the petrology of a large intra oceanic silicic eruption the sandy bay tephra kermadec arc southwest pacific abstract the pumiceous pyroclastic deposits known as the sandy bay tuff on macauley island in the kermadec group represent a medium scale silicic eruption in an oceanic subduction setting the sandy bay eruption occurred about 6310 yr bp forming a large submarine caldera in the summit of the mainly submarine and essentially basaltic macauley volcano the sandy bay magma contained

a late cretaceous and cenozoic reconstruction of the southwest pacific region tectonics controlled by subduction and slab rollback processes abstract a cenozoic tectonic reconstruction is presented for the southwest pacific region located east of australia the reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for pacific australia and lord howe rise pacific relative plate motion the reconstruction is based on a conceptual tectonic model in which the large scale structures of the region are manifestations of slab rollback and backarc extension processes the current paradigm proclaims that the southwestern pacific plate boundary was a west dipping subduction boundary only since the middle eocene the new reconstruction provides kinematic evidence that this configuration was already established in the late cretaceous and early paleogene from 82 to 52 ma subduction was primarily accomplished by east and northeast directed rollback of the pacific slab accommodating opening of the new caledonia south loyalty coral sea and pocklington backarc basins and partly accommodating spreading in the tasman sea the total amount of east directed rollback of the pacific slab that took place from 82 ma to 52 ma is estimated to be at least 1200 km a large percentage of this rollback accommodated opening of the south loyalty basin a north south trending backarc basin it is estimated from kinematic and geological constraints that the east west width of the basin was at least 750 km the south loyalty and pocklington backarc basins were subducted in the eocene to earliest miocene along the newly formed new caledonia and pocklington subduction zones this culminated in southwestward and southward obduction of ophiolites in new caledonia northland and new guinea in the latest eocene to earliest miocene it is suggested that the formation of these new subduction zones was triggered by a change in pacific australia relative motion at 50 ma two additional phases of eastward rollback of the pacific slab followed one during opening of the south fiji basin and norfolk basin in the oligocene to early miocene up to 650 km of rollback and one during opening of the lau basin in the latest miocene to present up to 400 km of rollback two new subduction zones formed in the miocene the south dipping trobriand subduction zone along which the solomon sea backarc basin subducted and the north dipping new britain san cristobal new hebrides subduction zone along which the solomon sea backarc basin subducted in the west and the north loyalty south fiji backarc basin and remnants of the south loyalty santa cruz backarc basin subducted in the east clockwise rollback of the new hebrides section resulted in formation of the north fiji basin the reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere a change in plate kinematics or slab mantle interaction

intra oceanic subduction related hydrothermal venting kermadec volcanic arc new zealand intra oceanic volcanic arcs mark the boundaries between converging lithospheric plates where subduction produces volcanic and tectonic activity that ensures a steady supply of magmatic heat and hydrothermal fluids to the seafloor here we report on the first broad and systematic survey of hydrothermal emissions generated along a submarine arc front more than half seven of 13 of the volcanoes surveyed along 260 km of the southern kermadec arc ne of new zealand are hydrothermally active our results indicate that volcanic arcs represent a previously unheeded but potentially extensive source of shallow 2 km water depth vent fields expelling fluids of a unique and heterogeneous composition into the oceans

backarc rifting constructional volcanism and nascent disorganised spreading in the southern havre trough backarc rifts sw pacific high resolution multibeam em300 and seabeam data of the southern havre trough sht combined with observations and sample collections from the submersible shinkai6500 and deep tow camera are used to develop a model for the evolution and magmatism of this backarc system the havre trough and the associated kermadec arc are the product of westward subduction at the pacific australian plate boundary detailed studies focus on newly discovered features including a seamount saito seamount and a deep graben ngatoroirangi rift 4000 m water depth floored with a constructional axial volcanic ridge 5 km in length and in excess of 200 m high both of which are characterised by pillow and lobate flows estimated at elongate volcanic ridges at 35 15 s and 34 30 s and backarc seamounts 35 30 s 178 30 e occur at the eastern margin of the sht similar seafloor morphology is observed in the central and western portions of the basin suggesting that recent volcanism may be broadly distributed across the backarc mass balance modelling indicates a maximum crustal thickness of 11 km to 4000 m spreading systems these discontinuous spreading systems are characterised by failed rifts rift segmentation and propagation successive episodes of magmatic intrusion into thinned faulted arc basement results in defocused asymmetrical accretion cross arc volcanic chains isolated volcanoes and underlying basement plateaus are interpreted to represent a cap of recent extrusives however they may also be composed entirely of newly accreted crust and the spatially extensive basement fabric of elongated volcanic ridges may be the surface expression of pervasive dike intrusion that has thoroughly penetrated and essentially replaced the original arc crust with newly accreted intrusives

solute provenance transport and denudation in a high arctic glacierized catchment recent understanding of chemical weathering in glacierized catchments has been focused on mid latitude alpine catchments comparable studies from the high latitudes are currently lacking this paper attempts to address this deficiency by examining solute provenance transport and denudation in a glacierized catchment at 78 n in the svalbard high arctic archipelago representative samples of snow glacier ice winter proglacial icing and glacier meltwater were obtained from the catchment during spring and summer 1993 and analysed for major ion chemistry seasonal variations in the composition of glacier meltwater occur and are influenced by proglacial solute acquisition from the icing at the very start of the melt season and subsequently by a period of discharge of concentrated snowmelt caused by snowpack elution weathering within the ice marginal channels that drain the glacier particularly carbonation reactions continues to furnish solute to meltwater when suspended sediment concentrations increase later in the melt season partitioning the solute flux into its various components sea salt crustal aerosol and atmospheric sources shows that c 25 of the total flux is sea salt derived consistent with the maritime location of the glacier and c 71 is crustally derived estimated chemical denudation 160 meq m 2 a 1 sea salt corrected cation equivalent weathering rate is somewhat low compared with other studied glacierized catchments estimates in the range 450 1000 meq m 2 a 1 which is probably attributable to the relatively short melt season and low specific runoff in the high arctic a positive relationship was identified between discharge and co 2 drawdown owing to carbonation reactions in turbid meltwater

submarine hydrothermal activity along the mid kermadec arc new zealand large scale effects on venting 1 the 2 500 km kermadec tonga arc is the longest submarine arc on the planet here we report on the second of a series of cruises designed to investigate large scale controls on active hydrothermal venting on this arc the 2002 nzaplume ii cruise surveyed 12 submarine volcanic centers along 580 km of the middle kermadec arc mka extending a 1999 cruise that surveyed 260 km of the southern kermadec arc ska average spacing between volcanic centers increases northward from 30 km on backarc crust along the ska to 45 km on backarc crust along the southern mka to 58 km where the mka joins the kermadec ridge volcanic cones dominate in the backarc and calderas dominate the kermadec ridge the incidence of venting is higher along the mka 83 10 of 12 volcanic centers than the ska 67 8 of 12 but the relative intensity of venting as given by plume thickness areal extent and concentration of dissolved gases and ionic species is generally weaker in the mka this pattern may reflect subduction of the 17 km thick oceanic hikurangi plateau beneath the ska subduction of this basaltic mass should greatly increase fluid loss from the downgoing slab initiating extensive melting in the upper mantle wedge and invigorating the hydrothermal systems of the ska conversely volcanic centers in the southern mka are starved of magma replenishment and so their hydrothermal systems are waning farther north where the mka centers merge with the kermadec ridge fewer but larger magma bodies accumulate in the thicker older crust ensuring more widely separated caldera dominated volcanic centers

new glass fission track ages of middle pleistocene tephras on yakushima island southern japan abstract the ages of two hornblende bearing middle pleistocene rhyolitic tephra beds on yakushima island southern japan have been determined by the glass itpft method koseda pyroclastic flow deposit is 0 58 0 08 ma and anbo tephra an is 0 78 0 09 ma the normal geomagnetic polarity of an demonstrates a stratigraphic position just above the bruhnes matuyama boundary both volcanic units are the result of large magnitude eruptions from a source that is most probably located at one of the calderas south of the central part of the kagoshima graben southern kyushu given the low total alkali content of their glass shards they must have been widely distributed across kyushu and central japan although no other occurrences are presently known new information on their mineralogy glass composition and age will greatly facilitate recognition of their distal correlatives although not explored in this study these two tephra beds will undoubtedly play a useful role in future studies aimed at elucidating the quaternary paleoenvironmental history of yakushima island

