multiple sources of quaternary tephra layers in the mariana trough abstract tephra layers in quaternary sediments stolper and newman 1994 model of binary mixing of source components with one endmember being the nmorb source of stolper and newman 1994 and the other one being a compositionally variable slab derived h2o rich component similar to the h2o component of stolper and newman 1994 this interpretation implies that the variation in k2o content in the arc derived glasses about a factor of 3 at 60 wt sio2 reflects a magmatic diversity that is inherent to the central mariana arc and is not necessarily reflective of temporal changes of the composition of arc magma sources

contrasting arc and morb like assemblages in the paleoproterozoic flin flon belt manitoba and the role of intra arc extension in localizing volcanic hosted massive sulphide deposits the flin flon belt trans hudson orogen manitoba and saskatchewan is the largest paleoproterozoic volcanic hosted massive sulphide vms district in the world with 118 7 million tonnes mt of z

petrology and geochemistry of lavas from the sumisu and torishima backarc rifts thirteen dredge hauls from the active sumisu and torishima rift grabens west of the izu bonin arc at about 30 31 n 140 e recovered a suite of tholeiitic basalts to sodic rhyolites volcanism occurs along tensional faults within and bounding the rift grabens and along the transfer zones between adjoining rift segments the sumisu and torishima rift lavas differ significantly from the lavas of the adjacent arc volcanic centers in having loweral2o3 na2o ba zr v ti andba ce and higher abundances of the rare earth elements the rift lavas also have characteristics of backarc basin basalts in that they are enriched in al2o3 and depleted in total iron and tio2 relative to mid ocean ridge basalt characteristics which are consistent with a higher water content in the source thus the model of a progressive change in backarc basin basalt composition from arc like to mid ocean ridge like as a function of evolution of the basin as has been suggested from many backarc regions is not generally applicable the comparison of the sumisu and torishima rift lavas with mariana backarc basin lavas indicates that backarc basin basalts differ in composition from one basin to another the comparison of these backarc basin suites with mid ocean ridge suites from similar axial depths indicates that the overall control over the spectrum of backarc basin basalt compositions may be different extents of melting of the mantle the sumisu and torishima rift lavas formed by a slightly higher extent of melting than the mariana backarc basin basalts a phenomenon which is related to the depth of the ridge segments furthermore the data suggest a systematically higher extent of melting in the arc lavas than in the backarc lavas for both of these arc backarc systems consistent with a greater flux of water beneath the arc than beneath the backarc region

geological modelling from field data and geological knowledge part ii modelling validation using gravity and magnetic data inversion the analysis of multiple data sets is required to select a realistic 3d geological model among an infinite number of possibilities an inverse method that aims at describing the 3d geometry of geological objects is presented the method takes into account the geology and the physical properties of rocks while respecting the topological properties of an a priori model the a priori model is built from the geological data set and its geometry is largely dependent upon assumptions about inaccessible geology at depth this method referred to as total litho inversion is a generalized 3d inversion that results in quantifying the lithology and the distribution of rock property in a probabilistic way its application is demonstrated through i a simple synthetic case and ii the relative distribution characterization of granites and diorites in an orogenic domain

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

benthic nutrient recycling in port phillip bay australia benthic chamber measurements of the reactants and products involved with biogenic matter remineralization oxygen ammonium nitrate nitrite phosphate silicate tco2and alkalinity were used to define solute exchange rates between the sediment and overlying water column of port phillip bay australia measurements at various sites throughout the bay conducted during the summers of 1994 and 1995 indicate that the variability in flux values within a site is comparable to year to year variability 50 four regions of the bay were distinguished by sediment properties and the northern region was identified as having 3 30 times greater nutrient regeneration rates than the other regions benthic recycling accounted for 63 and 72 of the annualized n and p input respectively to the entire bay as determined by summing benthic dissolved riverine atmospheric and dissolved effluent sources however bay wide sedimentary denitrification accounted for a loss of 63 of the potentially recyclable n this fraction is higher than many other coastal regions with comparable carbon loading denitrification efficiency is apparently not enhanced by benthic productivity nor by bio irrigation the rate of bio irrigation is negatively correlated with denitrification efficiency bio irrigation was studied using radon 222 and cscl spike injection chamber measurements radon fluxes from sediments in port phillip bay were enhanced over the diffusive flux by 3 16 times the modelled rate of loss of cs from chamber water was positively correlated with radon flux enhancement results both methods identify regions within port phillip bay that have particularly high rates of non diffusive pore water overlying water solute exchange

nitrogen sources and recycling at subduction zones insights from the izu bonin mariana arc 1 we report new chemical and nitrogen isotopic data from 29 volcanic and hydrothermal gas samples covering eight centers in the izu bonin mariana ibm arc to investigate the sources flux and mass balance of nitrogen at a cool convergent margin the majority of samples have high n2 he 1217 17 300 and low co2 n2 exc 78 937 implying addition of nitrogen from the subducting slab this inference is supported by positive i e sediment like 15n values up to 5 5 in most samples the exception to these trends is agrigan in the mariana arc with low n2 he 200 high co2 n2 exc 1500 and negative 15n mixing calculations suggest an average of 34 of the nitrogen in our samples is derived from subducted sediment or 75 after correction for atmospheric contamination sediment derived n2 fluxes estimated by three different methods range from 0 25 108 to 1 11 108 mol yr 1 n2 representing 4 17 of the total nitrogen input flux or 11 51 of the sedimentary nitrogen input flux the altered oceanic crust is identified as an important contributor to the arc nitrogen budget and the 15n of the residual nitrogen subducted into the mantle is estimated at approximately 1 9 despite similarities in gas chemistry and 15n values our conclusions regarding nitrogen recycling for ibm are markedly different than those for the nicaraguan segment of the central american arc and we suggest that thermal regime is the major control on nitrogen recycling within subduction zones the global nitrogen cycle is estimated to be in steady state suggesting either that subducted sediments are an unlikely source for heavy nitrogen in plume related rocks or secular variation in the isotopic composition of subducted sediments better constraints on nitrogen recycling at other arcs are required to test these conclusions

aspects of the marine nitrogen cycle of the chukchi sea shelf and canada basin abstract as a highly productive seasonally ice covered sea with an expansive shallow continental shelf the chukchi sea fuels high rates of sedimentary denitrification this contributes to its fixed nitrogen n deficit relative to phosphorus p which is among the largest in the global ocean making the chukchi sea severely n limited during the phytoplankton growth season here we examine aspects of the n cycle on the chukchi sea shelf and the downstream canada basin using nutrients dissolved oxygen o 2 and the stable isotopes of nitrate no 3 in the northward flow path across the chukchi shelf bottom waters experienced strong o 2 drawdown from which we calculated a nitrification rate of 1 3 mmol m 2 d 1 this nitrification was likely primarily in sediments and directly fueled sedimentary denitrification historically measured at similar rates we observed significant accumulations of ammonium nh 4 in bottom waters of the chukchi shelf up to 5 m which were inversely correlated with 15 n no 3 indicating a sediment source of 15 n enriched nh 4 this is consistent with a process of coupled partial nitrification denitrification cpnd which imparts significant 15 n enrichment and 18 o depletion to pacific origin no 3 this cpnd mechanism is consistent with a significant decrease in 18 o no 3 relative to bering sea source waters indicating that at least 58 of no 3 populating the pacific halocline was regenerated during its transit across the north bering and chukchi shelves rather than arriving preformed from the bering sea slope this pacific origin no 3 propagates into the canada basin and towards the north atlantic being significantly 15 n enriched and 18 o depleted relative to the underlying atlantic waters

