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

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

ocean island densities and models of lithospheric flexure estimates of the effective elastic thickness te of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones the anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere to erosion of the mechanical boundary layer by mantle plumes to pre existing thermal stresses and to overprinting of old volcanic loads by younger ones a fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads we suggest that load densities have been systematically overestimated in studies of lithospheric flexure potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism we illustrate the effect of underestimating load density with synthetic examples and an example from the marquesas islands this effect combined with the other effects listed above in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses

a model for the plate tectonic evolution of the east central pacific based on seasat investigations abstract a complete seasat dataset is used to investigate the small scale bathymetry of a 15 by 15 area of the east central pacific northeast of pitcairn island which was involved in the miocene ridge jump and reorientation two fracture zones fz1 and fz2 are mapped along 450 km at an azimuth of n95 e while the signature of the austral fracture zone disappears east of 135 w several seamounts are discovered or confirmed along the oeno henderson ducie lineament and the degree of isostatic compensation of a number of islands at the eastern end of the tuamotu group is studied islands of the northern branch have practically no signatures in the geoid while the southern ones must have formed on lithosphere at least 10 ma old on the basis of these constraints we present a model of the evolution of spreading since 40 ma b p in which the deactivation of the austral fracture zone and the transfer of its offset to fz1 in the late oligocene is interpreted as due to rift propagation triggered by a northern tuamotu hotspot we propose that the oeno ducie chain is the surface expression of a southern tuamotu hotspot deviated 15 in azimuth by the presence of a young and fresh fracture zone thus our model predicts that hotspots and ridge systems can have a moderate influence on each other s kinematics and surface expressions which does not however extend to permanent trapping

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

two way exchange between the easter mantle plume and the easter microplate spreading axis the conventional model whereby plume volcanism forms linear age progressive volcanic chains with the youngest activity occurring nearest a spreading axis at a hotspot has been challenged for the easter seamount chain1 4 whereas early work suggested the existence of a linear melting anomaly a hotline 1 2 more recent studies3 4 have proposed a hotspot near salas y gomez island connected with the easter microplate spreading system by an 800 km long volcanically active plume channel here we use geochemical geological and geochronological data to argue that the hotspot lies close to easter island moreover new isotopic data for lavas from the seamount chain provide evidence for bidirectional flow between the spreading axis and the plume thus supporting geophysical and fluid dynamical models of mantle flow in a plume spreading axis system5 7 material balance and flux considerations show the easter plume to be weak and cool compared with those beneath larger features such as iceland hawaii and the galapagos islands

progressive development of structures in a ductile shear zone in the singhbhum shear zone of eastern india successive generations of folds grew in response to a progressive ductile shearing during this deformation a mylonitic foliation was initiated and was repeatedly transposed the majority of fold hinges were formed in an arcuate manner at low angles to the y axis in an e w trending subhorizontal position and major segments of the fold hinges were then rotated towards the down dip northerly plunging x axis the striping and intersection lineations were rotated in the same manner the down dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations the consistent asymmetry of the folds the angular relations between c and s surfaces and the evidence of two dimensional boudinage indicate that the deformation was non coaxial but with a flattening type of strain with 1 2 the degree of non coaxiality varied both in space and time from the progressive development of mesoscopic structures it is concluded that the 2 3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale when a new set of shear lenses was superimposed on already sheared rocks the preexisting foliation generally lay at a low angle to the lenses no new folds developed where the acute angle was sympathetic to the sense of shear displacements where the acute angle was counter to the sense of shear the pre existing foliation lying in the instantaneous shortening field was deformed into a set of asymmetric folds

mantle plume noble gas component in glassy basalts from reykjanes ridge basalts from the reykjanes ridge contain noble gases delivered from the non degassed lower mantle by the iceland plume these lower mantle gases are thought to be a mixture of planetary and solar components as would be expected if the earth accreted from fine silicate particles

on the accuracy of marine gravity measurements we have assessed the accuracy of lamont doherty geological observatory s global marine gravity data bank by examining the crossover errors coes at intersecting ship tracks more than 63 000 coes were found having a standard deviation of 22 43 mgal mgal 10 5 m s 2 we use the coes to find and remove linear drifts and dc shifts present in the data set this adjustment reduces the standard deviation to 13 96 mgal coes generally decrease with latitude which we attribute to uncertainties in the eotvos correction high coes occur in areas of high gravity gradients these two features point to poor navigation as the principal source of error in marine gravity surveys coes have generally been decreasing during the last two decades reflecting improvements in instrumentation and quality of navigation a comparison of the shipboard gravity data to seasat derived gravity revealed a 9 mgal bias in the terrestrial data which probably reflects uncertainties in choice of reference field the adjusted data set was used to generate a gravimetric geoid for the nw atlantic ocean by removing this geoid from the seasat sea surface heights a residual geoid was obtained a special feature of this map is an elongate ene trend that appears to correlate with the edge of the gulf stream

planar non planar and refolded sheath folds in the phulad shear zone rajasthan india abstract progressive ductile shearing in the phulad shear zone of rajasthan india has produced a complex history of folding with development of planar non planar and refolded sheath folds there are three generations of reclined folds f 1 f 2 and f 3 with a striping lineation l 1 parallel to the hinge lines of f 1 the planar sheath folds of f 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices l 1 swerves harmoniously with the curving of f 1 hinge line there is a strong down dip mineral lineation parallel to the striping lineation in most places but intersecting it at apices of first generation sheath folds both the striping and the mineral lineation are deformed in u patterns over the hinges of reclined f 2 and f 3 folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non planar sheaths the reclined folds of all the three generations were deformed by a group of subhorizontal folds each generation of fold initially grew with the hinge line at a very low angle with the y axis of bulk non coaxial strain and was subsequently rotated towards the down dip direction of maximum stretching the patterns of deformed lineations indicate that the stretching along the x direction was extremely large much in excess of 6000 percent

the support mechanism of the young foundation seamounts inferred from bathymetry and gravity summary the youngest volcanoes of the foundation volcanic chain were built on a plate less than 5 myr near the pacific antarctic ridge par axis the progressive approach of the spreading ridge to the hotspot during the last 10 myr resulted in the building of the edifices upon a progressively younger plate estimates of the effective elastic thickness te using two independent methods show that the elastic thickness of the plate diminishes towards the spreading ridge from 5 3 km to 2 0 km the spatial variation of te correlates with a significant change in both the morphology and the volume of the volcanoes suggesting an important control of the elastic thickness on the volcanic processes the reduction in te also explains conveniently the reduction in the distance between the north and the south systems the double volcanic line that characterizes the young part of the foundation chain in the direction of young loading ages the south system formed at the top of a flexural bulge created by the emplacement of the north system the main volcanic line the low te estimate for the south system and the shape of the bouguer anomalies confirm this model the effective elastic thicknesses obtained in this study are well matched by isotherms between 200 c and 300 c and support the hypothesis of a weak plate three mechanisms possibly acted to lower the plate rigidity re heating of a normal plate by the hotspot north system local fracturing south system and the creation of a thin lithosphere at the par axis by the coupling of a hotspot and the ridge related melting zones near ridge volcanoes

