distribution and mechanism of neogene to present day vertical axis rotations pacific australian plate boundary zone south island new zealand remarkably little knowledge exists about mechanisms of vertical axis rotation in continental crust steeply dipping basement rocks in south island new zealand provide an opportunity to map the distribution of rotations across the pacific australian plate boundary zone and to delineate boundaries of rotated blocks in unusual detail we synthesize new structural data with new and existing paleomagnetic data with geodetic data and with patterns of neogene quaternary faulting in the strike slip marlborough fault system for the past 20 m y vertical axis rotations have been hinged about two crustal scale boundaries near the east coast the ne hinge accommodated 50 of early middle miocene clockwise rotation which caused deformation of the eastern ends of the alpine wairau and clarence strike slip faults the sw hinge has accommodated a further 30 50 of finite clockwise rotation since 4 ma and deflects active fault traces the locus of rotation has shifted southwestward astride a subduction margin that is lengthening in that direction rotating rocks are pinned to the south against a locked collision zone where the continental chatham rise impinges against the margin slip on inland strike slip faults is transformed seaward across a zone of fault termination into rigid body rotation of a large continental block that has been thrust eastward over the downgoing subducted slab of the pacific plate the rotation mechanism is a migrating hinge which resembles a flexed telephone book strike slip faults are translated through a brecciated hinge region that does not coincide with a fixed material line in the rock

thermal state of the taranaki basin new zealand the taranaki basin is an active margin basin that has been significantly affected by miocene subduction tectonics along the pacific australian plate boundary we have analyzed its present day thermal state using 354 bottom hole temperatures bhts from 115 wells distributed throughout the basin the measured temperatures were corrected using an exact solution to bullard s equation rather than the horner approximation thereby allowing for recovery dependence on well diameter and correction for some bhts at early time after circulation had ceased thermal conductivity measurements were completed on 256 samples from eight wells and matrix conductivities were determined for six end member lithologies by inversion formation conductivities are based on the conductivity and relative proportion of each end member component corrected bhts in situ thermal conductivity and estimates of sediment heat production were combined to compute the present day steady state heat flow the average heat flow is 60 mw m 2 but important geographic variations are present heat flow on the western platform is remarkably consistent at 53 60 mw m 2 attesting to its relative stability since the late cretaceous heat flow in the southern part of the basin is 65 70 mw m 2 due to as much as 3 km of late miocene erosion on the southern onshore and to the south of the peninsula heat flow is 50 3 mw m 2 possibly due to the heat sink effects of crustal thickening heat flow is highest at 74 mw m 2 on the northern peninsula adjacent to the taranaki volcanic zone suggesting a causal relationship between quaternary volcanism and high heat flow

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

late mesozoic and cenozoic palaeomagnetism of australia i a redetermined apparent polar wander path summary palaeomagnetic measurements have been carried out on one cretaceous and five cenozoic sedimentary sequences of australia of those one is in the carnarvon basin of north western australia and the others are in the otway basin of south eastern australia the new results are used together with those published previously to define a revised late mesozoic cenozoic apparent polar wander path for australia this path differs from earlier basalt derived paths by the absence of a large westward excursion and zig zag irregularities it is characterized instead by a substantially straight cenozoic trajectory a sharp bend in the late cretaceous and a non uniform rate of apparent polar wander the early tertiary segment of the new path lies north of the original paths thus eliminating a discrepancy that has been noted previously between the indian and australian palaeomagnetic data luyendyk rennick peirce and klootwyk peirce a re examination of the original data suggests that this discrepancy as well as two others in the australian results may have been caused principally by incomplete time averaging of remanence directions because the volcanics on which the results were based had been extruded episodically

landward and seaward directed thrusting accompanying the onset of subduction beneath new zealand a short 5 m y episode of deformation accompanied the onset of the present phase of subduction beneath northern and eastern new zealand in the earliest miocene at many places along the continental margin the structures produced were dominated by subhorizontal thrust sheets with tens of kilometres of displacement thrusting was directed both landward with or without involvement of the upper part of the downgoing plate and seaward the structures are different in style from those typical of active trench slopes which are predominantly seaward directed imbricate thrusts and associated folds the structures show that delamination of the downgoing plate occurred at some places and suggest that a high degree of coupling between upper and lower plates occurred at others

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

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

study of the high frequency spectrum of the anomalous gravity potential the analysis of the high frequency spectrum of the anomalous gravity potential is important for a number of current and emerging technologies in geodesy and geophysics among them is the precise determination of deflections of the vertical by inertial techniques the development of data processing methods in airborne gradiometry and the signal extraction methods for airborne gravimetry the spectral information required can be obtained by analyzing dense gravity data by fast fourier transform techniques an attempt has been made to use available data files in canada for this purpose the data consisted of a rather uniform set of 5 arc min 5 arc min point anomalies in most of the continental areas of canada and four sets of point gravity anomalies with an average data spacing of about 1 km in areas of limited extent the analysis showed that the gravity spectrum has to be resolved to about degree and order 7000 i e to a point spacing of about 3 km to resolve the gravity anomalies to 1 mgal thus a similar data spacing would be required in airborne gradiometry and gravimetry in order to get an equivalent resolution the derived spectrum was used to determine suitable degree variance models and covariance functions for the above applications the spectrum estimated from the 5 arc min 5 arc min gravity anomalies covering the nonmountainous areas of canada indicates that the degree variances of the anomalous potential decay as l 3 6 this means that a number of currently used covariance models do not agree with the results of this analysis a discussion on the choice of a more adequate model concludes the paper

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

inconsistent magnetic polarities between greigite and pyrrhotite magnetite bearing marine sediments from the tsailiao chi section southwestern taiwan abstract to establish a magnetobiostratigraphy for a 620 m thick middle to late pleistocene mudstone sequence in the lower gutingkeng formation of the tsailiao chi tlc section in southwestern taiwan we conducted paleomagnetic and mineral magnetic measurements together with sediment granulometry and calcareous nannofossil identification paleomagnetic samples from 65 sites revealed two types of thermal demagnetization 25 400 c behavior 1 single component stable characteristic remanence in magnetite and pyrrhotite dominated samples type s and 2 abrupt changes in polarity when samples with significant greigite concentrations were heated above 320 340 c type c the characteristic polarities derived from type s samples and from magnetite dominated type c samples obtained above 340 c are consistent with those determined from nannofossil biostratigraphy this implies that the nrm carried by magnetite and pyrrhotite is reliable the essentially antiparallel remanence components in type c samples below 340 c are attributed to greigite the almost antiparallel direction could have resulted from delayed formation of greigite but in this case the different direction of this component must have resulted from variable remanence lock in times alternatively the opposite polarities may result from self reversal which warrants further investigation pyrrhotite and greigite may have both formed authigenically but there is no clear explanation for the observed differences in direction

review of paleostrain stress directions in northland new zealand and of the structure of the northland allochthon abstract folding shearing and fracturing in the northland allochthon occurred prior to deposition of the upper oligocene miocene akarana supergroup observations on fold vergence indicate that the allochthon moved from nnw to sse part of the pre miocene post lower cretaceous deformation in the autochthonous basement consisted of thrusting and strike slip faulting with compression trending ne sw across northland peninsula nw se thrusting followed by nw se extension affected the miocene rocks nw se extension persisted into the quaternary

rock magnetic signature of gas hydrates in accretionary prism sediments sediments from two ocean drilling program leg 146 sites from the cascadia margin of western north america have magnetic properties indicating diagenesis of magnetic minerals associated with the presence of gas hydrates two indices combining coercivity remanence and susceptibility parameters djh jrs js hcr hc and ds jrs k hcr when combined with thermo magnetic data can be diagnostic of these changes at site 892 ds values are distinctly higher and more scattered above the bottom simulating seismic reflector bsr which marks the base of the hydrate stability zone within the hydrate stability zone at site 892 djh shows two trends an increase from about 50 mbsf to the bsr at 73 mbsf corresponding to an expected increase in hydrate concentration near the bsr and a second increase upwards from 50 mbsf to peak values at less than 21 mbsf associated with hydrate recovered in cores above 19 mbsf at site 889 890 djh increases downhole to about 285 mbsf substantially below the bsr at 225 mbsf high djh sediments within a low cl zone at this site have magnetic mineralogies which are dominated by fine grained magnetic sulfides whereas sediments from above and below this zone are characterised by magnetite magnetic sulfide mixes this trend at site 889 890 is consistent with an interpretation based on pore water geochemistry low cl and bottom water temperature that a fossil gas hydrate zone extended downwards to about 295 mbsf during the last glacial the observed changes in magnetic properties can be attributed to steps in the reduction series from magnetite through sd greigite to pyrite or to overgrowth of sd greigite to md size diagenetic growth of magnetic iron sulfides greigite and or pyrrhotite has been reported in other accretionary wedge sediments thermal demagnetization of multi component isothermal remanent magnetization mirm indicates the presence of a low coercivity magnetic mineral with an unblocking temperature tub between 310 and 350 c high jrs k ratios suggest that the low coercivity low unblocking temperature mineral is predominantly greigite rather than pyrrhotite a low to medium coercivity mineral with tub ca 580 c magnetite is also present in varying amounts hydrate apparently controls the presence of greigite by incorporating h2s shown to be present as a hydrate phase together with methane in hydrate recovered at site 892 release of h2s below the base of the hydrate layer allows overgrowth of greigite grains to md size or the conversion of some of the greigite to pyrite

