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

a mesozoic crustal suture on the gondwana margin in the new zealand region 1 marine crustal multichannel seismic data recorded offshore south island new zealand are used to provide constraints on the evolution of the gondwana margin in this region lower crustal and upper mantle seismic reflectivity off southeastern stewart island south of south island is interpreted in terms of a northeast dipping paleosubduction zone this inferred suture is considered to be related to the docking of the brook street oceanic island arc terrane to the gondwana margin in the triassic the junction of which is now largely covered by the median batholith a similar lower crustal upper mantle feature has been imaged off western south island both inferred sutures have similar strikes and are approximately collinear after restoration of the region for major cenozoic horizontal deformation it is proposed that they are parts of the same paleosuture that was oriented subparallel to the former gondwana margin and to the present paleozoic and mesozoic terranes forming south island

circulation on the central bering sea shelf july 2008 to july 2010 1 we examine the july 2008 to july 2010 circulation over the central bering sea shelf using measurements at eight instrumented moorings hindcast winds and numerical model results at sub tidal time scales the vertically integrated equations of motion show that the cross shelf balance is primarily geostrophic the along shelf balance is also mainly geostrophic but local accelerations wind stress and bottom friction account for 10 40 of the momentum balance depending on season and water depth the shelf exhibits highly variable flow with small water column average vector mean speeds 5 cm s 1 mean peak speeds in summer 3 6 cm s 1 10 30 cm s 1 are smaller than in winter and fall 6 12 cm s 1 30 70 cm s 1 low frequency flows 1 4 cpd are horizontally coherent over distances exceeding 200 km vertical coherence varies seasonally degrading with the onset of summer stratification because effects of heating and freezing are enhanced in shallow waters warm summers increase the cross shelf density gradient and thus enhance northward transport cold winters with increased ice production and brine rejection increase the now reversed cross shelf density gradient and enhance southward transport although the baroclinic velocity is large enough to influence seasonal transports wind forced ekman dynamics are primarily responsible for flow variations the system changes from strong northward flow with coastal convergence to strong southward flow with coastal divergence for northerly and easterly winds respectively under northerly and northwesterly winds nutrient rich waters flow toward the central shelf from the north and northwest replacing dilute coastal waters that are carried south and west

the moho in australia and new zealand abstract australia and new zealand share in part a history in the gondwana supercontinent australia has a long and complex tectonic history with the last major accretion in the early paleozoic whereas new zealand is still undergoing major plate boundary processes the australian continent is relatively well covered with both active and passive seismic techniques multiple sources of information are therefore available for building a model of moho depth results from on shore and off shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations moho picks from more than 10 500 km of full crustal reflection profiles provide valuable additional constraints the composite data set provides good sampling of much of australia though coverage remains low in some remote desert areas the various datasets provide multiple estimates of the depth to moho in many regions and the consistency between the different techniques is high some of the thinnest crust lies beneath the archean craton in the pilbara and in the neighbourhood of the simpson desert thick crust is encountered beneath parts of the proterozoic in central australia and beneath the paleozoic lachlan fold belt in southeastern australia there are a number of zones of sharp contrast in depth to moho notably in the southern part of central australia despite most of the continental material around new zealand being submerged moho data for this region is mainly onshore concentrating on the australia pacific plate boundary two major wide angle reflection transects provide the bulk of the active source data with just a few traditional reflection profiles offshore the plate boundary provides an abundance of local earthquakes for tomographic imaging and this data is supplemented with receiver functions from both portable and permanent networks onshore the combined coverage is as dense as that of australia although it could be argued that a higher spatial resolution is required to capture the nature of the moho of tectonically active new zealand three regions of thickened crust can be identified one beneath the southern alps another beneath fiordland and below the wanganui basin between the north and south island thin crust is identified west of the volcanic arc with extensive underplating below the back arc region

low frequency current regimes over the bering sea shelf abstract using direct current measurements made during the period 1975 81 we describe the general circulation over the southeastern bering sea and differentiate it by regimes related to depth and forcing mechanisms three regimes are present delineated by water depth z the coastal z 50 m the middle shelf 50 z 100 m and the outer shelf z 100 m these are nearly coincident with previously described hydrographic domains statistically significant mean flow 1 to 10 cm s 1 exists over the outer shelf generally directed toward the northwest but with a cross isobath component flow of similar magnitude 1 6 cm s 1 occurs in the coastal regime paralleling the 50 m isobath in a counterclockwise sense around the shelf mean flow in the middle shelf is insignificant kinetic energy at frequencies 0 5 cycle per day cpd is greater over the outer shelf than in the other two regimes suggesting that oceanic forcing is important there but does not affect the remainder of the shelf kinetic

satellite observations of circulation in the eastern bering sea satellite infrared observations of the eastern bering sea shelf and basin were analyzed from december 1979 to august 1981 a region of warm surface water which extends from the gulf of alaska through the aleutian passes into the bering sea was observed coincident with a band of warm surface water and with eddies along the shelf break the spatial and temporal relationships of these patterns imply a surface inflow through unimak and other western aleutian passes flow along the shelf break including eddies along the bering slope current and cyclonic flow in the basin the proposed generating mechanisms for eddies along the shelf break include topographic interactions baroclinic instabilities and rossby waves

