new visualizations of global tectonic plate motions and plate boundary interactions clear understanding of detailed lithospheric plate motions has been impeded by lack of a suitable means of graphical representation a series of coloured global maps are presented that reveal more detail in the patterns of both absolute and relative global plate motions the use of continuous colour to represent velocities overcomes the limitations of earlier maps that used isolated vectors at selected points to indicate plate velocities velocity magnitudes and directions for entire surfaces of plates were computed at a resolution of 0 5 and are shown on two separate maps relative motions between plates were decomposed into their shear and normal components and are plotted on separate maps continuous colour is again used to indicate both the directions and magnitudes of sinistral dextral and convergent divergent motions for all plate boundaries a final map of normalized velocity magnitudes for all plates reveals a global fast belt of plate motion that parallels a great circle aligned with the fastest portion of the pacific plate and orthogonal to the east pacific rise

japan sea a pull apart basin abstract recent field work in the hokkaido central belt and marine geology studies along the eastern margin of japan sea in addition to previously published data lead us to propose a new model of opening of the japan sea the synthesis of both on land and offshore structural data gives new constraints about the structural evolution of the system the rhombohedral shape of the japan basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early eo oligocene rifting of a pull apart basin accommodated along two large right lateral shear zones east of korea and west of northeast japan and sakhalin it is followed during upper oligocene lower miocene by the main opening of the japan basin as a mega pull apart then a back arc spreading probably related to the subduction process induced the creation of the yamato and tsushima basins at the end of lower miocene and in middle miocene clockwise rotation of southwest japan larger than 20 or major bending of honshu mainland deduced from paleomagnetic studies is unlikely at this time since 1 or 2 my b p to present compression prevails along the eastern margin of the japan sea the generation of marginal basins as pull apart basins along intracontinental strike slip faults is a mechanism which has been proposed by other authors concerning the south china sea the question then is whether the fragmentation of the asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one

back arc extension in the scotia sea the nature of back arc extension in the east scotia sea is re examined with the use of an enlarged geophysical data set well developed oceanic magnetic lineations confirm that the present spreading episode started about 8 ma ago that spreading is asymmetric and that the total rate increased from 50 to 70 m m a about 1 5 ma ago most of the currently active south sandwich volcanic island arc lies upon ocean floor only 6 8 m a old and generated at the current spreading ridge subsequent extension has not modified the curvature of the arc east west magnetic lineations of miocene age in the central scotia sea and contemporaneous low k arc tholeiites dredged from the eastern south scotia ridge discovery bank indicate a regime of coupled subduction and back arc extension preceding that occurring now a speculative model involving a series of collisions of parts of this earlier discovery trench with ridge crest sections of the south american antarctic plate boundary explains the transformation of this earlier regime into the present self contained sandwich plate regime the considerable small scale variability observed in the back arc region may be seen as an inevitable consequence of the action of the ridge trench collision mechanism the entire scotia sea could have formed by a similar kind of back arc extensional modification of the south american antarctic plate boundary

new insights into the tectonic evolution of the andaman basin northeast indian ocean multibeam swath bathymetry data acquired over an area of about 30 000 km2 together with magnetic and single channel seismic data have been analyzed to understand the tectonic evolution of the andaman basin northeast indian ocean swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc parallel seamount chains and n s trending fault systems and a relatively smooth eastern part a sw ne trending spreading ridge bisects the basin an offset of about 11 8 km at 94 21 e longitude divides the spreading ridge into sw and ne segments high resolution bathymetry and magnetic data enabled us to identify two finer scale segments over the sw part of the ridge that display distinct topographic fabric compared to the ne segment thin sediment cover and presence of axial seamounts characterize sw segments of the ridge whereas the ne segment has thick sediment cover and well maintained rift valley kinematics of basin evolution derived from the identification of the seafloor spreading magnetic anomalies suggest very slow initial opening rates of about 1 6 cm yr and an increase in the spreading rate to about 3 8 cm yr from anomaly 2 to present although this interpretation relies only on magnetic anomalies in the two westernmost segments it is suggested that the true seafloor spreading started about 4 ma in the andaman backarc basin rather than the 11 ma postulated previously we infer based on the integrated analysis of swath bathymetry magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the sagaing and semengko fault systems extension by seafloor spreading during the past 4 myr resulted in the formation of the deep andaman backarc basin this phase has also experienced westward propagation of the spreading center

the depth distribution of mantle anisotropy beneath the tonga subduction zone abstract shear wave splitting recorded by a temporary deployment of broadband seismometers located above the tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the tonga back arc region splitting parameters obtained for local s and teleseismic sks phases were modeled using a ray based method that predicts shear wave splitting on individual phase paths and determines the best fitting values of anisotropic strength orientation and depth extent splitting in teleseismic sks phases is identical to that in s phases from local earthquakes that occur at the base of the transition zone demonstrating that there is no significant splitting due to anisotropy in the lower mantle splitting times for local s phases do not vary significantly with depth and for models in which the strength of anisotropy is laterally uniform they rule out anisotropy in the transition zone finally the observed splitting indicates that anisotropy of more than 0 8 exists in the upper mantle with a fast symmetry axis at 60 w roughly parallel to the absolute plate motion vector of the subducting pacific lithosphere

propagating rift west of the fiji archipelago north fiji basin sw pacific a sea beam survey of an area located west of the fiji islands in the north fiji basin between 17 10 s and 18 s and 175 40 e and 176 40 e shows a complex structure characterized by two deep grabens bounding a central plateau in previous papers we interpreted this area as an intraoceanic strike slip deformation zone rejecting the possibility of oceanic spreading in the area by comparison with the structures observed in the galapagos 95 5 w area we now reinterpret the area as a southward propagating spreading system the analysis of structural and magnetic data and earthquake focal mechanisms confirms this new interpretation the full spreading rate for the north fiji basin ridge deduced from magnetic lineations varies from 50 to 80 mm yr for the west fiji spreading ridge the magnetic anomaly analysis gives a spreading rate of about 50 mm yr for the northern segment and slightly slower 40 mm yr for the southeastern segment using a 50 mm yr spreading rate the angles between the pseudofaults allow us to calculate the propagation velocity for an angle of 75 north of 17 35 s the propagation velocity is 33 mm yr the tip of the propagator is characterized by a 20 angle between the pseudofaults which implies a recent increase of propagation velocity to 141 mm yr the initiation of this propagation could be related to the emplacement of the north fiji fracture zone 1 to 1 5 m y ago resulting in the formation of the 16 50 s triple junction the main kinematic implication of the existence of a propagating rift west of the fiji islands is that in the north fiji basin the present day accretion is distributed on two parallel active spreading ridges one in the central part of the basin the other one in its southeastern quadrant such a phenomenon unusual on mid oceanic ridges except for microplates such as galapagos easter and juan fernandez may be more common in marginal basins because of their peculiar geodynamic setting

