the meandering gulf stream as seen by the geosat altimeter surface transport position and velocity variance from 73 to 46 w the surface geostrophic velocity field for the gulf stream region was analyzed for the position structure and surface transport of the gulf stream for 2 5 years of the geosat altimeter exact repeat mission synthetic data using a gaussian velocity profile were generated and fit to the sea surface residual heights to create a synthetic mean sea surface height field and profiles of absolute geostrophic currents most of cross track velocity variance was due to the meandering of the gulf stream rather than to rings or complicated velocity profiles which suggested that the simple model parameters are an efficient description of the gulf stream fluctuations an analysis of the model parameters and the actual geostrophic velocity profiles revealed two different flow regimes for the gulf stream connected by a narrow transition region coincident with the new england seamount chain the upstream region was characterized by relatively straight gulf stream paths long eulerian time scales and eastward propagating meanders the downstream region had more large meanders no consistent propagation direction and shorter eulerian time scales a 25 reduction in surface transport occurred in the transition region with a corresponding reduction in current speed and no change in gulf stream width a significant anomaly in surface transport near 69 w suggested the existence of small scale recirculation gyres there the highest variance empirical orthogonal functions of position and surface transport were significantly correlated with larger surface transports leading more northerly positions by a month

model for the evolution of the avalonian cadomian belt the inception and termination of the ca 670 550 ma avalonian cadomian orogenic cycle are interpreted in terms of the amalgamation and breakup of a late precambrian supercontinent synorogenic arc related volcano sedimentary and granitoid rocks are attributed to oblique southward directed subduction along the margin of the supercontinent after its amalgamation breakup of the supercontinent and inception of the iapetus cycle led to termination of subduction and its replacement by transform activity the resulting closure of late precambrian arc related basins and the development of early cambrian platform successions with acado baltic fauna accompanied the formation of iapetus but took place on the active margin of a preexisting ocean and do not record the evolution of its rifted european flank

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

the jeffers brook diorite granodiorite pluton style of emplacement and role of volatiles at various crustal levels in avalonian appinites canadian appalachians small appinite plutons ca 610 ma outcrop in the peri gondwanan avalon terrane of northern nova scotia with different structural levels exposed field mapping shows that the jeffers brook pluton is a laccolith emplaced along an upper crustal thrust zone likely in a dilational jog in a regional dextral strike slip system the oldest rocks are probably mafic sills which heated the area facilitating emplacement of intermediate magmas cross cutting relationships show that both mafic and intermediate magmas were supplied throughout the history of pluton emplacement the modal composition mineral chemistry and bulk chemistry of gabbro diorite tonalite granodiorite and granite have been studied in the main plutonic phases dykes and sills and mafic microgranular enclaves as with the type appinites in the scottish caledonides the pluton shows evidence of high water content the dominance of hornblende locally within pegmatitic texture vesicles and irregular felsic patches in enclaves and late aplite dykes analyzed mafic microgranular enclaves are geochemically similar to larger diorite bodies in the pluton tonalite granodiorite is distinct from the diorite in trace element geochemistry and radiogenic isotopes elsewhere to the east similar rocks of the same age form vertically sheeted complexes in major shear zones hornblende chemistry shows that they were emplaced at a deeper upper crustal level this implies that little of the observed geochemical variability in the jeffers brook pluton was developed within the pluton the general requirements to form appinites are proposed to be small magma volumes of subduction related magmas that reach the upper crust because of continual heating by mafic magmas moving through strike slip fault pathways and trapping of aqueous fluids rather than venting through volcanic activity

regional tilt of the mount stuart batholith washington determined using aluminum in hornblende barometry implications for northward translation of baja british columbia we have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum in hornblende ah barometer the method is used to correct previously published paleomagnetic data from the 93 96 ma mount stuart batholith of the cascades mountains washington state for the effects of postemplacement tilting ah barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressureestimation high contrastback scatteredelectronimagingwas usedtoensurethattheanalyzedhornblendeswerenotsignificantly affected by subsolidus alteration the success of the ah barometry is indicated by two observations first increases in the al content of the hornblendes are governed almost entirely by a pressure sensitive tschermak type substitution second amphibole plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions 650 c and that temperature has a negligible effect on our pressure estimates ah barometry results indicate that the depth of crystallization across the batholith decreases systematically from 0 3 gpa in the northwest to 0 15 gpa in the southeast consistentwithindependentbarometryforthecontactaureole ofthebatholithandregionalstructuralandstratigraphicrelations using a best fit planar tilt model and bootstrap analysis of uncertainties we estimate that the paleohorizontal plane has a strike of 43 630 4 and dip of 7 62 0 southeast 695 confidence our estimated paleohorizontal allows us to restore the paleomagnetic data of beck et al 1981 and to estimate the original paleolatitude of the mount stuart batholith beck et al found that the southern part of the batholith yielded a number of sites with a well defined high coercivity remanence the carrier of this remanence was not resolved but the following four lines of evidence stronglysuggestthatthepublisheddirectionswereacquiredshortly after emplacement of the batholith 1 the stable sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our ah results and concordant k ar ages for hornblende biotite pairs 2 the high coercivity component was always normal in polarity which is consistent with emplacement of the mount stuart batholith at the end of the cretaceous long normal 3 sites from the batholith and the contact aureole gave similar directions 4 the directions show no indication of tilt related smearing after restoration the paleomagnetic data indicate 42 6 11 clockwise rotation and 3100 6 600 km of northward offset

minas fault zone late paleozoic history of an intra continental orogenic transform fault in the canadian appalachians abstract the minas fault zone mfz defines the boundary between the avalon and meguma terranes in the canadian appalachians and is exposed in mainland nova scotia and southern new brunswick these terranes originated along the gondwanan margin but had accreted to laurentia by the middle devonian the surface trace of the mfz is adjacent to the southern margin of the latedevonian permian maritimes basin the late devonian late carboniferous evolution of the mfz involves several episodes of oblique dextral shear that resulted in basin formation and inversion and at various times the zone was the focus of magmatism regional fluid flow and mineralization in the late devonian early carboniferous asymmetric rifting accompanied by dextral shear produced two coeval sequences the horton group which is dominated by continental clastic strata and the fountain lake group which consists predominantly of bimodal volcanic rocks that overlie high level plutons emplaced along active shear zones the overall tectonic environment may have been dominated by dextral transtension along the southern margin of laurentia which corresponded with the northern flank of the rheic ocean a major change in the evolution of the minas fault zone occurred in the late mississippian early pennsylvanian and produced the e w chedabucto fault clockwise rotation of pre existing structures local zones of transtension and transpression as well as regional fluid flow and extensive mineralization this major change may reflect the onset of laurentia gondwana oblique collision the effects of which continued into the latest carboniferous with coeval development of flower structures and pull apart basins in zones of local transpression and transtension

