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

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

imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

lithospheric structure across oblique continental collision in new zealand from wide angle p wave modeling 1 oblique convergence between the australian and pacific plates in south island new zealand has resulted in crustal thickening and distributed deformation within both plates we measure this thickening and image the deformation with seismic wide angle data along a 600 km long transect that spans the plate boundary p wave arrival times from 34 000 rays are used to construct a two dimensional model for seismic velocity with depth crustal velocities average 6 1 and 6 2 0 23 km s for the australian and pacific sides of the boundary respectively upper mantle velocities average 8 1 0 25 km s distributed deformation is indicated by the following observations a reduction in upper crustal and midcrustal velocities by 4 west of the plate boundary ascribed to flexural bending stresses a velocity reduction of 8 immediately east of the alpine fault linked to high pore fluid pressures 17 km of crustal thickening to form a 44 1 4 km deep crustal root that is offset 10 20 km se of the highest mountains a velocity reversal below the midcrust in the eastern part of the crustal root caused either by heating or an artifact from seismic anisotropy strong thickening of a 7 0 0 4 km s fast lower crust within the crustal root and a low velocity zone in the australian upper mantle due to sampling the slow orientation of upper mantle anisotropy

india madagascar paleo fit based on flexural isostasy of their rifted margins the present study contributes new constraints on and definitions of the reconstructed plate margins of india and madagascar based on flexural isostasy along the western continental margin of india wcmi and the eastern continental margin of madagascar ecmm we have estimated the nature of isostasy and crustal geometry along the two margins and have examined their possible conjugate structure here we utilize elastic thickness te and moho depth data as the primary basis for the correlation of these passive margins we employ the flexure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elastic thickness gravity inversion and flexure inversion techniques are used to estimate the configuration of the moho crust mantle interface that reveals regional correlations with the elastic thickness variations these results correlate well with the continental and oceanic segments of the indian and african plates the present study has found a linear zone of anomalously low te 1 5 km along the wcmi similar to 1680 km which correlates well with the low te patterns obtained all along the ecmm we suggest that the low te zones along the wcmi and ecmm represent paleo rift inception points of lithosphere thermally and mechanically weakened by the combined effects of the marion hotspot and lithospheric extension due to rifting we have produced an india madagascar paleo fit representing the initial phase of separation based on the te estimates of the rifted conjugate margins which is confirmed by a close fit correlation of moho geometry and bathymetry of the shelf margins the matching of tectonic lineaments lithologies and geochronological belts between india and madagascar provide an additional support for the present plate reconstruction c 2014 international association for gondwana research published by elsevier b v all rights reserved

inverse modelling of elastic thickness by convolution method the eastern alps as a case example an unconventional scheme is used to estimate the flexural rigidity or equivalently the elastic thickness of the lithosphere given the topography and gravity data the flexural rigidity is the parameter that governs the flexural response of the lithosphere in the frame of the thin plate flexure model the scheme is an alternative to the widely used calculation of admittance of topography sea floor or continental topography and gravity bearing some advantages which are explained in the paper the scheme involves the inversion of the gravity data in order to formulate a model of the crust mantle interface cmi undulations in a second step the flexure parameter is then evaluated from the relation between topography and cmi variations instead of calculating the admittance function using a spectral analysis a set of point load response functions are used in order to retrieve the optimal flexure parameter this has two main advantages instabilities of the numerical admittance evaluation at wavenumbers with low spectral energy in the topography are overcome and the analysis can be made over an area which is not necessarily rectangular as required for the spectral analysis the proposed method allows a higher space resolution of elastic thickness than any spectral method for validation the numerical strategy is applied to the situation of a realistic synthetic model where all inputs and outputs are known a priori finally the spatial variations of the elastic thickness are studied in an area across the eastern alps 4 2002 elsevier science b v all rights reserved

three dimensional estimation of elastic thickness under the louisville ridge a three dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry this technique is applied to the louisville ridge system to study the tectonic history of the region the inversion is performed as both a first order approximation and a nonlinear relationship between gravity and topography based on parker s 1973 equation while the higher order effect on the gravity anomaly is nearly zero for most of the region the magnitude is significant over the summits of the ridge nevertheless the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region lowering the value by 1 2 km compared with the linear result the incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly resulting in false sidelobe structure of nearly 20 mgal for large seamounts our elastic thickness estimates are compared with the contradictory values obtained in previous studies by cazenave and dominh 1984 and watts et al 1988 we find an increasing elastic thickness along the chain from southeast to northwest with a discontinuity along the wishbone scarp the jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge

large scale crustal heterogeneities and lithospheric strength in cratons abstract the rheology and thermal structure of the continental lithosphere are intimately linked in old cratons the effective elastic thickness of the lithosphere has been estimated by various spectral inverse methods based on the correlation between topography and gravity anomalies estimates vary within a very large range from 40 km to 120 km depending on the method used in this paper we use forward models to account for lateral variations in mechanical properties and their effect on the equivalent elastic thickness eet of the lithosphere from these models which allow brittle elastic ductile rheologies and mechanical discontinuities faults we have calculated the strain stress distributions and displacement fields vertical integration of the stress permits a local determination of the effective elastic thickness the computed displacements were used to calculate related bouguer and free air gravity anomalies and compare them with the observations the analysis is applied to the 2000 ma kapuskasing uplift in the superior province of the canadian shield where the presence of a high density block in the upper crust is due to the upthrusting of midcrustal rocks along a major thrust fault the study shows that the stability of this structure on geologic time scales requires a strong lower crustal rheology a cold geotherm and the fault to be healed this study also shows that because of stress dependence of the non linear rheology crustal heterogeneities may cause significant 40 local reductions of the lithospheric strength away from the kapuskasing structure the average strength of the lithosphere remains high eet 100 km conventional methods for estimating the elastic thickness would not resolve such local strength reductions in cratons but would predict depending on the method used highly overestimated or instead underestimated eet

rapid weakening of subducting plates from trench parallel estimates of flexural rigidity abstract the negative buoyancy force of sinking lithosphere slabs is the principle driving force for subducting plates but transmission of this force to the subducting plate depends on the strength of the slab e g conrad and hager 2001 slab strength has been studied in the context of plate bending within subduction zones for a wide range of rheologies i e perfectly elastic perfectly viscous perfectly plastic brittle ductile layered because the applicability of these rheologic models cannot be distinguished based on trench perpendicular plate bending models forsyth 1980 a method was developed to directly measure variations in plate strength with distance from the trench and has found significant plate weakening within 100 km of the kermadec trench billen and gurnis 2005 using the same method we show that rapid plate weakening trenchward of the forebulge also exists at the tonga and japan izu bonin subduction zones the observed plate weakening provides further evidence for a plate rheology that leads to significant yielding loss of elastic strength and reduction in effective viscosity within the bending region of the subducting plate this rapid weakening within the shallow low curvature region of the plate may significantly decrease estimates of energy dissipation related to plate bending compared to recent calculations assuming high plate strength and constant plate curvature

