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

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

paleomagnetic evidence for motion of the hawaiian hotspot during formation of the emperor seamounts the bend in the hawaiian emperor chain is the best example of a change in plate motion recorded in a fixed hotspot frame of reference alternatively the bend might record primarily differences in motion of the hawaiian hotspot relative to the pacific lithosphere new paleomagnetic data from the emperor chain support the latter view althouth the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain the best available paleomagnetic data suggest pacific hotspots may have moved at rates comparable to those of lithospheric plates 30 mm yr 1 in late cretaceous to early tertiary times 81 43 ma if correct this requires a major change in how we view mantle dynamics and the history of plate motions in the early to mid cretaceous 128 95 ma hotspots in the atlantic moved at similar rates these episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion such as the past 5 m y

circum pacific arc flare ups and global cooling near the eocene oligocene boundary explosive eruptions from subaerial arc volcanoes can have significant environmental impact because of the discharge of ash and volatiles directly into the atmosphere and oceans the link between climate cooling and voluminous volcanic eruptions has remained speculative due to a lack of supporting evidence a compilation of 2814 k ar and 40 ar 39 ar age determinations from four circum pacific arcs indicates that periods of high volcanic output i e flare ups have been episodic and in some cases synchronous peak periods and subsequent lulls in arc magmatism over the past 50 m y have occurred coeval with major fluctuations in global climate including the eocene oligocene transition one of the most prominent global climate reorganizations in the cenozoic hundreds of intermediate to silicic eruptions occurred during an extremely vigorous period of circum pacific volcanism beginning in the late eocene which likely led to the production of sulfur aerosols in the stratosphere and fertilization of surface waters of the pacific ocean we provide a mechanism that may have been partly responsible for the climatic preconditioning that must have preceded and ultimately promoted antarctic ice sheet growth 34 m y ago

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

reconstruction of subduction and back arc spreading in the nw pacific and aleutian basin clues to causes of cretaceous and eocene plate reorganizations the eocene 50 45 ma major absolute plate motion change of the pacific plate forming the hawaii emperor bend is thought to result from inception of pacific plate subduction along one of its modern western trenches subduction is suggested to have started either spontaneously or result from subduction of the izanagi pacific mid ocean ridge or from subduction polarity reversal after collision of the olyutorsky arc that was built on the pacific plate with ne asia here we provide a detailed plate kinematic reconstruction of back arc basins and accreted terranes in the northwest pacific region from japan to the bering sea since the late cretaceous we present a new tectonic reconstruction of the intraoceanic olyutorsky and kronotsky arcs which formed above two adjacent oppositely dipping subduction zones at 85 ma within the north pacific region during another pacific wide plate reorganization we use our reconstruction to explain the formation of the submarine shirshov and bowers ridges and show that if marine magnetic anomalies reported from the aleutian basin represent magnetic polarity reversals its crust most likely formed in an 85 to 60 ma back arc basin behind the olyutorsky arc the olyutorsky arc was then separated from the pacific plate by a spreading ridge so that the 55 to 50 ma subduction polarity reversal that followed upon olyutorsky ne asia collision initiated subduction of a plate that was not the pacific hence this polarity reversal may not be a straightforward driver of the eocene pacific plate motion change whose causes remain enigmatic

rapid change in drift of the australian plate records collision with ontong java plateau earth s largest and thickest oceanic plateau the ontong java plateau is currently colliding with the australian plate but it has been difficult to date the start of this momentous event with much accuracy now a team from the university of queensland presents strong evidence for a collision starting about 26 million years ago the dating comes from geochronological data on hotspot volcanoes in the tasman sea east of australia which reveal a link between collision of the plateau with the melanesian arc and motion of the australian plate the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks in the pacific including the archetypal hawaiian chain suggesting that immense oceanic plateaus like ontong java can contribute to initiating rapid change in plate boundaries and motions on a global scale geochronological data on hotspot volcanoes in eastern australia are presented which reveal a strong link between collision of the plateau with the melanesian arc and motion of the australian plate the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks in the pacific including the archetypal hawaiian chain and thus provide strong evidence that immense oceanic plateaus can contribute to initiating rapid change in plate boundaries and motions on a global scale the subduction of oceanic plateaux which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood basalt provinces is an important factor in many current models of plate motion1 2 3 4 and provides a potential mechanism for triggering plate reorganization5 to evaluate such models it is essential to decipher the history of the collision between the largest and thickest of the world s oceanic plateaux the ontong java plateau and the australian plate but this has been hindered by poor constraints for the arrival of the plateau at the melanesian trench here we present 40ar 39ar geochronological data on hotspot volcanoes in eastern australian that reveal a strong link between collision of the greenland sized ontong java plateau with the melanesian arc and motion of the australian plate the new ages define a short lived period of reduced northward plate motion between 26 and 23 myr ago coincident with an eastward offset in the contemporaneous tracks of seamount chains in the tasman sea east of australia these features record a brief westward deflection of the australian plate as the plateau entered and choked the melanesian trench 26 myr ago from 23 myr ago australia returned to a rapid northerly trajectory at roughly the same time that southwest directed subduction began along the trobriand trough6 the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the pacific plate including the archetypal hawaiian chain7 and thus provide strong evidence that immense oceanic plateaux like the ontong java can contribute to initiating rapid change in plate boundaries and motions on a global scale

observations of geometry and ages constrain relative motion of hawaii and louisville plumes abstract the classic view of linear island chains as volcanic expressions of interactions between changing plate tectonic motions and fixed mantle plumes has come under renewed scrutiny in particular observed paleolatitudes from the emperor seamounts imply that the hawaii hotspot was 5 15 further north during formation of these seamounts and that rapid retardation of its southward migration was the primary agent forming the angular hawaii emperor bend supporting this view are predictions from fluid dynamic experiments that suggest the general mantle circulation may displace narrow mantle plumes consequently the surface locations of hotspots are not fixed and may have varied considerably in the past however the locations and ages of available rock samples place fundamental limits on the relative motion between the hawaii and louisville hotspots here we use such data to estimate empirical age progression curves for separate chains and calculate the continuous variations in hotspot separations through time while the data are sparse the inferred inter hotspot motion for ages 55 myr appears significant but the observed relative motion is only about half of what is predicted by mantle dynamics models to reconcile the observed paleolatitudes with our observed relative motion requires either a larger contemporaneous southward motion of the louisville hotspot than previously suggested or a moderate component of true polar wander

motion between the indian antarctic and african plates in the early cenozoic we used a three plate best fit algorithm to calculate four sets of euler rotations for motion between the india capricorn africa somali and antarctic plates for 14 time intervals in the early cenozoic each set of rotations had a different combination of data constraints the first set of rotations used a basic set of magnetic anomaly picks on the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir and fracture zone constraints on the cir and seir but did not incorporate data from the carlsberg ridge and did not use fracture zones on the swir the second set added fracture zone constraints from the region of the bain fracture zone on the swir which were dated with synthetic flowlines based on the first data set the third set of rotations used the basic constraints from the first rotation set and added data from the carlsberg ridge the fourth set of rotations combined both the swir fracture zone constraints and the carlsberg ridge constraints data on the indian plate side of the carlsberg ridge arabian basin were rotated to the capricorn plate before being included in the constraints plate trajectories and spreading rate histories for the cir and swir based on the new rotations document the major early cenozoic changes in plate motion on the cir and seir there was a large but gradual slowdown starting around chron 23o 51 9 ma and continuing until chron 21y 45 3 ma followed 2 or 3 myr later by an abrupt change in spreading azimuth which started around chron 20o 42 8 ma and which was completed by chron 20y 41 5 ma no change in spreading rate accompanied the abrupt change in spreading direction on the swir there was a continuous increase in spreading rates between chrons 23o and 20o and large changes in azimuth around chrons 24 and 23 and again at chron 21 unexpectedly we found that the two sets of rotations constrained by the carlsberg ridge data diverged from the other two sets of rotations prior to anomaly 22o when compared to rotations for the cir that are simultaneously constrained by data from all three branches of the indian ocean triple junction there is a progressively larger separation of anomalies on the carlsberg ridge with a roughly 25 km misfit for anomaly 23o and increasing to over 100 km for anomaly 26y these data require that there was previously unrecognized convergence somewhere in the plate circuit linking the indian capricorn and somali plates prior to chron 22o we quantify this motion by summing our new capricorn somalia rotations with previously published rotations for neogene india capricorn motion and for early cenozoic somali india motion based solely on carlsberg ridge data the most likely possibility is that there was motion within the somalia plate due to a distinct seychelles microplate as young as chron 22o the sense of the misfit on the carlsberg ridge is consistent with roughly 100 150 km of convergence across a boundary passing through the amirante trench and extending north to the carlsberg ridge axis between anomalies 26y and 22o alternatively there may have been convergence within the indian plate either along the western margin of indian or east of the cir in the region of the current capricorn indian diffuse plate boundary our work sharpens the dating of the two major eocene changes in plate motion recognized in the indian ocean

