new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

images of transequatorial f region bubbles in 630 and 777 nm emissions compared with satellite measurements images of transequatorial f region bubbles were obtained from magnetic field aligned airglow observations from haleakala hawaii for 9 nights in august 1988 bubbles of scale size 10 100 km were recorded with an intensified ccd detector attached to a filter wheel imager and sequential images were made in the plasma recombination emissions at 630 nm and 777 nm and in the background emissions the observations also yield information on the large scale structure of the ionosphere in the northern appleton anomaly region particularly on the latitude variations of plasma concentration and layer height measurements from the dmsp f8 and f9 satellites extend this to include the southern anomaly region the dmsp satellites were near 830 km in altitude and in near polar orbits at about 1800 and 2120 hours local time a comparison of the bubble images and the dmsp results with plasma concentration and drift velocities obtained from the san marco satellite allows us to put the measurements along the san marco track into the larger ionospheric context one to one correspondence is found between the presence of the optical bubbles and the presence of plasma depletions measured from the san marco satellite allowing identification on consecutive orbits of the same depletions this extends for up to 5 hours of the satellite measured information for individual depletions that is their vertical and horizontal drift velocities and the plasma concentration variations the images show the shapes of the depletions in apex height and longitude and their evolution horizontal drift velocities of the bubbles were determined by a correlation analysis bubble occurrence is compared with day to day changes in the equatorial fountain electric fields and transequatorial asymmetries caused by neutral winds and with measurements of radar backscatter from centimeter scale irregularities associated with bubbles in the kwajalein sector the development of 10 100 km scale size bubbles at a given time and location is clearly a consequence of the presence of even larger wavelike corrugations in the f layer provided there is also at least a moderately strong equatorial fountain

upb geochronology of the central ribeira belt brazil and implications for the evolution of the brazilian orogeny abstract the ribeira belt in southeastern brazil was generated during the brazilian orogeny the south american equivalent of the pan african orogeny 700 450 ma the central sector of the belt is characterised by a series of northwest vergent crustal scale thrust stacks emplaced under amphibolite grade conditions major shear zones separate four lithotectonic domains andrel ndia to the northwest juiz de fora paraiba and coastal domains analyses of zircon monazite and titanite from a variety of rock types from the four domains lead to the following conclusions all domains record an important tectono thermal event at 590 565 ma represented by partial melting granitoid intrusion and remobilisation of older basement gneisses and corresponding to thrusting and development of dextral shear zones also recorded are older metamorphic ages 611 604 ma of unknown significance only the paraiba and coastal domains in the south record a younger metamorphic event m2 at 535 520 ma most likely associated with another episode of thrusting and shear zone development the absence of m2 metamorphism in the andrel ndia and juiz de fora domains can be explained by post 535 520 ma emplacement of the paraiba domain late to post tectonic activity is represented by 503 492 ma metamorphism precursors of the basement gneisses are 2185 2134 ma old rocks generated during the transamazonian orogeny some of which contain a significant archean component with minimum ages of 2981 2846 ma the ages presented above define specific metamorphic events for the brazilian orogeny for the first time in southeastern brazil in addition together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central ribeira belt represent an extension of the units present in the southern sao francisco craton and indicate that the foreland was more extensive than the presently defined craton

constraining fault friction in oceanic lithosphere using the dip angles of newly formed faults at outer rises article history we investigate the mechanical properties of the oceanic lithosphere using earthquake focal mechanisms from subduction zone outer rises we study regions where faulting oblique to the pre existing mid ocean ridge fabric implies the formation of new faults the nodal plane dips of dip slip earthquakes on these faults are dominantly in the range 30 60 with a strong peak concentrated around 45 this distribution is inconsistent with the pattern that would result from high coefficients of friction e g 0 6 equivalent to normal faults forming at 60 and thrust faults at 30 we instead suggest that the observed distribution of dips implies that faults in the oceanic lithosphere have a low coefficient of friction due to either low friction clay minerals formed by hydrothermal alteration at the ridge or due to an intrinsic level of friction that is lower than that suggested by laboratory studies