fluid generation and pathways beneath an active compressional orogen the new zealand southern alps inferred from magnetotelluric data 1 forty one wideband magnetotelluric mt soundings were collected in a 150 km long transect across the southern alps of the central south island of new zealand an active compressional orogen decomposed mt impedance tensors vertical magnetic field relations and reconnaissance soundings at two locations off line imply an approximately two dimensional geometry here with average regional geoelectric strike of n40 e similar to surface geologic trends two independent two dimensional inversion algorithms were applied to the mt data and both imply a concave upward u shaped middle to lower crustal conductive zone beneath the west central portion of the island the average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along strike conductive strands narrower than the transverse magnetic cross strike mode mt data can resolve the deep crustal conductor under the southern alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust fluid interconnection and electrical conduction are promoted by shear deformation the conductor rises northwestward toward the trace of the alpine fault but attains a near vertical configuration at a depth of 10 km and reaches close to the surface 5 10 km inland of the fault trace itself the transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle ductile transition in uplifting schist and approach the surface through induced hydrofractures the high grade schist becomes resistive after depletion of fluids and continues to extrude toward the alpine fault shallow extensions of the deep high conductivity are coincident with modern hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance to the southeast high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent southern alps orogen which also exhibits evidence for expulsion of high temperature fluids the higher conductivity inferred along strike possible anisotropy could reflect more efficient fluid interconnection in this higher strain direction as well as possible contributions by sheared fluid deposited graphite conductivity of the uppermost mantle of the south island is low consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by p wave delay studies

elastic thickness structure of south america estimated using wavelets and satellite derived gravity data abstract we used a wavelet formulation of the classical spectral isostatic analysis to invert satellite derived gravity and topography bathymetry for elastic thickness te over south america and its surrounding plates to provide a homogeneous representation of the gravity field for this vast region we corrected free air anomalies derived from a combination of terrestrial marine gravity data with data from the grace and champ satellite missions model eigen cg03c by a simple bouguer slab using a smoothed representation of surface relief wavelengths 125 km the resulting bouguer anomaly compares well with terrestrial data acquired in the central andes and allows te to be confidently estimated for values greater than 10 km the te map resolves regional scale features that are well correlated with known surface structures and shows maximum values of 100 15 km over the archean neoproterozoic core of the continent decreasing to less than 30 km around continental margins several regions of the oceanic plates and continental margins have an elastic thickness less than 10 km we performed a quantitative analysis by comparing the elastic thickness with the thermal structure predicted from the age of oceanic crust and igneous metamorphic rocks this demonstrates that oceanic plates have been weakened by thermal interaction with hotspots and locally by fracturing and hydration near the trench we observe that only the nucleus of the continent has resisted the thermomechanical weakening induced by the rifting of africa and south america along the passive margin and the andean orogeny along the active margin this latter region shows along strike variations in te that correlate with the geotectonic segmentation of the margin and with the pattern of crustal seismicity our results reveal that the rigidity structure follows the segmentation of the seismogenic zone along the subduction fault suggesting a causal relationship that should be investigated in order to improve the understanding and predictability of great earthquakes and tsunamis

elastic thickness of the himalayan tibetan orogen estimated from the fan wavelet coherence method and its implications for lithospheric structure abstract this paper is concerned with the spatial variation of the elastic thickness t e in the himalayan tibetan orogen and its implications for the understanding of lithospheric structure and deformation new t e maps of the himalayan tibetan orogen are generated from the new combined satellite terrestrial gravity eigen6c3stat and topography etopo1 models using the fan wavelet coherence technique the high t e values 60 km are observed in the indian shield and the tarim basin while the low t e values 40 km are prevailing over most of the tibetan plateau we also find that the t e distributions estimated using the high and low central wavenumber i e k 0 wavelets are different in southern tibet by combining constraints from the current seismic and thermo rheological modeling this difference may reflect that the strength of the lithosphere in southern tibet mainly resides in the lithospheric mantle within most parts of central and northern tibet the t e values are relatively low 30 km indicating that the entire lithosphere is weak our analysis finds that the t e variations are probably associated with changes in lithosphere structure we interpret the abrupt changes in t e between southern and northern tibet to indicate the location of the underthrusting strong indian lithosphere beneath tibet our results suggest that the northern limit of the underthrusting indian plate reaches as far north as the hindu kush and the southwestern margin of the tarim basin near 70 80 e and extends along the bangong nujiang suture in the central 80 87 e eastward between 87 e 93 e it may move to the indus tsangpo suture indicating a delamination of indian mantle lithosphere there further to the east it may underthrust northward beneath the eastern qiangtang block

sediment loading on the western platform of the new zealand continent implications for the strength of a continental margin abstract mechanical properties of the new zealand lithosphere are determined by forward modelling the gravity and isostatic response of a plio pleistocene sediment loaded shelf margin the giant foreset formation on the western platform of new zealand is a 2 km thick 250 km long and 100 km wide sediment package both the downward flexed pliocene miocene boundary beneath the package and a 35 50 mgal peak to trough gravity anomaly over the sediment package are best matched with a flexural rigidity in the region of 5 6 10 22 1 5 10 23 n m effective elastic thickness t e 20 28 km the giant foresets are a particularly important load to model because unlike most sediment loaded passive margins there is a time gap of about 70 80 ma between rifting and subsequent deposition of the sedimentary load furthermore the width of the giant foreset load is sufficiently small to afford a reasonable resolution of t e

