new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

hydrothermal exploration of the fonualei rift and spreading center and the northeast lau spreading center we report evidence for active hydrothermal venting along two back arc spreading centers of the ne lau basin the fonualei rift and spreading center frsc and the northeast lau spreading center nelsc the ridge segments investigated here are of particular interest as the potential source of a mid water hydrothermal plume 1500 2000 m depth which extends more than 2000 km across the sw pacific ocean dispersing away from an apparent origin close to the most northeastern limits of the lau basin our results indicate the presence of at least four new hydrothermal plume sources three along the frsc and one on the nelsc the latter situated within 150 km of the maximum for the previously identified sw pacific regional scale plume however tdfe and tdmn concentrations in the southernmost frsc plume that we have identified only reach values of 19 and 13 nmol l and dissolved 3he anomalies in the same plume are also small both in relation to the sw pacific plume and to local background which shows evidence for extensive 3he enrichment throughout the entire lau basin water column our results reveal no evidence for a single major point hydrothermal source anywhere in the ne lau basin instead we conclude that the regional scale sw pacific hydrothermal plume most probably results from the cumulative hydrothermal output of the entire topographically restricted lau basin discharging via its ne most corner

crustal construction and magma chamber properties along the eastern lau spreading center abstract the l scan active source seismic tomography experiment maps the crustal structure and magmatic system along a 120 km long section of the back arc eastern lau spreading center elsc where the ridge undergoes abrupt changes in morphology and composition associated with increasing proximity to the tofua volcanic arc using this dataset we picked 197 000 p wave travel times from 57 seismic airgun lines recorded at 83 ocean bottom seismograph stations and inverted for a 3 d p wave velocity image the seismic images reveal a prominent but narrow seismic low velocity volume lvv located beneath all surveyed ridge segments consistent with the high temperatures and melt of a crustal magmatic system crustal magmatic systems thus underlie spreading axes even where previous seismic reflection surveys did not detect magma lens reflectors accounting for the heat source of high temperature hydrothermal vents in these areas the top of the lvv closely follows the ridge axis and steps across three overlapping spreading centers as the offset of the overlap increases the lvv becomes increasingly discontinuous across the ridge limbs surprisingly the lvv is as much as twice as wide but deeper in the northern part of the ridge system where the crust is thinner as compared to the lvv beneath the southern segments where the crust is relatively thicker the width of the lvv may be modulated by the degree of deep hydrothermal activity or temporal variations in melt supply and thus may not correlate directly with the average melt flux as indicated by crustal thickness over the past 4 myr the location of the ridge has swept across different mantle compositional domains and the crust produced at the ridge formed a zoned pattern the interpretation is that thick highly porous volcanic layers with felsic compositions cap regions of thick crust thinner volcanic layers of basaltic composition cap regions of thinner crust the zonal pattern indicates that the influence of slab derived water on crustal construction has substantially decreased over time

shallow seismicity and tectonics of the central and northern lau basin abstract the lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time as such crustal earthquakes within the lau basin derive from intrabasin tectonic volcanic and hydrothermal sources because of high mantle attenuation small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity however the 1994 labatts ocean bottom seismograph experiment in the lau basin recorded more than 100 000 local t and body phases allowing for a detailed examination of basin seismotectonics nearly 1000 events are located within the basin proper with another 2000 associated with the forearc and the aseismic front we identify two previously undocumented tectonic features a triple junction in the northwestern basin and a nanoplate in the central lau basin the single most seismically active feature within the lau basin is the lau extensional transform zone letz just north of the central lau spreading center significant seismicity also occurs near the northern extension of the eastern lau spreading center and along a southeast extension of the letz delineating the boundary of a previously unknown nanoplate swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics we see no evidence of deformation along the southern boundary of the niuafo ou microplate where 4 5 cm yr of strain is predicted it is possible that the plate boundary extends eastward from the fonualei rift fr tip towards the trench creating a niuatoputapu tonga plate division rather than westward implied by the currently accepted niuafo ou tonga plate system but more data are necessary from the fr to test that hypothesis instead we suggest that the niuafo ou tonga pole is 1 5 further north than previously proposed reducing the predicted strain in this region to

