roller bearing tectonic evolution of the juan fernandez microplate a combined gloria sidescan and hydrosweep multi narrow beam sonar survey shows striking evidence for fan shaped spreading systems bounding pseudofaults and compression ridges associated with 90 of clockwise rotation of the juan fernandez microplate in the past 4 myr a quantitative analysis of this evolution suggests that the microplate has rotated as a rigid roller bearing for most of its history driven at its edges by shear between the surrounding major plates the remarkable similarity between the juan fernandez and easter microplates despite their different tectonic settings suggests that this is a general model for microplate tectonics

evolution of the juan fernandez microplate during the last three million years during the spring of 1983 the r v thomas washington surveyed an area located north of the antarctic nazca pacific triple junction at 35 s magnetic and seabeam bathymetric data collected during the survey confirmed the existence of the juan fernandez microplate this paper presents an analysis of the magnetic anomaly data the western boundary of the microplate is a fast spreading center which has existed since 2 ma and where the accretion rate has been about 14 5 cm yr for the last 0 7 ma the eastern boundary of the microplate is characterized by a slow spreading center which separates the juan fernandez and nazca plates the accretion rate has been about 7 0 cm yr between 0 7 and 0 4 ma and about 1 6 cm yr for the last 0 4 ma the two spreading centers are connected in the north and south by transform faults between the juan fernandez and rapanui microplates the east pacific rise is well defined between 30 and 32 s in this region the axis displays a record accretion of about 17 2 cm yr south of the juan fernandez microplate one magnetic profile oceano 7008 indicates that the opening rate is about 12 0 cm yr on the antarctic pacific ridge the birth of the microplate is dated at about 2 ma when the western boundary started to accrete the evolution of the microplate corresponds to a transfer of accretion from the eastern boundary to the western axis this is revealed by the net decrease of the opening rate from 7 0 cm yr to 1 6 cm yr observed at the eastern ridge where a small jump occurred at 0 4 ma

oceanic equatorial waves and the 1991 93 el nino abstract equatorial kelvin and rossby waves associated with the 1991 93 el nino warm event were detected in temperature observations made by the tropical atmosphere ocean tao buoy array intraseasonal kelvin waves were a prominent part of equatorial thermocline depth variability and were well represented by a simple model consisting only of first and second mode baroclinic kelvin waves the second mode was essential to properly represent the observed amplitude thermocline depth variability at 5 n and 5 s was dominated by annual and interannual rossby waves which were found to have been largely wind forced in midbasin with little if any signal associated with eastern boundary reflection an evaluation of the wyrtki buildup hypothesis and the delayed oscillator hypothesis in connection with the 1991 93 events showed that a long lag about two years occurred between the arrival of the downwelling signal in the west and the subsequent initiation of el nino this was considerably longer than suggested

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

fast rift propagation along the east pacific rise near easter island during a 1983 seabeam cruise we surveyed the easter nazca spreading center which forms the eastern boundary of the easter microplate we have analyzed all available bathymetry and magnetic anomaly data to determine the present fine scale tectonic configuration and recent tectonic evolution of this spreading center the narrowing of the axial brunhes anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm yr assuming an average spreading rate of 120 mm yr there are indications of a short episode of faster propagation 350 mm yr prior to 0 4 ma these are the fastest rates of rift propagation yet documented the tip of the propagating rift is at about 25 s 112 25 w 180 km south of the previously hypothesized position the recent spreading direction is n85 w a large well defined sheared zone to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the easter pacific spreading center 360 km to the west is being replaced by the fast propagator

recent upward shift of the deep convection system in the japan sea as inferred from the geochemical tracers tritium oxygen and nutrients the thermohaline circulation system in the japan sea an almost land locked marginal sea in the northwest pacific is now in a transition state comparison of vertical profiles of tritium oxygen and nutrients between 1984 and 1998 demonstrates the following two points 1 the supply of dense surface seawater to the bottom layer 2 500 m in depth has almost ceased since 1984 and 2 the intrusion of surface seawater to an intermediate depth range between 700 m and 2 000 m in depth has been reinforced during this 14 year period such transitional upward shift of the thermohaline conveyor belt in the japan sea is thought to have separeted the bottom water from the conveyor belt and brought about a stagnant mode of the bottom water

microstructures and lattice fabrics in the hilti mantle section oman ophiolite evidence for shear localization and melt weakening in the crust mantle transition zone 1 experimental studies have shown that olivine aggregates with 4 melt are significantly weaker than melt free aggregates however questions remain as to the importance of melt weakening in nature in several studies melt weakening has been invoked to explain patterns of mantle flow in the oman ophiolite in this paper we reinvestigate evidence for melt weakening in the hilti mantle section using structural and microstructural methods the average olivine grain size increases with depth below the crust mantle boundary this is related to a change from equigranular to porphyroclastic microstructures at shallow levels to coarse porphyroclastic microstructures at depth a strong foliation and high degree of recrystallization in the upper part of the mantle section are interpreted as the strong imprint of localized deformation at stresses of 4 10 mpa lattice orientation data show that the high strain peridotites have recorded top to the west shear reversed shear senses were found deeper in the section since there is petrographic evidence for melt in the high strain zone strain localization was probably caused by a melt related weakening mechanism high melt contents required for melt weakening suggest that melt accumulated just below the crust mantle boundary we conclude that melt weakening is probably related to enhancement of grain boundary rather than intragranular deformation processes effective viscosities 2 1016 pa s may locally exist in the uppermost mantle beneath ridges if melt weakening occurs although our results agree with those of previous studies in the hilti massif we conclude that both the previously published active flow model and a ridge compression model can account for them

pito and orongo fracture zones the northern and southern boundaries of the easter microplate southeast pacific abstract the easter rapanui microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on earth is occurring today recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets during the january 1987 rapanui expedition on the n o jean charcot we conducted a sea beam magnetics gravity survey of a portion of the microplate boundaries the method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships the northern terminus of the east rift or eastern boundary of the microplate is expressed as a series of parallel nw se trending valleys including what appears to be with 5890 m depth the deepest active rift axis mapped in the pacific today pito rift the northern end of the pito rift merges with an e w to 083 narrow band of linear faults interpreted to be a transform fault between the nazca and easter rapanui plates the northern triple junction between the easter rapanui nazca and pacific plates is a rff type with the two transform faults colinear along an approximately e w direction the southwestern boundary of the easter rapanui microplate is marked by a series of en echelon offsets outlined by depressions which merge into an approximately e w zone where shear must be predominant the southern triple junction is a rrf junction with an overlapping ridge system the structural data acquired during the survey provide strong constraints for kinematic models of the microplate the structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate

systematics of ridge propagation south of 30 s abstract new high resolution geosat altimetry data south of 30 s reveal numerous propagating ridge wakes along intermediate and slow spreading ridges these new examples provide a large enough database to establish systematics of ridge propagation almost all active propagating ridges propagate down a regional along axis gravity or bathymetry gradient the sense of the propagating ridge offset right lateral vs left lateral is related to recent changes in spreading direction we find there is a significant difference between the propagation of ridges with an axial high morphology which propagate at greater than 50 of their full spreading rate and ridges with a median valley morphology which usually propagate at 25 of their spreading rate the axial high propagators leave behind an asymmetric wake the outer pseudofault appears as a continuous linear trough step while the sheared zone appears as a chain of small gravity bumps while we clearly see the propagating ridge wakes from offsets greater than 10 km at slow and intermediate spreading ridges at ridges spreading faster than 75 mm yr the amplitude of the wake topography decreases to the point where we no longer see these wakes in geosat altimetry data the systematics seen in this new data set support a fracture mechanics model for the dynamics of ridge propagation

