sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

noble gases in individual l chondritic micrometeorites preserved in an ordovician limestone we have measured the he and ne concentrations and isotopic ratios of individual sediment dispersed extraterrestrial chromite grains 63 180 mu m in diameter from an ordovician limestone in southern sweden in the same sediment many fossil meteorites were found and have been attributed to the l chondrite parent body breakup event similar to 470 ma ago in this analysis of 37 individual extraterrestrial chromite grains of l chondritic major element composition at least 35 similar to 95 contain surface implanted helium and neon of fractionated solar wind composition implying that these grains are relict parts of fossil micrometeorites of asteroidal origin similar to what has been observed in recent micrometeorites collected in the polar regions significant amounts of cosmogenic ne 21 were found in several grains resulting in cosmic ray exposure cre ages of up to similar to 77 myr these ages exceed both dynamical lifetimes for asteroidal micrometeorites of this size as well as cre ages observed in chromites from fossil meteorites from the same sediment beds significant contributions from terrestrial sources like nucleogenic and cosmic ray muon induced ne 21 can be excluded in the extraterrestrial chromites since 11 terrestrial chrome spinel grains from the same sediment beds did not contain any measurable ne 21 excesses having found micrometeorites of undisputed asteroidal origin with cosmic ray exposure ages of several 10 7 years implies that high cosmic ray exposure ages alone are not a good indicator of cometary origin in opposition to what has been suggested for recent micrometeorites and idps we propose instead that these grains collected their cosmogenic ne 21 while residing in the regolith layer of their parent body c 2009 elsevier b v all rights reserved less

plate kinematics in the western pacific derived from geodetic observations 1 plate kinematics have been derived from gps site velocities spanning australasia and the western pacific and define a framework in which deformation fields at plate boundaries can be investigated the australian plate appears to be rigid from cocos island in the west to noumea in the east and incorporating all of the australian continent a revision of the angular velocity vector of the pacific plate using six site velocities has led to the detection of significant relative motion between the pacific plate and sites in northern papua new guinea this provides strong evidence in support of a north bismarck plate between the south bismarck and pacific plates and we model the motion of the north bismarck plate with the angular velocity vector 45 0 s 126 4 e 0 85 myr these new angular velocity vectors model the rigid plate motions of a substantial part of the australasian and western pacific regions with an accuracy better than 3 mm yr

the micrometeorite flux to earth during the frasnian famennian transition reconstructed in the coumiac gssp section france abstract we have reconstructed the distribution of extraterrestrial chrome spinels in a marine limestone section across the frasnian famennian stratotype section at coumiac in southern france providing the first insights on the types of micrometeorites and meteorites that fell on earth at this time the data can test whether the small cluster of roughly coeval large impact structures is related to an asteroid breakup and shower with possible bearings also on the late devonian biodiversity crisis a total of 180 extraterrestrial spinel grains 32 m were recovered from 957 kg of rock noble gas measurements of individual grains show high solar wind content implying an origin from decomposed micrometeorites element analyses indicate a marked dominance of ordinary chondritic over achondritic grains similar to the recent flux the relation between h l and ll meteorites is 29 58 13 similar to the late silurian flux 31 63 6 but different from the distribution 45 45 10 in the recent and the cretaceous flux our data show no indication of a generally enhanced late devonian micrometeorite flux that would accompany an asteroid shower however in a single limestone bed that formed immediately before the upper kellwasser horizon that represents the main end frasnian species turnover event we found an enrichment of 10 ordinary chondritic grains 63 m per 100 kg of rock compared to the 1 3 grains per 100 kg that characterise background the anomalously abundant grains are of mixed h l and ll types and may be related to an enhanced flux of extraterrestrial dust during postulated minima in both the 405 ka and 2 4 ma earth orbit eccentricity cycles at the onset of the upper kellwasser event in the present solar system the dust accretion at earth is the highest at eccentricity minima because of the spatial distribution of dust bands of the zodiacal cloud besides this small grain anomaly the data here and in previous studies support a stable meteorite flux through the late silurian and devonian in contrast to the mid ordovician when achondritic meteorites that are rare on earth today were common followed by the influx of a flood of debris related to the breakup of the l chondrite parent body our accumulated data for six time windows through the phanerozoic indicate that the ordinary chondrites make up a major fraction in the meteorite flux since at least the mid ordovician we note that the sources in the asteroid belt of the h and l meteorites the two most common types of meteorites today and through much of the phanerozoic remain elusive

new insights on the tectonics along the new hebrides subduction zone based on gps results 1 at the new hebrides nh subduction zone ridges born by the subducting australia plate enter the trench and collide with the overriding margin results from gps surveys conducted on both sides of the trench and new bathymetry maps of the nh archipelago bring new light on the complex tectonics of this area convergence vectors present large variations that are not explained by australia pacific a p poles and that define four segments vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors slow convergence i e 30 40 mm yr is found at the central segment facing the d entrecasteaux ridge the southern segment moves faster than a p motion predicts 89 to 124 mm yr relatively to a western north fiji basin wnfb reference the northern and southern segments rotate in opposite directions consistently with the extension observed in the troughs east of both segments both rotations combine in central vanuatu into an eastward translation that bulldozes the central segment into the wnfb at 55 mm yr that model suggests that the motion of the central segment forced by the subduction collision of the d entrecasteaux ridge influences the motion of the adjoining segments the new caledonia archipelago is motionless with respect to the rest of the australia plate despite the incipient interaction between the loyalty ridge and the nh margin southeast of the interaction area convergence is partitioned into a 50 mm yr trench normal component accommodated at the trench and a 90 mm yr trench parallel component close to the a p convergence and presumably accommodated by a transform boundary at the rear of the nh arc

rapid forearc uplift and subsidence caused by impinging bathymetric features examples from the new hebrides and solomon arcs 1 isotopically dated corals from the central new hebrides and new georgia island group solomon islands indicate that both forearcs underwent rapid late quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to 8 mm yr while total plate convergence was only a few kilometers two mechanisms that might account for these rapid reversals in vertical motion include 1 a displacement mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate as the object moves deeper and vacates the base of the forearc the forearc subsides to near its original position and 2 a crustal shortening mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone rapid subsidence follows when the impinging object is broken or otherwise decoupled shallow interplate coupling becomes weak and the uplifted forearc extends and subsides the displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers but it fails to explain the geographic pattern short time frame and abruptness of the change from subsidence to uplift that we observe the crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone

ages of large lunar impact craters and implications for bombardment during the moon s middle age abstract standard lunar chronologies based on combining lunar sample radiometric ages with impact crater densities of inferred associated units have lately been questioned about the robustness of their interpretations of the temporal dependance of the lunar impact flux in particular there has been increasing focus on the middle age of lunar bombardment from the end of the late heavy bombardment 3 8 ga until comparatively recent times 1 ga to gain a better understanding of impact flux in this time period we determined and analyzed the cratering ages of selected terrains on the moon we required distinct terrains with random locations and areas large enough to achieve good statistics for the small superposed crater size frequency distributions to be compiled therefore we selected 40 lunar craters with diameter 90 km and determined the model ages of their floors by measuring the density of superposed craters using the lunar reconnaissance orbiter wide angle camera mosaic absolute model ages were computed using the model production function of marchi et al marchi s mottola s cremonese g massironi m martellato e 2009 astron j 137 4936 4948 we find that a majority 36 of 40 of our superposed crater size frequency distributions are consistent with the model production function a histogram of the original crater floor model ages indicates the bombardment rate decreased gradually from 3 8 ga until 3 0 ga implying an extended tail to the late heavy bombardment for large craters it also preliminarily suggests that between 3 0 and 1 0 ga bombardment may be characterized by long periods 600 myr of relatively few impacts lulls broken by a short duration 200 myr of relatively more impacts spike while measuring superposed craters we also noted if they were part of a cluster or chain named obvious secondary and analyzed these craters separately interestingly we observe a wide variety of slopes to the differential size frequency power law which demonstrates that there can be considerable variation in individual secondary crater field size frequency distributions finally four of the small superposed crater size frequency distributions are found to be inconsistent with the model production function possible reasons are resurfacing has modified these distributions unrecognized secondary craters and or the model production function has incorrect inputs such as the scaling law for the target terrain the degraded appearance of the superposed craters and indications of resurfacing suggest that the first cause is the most likely

extraterrestrial chromite in latest maastrichtian and paleocene pelagic limestone at gubbio italy the flux of unmelted ordinary chondrites the distribution of sediment dispersed extraterrestrial ordinary chondritic chromite ec grains 63 m has been studied across the latest maastrichtian and paleocene in the bottaccione gorge section at gubbio italy this section is ideal for determining the accumulation rate of ec because of its condensed nature and well constrained sedimentation rates in a total of 210 kg of limestone representing eight samples of 14 28 kg distributed across 24 m of the bottaccione section only 6 ec grains were found an average of 0 03 ec grains kg 1 in addition one probable pallasitic chromite grain was found no ec grains could be found in two samples at the cretaceous tertiary k t boundary which is consistent with the k t boundary impactor being a carbonaceous chondrite or comet low in chromite the average influx of ec to earth is calculated to 0 26 grain m 2 kyr 1 this corresponds to a total flux of 200 tons of extraterrestrial matter per year compared to 30 000 tons per year as estimated from os isotopes in deep sea sediments the difference is explained by the ec grains representing only unmelted ordinary chondritic matter predominantly in the size range from 0 1 mm to a few centimeters in diameter sedimentary ec grains can thus give important information on the extent to which micrometeorites and small meteorites survive the passage through the atmosphere the average of 0 03 ec grain kg 1 in the gubbio limestone contrasts with the up to 3 ec grains kg 1 in mid ordovician limestone that formed after the disruption of the l chondrite parent body in the asteroid belt at 470 ma the two types of limestone were deposited at about the same rate and the difference in ec abundance gives support for an increase by two orders of magnitude in the flux of chondritic matter directly after the asteroid breakup

the mw 7 5 1999 ambrym earthquake vanuatu a back arc intraplate thrust event 1 on 26 november 1999 a mw 7 5 thrust earthquake occurred in the vanuatu back arc seismic zone the aftershocks seismicity defines a west dipping plane down to a depth of 20 km consistent with the focal mechanism geometry the shallow focal depth of the main shock and the coincidence of the surface projection of the plane of aftershocks with a 600 m high sea bottom fault scarp indicate the fault plane ruptured up to the surface the size of the fault has been estimated to 50 25 km2 from the aftershocks distribution and modeling of coseismic vertical motion inferred from biological markers the inferred length indicates the rupture affected the whole ambrym segment but inversion of gps horizontal displacement data indicates most of the seismic moment has been released on a smaller 35 20 km2 fault surface with an average slip of 6 5 m foreshock and aftershock seismicity are organized in planar seismic zones parallel to the inferred main shock rupture plane this spatiotemporal distribution of imaged fault suggests that the thrust front propagates in times by eastward jumps to new intraplate faults occurring in the western edge of the north fiji basin the back arc present setting could eventually represent the very early stages of an incipient subduction zone with reverse polarity the intraplate fault model yields shortening by thickening of the back arc crust using crustal balanced sections to model the deformations over the last 1 8 m y period a maximum shortening of 55 km has been found

