oxygen diffusion in amphiboles the kinetics of oxygen isotope self diffusion in natural samples of hornblende tremolite and richterite have been measured samples were run under hydrothermal conditions using 18o enriched water profiles of 18o 16o 18o vs depth into the crystal were obtained using an ion microprobe the depths of sputtered holes were measured using an optical interferometer at 1000 bars 100 mpa water pressure the activation energies q and pre exponential factors d0 for diffusion parallel to c are d0 cm2 sec q kcal gm atom t c hornblende 1 20 1 10 741 6 650 800 tremolite 2 30 2 10 8 39 5 650 800 richterite 3 5 2 10 4 57 2 650 800 the diffusion coefficient d for hornblende at 800 c and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions an increase in water pressure from 200 to 2000 bars increases d by a factor of 2 7 for diffusion parallel to c at 800 c the d value for hornblende at 800 c is about 0 01 that for quartz and 0 001 that for anorthite as a result closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars it is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions

extension crustal structure and magmatism at the outer voring basin norwegian margin regional analysis of new 2d and 3d multichannel seismic data has improved interpretation of the crustal configuration and structural style along the norwegian margin five domains with different structural styles and evolutions are defined along the outer voring basin 1 the nyk high naglfar dome 2 the north gjallar ridge 3 the gleipne saddle 4 the south gjallar ridge 5 the ran ridge early campanian early paleocene and early to mid cretaceous extensional events are evidenced timing of deformation and structural styles observed along each segment reflect a lateral variation of the rifted system probably affected by magma tectonic processes correlation with the deep structures of the outer voring basin shows that the shallow structure in that basin is directly controlled by a deep seated strong high amplitude reflection the t reflection marking the top of a high velocity body v p 7 km s 1 the relation between the lower crustal architecture and the subsurface basin structures has implications for the margin evolution and for the nature of the high velocity body

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

geomorphological evidence of the influence of pre volcanic basement structure on emplacement and deformation of volcanic edifices at the cofre de perote pico de orizaba chain and implications for avalanche generation abstract pre volcanic structure of the basement influences volcanism distribution and avalanche generation in volcanic edifices therefore systematic studies of basement structure below volcanic chains are necessary to understand the deformation effects observed in the surface and vice versa based on a compilation of pre existing data interpretation of aerial photographs and satellite images and a collection of structural data we analyzed morphological and structural features of the cofre de perote pico de orizaba cp po volcanic chain and its basement we have identified three sets of regional lineaments that are related to basement trends 1 nw 55 se fractures are parallel to anticline folds observed in cretaceous rocks that originated during laramide shortening these folds present an abrupt morphology observed only in the eastern flank but that is likely to continue below the volcanic chain 2 ne 55 sw fractures are parallel to normal faults at the basement we infer that these basement faults confine the cp po chain within a stepped graben with a total normal displacement of about 400 m these faults have been active through time since they have affected volcanic deposits and induced the emplacement of monogenetic vents notably lineaments of monogenetic vents concentrate where the basement is relatively shallow 3 another set of faults oriented n s has been observed affecting the scarce basement outcrops at the western flank of the chain covered by lacustrine deposits lineaments measured in the volcanic edifice of pico de orizaba correlate with the regional trends in particular the ne 55 sw alignment of monogenetic vents and fractures at pico de orizaba suggest that the same dike trend exists within the volcanic edifice a normal fault with similar orientation was documented at the ne continuation of an alignment crossing the volcanic edifice along the jamapa canyon in the absence of magmatic activity related to collapses the displacement of ne 55 sw faults represents a potential triggering mechanism for generating avalanches at pico de orizaba volcano instability is enhanced by the presence of n s trending fractures crossing the entire volcanic edifice and e w fractures affecting only the present day cone we conclude that mechanical instability of the volcanic chain is influenced by the basement structure heterogeneity but further detailed studies are necessary at individual volcanoes to evaluate their effects on volcano deformation

an experimental study of strontium and lead diffusion in calcite and implications for carbonate diagenesis and metamorphism chemical diffusion of pb and sr has been measured in natural calcite under anhydrous conditions the source of diffusants for the sr and pb diffusion experiments were srco3 and pbo powders respectively experiments were run in sealed silica capsules containing finely ground calcite to ensure stability of the single crystal samples during diffusion anneals rutherford backscattering spectroscopy rbs was used to measure diffusion profiles the following arrhenius relations were obtained over the temperature range 440 800 c for sr 440 650 for pb dsr 2 1 x 10 13exp 132 6 kj mol 1 rt m2 s 1 dpb 5 8 x 10 15exp 117 9 kj mol 1 rt m2 s 1 diffusion of sr and pb is relatively rapid in comparison to rates for these elements in other minerals but fine scale zoning and isotopic and chemical signatures many be retained in calcites under many conditions if diffusion is the dominant process affecting alteration

oxygen diffusion in quartz diffusional oxygen exchange between quartz and water was studied by determining 18o profiles in the quartz using an ion microprobe inverse error function plots of the data are linear as required for a diffusion mechanism one natural and two synthetic samples were used and although they had different impurity concentrations oh na and al per 106 si atoms are 7 4000 35 835 and 67 127 respectively the d values were the same within experimental uncertainty diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700 c and diffusion normal to the rhombohedron is intermediate the data indicate a change in slope on an arrhenius plot at the boundary at 100 mpa 1 kbar water pressure the preexponential factors do in square centimeters per second and activation energies q in kilojoules per mole for quartz 500 550 c are as follows do 190 and q 284 parallel to c and do 8 10 2 and q 238 normal to the rhombohedron for quartz 600 800 c do 4 10 7 and q 142 parallel to c do 9 10 7 and q 155 normal to the rhombohedron and do 1 10 4 and q 234 normal to c d increases with ph 2 o and between 25 and 350 mpa log d versus log h 2 o has a slope of approximately 1 1 these d values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water free systems

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

temperature limits for preservation of primary calcite clumped isotope paleotemperatures abstract solid state reordering of c o bonds in the calcite lattice can alter the clumped isotope composition of paleotemperature archives such as fossil brachiopod shells without inducing significant changes in shell microstructure and trace element concentrations metrics commonly used to gauge preservation quality to correctly interpret the paleoenvironmental significance of clumped isotope derived paleotemperatures it is necessary to understand the temperature time domain in which solid state c o bond reordering is important we address this question using a combination of laboratory and natural geological experiments on paleozoic brachiopod shells the laboratory experiments involve heating fossil brachiopod calcite at different temperatures and times to directly observe rates of 13 c 18 o bond reordering the resulting arrhenius parameters are indistinguishable from values previously determined for an optical calcite with similar trace element compositions we develop an alternative kinetic model for reordering that accounts for non first order reaction progress observed during the initial several hundred minutes of laboratory heating experiments and show that the simplified first order approximation model passey and henkes 2012 predicts reaction progress equally well for temperatures and timescales relevant to sedimentary basins we evaluate our laboratory based rate predictions by studying brachiopod calcite from several sedimentary basins with independently constrained burial temperature histories specifically we use the laboratory derived arrhenius parameters to predict the evolution of brachiopod calcite clumped isotope compositions during successive one million year time steps reflecting the burial and exhumation temperature paths of each basin while this exercise is limited by the relatively large uncertainties in the temperature histories of these basins we find general correspondence within error between predicted and observed clumped isotope values we present simplified temperature time diagrams for calcite showing domains where primary clumped isotope compositions will be preserved partially reordered and fully reordered in conclusion calcite samples dwelling at 100 c or lower for 10 6 10 8 year timescales should not be affected by solid state c o bond reordering

assessment of grain scale homogeneity and equilibration of carbon and oxygen isotope compositions of minerals in carbonate bearing metamorphic rocks by ion microprobe abstract nineteen samples of metamorphosed carbonate bearing rocks were analyzed for carbon and oxygen isotope ratios by ion microprobe with a 5 15 m spot three from a regional terrain and 16 from five different contact aureoles contact metamorphic rocks further represent four groups calc silicate marble and hornfels 6 brucite marble 2 samples that contain a reaction front 4 and samples with a pervasive distribution of reactants and products of a decarbonation reaction 4 the average spot to spot reproducibility of standard calcite analyses is 0 37 2 standard deviations sd for 18 o and 0 71 for 13 c ten or more measurements of a mineral in a sample that has uniform isotope composition within error of measurement can routinely return a weighted mean with a 95 confidence interval of 0 09 0 16 for 18 o and 0 10 0 29 for 13 c using a difference of 6sd as the criterion only four of 19 analyzed samples exhibit significant intracrystalline and or intercrystalline inhomogeneity in 13 c at the 100 500 m scale with differences within individual grains up to 3 7 measurements are consistent with carbon isotope exchange equilibrium between calcite and dolomite in five of six analyzed samples at the same scale because of relatively slow carbon isotope diffusion in calcite and dolomite differences in 13 c can survive intracrystalline homogenization by diffusion during cooling after peak metamorphism and likely represent the effects of prograde decarbonation and infiltration all but 2 of 11 analyzed samples exhibit intracrystalline differences in 18 o up to 9 4 intercrystalline inhomogeneity in 18 o up to 12 5 and or disequilibrium oxygen isotope fractionations among calcite dolomite calcite quartz and calcite forsterite pairs at the 100 500 m scale inhomogeneities in 18 o and 13 c are poorly correlated with only a single mineral dolomite in a single sample exhibiting both because of relatively rapid oxygen isotope diffusion in calcite intracrystalline inhomogeneities in 18 o likely represent partial equilibration between calcite and fluid during retrograde metamorphism calcite is in oxygen isotope exchange equilibrium with forsterite in one of four analyzed samples in equilibrium with dolomite in none of six analyzed samples and in equilibrium with quartz in neither of two analyzed samples there are no samples of contact metamorphic rock with analyzed reactants and products of an arrested metamorphic reaction that are in oxygen isotope equilibrium with each other the degree of departure from equilibrium in analyzed samples is variable and is often related at least in part to alteration of 18 o of calcite during retrograde fluid rock reaction in situ sub grain scale carbon and oxygen isotope analyses of minerals are advisable in the common applications of stable isotope geochemistry to metamorphic petrology correlation of sub mm scale stable isotope data with imaging will lead to improved understanding of reaction kinetics reactive fluid flow and thermal histories during metamorphism

