historical development of the gravity method in exploration the gravity method was the first geophysical technique to be used in oil and gas exploration despite being eclipsed by seismology it has continued to be an important and sometimes crucial constraint in a number of exploration areas in oil exploration the gravity method is particularly applicable in salt provinces overthrust and foothills belts underexplored basins and targets of interest that underlie high velocity zones the gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies there is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets gravimeters have undergone continuous improvement during the past 25 years particularly in their ability to function in a dynamic environment this and the advent of

new standards for reducing gravity data the north american gravity database the north american gravity database as well as data bases from canada mexico and the united states are being revised to improve their coverage versatility and accuracy an important part of this effort is revising pro cedures for calculating gravity anomalies taking into ac count our enhanced computational power improved ter rain databases and datums and increased interest in more accurately defining long wavelength anomaly components users of the databases may note minor differences be tween previous and revised database values as a result of these procedures generally the differences do not impact the interpretation of local anomalies but do improve re gional anomaly studies the most striking revision is the use of the internationally accepted terrestrial ellipsoid for the height datum of gravity stations rather than the conven tionally used geoid or sea level principal facts of gravity observations and anomalies based on both revised and pre vious procedures together with germane metadata will be available on an interactive web based data system as well as from national agencies and data centers the use of the revised procedures is encouraged for gravity data reduc tion because of the widespread use of the global position ing system in gravity fieldwork and the need for increased accuracy and precision of anomalies and consistency with north american and national databases anomalies based on the revised standards should be preceded by the adjec tive ellipsoidal to differentiate anomalies calculated us ing heights with respect to the ellipsoid from those based on conventional elevations referenced to the geoid

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

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

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

ellipsoid geoid gravity geodesy and geophysics geophysics uses gravity to learn about the density variations of the earth s interior whereas classical geodesy uses gravity to define the geoid this difference in purpose has led to some confusion among geophysicists and this tutorial attempts to clarify two points of the confusion first it is well known now that gravity anomalies after the free air correction are still located at their original positions however the free air reduction was thought historically to relocate gravity from its observation position to the geoid mean sea level such an understanding is a geodetic fiction invalid and unacceptable in geophysics second in gravity corrections and gravity anomalies the elevation has been used routinely the main reason is that before the emergence and widespread use of the global positioning system gps height above the geoid was the only height measurement we could make accurately i e by leveling the gps delivers a measurement of height above the ellipsoid in principle in

along strike magma mixing beneath mid ocean ridges effects on isotopic ratios abstract the effects of mixing processes on the isotopic variability of mid ocean ridge basalts are studied the processes considered are porous flow dispersion and convective mixing in magma chambers porous flow dispersion is capable of mixing magmas over distances of only a few tens of meters convective mixing on the other hand is found to produce mixing over scales of kilometers to hundreds of kilometers calculations of convective mixing are carried out for continuous magma chambers where mixing is limited by convective processes and for discontinuous chambers where mixing is limited by chamber size preliminary comparison of our calculations with observations along the mid ocean ridges shows that the calculations are consistent with the existence of a correlation between bathymetry and isotopic ratio at long but not at short wavelengths they are also capable of explaining a decrease in isotopic variability with increasing spreading rate

apparent density mapping using entropic regularization we present a new apparent density mapping method on the horizontal plane that combines the minimization of the first orderentropywiththemaximizationofthezeroth order entropyoftheestimateddensitycontrasts theinterpretation modelconsistsofagridofvertical juxtaposedprismsinboth horizontal directions we assume that the top and the bottom of the gravity sources areflat and horizontal and estimate the prisms densitycontrasts theminimizationofthefirst order entropy favors solutions presenting sharp borders and the maximization of the zeroth order entropy prevents the tendency of the source estimate to become a single prism thus ajudiciouscombinationofbothconstraintsmayleadtosolutions characterized by regions with virtually constant estimated density contrasts separated by sharp discontinuities we apply our method to synthetic data from simulated intrusive bodies in sediments that presentflat and horizontal tops by comparing our results with those obtained with the smoothness constraint we show that both methods produce good and equivalent locations of the sources central positions however the entropic regularization delineates the boundaries of the bodies with greater resolution even in the case of 100 m wide bodies separated by a distance as small as 50 m both the proposed and the global smoothness constraints are applied to real anomalies from the eastern alps and from the matsitama intrusive complex northeastern botswana in the first case the entropic regularization delineates two sources with a horizontal and nearly flat top being consistent with the known geologic information in the second case both constraints produce virtually the same estimate indicating in agreement with results of synthetic tests thatthetopsofthesourcesareneitherflatnorhorizontal

petrogenesis and assembly of the don manuel igneous complex miocene pliocene porphyry copper belt central chile the 4 0 3 6 ma don manuel igneous complex dmic central chile provides a window into igneous processes involved in magma genesis associated with porphyry style copper mineralization this study uses petrographic petrological geochemical and isotopic data to examine the evolution of magmas from the mid to lower crustal source region to shallow emplacement the data provide evidence for progressive oxidation of magma during differentiation and ascent fractionation of cl from s through degassing and the late stage near solidus removal of cl from the system magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns hydrous derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3 5 and 5 km intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing these crystal cargoes represent samples of crystal mush entrained from different depths as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite testifying to magma mingling during ascent sulfur and chlorine contents of apatite within the different dmic units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine the hypabyssal nature of the dmic affords a detailed integrated record of magmatic differentiation processes occurring within trans crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry style copper mineralization

jackpine creek magnetic anomaly identification of a buried meteorite impact structure the meteorite impact process is quite a remarkable natural phenomenon when it occurs in a sedimentary basin it can create a trap generating structure rim and central uplift and reservoir rocks ejecta piles and breccia infill both at the same time

the empirical age depth relation and depth anomalies in the pacific ocean basin this paper presents a new age depth relation and a comprehensive depth anomaly map for the pacific ocean age depth and sediment thickness were averaged for each 1 1 area bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep hot spots were treated as thermal anomalies and data within 800 km of them were eliminated from further analysis an age depth equation was determined by fitting a least squares line to the remaining data the resulting depth versus age distribution confirms the linear dependence of depth on age up to 80 ma all older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted this flattening of the age depth distribution requires that a separate function be fit to data older than 80 ma as a result data representing the deepest seafloor older than 80 ma were selected for curve fitting and the age depth distribution was approximated by an exponential function it is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age depth data from the pacific ocean a depth anomaly map prepared by comparing observed depth with the depth predicted by the new age depth formula shows several newly recognized features including swells over the louisville ridge in the south pacific and in the gulf of alaska seamounts