tectonics and magma genesis in the northern north fiji basin abstract the north fiji basin is a complex back arc basin in which active seafloor spreading is occurring along two north south ridge systems lithospheric extension is also taking place along segments of the east west trending fiji fracture zone and south pandora ridge the fiji fracture zone is clearly a transform fault but the south pandora ridge is more difficult to interpret some characteristics are consistent with a slow spreading ridge whereas others suggest a transform fault basalts recovered on the south pandora ridge are enriched in high field strength and large ion lithophile elements and many of them resemble ocean island tholeiites or transitional alkali basalts the south pandora ridge is an enigmatic tectonic feature and the geochemistry morphology and seismicity are best explained in terms of a model involving slow spreading ridge segments within a broad transform domain at its southern end the central ridge of the north fiji basin has a bathymetric profile normally associated with fast spreading steady state mid ocean ridges and the basalts recovered on this part of the ridge are moderately evolved normal mid ocean ridge basalts morb in contrast the northern end of the ridge has a complex topography dominated by a series of deep linear troughs and grabens and basalts recovered from the floors of these troughs vary widely in chemistry from normal morbs to transitional basalts that are comparatively enriched in large ion lithophile and high field strength elements the central ridge appears to be propagating northwards into old north fiji basin crust and the northern extension of the ridge is an incipient spreading locus in the very earliest stages of development in areas of the basin where fast spreading large scale mantle upwelling leads to extensive melting of asthenosphere derived depleted mantle morb like compositions dominate the erupted rocks in contrast areas of slow or incipient spreading and other tectonic settings where a transient thermal disturvance occurs are characterized by basaltic magmatism that shows relative enrichment in large ion lithophile and high field strength minor and trace elements mixing of morb and enriched end member components either at source or as magmas gives rise to a spectrum of compositions that can be related to a range of tectonic environments in the back arc setting

propagation of back arc extension into the arc lithosphere in the southern new hebrides volcanic arc new geophysical data acquired during three expeditions of the r v southern surveyor in the southern part of the north fiji basin allow us to characterize the deformation of the upper plate at the southern termination of the new hebrides subduction zone where it bends eastward along the hunter ridge unlike the northern end of the tonga subduction zone on the other side of the north fiji basin the 90 bend does not correspond to the transition from a subduction zone to a transform fault but it is due to the progressive retreat of the new hebrides trench the subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the new hebrides arc and toward the south of the hunter ridge so that the direction of convergence remains everywhere orthogonal to the trench in the back arc domain the active deformation is characterized by propagation of the back arc spreading ridge into the hunter volcanic arc the n s spreading axis propagates southward and penetrates in the arc where it connects to a sinistral strike slip zone via an oblique rift the collision of the loyalty ridge with the new hebrides arc less than two million years ago likely initiated this deformation pattern and the fragmentation of the upper plate in this particular geodynamic setting with an oceanic lithosphere subducting beneath a highly sheared volcanic arc a wide range of primitive subduction related magmas has been produced including adakites island arc tholeiites back arc basin basalts and medium k subduction related lavas

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

ultralong period seismic study of the december 2004 indian ocean earthquake and implications for regional tectonics and the subduction process analysis of the earth s longest period normal modes shows that the december 2004 sumatra andaman earthquake was much larger mw 9 3 than ini tially inferred from surface wave data and involved slip on a much longer fault than initially inferred from body wave data the seismic moment and relative excitation of the normal modes indicate that the entire aftershock zone ruptured consistent with the large tsunami amplitudes in thailand sri lanka and india an apparent increase in seismic moment with period results from interference between parts of the fault the earthquake resulted from subduction of the indian plate beneath the burma microplate a sliver plate between the indian and sunda plates hence the rate and direction of convergence depends on the motion of the burma plate which is not well known convergence would be highly oblique if the rate of motion between burma and sunda is that inferred from spreading in the andaman sea and less if a slower rate is inferred from the sagaing fault the december earthquake was much larger than expected from a previously proposed relation based on the idea of seismic coupling in which such earthquakes occur only when young lithosphere subducts rapidly moreover a global reanalysis finds little support for this correlation hence we suspect that much of the apparent differences between subduction zones such as some trench segments but not others being prone to m w 8 5 events and hence oceanwide tsunamis may reflect the short earthquake history sampled this possi bility is supported by the variability in rupture mode at individual trench segments

seismic recurrence intervals and timing of aseismic subduction inferred from emerged corals and reefs of the central vanuatu new hebrides frontal arc the recognition and dating of corals that have been killed by tectonic uplift allow us to date paleoseismic uplifts in the vanuatu island arc we recognize corals that record paleouplifts by their similarity to those known to have died during contemporary sudden uplifts and date them 1 by counting annual coral growth bands only if part of the coral is alive at the time of collection or 2 by newly developed techniques for obtaining 230 th ages by mass spectrometry the mass spectrometric method produces isotopic ages with precisions of 3 to 9 years 2 in the 0 1000 years b p time range the 230 th ages in this time range appear to be accurate samples whose ages are known by counting coral growth bands give 230 th ages that are indistinguishable from their growth band ages by dividing the average increment of uplift for the latest holocene uplifts by the mean holocene uplift rate we can estimate average seismic uplift recurrence intervals for the past 6000 years the results for each of four central vanuatu arc segments are 1 north santo emerged 1 2 m in 1866 a d and 0 6 m 107 years later in 1973 a d the average coseismic uplift of 0 9 m and mean holocene uplift rate of 4 3 mm yr 1 suggest a longer recurrence interval of 212 years 2 south santo emerged 0 29 m in 1946 and 0 26 m 19 years later in 1965 including the related 1971 event here the mean holocene uplift rate is 5 5 mm yr 1 the uplift data suggest a longer average recurrence interval of about 51 years 3 north malekula emerged 1 23 m near 1729 a d and 1 05 m 236 years later in 1965 the mean holocene uplift rate of 2 7 mm yr 1 and mean coseismic uplift of 1 14 m for dated events suggest a longer recurrence interval of 422 years 4 part of southernmost malekula has uplifted continuously or episodically by about 0 35 m from about 1957 until at least mid 1983 a d the maximum uplift of 2 7 mm yr 1 occurs near a nest of small earthquakes both the earthquakes and rapid uplift suggest that interplate slip beneath south malekula may be continuous rather than episodic episodes of 0 35 m uplift would have to recur every 130 years to maintain the 2 7 mm yr 1 uplift rate in contrast we find no evidence of interseismic vertical movements for the other three blocks the most reasonable interpretation of these results is that the seismic recurrence intervals and processes for accommodation of slip are quite different on adjacent arc segments we have used the most widely accepted moment magnitude relationship to evaluate the accumulated seismic slip caused by large earthquakes occurring since 1920 in all four arc segments this analysis suggests that the seismically radiated moments account for less than one third to one tenth of the slip associated with plate convergence the similarity between the paleoseismic record of uplifts and the contemporary record of coseismic uplifts suggests that this analysis can be generalized to times before 1920 for the northern three segments of central vanuatu aseismic slip probably occurs in the same years as large earthquakes because the contemporary coral record records uplifts only in years having large historic earthquakes this suggests that aseismic slip is not continuous and does not occur at rates which vary slowly over the course of the earthquake cycle the south santo segment may have the highest proportion of seismic slip because the mean recurrence interval of 51 years is shortest and the mean holocene uplift rate of at least 5 5 mm yr 1 is the fastest

he and ne in individual chromite grains from the regolith breccia ghubara l5 exploring the history of the l chondrite parent body regolith abstract we analyzed he and ne in chromite grains from the regolith breccia ghubara l5 to compare it with he and ne in sediment dispersed extraterrestrial chromite sec grains from mid ordovician sediments these sec grains arrived on earth as micrometeorites in the aftermath of the l chondrite parent body lcpb breakup event 470 ma ago a significant fraction of them show prolonged exposure to galactic cosmic rays for up to several 10 ma the majority of the cosmogenic noble gases in these grains were probably acquired in the regolith of the lcpb meier et al ghubara an l chondritic regolith breccia with an ar ar shock age of 470 ma is a sample of that regolith we find cosmic ray exposure ages of up to several 10 ma in some ghubara chromite grains confirming for the first time that individual chromite grains with such high exposure ages indeed existed in the lcpb regolith and that the 10 ma cosmic ray exposure ages found in recent micrometeorites are thus not necessarily indicative of an origin in the kuiper belt some ghubara chromite grains show much lower concentrations of cosmogenic he and ne indicating that the 4 pi last stage exposure age of the ghubara meteoroid lasted only 4 6 ma this exposure age is considerably shorter than the 15 20 ma suggested before from bulk analyses indicating that bulk samples have seen regolith pre exposure as well the shorter last stage exposure age probably links ghubara to a small peak of ar 40 poor l5 chondrites of the same exposure age furthermore and quite unexpectedly we find a ne component similar to presolar ne hl in the chromite grains perhaps indicating that some presolar ne can be preserved even in meteorites of petrologic type 5 less

kuwae 1425 a d the forgotten caldera in vanuatu tongoa and epi islands once formed part of a larger landmass kuwae which was partly destroyed during a cataclysmic seismo volcanic event that is recorded in local folklore it led to the formation of a 12 kmlong and 6 km wide oval shaped submarine caldera with two distinct basins and a total area of 60 km2 at the level of the rim the age ofthis eniption 1420 1430 a d and the structure of the related collapse are discussed and a composite log 143 m of the pyroclastics surrounding the caldera is presented they comprise thick hydromagmatic deposits belonging to a terminal hydromagmatic phase of the pre caldera edifice which grade upwards into two major sequences of pyroclastic flow deposits clearly related to the caldera event collapse near the caldera edge was at least in the range 650 to 950 m and may have been as much as 800 to 1100 m the volume of rocks engulfed during the caldera formation is 32 39 km3 suggesting the same volume of magma was erupted even if two coalescent collapse structures were formed it is worth noting that the kuwae caldera is not a reactivated structure but the result of a single event of short duration which occurred in the first half of the fifteenth century this event is one of the seven biggest caldera forming events during the last 10 000 years and is comparable with the santorini minoan eruption and the crater lake eruption

asteroid breakup linked to the great ordovician biodiversification event the great ordovician biodiversification event coincides 470 million years ago with the break up of a large asteroid and the resultant frequent bombardment of earth with asteroid fragments

