a new mesozoic isochron chart of the northwestern pacific ocean paleomagnetic and tectonic implications a new mesozoic isochron chart of the northwestern pacific ocean has been completed by utilizing all the currently available data from 357 cruises our chart provides the first whole view of the mesozoic magnetic anomaly lineations in the northwestern pacific ocean and yields essential information for understanding the problems of the mesozoic magnetostratigraphic time scale and the evolutional history of the pacific plate the mesozoic magnetostratigraphic time scale must be recalibrated using not only the hawaiian lineation set but also other mesozoic lineation sets the lack of lineation m0 in the lineation sets other than the hawaiian lineation set indicates that a reorganization of the ridges except the pacific farallon ridge occurred between chrons m1 and m0 nearly 120 ma the eruption rate of the shatsky hotspot seems to be comparable to that of the reunion hotspot at their initial stage the growth of the pacific plate began in a small triangular region at about 190 ma

neotectonic deformation of the central chile margin deepwater forearc basin formation in response to hot spot ridge and seamount subduction 1 underlying the midslope valparaiso terrace the valparaiso basin constitutes a unique and prominent deepwater forearc basin on the central chile margin the basin contains a 3 3 5 km thick sediment fill of late cenozoic age the seismic stratigraphic architecture of the valparaiso basin fill shows that margin wide subsidence above the underthrusting juan fernandez ridge an aseismic volcanic swell crested with seamounts on the nazca plate characterized the basin evolution contemporaneously with margin subsidence and seaward rotation of the upper slope and shelf the outer part of the forearc was deformed by thrust faulting above the leading flank of underthrusting seamounts that rotated the middle slope to a more horizontal attitude and formed seaward vergent outer forearc highs subsidence of the margin above the underthrusting juan fernandez ridge is ascribed to processes of subduction erosion enhanced by factors related to subduction of the ridge that caused dismemberment of upper plate material along the shallow aseismic part of the plate interface and transfer of material to the downgoing nazca plate accelerated midslope subsidence 55 75 km landward of the trench coincides with the seaward limit of interplate seismicity suggesting that enhanced basal erosion is related to the transition from aseismic to stick slip subduction behavior since subduction of the juan fernandez ridge began near 33 s 10 ma ago the chile trench north of 33 s is inferred to have retreated 3 km myr and the midslope valparaiso basin subsided 0 3 0 5 km myr approximating analogous neogene rates for the peruvian margin

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

geochemistry of muong nong type tektites iv selected trace element correlations muong nong type tektites are distinct from other tektites in their shape size petrographical characteristics and chemistry volatile elements are enriched but other trace elements do not show large differences when compared to normal splash form tektites an analysis is made of nineteen muong nong type tektites for 40 50 elements fifteen nonvolatile elements are discussed in this paper it is shown that the muong nong type tektites are very restricted in chemical composition and are almost indistinguishable from the australasian tektites elemental ratios calculated from the data reported here show the same genetic connection all elementary abundances and ratios are fully consistent with terrestrial sediments the nonvolatile elements may record more about the conditions of the origin

a possible tektite strewn field in the argentinian pampa impact glass associated with 11 elongate depressions in the pampean plain of argentina north of the city of rio cuarto was suggested to be proximal ejecta related to a highly oblique impact event we have identified about 400 additional elongate features in the area that indicate an aeolian rather than an impact origin we have also dated fragments of glass found at the rio cuarto depressions the age is similar to that of glass recovered 800 kilometers to the southeast this material may be tektite glass from an impact event around 0 48 million years ago representing a new tektite strewn field

an expression of philippine sea plate rotation the parece vela and shikoku basins abstract the philippine sea plate located between the pacific eurasian and australian plates is the world s largest marginal basin plate the motion of the philippine sea plate through time is poorly understood as it is almost entirely surrounded by subduction zones and hence previous studies have relied on palaeomagnetic analysis to constrain its rotation we present a comprehensive analysis of geophysical data within the parece vela and shikoku basins two oligocene to miocene back arc basins which provide independent constraints on the rotational history of the philippine sea plate by means of their seafloor spreading record we have created a detailed plate model for the opening of the parece vela and shikoku basins based on an analysis of all available magnetic gravity and bathymetric data in the region subduction along the izu bonin mariana trench led to trench roll back arc rupture and back arc rifting in the parece vela and shikoku basins at 30 ma seafloor spreading in both basins developed by chron 9o 28 ma and possibly by chron 10o 29 ma as a northward and southward propagating rift respectively the spreading orientation in the parece vela basin was e w as opposed to ene wsw in the shikoku basin the spreading ridges joined by chron 6by 23 ma and formed a r r r triple junction to accommodate the difference in spreading orientations in both basins at chron 6no 20 ma the spreading direction in the parece vela basin changed from e w to ne sw at chron 5ey 19 ma the spreading direction in the shikoku basin changed from ene wsw to ne sw this change was accompanied by a marked decrease in spreading rate cessation of back arc opening occurred at 15 ma a time of regional plate reorganisation in se asia we interpret the dramatic change in spreading rate and direction from e w to ne sw at 20 1 3 ma as an expression of philippine sea plate rotation and is constrained by the spacing between our magnetic anomaly identifications and the curvature of the fracture zones this rotation was previously thought to have begun at 25 ma as a result of a global change in plate motions our results suggest that the philippine sea plate rotated clockwise by about 4 between 20 and 15 ma about a pole located 35 n 84 e this implies that the majority of the 34 clockwise rotation inferred to have occurred between 25 and 5 ma from paleomagnetic data may have in fact been confined to the period between 15 and 5 ma

forearc basins types geometries and relationships to subduction zone dynamics forearc basins are one of the fundamental sedimentary basins and they preserve various geological phenomena arising from dynamic interactions between subducting and overriding plates along convergent plate boundaries previous studies have proposed descriptive classification schemes for such basins and performed analog experiments and numerical simulations examining basin formation and evolution however the great variability of forearc basins means that a comprehensive understanding of these features remains elusive this paper proposes a new classification scheme for forearc basins from the viewpoints of material transfer between the two plates accretionary or nonaccretionary and the long term strain field in the basin compressional or extensional by using characteristics of 41 modern forearc basins geometric characteristics were also analyzed in order to reveal the relationship to subduction zone dynamics including material transfer at the plate boundary accretion erosion plate convergence configurations and kinematics and the mechanics of strain and its partitioning compressional accretionary type basins which are characterized by landward tilting strata and the landward migration of depocenters show a constant width thickness ratio indicating self similar growth of the basin as the outer wedge builds up extensional accretionary type basins develop normal faults in the basin probably due to a gravitational instability of the outer wedge an increase of sediment flux at the trench may grow the accretionary prism and thicken the subduction channel faster leading to a reduction of basal and internal shear strengths of the outer wedge subsidence caused by crustal thinning characterizes the nonaccretionary type basins which have thin trench fill sediments large forearc slopes steady widths of the outer wedge and variable width thickness ratios of the basins from these results the sediment flux at the trench and the plate boundary can be considered as a dominant factor that determines accretion or erosion of the outer wedge and uplift or subsidence of the outer arc high and that is also responsible for the style of deposition in the forearc basin an increase or decrease of the sediment flux may change the type geometry and deformation manner of the forearc basin the results of this study provide important clues to further our understanding of dynamic interactions between forearc basins and subduction zone processes

geochemistry and origin of libyan desert glasses abstract we analysed ten libyan desert glasses ldgs for chemical composition by icp aes and icp ms and four samples for their sr and nd isotopic compositions our results confirm the close compositional relationship of ldgs to mature sandstones the glasses are lree enriched la n sm n 4 2 4 5 and display significant negative eu anomalies eu eu 0 6 0 75 similar to post archaean sediments in addition most element abundances are correlated with ree contents the correlations are interpreted as mixing lines and strongly suggest that the parent material was composed of quartz grains coated with a mixture of kaolinite fe ti oxides and accessory phases significant pge concentrations were measured in the dark coloured part of an ldg sample its normalized pattem is chondritic demonstrating the presence of a meteoritic component in the glass several hypotheses have been proposed for the origin of ldgs however our results confirm that ldgs are impact derived glasses and have some petrographical and chemical features similar to those of muong nong type tektites neodymium and sr isotopic signatures of ldgs are consistent with their derivation by melting of mature sandstones derived from proterozoic materials such as the east saharan basement rocks the most likely target rocks are the quartz arenites of the jurassic cretaceous nubia group