the tectonomagmatic source of ore metals and volatile elements in the southern kermadec arc deciphering the ultimate source of chalcophile metals e g cu zn pb in volcanogenic massive sulfide vms deposits and the volatiles that help drive their formation is critical for understanding where how and why vms deposits form the southern kermadec arc volcanic front is known to be highly hydrothermally active and host to at least three vms deposits whereas the associated back arc system is apparently hydrothermally inactive although this may simply be due to a lack of exploration in this region we have analyzed major trace and volatile element concentrations in a suite of basaltic glasses and olivine hosted melt inclusions from volcanoes ridges and rifts of the southern kermadec arc volcanic front and havre trough back arc basin these data indicate clear compositional differences in the mantle beneath the arc front and the back arc with the arc front having higher extents of prior melt extraction and enrichment in volatile and metal elements from slab derived aqueous fluids the magmatic budget of pb is supplied to the mantle source by these slab derived fluids whereas magmatic concentrations of zn and cu are primarily controlled by the degree of partial melting with a cu bearing residual phase required in the mantle consequently the relative enrichment of chalcophile metals in the mantle derived melts is pb cu zn the magma volatile history recorded by the glasses and melt inclusions indicates that degassing during crystal fractionation is significant and notably leads to the reduction of sulfur in the evolving magma as oxidized sulfur degasses preferentially the apparent absence or retention of volatiles notably co 2 and so 3 in magma chambers lack of magmatic activity and absence of hot mafic dikes beneath volcanoes may be important factors in inhibiting the formation of active hydrothermal venting and by extension the potential for vms deposition

crustal structure and seismicity of the havre trough at 26 s an ocean bottom seismographic obs experiment was carried out to investigate the present day tectonics at the northern end of the havre trough a backarc basin in the southwestern pacific hypocenter distribution shows no seismicity within 50 km of the obs array but two active regions outside the trough one is to the northeast of the array at a bathymetric high between the lau basin and havre trough the other is to the southeast of the array beneath the landward slope of the kermadec trench p wavespeed structure obtained from the refraction survey is characterized by a thinned island arc crust with a thickness of 9 10 km many volcanic intrusions shown in reflection profiles that support extension tectonics are covered by undisturbed sediments these suggest little present activity related to rifting and or spreading

dike intrusion and displacement accumulation at the intersection of the okataina volcanic centre and paeroa fault zone taupo rift new zealand abstract we examine the relationship between dike emplacement and normal fault displacement along the south western margin of okataina volcanic centre new zealand at this location dikes including a basaltic dike up to 5 m wide at the ground surface intruded during the 1886 ad tarawera eruption formed over the last 22 kyr in the tarawera linear vent zone within a few kilometres of the paeroa fault zone displacement analysis and mapping of fault scarps indicate that fault and dike extension are complementary over time periods of tens of thousands of years extension that is primarily accommodated by faulting outside the volcanic complex is achieved by dike intrusion and faulting within it historical observations following the 1886 ad eruption and previously published active fault trenching collectively indicate however that dike intrusion generally does not induce slip within the paeroa fault zone close to the lateral dike tip we suggest that displacement in the paeroa fault zone outside the okataina volcanic centre is generally not triggered by eruptions and that this fault is principally tectonic in origin

from rifting to active spreading in the lau basin havre trough backarc system sw pacific locking unlocking induced by seamount chain subduction associated with pacific australia plate convergence the lau basin havre trough is an active back arc basin that has been opened since 5 5 ma by rifting and southward propagating oceanic spreading current back arc opening rates decrease from 159 mm yr 1 in the northern lau basin to 15 mm y 1 in the southern havre trough major tectonic changes occur at the transition between havre trough rifting and full oceanic spreading of the eastern lau spreading center elsc where the oblique to trench westward subducting louisville seamount chain lsc sweeps southwards along the tonga trench new swath bathymetry seismic reflection data and limited rock sampling in this area constrain a tectonic and kinematic back arc model that incorporates the effects of lsc subduction the elsc which extends southward to 24 55 s forms a deep rift valley propagating southward through older rifted arc basement present day seismicity and fresh and fractured pillow lavas at 23 42 s are consistent with rift valley neovolcanism conversely the northern havre trough has low seismicity and rifted volcanic basement ridges trending 25 45 oblique to the basin axis consistent with low levels of extensional tectonism and volcanism this latter structural fabric is interpreted as an early stage of rifting that is now locked due to compression on the arc exerted by lsc subduction while in the lau basin such effects have passed as the lsc swept along the tonga trench it is proposed that the lau havre back arc opening is controlled by tectonic constraints exerted at the limits of the system by the lsc subduction which determines the southward migration of the tonga arc pole of rotation and associated lau basin opening a discrete three stage back arc opening evolution is proposed comprising 1 an initial phase of back arc rifting along the whole length of the plate boundary beginning at 6 5 ma 2 a subsequent phase mostly present in the southern part of the back arc domain and still active in the havre trough of transpression and transtension starting at 4 ma in the north as the lsc starts to subduct and sweeps southward along the tonga trench and 3 a renewed opening phase in the northern segment of the back arc domain with rifting and spreading starting at 3 5 ma as subduction of the lsc along the northern tonga trench is progressively completed

oblique rifting along the central and southern kermadec arc front 30 36 s sw pacific we present a series of eleven structural maps of the central kermadec arc front between 30 s and 35 50s interpreted from multibeam bathymetric data these maps show the structural morphology of eighteen submarine arc volcanoes proximal to the havre backarc providing a critical window into the tectonomagmatic processes that operate at divergent arcs transitional to seafloor spreading each arc volcano unaffected by subaerial processes shows the structural imprint of oblique backarc rifting providing an exceptional opportunity to evaluate tectonic influences on edifice construction and destruction mapped seafloor structures include faults volcanic ridges aligned vents and cones cones aligned with faults elongate calderas and structurally controlled sector collapse in addition to the structural maps we present the results of quantitative analyses of fault orientation and growth behaviors we demonstrate that 50 of identified faults strike ne sw between 030 and 060 with a modal value of 047 evidence in support of an extension axis dextrally oblique to the nne trend of the rift further to this minimum and maximum values for the extension axis are calculated as 137 and 150 perpendicular to the modal strikes for all identified faults and for faults within the most oblique surveyed backarc rift domain respectively these values confirm moderate obliquity of opening along the havre rift with the sht opening slightly less obliquely than the nht consistent with accommodation of a lesser partitioned component of dextral pacific australian plate convergence

structural trends and back arc extension in the havre trough the havre trough is an active back arc ocean btsin located to the west of the kermadec trench arc system in the southwest pacific two recent cruises have provided reconnaissance swaths of sea beam bathymetry and gloria side scan sonar data through much of the havre trough as well as a detailed survey at 33 5 s allowing the overall structural character of the trough and the nature of crustal extension within the detailed survey area to be assessed the orientation of the tectonic fabric within the havre trough is consistently oblique to the basin as a whole trending 045 to 060 while the bounding kermadec and colville ridges trend 015 to 025 the oblique internal structure indicates that the havre trough has not opened perpendicular to its own trend within the detailed survey area a series of short 15 35 km long en echelon basins are interpreted as an active spreading center a lack of sediment cover and the dredging of fresh basalt from one of the 3500 4000 m deep basins confirms the youth of these features no recent extensional features are observed in a gloria crossing of the trough at 31 5 s suggesting that this part of the havre trough may currently be inactive

forearc structures and tectonic regimes at the oblique subduction zone between the hikurangi plateau and the southern kermadec margin the convergent southern kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic hikurangi plateau swath bathymetric data have been combined with other geophysical data to reveal that the south kermadec inner trench wall is segmented into distinct upper middle and lower margins the lower margin is a nongrowing compressionally unstable accretionary wedge that is dissected by transcurrent faults the wedge has been thinned and narrowed as a result of tectonic erosion the middle margin is stable by comparison and separated from the lower margin by the 220 km long awanui fault this fault is a landward dipping strike slip reverse fault the middle upper margin including east cape ridge ecr forms a deforming backstop with a sharp along strike change in tectonic regime south of 36 40 s transpression characterizes the ecr shear zone north of 36 40 s the ecr is marked by uplift by extension and farther north by subsidence it is suggested that subsidence and subsequent tectonic erosion of the margin north of 35 50 s in the wake of hikurangi plateau subduction have broken the link between once continuous segments of the ecr tonga platform the unstable nature of the southern kermadec wedge under compression together with a comparison of its taper characteristics with those of modern convergent margins implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime strike slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the hikurangi plateau

