easter microplate evolution geochemical evidence confirms that there is current active spreading on both the east and west rift zones of the easter microplate and northwards propagation of the east rift with the active tip at 25 s the east rift is influenced by lateral flow of plume derived material from a hotspot located beneath either sala y gomez or easter island spreading rate variation does not seem to be influential in controlling the composition of erupted basalts

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

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

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

kinematic framework of the cocos pacific plate boundary from 13 n to the orozco transform fault results from an extensive magnetic and seamarc ii survey during the summer of 1987 magnetic anomaly data were collected by surface ship as part of an extensive seamarc ii investigation of the east pacific rise epr from 13 n to the orozco transform the survey extended to either side of the rise axis onto seafloor at least 1 8 million years m y in age enabling the recent evolution of the structural and kinematic framework of the plate boundary to be studied in detail north of 13 50 n there has been a major perturbation in the evolution of the plate boundary swaths of lineaments that trend oblique to epr parallel topography form a north pointing v shaped discordant zone on the pacific and cocos plates that is broadly symmetric about the epr axis on the pacific plate a zone of discordant morphology 130 km long and between 6 and 14 km wide with a structural grain that is highly oblique to the present day spreading direction is observed on seafloor 0 9 1 8 m y in age a similar but more subtle feature of the same age is also present on the cocos plate these zones of discordant lineaments can be correlated with changes in the magnetic lineation pattern on the pacific plate the disturbed zone lies between anomalies j and 2 creating greater than normal distance between the anomalies on the cocos plate the disturbed zone is characterized by a distinct high amplitude northwestward trending magnetic anomaly the observed structural grain and the changes in the magnetic anomaly patterns associated with the disturbed zones are very similar to those observed at propagating ridges based on the magnetic anomalies a propagation rate of 10 8 cm yr in a n10 w direction is estimated for the past 1 8 m y a detailed examination of the structures developed within the disturbed zone on the pacifc plate indicates that the rift propagation in this area can best be explained by the model of wilson 1990 which involves cyclic rift failure with inward curvature of both rift tips plate adjustment to the propagation event is ongoing there is a pronounced change in the morphology of the rise axis with distance from the propagation event close to and for 100 km behind the propagator tip the epr crest is not well developed and is characterized by a series of low relief of 100 200 m ridges and troughs with a poorly defined neovolcanic zone further away 100 km from the propagator tip the rise crest is a single linear horst shaped ridge with a well developed narrow axial graben at the axis of the ridge regional morphologic and magnetic data suggest that this cycle of rift propagation may have begun approximately 2 5 million years before present m y b p near the eastern ridge transform intersection of the paleo o gorman transform and is continuing today at the orozco transform

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

a new shipboard three component magnetometer the three components of the geomagnetic field were measured at sea by stcm shipboard three component magnetometer with a relative accuracy of 50 25 nt the stcm was controlled by a microcomputer and the three geomagnetic components were calculated by reducing the magnetic field due to the ship s remanent and induced magnetic moments the only assumption being that the magnetic field is linearly related to the magnetic moments although stcm has no particular magnetic compensation device it can be set at any place on board of any kind of ship in the kh82 5 cruise the three component measurements were conducted along latitude 42 n north of the mendocino fracture zone where there are well defined magnetic anomaly lineations trending north and south stcm measured the north component anomalies with very small amplitudes compared to the east and downward component anomalies which shows that lineations trend north and south the three measured component anomalies are almost identical to the anomalies

sea beam deep tow investigation of an active oceanic propagating rift system galapagos 95 5 w we have tested and corroborated the propagating rift hypothesis with high resolution sea beam and deep tow data collected over the galapagos 95 5 w propagating rift system the propagating rift is continuously breaking through the cocos plate at a velocity of about 50 km m y with an azimuth of about 273 away from the galapagos hotspot this process transfers cocos lithosphere to the nazca plate including the preexisting spreading center which fails as it is gradually preempted by the propagating rift the spreading center azimuth is being changed by about 8 clockwise by this rift propagation the active propagating and failing rift axes overlap by about 15 km and are joined by a broad and anomalously deep zone of nonrigid plate deformation rather than by a transform fault although rift propagation is continuous on a scale of a few kilometers 50 000 years and lithospheric transferral from one plate to the other occurs mostly continuously as the preexisting rift fails it breaks into discrete segments the geometry of seafloor fabric and tectonic elements such as active and failed rifts the v shaped pattern of pseudofaults and the oblique tectonic fabric in the sheared zone of transferred lithosphere are observed as predicted this quantitative verification provides strong support for the propagating rift hypothesis