hotspotting principles and properties of a plate tectonic hough transform hotspotting is a geometric technique used to assess the self consistency of absolute plate motion models under the assumption of fixed hot spots geometric data constraints typically seamount locations and the plate motion model are used to determine the optimal locations of hot spots in a self consistent model these should match the assigned hot spot locations while self consistency is a desirable model attribute it is not sufficient by itself to guarantee a geologically meaningful model hotspotting is the study of seafloor flow lines and may be considered a pattern recognition algorithm similar to the hough transform flow line analysis can yield insight into the importance of particular geometric constraints and yield age predictions for undated features hotspotting cannot easily be used to derive improved rotation parameters but instead is used in concert with methods that determine plate motions to ensure the aforementioned self consistency

thermochronology quantifying exhumation history of the wudang complex in the south qinling orogenic belt central china the wudang complex located in the central part of south qinling has been inferred to be a segment of the yangtze craton involved in the orogen in this study the cooling exhumation history of the wudang complex is revealed through combined published geochronology data and new apatite fission track results three rapid exhumation episodes related to relevant geodynamic events have been identified previous 40 ar 39 ar and u th he data indicate that the most significant exhumation induced by the collision between the north and south china blocks occurred from c 237 to 220 ma after long term subsidence and sedimentation of the passive continental margin the second exhumation event related to the long distance effect of the pacific subduction occurred during the period from c 126 to 90 ma following the late cretaceous eocene peneplanation stage the final late cenozoic exhumation since c 15 ma may be attributed to the combined effect of the eastward growth of the tibetan plateau uplift and the asian monsoon

timing tectonic implications and genesis of gold mineralization in the xincheng gold deposit china c h o isotopes pyrite rb sr and zircon fission track thermochronometry abstract the xincheng gold deposit located at the northwestern edge of the jiaodong peninsula in the north china craton has a reserve of 96 t au and is among the largest in the jiaodong gold province here disseminated and stockwork style ores are hosted in mesozoic granitoids the mineralization and alteration are largely controlled by the regional jiaojia fault the c h o isotope values in the ores indicate that the ore forming fluids were derived mainly from magmatic sources new rb sr geochronological data on hydrothermal pyrite from the xincheng deposit presented in this study indicate that the main gold mineralization occurred between 126 ma and 122 ma the zircon fission track of convergent ages and decomposed divergent ages from the ores and altered rocks show four groups of ages 135 ma 124 ma 96 75 ma and 66 52 ma the 135 ma is related to the exhumation and cooling of the linglong granitoids the 124 ma is considered to represent the principal stage of gold mineralization and is consistent with the pyrite rb sr ages the younger ages of 96 75 ma and 66 52 ma resulted from subsequent annealing caused by later tectonic activity the timing of gold mineralization in the xincheng gold deposit broadly coincides with the switch in stress field in eastern china from extension to transpression resulting from the change in subduction direction of the pacific plate from ne sw to nw se and marked by extension related voluminous magmatism associated with the cretaceous giant igneous event in eastern china

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

pacific plate motion change at the time of the hawaiian emperor bend constrains the viscosity of earth s asthenosphere important constraints on asthenospheric viscosity come primarily from modeling the glacial rebound of the past 20 kyr but remain somewhat loose because of the intrinsic resolving power of these models we obtain narrower bounds by building on the notion that the asthenosphere also controls the ability to change plate motions over myr we focus on the pacific kinematic change at the time of the hawaiian emperor bend event which is linked to the coeval inception of subduction in the western pacific we sample plausible asthenospheric viscosity and thickness values by requiring the rate at which torque varied to generate the observed kinematics consistent with the nature of subduction initiation uncertainties on the bend event duration and the occurrence of pacific hot spots drift do not hamper our results that suggest that the asthenosphere viscous response to vertical shear over kyr is consistent with that to horizontal shear over myr

on the role of slab pull in the cenozoic motion of the pacific plate we analyze the role of slab pull acting on the pacific plate during its early tertiary change in motion slab pull forces are estimated by integrating the negative buoyancy of a 700 km long slab along a revised subduction boundary model adopting the muller et al 2008 seafloor age reconstructions our results indicate that torques predicted from a simple slab pull model match the pacific plate euler vectors during the tertiary fairly well the change of the pacific motion at similar to 50 40 ma appears to be driven by the onset of the izu bonin mariana system and soon afterwards by the tonga kermadec subduction zones

the early miocene 25 ma volcanism in the northern kyushu palau ridge enriched mantle source injection during rifting prior to the shikoku backarc basin opening the northern kyushu palau ridge kpr remnant conjugate arc of the izu ogasawara bonin mariana ibm active arc is dominated by basalt andesite except for the komahashi daini seamount where acidic plutonic rocks of 38 ma were recovered these mafic to intermediate volcanics are produced by the rifting volcanism in the proto ibm arc associated with spreading of the shikoku basin the hfse and hree contents and ratios of these volcanics indicate enriched source mantle composition compared to recent volcanic front the lile ratios exhibit similar characteristics to reararc volcanism of the recent izu arc and some enriched volcanics exhibit high abundance of sediment melt inputs based on these observations and compilations of the published data set the replacement event of the wedge mantle under the ibm arc occurred two times the first event occurred between 45 and 38 ma with pacific type mantle being replaced by depleted indian type mantle the second event occurred between 36 and 25 ma enriched mantle flowed from reararc side the slab component during the proto ibm arc rifting was a similar characteristic to recent reararc volcanism of the izu arc and sediment melt added in a local area

comment on a 40ar 39ar and u pb isotopic study of the ilimaussaq complex south greenland implications for the 40k decay constant and the duration of magmatic activity in a peralkaline complex by krumrei et al

generation of plate tectonics with two phase grain damage and pinning source sink model and toroidal flow abstract the grain damage and pinning mechanism of bercovici and ricard 2012 for lithospheric shear localization is employed in two dimensional flow calculations to test its ability to generate toroidal strike slip motion and influence plate evolution this mechanism posits that damage to the interface between phases in a polycrystalline material like peridotite composed primarily of olivine and pyroxene increases the number of small zener pinning surfaces which then constrain mineral grains to ever smaller sizes regardless of creep mechanism this effect allows a self softening feedback in which damage and grain reduction can co exist with a grain size dependent diffusion creep rheology moreover grain growth and weak zone healing are greatly impeded by zener pinning thereby leading to long lived relic weak zones the fluid dynamical calculations employ source sink driven flow as a proxy for convective poloidal flow upwelling downwelling and divergent convergent motion and the coupling of this flow with non linear rheological mechanisms excites toroidal or strike slip motion the numerical experiments show that pure dislocation creep rheology and grain damage without zener pinning as occurs in a single phase assemblages permit only weak localization and toroidal flow however the full grain damage with pinning readily allows focussed localization and intense plate like toroidal motion and strike slip deformation rapid plate motion changes are also tested with abrupt rotations of the source sink field after a plate like configuration is developed the post rotation flow and material property fields retain memory of the original configuration for extensive periods leading to suboptimally aligned plate boundaries e g strike slip margins non parallel to plate motion oblique subduction and highly localized weak and long lived acute plate boundary junctions such as at what is observed at the aleutian kurile intersection the grain damage and pinning theory therefore readily satisfies key plate tectonic metrics of localized toroidal motion and plate boundary inheritance and thus provides a predictive theory for the generation of plate tectonics on earth and other planets

late mesozoic early cenozoic deformation history of the yuanma basin central south china abstract the yuanma basin situated in central south china with more than 800 km far away from the active plate boundaries is a key region in deciphering the late mesozoic early cenozoic tectonic evolution of south china based on a field analysis of fault slip data collected from different rock units we document polyphase tectonic stress fields and address the changes in the orientation of the principal stresses and finally establish a five phase stress evolution of the basin during the late mesozoic early cenozoic the earliest one with e w compression and n s extension occurred in the mid late jurassic and resulted in regional folding and west directed thrusting along the eastern margin of the basin this deformational event constrains the timing of the significant intracontinental orogeny in south china to be mid late jurassic the subsequent nw se extensional regime was responsible for the initial opening and sedimentary infill of the basin during the earliest cretaceous which occurred coevally with the extensive early cretaceous magmatism volcanism and extensional doming in south china the tectonic regime then changed in the late early cretaceous to a compressional one with nw se compression and ne sw extension causing the inversion of this extensional basin this compression affected the whole south china leading to an apparent magmatic quiescence at 108 117 ma and sinistral strike slip faulting on the nne trending faults the n s extension prevailed in the late cretaceous and activated the east trending normal faults that controlled the late cretaceous subsidence this extensional phase seems to have lasted through the early paleogene and ended in the late paleogene by a compressional regime with ne sw compression and nw se extension as attested by the stratigraphic unconformity at the base of the neogene in the jianghan basin the temporary evolution of these tectonic stress fields documented in the yuanma basin provides a new insight into the influences of different plate tectonics exerting on the south china block over the late mesozoic to early cenozoic time we infer that the yuanma basin was influenced in the middle jurassic early cretaceous by the process of westward subduction of paleo pacific plate and in the late cretaceous early paleogene by a combined effect of the subduction along the west pacific margin and in the tethyan domain and finally in the late paleogene by the far field effects of india asia collision