compressional deformation north of the easter microplate a manned submersible and seafloor gravity investigation we have investigated an e w compressional ridge in 0 9 ma lithosphere of the nazca plate just north of the easter microplate on the east pacific rise using a combination of visual observation and seabed gravity measurements from four manned submersible dives three of the dives made a 26 km long traverse across the ridge during which 14 gravity measurements were made the observations confirm the presence of active thrust faulting on the 1 1 km high south face of the ridge however the gravity precludes the existence of a simple low angle thrust and is only compatible with significant crustal shortening and thickening if the block to the south of the thrust has a lower density than the overriding block visual and sonar observations of pervasive fine scale tectonism in this area lend some support to this idea alternatively there could be oblique thrusting on a high angle fault with relatively little crustal shortening in addition a small degree of crustal shortening may take place in the form of buckling of an elastic or plastic lithosphere though this cannot explain the strong asymmetry of the ridge which we believe must imply faulting

strike slip earthquakes in the oceanic lithosphere observations of exceptionally high apparent stress summary the radiated energies e s and seismic moments m0 for 942 globally distributed earthquakes that occurred between 1987 to 1998 are examined to find the earthquakes with the highest apparent stresses a e s m0 where is the modulus of rigidity the globally averaged a for shallow earthquakes in all tectonic environments and seismic regions is 0 3 mpa however the subset of 49 earthquakes with the highest apparent stresses a greater than about 5 0 mpa is dominated almost exclusively by strike slip earthquakes that occur in oceanic environments these earthquakes are all located in the depth range 7 29 km in the upper mantle of the young oceanic lithosphere many of these events occur near plate boundary triple junctions where there appear to be high rates of intraplate deformation indeed the small rapidly deforming gorda plate accounts for 10 of the 49 high a events the depth distribution of a which shows peak values somewhat greater than 25 mpa in the depth range 20 25 km suggests that upper bounds on this parameter are a result of the strength of the oceanic lithosphere a recently proposed envelope for apparent stress derived by taking 6 per cent of the strength inferred from laboratory experiments for young less than 30 ma deforming oceanic lithosphere agrees well with the upper bound envelope of apparent stresses over the depth range 5 30 km the corresponding depth dependent shear strength for young oceanic lithosphere attains a peak value of about 575 mpa at a depth of 21 km and then diminishes rapidly as the depth increases in addition to their high apparent stresses which suggest that the strength of the young oceanic lithosphere is highest in the depth range 10 30 km our set of high a earthquakes show other features that constrain the nature of the forces that cause interplate motion first our set of events is divided roughly equally between intraplate and transform faulting with similar depth distributions of a for the two types secondly many of the intraplate events have focal mechanisms with the t axes that are normal to the nearest ridge crest or subduction zone and p axes that are normal to the proximate transform fault these observations suggest that forces associated with the reorganization of plate boundaries play an important role in causing high a earthquakes inside oceanic plates extant transform boundaries may be misaligned with current plate motion to accommodate current plate motion the pre existing plate boundaries would have to be subjected to large horizontal transform push forces a notable example of this is the triple junction near which the second large aftershock of the 1992 april cape mendocino california sequence occurred alternatively subduction zone resistance may be enhanced by the collision of a buoyant lithosphere a process that also markedly increases the horizontal stress a notable example of this is the aleutian trench near which large events occurred in the gulf of alaska in late 1987 and the 1998 march balleny sea m 8 2 earthquake within the antarctic plate

tectonic evolution of the tehuantepec ridge the tehuantepec ridge is one of the most prominent lithospheric structures of the cocos plate yet its tectonic evolution has remained poorly constrained until now calculated ocean floor ages and spreading rates morphostructural analysis of the ridge and the surrounding ocean floor are used to infer the tectonic evolution and pattern of the tehuantepec ridge and associated structures the ocean floor age estimates south of the tehuantepec ridge suggest an age of 26 ma at the middle america trench a mean age difference across the tehuantepec ridge of 7 ma suggests that the tehuantepec ridge would be formed as a long transform fault on the guadalupe plate prior to 15 ma the full spreading rate estimates show that there might be a differential full spreading rate across the transform fault between 15 and 5 ma when the oceanic plate just south of clipperton fracture zone was decelerating between 15 and 10 ma then accelerating for the next 5 ma we propose a model in which between 13 and 8 ma temporarily transpressional intratransform spreading centers would have been existing as a consequence of the east pacific rise onset with an angle of 108 clockwise with respect to the ceased mathematician ridge our model estimates indicate that between 21 and 15 ma a prominent pseudofault was formed south of the tehuantepec ridge due to an unstable offset on the mathematician ridge that migrated northward the tehuantepec ridge and the pseudofault trace bound a deeper oceanic basin than the surrounding area with no corresponding anomalous depth on the western side of the east pacific rise the complete absence of a pseudofault trace and a deeper oceanic basin on the western side of the east pacific rise as well as the spreading rate changes suggest the existence of a microplate embedded into the cocos plate just south of the tehuantepec ridge also the asymmetric and sharp morphology of the tehuantepec ridge suggests that it may be the expression of a major transpressional structure along the former transform fault on the guadalupe plate 15 20 ma ago d 2005 elsevier b v all rights reserved

implications of updated magnetic anomalies for the late cretaceous tectonic evolution of walvis ridge in the south atlantic a reorganization of the mid atlantic ridge began before anomaly c34n 83 6 ma and ended before anomaly c30n 66 4 ma complicating tectonics of rio grande rise and older walvis ridge wr which formed together at the mid atlantic ridge this reorganization is poorly understood because magnetic anomalies c30n c34n are poorly defined near wr we interpreted these anomalies along westernwrto improve knowledge of rio grande rise wrtectonic development anomaly trends indicate that valdivia bank has an e w age progression perpendicular to that predicted by hot spot models anomaly spacing and width is irregular and anomalous near wr implying a series of ridge jumps and possibly a microplate between anomalies c34n and c32n eastward ridge jumps transferred microplate lithosphere to the south american plate this study shows that late cretaceous tectonic evolution of the rio grande rise wrlarge igneous provinces was more complex than previously understood

extending the astronomical polarity time scale into the miocene an astronomical time scale is presented for the late miocene based on the correlation of characteristic sedimentary cycle patterns in marine sections in the mediterranean to the 65 n summer insolation curve of la90 1 2 with present day values for the dynamical ellipticity of the earth and tidd dissipation by the moon this correlation yields ages for all sedimentary cycles and hence also for the recorded polarity reversals and planktonic foraminiferal and dinoflagellate events the tortonian messinian t m boundary placed at the first regular occurrence of the globorofuliu conomiozea group in the mediterranean is dated at 7 24 ma the duration of the messinian is estimated at 1 91 myr because the miocene pliocene boundary has been dated previously at 5 33 ma 3 the new time scale is confirmed by oar 3gar ages of volcanic beds and by the number of sedimentary cycles in the younger part of the mediterranean messinian