reductive diagenesis magnetite dissolution greigite growth and paleomagnetic smoothing in marine sediments a new view abstract in many anoxic sedimentary environments the onset of sulfate reduction and pyritization of detrital iron bearing minerals leads to a precipitous decline in magnetic mineral concentration during early diagenesis the usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution detailed rock magnetic and electron microscope analyses of rapidly deposited 7 cm kyr latest pleistocene holocene sediments from the continental margins of oman 22 22 4 n 60 08 0 e and northern california 38 24 8 n 123 58 2 w demonstrate that pyritization during early diagenesis also leads to the progressive down core growth of the ferrimagnetic iron sulfide greigite greigite growth begins with nucleation of large concentrations of superparamagnetic sp nanoparticles at the inferred position of the sulfate methane transition which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of sp particles looping of hysteresis parameters on a day plot records the dissolution of single domain sd titano magnetite and the formation of sp greigite which then slowly and progressively grows through its sd blocking volume and acquires a stable paleomagnetic signal this looping trend is also evident in data from several published records oregon margin korea strait japan sea niger fan argentine margin and the ontong java plateau indicating that these processes may be widespread in reducing environments our observations have profound implications for paleomagnetic records from sulfate reducing environments the paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10 s of kyr and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10 s to 100 s of kyr we therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis

shallow subduction tectonics in the raukumara peninsula new zealand as illuminated by earthquake focal mechanisms the raukumara peninsula affords an excellent opportunity to study the subduction process as subduction of the buoyant hikurangi plateau on the pacific plate has resulted in exposure of the forearc above the shallow part of the subduction thrust here we report on the focal mechanisms of 117 earthquakes of m l 2 4 4 9 and shallower than 80 km recorded during a 5 month deployment of 36 portable seismographs on the peninsula mechanisms have been constrained using both first motion polarity data and amplitudes of seismogram envelopes downdip tensional strain predominates in the subducted plate with taxes of events in both the upper and lower planes of the dipping seismic zone generally paralleling the local dip of the zone trenchward extensional strain is seen in the uppermost part of the overlying australian plate in line with geodetic and geological results this can be related to extension and gravity sliding of surficial rocks due to uplift of the raukumara range resulting from underplating of subducted sediment there is a marked change in earthquake mechanisms along strike in the lower part of the overlying plate and at the plate interface a concentration of low angle thrust events at the plate interface in the northeastern half of the peninsula suggests that the plate interface is less coupled there than to the southwest this along strike change in plate coupling corresponds closely to a change in the crustal structure of the overlying plate and also to a change in tectonic rotation domain determined paleomagnetically

shortening of an overriding plate and its implications for slip on a subduction thrust central hikurangi margin new zealand 1 at subduction plate boundaries strains accumulate within the overriding plate in response to motion of the down going plate to a first approximation margin normal interplate slip and shortening in the overriding plate sum to the total plate convergence therefore margin normal interplate slip can be estimated by measuring shortening in the overriding plate and total plate convergence here we quantify margin normal interplate slip by measuring shortening of the upper plate along a profile which extends up to 480 km across and approximately normal to the central hikurangi margin new zealand long term deformation is estimated for periods of millions of years using data from seismic reflection lines and cross sections while contemporary strain rates are derived from global positioning system gps geodetic velocities these data permit comparison of plate convergence rates determined by seafloor spreading data with the rates of contraction over short 7 years and long 2 5 and 5 myr periods of time on this part of the plate boundary total margin normal shortening in the overriding plate over the last 5 myr was 9 28 km and accounts for 6 19 of the total convergence 145 160 km at rates of 2 6 mm yr the remaining 80 or more of plate convergence has been accommodated by slip on the subduction thrust gps contraction along approximately the same profile indicates 14 16 mm yr of contemporary shortening across the terrestrial part of the upper plate which is approximately modeled by 80 interplate coupling on the subduction interface down to depths of 30 km the contrasting proportions of inferred interplate coupling for contemporary and geological data sets are consistent with stick slip behavior on the plate interface and with slip being principally achieved during intermittent large earthquakes the general shapes of short and long term shortening profiles are similar and suggest that the highest strains occur at or immediately east of the mohaka fault these strain profiles are consistent with a model where high strains in the upper plate are concentrated near the down dip end of the contemporary locked zone on the plate interface this model requires that the down dip end of the interseismic locked zone has on average been approximately stable over a period of at least 2 5 5 myr

widespread remagnetizations and a new view of neogene tectonic rotations within the australia pacific plate boundary zone new zealand 14 ma 1 than the presently observed rate of 3 4 ma 1 which is only likely to be characteristic of the tectonic regime established since 1 2 ma these new results are consistent with both long and short term deformation on the hikurangi margin being driven by realignment of the subducting pacific plate with collision of the hikurangi plateau in the late miocene potentially being key to both the initiation of tectonic rotations and the widespread remagnetization of neogene sediments however accommodating faster more coherent rotation of the hikurangi margin in neogene reconstructions of the new zealand plate boundary region particularly in the late miocene remains a challenge

complex polarity pattern at the former plio pleistocene global stratotype section at vrica italy remagnetization by magnetic iron sulphides the vrica section in calabria southern italy was the global stratotype for the pliocene pleistocene boundary until this boundary was redefined in 2009 several paleomagnetic investigations have been carried out at vrica to determine the age of the formerly defined pliocene pleistocene boundary which was a key calibration point for the astronomical polarity timescale apts each study has documented a complex polarity pattern at and above the top of the olduvai subchron and in relation to the existence of the so called vrica subchron when constructing the apts two alternative interpretations for the vrica section were proposed neither of which could be conclusively supported authigenic growth of magnetic iron sulphide minerals was proposed to explain the complex magnetic polarity record availability of a fresh 50 m sediment core enabled us to test this possibility our magnetostratigraphic record is similar to that of previous studies but it is also complex above the olduvai subchron we confirm abundant occurrences of authigenic greigite and pyrrhotite along with detrital titanomagnetite authigenic monoclinic pyrrhotite indicates growth significantly later than deposition and greigite can grow at any time during diagenesis depending on the availability of dissolved iron and sulphide the spatially variable magnetic polarity pattern at vrica is therefore interpreted to have resulted from post depositional magnetic iron sulphide formation at variable times tectonism along the calabrian arc provides a plausible mechanism for forcing reducing fluids through the sediments thereby supplying the dissolved ions needed to produce late diagenetic sulphide growth and remagnetization the complex magnetostratigraphic record at vrica was taken into account when the apts was developed and alternative interpretations result in a maximum age difference of 50 kyr for the upper olduvai reversal our results therefore do not undermine the apts rather they explain the complex magnetic polarity pattern at this globally important location and highlight the importance of remagnetization processes in such sediments

large tectonic rotations in a wide zone of neogene distributed dextral shear northeastern south island new zealand abstract the new zealand plate boundary zone in the northern part of south island new zealand comprises a series of active dextral strike slip faults referred to as the marlborough fault zone mfz which link oblique continental convergence farther south along the alpine fault to subduction of the pacific plate along the hikurangi margin new paleomagnetic data are presented which together with previous studies suggest coherent neogene clockwise rotation up to 130 about a vertical axis of crustal blocks in the mfz starting at 20 ma there is a striking correlation between the amount of observed rotation and the trend of mesozoic basement structures creating this part of the new zealand orocline in a zone of distributed dextral shear about 100 km wide which has persisted for about 20 ma in the southern part of the mfz crustal blocks appear to be on a scale of 1 10 km with boundaries that are subparallel to the basement strike this way deformation is accommodated by a combination of fault slip and rotation in a zone of more pervasive dextral shear resulting in coherent bending of the basement structure in the central part of the mfz this rotation appears to be that of elongate crustal blocks 50 km 10 km in size rotating 80 clockwise at an average rotation rate 4 ma in detail the rotation rate has decreased towards the present from 6 ma at 20 ma to

