evolution of oceanic lithosphere a driving force study of the nazca plate the role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the nazca plate results indicate that ridge push forces when distributed over that portion of the oceanic lithosphere giving rise to the force predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest in particular the component of stress normal to the ridge vanishes near the ridge for the new modeling predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on nazca intraplate stress stress states have also been calculated for a variety of slab pull models which include possible dependence on age rate and angle of subduction the predicted stresses vary substantially near the trench among these models with less variation farther from the trench combinations of ridge push and slab pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab pull over ridge push simple viscous modeling with only distributed ridge forces predicts relative motion between the nazca and south american plates that is in fairly poor agreement with the observed direction the fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the antarctic plate

formation and growth of deep mantle plumes summary if whole mantle convection occurs in the earth s mantle then the core mantle boundary constitutes the lower boundary layer for mantle convection this boundary layer appears to be unstable on a small scale and thus may be a source of plumes of hot matter which penetrate the mantle and occasionally even the lithosphere producing hot spots a finite amplitude numerical code is used to study the formation of such plumes and their growth through the mantle the plumes are restricted to being two dimensional sheets rather than cylinders the initial conditions consist of a steadily convecting mantle and plumes are produced by introducing a perturbation in the form of either a pulse or a steady stream of heat into the bottom of the mantle two main results are obtained 1 a critical perturbation size has been found for a mantle with a rayleigh number of lo small perturbations produce plumes which fail to penetrate the mantle and instead are swept up by the pre existing convective pattern while large perturbations succeed in penetrating the mantle and reaching the lithosphere the critical perturbation size is shown empirically to be proportional to the effective bouyancy and to a factor related to the shape of the perturbation a perturbed region 150km wide and 60 km deep should produce a successful plume when the temperature perturbation is 200k or more 2 deep mantle plumes appear to require on the order of 50 100myr to penetrate the mantle episodic plumes on shorter time scales appear unlikely a similar time is required for plumes forming in an initially static uniform temperature mantle

development of the accretionary prism along peru and material flux after subduction of nazca ridge results from ocean drilling program leg 112 indicated tectonic erosion of the peruvian convergent margin during subduction of the nazca ridge followed by renewed accretion against the erosional scar seismic images and swath mapping morphology show a regional geology shaped as subduction of nazca ridge migrated 800 km along the peru trench the accretionary prism is imaged at two stages of development since upper miocene time with depth migrated seismic records in which the back stop is well defined from these data material flux can be quantified after the ridge crest subducted and accretion dominated over erosion the prism grew rapidly to 10 to 15 km width during rapid growth the margin taper was large and about 60 percent of the sediment supply accreted as the prism growth slowed only 30 percent accreted despite an increased trench sediment supply since the convergence rate changed little the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted the decreased prism growth may be partially controlled by varying the structure of the back stop the force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary thus the shear stress transmitted across the plate boundary is insufficient to drive out of sequence thrust faults that would thicken the back stop or raise it beyond a limited height a constant back stop height limits accretion and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop nazca ridge subduction affected the convergent margin mass flux for about 8 ma mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest when growth of the accretionary prism slowed a greater terrigenous input through the trench was accommodated by sediment subduction the resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which may affect coupling

