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

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

dune field development interactions and boundary conditions for crescentic and stellate megadunes of the al liwa basin the empty quarter within the greater ar rub al khali empty quarter sand sea lies an internal depocentre the al liwa basin which comprises a variety of mega scale dune types crescentic dunes dominant the north of the basin while megadunes of stellate or star form are a major landform of the south eastern reaches their development into dune fields is determined by the style and rate of dune dune interactions the boundary conditions imposed by a multi modal wind regime fluctuating groundwater levels and sediment availability under an assortment of climatic conditions throughout the quaternary as a result dune field patterns are a collective response to these perturbations in space time and environment the r statistic is a collective measure of these responses and is a metric capable of identifying the degree of pattern maturity or self organization of the aeolian system and the pathways from which patterns evolve the spatial signature of the southerly located star dunes is characterized by two definitive patterns of organization the first one of complete spatial randomness the second a low degree of spatial uniformity in isolation these results appear to be unrelated to those for crescentic dunes of the region in which a significantly higher degree of pattern dispersion is the norm however when spatial statistical measures are integrated with the theoretical understanding of dune dune interactions and the involvement of environmental agents the complex morphodynamic pathways and linkages between regional dune fields is better understood in this case both constructive e g merging lateral linking and regenerative activity e g calving have played important roles in the development of dune size and associated adjustments in spacing and dune numbers and subsequently dune field patterns synergetic patterns are emblematic of this vast dunescape whereby transitional geographic morphologic dimensional and environmental modifications exist between the mega crescentic and mega stellate dunes of the empty quarter copyright 2012 john wiley sons ltd

gravity anomalies flexure and the elastic thickness structure of the india eurasia collisional system wehaveusedbouguergravityanomalyandtopographydatatodeterminetheequivalentelasticthicknessofthelithosphere te in the region of the india eurasia collisional system comparison of observed and modelled gravity anomalies along 1dimensional profiles suggest there are significant variations in te along strike of the himalaya foreland estimates decrease from a high of 70 km in the central region to 30 50 km in the east and west we have verified these inferences of spatial variations using a 2 dimensional non spectral interative flexure and gravity anomaly modelling technique the himalaya foreland forms a high te 40bteb100 km rigid block with a well defined edge as shown by the localisation of faulting and deformation along its northern margin other highteblocks occur to the north beneath the qaidam and sichuan basins the tibetan plateau forms a low te 0bteb20 km weak region that extends from the central part of the plateau into south western china tectonic styles in the india eurasia collisional system therefore involve both drigidt and dnon rigidt blocks where high te rigid blocks are present the stylesdominatedbyunderthrustingofthemorerigidblock wherethecollisionalzoneisnotconstrainedbyrigidblocks however the style appears to be dominated by lower crustal flow and a more continuous style of deformation d 2005 elsevier b v all rights reserved

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

an analysis of the altimetric geoid in various wavebands in the central pacific ocean constraints on the origin of intraplate features abstract seasat profiles were filtered in three different wavebands and interpolated to elaborate maps of the very short 25 110 km and short 80 320 km and 280 820 km wavelengths of the geoid anomalies for the south pacific for the very short wavelengths we mapped the geoid roughness i e the variations of amplitude of the anomalies two lines following present day plate motions and separated by 800 1000 km were identified the first is formed by the tuamotu archipelago the pitcairn crough alignment and the easter sala y gomez chain the second comprises the cook austral islands a seamount cluster mapped close to the east pacific rise south of 30 s and the chile rise alignments with a n150 e direction and corresponding to traces of old thermal activity were mapped on lithosphere older than 40 m y these features are also spaced of 800 1000 km the map for the 80 320 km waveband shows a pattern of 200 km wavelength undulations several trends are superimposed but the n95 e direction is dominant in the area we associate these geoid undulations to lithospheric phenomena such as fracturations and or small magmatic ridges the 280 820 km waveband reveals linear anomalies roughly following the direction of present day absolute plate motions the linear anomalies have a 400 500 km wavelength and are nearly continuous between the pacific and the nazca plates the topography is located on the highs but the anomalies generally precede the active ends of the volcanic chains this result favours the hot line hypothesis previously suggested by several authors we associate these bands to thermal heterogeneities probably due to a convective regime since the main topographic features shall be emplaced over the ascending limbs of two neighbouring cells the 800 1000 km spacing of the topography imposes a minimum size of 400 500 km for the cells so the convective cells cannot be limited to a thin layer beneath the lithosphere but will affect part of the upper asthenosphere the existence of hot lines aligned parallel to plate motions can explain the intermittent magmatic activity observed for the volcanic lineaments of the south pacific volcanic activity can be the result of the interaction between the hot lines and lithospheric zones of weakness and or of instabilities in the convective cells the regular spacing observed for topography formed before and after 40 m y suggests that the size of the cells is rather constant through time