morphometric analysis of the submarine arc volcano monowai tofua kermadec arc to decipher tectono magmatic interactions morphometric analysis of multibeam bathymetry and backscatter data is applied to monowai a submarine volcano of the active tofua kermadec arc to map and document the structure and evolution of the volcanic centre low rates of erosion and sedimentation and pervasive tectonic and magmatic processes allow quantification through detailed structural analysis and measurement of deformation the slope aspect curvature rugosity and hydrology flow tools of arcgis provide a robust structural interpretation and the development of a model of monowai evolution a nested caldera structure with a volume of 31 km3 and a stratovolcano of 18 km3 dominate the magmatic constructs the outer caldera is elongate along 125 and the inner caldera along 135 numerous parasitic cones and fissure ridges are also observed oriented at 039 and 041 respectively northeast trending faults with a regional average strike of 031 are widespread within this part of the backarc forming a nascent rift graben to the west of the monowai caldera complex the distribution of throw varies spatially reaching a maximum total along rift of 320 m and across rift of 120 m with greater throw values measured in the west elongation directions of the two nested calderas are near perpendicular to the trends of faults and fissure ridges the inner caldera is more orthogonal to the magmatic constructs fissure ridges and aligned vent cones and the outer caldera is approximately orthogonal to the regional fault fabric suggesting a strong interaction between magmatic and tectonic processes and the directions of the horizontal principal stress we present a detailed morphometric analysis of these relationships and the data are used to interpret the spatial and temporal evolution of the tectono magmatic system at monowai and classify the type of rifting as transtensional similar analysis is possible elsewhere in the kermadec backarc and within other regions of submarine volcanism

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

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

role of ridge jumps and ridge propagation in the tectonic evolution of the lau back arc basin southwest pacific gloria sidescan imagery of the central lau basin reveals a complex neotectonic framework in which the central lau spreading center is actively propagating southward at the expense of the eastern lau spreading center the latter is located about 75 km closer to the active tofua arc and the transfer of spreading motion across this distance is achieved via a relay spreading ridge segment new magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern lau spreading center between 5 ma and the present day the processes of arcward ridge migration ridge propagation and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back arc spreading and may be a common feature of this tectonic environment

the elution of ions through field and laboratory snowpacks it has been recognised for many years that the first fraction of meltwater from a melting snowpack contains a much higher concentration of ions than that of the bulk or average snow of which the pack is comprised foster 1 978 johannessen and henriksen 1978 this process leads to the so called acid flush in the spring and can have severe ecological effects on lakes and streams hagen and langeland 1 973 leivestad and muniz 1 976 national research council of canada 1 981 we have studied elution of ions through a snowpack on folgefonna near bergen norway and in the laboratory in the field we collected meltwater samples at hourly intervals for a period of 4 d at the start of the melt season and analyzed them for ph no so ci na mg2 ca2 and k 2 m cores were also collected at the beginning and end of the experiment in order to study bulk changes in snow composition the meltwaters showed a diurnal cycle with high ionic concentrations around noon with no and so levels rising by factors of up to four and six fold compared to the average concentrations on the first day cllevels peaked much later after the no and so levels had decreased and only reached 2 3 times their lowest concentrations ph values were strongly correlated with no and s02levels suggesting that a significant proportion of these ani ns wer in the form of strong acids the laboratory experiments involved slow melting of snow samples collected in the cairngorm mountains scotland and also showed that no and so and also mg2 and k ions were removed from the snow preferentially whilst na and cltended to remain longer the position of h within the ion elution sequence is unclear due to uncertainties in the absolute determination of ph in the field measurements but the laboratory experiments confirm the differential rates of elution shown in the norwegian snowpack and reaffirms its importance in the acidification of streams during spring

hot fingers in the mantle wedge new insights into magma genesis in subduction zones abstract quaternary volcanoes in the northeast japan arc can be grouped into 10 volcanic clusters striking transverse to the arc these have an average width of 50 km and are separated by parallel gaps 30 75 km wide this clustering of volcanic centres topographic profiles low velocity regions in the mantle wedge and local negative bouguer gravity anomalies along the japan sea side of the volcanic arc are closely correlated all these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined 50 km wide fingers each of the 10 fingers recognised extends from deep mantle 150 km below the back arc region towards the shallower mantle 50 km beneath the volcanic front quaternary volcanoes are built immediately above the hot mantle fingers the volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust accompanied by quaternary volcanic activity at the surface although volcanic activity is rare along the japan sea coast tomographic results show that hot low velocity mantle fingers exist within the mantle wedge the negative bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers these have not yet been erupted but have accumulated at the moho discontinuity

point counting technique for fission track dating of tephra glass shards and its relative standard error fission track dating of glass is potentially an efficient technique for tephrochronological studies however most glasses from quaternary tephra have very low spontaneous track densities and track counting is laborious the point counting technique is a simplified procedure for evaluating fission track areal densities in a glass shard population that allows the analyst to count larger numbers of tracks in significantly shorter times a field of view is coded as 1 if its center is on glass and is 0 otherwise and the total area of glass is estimated by counting the number of 1 s under simple assumptions the binomial distribution appears to provide a reasonable evaluation of the additional experimental error in a fission track density determination by this technique glass shard populations have been simulated assuming that glass areas are random samples from distributions analogous to real ones the effect of variation in glass area on the relative standard error of the estimated track density is found to be small the point counting technique significantly enhances the potential of fission track dating especially for very low track densities typical of most quaternary tephra as the additional error associated with its use is largely counterbalanced by the greater number of tracks that can be counted

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

structure and structural development of the havre trough sw pacific northeast of new zealand the pacific plate is obliquely subducted beneath the overriding australian plate major differences appear in the oblique convergence driven structures that occur along the plate boundary while strike slip component of convergence is accommodated in the forearc domain to the south at the eastern north island margin we here substantiate that at least part of the strike slip component is accommodated within the back arc havre trough to the north offshore new zealand new swath bathymetry and structural data collected in the havre back arc domain show that the basin as a whole displays oblique to basin axis structures that allow considering the basin to be the loci of extensive tectonics associated with dextral displacement of the kermadec arc block relative to the australian plate therefore where the two plates are oceanic the oblique convergence between pacific and australian plates is partitioned at the subduction trench for its normal to trench component and at the rear of the kermadec arc block for part or all of its along strike component where it is intimately associated with extensional rifting within the back arc domain the kinematic pattern of australian pacific plate boundary northeast of new zealand that straddles a limit between continental and oceanic lithosphere allows addressing the specificity of the structural development the deformation distribution and the assessment of movement components that occur in a back arc basin associated with oblique convergence

location and migration of miocene quaternary volcanic arcs in the sw pacific region abstract we report new ar ar and u pb ages from ten rocks in the sw pacific region our results 1 establish a northward late miocene australian plate movement rate of 57 mm a for the lord howe hotspot chain 2 reinforce the previously established widespread nature of early miocene subduction related volcanism in onland and nearshore northern new zealand 3 indicate that leucotonalite xenoliths from raoul island are the products of quaternary kermadec arc magmatism rather than being cretaceous pliocene basement a synthesis of available sw pacific data emphasises that while there is a reasonably complete record of subduction related volcanism from at least 23 ma to the present day the process of back arc basin formation is highly episodic and asymmetric subduction related arcs stabilised along the taupo kermadec tonga arc from 2 ma to the present along the taranaki colville lau trend from 17 6 ma and along the three kings ridge northland plateau northland trend from 23 18 ma south of latitude 25 s back arc basin opening occurred during the early miocene and quaternary arc volcanic episodes but does not appear to have accompanied the 17 5 ma volcanism

