population wide garnet growth zoning revealed by la icp ms mapping implications for trace element equilibration and syn kinematic deformation during crystallisation the strong partitioning of many trace elements into garnet and their slow diffusivities in both garnet and the rock matrix means that their distribution may record valuable petrogenetic information not documented by major elements in metamorphic rocks complex trace element growth zoning in garnet porphyroblasts from a garnet grade metapelite from the barrovian sequence of the sikkim himalaya is assessed using quantified la icp ms raster mapping coupled with x ray micro computed tomography the data document systematic changes in the zoning patterns from early to late nucleated crystals and also suggest that the ree y chemistry incorporated into garnet is dependent on persistent disequilibrium in the rock volume there is evidence for hree y diffusion haloes surrounding growing garnets and a heterogeneous hree y distribution in the rock matrix annuli superimposed on oscillatory zoning are not consistent with formation during some rock wide event but are dependent on the spatial disposition of the garnet annuli are interpreted to reflect an integrated history of varying growth rates and the incorporation of pre existing heterogeneities due to relatively slow matrix diffusivities conversely smooth zoning of many transition metals indicate that their distribution in garnet may be controlled by equilibrium partitioning between garnet and the rock matrix significant rotation of garnet porphyroblasts during growth is revealed due to immobility of cr over the duration of the crystallisation interval and overprinting of the heterogenous precursor cr distribution strain rate estimates derived from this zoning are on the order of 10 11 10 12 hbox s 1

crystal size distribution and composition of garnets in eclogites from the dabie orogen central china crystal size and spatial distributions of minerals in metamorphic rocks provide insight into their nucleation growth environment and the metamorphic evolution of their host rocks episodic nucleation and growth histories for garnets in eclogites from the dabie orogen are revealed by variations in crystal size geochemical zoning and mineralogy of inclusions in garnets the studied garnets show pseudo lognormal crystal size distributions csds prograde chemical zoning patterns indexed by mineral inclusions and are mainly distributed at random constant growth rate for each episode is proposed based on the mineralogy of inclusions in garnets and the chemical zoning patterns the csd shapes were evaluated in terms of a size dependent proportionate growth model and a thermally accelerated diffusion controlled model but neither is consistent with the geochemical data and the concordant occurrence of mineral inclusions in garnet initial increasing nucleation rate followed by a subsequent medial stage of nearly constant growth and finally declining nucleation rate is inferred from the csd data this study suggests that linking chemical analysis with textural analysis is crucial to avoid misleading interpretation solely through csd shapes of minerals in metamorphic rocks

a comparison of microstructural and chemical patterns in garnet from the fleur de lys supergroup newfoundland this study uses compositional analyses of garnet porphyroblasts to test a previously published microstructure based model of garnet growth in the fleur de lys supergroup x ray maps reveal significant compositional anomalies within garnet including zoning reversals and steepened compositional gradients these anomalies occur at the margin of the proposed first stage of garnet growth g1 and coincide with truncations of inclusion trails and changes in the inclusion assemblage intervals of reversed composition zoning and steepened compositional gradients across this boundary are interpreted to represent a hiatus in garnet growth possibly accompanied by garnet consumption during which changes in the garnet forming reaction p t conditions and deformation kinematics occurred the junction of the proposed second and third stages of garnet growth g2 and g3 coincides with the transition between successive crenulation cleavages without substantial microstructural truncations or changes in the inclusion assemblage the g2 g3 boundary is generally marked by uninterrupted normal zoning with subtle compositional anomalies in some samples this boundary may in fact record continuous garnet growth or alternatively mark a relatively short intra orogenic pause in garnet growth with minimal affect on zoning patterns individual porphyroblasts with contrasting inclusion trail microstructures also have different zoning patterns and this supports the previous recognition of contrasting growth histories between individual porphyroblasts a combined structural metamorphic model is presented that integrates the timing of garnet growth and foliation development reaction history and the p t t path in the fleur de lys supergroup

a structural metamorphic study of the broken hill block nsw australia the prograde pressure temperature p t path for the complexly polydeformed proterozoic broken hill block australia has been reconstructed through detailed structural analysis in conjunction with calculation of compositionally specific p t pseudosections of pelitic rock units within a high temperature shear zone that formed early in the tectonic evolution of the terrane whilst the overall p t path for the broken hill block has been interpreted to be anticlockwise the prograde portion of this path has been unresolved our results have constrained part of this prograde path showing an early heating event m1 at p t conditions of at least c 600 c and c 2 8 4 2 kbar associated with an elevated geothermal gradient c 41 61 c km 1 this event is interpreted to be the result of rifting at c 1 69 1 67 ga or at c 1 64 1 61 in the broken hill block early rifting was followed by an episode of lithospheric thermal relaxation and burial during which time sag phase sediments of the upper broken hill stratigraphy paragon group were deposited following sedimentation a second tectonothermal event m2 d2 occurred this event is associated with peak low pressure granulite facies metamorphism c 1 6 ga and attained conditions of at least 740 c at c 5 kbar a regionally pervasive high temperature fabric s2 developed during the m2 d2 event and deformation was accommodated along lithology parallel high temperature shear zones the larger scale deformation regime extensional or shortening of this event remains unresolved the m2 d2 event was terminated by intense crustal shortening during the olarian orogeny during which time the first mappable folds within the broken hill block developed