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

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

microplate tectonics new insights from micro blocks in the global oceans continental margins and deep mantle abstract any plate has a growth process from small to large the micro blocks or micro plates are sometimes the precursors of large plates the origin growth aborting extinction and residual process of micro blocks are of great significance for the study of plate tectonics and pre plate tectonics the micro block can be divided into continental oceanic and mantle micro blocks according to their compositions in this paper the micro blocks in the global oceans have been summarized according to the following five environments mid ocean ridge system subduction system transform fault system deep sea intraplate system and extension rift system we first propose a genetic classification of micro blocks comprising detachment derived rifting derived transform derived propagation derived ridge jumping derived subduction derived accretion derived collision derived and delamination derived micro blocks the different types of micro block boundaries such as active or fossil detachment fault subduction zone mid ocean ridge transform fault fracture zone transfer fault accommodation zone lithosphere scale strike slip fault pseudofault intra oceanic convergent zone overlapping spreading centre non transform offset rheological crustal or mantle discontinuity are systematically discussed for different micro blocks thus the number of triple junctions will be more than the 16 in the traditional plate tectonic theory a stability analysis of these triple junctions is the key to understanding the causes of micro blocks however the micro block has no ultimate cause so it is unnecessary to pursue one ultimate cause or initiation of plate tectonics these micro blocks within oceanic basins along oceanic margins or within the deep mantle can not only be used to develop deep ocean fine structural analysis and plate tectonic reconstruction but also to explain the causes of micro blocks in some orogens this will enrich the research of more detailed pre orogenic or syn orogenic evolution of orogenic belts and even extend to the study of early precambrian pre plate tectonic mechanisms the micro block can be a transition among microplate plate and terrane under a plate tectonic regime it can also be formed in inter sphere tectonic processes it helps better our understanding of regimes of cratonic basin formation and intra continental deformation which are the difficulties faced by the plate tectonics theory we speculate that the micro block tectonics theory is a unified tectonic theory of cross layer cross phase cross space time scale and cross planet

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

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

structural pattern of the galapagos microplate and evolution of the galapagos triple junctions a 13 000 km2 microplate intervenes to prevent the junction of the nazca cocos and pacific plates in the eastern equatorial pacific ocean west of the galapagos islands the microplate is rotating clockwise at 6 m y about an absolute pole 400 km southeast of its center so that although its northern margin has a similar motion to adjacent parts of the cocos plate its southern margin is 50 slower the perimeter of the galapagos microplate is best defined by bathymetry and has been surveyed with sea beam and examined by deep tow two of the three triple junctions on its margins are ridge ridge ridge rrr junctions 160 km apart on the crest of the east pacific rise epr the most like a classic rrr junction is the pacific nazca galapagos intersection at 1 10 n where the epr axis is joined by a narrow volcanic ridge 70 km long and 1 km high the cocos nazca galapagos junction at the west end of the cocos nazca spreading center is inferred to be of the rff type but the transform fault branches are represented by large nontransform offsets lack of stable transform faults is related to rapid westward migration of the junction caused by lengthening of the cocos nazca axis at 60 70 mm yr this rate which was once required to maintain a stable nazca cocos pacific rrr junction has persisted even after birth of the microplate about 1 ma continuing propagation of the cocos nazca axis has opened up the hess deep rift basin in the eastern epr flank in advance of the eruptive propagator tip this rift valley has a relief of more than 3 km from its uplifted rims to the floor where severe attenuation exposes basal layers of the epr crust the similar dietz deep rift basin at the southeastern margin of the microplate 70 km from hess deep was formed when a lengthening galapagos nazca spreading axis rifted through another part of the epr flank