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

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

east pacific rise at 21 n the volcanic tectonic and hydrothermal processes of the central axis abstract photographs obtained by the angus survey system at 21 n reveal many similarities to the geological processes delineated at other spreading centers and in particular those observed in the galapagos rift at 86 w the region of recent volcanism is restricted to a narrow zone zone 1 approximately 1 km wide this suggests that the width of the magma chamber feeding these flows is also narrow at the top variations in sediment cover are used to subdivide the flows within zone 1 into three sheet flow pillow flow pairs the youngest pair appears to have erupted form a linear fissure 8 km long running parallel to the valley axis this fissure is part of a larger en echelon pattern of eruptive fissures active hydrothermal vents associated with the youngest flows are situated directly above the apparent eruptive fissure the high 350 c temperature for some of the fluids exiting from the vents suggests a highly restricted circulation system involving at times little to no mixing with cold seawater the lava terrain in the remainder of zone 1 and bordering regions is characterized by extensive fracturing further suggesting a narrow upper dimension to the magma chamber and also that subsurface hydrothermal deposition of massive sulfides may lead to a reduction in the mixing processes

geology hydrothermal activity and sea floor massive sulfide mineralization at the rumble ii west mafic caldera abstract sea floor imagery volcanic rock massive sulfide and hydrothermal plume samples 3 he ph dissolved fe and mn and particulate chemistry have been collected from the rumble ii west volcano southern kermadec arc new zealand rumble ii west is a caldera volcano with an 3 km diameter summit depression bounded by ring faults with a resurgent central cone rocks recovered to date are predominantly mafic in composition i e basalt to basaltic andesite with volumetrically lesser intermediate rocks i e andesite on the basis of its size geometry volcanic products and composition rumble ii west can be classified as a mafic caldera volcano rumble ii west has a weak hydrothermal plume signature characterized by a small but detectable 3 he anomaly 25 time series light scattering data though obtained from vertical casts and tow yos do show that hydrothermal activity has increased in intensity between 1999 and 2011 massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite with lesser sphalerite pyrite and traces of galena the weak hydrothermal plume signal indicates that the volcano is in a volcanic hydrothermal quiescent stage compared to other volcanoes along the southern kermadec arc although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past of the volcanoes along the kermadec tonga arc known to host massive sulfides i e clark rumble ii west brothers monowai volcano 19 and volcano 1 the majority five out of six are dominantly mafic in composition and all but one of these mafic volcanoes form moderate size to large calderas to date mafic calderas have been largely ignored as hosts to sea floor massive sulfide deposits that 75 of the presently known massive sulfide bearing calderas along the arc are mafic in composition the dacitic brothers volcano is the exception has important implications for sea floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land

structure and petrology of newly discovered volcanic centers in the northern kermadec southern tofua arc south pacific ocean 1 the nzaplume iii expedition of september october 2004 to the northern kermadec southern tofua nkst arc between 28 52 s and 25 07 s resulted in the discovery of at least seven new submarine volcanic centers and a substantial caldera complex adjacent to the previously known monowai seamount the volcanic centers form a sublinear chain that coincides with the kermadec ridge crest in the south hinetapeka and diverges 45 km westward of the ridge crest in the north v just to the south of where the louisville ridge intersects with the arc all of the centers contain calderas or caldera like structures as well as multiple cones domes fissure ridges and vent fields all show signs of recent eruptive and current hydrothermal activity there are strong structural controls on edifice location with cones and fissure ridges typically associated with faulting parallel to the regional 12 strike of the arc front several of the calderas are ellipsoidal orientated northwest southeast in the general direction of least compressive stress sampled volcanic rocks representing the most recently erupted lavas are all low k tholeiites two of the centers gamble and rakahore yielded only high silica dacite to rhyolite 69 74 wt silica whereas two others monowai and v yielded only basalt to andesite 48 63 wt silica mineral assemblages are plagioclase pyroxene dominated with accessory fe ti oxides apatite olivine and quartz tridymite cristobalite typical of dry volcanic arc systems hornblende occurs only in a felsitic rhyolite from hinepuia volcanic center and zircon is absent glass contents range to 57 in basalts andesites mean 20 and 97 in andesites rhyolites mean 59 and other quench textures including swallow tailed plumose or dendritic crystal forms and crystallites are common most lavas are highly vesicular 63 mean 28 and have low volatile contents mostly 2 wt which together with the occurrence of tridymite or cristobalite indicates explosive eruption and rapid cooling exceptions are rocks from u volcanic center which have low vesicularity and low glass contents across a wide compositional range indicating effusive eruption disequilibrium mineral textures the frequent occurrence of xenoliths and xenocrysts and macroscopic evidence for magma mingling indicate that many of the lavas are hybrids having resided only a short time in upper crustal reservoirs prior to eruption silicic magmas are major components of nkst arc volcanism and caldera formation is the dominant eruptive style the scale of silicic magmatism is in marked contrast to the dominant basaltic andesitic magmatism in the southern kermadec arc with evidence from other arcs silicic magmatism is now recognized as a major feature of intraoceanic arcs globally

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

isothermal plateau fission track ages of hydrated glass shards from silicic tephra beds abstract the age of silicic tephra beds that have experienced a simple thermal history can be accurately and precisely determined by application of the isothermal plateau fission track technique to hydrated glass shards age estimates obtained by this means compare very well with those based on co existing mineral phases using the k ar 40ar 39ar external detector fission track and thermoluminescence techniques this concordance contradicts the popular belief that hydrated glass shards give unreliable ages distal tephra beds many of which are difficult to date because of their fine grainsize low abundance of crystals and common presence of detrital grains are readily dated by the isothermal plateau glass fission track method this new advance should greatly facilitate the development of detailed chronologies of tephra bearing sedimentary sequences located far from volcanic centres

the role of tephras in developing a high precision chronostratigraphy for palaeoenvironmental reconstruction and archaeology in southern kyushu japan since 30 000 cal bp an integration abstract tephras are important for the chronostratigraphy of palaeoenvironmental and archaeological records in southern kyushu because numerous tephra beds enable these records to be connected and dated precisely using tephrochronology a regional tephra based framework or lattice for the past 30 000 calendar cal years is proposed in the style of recent intimate projects elsewhere we review stratigraphic compositional age and distributional data for a range of tephras and comment on the relationship of several marker tephras to distinct palaeoenvironmental and cultural events in the southern kyushu region since 30 000 cal bp more than 90 visible tephra beds are recorded deriving from kakuto aira ata and kikai volcanic centres which incorporate large calderas and the tokara volcanic islands the tephra record is underpinned by two widespread tephras aira tn tephra at ca 30 000 cal bp and kikai akahoya tephra k ah ca 7300 cal bp in addition to at and k ah locally distributed marker tephra beds are related to the chronostratigraphy of terrestrial and marine palaeoenvironments including sea level and coastal environments vegetation and culture some tephras notably at k ah and sakurajima satsuma tephra sz s ca 12 800 cal bp have been recognized in marine and laminated lake sediments and hence dated with high precision thereby facilitating correlation of those records to global high resolution ice marine and terrestrial stratotypes sz s is a useful isochron for the last deglaciation in southern kyushu our integrated regional chronostratigraphic model for southern kyushu founded by tephras and with further potential via systematic identification of cryptotephras will enable change in regional palaeoenvironments and culture to be evaluated in national and global contexts

rapid microplate rotations and backarc rifting at the transition between collision and subduction using global positioning system velocities from convergent plate boundaries in papua new guinea new zealand tonga vanuatu and the marianas we note a spatial correlation between rapid tectonic block rotations and the transition from subduction to collision we present a mechanism for the block rotations in which the change from collision of a buoyant indentor to normal subduction exerts a torque on the upper plate microplate this work improves our understanding of the causes of rapid vertical axis rotations often observed in paleomagnetic studies we also show how collision induced rotations may lead to backarc rifting

