heat flow study of the emeishan large igneous province region implications for the geodynamics of the emeishan mantle plume abstract the emeishan large igneous province elip is widely considered to be a consequence of a mantle plume the supporting evidence includes rapid emplacement voluminous flood basalt eruptions and high mantle potential temperature estimates several studies have suggested that there was surface uplift prior to the eruption of the emeishan flood basalts additionally the plume s lateral extent is hard to constrain and has been variously estimated to be 800 1400 km in diameter in this study we analyzed present day heat flow data and reconstructed the permian paleo heat flow using vitrinite reflectance and zircon u th he thermochronology data in the elip region and discussed implications for the geodynamics of the emeishan mantle plume the present day heat flow is higher in the inner and intermediate zones than in the outer zone with a decrease of average heat flow from 76 mw m2 to 51 mw m2 thermal history modeling results show that an abnormal high paleo heat flow of 90 110 mw m2 was caused by the emeishan mantle plume activity based on the present day heat flow data we can calculate that there is lithospheric thinning in the central elip region which may be due to the destruction of the lithosphere by mantle plume upwelling and magmatic underplating the permian paleo heat flow anomaly implies that there was a temperature anomaly in the mantle the ascending high temperature mantle plume and the thinned lithosphere may have induced the large scale uplift in the elip region according to the range of the surface heat flow anomaly it can be estimated that the diameter of the flattened head of the emeishan mantle plume could have reached 1600 1800 km our research provides new insights into the geodynamics of the emeishan mantle plume through study of heat flow

seismic imaging of melt in a displaced hawaiian plume the precise location of the mantle plume upwelling beneath hawaii is debated seismic data reveal a thick layer of melt in the mantle beneath western hawaii implying that the upwelling plume may be deflected around an ancient resistive root beneath the island

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

submarine hydrogeology of the hawaiian archipelagic apron 2 numerical simulations of coupled heat transport and fluid flow we perform numerical simulations of buoyancy driven pore fluid flow in the hawaiian archipelagic apron and underlying oceanic crust in order to determine the extent to which heat redistributed by such flow might cause conductive heat flow measurements to underrepresent the true mantle heat flux we also seek an understanding of undulations observed in finely spaced heat flow measurements acquired north of oahu and maro reef with wavelengths of 10 to 100 km and amplitudes of 2 to 7 mw m 2 we find that pore fluid flow can impart significant perturbations to seafloor heat flow from the value expected assuming a constant mantle flux in the simplest scenario moat wide circulation driven by bathymetric relief associated with the volcanic edifice recharges a fluid system over the moat and discharges the geothermally heated water through the volcanic edifice the existing heat flow data are unable to confirm the existence of such a flow regime in that it produces prominent heat flow anomalies only on the steep flanks of the volcano where heat flow probes cannot penetrate however this flow system does not significantly mask the mantle flux for reasonable permeabilities and flow rates another numerical simulation in which the upper oceanic basement acts as a aquifer for a flow loop recharged at basement outcrops on the flexural arch and discharged within a permeable volcanic edifice is capable of almost uniformly depressing conductive heat flow across the entire moat by 15 large heat flow anomalies 20 mw m 2 are located over the recharge and discharge zones but are beyond the area sampled by our data presumably finely spaced heat flow measurements over the flexural arch could test for the existence of the predicted recharge zone we demonstrate that the prominent shorter wave undulations in heat flow across the oahu and maro reef moats are too large to be explained solely by relief in the upper oceanic basement more likely shallower large scale turbidites or debris flows also serve as aquifers within the less permeable moat sediments with our limited information on the structural geology of the moat permeability structure of its major geologic units and their variations in the third dimension we are not able to exactly match the spatial distribution of heat flow anomalies in our data but spectral comparisons look promising

anomalous heat flow near the crozet plateau and mantle convection abstract a heat flow study has been performed near the crozet plateau southern indian ocean where the geoid and bathymetry are anomalously high with respect to the nearby deep ocean basins data collected on 67 ma old oceanic crust agree with simple plate model predictions at the stations furthest removed from the plateau heat flow values increase significantly towards the centre of the feature suggesting a thermal origin for this uplift the ratio of geoid to bathymetry confirms that simple crustal thickening with airy local compensation cannot explain the observations the data are compatible with a model of dynamic convective support of the plateau by an underlying thermal plume in the earth s mantle

