strain softening along the mct zone from the sikkim himalaya relative roles of quartz and micas abstract in the darjeeling sikkim himalaya two distinct faults form the main central thrust mct the structurally higher mct1 and the lower mct2 each has accommodated translation greater than 100 km the lower mct2 places greater himalayan amphibolite grade paro lingtse gneiss over lesser himalayan greenschist grade daling metapelites the mct2 is folded by the underlying lesser himalayan duplex and is exposed at different structural positions of the fold at pelling the mct2 zone is exposed as a 373 m thick nw dipping fault zone that exposes 19 m of hanging wall mylonitized lingtse gneiss the lingtse protolith shows evidence of amphibolite grade plastic deformation features in quartz and feldspar within the hanging wall mylonite zone hwmz quartz and feldspar have undergone grain size reduction by different deformation mechanisms and feldspars are sericitized suggesting the presence of fluids during deformation we estimate a temperature of 300 c within the fault zone during fluid assisted retrogression and deformation reaction softening of feldspars produced a large proportion of intrinsically weak matrix this in combination with development of a strong foliation defined by parallel mica grains resulted in strain softening along the mct2 zone and concentrated the deformation along a thin zone or zones

formation of diamond with mineral inclusions of mixed eclogite and peridotite paragenesis two unusual diamonds were studied from kimberlites from china which contain both ultramafic and eclogitic mineral inclusions in the same diamond hosts diamond l32 contains seven fe rich garnets four omphacites and one olivine inclusion four olivine one sanidine and one coesite were recovered from diamond s32 both garnet and omphacite inclusions have similar compositions as those from other localities of the world and show basaltic bulk composition all the garnet and omphacite inclusions in diamond l32 have positive eu anomalies eu eu 1 64 1 79 these observations support the proposal that mantle eclogite is the metamorphic product of subducted ancient oceanic crust the mg mg fe ratio of the olivine inclusions from the two diamonds 91 92 are evidently lower than the normal olivine inclusions in diamonds from the same kimberlite pipe 92 95 the following model is proposed for the formation of diamonds with mixed mineral inclusions ascending diamond bearing eclogite recycled oceanic crust entrained in mantle plumes may experience extensive partial melting whereas the ambient peridotite matrix remains subsolidus in the diamond stable field this provides a mechanism for the transport of diamond from its original eclogitic host to an ultramafic one subsequent re growth of diamond in the new environment makes it possible to capture mineral inclusions of different lithological suites partial melts of basaltic sources may interact with the surrounding peridotite resulting in the relatively lower mg mg fe ratios of the coexisting olivine inclusions from the studied diamonds diamonds with mixed mineral inclusions demonstrate that plume activity also occurred in the archean cratons

measured changes in ocean surface roughness due to atmospheric boundary layer rolls a research aircraft was recently flown over the mid atlantic eight during a mild cold air outbreak to sample the planetary boundary layer at low levels of 15 20 m above the sea surface aircraft sensors measured near surface atmospheric turbulence and at the same time variations in sea surface roughness using laser and radar the aircraft turbulence observations indicate a coherent secondary flow consistent with boundary layer roll vortices of crosswind scale 1 5 2 0 km modulation in measured laser and radar inferred sea surface slope variance occurs at similar scales remarkable correlation is found between the radar backscatter and near surface wind speed data showing that the short wave slope variance is responsive to these large eddies associated small but periodic changes in wind direction do not appear to impact the short wave slope signature differentiation between short and intermediate scale wave roughness using aircraft laser and radar data leads us to conclude that observed fluctuations in intermediate scale wave slope were not directly linked to the local wind a synthetic aperture radar sar image was acquired by the canadian space agency s radarsat coincident with the aircraft measurements widespread streaking in the sar image suggests the surface impacts of atmospheric longitudinal rolls our aircraft results indicate that the dominant term dictating sar backscatter modulations at the eddy scale of 1 2 km is the fluctuation of the along wind velocity nominally associated with regions of near surface convergence or divergence wind speed fluctuations of 7 10 estimated from the sar and aircraft radar are consistent with the aircraft s measured variations in near surface wind speed