nd and sr isotopic compositions of tektite material from barbados and their relationship to north american tektites isotopic analyses of nd and sr on individual microtektites and a bulk microtektite sample from barbados show them to have a very well defined isotopic composition these data plot on an e sr e nd diagram precisely within the narrow field determined by north american tektites e sr 111 e nd 6 2 they yield an nd model age of 0 6 ae these results show that the microtektites from the oceanic beds of late eocene age are derived from the same target as the north american tektites and should be associated with the same event samples of the deep sea sediments in which the barbados microtektites occur are found to have isotopic signatures which appear to reflect ambient sea water and detrital sediments they cannot be the source of sr or nd in the tektites following the arguments of shaw and wasserburg 1982 we conclude that the target area which produced the north american tektite field was composed of sediments eocambrian or younger derived from very late precambrian crust glass beads from lake wanapitei crater are isotopically different from all other tektites e sr 960 e nd 31 4 and cannot be related to the north american tektites

introduction to the special issue on the 2003 eruption of anatahan volcano commonwealth of the northern mariana islands cnmi 1 introductionanatahan volcano situated in the commonwealthof the northern mariana islands cnmi erupted forthefirst timeinrecordedhistoryonmay10 2003 theeruption prompted a period of intensive study andcollaboration between researchers from a number ofgovernmental agencies institutes and universities andresultedinaspecialsessiondedicatedtotheeruptionatthe fall meeting of the american geophysical union agu in december of thesame year seeeos trans agu 84 46 p 1561 1563 this volume of thirteenpapersgrewoutoftheaguspecialsessionandcoversfour principal themes a geological studies of anata han carried out prior to the may 2003 event includingmapping of both the submarine and subaerial portionsof the volcano and monitoring of the local seismicityfollowing activity in 1990 b geophysical signals ofthe 2003 eruption c petrology and geochemistry ofthe eruptive products of the 2003 eruption and d impacts associated with the 2003 eruption in this briefintroduction we summarize the major findings asso ciatedwitheachofthesethemes wealsooffersugges tionstowhereupdatedmaterialandresultsofon goingstudies can be found at the time of writing january2005 anatahan volcano continues to be active albeit at a lower level compared to the spring of2003 so remains the focus of considerable researchand monitoring efforts 2 anatahan volcano background geologythe island of anatahan is situated about 120 kmnorth of saipan the capital of the cnmi it lies atthe southern end of the 1500 km long izu bonin mariana ibm arc system where the oldest oceaniccrust of the pacific plate is subducting beneath youn ger crust of the philippine plate the island representsthe summit of a large strato volcano which rises fromdepths of 3000 m below sea level to an elevation of788 m

identification of the projectile at the brent crater and further considerations of projectile types at terrestrial craters abstract impact melt samples from drill hole b1 59 at the 3 8 km diameter brent crater ontario have been analysed for siderophile trace elements indicative of meteoritic contamination samples from the basal melt zone at 823 857 m depth are enriched in ir os pd ni co cr and se over basement with the abundance pattern suggesting a chondritic projectile for brent from a ni cr correlation of 10 melt samples an l or ll chondrite is inferred the contribution of an ultramafic country rock alnoite in the melt is too small to significantly influence its ni cr ratio glass rich breccias from the allochthonous breccias filling the crater also contain a meteoritic component interelement ratios e g ni cr are however fractionated relative to the melt zone samples this as well as the low au content of all brent samples is probably a product of alteration additional data on impact melts from the 65 km diameter crater manicouagan still did not reveal a meteoritic component as also for the mistastin crater 28 km diameter where cr analyses set an upper limit of 1 of an achondritic projectile component in the melt irghizites tektite like glasses from the zhamanshin impact structure have been found to contain high ni and co concentrations and our data show that ir is also enriched it is however not possible to define the projectile type enrichment of an ivory coast tektite in ir is confirmed there are large differences in siderophile element concentrations among tektites with otherwise similar chemical composition there are now four known craters formed by chondrites clearwater east lapparjarvi wanapitei and brent with brent being the smallest of these for smaller craters the projectiles appear to be limited to iron or stony iron meteorites because of atmospheric destruction of relatively small stony meteorites it appears however that all major classes of meteorites are represented among the projectiles at terrestrial impact craters

neodymium and strontium isotopic study of australasian tektites new constraints on the provenance and age of target materials nd and sr isotopic studies of australasian tektites provide information on the age and provenance of the target materials and allow us to characterize the target area and the impact process leading to tektite formation nd values of australites splash form indochinites and muong nong type indochinites are indistinguishable within analytical uncertainty and average 11 5 u depleted mantle nd model ages fall within the narrow range of 1490 1620 ma indicating that the source material was derived dominantly from a proterozoic crustal terrane sr values are variable and are correlated with the geographic location of the tektite samples analyses of four muong nong type or layered indochinites from a single locality in eastern thailand yield an isochron age of 167 12 ma 2 a correlation of rb sr fractionation with sr model ages indicates that the last major rb sr fractionation event experienced by the target materials occurred 175 15 ma ago we interpret this age as the time of deposition of sedimentary target rocks and consider the compositional layering observed in muong nong type tektites to reflect compositional variability inherited from jurassic sediments the nd and sr isotopic data provide evidence that all australasian tektites were derived from a single sedimentary formation with a narrow range of stratigraphic ages close to 170 ma we suggest that all of the australasian tektites were derived from a single impact event and that the australites represent the upper part of a melt sheet ejected at high velocity whereas the indochinites represent melts formed at a lower level in the target material which were distributed closer to the area of impact the impact site is inferred to be within an area of jurassic sedimentary bedrock which spans the geopolitical boundaries between northern cambodia southern laos and southeastern thailand

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

beryllium 10 in australasian tektites constraints on the location of the source crater abstract by using accelerator mass spectrometry we have measured the 10be concentrations of 86 australasian tektites corrected to the time of tektite production 0 8 my ago the 10be concentrations 106 atom g range from 59 for a layered tektite from huai sai thailand to 280 for an australite from new south wales australia the average value is 143 50 when tektites are sorted by country their average measured 10be concentrations increase slowly with increasing distance from southeast asia the probable location of the tektite producing event from 59 9 for 6 layered tektites from laos to 136 20 for 20 splash form tektites from australia the lowest 10be concentrations for tektites fall on or within a contour centered off the shore of vietnam south of the gulf of tonkin 107 e 17 n but also encompassing two other locations in the area of northeastern thailand previously proposed for the site of a single tektite producing impact the 10be concentrations of layered tektites show only a weak anticorrelation r 0 3 with the numbers of relict crystalline inclusions loosely consolidated fine grained terrestrial sediments or recently consolidated sedimentary rocks are the most likely precursor materials dilution of sediments with other kinds of rock raises problems in mixing and is not supported by petrographic data sedimentary columns that have the right range of 10be concentrations occur off the coasts of places where sedimentation rates are high relative to those in the deep sea a single impact into such a region 15 to 300 m thick could have propelled near surface high 10be material farthest to australia while keeping the deeper lying low 10be layers closer to home we do not rule out however other proposed mechanisms for tektite formation

palaeoseismicity and pottery investigating earthquake and archaeological chronologies on the hajiarab alluvial fan iran for millennia humans have lived in locations that are highly vulnerable to large earthquakes often out of strategic or cultural necessity and or the proximity of these locations to resources necessary for survival despite the often catastrophic effects when large earthquakes occur recent history reveals that human nature is to rebuild rather than relocate implying that seismic activity is not a sufficient deterrent of population growth in tectonically vulnerable areas in order to investigate whether this was the case for ancient civilisations and thus perhaps a fundamental tenet of human behaviour a palaeo earthquake history was developed for the active cheskin and ipak faults in northwestern iran and compared with the well resolved archaeological history of the nearby sagzabad cluster settlements of zagheh 7170 6300 bp ghabristan 6215 4950 bp and sagzabad 4050 2350 bp combining new geologic geomorphic and chronologic datasets revealed the presence of a fault propagated anticline formed by large mw 6 5 7 0 earthquakes on a blind thrust fault that projects to seismogenic depth directly beneath the sagzabad cluster settlement sites large earthquakes with a return period of 90 probability under most faulting scenarios that the energy release from these earthquakes would have been of sufficient magnitude to generate peak horizontal acceleration pha values at the sagzabad cluster in excess of likely threshold values for complete settlement destruction poisson modelling assuming a time displacement repeating model for earthquake recurrence indicates a 66 42 probability of one two earthquakes that would generate pha 0 25 g occurring during occupation of zagheh a 79 55 probability for ghabristan and an 88 65 probability for sagzabad despite the near certainty that the residents of these holocene settlements experienced large destructive earthquakes the near continuous history of occupation at this area suggests that early humans were not apt to relocate in response to earthquake activity environmental e g alluviation stream channel avulsion climate change cultural and or political factors may have been more important drivers of settlement shifts and abandonment at the sagzabad cluster of iran