heat flow on the hawaiian swell and lithospheric reheating most oceanic volcanic centres caused by hotspot activity are surrounded by sea floor that is unusually shallow for its age1 2 there is increasing evidence that swells found on older sea floor like the hawaiian swell have formed from a broad scale reheating and thinning of the lithosphere as it passes over a mantle hotspot3 4 if the formation and subsequent disappearance of these swells are controlled by thermal processes they should have an above normal heat flux we present here the results from 95 new heat flow measurements on the hawaiian swell along the older part of the swell the heat flow is 20 25 higher than the normal heat flow for crust of this age this anomaly is consistent with the observed swell uplift the shape and amplitude of the heat flow anomaly require that the reheating be largely confined to the lower half of the lithosphere

heat flow on hot spot swells evidence for fluid flow 1 we examine variability observed in heat flow determinations collected on hot spot swells we find substantial scatter in heat flow at wavelengths of a few hundred kilometers and less at both hawaii and reunion where closely spaced data exist and large variability in the regional heat flow surveys at cape verde bermuda and crozet our preferred interpretation is that the observed variability is due to fluid flow the presence of fluid flow admits the possibility that some heat is lost through advection such that the mean observed heat flow is less than the actual mantle flux if so the full magnitude of sublithospheric thermal variations may not be observed this interpretation has important implications for understanding heat flow determinations made on hot spot swells and resulting geodynamic inferences we suggest fluid flow may be masking variations in sublithospheric heat flux making available surface heat flow values a poor discriminator between geodynamic models for hot spot swells future field programs methods of data analysis and models should be designed to help extract a low frequency mantle flux disguised by a high noise surface filter

heat flow over reunion hot spot track additional evidence for thermal rejuvenation of oceanic lithosphere we present 99 new heat flow determinations obtained with a regular sampling 2 3 km along two seismic profiles across the mascarene ridge indian ocean this aseismic ridge is volcanic relief due to the activity of a hot spot now responsible of the active volcanism of reunion island our sampling technique allows us to identify short wavelength processes such as slope driven water flows that affect heat flow on the ridge with the exception of these perturbed sites heat flow appears to be very uniform across the ridge and significantly higher 6 8 mw m 2 than the reference heat flow predicted for the seafloor age although this anomaly is low it confirms other evidence for thermal rejuvenation of the lithosphere based on gravity and bathymetry the set of information now available for the mascarene ridge seems to indicate a mixed mode of compensation with dynamical support below the front of the swell track and thermal support behind for the older part of the swell

the nicoya convergent margin a region of exceptionally low heat flow over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the nicoya peninsula is characterized by remarkably low heat flow values over the accretionary prism average 28 mw m and values in the trench and the ocean crust seaward of the trench average 14 mw m we attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits

submarine hydrogeology of the hawaiian archipelagic apron 1 heat flow patterns north of oahu and maro reef we present two profiles of collocated single channel seismic reflection and heat flow determinations across the hawaiian flexural moat one north of oahu and the other north of maro reef seismic reflection data are used as an aid in determining depth to basement and interpreting moat stratigraphy moat sediments are locally up to 2 km thick and result mostly from slumps debris avalanches and turbidites from volcanoes capping the hawaiian ridge each heat flow profile is 200 km long and consists of paired thermal gradient and conductivity measurements made at an interval of 1 2 km the mean heat flow unconnected for the cooling effect of sedimentation along the oahu profile is 63 9 mw m 2 s d 11 9 mw m 2 and along the maro reef profile is 59 8 mw m 2 s d 5 0 mw m 2 our preferred sedimentation correction model gives a corrected heat flow mean along the oahu profile of 74 2 mw m 2 s d 14 6 mw m 2 and for the maro reef profile of 64 0 mw m 2 s d 4 9 mw m 2 these values are not significantly different despite the fact that oceanic crust at maro reef is 19 myr older than at oahu the mean heat flow along the oahu profile is within one standard deviation of the global mean value for oceanic crust of the same age while the mean heat flow along the maro reef profile differs by greater than one standard deviation for the global mean however the mean heat flow along the maro reef profile is within the uncertainty of previous heat flow determinations made offswell at maro reef von herzen et al 1989 variability along each profile is significantly greater than the measurement uncertainty measurement uncertainty is 3 and 2 mw m 2 for the oahu and maro reef profiles respectively and is significantly greater along the oahu profile than the maro reef profile while this variability could result from fluid flow within the archipelagic apron or underlying crust simple moat wide circulation models do not appear to explain the dominant variability in heat flow heat flow means and variability of both profiles are inconsistent with simple conductive models of lithospheric reheating