cretaceous oblique extensional deformation and magma accumulation in the fosdick mountains migmatite cored gneiss dome west antarctica 1 the fosdick mountains west antarctica expose a 15 x 80 km migmatite cored gneiss dome consisting of migmatitic gneisses diatexite migmatite and subhorizontal leucogranite sheets the fosdick dome was emplaced and exhumed in the mid cretaceous due to oblique extension associated with the west antarctic rift system along the west antarctic new zealand segment of east gondwana the dome is bounded to the south by a dextral oblique detachment structure and to the north by an inferred dextral strike slip fault within the fosdick dome and in the detachment zone granite occupies leucosomes dikes sills and dilatant and shear structures the pattern of kilometer scale domains of migmatite and granite suggest that lithologic variations and heterogeneous deformation boudinage resulted in pressure gradients that enhanced melt flow and magma accumulation in the fosdick dome steep foliations are overprinted folded and transposed by subhorizontal fabrics the crosscutting relationship is interpreted as a transition from wrench deformation to oblique divergence steep structures in the dome host concordant subvertical leucosome and granite sheets yielding shrimp u pb zircon ages between ca 117 and 114 ma prevalent subhorizontal domains host large volumes of subhorizontal diatexite migmatite and granite sheets that yield u pb zircon ages between ca 109 and 102 ma these ages indicate a timescale for melt influx of approximately 15 ma and that the transition from wrench to oblique divergence may have occurred in as little as 5 ma granites with crystallization ages between ca 109 and 102 ma were also emplaced in the south fosdick detachment zone indicating that the detachment was active during oblique divergence shrimp u pb titanite ages between ca 102 and 97 ma for late to post tectonic diorite dikes are interpreted as emplacement ages and give a minimum age for gneissic foliation development during detachment faulting the fosdick mountains preserve a record of the middle to lower crustal response to a transition from wrench to oblique extensional deformation overprinting structural relationships show that a change in the angle of oblique extension can induce accumulation of subhorizontal magma sheets and lead to initiation of a detachment zone

spatial variations in effective elastic thickness in the western pacific ocean and their implications for mesozoic volcanism abstract we have used free air gravity anomaly and bathymetric data together with a moving window admittance technique to determine the spatial variation in oceanic elastic thickness t e in the western pacific ocean synthetic tests using representative seamounts show that t e can be recovered to an accuracy of 5 km for plates up to 30 km thick with increased accuracy of 3 km for t e 20 km the western pacific has a t e range of 0 50 km with a mean of 9 4 km and a standard deviation of 6 8 km the t e structure of the region is dominated by relatively high t e over the hawaiian emperor seamount chain intermediate values over the marshall islands gilbert ridge and marcus wake guyots and low values over the line islands mid pacific mountains caroline islands shatsky rise hess rise and musician seamounts plots of t e at sites with radiometric ages suggest that t e is to first order controlled by the age of the lithosphere at the time of loading in areas that backtrack into the south pacific isotopic and thermal anomaly sopita t e may be as low as the depth to the 180 120 c isotherm at least locally in the northern part of the study area including the hawaiian emperor seamount chain t e correlates with the depth to 310 120 c these best fitting isotherms imply peak rates of volcanism during 100 120 ma early cretaceous and 140 150 ma late jurassic the corresponding addition of 8 10 6 km 3 and 4 10 6 km 3 of volcanic material to the surface of the oceanic crust would result in long term sea level rises of 20 m and 10 m respectively the late jurassic volcanic event like the later early cretaceous event appears to have influenced the tectonic evolution of the pacific plate convergent boundaries resulting in increased volcanism and orogenesis

effective elastic thickness of south america and its implications for intracontinental deformation 1 the flexural rigidity or effective elastic thickness of the lithosphere te primarily depends on its thermal gradient and composition consequently maps of the lateral variability of te in continents reflect their lithospheric structure we present here a new te map of south america generated using a compilation of satellite derived grace and champ missions and terrestrial gravity data including egm96 and sagp and a multitaper bouguer coherence technique our te maps correlate remarkably well with other proxies for lithospheric structure areas with high te have in general high lithospheric mantle shear wave velocity and low heat flow and vice versa in this paper we focus on the te of the stable platform we find that old cratonic nuclei mainly archean and early middle proterozoic have in general high te 70 km while the younger patagonian phanerozoic terrane has much lower te 20 30 km suggesting that te is related to terrane age as has already been noted in europe within cratonic south america te variations are observed at regional scale relatively lower te occurs at sites that have been repeatedly reactivated throughout geological history as major sutures rift zones and sites of hot spot magmatism today these low te areas are surrounded by large cratonic nuclei they concentrate most of the intracontinental seismicity and exhibit relatively high surface heat flow and low seismic velocity at 100 km depth this implies that intracontinental deformation focuses within relatively thin hot and hence weak lithosphere that cratonic interiors are strong enough to inhibit tectonism and that the differences in lithospheric rigidity structure and composition between stable cratons and sites of intracontinental deformation are not transient and may have been maintained in some cases for at least 500 m y

tsunami run up on a plane beach in a tidal environment we analytically solve the nonlinear shallow water theory for the tsunami run up on a plane beach in the presence of tide and show that over a plane beach the tide in the nearshore zone can be considered static uniform in space and frozen in time with this assumption we find an exact analytical solution for the tsunami run up height as a function of the amplitude of the incident wave and analyse the influence of the tide on the tsunami run up characteristics we confirm these results by calculation of the total tsunami and tidal run up field using analytical solutions of the initial nonlinear shallow water equations

effective elastic thickness of zealandia and its implications for lithospheric deformation abstract zealandia was once located at the active gondwana margin and experienced complicated tectonic evolution from continental breakup to subduction and oblique slipping along the plate boundary here we investigate the lithospheric thermal and rheological properties via spatial variations in effective elastic thickness te using the topography sediment thickness and gravity data the fan wavelet coherence method is employed to recover te values the effects of dynamic topography and gravity in the subduction zone on the estimated te are considered and removed the final results reveal that the te variation pattern generally coincides well with the tectonic regimes relatively high te values up to 22 km are observed in north island and the southernmost part of south island while central and northern south island are dominated by smaller te values 15 km similar to those of the surrounding submerged plateaus we interpret the relatively high te values as indicating the combined lithospheric strength due to the contacting plates in the subduction zone to the north along the tonga kermadec subduction zone the lithosphere surprisingly exhibits anomalously high te values ranging from 22 to 36 km which contradicts the geological evidence of normal faults developed in the incoming pacific plate and the magmatism in the overlying australian plate that generally contribute to low lithospheric strength we speculate that the high te zone is related to the large bathymetry and gravity anomalies in the bending plate and controlled by non isostatic factors finally we find widespread low te values covering the entire arcs and oceanic basins in northern zealandia which may indicate that regional lithospheric thinning associated with delamination weakened the integrated strength of the lithosphere