rock magnetism and palaeomagnetism of the montalbano jonico section italy evidence for late diagenetic growth of greigite and implications for magnetostratigraphy summary the montalbano jonico mj section cropping out in southern italy represents a potential candidate to define the lower middle pleistocene boundary and it has been proposed as a suitable global stratotype section and point gssp of the ionian stage middle pleistocene the mj section is the only continuous benthic and planktonic 18o on land reference in the mediterranean area for the mid pleistocene transition spanning an interval between about 1240 and 645 ka combined biostratigraphy and sapropel chronology tephra stratigraphy and complete high resolution benthic and planktonic foraminiferal stable oxygen isotope records already provide a firm chronostratigraphic framework for the mj section however magnetostratigraphy was still required to precisely locate the brunhes matuyama transition and to mark the gssp for the ionian stage we carried out a palaeomagnetic study of a subsection ideale section of the mj composite section sampling 61 oriented cores from 56 stratigraphic levels spread over a ca 80 m thick stratigraphic interval that correlates to the oxygen isotopic stage 19 and should therefore include the brunhes matuyama reversal the palaeomagnetic data indicate a stable and almost single component natural remanent magnetization nrm a characteristic remanent magnetization chrm was clearly identified by stepwise demagnetization of the nrm the chrm declination values vary around 0 and the chrm inclination around the expected value 59 for a geocentric axial dipole field at the sampling locality this result indicates that the section has been remagnetized during the brunhes chron a preliminary study of 27 additional not azimuthally oriented hand samples collected at various levels from other parts of the mj composite section indicates that all the samples are of normal polarity and demonstrates that the remagnetization is widespread across the whole exposed stratigraphic sequence a series of specific rock magnetic techniques were then applied to investigate the nature of the main magnetic carrier in the study sediments and they suggest that the main magnetic mineral in the mj section is the iron sulphide greigite fe3s4 scanning electron microscope observations and elemental microanalysis reveal that greigite occurs both as individual euhedral crystals and in iron sulphides aggregates filling voids in the clay matrix therefore we infer that the remagnetization of the section is due to the late diagenetic growth of greigite under reducing conditions most likely resulting in the almost complete dissolution of the original magnetic minerals iron sulphide formation in the mj section can be linked to migration of mineralized fluids our inferred timing of the remagnetization associated with greigite growth represents the longest remanence acquisition delay documented in greigite bearing clays of the italian peninsula so far

shallow morphology of the subducted pacific plate along the hikurangi margin new zealand abstract along the east coast of the north island new zealand the pacific plate is subducted beneath the australian plate the seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks a synthesis of the microearthquake seismicity the depths of thrust events the depth of s to p conversions and deep reflection seismic studies leads to a coherent picture of the subducting slab both in terms of its mechanical structure and in terms of the general morphology the slab is initially subducted at a low angle of a few degrees is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth the radius of curvature varies along strike from about 280 km just north of wellington to 240 km north of hawke bay the slab shape between depths of 15 45 km is thus approximately conical it also approximates within the errors of hypocentre location a surface of constant gaussian curvature as would be geometrically expected from the simple bending of a thin spherical lithospheric shell observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the pacific plate with a lower level of activity within the subducted mantle normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane

seismic stratigraphy of a buried volcanic arc northland new zealand and implications for neogene subduction abstract for over 6 million years in the early miocene the northland peninsula of new zealand s north island was the site of a volcanic arc comprising north westerly trending twin belts of marine and subaerial andesitic volcanoes approximately 50 volcanic edifices from major massifs to small cones have been newly identified on marine seismic profiles of the western belt which is largely buried the volcanoes were important point sources of sediment and the volcaniclastic deposits of each of the major and intermediate sized volcanoes form distinct seismic sequences the three main elements of the volcanic sequence are i a main body ii a sloping apron and iii a wide almost flat ring plain the downlap of the oldest discernible internal clinoform reflector relative to a known regional marker reflector underlying it indicates the approximate time of onset of activity whereas downlap of the top bounding reflector relative to those of other volcanic sequences provides a relative order of extinction sequence stratigraphic principles and super position determine relative stratigraphic hierarchy the volcanoes studied here were active only in the early miocene with few exceptions most were active by 22 ma and extinct by 16 ma with many extinct by 19 ma there was no pattern in their order of inception or extinction although a few became dominant burying or surrounding their neighbours at the south eastern end of the chain there is a rapid change over a short distance in both age and orientation from the northwest trending early miocene volcanism in the northland basin to north east trending middle and late miocene volcanism in the contiguous taranaki basin the northland volcanoes are interpreted to have been an arc facing the south westwards subducting south fiji basin whereas the taranaki volcanoes are interpreted to have been part of a separate pacific facing volcanic arc the lau colville ridge

rates and mechanics of rapid frontal accretion along the very obliquely convergent southern hikurangi margin new zealand analysis of seismic reflection profiles swath bathymetry side scan sonar imagery and sediment samples reveal the three dimensional structure morphology and stratigraphic evolution of the central to southern hikurangi margin accretionary wedge which is developing in response to thick trench fill sediment and oblique convergence between the australian and pacific plates a seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby by coccolith and foraminifera biostratigraphy of three core and dredge samples and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds on the growth and mechanics of frontal accretion under variable convergence obliquity and on the amounts and rates of horizontal shortening the data place constraints on the partitioning of geological strain across the entire southern hikurangi margin the principal deformation front at the toe of the wedge is discontinuous and represented by right stepping thrust faulted and folded ridges up to 1 km high which develop initially from discontinuous protothrusts in the central part of the margin near 41 s where the convergence obliquity is 50 orthogonal convergence rate is slow 27 mm yr and about 75 of the total 4 km of sediment on the pacific plate is accreted frontally the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening at least 80 of this shortening has occurred within the last 0 4 0 1 m y at an average rate of 12 3 mm yr this rate indicates that the frontal 30 km of the wedge accounts for about 33 55 of the predicted orthogonal contraction across the entire plate boundary zone despite plate convergence obliquity of 50 rapid frontal accretion has occurred during the late quaternary with the principal deformation front migrating seaward up to 50 km within the last 0 5 m y i e at a rate of 100 km m y the structural response to this accretion rate has been a reduction in wedge taper and consequently internal deformation behind the present deformation front near the southwestern termination of the wedge where there is an along the margin transition to continental transpressional tectonics the convergence obliquity increases to 56 and the orthogonal convergence rate decreases to 22 mm yr the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward verging folds these structures have accommodated not more than 0 5 km of horizontal shortening at a rate of 1 mm yr which represents 5 of the predicted orthogonal shortening across the entire plate boundary in southern north island the landward vergent structural domain may represent a transition zone from rapid frontal accretion associated with low basal friction and high pore pressure ratio in the central part of the margin to the northern south island region where the upper and lower plates are locked or at least very strongly coupled

tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from new zealand detailed scanning electron microscope observations coupled with elemental microanalysis confirm the presence of a chemical remanent magnetization carried by authigenic greigite fe 3 s 4 in uplifted neogene marine sediments from the hikurangi margin of new zealand normal polarity samples from the studied section have declinations that are deflected 60 clockwise of reversed polarity samples indicating the presence of two distinct magnetizations separated by several million years of tectonic rotation about a vertical axis however although multiple generations of iron sulfide growth are observed petrographically we see no clear differences in the relative timing of greigite formation between samples carrying these two magnetizations not only can the diagenetic growth of greigite in fine grained marine sediments occur long after deposition obscuring tectonic and magnetostratigraphic information but such remagnetizations are also difficult to distinguish from a more primary signal in the absence of constraints from field tests our observations emphasize that considerable care is necessary when interpreting paleomagnetic data from greigite bearing sediments

large tectonic rotation of part of new zealand in the last 5 ma abstract detailed paleomagnetic data from the wairoa syncline a middle miocene to the present forearc basin on the east coast of the north island new zealand show that the rate of clockwise rotation for the last 5 ma has been 7 8 ma of which less than 1 5 ma can be explained by apparent polar wander due to motion of the australian or pacific plates this rotation is similar to a present day rate of 7 ma determined from geodetic data between 5 and 20 ma ago the rate of tectonic rotation is poorly determined and may be between 0 and 2 ma the change in the rate of rotation of the wairoa syncline around 5 ma is probably related to a markedly different tectonic style in the new zealand region within the last 5 ma associated with a change in position of the euler poles of rotation for the pacific australian plates