new hydrothermal activity and alkalic volcanism in the backarc coriolis troughs vanuatu the vanuatu australia vents expedition vave to the coriolis troughs in southern vanuatu during september 2001 aboard the rv franklin discovered a new hydrothermall vent field herein informally named nifonea and recent alkallic volcanic activity the nifonea field in the central vate trough was located by coincident light transmission and ch4 anomalies in a hydrothermal plume of 60 km2 extent best developed between 1600 and 1750 m depth at 150 m above the seafloor extensive hydrothermal fauna and yellow brown crusts and mounds cover an area of 1 km2 very fresh glassy variably vesicular sparsely phyric and aphyric basalt trachybasalt and basaltic trachyandesite with 5 6 wt combined alkalies at 51 53 sio2 and enriched light rare earth elements nb and zr samples were dredged from youthful curtain tube and sheet flows plus iron oxyhydroxide deposits the alkalic composition of lavas in this tectonic setting is unique and attributed to thin ocean crust being developed in an incipient rifting phase involving a relatively low percentage of source mantle melting the coriolis troughs are among earth s most youthful backarc basins and thus provide valuable insights to incipient rifting and hydrothermal processes 2005 geological society of america

recent tectonics associated with the subduction collision of the d entrecasteaux zone in the central new hebrides abstract recent data from the new hebrides vanuatu subduction zone have enabled a morphological description and tectonic interpretation to be made of the complex plate contact between the d entrecasteaux zone and the west torres massif on the one hand and the old santo malekula block on the other between latitudes 14 30 and 17 s no trench occurs owing to the presence of the old rigid santo malekula block at the edge of the overriding plate the outer limit of plate contact describes a tortuous line westward of the projected line of the adjacent trenches this may be explained by local collision of the north d entrecasteaux ridge and the south d entrecasteaux chain with the west flank of santo this collision is compensated for by lateral plastic flow of the inner wall downward into the small basins which separate the d entrecasteaux zone ridges a process which may be explained by plastic plane strain theory the westward jump of the plate contact zone here may result from pieces of the north d entrecasteaux ridge being cut off and incorporated in the frontal arc a review of the geological evidence and the shallow seismicity of the central new hebrides shows that radiant horizontal stresses resulting from this collision have generated a specific compressive stress regime which influences the whole width of the arc the resultant deformation shows a close resemblance to a theoretical strain field produced by the indentation of a long narrow plastic body cf the arc by a rigid flat die cf the d entrecasteaux zone thus the central new hebrides block is both elastically bent under vertical stresses and plastically deformed and pushed eastwards by horizontal stresses recent uplift of the eastern chain can be explained by this model in contrast at the rear of both the northern and southern sections of the arc the tectonics are mainly controlled by a tensional stress regime the strong plate interaction due to the subduction of the d entrecasteaux zone is thought to be responsible for the compressive stress regime across the central new hebrides the same cause can be invoked to account for the tensional stress regime observed in north and south new hebrides rear arc regions

the ordovician chondrite from brunflo central sweden i general description and primary minerals abstract the brunflo chondrite was found in the rodbrottet quarry near brunflo central sweden lat 63 5 n long 14 53 e the meteorite ca 11 cm 9 5 cm occurs in a polished slab of middle ordovician limestone which based on its content of conodonts belongs to the aserian segerstad limestone the meteorite fell in the iapetus ocean close to its eastern coast line the original minerals except chromite are replaced by secondary minerals mainly calcite remarkably even minute details of textures and structures are preserved in some areas the various types of chondrules and their frequencies could therefore be studied they are similar to present falls of ordinary chondrites of petrologic types 3 and 4 chromite occurs as up to ca 0 1 mm large grains microprobe analyses of 59 grains show that except for zn the composition is fairly constant and similar to h4 h5 chromites the zn content varies from 0 2 to 6 8 wt zno mean ca 0 6 wt two analyses show that also chrome spinel is present tentatively brunflo is classified as h4 h5

extraterrestrial chromite in the resurge deposits of the early late ordovician lockne crater central sweden the distribution of extraterrestrial chromite grains 63 mu m has been studied in the resurge deposit of the early late ordovician 458 ma lockne impact structure in central sweden a 1 kg sample of resurge sediment from the rim of the crater is extremely rich in extraterrestrial chromite in total the sample contains 125 chromium spinel grains kg 1 most grains being chromite due to post depositional alterations the origin of several of the grains is dubious although it is clear that the major part of the chromites is of extraterrestrial origin 75 grains kg 1 most likely derived from the impactor the alterations are primarily due to the hydrothermal system induced by the impactor with zinc enrichment in the chromites as the most common characteristic the element chemistry of the least altered chromite grains indicates that the impactor was an ordinary l chondrite this concurs with a suggested increase in the flux of l chondritic asteroids to earth following the disruption of the l chondrite parent body at ca 470 ma more than 170 impact craters are known from around the earth but only few of the impactors have been identified mainly due to the low survival rate of projectile material further studies of impact craters will reveal if survival of extraterrestrial chromite is unique for the lockne structure c 2006 elsevier b v all rights reserved less

contemporary global horizontal crustal motion summary an analysis of satellite laser ranging slr data to the lageos satellite has yielded improved estimates of the horizontal motion for a subset of 34 tracking sites within the global tracking network the analysis called sl8 3 utilized data acquired between 1980 january and 1993 june by the global network composed of 71 sites the solution design provides for the simultaneous estimation of site positions and their velocities within a pre defined kinematic frame the solution is statistically rigorous and retains the full correlation information content least squares estimates of relative poles of rotation which are used to model the motion of one plate relative to another were made based on the slr estimated velocities for sites known to be well away from deformation zones the resulting slr based relative rotation poles differ slightly from those of nuvel 1 but in general indicate that the magnitude of the slr implied velocities is slower than those implied by nuvel 1 consistent with the 4 5 per cent slowing in relative spherical rates noted in earlier comparisons spherical rates between sites in western north america support models of extension in the basin and range province and the rotation of the sierra nevada microplate an analysis of the spherical rates crossing the north atlantic shows that sl8 3 estimated extension between north america eurasia sites is generally smaller than those implied by nuvel 1 meanwhile sl8 3 rates between north america africa sites are in better agreement with nuvel 1 although they are not so well determined the maintenance and ongoing monitoring of global slr site kinematics provides a well defined global reference which will aid in combination global kinematic solutions where information from other technologies are merged e g very long baseline interferometry and global positioning system and in providing the context for densification studies of regional kinematics derived from terrestrial and global positioning system observations

asteroidal source of l chondrite meteorites abstract establishing connections between meteorites and their parent asteroids is an important goal of planetary science several links have been proposed in the past including a spectroscopic match between basaltic meteorites and 4 vesta that are helping scientists understand the formation and evolution of the solar system bodies here we show that the shocked l chondrite meteorites which represent about two thirds of all l chondrite falls may be fragments of a disrupted asteroid with orbital semimajor axis a 2 8 au this breakup left behind thousands of identified 1 15 km asteroid fragments known as the gefion family fossil l chondrite meteorites and iridium enrichment found in an 467 ma old marine limestone quarry in southern sweden and perhaps also 5 large terrestrial craters with corresponding radiometric ages may be tracing the immediate aftermath of the family forming collision when numerous gefion fragments evolved into the earth crossing orbits by the 5 2 resonance with jupiter this work has major implications for our understanding of the source regions of ordinary chondrite meteorites because it implies that they can sample more distant asteroid material than was previously thought possible

the origin of the brunflo fossil meteorite and extraterrestrial chromite in mid ordovician limestone from the garde quarry jamtland central sweden the brunflo fossil meteorite was found in the 1950s in mid ordovician marine limestone in the garde quarry in jamtland it originates from strata that are about 5 million years younger than similar limestone that more recently has yielded 50 fossil meteorites in the thorsberg quarry at kinnekulle 600 km to the south based primarily on the low tio2 content about 1 8 wt of its relict chromite the brunflo meteorite had been tentatively classified as an h chondrite the meteorite hence appears to be an anomaly in relation to the kinnekulle meteorites in which chromite composition chondrule mean diameter and oxygen isotopic composition all indicate an l chondritic origin reflecting an enhanced flux of meteorites to earth following the disruption of the l chondrite parent body 470 ma new chondrule size measurements for the brunflo meteorite indicate that it too is an l chondrite related to the same parent body breakup chromite maximum diameters and well defined chondrule structures further show that brunflo belongs to the l4 or l5 type chromites in recently fallen l4 chondrites commonly have low tio2 contents similar to the brunflo chromites adding support for brunflo being an l4 chondrite the limestone in the circle quarry is relatively rich about 0 45 grain kg 1 in sediment dispersed extraterrestrial chromite grains 63 mu m with chemical composition similar to those in l chondrites and the limestone 1 3 grains kg 1 at kinnekulle suggesting that the enhanced flux of l chondrites prevailed although somewhat diminished at the time when the brunflo meteorite fell less

coseismic and long term vertical displacement due to back arc shortening central vanuatu offshore and onshore data following the mw 7 5 26 november 1999 ambrym earthquake 1 this paper presents a combined analysis of data collected both onshore and offshore in the epicentral region of the mw 7 5 26 november 1999 ambrym earthquake this offshore event occurred at the southern end of the back arc thrust belt of the vanuatu subduction zone east of the island of ambrym and resulted in permanent uplift of the eastern tip of the island the data presented include 1 interpretation of older seismic lines across the tectonic front 2 compilation of aftershock sequences 3 postseismic bathymetry from multibeam surveys 4 measurements of permanent coseismic ground motion using desiccated red algae uplifted beaches and uplifted coral colonies and 5 dating of raised corals exposed along the coastal cliff the active scarp of the main fault appears 13 km east of the eastern tip of ambrym 40 km long 2 km depth 950 m maximum step uplift rates of 3 5 4 mm yr for the past 8000 years are calculated by dating raised corals this leads to a longer seismic recurrence interval of 375 years for events similar to that of 1999 assuming no interseismic subsidence postseismic ground motion was monitored using stainless steel marks set at three sites along the uplifted coast in january 2000 no significant motion could be recorded during the 2 years following the event but slight subsidence cannot be ruled out finally our results confirm that the back arc belt of central vanuatu is an active tectonic crustal wedge propagating eastward that accommodates more shortening than the vanuatu subduction zone to the west

kinematics of active spreading in the central north fiji basin southwest pacific abstract based on a synthesis of magnetic and bathymetric data we re evaluate the kinematics of the recent opening of the central part of the north fiji basin nfb the westward motion of the pacific plate along the left lateral north fiji fracture zone nffz results in the opening of two n s trending spreading ridges located at 173 30 e and 176 e both ridges show complex features such as propagating rifts ridge jumps and overlapping spreading centres their spreading rates are similar 7 6 to 4 cm yr 1 across the western ridge 5 5 cm yr 1 across the eastern one while the nffz is purely strike slip to the east of the eastern ridge it becomes more complex to the west it changes from transpressional to transtensional then to purely transform and finally joins the western n s ridge in a rrr type triple junction our kinematic analysis shows that most of the left lateral motion along the nffz is transferred to the eastern ridge at the rtf type 176 e triple junction it suggests that the western n s ridge is probably connected to the north with another left lateral transform possibly the south pandora ridge