preliminary results from a geophysical study across a modern continent continent collisional plate boundary the southern alps new zealand abstract the southern alps of south island new zealand is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement a joint us nz geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three dimensional model of the deformation the measurements undertaken include active source and passive seismology magnetotelluric and electrical studies and petrophysics preliminary models for the active source seismic measurements across south island confirm in general terms a thickened crust under the southern alps a high velocity lower crustal layer and a major crustal discontinuity associated with the alpine fault the anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements shear wave splitting and resistivity the mid crust under the southern alps coincides with a major electrical conductivity high which possibly corresponds to fluid in the crust the top lies at about 15 km close to the base of shallow seismicity east of the alpine fault offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to south island these data are showing complex structure particularly off western and southeastern south island the complexity in structure high quality data and consistency in results from several techniques indicates that the south island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular and the continental lithosphere in general

influence of the 3d distribution of q and crustal structure on ground motions from the 2003 mw 7 2 fiordland new zealand earthquake the m w 7 2 earthquake of 21 august 2003 provides an unprecedented opportunity to evaluate the applicability of the new zealand acceleration response spectrum attenuation model mcverry et al 2006 in the fiordland region and to consider effects from 3d q heterogeneity this was the largest shallow earthquake in new zealand in the past 40 yrs it was well recorded by the geonet seismograph and strong motion network we compare the observed 5 damped response spectral accelerations sas to the new zealand model values for both crustal reverse and subduction interface earthquakes on average both models underpredict the observed sas by approximately 0 4 in lnsa for 0 400 km distance however the data show great variability using the south island 3d q model we compute the path averaged attenuation rate cq for synthetic fiordland hypocenters to distributed sites compared to the standard model the relatively high q in south island produces lower attenuation rates although the low q crustal root of the southern alps produces a 10 times greater attenuation rate for certain paths the average attenuation rate estimated from the sa data is similar to cq for frequencies 5 hz and constant above 5 hz for an acceleration model for fiordland earthquakes 0 60 km depth an applicable anelastic attenuation rate is the mean cq from the synthetic hypocenters which is 0 006 km 1 at 2 5 hz for the observed sas the most significant regional effect is high relative sas at some sites that may be due to postcritical moho reflections from the southern alps crustal root

wind driven ice drift in a shallow sea the free drift equations are solved for ice floes in a shallow sea of neutral stratification typical of many high latitude continental shelves solutions for ice drift and current velocity are obtained as a function of wind stress ice thickness and water depth the ocean is modeled by second order closure which allows continuous solutions from 5 m total depth to deep water results are presented with drag coefficients for the air ice ice water and water bottom interfaces specified from recent surveys from the bering sea shelf a region with broad areas of water depths between 20 and 50 m the solution shows little dependence on water depth for depths greater than 30 m this occurs because turbulent mixing is a decreasing function of water depth and offsets other influences of finite depth however for water depths less than 30 m ice velocities can change rapidly with wind speed and water depth and the presence of turbulence from tidal shear is very important for coupling wind driven ice drift to the bottom for the deep water limit the second order closure solution confirms analytic solutions that indicate an increase of 20 in the ratio of ice speed to wind speed as the wind speed increases from 10 to 25 m s

on the bering sea ice edge front an oceanographic field program was carried out along the bering sea ice edge in winter 1982 83 to investigate the temperature salinity front associated with and paralleling the winter ice edge currents were measured using taut wire moorings that were in place over winter and an intensive ctd survey was carried out of the frontal structure in midwinter february march the midwinter front separated cold 0 c more saline 32 5 outer shelf water the front had northwestward baroclinic currents paralleling the front ice edge with near surface speeds of 5 6 cm s and an associated transport of about 0 3 106 m3 s these baroclinic currents were superimposed on a regional barotropic along isobath flow resulting in north westward midwinter upper layer flows with monthly mean speeds exceeding 10 cm s an upper layer flow component was directed beneath the ice and provided heat adequate to melt the ice and limit advance of the edge the resulting low salinity meltwater was mixed vertically to maintain the front by wind acting across a highly mobile hydrodynamically rough ice cover subtidal fluctuations were not significantly affected by the front or the ice cover and local winds accounted for less than one quarter of the observed current variability the remainder being probably due to regional variations in the barotropic flow

the bering slope current system revisited mean circulation and water properties within the aleutian basin of the bering sea are investigated using hydrographic and subsurface park pressure displacement data from a regional array of 14 profiling ctd floats after 10 days drifting at 1000 dbar each float measures temperature and salinity profiles as it rises to the surface and then transmits these data via satellites which also make several fixes of the float surface positions before it sinks again every fourth cycle the floats sink from 1000 dbar to a 2000 dbar target just prior to ascent to measure deeper profiles the 1000 dbar displacements estimated from the float surface position fixes reveal a coherent few centimeters per second northwestward flow along the northeastern boundary the deep signature of the bering slope current middepth water property distributions are consistent with cyclonic advection of warm water from the south around the basin eastward in the aleutian north slope current and then northwestward in the bering slope current geostrophic transport estimates relative to 1000 dbar also show cyclonic motion although with significant noise likely owing to the influence of mesoscale eddies the mean alongslope geostrophic transport of the bering slope current is determined between 0 and 1900 dbar relative to 1000 dbar and then combined with mean along slope velocities at 1000 dbar the result is an absolute geostrophic transport estimate with 95 confidence intervals for the along slope current offshore of the 1000 m isobath and between 0 and 1900 dbar of 5 8 61 7 3 106 m3 s 21