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

a reassessment of spectral te estimation in continental interiors the case of north america 1 conventional spectral te studies use the real part of the admittance between gravity anomalies and topography or alternatively the square of the magnitude of the coherency i e coherence here we show the utility of treating both the admittance and coherency as complex quantities inverting the real parts to estimate te we use the imaginary parts to tell if the inversion is biased by noise one method inverts the square of the real coherency with the internal to total load ratio f derived as a function of wave number directly from the gravity and topography the other method inverts the real part of the admittance assuming that f is wave number independent we test the methods using synthetic elastic plate models loaded at the surface and moho in such a way that the final relief is the actual north american topography in some of the models we add gravity noise generated by a model having both surface and internal loads such that the final topography is zero and find that both methods are susceptible to noise application of the two methods to north america gives te maps showing substantial agreement except in regions affected by noise but these are not a dominant part of the total area given the suggested mechanisms by which noise might arise it is not surprising that it is not a more widespread feature of the north american craton importantly both methods show that large parts of the canadian shield are characterized by te 100 km

crustal structure of the basin and ridge system west of new caledonia southwest pacific from wide angle and reflection seismic data 1 during the zoneco 11 marine geophysical survey september 2004 two deep reflection seismic profiles recorded by ocean bottom seismometers were acquired in the offshore domain west of new caledonia the northern profile crosses the new caledonia basin the fairway ridge the fairway basin and the lord howe rise the southern profile crosses the norfolk rise south of new caledonia the new caledonia basin the fairway ridge and basin and ends at the foot of lord howe rise on the northern profile the lord howe rise has a crustal thickness of 23 km and exhibits seismic velocities and velocity gradients characteristic of continental crust the crust thins to 12 15 km in the neighboring fairway basin which is interpreted to be of thinned continental origin based on the seismic velocities the crustal thickness of the fairway rise is 22 km and it is also interpreted to be of continental origin the new caledonian basin is underlain by crust of 10 km thickness which shows unusally high velocities between 7 0 and 7 4 uncharacteristic for either thinned continental or oceanic crust on the southern profile the norfolk rise is also found to be of continental nature here the new caledonia basin shows velocities crustal thickness and basement roughness characteristic of typical oceanic crust the crust in the fairway basin shows higher velocities than on the northern profile which could be caused by volcanic intrusions into the crust during extension a deep reflector in the upper mantle was imaged underneath the new caledonian basin on the northern profile

reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents we use seismic reflection and refraction data to determine crustal structure to map a fore arc basin containing 12 km of sediment and to image the subduction thrust at 35 km depth seismic reflection megasequences within the basin are correlated with onshore geology megasequence x late cretaceous and paleogene marine passive margin sediments megasequence y a similar to 10 000 km 3 submarine landslide emplaced during subduction initiation at 22 ma and megasequence z a neogene subduction margin megasequence the moho lies at 17 km beneath the basin center and at 35 km at the southern margin beneath the western basin margin we interpret reflective units as deformed gondwana fore arc sediment that was thrust in cretaceous time over oceanic crust 7 km thick raukumara basin has normal faults at its western margin and is uplifted along its eastern and southern margins raukumara basin represents a rigid fore arc block 150 km long which contrasts with widespread faulting and large neogene vertical axis rotations farther south taper of the western edge of allochthonous unit y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward as proposed for the correlative onshore allochthon spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore arc crustal thickness the trench slope has many small extensional faults and lacks coherent internal reflections suggesting collapse of indurated rock rather than accretion of 1 km of sediment from the downgoing plate the lack of volcanic intrusion east of the active arc and stratigraphic evidence for the broadening of east cape ridge with time suggests net fore arc accretion since 22 ma we propose a cyclical fore arc kinematic rock moves down a subduction channel to near the base of the crust where underplating drives rock uplift oversteepens the trench slope and causes collapse toward the trench and subduction channel cyclical rock particle paths led to persistent trench slope subsidence during net accretion existing global estimates of fore arc loss are systematically too high because they assume vertical particle paths citation sutherland r et al 2009 reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents tectonics 28 tc5017 doi 10 1029 2008tc002356

tsunami run up and draw down on a plane beach tsunami run up and draw down motions on a uniformly sloping beach are evaluated based on fully nonlinear shallow water wave theory the nonlinear equations of mass conservation and linear momentum are first transformed to a single linear hyperbolic equation to solve the problem with arbitrary initial conditions we apply the fourier bessel transform and inversion of the transform leads to the green function representation the solutions in the physical time and space domains are then obtained by numerical integration with this semi analytic solution technique several examples of tsunami run up and draw down motions are presented in particular detailed shoreline motion velocity field and inundation depth on the shore are closely examined it was found that the maximum flow velocity occurs at the moving shoreline and the maximum momentum flux occurs in the vicinity of the extreme draw down location the direction of both the maximum flow velocity and the maximum momentum flux depend on the initial waveform it is in the inshore direction when the initial waveform is predominantly depression and in the offshore direction when the initial waves have a dominant elevation characteristic

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

global gravity bathymetry and the distribution of submarine volcanism through space and time 1 the seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin relatively few of these features 0 1 however have been dated and so little is known about their tectonic setting one parameter that is sensitive to whether a seamount formed on near or far from a mid ocean ridge is the elastic thickness te which is a proxy for the long term strength of the lithosphere most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of te and then comparing the calculated and observed gravity anomaly the problem with such an approach is that bathymetry data are usually limited to single beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape we therefore use the satellite derived gravity anomaly to predict the bathymetry for different values of te by comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the wessel global seamount database we have obtained 9758 te estimates from a wide range of submarine volcanic features in the pacific indian and atlantic oceans comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate te and its upper and lower bounds te at sites where there is both a sample and crustal age show considerable scatter however and there is no simple relationship between te and age nevertheless we are able to tentatively assign a tectonic setting to each te estimate the most striking results are in the pacific ocean where a broad swath of on ridge volcanism extends from the foundation seamounts and ducie island easter island ridge in the southeast across the equator to the shatsky and hess rises in the northwest interspersed among the on ridge volcanism are flank ridge and off ridge features the indian and atlantic oceans also show a mix of tectonic settings off ridge volcanism dominates in the eastern north atlantic and northeast indian oceans while flank ridge volcanism dominates the northeastern indian and western south atlantic oceans we have been unable to assign the flank ridge and off ridge estimates an age but the on ridge estimates generally reflect we believe the age of the underlying oceanic crust we estimate the volume of on ridge volcanism to be 1 1 106 km3 which implies a mean seamount addition rate of 0 007 km3 yr 1 rates appear to have varied through geological time reaching their peak during the late early cretaceous and then declining to the present day