modes of continental lithospheric extension experimental verification of strain localization processes abstract three modes of continental lithospheric extension narrow rift wide rift and core complex were studied by means of analogue experiments the experimental method is based on models which simulate an idealized lithospheric strength profile and are scaled for gravity various styles of lithospheric extension were induced by systematic variation of brittle upper crustal thicknesses narrow rifting occurs during stable lithospheric extension when the resistance of the brittle upper crust dominates the deformation process wide rifting results from unstable lithospheric extension when the brittle upper crust is thin and ductile flow in the lower crust allows expansion of the deformation zone periodic instabilities with two wavelengths dominate wide rifting core complex mode extension is transitional between unstable and stable lithospheric extension it begins as a distributed zone of unstable extension due to thin upper brittle crust but lower crustal flow induced by the gravity force causes a relocalization of strain into narrow zones transitions between the three modes of extension are characterized in terms of competition between the strength of lithosphere and gravity forces as well as between stable and unstable deformation processes

last stage of the japan sea back arc opening deduced from the seismic velocity structure using wide angle data the japan sea is one of the most well studied back arc basins in the northwestern pacific the seismic crustal model however has been inadequate to elucidate the detailed opening model of the japan sea in 2002 to clarify the late stage of the formation style of the japan sea opening a seismic experiment using 35 ocean bottom seismographs obss an air gun array and a multichannel hydrophone streamer was undertaken in the areas from the southwestern yamato basin the oki ridge and the southwestern oki trough to the coast of the southwestern japan island arc the crusts beneath the southwestern yamato basin and the oki ridge are estimated as having approximately 13 km and 19 5 km respectively the upper and lower crusts of the southwestern yamato basin are approximately 3 2 km and 8 km thick respectively those of the oki ridge are approximately 8 2 km and 10 5 km thick respectively the upper crust of the oki ridge thickens more steeply than that of the southwestern yamato basin however the lower crust thickens more gently the crustal structure of the southwestern yamato basin shows the extended continental crust accompanied with the opening of the japan sea a remarkable structural characteristic the upper crust being thinner than the lower crust caused by listric or complicated normal faults developed in the upper crust of the southwestern yamato basin this deformed upper crust is a common structural characteristic in the southern japan sea which includes the yamato basin the southern yamato basin including the southwestern yamato basin has the thinnest upper and lower crusts in the japan sea for that reason it is suggested that the southern yamato basin had the strongest deformation by a back arc opening and that the period of the opening in the southern yamato basin had been longest in the southern japan sea the formation process of the southern yamato basin is inferred to have two stages rifting and extension of continental crust separating the northeastern and southwestern japan island arcs from the asian continent and further the extension affected by the rotation of the southwestern japan island arc

the structure of the lower crust at the argentine continental margin south atlantic at 44 s abstract it is well established that the argentine passive margin is of the rifted volcanic margin type this classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope manifested by seismic data as a seaward dipping reflector sequence sdrs here we investigate the deep structure of the argentine volcanic margin at 44 s over 200 km from the shelf to the deep oceanic argentine basin we use wide angle reflection refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times the resulting p wave velocity depth model confirms the typical volcanic margin structure an underplated body is resolved as distinctive high seismic velocity vp up to 7 5 km s feature in the lower crust in the prolongation of a seaward dipping reflector sequence a remarkable result is that a second isolated body of high seismic velocity vp up to 7 3 km s exists landward of the first high velocity feature the centres of both bodies are 60 km apart the high velocity lower crustal bodies likely were emplaced during transient magmatic volcanic events accompanying the late rifting and initial drifting stages the lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the south atlantic rift evolved by instantaneous breakup of longer continental margin segments these segments are confined by transfer zones that acted as rift propagation barriers a lower crustal reflector was detected at 3 to 5 km above the modern moho and probably represents the lower boundary of stretched continental crust with this finding we suggest that the continent ocean boundary is situated 70 km more seaward than in previous interpretations

mid crustal flow during tertiary extension in the ruby mountains core complex nevada structural analysis and geochronologic data indicate a nearly orthogonal late eocene oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer thick extensional mylonitic shear zone of the ruby mountains metamorphic core complex nevada new u pb radiometric dating indicates that the development of a northward trending lineation in the infrastructure is partly coeval with the development of a pervasive west northwest trending lineation in the mylonitic shear zone u pb monazite data from the leucogranite orthogneiss of thorpe creek indicate a crystallization age of ca 36 39 ma zircon fractions from a biotite monzogranite dike yield an age of ca 29 ma the three dated samples from these units exhibit a penetrative approximately north south trending elongation lineation this lineation is commonly defined by oriented bundles of sillimanite and or elongated aggregates of quartz and feldspar indicating a synmetamorphic and syndeformational origin the elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic nonmylonitic infrastructural core of the northern ruby mountains a portion of this midcrustal flow is coeval with the well documented west northwest sense of slip in the structurally overlying kilometer thick mid tertiary mylonitic shear zone lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure suggesting fundamental plastic decoupling between structural levels in this core complex furthermore the infrastructure is characterized by overlapping oppositely verging fold nappes which are rooted to the east and west one of the nappes may be synkinematic with the intrusion of the late eocene orthogneiss of thorpe creek in addition the penetrative elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes suggesting a kinematically related evolution the area is evaluated in terms of a whole crust extension model magmatic underplating in the lower crust stimulated the production of late eocene early oligocene granitic magmas which invaded metasedimentary and mesozoic granitic rocks of the middle crust the midcrustal rocks weakened by the magmatic heat influx acted as a low viscosity compensating material decoupled from an extending upper crust the fold nappes and lineation trends suggest large scale flow of the weakened crust into the study area the inflow pattern in the migmatitic infrastructure can be interpreted as a manifestation of midcrustal migration into an area beneath a domain of highly extended upper crustal rocks at present the inferred eocene early oligocene phase of upper crust extension remains unknown but available data on relative and geochronologic timing are not inconsistent with our model of return flow into an area already undergoing large scale upper crustal extension

styles of lithospheric extension controlled by underplated mafic bodies abstract the role of underplated mafic bodies upmb in the localization of deformation is examined using a two dimensional thermo mechanical finite element model rheological heterogeneity brought about by the upmb is linked with the following two main physical effects the upmb material is assumed to have 1 an anomalous high temperature and 2 a mafic crustal rock composition that is intrinsically weaker than the mantle the thermal effect will disappear rather quickly but the rock composition can have an effect at any stage of extension we show that the upmb has a strong influence on the style of lithospheric extension which depends on the thermal condition of the uppermost mantle at the time when the upmb is emplaced since the strength contrast between the weakened and non weakened regions is the most important factor the upmb works more efficiently on the localization of deformation for a colder uppermost mantle dependence of the localization of deformation on the temperature and thickness of the upmb i e more significant localization for the upmb with higher temperature and greater thickness also depends on the thermal condition of the uppermost mantle however the width of the upmb has a strong influence on the localization for any thermal condition of the uppermost mantle a greater amount of thinning is distributed into a narrower weakened region such a model behaviour implies that various styles of lithospheric extension including inward or outward migration of deformation and asymmetric extension can be simply obtained by considering the emplacement of the upmb which also plays an important role in controlling the onset of continental break up

crustal structure of eastern nevada from cocorp deep seismic reflection data the western nevada segments of the cocorp 40 n deep seismic reflection survey of the north american cordillera reveal the geometry of structures of cenozoic and possibly earlier ages to travel times of 10 s corresponding to depths of 30 km the most striking feature of the data is a band of prominent reflections typically at traveltimes of 9 5 to 10 5 s that are present discontinuously across the entire data set few reflections are observed from beneath the base of this reflective zone which is interpreted as the crust mantle transition this reflection mono is inferred to be continuous across the survey area varying gradually in depth but without resolvable offsets it appears to have taken its present form or position during basin range crustal extension the middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west these reflections may represent intrusions or shear zones related to basin range or pre basin range extension but some are likely to be inherited from earlier compressional deformation reflections from the upper crust are interpreted as images of basin fill strata basin range normal faults and mesozoic and paleozoic thrusts related to back arc thrusting and accretion of oceanic and arc related rocks

structural features in a brittle ductile wax model of continental extension structural features produced during the rifting of continents depend on the layered rheological properties of the crust and lithosphere and in particular on the presence of any transitions between brittle and ductile behaviour1 here we use a wax model to explore the gross structural response of continental lithosphere under pure shear extension in the presence of a continuous brittle ductile transition the wax models were deformed under various boundary conditions to reflect a variety of different regions most notably the basin and range province of north america our experiments show the development of listric normal faults structures common to regions of continental extension we also observe the formation of distributed and discrete rifting and intrusion and occlusion of the upper brittle layer by the ductile lower layer the factor controlling deformation style in each case appears to be the relative thickness of the brittle and ductile layers although a relatively high rate of strain generally promotes discrete rifting

forced folding above shallow magma intrusions insights on supercritical fluid flow from analogue modelling abstract magma emplacing at shallow crustal levels may cause significant deformation in the overlying country rock i e forced folding fracturing and faulting both at a local and or regional scale to get insights into these processes we investigated in the laboratory the development of forced folds and associated fracture fault networks an analogue magma simulated by polyglycerols was intruded into a sand pack representing the brittle crust the scaled analogue models reproduced different 3d deformation structures depending on the model parameters e g magma viscosity injection rate volumetric flux and the rheology and thickness of the host and cover rocks however all models support the observation that the emplacement of shallow magmatic bodies may result in the growth of dome shaped forced folds and associated development of tensional and compressional deformation in the host rock although the models involve simplifications these results provide useful hints for geothermal research as fractures and faults associated with magma emplacement can significantly influence the distribution and migration of superhot geothermal fluids these structures can therefore be considered potential targets for geothermal and or ore deposit exploration in this perspective the results of analogue models may provide useful geometric constraints for field work numerical modelling and particularly seismic interpretation allowing production and a better understanding of integrated conceptual models concerning the circulation of supercritical fluids

