surface eddy momentum flux and velocity variances in the southern ocean from geosat altimetry abstract satellite altimetry has previously been used to map the magnitude of the surface eddy variability of the global oceans but the direction of the time variable velocities have been more difficult to determine here a technique is presented for resolving both magnitude and direction of residual surface geostrophic velocities at geosat altimeter crossover points providing a two year time series with a temporal resolution of 17 days and horizontal resolution of around 100 km the time series of residual velocity components are then used to determine surface eddy statistics in the southern ocean and to investigate the role of transient eddies in the southern ocean momentum balance the surface eddy statistics from geosat crossover points show a complex spatial distribution in the surface reynolds stresses u 2 v 2 u v in contrast to the assumptions of isotropic variability in previous analyses of altimeter data velocity variance ellipses are found to be distinctly anisotropic in many regions

thermal history of the h chondrite parent body implications for metamorphic grade and accretionary time scales abstract multiple temperature age constraints for eight h chondrite samples have been used to provide insight into the thermal history of their parent body through combination with numerical models of thermal evolution assuming internal heating by 26al and conductive cooling the effect of spreading accretion out over time is the principal focus of this work a wide range of body size and date of accretion is systematically tested for different values of accretion rate in order to quantify and illustrate the parameter space that is consistent with the available thermo chronological data we conclude that the h chondrite samples considered have a thermal history consistent with a parent body that at some stage had a concentric onion shell internal structure that body had a radius no larger than 130 km and accretion most probably took place over a time interval on the order of 0 0 0 2 myr approximately 2 myr after cai condensation in any case the time interval of accretion is unlikely to have been more than 0 5 myr supporting evidence in favour of rapid accretion possibly through reassembly of the fragments of an earlier generation of bodies furthermore the h chondrites studied here are inferred to have come from a wide depth range within the body where they experienced metamorphism indicating that preservation of the onion shell structure is unlikely the presence of an insulating regolith does not modify this conclusion as appropriate thermal histories for the three h6 samples considered cannot be reproduced at depths near the surface asteroid 6 hebe may be the parent body of the h chondrites but the high bulk density of the latter is difficult to reconcile with a rubble pile structure of pure h chondrite material finally optimized thermal histories are used to constrain the temperatures characterizing boundaries between petrological types 800 1000 and 1140 k for the h3 4 h4 5 and h5 6 boundaries respectively in detail the type 6 samples studied here are inferred to have experienced a similar peak temperature on the order of 1180 k but spent different times at that temperature while samples of lower metamorphic grade experienced different peak temperature but spent more or less the same time within 10 k of peak temperature on the order of 1 ma these results constitute a quantitative framework within which variations in petrographic and textural properties may be interpreted

tephra beds in deep sea cores off northern new zealand implications for the history of taupo volcanic zone mayor island and white island volcanoes twenty piston cores were collected from water depths of 650 3300 m from offshore bay of plenty north island new zealand they contain tephra from 11 okataina volcanic centre and 4 taupo volcanic centre rhyolite eruptions during the last 50 kyr that produce a tephrostratigraphic framework across the tectonically and volcanically complex region of southern havre trough alderman trough and the bay of plenty continental shelf this allows correlation between offshore and onshore sequences up to 200 km from source volcanoes covering much of marine isotope stages 3 and 2 the framework temporally constrains poorly dated and newly recognised volcanic events macroscopic tephra layers from the peralkaline mayor island volcano are documented for the first time at pre 50 post 50 40 5 37 4 22 2 and 14 2 ka in addition to the well known 7 ka tuhua event these macroscopic layers represent some of the most explosive events from this volcano and provide new marker horizons they are dispersed up to 90 km north east to east of the edifice minor tephra dispersal is also recorded at 17 8 25 5 35 and pre 50 ka the tephra contain high sio2 73 75 5 wt glass and subordinate basaltic components and each tephra represents a distinct magma batch that can be fingerprinted the mayor island tephra form two temporal trends toward less evolved magma compositions punctuated by a large caldera forming event at 36 ka the historically active andesitic white island volcano does not have widely dispersed tephra and the oldest primary deposit found is 21 ka five pre 50 ka rhyolite eruptions from an unknown taupo volcanic zone source provide evidence for explosive activity in a time interval poorly documented on land the cores demonstrate the patchy and uneven preservation of large magnitude tephra falls caused by local faulting bioturbation and ponding in bathymetrically complex regions reworked tephra layers are common and often lack indicative lithological features such units could easily be misinterpreted as primary events without micro beam geochemical analyses of glass shards

tectonic rotations about vertical axes during the last 4 ma in part of the new zealand plate boundary zone the declinations of the primary magnetization of sedimentary rocks in the northern part of the new zealand plate boundary zone after thermal or alternating field cleaning have been used to determine tectonic rotations about vertical axes of rigid crustal blocks the pattern of rotations during the last 4 ma combined with structural style and continuity defines seven structural domains each ca 100 100 200 km across which contain crustal blocks up to 100 km across and 20 km thick large crustal blocks tens of km across in two of these domains have rotated clockwise more than 20 relative to one of the margins of the plate boundary zone in the last 4 ma their behaviour appears to be controlled by the nature of the plate boundaries such as the presence of an underlying subducted slab and the strength of the crust at the back of the overlying crustal wedge small crustal blocks 10 km across may have rotated clockwise through angles greater than 20 during the last 4 ma floating on an underlying zone of more distributed deformation the tectonic rotations of the large crustal blocks and the nature of the deformation at their boundaries combined with an interpolation of the relative plate positions can be used to reconstruct the plate boundary zone at ca 4 ma

volatile transport of metals and the cu budget of the active white island magmatic hydrothermal system new zealand abstract evidence of metal transport by volcanic gases is found amongst others in volcanic plumes and sublimates growing around fumaroles the transport mechanisms i e volatile metal compounds are however not well constrained here we present results on the volatile transport of metals at the scale of a magmatic hydrothermal system white island volcano we studied metal emissions from the main vent through the crater lake covering the conduit volcanic gases injected in the lake are responsible for the waters and sediments enrichment in various metals while elemental sulfur forming at the bottom appears as the main sink for chalcophile metals metal chloride complexes are the dominant species in the hot and acidic crater lake waters nearby low temperature fumarolic gases are lower in metals than higher temperature gases reported in the 1970s emphasizing precipitation at depth and sequestration by condensing elemental sulfur efficient gaseous transport of metals by hcl and hbr is revealed by thermodynamic modeling whereas the role of sulfur is restrained to controlling redox conditions a copper budget for white island magmatic hydrothermal system is assessed taking into consideration all the potentials sinks about 3900 tons of copper are estimated to be retained in the subsurface environment every year over the 10 000 years lifetime of the magmatic hydrothermal system this adds up to 40 megatons depicting white island as a potential cu porphyry forming deposit

initial sizes of planetesimals and accretion of the asteroids abstract the present size frequency distribution sfd of bodies in the asteroid belt appears to have preserved some record of the primordial population with an excess of bodies of diameter d 100 km relative to a simple power law the survival of vesta s basaltic crust also implies that the early sfd had a shallow slope in the range 10 100 km morbidelli a bottke w f nesvorny d levison h f 2009 icarus 204 558 573 were unable to produce these features by accretion from an initial population of km sized planetesimals they concluded that bodies with sizes in the range 100 1000 km and a sfd similar to the current population were produced directly from solid particles of sub meter scale without experiencing accretion through intermediate sizes we present results of new accretion simulations in the primordial asteroid region the requisite sfd can be produced from an initial population of planetesimals of sizes 0 1 km smaller than the usual assumption of km sized bodies the bump at d 100 km is produced by a transition from dispersion dominated runaway growth to a regime dominated by keplerian shear before the formation of large protoplanetary embryos thus accretion of the asteroids from an initial population of small sub km planetesimals cannot be ruled out

