a melt inclusion study of the toba tuffs sumatra indonesia the toba caldera in northern sumatra indonesia is the site of earth s largest quaternary volcanic eruption this eruption dated at 74 ka produced the 2800 km3 youngest toba tuff ytt and ash fall quartz bearing silicic pumices ranging from 68 to 77 wt sio2 indicate that the ytt magma was zoned compositionally and mineralogically prior to the ytt eruption two other silicic quartz bearing tuffs known as the middle toba tuff mtt and the oldest toba tuff ott were erupted from toba at 0 501 ma and 0 840 ma respectively although the volatile contents of the toba magmas are poorly constrained aerosols generated by the ytt eruption are generally thought to have caused a global volcanic winter an evaluation of the pre eruptive dissolved gas contents of the ytt magma is fundamental to understanding the global effects of this eruption we used melt inclusions in quartz crystals to determine the dissolved h2o co2 s cl and f contents of the ytt mtt and ott magmas quartz crystals selected from pumice blocks and welded tuffs that spanned the compositional ranges of the three units were chosen for study major and trace element analyses of melt inclusions were also conducted to characterize melt evolution and provide context to the volatile data melt inclusions from the ytt mtt and ott are rhyolitic in composition 73 77 wt sio2 and have overlapping indistinguishable major element trends inclusions from the large eruptions ytt and ott have similar geochemical relationships where the most evolved melt inclusions occur in the least silicic bulk rock samples and are more evolved than their matrix glasses in contrast the least evolved melt inclusions occur in the most silicic samples and either overlap with or are less evolved than their matrix glasses these patterns can be explained if quartz crystals in the least silicic magmas were inherited from more evolved melt while those in the most silicic melt formed in situ several lines of evidence suggest that crystal settling of early formed quartz crystals in long lived magma bodies was an important process in the ytt and ott magmas trace element data from the ytt melt inclusions have wide ranges rb 250 830 ppm ba 10 485 ppm and mostly support these models however incompatible trace elements are enriched 2 3 over their matrix glass compositions in all samples in the smaller mtt melt inclusions overlapped with the compositional range of the bulk rock samples and are consistent with in situ crystallization ftir analyses indicate that the ytt mtt and ott melts contained about 4 0 5 5 2 0 5 5 and 2 0 5 5 wt h2o respectively co2 contents in all units are generally 100 ppm and cl is 2000 ppm water gradients occur in all 3 tuffs cl gradients are evident only in the ytt and the least silicic samples from all units had no detectable co2 sulfur contents in melt inclusions are low in all samples 32 ppm and overlap with matrix glass s contents applying broad degassing constraints about 1014 g of h2so4 aerosols were likely loaded into the atmosphere during the ytt eruption 2 orders of magnitude less than previous petrologic estimates considerably more cl and f about 1015 g of each may have been part of the ytt aerosol cloud

newly discovered eastern dispersal of the youngest toba tuff volcanic glasses with minor mafic mineral fragments such as biotite and hornblende found in deep sea sediments of the south china sea basin scsb have been clearly identified as eruptive products of the youngest toba tuff ytt northern sumatra indonesia the tephra layer occurs between marine oxygen isotopic event 5 1 79 3 ka and event 4 22 64 1 ka with an interpolated age of 74 0 ka which is in good consistence with previous radiometric dating 73 75 ka and ice core dating 71 5 ka of the ytt the tephra consists predominantly of bubble wall shards with minor elongated vesicles of pumice fragments geochemical characteristics of the tephra such as high total alkali content high 87 sr 86 sr ratio and uniformity of their compositions all suggest that the recovered tephra is of the youngest toba tuff this finding supports an extended dispersal of coarse 63 mm glass shards over 1500 km northeast of the toba caldera a direction opposite to what previously conceived while providing a better documentation of the distribution extent of the toba ash this report points to the need to reestimating the eruptive volume of the ytt and re evaluating its environmental impact q 2000 elsevier science b v all rights reserved

carbonate deposition in mixed siliciclastic carbonate environments on top of an orogenic wedge late cretaceous northern calcareous alps austria abstract in the middle turonian to lower campanian lower gosau subgroup lgs of the northern calcareous alps austria in the highstand systems tract of mixed siliciclastic carbonate depositional sequences small carbonate shelves locally developed that were mainly controlled by a differentiated morphology of the substratum and siliciclastic input onto a storm dominated shelf the lgs was deposited above thrust nappes of the alpine orogen the transgressive systems tracts of depositional sequences consist of local fan delta or fluvial deposits overlain by a succession mainly of siliciclastics that records deepening from paralic to outer shelf environments locally rudist biostromes and coral rudist mounds accumulated in lagoonal bay to inner shelf environments but were buried by paralic siliciclastics along coastal sectors of low siliciclastic input because of a high relief truncation surface at the base of the lgs the transgressive systems tracts consist of an upward deepening succession of clastic carbonates deposited from gravelly to rocky carbonate shores the highstand systems tracts are dominated by siliciclastics and record shoaling to inner shelf depths and near the basin margins to lagoonal to marsh environments during highstand conditions delta progradation combined with shoreline compartmentalization by headlands locally led to establishment of inner shelf compartments of low siliciclastic input in these compartments carbonate shelves developed along strike the carbonate shelves were up to more than 10 km in length beyond limits of larger outcrops and about 1 km to possibly 10 km wide down dip the carbonate shelves consisted of a an inner shelf belt with coral rudist mounds rudist biostromes and bioclastic sand bodies b a dissipative shore zone of bioclastic sand bodies c open lagoons bays with radiolitid biostromes and d narrow micro tidal flats or lithoclastic bioclastic beaches up section inner shelf to shore zone carbonate parasequence tracts consist of a coral rudist mound a rudist biostrome and of shore zone bioclastic limestones parasequence tracts deposited in lagoons bays commonly shoaled incompletely and mainly consist of more or less marly limestones deposited in shallow subtidal environments with radiolitid biostromes substrata of bioclastic sand or lime mud and with local mass accumulations of gastropods or of epibenthic non rudist bivalves the carbonate shelf successions are up to 100 m thick and in vertical section consist of stacked parasequence tracts that become thinner up section and record a shoaling of mean depositional water depth carbonate deposition was confined to the actual inner shelf to tidal flat beach compartment down dip the coral rudist mounds scattered along the seaward fringe of the carbonate shelves graded into small haloes composed mainly of disoriented fragmented rudists and corals larger carbonate slope depositional systems were not individuated and the carbonate shelves interfingered with and pinched out into inner shelf siliciclastics the development of larger carbonate slopes was prevented by the low relief of the coral rudist mounds combined with high input and effective dispersal of siliciclastics ahead carbonate shelf progradation over a possible distance of up to a few kilometres was linked with aggradation of shelf siliciclastics

synopsis of late palaeozoic and mesozoic terrane accretion within the cordillera of western north america and discussion establishing the paleogeographic origin of most of the terranes within the cordillera remains an ellusive goal despite more than 10 years of multidisciplinary research the home port of any major terrane has not been identified unequivocally even most continental fragments that show affinities to north america cannot be repositioned confidently along the cordilleran margin and some continental fragments e g chulita probably are not north american in origin cordilleran oceanic terranes including island arcs seamounts off ridge islands and scraps of ocean basins are especially difficult to reposition because panthalassa has been destroyed faunal studies with emphasis on palaeobiogeographic affinities are the most useful particularly when coupled with analyses of faunal diversity and endemism such studies suggest that some terranes previously thought to have formed near the cordillerran margin were situated thousands of kilometres to the west and were separated from the continent by broad ocean basins rather than by a narrow marginal sea

the application of icp ms methods to tephrochronological problems pearce nicholas westgate j a perkins w t preece s j 2004 the application of icp ms methods to tephrochronological problems applied gechemistry 19 3 pp 289 322 rae2008

fresh pumice from the central indian basin a krakatau 1883 signature the occurrence of pumice in the central indian basin cib has been a controversy some are of the opinion that these pumices are erupted from a volcano along the ninetyeast ridge while others suggest the indonesian arc volcanism as their probable source recently it is also suggested that the pumice are the products of in situ silicic volcanism in the cib this study was initiated in order to resolve this controversy about the origin of the fresh pumice from the cib the fresh pumice pieces of different sizes and shapes were collected from the surficial sediments of the cib from an average depth of about 5000 m 12 s 76 e glass is the main phase comprising about 95 volume and resembles the matrix glass in krakatau 1883 dacitic pumice which is also very poor 5 in the modal mineral composition the mineral and glass analyses were carried out with the help of the electron microprobe the orthopyroxenes clinopyroxenes plagioclases and spinels are the mineral phases present in these pumices the pyroxenes have similar wo en and fs components to those previously reported from the 1883 krakatau eruption products geochemically these fresh pumices have a high sio2 content 71 and high total alkali content 8 similar to 1883 eruption products of krakatau thus based on the similarities in mineralogy and glass chemical composition it is inferred that these fresh pumices are the products of the fierce 1883 eruption of the krakatau volcano in the sunda strait of the indonesian arc

late palaeogene quaternary geology of halmahera eastern indonesia initiation of a volcanic island arc the late palaeogene quaternary stratigraphy of halmahera is described and new formation names are proposed based on recent field investigations of the ne and central part of the island this stratigraphic information provides new insights into the neogene history of halmahera and the development of the present island arc the late palaeogene and younger rocks rest unconformably on an ophiolitic basement complex which formed part of a late cretaceous early tertiary fore arc after volcanic arc activity ceased in the eocene the former fore arc terrane was uplifted and deeply eroded in the late palaeogene some of the late palaeogene early miocene river valleys are currently being re excavated by the present rivers slow subsidence began in the mid late oligocene and by the end of the miocene all eastern halmahera was the site of shallow water carbonate deposition there is no evidence for arc volcanism in central halmahera at this time and the reported oligo miocene volcanism in nearby regions is interpreted as volcanism related to the sorong fault system the miocene shallow water region subsided rapidly in the early pliocene and the sedimentary basin formed was filled with marls succeeded by siliciclastic turbidites with increasing amounts of calc alkaline volcanic debris from a pliocene volcanic arc built on the western arms of halmahera probably on the eroded early tertiary arc this phase of rapid subsidence in the pliocene back arc region resulted from the initiation of subduction of the molucca sea lithosphere eastwards beneath halmahera differential subsidence on nw se and ne sw sets of faults in the region immediately behind the active arc led to the formation of deep sediment filled basins adjacent to the eastern arms a major deformation event in the pleistocene resulted in folding and local thrusting at the junction between eastern and western halmahera and volcanism ceased in the pliocene arc the third halmahera arc the quaternary arc currently active in the northern part of the islands began activity within the last 1 ma and is built upon the deformed and partly eroded pliocene arc the pleistocene deformation event and shift in position of the arc are interpreted as the result of the interaction of the eastward dipping molucca sea plate with adjacent plates either with a fragment of the australian continent in the sorong fault zone and or with the philippine sea plate beneath northern halmahera

