northland ophiolite new zealand and implications for plate tectonic evolution of the southwest pacific the northland ophiolite and coeval ocean floor sedimentary rock form the northland allochthon obducted from the northeast onto new zealand in the late oligocene the ophiolite was probably emplaced as a single sheet and separated into individual massifs during subse quent movement of the allochthon chemically the bulk of the igneous rocks are normal mid ocean ridge basalts but the ophiolite also includes a younger suite of hornblende modal within plate alkalic rocks believed to represent seamounts two alternative models one involving a subduction flip the other involving continuous westward subduction are proposed to account for the obduction of the upper part of the northland ophiolite the oceanic crust from which the ophiolite originated was formed simultaneously with tasman sea spreading on the western rim of a once much larger south fiji plate assemblage

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

structure and breakup history of the rifted margin of west antarctica in relation to cretaceous separation from zealandia and bellingshausen plate motion 1 geophysical data acquired using r v polarstern constrain the structure and age of the rifted oceanic margin of west antarctica west of the antipodes fracture zone the 145 km wide continent ocean transition zone cotz of the marie byrd land sector resembles a typical magma poor margin new gravity and seismic reflection data indicates initial continental crust of thickness 24 km that was stretched 90 km farther east the bellingshausen sector is broad and complex with abundant evidence for volcanism the cotz is 670 km wide and the nature of crust within the cotz is uncertain margin extension is estimated to be 106 304 km in this sector seafloor magnetic anomalies adjacent to marie byrd land near the pahemo fracture zone indicate full spreading rate during c33 c31 80 68 myr of 60 mm yr 1 increasing to 74 mm yr 1 at c27 62 myr and then dropping to 22 mm yr 1 by c22 50 myr spreading rates were lower to the west extrapolation towards the continental margin indicates initial oceanic crust formation at around c34y 84 myr subsequent motion of the bellingshausen plate relative to antarctica 84 62 myr took place east of the antipodes fracture zone at rates 40 mm yr 1 typically 5 20 mm yr 1 the high extension rate of 30 60 mm yr 1 during initial margin formation is consistent with steep and symmetrical margin morphology but subsequent motion of the bellingshausen plate was slow and complex and modified rift morphology through migrating deformation and volcanic centers to create a broad and complex cotz

the metamorphic sole of new caledonia ophiolite 40ar 39ar u pb and geochemical evidence for subduction inception at a spreading ridge amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of new caledonia termed peridotite nappe recrystallized in the high temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the eocene metamorphic complex amphibolites mostly display the geochemical features of morb with a slight nb depletion and thus are similar to the youngest late paleocene eocene babb components of the allochthonous poya terrane thermochronological data from hornblende 40ar 39ar zircon and sphene u pb suggest that these mafic rocks recrystallized at 56ma using various geothermobarometers provides a rough estimate of peak recrystallization conditions of 0 5gpa at 800 950c the thermal gradient inferred from the metamorphic assemblage 60 ckm 1 geometrical relationships and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction obduction system and recrystallized when they subducted below young and hot oceanic lithosphere they were detached from the down going plate and finally thrust onto unmetamorphosed poya terrane basalts this and the occurrence of slab melts at 53ma suggest that subduction inception occurred at or near to the spreading ridge of the south loyalty basin at 56ma 2012 american geophysical union all rights reserved

late cretaceous cenozoic tectonic development of the southwest pacific region abstract a new model of the plate tectonic development of the southwest pacific integrates the continental geology of new zealand with the age structure of the surrounding oceanic crust revealed previously from magnetic anomaly lineations the model differs from previous ones in that the onland geology of new zealand is used to constrain the tectonic development in two important ways 1 the modern australia pacific plate boundary did not transect the new zealand sector until 23 million years ago 2 there has been a total of only c 500 km of dextral displacement on the plate boundary through new zealand the model is described with reference to a series of paleotectonic maps drawn to represent the setting at the times of anomalies 32 24 7 and 5 novel interpretations based on these reconstructions include the following during the late cretaceous the campbell fault was a continental transform fault between the tasman sea spreading centre and the bounty rift its 330 km of dextral displacement accompanying 25 of counterclockwise rotation of a campbell plateau block when the eastward propagating southeast indian ridge broke into the south tasman sea 57 million years ago it continued to propagate directly into southern south island and generated late eocene oligocene continental rifting through western new zealand at the same time back arc spreading in the norfolk basin caused continental rifting along a similar trend in western north island the australia pacific plate boundary originated as a transform fault between the southeast indian ridge and the pre existing kermadec trench it developed in response to the sudden cessation of spreading on the segment of the southeast indian ridge east of a fracture zone here named fracture zone z two implications arise from this model regarding the integrity of antarctica 1 1 during the cretaceous marie byrd land was probably no more than 200 km northwest of its present position 2 2 the proposition made in some earlier plate tectonic reconstructions that if the alpine fault did not form until after the late eocene there must be a late cretaceous early tertiary plate boundary within antarctica is most probably valid

the oroclinal bend in the south island new zealand abstract most of the south island of new zealand lies within an eocene recent continental shear zone related to pacific australia plate motion macroscopic finite strain in this shear zone has in the past been tracked through the deformation of the dun mountain ophiolite belt this paper identifies additional sub vertical basement strain markers including buller takaka terrane boundary darran suite and jurassic volcanic belt within the median batholith taieri wakatipu goulter synform axial trace esk head melange and bedding form surfaces within the buller takaka and torlesse terranes an analysis of the oroclinal bend over the entire zealandia continent shows that it is a composite feature involving pre as well as post eocene bending of basement structures satisfactory paleogeographic reconstructions of zealandia cannot be made without the use of substantial regional scale non rigid intracontinental deformation

geologic and kinematic constraints on late cretaceous to mid eocene plate boundaries in the southwest pacific abstract starkly contrasting tectonic reconstructions have been proposed for the late cretaceous to mid eocene 85 45 ma evolution of the southwest pacific reflecting sparse and ambiguous data furthermore uncertainty in the timing of and motion at plate boundaries in the region has led to controversy around how to implement a robust southwest pacific plate circuit it is agreed that the southwest pacific comprised three spreading ridges during this time in the southeast indian ocean tasman sea and amundsen sea however one and possibly two other plate boundaries also accommodated relative plate motions in the west antarctic rift system wars and between the lord howe rise lhr and pacific relevant geologic and kinematic data from the region are reviewed to better constrain its plate motion history during this period and determine the time dependent evolution of the southwest pacific regional plate circuit a model of 1 west dipping subduction and basin opening to the east of the lhr from 85 55 ma and 2 initiation of northeast dipping subduction and basin closure east of new caledonia at 55 ma is supported west dipping subduction and basin opening were not driven by convergence as has previously been proposed our plate circuit analysis suggests that between at least 74 ma and subduction initiation at 55 ma there was little net relative motion between the pacific plate and lhr

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

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

subduction kinematics and dynamic constraints the kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings two reference frames are used here to describe the simple 2d kinematics of subduction zones in the first the upper plate is assumed fixed whereas in the second frame upper and lower plates move relative to the mantle relative to a fixed point in the upper plate u the transient subduction hinge h can converge diverge or be stationary similarly the lower plate l can converge diverge or be stationary the subduction rate vs is given by the velocity of the hinge h minus the velocity of the lower plate l vs vh vl the subduction rate 1 increases when h diverges and 2 decreases when h converges combining the different movements at least 14 kinematic settings can be distinguished along the subduction zones variable settings can coexist even along a single subduction zone as shown for the 5 different cases occurring along the apennines subduction zone apart from few exceptions the subduction hinge converges toward the upper plate more frequently along e or ne directed subduction zone whereas it mainly diverges from the upper plate along w directed subduction zones accompanying backarc extension before collision orogen growth occurs mostly at the expenses of the upper plate shortening along e ne directed subduction zones whereas the accretionary prism of w directed subduction zones increases at the expenses of the shallow layers of the lower plate the convergence shortening ratio is 1 along e or ne directed subduction zones whereas it is 1 along accretionary prisms of w directed subduction zones backarc spreading forms in two settings along the w directed subduction zones it is determined by the hinge divergence relative to the upper plate minus the volume of the accretionary prism or in case of scarce or no accretion minus the volume of the asthenospheric intrusion at the subduction hinge since the volume of the accretionary prism is proportional to the depth of the decollement plane the backarc rifting is inversely proportional to the depth of the decollement on the other hand along e or ne directed subduction zones few backarc basins form e g aegean andaman and can be explained by the velocity gradient within the hangingwall lithosphere separated into two plates when referring to the mantle the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames the subduction hinge is mostly stationary being the slab anchored to the mantle along w directed subduction zones whereas it moves w or sw ward along e or ne directed subduction zones surprisingly along e or ne directed subduction zones the slab moves out of the mantle i e the slab slips relative to the mantle opposite to the subduction direction kinematically this subduction occurs because the upper plate overrides the lower plate pushing it down into the mantle as an example the hellenic slab moves out relative to the mantle i e sw ward opposite to its subduction direction both in the deep and shallow hotspot reference frames in the shallow hotspot reference frame upper and lower plates move westward relative to the mantle along all subduction zones this kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions w directed subduction zones rather provide about 2 3 times larger volumes of lithosphere re entering into the mantle and the slab is pushed down this opposite behavior is consistent with the down dip extension seismicity along e ne directed subduction zones and the frequent down dip compression along the w directed subduction zones subduction kinematics shows that plate velocity is not dictated by the rate of subduction along the w directed subduction zones the rate of subduction is rather controlled i by the hinge migration due to the slab interaction with the easterly trending horizontal mantle wind along the global tectonic mainstream ii by the far field plate velocities and iii by the value of negative buoyancy of the slab relative to the country mantle alternatively e ne nne directed subduction zones have rates of sinking chiefly determined i by the far field velocity of plates and ii by the value of negative buoyancy of the slab relative to the country mantle along this type of subduction the subduction hinge generally advances e ne ward toward the upper plate decreasing the subduction rate but it moves w sw ward relative to the mantle all this indicates that subduction zones have different origin as a function of their geographic polarity and the subduction process is more a passive feature rather than being the driving mechanism of plate motions a rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones

seismic stratigraphic record of transition from mesozoic subduction to continental breakup in the zealandia sector of eastern gondwana abstract the southwest pacific between australia new zealand and new caledonia is a block of continental crust zealandia that moved away from australia and antarctica after a long period of subduction beneath eastern gondwana we use 100 000 line km of seismic reflection profiles to identify intra continental basins related to the gondwana active margin overlain with erosional unconformity by retrogradational strata we interpret this regional scale first order unconformity the eastern gondwana composite surface and seismic stratigraphic megasequence pattern to represent the transition from subduction to continental breakup and separation of eastern gondwana rocks that make up the lower seismic stratigraphic megasequence zealandia 3 permian to early cretaceous have been drilled near new zealand and can be correlated with murihiku supergroup rocks farther north on the lord howe rise we correlate this megasequence with triassic to jurassic rocks of the clarence moreton basin of eastern australia we reinterpret the fairway aotea basin to be underlain by a wedge of zealandia 3 deposits thrust in a retro arc like foreland basin that was active immediately prior to breakup the lower rift and retrogradational megasequence zealandia 2 late cretaceous to eocene we correlate with the pakawau and kapuni groups of taranaki basin new zealand and the formation a charbon and phtanites of new caledonia the boundary with the shallowest megasequence zealandia 1 late eocene to present is the eocene oligocene unconformity which is primarily overlain by pelagic carbonate rocks this regional unconformity likely represents the onset of the modern tonga kermadec subduction system

discovery of early cretaceous rocks in new caledonia new geochemical and u pb zircon age constraints on the transition from subduction to marginal breakup in the southwest pacific abstract new u pb dating of detrital zircon and geochemical features of permian mesozoic arc derived volcanic rocks and volcaniclastic turbidites graywackes when compared with those of the volcanic rocks associated with unconformable late cretaceous shallow water sediments reveal that subduction in new caledonia once thought to be extinct in the late jurassic ca 150 ma was still active at least from ca 130 to 95 ma the accumulation of volcanic arc derived sediments during the late early cretaceous suggests that as in new zealand active margin activity went on for a short time in spite of the assumed subduction jamming by the hikurangi plateau at ca 100 ma meanwhile the rift related magmatic activity that preceded the marginal breakup migrated eastward from ca 130 ma 130 95 ma in eastern australia to 110 ma 110 82 ma in new zealand and finally to ca 89 ma 89 83 ma in new caledonia and generated large volumes of silicic magma in contrast marginal basins opened synchronously at

new 40ar 39ar age progression for the louisville hot spot trail and implications for inter hot spot motion 1 in this study we present 42 new 40ar 39ar incremental heating age determinations that contribute to an updated age progression for the louisville seamount trail louisville is the south pacific counterpart to the hawaiian emperor seamount trail both trails representing intraplate volcanism over the same time interval 80 ma to present and being examples of primary hot spot lineaments our data provide evidence for an age progressive trend from 71 to 21 ma assuming fixed hot spots this makes possible a direct comparison to the hawaiian emperor age progression and the most recent absolute plate motion apm model wk08g of wessel and kroenke 2008 we observe that for the louisville seamount trail the measured ages are systematically older relative to both the wk08g model predictions and hawaiian seamount ages with offsets ranging up to 6 myr taking into account the uncertainty about the duration of eruption and magmatic succession at individual louisville volcanoes these age offsets should be considered minimum estimates as our sampling probably tended to recover the youngest lava flows these large deviations point to either a contribution of inter hot spot motion between the louisville and hawaiian hot spots or to a more easterly location of the louisville hot spot than the one inferred in the wk08g model both scenarios are investigated in this paper whereby the more eastern hot spot location 52 0 s 134 5 w versus 52 4 s 137 2 w reduces the average age offset but still results in a relatively large maximum offset of 3 7 myr when comparing the new ages to the apm models s04p s04g by steinberger et al 2004 that attempt to compensate for the motion of hot spots in the pacific hawaii or globally hawaii louisville reunion and walvis the measured and predicted ages are more in agreement showing only a maximum offset of 2 3 myr with respect to the s04g model at face value these more advanced apm models which consider both plate and hot spot motions therefore provide a better fit to the new louisville age data the fit is particularly good for seamounts younger than 50 ma a period for which there is little predicted motion for the louisville hot spot and little inter hot spot motion with hawaii however discrepancies in the louisville age distance record prior to 50 ma indicate there is an extra source of inter hot spot motion between louisville and the other pacific hot spots that was not corrected for in the global s04g model finally based on six new 40ar 39ar age dates the 169 w bend in the louisville seamount trail seems to have formed at least 3 myr before the formation of the hawaiian emperor bend the timing of the most acute parts of both bends thus appears to be asynchronous which would require other processes e g plume motions than a global plate motion change between 50 and 47 ma to explain these two observations

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

cretaceous demise of the moa plate and strike slip motion at the gondwana margin radiolaria and paleomagnetic data from deep sea drilling project sites 595 and 596 indicate that early cretaceous seafloor was formed at moderate to high latitudes in the pacific this inference requires a cretaceous spreading ridge south of the pacific phoenix ridge and substantial dextral strike slip motion at the gondwana margin the previously unrecognized southern oceanic plate called here the moa plate was obliquely subducted beneath the gondwana margin and material accreted from it is now part of the eastern province of new zealand

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

on the fit of broken ridge and kerguelen plateau abstract we present revised finite rotation parameters for australia and east antarctica in the early tertiary incorporating new magnetic data off of wilkes land antarctica reconstructions based on these and recently published rotation parameters document a relatively stationary position of broken ridge abutted against central kerguelen plateau between chron 34 83 ma and chron 20 43 ma prior to the commencement of seafloor spreading between the two late cretaceous features these reconstructions are significant as they are the first which are consistent with geological and geophysical constraints in not producing an overlap of broken ridge and kerguelen plateau between chron 34 and chron 24 53 ma and not requiring significant extension or strike slip motion within kerguelen plateau we also find that the finite rotations for chrons 20 to 34 outline the area of rough topography inferred from the satellite derived free air gravity for the diamantina zone and labuan basin a consequence of these rotations is that there is considerable overlap of continental crust in the region of tasmania the south tasman rise and east antarctica in the late cretaceous solutions to this problem may imply either post santonian 79 ma extension or transform motion in the bass strait or late cretaceous or cenozoic extension in wilkes land the reconstructions also agree with prior estimates of roughly 150 km of motion between east and west antarctica since chron 20

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

new sw pacific tectonic model cyclical intraoceanic magmatic arc construction and near coeval emplacement along the australia pacific margin in the cenozoic various reconstructions of the sw pacific for the late cretaceous and cenozoic suggest that northeast dipping subduction began in the south loyalty basin slb at 55 50 ma and that subsequent closure of the slb resulted in the diachronous emplacement of cretaceous paleocene ophiolitic nappes onto the norfolk ridge in new caledonia at 40 34 ma and in northland new zealand around 24 21 ma a fundamental problem with these models is that they do not account for the fact that ne dipping subduction had already been established offshore papua new guinea by at least 65 60 ma which resulted in the emplacement of the papuan ultramafic belt pub ophiolite at 59 58 ma a second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the loyalty arc and three kings ridge finally reconstructions of the northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the northland allochthon is late cretaceous paleocene when in fact it is oligocene a new model is presented whereby the pub new caledonia and northland ophiolites formed and were emplaced in a cyclical fashion above an extensive ne dipping cenozoic intraoceanic arc system which diachronously propagated n s along the entire eastern margin of the australian plate these infant arc ophiolites represent fragments of suprasubduction zone lithosphere sszl generated in the earliest stages of magmatic arc formation that were emplaced shortly after 20 m y as a result of forearc australian plate collision subduction inception was the result of subsidence of older morb like lithosphere generated within an extensive back arc basin to the east of the norfolk ridge during the earliest stages of slb formation above a southwest dipping pacific plate during emplacement of each ophiolite a crustal fragment of the older lithosphere was scraped off the ne dipping slab and subsequently back thrust beneath each ophiolite during its emplacement