the geometry of aleutian subduction three dimensional kinematic flow model we construct a steady state kinematic model of slab flow along the aleutian arc assuming the slab s geometry is known the slab geometry is constrained by local network data where the slab is seismically active and by p wave residual sphere analysis of 11 earthquakes and one nuclear explosion where it is aseismic the slab is approximated by a thin newtonian sheet subducting in a less viscous and incompressible mantle particles enter the trench at known rates of relative plate motion and are required to subduct along the predefined slab geometry for discrete but small 0 1 m y time steps we determine the speed and direction of each slab particle that minimizes dissipation power as the spherical shell oceanic lithosphere deforms to obtain the observed slab shape because the dip of the slab in the central aleutians is steep relative to the radius of curvature of the trench the slab must stretch along arc to obtain its present configuration our kinematic model represents one internally consistent flow that results in the observed geometry furthermore it provides a lower bound on the total amount of internal in plane deformation required by the shape and an estimate of stream lines and of the orientation and spatial distribution of the deformation rate the predicted in plane strain rate field is dominated by along arc extension reaching values as large as 10 15s 1 in the central aleutians and decaying by an order of magnitude to the east and west the slab accumulates an in plane strain of 10 by the time it reaches the depth of the seismicity cutoff along arc variations in both the magnitude and orientation of calculated strain rates are consistent with the spatial distribution of seismic moment release and source mechanism orientations of intermediate depth earthquakes suggesting that in order to understand intermediate focus earthquakes one must consider along arc deformation three dimensional analyses of subduction in a spherical geometry such as the present study are required to model along arc effects relative plate motions along the trench vary from nearly normal in the eastern aleutians to transform farther west coincident with this change the maximum depth of seismicity within the descending slab decreases from 250 to 50 km and active andesitic volcanism ceases the kinematic flow model predicts that the slab subducted obliquely along the central portion of the aleutian arc is transported laterally westward producing cold slab material to a depth of at least 300 km along the entire arc after only 15 m y of subduction the lack of volcanic activity in the western aleutians is apparently related to the lateral transport of the slab perhaps indicating that the slab is dehydrated and incapable of producing the volatiles necessary for arc magmatism the along arc variation in the maximum depth of seismicity can be explained thermally we construct temperature estimates within the slab using a quasi three dimensional finite difference scheme including entrained mantle flow stream lines from the kinematic model and the time history of the age of the subducting lithosphere as a function of position along the trench the theoretical temperature at the seismicity cutoff depth is remarkably close to a constant fraction of the depth dependent mantle solidus along the entire arc this observation is consistent with the hypothesis that seismicity terminates because of the initiation of high temperature steady state creep and provides an explanation for the lack of intermediate focus earthquakes in the western aleutians even though slab material exists there

lunar magnetism polar displacements and primeval satellites in the earth moon system the discovery that rocks of the lunar crust possess remanent magnetism is a surprising result in view of the fact that the present moon does not exhibit a magnetic field nor was there once evidence of an iron core in which one could be generated this review looks at the concept of lunar palaeomagnetism and discusses its implications for lunar science as a whole

scotia sea tectonics from high resolution satellite gravity the release of altimetric data from the geosat geodetic mission by the us navy 1 is leading to a much improved understanding of tectonics in the southern ocean a region in which remoteness and adverse physical conditions have limited the acquisition of geophysical data by research ships the scotia sea is an outstanding example of back arc spreading which is revealed in some detail by free air gravity anomaly maps derived from the latest release of data acquired south of 30 s 2 sea surface height data for this region have been reduced to a grid of free air gravity anomalies and are illustrated here by means of colour shaded relief and contour maps the new data confirm the existence of a number of inactive spreading ridges within the scotia sea and surrounding small basins the amplitudes and wavelengths of gravity anomalies over these ridges conform in general to the expected relationship with spreading rate except in the central scotia sea where a proposed miocene slow spreading ridge appears to have left no clear signature the spreading ridge axis in the east scotia sea comprises seven or more segments separated by small mainly sinistral offsets and exhibits a median valley with depths of 200 1000 m that is reflected in free air lows of 10 40 mgal near both its northern and southern termini the gravity signature of the ridge becomes less distinct with a less pronounced axial low the northernmost segments of the ridge are displaced in a right lateral sense by a feature which appears to represent a southward migrating non transform offset whereas the process of spreading in drake passage and the east scotia sea was comparable to mid ocean ridges that in the central scotia sea may have been disorganized as observed in some western pacific back arc basins

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

petrology of mariana trough and lau basin basalts the crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid ocean ridge basalts n morb but some subtle but distinctive differences exist these are largely due to differences in volatiles fe oxidation ratio abundances of large ion lithophile elements lil and some high field strength hfs elements isotope data for sr pb nd he and o are variable but broadly are morb like we present data contrasting mariana trough mtb and lau basin basalts lbb showing how they differ in certain trace element and isotopic signatures and compare both to n morb relative to n morb and lbb the mtb are enriched in alkalis alkaline earths light rare earth elements ree h2o and fe fe even for primitive samples while having morb type he 3 4 7 8 to 8 4 ra sr 87 86 mean 0 70293 and nd e 10 2 the lau basin is zoned with a central area having n morb characteristics for h2o lil ree nd e 8 1 to 9 0 and o but a depletion in hfs elements sr is more radiogenic mean 87 86 0 70347 and 3he 4he 3 4 7 2 to 11 0 ra is high relative to n morb the lau basin has a zone bordering the adjacent island arcs which is like mtb in both back arc basins relatively primitive unfractionated basalts with mg 100 mg fe 62 are found which resemble postulated primitive n morb calculated parental magmas obtained by adding xenocrystic olivine plot near or on 3 and 4 phase boundaries at 1 1 5 gpa for the ol di si system low p fractional crystallization and magma mixing explain most of the observed rock types possible explanations for differences between mtb and lbb include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere the long and complex history of mariana trough mantle sources led to a metasomatized mantle which gives rise to the distinctive mtb in contrast the lau basin is dominated by basalts derived from a source depleted in volatiles lil and hfs elements a previous melting history is implied mixing with a mantle component enriched in 87sr and 3he may be due to mantle counterflow driven by the subduction process

geophysical evidence for post miocene rotation of the island of viti levu fiji and its relationship to the tectonic development of the north fiji basin geophysical studies of the island of viti levu fiji show largeamplitude magnetic anomalies of up to 1500 nt to be located above the tavua volcanic center field sampling of the volcanic rocks from the intrusive and extrusive centers of the volcano show the rock samples to have initial intensities of natural remanent magnetization of up to 380 a m radiometric dating of fifteen oriented samples show an age range of 2 9 to 7 0 m y b p for the suite of rocks collected from the mba basaltic group paleomagnetic analysis of the dated rocksshows declination vectors ranging from 325 9 for an age of 2 9 0 2 m y b p to 290 4 for an age of 7 0 0 4 m y b p as a group the paleomagnetic declination values of the samples systematically decrease with age this decrease is attributed to progressive anticlockwise rotation of viti levu during the process of formation of the north fiji basin a linear least squares analysis of the paleomagnetic data indicates that the island has rotated in an anticlockwise direction through 90 during the past 7 m y at a rate of 13 7 per million years declination values as low as 245 measured for some sites not dated suggest that anticlockwise rotation of viti levu and development of the north fiji basin could have commenced as early as 10 m y b p following the breakup of the ancient solomons new hebrides fiji lau island arc large rotations of microplates may have occurred along other island arcs and marginal basin systems elsewhere in the western pacific and such rotational histories may reflect in detail the evolutionary development of these arcs and basins

lunar paleointensity measurements implications for lunar magnetic evolution abstract we analyze published and new paleointensity data from apollo samples to reexamine the hypothesis of an early 3 9 3 6 ga lunar dynamo our new paleointensity experiments on four samples use modern absolute and relative measurement techniques with ages ranging from 3 3 to 4 3 ga bracketing the putative period of an ancient lunar field samples 60015 anorthosite and 76535 troctolite failed during absolute paleointensity experiments samples 72215 and 62235 impact breccias recorded a complicated multi component magnetic history that includes a low temperature 500 c component associated with a low intensity 2 t similar multi component behavior has been observed in several published absolute intensity experiments on lunar samples additional material from 72215 and 62235 was subjected to a relative paleointensity experiment a saturation isothermal remanent magnetization or sirm experiment neither sample provided unambiguous evidence for a thermal origin of the recorded remanent magnetization we test several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples specifically an overprint from exposure to a small magnetic field an isothermal remanent magnetization results in multi component behavior similar to absolute paleointensity results from which we could not recover the correct magnitude of the original thermal remanent magnetization in light of these new experiments and a thorough re evaluation of existing paleointensity measurements we conclude that although some samples with ages of 3 6 to 3 9 ga are strongly magnetized and sometimes exhibit stable directional behavior it has not been demonstrated that these observations indicate a primary thermal remanence particularly problematic in the interpretation of lunar sample magnetizations are the effects of shock as relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable consequently current paleointensity measurements do not support the existence of a 3 9 3 6 ga lunar dynamo with 100 t surface fields a result that is in better agreement with satellite measurements of crustal magnetism and that presents fewer challenges for thermal evolution and dynamo models