petrologic evolution of krakatau indonesia implications for a future activity abstract krakatau volcano is located along a n35e volcanic lineament running through the sunda straits indonesia its last activity has been characterized by successive phases each beginning with the construction of a cone and ending with its destruction and the formation of a caldera the two last pre and post 1883 cycles are well known but the more ancient ones are not so clearly defined lavas of krakatau belong to an andesitic series in which fractional crystallization plays the most important role the petrologic evolution is characterized by a cyclicity in good agreement with the structural evolution the succession is regular basalts basic andesites acid andesites dacites a gap between acid and basic andesites occurs in each cycle the destructive stages correspond to the occurrence of dacitic terms the anak cycle was characterized from 1927 to 1979 by basalts and basic andesites the 1981 eruption involved a more differentiated magma close to dacitic detailed study of the petrologic evolution since 1883 emphasizes the predominant role of fractional crystallization this process occurred during a very short period between 1979 and 1981 separation of labradorite augite olivine and magnetite from parental basic andesite may generate the dacitic descendant in a shallow reservoir p h 2 o estimated about 0 5 kbar implications for a future activity are considered

potential contributions of ancient oceanic islands to evolutionary theory before much was known about the geology of continental margins and the occurrence of oceanic islands in suspect terranes tests of theories related to island biogeography were undertaken mostly by ecologists and in paleontological investigations of non insular biota a significant new source of paleontological data with which to test models for species equilibrium over evolutionary versus ecological time scales and other important problems in evolutionary theory exists within circum oceanic belts where ancient oceanic islands are preserved as allochthonous terranes research on faunas from these areas simplifies attempts to apply the theory of island biogeography to the fossil record and to trace the history of island populations over evolutionary time scales paleontologic data from a paleozoic island arc alexander terrane show significant fluctuations in the taxonomic diversity of brachiopods and thus question one of the main assertions of the dynamic equilibrium model these data also support the con

biogeographic complexity in triassic bivalves of the wallowa terrane northwestern united states oceanic islands not continents provide the best analogues high levels of endemism and complex overlapping biogeographic patterns characterize modern molluscan faunas of the hawaiian islands and the triassic bivalve fauna of the wallowa volcanic arc terrane in hells canyon oregon such biogeographic complexities and high levels of endemism in many modern and mesozoic island settings constrain the use of faunal data as a primary basis for paleogeographic reconstruction of accreted terranes large taxonomically diver samples are required to identify genuine biogeographic patterns in these insular settings selective use of individual species genera or families to reconstruct terrane paleogeography may give misleading results

deposition and diagenesis of miocene arc fringing platform and debris apron carbonates southwestern viti levu fiji abstract carbonate deposition was widespread in the eastern melanesian arc during the late early to early middle miocene in southwestern viti levu fiji limestones of this age include a massive to crudely bedded reef or platform facies and a well bedded horizon of redeposited limestone rudites and calcarenite turbidites interbedded with hemipelagic lime mudstones the redeposited limestones are considered to represent a series of carbonate debris aprons deposited in relatively deep water in the early middle miocene the subsurface equivalents of the reef platform limestones are likely to form the best potential hydrocarbon reservoir rocks in fiji carbonate deposition coincided with an increase in epiclastic sediment yield to the forearc basin indicating increased emergence and erosion of the arc it is suggested that by bringing larger submerged areas into the photic zone uplift of the arc was the main factor allowing widespread shallow water carbonate deposition at this time primary pore space in the reef platform carbonates was largely occluded in the marine phreatic zone by microcrystalline cements isopachous fibrous or blocky cement fringes and micritic internal sediments remaining primary pores and secondary biomouldic pores were filled by equant sparry calcite during meteoric phreatic and burial diagenesis reef or platform derived limestone clasts are abundant in the redeposited limestones and there is evidence of an early to middle miocene depositional hiatus accompanied by early subaerial exposure within the largest platform carbonate body it is therefore suggested that the debris apron carbonates were deposited as the result of emergence and erosion of the nearby shallow water limestones which may have been related to sharp but short lived eustatic sea level falls at 16 and 15 ma

deformation across the mantle transition zone a theoretical mineral physics view abstract the dynamics of the earth s mantle is still poorly constrained due to the lack of understanding the transfer of matter between the upper and the lower mantle and their convective vigor the transition zone tz might play a crucial role as the interface connecting the upper to the lower mantle here we examine the rheology of the main tz minerals wadsleyite ringwoodite and majorite garnet based on a mineral physics approach using the results of lattice friction modeling and dislocation glide mobilities together with the available data on self diffusion in the tz minerals we quantify their plastic deformation by diffusion and dislocation creep from theoretical plasticity models we show that pure climb creep is expected to contribute to the plasticity of the tz without the need of significant diffusion related hydrolytic weakening matching well the geophysical observations our model results predict that crystallographic preferred orientations cpo might only develop along with stress concentrations as present around cold subducting slabs which can be locally weaker than the surrounding tz despite their lower temperatures

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

the sandstone memory of the evolution of a triassic volcanic arc in the southern alps italy the erosion of a southern volcanic belt provided the bulk of the fine grained and moderately sorted siliciclastic detritus in the whole deltaic to lagoonal carnian upper triassic sequence of the bergamasc alps volcanic input began in the early carnian calcare metallifero bergamasco and became prominent in early middle val sabbia sandstone and late carnian times s giovanni bianco fm after an intervening period of diminished supply gorno fm the mineralogical and chemical composition of the sandstones compares closely with that of siliciclastics derived from modern pacific type magmatic arcs and it testifies to an evolution of the source rocks from andesites and dacites during the deposition of the val sabbia sandstone towards rhyodacitic ignimbrites for the s giovanni bianco fm the marked reduction in feldspars at the top of the sequence contrasts with the classical trend from undissected to dissected magmatic arc provenance and is ascribed to the progressive deepening of erosion into middle triassic felsic volcanics an increase in undulatory and polycrystalline quartz grains in the late carnian shows that the erosion did reach deeper into the crystalline roots of the arc although non volcanic detritus never exceeded 10 after a terminal pyroclastic event in middle late carnian times suggested by the sudden appearance of pumiceous vitric clasts at the top of the val sabbia sandstone the mid triassic orogenic magmatism ended and was followed by the tensional stage which led to the birth of neotethys

effects of water depth on pumice formation in submarine domes at sumisu izu bonin arc western pacific domes of the sumisu volcanic complex western pacific having summits at ocean depths of 1100 600 245 and 95 m are mantled with compositionally identical rhyolitic pumice that has similar vesicularity but that varies systematically in distribution size and surface texture suggesting that facies and morphology can serve as useful indicators of eruption depth at depths 500 m the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant meters to tens of meters polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace at 500 m depths the pumice occurs as an apron of blocky giant and smaller rough textured clasts enclosed by quenched margins and pockmarked by coarse cm vesicles no carapace pumice occurs and the summit is composed of craggy dense dome rock these shallower water pumice clasts resemble those spalled from historic submarine dome forming eruptions that buoyed to the sea surface we interpret spalling to result from vent derived weak volatile driven explosions that take place at water depths 500 m our study shows that an increase in hydrostatic pressures over a range of 12 mpa reduces volatile driven explosivity for subaqueous rhyolitic dome forming eruptions but does not affect vesicularity we conclude that meter size highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m and its morphology can be used to distinguish between explosive and effusive origins

the high pressure single crystal elasticity of pyrope grossular and andradite high pressure brillouin scattering experiments were conducted on three natural singlecrystal garnets with near end member compositions pyrope mg3al2si3o12 grossular ca3al2si3o12 and andradite ca3fe 31 2si3o12 acoustic velocities of the samples were measured from the samples at pressures to 10 gpa in a diamond anvil cell with a methanolethanol water pressure medium elastic stiffness constants cij and their pressure dependencies cij p were calculated as were aggregate elastic moduli ks and g and their respective pressure derivatives the cauchy relations changed with pressure for both grossular and andradite suggesting that assuming these are constant is not valid for extrapolation to high pressures for these and possibly other minerals the variation in ks dp and g p was sensitive to composition and required a complex mixing model for extrapolation of end member elasticity values from garnets of intermediate compositions

paleoceanography of the tropical eastern pacific ocean the east pacific barrier epb is the most effective marine barrier to dispersal of tropical shallow water fauna in the world today the fossil record of corals in the eastern pacific suggests this has been true throughout the cenozoic in the cretaceous the epb was apparently less effective in limiting dispersal equatorial circulation in the pacific then appears to have been primarily east to west and the existence of oceanic atolls now drowned guyots in the eastern pacific probably aided dispersal similarly in the middle and early mesozoic and late paleozoic terranes in the central tropical pacific likely served as stepping stones to dispersal of tropical shelf faunas reducing the isolating effect of an otherwise wider pacific ocean panthalassa

abyssal sediment erosion in the central indian basin evidence from radiochemical and radiolarian studies abstract radiochemical analyses of three spade cores collected around 78 e between 10 s and 12 s from the abyssal depths in the central indian basin yield an average accumulation rate of 2 mm ka the ratios of 230th flux in the sediments to its production rate in the overlying water column fa fp are extremely low 0 2 this fact and the absence of collosphaera invaginata first appearance datum 150 200 ka an index radiolarian species of neogene radiolarian zone 1 indicate intense erosion and lateral transport of younger sediments from this region the radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent antarctic bottom water entering the central indian basin through the northern saddles of the ninety east ridge