plate motions in the north pacific the 43 ma nonevent the hawaiian emperor seamount chain in the north pacific ocean is commonly considered to have been produced by motion of the pacific plate over a hotspot if the hotspot is assumed to have remained fixed in a mantle frame of reference the 60 change in trend between the hawaiian and emperor portions of the chain results from a change in motion direction of the pacific plate relative to the mantle at 43 ma during middle eocene time such a large and abrupt direction change should result in significant plate reorganizations and tectonic events in the pacific and surrounding plates but no evidence for these events can be found in addition in this paper it is shown that there were no changes in relative plate motions between pacific area plates that can be associated with the hawaiian emperor bend this is done by calculating motion of the pacific farallon and kula plates relative to asia and north america using the plate circuit farallon pacific antarctica africa north america asia with no major regional tectonic event at the time of the bend it is suggested that there was no plate reorganization at 43 ma and that the emperor portion of the seamount chain was formed by a nonstationary hotspot

reconciling late neogene pacific absolute and relative plate motion changes new models of pacific absolute plate motion relative to hot spots and models of relative plate motion involving the pacific plate all agree there was a significant change in the late neogene chron 3a 5 89 ma reflecting a more northerly absolute motion than previously determined as pacific absolute plate motion became slightly more northerly left stepping transform segments came under compression some left stepping segments became microplates with clockwise rotation others show clear evidence of compressional deformation conversely right stepping transforms came under tension and many developed intratransform spreading centers or show similar evidence for transform magmatism several large left stepping transform offsets represent portions of the nazca plate protruding into the pacific and as such act as obstacles to the more northerly pacific absolute plate motion we suggest these obstructions act to enhance the generally tensile equatorial pacific stress regime caused by distant slab pull as a consequence the greater french polynesia region has experienced diffuse volcanism that increased considerably following the chron 3a plate motion change we propose that since the hawaii emperor bend time the distant pacific slab pull and the friction between the large buoyant ontong java plateau and the northern margin of the australia plate have produced episodes of increased tensile stresses in the equatorial pacific we believe these stress excursions are responsible for much of the intraplate volcanism observed in a wide triangular region from samoa to the east pacific and pacific antarctic rises

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

nonlinear 40ar 39ar age systematics along the gilbert ridge and tokelau seamount trail and the timing of the hawaii emperor bend over the last three decades the first order correlation in morphology and orientation of seamount trails has been called upon to support the concept of a fixed pacific hot spot frame of reference and to explain the hawaii emperor bend heb by a dramatic change in pacific plate motion in this paper however we present 40ar 39ar ages for the gilbert ridge and tokelau seamounts pacific that show similar changes or bends in their orientation but at different geological times up to 20 myr earlier than the heb changes in pacific plate motion alone cannot explain these observations because these asynchronous bends should have been reflected in the morphology of each of these seamount trails together with the lack of linear age progressions and inconsistent apparent local plate velocities of 131 and 87 mm yr we rule out a fixed hot spot origin for the gilbert ridge and tokelau seamount trails instead we invoke secondary or alternate processes to explain the complex age systematics and morphologies in these seamount trails we propose here that the heb type bends in these seamount trails were likely formed by short term jerk like plate extensions in the studied southwestern region of the pacific plate reactivating a preconditioned lithosphere that can be characterized by a complex structure and precursory magmatic impingements the remarkable differences observed in these colinear seamount trails fundamentally question the existence of heb type bends in the formation of pacific volcanic lineaments they also show us that applying geometric and morphologic observations alone is insufficient in constraining past plate motions nevertheless the need and search for alternate volcano tectonic mechanisms offer opportunities to better understand intraplate volcanism in general

storm related taphofacies and paleoenvironments of malvinokaffric assemblages from the lower middle devonian in southwestern gondwana abstract taphofacies of malvinokaffric macroinvertebrate bearing rocks from the uppermost pragian to lower givetian in southwestern gondwana are evaluated during the lower middle devonian the parana basin southern brazil was positioned at high latitudes in southwestern gondwana 60 to 80 s integrated litho and taphofacies interpretations in siliciclastic environments presented here provide paleoecological and paleoenvironmental insights the six taphofacies recognized here showed changing depositional conditions in paleoenvironments ranging from shoreface to offshore shelf settings they vary in recurrence and abundance but all facies exhibit evidence of storm events winter storms are to be expected at latitudes 60 to 80 s under cool climate zone storm related taphofacies recurrence during the latest pragian to early givetian points to two agents of sedimentation direct wave erosion by winter storms and sedimentary processes related to combined flow during deposition e g turbidity currents and oscillatory flow winter storms are considered more capable of affecting sediment transport than hurricanes due to longer duration and larger coverage area in higher latitudes therefore winter storms of high latitudes should generate more persistent currents in space and time finally the taphofacies lithofacies and high latitude paleogeographic context of the parana basin suggest the action of storm influence in the genesis of all lower middle devonian malvinokaffric assemblages here recognized

the bent hawaiian emperor hotspot track inheriting the mantle wind bends in volcanic hotspot lineaments best represented by the large elbow in the hawaiian emperor chain were thought to directly record changes in plate motion several lines of geophysical inquiry now suggest that a change in the locus of upwelling in the mantle induced by mantle dynamics causes bends in hotspot tracks inverse modeling suggests that although deep flow near the core mantle boundary may have played a role in the hawaiian emperor bend capture of a plume by a ridge followed by changes in sub pacific mantle flow can better explain the observations thus hotspot tracks can reveal patterns of past mantle circulation

radiometric ages for basement rocks from the emperor seamounts odp leg 197 1 the hawaiian emperor seamount chain is the type example of an age progressive hot spot generated intraplate volcanic lineament however our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago improvements in instrumentation sample preparation methods and new material obtained by recent drilling warrant a reexamination of the age relations among the older hawaiian volcanoes we report new age determinations 40ar 39ar incremental heating method on whole rocks and feldspar separates from detroit sites 1203 and 1204 nintoku site 1205 and koko site 1206 seamounts ocean drilling program odp leg 197 and meiji seamount deep sea drilling project dsdp leg 19 site 192 plateaus in incremental heating age spectra for site 1203 lava flows give a mean age of 75 8 0 6 2 ma which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments site 1204 lavas produced discordant spectra indicating ar loss by reheating and k mobilization six plateau ages from lava flows at site 1205 give a mean age of 55 6 0 2 ma corresponding to chron 24r drilling at site 1206 intersected a n r n magnetic polarity sequence of lava flows from which six plateau ages give a mean age of 49 1 0 2 ma corresponding to the chron 21n 22r 22n sequence plateau ages from two feldspar separates and one lava from dsdp site 192 range from 34 to 41 ma significantly younger than the cretaceous age of overlying sediments which we relate to postcrystallization k mobilization combined with new dating results from suiko seamount dsdp site 433 and volcanoes near the prominent bend in the lineament sharp and clague 2002 the overall trend is increasing volcano age from south to north along the emperor seamounts consistent with the hot spot model however there appear to be important departures from the earlier modeled simple linear age progression which we relate to changes in pacific plate motion and the rate of southward motion of the hawaiian hot spot