fractured controlled fluid flow and metasomatism in the contact aureole of the marulan batholith new south wales australia the late silurian bungonia limestone was contact metamorphosed adjacent to the early devonian glenrock granodiorite at 500 580 c and 0 7 2 2 kbar at 60 m from the granodiorite contact the limestone is pure calcite marble closer to the contact it was extensively metasomatised to form metre scale dykes of garnet and clinopyroxene massive wollastonite skarns decimetre thick veins dominated by vesuvianite and distinct breccia zones that contain millimetre wide veinlets of vesuvianite garnet these skarns and veins represent channel ways for aqueous si mg fe al bearing metasomatic fluids of magmatic origin the 18o values of the marble are typically little reset 20 even within millimetres of the skarn breccias and only within tens of centimetres of the thicker skarns the pattern of metasomatism and isotope resetting observed around and within the skarn zones does not accord with simple flow models that assume fluid flow from the granodiorite into marble that initially contained a connected fluid filled porosity instead the data are most compatible with hydrofracture of corrosive metasomatic fluids into initially dry rocks in short duration flow events

centrifuge modelling of the influence of crustal fabrics on the development of transfer zones insights into the mechanics of continental rifting architecture abstract centrifuge analogue experiments are used to model the reactivation of pre existing crustal fabrics during extension the models reproduced a weakness zone in the lower crust whose geometry was varied in order to investigate its role in controlling the architecture of rift segments and related transfer zones the typical rift system geometry was characterised by two offset rift segments connected by a major transfer zone in which boundary faults were oblique to the extension vector and displayed a significant transcurrent component of movement the transfer zone was also characterised by cross basin faults with both trend and strike slip component of movement opposite to that displayed by the master faults typically different structural patterns were obtained by changing the offset angle between the rift segments supporting that the structural pattern at transfer zones is strongly influenced by the orientation of pre existing discontinuities with respect to the stretching vector in the models the aspect ratio ratio of length vs width of the transfer zone shows a positive correlation with the offset angle i e the more the inherited fabric is parallel to the extension direction the longer and narrower the transfer zones in case of staircase offset of the rift segments 90 the structural pattern was characterised by two isolated rift depressions linked by a narrow transfer zone in which border faults with alternating polarity overlapped prominent rise of the ductile lower crust was also observed at the transfer zone many of these geometrical features display striking similarities with natural rift systems the results of the current experiments provide useful insights into the mechanics of continental rift architecture supporting that rift propagation width and along axis segmentation may be strongly controlled by the reactivation of pre existing pervasive crustal fabrics

dynamic models of continental rifting with melt generation abstract active or passive continental rifting is associated with thinning of the lithosphere ascent of the asthenosphere and decompressional melting this melt may percolate within the partially molten source region accumulate and be extracted two dimensional numerical models of extension of the continental lithosphere asthenosphere system are carried out using an eulerian visco plastic formulation the equations of conservation of mass momentum and energy are solved for a multi component crust mantle and two phase solid melt system temperature pressure and stress dependent rheologies based on laboratory data for granite pyroxenite and olivine are used for the upper and lower crust and mantle respectively rifting is modelled by externally prescribing a constant rate of widening with velocities between 2 5 and 40 mm yr a typical extension experiment is characterized by 3 phases 1 distributed extension with superimposed pinch and swell instability 2 lithospheric necking 3 continental break up followed by oceanization the timing of the transition from stages 1 to 2 depends on the presence and magnitude of a localized perturbation and occurs typically after 100 150 km of total extension for the lithospheric system studied here this necking phase is associated with a pronounced negative topography rift valley and a few 100 m of rift flanks the dynamic part of this topography amounts to about 1 km positive topography this means if rifting stops e g due to a drop of external forces immediate additional subsidence by this amount is predicted solidification of ascended melt beneath rift flanks leads to basaltic enrichment and underplating beneath the flanks often observed at volcanic margins after continental break up a second time dependent upwelling event off the rift axis beneath the continental margins is found producing further volcanics melting has almost no or only a small accelerating effect on the local extension value value for a constant external extension rate melting has an extremely strong effect on the upwelling velocity within asthenospheric wedge beneath the new rift this upwelling velocity is only weakly dependent on the rifting velocity the melt induced sublithospheric convection cell is characterized by downwelling flow beneath rift flanks melting increases the topography of the flanks by 100 200 m due to depletion buoyancy another effect of melting is a significant amplification of the central subsidence due to an increase in localized extension subsidence modelled magma amounts are smaller than observed for east african rift system increasing the mantle temperature as would be the case for a large scale plume head better fits the observed magma volumes if extension stops before a new ocean is formed melt remains present and convection remains active for 50 100 myr and further subsidence is significant

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

the effect of dissolved magnesium on diffusion creep in calcite we experimentally tested a series of synthetic calcite marbles with varying amounts of dissolved magnesium in a standard triaxial deformation machine at 300 mpa confining pressure temperatures between 700 and 850 c stresses between 2 and 100 mpa and strain rates between 10 7 and 10 3 s 1 the samples were fabricated by hot isostatic pressing of a mixture of calcite and dolomite at 850 c and 300 mpa the fabrication protocol resulted in a homogeneous fine grained high magnesian calcite aggregate with minimal porosity and with magnesium contents between 0 07 and 0 17 mol mgco3 at stresses below 40 mpa the samples deformed with linear viscosity that depended inversely on grain size to the 3 26 0 51 power suggesting that the mechanisms of deformation were some combination of grain boundary diffusion and grain boundary sliding because small grain sizes tended to occur in the high magnesium calcite the strength also appeared to vary inversely with magnesium content however the strength at constant grain size does not depend on the amount of dissolved magnesium and thus the impurity effect seems to be indirect at stresses higher than 40 mpa the aggregates become non linearly viscous a regime we interpret to be dislocation creep the transition between the two regimes depends on grain size as expected the activation energy for diffusion creep is 200 30 kj mol and is quite similar to previous measurements in natural and synthetic marbles deformed at similar conditions with no added magnesium

experimental modelling of thin skinned shortening around magmatic intrusions magmatic intrusions can trigger thin skinned compression of the adjacent sedimentary cover by three processes a gravity gliding away from the topographic dome resulting from the ascending magma b fluid push from the rear resulting from forceful intrusion then lateral spreading of the magma and c increased loading by volcanic accumulation the applicability of the first two mechanisms gravity gliding and fluid push was tested using dynamically scaled experiments model results help to elucidate the kinematics and structural evolution of thrusts and folds formed by such processes and determine which geological parameters control the deformation style the results show that the presence of a weak layer within the sedimentary overburden is essential to form thrust and fold belts around the intrusion experiments suggest that although gravity gliding can dominate the early stages of deformation most of the deformation is caused by lateral spreading of the fluid magma pushing the adjacent sedimentary rocks models also suggest that true laccoliths can form only if the sedimentary section comprises a basal low viscosity stratum comparison with natural examples allows the evaluation of the applicability of experimental results

dynamic models of continental rift valley systems abstract in experimental scale models simulating the major continental rift valley systems rifting is caused by diapiric uprise of viscous material induced by a centrifugal body force this force plays the same role as gravity in nature the scale models show that bending alone as a mechanism of rifting is unrealistic rather the rift structures form as a result of the combined effect of gentle bending and lateral stretching of the crust the models also show that graben structures could form at any stage of development of an ascending and laterally spreading diapiric body depending upon the density and viscosity contrast of the body and its surroundings and the strength of the model crust the close resemblance in structure of the models with natural rift systems suggests a close similarity of dynamics and driving forces

do calcite rocks obey the power law creep equation abstract the power law creep equation e n exp q rt is commonly used to relate strain rate e stress and temperature t for thermally activated dislocation creep in rocks when triaxial deformation experiments on marble and limestone samples are performed at temperatures of 400 1050 c to strains 3 and 10 7 s 1 the variations in strength among different rocks at nominally identical conditions are much larger than the experimental uncertainty during dislocation creep the strengths of various limestones and marbles decrease with increasing grain size similar to the hall petch effect in metals the stress sensitivity of strain rate n ln e ln and the temperature sensitivity of strain rate q r ln e 1 t differ greatly for the various calcite aggregates there is a systematic dependence of n and q on stress grain size and perhaps temperature and there is no interval in stress where n is constant thus the steady state power law equation is an inadequate description of dislocation creep in calcite rocks to improve the constitutive law it may be necessary to include at least one additional state variable that scales with grain size

ion probe and fluid inclusion evidence for co seismic fluid infiltration in a crustal detachment we have investigated the geochemical pattern of fluid infiltration in the extensional detachment of tinos island cyclades greece ion microprobe o isotope analyses and fluid inclusion studies have been conducted in strain fringes developing around pyrite blasts in the mylonite of the shear zone micro scale traverses in quartz and calcite fibres show that 18o increases from 17 18 to 20 21 in 1 mm towards the blast drops of 3 in 200 m then rises again in the direction of growth 18o variations are interpreted as transient influxes of exotic fluids into the shear zone between periods of closed system buffering by the host rock fluid inclusions trapped in the fibres show fluctuating salinities 0 4 wt nacl eq and densities that reflect drops of the pore pressure from lithostatic 1 to hydrostatic 0 4 values during fringe growth isotopic and microthermometric data are consistent with models of seismic pumping developed for compressive shear zones we therefore suggest that co seismic pore pressure variations developed suction forces sufficient to drive large scale fluid migration in the tinos detachment as in convergent tectonic settings