magnetic and tectonic studies of the dueling propagating spreading centers at 20 40 s on the east pacific rise evidence for crustal rotations we present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20 40 s on the east pacific rise a majority of the data used were collected during a cruise aboard the r v moana wave during which continuous seamarc ii coverage was obtained these data were combined with additional data to produce an anomaly map which extends to anomaly 2 aged crust a three dimensional inversion in the presence of bathymetry was carried out for the area the resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity the results indicate consistent asymmetric spreading faster to the east discontinuous high magnetizations in the discordant zone associated with the discontinuity and southward migration of the feature at a rate of 90 100 mm yr between jaramillo and brunhes time 0 95 to 0 73 ma with slowing during the brunhes to less than 10 mm yr there is also an overlapping jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the nazca to the pacific plate in addition areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector results from this second model support the presence of highly rotated pre brunhes nazca crust within brunhes pacific crust which has been deformed by bookshelf faulting this indicates at least two episodes of crustal transfer from the nazca plate to the pacific plate the discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the nazca plate between the discontinuity and the easter microplate to the south the detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges

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

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

geochemical evidence for low magma supply and inactive propagation at the galapagos 93 25 w overlapping spreading center 1 major and trace element concentrations sr nd and pb isotopic ratios and plagioclase and olivine compositions of samples from 92 35 w to 94 10 w on the galapagos spreading center gsc constrain magmatic processes associated with the 93 25 w overlapping spreading center osc significant variations in parental magma compositions and extent of low pressure fractionation over short along axis distances suggest small discontinuous or poorly mixed magma chambers mineralogical petrological and geochemical data indicate this osc is characterized by low overall melt supply regional along axis geochemical gradients along the western gsc e g in 206pb 204pb nb zr and la 8 sm 8 are interrupted by increased variability within the osc consistent with less efficient mixing of melts in this region despite a long term history of westward propagation bathymetric and geochemical data are inconsistent with recent propagation of this offset the eastern limb appears to have recently retreated and left behind a series of abandoned ridges within the overlap zone the eastern limb shows two progressive westward trends of decreasing ratios of highly to moderately incompatible elements one outside and one within the overlap zone the overlapping portion of the western limb also has low ratios of these elements these patterns are consistent with previously depleted mantle being preferentially melted beneath this offset

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

hot spot ridge crest convergence in the northeast pacific evolution of the juan de fuca ridge during the past 7 m y has been reconstructed taking into account both the propagating rift history and migration of the spreading center in the absolute fixed hot spot reference frame northwestward migration of the spreading center at a rate of 30 km m y has resulted in progressive encroachment of the ridge axis on the cobb hot spot and westward jumping of the central third of the ridge axis more recently than 0 5 ma seamounts in the cobb eickelberg chain are predicted to display systematic variations in morphology and petrology and a reduction in the age contrast between the edifice and underlying crust as a result of the ridge axis approach relative seamount volumes also indicate that magmatic output of the hot spot varied during this interval with a reduction in activity between 2 5 and 4 5 ma compared with relatively more robust activity before and after this period spatial relationships determined in this reconstruction allow hypotheses relating hot spot activity and rift propagation to be evaluated in most cases rift propagation has been directed away from the hot spot during the time period considered individual propagators show some reduction in propagation rate as separation between themore propagating rift tip and hot spot increases but cross comparison of multiple propagators does not uniformly display the same relationship no obvious correlation exists between propagation rate and increasing proximity of the hot spot to the ridge axis or increasing hot spot output taken together these observations do not offer compelling support for the concept of hot spot driven rift propagation however short term reversals in propagation direction at the cobb offset coincide with activity of the heckle melting anomaly suggesting that local propagation effects may be related to excess magma supply at the ridge axis less