stable isotope compositions and water contents of boninite series volcanic rocks from chichi jima bonin islands japan measurements of stable isotope compositions and water contents of boninite series volcanic rocks from the island of chichi jima bonin islands japan confirm that a large amount 1 6 2 4 wt of primary water was present in these unusual magmas an enrichment of 0 6 in18o during differentiation is explained by crystallization of18o depleted mafic phases silicic glasses have elevated 18o values and relatively low d values indicating that they were modified by low temperature alteration and hydration processes mafic glasses on the other hand have for the most part retained their primary isotopic signatures since eocene time primary d values of 53 for boninite glasses are higher than those of morb and suggest that the water was derived from subducted oceanic lithosphere

nitrogen and trace elements in tektites and impact glasses abstract nitrogen and several trace elements have been studied in samples of one muong nong tektite irghizites tektite like glass zhamanshinites impactites and several obsidian glasses a detailed investigation of inter and intra layer samples from the muong nong tektite shows a chemical heterogeneity both within and between the layers in general the elemental variations seem to be due to a varying feo content irghizites both sio2 rich and sio2 poor are compositionally more homogeneous than the zhamanshinites the heterogeneous distribution of nitrogen observed in the muong nong tektite and the zhamanshinites is unlike that in any other tektite studied so far this shows clearly that zhamanshinites have suffered lower peak temperatures than irghizites and muong nong tektites lower ones than normal splash form tektites available chemical and petrological data on muong nong tektites suggest that they were formed by in situ melting of soil

noble gases in muong nong type tektites and their implications we have measured the elemental abundances and isotopic compositions of noble gases in muong nong type tektites from the australasian strewn field by crushing and by total fusion of the samples we found that the abundances of the heavy noble gases are significantly enriched in muong nong type tektites compared to those in normal splash form tektites from the same strewn field neon enrichments were also observed in the muong nong type tektites but the ne ar ratios were lower than those in splash form tektites because of the higher ar contents in the former the absolute concentrations of the heavy noble gases in muong nong type tektites are similar to those in impact glasses the isotopic ratios of the noble gases in muong nong type tektites are mostly identical to those in air except for the presence of radiogenic 40ar the obtained k ar ages for muong nong type tektites were about 0 7 myr similar to ages of other australasian tektites the crushing experiments suggest that the noble gases in the muong nong type tektites reside mostly in vesicles although xe was largely affected by adsorbed atmosphere after crushing we used the partial pressure of the heavy noble gases in vesicles to estimate the barometric pressure in the vesicles of the muong nong type tektites likely muong nong type tektites solidified at the altitude between the surface and a maximum height of 8 30 km lower than that for splash form tektites

layered tektites a multiple impact origin for the australasian tektites abstract high precision neutron activation studies of southeast asian tektites confirm their close compositional relationship to well mixed continental sediments in confirmation of earlier studies concentrations of the volatile metals zn ga as and sb are 2 4 times lower in splash form tektites than in layered or muong nong type tektites the th u ratio is higher in splash form than in layered tektites this implies loss of about 30 of the initially present u from the splash form tektites volatile contents of layered tektites are similar to those in continental sediments indicating minor to negligible loss during their formation a remarkably coherent relationship between cr and co and an associated ni co correlation indicate the variable presence of a minor ultramafic component having an atypically low cr co ratio there is compelling evidence indicating that the layered tektites formed as sheets or pools of melt because there are severe difficulties with the impact transport of melt 600 km their distribution across a field 1140 km in length is inconsistent with their formation in a single crater maintenance of the melt in a liquid state required that it remained in the fireball until it neared the ground thus layered tektites should be deposited within a few crater radii of their parent craters and many craters are required to account for their distribution across a field 1140 km long confirmation of this model can be obtained by discovering the predicted km size craters or by demonstrating that extensive and widely separated regions yield only layered tektites and were thus covered with continuous melt sheets ejected from independent craters

coesite and shocked quartz discovered in the australasian and north american microtektite layers samples from the north american australasian and ivory coast microtektite layers were examined for unmelted impact ejecta 125 m size coesite and shocked quartz identified by x ray diffraction were found at all the sites where a well defined north american microtektite layer had previously been documented coesite and shocked quartz were also foundin seven out of 33 cores from the australasian strewn field however no coesite or shocked quartz was found associated with the ivory coast microtektite layer stishovite was also found at three of the australasian sites all of the australasian cores containing coesite and shocked quartz were taken from within 2000 km of indochina which is believed by most to be the source area for the australasian tektites and microtektites six of the seven cores have the highest concentrations of microtektites ever determined for australasian microtektite bearing cores the seventh core did not contain any microtektites which may be consistent with the hypothesis that the microtektites were deposited in a raylike pattern the discovery of coesite and impact ejecta associated with both the north american and australasian microtektite layers provides further support for the hypothesis of terrestrial impact origin of tektites

geophysical characteristics of the southern mariana trough 11 50 n 13 40 n despite slow opening rates generally inferred for the mariana trough the southernmost part of the basin has fast spreading geophysical and morphologic characteristics that are unlike the features of the basin to the north a side scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11 50 n and 13 40 n and identifies the following characteristics the ridge across axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross sectional area variations of 1 to 7 km2 the along axis mantle bouguer gravity gradient is 0 2 mgal km axial segmentation occurs as overlapping axes and small deviation in trend no transform fault offsets exist despite significant changes in the trend of the spreading center characteristics of the surrounding basin include shallower overall depth than in the north no well developed frontal arc high in the southernmost trough the close proximity of submarine arc type volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank these characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin we propose that these effects are related to 1 the geometry of trench rollback in the southern trough leading to trench parallel extension generating inward radiating extensional faults 2 decoupling of the trench parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin and 3 augmentation of the spreading center s magmatic budget by arc magmatic sources contributing to its fast spreading character although these effects may be accentuated in the southern mariana trough by the geometry of trench rollback and position of the slab which here underlies the spreading center they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid ocean ridges

na k ion exchange reaction between rhyolitic glass and na k c1 aqueous solution under hydrothermal conditions abstract hydrothermal experiments on ion exchange reactions were carried out using synthetic and natural rhyolitic glasses and na k cl aqueous solution at 200 300 c and 1000 bars the rhyolitic glass did not crystallize during the reaction even under hydrothermal conditions and it behaved as a na k ion exchanger the na k ion exchange properties of the glass were strongly influenced by its thermal history quenched glass preferentially fixed k rather than na and glass obtained by slow cooling showed the opposite trend the present experimental results show clearly that glass as well as minerals can control the composition of the coexisting aqueous solution in hydrothermal systems the present results also suggest that the chemical composition of coastal thermal waters in japan is controlled by the two stage episode of reaction between acidic volcanic rocks and seawater

the bam iran earthquake of december 26 2003 from an engineering and seismological point of view abstract this paper is devoted to studying the destructive bam earthquake from an engineering seismological point of view on december 26 2003 a large earthquake of magnitude 6 7 m s shook the bam district located in southeastern iran about 80 of buildings totally collapsed several dozen villages were destroyed and tens of them were severely damaged more than 45 000 people were killed and 30 000 were injured according to the macroseismic data focal depth of the event was estimated at 7 2 km the event was associated with a 9 km surface rupture of the bam fault the maximum residual strike slip displacement is 5 cm which is left lateral the highest recorded peak ground acceleration is at bam station 0 98 g vertical and 0 87 g horizontal located about 1 5 km from the surface rupture the macroseismic analyses show that the epicenter of the quake is 1 5 km east of bam all of the masonry buildings totally collapsed in the epicentral area many semi engineered structures were heavily damaged and some of them collapsed engineered structures constructed according to the seismic code of iran have shown very good structural behavior without significant damage finally the earthquake intensity at the epicenter is determined as ix on the msk scale which strongly decreases with distance from the surface ruptures