magnetic anomaly map of ori massif and its implications for oceanic plateau formation abstract many oceanic plateaus have been emplaced at or adjacent to mid ocean ridges to explain plateau volume and thickened crust compared to normal oceanic crust hotspot ridge interaction is commonly assumed but the manner of interaction remains unclear the shatsky rise oceanic plateau is a large volcanic mountain that formed at a triple junction during late jurassic and early cretaceous time recent drilling and seismic investigations suggest that the intermediate edifice in the rise ori massif is a central volcano paradoxically magnetic lineations were traced across parts of ori massif implying formation at a spreading ridge in this study we examined magnetic anomalies over and around ori massif to obtain insights about the formation of this volcanic edifice magnetic data from 21 cruises were corrected combined and gridded to construct a magnetic anomaly map forward and inverse magnetic modeling was done to investigate the magnetic structure of ori massif the results imply that this large volcanic edifice is predominantly characterized by linear magnetic anomalies resulting from alternating normal and reversed polarity magnetization blocks analogous to magnetic anomalies recorded by spreading ridges this magnetic structure is not expected for a central volcano that was built by long runout lava flows implying that ori massif eruptions must have been constrained near the ridge axis magnetic bights on the north and south boundaries of ori massif imply that it was bracketed by triple junctions indicating complex ridge tectonics during the formation of shatsky rise the surprising finding that ori massif is traversed by coherent linear magnetic anomalies indicates that oceanic plateaus can record seafloor spreading magnetic anomalies despite large crustal thickness other oceanic plateaus also record linear magnetic anomalies implying a link between divergent plate boundaries and oceanic plateau volcanism

southeast pacific tectonic evolution from early oligocene to present plate tectonic reconstructions of the nazca antarctic and pacific plates are presented from late oligocene to present these reconstructions document major plate boundary reorganizations in the southeast pacific at dirons 6c 24 ma 6 o 20 ma and 5a 12 ma and a smaller reorganization at chron 3 o 5 ma during the chron 6 o reorganization it appears that a ridge propagated into crust north of the northernmost pacific antarctic ridge between the chiloe fracture zone fz of the chile ridge and agassiz fz of the pacific nazca ridge which resulted in a northward jump of the pacific antarctic nazca pac ant naz mid ocean triple junction during the chron 5a reorganization the chile ridge propagated northward from the valdivia fz system to the challenger fz through lithosphere formed roughly 5 myr earlier at the pacific nazca ridge during this reorganization a short lived microplate the friday microplate existed at the pac ant naz triple junction the pac ant naz triple junction jumped northward 500 km as a result of this reorganization from a location along the valdivia fz to a location along the challenger fz the chron 5a reorganization also included a change in spreading direction of the chile and pacific antarctic ridges the reorganization at chron 3 o initiated the formation of the juan fernandez and easter microplates along the east pacific rise the manner of plate boundary reorganization at chron 6 o and chron 5a and possibly today at the juan fernandez microplate included a sequence of rift propagation transfer of lithosphere from one plate to another microplate formation and microplate abandonment and resulted in northward migration of the pac ant naz triple junction the associated microplate differs from previously studied microplates in that there is no failed ridge

dual overflows into the deep sulu sea 1 the sulu sea isolated from the neighboring ocean below 570 m is nearly isothermal below 1250 m but with a marked salinity increase with depth the source of the deep sulu sea water has been attributed to south china sea water overflowing the 570 m topographic sill of panay strait however the panay overflow estimated as 0 32 106 m3 sec is an unlikely source for the saltier water sulu sea deep water we propose that deep sulu sea ventilation is derived from the south from the sulawesi sea through sibutu passage sulawesi sea water between 245 to 527 m is mixed and heaved over the sibutu passage 234 m sill by the energetic tidal environment oxygen concentrations within the deep sulu sea suggest that the sulawesi overflow is 0 15 106 m3 sec with a residence time of sulu sea deep water of 60 years the deep tropical sulu sea has the unique distinction of being ventilated from two separate sources whose ratio may fluctuate across a range of temporal scales associated with regional thermocline depth changes

geochemical implications for the mechanism of deep convection in a semi closed tropical marginal basin sulu sea abstract in 1996 and 2002 we measured the vertical profiles of potential temperature salinity dissolved oxygen nutrients tritium only in 1996 and helium isotopes only in 2002 in the sulu sea a small semi closed tropical marginal basin west of the philippines with a maximum depth of 5000 m judging from these data in combination with those for the south china sea adjacent to the sulu sea we conclude that the deep convection in the sulu sea is controlled by a combination of the following three processes i quasi steady inflow of the south china sea intermediate water through the mindoro strait sill depth 420 m ii occasional inflow of denser intermediate water through the mindoro strait from the south china sea when the thermocline in the south china sea is uplifted and iii formation of less dense bottom water by means of geothermal heating which in turn causes overturning of the sulu sea bottom water below a depth of 3000 m from 1996 to 2002 the bottom water of the sulu sea was stagnant its temperature increased by 0 003 c and its dissolved oxygen concentration decreased by 5 8 mol kg 1 the 6 years study results show that the temperature increase can be attributed to the terrestrial heat flow from the seafloor

edge driven microplate kinematics it is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate s motion relative to the bounding major plates are frequently located close to its margins with those plates close to the tips of propagating rifts we propose a class of edge driven block models to illustrate how slip across the microplate margins block rotation and propagation of rifting may be related to the relative motion of the plates on either side an important feature of these edge driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates according to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time pseudofault geometries and matching rotations of the easter microplate show that for most of its 5 m y history block rotation could be driven by the drag of the nazca and pacific plates on the microplate s edges rather than by a shear flow of mantle underneath

block rotation and deformation by strike slip faults 2 the properties of a type of macroscopic discontinuous deformation study of the general features of deformation by systems of subparallel strike slip faults shows that the governing constraints are kinematic the fault blocks must remain in contact with each other and the deformed area must fit with its surroundings as a result fault blocks that move laterally without significant internal deformation also rotate about vertical axes relative to boundaries of the fault domain by an amount that is quantitatively related to fault slip spacing and orientation areas where left and right lateral faults move simultaneously are usually divided into domains that contain faults of one kind only deformation of such multidomain areas is nonhomogeneous but the differences between domains can compensate for each other so that the regional deformation is quite simple strike slip faulting combined with block rotation is an efficient mechanism of deformation because modest offsets and rotations change the linear dimensions of faulted areas by several tens of percents which may often greatly exceed the results of other types of coeval deformation paleomagnetic data can provide an independent measure of block rotations and should be used to supplement structural data block rotation changes the initial angles between right and left lateral faults and also their orientation relative to the stress field the faults probably tend to rotate away from the axis of maximum compression rotated faults continue to move because they are weak surfaces since the deformation depends on geometric factors such as fault orientation and domain geometry it is not simply related to the stress field these properties including rotation of material units characterize discontinuous deformation in general

deep water renewal by turbidity currents in the sulu sea the ventilation of the intermediate and deep layers of the ocean is governed by convection dense water formed at the sea surface through air sea interaction processes mainly at high latitudes sinks to great depths convection determines the heat salt and dissolved gas budgets of the ocean s interior and therefore exerts a significant influence on the earth s climate convective processes can also be initiated by the flow of dense water over submarine sills separating ocean basins with water masses of different buoyancy here we report the observation of relatively light deep water in the overflow region near the sill in the northern part of the 5 000 m deep sulu sea because of its buoyancy this water could not have reached the sea floor of its own accord turbidity currents induced by changes in water density that result from sediment influx occur in this region at intervals of several decades as indicated by the frequent deposition of graded sequences of silty mud on the abyssal plain we suggest that such turbidity currents combined with the effects of classical hydrological plume convection are capable of bringing light water to large depths in tropical regions where normal oceanic convection is relatively weak this mechanism may contribute significantly to the ventilation of deep basins