ocean and atmosphere storm tracks the role of eddy vorticity forcing abstract signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period 1992 2002 in the atmosphere there are localized regions of eddy variability referred to as storm tracks at the entrance of the storm track the eddies grow providing a downgradient heat flux and accelerating the mean flow eastward at the exit and downstream of the storm track the eddies decay and instead provide a westward acceleration in the ocean there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the antarctic circumpolar current within these regions of high eddy kinetic energy there are more localized signals of high eady growth rate and downgradient eddy heat fluxes as in the atmosphere there are localized regions in the southern ocean where ocean eddies provide statistically significant vorticity forcing which acts to accelerate the mean flow

the early impact histories of meteorite parent bodies we have developed a statistical framework that uses collisional evolution models shock physics modeling and scaling laws to determine the range of plausible collisional histories for individual meteorite parent bodies it is likely that those parent bodies that were not catastrophically disrupted sustained hundreds of impacts on their surfaces compacting heating and mixing the outer layers it is highly unlikely that many parent bodies escaped without any impacts processing the outer few kilometers the first 10 20 myr were the most important time for impacts both in terms of the number of impacts and the increase of specific internal energy due to impacts the model has been applied to evaluate the proposed impact histories of several meteorite parent bodies up to 10 parent bodies that were not disrupted in the first 100 myr experienced a vaporizing collision of the type necessary to produce the metal inclusions and chondrules on the cb chondrite parent around 1 5 of bodies that were catastrophically disrupted after 12 myr sustained impacts at times that match the heating events recorded on the iab winonaite parent body more than 75 of 100 km radius parent bodies which survived past 100 myr without being disrupted sustained an impact that excavates to the depth required for mixing in the outer layers of the h chondrite parent body and to protect the magnetic field on the cv chondrite parent body the crust would have had to have been thick approximately 20 km to prevent it being punctured by impacts

sea beam deep tow investigation of an active oceanic propagating rift system galapagos 95 5 w we have tested and corroborated the propagating rift hypothesis with high resolution sea beam and deep tow data collected over the galapagos 95 5 w propagating rift system the propagating rift is continuously breaking through the cocos plate at a velocity of about 50 km m y with an azimuth of about 273 away from the galapagos hotspot this process transfers cocos lithosphere to the nazca plate including the preexisting spreading center which fails as it is gradually preempted by the propagating rift the spreading center azimuth is being changed by about 8 clockwise by this rift propagation the active propagating and failing rift axes overlap by about 15 km and are joined by a broad and anomalously deep zone of nonrigid plate deformation rather than by a transform fault although rift propagation is continuous on a scale of a few kilometers 50 000 years and lithospheric transferral from one plate to the other occurs mostly continuously as the preexisting rift fails it breaks into discrete segments the geometry of seafloor fabric and tectonic elements such as active and failed rifts the v shaped pattern of pseudofaults and the oblique tectonic fabric in the sheared zone of transferred lithosphere are observed as predicted this quantitative verification provides strong support for the propagating rift hypothesis

seismotectonics and present day relative plate motions in the tonga lau and kermadec havre region abstract an updated compilation of shallow seismicity in the tonga kermadec arc and back arc region including especially 304 focal mechanisms from centroid moment tensor solutions files provides new insights on the present day tectonics and allows the relative motions between the pacific indo australian and tonga kermadec plates to be quantified azimuths of slip vectors along the trench change at 19 s in the southern domain 35 19 s the mean azimuth ranges from n280 e to n285 e while in the northern domain it trends n276 277 e this 19 s boundary also exits in the back arc domain south of 19 s from 35 s to 20 s the direction of back arc extension is respectively n135 e n122 e and n111 e within the southern havre trough northern havre trough and southern lau basin in the southern lau basin the n111 e opening direction is compatible with the trend of the newly mapped valu fa ridge north of 19 s in the northern lau basin 16 18 s a n93 e trending crustal extension occurs along the n135 e peggy ridge and along an inferred n5 10 e spreading zone between these two extensional features a fourth plate the northern lau microplate is present at 18 19 s a ridge ridge ridge triple junction accommodated by n95 115 e right lateral strike slip fracture zones is proposed at the complex transitional zone between the southern and the northern parts of the lau basin the fiji north tonga fracture zone which bounds northward the lau back arc basin is composed of two segments a n95 e eastern segment extending from the northern end of the tonga trench to 178 30 w and a n75 e western segment lying north of the fiji platform a ridge fracture fracture triple junction is inferred at 178 30 w 15 45 s between the northern end of the peggy ridge and the two segments of the fiji north tonga fracture zone using the model rm 2 of minster and jordan 1978 and the directions of back arc spreading and convergence at the trench relative plate motions are estimated the extension rates in the back arc domain increase northward and are respectively 0 8 2 1 and 8 cm y at 33 s 28 s and 24 s farther north parallelism between directions of different motions does not allow the calculation of any accurate result therefore north of 24 s we used a spreading rate of 8 cm y which corresponds to the 24 s result and also to the value deduced from magnetic anomalies the consumption rates along the tonga kermadec trench are respectively 7 2 9 6 16 4 17 1 and 17 8 cm y at 33 s 28 s 24 s 20 s and 17 s both the back arc opening rate and the consumption rate of the pacific plate sharply increase between the northern havre trough and the southern lau basin this limit coincides with the louisville ridge trench junction along the n95 e eastern segment of the fiji north tonga fracture zone a pure left lateral strike slip motion parallel to the n93 e opening tectonics within the northern lau basin occurs in contrast along the n75 e western segment of the fiji north tonga fracture zone a left lateral strike slip motion of 7 cm y should be accompanied by a n135 e extensional motion of 3 5 cm y

crustal seismic reflection measurements across the northern extension of the taupo volcanic zone north island new zealand abstract a crustal seismic reflection profile across the offshore taupo volcanic zone otvz has been reprocessed to improve the lower crust upper mantle image the otvz is underlain by several strong sub horizontal crustal reflections that may correspond to shear zones or mafic sills a reflector interpreted to correspond to moho lies at a depth of about 8 8 5 s twt seconds two way traveltime about 20 km under the western margin of the raukumara peninsula and rises to a depth of about 6 s twt about 16 km under the active eastern part of the otvz and can be traced at about this depth across most of the otvz underlying this inferred moho are two diffuse zones of discontinuous reflectivity one underlying the active eastern otvz and a less well defined one underlying the landward extension of the havre trough they extend from about 15 to 35 km depth within the mantle wedge and are inferred to be imaging a zone of depleted upper mantle associated with crustal extension a localised zone of deep 30 50 km reflectivity and seismicity occurs under the eastern otvz margin and may relate to fluid flow from the hikurangi plateau being subducted further east

