the top of the succession indicate a major period of volcanism within the rift oceanic island arc rift sequences are characterised by highly variable sedimentary facies distribution facies distribution over the 400 km extent of the gamilaroi terrane is also highly variable modern arc rift zones are divided into a series of smaller separate elongate basins providing an explanation for the high facies variability observed in the field

evolution and petroleum geology of amlia and amukta intra arc summit basins aleutian ridge abstract amlia and amukta basins are the largest of many intra arc basins formed in late cenozoic time along the crest of the aleutian arc both basins are grabens filled with 2 5 km of arc derived sediment a complex system of normal faults deformed the basinal strata although initial deposits of late micocene age may be non marine in origin by early pliocene time most of the basinfill consisted of pelagic and hemipelagic debris and terrigenous turbidite deposits derived from wavebase and subaerial erosion of the arc s crestal areas late cenozoic volcanism along the arc commenced during or shortly after initial subsidence and greatly contributed to active deposition in amlia and amukta basins two groups of normal faults occur major boundary faults common to both basins and intra basin faults that arise primarily from arc parallel extension of the arc the most significant boundary fault amlia amukta fault is a south dipping growth fault striking parallel to the trend of the arc displacement across this fault forms a large half graben that is separated into the two depocentres of amlia and amukta basins by the formation of a late cenozoic volcanic centre seguam island faults of the second group reflect regional deformation of the arc and offset the basement floor as well as the overlying basinal section intra basin faults in amlia basin are predominantly aligned normal to the trend of the arc thereby indicating arc parallel extension those in amukta basin are aligned in multiple orientations and probably indicate a more complex mechanism of faulting displacement across intra basin faults is attributed to tectonic subsidence of the massif aided by depositional loading within the basins in addition most intra basin faults are listric and are associated with high growth rates although the hydrocarbon potential of amlia and amukta basins is difficult to assess based on existing data regional considerations imply that an adequate thermal history conducive to hydrocarbon generation has prevailed during the past 6 5 my the possibility for source rocks existing in the lower sections of the basins is suggested by exposures of middle and upper miocene carbonaceous mudstone on nearby atka island and the implication that euxinic conditions may have prevailed during the initial formation of the basins large structures have evolved to trap migrating hydrocarbons but questions remain concerning the preservation of primary porosity in a sedimentary section rich in reactive volcaniclastic debris

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

contrasting early and late mesozoic petrotectonic evolution of northern california devonian middle jurassic terrane assemblies in the klamath mountains and sierra nevada foothills consist chiefly of ophiolite chert argillite sequences mafic ultramafic complexes are oceanic whereas associated fine grained deep water terrigenous sediments were derived mainly from adjacent previously docked klamath sierran terranes coeval calc alkaline arc rocks are volumetrically rare geologic and petrochemical relations suggest a rifted arc origin for klamath mafic metavolcanic units inter layered with distal turbidites in the 170 200 ma north fork terrane detrital zircon u pb ages indicate that the clastic debris had a regional eastern klamath source the eastern hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby eastern klamath antelope mountain quartzite the seaward 200 ma rattlesnake creek terrane is an ophiolitic melange with north fork petrotectonic affinities the north fork eastern hayfork rattlesnake creek amalgam correlates with the calaveras complex and the outboard jura triassic arc belt in the sierran foothills geochemical bulk rock and zircon u pb age data support interpretation of the 200 ma jura triassic arc as an adjacent offshore mafic belt overlying a 300 ma ophiolitic basement these oceanic complexes were sutured against the central metamorphic belt eastern klamath feather river northern sierra terrane backstop before deposition and deformation of the outboard upper jurassic galice and mariposa formations klamath sierran terrane assemblies reflect 230 m y of transpression transtension involving only minor episodes of subduction producing ubiquitous ophiolite chert argillite lithologies and rare felsic arc rocks in contrast the late jurassic to largely cretaceous klamath sierra nevada quartzo feldspathic volcanic plutonic arc attests to massive calc alkaline magmatism attending a strong eastward component of underflow by the farallon plate the coeval galice mariposa formations followed by the cretaceous great valley forearc and franciscan trench deposits are first cycle felsic debris shed mainly from the klamath sierran arc these units record 70 m y of rapid sialic crustal growth attending major periods of approximately margin normal convergence this profound transition in northern california included devonian middle jurassic rifting drifting and stranding of ophiolite chert argillite terranes along an adjacent curvilinear continental margin then nearly head on cretaceous subduction that resulted in massive calc alkaline igneous activity the erosion of which generated the felsic great valley group forearc basin and franciscan complex trench clastic sedimentary units