gridding aeromagnetic data using longitudinal and transverse horizontal gradients with the minimum curvature operator the measurement of gradients of the earth s magnetic field from an airborne platform has been advocated for many years in 1954 wickerham proposed the use of what is essentially a gradient along the flight direction longitudinal gradient hood s 1965 suggestion was to measure the vertical gradient using a total field magnetometer mounted in a tail stinger and a second magnetometer in a towed bird below the tail stinger in fact the system finally built by hood s group at the geological survey of canada used two tail stingers separated by 2 m the advantages of this type of total field vertical gradiometry were advocated so convincingly that vertical gradiometers was also developed for helicopter surveying one of the key developments that allowed the collection of high quality vertical gradient data from a fixed wing aircraft was the development of aircraft compensation schemes for gradiometer arrays subsequent installations generally measured a vertical gradient and a transverse gradient perpendicular to the flight direction the longitudinal gradient is either calculated by taking the spatial derivative of a single sensor traverse or by averaging two wing tip sensors and subtracting this from a tail sensor marcotte et al 1992 give a number of examples where having both horizontal gradients can help in the interpretation of magnetic data they suggest that both gradients can be used to 1 determine whether the geology is two dimensional 2 estimate the strike direction and 3 help calculate the vertical gradient interestingly marcotte et al demonstrate that the vertical gradient derived from the horizontal gradients gives data with noise levels one tenth of that seen on the measured vertical gradients other advantages of measured horizontal gradients are that they can be used 1 to derive a total field that is free of diurnal effects 2 to derive the magnetic gradient tensor 3 to estimate the depth

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

the apollo 17 gravity measurements on the moon the only gravity measurements made on the surface of a heavenly body were on the moon during the apollo 17 mission in 1973 an obvious objective was to meet the challenge of making gravity measurements on an extra terrestrial body more specific objectives were to make an earth moon gravity tie and to investigate the buried structure of the taurus littrow valley the apollo 17 landing site this valley lies between the north massif and the south massif and the nature of the valley fill was to be determined

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

regularized two dimensional fourier gravity inversion method with application to the silent canyon caldera nevada a modification of the 2 d fourier gravity inversion method includes regularization and a linear density variation with depth explicit downward continuation in the fourier inversion of gravity observations from mass distributions at depth produces instability in the presence of noise and shallow mass distributions a data adaptive regularization filter tapers growth of the exponential continuation function an empirical relationship between the regularization filter parameter and a parametric model of potential field spectra results in automatic selection of the filter parameter for a given continuation depth inversion of synthetic data from a random noise contaminated basin type model produces a depth model that agrees with the synthetic structure with an rms error commensurate with the data noise a model of the silent canyon caldera buried beneath pahute mesa at the nevada test site results in a gravity field that agrees with the observations to within a 4 percent rms error the caldera gravity mode

three dimensional crustal structure in central taiwan from gravity inversion with a parallel genetic algorithm the genetic algorithm method is combined with the finite element method for the first time as an alternative method to invert gravity anomaly data for reconstructing the 3d density structure in the subsurface the method provides a global search in the model space for all acceptable models the computational efficiency is significantly improved by storing the coefficient matrix and using it in all forward calculations then by dividing the region of interest into many subregions and applying parallel processing to the subregions central taiwan a geologically complex region is used as an example to demonstrate the utility of the method a crustal block 120 150 km 2 in area and 34 km in thickness is represented by a finiteelement model of 76 500 cubic elements each 2 2 2 km 3 in size an initial density model is reconstructed from the regional 3d tomographic seismic velocity using an empirical relation between velocity and density the difference between the calculated and the observed gravity anomaly i e the residual anomaly shows an elongated minimum of large magnitude that extends along the axis of the taiwan mountain belt among the interpretive models tested the best model shows a crustal root extending to depths of 50 to 60 km beneath the axis of the western central and eastern central ranges with a density contrast of 400 or 500 kg m 3 across the moho both predictions appear to be supported by independent seismological and laboratory evidence

vertical resolution gravity versus vertical gravity gradient can gravity really detect a 200 m throw fault at a depth of 3000 m such a question may be frequently asked when we interpret gravity data at present with gravity gradiometry available and the exploration activity in areas such as the gulf of mexico another parallel question is raised

resolving the k 2 salt structure in the gulf of mexico an integrated approach using prestack depth imaging and full tensor gravity gradiometry while potential field techniques have long been applied in regional studies and for mineral exploration the recently commercialized full tensor gradiometry ftg system possesses the resolution and sensitivity required in detailed mapping for oil and gas exploration even for deep objectives used in combination with prestack depth imaging this provides a potent mapping capability especially in areas of structural complexity we illustrate this with a subsalt case study from the gulf of mexico where an integrated approach using wave equation depth imaging and ftg in version succeeds in resolving the base of salt in an area where kirchhoff depth imaging alone fails

regional variations in subsidence rate of oceanic plates a global analysis a global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently each plate has been divided into regions bounded by major tectonic features a total of 32 regions have been considered bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region except for three regions western pacific between 10 s and 40 n north american plate between 24 n and 38 n and african plate between 10 n and 24 n where ocean floor older than 80 100 ma flattens with age in other regions depth increases linearly with age1 2 subsidence rate over each region has been computed by linear regression large variations up to a factor 2 are reported from one region to another asymmetrical subsidence is also observed in a number of areas however the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography shallow ridge subsiding quickly and deep ridge subsiding slowly hence seafloor depth subsides at a rate modulated by the value of the initial axial depth this observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid ocean ridges rather than lateral temperature variations inside the asthenosphere involving small scale convection

trace element and isotopic effects arising from magma migration beneath mid ocean ridges abstract the trace element concentrations and isotopic ratios in the magma erupted on mid ocean ridges may differ from those in the source material due to physical effects such as porous flow dispersion exchange of trace elements between the fluid and solid phases during magma migration and convective mixing in magma chambers these differences are in addition to those produced by better known processes such as fractional cyrstallization and partial melting the effects of the three former processes are described it is predicted that magma typically reaches the sub ridge magma chambers with a spatial heterogeneity only slightly reduced from that of the source material but with a subdued variation in time convective mixing then further reduces the spatial heterogeneity application of the results for convective mixing to a recent fourier analysis of 87 sr 86 sr variations along the mid atlantic ridge suggests that the falloff in amplitude of variation observed with decreasing wavelength in the mid atlantic ridge data cannot be explained by convective mixing in magma chambers instead it is postulated that this falloff is due to the mechanics of the production and or the solid state convective mixing of chemical and isotopic heterogeneities in the solid mantle