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

from the mid ordovician into the late silurian changes in the micrometeorite flux after the l chondrite parent breakup we present the first reconstruction of the micrometeorite flux to earth in the silurian period we searched 321 kg of condensed marine limestone from the late silurian cellon section southern austria for refractory chrome spinel grains from micrometeorites that fell on the ancient sea floor a total of 155 extraterrestrial spinel grains 10 grains 63 m and 145 in the 32 63 m fraction were recovered for comparison we searched 102 kg of similar limestone from the mid ordovician komstad formation in southern sweden this limestone formed within 1 ma after the breakup of the l chondrite parent body lcpb in the asteroid belt in the sample we found 444 extraterrestrial spinel grains in the 63 m fraction and estimate a content of at least 7000 such grains in the 32 63 m fraction our results show that in the late silurian 40 ma after the lcpb the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude almost down to background levels among the ordinary chondrites the dominance of l chondritic micrometeorites has waned off significantly from 99 in the post lcpb mid ordovician to 60 in the late silurian with 30 h and 10 ll chondritic grains in the late silurian primitive achondrite abundances are similar to today s value contrasting to the much higher abundances observed in pre lcpb mid ordovician sediments

current tectonics of the tonga new hebrides region abstract this paper proposes a present day tectonic map of the tonga new hebrides region the result of combining data from bathymetry satellite derived gravity seismicity magnetic measurements and gps observations focal mechanisms and a large amount of bathymetric data collected during the last decade in the lau and north fiji back arc basins indicate a deformation distributed over numerous spreading ridges rather than diffuse and shear dominated as was previously thought the region is also characterised by a large variation of both consumption rate along the arcs and opening rate along the back arc basins spreading centres geometry and rate of spreading in the lau and north fiji back arc basins and segmentation of the tonga and new hebrides arcs are closely related to and significantly influenced by the subduction of aseismic ridges namely the louisville d entrecasteaux and loyalty ridges where subducting aseismic ridges enter the trenches we observe slower convergence arc transverse strike slip faulting thrusting at the rear of arcs and slow or absent back arc spreading rapid subduction correlates with fast back arc opening sometimes parallel spreading ridges in the back arc domain are required to accommodate the fast motion of convergence

tsunami hazards along the eastern australian coast from potential earthquakes results from numerical simulations australia is surrounded by the pacific ocean and the indian ocean and thus may suffer from tsunamis due to its proximity to the subduction earthquakes around the boundary of australian plate potential tsunami risks along the eastern coast where more and more people currently live are numerically investigated through a scenario based method to provide an estimation of the tsunami hazard in this region we have chosen and calculated the tsunami waves generated at the new hebrides trench and the puysegur trench and we further investigated the relevant tsunami hazards along the eastern coast and their sensitivities to various sea floor frictions and earthquake parameters i e the strike the dip and the slip angles and the earthquake magnitude rupture length the results indicate that the puysegur trench possesses a seismic threat causing wave amplitudes over 1 5 m along the coast of tasmania victoria and new south wales and even reaching over 2 6 m at the regions close to sydney maria island and gabo island for a certain worse case while the cities along the coast of queensland are potentially less vulnerable than those on the southeastern australian coast

the fusion crusts of stony meteorites implications for the atmospheric reprocessing of extraterrestrial materials fusion crusts develop on all meteorites during their passage through the atmosphere but have been little studied we have characterized the textures and compositions of the fusion crusts of 73 stony meteorites to identify the nature of meteorite ablation spheres mas and constrain the processes operating during the entry heating most chondrite fusion crusts are porphyritic and are dominated by olivine glass and accessory magnetite whereas those of the achondrites are mainly glassy chondrite fusion crusts contain sulphide droplets with high ni contents 55 wt the partially melted substrate of ordinary chondrites underlying the outer melted crusts are dominated by silicate glass and composite metal sulphide and cr bearing fe oxide droplets that form as coexisting immiscible liquids enstatite chondrite substrates contain cr and mn bearing sulphides the substrates of the carbonaceous chondrites comprise a sulphide enriched layer of matrix the compositions of melted crusts are similar to those of the bulk meteorite however differences from whole rock suggest that three main processes control their chemical evolution 1 the loss and reaction of immiscible fe rich liquids 2 mixing between substrate partial melts and bulk melts of the melted crust and 3 the loss of volatile components by evaporation and degassing data from fusion crusts suggest that mas produced at low altitude have compositions within the range of those of silicate dominated cosmic spherules that are formed by the melting dust particles meteorite ablation spheres produced at high altitude probably have compositions very different from bulk meteorite and will resemble cosmic spherules derived from coarse grained precursors

the phanerozoic impact cratering rate evidence from the farside of the moon the relatively recent 1 b y flux of asteroids and comets forming large craters on the earth and moon may be accurately recorded by craters with bright rays on the moon s farside many previously unknown farside rayed craters are clearly distinguished in the low phase angle images returned by the clementine spacecraft some large rayed craters on the lunar nearside are probably significantly older than 1 ga rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity most of the farside crust has a more homogeneous composition and only immature rays are visible the size frequency distribution of farside rayed craters is similar to that measured for eratosthenian craters up to 3 2 b y at diameters larger than 15 km the areal density of farside rayed craters matches that of a corrected tabulation of nearside copernican craters hence the presence of bright rays due to immature soils around large craters provides a consistent time stratigraphic basis for defining the base of the copernican system the density of large craters less than 3 2 b y old is 3 2 times higher than that of large farside rayed craters alone this observation can be interpreted in two ways 1 the average cratering rate has been constant over the past 3 2 b y and the base of the copernican is 1 ga or 2 the cratering rate has increased in recent geologic time and the base of the copernican is less than 1 ga we favor the latter interpretation because the rays of copernicus 800 850 m y old appear to be very close to optical maturity suggesting that the average copernican cratering rate was 35 higher than the average eratosthenian rate other lines of evidence for an increase in the phanerozoic 545 ga cratering rate are 1 the densities of small craters superimposed on copernicus and apollo landing sites 2 the rates estimated from well dated terrestrial craters 120 m y and from present day astronomical observations and 3 the proterozoic rate suggested by the crater record of australia the hypothesis most consistent with several key observations is that the cratering rate has increased by 2x during the past 300 m y

a search for h chondritic chromite grains in sediments that formed immediately after the breakup of the l chondrite parent body 470 ma ago abstract a large abundance of l chondritic material mainly in the form of fossil meteorites and chromite grains from micrometeorites has been found in mid ordovician 470 ma old sediments globally the material has been determined to be ejecta from the l chondrite parent body breakup event a major collision in the asteroid belt 470 ma ago in this study we search the same sediments for h chondritic chromite grains in order to improve our understanding of the extraterrestrial flux to earth after the asteroid breakup event we have used sims in conjunction with quantitative sem eds to determine the three oxygen isotopic and elemental compositions respectively of a total of 120 randomly selected sediment dispersed extraterrestrial chromite grains mainly representing micrometeorites from 470 ma old post breakup limestone from the thorsberg quarry in sweden and the lynna river site in russia we show that 99 or more of the grains are l chondritic whereas the h chondritic fraction is 1 or less the l h chondrite ratio after the breakup thus was 99 compared to 1 1 in today s meteoritic flux this represents independent evidence in agreement with previous estimates based on sediment dispersed extraterrestrial chromite grain abundances and sedimentation rates of a two orders of magnitude higher post breakup flux of l chondritic material in the micrometeorite fraction finally we confirm the usefulness of three oxygen isotopic sims analyses of individual extraterrestrial chromite grains for classification of equilibrated ordinary chondrites the h and l chondritic chromites differ both in their three oxygen isotopic and elemental compositions but there is some overlap between the groups in chromite tio 2 is the oxide most resistant to diagenesis and the combined application of tio 2 and oxygen three isotope analysis can resolve uncertainties arising from the compositional overlaps

constraints on macquarie ridge tectonics provided by harvard focal mechanisms and teleseismic earthquake locations in this study we evaluate teleseismically determined focal mechanisms and epicenters for earthquakes along the macquarie ridge complex mrc from 45os to 61 os and 155oe to 168oe a region characterized by some previous investigators as undergoing subduction initiation from 65 centroid moment tensors reported by harvard we develop statistical guidelines for choosing 26 which represent better determined more reliable focal mechanisms for tectonic analysis although thrust mechanisms occur in the north near fiordland elsewhere along the mrc the better determined mechanisms virtually all indicate that present day motion along most of the mrc is strike slip this is consistent with sidescan sonar and multichannel reflection data collected between 50os and 57os on 1994 and 1996 cruises the active plate boundar3 zone appears to be quite narrow 5 km wide and no active compressional features can be observed on the seafloor if we determine a rotation pole for plate boundary motion using only slip vectors from better determined harvard mechanisms along the mrc the best fitting instantaneous pole is at 57 4os 179 4oe about 2 5 o north of the nuvel i australian pacific pole which averages motion over the last 3 0 my if the mrc pole was formerly farther south than at present this could explain the existence of relict features associated with crustal shortening such as bathymetric highs and troughs yet the absence of active features such as thrust faults etc suggests ongoing compression or subduction initiation we also carefully read arrival times for p phases for 53 earthquakes at 16 teleseismic stations selected to represent a range of azimuths surrounding the earthquakes we relocated these earthquakes using standard joint epicentral determination jed methods while most of the better quality relocations lie on or very close to the australia pacific boundar3 as determined on the 1994 cruise a few epicenters occur well away from the boundar3 apparently on cenozoic fracture zones thus on the macquarie ridge complex and other major strike slip boundaries e g in california it appears that the very largest earthquakes occur on the principal plate boundar3 fault but that other earthquakes some quite large may occur away from the boundary along zones of preexisting weakness

sediment dispersed extraterrestrial chromite traces a major asteroid disruption event abundant extraterrestrial chromite grains from decomposed meteorites occur in middle ordovician 480 million years ago marine limestone over an area of 250 000 square kilometers in southern sweden the chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to earth during the mid ordovician as previously indicated by fossil meteorites extraterrestrial chromite grains in mid ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar system history

global plate velocities from the global positioning system we have analyzed 204 days of global positioning system gps data from the global gps network spanning january 1991 through march 1996 on the basis of these gps coordinate solutions we have estimated velocities for 38 sites mostly located on the interiors of the africa antarctica australia eurasia nazca north america pacific and south america plates the uncertainties of the horizontal velocity components range from 1 2 to 5 0 mm yr with the exception of sites on the pacific and nazca plates the gps velocities agree with absolute plate model predictions within 95 confidence for most of the sites in north america antarctica and eurasia the agreement is better than 2 mm yr we find no persuasive evidence for significant vertical motions 3 standard deviations except at four sites three of these four were sites constrained to geodetic reference frame velocities the gps velocities were then used to estimate angular velocities for eight tectonic plates absolute angular velocities derived from the gps data agree with the no net rotation nnr nuvel 1a model within 95 confidence except for the pacific plate our pole of rotation for the pacific plate lies 11 5 west of the nnr nuvel 1a pole with an angular speed 10 faster our relative angular velocities agree with nuvel 1a except for some involving the pacific plate while our pacific north america angular velocity differs significantly from nuvel 1a our model and nuvel 1a predict very small differences in relative motion along the pacific north america plate boundary itself our pacific australia and pacific eurasia angular velocities are significantly faster than nuvel 1a predicting more rapid convergence at these two plate boundaries along the east pacific rise our pacific nazca angular velocity agrees in both rate and azimuth with nuvel 1a