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

paleozoic ophiolitic assemblages within the southern new england orogen of eastern australia implications for growth of the gondwana margin several ophiolitic assemblages occur in the southern new england orogen the development of these rocks and their relations to the rest of the orogen have major implications for the tectonic evolution of eastern gondwana a major narrow but elongate belt of early cambrian supra subduction zone ophiolite crops out along and near the peel manning fault system and is juxtaposed against younger arc and subduction complex terranes no pre permian links with the rest of the new england orogen have been established for this terrane it may represent portions of lachlan fold belt basement which underlies younger westward overthrust new england terranes and has been diapirically emplaced at higher crustal levels as serpentinite matrix melange middle to late devonian ophiolitic rocks in the yarras complex comprise basement to the birpai subterrane and represent a crustal cross section through a rifted island arc correlatives of this terrane also occur within the more extensive gamilaroi terrane to the west of which deeper crustal levels are not exposed the various components of serpentinite matrix melange in the ngamba terrane at port macquarie superficially appear to represent a dismembered ophiolite association however the various components of the melange exhibit a wide range of ages metamorphism and tectonic affinities rendering a genetically related origin unlikely this terrane includes fragments of ocean floor accreted into a late devonian subduction complex which was later affected by early carboniferous forearc serpentinite diapirism and high mg series magmatism zircon inheritance in triassic dikes which intrude the melange attest to the development or later emplacement of this forearc region over an older lachlan fold belt basement ultramafic rocks of the bundjalung terrane in the east of the new england region probably formed at deep levels in an intraoceanic island arc and are intruded by boninitic dikes the tectonic development of the neo was significantly more complicated than has been suggested in earlier published models periodic accretion of island arc systems some of which are now represented by supra subduction zone ophiolites to the eastern margin of gondwana suggests multiple phases of subduction with the possibility of polarity reversals throughout the history of accretion lateral accretion was not the only means by which gondwana continental crust grew and there was considerable postaccretion continentward overthrusting of younger terranes copyright 1994 by the american geophysical union

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

helium and argon isotope systematics of the central lau basin and valu fa ridge evidence of crust mantle interactions in a back arc basin abstract we report helium and argon isotope analyses of fresh lau basin volcanic glasses from six sites within the central basin 18 s six localities along the valu fa ridge 21 22 4 s and a single site from the northeastern basin 15 s central basin basalts have 3 he 4 he ratios r between 8 2 8 5 r a r a air 3 he 4 he 40 ar 36 ar ratios significantly greater than atmosphere up to 4900 and 4he contents from 3 5 9 4 10 6cm3 stp g similar to n morbs worldwide these results are consistent with trace element and other radiogenic isotope data on these samples which indicate derivation from a depleted mantle source region in contrast evolved lavas from the valu fa ridge all have r r a and show evidence of mixing with a component rich in radiogenic helium there is a clear relationship between the 3 he 4 he ratios of these samples and their chemistry basaltic andesites have 6 02 r c r a andesites have lower ratios 2 37 2 65 ra and a dacite has the lowest value of the entire sample suite 1 19 ra all lavas have 40 ar 36 ar ratios similar to the atmospheric value and low helium concentrations from 3 11 10 8cm3 stp g or between 30 and 300 times less than the central basin basalts although the helium isotope results of the valu fa lavas mirror the shift to more radiogenic values of other isotope systems e g 206 pb 204 pb 87 sr 86 sr which indicate addition of subducted sediment to these magma sources we find no evidence that the radiogenic helium has a mantle or slab derivation or is in any way coupled to these other tracers instead the most plausible mechanism to explain its incorporation into the valu fa lavas is by assimilation of old lau crust in the near surface environment by previously degassed magma we argue that this mechanism has general applicability and can explain a number of hitherto apparently paradoxical geochemical features of some back arc and ocean ridge lavas such as their high volatile and lil element contents with low rare gas concentrations and their mantle 3 he 4 he ratios with hydrated oceanic crustal d h values the realisation that the helium and argon systematics of the valu fa lavas are controlled by crust mantle interactions has important implications for distinguishing between a number of models proposed for the formation of such evolved lavas and we show that fractional crystallisation processes can most readily account for the low concentrations of and systematic trends in the mantle derived helium and argon component of these lavas in addition because pre existing crust in the lau basin must be old and or altered enough to supply the radiogenic helium and atmospheric like 40 ar 36 ar component to the valu fa lavas the occurrence of crust mantle interactions implies that old forearc crust may have been trapped within the lau basin such a scenario has a clear bearing on ideas of the tectonic development of the basin finally because of the potential of crust mantle interactions to modify 3 he 4 he and 40 ar 36 ar ratios of mantle derived melts we assess the implications for using he and ar tracers to characterise mantle sources in arcs back arcs and spreading ridges and consider the consequences for the combined use of rare gases with other radiogenic isotopic tracers of magma provenance at such settings the basaltic andesite from the northeastern basin may also be influenced by the same kind of crustal interaction as the valu fa lavas as it falls within the 3 he 4 he range 6 9ra of the other basaltic andesites interestingly other helium isotope studies indicate that this part of the basin is characterised by a wide range in 3 he 4 he ratios from morb values up to 22ra the low 3 he 4 he ratio of the basaltic andesite therefore serves to illustrate the possible effects of magma chamber processes on the rare gas and other volatile characteristics of hotspot lavas an observation which is important not only for this part of the lau basin but for other localities worldwide