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

evolution and strength of pacific fracture zones previous studies have shown that pacific fracture zones are strong in some locations sustaining the stresses associated with differential subsidence across a locked fault while at other locations they show signs of weakness acting as preferential conduits for volcanism or supporting anomalously low shear stresses we find that half of all geosat crossings of central pacific fracture zones are inconsistent with a flexurally maintained scarp we test two hypotheses for the origin of such anomalous crossings 1 anomalous structures formed at the transform fault during times of changes in plate motions and preserved at strong fracture zones and 2 anomalous structures representing a posttransform response of the fracture zone to subsequent tectonic activity we find that three quarters of the anomalous crossings occur over the parts of fracture zones that formed during or immediately subsequent to times of changes in spreading directions with the exception of several locations overprinted by hot spot volcanism these same crossings show no obvious correlation with regional tectonic processes these observations suggest that most anomalous fracture zone topography is inherited from the transform fault and is not a product of subsequent activity consistent with the hypothesis that fracture zones generally remain strong throughout their lifetimes

late cretaceous to recent plate motions in western south america revisited abstract the andean cordillera has evolved since the late cretaceous in the context of subduction of oceanic lithosphere beneath continental lithosphere making the kinematics between south america and its adjacent oceanic plates in the pacific basin valuable to analyze the development of the andean orogen the latest cretaceous cenozoic convergence history in western south america may be divided into three stages the youngest stage 1 25 0 ma is characterized by ene directed convergence of the nazca plate toward most of south america and by e w subduction of the antarctic plate beneath southern patagonia the nazca south america convergence rate in stage 1 shows a continuous decrease from the highest values in the cenozoic 15 cm yr to the present day values from gps measurements 7 cm yr stage 2 47 28 ma is characterized by ne directed subduction of farallon with the convergence rate remaining almost constant during the entire interval in those times obliquity was dextral in chile sinistral in southern peru while almost head on convergence occurred in central and northern peru during latest cretaceous to early eocene times stage 3 the farallon plate was subducted beneath peru and the phoenix plate was subducted farther south where a triple junction migrated southward along the chilean margin the subduction of the farallon plate was rather slow with variable direction imposed by the position of the triple junction whereas subduction of the phoenix plate was rapid 10 cm yr and ese directed we present a working hypothesis suggesting no major changes in the age of subducted lithosphere in the chile trench from middle eocene to late oligocene followed by subduction of progressively older oceanic lithosphere in the early neogene and progressively younger lithosphere during the late neogene and the quaternary in addition it is shown that south american motion as predicted by available hotspot models has insufficient resolution to be applied to the analysis of cenozoic andean deformation

geochemistry and tectonic significance of the tangihua ophiolite complex new zealand abstract the ophiolitic igneous complexes of the tangihua complex of northland new zealand consist of massive and pillowed basaltic lava sequences with intercalated sediments and lesser gabbro microgabbro and basaltic intrusions the complex also includes minor felsic derivatives younger alkalic intrusions which exhibit within plate characteristics and rare ultramafic rocks the lavas are relatively homogeneous and are dominantly tholeiitic basalts with minor calc alkaline and alkaline affinities the complex nature of the chemistry in particular those lavas with transitional arc signatures and the presence of non arc lavas is clear evidence that these volcanics formed in a suprasubduction zone environment the combined geochemical and tectonic constraints suggest that the tangihua complex formed in either a transitional zone between an arc and a back arc setting or in a zone of migration from arc to back arc volcanism as such the field age and geochemical data support the model in which these rocks formed suprasubduction zone crust and are fragments recording some or all stages of arc initiation arc rifting and back arc basin formation the orientation of the arc and back arc volcanic rocks in relation to the over tectonic regime suggests subduction towards the west recent work in the sw pacific shows that late cretaceous ophiolites are present in papua new guinea new caledonia and new zealand the formation of these ophiolitic systems supports the presence of a large scale tectonic regime convergence and consequent westward subduction stretching from papua new guinea to new zealand during the late cretaceous to early palaeocene

the origin and p t evolution of peridotites and serpentinites of ne new caledonia prograde interaction between continental margin and the mantle wedge the pouebo and diahot terranes of ne new caledonia mostly comprise eclogite to blueschist facies metabasite and metasedimentary rocks that experienced c 40 ma metamorphism this eocene high p event has been linked with the sw directed obduction of the new caledonian ophiolite an extensive ultramafic nappe that dominates outcrop in the south of the island in the north ultramafic lithologies are found only as sheets or discrete lenticular masses interleaved with but separated from the eclogites and blueschists by foliated talc chlorite serpentine carbonate bearing rocks the base of the largest and best preserved ultramafic body at yambe is marked by a distinctive 2 m thick layer of high p mylonite that preserves evidence for early blueschist facies conditions s1 as inclusions in eclogite facies minerals textural evidence preserved in olivine bearing serpentinites and their bounding mafic mylonites suggest that the ultramafic bodies were emplaced within the structurally highest levels of the high p terrane as serpentinite tectonites sourced from hydrated mantle formerly in the hangingwall of the eocene subduction zone serpentinite emplacement accompanied burial of the ne new caledonian margin at t 500 c and p 16 kbar the ultramafic fragments were buried to depths of 50 60 km in the subduction zone where olivine was stable and coarse grained garnet omphacite rich assemblages developed in low strain domains within enclosing mylonites host metabasic and metasedimentary rocks from the structurally highest portions of the high p belt have a prograde record identical to that of the ultramafic tectonites the early emplacement and similar p t history of host rocks and ultramafic masses suggest that ne new caledonia preserves a fossil slab mantle wedge boundary reactivated during exhumation

history of the cretaceous osbourn spreading center the osbourn trough is a fossil spreading center that rifted apart the manihiki and hikurangi plateaus during cretaceous time previous models of the osbourn spreading center are based on data collected near the trough axis and therefore only constrain the history of the osbourn spreading center during the last few ma of spreading our data set includes multibeam data collected northward to the manihiki plateau allowing us to examine seafloor morphology created during the entire active period of the osbourn spreading center as well as several additional multibeam data sets that provide the opportunity to examine the relationship between the osbourn paleospreading center and the cretaceous pacific phoenix ridge the axial gravity of the trough is similar to the gravity found at other extinct slow intermediate spreading rate ridges magnetic field measurements indicate that spreading at the trough ceased during chron c34 abyssal hill trends indicate that spreading during the early history of the osbourn spreading center occurred at 15 20 the east west component of this spreading explains the modern east west offset of the manihiki and hikurangi plateaus spreading rotated to 2 5 prior to extinction abyssal hill rms amplitudes show that a decrease in spreading rate from 7 cm yr to 2 6 cm yr full spreading rate occurred 2 6 ma prior to ridge extinction our data analysis is unable to determine the exact spreading rate of the osbourn spreading center prior to the slowing event the temporal constraints provided by our data show that the osbourn spreading center ceased spreading prior to 87 ma or 93 ma depending on whether the manihiki and hikurangi plateaus rifted at 115 ma or 121 ma our model resolves the conflict between regional models of osbourn spreading with models based on trough characteristics by showing that spreading at the osbourn spreading center was decoupled from pacific phoenix spreading

two stage rifting of zealandia australia antarctica evidence from 40ar 39ar thermochronometry of the sisters shear zone stewart island new zealand the sisters shear zone is a newly discovered late cretaceous detachment fault system exposed for 40 km along the southeast coast of stewart island southernmost new zealand footwall rocks consist of variably deformed ca 310 and 105 ma granites that range from undeformed to protomylonite mylonite and ultramylonite the hanging wall includes non marine conglomerate and brittley deformed granite k feldspar thermochronometry of the footwall indicates moderately rapid cooling 20 30 c m y due to tectonic denudation over the interval ca 89 82 ma return to slow cooling at 82 ma coincides with the age of the oldest seafloor adjacent to the campbell plateau and reflects the mechanical transition from continental extension to lithospheric rupture and formation of the pacific antarctic ridge our findings support a two stage rift model for continental breakup of this part of the gondwana margin stage one ca 101 88 ma is the northward propagation of continental extension and the tasman ridge as recorded in mylonite dredged from the ross sea and the paparoa core complex stage two ca 89 82 ma is extension between the campbell plateau and west antarctica leading to formation of the pacific antarctic ridge

revised history of early tertiary plate motion in the south west pacific reanalysis of early tertiary magnetic anomalies on the pacific plate south of the campbell plateau indicates that in early tertiary time there was a previously unrecognized triple junction of the pacific the antarctic and a third plate now beneath the bellingshausen sea corresponding revised parameters for pacific antarctica motion do not require early tertiary displacements within antarctica or new zealand but suggest a change in direction of the pacific plate over the hotspots of approximately the same sense and timing as the hawaiian emperor bend