velocity and anisotropy structure at the hikurangi subduction margin new zealand from receiver functions summary we examine seismic velocity and anisotropy at the interface between the australian and the subducting pacific plate in southern north island new zealand using receiver functions from teleseismic earthquakes recorded on two l shaped seismic arrays iris station snzo makes up the corner of the southern array velocity models are tested with a ray theoretical method that allows both dipping boundaries and anisotropy assuming a 2 d structure beneath each array the strongest arrival on the northern array consistently appears at 2 s the arrival is modelled by a jump in velocity at a flat boundary at 12 km which may be caused by a change in metamorphic grade this prominent arrival correlates with a weaker arrival at 1 5 s 7 km at station snzo isotropic layers dipping with the slab can explain many features in the first 20 s of the records but anisotropy is needed to explain both the transverse receiver functions and a change in the polarity of a prominent ps arrival at 3 s delay relative to p o nthe radial component this arrival is consistent with a change between an upper layer that is either isotropic or anisotropic with a horizontal symmetry axis and a supraslab metasediment layer having anisotropy with av ertical slow axis of symmetry there is tradeoff between boundary dip the plunge of the axis of symmetry layer thickness and the amount of anisotropy a dip of 20 shallowing to 15 under the tararua array thickness of 4 km and a vertical symmetry axis fits the backazimuth variation of the amplitudes and polarities of the arrivals with anisotropy of 4 6 per cent observed layer multiples are smaller than synthetic predictions to explain this discrepancy either layer boundaries are rough attenuation is strong or the layer boundaries are more gradational at the periods 1 s used in this study compared to previous studies 5 s

a census of mesoscale eddies in shelikof strait alaska during 1989 over a 5 month period in the spring and summer of 1989 12 mesoscale eddies were identified crossing a section of moored current meters in the shelikof sea valley this represents the first census of mesoscale eddies on the northwestern gulf of alaska continental shelf the majority of eddies 7 were observed in may and june anticyclonic eddies outnumbered cyclonic eddies two to one but all had characteristics similar to eddies previously observed in the region analysis of the current meter time series revealed that the mean speeds volume transport and the structure of the flow at the present location were similar to current meter and hydrographic measurements during other years in the sea valley we suggest that 1989 was a typical year in terms of eddy formation as well the proliferation of mesoscale eddies in this region in spring has an impact on the larval dispersal and eventual recruitment of walleye pollock

coupled wind forced controls of the bering chukchi shelf circulation and the bering strait throughflow ekman transport continental shelf waves and variations of the pacific arctic sea surface height gradient abstract we develop a conceptual model of the closely co dependent bering shelf bering strait and chukchi shelf circulation fields by evaluating the effects of wind stress over the north pacific and western arctic using atmospheric reanalyses current meter observations satellite based sea surface height ssh measurements hydrographic profiles and numerical model integrations this conceptual model suggests bering strait transport anomalies are primarily set by the longitudinal location of the aleutian low which drives oppositely signed anomalies at synoptic and annual time scales synoptic time scale variations in shelf currents result from local wind forcing and remotely generated continental shelf waves whereas annual variations are driven by basin scale adjustments to wind stress that alter the magnitude of the along strait meridional pressure gradient in particular we show that storms centered over the bering sea excite continental shelf waves on the eastern bering shelf that carry northward velocity anomalies northward through bering strait and along the chukchi coast the integrated effect of these storms tends to decrease the northward bering strait transport at annual to decadal time scales by imposing cyclonic wind stress curl over the aleutian basin and the western subarctic gyre ekman suction then increases the water column density through isopycnal uplift thereby decreasing the dynamic height sea surface height and along strait pressure gradient storms displaced eastward over the gulf of alaska generate an opposite set of bering shelf and aleutian basin responses while ekman pumping controls canada basin dynamic heights proshutinsky et al 2002 we do not find evidence for a strong relation between beaufort gyre sea surface height variations and the annually averaged bering strait throughflow over the western chukchi and east siberian seas easterly winds promote coastal divergence which also increases the along strait pressure head as well as generates shelf waves that impinge upon bering strait from the northwest

deformed neogene basins active faulting and topography in westland distributed crustal mobility west of the alpine fault transpressive plate boundary south island new zealand abstract tectonic activity in the south island of new zealand is dominated by the alpine fault component of the australia pacific plate boundary west of the alpine fault deformation is recorded by paleogene neogene basins coeval with the evolution of the right lateral transpressive plate margin initial tectonic setting was controlled by n s normal faults developed during late cretaceous and eocene early miocene rifting following inception of the alpine fault c 25 ma reverse reactivation of the normal faults controlled tectonic segmentation that became apparent in the cover sequences at c 22 ma based on restored transects tied to stratigraphic sections seismic lines and wells we reconstruct the vertical mobility of the top basement unconformity west of alpine fault from c 37 35 ma to 22 ma subsidence was controlled by extensional faulting after 22 ma the region was affected by differential subsidence resulting from eastward crustal flexure towards the alpine fault boundary and or components of transtension transition from subsidence to uplift started at c 17 ma within a belt of basement pop ups separated by subsiding basins localised in the common footwall of oppositely dipping reverse faults from 17 to 7 3 ma reverse fault reactivation and uplift migrated to the wsw persistent reverse reactivation of the inherited faults in the present stress field is reflected by the close match between tectonic block segmentation and topography filtered at a wavelength of 25 km i e at a scale comparable to crustal thickness in the region however topography filtered at wavelength of 75 km shows marked contrasts between the elevated tasman ranges region relative to regions to the south variations in thickness and rigidity of the australian lithosphere possibly control n s longitudinal changes consistent with our estimates of increase in linear shortening from the tasman ranges to the regions located west of the alpine fault bend