plume ridge interaction revisited evidence for melt mixing from he ne and ar isotope and abundance systematics abstract submarine volcanic glass he ne and ar data from the pacific antarctic rise at its intersection with the foundation seamount chain have been obtained to further elucidate one of the main outstanding problems in plume ridge interaction studies the physical state in which mantle flow and mixing occur in those settings mixing of plume mantle and mid ocean ridge basalt source material may either be in the form of two solids of solid melt or of two melt phases only the positive correlation of 4 he 40 ar with 206 pb 204 pb as well as the negative correlation of 4 he 40 ar with 40 ar and 4 he concentrations combined with a lack of correlation between 4 he 40 ar and ridge depth or mgo concentration indicate that the enriched mantle plume material has been extensively degassed prior to mixing with the mid ocean ridge source material degassing of the plume source and mid ocean ridge source material prior to mixing is also suggested by the he versus pb isotope systematics however the occurrence of degassing processes requires the existence of melts implying that mixing between the plume and the mid ocean ridge material occurs in the physical form of melts

melting dehydration and the geochemistry of off axis plume ridge interaction 1 we present results from a series of two dimensional numerical experiments in which synthetic melt compositions are calculated for a system in which a thermally buoyant off axis mantle plume interacts with a nearby ridge axis spatial gradients in synthetic melt properties are compared to observed spatial gradients in geochemistry from the easter salas y gomez system in an effort to constrain the dynamics of mantle flow in off axis plume ridge systems results indicate that observed gradients in radiogenic isotopic ratios between the ridge axis and the plume require significant heating of the ambient mantle adjacent to the plume this heating allows ambient mantle to melt off axis but also attenuates the flow from the plume to the ridge when increases in viscosity due to dehydration during melting are considered spatial gradients in the geochemical properties of synthetic melts become extremely sharp at odds with the observational data this may indicate that viscosity increases due to dehydration are not significant in off axis plume ridge systems

the dynamics of off axis plume ridge interaction in the uppermost mantle two decades of geochemical and geophysical observations have led to the plume channel model whereby buoyant off axis mantle plumes feed and interact with diverging plates at mid ocean ridge axes which act as sinks of upper mantle material here we present results from two dimensional 2 d numerical experiments which incorporate the essential physics and fluid dynamic aspects of the plume ridge upper mantle system in order to test the feasibility of this plume ridge interaction model specifically experiments test the relative importance of physical effects such as strong viscosity variations and thermal and chemical buoyancy forcing in plume ridge dynamics results indicate that both transient and steady state connections may be established between off axis plumes and ridges for a range of realistic mantle conditions the presence of a strongly sloping rheological boundary layer rbl is a necessary condition for long term communication between an off axis buoyant upwelling and a spreading ridge the flux of buoyant material to the ridge is also shown to increase with increasing plume to mantle density contrast decreasing plume viscosity and smaller plume ridge separation distances plume ridge interaction regimes are defined based on the competing effects of plate driven and buoyancy driven flow thermal erosion of the viscous lithosphere strongly inhibits long term plume ridge interaction by enhancing the plates ability to deflect the plume both head and conduit away from the ridge axis

time space mapping of easter chain volcanism abstract new 40ar 39ar and published k ar ages show that the locus of volcanism along the easter volcanic chain evc has shifted systematically from the nazca ridge at about 26 m y to the recently active sala y gomez island easter island region this indicates a plume rather than a hotline i e mantle roll origin for the evc the time space distribution of ages combined with published ages for the galapagos and juan fernandez volcanic chains is used to reconstruct nazca plate velocities over the past 26 m y a plume now located in the region of sala y gomez island is most compatible with these data west of the plume the evc records neither nazca nor pacific plate motions this section of the evc may be related to westward channeling of plume material to the pacific nazca spreading boundary region

tectonic wedging along the rear of the offshore taiwan accretionary prism abstract the structural geometry kinematics and density structure along the rear of the offshore taiwan accretionary prism were studied using seismic reflection profiling and gravity modeling deformation between the offshore prism and forearc basin at the point of incipient collision and southward into the region of subduction has been interpreted as a tectonic wedge similar to those observed along the front of mountain ranges this tectonic wedge is bounded by an east dipping roof thrust and a blind west dipping floor thrust an east dipping sequence of forearc basin strata in the hanging wall of the roof thrust reaches a thickness in excess of 4 km near the tip of the interpreted tectonic wedge section restoration of the roof sequence yields an estimate of 4 km of shortening which is small compared with that inferred in the collision area to the north based on the variation in distance between the apex of the prism and the island arc previous studies propose that either high angle normal faulting or backfolding has exhumed the metamorphic rocks along the eastern flank of the central range in the collision zone on land to better constrain the initial crustal configuration we tested 350 crustal models to fit the free air gravity anomaly data in the offshore region to study the density structure along the rear of the accretionary prism in the subduction and initial collision zones before the structures become more complex in the collision zone on land the gravity anomaly observed in the region of subduction 20 2 n can be modeled with the arc basement forming a trenchward dipping backstop that is overlain by materials with densities in the range of sedimentary rocks near the point of incipient collision 20 9 n however the free air gravity anomaly over the rear of the prism is approximately 40 mgal higher compared with the region of subduction and requires a significant component of high density crustal rocks within the tectonic wedge these results suggest that the forearc basement may be deformed along the rear of the prism associated with the onset of collision but not in the subduction region further to the south

development of successive generations of shear zones in the precambrian basement of schirmacher hills east antarctica abstract in the precambrian basement of the schirmacher hills ductile shear zones have developed at several stages over a wide time span of tectonothermal history the earliest shear zones were formed under granulite facies conditions producing well foliated gneiss the majority of the ductile shear zones developed under amphibolite facies conditions the sheared rock is a mylonite characterized by drastic grain refinement of the constituent minerals the shear zones under amphibolite facies conditions are of four broad categories an early set subparallel to the gneissic foliation an early and a late cross cutting types shear zones which developed within massive rocks and lastly those which developed at the contacts of strong rheological contracts the last phase of deformation produced discrete shear fractures all the different phases of ductile shearing were broadly synchronous with emplacement of quartzofeldspathic materials pegmatite was emplaced even along some of the discrete shear fractures the magnitude of shear strain could be measured from the orientations of newly formed foliations and also from the rotation of early foliations

isotopic constraints on the easter seamount chain source new isotopic data sr nd and pb for lavas from easter and salas y gomez islands and nine seamounts along the length of the easter seamount chain esc to the east of these islands exhibit a regular behavior that is consistent with two component mixing between a morb like source and more enriched mantle the easter plume source characterized by relatively radiogenic pb and unradiogenic sr based on the linearity of mixing trends on the isotope diagrams the plume source appears to have maintained approximately the same isotopic composition over the 20 to 30 ma history of the easter plume sampled by the seamounts in current nomenclature the plume source resembles a mix of predominantly himu mantle with a small amount of emi the proportions of morb and plume source mantle sampled by the lavas erupted along the esc vary in a systematic way with easter island and seamounts to the west containing larger fractions of the morb component than salas y gomez island and the older seamounts to the east this variability is probably a reflection of the nearness to the spreading center and therefore lithospheric thickness at the time of volcanism