structure and growth of the izu bonin mariana arc crust 1 seismic constraint on crust and mantle structure of the mariana arc back arc system 1 a high resolution seismic velocity model is presented for the crust and upper mantle of the mariana arc back arc system mabs based on active source seismic profiling the major characteristics are 1 slow mantle velocity of 8 km s 1 in the uppermost mantle especially and deep reflectors under the mariana arc ma and the west mariana ridge wmr 2 a deep reflector in the upper mantle beneath the relative thick crust of the mariana trough mt axis 3 distribution of lower velocity lower crusts 6 7 6 9 km s 1 beneath the volcanic front and adjacent to the mt and 4 high velocity lower crust 7 2 7 4 km s 1 beneath the boundary regions between the ma and mt and between the wmr and the parece vela basin pvb adding to structural characteristics of crust and upper mantle beneath the mabs of the characteristics described above characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the moho considering that the wmr is extinct arc currently on the other hand characteristic 2 suggests that the origin of deep reflectors beneath the mt axis might be lower velocity materials due to the diffractive signals with strong amplitudes characteristic 3 suggests that the lower velocity lower crust advanced crustal growth and characteristic 4 suggests that the high velocity lower crust beneath arc back arc transition zone is composed of mafic ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite

the seismic structure and dynamics of the mantle wedge seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction slab dehydration arc volcanism and backarc spreading the mantle wedge is characterized by a low attenuation forearc an inclined zone of low velocity and high attenuation underlying the volcanic front and a broad region of low velocity and high attenuation beneath the backarc spreading center when present seismic velocities bathymetry and basalt chemistry suggest mantle temperature variations of 100 c between different backarc regions rock physics experiments and geodynamic modeling are essential for interpreting seismic observations seismic anisotropy indicates a complex pattern of mantle flow that can be modeled with along strike flow in a low viscosity channel beneath the arc and backarc comparison of geodynamic models with seismic tomographic results using experimentally derived relations between velocity attenuation and temperature suggests the existence of small melt fractions in the

seismic attenuation tomography of the tonga fiji region using phase pair methods the anelastic structure of the region surrounding the tonga slab and lau back arc spreading center in the southwest pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs two differential attenuation methods determine t over the frequency band 0 1 to 3 5 hz for earthquakes in the tonga slab the s p method measures the difference in spectral decay between p and s waves arriving at the same station the p p method measures the difference in spectral decay for p waves with different paths through the upper mantle eight hundred sixty phase pairs are used to invert for two dimensional 1 q structure using a nonnegative least squares algorithm a grid search method determines the q q ratio most compatible with both the s p and p p differential measurements the highest attenuation q 90 is found within the upper 100 km beneath the active portions of the lau basin extending westward to the lau ridge these regions probably delineate the source region for the back arc spreading center magmas expected to be within the upper 100 km based on petrological considerations the high attenuation regions also correlate well with zones of low p wave velocity determined by regional velocity tomography somewhat lower attenuation is found beneath the fiji plateau than beneath the lau basin the entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km the slab is imaged as a region of low attenuation q 900 material a q q ratio of 1 75 provides the best fit between the s p and p p data sets upon inversion spectral stacking shows no frequency dependence within the frequency band analyzed

seismic constraints of the formation process on the back arc basin in the southeastern japan sea to clarify the formation process of the back arc basin in the japan sea which is located next to the northwestern pacific a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern yamato basin to the coast of the northeastern japan island arc off awa shima island the crust beneath the northern yamato basin off awa shima island is approximately 16 km thick the upper and lower crusts are respectively about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient in the basin there are very few units with p wave velocity of 5 4 6 0 km s corresponding to the continental upper crust the crustal structure of the northern yamato basin has characteristics of thicker oceanic crust the high velocity lowermost crust in the northern yamato basin with 7 2 7 4 km s might show melt formed by a slightly high mantle temperature during the opening of the basin however the crust beneath the areas from the sado ridge to the coast which is approximately 25 26 km thick is slightly thinner than that of the continental crust and island arc crust the crustal structure beneath this area is inferred to be a rifted continental and or a rifted island arc crust