the lithosphere asthenosphere boundary beneath the south island of new zealand abstract lithosphere asthenosphere boundary lab properties beneath the south island of new zealand have been imaged by sp receiver function common conversion point stacking in this transpressional boundary between the australian and pacific plates dextral offset on the alpine fault and convergence have occurred for the past 20 my with the alpine fault now bounded by australian plate subduction to the south and pacific plate subduction to the north using data from onland seismometers especially the 29 broadband stations of the new zealand permanent seismic network geonet we obtained 24 971 individual receiver functions by extended time multi taper deconvolution and mapped them to three dimensional space using a fresnel zone approximation pervasive strong positive sp phases are observed in the lab depth range indicated by surface wave tomography these phases are interpreted as conversions from a velocity decrease across the lab in the central south island the lab is observed to be deeper and broader to the northwest of the alpine fault the deeper lab to the northwest of the alpine fault is consistent with models in which oceanic lithosphere attached to the australian plate was partially subducted or models in which the pacific lithosphere has been underthrust northwest past the alpine fault further north a zone of thin lithosphere with a strong and vertically localized lab velocity gradient occurs to the northwest of the fault juxtaposed against a region of anomalously weak lab conversions to the southeast of the fault this structure could be explained by lithospheric blocks with contrasting lab properties that meet beneath the alpine fault or by the effects of pacific plate subduction the observed variations in lab properties indicate strong modification of the lab by the interplay of convergence and strike slip deformation along and across this transpressional plate boundary

three dimensional lithospheric structure below the new zealand southern alps uppermost mantle seismic structure below the southern alps in south island new zealand is investigated by teleseismic p wave travel time residual inversion the three dimensional tomographic images show a near vertical high velocity 2 4 structure in the uppermost mantle that directly underlies thickened crust along the nnessw axis of the southern alps the center of the high velocity anomaly lies to the east of the alpine fault which bounds pacific and australian plate rocks the oblique collision of these plates resulted in the uplift of the southern alps during the past 5 7 m y also a high velocity anomaly 3 5 corresponding to the hikurangi subduction zone lies to the northeast of the southern alps anomaly and low velocity anomalies 3 underlying parts of northwestern and southern south island may be signatures of late tertiary extension and volcanism the data consist of teleseismic arrival times from the new zealand national seismograph network and arrival times recorded during the 1995 1996 southern alps passive seismic experiment crustal heterogeneity was accounted for by back projecting the rays through an independently obtained three dimensional crustal velocity and moho depth model the southern alps uppermost mantle velocity anomalies are most simply explained by lithospheric thickening below the center of convergence accompanied by thinning and asthenospheric upwelling adjacent to the region of convergence

crustal and upper mantle seismic structure of the australian plate south island new zealand abstract seismic reflection and refraction data were collected west of new zealand s south island parallel to the pacific australian plate boundary the obliquely convergent plate boundary is marked at the surface by the alpine fault which juxtaposes continental crust of each plate the data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary arrival times of wide angle reflected and refracted events from 13 recording stations are used to construct a 380 km long crustal velocity model the model shows that beneath a 2 4 km thick sedimentary veneer the crust consists of two layers the upper layer velocities increase from 5 4 5 9 km s at the top of the layer to 6 3 km s at the base of the layer the base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end the lower layer velocities range from 6 3 to 7 1 km s and are commonly around 6 5 km s at the top of the layer and 6 7 km s at the base beneath the lower layer the model has velocities of 8 2 8 5 km s typical of mantle material the mohorovicic discontinuity moho therefore lies at the base of the second layer it is at a depth of around 30 km but shallows over the south central third of the profile to about 26 km possibly associated with a southwest dipping detachment fault the high variable sub moho velocities of 8 2 km s to 8 5 km s are inferred to result from strong upper mantle anisotropy multichannel seismic reflection data cover about 220 km of the southern part of the modelled section beneath the well layered oligocene to recent sedimentary section the crustal section is broadly divided into two zones which correspond to the two layers of the velocity model the upper layer down to about 7 9 s two way travel time has few reflections the lower layer down to about 11 s two way time contains many strong subparallel reflections the base of this reflective zone is the moho bi vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening these structures and the strong northeast dipping reflections beneath the moho towards the north end of the mcs line are interpreted to be caused by paleozoic north dipping subduction and terrane collision at the margin of gondwana deeper mantle reflections with variable dip are observed on the wide angle gathers travel time modelling of these events by ray tracing through the established velocity model indicates depths of 50 110 km for these events they show little coherence in dip and may be caused side swipe from the adjacent crustal root under the southern alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root