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

tectonics of the nazca antarctic plate boundary abstract we have constructed a new bathymetric chart of part of the chile transform system based mainly on an r v endeavor survey from 100 w to its intersection with the east ridge of the juan fernandez microplate at 34 30 s 109 15 w a generally continuous lineated trend can be followed through the entire region with the transform valley being relatively narrow and well defined from 109 w to approximately 104 30 w the fracture zone then widens to the east with at least two probable en echelon offsets to the south at 104 and 102 w six new strike slip mechanisms along the chile transform and one normal fault mechanism near the northern end of the chile rise inverted together with other plate motion data from the eastern portion of the boundary produce a new best fit euler pole for the nazca antarctic plate pair providing tighter constraints on the relative plate motions

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

the chile ridge a tectonic framework a new chile ridge tectonic framework is developed based on satellite altimetry data shipboard geophysical data and primarily 38 500 km of magnetic data gathered on a joint u s chilean aeromagnetic survey eighteen active transforms with fossil fracture zones fzs including two complex systems the chile fz and valdivia fz systems have been mapped between the northern end of the antarctic nazca plate boundary chile ridge at 35 s and the chile margin triple junction at 47 s chile ridge spreading rates from 23 ma to present have been determined and show slowdowns in spreading rates that correspond to times of ridge trench collisions the valdivia fz system previously mapped as two fzs with an uncharted seismically active region between them is now recognized to be a multiple offset fz system composed of six fzs separated by short ridge segments 22 to 27 km long at chron 5a 12 ma the chile ridge propagated from the valdivia fz system northward into the nazca plate through crust formed 5 myr earlier at the pacific nazca ridge evidence for this propagation event includes the friday and crusoe troughs located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults this propagation event led to the formation of the friday microplate which resulted in the transferal of crust from the nazca plate to the antarctic plate and in a 500 km northward stepwise migration of the pacific antarctic nazca triple junction rift propagation microplate formation microplate extinction and stepwise triple junction migration are found to occur during large scale plate motion changes and plate boundary changes in the southeast pacific

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

pre cretaceous tectonic evolution of the pacific plate and extension of the geomagnetic polarity reversal time scale with implications for the origin of the jurassic quiet zone abstract linear magnetic anomalies resulting from seafloor spreading were mapped in the vicinity of the magnetic bight in the western pacific ocean new aeromagnetic data allowed the magnetic bight to be more accurately mapped from m21 to m28 and enabled the identification of low amplitude magnetic lineations in the jurassic quiet zone these lineations were formed by magnetic field reversals prior to m29 a revised jurassic geomagnetic polarity reversal time scale was constructed with nineteen reversals older than m29 numbered m30 m38 these reversals extend the record of geomagnetic polarity back in time by approximately 8 ma and are important constraints on the origin of the quiet zone in particular they imply that the jurassic was not a period of constant normal polarity an explanation offered by some authors further they cast doubt on a model of systematically decreasing geomagnetic field strength with increasing age during this period the early history of the northern pacific plate and pacific farallon izanagi p f i triple junction was traced by mapped magnetic isochrons the pacific plate seems to have evolved from a small plate that formed near the phoenix farallon izanagi triple junction about 180 188 m y ago at approximately 17 n 160 e in present coordinates until m21 time the evolution of the northern pacific plate was relatively simple and the p f i triple junction migrated north northwest with respect to the pacific

the foundation seamounts tectonic setting of a newly discovered seamount chain in the south pacific abstract large areas of the south pacific remain unsurveyed because they are so remote a major volcanic chain the foundation seamounts lies between 33 s and 39 s and between 111 w and 125 w poorly defined on conventional bathymetric charts it becomes very apparent when combining an analysis of conventional geophysical data with the satellite altimeter data the length of the seamount chain is 1350 km and the volcanoes are clustered within a 180 km wide band trending approximately n70w which is the direction of absolute motion of the pacific plate over a fixed reference system some of these volcanoes are so shallow that they may have emerged above the sea surface at an earlier time no dates are known for the seamounts surrounding the foundation seamounts chain the tectonic setting is that of the pacific plate at the time when the strike of the spreading ridge changed from a northwesterly direction to the present northeasterly one this episode of spreading reorganization involved the existence of a short lived microplate west of the pacific antarctic ridge a captured segment of nazca plate accounts for the large discrepancies in the magnetic anomalies offsets along the agassiz fracture zone large propagators like the adventure and kurchatov troughs are described the newly discovered western segment of the chile fracture zone is the pacific plate match of the chile fracture zone on the nazca plate