exsolution of thortveitite yttrialite and xenotime during low temperature recrystallization of zircon from new caledonia and their significance for trace element incorporation in zircon recrystallized zircon grains in a phengite epidote chlorite schist from north eastern new caledonia contain as inclusions a mineral assemblage consisting of celadonite kaolinite quartz fe oxy hydroxide smectite chlorite xenotime y thortveitite yttrialite and allanite ce this assemblage formed during low temperature 100 c seafloor alteration of a plagioclase rich mafic rock and represents the first documented evidence of this alteration event in the high p belt of northeastern new caledonia the survival of this low temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions thortveitite sc2si2o7 and yttrialite y2si2o7 inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized the altered zircon cores generally have lower trace element contents than the pristine igneous zircon cores and we suggest that thortveitite yttrialite and xenotime y formed as a result of trace element expulsion from zircon during low temperature recrystallization trace element concentrations in the zircon cores indicate that trivalent cations ree y sc in zircon cannot be charge balanced by xenotime substitution alone pentavalent cation concentrations nb as are also insufficient for charge balance the presence of thortveitite sc2si2o7 and yttrialite y2si2o7 suggests that trivalent cations in zircon might be charge balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as h occurring interstitially in the zircon lattice

a marine geophysical study of the wilkes land rifted continental margin antarctica summary the wilkes land margin of east antarctica conjugate to the southern australian margin is a non volcanic rifted margin that formed during the late cretaceous during 2000 01 and 2001 02 geoscience australia acquired 10 000 line km of seismic reflection and refraction magnetic anomaly and gravity anomaly data over the margin we have used the seismic data to estimate the sediment thickness along the margin the data reveal a deep 11 km rift basin that contains over 9 km of sediments seaward of the totten glacier western wilkes land the limits of oceanic and continental crust that underlies the thick post rift and the significantly thinner pre and syn rift sediments are equivocal seismic reflection data suggest a 30 100 km wide continent ocean transition zone cotz along the margin the cotz extends over 400 km seaward of the shelf break off the eastern wilkes land terre ad elie sector

life of the bellingshausen plate the onset of independent bellingshausen plate movement in the south pacific is estimated at just after chron c33o this event closely follows a change in the offset sense on the transform arm of the late cretaceous pacific phoenix antarctic triple junction and accompanies the creation and or lengthening by different mechanisms of long offsets on the pacific antarctic ridge at the udintsev tharp heezen and v transforms as the triple junction stepped away from the ridge at around chron c27 the bellingshausen plate ceased to move independently and the pacific antarctic ridge inherited the set of long offset transforms shortly afterwards the gravity fabric on the flanks of much of the ridge displays evidence for a spreading rate decrease possibly related to transpression on the long offset transforms we speculate on a link between these events and regional tectonic changes around chron c27

regional implications of u pb shrimp age constraints on the tectonic evolution of new caledonia abstract u pb shrimp ages for zircons from plagiogranites indicate a late carboniferous age of formation for ophiolitic basement of the koh terrane in the central chain mountains of new caledonia samples from ophiolites at koh and koua yielded ages of 302 7 ma and 290 5 ma respectively the similarity of these ages to those of plagiogranites in the dun mountain ophiolite belt maitai terrane of new zealand and the comparable structural positions of the two terranes potentially constrain reconstructions of the cretaceous sw pacific margin detrital zircons from late mesozoic sediments that overlap the koh terrane also provide potential constraints on the location of new caledonia prior to its break up and dispersal from eastern australia

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

west antarctic paleotopography estimated at the eocene oligocene climate transition 1 one generally recognized limitation of models for rapid growth of antarctic ice near the eocene oligocene transition is that they are based on present topography corrected only for removal of modern ice for west antarctica this results in large areas below sea level that would not readily host ice in the recently active west antarctic rift system other factors may have contributed to significant vertical motions since the eocene additional corrections for thermal contraction resulting from tectonic extension and for erosion and sedimentation since 34 ma restore most of west antarctica to above sea level increasing total antarctic land area by 10 20 because ice sheets have convex slopes the potential increase in ice volume is larger than the increase in land area accounting for large changes in west antarctic topography may resolve conflicts between ice models and data by demonstrating the possibility of an early large west antarctic ice sheet

nature of the continent ocean transition zone along the southern australian continental margin a comparison of the naturaliste plateau sw australia and the central great australian bight sectors we document the interpretation of three crustal sections from coincident deep seismic reflection gravity and magnetic data acquired on australia s southern margin one section from the naturaliste plateau and the diamantina zone and two in the great australian bight gab interpretations are based on an integrated study of deep multichannel seismic gravity and magnetic data together with sparse sonobuoy and dredging information all interpreted sections of the margin show a transition from thinned continental crust through a wide continent ocean transition zone cotz in the gab the transition is to slow sea floor spreading oceanic crust that dates from breakup in the campanian c 83 ma in the naturaliste diamantina margin the earliest oceanic crust is undated the cotz on these margins is geologically and geophysically complex but interpretation of all data including dredge hauls is consistent with the presence of a mixture of modified continental lower crust breakup related volcanics and exhumed continental mantle serpentinized detachment faults are not well imaged but have been inferred from high amplitude magnetic signatures interpreted to arise from magnetite associated with the hydration of peridotites alternative models for the structure of the cotz involving either mafic underplating or aborted sea floor spreading have been explored but are considered unlikely on this margin similarity in the final architecture of these margins has major implications for the nature of rifting in the southern rift system and may point to the entire 4000 km long system being non volcanic in character second order differences in geometry and morphology of the two areas studied are unlikely to be a function of strain rate instead they probably reflect complexities owing to the multiple tectonic events that occurred during final gondwanide fragmentation the most dramatic of these is the impact of hotspot activity in the kerguelen plateau which commenced some 50 ma prior to final breakup in that sector

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

the initiation and evolution of the transpressional straight river shear zone central fiordland new zealand abstract structural data and u pb geochronology on zircon from central fiordland new zealand show the role of pre existing structural heterogeneities in the kinematic evolution of a newly discovered zone of transpression the straight river shear zone consists of steep zones of high strain that are superimposed onto older fabrics across a 10 80 km region the older foliation formed during two periods of tectonism contraction and magmatism of mostly carboniferous 312 306 ma age and early cretaceous batholith emplacement ending by 113 4 1 7 ma followed by extension that ceased by 88 4 1 2 ma the primary mechanism for the formation of steep shear zone foliations was the folding of these older fabrics conjugate crenulation cleavages associated with the folding record shortening at high angles to the shear zone boundaries fold axial surfaces and axial planar cleavages strike parallel to the shear zone with increasing strain as they progressively steepened to subvertical in most areas shear sense flips from oblique sinistral east side down component to oblique dextral west side down across zones of intermediate and high strain high strain zones display subvertical mineral lineations steep strike slip faults and shear sense indicators that record strike slip motion across the steep lineations these patterns reflect triclinic transpression characterized by narrow zones of mostly strike slip deformation and wide zones of mostly contraction zones of high strain align with offshore traces of late tertiary strike slip faults suggesting that a previously undocumented component of late tertiary shortening and strike slip motion is accommodated within fiordland

tectonic and denudational history of the rennick graben north victoria land implications for the evolution of rifting between east and west antarctica 1 the tectonic depression of the rennick graben northern victoria land antarctica is a key element for deciphering the cenozoic separation history between east and west antarctica and evolution of the west antarctic rift system the geometry kinematics and timing of the boundary faults of the rennick graben are presented on the basis of a detailed structural study of the major fault segments combined with apatite fission track data the structural architecture of the rennick graben is controlled by distributed and partitioned right lateral strike slip faulting fission track data indicate that the last significant denudation stage of the rennick graben shoulders started from eocene times with increasing cooling and denudation since 40 50 ma a direct structural connection is then established between cenozoic denudation and the activation of the dextral strike slip regime in north victoria land during the cenozoic in this framework the rennick graben is interpreted as a localized tectonic depression generated as a displacement accommodation area of the regionally sized dextral strike slip shearing the results of this study 1 invalidate the commonly assumed link between mesozoic ross sea rifting and opening of the rennick graben 2 argue against orthogonal rifting models for the cenozoic evolution of the ross sea region and 3 provide constraints to the modalities and timing of the separation between the east antarctic craton and the west antarctica extensional province

kinematic vorticity and tectonic significance of superposed mylonites in a major lower crustal shear zone northern fiordland new zealand abstract new structural metamorphic finite strain and kinematic vorticity data from a 4 km wide subvertical shear zone in fiordland new zealand reveal a history of deformation reflecting different tectonic regimes an analysis of ductile fabrics within the shear zone and its wall rocks shows two distinctive stages of amphibolite facies mylonitic deformation dasz2 and dasz3 that are superimposed on older paleozoic or early mesozoic structures variations in strain intensity and well defined shear zone boundaries have allowed us to examine the progressive development of lasz2 sasz2 and lasz3 sasz3 fabrics and compare the types kinematics and conditions of deformation that produced them mineral assemblages defining lasz2 sasz2 provided calculated peak conditions of 11 9 1 1 kbar and 581 34 c indicative of lower crustal depths mineral assemblages defining lasz3 sasz3 provided calculated peak conditions of 8 7 1 2 kbar and 587 42 c finite strain and kinematic vorticity studies show that dasz2 involved ductile normal faulting and crustal thinning leading to decompression and exhumation of lower crustal rocks at some time during the cretaceous rifting of ancestral new zealand from gondwana dasz3 represents an episode of mid crustal dextral transpression that may have resulted from latest mesozoic or cenozoic oblique convergence reactivation of the subvertical sasz3 foliation by cataclastic shear zones and brittle faults dasz4 was accompanied by limited recrystallization at greenschist facies conditions dasz4 shear zones record upper crustal dextral strike slip faulting that resembles late tertiary deformation patterns associated with the australian pacific transform plate boundary