multistage magma mixing revealed in phenocryst zoning of the yunokuchi pumice akagi volcano japan detailed study of the texture and chemical zoning of phenocrysts in the compositional and thermal histories at the sites a plinian fall deposit yunokuchi pumice from akagi volcano where they grew only recently have such studies been japan permits reconstruction of the subvolcanic magma chamber undertaken focusing on magma chambers as the sites hypersthene phenocrysts are complexly zoned some have spongy of phenocryst formation tait 1988 de silva 1989 cellular textures whereas others exhibit simple reverse zoning augite umino iwano 1992 fodor et al 1993 nakada et phenocrysts have texturally similar cores but al and ti vs mgal 1994 the magma chambers depicted by these number zoning patterns are correlatable with hypersthene cores studies are mainly composed of phenocryst poor magma most plagioclase phenocrysts have a uniform calcic core enclosed surrounded by a comparatively thin crystallizing boundby a normally zoned rim others have variable an contents and ary layer phenocrysts are interpreted to have crysmottled textures these characteristics and melt glass compositions tallized either within the main body of magma or in suggest a complex multistage mixing of low t 940 960 c situ within the boundary layer highly silicic and high t 980 1060 c less silicic magmas combination of geophysical and petrological data stratigraphic variations of the phenocryst types show that the earlier together with fluid dynamical simulations of magmatic erupted magma inherited phenocrysts from more high t magma processes beneath mid oceanic ridges has shown that a components we propose the presence of an inhomogeneous mushy mush of crystals and interstitial melt 25 55 vol chamber filled with a highly silicic magma which was recharged crystals probably makes up the majority of the magma periodically by less silicic magmas the large viscosity contrast and chamber and only a small volume exists as a meltdensity balance of the new magma relative to the mush controlled enriched lens e g sinton detrick 1992 nicolas et al the efficiency of mixing and the position of emplacement of the high1993 barth et al 1994 likewise it is highly probable t magma in the mush tapping the chamber first expelled the that magma chambers beneath island arc volcanoes fluidal mixed magma followed by progressive erosion of the mush largely consist of a mush of crystals and interstitial melt as the eruption continued recent studies on arc volcanic products have presented evidence for the existence of such mushy magma chambers these include an estimation of very high crystallinity

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

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

chemical and isotopic diversity in basalts dredged from the east pacific rise at 10 s the fossil galapagos rise and the nazca plate abstract we present petrographic chemical and isotopic data for fresh lava samples dredged from three regions 1 the fossil galapagos rise 2 an elongate volcano near this extinct spreading center and 3 the east pacific rise at 10 s the samples from the galapagos rise are among the first samples from any fossil spreading center to be analyzed alkalic picrites from the elongate seamount and transitional basalts from the east pacific rise are both somewhat unusual rock types considering their respective tectonic environments the dredges from the east pacific rise at about 10 s recovered unusual transitional light rare earth element lree enriched basalts which show a range of fractionation on the basis of their chemical and isotopic abundances it is unlikely that the lavas are related by a single simple process of magmatic differentiation we suggest that the mantle source region of these basalts was chemically and isotopically heterogeneous the chemistry of lree depleted tholeiitic basalt dredged from near the axis of the extinct galapagos rise indicates complex petrogenesis and differentiation the presence of tholeiitic basalts here indicates that unlike the guadalupe and mathematician fossil ridges the galapagos rise has not been the site of voluminous post abandonment alkalic volcanism alkalic basalts of picritic bulk composition dredged from an elongate seamount near the galapagos rise do not represent liquid compositions instead we suggest that these alkalic liquids contain added olivine and plagioclase xenocrysts although most of the samples analyzed are very fresh a few have been altered the latter exhibit characteristic chemical and isotopic effects of seawater alteration

intraplate en echelon volcanic ridges in the south pacific west of the easter microplate satellite altimetric data for the south central pacific ocean show linear geoid anomalies extending from pitcairn island 130 w 24 s to the western border of the easter microplate 118 w 26 s previously these anomalies have been interpreted as a submarine volcanic chain more than 1300 km long inclusive of henderson island ducie island and crough seamount and constructed on young 30 ma oceanic lithosphere recent bathymetric data and side scan sonar images gloria from the eastern part of this volcanic chain suggest that the anomalies correspond to a succession of elongate ridges arranged in a right stepping en echelon pattern along a fracture zone at least five volcanic ridges were recognized during the so80a cruise 1992 between 123 w and 118 w at 25 s trending 275 280 parallel to plate motion the ridges range from 70 to 200 km long 20 to 35 km wide and 1100 to 2900 m high their volumes 600 to 1500 km3 are equivalent to those of large intraplate submarine volcanoes from the polynesian hotspots sediment thickness and the degree of alteration of samples increase progressively from east to west suggesting that the chain grew from a volcanic centre located near 118 w 25 s close to the west rift that forms the western border of the easter microplate the ridges are built along a fracture zone of the pacific plate trending n100 110 which shifts the magnetic anomalies more than 300 km to the left the origin of this fracture zone is not well assessed but it strikes in the continuity of the present day orongo transform fault forming the southwestern boundary between the easter and pacific plates east of 114 50 w the crests of the volcanic ridges are oriented n055 075 at angles of 30 50 with respect to the fracture zone the arrangement of both features could be compared to that of right stepping opening cracks volcanic ridges along a sinistral shear zone transform fault an alternative model is based on the orientation of the abyssal hills which is almost perpendicular to that of the ridges especially in the eastern part of the volcanic alignment the origin of the en echelon ridges could also be related to tension and fracturing in the youngest lithosphere along leaky transform faults