a seismic reflection image for the base of a tectonic plate a high resolution image for the base of an oceanic plate that is subducting beneath north island new zealand reveals a channel which is interpreted as a sheared zone of ponded partial melts or volatiles this low viscosity channel decouples the plate from mantle flow beneath allowing plate tectonics to work tim stern and co authors use explosion generated seismic waves to form a high resolution image of the base of an oceanic plate that is subducting beneath north island new zealand they find an abrupt seismic wave speed transition less than a kilometre thick at a depth of 100 km dipping at an angle of 15 the presence of a parallel 10 km deeper reflection event shows that the decrease in compressional wave speed is confined to a channel at the base of the plate which the authors interpret as a sheared zone of ponded partial melts or volatiles they conclude that this is independent evidence for a low viscosity channel at the lithosphere asthenosphere boundary that decouples plates from mantle flow beneath and allows plate tectonics to take place plate tectonics successfully describes the surface of earth as a mosaic of moving lithospheric plates but it is not clear what happens at the base of the plates the lithosphere asthenosphere boundary lab the lab has been well imaged with converted teleseismic waves1 2 whose 10 40 kilometre wavelength controls the structural resolution here we use explosion generated seismic waves of about 0 5 kilometre wavelength to form a high resolution image for the base of an oceanic plate that is subducting beneath north island new zealand our 80 kilometre wide image is based on p wave reflections and shows an approximately 15 dipping abrupt seismic wave speed transition less than 1 kilometre thick at a depth of about 100 kilometres the boundary is parallel to the top of the plate and seismic attributes indicate a p wave speed decrease of at least 8 3 per cent across it a parallel reflection event approximately 10 kilometres deeper shows that the decrease in p wave speed is confined to a channel at the base of the plate which we interpret as a sheared zone of ponded partial melts or volatiles this is independent high resolution evidence for a low viscosity channel at the lab that decouples plates from mantle flow beneath and allows plate tectonics to work

continental crust under compression a seismic refraction study of south island geophysical transect i south island new zealand 1 the 1996 south island geophysical transect sight active source seismic survey was designed to show the style of lithospheric thickening due to late cenozoic oblique convergence across the australian pacific plate boundary in new zealand as part of this study two seismic refraction lines were shot across central south island and offshore extensions of the continental crust in the tasman sea and pacific ocean we present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects a tomographic inversion of 62 563 travel times from crustal and upper mantle refractions and wide angle reflections resulted in a model with a two layer crust upper crustal velocities were between 5 9 and 6 3 km s and lower crustal velocities were between 6 5 and 7 0 km s continental compression has locally reduced the seismic velocities in the pacific plate crust by 0 2 0 3 km s a possible effect of high strain and fluids in the crust the thickening of the crust from 28 km at the east coast of south island to 37 km beneath the southern alps can account for about 25 of the 80 110 km shortening of pacific plate crust while the rest must be accounted for by rapid erosion of mesozoic sedimentary rocks on the west side of the orogen in our model the lower crust forms a continuous 2 6 km thick layer beneath central south island the asymmetric topography of the southern alps is reflected in the crustal root which has a steeper flank at the west coast this observation is consistent with westward underthrusting of pacific lower lithosphere beneath south island that has been suggested in earlier studies

dynamic topography above retreating subduction zones dynamic topography provides a measure of stresses within the earth s interior dense slabs induce an upper mantle flow that deflects the surface of the earth downward above them by combining a simple theoretical stokeslet model of subduction gravity modeling and seismic tomography i show that a significant fraction as much as 2000 m of the topographic variations observed above the scotia mariana and hellenic subduction systems appears to be dynamically induced by stresses related to the underlying subduction

dynamic uplift in a transpressional regime numerical model of the subduction area of fiordland new zealand abstract bending of the downgoing plate in subduction zone typically leads to an offshore peripheral bulge this leads to dynamic uplift generated by the elastic bending of the subducted slab and is generally enough to support the topography of the bulge in a non isostatic manner and produce a positive gravity anomaly the southwest region of the south island of new zealand fiordland is characterized by high elevation and a large positive bouguer gravity anomaly this combination of high topography with high bouguer gravity argues against isostatic equilibrium and suggests an additional support mechanism earthquakes as deep as 150 km a deformed benioff zone and inferences from plate reconstructions all support a tectonic model where the eastern margin of the australian plate is subducting beneath fiordland and is sharply bent this bending of the australian plate provides the needed non isostatic support for fiordland topography and generates the observed gravity anomaly although the peripheral bulge in subduction zones is generally localized offshore the positive gravity anomaly bouguer and free air in fiordland is onshore close to the shoreline and generally corresponds spatially with high elevations here we propose a mechanism that allows the subducted sliver of slab to be decoupled from the main australian plate and strongly bent beneath fiordland we test this scenario with a finite element model the model allows us to study the flexural response of a subducting elastic slab bent by lateral compression into a shape similar to the one inferred from seismicity we test how different plate geometries and plate boundary forces influence the flexural dynamic support of fiordland topography providing important constraints on the local plate dynamics the model results show that for a tectonically reasonable combination of plate geometries and boundary forces the deformation of the lithosphere produces the observed topography and gravity signature in particular we find that the bending of the subducted australian plate can supply the needed uplift and support for the topography of fiordland however a weak area west of but nearby the fiordland shoreline perhaps a fault or tear is needed to decouple the subducted sliver confine the bulge and localize the uplift within fiordland

mechanism for normal faulting in the subducting plate at the mariana trench we investigate the mechanisms of normal fault initiation and evolution in the subducting pacific plate near the mariana trench through bathymetry analysis and geodynamic modeling we model the subducting plate as an elastoplastic slab subjected to tectonic forcing at the trench including vertical load bending moment and horizontal tensional force in our simulations normal faults initiate within the outer rise region and reach maximum throw toward the trench this result holds over a wide range of tectonic forcing and is consistent with observations of the challenger deep region where multibeam bathymetry data indicate faults initiate near the outer rise at 70 110 km from the trench and reach maximum throw at 10 35 km from the trench however models require a horizontal tensional force with magnitude comparable to axial vertical load to jointly explain the observed seafloor bathymetry location of maximum normal fault throw and prevalence of normal faults dipping toward the trench

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

effective elastic thickness and mechanical anisotropy of south china and surrounding regions abstract south china and surrounding regions extend from the eastern tibetan plateau through the tectonically stable sichuan basin and the broad mesozoic magmatic and fold belt to the trench arc basin system in the western pacific which provide an ideal place to study deformation of the continental lithosphere under long term magmatism and oceanic subduction we obtained the effective elastic thickness t e and its anisotropy of south china and surrounding regions from the analysis of coherence between topography and satellite gravity using wavelet methods the t e values of the study area vary from 2 to 75 km with relatively low t e values 30 km along the tectonic boundaries the north south gravity lineament nsgl and seismic zones and in the regions with high surface heat flow the evenly low t e values in the lower yangtze region and the cathaysia block can be attributed to the long lived subduction of the paleo tethys and the paleo pacific ocean basins beneath the south china block scb the nsgl in the scb may separate the unmodified high t e and thermally weakened low t e continental lithosphere due to oceanic subduction despite different distances to the tectonic boundaries earthquakes occur more frequently in regions with t e values of 10 30 km implying strain concentration in the low t e regions a positive correlation between seismic activity and the magnitude of t e anisotropy suggests that a highly anisotropic mechanical structure will promote strain localization and brittle failure in the lithosphere the poor correlation between the weak axis of t e anisotropy and the dynamic indicators of the present tectonic regime the shear wave splitting direction the maximum horizontal compressive stress direction confirms that t e anisotropy mainly reflects tectonic inheritance of the continental lithosphere

