late eocene and maastrichtian paleomagnetic poles for the pacific plate implications for the validity of seamount paleomagnetic data abstract mean paleomagnetic poles for the pacific plate have been calculated for the late eocene 39 ma and the maastrichtian 69 ma the former is located at 77 6 n 7 6 e the latter at 69 9 n 0 9 e although these pole positions are little changed from previous calculations they are better constrained with additional data slightly less than half of the data are derived from the inversion of seamount magnetic fields providing an excellent opportunity to compare such data with other paleomagnetic data of the same age as no significant systematic difference between the two types of data is evident it is inferred that most seamount paleomagnetic data are probably useful indicators of the paleomagnetic field direction

contribution of induced magnetization to magnetization of seamounts a fundamental assumption in modelling seamount magnetic anomalies is that the contribution of induced magnetization is negligible the general consistency of seamount and non seamount palaeopoles scarcity of poles skewed toward the present field direction and the high ratio of remanent to induced magnetization koenigsberger ratio of many oceanic basalts have been cited as evidence supporting this assumption1 2 recent discussions concerning the dominance of normally magnetized seamounts have focused attention on the possible role of viscous and induced magnetization in seamount anomalies3 6 here we determine natural remanent magnetization initial volume susceptibility and the koenigsberger ratio for more than 2 000 samples from a subaerially exposed seamount section on la palma canary islands table 1 by contrast to results from the oceanic crust and ophiolites these data indicate that a variety of rock types are potential magnetic sources the significant induced component of intrusives underscores the importance of the lithological distribution in determining the character of seamount magnetic anomalies the la palma data together with a plausible lithological distribution indicate that induced magnetization may account for one sixth of seamount magnetization

correlating magnetic field polarity changes with geologic phenomena two reported correlations are discussed in this paper the first claims a correlation between magnetic polarity and oxidation state of igneous rocks and the second claims that the hawaiian hot spot chain exhibits a significantly dominant normal polarity bias on the basis of mechanistic arguments and examination of the sample distribution the first correlation is shown to be due to a data artifact although the second correlation is supported by magnetic modeling of seamounts given in this paper it too may be due to a data artifact the most likely alternative explanation is that it originates from a bias imparted by viscous remanent magnetization vrm in the brunhes chron the role of vrm may be far more important in determining large scale magnetic anomalies than is generally believed

constraints on age and construction process of the foundation chain submarine volcanoes from magnetic modeling abstract the interaction between the foundation hotspot and the pacific antarctic ridge south pacific is one of the two known cases where a ridge is approaching a hotspot this ridge hotspot relative movement results in a change in the morphology of the volcanoes along the chain as the age of the lithosphere at the time of edifice formation progressively diminishes four hundred kilometers west of the pacific antarctic ridge axis volcanism is distributed along two sub parallel lines of volcanoes separated by distances diminishing from 100 km at the west to 50 km near the axis of the ridge the magnetic anomalies mapped on this part of the chain show a pattern clearly dominated by the magnetic signature of the seamounts the anomalies were forward modeled using a remanent magnetization of normal or reversed polarity within the volcano topography the resulting pattern of normal and reversed magnetization is consistent with the intervals of the geomagnetic polarity time scale for the last 5 ma the age of the oldest studied volcano these magnetic anomalies therefore represent an independent means of dating the volcanic edifices the ages deduced from the modeling are consistent with the hypothesis of volcanoes built by a hotspot on the fast moving pacific plate and are in good agreement with the published radiometric dates although these dates tend to correspond to the late constructional stages of the edifices the differences between magnetic and radiometric ages suggest that the average time span to build a volcano of the foundation chain is about 1 million years in agreement with other intraplate edifices the difference between the magnetic ages of the seamounts and of the underlying crust both deduced from the magnetic anomaly analysis show that the ridge has approached the hotspot at a rate of 40 km ma