tectonic rotation about the termination of a major strike slip fault marlborough fault system new zealand the marlborough fault system comprises a series of major right lateral strike slip faults that link the alpine fault to the hikurangi subduction zone in the pacific australia plate boundary zone in new zealand all of the major active faults of the marlborough fault system have continuous traces except for the clarence fault which terminates abruptly near the ward syncline paleomagnetic data from upper miocene and lower pliocene sedimentary rocks between the marlborough faults indicate a consistent post early pliocene regional clockwise rotation of about 20 an additional rotation of about 10 is evident at many localities near the termination of the clarence fault it is proposed that the additional rotation relative to the rest of the region is caused by a velocity gradient that likely exists between the active awatere fault and the termination of the clarence fault the existence of such a velocity gradient is consistent with inferences from geodetic strain data which suggest that one third of the displacement in the marlborough fault system occurs between the awatere and clarence faults the kinematics of rotation can be modelled by treating the area as a rigid block that pivots about the termination of the clarence fault the block rotation model satisfies field constraints on the styles of deformation observed at the boundaries of the block and is consistent with available paleomagnetic and geodetic data

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

apparent magnetic polarity reversals due to remagnetization resulting from late diagenetic growth of greigite from siderite summary a mixed polarity zone representing alternations between remagnetized and non remagnetized strata has been documented within the lower few metres of the crp 1 core ross sea antarctica detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single domain particles of a ferrimagnetic iron sulphide mineral while the reversed polarity magnetization of non remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite fe3s4 based on microtextural relationships it is not possible to determine the relative timing of formation for much of the greigite however a significant proportion of the greigite has grown on the surface of authigenic siderite feco3 grains that occur as microconcretions and as cement surrounding detrital matrix grains in such cases microtextural relationships indicate that siderite post dates early diagenetic pyrite and that greigite post dates the siderite siderite usually forms in environments with abundant dissolved iron and carbonate but without dissolved pore water h2s this set of geochemical conditions occurs in methanic settings below the sulphate reduction zone in which early diagenetic pyrite forms we interpret the observed remagnetization of the lower part of the crp 1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide thereby causing precipitation of greigite the distribution of siderite and associated greigite in the lower part of the crp 1 core is patchy which accounts for the apparent alternation of polarities this study is part of a growing catalogue of remagnetizations involving greigite which suggests that occurrences of greigite should be treated with caution in palaeomagnetic and environmental magnetic studies

magnetostratigraphic chronology of late miocene to early pliocene biostratigraphic and oceanographic events in new zealand successions of uplifted shallow marine sediments in new zealand were among the first to provide evidence of latest miocene climatic deterioration and were also among the first onshore successions where late miocene to early pliocene magnetostratigraphic records were established a revised chronology of late miocene to early pliocene events has been determined from new magnetostratiigraphic results from two important successions at blind river and upton brook south island new zealand lat 41 45 s long 174 05 e magnetostratigraphic results from upton brook are the fast from strata containing the kapitean index mollusks and provide the first reliable estimate of the age of the tongaporutuan kapitean boundary a coherent chronology from blind river and upton brook indicates that two synchronous low sea level and cool paleoclimatic events occurred in the late miocene kapitean stage this inference is supported from other well known coeval successions in the new zealand region the revised new zealand chronology supports recent determinations from the mediterranean that indicate a two stage messinian salinity crisis and a 5 2 ma age for the miocene pliocene boundary data from new zealand the mediterranean and ocean cores around the world suggest that these latest miocene events are related to glacio eustatic sea level fluctuations a 6 05 ma age for the first occurrence datum of globorotalia conomiozea indicates that significant diachroneity exists in the placement of this datum between the southwest pacific and the mediterranean confirming the observation that g conomiazea is unsuitable for correlation between ocean basins

a three dimensional image of shallow subduction crustal structure of the raukumara peninsula new zealand summary earthquake arrival time data from a 36 station deployment of portable seismographs on the raukumara peninsula have been used to determine the 3 d vp and vp vs structure of this region of shallow subduction a series of inversions have been performed starting with an inversion for 1 d structure then 2 d and finally 3 d this procedure ensures a smooth regional model in places of low resolution the subducted plate is imaged as a northwest dipping feature with vp consistently greater than 8 5 km s 1 in the uppermost mantle of the plate structure in the overlying plate changes significantly along strike in the northeast there is an extensive low velocity zone in the lower crust underlying the most rapidly rising part of the raukumara range it is bounded on its arcward side by an upwarp of high velocity a viable explanation for the low velocity zone is that it represents an accumulation of underplated subducted sediment while serpentinization of the uppermost mantle may be responsible for the adjacent high velocity region the low velocity zone decreases and the adjacent high velocity region is less extensive in the southwest this change is interpreted to be related to a change in the thickness of the crust of the overlying plate in the northeast the crust is thinner and subducted sediment ponds against relatively strong uppermost mantle while in the southwest the crust is thicker and the relatively weak lower crust allows sediment subduction to greater depths a narrow zone of high vp vs parallels the shallow part of the plate interface this suggests elevated fluid pressures with the distribution of earthquakes about this zone further suggesting that these pressures may be close to lithostatic the plate interface at 20 km depth beneath the raukumara peninsula may thus be a closed system for fluid flow similar to that

magnetite dissolution diachronous greigite formation and secondary magnetizations from pyrite oxidation unravelling complex magnetizations in neogene marine sediments from new zealand detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of neogene marine sediments from the east coast of new zealand is dominated by the authigenic iron sulphide greigite the greigite is present as a mixed population of stable single domain and superparamagnetic grains which is consistent with authigenic growth from solution this growth can result from pyritization reactions soon after deposition which also leads to dissolution of most detrital magnetite however where constrained by field tests our data suggest that remanence acquisition can occur 1 myr after deposition and can vary in timing at the outcrop scale strong viscous overprints result from oxidation of the iron sulphides probably during percolation of oxic ground water this process can sometimes destroy any ancient remanent magnetization this complex magnetic behaviour particularly the presence of late forming magnetizations carried by greigite means that the remanence in new zealand cenozoic sediments and in similar sediments elsewhere cannot be assumed to be primary without confirmation by field tests the reversals test should be employed with caution in such sediments as patchy remagnetizations can lead to false polarity stratigraphies

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

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

large tectonic rotations since the early miocene in a convergent plate boundary zone south island new zealand abstract a palaeomagnetic study in part of the new zealand plate boundary zone provides new constraints on the temporal and spatial distribution of neogene and quaternary tectonic rotations thermal demagnetization of samples from cretaceous basaltic dykes palaeocene oligocene micritic limestone and miocene and pliocene siltstones in the marlborough region south island have defined stable high temperature magnetic components which are interpreted as the primary magnetization declination anomalies after tectonic corrections are interpreted as rigid body rotations about a vertical axis of sample sites relative to the pacific plate all palaeomagnetic data from marlborough cluster into three main groups a 60 100 clockwise rotation affected palaeocene to middle miocene sedimentary sequences across marlborough between 18 ma and 8 ma coeval with a phase of low angle thrusting the absence of this rotation in a late cretaceous dyke swarm defines the present western limit of the early rotating zone a regional 20 clockwise rotation occurred in the last 4 ma during the development of the marlborough fault system in a zone of dextral transpression although locally clockwise rotations 40 may have occurred near some of the major dextral strike slip faults however a negligible rotation is observed in the same period in the region to the southeast of the major kekerengu dextral strike slip fault which appears to have acted as a hinge zone accommodating relative rotation by dextral strike slip on an arcuate fault bending and internal deformation the observed tectonic rotations record the overall clockwise rotation of the trend of the southern end of the hikurangi margin from w to nw in the early miocene to ne today determined independently from the long term relative plate motion data for the pacific and australian plates