baltica a synopsis of vendian permian palaeomagnetic data and their palaeotectonic implications torsvlk t h smethurst m a van der voo r trench a abrahamsen n and halvorsen e 1992 baltlca a synopsis of vendlan permmn palaeomagnetic data and their palaeotectonlc implications earth sci rev 33 133 152 in light of recent additions to the palaeozoic palaeo magnetlc data base particularly for the ordovlclan era a revised apparent polar wander apw path for baltica has been constructed following a rigorous synthesis of all late precambrlan perm lan data the apw path is characterized by two prominent loops firstly a late precambrlan camb rlan loop probably relating to a rifting event and secondly a younger loop relating to a mid silurian scandlan colhsion event these features imply major change in plate tectonic reconfiguratlon baltlca probably represented an individual continental unit m early palaeozoic times and was positioned m high southerly latitudes in an inverted geographic orientation in such a reconstruction baltlca was separated from the northern margin of gondwana by the tornqulst sea and from laurentla by the iapetus ocean the tornqulst zone is thus interpreted as a passive or dextral transform margin during the early palaeozoic while undergoing counter clockwise rotations up to 1 6 ma baltica drifted northward through most ol the palaeozoic except for a short period of southerly movement in late silurian early devonian times after colhslon with laurentla rapid movements in latitude up to 9 cm yr are noted m late precambrlan earl y palaeozoic times and slgmficant decrease in velocities throughout palaeozoic time probably reflect the progressive amalgamation of a larger continent by early devonian euramerlca and permian pangea times the tornqulst sea had a principal component of palaeo east west orientation hence it is difficult to be precise in the timing of when micro continents such as eastern avalonla and the european masslfs ultimately collided along the southwestern margin of baltica these micro continents are considered to have been peripheral to gondwana m high southerly latitudes during the early ordovlclan eastern avalonla clearly had rifted off gondwana by llanvlrn llanded o times and may have collided with baltlca during late ordovlclan times although the present available sdunan palaeomagnetlc data from eastern avalonia may suggest colhslon in late silurian times across the lapetus facing margin of baltica laurentla was s tuated in equatorml to southerly latitudes during most of the lower palaeozoic these continents collided in mid silurian times l e a first collision between southwestern norway and greenland scotla nd which gave rise to the early scandlan orogeny 425 ma in southwestern norway possible followed by a later but less dramatic scandlan event in northern norway at around 410 ma since baltlca was geographically reverted m early palaeozoic times the colhsional margin could not have been a margin that once rifted off laurentm as assumed in a number of plate tectonic models

noble gases in fossil micrometeorites and meteorites from 470 myr old sediments from southern sweden and new evidence for the l chondrite parent body breakup event we present noble gas analyses of sediment dispersed extraterrestrial chromite grains recovered from similar to 470 myr old sediments from two quarries hallekis and thorsberg and of relict chromites in a coeval fossil meteorite from the gullhogen quarry all located ill southern sweden both the sediment dispersed grains and the meteorite gullhogen 001 were generated in the l chondrite parent body breakup about 470 myr ago which was also the event responsible for the abundant fossil meteorites previously found in the thorsberg quarry trapped solar noble gases in the sediment dispersed chromite grains have partly been retained during similar to 470 myr of terrestrial residence and despite harsh chemical treatment in the laboratory this shows that chromite is highly retentive for solar noble gases the solar noble gases imply that a sizeable fraction of the sediment dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites the grains in the oldest sediment beds were rapidly delivered to earth likely by direct injection into all orbital resonance in the inner asteroid belt whereas grains in younger sediments arrived by orbital decay due to poynting robertson p r drag the fossil meteorite gullhogen 001 has a low cosmic ray exposure age of similar to 0 9 myr based on new he and ne production rates in chromite determined experimentally this age is comparable to the ages of the fossil meteorites from thorsberg providing additional evidence for very rapid transfer times of material after the l chondrite parent body breakup

late eocene 3he and ir anomalies associated with ordinary chondritic spinels during the late eocene there was an enigmatic enhancement in the flux of extraterrestrial material to earth evidence comes from sedimentary 3he records indicating an increased flux of interplanetary dust during ca 2 myr as well as two very large impact structures popigai 100 km diameter and chesapeake bay 40 85 km that formed within 10 20 kyr at the peak of the 3he delivery the massignano section in italy has one of the best sedimentary records of these events including a well defined 3he record an ir rich ejecta bed related to the popigai impact event and two smaller ir anomalies recently we showed that the popigai ejecta is associated with a significant enrichment of chromite grains 63 m with an h chondritic elemental composition 17 grains in 100 kg of rock most likely these grains are unmelted fragments from the impactor slightly higher up ca 20 cm in the section where a small ir anomaly possibly related to the chesapeake bay impact has been measured we found a weak enrichment in l chondritic grains 8 grains in 208 kg of rock here we report an extended data set increasing the total amount of sediment dissolved in acid and searched for extraterrestrial chromite grains from 658 to 1168 kg in altogether 760 kg of background sediment from 17 levels over 14 m of strata outside the interval corresponding to the popigai and chesapeake bay impacts we only found 2 extraterrestrial chromite grains both grains have l chondritic compositions and were found in a 100 kg sample from the ca 10 25 m level in the section where the second of the smaller ir anomalies has been reported a correlation appears to exist between ir 3he and chromite from ordinary chondrites we also report oxygen three isotope measurements of the extraterrestrial chromite grains associated with the popigai ejecta and confirm an h chondritic composition the new results strengthen our scenario that the upper eocene 3he and ir enrichments originate from the asteroid belt rather than the oort cloud as originally proposed when the 3he anomaly was discovered the generally low background concentrations of extraterrestrial chromite through the section speak against any major single asteroid breakup event such as in the mid ordovician after the break up of the l chondrite parent body instead the data reconcile with a small possibly a factor of 2 3 increase in the flux of extraterrestrial material to earth but of both h and l chondritic composition we also report the composition of all the 2310 terrestrial chrome spinel grains recovered and show that their chemical composition indicates a dominantly regional ophiolitic source four anomalous chrome spinel grains with high ti and v concentrations were found in the popigai ejecta these grains originate from siberian traps basalts in the popigai crater at the time of impact less

in situ multiphase u pb geochronology and shock analysis of apatite titanite and zircon from the lac la moinerie impact structure canada in situ u pb geochronology has been performed via laser ablation inductively coupled mass spectrometry on shocked apatite titanite and zircon from the 8 km diameter lac la moinerie impact structure canada the three analyzed phases are inherited from target rocks that were transformed to impact melt bearing breccias and clast laden impact melts apatite yields an array of u pb ratios populating a region between a younger regression with a refined age of 453 5 ma and an older regression with a lower intercept age of 1708 10 ma titanite defines a younger regression with an age of 444 15 ma and an older array with a lower intercept at 1844 24 ma zircon yields an upper intercept age of 1810 13 ma and a lower intercept age of 433 21 ma within error of apatite and titanite the oldest ages from all three phases define paleoproterozoic igneous metamorphic events in granitoid target rocks of the de pas suite isotopic resetting in apatite and titanite with relatively high pb diffusion rates was controlled by a combination of proximity to impact generated superheated melt and the formation of fast diffusion pathways facilitated by shock effects and dynamic recrystallization zircon resetting was controlled by shock effects and metamictization which resulted in significant pb loss we outline an approach for discriminating true impact ages from metamict zircons with high common pb that have experienced post impact pb loss lower intercept ages of all phases constrain the age of formation for the lac la moinerie impact structure with apatite yielding the most precise age of 453 5 ma with an age of 453 ma lac la moinerie joins a growing cluster of more than 12 terrestrial impact structures formed in the middle to late ordovician with potential connection to the l chondrite parent body breakup event and a period of enhanced bombardment on earth

a rain of ordinary chondritic meteorites in the early ordovician forty fossil meteorites with a total original mass of v7 7 kg have been recovered in the first systematic search for fossil meteorites pursued in an active quarry in lower ordovician 480 ma marine limestone in southern sweden the meteorites represent at least 12 different falls over a seafloor area of v6000 m 2 during 91 75 myr making the quarry one of the most meteorite dense areas known in the world geochemical analyses of relict chromite grains indicate that all or most of the meteorites are ordinary chondrites and probably l chondrites mechanisms for meteorite delivery from the asteroid belt to earth were the same 480 ma as today however the flux was one to two orders of magnitude higher most likely reflecting the disruption of the l chondrite parent body at about that time this is a major event in late solar system history which may also have led to an enhanced flux of asteroids to earth during v30 myr fl 2001 elsevier science b v all rights reserved

petrographic classification of middle ordovician fossil meteorites from sweden the maximum diameter of chromite fecr2o4 grains within l chondrites reflects the petrographic type of the sample on the basis of our measurements of nine recent l chondrites l3 chromite d mac 34 50 mu m l4 87 150 mu m l5 76 158 mu m and l6 253 638 mu m this variation reflects the crystallization of the chromite grains during parent body thermal metamorphism we use this calibration to classify six fossil meteorites from the middle ordovician in sweden as type 3 or 4 to 6 the high flux of l chondrites at 470 ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent l chondrite falls the fossil meteorites have in places preserved recognizable chondrule textures including porphyritic olivine barred olivine and radiating pyroxene a large relict clast and fusion crust have also been tentatively identified in one fossil meteorite apart from chromite all of the original meteorite minerals have been replaced by carbonate and sheet silicate and sulfate during diagenesis within the limestone host the preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules the range 0 4 0 6 mm is consistent with measurements made in the same way on recent l chondrites less

the itrf96 realization and its associated velocity field this text intends to present and to analyse the quality of the new 1996 edition of the international terrestrial reference system called the itrf96 the itrf96 represents a key step forward in the activity of the iers terrestrial system determinations a new physical combination model has been developed as well as a new stochastic model for the first time the level of accuracy has reached 1 cm or better in position for about 50 of itrf96 stations and 30 of the stations have their velocities determined free of any aditional tectonic plate model assumption with an uncertainty better than 3 mm yr the itrf96 also provides a more complete solution a new software has been developed which makes available the itrf96 full variance covariance matrix whereas this information was restricted to station positions in the previous itrf94 edition moreover the network coverage is clearly the largest set of globally distributed terrestrial space geodesy stations that have ever been computed 521 stations located among 290 sites in addition station velocities are also determined at a very accurate level allowing possible determination of kinematic plate motion models free of any geological assumption