age dependent variation in the magnetization of seamounts the magnetization of pacific seamounts shows a surprisingly large variation when viewed as a function of time similar to normal upper oceanic crust the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation this high magnetization decays rapidly with time resulting in a factor of 5 reduction in intensity in less than 500 000 years because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years these capping rocks can be locally abundant i e hawaii are exposed to a different post formation environment and can have a magnetic mineralogy that is different from submarine volcanoes in spite of these differences the magnetization of subaerially erupted rocks of the hawaii emperor chain also may show a similar rapid reduction in intensity with age finally seamounts of cretaceous age show a factor of 2 higher average magnetization when compared to tertiary seamounts similar to the previously observed increased magnetization of cretaceous seafloor basalts recovered from deep sea drilling project ocean drilling project cores this increase in cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some but not all of the ocean basins prior and subsequent to the cretaceous normal superchron the processes potentially responsible for the elevated cretaceous magnetization values include a high geomagnetic field intensity large scale variations in whole rock major element geochemistry of the extrusive rocks or a systematic acquisition of a secondary magnetization during the cretaceous normal superchron

nonuniform magnetization of jasper seamount paleopoles derived from seamounts have been used to reconstruct the tectonic history of ocean basins however the interpretation of seamount magnetization models and the validity of seamount paleopoles may be affected by inhomogeneous magnetization multibeam bathymetric data sea surface and deep tow magnetic field data and paleomagnetic analyses of dredged samples were used to examine the origin of nonuniform magnetization within jasper seamount 30 27 n 122 44 w models indicate that the seamount is predominantly reversely magnetized with local zones of normal polarity as corroborated by deep tow measurements lithologies likely to be volumetrically important in a seamount edifice show highly variable magnetic properties basalts have high intensities 0 5 27 0 a m high koenigsberger ratios q and low viscous remanence vrm acquisition low q ratios and high vrm acquisition coefficients of coarse grained material and volcaniclastics suggest that they may have substantial viscous and induced components models for jasper are characterized by low uniform intensities and far sided paleopoles the shallow model inclinations may be attributed to nondipolar components in the time averaged geomagnetic field the low intensities of the uniform models and the large nonuniform component in the seminorm solutions imply a complex distribution of magnetization sources within jasper this nonuniformity may result from either lithological variability or construction of the seamount spanning two or more polarity intervals

evidence for deuteric magnetization in hydrothermally altered mesozoic basaltic rocks from east antarctica abstract mesozoic basaltic lavas and dykes from vestfjella east antarctica are dominated by well grouped single component palaeomagnetic directions of normal polarity reversed magnetizations heavily overprinted by normal polarity components were encountered in a few lava flows and dykes only thermal demagnetization was successful in separating tight distributions of reversed polarity directions interpreted to represent a deuteric magnetization the region has been exposed to regional hydrothermal alteration to the epidote prehnite metamorphic facies t 300 c titanomagnetite grains in high temperature oxidation classes ii iii show alteration features typical for hydrothermally altered basalts partial to complete replacement of both ilmenite and magnetite to sphene is common in the most altered rocks which also exhibit varying degrees of decomposition of the opaque minerals the lavas display two major but distinctly different thermomagnetic curves the kink type previously reported from rocks exposed to the epidote metamorphic facies and curves dominated by a paramagnetic contribution defining a magnetite curie point it is concluded that the basalts have retained palaeomagnetic directions acquired during the deuteric cooling phase implying that granulation sphene formation and fe depletion of decomposed titanomagnetite grains in classes ii iii do not necessarily cause complete remagnetization or the acquisition of significant secondary remanent magnetizations

large scale motion between pacific and atlantic hotspots studies of true polar wander tpw the rotation of the solid earth with respect to the spin axis1 have suggested that there has been 10 15 of relative motion over the past 130 myr refs 2 4 in such studies the orientation of the spin axis is recovered from continental palaeomagnetic poles corrected for relative plate motions and compared with a deep mantle reference frame defined by hotspot locations but deducing relative plate motions becomes increasingly difficult for older mesozoic time periods hindering tests of tpw on timescales comparable to those of large scale mantle convection moreover the assumption of hotspot fixity is controversial5 6 we examine here a more direct approach7 8 using palaeolatitudes derived from pacific guyots contrary to predictions from tpw models these data suggest only minor latitudinal shifts of pacific hotspots during the cretaceous period instead of tpw relative motion between the atlantic and pacific hotspot groups9 is required at a velocity of approximately 30 mm yr 1 more than 50 larger than previously proposed5