87sr86sr and ree variations along the easter microplate boundaries south pacific application of multivariate statistical analyses to ridge segmentation abstract we report 87 sr 86 sr ree sc v cr and co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the easter microplate emp the latitudinal 87 sr 86 sr variation along the east and west rifts closely parallels that of the pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the east pacific rise epr the data also further support the binary mantle mixing model previously proposed we evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses the data base includes major and trace elements ranging from compatible e g cr to highly incompatible e g la and the pb and sr isotope ratios multivariate classification of these geochemical variables in q mode sample space allows to clearly distinguish major element grouping from those of moderate to highly incompatible elements and the isotope ratios reflecting mantle source heterogeneities present in the region the power of this multivariate geochemical classification is best represented in two dimensional space by combining principal components analysis pca with cluster analysis ca illustrated in the form of a dendrogram the method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities using the variance covariance matrix and correlation matrix in r mode we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation both discriminant analysis and pca further indicate a close relationship between the geochemical anomaly located on the east rift of the emp and sala y gomez and easter island to a lesser extent the tuamotu chain whereas there is no close affinity between the emp and the more distant juan fernandez san felix or society islands in the isotopic space considered this provides further support that the geochemical anomaly located on the east rift of the emp may be related to the influence of a plume located either beneath the east rift or in the vicinity of sala y gomez or easter island in the last two models the plume would be laterally deflected at shallow depth towards the epr emp spreading complex an analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle the flow between the plume and the east rift of the emp must be more confined or channelled the possibility that the plume would be located directly beneath the east rift cannot be ruled out but appears less likely

en echelon volcanic elongate ridges connecting intraplate foundation chain volcanism to the pacific antarctic spreading center abstract the development of volcanic elongate ridges vers could be controlled in many cases by complex processes related to interaction between mantle plumes and spreading centers we report here measured ages for the foundation chain se pacific showing that volcanism along morphologically distinct vers can develop occasionally as rapidly formed continuous lines of coeval volcanism extending from a region of intraplate volcanism to the pacific antarctic spreading center but a significantly more dominant trend is for coeval yet structurally disconnected segments of foundation chain vers to develop in a series of en echelon ne sw elongate zones of coeval hotspot volcanism these elongate zones developed at intervals of approximately 1 myr while maintaining a basically steady state orientation and size as the pacific antarctic spreading center migrated continually closer to the foundation plume hotspot we infer from this new information that foundation chain ver development was controlled by an interplay between short lived local factors e g location of nearest spreading center segment lithospheric stress and long lived attributes of the foundation plume hotspot e g size orientation periodicity

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

the sierra ballena shear zone in the southernmost dom feliciano belt uruguay evolution kinematics and deformation conditions the sierra ballena shear zone sbsz is part of a high strain transcurrent system that divides the neoproterozoic dom feliciano belt of south america into two different domains the basement on both sides of the sbsz shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization grain boundary migration and fluid assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation age estimate of this episode is 658 ma the second stage corresponds to the start of transpressional deformation and the nucleation and development of the sbsz during this stage pure shear dominates the deformation and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites this event dates to 658 600 ma based on the age of these intrusions the third stage was a second transpressional event at about 586 to 560 ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening deformation in the sbsz took place during the late stages under regional low grade conditions as indicated by the metamorphic paragenesis in the supracrustals of the country rocks granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar a transition from magmatic to solid state microstructures is also frequently observed in syntectonic granites mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites which resulted in shear zone weakening quartz microstructures characteristic of the transition between regime 2 and regime 3 grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions 550 400 c on the other hand the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high t solid state deformation during cooling of the dikes

correlated geophysical geochemical and volcanological manifestations of plume ridge interaction along the galapagos spreading center 1 as the galapagos hot spot is approached from the west along the galapagos spreading center there are systematic increases in crustal thickness and in the k ti nb zr 3he 4he h2o and na2o content of lavas recovered from the spreading axis these increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth residual mantle bouguer gravity anomaly magma chamber depth average lava mg ca al ratio and the frequency of point fed versus fissure fed volcanism magma chamber depth and axial morphology display a threshold effect in which small changes in magma supply result in large changes in these variables these correlated variations in geophysical geochemical and volcanological manifestations of plume ridge interaction along the western galapagos spreading center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes and the consequences of these variations on magma supply axial thermal structure basalt chemistry and styles of volcanism

the structural evolution of sheath folds a case study from cap de creus abstract it has long been recognised that within zones of intense non coaxial deformation fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds however there is a surprising lack of detailed and systematic field analysis of such evolving sheath folds this case study therefore focuses on the sequential development of cm scale curvilinear folds in the greenschist facies el llimac shear zone cap de creus spain this simple shear dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite increasing amounts of fold hinge curvature are marked by hinge segments rotating into sub parallelism with the mineral lineation lm whilst the acute angle between the axial planar hinge girdle and foliation also displays a sequential reduction although lm bisects the noses of sheath folds it is also clearly folded and wrapped around the sheath hinges lm typically preserves a larger angle with the fold hinge on the lower limb l compared to the upper u limb l u suggesting that lm failed to achieve a steady orientation on the lower limb adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature or alternatively opposing directions of antithetic curvature such patterns reflect original buckle fold geometries coupled with the direction of shearing the ratio of long short fold limbs decreases with increasing hinge curvilinearity indicating sheath folds developed via stretching of the short limb rather than migrating or rolling hinge models this study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge line curvilinearity compared to adjacent return hinges this may provide a useful guide to bulk shear sense

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

effective elastic thickness of the lithosphere along the easter seamount chain bathymetry and gravity data collected during legs 5 6 and 7 of the 1993 gloria expedition and the recently released 2 min altimetry derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the easter seamount chain esc forward modeling admittance and coherence methods yield consistent restfits with the exception of the eastern and western ends of the esc the effective elastic thickness along the chain is 1 4 km the thin elastic thickness for the majority of the esc seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the nazca plate seafloor age the elastic thickness southeast of the nazca fracture zone is 6 km apparently because of the seafloor age discontinuity across the fracture zone the elastic thickness near the san felix island at the eastern end of the esc is even greater 11 km which is compatible with the estimated seafloor age at the time of loading a slight increase in the effective elastic thickness of the far western part of the esc suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center these findings combined with published geochemistry support a hotspot origin for the esc complicated by large scale plate boundary reorganizations and channeling of plume material to the east pacific rise

quantitative analysis of surface coseismic faulting and postseismic creep accompanying the 2003 mw 6 5 chengkung earthquake in eastern taiwan 1 on the basis of field observations and geodetic measurements we analyzed coseismic and postseismic deformation of the 2003 mw 6 5 chengkung earthquake across the rupture trace of the chihshang fault which lies along the suture zone between the philippine sea plate and the eurasian plate in eastern taiwan at three of our investigation sites along the chihshang fault the earthquake deformation was exhibited by fresh fractures within the 150 to 200 m wide surface fault zone reactivating preexisting fractures in most cases in addition to daily recorded creep meter data geodetic measurements including leveling distance and angle electronic distance meters and gps measurements were carried out during surveys 20 25 days before 20 25 days after and 120 125 days after the earthquake the near fault surface deformation is mainly characterized by anticlinal folding in the hanging wall and minor gentle synclinal folding in the footwall the geodetic data show that within our 150 to 250 m wide networks the coseismic deformation of the main shock produced only about 1 2 cm of horizontal shortening and vertical offset across the fault zone a larger additional displacement of about 7 9 cm for both the horizontal shortening and the vertical offset occurred as postseismic creep during the 120 125 days following the main shock we interpret the predominant folding and the rather large postseismic creep as a result of strong velocity strengthening along the fault plane near the surface which caused locking or coupling effect during the coseismic rupturing the depth of locked segment along which the coseismic and postseismic slip decreased dramatically upward is estimated to be 25 100 m depending on the site we interpreted the velocity strengthening and coupling effect at the shallow level as viscoelastic behavior of unconsolidated deposits in the footwall and melange mudstone in the hanging wall by incorporating far fault continuous gps data it was found that minor but significant deformation occurred outside of the surface ruptures zone of the chihshang fault in addition a possible back thrust or back fold might have occurred in the hanging wall we note that the chengkung earthquake occurred during the dry season this is consistent with the fact that the mechanical coupling of the fault at shallow depth is higher during drier periods inferred from the previous seasonal creep data at outcrop scale regarding damage of construction such as a concrete retaining wall most of the coseismic and postseismic horizontal shortening was essentially absorbed by multiple distributed brittle ruptures which deserves attention in terms of earthquake hazard mitigation