thermomechanical erosion of the lithosphere by mantle plumes a simplified semianalytic theory is presented that describes the process of lithosphere thinning through thermomechanical erosion by a hot mantle upwelling or plume the theory compares well with published numerical models but predicts that the rate of thinning is very dependent on the lateral scale of the upwelling and on the viscosity through the lithosphere asthenosphere transition a plume tail is likely to cause rapid local thinning reducing the lithosphere thickness substantially within a few million years plume heads or broadly spreading plume tail material are unlikely to cause significant thinning by this mechanism alone these results are consistent with geophysical evidence for localized thinning under the hawaiian volcanic chain but not under the broader hawaiian swell they are also qualitatively consistent with the distribution of basaltic volcanism in oceanic and continental hotspot tracks the theory predicts that thin continental lithosphere may be further thinned sufficiently by a plume tail to cause crustal melting within about 20 m y thus this mechanism may account for some delayed silicic magmatism on some slow continental plates higher thinning rates would have occurred in the archean

cross correlation weighted migration towards high resolution mapping of mantle heterogeneities summary we present a new type of born migration scheme to obtain a high resolution image of smallscale scatterers in the mantle seismic migration is a technique that projects scattered energy back to its origin by shifting and stacking seismograms using theoretical traveltimes migration methods in global seismology because of the limited distribution of sources and receivers tend to suffer from considerable smearing owing to isochronal artefacts this type of artefacts is caused by migration grids that have the set of theoretical traveltimes from a given source to all receivers similar to that for a true scattering point previously the information of slowness and backazimuth was explicitly explored to reduce such migration artefacts here we propose a simpler and more computationally efficient method but with similar effects in which the coherency of shifted seismograms measured by cross correlation is exploited to suppress isochronal artefacts the new migration scheme makes it possible to compute a large number of migrated images efficiently and thus evaluate the robustness of observed scattering objects by statistical approaches such as bootstrap resampling

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

inferring multiscale structure in atmospheric turbulence using satellite based synthetic aperture radar imagery this paper describes the multiscale structure in synthetic aperture radar sar backscatter seen in an image of a cold air outbreak as being due to the influence on the surface of the ocean of three scales of interacting atmospheric turbulence a low resolution pixels have side lengths of 100 m sar image shows the first class of backscatter pattern to be broken lines of enhanced radar backscatter of order 1 db above the background with an average cross wind scale of 5 km and downwind lengths of many tens of kilometers these are associated with quasi two dimensional roll vortices the second class of radar backscatter pattern is two dimensional backscatter regions whose linear aggregrations constitute the lines of backscatter they are on average 1 km wide in the cross wind direction and are 2 5 km long in the downwind direction a higher resolution image pixels have side lengths of 12 5 m of the same area shows a third class of structure smaller scale regions of enhanced backscatter up to several hundred meters in length downwind and greater than 100 m across wind that are associated with 7 db variations in in the unaveraged image the modulated fine scale backscatter structure compares favorably in both scale and effect to observations of the modulation of microfronts in the atmospheric surface layer by kilometer scale inactive eddies the latter being of uncertain origin therefore the hypothesis offered here is that the fine scale sar structure is caused by the influence of atmospheric microfronts on the ocean surface while the two dimensional kilometer scale backscatter features are the signature of inactive eddies linearly aggregated by or possibly intrinsic to the roll vortices

scratching the surface estimating dimensions of large igneous provinces a study of five major basaltic provinces including oceanic plateaus volcanic passive margins and continental flood basalts shows that they are voluminous constructions of extrusive igneous rock underlain by intrusive rock crustal thickness ranges from 20 to 40 km and lower crust is characterized by high 7 0 7 6 km s seismic velocities volumes and emplacement rates derived for two oceanic plateaus the ontong java and kerguelen broken ridge reveal short lived pulses of increased global crustal production and suggest an origin involving the lower mantle the ontong java rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid ocean ridge system despite the importance off large igneous provinces in studies of mantle dynamics and the global environment scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features