the grand brule exploration drilling new data on the deep framework of the piton de la fournaise volcano part 1 lithostratigraphic units and volcanostructural implications abstract the sinking of a deep drill hole 3003 5 m for geothermal exploration in the eastern part of the piton de la fournaise volcano on reunion island has revealed the underlying lithostratigraphy six principal sequences have been distinguished a 0 440 m subaerial basaltic flows b 440 556 m a transitional series characterized by an alternation of subaerial basalts and submarine deposits vitric basalt and hyaloclastic breccia c 556 616 m submarine basalts lavas and hyaloclastites d 616 890 m subaerial basalts e 890 1010 m submarine basaltic deposits with abundant argillized hyaloclastites f 1010 3003 5 m a layered intrusive complex comprising 1010 1129 m sheets of gabbro injected into the lavas and hyaloclastites 1129 1920 m a zoned intrusion of olivine gabbro and dolerite 1920 3003 5 m stratified cumulate gabbro wehrlite dunite the occurrence of the first submarine deposits at 440 m 268 m below present sea level and the intercalation with them of subaerial basalts down to 890 m 728 m below present sea level tend to support a hypothesis of sliding into the sea during subsidence of the eastern flank of the fournaise massif the intrusive complex intersected from 1010 m down is regarded as a former shallow magma chamber a large positive gravimetric anomaly 40 50 mgal centered over grand brule and attributed to the intrusive complex indicates that its roof forms an ellipse 3 20 km elongated n s stratigraphic and morphological structural evidence on land and on the sea floor and the results of the mineralogical study of this very large magma chamber suggest that it belongs to an ancient edifice proto fournaise whose activity preceded the construction of the present fournaise and ended at about 0 6 ma on land the evidence for this earlier edifice is to be found in the zeolitised basalts and differentiated lavas hawaiites forming the basement to the phase 1 lava flows certain subsidence structures on the present fournaise are interpreted as resulting from settling of the proto fournaise volcano the destabilization of the east flank grand brule area of the currently active volcano and the location of the ne and se rift zones could be attributed to the same phenomenon

seamount magnetic polarity and cretaceous volcanism of the pacific basin in this paper we examine the magnetic polarity for pacific basin seamounts exclusive of the hawaiian ridge on pacific crust older than 65 ma seamounts of normal polarity predominate on crust younger than 65 ma seamounts exhibit normal and reversed polarity as well as mixed polarity we show that viscous components do not dominate seamount magnetization and we suggest that a large number of normal seamounts were formed during a major magmatic event postulated for the period 110 70 ma this period overlaps with the long cretaceous normal period 118 84 ma and corresponds to 85 normal polarity for earth9s magnetic field the correspondence of a period of high magmatic activity with a long period of normal polarity supports the possible correlation between those processes responsible for magma production and for magnetic field generation

introduction to special section magmatism and extension the relationship between magmatism and the formation of continental rift zones is the subject of much controversy in particular the cause and effect relationships between magmatism and extension and the mode of generation of magma during the process of extension are still hotly debated this controversy served as the theme of a symposium on cenozoic magmatism in the colorado river extensional corridor and adjacent areas and a field trip held as part of the geological society of america cordilleran rocky mountain section meeting in reno nevada metcalf et al 1993 it was clear from data presented at the symposium that a considerable amount of new information has become available regarding magmatism and extension since the last special section on this topic published by the journal of geophysical research june 1989