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

mechanical relation between crustal rheology effective fault friction and strike slip partitioning among the xiaojiang fault system southeastern tibet the north south trending xiaojiang fault system accommodates 10 12 mm yr sinistral motions between southeastern tibet and south china in the south segment the fault system composes mainly of four parallel strike slip faults namely from west to east the luzhijiang fault the yimen fault the puduhe fault and the xiaojiang fault geological and seismological observations have shown that these strike slip faults are all of active while the slip rates of the luzhijiang the yimen and the puduhe faults are much less than that of the xiaojiang fault we use finite element modeling to explore the mechanical relation between crustal rheology effective fault friction and long term slip rate partitioning among the four parallel faults the individual faults are simplified as vertical discontinuities embedded in the crust as geophysical explorations have predicted a large number of models are tested associating with variations of the crustal rheolohy and the effective fault friction of individual faults result shows that if crust bounding the faults trends to behave like rigid blocks and decoupled mechanically from underlying layer the modeled result is hard to approximate slip rates of the individual faults to better fit slip rates of the individual faults viscous deformation of the lower crust should be included with a heterogeneously viscous lower crust model that is built upon thermal structure of the heat flow data associating with relatively low effective friction of the xiaojiang fault the modeled results fit the geological slip rates well with 1 1 5 mm yr for the luzhijiang the yimen and the puduhe faults and 6 6 5 mm yr for the xiaojiang fault thus in the southward movement of the tibetan plateau around the eastern himalayan syntaxis slip partitioning among the xiaojiang fault system should be related to viscous deformation of the lower crust associated with different strength of the individual faults highlighting that deformation of this fault system is coupled mechanically between the frictional upper crust and the viscous lower crust

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

upper mantle q structure beneath the east pacific rise a model of q as a function of depth has been determined for the upper mantle beneath the east pacific rise the model is appropriate for shear waves with periods from roughly 15 to 30 s the amplitudes of multibounce sh phases which propagate within the upper mantle were used to constrain the q structure the multibounce phase amplitudes were measured with respect to the s wave amplitude for each seismogram the data consist of s ss sss and ssss waves for distances from 30 to 80 the sources are earthquakes which occurred on transform faults associated with the east pacific rise and the seismograms were recorded by world wide standard seismograph network and canadian seismic network stations in western north america mantle beneath oceanic crust less than 15 ma and tectonically active continental crust is sampled since the s wave data primarily sample the deep mantle the resulting upper mantle q model depends on the q structure assumed for the lower mantle we assumed a constant q of 312 for the mantle beneath 650 km depth with this assumption the upper mantle phase amplitudes require a q of about 70 in the upper 150 km of the mantle from 150 to 400 km depth q increases to 180 the q in the transition zone from 400 to 650 km depth was found to be 180 significantly lower than the lower mantle q the resulting q model is consistent with several surface wave studies of mantle attenuation beneath the pacific however our results predict a qscs 210 which is significantly higher than many direct measurements of scs attenuation indicate furthermore the large range in measured qscs is inconsistent with lateral q variations being confined to the upper mantle if the region we have studied represents the low q extreme of upper mantle we conclude that large lateral variations in q exist in the lower mantle

characteristics of the active xiaojiang fault zone in yunnan china a slip boundary for the southeastward escaping sichuan yunnan block of the tibetan plateau abstract the xiaojiang fault zone within the yunnan province china is one of the most active fault zones of the southeastern edge of the tibetan plateau this n s trending 400 km long fault zone defines the southern part of the eastern boundary of the sichuan yunnan block which is escaping southeastward from the tibet plateau and rotating around the eastern himalayan syntaxis the middle section of the fault zone splits into two branches each branch fault consists of several en echelon faults basins of various ages have developed on the step overs between the secondary faults the left lateral strike slip rate along the eastern branch is up to 9 mm a and that on the western branch is 7 mm a more than 10 m 6 historic earthquakes have occurred along the fault zone the largest was the 1833 songming earthquake of m 8 four earthquake rupture zones were discovered along the fault zone 15 trenches have been excavated along every major segment of the fault zone and 4 5 paleoearthquakes have been distinguished along the xiaojiang fault zone the average strong earthquake recurrence interval is 2000 2500 years