the late archaean melita complex eastern goldfields western australia shallow submarine bimodal volcanism in a rifted arc environment abstract the melita volcanic complex is a late archaean bimodal rhyolite basalt volcanic succession within the gindalbie terrane in the eastern goldfields province of the yilgarn craton the melita complex has been dated by ion probe at 2683 3 ma 95 and forms part of a distinctive 2681 2692 ma volcanic association that records bimodal basalt rhyolite and calc alkaline intermediate silicic volcanism at several discrete volcanic centres and which locally hosts volcanic massive sulphide mineralisation teutonic bore approximately 3 km of stratigraphic thickness is exposed in the melita area the upper 1 1 5 km of the exposed succession is dominated by subaqueously resedimented volcaniclastic sandstones and breccias rhyolite flows and sills primary subaerial pyroclastic deposits including ignimbrites have not been identified in this study although subaerial explosive activity is indicated by the occurrence of accretionary lapilli and the abundance of vitric material shards and pumice fragments in resedimented deposits the lower part of the succession is dominated by pillowed to massive basalt lavas and in situ and resedimented mafic hyaloclastites mafic extrusive and intrusive rocks are tholeiitic with trace element concentrations similar to modern arc tholeiites felsic volcanic rocks at melita are dacite to high silica rhyolite they are highly enriched in incompatible elements particularly high field strength element enriched compared to other felsic associations in the eastern goldfields province representing evolved partial melts of heterogeneous intermediate arc type crust the volcanic facies and geochemistry of volcanic rocks at melita are consistent with those observed in modern intra arc or arc rift settings and the succession is interpreted to represent the initial stages of back arc rifting within a complex convergent margin

ree patterns nd sm and u pb ages of the metamorphic rocks of the diagorou darbani greenstone belt liptako sw niger implication for birimian palaeoproterozoic crustal genesis

three dimensional crustal structure in central taiwan from gravity inversion with a parallel genetic algorithm the genetic algorithm method is combined with the finite element method for the first time as an alternative method to invert gravity anomaly data for reconstructing the 3d density structure in the subsurface the method provides a global search in the model space for all acceptable models the computational efficiency is significantly improved by storing the coefficient matrix and using it in all forward calculations then by dividing the region of interest into many subregions and applying parallel processing to the subregions central taiwan a geologically complex region is used as an example to demonstrate the utility of the method a crustal block 120 150 km 2 in area and 34 km in thickness is represented by a finiteelement model of 76 500 cubic elements each 2 2 2 km 3 in size an initial density model is reconstructed from the regional 3d tomographic seismic velocity using an empirical relation between velocity and density the difference between the calculated and the observed gravity anomaly i e the residual anomaly shows an elongated minimum of large magnitude that extends along the axis of the taiwan mountain belt among the interpretive models tested the best model shows a crustal root extending to depths of 50 to 60 km beneath the axis of the western central and eastern central ranges with a density contrast of 400 or 500 kg m 3 across the moho both predictions appear to be supported by independent seismological and laboratory evidence

three dimensional magnetic imaging of the chicxulub crater although few magnetization measurements are available for the structural elements of the chicxulub impact crater the magnetization intensities of the melt sheet upper breccia unit and central uplift are 3 4 orders of magnitude greater than the 3 to 4 km thick carbonate and evaporite stratigraphy covering the yucatan block this allows three dimensional modeling of the crater s structure by inversion using a two layer model two layers are separately inverted by dividing the crater s magnetic field expression into 40 km wavelength components the upper layer average depth 2 km models the distribution of highly magnetized zones in the crater s melt sheet the lower layer average depth 5 km represents relief on the yucatan block s basement surface and effectively maps the crater s 50 km diameter central uplift and possibly the expression of the surrounding collapsed disruption cavity fill the shallower magnetized zones consist of two generally concentric distributions at radii of 20 and 45 km these highly magnetized zones are thought to result from hydrothermal systems localized at the edge of the central uplift and the collapsed disruption cavity having produced magnetic phases during alteration of the melt sheet

early diagenesis and accumulation of redox sensitive elements in east siberian arctic shelves abstract the arctic shelf makes up half of the area within the arctic ocean it plays an important role in the global carbon cycle whilst its part in the recycling process of redox sensitive elements rses is still not clear this study has selected nine sediment cores 30 to 40 cm in length covering a wide area over the east siberian arctic shelves esas and analyzed a suite of rses s mn mo u v cd in the sediments along with al fe total organic carbon toc grain size and sedimentation rate the redox environment in surface sediments was clearly divided based on the s and mn contents of the cores ranging from being more oxidized on the western east siberian sea shelf to more reduced on the chukchi sea shelf toc also showed an increasing trend eastward in the study region statistically significant correlations were found between cd and s v and fe mo and mn and u and toc implying their close relationships during early diagenetic processes at the sediment water interface across the study area authigenic accumulation occurred for all rses except mn the estimated authigenic accumulation fluxes for mo u v and cd over the entire esas are 0 21 0 09 0 086 0 043 11 0 9 2 and 0 019 0 014 108 mol y 1 respectively which are comparable to values reported in suboxic marginal sediments for mo u and cd but exceptionally high for v although the sampling resolution was coarse and there are uncertainties in the sedimentation rates and detrital background values of v we propose the authigenic accumulation flux for v on the broad and shallow esas may be a significant but missing sink term for the global v cycle

physical volcanology and geochemistry of a late archaean volcanic arc kurnalpi and gindalbie terranes eastern goldfields superterrane western australia abstract late archaean 2715 2704 ma calc alkaline volcanic complexes in the kurnalpi terrane are identical in facies and geochemistry to modern intra arc volcanic complexes preserved volcanic rocks and associated feldspathic sedimentary rocks represent proximal to medial submarine facies of submarine to locally emergent intra oceanic arc related volcanoes the least altered volcanic rocks are dominantly andesite but range from basalt and basaltic andesite to rare rhyolite and are the product of magmas similar in composition to those of modern arcs intermediate magmas were most likely derived from large ion lithophile element lile enriched mafic magmas by fractional crystallisation with or without assimilation processes the kurnalpi terrane thus provides an example of a late archaean volcanic arc in which hydrated mantle wedge melting was the main mechanism for magma genesis ttd suite volcanism derived from slab melting was either absent or played a subordinate role the adjacent gindalbie terrane contains younger 2692 2680 ma bimodal basalt rhyolite volcanic complexes with associated quartz rich sedimentary rocks and calc alkaline intermediate silicic volcanic complexes volcanosedimentary basement to the gindalbie volcanic sequences is deeply eroded equivalents of the calc alkaline volcanic complexes their associated sedimentary rocks and younger basaltic sequences found in the kurnalpi terrane gindalbie basalt rhyolite successions locally contain volcanic hosted massive cu zn mineral deposits rhyolites derived by low pressure fractional crystallisation or melting of andesites are more abundant than in the kurnalpi volcanic complexes facies associations and the local occurrence of distinctive incompatible element enriched mildly peralkaline a type rhyolites are consistent with rifting of a mature arc system the two terranes most likely represent volcanic evolution of the same late archaean volcanic arc but are different tectonic fragments that were welded together by 2660 ma