age and provenance of the target materials for tektites and possible impactites as inferred from sm nd and rb sr systematics sm nd and rb sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects tektites have large negative e nd 0 values and are uniform within each tektite group while the e sr 0 are large positive values and show considerable variation within each group chemical trace element and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust each tektite group is characterized by a uniform nd model age t chur nd interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites 1 1 15 ae for australasian tektites 2 1 91 ae for ivory coast tektites 3 0 9 ae for moldavites 4 0 65 ae for north american tektites and 5 0 9 ae for high si irghizites sr model ages t ur sr are variable within each group reflecting rb sr fractionation and in the favorable limit of very high rb sr ratios approach the time of sedimentation of the parent material which melted to form the tektites australasian tektites are derived from 0 25 ae sediments moldavites from 0 0 ae sediments ivory coast tektites from 0 95 ae sediments possible parent sediments of other tektite groups have poorly constrained ages our data on moldavites and ivory coast tektites are consistent with derivation from the ries and bosumtwi craters respectively irghizites are isotopically distinct from australasian tektites and are probably not related sanidine spherules from a cretaceous tertiary boundary clay have initial e nd 2 e sr 5 and are not derived from old continental crust or meteoritic feldspar they may represent a mixture of basaltic oceanic crust and sediments implying an oceanic impact these isotopic results are also consistent with a volcanic origin for the spherules

on the motion of hawaii and other mantle plumes abstract the hawaiian hotspot and its prominent bend at 47 ma have figured prominently in the development of ideas on the nature of mantle plumes and plate motion here i review paleomagnetic analyses associated with ocean drilling program odp leg 197 which together with studies of plate circuits and geodynamic modeling results indicate southward motion of the hawaiian hotspot during formation of the emperor seamounts these analyses confirm the long posited idea that mantle plumes should be influenced by mantle flow moreover these results indicate that late cretaceous early paleogene hotspot motion was the dominant factor in forming the emperor track and thus the famous bend morphology an important corollary of this finding on hotspot motion is that long term polar wander of earth which has been inaccurately assessed by viewing paleomagnetic data in a fixed hotspot reference frame has been far less than previously thought salient points for future research raised by the odp leg 197 paleomagnetic results include the scale of mantle flow recorded by hotspot track segments and the underlying processes that cause track segments to be dominated by mantle flow rather than plate motion for given time intervals

imaging changes in morphology geometry and physical properties of the subducting pacific plate along the izu bonin mariana arc abstract recent studies on slab structure in the northwest pacific have imaged variable geometries of the subducted oceanic lithosphere along the plate margin tomographic images show portions of the subducted pacific plate penetrate vertically into the lower mantle below the mariana arc whereas below the izu bonin arc the slab appears to be lain down horizontally on top of the 670 km discontinuity using new technology to grid and visualize regional seismic tomography data even more detail about the structure of the subducting plates can be extracted from the three dimensional images we investigate the morphology and geometry of the subducting pacific plate as it changes between horizontal to nearly vertical using p wave tomographic images the most profound result from these new models is the presence of a distinct change of seismic property in the downgoing pacific slab beneath the izu bonin arc at a depth of 350 400 km the position of this anomaly corresponds to a region north and west of the ogasawara plateau we propose the change in morphology and physical property of the slab is related to the distortion of the pacific plate geometry as its shape converts from near horizontal to vertical together with the subduction of the ogasawara plateau

compressive episodes and faunal isolation during rifting southwest iberia abstract evidence for three short lived compressive episodes of late carixian late callovian early oxfordian and tithonian berriasian ages that lasted 5 m yr and occurred during the process of rifting of the algarve basin is presented these tectonic inversion episodes are described at outcrop and cartographic scales and have been dated with the accuracy provided by the ammonoid scale an uplift event of late toarcian aalenian age of undetermined tectonic origin is also described we show that these four tectonic episodes coincide in time with important ecological events such as the onset of migration and or the segregation of boreal and tethyan ammonite species and the confinement of the algarve basin stratigraphic and paleoecological data from the algarve and lusitanian basins are compared and discussed together with eustatic and tectonic information we propose that the tectonic inversion episodes that caused uplift are the origin of the mesozoic sedimentary gaps and the intermittent opening and

cerium anomalies in lateritic profiles the ree geochemistry and mineralogy have been studied in four lateritic profiles one derived from a syenite akongo sw cameroon the others being developed on a gneissic basement and located along a soil toposequence goyoum e cameroon there is a fractionation between lree and hree in the lateritic samples during weathering the weathered residual products being enriched in lree from la to eu and depleted in hree from gd to lu sampled waters are enriched in hree in relation to the syenite host rock a positive ce anomaly has been found systematically at the top of the saprolite beneath a zone of iron oxide accumulation in the laterite up to 2000 ppm ce may be present in the akongo profile cerianite ceo2 is present as very fine coatings in non ferruginous clayey domains primary ree bearing accessory minerals are weathered at the bottom of the profile specifically allanite and apatite are transformed into florencite and rhabdophane but these phases have no ce anomaly all the data are interpreted as the result of the following processes 1 1 ree leaching in a reducing environment 2 2 oxidation of ce3 to ce4 in an oxidizing environment 3 3 deposition of cerium as cerianite whereas the other ree remain in solution

pacific plate subduction beneath the central mariana and izu bonin fore arcs new insights from an old margin 1 multichannel seismic mcs profiles and bathymetric data from the central mariana and izu bonin subduction systems image the subducting pacific plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc subducting oceanic crust varies along the mariana margin from 5 3 to 7 km thick and is covered by 0 5 2 km thick sediments and numerous seamounts oceanic crustal thickness east of the izu bonin trench is 6 km faulting resulting from flexure of the incoming pacific plate begins up to 100 km east of the trench axis near the 6 km depth contour the plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes 25 incoming crust breaks along new faults with a trench parallel strike the mariana trench axis is commonly a graben that accommodates an abrupt change within 8 beneath the outer fore arc we infer that the plate fails there rather than simply bends under the applied loads along portions of the mariana margin subducting seamounts displace the trench axis westward and uplift the toe of the slope surprisingly west of the toe there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction nor of significant subduction erosion or sediment underplating mcs profiles across the base of the mariana inner trench slope provide evidence for both complete subduction and small scale accretion of pacific plate sediments however we found no evidence for long term sediment accretion the subducting plate dips 9 12 beneath serpentinite seamounts on the izu bonin and mariana fore arcs along the mariana margin the majority of these seamounts are located 50 70 km west of the trench where the mantle wedge is 3 7 km thick between 8 10 km thick fore arc crust and the top of the subducting plate the apparent lack of significant deformation of the mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben

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

on the nature of the ni rich component in splash form australasian tektites abstract the australasian tektite strewn field is exceptional not only as the largest and most recent but also as the only strewn field without an identified source impact crater therefore scenarios without the formation of an impact crater such as a low altitude cometary airburst have proven hard to discard here new geochemical evidence is presented for mixing of projectile and target material which implies the formation of an australasian tektite related impact crater first ninety two australasian tektites were grouped according to their cr co and ni concentrations based on this data australasian tektites with the highest ni contents 200 g g occur more than 1500 km south southeast sse of the northern indochina region with the highest concentration of ni rich tektites in south vietnam the islands of borneo belitung and java and reports in literature for ni rich tektites in central australia the tektites with the highest cr and ni abundances often also show highly siderophile element hse enrichments of up to 4 ng g ir the most ni rich samples exhibit broadly chondrite relative hse proportions however a chondritic impactor contribution appears to be inconsistent with the observed ni cr ni co and cr co ratios a previously suggested significant terrestrial mantle contribution can also not explain the siderophile element enrichments in combination with relatively low feo tot