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

the development of two oceanic microplates easter and juan fernandez microplates east pacific rise in this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid ocean ridge microplates on the east pacific rise we quantify their development in terms of the principal dimensions of the microplates and their boundaries and the temporal changes in these dimensions the results are discussed in the framework of the roller bearing microplate model of schouten and others which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates we test the specific predictions of this model for the geometry of the pseudofaults traces of the propagating tips of the microplates spreading boundaries and the histories of rotation of the microplates both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling no slip or deformation between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries juan fernandez microplate was also rotating at the predicted rate just before 2 ma when it was driven by the nazca pacific couple rather than the nazca antarctic couple that currently applies however earlier rotations were slower than predicted implying some degree of decoupling or deformation at earlier times the microplates appear to have grown from moderate sized c 100 km probably left stepping offsets of the east pacific rise since about 5 ma the nature of these offsets is currently uncertain but they may have been propagating rifts which changed from shearing by bookshelf faulting to rigid block microplate rotation on reaching a critical size once rigid block rotation began the microplates grew by seafloor spreading accompanied by propagation of their bounding spreading centres the effects of the neighbouring easter mantle plume and pacific nazca antarctic triple junction can be discerned in the bathymetry and ridge propagation rates but do not appear to have had a major influence on the microplates development

migrating transform zone and lithospheric transfer at the galapagos 95 5 w propagator at the 95 5 w propagator system along the cocos nazca plate boundary the spreading axis is offset left laterally about 35 km 1 m y this offset migrates westward 52 km m y relative to points along the nearly east west trending ridge axis transferring lithosphere from the cocos to the nazca plate as it does so the required 58 km m y of right lateral motion across this migrating transform is apparently not accommodated by a throughgoing north south transform fault but rather by an approximately 15 to 20 km wide zone of shear deformation dominated by eastward curving of east west structures to a northwest southeast trend this pattern of curvature the coinciding zone of high seismicity and the abundance of talus in the area suggest that shear is distributed over this entire zone the deformation is inferred to be taken up by a bookshelf mechanism whereby faults oriented at high angles to the shear couple have left lateral slip antithetic to the right lateral shear couple rotating individual blocks clockwise these are interpreted as reactivated abyssal hill bounding normal faults probably dipping 45 60 northeast southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of de steiguer ridge in the zone of transferred lithosphere these faults have apparently had a very complex history initially normal faults formed as abyssal hills were created at the spreading axis and subsequently they had left lateral strike slip motion in the transform zone and reverse motion uplifting de steiguer ridge strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires 0 4 m y

the kinematic evolution of the macquarie plate a case study for the fragmentation of oceanic lithosphere abstract the tectonic evolution of the southeast indian ridge seir and in particular of its easternmost edge has not been constrained by high resolution shipboard data and therefore the kinematic details of its behavior are uncertain using new shipboard magnetic data obtained by r vib araon and m v l astrolabe along the easternmost seir and available archived magnetic data we estimated the finite rotation parameters of the macquarie antarctic and australian antarctic motions for eight anomalies 1o 2 2ay 2ao 3y 3o 3ay and 3ao these new finite rotations indicate that the macquarie plate since its creation 6 24 million years ago behaved as an independent and rigid plate confirming previous estimates the change in the australian antarctic spreading direction from n s to nw se appears to coincide with the formation of the macquarie plate at 6 24 ma analysis of the estimated plate motions indicates that the initiation and growth stages of the macquarie plate resemble the kinematic evolution of other microplates and continental breakup whereby a rapid acceleration in angular velocity took place after its initial formation followed by a slow decay suggesting that a decrease in the resistive strength force might have played a significant role in the kinematic evolution of the microplate the motions of the macquarie plate during its growth stages may have been further enhanced by the increased subducting rates along the hjort trench while the macquarie plate has exhibited constant growth by seafloor spreading

episodic triple junction migration by rift propagation and microplates we describe a model for episodic open ocean triple junction migration based on observations of actual triple junction evolutions migration progresses by repeated episodes of rift propagation microplate formation and microplate accretion to an adjacent larger plate these episodes may be highly variable in space and time depending on triple junction geometry velocity triangle and other factors affecting local thermal and rheological conditions resulting tectonic features may include an abandoned transform fault straight and potentially curving pseudofaults sheared and potentially rotated abyssal hill fabric and a paleomicroplate with no associated failed rift this model developed mainly from the evolution of the pacific antarctic nazca triple junction may be relevant for other types of triple junctions such as the bouvet and azores triple junctions and ridge ridge ridge triple junctions in the indian and pacific oceans episodic migration is found to occur even when the triple junction is kinematically stable this model highlights the difference between predicted kinematic stability of triple junctions and observations of their true tectonic history

speed limit for oceanic transform faults oceanic transform faults with slip rates greater than 145 km m y do not currently exist along the east pacific rise where sea floor spreading rates range from 145 to 160 km m y instead offsets of the the very fast spreading east pacific rise are accommodated by microplates propogating rifts or overlapping spreading centers this suggests that there might be a speed limit above which transform faults do not exist a physical reason for a speed limit is not known but it might be related to unstable stress fields near the rifts tips causing them to episodically propagate and prevent a transform fault from being formed the spreading rates quoted are from our new 0 0 73 ma relative motion model for the pacific and nazca plates

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

tectonic volcanic segmentation and controls on hydrothermal venting along earth s fastest seafloor spreading system epr 27 32 s 1 we have collected 12 khz seabeam bathymetry and 120 khz dsl 120 side scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading 150 km myr part of the global mid ocean ridge system the southern east pacific rise between the easter and juan fernandez microplates this area is presently reorganizing by large scale dueling rift propagation and possible protomicroplate tectonics fracture patterns observed in the side scan data define structural segmentation scales along these ridge segments these sometimes but not always correlate with linear volcanic systems defining segmentation in the seabeam data some of the subsegments behave cohesively with in phase tectonic activity while fundamental discontinuities occur between other subsegments we also collected hydrothermal plume data using sensors mounted on the dsl 120 instrument package as well as ctdo tow yos to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the pacific nazca axis here we report the first rigorous correlation between coregistered hydrothermal plume and high resolution marine geophysical data on similar scales and over multisegment distances major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low these correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest resulting in recent eruptions that have paved over the neovolcanic zone areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas increased crustal permeability as gauged by increased fracture density does not enhance hydrothermal venting in this area axial summit troughs and graben are rare probably because of frequent volcanic resurfacing in this superfast spreading environment and are not good predictors of hydrothermal activity here many of the hydrothermal areas are found in inflated areas near the ends of segments suggesting that abundant magma is being supplied to these areas