800 km long n s spreading system of the north fiji basin abstract the 800 km long n s spreading system in the north fiji basin consists of six contiguous fan shaped rift segments each 100 200 km long with various structural styles such as structural overprinting triple junction rift propagation voluminous magmatism and transform or strike slip fault influence deformation of the plate boundary system within the young hot weak lithosphere occurred under local stress conditions resulting in a unique rift segmentation distinct from that of midoceanic ridges spreading across a series of short and variably oriented segments produces series of fanning spreading centers whose rotation pole is located at the end of each segment the surrounding seafloor basement has adjusted to this setting with non rigid deformation or with fragmentation furthermore the changing tectonic framework caused by the arc rotation probably makes it difficult to keep the same stress condition for a long period small scale fan shaped sea floor spreading is short lived and results in a rapidly evolving plate boundary geometry this suggests a more complicated tectonic style for marginal basins than mid ocean ridge spreading centers

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

shallow seismicity and tectonics of the central and northern lau basin abstract the lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time as such crustal earthquakes within the lau basin derive from intrabasin tectonic volcanic and hydrothermal sources because of high mantle attenuation small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity however the 1994 labatts ocean bottom seismograph experiment in the lau basin recorded more than 100 000 local t and body phases allowing for a detailed examination of basin seismotectonics nearly 1000 events are located within the basin proper with another 2000 associated with the forearc and the aseismic front we identify two previously undocumented tectonic features a triple junction in the northwestern basin and a nanoplate in the central lau basin the single most seismically active feature within the lau basin is the lau extensional transform zone letz just north of the central lau spreading center significant seismicity also occurs near the northern extension of the eastern lau spreading center and along a southeast extension of the letz delineating the boundary of a previously unknown nanoplate swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics we see no evidence of deformation along the southern boundary of the niuafo ou microplate where 4 5 cm yr of strain is predicted it is possible that the plate boundary extends eastward from the fonualei rift fr tip towards the trench creating a niuatoputapu tonga plate division rather than westward implied by the currently accepted niuafo ou tonga plate system but more data are necessary from the fr to test that hypothesis instead we suggest that the niuafo ou tonga pole is 1 5 further north than previously proposed reducing the predicted strain in this region to

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

seismicity and crustal structure of the fiji platform southwest pacific data from a telemetered seismic network in fiji provide new constraints on the crustal structure and active deformation within and around the fiji platform an unusual fragmented arc complex situated within the pacific indo australian plate boundary seismic refraction data indicate that crustal thickness in fiji is typical of island arcs averaging about 15 20 km with relatively low midcrustal seismic velocities vp 6 0 km s characteristic of felsic arc related rocks the anticipated higher velocity lower crustal layer is either thin or absent mantle velocity beneath the platform is low 7 6 km s in the same range as that observed beneath the neighboring marginal basins and island arcs the record of crustal seismicity based on historical teleseismic and microearthquake catalogs indicates significant activity both within and along the boundaries of the fiji platform earthquakes located along the edges of the platform are associated with major zones of back arc deformation of the north fiji and lau basins the fiji fracture zone to the north the hunter fracture zone to the south and a deformed zone in the eastern portion of the north fiji basin to the west earthquakes within the platform intraplate events occur more sporadically than those along its margins areas of significant intraplate activity include the taveuni northeastem vanua levu area koro sea the baravi basin and central and south eastern viti levu earthquake depths within the platform are concentrated in the upper 13 16 km of lithosphere as in other areas of extensional strike slip tectonics in fiji however the seismicity apparently extends through the entire crustal section the large 1953 intraplate suva earthquake ms 6 75 was caused by strike slip faulting under the same east west extensional stress field that is deforming the north fiji and lau basins microearthquakes within and around the island of viti levu appear to occur as a result of the same stresses these earthquakes occur in discrete nests that are tightly clustered in space and time they are related to late cenozoic faults that cut the mio pliocene rocks of viti levu the characteristics of its crustal structure the anomalous mantle velocity and the active deformation within the fiji platform distinguish it from microcontinental blocks that have been observed elsewhere the seismicity and focal mechanisms within the fiji platform demonstrate that it is not technically isolated from the active deformational processes that pervade the neighboring arcs and back arc basins

double saloon door tectonics in the north fiji basin abstract fiji platform rotated counter clockwise from at least 10 2 ma until 1 56 ma while vanuatu arc rotated clockwise from 12 10 ma until the present a revised model incorporating these opposite rotations explains the distribution of magnetic anomalies in the north fiji basin nfb the conjugate margin of southwest fiji platform is southeast vanuatu arc previous models which associate nw oriented anomalies off southwest fiji with similarly oriented anomalies northeast of vanuatu arc are therefore wrong secondly these models propose a nw oriented spreading ridge extending to 19 s 177 5 e almost 500 km southeast of the mapped extension of rifted island arc crust on vanuatu arc this creates an unacceptable overlap with fiji platform in pre rift reconstructions thirdly anomalies off sw fiji which are nw oriented in their present day position were oriented nne during initial breakup rather than aligning with nw oriented anomalies in the western nfb they are matched by nne oriented anomalies off se vanuatu arc with further rotation these conjugate anomaly sets form a fan shape in the southern nfb fourthly previous models which recognise a triple junction only from 3 ma do not explain early wnw ese separation of vanuatu arc and fiji platform required by well documented opposite rotations nfb characteristics which match the double saloon door tectonic model include opposite rotations of island arc terranes backarc seafloor spreading which is both arc parallel and arc perpendicular and rifts propagating north south northeast and northwest features which do not match the double saloon door model include the north fiji fracture zone and the west fiji spreading centre both initiated post 1 5 ma when fiji platform stopped rotating and only one terrane vanuatu arc continued to rotate the nfb developed via double saloon door tectonics from 12 10 ma until 1 5 ma whereas post 1 5 ma opening constitutes single saloon door rotation

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

arc deformation and marginal basin opening japan sea as a case study we discuss the opening mechanism of the japan sea in miocene time using 1 tectonic and published paleomagnetic data along the eastern margin from the north of hokkaido island to sado island 2 a mechanical model which is tested by small scale physical modeling and 3 crustal structure and bathymetric features in the japan sea which constrain our kinematic model and preopening reconstructions our main conclusions are the following the eastern margin of the japan sea was as a whole a dextral shear zone about 100 km wide this conclusion is supported by the existence of a ductile dextral shear zone in central hokkaido hidaka mountains and associated brittle deformation in western hokkaido and northeastern honshu the stress field during the opening which ended about 12 ma ago at the end of the middle miocene changes from right lateral transpression in the north to right lateral transtension in the south the western margin along the korean peninsula during the same period also was an active dextral shear zone paleomagnetic results indicate that clockwise rotations occurred in the south during the opening and counterclockwise rotations in the north we propose a model of right lateral pull apart deformation with clockwise rotations of rigid blocks in the southern transtensional domain and counterclockwise rotations in the transpressional one small scale physical models show that the clockwise rotation in transtension is possible provided that the eastern boundary pacific side is free of stress the opening stopped and compression subsequently began about 12 ma ago finally we show that the dextral shear which is distributed over the whole japan sea area is accommodated by n s trending right lateral faults and rotation of blocks located between these right lateral faults