an exact solution for the gravity curvature bullard b correction the complete bouguer reduction includes in addition to the simple bouguer slab correction bullard a both curvature bullard b and terrain bullard c corrections a new closed form formula for the curvature correction is derived for which the calculated values differ from those published by swick by more than 0 5 mgal for high elevations these corrections reduce those of an infinite slab bullard a to that of a spherical cap having a surface radius of 166 7 km the spherical cap produces a lesser effect than the infinite slab because of the truncation of that part of the slab above the earth and extending to infinity but it produces a greater effect than the slab because of subslab earth resulting from curvature the physical significance of the correction lies in the combination of these two differences which are each a function of elevation the bullard b surface radius 166 7 km outer radius of the hayford bowie system is reaffirmed by the exact formula to be appropriate for exploration su

triassic mid ocean ridge basalts from the argolis peninsula greece new constraints for the early oceanization phases of the neo tethyan pindos basin

correction of topographic distortions in potential field data a fast and accurate approach the equivalent source concept is used in the wavenumber domain to correct distortions in potential field data caused by topographic relief the equivalent source distribution on a horizontal surface is determined iteratively through forward calculation of the anomaly on the topographic surface convergence of the solution is stable and rapid the accuracy of the fourier based approach is demonstrated by two synthetic examples for the gravity example the rms error between the corrected anomaly and the desired anomaly is 0 01 mgal which is less than 0 5 percent of the maximum synthetic anomaly for the magnetic example the rms error is 0 7 nt which is less than 1 percent of the maximum synthetic anomaly the efficiency of the approach is shown by application to the gravity and aeromagnetic grids for kansas for gravity data with a maximum elevation change of 500 m reducing to a horizontal datum results in a maximum correction in gravity anomaly amplitude of up to 2 6 mgal for aeromagnetic data the m

interaction of mantle plumes and migrating mid ocean ridges implications for the galapagos plume ridge system we investigate the three dimensional interaction of mantle plumes and migrating mid ocean ridges with variable viscosity numerical models numerical models predict that along axis plume width w and maximum distance of plume ridge interaction xmax scale with q u where q is plume source volume flux and u is ridge full spreading rate both w and xmax increase with buoyancy number b which reflects the strength of gravitational versus plate driven spreading scaling laws derived for stationary ridges in steady state with near ridge plumes are consistent with those obtained from independent studies of ribe 1996 in the case of a migrating ridge the distance of plume ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate given the mildly buoyant and relatively viscous plumes investigated here the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow from observed plume widths of the galapagos plume migrating ridge system our scaling laws yield estimates of galapagos plume volume flux of 5 16 106km3 m y 1 and a buoyancy flux of 2 103 kg s 1 model results suggest that the observed increase in bathymetric and mantle bouguer gravity anomalies along cocos plate isochrons with increasing isochron age is due to higher crustal production when the galapagos ridge axis was closer to the plume several million years ago the anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly 50 100 c and moderate radius 100 200 km predictions of the along axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but instead may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source

spherical earth gravity and magnetic anomaly analysis by equivalent point source inversion abstract to facilitate geologic interpretation of satellite elevation potential field data analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation a powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion linear transformations of the equivalent source field lead to corresponding geoidal anomalies pseudo anomalies vector anomaly components spatial derivatives continuations and differential magnetic pole reductions a number of examples using 1 averaged surface free air gravity anomalies and pogo satellite magnetometer data for the united states mexico and central america illustrate the capabilities of the method

constraints on the magmatic evolution of the oceanic crust from plagiogranite intrusions in the oman ophiolite we present major and trace element as well as sr nd and hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the oman ophiolite the rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt sio2 i e from gabbros to tonalites the samples are grouped into a ti rich and relatively light rare earth element lree enriched p1 group ce yb n 0 7 resembling the early v1 lavas and a ti poor and lree depleted p2 group ce yb n 0 7 resembling the late stage v2 lavas based on the geochemical differences and in agreement with previous structural and petrographic models we define phase 1 p1 and phase 2 p2 plutonic rocks felsic magmas in both groups formed by extensive fractional crystallization of olivine clinopyroxene plagioclase apatite and ti magnetite from mafic melts the incompatible element compositions of p1 rocks overlap with those from mid ocean ridges but have higher ba nb and th nb trending towards the p2 rock compositions and indicating an influence of a subducting slab the p2 rocks formed from a more depleted mantle source but show a more pronounced slab signature these rocks also occur in the southern blocks with the exception of the tayin block of the oman ophiolite implying that the entire ophiolite formed above a subducting slab initial nd and hf isotope compositions suggest an indian morb type mantle source for the oman ophiolite magmas isotope compositions and high th nb in some p2 rocks indicate mixing of a melt from subducted sediment into this mantle

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

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

high latitude thermospheric vertical wind activity from dynamics explorer 2 wind and temperature spectrometer observations indications of a source region for polar cap gravity waves 1 the large scale distribution of thermospheric vertical wind activity from 250 to 650 km altitude was studied using observations from the wind and temperature spectrometer on the dynamics explorer 2 satellite we calculated the vertical velocity standard deviation vz within a sliding window of width 120 s corresponding to an along track distance of 900 km maps of vz in local magnetic time and invariant latitude reveal a high latitude region of enhanced activity that largely fills the polar cap maximizing in the midnight dawn sector separating the data by altitude suggests that most of the vertical wind energy present at 250 450 km is dissipated within a few hundred vertical kilometers northern and southern hemisphere high latitude vz fields were found to be very similar indicating no significant hemispheric differences no strong correlation was found between solar illumination and vz at high latitudes although the dayside vertical wind activity may be slightly reduced compared to night and twilight intervals however a clear relationship to geomagnetic activity as measured by the ae index was found vertical wind activity increases with increasing ae we interpret these results as a possible signature of polar cap gravity waves with sources in or near the midnight dawn auroral oval we use the probability density functions of vz separated by ae to infer the temporal characteristics of the wave source and consequently to provide preliminary estimates of the production rates of polar cap gravity waves

mantle dynamics and generation of a geochemical mantle boundary along the east pacific rise pacific antarctic ridge a large scale mantle compositional discontinuity was identified along the east pacific rise epr and the pacific antarctic ridge par with an inferred transition located at the epr 23 degrees s 32 degrees s because of the epr easter hotspot interactions in this area the nature of this geochemical discontinuity remains unclear iodp sites u1367 and 1j1368 drilled into the ocean crust that was accreted at similar to 33 5 ma and similar to 13 5 ma respectively between 28 degrees s and 30 degrees s on the epr we use lavas from sites u1367 and u1368 to track this mantle discontinuity away from the epr the mantle sources for basalts at sites u1367 and u1368 represent respectively northern and southern pacific mantle sub domains in terms of sr nd pb hf isotopes the significant isotopic differences between the two iodp sites are consistent with addition of ancient subduction processed ocean crust to the south pacific mantle sub domain our modeling result shows that a trace element pattern similar to that of u1368 e morb can be formed by melting a subduction processed typical n morb the trace element and isotope compositions for site u1368 morbs can be formed by mixing a himu mantle end member with site u1367 morbs comparison of our data with those from the epr par shows a geochemical mantle boundary near the easter microplate that separates the pacific upper mantle into northern and southern sub domains on the basis of reconstruction of initial locations of the ocean crust at the two sites we find that the mantle boundary has moved northward to the easter microplate since before 33 5 ma a model in which along axis asthenospheric flow to where asthenosphere consumption is strongest explains the movement of the apparent mantle boundary crown copyright c 2013 published by elsevier b v all rights reserved