strike slip intraplate earthquakes in the western philippine sea plate abstract on 26 april 2010 a strike slip earthquake mw 6 5 occurred in the western philippine sea plate we deployed 14 ocean bottom seismometers to record the corresponding aftershocks to acquire information regarding these intraplate events our results show that the aftershocks were located along two linear features that intersect with an angle of approximately 120 and are considered a conjugate fault set the p axis of the mainshock focal mechanism is consistent with the compressive stress direction induced by the arc continent collision occurring in eastern taiwan the pre existing oceanic fracture zones and tectonic fabrics do not appear to be reactivated based on the distinct rupture directions determined from the relocated aftershocks however the abrupt halt of the aftershocks at the border of the fracture zone suggests that pre existing weak zones could act as a barrier to rupture propagation moreover most large earthquakes have occurred near fracture zones indicating that the pre existing weakness may favor the generation of earthquakes compared to the other portion of the oceanic plate due to the relatively low rock strength of this zone

ionospheric conditions affecting the evolution of equatorial plasma depletions model studies have been carried out to specify the ionospheric conditions and electrodynamical processes required to describe the geometry of westward tilting and bifurcating equatorial plasma depletions inferred from airglow imaging measurements taken on ascension island in early 1981 the results indicate that if the westward tilts are to be associated with eastward plasma drifts that decrease with altitude above the f peak this shear must be set up by a zonal wind pattern of decreasing speed at increasing distances from the equator below the peak calculations of flux tube integrated pedersen conductivity including both e and f region contributions are in agreement with the simulations of zalesak et al 1982 that show bifurcated depletions to be associated with a mild gradient in the bottomside conductivity profile

confirming sharp bending of the pacific plate in the northern japan trench subduction zone by applying a traveltime mapping method abstract many destructive earthquakes have occurred in the northern japan trench region owing to the subduction of the pacific plate two plate bending points where the dip of the subducting plate changes markedly have been found and correlations between the bending points and the updip limits of large earthquake rupture zones have been previously found in the regions off aomori 40 n and off miyagi 38 n in this study we investigate whether sharp plate bending is also present in the region off iwate 39 n located approximately halfway between the off aomori and off miyagi regions we analysed the existing off iwate wide angle seismic experimental data by applying a new method in which observed reflection phases are directly projected onto the depth distance domain from the time distance domain as a result we found a plate bending point in the region off iwate this is the first time such a point has been found there distances between the three bending points and the trench axis are almost uniform and sharp plate bending is a common feature in the northern japan trench subduction zone the bending axis which is defined by connecting bending points approximately coincides with the updip limit of the rupture zones of large interplate earthquakes seismic swarm activities have been observed inside the subducting oceanic crust along the bending axis and we infer that some of these activities are caused by bending of the subducting plate in addition p wave velocities of the upper plate vary across the bending axis implying that the position of the bending axis is closely related to physical properties of the upper plate and to seismic activities

neogene subsidence pattern in the multi episodic extension systems insights from backstripping modelling of the okinawa trough abstract extensional systems of back arc basins have been widely recognized and studied in se asia however the interactions between extensional tectonics and spatial temporal distributions of tectonic subsidence in an active back arc basin are poorly known using a backstripping technique we analysed 33 multichannel seismic profiles to calculate the neogene tectonic subsidence across the entire okinawa trough ot for the first time reconstruction of the tectonic subsidence shows that the ot can be roughly divided into four subsidence stages with variable rate by comparing the temporal spatial distribution of the intensive subsidence area we confirmed that rifting events first occurred in the northern segment not and middle segment mot from the miocene to late pliocene and then propagated to the southern segment sot after the late pliocene the factors controlling the rapid tectonic subsidence over the entire ot can be classified into two categories 1 ne to nne trending normal faulting related to extension of the ot and nw to nww trending strike slip faulting i e the tokara fault and the miyako fault related to subduction of the philippine sea plate under the eurasian plate 2 the anomalous rapid subsidence occurred in the southernmost part of the sot is likely related to the subducted gagua ridge which caused magma intrusion resulting from partial melting we suggest that magmatic bodies produced by partial melting of subducted materials intruded into the crust and created a dense lithosphere causing this rapid tectonic subsidence