comprehensive analysis of marine magnetic vector anomalies we present a comprehensive description of the analysis of marine magnetic vector anomalies the marine magnetic vector field can be measured with a shipboard three component magnetometer stcm the stcm has the advantage of measuring three components but it suffers from several types of noise sources because the principles of its measurement are more complex than that of a proton precession magnetometer we have estimated the quality of stcm data constructed an optimal filter and evaluated the reliability of the filtered data then we have organized an analytical procedure composed of four steps 1 the discrimination between two and three dimensional magnetic sources 2 the determination of magnetic boundaries 3 the calculation of the boundary strikes and 4 the estimation of the boundary magnetization contrasts we show examples of these procedures using the data near the east pacific rise 28 s 31 s collected during leg 5 of the gloria expedition all the results of the above processing are merged in order to illustrate the distinct pattern of the seafloor magnetization in the surveyed area

history of rift propagation and magnetization intensity for the cocos nazca sspreading center analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the cocos nazca spreading center forward models of the magnetic anomalies accounting for spreading rate variations ridge jumps asymmetric spreading magnetization intensity variations and bathymetry show excellent agreement with observed anomalies spreading rates can be constrained to a common finite rotation history through anomaly 4a with three changes in rates rate changes at about 1 5 ma and 4 1 ma correspond to changes in rate gradients and occur during the well calibrated part of the reversal timescale so they can unquestionably be identified as true changes in plate motion a 15 rate decrease at about 5 2 ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale the change at 4 1 ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0 6 ma rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis propagation has been predominantly away from the hotspot with jumps predominantly south ward propagation rates have ranged from 30 to 120 mm yr commonly near 70 mm yr origin of most propagation sequences is difficult to interpret but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot magnetic anomaly amplitude appears to be a reliable tracer of fe content of lavas several generalizations can be drawn about along axis variations in magnetization intensities since 8 ma high magnetizations are only observed at the far ends relative to the galapagos hotspot of segments at least 150 km long offset at the end of a high magnetization segment is at least 15 km and there are no offsets larger than 30 45 km between high magnetization segments and the reconstructed position of the hotspot we interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along axis flow of evolved magma the gradients in supply result from subaxial flow of hotspot derived asthenosphere in a narrow conduit this flow is only partly obstructed by an offset of 20 30 km but entirely blocked by an offset of 50 km

sensitivity of sediment geochemical proxies to coring location and corer type in a large lake implications for paleolimnological reconstruction we compared a suite of geochemical proxies in sediment cores collected in 1982 1988 1991 and 2003 from sites near the depocenter of lake erie to evaluate the reliability of paleoenvironmental reconstructions derived from lacustrine sediments our proxies included the concentrations and carbon isotopic compositions of organic and inorganic carbon toc caco3 13corg and 13ccaco3 augmented by organic c to total n ratios corg ntot 15n and carbonate 18o values 18ocaco3 the three coring sites were clustered within 12 km two types of corers a box corer and a benthos gravity corer were used for the 1991 sampling campaign the variance of most proxies was accounted for not only by temporal environmental changes but also by coring locations and corer type indicating that sediment spatial heterogeneity and differences in sediment recovery due to the use of different corers also played a part in determining the geochemical compositions of these cores the toc 13corg and 13ccaco3 values showed decadal temporal patterns that were consistent between the multiple sampling campaigns in contrast the 15n corg ntot caco3 and 18ocaco3 exhibited across core differences in their temporal variations making it difficult to extract consistent environment information from different cores our findings suggest that in addition to temporal environmental changes high resolution paleolimnological reconstruction is sensitive to many factors that could include spatial sediment heterogeneity discontinuous sedimentation processes bioturbation sediment dating uncertainty and artifacts associated with analytical and coring procedures therefore multiple core sampling and analysis are important in reliably reconstructing environmental changes particularly for large heterogeneous lacustrine basins

40ar 39ar dating of the brunhes matuyama geomagnetic field reversal magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale gpts to date events in the range 0 to 5 million years ago a critical tie point on the gpts is the potassium argon age of the most recent brunhes matuyama geomagnetic field reversal astronomical values for the forcing frequencies observed in the oxygen isotope record in ocean drilling project site 677 suggest that the age of this last reversal is 780 ka thousand years ago whereas the potassium argon based estimate is 730 ka results from 4039 ar incremental heating studies on a series of lavas from maui that straddle the brunhes matuyama reversal give an age of 783 11 ka in agreement with the astronomically derived value the astronomically based technique appears to be a viable tool for dating young sedimentary sequences

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

propagating rift model for the v shaped ridges south of iceland 1 we present new marine geophysical data which constrain the seafloor spreading history of the reykjanes ridge near iceland and the origin of its flanking v shaped topographic and gravity ridges contrary to the geometry assumed in pulsing plume models the v shaped ridges are not symmetric about the reykjanes ridge axis and seafloor spreading has not been symmetric about a stable axis thus existing models must at least be modified to include an additional asymmetry producing mechanism the best understood and documented such mechanism is rift propagation one possibility is that plume pulses drive the propagators however rift propagation also produces v shaped wakes with crustal thickness variations suggesting the possibility that a pulsing iceland plume might not be necessary to explain the reykjanes v shaped ridges scarps and troughs

the serpukhovian in britain use of foraminiferal assemblages for dating and correlating analysis of foraminiferal assemblages from the visean serpukhovian boundary interval in britain has led to the recognition that levels correlated with the first occurrence of lochriea ziegleri at the base of the serpukhovian stage can be established by the foraminifer neoarchaediscus gregorii in england and scotland which first occurs from the single post and cockleshell limestones in northern england and laterally equivalent levels as well as asteroarchaediscus bashkiricus and a rugosus except for south wales contrary to some previous studies the base of the serpukhovian does not equate with the base of the namurian pendleian substage but lies approximately at the early late brigantian boundary based on the first occurrence of neoarchaediscus postrugosus four foraminiferal assemblages are distinguished in the early serpukhovian 7 10 and four assemblages in the late serpukhovian 11 14 despite the contrast in facies it is now recognised for the first time that throughout the midland valley of scotland northern england and south wales the foraminiferal assemblages from shallow water platform facies are completely compatible with the ammonoid subzones from deep water basinal facies with no apparent mismatches there appears to be close comparability of foraminiferal assemblages and first appearance datums of marker species with most of the international foraminiferal zonal schemes in russia supplementary material https doi org 10 6084 m9 figshare c 5240408

the geometry of propagating rifts abstract the kinematics of two different processes are investigated both of which have been described as rift propagation courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin the term localisation is instead used here to describe this process to distinguish it from hey s type of propagation localisation generally leads to rotation of the direction of magnetisation to hey propagation means the extension of a rift into the undeformed plate beyond a transform fault detail surveys of the galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault but by a zone which is undergoing shear the principal deformation is simple shear and the kinematics of this deformation are investigated in some detail the strike of most of the lineations observed in the area can be produced by such deformation the mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth

deep sea magnetic vector anomalies over the hakurei hydrothermal field and the bayonnaise knoll caldera izu ogasawara arc japan 1 we conducted deep sea magnetic measurements using autonomous underwater vehicles in the bayonnaise knoll caldera the izu ogasawara island arc which hosts the large hakurei hydrothermal field we improved the conventional correction method applied for removing the effect of vehicle magnetization thus greatly enhancing the precision of the resulting vector anomalies the magnetization distribution obtained from the vector anomaly data shows a 2 km wide belt of high magnetization trending nnw sse going through the caldera and a low magnetization zone 300 m by 500 m in area extending over the hakurei site comparison between the results obtained using the vector anomaly and the total intensity anomaly shows that the magnetic field is determined more accurately especially in areas of sparse data distribution when the vector anomaly rather than the total intensity anomaly is used we suggest a geologically motivated model that basaltic volcanism associated with the back arc rifting occurred after the formation of the caldera resulting in the formation of the high magnetization belt underneath the silicic caldera the hakurei hydrothermal field lies in the intersection of the basaltic volcanism belt and the caldera wall fault suggesting a mechanism that hot water generated by the heat of the volcanic activity has been spouting out through the caldera wall fault the deposit apparently extends beyond the low magnetization zone climbing up the caldera wall this may indicate that hot water rising from the deep through the alteration zone is transported laterally when it comes near the seafloor along fissures and fractures in the caldera wall

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

extension of the astronomically calibrated polarity time scale to the miocene pliocene boundary the early pleistocene to late pliocene astronomically calibrated time scale of shackleton et al 1 and hilgen 2 is extended to the miocene pliocene boundary this is done by correlating the detailed record of caco3 cycles in the trubi and the lower part of the overlying narbone formation rossello composite section sicily to the astronomical record using 1 inferred phase relations between these caco3 cycles and the corresponding orbital cycles of precession and eccentricity and 2 calibration points provided by the previously established astronomical calibration of sapropelitic layers which occur in the topmost part of the caco3 record 2 this correlation allows all small scale caco3 cycles in the rossello composite section to be coded after the correlative precession peak and to be dated astronomically with an accuracy of 1 ky in combination with the available high resolution magnetostratigraphy it further allows the construction of an astronomically calibrated geomagnetic polarity time scale for the major part of the gilbert and gauss chrons this new time scale gives ages for the reversal boundaries of the kaena 3 04 3 11 ma and mammoth 3 22 3 33 ma the gilbert gauss chron boundary 3 58 ma and the reversal boundaries of the cochiti 4 18 4 29 ma nunivak 4 48 4 62 ma sidufjall 4 80 4 89 ma and thvera 4 98 5 23 ma these ages are significantly older than the ages given by conventional time scales age discrepancies gradually increase with increasing age the age of 5 32 ma for the miocene pliocene boundary as defined by the base of the trubi on sicily is almost 0 5 m y 9 5 older than the presently accepted age of 4 86 ma 3 these considerable discrepancies with the conventional time scale can either be explained by an consistent error in the decay constants used in k ar radiometric dating or more likely by diffusional loss of radiogenic argon

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

mantle viscosity beneath the galapagos 95 50 w propagating rift detailed geophysical surveys in the vicinity of the galapagos 95 5 w propagating rift tip establish the opening history of the rift and its velocity of propagation these data together with a theory for mantle upwelling into slowly widening lithospheric cracks constrain the viscosity of the asthenosphere beneath the propagating rift to be less than about 1017 to 1018 pa s

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

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

the east ridge system 28 5 32 s east pacific rise implications for overlapping spreading center development abstract we report here on geophysical data from the east ridge and surrounding areas of the large offset overlapping spreading centers oscs that accommodate pacific nazca opening between 28 5 and 32 s the east ridge overlaps and is offset from the west ridge system by 120 km forming the largest known pair of oscs in this area spreading rates reach the fastest currently active on earth of 149 mm yr although the east ridge is composed of 4 morphologically defined segments separated by 3 small oscs other geophysical characteristics imply 1 upwelling segment all the active ridge segments in this area including the propagating tips of the east and west ridges form relative topographic highs with respect to the flanking sea floor however identified abandoned ridge tips form deeps we interpret these data in terms of a model in which the propagating segment represents an overshoot of a surficial rupture of the brittle lithospheric layer only partially coupled to the diverging flow of a more broadly distributed ductile deformation zone ddz surrounding the steady state ridges and crossing the offset between the oscs the topographic high of the propagating segment may be maintained primarily by along axis melt migration from the stable spreading segments rather than by direct upwelling from beneath the ridge the large overlapping ridges are inherently unstable and continued extension causes the overlapping axes to become offset from the stably spreading segments cut off from the supply of melt and replaced by a new set the failed rift tips for a period of time overlie the broad ddz and preferentially undergo continued extension and subsidence the ddz surrounding the ridge axes may be very broad in this area because of the very fast spreading rate creating a very thin lithosphere susceptible to perturbation by relatively small mantle heterogeneities advected near the ridge axis leading to the formation of the smaller oscs observed

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 detailed study of the cobb offset of the juan de fuca ridge evolution of a propagating rift the cobb offset on the northern portion of the juan de fuca ridge has been identified as the tip of a northward propagating rift hey and wilson 1982 map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the offset and permit detailed modeling of the recent evolution within this transform zone the axis of recent spreading on the southern ridge segment bends from the normal ridge trend n20 e to a n s trend north of 47 15 n the spreading axis on the northern ridge segment generally defines a n20 e trend except at the southern terminus where the spreading center is offset slightly to the east the two spreading centers overlap by about 33 km in the offset vicinity and there is evidence of recent volcanism on both segments present ridge axis morphology exhibits a transitional sequence from a symmetrical axial high along the more normal portions of each ridge segment to a grabenlike depression as the tip is approached the magnetic anomaly patterns observed in the cobb offset vicinity are not consistent with the patterns predicted by models of continuous northward propagation the magnetic anomaly patterns of the brunhes epoch require an event of rapid northward propagation about 0 7 m y b p followed by a more gradual southward propagation in the middle brunhes epoch most recently the spreading center on the southern ridge has extended northward to its present configuration prior to the brunhes epoch modeling of the magnetic anomaly patterns does not indicate a unique solution however net propagation has been northward we present alternative models for the period beginning 1 7 m y b p in the first model the cobb offset has evolved by a series of northward and southward events of propagation with net advance to the north in the second model stable asymmetric spreading from overlapping ridge segments has evolved into a transform fault offset by gradual decay of the northern ridge both models indicate that propagation has been discontinuous and relatively rapid compared to spreading rate and that realignment of ridge segments near the propagator tip has been an important component

tectonics of the southern tip of the parece vela basin philippine sea plate we report new geophysical and petrological data collected at the southern tip of the parece vela basin in the philippine sea the parece vela basin which was formed as a backarc basin behind proto mariana arc trench system from late oligocene to middle miocene provides us a good opportunity to study the nature of successive backarc basin formations in the philippine sea and the relationship between are and backarc magmatisms regional bathymetric map derived from satellite altimetry shows that the southern tip of the basin now located just west of the yap arc trench system has unique morphological and tectonic features which include 1 the absence of spreading center or its trace 2 shallow average depth and 3 enigmatic curved structures our newly collected high resolution bathymetric data reveal that the spreading fabric similar to the central parece vela basin exists to the north of 9 degrees 20 n thus it appears that the present day yap arc and backarc region represent the western half of the seafloor that was produced by the early e w and the following ne sw spreading in the northern and central parece vela basin and that the eastern counterpart now lies west of the west mariana ridge unlike the northern parece vela basin there appears to be no evidence for a systematic propagation of spreading center in the southern part instead two rift segments one which extends from the central parece vela basin and the other which lies within the western remnant arc kyushu palau ridge overlap at the southern tip of the basin producing a complex seafloor that includes curvilinear deeps and deformed topographic highs c 2007 elsevier b v all rights reserved

kinematics of overlapping rift propagation with cyclic rift failure existing kinematic models for propagation of oceanic spreading ridges that incorporate overlap between ridge segments fail to describe detailed observations of the failed segments i present a new model which discards the assumption of steady state behavior of the failing rift permitting inward curvature of both rift tips in the overlap region the shape of an inward curving failing rift must continuously change but is assumed to cyclically return to its original shape by discrete inward ridge jumps other assumptions of symmetric spreading and uniform simple shear deformation between the overlapping rift tips are retained from previous models inward curvature of failed rift structures provides much better agreement with observations and is consistent with tensile fracture theory if the offset between ridge segments is small enough the inward jumps of the failing rift will cut across deformed structures originally formed at the propagating tip possibly generating seafloor fabrics that crosscut each other at nearly right angles observations of such structures near the gorda ridge can be explained by a model incorporating variable cyclieity of the failing rift chrons curve inward toward the ridge faithfully recording the shape of the propagating tip in an undeformed part of the plate isochrons also curve toward the ridge along the inner pseudofault but in the original formulation of the rules of plate tectonics transform faults were assumed to be fixed relative to each other and the ridge segments they offset the propagating rift model 1 which discards this assumption has been very successful in describing oblique offsets in magnetic anomalies as a result of the lengthening of a ridge segment through time at the expense of the adjacent segment detailed observations of active propagating rifts suggest that the assumption that ridges are necessarily offset by discrete transform faults should also be discarded with the segments instead offset by zones of distributed shear with widths of 10 or more km 2 3

overlapping spreading centers implications from crack growth simulation by the displacement discontinuity method overlapping spreading centers osc s are a fundamental aspect of accretionary processes at intermediate and fast spreading centers and typically occur at deep points along the axial depth profile they have a characteristic geometry consisting of two en echelon overlapping curving ridges separated by an elongated depression the length to width ratio of this overlap basin is typically 3 1 we have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity a boundary element displacement discontinuity method was used to solve this problem our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region for small osc s there is a very good agreement between calculations and observations but for large ones the overlap basin tends to be longer than our predicted results indicate this suggests that the assumptions made in the model i e perfectly elastic isotropic and homogeneous medium are perhaps valid for the brittle lid above the magma chamber that underlies osc s with small offsets 2 km but oversimplified for osc s with large offsets our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach the deflection will be the greatest for small misalignments of the fracture systems thus even minor misalignments of the spreading centers may result in the development of osc s where the misalignment is less than the width of the cracking front the fracture systems may meet head on creating a saddle point along the axial depth profile our results support the hypothesis suggested by macdonald et al 1984 in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned

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

morphotectonic fabric of the orozco transform fault results from a sea beam investigation we have used data collected with the sea beam system to produce a detailed regional bathymetric map of the medium to fast slipping 90 mm yr orozco transform fault the morphotectonic fabric of the 90 50 km transform domain is spatially heterogeneous and is composed of relatively shallow areas 3200 m the plate boundary geometry is clearly identified morphologically and is characterized by a series of linked basins and troughs that approximate a zed pattern in plan view east west trending basins define the northern and southern boundaries of the transform domain and these troughs are thought to mark the location of strike slip tectonism a central north south trending ridge with an axial valley called here the mid orozco spreading center is thought to represent a short 30 km ridge segment that links two transform faults morphotectonic relationships developed along the southeastern flank of the survey area suggest that this plate boundary geometry has recently become unstable and is presently undergoing a transformation as the tip of the east pacific rise axis propagates northward across the transform domain bypassing the mid orozco spreading center if the southern ridge axis continues to propagate northward it will link with the aseismic extension of the northern strike slip strand and the present day plate boundary will evolve into a simple ridge transform ridge r t r geometry with an offset of approximately 70 km

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

possible perturbations to hotspot traces and implications for the origin and structure of the line islands perturbations to hotspot signals recorded on the surface of the earth will produce hotspot traces with characteristics more complex than those of simple time progressive volcanic chains some of these perturbations are a consequence of normal plate tectonics some are observed interactions of hotspot magmatism with lithospheric structure and processes and others are hypothetical the line islands volcanic chain has a complex structure and some characteristics that cannot be reconciled with those expected of a simple time progressive hotspot trace two models for the development of the line islands are presented model 1 assumes that the line islands were formed between 130 and 67 ma and model 2 assumes an age span of 90 to about 15 ma and incorporates the tuamotu islands as an extension of the line islands in model 1 most of the complex structure and subsidence of the chain can be explained in terms of perturbations to the hotspot signal model 2 gives a good geometric fit to the volcanic chain but fails to explain the anomalous subsidence of the line islands

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

propagating rift explanation for the tectonic evolution of the northeast pacific the pseudomovie abstract we have developed a new computer graphic technique to use in reconstructing plate tectonic evolution and to make movies illustrating this evolution using this technique we have deciphered the evolutionary history of the juan de fuca area in the northeast pacific and show that a combination of seafloor spreading and rift propagation can explain this seemingly complex area as a simple consequence of the interaction of two rigid plates

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

tectonic history of the juan de fuca ridge over the last 40 million years interpretation of a new compilation of magnetic anomaly data for north of the mendocino fracture zone and younger than chron 15 39 ma indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation reorganizations of the propagation pattern occurred at 30 ma in response to a counterclockwise change in direction of relative motion and at 19 ma in response to a clock wise change in addition there are significant changes in the gradient of the spreading rate apparently without major changes in motion direction at 27 and 24 ma the change at 30 ma is apparently the result of the separation of the juan de fuca plate from the rest of the farallon plate along the pioneer fracture zone the pattern of absolute motion after 30 ma does not generally support menard s pivoting subduction model and suggests that juan de fuca plate motion is being significantly affected by forces transmitted across the mendocino transform fault

geomorphology and structural segmentation of the crest of the southern pacific antarctic east pacific rise geomorphology of the boundary between pacific and antarctic plates was mapped with a sea beam multibeam echosounder and a seamarc ii bathymetric side scan sonar from the southern end of juan fernandez microplate at 35 s to heezen transform at 56 s there are six spreading center systems separated by two large left stepping nontransform offsets at 36 5 s and 41 5 s two right stepping transform fault systems and a left stepping hybrid structure with equally long strike slip and nontransform offsets axial rift zones spreading axes are generally within 1 of normal to the relative motion predicted by current plate rotation models most axial rift zones crop out along the crests of typical east pacific rise epr axial ridges which commonly have flat rather than humped long profiles about 2 of this fast spreading 84 100 mm yr rise crest has an axial rift valley instead of an axial ridge the longest rift valley segment is midway between menard and vacquier transforms near 51 s where an anomalously deep rise crest may mark a zone of below average mantle upwelling nontransform offsets structurally similar to those on the tropical epr but mostly formed by differential asymmetric spreading subdivide the four longest spreading center systems on the northern transform free two thirds of the rise net rift propagation at migrating offsets has been into the faster pacific plate this causes right steps to migrate north left steps to migrate south and crust to be transferred to the east flank this pattern has prevailed for several million years judging from the oblique fracture zones pseudofaults mapped on the rise flanks with older marine data and geosat altimetry on this rise crest the difference in stress on two plates with very different velocities may control the direction of offset migration

seamarc ii survey of a propagating limb of a large nontransform offset near 29 s along the fastest spreading east pacific rise segment seamarc ii side scan and bathymetric data combined with magnetic anomaly and sea beam bathymetric data define a large 90 120 km nontransform offset of the east pacific rise epr between the easter and juan fernandez microplates near 29 s where one or more transforms had been suggested by the seismicity pattern on the basis of morphology structural and tectonic features three major regions are delineated an eastern spreading axis a western spreading axis and a central transfer zone these two spreading axes overlap by at least 35 km and possibly as much as 100 km the eastern axis is characterized by an axial ridge that narrows and deepens toward the tip 28 38 s 112 08 w approaching the tip the spreading axis azimuth changes from approximately n5 e to n25 w and axial magnetic anomaly values increase to over 700 nt 37 km south of the rift tip a pair of deep 600 m bathymetric troughs on either side of the ridge axis trending n11 w and n44 w converge on the ridge tip and define the outermost boundaries of ridge parallel structures these structures are interpreted as pseudofaults resulting from northward rift propagation the western spreading axis is approximately 2400 m deep and corresponds to a young volcanic area the central lithospheric transfer zone west of the inner pseudofault of the eastern spreading axis is characterized by oblique abyssal hill and magnetic fabric interpreted to result from shear between overlapping spreading centers and a dispersed pattern of teleseismic earthquake epicenters the eastern spreading axis is propagating rapidly northward at up to 265 km m y based on a 158 km m y spreading rate and the 33 angle between the pseudofaults however complex patterns of seafloor fabric which differ from that predicted by the simple propagating rift model suggest that multiple propagation events may have occurred furthermore southward propagation of the western limb may be dominant in the long term the absence of transform faults along this superfast spreading epr segment documents that transforms do not presently exist at such superfast spreading rates

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

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

pliocene change in pacific plate motion the absolute motion of the pacific plate has been nearly constant during the past 15 myr but epp1 recognized that some change took place between 15 myr and the present and tentatively chose 1 myr for the time of the change cox and engebretson2 found a change in pacific absolute motion based on hotspot tracks and a change in pacific antarctic relative motion the pliocene deformation in california and the juan de fuca rotations led them to postulate a 5 myr age for the change the manner and timing of the change are explored more thoroughly here and we put the time for the change in motion of the pacific plate at between 5 and 3 2 myr basing the younger limit on the correlation of computed hotspot tracks with pacific and nazca hotspot traces the change may explain several observations on the neighbouring continental and oceanic plate boundaries including direct adjustments at the pacific spreading ridges a likely mechanism for the change is a large clockwise torque caused by acceleration of the sinking slab beneath the kurile trench where a fossil spreading ridge is inferred to have been subducted

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

evidence from earthquakes for bookshelf faulting at large non transform ridge offsets migrating non transform offsets which occur along mid ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring doomed rift segment1 3 represent a significant modification of the plate tectonic paradigm4 the migrating offset zone between the two spreading segments is clearly a locus of significant deformation but the mechanism of this deformation has been controversial here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2 4 7 earthquakes at large non transform offsets at medium and fast spreading rates show strike slip mechanisms with nodal planes rotated relative to the expected transform fault orientation one set of nodal planes presumably corresponding to the fault planes is parallel to curved sea floor lineaments which show increased rotation as a function of position in the deforming zone a bookshelf faulting model in which initially ridge parallel faults are rotated by simple shear is consistent with the observed lineament orientations focal mechanisms and earthquake distributions