the 6 8 aug 2019 eruption of volcano f in the tofua arc tonga abstract in august 2019 a large raft of pumice appeared in the territorial waters of tonga as in many other cases this pumice raft was the only surface expression of a major submarine volcanic eruption discolored water and reconstruction of the drift path of the pumice raft using satellite imagery points towards volcano f in the tofua arc nw of the island of vava u as the most likely volcanic source here we present imagery from esa s sentinel 2 satellite that captured the start of the submarine eruption on 6 august 2019 and the waning of the eruption on 8 august followed by observations of the drifting pumice raft until 14 august this start time is consistent with t phase records at the seismic stations on niue island and rarotonga and the signal delay time of 733 s between the two stations is consistent with an origin at or at least near volcano f on 8 august a 136 7 km2 large raft of pumice appears at the sea surface the modelled minimum raft volume is 8 2 41 0 106 m3 which is equivalent to 2 5 12 3 106 m3 dense rock the eruption thus corresponds to a volcanic explosivity index vei 2 3 eruption in the submarine environment prior to the volcanic eruption a series of earthquakes close to volcano f was recorded the series started on 5 august with a mb 4 7 event followed by at least six shallow earthquakes mb 3 9 on 6 august in december 2018 and january 2019 we surveyed the seafloor around volcano f with multibeam sonar combining our data with pre existing information we present the first comprehensive bathymetric map of the volcanic edifice and its geologic setting we show that volcano f represents a major arc volcanic complex that is situated in an extensional setting the basal diameter of the volcanic apron is 50 km with a large central 8 7 x 6 km caldera with a floor at 700 m water depth the top of the post caldera constructional cone complex had a summit depth of 35 m below sea level in 2004 the volcano shows geochemical differences to the adjacent arc volcanoes on fonualei and late islands the volcano s pristine volcanic morphology and two documented eruptions 2001 and 2019 indicate a highly active volcanic system that warrants further scientific attention

quantitative analysis of rock stress heterogeneity implications for the seismogenesis of fluid injection induced seismicity abstractwe compared elastic rock heterogeneity measured by borehole logging to the occurrence of seismic events caused by hydraulic fracturing of the corresponding rock sections our observations made from two hydraulic fracturing case studies suggest that elastic rock heterogeneity controls the occurrence of fluid injection induced seismicity the seismic events occurred preferentially in rock sections characterized by low poisson s ratio and high young s modulus based on analytic solutions of stress fluctuations in elastically heterogeneous media in equilibrium to a homogeneous far field stress we quantified the relation between elastic rock heterogeneity and stress changes leading to fracture opening and reactivation in two end member models of 1d and 3d heterogeneity we found that significant fluctuations of rock stress originated from elastic rock heterogeneity moreover we found that stress changes leading to fracture opening and reactivation in rocks undergo scale invariance spatial fluctuation

physical properties of crustal and upper mantle rocks with regards to lithosphere dynamics and high pressure mineralogy this paper focusses on the physical properties of rocks and property changes occurring during geodynamic processes it addresses a number of outstanding petrophysical problems in the framework of crustal and mantle deformation the major themes to be discussed will be the seismic signature of crustal and mantle shear zones the nature of crustal and mantle anisotropy and the contribution of anisotropic rocks to seismic reflectivity in particular seismic shear waves traversing anisotropic strata can reveal important information on the spatial orientation of deformed structures because the measured seismic anisotropy has a structural origin emphasis will be placed on the relations between shear wave splitting and fabric anisotropy fabric anisotropy and strain and strain and geologic tectonic processes the second part of this paper discusses the effect that metamorphic phase transitions in crustal and mantle rocks may have on rock physical properties velocity density specifically the possible role that the gabbro eclogite transition could have in crustal delamination and recycling of material into the mantle will be considered