diffusion creep of fine grained garnetite implications for the flow strength of subducting slabs creep experiments were performed on synthetic fine grained garnetite to investigate the flow strength of the earth s subducting slabs experiments were conducted at temperatures t of 1373 1543 k and total pressure p of 0 1 mpa in controlled atmospheres of f 02 10 17 10 8 mpa the mechanical data indicate a grain size sensitive diffusion flow and the creep behavior can be described by an equation of the form e 5 32 3 10 10 6 td 2 5 0 3 f 0 o2 1 1 0 2 exp 347 46 kj mol rt where t in kelvin d in meter and f o2 in mpa based on the diffusivities d calculated from creep and diffusion experiments we proposed that grain boundary diffusion is the dominant mechanism for high temperature creep of the fine grained garnetite normalized creep strength of the garnetite is found to be comparable to those of feldspar and olivine in diffusion creep regime suggesting that garnetite may not form a strong layer in the subducted oceanic lithosphere if it deforms by grain boundary diffusion creep

trace element composition of single glass shards in distal minoan tephra from sw turkey volcanic glass from the seventeenth century bc c 3600 bp minoan eruption of santorini thera has been recovered from a 4 cm thick tephra deposit at golhisar golu a small lake in sw turkey analysis of single grains of this glass by laser ablation inductively coupled plasma mass spectrometry la icp ms provides for the first time an accurate determination of the trace element composition of the juvenile glass component of this eruption free from contaminants such as phenocrysts or detrital material individual glass shards 150 m show a large degree of variation in their trace element composition indicative of substantial small scale compositional heterogeneity in the magma this is a result of the proximity of crystallizing phases and provides insights into the chemical evolution of the minoan magma the range of elemental compositions displayed by individual glass shards combined with the consistency of many trace element ratios are characteristics of the minoan glass these features provide a trace element signature for the minoan tephra which will be of value in stratigraphic correlation across the eastern mediterranean

volcaniclastic and carbonate sedimentation in late paleozoic island arc deposits eastern klamath mountains california the carboniferous baird formation and early permian mccloud limestone eastern klamath mountains california record depositional relations of volcaniclastic and carbonate sediments within an island arc setting pennsylvania parts of the baird include interbedded mudstone sandstone and pebbly sandstone of volcaniclastic composition these beds accumulated by gravity displacement as volcaniclastic submarine fans a locally conformable transition from the baird to the overlying wolfcampian mccloud limestone is marked by limestone conglomerate of debris flow origin interbedded with volcaniclastic sediments and skeletal wackestone this slope related sequence is overlain by skeletal wackestone and interbedded skeletal packstones and the section represents a distally steepened carbonate ramp that prograded over volcaniclastic slope deposits in other areas diachronous bodies of mccloud limestone disconformably overlie upper mississippian volcanics and volcaniclastics of the baird formation laterally variable age relations of the mccloud and baird indicate that platforms developed at different times on elevated arc surfaces prograding locally over relic volcaniclastic slopes and accumulating thick carbonate sequences through platform subsidence

subduction zone rheology rheological flow laws can be obtained from studies using multi anvil high pressure systems with synchrotron based piezometers and strain metrics the high flux x ray source provides minute scale time resolution with accurate measurement of diffraction patterns and direct sample images measurements of length changes with an accuracy of one part in 10 4 are being developed and will provide a new generation of rheological tools flow laws derived from peak broadening agree well with literature data for corundum spinel and olivine properties of several mantle phases are compared for the temperature and pressure regime appropriate to a subducting slab temperature dependence of these properties exhibits a strong temperature insensitive low temperature region a thermally softened region and a weak high temperature region the middle of these could be related to the seismogenic zone of a subduction zone the progression of the temperature for softening with mineral phase suggests that earthquakes deeper than 400 km correspond to higher temperatures than for olivine in the upper 400 km plastic instabilities are suggested by these data as the origin of deep earthquakes 2001 elsevier science b v all rights reserved

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

comparison of terrane accretion in modern southeast asia and the mesozoic north american cordillera the western part of the north american cordillera has been divided into numerous suspect terranes some of these terranes have traveled hundreds to thousands of kilometres between the time of their formation and their final incorporation into the accretionary belt of the cordillera most of the terranes appear to be relics of island arcs oceanic plateaus and islands continental margin fragments and complex accretionary terranes the latter including melange belts ophiolite fragments and thrust faulted forearc provinces because the terranes have complex histories and diverse stratigraphies it has been difficult to envisage an actualistic setting for the mesozoic cordillera here we propose an analogy with the tectonic setting of the indo pacific region from the tonga trench on the east to eastern indonesia on the west this region comprises several distinct island arcs several large oceanic plateaus numerous accretionary terranes melange zones ophiolite fragments and a variety of continental fragments all moving toward the central collision zone like the cordillera this part of the indo pacific region is undergoing oblique convergence driven by relatively high rates of movement between the oceanic plates and the collision zone also analogous to the cordillera is the formation of a foreland fold and thrust belt developing between the collision zone and the australian continent and a zone of basement rooted foreland folds suggestive of an early laramide style of deformation

the origin of ferro manganese oxide coated pumice from the central indian ocean basin pumice clasts partially and fully coated with ferro manganese oxide from the central indian ocean basin ciob were analysed for major trace and rare earth elements and glass and mineral grain chemistry to assess their possible source these pumice clasts have 95 glassy matrix with 5 of plagioclase ortho pyroxene amphibole and spinel mineral grains pumice is rhyolitic with high content of silica and total alkalis and show strong light rare earth element enrichment with la 100 times enriched compared to chondritic abundance and exhibits a pronounced negative eu anomaly both the partially and fully coated pumice clasts have a similar composition but this and their glass and mineral orthopyroxene chemistry differs from all tuffs known to have erupted from the toba caldera complex the partial to complete fe mn oxide coating suggests the pumice probably predates activity from the toba caldera their general composition is consistent with a volcanic arc origin and show similarities to rhyolitic eruptives from sumatra the distribution of partially and fully coated pumice in the ciob overlaps with that of uncoated pumice from the voluminous ytt eruption suggesting that other large rhyolitic calderas active in sumatra indonesia similarities in composition to ash layers from the ciob suggest a possible late miocene late pleistocene age for these pumices

elasticity of a majorite pyrope solid solution the elastic properties of a solid solution consisting of 59 mole percent pyropic garnet and 41 mg sub 4 si sub 4 o sub 12 majorite a high pressure form of pyroxene have been measured by brillouin spectroscopy under ambient conditions values of the adiabatic bulk modulus k sub s 164 plus minus 15 gpa and shear modulus g 89 4 plus minus 3 7 gpa are similar to those for low pressure aluminosilicate garnets the enstatite majorite reaction is characterized by a large 49 increase in k sub s and a much smaller 13 increase in g the properties of low pressure garnets suggest that the contrast in shear moduli for this reaction is probably larger in ca rich systems taken at face value the stp properties of these phases imply that large quantities of ca rich clinopyroxene are needed in order to satisfy seismologically determined shear modulus gradients in the transition zone this suggestion is however subject to considerable uncertainties in the pressure and temperature derivatives of elastic properties for mantle minerals and the velocity structure of the transition zone

all toba tephra occurrences across peninsular india belong to the 75 000 yr b p eruption abstract a controversy currently exists regarding the number of toba eruptive events represented in the tephra occurrences across peninsular india some claim the presence of a single bed the 75 000 yr old toba tephra others argue that dating and archaeological evidence suggest the presence of earlier toba tephra resolution of this issue was sought through detailed geochemical analyses of a comprehensive suite of samples allowing comparison of the indian samples to those from the toba caldera in northern sumatra malaysia and importantly the sedimentary core at odp site 758 in the indian ocean a core that contains several of the earlier toba tephra beds in addition two samples of toba tephra from western india were dated by the fission track method the results unequivocally demonstrate that all the presently known toba tephra occurrences in peninsular india belong to the 75 000 yr b p toba eruption hence this tephra bed can be used as an effective tool in the correlation and dating of late quaternary sedimentary sequences across india and it can no longer be used in support of a middle pleistocene age for associated acheulian artifacts

biogeographic constraints for tectonic reconstructions of the pacific region suspect terranes in western north america contain permian and triassic genera that were endemic to the tethyan region trend surface analysis of modern biogeographic data constitutes a new approach to understanding the origin of these exotic faunas we use the generic diversity of living reef corals together with paleomagnetic results from quaternary lavas to calculate the present locations of easter island hawaii and tahiti the results demonstrate that the eastward decline in coral diversity may be used as a measure of longitude similar data from the fossil record should constrain paleogeographic reconstructions of the pacific region

a possible submarine volcano near the central part of ninety east ridge indian ocean abstract floating pumice masses have been observed several times near the central part of ninety east ridge indian ocean in 1879 and in 1883 the author is of the opinion that the source of these pumice masses cannot have been the volcano krakatau in the sunda strait as has been suggested earlier by other authors but that they originated in situ examples of floating pumice observed over submarine volcanoes lying at depths of 1500 m or even more are mentioned briefly by other authors suggesting that eruptions in deep water may indeed lead to its formation the suspected submarine volcano may coincide with an elongated but rather flat elevation a little north of the central part of ninety east ridge the center of this elevation is located approximately at 6 05 s 89 10 e a roughly nw se profile of this area shows the presence of this shield like feature very near the site where floating pumice was seen on three occasions the inferred submarine volcanic center is lying within the belt of heat flow anomaly called belt i a particular hot spot with an unusually high heat flow value 7 61 hfu is located southeast of the feature in question the heat flow values measured in belt i are well above the world average this belt may be the result of the presence of linearly arranged magma chambers at a relatively shallow depth probably within the oceanic crust this belt has a length of 1100 km the seismic activity of the area investigated is similarly remarkable numerous powerful shocks have taken place there belt i represents the axis of the seismic zone extending from points b to c fig 5 the seismic belt is regarded as a zone of weakness within which fractures can originate permitting rock melts to emerge from depth towards the surface