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

the constitution and structure of the lunar interior the current state of understanding of the lunar interior is the sum of nearly four decades of work and a range of exploration programs spanning that same time period missions of the 1960s including the rangers surveyors and lunar orbiters as well as earth based telescopic studies laid the groundwork for the apollo program and provided a basic understanding of the surface its stratigraphy and chronology through a combination of remote sensing surface exploration and sample return the apollo missions provided a general picture of the lunar interior and spawned the concept of the lunar magma ocean in particular the discovery of anorthite clasts in the returned samples led to the view that a large portion of the moon was initially molten and that crystallization of this magma ocean gave rise to mafic cumulates that make up the mantle and plagioclase flotation cumulates that make up the crust smith et al 1970 wood et al 1970 this model is now generally accepted and is the framework that unifies our knowledge of the structure and composition of the moon the intention of this chapter is to review the major advances that have been made over the past decade regarding the constitution of the moon s interior much of this new knowledge is a direct result of data acquired from the successful clementine and lunar prospector missions as well as the analysis of new lunar meteorites as will be seen results from these studies have led to many fundamental amendments to the magma ocean model much of what we know from sample analyses has been previously summarized elsewhere and only their most important aspects will be discussed in this chapter the reader is referred to the relevant chapters in the books basaltic volcanism on the terrestrial planets basaltic volcanism study project 1981 the

the offshore morpho structure and tsunami sources of the viti levu seismic zone southeast viti levu fiji abstract new high resolution seabat 8160 multibeam bathymetry data reveal in detail the complex offshore morphology over part of the viti levu seismic zone vlsz in southeast viti levu the mapped seafloor is scarred by a complex network of linear submarine canyons and numerous submarine slides the submarine canyons are morphologic expressions of graben structures in the underlying basement rock submarine canyon faults are continuous with nearshore shelf faults and onshore fault controlled valleys and structural lineaments together these define continuous onshore offshore fault zones which combined form a structurally complex fault mesh of composite origin we attribute the development of the principal fault sets in the fault mesh to progressive distributed faulting in response to anticlockwise tectonic block rotations within the fiji platform during the neogene low occurrence of large earthquakes and a diffused pattern of seismicity currently observed in the vlsz may reflect complex coseismic faulting through the fault mesh the abundance of submarine slides over the vlsz which typically occur on the outer barrier reef edge as well as the submarine canyon heads and walls and mid slope imply that seismic shaking may be an important factor for triggering slope failure morphometric analysis and empirical modelling of the slides show that slides triggered in shallow water depths within 5 km of the coastline at the outer barrier reef edge and submarine canyon heads produce the largest near field tsunami amplitudes we interpret that such slides represent a significant local tsunami hazard

arc stresses determined by slabs implications for mechanisms of back arc spreading there is an anti correlation in stress field between the back arc area and the shallow portion of the slab generally slabs showing down dip compression have tensional back arcs and the reverse we explain this anti correlation by a force balance in the fore arc wedge between the slab pull collision force in the upper plate and the ridge push back arc spreading can occur as a passive response to the down dip compressional slab the mariana kyushu and aegean arcs are the exceptions to the anti correlation in these arcs the back arc is tensional although the slab is in down dip tension this may be because the fore arc is driven by the mantle drag toward the trench resulting in compression balanced with the slab pull the flow in the mantle would cause back arc spreading in this case

shear wave splitting in a young continent continent collision an example from eastern turkey an edited version of this paper was published by the american geophysical union agu copyright 2003 agu see also http www agu org pubs crossref 2003 2003gl017390 shtml http atlas geo cornell edu turkey publications sandvol et al 2003b htm

combined geological and gravimetric mapping and modelling for an improved understanding of observed high resolution gravity variations a case study for the global geodynamics project ggp station moxa germany geodynamic observatories around the globe continuously monitor signals like gravity tilt and strain as a function of time however global signals are often masked by local effects caused by the direct surroundings of the station including the local geological setting this link is well established for superconducting gravimeters sg that observe the gravity field variations at very high resolution an enhancement of the sg time series by the application of local correction is exemplarily shown here in a very practicable procedure for the geodynamic observatory moxa germany we show how the combination of geological and gravimetrical mapping and modelling around moxa results in a significant correction of the original gravity data as can be proven by comparison with the satellite derived gravity field detailed geological mapping of the observatory surroundings including measurements of fold axes foliations and joints was the basis of the present study the fold structure in the area of interest was interpreted by geometrical 3d modelling the complete representation of different rock types in space also provides a better understanding of the local hydro geological situation using a combination of the 3d geological model with the high resolution bouguer map of the observatory surroundings we developed a 3d density model this model enables the correction of small gravity effects caused by mass variations close to the sg in particular local hydro geological mass changes thus the procedure presented here based on a close connection of geology and gravimetry is a powerful tool for the reduction of local gravity effects on sg recordings it should be applicable to sg stations worldwide where similar hydrologically driven mass changes can be assumed

active tectonism within the fiji platform southwest pacific new observational data on the velocity structure seismicity and state of stress of the large submarine platform that includes the fiji islands demonstrate that the platform is an active participant in the pervasive regional deformation of the pacific indian australian plate boundary refraction data collected within the fiji platform demonstrate that its crustal thickness 15 25 km and seismic velocity structure are comparable to those of active island arcs uppermost mantle velocity beneath the platform is anomalously low 7 55 km s in the same range as that observed beneath the neighboring marginal basins and island arcs shallow seismicity is observed both within and along the boundaries of the fiji platform moderate to large earthquakes within the platform occur more sporadically than those along its margins these intraplate events are separated by long periods of quiescence earthquake depths within the platform are concentrated in the upper 13 16 km as in other areas of extensional strike slip tectonics both the large 1953 suva earthquake m s 6 75 and microearthquakes within and around the island of viti levu occur by strike slip faulting under the same stress field that is deforming the north fiji and lau basins the low upper mantle velocities presence of widespread seismicity and neogene volcanism and state of stress of the fiji platform contradict the notion of a stable land mass surrounded by zones of deformation in the neighboring back arc basins

a review of lunar paleointensity data and implications for the origin of lunar magnetism the saturation remanence normalization method of paleointensity estimation has been applied to more than 50 lunar samples and has been calibrated in terms of standard intensity determinations done on 14 samples these results suggest that a lunar surface field comparable in intensity to the earth s magnetic field existed from about 3 6 to 3 8 ae absolute paleointensity estimates and lunar surface field strength determinations are consistent with this observation the data presented here differ from previous descriptions of the variation in the lunar field with time in that a the oldest samples 3 9 ae are among the most weakly magnetized and b an order of magnitude drop in paleointensity is suggested between the time of extrusion of the apollo 11 low and high potassium basalts the limited range in age of the strongly magnetized lunar samples and the absence of shocked samples in this set argues against a solar source for the lunar paleofield or for fields arising from transient phenomena such as impacts the coincidence of this high field era with the termination of basin forming impacts and the beginning of mare basalt extrusion suggests a common cause a close approach of the moon to the earth at about 3 8 ae might account for all three phenomena the confined duration of the strong lunar field may allow for the assignment of relative ages for certain lunar samples and surface features

two modes of back arc spreading back arc spreading is initiated with rifting along the volcanic zone of an island arc under extensional tectonics the width of the volcanic zone of the arc which is closely related to the dip angle of the subducting slab should cause a variation in the style of initial rifting and subsequent back arc spreading two modes of back arc spreading single rift type and multirift type are introduced from the comparative study of the bonin arc and the japan sea divergent tectonics in an island arc above a steeply dipping slab will cause a single rift system in a narrow volcanic zone but will cause a multirift system in a broad volcanic zone which is related to a shallow dipping angle of subduction

evidence for a holocene high sea level stand vanua levu fiji abstract two distinct elevations of emerged notches can be recognized on the southeastern coast of vanua levu the higher notch is 2 22 to 3 30 m above low tide level and the lower 1 52 to 1 65 m above low tide level many emerged holocene corals encrust higher notch floors and benches or occur as microatolls and from these radiocarbon ages ranging from 6000 to 3400 yr b p were obtained the higher notches are significantly higher than present high tide level even allowing for relative tectonic uplift this is supported by elevations of emerged microatolls and their 14 c ages thus mean sea level of the mid holocene was higher than that at present

anisotropy and flow in pacific subduction zone back arcs we have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the tonga southern kuril japan and izu bonin subduction zones using shear wave splitting in s phases from local earthquakes and in teleseismic core phases such as sks the observed splitting in all four subduction zones is consistent with a model in which the lower transition zone 520 660 km and lower mantle are isotropic and in which significant anisotropy occurs in the back arc upper mantle the upper transition zone 410 520 km beneath the southern kurils appears to contain weak anisotropy the observed fast directions indicate that the geometry of back arc strain in the upper mantle varies systematically across the western pacific rim beneath izu bonin and tonga fast directions are aligned with the azimuth of subducting pacific plate motion and are parallel or sub parallel to overriding plate extension however fast directions beneath the japan sea western honshu and sakhalin island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing models of back arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in tonga and izu bonin but further three dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern kurils and japan the fast directions in the southern kurils and japan may require strain in the back arc mantle that is driven by regional or global patterns of mantle flow