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

evolution of tectono sedimentary systems in the kumano basin nankai trough forearc abstract sedimentary deposits in the distal kumano forearc basin of the nankai accretionary margin off kii peninsula japan have been imaged using three dimensional 3d seismic data the seismic data along with logging and core data from the integrated ocean drilling program iodp show that the unconformity between the accretionary prism and overlying forearc sediments is time transgressive the unconformity at site c0002 separates 5 ma prism rocks from 3 65 ma basin deposits at site c0009 it separates 5 6 ma prism from 3 8 ma basin sediments acoustic reflections in the basal deposits are sub parallel to the underlying accretionary prism the acoustic facies varies in thickness from 50 to 750 m the mudstone deposits and laterally equivalent turbidites are interpreted as lower trench slope deposits the condensed slope sediment ss section decreases in age from 3 5 to 1 5 ma at site c0002 to 1 5 0 9 ma at c0009 acoustic sequences within the lower forearc basin lfb contain higher proportions of silt and sand turbidites and progressively onlap the ss unit along a low angle discontinuity kl in a landward direction because of the landward onlap of the lfb unit the oldest lfb strata at c0002 are older than 1 67 ma whereas those at c0009 are younger than 0 9 ma thus the amount of time missing or characterized by condensed sedimentation across the kl unconformity decreases in duration in the landward direction the landward onlapping sequences tilt progressively landward in response to regional uplift along an out of sequence thrust oost mega splay fault regional tilting shifted the basin s depocenter progressively landward expanding that part of the basin from 10 km in width to 30 km the onset of sand silt turbidite deposition in the distal basin began after more accommodation space was created by the uplift of the outer ridge along the splay fault at 1 9 ma conversely turbidites of the upper forearc basin ufb progressively onlap lfb in a seaward direction furthermore the respective thicknesses of the lfb and ufb units switch from the seaward side of the basin c0002 farther landward c0009 the lfb unit is 800 m thick in the seaward region whereas it is only 200 300 m thick in the landward region the ufb unit is

the zhamanshin impact structure kazakhstan a comparative geochemical study of target rocks and impact glasses abstract the zhamanshin impact structure which is about 1 myr old has a diameter of 14 km and is situated in the semi arid region of kazakhstan 48 24 n 60 58 e it has a heterogeneous suite of target rocks including predominantly crustal lithologies e g clays and siltstones with minor ultramafics zhamanshin is known for its unique association of impact glasses including basic and acidic varieties of zhamanshinites and tektite like and a few aerodynamically shaped irghizites the origin of both of these impact glasses has long been debated which is complicated by incomplete sampling of target lithologies at the zhamanshin site and a limited number of isotopic analyses however such studies are a prerequisite for a comprehensive discussion of the mechanisms that formed the unique association of different impact glasses in one impact event we present major and trace element contents as well as combined sr nd isotope data for target rocks and impact glasses from the zhamanshin impact structure these data for the first time include paleogene clays and siltstones from a core drilled in the vicinity of the crater and cutting through all major target lithologies the core samples contain important source lithologies for the impactites from the zhamanshin area mixing calculations based on geochemical data and sr nd isotope signatures indicate that irghizites and si rich zhamanshinites can be produced from variously homogenized mixtures of mainly clays and siltstones with minor additions of ultrabasic rocks based on highly siderophile element hse and os isotope data including the first analyses of the clay and siltstone lithologies we calculated a hypothetical os composition of the irghizite precursor allowing us to approximate a chondritic admixture to the irghizites of roughly 1 of a chondritic component this confirms previous suggestions about the amount of extraterrestrial components a new hse and os isotope dataset for five zhamanshinites reveals on average crust like hse concentrations and os isotope compositions confirming earlier suggestions of a lack of meteoritic admixtures to these impact glasses

island arc tectonics abstract tectonic stress states in arc backarc systems have been examined on simplified model structures corresponding to the chilean mariana and intermediate arc types two sources of tectonic stress are assumed the stress due to lateral density variations and the stress transmitted from the subducting oceanic plate to the forearc the average vertical stress zz of the former type has been calculated for each isostatically compensated model lithospheric plate using the difference from a reference structure from the sign of xx zz one can estimate whether the tectonic situation is extentional or compressional if we assume that xx is caused by mechanical coupling between the subducting oceanic slab and is laterally constant in one arc backarc system we can estimate at least the upper or lower boundary of the mechanical coupling for the chilean type arcs where the truly great interplate earthquakes occur and no backarc spreading takes place the transmitted average stress must be larger than 660 bar for the mariana type arcs where no large interplate earthquakes are recorded and spreading takes place in the backarc region xx should be smaller than 100 bar in any arc backarc system the most extentional feature is expected at the highest topography of the volcanic arc once spreading has started the viscosity under the backarc region drops dramatically and the spreading will continue in the backarc even if its topography is subdued in comparison with the now separated main and remnant arcs

sedimentary volcanic and tectonic processes of the central mariana arc mariana trough back arc basin formation and the west mariana ridge 1 we present new multichannel seismic profiles and bathymetric data from the central marianas that image the west mariana ridge wmr remnant arc both margins of the mariana trough back arc basin the modern arc and eocene frontal arc high these data reveal structure and stratigraphy related to three periods of arc volcanism and two periods of arc rifting we interpret the boundary between accreted back arc basin and rifted arc crust along the mariana trough and support these findings with drilling results and recent seismic refraction and gravity studies we show that with the exception of a few volcanoes behind the volcanic front that straddle the boundary between crustal types the modern mariana arc is built entirely on rifted arc crust between 14 and 19 n our data indicate that there is more accreted back arc seafloor to the west of the mariana trough spreading axis than to the east confirming previous evidence for an asymmetric basin the rifted margin of the wmr remnant arc forms a stepped pattern along the western boundary of the mariana trough between 15 30 and 19 n in this region linear volcanic cross chains behind the wmr are aligned with the trend of mariana trough spreading segments and the wmr ridges extend into the back arc basin along the same strike these ridges are magmatic accommodation zones which to the north along the izu bonin arc punctuate tectonic extension for the wmr we hypothesize that rift basins are more commonly the sites where spreading segment offsets nucleate whereas magmatic centers of spreading segments are sites where magmatism continues from arc volcanism through rifting to back arc spreading the mariana trough is opening nonrigidly and is characterized by two predominant abyssal hill trends nnw sse in the north and n s in the south between the only two basin crossing fracture zones at 15 5 and 17 5 n s axes propagated north at the expense of nnw axes

3 d configuration of subducted lithosphere in the western pacific summary computer graphics techniques are used to portray the shape of the subducted lithosphere as seen from different views the configuration of the subducted lithosphere is represented by a surface that approximately fits the upper envelope of the slab like spatial distribution of earthquake hypocentres in the benioff wadati zone regions considered include tonga kermadec izu bonin mariana japankuril kamchatka and sunda banda in the western pacific these regions include all known zones of deep focus earthquakes except the tectonically complex regions of new hebrides solomon new britain molucca sea phillipines the mediterranean and south america shown at the same scale and with the effect of earth s sphericity included the views are very useful for illustrating variations in the configuration of the subducted lithosphere between different regions and within the same region the results illustrate the segmentation of the upper several hundred kilometres of subducted lithosphere into major units of relatively uniform configuration these segments are also clearly defined by major features of surface morphology of the trench and upper plate the distribution of subduction related volcanism and by the intersection with the subduction zone of major bathymetric features of the subducting plate in contrast the deeper portions of the slabs have a more complex and variable configuration discordances between the deep and shallow structures are often important on a regional scale and suggest that there are relative horizontal movements between the surface trace of the subduction zone and the material at depth into which the subducted lithosphere is sinking these relative movements appear to be intimately associated with the tectonic evolution of the region and indicate a regional aspect of subduction that is often overlooked in an attempt to explain the global variability of the shapes of subducted lithosphere although earthquake activity ceases at 680 km possibly because of a phase transition the geometry of the deep portion of subducted lithosphere suggests that subducted lithosphere does penetrate to greater depth

a seismic stratigraphic analysis of mariana forearc basin evolution new seismic data collected in the 14 5 18 5 n mariana segment of the izu bonin mariana island arc system image six seismic stratigraphic sequences that can be mapped throughout the inner forearc these sediments were most likely deposited from 35 ma to the present the oldest stratigraphic units 1 2 and 3 are syn rift volcaniclastic deposits unit 4 deposits accumulated during a period of mild structural inversion which resulted in several isolated reverse faulted anticlines within the forearc sedimentary basin a late period of extensional deformation began near the end of unit 5 deposition and continued through unit 6 sedimentation to the present seismic lines show that the basement of the forearc is composed of large rotated fault blocks and half grabens with ne nw and nne trends fault offset calculations show that basement faults with dips between 45 and 50 account for only 4 total extension in the forearc south of 16 3 n normal growth faults initiated during basement extension offset the frontal arc high from a deep forearc basin from correlations with the known geologic history we hypothesize that extension during deposition of units 1 through 3 corresponds to rifting of the eo oligocene arc between 35 ma and 29 ma older deposits being too thin to be seismically resolvable localized compression during unit 4 accumulation occurred some time after eo oligocene rifting in the early miocene late stage normal faulting near the end of deposition of unit 5 and throughout unit 6 accumulation may be associated with the opening of the mariana trough backarc basin from 8 ma years to the present there is a higher density of these later faults in the inner forearc between 15 5 and 17 n and in the outer forearc between 14 n and 18 n recent extension is at least partially accommodated by reactivation of older basement faults with the same ne nw and nne trends stratigraphic relationships indicate that the inner forearc south of 16 3 n has differentially subsided and tilted trenchward possibly as a result of a recent change in subducting slab geometry or subducted relief under the forearc