petrology of mariana trough and lau basin basalts the crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid ocean ridge basalts n morb but some subtle but distinctive differences exist these are largely due to differences in volatiles fe oxidation ratio abundances of large ion lithophile elements lil and some high field strength hfs elements isotope data for sr pb nd he and o are variable but broadly are morb like we present data contrasting mariana trough mtb and lau basin basalts lbb showing how they differ in certain trace element and isotopic signatures and compare both to n morb relative to n morb and lbb the mtb are enriched in alkalis alkaline earths light rare earth elements ree h2o and fe fe even for primitive samples while having morb type he 3 4 7 8 to 8 4 ra sr 87 86 mean 0 70293 and nd e 10 2 the lau basin is zoned with a central area having n morb characteristics for h2o lil ree nd e 8 1 to 9 0 and o but a depletion in hfs elements sr is more radiogenic mean 87 86 0 70347 and 3he 4he 3 4 7 2 to 11 0 ra is high relative to n morb the lau basin has a zone bordering the adjacent island arcs which is like mtb in both back arc basins relatively primitive unfractionated basalts with mg 100 mg fe 62 are found which resemble postulated primitive n morb calculated parental magmas obtained by adding xenocrystic olivine plot near or on 3 and 4 phase boundaries at 1 1 5 gpa for the ol di si system low p fractional crystallization and magma mixing explain most of the observed rock types possible explanations for differences between mtb and lbb include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere the long and complex history of mariana trough mantle sources led to a metasomatized mantle which gives rise to the distinctive mtb in contrast the lau basin is dominated by basalts derived from a source depleted in volatiles lil and hfs elements a previous melting history is implied mixing with a mantle component enriched in 87sr and 3he may be due to mantle counterflow driven by the subduction process

seasonal and spatial patterns of sedimentary denitrification rates in the chukchi sea abstract recent constructions of the global nitrogen budget estimate that at least half of the ocean s fixed nitrogen is lost by sedimentary denitrification the majority of which occurs in continental shelves the arctic contains approximately 20 of the world s continental shelf suggesting it is a substantial contributor to the global sedimentary denitrification rate during two cruises in the summer and spring of 2002 and 2004 respectively denitrification rates were calculated from the downward diffusive flux of nitrate in the shelf and slope sediments of the chukchi sea in the western arctic additionally in the spring of 2004 denitrification rates were determined by whole core incubations in which the flux of nitrogen gas out of the sediments was measured measurements were made along three transects crossing the shelf and slope 50 3000 m each transect having different overlying water characteristics denitrification rates generally decreased with increasing water depth rates varied from about 1 6 mmol n m 2 d 1 for the shallow water sediments to undetectable in deep water sediments rates showed little variation between the two seasons however rates were found to correspond with differences in annual overlying primary productivities and particulate organic carbon export fluxes an extrapolation to the whole arctic yielded an average arctic sedimentary denitrification rate of 13 tg n yr 1 taken in the context of the global nitrogen budget it is about 4 13 of the total sink of fixed nitrogen in the ocean

3 d constrained inversion of gravimetric data to map the tectonic terranes of southeastern laurentia using simulated annealing abstract inversion of gravity anomaly data can be an effective tool in reconstructing the geologic history of a region by segregating tectonic terranes with different physical properties recent advances in computational capabilities coupled with lithospheric scale seismic experiments provide the opportunity to conduct 3d regional scale constrained inversions of gravity data using global optimization algorithms here a method for inverting gravity anomaly data by combining a finite element method based forward calculation with a simulated annealing algorithm is introduced the method is applied to the southeastern laurentia region which is comprised of an amalgamation of crustal blocks spanning a tectonic history of over 1 5 ga the method better defines the gross crustal density distribution of the region and proves to be efficient in spatially discriminating between the different tectonic terranes the tectonic terranes fall within four distinct groups based on the average crustal density each of which seem to have undergone a distinct geologic history

the pyrenean architecture as revealed by teleseismic p to s converted waves recorded along two dense transects between 2011 and 2013 two dense transects were deployed across the central and western pyrenees to get better constraints on the deep lithospheric architecture and discriminate the competing models of the structure and formation of the pyrenees each transect recorded the regional and global seismicity during a period of approximately 1 yr here we exploit the records of teleseismic compressional waves and of their conversions to shear waves on internal discontinuities in order to map lithospheric interfaces beneath the two transects the migrated sections obtained by performing common conversion point stacks are in remarkable agreement with the results of the ecors pyrenees and ecors arzacq deep seismic surveys however the migrations of converted waves reveal new details of the deep lithospheric architecture that could not be seen with the active source experiments the new images provide clear and definite evidence for the subduction of a thinned iberian crust down to at least 70 km depth a result that has important implications for the formation of the pyrenees the subduction of the iberian lithosphere leads to reconsider the amount of convergence between iberia and europe during the cenozoic a recent regional p wave tomography relying on the data of the pyrope and iberarray temporary experiments revealed the segmentation of lithospheric structures by inherited hercynian ne sw transfer faults that were reactivated during the albian rifting our migration images are consistent with this model and give further support to the idea that the pyrenees were produced by the tectonic inversion of a segmented hyperextended rift that was buried by subduction beneath the european plate

backarc volcanism along the en echelon seamounts the enpo seamount chain in the northern izu ogasawara arc 1 we report results of detailed petrological analyses of volcanic rocks from the enpo seamount chain that reveal the characteristics of volcanism in a backarc en echelon seamount chain in an oceanic island arc setting we identified two types volcanic rock suites in the enpo seamount chain based on bulk rock trace element chemistry and compositions of chrome spinels included in olivine phenocrysts of primitive basalts more enriched suites mes have enriched hfse compositions such as nb ta higher nb zr values and low cr in spinel less enriched suites les have depleted hfse compositions lower nb zr values and high cr in spinel these results require distinct mantle sources with different for the volcanic rocks constituting the enpo seamount chain mineralogical and geochemical analyses show that petrological variations within les lavas are explained by fractionation with open system magma mixing in contrast variations between mes lavas require different mechanisms of magma genesis because petrological variation within this suite cannot be explained by fractionation mes basalts also exhibit mineralogical evidence for magma mixing volcanism on the enpo seamount chain is characterized by complex spatial and temporal relationships between mes lavas consisting of subalkaline basalt alkaline andesite and hornblende water rich andesite and les lavas consisting of subalkaline basalt and fractionated andesite we suggest that these features are characteristic of variability within magma systems that feed backarc seamount chains in oceanic island arcs

plumes in central panthalassa deductions from accreted oceanic fragments in japan accretion of a large number of seamounts or fragments of huge oceanic plateaus is recorded in the geology of japan major accretion occurred twice a late early carboniferous seamount chain accreted in latest permian to middle jurassic time and a late jurassic oceanic plateau accreted in the early cretaceous the late jurassic plateau now in northern japan and sakhalin is here named the sorachi plateau and a tithonian age for it is well documented on the basis of microfossils from pelagic cherts and limestones petrochemical characteristics of basalts suggest two kinds of sources depleted mantle under oceanic lithosphere and enriched mantle related to plume upwelling all the accreted fragments in japan can be traced back to the central part of the panthalassa by using paleomagnetic data and plate trajectory analysis the timing of generation of the accreted seamount chain and the plateau is consistent with a time of global change their place of origin in the mid pan thalassa retains a positive residual geoid anomaly and slow vp in the lower mantle which suggest a past upwelling of hot material from the core mantle boundary

evolution and interaction of migrating cross arc volcanism and backarc rifting an example from the southern havre trough 35 20 37 s the active southern havre trough 35 20 37 s backarc basin is interpreted to form from the evolution and interaction of migrating cross arc magmatism and the progressive development of longitudinal rift grabens the proposed migration of the proto kermadec arc front from the colville arc at 5 ma to the active kermadec arc margin is recorded by the construction of arc edifices trailed across the intervening and contemporaneously rifting backarc complex migration trails are identified for at least four arc volcano sources the rumble v arc migration trail is the most prominent forming a continuous high standing magmatic arc ridge these postulated arc trails segment the backarc region and initially limit rift development to the intervening proto arc crust blocks early rifting between the arc volcanoes forms fully developed rift grabens which with progressive basin widening propagate longitudinally across the trails of migrating constructional arc magmatism a model of the balance between rates of constructional arc magma production mr and destructive back arc rifting extension ve is proposed when ve is high mr is insufficient to keep abreast of destructional rifting resulting in small isolated arc massifs quickly dismembered by rifting conversely when ve is low mr is sufficiently greater than rifting to produce a continuous high standing cross arc ridge which segments longitudinal rift development migrating arc magmatism will be best observed and preserved in rifting backarc basins when mr is 600 km3 m y 1 100 km 1 of plate boundary and ve is 25 mm a 1