single crystal x ray structure study of synthetic pyrope almandine garnets at 100 and 293 k the crystal structures of synthetic pyrope mg3ai2si3012 almandine feai2si3012 and the solid solution garnet compositions py8o alm2o py6o alm4o and py2o alm8o have been refined in space group ia1d from high precision x ray diffraction data with sin 0 0 4 a i measured at 100 and 293 k there is no indication of lower symmetry for pyrope almandine or solid solution members experimentally determined atomic coordinates and displacement parameters for the solid solution compositions are in good agreement with those linearly interpolated from the end members thus there are no apparent structural features that could account for substantial nonideal enthalpies of mixing in the system pyrope almandine the tetrahedral rotation angle is inversely correlated with the x o distance fe2t substitution on the eight coordinated x site of pyrope or increasing temperature decreases the rigid tetrahedral rotation in garnet the large and anisotropic displacement parameters for the x site cations in garnet are mainly a result of anisotropic thermal vibrations along the longer x o bonds which produce nonrigid polyhedral behavior for the dodecahedral site the tetrahedra and octahedra behave as rigid bodies these strong vibrations of the former give rise to the relatively large heat capacities and third law entropies in garnet previous proposals concerning subsite dodecahedral ordering in pyrope must be revised

evolution of a permo triassic sedimentary melange grindstone terrane east central oregon the grindstone terrane in east central oregon is one of the few areas in western north america where large blocks of unmetamorphosed devonian mississippian and permian limestones are inter mixed with permian and lower triassic radiolarian chert and pennsylvanian permian and triassic volcaniclastic rocks although originally described as parts of a coherent succession we interpret the grindstone rocks to be a sedimentary melange composed of paleozoic limestone slide and slump blocks that became detached from a carbonate shelf fringing a volcanic knoll or edifice in late permian to middle triassic time and were intermixed with permian and triassic slope to basinal clastic and volcaniclastic rocks in a forearc basin setting paleogeographic affinities of the grindstone limestone faunas and volcaniclastic debris in the limestone and clastic rocks all indicate deposition in proximity to an island arc system near the north american craton the grindstone terrane deposits are unconformably overlain by upper triassic to middle jurassic sequences of the izee terrane although lithologic and faunal differences indicate that the grindstone and izee terranes together represent a tectonic block separate from the adjacent baker terrane all three terranes were juxtaposed by late triassic or early jurassic time we recommend reduction to informal status for the coffee creek mississippian spotted ridge pennsylvanian and coyote butte permian formations because 1 they cannot be mapped or traced beyond limited areas and 2 these older rocks are chaotically inter mixed with younger chert and volcaniclastic rocks new biostratigraphic data indicate that the bulk of the spotted ridge volcaniclastic rocks represent a part of the triassic vester formation of the izee terrane

elasticity of majorite and a majorite pyrope solid solution to high pressure implications for the transition zone 1 high seismic velocity gradients in the transition zone have been attributed to large pressure derivatives of the elastic moduli of transition zone minerals such as majorite garnet here we present brillouin scattering measurements of the elasticity of polycrystalline mg majorite mj100 mg4si4o12 and a mj50py50 majorite pyrope solid solution to pressures similar to those at the top of the transition zone 15 1 1 gpa and 12 8 1 gpa respectively the pressure derivatives of the adiabatic bulk ks and shear moduli of both mj50py50 and mj100 are 4 2 3 and 1 4 2 respectively and are equal to those of pyrope within the experimental uncertainties we conclude that neither the adiabatic compression of majorite garnet nor any other likely transition zone phase is able to produce the high velocity gradients observed seismically high velocity gradients are consistent with gradual transformations between minerals with very different elastic properties the transformation of 25 or more of clinopyroxene into majorite above 520 km and the formation of ca silicate perovskite below 520 km could produce the observed seismic gradients

island arc sedimentation in the middle devonian kennett formation eastern klamath mountains california the kennett formation is a unit of intra arc sediments which was deposited during the final stages of early to middle devonian island arc construction in the eastern klamath mountains california nearshore bioturbated volcaniclastics with a shallow marine fauna also contain rare debris of land plants indicating island source areas these sediments grade to volcaniclastic turbidites of slope origin which in turn grade to thin bedded siliceous mudstone representing a basin floor environment limestone lenses at the top of the kennett are debris deposits with matrix supported intraclasts and overturned colonial skeletons which were derived from shallow carbonate banks sedimentary facies of the kennett indicate a mature arc topography of islands submerged highs and intra arc basins basins were probably developed by tectonic extension of the arc during the early devonian

plastic deformation of garnets systematics and implications for the rheology of the mantle transition zone abstract plastic properties of materials with garnet structure have been studied under wide temperature conditions ranging from room temperature to 95 of the melting temperatures using uniaxial compression and hot microhardness tests garnets studied include single crystals of oxide garnets y 3 al 5 o 12 gd 3 ga 5 o 12 and y 3 fe 5 o 12 and silicate garnets various solid solutions including grossular almandine andradite pyrope spessartine and uvarovite both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions t t m and and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the earth s mantle based on both stress dip tests and microstructural observations it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes this conclusion is consistent with the high peierls stress inferred from the hot hardness tests the high peierls stress in garnets is presumably due to the large unit cell i e long burgers vectors and or the bcc packing which are common to all garnets we postulate therefore that the present results can be applied to the strength of high pressure garnet majorite and suggest that garnet rich layers in the earth such as subducted oceanic crust in the transition zone or a possibly garnet rich bottom part of the transition zone will be considerably stronger than surrounding regions

biological pumping upwards of the coarse fraction of deep sea sediments abstract in a restricted part of an area intensively sampled by box coring approximately 50 samples from a 2 4 km rectangle a stiff muddy glacial gravel till is encountered at a depth of 30 40 cm x radiographs of the upper 15 to 20 cm of the cores reveal gravel in overlying foraminiferal mud only from the area where there is the deeper till layer the localization strongly suggests that the material in the top of the cores has its source in the deeper layer i postulate that burrowing organisms move gravel upward by pushing aside the particles that are too large to be eaten because the strength of the enclosing mud decreases upwards the gravel particles move preferentially upward the upward decrease in shear strength of he sediment is rapid at an interface occurring 4 cm below the surface on average thus hydrostatic pressures of 20 to 200 g cm2 exerted by vermiform burrowing animals act within an environment of asymmetrical sediment property permitting an asymmetrical response to the uniformly directed body pressure this asymmetrical response a pumping mechanism affecting material that does not pass through the gut is contrasted with the mixing of finer sediments that are eaten and excreted documented by many other workers dimensional arguments suggest the flux of material is related to the ratio of biological mixing rate to sedimentation rate the mechanism is also applicable to maintenance of mn nodules and their nuclei at the surface if macrofossils have behaved like gravel some s ell beds may have been generated and some stratigraphic anomalies caused by this means

development of a late visean mississippian mixed carbonate siliciclastic platform in the guadalmellato valley southwestern spain abstract the penas rubias syncline southwestern spain exposes a well preserved shallow water platform succession containing a spectrum of facies corresponding to inner platform intertidal to supratidal environments across to deeper water middle to outer platform environments nineteen microfacies are recognized which are grouped into seven facies association corresponding to siliciclastic deltaic bars mixed carbonate siliciclastic shoals carbonate mud mound boundstones background platform carbonates background platform siliciclastics mixed tempestites and deep water siliciclastic shales and sandstones the age of the succession was determined mainly by foraminiferans and calcareous algae which permit the succession to be assigned to the late brigantian latest visean this upper brigantian platform is the only record of sedimentation of this age in the region and thus is key for interpreting the sedimentary and tectonic evolution of the carboniferous rocks in sierra morena biotic and sedimentological features were analyzed in order to assess the controls on the sedimentation several factors have influenced sedimentological changes turbidity subsidence siliciclastic discharges storms and bioturbation the siliciclastic discharges exerted a considerable control on the basal deposits mostly in their percentage of quartz sand grains and as microconglomerates however they did not develop as large deltaic deposits and their influence can be considered as virtually negligible in regards to the remaining part of the succession turbidity as a result of higher percentage of silt and mud in suspension seems to be the main factor controlling the change between the older intertidal deposits in the inner platform to the younger subtidal deposits of the middle and outer platform as a result of the increase of the mud and silt in suspension facies changed first to marlstones and nodular argillaceous limestones and second to predominantly calcimicrobial boundstones and shales in the uppermost part of the carbonate succession as well as showing a marked change from photic controlled benthic faunal and microfloral assemblages to assemblages more tolerant or better adapted to muddier dysphotic substrates bioturbation is also interpreted as one of the main controls influencing the different type of boundstones in the middle platform permitting the vertical growth of dome shaped mud mounds or as sheet like deposits storm influences seem to have exerted some control on the positive relief of the dome shaped boundstones which are usually capped by tempestites the differential subsidence observed in the northwestern sector of the platform allowed the accumulation of many stacked dome shaped mounds a feature not recognized in southeastern parts the general stratigraphical sequence seems to be controlled by eustasy and synsedimentary tectonics the overall succession exhibits a pronounced deepening upwards transgressive sequence from siliciclastic delta bars mixed shoals carbonates and shales of the middle platform with the growth of calcimicrobial boundstones and dark green shales passing up into black shales at the top of the sequence in the outer platform and possibly submarine slope settings although this transgressive sequence is in harmony with the 3rd order glacioeustatic cycle defined for the late brigantian in the western palaeotethys all these brigantian rocks accumulated in a synsedimentary extensional regime which is related to the initiation of a sinistral strike slip regime previously recognized as affecting only serpukhovian and younger rocks in sierra morena these synsedimentary faults allow us to recognize significant lateral variations in thickness over short distances