muong nong type tektites from the moldavite and north american strewn fields muong nong type tektites are a very important group of tektites because they differ from splash form tektites in several important characteristics e g volatile element enrichment major and minor element inhomogeneities bubbles inclusions layered and sometimes brecciated structure they seem to have experienced a lower peak temperature and pressure thus preserving the precursor material much better than the splash forms do they lead to important conclusions about the impact process especially regarding the behaviour of the trace elements during impact melting to date they have been found only within the australasian strewn field finding such tektites within other strewn fields would be very important in proving a genetic relationship between them and splash form tektites two samples one bediasite from the north american strewn field and one moldavite from the czechoslovakian strewn field that show some muong nong features layered structure have been analyzed for major and minor as well as trace elements to test for the presence of chemical muong nong signatures although these three samples show a somewhat larger internal variation of major elements than normal splash form tektites this variation is not as large as that of typical muong nong tektites likewise there is no enrichment in volatiles at least no significant one which is one of the most obvious characteristics of muong nong type thus these samples deviate to some extent from splash forms but do not constitute real muong nong tektites so no genetic relationship between muong nong tektites and normal splash forms has yet been established beyond doubt

distinct regional differences in crustal thickness along the axis of the mariana trough inferred from gravity anomalies we have compiled extensive gravity and bathymetry data for the whole mariana trough which were collected during several japanese scientific cruises over the last few years this study aims to clarify the lateral distribution of the local differences in geochemical signatures which have been observed locally in the mariana trough shipboard free air gravity anomaly data from eight japan agency for marine earth science and technology jamstec cruises were compiled with those crossover errors of 2 85 mgal mantle bouguer anomalies mba were calculated by subtracting the predictable gravity signal due to the seawater crust and crust mantle density boundaries the crustal thickness variation along the spreading axis was estimated from the mba different features in crustal thickness its variation and segment length for each segment allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the mariana trough segment in region a to the north of 20 35 n shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources a variety of crustal thickness values and of along axis crustal thickness variations in region b between 15 38 n and 20 35 n suggests two types of segments one is similar to a slow spreading ridge segment that has a plume like mantle upwelling under the spreading axis and the other is a magma starved segment region c between 14 22 n and 15 38 n is a less magmatic region individual crustal thickness averages of 3 4 4 1 km region d to the south of 14 22 n has higher individual crustal thickness averages of 5 9 6 9 km suggesting higher magmatic activity with a sheet like mantle upwelling under the spreading axis different features in the mba for off axis areas suggest that these four regions have existed since the mariana trough started spreading moreover comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust which are locally distributed in both regions b and c probably result from mantle source depleted of water and incompatible elements this suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the mariana trough

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

heterogeneity within the subducting pacific slab beneath the izu bonin mariana arc evidence from tomography using 3d ray tracing inversion techniques distinct zones of seismic heterogeneity along the izu bonin mariana arc have been investigated using complimenting regional bulk sound shear wave speed and p wave tomographic images the distribution of seismic anomalies and the inferred geometry of the subducting pacific plate have been modelled in unprecedented detail using joint tomography and new p wave model using a 3d inversion algorithm the use of 3d ray tracing techniques and smaller cell parameterizations have greatly enhanced the resolution of gradients therefore the models show much more detail about the structure and physical properties of the subduction zone the well defined features from the multiple wave speed images are used to elucidate the distinct morphology change between slab beneath the izu bonin horizontal slab and mariana vertical arcs and the distribution of physical properties in the mantle and the subducting oceanic lithosphere changes in physical properties within the slab tear at the southern end of the izu bonin arc identified as a bgap or thinningq in the tomographic images could be the result of the distortion of the pacific plate as its shape transforms between near horizontal to near vertical a decrease in the rigidity and strength of the lithosphere the subduction of the marcus necker ridge change in subduction velocity or a combination of all these factors strong slow anomalies exist both in the mantle wedge above and below the slab beneath the izu bonin arc but are not present beneath the mariana arc at the junction between the izu bonin and mariana arcs it appears that the there are two separate pieces of the pacific plate the torn slab north of 268 n and a buckled near vertically dipping slab south of the tear the transition between the two distinct slab morphologies coincides with the location of the ogasawara plateau and the trench d 2005 elsevier b v all rights reserved

beryllium 10 in australasian tektites evidence for a sedimentary precursor each of seven australasian tektites contains about 1 x 10 sup 8 atoms of beryllium 10 half life 1 53 x 10 sup 6 years per gram cosmic ray bombardment of the australites cannot have produced the measured amounts of beryllium 10 either at the earth s surface or in space the beryllium 10 contents of these australites are consistent with a sedimentary precursor that adsorbed from precipitation beryllium 10 produced in the atmosphere the sediments must have spent several thousand years at the earth s surface within a few million years of the tektite producing event

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

noble gases and k ar ages in aouelloul zhamanshin and libyan desert impact glasses abstract elemental abundances of noble gases and isotopic compositions of ne and ar have been determined in seven aouelloul impact glasses four zhamanshin impact glasses and one libyan desert glass sample similar to most other natural terrestrial glasses a high ne ar ratio was observed in all impact glasses the ne concentrations in the impact glasses are of the same order of magnitude as those in tektites but the concentrations of ar and the heavy noble gases kr and xe are significantly higher than those in the tektites as a result the ne ar ratios in the impact glasses are lower than in tektites the isotopic composition of ne in the impact glasses is identical to that of air as also observed in tektites suggesting that the ne is atmospheric in origin from the k content of the glasses we calculated k ar ages of 10 15 ma for aouelloul and 0 7 1 0 ma for zhamanshin impact glasses for zhamanshin glasses these ages are in good agreement with other radiogenic and fission track ages while for aouelloul our k ar ages agree with older k ar ages reported before but not with the younger 3 2 ma fission track ages

chemical composition of north american microtektites and tektite fragments from barbados and dsdp site 612 on the continental slope off new jersey abstract we have determined the major element and trace element content of tektite fragments and microtektites found in deep sea sediments from barbados and dsdp site 612 on the continental slope off new jersey the major element and trace element contents were determined using energy dispersive x ray analysis and instrumental neutron activation analysis respectively the compositions are consistent with the conclusion that the samples from both occurrences belong to the north american tektite strewn field the chemistry of the samples from both occurrences is however not identical with the bediasite or georgianite chemistry there are differences especially in the lower sodium and higher k and cr and possibly higher volatile element contents of the tektite fragments from dsdp site 612 the differences in chemistry between bediasites georgianites and the tektites from the sites analyzed here show that there are geographical variations in composition in the north american strewn field like in the czechoslovakian or australasian strewn field the relatively high volatile element contents e g sb and au of the site 612 tektite fragments together with the occurrence of mineral inclusions and a layered structure of some specimens indicates that these specimens are or close to muong nong type tektites if the dsdp site 612 tektites are muong nong type then the source crater has to be close to that site and is therefore probably on the continental shelf as had previously been suggested tektites on land usually occur in deposits which are much younger than the tektites the so called age paradox the association of tektites and microtektites together in one layer with a stratigraphic age that is consistent with the radiometric age would seem to resolve this paradox

tektites size estimates of their source craters and implications for their origin abstract from estimates of the total masses of tektites in three strewnfields calculations by orphal et al 1980 of the amount of melt that could be ejected from impact craters and equations relating kinetic energy of impact to crater diameter it is possible to calculate minimum diameters of lunar craters capable of ejecting the liquid masses that could have formed the various tektite strewnfields no lunar craters of the requisite sizes have been found that are young enough to correlate with the dates of formations of the strewnfields and it seems clear that the moon must be eliminated as a source of tektites on the earth it is concluded that the associations of the ivory coast tektites with the bosumtwi crater and the moldavites with the rieskessel are real and the tektites are of terrestrial origin it follows that if the ivory coast tektites came from the 10 5 km wide bosumtwi crater the larger masses in the australasian and north american strewnfields came from craters 17 km in diameter and between 33 and 65 km in diameter respectively no crater has yet been proven to be the parent of the australisian tektites the large crater that formed the north american tektites may not yet have been found although the mistastin lake crater may eventually be proven to be the source