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

a process oriented approach to modeling the gravity signature of continental margins the gravity anomaly at a continental margin can be regarded as the result of all the processes that have shaped it through time these include rifting sedimentation and magmatic underplating this article discusses a new process oriented approach to gravity interpretation which uses seismic constraints on sediment thickness and the underplate at continental margins to estimate the gravity anomaly that is associated with rifting sedimentation and magmatic underplating by comparing the calculations to the observed free air gravity edge effect anomaly we have been able to determine the elastic thickness and crustal structure and to provide an estimate of the location of the ocean continent transition since crustal structure is a proxy for the amount of heating that occurred at the time of rifting the approach has important implications for the deep water hydrocarbons industry

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

gravity anomaly separation by wiener filtering we introduce a gravity anomaly separation method based on frequency domain wiener filtering gravity anomaly separation can be effected by such wavelength filtering when the gravity response from the geologic feature of interest the signal dominates one region or spectral band of the observed gravity field s power spectrum the wiener filter is preferable to a conventional band pass filter because geologic information from the study area can be incorporated to a greater extent in specifying the filter s transfer function our method differs from previous wiener filtering schemes in that it provides through direct modeling of known geology e g outcrop and borehole data a more objective estimate of the signal power spectrum required for defining the transfer function of the filter we illustrate the technique first with synthetic data and then with a field example from the southern paradox basin the paradox basin example reveals the limitation inherent to all wavelength filtering which results f

global derivation of marine gravity anomalies from seasat geosat ers 1 and topex poseidon altimeter data summary marine gravity anomalies over the area 82 s to 82 n and 0 e to 360 e on a 2 x 2 grid have been derived from seasat geosat ers 1 and topex poseidon altimeter data the inverse vening meinesz formula with a 1 d fft method was used to compute gravity anomalies from gridded north south and west east geoid gradients in a remove restore procedure with the egm96 gravity model as the reference field this paper presents a new gridding technique and an altimeter data management system that help to extract data efficiently in the 12 areas where ship measured gravity and satellite derived gravity were compared rms agreements of 5 14 mgal were obtained special discussions of the gravity anomalies over selected inland seas the arctic antarctica and coastal waters are presented the derived gravity anomalies show detailed tectonic structures of the world ocean the global data set will be continuously updated with more altimeter data included in the derivation

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

on bottom gravity profile across the east pacific rise crest at 21 north twenty on bottom gravity stations were taken with a lacoste romberg model g gravity meter inside the submersible alvin over a 6 km long cross strike profile which straddles the east pacific rise spreading center at 21 n the residual bouguer anomaly shows a 1 5 mgal low with a total half width of 1 7 km centered about the spreading axis and hydrothermal vents uncertainties in the ocean depth are believed to contribute a standard error of about 0 3 to 0 4 mgal for the gravity data the observed elevation factor of 0 1552 mgal m indicates that the upper ocean crust bulk density is 2 62 0 09g cm3 on the other hand 90 rock samples give a density of 2 90 0 01g cm3 this suggests that the largescale porosity of the upper crust is 14 percent the gravity anomaly was inverted to find the ideal body with the smallest maximum density contrast which satisfies the data and their associated errors this body extends from the sea floor to 2 500 m depth and has a density contrast of 0 043g cm3 if the density contr

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

the decompensative gravity anomaly and deep structure of the region of the rio grande rift an isostatic correction is commonly made to bouguer anomaly gravity data to remove the gravity effect of isostatic compensation of topographic loads in the ussr a decompensative correction has then been made to the isostatic gravity anomaly to remove the gravity effect of isostatic compensation of geologic loads as well under an hypothesis of local as opposed to regional compensation the gravity effect of a shallow geological body can be separated from that of its inferred deep compensating root by deconvolution by contrast with the procedure originally defined by zorin we employ here calculations in the wave number domain leading to an efficient and exact solution the decompensative transfer function acts to increase amplitude of the isostatic anomaly at all wavelengths but especially at the low end of the isostatic anomaly spectrum in the 200 600 km wavelength region where anomaly amplitudes may increase by as much as a factor of 2 in a 1200 1200 km region centered on the rio grande rift the decompensative correction ranges from about 35 to 25 mgal the rift is not particularly apparent in the decompensative correction nor is the rift any more apparent in the decompensative anomaly than in the isostatic anomaly the decompensative anomaly however highlights an arcuate gravity low and a system of gravity highs inferred to reflect prerift welts of mass concentration which have indirectly influenced the position of the rift and its segmentation and zones of accommodation under the assumptions made if the decompensative anomaly is subtracted from the bouguer anomaly then the residual is the gravity anomaly field of deep structure without gravity effects of shallow sources in the upper crust we suppose this gravity field to be dominated by the gravity effects of the density contrast across the crust mantle boundary moho and the density contrast at the lithosphere asthenosphere boundary using available seismic data to weakly constrain the moho surface we invert the residual gravity field for topography of the base of the lithosphere lithosphere is found to be 200 km thick in the high plains 40 50 km in the eastern great basin 75 100 km in the colorado plateau and as thin as 40 km in the southern rio grande rift in the area studied the thickness of the lithosphere is everywhere greater than that of the crust an asthenosphere bulge closely tracks the axis of the rift and is symmetrical to it by contrast with asymmetry that might be inferred from broad features of the bouguer anomaly the separation of gravity effects made possible by the decompensative correction shows how the rift is fundamentally controlled by thinning of the lithosphere yet in detail is deflected by long lived tectonic welts in the shallow brittle crust