distribution abundance biomass and diversity of benthic infauna in the northeast chukchi sea alaska relation to environmental variables and marine mammals abstract in summer 2009 and 2010 as part of chukchi sea offshore monitoring in drilling area chemical and benthos comida cab program we performed a quantitative assessment of the biomass abundance and community structure of benthic infaunal populations of the northeastern chukchi sea this analysis documented a benthic species inventory of 361 taxa collected from 142 individual van veen grab samples 0 1 m 2 at 52 stations infaunal abundance was dominated by polychaeta mollusca and crustacea large concentrations of bivalves up to 1235 m 2 920 2 gww m 2 were collected south of hanna shoal where flow from two water masses converge and deposit labile carbon to the seafloor as indicated by low surface sediment c n ratios amphipods up to 1640 m 2 26 0 gww m 2 and polychaetes up to 4665 m 2 114 7 gww m 2 were documented from multiple stations west of and within barrow canyon this high productivity was most likely due to the canyon effect where marine and coastal detrital carbon supplies are channeled by the canyon structure enhancing carbon deposition and flux which supports rich benthic communities within the canyon and surrounding areas to examine the relationships between infaunal distributions of all collected taxa with the physical environment we used a biota and environment matching bio env routine a combination of water depth bottom water temperature and salinity surface sediment total organic nitrogen ton and sediment c n molar ratios correlated closest with infaunal abundance distribution 0 54 indicating that multiple factors influence the success of benthic communities bio env routines produced similar correlation results when performed on targeted walrus prey items bivalves 0 50 polychaetes 0 53 but gray whale prey items amphipods were not strongly correlated to any combination of physical environmental factors 0 24 distributions of primary prey items for gray whales amphipods and walruses bivalves gastropods and polychaetes were compared with gray whale and walrus distribution as described by sightings from the 2009 and 2010 aerial survey component of comida in general concentrations of walruses and their prey occurred in a swath located south of hanna shoal and on the shoal itself although the large differences in sea ice distribution between the two study years showed that walrus distributions were closely linked to sea ice location other areas within barrow canyon and the shelf west of the canyon showed high concentrations of benthic amphipods that were coincident with gray whale sightings as quantified by comida aerial surveys overall data collected on this project indicate that the northeast chukchi sea supports a highly productive and diverse benthic ecosystem that is of significant importance to higher trophic level megafauna

an archean oceanic felsic dyke swarm in a nascent arc the hunter mine group abitibi greenstone belt canada abstract the 2730 ma old hunter mine group hmg a dominantly felsic subaqueous volcanic sequence was formed during early arc construction in the abitibi greenstone belt quebec canada the western part of the hmg contains a felsic dyke swarm up to 1 5 km wide and traceable up section for 2 5 km five distinct generations were identified 1 aphanitic to feldspar phyric dykes 2 quartz feldspar phyric dykes with nd values suggest that an older sialic substrate was not involved in their genesis we infer that the felsic rocks were generated by melting of mafic oceanic crust the swarm was emplaced during nascent oceanic island arc development and was related to rifting of the arc the conformably overlying morb like basalts and basaltic komatiites of the stoughton roquemaure group used the same conduits and further indicate splitting of the arc hmg and associated parts of the abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western pacific

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

alvin seabeam studies of the sumisu rift izu bonin arc abstract bimodal volcanism normal faulting rapid sedimentation and hydrothermal circulation characterize the rifting of the izu bonin arc at 31 n analysis of the zigzag pattern in plan view of the normal faults that bound sumisu rift indicates that the extension direction 080 10 is orthogonal to the regional trend of the volcanic front normal faults divide the rift into an inner rift on the arc side which is the locus for maximum subsidence and sedimentation and an outer rift further west transfer zones that link opposing master faults and or rift flank uplifts further subdivide the rift into three segments along strike volcanism is concentrated along the ene trending transfer zone which separates the northern and central rift segments the differential motion across the zone is accommodated by interdigitating north trending normal faults rather than by ene trending oblique slip faults volcanism in the outer rift has built 50 700 m high edifices without summit craters whereas in the inner rift it has formed two multi vent en echelon ridges the largest is 600 m high and 16 km long the volcanism is dominantly basaltic with compositions reflecting mantle sources little influenced by arc components an elongate rhyolite dome and low temperature hydrothermal deposits occur at the en echelon step in the larger ridge which is located at the intersection of the transfer zone with the inner rift the chimneys veins and crusts are composed of silica barite and iron oxide and are of similar composition to the ferruginous chert that mantles the kuroko deposits a 1 2 km transect of seven alvin heat flow measurements at 30 48 5 n showed that the inner rift bounding faults may serve as water recharge zones but that they are not necessarily areas of focussed hydrothermal outflow which instead occurs through the thick basin sediments the rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine arc calderas such as sumisu and south sumisu volcanoes

pathways of organic carbon oxidation in three continental margin sediments we have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of denmark and norway rates of nh4 and sigma co2 liberation sediment incubations were used with o2 penetration depths to conclude that o2 respiration accounted for only between 3 6 17 4 of the total organic carbon oxidation dentrification was limited to a narrow zone just below the depth of o2 penetration and was not a major carbon oxidation pathway the processes of fe reduction mn reduction and sulfate reduction dominated organic carbon mineralization but their relative significance varied depending on the sediment where high concentrations of mn oxide were found 3 4 wt mn only mn reduction occurred with lower mn oxide concentrations more typical of coastal sediments fe reduction and sulfate reduction were most important and of a similar magnitude overall most of the measured o2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon we suspect that the importance of o2 respiration in many coastal sediments has been overestimated whereas metal oxide reduction both fe and mn reduction has probably been well underestimated

petrology structural setting timing and geochemistry of cretaceous volcanic rocks in eastern mongolia constraints on their tectonic origin abstract cretaceous volcanism occurred over a wide region of eastern asia hypotheses for its development include intra continental rifting continental arc magmatism plume activities and delamination of a previously thickened mantle lithosphere one of the major obstacles in differentiating the above model is the lack of systematic studies of cretaceous volcanic rocks in eastern mongolia and far east russia in this study we address this issue by examining the timing and geochemistry of cretaceous volcanic rocks in eastern mongolia our 40 ar 39 ar dating of volcanic rocks together with the existing k ar ages of volcanic rocks indicate that cretaceous volcanism in our study area occurred between 120 ma and 104 ma our field investigation and geochemical analysis reveal three types of volcanic rocks in the study area 1 a bimodal series 2 a shoshonite series and 3 a basanite series geochemical analysis of the volcanic rocks indicates a highly heterogeneous source which is characterized by a mixture of depleted enriched and metasomatized mantle at least three models may explain the timing geochemistry and regional tectonic setting of cretaceous volcanism in eastern mongolia 1 partial delamination of a previously thickened mantle lithosphere generated by arc magmatism and collision tectonics 2 back arc extension and its related mantle upwelling due to rapid collapse of a flat subduction slab and 3 mantle avalanche of a thick pile of cold subducted paleo asian oceanic plates initially trapped at the 670 km endothermic phase transition triggered by westward pacific subduction causing mixing of several mantle sources testing the above models require further studies on the spatial and temporal relationships between cretaceous volcanism and structural history of ne asia in addition a careful restoration of the position and history of past subducted slabs of the paleo asian oceans may help differentiate the above competing models in the future