nd and sr isotopic characterization of the wrangellia terrane and implications for crustal growth of the canadian cordillera nd and sr isotopic data are reported from samples of most of the major lithologic units of the wrangellia terrane exposed on vancouver island and the queen charlotte islands initial epsilon nd values range from 1 0 to 7 3 and initial 87 sr 86 sr ratios range from 0 70323 to 0 70481 these ratios are similar to those of modern island arcs and demonstrate the unevolved juvenile nature of this terrane these data indicate that wrangellia probably resided in an intra oceanic environment until its accretion to north america nd crustal residence ages for clastic sedimentary rocks of wrangellia mean t cr 0 77 pm 0 33 ga t dm 0 57 pm 0 35 ga are much closer to their stratigraphic ages than is the case for the ages of many other phanerozoic sedimentary rocks thus providing strong evidence for phanerozoic crustal addition to north america early proterozoic crustal components cannot comprise more than about 6 of wrangellia therefore the terrane consists of 94 new mantle d

rheological control of oceanic crust separation in the transition zone mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle we have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle confirming previous results the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling however a soft layer may exist between the strong garnet crust and cold slab interior models employing a simplified newtonian sandwich model show that this thin weak layer can effectively decouple the crust and slab once entrained into the lower mantle the then lighter crust can rise sufficiently fast as a rayleigh taylor instability to avoid further entrainment these results suggest that the crustal component of slabs may be trapped at 670 km depth leading to a garnet enriched transition zone

elasticity of pyrope and majorite pyrope solid solutions to high temperatures abstract majorite garnet solid solutions are major mineral phases in the earth s upper mantle and transition zone here we present the first brillouin scattering measurements of the elasticity of majorite mj mg 4 si 4 o 12 pyrope py mg 3 al 2 si 3 o 12 solid solutions mj 50 py 50 and mj 80 py 20 and single crystal elasticity of pure synthetic pyrope at temperatures up to 800 c the temperature derivatives of the adiabatic bulk k s and shear moduli for all compositions along the mj py join are the same within the experimental uncertainties k s t 14 0 14 5 20 mpa k t 8 3 9 2 10 mpa k the temperature dependence of the acoustic velocities for mj py solid solutions is about half that of other major transition zone minerals this implies that temperature variations in the transition zone inferred from lateral velocity heterogeneity can be significantly underestimated if the properties of majoritic garnet are not taken into account

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

significance of tethyan fossils in the american cordillera equatorial faunas of the ancient tethyan seaway which extended from western europe to southeastern asia comprise some of the most diverse marine taxa in the fossil record comparable or identical tethyan species that occur far from the tethyan seaway in paleozoic and mesozoic rocks of the north and south american cordillera have long been considered as a major biogeographic anomaly two leading theories to account for the occurrence of these anomalous tethyan faunas in the cordillera are that they were transported long distances to the east on tectonic blocks suspect terranes that originated near the tethys or that they migrated westward via undiscovered marine corridors through continental areas of pangea an alternative model is that these tethyan fauna were pantropic species that extended with attenuated diversities into the eastern proto pacific ocean this pantropic model can better account for the distribution patterns of many paleozoic and early mesozoic tethyan species in the american cordillera and provides a steady state hypothesis against which the other models can be tested the distribution of pre cretaceous tethyan faunas is similar to the known pantropic distribution of many cretaceous and cenozoic tropical biotas during the cenozoic taxa were most diverse in the tethys and indo west pacific regions but extended with attenuated diversity to many parts of the equatorial pacific ocean including the west coasts of north and south america the eastern pacific occurrence of many indo west pacific species provides a modern analog for the occurrence of many anomalous tethyan fossils in the american cordillera

uranium thorium isotopes and transition metal fluxes in two oriented manganese nodules from the central indian basin implications for nodule turnover transition metal fluxes to the top and bottom of two oriented manganese nodules ss 657 and sk 176 were determined by combining radiochemical and geochemical analyses distinct differences in transition metal fluxes 230th contents 230th232th and mnfe ratios and silicate mineralogy in the top and bottom of nodule ss 657 suggest that since the time of inception of oxyhydroxide accretion 1 m y b p it has been growing with a fixed orientation on the seafloor in nodule sk 176 in contrast nearly uniform accumulation rates 230thexc and 230thexc232th extrapolated to surface activities transition metal fluxes and increasing diagenetic influence outwards in the top and bottom are thought to be suggestive of rotation of the nodule several times over time intervals which are smaller than the time resolution involved in u th isotope dating techniques

dispersed remnants of a northeast pacific fringing arc upper paleozoic terranes of permian mccloud faunal affinity western u s two fragmentary subparallel belts of terranes within the western north american cordillera contain upper paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting permian faunal affinity these two belts are a volcanic arc related successions of permian mccloud faunal affinity mccloud belt and b subduction related accretionary complexes of permian tethy an faunal affinity cache creek belt this paper supports the hypothesis that the fragmentary terranes of the mccloud belt once constituted parts of a northeast pacific fringing arc system and in constrast to some earlier interpretations concludes that the volcanic arc evolved above an eastward dipping subduction zone the absolute distance between this arc and western north america during the late paleozoic cannot be constrained however there is little evidence to suggest closure of a major 10 km ocean basin or protracted periods of westward dipping subduction parts of devonian to permian volcanic island arc sequences of the western u s cordillera are represented in the northern sierra eastern klamath bilk creek grindstone and chilliwack terranes these scattered volcanic arc remnants share several fundamental characteristics 1 the sequences were constructed across continental affinity basement assemblages 2 they underwent similar tectonic evolution during late paleozoic time such as coeval pulses in volcanism and related depositional histories 3 they contain early permian mccloud type fauna of distinctive biogeographic affinity 4 mccloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the cache creek belt which contain fragments of upper triassic blueschist and permian limestone blocks bearing tethyan permian fusulinids and corals based on the presence of distinctive early permian mccloud fauna the island arc remnants discussed in this paper are referred to as the mccloud belt the similarities between mccloud belt terranes suggest that they were once parts of a long lived late paleozoic volcanic arc complex the stratigraphy of these terranes records the tectonic evolution of the volcanic arc volcanism within the paleozoic arc began as early as the early devonian and was succeeded or accompanied by carbonate and epiclastic sedimentation during the middle and late devonian an influx of detritus derived from sedimentary and metasedimentary rocks during the late devonian and mississippian records the uplift and erosion of arc basement sequences an event that was at least locally accompanied by ongoing volcanism a pulse in volcanism during the pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and locally subaerial exposure which persisted into the early early permian elsewhere during the early permian shallow water carbonate deposition was established with only minor influx of volcanogenic debris during the late permian and locally as early as late early permian a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas most of these sequences are overlain by upper triassic carbonate platformal sequences arc volcanism was recorded at least locally until the middle jurassic paleomagnetic data are used to constrain a schematic pre cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages the inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone with a long lived paleogeographic relation to north america the current structural position of these terranes is largely the result of mesozoic and locally younger deformation

dislocation microstructures in majorite garnet experimentally deformed in the multi anvil apparatus majorite garnets with the composition mj70py30 have been deformed in the multi anvil apparatus at 17 gpa and 2000 c the microstructure has been characterized by transmission electron microscopy it is shown that under these conditions majorite garnet is ductile dislocations with and burgers vectors are observed with a density 1 5 1012 m 2 the absence of clear glide planes and the occurrence of subgrain boundaries suggest the importance of diffusion and climb in the plasticity of majorite garnets in mantle transition zone conditions

late pleistocene holocene acceleration of uplift rate in southwest erromango island southern vanuatu south pacific relation to the growth of the vanuatuan mid sedimentary basin late quaternary and holocene raised coral reefs are well developed in southwestern erromango island which lies in the frontal arc area of the vanuatuan island arc eight uranium series ages and one 14 c age from samples from coral reefs at three localities range in age from 4 800 b p to about 320 000 b p six of the samples dated are from the matiwo point area here the youngest reef has given a 230 th 234 u age of 4 800 b p and a slightly older reef 4 3 m higher in elevation has a 14 c age of 5 270 b p inland of a cliff the youngest three of four northeastward tilted raised reefs have given 230 th 234 u ages ranging from 104 000 b p to about 320 000 b p these data indicate accelerating uplift rates for southwest erromango during the periods 320 000 133 000 b p 133 000 6 000 b p and 6 000 0 b p average uplift rates were 0 33 mm yr 0 65 mm yr and about 1 mm yr respectively these data are interpreted to indicate the growth of the mid sedimentary basin which lie

new occurrence of youngest toba tuff in abyssal sediments of the central indian basin abstract volcanic glass and pumice found in siliceous abyssal sediments of the central indian basin south of the equator have previously been assigned various origins including intra basin volcanism indonesian arc and krakatau rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the youngest toba tuff erupted at 74 ka from northern sumatra the correlation extends the distribution of coarse 63 m glass shards some 1500 km south of the previously known fallout zone and into the southern hemisphere reaching latitudes of ca 14 s this provides evidence for bi hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption such dispersal could have facilitated the global impact of the eruption

relative strength of the pyrope majorite solid solution and the flow law of majorite containing garnets even though the garnet phase is the second most abundant phase in the upper mantle and transition zone no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions here we report the results of constant strain rate and stress relaxation experiments on garnets in the pyrope majorite solid solution which constrain the strength of majoritic containing garnets relative to pyrope as a function of majorite content and temperature we find that at temperatures below 650 degrees c both pure pyrope and majoritic garnets have the same strength conversely above 650 degrees c we find that majoritic garnets are initially stronger than pure pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than pyrope above approximately 800 degrees c we develop a flow law for the entire pyrope majorite solid solution as a function of temperature and majorite content c 2010 elsevier b v all rights reserved