tracking repeated subduction of the hikurangi plateau beneath new zealand abstract the hikurangi plateau has been subducted beneath new zealand twice firstly at ca 100 ma during n s convergence with gondwana and currently during e w convergence between the pacific and australian plates here we use a nationwide 3 d seismic velocity model derived from seismic tomography studies and seismicity relocated with this model to determine the location of the subducted western and southern edges of the plateau in the hikurangi subduction zone in the north island and northern south island we identify the western edge in the 37 140 km depth range based on the distribution of seismic velocities in the top of the subducted pacific plate changes in the thickness of the dipping seismic zone and steepening of the subducted plate downdip of the buoyant plateau in the central south island where the hikurangi plateau is subducting for a second time the plateau dips less steeply its southern edge is well defined by the collision of the delaminated lower crust of the plateau with the northern fiordland subduction zone where it bends the younger subducted australian plate to vertical the hikurangi plateau was originally rifted from the ontong java plateau and the subducted portions of the hikurangi plateau identified here fill a gap in reconstructions of this large igneous province

seismic reflections from the mantle represent relict subduction zones within the continental lithosphere in many areas of the world the continental lithospheric upper mantle contains bright continuous regionally extensive seismic reflectors despite the increasingly common observation of such mantle reflectors on deep seismic reflection profiles their significance and geologic origin remain obscure we report results from a series of seismic experiments acquired across two of the brightest of these reflectors and provide new constraints upon the composition and thickness of the reflecting region these new seismic data reveal regionally extensive dipping and subhorizonal slabs of high velocity 8 4 km s high density 3500 kg m 3 material several kilometres 2 km thick entrained within otherwise unremarkable upper mantle the geometry physical properties and geologic setting of these mantle reflectors suggest that they represent fragments of eclogitic oceanic crust a relict of pre caledonian oceanic subduction now preserved within the lower continental lithosphere such relict subduction zones appear to be widespread within the continental lithosphere and to exert an important influence upon the location and style of subsequent continental deformation

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

trapped motion around the pribilof islands in the bering sea observations in the region of the pribilof islands and canyon pic reveal a clockwise circulation around the group of islands and around each of the two largest islands st paul and st george six current meters deployed around st paul island revealed a steady clockwise flow around the island which was strongest south of the island and weakest to the east we use a high resolution tidal model in the pic region to show that this flow pattern results from tidal rectification over the shallow topography tides tidal residual currents of 10 15 cm s were predicted by the model compared to mean currents of 5 20 cm s observed at the mooring sites both diurnal and semidiurnal tidal oscillations contribute to the clockwise circulation around the islands in the diurnal band the enhanced currents occur also at the shelf slope where a tidal wave generates a shelf wave due to resonance with topography in the pic region the main shelf wave occurs in the pribilof canyon where both observations and measurements show enhancement of the diurnal tidal currents

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

summertime synoptic variability of frontal systems in the northern bering sea hydrographic observations in the northern bering sea during august and september 1987 indicate the presence of a front dividing relatively warm fresh bering shelf water from relatively cool saline anadyr water along the western and northern coasts of st lawrence island near anadyr strait a buoyant layer 20 m thick with surface salinities as low as 29 5 practical salinity units and a maximum temperature of 10 c was present adjacent to the island the surface outcrop of the front migrated 80 km north during the nine day time period of the hydrographic observations surface thermal patterns suggest that this front may extend the length of the northern coastline of st lawrence island during the summer the front veers north and passes through the bering strait where temperature differences as large as 6 c exist across the strait an examination of the water mass properties of the bering sea suggests that the buoyant water north of st lawrence island is bering shelf water which has been carried northward through anadyr strait the baroclinic transport assuming no flow at the bottom associated with the front is 0 07 sv which is roughly a third of the seasonal increase in transport through the bering strait in the summer

coupled nitrification denitrification in sediment of the eastern bering sea shelf leads to 15n enrichment of fixed n in shelf waters 1 we studied the nitrogen biogeochemistry of the ice covered eastern bering sea shelf using the isotope ratios 15n 14n and 18o 16o of no3 and other n species the 15n 14n of late winter no3 on the shelf decreases inshore and is inversely correlated with bottom water nh4 consistent with an input of low 15n 14n no3 from partial nitrification of nh4 remineralized from the sediments an inshore 15n 14n increase in total dissolved n tdn suggests that 1 the sediment derived nh4 is elevated in 15n due to the same partial nitrification that yields the low 15n 14n no3 and 2 15n deplete no3 from partial nitrification within the sediments is denitrified to n2 the proportion of newly nitrified no3 on the shelf evidenced by an inshore decrease in no3 18o 16o is correlated with the n deficit further implicating nitrification coupled to denitrification however a simple n isotope budget indicates a comparable rate of denitrification supported by diffusion of no3 into the sediments the isotopic impact of benthic n loss is further demonstrated by a correlation between the 15n 14n of shelf surface sediment and the n deficit of the overlying water column both of which increase inshore and northward as well as by arctic no3 isotope data indicating that the fixed n transported through bering strait has a 15n 14n higher than is found in the open bering sea the significant net isotope effect of benthic n loss on the bering shelf 6 8 is at odds with previous assumptions regarding the global ocean s n isotope budget