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

eocene arc continent collision in new caledonia and implications for regional southwest pacific tectonic evolution new caledonia preserves evidence that constrains models for the tectonic evolution of the southwest pacific region onland geology reflects four main tectonic phases 1 early mesozoic development of subduction related terranes and their accretion to the gondwana margin 2 cretaceous passive margin development and sea floor spreading during the gondwana breakup 3 foundering of an oceanic basin and the eocene arrival of thinned gondwana margin crust at a southwest facing subduction zone resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast and 4 detachment faulting during extensional collapse resulting in unroofing of metamorphic core complexes the last phase explains supposedly anomalous metamorphic gradients in the northeast of the island

new continental margin magnetic anomalies of east antarctica over the past decade australian norwegian and russian marine surveys have collected integrated seismic gravity and magnetic data in the southern indian ocean the more than 350 000 line km of new airborne and marine magnetic observations for the east antarctic continental margin have been compiled into an improved definition of crustal magnetic anomaly patterns this compilation provides important new constraints on the breakup processes and igneous activity related to the formation of the passive margin of east antarctica the eastern sector of the map from bruce rise in the west to the d urville sea in the east is largely dominated by seafloor spreading magnetic anomalies the adelie rift block of highly stretched and extensively faulted continental crust is associated with a smooth anomaly fabric abrupt magnetic anomaly changes along the oceanic continent transition in the cooperation sea including the enderby basin anomaly extend for more than 1680 km from the kerguelen plateau towards the cosmonaut sea three sectors of the east antarctic continental margin exhibit pronounced disparities in the anomaly patterns that strongly suggest different modes of seafloor formation strong positive seafloor magnetic anomalies mark the southern margin of the kerguelen plateau the maud rise and adjacent areas in the riiser larsen sea the new compilation suggests that at least 300 km of the enderby basin and shackleton basin may be part of the cretaceous kerguelen volcanic province and possibly maps an abandoned fossil spreading center in the central enderby basin the majority of the published age models for the enderby basin and australian sector of the east antarctic margin are not in agreement with the structural grain of magnetic anomalies in the newly compiled map

cretaceous fore arc basalts from the tonga arc geochemistry and implications for the tectonic history of the sw pacific the tonga fore arc preserves a complex history of subduction initiation back arc basin formation and arc volcanism which has extended from the cretaceous to the present in this paper we discuss the geochemistry of a cretaceous basalt dolerite gabbro suite recovered in two dredges from the tonga fore arc at 19 s the geochemistry of the tonga fore arc suite is unlike that of the uniformly depleted morb basalts of the subducting pacific plate and therefore is unlikely to be accreted from pacific cretaceous crust the 102 ma age obtained for one tongan fore arc dolerite is contemporaneous with a major phase of cretaceous subduction related volcanism recorded both in detrital zircon age populations and associated volcanics from new caledonia and new zealand we believe that the tonga fore arc basalts are a remnant of a hypothesized once extensive cretaceous back arc basin called the east new caledonia basin which we propose to have existed from 102 to 50 ma the allochthonous poya terrane of new caledonia is geochemically very similar to the tonga fore arc basalts and represents a younger 84 55 ma remnant of the same basin subduction related cretaceous volcanics from the sw pacific representing both arc and back arc settings all appear to have similar zr nb values suggesting a common mantle component in their petrogenesis the tonga fore arc basalts are also similar to fore arc basalts recovered from the izu bonin mariana fore arc but unlike these basalts they are not associated with subduction initiation

basalts erupted along the tongan fore arc during subduction initiation evidence from geochronology of dredged rocks from the tonga fore arc and trench a wide variety of different rock types were dredged from the tonga fore arc and trench between 8000 and 3000 m water depths by the 1996 boomerang voyage 40ar 39ar whole rock and u pb zircon dating suggest that these fore arc rocks were erupted episodically from the cretaceous to the pliocene 102 to 2 ma the geochemistry suggests that mor type basalts and dolerites were erupted in the cretaceous that island arc tholeiites were erupted in the eocene and that back arc basin and island arc tholeiite and boninite were erupted episodically after this time the ages generally become younger northward suggesting that fore arc crust was created in the south at around 48 52 ma and was extended northward between 35 and 28 ma between 9 and 15 ma and continuing to the present day the episodic formation of the fore arc crust suggested by this data is very different to existing models for fore arc formation based on the bonin marianas arc the bonin marianas based models postulate that the basaltic fore arc rocks were created between 52 and 49 ma at the beginning of subduction above a rapidly foundering west dipping slab instead a model where the 52 ma basalts that are presently in a fore arc position were created in the arc back arc transition behind the 57 35 ma loyalty three kings arc and placed into a fore arc setting after arc reversal following the start of collision with new caledonia is proposed for the oldest rocks in tonga this is followed by growth of the fore arc northward with continued eruption of back arc and boninitic magmas after that time

formation and emplacement of the northland ophiolite northern new zealand sw pacific tectonic implications petrological geochemical geochronological and palaeomagnetic data for rocks of the northland ophiolite terrane of northern new zealand suggest that it formed in a suprasubduction zone setting between c 29 and 26 ma at c 35 s close to its late oligocene obduction site cretaceous igneous rocks formerly considered to be part of the ophiolite probably represent the basement upon which the ophiolite was emplaced and are probably part of the mount camel arc related terrane the ophiolite is believed to have been generated in the southeastern south fiji basin close to a nw se oriented transform fault located to the sw of the vening meinesz fracture zone and was probably emplaced in response to the collision of the hikurangi plateau with eastern new zealand at the end of the oligocene this collision would have involved a major adjustment on the transform fault thereby allowing a portion of the upper crustal section of the southern south fiji basin to be emplaced southwestward onto northern new zealand as well as the coeval emplacement of the east cape allochthon to the south concomitant subduction of the lower crust mantle section led to the initiation of arc volcanism that resulted in the northland lower miocene volcanic plutonic suite

intra panthalassa ocean subduction zones revealed by fossil arcs and mantle structure the vast panthalassa ocean once surrounded the supercontinent pangaea subduction has since consumed most of the oceanic plates that formed the ocean floor so classic plate reconstructions based on magnetic anomalies can be used only to constrain the ocean s history since the cretaceous period 1 2 and the triassic jurassic plate tectonic evolution of the panthalassa ocean remains largely unresolved 3 4 geological clues come from extinct intra oceanic volcanic arcs that formed above ancient subduction zones but have since been accreted to the north american and asian continental margins 4 here we compile data on the composition the timing of formation and accretion and the present day locations of these volcanic arcs and show that intra oceanic subduction zones must have once been situated in a central panthalassa location in our plate tectonic reconstructions 5 7 to constrain the palaeoposition of the extinct arcs we correlate them with remnants of subducted slabs that have been identified in the mantle using seismic wave tomographic models 8 9 we suggest that a series of subduction zones together called telkhinia may have defined two separate palaeo oceanic plate systems the pontus and thalassa oceans our reconstruction provides constraints on the palaeolongitude and tectonic evolution of the telkhinia subduction zones and panthalassa ocean that are crucial for global plate tectonic reconstructions and models of mantle dynamics the spatial uncertainty in reconstructions of oceanic basins increases the further we go back in time because oceanic plates subduct plate reconstructions for the triassicjurassic history of the panthalassa ocean once surrounding the pangaea supercontinent are based on the premise that the pacific plate formedinthemiddlejurassicasatriangle 2 originatingfromatriple junction between the farallon phoenix and izanagi plates fig 1 the preserved area of jurassic palaeoplates represents only a small portion 10 of the oceanic crust of the jurassic panthalassa ocean the remaining part along with pre jurassic oceanic plate configurations is entirely unconstrained however the geology of the circum panthalassa continental margins provides evidence that early mesozoic subduction related volcanism occurred in the panthalassa ocean 4 10 on subduction continental fragments volcanic arcs ophiolites and ocean floor sediments collided with or accreted to the continental margins of in particular western north america far east asia and japan marine microfossils in these sediments and palaeomagnetic data constrain their age and palaeolatitude but their palaeolongitudes remain undetermined palaeogeographic reconstructions therefore show uncorrelated intra panthalassa exotic terranes essentially

stratigraphy of the southern norfolk ridge and the reinga basin a record of initiation of tonga kermadec northland subduction in the southwest pacific abstract seismic stratigraphic interpretations of a large new 2d seismic reflection dataset from the reinga basin region northwest of new zealand constrain the history of cretaceous fragmentation of gondwana and cenozoic initiation of tonga kermadec northland subduction the southern norfolk ridge system lay in a proximal location to plate boundaries that were active during both cretaceous and cenozoic events and persistent marine conditions led to a relatively complete record of sedimentation cretaceous extension was followed by regional subsidence and transgression two cenozoic contractional events are separated by substantial 1 km subsidence late eocene contraction led to reverse faulting and folding between new caledonia and the southern norfolk ridge system and topographic highs created in the north western sector were locally eroded by wave abrasion reinga basin was located at the southern tip of this event which had only limited impact in adjacent northern new zealand regional oligocene and miocene subsidence was contemporaneous with late oligocene and early miocene emplacement of nappes in northern new zealand and the onset of arc volcanism early miocene contraction in the reinga basin led to formation of the wanganella ridge these events can be related to calculated plate motions and are shown to be consistent with models of induced subduction nucleation that require c 150 km of convergence the first contraction is associated with regional inception of tonga kermadec subduction whereas oligocene to miocene events and the onset of hikurangi northland subduction were in response to a local change in plate boundary displacement rate and consequent linkage between subduction north of new zealand with alpine fault formation farther south