gravity models of the albuquerque basin rio grande rift new mexico as part of a hydrologic study gravity anomalies over the albuquerque basin were modeled to determine the thickness of the low density neogene basin deposits the major aquifer of the basin the models are constrained by substantial density log and well data fifteen two dimensional 2 d models include four major density units the upper unit density 2 20g cm3 corresponds to the neogene syn rifting deposits thickness of this unit averages about 1 5 km and exceeds 2 0 km in a strip along the east side of the basin the thickness is about 1 0 km between the albuquerque and the santo domingo basins at the south end of the albuquerque basin the thickness is less than 0 5 km the configuration of the remaining density units less constrained by well data raises significant questions about the history of the basin that cannot be answered with existing data

horizontal gravity gradient an aid to the definition of crustal structure in north america a map of the magnitude of the horizontal bouguer gravity gradient over the north american continent is used to delineate lateral discontinuities in upper crustal density and or thickness associated with such processes as suturing and rifting the usefulness of gradient trends in mapping major structural boundaries which are sometimes poorly exposed or completely buried is demonstrated by examples such as the buried southward extension of the grenville front and buried boundaries of the superior province gradient trends also draw attention to poorly known structures which may have major tectonic significance and to a continent wide structural fabric which may provide a record of the tectonic growth of the north american continent

gravity study of the pitcairn easter hotline shipboard free air gravity and bathymetric anomalies with an extension of 400 km were identified across the pitcairn easter hotline in the south pacific the anomalies are associated with one of the positive geoid undulations observed in the area from satellite data several smaller topographic features volcano tectonic ridges oriented n65 e are superimposed on the topographic high admittance computations and direct modeling show that the swell topography is compensated by a low density zone within the lithosphere 4 to 8 km below the crust the volcano tectonic ridges are locally compensated in a classical airy sense the swell and the associated ridges were probably created by the action of a thermal anomaly resulting from the interaction of the easter island hotspot and of the easter microplate accretion centers

hf radar signatures of the cusp and low latitude boundary layer continuous ground based observations of ionospheric and magnetospheric regions are critical to the geospace environment modeling gem program it is therefore important to establish clear intercalibrations between different ground based instruments and satellites in order to clearly place the ground based observations in context with the corresponding in situ satellite measurements hf radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis in this paper we report on an intercalibration study made using the polar anglo american conjugate radar experiment radars located at goose bay labrador and halley station antarctica and the defense meteorological satellite program dmsp satellites the dmsp satellite data are used to provide clear identifications of the ionospheric cusp and the low latitude boundary layer llbl the radar data for eight cusp events and eight llbl events have been examined in order to determine a radar signature of these ionospheric regions this intercalibration indicates that the cusp is always characterized by wide complex doppler power spectra whereas the llbl is usually found to have spectra dominated by a single component the distribution of spectral widths in the cusp is of a generally gaussian form with a peak at about 220 m s the distribution of spectral widths in the llbl is more like an exponential distribution with the peak of the distribution occurring at about 50 m s there are a few cases in the llbl where the doppler power spectra are strikingly similar to those observed in the cusp

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

galapagos hot spot spreading center system 2 87sr 86 sr and large ion lithophile element variations 85 w 101 w thirty eight samples of basalts dredged from the galapagos spreading center gsc between 85 w and 101 5 w longitudes have been analyzed for k rb cs ba and sr contents as well as for 87sr 86sr ratios basalts between 85 w and 89 w segment of the gsc range from 0 7025 to 0 7028 in 87sr 86sr and those between 95 5 w and 101 5 w range from 0 7026 to 0 7027 these basalts are characterized by lree depleted patterns basalts from 89 w to 95 5 w segment having slightly lree depleted to lree enriched patterns la sm e f between 0 8 and 2 4 range from 0 7027 to 0 7031 in 87sr 86sr the longitudinal 87sr 86sr variation shows a maximum at about 92 w where the gsc is nearest to darwin island the alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles but these occur at about 91 w the apparent shift of 87sr 86sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting the highest 87sr 86sr from the gsc is only slightly lower than the highest 87sr 86sr for tholeiitic basalts from the galapagos archipelago several possible hypotheses have been examined to account for these geochemical and sr isotopic variations in the light of the presently available data the most promising explanation seems to be a binary mixing of hot spot or plume type material with the source of normal mid ocean ridge basalts and resulting rift propagation both end members seem to be heterogeneous and therefore it is at present difficult to put severe constraints on this binary mixing nevertheless the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91 w and 92 5 w fracture zones toward the rift

a terracing operator for physical property mapping with potential field data the terracing operator works iteratively on gravity or magnetic data using the sense of the measured field s local curvature to produce a field comprised of uniform domains separated by abrupt domain boundaries the result is crudely proportional to a physical property function defined in one profile case or two map case horizontal dimensions this result can be extended to a physical property model if its behavior in the third vertical dimension is defined either arbitrarily or on the basis of the local geologic situation the terracing algorithm is computationally fast and appropriate to use with very large digital data sets where gravity and magnetic data are both available terracing provides an effective means by which the two data sets can be compared directly results of the terracing operation somewhat resemble those of conventional susceptibility or density mapping in contrast with conventional susceptibility mapping however the terraced function is a true step function which cannot be depicted by means of contour lines magnetic or gravity fields calculated from the physical property model do not in general produce an exact fit to the observed data by intent the terraced map is more closely analogous to a geologic map in that domains are separated by hard edged domain boundaries and minor within domain variation is neglected the terracing operator was applied separately to aeromagnetic and gravity data from a 136 km x 123 km area in eastern kansas results provide a reasonably good physical representation of both the gravity and the aeromagnetic data superposition of the results from the two data sets shows many areas of agreement that can be referenced to geologic features within the buried precambrian crystalline basement the emerging picture of basement geology is much better resolved than that obtained either from the scanty available drill data or from interpretation of the geophysical data by inspection

monitoring fluid movement with the borehole gravity meter use of the borehole gravity bhg meter as a detector of density distribution suggests that a reservoir could be monitored for bulk density changes caused by redistribution of fluids after hydrocarbons have been partially extracted from it in a simple situation in which liquid hydrocarbons are being extracted from a well an overlying gas zone might begin to penetrate toward the perforations from above while an underlying water zone formation might begin to penetrate toward the perforations from below one idealized reservoir and one real reservoir are modeled for bhg meter responses in prepump and postpump reservoir conditions results suggest that the bhg meter density differences could be detected if the survey had a high degree of quality control

high latitude joule heating response to imf inputs 1 we evaluate the response of the high latitude joule heating to orientation and magnitude of the interplanetary magnetic field imf approximately 9000 individual joule heating patterns derived from data assimilation for the northern hemisphere were used to develop averaged and hemispherically integrated joule power maps for the northern hemisphere north of 40 magnetic latitude hemispherically integrated joule heating increases with imf magnitude when the imf is southward but is relatively unchanged with increased imf magnitude when the imf is north only a small imf by effect is evident in the joule heating patterns we quantify the imf effects with linear fits of integrated joule heating as a function of imf magnitude for eight different imf clock angle bins in the gsm y z plane during extreme northward imf conditions joule heating is restricted to the high latitude dayside during extreme southward imf conditions joule heating is located predominantly in the auroral region with increased heating in the morning sector compared to the evening sector this additional heating may lead to the increased incidence of gravity waves reported in the morning sector our estimate of the increase of hemispherically integrated joule heating with imf magnitude during southward imf periods is 13gw nt