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

tectonic evolution of the northern pacific plate and pacific farallon izanagi triple junction in the late jurassic and early cretaceous m21 m10 abstract magnetic lineations in the northwestern pacific near the shatsky plateau were interpreted to trace the tectonic history of the northern pacific plate from the time of anomalies m21 to m10 at m22 time the pacific farallon izanagi p f i triple junction was located southwest of the plateau and migrating north northwest with respect to the pacific plate shortly thereafter the stress field acting on the three plates changed significantly and the pacific lzanagi ridge reacted by pivoting 24 clockwise from m21 to m19 time several ridge propagation events occurred while the ridge adjusted to the new spreading regime to the east of the m22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the pacific farallon ridge the geometry of magnetic isochron m21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates differences in the spreading rate of the japanese magnetic lineations north of the plateau imply that one microplate which we call the shatsky microplate existed until about m15 m13 time it was probably captured by either the farallon or the izanagi plate and subsequently destroyed by subduction after m15 the p f i triple junction was reformed on seafloor that is now beneath the shatsky plateau the junction drifted generally northeast either in a ridge ridge ridge or in a ridge fault fault configuration perhaps alternating between the two from m11 m10 time the stress field on the three plates changed again and the pacific farallon ridge southeast of the plateau reoriented 15 clockwise at this time the p f i junction assumed a ridge ridge ridge geometry at the northeast end of the plateau

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

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

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

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

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

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

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

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

field study of a highly active fault zone the xianshuihe fault of southwestern china the xianshuihe fault of western sichuan province china is one of the world s most active faults having produced 4 earthquakes during this century of magnitude 7 along a 350 km length of the fault at least 8 such events have occurred since 1725 in the more limited 150 km long segment including luhuo and daofu major earthquakes in 1904 1923 1973 and 1981 m 7 7 7 6 6 9 were associated with overlapping surficial fault ruptures and with individual left lateral displacements as large as 3 6 m field studies indicate that this high degree of activity is typical of the fault s longer term history the holocene left lateral slip rate on the north western segment of the fault has been 15 5 mm yr decreasing to about 5 mm yr on its southeastern segment based on radiometrically dated offset stream channel and terrace deposits and on offset glacial moraines physiographic features of active faulting are fully as diagrammatic as those of california s san andreas fault mainly because of high altitude preservation and the absence of cultural modification on this eastern margin of the tibetan plateau detailed en echelon tensional and pushup features resulting from surface ruptures in 1973 1955 1923 and 1893 can still be recognized today and new data have been collected bearing on the offsets and fault rupture lengths during these and other events the locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the xianshuihe fault obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events whereas segmentation control for others remains obscure based on the historic record repeat times estimated from slip rates and current seismic gaps two segments are particularly likely sites for m 7 earthquakes in the near future the 65 km long segment between daofu and qianning and the 135 km long segment bracketing kangding continuing creep has been documented along some segments of the fault and this together with the high degree of activity and other unique attributes makes the xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard evaluation studies

is the shanxi rift of northern china extending 1 the shanxi rift in northern china is marked by intensive seismicity including many devastating historic earthquakes geological and seismological evidence show 0 5 1 6 mm yr extension across the rift and previous gps results indicated an extension rate of 4 2 mm yr we show here newly compiled gps data that indicate coherent crustal motion and no clear sign of extension across the rift we reconcile the discrepancy between geological observations and gps results in a simple viscoelastic finite element model with timescale dependent crustal deformation the gps velocities can be fit by a model with a near elastic upper crust consistent with predominantly interseismic deformation the geological rate of extension is predicted when viscous creep of the crust is dominant driven by the gravitational potential energy arising from the heterogeneity of crustal structure