quantitative models linking igneous amphibole composition with magma cl and oh content minerals of the amphibole group are found in igneous rocks on earth and other rocky bodies since the o 3 site of amphibole can contain oh o 2 f and cl amphibole composition provides important information about water and halogen contents oxidation state and other features of its formation and alteration environments however the complexity of amphibole crystal chemistry means this information is difficult to extract furthermore it has been regular practice in the era of the electron microprobe to neglect h and fe 3 analyses for amphibole critically reducing the amount of information available in amphibole analyses in the literature we have assembled models and insights from previous work to create a methodology that allows the estimation of magmatic h 2 o and cl contents from existing amphibole analyses since the methodology requires use of a cation norm we begin with a deeper investigation of the consequences of different cation normalization schemes for amphibole analyses and provide grounds for deciding which scheme best fits a given amphibole analysis we then show how the existing model of popp and coworkers can be reversed to estimate the oh and o 3 o 2 contents of amphiboles in synthesis experiments using a synthetic data set collected from the literature 39 amphibole analyses we calibrate a partitioning model for the oh cl competition on the o 3 site of igneous amphiboles k cl x cl x oh amphibole cl oh melt ln k cl 6 59 k na a 0 679 mg 0 487 f 6 e where x signifies a mole fraction of an anion on the o 3 site cl and oh signify melt mole fractions of the anion in question on a one oxygen stolper zhang basis k na a mg and 6 fe signify the amount of each component in the amphibole in atoms per amphibole formula unit with a denoting vacancies on the amphibole a site we then combine the popp et al model with our new model to link the occupancy of the amphibole o 3 site and other crystal chemical parameters to the cl and h 2 o content of melts crystallizing amphibole the competition between oh and cl for this site can be used to calculate the melt and amphibole oh h 2 o contents as well as the speciation of fe in amphibole provided that the cl content of both the amphibole and its coexisting melt is known without analyzing either phase for h or fe 3 fe 2 while the models in this paper should be recalibrated using future experimental data this work shows that considerably more information about the volatile contents of magmas can be gleaned from amphibole than previously shown and provides additional information about the crystal chemistry of amphibole and how it affects cl partitioning into minerals of this group

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

whole rock geochemical variations and evolution of the arc derived murihiku terrane new zealand arc flank volcaniclastic sedimentation in the murihiku terrane of new zealand lasted about 120 million years from late permian to early cretaceous time despite the effects of pervasive zeolite facies alteration whole rock geochemical parameters for sandstones siltstones and tuffs record changes in source rock composition both in time and along the length of the depositional basin sandstones are considered to give a more reliable indication of the state of evolution of the source volcanic arc than do the siltstones the siltstones commonly contain detrital white mica flakes that are generally lacking in the sandstones and are possibly of distal continental origin some also contain very fine felsic ash particles average abundances and normalized multi element diagrams are used to estimate proportions of three model end member source constituents average upper continen tal crust ucc high k rhyolite rhy and basaltic andesite and sandstone provenance for the southland syncline sector changed from a predominantly basaltic andesite source in late permian to early middle triassic time for example ucc rhy and 0 17 83 in the early to early middle triassic to highly felsic in the middle to late triassic reaching ucc rhy and 2 74 24 in the late triassic oretian stage a ucc component became increasing significant from latest triassic upward and the proportion of mafic to felsic volcanism increased again with ucc rhy and 15 30 35 in the middle jurassic temaikan stage mix modelling suggests that along arc source propor tions varied with greater mafic and upper continental crust contributions in the northern kawhia seg ment than in the southland segment these patterns may be explained by deposition at an oceanic aleutian type arc margin with transition to a continental oceanic arc character induced either by arc evolution and dissection forearc sliver translation or underplating of rafted microcontinental fragments

linking the late cretaceous to paleogene pacific plate and the atlantic bordering continents using plate circuits and paleomagnetic data 1 late cretaceous to paleogene paleomagnetic data from the pacific plate the emperor seamounts can be compared with data from the atlantic bordering continents through the use of plate circuit reconstructions here we summarize the uncertainties in all data sets and present formal tests we report agreement between late cretaceous pacific paleomagnetic data and predictions based on the estimates of non pacific pole positions from synthetic apparent polar wander paths this congruency points to the veracity of the plate circuits and the accuracy of the paleomagnetic estimates in contrast to the agreement seen for the late cretaceous small discrepancies are observed in the comparisons of the pacific paleogene data and predictions from synthetic apparent polar wander paths such a disparity in a younger time interval is unexpected given the agreement of the late cretaceous data the possibility that minor temporally variable nondipole field components contribute to the discrepancy cannot be completely discounted however an alternative and more straightforward explanation is suggested by further comparisons of the mean non pacific paleomagnetic data and the highest quality poles that contribute to the means in particular we note that 1 the pacific paleogene data are in full agreement with coeval poles from north america meeting strict reliability criteria and 2 the non pacific paleogene poles of synthetic apparent polar wander paths are dominated by results from the north atlantic igneous province naip but taken as a whole the naip data fail a paleomagnetic reversal test hence minor discrepancies between paleocene paleomagnetic data from the pacific and atlantic hemispheres may point to limitations of the latter which incorporate a relatively large number of older lower quality data these findings call for renewed data collections utilizing comprehensive rock magnetic and paleomagnetic demagnetization procedures to improve resolution of paleocene non pacific data the uncertainties of the pacific paleolatitude data and the non pacific reference poles are larger than the differences related to the use of the two alternative plate circuits through east to west antarctica and through australia to the lord howe rise that link the atlantic and pacific hemispheres while no preference can thus be given to either plate circuit their overall consistency with paleomagnetic data suggests that they can be used to investigate long term motion of the pacific plate since late cretaceous times

new insight into the dynamic development of the southern alps new zealand from detailed thermochronological investigation of the mataketake range pegmatites

andean analogue for late carboniferous volcanic arc and arc flank environments of the western new england orogen new south wales australia abstract late carboniferous continental conglomerates interbedded with silicic ignimbrite sheets outcrop along more than 400 km of the western margin of the southern portion of the new england orogen farther east the coeval sedimentary facies are volcanogenic shallow marine and turbidite deposits the volcanic source terrain no longer exposed was located to the west of the existing conglomerate ignimbrite sequences and was underlain by continental crust which is in part represented by the northern lachlan fold belt the regional late carboniferous palaeogeography was similar to that of the present day western continental margin of south america the geology of the oceanward flank of the andean arc in northern chile and a section of the late carboniferous continental sequence near currabubula are comparable in detail the andean stratovolcanoes and ignimbrite centres thus provide the means of reconstruction of the late carboniferous volcanic source terrain the geological record of both of these continental margin volcanic arcs preserved in deposits of the arc flanks is shaped by volcanism especially the eruption of voluminous ignimbrites and by uplift deformation and glaciation centered on the arc for the arc sections considered diversity in the flank sequences arises because these controls vary in importance spatially and during the life of the arc 20 30 ma for the entire andean arc arc parallel variations in the sites of active volcanism and its character appear to be related to differences in the continental crust thickness and the circumstances of subduction of oceanic crust particularly the dip of the benioff zone by analogy variation in the age duration and style of volcanic activity along the late palaeozoic magmatic arc of the western new england orogen perhaps reflects the former existence of significant differences in crust thickness and in the angle of subduction

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

coda q distribution in the iberian peninsula summary several attenuation studies have established a frequency dependence law of the anelastic attenuation factor q in the form q qo f fo v for the approximate 1 10 hz frequency range we propose a method that leads to the determination of q which is a function of the reference frequency fo and the real exponent v with a single station to carry out the problem we determine a set of master curves as a function of y we discuss the method and the different features of the master curves when it is applied to the complicated regions of the iberian peninsula and to several instruments with different responses using this new method and the seismographic stations available in the iberian peninsula we have mapped iso qo lines at a reference frequency of 1 hz applying inversion methods the q values determined for iberia vary between about 100 and about 600 values close to 100 correspond to the southern part of iberia in general q values increase from south to north with values about 600 near the nw part of iberia the pyrenees mountains and adjacent areas present q values between about 200 and about 350 these results suggest a strong qo lateral variation in iberia a considerable frequency dependence of coda q has also been determined the v values vary between 0 3 and 0 8 the q values obtained in the iberian peninsula show very good agreement with several q values obtained in other regions of the world comparison between the iso q lines and other geophysical parameters like regional variations of p velocities heat flow isoseismal intensity distribution and crustal thickness indicates that lower q values are associated with higher isoseismal intensity attenuation higher heat flow lower p velocities and thinner crust