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

ion elution through shallow homogeneous snow controlled melting of snowpacks in the laboratory demonstrates the importance of the initial distribution of chemical species within the pack mesoscale and within the snow grains microscale to the timing and relative concentrations of meltwater released from the base of the pack acid and salt tracers were applied to homogeneous snow in order to investigate these effects species deposited near the top of the pack eluted out considerably before those more uniformly distributed with depth also about 80 of the salts dosed at the top of a 0 4 m pack were removed in the first 20 of the meltwater versus 70 of a salt dosed at middepth both the top and middepth tracers were present in the first fractions of meltwater draining from the pack however indicating that there is partial mixing of percolating meltwater with solutes in an immobile surface phase on grains deeper in the pack concentration of species on the outside of grains by seasonal metamorphism and on the outside of multigrain particles and clusters during melt freeze will enhance their early removal different ions that were present in the snow with the same mesoscale and microscale conditions prior to melt came out at the same relative concentrations i e they did not exhibit preferential elution

mifil a method to characterize seafloor swells with application to the south central pacific 1 we propose a filtering method to characterize large scale depth anomalies the mifil method for minimization and filtering requires two stages a first one to roughly remove the volcano component by minimizing the depth anomaly and a second one to smooth the shape and totally remove the small spatial length scale remaining topography using a median filter the strength of this method directly applicable on two dimensional grids is that it does not require any assumption on the location amplitude or width of the large scale feature to characterize except its minimal width we only consider the spatial length scale of the features to remove application to volcanic chains of the south central pacific is presented and the results lead to a better understanding of the tectonics and volcanism emplacement of the zone the society is the only classical hot spot that corresponds to the simple interaction of a plume with the lithosphere and for which a buoyancy flux of 1 58 0 15 mg s 1 is obtained the marquesas volcanic chain although quite comparable presents a swell morphology that prevents such interpretation and quantification for the tuamotu and cook austral volcanic chains no reliable quantification can be made because the depth and geoid anomalies are caused by several phenomena occurring at different depths that cannot be separated

crustal structure of the kermadec arc from mango seismic refraction profiles three active source seismic refraction profiles are integrated with morphological and potential field data to place the first regional constraints on the structure of the kermadec subduction zone these observations are used to test contrasting tectonic models for an along strike transition in margin structure previously known as the 32 s boundary we use residual bathymetry to constrain the geometry of this boundary and propose the name central kermadec discontinuity ckd north of the ckd the buried tonga ridge occupies the forearc with vp 6 5 7 3 km s 1 and residual free air gravity anomalies constrain its latitudinal extent north of 30 5 s width 110 20 km and strike 005 south of 25 s south of the ckd the forearc is structurally homogeneous down dip with vp 5 7 7 3 km s 1 in the havre trough backarc crustal thickness south of the ckd is 8 9 km which is up to 4 km thinner than the northern havre trough and at least 1 km thinner than the southern havre trough we suggest that the eocene arc did not extend along the current length of the tonga kermadec trench the eocene arc was originally connected to the three kings ridge and the ckd was likely formed during separation and easterly translation of an eocene arc substrate during the early oligocene we suggest that the first order crustal thickness variations along the kermadec arc were inherited from before the neogene and reflect mesozoic crustal structure the cenozoic evolution of the tonga kermadec hikurangi margin and along strike variations in the duration of arc volcanism

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

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

hikurangi plateau a cretaceous large igneous province in the southwest pacific ocean the first dredge haul of basement rocks obtained from the hikurangi plateau in the southwest pacific ocean consists dominantly of volcanic and volcaniclastic rocks of probable pre late cretaceous age all samples have undergone extensive seafloor weathering to phyllosilicate and zeolite bearing assemblages petrography mineral chemistry and whole rock element concentrations and ratios of the least altered lavas e g tio2 1 3 1 6 wt cr 132 224 ppm zr y 2 9 3 6 zr nb 19 29 ti v 22 29 suggest a restricted igneous compositional range broadly comparable with normal to enriched mid ocean ridge basalt however isotopic compositions of leached samples 87sr 86sri 0 70361 0 70374 endi 5 7 6 2 are comparable with oceanic island basalt suites collectively these petrological characteristics are similar to rocks from large igneous provinces of cretaceous age in the western pacific ocean e g manihiki and ontong java plateaus consequently we interpret the hikurangi plateau as another of these early cretaceous basaltic oceanic plateaus speculatively parts of the obducted tangihua and matakaoa igneous complexes of the north island may have been derived from the hikurangi plateau as the plateau collided with the new zealand continent in the early miocene

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

controls on segmentation and morphology along the back arc eastern lau spreading center and valu fa ridge back arc spreading centers increasingly depart from mid ocean ridge mor characteristics with proximity to the arc volcanic front the close association of these departures with slab derived materials in erupted lavas suggests that subduction related chemical effects are their primary cause the eastern lau spreading center elsc and valu fa ridge vfr are type examples of this process together they constitute a first order spreading center in the lau back arc basin that progressively converges on the tofua arc volcanic front from north to south here we use ship multibeam and deep towed side scan sonar data to examine variations in axial morphology and volcanism at the second and third order segment scale along these ridges and develop a model for the processes that control them closest to the arc vfr and the southern segment of the elsc shoal toward second order segment ends in contrast to mors northward and beyond 70 km from the arc the axis becomes abruptly deeper and flatter and no longer shoals toward second order segment ends at vfr along axis topographic highs correlate with the location of arc volcanoes along slab flow lines these correlations are weaker along the southernmost elsc segment and absent along elsc segments farther north the observations show a modulation of back arc segmentation with arc proximity that rapidly diminishes with distance they support a model of the mantle wedge with a strongly hydrous domain within 70 km of the arc within which the arc and ridge interact and a much less hydrous domain farther from the arc without evident arc ridge interactions

contrasting pyroclast density spectra from subaerial and submarine silicic eruptions in the kermadec arc implications for eruption processes and dredge sampling pyroclastic deposits from four caldera volcanoes in the kermadec arc have been sampled from subaerial sections raoul and macauley and by dredging from the submerged volcano flanks macauley healy and the newly discovered raoul sw suites of 16 32 mm sized clasts have been analyzed for density and shape and larger clasts have been analyzed for major element compositions density spectra for subaerial dry type eruptions on raoul island have narrow unimodal distributions peaking at vesicularities of 80 85 whereas ingress of external water wet type eruption or extended timescales for degassing generate broader distributions including denser clasts submarine erupted pyroclasts show two different patterns healy and raoul sw dredge samples and macauley island subaerial emplaced samples are dominated by modes at 80 85 implying that submarine explosive volcanism at high eruption rates can generate clasts with similar vesicularities to their subaerial counterparts a minor proportion of healy and raoul sw clasts also show a pink oxidation color suggesting that hot clasts met air despite 0 5 to 1 km of intervening water in contrast macauley dredged samples have a bimodal density spectrum dominated by clasts formed in a submarine eruptive style that is not highly explosive macauley dredged pyroclasts are also the mixed products of multiple eruptions as shown by pumice major element chemistry and the sea floor deposits reflect complex volcanic and sedimentation histories the kermadec calderas are composite features and wide dispersal of pumice does not require large single eruptions when coupled with chemical constraints and textural observations density spectra are useful for interpreting both eruptive style and the diversity of samples collected from the submarine environment

new multibeam mapping and geochemistry of the 30 35 s sector and overview of southern kermadec arc volcanism new multibeam mapping and whole rock geochemistry establish the first order definition of the modem submarine kermadec arc between 30 degrees and 35 degrees s twenty two volcanoes with basal diameters 5 km are newly discovered or fully mapped for the first time giggenbacb macauley havre haungaroa kuiwai ngatoroirangi sonne kibblewhite and yokosuka for each large volcano edifice morphology and structure surficial deposits lava fields distribution of sector collapses and lava compositions are determined macauley and havre are large silicic intra oceanic caldera complexes for both concentric ridges on the outer flanks are interpreted as recording mega bedforms associated with pyroclastic density flows and edifice foundering other stratovolcanoes reveal complex histories with repeated cycles of tectonically controlled construction and sector collapse extensive basaltic flow fields and the development of summit craters and or small nested calderas combined with existing data for the southernmost arc segment we provide an overview of the spatial distribution and magmatic heterogeneity along similar to 780 km of the kermadec arc at 30 degrees 36 degrees 30 s coincident changes in arc elevation and lava composition define three volcano tectonic segments a central deeper segment at 32 degrees 20 34 degrees 10 s has basement elevations of 3200 m water depth and relatively simple stratovolcanoes dominated by low k series basalt basaltic andesite in contrast the adjoining arc segments have higher basement elevations typically 3 km coincide with the shallower arc segments the dominant mode of large caldera formation is interpreted as mass discharge pyroclastic eruption with syn eruptive collapse hence the shallower arc segments are characterized by both the generation of volatile enriched magmas from crustal melting and a reduced hydrostatic load allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions proposed initiation parameters for submarine pyroclastic eruptions are water depths 70 wt sio2 and a high discharge rate c 2005 elsevier b v all rights reserved