the late accretion and erosion of vesta s crust recorded by eucrites and diogenites as an astrochemical window into the formation of jupiter and the early evolution of the solar system abstract the circumsolar disc was the birthplace of both planetesimals and giant planets yet the details of their formation histories are as elusive as they are important to understand the origins of the solar system for decades the limited thickness of vesta s basaltic crust revealed by the link between the asteroid and the howardite eucrite diogenite family of meteorites and its survival to collisional erosion offered an important constraint for the study of these processes some results of the dawn mission however cast doubts on our understanding of vesta s interior composition and of the characteristics of its basaltic crust weakening this classical constraint in this work we investigate the late accretion and erosion experienced by vesta s crust after its differentiation and recorded in the composition of eucrites and diogenites and show that it offers an astrochemical window into the earliest evolution of the solar system in our proof of concept case study focusing on the late accretion and erosion of vesta s crust during the growth and migration of jupiter the water enrichment of eucrites appears to be a sensitive function of jupiter s migration while the enrichment in highly siderophile elements of diogenites appears to be particularly sensitive to the size frequency distribution of the planetesimals the picture depicted by the enrichments created by late accretion in eucrites and diogenites is not qualitatively affected by the uncertainty on the primordial mass of vesta crustal erosion instead is more significantly affected by said uncertainty and vesta s crust survival appears to be mainly useful to study violent collisional scenarios where highly energetic impacts can strip significant amounts of vestan material while limitedly contributing to vesta s late accretion while our proof of concept case study is based on a simplified physical model and explores only a limited set of scenarios our results suggest that the astrochemical record of the late accretion and erosion of vesta s crust provided by eucrites and diogenites can be used as a tool to investigate any process or scenario associated to the evolution of primordial vesta and of the early solar system

analysis of late holocene faulting within an active rift using lidar taupo rift new zealand abstract high quality light detection and ranging lidar data collected across the rangitaiki plains the fastest extending section of the onshore taupo rift new zealand reveal 122 active fault traces and provide new constraints on displacements displacement rates and paleoearthquakes of the normal faults in the rift the identified lineaments are scarps that record vertical offset of geomorphic surfaces e g beach ridges meander channel floors river terraces etc and trend parallel or sub parallel to other active faults in the rift these lineaments are interpreted to be active faults active fault traces trend 060 and their lengths range from 0 25 to 6 km they mainly traverse and displace a diachronous landscape of

impact excavation on asteroid 4 vesta hubble space telescope results hubble space telescope images of asteroid 4 vesta obtained during the favorable 1996 apparition show an impact crater 460 kilometers in diameter near the south pole color measurements within the 13 kilometer deep crater are consistent with excavation deep into a high calcium pyroxene rich crust or olivine upper mantle about 1 percent of vesta was excavated by the crater formation event a volume sufficient to account for the family of small vesta like asteroids that extends to dynamical source regions for meteorites this crater may be the site of origin for the howardite eucrite and diogenite classes of basaltic achondrite meteorites

structural control of volcanism and caldera development in the transtensional taupo volcanic zone new zealand abstract the taupo volcanic zone tvz new zealand comprises several segments with variable components of extension and is characterised by longitudinal segmentation into a central zone dominated by rhyolitic calderas and extremities with only andesitic composite volcanoes modern

shallow structure and active tectonism of an offshore continental back arc spreading system the taupo volcanic zone new zealand the taupo volcanic zone tvz the predominant active volcano tectonic structure within the new zealand sub continent marks the zone of quaternary back arc spreading and calc alkaline volcanism associated with the convergence of the pacific and australian plates at the hikurangi plate boundary margin north island new zealand new bathymetric shallow seismic reflection magnetic anomaly and gloria data presented here accurately define for the first time the extent structure and tectonism of the poorly known offshore portion of the tvz these data show that nearly half of the entire ca 40 km wide tvz lies offshore extending northeast for some 150 km seaward to 36 45 s the southeastern and northwestern boundaries of the offshore tvz are significant tectonic and bathymetric features which comprise inward facing normal fault zone the white island and tauranga fault zones respectively three essentially parallel tectono morphological units are identified along the length of the offshore tvz they comprise from southeast to northwest a frontal non volcanic graben a volcanic ridge and a volcanic back arc graben the spatial position of these units with respect to the hikurangi subduction margin is not a simple arc and back arc relationship tectonism within the offshore tvz although pervasive increases from northwest to southeast so that the area of highest extension and subsidence of the non volcanic graben occurs in front of the volcanic ridge geophysical data also suggest the possible tectonic ablation of blocks of the forearc hinterland into the offshore tvz along its southeastern margin the southern havre trough and northern tvz are offset sinistrally by 45 50 km they overlap by 40 km and differ in orientation by ca 7 dextral displacement between these two extensional regimes is accomodated by a series of dextrally oblique and en echelon bookshelf faults the orientation of extensional structures within the offshore tvz is also dextrally oblique by 10 15 to the regional 030 trend of the bounding faults and agree with axes of maximum extension determined elsewhere from geodetic and seismological data

thermogenic hydrocarbons from the offshore calypso hydrothermal field bay of plenty new zealand hydrothermal gases from shallow seafloor vents in the bay of plenty new zealand contain co2 ch4 and the higher gaseous hydrocarbons up to i n c4h10 the gases are similar to those discharged at fumaroles on the nearby white island carbon isotope compositions for co2 fall between 3 4 and 5 5 pdb and reflect a shallow magmatic carbon source the 13c values of ch4 range from 24 6 to 28 9 pdb and the d values vary between 122 and 135 smow the ch4 isotope values and the presence of the higher hydrocarbon compounds such as c2h6 and c3h8 with 13c values near 20 pdb suggest hydrocarbon production by high temperature maturation of sedimentary organic matter and mixing 1 1 of the thermogenic ch4 with abiogenic ch4 long chained hydrocarbons occur in dredged samples close to the active vents their n alkane distribution has a high to moderate odd even predominance and an extensive hopane series indicative of higher land plant waxes and prokaryotic membranes in the source substantial amounts of unsubstituted polynuclear aromatic hydrocarbons pah mark the transition from aliphatic to aromatic dominated bitumens consistent with extensive source maturation resulting from thermal stress the bitumens are interpreted as pyrolysates derived from buried near coastal vegetation and terrestrial detritus under various thermal regimes mixed with immature seafloor organic matter

nature and tectonic setting of the southern kermadec submarine arc volcanoes an overview abstract the 1000 km long kermadec subduction system northern new zealand and adjacent havre trough back arc basin associated with the pacific australian plate boundary is an archetypal example of an active island arc convergent margin although the position and tectonic setting of the frontal arc is critical to understanding tectonism and magmatism within an arc back arc system this has not been unequivocally established for the kermadec subduction system swath surveys from the 35 37 s segment of the kermadec system reveal that the frontal arc comprises seven large arc stratovolcanoes rumble ii iii iv silent ii rumble v tangaroa and clark the frontal arc continues south with the whakatane volcano sited at the northern extremity of the taupo volcanic zone of these eight volcanoes the latter four are newly identified and all have constructional volumes varying between 50 and 260 km 3 all of these arc edifices are sited 15 25 km west of the kermadec frontal ridge bathymetric data similarly indicate that other kermadec arc volcanoes between 32 and 35 s also lie west of the ridge axis within the back arc region while north of 32 s the arc volcanoes lie along the frontal ridge this apparent variation in the tectonic position of the frontal arc coincides with other changes in the morphology of the subduction system north and south of 32 s and as suggested by pelletier and dupont 1990 may reflect differences in the length of the underlying subducted slab the 35 37 s frontal arc segment is sited within 35 45 km of the present day back arc rift axis and may control the initial loci of rift graben evolution sidescan imagery and seafloor photography of the southern kermadec arc volcanoes reveal a complex volcanic stratigraphy with intertwined pillow lavas proximal pillow breccias highly vesicular scoriaceous lapilli and pervasively mass wasted volcaniclastic and epiclastic detritus all volcanic deposits interdigitate with pelagic sediments a single k ar dated andesite from whakatane volcano suggests that the history of the present frontal arc may extend back to at least 0 77 ma while rumble iii and v volcanoes are the major although not exclusive sites of geologically recent edifice construction the historical acoustic record of southern kermadec submarine eruptions since 1958 is now regarded as equivocal with the july 1986 eruption of rumble iii the only unambiguous volcanic activity recorded in modern times

elemental mercury at submarine hydrothermal vents in the bay of plenty taupo volcanic zone new zealand hot springs in active geothermal areas such as yellowstone national park the geysers geothermal field in california and the taupo volcanic zone in new zealand are notably enriched in the trace metals au ag as sb and hg such near surface hot springs have formed many of the world s important deposits of gold and silver and some of the largest deposits of mercury the majority of these are associated with continental geothermal systems in subaerial environments here we report the discovery of active mercury depositing hot springs in a submarine setting at nearly 200 m water depth within the offshore extension of the taupo volcanic zone of new zealand these vents contain the first documented occurrence of elemental mercury on the sea floor and provide an important link between offshore hydrothermal activity and mercury depositing geothermal systems on land the discovery has implications for mercury transport in sea floor hydrothermal systems and underscores the importance of submarine volcanic and geothermal activity as a source of mercury in the oceans