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

apparent magnetization mapping using entropic regularization a new apparent magnetization mapping method on the horizontal plane combines minimization of first order entropy with maximization of zeroth order entropy of the estimated magnetization the interpretation model is a grid of vertical juxtaposed prisms in both horizontal directions to estimate the magnetization of the prisms assume that the top and bottom of the magnetic sources are horizontal minimization of the first order entropy favors solutions with sharp borders and the maximization of zeroth order entropy prevents the tendency of the estimated source to become a single prism with large magnetization thus a judicious combination of both constraints can lead to solutions characterized by regions with virtually constant magnetizations separated by sharp discontinuities this is applied to synthetic data from simulated intrusive bodies in sediments that have horizontal tops by comparing the results with those obtained with the common tikhonov regularization smoothness constraint method it is shown that both methods produce good and equivalent locations of the central positions of the sources however entropic regularization delineates the boundaries of the bodies with greater detail both the proposed and the smoothness constraints are applied to real anomaly data over a magnetic skarn in butte valley nevada u s a entropic regularization produced an estimated magnetization distribution with sharper boundaries smaller volume and higher apparent magnetization as compared with results produced by incorporating the smoothness constraint

the theory of the bouguer gravity anomaly a tutorial although the corrections which bring about a bouguer gravity anomaly are well established the reasons for doing them are not well understood one cause of this common misunderstanding is that the subject has been poorly presented in many of the basic texts this claim was quantified by t r lafehr a major figure in gravity exploration for 30 years in 1991 when he wrote in geophysics that nine of 15 english language textbooks contain erroneous statements on this subject

high resolution mean sea surface computed with altimeter data of ers 1 geodetic mission and topex poseidon the remote sensing satellite ers 1 launched in 1991 to study the earth s environment was placed on a geodetic 168 day repeat orbit between 1994 april and 1995 march to map through altimetric measurements the gravity field over the whole oceanic domain with a resolution of 8 km at the equator in both along track and cross track directions we have analysed the precise altimeter data of the geodetic mission and by also using one year of topex poseidon altimeter data we have computed a global high resolution mean sea surface the various steps involved in pre processing the ers 1 data consisted of correcting the data for environmental factors editing and reducing through crossover analyses the radial orbit error which directly affects sea surface height measurements for this purpose we adjusted sinusoids at 1 and 2 cycle rev 1 along the ers 1 profiles in order to minimize crossover differences between ers 1 and yearly averaged topex poseidon profiles in effect the orbit of topex poseidon is very accurately determined within 2 3 cm for the radial component so topex poseidon altimeter profiles can serve as a reference to reduce the ers 1 radial orbit error the ers 1 residual orbit error was further reduced through a second crossover analysis between all ascending and descending profiles of the geodetic mission the along track ers 1 and topex poseidon data were then interpolated over the whole oceanic domain on a regular grid of 1 16 1 16 size the mapping of the gridded sea surface heights reveals the very fine structure of the marine geoid up until now unknown at a global scale this new data set will be most useful for marine geophysical and tectonic investigations

apparent density mapping and 3d gravity inversion in the eastern alps1 power spectrum analysis of the bouger gravity values in the eastern alps suggests that the gravity field may be separated into long and short wavelength components the long wavelength component is assumed to be caused by alpine crustal thickening this long wavelength component was subjected to a gravimetric single density interface inversion procedure giving a gravimetric mohorovicic model which is generally in good agreement with moho depths derived by refraction and reflection seismology the residual high frequency gravity component correlates well with the main surface geological units in the eastern alps apparent density mapping by applying an inverse density deconvolution filter to the short wavelength gravity component gives density values for the upper crust which correspond well with averaged density values from rock samples

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

sunshading geophysical data using fractional order horizontal gradients the interpretation of linear features in geophysical imagery is of considerable importance because they can correspond to dykes faults or other geologic discontinuities one popular tool commonly used for their enhancement is sunshading which considers the data values as if they were a topographic surface that is illuminated by a light source or sun which has a specified azimuth and elevation the reflectance of the light by the surface is calculated and imaged features whose strike direction is orthogonal to the sun azimuth reflect the light strongly and are consequently enhanced while those lying parallel to the sun direction do not reflect the light and become less apparent as a result the reflectance algorithm uses the first horizontal gradients of the data calculated in two orthogonal directions horn 1982

lateral flow of hot plume material ponded at sublithospheric depths local buoyancy forces drive the flow of ponded plume material along the base of the lithosphere this processes complicates the geological effects of plumes beyond those of simple passage of plates over point heat sources variations in the depth to the base of the lithosphere significantly affect lateral flow the flow rate is modified to a moderate extent by locally thin lithosphere beneath midplate hotspots such as hawaii the effect is relevant when numerical models are rescaled to better fit the observed uplift and the observed distance from the hotspot to the nose of the swell variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes effectively forming an upside down drainage pattern plume material flows upslope toward ridge axes from off axis plumes and along ridge axes from on axis plumes at sufficiently low spreading rates thinner lithosphere exists above plume material than above normal asthenosphere the edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material pressure release melt is associated with the upslope flow of plume material it is expected to cause volcanism away from the plume and to sometimes supply melt depleted plume material to ridge axes that are upslope from the region of voluminous melting