tectonic volcanic and hydrothermal features of a nascent rift graben in the southern okinawa trough abstract high resolution geophysical mapping using an autonomous underwater vehicle auv and surface vessel revealed tectonic volcanic and hydrothermal features of a nascent rift graben in the southern okinawa trough ship based bathymetry identified ene wsw trending lineaments at a water depth of 1900 2000 m that form a rift graben the ishigaki rift which is 50 km long 7 km wide and has relief of 100 m shipboard em122 multibeam sonar detected acoustic bubble plumes and high backscatter intensity on the seafloor along the rift the auv high resolution bathymetry 2 m and sidescan imagery revealed the presence of a ropey texture with high acoustic reflectivity interpreted to be relatively young lava flows evidence of hydrothermal venting was observed at two previously unknown sites along the rift steep sided spires up to 23 m in height and a mound discharging smoke like acoustic plumes were identified the auv temperature sensor observed sharp temperature increases of up to 0 13 c around the plumes even at the vehicle s altitude of 100 m above the seafloor the presence of relatively young lava flows and indicators of high temperature hydrothermal venting and gas bubbles suggest the ishigaki rift is magmatically active compared with adjacent active back arc rifts the ishigaki rift has a shorter length narrower width and lower relief and occurs at a shallower water depth suggesting that the rift is relatively young and immature considering that the ishigaki rift is located near the arc volcanic front and that other mature rifts are typically located farther from the front it is suggested that a new axial rift system is evolving within the okinawa trough and that back arc rifting processes may be gradually migrating arcward in the trough during the quaternary

zircon u pb dating using la icp ms quaternary tephras in yakushima island japan abstract zircon u pb dating using la icp ms was applied to three quaternary tephras in yakushima island southern japan the anbo tephra the koseda pyroclastic flow deposit ksd and the kikai tozurahara k tz tephra the obtained u pb ages were 0 73 0 04 ma error shown as 95 confidence level for the anbo 0 63 0 04 ma for the ksd and 0 17 0 05 ma for the k tz these ages are consistent with or slightly older than those reported previously which demonstrates the applicability of the u pb method to date zircons as young as 0 1 ma the k tz contains 0 7 0 6 ma zircons assumed to be derived from the anbo and ksd tephras because the k tz was originated from the kikai caldera 30 km north of yakushima island both the anbo and ksd were likely also derived from the same caldera system therefore our data and interpretation will contribute to elucidate the magmatic history of the kikai caldera whose latest eruption devastated prehistoric human settlements of southern kyushu japan and provide an improved chronostratigraphic constraint for japan

southwest pacific absolute plate kinematic reconstruction reveals major cenozoic tonga kermadec slab dragging tectonic plates subducting at trenches having strikes oblique to the absolute subducting plate motion undergo trench parallel slab motion through the mantle recently defined as a form of slab dragging we investigate here long term slab dragging components of the tonga kermadec subduction system driven by absolute pacific plate motion to this end we develop a kinematic restoration of tonga kermadec trench motion placed in a mantle reference frame and compare it to tomographically imaged slabs in the mantle estimating tonga kermadec subduction initiation is challenging because another new caledonia subduction zone existed during the paleogene between the australia and pacific plates we test partitioning of plate convergence across the paleogene new caledonia and tonga kermadec subduction zones against resulting mantle structure and show that most if not all tonga kermadec subduction occurred after ca 30 ma since then tonga kermadec subduction has accommodated 1 700 to 3 500 km of subduction along the southern and northern ends of the trench respectively when placed in a mantle reference frame the predominantly westward directed subduction evolved while the tonga kermadec trench underwent 1 200 km of northward absolute motion we infer that the entire tonga kermadec slab was laterally transported through the mantle over 1 200 km such slab dragging by the pacific plate may explain observed deep slab deformation and may also have significant effects on surface tectonics both resulting from the resistance to slab dragging by the viscous mantle

tonga ridge and lau basin crustal structure from seismic refraction data 1 the crustal structure across the tonga lau arc back arc system from the lau ridge to the pacific plate 178 170 w 18 19 s is modeled using data from an 840 km long air gun refraction line over 19 ocean bottom seismometers and one land station the data reveal that the pacific plate crust is 5 5 km thick with a velocity structure similar to that found at the present day east pacific rise epr beneath tonga ridge an intermediate velocity layer 6 7 km s is up to 7 5 km thick and has a velocity depth distribution similar to andesitic rocks found in continental crust the crust is abnormally thin 4 km at the boundary between the tonga ridge and the lau basin at the east end of lau basin the crust is 5 5 6 5 km thick and resembles crust formed at the epr except for a thicker sheeted dike section 2 3 km and thinner lower crust 2 km the lau basin crust thickens to 7 8 km near the central lau spreading center clsc mostly through thickening of the lower crust the crust thickens again to 8 5 9 5 km at 50 km west of the clsc mostly through thickening of the midcrust in the thick westernmost section the crustal structure is uniform even though one part of this section formed through extension of arc type crust while the rest was created at an oceanic spreading center the relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center

opposing trends in crustal thickness and spreading rate along the back arc eastern lau spreading center implications for controls on ridge morphology faulting and hydrothermal activity abstract along back arc ridges located near the arc volcanic front crustal thickness is observed to vary systematically with arc proximity independent of spreading rate this effect is especially well expressed by the 400 km long eastern lau spreading center elsc where approaching the arc volcanic front from 100 to 40 km crustal thickness nearly doubles 5 5 9 km as spreading rates decrease by more than half 97 39 mm yr the crustal thickness variations at the elsc appear to result primarily from changes in mantle wedge composition caused by subduction we investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology faulting and hydrothermal activity as part of the first phase of ridge2000 integrated studies in the lau back arc basin we used deep towed side scan sonar instruments dsl120a and imi30 to continuously map the near axis region within broader coverage ship multibeam bathymetry and acoustic imagery swaths an array of optical sensors maprs concurrently surveyed the near bottom water for hydrothermal plume anomalies hydrocasts made at identified plume sites confirmed their hydrothermal origin the data show that as spreading rates increase and crustal thicknesses decrease along the elsc 1 morphology transitions from shallow peaked volcanic highs to a deeper flat axis 2 faults become larger and more widely spaced 3 hydrothermal activity as measured by plume incidence increases and appears to reach levels higher than the global mid ocean ridge trend with spreading rate the observations indicate that crustal thickness magma supply has a greater control on ridge morphology and faulting than spreading rate even at fast rates where the thermal lithosphere should be thin however heat input from mantle advection proportional to spreading rate perhaps in combination with increased fault permeability appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the elsc the observed effects may have generic implications for controls of seafloor spreading characteristics at mid ocean ridges

volcanic evolution of the south sandwich volcanic arc south atlantic from multibeam bathymetry abstract new multibeam bathymetry data are presented for the south sandwich intra oceanic arc which occupies the small sandwich plate in the south atlantic and is widely considered to be a simple end member in the range of intra oceanic arc types the images show for the first time the distribution of submarine volcanic tectonic and erosional depositional features along the whole length of the 540 km long volcanic arc allowing systematic investigation of along arc variations the data confirm that the volcanic arc has a simple structure composed of large volcanoes which form a well defined volcanic front but with three parallel cross cutting seamount chains extending 38 60 km from near the volcanic front into the rear arc there is no evidence for intra arc rifting or extinct volcanic lines topographic evidence for faulting is generally absent except near the northern and southern plate boundaries most of the volcanic arc appears to be built on ocean crust formed at the associated back arc spreading centre as previously proposed from magnetic data but the southern part of the arc appears to be underlain by older arc or continental crust whose west facing rifted margin facing the back arc basin is defined by the new bathymetry the new survey shows nine main volcanic edifices along the volcanic front and ca 20 main seamounts the main volcanoes form largely glaciated islands with summits 3 0 3 5 km above base levels which are 2500 3000 m deep in the north and shallower at 2000 2500 m deep in the south some of the component seamounts are interpreted to have been active since the last glacial maximum and so are approximately contemporaneous with the volcanic front volcanism seven calderas all either submarine or ice filled have been identified adventure volcano a newly discovered submarine volcanic front caldera volcano is described for the first time all but one of the calderas are situated on summits of large volcanoes in the southern part of the arc and most are associated with current or historic volcanic or hydrothermal activity shallow shelves around the islands are generally 1 10 km wide submerged banks up to 1100 m deep are interpreted as subsided erosional surfaces seamounts and emergent volcanoes experienced a range of mass wasting processes including by landsliding and smaller mass flows