modelling the extension of heterogeneous hot lithosphere abstract the consequences of weak heterogeneities in the extension of soft and hot lithosphere without significant previous crustal thickening has been analysed in a series of centrifuge models the experiments examined the effects of i the location of heterogeneities in the ductile crust and or in the lithospheric mantle and ii their orientation perpendicular or oblique to the direction of bulk extension the observed deformation patterns are all relevant to the so called wide rifting mode of extension weak zones located in the ductile crust exert a more pronounced influence on localisation of deformation in the brittle layer than those located in the lithospheric mantle the former localise faulting in the brittle crust whereas the latter tend to distribute faulting over a wider area this latter behaviour depends in turn upon the decoupling provided by the ductile crust localised thinning in the brittle crust is accompanied by ductile doming of both crust and mantle domains of maximum thinning in the brittle crust and ductile crust and mantle are in opposition lateral differences in brittle crust thinning are accommodated by lateral flow in the ductile crust and mantle this contrasts with cold and strong lithospheres whose high strength sub moho mantle triggers a necking instability at the lithosphere scale this also differs from the extension of thickened hot and soft lithospheres whose ductile crust is thick enough to give birth to metamorphic core complexes thus for the given lithospheric rheology the models have relevance to backarc type extensional systems such as the aegean and the tyrrhenian domains

dynamics of periodic instabilities during stretching of the continental lithosphere view from centrifuge models and comparison with natural examples 1 analogue models are used to investigate the development of periodic instabilities during a wide rifting mode of continental extension the results of modeling support that unstable lithospheric extension is accompanied by development of regularly spaced structures at two characteristics wavelengths a crustal shorter wavelength and mantle longer wavelength both wavelengths are dependent on the brittle ductile decoupling i e dominance of the brittle rheology of the upper crust over the ductile behavior as an increase in the brittle layer thickness and a decrease in the extension velocity decreasing the ductile resistance lead to an increase in both crustal and mantle wavelengths the characteristic wavelengths also increase with increasing the amount of total extension whereas variations in the thickness of the ductile lower crust have minor effects on the spacing of structures these results are similar to previous analytical models and match well the wavelength of structures in regions undergoing wide rifting as the basin and range the aegean the west antarctic rift system and examples from the extensional systems of the china mongolia regions

carbonate clumped isotope bond reordering and geospeedometry carbonate clumped isotope thermometry is based on the preference of 13c and 18o to form bonds with each other at elevated temperatures such bond ordering is susceptible to resetting by diffusion of c and o through the solid mineral lattice this type of bond reordering has the potential to obscure primary paleoclimate information but could also provide a basis for reconstructing shallow crustal temperatures and cooling rates we determined arrhenius parameters for solid state reordering of c o bonds in two different calcites through a series of laboratory heating experiments we find that the calcites have different susceptibilities to solid state reordering reaction progress follows a first order rate law in both calcites but only after an initial period of non first order reaction that we suggest relates to annealing of nonequilibrium defects when the calcites are first heated to experimental temperature we show that the apparent equilibrium temperature equations or closure temperature equations for carbonate clumped isotope reordering are analogous dodson s equations for first order loss of daughter isotopes for each calcite the sensitivity of apparent equilibrium temperature to cooling rate is sufficiently high for inference of cooling rates within a factor of 5 or better for cooling rates ranging from tens of degrees per day to a few degrees per million years however because the calcites have different susceptibilities to reordering each calcite defines its own cooling rate apparent equilibrium temperature relationship the cooling rates of carrara marble inferred from carbonate clumped isotope geospeedometry are 10 6 10 3 degrees per annum and are in broad agreement with rates inferred from thermochronometric methods cooling rates for 13c depleted calcites from the late neoproterozoic doushantou cap carbonates in south china are on the order of 102 104 degrees per annum consistent with rapid cooling following formation of these calcites by a short lived hydrothermal event bristow et al 2011 nature v 474 p 68 71 most of the uncertainty in these estimates relates to uncertainty in arrhenius parameters for different calcites thus while the carbonate clumped isotope geospeedometer shows promise for recording cooling rates in settings and lithologies where other geospeedometers may not be applicable the uncertainty in cooling rate will be large without independent knowledge of the reordering kinetics of each study material thus the full potential of the method will only be realized if reordering kinetics can be accurately determined for each study material or predicted on the basis of mineral composition texture or other observable parameters

fast diffusion along mobile grain boundaries in calcite experimental measurements of grain boundary diffusion are usually conducted on static boundaries despite the fact that grain boundaries deep in the earth are frequently mobile in order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44ca from a layer of 44ca enriched calcite powder during the static recrystallization of a single crystal of calcite at 900 c a region about 500 m wide adjacent to the powder layer is heterogeneously enriched in 44ca and complex zoning patterns including sharp steps in composition and continuous increases and decreases in 44ca content are developed in metamorphic rocks these would normally be interpreted in terms of changes in pressure or temperature rayleigh fractionation or episodic fluid infiltration these explanations cannot apply to our experiments and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate we have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width dgb to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer the value of dgb in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions in order to allow the scale of chemical equilibrium and hence textural evolution to be predicted under both experimental and geological conditions we present quantitative diffusion regime maps for static and mobile boundaries in calcite using both published values and our new values for grain boundary diffusion in mobile boundaries enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of earth materials for geochronology and for interpretations of the length and time scales of chemical mass transport moreover zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones rather than regions of anomalous mineralogy water or melt content as is generally suggested

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

diffusion of a multi species component and its role in oxygen and water transport in silicates an important but poorly understood factor that affects diffusion rates is the role of speciation during diffusion of a multi species component the diffusion of such a component is complicated by the different diffusion coefficient of each species and the interconversion reactions among the species these complexities can be treated by a diffusion equation that incorporates the diffusive fluxes of all species contributing to the concentration of the component the effects of speciation on the diffusion of the component can be investigated experimentally in some simple cases by measuring concentration profiles of all species developed during diffusion experiments or by studying some of their other consequences experimental data on water diffusion in rhyolitic glasses indicate that although dissolved water is present as two species h 2o molecules and oh groups molecular h 2o is the dominant diffusing species at very low to high water concentrations this explains the apparently complex behavior of water diffusion experimental data on oxygen diffusion in some silicates using 18 o tracers in the form of h 2 18 o are consistent with the idea that 18 o transport is dominated by diffusion of h 2o molecules even at lower water contents ppm or less this explains why oxygen transport depends on the presence of water and generally depends on water fugacity linearly for this mode of oxygen transport there is a simple theoretical relationship between the effective total oxygen diffusion coefficient and the total water diffusion coefficient that is a function of only the water concentration of the silicate at low water content this relationship appears to describe quantitatively the existing data over a wide range in water contents and diffusion coefficients in several phases

magma induced strain localization in centrifuge models of transfer zones abstract scaled centrifuge experiments have been used to investigate the dynamic relations between deformation and magma distribution in rift related transfer zones the physical models were built using suitable analogue materials such as sand to represent the brittle upper crust various kinds of silicone mixtures to simulate the lower crust and upper mantle and glycerol to reproduce magma models simulated the development of transfer zones across pre existing glycerol reservoirs placed at the base of the analogue continental crust in plan view different geometries dimensions and positions of subcrustal reservoirs were reproduced in three different sets of experiments to compare results models were also performed without magma simulating glycerol set 1 experiments incorporating a narrow rectangular glycerol reservoir show that the low viscosity material is able to localise deformation into the overlying crust giving rise to discrete transfer zones this concentrated surface deformation corresponds at depth to major magma accumulation set 2 experiments with an initial wide squared glycerol reservoir show instead that deformation is distributed across the whole model surface corresponding at depth to relatively minor magma accumulation set 3 experiments explored various positions of a small squared reservoir that invariably localised faulting in the overlying analogue brittle crust at the onset of model deformation the overall model behaviour suggests that magma distribution at depth can effectively control the strain distribution in the overlying crust and the deformative pattern of transfer zones strain distribution in turn may control magma emplacement as localized deformation would favour major accumulation of magma at transfer zones coupled to a strong thermal weakening of the country rocks this process may ultimately lead to a positive feedback interaction between magma and deformation

steep oxygen isotope gradients at marble metagranite contacts in the northwest adirondack mountains new york usa products of fluid hosted diffusion steep gradients in oxygen isotope ratios are recorded from two contacts between grenville marbles calcite 18o 21 24 and metamorphosed granitic intrusives whole rock 18o 11 12 in the northwest adirondacks oxygen and carbon isotope fractionations between coexisting minerals in the marbles and the metagranites are consistent with isotopic equilibration at peak metamorphic temperatures the 18o profiles are sigmoidal with inflection points at the contacts and are developed over 5 6 m quantitative fluid infiltration diffusion modeling suggests that the isotopic gradients are best explained as the result of fluid hosted diffusion for porosities of 10 3 to 10 6 the profiles could have formed in 103 107 years via an interconnected grain boundary fluid the granites were probably emplaced at shallow crustal levels prior to the regional grenville metamorphism and isotopic exchange is most likely to have occurred at that time contact metamorphism at these localities involved devolatilisation of the marbles and the time estimates may reflect the periods over which fluid producing reactions took place and an enhanced porosity was maintained in the marbles there is a progressive increase in the percentage of diopside and k feldspar at the expense of phlogopite and tremolite towards the metagranites these mineralogical changes probably occurred due to the temperature increase associated with granite intrusion and diffusion of h2o from the granite the two localities lie on either side of the regional metamorphic amphibolite to granulite facies transition the preservation of isotopic profiles formed during contact metamorphism suggests that there was little pervasive fluid flow at this crustal level during the regional metamorphism or at any time thereafter steep isotopic gradients are present at orthogneiss marble contacts elsewhere in the adirondacks suggesting that this conclusion may be generally applicable to this terrain

a preliminary stable isotope study on mogok ruby myanmar the primary occurrence of ruby in the mogok area northern myanmar is exclusively found in marble along with spinel forsterite bearing marble and phlogopite graphite marble these marble units are enclosed within banded biotite garnet sillimanite oligoclase gneisses samples of these marbles collected for c o stable isotope analysis show two trends of 13c 18o variation resulting most likely from fluid rock interactions ruby bearing marble and phlogopite graphite marble followa trend with coupled c o depletion whereas spinel forsterite bearing marble follows a 18o depletion trend with relatively constant 13c values ruby formation might have resulted from co2 rich fluid rock interaction while spinel forsterite bearing marble was genetically related to co2 poor fluid rock interaction both fluids may have arisen from external sources based on graphite raman spectral thermometry the estimated temperature for phlogopite graphite marble and probably ruby bearing marble was lower than 607 c and for spinel forsterite bearing marble lower than 710 c contrasting c o diffusion between graphite ruby spinel forsterite and calcite local variations of isotopic compositions of newly formed minerals as a result of non pervasive fluid infiltration and open system isotopic disturbance during cooling may have affected c o isotopic fractionations between minerals the estimated high formation temperatures for ruby and spinel forsterite imply that the parental fluidsmay have been related to nearby igneous intrusions and or metamorphic processes whether these two types of fluid were genetically related is unclear based on the present data