the fixed hotspot hypothesis and origin of the easter sala y gomez nazca trace reconstructions of the pacific and nazca plates suggest that both the nazca and tuamotu ridges originated from a melting anomaly which existed beneath the pacific farallon ridge during the time interval between anomalies 19 and 11 as would be predicted from the fixed hotspot hypothesis the hotspot hypothesis interpreted according to morphology of the easter sala y gomez nazca trace would further suggest that the hotspot was located entirely beneath the nazca plate during the time interval between anomalies 11 and 2 at the end of this interval the eastern boundary of the easter plate formed centered above the hotspot according to this scheme the present location of the hotspot is about 200 km west of easter island however the easter model is incompatible with the hotspot model for the hawiian emperior chain if the hotspots are fixed relative to one another as the age of the point on the tuamotu ridge which corresponds with the hawaiian emperor bend 53 to 55 m y is significantly greater than the age of the bend 43 m y the fixed hostpot hypothesis can be satisfied by the two traces only if one or a combination of the following conditions is met the kinematic age of the hawaiian emperor bend is substantially greater than the isotopically dated age the magnetic anomaly time scale is in significant error and or magnetic anomaly identifications in the vicinity of the tuamotu ridge are erroneous it might appear that other mechanisms for the origin of hotspot traces are preferable to the hotspot hypothesis the most promising of these involves propagating fractures resulting from intraplate stress this hypothesis has the advantage of accommodating anomalies in some hotspot traces such as persistence of igneous activity long after passage of the trace over the hotspot

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

evidence for diffuse extension of the pacific plate from pukapuka ridges and cross grain gravity lineations satellite altimeter measurements of marine gravity reveal 100 to 200 km wavelength lineations over a wide area of the pacific plate oriented roughly in the direction of absolute plate motion at least three mechanisms have been proposed for their origin small scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere diffuse n s extension of the lithosphere resulting in lineated zones of extension boudins and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism recently several chains of linear volcanic ridges have been found to be associated with the gravity lineations following ridgelike gravity signatures apparent in high resolution geosat gravity measurements we surveyed a series of volcanic ridges that extend northwest from the east pacific rise flank for 2600 km onto 40 ma seafloor our survey data as well as radiometric dates on samples we collected from the ridges provide tight constraints on their origin 1 individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion 2 the ridges formed along a band 50 to 70 km wide in the trough of one of the more prominent gravity lineations 3 radiometric dates of the largest ridges show no hotspot age progression moreover the directions predicted for minihotspot traces older than 24 ma do not match observed directions of either the gravity lineations or the ridges based on this last observation we reject the minihotspot model the occurrence of the ridges in the trough of the gravity lineation is incompatible with the small scale convection model which would predict increased volcanism above the convective upwelling we favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere however the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model perhaps the fault scarps are buried under more than 30 m of sediments and or covered by the elongated ridges finally we note that if ridge push force is much smaller than trench pull force then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

helium isotope ratios in easter microplate basalts abstract 3 he 4 he ratios in easter microplate basalt glasses show clear evidence of the effects of a mantle plume the east rift of the microplate between 26 and 28 s identified by la sm sr and pb isotopes and ridge crest elevation as the region of maximum plume influence has 3 he 4 he ratios spanning the entire range from 7 5 to 11 7 r a the easter microplate is the only section of the entire east pacific rise that is associated with a known hotspot track mantle plume and has elevated 3 he 4 he ratios although most of the west rift basalts contain morb helium 8 0 8 7 r a the basalt closest to the east rift has an elevated 3 he 4 he ratio 11 3 r a consistent with a significant plume component the diversity in isotopic signatures also indicates that homogenization of isotopic anomalies does not occur even in this region of super fast spreading the overall 3 he 4 he 206 pb 204 pb and 3 he 4 he 87 sr 86 sr trends have positive correlations although the high 3 he 4 he 87 sr 86 sr and 206 pb 204 pb component is not a single value but rather a broad band the apparent decoupling between the he and sr isotope distribution is modeled in the context of a plume source migrating ridge sink during channeling of the plume toward the ridge helium is preferentially lost from the center of the channeled plume resulting in lower he pb and he sr concentration ratios in the high 3 he 4 he component mixing trajectories in he sr isotopic space between a lile depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may explain the isotope systematics of plume ridge interactions in the context of modern theories of plume dynamics

patterns of deformed early lineations over later folds formed by buckling and flattening abstract the patterns of deformed early lineations l1 over later folds f2 can be classified into several morphological types depending on the nature of variation of l1 f2 over the folds the field relations indicate that the folds under consideration are neither shear folds nor parallel folds modified by flattening the lineation patterns are therefore interpreted in terms of an empirical model of simultaneous buckling and flattening in which it is assumed that i the central surface of the folded layer remains a sine curve in transverse profile ii the ratio of rates of buckle shortening to homogeneous strain is proportional to sin 2a with a as the dip angle and iii the progressive deformation is coaxial with the z axis of bulk strain parallel to the planar segments of the early folds the model gives an insight into the relative importance of different physical factors which control the development of dissimilar lineation patterns not all lineation patterns are explicable by this simplified model thus complex patterns with variable l1 f2 along the fold axis may develop by a progressive rotation of the geometrically defined fold hinge through successive material lines the theoretical results have been applied to interpret the lineation patterns in central rajasthan india it is concluded that l1 was initially very close to the e ese trending subhorizontal z axis of bulk deformation during f2 folding and that the x axis was subhorizontal or gently plunging with a n nne trend

new observations on mid plate volcanism and the tectonic history of the pacific plate tahiti to easter microplate abstract we describe the geology and tectonics of a continuous swathe of seafloor between tahiti and the western edge of the easter microplate imaged by gloria and sea beam on two separate cruise transits in 1987 and 1988 the data reveal that mid plate volcanism is common in this region even on deep seafloor hundreds of kilometres from major lines of seamounts and islands this supports the idea of a thin weak lithosphere over the pacific superswell and the idea that the tops of major mantle plumes may spread out over diameters of the order of 1000 km the mid plate volcanism occurs in two distinct forms over most of our traverse it appears as fields of relatively young and acoustically strongly backscattering lava flows often accompanied by groups of numerous small circular volcanoes east of 122 w about chron 5a however we observed a distinct form major sharp crested constructional volcanic ridges many tens of kilometres long individually trending ene but lying en echelon along an e w regional trend these ridges appear morphologically identical to the cross grain ridges seen elsewhere in the pacific we attribute their formation to magma supplied from the regionally hot mantle leaking along tectonic lines of weakness however although these ridges are parallel to fracture zone trends seen farther west they are morphologically very different from any known fracture zone moreover individual ridges are somewhat oblique to the tectonic spreading fabric around them and so do not seem to follow actual fracture zone traces the whole line of en echelon ridges lies along part of the predicted trace of fracture zone 2 of okal and cazenave 15 and is probably its morphological expression however nowhere did we see a convincing conventional fracture zone trace in or following the predicted position or orientation we suggest instead that magma from an independent source has used lines of weakness along minor fracture zones to produce these en echelon features the austral fracture zone is the only major fracture zone crossed in our transit and here is characterised by four fossil transform strands its marked position on the aapg and gebco maps is found to be in error finally we found that the expected change from nnw to nne trending spreading fabric at chron 6c did not occur in a clear cut way as predicted by earlier tectonic histories of the pacific instead the post chron 6c fabric oscillates in a confused way between nne and nnw suggesting to us that this area has been characterised by an unstable plate boundary probably associated with a succession of propagating rifts or microplates from chron 6c to the present