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

bookshelf faulting and horizontal block rotations between overlapping rifts in southern afar lateral slip on initially rift parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts it occurs in southern afar where clockwise block rotations result from distributed dextral shear between the overlapping ghoubbet asal manda inakir and manda hararo abhe bad rifts faulting observed during the 1969 serdo earthquakes and on spot images is consistent with the shear being taken up by left lateral slip on steep nw se striking faults which formed as normal faults before extensional strain became localized in the two rifts this bookshelf faulting accounts quantitatively for the 14 5 7 5 rotation documented by paleomagnetism in the 1 8 0 4 ma old afar stratoid basalts given the 17 5 5 mm yr rate of separation between arabia and somalia

a lower crustal bright spot under the bay of plenty north island new zealand abstract marine seismic reflection measurements across the central bay of plenty northern north island new zealand detected a strong seismic reflection in the lower crust at a depth of about 10 km 5 s twt this reflection is observed on the stacked seismic section for about 8 5 km across and below a young actively rifting basin that lies along the offshore extension of the active taupo volcanic zone the reflection is unusual in that its maximum extent on any shot record is about 1250 m about half the fresnel zone diameter suggesting a series of point reflectors or reflectors of limited extent wave equation modelling indicates that the reflection event probably represents a positive increase in acoustic impedance and thus does not represent a magma body or partial melt as has been suggested previously the preferred model suggests a thin layer such as a mafic sill with the large reflectivity and apparent wavelength caused by wavelet tuning similar to that deduced for the surrency bright spot in the usa a mafic sill would be consistent with the geological setting although other explanations such as a highly altered shear zone cannot be excluded from the restricted extent of the reflections on the shot gathers and their spatial relationship we infer that the sill is fragmented

subduction evolution and mantle dynamics at a continental margin central north island new zealand 1 central north island new zealand provides an unusually complete geological and geophysical record of the onset and evolution of subduction at a continental margin whereas most subduction zones are innately two dimensional north island of new zealand displays a distinct three dimensional character in the back arc regions specifically we observe mariana type subduction in the back arc areas of central north island in the sense of back arc extension high heat flow prolific volcanism geothermal activity and active doming and exhumation of the solid surface evidence for emplacement of a significant percent of new lithosphere beneath the central north island comes from heat flux of 25 mw km of strike of volcanic zone and thinned crust underlain by rocks with a seismic wave speed consistent with underplated new crust seismic attenuation qp 1 is high 240 and rhyolitic and andesitic volcanism are widespread almost complete removal of mantle lithosphere is inferred here in pliocene times on the basis of the rock uplift history and upper mantle seismic velocities as low as 7 4 0 1 km s in contrast southwestern north island exhibits chilean type back arc activity in the sense of compressive tectonics reverse faulting low heat flow thickened lithosphere and strong coupling between the subducted and overriding plates this rapid switch from mariana type to chilean type subduction occurs despite the age of the subducted plate being constant under north island moreover stratigraphic evidence shows that processes that define the extensional back arc area the central volcanic region are advancing southward into the compressional system wanganui basin at about 10 mm yr we link the progression from one system to another to a gradual and viscous removal of thickened mantle lithosphere in the back arc regions thickening occurred during the miocene within the taranaki fault zone the process of thickening and convective removal is time and temperature dependent and has left an imprint in both the geological record and geophysical properties of central north island which we document and describe

normal fault growth and linkage in the whakatane graben new zealand during the last 1 3 myr determination of fault growth rates and fault network evolution at time scales from 104 106 years has been hampered by a lack of a well constrained stratigraphic succession that provides a high fidelity record of fault development over these time periods here we show how seismic reflection data of different spatial resolutions can be used to constrain the linkage history and displacement rate variations of a single major fault we present data collected in the offshore whakatane graben bay of plenty new zealand where intense normal faulting occurs as a result of active back extension the focus of our study is the rangitaiki fault a linked segmented normal fault which is the dominant active structure in the graben the total linked fault length is c 20 km long and has a displacement of up to 830 130 m in the top 1 5 km of sediments the fault has been actively growing for the last 1 34 0 51 ma and has developed from isolated fault segments to a fully linked fault system initially the dominant process of fault growth was tip propagation with an average and maximum displacement rates of 0 52 0 18 mm yr 1 and 0 72 0 23 mm yr 1 respectively interaction and linkage became more significant as the fault segments grew towards each other resulting in the fault network becoming fully linked between 300 and 18 ka following fault segment linkages the average displacement rate of the fault network increased by almost three fold to 1 41 0 31 mm yr 1 while the maximum displacement rate increased to 2 72 0 61 mm yr 1 this is the first time that the growth rate of unlinked fault segments has been resolved and has been shown to be slower than in the subsequent linked fault system

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

asymmetric rifting in a continental back arc environment north island new zealand marine seismic reflection data have been used to investigate the structure and evolution of the continental back arc basin forming the volcanic region of central north island new zealand the seismic reflection data defines two parts to the volcanic region in the west an older region showing narrow back tilted blocks and a younger rift basin delineated by the white island and tauranga fault zones to the east the younger rift coincides with the offshore trend of the active taupo volcanic zone tvz listric faulting of basement in the eastern part gives rise to about 80 extension for the basin which is consistent with the thinning of the crust from 25 km along the margins of the basin to 15 km under the basin basement in the region is interpreted to be highly extended greywacke the nature of the faulting and the seismic stratigraphy demonstrates that the extension has been asymmetric in nature being progressively younger towards the plate boundary in the east active faulting offsets the seafloor in the younger eastern part of the basin eight major sedimentary sequences delineated by unconformities are recognised in the basin these sequences based on correlation with major onshore rhyolitic eruptions are considered to be up to about 1 6 ma old a major unconformity within the sequences may coincide with a hiatus in volcanic activity from 0 7 0 4 ma ago a caldera structure is tentatively identified under the western part of the younger rift basin offshore tvz from seismic and other data its age is not well constrained but an age corresponding to the initiation of the younger extension episode 1 6 ma ago is preferred older volcanic sequences underlie the oldest tvz volcanic units to the west of the younger basin and probably correspond to the pliocene miocene volcanics onshore in the coromandel peninsula