late cretaceous infant intra oceanic arc volcanism the central pontides turkey petrogenetic and tectonic implications abstract a tectonic slice of an arc sequence consisting of low grade metavolcanic rocks and overlying metasedimentary succession is exposed in the central pontides north of the izmir ankara erzincan suture separating laurasia from gondwana derived terranes the metavolcanic rocks mainly consist of basaltic andesite andesite and mafic cognate xenolith bearing rhyolite with their pyroclastic equivalents which are interbedded with recrystallized pelagic limestone and chert the metasedimentary succession comprises recrystallized micritic limestone with rare volcanogenic metaclastic rocks and stratigraphically overlies the metavolcanic rocks the geochemistry of the metavolcanic rocks indicates an arc setting evidenced by depletion of hfse ti p and nb and enrichment of fluid mobile lile identical trace and rare earth elements compositions of basaltic andesites andesites and rhyolites suggest that they are cogenetic and derived from a common parental magma the arc sequence crops out between an albian turonian subduction accretionary complex representing the laurasian active margin and an ophiolitic melange absence of continent derived detritus in the arc sequence and its tectonic setting in a wide cretaceous accretionary complex suggest that the kosdag arc was intra oceanic zircons from two metarhyolite samples give late cretaceous 93 8 1 9 and 94 4 1 9 ma u pb ages these ages are the same as the age of the supra subduction ophiolites in western turkey which implies that that the kosdag arc may represent part of the incipient arc formed during the generation of the supra subduction ophiolites the low grade regional metamorphism in the kosdag arc is constrained to 69 9 0 4 ma by 40 ar 39 ar muscovite dating indicating that the arc sequence became part of a wide tethyan cretaceous accretionary complex by the latest cretaceous non collisional cessation of the arc volcanism is possibly associated with southward migration of the magmatism as in the izu bonin mariana arc system