an overgrowth model to explain multiple dispersed high mn regions in the cores of garnet porphyroblasts the central portions of garnet porphyroblasts from harpswell neck maine exhibit small discrete regions of high mn content dispersed in three dimensions orientation contrast imaging oci and quantitative crystallographic orientation measurements from electron backscatter diffraction ebsd patterns show that most garnet porphyroblasts contain no variations in crystallographic orientation it is difficult to reconcile these data with a model in which each high mn region represents a discrete garnet nucleus an alternative hypothesis invokes a model of garnet growth in which precursor phases rich in mn are overgrown and their mn is incorporated locally into the garnet structure although this model requires length scales for equilibration of mn during the early stages of garnet growth that are shorter than any previously documented that inference is consistent with the very low temperatures of garnet nucleation in these rocks and with compositional evidence for progressively larger scales of equilibrium as reaction progressed very short length scales for equilibration of mn identified in this study for the lowest grade portion of the history of the rocks call into question the common assumption of rock wide equilibrium for mn during garnet growth at low temperature the absence of rock wide equilibrium for mn at low grade has potential negative implications for all common interpretive methods based in phase equilibria including thermobarometric studies and thermodynamic analysis of the origin and meaning of zoning in low grade garnet

three dimensional imaging of active structures using earthquake aftershocks the northridge thrust california abstract aftershocks of the 1994 northridge earthquake m 6 8 allow us to image the structure beneath the san fernando valley in northwestern los angeles in three dimensions we combine aftershocks and geological data to build an image of the three dimensional 3 d geometry of the previously unrecognized north vergent northridge blind thrust to a depth of 21 km the most striking feature of the imaged fault is mega corrugations oriented parallel to the mean aftershock slip vector with most of the 1994 slip confined to west of the largest corrugation we also image the partially overlying south vergent san fernando thrust which broke to the surface in a complex rupture in 1971 m 7 1 both thrusts produce fault related folding because of either fault propagation or fault bends however this deeper folding is masked by overlying complex deformation in the cover which is one reason why the northridge thrust was not identified until it ruptured in 1994 we use trishear fold modeling based on our 3 d fault geometry to evaluate possible folding due to slip on the northridge thrust as well as its interaction with the overlapping san fernando thrust and with shallow structures in the cover this example illustrates the importance of earthquake data to structural geology and the value of its 3 d integration with surface and near surface geological data

chronology and rates of faulting of ventura river terraces california new evidence concerning the chronology of four late pleistocene terraces of the ventura river near oak view california that are vertically offset and tilted by reverse and reverse flexural slip faults provides a means of estimating rates of fault movement and downcutting by the ventura river radiocarbon ages of charcoal contained within terrace deposits date two of the terraces correlation of soils that have developed on terrace deposits and extrapolation of the rate of vertical displacement of the arroyo parida santa ana fault not a flexural slip fault date the others resulting age estimates for the four main late pleistocene terraces are qt5b 30 000 qt6a 38 000 qt6b 54 000 and qt6c 92 000 yr b p vertical slip rates on flexural slip faults range from 0 3 to 1 1 mm yr and apparently are related to the rate form and mechanics of folding of the north limb of the ayers creek canada larga syncline average rates of downcutting of the ventura river for several intervals during the late pleistocene estimated from the chronology and relative elevation of river terraces north of the arroyo panda santa ana fault upstream from the zone of faulting vary from 0 5 to 1 3 mm yr the range in rates probably reflects variations in local uplift as well as adjustments to changing eustatic sea level climatic conditions and or regional deformation of the western transverse ranges the average rate of downcutting of the ventura river north of the zone of flexural slip faulting is 0 8 mm yr compared with 1 2 to 2 2 mm yr in the deformation zone immediately to the south this apparently indicates that during tectonic deformation there is an approximate balance between the rate of uplift due to faulting and folding and the rate of downcutting by the fluvial system

petrology and geochemistry of siliciclastic rocks of mixed feldspathic and ophiolitic provenance in the northern apennines italy provenance in clastic sedimentary rocks in the northern apennines is determined by integrated petrographic and geochemical criteria thus mafic poor arkoses in the gottero formation senonian paleocene deposited prior to the alpine orogeny were derived from granitic provenance in the sardinian corsican massif in the gottero formation chemical criteria distinguish between very low input of detrital mafic material and co c and ni added entirely by adsorption on clay minerals post alpine orogeny feldspathic sandstones in the macigno formation mid oligocene lower miocene and cervarola formation lower miocene reflect a more diverse provenance of granitic metamorphic ophiolitic and sedimentary rocks in the apennines and alps the macigno and cervarola rocks are distinguished by serpentine gabbro and diabase fragments and higher chlorite contents than in the gottero rocks chemically the macigno and cervarola rocks contain distinctive signatures expressed in higher mgo co cr and ni contents of the mafic ultramafic detritus a wider range and more scattered cr and ni variation and higher niggli ti for a given niggli al alk content the mgo co cr and ni abundances indicate up to 10 ultramafic ophiolitic detritus in the macigno and cervarola sandstones and up to 17 ophiolitic detritus in associated shales estimated by comparison of mixtures of feldspathic sands with ligurian peridotites up to 10 basaltic ophiolitic detritus in these sediments is suggested by cao and fe2o3 total abundances petrographic data indicate that a major part of the detritus in the macigno and cervarola formations is from alpine granitic basement of calc alkaline granite to tonalite plutons and gneisses but apart from higher ba in macigno and cervarola sandstones than in the gottero sandstones this is difficult to identify chemically the trace elements cr ni ba rb ce la y co cu ga pb and zn generally have positive correlations with niggli al alk a chemical surrogate measure of sheet mineral clays micas and chlorites content whereas sr which is associated with carbonate and feldspar shows negative correlation with al alk an olistostrome unit contains the most weathered detritus but otherwise shows similar chemical variation to the other formations methods to 1 identify separately mafic and ultramafic inputs 2 recognize in shales co cr and ni derived from ophiolitic sources and non ophiolitic sources 3 estimate the minimum mafic ultramafic proportion in a sediment may have general application elsewhere in sedimentary and metasedimentary rocks