sedimentary architecture of a plio pleistocene proto back arc basin wanganui basin new zealand the sedimentary architecture of active margin basins including back arc basins is known only from a few end members that barely illustrate the natural diversity of such basins documenting more of these basins types is the key to refining our understanding of the tectonic evolution of continental margins this paper documents the sedimentary architecture of an incipient back arc basin 200 km behind the active hikurangi subduction margin north island new zealand the wanganui basin wb is a rapidly subsiding plio pleistocene sedimentary basin located at the southern termination of the extensional back arc basin of the active central volcanic region tvz the wb is asymmetric with a steep thrust faulted outer arc ward margin and a gentle inner craton ward margin it contains a 4 km thick succession of plio pleistocene sediments mostly lying offshore composed of shelf platform sediments it lacks the late molasse like deposits derived from erosion of a subaerial volcanic arc and basement observed in classical back arc basins detailed seismic stratigraphic interpretations from an extensive offshore seismic reflection data grid show that the sediment fill comprises two basin scale mega sequences 1 a pliocene 3 8 to 1 35 ma sub parallel regressive bpre growthq sequence that overtops the uplifted cratonward margin above the reverse taranaki fault and 2 a pleistocene 1 35 ma to present divergent transgressive bsyn growthq sequence that onlaps i the craton ward high to the west and ii uplifted basement blocks associated with the high angle reverse faults of the arc ward margin to the east along strike the sediments offlap first progressively southward mega sequence 1 and then southeastward mega sequence 2 with sediment transport funnelled between the craton and arc ward highs towards the hikurangi trough through the cook strait the change in offlap direction corresponds to the onset of arc ward thrust faulting and the rise of the axial ranges at ca 1 75 ma resulting in 5100 5700 m of differential subsidence across the fault system sedimentation has propagated south to southeast ward over the last 4 myrs at the tip of successive back arc graben volcanic arcs and the associated thermally uplifted parts of the north island following the southward migration of the hikurangi subduction margin subsidence occurred by mantle flow driven flexure the result of active down drag of the lithosphere by locking of the hikurangi subduction interface and sediment loading the wb is considered to be a proto back arc basin that represents the intermediate stage of evolution of an epicratonic shelf platform impacted by active margin processes

eddy momentum flux and its contribution to the southern ocean momentum balance a large amount of momentum is transferred to the southern ocean by strong westerly winds analytical and numerical models have suggested that transient eddies may be important in transporting this momentum away from the region of wind forcing either horizontally1or vertically downwards where it is balanced by bottom topographic drag2 5 there are however few long term in situ observations of horizontal eddy momentum flux6 8 and no large scale measurements of vertical eddy fluxes to test these models as a result the momentum balance of the antarctic circumpolar current acc remains uncertain and the role of eddies controversial here we use geosat satellite altimeter data to resolve directional eddy kinetic energy and horizontal eddy momentum flux in the acc on fine spatial and temporal scales the complex spatial distribution of surface eddy momentum flux is strongly influenced by bottom topography the horizontal eddy momentum flux tends generally to concentrate the mean flow although some regions of divergence are observed our results show that the zonally averaged horizontal eddy momentum flux from transient eddies is an order of magnitude too small and in the wrong direction to directly balance the eastward momentum input from wind1

evolution of the taupo hikurangi subduction system abstract the present day taupo hikurangi subduction system is a southward extension of the tonga kermadec arc system into a sediment rich continental margin environment it consists of a shallow structural trench the hikurangi trough a 150 km wide imbricate thrust controlled accretionary borderland the continental slope shelf and coastal hills of eastern north island a frontal ridge the main greywacke ranges of north island and a volcanic arc and marginal basin the taupo volcanic zone structural elements become progressively more elevated and subduction more oblique towards the south the whole nne trending system is truncated at a largely strike slip transform boundary that extends along the southwestern part of the hikurangi trough and the hope fault of south island to the main alpine fault the volcanic arc is 200 270 km from the structural trench and comprises a nne trending chain of andesite dacite volcanoes extending along the eastern side of the taupo volcanic zone most of the andesites are olivine bearing and have been erupted within the last 50 000 years it is suggested the taupo hikurangi margin has evolved by rotation of accretionary elements from an original nw trending subduction system north of new zealand the older elements of the prism were associated with subduction of a re entrant of the pacific plate and perhaps the south fiji basin in mid tertiary times they subsequently became separated from their nw trending volcanic arc by dextral strike slip movement along curved faults east of the main greywacke ranges during the plio pleistocene oblique subduction and accretion intensified as the taupo hikurangi margin rotated into line with the nne trending kermadec system and a marginal basin was developed along a similar trend to form the taupo volcanic zone within the last 50 000 years olivine bearing andesite volcanism has commenced along the eastern side of the taupo volcanic zone

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

magma origin and evolution of white island whakaari volcano bay of plenty new zealand white island is an active composite stratovolcano in the bay of of the north island the pacific plate is being subducted plenty new zealand that comprises many small volume westwards under the australian plate at a rate of 50 mm yr to form the taupo hikurangi arc trench with phenocryst contents of 15 44 minor high mg basaltic system volcanic activity is now concentrated in the andesite explosive eruptions such as those of 1976 1992 may taupo volcanic zone tvz the youngest expression have occurred at intervals throughout the history of white island of 22 ma cenozoic continental margin arc volcanism in but are rarely preserved these alternate with major episodes of the north island of new zealand cole 1990 tvz andesite dacite lava extrusion the high mg magmas form by fig 1 inset is at the southern end of the 2800 km hydrous melting of mantle metasomatized by fluids from the tonga kermadec taupo volcanic arc system and the dehydrating slab at the slab mantle wedge interface that rise lau basin havre trough ngatoro basin back arc basin rapidly to shallow magma chambers 2 7 km where limited system southern havre trough and tvz are offset mixing and contamination occurs before eruption some of this sinistrally by 40 50 km and wright 1992 believes the magma remains in the magma chamber where it interacts with the offset to be accommodated by a series of dextrally oblique crystal mush from which it inherits phenocrysts to form so called and en echelon bookshelf faults the southwestern limit dirty lavas total phenocryst content of these lavas is correspondingly of this fault system is placed 20 km north of white higher as more magma is intruded into the chamber the heat flux island and has nw se orientation fig 1 inset will increase and melt fraction will eventually rise to the surface to tvz extends offshore into the bay of plenty as a form high silica andesite dacite magma clean lavas with fewer graben structure of 45 km width fig 1 bounded by the inherited phenocrysts similar multi magma chamber plumbing tauranga fault zone to the west and the white island systems with complex evolution involving fractionation and confault zone to the east wright 1992 the eastern side tamination probably occur in most andesite dacite arc volcanoes of tvz coincides with the current active volcanic front and a broadly linear alignment of andesitic volcanoes extends from ruapehu in the southwest to whakatane

wind driven flow over topography in a zonal β plane channel a quasi geostrophic model of the antarctic circumpolar current abstract the paper gives a detailed account of the dynamical balance of a wind driven zonally unbounded flow over topography the problem is investigated with a quasi geostrophic plane channel with two layers and eddy resolution the channel has a width of 1500 km and a zonal periodicity of 4000 km apart from the dimensions the model structure is similar to the one used by mcwilliams et al the experiments with this model address the problem of the relative role of transient and standing eddies as well as bottom friction and topographic form stress in the balance of a current driven by a steady surface windstress the response of the system is investigated for different values of the friction parameter and various locations of topographic obstacles in the bottom layer of the channel the principal momentum balance emerging from these experiments supports the concept of munk and palmen for the dynamics of the antarctic circumpolar current which proposes that the momentum input by windstress is transfe

evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin abstract chondrules are the major high temperature components of chondritic meteorites which are conventionally viewed as the samples from the very first generation of undifferentiated planetesimals growing evidences from long and short lived radionuclide chronologies indicate however that chondrite parent asteroids accreted after or contemporaneously with igneous activities on differentiated asteroids questioning the pristine nature of chondrites here we report a discovery of metal bearing olivine aggregates with granoblastic textures inside magnesian porphyritic type i chondrules from the cv carbonaceous chondrite vigarano formation of the granoblastic textures requires sintering and prolonged high temperature 1000 c annealing conditions which are not expected in the solar nebula during chondrule formation but could have been achieved on parent bodies of olivine rich differentiated or thermally metamorphosed meteorites the mineralogy and petrography of the metal olivine aggregates thus indicate that they are relict dunite like lithic fragments which resulted from fragmentation of such bodies the very old pb pb absolute ages and al mg relative model ages of bulk cv chondrules suggest that such planetesimals may have formed as early as the currently accepted age of the solar system 4567 2 0 6 ma