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

structural geometry microstructural and strain analyses of l tectonites from paleoproterozoic orthogneiss insights into local transport parallel constrictional strain in the sikkim himalayan fold thrust belt paleoproterozoic lingtse orthogneiss shows three dominant structural associations in the sikkim himalayan fold thrust belt ftb a these rocks locally define the pelling thrust pt zone that lies in the footwall of the main central thrust mct b they occur as linear almost n s trending imbricates within the pt sheet and c they form a n s trending 13 5 km long 2 km wide discontinuous linear mylonitic klippen within the daling formation of the lower lesser himalayan sequence lhs that lie east of the n s trending teesta valley the klippe records n s trending strong l tectonites that are spatially associated with ls and sl tectonites the stretching lineations are oriented along the transport direction these rocks are exposed along three mylonite zones from north to south these are the phodong mylonite zone the tumin eingee mylonite zone and the rumtek mylonite zone all the three mylonite zones are overall folded along gently plunging transport parallel n s trending synform with the strongest l fabric developed along the hinge zone and its dominance decreases progressively toward the eastern and western structural contacts of the klippen the highest constrictional strain is recorded in phodong and tumin eingee mylonite zones and decreases south toward rumtek mylonite zone the long axes x of the quartz strain ellipsoids show n s trend and gentle plunge that agrees with the stretching lineations observed in the field additionally the xy plane of the generated ellipsoids overall agree with the observed variation of the mylonitic foliation planes in the field from all the three locations microstructural analysis from the mylonite klippe suggests that quartz and feldspar have undergone 35 and 76 grain size reduction with respect to the protolith dislocation creep and microfracturing are the dominant deformation mechanisms in quartz and feldspar respectively the grain size reduction is more prominent near the eastern and the western boundaries as they mark structural contacts between the l tectonites and the daling phyllites strain is partitioned between quartz and feldspar in the mylonite zones with quartz recording constrictional strain while feldspar records flattening strain we explain this variation due to their different deformation mechanisms shear sense analysis of feldspar porphyroclasts suggest dominantly transport parallel flow along with a component of transport perpendicular flow in the mylonite zones existing models on mechanism of l tectonite formation and transport parallel folds do not completely explain our observations we propose the possible existence of a lateral ramp beneath the pt sheet to explain the generation of l tectonites and transport parallel fold in the study area

mantle temperature anomalies along the past and paleoaxes of the galapagos spreading center as inferred from gravity analyses to better understand the effects of hot spots on mid ocean ridge thermal structure we investigate the subsurface density structure of the galapagos spreading center and nearby lithosphere using shipboard gravity and bathymetry data we obtain maps of mantle bouguer anomalies mba by removing from the free air gravity the attractions of seafloor topography and a 6 km thick model crust comparison of observed and theoretical mba profiles along isochrons for ages 0 0 7 7 ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle this result is consistent with the notion that seafloor topography along the galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs along the ridge axis the mba decreases from the east and west toward the galapagos hot spot by 90 mgal reaching a minimum nearest the hot spot at 91 w seafloor topography mirrors the mba along axis increasing by 1 1 km toward the hot spot these variations in mba and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50 25 c toward the hot spot the predicted crustal thickening of 2 4 km nearest the hot spot accounts for 70 75 of the along axis mba and bathymetry anomalies mantle density variations account for the rest of the anomalies from the crustal isochron of age 7 7 ma to the present day axis the along isochron amplitudes of mba decrease from 150 to 90 mgal the corresponding along isochron bathymetry anomalies decrease from 1 7 to 1 1 km these observations along the paleoaxes of the galapagos spreading center indicate that the axial temperature anomaly was 70 hotter in the past 86 25 c and has steadily decreased to 50 25 c as the ridge axis migrated away from the galapagos hot spot these along isochron temperature anomalies however have remained well below that estimated for the hot spot itself 200 c indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10 20 times greater than that along the ridge axis over the past 7 7 m y