the tavua caldera fiji a complex shoshonitic caldera formed by concurrent faulting and downsagging the shoshonitic tavua volcano in viti levu fiji developed from a submarine to a subaerial environment and comprises lava flows dominantly of absarokite composition with debris flows and conglomerates on its flanks the tavua caldera formed in the centre of the volcano within the caldera avalanche deposits are overlain by the turtle pool formation which consists of lacustrine sediments shoshonite lavas and hydrovolcanic pyroclastic lapilli tuffs subsequently the inner caldera formed in the centre of the tavua caldera this was infilled by the morrisons pool formation which comprises lavas and pyroclastic rocks of banakite composition with minor intercalated sediment thus the magmas became progressively more evolved in addition to normal movement along regional and concentric faults subsidence which exceeds 3 km was accomplished via substantial downsagging evidence for this is the consistent centripetal dip of caldera fill material which locally exceeds 50 in parts of the turtle pool formation centripetal dips are also recognized in pre caldera rocks immediately outside the tavua caldera subsidence was episodic caused by periodic non explosive withdrawal of underlying magma to erupt on the flanks and in part within the caldera downsagging and fault movement were responsible for approximately equal amounts of subsidence and their interaction produced a caldera with the shape of a stepped funnel

arc dynamics and tectonic history of fiji based on stress and kinematic analysis of dikes and faults of the tavua volcano viti levu island fiji 1 stress analysis of dikes and kinematic analysis of faults within the tavua volcano of viti levu fiji indicate an apparent migration of the causative maximum principal stress axis 1 from nw toward north between 5 0 and 3 9 ma the dikes and faults are part of successive structural and magmatic events at the arc scale we present data on the chronological succession of stress indicators in the tavua volcano based on overprinting relationships and published geochronology which also equate with a counterclockwise rotation of fiji assuming an approximately constant n s regional compressive stress orientation during this time the latter interpretation is consistent with paleomagnetism studies that require substantial counterclockwise rotation of the fiji platform since fragmentation of the formerly continuous outer melanesian arc during the latest miocene to earliest pliocene the stress data suggest that the fiji platform has undergone an 50 rotation since 5 ma given that rotation apparently stopped at 3 ma then particularly fast rotation of 25 myr 1 occurred from 5 ma to 3 ma recent seismicity data indicate the platform is currently subject to n s compression the history of late miocene and pliocene events in the region can be related to the consequences arising from collision of an island arc with an oceanic plateau

right lateral shear along the northwest pacific margin and the india eurasia collision right lateral shear along the eastern margin of asia from the eocene to the present has led to the opening of pull apart basins intracontinental such as the bohai basin or oceanic such as the japan sea we suggest in this paper that this right lateral shear is a consequence of indentation of asia by india as in small scale analog experiments we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left lateral shear zones tien shan baikal stanovoy for the northern one and qin ling for the southern one we discuss the compatibility of this mechanism which involves a rather small amount of extrusion with the fast eastward expulsion described for southeast asia we re emphasize the role played in the opening of marginal basins by the pacific subduction as a free boundary to the east

kinematics of active spreading in the central north fiji basin southwest pacific abstract based on a synthesis of magnetic and bathymetric data we re evaluate the kinematics of the recent opening of the central part of the north fiji basin nfb the westward motion of the pacific plate along the left lateral north fiji fracture zone nffz results in the opening of two n s trending spreading ridges located at 173 30 e and 176 e both ridges show complex features such as propagating rifts ridge jumps and overlapping spreading centres their spreading rates are similar 7 6 to 4 cm yr 1 across the western ridge 5 5 cm yr 1 across the eastern one while the nffz is purely strike slip to the east of the eastern ridge it becomes more complex to the west it changes from transpressional to transtensional then to purely transform and finally joins the western n s ridge in a rrr type triple junction our kinematic analysis shows that most of the left lateral motion along the nffz is transferred to the eastern ridge at the rtf type 176 e triple junction it suggests that the western n s ridge is probably connected to the north with another left lateral transform possibly the south pandora ridge

ductile deformation of passive margins a new mechanism for subduction initiation 1 the onset of subduction at passive margins has been extensively investigated and debated however the force constellations and mechanisms that enable the development of a subduction system from a passive margin remain unclear this study presents new insights into the conditions and processes by which lateral density differences between oceanic and continental lithospheres in passive margins may lead to initiation of a low angle subduction system the presented study consists of 1 analytical calculations of flow fields generated in passive margins and 2 analogue experiments of mature passive margins performed in a centrifuge the analytical formulation predicts temporal and spatial evolution of an interface between the oceanic and continental lithospheres and demonstrates that oceanic underthrusting may occur by rotation of this interface the analogue experiments show that incipient subduction may develop by ductile deformation within the lithosphere involving no sliding along the ocean continent interface so that the frictional resistance between the plates need not be overcome the force induced by the negative buoyancy of the oceanic plate with respect to the asthenosphere is found to be in some cases irrelevant to subduction nucleation results of both the analogue experiments and the analytical calculation are compared to the south east australian passive margin and show an excellent fit to its geometry and stress distribution the proposed mechanism is also applied to the only two cases of subduction of atlantic tectonic system the lesser antilles and the south sandwich subduction systems

the early magnetic field and primeval satellite system of the moon clues to planetary formation from the stable remanent magnetization of the apollo igneous rocks and high grade breccias the existence of a primeval lunar magnetic field was inferred the palaeointensities of the samples rise rapidly to a maximum at 3 9 ga then decrease exponentially to 3 2 ga strongly suggesting that the moon had a field generated in a core the existence of which was inferred from its non hydrostatic figure modelling of the apollo 15 and 16 subsatellite magnetic anomalies by p j coleman l l hood and c t russell gave palaeomagnetic directions of crustal strata this enabled n pole positions to be calculated which were empirically found to form three bipolar groups the mean poles of which define on the core dynamo hypothesis three axes of rotation different from the present these were dated as pre nectarian lower nectarian and upper nectarian imbrian multi ring basins of these ages were found to lie close to the corresponding palaeo equators the impacting bodies were therefore satellites not asteroids or comets their velocities before collision can be shown from basin asymmetries to be nearly equatorial the consequent changes in the moment of inertia tensor by basin formation caused these successive reorientations of the moon relative to its axis of rotation in space the three mean poles form a 90 spherical triangle the explanation is that the moon had three satellites the orbits of each decayed they broke up at the roche limit into smaller bodies which produced impact basins near the equator the moon then reorientated according to euler s principle before the next group of impacts lunar palaeomagnetism and especially the inferences that the moon has an iron core that segregated late and had a primeval satellite system may provide important constraints on theories of lunar and planetary formation

revisiting the field geology of taurus littrow abstract integration of apollo 17 field observations and photographs sample investigations lunar reconnaissance orbiter camera images chandrayaan 1 moon mineralogy mapper m3 spectra and miniature radio frequency mini rf s band radar images provides new insights into the geology of the valley of taurus littrow on the moon connecting the various remote observations to sample data enables a set of new conclusions to be drawn regarding the geological evolution of the valley structural considerations and published and recalculated 40ar 39ar analyses of samples from the north massif and the sculptured hills indicate that the crisium basin formed about 3 93 ga the serenitatis basin about 3 82 ga and the imbrium basin no earlier than 3 82 ga and no later than the average of 3 72 ga for 33 age dates from samples of the valley s mare basalts strong evidence continues to support the conclusion of others lucchitta 1972 spudis et al 2011 fassett et al 2012 that the sculptured hills physiographic unit consists of imbrium ejecta interpretation of m3 spectral data and apollo 17 samples indicate that rock units of the sculptured hills consist of a largely coherent mg suite pluton lroc nac stereo images and mini rf data indicate the presence of several exposed pyroclastic fissures across the sculptured hills rim boulders at camelot crater constitute nearly in situ wall rocks of that crater rather than ejecta and provide an opportunity for investigations of remanent magnetic field orientation at the time of the eruption of late mare basalt lavas in the valley paleomagnetic field orientation information also may be obtained relative to melt breccia contacts in north massif boulders that suggest original horizontal orientations lroc images indicate the existence of two temporally separate light mantle avalanche deposits the origin potential flow mechanisms and geology of the youngest avalanche from the south massif have been clarified the existence of two distinct light mantle avalanches raises doubt about the association of either light mantle avalanche with secondary impacts related to the tycho impact event alternatively the lee lincoln thrust fault appears to have triggered the second light mantle avalanche between 70 and 110 ma a simple structural analysis shows that this thrust fault dips 20 25 to the southwest where it crosses the north massif and to the west where it crosses the valley floor mini rf data reveal a line of reduced reflections roughly perpendicular to contours on the north massif about 3 km to the east of the lee lincoln fault although this line is possibly an older ancillary fault lroc nac stereo images indicate that it may be best explained as a pyroclastic fissure a debris flow of dark apparent pyroclastic ash lies below the southeast end of the potential fissure finally young lunar impact glass sample 70019 has been precisely located within lroc nac images and oriented for the first time using 60 mm f l sample documentation photographs sample 70019 can now be employed in lunar paleomagnetic field orientation studies