lithosphere delamination with foundering of lower crust and mantle caused permanent subsidence of new caledonia trough and transient uplift of lord howe rise during eocene and oligocene initiation of tonga kermadec subduction western pacific 1 we use seismic reflection and rock sample data to propose that the first order physiography of new caledonia trough and norfolk ridge formed in eocene and oligocene time and was associated with the onset of subduction and back arc spreading at the australia pacific plate boundary our tectonic model involves an initial cretaceous rift that is strongly modified by cenozoic subduction initiation hence we are able to explain 1 complex sedimentary basins of inferred mesozoic age 2 a prominent unconformity and onlap surface of middle eocene to early miocene age at the base of flat lying sediments beneath the axis of new caledonia trough 3 gently dipping variable thickness and locally deformed late cretaceous strata along the margins of the trough 4 platform morphology and unconformities on either side of the trough that indicate a phase of late eocene to early miocene uplift to near sea level followed by rapid oligocene and miocene subsidence of 1100 1800 m and 5 seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of new caledonia trough by 1800 2200 m since eocene time the cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab this created a deep 2 km enclosed oceanic trough 2000 km long and 200 300 km across in eocene and oligocene time as the lower crust detached with simultaneous uplift and local land development along basin flanks disruption of late cretaceous and paleogene strata was minimal during this cenozoic phase and involved only subtle tilting and local reverse faulting or folding basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into norfolk basin we suggest that delamination of the lithosphere with possible mixing of the lower crust back into the mantle is more widespread than previously thought and may be commonly associated with subduction initiation such as cenozoic events in the mediterranean and western pacific

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

constraints on subducting plate strength within the kermadec trench four specially designed surveys parallel to the kermadec trench allow localized estimates of plate strength within the subducting pacific plate to be made the transfer function between topography and gravity is estimated for five trench parallel ship tracks at distances of 25 110 km from the trench axis we find a clear reduction in the magnitude and peak wavelength of the transfer function from the outer rise to the trench axis the change in the transfer function indicates a decrease in plate strength and is consistent with a reduction in flexural rigidity by 3 5 orders of magnitude or a decrease in effective elastic thickness by more than 15 km such a large magnitude decrease in the effective elastic strength suggests that the plate has little or no elastic strength within the trench and that viscous stresses play an important role in transferring slab pull forces to the subducting plate and regulating plate speeds in subduction zones

practical implications of the geometrical sensitivity of elastic dislocation models for field geologic surveys geodetic observations of both coseismic and interseismic surface deformation fields in the vicinity of subduction zones are frequently interpreted using simple elastic dislocation models edms the geometry of the megathrust interface used in these models has a first order effect on their predicted surface deformation here we systematically explore the sensitivity of the surface velocity field predicted by edms both early and late in the seismic cycle to parameterizations of megathrust interface geometry effective subducting plate thickness and gradual transitions in apparent plate coupling we focus on how these parameterizations affect the hingeline the location where vertical velocities switch from subsidence to uplift as well as the location of the peak uplift rates we find that these surface observables are much less sensitive to uncertainties in dip at the downdip end of the seismogenic zone for realistic curved faults in comparison to planar faults for realistic megathrust geometries planar or curved having gradual transitions in apparent plate coupling we find that the extent of locking is best approximated at the surface by the location of peak uplift rates therefore the common notion based on shallow dipping planar faults that the hingeline is located directly above the maximum depth extent of the locked plate interface is generally incorrect using the hingeline as the basis for coupling may lead to a significant underestimation of seismic hazard early in the cycle as well as during the interseismic period this analysis also demonstrates the importance of considering both vertical and horizontal velocities for determining seismic source extents as well as interseismic coupling on the megathrust the tradeoffs presented here between the geometry of the megathrust and fault coupling along its surface can assist in the planning of campaign gps or field geologic surveys and help improve seismic hazard estimates in active subduction zones

variations in oceanic plate bending along the mariana trench we quantify along trench variations in plate flexural bending along the mariana trench in the western pacific ocean a 3 d interpreted flexural deformation surface of the subducting pacific plate was obtained by removing from the observed bathymetry the effects of sediment loading isostatically compensated topography based on gravity modeling age related lithospheric thermal subsidence and residual short wavelength features we analyzed flexural bending of 75 across trench profile sections and calculated five best fitting tectonic and plate parameters that control the flexural bending results of analysis revealed significant along trench variations the trench relief varies from 0 9 to 5 7 km trench axis vertical loading v 0 from 0 73 x 10 12 to 3 17 x 10 12 n m and axial bending moment m 0 from 0 1 x 10 17 to 2 7 x 10 17 n the effective elastic plate thickness seaward of the outer rise region t e m ranges from 45 to 52 km while that trench ward of the outer rise t e m ranges from 19 to 40 km this corresponds to a reduction in t e of 21 61 the transition from t e m to t e m occurs at a breaking distance of 60 125 km from the trench axis which is near the outer rise and corresponds to the onset of observed pervasive normal faults the challenger deep area is associated with the greatest trench relief and axial vertical loading while areas with seamounts at the trench axis are often associated with more subtle trench relief smaller axial vertical loading and greater topographic bulge at the outer rise c 2014 elsevier b v all rights reserved

pliocene exhumation and landscape evolution of central north island new zealand the role of the upper mantle 1 mudstone porosity analyses show that over 2 km of exhumation has occurred in central north island new zealand since 5 ma this combined with a coeval surface uplift of 500 m indicates a maximum 2 5 km of rock uplift in central north island the magnitude and wavelength 400 km of the rock uplift is similar to some topographic doming observed in continental areas associated with mantle upwelling yet central north island is recognized as a region of back arc extension marine terraces 100 150 km from the center of maximum exhumation show uplift rates of 0 2 0 3 mm yr 1 consistent with about 1 km of exhumation if averaged over 4 5 myr which compares well with mudstone porosity analyses seismic attenuation and slow pn velocities indicate that a thermal anomaly in the upper mantle is the source of this rock uplift the rock uplift may be explained by the replacement of a 100 km thick mantle lid by asthenosphere with a density 66 kg m 3 less than the lid about half this density contrast can be explained by thermal expansion and the rest is ascribed to partial melts in the upper mantle southward migration for the locus of maximum rock uplift is consistent with the zone of maximum incision erosion being 50 km south of the southernmost volcano such a migration is in keeping with other geological markers of volcanic and thermal phenomena and further strengthens the link of landscape evolution at active continental margins with processes in the mantle