rapid subsidence of the kikai seamount inferred from drowned pleistocene coral limestone implication for subduction of the amami plateau northern philippine sea abstract carbonate rock cores drilled on the kikai seamount northern philippine sea are examined for better understanding of tectonic history of the northern philippine sea the kikai seamount the summit of which is at 1960 m water depth is an isolated high on the northwestern part of the amami plateau formed by subduction related arc volcanism and is situated close to the axis of the ryukyu trench in front of the ryukyu arc sw japan the seamount is capped with shallow water carbonates such as coral rudstone detailed examinations of lithology larger foraminiferal assemblages and sr isotope composition reveal that the core material comprises miocene carbonates unconformably overlain by early pleistocene carbonates it indicates rapid subsidence of the kikai seamount since the early pleistocene the most probable cause of rapid subsidence is collision and subduction of the amami plateau laden with the kikai seamount the rapid subsidence may have started when the western corner of the plateau reached the ryukyu trench and began subduction beneath the ryukyu arc the onset of the subsidence is likely to be controlled by a motion change in the philippine sea plate the latest change in subduction direction from north to northwestward into northwestward to west has been believed to have occurred at 1 2 ma during latest pliocene to early pleistocene time the change of direction resulted in the shift from oblique into right angle subduction of the plate beneath the ryukyu arc and also the onset of the collision and subduction of the amami plateau

geometry of depleted plasma regions in the equatorial ionosphere an image intensified all sky 6300 a airglow imaging system operating on ascension island is early 1981 has been used to assemble a photographic data base of several hundred airglow depletion patterns by mapping the airglow depletion features along geomagnetic field lines to the equatorial plane the geometrical characteristics of plasma depleted regions may be described over the 400 to 1200 km altitude region above the equator such an analysis revealed that single or linear depletions tilted toward the west with an average skewness of 0 6 longitude west 100 km altitude corresponding to a zenith angle tilt of 40 at a height of 800 km the tilts increased with local time during the evening hours showing that an effective velocity shear of 0 05 m s km is required to account for the observed evolution in several cases the depletion geometry showed a wishbone or bifurcated pattern such features had a mean height of bifurcation of 740 km and tilt factors very similar to those found for linear depletions observed during the same local time periods

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

crustal structure of the southern okinawa trough symmetrical rifting submarine volcano and potential mantle accretion in the continental back arc basin back arc basins are a primary target to understand lithospheric evolution in extension associated with plate subduction most of the currently active back arc basins formed in intraoceanic settings and host well developed spreading centers where seafloor spreading has occurred however rift structure at its initial stage a key to understand how the continental lithosphere starts to break in a magma rich back arc setting is poorly documented here we present seismological evidence for structure of the southern okinawa trough an active rift zone behind the ryukyu subduction zone we find that the southern okinawa trough exhibits an almost symmetric rift system across the rift axis yaeyama rift and that the sedimentary layers are highly cut by inward dipping normal faults the rift structure also accompanies a narrow 2 7 km wide on axis intrusion resulted from passive upwelling of magma on the other hand an active submarine volcano is located 10 km away from the rift axis the p wave velocity vp model derived from seismic refraction data suggests that the crust has been significantly thinned from the original 25 km thick arc crust and the thinnest part with 12 km thickness occurs directly beneath the rift axis the velocity model also reveals that there exists a thick layer with vp of 6 5 7 2 km s at lower crustal levels and may indicate that mantle materials accreted at the bottom of the crust during the crustal stretching the abrupt crustal thinning and the velocity depth profile suggest that the southern okinawa trough is at a transitional stage from continental rifting to seafloor spreading