tectonic accretion and underplating of mafic terranes in the late eocene intraoceanic fore arc of new caledonia southwest pacific geodynamic implications this paper deals with the tectonic events that result in the accretion of mafic terranes in the fore arc region and a close juxtaposition of ultramafic rocks low grade and high grade mafic terranes in many collisional orogens the example is taken from new caledonia where tectonic accretion subduction underplating and obduction of mafic terranes took place during the late eocene in an intra oceanic forearc setting the late eocene tectonic complex comprised three major terranes an overlying ultramafic mainly harzburgitic allochthon named the ophiolitic nappe an intermediate mafic mainly basaltic off scraped melange composed of kilometre scale slices of oceanic upper crust called the poya terrane parts of which have been metamorphosed into an eclogite blueschist facies complex the pouebo terrane and a lower continental basement formed by the norkolk ridge terranes based upon exhaustive sampling of the mafic terranes and field surveys our tectonic micropaleontologic and geochemical data reveal that poya and pouebo terranes rocks originally formed within one single campanian to late paleocene oceanic basin floored by tholeiitic basalt associated with some minor seamount related intraplate alkali basalt the tholeiitic basalt displays a continuous range of compositions spanning between undepleted and depleted end members the former being volumetrically predominant the overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin the south loyalty basin which almost completely disappeared during eocene convergence the opening of this basin originally located to the east of the norfolk ridge was synchronous with that of tasman sea basin as a consequence of oceanward migration of the west dipping pacific subduction zone establishing the origin of the ultramafic ophiolitic nappe is beyond the scope of this paper however it appears to be genetically unrelated to the mafic poya and pouebo terranes although it was located in the late eocene fore arc the ophiolitic nappe and the corresponding oceanic lithosphere originated before the late cretaceous to the east of the south loyalty basin in a back arc setting or alternatively in a much older trapped basin for reasons that remain unclear a new east dipping subduction started in the eocene and consumed most of the south loyalty basin forming the intra oceanic loyalty arc due to a changing subduction regime underplating of the diahot terrane the mafic slices that now form the poya terrane were tectonically accreted in the loyalty fore arc region and remained under low pressure low temperature conditions possibly at the subsurface until the norfolk ridge reached the subduction zone diachronously this resulted in the final obduction of the fore arc area the two step obduction involved first the mafic complex forming the poya terrane and thereafter the lithospheric mantle that now forms the ophiolitic nappe in contrast pieces of the accretionnary complex were dragged down into the subduction zone underplated at depth ca 70 km and metamorphosed into high temperature eclogite to form the pouebo terrane metamorphics that display the same geochemical features as the poya terrane basalt a mid to late eocene syntectonic piggy back sedimentary basin the nepoui flysch basin mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore arc region due to the accretion and underplating of mafic ocean related and other material in contrast a slightly younger foreland basin located upon the norfolk ridge the priabonian bourail flysch basin received a massive input of detrital material derived from the norfolk ridge itself and a time increasing amount of mafic poya derived material that recorded the first step of obduction thereafter the bourail flysch was overthrust by the poya terrane and finally by the ophiolitic nappe at the same time buoyancy driven uplift and exhumation of the high pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone finally for a short time a new subduction started along the west coast of new caledonia and generated small amounts of active margin related magmas these events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre collision analogue of mafic ultramafic belts found in many collisional mountain ranges

transform and rift structure of paleogene crust near resolution ridge tasman sea southwest new zealand 1 multibeam bathymetry seismic reflection magnetic anomaly and gravity anomaly data show that most of the resolution ridge system is tasman sea oceanic crust deformed by eocene rift faulting associated with the initiation of the modern australia pacific plate boundary resolution ridge the most eastern ridge of the system is inferred to be continental crust that was plucked from the southwest corner of the campbell plateau during eocene rift propagation rift propagation proceeded through sequential northward steps between weak points in the lithosphere young fossil spreading centers fracture zones and the western margin of the continental campbell plateau fracture zones guided the large scale geometry of the rift but there is not always a close surface correspondence between transform and rift faults this may be because transform faults were subvertical whereas rift faults evolved to be moderately dipping the very good correspondence of fracture zones rift faults and magnetic anomalies from the resolution ridge system with their conjugates southwest of the campbell plateau allows precise prerift reconstruction of the australia pacific plate boundary at a time circa 48 ma of significant global plate reorganization and profound change in pacific plate motion this precise plate reconstruction better constrains plate motion chains that connect the pacific and indo atlantic hemispheres thus contributing to debate of global scientific issues such as what caused the bend in the emperor hawaii seamount chain

link between ssz ophiolite formation emplacement and arc inception northland new zealand u pb shrimp constraints cenozoic sw pacific tectonic implications abstract new u pb age data from zircons separated from a northland ophiolite gabbro yield a mean 206 pb 238 u age of 31 6 0 2 ma providing support for a recently determined 28 3 0 2 ma shrimp age of an associated plagiogranite and 29 26 ma 40 ar 39 ar ages n 9 of basalts of the ophiolite elsewhere miocene arc related calc alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206 pb 238 u ages of 20 1 0 2 and 19 8 0 2 ma the ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122 104 ma the early cretaceous zircons in the arc andesites are interpreted as xenocrysts from the mt camel basement terrane through which magmas of the northland miocene arc lavas erupted the inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation we postulate that the tholeiitic suite of the ophiolite represents the crustal segment of ssz lithosphere sszl generated in the southern south fiji basin sfb at a northeast dipping subduction zone that was initiated at about 35 ma the subduction zone nucleated along a pre existing transform boundary separating circa 45 20 ma oceanic lithosphere to the north and west of the northland peninsula from nascent back arc basin lithosphere of the sfb construction of the sszl propagated southward along the transform boundary as the sfb continued to unzip to the southeast after subduction of a large portion of oceanic lithosphere by about 26 ma and collision of the sszl with new zealand compression between the australian plate and the pacific plate was taken up along a new southwest dipping subduction zone behind the sszl renewed volcanism began in the oceanic forearc at 25 ma producing boninitic like ssz and within plate alkalic and calc alkaline rocks rocks of these types temporally overlap ophiolite emplacement and subsequent miocene continental arc construction

the geochemistry of tholeiitic and alkalic plutonic suites within the northland ophiolite northern new zealand magmatism in a back arc basin abstract the northland ophiolite of new zealand consists mainly of tholeiitic basalts with subordinate alkalic volcanic rocks the alkalic rocks occur together with but are generally younger than the tholeiitis both the tholeiitic and alkalic suites include minor plutonic intrusives which range from gabbro to plagiogranite variations within the tholeiitic plutonic suite can be modelled in terms of low pressure crystal fractionation the evolution of the alkalic suite is more problematic because each massif appears to be derived from a slightly different parental magma new trace element chemistry presented in this study shows that the tholeiitic rocks formed from a mantle source that was compositionally modified by supra subduction zone processes but that the alkalic suite has a different chemical signature nevertheless both suites of rocks are interpreted as having formed in a back arc spreading system in this environment the addition of fertile mantle to the supra subduction wedge facilitated by subduction roll back provided a source for the alkalic rocks with mixed geochemical characteristics

zoned cretaceous and cenozoic garnet and the timing of high grade metamorphism southern alps new zealand new garnet sm nd and lu hf and existing rb sr 40ar 39ar u pb and sm nd ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the alpine schist belt and southern alps orogen new zealand are diachronous and partly cenozoic in age the dominant metamorphic isograds in the alpine schist formed during crustal thickening at a previously unsuspected time at c 86 ma immediately prior to the opening of the tasman sea at c 84 82 ma obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism and fabric elements can be highly diachronous a detailed timing history for the growth of a single garnet is recorded by a sm nd garnet whole rock age of 97 8 8 1 ma for the inmost garnet core zone 1 lu hf ages of 86 2 0 2 ma and 86 3 0 2 ma for overgrowth zones 2 and 3 a step leach sm nd age of 12 37 ma for zone 4 and growth of the garnet rim zone 5 over the alpine fault mylonite foliation during the modern phase of oblique collision that began at c 5 6 ma plate convergence along the new zealand portion of the gondwana margin continued after c 105 ma almost certainly culminating in the oblique collision of a large oceanic plateau hikurangi plateau the metamorphism of the alpine schist at c 86 ma is evidence of that hit the mid to late cretaceous extension that is widespread elsewhere in the new zealand region is attributed to upper plate extension and slab roll back the effects of the collision with the hikurangi plateau may have contributed to the changing plate motions in the region leading up to the opening of the tasman sea at c 82 ma

west antarctic rift system in the antarctic peninsula 1 decades after the recognition of the west antarctic rift system and in spite of its global importance the location and nature of the plate boundary it formed at are unknown east of the byrd subglacial basin alternative constructions of the circuit of south pacific plate boundaries suggest the presence of either a transcurrent plate boundary or a continuation of the extensional rift system we identify george vi sound a curved depression separating alexander island from palmer land as the easternmost basin of a rift system that terminated at a triple junction with the antarctic peninsula subduction zone the history of the triple junction s third transform arm suggests extension started around 33 5 30 ma a more speculatively identified basin further west may have formed earlier during the same episode of rifting starting around 43 ma proposals of earlier cenozoic relative motion between east and west antarctica cannot be verified from this region