large rotation of the easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor one of the goals of the pito93 cruise along the perimeter of the easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study using a new orienting device the geocompass 14 oriented blocks were obtained during 6 dives conducted using the ifremer dsrv nautile paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and af demagnetizations and reveals a mean characteristic remanent magnetization chrm direction within each block with both normal and reverse polarities after elimination of data from 5 blocks situated outside the microplate and one block which bears an anomalous direction we have computed a mean chrm direction at dm 49 0 im 39 0 k 10 2 95 19 8 n 7 although the mean inclination is in agreement with the inclination of the recent dipole magnetic field d 0 i 43 the average declination is significantly different from this direction which we interpret as due to a bulk clockwise rotation of 48 5 11 after the bivariate average of the easter microplate over the past 2 3 ma this value is in excellent agreement with previous estimates based on remote geophysical measurements and models

the pliocene seamount series of la palma canary islands a pliocene submarine series of alkali basaltic pillow lavas hyaloclastites and breccias a a sheeted dike swarm b and a basal suite of gabbro and ultramafic rocks c from la palma canary islands is interpreted as a cross section through an uplifted seamount this series has been tilted to its present orientation of 50 230 plunge and azimuth probably by upwarping due to intrusions in the central portion of the island the basal plutonic complex c also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series the plutonic suite c is overlain abruptly by more than 1800 m of sills b 0 4 1 m thick on average with minor screens of lavas and breccias extrusives a form a 1750 m thick sequence of pillow lavas breccias and hyaloclastites the clastic rocks increase in abundance upward and are of four main types 1 breccias consisting of partly broken pillows formed nearly in situ 2 heterolithologic pillow fragment breccias 3 hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows 2 and 3 being interpreted to have been resedimented and 4 pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal highly vesicular pillows and lava stringers and local bombs probably formed by cracking and bleeding of gas rich expanding pillow lava and some shallow submarine subaerial lava fountaining the extrusive series is chemically and mineralogically crudely zoned with the most differentiated rocks metatrachytes and mugearites at the base and most picritic lavas occurring near the top of the series subsequent to emplacement the entire extrusive and intrusive series has been hydrothermally altered the lower part to greenschist and the upper part to smectite zeolite facies mineral assemblages the la palma succession combined with evidence from surface studies of seamounts suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions the nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth the critical depth depending on magma and thus volatile composition changes in slopes of a seamount influence depositional processes based on these factors at least three major depositional sites develop as a seamount grows summit flank and apron facies nonexplosive extrusive processes prevail in the deep water stage dominantly producing pillow lavas 75 these consist of individual pillow volcanoes up to 200 m high with large pillows near the base and decreasing pillow size toward the top of a volcano pillow breccias and pillow fragment breccias comprise approximately 20 of this facies the deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material the shallow water shoaling stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles about 800 m for the alkali basaltic seamount of la palma resulting in formation of mainly clastic rocks 70 in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining resedimented heterolithologic pillow fragment breccias lapilli breccias and hyaloclastites are deposited on the flanks and aprons of the seamount pillow lavas comprise 30 of these deposits

the bahia seamounts test of a hotspot model and a preliminary south american late cretaceous to tertiary apparent polar wander path abstract detailed bathymetric and magnetic data collected over the bahia seamounts in the western south atlantic brazil basin are analyzed six paleomagnetic poles are determined one from three seamounts sharing a ridge and five from isolated seamounts two poles fall near a continental late cretaceous south american pole and are assumed to be of late cretaceous age another seamount has a preliminary 40 ar 39 ar age of 62 4 ma we check this age and estimate the ages of other seamounts by rotating their poles into north american coordinates and comparing them with north american paleomagnetic reference poles these ages and the characteristic magnetic polarities of all bahia seamounts are used to test hotspot models of formation of the seamount group the northern and central bahia seamount chains could have formed from a single hotspot the southern bahia chain may require a second hotspot the two postulated hotspots were probably concurrent and separated by 150 to 200 km the paleomagnetic poles trace out a smooth curve from the late cretaceous poles to a pole that is indistinguishable from the spin axis which we estimate as having an early eocene age the pole ages remain preliminary until better age dates are found and rocks are recovered from more seamounts