comportement du systeme u pb dans la monazite et chronologie de la deformation et du metamorphisme des metasediments du domaine de kisseynew orogene trans hudsonien manitoba canada the kisseynew domain is the central unit of the reindeer zone of the paleoproterozoic trans hudson orogen in manitoba and saskatchewan the southern flank of the domain is a transition zone betwee

early proterozoic deformation along the southern margin of the kisseynew gneiss belt trans hudson orogen a 30 ma progressive deformation cycle fold axes and lineations across the southern margin of the kisseynew gneiss belt were progressively reoriented from northwesterly to northeasterly during an early proterozoic southwest directed compressive deformation cycle partitioning of deformation resulted in a concentration of non coaxial strain along the boundary between the kisseynew gneiss belt and flin flon snow lake belt the structural evolution was complicated by streamlining of the movement vector around the comparatively rigid flin flon snow lake belt the relationship of metamorphic assemblages to kinematic fabrics indicates that deformation was coeval with a prograde retrograde metamorphic cycle progressive deformation occurred during a period of 30 ma which is consistent with phanerozoic orogenic time scales

aseismic zone and earthquake segmentation associated with a deep subducted seamount in sumatra the subduction of topographic features has been linked both to earthquake generation and the segmentation of faults seismic imaging reveals a seamount subducted to 40 km depth below sumatra that is associated with an aseismic zone suggesting that at this location the seamount reduces coupling of the slab and overriding plate

evolution of the snow lake portion of the palaeoproterozoic flin flon and kisseynew belts trans hudson orogen manitoba canada abstract understanding growth of early precambrian crust requires knowledge of the evolution of granite greenstone and metasedimentary belts two of the fundamental building blocks of early precambrian cratons u pb geochronologic data from the snow lake area which straddles the flin flon granite greenstone and kisseynew metasedimentary belts of the trans hudson orogen documents a 60 70 ma period of palaeoproterozoic crust formation from inception of these belts in ocean basins through their eventual structural telescoping during ocean basin consumption and collision with bounding archaean cratons earliest volcanic and plutonic rocks of the flin flon granite greenstone terrane at snow lake formed 1 89 ga mainly in a juvenile oceanic arc tectonic setting sedimentary rocks of the kisseynew belt previously thought to be contemporaneous with snow lake area arc volcanism are up to 30 50 ma younger 1 86 1 84 ga and postdate deformation associated with 1 88 1 87 ga arc arc collisions and 1 87 1 84 ga calc alkaline plutonism that affected much of the western flin flon belt deformation at snow lake occurred during two widely spaced episodes both of which effect the post 1 86 1 84 ga turbidite sequence and thus postdate 1 88 1 87 ga intraoceanic accretion in the western flin flon belt the older deformational episode involved isoclinal folding formed during 1 84 ga convergence of the kisseynew and flin flon belts and thrusting of kisseynew sedimentary lithologies southwards onto the flin flon volcanic rocks the younger deformational episode occurred 15 25 ma later and involved further folding and southwest thrusting accompanied by a 1 81 ga high grade thermal metamorphic episode centred in the kisseynew belt only minor increase of crustal material in the form of late intrusions were added following this episode

field aligned observations of transequatorial bubbles from rarotonga in 1969 70 a raster scanning photometer with 0 5 field of view was used in 1969 70 to make images in 6300 a and 5577 a emission from rarotonga of tropical f region trans equatorial bubbles or depletions from this site the f region plasma structures in the emission layer can be viewed in a magnetic field aligned configuration a spatial resolution of approximately 15 km was achieved and revealed a variety of shapes in the bubble cross sections including bifurcations and pinched off bubbles eastward drift velocities averaged 125 ms 1 tilts were usually westward at about 45 in the equatorial plane but some were near zero and some were eastward the tops reached apex heights of over 800 km the structures appeared a little less diffuse in 5577 a emission than in 6300 a owing to less diffusion of o s with its 0 75 sec lifetime than o d with its 150 sec lifetime