tectonic evolution of ridge axis discontinuities by the meeting linking or self decapitation of neighboring ridge segments recent deep tow sea beam and seamarc ii studies indicate that ridge axis discontinuities along the east pacific rise evolve in several different ways where magmatic pulses along the spreading center happen to meet head on a low point or saddle point occurs along the axial depth profile of the spreading center where the magmatic pulses misalign overlapping spreading centers develop and two distinct evolutionary paths are possible one spreading center tip may cut through to and link with the adjacent en echelon ridge chopping off the opposing ridge tip as proposed by macdonald and fox 1983 in addition either spreading center may cut inside or outside of itself repeatedly decapitating its own ridge tip crack propagation studies show that the crack propagation force g drops significantly when the ratio of crack overlap to crack offset l w exceeds 3 applying this relation to spreading centers we suggest that propagation of the individual spreading center tips may stall when l w is greater than 3 if linkage has not yet occurred then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip decapitating the ridge tip in a process we call self decapitation subsequent magmatic pulses may be deflected or derailed from the existing path of the ridge because freezing of the axial magma chamber near ridge axis discontinuities creates a core of coherent unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber in addition the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge for propagating rifts which represent first order changes in plate boundary geometry l w remains 1 5 so that g is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced

dissolved rare earth elements and hydrography in the sulu sea because most southeast asian basins are flushed rapidly by waters from the western pacific the effects of respiration and silica dissolution within those basins are hardly discernible based on general hydrographic and geochemical observations broecker et al 1987 however a different situation is expected for the sulu sea because its deep water is isolated from other deep sea basins by shallow sills of 400 m depth hence any heterogeneity of elemental distributions within the basin must be ascribed to vertical physical and biogeochemical processes we have obtained detailed vertical profiles of dissolved rare earth elements rees together with conductivity temperature depth ctd and hydrographic measurements in the sulu sea during the 1996 97 hakuho maru cruise vertical profiles clearly indicate that the ree iii s are enriched in the deep water due to regeneration and are involved in vertical biogeochemical cycling the light ree iii s la pr and nd showed a subsurface maximum at 500 m and a minimum 1500 m in depth none of the hydrographic properties or nutrients shows such features therefore the ree iii s and their elemental ratios may be useful as tracers of water masses the relationship of ree iii s versus dissolved si suggests that the ree iii s are regenerated in a delayed fashion relative to si and it is likely that their dissolution largely occurs at the bottom interface the mean residence time of the deep water is estimated to be 300 6 150 yr consequently the calculated benthic flux of ree iii s becomes roughly comparable with the river and atmospheric fluxes because of oxidation to ce iv the ce profile shows a decrease from 6 pmol kg at the surface to a constant value at 3 9 pmol kg below 1500 m unlike the other rees the rees in the sulu sea are enriched in the middle rees and ce relative to those of the north pacific deep water and must derive from local sources around the sulu sea copyright 1999 elsevier science ltd

modelling the plate driving mechanism this paper demonstrates how sub lithospheric loading gives rise to plate interior stresses and plate boundary forces the stressing of the lithosphere resulting from plume heads in the upper mantle hot spots from low density asthenospheric upwelling beneath ocean ridges and from dense subducting slabs has been modelled by finite element analysis it is first demonstrated how a sub lithospheric load exemplified by a sub continental plume head produces local deviatoric stress in the strong upper lithosphere within a plate interior region it is shown that a 500 km wide region of anomalous density averaging 8 kg m 3 between 100 km and 400 km depths gives rise to a deviatoric tension of 75 mpa in a 20 km thick strong layer forming the upper lithosphere above the deep buoyant load the large stresses in the strong layer are caused by the shear drag and excess pressure exerted by the buoyant load when such a stress system is cut across by a zone or plane of weakness then plate boundary forces and distant plate interior stresses are produced a weak zone beneath the crest is incorporated in a model of a normal ocean ridge and this yields a ridge push force of 2 5 10 12 nm 1 referenced to old ocean floor and deviatoric compression of about 40 mpa in old ocean floor a model of a ridge underlain by an anomalously low density upper mantle plume head yields a much larger ridge push force and large deviatoric compressions of nearly 100 mpa extend into the bordering continents subduction pull slab pull and trench suction of about 4 5 10 12 nm 1 is modelled for a 300 km deep slab separated from the overriding plate by a weak fault and collision pull is shown to result from downbulging lower lithosphere beneath collision mountain ranges

plate tectonic reconstructions of the juan fernandez microplate transformation from internal shear to rigid rotation side scan sonar swath bathymetry and magnetic anomaly date define a detailed three phase history of the juan fernandez microplate the 6 m y history is presented in a series of discrete time steps to document the growth and reorganization of propagating spreading centers and structural feature and microplate kinematic evolution prior to the microplate the east pacific rise at the pacific antarctic nazca triple junction was offset by a long transform fault zone likely the fastest slipping transform on earth at anomaly 3a time the microplate originated from an intratransform setting between anomaly 3a 5 95 ma and anomaly 3 5 24 ma time its early development resembled a large propagating rift system and microplate core structures suggest the entire offset zone may have experienced deformation fast propagation of the east ridge dominated microplate growth until 2 6 1 9 ma when seafloor spreading became the dominant growth process the microplate rotation rate increased threefold from 9 to 29 m y 1 average from phase 1 4 2 2 6 ma to phase 2 2 6 1 1 ma of the microplate s history then reduced fourfold 29 to 7 m y 1 average phase 3 1 1 ma to present phases 2 and 3 of the microplate s rotational history support the edge driven model for microplate kinematics of schouten and others to a good approximation the pacific nazca shear couple drove microplate rotation during phase 2 but development of the southeastern boundary enabled a transfer to the nazca antarctic plate pair phase 3 west ridge propagation and reorganization of the south western boundary may have decoupled the pacific plate from the microplate thus facilitating the shear couple transfer the recent continued deceleration in microplate rotation rate and westward migration of the pacific antarctic ridge axis relative to the microplate may indicate that the process of microplate death has begun we speculate that the juan fernandez microplate will accrete to the antarctic plate perhaps within the next million years like the extinct friday microplate has done thereby accomplishing another northward migration of the pacific antarctic nazca triple junction our reconstructions illustrate that the easter and juan fernandez microplates are more similar than previously thought in terms of their origin growth rift propagation ridge segmentation and overall tectonic evolution

a block model of distributed deformation by faulting it is clear from palaeomagnetic evidence that large and hitherto unexpected rotations about a vertical axis are common in regions of distributed continental deformation we propose a simple two dimensional model to illustrate how fault movement and block rotation within a zone of distributed deformation may be related to the relative motion of the rigid plates that bound the zone one surprizing feature of this model is that the component of strike slip motion on the faults within the deforming zone is in the opposite sense to the strike slip component across the zone as a whole thus slip vectors within the zone are not the same as that between the bounding rigid plates in contrast to deformation at oceanic plate boundaries examples of active fault geometries similar to those of our model can be seen in greece iran and the western usa