modeling the evolution of the parent body of acapulcoites and lodranites a case study for partially differentiated asteroids abstract the acapulcoites and lodranites are rare groups of primitive achondrites that originate from a common parent body and are of particular interest since they experienced only partial melting we calculated thermal evolution and differentiation models of the parent body of the acapulco lodran meteorite clan the models were compared to the maximum metamorphic temperatures differentiation degree and thermo chronological data available an optimized set of parameters which fits to the data was determined a radius of 260 km a formation time of 1 7 ma after cais and an initial temperature of 250 k the burial depths derived are 7 13 km the respective layers experienced minor melting and small scale melt migration matching the differentiation degree of the meteorites the resulting structure has an iron core a silicate mantle a partially differentiated layer and an undifferentiated outer shell our results indicate a larger size an earlier formation time and a formation closer to the sun of the parent body of acapulcoites and lodranites than typical estimates for ordinary chondritic parent bodies consistent with a stronger thermal metamorphism the burial depths support excavation by a single impact the presence of core and mantle indicates that these meteorites could share a common parent body with differentiated stony and iron meteorites

a global analysis of sverdrup balance using absolute geostrophic velocities from argo using observations from the argo array of profiling floats the large scale circulation of the upper 2000 decibars db of the global ocean is computed for the period from december 2004 to november 2010 the geostrophic velocity relative to a reference level of 900db is estimated from temperature and salinity profiles and the absolute geostrophic velocity at the reference level is estimated from the trajectory data provided by the floats combining the two gives the absolute geostrophic velocity on 29 pressure surfaces spanning the upper 2000db of the global ocean these velocities together with satellite observations of wind stress are thenusedtoevaluatesverdrupbalance thesimplecanonicaltheoryrelatingmeridionalgeostrophictransport to wind forcing observed transports agree well with predictions based on the wind field over large areas primarily in the tropics and subtropics elsewhere especially at higher latitudes and in boundary regions sverdrup balance does not accurately describe meridional geostrophic transports possibly due to the increased importance of the barotropic flow nonlinear dynamics and topographic influence thus while it provides an effective framework for understanding the zero order wind driven circulation in much of the global ocean sverdrup balance should not be regarded as axiomatic

eddy mean flow interaction in the decadally modulating kuroshio extension system abstract satellite altimeter sea surface height ssh data of the past 16 years are used to investigate the decadal changes of the kuroshio extension ke system that oscillated between a stable and an unstable dynamic state during the stable state of 10 1992 06 1995 and 01 2002 12 2004 the ke jet was intense and had a northerly zonal mean path and a well defined southern recirculation gyre during the unstable state of 07 1995 12 2001 and 01 2005 present the ke jet had a reduced eastward transport and a more southerly flow path transitions between the two dynamic states are caused by the basin scale wind stress curl forcing in the eastern north pacific related to the pacific decadal oscillations pdos or the north pacific gyre oscillations npgos during the positive pdo or negative npgo phase the intensified aleutian low generates negative ssh anomalies in the eastern north pacific through ekman divergence as these wind induced negative ssh anomalies propagate to the west as baroclinic rossby waves they weaken the zonal ke jet and shift its path southward as its path is pushed southward 32 n the deep reaching ke jet has to ride over the shallow shatsky rise generating localized disturbances that lead to their subsequent development along the ke jet west of the shatsky rise the sequence opposite to that listed above occurs when the pdo npgo changes to its negative positive phase after the ke system transitions from a stable to unstable state the enhanced eddy interaction is found to strengthen both the southern recirculation gyre and quasi stationary meanders along the ke s upstream path this nonlinear eddy interaction is important in determining the amplitude of the observed decadal ke variability

on the patterns of wind power input to the ocean circulation pathways of wind power input into the ocean general circulation are analyzed using ekman theory direct rates of wind work can be calculated through the wind stress acting on the surface geostrophicflow however because that energy is transported laterally in the ekman layer the injection into the geostrophic interior is actuallycontrolledby ekmanpumping with a patterndeterminedby the windcurl ratherthanthe wind itself regions of power injection into the geostrophic interior are thus generally shifted poleward compared to regions of direct wind power input most notably in the southern ocean where on average energy enters the interior 108 south of the antarctic circumpolar current core an interpretation of the wind power input to the interior is proposed expressed as a downward flux of pressure work this energy flux is a measure of the work done by the ekman pumping against the surface elevation pressure helping to maintain the observed anomaly of sea surface height relative to the global mean sea level

acceleration and overturning of the antarctic slope current by winds eddies and tides all exchanges between the open ocean and the antarctic continental shelf must cross the antarctic slope current asc previous studies indicate that these exchanges are strongly influenced by mesoscale and tidal variability yet the mechanisms responsible for setting the asc s transport and structure have received relatively little attention in this study the roles of winds eddies and tides in accelerating the asc are investigated using a global ocean sea ice simulation with very high resolution 1 48 grid spacing it is found that the circulation along the continental slope is accelerated both by surface stresses ultimately sourced from the easterly winds and by mesoscale eddy vorticity fluxes at the continental shelf break the asc exhibits a narrow 30 50 km swift 0 2 m s 1 jet consistent with in situ observations in this jet the surface stress is substantially reduced and may even vanish or be directed eastward because the ocean surface speed matches or exceeds that of the sea ice the shelfbreak jet is shown to be accelerated by tidal momentum advection consistent with the phenomenon of tidal rectification consequently the shoreward ekman transport vanishes and thus the mean overturning circulation that steepens the antarctic slope front asf is primarily due to tidal acceleration these findings imply that the circulation and mean overturning of the asc are not only determined by near antarctic winds but also depend crucially on sea ice cover regionally dependent mesoscale eddy activity over the continental slope and the amplitude of tidal flows across the continental shelf break

demise of one volcanic zone and birth of another a 12 m y marine record of major rhyolitic eruptions from new zealand ocean drilling program sites 1123 and 1124 provide an unprecedented 12 m y record of major rhyolitic eruptions from the coromandel and taupo volcanic zones of new zealand macroscopic tephras n 197 were dated using magnetostratigraphy supplemented by geochemical correlation with subaerial tephra isothermal plateau fission track ages and orbitally tuned stable isotope data eruptions began in the coromandel volcanic zone ca 12 ma 1 6 1 m y earlier than previously known thereafter volcanism was fairly continuous with a tempo and intensity that increased through the late miocene pliocene and into the quaternary when the taupo volcanic zone formed the transition from the coromandel to the taupo zone previously placed as ca 4 2 ma was seamless without obvious breaks or changes in ash composition this well dated history of long lived and productive volcanism allows for more confident correlation with other circum pacific tephra records thus helping confirm the occurrence of widespread coeval eruptions throughout the region

the importance of experiments constraints on chondrule formation models abstract we review a number of constraints that have been placed on the formation of chondrules and show how these can be used to test chondrule formation models four models in particular are examined the x wind model sudden exposure to sunlight 0 1 au from the proto sun with subsequent launching in a magnetocentrifugal outflow solar nebula lightning nebular shocks driven by eccentric planetesimals and nebular shocks driven by diskwide gravitational instabilities we show that constraints on the thermal histories of chondrules during their melting and crystallization are the most powerful constraints and provide the least ambiguous tests of the chondrule formation models such constraints strongly favor melting of chondrules in nebular shocks shocks driven by gravitational instabilities are somewhat favored over planetesimal bow shocks