high resolution seismic reflection profiling of the santa monica fault zone west los angeles california high resolution seismic reflection data obtained across the santa monica fault in west los angeles reveal the near surface geometry of this active oblique reverse left lateral fault although near surface fault dips as great as 55 cannot be ruled out we interpret the fault to dip northward at 30 to 35 in the upper few hundred meters steepening to 65 at 1 to 2 km depth a total of 180 m of near field thrust separation fault slip plus drag folding has occurred on the fault since the development of a prominent erosional surface atop 1 2 ma strata in the upper 20 to 40 m strain is partitioned between the north dipping main thrust strand and several closely spaced near vertical strike slip faults observed in paleoseismologic trenches the main thrust strand can be traced to within 20 m of the ground surface suggesting that it breaks through to the surface in large earthquakes uplift of a 50 000 year old alluvial fan surface indicates a short term dip slip rate of 0 5 mm yr similar to the 0 6 mm yr dip slip rate derived from vertical separation of the oxygen isotope stage 5e marine terrace 3 km west of the study site if the 0 6 mm yr minimum dip slip only rate characterizes the entire history of the fault then the currently active strand of the santa monica fault probably began moving within the past 300 000 years

diffusion limited ree uptake by eclogite garnets and its consequences for lu hf and sm nd geochronology garnets from the zermatt saas fee eclogites contain narrow central peaks for lu yb tm er and at least one additional small peak towards the rim the ree sm eu gd tb dy are depleted in the cores but show one prominent peak close to the rim these patterns cannot be modeled using rayleigh fractionation accompanied by mineral breakdown reactions instead the patterns are well explained using a transient matrix diffusion model where ree uptake is limited by diffusion in the matrix surrounding the porphyroblast observed profiles are well matched if a roughly linear radius growth rate is used the secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism the model predicts anomalously low 176lu 177hf and 147sm 144nd ratios in garnets where growth rates are fast compared to diffusion of the ree and these results have important implications for lu hf and sm nd geochronology using garnet

cathodoluminescence imaging and titanium thermometry in metamorphic quartz cathodoluminescence cl of quartz from metamorphic rocks representing a range of conditions from the garnet grade to the migmatite grade reveals a variety of textures that is a function of metamorphic grade and deformation history ti concentrations determined by electron microprobe and ion microprobe generally correlate with cl intensity blue wavelengths and application of the ti in quartz thermometer titaniq reflects the temperature of quartz growth or recrystallization and in some settings modification by diffusion quartz from garnet grade samples is not visibly zoned records temperatures of 425 475 c and is interpreted to have recrystallized during fabric formation quartz grains from staurolite grade samples are zoned in cl with markedly darker cores and brighter rims some of which are interpreted to have been produced by the dominant staurolite producing reaction whereas others are interpreted as having formed by diffusion of ti into quartz rims quartz from the matrix of kyanite and sillimanite grade samples are generally unzoned although locally displays slightly brighter rims higher ti quartz inclusions within garnet and staurolite have distinctly brighter rims which are interpreted as having been produced by diffusive exchange with the host mineral quartz from migmatite grade samples displays highly variable cl intensity which is dependent on the location of the grain matrix grains in melanosomes are largely unzoned or rarely zoned with darker cores leucosome quartz is strongly zoned with bright cores and dark rims and is interpreted as having formed during crystallization of the melt locally within the leucosome is observed oscillatory zoned quartz which is interpreted as a subsolidus recrystallization to achieve strain relaxation quartz inclusions within garnet or plagioclase crystals often show bright domains separated by zones of dark cl these enigmatic textures possibly reflect local melting fluxed by fluid inclusions temperatures calculated from the ti in quartz thermometer are a function of the metamorphic grade of the sample the textural setting of the quartz the reaction history and the deformation history of the rock the titaniq temperatures can be used to constrain the conditions at which various metamorphic processes have occurred