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

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

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

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

errors in minimum plate velocity determined from paleomagnetic data in prior studies paleomagnetic data have been used to determine ancient minimum velocities of the continents the greatest minimum velocities 50 km m y exceed the present day absolute velocities of the major continents this result is important because the observation that present day oceanic plates move faster than present day continental plates can be treated as a major constraint on plate dynamics however these prior studies are limited because they ignore known errors in the paleomagnetic data in this paper we present a technique for incorporating these errors and using them to estimate 95 confidence limits on the minimum velocities this new technique was applied to pennsylvanian to cretaceous paleomagnetic data from north america and to the ancient geometry of laurasia when errors are considered minimum velocities are 10 to 50 km m y lower than when errors are ignored additional uncertainties owing to a hypothesized clockwise rotation of the colorado plateau a source of many of the data are considered and found to alter the minimum velocities only negligibly allowing for uncertainties in the pole positions uncertainties in ages and the uncertainty due to possible rotation of the colorado plateau we estimate that laurasia moved at least 48 km m y from early jurassic to early cretaceous time and at least 43 km m y from pennsylvanian through late triassic time there is only a 5 risk that the true minimum velocities are lower than these lower bounds these velocities are less than the peak velocities over short intervals found in prior analyses but exceed the average velocities previously found for triassic and jurassic time these newly estimated velocities are still much higher than the present velocities of the continents and as in the prior studies imply that the slow motion characteristic of major continents at present is not a fundamental attribute of plate motions

intraplate seismicity of the southern part of the pacific plate we present a compilation of all the known seismicity of the part of the pacific plate located south of the equator this includes historical earthquakes dating back to 1937 in addition to the gilbert islands swarm of 1981 1983 we identify 60 events at 32 epicenters in the magnitude range 4 3 6 9 our results indicate that the level of seismicity of the southern pacific plate is comparable to that of other plates including continent bearing ones most of the seismic activity is concentrated in the youngest parts of the plate but a few large events are present in cretaceous lithosphere with the exception of events from the gilbert islands swarm earthquakes located in lithosphere 35 ma and older are usually compatible with the release of horizontal compressional stress due to the gravitational process known as ridge push in younger areas the situation is much less clear cut the final geometry of the focal mechanisms is controlled by local tectonics in particular due to the miocene reorientation of the spreading at the east pacific rise the bulk of the pacific plate is particularly vulnerable to ridge push and this explains the numerous strike slip events and the absence of magnitude 6 events in most of the plate correlations of epicenters with zones of weakness or bathymetric features are frequent but not universal finally a number of areas with high seismic release are identified as potential targets for future exploration

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

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

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

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

significance of a molluscan fauna to the physiographic history of the calgary area alberta a newly discovered molluscan fauna provides an approximate age for a previously undated gravel sequence capping nose hill a relict plateau adjacent to the bow river in calgary alberta and constrains the age of the physiographic relief in the area the fauna consists of a mixed terrestrial aquatic assemblage whose composition indicates a paleoclimatic regime similar to that of the area today the fossiliferous sediment interpreted as a pond deposit is a pebbly mud within a thick braid plain gravel sequence isoleucine epimerization in the molluscs is not inconsistent with an early pleistocene or older age for the deposit the nose hill gravels and an equivalent sequence capping another outlier broadcast hill located on the other side of the bow river define an originally extensive paleo plains surface fluvial dissection of the surface including 175 m of incision by the bow river and resultant separation of the two outliers occurred within the last one to two million years