mg diffusion in anorthite implications for the formation of early solar system planetesimals we have measured the self diffusion coefficients of mg ca and sr in anorthitic plagioclase in order to assess the potential of mg isotopic heterogeneities in early solar system planetesimals to survive thermal metamorphism diffusion couples were constructed from polished single crystals of natural anorthite and synthetic isotopically enriched anorthite glass couples were annealed at atmospheric pressure and 1200 1400 c and isotopic concentration profiles were measured with an ion microprobe the results show that mg diffusion in anorthite is surprisingly fast with d mg being over 2 orders of magnitude greater than d sr this indicates that the diffusion coefficient of mg in anorthite cannot be approximated with that for sr mg diffusion in the c direction is also slightly faster than in the b direction while ca and sr diffusion appear to be isotropic the results provide important constraints on the thermochronological history of anorthite bearing mineral assemblages that preserve radiogenic 26 mg excesses in a planetesimal heated by the decay of 26 al the temperature at any point depends on the planetesimal size time of formation thermal conductivity and depth within the planetesimal given sufficient heating 26 mg heterogeneities produced by the in situ decay of 26 al in ca al rich inclusions cais and chondrules will be erased by diffusive equilibration using the self diffusion coefficient for mg in anorthite measured in this study we show that the common occurrence of 26 mg excesses in these inclusions requires that they must be stored in small 15 km bodies or the outermost rims of larger bodies for the first 1 2 million years of the solar system s history for early formed bodies larger than 15 km most of the mass will have been heated sufficiently for any radiogenic 26 mg to have been diffusively homogenized in the mg rich planetary environment

patterns of tectonic fault interactions captured through geostatistical analysis of microearthquakes 1 historical earthquakes are often strongly clustered in space and time this clustering has been attributed to static stress triggering associated with tectonic fault interactions and or to fluid migration discrimination between these two models requires detailed information on the timing location and size of earthquakes the matata earthquake sequence in the taupo rift new zealand provides a unique opportunity to chart spatial and temporal patterns of earthquakes along individual faults and their relations to other faults in the system over timescales of days to years this is possible because there are 2563 accurately relocated within 5 5 earthquakes that occur over thousand year timescales e g 60 kyr

origin and history of ureilitic material in the solar system the view from asteroid 2008 tc3 and the almahata sitta meteorite asteroid 2008 tc3 approximately 4 m diameter was tracked and studied in space for approximately 19 h before it impacted earth s atmosphere shattering at 44 36 km altitude the recovered samples 680 individual rocks comprise the meteorite almahata sitta ahs approximately 50 70 of these are ureilites ultramafic achondrites the rest are chondrites mainly enstatite ordinary and rumuruti types the goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid almahata sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts however we calculate that the mass of fallen material was 0 1 of the pre atmospheric mass of the asteroid based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples we infer that the lost material was mostly ureilitic therefore 2008 tc3 probably contained only a few percent nonureilitic materials similar to other polymict ureilites except less well consolidated from available data for the ahs meteorite fragments we conclude that 2008 tc3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites we therefore suggest that the immediate parent of 2008 tc3 was the immediate parent of all ureilitic material sampled on earth we trace critical stages in the evolution of that material through solar system history based on various types of new modeling and re evaluation of published data we propose the following scenario 1 the ureilite parent body upb accreted 0 5 0 6 ma after formation of calcium aluminum rich inclusions cai beyond the ice line outer asteroid belt differentiation began approximately 1 ma after cai 2 the upb was catastrophically disrupted by a major impact approximately 5 ma after cai with selective subsets of the fragments reassembling into daughter bodies 3 either the upb before breakup or one of its daughters after breakup migrated to the inner belt due to scattering by massive embryos 4 one daughter after forming in or migrating to the inner belt became the parent of 2008 tc3 it developed a regolith mostly 3 8 ga ago clasts of enstatite ordinary and rumuruti type chondrites were implanted by low velocity collisions 5 recently the daughter was disrupted fragments were injected or drifted into earth crossing orbits 2008 tc3 comes from outer layers of regolith other polymict ureilites from deeper regolith and main group ureilites from the interior of this body in contrast to other models that have been proposed this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 tc3 and other polymict ureilites

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

dynamical and collisional constraints on a stochastic late veneer on the terrestrial planets abstract given their tendency to be incorporated into the core during differentiation the highly siderophile elements hses in earth s mantle are thought to have been accreted as a late veneer after the end of the giant impact phase bottke et al bottke w f walker r j day j m d nesvorny d elkins tanton l 2010 science 330 1527 proposed that the large earth to moon hse abundance ratio can be explained if the late veneer was characterized by large d 1000 4000 km impactors here we simulate the evolution of the terrestrial planets during a stochastic late veneer phase from the end of accretion until the start of the late heavy bombardment 500 myr later we show that a late veneer population of 0 05m dominated by large d 1000 km bodies naturally delivers a 0 01m veneer to earth consistent with geochemical constraints the eccentricities and inclinations of the terrestrial planets are excited by close encounters with the largest late veneer bodies we find the best agreement with their post veneer orbits if either a the terrestrial planets pre veneer angular momentum deficit amd0 was less than about half of the current one amdnow or b amd0 amdnow and the veneer was limited to smaller dmax 2000 km bodies veneer impacts on venus earth and mars were mostly accretionary but on mercury and the moon they were mostly erosive in 20 of simulations an energetic impact occurred that could have removed 25 of mercury s mass thereby increasing its iron mass fraction we show that due to the erosive nature of larger impacts the moon cannot accrete any material from objects larger than 500 1000 km the large earth to moon hse abundance ratio is naturally explained if the late veneer included large impactors d 500 1000 km regardless of their size distribution as long as most of earth s veneer came from large bodies the spin angular moment imparted by stochastic late veneer impacts was far in excess of the current ones for mercury and venus meaning that their post veneer spin rates were much faster the late veneer if it included large impactors was an accretionary phase for venus earth and mars but an erosive phase for mercury and the moon

observations of seismic volcanic signals at stromboli volcano italy abstract digital recordings of ground motion during a seismic operation organized at stromboli volcano italy from may 31 1986 to june 26 1986 have been studied the amplitude of background seismic noise is greater at the summit than at the base of the volcano and the spectra calculated on different days are dominated by at least two frequencies 2 0 and 6 0 hz with subdominant frequencies at 4 1 4 8 and 8 hz according to their waveform and their spectral content three types of discrete volcanic signals are distinguishable the first type is characterized by a dominant frequency at 2 hz and a subdominant frequency at 4 1 hz the second type also has a dominant peak frequency at 2 hz with two important subdominant frequencies at 5 and 8 hz the type 3 has a sharp dominant frequency at 6 hz and subdominant frequency at 3 2 hz the frequencies observed for discrete signals and ground noise suggest that they are produced by the same repeated mechanism which probably has a volcanic origin the discrete signals are linked to explosions at the active vents within the craters

subduction and back arc activity at the hikurangi convergent margin new zealand the hikurangi margin is a region of oblique subduction with northwest dipping intermediate depth seismicity extending southwest from the kermadec system to about 42 s the current episode of subduction is at least 16 20 ma old the plate convergence rate varies along the margin from about 60 mm a at the south end of the kermadec trench to about 45 mm a at 42 s the age of the pacific lithosphere adjacent to the hikurangi trench is not known