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

faulting associated with large earthquakes and the average rate of deformation in central and eastern asia we determine approximate average rates of deformation in portions of eastern and central asia for the last 80 years these rates are based on estimates of source dimensions average displacements seismic moments and orientations of faults of all large earthquakes m 7 5 and as many other major earthquakes m 7 0 as possible in asia since 1900 a major part of this study is the compilation of these data for separate parts of asia we combine seismic moment tensors to estimate the average rotational strain or deformation tensors for these regions for the last 80 years we estimate that shortening across the tien shan has occurred at 11 mm yr between 5 and 19 mm yr left lateral shear on easterly trending planes in mongolia in northwestern tibet gansu and ningxia and on the xianshuihe fault in western sichuan has been very rapid more than 10 mm yr in each region and apparently as rapid as 40 mm yr in northwestern tibet gansu and ningxia slower rates of a few to 10 mm yr appear to characterize east west extension and conjugate north south shortening of tibet the earthquake history of the himalaya is consistent with rates of underthrusting of more than 10 mm yr but is probably less than the convergence rate of india with eurasia of about 50 mm yr the overall strain field is consistent with a large part of india s penetration into eurasia being absorbed by the extrusion of material out of india s way as a result southeast china moved at about 21 mm yr east southeast with respect to eurasia in the last 80 years with an uncertainty of about a factor of 2 in this estimated rate

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

lower friction of the xianshuihe xiaojiang fault system and its effect on active deformation around the south eastern tibetan margin we use three dimensional mechanical modelling with fault as coulomb type frictional surface to explore the active deformation of the xianshuihe xiaojiang fault system in south eastern tibet crustal rheology is simplified as an elastoplastic upper crust and a viscoelastic lower crust far field gps velocities and quaternary fault slip rates are used to constrain the model results numerical experiments show that effective fault friction lower than 0 1 0 08 leads to a high slip rate that fits well with geological estimates of the slip rate on the fault system associating with the modelled fault slip rate strain in the surrounding crust distributes broadly and is partitioned into strike slip and thrust senses this means that in the indianeurasia convergence accommodation of the large fraction of sinistral motion on the fault system is achieved mainly due to its lower fault friction this in turn affects crustal deformation around the south eastern tibetan margin resulting in negligible compression across the longmen shan

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

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

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

tectonic history and new isochron chart of the south pacific we have developed an internally consistent isochron chart and a tectonic history of the south pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data highly accurate vertical deflection profiles 1 2 rad derived from 22 repeat cycles of geosat altimetry reveal subtle lineations in the gravity field associated with the south pacific fracture zones these fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas the high density of geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map the tectonic fabric was then used to derive new poles of rotation for 12 selected times in the late cretaceous and cenozoic from our reconstructions we estimated the former location of the spreading centers in order to derive a new set of isochrons interpreted unes of equal age on the ocean floor we believe that the use of new geosat altimeter data in combination with a multi plate reconstruction has led to an improvement in our understanding of south pacific tectonics

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

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen

petrogenesis of axial lavas from the southern chile ridge major element constraints we present major element glass data for 163 rock samples collected from four ridge segments of the southern chile ridge between the chiloe fracture zone and the chile margin triple junction including the segment currently being subducted at the chile trench segment 1 the subridge mantle is heterogeneous at small spatial scales normal mid ocean ridge basalts n morb recovered from all four ridge segments have experienced variable extents of low pressure fractionation but have been generated by relatively uniform extents f and initial pressures po of melting of a slightly heterogeneous depleted source type 1 e morb found only on segment 4 have trace element affinities to some ocean island basalts display a large range of major element variations at constant and high mgo and are spatially associated with n morb type 2 e morb have trace element affinities with suprasubduction zone settings they are found at two segment 1 sites and along most of segment 3 in order to minimize fractionation and source heterogeneity effects and assess melting conditions e morb compositions were double backtracked to 8 wt mgo and a k ti ratio of 0 1 although the magnitudes of f and po are model dependent we find that n morb and both types of e morb were generated under similar melting conditions these observations indicate that spreading rate and mantle temperature exert primary control on the southern chile ridge thermal regime we see no influence of ridge subduction on the major element systematics and melting conditions of segments closest to the trench