tectonic evolution of the pacific phoenix farallon triple junction in the south pacific ocean analysis of multibeam and gravity data reveals the tectonic history of the mid cretaceous 119 107 ma penrhyn basin in the equatorial south pacific ocean the basin formed during a period of heightened geodynamic activity and cessation of magnetic reversals similarities in the geometry of the tongareva triple junction and the rodriguez triple junction in the indian ocean make this study an interesting comparison between modern and ancient tectonics changes in abyssal hill trends during the formation of the basin suggest either a change in the location of the euler pole describing the relative motion between the pacific and farallon plates or a significant period of oblique spreading interaction between the local stress field associated with the break up of the manihiki plateau and the regional stress field controlling major plate motions complicated the tectonic evolution of the penrhyn basin construction of velocity triangles from abyssal hill trends and measurements of the triple junction trace suggests that the triple junction oscillated between ridge ridge ridge and ridge ridge fault configurations at least two reorganizations in the geometry of the triple junction occurred within 10 ma of the initial rifting of the manihiki plateau both changes in triple junction geometry coincide with discontinuities in the triple junction trace and result in right lateral displacements of the triple junction trace changes in the bathymetric expression of the triple junction trace suggest a period of triple junction propagation controlled by rift propagation shortly after the change in euler pole location

magnetization of the la palma seamount series implications for seamount paleopoles paleopoles determined from seamount magnetic anomalies constitute the major data source for the pacific apparent polar wander path but relatively little is known about the processes of remanence acquisition in seamounts since magnetic anomalies reflect both natural remanence nrm and the induced field it is important first to assess whether the nrm is likely to represent an original field direction and second to constrain the magnitude of the induced component to this end we present paleomagnetic data from an uplifted subaerially exposed section through a seamount on la palma canary islands the pliocene seamount series of la palma comprises a 6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts the structural tilt of the seamount series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting e g viscous remanence the nrm provides a poor indication of the original magnetization direction although the characteristic magnetization of many la palma samples is compatible with the original pretilt direction hydrothermal alteration has resulted in the production of ti poor magnetite and an increasing contribution of hematite with increasing degree of alteration more importantly well defined magnetization directions which deviate from any reasonable geomagnetic direction at la palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization based on a comparison of the magnitude of low stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization we conclude that viscous and induced magnetization probably account for 15 25 of the total magnetization of seamounts the resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4o to 16 o for cretaceous seamounts in the pacific

jasper seamount structure seafloor seismic refraction tomography the velocity structure of jasper seamount was modeled using one and three dimensional inversions of p wave travel times the results represent the first detailed seismic images of a submerged intraplate volcano two seismic refraction experiments were completed on jasper seamount incorporating ocean bottom seismometers and navigated seafloor shots the p wave travel times were first used to compute a one dimensional velocity profile which served as a starting model for a three dimensional tomographic inversion the seamount p velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths this suggests that jasper seamount is constructed predominantly of extrusive lavas with high average porosity the velocity models confirm morphological predictions jasper seamount is a shield volcano with rift zone development high seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks comparisons between p velocity models of jasper seamount and the island of hawaii reveal that these two shield volcanoes are not structurally proportional jasper seamount is far smaller than hawaii yet both volcanoes exhibit an outer extrusive layer of similar thickness this suggests that seamount size influences the intrusive extrusive proportions density equilibrium between melt and country rock may explain this behavior