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

comment to distribution and geochronology of the oregon plateau u s a flood basalt volcanism the steens basalt revisited by m e brueseke et al j volcanol geotherm res 161 2007 187 214 abstract the 40 ar 39 ar stepheating results of brueseke et al 2007 are critically evaluated firstly for possible alteration of the samples dated steps passing the alteration index test baksi 2007a can then be used to derive plateau ages which must satisfy rigorous statistical tests of the eleven samples studied by brueseke et al 2007 only four yield plateau values that reflect eruption ages these suggest steens basalt was erupted within a relatively short period of time 0 2 m y considerably less than the 1 m y suggested by the authors the seeming difference between the plateau ages for the steens mountain basalts obtained by these authors and two earlier attempts is resolved by a proper adjustment of ages at various laboratories for interlaboratory calibration differences and by rejecting ages from altered samples the age of the important steens mountain geomagnetic polarity transition is 16 59 0 03 ma

drilling the solid earth global geodynamic cycles and earth evolution the physical and chemical evolution of the earth is driven by geodynamic cycles that are global in scale operating over 4 57 ga of earth s history some processes are truly cyclic e g the wilson cycle while others are irreversible e g core formation heat and mass transfer between the lowermost mantle e g core mantle boundary and the surface drives these global geodynamic processes subduction of lithospheric plates transfers cool fractionated material into the lower mantle and leads indirectly to the formation of new oceanic lithosphere while the rise of thermochemical plumes recycles the remnants of these plates back to the surface driven by heat transfer across the core mantle boundary these global geodynamic cycles are responsible for hotspot volcanism the formation of continental crust collisional orogenies continental rifting subduction zone processes arcs accretionary prisms and ore deposits each of these presents opportunities for investigation by continental scientific drilling in addition these cycles affect other processes that are targets of continental scientific drilling the origin and evolution of life and an oxygenated atmosphere the impact of large volcanic eruptions on climate and geological hazards such as earthquakes and volcanic eruptions in this paper we present the scientific rationale for continental scientific drilling to study global geodynamic processes review past successes in this realm that were sponsored in part by icdp and suggest potential new targets for drilling campaigns that focus on solid earth evolution this paper builds on discussions at the 2013 icdp science meeting on the future of continental scientific drilling held in potsdam in november 2013

deformation related volcanism in the pacific ocean linked to the hawaiian emperor bend ocean islands seamounts and volcanic ridges are thought to form above mantle plumes yet this mechanism cannot explain many volcanic features on the pacific ocean floor and some might instead be caused by cracks in the oceanic crust linked to the reorganization of plate motions a distinctive bend in the hawaiian emperor volcanic chain has been linked to changes in the direction of motion of the pacific plate movement of the hawaiian plume or a combination of both however these links are uncertain because there is no independent record that precisely dates tectonic events that affected the pacific plate here we analyse the geochemical characteristics of lava samples collected from the musicians ridges lines of volcanic seamounts formed close to the hawaiian emperor bend we find that the geochemical signature of these lavas is unlike typical ocean island basalts and instead resembles mid ocean ridge basalts we infer that the seamounts are unrelated to mantle plume activity and instead formed in an extensional setting due to deformation of the pacific plate 40ar 39ar dating reveals that the musicians ridges formed during two time windows that bracket the time of formation of the hawaiian emperor bend 53 52 and 48 47 million years ago we conclude that the hawaiian emperor bend was formed by plate mantle reorganization potentially triggered by a series of subduction events at the pacific plate margins

pacific plate apparent polar wander hot spot fixity and true polar wander during the formation of the hawaiian island and seamount chain from an analysis of the skewness of magnetic anomaly 20r 44 ma

cenozoic evolution of the tan lu fault zone east china constraints from seismic data abstract the tan lu fault zone tlfz is a continental scale strike slip fault zone in the east china that contains rich and significant information about the mesozoic cenozoic evolution of eastern asia it experienced a long term and complex superimposed deformation characterized by mesozoic sinistral and cenozoic dextral motions the fault has a well documented mesozoic sinistral displacement history but its cenozoic history is poorly known in this study based on the observations of the interrelationships of fault kinematics fault geometry and concomitant sedimentation the cenozoic evolution of the tlfz is constrained by abundant seismic data from oil fields numerous bends along the cenozoic master fault sensitively record changes in the stress field during strike slip motion taking into account the cenozoic evolution of the bohai bay basin east china the evolutionary history of these strike slip bends is investigated and the results indicate that dextral motion along the tlfz began at about 40 ma furthermore the cenozoic evolutionary history of the tlfz is suggested to include an extensional deformation with a possible weak sinistral shear from 65 to 40 ma from 40 to 0 ma dextral motion occurred due to a change in the subduction direction of the pacific plate relative to the eurasian plate causing the fault to experience dextral shear with a component of extensional deformation dextral motion was most intense from 40 to 25 ma and then the dextral direction may have suddenly changed from a nearly ne sw trend to a nearly n s trend at around 25 ma dextral motion was relatively enhanced again after 12 ma and was particularly strong after 5 ma episodic and ephemeral thrusting deformation might have occurred during the quaternary the cumulative cenozoic dextral displacement of the tlfz is far smaller than its mesozoic sinistral strike slip displacement may be no more than 21 km

cenozoic exhumation history of south china a case study from the xuefeng mt range abstract new apatite fission track aft dating was applied to the xuefeng mt range and yuanma basin to constrain the cenozoic exhumation process of the southeastern yangtze block south china the analyzed samples in this study have aft ages ranging from 27 9 2 5 to 61 5 5 9 ma which are younger than the deposition or crystallization ages of the host rocks the aft analysis and thermal history modeling indicate that both the xuefeng mt range and the yuanma basin underwent significant exhumation during the early cenozoic ca 60 40 ma these samples were rapidly exhumed to near the surface during this period our results suggest that an important tectonic event occurred along or near regional fault zones e g the qinhang fault in south china during the early cenozoic ca 60 40 ma however it is difficult to relate this event to the eastern sichuan fold belt which is much older and is characterized by large scale folding and thrusting combined with fieldwork in the yuanma xupu and xinning basins we refute the cretaceous pan yangtze basin that was proposed to have been separated by the uplifted xuefeng mt range after the late cretaceous the exhumation stage from ca 60 ma to 40 ma was an important period during which plate movements across the eastern asian and pacific regions were reorganized the early cenozoic tectonothermal event in south china can be attributed to a change in the direction and speed of the subduction of the pacific plate beneath the eurasian plate an oligocene miocene cooling event was also recorded in the eastern xuefeng mt range which we tentatively attribute to the activity of dextral faults in this area as a far field effect of the collision between the indian and eurasian plates

revision of paleogene plate motions in the pacific and implications for the hawaiian emperor bend reply understanding the relative motion between the pacific plate and its neighboring plates in the paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the pacific ocean basin the history of circum pacific subduction and the cause of the hawaiian emperor bend heb we quantitatively model the farallon vancouver pacific antarctic seafloor spreading history from 67 to 33 ma based on a comprehensive synthesis of magnetic anomaly and fracture identifications we find a well constrained increase from 75 5 mm yr to 101 5 mm yr in pacific farallon full spreading rates between 57 6 ma and 55 9 ma followed by a stepwise increase to 182 2 mm yr from 49 7 to 40 1 ma the increases in pacific farallon spreading rates are not accompanied by any statistically significant change in spreading direction the 57 6 55 9 ma surge of pacific farallon spreading reflects an eastward acceleration in farallon plate motion as it precedes west pacific subduction initiation and is not associated with any significant change in pacific antarctic spreading we interpret the increase in pacific farallon spreading rates after ca 50 ma as a consequence of further acceleration in farallon plate motion we find no indication of a major change in pacific plate absolute motion at this time our model suggests that changes in relative motion direction between the pacific and farallon and pacific and antarctic plates were insignificant around the formation time of the heb ca 47 5 ma and the bend is largely a consequence of hawaiian hotspot motion which ceased rapid motion after 47 ma

plate tectonics radiogenic isotopic tracers and paleoceanography the case of the manganese crusts in the pacific abstract hydrogenous ferromanganese crust deposits on seafloor and volcanic seamounts are archives of past isotopic compositions of the ocean isotopic compositions of long life radiogenic isotopes that are dissolved in the ocean such as nd and pb are mixtures of different erosion products continents volcanics arcs ridges etc transported by oceanic currents or winds isotopic studies on mn crusts are thus potential paleogeographic and paleoceanographic tracers however global paleogeography can drastically evolve over the period of growth of crust as well as the latitude longitude and water depth of the mn crust thus each crust sampled a succession of geographic sites related to its spatial evolution through time for this reason valid information about the past ocean isotopic composition can only be extracted from mn crust under the condition that the successive paleogeographic and paleodepth positions can be reconstructed for each mn crust we followed this approach for six hydrogenous mn crusts from the pacific ocean that have recorded pb and nd isotopic signals for the past 80 myr the evolution of the isotopic data was considered within a proper geodynamical framework that takes into account the paleopositions of the pacific plate and surrounding continents of the hot spots and ridges as well as the paths of the six crusts the differences and similarities between the pb and nd revealed by our analysis emphasize their complementarity as paleoceanographic tracers the nd isotopes are strongly influenced by the exchange between indian and pacific waters specifically by the progressive closure of the indonesian gateway the closed central pacific characteristics are defined from the extrapolation of the isotopic trend this highlights the important role played by erosion of volcanic arcs in the nd isotopic value of seawater our approach also stresses the influence of local volcanic activity ridges or active hot spot for lead isotopes