heat and fluid flux through sediment on the western flank of the mid atlantic ridge a hydrogeological study of north pond a comprehensive survey of heat flow sediment thickness and bathymetry in a large intermontane sediment pond on the western flank of the mid atlantic ridge north pond shows that only 20 to 25 of the heat escaping from the lithosphere flows through the sediments in the pond even though they completely cover a 70 km2 area of the seafloor three in situ pore pressure gradient measurements gave values that were negative relative to hydrostatic indicating that there is drawdown of water through the sediment at rates of 1 to 5 mm yr north pond is a recharge zone for hydrothermal circulation that is probably driven by lateral pressure gradients produced by topographic relief

two station measurements of rayleigh wave group velocity along the hawai ian swell two station measurements of rayleigh wave group velocity between midway atoll and o ahu provide a useful constraint on lithospheric thickness along the hawai ian swell comparison of the observed dispersion curve with age dependent regionalized dispersion curves suggests that the swell acts as 50 110 m y old lithosphere and not as 20 50 m y old lithosphere as is predicted by the currently favored hypothesis for the swell s formation a preliminary isotropic shear velocity model for the swell suggests that the lithosphere may be as thick as 100 km this value is difficult to reconcile with the lithosphere being substantially thinned during its passage over the hawai ian hotspot

geoid and topography associated with sublithospheric convection negligible contribution from deep currents abstract the sources of geoid and topography anomalies in a convecting upper mantle with realistic temperature and pressure dependent viscosity are studied the results differ from those obtained in previous studies with layered depth dependent viscosity for a newtonian viscosity the topography and the geoid anomalies coming from below the low viscosity zone i e deeper than 250 km amount to 30 m and 10 cm respectively while they are reduced to 10 m and 10 cm for a non newtonian viscosity it represents at most 6 of the complete signal and affects very little the geoid over topography ratio the weakness of the contribution from the lower part of the cell is due to both small kernels and small lateral temperature variations at depth yet a linear regression of the complete geoid signal versus topography yields an apparent compensation depth shallower than the base of the lithosphere the damping of the vertical stress through a lithosphere with a depth dependent viscosity implies a departure from isostasy and is the cause of the shallow apparent compensation depths found in our model

heat flow and depth versus age for the mesozoic northwest atlantic ocean results from the sohm abyssal plain and implications for the bermuda rise abstract heat flow in the sohm abyssal plain is measured to be 53 mw m 2 at an age of 163 ma this is 25 higher than predicted by conductive cooling models even though the sediment corrected basement depth of 6 5 km at this location is normal for its age an analysis of existing heat flow depth and geoid anomalies in the northwest atlantic shows that there is little correlation between heat flow and depth throughout the entire region depth and geoid are clearly related to the bermuda swell while the associated heat flow anomaly once adjusted for variations with age is limited to 5 mw m 2 and only decays to the south this means that the bermuda swell is probably not caused by extensive thermal reheating within the lithosphere but instead by dynamic uplift at its lower boundary due to the convective upwelling of a mantle plume the regionally high heat flow in the northwest atlantic may be a thermal remanent of previous plumes which passed beneath this region early in its history therefore depth and heat flow anomalies from this region cannot be used to provide constraints on steady state parameters of the lithosphere such as the presence or absence of a long term boundary layer at its base

heat flow observations on the bermuda rise and thermal models of midplate swells the bermuda rise is a broad topographic swell which is apparent in both residual depth and geoid anomaly maps of the western north atlantic the magnitudes of the depth and geoid anomalies associated with the bermuda rise are similar to the anomalies associated with other swells surrounding recent volcanic islands e g hawaii suggesting that despite the lack of recent volcanism on bermuda the rise has a similar origin to other midplate swells results are reported from 171 new heat flow measurements at seven carefully selected sites on the bermuda rise and the surrounding seafloor off the bermuda rise the basement depths are generally shallower and the heat flow higher than either the plate or boundary layer models predict with the measured heat flow apparently reaching a uniform value of about 50 mw m 2 on 120 m y old crust on the bermuda rise the heat flow is significantly higher 57 4 2 6 mw m 2 than off the swell 49 5 1 7 mw m 2 the magnitude of the anomalous heat flux 8 10 mw m 2 is comparable to that previously found along the older portion of the hawaiian swell near midway the existence of higher heat flow on both the hawaiian swell and bermuda rise indicates that these features fundamentally have a thermal origin the differences in the shape uplift and subsidence histories of the hawaiian swell and bermuda rise can be quantitatively explained by the different absolute velocities of the pacific and north american plates moving across a distributed heat source in the underlying mantle two dimensional numerical convection models indicate that the observed depth geoid and heat flow anomalies are consistent with simple convection models in which the lower part of the thermally defined plate acts as the upper thermal boundary layer of the convection