current tectonics of the tonga new hebrides region abstract this paper proposes a present day tectonic map of the tonga new hebrides region the result of combining data from bathymetry satellite derived gravity seismicity magnetic measurements and gps observations focal mechanisms and a large amount of bathymetric data collected during the last decade in the lau and north fiji back arc basins indicate a deformation distributed over numerous spreading ridges rather than diffuse and shear dominated as was previously thought the region is also characterised by a large variation of both consumption rate along the arcs and opening rate along the back arc basins spreading centres geometry and rate of spreading in the lau and north fiji back arc basins and segmentation of the tonga and new hebrides arcs are closely related to and significantly influenced by the subduction of aseismic ridges namely the louisville d entrecasteaux and loyalty ridges where subducting aseismic ridges enter the trenches we observe slower convergence arc transverse strike slip faulting thrusting at the rear of arcs and slow or absent back arc spreading rapid subduction correlates with fast back arc opening sometimes parallel spreading ridges in the back arc domain are required to accommodate the fast motion of convergence

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

palaeozoic arc growth deformation and migration across the lachlan fold belt southeastern australia abstract the palaeozoic lachlan fold belt lfb is a low pressure metamorphic belt characterized by greenschist facies rocks and infolded volcanic sequences which indicate that the belt has not undergone substantial uplift at any stage porphyroblast matrix relationships in one of the highest grade metamorphic zones of the lfb the omeo complex indicate that the thermal peak was reached before or was synchronous with the earliest deformation similar to that in high grade metamorphic zones of the northern arunta inlier central australia in both areas the metamorphism is of low p high t type centred on major granite intrusions as regional aureoles and characterized by anti clockwise p t t paths however in the outer lower grade parts of the aureoles peak metamorphic conditions post date the earliest deformation this diachronous relationship indicates that 1 outward migration of a thermal front was centred on major granitoid intrusions batholiths 2 deformation propagated outward more rapidly than migration of the thermal front and 3 plastic and ductile deformation began only after heating around the batholiths thus orogeny was initiated after and localized by heat focusing in the mid crust associated with batholith emplacement therefore the early deformation is a result of thermal softening it is typically subhorizontal ductile shear at mid crustal levels but is characterized by upright to inclined folds at upper crustal levels in the latter environment the typically discordant contact aureole plutons are often interpreted as post tectonic granitoids but they were part of the ongoing mid crustal thermal perturbation that induced regional greenschist facies metamorphism which overprinted the early formed upright folds as it migrated to upper crustal levels orogeny migrated generally eastward through the lfb it began with the early silurian benambran orogeny centred on the wagga metamorphic belt wmb and terminated with the carboniferous kanimblan orogeny which was most intense in the hill end trough eastward migration of orogeny across central victoria part of the west lfb began in the ballarat bendigo zone during the early devonian and terminated in the melbourne zone in the middle devonian orogeny was centred on the meridional batholiths of the lfb each of which are considered to represent the transitory axis of an ancient magmatic arc stepwise but generally eastward arc migration caused the eastward migration of orogeny a tectonic model based on the modern southwest pacific arc system can be applied to the lfb following cambrian intra oceanic arc growth associated with west dipping subduction an ordovician marginal sea developed partly on stretched cambrian crust and was flooded by an extensive turbiditic wedge closure of the eastern part the east lfb began in the early silurian after a magmatic arc developed over the wmb possibly associated with an east dipping subduction zone after re establishment of west dipping subduction in the middle silurian absolute motion retreat of the upper australian plate caused by oblique plate convergence resulted in dextral transtension and the generation of siluro devonian inter arc and back arc basins transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard eastward arc system periodic compression within the evolving arc and back arc system resulted in a series of east younging east verging linear siluro devonian fold and thrust belts which were localized adjacent to elongate batholiths that represent the relict arc systems local westward jumps in deformation and plutonism occurred the most significant is the 600 km westward jump into the west lfb caused by closure of the remainder of the passive ordovician back arc basin melbourne and bendigo ballarat zones west dipping subduction beneath the basin produced a low pressure fold thrust belt similar to the east lfb though the granitoids have a more primitive isotopic character indicating closure of thin dominantly oceanic crust in the early to middle devonian localized deformation induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins is likely to produce anticlockwise p 7 t paths and may be a typical feature of southwest pacific style tectonism

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

seismotectonics of the fiji plateau and lau basin abstract focal mechanism solutions of shallow earthquakes in the fiji plateau and lau basin provide significant contributions to the understanding of the recent tectonic movements beneath these marginal seas left lateral shear zones are identified on the linear shallow seismic zone extending from north of the fiji islands to the northern end of the lau basin in the central area of the fiji plateau and in the northern part of the lau basin nearly nw se trending opening tectonics is suggested nearly e w striking compressional tectonics is inferred near the western edge of the hazel holme fracture zone

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

deep earthquakes beneath the fiji basin sw pacific earth s most intense deep seismicity in stagnant slabs abstract previous work has suggested that many of the deep earthquakes beneath the fiji basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position since nowhere else in the earth do so many earthquakes occur in slabs stagnated in the transition zone these earthquakes merit closer study accordingly we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well located deep events in the geographic area between the bottoms of the main vanuatu and tonga wadati benioff zones two regions of deep seismogenesis are recognized there i 163 deep shocks have occurred north of 15 s in the vityaz group from 1949 to 1996 these seismological observations and the absence of other features characteristic of active subduction suggest that the vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone ii another group of nearly 50 outboard deep shocks occur between about 450 and 660 km depth west of the complexly buckled and offset western edge of the tonga wadati benioff zone their geometry is in the form of two or possibly three small circle arcs that roughly parallel the inferred motion of tonga trench migration earthquakes in the southernmost of these arcs occur in a recumbent high seismic wavespeed slab anomaly that connects both to the main inclined tonga anomaly to the east and a lower mantle anomaly to the west van der hilst r 1995 complex morphology of subducted lithosphere in the mantle beneath the tonga trench nature vol 374 pp 154 157 both groups show complexity in their focal mechanisms the major question raised by these observations is the cause of this apparent temporary arrest in the descent of the tonga slab into the lower mantle we approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered or was buckled segmented and laid out in the transition zone

the case for a younger imbrium basin new 40ar 39ar ages of apollo 14 rocks petrographically and chemically well characterized rocks of the apollo 14 landing site 12 impact melt breccias 3 clast free impact melt rocks 2 granulitic rocks 1 cataclastic noritic anorthosite 1 shocked gabbronorite 1 mare basalt and 1 plagioclase single crystal have been dated by the 40ar 39ar technique the samples represent two different formations at the apollo 14 site 1 small lithic clasts 10 specimens from fragmental breccia 14063 ejected from the 26 ma old cone crater and one clast mare basalt from cone crater soil 14140 and 2 large rock samples 10 specimens collected throughout the regolith of the landing site the first group of samples has exposure ages close to 26 ma and 40ar 39ar ages ranging from 3 86 to 4 09 ga which indicates that breccia 14063 was assembled 3 86 ga ago the second group of samples displays lower 40ar 39ar ages ranging from 3 73 to 3 85 ga we interpret breccia 14063 as part of a nectarian megabreccia unit which was incorporated into the fra mauro formation as a megablock by secondary mass wasting of the ejecta blanket of a local pre imbrian crater the remaining samples are thought to represent clasts of the apollo 14 subregolith basement megabreccia defined as the fra mauro formation consequently the reported ages of these clasts confirm previous suggestions that the fra mauro formation and therefore the imbrium basin is only 3 75 ga old and not 3 85 ga as commonly assumed