finite extension and low angle normal faulting andersonian theory for infinitesimal strains predicts that normal faults will form at high angles to the surface in extensional terranes in contrast low angle normal faults are sometimes observed on which tens of kilometers of slip has occurred and moderate angle faults with slip of 5 to 15 km are common a simple extension of the theory to account for the stresses required to drive finite deformation shows that low angles are preferred for large extension on individual faults the formation of low angle faults may require inhomogeneities in stress or material properties but once slip begins on a low angle fault it is likely to continue whereas slip on high angle faults is likely to end after modest slip to be replaced by the formation of new high angle faults

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

accessory minerals and the geochemical evolution of crustal magmatic systems a summary and prospectus of experimental approaches it has recently been recognized that accessory minerals in continental rocks may contain a substantial fraction of the whole rock complement of trace elements and geochemically important isotopes this observation is in itself interesting but its importance is magnified many fold when one considers the possible consequences to the geochemical rearrangement that accompanies crustal melting if for example an accessory phase is insoluble in early formed partial melts it may effectively retain in the residue many incompatible elements e g ree and daughter isotopes e g radiogenic pb that would otherwise be partitioned into the melt in addition should an unmelted accessory mineral e g zircon be entrained in a departing melt fraction it may carry with it radiogenic components pb nd143 hf176 characteristic of the source region the effectiveness of phases such as zircon apatite sphene allanite epidote and monazite in the roles noted above can only be gauged from knowledge of the three fundamental accessory phase parameters i e 1 the solubilities of the accessory minerals in crustal melts 2 the equilibrium mineral liquid partition coefficients for the trace element and isotopes of interest and 3 the diffusivities that govern the rates at which equilibrium will be approached in general these parameters can only be measured in laboratory experiments and relatively few data have been accumulated to date it does seem clear however that apatite generally is residual insoluble during crustal melting and zircon can be depending on melting conditions and abundance of zircon in the source petrographic and geochemical studies indicate that monazite allanite and epidote may also be highly insoluble in some cases but the specific circumstances that promote insolubility remain unknown the few experimentally determined partition coefficients now available for apatite and zircon agree qualitatively with natural rock data on mineral separates however preliminary diffusion measurements and inferences from natural systems suggest that trace element and isotopic equilibrium between undissolved accessories and crustal melts is in fact rarely approached ironically it is this very disequilibrium in the form of preserved diffusion profiles that may afford the most information on time temperature histories of melting events continued experimental interest in the fundamental accessory phase parameters will eventually provide the tools that will enable rigorous assessment not only of the timing and duration of crustal melting episodes but also of the extent of melting and re distribution of trace elements and radiogenic isotopes

activation of very low frequency earthquakes by slow slip events in the ryukyu trench long term activity of shallow very low frequency earthquakes vlfes in the ryukyu trench unassociated with recent large thrust earthquakes was analyzed using a broadband seismometer network the distribution of shallow vlfes was divided into three large clusters the activity of the vlfes is modulated by repetitive slow slip events and the activity of these vlfes increases to 2 3 times its ordinary rate at 10 20 days after the onset of the slow slip events the activation of vlfes could be generated by small increases on the order of approximately 0 1 0 5 kpa in coulomb failure stress suggesting that stress in the plate interface where the vlfes occur is frequently released in small amounts moreover the distribution of vlfes is complementary to the historical tsunami source area and locked area the distribution of the vlfes indicates heterogeneity in interplate coupling along the trench