early evolution of a young back arc basin in the havre trough back arc basins are found at convergent plate boundaries nevertheless they are zones of significant crustal extension that show volcanic and hydrothermal processes somewhat similar to those of mid ocean ridges accepted models imply the initial rifting and thinning of a pre existing volcanic arc until seafloor spreading gradually develops over timescales of a few million years the havre trough northeast of new zealand is a unique place on earth where the early stages of back arc basin formation are well displayed in the recent geological record here we present evidence that in this region rifting of the original volcanic arc occurred in a very narrow area about 10 15 km wide which could only accommodate minimal stretching for a very short time before mass balance required oceanic crustal accretion an initial burst of seafloor spreading started around 5 5 5 0 million years ago and concluded abruptly about 3 0 2 5 million years ago after which arc magmatism dominated the crustal accretion the sudden transition between these different tectonomagmatic regimes is linked to trench rollback promoted by gradual sinking of the subducting lithosphere which could have diverted the arc flux outside the region of seafloor spreading and induced the vertical realignment of surface volcanism with the source of arc melts at depth the havre tough back arc basin new zealand formed rapidly and in two phases initial limited seafloor spreading was followed by a transition to arc magmatism as shown by geophysical data and modelling

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

submarine lava flow emplacement and faulting in the axial valley of two morphologically distinct spreading segments of the mariana back arc basin from wadatsumi side scan sonar images 1 high resolution deep tow side scan sonar data were collected over two distinct spreading segments in the central part of the mariana back arc basin these data allow mapping of small fissures and faults and the distinguishing of hummocky from smooth lava flows using these data we observe spatial variations in seafloor deformation and volcanic activity within each segment and also significant differences in the degree of tectonic deformation between the two segments one segment characterized by an hourglass shape suggestive of magmatic processes dominating over tectonic processes is in fact currently dominated by intense deformation rather than volcanism the other segment which exhibits morphology wide and deep linear axial valley typical of magma starved segments is subjected to very limited deformation and is covered by mostly unfaulted recent flows each segment also displays along axis variations in the degree of tectonic deformation and in lava flows freshness we observe a decrease of lava effusion rate from segment centers toward their ends we also investigated the apparent asymmetry of the mariana basin on the southern segment of the study area azimuths of tectonic structures are divided into two groups one segment parallel and one 15 oblique to strike of the segment these two trends of linear features developed synchronously with volcanic activity currently oblique structures seem to be the most active ones the tectonic structures that are parallel to the overall valley trend are distributed over the entire valley whereas the oblique structures are only located proximal to the eastern valley wall they are likely related to changes in the local stress field related to the obliquely trending eastern axial valley wall asymmetric character such as nonuniform spacing and throw of faults was not observed these observations suggest that the asymmetry of the basin is not due to asymmetric spreading but rather to eastward ridge jumps of several kilometers such small amplitude ridge jumps likely occurred frequently during basin development because the asymmetric character of the basin is strongly pronounced and no abandoned rift valley is recognizable within the entire basin

bathymetry and magnetic anomalies in the havre trough and southern lau basin from rifting to spreading in back arc basins abstract the havre trough and lau basin are active back arc basins related to pacific australian plate convergence these back arc basins provide insight into the evolution of rifting through various stages including a final stage of oceanic spreading we carried out a swath bathymetry and magnetic anomaly survey focused on the less studied havre trough and southern lau basin the western portion of the whole havre trough is deeper and more thickly sedimented whereas shallow bathymetry and volcanic edifices are dominant in the east we suggest that crustal rifting progresses asymmetrically with respect to the trough axis the active tectono magmatic zone migrates trenchward during ongoing widening rifting along normal faults trending in the longitudinal trough strike occurs in the southern havre trough volcanism is restricted to a line along the normal faults circular or elongated knolls in the northern havre trough indicate that en echelon type segmented rifting of short strike length and successive volcanic intrusion takes place volcanism in this area is dominated by diapir like magma intrusion into an extended and thinned arc crust the southernmost tip of the spreading center of lau basin is defined by the deep graben striking at n20 e located at 24 00 s 177 10 w

deep drilling of glaciers in eurasian arctic as a source of paleoclimatic records abstract results of the deep drilling 80 100 m of eurasian arctic glaciers svalbard severnaya zemlya and franz josef land from the middle of the 1970s to the end of the 1990s have been discussed all spectra of the techniques used for ice core and borehole studies results of ice core dating and major paleoclimatic conclusions have been analyzed particular emphasis has been placed on the problem of accuracy and comparison of dates suggested by different authors new integrated time scale for eurasian arctic glaciers based on the ice core dating of vetreniy ice cap graham bell island and franz josef land which covers the last 700 800 years has been developed methods and results of the new paleo temperature reconstructions have been analyzed

insights on the spreading of the tyrrhenian sea from the magnetic anomaly pattern introductionthe tyrrhenian sea is interpreted as aback arc basin related to the tectonicprocesses that have led to the buildingof the apennine chain here strongextensional phenomena have occurredsince the upper miocene malinvernoand ryan 1986 producing an asym metric basin with an overall triangularshape the rifting process driven bythe passive sinking of the ionianlithosphere subducting beneath cala bria and the related volcanismmigrated from west to east over time doglioni 1991 generating two smalloceanized basins the vavilov basinand later the marsili basin which arecharacterized by the presence of thehomonymous volcanoes by a mohodepth of about 10 km and by maximaof gravity eld and heat ux e g rapolla et al 2008 there are chem ical di erences between the basalts ooring these two basins oceanisland basalts in the vavilov basin island arc basalts in the marsilibasin these di erences have beenrelated to the di erent positions of thebasins with respect to the subductingionian plate serri et al 2001 the presence of oceanic like crust inthe tyrrhenian sea created by therifting process in pliocene pleistocenetimes and proved by drilling results mascle et al 1988 kastens andmascle 1990 has stimulated someresearchers to study the magneticanomalies in this area looking for astriped pattern similar to that charac terizing ocean environments theidenti cation of striped magneticanomalies could constrain the tempo ral evolution of the tyrrhenian seaopening the most complete magnetic anom aly dataset in the tyrrhenian area agip 1981 was measured at about1460 m above sea level a s l thepresence of di use magmatic events isclearly shown by the magnetic anom alies however in the vavilov andmarsili areas the magnetic signaturedoes not seem to show an obviousstriped pattern although it is clearlyvisible in other back arc basins e g hsu et al 2004 the basins youngage their limited size and asymmetricextension may explain the lack of astriped magnetic anomaly pattern nevertheless the basins evolved in atime span that allowed the formationof igneous susceptive rocks undereither a normal or a reversed directionof the geomagnetic eld in this arti cle the pattern of normal and reversedabstract

isotopic characteristics of neogene quaternary tephra from iodp site u1438 a record of explosive volcanic activity in the kyushu ryukyu arc a m acknowledges support from the swiss national science foundation grant 200020 135511 to othmar muntener and grant p2lap2 171819 to a m and swiss iodp this work was also supported by grants to g m y and m b from nsf oce 1537135 and iodp

tephrostratigraphy and provenance from iodp expedition 352 izu bonin arc tracing tephra sources and volumes from the oligocene to recent provenance studies of widely distributed tephras integrated within a well defined temporal framework are important to deduce systematic changes in the source scale distribution and changes in regional explosive volcanism here we establish a robust tephro chronostratigraphy for a total of 157 marine tephra layers collected during iodp expedition 352 we infer at least three major phases of highly explosive volcanism during oligocene to pleistocene time provenance analysis based on glass composition assigns 56 of the tephras to a japan source including correlations with 12 major and widespread tephra layers resulting from individual eruptions in kyushu central japan and north japan between 115 ka and 3 5 ma the remaining 101 tephras are assigned to four source regions along the izu bonin arc one of exclusively oligocene age is proximal to the bonin ridge islands two reflect eruptions within the volcanic front and back arc of the central izu bonin arc and a fourth region corresponds to the northern izu bonin arc source first order volume estimates imply eruptive magnitudes ranging from 6 3 to 7 6 for japan related eruptions and between 5 5 and 6 5 for ibm eruptions our results suggest tephras between 30 and 22 ma that show a subtly different izu bonin chemical signature compared to the recent arc after a 11 m y gap in eruption tephra supply from the izu bonin arc predominates from 15 to 5 ma and finally a subequal mixture of tephra sources from the palaeo honshu and izu bonin arcs occurs within the last 5 ma