new evidence for the hawaiian hotspot plume motion since the eocene abstract a thick mound of fossiliferous sediments reflecting high biogenic productivity at the equator can be used to determine latitudinal motion of the pacific lithospheric plate plate motion estimates based on the latitudinal movement of equatorial facies are independent of paleomagnetic data and hotspot tracks and thus permit further testing of kinematic models we have determined the northward motion of the pacific plate for the last 53 myr based on the position of the paleoequator as shown by equatorial sediment facies between 26 and 69 dsdp and odp sites sample the past 53 myr in the tropical pacific based on the mapped patterns of accumulation rates in these sites we were able not only to determine the position of the paleoequator but also to estimate the equatorial great circle and hence the relative position of the spin axis since the early eocene the northward motion of the pacific plate inferred from the change in latitude of dated hawaiian chain seamounts relative to the hawaiian hotspot is consistently higher than that deduced from the analyses of equatorial sediment facies such a difference results from a latitudinal shift of the hawaiian hotspot during the last 53 myr all together our observations and recent paleomagnetic results from the detroit nintoku and koko seamounts j a tarduno r a duncan d w scholl r d cottrell b steinberger t thordarson b c kerr c r neal f a frey m torii m c carvallo the emperor seamounts southward motion of the hawaiian hotspot plume in earth s mantle science 301 2003 1064 1069 1 are consistent with a progressive southward motion of the hawaiian mantle plume since the late cretaceous our results suggest that the hawaiian hotspot moved at 32 mm yr to the se during the past 43 million years and that the pacific plate moved 12 northward since 53 ma at an average rate of 25 mm yr

improved late cretaceous and early cenozoic paleomagnetic apparent polar wander path for the pacific plate understanding of pacific plate tectonics and geodynamics is aided by refinement of the plate s apparent polar wander path apwp we improved the late cretaceous and early cenozoic apwp by analyzing a large diverse paleomagnetic data set that combines core sample seamount magnetic anomaly model and marine magnetic anomaly skewness data our preferred apwp has five mean paleomagnetic poles representing the oligocene 30 ma late 39 ma and early 49 ma eocene and paleocene 61 ma epochs and the maastrichtian 68 ma stage along with a published 80 ma pole the apwp shows a stillstand from 80 to 49 ma punctuating the large overall northward drift of the plate the two youngest poles imply resumption of northward motion during mid eocene time with another change of polar motion after 30 ma if unaffected by other phenomena e g true polar wander or change in time averaged magnetic field geometry the stillstand implies negligible northward plate motion during the period of emperor seamounts formation contrary to most accepted plate motion models the stillstand is consistent with paleomagnetic data from the emperor seamounts which imply southward motion of the hawaiian melting anomaly it also implies significant westward drift of the hotspot if the pacific plate was moving west at rates similar to the later cenozoic in addition changes in polar wander after 49 ma are consistent with changes of north pacific plate boundaries 2007 elsevier b v all rights reserved

magnetic structure of the upper kilometer of the marine crust at deep sea drilling project hole 504b eastern pacific ocean deep sea drilling project hole 504b is located in 5 9 m y crust in the eastern pacific ocean about 200 km south of the costa rica rift at 1076 m subbasement it is the deepest penetration of marine crust yet achieved we present here magnetic data from this hole especially from the recently cored leg 83 section which constitutes the lowermost 500 m these data when combined with those of other studies yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available the basement rocks from hole 504b can be divided into three major magnetic units the upper units the top 500 m or so of basement are essentially similar to other shallow marine basement sections consisting of a mixture of various types of extrusive basalt low temperature altered titanomagnetite is the dominant magnetic carrier and the magnetic properties of these rocks are comparable to other shallow marine basalts below the upper units is a 200 m transition zone consisting of a mixture of extrusives and dikes high temperature hydrothermal alteration much of it greenschist facies grade has caused oxidation exsolution of primary titanomagnetite as well as its partial or total replacement by silicates the dominant magnetic carrier is an fe rich titanomagnetite magnetically indistinguishable from pure magnetite the combination of oxidation exsolution and silicate replacement results in a very low natural remanent magnetization nrm the remaining 300 m are in the upper portion of the sheeted dike complex primary titanomagnetite is exsolved but alteration is less intense than in the transition zone nrm values are substantially higher than those of the transition zone due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite which occurs as a silicate alteration product the nrm magnitude is sufficiently high that the sheeted dike complex in this location at least may make a significant if not substantial contribution to the magnetic anomaly overall the magnetic properties of this hole especially the lower 500 m are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling as hydrothermal temperatures do not appear to have exceeded 400 c the nrm in the lower two sections is apparently a chemical rather than a thermal remanence as such models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust a better understanding of the nature of postemplacement alteration in the marine crust is required especially its characteristics at high temperatures and its effects on magnetic properties