quantifying dextral shear on the bristol granite mountains fault zone successful geologic prediction from kinematic compatibility of the eastern california shear zone abstractfor regional kinematic compatibility to be a valid boundary condition for continental tectonic reconstructions there must be tests that validate or invalidate kinematic model predictions in several reconstructions of western north america the displacement history of the mojave block continues to be unresolved the magnitude of displacement along the bristol granite mountains fault zone bgmfz which is the eastern margin of the eastern california shear zone ecsz in the mojave block is a key example of a long standing kinematic prediction that has defied a positive field test until now the ecsz is a network of late neogene and quaternary right lateral strike slip faults that extend from the gulf of california north through the mojave desert linking pacific north america plate motion with basin and range extension this network of faults accounts for 15 of post 16 ma plate transform motion geologic estimates of net dextral offset along the mojave portion of the ecsz 53 6 km are appro

ice drift and regional meteorology in the southern bering sea results from mizex west in february 1983 a multifaceted study of the air ice ocean interaction was made in the southern bering sea the study mizex west addressed a broad spectrum of physical problems related to the bering sea marginal ice zone miz as a part of that study an array of eight argos tracked floes on a scale of 50 km provided information on the mesoscale behavior of the ice pack wind and current measuring platforms on two of the floes gave detailed information on the forces on individual floes which were compared with the larger scale motions under relatively steady northeast winds and with weak or negligible regional currents the floes accelerated considerably as they crossed the miz the array showed little distortion even though it skirted around st matthew island and changed trajectory direction by over 90 during the 12 day study period similarly individual floes remained within 20 of their original orientation although their angular motions were erratic and often rapid a baroclinicly forced acceleration of the wind over the miz contributed to floe acceleration the relative currents at 2 and 6 m under the ice were highly variable and random in time although well correlated in depth one of the floes was much smoother than the other and higher winds and currents were noted at the smoother floe the motion of the floes reflect strong coupling to the currents at tidal and lower frequencies and low frequency greater than 6 hours response to the wind both floes drifted to the right of the mean wind by approximately 30 at about 4 of the wind speed at 3 m and relative wind and current directions were within 20 of being colinear traits that are consistent with free drift hypothesis using quadratic drag representations for wind and current stress the near colinearity allowed calculation of the ratio of the air ice to ice water drag coefficients ca cw relative to measurement heights of 3 m and 2 m for the rough floe this ratio changed in mean from 0 06 to 0 2 in its movement to the miz for the smooth floe the variation in the ratio was less from 0 14 to 0 2 however for the rough flow the angle between wind and current was generally less than 180 and for the smooth flow it was greater it is possible that another type of drag related to the bow effect of the floe pushing through the water can account for this observation finally the motion of the ice floes and ice edge are compared by examination of satellite photos two regions of the ice pack one thick with rafting and rubble one thin and broken showed a ratio of edge velocity to floe velocity of 0 64 and 0 43 respectively thinner ice overtakes the edge and melts more rapidly than the thicker ice a simple thermodynamic model explains this observation

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

sub crustal earthquakes within the australia pacific plate boundary zone beneath the southern alps new zealand abstract sub crustal earthquakes have been observed sporadically for 40 years in the central south island of new zealand we report on 20 events recorded between december 2008 and february 2012 near the alpine fault in the continental collision zone between the australian and pacific plates a subset of 18 events at depths of 47 74 km occurs south of mt cook and together with recently reported tremor locations indicates along strike variations in deformation behaviour along the plate boundary the sub crustal earthquakes south of mt cook increase in depth frequency and size southwards towards the puysegur subduction zone focal mechanisms could be determined for 14 earthquakes and exhibit predominantly strike slip and reverse faulting solutions stress inversion analysis of the focal mechanisms yields a stress field favouring oblique reverse faulting we interpret the geographic and vertical distributions of these sub crustal events in relation to a previously proposed tectonic model of a remnant passive margin that formed south of new zealand in the eocene and was overridden when dextral strike slip motion initiated on the alpine fault we infer that sub crustal earthquakes occur along the leading edge of this structure which is attached to the continental australian crust

plate boundary deformation in south island new zealand is related to inherited lithospheric structure abstract the alpine fault is the main active structure in the oblique continental collision zone of south island new zealand it is continuous at the surface for 800 km and accommodates 70 of current plate motion a 460 km offset of basement rocks suggests it has accommodated 50 of plate displacement since 45 ma geophysical geological and contemporary kinematic data are successfully modeled by slip on a narrow alpine fault zone extending into the lower crust the data are consistent with plate boundary deformation occurring on either northwest or southeast dipping shear zones in the upper mantle or with widely distributed strain in the upper mantle we propose that the position of the alpine fault and any associated lithospheric discontinuity is controlled by an eocene passive margin that separated palaeozoic continental lithosphere of the challenger plateau from much younger oceanic lithosphere strike slip motion since 25 ma has translated the passive margin into the continental collision zone resulting in subduction of oceanic lithosphere beneath south island and progressive localisation of shear strain near the alpine fault the eocene rift boundary formed by exploiting cretaceous oceanic transform faults and an older discontinuity within the new zealand continent we propose that a precursor to the alpine fault could be as old as palaeozoic inherited structure has clearly controlled the first order deformation pattern through new zealand since at least 100 ma and demonstrates that ancient faults can play an important role in determining lithospheric scale patterns of deformation at continental plate boundaries