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

arctic shelves as platforms for biogeochemical activity nitrogen and carbon transformations in the chukchi sea alaska abstract continental shelves comprise 50 of marine denitrification the hanna shoal region part of the continental shelf system in the northeast chukchi sea alaska is recognized for its high biodiversity and productivity we investigated the role of sediments in organic matter decomposition and nutrient cycling at five stations on the shallow hanna shoal in particular we asked 1 how much sediment organic matter is remineralized in the chukchi sea and what factors drive this degradation 2 do sediments function as a net source for fixed nitrogen thus fueling primary production in the overlying water or as a net sink for fixed nitrogen thereby removing it from the system and 3 what is the balance between sediment nh4 uptake and regeneration and what factors drive nh4 cycling we conducted dark sediment core incubations to measure sediment o2 consumption net n2 and nutrient nh4 no3 no2 po43 fluxes and rates of sediment nh4 cycling including uptake and regeneration rates of sediment o2 consumption and nh4 and po43 efflux suggest that high organic matter remineralization rates occurred in these cold 2 c sediments we estimated that total organic carbon remineralization accounted for 20 57 of summer export production measured on the chukchi shelf net n2 release was the dominant nitrogen flux indicating that sediments acted as a net sink for bioavailable nitrogen via denitrification organic carbon remineralization via denitrification accounted for 6 12 of summer export production which made up 25 of the total organic carbon oxidized in hanna shoal sediments these shallow productive arctic shelves are hotspots for organic matter remineralization

pre spread rifting and heterogeneous volcanism in the southern havre trough back arc basin abstract the havre trough is an active back arc basin associated with pacific australian plate convergence two segments within the southern havre trough sht surveyed with gloria sea beam and sys09 swath data now provide the first detailed data to reliably document the structure and morphology of this complex back arc region at the southern segment 35 40 37 s along a strike length of some 120 km the sht structure comprises a series of segmented en echelon generally flat floored sediment filled axial rift grabens flanked by ridge and basin rift block topography prominent fault escarpments with relief of 500 1000 m and a marked zig zag arrangement mark the outer margins of the rift system within the rift grabens extensional tectonism is pervasive extensive shallow seismicity within the sht can only partly be associated with the rift graben system at present youthful constructional volcanic terrains within the rift grabens are not common and invariably have a restricted areal extent and a relief an identical rift morphology has been identified at the northern 33 34 s site data from both sites show individual rifts are 15 35 km in length 6 10 km wide and have axial depths of 3100 4000 m apart from at least one 15 20 km wide cross trough ridge available data indicate the rift graben system is contiguous between 33 s and 37 s this rift graben morphology by analogy with other known rifting back arc basins is considered here not to be consistent with true oceanic back arc spreading rather it is argued that the sht possibly indeed all of the havre trough is undergoing back arc rifting prior to true spreading further the rifting tectonic fabric within the sht and in the onshore taupo to the south extension is consistently dextrally oblique relative to the bounding colville and kermadec ridges the degree of obliquity increases from south to north if accepted that the sht is rifting rather than spreading magnetic anomaly data within the trough do not record the accretion of new oceanic crust and consequently can no longer be used to establish the spreading history of the sht the sht magnetic anomalies are here interpreted as pseudo linear magnetic anomalies resulting from the generally irregular spatial and temporal emplacement of magnetic sheeted lava and dike intrusives between older low magnetisation arc basement rift blocks flanking the axial rift proxy data from onshore north island new zealand and offshore sht suggest that the rate of extension and age of initiation of back arc rifting are between 35 and 37 s respectively 15 20 mm yr 1 and 5 ma

structural development of sumisu rift izu bonin arc geophysical swath mapping multichannel seismic profiling and ocean drilling data are used to document the structural evolution of sumisu rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc the 120 km long 30 50 km wide sumisu rift is bounded to the north and south by structural and volcanic highs west of the sumisu and torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes the zig zag pattern of normal faults average strikes 337 and 355 indicates extension oriented 076 10 orthogonal to the volcanic arc three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift subsidence patterns differential strain across the transfer zones is accommodated by interdigitating rift parallel faults and sometimes by cross rift volcanism rather than by strike or oblique slip faults from estimates of extension 2 5 km the age of the rift 2 ma and the accelerating subsidence we infer that sumisu rift is in the early synrift stage of back arc basin formation following an early sag phase half graben formed with synthetically faulted structural rollovers facing large offset 2 2 5 km throw border fault zones in the three northern rift segments the largest faults are on the arc side and dip 60 75 w whereas in the southern segment they are on the west side and dip 25 50 e the present full graben stage is dominated by hanging wall antithetic faulting basin widening by footwall collapse and a concentration of subsidence in an inner rift the hanging wall collapses but not necessarily as a result of border fault propagation from adjacent rift segments whereas the border faults may penetrate the theologically weak lithosphere te 3 km many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor back arc volcanism usually erupted along faults occurs in the rift and along the protoremnant arc during both stages where drilled the arc margin has been uplifted 1 1 0 5 km concurrently with 1 1 km of rift basin subsidence extremely high sedimentation rates up to 6 m kyr in the inner rift have kept pace with synrift faulting created a smooth basin floor and resulted in sediment thicknesses that mimic the differential basin subsidence a linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting rifts are better developed between the arc edifices intrusions may be accommodating extensional strain adjacent to the arc volcanoes no obvious correlations are observed between the rift structures and preexisting cross arc trends

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

compositional variations of chromite and solid inclusions in ophiolitic chromitites from the southeastern turkey implications for chromitite genesis abstract chromitites associated with upper mantle peridotites of ophiolites from southeastern turkey are lenticular bodies or veinlets made up of massive banded and disseminated chromite chromite shows a wide range of cr cr cr al varying between 0 42 and 0 81 and mg mg mg fe2 of 0 45 to 0 75 geochemically they are divided into two compositional groups such as high cr cr 0 7 and high al cr

supra subduction zone extensional magmatism in vermont and adjacent quebec implications for early paleozoic appalachian tectonics metadiabasic intrusions of the mount norris intrusive suite occur in faultbounded lithotectonic packages containing stowe moretown and cram hill formation lithologies in the northern vermont rowe hawley belt a proposed ordovician arc trench gap above an east dipping subduction zone rocks of the mount norris intrusive suite are characteristically massive and weakly foliated have chilled margins contain xenoliths and have sharp contacts that both crosscut and are parallel to early structural fabrics in the host metasedimentary rocks although the mineral assemblage of the mount norris intrusive suite is albite 1 actinolite 1 epidote 1 chlorite 1 calcite 1 quartz intergrowths of albite 1 actinolite are probably pseudomorphs after plagioclase 1 clinopyroxene the metadiabases are subalkaline tholeiitic hypabyssal basalts with preserved ophitic texture a backarcbasin tectonic setting for the intrusive suite is suggested by its lree light rare earth element enrichment negative nbta anomalies and ta yb vs th yb trends although no direct isotopic age data are available the intrusions are broadly ordovician because their contacts are clearly folded by the earliest acadian silurian devonian folds field evidence and geochemical data suggest compelling along strike correlations with the coburn hill volcanics of northern vermont and the bolton igneous group of southern quebec isotopic and stratigraphic age constraints for the bolton igneous group bracket these backarc magmas to the 477 458 ma interval a tectonic model that begins with east dipping subduction and progresses to outboard west dipping subduction after a syncollisional polarity reversal best explains the intrusion of deformed metamorphosed metasedimentary rocks by backarc magmas