extension in the northern mariana inner forearc seismic reflection seamarc ii acoustic imagery and bathymetric data across the northern mariana island arc reveal dense high angle normal faulting of the arc and inner forearc faults with throws up to 780 m cut a thick sequence of stratified but otherwise undeformed sediments the major faults display a sinuous or zigzag trace that is commonly associated with rifting this pattern can be indicative of a single event of triaxial strain that requires two contemporaneous conjugate fault pairs in orthorhombic symmetry interpretation and statistical analyses of the data show activity since the last sedimentary deposition and concurrent fault development of two distinct fault populations one associated with the backarc basin s east boundary fault and the other with extension in the forearc extensional faults along the eastern side of the backarc basin near the active arc at 22 n where active rifting is occurring have approximately the same trend as the east boundary fault in the survey area we estimate the direction of extensional minimum principal strain for the east boundary fault at 22 n to be 39 1 orthogonal to the extensional strain direction measured in the inner forearc the change in strain is distinct and abrupt occurring across a 20 km wide region along the arc massif within this narrow region of transition the intermediate and minimum principal strain directions are nearly equal in magnitude with orientations midway between the dominant forearc and backarc basin principal strain directions statistical analyses show that the strain from either backarc or forearc regime is not transmitted across this narrow region the two opposing strain regimes coexist virtually juxtaposed the coexistence of these two orthogonal extensional fault regimes within the overriding plate between the mariana backarc basin and inner forearc may be explained by assuming that a weak transitional region exists along the east boundary fault zone where stresses can be more easily relieved this allows active basin wide extension in the backarc basin while the forearc data are consistent with models that require radial forearc deformation resulting from increasing arc trench curvature of the northern half of the mariana island arc

australasian microtektites in the south china sea and the west philippine sea implications for age size and location of the impact crater abstract microtektites from two deep sea cores in the south china sea and the west philippine sea are identified as belonging to the australasian tektite strewn field based on the morphology chronostratigraphic occurrence and geographical location of these microtektites the higher concentrations of microtektites 1000 cm2 in the marginal seas of the western pacific with the peak concentration in the south china sea support the hypothesis of a large impact crater in indochina these two new occurrences lead to a more precise dating of the impact event at 793 ka whereas the size of the australasian source crater on the indochina peninsula is estimated to be 90 1 16 km introduction the australasian strewn field covering at least one tenth of the earth s surface is the largest and the youngest 0 77 0 02 ma izett and obradovich 1992 of the four known tektite strewn fields microtektites belonging to the australasian strewn field have been found in more than 40 deep sea cores throughout much of the indian ocean the western equatorial pacific ocean and the sulu celebes and philippine seas glass

impact glasses from zhamanshin crater u s s r chemical composition and discussion of origin three silica rich zhamanshinites and one irghizite from the zhamanshin impact crater northern aral area u s s r have been analyzed for up to 40 major minor and trace elements all data point to a clear distinction between these impact glasses and other tektites or impact glasses e g from the australasian strewn field halogens are generally enriched in the irghizites and zhamanshinites when compared to normal splash for tektites with zhamanshinites enriched more than irghizites the same holds also for the alkali metals and a number of other volatile elements like sb and as nickel and cobalt are enriched in the irghizite sample to a considerable degree suggesting meteoritic contamination this view is also supported by gold and selenium data but for quantifications other siderophile elements need to be considered chromium is not a valid indicator of meteoritic contamination because smallamounts of ultra basic igneous material may completely alter the picture the rare earth elements do show a sedimentary pattern consistent with two or three different source materials and a variation which is probably mostly due to dilution with silica rich materials the peak pressure and temperature experienced by the irghizites was lower than for australites or other splash form tektites and even lower for zhamanshinites which is revealed by the content of volatile elements and lesser homogeneity

deformation and faulting of subduction overriding plate caused by a subducted seamount we conducted numerical experiments to simulate elastoplastic deformation of the overriding plate caused by a subducted seamount calculations revealed development of a distinct pair of fault like shear zones including a landward dipping forethrust fault initiated from the seamount top and a seaward dipping backthrust fault from the landward base of the seamount significant dome shaped surface uplift was predicted above the thrust faults lesser developed seaward dipping backthrust faults were calculated to develop under certain conditions the overriding plate was calculated to deform in two stages in stage i elastic deformation leads to the formation of fault like shear zones after major faults have cut through the entire plate plastic deformation on faults dominates stage ii on the subduction interface compressional normal stress was calculated to increase on the landward leading flank of the seamount and decrease on the seaward trailing flank these changes together with associated stress singularities at seamount edges could affect earthquake processes

centroid moment tensor solutions for deep earthquakes predating the digital era discussion and inferences abstract we present a discussion of 139 new centroid moment tensor solutions for deep earthquakes covering the wwssn 1962 1976 and earlier years obtained by applying the standard harvard inversion algorithm to data sets of seismograms digitized from analog records our solutions fill several gaps in seismicity reflecting undersampling by the harvard catalogue while most focal mechanisms previously published for wwssn era earthquakes are in good agreement with our inverted solutions focal geometries for older events appear unreliable we confirm that the region of most intense moment release is around 650 km and that a minimum exists at 500 km however the local maximum around 450 km may be less marked than previously suggested the bonin marianas subduction zone exhibits a different pattern with an overall maximum of seismic activity at 420 km all other subduction zones demonstrate the potential for seismicity at least at the 1027 dyn cm level finally while non double couple components are found systematically in our enhanced data set they correlate neither with depth nor with earthquake size suggesting that their presence does not necessarily relate to the exiguity of the space available for rupture at the tip of subduction zones

no evidence for a 0 8 0 9 m y old micro australite layer in deep sea cores abstract fission track and 40 ar 39 ar dating of australasian tektites indicates that the australites are older than the rest of the australasian tektites some authors have suggested therefore that there should be two microtektite layers in the australasian region a younger layer approximately synchronous with the brunhes matuyama geomagnetic reversal boundary previously described in the literature and an older layer of australian microtektites micro australites below the brunhes matuyama reversal boundary and perhaps associated with the end of the jaramillo event fifteen cores already known to contain a layer of australasian microtektites at or slightly above the brunhes matuyama reversal boundary were searched for an 0 8 to 0 9 m y old micro australite layer no evidence for this layer was found thus the data do not support the conclusion that the australasian tektites were produced by two impact events at different times

implication of welded breccia in muong nong type tektites tektites are considered to have originated 1 as ejecta from terrestrial cometary impacts1 2 or from terrestrial meteorite or asteroid impacts3 4 2 in cometary impacts in which splash form tektites were ejected and muong nong type tektites were produced in their place5 6 or 3 as volcanic ejecta from the moon7 welded breccias fig 1 have recently been reported in several muong nong type tektites from thailand8 two additional welded breccia structures of a more complex nature are described here which show evidence of having experienced two distinct periods of high temperatures during their formation separated by a minimum of several minutes of cooling through the annealing range this implies a prolonged multi stage formation process entirely at the site of origin in contrast theories of ejection of tektite glass from terrestrial meteorite impact craters or cometary collision events imply instant formation

asynchronous bends in pacific seamount trails a case for extensional volcanism the gilbert ridge and tokelau seamounts are the only seamount trails in the pacific ocean with a sharp 60 bend similar to the hawaii emperor bend heb these two bends should be coeval with the 47 million year old heb if they were formed by stationary hot spots and assuming pacific plate motion only new 40ar 39ar ages indicate that the bends in the gilbert ridge and tokelau seamount trail were formed much earlier than the heb at 67 and 57 million years ago respectively such asynchronous bends cannot be reconciled with the stationary hot spot paradigm possibly suggesting hot spot motion or magmatism caused by short term local lithospheric extension

geochemistry of darwin impact glass and target rocks abstract we have analyzed the major and trace element composition of 18 darwin glass samples and 7 target rocks sandstones shales and a quartzite from the darwin crater area on the basis of our data and using statistical methods 3 chemically distinct groups of darwin glass were identified a low fe al lfe al or average darwin glass group b hfe al group and c hmg na group the glasses of group c also show anomalous enrichments of several elements e g cr mn co and ni electron microprobe studies show that the glasses are inhomogeneous on the micrometer scale which is typical for impact glasses the geochemistry of all glasses is very similar to terrestrial sediments and thus supports the impact origin model we have performed mixing calculations which show that in general darwin glass can be formed by melting and mixing local target rocks the best fit is obtained for a mixture of 30 quartzite 60 shale b3 dg and 10 shale b1 dg some major element contents do not agree exactly which is most probably due to the limited selection of target rocks that were available for our study the analyses and mixing models demonstrate that volatile elements e g zn ga sb and the alkalies have been lost during production of the impact glasses which can be expected because of the high formation temperature we have furthermore tried to explain the enrichments of cr mn co and ni in group c glasses by contributions from a non sedimentary source e g ultrabasic rocks or from the impacting body none of the mixtures provides a satisfactory fit darwin glass does not show any significant ir enrichments admixture of material from iron meteorites gives too high fe co and ni and too low cr and mn contents chondritic contaminations would yield ir abundances in the glass that are several orders of magnitude above the observed levels better fits are obtained for an achondritic contamination but again give excess ir an ultrabasic contribution gives better results except for higher mg but no such rocks are known from the target area thus at the present time we are not able to explain the enrichments of cr mn co and ni in glasses of group c in a satisfactory way