change of tectono stratigraphic regime in the australian plate during the 99 ma mid cretaceous and 43 ma mid eocene swerves of the pacific the clockwise bend at 99 ma mid cretaceous in linear volcanic chains in the tropical pacific coincides with a change from pre 99 ma head on chilean type subduction of the pacific plate beneath eastern gondwana to 99 43 ma sinistral oblique mariana type subduction and strike slip breakup by simple sea floor spreading between australia and antarctica and by backarc spreading in the southwest pacific the 99 ma breakup of australia from antarctica is documented by a mid cretaceous unconformity this tectono stratigraphic change founded modern australia with a mountain chain along an upper plate margin in the east and lowlands on the lower plate margin in the south the counterclockwise bend at 43 ma mid eocene the emperor hawaiian bend coincides with the onset of structure in the challenger rift of new zealand the eromanga cooper basin of central australia and the oil shale grabens of coastal queensland

reconstructions of the continents around the north atlantic at about the 60th parallel abstract late carboniferous early tertiary apparent polar wander apw paths 300 40 ma for north america and europe have been tested in various reconstructions these paths demonstrate that the 500 fathom bullard et al fit is excellent from late carboniferous to late triassic times but the continental configuration in northern pangea changed systematically between the late triassic ca 214 ma and the mid jurassic ca 170 ma due to pre drift extension best fit north atlantic reconstructions minimize differences in the late carboniferous early jurassic and late cretaceous tertiary segments of the apw paths but an enigmatic difference exists in the paths for most of the jurassic whereas for the early cretaceous the data from europe are nearly non existent greenland s position is problematic in a bullard et al fit because of a late triassic early jurassic regime of compression 300 km that would be inherently required for the norwegian shelf and the barents sea but which is geologically not defensible we suggest a radically new fit for greenland in between europe and north america in the early mesozoic this fit keeps greenland locked to europe for the late paleozoic early mesozoic and maintains a reconstruction that better complies with the offshore geological history of the norwegian shelf and the barents sea pre drift a24 extension amounted to approximately 450 km on the mid norwegian shelf but with peak extension in the late cretaceous

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

geochemistry and tectonic setting of matakaoa volcanics east coast allochthon new zealand suprasubduction zone affinity regional correlations and origin in north eastern new zealand the late cretaceous to eocene submarine volcanic rocks of the east coast allochthon referred to as matakaoa volcanics formed a dominantly mafic melange with minor abyssal sediments during the subduction of oceanic crust accreted and melanged pillow lavas and dolerites display the geochemical and isotopic features of tholeiites with some diversity referred to as midocean ridge basalts island arc tholeiites and back arc basin basalts the association of supra subduction tholeiitic magmas abyssal sediments and polymetallic volcanogenic sulphide deposits is typical of many back arc basins therefore a correlation with the alkaline and transitional basalts of the hikurangi manihiki ontong java plateau may be ruled out these petrological features and the fossil ages as well are closely similar to those of the tangihua complex northland allochthon and allow matakaoa and tangihua mafic volcanic rocks now separated by the modern havre trough to be correlated within one single late cretaceous to eocene 2 basin the consideration of possible younger ages oligocene is also discussed the occurrence of late cretaceous eocene oceanic crust with supra subduction zone affinities to the east of the norfolk ridge poses the problem of the intervening southwest dipping subduction the missing late cretaceous eocene arc and their relationship with southeast gondwana marginal break up

toward a self consistent high resolution absolute plate motion model for the pacific the hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes given geometry and ages from several trails one can reconstruct absolute plate motions apm that provide valuable information about past and present tectonism paleogeography and volcanism most apm models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles opening angles and time intervals unlike relative plate motion rpm models such apm models suffer from oversimplicity self inconsistencies inadequate fits to data and lack of rigorous uncertainty estimates in addition they work only for fixed hot spots newer methods are now available that overcome many of these limitations we present a technique that provides high resolution apm models derived from stationary or moving hot spots given prescribed paths the simplest model assumes stationary hot spots and an example of such a model is presented observations of geometry and chronology on the pacific plate appear well explained by this type of model because it is a one plate model it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the emperor chain whether there was significant relative motion within the hot spots under the pacific plate during the last 70 m y is difficult to quantify given the paucity and geological uncertainty of age determinations evidence in support of plume drift appears limited to the period before the 47 ma hawaii emperor bend and apart from the direct paleolatitude determinations may have been somewhat exaggerated

geochronology and geochemistry of high pressure granulites of the arthur river complex fiordland new zealand cretaceous magmatism and metamorphism on the palaeo pacific margin the arthur river complex is a suite of gabbroic to dioritic orthogneisses in northern fiordland new zealand the arthur river complex separates rocks of the median tectonic zone a mesozoic island arc complex from palaeozoic rocks of the palaeo pacific gondwana margin and is itself intruded by the western fiordland orthogneiss new shrimp u pb single zircon data are presented for magmatic metamorphic and deformation events in the arthur river complex and adjacent rocks from northern fiordland the arthur river complex orthogneisses and dykes are dominated by magmatic zircon dated at 136 129 ma a dioritic orthogneiss that occurs along the eastern margin of the complex is dated at 154 4 3 6 ma and predates adjacent plutons of the median tectonic zone rims on zircon cores from this sample record a thermal event at c 120 ma attributed to the emplacement of the western fiordland orthogneiss migmatitic palaeozoic orthogneiss from the arthur river complex 346 6 ma is interpreted as deformed wall rock very fine rims 5 20 lm also indicate a metamorphic age of c 120 110 ma a post tectonic pegmatite 81 8 1 8 ma may be related to phases of crustal extension associated with the opening of the tasman sea the arthur river complex is interpreted as a batholith emplaced at mid crustal levels and then buried to deep crustal levels due to convergence of the median tectonic zone arc and the continental margin

mesozoic history of the fairway aotea basin implications for the early stages of gondwana fragmentation the fairway ridge is a buried continental structure that separates the fairway basin from the new caledonia basin the proposed cretaceous age of the fairway basin has remained highly hypothetical to date deep offshore petroleum exploration wells revealed well dated mesozoic carbonaceous sedimentary rocks in the taranaki basin at the southern end of the aotea basin in this paper we use geophysical data to confirm the continuity of the 2000 km long fairway aotea basin connecting new caledonia to new zealand and prove its early late cretaceous age analysis of seismic reflection profiles together with newly compiled gravity and magnetic maps reveals late cretaceous ne sw trending lineaments projecting northeastward from major tasman sea fracture zones and the bellona trough which demonstrate that the opening of the fairway aotea basin predates the opening of the tasman sea this result combined with observations of the mesozoic regional geology suggests that the lord howe fairway and norfolk ridges are part of a remnant late early cretaceous continental arc which was fragmented into three pieces by the late early to early late cretaceous this event might be contemporaneous with a plate motion change between the gondwana and pacific plates and or the arrival of the hikurangi plateau in the subduction zone around 105 ma which caused the cessation of subduction along this plate boundary we interpret either of those two events as being possible trigger events for the post early cretaceous fragmentation of the eastern gondwana margin in a slab retreat process

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

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

arc continent collisional orogenesis in the sw pacific and the nature source and correlation of emplaced ophiolitic nappe components abstract southwest pacific ophiolitic nappes emplaced upon the former margin of eastern australia provide a record of cyclical episodes of arc continent collisional orogenesis that occurred throughout the paleogene cenozoic nappes in papua new guinea new caledonia and northland new zealand consist of two key components 1 a basal and apparently older late cretaceous eocene metamorphosed normal to weakly enriched morb like basaltic ocean floor terrane and 2 an uppermost supra subduction zone ssz ophiolite that represents basement of the associated magmatic forearc a detailed comparison of geochemical characteristics of unequivocal ssz basalts with those considered as representative of the basaltic ocean floor terranes illustrates that formation of the majority of the latter is consistent with melting of a source transitional to normal n and enriched e morb these slight ree enriched or transitional t morb have generally been considered as the result of seafloor spreading in a back arc basin bab or basins to the east of australia in the late cretaceous paleocene 85 55 ma the enrichment recorded by the bab basalts is likely a result of upwelling plumes associated with initial fragmentation of eastern gondwana that left their signature on the mantle source consequently tapped by the bab basalts a subset of slightly more enriched basalts may owe their enrichment to source contamination minor assimilation of thinned gondwana derived continental crust associated lree depleted n mor basalts previously considered as part of the basaltic ocean floor terranes on the other hand are similar to ssz basalts of the northland and papuan ultramafic belt ophiolites refinement of existing tectonic models is hindered by ambiguities in the stratigraphic relationships of the underlying basaltic ocean floor terranes with the overlying ssz ophiolites a paucity of age data and uncertainties in the tectonomagmatic affinities of the underlying basalts nonetheless the geochemical characteristics displayed by lree depleted basalts of the basaltic ocean floor terranes suggest that these may instead represent younger magmatic forearc basement components subsequently overthrust by lower sections of a genetically related ophiolite whether these and for that matter the majority of basalts of the basaltic ocean floor terranes indeed represent complementary forearc crust to the ssz ophiolite as opposed to a slice of late cretaceous eocene bab floor backthrust beneath the ssz ophiolite upon emplacement will have to await the acquisition of corresponding radiometric age data