origin of striping lineation and transposition of linear structures in shear zones abstract striping lineation occurs as fine colour stripes on foliation surfaces of strongly sheared rocks that have a mylonitic colour banding although described as intersection lineation striping lineation in mylonites does not show two different intersecting s surfaces unlike ordinary bedding cleavage intersection striping lineation is a typical product of ductile shearing the colour stripes may have formed at a very early stage of deformation as intersections of a bedding or a colour banding on the foliation surface at a later stage the bedding or the colour banding ceased to be reoriented by external rotation the foliation became the only active s surface and the deformation of the colour stripes was controlled entirely by the nature of the strain after the appearance of a layer trace on the axial plane cleavage in an early stage of deformation the later history of its changing width and orientation becomes independent of changes in the thickness and orientation of the layer in strongly sheared rocks the nature of the original layering is greatly modified hence a colour striping can remain with very small width even when the foliation and bedding have become parallel the striping lineations in the phulad shear zone in western india and singhbhum shear zone in eastern india are often transposed and retransposed in response to folding sheath folding and strong stretching the process is very similar to the development of transposed foliation

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

geochemical tracers of mantle processes the mantle can be sampled directly only very rarely geochemists have thus come to rely heavily on mantle derived magmas to study the composition and evolution of the mantle only those compositional features that are unaffected by magmatic processes are useful as tracers of mantle processes these include radiogenic isotope ratios such as those of he sr hf and os stable isotope ratios and ratios of highly incompatible elements or elements of similar incompatibility such as ba nb or pb ce the term incompatible denotes a preference of the element for a melt over mantle minerals highly incompatible elements will partition entirely into the melt under most circumstances so that the ratio of two such elements in a basalt will be virtually identical to that ratio in its source this is also true to a lesser degree of ratios such as la sm and zr nb as zr and sm are not highly incompatible elements

a history of the easter microplate 5 25 ma to present gloria and other geophysical data are used to interpret the morphology and tectonic pattern of the easter microplate we model magnetic anomaly data and calculate spreading rates for use in euler pole calculations we calculate three plate closure and best fit poles for nazca pacific nazca easter and pacific easter plate motions and compare the results with earthquake focal mechanisms and seismicity we use the three plate closure poles to rotate digital gloria and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate s east rift first started to propagate northward at around 5 25 ma the microplate has evolved in two stages the first is from 5 25 ma to around 2 5 ma and involved continuous northward propagation of the east rift while the west rift became segmented along new transform faults as it adjusted to keep the overall nazca pacific velocities constant the transpressive northern boundary migrated with the east rift tip while the transtensional southern boundary remained more or less still at some time after 2 5 ma the east rift ceased northward propagation and the southwest rift began to open up along the transtensional pacific easter transform a convergent northern boundary formed involving southward thrusting of the nazca plate over the microplate no further nazca plate lithosphere was transferred from north of the microplate to the microplate interior and the pito deep region opened by pervasively rifting apart preexisting nazca plate lithosphere

directional median filtering for regional residual separation of bathymetry 1 we present a new spatial filter designed to separate short length scale bathymetric features from regional bathymetry the directional median dim filter divides a given filter circle into n bow tie sectors allocates data points inside the filter circle to each sector based on their relative position within the circle estimates a median for each sector and returns the lowest of these n medians this approach prevents dim filtering from choosing the biased medians near the features on a sloping regional trend which is a serious artifact of standard median filtering as the bathymetry comprises diverse length scale features the separated results vary with the choice of filter width such variations are spatially distributed using a finite range of filter widths we evaluate the spatial distribution of variations in the dim based separation by estimating their median absolute deviation mad at each data point the distribution of mad values is indicative of uncertainties inherent in the separation we demonstrate the effectiveness of dim filtering with both synthetic and actual bathymetric data

destruction of luzon forearc basin from subduction to taiwan arc continent collision abstract along offshore to the east of southern taiwan different stages of subduction and collision occur simultaneously along strike of the convergent boundary as a result the evolution of the luzon arc and its forearc basin can be studied from the younger subduction zone to the south to the collision zone to the north examining more than 8000 km of seismic lines we analyzed the seismic stratigraphy of strata in a forearc basin and its successive basins in the collision zone to study the processes related to arc collapse and forearc basin closure the study area presents three evolutional stages intra oceanic subduction initial arc continent collision and arc continent collision we divided 9 seismic sequences in the forearc basin and found older sub parallel basin fill sequences 4 9 and younger divergent sequences 1 3 isochron maps of the sequences were used to interpret different deformation modes and their areal extends on the arc side of the basin of the subduction and initial collision zones we found relatively undisturbed strata showing little arc deformation on the trench side the growth strata in sequences 1 through 3 are the result of recent tectonic wedging along the rear of the accretionary prism tectonic wedging and back thrusts incorporate the forearc strata into the rear of the accretionary prism until they close the forearc basin at a region with a 2200 m basement relief this relief is not caused by active deformation as young flat forearc strata lap onto it and mark the transition from initial collision to collision where many growth strata to the north suggest abrupt increase in active arc basement deformation the 1 deforming basement 2 back thrusts and 3 other sedimentary processes affect the architecture of the successive basins in the collision zone until the arc is juxtaposed to the rear of the fold and thrust belt on land

flexure modelling at seamounts with dense cores summary the lithospheric response to seamounts and ocean islands has been successfully described by deformation of an elastic plate induced by a given volcanic load if the shape and mass of a seamount are known the lithospheric flexure due to the seamount is determined by the thickness of an elastic plate te which depends on the load density and the age of the plate at the time of seamount construction we can thus infer important thermomechanical properties of the lithosphere from te estimates at seamounts and their correlation with other geophysical inferences such as cooling of the plate whereas the bathymetry i e shape of a seamount is directly observable the total mass often requires an assumption of the internal seamount structure the conventional approach considers the seamount to have a uniform density e g density of the crust this choice however tends to bias the total mass acting on an elastic plate in this study we will explore a simple approximation to the seamount s internal structure that considers a dense core and a less dense outer edifice although the existence of a core is supported by various gravity and seismic studies the role of such volcanic cores in flexure modelling has not been fully addressed here we present new analytic solutions for plate flexure due to axisymmetric dense core loads and use them to examine the effects of dense cores in flexure calculations for a variety of synthetic cases comparing analytic solutions with and without a core indicates that the flexure model with uniform density underestimates te by at least 25 per cent this bias increases when the uniform density is taken to be equal to the crustal density we also propose a practical application of the dense core model by constructing a uniform density load of same mass as the dense core load this approximation allows us to compute the flexural deflection and gravity anomaly of a seamount in the wavenumber domain and minimize the limitations recognized from the analytic tests then the dense core model is applied to predict the lithospheric flexure beneath howland island in the tokelau seamount chain these results are compared with the predictions of the uniform density model based on age dating of howland and the age of the seafloor traditional te versus age curves predict the elastic plate thickness beneath the seamount to be around 20 km which is comparable to the best dense core model of te 26 km however the best uniform density model is found at te 12 km which is significantly less than the predicted from our investigations of synthetic and real seamount cases we conclude that the dense core model approximates the true mass distribution of a seamount better than the uniform density model finally we suggest that the role of underplating in flexure modelling may need to be reexamined because the dense core model predicts substantially less deflections than the uniform density model without requiring additional buoyancy caused by underplated material