exploration history and geology of the diamondiferous ultramafic saptarshi intrusions madhya pradesh india abstract the saptarshi field comprises a suite of at least eight known diamondiferous pipes and dykes within rio tinto s bunder project on the bundelkhand craton of madhya pradesh india the discovery made in 2004 followed regional scale area selection gravel sampling and subsequent ground geophysics soil geochemistry and prospecting the geology and petrography of the largest and most economically prospective pipe atri suggests that near the surface it forms a large vent comprised of two distinct lobes filled predominantly with primary pyroclastic rocks atri is composed of relict olivine phlogopite apatite minor spinel and ti oxides the rock is altered no primary olivine and little phlogopite remains instead much of the rock is now composed of serpentine chlorite carbonate other occurrences within the saptarshi field include small pipes and altered dykes atri is one example of the diverse assemblage of potassic rocks found on the indian subcontinent many parallels may be drawn with the nearby diamondiferous majhgawan pipe including mineral chemistry and petrography atri has many characteristics in common with olivine lamproite but many subtle deviations from type lamproite are observed we suggest that atri represents a local expression of potassic magmatism unique to the bundelkhand craton of india and is the product of partial melting of modally diverse veins in ancient metasomatized lithospheric mantle these rocks are not readily classified according to existing schemes devised for potassic rocks known from other petrological provinces a practical and workable scheme has not been devised that recognizes the unique characteristics of the diversity of lamproite like rocks the authors tentatively classify atri as a diamondiferous ultramafic intrusion with lamproitic affinities recognizing its obscured primary mineralogy its petrographic and textural characteristics and the important implications this classification has with respect to exploration the 1100 ma age of the saptarshi intrusions imply that they are part of a larger igneous province which includes the nearby majhgawan and hinota diamondiferous pipes this age temporally relates to the lamproites of western australia rio tinto completed an order of magnitude oom scoping study comprising large diameter and slim line delineation drilling and surface bulk sampling on the project in 2008

effect of margin curvature on plate deformation in a 3 d numerical model of subduction zones summary the large amount of data acquired along most subduction zones underlines the importance of three dimensions in the subduction mechanism in this work we use a fully 3 d mechanical numerical model to analyse the impact of the plate boundary geometry on the deformation of the upper plate the model consists of two initially horizontal independent plates overlying an inviscid fluid and continuously pushed towards each other the initial geometry of the contact zone between the two plates controls the stress regime and the strain pattern in the upper plate the case with a linear plate boundary and orthogonal convergence is first considered and used as a reference model with this simple model the effects of some important parameters i e the interplate friction and the lithosphere asthenosphere density contrast are investigated finally cases with curved plate boundaries are considered and the results are used to shed light on the importance of the dip direction of the interplate plane for the upper plate strain pattern unnecessary oceanward convexity causes an accumulation of the subducted material beneath the upper plate and induces an important uplift of the convex area on the contrary the material escapes from an oceanward concavity and provokes subsidence in the forearc zone such a behaviour induces preferential zones of weakeness in the overriding plate and may allow for explaining some local stress regime variations along convergent margins

stratigraphy and geochemistry of the lac brompton ophiolite canada evidence for extensive forearc magmatism and mantle exhumation in the southern quebec ophiolite belt three ophiolites are currently recognized in the southern quebec appalachians from south to north the mont orford ophiolite dominated by tholeiitic to transitional alkaline basalt and diabase a

episodic formation of neotethyan ophiolites tibetan plateau snapshots of abrupt global plate reorganizations during major episodes of supercontinent breakup abstract neotethyan ophiolites along the bangong nujiang and yarlung tsangpo sutures tibetan plateau consist of mantle peridotite cumulates isotropic gabbro diabase plagiogranite and basaltic lava they comprise morb iab oib and boninitic rocks and mostly belong to supra subduction zone ssz type compositionally different rocks crystallized synchronously over a distance of 2000 km along the bangong nujiang and yarlung tsangpo sutures during the short periods of 190 182 and 132 120 ma respectively resembling the izu bonin mariana ibm proto arc the early jurassic bangong nujiang ophiolites formed in forearc settings during subduction initiation while the middle jurassic early cretaceous bangong nujiang ophiolites and the early cretaceous yarlung tsangpo ophiolites formed in forearc settings at pre existing subduction zones crystallization of the neotethyan ssz ophiolites from the eastern mediterranean middle east tibetan plateau to southeast asia and formation of the associated metamorphic rocks temporally coincided with major supercontinent breakup events 1 initial breakup of pangea in central atlantic and westernmost tethys 190 ma 2 separation of west and east gondwana 170 160 ma and initial opening of piemont ligurian tethys 175 164 ma 3 initial breakups of east and west gondwana 135 125 ma and 4 final breakup of gondwana 104 96 ma far field forces associated with abrupt global plate reorganizations may have triggered subduction initiation or transient interplate coupling decoupling at pre existing subduction zones leading to episodic formation of the neotethyan ssz ophiolites and rapid exhumation of metamorphic rocks in subduction zones the formation events of neotethyan ssz ophiolites and associated metamorphic rocks represent snapshots of global plate reorganizations during the major episodes of supercontinent breakup