the kinematics of northern south island new zealand determined from geologic strain rates relative motions within the distributed plate boundary zone of northern south island new zealand are determined through an inversion of geologic strain rate estimates the quaternary fault slip rate estimates define the shear strain rates and rock uplift rates provide information on the horizontal divergence rates an erosion rate to rock uplift rate ratio along with a crustal compensation factor is estimated in order to convert rock uplift rates to horizontal divergence rates because of the uncertainty in erosion rates horizontal divergence rates are given a large standard error of the three horizontal strain rate components obtained from these data completely define the horizontal velocity gradient tensor strain rate distributions are matched with spline polynomial functions which can be constrained to behave rigidly within specified regions such as the pacific or australian plates inversion of the strain rate distribution assuming uniform erosion rates across the northern south island yields a velocity field that has small differences in both magnitude 10 larger and direction with the nuvel 1a plate motion model between pacific and australian plates a revised strain rate data set obtained from a variable erosion model in which erosion rates are a linear function of the log of the average annual rainfall magnitudes yields a velocity field with expected directions that are indistinguishable from the nuvel 1a plate motion model between pacific and australian plates but velocity magnitudes are still 10 15 higher than the plate motion model therefore the average values of slip rate on strike slip faults in marlborough required by the nuvel 1a plate motion model are typically close to the low end of the published range of slip values for those structures the major strike slip structures within the marlborough region are accommodating 80 100 of the total plate motion between australia and pacific plates on northern south island as much as 20 of the relative motion could be accommodated by shear generated by folding or thrust faulting with a single orientation between the major structures the magnitude of plate motion contributed by the major faults indicates that the distributed geodetic shear strains measured across northern south island and discussed by bibby 1981 and walcott 1984 are not a form of irrecoverable strain but rather a feature of the strain field that will eventually be released primarily as concentrated slip on the strike slip structures within the region strain modeling indicates that the earthquake moment release over the last 150 years within the northwest nelson province which is west of the marlborough region is at least half an order of magnitude higher than the long term rate of moment release overall the northwest nelson region is playing only a minor role in taking up the long term plate motion

late neogene exhumation patterns in taranaki basin new zealand evidence from offset porosity depth trends taranaki basin new zealand is located adjacent to the australian pacific plate boundary where the tectonic regime changes from dominantly subduction related to the north to transpression related along the alpine fault to the south during the neogene burial and exhumation varied extensively in both time and space in response to subsidence and uplift along this evolving plate boundary zone the basin can be divided into two regions 1 the western platform outside the plate boundary deformation zone where no uplift nor exhumation has occurred and 2 the eastern mobile belt which lies inside the deformation zone and has been variably uplifted and exhumed exhumation magnitudes for sedimentary deposits of the basin are estimated from offset porosity depth trends the analysis is based on correlating sonic log travel times with compensated density logs permitting sonic travel time to be used as a proxy for porosity twelve western platform wells are used to define a standard exponential porosity depth trend with an extrapolated surface porosity of 50 and an exponential decay factor of 2265 m valid for mudstone shale sections spanning a depth range of 300 3000 m this curve is a calibration curve against which offset porosity depth trends from wells in exhumed regions can be compared to determine apparent exhumation amounts porosity depth trends for 43 eastern mobile belt wells are offset 0 2800 m shallower relative to the western platform trend but generally are parallel to it in the southern region of the eastern mobile belt net exhumation amounts which are the sum of the porosity depth trend offset and depth to unconformity range from 850 to 3000 m most of this exhumation occurred on discrete contractional structures in late miocene to early pliocene time probably associated with an increased rate of convergence across the alpine fault system to the south in the eastern region of the eastern mobile belt net exhumation progressively increases from 0 m in the south to 1800 m in the northeast part of the basin consistent with the 2 4 sw structural tilt of onshore strata the regional and consistent pattern of exhumation in the eastern region contrasts with the discrete structure related exhumation in the southern region and is consistent with models of thermal and flexural uplift

toward an understanding of the multicomponent magnetization of uplifted neogene marine sediments in new zealand new zealand contains many sequences of tectonically uplifted miocene pliocene marine sediments which have been central to the development of a biostratigraphic framework for temperate southern latitudes chronological control is almost invariably dependent on magnetostratigraphy interpretation of the natural remanent magnetization nrm of these sediments is however notoriously difficult because of the weak magnetization secondary components that resist demagnetization and thermal instability of the clay minerals a new paleomagnetic study of wanganui basin sediments has yielded unusually well defined demagnetization data from which systematic characterization explanation and interpretation of the multicomponent nature of the nrm have been possible in general the magnetization consists of three components 1 a low 250 c t b component that is considered to be diagenetic in origin this interpretation of the intermediate and high t b components is supported by rock magnetic data which show that specimens carrying strong high t b components have a distinctly different spectrum of ferrimagnetic grains specifically a population with much higher coercivities than is seen in specimens which lack a high t b component it is argued that these high coercivity high t b grains are secondary in origin leading to the conclusion that the intermediate t b component is the primary detrital magnetization consistency of the paleomagnetic data within and between sites also supports this conclusion it is suggested that failure to recognize these three components has in some cases in the past led to misidentification of the primary magnetization using this hypothesis of the multi component magnetization the turakina river data of mcguire 1989 have been reinterpreted yielding a high resolution magnetostratigraphy from the upper gilbert chron 3n circa 4 ma to the lower matuyama chron 2r the correlation of this magnetostratigraphy with the geomagnetic polarity timescale is supported by biostratigraphic constraints

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

palaeomagnetism and rock magnetism in the upper pliocene valle ricca rome italy section summary a revision of the magnetostratigraphy in the valle ricca monterotondo rome pliopleistocene section was carried out this section consisting mainly of blue grey marly clays was previously investigated for palaeomagnetic effects by arias et al 1980 1990 who detected a normal polarity interval close to a volcanic ash layer which was correlated to the reunion subchron this interpretation conflicts however with new biostratigraphic data and palaeomagnetic observations in order to investigate the possible origin of these discrepancies we performed a detailed sampling for palaeomagnetic and rock magnetic analyses in the controversial interval around the volcanic ash layer the present results show strong variations in the rock magnetic parameters and suggest significant changes in the magnetic mineralogy two levels with a high intensity low temperature normal component of magnetization were distinguished at these two levels the rock magnetic parameters are typical of greigite and a widespread occurrence of framboids of micron sized iron sulphide grains was found by mineralogical analyses a high temperature reversed component of magnetization carried by magnetite was isolated for the whole interval investigated we demonstrate that the previously recognized normal polarity interval is the effect of changes in the magnetic mineralogy and it does not represent a true reversal of the earth s magnetic field the origin of the low temperature normal component of magnetization is discussed in light of the present knowledge about greigite formation and remanence acquisition

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

bending within the axial tectonic belt of new zealand in the last 9 myr from paleomagnetic data abstract tongaporutuan stage late miocene 11 6 myr sedimentary rocks at two localities within the axial tectonic belt of new zealand have a direction of magnetisation in which the declination is 020 11 and 030 8 d 95 the directions of magnetisation in rocks of similar age outside the belt have a declination like that of the earth s present axial dipole 360 the clockwise deviation of the declination is inferred to indicate bending of the rocks within the zone of deformation between the pacific and indian plates due to the oblique convergence of these plates

paleomagnetic constraints on cenozoic deformation along the northwest margin of the pacific australian plate boundary zone through new zealand 1 new zealand straddles the boundary between the australian and pacific plates a zone of oblique continental convergence and transform motion the actively deforming region offers a unique opportunity to study the dynamics of deformation including vertical axis rotation of rigid blocks within a transcurrent plate boundary zone we present and interpret paleomagnetic data from three new and three previously published sites from the nw part of the south island nw nelson region where sedimentary strata dated between 36 and 10 ma overlie the crystalline paleozoic basement assemblages of the gondwana margin compared with reference directions from the australian apparent polar wander path none of the results provide evidence of post eocene vertical axis rotation this suggests that for the past 36 myr nw nelson has remained a strong coherent block that has moved as a contiguous part of the australian plate this is in marked contrast to the strongly rotated nature of more outboard accreted terranes to the east for example the hikurangi margin in the north island nw of the alpine fault and the marlborough region in the ne of the south island se of the alpine fault have both undergone diverse clockwise rotations of up to 140 since the early paleogene the nw tip of the south island seems to have acted as a rigid backstop relative to these more complex oroclinal deformations we infer that because of its relatively stiff bulk rheology it has not been drawn into the distributed plate boundary rotational deformation associated with the new zealand orocline

a seismic crustal section across the east cape convergent margin new zealand abstract crustal seismic reflection measurements were made along a 250 km profile across the pacific australian plate boundary north of new zealand data were recorded to 16 s two way time and have defined a major sedimentary basin corresponding to the southern end of the raukumara basin a fore arc basin reaching a depth of about 13 km this basin contains a lower generally conformable sequence of sediments and an upper sequence dominated by several major uncomformities the eastern part of this basin is cut by several recently active eastward dipping high angle thrust faults which are interpreted to be caused by the recent convergence at the adjacent pacific australian plate boundary the eastern margin of the basin is marked by a sharp change in character on the seismic data we interpret this feature as a terrane boundary coinciding with a major transcurrent fault which has juxtaposed a detached fragment of an island arc fore arc with a thin 15 km crust to the east against a passive margin sedimentary sequence to the west the southern raukumara basin is floored by cretaceous age oceanic crust fragments of which were obducted onto the north island about 22 ma ago the area has had a complex history a schematic model for the evolution of this margin during the past 22 ma is developed suggesting that the raukaumara basin has developed as a combination of fore arc basin processes and oblique crustal extension