tectonic implications of 40ar 39ar hornblende ages from late proterozoic cambrian plutons in the avalon composite terrane nova scotia canada the avalon composite terrane in nova scotia mainly represents a late proterozoic rifted cratonic magmatic arc complex the geo chemistry of the volcanic rocks indicates that most of southeastern cape breton island is underlain by a volcanic arc whereas the antigonish and cobequid highlands expose intra arc rift sequences in cape breton island late proterozoic rocks are both volcanic and plutonic south of the macintosh brook fault transecting the boisdale hills whereas they are mainly plutonic rocks north of the fault these igneous units are unconformably overlain by subaerial to shallow marine cambrian ordovician sedimentary sequences containing an atlantic fauna that are interbedded with early to middle cambrian within plate continental rift volcanic rocks 4o ar 39 ar plateau ages recorded by hornblende from plutons spatially associated with these volcanic rocks correspond to three groupings 1 ca 600 635 ma in cape breton island south of the macintosh brook fault 2 ca 525 555 ma in cape breton island north of the macintosh brook fault boisdale hills north mountain and creignish hills 3 ca 600 625 ma in the antigonish and cobequid highlands based upon the low metamorphic grade of the late proterozoic and early paleozoic rocks and the low pressures recorded in contact aureoles and plutons it is inferred that these plutons were emplaced at relatively shallow crustal levels and therefore the 40 ar 39 ar plateau ages probably closely date their emplacement interpretation of the age data suggests that plutonism in nova scotia associated with the rifted magmatic arc lasted from ca 635 600 ma igneous activity continued between ca 555 525 ma and may have been related to the cambrian within plate rift volcanism

topex poseidon derived space time variations of the kuroshio current applications of a gravimetric geoid and wavelet analysis summary using topex poseidon t p altimeter data and a gravimetric geoid we derived 7 year time series of kuroshio s volume transport axis velocity width and axis locations northeast and southeast of taiwan for northeastern and southeastern taiwan the mean volume transports are 19 and 26 sv the mean widths are 112 and 96 km and the mean axis velocities are 50 and 82 cm s 1 respectively the t p derived volume transports agree well with in situ result drifter d21160 almost passes through the t p derived axes of the kuroshio the time series show intraseasonal semi annual annual and interannual variations but the frequency characteristics at the two locations are different owing to monsoon bottom topography and other factors wavelet analyses show that the amplitudes and phases of the frequency components vary with time a biennial component is found in almost all time series and may be excited by the tropical quasi biennial oscillation of the atmosphere the volume transport northeast of taiwan has a positive correlation 0 6 with enso with a 1 month lag and both the volume transport and velocity southeast of taiwan have a negative correlation 0 6 with enso with a 9 10 months lead

anatomy and evolution of a pull apart basin stellarton nova scotia the stellarton basin is a late paleozoic pull apart basin located close to the meguma avalon terrane boundary in the canadian appalachians at the stepover between the cobequid and hollow strike slip faults the basin contains 3 km of rapidly deposited pennsylvanian clastic sedimentary rocks representing lacustrine and deltaic environments extensively documented through coal related mining and drilling coal seams and oil shales allow stratigraphic correlation within the basin permitting reconstruction of basin subsidence and structural evolution coal seams represent approximately paleo horizontal surfaces thickness variations corrected for tilt and compaction in the most coal rich part of the basin fill show that the south basin margin subsided rapidly during deposition acting as a trap for coarse sediment and allowing coal forming mires to develop in the north abundant soft sediment deformation structures reflect synsedimentary tectonic activity north striking normal faults dissected the basin fill during and soon after deposition early in the diagenetic history contouring of mine plans allows fault heaves to be identified and also shows that both coal seams and faults were folded by east to northeast trending folds consistent with 1 an environment of deformation involving dextral strike slip motion and 2 clockwise rotation of fault blocks during progressive strain later north northeast striking contractional structures within the basin and a positive flower structure at the northwest margin probably represent a transition from transtension to transpression the basin illustrates the role of rotational progressive strain in the reorientation of structures generated during transtension it displays evidence for changes in tectonic style through time from transtension to transpression that may typify basins developed on major strike slip faults

ordovician appinites in the wugongshan domain of the cathaysia block south china geochronological and geochemical evidence for intrusion into a local extensional zone within an intracontinental regime abstract palaeozoic mafic igneous rocks are potentially significant in constraining the tectonic nature and evolution of the kwangsian orogeny in the eastern south china block yet they have received little attention because of their limited outcrop geochemistry and geochronology was carried out on newly identified ordovician ultramafic mafic appinites in the wugongshan domain of the cathaysia block seven appinite samples yielded 206pb 238u crystallisation ages ranging from 452 4 ma to 473 3 ma abundant 480 500 ma zircon xenocrysts and or inherited zircons were found in the appinites possibly indicating an earlier magmatism episode in the early palaeozoic period the wugongshan appinites are ultramafic to mafic in composition and the ultramafic rocks display features of cumulates high concentrations of mgo fe2o3t cr ni and low concentrations of total alkali and total rare earth elements ree the appinite geochemistry displays relatively flat chondrite normalised ree patterns with slight enrichment in light ree and weak negative eu anomalies enrichment in large ion lithophile elements such as rb k and weak depletion in nb ta in primitive mantle normalised trace element patterns we suggest that the wugongshan appinites likely originated from an ancient metasomatised mantle and that crustal assimilation fractional crystallisation afc magma mingling and hydration were involved in the petrogenetic process based on the combination of geochemistry crust like bulk sr nd and zircon hf isotopic compositions end t 8 2 to 3 2 initial 87sr 86sr ratios of 0 7067 0 7144 zircon ehf t values peaking at 9 to 3 and regional geological data further considering the alignment and chronology of the appinites we suggest that the appinitic magmas probably were emplaced along the jiangshan shaoxing fault in a local extensional zone in an intracontinental regime in the early palaeozoic

a comparative analysis of pre silurian crustal building blocks of the northern and the southern appalachian orogen the new york promontory serves as the divide between the northern and southern segments of the appalachian orogen antiquated subdivisions distinct for each segment implied that they had lithotectonic histories that were independent of each other using new lithotectonic subdivisions we compare first order features of the pre silurian orogenic building blocks in order to test the validity of the implication of independent lithotectonic histories for the two segments three lithotectonic divisions termed here the laurentian iapetan and the peri gondwanan realms characterize the entire orogen the laurentian realm composed of native north american rocks is remarkably uniform for the length of the orogen it records the multistage neoproterozoic early paleozoic rift drift history of the appalachian passive margin formation of a taconic seaway and the ultimate demise of both in the middle ordovician the iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the iapetus ocean between laurentia and gondwana in the northern segment the realm is divisible on the basis of stratigraphy and faunal provinciality into peri laurentian and peri gondwanan tracts that were amalgamated in the late ordovician south of new york stratigraphic and faunal controls decrease markedly rock associations are not inconsistent with those of the northern appalachians although second order differences exist exposed exotic crustal blocks of the peri gondwanan realm include ganderia avalonia and meguma in the north and carolinia in the south carolinia most closely resembles ganderia both in early evolution and late ordovician silurian docking to laurentia our comparison indicates that to a first order the pre silurian appalachian orogen developed uniformly starting with complex rifting and a subsequent drift phase to form the appalachian margin followed by the consolidation of iapetan components and ending with accretion of the peri gonwanan ganderia and carolinia this deduction implies that any first order differences between northern and southern segments post date late ordovician consolidation of a large portion of the orogen

tectonic influence on late proterozoic avalonian magmatism an example from the greendale complex antigonish highlands nova scotia canada

seasat altimeter observations of dynamic topography in the gulf stream region individual seasat altimeter profiles in the western north atlantic have been differenced with the best available geoid model to remove the gravitational component the resulting sea surface height profiles compare remarkably well with independent oceanographic observations the gulf stream is clearly apparent in each profile as a 1 to 2 m step and known positions of cyclonic and anticyclonic rings correspond with depressions and elevations respectively with amplitudes as large as 95 cm some of the most important altimeter data for analyzing the dynamic ocean signal were gathered during the last month of the mission when the same ground track was repeated every 3 days these data allow detailed examination of time varying ocean phenomena since the gravitational component is time invariant one set of collinear passes clearly shows a cyclonic ring as it moves out from underneath the spacecraft s track an event which was simultaneously observed with the aid of a satellite tracked surface buoy another striking feature seen in the altimeter data set is the apparent variability exhibited by the gulf stream on time scales of a few days surface transport indicated by the sea surface height difference across the stream varied by nearly 30 and over the entire 3 month period much larger fluctuations were observed suggesting significant changes in total mass transport altimetry may provide an effective means of determining the time and space scales associated with these variations

partial melting of an ancient sub continental lithospheric mantle in the early paleozoic intracontinental regime and its contribution to petrogenesis of the coeval peraluminous granites in south china abstract the appinite granite association has been found in various tectonic regimes related to recent subduction arc continent or continent continent collision and post collision orogen and appinites generally originate from recently subduction modified lithospheric mantle we conducted a study on a rarely reported appinite granite association formed in an intracontinental regime the zhangjiafang qinglongshan complex zqc in which the appinites were derived from an ancient sub continental lithospheric mantle sclm the zqc is located in the western wugongshan domain and consists of basaltic to intermediate appinites and granitoids ten dated samples including massive and gneissoid granitoids hornblende gabbro and diorite give zircon 206pb 238u ages ranging from 444 3 ma to 452 4 ma indicating that these various lithologies were emplaced synchronously the basaltic appinites show radiogenic 87sr 86sri 0 71016 0 71431 and negative end t 6 1 to 8 9 and zircon ehf t 4 2 to 7 5 values combined with regional geological background an origin from the neoproterozoic metasomatised sclm can be inferred for the appinites in the wugongshan domain the granitoids are peraluminous and almost high k calc alkaline to shoshonitic they exhibit a wide range of isotopic compositions 87sr 86sri 0 70828 0 71857 end t 6 2 to 10 5 zircon ehf t 9 5 to 26 6 some of the granitoids display the most evolved sr nd isotopic signatures among all the studied lithologies which are consistent with those of the middle to upper crust suggesting a pure crustal origin other granitoids show relatively mafic composition and less evolved isotopic signature the intermediate appinites have intermediate chemical compositions between those of the basaltic appinites and granitoids and similar sr nd isotopic compositions to those of the basaltic appinites that have relatively evolved composition and isotopic signature the petrographical and the geochemical studies as well as magma mixing and mingling between mafic magma and felsic magma observed in field work indicate that the zqc was formed by several complex magmatic processes including assimilation and fractional crystallization afc of a basaltic magma derived from an enriched mantle magma mixing and mingling fractional crystallization of a crust derived felsic magma the contribution of the neopreoterozoic subduction modified lithospheric mantle to the early paleozoic granitoids in south china has been poorly constrained due to its evolved isotopic signatures such as negative end t and ehf t values high initial ratio of 87sr 86sr and few studies on the early paleozoic basaltic felsic rock association our work here suggests that the wugongshan appinite granite association was formed in an intracontinental orogen and the ancient enriched mantle may have had a significant chemical contribution to the formation of some of the paleozoic peraluminous granites in south china

sea level variabilities in the gulf stream between cape hatteras and 50 w a geosat study sea level residuals relative to a 2 year mean sea level in the gulf stream downstream of cape hatteras between 80 w and 50 w longitude are studied using geosat altimetry between november 1986 and december 1988 spectral slopes computed at four nominal locations along the gulf stream 72 w 65 w 60 w and 55 w range between 1 5 and 0 9 significantly less steep than the 2 5 spectral slope found at bermuda because of increased short wavelength energy the spectra tend to become more white going eastward but this trend is not statistically significant energy at the annual cycle decreases eastward from around 40 of the total variance at 72 w to 20 of the total variance at 55 w maximum residual geostrophic sea surface velocities associated with the annual cycle were of the order of 70 cm s comparison between the gulf stream path from sea level residual maps and the north wall edge observed from noaa infrared ir imagery shows an rms difference of 49 km between 75 w and 60 w longitude but not all sea level maps show a clearly defined gulf stream using the ir estimates for gulf stream position sea level residuals were modeled as the meandering of a sea level front shaped as 0 5 m tanh x x0 50 km sin with the angle between the geosat track and the gulf stream edge and x0 is based on the observed ir north wall positions this showed that the annual signal in terms of the location and phase of its maxima is dominated by the meandering of the gulf stream

sm nd isotopic systematics as tectonic tracers an example from west avalonia in the canadian appalachians abstract the tectonothermal evolution of west avalonia in mainland nova scotia appalachian orogen provides an excellent example of the application of sm nd isotopic analyses to tectonic studies regional syntheses indicate that west avalonia originated in the neoproterozoic as one of a number of terranes collectively known as peri gondwanan terranes along the gondwanan margin these terranes are characterized by late neoproterozoic voluminous arc related magmatism west avalonia became detached from gondwana in the early ordovician was accreted to laurentia by the silurian and was deformed by post accretionary strike slip tectonics in the late devonian and carboniferous associated with the amalgamation of pangea all stages of this evolution can be recognized in the sm nd isotopic signature in various neoproterozoic and paleozoic volcanic and sedimentary successions in mainland nova scotia crustally derived felsic volcanic rocks of late neoproterozoic ca 615 ma early cambrian and early silurian ages define an envelope that provides gross constraints on the sm nd isotopic composition of the underlying basement source e nd values range from 0 78 to 2 92 in the neoproterozoic rocks from 2 0 to 5 0 in the cambrian rocks and from 0 1 to 2 5 in the silurian rocks t dm model ages for each of these suites are remarkably similar they typically range from 0 9 to 1 2 ga suggesting that the sm nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source this sm nd envelope provides a reference by which other suites may be compared and interpreted for example turbidites in a late neoproterozoic volcanic arc basin have sm nd compositions that lie within this envelope suggesting that they are derived from the coeval volcanic rocks that flank the basin the genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre date the main arc phase two felsic complexes have compositions that lie within the envelope and have similar model ages the presence of this signature in early avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca 1 0 to 1 2 ga that formed juvenile proto avalonian crust by implication the main arc phase in avalonia recycled this crust sm nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre dated avalonian arc activity instead the data indicate derivation from two distinct sources ancient gondwanan basement and juvenile avalonian arc suggesting deposition in a western pacific type back arc environment this re interpretation has important implications for neoproterozoic reconstructions the sm nd isotopic composition of west avalonian basement contrasts with that of other peri gondwanan terranes for example similarly aged felsic rocks in the cadomian terrane of nw france are characterized by more negative e nd values and older t dm model ages 1 0 1 9 ga these data together with detrital u pb data suggest that west avalonia was developed along the amazonian margin whereas cadomia was developed along the west african margin of gondwana sm nd isotopic analyses of early silurian clastic rocks in mainland nova scotia show fundamental differences with unconformably underlying neoproterozoic and early paleozoic avalonian successions these data have e nd values ranging from 5 7 to 6 1 with t dm model ages of about 1 4 to 1 7 ga indicating that the clastic rocks cannot have been derived from underlying successions a variety of geologic data and regional constraints suggests that they may have been derived from the caledonide orogen of western europe thereby providing a minimum age for the accretion of west avalonia to laurentia baltica sm nd isotopic analyses from late devonian early carboniferous continental clastic rocks deposited along the southern flank of avalonia in mainland nova scotia also show little influence from avalonian rocks and are compatible with derivation from the meguma terrane immediately to the south these data are consistent with regional synthesis which indicate uplift of the meguma terrane relative to west avalonia during dextral transpression

high pressure metamorphism in mafic schist from northern vermont the only confirmed occurrence of glaucophane and omphacite in the appalachian mountains of new england is in mafic schist from tillotson peak 44 48 72 33 north central vermont mineral assemblages observed all with epidote sphene magnetite phengitic muscovite carbonate calcite or dolomite apatite sulfide are 1 actinolite glaucophane chlorite garnet quartz 2 actinolite glaucophane chlorite albite garnet 3 actinolite glaucophane omphacite garnet 4 glaucophane chlorite garnet quartz 5 glaucophane omphacite quartz 6 albite glaucophane chlorite quartz and 7 albite paragonite chlorite quartz electron microprobe investigations show that plagioclase chlorite sphene white mica carbonate and magnetite grains are relatively homogeneous however amphibole pyroxene epidote and garnet grains are zoned indicating incomplete equilibration and changing physical conditions during metamorphism glaucophane in assemblage 6 has the general formula rim composition na 1 9 m4 ca 0 1 mg 1 8fe 1 2 2 al 1 0 vi fe 0 3 3 si 7 9 al 0 1 iv o 22 oh 2 glaucophane in assemblages 1 to 5 is somewhat poorer in the glaucophane endmember and richer in actinolite ca rich amphibole grains are zoned extensively with barroisite cores and actinolite rims omphacite grains in assemblages 3 and 5 have the general formula rim composition na 0 5 ca 0 5 mg 0 4 fe 0 1 2 al 0 4 vi fe 0 1 3 si 2o 6 in spite of the significant zoning the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition correlation of mineral compositions and assemblages with mineral stability data suggests high pressure facies series metamorphism at about 9 2 kb and 450 100 c with p h2o p total it is proposed that the metamorphism occurred during the ordovician

dynamic topography estimates using geosat data and a gravimetric geoid in the gulf stream region summary a gravimetric geoid undulation on a 3 x 3 grid has been calculated in the gulf stream region 30 5 i 45 278 5 i 5 318 these undulations were calculated using two 360 potential coefficient models land ship and altimeter derived gravity anomalies and bathymetric data least squares collocation and fast fourier transform procedures were used with various data selection and gridding procedures results from five different undulation computations are described with comparisons made with synthetic geoid undulations along geosut tracks in the region the standard deviation of the undulation differences was f14cm when a cubic polynomial was used to model long wavelength errors a point verification of the geoid undulation at the laser tracking station on bermuda was also made with a discrepancy ground truth minus model undulation of 40 cm within the predicted standard deviation the gravimetric geoid undulation was used to compute dynamic topography along geosat tracks for comparison with existing estimates based on hydrographic data in the gulf stream region the agreement between these two estimates is on the order of f15 cm although the discrepancies can reach 60 cm the larger differences are usually associated with a location on a track that passes near a seamount where the gravity data may be inadequate to represent the high frequency variations in the geoid undulations this effect will be represented in the undulation standard deviations that have been calculated using the least squares collocation procedure the average accuracy is fl6cm with the range from f14 to f48cm the dynamic height derived from the gravimetric undulations and altimeter data has been used to calculate characteristics of the gulf stream width velocity centre location height profile using an implied velocity model for the set the results are compared with previous estimates with generally good agreement however the maximum velocities and the jump function are approximately 30 per cent larger than previous studies that used an average of altimeter tracks to define the geoid undulation the dynamic topography was calculated for the entire region using a mean sea surface based on geos 3 seasat and geosat data the results are compared with two hydrographic estimates due to martel wunsch and letraon mercier the agreement is at the f25cm level with the best correlation coefficient reaching 0 72 with the martel wunsch model

mean sea surface and geoid gradient comparisons with topex altimeter data cycles 4 to 54 of topex data have been analyzed through comparisons with the mean sea surface given on the distributed geophysical data record gdr two inverted barometer correction procedures were considered for the data reduction one used a constant atmospheric pressure for all data while the one adopted for use for most computations introduced a cycle average pressure the maximum difference between the two estimates was 3 0 cm with a clear annual signal with the modified correction the topex sea surface was compared to the ohio state university osu mean sea surface given on the gdr to estimate three translations x 2 3 cm y 25 0 cm z 0 3 cm and a bias 43 3 cm between the two surfaces the only significant translation is y which indicates the reference frame of the topex system differs from that used in the osu mean sea surface system the bias between the topex mean sea surface and the osu mean sea surface was used to estimate an equatorial radius of 6 378 136 55 m based on an 18 cm bias estimate of the topex altimeter examination of the average difference by cycle between the topex sea surface and the osu mean sea surface suggested a bias change of 3 1 2 2 mm yr with a positive sign indicating the average ocean surface is rising or the altimeter measured distance is decreasing models were implemented that solved directly for a bias bias rate annual semiannual and tide correction terms these latter two terms were represented by a degree 8 spherical harmonic expansion with the tide corrections being to the components of the cartwright and ray tide model given on the gdr the most complete solution gave a bias rate of 3 6 0 6 mm yr when the osu mean sea surface was used and 3 2 0 4 mm yr when the topex data were used to define a mean sea surface when the bias and bias rate terms were included in the estimation process the annual semiannual terms and the tide terms estimate changed slightly the greatest change 1 mm took place in the annual term the computations indicated that a simultaneous solution for bias bias rate and annual semiannual terms gave the most accurate results nonsimultaneous solutions led to slightly different bias rate values the root mean square difference between the topex sea surface and the osu mean sea surface after translation and bias correction was 17 cm for a typical cycle however some locations were identified where the difference could reach 2 3 m and were repeated over several cycles indicating errors in the mean sea surface most of the large differences occur in regions lacking altimeter data prior to the topex poseidon mission and or areas of significant bathymetrie signature geoid gradients are needed for the reduction of individual track data to a reference track the accuracy of the determination of such gradients was determined through the comparison of predicted from a mean sea surface along track gradients to the observed gradients among four mean sea surfaces tested the best agreement 0 9 cm km was found with the osu mean sea surface placed on the topex geophysical data record

nomenclature of amphiboles additions and revisions to the international mineralogical associations amphibole nomenclature the introduction of a fifth amphibole group the na ca mg fe mn li group defined by 0 50 b mg fe 2 mn 2 li 1 50 and 0 50 b ca na 1 50 apfu atoms per formula unit with members whittakerite and ottoliniite has been required by recent discoveries of b lina amphiboles this and other new discoveries such as sodicpedrizite which is herein slightly but significantly changed from the original idealized formula necessitate amendments to the ima 1997 definitions of the mg fe mn li calcic sodic calcic and sodic groups the discovery of obertiite and the finding of an incompatibility in the ima 1997 subdivision of the sodic group requires further amendments within the sodic group all these changes which have ima approval are summarized

a study of the kuroshio s seasonal variabilities using an altimetric gravimetric geoid and topex poseidon altimeter data the first year of topex poseidon t p altimeter data were used to study the seasonal variabilities of the kuroshio current the intercomparison between the t p and gauge sea levels at selected tide gauge stations around taiwan shows that the two have a correlation of about 0 9 at the immediate vicinity of the deep ocean and both show an annual cycle with an amplitude of 15 cm a 3 3 geoid for the western pacific was constructed by least squares collocation using gravity anomalies and sea surface gradients derived from seasat geosat ers 1 and t p altimetry to account for the oceanographic signal in the altimeter data we derived a covariance function for the gradients of the sea surface topography sst based on a global averaging concept and a spherical harmonic expansion of the levitus sst to degree 50 the accuracy of the geoid ranges from 5 to 40 cm the sst values derived from the geoid and the data along t p s descending tracks were fitted to the hyperbolic functions in a two step procedure to find the kuroshio s seasonal axes and the parameters that describe its characteristics the mean variabilities in the kuroshio s path maximum velocity baroclinic transport and width are 22 km 19 cm 8 sv and 27 km respectively the averaged percentage variability of all quantities is 25 all the variabilities are relatively large near the northeast coast of taiwan and the starting point of the kuroshio extension the large scale circulations over the western pacific were obtained by median filtering the sst at the 1334 km spatial scale the pacific s subtropical gyre is clearly visible and it shows different features over the four seasons and its kinetic energy shows possible correlations with the determined parameters over the south china sea a warm ring with a radius of 300 400 km and a center at 15 n 113 e was detected in the spring and summer

microchemistry of amphiboles near the roof of a mafic magma chamber insights into high level melt evolution abstract the late neoproterozoic greendale complex located within the avalon terrane of nova scotia is a suite of appinitic rocks ranging from ultramafic to felsic in composition that were intruded during regional ensialic arc magmatism and crystallized at shallow crustal levels under conditions of high ph 2 o amphibole is the dominant mafic mineral in ultramafic to mafic rocks and displays the extraordinary variability in texture and modal abundance that is characteristic of appinite suites these features allow sensitivity of amphibole composition major trace and ree to the evolution of water rich magma to be investigated all amphiboles in mafic and ultramafic rocks are calcic with ca na b 1 34 and si iv between 6 1 and 7 3 they predominantly range in composition from tschermakite to magnesiohornblende and display a dominance of edenite na k a al iv si iv substitution although each sample exhibits remarkably uniform mg over a wide range in si of up to one formula unit the mafic rock amphiboles are characterized by lower 0 5 to 0 7 mg compared to the ultramafic rocks 0 7 and 0 9 ree profiles are bow shaped and are characterized by depletion in lree la sm 0 61 a slight depletion in hree gd yb 1 55 as well as a negative eu anomaly which is attributed to co precipitation of plagioclase ree and trace element profiles of ultramafic amphiboles are divided into two groupings group a amphiboles occur in all specimens analyzed and their ree profiles are very similar to the whole rock analyses of the mafic rocks and to those predicted from amphibole melt partition coefficients in contrast group b amphiboles display relative enrichment in light rees la sm 2 05 have lower ree and lack a negative eu anomaly relative to sm and gd group b amphiboles are more enriched in th and u and show a more pronounced depletion in nb ti y and hree group b amphiboles probably grew in a reaction relationship with olivine and pyroxene and their contrasting profiles suggest that partition coefficients traditionally used for petrogenetic modeling may not be appropriate for such amphiboles groups a and b are virtually indistinguishable with respect to the major elements used for amphibole classification suggesting that ree and selected trace element data when combined with textural observations may provide important additional insights into phase equilibria and conditions of crystallization

u pb geochronological constraints on precambrian stratified units in the avalon composite terrane of nova scotia canada tectonic implications concordant u pb ages of detrital zircons in several proterozoic units in the avalon composite terrane of nova scotia along with previously published data indicate that deposition of low and high grade stratified units in the cobequid highlands gamble brook formation and central cape breton island variously assigned to the george river group or george river metamorphic suite and the bras d or gneiss started after 977 ma extended through a period of volcanic activity between 694 and 637 ma and ended before the 629 612 600 ma volcanic activity associated with deposition of the georgeville and jeffers groups in the antigonish and cobequid highlands and the east bay hills and coxheath groups in southern cape breton island these age data along with u pb analyses of metamorphic and igneous zircon and monazite also constrain three late proterozoic tectonothermal events i oblique sinistral ductile shear deformation and greenschist amphibolite facies metamorphism probably occurred between 694 an

neoproterozoic rosetta gabbro from northernmost arabian nubian shield south jordan geochemistry and petrogenesis abstract an ediacaran mafic intrusion of south jordan is a distinctive appinitic igneous rock with a possibly unique texture characterized by spherical clots up to 40 mm in diameter composed of amphibole cores from which plagioclase euhedra radiate we call it the rosetta gabbro it is exposed as a small ca 750 m 2 outcrop in the neoproterozoic basement of south jordan a second outcrop of otherwise similar gabbro is located about 400 m to the north of the rosetta gabbro but it lacks the distinctive texture the rosetta gabbro could represent a magma pipe it intrudes the aqaba complex 600 ma granitoids and metasediments of the janub metamorphic complex 633 617 ma the gabbro is an ol to qz tholeiite with the following chemical characteristics sio 2 46 2 47 8 wt al 2 o 3 16 4 17 7 wt tio 2 1 70 2 82 wt na 2 o 1 27 2 83 wt k 2 o 0 82 1 63 wt mg 58 63 ree 70 117 ppm la yb 6 to 8 and eu eu 1 05 1 2 the investigated gabbro has the geochemical features of a continental flood tholeiitic basalt emplaced in a within plate tectonic setting two varieties of amphiboles are found 1 large 3 5 mm brown ferri titanian tschermakite k 0 09 na 0 28 na 0 20 ca 1 80 mn 0 04 fe 3 1 1 mg 2 34 fe 2 0 90 ti 0 29 al 0 22 al 1 85 si 6 15 o 22 oh 1 95 of the calcic amphibole group which is riddled with opaques and 2 acicular yellowish light green ferrian magnesiohornblende k 0 04 na 0 153 ca 1 755 na 0 245 fe 3 0 66 mn 0 01 fe 2 1 01 mg 3 03 ti 0 06 al 0 22 al 1 03 si 6 97 o 22 oh 1 95 scattered flakes of phlogopite also occur tabular radiating plagioclase an 64 79 are complexly twinned with broad lamellae that show no zoning laser ablation icp ms analyses of amphibole and plagioclase reveal considerable variation in trace element abundance in spite of more subtle major element variations except for tio 2 in amphibole the ree in the amphibole shows an order of magnitude variation with a concave downward pattern and a positive eu anomaly eu eu 0 6 2 though far less pronounced compared to the eu eu 5 45 of plagioclase the 3d dandelion like texture of the rosettas is broadly similar to chrysanthemum stone which is a diagenetic growth in sedimentary rock but we can find no description of similar textures in igneous rocks the formation of the rosettas is thought to reflect loss of magmatic water resulting in supersaturation of plagioclase which grew rapidly around amphibole and may have floated in the magma this implies magmatic evolution in shallow 10 to 12 km deep crust where temperatures were nevertheless in the range of ca 750 to 900 c

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

gulf stream surface transport and statistics at 69 w from the geosat altimeter geosat altimeter height measurements for the gulf stream for three subtracks from the exact repeat mission were processed to remove the geoid and mean gulf stream height a gaussian model for the gulf stream velocity profile was used to predict the form of the residual height profiles seen by geosat and the mean height profile using estimates of the width and position derived from the height residuals estimates of the maximum surface velocity and the surface transport were obtained by a least squares fit of the synthetic height profiles to the data location statistics from the geosat data were in good agreement with values obtained from infrared images of the gulf stream maximum surface velocities were typically between 1 2 and 2 0 m s 1 the time series showed that surface transport is correlated primarily with the magnitude of the surface velocities although an anomalously large increase in surface transport in the winter of 1987 1988 was caused by an increase in the width of the gulf stream an analysis of the average annual signal in transport showed maximum values in the late fall and minimum transport in the late spring maximum minimum surface transport generally occurred when the gulf stream was north south of its mean position

geochemical and nd pb isotopic characteristics of the tethyan asthenosphere implications for the origin of the indian ocean mantle domain abstract it is unclear why the pb nd and sr isotopic composition of the modern mid ocean ridge basalts morb from the indian ocean is different from that of the north atlantic and pacific oceans a possible explanation for this is that the indian morb type isotopic signature is a long lived regional feature of the mantle as evidently shown by the isotopic composition of the 350 ma morb like mian lue northern ophiolite which was formed in the same region presently occupied by the indian ocean however this hypothesis is in conflict with the lack of indian morb type isotopic signature in a number of 150 ma tethyan and indian ocean crusts to further constrain the origin of the indian morb type isotopic signature we analyze the geochemical and pb nd and sr isotopic composition of representative mafic rocks from four tethyan ophiolites ranging in age from 90 to 360 ma the sr isotopic composition of the samples is unreliable due to alteration but the age corrected nd and pb isotopic ratios and geochemical data indicate that these tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear indian morb type isotopic signature we therefore conclude that the bulk of the indian suboceanic mantle was most probably inherited from the tethyan asthenosphere a few regions in both the tethyan and indian oceans however are most probably underlain by pacific and north atlantic morb type mantle and vice versa because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins the indian morb type isotopic signature of the western pacific marginal basin crusts could be due to either flow of the indian ocean mantle into the western pacific or to endogenous production of such an isotopic signature from delaminated east asian sublithospheric materials during closure of the tethys ocean

regional tilt of the mount stuart batholith washington determined using aluminum in hornblende barometry implications for northward translation of baja british columbia discussion and reply

the scales of mantle convection seismic topographic and gravity data show that there are two important scales of mantle convection these are associated with spherical harmonic degrees l 2 and 6 the l 2 pattern corresponds to the pattern of subduction cooling since the breakup of pangea most hotspots and ridges occur in the half of the globe unaffected by this cooling and all large igneous provinces were generated over this part of the mantle the l 2 distribution of upwellings and downwellings is likely to be a long lived feature of the earth cold regions of the mantle repeatedly attract continents and subduction reinforces the coldness an l 1 pattern of convection is probably related to supercontinents and their breakup degree 6 convection shows up in the spectrum of hotspots and upper mantle tomography and in the correlation of tomography and topography with the geoid cratons with their deep cold keels control the l 6 pattern and may even cause it by their role in establishing lateral temperature gradients and relief at the top of the convecting mantle subduction zones reinforce the craton pattern downwellings preferentially occur under cratons upwellings and hotspots occur at complementary locations supercontinents and their associated subduction zones l 1 constantly assemble and reassemble in the african atlantic hemisphere and the continental fragments in the dispersed state periodically settle into the polar band of geoid lows l 2 and 6 that now includes the americas antarctica australia and india if cratons control the l 6 pattern of convection and many patterns of l 6 are possible e g sectoral zonal checker board then upper mantle convection may reorganize roughly every 30 ma as cratons move about the hotspot spectrum is probably related to lithospheric extension as well as to broad upwellings a third and smaller scale of convection order 400 1000 km in dimension is just below the resolution of global tomography but shows up in the gravity field geoid and topography this scale is controlled by the depth of an endothermic phase change which tends to stratify mantle convection and the thickness of the upper mantle low viscosity zone convective domains of this dimension are also implied by the scales of chemical homogeneity lengths of rifts ridges and seamount chains fracture zone spacing and mid ocean ridge segmentation the temperature geochemistry and fertility of these upper mantle scale domains is controlled by their previous history of subduction continental insulation or refrigeration and processing by ridges fertile or volatile rich or hot cells can be mistaken for plumes hotspot swells are typically of the dimension that we argue is a characteristic upper mantle scale rather than a deep mantle plume scale geochemical domains of various sizes exist in the upper mantle they are broad scale features rather than point sources as in plume theories lithospheric dynamics and geometric focusing not mantle dynamics control the dimensions of so called hotspot eruptives

ach uaine hybrid appinite pipes evidence for mantle derived shoshonitic parent magmas in caledonian granite genesis the ach9uaine hybrids of northern scotland form a suite of caledonian minor intrusions with a wide range of cogenetic compositions ranging from mantle derived mafic shosonite chemically similar to the lamprophyres of the region through syenites comparable to the rare caledonian syenites to granites typical of the local newer granites they therefore provide a hitherto unsuspected direct link between these major caledonian magma types and suggest that the newer granites in the northern highlands may have mantle derived parental components of subduction related shoshonitic affinity shoshonite to granite fractionation is thought to have proceeded within a compositionally stratified caledonian magma chamber chemical data including rare earth element analyses demonstrate the control of crystal fractionation but the wide range of silica saturation may imply a significant crustal input

estimating the surface transport of meandering oceanic jet streams from satellite altimetry surface transport estimates for the gulf stream and kuroshio extension abstract a method has been developed to detect the location width maximum velocity and suffice transport of a meandering oceanic jet stream from satellite altimetry ale method is based on a nonlinear least squares scheme in which gaussian shaped velocity profiles are fitted to explain the observed current variability in the meandering region the method has been applied to 25 geosat tracks in the kuroshio extension and gulf stream for the first you of the geosat exact repeat mission a reasonably smooth circulation pattern has emerged for the surface transport for both currents at opposed to the widely varying direct transport estimates after leaving the coast both currents have surface strength of the order of 90 103 m2 a 1 equivalent to a 90 cm height jump i e the sea surface height difference across the jet they continue to gain strength downstream until a maximum of the order of 130 10 3 m2 a 1 is reached near 150 e for the kuroshio and 63 w for the gulf stream the gulf stream the

development of oceanic detachment and asymmetric spreading at the australian antarctic discordance 1 the largest known oceanic detachment terrains occur in segment b3 of the australian antarctic discordance aad using newly collected bathymetry magnetic and gravity data we show that segment b3 is divided into two contrasting second order segments the western subsegment b3w is characterized by well ordered ridge parallel abyssal hills and low mantle bouguer gravity anomalies the eastern subsegment b3e displays rough chaotic morphology and includes several megamullions characterized by high mantle bouguer gravity anomaly values the crust is estimated to be thinner by a maximum of 3 km in southern b3e the combination of chaotic morphology with thinner crust supports the idea that the megamullions are exposed footwalls of oceanic detachments megamullion terrains are characterized by higher magnetization than adjacent terrains most likely as a result of serpentinization of peridotite exposed at the detachment surfaces detachment surfaces constitute up to 70 of the total area of both ridge flanks younger than 2 ma in b3e indicating that oceanic detachments have played a major role in its development spreading in b3e has been extremely asymmetric with higher apparent rates associated with the large detachment surfaces where up to 75 of the total extension occurred similar asymmetric spreading on oceanic detachments is also recognized in segment b4 suggesting that this is the dominant mode of extension associated with cold mantle and low magma supply in this deepest part of the aad where it is confined to a mere 100 km long section of the aad spreading axis

natural experimental charges an ion microprobe study of trace element distribution coefficients in glass rich hornblendite and clinopyroxenite xenoliths abstract igneous glasses found in hornblende and clinopyroxene rich xenoliths represent former melt pools and contain important information about magmatic processes in mafic alkaline magmas rare glass bearing hornblendite and clinopyroxenite xenoliths entrained in intraplate alkali basalts from germany and israel contain up to 11 interstitial glass surrounded by euhedral crystals of mafic minerals hornblende clinopyroxene phlogopite these xenoliths represent natural experimental charges of alkaline magma quenched by rapid eruption whilst crystallising mafic mineral phases bulk rock compositions of the xenoliths are basanitic considered as representing the starting compositions of the natural experiment whereas the interstitial glasses are chemically more evolved highly alkaline foidites as a result of closed system crystallisation of mafic minerals ion probe trace element data for glass and mineral phases hornblende clinopyroxene phlogopite and apatite yield in situ distribution coefficients kds between magma and minerals the results generally confirm distribution coefficients derived from experimental data and from melt bearing peridotite xenoliths but tend to be slightly higher perhaps because of the evolved nature of the melt disruption of these xenoliths which must have been semi consolidated when entrained may help to explain the origin of clinopyroxene amphibole and phlogopite megacrysts that are commonly observed in alkali basalts

terrestrial and planetary gravity fields the determination of the gravity field together with knowledge of the surface topography provides one of the primary means of inferring the density structure and dynamics of planetary interiors phillips and lambeck 1980 hager 1985 after removing the topographic gravity signal the distribution of internal density anomalies caused by thermal or compositional differences can be studied gravity field models can also be used to study the compensation of surface topography which can provide information on the mechanical properties and state of stress of the lithosphere the determination of these models has been accomplished using a wide variety of different measurement types and solution techniques the near earth measurement types can be divided into satellite tracking measurements surface gravity measurements and satellite altimeter measurements in both near earth and interplanetary contexts tracking data are used to measure gravitational perturbations affecting satellites with the accuracy and spatial temporal distribution of the data being the most important factors in the resulting field accuracy and resolution surface gravity data provide a more direct measurement of the gravity field but acquiring data uniformly over the earth has always been difficult satellite altimetry provides precise measurements of the marine gravity field provided that satellite orbit errors and non geoidal sea surface height variations can be adequately modeled comprehensive gravity field solutions must incorporate these disparate data types in order to estimate mathematical parameters describing the gravity field such as spherical harmonic coefficients the proper combination of these data along with the desire to acquire an accurate representation of the model errors require that complex procedures be developed for computing these solutions

geochemistry of the neoproterozoic tilemsi belt of iforas mali sahara a crustal section of an oceanic island arc abstract the pre pan african neoproterozoic tilemsi belt of the western iforas mali sahara comprises a predominantly bimodal mafic and felsic volcanic assemblage associated with numerous intrusions mainly of mafic composition the volcanics include island arc tholeiites and the calc alkali rocks rhyolites dacites and andesites the volcanic domain shows similarities to recent oceanic island arc systems as well as to archaean greenstone belts the tilemsi magmatic sequence represents an example of a section through the middle and upper levels of a neoproterozoic intra oceanic island arc crust that is composed nearly exclusively of igneous material the middle crustal section is mainly of mafic composition while the upper crustal level contains more silica rich plutonic volcanic and volcaniclastic assemblages the mafic intrusions include a large proportion of cumulates indicating that fractional crystallization played an important role during the evolution of the oceanic arc suites compositionally the arc changed through time from an immature to a relatively developed system the rocks became more silica and incompatible element rich with higher sr isotopic ratios this trend reflects the increasing growth and maturity of the arc

regional significance of new u pb age data for neoproterozoic igneous units in avalonian rocks of northern mainland nova scotia canada gondwanan neoproterozoic tectonothermal events pan african and brasiliano are represented in northern mainland of nova scotia by volcanic and sedimentary rocks assigned to the jeffers and georgeville groups and by gabbroic to granitoid plutons these rocks comprise part of avalonia an exotic terrane in the appalachian orogen that was deposited in an arc related environment along the periphery of gondwana prior to accretion to laurentia lavas sampled in the basal units of the jeffers and georgeville groups yielded slightly discordant u pb zircon and monazite data that fall on chords with upper intercept ages of 628 ma and 617 7 1 6 ma respectively syntectonic to late syntectonic plutons intruded into these groups yielded u pb zircon ages of 606 6 1 6 ma and 603 9 5 ma the former intrusion also yielded a concordant titanite age of 607 3 ma when combined with previously published ages these data indicate that the back arc deposition recorded in these groups lasted 10 15 million years 628 613 ma and was closely followed by c 613 595 ma metamorphism intrusion and heterogeneous strike slip related deformation assuming no significant shuffling of fault blocks the relative locations of the cobequid antigonish back arc basin and the southern cape breton island volcanic arc are consistent with their genesis above a north west dipping subduction zone the age range of arc related magmatism in nova scotia is similar to that of avalonian rocks in southeastern newfoundland and britain lending support to hypotheses of neoproterozoic linkages

preliminary estimates of gulf stream characteristics from topex data and a precise gravimetric geoid topex sea surface height data has been used with a gravimetric geoid to calculate sea surface topography across the gulf stream this topography was initially computed for nine tracks seven descending and two ascending on cycles 21 to 29 due to inaccurate geoid undulations on one track results for eight tracks are reported in the paper the sea surface topography estimates were used to calculate parameters that describe gulf stream characteristics from two models of the gulf stream one model was based on a gaussian representation of the velocity while the other was a hyperbolic representation of velocity or the sea surface topography the parameters of the gaussian velocity model fit were a width parameter a maximum velocity value and the location of the maximum velocity the parameters of the hyperbolic sea surface topography model were the width the height jump position and the sea surface topography at the center of the stream both models were used for the eight tracks and nine cycles studied comparisons were made between the width parameters the maximum velocities and the height jumps for one track d04 near 36 n 73 w the average width parameter was 69 15 km based on the gaussian velocity fitting and 75 14 km based on the hyperbolic sea surface topography fit with a 10 km smoothing function along the same track the average maximum velocity was 164 30 cm s using the velocity fit and 163 37 cm s with the hyperbolic sea surface topography fit some of the parameter estimates were found to be highly 0 9 correlated when the hyperbolic sea surface topography fit was carried out but such correlations were reduced for either the gaussian velocity fits or the hyperbolic velocity model fit a comparison of the parameters derived from 1 year of topex data showed good agreement with values derived by kelly 1991 using 2 5 years of geosat data near 38 n 66 w longitude width 78 22 km this paper 82 25 km kelly height jump 115 28 cm 123 19 cm maximum velocity 171 59 cm s 1 179 46 cm s 1 the accuracy of the geoid undulations used in the calculations was of the order of 16 cm with the accuracy of a geoid undulation difference equal to 15 cm over a 100 km line in areas with good terrestrial data coverage the north edge of the gulf stream was studied with topex data from cycles 1 to 37 the north edge was located at the position corresponding to the half width of the gulf stream north of the center of the stream along each track with all parameters estimated from the gaussian velocity fit procedure the location of the north edge along one track generally agreed with the location defined by national oceanic and atmospheric administration national ocean survey noaa nos maps based on advanced very high resolution radiometer avhrr data and with a north wall location provided by cornillon at 2 day intervals the results for this topex track showed the north edge at the highest 37 8 n latitude in october november 1992 falling to the lowest 36 9 n latitude in february march 1993 after which the stream moves north a second track 550 km to the east of the first track showed consistent agreement with the cornillon data for the edge location when such data were available in one case with the noaa data there was a 77 km discrepancy between the topex and noaa estimates of the north edge analysis of the sea surface topography suggested the noaa avhrr location may be inaccurate at this one location and time this paper demonstrates that our knowledge of geoid undulations and undulation differences in a portion of the gulf stream region is sufficiently accurate to determine characteristics of the jet when used with topex altimeter data the method used here has not been shown to be more accurate than methods that average altimeter data to form a reference surface used in analysis to obtain the gulf stream characteristics however the results in the paper show the geoid approach may be used in areas where lack of current meandering reduces the accuracy of the average surface procedure

measurements of dynamic topography during synop utilizing a geosat synthetic geoid dynamic topography estimates were obtained for the gulf stream region employing a previously derived synthetic geoid and geosat radar altimeter measurements ascending tracks passed through the central and eastern arrays of the synoptic ocean prediction experiment by utilizing the geoid estimate instead of the mean altimetric signal the axis position and velocity can be computed a time series of the dynamic height profiles crossing the gulf stream and a table of the positions of the axis of the stream along two tracks are given time series of the maximum velocity perpendicular to the ground track were computed for both arrays it was found that there was a seasonal cycle in this time series with a maximum in november

chaotic topography mantle flow and mantle migration in the australian antarctic discordance oceanic crust formed over the past 30 million years at the australian antarctic discordance aad is characterized by chaotic sea floor topography reflecting a weak magma supply from an unusually cold underlying mantle during the past 3 4 million years however a source of increased magma supply coinciding with the known indian pacific mantle isotopic boundary has propagated into the eastern aad displacing the chaotic terrain and replacing it with normal sea floor pacific mantle reached the eastern boundary of the aad at least 7 million years ago but it was not until 3 4 million years ago that lavas derived from pacific mantle were first erupted within the aad this long hiatus combined with the ridge transform geometry across the aad boundary constrains the locus of mantle migration to a narrow relatively shallow region directly beneath the spreading axis of the southeast indian ridge

westward migration of pacific ocean upper mantle into the southern ocean region between australia and antarctica the southern ocean south of the great australian bight hosts a globally anomalous region of mid ocean ridge the australian antarctic discordance in addition to its recognized morphological and geophysical anomalies the discordance represents the current on axis location of a proposed isotopic boundary between indian ocean and pacific ocean upper mantle convective regimes previously defined from analyses of 4 ma mid ocean ridge basalts morb pb sr and nd isotope analyses of 36 66 ma seafloor dredged from either side of the northward extrapolation of the australian antarctic discordance show that 36 ma seafloor to the east has an indian ocean morb isotopic signature this indicates that the indian pacific isotopic boundary does not extend directly north of the ridge toward the southern margin of australia progressive westward migration of an arcuate shaped front of pacific ocean upper mantle therefore appears to be a consequence of australian antarctic rifting and southern ocean opening suggesting that the current location of the isotopic boundary within the discordance may be entirely coincidental

tectonic control on the origin and orientation of igneous layering an example from the greendale complex antigonish highlands nova scotia canada layering in appinites hornblende rich gabbros of the ca 610 ma greendale complex antigonish highlands nova scotia is defined by variations in texture and modal abundances of hornblede and plagioclase and ranges in width from 5 to 50 cm regional studies indicate that the complex was probably emplaced during late precambrian dextral shear on northeast trending faults by brittle failure at the roof of the magma chameber the origin and orientation of the layers may have been controlled by the regional tectonic setting the layering is steep to vertically dipping and varies in strike from 080 to 180 clockwise with concentrations at 090 and 160 the 090 layers are dilational and are thought to have developed perpendicular to north south extension along the extensional plane of the instantaneous strain ellipsoid associated with progressive dextral shear the 160 layers commonly display boudinage they may have rotated clockwise from an original 090 attitude toward the plane of flattening and hence into a stretching field during progressive dextral shear late stage cogenetic felsic veins fill conjugate shear fractures consistent with a dextral shear regime this study emphasizes the potential importance of considering regional tectonic setting when analyzing the origin and orientation of igneous fabrics