petrologic and stable isotopic studies of a fossil hydrothermal system in ultramafic environment chenaillet ophicalcites western alps france processes of carbonate cementation abstract the late jurassic chenaillet ophiolitic complex western alps represents parts of an oceanic core complex of the liguria piemonte domain a model for the origin and evolution of the chenaillet ophicalcites based on textural and isotopic characterization is presented the chenaillet ophicalcites correspond to brecciated serpentinized peridotites that record seafloor shallow serpentinization at a minimum temperatures of 150 c followed by authigenic carbonation carbonation starts with a network of micrometric to millimetric pre or syn clast formation calcite veins accompanied by a pervasive carbonation of residual olivine and serpentine inside the serpentinite mesh core a matrix of small calcite 13 c vpdb values that range between 5 and 0 4 the lower values were obtained for calcite within the serpentinite clasts the 18 o vsmow values have a range between 11 and 16 in carbonated clasts the 18 o values in the matrix are fairly homogeneous with an average at 12 and the late calcite veins have values between 12 5 and 15 5 these values suggest a relatively high temperature of formation for all the carbonates carbonates within clast are mainly characterized by a formation temperature in the range of 110 c to 180 c assuming a 18 o value of seawater of 0 the matrix forms at a temperature of ca 165 c late veins are characterized by a formation temperature ranging between 120and 155 c we propose a model where serpentinization is followed by discrete carbonation then brecciation and cementation as a consequence of continuous hydrothermal fluid circulation in the serpentinite basement this is comparable to observations made in the stockwork of present day long lived oceanic hydrothermal systems

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

volcanomagnetic signals associated with the 2001 flank eruption of mt etna italy 1 geomagnetic changes were observed during the 2001 flank eruption at mt etna after differential magnetic fields were filtered from the seasonal thermic noise by using temperature data we recognized three stages in the total intensity changes related to different volcanic events a rapid variation of 4 nt associated with july 12 seismic swarm recorded beneath the summit craters b step like variation of 7 nt occurring during the quick propagation of fractures in the south flank which was followed by the off set of 2 nt before july 17 when the lava outpoured from fissures and c quick variation of 3 nt coincident with july 19 eruptive fissures opening up in the valle del leone these observations are consistent with those calculated from volcanomagnetic models in which the magnetic changes are generated by stress redistribution due to magmatic intrusions at different depth and by the thermal demagnetization at a rather shallow depth

modelling the lithospheric rheology control on the cretaceous rifting in west antarctica small scale analogue models were used to investigate the process of cretaceous orthogonal extension in the west antarctic rift system the models considered the transition from the east antarctic craton to a weaker lithosphere and the results support previous hypotheses about the strong control exerted by lateral variations in lithospheric structures on the process of extension strain was mostly accommodated at the boundary between the two types of lithosphere with a relative uplift of the cratonic block which remained essentially undeformed conversely the weaker lithosphere showed wide rifting style geometry locally associated with core complex like structures in agreement with the natural prototype this tectonic scenario led to a long lasting extension without continental break up and to the absence of relevant surface magmatism 2007 blackwell publishing ltd

insights into the collection and emplacement of granitic magma based on 3d seismic images of normal fault related salt structures 3d seismic data from salt structures in the southern north sea show how viscous fluids tend to migrate towards and flow upwards along the footwalls of active normal faults where zones of low stress are inherent this process provides one potential solution for the question of how enough space is created to accommodate large granitic bodies in the crust during episodes of regional extension granitic melt may collect in the lower crust below normal detachments in domed areas from which magma may be sourced subsequently in the upper crust emplacement can occur in fault bounded laccoliths at rates commensurate with other geological processes

ree diffusion in calcite chemical diffusion of four rare earth elements la nd dy and yb has been measured in natural calcite under anhydrous conditions using rare earth carbonate powders as the source of diffusants experiments were run in sealed silica capsules along with finely ground calcite to ensure stability of the single crystal samples during diffusion anneals rutherford backscattering spectroscopy rbs was used to measure diffusion profiles the following arrhenius relations were obtained over the temperature range 600 850 c dla 2 6 10 14 exp 147 14 kj mol 1 rt m2 s 1 dnd 2 4 10 14 exp 150 13 kj mol 1 rt m2 s 1 ddy 2 9 10 14 exp 145 25 kj mol 1 rt m2 s 1 dyb 3 9 10 12 exp 186 23 kj mol 1 rt m2 s 1 in contrast to previous findings for refractory silicates e g zircon differences in transport rates among the ree are not pronounced over the range of temperature conditions investigated in this study diffusion of the ree is significantly slower than diffusion of the divalent cations sr and pb and slower than transport of ca and c at temperatures above 650 c fine scale zoning and isotopic and ree chemical signatures may be retained in calcites under many conditions if diffusion is the dominant process affecting alteration

fault kinematics and past plate motions at a convergent plate boundary tertiary shimanto belt southwest japan early and late stage outcrop scale faults have been documented in paleogene strata of the muroto peninsula of southwest japan the former were active early in the consolidation history of the sediment and are crosscut by the latter which were active after regional scale imbrication folding and penetrative deformation i e regional spaced pressure solution cleavage orientation distributions of striae and kinematic axes indicate that the two generations of faults reflect distinct kinematic regimes the orientations and crosscutting relations indicate a counterclockwise viewed downward rotation of the principal shortening direction between early and late stage faulting existing fossil and radiometric age data suggest this transition occurred during eocene oligocene time the fault kinematic data are interpreted to reflect a change in relative plate motions between eurasia and an oceanic plate subducting beneath eurasia in paleogene time these results coupled with existing paleothermal and paleopressure data are consistent with the presence of a spreading ridge in the western pacific basin during this time

crustal structures of the rhinegraben and the massif central grabens an experimental approach two of the most improtant segments of the west european rift the rhinegraben and the massif central grabens show in plan and cross section a very different crustal structure

oxygen grain boundary diffusion in natural and hot pressed calcite aggregates abstract oxygen grain boundary diffusion rates were experimentally determined in natural solnhofen limestone and hot pressed calcite aggregates at 300 to 500 c and 100 mpa water pressure the solnhofen limestone was pre annealed for 24 h at 700 c and 100 mpa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments the synthetic calcite aggregates were formed by hot isostatic pressing of an intimate mixture of 1 2 m calcite grains plus 5 wt 0 3 m alumina grains at 700 c and 200 mpa for 3 10 days values for the product of the grain boundary diffusion coefficient d and the effective grain boundary diffusion width were determined from 18 o concentration profiles measured with an ion microprobe there is no measurable difference between d values obtained for pre annealed solnhofen samples and hot pressed calcite aggregates the temperature dependence for oxygen grain boundary diffusion at 100 mpa is described by the arrhenius parameters d 0 3 8 10 14 m 3 s and q 127 17 kj mol comparison of the results of this study with previously published data show that the rate of oxygen grain boundary diffusion in calcite aggregates is four to five orders of magnitude greater than that of volume diffusion in calcite single crystals and the activation energy is less grain boundary diffusion of oxygen in calcite aggregates is five to six orders of magnitude greater than that of calcium when the latter is extrapolated to the temperature range of this study in addition oxygen grain boundary diffusion rates in calcite aggregates are about a factor of 4 10 and 100 times greater than that in feldspar and quartz aggregates respectively under similar experimental conditions

development of the north viking graben inferences from laboratory modelling abstract the north viking graben is a classic sedimentary basin in the north sea that has been thoroughly studied over the last decade despite the amount and quality of seismic and well data now available the processes of extension remain unclear and the relationship between sedimentation and deformation is still a matter of debate we present the results of small scale laboratory modelling the models are made with sand and silicone putty to simulate brittle and ductile behaviour on a crustal scale the experiments were designed to study the effects of a the degree of coupling between brittle and ductile layers b the obliquity of extension and c the reactivation of inherited crustal scale structures the results are used to discuss the symmetry of extensional processes on crustal and lithospheric scales as well as the relationship between sedimentation and deformation it is proposed that the asymmetry of faulting in the upper crust and the nearly symmetric thinning of the whole crust are not mutually exclusive a model involving nearly pure shear on the lithospheric scale is used to explain the development of an asymmetrical basin at shallow level in addition this model accounts for the nearly horizontal stratification of lower cretaceous sediments the basal cretaceous unconformity which caps many of the normal faults and the localization of deformation to the west of the graben during the lower cretaceous

oxygen self diffusion in calcite dependence on temperature and water fugacity oxygen self diffusion in natural calcite single crystals was studied hydrothermally at 400 800 c and 10 350 mpa confining pressure diffusion coefficients d were determined from18o concentration profiles measured with an ion microprobe at 100 mpa confining pressure the arrhenius parameters yield an activation energy q 173 6 kj mole and pre exponential factor d0 7 10 9 m2 s over the temperature range 400 800 c and there is no measurable anisotropy constant temperature experiments at 700 c indicate that there is a strong linear correlation slope 0 9 of d with water fugacity over the range 4 240 mpa consistent with the interpretation that in the presence of water the dominant oxygen bearing transport species in calcite is molecular water a sample containing 1180 ppm mn shows a marked increase in observed d values at temperatures below 600 c however when pre annealed at 700 c the d values obtained at 550 c are within a factor of four of the value obtained for the low 100 ppm mn sample the results of this study confirm that in contrast to carbon oxygen diffusion rates in calcite are greatly enhanced when water is present the difference in the effect of water on doxygen and dcarbon in calcite may provide valuable information for evaluating the role of fluids in the thermal histories of calcite bearing rocks