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

inconsistent magnetic polarities in magnetite and greigite bearing sediments understanding complex magnetizations in the late messinian in the adana basin southern turkey 1 we present paleomagnetic rock magnetic and scanning electron microscope data from three upper messinian stratigraphic sections from the adana basin southern turkey the collected samples are from fine grained units which were deposited during the messinian salinity crisis within subchron c3r paleomagnetic results reveal an inconsistent polarity record related to a mixture of magnetite and greigite that hinders determination of a reliable magnetostratigraphy three classes of samples are recognized on the basis of paleomagnetic results the first is characterized by a single magnetization component with normal polarity that is stable up to 530 580 c and is carried by magnetite the second is characterized by a single magnetization component with reversed polarity that is stable up to 330 420 c this magnetization is due to greigite which developed after formation of slumps and before tectonic tilting of the studied successions the third is characterized by reversed polarity which is stable up to 530 580 c we interpret this component as a primary magnetization carried by fine grained and magnetically stable detrital magnetite results indicate that in the adana basin the assumption that a primary magnetization is carried by magnetite and a magnetic overprint carried by greigite does not hold because a late magnetic overprint has also been found for magnetite bearing samples our data illustrate the complexity of magnetostratigraphic reconstructions in successions characterized by variable mixtures of magnetic minerals with different magnetic stability that formed at different stages we demonstrate the need to perform detailed magnetic mineralogy analyses when conducting magnetostratigraphic studies of clay rich sediments from marine or lacustrine environments

field evidence of the viscous sublayer in a tidally forced developing boundary layer field observations of boundary layer development within a tidally forced estuary revealed evidence of an observable viscous sublayer evidence is provided by several independent measures of the flow field including hydrodynamic smoothness an immobile bed and characteristic velocity constant stress and higher order moment structures this investigation reports what may be the second comprehensive observation of the viscous sublayer in a marine environment and what could be the first observation of a momentum balance that includes the viscous sublayer within a shallow estuarine environment hydrodynamic observations were made in a straight channel within the great bay estuary of new hampshire over a flat sandy mud with low water depth of 1 5 m at the sampling location beyond quantifying the role of the benthic boundary layer in nutrient dynamics these observations are useful to provide insight into very near boundary stress estimates leading to incipient motion in estuarine and coastal environments

structural evolution along the inner forearc of the obliquely convergent hikurangi margin new zealand the accretionary margin of the hikurangi forearc on the southeast coast of the north island of new zealand is part of the leading edge of the australian plate which is overriding the obliquely converging pacific plate we investigate the last 10 m y of deformation history of the innermost western quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the eketahuna area on the east coast of the north island the eketahuna area is ideal for such a study because emergence of the margin in the quaternary has exposed a complete late neogene rock record this record has allowed us to chronicle the strain history from 10 ma to about 2 5 ma this forearc region was the locus of subsidence and marine deposition in the latest pliocene this part of the margin began to shorten through folding and reverse faulting bringing an end to basin filling the period of shortening was brief and by the late pleistocene reverse faulting had ceased and was immediately succeeded by dextral strike slip faulting in some cases along the same faults presently the dominant strain regime in the inner quarter of the forearc is strike slip faulting this structural history illustrates that over time the pattern of strain partitioning has changed in the hikurangi forearc the switch from crustal shortening to dextral shear along the major faults in this area in the last 1 m y may be a response to more than 10 of clockwise rotation in the southern hikurangi forearc in pliocene and pleistocene time this rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging australian and pacific plates in this area at the southernmost end of the kermadec hikurangi subduction system the inboard portion of this young accretionary margin is exceptionally well exposed today probably in part because of the late neogene subduction of relatively thick buoyant crust of the hikurangi chatham plateau

post deformational growth of late diagenetic greigite in lacustrine sediments from southern italy 1 paleomagnetic rock magnetic and micro textural analyses from a middle pleistocene lacustrine sequence in the southern apennines italy indicate the presence of greigite and magnetite as the main magnetic minerals at different stratigraphic levels in all cases a normal polarity characteristic remanent magnetization chrm was observed in agreement with an 40ar 39ar age of 0 712 0 018 ma from volcaniclastic sanidine after correction for bedding tilt chrm directions carried by greigite do not coincide with the expected geocentric axial dipole field direction at the site latitude whereas the magnetite chrms directions do the data indicate that the greigite magnetization was acquired after tilting and after lock in of the magnetite chrm the estimated delay for the remanence carried by greigite with respect to deposition is 300 ka scanning electron microscope analyses indicate that alteration of detrital volcanic minerals has occurred and that authigenic greigite is generally present in agglomerates and around volcanic grains this observation is consistent with a late diagenetic origin of greigite due to anoxic conditions and availability of dissolved sulfide associated with decomposition of organic matter in the paleolake documentation of a late diagenetic magnetization confirms that care should be taken when using greigite bearing sediments for magnetostratigraphic and tectonic studies

cenozoic tectonic evolution of the new zealand plate boundary zone a paleomagnetic perspective abstract new zealand straddles the obliquely convergent boundary between the pacific and australian plates with subduction of pacific plate along the hikurangi margin finite plate reconstructions predict 800 km of relative plate motion since 43 ma with 80 90 clockwise rotation of the hikurangi margin since 20 ma at an average rate of 4 4 5 myr and the short term deformation

gravity anomalies and sea surface heights derived from a combined geos 3 seasat altimeter data set gravity anomalies and sea surface heights have been computed on a 0 125 grid in the ocean areas from a combined geos 3 seasat altimeter data set the basic estimation procedure used least squares collocation where model covariance functions are tailored to individual areas through altimeter residual variance scaling preliminary tests led to production prediction procedures using a reference model defined by a set of potential coefficients complete to degree 180 comparisons of the predicted anomalies with ship derived values showed agreement varying from 9 to 30 mgal no correction to the altimeter implied sea surface heights was made for sea surface topography effects the maximum anomaly predicted is 396 mgal near hawaii and the most negative anomaly is 361 mgal over the puerto rican trench the actual resolution of the predicted quantities was estimated to be about 0 19 based on a power spectrum analysis the resolution is limited by our data selection process which uses 300 data points from a thinned altimeter data set for the prediction of the 0 125 in a 3 3 block with one data selection and matrix inversion the predicted data are used to compute 104 potential coefficient spectrums using flat earth approximations developed by forsberg the spectra were classified in terms of smooth mild or rough areas the great majority 78 of the spectra were in the smooth classification good agreement was found from spectra computed from topographically reduced land data and the mean ocean data up to degree 800 we found that the spectra decayed as l 3 6 as compared to the decay of l 3 implied by the kaula model the gridded data was used to compute 1 1 and 0 5 0 5 mean values the 1 mean anomalies were compared to terrestrial data where an rms difference of 7 mgal was found in comparing 10 139 values these new values have allowed us to identify 1 mean anomalies based on terrestrial estimates that are in substantial error