three dimensional full waveform inversion of short period teleseismic wavefields based upon the sem dsm hybrid method we present a method for high resolution imaging of lithospheric structures based on full waveform inversion of teleseismic waveforms we model the propagation of seismic waves using our recently developed direct solution method spectral element method hybrid technique which allows us to simulate the propagation of short period teleseismic waves through a regional 3 d model we implement an iterative quasi newton method based upon the l bfgs algorithm where the gradient of the misfit function is computed using the adjoint state method compared to gradient or conjugate gradient methods the l bfgs algorithm has a much faster convergence rate we illustrate the potential of this method on a synthetic test case that consists of a crustal model with a crustal discontinuity at 25 km depth and a sharp moho jump this model contains short and long wavelength heterogeneities along the lateral and vertical directions the iterative inversion starts from a smooth 1 d model derived from the iasp91 reference earth model we invert both radial and vertical component waveforms starting from long period signals filtered at 10 s and gradually decreasing the cut off period down to 1 25 s this multiscale algorithm quickly converges towards a model that is very close to the true model in contrast to inversions involving short period waveforms only which always get trapped into a local minimum of the cost function

three dimensional gravity anomaly inversion in the pyrenees using compressional seismic velocity model as structural similarity constraints we explore here the benefits of using constraints from seismic tomography in gravity data inversion and how inverted density distributions can be improved by doing so the methodology is applied to a real field case in which we reconstruct the density structure of the pyrenees along a southwest northeast transect going from the ebro basin in spain to the arzacq basin in france we recover the distribution of densities by inverting gravity anomalies under constraints coming from seismic tomography we initiate the inversion from a prior density model obtained by scaling a pre existing compressional seismic velocity vp model using a nafe drake relationship the vp model resulting from a full waveform inversion of teleseismic data gravity data inversions enforce structural similarities between vp and density by minimizing the norm of the cross gradient between the density and vp models we also compare models obtained from 2 5d and 3d inversions our results demonstrate that structural constraints allow us to better recover the density contrasts close to the surface and at depth without degrading the gravity data misfit the final density model provides valuable information on the geological structures and on the thermal state and composition of the western region of the pyrenean lithosphere

dinitrogen oxygen and nutrient fluxes at the sediment water interface and bottom water physical mixing on the eastern chukchi sea shelf abstract we measured fluxes of net dissolved n2 and o2 no 3 and po 4 3 at the sediment water interface in whole cores in relation to bottom water temperature and salinity in late summer 2010 on the shelf of the eastern chukchi sea cold 1 68 c and high salinity 33 72 bottom water characteristic of pacific winter water overlaid the sediment over the northeastern region of the shelf while relatively warmer 8 33 c and less saline 29 88 water with properties of alaska coastal water covered the seabed in the southeastern region latitudinal variability of gas fluxes was not linked to differences in temperature or salinity relatively higher n2 efflux was measured for sediment located at the water mixing front between cold and warm bottom waters benthic o2 consumption in the southeastern region was relatively intense as were no 3 and po 4 3 effluxes from the sediments into the water column overall sediments in the chukchi sea are sites of intense denitrification but these losses appear compensated by benthic no 3 regeneration as no 3 efflux from sediments into the overlying water exceed those of n2 exposure of sediment cores to incoming light did not affect benthic fluxes significantly compared to dark conditions

volcanism in nascent back arc basins behind the shichito ridge and adjacent areas in the izu ogasawara arc northwest pacific evidence for mixing between e type morb and island arc magmas at the initiation of back arc rifting mineral chemistry major and trace elements and 87sr 86sr ratios are presented for 29 igneous rocks dredged from the northern portion of the izu ogasawara arc these rocks are compositionally bimodal basement gabbro and trondhjemite from the arc are extremely poor in k2o 0 05 0 19 and rb 0 48 0 62 ppm and their ree patterns and sr isotope ratios indicate that there are island arc tholeiites quaternary volcanic rocks from the present volcanic front shichito ridge active arc back arc seamounts east side inactive arc and torishima knoll between the two back arc depressions incipient back arc basins behind the active arc have the same geochemical characteristics as the above plutonic rocks though they are not as depleted in k and rb rhyolite pumice from the backarc depression is also the depleted island arc tholeiite whereas basalts from the depression have compositions that are transitional between morb and island arc tholeiites in trace element ti ni cr v y and zr and mineral chemistries the back arc depression basalts have relatively high ban cen 0 66 1 24 cen ybn 1 1 1 9 and k ba 45 105 and low 87sr 86sr 0 70302 0 70332 and ba sr 0 1 0 2 which are similar to other back arc basin basalts and e type morb but are quite unlike the depleted island arc tholeiites the diverse trace element and sr isotope compositions of basalt andesite from the back arc depressions imply the interplay between e type morb and island arc tholeiite these chemical characteristics and the relationships of ce yb n vs ba ce n and ce yb n vs 87sr 86sr suggest that the back arc depression magmas are generated by mixing of e type morb and depleted island arc tholeiite magmas geochemical characters of the associated rhyolite from the depression are compatible with partial melting of lower crust

robust 3d gravity gradient inversion by planting anomalous densities we have developed a new gravity gradient inversion method for estimating a 3d density contrast distribution defined on a grid of rectangular prisms our method consists of an iterative algorithm that does not require the solution of an equation system instead the solution grows systematically around userspecified prismatic elements called seeds with given density contrasts each seed can be assigned a different density contrast value allowing the interpretation of multiple sources with different density contrasts and that produce interfering signals in real world scenarios some sources might not be targeted for the interpretation thus we developed a robust procedure that neither requires the isolation of the signal of the targeted sources prior to the inversion nor requires substantial prior information about the nontargeted sources in our iterative algorithm the estimated sources grow by the accretion of prisms in the periphery of the current estimate in addition only the columns of the sensitivity matrix corresponding to the prisms in the periphery of the current estimate are needed for the computations therefore the individual columns of the sensitivity matrix can be calculated on demand and deleted after an accretion takes place greatly reducing the demand for computer memory and processing time tests on synthetic data show the ability of our method to correctly recover the geometry of the targeted sources even when interfering signals produced by nontargeted sources are present inverting the data from an airborne gravity gradiometry survey flown over the iron ore province of quadrilatero ferrifero southeastern brazil we estimated a compact iron ore body that is in agreement with geologic information and previous interpretations

mineralogical and geochemical characteristics of hydrothermal alteration of basalt in the kuroko mine area japan implications for the evolution of a back arc basin hydrothermal system abstract basalt in the furutobe district of the kuroko mine area in japan is characterized by abundant chlorite and epidote fluid inclusion studies indicate that chlorite is formed at lower temperatures 230 250 c than epidote 250 280 c the seawater basalt mass ratio for the early chlorite rich alteration was high max 40 but that for the later alteration was low 0 1 1 8 the cao na2o and sio2 of the bulk rock correlate negatively with mgo while feo and fe correlate positively with mgo these changes in the characteristic features of hydrothermal alteration from early to late are generally similar to those for a mid ocean ridge geothermal system accompanying basalt alteration the mgo feo ratios of chlorite and actinolite and the fe2o3 concentration of epidote from the basalt are greater than those of mid ocean ridge basalt probably owing to the differences in the fe2o3 feo and mgo feo ratios of the parent rocks the lower cao concentration and the higher na2o concentration of the bulk rock compared with altered mid ocean ridge basalt can be interpreted in terms of the difference in original bulk rock compositions the furutobe basalt as well as other submarine back arc basalts contains more vesicles filled with hydrothermal minerals epidote calcite quartz chlorite pyrite than do the mid ocean ridge basalts the abundance of vesicles plays an important role in controlling the secondary mineralogy and geochemistry of hydrothermally altered submarine back arc basin basalts