structural transect of the western transverse ranges california implications for lithospheric kinematics and seismic risk evaluation a retrodeformable cross section that integrates surface and subsurface data across the western transverse ranges california illustrates an actively developing fold and thrust belt that began forming at 2 3 ma high level thrusts are interpreted to root in a mid crustal detachment at 12 15 km depth which coincides with the maximum depth of earthquakes the cross section documents 53 km of convergence above the mid crustal detachment dividing this by the time since onset of deformation yields convergence rates of 17 6 26 5 mm yr the high density lithospheric anomaly beneath the transverse ranges is related to subduction of lower crust and lithosphere below the mid crustal detachment to balance the shallow crustal shortening thrust ramps coincide with zones of high seismicity in the transverse ranges this suggests that ramp regions have the highest potential for compressive earthquake events e g the recent whittier narrows earthquake of october 1 1987 m sub l 5 9 occurred along the eastern part of a ramp zone

vertical deformation of the japanese islands 1996 1999 1 the interseismic vertical deformation field of the japanese islands is derived from the global positioning system gps data between 1996 and 1999 although vertical components of the gps velocities have rarely been used as important information because of their higher noise level information on the vertical deformation field will enable us to separate rigid plate motions from deformation due to interplate coupling because horizontal displacements contain both effects whereas vertical displacements contain only the interplate coupling effect the results show that 1 the precision of the vertical velocity is generally better than 2 mm yr 2 the existence of clear uplift peaks of about 8 mm yr in the shikoku and tokai regions is consistent with strong coupling with a gradual brittleplastic transition zone at the nankai and the suruga trough respectively 3 clear uplift peaks do not seem to exist in the tohoku region because the japan trench plate boundary is too far oceanward from the gps sites 4 significant uplift is obtained in the western part of hokkaido which may be associated with the long term postseismic deformation due to the 1983 japan sea and the 1993 hokkaido nansei oki m 7 6 earthquakes and 5 uplift in northeastern japan along the japan sea coast is apparently associated with distributed compression due to incipient subduction

tectonic contraction across los angeles after removal of groundwater pumping effects after the 1987 whittier narrows and 1994 northridge earthquakes revealed that blind thrust faults represent a significant threat to metropolitan los angeles a network of 250 continuously recording global positioning system gps stations was deployed to monitor displacements associated with deep slip on both blind and surface faults here we augment this gps data with interferometric synthetic aperture radar imagery to take into account the deformation associated with groundwater pumping and strike slip faulting after removing these non tectonic signals we are left with 4 4 mm yr 1 of uniaxial contraction across the los angeles basin oriented n 36 degrees e perpendicular to the major strike slip faults in the area this indicates that the contraction is primarily accommodated on thrust faults rather than on the northeast trending strike slip faults we have found that widespread groundwater and oil pumping obscures and in some cases mimics the tectonic signals expected from the blind thrust faults in the 40 km long santa ana basin groundwater withdrawal and re injection produces 12 mm yr 1 of long term subsidence accompanied by an unprecedented seasonal oscillation of 55 mm in the vertical direction and 7 mm horizontally

discrepancy between geological and geodetic deformation rates in the ventura basin space geodesy provides a new tool for measuring neotectonic deformation which when combined with geological and geophysical observations and models can be used to infer rates and styles of deformation in the ventnra basin region of southern california the geodetically measured convergence rates of 7 10 mm yr 1 ref 1 are significantly lower than the 20 25 mm yr 1 estimated from geological data2 3 here we investigate the possible implications of this discrepancy it might be for example that temporal variations cause the short and long term convergence rates to differ but if this were the case we would expect to observe the long term rate by extending the geodetic measurements over a broader region yet only 11 3 mm yr 1 of regional convergence is observed4 our preferred explanation of the discrepancy is that the geological models require revision and to this end we present two deformation models that satisfy both the geological and geodetic constraints our modelling suggests that the faults bounding the basin are locked at the surface but are slipping at depths below 2 5 km

mechanical modeling of compressional basins origin and interaction of faults erosion and subsidence in the ventura basin california using geophysical observations and some basic geological ideas on the present structure of the ventura basin we model its possible mechanical evolution during the last 4 million years we use a two dimensional finite element model with frictional and elastoviscoplastic rheologies that allow strain localization this allows us to study the mechanical origin of the san cayetano thrust fault scf and its interaction with the oak ridge fault orf the results of our modeling indicate that the geometry and activity of a shear zone which mimics the san cayetano fault are conditioned both by the preexistence of the oak ridge fault and by the shape of the brittle ductile transition furthermore the presence of a decollement level is shown to favor the development of the san cayetano fault shear zone as a low angle structure the value of the effective friction on the oak ridge fault is evaluated from this model possible values of the viscosity of the sediments are also estimated it is shown that low sediment viscosity may completely hinder the activity of such a fault at the surface while slip at depth continues the stratigraphic profile of the basin is also shown to depend on the relative strengths of the orf and scf which also provide an explanation for the shortening and rapid subsidence of the basin

three dimensional patterns of garnet nucleation and growth three dimensional compositional zoning patterns in garnet zone garnets from harpswell neck maine united states suggest complex patterns of nucleation and growth compositional zoning is often asymmetric and core compositions do not always coincide with the geometric center of the garnet in detail garnet zoning patterns show multiple isolated high mno areas that we interpret as sites of garnet nucleation if it is assumed that garnet growth is close to equilibrium and the mn zoning is not significantly modified by diffusion then the mno contours or isosurfaces can serve as a proxy for patterns of garnet nucleation and growth computer generated three dimensional animations of these patchwork mno zoning patterns reveal that many garnets contain multiple nuclei that grew and coalesced to form a single crystal new nuclei often appeared next to preexisting porphyroblasts and became incorporated into the growing crystal contrary to the widely held idea that garnets grow radially outward from a single nucleus we propose that many of the garnets examined in this study grew by the coalescence of multiple nuclei