the silurian nautiloid bearing strata of the cellon section carnic alps austria color variation related to events abstract silurian nautiloid bearing strata llandovery p idoli from the cellon section of the carnic alps austria are compared using the innovative perspective of the color of the enclosing sediment of these fossiliferous beds with coeval regional occurrences along the north gondwana margin prague basin sardinia morocco particular attention was placed on distinguishing and comparing the colors of these levels and their general nautiloid faunal composition in order to identify the prevailing features of their occurrence at precise stratigraphic intervals red brown strata usually reflect time rich or slowly accumulated deposits from a diverse in situ fauna while black gray strata are related to single event beds due to episodic deposition probably influenced by currents other single event beds of distinctive pink gorstian in age and purple strata ludfordian in age may prove useful as marker horizons the overall data set available in relation to these diversely colored nautiloid bearing horizons at the cellon section at precise time slices when compared with global eustatic conditions and biotic events has highlighted that these deposits reflect both local phenomena within the depositional basin and regional depositional episodes these include apparent mass mortality events and the cephalopod limestone biofacies that may be considered as time specific facies

late pleistocene evolution of the mixed siliciclastic and carbonate southwestern new caledonia continental shelf lagoon abstract the main island of new caledonia nc is surrounded by the second largest barrier reef in the world after the australian great barrier reef the upper pleistocene evolution of this barrier reef lagoon complex offers the opportunity to examine the response of a modern mixed tropical siliciclastic carbonate system to relative sea level changes and to better constrain the relative role of lagoon backfilling and paleo drainage system in sediment transfer across the shelf this paper mainly focuses on the results of chronostratigraphic and sedimentological interpretations of new seismic bathymetric and coring data collected in the sw nc lagoon several significant points are highlighted from this study and allow the construction of a refined model which includes the evolution of both the sw nc mixed shelf and lagoon new results show that the nc lagoon has not recorded a reciprocal sedimentation during glacial interglacial sea level cycles but a unique mixed regime strong contrasts are observed and these depend above all on the two lagoon domains the outer coral plateau and the inner lagoon depression corresponding to a semi closed proximal basin which is separated from the plateau by a basement ridge in the depression clastic sediments remain dominant from early flooding to highstand stages while carbonate sedimentation continued through time across the coral plateau this spatial contrast between terrigenous and carbonate deposits was maintained throughout various sea level cycles thus even if spatial segregation is also observed for other examples of mixed shelf environments the sw nc shelf appears quite unique in comparison to the other mixed systems when considering conceptual models of reciprocal sedimentation moreover the presence of a semi enclosed lagoon depression allows the preservation of larger volume of terrigenous deposits this semi filled geometry of the lagoon is assumed to be the result of several factors a significant subsidence that offers accommodation for aggrading deposits evacuation through channels of sediments mobilized during storm events the tidal currents and fairweather wave dynamics lastly the inherited morphologic and structures may explain the geomorphic evolution of the escarpment margin type of the sw nc rimmed shelf and its differentiation with the opposite se nc ramp type shelf

interseismic and coseismic motions in gps series related to the ms 7 3 july 13 1994 malekula earthquake central new hebrides subduction zone on july 13 1994 an earthquake ms 7 3 occurred at malekula in the new hebrides archipelago the gps data collected across the new hebrides trench between 1990 and 1996 were processed in order to separate the interseismic and coseismic motions from the drifts related to the convergence with the australian plate the gps derived coseismic displacements at the gps site in malekula are 49 15 mm southward 230 30 mm westward and 170 37 mm downward when the cmt derived displacements are 50 mm southward 210 mm westward and 150 mm downward taking into account the interseismic strain accumulation 25 mm yr at the source established from historical seismicity 7 5 mm yr at the gps site the strain free convergence rate at malekula is 49 3 mm yr other gps derived convergence rates are 95 1 mm yr at efate and 37 2 mm yr at santo these rates imply a regional right lateral motion between the efate and the santo malekula segments in contrast the focal mechanism of the earthquake mostly indicates a left lateral motion therefore we hypothesize that the earthquake is related to variations in the interplate coupling along the converging boundary of the santo malekula segment

probabilistic seismic hazard model for vanuatu abstract this study develops a preliminary probabilistic seismic hazard model for vanuatu the area of investigation formerly referred to as the new hebrides lies in the center of a chain of partly vulcanologically active islands that mark the present day boundary between the australia india plate and the microplate of the north fiji basin the seismicity of the vanuatu arc is dominated by an east dipping subduction zone which shows striking structural anomalies in the central part between 14 and 18 s our historical catalog contains 7519 events within the vanuatu region for the period from 1964 to 2003 drawn from the global teleseismic catalogs by the united states geological survey national earthquake information center usgs neic see data and resources section and engdahl et al 1998 as a measure of seismic hazard we use horizontal peak ground acceleration pga and horizontal spectral ground acceleration sga at a period of 1 sec the hazard estimates are based on a logic tree approach to account for the epistemic uncertainties associated with our analysis our results suggest that the entire island arc experiences a high and uniform seismic hazard typical values for pgas range from 0 65 g to 0 77 g with a 10 probability of exceedence in 50 yr for port vila the capital and largest city in vanuatu we additionally present a pga hazard curve and a uniform hazard spectrum over the period range 0 1 2 sec

geodetic measurements of convergence across the new hebrides subduction zone between 1990 and 1994 geodetic measurements gps observations have been conducted across the new hebrides subduction zone where the australia plate subducts under the new hebrides arc this paper establishes convergence rate variations along the trench in the south at tanna the relative motion is oriented n244 4 and has a uniform rate of 11 7 0 8 cm yr the rate at efate is 10 3 0 9 cm yr oriented n242 4 both azimuths very well compare with slip vectors of the last major earthquakes in the north the rate at santo is only 3 6 1 2 cm yr oriented n253 26 the difference in the convergence rates between santo on the one hand and efate and tanna in the other hand points to a right lateral shear zone between santo and efate at santo where the plate coupling is very high the very low convergence rate might be related to the absence of recent strong earthquakes no significant variations are detected for the baselines within the australia plate

breakdown and reformation of the intertropical convergence zone in a moist atmosphere abstract the effects of moisture on the intertropical convergence zone itcz over the eastern pacific on the synoptic time scale are investigated using an intermediate complexity atmospheric circulation model the quasi equilibrium tropical circulation model qtcm1 on an aquaplanet the dry simulation shows results consistent with those of simple dynamic models except that a slightly stronger heating rate is needed owing to different model designs in the moist simulations the most important result is the formation of a tail southwest of a vortex during and after the itcz breakdown this tail may extend zonally more than 60 longitude and last for more than two weeks in an idealized simulation in the eastern north pacific this phenomenon is often observed in cases that involve easterly waves in a sense the formation of the tail suggests a possible mechanism that forms an itcz efficiently this study shows that the surface convergent flow induced by a disturbance initializes a positive wind evapor

insights into the evolution of the yenkahe resurgent dome siwi caldera tanna island vanuatu inferred from aerial high resolution photogrammetry the yenkahe dome tanna island vanuatu is one of the most spectacular examples of presently active post caldera resurgence exhibiting a very high uplift rate over the past 1000 years 156 mm year on average although numerous inhabited areas are scattered around the dome the dynamics of this structure and associated hazards remain poorly studied because of its remote location and dense vegetation cover a high resolution photogrammetric campaign was carried out in november 2011 over the dome georeferenced photographs were treated by structure from motion and multiple view stereophotogrammetry methods to produce a 3d digital surface model dsm of the area and its associated orthophotograph this dsm is much more accurate than previously available srtm and aster digital elevation models dems particularly at minimal coastline and maximal altitudes yasur culmination point 390 m while previous mapping relied mostly on low resolution dems and satellite images the high precision of the dsm allows for a detailed structural analysis of the yenkahe dome notably based on the quantification of fault displacements the new structural map inferred from the 3d reconstruction and morphological analysis of the dome reveals a complex pattern of faults and destabilization scars reflecting a succession of constructive and destructive events numerous landslide scars directed toward the sea highlight the probable occurrence of a tsunami event affecting the south eastern coast of tanna simulations of landslide triggered tsunamis show the short time propagation of such a wave 1 2 min which could affect coastal localities even following relatively small destabilized volumes a few million cubic meters

motion and rigidity of the pacific plate and implications for plate boundary deformation 1 using up to 11 years of data from a global network of global positioning system gps stations including 12 stations well distributed across the pacific plate we derive present day euler vectors for the pacific plate more precisely than has previously been possible from space geodetic data after rejecting on statistical grounds the velocity of one station on each of the pacific and north american plates we find that the quality of fit of the horizontal velocities of 11 pacific plate pa stations to the best fitting pa euler vector is similar to the fit of 11 australian plate au velocities to the au euler vector and 20 better than the fit of nine north american plate na velocities to the na euler vector the velocities of stations on the pacific and australian plates each fit a rigid plate model with an rms residual of 0 4 mm yr while the north american velocities fit a rigid plate model with an rms velocity of 0 6 mm yr our best fitting pa au relative euler vector is located 170 km southeast of the nuvel 1a pole but is not significantly different at the 95 confidence level it is also close 70 km in position and 3 in rate to a pole derived from transform faults identified from satellite altimetry suggesting that the vector has not changed significantly over the past 3 myr our relative euler vector is also consistent with all known geological and geodetic evidence concerning the au pa plate boundary through new zealand the gps sites offshore of southern california are presently moving 4 5 1 mm yr relative to predicted pacific velocity with their residual velocities in approximately the opposite direction to pa na relative motion likewise the easternmost sites in south island new zealand are moving 3 1 mm yr relative to predicted pacific velocity with the residuals in approximately the opposite direction to pa au relative motion these velocity residuals are in the same sense as predicted by elastic strain accumulation on known plate boundary faults but are of a significantly higher magnitude in both southern california and new zealand implying that the plate boundary zones in both regions are wider than previously believed index terms 1206 geodesy and gravity crustal movements interplate 8155 1208 geodesy and gravity crustal movements intraplate 8110 1243 geodesy and gravity space geodetic surveys 8158 tectonophysics evolution of the earth plate motions present and recent 3040 8159 tectonophysics evolution of the earth rheology crust and lithosphere keywords pacific plate motion plate rigidity plate boundary deformation

a probabilistic tsunami hazard study of the auckland region part i propagation modelling and tsunami hazard assessment at the shoreline regional source tsunamis represent a potentially devastating threat to coastal communities in new zealand yet are infrequent events for which little historical information is available it is therefore essential to develop robust methods for quantitatively estimating the hazards posed so that effective mitigation measures can be implemented we develop a probabilistic model for the tsunami hazard posed to the auckland region of new zealand from the kermadec trench and the southern new hebrides trench subduction zones an innovative feature of our model is the systematic analysis of uncertainty regarding the magnitude frequency distribution of earthquakes in the source regions the methodology is first used to estimate the tsunami hazard at the coastline and then used to produce a set of scenarios that can be applied to produce probabilistic maps of tsunami inundation for the study region the production of these maps is described in part ii we find that the 2 500 year return period regional source tsunami hazard for the densely populated east coast of auckland is dominated by events originating in the kermadec trench while the equivalent hazard to the sparsely populated west coast is approximately equally due to events on the kermadec trench and the southern new hebrides trench