crustal intrusion beneath the louisville hotspot track we report here the first detailed 2d tomographic image of the crust and upper mantle structure of a cretaceous seamount that formed during the interaction of the pacific plate and the louisville hotspot results show that at 1 5 km beneath the seamount summit the core of the volcanic edifice appears to be dominantly intrusive with velocities faster than 6 5 km s the edifice overlies both high lower crustal 7 2 7 6 km s and upper mantle 8 3 km s velocities suggesting that ultramafic rocks have been intruded as sills rather than underplated beneath the crust the results suggest that the ratio between the volume of intra crustal magmatic intrusion and extrusive volcanism is as high as 4 5 in addition the inversion of moho reflections shows that the pacific oceanic crust has been flexed downward by up to 2 5 km beneath the seamount the flexure can be explained by an elastic plate model in which the seamount emplaced upon oceanic lithosphere that was 10 myr at the time of loading intra crustal magmatic intrusion may be a feature of hotspot volcanism at young hot oceanic lithosphere whereas magmatic underplating below a pre existing moho may be more likely to occur where a hotspot interacts with oceanic lithosphere that is several tens of millions of years old

the runup of solitary waves this is a study of the runup of solitary waves on plane beaches an approximate theory is presented for non breaking waves and an asymptotic result is derived for the maximum runup of solitary waves a series of laboratory experiments is described to support the theory it is shown that the linear theory predicts the maximum runup satisfactorily and that the nonlinear theory describes the climb of solitary waves equally well different runup regimes are found to exist for the runup of breaking and non breaking waves a breaking criterion is derived for determining whether a solitary wave will break as it climbs up a sloping beach and a different criterion is shown to apply for determining whether a wave will break during rundown these results are used to explain some of the existing empirical runup relationships

crust and upper mantle structure of a continental backarc central north island new zealand summary near vertical and wide angle seismic data provide evidence for a gradational crust mantle boundary in a depth range of 15 20 km beneath the central volcanic region cvr new zealand this volcanic area includes the taupo volcanic zone and is a direct extension of tonga kermadec oceanic backarc spreading into continental lithosphere long range seismic refraction data show velocities of 6 km s 1 and less within the top 15 km of the crust of the cvr at a depth of 15 km compressional seismic velocities increase to 6 8 km s 1 and then to 7 4 0 2 km s 1 at 20 km depth these 7 4 km s 1 seismic wave speeds are interpreted as anomalous upper mantle as beneath this level passive seismic studies show similar pn wave speeds that increase slowly to 7 8 km s 1 at about 80 km deep we interpret rocks between 15 and 20 km to be a layer of new crust formed by underplating the strongest reflection observed and what might also be interpreted as a reflection moho is from the top of the proposed underplated layer at 15 km depth at 20 km depth no such distinct reflection is observed rather wide angle reflection data show a continuum of low level reflectivity between 15 and at least 35 km depth indicating some heterogeneity and or structure within the lower crust and upper mantle thus the transition from lower crust to upper mantle is broad and a conventional reflection moho does not exist beneath the cvr buoyancy force calculations based on rock uplift for the central north island indicate that the subjacent mantle to a depth of 80 100 km is 70 kg m 3 or 2 per cent less dense than normal mantle best estimates attribute half of this density anomaly to the effects of increased temperature with additional contributions from partial melt 1 2 per cent and melt residuum

the hikurangi plateau abstract recently collected geophysical data reveal the basement and sedimentary structure of the hikurangi plateau presently being subducted beneath the north island of new zealand we have subdivided the plateau into 6 areas on the basis of basement structure two basins separated by a basement high trend subparallel to the chatham rise probably filled with mesozoic sediments another is parallel with the present plate margin along the east coast of the north island and is filled with late cenozoic sediments the northern part of the plateau is characterised by shallow volcanic basement the southeast part of the region is complex but has several kilometers of sediments deformed by volcanic intrusions at least two episodes of volcanism have occurred the older probably mesozoic episode produced numerous large isolated edifices some exposed as seamounts and others buried by sediments activity resumed in the neogene resulting in the emplacement of smaller knolls and widespread intrusions variation in crustal thickness north of the chatham rise supports the hypothesis of a convergent margin along the north flank of the chatham rise in the mesozoic the structure of the hikurangi plateau may explain the origin of the postulated physiographic variations and segmentation of the downgoing slab along the active kermadec hikurangi subduction margin

effective elastic thickness variations along the andean margin and their relationship to subduction geometry 1 we present a new map of the spatial variations in effective elastic thickness te along the andes estimated using bouguer coherence the te variations reflect interactions between subducting slab and preexisting terrane structure in the forearc conductive cooling of the continent by the subducting slab exerts primary control on rigidity resulting in te that is highest 40 km where the oceanic lithosphere is oldest and coldest 20 s in the central andes te is relatively low 20 km along the volcanic chain and the altiplano and puna plateaus we interpret this weakening to reflect a high geothermal gradient maintained by advective magmatic processes a shallow and hot asthenosphere and a very weak lower crust throughout this region east of the plateaus high te delineates underthrusting of the brazilian shield finally north and south of the plateaus flat subduction areas are characterized by high te high shear wave velocity thick thermal lithosphere and low heat flow indicating that continental lithosphere there is thicker colder and stronger on the basis of these relationships we suggest that variations in slab dip along the margin relate to variations in structure of the continental lithosphere in particular we propose that upper plate structure influences the width and viscosity of the asthenospheric wedge which control the suction moment responsible for the subduction angle at depths 70 100 km for example when oceanic lithosphere subducts beneath thin continental lithosphere the low viscosity asthenosphere allows the slab to detach from the continent and sink into the mantle at normal angles however when oceanic lithosphere subducts close or beneath thick and strong continental lithosphere the asthenospheric wedge narrows and corner flow drags high viscosity mantle from the base of the thick 150 km cold continent into the wedge suction forces increase with both narrowing of the wedge and its increasing viscosity we estimate the asthenospheric viscosity underneath thick cold continents to be 1020 pa s sufficient to induce flat subduction later after prolonged hydration and weakening of the continent s base asthenospheric flow into the wedge may resume allowing the slab to sink again into the mantle at normal angles