global eocene tectonic unrest possible causes and effects around the north american plate abstract many of our planet s crises were the result of sudden changes in plate tectonic configuration or catastrophic outbursts of volcanism caused by mantle plume impingement at the base of the lithosphere at the paleocene eocene boundary and in the early eocene several mantle plumes continental collision and mid ocean ridge subduction triggered a series of changes in seafloor spreading dynamics we have constructed a detailed global model of oceanic lithosphere age and spreading rates for the 60 to 35 ma interval we revise evidence for changes in seafloor spreading direction in the north atlantic arctic and ne pacific oceans at least two periods of spreading rate highs which are separated by sharp value decrease occurred along the entire eastern north american plate boundary from c25 to c18 time c 57 to 40 ma the collision and incipient subduction of the early eocene siletzia oceanic lip may have caused the sharp decrease in spreading rate at c23 time in the labrador sea and north of charlie gibbs fracture zone the post c23 rapid farallon slab break off and subsequent upper mantle flow upwelling may have led to further variations in north atlantic spreading rates at c22 21 time eastward pacific subduction may have resumed at c 43 ma as indicated by a steady ne pacific seafloor spreading regime which resumed at or shortly after c21 the north atlantic realm shows a delayed response to tectonic events west of north america with an increase in spreading rate south of charlie gibbs fracture zone from c20 to c18 time followed by a steady decrease until the oligocene north american late paleocene early eocene kimberlite magma that erupted more than 1000 km from its western plate boundary constitutes additional evidence that tectonic stresses due to changes in the mantle lithosphere interactions may have affected the entire plate and therefore also its eastern boundaries

the tectonic fabric of the ocean basins 1 we present a global community data set of fracture zones fzs discordant zones propagating ridges v shaped structures and extinct ridges digitized from vertical gravity gradient vgg maps we use a new semi automatic fz tracking program to test the precision of our hand digitized traces and find a mean absolute deviation of less than 3 4 km from the raw vgg minima that most clearly delineate each feature and less than 5 4 km from the fz location predicted by fitting model profiles to the vgg data that represent the morphology of the individual fzs these offsets are small considering gravity data only provide an approximation for the underlying basement morphology we further investigate the origin of non fz seafloor fabric by combining published abyssal hill heights computed from gravity anomalies with global half spreading rates a residual abyssal hill height grid with spreading rate effects removed combined with our interpreted tectonic fabric reveals several types of seafloor fabric distinct from typical abyssal hills where discordant zones do not overprint abyssal hill signals residual abyssal hill height anomalies correspond to seafloor that accreted near mantle thermal anomalies or zones of melt depletion our analysis reveals several areas where residual abyssal hill height anomalies reflect pseudo faults and extinct ridges associated with ridge propagation and or microplate formation in the southern pacific ocean

cretaceous cenozoic tectonic history of the jiaojia fault and gold mineralization in the jiaodong peninsula china constraints from zircon u pb illite k ar and apatite fission track thermochronometry the jiaojia fault jjf in the jiaodong area of eastern china is an important nne trending structure that is subsidiary to the regional tancheng lujiang tan lu fault zone and hosts 1200 t of gold reserves contained in disseminated and stockwork ore dominantly in the footwall of the fault we present new zircon u pb apatite fission track and illite k ar data along the jjf and have delineated its tectonic history focusing on its formation and reactivation zircon u pb dating shows that the shangzhuang granite is a composite body with ages between 132 1 and 127 1 ma illite k ar ages for the fault s gouge range from 83 2 to 68 2 ma and the measured apatite fission track ages for ores are between 55 and 21 ma previous zircon u pb geochronology and structural studies suggest that the jjf was originally activated in the jurassic during 160 150 ma as a sinistral fault the jjf was a normal fault in the early cretaceous due to nw se orientated tension and ne sw compression which lasted from 135 to 120 ma this was followed by sinistral strike slip faulting due to nw se compression and ne sw tension during 120 110 ma and it changed to normal displacement at ca 110 ma our apatite fission track data analysis and thermal modeling of representative samples suggest that there was a subsequent dextral reactivation of the fault at ca 55 ma previous age data of ca 130 110 ma for gold mineralization along the jjf coincides with the early cretaceous magmatism and is coeval with the transition from normal faulting to sinistral strike slip faulting of the jjf in early cretaceous which is interpreted to be due to changing direction of the subducting pacific plate

high resolution 40ar 39ar geochronology of the louisville seamounts iodp expedition 330 drill sites implications for the duration of hot spot related volcanism and age progressions

regional sandstone type uranium mineralization rooted in oligo miocene tectonic inversion in the songliao basin ne china abstract the tectonic inversion of the songliao basin during the oligo miocene may have played an important role in controlling the development of sandstone type uranium deposits suds here we investigate drill holes along a southeast to northwest section in this basin based on apatite fission track aft and zircon fission track zft techniques we present 50 data from 15 deep boreholes at different depths between 665 and 3956 m and different structural units including grabens and horsts formed in the early cretaceous beneath the basin the results of the effective aft ages are 100 11 to 2 3 0 4 ma p x2 5 and zft ages are 97 5 20 4 ma including binomial peak ages these results reveal that the basin underwent two distinct stages of rapid cooling after late cretaceous in the first stage during the late cretaceous early paleogene 80 50 ma tectonic uplift occurred in all of the structural units including grabens and horsts which was marked by an unconformity between the latest cretaceous mingshui and the eocene yi an formations in the second stage during the oligo miocene 40 10 ma tectonic uplift occurred mainly in the grabens but not in the horsts corresponding with a few sediments of the neogene da an and taikang formations we propose that the folds and the thrust faults mostly characterize in the second stage indicating a major tectonic inversion in the basin the shifting of the two stages was probably in response to differences in the subduction angles and directions of motion of the paleo pacific plate from the southeast combined with previous information it was demonstrated that most of the u mineralization ages are younger than 40 ma with a peak in the miocene or later

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

eocene 46 44 ma onset of australia pacific plate motion in the southwest pacific inferred from stratigraphy in new caledonia and new zealand

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

abyssal hill deflections at pacific antarctic ridge transform intersections nearly complete coverage of shipboard multibeam bathymetry data at the right stepping menard and pitman fracture zones allowed us to map abyssal hill deviations along their traces in this study we distinguish between 1 j shaped curvatures at their origin where modeling is addressing primary volcanism and faulting following a curved zone and 2 straight abyssal hills getting bent in anti j shaped curvatures in response to increased coupling across the transform fault after they were formed we compared the mapped abyssal hill deflections to a detailed plate motion model for the pacific antarctic ridge to test how abyssal hill curvature correlates to changes in plate motion direction which lead to periods of transtension or transpression this test was based on the number and size of the abyssal hill deflections the observations show a high abundance of j shaped abyssal hills during periods of significant clockwise change in plate motion direction which leads to transtension the tip of the ridge axis can deflect up to 60 into the transform fault in response to changes in the stress field at ridge transform intersections this is observed in particular at the pitman fracture zone where there has been a 15 clockwise rotation of the spreading direction azimuth during the last 9 myr in addition we observed anti j shaped curvatures at menard pitman and heirtzler fracture zones during periods of transpression when increased coupling across an oceanic transform fault is partially accommodated by distributed strike slip deformation rather than solely by discontinuous displacement at the transform fault anti j shaped deflections typically develop in seafloor less than 2 myr old when the oceanic lithosphere is thin

chert in the pacific biogenic silica and hydrothermal circulation abstract data on sections drilled by dsdp and odp in the pacific have been compiled for 187 sites 105 of which contain chert the spatial pattern of siliceous deposits in the pacific generally follows the pattern of biologic productivity there is some evidence that enhanced vertical diffusivity in the ocean during the eocene warm climate gave rise to siliceous deposits on and near bathymetric highs and basin margins the most common occurrence of chert is in eocene and older sections however cherts do occur in sections as young as 7 ma in the sites investigated over half of the youngest chert occurrences in sedimentary sections are within 150 m of basement nearly all the chert occurrences on crust younger than 60 ma are within 150 m of basement the deficit in heat flow over ocean crust younger than 60 ma is explained as the result of heat advection by hydrothermal waters circulating in the upper part of the ocean crust these waters appear to dissolve biogenic silica and deposit it as chert in the lower parts of sedimentary sections diffusion of these waters into the lower sediment section is enhanced by small scale faulting this faulting may be associated with intra plate stresses that develop during ridge jumps and reorientations of plate motion the relatively warm bottom waters of the early cenozoic may have also enhanced the dissolution of biogenic silica during hydrothermal circulation in the upper oceanic crust and basal sediment section