magma underplating of continental crust experiments conducted over the past decade have shown that common mantle melting products have densities at moho pressures that exceed the density of average continental crust in a situation of perfect gravitational equilibrium they would not rise to or near the surface the exact behaviour reflects the coupling of crust to the mantle strong at early periods of magmatic cycle but weak at more evolved stages where crustal melting and degassing commences spreading of underplate magmas in rift zones above subduction zones or even hot spots may perturb the state of the crust over very large areas where dense magmas occur beneath normal 30 km continental crust they may crystallize to form gabbro peridotiteanorthosite complexes dense crustal components may founder with melting assimilation and degassing producing very high temperature fluids rich in h2o co2 co and s halogen species depending on compositions the process acts as density filter cleaning the crust and lowering its mean density this process must have been particularly efficient during the archean when komatiite magmas were common opportunities for magma contamination and mixing processes must abound in situations of large scale underplating when crust is overthickened underplate magmas may crystallize to products with eclogitic affinity with the possibility of their foundering in normal mantle

mantle exhumation at passive margins abstract laboratory models of lithosphere necking have been used to study the modes of passive margin formation and related mantle exhumation at continent ocean boundary four layer models were constructed with sand and silicone putty to represent the brittle and ductile layers respectively of both crust and mantle it is shown that necking of the whole lithosphere model is nearly symmetrical pure shear but that asymmetrical structures simple shear develop internally due to heterogeneous boudinage and or faulting of brittle layers relative movements between brittle layers is accommodated by layer parallel shear in the ductile layers defining a set of conjugate shear zones with increasing stretching lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of boudins in the brittle layer the experimental results are used to propose a general model of passive margin formation leading to mantle exhumation they are also compared to the situation observed on the west iberia margin the exhumation of lithosphere mantle is the result of a bulk pure shear at lithospheric scale shearing of exhumed mantle rocks does not correspond to detachment faults cross cutting the whole lithosphere at the onset of rifting but results from heterogeneous stretching and boudinage of the high strength sub moho mantle

synextensional granitoids and detachment systems within cycladic metamorphic core complexes aegean sea greece toward a regional tectonomagmatic model within deforming continental regions where metamorphic core complexes mccs and synextensional granitoids are closely associated deciphering the link between detachment faulting and magmatism often remains complex as 1 the rheological weakness of magma may stimulate mechanisms of strain localization and conversely 2 tectonic processes may open close drains where magmas can intrude here we tackle this issue by focusing on the cyclades with the comparison of five granitoid cored mccs tinos mykonos ikaria naxos and serifos and their flanking detachment systems in this region granitoids were emplaced into the middle upper crust over a relatively short time period 15 9 ma while metamorphic domes were largely exhumed after more than 10 myr of extension none of those intrusions thereby proves to be a real candidate for the genesis of mccs but would rather be a consequence of a warmer regime during lithospheric thinning however all collected structural and kinematic data converge toward a regional scheme in which magmatic activity played a more pivotal role than previously postulated indeed late evolution stages of mccs were dynamically impacted by intrusions along which local and transient heterogeneities of the mechanical strength occurred interfering with the sequential development of detachments during their tectonically controlled emplacement magmatic products intruded already formed detachments at depth locally inhibiting their activity associated with a contemporaneous upward migration of extensional deformation that tended to localize through time within intrusion roofs along rheological discontinuities the newly formed detachments are expressed within granitoids through a continuum of deformation from comagmatic to ductile conditions followed by cataclasis along detachments

the temperature of formation of carbonate in martian meteorite alh84001 constraints from cation diffusion we have measured the rates of chemical diffusion of mg in calcite and ca in magnesite and used these new data to constrain the formation temperature and thermal history of carbonates in the martian meteorite alh84001 our data have been collected at lower temperatures than in previous studies and provide improved constraints on carbonate formation during relatively low temperature processes 400 c mea sured log d0 values for chemical diffusion of mg in calcite and ca in magnesite are 216 0 6 1 1 and 27 8 6 4 3 m 2 s and the activation energies ea are 76 6 16 and 214 6 60 kj mol respectively measured diffusion rates of mg in calcite at temperatures between 400 and 550 c are substantially faster than expected from extrapolation of existing higher temperature data suggesting that different mechanisms may govern diffusion of mg at temperatures above and below 550 c we have used these data to constrain thermal histories which will allow the 1 mm variations in ca mg composition in alh84001 carbonates to survive homogenization by diffusion our results are generally consistent with models for formation of carbonates in alh84001 at low temperatures for initial cooling rates of between 10 21 and 10 3 ma our results demonstrate that carbonates formed at temperatures no higher than 400 c and most probably less than 200 c this range of cooling rates is similar to those observed within the terrestrial crust and suggests that formation of the carbonates by igneous metamorphic or hydrothermal or other processes in the martian crust most plausibly occurred at temperatures below 200 to 400 c models that suggest alh84001 carbonates formed during a martian impact event are also constrained by our data the thermal histories of terrestrial impact structures suggest that cooling rates sufficiently rapid to allow preser vation of the observed carbonate zoning at formation temperatures in excess of 600 c 10 7 c ma occur only within the uppermost melt rich portions of an impact structure material deeper within the impact structure where cooling is dominated by uplifted crustal material and where much of the formation of hydrothermal minerals is concentrated cools much slower typically at rates of 10 2 to 10 3 ma carbonates formed within this region would also only preserve 1 mm compositional zoning at formation temperatures of less than 200 to 400 c copyright 2001 elsevier science ltd

correlation between oh concentration and oxygen isotope diffusion rate in diopsides from the adirondack mountains new york the concentration of structural oh in diopside was determined for four granulite facies siliceous marble samples from the adirondack mountains new york using ftir spectroscopy single crystal polarized ir spectra were measured on 100 and 010 sections of diopside the relative intensities of four oh bands in the 3700 3200 cm 1 region vary among the samples with the 3645 cm 1 band dominating the spectra of diopside from a xenolith at cascade slide total oh content in the diopsides ranges from 55 to 138 ppm h 2 o by weight the oh concentration in diopside increases monotonically with increasing f h2o for the sample as estimated using oxygen isotope systematics for diffusion rates in these samples from edwards and valley 1998 there is no significant variation in oh content within a single diopside grain or among diopside grains from the same hand sample charge coupled substitution with m 3 and ti 4 in the crystal structure may have allowed retention of oh in the diopside structure during and after peak metamorphism 750 c 7 8 kbar the cascade slide diopsides have an fe 3 fe 2 of 0 98 compared to fe 3 fe 2 0 to 0 05 for the other samples implying that some loss of hydrogen through oxidation of fe was possible in this sample this is the first study we know of which shows that the oh content in anhydrous minerals from natural samples correlates to the rate of oxygen isotope diffusion

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

marked along strike variations in dip of normal faults the lokichar fault n kenya rift a possible cause for metamorphic core complexes abstract the lokichar fault is a major boundary fault in the northern kenya rift mapped from seismic reflection data the fault is a mixture of high 45 60 low 20 45 and very low 12 20 angle segments the areas of least displacement up to a maximum 10 km heave are the very low angle fault segments 12 20 the southern higher angle fault segment has a maximum estimated heave of about 20 km initiation of normal faults at a low angle cannot be easily explained by rock mechanics theory common explanations for such faults include 1 rotation of higher angle faults by the domino faulting model 2 rotation of large displacement faults by isostatic instability created by the faulting rolling hinge models and 3 reactivation of low angle pre existing fabrics the lokichar fault geometry is inconsistent with any of the above explanations the very low angle segments coincide with regions of intense igneous intrusive activity re orientation of the stress axes from the simple andersonian condition could permit normal faults to form at a lower angle this may happen around intrusive complexes or by setting up a basal shear stress between flowing and static crust if faults associated with metamorphic core complexes were associated with marked along strike changes in fault angle the resulting variations in footwall uplift could give rise to the antiformal metamorphic core complex geometry

crustal structure of the main ethiopian rift from gravity data 3 dimensional modeling abstract a three dimensional interpretation of the newly compiled bouguer anomaly map of the main ethiopian rift is presented a high resolution 3 d model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben the bouguer anomalies along the axial portion of the rift floor as deduced from the results of the regional and residual separation are mainly caused by the deep seated structures the inferred zone of intrusion which is the main subject of the present study coincides with the maximum gravity anomaly of the rift floor the intrusion is displaced at several sectors along the east west direction and the two major displacements coincide with the locations of the major rift offsets on the surface because of the asthenospheric uplift the crust under the main ethiopian rift is slightly thinned the zone of crustal thinning 31 km coincides with the location of the intrusion beneath the rift floor and the maximum of which is attained in the northern and central sectors of the graben the trend of the crustal thinning zone which is from south to north is the same as the one obtained in the afar depression the southeastern and western plateaus on the other hand show by far the largest crustal thickness in the region 38 51 km in contrast to the afar depression where the crust is partly oceanized the thickness and density of the crust suggest that the main ethiopian rift is underlain by a purely continental crust the deep and relatively large nature of the intrusion leads to the conclusion that a large scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben

oxygen diffusion in magnetite the diffusion rate of oxygen in magnetite fe3o4 has been measured under hydrothermal conditions between 800 and 500 c at 1000 bars pressure and yields the arrhenius relation d 3 5 10 6 exp 45 000rt kcal g atom o the diffusion rate was found to be the same within the estimated uncertainty of a factor of 2 for measurements at 100 1000 and 2000 bars 20 200 mpa water pressure and for oxygen fugacities buffered by magnetite hematite and approximately ninio the activation energy and diffusion mechanism probably change to that given by castle and surman 10 at temperatures below 460 c the oxygen diffusion rate in magnetite is between those of quartz and hornblende closure temperatures fall in the range 380 c for 0 1 mm radius grains cooling at 1 c m y to 675 c for 1 mm radius and cooling at 1000 c m y if the present data are used but will be lower for the fine grain sizes and slow cooling rates if the castle and surman data are used

red sea detachment and basement core complexes in eritrea eritrea s war of independence precluded checking a 12 yr old prediction that a low angle detachment along which the red sea opened might be exposed on the western shore following eritrea s success we have found a suite of regional scale detachments along which northeast southwest extension began in rocks that were ductile at depth but became increasingly brittle as they cooled adiabatically on the way to exhumation the only signs of red sea detachment in the rift shoulder are steep and low angle faults offsetting oligocene dikes and lavas by contrast low angle ductile mylonites with a consistent top to the basin sense of shear were superimposed on steep pan african fabrics in the coastal lowlands these mylonites define core complexes beneath hanging walls of embrittled mylonites and synextensional dikes in which panels of domino fault blocks face both toward and away from the basin over low angle semi brittle detachments both the early ductile and later semi brittle low angle detachments were extended by a later set of grabens that probably sole out to an even younger detachment beneath the coastal lowlands

dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism a set of experimental small scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers initially underplated at the base of the crust the models are driven by a centrifugal body force which simulates the role of gravity in nature the models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension the experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers in particular extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults in the overlying brittle layer associated with the ductile domal uplift stretching of the continental crust induces a reactive migration of viscous layers into the footwall of major normal faults where magma accumulation at the core of the domes takes place this behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults a situation that is often observed in core complex structures although the models presented here are intended to simulate magma emplacement during extension of a two layer brittle ductile system lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts e g the ethiopian rift may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones in the suggested model such volcanoes are related to large basaltic magma reservoirs located at the base of the crust to accommodate the space vacated by lateral flow and thinning of the ductile crust the similarity in structure of the models with natural examples of continental extension modes such as rift systems and core complexes may suggest a close similarity of dynamic processes

formation of the double rift system in the thaumasia highlands mars 1 the thaumasia highland rifts are two complex graben structures in the thaumasia highland belt located in the southern thaumasia region mars the rifts are arranged 90 130 km apart in a nearly subparallel setting and represent a very unusual style of crustal deformation we have investigated the mechanism responsible for this peculiar style of rifting and found strain localization by volcanic weak zones to be a major factor for fault pattern formation indicating widespread prerift and synrift volcanic activity in the region this result is consistent with the high heat flow during rift emplacement as estimated from rift flank uplift our results indicate that the onset of volcanism predates the rift formation process and that magmatism and the associated lithospheric weak zones control the rift locations the simulations performed here are in good agreement with the observations for extension in the nw se to nnw sse direction and we favor a formation scenario involving passive rifting in a magmatectonically active environment

stable isotope evidence of heterogeneous fluid infiltration at the ubehebe peak contact aureole death valley national park california stable isotope ratios of carbon and oxygen are used to define quantitatively the effects of magmatic fluid infiltration in marbles contact meta morphosed by the 173 ma ubehebe peak quartz monzonite death valley national park california in previous studies of fluid infiltration quantitative interpretation of aureole wide isotopic data has been difficult due to small data sets for this study sampling strategies were developed to obtain a data set that was large and unbiased enough to be statistically representative of the carbonates north of the ubehebe peak intrusion a total of 357 samples of marble were analyzed for bulk carbonate isotopic ratios separate analyses of coexisting calcite and dolomite were also obtained for an additional 31 samples unmetamorphosed samples 1900 3000 m from the intrusion have 18 o permil smow values of 25 5 0 8 1 and 13 c permil pdb values of 0 4 0 6 1 samples in the tremolite zone 750 1900 m have 18 o values ranging from 19 4 to 27 7 permil with a median value of 25 2 permil and 13 c ranges from 5 1 to 0 5 permil with a median value of 0 9 permil forsterite zone samples 0 750 m have isotopic ratios shifted to values as low as 11 1 permil 18 o and 9 1 permil 13 c despite this shift most forsterite zone samples retain sedimentary isotopic compositions with median 18 o values of 25 0 permil and 13c values of 1 2 permil 18o values for igneous minerals show no evidence for interaction with heated meteoric or metamorphic fluids the shifts in isotopic compositions within the marbles are interpreted to be the result of magmatic infiltration the effects of this infiltration were quantified by identifying samples with isotopic alteration that can only be attributed to infiltration the results show that magmatic fluid infiltration was limited in extent and very heterogeneous there is no evidence for infiltration of isotopically reactive fluids beyond 850 m from the intrusive contact and within this 850 m zone only 28 percent of the samples have been infiltratively altered with respect to 18 o and 20 percent are depleted in 13 c compositions the isotopic data when evaluated in conjunction with geostatistical and petrologic data indicate that the geometry of the hydrothermal flow system was mainly vertical and away from the pluton infiltration was restricted to large nearly vertical tube like zones of increased permeability these higher permeability zones likely reflect an initial heterogeneity of the host rocks and show no significant evidence for reaction enhanced permeability given the heterogeneity of the system and a lack of knowledge about many basic parameters controlling fluid infiltration it is shown that the best method of calculating the amount and composition of the infiltrating fluid may be a mass balance approach fluid rock ratio the application of mass balance models is discussed and shown to be valid only under limited conditions since infiltration at ubehebe peak was largely vertical the observed isotope alteration patterns represent an infiltration side and not a front because this precludes the use of traditional mass balance calculations a new infiltration side inside model is proposed that allows the isotopic data to be evaluated the inside model uses the ratio of the areal amounts of infiltrative alteration to calculate a fluid composi tion fluid amounts cannot be obtained from this model results for the ubehebe

squeeze box in a centrifuge abstract a squeeze box which operates in a centrifuge is presented in the experimental set up the centrifugal body force mimics the role of gravity in nature and at the same time drives a hydraulic press which simulates lateral forces the rate of lateral shortening is controlled by varying the loads placed on the hydraulic press utilization of a squeeze box in a centrifuge enhances the range of scaling and enables the use of coherent model materials without violating similitude requirements in the traditional squeeze box which operates under normal gravity similitude requirements severely limit the choice of model materials as a test of the apparatus simple experiments were carried out comparing models shortened in a centrifuge with similar models shortened under normal gravity the realistic form of decollement structures e g folds faults and diapirs shortened in a centrifuge in these tests as well as in other tests which will be reported later show the potential of this new technique for rigorous modelling of geodynamic systems which evolve under the influence of both gravity and a push from the rear the set up can also be used as a simple shear box in a centrifuge after a slight adjustment

constraints on the petrologic structure of the subduction zone slab mantle interface from franciscan complex exotic ultramafic blocks ultramafic blocks within mud matrix melange of the franciscan complex california preserve a series of metasomatic mineral zones generated by infiltration of si rich hydrous fluids during subduction we describe the petrology and geochemistry of the metasomatic zones and compare them to current model predictions for the metasomatism of the mantle wedge by subduction zone fluids fluid flow affected a cr spinel lherzolite protolith to form first serpentinite then a talc dominated rock and finally an amphibole rich assemblage a diverse suite of accessory minerals in the amphibole rich zone titanite clinozoisite zircon apatite suggests that the trace element signature of subduction zone fluids may be fractionated in this zone oxygen isotopic evidence suggests that the ultramafic blocks equilibrated with metasomatic fluids during serpentinization and that subsequent reactions occurred in equilibrium with these fluids in a temperature range of 450 500 c whole rock geochemistry indicates mobility of many elements into and out of the blocks during metasomatism including elements such as ti which are currently considered to have low solubilities in such fluids taken as a whole the blocks appear to preserve the metasomatic structure of the slab mantle interface in subduction zones and imply that the chemistry of slab derived fluids is modified as they pass through these metasomatic zones in the mantle wedge our results suggest that the primary composition of subduction zone fluids is not likely reflected by arc magmas instead we propose that arc magmas are derived from regions of the mantle fluxed by fluids residual to the metasomatic processes we observe

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

fluid enhanced low temperature plasticity of calcite marble microstructures and mechanisms postmetamorphic carbonate fault rocks from the damara orogen namibia show the following features 1 obvious contrasts between centimeter scale grains in the clasts and nanometer to micrometer scale grains in the matrix and between macroscopic cataclastic and microscopic mylonitic microstructures 2 coexistence of tangled dislocations and organized dislocation walls 3 occurrence of subgrains along margins of clasts and their transition into dynamically recrystallized grains in the matrix and 4 new grains in grain sizes of a few nanometers to 3 m in diameter the coexistence of brittle and ductile microstructures is attributed to comprehensive intragranular twinning and kinking fracturing and subsequent dislocation remobilization or reorganization and recrystallization fracturing is triggered by dislocation pileups due to dynamic loading twinning and kinking it also generates free dislocations and tangled dislocations fractures provide fluid paths increase fluid rock interfaces and enhance the possibility of fluid rock interaction fracturing is subsequently accommodated by low temperature plasticity that is attributed to hydrolytic weakening i e fluid enhanced recovery and dynamic recrystallization due to the infiltration of fluids into the deforming grains during hydrolytic weakening remobilized free dislocations and tangled dislocations climb toward incoherent boundary like fractures the dislocations are reorganized into dislocation walls that commonly constitute parts of subgrains developing into new grains we conclude that 1 fluids may increase the rate of dislocation glide and dislocation climb and may also enhance the recovery of strain hardened rocks to accommodate fracturing processes in calcite marbles at low temperatures and 2 calcite marble may have low temperature plasticity and may undergo crystal plastic deformation due to hydrolytic weakening at shallow crustal levels

phase equilibrium and stable isotope constraints on the formation of metasomatic garnet vesuvianite veins sw adamello n italy metasomatic garnet vesuvianite veins occur within the contact metamorphic marble sequence of the lower triassic prezzo formation in a narrow 1 5 m wide zone along an intrusive marble granodiorite contact at the southwestern border of the tertiary adamello batholith the metasomatic mineral assemblage is comprised of garnet vesuvianite clinopyroxene wollastonite and pyrrhotite which were precipitated from the vein forming fluid in a preexisting calcite matrix at conditions of about 2800 bars and 630 c the veins are enriched in silicon aluminum iron magnesium titanium and depleted in calcium with respect to the unaltered contact metamorphic marble graphite which is present in the unaltered prezzo marble is absent in the veins irregularly shaped mineralogically distinct zones with different degrees of silicification can be distinguished within the veins the isotopic compositions of calcite cc in the unaltered marble are about 18o smow standard mean ocean water 21 0 and 13c pdb peedee belemnite 0 0 they are reset to significantly lower values within the veins where 18occ is 15 0 to 16 0 and 13ccc is 4 5 to 3 5 the isotopic front coincides with an abrupt change in the microscopic texture of matrix carbonate which occurs at the sharp boundary between graphite bearing and graphite free material within the veins the oxygen isotope fractionation between calcite and garnet gar varies systematically with distance from highly silicified zones the variations in 18occ gar are as large as 2 on a millimeter scale indicating garnet calcite isotopic disequilibrium vein formation was due to the infiltration of a water rich fluid of magmatic provenance into the carbonate country rock along fractures removal of graphite from the wall rock by dissolution through the metasomatic fluid induced recrystallization of matrix calcite permeability was enhanced during calcite recrystallization facilitating material transport into the wall rock and metasomatic alteration vein garnet was precipitated in isotopic equilibrium with the metasomatic fluid the isotopic composition of preexisting calcite was initially out of equilibrium with the vein forming fluid and it was shifted towards equilibrium by surface reaction controlled calcite fluid isotopic exchange during calcite recrystallization due to the short lifetime of the metasomatic system calcite fluid isotopic equilibrium was generally not attained within the veins oxygen and carbon transport was fast relative to mineral fluid exchange of their isotopes and the geometry of the isotopic pattern is largely controlled by the kinetics of mineral fluid exchange