chemical and isotopic variations along the superfast spreading east pacific rise from 6 to 30 s chemical data of 39 fresh basaltic glasses from the east pacific rise epr between 6 and 30 s and pb sr and nd isotopic compositions of 12 basalt glasses are presented major and trace element data indicate a wide compositional range including primitive basalts mg 0 67 and highly evolved feti basalts mg 0 34 molmg mg fe2 the compositional range can be attributed to low pressure fractional crystallization fractionation corrected major element concentrations provide evidence for varying mantle melting conditions calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar at temperatures between 1430 and 1280 c and total degrees of partial melting between 17 and 20 by weight leached and hand picked basalt glasses display large variations in 87sr 86sr 0 70235 0 70270 143nd 144nd 0 51312 0 51323 and 206pb 204pb 18 064 18 665 but are similar to other n type morb from the epr the isotopic ratios of basalts from 13 to 23 s show strong correlations and delineate two systematic trends from 23 to 17 s 87sr 86sr and pb isotope ratios increase and 143nd 144nd decrease in agreement with previous results mahoney et al 1989 a reverse trend is indicated by basalts from 17 to 13 s however k ti and la sm n continuously increase from 23 to 13 s this opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17 s north of 13 5 s garrett fracture zone isotopic data show no systematic variation with ridge location and display an overall weaker covariation the results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions p t f f degrees of melting there is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30 s we interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components in contrast variations in trace elements are attributed to young mantle differentiation processes

approximating edges of source bodies from magnetic or gravity anomalies cordell and grauch 1982 1985 discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks the final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes we attempt to automate their final step our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources the method applies to both local surveys and to continent wide compilations of magnetic and gravity data e g zietz 1982 simpson et al 1983a kane et al 1982

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

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

density mapping from gravity data using the walsh transform one of the main purposes of geophysical mapping is the identification of units that can be related to known geology on a regional scale aeromagnetic and gravity maps are the most useful tools presently available although other techniques such as conductivity mapping palacky 1986 or remote sensing watson 1985 are very helpful in locating lithologic boundaries interpretation now makes extensive use of enhanced maps susceptibility maps for magnetic data density maps for gravity data first and second vertical derivative and horizontal gradient maps for both types of data the objective of susceptibility and density mapping is to transform the potential field data into a physical property map for physical property mapping some hypotheses and simplifications are made the earth model is assumed to consist of right rectangular prisms of finite gravity or infinite magnetics depth extent for ease of data processing the potential field is interpolated onto a regular rectangular array so that

climatologies of nighttime upper thermospheric winds measured by ground based fabry perot interferometers during geomagnetically quiet conditions 2 high latitude circulation and interplanetary magnetic field dependence we analyze upper thermospheric 250 km nighttime horizontal neutral wind patterns during geomagnetically quiet kp 3 conditions over the following locations south pole 90 s halley 76 s 27 w millstone hill 43 n 72 w sondre stromfjord 67 n 51 w and thule 77 n 68 w we examine the wind patterns as a function of magnetic local time and latitude solar cycle day of year and the dawn dusk and north south components of the interplanetary magnetic field imf b y and b z in magnetic coordinates the quiet time high latitude wind patterns are dominated by antisunward flow over the polar cap with wind speeds that generally increase with increasing solar extreme ultraviolet euv irradiation the winds are generally stronger during equinox than during winter particularly over the south pole in the direction of eastern longitudes imf b y exerts a strong influence on the wind patterns particularly in the midnight sector during winter b y positive winds around midnight in the northern southern hemisphere are directed more toward the dusk dawn sector compared to corresponding b y negative winds this behavior is consistent with the b y dependence of statistical ionospheric convection patterns the strength of the wind response to b y tends to increase with increasing solar euv irradiation roughly in proportion to the increased wind speeds quiet time b y effects are detectable at latitudes as low as that of millstone hill magnetic latitude 53 n quiet time b z effects are negligible except over the magnetic polar cap station of thule

skeletonization of aeromagnetic data skeletonization is a syntactic pattern recognition method that is applied to gridded data to produce an automatic line drawing with an associated event catalog previous implementations of skeletonization have been tailored for seismic data here we modify that technique to render it more suitable for other types of gridded data with particular emphasis on aeromagnetic maps a modification from previous schemes is the use of a two pass approach to reduce the effects of an otherwise problematic directional bias that discriminates against events oriented parallel to columns of the grid the method can be used effectively for filtering aeromagnetic data on the basis of strike direction event linearity event amplitude and polarity it is based on the delineation of peak trough pairs cycles which are traced throughout the grid to form contiguous events cycles and events are characterized by attributes that include amplitude polarity and pulse width events are further characterized by length average strike direction and linearity the event attributes are stored in a catalog thus enabling one to perform attribute based analysis and data filtering we illustrate our algorithm using two regional aeromagnetic examples from different parts of the canadian shield the first from the great slave lake shear zone is dominated by linear anomaly trends produced by faults and mafic dikes the second from the manicouagan region of northeastern quebec contains abundant subcircular and arcuate anomaly patterns caused by large intrusive complexes and a meteorite impact structure

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

three dimensional gravity inversion using simulated annealing constraints on the diapiric roots of allochthonous salt structures in the northern continental slope of the gulf of mexico large oil and gas reservoirs are often found beneath sheetlike allochthonous salt structures that are laterally extensive some of these salt structures retain their diapiric feeders or roots beneath them these hidden roots are difficult to image seismically in this study we develop a method to locate and constrain the geometry of such roots through 3 d inverse modeling of the gravity anomalies observed over the salt structures this inversion method utilizes a priori information such as the upper surface topography of the salt which can be delineated by a limited coverage of 2 d seismic data the sediment compaction curve in the region and the continuity of the salt body the inversion computation is based on the simulated annealing sa global optimization algorithm the sa based gravity inversion has some advantages over the approach based on damped least squares inversion it is computationally efficient can solve underdetermined inverse problems can more easily implement complex a priori information and does not introduce smoothing effects in the final density structure model we test this inversion method using synthetic gravity data for a type of salt geometry that is common among the allochthonous salt structures in the gulf of mexico and show that it is highly effective in constraining the diapiric root we also show that carrying out multiple inversion runs helps reduce the uncertainty in the final density model

gravity instruments past present future geophysicists and geodesists have traditionally demanded high levels of accuracy from gravity sensors to make notoriously difficult measurements of extremely small differences in the earth s gravity field the earth s gravity field ranges from about 983 000 mgal at the poles to about 978 000 mgal at the equator yet we demand accuracies in the range of less than 1 mgal for typical surface measured exploration targets this means resolution of 1 part per million compared with the earth s gravitational field