contrasting crustal production and rapid mantle transitions beneath back arc ridges the eastern lau spreading centre located on the lau back arc basin hydrothermal vent system between fiji and tonga is a focus site of the ridge 2000 research programme of the us national science foundation the first large scale seismic study of the area reveals large and abrupt across and along strike changes in crustal properties implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes the authors conclude that stable broad triangular upwelling regions as inferred for mid ocean ridges cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which the material is rapidly released resulting in rapid changes in the character of the crust formed at the ridge this study shows that across strike and along strike changes in crustal properties at the eastern lau spreading centre are large and abrupt implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes it is concluded that stable broad triangular upwelling regions as inferred for mid ocean ridges cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which that material is rapidly released resulting in rapid changes in the character of the crust formed at the ridge the opening of back arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading1 during this process the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced with high volatile contents derived from the subducting plate 2 3 4 5 current models4 6 7 8 predict gradational rather than abrupt changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction influenced mantle in contrast here we show that across strike and along strike changes in crustal properties at the eastern lau spreading centre are large and abrupt implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes with incremental separation of the ridge axis from the volcanic front of as little as 5 km seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills upper crustal seismic velocities abruptly increase by over 20 and gravity anomalies and isostasy indicate crustal thinning of more than 1 9 km we infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge such that stable broad triangular upwelling regions as inferred for mid ocean ridges9 10 cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which that material is rapidly released from the upwelling zone resulting in rapid changes in the character of the crust formed at the ridge

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

effect of latent heat of freezing on crustal generation at low spreading rates lithospheric structure changes at low spreading rates 40 mm a 1 such that steady state molten rock cannot exist above the normal moho depth of 6 km dredge statistics of recovered lithologies indicate that the percentage of basalt is significantly decreased compared to faster ridges and that the mode of crust generation is variable one important effect on lithospheric structure is the latent heat released when basaltic magma freezes within the mantle instead of being transported all the way to the surface using thermal models we show that freezing of melt at mantle depths buffers temperature due to the latent heat of crystallization two quasi stable seafloor spreading patterns are imposed on the model 1 upwelling of magma along a narrow axial zone and 2 pure shear extension over a broad zone the variability in crustal structure predicted by these models explains variability observed in dredge lithology statistics at eight slow to ultraslow spreading ridge segments for example the gakkel eastern volcanic zone and sparsely magmatic zone are both spreading at 12 mm a 1 but the former is composed of 7 peridotite whereas the latter is 46 peridotite this difference can be explained by a change from a narrow axis regime with 2 km thick crust to a wide axis regime with a crust mantle mix overall dredge statistics and thermal models suggest that some but not all latent heat of ascending magmas is released at mantle depths and that various ridge axial morphologies can occur

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

body wave attenuation in the tonga subduction zone attenuation p and s waves of in the tonga subduction zone is estimated from locally recorded data using a spectral ratio method the data consist of three component digital seismograms of 77 earthquakes recorded at two stations in fiji and one in tonga the stations in the fiji islands lie above the deep earthquakes in the subduction zone while the tonga station is close to the top of the subducted slab variations in the high frequency spectral decay rate define four attenuation provinces paths from earthquakes deeper than 350 km to stations on viti levu fiji and the lau ridge traverse mantle with normal attenuation qp 570 qs 200 400 the subducting tonga slab has very low attenuation qp 2000 qs 1800 relative to the surrounding mantle the wedge of mantle above the slab at depths shallower than 350 km and east of the lau ridge is extremely attenuating for p waves qp 240 several paths from main thrust zone events to the station in tonga show intermediate attenuation qp 300 500 in the forearc region the observed spectra do not require q to be frequency dependent in the 1 0 hz to 7 6 hz band in this region

influence of pressure and mineralogy on seismic velocities in oceanic gabbros implications for the composition and state of the lower oceanic crust 1 we have analyzed the relationship between p wave velocities measured at pressures of 40 100 and 200 mpa and modal mineralogy in oceanic gabbro samples from ocean drilling program holes 735b 894g and 923a at all pressures increasing velocities correspond to increasing pyroxene contents and decreasing alteration phyllosilicate and amphibole content but indicate little or no variation of olivine content in part because the olivine compositions are in the range fo65 to fo73 a voigt reuss hill inverse model reveals that the effective bulk density and elastic moduli of olivine in the gabbros are low even relative to fo73 possibly because the olivine grains contain ubiquitous networks of cracks even if the cracks are not present in situ on average gabbros with velocities typical of seismic layer 3 6 7 7 0 km s 1 contain 5 15 alteration products including 5 15 amphibole and 0 5 phyllosilicates these results suggest that the lower crust is on average slightly to moderately altered