a history of the easter microplate 5 25 ma to present gloria and other geophysical data are used to interpret the morphology and tectonic pattern of the easter microplate we model magnetic anomaly data and calculate spreading rates for use in euler pole calculations we calculate three plate closure and best fit poles for nazca pacific nazca easter and pacific easter plate motions and compare the results with earthquake focal mechanisms and seismicity we use the three plate closure poles to rotate digital gloria and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate s east rift first started to propagate northward at around 5 25 ma the microplate has evolved in two stages the first is from 5 25 ma to around 2 5 ma and involved continuous northward propagation of the east rift while the west rift became segmented along new transform faults as it adjusted to keep the overall nazca pacific velocities constant the transpressive northern boundary migrated with the east rift tip while the transtensional southern boundary remained more or less still at some time after 2 5 ma the east rift ceased northward propagation and the southwest rift began to open up along the transtensional pacific easter transform a convergent northern boundary formed involving southward thrusting of the nazca plate over the microplate no further nazca plate lithosphere was transferred from north of the microplate to the microplate interior and the pito deep region opened by pervasively rifting apart preexisting nazca plate lithosphere

intratransform origins of mid ocean ridge microplates we present evidence that the easter and juan fernandez microplates originated by rift propagation from within transform fault zones at locations far from ridge transform intersections an intratransform spreading center may have served as the focal point for each new propagation both events appear coincident with a circum pacific plate reorganization dated at 5 6 ma which may have been related to a change in pacific absolute plate motion and a counterclockwise rotation of pacific nazca relative plate motion this intratransform model may be applicable to the generation of other microplates and may thus serve as a general model for microplate origins

wilkes transform system and nannoplate a recent hydrosweep survey of the wilkes transform system the second fastest slipping transform on the mid ocean ridge system shows it to be a highly complex and continually evolving plate boundary an area 50 km 2 north of the eastern part of the wilkes transform appears to be rotating counterclockwise in accordance with the theory of edge driven microplate kinematics we have called this region a nannoplate to distinguish it both as a separate small area of lithosphere between the pacific and nazca plates and as a smaller and less stable phenomenon than a microplate

a history of the selkirk paleomicroplate abstract located in the south pacific between latitudes 33 s and 38 s and longitudes 120 w and 125 w the extinct selkirk microplate was first described as originating from lithosphere transferred from the nazca plate to the pacific plate earth planet sci lett 113 1992 293 multibeam bathymetry data backscatter imagery and magnetic data obtained in 1997 during the foundation hotline cruise permit a more complete and detailed characterisation of the evolution of this paleomicroplate the western boundary of the paleomicroplate is a fossil spreading axis composed by two segments presenting two southward propagations and fan shaped opening the eastern boundary of the paleomicroplate is formed by a northward propagating rift in the present day only the western part of the fan shaped sequence is present in the pacific plate according to satellite altimetry data the northern boundary is a complex compressive domain formed by n110 ridges and troughs the southern boundary is located along the mocha fracture zone our magnetic interpretation suggests that this microplate has been active between chron 6c 24 ma and chron 6ar anomaly 6a reverse period 20 8 ma the last stage of the selkirk microplate evolution is marked by a compression in the southern boundary and a sinistral shearing along an associated n45 structure located in the microplate these observations imply the transfer of the eastern southern boundary of the microplate previously located along the mocha fracture zone to these complex structures the observed structures imply a change of the microplate rotation axis for this last stage leading to the locking of the microplate and the transfer of all the spreading to the eastern boundary

the bauer scarp ridge jump a complex tectonic sequence revealed in satellite altimetry abstract we investigate here the ridge jump that led to abandonment of the galapagos rise and formation of the bauer scarp during the initiation of the present day configuration of the east pacific rise since the lower miocene we use recently available high resolution satellite derived gravity data to investigate in detail the tectonic structure of the eastern pacific from the equator to 20 s with this data we identify fracture zones abandoned spreading ridges scarps and other seafloor features that provide evidence for discerning tectonic history based on our structural interpretation of the satellite derived gravity field we make the following conclusions 1 the galapagos rise spreading center appears to have originated by opening of the marquesas mendana transform complex as a result of the change in spreading direction following breakup of the farallon plate 2 the galapagos rise was not the sole locus of spreading following plate reorganization at 20 ma through to the initiation of the bauer scarp at 8 ma as had been previously hypothesized rather it and a second western spreading axis were likely active concurrently forming a counterclockwise rotating bauer microplate at a much earlier stage than thought previously 3 the bauer scarps are pseudofaults associated with northward rift propagation propagation proceeded in several stages a first propagator emanating from the garrett transform complex stalled at the future location of the wilkes transform creating an area of complex morphology near its northern tip a second propagator also emanating from the garrett complex followed in the first s wake and broke through the complex region at this point the propagation proceeded very rapidly to the northern end of the bauer microplate the gallego fracture zone later to become the yaquina transform fault ridge propagation continued north in two more stages creating the gofar and quebrada transforms at the terminus of each stage

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

the relationship between seismicity and fault structure on the discovery transform fault east pacific rise there is a global seismic moment deficit on mid ocean ridge transform faults and the largest earthquakes on these faults do not rupture the full fault area we explore the influence of physical fault structure including step overs in the fault trace on the seismic behavior of the discovery transform fault 4s on the east pacific rise one year of microseismicity recorded during a 2008 ocean bottom seismograph deployment 24 377 0 ml 4 6 earthquakes and 24 years of mw 5 4 earthquakes obtained from the global centroid moment tensor catalog are correlated with surface fault structure delineated from high resolution multibeam bathymetry each of the 15 5 4 mw 6 0 earthquakes that occurred on discovery between 1 january 1990 and 1 april 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross correlation technique microseismicity was relocated using the hypodd relocation algorithm the western fault segment of discovery dw is composed of three zones of varying structure and seismic behavior a zone with no large events and abundant microseismicity a fully coupled zone with large earthquakes and a complex zone with multiple fault strands and abundant seismicity in general microseismicity is reduced within the patches defined by the large repeating earthquakes while the extent of the large rupture patches on dw correlates with physical features in the bathymetry step overs in the primary fault trace are not observed at patch boundaries suggesting along strike heterogeneity in fault zone properties controls the size and location of the large events

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

near real time merging of gloria and hydrosweep pseudo sidescan images a method has been developed for merging gloria and hydrosweep data into single combined sidescan type images by adding a shaded relief representation of the seafloor topography to the near nadir part of the gloria image which is usually poorly insonified several examples are presented taken from a variety of seafloor terrains

the juan fernandez microplate north of the pacific nazca antarctic plate junction at 35 s abstract results of the r v thomas washington pascua 3 expedition provide evidence for the existence of the juan fernandez microplate just north of the junction between the east pacific rise epr and the chile fracture zone prior to pascua 3 the microplate in the region had been hypothesized from the pattern of seismicity the eastern and western boundaries of the juan fernandez microplate are well defined and represent north south trending spreading centers characterized by very slow and very fast rates of accretion respectively in agreement with the rates the eastern boundary is represented by a rift valley and the western boundary by an epr type axial ridge the northern boundary of the juan fernandez microplate is a 100 trending wide fracture zone complex which may have resulted from northward transform fault migration the fracture zone fails to meet the zone of accretion at the pacific nazca juan fernandez triple junction in this area the zone of accretion displays a double ridge with a large overlap the southern boundary of the juan fernandez microplate is still poorly constrained the plate geometry derived from seabeam differs from that derived by anderson fontana et al 1986 14 from a plate motion inversion scheme using primarily earthquake first motion solutions together with limited bathymetric and magnetic data