petrology and geochemistry of the island of sarigan in the mariana arc calc alkaline volcanism in an oceanic setting isotopic studies of rocks from oceanic island arcs such as the marianas indicate that little if any recycling of continental material e g oceanic sediments occurs in these arcs because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs an important question is whether the andesites of continental arcs are produced by a fundamentally different more complex mechanism than the lavas of oceanic arcs an excellent opportunity for study of this question is provided by the island of sarigan in the mariana active arc on which calc alkaline andesites including hornblende bearing types are exposed along with more mafic lavas

rifting history of the northern mariana trough seamarc ii and seismic reflection surveys seamarc ii sidescan sonar data and seismic reflection profiles of the northernmost portion of the mariana trough suggest a rifting rather than a seafloor spreading origin for lithosphere within this active backarc basin of the western pacific variations in morphology along the strike of the backarc basin suggest both a spatial and a temporal variation in the style of extension of the basin the northern mariana trough widens from 70 km near 23 n to 135 km near 21 30 n major active fault zones bound both sides of the backarc basin active rifting and volcanism are predominant in a roughly 20 km wide active rift zone along the eastern arc side of the basin from 24 n to 22 n the locus of active rifting has remained near the volcanic front of the arc during extension and is probably controlled by lithospheric weakness which is greatest at the volcanic front the zone of rifting intersects the active volcanic arc at nikko seamount the distinction between the mechanism for formation of the mariana trough proposed here and that of slow mid ocean ridge spreading is that no true plate boundary has developed in the northernmost mariana trough rather the backarc basin is composed of arc lithosphere intruded by arc and backarc basin magmas volcanism both in the rift zone and throughout the basin is fault controlled south of 22 n the zone of active rifting diverges westward from the arc as the transition to true seafloor spreading is approached the transition to true mid ocean ridge spreading occurs to the south of the survey area described here

seismicity and structure of akutan and makushin volcanoes alaska using joint body and surface wave tomography joint inversions of seismic data recover models that simultaneously fit multiple constraints while playing upon the strengths of each data type here we jointly invert 14 years of local earthquake body wave arrival times from the alaska volcano observatory catalog and rayleigh wave dispersion curves based upon ambient noise measurements for local vp vs and hypocentral locations at akutan and makushin volcanoes using a new joint inversion algorithm the velocity structure and relocated seismicity of both volcanoes are significantly more complex than many other volcanoes studied using similar techniques seismicity is distributed among several areas beneath or beyond the flanks of both volcanoes illuminating a variety of volcanic and tectonic features the velocity structures of the two volcanoes are exemplified by the presence of narrow high vp features in the near surface indicating likely current or remnant pathways of magma to the surface a single broad low vp region beneath each volcano is slightly offset from each summit and centered at approximately 7 km depth indicating a potential magma chamber where magma is stored over longer time periods differing recovery capabilities of the vp and vs data sets indicate that the results of these types of joint inversions must be interpreted carefully

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

origin and composition of carbonate buildups and associated facies in the fort payne formation lower mississippian south central kentucky an integrated sedimentologic and paleoecologic analysis an integrated sedimentologic paleoecologic and taphonomic analysis is used to test competing hypotheses for the origin of the fort payne formation of south central kentucky characteristics of eight distinct facies both carbonate and siliciclastic support the hypothesis that the lower mississippian fort payne formation is a progradational shallowing upward basin filling sequence five facies are considered in detail including the autochthonous fossitiferous green shale facies two autochthonous carbonate facies wackestone buildups and crinoidal packstone buildups and two transported carbonate facies channelform packstone facies and sheetlike packstone facies the two types of carbonate buildups are independent features but both developed on mounds of green fossiliferous shale the fort payne wackestone buildups are interpreted to be waulsortian buildups the fort payne waulsortian and associated facies were deposited in the lower portion of the photic zone which is phase d the shallowest phase of the lees and others 1985 waulsortian model the source of carbonate mud in the buildups must have been autochthonous all autochthonous facies had distinct faunas dominated by crinoids and bryozoans the channelform and sheetlike packstone facies are interpreted to be proximal and distal portions respectively of carbonate apron deposits both are composed principally of crinoid packstones and represent deeper water portions of the fort payne basin filling sequence the siltstone facies records background sedimentation