high resolution imaging of the pyrenees and massif central from the data of the pyrope and iberarray portable array deployments the lithospheric structures beneath the pyrenees which holds the key to settle long standing controversies regarding the opening of the bay of biscay and the formation of the pyrenees are still poorly known the temporary pyrope and iberarray experiments have recently filled a strong deficit of seismological stations in this part of western europe offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution here we report the results of the first tomographic study of the pyrenees relying on this rich data set the important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections beneath the massif central the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the pyrenees in contrast they reveal the segmentation of lithospheric structures mainly by two major lithospheric faults the toulouse fault in the central pyrenees and the pamplona fault in the western pyrenees these inherited hercynian faults were reactivated during the cretaceous rifting of the aquitaine and iberian margins and during the cenozoic alpine convergence therefore the pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the european plate

isotopic and trace element constraints on mixing and melting models of marginal basin volcanism bransfield strait antarctica bransfield strait is a narrow marginal basin separating the south shetland islands from the northern end of the antarctic peninsula quaternary volcanism occurs in the strait as subaerial and submarine volcanoes aligned on the inferred axis of rifting and as two subaerial off axis volcanoes on the northern margin of the strait the bransfield strait lavas are similar to published analyses from other marginal basins ranging from basalts and basaltic andesites to trachytes they exhibit moderate enrichments in alkali and alkaline earth elements relative to high field strength elements that are typical of many back arc basin basalts the seamount basalts have trace element chemistry similar to enriched mid ocean ridge basalts e morb but with variously higher alkali and alkaline earth element concentrations and frequently lower nb low pressure fractional crystallization accounts for most of the compositional variation within individual volcanoes but it does not explain intervolcano differences even though the volcanoes are closely spaced and presently or recently active melting of depleted mantle mixed with 0 5 2 crust or sediment explains the isotopic and trace element variations not accounted for by magma chamber processes the off axis volcanoes are the products of less partial melting than the on axis volcanoes one of the off axis volcanoes also contains more of an enriched component in its source some of the lavas have rb sr too low to account for their 87sr 86sr perhaps due the loss of rb during mantle metasomatism or interaction between slab derived fluids and the mantle source of the basalts strontium neodymium and 207pb 204pb isotopic signatures remained constant during the transition from tertiary island arc volcanism to quaternary marginal basin volcanism but 206pb 204pb increased high cen smn of basalts from some volcanoes requires residual garnet in the source thus the young 15 25 ma at the trench subducted slab is either deeper than 60 km beneath the volcanoes or it has fractured and is no longer a coherent barrier to melts generated below it

geological setting of the rapu rapu gold rich volcanogenic massive sulfide deposits albay province philippines gold rich fe cu zn volcanogenic massive sulfide deposits occur within strata of probable jurassic age on rapu rapu island in albay province philippines massive sulfides at the ungay malobago and hixbar deposits are spatially associated with dacitic volcanic rocks within a highly deformed sequence of mafic volcanic and quartzofeldspathic sedimentary rocks the massive sulfide deposits formed at the stratigraphic contact between footwall dacites and hangingwall mafic volcanic and quartzofeldspathic rocks the deposits and their host strata have undergone regional metamorphism with strong penetrative deformation metamorphic mineral assemblages and textural evidence suggest that peak metamorphism was upper greenschist to lower amphibolite grade and syn d1 deformation based on the age of regional metamorphism deformation is inferred to be mid tertiary in age deformation at rapu rapu resulted in reorientation of the strata into a broad antiform with strong shallow plunging elongation fabrics overturning of the volcanic sequence that hosts the ungay malobago deposit and complex folding of the mineralized zones the present highly linear form of the ungay malobago deposit is mainly a product of this ductile strain

denitrification in continental shelf sediments has major impact on the oceanic nitrogen budget denitrification rates in sediments within the oxygen deficient waters off mexico and from the gulf of maine were investigated on the basis of interstitial nutrient profiles nitrate fluxes into the sediments were calculated from gradients across the sediment water interface and vertical molecular diffusion coefficients and averaged 0 151 mexico and 0 0920 gulf of maine pmol no 3 cm 2 s 1 these are minimum values since these gradients may have been underestimated in the gulf of maine bottom water irrigation by macrobenthos increases the nitrate supply well above this estimate in addition only 15 22 of the expected ammonium is present in gulf of maine sediments perhaps because of removal by a rapid coupling of nitrification with denitrification this large apparent loss of the regenerated ammonium appears to be ubiquitous in shelf sediments with oxygenated bottom water the global denitrification rate in continental shelf sediments was reassessed to be 50 tg n yr 1 1 tg 1012 g demonstrating that sediments are an important sink for oceanic nitrogen globally current nitrogen losses from the oceans may exceed inputs by 60 90 tg n yr 1 over the glacial interglacial cycle the global sedimentary denitrification rate probably varied commensurately with the changing continental shelf area an oscillating oceanic nitrogen budget over these time scales could occur given the sequence of 1 scouring and dumping of terrestrial nitrogen into the oceans during glacial advance 2 removal of oceanic combined nitrogen to the atmosphere by denitrification following glacial retreat and 3 reincorporation of this n into terrestrial biomass during the interglacial period

a three dimensional model of the pyrenees and their foreland basins from geological and gravimetric data abstract we construct a three dimensional geological model of the pyrenees and their foreland basins with the geomodeller this model which accounts for different sources of geological and geophysical informations covers the whole pyrenees from the atlantic ocean to the mediterranean sea and from the iberian range to the massif central down to 70 km depth we model the geological structure with a stratigraphic column composed of a superposition of layers representing the mantle lower middle and upper crusts the sedimentary basins are described by two layers which allow us to make the distinction between mesozoic and cenozoic sediments which are characterized by markedly different densities and seismic velocities since the pyrenees result from the convergence between the iberian and european plates we ascribe to each plate its own stratigraphic column in order to be able to model the imbrication of iberian and european crusts along this fossile plate boundary we also introduce two additional units which describe the orogenic prism and the water column in the bay of biscay and in the mediterranean sea the last ingredient is a unit that represents bodies of shallow exhumed and partly serpentinized lithospheric mantle which are assumed to produce the positive bouguer gravity anomalies in the north pyrenean zone a first 3d model is built using only the geological information coming from geological maps drill holes and seismic sections we use the potential field method implemented in geomodeller to interpolate these geological data this model is then refined in order to better explain the observed bouguer anomalies by adding new constraints on the main crustal interfaces the final model explains the observed bouguer anomalies with a standard deviation less than 3 4 mgal and reveals anomalous deep structures beneath the eastern pyrenees

the deep roots of the western pyrenees revealed by full waveform inversion of teleseismic p waves imaging the architecture of mountain roots is required to understand the support of topography and for kinematic reconstructions at convergent plate boundaries but is still challenging with conventional seismic imaging approaches here we present a three dimensional model of both compressional and shear velocities in the lithosphere beneath the western pyrenees southwest europe obtained by full waveform inversion of teleseismic p waves this tomographic model reveals the subduction of the iberian crust beneath the european plate and the european serpentinized subcontinental mantle emplaced at shallow crustal levels beneath the mauleon basin the rift inherited mantle wedge acted as an indenter during the pyrenean convergence these new results provide compelling evidence for the role of rift inherited structures during mountain building in alpine type orogens

optimum expression for computation of the gravity field of a homogeneous polyhedral body1 the formula for the computation of the gravity field of a homogeneous polyhedral body is derived and transformed into the form which is most suitable when considering speed and simplicity of numerical calculation

a hybrid method to compute short period synthetic seismograms of teleseismic body waves in a 3 d regional model we present a hybrid method to simulate the propagation of short period teleseismic body waves through 3 d regional models the incident wavefield is computed in an spherically symmetric reference earth model based on the direct solution method the global and regional wavefields are matched at the boundaries of the regional mesh in the regional domain we implement a spectral element method with absorbing boundaries to cancel the outgoing scattered wavefield the hybrid method is successfully benchmarked against the direct solution method in the reference earth model iasp91 the potential of the method is illustrated by computing short period p wave synthetic seismograms in a 3 d model with a 20 km moho offset and or topography on the free surface focusing on the actual example of the pyrenees