using mechanical modeling to constrain fault geometries proposed for the northern marmara sea 1 in recognition of potential seismic hazard along the north anatolian fault within the marmara sea geophysical surveys have recently been conducted to collect high resolution bathymetry and seismic reflection data despite the high quality of the data fault interpretations by different authors are varied and a conclusive fault geometry within the marmara sea remains elusive this paper presents a mechanical modeling method for constraining proposed fault geometry interpretations in the model crustal faults within the marmara sea slip in accordance with gps constrained slip rates along deep plate boundary dislocations we test three fault models proposed by others and show that an interpretation with a series of pull apart basins along a master strike slip fault best produces the observed deformation pattern within the marmara sea the locations and relative subsidence of the basins along the northern marmara trough are well matched the computed slip vectors on the faults in the best fitting model indicate that except for small faults bounding the tekirdag basin the faults have dominantly right lateral slip with rakes within 15 of horizontal on the local scale this suggests that interaction between the segmented strike slip faults is the dominant mechanism producing the observed vertical deformation in the marmara sea on the regional scale if the marmara sea faults behaved in this manner over geologic timescales then right lateral plate boundary motion without an additional component of north south extension associated with the aegean sea is sufficient to produce the marmara sea pull apart basin morphology

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

microstructure and crystallographic preferred orientation of polycrystalline microgarnet aggregates developed during progressive creep recovery and grain boundary sliding abstract optical microscopy electron probe microanalysis and electron backscatter diffraction methods have been used to examine a broad range of garnet microstructures within a high strain zone that marks the western margin of a major transpression zone in the southern new england appalachians garnet accommodated variable states of finite strain expressed as low strain porphyroclasts type 1 high strain polycrystalline aggregates type 2 and transitional morphologies type 3 that range between these end members type 1 behaved as rigid porphyroclasts and is characterized by four concentric ca growth zones type 2 help define foliation and lineation are characterized by three ca zones and possess a consistent bulk crystallographic preferred orientation of 100 symmetrical to the tectonic fabric type 3 show variable degrees of porphyroclast associated with aggregate where porphyroclasts display complex compositional zoning that corresponds to lattice distortion low angle boundaries and subgrains and aggregate cpo mimics porphyroclast orientation all aggregates accommodated a significant proportion of greenschist facies deformation through grain boundary sliding grain rotation and impingement and pressure solution which lead to a cohesive behavior and overall strain hardening of the aggregates the characteristic cpo could not have been developed in this manner and was the result of an older phase of partitioned amphibolite facies dislocation creep recovery including chemical segregation and recrystallization of porphyroclasts this study demonstrates the significance of strain accommodation within garnet and its affect on composition under a range of pt conditions and emphasizes the importance of utilizing ebsd methods with studies that rely upon a sound understanding of garnet

interseismic strain accumulation and anthropogenic motion in metropolitan los angeles 1 we use global positioning system gps geodesy and synthetic aperture radar sar interferometry to distinguish between interseismic strain accumulation and anthropogenic motion in metropolitan los angeles we establish a relationship between horizontal and vertical seasonal oscillations of the santa ana aquifer use this relationship to infer cumulative horizontal anthropogenic motions from cumulative vertical motions caused by water and oil resource management and estimate horizontal interseismic velocities corrected for anthropogenic effects vertical anthropogenic rates from 1992 to 1999 are slower than 3 mm yr 1 in the santa ana and san gabriel aquifers and faster than 5 mm yr 1 in the chino aquifer and in many oil fields inferred horizontal anthropogenic velocities are faster than 1 mm yr 1 at 18 of 46 gps sites northern metropolitan los angeles is contracting with the 25 km south of the san gabriel mountains shortening at 4 5 1 mm yr 1 95 confidence limits the thrust fault in an elastic edge dislocation model of the observed strain is creeping at 9 2 mm yr 1 beneath and north of a position 6 2 km deep and 8 8 km north of downtown los angeles the model fault is near the los angeles segment of the puente hills thrust but south of the sante fe springs segment of the thrust disagreement between the 6 km locking depth in the model and the 15 km seismogenic depth inferred from earthquakes suggests that the elastic continuum model may be unsatisfactory models with different stiffnesses of sedimentary basin and crystalline basement must be investigated

evolution of the hazelton arc near terrace british columbia stratigraphic geochronological and geochemical constraints on a late triassic early jurassic arc and cu au porphyry belt understanding the development of island arcs that accreted to the north american craton is critical to deciphering the complex geological history of the canadian cordillera in the case of the hazelton arc part of the stikine terrane or stikinia in northwestern british columbia understanding arc evolution also bears on the formation of spatially associated porphyry cu au epithermal and volcanogenic massive sulfide deposits the hazelton group is a regionally extensive long lived and exceptionally thick upper triassic to middle jurassic volcano sedimentary succession considered to record a successor arc that was built upon the paleozoic and triassic stikine and stuhini arcs in central stikinia near terrace british columbia the lower hazelton group telkwa formation comprises three volcanic intrusive complexes mt henderson mt o brien and kitselas that at their thickest constitute almost 16 km of volcanic stratigraphy basal telkwa formation conglomerates and volcanic rocks were deposite