ontong java plateau and late neogene changes in pacific plate motion aided by a new geometrical technique called hot spotting we recently proposed a new model for the absolute motion of the pacific plate this model called wk97 assumed the hawaiian hotspot is under loihi and that there was a change in plate motion at 3 6 ma the model located the louisville hotspot close to the hollister ridge a shallow volcanic ridge associated with a geoid high seismicity and recent volcanism most of the tectonic implications of wk97 however were not addressed furthermore although several recent studies suggest that the geochemistry of hollister lavas may indicate a mixture between louisville melt material and pacific antarctic ridge mid ocean ridge basalt these studies strongly disagree with wk97 and consider it seriously flawed here we discuss numerous tectonic predictions of wk97 and present a large amount of observational evidence in support of it we find that a late neogene collision between the ontong java plateau and the northern margin of the australia plate appears to have altered the motion of the pacific plate as inferred from hotspot volcanism by forcing it to rotate counter clockwise this rotation about a low latitude pole appears to have induced right lateral shear stress along the entire pacific plate divergent boundary which resulted in the formation of extensional transform faults microplates and propagating ridges this change in motion also appears to have triggered concomitant circum pacific tectonism including trench migration and back arc rifting the wk97 suggests that long lived plume fed volcanism on the pacific plate appears very limited perhaps only to five to six hotspots hawaii louisville cobb caroline and perhaps marquesas and bowie the geometry of these chains are consistent with our model despite some evidence for volcanic age progressions most other pacific neogene hotspots are most likely crack spots i e sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses indeed several volcanic ridges such as hollister ridge near the louisville hotspot appear to have formed in response to stresses following the change in absolute plate motion wk97 also obviates the need for simultaneous different directions of mantle flow in the south central pacific transpression at the san andreas and alpine strike slip faults and aleutian arc explosivity since 6 ma also appear to be consequences of the changes in pacific plate motion furthermore both the wk97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion our findings support slab pull as the dominant plate tectonic driving force highlight the rheological difference between strong oceanic lithosphere and weak continental crust and suggest that wk97 presently provides a unifying albeit preliminary framework for relating both intraplate and circum pacific tectonism and volcanism in the late neogene

southeastern boundary of the juan fernandez microplate braking microplate rotation and deforming the antarctic plate data from a 1991 multibeam sidescan sonar and geophysical survey of the juan fernandez microplate at the pacific nazca antarctic triple junction strongly support a model of edge driven microplate rotation they reveal the nature of the microplate s southeastern boundary with the antarctic plate and show that plate boundary interactions are capable of significantly altering plate motions microplate rotation slowed progressively from 30 m y to 10 m y between anomaly 2 time 1 9 ma and the early brunhes 0 6 ma magnetics bathymetry and morphology suggest that this deceleration resulted from coupling across a rapidly evolving juan fernandez antarctic jf a plate boundary over the past 2 m y the jf a plate boundary has lengthened changed orientation and migrated to the southeast into the antarctic plate this process deformed and transferred to the juan fernandez plate about 3000 km2 of the antarctic plate kinematic and morphologic observations support interpreting curved lineaments within this deformed zone as originally n s abyssal hills generated at the pacific antarctic ridge that were later sheared by dextral slip bookshelf faulting within the migrating sinistral shear zone of the jf a boundary deformation within this system documents that shear zone migration not rift propagation is the key process associated with development of curved lineaments and transferred lithosphere the jf a plate boundary shear zone may have been quite narrow 10 20 km despite its migration through the antarctic plate narrow migrating plate boundaries are also observed at other systems such as the galapagos 95 5 w migrating offset which also has a width of 20 km such narrow deformation zones may be common along migrating plate boundaries in various tectonic environments triple junction systems in this area appear to represent a suite of plates and triple junctions crossing a range of scales the observations here support models for triple junctions having complex evolutionary histories involving rapid changes in plate boundary configuration and stress field

recent dueling propagation history at the fastest spreading center the east pacific rise 26 32 s the east pacific rise epr 26 32 s located between the easter microplate and the juan fernandez microplate has the world s fastest spreading rate and tectonics characterized by dueling ridge propagation a glori b and sea beam 2000 side scan bathymetry survey together with other geophysical data collection was conducted aboard the r v melville covering an approximately 5 by 5 area centered on this part of epr the side scan and bathymetric data reveal the complete geometry of the propagator system the western and eastern ridges both curve inwardly and are overlapped 120 km with an offset to overlap ratio of nearly one several abandoned ridges and rifts are clearly mapped on the east flank and a few failed rifts are also mapped near the western ridge tip a fracture zone discovered south of easter island indicates a stage of stable ridge transform intersection in the past an inversion with magnetic anomaly vectors was carried out for the area a resultant magnetic boundary strike map was interpreted with the downward component profiles to construct a magnetic isochron map the spreading and propagation rates were estimated from the magnetic isochrons the net southward propagation rate of the western ridge is 120 km m y for the last 1 9 m y the spreading rate of the eastern ridge has been 150 km m y and its current propagation rate is estimated as 500 km m y toward the north intense asymmetric spreading as high as 30 is observed at both ridges together with the side scan and bathymetric data the magnetic lineations provide significant constraints on the evolution of the propagator system and a propagation model with cyclic rift failure was applied to model the tectonic evolution during the last 2 m y a normal ridge transform intersection evolved into a nontransform overlapping offset 1 95 ma and the eastern and the western ridges have propagated alternately the dueling propagation history is characterized by the dominant southward propagation of the western ridge and the transitory propagation events of the eastern ridge with a variety of overlap lengths widths and ridge curvatures in the transition zone

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

heterogeneous mantle complex crustal processes and obduction kinematics in a unified pindos vourinos ophiolitic slab northern greece

comparisons of gravity anomalies at pseudofaults fracture zones and nontransform discontinuities from fast to slow spreading areas published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading we investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments we find that altimetry based gravity anomalies on the mathematician bauer easter juan fernandez and northern chile ridge pseudofaults located in 75 150 mm yr full rate seafloor spreading environments are similar in amplitude and form to atlantic fracture zones with 20 30 mm yr spreading rates a 5 15 mgal positive mantle bouguer anomaly is observed on the pseudofault bounding the eastern juan fernandez microplate comparable to those at some similar age offset nontransform discontinuities in slow spreading environments our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust the latter of which remains frozen at pseudofaults we predict that any pseudofaults with age offsets more than 1 m y and not coincident with hotspot volcanism will be associated with thin and possibly unusually dense crust even in superfast seafloor spreading environments