flexural isostasy and loading sequence of the dokdo seamounts on the ulleung basin in the east sea sea of japan abstract the isostatic compensation and loading sequence of the dokdo seamounts on the ulleung basin in the east sea are studied by a flexure model using gravity and bathymetric data the dokdo seamounts comprise dokdo dok island lying as a cluster of emerged small islets surmounting a large submerged volcanic edifice and two voluminous tablemounts simheungtaek and isabu in spite of their similarity in topographic expression a large difference of about 50 mgal between the observed gravity anomaly peaks over dokdo and the isabu tablemount implies different amounts of compensation a comparison between the observed and calculated gravity anomalies of the seamounts shows that the flexural compensation investigated in this study predicts well the observed data the flexure model suggests that the dokdo seamounts might be composed of high density volcanic rocks modeling results show that the flexural rigidity or effective elastic thickness of the lithosphere under dokdo is stronger or thicker than that under the isabu tablemount it implies that the age of lithosphere at the time of loading for isabu was younger than that for dokdo the effective elastic thicknesses associated with the dokdo seamounts approximately follow 200 400 c isotherms in the cooling plate model the results of this study and the former age dating of dokdo suggest that the isabu tablemount was formed first followed by dokdo after the opening of the east sea ceased

the three dimensional shape and localisation of deformation within multilayer sheath folds abstract sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non coaxial deformation where layering becomes rheologically significant then active sheath folds may form in which the inner nose of the sheath decouples along detachments and undergoes a translation relative to the outer folded layers that surround and envelope the nose we present a systematic 3 d analysis of multiple folded layer geometries in a serially sectioned natural sheath fold weak layers which are reactivated as detachments define different shaped sheath folds relative to other layers with the sense of cut off along detachments reversing across the axial surface as the inner fold nose has protruded into the surrounding sheath envelope detachment layers are more tightly folded meaning that such active sheath folds are non similar shapes the obliquely oriented bifurcating en echelon nature of the fold hinges developed in adjacent layers suggest that pre cursor folds formed with pronounced 3 d obliquity relative to the subsequent shear plane mineral lineations folded around sheath closures display asymmetric star burst patterns consistent with recrystallisation during active folding and hinge rotation we show that the eye fold shapes exposed in any 2 d y z slice can be used to predict the geometry of marker horizons back along the x axis in the third dimension this self similarity may be of value when tracing stratiform mineralised horizons in large scale sheath folds

using submarine lava pillars to record mid ocean ridge eruption dynamics abstract submarine lava pillars are hollow glass lined basaltic cylinders that occur at the axis of the mid ocean ridge and within the summit calderas of some seamounts typically pillars are 1 20 m tall and 0 25 2 0 m in diameter with subhorizontal to horizontal glassy selvages on their exterior walls lava pillars form gradually during a single eruption and are composed of lava emplaced at the eruption onset as well as the last lava remaining after the lava pond has drained on the deep sea floor the surface of a basaltic lava flow quenches to glass within 1 s thereby preserving information about eruption dynamics as well as chemical and physical properties of lava within a single eruption investigation of different lava pillars collected from a single eruption allows us to distinguish surficial lava pond or lava lake geochemical processes from those operating in the magma chamber morphologic major element petrographic and helium analyses were performed on portions of three lava pillars formed during the april 1991 eruption near 9 50 n at the axis of the east pacific rise modeling results indicate that the collected portions of pillars formed in 2 5 h suggesting a total eruption duration of 8 20 h these values are consistent with observed homogeneity in the glass helium concentrations and helium diffusion rates major element compositions of most pillar glasses are homogeneous and identical to the 1991 flow but slight chemical variations measured in the outermost portions of some pillars may reflect post eruptive processes rather than those occurring in subaxial magma bodies because lava pillars are common at mid ocean ridges mors the concepts and techniques we present here may have important application to the study of mor eruptions thereby providing a basis for quantitative comparisons of volcanic eruptions in geographically and tectonically diverse settings more research is needed to thoroughly test the hypotheses presented here

observations of flexure and the rheology of oceanic lithosphere the principal evidence for the long term i e 1 myr mechanical behaviour of the oceanic lithosphere has come from studies of how it deforms in response to large loads such as volcanoes and sediments a model widely used to explain the deformation is an elastic plate in which flexural rigidity depends on the thermal age of the lithosphere at the time of loading an elastic model however is time invariant and does not take into account temporal changes that may occur in the flexural rigidity as a consequence of loading there is evidence for example that the flexural rigidity of the oceanic lithosphere also depends on load age being large at short times and small at long times thus competing effects may exist between thermal cooling which strengthens the lithosphere and some form of load induced stress relaxation which weakens it in order to investigate the relative roles of these processes we have developed a multilayered viscoelastic model which is based on the results of experimental rock mechanics that creep in the lithosphere is a thermally activated process and on a thermal structure that is given by the plate cooling model by comparing the predictions of the model with a new compilation of flexural rigidity estimates we have found that if the upper mantle viscosity is 1020 pa s the activation energy that best describes the long term mechanical behaviour of the oceanic lithosphere is 120 kj mol 1 this parameter pair explains the dependence of flexural rigidity on both plate and load age it also helps account for the subsidence and uplift history of oceanic islands and the stratigraphic patterns that develop in the flexural moats that flank them at atolls a multilayered viscoelastic model explains the rapid subsidence that follows shield building and does not require that the age of the lithosphere that supports a volcano is thermally re set to a younger value our studies suggest that while the oceanic lithosphere has viscoelastic properties the viscosity of its upper layers is so much higher than that of its lower layers that in effect it behaves as a thin elastic plate on long timescales there will therefore be a certain permanence to the observations of oceanic flexure such that they may be used with some confidence to evaluate the tectonic setting of individual features of the seafloor and in some cases their age

geology of young submarine volcanoes west of easter island southeast pacific abstract submarine volcanoes probably related to the easter hotspot were found between the easter microplate and easter island near 26 15 s 111 w hydrosweep swath mapping accompanied by deep towed camera observations and sampling revealed the presence of fresh lava flows confined to two distinct fields the previously known ahu and the newly discovered umu volcanic fields covering a combined area of more than 4300 km 2 these lava fields form constructional ridges with volcanic cones as well as isolated seamounts another large volcanic area probably also belonging to the easter hotspot was found 100 km south of the ahu umu region thus the young hotspot region has a diameter of about 200 km and its centre probably lies underneath the umu volcanic field rather than under ahu volcanic field the fresh volcanics recovered from these areas are olivine and plagioclase phyric enriched and more rarely depleted tholeiites the enriched lavas are comparable to e morb from the easter microplate spreading axes and volcanic ridges and seamounts west of the microplate