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

depleted mantle wedge and sediment fingerprint in unusual basalts from the manihiki plateau central pacific ocean comment and reply comment ingle et al 2007 1 reported 40ar 39ar ages for manihiki plateau rocks and suggested some of them were contemporaneous 120 ma with lavas from the ontong java and hikurangi plateaus each some 2500 km distant ingle et al 2007 1 their fig 1 critical examination of the radiometric

widespread compression associated with eocene tonga kermadec subduction initiation eocene onset of subduction in the western pacific was accompanied by a global reorganization of tectonic plates and a change in pacific plate motion relative to hotspots during the period 52 43 ma we present seismic reflection and rock sample data from the tasman sea that demonstrate that there was a period of widespread eocene continental and oceanic compressional plate failure after 53 48 ma that lasted until at least 37 34 ma we call this the tectonic event of the cenozoic in the tasman area tecta its compressional nature is different from coeval tensile stresses and back arc opening after 50 ma in the izu bonin mariana region our observations imply that spatial and temporal patterns of stress evolution during western pacific eocene subduction initiation were more varied than previously recognized the evolving eocene geometry of plates and boundaries played an important role in determining regional differences in stress state

mesozoic multiphase magmatism at the xinan cu mo ore deposit zijinshan orefield geodynamic setting and metallogenic implications abstract the xinan cu mo deposit newly discovered in the zijinshan au cu mo orefield the largest porphyry epithermal system in se china is featured by the presence of abundant multi phase granitoids which reflects the complex mesozoic tectono magmatic evolution in the region new and published la icp ms zircon u pb age data reveal that the mesozoic zijinshan magmatism occurred in two major phases 1 middle to late jurassic ca 169 150 ma forming the zijinshan complex granite and the xinan monzogranite 2 late early cretaceous to earliest late cretaceous ca 112 98 ma forming the shimaoshan volcanic rocks sifang granodiorite and the xinan fine grained granodiorite porphyry porphyritic granodiorite and late aplite dykes additionally a possible earliest cretaceous magmatism ca 141 ma may have occurred based on inherited zircon evidence major and trace element geochemistry indicates that all the zijinshan igneous rocks show subduction related geochemical affinities zircon ce 4 ce 3 values of the late early cretaceous to earliest late cretaceous granitoids ce 4 ce 3 190 1706 are distinctly higher than the middle to late jurassic ones ce 4 ce 3 27 457 suggesting that the former were derived from more oxidized parental magma the middle to late jurassic zijinshan complex granite and monzogranite have e hf t values of 8 02 to 10 00 with the two stage hf model ages t dm2 of 1 72 to 1 84 ga similar to the paleoproterozoic metamorphosed cathaysia block basement suggesting that they were derived from partial melting of the basement the late early cretaceous to earliest late cretaceous sifang granodiorite and xinan fine grained granodiorite porphyry porphyritic granodiorite and aplite dykes contain higher and wider range of e hf t values 0 66 to 6 05 with t dm2 of 1 12 to 1 56 ga indicating that they were also partial melting product of the cathaysia basement but with more mantle and or juvenile mafic lower crustal input we propose that the zijinshan orefield was in a compressive pacific subduction related tectonic setting during the middle to late jurassic the regional tectonic regime may have changed to extensional in the late early cretaceous to earliest late cretaceous during which the pacific plate subduction direction change and the accompanying subduction roll back and slab window opening occurred the tectonic regime transition high oxygen fugacity and mantle mafic lower crustal materials involvement in the late early cretaceous to earliest late cretaceous may have generated the zijinshan porphyry related au cu mo mineralization

40ar 39ar geochronology and the paleoposition of christmas island australia northeast indian ocean abstract the christmas island seamount province is an extensive zone of volcanism in the northeast indian ocean consisting of numerous submerged seamounts and flat topped guyots within this region lies two subaerial island groups christmas island and the cocos keeling archipelago christmas island has experienced multiple episodes of volcanism that are exposed sporadically along its coastline here we dated these volcanics using 40ar 39ar geochronology and analysed them for paleomagnetism the oldest exposed volcanism occurred in the eocene between 43 and 37 ma this is followed by a time gap of 33 million years before the eruption of a younger episode of pliocene age 4 32 0 17 ma it has however been suggested by previous workers that there is a much older late cretaceous event beneath the limestone which is unexposed in addition this study conducted the first paleomagnetic analysis of samples from christmas island to determine its paleoposition and the paleomagnetic polarity of the sampled sites two normal and two reversal magnetic events have been recorded that agree with the geomagnetic reversal timescales late eocene 38 39 ma palaeomagnetic data suggest a palaeolatitude of 43 5 11 2 9 0 which is further south than palaeolatitudes around 30 s derived from existing plate reconstruction models for the australian plate however the late eocene palaeomagnetic data are limited only two sites and secular variation may not have been averaged out during the pliocene ca 4 ma we estimate a palaeolatitude of approximately 13 s the presence of the late eocene ages at christmas island correlates well with the cessation of spreading of the wharton ridge 43 ma the initiation of spreading along the south east indian ridge and the transit of christmas island over a broad low velocity zone in the upper mantle this suggests that changes in stress regimes following the tectonic reorganisation of the region prior to 43 ma may have allowed deeper origin mantle melts to rise similarly changes in the plate s stress regime at the flexural bulge of the sunda java subduction zone may be implicated in renewed melting at 4 ma suggesting that tectonic stresses have exerted a first order effect on the timing and emplacement of volcanism at christmas island

late paleogene sinistral strike slip system along east qinling and in southern north china implications for interaction between collision related block trans rotation and subduction related back arc extension in east china abstract we present in this paper a compilation of paleogene rift and strike slip structures in east qinling and southern north china based on a synthesis of published stratigraphic structural and thermochronological data which aids in delineating a broad 500 km wide kinematically interlinked eocene oligocene transtensional system this transtensional system was bifurcated from two discrete strike slip fault zones in ne tibetan plateau the overall fault patterns indicate a clockwise rotation of south china relative to north china during the cenozoic possibly driven by dextral shearing along the eastern tibetan margin this collision related trans rotation of south china occurred coevally with late paleogene intracontinental rifting and volcanism in east china in a subduction related back arc extensional setting these two tectonic processes interacted with each other giving birth to cenozoic rift systems and contrasting morpho structural features in east asia

initiation and origin of dextral deformation at mid eocene in the western bohai bay basin east china abstract the cenozoic back arc rift basins along the continental margin of east asia are featured by both extensional and dextral deformation systems here we investigate the timing and interaction of these two types of systems based on seismic data stratigraphic isopach maps and drilling data in the jizhong depression in the northwestern bohai bay basin bbb east china the results show the rifting was initiated as early as the eocene while the dextral deformation did not occur until ca 42 ma the jizhong depression changed from a simple rift basin with boundary faults to the west to a geometrically more complex dextral transtensional basin at ca 42 ma this event was roughly concordant with the initial dextral slip on the tan lu fault lying at the eastern bbb and together they transformed the bbb into a complex pull apart rift basin the tectonic reorganization of the bbb was probably the response to the change from an orthogonal to an oblique subduction of the pacific plate beneath east asia at that time

mantle flow reversals in cylindrical earth models abstract we employ a two dimensional model of mantle convection in a cylindrical shell to study the influence of curvature on the phenomenon of spontaneous flow reversal which has been found previously only in plane layer models in cartesian geometry our model includes rigidly moving plates with weak zones at each end and uniformly distributed internal heat sources surface plates in this model are passive in that their lateral velocities reflect the overall buoyancy in the underlying mantle and at each time step match the average surface velocity that would occur in the absence of plates our principal finding is that flow reversals similar to those in plane layers are also found in our cylindrical shell models thereby attesting to the robustness of this feature we conduct systematic investigations of the impact on the flow reversal behavior of varying degree of curvature plate thickness internal heating rate and aspect ratio of the convection cells flow reversals are driven by a build up of internally heated material adjacent to a major mantle downwelling when thermal instabilities develop in the upper boundary layer they develop into intermediate sinking plumes which disrupt the build up of hot material near sinking plumes accordingly parameter values which tend to stabilize the upper thermal boundary layer low degree of curvature high plate thickness small aspect ratio and intermediate internal heating rate favor regular flow reversals

models of hawaiian volcano growth and plume structure implications of results from the hawaii scientific drilling project the shapes of typical hawaiian volcanoes are simply parameterized and a relationship is derived for the dependence of lava accumulation rates on volcano volume and volumetric growth rate the dependence of lava accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the hawaiian plume with the eruption rate determined from a specified radial dependence of magma generation in the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit the timescale of volcano growth is t 2 r plate where r is the radius of the melting zone of the circular plume and plate is the velocity of the pacific plate the growth progress of a volcano can be described by a dimensionless time t t plate 2r where t 0 is chosen to be the start of volcano growth and t 1 approximates the end of shield growth using a melt generation rate for the whole plume of 0 2 km 3 yr a plume diameter of 50 km and a plate velocity of 10 cm yr we calculate that the lifetime of a typical volcano is 1000 kyr for a volcano that traverses the axis of the plume the standard dimensions are a volume of 57 000 km 3 a summit thickness of 18 km a summit elevation of 3 6 km and a basal radius of 60 km the volcano first breaches the sea surface at t 0 22 when it has attained only 5 of its eventual volume 80 of the volume accumulates between t 0 3 and t 0 7 typical lava accumulation rates start out over 50 m kyr in the earliest stages of growth from the seafloor and level out at 35 m kyr from t 0 05 until t 0 4 from t 0 4 to t 0 9 the submarine lava accumulation rates decrease almost linearly from 35 m kyr to 0 subaerial accumulation rates are about 30 lower the lava accumulation rate is a good indicator of volcano age a volcano that passes over the plume at a distance 0 4r off to the side of the plume axis is predicted to have a volume of about 60 of the standard volcano a lifetime about 8 shorter and lava accumulation rates about 15 20 smaller the depth age data for mauna kea lavas cored by the hawaii scientific drilling project are a good fit to the model parameters used given that mauna kea appears to have crossed the plume about 15 20 km off axis the lifetime of mauna kea is estimated to be 920 kyr mauna loa is predicted to be at a stage corresponding to t 0 8 kilauea is at t 0 6 and loihi is at t 0 16 the model also allows the subsurface structure of the volcanoes the interfaces between lavas from different volcanoes to be modeled radial geochemical structure in the plume may be blurred in the lavas because the volcanoes capture magma from a sizeable cross sectional area of the plume this inference is qualitatively born out by available isotopic data the model predicts that new hawaiian volcanoes are typically initiated on the seafloor near the base of the next older volcano but generally off the older volcano s flank