deformation induced garnet zoning compositional zoning patterns in garnet porphyroblasts from kyanite bearing samples of the devonian littleton formation north central massachusetts reveal complex patterns of growth that are related to multiple deformation and metamorphic events garnet porphyroblasts exhibit asymmetrical and irregular zoning patterns in xmn xca and fe fe mg zoning reversals in mn and fe fe mg and patch distribution in ca appear to occur around the boundaries of the textural zones also the compositions of the garnet at the textural boundaries are variable for all traverses these observations suggest that the garnet zoning was not only modified from diffusion processes but was also influenced by pre existing microfabrics through the effects of preferential dissolution and resorption in partial disequilibrium relationships between chemical and textural truncations indicate that the zoning patterns of garnet were strongly modified from preferential dissolution and precipitation during the development of successive foliations that occurred in zones of high strain stress cleavage seams and zones of low strain stress respectively

getting the inside story using computed x ray tomography to study inclusion trails in garnet porphyroblasts this study demonstrates how high resolution x ray computed tomography can be used to determine the geometry of curved inclusion trails in garnet porphyroblasts for the first time the three dimensional geometry of these features is shown as an animation a stereo pair a vrml model and two dimensional sections the spatial resolution of the data in this study approximately 10 m is high enough to resolve inclusions and finer resolutions are possible comparisons of scan imagery with thin section and microprobe data demonstrate that many typical inclusion phases in garnet porphyroblasts can be clearly distinguished including quartz ilmenite and monazite a sodium metatungstate solution with a density of 1 19 g cm3 was used for a wedge calibration during scanning reducing artifacts to a minimum and allowing maximum image contrast the overall conclusion of this study is that high resolution x ray computed tomography is a quick non destructive technique that is invaluable in the study of porphyroblast microstructure the three dimensional data generated by this technique can be compared with geometries predicted by the varying theoretical models for the formation of curved inclusion trails in garnet porphyroblasts they can also be analyzed to determine the three dimensional orientation of curvature axes in individual porphyroblasts which has not been possible until now

application of three fault growth criteria to the puente hills thrust system los angeles california usa abstract three dimensional mechanical models are used to evaluate the performance of different fault growth criteria in predicting successive growth of three echelon thrust faults similar to the segments of the puente hills thrust system of the los angeles basin california four sequential boundary element method models explore the growth of successive echelon faults within the system by simulating snapshots of deformation at different stages of development these models use three criteria 1 energy release rate 2 strain energy density and 3 navier coulomb stress to characterize the lateral growth of the fault system we simulate the growth of an echelon thrust fault system to evaluate the suitability of each of these criteria for assessing fault growth each of these three factors predicts a portion of the incipient fault geometry i e location or orientation however each provides different information in each model energy release rate along the westernmost leading tip of the puente hills thrust drops with growth of the next neighboring fault this result supports the overall lateral development of successive echelon segments within each model regions of high strain energy density and navier coulomb stress envelope at least a portion of the next fault to develop although the strain energy density has stronger correlation than navier coulomb stress to the location of incipient faulting in each model one of the two predicted planes of maximum navier coulomb stress ahead of the leading fault tip matches the strike but not the dip of the incipient fault plane recreating part of the fault orientation the incipient fault dip is best predicted by the orientation of the strain energy density envelopes around the leading fault tip furthermore the energy release rate and pattern of strain energy density can be used to characterize potential soft linkage overlap or hard linkage connection of echelon faults within the system

chemical weathering and provenance evolution of holocene recent sediments from the western indus shelf northern arabian sea inferred from physical and mineralogical properties abstract we present a multi proxy mineral record based on x ray diffraction and diffuse reflectance spectrophotometry analysis for two cores from the western indus shelf in order to reconstruct changing weathering intensities sediment transport and provenance variations since 13 ka core indus 10 is located northwest of the indus canyon and exhibits fluctuations in smectite illite chlorite ratios that correlate with monsoon intensity higher smectite illite chlorite and lower illite crystallinity normally associated with stronger weathering peaked during the early mid holocene the period of maximum summer monsoon hematite goethite and magnetic susceptibility do not show clear co variation although they both increase at indus 10 after 10 ka as the monsoon weakened at indus 23 located on a clinoform just west of the canyon hematite goethite increased during a period of monsoon strengthening from 10 to 8 ka consistent with increased seasonality and or reworking of sediment deposited prior to or during the glacial maximum after 2 ka terrigenous sediment accumulation rates in both cores increased together with redness and hematite goethite which we attribute to widespread cultivation of the floodplain triggering reworking especially after 200 years ago over holocene timescales sediment composition and mineralogy in two localities on the high energy shelf were controlled by varying degrees of reworking as well as climatically modulated chemical weathering