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

upper mantle heterogeneities and dynamics the width of geochemical and residual elevation anomalies along the mid ocean ridge system decrease as both the distance between migrating ridge segments and related hotspots and the local geochemical dispersion in the anomalies increase these observations provide further evidence that flows of mantle plumes in the upper mantle are deflected by migrating ridge sinks once they override hotspots

tectonic evolution of late cenozoic arc continent collision in taiwan the island of taiwan is an active orogen formed by the collision between the luzon arc and the asian continent the kinematic progression of the arc continent collision can be reconstructed by superimposing the restored paleopositions of luzon arc upon the precollisional asian continental margin the geological history of the collision can be interpreted from the rock records of the mountain ranges of taiwan by incorporating geological information into plate kinematics the collision can be attributed to the northwesterly impingement of the luzon arc upon the continental margin in the last 12 million years during the initial stage of the collision some of the continental materials might have been metamorphosed in the deep subduction zone but no distinct effects can be perceived in the sedimentary record in the mio pliocene time about 5 ma the accretionary wedge grew large enough to become a sediment source for the luzon forearc basin and to induce foreland subsidence on the continental margin in the early late pliocene about 3 ma drastic collision caused rapid uplift of the collision orogen that shed voluminous orogenic sediments into the forearc and foreland basins continued collision progressively accreted the forearc and foreland basins to the collision orogen from northmore to south to the present configuration less

evaluation of the bgm 3 sea gravity meter system onboard r v conrad the first bell aerospace bgm 3 marine gravity meter system available for academic use was installed on r v robert d conrad in february 1984 the bgm 3 system consists of a forced feedback accelerometer mounted on a gyrostabilized platform its sensor requiring no cross coupling correction is a significant improvement over existing beam and spring type sea gravimeters such as the gss 2 a gravity survey over the wallops island test range together with the results of subsequent cruises allow evaluation of the precision accuracy and capabilities of the new system over the test range the bgm 3 data were compared directly to data obtained by a gss 2 meter onboard r v conrad the rms discrepancy between free air gravity anomaly values at intersecting ship tracks of r v conrad was 0 38 mgal for bgm 3 compared to 1 60 mgal for the gss 2 moreover bgm 3 s platform recovered from abrupt changes in ship s heading more rapidly than did the platform of gss 2 the principal factor limiting the accuracy of se

slip distribution on a thrust fault at a plate boundary the 2003 chengkung earthquake taiwan summary the 2003 chengkung earthquake mw 6 8 occurred on the east dipping chihshang fault of eastern taiwan a listric fault with dominant reverse motion a joint inversion taking into account geodetic gps and seismological accelerometric and teleseismic data is performed this modelling highlights the rupture process behaviour on the fault plane at depth the coseismic rupture developed essentially between 12 and 26 km depth expanding laterally by about 30 km towards the ssw two slip patches dominated with slip values exceeding 1 5 m during the 30 s of the whole process the rupture propagation decelerated with time starting at 3 4 0 3 km s 1 in the two main slip patches and ending with velocities below 1 6 0 1 km s 1 the 48 hr of aftershocks that followed the main shock are distributed essentially inside the coseismic slip patches on the fault surface except in the deepest southern part of the fault where slip is less resolved the results of our modelling illustrate the particular behaviour of the 2003 chengkung earthquake where the coseismic slip rapidly diminished towards the surface in good agreement with the observation of fast post seismic slip followed by interseismic creep along the chihshang fault

evidence from en echelon cross grain ridges for tensional cracks in the pacific plate sea floor topography in the pacific is mainly aligned with original spreading directions1 but is overprinted by alignments created by mid plate processes spreading produces abyssal hills and fracture zones and mid plate volcanism generates seamounts isolated or in chains a different category of topography the cross grain discovered in geoid height data collected by the seasat radar altimeter2 comprises linear troughs and swells spaced 200 km apart oblique to fracture zones and abyssal hills but parallel to the hawaiian chain three models have been proposed for the cross grain small scale convection organized into longitudinal rolls by the shear of the pacific plate2 compressive buckling3 and lithospheric boudinage resulting from plate wide tensile stresses4 5 none of the previously available data ruled out any of these models here we report multi beam bathymetric data revealing long narrow en echelon ridges along the cross grain interpreted as evidence of tension cracks in the pacific plate

analysis of transpressional deformation from geometrical evolution of mesoscopic structures from phulad shear zone rajasthan india abstract transpressional deformation in ductile shear zones may be analyzed by simultaneous wall parallel simple shear with strain rate xy along with three dimensional coaxial deformation with strain rates x y and z several types of transpression may be distinguished on the basis of relative values of strain rate ratios x a y b and z c it is possible to identify the nature of transpressional deformation from a detailed analysis of shear zone structures in the phulad shear zone of rajasthan india evidence of general flattening a thrusting sense of shear occurrence of transport parallel stretching lineation frequent occurrence of sheath folds with apical direction parallel to the stretching lineation and occurrence of u shaped lineation patterns indicate that the deformation was transpressional with b a ratio ranging between 1 and 2 the bulk deformation was not dominated by simple shear but involved both simple and pure shear with extrusion along the transport direction much greater than along the vorticity vector rotation of long tectonic clasts caused reorientation of monoclinic rolling structures so that strongly monoclinic structures with opposite senses of asymmetry in different domains appear on subhorizontal outcrops parallel to the vorticity vector although the component of simple shear is zero on these surfaces

the structure of the nazca ridge and sala y gomez seamount chain from the dispersion of rayleigh waves summary we report single station measurements of group velocity for fundamental mode rayleigh waves that propagated along two aseismic ridges in the eastern pacific ocean basin the nazca ridge and the sala y gomez chain we find that the nazca ridge dispersion curves when regionalized to isolate the ridge contribution indicate group velocities of about 3 5 km s for short periods these values are slower than predicted for the geometry of the propagation paths interpreting this velocity heterogeneity is complicated by the recognition that the nazca ridge is about 1000 km longer than previously thought but models of vyv for the ridge derived by inverting the dispersion curves reveal abnormally thick 18 f 3 km oceanic crust along its longitudinal axis this value compares favourably with previously published results over the tuamotu plateau thus strengthening the hypothesis that the nazca ridge and tuamotu plateau are mirror images formed by a hotspot coincident with the farallon pacific spreading centre before 20 ma in contrast the measurements along the sala y gomez chain are consistent with typical oceanic crust and mantle when these values are inverted they reveal no thick crust 6 km along the sala y gomez chain we conclude from this result that the chain was likely formed as a leaky fracture zone not as a simple hotspot track

morphology and distribution of seamounts surrounding easter island we investigate the morphology and distribution of a seamount population on a section of seafloor influenced by both superfast seafloor spreading and hotspot volcanism the population under investigation is part of a broad chain of seamounts extending eastward from the east pacific rise near easter island in order to define the morphological variability of the seamounts basal shape cross sectional area volume flatness and flank slope are plotted against height for 383 seamounts with heights greater than 200 m based on bathymetry data collected by glori b and seabeam 2000 during three cruises onboard the r v melville in the spring of 1993 nearly complete swath mapping coverage of the seamounts is available for the analysis of size and shape distribution we quantitatively describe the seamount population of this active region in which seamounts cover 27 of the seafloor and account for 4 2 of the total crustal volume over 50 of the total volume 61 000 km3 of seamounts used in this study is made up by the 14 largest seamounts and the remaining volume is made up by the 369 smaller seamounts 200 m in height our analysis indicates there are at least two seamount populations in the easter island salas y gomez island 25 29 s 113 104 w study area one population of seamounts is composed of short seamounts 1200 m shield like pointy cones flatness 1200 m originate exclusively from a hotspot source but only a portion of the smaller volcanoes 1200 m are formed from a hotspot source the remainder would be presumably formed by a normal mantle or mixed source