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

asynchronous bends in pacific seamount trails a case for extensional volcanism the gilbert ridge and tokelau seamounts are the only seamount trails in the pacific ocean with a sharp 60 bend similar to the hawaii emperor bend heb these two bends should be coeval with the 47 million year old heb if they were formed by stationary hot spots and assuming pacific plate motion only new 40ar 39ar ages indicate that the bends in the gilbert ridge and tokelau seamount trail were formed much earlier than the heb at 67 and 57 million years ago respectively such asynchronous bends cannot be reconciled with the stationary hot spot paradigm possibly suggesting hot spot motion or magmatism caused by short term local lithospheric extension

paleocene pacific plate reorganization mirrored in formation of the suvarov trough manihiki plateau the suvarov trough is a graben structure that deviates from the danger islands troughs within the manihiki plateau a large igneous province lip located in the central pacific new high resolution seismic reflection data provide evidence that the graben formed in two phases during the paleocene 65 45 ma in a first phase extension occurred in south westward direction pulling apart the northern part of the suvarov trough and a parallel trending unnamed trough in a second phase a change of extensional force direction occurred from southwest to west northwest forming the southern part of the suvarov trough that extends onto the high plateau the formation of the suvarov trough is accompanied by a series of normal fault systems that apparently formed simultaneously comparing the seismic results to existing pacific paleo strain reconstructions the timing of increased strain and local deformation direction fits well to our findings we thus suggest that the multiple strike directions of the suvarov trough represent an extensional structure that was caused by the major stepwise pacific plate reorganization during the paleocene

episodic formation of neotethyan ophiolites tibetan plateau snapshots of abrupt global plate reorganizations during major episodes of supercontinent breakup abstract neotethyan ophiolites along the bangong nujiang and yarlung tsangpo sutures tibetan plateau consist of mantle peridotite cumulates isotropic gabbro diabase plagiogranite and basaltic lava they comprise morb iab oib and boninitic rocks and mostly belong to supra subduction zone ssz type compositionally different rocks crystallized synchronously over a distance of 2000 km along the bangong nujiang and yarlung tsangpo sutures during the short periods of 190 182 and 132 120 ma respectively resembling the izu bonin mariana ibm proto arc the early jurassic bangong nujiang ophiolites formed in forearc settings during subduction initiation while the middle jurassic early cretaceous bangong nujiang ophiolites and the early cretaceous yarlung tsangpo ophiolites formed in forearc settings at pre existing subduction zones crystallization of the neotethyan ssz ophiolites from the eastern mediterranean middle east tibetan plateau to southeast asia and formation of the associated metamorphic rocks temporally coincided with major supercontinent breakup events 1 initial breakup of pangea in central atlantic and westernmost tethys 190 ma 2 separation of west and east gondwana 170 160 ma and initial opening of piemont ligurian tethys 175 164 ma 3 initial breakups of east and west gondwana 135 125 ma and 4 final breakup of gondwana 104 96 ma far field forces associated with abrupt global plate reorganizations may have triggered subduction initiation or transient interplate coupling decoupling at pre existing subduction zones leading to episodic formation of the neotethyan ssz ophiolites and rapid exhumation of metamorphic rocks in subduction zones the formation events of neotethyan ssz ophiolites and associated metamorphic rocks represent snapshots of global plate reorganizations during the major episodes of supercontinent breakup

structural architecture and tectonic evolution of the cenozoic zhanhua sag along the tan lu fault zone in the eastern north china reconciliation of tectonic models on the origin of the bohai bay basin abstract the zhanhua sag a major depocenter within the larger bohai bay basin bbb along the tan lu fault zone tlfz in the eastern north china developed through two successive phases with different stress regimes during the paleogene the kinematics and the slip history of the nw se and n s striking normal fault systems strongly controlled the internal structure and stratigraphy of the paleocene lower eocene strata in the zhanhua sag during phase 1 isopach maps of the paleocene lower eocene stratigraphy show 4000 m thick clastic rock sequences in these structurally controlled depocenters the average dip slip rates of the nw se and n s striking faults were 78 96 and 50 82 m m y respectively in the paleocene early eocene but decreased to 45 87 and 35 81 m m y in the middle late oligocene the ne sw striking fault systems and their slip history controlled the development of the middle eocene oligocene depocenters and their strata during phase 2 the average dip slip rate of the ne sw striking faults was 10 47 m m y in the paleocene early eocene but increased to 93 49 m m y in the middle late oligocene the ne sw oriented faults cut across and displace the nw se striking faults indicating their relatively younger ages these fault patterns and kinematics recorded in the zhanhua sag reflect an abrupt change in extension directions from ene wsw to nw se before and after the middle eocene that was related to a reversal in the slip direction from sinistral to dextral along the tlfz this slip reversal was in turn a result of a major change in the subduction direction of the pacific plate beneath eastern asia and in the mode of strain partitioning across the tlfz in the continental upper plate

a revised kinematic model for the relative motion between pacific oceanic plates and north america since the late cretaceous 1 the convergence between the pacific farallon and kula oceanic plates and the north american plate has provided a driving force for tectonic processes that have shaped the western margin of north america yet reconstructions of plate motion in the pacific basin have traditionally relied on the assumption of fixed hot spots in the last decade substantial cretaceous to paleogene motion between the pacific and atlantic hot spots has been recognized providing motivation to reevaluate the history of relative motion between pacific oceanic plates and north america by means of plate circuit reconstructions this paper presents new kinematic models based on two alternative plate circuits linking the pacific plate to north america since late cretaceous time when compared to models relying on fixity of pacific and indo atlantic hot spots our reconstructions suggest that the pacific kula and pacific farallon ridges were 600 1000 km closer to the western margin of north america and hence that the farallon and kula plates were smaller in late cretaceous to middle eocene time these findings cast significant doubt on the viability of a configuration of the farallon kula ridge that juxtaposes the kula plate with the southern part of the paleo north american margin during this time interval the results of our new reconstructions also suggest more oblique convergence between the farallon or kula plate and north america from the late cretaceous to the middle eocene related to the combination of dominantly northwest motion of the pacific plate and steady southwest motion of north america this provides an efficient mechanism for the coast parallel translation of some accreted terranes of north america it may also explain the structure of geological units in western north america such as the central belt of the franciscan complex which show a dominant coast parallel fabric formed during an interval coeval with the episode of oblique convergence

high precision dating and correlation of ice marine and terrestrial sequences spanning heinrich event 3 testing mechanisms of interhemispheric change using new zealand ancient kauri agathis australis robustly testing hypotheses of geographic synchroneity of abrupt and extreme change during the late pleistocene 60 000 to 11 650 years ago requires a level of chronological precision often lacking in ice marine and terrestrial sequences here we report a bidecadally resolved new zealand kauri agathis australis tree ring sequence spanning two millennia that preserves a record of atmospheric radiocarbon 14c during ice rafted debris event heinrich event 3 he3 in the north atlantic and antarctic isotope maximum 4 aim4 in the southern hemisphere using 14c in the marine cariaco basin and 10be preserved in greenland ice the kauri 14c sequence allows us to precisely align sequences across this period we observe no significant difference between atmospheric and marine 14c records during he3 suggesting no stratification of surface waters and collapse in atlantic meridional overturning circulation amoc instead our results support recent evidence for a weakened amoc across at least two millennia of the glacial period our work adds to a growing body of literature confirming that heinrich events are not the cause of stadial cooling and suggests changes in the amoc were not the primary driver of antiphase temperature trends between the hemispheres decadally resolved 14c in ancient kauri offers a powerful new and complementary approach to polar ice core ch4 alignment for testing hypotheses of abrupt and extreme climate change