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

timing of the martian dynamo on mars the strong magnetization in the highland crust of the southern hemisphere and the absence of magnetic anomalies at the hellas and argyre impact basins have been taken as signs that the core dynamo that once drove the planet s magnetic field turned off more than 4 billion years gyr ago here we argue instead that the martian dynamo turned on less than 4 gyr ago and turned off at an unknown time since then high spatial resolution magnetometry in both martian hemispheres is needed to reveal the true history of the martian dynamo

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

where and why do large shallow intraslab earthquakes occur we try to find how often and in what regions large earthquakes m 7 0 occur within the shallow portion 20 60 km depth of a subducting slab searching for events in published individual studies and the harvard university centroid moment tensor catalogue we find twenty such events in e hokkaido kyushu sw japan s mariana manila sumatra vanuatu n chile c peru el salvador mexico n cascadia and alaska slab stresses revealed from the mechanism solutions of these large intraslab events and nearby smaller events are almost always down dip tensional except for e hokkaido manila and sumatra the upper plate shows horizontal stress gradient in the arc perpendicular direction we infer that shear tractions are operating at the base of the upper plate in this direction to produce the observed gradient and compression in the outer fore arc balancing the down dip tensional stress of the slab this tectonic situation in the subduction zone might be realized as part of the convection system with some conditions as shown by previous numerical simulations 2003 elsevier b v all rights reserved

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

a possible triple junction at 14 50 s on the north fiji basin ridge southwest pacific auzende j m grficia mont e bendel v huchon p lafoy y lagabrielle y de alteriis g and tanahashi m 1994 a possible triple junction at 14 50 s on the north fiji basin ridge southwest pacific in j m auzende and t urabe editors north fiji basin sxarner french japanese program mar geol 116 25 35 recent full coverage bathymetric and geophysical surveys yokosuka 90 and 91 carried out in the framework of the starmer japanese french joint project reveal details of the structure of the n160 segment of the north fiji basin ridge despite its intermediate spreading rate this segment shows a slow spreading type morphology with steep 1000 m high walls reaching an axial depth of 4000 m its northern tip at 14 50 s is a complex area involving several plates boundaries this domain can be interpreted either as a complex boundary between two plates or as a new triple junction symmetrical with the 16 50 s triple junction characterizing its southern tip in either case the complexity of the spreading ridge geometry in this area illustrates the instability of the accretionary geometry due to the deformation of the whole north fiji basin between the much larger indo australian and pacific plates

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

geological evidence for a global tectonic polarity inferences about plate motions over the past 40 ma based on structural and geodetic data suggest that there is a coherent movement direction on a global scale moreover a westward drift of the lithosphere relative to the asthenosphere is indicated by plate motions in the hot spot reference frame independent geological observations such as the different styles of deformation in thrust belts associated with subduction following or opposing the relative eastward or northeastward mantle flow also support this global tectonic polarity unfortunately most hot spot reference frames are based on assumptions which strongly limit their validity for example many combine magmatic sources originating from different depths in the mantle including hot spots located along oceanic ridges which are not fixed relative to one another using a hot spot reference frame filtered to exclude shallow hot spots and anomalous wet spots we would predict a much higher relative westward drift of the lithosphere plate motions are not random rather that they are controlled by the relative motion between lithosphere and asthenosphere and by gradients in lithospheric thickness and composition triangular shaped plates may experience rotations resulting from differential drag at the base of the lithosphere with respect to the asthenosphere e g the clockwise rotation of south america plates appear to follow an undulatory pattern of motion on a global scale and their poles of rotation fall into a main cone located near the earth s rotation poles plate rotations induced by triangular shape differential basal drag or lithospheric anisotropies along subduction zones may generate the scattered distribution of euler poles this might have inhibited the recognition of a coherent global tectonic pattern

the influence of plate motions on three dimensional back arc mantle flow and shear wave splitting both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western pacific subduction zones the goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting we calculated three dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow induced strain predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain a parameter study shows that both finite strain and textural anisotropy developed within three dimensional plate coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere flow models incorporating plate motion boundary conditions for the tonga southern kuril and eastern aleutian subduction zones produce splitting parameters consistent with observations from each region trench parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

geological structure of the central spreading system north fiji basin abstract seabeam mapping single channel and multichannel seismic profiling sonobuoy obsh refraction measurements and geomagnetic anomaly observations obtained during the cruises kaiyo87 and 88 in the north fiji basin reveal the detailed structure and history of jumps of young spreading systems currently active spreading systems north northwest and south southwest of a triple junction at about 17 s 174 e where active hydrothermal activity was discovered during kaiyo88 jumped to their present positions possibly in the late quaternary the spreading ridge south southwest of the triple junction which is oriented n15 20 e has a 0 7 3 5 km wide axial graben and a 15 35 km wide trapezoidal morphological profile it cuts into a wide n40 60 e oriented graben whose side walls themselves sharply cut previously formed structures including a high seamount which is now separated into three highs

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

seismicity and state of stress within the overriding plate of the tonga kermadec subduction zone 1 to reassess the main tectonic units and to quantify the slip partitioning within the overriding plate of the tonga kermadec subduction zone a seismotectonic study was performed using global seismicity and focal mechanisms catalogs 1 new tectonic features were identified within the lau basin and the volcanic arc by remarkable shallow hypocenters alignments 2 the centroid moment tensor solutions catalog was processed in order to map the stress tensor variation in the upper plate we found the tectonic features characterized by a diffuse seismicity are subjected to a composite stress regime and they are interpreted as diffuse immature plate boundaries controlled by the high thermal anomaly lying beneath the lau basin 3 we quantified the margin parallel rates of motion using the azimuth of the maximum compressive stress component computed within the interplate zone the results highlight a major tectono kinematic segmentation related to the subduction of the louisville seamount chain

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

diffuse back arc deformation in the southwestern pacific earthquake distribution and focal mechanisms from the lau and north fiji back arc basins indicate a diffuse and shear dominated system of deformation at odds with traditional plate tectonic models of lithospheric accretion in back arc basins the entire back arc region between the tonga and new hebrides arcs is more realistically modelled as a giant internally deformed pull apart basin formed along a left step in the transform boundary between the pacific and indo australian plates

role of mantle flow at the north fiji basin insights from anomalous topography the north fiji basin is a complex back arc system situated between the opposite verging tonga and new hebrides subduction zones on the basis of two independent calculations we find that the back arc region is anomalously high with topography residuals reaching up to 2000 m above what should be isostatically supported our study aims to test whether a hot buoyant upwelling is required beneath the north fiji basin to support the observed high elevation the mantle thermal structure beneath the north fiji basin is inferred from the global tomographic model s20rts where seismic velocity anomalies are converted to density temperature anomalies this thermal structure is input into a three dimensional mantle convection code citcoms and the flow induced surface topography is calculated and compared with the observed anomalous topography in a series of experiments we eliminate various portions of the mantle structure to consider what type of mantle flow can support the anomalous topography the experiments suggest that the observed topography in the north fiji basin is consistent with the support of hot buoyant upper mantle upwelling and specifically rising material in the depth range from 70 to 120 km is the most critical component a relatively thin weak lithosphere is required to produce the high amplitude and short wavelength topography residuals and the presence of discrete breaks in the plate produces further localization of flow induced topography