complete gravity study of piton de la fournaise volcano reunion island abstract we present an interpretation of all available gravity data on piton de la fournaise volcano including a new set collected in the summit area using a linear programming inversion method we determine the extension of the grand brule ultramafic body drilled in 1985 and we propose to interpret it as a frozen magma chamber related to a disappeared volcano we also present evidence for superficial mass excess under the enclos fouque and plaine des sables calderas that we link to the present piton de la fournaise volcanism the detailed gravity survey of the summit area allows us to emphasize the great importance of dyke complexes associated with the ne and se rift zones and with the shallow magma chamber a negative anomaly in the summit area is interpreted as a relative minimum between the positive anomaly of grand brule and those of the western part of the enclos fouque and plaine des sables calderas

hawaiian hotspot volcanism mainly during geomagnetic normal intervals according to their airborne detected magnetic anomalies volcanoes forming the islands banks and seamounts along the hawaiian emperor seamount chain erupted predominantly during intervals of normal polarity of earth9s magnetic field for the past 10 m y of hotspot activity when the field was 50 normal and 50 reversed 19 of 23 volcanic edifices from hawaii to necker island were dominantly normal one probably was normal and three were dominantly reversed statistical probabilities consideration of induction in the present normal field and possibilities of field reversals during rapid edifice building nevertheless show that eruptions were not random with respect to the magnetic field similar conclusions where data are fewer are good for the older hawaiian ridge 29 of 36 normal and are fair for the emperor seamounts 3 of 3 known normal the correspondence suggests that hotspot plumes are generated near the core mantle boundary spreading center and magmatic arc volcanism on the other hand are mainly upper mantle processes the jurassic and cretaceous long normal invervals however were times of such high heat flux that sea floor spreading as well as hotspot activity increased larger hotspots under continental lithosphere during those normal intervals may have initiated continental rifting

cretaceous paleomagnetic apparent polar wander path for the pacific plate calculated from deep sea drilling project and ocean drilling program basalt cores abstract the apparent polar wander path apwp of the pacific plate still has many uncertainties owing to the fact that paleomagnetic data are difficult to obtain for oceanic plates after more than three decades of coring by the deep sea drilling project dsdp and ocean drilling program odp there are now a large number of reliably dated basalt cores recovered from the pacific plate and this provides an opportunity to determine paleomagnetic poles based on igneous rock samples considered by many scientists to be the most reliable data type cretaceous pacific plate basalt core data were compiled corrected using a standard technique divided into groups based on age and combined to calculate five mean paleomagnetic poles with ages of 80 92 112 121 and 123 ma the latter two being for two different coeval regions in all pole analyses the lack of azimuthal orientation for cored samples leads to large uncertainties in pole locations along a nearly east west direction this difficulty was mitigated by using declination data from magnetic anomaly inversions of dated pacific seamounts for azimuth constraint the two nearly same age poles were calculated because paleocolatitudes from ontong java plateau ojp are discordant compared to those from other pacific locations i interpret the discordant ojp results to indicate that the plateau is on crust that had an early history as an independent plate the other poles 80 92 112 and 123 ma fall on a northeast trending line that suggests slow apparent polar wander during the early and mid cretaceous followed by rapid polar wander between 92 and 80 ma comparison of the 123 ma pole with previously published paleomagnetic data of jurassic age implies southward apparent polar wander followed by a turnaround because the 123 ma pole is the farthest from the geographic pole it implies the turnaround happened near that time and that the pacific plate has moved 40 northward since then the 80 ma pole stands 17 from the geographic pole indicating that 60 of the northward drift occurred prior to that time and 40 afterwards