the termination of the southern new hebrides subduction zone southwestern pacific abstract recent data bathymetric map and petrological investigations as well as reappraisal of local seismological events for more than 20 years allow us to present a new geodynamic interpretation of the southern new hebrides subduction zone a multidisciplinary approach to this complex area where transform movements relay subduction process clearly shows the interrelationships between intermediate seismicity lithospheric plates tectonics and volcanic activity according to our model two hinge zones tearing the downgoing slab along directions which parallel the convergence vector n70 e reasonably account for most seismological and petrological data from the region the approach of the loyalty islands ridge towards the trench and its increasing influence on the subduction regime is beginning to complicate this scheme

source process of the 21 april 1997 santa cruz island earthquake mw 7 8 we apply an empirical green s function method to recordings of teleseismic body waves from the april 21 1997 santa cruz island earthquake the far field moment rate functions reveal that rupture occured on a shallow dipping fault plane dip 17 the main rupture consists of 6 large subevents and covers a total area of about 5000 km while the hypocenter is fixed at 33 km depth the main slip is located above 20 km depth within the accretionary wedge the source duration is about 70 sec and the average rupture velocity is found to be 1 9 km sec this event caused a local tsunami and although it is not possible to determine whether it was caused by tectonic faulting or submarine shaking the fact that most of the moment release occured at shallow depths is consistent with either of these possibilities and points to the utility of the procedure outlined here in possible rapid characterization of tsunami potential

meteorite flux to earth in the early cretaceous as reconstructed from sediment dispersed extraterrestrial spinels we show that earth s sedimentary strata can provide a record of the collisional evolution of the asteroid belt from 1652 kg of pelagic maiolica limestone of berriasian hauterivian age from italy we recovered 108 extraterrestrial spinel grains 32 250 m representing relict minerals from coarse micrometeorites elemental and three oxygen isotope analyses were used to characterize the grains providing a first order estimate of the major types of asteroids delivering material at the time comparisons were made with meteorite flux time windows in the ordovician before and after the l chondrite parent body breakup in the early cretaceous 80 of the extraterrestrial spinels originated from ordinary chondrites the ratios between the three groups of ordinary chondrites h l ll appear similar to the present 1 1 0 2 but differ significantly from ordovician ratios we found no signs of a hypothesized baptistina ll chondrite breakup event about 10 of the grains in the maiolica originate from achondritic meteorite types that are very rare 1 on earth today but that were even more common in the ordovician because most meteorite groups have lower spinel content than the ordinary chondrites our data indicate that the latter did not dominate the flux during the early cretaceous to the same extent as today based on studies of three windows in deep time we argue that there may have been a gradual long term a few hundred million years turnover in the meteorite flux from dominance of achondrites in the early phanerozoic to ordinary chondrites in the late phanerozoic interrupted by short term a few million years meteorite cascades from single asteroid breakup events less

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

accretion rates of meteorites and cosmic dust in the early ordovician abundant fossil meteorites in marine condensed lower ordovician limestones from kinnekulle sweden indicate that accretion rates of meteorites were one to two orders of magnitude higher during an interval of the early ordovician than at present osmium isotope and iridium analyses of whole rock limestone indicate a coeval enhancement of one order of magnitude in the influx rate of cosmic dust enhanced accretion of cosmic matter may be related to the disruption of the l chondrite parent body around 500 million years ago

analysis of partially emerged corals and reef terraces in the central vanuatu arc comparison of contemporary coseismic and nonseismic with quaternary vertical movements in the central vanuatu arc living and recently deceased reef corals act as natural tide gauges which have allowed us to map vertical tectonic deformation patterns as corals grow the density of the aragonite coral skeletons varies on an annual cycle producing annual growth bands similar to tree rings using coral growth bands we can determine the year coral surfaces died due to emergence we interpret four major coral emergence events as coseismic uplifts that occurred near the epicenters and times of large shallow earthquakes on january 5 1946 ms 7 3 august 11 1965 ms 7 5 october 27 1971 ms 7 1 and december 29 1973 ms 7 5 the 1965 and 1973 events caused maximum uplifts of 120 and 60 cm respectively in the frontal arc also related to these events are uplifts of 10 cm and 6 cm in the back arc on pentecost and maewo islands which lie east of the volcanic chain and the primary forearc zones of uplift and subsidence similar secondary zones of uplift occurred with the great 1960 chile and 1964 alaska earthquakes the amplitude of these secondary uplifts is significantly larger than that predicted by models having a single fault in an elastic half space however the amount of secondary uplift is comparable to that predicted if the fault occurs in a plate of constant thickness overlying a viscoelastic half space at various places in 1957 1969 1970 1977 and 1978 1981 there was about 5 10 cm of emergence not associated with major earthquakes which may indicate nonseismic tectonic uplift however oceanographically lowered sea levels as in el ninos may have determined the times when corals died and recorded these events nevertheless the accumulation of emergence its persistence the limited geographic extent of each event and occurrence in areas of rapid holocene uplift suggest that the causes of the uplifts are tectonic these events suggest that in some areas a third or more of the total accumulated uplift in central vanuatu takes place as aseismic motion however in some areas we find only coseismic emergence in central vanuatu contemporary coseismic vertical deformation holocene uplift and topography have remarkably similar patterns this suggests that the mechanisms and processes causing vertical deformation have varied little over the last 106years apparently the topography structure and seismotectonics are controlled by the subduction of the d entrecasteaux ridge a major bathymetric feature underthrusting this part of the arc the influence of this ridge may have been especially extensive because it migrates very slowly along the arc trend and thus it interacts for a long time with a single portion of the arc system our previous studies of reef terraces indicated the existence of at least four seismotectonic arc segments or blocks along the santo malekula interval of the arc and our present results further support this conclusion each block has uplifted at different times by different amounts at different rates and tilted in a different direction boundaries between the north santo and the south santo segment and between the north malekula and the south malekula segment correlate with the north and south flanks of the d entrecasteaux ridge as does the absence of a physiographic trench west of santo

geodynamics of an arc ridge junction the case of the new hebrides arc north fiji basin abstract detailed surveys bathymetry magnetism seismicity and focal mechanism solutions recorded on the junction between the southern new hebrides arc and the north fiji basin enlighten the geodynamic complexity of the area the presently active c n s oriented spreading structures of the north fiji basin are superimposed on ancient ones which appeared during a n135 e spreading episode active before 3 ma a recent southward extension of the new hebrides arc occurred c 2 ma ago structural and geophysical consequences of both events partly obscure the present arc ridge junction for example back arc troughs do not exist to the south of the former arc termination n45 e structural directions present all over the studied area are interpreted as older transform faults related to the n135 e spreading axis which also left recognizable nw se magnetic anomalies however the new hebrides arc north fiji basin junction remains geodynamically unstable due to the concomitance of convergent strike slip and divergent movements their respective crustal expressions i e arc volcanism lateral displacements and seafloor spreading are examined and considered in their regional environment a provisional model of the arc ridge junction that accounts for most of the parameters analysed above is presented

relative motions of the australian pacific and antarctic plates estimated by the global positioning system gps measurements spanning 3 years have been used to determine velocities for 7 sites on the australian pacific and antarctic plates the site velocities agree with both plate model predictions and other space geodetic techniques we find no evidence for internal deformation of the interior of the australian plate wellington new zealand located in the australian pacific plate boundary zone moves 20 5 mm yr west southwest relative to the australian plate its velocity lies midway between the predicted velocities of the two plates relative euler vectors for the australia antarctica and pacific antarctica plates agree within one standard deviation with the nuvel 1a predictions

a late miocene dust shower from the break up of an asteroid in the main belt throughout the history of the solar system earth has been bombarded by interplanetary dust particles idps which are asteroid and comet fragments of diameter approximately 1 1 000 microm the idp flux is believed to be in quasi steady state particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution dynamical ejection and planetary or solar impact because idps are rich in 3he seafloor sediment 3he concentrations provide a unique means of probing the major events that have affected the idp flux and its source bodies over geological timescales here we report that collisional disruption of the 150 km diameter asteroid that created the veritas family 8 3 0 5 myr ago also produced a transient increase in the flux of interplanetary dust derived 3he the increase began at 8 2 0 1 myr ago reached a maximum of approximately 4 times pre event levels and dissipated over approximately 1 5 myr the terrestrial idp accretion rate was overwhelmingly dominated by veritas family fragments during the late miocene no other event of this magnitude over the past approximately 10 8 yr has been deduced from main belt asteroid orbits one remarkably similar event is present in the 3he record 35 myr ago but its origin by comet shower or asteroid collision remains uncertain

fifteen years of thermal activity at vanuatu s volcanoes 2000 2015 revealed by mirova abstract the vanuatu archipelago consists of 80 islands and hosts 5 subaerial volcanoes yasur lopevi ambrym aoba and gaua that have shown sign of activity during the past decade in this contribution we provide a 15 years long datasets 2000 2015 of the thermal activity recorded at these active volcanoes by means of mirova middle infrared observation of volcanic activity a new volcanic hotspot detection system based on modis data the analyzed volcanoes are characterized by a spectrum of volcanic activities whose thermal signature has been tracked and carefully analyzed these include strombolian vulcanian explosions at yasur lava flows at lopevi lava lakes at ambrym surtseyan type eruptions within the voui crater lake of aoba and ash dominated eruptions with strong degassing at gaua the collected data reveal several details of the long term eruptive dynamics at single sites such as a monthly long pulse in thermal emissions at yasur volcano as well as at the two active craters of ambrym benbow and marum heating cycles within aoba crater lake and intermittent pressurized eruptions at lopevi volcano has also been detected and shed light in the eruptive dynamics of the analyzed volcanoes in addition we were able to track a two years long intensification of thermal output at benbow crater ambrym that preceded the occurrence of the first intra caldera eruptions of this volcano since 1989 we emphasize how the data provided by mirova represent a new safe and affordable method for monitoring in near real time a large spectrum of volcanic activities taking place at vanuatu and other volcanic areas

the micrometeorite flux to earth during the earliest paleogene reconstructed in the bottaccione section umbrian apennines italy based on sediment dispersed extraterrestrial spinel grains in the bottaccione limestone section in italy we reconstructed the micrometeorite flux to earth during the early paleocene from a total of 843 kg of limestone 86 extraterrestrial spinel grains 12 grains 63 m and 74 in the 32 63 m fraction have been recovered our results indicate that the micrometeorite flux was not elevated during the early paleocene ordinary chondrites dominated over achondritic meteorites similar to the recent flux but h chondrites dominated over l and ll chondrites 69 22 and 9 respectively this h chondrite dominance is similar to that recorded within an enigmatic 3he anomaly 70 27 and 3 in the turonian but different from just before this 3he anomaly and in the early cretaceous where ratios are similar to the recent flux 45 45 and 10 the k ar isotopic ages of recently fallen h chondrites indicate a small impact event on the h chondrite parent body 50 to 100 ma ago we tentatively suggest that this event is recorded by the turonian 3he anomaly resulting in an h chondrite dominance up to the paleocene our sample spanning the 20 cm above the cretaceous paleogene k pg boundary did not yield any spinel grains related to the k pg boundary impactor less