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

magnetostratigraphic lithostratigraphic and tephrostratigraphic constraints on lower and middle pleistocene sea level changes wanganui basin new zealand wanganui basin north island new zealand contains a complex sedimentary record of lower and middle pleistocene sea level changes palaeomagnetic results allow identification of the matuyama brunhes transition the jaramillo subchron and the cobb mountain subchron correlations of rhyolitic tuff horizons across the basin are consistent with magnetostratigraphic correlations between sections isothermal plateau fission track itpft ages of 1 05 0 05 ma and 1 63 0 15 ma on two tuffs potaka pumice and pakihikura pumice respectively are consistent with the interpreted magnetostratigraphy and also with the astronomically tuned timescale of odp site 677 magnetostratigraphy and itpft ages allow correlation of sedimentary cycles at wanganui with odd numbered oxygen isotope stages 17 31 in deep sea cores the stratotype section for the new zealand castlecliffian stage is shown to be incomplete relative to other studied sections in the basin below stage 31 the character of the cyclothems changes at wanganui from marine dominated to a greater representation of non marine and estuarine strata this change may be partly related to a change in amplitude and frequency of climatic cycles identified in oxygen isotope records but the primary cause is likely to be increased basinal uplift

diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems mineral magnetic results and electron microscope observations from gas hydrate bearing marine sediments cored at southern hydrate ridge during ocean drilling program leg 204 sites 1244 to 1252 cascadia margin offshore oregon demonstrate that authigenic greigite and pyrrhotite formed as a byproduct of microbially mediated diagenetic reactions in the sulphate the anaerobic oxidation of methane aom and the methanic gas hydrate zones geochemical conditions favourable for formation and preservation of greigite and pyrrhotite appear to be a limited source of sulphide whether it derives from microbially driven sulphate reduction in the sulphate zone in the aom zone or in deep sediments undergoing aom so that pyritization reactions are not driven to completion our results indicate that rock magnetic identification of greigite and pyrrhotite should be useful for detecting ancient gas hydrate systems in the marine sedimentary record because it can enable rapid screening of ancient sediments for potential horizons where methane and disseminated gas hydrates might have occurred formation of authigenic greigite and pyrrhotite at different depths within the gas hydrate stability zone also implies that the magnetization of the host sediments will have been acquired at variable times which is likely to compromise paleomagnetic results from greigite and pyrrhotite bearing marine sediments

contradictory magnetic polarities in sediments and variable timing of neoformation of authigenic greigite abstract in several recent published studies paleomagnetic results from greigite bearing sediments reveal characteristic remanences that are anti parallel to those carried by coexisting detrital magnetic minerals and polarities that are opposite to those expected for the age of the rock unit these observations have important implications for the reliability of paleomagnetic data from greigite bearing sediments we have investigated the origin of such contradictory magnetic polarities by studying the formation mechanisms of greigite in mudstones from the lower gutingkeng formation southwestern taiwan scanning electron microscope observations indicate that the gutingkeng greigite has three modes of occurrence including nodular framboidal and matrix greigite microtextural observations including transection of bedding by iron sulfide nodules with no deviation of sediment textures the presence of partially dissolved edges around detrital and early diagenetic phases and neoformation of greigite and fe rich clays around detrital phyllosilicates indicate that all three types of greigite have a diagenetic origin that post dates early diagenetic pyrite in addition paleomagnetic data yield contradictory polarities even for greigite bearing sister samples from the same stratigraphic horizon the data are collectively interpreted to indicate that neoformation of the gutingkeng greigite occurred after partial dissolution of syngenetic or early diagenetic pyrite the timing of greigite formation can apparently vary enough to give contradictory polarities for different greigite components even within a single stratigraphic horizon direct petrographic observation of authigenic magnetic iron sulfide phases as carried out in this study can provide important constraints on formation mechanisms and timing of remanence acquisition for these minerals and suggests that care should be taken when interpreting magnetostratigraphic data from greigite bearing sediments

end member modelling as an aid to diagnose remagnetization a brief review remagnetization of a palaeomagnetic signal is difficult to recognize independently of directional information the situation becomes more complex when remagnetized rocks pass palaeomagnetic field tests for example when the remagnetization of a rock sequence has occurred before folding it is evident that palaeogeographic reconstructions are seriously flawed when actu ally remagnetized rocks are not identified as such here we discuss the merits and pitfalls of so called end member modelling of acquisition curves of the isothermal remanent magnetization irm to recognize remagnetized strata the technique requires no a priori information about the irm acquisition curves the algorithm unmixes a set of irm acquisition curves into a number of invariant curves termed end members and calculates the mixing proportions of the end members for each sample since primary natural remanent magnetization nrm and remag netized nrm are acquired by different processes their signatures can be recognized from subtle differences in the magnetic properties we illustrate the potential of the approach by three case studies one from spain and two from turkey in which the magnetic properties of remagnetized and non remagnetized rocks are evaluated the natural remanent magnetization nrm of a rock is described asremagnetized when itrepresents apalaeomagnetically younger age thanthe sediment age of the rock formation under study remagneti zation interferes with palaeogeographic reconstruc tions asisincreasinglybeingappreciated the late palaeozoic remagnetization event was firmly rec ognized in the appalachians in the usa in the early 1980s e g mccabe et al 1983 see the review of van der voo torsvik 2012 while the implica tionsoflatepalaeozoicremagnetizationineuropean

cretaceous termination of subduction at the zealandia margin of gondwana the view from the paleo trench abstract during the permian to cretaceous zealandia occupied a position on the proto pacific facing convergent margin of gondwana subduction on this margin ceased somewhere between 105 ma and perhaps 70 ma but the timing of this tectonic transition remains controversial resolution of this uncertainty is important for tectonic reconstructions of the southwest pacific and for global plate tectonic models here we revisit the problem by reference to new stratigraphic and geochemical data from the east coast basin of new zealand which occupied a position adjacent to and possibly superimposed on the relict cretaceous subduction trench at the time subduction ceased this basin is expected to preserve direct structural and stratigraphic evidence for or against late cretaceous subduction in western parts of the east coast basin the western sub belt a conspicuous unconformity separates undoubted accretionary prism rocks of the torlesse composite terrane from younger cretaceous cover units strata beneath and overlying this unconformity vary in age from place to place but abundant paleontological data and published ages of detrital zircons some reinterpreted herein indicate that exposed torlesse rocks are nowhere younger than 100 ma overlying zealandia megasequence cover strata are mostly younger than 110 ma between 110 and 85 ma the entire length of the western sub belt reveals complex stratigraphic architecture of relatively small scale depocentres subjected to alternating episodes of subsidence and local uplift and erosion there is widespread evidence for compression on the western sub belt over this period although the overall amount of shortening is likely to be relatively modest in contrast the eastern sub belt preserves a record of near continuous and apparently simple deposition over the same interval of time we see little and somewhat equivocal evidence for extension in either sub belt through the late cretaceous superimposed on this general pattern we observe discrete widespread or basin wide tectonic events at 95 ma and within the intervals 86 83 ma and 83 81 ma indicated by the presence of unconformities and sedimentary facies changes importantly all these events can be correlated with coeval events recorded elsewhere in zealandia suggesting that the east coast basin was subject to the same tectonic regime as the rest of zealandia and shared a common history during the mid to late cretaceous integrating these findings with data from elsewhere in zealandia we argue that there is diverse evidence to indicate that subduction finished along the new zealand segment of the gondwana margin by 100 ma that said the causes of on going late cretaceous compression in the east coast basin and in other parts of zealandia are not well constrained although several possible explanations are plausible there are few analogue abandoned subduction margins described in the literature however the situation in the south shetland trench shows strong similarities with patterns and complexities observed in the east coast basin

greigite from carbonate concretions of the ediacaran doushantuo formation in south china and its environmental implications abstract greigite fe 3 s 4 is a ferrimagnetic iron sulfide that commonly forms as a precursor of pyrite in anoxic environments where the supply of reactive fe outpaces that of sulfide h 2 s and hs because of its metastability and sensitivity to redox changes during burial greigite has been rarely documented in rocks older than the cretaceous here we report well preserved greigite in carbonate concretions of the upper doushantuo formation ca 551 ma in the yangtze gorge area south china greigite in the carbonate concretions coexists with anhedral and framboidal pyrite and is distributed in clay rich carbonates with card house microtextures and dolomitic spherical structures indicative of early diagenetic formation during shallow burial preservation of greigite in carbonate concretions of the upper doushantuo formation implies that these concretions maintained a closed micro system since their formation and that they provide information about the ancient depositional and early diagenetic environments

diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments south island new zealand abstract detailed magnetostratigraphic studies of late neogene siliciclastic sediments of the awatere group south island new zealand 41 45 s 174 05 e have revealed a wide range of palaeomagnetic behaviour examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic x ray diffraction and electron microprobe analyses these analyses indicate that the ferrimagnetic iron sulphide minerals greigite and pyrrhotite are responsible for a stable and intense magnetic remanence in fine grained sediments whereas titanomagnetite is the only remanence bearing mineral identified in coarser grained sediments which are less strongly and less stably magnetised than the fine grained sediments detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation which occurs commonly in rapidly deposited anoxic sediments that support active sulphate reduction and h 2 s formation preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process probably due to the low permeability of the fine grained sediments and consumption of available h 2 s before full reaction to pyrite occurred relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution

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

the decay of the spectrum of the gravitational potential and the topography for the earth summary the spectrum of the earth s gravitational potential and topography as represented by spherical harmonic expansions to degree 180 have been computed modelling the decay in the form of al values of a and for several degree l ranges were computed for degree range 5 180 was 2 54 for the potential and 2 16 for equivalent rock topography the potential decay was somewhat slower than that i e 3 implied by kaula s rule however at high degree ranges the values were larger 3 20 for degrees 101 180 agreeing better with recent determinations from terrestrial gravity data and geoid undulations implied by satellite altimetric data the values imply that the potential decays faster at higher l values the values of for topography were fairly uniform around 2 which agrees with a suggestion made by vening meinesz in 1951 we also found that the value for the earth s potential agrees well with the value implied by the topography with airy isostatic compensation with the depth of compensation equal to 30 km however the magnitude of the power implied by the topographic isostatic potential was approximately one third of the observed potential

the opening of cook strait interglacial tidal scour and aligning basins at a subduction to transform plate edge abstract cook strait the central seaway through the axial ranges of new zealand has many inferred origins a compilation of marine geological and geophysical datasets suggests that cook strait developed when five sedimentary basins at a rapidly changing obliquely convergent plate boundary were moved into line and were linked by strong tidal scour in middle pleistocene times the basins relate partly to oblique subduction north of cook strait and partly to intercontinental transform to the south prior to opening muddy subduction pull down and foreland basins in the northwest were separated from equally quiet water rotating forearc to transform basins in the southeast the land barrier between them narrowed and was finally breached as subduction related basins migrated southwards and transform related basins extended northwards in response to rotation and divergent branching of a major transcurrent fault following breaching a history of alternating scour and quiet water deposition is recorded by a series of deep irregular unconformities in muddy basin fill scour is correlated with interglacial periods of high sea level when land barriers were submerged and strong tides caused by a 140 phase difference at either end of the strait eroded muddy sediments deposited in glacial periods when an emergent landbridge limited tidal exchange

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

scsp observed on north island new zealand implications for subducting plate structure seismic phase conversions provide important constraints on the layered nature of subduction zone structures recordings from digital stations in north island new zealand have been examined for converted scs to p scsp arrivals from deep 150 km tonga kermadec earthquakes to image layering in the underlying hikurangi subduction zone consistent p wave energy prior to scs has been identified from stations in eastern and southern north island where the subducted plate interface is at a depth of between 15 and 30 km two scs precursors are observed ray tracing indicates that the initial precursor scsp1 corresponds to conversion from the base of an 11 14 km thick subducting pacific crust the second precursor is interpreted as a conversion from the top of the subducting plate the amplitude ratio scsp1 scs increases from 0 10 to 0 19 from northern to southern north island this is within the range expected from a simple first order velocity discontinuity at an oceanic moho a 1 2 km thick layer of low velocity sediment at the top of the subducting plate is required to explain the remaining scsp waveform our results imply that the abnormally thick hikurangi chatham plateau has been subducting beneath new zealand for at least 2 9 myr thus explaining the high uplift rates observed across eastern north island

3 d modelling of seamount topography from satellite altimetry we develop a complete set of algorithms to perform 3d modelling of seamount bathymery from satellite altimetry the first stage of the data processing consists in gridding the geoid to account for the long wavelength errors geoid heights are first bias adjusted at cross overs then a collocation on a regular grid is performed accounting for the altimeter errors in a second stage geoid heights are converted into bathymetry no simplifying assumption on the shape and location of the bathymetry highs is necessary bathymetric uncertainties due to the data sampling and the parameters of the mechanical and crustal models are evaluated

multiple mechanisms of remagnetization involving sedimentary greigite fe3s4 sedimentary greigite fe3s4 is being increasingly implicated as the carrier of late diagenetic remagnetizations in fine grained marine and terrestrial sediments we have conducted detailed scanning electron microscope observations on polished sections from such sediments coupled with elemental microanalysis to identify greigite and to characterize the mode of occurrence and the relationships between the observed greigite and other authigenic and detrital mineral phases this detailed observational work in conjunction with recently published work has enabled identification of several mechanisms for remagnetization involving greigite 1 neoformation of greigite on the surfaces of early diagenetic framboidal and nodular pyrite 2 greigite growth within cleavages of detrital sheet silicate grains 3 neoformation of greigite on the surfaces of authigenic clays smectite illite 4 greigite nucleation on the surfaces of siderite 5 greigite growth on the surface of gypsum that resulted from earlier oxidation of nodular pyrite many other mechanisms for greigite neoformation are conceivable the key variables are appropriate redox conditions for generation of sulphide and availability of iron from the dissolution of a wide range of possible reactive detrital and authigenic iron bearing minerals documentation of a wide range of mechanisms for neoformation of greigite provides compelling evidence that sediments containing greigite should be routinely suspected of remagnetization which will complicate or compromise studies of environmental magnetism and geomagnetic field behaviour

geochemical and isotopic development of the coromandel volcanic zone northern new zealand since 18 ma abstract cenozoic subduction related volcanism in northern new zealand developed from mainly basaltic and andesitic volcanisms in the early miocene to mainly silicic volcanism in the present taupo volcanic zone the first explosive silicic eruptions occurred at 12 ma and large silicic calderas formed from 8 ma this transition is recorded by the 18 1 9 ma coromandel volcanic zone cvz succession which provides an outstanding opportunity to investigate the origins of the modern north island volcanism new major and trace element data on a rock suite representing cvz andesites rhyolites and basalts indicate a consistent subduction related origin for the entire succession the cvz andesites generally define a consistent medium potassic differentiation trend however distinct andesites with relatively sodic compositions occur at the bases of two consecutive eruptive series at 18 and 10 ma the distinct andesites are geochemically similar to basalts that erupted sporadically near the oceanic margin of the cvz cvz rhyolites are medium to high potassic peraluminous rocks with initial 87sr 86sr values comparable to or somewhat more radiogenic than coeval andesites 0 7049 their bulk rock composition suggests derivation from sources comparable to least differentiated andesites or basalts we interpret the consecutive andesitic successions to represent stages in cvz development during which distinct arc segments developed consecutively the early stage in each segment yielded mainly intermediate volcanism whereas in the later stage volcanism locally proceeded rapidly within 1 myr of initial andesitic activity to silicic activity taken together the succession records the rapid development and migration of arc system in a dynamic plate boundary setting

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

temporal stability of deformation rates comparison of geological and geodetic observations hikurangi subduction margin new zealand abstract we examine how well contemporary horizontal velocity fields from global positioning system gps geodetic measurements describe long term deformation rates million year rates of extension shortening strike slip and vertical axis rotations are compared with modelling of gps velocities from the last 10 15 years in the hikurangi subduction margin new zealand within measurement uncertainties of mainly 50 the rates of deformation and vertical axis rotations for each time interval are usually indistinguishable the comparable deformation rates are consistent with the notion of near constant rates since 1 5 ma during this time oblique relative plate motion was partitioned with the majority 85 of margin normal motion accommodated on the plate interface and most 60 margin parallel motion by strike slip faulting and vertical axis rotations in the upper plate prior to 1 5 ma gps velocities cannot be reliably used to describe deformation and rotations of the hikurangi margin because at this time the taupo rift commenced opening the boundary between raukumara and wairoa rotational domains was abandoned and the rates of upper plate shortening increased the broad compatibility of gps and geological deformation and rotation rates since 1 5 ma suggests that the build up of contemporary elastic strains in the north island measured by gps is largely uniform for much of the duration of each seismic cycle the good agreement between the gps and geological deformation rates in the north island is consistent with the view that contemporary elastic strains will mainly be converted to permanent dip slip on the subduction thrust and to displacement on upper plate faults during future large magnitude earthquakes

magnetic properties of sedimentary greigite fe3s4 new data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite fe3s4 several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite such discriminants are necessary because greigite displays a similar or higher range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility mrs which are accompanied by hysteresis ratios of mrsms 0 5 and bcr bc 1 5 ms is the saturation magnetization bc is the coercive force and bcr is the coercivity of remanence greigite has no low temperature phase transition whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 k and unoxidized magnetite displays the verwey transition at 118 k low temperature measurements may therefore prove valuable in distinguishing between magnetite pyrrhotite and greigite although the absence of a low temperature transition cannot be taken as definitive evidence for the presence of greigite greigite also displays characteristic high temperature behaviour with a major drop in magnetization between 270 and 350 c determination of the mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite subsequent low and high temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals