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

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

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

easter microplate dynamics 1 we use two dimensional elastic finite element analysis supplemented by strength estimates to investigate the driving mechanism of the easter microplate modeled stresses are compared with the stress indicators compiled from earthquake focal mechanisms and structural observations the objective is to constrain the tectonic forces that govern the easter microplate rotation and to test the microplate driving hypothesis proposed by schouten et al 1993 we infer that the mantle basal drag cannot drive the microplate rotation but opposes it and that the asthenospheric viscosity is no more than about 1 1018 pa s at most the basal drag comprises 20 of the force resisting microplate rotation the outward pull of the main plates can drive the rotation by shear drag applied along the northern and southern boundaries of the microplate however we propose an additional driving force which arises from the strong variation of the ridge resistance force along the east and west rifts so that the main driving torques come from the pull of the major plates acting across the narrowing and slowing rifts this requires the strength to increase substantially toward the rift tips due to thickening of the brittle lithosphere as the spreading rate slows

statistical physics seismogenesis and seismic hazard the scaling properties of earthquake populations show remarkable similarities to those observed at or near the critical point of other composite systems in statistical physics this has led to the development of a variety of different physical models of seismogenesis as a critical phenomenon involving locally nonlinear dynamics with simplified rheologies exhibiting instability or avalanche type behavior in a material composed of a large number of discrete elements in particular it has been suggested that earthquakes are an example of a self organized critical phenomenon analogous to a sandpile that spontaneously evolves to a critical angle of repose in response to the steady supply of new grains at the summit in this stationary state of marginal stability the distribution of avalanche energies is a power law equivalent to the gutenberg richter frequency magnitude law and the behavior is relatively insensitive to the details of the dynamics here we review the results of some of the composite physical models that have been developed to simulate seismogenesis on different scales during 1 dynamic slip on a preexisting fault 2 fault growth and 3 fault nucleation the individual physical models share some generic features such as a dynamic energy flux applied by tectonic loading at a constant strain rate strong local interactions and fluctuations generated either dynamically or by fixed material heterogeneity but they differ significantly in the details of the assumed dynamics and in the methods of numerical solution however all exhibit critical or near critical behavior with behavior quantitatively consistent with many of the observed fractal or multifractal scaling laws of brittle faulting and earthquakes including the gutenberg richter law some of the results are sensitive to the details of the dynamics and hence are not strict examples of self organized criticality nevertheless the results of these different physical models share some generic statistical properties similar to the universal behavior seen in a wide variety of critical phenomena with significant implications for practical problems in probabilistic seismic hazard evaluation in particular the notion of self organized criticality or near criticality gives a scientific rationale for the a priori assumption of stationarity used as a first step in the prediction of the future level of hazard the gutenberg richter law a power law in energy or seismic moment is found to apply only within a finite scale range both in model and natural seismicity accordingly the frequency magnitude distribution can be generalized to a gamma distribution in energy or seismic moment a power law with an exponential tail this allows extrapolations of the frequency magnitude distribution and the maximum credible magnitude to be constrained by observed seismic or tectonic moment release rates the answers to other questions raised are less clear for example the effect of the a priori assumption of a poisson process in a system with strong local interactions and the impact of zoning a potentially multifractal distribution of epicentres with smooth polygons the results of some models show premonitory patterns of seismicity which could in principle be used as mainshock precursors however there remains no consensus on both theoretical and practical grounds on the possibility or otherwise of reliable intermediate term earthquake prediction

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

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

a mechanism for lack of sea ice reversibility in the southern ocean we find evidence that ocean processes during global warming may result in irreversible changes to the antarctic sea ice whereas the arctic sea ice changes appear to be reversible increased forcing gives rise to strong heat uptake in the southern ocean and existing pathways provide an increased transport of heat to the weddell sea as atmospheric concentrations of co2 are returned to preindustrial levels the antarctic ice extent at first recovers but a rapid change in the position of the an ocean front in the south atlantic maintains the heat transport into the weddell sea a cooling surface initiates deep convection accessing the stored heat resulting in a substantial loss of sea ice which has not recovered after a further 150 years at preindustrial co2

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

the mesozoic tectonic history of the magellan microplate in the western central pacific magnetic lineation mapping in the western central pacific has revealed a pair of opposite sensed fanned lineation patterns that define the accretionary boundaries of the fossil magellan microplate this tectonic synthesis results from extensive magnetic mapping of two new lineation patterns over a large area and extended mapping of previously identified lineations the entire evolutionary history of the magellan microplate is well constrained to a 9 m y period in the early cretaceous by synchronous spreading patterns and associated geologic data during this period the microplate grew and evolved as a generally rectangular structure to a final size of 700 km 600 km with spreading centers on two opposing sides and transform faults on the other two sides the lifetime and size of the magellan microplate are somewhat longer and larger respectively than presently active microplates on the east pacific rise however these modern structures are still evolving and growing and the tectonic behavior of the modern and cretaceous systems appears to be similar study of both active and fossilized microplates should provide additional insights on their common tectonic histories in particular we show that the magellan trough spreading center behaved as an asymmetric accretionary plate boundary that can be described with two separate poles of motion very close to this spreading center during much of its history the magellan trough spreading center then failed as a result of a larger ridge reorganization at the triple junction of the pacific farallon and phoenix plates at ml0n time microplate activity ceased when the microplate became welded to the pacific plate at m9 time

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

formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate application to the 95 5 w galapagos propagator the geometry of oceanic spreading centers can change by rift propagation in which a new ridge segment propagates and preempts a preexisting one as the locus of relative motion between the two plates shifts the dying and growing ridge segments are joined by a boundary usually a transform which migrates with time causing seafloor to be transferred from one plate to the other as a result originally ridge parallel isochrons in a zone between the growing and failed rifts can be reoriented to trends oblique to both the ridge and spreading direction an attractive feature of this model is that the reorientation occurs by strictly rigid plate tectonics since relative motion occurs only between points on different plates recent surveys of the galapagos propagating rift system at 95 w show seafloor fabric curving away from the growing rift and toward the failed rift these observations have been interpreted as requiring a zone of distributed simple shear between the two rifts such behavior would represent a local deviation from rigid plate tectonics since relative motion would be distributed over a finite region not part of either of the two plates here we explore possible rigid plate models for the formation of curved seafloor lineaments by rift propagation we find that such fabric can be generated by changes either in the velocity rate or direction of rift propagation or in the spreading rate during rift propagation the curvature of various tectonic elements is diagnostic of the different possibilities the geometry observed at the galapagos can result from either rift propagation acceleration or a spreading rate decrease during the last few hundred thousand years the reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate propagator acceleration and spreading rate slowing which produce similar structural trends could in principle be distinguished using spreading rate data at galapagos however since either would have occurred within the present period of normal polarity both are consistent with the magnetic anomaly data we thus conclude that the data interpreted as requiring a shear zone are equally consistent with two distinct models based on rigid plate tectonics it may however be possible to discriminate between rigid and shear models using fabric observations at other propagators the shear model requires that the sense of curvature at galapagos be a ubiquitous feature of active propagators in contrast the rigid plate tectonic model allows for fabric with a variety of orientations including reverse curvature formed by either propagation deceleration or increasing spreading rate and linear fabric formed by constant propagation and spreading velocities

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

controls on subsurface methane fluxes and shallow gas formation in baltic sea sediment aarhus bay denmark shallow gas accumulates in coastal marine sediments when the burial rate of reactive organic matter beneath the sulfate zone is sufficiently high and the methanogenic zone is sufficiently deep we investigated the controls on methane production and free methane gas accumulation along a 400 m seismo acoustic transect across a sharp transition from gas free into gas bearing sediment in aarhus bay denmark twelve gravity cores were taken in which the pore water was analyzed for inorganic solutes while rates of organic carbon mineralization were measured experimentally by so42 s 35 radiotracer method the thickness of organic rich holocene mud increased from 5 to 10 m along the transect concomitant with a shallowing of the depth of the sulfate methane transition from 4 m to 2 5 m in spite of drastic differences in the distribution of methane and sulfate in the sediment along the transect there were only small differences in total mineralization and methanogenesis was only equivalent to about 1 of sulfate reduction shallow gas appeared where the mud thickness exceeded 8 9 m rates of methanogenesis increased along the transect as did the upward diffusive flux of methane interestingly the increase in the sedimentation rate and holocene mud thickness had only a modest direct effect on methanogenesis rates in deep sediments this increase in methane flux however triggered a shallowing of the sulfate methane transition which resulted in a large increase in methanogenesis at the top of the methanogenic zone thus our results demonstrate a positive feedback mechanism that causes a strong enhancement of methanogenesis and explains the apparently abrupt appearance of gas when a threshold thickness of organic rich mud is exceeded c 2016 elsevier ltd all rights reserved

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

late jurassic early cretaceous evolution of the eastern indian ocean adjacent to northwest australia over 9700 km of new aeromagnetic data were acquired off the northwest coast of australia and combined with existing magnetic data to map magnetic isochrons in the eastern indian ocean the isochrons were used to constrain a tectonic model of the evolution of the seafloor in the argo cuvier and gascoyne abyssal plains a complete set of anomalies from m26 through m16 was found in the argo abyssal plain trending generally n70 e spreading commenced in the center of the basin at or prior to m26 and propagated outward until at least m24 time anomalies m10 mo recording the separation of australia and india were found in the cuvier and gascoyne abyssal plains with a trend of about n30 e a significant crustal age discontinuity occurs in the vicinity of the joey rise where the two lineation sets converge because there appears to be no overlap of isochron ages in the two groups it is not necessary to postulate that a triple junction existed off northwest australia as has been previously suggested at m4 m5 time a 10 clockwise change in spreading direction occurred on the cuvier gascoyne spreading system this event triggered ridge jumps that transferred two pieces of the indian plate to the australian plate overlapping spreading on the forming and dying ridges curved fracture zones and lineations as well as fanned lineation trends suggest that the ridge jumps occurred by ridge propagation and that the transferred lithospheric blocks behaved as microplates for a brief interval of approximately 1 2 m y

seismotectonics in the republic of djibouti linking the afar depression and the gulf of aden abstract the arta seismological array around the gulf of tadjurah monitored earthquake activity for over 15 years epicentral locations and focal mechanisms are compared with tectonic evidence from surface breaks traces of active faults and the spatial distribution of swarms of aftershocks strike slip motion on near vertical faults predominates in the gulf left lateral strike slip on n150 e oriented faults is suggested at variance with the emphasis in previous models on either 1 a continuous plate boundary with spreading centres connected by ene trending right lateral transform faults 2 a propagating rift oriented approximately east west or 3 a few broad zones of extension between n150 e normal faults left lateral strike slip motion occurring on a series of parallel faults results in clockwise rotation of the blocks between the faults the type of deformation has similarities to that further west but is not obviously controlled by a distinct pair of overlapping rifts as proposed inland by tapponnier et al 1990 it is suggested that the stresses induced by the northeasterly drift of arabia alone may deform the southern part of the more easterly oriented zone of lithosphere derived of plume and ridge which separates arabia from africa

late cretaceous and paleogene tectonic evolution of the north pacific ocean the late cretaceous history of the northern pacific ocean has not been adequately deciphered largely because a major plate reorganization occurred during the cretaceous magnetic quiet interval using primary data to reconstruct plate motions from fracture zone trends and late cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region especially the emperor and chinook troughs the primary event in our reconstruction is the subduction of the old northwest pacific triple junction new relative plate motions imposed by formation of convergent boundaries along both the northern pacific and farallon plates caused the farallon plate to crack this subdivision occurred 82 m y ago and resulted in the formation of the kula and chinook plates the chinook plate was bounded on the north by the chinook kula ridge the western arm of the great magnetic bight on the west by the southern emperor trough a slowly spreading rift valley on the south by the mendocino transform and on the east by the chinook farallon ridge the northern emperor trough formed the initial western boundary of the kula plate from 82 m y ago to 50 m y ago this configuration was stable the chinook plate expanded the chinook kula ridge migrated north and the kula plate was subducted with the subduction of this ridge the chinook plate adhered to the pacific plate and the pacific farallon ridge became the north south trending feature reflected by anomalies 22 and younger this reconstruction obviates the awkward late cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region to achieve closure of vector triangles representing the relative late cretaceous plate motions some extension must occur presumably across the eastern mendocino fracture zone between the northern and southern portions of the farallon plate these two plates apparently were independent units between 105 and 50 m y ago the initiation and cessation of rapid north farallon north american convergence 82 and 50 m y ago may have determined the nature and duration of the laramide orogeny

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

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

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

bookshelf faulting and horizontal block rotations between overlapping rifts in southern afar lateral slip on initially rift parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts it occurs in southern afar where clockwise block rotations result from distributed dextral shear between the overlapping ghoubbet asal manda inakir and manda hararo abhe bad rifts faulting observed during the 1969 serdo earthquakes and on spot images is consistent with the shear being taken up by left lateral slip on steep nw se striking faults which formed as normal faults before extensional strain became localized in the two rifts this bookshelf faulting accounts quantitatively for the 14 5 7 5 rotation documented by paleomagnetism in the 1 8 0 4 ma old afar stratoid basalts given the 17 5 5 mm yr rate of separation between arabia and somalia

intense local toroidal motion generated by variable viscosity compressible convection in 3 d spherical shell strong dynamical effects arising from lateral viscosity variations are found near the heads of fast upwelling plumes approaching the top boundary for unsteady 3 d compressible convection in a spherical geometry at an average rayleigh number of o 106 the temperatures at the center of fast upwelling plume are very hot due to the positive feedback interaction between viscous dissipation and temperature dependence of the viscosity although the laterally averaged ratio of toroidal to poloidal energy varies only around 10 the local toroidal velocities can reach about 40 of the poloidal velocities the strongest toroidal motions always take place right above or close to the fast upwelling plume heads and behave as vortical flows with time this finding may explain the origins of microplate rotation in regions with hotspot activity e g easter island which shows no direct relation to the transform faulting system near the mid ocean ridge

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

geodetic gps measurements in south iceland strain accumulation and partitioning in a propagating ridge system 1 gps observations in south iceland between 1994 and 2003 are compared with two dimensional elastic half space and viscoelastic coupling models for two parallel rift zones representing the western and eastern volcanic zones wvz evz gps data from the hreppar block between the wvz and evz fit a rigid block model within uncertainties spreading rates across the wvz increase from 2 6 0 9 mm yr in the northeast to 7 0 0 4 mm yr in the southwest conversely spreading rates in the evz decrease from 19 0 2 0 mm yr in the northeast to 11 0 0 8 mm yr in the southwest the direction of ridge propagation summed extension rates across the two rift zones are approximately constant and equal to the total plate rate 18 20 mm yr consistent with a simple propagating ridge model whereby the wvz is deactivating in the direction of evz propagation the coupling model confirms results from the simple elastic half space model including relatively shallow locking depths 5 km beneath the rift zones and allows for an estimate of mean viscosity 1019 1020 pa s beneath the elastic layer the location of maximum surface velocity gradient in the evz presumably the locus of subsurface magma accumulation and future rifting does not coincide with the 1783 1784 lakagigar fissure eruption but is 20 km to the west on the bar abunga veidivotn fissure swarm this had a small volume eruption in 1862 1864 but a major eruption in 1477 a d

large rotation of the easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor one of the goals of the pito93 cruise along the perimeter of the easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study using a new orienting device the geocompass 14 oriented blocks were obtained during 6 dives conducted using the ifremer dsrv nautile paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and af demagnetizations and reveals a mean characteristic remanent magnetization chrm direction within each block with both normal and reverse polarities after elimination of data from 5 blocks situated outside the microplate and one block which bears an anomalous direction we have computed a mean chrm direction at dm 49 0 im 39 0 k 10 2 95 19 8 n 7 although the mean inclination is in agreement with the inclination of the recent dipole magnetic field d 0 i 43 the average declination is significantly different from this direction which we interpret as due to a bulk clockwise rotation of 48 5 11 after the bivariate average of the easter microplate over the past 2 3 ma this value is in excellent agreement with previous estimates based on remote geophysical measurements and models

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

statistical tests for closure of plate motion circuits the standard method of testing whether data describing the relative motion of three plates meeting at a triple junction are consistent with rigid plate tectonics uses the condition of plate circuit closure the euler vectors or angular velocities describing the relative motion of each plate pair must sum to zero deviations from closure suggest that the data are systematically in error or that the plate boundaries have been identified incorrectly or that the plates are non rigid we examine two methods one based on a chi square test and the other based on an f ratio test of testing whether plate motion circuits close we find the latter test is useful but the former is of limited value because of the common practice of systematically overestimating errors in plate motion data we apply the f ratio test to minster and jordan s data for the boundaries dividing the pacific cocos and nazca plates which prove consistent with plate circuit closure application of the test to their data for the boundaries dividing the australian antarctic and african plates shows nonclosure even after deletion of data from the carlsberg ridge which recently has been suggested to divide the african plate from an indo arabian plate this nonclosure suggests systematic errors in the data or internal deformation of one or more of the plates meeting at the indian ocean triple junction

the tectonic and geomagnetic significance of paleomagnetic observations from volcanic rocks from central afar africa deformation throughout afar over the past 2 myr has been characterized by widespread and intense crustal fragmentation that results from inhomogeneous extension across the region in eastern afar this situation has evolved to localized extension associated with the westward propagation of the gulf of aden gulf of tadjurah seafloor spreading system into the asal ghoubbet rift during the gradual process of rift propagation and localization crustal blocks in eastern afar sustained clockwise rotations of 11 to better understand the processes of rift propagation and localization and how they affect the rest of afar we have collected and analyzed over 400 oriented paleomagnetic samples from 67 lava flows from central and southern afar unlike eastern afar the mean paleomagnetic direction from central afar indicates that vertical axis rotations are statistically insignificant 3 6 4 4 though small clockwise rotations 8 are permitted thus propagation and localization in central afar have not had the same influence in causing crustal block rotations or perhaps more likely have not reached the same stage of evolution as seen in eastern afar in addition several of the lava flows record intriguing geomagnetic field behavior associated with polarity transitions excursions or large secular variation events interestingly the transitional or anomalous virtual geomagnetic poles vgps tend to cluster in two nearly antipodal regions one in the northern pacific ocean and the other in the southwest indian ocean one lava flow has recorded both of the antipodal transitional components with the two components residing in magnetic minerals with unblocking temperatures above and below 500 c respectively reheating and partial remagnetization by the overlying flow cannot explain either of the transitional directions because both differ significantly from that of the reversely magnetized overlying flow the high temperature component gives a vgp in the northern pacific whereas the lower temperature component gives a nearly antipodal vgp south of cape town south africa hence the configuration of the geomagnetic field appears to have jumped nearly instantaneously from a northern hemisphere transitional state to a southern hemisphere one during this normal to reverse polarity transition

mapping the miocene farallon ridge jump on the pacific plate a seismic line of weakness abstract bathymetric and magnetic data are used to obtain estimates on the pacific and nazca plates of the boundaries separating lithosphere generated at the old farallon ridge from the more recent one created at the present day east pacific rise an excellent correlation is found with the sites of known teleseismically recorded intraplate seismicity suggesting that these boundaries which are lines of age discontinuity in the plate must be zones of weakness of the lithospheric plate in particular the so called region c identified by okal et al as a major site of seismic release sits on a small piece of farallon plate in the immediate vicinity of the northern extension of the fossil roggeveen rise cut across by the east pacific rise during the ridge jump

block rotation and continental extension in afar a comparison to oceanic microplate systems the reorganization of oceanic spreading centers separating major plates often appears to occur by a process in which discrete microplates form and evolve by rift propagation to see whether such microplate behavior has implications for continental rifting we investigate the application of a microplate model to the afar region at the nubia somalia arabia triple junction studies of marine magnetic anomalies volcanic ages bathymetry and seismicity suggest that the westward propagating gulf of aden spreading center has propagated into eastern afar within the past 2 m y causing rifting and extension within the continent we derive constraints on the extension history from the geometry and timing of rift formation and from paleomagnetic data indicating that pliocene to pleistocene age rocks have undergone a clockwise rotation of 11 we suggest that the history of rifting the rotation and several other features of the regional geology can be described by combining features of an oceanic microplate model and the concept of rift localization previously proposed for afar in this scenario motion occurring on several rifts within an extensional zone preceding the propagating spreading center is gradually transferred to a single rift while motion is transferred the overlap region between the growing and dying rifts acts as one or more microplates or blocks that rotate relative to the surrounding major plates the rifting history and rotations in eastern afar are thus related to the rift propagation and localization that occurs as the plate boundary evolves provided the constraints we use are appropriate our model better describes the regional kinematics than alternative block models including one based on bookshelf faulting if the tectonics of afar are typical for continental breakup they have interesting implications for the geometry of passive margins in particular asymmetric rifted margins can be produced if the final location of the rift axis is not at the center of the zone of initially disrupted lithosphere additionally if the rate of rift propagation and the rate and location of rift localization are not uniform then along axis structural variations will result

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

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

tectonics of rift propagation into a continental margin western woodlark basin papua new guinea oblique convergence along the irregular boundary between the pacific and indo australian plates in the sw pacific has resulted in a transpressional regime the drag of the overriding pacific plate has produced a breakup of the solomon microplate from the indo australian plate and has induced its rotation within the mega shear zone between the two major plates the microplate is being separated from the indo australian plate by passive rifting and seafloor spreading in the woodlark basin which is propagating westward along the continental margin of papua new guinea the present day pole of the microplate rotation with respect to indo australian plate is estimated to be near the owen stanley fault zone which represents the suture after paleocene eocene arc continent collision the average rate of the seafloor spreading propagation is estimated to be 150 mm yr 1 over the last 3 4 ma the transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault the style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks

oscillatory spreading explanation of anomalously old uplifted crust near oceanic transforms rocks significantly older than their theoretically predicted age have been recovered in the vicinity of some atlantic transform zones rocks of palaeocene 55 58 myr and possibly even of cretaceous age are found near the vema transform on tectonically uplifted crust which should not be older than 30 myr we present here a model which might explain the presence of uplifted anomalously old crust near the vema transform zone tz the proposed model involves axial rift propagation and reorientation and migration of the transform as a result of a change in the regional stress field during transform migration a crustal wedge formerly part of the african plate is transferred to the south american plate the ensuing oscillatory spreading can explain the anomalous age of the uplifted crustal block compression resulting from rift propagation and transform migration provides a mechanism for crustal uplift

constraints on rift propagation history at the cobb offset juan de fuca ridge from numerical modeling of tectonic fabric abstract rift propagation has been proposed as the mechanism for the reorganization of the juan de fuca ridge during the cenozoic in particular magnetic anomalies in the cobb offset region of this ridge have been interpreted in terms of a complex dueling propagator history in which rift propagation has occurred alternately in opposing directions on adjacent ridge segments although the overall propagation history has been inferred from the magnetic data these data are limited in their resolution for the detailed history by the length of time during which magnetic polarity remains constant we thus examine whether more detailed resolution within polarity chrons can be obtained using seafloor tectonic fabric from acoustic imagery by modeling lineations as isochrons synthetic fabric can be generated for a given rift propagation history and the fit of the model to the data can be compared numerically for the cobb we used a propagation history model with a given timing and direction of propagation and generated synthetic fabric for different propagation rates the fabric is best fit for propagation rates between 400 and 900 mm yr slower 900 mm yr propagation rates yield slightly poorer fits the fabric results are thus in accord with the interpretation based on magnetics alone that the rates of rift propagation significantly exceeded the spreading rates an interesting feature of the model is that the complex tectonic fabric is reasonably well fit on this scale 1 km by simple rift propagation models which do not invoke shear zone behavior and thus deviate from rigid plate tectonics

control on seafloor spreading geometries by stress and strain induced lithospheric weakening abstract seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults few regions exhibit different patterns such as the east pacific rise epr which additionally displays overlapping spreading centres oscs and microplates we introduce a dynamical model using two independent scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins the elastic modulus that is affected by the damage determines the type of localized deformation damage reducing the bulk modulus tends to result in tensile fractures while a reduction in shear modulus leads to shear fractures the damage source determines the fracture orientation material weakening in tension results in fractures perpendicular to the most tensile principal stress while shear weakening results in two conjugate fractures at 45 relative to the applied stress strain or energy dependent damage results in propagating localized fractures stress dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones starting from small perturbations with reduced elastic moduli as nucleation points all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress the orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones oscs form when the shear damage leading to transform faults is suppressed while microplate formation requires additional reduction of the shear modulus by tensile energy oblique spreading at 45 recently discovered along ultraslow spreading ridges occurs when both moduli are weakened by shear energy a parameter regime exists in which ridge transform patterns develop at low applied tension while microplates form at higher stresses these results indicate that at least three different micromechanical processes operate with different evolution rates oscs and oblique spreading require different material properties

investigation of spreading center evolution by joint inversion of seafloor magnetic anomaly and tectonic fabric data spreading center segments that have experienced a complex tectonic history including rift propagation may have a complicated signature in bathymetric and magnetic anomaly data to gain insight into the history of such regions we have developed techniques in which both the magnetic anomaly patterns and seafloor fabric trends are predicted theoretically and the combined predictions are compared numerically with the data to estimate best fitting parameters for the propagation history fitting functions are constructed to help determine which model best matches the digitized fabric and magnetic anomaly data such functions offer statistical criteria for choosing the best fit model we use this approach to resolve the propagation history of the cobb offset along the juan de fuca ridge in this example the magnetic anomaly data prove more useful in defining the geometry of the propagation events while the fabric with its greater temporal resolution is more useful for constraining the rate of propagation it thus appears that joint inversion of magnetic and seafloor fabric data can be valuable in tectonic analyses

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

origin and models of oceanic transform faults abstract mid ocean ridges sectioned by transform faults represent prominent surface expressions of plate tectonics a fundamental problem of plate tectonics is how this pattern has formed and why it is maintained gross scale geometry of mid ocean ridges is often inherited from respective rifted margins indeed transform faults seem to nucleate after the beginning of the oceanic spreading and can spontaneously form at a single straight ridge both analog and numerical models of transform faults were investigated since the 1970s two main groups of analog models were developed thermomechanical freezing wax models with accreting and cooling plates and mechanical models with non accreting lithosphere freezing wax models reproduced ridge ridge transform faults inactive fracture zones rotating microplates overlapping spreading centers and other features of oceanic ridges however these models often produced open spreading centers that are dissimilar to nature mechanical models on the other hand do not accrete the lithosphere and their results are thus only applicable for relatively small amount of spreading three main types of numerical models were investigated models of stress and displacement distribution around transforms models of their thermal structure and crustal growth and models of nucleation and evolution of ridge transform fault patterns it was shown that a limited number of spreading modes can form transform faults microplates overlapping spreading centers zigzag ridges and oblique connecting spreading centers however the controversy exists whether these patterns always result from pre existing ridge offsets or can also form spontaneously at a single straight ridge during millions of year of accretion therefore two types of transform fault interpretation exist plate fragmentation structures vs plate accretion structures models of transform faults are yet relatively scarce and partly controversial consequently a number of first order questions remain standing and significant cross disciplinary efforts are needed in the future by combining natural observations analog experiments and numerical modeling

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

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

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

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

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

global tectonics and space geodesy much of the success of plate tectonics can be attributed to the near rigidity of tectonic plates and the availability of data that describe the rates and directions of motion across narrow plate boundaries m 1 to 60 kilometers wide nonetheless many plate boundaries in both continental and oceanic lithosphere are not narrow but are hundreds to thousands of kilometers wide wide plate boundary zones cover m 15 percent of earth s surface area space geodesy which includes very long baseline radio interferometry satellite laser ranging and the global positioning system is providing the accurate long distance measurements needed to estimate the present motion across and within wide plate boundary zones space geodetic data show that plate velocities averaged over years are remarkably similar to velocities averaged over millions of years