elastic wave velocities of pyrope majorite garnets py62mj38 and py50mj50 to 9 gpa abstract polycrystalline specimens of two pyrope majorite garnets 2 py 62 mj 38 and py 62 mj 38 were synthesized in a 2000 ton uniaxial split sphere apparatus ussa 2000 at pressures of 16 gpa and temperatures of 1670 k for run durations of about 2 h using homogeneous glasses as starting materials ultrasonic interferometric measurements on these specimens were conducted at pressures up to 9 gpa at room temperature using a 1000 ton uniaxial split cylinder apparatus usca 1000 with bi as an internal pressure marker from the measurements of the travel times for compressional p and shear s waves the velocities and elastic moduli are calculated using length density changes determined from the travel time data third order eulerian finite strain analysis of these data yield the longitudinal l and shear g moduli and their pressure derivatives m 0 m p t for py 62 mj 38 l 0 291 6 gpa and l 0 9 1 0 6 g 0 90 1 gpa and g 0 1 9 0 2 from which we calculate the values for the adiabatic bulk modulus k l 4 3 g to be k 0 171 5 gpa and k 0 6 2 0 5 these new data for the shear modulus and its pressure derivative are in agreement with the previous values for the same composition rigden s m gwanmesia g d liebermann r c 1994 elastic wave velocities of a pyrope majorite garnet to 3 gpa phys earth planet inter 86 35 44 but the p wave velocity the bulk modulus and its pressure derivative are somewhat higher for the py 50 mj 50 composition these values are l 0 289 6 gpa l 0 9 2 0 7 g 0 89 1 gpa g 0 2 1 0 2 k 0 170 5 gpa and k 0 6 4 0 5 the high pressure derivatives of k and g for these py mj garnets may be important in explaining the high velocity gradients observed in seismic models of the transition zone of the earth s mantle

plastic flow of pyrope at mantle pressure and temperature despite the abundance of garnet in deforming regions of the earth such as subduction zones its rheological properties are not well defined by laboratory measurements here we report measurements of steady state plastic properties of pyrope in its stability field temperature up to 1573 k pressure up to 6 8 gpa strain rate approx 10 5 s 1 using a deformation dia apparatus d dia coupled with synchrotron radiation synthetic pyrope py100 and natural pyrope py70alm16gr14 are both studied in a dry environment transmission electron microscopy tem investigation of the run products indicates that dislocation glide assisted by climb within grains and dynamic recrystallization for grain boundary strain accommodation is the dominant deformation process in pyrope both synthetic and natural pyropes stress and strain rate data as measured in situ by x ray diffraction and imaging are best fitted with the single flow law

paleo oceanographic and tectonic settings of early miocene reefs and associated carbonates of offshore southeast asia the late oligocene to early middle miocene was a period of widespread reef and associated carbonate deposition over a wide latitudinal range in the offshore region of southeast asia a generally rising eustatic sea level during this period allowed the accumulation of thick reefal units an expansion of the coral algal regional facies belt north to japan and south to new zealand occurred during the early miocene as the result of a reduced oceanic latitudinal temperature gradient an episode of enhanced organic carbon burial is reflected in a positive dgr13c excursion from 17 5 to 13 5 ma increasing paleo oceanographic equability may have resulted in reduced ocean circulation with the resultant development of an expanded oxygen minimum zone marine oil prone or anic carbon was deposited in silled anoxic basins both bounded by and within island arc systems a combination of tectonic and paleo oceanographic factors therefore created conditions that favored contemporaneous deposition of organic carbon rich source beds and reef reservoirs during the latest early to early middle miocene paleogeographic and facies reconstructions for an 18 m y interval indicate that deposition occurred in three tectonic settings 1 passive continental margins 2 convergent plate boundaries and 3 obliquely convergent plate boundaries in all these settings the vertical motion histories of fault bounded blocks and of larger tectonic elements within which they are contained directly controlled the geographical distribution of carbonate buildups their scal and morphology duration diagenesis and relationship to siliciclastics

permian longitudes of wrangellia stikinia and eastern klamath terranes based on coral biogeography trend surface analysis of biogeographic data and probabilistic estimates of diversity and similarity provide a new approach to understanding the origin of north american suspect terranes results from permian corals indicate that 1 the eastern boundary of the tethyan coral province was about 2000 km west of the north american craton 2 eastern klamath and stikinia were close to one another and were up to 6700 km west of north america during the early permian 3 wrangellia was situated to the southeast of stikinia as much as 5000 km west of the craton 4 the late permian location of the eastern klamath terrane was 3000 5800 km west of the craton and between 11 and 21 n and 5 the eastern klamath terrane moved westward during the permian possibly as a result of back arc spreading

depositional history of an upper triassic drowned carbonate platform sequence wallowa terrane oregon and idaho the upper triassic martin bridge limestone exposed in hells canyon along the idaho oregon border is a thick sequence of limestone and dolostone the allochthonous nature and low latitude origin of the martin bridge and underlying seven devils group which along with coeval rocks exposed in the wallowa mountains make up the wallowa terrane are well established the martin bridge limestone in hells canyon was deposited initially under supratidal and intertidal conditions and then in a shallow subtidal environment absence of terrigenous or volcanic sediments indicates that these carbonates were isolated from a source of such sediment although potential reef building organisms are present in the martin bridge limestone no reef structures are exposed in hells canyon the shelf edge environment apparently was dominated by relatively coarse grainstone and packstone shoals and banks stratigraphic evidence from the wallowa mountains and near riggins idaho indicates that the martin bridge carbonate platform was eventually drowned and overlain by fine grained argillaceous and siliceous clastic sediments decreased benthic carbonate production due to climatic deterioration initiated by northward drift and increased clastic and volcaniclastic input from adjacent terranes and the wallowa terrane itself facilitated drowning of the carbonate platform

the collision zone between the north d entrecasteaux ridge and the new hebrides island arc 1 sea beam morphology and shallow structure sea beam bathymetric data closely spaced single channel seismic reflection sections and geopotential field data were collected aboard the r v j charcot over the collision zone between the north d entrecasteaux ridge ndr and the new hebrides island arc the ndr trends east at a small angle 14 to the plate convergence direction so that the ridge creeps northward along the trench at about 2 5 cm yr requiring a continual structural adjustment of the accretionary complex in this report we study the shallow structure tectonic erosion and consequent mass wasting of the accretionary complex all of which were produced by the collision of the ndr in a complex deformational environment the accretionary complex in this collision zone can be divided into northern central and southern parts the northern part of the collision zone which lies north of the leading flank of the ridge shows a lobate lower accretionary complex that is structured by east to southeast dipping thrust and reverse faults and n60 w trending strike slip faults a middle accretionary complex that bulges seaward and appears to be rotated 45 e from the regional arc trend and an upper accretionary complex that is shaped by slumps and a canyon network the central part of the collision zone which directly overlies the crest of the ridge forms a broad shallow protrusion that is bounded on its northern and southern sides by steep scarps rocks forming this protrusion have been uplifted possibly by as much as 1500 2500 m and tilted to the north causing northward block sliding along an extensional detachment surface the southern part of the collision zone which lies in the wake of the ridge is deformed by large slumps and normal faults that trend parallel to the ridge axis rocks forming this southern part collapse causing widespread mass wasting these geophysical data indicate that normal trench convergence has apparently produced only a few trench parallel structures confined to the toe of the accretionary complex whereas the along trench motion of the ndr has resulted in well developed structures that extend obliquely across the arc slope and form an asymmetric tectonic pattern the area of the accretionary complex that is disturbed by collision appears to be about twice as wide as the width of the ndr and the time required to heal the disturbance may be about 0 8 m y

ash layers and pumice in the central indian basin relationship to the formation of manganese nodules abstract manganese nodules in areas of biosiliceous sedimentation in the central indian basin cib have three main types of nuclei 1 fragments of altered ash layer 2 felsic pumice and 3 indurated residual sediments red clay type morphological variations among nodules are due to the form of the nuclei the type of accretionary process diagenetic or hydrogenetic accretion and thickness of the crust piston cores in the same areas sampled up to four tephra layers interstratified in red clay all of which underlie condensed series of pliocene to recent siliceous oozes the in situ tephra layers and fragments of tephra in the core of the nodules are similar in composition they are completely altered to smectite and smectite zeolite tephra horizons are 10 35 mm thick and have sharp contacts with the red clay rare earth element ree patterns zr hf and nb content and ti al ratios strongly suggest that this altered volcaniclastic material originated as distal fallout from the indonesian volcanic arc iva and was deposited in late neogene time positive ce ce anomalies indicate that alteration of this tephra may have occurred at the sediment water interface prior to burial under hydrogenous conditions rhyolite pumicelapilli are water laid and are variably replaced by fe mn oxides inducing a higher metal content in the bulk nodules pumice constitutes one of the main types of nuclei of diagenetic nodules that are found in the deep sea troughs and plains where biosiliceous deposits are thick petrological characteristics of the pumice suggest that volcanic eruptions from toba in northern sumatra are the most likely source tectonic deformation of the indian plate may have affected the sedimentary sequence causing uplift and outcrop of ash layers on the topographic highs and slopes eroded fragments of these ash layers and waterlaid pumice fragments have contributed significantly to the large extent of mn nodules in the cib

the kinetics of the reaction of majorite plus ferropericlase to ringwoodite implications for mantle upwellings crossing the 660 km discontinuity we have measured the kinetics of reaction between mgo and majoritic garnet at 20 gpa and 1773 2123 k as a proxy for the reaction between perovskite and ferropericlase during mantle upwelling across the 660 km seismic discontinuity ringwoodite forms a layer between mgo and garnet and in the case of aluminous garnets the interface between ringwoodite and garnet develops a fingering instability resulting in a complex intergrowth at this interface by contrast the mgo ringwoodite interface is always planar for an initial planar mgo garnet interface two thicknesses are therefore defined 1 a layer thickness x1x1 which is the maximum thickness of ringwoodite which forms a plane parallel bounded layer next to the mgo and 2 an interface thickness x2x2 which is the maximum extent of the intergrowth region away from the ringwoodite layer the growth of both of these regions can be described by apparent rate constants kiki which are arrhenius with view the mathml sourceln k10 6 36 0 25 m2 s and e1 456 40 kj mole1 456 40 kj mol for the ringwoodite layer and view the mathml sourceln k20 9 2 3 3 m2 s and e2 371 53 kj mole2 371 53 kj mol for the intergrowth region the fingering instability is caused by the incompatibility of aluminium in ringwoodite and its low chemical diffusivity in garnet which results in an increase of surface area at the ringwoodite garnet interface to minimise the aluminium concentration at the interface the intergrowth region contains a fine grained mixture of ringwoodite and garnet which coarsens very slowly with time this might result in a transient weakening of upwelling regions of mantle just above the 660 km seismic discontinuity allowing some viscous decoupling between the upper and lower mantle