garnet polycrystals and the significance of clustered crystallization polycrystalline garnets are common in metamorphic rocks and may form as a result of close spacing of nuclei if clustering is early or impingement of larger grains if clustering occurs later in the growth history the timing of clustering relative to garnet growth is relevant to understanding the formation and evolution of porphyroblasts and evaluating the significance if any of clustering electron backscattered diffraction ebsd analysis of garnet bearing metamorphic rocks reveals the presence of polycrystalline garnet in nine localities examined in this study the northern appalachians vermont maine new york usa north american cordillera north cascades range washington snake range nevada usa western rocky mountains british columbia canada southern menderes massif turkey santander massif colombia and the sanandaj sirjan zone hamadan iran in some samples polycrystals comprise 20 30 of garnets analyzed and chemical and textural evidence suggests that early coalescence of garnet polycrystals is common some early coalescing polycrystals exhibit growth zoning that is concentric about the geometric center of the polycrystal in thin section these garnets may be undetectable as polycrystals based on morphology or zoning in some polycrystals zoning is unrelated to the location of internal grain boundaries in others fe mn mg zoning has a different pattern than that of ca zoning patterns may vary on the scale of a single thin section in addition some polycrystals are characterized by high angle misorientation boundaries that may be in special non random orientations an observation that indicates that these polycrystals are not random clusters of grains the presence of internal grain boundaries may affect diffusion pathways and length scales and may facilitate communication of porphyroblast interiors with matrix phases thereby influencing reaction history of the rock and the composition zoning of garnet

periodic slow earthquakes from the cascadia subduction zone continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking and they are thus known as slow or silent earthquakes because subduction zones generate the largest earthquakes

the geochemistry of dsdp sediments from site 223 indian ocean abstract detailed major and trace element chemistry is presented for 41 sediment samples from dsdp site 223 borehole cores a marked change in chemical and mineralogical character is shown at the end of the early miocene epoch which relates to tectonic events and associated changes in sedimentary regime enrichment in the contents of such elements as mg cr and ni compared with average values for fine grained sediments occurs throughout the sequence and is particularly marked in the upper group of samples a basic ultrabasic provenance is suggested the oman ophiolites leaching with combined acid reducing agent indicated typical lithogenous character ordering for the elements and emphasised the enrichment of mg cr ni and li cu zn pb fe and ti over values for near shore muds and terrigenous material factor analysis on the bulk chemical data identifies the main lithogenous and biogenous components subdividing the latter it separates the upper and lower group of chemically dissimilar sediments and delineates a mn hydroxide phase it also shows the essentially independent roles of na ba and p

effects of nonplanar fault topology and mechanical interaction on fault slip distributions in the ventura basin california to assess the control of fault geometry and mechanical interactions on fault slip distributions in a complex natural system we present results from three dimensional mechanical models incorporating both nonplanar and rectangular planar representations of active faults within the ventura basin region of southern california we find that the incorporation of geologically constrained nonplanar fault surfaces into numerical models of active deformation results in a better match to available geologic slip rate data than models utilizing rectangular planar fault surfaces the model results demonstrate that nonplanar fault geometry and mechanical interactions exert a strong control on resultant slip distributions additionally we find that slip rates at most locations along the surface trace of ventura faults are not likely to rep resent average values for the entire fault surface we propose that results from three dimensional mechanical models using realistic i e nonplanar fault geometry can be used to both predict slip rates at specific locations and determine whether existing site specific slip rate estimates are representative of average fault slip rates although geo metric irregularities along fault surfaces should resist slip planar faults can have lesser slip than nonplanar faults due to the differing mechanical interactions among nearby faults in the two representations this suggests that models using simplified or planar fault geometry are likely to inaccurately simulate regional deformation we assert that detailed knowledge of three dimensional fault shape as well as the geometry and con figuration of deep fault intersections is essential for accurate seismic hazard char acterization of regions of complex faulting such as the ventura basin of southern california

principal component analysis of geodetically measured deformation in long valley caldera eastern california 1983 1987 typical geodetic measurements of deformation consist of repeated surveys of a particular geodetic network such deformation data can be interpreted as a consequence of one or more self coherent sources by means of principal component analysis a self coherent source is defined as any source that produces deformation that is time and space separable principal component analysis then gives the time and space factors that characterize the deformation attributed to each self coherent source geodetic measurements of deformation at long valley caldera provide two examples of the application of principal component analysis a 40 line trilateration network surrounding the caldera was surveyed in midsummer 1983 1984 1985 1986 and 1987 principal component analysis indicates that the observed deformation can be represented by a single coherent source the time dependence for that source displays a rapid rate of deformation in 1983 1984 followed by less rapid but uniform rate in the 1984 1987 interval the spatial factor seems consistent with expansion of a magma chamber beneath the caldera plus some shallow right lateral slip on a vertical fault in the south moat of the caldera an independent principal component analysis of the 1982 1983 1984 1985 1986 and 1987 leveling across the caldera requires two self coherent sources to explain the deformation the deformation pattern produced by the larger of these two sources appears to be roughly consistent with that found from the trilateration data the deformation due to the second source is a nearly uniform tilt in the uplift profile presumably that tilt is simply an artifact of systematic error in the leveling