seismotectonics and present day relative plate motions in the new hebrides north fiji basin region abstract relative motions between the pacific plate p indo australian plate ia new hebrides nh arc microplate and the north fiji basin nfb microplate are estimated using shallow seismicity 276 focal mechanism solutions bathymetry magnetism and the rm 2 plate model of minster and jordan 1978 within the nfb we define the western nfb wnfb eastern nfb enfb and southern nfb snfb microplates a 8 cm y spreading in a n72 e direction occurs along the n s trending spreading ridge of nfb wnfb enfb boundary along the enfb ia boundary located at 176 e a 3 cm y extension in a n108 e direction is proposed the wnfb p boundary which includes the hazel holme extensional zone hhez is complex and a general n25 e extension 2 cm y is inferred the fiji fracture zone ffz is composed of two segments along the eastern zone p ia boundary trending n80 e a left lateral strike slip motion occurs 9 6 cm y accommodated by minor n109 e extension pull apart while along the western zone enfb p boundary a n84 e trending left lateral strike slip motion 7 cm y is inferred the wnfb snfb boundary corresponds to a n70 e broad fracture zone along which an e w left lateral strike slip motion is proposed the snfb microplate moves rapidly eastwards 10 5 cm y at 172 e and is almost attached to the ia plate within the nfb an improved model is discussed which includes deformations within the ia and p plates and rotations with euler poles close to the nfb along the new hebrides trench the consumption rates are respectively 16 15 9 and 12 cm y at 11 12 5 15 5 and 20 s the minimum rate occurs where the d entrecasteaux ridge collides with the new hebrides arc rates of extension at the rear of the nh arc are 7 5 5 and 2 cm y at 11 12 5 and 20 s respectively back arc compression occurs between 13 30 s and 17 s the western end of the hazel holme extension zone coincides with the 13 30 s boundary separating the nh back arc compressive belt and the northern nh back arc troughs south of 21 22 s the trend of the slip vector changes along the nh trench from n70 e to n20 w this implies a n s motion of convergence along the e w trending southern part of the nh trench motion snfb ia of 1 5 cm y at 172 e and the existence of a n70 e trending left lateral fracture zone crossing the southern part of the nfb this fracture should be considered as a major plate boundary

the influence of topographic and lithologic features on horizontal deformations summary a number of studies have been carried out in order to determine the magnitude of effects originating from the surroundings of an observation site which influence seismometer records or deformation observations with tilt or strainmeters the investigations generally comprise thermal atmospheric topographic and cavity effects as well as ocean loading recently these studies have been continued and extended using finite element fe modeling the effects are investigated exemplarily using air pressure variations as loading signal the observatory sites differ in their local surroundings based on this the influence on topographic and lithologic features is investigated with respect to air pressure changes the models developed reflect different topographies around stations wide and narrow valleys hill flanks and flat areas at the foot of the hill flank galleries are integrated into the models the topographic and the gallery geometry influence are systematically investigated with regard to e g rock coverage above an observation site changes in the hill slope width of a valley and lithology the deformations are only caused by air pressure changes and not associated with gravity changes due to an advancing pressure front for loading different air pressure scenarios are simulated with an uniform load and a pressure front which moves over the model in different directions as an elastic rheology is considered the effects can be scaled to actual occurring air pressure variations initial studies focus on the cavity effect to estimate the order of magnitude to separate the pure cavity effect from other influences studies of topographic effects e g an increasing slope angle yield a non linear increase in the strain component with amplitudes up to 2 nstrain and tilt changes up to 2 nrad for a uniform air pressure load of 1 hpa for prem parameterized models different lithologies can result in five times larger deformations for moving high pressure areas changes emerge in strain of up to 2 nstrain and in tilt of up to 4 2 nrad the results obtained improve the interpretation of deformations observed with regard to fundamental geodynamic processes and contribute to the selection of future observation sites

plate tectonics and aspherical earth structure the importance of poloidal toroidal coupling we derive poloidal flow internal loading green functions for incompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of two adjacent spherical shells having different viscosities from the green function we obtain kernels connecting the surface divergence the geoid and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques and with these kernels we argue that both the surface divergence and geoid fields consisting of harmonic degrees 2 5 may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km we point out however that the coupling from poloidal to toroidal flow which is required to understand surface velocity spectra may allow these observations to be understood in terms of a viscosity increase at depth which is smaller than required by nonhydrostatic geoid data in the context of pure poloidal models using the observed kinetic energy in the surface plate motions as a constraint allows us to infer a value for the steady state upper mantle viscosity of 2 0 0 5 1021 pa s we comment on the implication of the difference between this number and the somewhat lower value 1 1021 pa s which has been derived on the basis of analyses of signatures of the glacial isostatic adjustment process

newly identified segments of the pacific australia plate boundary along the north fiji transform zone abstract the north fiji transform zone a 1500 km long and 200 km wide transform segment of the pacific australia plate boundary is one of the major transform fault systems of the earth new data collected during the alaufi cruise march 2000 on board the r v l atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary three spreading centers or extensional zones the north cikobia spreading center the futuna spreading center and the southeast futuna volcanic zone and a strike slip fault zone the futuna transform fault have been discovered over a distance of 500 km along the eastern north fiji transform zone from the north of the fiji platform to the east of the futuna archipelago the futuna transform fault oriented 100 has been mapped over a distance of 250 km it must be considered to be an important tectonic element of the transform plate boundary pure strike slip as well as transpression and transtension motions are responsible for the complex morphology of this feature the uplifted futuna alofi ridge represents a major compressional relay along the futuna transform fault the futuna spreading center trending 20 30 is composed of a series of en echelon left stepping spreading segments it represents a 200 km long extensional relay between the futuna transform fault and the western part of the north fiji transform zone the fiji transform fault which bounds the fiji platform to the north the opening rate at the futuna spreading center is estimated at 4 cm yr although the north cikobia spreading center and the southeast futuna volcanic zone have been only partly mapped bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features a spreading rate of 2 cm yr is inferred at the north cikobia spreading center therefore the north fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones the three active extensional zones have an ene wsw to nne ssw orientation while compressive features along the futuna transform fault are nw se to nnw sse oriented in accordance with the present day left lateral transform motion along this part of the pacific australia plate boundary

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

deep structure and evolution of the harz mountains results of three dimensional gravity and finite element modeling a new bouguer anomaly map is presented for the region of the entire harz mountains based on more than 60 000 gravity values the various gravity anomalies are discussed and interpretation is carried out by high resolution 3 d gravity modeling one of the main subjects of interest in the investigation is the northern boundary fault zone of the harz mountains separating the mesozoic sediments in the north from the palaeozoic rocks of the harz in the south dip and vertical displacement are determined for this fault zone mean values are 3400 m and 70 respectively gravity modeling shows that the brocken and the ramberg granites are distinctly different the brocken granite is shallow whereas the ramberg granite has a maximum depth of 8 5 km and a n s dimension of 37 km the prominent benneckenstein gravity high is explained by two different models one based on a granodioritic intrusion 2900 kg m3 with a center depth of 14 km and the other based on phyllites 2740 kg m3 on a depth of 3 4 km studies on the geodynamic evolution of the harz mountains are carried out using the finite element method on the basis of a 3 d model vertical displacements that can be related to horizontal forces are computed for the period of the variscan orogeny an uplift of 600 m in the harz area is calculated for late cretaceous and tertiary 400 m are determined the total amount of 1000 m is about 12 of the vertical displacement of the northern boundary fault zone of the harz mountains shown by the gravity modeling these results do not contradict geological ideas

primeval displacements of the lunar pole abstract the hypothesis that the magnetic field which magnetized the lunar crust was generated by the dynamo process in a small fluid iron core can now be tested because the coriolis force was a dominant term in the equations of motion in this core the mean lunar field was aligned along the ancient axis of rotation from hood s modelling of the magnetic anomalies in the lunar crust mapped by the apollo 15 and 16 subsatellites the palaeo directions of this field have been determined from them palaeopole positions have been determined and are found to be grouped with respect to age the palaeoequators corresponding to ages 4 ga and 3 85 ga show close relationships with the circular maria or mascons on the near side and multi ring basins of corresponding age on the far side the polar displacements indicated from lunar palaeomagnetism can be explained by the changes in the moment of inertia tensor consequent on the excavation and later flooding of these circular mare small moons in the primeval earth moon system are inferred to be the impacting bodies