chemical aspects of alteration of acidic tuffs and their application to siliceous deposits in order to evaluate fundamental chemical characteristics of acidic tuffs and to determine which of these characteristics in volcanogenic sediments is preserved during weathering processes we have analysed 41 pairs of ash samples each pair consists of 1 raw samples obtained from ash layers which consist of a mixture of volcanic glasses and clays and 2 glass shard aggregations extracted from the raw samples the migration of elements in the samples during alteration was examined using the analysed values of these pairs the results showed that certain compositional ratios of the glass shards do not basically change during alteration several diagrams showing these essentially constant ratios in acidic 65 sio2 volcanic glasses are presented these diagrams are helpful in identifying altered siliceous deposits derived from acidic tuffs and in discriminating such deposits from biogenically derived cherts

plastic deformation of silicate garnets ii deformation microstructures in natural samples abstract we have used transmission electron microscopy to characterize the deformation microstructures in natural garnets from several localities eclogites from sesia lanzo alps eclogites and garnet amphibolite from braganca portugal garnet pyroxenite from lherz france and eclogites from yakutia pipe siberia two characteristic microstructures have been identified the first found in eclogites from the alps consists of microplasticity associated with microcracking that suggests brittle behavior the remaining samples show a microstructure characteristic of dislocation creep with recovery sub grain boundaries the transition between these deformation regimes occurs at 600 c i e at the boundary between group c and group b eclogites the deformation microstructures suggest that the dislocations experience strong lattice friction below 600 c whereas at high temperature diffusion assists dislocation glide and climb recovery we observed the following glide systems in the whole temperature range 1 2 111 110 1 2 111 112 1 2 111 123 100 010 and 100 011 no correlation could be established between the dislocation microstructure and the hydrous component or the chemistry of the garnets among the pyralspites comparison of the natural deformation microstructures and those generated in high pressure experiments voegele et al 1998 part i validates the extrapolation of experimental data to nature

olivine flow mechanisms at 8 gpa abstract the mechanisms responsible for high temperature olivine deformation are investigated at a pressure of 8 gpa and temperatures up to 1780 k san carlos olivine specimens of different average grain sizes 0 5 and 5 m were deformed simultaneously between hard alumina pistons during relaxation experiments these experiments are carried out in a multi anvil high pressure apparatus coupled with synchrotron x ray radiation the different grain size specimens experienced identical p t stress condition at any given time a new method for measuring strains and strain rates 10 6 s 1 of specimens at high pressure is documented this method uses time resolved in situ x ray imaging and an image analysis computation the microstructures of run products recovered after being quenched at different temperatures were characterized by transmission electron microscopy tem we find that high temperature olivine flow is grain size insensitive at 8 gpa which suggests that dislocation creep dominates olivine deformation at high pressure this result is confirmed by the tem investigation of our deformed specimens in which we find evidences of the activation of olivine dislocation slip systems specimen microstructures are consistent with dynamic recrystallization as an assisting process in olivine deformation during the high pressure experiments extrapolation of our results to the low stress level and large grain size expected in the mantle suggests that dislocation creep assisted by dynamic recrystallization may also dominate natural olivine deformation in the upper mantle

upper triassic sponges sphinctozoa from southern yukon stikinia terrane the first tethyan type upper triassic sponge reefs in north america have recently been discovered in the southern yukon in stikinia terrane sphinctozoa from these reefs and inter reef limestones are represented by 18 species and 12 genera belonging to six families the genus yukonella with one species y rigbyi and the species polytholosia ramosa and polycystocoelia norica are described for the first time some of the yukon sponges were previously known only from the tethyan realm colospongia dictyocoelia polycystocoelia and uvanella others from tethyan and north american localities ascosymplegma follicatena paradenigeria and salzburgia one polytholosia cylindrica is probably endemic to north america the mixed affinities of the yukon sponges may reflect the paleogeographic origin of stikinia as an island in the ancestral pacific ocean between tethys and the north american craton

oceanic island arc stratigraphy in the caribbean region don t take it for granite abstract a popular misconception of island arcs is that they consist mostly of intrusive rocks belonging to the granite family and volcanic rocks study of exhumed caribbean island arcs indicates that they are composed mostly of sedimentary rocks such as epiclastic volcanic materials and true plutonic and volcanic rocks are relatively minor a simple model for the stratigraphic development of oceanic island arcs is proposed based on observations in the caribbean region and intuitive reasoning the lowermost part of the model arc is oceanic crust oceanic arc depositional system 1 or oads i built atop of the oceanic crust is the arc sequence including volcaniclastic strata lava flows with feeder dikes and plutons oceanic arc strata consist of a lower pyroclastic absent volcanic unit oads ii formed by submarine volcanism and an upper unit oads iii bearing volcanic pyroclastic and epiclastic rocks referred to here as the pyroclastic present association or unit both oads ii and iii may contain intercalated pelagic and or biogenic sediments such as limestones and cherts submarine volcanic rocks include pillowed and sheet lava flows lithic and hyaloclastic breccias and resedimented lithic and hyaloclastic debris oads iii can be recognized by the presence of shallow water fossils stream or beach worn volcanic clasts and other epiclastic materials even in deep water environments where such materials are resedimented shallow water materials and epiclastic strata are never found in oads ii thickness of the units vary in accordance with initial water depth of oceanic crust prior to arc formation amount of thermally induced uplift during early arc rifting subsidence due to loading and differential basinal subsidence or rates of volcanic aggradation examples of arc sequences from aruba bonaire curacao grenada martinique the virgin islands and puerto rico show significant variability aruba and curacao show incomplete sequences of arc growth and may reflect arc development on thick oceanic crust for example a plateau submarine volcanic arc facies associations oads ii occur only in the northern virgin islands water island formation and bonaire washikemba formation typically great structural thicknesses 5 10 km of arc related strata are preserved although condensed arc sequences built upon accreted terranes are present locally southwest puerto rico grenada on grenada the active magmatic platform is underlain by overthrust forearc basin strata indicating that magmatic platforms may grow by means other than volcanic aggradation

carbonate bank sedimentation in a volcaniclastic arc setting lower carboniferous limestones of the eastern klamath terrane california abstract carboniferous volcaniclastic arc deposits of the eastern klamath terrane california include late visean namurian limestone lenses that formed as small carbonate banks the limestone lenses within the bragdon and baird formations reach 17 m in thickness and 1 2 km in length slope deposits consist of argillaceous spiculitic wackestone and bank edge deposits include ooid grainstone striatifera packstone argillaceous phylloid algal packstone and argillaceous skeletal packstone bank interior deposits include skeletal wackestone packstone and argillaceous sandy mudstone the limestone lenses overlie proximal deltaic deposits of thick bedded volcaniclastic sandstone and conglomerate carbonate banks developed on delta lobes during intervals of minimal clastic sedimentation ossibly related to sea level rise and volcanic quiescence the carbonate banks were short lived depositional systems and they were covered by prograding deposits of younger volcaniclastic sands

paleogeographic reconstruction of accreted oceanic rocks akiyoshi southwest japan the depositional setting of an accreted oceanic rock succession of limestone chert and greenstone is reconstructed by using the stratigraphy age and lithologic features in the akiyoshi terrane southwest japan stratigraphic and radiolarian paleontological results identify four coeval oceanic rock units ranging in age from late early carboniferous to late middle permian sediments on the top the upper flank the lower flank and the foot of the seamount and on the surrounding ocean floor are represented by reef limestone detrital limestone with spicular chert spicular chert with displaced limestone and radiolaria bearing chert respectively all units rest on greenstone radiolaria bearing chert is interpreted as a more distal facies than spicular chert these rocks are interpreted as sediments deposited on and around a basaltic seamount formed in an open ocean realm that had no input of terrigenous materials

intra arc sedimentation and tectonism late paleozoic evolution of the eastern klamath terrane california rocks of the eastern klamath terrane redding section in northern california record deposition magmatism and tectonism in an island arc setting volcanism and sedimentation persisted for more than 220 m y spanning devonian to middle jurassic time and yielding thick successions of water laid volcanic debris and lavas with interstratified carbonate and clastic sequences this study focuses on the upper paleozoic rocks within the section the oldest rocks studied within the upper devonian and mississippian brag don formation contain epiclastic debris that was deposited in a submarine fan setting sandstone compositions indicate that quartzose sedimentary and metasedimentary rocks formed the source area for epiclastic debris and that volcanism persisted throughout sedimentation transitional shallow water facies in the upper part of the bragdon interfinger with the overlying baird formation which contains abundant volcanic debris volcanogenic shelf and strandline deposits characterize the lower part of the formation and volcaniclastic turbidites characterize the upper part the baird formation is locally interfingered with the overlying lower permian mccloud limestone which records two stages of early permian carbonate platform development early wolfcampian and late wolfcampian to early leonardian only the younger of the two platforms is present within the study area and it contains more distal facies than those preserved to the south the strata contain abundant fusulinids and corals which provide age constraints for carbonate platform development and form the basis for regional biogeographic correlation the time transgressive upper permian bollibokka group nosoni and dekkas formations succeeds the mccloud limestone comprising thick sequences of volcanogenic debris intercalated lava and in the upper part lenses of shallow water limestone the upper permian strata reflect volcanism and rapid subsidence within the are all upper paleozoic units were intruded by small gabbroic bodies and diabase dikes which may be partly cogenetic with upper permian basalts in the upper dekkas formation the upper paleozoic succession records local pulses of magmatism and variable subsidence rates within the eastern klamath arc mississippian epiclastic rocks reflect uplift of the volcanic arc basement which contained rocks similar to those now exposed in the adjacent yreka terrane or shoo fly complex pulses in volcanism were restricted to mid carboniferous and late permian time diminished volcanism during the early permian allowed development of carbonate platforms rapid fluctuations of sedimentation and sub sidence rates pronounced lateral facies and age variations accumulation of thick volcanic debris aprons during episodic volcanism and sporadic persistence of volcanism during apparent lulls characterized deposition within this succession and are common processes in modern volcanic arcs the stratigraphic record is composed predominantly of volcanogenic rocks and lesser carbonates reflecting events and processes within the arc itself evidence for nearby basins or landmasses is sparse the mississippian epiclastic rocks however which were derived from the arc basement reflect an ultimate sialic crustal source comparison of volcanic arc evolution with the late paleozoic evolution of continental margin and basinal rocks to the east indicates possible paleogeo graphic ties between the eastern klamath volcanic arc and rocks of the golconda allochthon or the north american continent