the proportion of remineralized nitrate on the ice covered eastern bering sea shelf evidenced from the oxygen isotope ratio of nitrate 1 we present measurements of nitrate and its natural abundance oxygen isotope composition 18o 16o in the water column of the broad and shallow eastern bering sea shelf in the late winter and early spring of 2007 and 2008 in both years nitrate concentrations showed a characteristic decrease from 25 m at the slope to 5 m inshore the 18o 16o ratio of nitrate 18ono3 versus smow also decreased from 3 2 at the slope to 1 5 inshore in 2007 and to as low as 1 inshore in 2008 indicating that nitrate inshore was nitrified at least once since having been entrained as nitrate from the slope the shoreward decrease was less pronounced in 2007 due to 18o enrichment of nitrate from incident phytoplankton assimilation in the ice covered water column whereas little to no algal growth in the water column was evident in 2008 by comparing the 18o of nitrate to that of ambient water in spring 2008 we estimate the fraction of nitrate that was remineralized in situ rather than recently advected from the slope these estimates indicate that 20 100 of the nitrate in winter water of the middle and inner shelves derives from regeneration directly on the shelf rather than from the seasonal entrainment of slope waters with recycling being the dominant mode of seasonal nitrate recharge from the 70 m isobath shoreward these observations indicate substantial nutrient recycling on the shallow shelf which has direct implications for the extent of fixed n loss to benthic denitrification and the fertility of the eastern shelf

continental subduction and three dimensional crustal structure the northern south island new zealand the three dimensional vp and vp vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment and 3146 earthquakes have been relocated between 40 km and 100 km depth the subducted plate is imaged as a relatively low velocity feature in the uppermost mantle reflecting the continental nature of the subducted crust in this region an increase in amplitude of this low velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction velocity variations within the subducted and overlying plates show some spatial correlation this suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath marlborough is currently not accommodating much active subduction in the overlying plate the awatere fault is a major structural feature associated with a low velocity zone extending to 23 km depth there is a marked change in structure near this fault with seismic velocities being lower to the southeast a relatively high level of seismicity occurs in this region of lower seismic velocities suggesting a relationship between the two a possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust the low velocity region in the overlying plate coincides with the region of most intense active deformation suggesting it is relatively weak

continent scale strike slip on a low angle fault beneath new zealand s southern alps implications for crustal thickening in oblique collision zones new zealand s southern alps lie adjacent to the continent scale dextral strike slip alpine fault on the boundary between the pacific and australian plates we show with a simple 2 d model of crustal balancing that the observed crustal root and erosion expressed as equivalent crustal shortening is up to twice that predicted by the orthogonal plate convergence since 11 ma and even since 23 ma when the alpine fault formed we consider two explanations for this involving a strong component of motion along the length of the plate boundary zone geophysical data indicate that the alpine fault has a listric geometry flattening at mid crustal levels and has accommodated sideways underthrusting of australian plate crust beneath pacific plate crust the geometry of the crustal root together with plate reconstructions requires the underthrust crust to be the hyperextended part of an asymmetric rift system which formed over 500 km farther south during the eocene the narrow remnant part today forms the western margin of the campbell plateau at 10 ma the hyperextended margin underwent shallow subduction in the puysegur subduction zone and then was dragged over 300 km along the length of the southern alps beneath a low angle 20 section of the alpine fault we speculate that prior to 10 ma more distributed lower crustal shortening and thickening occurred beneath the southern alps accommodating southward extrusion of continental crust in the northern part of the plate boundary zone providing a mechanism for clockwise rotation of the hikurangi margin

crustal structure and apparent tectonic underplating from receiver function analysis in south island new zealand we utilize seismic converted phases on more than 700 receiver functions calculated for 42 stations in the south island new zealand to infer crustal and uppermost mantle structure we determine the crustal thickness from direct observations of conversion from the moho interface and infer zone of the maximum thickness being located along the axis of the southern alps just east from the alpine fault the crustal root widens from north to south in the direction perpendicular to the alpine fault and appears to have an asymmetric structure stations in the alpine portion of island show evidence for prominent midcrustal conversions significant crustal thickening is developed in response to both the convergent component of the motion on the alpine fault and subduction in the fiordland region we propose two models for a strong uppermost mantle conversion that occurs at depths between 33 and 83 km on 16 stations and forms a large continuous feature along the east coast and in the central portions of the south island our preferred model attributes upper mantle conversion to tectonically underplated oceanic crust formed by late oligocene miocene spreading between the australian and pacific plates which was detached from the australian plate and tectonically underplated under the south island an alternative model attributes the upper mantle conversions to long lived seismic fabric created by subduction of the gondwanaland margin

the crust and upper mantle discontinuity structure beneath alaska inferred from receiver functions in this study three receiver function stacking methods are used to study the detailed crust and upper mantle structure beneath south central alaska we used teleseismic waveform data recorded by 36 stations in the broadband experiment across the alaska range beaar and 4 permanent stations in alaska h stacking method using p to s converted wave and its multiply reflected waves between the earth s surface and the moho discontinuity is adopted to estimate the crustal thickness h and average crustal vp vs ratio in this region the receiver function results for 24 stations show that the crustal thickness under alaska ranges from 26 0 to 42 6 km with an average value of 33 8 km and the vp vs ratio varies from 1 66 to 1 94 with an average value of 1 81 which corresponds to an average poisson s ratio of 0 277 with a range from 0 216 to 0 320 high poisson s ratios under some stations are possibly caused by partial melting in the crust and the uppermost mantle common converted point ccp stacking results of receiver functions along three lines show clear moho and slab images under this subduction zone the depths of the slab from our ccp stacking images are consistent with those estimated from the wadati benioff zone wbz in the area between two stations dh2 147 8 w 63 3 n and dh3 147 1 w 63 0 n a moho depth offset of about 10 km is found by both the h and ccp stacking techniques common depth point cdp stacking of receiver functions shows not only the 410 520 and 660 km discontinuities but also significant variations 30 to 15 km in the transition zone thickness under the southwest and southeast parts of the study region the transition zone becomes thinner by 20 30 km indicating that the temperature there is 150 200 k higher than that of the normal mantle 2004 elsevier b v all rights reserved