estimation of current plate motions in papua new guinea from global positioning system observations plate tectonic motions have been estimated in papua new guinea from a 20 station network of global positioning system sites that has been observed over five campaigns from 1990 to 1996 the present velocities of the sites are consistent with geological models in which the south bismarck woodlark and solomon sea plates form the principal tectonic elements between the pacific and australian plates in this region active spreading is observed on the woodlark basin spreading centre but at a rate that is about half the rate determined from magnetic reversals the other major motions observed are subduction on the new britain trench seafloor spreading across the bismarck sea seismic lineation convergence across the ramu markham fault and left lateral strike slip across the papuan peninsula these motions are consistent with a 8 2 myr 1 clockwise rotation of the south bismarck plate about a pole in the huon gulf and a rotation of the woodlark plate away from the australian plate second order deformation may also be occurring in particular manus island and northern new ireland may be moving northward relative to the pacific plate at 5 8 mm yr 1 significant at the 95 but not at the 99 confidence level which may suggest the existence of a north bismarck plate

a russian record of a middle ordovician meteorite shower extraterrestrial chromite at lynna river st petersburg region numerous fossil meteorites and high concentrations of sediment dispersed extraterrestrial chromite ec grains with ordinary chondritic composition have previously been documented from 467 1 6 ma middle ordovician darriwilian strata these finds probably reflect a temporarily enhanced influx of l chondritic matter following the disruption of the l chondrite parent body in the asteroid belt 470 6 ma in this study a volkhovian kundan limestone marl succession at lynna river northwestern russia has been searched for ec grains 63 mu m eight samples forming two separate sample sets were collected five samples from strata around the asaphus expansusa raniceps trilobite zone boundary in the lower middle kundan yielded a total of 496 ec grains in 65 5 kg of rock average 7 6 ec grains kg 1 but up to 10 2 grains kg 1 these are extremely high concentrations three orders of magnitude higher than background levels in similar condensed sediment from other periods ec grains are typically about 50 times more abundant than terrestrial chrome spinel in the samples and about as common as terrestrial ilmenite three stratigraphically lower lying samples close to the a lepidurusa expansus trilobite zone boundary at the volkhov kunda boundary yielded only two ec grains in 38 2 kg of rock 0 05 grains kg 1 the lack of commonly occurring ec grains in the lower interval probably reflects that these strata formed before the disruption of the l chondrite parent body the great similarity in ec chemical composition between this and other comparable studies indicates that all or most ec grains in these russian mid ordovician strata share a common sourcethe l chondrite parent body less

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

seismotectonics and present day relative plate motions in the new hebrides north fiji basin region abstract relative motions between the pacific plate p indo australian plate ia new hebrides nh arc microplate and the north fiji basin nfb microplate are estimated using shallow seismicity 276 focal mechanism solutions bathymetry magnetism and the rm 2 plate model of minster and jordan 1978 within the nfb we define the western nfb wnfb eastern nfb enfb and southern nfb snfb microplates a 8 cm y spreading in a n72 e direction occurs along the n s trending spreading ridge of nfb wnfb enfb boundary along the enfb ia boundary located at 176 e a 3 cm y extension in a n108 e direction is proposed the wnfb p boundary which includes the hazel holme extensional zone hhez is complex and a general n25 e extension 2 cm y is inferred the fiji fracture zone ffz is composed of two segments along the eastern zone p ia boundary trending n80 e a left lateral strike slip motion occurs 9 6 cm y accommodated by minor n109 e extension pull apart while along the western zone enfb p boundary a n84 e trending left lateral strike slip motion 7 cm y is inferred the wnfb snfb boundary corresponds to a n70 e broad fracture zone along which an e w left lateral strike slip motion is proposed the snfb microplate moves rapidly eastwards 10 5 cm y at 172 e and is almost attached to the ia plate within the nfb an improved model is discussed which includes deformations within the ia and p plates and rotations with euler poles close to the nfb along the new hebrides trench the consumption rates are respectively 16 15 9 and 12 cm y at 11 12 5 15 5 and 20 s the minimum rate occurs where the d entrecasteaux ridge collides with the new hebrides arc rates of extension at the rear of the nh arc are 7 5 5 and 2 cm y at 11 12 5 and 20 s respectively back arc compression occurs between 13 30 s and 17 s the western end of the hazel holme extension zone coincides with the 13 30 s boundary separating the nh back arc compressive belt and the northern nh back arc troughs south of 21 22 s the trend of the slip vector changes along the nh trench from n70 e to n20 w this implies a n s motion of convergence along the e w trending southern part of the nh trench motion snfb ia of 1 5 cm y at 172 e and the existence of a n70 e trending left lateral fracture zone crossing the southern part of the nfb this fracture should be considered as a major plate boundary

temporal stability of deformation rates comparison of geological and geodetic observations hikurangi subduction margin new zealand abstract we examine how well contemporary horizontal velocity fields from global positioning system gps geodetic measurements describe long term deformation rates million year rates of extension shortening strike slip and vertical axis rotations are compared with modelling of gps velocities from the last 10 15 years in the hikurangi subduction margin new zealand within measurement uncertainties of mainly 50 the rates of deformation and vertical axis rotations for each time interval are usually indistinguishable the comparable deformation rates are consistent with the notion of near constant rates since 1 5 ma during this time oblique relative plate motion was partitioned with the majority 85 of margin normal motion accommodated on the plate interface and most 60 margin parallel motion by strike slip faulting and vertical axis rotations in the upper plate prior to 1 5 ma gps velocities cannot be reliably used to describe deformation and rotations of the hikurangi margin because at this time the taupo rift commenced opening the boundary between raukumara and wairoa rotational domains was abandoned and the rates of upper plate shortening increased the broad compatibility of gps and geological deformation and rotation rates since 1 5 ma suggests that the build up of contemporary elastic strains in the north island measured by gps is largely uniform for much of the duration of each seismic cycle the good agreement between the gps and geological deformation rates in the north island is consistent with the view that contemporary elastic strains will mainly be converted to permanent dip slip on the subduction thrust and to displacement on upper plate faults during future large magnitude earthquakes

quaternary uplift of the torres islands northern new hebrides frontal arc comparison with santo and malekula islands central new hebrides frontal arc coral reef terraces on the torres islands have recorded the quaternary uplift of part of the northern new indian plate underthrusts the arc from the west 14c and 230thp34u ages for torres fossil corals indicate that for approximately the past 100 000 years the islands uplifted at a constant rate ranging geographically from 0 7 to 0 9 yr the occurrence of similar uplift rates and eastward tilting directions and the absence of major crosscutting structures or rupture zone boundaries suggest that the islands may be part of a single tectonic block or arc segment the frontal arc morphology in the torres area is typical of frontal arcs farther south the central new hebrides frontal arc has no physiographic trench at the plate boundary west of santo and malekula islands and the axes of the maximum uplift rate are at the seaward edge of the plate the d entrecasteaux ridge dr a massive e w trending bathymetric feature on the indian plate is underthrusting santo and malekula the eastern parts of santo and malekula are physiographically equivalent to the torres islands and seem to have been on a former axis of maximum uplift rate however in response to subduction of the dr the uplift axes of santo and malekula may have shifted westward in a few hundred thousand years the new uplift axes caused the western parts of these islands to achieve their present topographic dominance over the earlier uplift axes holocene uplift rates on southern santo and northern malekula are about twice the average rates of the previous 100 000 years the accelerated holocene uplift could be explained if a particularly prominent part of the dr has recently underthrust santo and malekula d hebrides frontal arc these isles lie about midway between the volcanic chain and trench axis where the

extraterrestrial chromite in middle ordovician marine limestone at kinnekulle southern sweden traces of a major asteroid breakup event the distribution of sediment dispersed extraterrestrial chromite grains and other cr rich spinels 63 mu m has been studied in middle ordovician orthoceratite limestone from two quarries at kinnekulle southern sweden in the thorsberg quarry all similar to 3 2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment dispersed extraterrestrial chromite grains typically 1 3 grains are found per kilogram of limestone in the nearby hallekis quarry the same beds show similarly high concentrations of extraterrestrial chromite grains but in samples representing the 9 m downward continuation of the section exposed at this site only 5 such grains were found in a total of 379 kg of limestone the extraterrestrial equilibrated ordinary chondritic chromite grains call be readily distinguished by a homogeneous and characteristic major element chemistry including 2 0 3 5 wt tio2 and stable v2o3 concentrations close to 0 7 wt terrestrial cr rich spinels have a wide compositional range and co exist with extraterrestrial chromite in some beds these grains may be derived for example from mafic dykes exposed and weathered at the sea floor considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied instead the difference may be primarily related to all increase in the ancient flux or extraterrestrial matter to earth in connection with the disruption of the l chondrite parent body in the asteroid belt at about this time the coexistence in some beds of high concentrations of chondritic chromite and terrestrial cr rich spinels however indicates that redistribution of heavy minerals on the sea floor related to changes in sea level and sea floor erosion and currents must also be considered less

extraterrestrial chromite distribution across the mid ordovician puxi river section central china evidence for a global major spike in flux of l chondritic matter previous studies of mid ordovician limestone in sweden have shown that over a stratigraphic interval representing a few million years there is a two orders of magnitude enrichment in fossil l chondritic meteorites empty set 1 21 cm and sediment dispersed extraterrestrial chromite ec grains 63 mu m this has been interpreted as a dramatic increase in the flux of l chondritic matter to earth following the breakup of the l chondrite parent body which based on ar ar gas retention ages 470 6 ma of recently fallen meteorites occurred at about this time here we show that the general trend in the distribution of sediment dispersed ec grains can be reproduced in the puxi river section in central china a total of 288 kg of limestone was searched for chrome spinels in samples spanning the lower 8 m of the section representing the paroistodus originalis and lenodus antivariabilis conodont zones a total of 110 kg of limestone yielded only one ec grain similarly to the swedish sections ec grains begin to be common in the overlying l variabilis zone and remain common throughout the upper 9 m of the section representing the l variabilis yangtzeplacognathus crassus and l pseudoplanus zones in this part of the section 178 kg of limestone yielded 290 ec grains with an average chemical composition very similar to chromite from recent l chondrites in most of the beds over this interval one finds 1 4 ec grains per kilogram rock a clear two orders of magnitude enrichment relative to the lower part of the section small bed by bed variations in the ec content over the upper interval most likely reflect small variations in sedimentation rates the puxi river section contains only very rare terrestrial chrome spinel grains which can be distinguished already by their rounded abraded appearance compared to the angular pristine extraterrestrial spinels in the mid ordovician based on paleoplate reconstructions the puxi river site was positioned at mid latitudes on the southern hemisphere a couple of thousand kilometers east of the swedish sites the prominent enrichment of ec grains over the same stratigraphic interval in china and sweden is supporting evidence for a dramatic increase in the flux of l chondritic matter to earth shortly after the disruption of the l chondrite parent body in the asteroid belt c 2010 elsevier inc all rights reserved less