stratigraphic evolution of the paleozoic stikine assemblage in the stikine and iskut rivers area northwestern british columbia the stikine assemblage the basement of stikinia extends 500 km along the western flank of the intermontane belt east of younger coast belt plutons four different stratigraphic successions are characteristic of lower to middle devonian carboniferous and permian rocks in the stikine and iskut rivers area west of forrest kerr creek are penetratively deformed lower to middle devonian island arc volcaniclastic rocks coralline limestone and felsic tuff fringing carbonate buildups in an arc setting are best illustrated in the sequence at round lake where lower carboniferous mafic dominated bimodal submarine volcanic rocks grade upward into two distinctive coarse echinoderm limestone units and medial siliceous siltstone and limestone conglomerate conodont colour alteration indices for lower carboniferous rocks near newmont lake indicate an anomalously low temperature thermal history upper carboniferous permian polymictic volcanic conglomerate and lower permian limestone overlie these strata there t

weakening of calcium iridate during its transformation from perovskite to post perovskite magnesium silicate perovskite the dominant mineral in the lower mantle is thought to transform into a post perovskite phase in the mantle s lowermost region laboratory experiments suggest substantial weakening could occur during the transformation from perovskite to post perovskite which could explain the anomalous physical properties of the lowermost mantle

evidence of displaced terranes from permian to jurassic faunas around the pacific margins for the permian to jurassic time interval tethyan marine invertebrate faunas which are generally accepted as having lived at low latitude can be distinguished from less diverse higher latitude faunas displacement of these low latitude faunas to positions of high latitude around the pacific margins therefore provides key evidence for the movement of so called suspect or displaced terranes the fullest story so far has been worked out for the western margin of n america as far north as southern alaska but there is also convincing evidence for the substantial northward movement of continental segments along the ne asian margin as regards the southern hemisphere the torlesse terrane of new zealand appears to have moved a considerable distance southwards but there is no fauna1 evidence of comparable far travelled suspect terranes along the andean margin of south america not only tethyan and boreal but e and w pacific elements are detectable in the faunas and the distribution of these argues for a panthalassia studded with islands of various sizes fringing the adjacent continents rather than for the disintegration of a single continent pacifica

geochronology of drowned hawaiian coral reefs abstract two deeply submerged coral reefs were sampled with the hurl submersible pisces v off the islands of hawaii and maui an ancient fringing reef at 950 m depth on the kohala slope of hawaii was observed from deep towed camera photographs and during submersible operations a second reef at 1650 m depth on the haleakala slope of maui was observed on side scan images deep towed camera photographs and during submersible observations ages of the two reefs were determined by electron spin resonance dating with a signal intensity for aragonitic coral at g 2 0007 used to ascertain the accumulated dose by the additive dose method uranium concentration was measured by neutron activation analysis the chronology of the reefs indicates that drowning coincided with periods of rapid global climate change at the termination of continental glaciation at approximately 250 ka and 340 ka the drowning events correlate with oxygen isotope stage boundaries 7 8 and 9 10 these data support models of reef drowning on subsiding volcanic edifices

stratigraphy of the toba tuffs and the evolution of the toba caldera complex sumatra indonesia during the past 1 2 m y a magma chamber of batholithic proportions has developed under the 100 by 30 km toba caldera complex four separate eruptions have occurred from vents within the present collapse structure which formed from eruption of the 2800 km3 youngest toba tuff ytt at 74 ka eruption of the three older toba tuffs alternated from calderas situated in northern and southern portions of the present caldera the northern caldera apparently developed upon a large andesitic stratovolcano the calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the ytt samosir island and the uluan block are two sides of a single resurgent dome that has resurged since eruption of the ytt samosir island is composed of thick ytt caldera fill whereas the uluan block consists mainly of the oldest toba tuff ott in the past 74000 years lava domes have been extruded on samosir island and along the caldera s western ring fracture this part of the ring fracture is the site of the only current activity at toba updoming and fumarolic activity the toba eruptions document the growth of the laterally continuous magma body which eventually erupted the ytt repose periods between the four toba tuffs range between 0 34 and 0 43 m y and give insights into pluton emplacement and magmatic evolution at toba

geology and paleobiology of islands in the ordovician iapetus ocean review and implications a history of islands in the iapetus ocean and their destruction is recorded by widespread occurrences of lower to upper ordovician volcanic and epiclastic rocks and some limestone through the axial parts of the appalachian caledonide orogen in north america and northwestern europe assemblages of arenigian and llanvirnian benthic shelly fossils chiefly brachiopods from these rocks constitute the celtic biogeographic province that is distinguished by its endemic genera and by some that are older than in other biogeographic provinces some that are younger than elsewhere and others that have not hitherto been found together evidence from these fossils and from the geology of the rocks pertaining to them suggests that north american terranes that included islands of arenigian llanvirnian age lay at middle to high latitudes both in mid ocean and within the armorican continental margin whereas coeval rocks in west central ireland and norway that have toquima table head shelly fossils were originally part of the laurentian continental margin eradication of the celtic assemblage and the reduction of ordovician faunal provinciality in the appalachian caledonide region coincide with llanvirnian obduction subduction of hundreds of kilometres of iapetus ocean crust late llanvirnian and younger ordovician rocks in the remnant basin including some of peri insular origin have brachiopods of scoto appalachian affinities no discrete islands of ashgillian age have been recognized although fossiliferous volcaniclastic and polymictic conglomerates indicate intermontane basins in some places shelly fossils in these rocks and those in the siltstones and limestones from the gaspe peninsula are like those of ashgillian age elsewhere

elastic wave velocities of a pyrope majorite garnet to 3 gpa abstract the pressure dependence of the elastic wave velocities for a polycrystalline specimen 100 theoretical density of majorite pyrope garnet py 62 mj 38 has been determined at room temperature to 3 gpa by ultrasonic phase comparison interferometry pressure derivatives of the adiabatic bulk modulus dk s dp 5 3 and shear modulus dgdp 2 0 are derived from the travel times of the compressional p and shear s waves these derivatives clearly demonstrate that pressure has a more pronounced effect on the elasticity of the majorite rich garnet than on the pure pyrope end member presumably in response to the coupled substitution of si and mg for al on the octahedral site in the garnet structure when combined with plausible values for the as yet unmeasured temperature derivatives these new data can be used to calculate velocity depth profiles along a 1400 c adiabat for the range of depths in which majorite is expected to be stable in the transition zone of the earth s mantle 450 600 km for shear waves these majorite gradients match those from regional seismic models much better than those for a pure olivine mantle more reliable calculations of elastic wave velocities at mantle temperatures and pressures await measurement of the temperature derivatives of elastic modulus

drift pumice at christmas island and hawaii evidence of oceanic dispersal patterns the use of pumice as a geological tracer for oceanic dispersal patterns was evaluated by comparing the elemental signatures of drift pumice collected from christmas island and hawaii in relation to prevailing oceanic currents and pumice source areas both sites lie in the middle of the pacific ocean far from sources of volcanic pumice hawaii lies in the persistent westward flowing north equatorial current christmas island in contrast is influenced by the highly variable westward flowing south equatorial current and the eastward flowing equatorial counter current pumice is very abundant at christmas island in the beach drift pumice from christmas island is derived from the western pacific ocean krakatau southwestern pacific ocean tonga trench east pacific ocean mexico south atlantic ridge and an unknown source in contrast pumice is rare in hawaii pumice from hawaii originates primarily from the south sandwich islands mexico isla san benedicto and krakatau the currents that control dispersal of pumice also control dispersal of larvae and rafted organisms christmas island has a higher coral diversity 31 genera 81 species than hawaii 17 genera 50 species hawaii receives only small amounts of pumice drift from a limited area to the east and has a more restricted coral diversity while christmas receives massive amounts of pumice and presumably larvae and rafted organisms from the area of high coral diversity to the west

oman exotics oceanic carbonate build ups associated with the early stages of continental rifting the so called oman exotic limestones form isolated masses from boulder size to 1 000 m thick of middle to upper permian and upper triassic fossiliferous limestones and crop out within imbricated thrust slices beneath the semail ophiolite in the oman mountains permian exotics are of dominantly reef and forereef facies whereas triassic exotics are more typically of back reef and lagoonal facies they are commonly associated with a substrate of alkalic and transitional tholeiitic basalts and are interpreted as a series of reef associated carbonate buildups deposited in part on oceanic islands or seamounts close to the site of initial rifting of the oman continental margin

new developments in deformation experiments at high pressure although the importance of rheological properties in controlling the dynamics and evolution of the whole mantle of earth is well recognized experimental studies of rheological properties and deformation induced microstructures have mostly been limited to low pressure conditions this is mainly a result of technical limitations in conducting quantitative rheological experiments under high pressure conditions a combination of factors is changing this situation increased resolution of composition and configuration of earth s interior has created a greater demand for well resolved laboratory measurement of the effects of pressure on the behavior of materials higher strength materials have become readily available for containing high pressure research devices and new analytical capabilities in particular very bright synchrotron x ray sources are now readily available to high pressure researchers one of the biggest issues in global geodynamics is the style of mantle convection and the nature of chemical differentiation associated with convectional mass transport although evidence for deep mantle circulation has recently been found through seismic tomography e g van der hilst et al 1997 complications in convection style have also been noted they include 1 significant modifications of flow geometry across the mantle transition zone as seen from high resolution tomographic studies fukao et al 1992 masters et al 2000 van der hilst et al 1991 and 2 complicated patterns of flow in the deep lower mantle 1500 2500 km perhaps caused by chemical heterogeneity kellogg et al 1999 van der hilst and karason 1999 these studies indicate that while large scale circulation involving the whole mantle no doubt occurs significant deviations from simple flow geometry are also present two mineral properties have strong influence on convection 1 density and 2 viscosity rheology contrasts in the past the effects of density contrast have been emphasized honda et al 1993 kellogg et al 1999 tackley et al 1993 and the influence of