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

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

ridge subduction at an erosive margin the collision zone of the nazca ridge in southern peru the 1 5 km high obliquely subducting nazca ridge and its collision zone with the peruvian margin have been imaged by wide angle and reflection seismic profiles swath bathymetry and gravity surveying these data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust the lowermost layer contributes 10 12 km to the total crustal thickness of the ridge the sedimentary cover is 300 400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges at the collision zone of ridge and margin the following observations indicate intense tectonic erosion related to the passage of the ridge the thin sediment layer on the ridge is completely subducted the lower continental slope is steep dipping at 9 and the continental wedge has a high taper of 18 tentative correlation of model layers with stratigraphy derived from ocean drilling program leg 112 cores suggests the presence of eocene shelf deposits near the trench continental basement is located 15 km landward of the trench normal faults on the upper slope and shelf indicate extension a comparison with the peruvian and northern chilean forearc systems currently not affected by ridge subduction suggests that the passage of the nazca ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long term erosive regime

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

tectonic deformation of the andes and the configuration of the subducted slab in central peru results from a microseismic experiment summary a microearthquake survey was conducted in the central andes of peru east of the city of lima to study the seismicity and style of tectonic deformation of the peruvian andes although most of the stations forming the temporary seismographic network were located on the high andes the vast majority of the microearthquakes recorded occurred to the east of the mountain belt on the huaytapallana fault in the eastern cordillera and beneath the western margin of the sub andes thus the sub andes appear to be the physiographic province subject to the most intense seismic deformation focal depths of the crustal events in this region range generally from 15 to 35 km and some events beneath the sub andes appear to be as deep as 40 50 km the fault plane solutions of events in the sub andean margin show thrust faulting on steep planes oriented roughly north south similar to that observed in teleseismic earthquakes studied using body wave modelling the huaytapallana fault in the cordillera oriental also shows relatively high seismicity along a ne sw trend that agrees with the fault scarp and the east dipping nodal plane of two large earthquakes that occurred on this fault on 1969 july 24 and october 1 microearthquakes of intermediate depth recorded during the experiment show a flat seismic zone about 25 km thick at a depth of about 100 km this agrees with recent observations showing that beneath peru the slab first dips at an angle of about 30 to a depth of 100 km and then flattens following a quasi horizontal trajectory fault plane solutions of intermediate depth microearthquakes have horizontal t axes oriented east west suggesting slab pull is the dominant force in the downgoing slab

the crust and uppermost mantle structure of southern peru from ambient noise and earthquake surface wave analysis abstract southern peru is located in the northern central andes which is the highest plateau along an active subduction zone in this region the nazca slab changes from normal to flat subduction with the associated holocene volcanism ceasing above the flat subduction regime we use 6 s to 67 s period surface wave signals from ambient noise cross correlations and earthquake data to image the shear wave velocity v s v structure to a depth of 140 km a mid crust low velocity zone is revealed and is interpreted as partially molten rocks that are part of the andean low velocity zone it is oblique to the present trench and possibly indicates the location of the volcanic arcs formed during the steepening of the oligocene flat slab beneath the altiplano plateau the recently subducted slab beneath the forearc shows a decrease in velocity from the normal to flat subduction regime that might be related to hydration during the formation of the nazca ridge which in turn may contribute to the buoyancy of the flat slab the mantle above the flat slab has a comparatively high velocity which indicates the lack of melting and thus explains the cessation of the volcanism above a velocity contrast from crust to uppermost mantle is imaged across the cusco vilcanota fault system and is interpreted as the boundary between two lithospheric blocks

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

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

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

crustal structure of the peruvian continental margin from wide angle seismic studies keine zusammenfassung in englischer sprache vorhanden no summary in english language