metalliferous sediments from the h m s challenger voyage 1872 1876 abstract the legendary cruise of h m s challenger 1872 1876 around the globe must always occupy an eminent place in the annals of oceanography as being the first systematic attempt made on a global scale to explore the ocean this expedition made fundamental discoveries in biology and geology which have not been surpassed by any later scientific cruise sediment with high content of metals later called metalliferous was among the enigmatic findings taken onboard although the nature of metalliferous sediments is well known today the very first sampled sediments of this type have not been studied to date motivated by the historical value of challenger s metalliferous sediment collection we undertook an investigation addressing two questions 1 the composition of sediments from seafloor for which we have very limited data 2 sr nd pb fe zn isotope signature of these sediments collected before the substantial human impact on the ocean during the 20th century the se pacific metalliferous sediments sampled by the challenger s explorers are of 2 types 1 metalliferous oozes blanketing ridge crests and flanks down to the calcite compensation depth ccd and 2 stripped of caco 3 metalliferous sediments located beneath the ccd in the deeps near the mid ocean ridges the abiogenic part of these sediments is composed mainly of poorly crystalline to x ray amorphous fe mn oxyhydroxides and an amorphous silicate phase these sediments have geochemical features similar to those of all the other metalliferous sediments very high fe and mn content on abiogenic basis very low al al fe mn and high content on abiogenic basis of as ba be bi cd co cu mo ni pb sb th tl u v w y zn and zr their ree distribution patterns are similar to that of deep seawater and show weak signs of hydrothermal imprint weak positive or no eu anomaly seawater and or terrigenous input from south america control the sr nd pb isotope signature of the challenger metalliferous sediments and have almost completely obliterated any original morb derived hydrothermal signal zn isotopes are mainly contributed from seawater although other zn sources hydrothermal fluid and detrital aluminosilicates barite and volcanic glass are necessary to fully explain zn isotope ratios fe isotopes indicate relatively slow fe 2 to fe 3 oxidation in the non buoyant plume thus producing relatively lighter fe isotope signature of the feooh particles that formed the studied metalliferous sediments

tectonics and evolution of the juan fernandez microplate at the pacific nazca antarctic triple junction in this study magnetic and bathymetric data from an r v endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the juan fernandez microplate at the pacific nazca antarctic triple junction magnetic lineations along the east ridge of the microplate fan to the south indicating that the nazca juan fernandez pole is near the northern end of this feature relative motions of the juan fernandez pacific nazca antarctic four plate system are determined using relative plate motion inversions the resulting tectonic model predicts that the northern boundary of the juan fernandez microplate is a zone of compression and that the west ridge and southwestern boundary are spreading obliquely the southeastern boundary is predicted to be obliquely divergent but present data fail to constrain its geometry we also present a schematic evolutionary model which relates the formation of the juan fernandez microplate to differential spreading rates at the triple junction and discuss related complexities the future evolution of the fourplate system depends on whether one or both ridges remain active

exhumation of the lower crust during crustal shortening an alice springs 380 ma age for a prograde amphibolite facies shear zone in the strangways metamorphic complex central australia foliated garnet bearing amphibolites occur within the west bore shear zone cutting through granulite facies gneisses of the strangways metamorphic complex in the amphibolites large euhedral garnet up to 3 cm occurs within fine grained recrystallized leucocratic diffusion haloes of plagioclase quartz the garnet and their haloes include a well developed vertical foliation also present in the matrix this foliation is the same as that cutting through the unconformably overlying neoproterozoic heavitree quartzite the textures indicate syn to late kinematic growth of the amphibolite facies mineral assemblages all mineral assemblages record an arrested prograde reaction history noteworthy is the growth of garnet at the expense of hornblende and plagioclase and the breakdown of staurolite hornblende to give plagioclase gedrite these dehydration reactions indicate increasing p t conditions during metamorphism and suggest heating towards the end of a period of intense deformation temperature estimates for the garnet amphibolite and related staurolite hornblende assemblages from the shear zone are about 600 c pressure is estimated at about 5 kbar an sm nd isochron gives an age of 381 7 ma for the peak metamorphism and associated deformation this age determination confirms that amphibolite facies conditions prevailed during shear zone development within the strangways metamorphic complex during the alice springs orogeny these temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient the alice springs orogeny was associated with significant crustal thickening allowing exhumation of the granulite facies palaeoproterozoic lower crust along strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the alice springs orogeny

microplate and shear zone models for oceanic spreading center reorganizations the reorganization of oceanic spreading centers is often accomplished through the process of rift propagation we first examine general rift propagation geometries for the simple two plate case in which the propagating rift instantaneously reaches full spreading rate while the other rift dies instantaneously such that the propagating and dying rifts never overlap a velocity space representation provides a framework for describing the geometry of rift propagation and its various tectonic elements and shows how both the failed rift and isochrons between the two rifts are reoriented as a consequence of purely rigid plate tectonics we then discuss the more complex geometries that result when both growing and dying rifts spread simultaneously during propagation such that a technically independent overlap region like the easter or juan fernandez plates along the east pacific rise is produced we examine the effect of the propagation rate and rise time of spreading on the evolution of overlap regions between dual oceanic spreading centers the tectonics and evolution of such regions can be modeled by assuming that they act either as rigid microplates or as zones of distributed shear rigid plate models are suggested by the reasonable success of euler vectors derived from inversions of plate motion data spreading rates transform azimuths and slip vectors both in describing available data and in predicting subsequently acquired data on the other hand recent structural data showing fabric oblique to the surrounding spreading centers might suggest shearing rather than rigid behavior in the interior of the easter microplate we use schematic evolutionary simulations for an easter style geometry to examine whether plate motion data and orientations of preexisting and present structures can be used to discriminate between rigid and shear models in a microplate model rigid plate tectonics requires that relative motion occur only along the boundaries of the overlap region while structures within the overlap zone rotate rigidly in contrast in a shear model relative motions at the boundaries differ from those predicted by rigid plate tectonics and the angular relationships of structures within the overlap region are altered by internal deformation thus inversion of relative motion data yields different results for rigid and shear behavior similarly under favorable circumstances structural trends could be diagnostic of the different cases we find that much of the easter structural data suggestive of shearing are also consistent with rigid plate tectonics we conclude that microplate models offer useful testable hypotheses for describing overlap systems and that shearing need be invoked only to explain features not describable by rigid plate tectonics furthermore the approaches used here to study current overlap regions should be useful in the analysis of similar regions formed during past oceanic plate boundary reorganizations

a major helium 3 source at 15 s on the east pacific rise an extensive plume of water enriched with helium 3 has been discovered in the deep pacific ocean at latitude 15 s on the east pacific rise in the core of the plume at a depth of 2500 meters over the ridge crest the helium 3 helium 4 ratio is 50 percent higher than the ratio in atmospheric helium indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems the helium 3 plume is completely absent east of the rise but it can be traced over 2000 kilometers to the west above a newly observed physical feature a density discontinuity here caled the ridge crest front the injected plume provides a unique deep sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west the striking intensity and lateral extent of this helium 3 anomaly compared to observations at known oceanic hydrohrmal sites suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15 s on the east pacific rise

intraplate thrusting near the easter microplate gloria long range sidescan sonar images from a region of the nazca plate immediately to the north of the easter microplate have revealed a prominent series of anomalous ridges these extend from lat 22 s long 114 w near the east pacific rise to 23 s 112 w near pito deep at the northeast corner of the microplate they are 20 to 50 km long and trend east west subparallel to the pacific nazca spreading direction swath bathymetric data indicate that the ridges dip steeply to the south and gently to the north and in places reliefs exceed 1 km earthquake focal mechanisms and predicted plate motions point to thrust faulting as the most likely mechanism for producing these ridges and their morphology suggests that the crust to the north has over ridden that to the south the nazca easter euler pole predicts a maximum present day convergence of about 50 km m y at the western end of the easter microplate9s northern boundary and we estimate that about one third of this has been taken up by the intraplate thrusts during the past 2 5 m y