crustal structure of the niuafo ou microplate and fonualei rift and spreading center in the northeastern lau basin southwestern pacific the northeastern lau basin is one of the fastest opening and magmatically most active back arc regions on earth although the current pattern of plate boundaries and motions in this complex mosaic of microplates is reasonably understood the internal structure and evolution of the back arc crust are not we present new geophysical data from a 290 km long east west oriented transect crossing the niuafo ou microplate back arc the fonualei rift and spreading center frsc and the tofua volcanic arc at 17 20 s our p wave tomography model and density modeling suggest that past crustal accretion inside the southern frsc was accommodated by a combination of arc crustal extension and magmatic activity the absence of magnetic reversals inside the frsc supports this and suggests that focused seafloor spreading has until now not contributed to crustal accretion the back arc crust constituting the southern niuafo ou microplate reveals a heterogeneous structure comprising several crustal blocks some regions of the back arc show a crustal structure similar to typical oceanic crust suggesting they originate from seafloor spreading other crustal blocks resemble a structure that is similar to volcanic arc crust or a hydrous type of oceanic crust that has been created at a spreading center influenced by slab derived water at distances 50 km to the arc throughout the back arc region we observe a high velocity vp 7 2 7 5 km s 1 lower crust which is an indication for magmatic underplating which is likely sustained by elevated upper mantle temperatures in this region

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

p wave attenuation structure of the lau back arc basin and implications for mantle wedge processes abstract we image 3 d seismic p wave attenuation structure of the tonga subduction zone and the lau back arc basin using local earthquake waveforms recorded by land and ocean bottom seismographs of the 2009 2010 lau spreading center imaging project the amplitude spectra of p waves from local earthquakes are inverted for the path average attenuation operator t with a variety of frequency dependence exponent values analysis shows that the data are best fit by the assumption of a weak frequency dependence 0 3 consistent with laboratory results modeled as the effects of grain boundary sliding the t measurements are inverted for a 3 d tomographic q p 1 model the results show high p wave attenuation within the upper 100 km of the back arc mantle with a sharp boundary at the tofua arc between high attenuation sub arc mantle and the low attenuation fore arc perhaps the highest p wave attenuation q p 1 0 036 or q p 28 known in the mantle occurs at 20 60 km depth beneath the back arc spreading centers high attenuation anomalies form an inclined zone dipping from the spreading centers to the west away from the slab indicating abnormally high temperature and the existence of hydrous partial melt these observations suggest that hot materials supplied from the australian mantle upwell along with the mantle wedge flow pattern triggering extensive decompression melting near the back arc spreading centers the along strike variation of p wave attenuation is different from that of sv velocity possibly implying the different effects of partial melt on seismic attenuation and velocity

geology and kinematics of the niuafo ou microplate in the northern lau basin we present a revised three plate kinematic model for the lau basin focusing on new interpretations of the geologic characteristics of the niuafo ou microplate boundaries it builds upon the three plate model by zellmer and taylor 2001 while resolving some geologic discrepancies in that model along the southern branch of the mangatolu triple junction and the fonualei rift and spreading center which together form the niuafo ou tonga plate boundary our model predicts slow to ultraslow opening rates 32 8 mm yr southward consistent with axial morphology and magnetic profile modeling our interpretation of the seafloor magnetization pattern and flank morphology along the central lau spreading center clsc indicates that opening rates along the australia niuafo ou boundary are higher than predicted in the previous model thus compensating for slower rates along the niuafo ou tonga boundary lastly we show geologic evidence that peggy ridge pr interpreted previously as an active transform fault and therefore a strong azimuthal constraint on niuafo ou plate motion is inactive and that the active plate boundary is the adjacent lau extensional transform zone extending from the clsc to its intersection with the northwest lau spreading center the removal of the pr azimuthal constraint allows us to determine an euler pole for australia niuafo ou opening that satisfies three plate closure criterion and develop a geologically consistent model of lau basin kinematics while the final model focuses on constraining current plate velocities assuming rigid plate eulerian rotations the geologic analyses presented herein also provide insight into long term basin evolution which appears to involve significant nonrigid plate kinematics