geochemistry of tektites and impact glasses glass produced by melting during impact events is called impact glass or impactite although impact crate ring produces a great variety of shock metamorphosed rocks all of which are called impactites by various authors to a differing extent we restrict the use of this term here to material completely converted into glass the degree of metamorphism seen in different classes of impact materials starting with those showing simple shock effects shock lithification shatter cones diaplectic glasses coesite stishovite lechatelierite impact breccias glass depends on the peak pressure and temperature the rock has experienced as a result of mixing brecciation and similar processes impact glasses also tend to include fragments of less shocked or less homogeneous metamorphosed rocks we limit our discussion here however to the more homogeneous members of the terrestrial impactite family a discussion of lunar impact glasses was given by delano et al 1981 another group of natural glasses are tektites which at first glance are similar to obsidian most tektites are very homogeneous often aerodynam ically shaped spherically symmetric objects that are several centimeters in size and are found in various areas referred to as tektite strewn fields in reflected light they are usually black but in transmitted light they show variations of grey brown and green the surface features gouges furrows grooves pits striations clearly distinguish them from terrestrial volcanic glasses and have probably originated from solution etching and weathering effects on earth baker 1963 lamarche et a1 1984 and from surface stresses occurring during their cooling tektites found in geographically adjacent

the west philippine basin and the initiation of subduction revisited 1 new swath bathymetry plus existing geophysical data reveal that the direction of west philippine basin wpb seafloor spreading rotated 100 counter clockwise between 49 ma and 33 ma the curvilinear and multi stranded mindanao fracture zone separates the wpb from the palau basin to the south wpb opening was contemporaneous with and behind early izu bonin mariana ibm subduction whose arc volcanism began by 50 ma this produced over 1000 km of arc parallel spreading or stretching plus magmatism in the mariana segment of the eocene ibm arc forearc the initial ibm subduction cut across rather than followed pre existing structures remnant arcs fracture zones and spreading fabric new models of subduction initiation are required to reproduce such characteristics

seamount subduction erosion in the nankai trough and its potential impact on the seismogenic zone seamount subduction along subduction zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate however tectonic erosion processes have not been examined in detail with high quality three dimensional 3 d seismic reflection imaging in 1999 we acquired 3 d seismic reflection data from the nankai trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure fortuitously these data reveal a small to 1 km high basement ridge that has subducted to 7 km subseafloor updip from the basement ridge a 1 km thick sequence of sediment from the base of the accretionary wedge appears to be missing we interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction tectonic erosion has removed more than 25 km 3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone the tectonically eroded sediments are presumed to enhance fault zone fluid content potentially reducing fault zone effective stress and may temporarily inhibit earthquake rupture potential after the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating

source of the australasian tektite strewn field a possible off shore impact site although there is a preponderance of evidence that tektites were formed by asteroid impacts on the earth no source crater has been found for the largest and youngest of the strewn fields the australasian strewn field we have examined a combined seasat geos 3 altimeter data set of sea surface heights in the northern portion of the australasian strewn field for negative gravity anomalies on the continental shelf and slope which might be related to the source crater for these tektites a large negative anomaly which we call the qui nhon slope anomaly is a sea surface depression of approximately 1 5 meters over an area of 100 km diameter centered at 13 47 n latitude and 110 37 e longitude it corresponds to a gravity anomaly of about 50 mgal a surface ship gravimeter track across most of the structure confirms the satellite measurements this site is on the continental slope in 1 to 3 km of water a sea floor topographic depression of more than 200 meters occurs in the southeast quadrant of the anomaly the center of the structure is 175 km offshore and is less than 300 km from the muong nong substrewn field and the prolific dalat site perhaps the largest concentration of tektites within the australasian field due to its location size and magnitude we propose this anomaly may be due to the impact structure that produced the australasian strewn field

boron content and isotopic composition of tektites and impact glasses constraints on source regions abstract abundances of li be and b as well as boron isotopic compositions were determined in twenty seven tektite and impact glass samples using an ion microprobe samples included tektites from the australasian north american and ivory coast strewn fields and aouelloul and darwin impact glasses variations of b abundance and isotopic composition in a flanged australite were also studied 11 b variations of only a few permil were found within the australite flange the isotopic composition shows no correlation with the b contents or with the distance from the rim of the flange the mean 11 b value for the flanged australite is very similar to that of muong nong type tektites 1 9 1 9 thus vapor fractionation has been unimportant during tektite formation this is supported by the observation that b contents and the 11 b values of the different samples from the australasian tektite strewn field are not correlated with each other most tektites show a rather limited range of 11 b values 9 3 1 5 to 2 7 1 5 o which is small compared to the range observed for common terrestrial rocks 30 to 40 the b abundance and isotopic data can be used to place constraints on the tektite source rocks australasian tektites have high b and li abundances only clay rich sediments such as pelagic and neritic sediments as well as river and deltaic sediments have b contents up to 100 ppm and 11 b values that are in agreement with the range shown by australasian tektites 4 9 to 1 4 10 be and rb sr data indicate continental crustal source rocks and exclude pelagic and neritic sediments however deltaic sediments e g from the mekong river which are of continental crustal origin agree with 10 be rb sr and b data and support a possible source locality close to the coast of se indochina in the south china sea on the other hand one bediasite sample has a very high 11 b value of 15 1 2 1 requiring the presence of marine carbonates or evaporites among the source rocks this finding supports an offshore locality for the north american tektite source crater

geodetic evidence of back arc spreading in the mariana trough 1 repeated gps surveys in the mariana islands show that the mariana block is moving apart from the philippine sea plate the velocities of the islands relative to the philippine sea plate range from about 15 mm yr in the northern islands to about 45 mm yr near guam the data also suggest convergence rates for the mariana forearc with respect to the pacific plate of 35 45 mm yr at 19 n increasing to 55 70 mm yr at 13 5 n in addition the velocity vectors show a slight north south expansion of the arc the estimated location of the euler pole of the mariana forearc with respect to the philippine sea plate is well south of the geographical point where the back arc basin narrows to zero width

bediasite source materials a solution to an endmember mixing problem exploiting closed data abstract previous tektite studies proposed parent materials ranging from single lithologies to various three endmember mixtures of igneous and sedimentary rocks in part this breadth of proposed parent materials resulted from the unavailability of proper statistical methods for analyzing percentage or closed data however new statistical techniques penetrate these data and the bediasites being random samples of endmember mixtures afford opportunities to establish a paradigm for endmember identification determine the minimum number of lithologic geochemical endmembers contributing to the bediasite compositions and estimate the major element chemistries of those endmembers we conclude that both the major element chemistry and the correlations among major element ratios for bediasites 1 1 cannot be explained solely as an effect of selective volatilization of a single source material 2 2 require a minimum of two contributing endmember materials and 3 3 suggest a binary mixture of endmembers chemically similar to a subarkose and a ferro aluminum residual clay

seismicity and focal mechanisms associated with the subduction of the juan de fuca plate beneath the olympic peninsula washington abstract two and a half years of data from an expanded telemetered seismic network on the olympic peninsula reinforce the conclusion that the juan de fuca plate is subducting beneath washington however the rate of seismicity is presently much lower than typical rates at other convergent margins the located earthquakes can be divided into two groups on the basis of depth at the coast a shallow zone extends to a depth of approximately 20 km while a deep zone begins at 25 km on the eastern edge of the peninsula the shallow zone has thickened to 30 km and the depth to the top of the deep zone has increased to 45 km the rate of crustal seismicity is much lower on the peninsula than in the adjacent puget sound region the deep zone is 5 to 10 km thick and dips to the east at an angle of about 11 the deep events appear to be occurring in the upper part of the subducting oceanic lithosphere focal mechanisms were constructed for 16 events in the suite of deep events with magnitudes ranging from 1 6 to 4 3 the solutions show primarily normal faulting with the p axes normal to the slab and the t axes displaying a range of angles within the slab with the average t axis in the downdip direction small events with down dip tension in the downgoing slab are consistent with the slab sinking under its own weight several strike slip events may be caused by a shearing within the slab these features are consistent with ongoing subduction of the juan de fuca plate beneath north america