global and local isostatic coherence from the wavelet transform 1 a method to compute the variations in lithospheric elastic thickness te has been developed using the wavelet transform the technique which uses a superposition of two dimensional morlet wavelets in a geometry named a fan wavelet is designed to yield isotropic yet complex wavelet coefficients for the co and cross spectra of gravity and topography data these are then used to compute a spatially varying isostatic coherence from which both global and local estimates may be obtained we applied the method to synthetic gravity and topography generated for a thin elastic plate of uniform thickness 20 km yielding an apparent spatially variable te of 24 5 3 5 km the estimated global coherence for this model appears to fit the theoretical prediction as well as fourier transform based estimates and is smoother than these we also computed the wavelet coherence and hence spatially varying te for a plate of non uniform thickness yielding a difference with the model of 2 0 1 7 km

marine magnetic anomalies over the lord howe rise and the tasman sea implications for the magnetization of the lower continental crust abstract marine geophysical and geological data provide evidence for a continental origin of the lord howe rise a submarine plateau east of australia our marine geomagnetic data reveal a long wavelength anomaly associated with the western margin of the rise forward and inverse modeling shows that the anomalies can be best explained by strong magnetization of the lower crust of the lord howe rise a geomagnetic map of the northern tasman sea that includes dampier ridge another elongated piece of presumably continental crust shows positive anomalies over areas of continental origin and negative ones over the deep sea basin this indicates higher values of the product of magnetization times thickness for the continental crust than for the oceanic crust three dimensional model calculations show that the high magnetization in the lower crust of eastern australia is limited to a maximum depth of 15 20 km this is in accordance with geothermal results that indicate the curie isotherm is at a shallow depth there the anomalies of the tasman sea are negative due to the negative part of the edge anomaly on the seaward side of the continental margin it is not necessary to hypothesize strong reversed magnetization in the lower oceanic crust and upper mantle

origin of the louisville ridge and its relationship to the eltanin fracture zone system we have combined shipboard and seasat altimeter derived data in an intergrated geological and geophysical study of the louisville ridge a 3500 km long seamount chain extending from the tonga trench to the eltanin fracture zone a break in the smooth trend of the ridge at latitude 37 5 s has been recognized in both bathymetric and altimetric data the 40ar 39ar dating of rocks dredged either side of the break suggest that it is analogous to the bend in the hawaiian emperor seamount chain although the general trend of the ridge can be fit by small circles about pacific absolute motion poles determined from other seamount chains the new bathymetric and age data allow us to refine pacific absolute motion poles the continuity in smooth trend of the ridge and the eltanin fracture zone suggests some relationship between them however a major offset developed on this transform between 60 and 80 ma prior to the oldest dated rocks from the ridge although magmatism was more or less continuous on the ridge during 28 60 ma it probably occurred on crust with little or no offset thus magmatism appears to have been little influenced by the developing fracture zone by 28 ma the distance between the magmatic source and the fracture zone had decreased sufficiently for a portion of the ridge to have been emplaced on crust with an offset after about 12 ma however volcanic activity on the louisville ridge apparently waned despite a possible influence on the magmatism of the fracture zone

a new look at the new caledonia basin an extension of the taranaki basin offshore north island new zealand abstract the new caledonia basin is a major bathymetric trough extending north westwards from taranaki and continuing past new caledonia this paper is concerned with the 250 km of the feature closest to new zealand and adjacent to the taranaki shelf the new caledonia basin appears to have formed as the surface expression of a failed rift system gravity models constrained by seismic reflection and refraction data indicate thinned crust below the basin the date of inception of rifting is not known but it must be pre cenozoic and may be pre cretaceous new seismic reflection data reveal tensional block faulting within the basin and on the flanks of the challenger plateau and the west norfolk ridge offset gravity anomalies and apparent compressional faulting observed on seismic data provide evidence for transverse possibly transform faulting accompanying and facilitating oblique extension more than 1 5 s twt of sediments were deposited during rifting this is equivalent to a thickness of up to 3 km they are likely to consist of cretaceous clastics and volcaniclastics deposited in fluvial and marginal marine environments terrigenous clays and mudstones probably cap the rift sequence post rift sedimentation commenced with thick authigenic limestone reflecting regional thermal subsidence and remoteness from a clastic source there is little record of tectonic activity until at least the late eocene and probably not until the early miocene when volcanic activity began seismic sections show domal uplift at the base of the challenger plateau and the formation of several large submarine volcanoes this activity may have been related to renewed rifting in the bellona trough an average thickness of approximately 0 5 km of plio pleistocene sediments onlap the margins of the basin channels and fans are evident on the seismic data and account for the distribution of a significant portion of the sediment eroded from the rising southern alps the fragmentation of this part of the gondwana margin resembles the evolution of the north east atlantic in particular the structure of the rockall trough closely resembles that of the new caledonia basin hydrocarbon source rocks may be present in the rift sequence which has been buried to a depth great enough for thermal maturation structural and stratigraphic traps may exist at a number of levels

runup of tsunami waves in u shaped bays the problem of tsunami wave shoaling and runup in u shaped bays such as fjords and underwater canyons is studied in the framework of 1d shallow water theory with the use of an assumption of the uniform current on the cross section the wave shoaling in bays when the depth varies smoothly along the channel axis is studied with the use of asymptotic approach in this case a weak reflection provides significant shoaling effects the existence of traveling progressive waves propagating in bays when the water depth changes significantly along the channel axis is studied within rigorous solutions of the shallow water theory it is shown that traveling waves do exist for certain bay bathymetry configurations and may propagate over large distances without reflection the tsunami runup in such bays is significantly larger than for a plane beach

quantitative tectonic reconstructions of zealandia based on crustal thickness estimates zealandia is a key piece in the plate reconstruction of gondwana the positions of its submarine plateaus are major constraints on the best fit and breakup involving new zealand australia antarctica and associated microplates as the submarine plateaus surrounding new zealand consist of extended and highly extended continental crust classic plate tectonic reconstructions assuming rigid plates and narrow plate boundaries fail to reconstruct these areas correctly however if the early breakup history shall be reconstructed it is crucial to consider crustal stretching in a plate tectonic reconstruction we present a reconstruction of the basins around new zealand great south basin bounty trough and new caledonia basin based on crustal balancing an approach that takes into account the rifting and thinning processes affecting continental crust in a first step we computed a crustal thickness map of zealandia using seismic seismological and gravity data the crustal thickness map shows the submarine plateaus to have a uniform crustal thickness of 20 24 km and the basins to have a thickness of 12 16 km we assumed that a reconstruction of zealandia should close the basins and lead to a most uniform crustal thickness we used the standard deviation of the reconstructed crustal thickness as a measure of uniformity the reconstruction of the campbell plateau area shows that the amount of extension in the bounty trough and the great south basin is far smaller than previously thought our results indicate that the extension of the bounty trough and great south basin occurred simultaneously