improved late cretaceous and early cenozoic paleomagnetic apparent polar wander path for the pacific plate understanding of pacific plate tectonics and geodynamics is aided by refinement of the plate s apparent polar wander path apwp we improved the late cretaceous and early cenozoic apwp by analyzing a large diverse paleomagnetic data set that combines core sample seamount magnetic anomaly model and marine magnetic anomaly skewness data our preferred apwp has five mean paleomagnetic poles representing the oligocene 30 ma late 39 ma and early 49 ma eocene and paleocene 61 ma epochs and the maastrichtian 68 ma stage along with a published 80 ma pole the apwp shows a stillstand from 80 to 49 ma punctuating the large overall northward drift of the plate the two youngest poles imply resumption of northward motion during mid eocene time with another change of polar motion after 30 ma if unaffected by other phenomena e g true polar wander or change in time averaged magnetic field geometry the stillstand implies negligible northward plate motion during the period of emperor seamounts formation contrary to most accepted plate motion models the stillstand is consistent with paleomagnetic data from the emperor seamounts which imply southward motion of the hawaiian melting anomaly it also implies significant westward drift of the hotspot if the pacific plate was moving west at rates similar to the later cenozoic in addition changes in polar wander after 49 ma are consistent with changes of north pacific plate boundaries 2007 elsevier b v all rights reserved

stress field interactions between overlapping shield volcanoes borehole breakout evidence from the island of hawai i usa acknowledgments this pta2 borehole investigation was funded by the international continental scientific drilling program icdp and by vmapp volcanic margin petroleum prospectivity project vbpr dougalearth tgs in collaboration with the humu ula groundwater research project d a j and s p are partly funded through a norwegian research council centres of excellence project project number 223272 ceed we thank marco groh for the logging operations we thank two anonymous reviewers for the comments and suggestions we are particularly grateful to the associate editor mike poland for his valuable comments and his critical review that greatly improved the manuscript

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

tests of fixity of the indo atlantic hot spots relative to pacific hot spots rates of inter hot spot motion have been debated for decades herein we present updated predictions for the tracks of the tristan da cunha reunion and iceland hot spots assuming them to be fixed relative to pacific hot spots uncertainties in pacific hot spot rotations which include uncertainties in the current locations of hot spots of 100 200 km are combined with uncertainties in relative plate motions accumulated through the plate circuit to obtain the final uncertainty in the predicted positions including uncertainties of 150 200 km in the current locations of the indo atlantic hot spots improvements to reconstruction methods to relative plate reconstructions to age dates along the tracks and to the geomagnetic reversal timescale lead to significant changes from prior results when compared with the observed tracks the predicted tracks indicate nominal rates of motion of only 2 6 mm a 1 of these indo atlantic hot spots relative to pacific hot spots over the past 48 ma within the uncertainties the rates range from no motion to rates as high as 8 13 mm a 1 for reconstructions prior to 48 ma b p however the apparent rates of inter hot spot motion are much larger 46 55 20 mm a 1 if the motion occurred entirely between 68 ma b p and 48 ma b p either hot spots moved rapidly before 48 ma b p and slowed drastically at 48 ma b p or global plate circuits through antarctica become less reliable as one goes increasingly further into the past most paleomagnetic data favor the latter explanation

constraints on past plate and mantle motion from new ages for the hawaiian emperor seamount chain 1 estimates of the relative motion between the hawaiian and louisville hot spots have consequences for understanding the role and character of deep pacific mantle return flow the relative motion between these primary hot spots can be inferred by comparing the age records for their seamount trails we report 40ar 39ar ages for 18 lavas from 10 seamounts along the hawaiian emperor seamount chain hesc showing that volcanism started in the sharp portion of the hawaiian emperor bend heb at 47 5 ma and continued for 5 myr the slope of the along track distance from the currently active hawaiian hot spot plotted versus age is constant 57 2 km myr between 57 and 25 ma in the central 1900 km of the seamount chain including the heb this model predicts an age for the oldest emperor seamounts that matches published ages implying that a linear age distance relationship might extend back to at least 82 ma in contrast hawaiian age progression was much faster since at least 15 ma and possibly as early as 27 ma linear age distance relations for the hawaii emperor and louisville seamount chains predict 300 km overall hot spot relative motion between 80 and 47 5 ma in broad agreement with numerical models of plumes in a convecting mantle and paleomagnetic data we show that a change in hot spot relative motion may also have occurred between 55 ma and 50 ma we interpret this change in hot spot motion as evidence that the heb reflects a combination of hot spot and plate motion changes driven by the same plate mantle reorganization

re evaluating the geochronology of the permian tarim magmatic province implications for temporal evolution of magmatism the permian tarim magmatic province has 118 published ages ranging from 358 to 205 ma but the timing of mafic magmatism is not well constrained we report two new secondary ion mass spectrometry u pb zircon dates on the halahatang trachydacite and wajilitag olivine clinopyroxenite which are 287 2 2 0 ma and 283 2 2 0 ma respectively the trachydacite overlies the uppermost basalt and constrains the latest eruption age of basalt in northern tarim the latter is the first high resolution date for the wajilitag mafic layered intrusion by screening all published ages we identified 22 robust ages ranging from 290 9 4 1 to 261 7 1 8 ma the robust ages together with our new data reveal a protracted period of mafic magmatism at c 283 and c 267 ma silicic magmatism occurred from 291 to 272 ma although the current known volume of tarim basalt is too small to qualify as a large igneous province the eroded and intrusive components as well as pyroclastic deposits and silicic lavas may increase the estimated volume further work is required to refine the duration of magmatism and the volume estimate of the province supplementary material a list of all the published dates raw data of published 40 ar 39 ar dates raw data of published u pb dates published geochemistry data used for calculation and complete details of the data evaluation are available at http www geolsoc org uk sup18862

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

contribution of global potential field data to the tectonic reconstruction of the rio grande rise in the south atlantic abstract the application of advanced enhancement techniques for geophysical anomalies to global gravity wgm2012 and magnetic emag2 models sheds light on the complex tectonic evolution of the rio grande rise rgr in the southern south atlantic long wavelength bouguer gravity lows indicate a thicker crust beneath of the ridge whose nature can be related to a microcontinent or an excess of volcanism within the oceanic realm recently dredged continental rocks reinforce the hypothesis of a microcontinent or at least slivers of continental crust however the reserval magnetic pattern of the processed magnetic anomalies provide no evidence of aborted spreading center similar to the well studied jan mayen microcontinent and the surrounding inactive aegir and active kolbeinsey ridges in the north atlantic ocean the reversal magnetic anomalies show a series n s trending parallel stripes roughly follow the current south american coastline and segmented by e w oriented oceanic fracture zones fzs the magnetic stripes are bended eastwards at the rgr showing a more complex magnetic pattern similar to that in the iceland the aborted cruzeiro do sul rift csr and the jean charcot chain jcc are structures that cross the rgr and contribute to the understanding of the tectonic evolution of the south atlantic ocean nw se oriented extensive gravity lows and bathymetric valleys which mark the csr are segmented by e w trending magnetic lineaments related to fzs this structural configuration suggests that the extensional event which formed the rift and the seamounts chain was replaced by strike slip movements along the fzs in addition we constructed a plate kinematic model for the evolution of the rgr based on bathymetric free air and bouguer gravity and magnetic data our model comprises five main stages of the rgr formation and evolution between late cretaceous and paleocene ca 95 60 ma separated by published seafloor isochrones the proposed model suggests that the rgr was built at the mid atlantic ridge by increased magmatism probably related to the tristan da cunha hotspot

plate motion changes the hawaiian emperor bend and the apparent success and failure of geodynamic models abstract geodynamic models explain present day plate motions in terms of mantle buoyancy forces arising from subducted lithosphere and lithospheric thickening or from velocity anomalies mapped by seismic tomography however such models do not account for sudden plate tectonic reorganizations such as the dramatic change in pacific plate motion implied by the sharp bend in the hawaiian emperor seamount chain about 43 million years ago candidate mechanisms for the hawaiian emperor bend such as subduction initiation ridge subduction or the possibly time coincident collision between india and asia remain weakly formulated and largely untested we test the india asia collision hypothesis using a model for cenozoic plate motions driven mainly by buoyancy forces introduced at paleo subduction zones and we show that pacific plate motion is virtually unaffected by the development of compressional stresses along the india asia margin geologic evidence suggests subduction initiation at about 43 48 ma along a transform boundary on the western pacific plate margin but this mechanism is difficult to test we speculate that transform boundaries may guide plate motions for long periods of time and that rapid plate motion changes may result from the creation or destruction of major transform faults speculations aside our results show that the character of global plate motion changes is not adequately explained by current geodynamic models