characteristics and variability of the inner front of the southeastern bering sea the inner front of the southeastern bering sea shows marked spatial variability in frontal characteristics created by regional differences in forcing mechanisms differences in forcing mechanisms sea ice advance retreat and storm strength and timing and early spring water properties result in strong interannual variability in biological chemical and physical features near the front we have developed a simple model based on surface heat flux and water column mixing to explain the existence of cold belts cont shelf res 19 14 1999 1833 associated with such fronts hydrography fluorescence and nutrient observations show that pumping of nutrients into the euphotic zone occurs and this can prolong primary production at the inner front the effectiveness of this process depends on two factors the existence of a reservoir of nutrients in the lower layer on the middle shelf and the occurrence of sufficient wind and tidal energy to mix the water column

mapping the moho beneath the southern alps continent continent collision new zealand using wide angle reflections 1 a wide angle stack using all onshore offshore and onshore seismic data from the sight 96 experiment provides a direct three dimensional 3 d image of the moho below the continental collision zone through the south island of new zealand a bright reflector sequence up to 4 s thick the base of which coincides with the pmp modeled moho extends throughout most of the lower crust and bends downward asymmetrically beneath the southern alps the crustal root reaches a maximum depth at 15 s 45 km beneath the regional 80 mgal bouguer gravity low and is flanked east and west by shallower moho where the average crustal thickness is 20 km the 3 d structure of the crustal root in the central south island is moderately well resolved by the sight 96 experiment

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

crust and mantle thickening beneath the southern portion of the southern alps new zealand summary an 8 54 0 20 km s 1pn speed is estimated on a line oriented ca n5 e from the alpine fault beneath the southern alps of south island based on a refraction experiment that uses the fiordland earthquake of august 2003 as a source this high pn speed results from both strong anisotropy in the mantle lid of 7 13 per cent and a high pn speed average of 8 3 0 3 km s 1 a maximum crustal thickness of 48 4 km is calculated for the southern south island near the town of wanaka this represents a crustal root of about 18 km compared to measured crustal thicknesses at the east and west coasts of the south island the average topography in the southern southern alps is of the order of 1000 m which is less than half that predicted by airy isostasy for an 18 km crustal root as recently proposed for the central south island 120 km to the north we propose that thickened cold and therefore more dense mantle lithosphere exists beneath southern south island and that this excess of mass is an effective load that pulls down the overlying crust the load is similar to that beneath the central southern alps despite the predicted convergence across the alpine fault there being nearly twice that at wanaka gravity modelling of crustal structure along a profile through wanaka suggests that this mass excess has a minimum density contrast of 35 5 kg m 3 between thickened mantle and asthenosphere assuming an across moho density contrast of 300 kg m 3 we speculate that the reason for the enhanced thickening beneath wanaka is that the subducted australian plate at the southwestern corner of the south island acts like a backstop onto which pacific mantle collides at 26 mm yr 1 ca 3 4 the full plate speed

insights into the structure and tectonic history of the southern south island new zealand from the 3 d distribution of p and s wave attenuation

continental and oceanic crustal structure of the pampean flat slab region western argentina using receiver function analysis new high resolution results summary the pampean flat slab of central chile and argentina 30 32 s has strongly influenced cenozoic tectonics in western argentina which contains both the thick skinned basement cored uplifts of the sierras pampeanas and the thin skinned andean precordillera fold and thrust belt in this region of south america the nazca plate is subducting nearly horizontally beneath the south american plate at 100 km depth to gain a better understanding of the deeper structure of this region including the transition from flat to normal subduction to the south three iris passcal arrays of broad band seismic stations have been deployed in central argentina using the dense siembra array combined with the broader charge and esp arrays the flat slab is imaged for the first time in 3 d detail using receiver function rf analysis a distinct pair of rf arrivals consisting of a negative pulse that marks the top of the oceanic crust followed by a positive pulse which indicates the base of the oceanic crust can be used to map the slab s structure depths to moho and oceanic crustal thicknesses estimated from rf results provide new more detailed regional maps an improved depth to continental moho map shows depths of more than 70 km in the main cordillera and 50 km in the western sierras pampeanas that shallow to 35 km in the eastern sierras pampeanas depth to moho contours roughly follow terrane boundaries offshore the hotspot seamount chain of the juan fernandez ridge jfr is thought to create overthickened oceanic crust providing a mechanism for flat slab subduction by comparing synthetic rfs based on various structures to the observed rf signal we determine that the thickness of the oceanic crust at the top of the slab averages at least 13 19 km supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat the overthickened region is broader than the area directly aligned with the path of the jfr however and indicates along with the slab earthquake locations that the flat slab area is wider than the jfr volcanic chain observed in the offshore bathymetry further rfs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench parallel faults one explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted alternatively it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region