lithospheric deformation induced by loading of the hawaiian islands and its implications for mantle rheology the long term rheological properties of the lithosphere are fundamental for understanding both surface tectonics and mantle dynamics on earth in this study we have developed 3 d finite element models for computing the load induced surface deformation and stress for lithosphere and mantle with realistic nonlinear viscoelastic rheology including the frictional sliding low temperature plasticity and high temperature creep we have determined the lithospheric deformation and stress due to volcano loading in the hawaiian islands region for the last few million years by comparing model predictions with seismic observations of the depth to the top of oceanic crust and depth dependence of seismicity in the hawaiian islands region we have sought to constrain lithospheric rheology our calculations show that the load induced surface deformation is controlled by low temperature plasticity and frictional sliding but is insensitive to high temperature creep lithospheric strength predicted from laboratory derived low temperature plasticity needs to be reduced significantly and a frictional coefficient f ranging from 0 1 to 0 7 is required in order to account for the observations however f 0 1 weakens the shallow part of the lithosphere so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity our results therefore suggest a value for f between 0 25 and 0 7 finally the maximum stress that accumulates in the deformed lithosphere beneath the hawaiian islands is about 100 200 mpa for models that match the observations and this stress may be viewed as the largest lithospheric stress on earth

mechanical erosion and reheating of the lithosphere a numerical model for hotspot swells it is currently debated if either thermal erosion of the lithosphere or dynamical support is the source of topography and geoid anomalies the origin of tins controversy lies probably in the difficulty to model simultaneously these two effects for this purpose we have studied the time dependent behavior of two dimensional convection with a temperature and pressure dependent viscosity the use of a control volume method allows us to define a rigid zone simulating the mechanical lithosphere the interface between the lithosphere and the convective mantle is determined by a viscosity cutoff first some experiments model the rise of a plume below the lithosphere in order to observe the evolution of the uplift and thus to appreciate the various processes involved in the swell formation before the plume reaches the base of the thermal lithosphere an uplift a few hundred meters in amplitude develops which can only be ascribed to a pure dynamical support the major uplift occurs when the ductile part of the lithosphere the convective boundary layer is squeezed by the plume the reheating of the mechanical lithosphere takes place after this transient stage of dynamical erosion however this late process is very slow but can magnify the amplitude of the swell if the lithosphere stays long enough above the plume these results shed some light on the different mechanisms occurring during the swell formation but the configuration modeled does not correspond to the one expected for actual hotspot swells they feature plume rising up to the lithosphere while natural situations correspond to lithosphere drifting above preexisting plumes an experiment with a moving lithosphere was run and shows that thermal erosion does not affect significantly a moving lithosphere even for relatively slow drifting velocities few centimeters year indeed the thermal structure of the lithosphere is not modified above the 800 c isotherm except for a motionless plate in this case the resulting swell should be greater this could explain why azores crozet or cap verde swells are so high on the other hand the shape of a swell over a moving lithosphere is strikingly reminiscent of the hawaiian swell

asymmetric shallow mantle structure beneath the hawaiian swell evidence from rayleigh waves recorded by the plume network author posting the author s 2011 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 187 2011 1725 1742 doi 10 1111 j 1365 246x 2011 05238 x

earth science through thick and thin the sea floor around the hawaiian island chain is unusually shallow new seismic evidence suggests that this up raised swell is partly due to heating and thinning of the lithosphere beneath

crustal thickness along the western galapagos spreading center and the compensation of the galapagos hotspot swell wide angle refraction and multichannel reflection seismic data show that oceanic crust along the galapagos spreading center gsc between 97 w and 91 25pw thickens by 2 3 km as the galapagos plume is approached from the west this crustal thickening can account for v52 of the 700 m amplitude of the galapagos swell after correcting for changes in crustal thickness the residual mantle bouguer gravity anomaly associated with the galapagos swell shows a minimum of 325 mgal near 92 15pw the area where the gsc is intersected by the wolf darwin volcanic lineament wdl the remaining depth and gravity anomalies indicate an eastward reduction of mantle density estimated to be most prominent above a compensation depth of 50 100 km melting calculations assuming adiabatic passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of v30 c the associated thermal expansion and increase in melt depletion reduce mantle densities but to a degree that is insufficient to explain the geophysical observations the largest density anomalies appear at the intersection of the gsc and the wdl our results therefore require the existence of compositionally buoyant mantle beneath the gsc near the galapagos plume possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the galapagos islands that is later transported to the gsc our estimate for the buoyancy flux of the galapagos plume 700 kg s 31 is lower than previous estimates while the total crustal production rate of the galapagos plume 5 5 m 3 s 31 is comparable to that of the icelandic and hawaiian plumes

genesis of flood basalts from eclogite bearing mantle plumes on the basis of the geochemistry of flood basalts we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core mantle boundary we present finite element models of the melting in starting plumes plume heads in both continental and oceanic regions starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt or any melt at all for reasonable plume head temperatures 1400 1600 c or lithosphere ages 6 25 100 m y this result is in contrast to the results of farnetani and richards 1994 whose models require higher plume head temperatures 1600 1700 c and a weak mobile lithosphere in order to generate volumes of melt similar to flood basalts plumes composed of 15 wt eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings the maximum extent of melting in these models is about 10 30 this agrees with the model of hofmann and white 1982 who argued that ocean island basalts are derived from subducted oceanic crust if the embedded eclogite thermally reequilibrates with the surrounding plume material then melt volumes and total extents of melting increase by a additional factor of 2 3 a simple two dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume related melting occurs over 50 60 m y a much longer period than for flood basalts

seismic stratigraphy of the flexural moat flanking the hawaiian islands the elastic thickness of oceanic lithosphere based on flexure studies in the region of surface loads is two to three times smaller than its seismic thickness1 thus as a load ages the thickness of the mechanically supportive part of the lithosphere must decrease from its short term seismic thickness to its long term mechanical thickness extrapolation of data from experimental rock mechanics suggests that thinning occurs by brittle failure in the uppermost part of the lithosphere and thermally activated creep in its lower part2 modelling shows that for load ages less than about 1 myr flexural moats change shape and the peripheral bulges migrate in towards the load3 these transient changes in flexure are best manifest we believe in the stratigraphical sequences that infill flexural moats the largest thicknesses of sediments occur in moats flanking orogenic belts4 5 however the stratigraphical record in these basins is complicated by the effects of compaction sub aerial erosion and sea level changes and by the fact that the loading history extends over very long periods of time here we describe a multichannel seismic reflection profile across a flexural moat flanking the volcanic load of oahu in the hawaiian islands the advantages of oahu are that it is located in the interior of a plate the load is 2 7 myr old and formed within 1 myr6 and the flexural moat was not significantly affected by sea level changes and erosion we show that the observed stratigraphical patterns cannot be explained by changes in material influx load distribution and rapid relaxation of the lithosphere to its long term mechanical thickness rather the observed patterns suggest a prolonged relaxation 1 myr due to either local or regional re heating of the lithosphere this re heating has important implications for the relationship between elastic thickness and age of the lithosphere at the time of loadings

anomalous heat flow and geoid across the cape verde rise evidence for dynamic support from a thermal plume in the mantle summary we describe results from 200 heat flow measurements across the cape verde rise in the north atlantic they show that the heat flow through normal 125 myr old crust is 45 5 3 4mwm 2 close to the prediction for a lithospheric plate model heat flow increases towards the centre of the rise reaching a maximum of 16 4 mw m 2 above the normal oceanic value the concommitant geoid anomaly at the centre of the rise is 7 6 0 3 m and the depth anomaly is 1900 200 m the anomalous heat flow geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms lithospheric reheating models alone whether involving transient or sustained thinning of the lithosphere fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the cape verde rise axisymmetric convection models provide a better fit to the observed data and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle uplift caused by expansion of the solid lithosphere is of smaller importance the observed geoid bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2 4 times 1020 pa s and a coefficient of thermal expansion in the range 2 0 2 5 times 10 5 k 1 the effect of melting in the ascending plume is to buffer the heat flow anomalies in the case of the cape verde rise partial melting is minimal but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20 25 mw m 2

thermal origin of the hawaiian swell heat flow evidence and thermal models the lithospheric reheating hypothesis for the origin of the hawaiian swell has been tested by heat flow measurements along the swell between hawaii and midway ninety five measurements at eight carefully selected sites show that the heat flux on the swell increases systematically from near normal values 52 2 mw m 2 near hawaii to values near midway 59 2 mw m 2 about 20 25 higher than normal for lithosphere of that age 110 m y this heat flow pattern is qualitatively consistent with a deep seated thermal anomaly created in a lithospheric plate moving across a fixed heat source modeling of the heat flow and bathymetry data on the hawaiian swell strongly suggest that the swell is caused by a rapid heating within 40 to 50 km depth of normal 90 m y old lithosphere over a relatively broad 500 1000 km region near hawaii this model is also consistent with gravity and geoid anomalies associated with the swell which require a low density region in the lower lithosphere and with flexure studies of seamounts in the hawaiian chain which indicate that the rigidity of the lithosphere is not significantly reduced by the reheating appendices a c are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 005 1 00 payment must accompany order

petrology geochemistry zircon u pb dating and lu hf isotope of granitic leucosomes within felsic gneiss from the north qaidam uhp terrane constraints on the timing and nature of partial melting abstract granitic leucosomes are widely distributed within felsic gneiss in the north qaidam ultrahigh pressure uhp metamorphic terrane in western china which is crucial to understanding the relationships between partial melting metamorphic evolution and orogenic processes we have applied petrology whole rock geochemistry and sr nd isotope zircon u pb geochronology trace element composition and lu hf isotope of these granitic leucosomes to determine the nature and timing of partial melting of these rocks anatexis of the felsic gneiss is evidenced by 1 highly cuspate elongated feldspar grains along quartz quartz and quartz feldspar boundaries 2 cuspate wedge shaped pockets of k feldspar quartz plagioclase muscovite along the boundaries of quartz and or plagioclase and 3 felsic veinlets of k feldspar quartz plagioclase muscovite along grain boundaries major elements feo t mno mgo and tio 2 as well as lrees hrees and hfses are mainly retained in the melanosomes whereas the large ion lithophile elements liles e g rb ba k sr pb are preferentially partitioned into the granitic leucosomes three discrete u pb ages are recorded in the zoned zircons from the melanosomes and granitic leucosomes the inherited magmatic pre metamorphic zircon cores from melanosomes and granitic leucosomes contain quartz feldspar inclusions and record a neoproterozoic protolith age of approximately 950 ma the unzoned zircon mantles in the melanosomes and granitic leucosomes show characteristics similar to metamorphic zircons in terms of such as remarkably flat heavy rare earth element hree patterns an absence of obviously negative eu anomalies and low th u ratios these zircon mantles record an eclogite facies metamorphic age of 444 449 ma the last discrete age at 433 435 ma is preserved in anatectic zircon rims which display pronounced oscillatory zoning and contain felsic mineral inclusions of k feldspar plagioclase quartz the anatectic zircons have negative eu anomalies and pronounced positive ce anomalies and their low th u ratios and lu hf isotope features that are indistinguishable from those of the metamorphic domains the similarity in lu hf signatures indicates a closed system whereby no hf was added either from the outside via fluids or surprisingly from the pre metamorphic cores during partial melting muscovite relicts coexist with cuspate feldspars in the granitic leucosomes suggesting that the anatectic melts originate from dehydration melting due to breakdown of the muscovite in the felsic gneiss in conclusion an integrated study of the petrology geochronology and geochemistry demonstrates that partial melting of felsic gneiss in the nqd uhp terrane could mainly occurred during the initial retrogression stage and anatectic melt crystallized probably under granulite facies conditions

dominant influence of volcanic loading on vertical motions of the hawaiian islands abstract rates of island vertical motion above intra plate hotspots record the processes that arise from interactions between lithospheric plates and mantle plumes to assess the contribution of mantle and lithospheric processes to surface motion above the hawaiian hotspot we compare simple models of lithospheric deformation to vertical motion rates measured from dated paleoshorelines and tide gauge records in the hawaiian islands our analysis shows that observed uplift and subsidence rates mainly record the flexural response of the lithosphere to volcanic loads the effective elastic plate thickness that best fits the spatial distribution of subsidence and uplift rates is 40 km consistent with previous estimates based on total vertical deflection because volcanic loading dominates the vertical motion signal hawaiian islands appear to follow a predictable trajectory of vertical motion when they reside within one flexural half wavelength of the active volcanic center islands initially subside at rapid and decreasing rates in the first 1 myr following their construction uplift relatively slowly 1 2 5 myr following their construction and eventually subside again but at slow rates within 5 myr of their construction this observed pattern of uplift and subsidence is consistent with the pattern of vertical motion predicted to result from volcanic loading at the hawaiian hotspot lithospheric migration over the long wavelength topographic swell associated with the hawaiian hotspot has a comparatively minor influence on island uplift and subsidence its contribution to island vertical motion is not readily observed in our data with the possible exception of some uplift observed in the past 500 kyr on o ahu that might correspond to non steady state behavior of the hawaiian plume

constraints on lithospheric thermal structure for the indian ocean from depth and heat flow data models for the thermal evolution of oceanic lithosphere are primarily constrained by variations in seafloor depth and heat flow with age these models have been largely based on data from the pacific and atlantic ocean basins we construct seafloor age relations for the indian ocean which we combine with bathymetric sediment isopach and heat flow data to derive curves for depth and heat flow versus age comparison of these curves with predictions from three thermal models shows that they are better fit by the shallower depths and higher heat flow for the gdh1 model which is characterized by a thinner and hotter lithosphere than previous models

a two dimensional seismic investigation of crustal structure under the hawaiian islands near oahu and kauai two expanding spread profiles esps were analyzed in detail to investigate the deep structure of the volcanoes and the underlying oceanic crust near oahu one esp was located on the southeastern submarine flank of kauai esp 10 and the other was located directly over the ridge axis between oahu and molokai esp 5 two dimensional models of the entire crust were obtained by iterative comparison of both travel times and relative amplitudes of arrivals on the ray theoretical synthetic seismograms with those in the data a 2 4 km vertical offset in the preexisting crust inferred to be a large fault zone was found near the midpoint of esp 10 this fault may form the southern wall of a graben or half graben in which kauai sits the layering and seismic velocities of the preexisting oceanic crust near kauai have remained unmodified by the load of the volcanic pile esp 5 is located over a shallow 4 km thick basin filled with volcaniclastics and possibly volcanic flows bordered by the islands of oahu and molokai penguin bank and two buried rift zones under this basin the main volcanic edifice is 4 to 5 km thick the top of the preexisting oceanic crust is 8 to 9 km below sea level and is 3 to 4 km lower than the crust away from the islands the underlying oceanic crust is 6 to 7 km thick excluding the moho transition zone which is of normal thickness and at a depth from 15 to 16 km the existence of a large subcrustal plutonic body is refuted

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

a heat flow survey on anomaly m0 south of the bermuda rise abstract we have obtained a suite of 42 closely spaced acoustically navigated heat flow measurements on well sedimented crust of anomaly m0 age 109 ma in the northwest atlantic ocean 25 n 68 w 950 km south of bermuda the mean and standard deviation of the values obtained are 1 13 hfu cal cm2 s 47 3 mw m2 and 0 05 hfu 2 1 mw m2 respectively some of the variability is accounted for by refractive effects of the basement topography drill core data and our modelling suggest that the thermal conductivity contrast between sediments and basement rocks in this region is less than a factor of 1 6 the mean heat flow is close to the 1 1 hfu 46 mw m2 predicted by both the plate and boundary layer cooling models of the oceanic lithosphere this is the first detailed comparison with theoretical cooling models on old atlantic ocean crust since the difference in surface heat flow 0 15 hfu predicted by the two cooling models for the oldest observed oceanic lithosphere 180 ma is also not much larger than the range of uncertainty in our observations discrimination between the two models on the basis of surface heat flow data alone may prove difficult

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

origin of intraplate volcanoes from guyot heights and oceanic paleodepth the height of a guyot as measured from the surrounding regional sea floor to the vol cano s slope break records the water depth at the time the guyot submerged thus guyot heights may be used as indicators of the paleodepth of the surrounding ocean floor we compile data on the heights of 68 intraplate guyots and atolls in the pacific ocean as well as 46 volcanic islands in the pacific atlantic and indian oceans we find that guyot heights generally increase with the age of the lithosphere upon which they werc cmplaced although therc is a large amount of scattcr in nearly all cases seamount height and thus seafloor paleodepth is less than expzcted of normal seafloor these results suggest that most of the volcanoes in this study formed on anomalously shallow seafloor consistent with formation at hotspots to characterize thermal anomalies associ ated with these hotspot swells we model guyot heights by calculating the isostatic uplift predicted for normal lithosphere that has been partly reheated and is underlain by anomalously hot mantle this model is able to explain the anomalous water depth at most of the seamounts with hotspot thermal anomalies of 100 30o0c the heights of a few volcanic chains however are not anomalously low suggesting that these volcanoes are not associated with hotspots in addition the observed trend of hawaiian emperor guyot heights as well as the subdued morphology and gravity signature of the oldest emperor seamounts supports our hypothesis that cretaceous age meiji seamount may have formed on or near a spreading center

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

thermoelastic stress in oceanic lithosphere due to hotspot reheating we investigate the effect of hotspot reheating on the intraplate stress field by modeling the three dimensional thermal stress field produced by nonuniform temperature changes in an elastic plate temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere a thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements the stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series wherein each term is a source or an image modified from a closed form half space solution we apply the thermal stress solution to midplate swells in oceanic lithosphere with various thermal structures and plate velocities our results predict a stress field with a maximum deviatoric stress on the order of 100 mpa 1 kbar covering a broad area around the hotspot plume the predicted principal stress orientations show a complicated geographical pattern with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere although stress data near oceanic swells are limited the source parameters of intraplate earthquakes near several hotspots are consistent with the thermal stress model these results indicate that thermal stress due to reheating may be an important contributor to stress fields near hotspots in old oceanic lithosphere

towards determining the thermal state of old ocean lithosphere heat flow measurements from the blake bahama outer ridge north westem atlantic summary detailed suites of heat flow measurements have been completed at four sites on and to the east of the southern end of the blake bahama outer ridge in the north west atlantic on ocean crust which ranges in age from roughly 115 to 155 ma the principal purpose of these measurements was to compare instrumental techniques and to examine the decay of heat flow with age on old ocean floor multi penetration gradient measurements were made at the youngest site with a 2 5 m outrigger probe and multipenetration heat flow measurements at the other three sites with a new 3 5 m violin bow type instrument in addition at all four sites the gradient was measured using outriggers on a 9 m piston or gravity corer and conductivities were measured on the sediment recovered the average of thermal conductivities measured on cored sediments agreed to better than 2 per cent with the average of conductivities measured in situ at the oldest three sites such good agreement is surprising given the problems associated with making measurements on sediments recovered in cores the greatest measurement errors in determining heat flow were found to be caused by temperature disturbances generated by the piston coring process gravity coring reduced these disturbances but provided a less representative section for measuring thermal conductivity the violin bow probe which measures average interval conductivities in situ provided the most reliable heat flow determinations the accuracy of these determinations was limited primarily by the presence of thermal disturbances due to bottom water temperature variations the heat flow averages at all four sites are virtually identical they range from 47mw m 2 1 12 cal cm 2s 1 to 49mw m 2 1 17 cal cm 2s 1 with standard errors of less than 2 mw m 2 0 05 cal cm 2s 1 the constancy and the value of heat flow over lithosphere of this age and the local shallowing of depth with increasing age in the region containing the heat flow sites cannot be explained by either a simple plate or a boundary layer lithosphere cooling model the unexpectedly high heat flow and shallow depths may result from lithospheric thinning and perhaps dynamic forces associated with a thermal perturbation beneath the plate

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

heat flow and the thermal origin of hot spot swells the hawaiian swell revisited we present 150 new heat flow measurements obtained at eight sites along a 1230 km long profile across the hawaiian swell about 700 km ese of midway island most of the measurements include in situ thermal conductivity determinations which helped to reduce the statistical uncertainties 95 confidence at all sites to 59 mw m 2 about 20 higher than the predicted heat flow for 100 ma seafloor based on simple lithospheric cooling models thus even though there is a small increase in heat flow with age along the swell when compared with the off swell heat flux the anomalous heat flow associated with the hawaiian swell is probably of the order of 5 10 mw m 2 and arguably may not exist at all a previous investigation von herzen et al 1982 may have overestimated the magnitude of the heat flow anomaly associated with the hawaiian swell by comparing heat flux measurements on the swell with values expected for simple lithospheric cooling models the heat flow anomalies associated with the bermuda rise and the cape verde rise may also be smaller than previously estimated given the uncertainties in the heat flux on the normal seafloor surrounding these swells the lack of a significant heat flow anomaly associated with the bathymetric expression of the hawaiian swell is inconsistent with simple models of lithospheric reheating dynamic support accompanied by modest temperature increases 80 ma oceanic lithosphere

reheating of old oceanic lithosphere deductions from observations deep wide oceanic basins are the only regions of old seafloor where depth is truly representative of thermal isostasy when the depths of these basins are corrected for the effect of sediment accumulation and variation in crustal thickness the principal non thermal factors have been eliminated we collect the most precise and reliable values of heat flow for the same basins from multi penetration measurements with in situ thermal conductivity or deep sea drilling thermal gradients backed up by surface surveys the 9 data points that result from this selection process have been plotted on a depth versus heat flow graph and compared to published thermal models of lithosphere when considered without regard to age all the points fall at greater depths than predicted by the plate models with constant temperature lower boundaries and remarkably close to boundary layer cooling with parameters determined from the pre 80 ma depth and heat flow history of the ocean floor they are differentiated by their heat flows not much by their depths and the order they plot in along the heat flow axis is random with respect to crustal age modeling of discrete reheating events shows that near boundary layer conditions are re established after about 40 myr but corresponding to a younger than real age the data therefore favor discrete reheating events rather than a continuously hot basal boundary as implicitly assumed by the plate model lithospheric reheating appears to start only on ocean floor 100 ma the data alone cannot discriminate between a few discrete reheating events due to convective peel off at the base of the lithosphere or one or more catastrophic events however the distribution of points from the blake bahama basin is more consistent with distal reheating associated with the bermuda hotspot than local convective peel off

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006