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

a clinopyroxene basalt geothermobarometry perspective of columbia plateau nw usa miocene magmatism this article describes investigations into the pre eruptive evolution of columbia river basalt group magmas to constrain pressures and temperatures of ascending magmas and plumbing conditions

periodic instabilities during compression or extension of the lithosphere 1 deformation modes from an analytical perturbation method we argue that the plastic rheology of the lithosphere rather than its recoverable elastic properties is responsible for the systematic development of periodic instabilities during compression or extension we use a linear perturbation model with analytical solutions to calculate the instability modes for various rheologies the growth of such periodic instabilities is enhanced by the highly nonlinear stress strain rheologies encountered in the brittle layers of the lithosphere on the contrary ductile layers deforming according to high temperature creep flow laws tend to inhibit these instabilities for oceanic domains we assume that the only brittle layer is the upper part of the lithosphere in compression the only valid instability is a buckling whose wavelength is around 4 times the thickness of the brittle layer calculated wavelengths and growth rates are consistent with observations available for the indian ocean for continental domains a reasonable assumption is the existence of two plastic layers one in the upper crust the other in the upper mantle for moho temperatures between 450 c and 600 c in compression and extension two instabilities develop a long wavelength instability involving the whole lithosphere coupling mode and a short wavelength instability involving the crust and controlled by the upper brittle layer intrinsic crustal mode in compression the coupling mode is a whole lithosphere buckling with a wavelength about 4 times the thickness of the active lithosphere the two plastic layers plus the intermediate ductile layer in extension the coupling mode is a boudinage of opposite phase in the two plastic layers and a folding of the intermediate ductile layer the intrinsic crustal mode is a crustal buckling in compression crustal boudinage in extension neither deflects the moho the intrinsic crustal mode is favored by an increase in thermal gradient and by a decrease in strength of the ductile lower crust

oxygen isotope thermometry speedometry and hygrometry apparent equilibrium temperature versus closure temperature rather than indicating formation peak temperature oxygen isotope fractionations preserved in mineral assemblages of slowly cooled plutonic and metamorphic rocks yield apparent equilibrium temperatures tae the isotopic fractionations and tae values deliver information about cooling history as the extent of diffusive exchange of oxygen isotopes during cooling is controlled by the cooling time scale or cooling rate despite that several models such as the fast grain boundary fgb model have been developed to simulate oxygen isotope exchange between coexisting minerals during cooling extraction of cooling rate remains far from straightforward on the other hand there is a well defined quantitative relationship between the dodson closure temperature tc and the cooling rate but tc cannot be directly measured based on simulation results of existing models for a variety of rock systems including open systems with an infinite fluid reservoir closed systems with negligible fluid participation and semi open systems with moderate fluid participation this study demonstrates that tae of the mineral pair with the largest equilibrium isotope fractionation pleif is always bounded by their tc values regardless of how mineral proportions vary or how significant a role fluid has played in isotopic exchange if the two tc values happen to be similar tae will serve as a good approximation of both tc provided that the equilibrium fractionation factor has been precisely determined as a function of temperature one such pair is quartz magnetite by contrast a mineral pair with similar tc but relatively small fractionation is susceptible to the disturbance from other minerals hence does not always have tae confined within their tc range the relationship of tae tc correspondence for pleif with similar tc can be used to constrain either cooling rate i e as a speedometry method or oxygen isotope diffusivity if one of them has been independently determined in the latter case the inferred oxygen diffusivity may be an index of water fugacity i e as a hygrometry method when compared with experimental diffusivity values measured under different fluid conditions

asymmetric rifts due to asymmetric mohos an experimental approach large scale asymmetries in structures and in the uplift width and volcanism of pairs of conjugate passive margins have been documented in various parts of the world these asymmetries reflect the dominant mode of extension preceding the break up of the continental lithosphere of the many parameters involved the pre rift crustal structure has been shown to play a fundamental role in controlling the process of continental rifting in this work we have used analogue models to show how initial asymmetric moho configurations are prone to produce asymmetric rifting with prominent asymmetries in the amount of extension accommodated by boundary faults in the patterns of lithospheric thinning asthenospheric upwelling and in the trajectories of magma migration after continental break up these deformative patterns are expected to lead to the development of a pair of asymmetric passive margins with one narrow strongly volcanic margin and a wider less volcanic or with areas of concentrated volcanic activity counterpart

detachment faulting at ocean core complexes intracrustal reflections from a cretaceous inside corner are interpreted as a detachment fault that dips gently within the basement and can be followed up the flank of a domed high the fault heave is sufficient for ridge normal extension to have exhumed deep crustal and probably mantle rocks as sampled at some inside corner domed massifs the morphology of these massifs closely resembles that of the cretaceous high implying that the structures are analogous and supporting the notion that modern inside corner massifs are oceanic core complexes formed by unroofing the footwalls of similar detachment faults partially exposed as corrugated slip surfaces this is the first demonstration of the overall geometry of an oceanic core complex detachment fault

effect of water activity on reaction kinematics and intergranular transport insights from the ca oh 2 mgco3 caco3 mg oh 2 reaction at 1 8 gpa the kinetics of the irreversible reaction ca oh 2 mgco3 caco3 mg oh 2 were investigated at high pressures and temperatures relevant to metamorphic petrology using both in situ synchrotron x ray diffraction and post mortem analysis of reaction rim growth on recovered samples reaction kinetics are found to strongly depend on water content comparable bulk reaction kinetics are obtained under water saturated excess water c 10 wt and under intermediate 0 1 1 wt water conditions when temperature is increased by c 300 k in contrast similar reaction kinetics were observed at 673 k and 823 k between intermediate and dry experiments respectively where dry refers to a set of experiments with water activity below 1 0 no free water as buffered by the cao ca oh 2 assemblage given the activation energies at play this gap corresponding to the loss of no more than 1 wt of water by the assemblage leads to a difference of several orders of magnitude in reaction kinetics at a given temperature further analysis at the microscopic scale of the intermediate and dry condition samples shows that intergranular transport of calcium controls the reaction progress grain boundary diffusivities could be retrieved from the classic treatment of reaction rim growth rate in turn once modeled this rate was used to fit the bulk kinetic data derived from x ray powder diffraction offering an alternative means to derive calcium diffusivity data based on a comparison with effective grain boundary data for ca and mg from the literature it is inferred that both dry and intermediate datasets are consistent with a water saturated intergranular medium with different levels of connectivity the very high diffusivity of ca in the caco3 mg oh 2 rims in comparison with that of mg in enstatite rims found by earlier workers emphasizes the prominent role of the interactions between diffusing species and mineral surfaces in diffusion kinetics furthermore we show that the addition of water is likely to change the relative diffusivity of mg and ca in carbonate aggregates from a qualitative point of view we confirm in a carbonate bearing system that small water content variations within the 0 1 wt range have tremendous effects on both intergranular transport mechanisms and kinetics we also propose that the water content dependent diffusivity of major species mg ca in low porosity metamorphic rocks is strongly dependent on the interaction between diffusing species and mineral surfaces this parameter which will vary from one rock type to another needs also to considered when extrapolating p t t xh2o laboratory diffusion data to metamorphic processes

stable isotope thermometry at high temperatures determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by urey in theory this should be simple calibrated mineral pairs are common analysis is rapid and there is no significant pressure correction however in spite of widespread application the promise of reliable thermometry has been elusive stable isotope temperatures in metamorphic and igneous rocks are often controversial is the fractionation between two phases a thermometer speedometer rate dependant hygrometer p h2o dependant or chimera a number of factors have contributed to this uncertainty including incomplete and sometimes conflicting calibration of isotope fractionation factors limited understanding of the kinetics of mineral exchange and the lack of microanalytical techniques this situation has improved dramatically over the past decade permitting reliable and detailed thermal histories to be inferred in a landmark paper bottinga and javoy 1975 empirically calibrated the oxygen isotope fractionation factors among ten rock forming minerals at t 500 c their compilation shows that 84 of analyzed samples contain at least three minerals meeting a concordance test for equilibrium they concluded the great majority of igneous and metamorphic rocks has conserved a state of oxygen isotope exchange equilibrium if correct this bodes well for thermometry however this conclusion was contested by deines 1977 who critically examined the existing data and concluded that less than half of the rocks analyzed to date would yield concordant 18o derived temperatures while seeming irreconcilable both conclusions are based on fact all rocks contain some degree of disequilibrium but many also preserve a thermometric record it is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted the organization of this paper will be to discuss the assumptions

the origin of large local uplift in extensional regions large localized uplift is commonly observed in continental regions undergoing extension these observations can be modelled by planar high angle normal faulting of an elastic upper crust overlying an inviscid lower crust isostasy provides the necessary driving force the model quantifies the role of flexural rigidity density variations in the crust and erosion and deposition of sediment