discovery of hydrothermal sulfide mineralization from southern kermadec arc volcanoes sw pacific abstract we report the discovery of hydrothermal sulfide mineralization within the summit caldera of two southern kermadec frontal arc volcanoes brothers and rumble ii west of the 1200 km long kermadec havre arc back arc system the brothers and rumble ii west calderas both with re surgent domes and comprising mostly dacite and basalt andesite host rocks respectively rise to water depths of 1500 m these calderas comprise mostly effusive lavas and volcaniclastic deposits including talus breccias along their inner caldera walls these two southern kermadec hydrothermal sites have similar geological settings and mineralogy to other arc front vent sites two principle ore assemblages are identified comprising i chalcopyrite pyrite barite and ii sphalerite marcasite barite pyrite the most prevalent ore texture consists of massive chalcopyrite pyrite sphalerite commonly overprinting barite early marcasite cores within large euhedral pyrites and trails of pyrite inclusions within chalcopyrite are indicative of recrystallization the ore mineralogy is consistent with postulated venting temperatures of 300 c sulfide geochemistry is consistent with the ore mineralogy with concentrations of cu fe and zn up to 15 3 19 1 and 18 8 wt respectively the sulfide geochemistry is distinct however from comparable western pacific vent sites and may reflect source heterogeneity due to proximity to continental new zealand and sediment recycling along the southern kermadec trench arc system the recovery of two partial caridean vent shrimps suggests that present day hydrothermal venting is occurring within the brothers caldera

high resolution record of quaternary explosive volcanism recorded in fluvio lacustrine sediments of the uwa basin southwest japan abstract the identification of widespread tephras is important not only for understanding volcanic processes but also for age estimation and establishing stratigraphic relationships many widespread tephras have been erupted from six active and buried caldera volcanoes in the sw japan ryukyu volcanic arcs to reconstruct high resolution tephrostratigraphy we analyzed core ut from the uwa basin on western shikoku island and correlated the tephras in the core with the proximal source deposits that we sampled this 120 m long core composed of fluvio lacustrine deposits spanning the middle pleistocene to holocene we recognized and characterized 52 visible tephra and tephric sediment layers in the core on the basis of their stratigraphy the petrographic features and the major element composition of glass shards we successfully correlated 15 tephras with the following previously identified widespread tephras mainly from volcanoes on kyushu kikai akahoya 7 3 ka aira tn 29 ka aso 4 87 ka kikai tozurahara 95 ka aso abcd 100 ka ata 105 110 ka aso 3 112 7 ka calculated in this study aso 2 140 ka ata toihama 240 ka aso 1 250 270 ka tky ng1 290 300 ka kakuto 330 340 ka oda 420 450 ka hiwaki 570 580 ka and yufugawa 600 ka above 50 m depth all known widespread tephras from kyushu younger than 350 ky were included in the sequence that has a fairly constant sedimentation rate of about 0 14 mm yr this high resolution tephrostratigraphy was correlated with those of regional long core records from the lake biwa in the central japan and offshore of shikoku that contributes to establish onshore onshore and onshore offshore correlations in southwest japan in the sediments each of the following tephras from caldera forming eruptions aso 4 aso abcd ata aso 3 aso 2 and aso 1 occurred above multiple compositionally similar tephra layers erupted from the same volcano over a long time period this high resolution record is therefore an important contribution to the correlation of regional stratigraphy and reconstruction of both the long term eruptive history and the magma evolution of volcanoes in the sw japan and ryukyu volcanic arcs

national seismic hazard model for new zealand 2010 update a team of earthquake geologists seismologists and engineering seis mologists has collectively produced an update of the national probabilistic seismic hazard psh model for new zealand national seismic hazard model or nshm the new nshm supersedes the earlier nshm published in 2002 and used as the hazard basis for the new zealand loadings standard and numerous other end user applica tions the new nshm incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new new zealand based and international scaling relationships for the parameterization of the faults the dis tributed seismicity model has also been updated to include post 1997 seismicity data a new seismicity regionalization and improved methodology for calculation of the seismicity parameters probabilistic seismic hazard maps produced from the new nshm show a similar pattern of hazard to the earlier model at the national scale but there are some significant reductions and increases in hazard at the regional scale the national scale differences between the new and earlier nshm appear less than those seen between much earlier national models indicating that some degree of consis tency has been achieved in the national scale pattern of hazard estimates at least for return periods of 475 years and greater online material table of fault source parameters for the 2010 national seismic hazard model

enigmatic formation of the norfolk basin sw pacific a plume influence on back arc extension 1 the norfolk basin is a small back arc basin in the sw pacific with an unknown age and origin for its formation moreover the process behind its formation as a back arc basin in a region of extensive volcanism has previously never been resolved we have conducted a detailed interpretation of recently acquired swath bathymetry gravity and magnetic data from the northern norfolk basin faust 2 survey together with existing data from the region to address the age and origin of the norfolk basin and propose a new model for basin evolution the norfolk basin is characterized by complex bathymetry which formed during three distinct tectonic phases 1 regions of elevated topography termed plateaus show characteristics of thickened oceanic crust formed during the cretaceous normal superchron in either a back arc basin environment or as a trapped piece of pacific phoenix oceanic crust 2 regions of deep bathymetry termed basins separate the cretaceous plateaus and show basement textured by seafloor spreading fabric of miocene age 3 widespread intraplate plume related volcanic activity post dating the formation of the plateaus but predating the formation of the basins a miocene model for the formation of the basin regions in the norfolk basin requires a 700 km landward jump in the location of back arc spreading even with continued subduction at the tonga kermadec trench and the seaward migration of the subduction hinge the cessation of spreading in the basin regions would have been followed by an eastward jump of back arc spreading by about 1000 km into the current lau basin we propose that the driving force for such an enormous landward jump in back arc spreading is active rifting driven by a mantle plume this scenario is supported by the extensive miocene intraplate volcanism observed in the norfolk basin and the elevated bathymetry of the entire region in this model mantle driven uplift and excess volcanism would have favored back arc rifting landward rather than seaward of the existing south fiji basin in an extensional regime on the over riding plate caused by the continued roll back of the subduction hinge this scenario is supported by the numerous miocene volcanic cones in the norfolk basin which are mainly located on the preexisting plateaus probably predating the formation of the basins as well as their plume melt signature the norfolk basin appears to be the first known example of a major back arc basin whose position relative to the slab was not controlled by subduction driven mantle flow and melting but by plume tectonics this requires a re evaluation of the traditional back arc basin theory in supra subduction zones

morphology and history of the kermadec trench arc backarc basin remnant arc system at 30 to 32 s geophysical profile microfossil and k ar data abstract knowledge of the time span of arc activity essential for correct tectonic reconstructions has been lacking for the kermadec arc system but is supplied in this paper through study of microfossils contained in dredge samples and k ar ages on dredged basalt clasts the kermadec system at south latitudes 30 to 32 in the southwest pacific comprises from west to east the colville ridge remnant arc havre trough backarc basin kermadec ridge active arc and kermadec trench site of west dipping subduction of pacific plate lithosphere beneath the australian plate data are presented from two traverses dredge magnetic single channel seismic across the whole system an important transverse tectonic boundary the 32 s boundary lies between the two traverse lines and separates distinct northern 32 25 s and southern 32 36 s sectors the northern sector is shallower and well sedimented with broad ridges and a diffuse backarc basin the southern sector is deeper with narrow ridges and steep escarpments facing inwards to a little sedimented rifted backarc basin the kermadec ridge slopes smoothly trenchward to a mid slope terrace forearc basin with minor sediment fill at 5 6 km water depth a steeper 10 24 and more rugged lower trench slope is mantled with new zealand sourced rhyolitic vitric mud diamictons containing locally derived mafic volcanic clasts one clast is of late miocene age k ar age 7 84 ma the arc kermadec ridge is capped by active volcanoes very young k ar ages