deep structure of pyrenees range sw europe imaged by joint inversion of gravity and teleseismic delay time we present a new model of the lithosphere and asthenosphere structure down to 300 km depth beneath the pyrenees from the joint inversion of recent gravity and teleseismic data unlike previous studies crustal correction was not applied on teleseismic data in order i to preserve the consistency between gravity data which are mainly sensitive to the density structure of the crust lithosphere and traveltime data and ii to avoid the introduction of biases resulting from crustal reductions the density model down to 100 km depth is preferentially used here to discuss the lithospheric structure of the pyrenees whereas the asthenospheric structure from 100 to 300 km depth is discussed from our velocity model the absence of a high density anomaly in our model between 30 and 100 km depth except the labourd density anomaly in the northern part of the pyrenees seems to preclude eclogitization of the subducted iberian crust at the scale of the entire pyrenean range local eclogitization of the deep pyrenean crust beneath the western part of the axial zone west of andorra associated with the positive central density anomaly is proposed the pyrenean lithosphere in density and velocity models appears segmented from east to west no clear relation between the along strike segmentation and mapped major faults is visible in our models the pyrenees lithosphere segments are associated with different seismicity pattern in the pyrenees suggesting a possible relation between the deep structure of the pyrenees and its seismicity in the upper crust the concentration of earthquakes localized just straight up the central density anomaly can reflect the subsidence and or delamination of an eclogitized pyrenean deep root the velocity model in the asthenosphere is similar to previous studies the absence of a high velocity anomaly in the upper mantle and transition zone i e 125 to 225 km depth seems to preclude the presence of a detached oceanic lithosphere beneath the european lithosphere

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

evolution of the mariana convergent plate margin system the mariana convergent plate margin system of the western pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology the system s relative geologic simplicity and the well exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins the mariana convergent margin s long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the philippine sea plate the trenchward edge of the overriding plate has a relatively sparse sediment cover rocks outcropping on the trench s inner slope are typical of the early formed suprasubduction zone s lithosphere and have been subjected to various processes related to its tectonic history pervasive forearc faulting has exposed crust and upper mantle lithosphere many large serpentinized peridotite seamounts are within 100 km of the trench axis from these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids ocean drilling recovered suprasubduction zone lava sequences erupted since the eocene that suggest that the forearc region remains volcanologically dynamic seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution large portions of uplifted southern forearc are exposed at the larger islands active volcanoes at the base of the eastern boundary fault of the mariana trough vary in size and composition along strike and record regional differences in source composition their locations along strike of the arc are controlled in part by cross arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin the western boundary is the west mariana ridge the western portion of the volcanic arc active prior to formation of the mariana trough the trough evolved in a two stage extension process of rifting and subsequent seafloor spreading the back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid ocean type spreading center south of 20 n the change from initial rifting to true seafloor spreading is also evident across the mariana trough from rifted topography near the west mariana ridge to spreading ridges in the central to eastern basin south of 20 n this morphologic change indicates an early stage of extension with basin and range type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries the spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys the center of extension intersects the active volcanic front differently at either end of the mariana trough in the north extension is by rifting of arc lithosphere where it intersects the arc in the south a major strike slip fault extends from the trench axis across the forearc through the volcanic arc and into the back arc basin arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center the complexity of interrelated tectonism and magmatism in this convergent margin is daunting but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments

geochemistry and geodynamic setting of volcanic and plutonic rocks associated with early archaean volcanogenic massive sulphide mineralization pilbara craton abstract early archaean volcanic rocks and syn volcanic granites at strelley east pilbara craton western australia were formed during an extensional tectonic phase associated with an active convergent continental margin the volcanic sequence comprises a bimodal suite of fractionated sub alkalic tholeiitic basalts andesites and dacites siliceous laminites and volcanogenic massive sulphide vms deposits in the footwall and hangingwall calc alkaline dacites the footwall rocks are characterized by th lapm 1 variably fractionated lree and relatively high y and yb 33 and 3 ppm respectively negative normalized anomalies of nb p ti are consistent with derivation via slab dehydration fusing the mantle wedge the co magmatic and syn volcanic strelley granite is enriched in lile relative to hfse and has distinctively low al2o3 11 14 and la yb n 4 10 suggesting a relatively mature and cool subducted oceanic lithosphere as for the majority of phanerozoic arc granitoids and convergent margin magma compositions the chemical composition of the footwall basaltic to dacitic rocks and the sub volcanic strelley granite display signatures highly prospective for vms mineralization large ve eu anomalies low zr y ratios high level magmatic fractionation comparable with the noranda fiiia type suite the hangingwall dacites have more pronounced hree fractionation than footwall counterparts and lower y and yb the chemistry of the footwall volcanic rocks and granite reflect typical phanerozoic conditions of steep subduction hydrous metasomatism in the mantle wedge and fractionation similar characteristics are also evident in plutonic volcanic suites associated with vms deposits in the late archaean as at noranda canada early extension with nascent back arc development coupled with steep subduction is considered critical to the development of vms deposits

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

three dimensional gravity inversion using simulated annealing constraints on the diapiric roots of allochthonous salt structures in the northern continental slope of the gulf of mexico large oil and gas reservoirs are often found beneath sheetlike allochthonous salt structures that are laterally extensive some of these salt structures retain their diapiric feeders or roots beneath them these hidden roots are difficult to image seismically in this study we develop a method to locate and constrain the geometry of such roots through 3 d inverse modeling of the gravity anomalies observed over the salt structures this inversion method utilizes a priori information such as the upper surface topography of the salt which can be delineated by a limited coverage of 2 d seismic data the sediment compaction curve in the region and the continuity of the salt body the inversion computation is based on the simulated annealing sa global optimization algorithm the sa based gravity inversion has some advantages over the approach based on damped least squares inversion it is computationally efficient can solve underdetermined inverse problems can more easily implement complex a priori information and does not introduce smoothing effects in the final density structure model we test this inversion method using synthetic gravity data for a type of salt geometry that is common among the allochthonous salt structures in the gulf of mexico and show that it is highly effective in constraining the diapiric root we also show that carrying out multiple inversion runs helps reduce the uncertainty in the final density model

seismological study of lau back arc crust mantle water magmatic differentiation and a compositionally zoned basin abstract on the basis of seismic tomography analyses a structural model of crust formed along the eastern lau spreading center elsc is presented as evidence for a transition from a hydrous type of oceanic crust to a more typical oceanic crust the seismic data indicate that as the spreading center moved away from the active arc the crust thinned from 8 9 km to 7 km the lower crust changed from high p wave velocity values 7 2 7 4 km s to typical values for oceanic crust 7 0 7 2 km s and the upper crustal volcanic layer changed from a thick low velocity layer to a thinner layer with more typical wave speeds the seismic results in combination with other geophysical and geochemical data suggest that crustal formation along the elsc is strongly controlled by the influence of slab water when a spreading center is near the active arc water from the downgoing slab is entrained in the melting zone beneath the ridges where it enhances melting thereafter the water enhances crustal differentiation within sub ridge magma chambers this creates an anomalous hydrous form of oceanic crust with a thick felsic volcanic layer and a mafic ultramafic lower crust features that are not typically observed in crust formed at mid ocean ridges the lau basin has a zoned structure with an abrupt transition from this type of oceanic crust to more typical oceanic crust which resulted from a rapid change in the influence of slab water as the ridge moved away from the arc the abundance and high rate of production of the hydrous crust suggests that such crust may make up a significant proportion of the arc like crust that forms continents

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle