the tertiary structural and thermal evolution of the central alps compressional and extensional structures in an orogenic belt the western alpine arc has been created during the cretaceous and the tertiary orogenies the interference patterns of the tertiary structures suggest their formation during continental collision of the european and the adriatic plates with an accompanying anticlockwise rotation of the adriatic indenter extensional structures are mainly related to ductile deformation by simple shear these structures developed at a deep tectonic level in granitic crustal rocks at depths in excess of 10 km in the early palaeogene period of the tertiary orogeny the main tertiary nappe emplacement resulted from a nw thrusting of the austroalpine penninic and helvetic nappes heating of the deep zone of the upper cretaceous and tertiary nappe stack by geothermal heat flow is responsible for the tertiary regional metamorphism reaching amphibolite facies conditions in the lepontine gneiss dome geothermal gradient 25 c km the tertiary thrusting occurred mainly during prograde metamorphic conditions with creation of a penetrative nw se oriented stretching lineation xi finite extension parallel to the direction of simple shear earliest cooling after the culmination of the tertiary metamorphism some 38 ma ago is recorded by the cooling curves of the monte rosa and mischabel nappes to the west and the suretta nappe to the east of the lepontine gneiss dome the onset of dextral transpression with a strong extension parallel to the mountain belt and the oldest s vergent backfolding took place some 35 to 30 ma ago during retrograde amphibolite facies conditions and before the intrusion of the oligocene dikes north of the periadriatic line the main updoming of the lepontine gneiss dome started some 32 30 ma ago with the intrusion of the bergell tonalites and granodiorites concomitant with s vergent backfolding and backthrusting and dextral strike slip movements along the tonale and canavese lines argand s insubric phase subsequently the center of main updoming migrated slowly to the west reaching the simplon region some 20 ma ago this was contemporaneous with the westward migration of the adriatic indenter between 20 ma and the present the western aar massif toce culmination was the center of strong uplift the youngest s vergent backfolds the glishorn anticline and the berisal syncline fold the 12 ma rb sr biotite isochron and are cut by the 11 ma old rhone simplon line the discrete rhone simplon line represents a late retrograde manifestation in the preexisting ductile simplon shear zone this fault zone is still active today the oligocene neogene dextral transpression and extension in the simplon area were concurrent with thrusting to the northwest of the helvetic nappes the prealpes 35 15 ma and with the jura thin skinned thrust 11 3 ma it was also contemporaneous with thrusting to the south of the bergamasc 35 5 ma and milan thrusts 16 5 ma the oligocene neogene dextral transpression in the central alps is of the same age as the thrusting to the west and southwest in the external french part and to the east in the internal italian part of the alpine arc of the western alps

metamorphic reaction rates at 650 800 c from diffusion of niobium in rutile abstract the ability to quantify the rates at which metamorphic reactions occur is critical to assessing the extent to which equilibrium is achieved and maintained in a variety of dynamic settings here we investigate the kinetics of rutile replacement by titanite during amphibolite facies overprinting of eclogite garnet amphibolite and anorthosite from catalina island ca the tromso nappe norway the north qaidam terrane china and the guichicovi complex mexico trace element concentration profiles across rutile rimmed by titanite as determined by laser ablation inductively coupled plasma mass spectrometry la icp ms reveal nb zoning in rutile that we interpret as the result of nb back diffusion from the rutile titanite boundary we present new field based reaction rates calculated from grain boundary velocities which in turn were calculated using a 1 d diffusion model for nb back diffusion into rutile during titanite replacement over the temperature range 670 770 c our data are consistent with or slightly faster than previous estimates of field based reaction rates for regional metamorphism and extend the temperature and compositional range over which regional metamorphic reaction rates are known

site characterization by geophysical methods in the archaeological zone of teotihuacan mexico abstract in the present investigation a geophysical study was carried out in the eastern flank of the pyramid of the sun to define potential continuations in this direction of a tunnel discovered beneath the western main entrance of this building this man made structure is one of the many extraction tunnels hollowed by the ancient teotihuacans to obtain construction materials to build their city total field and high resolution vertical gradient magnetic surveys were carried out the spectral analysis of the total magnetic field enabled us to estimate the thickness of the alluvial cover over the basaltic flow as 3 3 m it was also found that the main contribution to the observed magnetic field comes from the pyroclastic flow that covers the area of study the horizontal gradient filter was applied to the low pass filtered magnetic field to enhance magnetic contacts and structural boundaries inferred magnetic trends were related to fracture patterns within the basalts and pyroclasts and low gradients provided an evidence of voids or tunnels the euler deconvolution method was applied as an attempt to confirm the above results using a structural index s 0 we determined the boundaries of main magnetic contacts as well as the interface between the basaltic flow and las varillas tunnel euler depths were found to range between 3 to 6 m which represent the mean thickness of the basaltic flow two parallel ground probing radar gpr profiles were surveyed in the nw se direction one passes on top of the known location of a tunnel las varillas and a second one 10 m to the north approximately the tunnel s roof is well outlined at depths between 3 5 and 4 m on the other hand little evidences of other buried tunnels or extensions of the known one were found the second profile depicts a more complex morphology for the pyroclastic sheet anomalies related to basaltic flow or eruptive centres are clearly observed both profiles depicted the sedimentary base at 3 m on average a resistivity profile was undertaken along the first gpr profile a resistivity image was obtained that showed the vertical and lateral distribution of the true resistivity high resistivity values were associated with the tunnel location las varillas its geometry could also be inferred its top is found at about 4 m and extends 20 m in the profile direction to the west unfortunately the depth to its base could not be estimated since profile length was too short the sediment pyroclastic flow interface could also be delimited at a depth of 3 m finally a vertical magnetic field profile taken along the same surveyed line gpr and resistivity was inverted applying a two dimensional algorithm the initial model was estimated from gpr and resistivity interpretations a simple model of las varillas tunnel was computed which reasonably well satisfied geological and geophysical considerations

coupled mass transport and kinetically limited isotope exchange applications and exchange mechanisms recent studies provide theoretical models for kinetically limited isotope exchange during fluid flow however few have attempted to apply these models to natural systems in this paper we interpret stable isotopic data from fossil hydrothermal systems in two different tectonic settings by using combined equations of mass transport and kinetically limited isotope exchange our models provide alternative interpretations for the geometry of flow responsible for the development of 18 o depletion annuli surrounding epizonal intrusions and for the geometry of flow in shallow level normal faults our models also provide information about relative isotope exchange rates and by extension mechanisms of isotope exchange our results indicate that dynamic recrystallization in fault zones promotes exchange by surface reaction whereas exchange in chemically nonreactive and undeformed minerals may be limited by solid state diffusion

correlations between fluid composition and deep seated structural style in the footwall of the simplon low angle normal fault switzerland structural and fluid inclusion analyses reveal a correlated change in structural style and fluid composition along strike in footwall mylonites of the simplon low angle normal fault of switzerland in the south early postmylonitic structures are semibrittle shears and brittle faults synkinematic fluid inclusions within the faults contain a carbonic component whereas those in the shears lack co 2 to the north early postmylonitic structures are extension veins and kinkbands and synkinematic fluid inclusions are aqueous entrapment conditions for all fluid inclusions are estimated to be 500 c and 5 kbar we speculate that different wetting characteristics of carbonic versus aqueous fluids influenced the mechanical behavior of the rocks nonwetting carbonic fluids locally increased fluid pressure and induced brittle failure resulting in development of abundant closely spaced brittle faults in areas lacking carbonic fluids ductile deformation continued to lower pressures and temperatures

constancy of silicate mineral weathering rate ratios between natural and experimental weathering implications for hydrologic control of differences in absolute rates abstract inverse models which apportion watershed effluxes geochemical mass balance over estimated mineral surface area give weathering rates for individual silicate minerals e g feldspar 1 3 orders of magnitude slower than laboratory rates comparisons of rates calculated from several recent watershed geochemical mass balance studies with experimentally determined rates reveal that the ratio of mineral weathering rates for a given silicate mineral pair determined in an internally consistent field study equals the ratio of rates for the same minerals in any internally consistent set of laboratory experimental data thus the coefficient which corrects the weathering rate for any mineral from a laboratory data set to a field setting is identical to the corresponding coefficient for any other mineral in the same data sets the correction factor linking experimental and natural rates must be independent of composition physical e g hydrologic controls rather than compositional or chemical controls cause the difference between weathering rates in nature and the laboratory because flow in natural weathering profiles is spatially heterogeneous not all of the potentially available surface in natural systems actually participates in reactions with pore fluids

high precision microsampling and preparation of zoned garnet porphyroblasts for sm nd geochronology abstract zoned garnet porphyroblasts contain one of the longest continuous records of tectonic processes with growth histories spanning many millions of years well known challenges exist in the physical preparation purification and geochronologic analysis of zoned garnets which have limited the progress and application of this potentially powerful geochronologic tool here we present details of an integrated methodology specifically for the acquisition of high resolution sm nd geochronologic records from zoned garnets that overcome these challenges we develop and demonstrate our methods on large test garnets from a shear zone in the tauern window in western austria chemically contoured microdrilling permits physical sampling of many discrete age annuli each as narrow as 100 m within a single garnet refined hno 3 hf hclo 4 partial dissolution methods cleanse coarse crushed 75 150 m size garnet annuli of detrimental inclusions yielding high 147 sm 144 nd ratios 0 9 to 4 6 and consequently accurate and precise ages experiments on powdered garnet 147 sm 144 nd 143 nd 144 nd with 10 20 ppm 2 rsd precision good enough for 147 sm 144 nd 1 0 in total this integrated sampling and analytical protocol permits the acquisition of 3 11 discrete high precision

automatic interpretation of gravity gradiometric data in two dimensions vertical gradient1 the magnetic and gravity field produced by a given homogeneous source are related through poisson s equation starting from this consideration it is shown that some 2d interpretation tools widely applied in the analysis of aeromagnetic data can also be used for the interpretation of gravity gradiometric data vertical gradient this paper deals specifically with the werner deconvolution analytic signal and euler s equation methods after a short outline of the mathematical development synthesized examples have been used to discuss the efficiency and limits of these interpretation methods these tools could be applied directly to airborne gravity gradiometric data as well as ground gravity surveys after transformation of the bouguer anomalies into vertical gradient anomalies an example is given of the application of the werner deconvolution and euler s equation methods to a microgravity survey

satellite observations of circulation in the eastern bering sea satellite infrared observations of the eastern bering sea shelf and basin were analyzed from december 1979 to august 1981 a region of warm surface water which extends from the gulf of alaska through the aleutian passes into the bering sea was observed coincident with a band of warm surface water and with eddies along the shelf break the spatial and temporal relationships of these patterns imply a surface inflow through unimak and other western aleutian passes flow along the shelf break including eddies along the bering slope current and cyclonic flow in the basin the proposed generating mechanisms for eddies along the shelf break include topographic interactions baroclinic instabilities and rossby waves

an observation of the surface circulation in a gulf stream frontal perturbation in the period of 3 16 june 1979 a satellite tracked free drifting buoy passed through a gulf stream frontal perturbation and was entrained into a warm filament on the shoreward side of the perturbation significant speed convergence was noted as the buoy passed through the perturbation and approached the counterclockwise turn into the warm filament in the warm filament the buoy drifted to the southwest at speeds of about 0 1 to 0 3 m sec eventually the buoy drifted out of the filament and went westward in an apparent response to wind induced surface drift

gulf stream and wind induced current variability on the georgia continental shelf winter 1978 low frequency longshore current fluctuations on the continental shelf off georgia and their relationships with local atmospheric forcing and gulf stream displacement were examined for a 3 month period from january to april 1978 acoustic travel time and bottom pressure measurements at a station on the continental slope were used to determine the depth of the main thermocline as an indicator of gulf stream displacement on the middle shelf current variability was dominated by local wind forcing at periods longer than 2 days with very little gulf stream influence longshore wind stress was the main driving force at periods longer than 4 days while cross shore wind contributed at shorter periods in contrast on the outer shelf current fluctuations in the upper layer were highly coherent with gulf stream displacement at a 12 day period and marginally coherent with longshore wind at a 6 day period linear regression analysis showed that gulf stream and local wind forcing accounted for most of the fluctuations in the upper layer over the shelf break for time scales greater than 5 days and at around 2 days a low multiple coherence window at 2 8 5 days was probably due to gulf stream frontal eddies in the lower layer over the shelf break the current fluctuations had a character intermediate between the upper layer currents gulf stream dominated and mid shelf currents wind dominated

chemical interactions between a sedimentary diapir and surrounding magma evidence from the phepane dome and bushveld complex south africa the phepane dome is a circular outcrop of metasedimentary rock within the eastern lobe of the bushveld complex hypothesized to have formed as a diapir when underlying wallrock rose into the overlying magma body interactions between the metasedimentary rock of the phepane dome and the magma of the bushveld complex were investigated through measurements of oxygen and lithium isotopic compositions determination of mineral modes and major element mineral compositions cathodoluminescence imaging and dihedral angle analysis evidence from cathodoluminescence imaging and dihedral angle analysis suggest that heat transfer during diapirism caused partial melting and complete recrystallization of the phepane dome metasedimentary rock oxygen isotope analysis of samples from traverses spanning the contact between metasedimentary and igneous rocks demonstrates that relatively minimal exchange of oxygen over distance 4 m occurred across the contact between the phepane dome and the surrounding bushveld magma the lithium concentrations and isotopic compositions of metasedimentary rock are significantly different from the associated igneous rocks lithium isotope analysis of samples from traverses across the contact demonstrates exchange of li over somewhat greater distances 60 m than oxygen consistent with evidence that suggests a higher diffusivity of li than most major elements models of oxygen diffusion through intergranular melt and aqueous fluid are used to place maximum constraints on the duration of diffusive exchange across the contact resulting in estimates ranging from 5 kyr to 5 myr these values are consistent with previous estimates of the duration of crystallization of the bushveld complex and phepane diapir development

partial equilibrium of radiogenic and stable isotope systems in garnet peridotite during ultrahigh pressure metamorphism a critical question in radiometric dating of metamorphic rocks is whether minerals used to define internal isochrons have achieved isotopic equilibrium during a given metamorphic event followed by preservation of the equilibrium afterward a garnet peridotite at zhimafang in the sulu terrane of eastern china shows incongruent u pb rb sr and sm nd ages zircon shrimp u pb dating yielded an isochron age of 224 8 ma pointing to zircon growth during triassic uhp metamorphism two mineral assemblages gave consistent rb sr internal isochron ages of 201 4 and 205 4 ma but much older sm nd internal isochron ages of 376 16 and 378 24 ma apparently the sm nd radiometric system fails to reequilibrate among the dated minerals during the triassic uhp metamorphic event for the garnet peridotite this result is confirmed by the state of oxygen isotope equilibrium or disequilibrium among the constituent minerals in the garnet peridotite in particular garnet is not in oxygen isotope equilibrium with any other of the analyzed minerals the degree of oxygen isotope disequilibria among the other minerals varies from pair to pair oxygen isotope equilibrium is observed only between orthopyroxene and olivine for both samples and between phlogopite and clinopyroxene for one sample it appears that the u pb rb sr sm nd and o isotope systems in minerals of the garnet peridotite are in partial equilibrium during uhp metamorphism i e equilibrium with respect to some isotopes and minerals but not all based on available experimental diffusion data our study of the combined u pb rb sr sm nd and o isotope systems in the garnet peridotite demonstrates that the time scale for the uhp metamorphism and subsequent hp eclogite facies recrystallization is possibly in the range of about 12 to 26 ma which was not long enough for the reequilibration of the sm nd and o isotope systems but just long enough for the reequilibration of the rb sr isotope systems therefore the rate of sr diffusion in phlogopite thus sr isotope reequilibration between the mafic minerals is constrained to be faster than rates of nd and o diffusion in garnet thus sm nd and o isotope reequilibration between the same minerals under conditions of subduction zone metamorphism

effect of bathymetric curvature on gulf stream instability in the vicinity of the charleston bump abstract the effect of the isobathic curvature on the development and evolution of gulf stream frontal waves meanders and eddies in the vicinity of the charleston bump a topographic rise on the upper slope off charleston south carolina referred to as cb hereinafter is studied using the hybrid coordinate ocean model hycom baroclinic and barotropic energy transfers from the gulf stream to its meanders and eddies that appear as cold and warm anomalies are computed for four different cases in case i the curvature of the isobaths is artificially reduced and the cb is removed from the bathymetry in this simulation the simulated gulf stream meanders were barely noticeable in the study region energy transfer from the gulf stream to meanders and eddies was negligible in case ii the curvature of the isobaths was the same as in case i but a bump of the scale of the cb was added to the bathymetry in this simulation gulf stream meanders were amplified while passing over the cb in case iii the cb w

strontium and oxygen isotope decoupling in the hercynian trois seigneurs massif pyrenees evidence for fluid circulation in a brittle regime systematic shifts of oxygen isotopic compositions in the higher grade parts of the high temperature low pressure hercynian metamorphic sequence exposed in the trois seigneurs massif have previously been explained as a result of an influx of surface derived water during the prograde part of the metamorphic cycle it has been suggested that this caused a regional lowering of 87sr 86sr in the metamorphic sequence mapping of strontium isotopic compositions across a 15 m meta carbonate horizon in the higher grade pelite psammite sequence shows that strontium isotopic compositions were homogenised over length scales of metres or less during the hercynian metamorphism which brought the carbonate and pelite psammite to oxygen isotopic equilibrium with a common fluid comparison of model pre hercynian 87sr 86sr profiles across the carbonate based on a depositional diagenetic age of 450 ma and initial 87sr 86sr ratio of 0 7086 given by 10 m length scale averaging with the post hercynian 87sr 86sr profile calculated from analysed 87sr 86sr and rb sr compositions implies strontium isotopic diffusion distances of ca 0 4 m in the carbonate and ca 7 m in the pelite psammite the limited sr isotopic diffusion distance of 0 4 0 7 m within the carbonate is compatible with pervasive oxygen isotopic exchange over distances restricted to 4 15 m if fluid strontium concentrations were between 4 and 50 ppm the strontium isotopic transport distances are not compatible with pervasive oxygen isotopic alteration over the observed 5 km regional scale either the flow was perfectly layer parallel or more probably the regional scale alteration of oxygen took place by fluid circulation in the brittle regime early in or prior to the hercynian metamorphic event flow along cracks with incomplete diffusive exchange between fluid and wall rock would allow greater decoupling of oxygen and strontium isotopic transport than pervasive advective transport with local fluid solid equilibrium

seismic activity and structural features in the charlevoix region quebec the charlevoix region is historically the most active earthquake zone in eastern canada understanding the links between its seismicity and the faults of the region is important for the assessment

rate and mechanism in prograde metamorphism for a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature disequilibrium depends on the slowest of three sequential steps a surface detachment of reactant minerals b transport of material to the site of mineral growth and c nucleation and growth of the product mineral we have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism the dissolution and growth steps are linear functions of the driving force g of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself

evaluating chemical equilibrium in metamorphic rocks using major element and sm nd isotopic age zoning in garnet townshend dam vermont usa abstract a quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet sm nd ages suggests that major element matrix equilibrium was maintained to a first order throughout a ca 40 cm wide rock sample during garnet growth however cm scale sm nd isotopic heterogeneity limits the sm nd age precision required to evaluate more subtle age differences within individual garnet crystals central wafers from 1 3 cm diameter garnet grains within a 1 21 104 cm3 block of pelitic schist were used to document concentric growth zoning of major elements with decreasing mn and ca and increasing fe and mg from cores to rims garnets also preserve growth zoning patterns for hree and mree and show evidence for resorption and partial recrystallization of the outermost rims similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium p t x conditions for major elements over the sample volume comparison of 28 rock sm nd isotope values from the sample indicates substantial cm scale heterogeneity which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation therefore sm nd isotopic compositions of thirty eight concentric core to rim garnet segments from ten large 1 3 cm garnets and two small 1 4 mm bulk garnets with narrow ranges of mn content are paired with sixteen matrix whole rock sm nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383 1 6 8 ma to 324 5 3 3 ma four of the garnets have anomalously young rims that likely result from post growth alteration chlorite quartz and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post growth resorption recrystallization effects excluding these young rims yields a range of ages from 383 1 6 8 oldest core to 374 9 1 8 ma youngest rim sm nd age precisions 1 5 m y and high mswd result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix however garnet cores with high mn n 7 mantles with intermediate mn n 14 and rims with low mn n 8 including the 2 smaller bulk garnet analyses define three distinct multi grain isochrons of 380 3 2 0 ma n 23 mswd 14 377 3 1 4 ma n 30 mswd 18 and 376 5 1 0 ma n 24 mswd 18 respectively yielding an average garnet growth duration of 3 8 2 2 m y these three composite mn age zones define a mn vs age relationship that reflects depletion of mn in the rock matrix as it is sequestered by growing garnet correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained to a first order throughout the ca 4 m y duration of garnet growth

surface current patterns suggested by suspended sediment distribution over the outer continental margin bering sea abstract samples of total suspended matter tsm were collected at the surface over the northern outer continental margin of the bering sea during the summers of 1980 and 1981 volume concentrations of surface tsm averaged 0 6 and 1 1 mg l 1 for 1980 and 1981 respectively organic matter largely plankton made up about 65 of the near surface tsm for both years distributions of tsm suggested that shelf circulation patterns were characterized either by meso and large scale eddies or by cross shelf components of flow superimposed on a general northwesterly net drift these patterns may be caused by large submarine canyons which dominate the physiography of this part of the bering sea continental margin

a new concept in euler deconvolution of isolated gravity anomalies euler s homogeneity equation has been used to develop a new technique to interpret the gravity anomalies over some simple geometrical sources namely a finite horizontal line vertical line a finite vertical ribbon a semicircular dome basin and an isosceles triangle approximating an anticline syncline a linear over determined system of equations has been solved to compute the depth the horizontal location and the structural index all treated as free parameters the concept of a variable structural index provides better depth estimates and helps to identify the source geometry nomograms have been prepared to compute an additional model parameter namely the horizontal vertical extent of a line the vertical extent of a ribbon and the radius of a dome basin the efficacy of the proposed method has been evaluated using two real field examples

morphology of intergranular pores and wetting angles in pelitic schists studied by transmission electron microscopy in pelitic schists composed mainly of quartz and albite grains the morphology of intergranular pores which were filled with water was studied by transmission electron microscopy tem although some pores are defined by crystallographic planes f face most of their form has an ideal shape determined by interface tensions between grains and fluid high resolution tem observations demonstrate that pore free regions at grain boundaries are tight even at the nanometer scale showing that the wetting angle is larger than 0 in this rock the pore distribution in two grain junctions can be compared to a necklace microstructure developed by instability of a fluid film along the boundary induced by microcracking wetting angles for pores located at grain edges of quartz and albite decrease in the order albite albite quartz quartz and quartz albite the quartz quartz wetting angle in a calcite free sample is smaller than that in a calcite containing sample this angle also changes due to grain misorientation our results confirm that solid solid and solid fluid interfacial energies control the geometry of intergranular fluid in natural rocks

mesoscale oceanographic features associated with the central bering sea ice edge february march 1981 midwinter 1981 observations from the central bering sea shelf showed a hydrographic structure which was two layered in temperature salinity and density this structure was confined to a 100 km wide band which coincided approximately with the ice edge the colder lower salinity upper layer was continuous in its t s properties with homogeneous water to the north beneath the ice the warmer more saline lower layer was similarly continuous with water to the south near the shelf break a northwestward baroclinic flow of the surface relative to 75 dbar was associated with the layered structure the layered structure and associated flow appear to be regular winter features associated with the ice edge in this region input of low salinity water from melting of ice along the edge is adequate to maintain the layered structure and its associated baroclinic circulation against the effects of tidal and wind mixing the resulting observed oceanographic features were similar to those which have been found in association with midshelf and shelf break fronts in other regions and appear to define a dynamically controlled boundary between the cold low salinity water on the northern shelf and the warmer more saline water near the shelf break

observations of gulf stream induced and wind driven upwelling in the georgia bight using ocean color and infrared imagery ocean color and infrared imagery from aircraft and satellite sensors are combined with in situ measurements of currents chlorophyll temperature salinity coastal winds and sea level to study upwelling events in the georgia bight where the gulf stream and continental shelf waters strongly interact these interactions were observed over an 8 day period in april 1980 during which five independent upwellings occurred the upwellings included a nearshore wind driven upwelling enhanced by topographic effects three gulf stream filament induced upwellings and what appears to have been a gulf stream meander induced event the chlorophyll distributions as inferred from the imagery are found to be reliable tracers which indicate filamentary circulation and propagation and advective routes by which nearshore water moves offshore the propagation of two of the filaments and the meander are studied in detail using the imagery and mooring array temperature time series both techniques yield nearly identical results for the phase speeds of each event there is evidence that one of the filaments accelerated as it entered into the charleston bump area where the gulf stream deflects offshore the other filament exhibited a steady deceleration as it moved past cape canaveral and eventually lost any semblance of wave motion also includedmore are data from april 1979 which offer corroborating evidence of filamentary structure and circulation intercomparisons of field measurements and the nimbus 7 coastal zone color scanner estimates of surface pigment concentrations show excellent agreement over the range of 0 1 7 0 mg m sup 3 54 references 22 figures 6 tables less

strontium isotope zoning in garnet implications for metamorphic matrix equilibration geochronology and phase equilibrium modelling in principle garnet growth rates may be calculated from 87 rb 86 sr and 87 sr 86 sr measurements in garnet subsamples and the surrounding rock matrix because of low rb sr garnet should passively record the matrix decay of 87 rb to 87 sr as a progressive increase in 87 sr 86 sr from core to rim this concept was tested by collecting rb sr data for five garnet grains from four major orogenic belts eastern vermont c 380 ma western new hampshire c 320 ma southern chile c 75 ma and northwestern italy c 35 ma both normal sr isotope zoning increasing 87 sr 86 sr from core to rim and inverse sr zoning decreasing 87 sr 86 sr from core to rim were observed garnet and matrix iso tope data commonly yielded grossly inaccurate model ages incomplete rb and sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v measured zoning patterns and the age disparities initially the reactive matrix is dominated by rapidly equilibrating rb rich mica which imparts high 87 sr 86 sr values in garnet cores progressive participation of slower equilibrating sr rich plagioclase buffers or even reduces 87 sr 86 sr possibly leading to flat or decreas ing 87 sr 86 sr from garnet cores to rims unusually high 87 sr 86 sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages so metamorphic ages growth rates and associated heating and loading rates are likely suspect although rb sr may be the most suscep tible because of the profound disparities between mica and feldspar zircon reactivity might influence the lu hf system by up to a few per cent the sm nd system seems generally immune to these effects pseudosection analysis and conventional garnet geochronology which presume complete matrix equilibration during metamorphism may require modification to account for differences between whole rock v reactive matrix compositions

paleorift structure constrained by gravity and stratigraphic data the statherian arai rift case abstract gravimetric and stratigraphic data were used to investigate the paleoproterozoic arai paleorift a failed statherian continental rift located in the western margin of the sao francisco craton where basement and cover were affected by the neoproterozoic brasiliano orogeny euler deconvolution tilt total horizontal gradient amplitude and upward continuation technics were applied to terrestrial gravimetric data in order to investigate the rift s main faults location direction and depth allowing to identify its main horsts grabens volcanic and plutonic centers we found that rift faults occur to a maximum depth of ca 38 km but major fault throw occurs from 4 to 8 km deep and attenuates from 8 to 12 km probably the brittle ductile transition zone at the time of rifting practically disappearing at 20 km stratigraphic data and basement mapping were used in order to constraint gravimetric results we classify the arai rift as a passive three armed failed rift narrow to divergent type that produced preferably anorogenic rapakivi related magmas most of it still lodged in the crust from surface down to ca 19 km deep and subsidiary mafic magmatism the results indicate the deep occurrence of low density magmas beneath the rift s main axis detected up to 20 km deep correlation to other global statherian rifts show that the sao francisco craton was strongly affected by taphrogenesis during the statherian together with siberia north america and north china cratons finally by comparing our results to recent rifts we found that the ethiopian rift s morphology is quite similar to the arai surrounding the tanzanian craton the cenozoic east africa rift system morphology is compared to the arai espinhaco rift system which surrounds the sao francisco craton the major contribution of this paper is the recognition of arai paleorift surface and subsurface morphology up to now unknown over an area of ca 45 000 km2

avoidable euler errors the use and abuse of euler deconvolution applied to potential fields window based euler deconvolution is commonly applied to magnetic and sometimes to gravity interpretation problems for the deconvolution to be geologically meaningful care must be taken to choose parameters properly the following proposed process design rules are based partly on mathematical analysis and partly on experience 1 the interpretation problem must be expressible in terms of simple structures with integer structural index si and appropriate to the expected geology and geophysical source 2 the field must be sampled adequately with no significant aliasing 3 the grid interval must fit the data and the problem neither meaninglessly overgridded nor so sparsely gridded as to misrepresent relevant detail 4 the required gradient data measured or calculated must be valid with sufficiently low noise adequate representation of necessary wavelengths and no edge related ringing 5 the deconvolution window size must be at least twice the original data spacing line spacing or observed grid spacing and more than half the desired depth of investigation 6 the ubiquitous sprays of spurious solutions must be reduced or eliminated by judicious use of clustering and reliability criteria or else recognized and ignored during interpretation 7 the process should be carried out using cartesian coordinates if the software is a cartesian implementation of the euler deconvolution algorithm most accessible implementations are cartesian if these rules are not adhered to the process is likely to yield grossly misleading results an example from southern africa demonstrates the effects of poor parameter choices

error estimation and optimization of gravity surveys1 abstract gravity data are often acquired over long periods of time using different instruments and various survey techniques resulting in data sets of non uniform accuracy as station locations are inhomogeneously distributed gravity values are interpolated on to a regular grid to allow further processing such as computing horizontal or vertical gradients some interpolation techniques can estimate the interpolation error although estimation of the error due to interpolation is of importance it is more useful to estimate the maximum gravity anomaly that may have gone undetected by a survey this is equivalent to the determination of the maximum mass whose gravity anomaly will be undetected at any station location given the data accuracy at each station assuming that the maximum density contrast present in the survey area is known or can be reasonably assumed from a knowledge of the geology the proposed procedure is as follows at every grid node the maximum mass whose gravity anomaly does not disturb any of the surrounding observed gravity values by more than their accuracies is determined a finite vertical cylinder is used as the mass model in the computations the resulting map gives the maximum detection error and as such it is a worst case scenario moreover the map can be used to optimize future gravity surveys new stations should be located at or near map maxima the technique is applied to a set of gravity observations obtained from different surveys made over a period of more than 40 years in the abitibi greenstone belt in eastern canada

thermobarometry of garnet clinopyroxene hornblende granulites from the kapuskasing structural zone amphibole mixing properties and standard state properties for the end members tremolite tschermakite pargasite ferrotremolite ferrotschermakite and ferropargasite have been refined through analysis of relevant phase equilibrium experiments coexisting cummingtonite hornblende pairs and natural mineral parageneses when merged with the mineral properties given by r g berman these amphibole properties allow the calculation of numerous equilibria suitable for thermobarometry in a wide range of bulk compositions at amphibolite to granulite metamorphic grades the method was applied to texturally equilibrated garnet pyroxene hornblende gneisses and compared both to results based on anhydrous assemblages and to those for garnet biotite gneisses from the kapuskasing structural zone thermometer resetting is apparent only in garnet biotite gneisses whereas touching garnet pyroxene pairs in mafic gneiss show little evidence of retrograde fe mg exchange computed metamorphic temperatures garnet clinopyrox

field measurement of high temperature bulk reaction rates i theory and technique knowledge of metamorphic reaction rates is crucial to accurately interpret rock and mineral chemistry the local equilibrium assumption used in geochronology geothermobarometry and material flux estimates requires that local reaction rates among system phases are fast relative to local rates of p t x change natural metamorphic reaction rates are essentially unknown and difficulties and disagreements exist regarding extrapolations of existing laboratory data to natural conditions growing recognition of natural effects that could be attributed to slower reaction rates justifies the need for a field based quantification of reaction rates to assess the accuracy of lab based predictions we describe in detail the theory and methodology of a technique for extracting bulk reaction rates directly from isotopic data derived from natural samples baxter and depaolo 2000 reaction rates measured using this technique may be judiciously applied to isotopic exchange or net reaction kinetics in other natural systems the technique requires collection of whole rock and garnet 87sr 86sr data along a sampling traverse normal to a lithologic contact where there was prior to metamorphism a sharp isotopic discontinuity garnet data provide information on syn metamorphic conditions and the time interval for the exchange process forward modeling of the reactive transport process using numerical methods and the equations for diffusive reactive transport allow determination of the reaction rate and bulk sr diffusivity that provides the best fit to the data measurement of reaction rates is best constrained if an isotopic step is preserved at the contact the size of which is directly proportional to the bulk reaction rate this contribution is intended to serve as a template for future use and development of this technique to acquire natural reaction rate data from metamorphic systems

field measurement of high temperature bulk reaction rates ii interpretation of results from a field site near simplon pass switzerland bulk metamorphic reaction rates have been measured using sr isotopes as tracers of reactive transport processes at a field site near simplon pass switzerland employing the technique described in part i of this study baxter and depaolo 2002a the reaction rate r inferred similar to the value first reported in baxter and depaolo 2000 is 1 4 1 1 0 4 x 10 7 yr 1 or r 7 0 5 5 2 0 x 10 9 g cm 2 yr normalized to the geometric surface area of plagioclase which is several orders of magnitude slower than extrapolations of laboratory data this rate suggests that attainment of local equilibrium may require 10 s of millions of years and consequently equilibrium may not be closely approached in dynamic metamorphic systems if significant changes in p t x occur over shorter timescales these data suggest that there is a problem in current extrapolations or applicability of laboratory kinetic data to natural high temperature systems possible reasons for this discrepancy include differences in reactive surface area reactant supply limited by slow diffusion of major elements or an armoring effect by which access to grain interiors is limited on the basis of microchemical patterns in plagioclase we identify grain boundary migration gbm as the rate limiting process for accessing grain interiors and thus for bulk reaction calculation of reaction quotients for six balanced mineral reactions suggests local disequilibrium for garnet with respect to its ca content the characteristic equilibrium lengthscales le for sr and ca are both about 1 meter equilibrium based geochemical tools such as isochron geochronology or geothermobarometry are compromised within a distance le of the lithologic contact bulk sr diffusivity measured at the field site changes from 10 9 to 10 7 m yr suggesting that the characteristics of the intergranular transporting medium itm changed significantly during the early retrograde history of the field site

the fractal dimension of gravity data sets and its implication for gridding gravity survey station locations are in general inhomogeneously distributed this inevitably results in interpolation errors in the computation of a regular grid from the gravity data the fractal dimension of the station distribution can be used to determine if an interpolated map is aliased at a specific wave length and moreover it is often possible to determine an optimum gridding interval synthetic distributions of gravity station locations have been used for theoretical studies and it is found that for randomly distributed data there is a range of sizes for which the spatial data distribution has a fractal dimension of 2 that is euclidean the minimum length scale at which the distribution ceases to be euclidean is the optimum interpolation interval obeying shannon s sampling theorem for dimensions less than 2 the optimum interpolation interval is the shortest length at which the scaling regime is constant in this case the gravity field cannot be interpolated without introducing some aliasing as the fractal dimension characterizes the data distribution globally over the whole study area the actual gridding interval in some cases will be smaller in order to represent short wavelength features properly in the more densely sampled sub areas but this may generate spurious anomalies elsewhere the proposed technique is applied to the station distribution of the canadian national gravity data base and a series of sub areas a fractal dimension of 1 87 is maintained over a range of sizes from 15 km to over 1600 km although aliasing occurs since the gravity field certainly contains much shorter wavelength anomalies aliasing errors may be minimized by selecting the proper interpolation interval from the fractal analysis

comment on a crustal thickness map of africa derived from a global gravity field model using euler deconvolution by getachew e tedla m van der meijde a a nyblade and f d van der meer summary we discuss a recent publication by tedla et al which presents a crustal thickness map of africa determined by euler deconvolution of gravity in our comment we first outline the limitations of the data set used by the authors and the deleterious effects of working with free air gravity next we discuss the grievous parameter choices grid interval euler window size structural index and incorrect assumptions made by the authors and their serious effects on the results finally we demonstrate with two examples why their results are not helpful in understanding crustal thickness and consequently the tectonic history this is not specific to africa but also true for the application to other parts of the earth

an appraisal of the serra da cangalha impact structure using the euler deconvolution method the applicability of the euler deconvolution method in imaging impact crater structure vis vis delineation of source depth of the circular magnetic anomaly and or basement depth beneath the crater is addressed in this paper the efficacy of the method has been evaluated using the aeromagnetic data obtained over the serra da cangalha impact crater northeastern brazil the analyses of the data have provided characteristic euler deconvolution signatures and structural indices associated with impact craters also through the interpretation of the computed euler solutions our understanding of the structural features present around the impact structure has been enhanced the euler solutions obtained indicate shallow magnetic sources that are interpreted as possibly post impact faults and a circular structure the depth of these magnetic sources varies between 0 8 and 2 5 km while the precambrian basement depth was found at 1 5 km this is in good agreement with the estimates of the precambrian basement depth of about 1 1 km calculated using aeromagnetic data the reliability of the depth solutions obtained through the implementation of the euler method was confirmed through the use of the existing information available in the area and the result of previous studies we find that the euler depth solutions obtained in this study are consistent with the results obtained using other methods

ion microprobe study of marble from naxos greece grain scale fluid pathways and stable isotope equilibration during metamorphism in metamorphic terranes modification of the stable isotope compositions of marbles by fluid infiltration has been extensively modeled to try to constrain both the magnitudes and time scales of metamorphic fluid fluxes we have made a study of key localities on the aegean island of naxos greece in which we combined textural observations with micro scale trace element and stable isotope analysis to test the assumptions upon which modeling of metamorphic fluid flow is based and to identify fluid transport mechanisms integration of ion microprobe analyses of 18 o 16 o 13 c 12 c mn and fe with detailed cathodoluminescence imaging provides evidence that water rich fluids were channeled along cracks and grain boundaries within marble with limited equilibration between the fluid and rock very low values of 18 o smow down to 2 along grain boundaries indicate a meteoric source for fluids rather than a metamorphic source in contrast to previous studies in this example the simplifying assumptions made previously in chromatographic theory to constrain metamorphic fluid fluxes time scales and porosities from bulk isotope data are incorrect by making micro scale measurements we can determine infiltration mechanisms and distinguish separate fluid flow events these observations and measurements provide a basis for constructing more realistic models of metamorphic fluid flow

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

peristerite exsolution in metamorphic plagioclase from the lepontine alps an analytical and transmission electron microscope study transmission and analytical electron microscopy were used to examine relationships between microstructures and compositions in greenschist and amphibolite facies metamorphic plagioclase albite and oligoclase from the lepontine alps switzerland and italy two kinds of exsolution microstructures related to the peristerite miscibility gap an1 25 were observed lamellae in bulk compositions ranging from a few mole percent anorthite to an10 15 and tweeds in bulk compositions from almost pure albite to an15 18 lamellae are typically 15 to 35 nm thick individual lamellae in crystallographically homogeneous or tweedy areas commonly have highly irregular spacings or end at dislocations or subgrain boundaries suggesting formation by heterogeneous nucleation tweeds are characteristically diffuse and probably formed by spinodal decomposition many tweeds have one exsolution direction that is consistently sharper or coarser than the other tweeds and lamellae may be intergrown in patches with irregular curving boundaries some of which define narrow stripes several micrometers long exsolution directions in these tweeds are approximately parallel and perpendicular to the lamellae the perpendicular direction is usually less diffuse or more regular than the parallel direction and may continue between widely spaced individuals in patches of lamellae tweeds in these intergrowths appear to be slightly more sodic than adjacent areas with lamellae except near fractures and in areas with high dislocation densities differences in microstructures within a single grain almost invariably reflect differences in composition microstructural variability within single grains and among different grains from the same hand sample was so large that it was impossible to identify systematic relationships between microstructures and metamorphic grade

numerical simulation of a mesolow over a gulf stream filament a three dimensional mesoscale numerical model is used to investigate mesoscale circulation over a gulf stream filament two numerical experiments are performed with different initial uniform ambient wind speeds u 0 1 m s 1 3 5 m s 1 and 7 m s 1 for a typical winter day it is found that for both low and moderate winds a closed mesoscale circulation forms over the gulf stream filament when the gulf stream filament was removed the model did not predict a mesoscale circulation the modeled circulation over the filament is in agreement with the observations suggesting that the atmospheric circulations over the filaments may be an important mechanism in the u s east coast cyclogenesis

the correlation of reaction and isotope fronts and the mechanism of metamorphic fluid flow infiltration of a metabasite sill from islay scotland by an h2o co2 fluid caused 1 modification of delta 18 o and 2 carbonation at the sill margins maps of delta 18 o and reaction progress were constructed from a 20 x 47 7 metre sample grid across

movement of the surface front induced by kuroshio frontal eddy analysis of successive satellite thermal images in tosa bay japan along the kuroshio showed that a cyclonic frontal eddy is formed due to the topographic effect of cape ashizuri when the kuroshio axis moves offshore unlike those found in the south atlantic bight sab along the gulf stream the frontal shape induced by the eddy is asymmetric comparison with the previous description on the eddy in the sab shows a possibility that cold nutrient rich deep water is not easily pumped up by the upwelling induced by the eddy in tosa bay and the surface temperature distribution is not affected much time series of water temperatures recorded on a ferryboat show that this phenomenon occurs about three times in a month but is not periodic

diffusion in solid earth systems recent years have seen a rapid expansion in the acquisition and use of information on diffusive transport in phases relevant to the solid earth crystals melts and fluids although far from complete the data base on diffusion coefficients is now sufficiently large that broad constraints can be placed upon the length and time scales of many natural transport phenomena in which diffusion plays a role conversely observations of diffusion progress in specific natural samples can be used to extract time temperature information for a variety of geologic and geochemical processes ranging from sediment burial and crustal erosion to fluidmediated reactions and biosignature retention despite this undeniable progress several major challenges remain that largely define the frontiers of research in solid earth diffusion perhaps foremost among these is the need to address and understand the multi scale multi path aspects of diffusion in many systems a complication that is not limited to polyphase materials individual mineral grains can exhibit clear indications of multi path behavior even when visible evidence of such paths is lacking many other diffusion frontiers are linked in one way or another to this multi scale issue they include diffusion of molecular h2o and the effect of h species on diffusion in minerals and rocks diffusive fractionation of multiple isotopes of a single element diffusion at the extreme conditions of the deep earth reconciliation of observations from natural samples and laboratory studies and development of theoretical approaches to predict

fluid flow and diffusion in the waterville limestone south central maine constraints from strontium oxygen and carbon isotope profiles oxygen carbon and strontium isotopic profiles across the margin of the waterville limestone member are used to investigate advective and diffusive transport during metamorphism of the waterville formation in south central maine usa rb sr isotopic systematics were homogenized on the 10 cm hand specimen scale at ages that are within error of the 366 6 ma rb sr whole rock age of the syn metamorphic hallowell pluton this is consistent with a plutonic heat source for this low pressure andalusite and sillimanite grade acadian metamorphic terrane advective displacements of all three isotope profiles at the garnet grade blue rock quarry indicate fluid flow to the east into the limestone and the oxygen isotope profile implies a time integrated fluid flux of 3 2 1 5 m 3 m 2 2 error this cross layer flux is insufficient to cause the observed reaction progress of the muscovite ankerite quartz to biotite anorthite in plagioclase calcite reaction in the 100 m thick waterville limestone member and much of the fluid flow responsible may have been layer parallel the isotope profiles indicate advective diffusive homogenization over distances of 1 5 m 13 c to 6 m 18 o and such homogenization distances are difficult to reconcile with observations of order of magnitude variations in reaction progression the centimetre scale or less it is possible that infiltration occurred during events short lived compared with diffusion that the reactions started at different temperatures dependent on bulk composition or that diffusion of water from layers with less reactants to layers with more reactants was important in driving the biotite producing reaction however variations of fluid composition inferred from the mineral assemblages are apparently inconsistent with diffusion driving reactions progress and models of precursor assemblages do not indicate significant compositional control of the temperature of the first appearance of biotite in the rocks irrespective of the details of flow and diffusive exchange on the centimetre scale the average reaction progress in the waterville limestone member requires significant layer parallel fluid fluxes

transport mechanisms by fluid flow in metamorphic rocks oxygen and strontium decoupling in the trois seigneurs massif a consequence of kinetic dispersion

natural constraints on metamorphic reaction rates abstract quantitative constraints on the rates at which metamorphic reactions proceed in nature are now available from several sources most common are predictions made on the basis of laboratory kinetic data however the applicability of such laboratory based predictions has long been questioned and many observations in the field now suggest much slower rates here published quantitative field based constraints on high temperature 400 c reaction rates are assembled from a variety of sources reaction rates attending regional metamorphism are four to seven orders of magnitude slower than most laboratory based predictions a general rate law for regional metamorphism has been derived which best describes these field based data log 10 r net 0 0029 t 9 6 1 where r net is the net reaction rate g cm 2 a and t is temperature c at the same time natural reaction rates attending contact metamorphism differ from laboratory based predictions by less than two orders of magnitude and are in close agreement at higher temperatures thus while existing laboratory based kinetic data may be judiciously applied to some contact metamorphic systems laboratory based kinetic predictions clearly misrepresent regional metamorphism to explain this kinetic discrepancy regional metamorphic reaction rates may be limited by slow intergranular transport due to comparatively limited or transient availability of aqueous fluid in the intergranular medium the general field based rate law may be applied to regional metamorphic and other environments i e ultrahigh pressure or ultrahigh temperature metamorphism if similar system characteristics mainly low aqueous fluid content can be inferred

a crustal thickness map of africa derived from a global gravity field model using euler deconvolution summary we develop a new continental scale crustal model for africa by modelling the free air gravity anomaly eigen gl04c which was developed from 30 months of grace level 1b data covering the period from 2003 february to 2005 july and surface gravity data from seven different sources from this gravity model crustal thickness is estimated using 3 d euler deconvolution a method that does not rely on ap rioridepth and density constraints the results are in good agreement i e within 5km of seismically determined moho depth estimates from across the continent except for narrow tectonic regions such as rift valleys and areas where seismic velocity models of the crust indicate a gradational moho the results show that crustal thickness is fairly homogeneous with an average crustal thickness for the whole continent of 39 2 sd km the average moho depth for most terrains is within 5km of the continental average and there is little variability between terrains of different age the average thickness for archean proterozoic and palaeozoic crust is 39 39 and 41km respectively crustal thickness in sedimentary basins across northern and central africa varies between 33 and 36km through comparison with global averages for similar aged terrains we find that african crustal thickness does not deviate significantly from the thickness of crust in other parts of the world

short note source geometry identification by simultaneous use of structural index and shape factor a cross plot of the shape factors and the structural indices determined from gravity anomalies over various idealized sources namely horizontal vertical lines and vertical ribbons with various strike lengths and depth extents forms a closed loop different segments of this loop termed the source geometry identification loop sgil correspond to different source geometries combined use of the structural index and the shape factor determined from an isolated gravity anomaly reduces the ambiguity in characterizing the source geometry a simulated example and three field examples namely a cuban chromite anomaly an indian example over manganese ore and a sulphide ore from quebec have been analysed by the proposed method in order to identify their respective source geometries

weighted euler deconvolution of gravity data euler deconvolution is used for rapid interpretation of magnetic and gravity data it is particularly good at delineating contacts and rapid depth estimation the quality of the depth estimation depends mostly on the choice of the proper structural index and adequate sampling of the data the structural index is a function of the geometry of the causative bodies for gravity surveys station distribution is in general irregular and the gravity field is aliased this results in erroneous depth estimates by weighting the euler equations by an error function proportional to station accuracies and the interstation distance it is possible to reject solutions resulting from aliasing of the field and less accurate measurements the technique is demonstrated on bouguer anomaly data from the charlevoix region in eastern canada

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

gulf stream meanders in the south atlantic bight 1 scaling and energetics as the gulf stream flows along the outer edge of the southeastern united states continental shelf it meanders on and offshore creating frontal eddies which influence the exchange of water between the shelf and the adjacent atlantic ocean on weekly timescales the dynamics controlling the development of such features in this region the open quotes south atlantic bight close quotes or sab is the subject of this pair of papers in the present paper the authors provide the background on the subject summarize the length time and velocity scales associated with these features and discuss meander energetics as simulated by a numerical model the scaling indicates that significant ageostgrophic dynamics may be crucial to the development of these flows and that the semigeostrophic equations may provide the simplest description of the system these nonlinear dynamics may permit complex interactions among meander modes and with topography resulting in the observed behavior of the gulf stream in the sab the princeton ocean model was tuned for conditions which prevail in the southern south atlantic bight and idealized numerical simulations of gulf stream meanders and associated frontal eddies were performed the system was found to be sensitive to both the amplitude and period of a small perturbationmore applied at the southern boundary the energetics indicate that the development of these features is controlled by a mixture of both baroclinic and barotropic instabilities of the mean gulf stream flow tantalizing hints of a period doubling phenomenon are present in the energy conversion time series 47 refs 12 figs 4 tabs less

structural setting of northern tunisia insights from gravity data analysis jendouba case study detailed gravity data in conjunction with available surface geology are analyzed to infer the organization of the underlying structures in jendouba area gravity data analysis benefits from the gravity bouguer anomaly upward continuations residual distribution derivatives and euler deconvolved maps the main results display a positive amplitude gravity anomaly as the response of triassic evaporitic bodies and important ne trending features at the boundaries between the triassic outcrops and their enveloping strata integration of gravity geological and structural maps let to the identification of major structural directions and trends of the study area it confirms some structural elements gathered from outcrops it defines also new ones

simulation of frontal eddies on the east florida shelf 1 frontal eddies are typical features on the cyclonic side of the florida current fc along the east florida shelf efs producing characteristic shingle patterns in sea surface temperature intensive observations and idealized numerical analyses have described such eddies over the past three decades here simulations from a high resolution 2 to 10 km curvilinear coastal ocean circulation model demonstrate their structure and evolution with realistic bottom topography and fc over an extended domain including the straits of florida and the entire efs simulations agree with observations in estimating translation speed recurrence period strength characteristic length scales and overall circulation patterns in addition the simulations demonstrate the interactions between the frontal eddies and the meander crests and bottom topography as they translate along the efs overall a first level of model validation has been established that will facilitate consideration of the forecast problem for frontal eddies associated with the fc on the efs

subsurface geology of the ariana region diapir zone northern tunisia by means of gravity analysis gravity data were used to investigate subsurface geology in the ariana region from the diapir zone in tunisia our study area is located in the north eastern part of the maghrebides affected by the alpine orogeny despite the smooth topography and the low density of the quaternary series integrated geological data and gravity responses bouguer anomaly and derivative mapping help decipher the geological heterogeneities beneath the quaternary overburden the interpretation of the gravity data points to alignments of positive anomalies delineating two major orthogonal features that are thought to characterize the subsurface deep structures 1 a lineament striking north west at the emplacement of an actual horst bordered by a buried fault system directed n140 and 2 a second counterpart striking north east and is clearly expressed in the surface outcrops these principal structural features were described as the result of a regional tectonic compressive phase dating the miocene this phase was marked by reactivation and oblique slip displacements of regional faults some of which n070 are reverse and south east vergent further faulting enhanced the ascension of triassic evaporites in diapir structures this tectonic evolution may be integrated in the regional tectonic scheme of alpine orogeny induced by a collision between african and european plates

investigation of crustal thickness in eastern anatolia using gravity magnetic and topographic data the tectonic regime of eastern anatolia is determined by the arabia eurasia continent continent collision several dynamic models have been proposed to characterize the collision zone and its geodynamic structure in this study change in crustal thickness has been investigated using gravity magnetic and topographic data of the region in the first stage two dimensional low pass filter and upward analytical continuation techniques were applied to the bouguer gravity data of the region to investigate the behavior of the regional gravity anomalies next the moving window power spectrum method was used and changes in the probable structural depths from 38 to 52 km were determined the changes in crustal thickness where free air gravity and magnetic data have inversely correlated and the type of the anomaly resources were investigated applying the euler deconvolution method to bouguer gravity data the obtained depth values are consistent with the results obtained using the power spectrum method it was determined that the types of anomaly resources are different in the west and east of the 40 e longitude finally using the obtained findings from this study and seismic velocity models proposed for this region by previous studies a probable two dimensional crust model was constituted

survey of a gulf stream frontal filament an assortment of data is used to map and characterize a gulf stream filament during 21 25 april 1980 a filament was captured as it moved through an array of moored current meters on the northeast florida continental shelf current and temperature time series along with satellite vhrr and aircraft art are used to reconstruct the event the event is found to consist of an anticyclonically rotating warm tongue inshore of a cold dome the rotational scheme of the current in the warm tongue is opposite to previous hypotheses upwelling occurs in both the cold dome region and below the warm tongue

gravity characterization of the la rioja valley basin argentina thelariojavalleyisasedimentarybasininthepampean ranges a region of basement uplifts and broad valleys in the andean foreland the present study advances understanding ofthegenesisofthebasinbyprovidingsubsurfacedataonits geometry and characteristics gravity and topographic data including a newly completed gravity survey were integrated tocreateacompletebougueranomalymap eulerandwerner deconvolution techniques sum with traditional potentialfieldsmethodsandsparseindependentdatafromseismicand magnetotelluric methods to test and constrain the subsurface geophysical characterization of the la rioja valley basin the residual anomaly obtained through upward continuation was inverted to obtain the best 3d gravity model of the la riojavalley basin which shows the first order geometry ofthebasin thisrevealsthatthebasinisasymmetric withup to 6000 m of low density basinfill in a depocenter located in thewest centralsectorofthevalley second orderfeaturesof thebasinincludeasoutheast trendingbasementhigh likelya fault zone that subdivides the basin which has been revealed by a suite of analytical methods 3d model of depth to basement a 2d inverse model and werner deconvolution solutions

grain scale stable isotope disequilibrium during fluid rock interaction 2 an example from the penninic austroalpine tectonic contact in eastern switzerland at the tectonic contact between the penninic platta nappe and the lower austroalpine err unit in eastern switzerland carbon and oxygen isotope fronts are preserved at the interface of serpentinites and low grade regional metamorphic metacarbonates the carbon and oxygen isotope compositions of calcite are delta 13 c 2 3 per mil pdb and delta 18 o 21 0 per mil smow in the unaltered metacarbonates at a distance of about 20 m from the lithologic contact and they are delta 13 c 0 0 per mil and delta 18 o 14 0 per mil at the metacarbonate serpentinite interface the concomitant shift in the oxygen isotope composition of quartz is from 24 0 to 17 0 per mil a peculiarity of the isotope pattern is the systematic variation of the quartz calcite oxygen isotope fractionations across the isotopic front quartz calcite fractionations are relatively small nearly equal 3 0 per mil in the unaltered metacarbonate and at the metacarbonate serpentinite interface and they increase in the transitional zone in between with a maximum of 7 1 per mil at a distance of about 2 m from the contact this pattern is interpreted as a result of advective dispersive material transport across the lithologic contact and coupled first order kinetic mineral fluid exchange the oxygen isotope systematics indicate that calcite fluid isotope exchange was fast and local calcite fluid equilibrium prevailed during fluid rock interaction in contrast quartz fluid exchange was relatively sluggish and the quartz calcite oxygen isotope fractionations deviated significantly from their equilibrium values front geometries suggest that dispersive processes contributed substantially to material transport across the metacarbonate serpentinite contact cross layer advective flow was minor with a maximum time integrated volumetric flux of 21 m 3 m 2 from the influence of quartz fluid exchange kinetics on the intermineral fractionations quantitative relations among transport velocities the rates of mineral fluid exchange and the duration of fluid rock interaction are derived these relations are then combined with geologic and experimental data to identify feasible scenarios of fluid rock interaction

can metamorphic reactions proceed faster than bulk strain available constraints on metamorphic reaction rates derived from the study of natural systems are similar to or slightly lower than the bulk strain rates measured in the same rocks here we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain grain boundary migration accommodated dislocation creep gbmdc or grain boundary diffusion creep gbdc i e pressure solution both of which involve dissolution precipitation as we define it will occur simultaneously with mineral reactions involving dissolution precipitation in the presence of a non zero deviatoric stress the exact relationships between reaction and strain are different depending on whether gbmdc or gbdc is controlling strain but the mechanistic link exists in both cases we present theoretical arguments which show that bulk strain by gbmdc or gbdc which may additionally be accommodated by processes not involving dissolution precipitation such as dislocation glide and climb or grain boundary sliding should in most cases be somewhat faster than the bulk reaction rates as observed with few exceptions for natural metamorphic systems undergoing plastic deformation strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks the data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate

oxygen isotope exchange processes and disequilibrium between calcite and forsterite in an experimental c o h fluid oxygen isotope exchange between calcite and forsterite were investigated in the presence of a large amount of supercritical fluid the experiments were conducted in standard cold seal pressure vessels at 680 c and 500 mpa in the t p x sub co sub 2 stability field of the calcite forsterite assemblage for 2 5 10 20 40 and 80 days respectively the weight ratio of mineral to fluid in the starting mixture is 1 46 the fluid was a mixture of h sub 2 o and co sub 2 with the mole fraction of co sub 2 being 0 1 the results show that the oxygen isotope exchange between the minerals was accomplished via mineral fluid exchange by a dual mechanism i e initial rapid exchange due to ostwald ripening of both calcite and forsterite followed by a slower diffusion controlled process furthermore for the given fluid composition calcite shows a greater rate of dissolution recrystallization and oxygen isotope exchange with fluid than forsterite as a result oxygen isotope fractionations between calcite and forsterite and between the minerals and the fluid can simply pass the equilibrium fractionations with time and even lead to crossover behavior once diffusion becomes a primary mechanism for further isotope exchange in the three phase system the rate of oxygen diffusion in calcite ismore equal to or slightly less than that in forsterite less

fast mapping of magnetic basement depth structure and nature using aeromagnetic and gravity data combined methods and their application in the paris basin assessment of deep buried basin basement relationships using geophysical data is a challenge for the energy and mining industries as well as for geothermal or co2 storage purposes in deep environments few methods can provide geological information magnetic and gravity data remain among the most informative and cost effective methods here in order to derive fast first order information on the basement basin interface we propose a combination of existing and original approaches devoted to potential field data analysis namely we investigate the geometry i e depth and structure and the nature of a deep buried basement through a case study sw of the paris basin joint processing of new high resolution magnetic data and up to date gravity data provides an updated overview of the deep basin first the main structures of the magnetic basement are highlighted using euler deconvolution and are interpreted in a structural sketch map the new high resolution aeromagnetic map actually offers a continuous view of regional basement structures and reveals poorly known and complex deformation at the junction between major domains of the variscan collision belt second werner deconvolution and an ad hoc post processing analysis allow the extraction of a set of magnetic sources at or close to the basin basement interface interpolation of these sources together with the magnetic structural sketch provides a werner magnetic basement map displaying realistic 3d patterns and basement depths consistent with data available in deep petroleum boreholes the last step of processing was designed as a way to quickly combine gravity and magnetic information and to simply visualize first order petrophysical patterns of the basement lithology this is achieved through unsupervised classification of suitably selected gravity and magnetic maps and as compared to previous work provides a realistic and updated overview of the cartographic distribution of density magnetization of basement rocks altogether the three steps of processing proposed in this paper quickly provide relevant information on a deep buried basement in terms of structure geometry and nature through petrophysics notwithstanding limitations of the proposed procedure are raised in the case of the paris basin for instance this study does not provide proper information on pre mesozoic basins some of which have been sampled in deep boreholes

characteristics of currents over the continental slope of the eastern bering sea between september 1989 and 1990 twenty six current records were collected by instruments on eight moorings located in pribilof and zhemchug canyons and at a site midway between these features these records provide the first long term eulerian measurements from the slope and mid slope of the eastern bering sea results from the current records together with water property observations permit a characterization of the bering slope current moderate flow 2 to 18 cm s 1 followed the bathymetry toward the northwest and existed primarily in the upper 300 m wind and current energy increased in winter but vector mean current did not increase at all sites wind forcing accounted for only a small fraction of the current fluctuations at one mid slope location in pribilof canyon bathymetry resulted in rectification of the strong daily tidal current estimates of heat and salt fluxes indicate some significant shoreward transport however this flux did not occur preferentially in the canyons

prograde temperature time evolution in the barrovian type locality constrained by sm nd garnet ages from glen clova scotland abstract the timing of garnet growth during metamorphism associated with the grampian orogeny in the sillimanite zone of the barrovian type locality in glen clova scotland was determined by sm nd geochronology two high precision garnet whole rock ages were achieved by employing hf partial dissolution of garnet separates to optimize purity multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data an early stage at 472 9 2 9 ma during d 2 deformation under garnet zone conditions c 500 550 c and a later stage at 464 8 2 7 ma during or slightly after d 3 deformation mostly under sillimanite zone conditions peak temperature of c 660 c but possibly including some growth during kyanite zone conditions when combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of barrow9s zones were achieved roughly contemporaneously the difference between garnet zone and sillimanite zone peak temperature attainment is 2 8 3 7 ma the near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over thickened crust we suggest that local igneous intrusions with synmetamorphic ages provided that additional heat

fluid flow and chemical reaction kinetics in metamorphic systems the treatment and effects of chemical reaction kinetics during metamorphism are developed along with the incorporation of fluid flow diffusion and thermal evolution the interplay of fluid flow and surface reaction rates the distinction between steady state and equilibrium and the possible overstepping of metamorphic reactions are discussed using a simple analytic model this model serves as an introduction to the second part of the paper which develops a reaction model that solves the coupled temperature fluid flow chemical composition differential equations relevant to metamorphic processes consideration of stable isotopic evidence requires that such a kinetic model be considered for the chemical evolution of a metamorphic aureole a general numerical scheme is discussed to handle the solution of the model the results of this kinetic model allow us to reach several important conclusions regarding the factors controlling the chemical evolution of mineral assemblages during a metamorphic event 41 refs 19 figs 5 tabs

rates and mechanisms of isotopic exchange background the proliferation of stable isotope laboratories in recent years has led to a marked increase in the routine use of stable isotope analyses in geochemical studies this trend is likely to continue because of the development of micro analytical techniques e g laser ablation ion microprobe femtomole carrier gas methods etc for stable isotope analysis and the potential of these techniques to reveal the detailed sequence of thermal and fluid histories preserved in the rock record see mckibben et al 1998 equilibrium isotope fractionation factors and rates of isotopic exchange form the cornerstones for the interpretation of stable isotope data from natural systems indeed determination of the distribution of the stable isotopes of c o h s in gases fluids and minerals has become a standard and extremely powerful approach in elucidating the temperatures material fluxes rock and fluid origins and time scales associated with ancient and active fluid rock interaction processes in the earth s crust see valley et al 1986 although the thermodynamic principles underlying isotopic fractionation behavior were developed more than fifty years ago by urey 1947 the calibration of fractionation factors has proven to be difficult during the last four decades a great deal of progress has been made in determining the equilibrium fractionations of 18o 16o d h 13c 12c and 34s 32s among fluid and mineral components at high temperatures 400 c both from experimental exchange studies theoretical calculations using vibrational spectra semi empirical calculations using bond type considerations and empirical calibrations based on natural assemblages see references in o neil 1986 kyser 1987 criss 1991 1999 chacko et al this volume of these the experimental method is the most direct because it involves the least number of a priori assumptions despite the utility of the equilibrium approach in quantifying the isotopic behavior in select water rock

drifting buoy observations of a loop current anticyclonic eddy the loop current penetrated deep into the gulf of mexico in early 1989 after several eddy formation and reattachment cycles a southwestward propagating anticyclonic eddy was formed during the summer the loop current and subsequent eddy produced strong currents over widespread areas on the louisiana continental slope prompting a series of current measurement programs because of the lack of satellite infrared coverage in the gulf of mexico during the summer the trajectories of 53 argos tracked drifting buoys deployed in the gulf of mexico in 1989 were assembled to determine the synoptic history of the loop current and anticyclonic eddy during this event ten of the most critical summertime buoy trajectories are discussed here in addition the trajectories of three of the buoys simultaneously deployed at different radii in the eddy were chosen for analysis with a kinematic feature model the model assumes the looping buoy motion is generated by an elliptical orbit around a translating eddy center the buoy trajectories were used to determine time series of the feature model parameters including eddy center shape and circulation characteristics the time series results indicate that the eddy actually propagated in a series of short sprints separated by longer stalls during the sprints the eddy propagated as an elliptical but otherwise symmetric solid body during the stalls significant asymmetries developed the asymmetries are consistent with attachment to the loop current during the first stall the effects of topographic rossby wave dispersion during the second and the effects of planetary rossby wave dispersion during the third the results provide both a framework for the interpretation of in situ observations and a detailed evolutionary history for numerical modelers

syndeformational recrystallization dynamic or compositionally induced dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change in chemical composition driven by strain energy in the form of dislocations driving potentials additional to internal strain energy may contribute to the recrystallization of naturally deformed minerals which form solid solutions such as feldspar amphiboles and pyroxenes if they change their composition during recrystallization to estimate the relative importance of these driving potentials the chemical composition of porphyroclasts and recrystallized grains of plagioclase clinopyroxene and hornblende have been investigated in samples from a high grade shear zone of the ivrea zone italy the plagioclases show two different recrystallization microstructures bulging recrystallization at grain boundaries and discrete zones of recrystallized grains across porphyroclasts probably involving fracturing deformation took place under amphibolite facies conditions on a retrograde p t path porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their or content and yield a g between mean host grain and mean recrystallized grain composition at fixed p t conditions of approximately 5 joules 10 4 m3 extreme compositional variations yield approximately 60 j 10 4 m3 the increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100 joules 10 4 m3 for high values of dislocation densities of 1014 m 2 thus the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but may be as great as that of deformational energies for larger chemical differences in the other type of microstructure porphyroclasts and recrystallized grains in discrete zones differ in their anorthite content the maximum g induced by the compositional disequilibrium is on the order of 100 j 10 4 m3 this maximum value is of the same magnitude as the g derived from high dislocation densities of 1014 m 2 the resulting combined g is approximately twice as high as for deformational g alone and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures the recrystallization mechanism depends on the nature of the driving potential grain boundary migration gbm and heterogeneous nucleation can release gibbs free energy induced by compositional disequilibrium whereas this is not likely for subgrain rotation therefore only gbm and heterogeneous nucleation may link metamorphism and deformation so that syndeformational recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction

models of garnet differential geochronology abstract rayleigh distillation models are developed to describe theoretical growth zoning of lu hf rb sr sm and nd in typical garnet crystals from metapelites and metabasites effects of diffusion limited transport within the matrix and intracrystalline diffusion are also considered qualitatively theoretical zoning profiles show strong depletions of lu in garnet rims compared to cores but virtually invariant hf rb sr sm and nd profiles generally consistent with natural profiles for lu and hf and previously published models theoretical isochron diagrams for lu hf exhibit distinctive arcuate distributions and high mswds consistent both with himalayan data and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude predicted isochron diagrams for sm nd and rb sr exhibit vertical arrays for garnet and high mswds that are generally lower than for lu hf in metapelites inherent chronologic resolution for bulk separates is best for lu hf in metapelites and rb sr but analytical considerations favor rb sr or sm nd for chronologic zoning studies diffusion limited transport in the rock matrix strongly influences zoning profiles but does not change the main trends on isochron diagrams intracrystalline diffusion will initially rotate lu hf isochrons to steeper slopes giving older apparent ages the natural himalayan data indicate growth of garnet in one rock from the greater himalayan sequence at 34 ma consistent with previously measured monazite ages from the same rock data from another himalayan rock suggest polymetamorphism that includes a paleozoic component

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