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

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

spreading rate spreading obliquity and melt supply at the ultraslow spreading southwest indian ridge we use bathymetry gravimetry and basalt composition to examine the relationship between spreading rate spreading obliquity and the melt supply at the ultraslow spreading southwest indian ridge swir we find that at regional scales more than 200 km melt supply reflects variations in mantle melting that are primarily controlled by large scale heterogeneities in mantle temperature and or composition focusing on adjacent swir regions with contrasted obliquity we find that the effect of obliquity on melt production is significant about 1 5 km less melt produced for a decrease of 7 mm a to 4 mm a in effective spreading rates esr but not enough to produce near amagmatic spreading in the most oblique regions of the ridge unless associated with an anomalously cold and or depleted mantle source our observations lead us to support models in which mantle upwelling beneath slow and ultraslow ridges is somewhat focused and accelerated thereby reducing the effect of spreading rate and obliquity on upper mantle cooling and melt supply to explain why very oblique swir regions nonetheless have large outcrops of mantle derived ultramafic rocks and in many cases no evidence for axial volcanism cannat et al 2006 dick et al 2003 we develop a model which combines melt migration along axis to more volcanically robust areas melt trapping in the lithospheric mantle and melt transport in dikes that may only form where enough melt has gathered to build sufficient overpressure these dikes would open perpendicularly to the direction of the least compressive stress and favor the formation of orthogonal ridge sections the resulting segmentation pattern with prominent orthogonal volcanic centers and long intervening avolcanic or nearly avolcanic ridge sections is not specific to oblique ridge regions it is also observed along the swir and the arctic gakkel ridge in orthogonal regions underlain by cold and or depleted mantle

seismological study of lau back arc crust mantle water magmatic differentiation and a compositionally zoned basin abstract on the basis of seismic tomography analyses a structural model of crust formed along the eastern lau spreading center elsc is presented as evidence for a transition from a hydrous type of oceanic crust to a more typical oceanic crust the seismic data indicate that as the spreading center moved away from the active arc the crust thinned from 8 9 km to 7 km the lower crust changed from high p wave velocity values 7 2 7 4 km s to typical values for oceanic crust 7 0 7 2 km s and the upper crustal volcanic layer changed from a thick low velocity layer to a thinner layer with more typical wave speeds the seismic results in combination with other geophysical and geochemical data suggest that crustal formation along the elsc is strongly controlled by the influence of slab water when a spreading center is near the active arc water from the downgoing slab is entrained in the melting zone beneath the ridges where it enhances melting thereafter the water enhances crustal differentiation within sub ridge magma chambers this creates an anomalous hydrous form of oceanic crust with a thick felsic volcanic layer and a mafic ultramafic lower crust features that are not typically observed in crust formed at mid ocean ridges the lau basin has a zoned structure with an abrupt transition from this type of oceanic crust to more typical oceanic crust which resulted from a rapid change in the influence of slab water as the ridge moved away from the arc the abundance and high rate of production of the hydrous crust suggests that such crust may make up a significant proportion of the arc like crust that forms continents

magmatic tectonic conditions for hydrothermal venting on an ultraslow spread oceanic core complex hydrothermal venting an important cooling mechanism of the earth supports a diverse array of seafloor and sub seafloor ecosystems that are sustained by large thermal and chemical fluxes vents have been found along even the slowest and coldest spreading centers calling into question the driving heat source for these vents the ultraslow spreading mid cayman spreading center in the caribbean sea which hosts the axial flank von damm vent field vdvf provides an opportunity to probe the mechanisms for venting at ultraslow spreading rates using active source seismic data from the 2015 cayseis cayman seismic experiment we determined the seismic velocities in the large massif beneath the vdvf we propose that this massif was produced by a pulse of on axis magmatism at ca 2 ma which was then followed by exhumation cooling and fracturing a low seismic velocity anomaly 5 km below the vdvf is evidence for either a cracking front mining lithospheric heat or intrusive magmatic sills both of which could drive ongoing deep hydrothermal fluid circulation we conclude that the transient magmatism and variable crustal thickness at ultraslow spreading centers create conditions for long lived hydrothermal venting that may be widespread and other vdvf like vents may be common in these areas