a geochemical study of off axis seamount lavas at the valu fa ridge constraints on magma genesis and slab contributions in the southern tonga subduction zone abstract new geochemical data are presented for fresh lavas from off axis seamounts east and west of the valu fa ridge and from seamounts south of the propagating rift tip of the valu fa ridge most seamount lavas are basaltic and thus more primitive than the mostly andesitic to dacitic lavas from the adjacent valu fa ridge slight differences in fluid mobile element ratios and in sr isotope ratios suggest small scale heterogeneities of the slab component in this region with an apparently higher sedimentary input into the off axis seamount lavas than into the island arc magmas off axis volcanoes near back arc basin spreading centres thus resemble off axis seamounts adjacent to mid oceanic ridges which also show more extreme compositions than the well homogenized mid ocean ridge basalts the geochemical variations support a diapiric ascent of the hydrated or partially molten mantle leading to separate magma supply systems for the different volcanic structures the mantle wedge between the valu fa ridge and the island arc appears to be uniformly depleted in fluid immobile incompatible elements compared to average depleted mid ocean ridge basalts whereas lavas from seamounts west of the valu fa ridge show less depletion in agreement with a depletion event of the whole mantle wedge in the backarc melting zone seamount lavas south of the valu fa ridge propagating rift resemble the valu fa lavas indicating an increased magma production caused by the influx of water into the mantle

characterization of geological boundaries using 1 d wavelet transform on gravity data theory and application to the himalayas we investigate the use of the continuous wavelet transform for gravity inversion the wavelet transform operator has recently been introduced in the domain of potential fields both as a filtering and a source analysis tool here we develop an inverse scheme in the wavelet domain designed to recover the geometric characteristics of density heterogeneities described by simple shaped sources the 1 d analyzing wavelet we use associates the upward continuation operator and linear combinations of derivatives of any order in the gravity case we first demonstrate how to localize causative sources using simple geometric constructions both the upper part of the source and the whole source can be studied when considering low or high altitudes respectively the homogeneity degree of the source is deduced without prior information and allows us to infer its shape introducing complex wavelets we derive analytically the scaling behavior of the wavelet coefficients for the dyke and the step sources the modulus term is used in an inversion procedure to recover the thickness of the source the phase term provides its dip this analysis is performed on gravity data we measured along a profile across the himalayas in nepal good agreement of our results with well documented thrusting structures demonstrates the applicability of the method to real data also deeper less constrained structures are characterized

gravity inversion of basement relief using approximate equality constraints on depths we present a gravity interpretation method for estimating the relief of an arbitrary interface separating two homogeneous media the upper medium is discretized into rectangular juxtaposed prisms whose thicknesses represent the depths to the interface and are the parameters to be estimated from the gravity anomaly the density contrast of each prism is assumed to be constant and known to stabilize the inversion we introduce two kinds of constraints on the depths the first one requires proximity between the observed and computed depths at isolated points such as those obtained from boreholes absolute equality constraint the second one requires that groups of depths approximately follow an established linear relationship among the depths relative equality constraint both kinds of constraints are imposed in the least squares sense we illustrate the method performance by applying it to a synthetic anomaly produced by a simulated basement relief consisting of four narrow and adjacent structural lows

what are the limitations of satellite altimetry radar altimeter measurements of the marine geoid collected during the seasat altimeter mission gave geophysicists hope of uncovering the gravity field over all the ocean basins however because of insufficient track density it has taken 16 years for the full potential of the satellite altimeter to be realized the high density coverage obtained by ers 1 during its geodetic mapping phase april 1994 march 1995 prompted the u s navy to declassify all of the geosat altimeter data in june 1995 the combination of these two high density data sets provided the first global view of all the ocean basins at a wavelength resolution of 20 30 km

on the normal vertical gradient of gravity by j h karl geophysics 48 1011 1013 july 1983 in his reply to c j swain s 1984 discussion karl states that no one has disagreed with his proposed 0 265 mgal m average value for the normal gravity gradient and that his global terrain model can be used to challenge the validity of the traditional approach to data reduction our investigations show that karl is in error on both counts and we hope that the following analyses will help toward a clearer understanding of this question

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

standardization in gravity reduction gravity reduction standards are needed to improve anomaly quality for interpretation and to facilitate the joining together of different data sets to the extent possible data reduction should be quantitative objective and comprehensive leaving ambiguity only to the interpretation process that involves qualitative subjective and geological decisions the term bouguer anomaly describes a field intended to be free of all nongeologic effects not modified by a partial geologic interpretation measured vertical gradients of gravity demonstrate considerable variation but do not suggest as often reported that the normal free air gradient is in error or needs to be locally adjusted such gradients are strongly influenced by terrain and to a lesser extent by the same geologic sources which produce bouguer anomalies a substantial body of existing literature facilitates the comprehensive treatment of terrain effects which may be rigorously implemented with current computer technology although variations in topographic rock density are a major source of bouguer anomalies a constant density appropriate to the area under investigation is normally adopted as a data reduction standard leaving a treatment of the density variations to the interpretation a field example from british columbia illustrates both the variations in vertical gravity gradients which can be encountered and the conclusion that the classical approach to data reduction is practically always suitable to account for the observed effects standard data reduction procedures do not and should not include reduction to datum the interpreter must be aware however that otherwise smooth regional bouguer anomalies caused by regional sources do contain high frequency components in areas of rugged topography

fast rift propagation along the east pacific rise near easter island during a 1983 seabeam cruise we surveyed the easter nazca spreading center which forms the eastern boundary of the easter microplate we have analyzed all available bathymetry and magnetic anomaly data to determine the present fine scale tectonic configuration and recent tectonic evolution of this spreading center the narrowing of the axial brunhes anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm yr assuming an average spreading rate of 120 mm yr there are indications of a short episode of faster propagation 350 mm yr prior to 0 4 ma these are the fastest rates of rift propagation yet documented the tip of the propagating rift is at about 25 s 112 25 w 180 km south of the previously hypothesized position the recent spreading direction is n85 w a large well defined sheared zone to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the easter pacific spreading center 360 km to the west is being replaced by the fast propagator

gravity analysis of salt structures an example from the el kef ouargha region northern tunisia triassic outcrops in the atlassic zone of northern tunisia may be modelled in two ways salt bodies piercing through cretaceous terrains or triassic salt flows stratified within an albian series both models find support from gravity data and are debatable to evaluate the mass distribution changes with depth the bouguer anomaly of the el kef ouargha region was successively decomposed into regional and residual components to construct multiple pseudo depth slices and apparent density maps analyses of gravity lows clearly show a vertical continuity of less dense materials below the triassic salt outcrops these features can be explained by salt diapirism during mesozoic and cenozoic further gravity data tend to indicate less dense materials below aptian outcropping in jebel aite oued bou adila thus suggesting triassic materials occurring at depth in addition dense entities were recognized under mio pliocene and quaternary deposits which are thought to correspond to cretaceous paleoshoals currently collapsed by non outcropping faults our findings lend support to a diapir model intruding overburden rather than the salt glacier model stratified in the albian series proposed by some authors as the genetic structural model for triassic material bearing series in the north of tunisia