numerical investigations of the mantle plume initiation model for flood basalt events recent studies have linked both the eruption of continental flood basalts and the emplacement of large oceanic plateaus to the initial phase of mantle plume activity i e the plume head model here we develop this concept further through numerical models of a large thermal diapir rising through the mantle and impinging upon the base of the lithosphere a finite element analysis in axisymmetric geometry is used to model the dynamics of solid state convective flow and heat transport in the mantle and an anhydrous batch melting model is used to estimate melt volumes and compositions obtained from partial melting in the shallow mantle we explore the effects of a number of model variables including mantle viscosity structure plume temperature and diameter and the mechanical response of the lithosphere our study emphasizes cases in which the lithosphere does not undergo significant extension prior to large scale melting a standard model is found to be consistent with some observed characteristics of flood basalt plume initiation events this model includes an initial diapir of radius 400 km and initial excess temperature 350oc rising through a mantle of viscosity 102 pa s and impinging beneath a model oceanic lithosphere 100 ma in age if lithospheric extension does not occur melting is almost entirely sublithospheric the melt volume produced is 3 5x106 km 3 and its composition has high mgo content 15 20 wt a time sequence of precursory central axial uplift of several kilometers followed by major melting volcanism is a robust feature of all the models regardless of mantle viscosity structure or lithospheric response low viscosity in the upper mantle compresses the timescale for these events and results in a constriction of the plume head initially impinging on the lithosphere the radial extent of the melting region about 500 km at the time of maximum melt production is only slightly affected by the upper mantle viscosity for nonrifting lithosphere no significant melt generation occurs for initial plume excess temperatures less than about 300oc the initial plume radius has only a modest effect upon the volume of melt obtained the most important aspect of lithospheric response is whether or not the uppermost crust and lithosphere above 40 km depth undergo extension otherwise the ductile strength of the lower lithosphere has little effect if the lithosphere is allowed to extend freely the melt volume increases by about an order of magnitude and it is comparable with some of the largest oceanic plateaus such as ontong java if continental or oceanic flood basalt events occur on old nonrifting lithosphere the primary melts are likely to be much more mgo rich than the basalts erupted at the surface this is consistent with abundant evidence for olivine fractionation in many flood basalt provinces along with the occurrence of picrite lavas and we hypothesize that fractionation occurs at a density trap at the crust mantle boundary moho at this stage and following it is likely that significant lower crustal and mantle lithospheric contamination of mantle plume derived magmas may occur both isotopically and in the trace elements the sequence and timing of both precursory and post magmatic events in the vast well exposed oceanic flood basalt terrane known as wrangellia se alaska british columbia are consistent with the predictions of our model the uplift predicted by our models is of order 2 4 km if we require that mantle plumes are hot enough to melt beneath unrifted lithosphere effects not modeled here such as small scale convection and hydrous melting will tend to reduce the model excess temperature and uplift in order to better understand the relation between mantle plumes and large igneous provinces several important advances are required implementation of a fractional melting model some realistic assessment of melt transport through the mantle and crust and a fully three dimensional convection model

directional attributes of the ocean surface wind stress vector in a recent paper by geernaert 1988 it was suggested that the stress and wind vectors are rarely aligned and that the deviation of the stress vector direction from the wind vector is related to the surface heat flux it was argued that since cold air advection over the open ocean generally corresponds to upward heat flux the consequent rotation of the winds to the right with height during cold air advection could be responsible for observations of the stress vector to be to the right of the wind flow for unstable stratifications conversely observations of the stress vector to the left of the flow for stable stratifications were suggested to be associated with the turning of the wind vector to the left of the flow for warm air advection or downward heat flux in that study only 30 of the statistical variance of the angle between the wind and wind stress vectors could be explained by air sea heat flux and it was suggested that the orbital velocity spectrum of surface waves i e data unavailable in that study could be important for explaining at least part of the remaining variance in this study we report on measurements of the wind stress vector and wave directional spectrum collected during the synthetic aperture radar x band ocean nonlinearities experiment conducted on the chesapeake light tower during 1988 the experiment yielded data reported herein that provide convincing evidence that the stress vector direction during light winds and near zero air sea heat flux is governed in large part by the direction of long waves on the ocean surface further the measurements suggest that the stress vector has a direction which is a blend between the wind direction and the swell direction

effect of large eddies on atmospheric surface layer turbulence and the underlying wave field to improve existing models for air sea interaction a better understanding of the energy transfer across the boundary layer and in particular of the coupling of large atmospheric eddies with the air sea interface is needed recent investigations have already shown a possible coupling of large structures in atmospheric turbulence and surface ripples this was done using synthetic aperture radar sar imagery of ocean surface and almost simultaneous advanced very high resolution radiometer avhrr imagery of cloud streets at a cold air outbreak the intent of our study is to further validate this hypothesis in a general case of coastal circulation for this purpose we analyze a suite of collocated simultaneous records of airflow radar return and surface elevations from a coastal platform we investigate the influence of large eddies 20 60 min on the turbulent properties of the airflow in the first 2 m above the ocean surface the analysis shows very prominent peaks in the magnitude of 12 to 16 min fluctuations which are further modulated in 20 to 40 min intervals these scales and modulations are characteristic for all variables of interest here the detected scales and their modulation suggest significant interaction of surface layer within the first 1 2 m with large eddies of scales of o 1 and o 10 km the intermittent structure of turbulence responds by alternating contributions from bursts and sweeps the frequency of occurrence of bursts and sweeps also reveals the influence of large structures the instantaneous cross correlation between the shorter scales of momentum flux and radar return corresponding to the individual burst events can be 4 times as strong as the overall cross correlation

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

noble gas isotopic systematics of fe ti v oxide ore related mafic ultramafic layered intrusions in the panxi area china the role of recycled oceanic crust in their petrogenesis olivine and clinopyroxene grains have been separated from four large fe ti v oxide ore bearing intrusions panzhihua hongge baima and taihe in the panxi area emeishan large igneous province southwest china for he and ar isotope studies the samples examined revealed extremely low 3 he 4 he ratios 0 078 4 34 ra with the mean value 0 78 ra for gases extracted by stepwise heating this feature combined with low 40 ar 36 ar ratios can be interpreted as due to addition of subduction related fluids and melts that had been stored in the lithospheric mantle for long periods considering the regional geologic history such addition can be attributed to the paleo subduction that occurred along the western margin of the yangtze block during the neoproterozoic the subducted oceanic crust beneath the panxi area underwent eclogite facies metamorphism and subsequent exhumation the infiltration of subduction related melts and fluids into the lithospheric mantle led to enriched isotopic signatures from that of the slightly depleted asthenopheric mantle which has been suggested by the sr nd and pb isotopic data of the emeishan basalts and picrites in addition considerable amounts of eclogitic melts produced by partial melting of eclogite facies oceanic crust extensively contaminated the lithospheric mantle during the late permian partial melting of an upwelling mantle plume that contained an eclogite or pyroxenite component generated the parental fe rich magma that supplied the ore bearing intrusions the combination of these factors may have been the crucial reason that many world class fe ti v oxides deposits are clustered in the panxi area

density of morb eclogite in the upper mantle in order to study the role of eclogite subducted oceanic crust in geodynamics we carried out high pressure experiments on natural mid oceanic ridge basalt morb using a multi anvil apparatus we determined phase relations of morb as a function of pressure and temperature in the temperature range of 1200 1700 c and pressures from 5 to 14 gpa the present results differ from a previous high pressure study of morb earth planet sci lett 77 1986 245 conducted at 1200 c as follows 1 coesite coe or stishovite st appears throughout the studied pressure range and 2 the clinopyroxene garnet cpx gar ratio is smaller particularly in the low pressure range 12 gpa our results show that the calculated bulk density of subducted oceanic crust increases discontinuously at around 9 gpa due to the coesite stishovite phase transition the density is considerably higher than estimates based on the previous experiments at pressures less than 12 gpa thus the role of eclogite in driving subduction should be more important than previously thought on the other hand the density difference between peridotite and eclogite in ascending mantle plumes becomes smaller as temperature increases a result of the increase in clinopyroxene garnet sio2 ratio in eclogite with increasing temperature as eclogite is still denser than peridotite even under subsolidus conditions recycled oceanic crust entrained in ascending plumes must be sustained by the upward flow of ambient peridotite in mantle plumes

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

the timing of crustal extension and the eruption of continental flood basalts the association between mantle plumes continental flood basalt eruption and the extension and rifting of the crust is well recognized1 3 but there has always been controversy as to whether the plume or the rifting was the initiating factor4 5 was rifting triggered by doming above the thermal plume2 3 6 or was rifting the driving force preceding the associated volcanicity7 8 here i show that recent flow by flow mapping of two continental flood basalt provinces the columbia river basalt group and the deccan basalt group allows one to infer the relative timing of volcanicity and crustal extension in both provinces eruption of the main tholeiitic phase preceded significant extension and crustal thinning this suggests that the primary event in the formation of these provinces was the upwelling of a mantle plume and that the subsequent crustal extension rifting and decompression was in part a consequence of the initiation of that plume

can a sheet like low velocity region form an elongated large igneous province 1 high resolution tomography of the lower mantle has revealed the existence of another chemically distinct region with low velocity and a sheet like structure beneath the western pacific on the other hand large igneous provinces lips sometimes have elongated shapes if a sheet like upwelling reaches the earth s surface while maintaining its shape an elongated lip may form in order to test this hypothesis we perform a series of experiments and investigate the stability of a buoyant sheet the experimental results show that the buoyant fluid accumulates at the top of the sheet to form a buoyant cylinder the gravitational instability divides the cylinder into several plume heads we develop a model to explain the growth of the buoyant cylinder and the time scale until instability begins our model shows that a thin sheet like upwelling with a width of 200 km a small density difference from the ambient mantle 10 kg m 3 a nd a high supply rate of buoyant fluid 0 1 m yr 1 can reach the earth s surface while maintaining its shape we thus infer that lips with an elongated shape can be generated by sheet like upwellings the width of the observed sheet like low velocity region beneath the western pacific is 500 km and is marginally sufficient to form an elongated lip

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

the evolution of the mantle s chemical structure abstract the geochemistry of flood basalts and their associated picrites and of komatiites and their associated basalts combined with a theoretical model for the structure of mantle starting plumes can be used to decipher key elements of the geochemical structure of the deep mantle and show how it has varied through time we argue that the thermal boundary layer above the core consisted mainly of depleted mantle similar to the present morb source during the archaean and this was largely replaced between 2 7 and 2 0 billion years ago by enriched mantle to form the oib source we suggest that this change in the nature of the hotspot source reflects a fundamental change in the dominant component of downward convection from cold plumes breaking away from beneath a stable lithosphere during the pre archaean to subduction of lithosphere in the archaean and post archaean mantles

geometry and kinematics of the darjeeling sikkim himalaya india implications for the evolution of the himalayan fold thrust belt abstract the darjeeling sikkim himalaya lies in a zone where the convergence vector between the indian and eurasian plates is almost perpendicular to the himalayan deformation front current outcrop pattern preserves high grade hanging wall rocks of the main central thrust mct and the pelling thrust pt within 5 km of the mountain front u pb shrimp analysis on zircon grains from mct sheet record two 206pb 238u age groups of ca 2575 and ca 943 ma and from the pt 1835 13 ma the lesser himalayan duplex in the footwall of the mct is composed of two duplex systems a structurally higher hinterland dipping duplex and a lower duplex system that varies from hinterland dipping in the north to an antiformal stack in the middle and foreland dipping duplex in the south the foreland dipping horses and reactivation of the roof thrust resulted in large translation of the overlying mct and pelling thrust sheets the duplex also resulted in a plunge culmination forming a broad n s trending anticline a transport parallel restorable balanced cross section reveals a minimum shortening of 450 km 81 south of the south tibetan detachment system and an average long term shortening rate 20 mm yr although the study area lies in a zone of high convergence angle with the deformation front the surface structures do not record the greatest shortening of the orogen the shillong plateau and a tranverse zone lying southeast of the study area have accommodated a part of the convergence related shortening thereby under representing the shortening in the sikkim himalayan fold thrust belt lateral variations in the original width of the lesser himalayan basin may have played a role in partitioning the shortening in the fold thrust belt

shortening structures in eastern and north western himalayan rocks w schwan p s saklani series editor today and tomorrow s printers and publishers new delhi 1980 62 pp 12 00

basalts generated by decompressive adiabatic melting of a mantle plume a case study from the isle of skye nw scotland

isotopic evidence for the origin of the manihiki and ontong java oceanic plateaus abstract pb nd and sr isotopic results for lavas of the cretaceous ontong java and manihiki oceanic plateaus fall well within the modern day oceanic island or hotspot field the data provide no evidence of old continental basements but indicate a major involvement of kerguelen type or em i like mantle in the sources of both plateaus which appear to have probably been formed at least in part by hotspots however the presently active hotspots that pacific plate reconstructions suggest might have been possible plateau sources lack kerguelen type isotopic compositions either these hotspots did not participate in the formation of the two plateaus or if they did kerguelen type material must have been volumetrically much more important early in their existence two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical geochronological and geophysical data the first holds that the plateaus formed cataclysmically in association with surfacing plume heads the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation a near ridge origin appears to be indicated by evidence that most of the pacific plateaus were built largely on relatively young ocean crust however we suggest that a near ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation which should be substantially greater than for ordinary hotspots in either case the lack of hotspot tracks seamount chains attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near ridge setting

petrogenesis and tectonic implications of the iron rich tholeiitic basalts in the hutuo group of the wutai mountains central trans north china orogen abstract the hutuo group located in the wutai mountains area is characteristic of paleoproterozoic strata in the north china craton and plays an important role in establishing evolutionary processes in the trans north china orogen we present petrological observations along with geochemical and nd isotopic data for basalts in the qingshicun and hebiancun formations of the hutuo group these basalts are enriched in tfeo 11 97 18 01 wt consistent with occurrences of iron rich basalt world wide they also display relatively high mgo 4 95 12 25 wt ni 79 121 ppm and cr 37 101 ppm and low in sio 2 41 37 51 95 wt therefore the iron rich character of the hutuo group basalts originated with their parental magma rather than via shallow crustal fractionation at low oxygen fugacity as inferred for many high iron basalts the hutuo group basalts exhibit weak to moderate differentiation of light ree over heavy ree la lu cn 2 61 6 79 with minor or slight eu anomalies eu eu 0 77 1 41 and also show the nb and ti troughs without obvious zr and hf anomalies the nd t values of the basalts range from 1 23 to 0 81 with t dm of 2489 2664 ma the basalts most plausibly originated from the fertile subcontinental lithosphere mantle and in the case of the qingshicun formation basalts also underwent the crustal contamination basalts in the hutuo group also have the high zr hf ratios and present high ti v values of 24 55 and zr y ratios ranging from 4 6 to 13 6 suggesting that they were emplaced in a within plate setting considering the sedimentary associations and detrital fragments in the hutuo and gantaohe groups and bimodal magmatism along the trans north china orogen we propose that 2 2 2 0 ga geological events within the orogen were rift related but did not result from subduction or island arc processes

chemical discrimination of columbia river basalt flows 1 the homogeneity and distinctive chemical composition of individual columbia river basalt flows are illustrated in a series of chemical plots and the procedure for flow identification is outlined the unique characterization of many individual eruptions permits units to be mapped from feeder dike to flow margins across the columbia plateau for hundreds of kilometers and so provides critical constraints on the physical and chemical evolution of this continental flood basalt province

low viscosity mantle blobs are sampled preferentially at regions of surface divergence and stirred rapidly into the mantle the persistence of mantle heterogeneity depends on its density and viscosity and as well as the characteristics of the convecting mantle such as the vigour and possible stratification of convection low viscosity regions stretch and deform more rapidly than their high viscosity counterparts low viscosity regions are also drawn into and hence preferentially sampled at regions of surface divergence such as mid ocean ridges and plume fed hotspots low viscosity heterogeneities in the form of blobs are thus more rapidly stirred into a convecting mantle discrete blobs of primitive or enriched mantle are unlikely to persist for billions of years

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

constraining the geometries of small scale heterogeneities a case study from the mariana region a new array based method for seismic waveform data is developed to detect small scale heterogeneities in the mantle and constrain their possible locations and sizes detecting small scale heterogeneities using scattered waves has been challenging because scattered phases usually suffer from low signal to noise ratios and determining the shapes of such heterogeneities is even more difficult because of isochronal artifacts in the proposed method the problem of the low signal to noise ratio is addressed by adopting dual bootstrap stacking which can detect weak signals even with a limited number of seismic traces as well as by ensuring reproducibility among multiple events the locations of detected scatterers and their sizes and shapes are then estimated using a new kind of hierarchical cluster analysis that could minimize the effect of isochronal artifacts the new method is applied to a previously analyzed data set of mariana earthquakes to facilitate a comparison with conventional methods in the mid to lower mantle beneath the mariana trench region a total of 39 scatterers are identified via s to p scattering with their potential volumes ranging from 8 104 to 1 106 km3

mantle plumes and flood basalts enhanced melting from plume ascent and an eclogite component new numerical models of starting plumes reproduce the observed volumes and rates of flood basalt eruptions even for a plume of moderate temperature arriving under thick lithosphere these models follow the growth of a new plume from a thermal boundary layer and its subsequent rise through the mantle viscosity structure they show that as a plume head rises into the lower viscosity upper mantle it narrows and it is thus able to penetrate rapidly right to the base of lithosphere where it spreads as a thin layer this behavior also brings the hottest plume material to the shallowest depths both factors enhance melt production compared with previous plume models the model plumes are also assumed to contain eclogite bodies inferred from geochemistry to be recycled oceanic crust previous numerical models have shown that the presence of nonreacting eclogite bodies may greatly enhance melt production it has been argued that the eclogite derived melt would react with surrounding peridotite and refreeze however recent experimental studies indicate that eclogite derived melts may have reached the earth s surface with only moderate or even minor modification combined with an assumed 15 vol component of eclogite our models yield a sharp peak in melting of about 1 3 myr duration and volumes of melt that encompass those observed in flood basalt provinces

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

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

viewing a cold air outbreak using satellite based synthetic aperture radar and advanced very high resolution radiometer imagery in this phenomenological study we compare nearly simultaneous visible wavelength advanced very high resolution radiometer avhrr and synthetic aperture radar sar images of a cold air outbreak that occurred off the ice pack in the bering sea the visible image shows the spatial evolution of the cloud structure from the ice edge to several hundred kilometers offshore near the ice edge the clouds form regular linear features known as cloud streets these streets are spaced about 5 km apart 5 times the depth of the boundary layer by 200 km offshore the visible image shows nascent closed cell mesoscale cellular convection mcc with connected disks and smears of white clouds surrounded by arcs and rings of dark cloud free regions the diameters of the disks range between 10 and 15 km the sar image shows the same broadly defined spatial pattern but with a substantial increase in detail uncorrelated with the avhrr image in the area of the cloud streets shown in the avhrr image are two dimensional elongated regions of enhanced radar backscatter called sar streaks with the same cross wind spacing as the cloud streets this suggests the presence of quasi two dimensional roll vortices in the atmospheric boundary layer downstream in the area of proto mcc are broadly spaced expanses of enhanced backscatter that form two dimensional patterns such as connected arcs that have structure and scales similar to the spaces between the clouds the spatial evolution of the streaks and streets broadly conforms to the patterns of downdrafts and updrafts respectively exhibited in modeling studies of cold air outbreaks however the comparison with modeling studies is more successful for the cloud streets than for the sar streaks we suggest that this difference is because the clouds in the avhrr images give a time integrated signal of large scale quasi two dimensional atmospheric turbulence whereas mesoscale radar patterns of surface roughness give a relatively instantaneous view of fundamentally three dimensional structure within the atmospheric boundary layer