mapping a bubble at dip equator and anomaly with oblique ionospheric soundings of range spread f multiple ionospheric sounders are employed in the first attempt to detect and map remotely an equatorial bubble near the dip equator and simultaneously near the appleton anomaly crest the sounders also provide latitudinal profiles of electron density through 30 degrees dip latitude diplat conditions are solar maximum and low kp using oblique range spread f rsf echoes from the bubble boundaries four sounders located near the dip equator detect a single isolated bubble at ranges as great as a 1020 km and track it for 2 25 hours at 5 min intervals as it drifts eastward through 15 degrees geomagnetic longitude mlong maximum velocity is 185 m s and width is 375 km near the northern anomaly crest a sounder at 20 3 degrees diplat observes the western boundary for a duration of 1 hour concurrently with and magnetically conjugate to the equatorial observations the boundary here has nearly the same drift velocity and mlong as at the equator implying that the boundary location and velocity are nearly independent of altitude above the dip equator subsequently in the south a sounder at 19 9 degrees diplat observes the bubble to have a velocity approximately half that seen by other stations and a mlong that is at least 3 degrees more west of the others this difference may be related to the north south asymmetry of the anomaly crest which is at the location of the northern sounder but equatorward of the southern sounder although the two are at the same diplat the onset of the bubble is recorded in the first appearance of rsf at 1915 local mean time 20 min later the bubble boundary appears and begins accelerating eastward the movement occurring within about 5 min of electric field reversal as inferred from f layer vertical motion 30 refs 8 figs less

subduction of thick oceanic plateau and high angle normal fault earthquakes intersecting the slab the role of seamounts on interplate earthquakes has been debated however its impact on intraslab deformation is poorly understood here we present unexpected evidence for large normal fault earthquakes intersecting the slab just ahead of a subducting seamount in 1995 a series of earthquakes with maximum magnitude of 7 1 occurred in northern ryukyu where oceanic plateaus are subducting the aftershock distribution shows that conjugate faults with an unusually high dip angle of 70 80 ruptured the entire subducting crust seismic reflection images reveal that the plate interface is displaced over 1 km along one of the fault planes of the 1995 events these results suggest that a lateral variation in slab buoyancy can produce sufficient differential stress leading to near vertical normal fault earthquakes within the slab on the contrary the upper surface of the seamount plate interface may correspond to a weakly coupled region reflecting the dual effects of seamounts plateaus on subduction earthquakes

equatorial spread f found in 5577 a and 6300 a airglow observations from hawaii 1 we have searched for f region ionospheric disturbances in a database of airglow observations from mt haleakala hawaii the airglow scan observations were made with narrowband filters of oi 5577 a and oi 6300 a for 164 nights in the period of april 1965 through october 1968 the oi 6300 emissions are produced mostly via recombination of o2 in the f region whereas the oi 5577 emissions are generated both in the e and f regions the database has unique significance since it covers more than three years of observations in the 1960s when there was no other program that systematically monitored ionospheric airglows we find disturbances in the oi airglow brightness from which we constructed projected maps of airglow distributions at the f region altitude the projected maps clearly show wavelike disturbances whose crests are mostly aligned along a north south direction structures seen even in the raw scan data the disturbances were found exclusively in the southerly azimuth toward the equator implying that they are one of the equatorial ionospheric irregularities known as equatorial spread f esf the esf disturbances occurred 19 nights out of a total of 161 nights of azimuth scanning observations with a maximum in september the disturbances seem to arise regardless of geomagnetic activities the seasonal variation of the occurrence is consistent with recent analyses of satellite data in which esf is suggested to be caused by gravity waves from the troposphere where they are generated by strong convection in intertropical convergence zone

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

weak interplate coupling by seamounts and repeating m 7 earthquakes subducting seamounts are thought to increase the normal stress between subducting and overriding plates however recent seismic surveys and laboratory experiments suggest that interplate coupling is weak a seismic survey in the japan trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude m 7 both observed seismicity and the pattern of rupture propagation during the 1982 m 7 0 event imply that interplate coupling was weak over the seamount a large rupture area with small slip occurred in front of the seamount its northern bound could be determined by a trace of multiple subducted seamounts whereas a subducted seamount itself may not define the rupture area its width may be influenced by that of the seamount

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

insights into the structure and tectonic history of the southern south island new zealand from the 3 d distribution of p and s wave attenuation