a back arc basin formed within continental lithosphere the central volcanic region of new zealand abstract the central volcanic region is an area of quaternary volcanism and tensional tectonics within the north island of new zealand that appears to be a direct continuation of a young oceanic back arc basin the havre trough into the continental structure of new zealand volcanism is predominantly rhyolitic yet the region possesses many of the geophysical characteristics often associated with oceanic back arc basins upper mantle seismic velocities are low pn and sn velocities of 7 4 and 3 95 km s respectively the crust is anomalously thin for its continental setting and the heat flow almost all of which is expressed in the discharge of hot water through geothermal systems is about 700 mw m2 i e some twelve times greater than normal continental heat flow in order to account for such a high heat flow a large scale mass transfer of heat is required it is proposed therefore that the central volcanic region represents a site of active back arc spreading within a continental lithosphere

the kinematics of the plate boundary zone through new zealand a comparison of short and long term deformations summary surveys of shear strain rates derived from repeated triangulation cover about one quarter of the area of the plate boundary zone through new zealand and are well distributed within it using a method developed by haines relating shear strains to the total strain and velocity fields the dilatational and rotational strain rates and the velocities within the deformed zone are derived the plate boundary zone is shown to be two dimensional for the most part in that gradients of the velocity along the length of the zone are small or zero the shear strain rates are not uniform however they reach a maximum along a central axis and decrease gradually to zero toward the sides nor are the two independent components of strain one involving shortening or extension perpendicular and the other shear parallel to the zone equally developed everywhere there is a clear geographic separation with shortening developed over the subduction thrust and shear developed behind in a zone of wrench faulting time variations in the strain field in part of the region above the subduction interface are related to changes in the degree of coupling across the interface a velocity field is computed the relative velocity of the two plates from this data agree very well with that derived from seafloor spreading data these strain and velocity fields derived from geodetic studies give the short term kinematics of the plate boundary zone the long term kinematics of the zone are estimated from 1 uplift and subsidence data which are related to changes in crustal thickness and hence to dilatational strain and 2 variation in the declination of primary magnetization in rocks of known age related to the rotational strain the rate of rotational strain has increased in time from about 2 to 3 myr 1 10 myr ago to about 9 myr 1 today the increase can be understood as a secondary effect of a developing compressional component increasing the coupling between the plates so that more of the motion parallel to the zone is accommodated by distributed shear and less by aseismic slip the rates of strain averaged over the last 1 myr or so and at a scale length of a few tens of kilometres are closely similar in distribution and magnitude to the strain rates derived from short term kinematics while therefore at scale lengths of a few hundred metres or so faulting is very important and the deformation is discontinuous with unstrained blocks faulted with large but uncertain slip against adjacent blocks the deformation at these longer scale lengths is continuous and regular the two consequences of this study are that 1 strain from geodetic data can be directly related to the average long term deformation and therefore can be an important tool in the study of deformation in regions of current tectonic activity and 2 at scale lengths of a few tens of kilometres the deformation of a region can be described as the deformation of a continuum and we can ignore the effects of faulting

holocene sediment lithofacies and dispersal systems on a storm dominated back arc shelf margin the east coromandel coast new zealand abstract holocene sediment lithofacies and dispersal systems on the east coromandel shelf new zealand are mainly characterised by an accommodation dominated regime in which autochthonous siliciclastic sediments were reworked through erosional shoreface retreat during the post glacial marine transgression 12 0 6 5 ka stabilisation of sea level at its present position ca 6 5 ka initiated onshore reworking of the autochthonous deposits into fine grained regressive barrier and shoreface sands while coarser sands remained offshore to form an erosional lag inner shelf deposit modern episodes of shoreface erosion rework fine and coarse autochthonous sands offshore and northwards into very large 0 5 2 5 m 250 1500 m submarine dunes the submarine dunes are similar in form to sand ridges on the north american atlantic shelf but with crests striking perpendicular to both the shoreline and generating flows allochthonous siliciclastic lithofacies are also important aspects of east coromandel shelf sedimentation and are transported offshore from infilled estuary systems to form very fine grained upper shoreface and muddy mid shelf sands the regional geology has a strong influence on shelf lithofacies and dispersal systems off the east coromandel coast southern shelf regions are associated with deep back arc basins which have formed major late cenozoic depocentres for sediments sourced from the taupo volcanic zone further north a shallow volcanic platform extends out across the shelf and forms a barrier to any large scale along shelf dispersal of sediments consequently lithofacies in northern shelf regions tend to be highly variable and reflect local catchment lithologies

northwest africa 5958 a weakly altered cm related ungrouped chondrite not a ci3 northwest africa nwa 5958 is a carbonaceous chondrite found in morocco in 2009 preliminary chemical and isotopic data leading to its initial classification as c3 0 ungrouped have prompted us to conduct a multitechnique study of this meteorite and present a general description here the petrography and chemistry of nwa 5958 is most similar to a cm chondrite with a low degree of aqueous alteration apparently under oxidizing conditions and evidence of a second limited alteration episode manifested by alteration fronts the oxygen isotopic composition with delta o 17 4 3 parts per thousand is more o 16 rich than all cm chondrites indicating along with other compositional arguments a separate parent body of origin we suggest that nwa 5958 be reclassified as an ungrouped carbonaceous chondrite related to the cm group

efficient early global relaxation of asteroid vesta abstract the asteroid vesta is a differentiated planetesimal from the accretion phase of solar system formation although its present day shape is dominated by a non hydrostatic fossil equatorial bulge and two large mostly unrelaxed impact basins vesta may have been able to approach hydrostatic equilibrium during a brief early period of intense interior heating we use a finite element viscoplastic flow model coupled to a 1d conductive cooling model to calculate the expected rate of relaxation throughout vesta s early history we find that given sufficient non hydrostaticity the early elastic lithosphere of vesta experienced extensive brittle failure due to self gravity thereby allowing relaxation to a more hydrostatic figure soon after its accretion vesta reached a closely hydrostatic figure with 40 200 my after formation depending on the assumed depth of megaregolith the veneneia and rheasilvia giant impacts which generated most non hydrostatic topography must have therefore occurred 40 200 my after formation based on crater retention ages topography and relation to known impact generated features we identify a large region in the northern hemisphere that likely represents relic hydrostatic terrain from early vesta the long wavelength figure of this terrain suggests that before the two late giant impacts vesta had a rotation period of 5 02 h 6 3 faster than present while its spin axis was offset by 3 0 from that of the present the evolution of vesta s figure shows that the hydrostaticity of small bodies depends strongly on its age and specific impact history and that a single body may embody both hydrostatic and non hydrostatic terrains and epochs

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

thermal evolution model for the h chondrite asteroid instantaneous formation versus protracted accretion abstract we present a model of the thermal evolution of asteroids assuming an onion shell model for the h chondrite parent body we obtain constraints for the h chondrite asteroid parameters by fitting empirical h chondrite cooling ages of estacado guarena kernouve mt browne richardton allegan nadiabondi ste marguerite and forest vale by using a genetic algorithm for parameter optimisation the model improves previous calculations on the thermal history calculated in the instantaneous accretion approximation considering sintering and porosity dependent heat conduction the model is extended to include a finite growth time of the parent body to study whether the meteoritic record constrains the duration of the growth phase of the parent body where it assembles most of its mass it is found that only short accretion times of up to 0 1 ma are compatible with the empirical data on h chondrite cooling histories best fit models yield excellent agreement with the cooling age data particularly they indicate that i 26 al was the major heat source driving metamorphism while 60 fe contributed rather marginally ii maximum temperatures remained below partial melting temperatures throughout the body indicating that no partial differentiation occurred on the h chondrite parent asteroid iii the h chondrite asteroid formed 2 ma after cais briefly after most ordinary chondrite chondrules formed if 26 al abundance defines a chronological sequence