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

opening and closing of folds in superposed deformations abstract during superposed deformation an early fold f1 may either tighten or open out opening out is possible if during the second deformation there is a bulk extension across the f1 axial plane the theoretical model indicates that the rate of opening out is largely controlled by the initial tightness of the fold the results of the numerical model are in agreement with analog experiments of superposed buckling and show that if there is a relatively small range of variation of initial tightness of f1 folds the range of fold tightness becomes much larger after the opening out of the folds in the second deformation consequently the refolded structure may show a close association of different modes of superposed buckling in a type 2 fold interference the continued opening out of f1 invariably causes a replacement of the initial f1 hinge line by a new and more sinuous hinge line when the shortening direction p2 of the second deformation was at an acute angle to the f1 axis depending upon their local orientation with respect to p2 the f1 folds were tightened in some places and were opened out elsewhere when the enveloping surface is inclined in the direction of maximum stretching of second deformation segments of f1 and f2 hinge lines may both rotate to lie in the extension field and both f1 and f2 may open out with progressive deformation if f1 has a low pitch and p2 is parallel to the f1 axis the two sets of hinge lines may become subparallel at an advanced stage of deformation the resulting structure is morphologically similar to non planar sheath folds

rough versus smooth topography along oceanic hotspot tracks observations and scaling analysis some hotspot tracks are topographically smooth and broad nazca carnegie cocos galapagos walvis iceland while others are rough and discontinuous easter sala y gomez tristan gough louisville st helena hawaiian emperor smooth topography occurs when the lithospheric age at emplacement is young favoring intrusive magmatism whereas rough topography is due to isolated volcanic edifices constructed on older thicker lithosphere the main controls on the balance of intrusive versus extrusive magmatism are expected to be the hotspot swell volume flux qs plate hotspot relative speed v and lithospheric elastic thickness te which can be combined as a dimensionless parameter r qs v 1 2 te which represents the ratio of plume heat to the lithospheric heat capacity observational constraints show that except for the ninetyeast ridge r is a good predictor of topographic character for r 3 they are smooth and dominated by intrusion

speed limit for oceanic transform faults oceanic transform faults with slip rates greater than 145 km m y do not currently exist along the east pacific rise where sea floor spreading rates range from 145 to 160 km m y instead offsets of the the very fast spreading east pacific rise are accommodated by microplates propogating rifts or overlapping spreading centers this suggests that there might be a speed limit above which transform faults do not exist a physical reason for a speed limit is not known but it might be related to unstable stress fields near the rifts tips causing them to episodically propagate and prevent a transform fault from being formed the spreading rates quoted are from our new 0 0 73 ma relative motion model for the pacific and nazca plates

geochemical constraints on magma sources and mixing processes in easter microplate morb se pacific a case study of plume ridge interaction the easter microplate is bounded in the east and west by two large overlapping spreading centres called the east and west rift respectively the east rift is located close to the easter hotspot the site of young intraplate volcanism above a mantle plume easter hotspot material creates a geochemical anomaly on the east rift spreading axis of the microplate and mainly northward directed material flow occurs beneath the propagating east rift the incompatible element depleted mantle of the easter microplate consists of two different sources one of which mixes with enriched easter hotspot material forming the east rift mid ocean ridge basalt morb the other depleted mantle source is present beneath the west rift and mixes with an enriched endmember apparently distinct from the easter hotspot source while the easter hotspot material leads to a bathymetric anomaly on the east rift no increased degree of melting is observed at the point of maximum inflow of enriched plume material implying low excess temperatures 50 c of the mantle the mixing processes between the easter hotspot and morb material probably take place in the partially molten asthenosphere during the horizontal flow of the plume material melting above the plume and at the spreading axes in the easter microplate region occurs over a range of pressures but there is no sign of melts from enriched garnet pyroxenite

ore formation mechanism of the weilasituo tin polymetallic deposit ne china constraints from bulk rock and mica chemistry he ar isotopes and re os dating abstract china s w sn resources are dominant on a global scale a series of sn polymetallic deposits has been found in recent years at depth on the flanks of known volcanic subvolcanic cu pb zn ag deposits in the southern great xing an range ne china the large scale weilasituo tin polymetallic deposit formed at the depth of the weilasituo cu zn deposit is most typical of these types of deposit tin polymetallic orebodies are hosted in the early paleozoic xilingol complex and their mineralization is closely related to early cretaceous quartz porphyry in this contribution bulk rock and mica chemical compositions re os geochronology and he ar isotopic analyses are described in addressing the ore formation mechanism of the deposit geochemical characteristics indicate that the ore bearing quartz porphyry corresponds to the i type granite serie five types of mica have been identified in the study area based on detailed petrography and epma and la icp ms analyses i early magmatic fe li mica in albitized quartz porphyry ii late magmatic fe li mica in mineralized quartz porphyry iii hydrothermal fe li mica in a cryptoexplosive breccia pipe distributed around breccia fragments iv fe li mica in quartz veins and v fe li mica in plagioclase gneiss wall rock magmatic zinnwaldite from the upper mineralized part of the quartz porphyry is enriched in nb 100 160 ppm and ta 100 150 ppm hydrothermal zinnwaldite from the explosive breccia quartz veins and surrounding gneiss is unusually rich in mg up to 3 4 wt mgo and sn up to 200 ppm 3he 4he ratios of the weilasituo tin polymetallic and cu zn deposits are in the ranges 3 38 4 91 ra and 4 8 4 9 ra respectively indicating ore forming fluids sourced from mixed crustal and mantle materials molybdenite re os dating of w mineralization yielded an isochron age of 129 0 4 6 ma similar to the porphyry crystallization and mineralization ages of the weilasituo tin polymetallic deposit and also consistent with the mineralization age of the cu zn deposit we propose a new metallogenic model with all subtypes of w sn cu pb zn ag mineralization in the weilasituo area belonging to a single complex metallogenic system compared with other ore districts ore related granitoids in the southern great xing an range display relatively uniform emplacement ages and were derived from partial melting of juvenile lower crust rather than old basement their ore forming fluid source was greater influenced by mantle fluids than which found in south china

multiple episodes of volcanism in the southern austral islands flexural constraints from bathymetry seismic reflection and gravity data 1 at the southeastern end of the cook austral island chain in the vicinity of the currently active macdonald seamount multiple episodes of volcanism have left a diverse population of seamounts multichannel seismic reflection and shipboard gravity data provide observational constraints on the magnitude and wavelength of flexure and multibeam bathymetry obtained by r v ewing and f s sonne within the study area provides full spatial coverage of all volcanic loads three dimensional flexural modeling of closely spaced loads emplaced at discrete times performed both with analytical models of idealized loads and with fourier domain solutions of the observed volcanic loads partitioned into the younger and older seamounts is compared with seismic and gravity data this modeling provides a more complete view of the volcanic history than radiometric dating and geochemical analysis of sparse dredge samples alone wide angle seismic refraction data from ocean bottom hydrophones obhs and expendable sonobuoys are also consistent with the flexural modeling results volume estimates of the different volcanic episodes show that one half to two thirds of the material added to the abyssal seafloor by midplate volcanic processes is due to older volcanism that erupted on young lithosphere while the younger higher seamounts contributed the remainder

heave of a surficial rock layer due to pressures generated by injected fluids 1 this paper examines the problem of heave developed in a surface or caprock layer of a storage formation due to the injection of pressurized fluids at depth the problem is of interest to the deep geologic disposal of hazardous wastes and the geologic sequestration of carbon dioxide by injection the paper develops a mathematical model that assumes elastic behavior of the storage medium while the surface rock layer is modeled as a thin plate elastic solutions are developed for the axisymmetric flexure deflections of the surface layer during internal pressurization over a flat circular region the model provides a convenient tool for the preliminary estimation of the influence of injection pressures on the integrity of surface rock layers