metamorphic history suggested by garnet growth chronologies in the isua greenstone belt west greenland abstract compositional maps for garnets from the five tectonic domains of the isua greenstone belt in west greenland are used to identify episodes of garnet growth during the metamorphism of the greenstone belt three distinct episodes of garnet growth are recorded in domains iii v two in domain ii and one in domain i a wide variety of garnet textures are observed including growth zoning post growth homogenisation replacement rim resorption and oscillatory zoning there is no simple correlation of garnet growth events among domains indicating that at least some events are related to localised metasomatic processes in the greenstone belt however the relative growth chronology combined with the available geochronology indicates that the observed metamorphic processes span both the early and late archaean

anisotropic structure under a back arc spreading region the taupo volcanic zone new zealand 1 we investigate anisotropy surrounding a continental back arc spreading region the central volcanic region cvr in the north island of new zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy s phases from local earthquakes yield consistent trench parallel fast directions in the southern compressional region but varied directions in the northern extensional region in the forearc east of the cvr is nearly trench parallel suggesting shear in the mantle or trench parallel flow in the western cvr local phases from 50 to 80 km depth give parallel to extension 120 and phases from 150 to 220 km depth yield similar 150 these results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100 150 km depth in the eastern cvr local phases show mixed suggesting that it is a transition between the western cvr and the forearc trench parallel for shallow local events within the cvr may be caused by fluid filled cracks trench parallel for deeper local events and for sks phases can be explained by standard olivine fabrics developed by lithospheric shearing by trench parallel flow and by fossil anisotropy in the subducting oceanic lithosphere alternatively in the cvr they could be caused in part by hydrous minerals at the slab mantle upper interface which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab we suggest that trench perpendicular flow in the extensional region helps to drive fluids away from the slab while in the compressional region trench parallel flow fails to distribute the fluids explaining the location of changes in geophysical properties

formation of garnet clusters during polyphase metamorphism pre alpine garnets of variscan age from metapelitic basement units in northern italy were strongly retrogressed at near surface conditions prior to alpine contact metamorphism the replacement by sheet silicates caused a significant volume increase during retrogression producing pervasive fracturing up to several hundreds of angular fragments formed from each crystal electron backscatter diffraction analysis documents a maximum misorientation of 22 of some fragments as a result of local rotation during fracturing new garnet growth is observed on the garnet fragments during contact metamorphic overprinting resulting in garnet clusters fragments can be identified due to calcium rich domains fragment orientations were inherited and only minor new nucleation occurred these garnets develop features typically associated with multiple nucleation models but here they reflect multiple metamorphic events we propose that clusters can be indicative of multiple metamorphic events which were separated by a period of intense retrograde alteration

geomorphic indicators of active fold growth south mountain oak ridge anticline ventura basin southern california south mountain oak ridge near ventura california is an asymmetric anticlinal uplift forming at the present time above the active buried oak ridge reverse fault shortening along the oak ridge fault accumulated largely in quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0 5 m y tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth stream gradient indices are relatively high in the northern eroded fold scarp of the ridge a pattern consistent with the existence of active rapid slip on the oak ridge fault mountain front sinuosity along the northern slope of the anticlinal ridge roughly decreases from 2 to 1 toward the westernmost 10 km of observed surface folding valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from 1 5 to 0 5 values of the hypsometric integral along the northern flank increase significantly from 0 35 to 0 4 maximum 0 55 from east to west drainage density varies from 4 to 6 km km2 along both flanks of the south mountain oak ridge anticline entrenchment of streams into the southern backlimb of the fold along the westernmost 9 km of the structure decreases from 20 m to 1 m from east to west apparent backlimb rotation as measured by dip of strata along the westernmost 7 km of the fold decreases from east to west from 35 to 20 fold growth of the south mountain oak ridge anticline occurred during the past 0 5 ma following deposition of the saugus formation lateral and vertical fold growth were likely produced by westward decrease in fault slip along the buried oak ridge fault

diffusion control of garnet growth harpswell neck maine usa a detailed analysis of chemical zoning in two garnet crystals from harpswell neck maine forms the basis of an interpretation of garnet nucleation and growth mechanisms garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation giving rise to complex two dimensional zoning patterns depending on the orientation of the thin section cut contours of ca zoning cross those of mn fe and mg indicating a lack of equilibrium among these major garnet constituents zoning of fe mg and mn is interpreted to reflect equilibrium with the rock matrix whereas ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal image analysis reveals that the growth of garnet is more rapid along triple grain intersections than along double grain boundaries moreover different minerals are replaced by garnet at different rates the relative rate of replacement by garnet along double grain boundaries is ordered as muscovite chlorite plagioclase quartz flux calculations reveal that replacement is limited by diffusion of si along double grain boundaries to or from the local reaction site it is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet with si and al transport being rate limiting in these samples

geochemical study of organic matter rich cycles from the kimmeridge clay formation of yorkshire uk productivity versus anoxia abstract in this contribution we study two meter scale cycles from the kimmeridge clay formation cored near marton yorkshire which shows cyclic organic matter om distribution our aim is to try to understand the factors responsile for om accumulation the first cycle called lower cycle shows a total organic carbon toc content between 1 and 10 whereas the second called upper cycle shows a toc content varying between 5 and 35 the geochemical composition major elements and trace elements the organic geochemistry toc hi palynofacies and mineralogy of the sediments have been determined in both cycles the cyclicity is expressed through variations in the nature and in the relative abundance of the various types of organic matter constituents furthermore dilution effects by inorganic components of the sediment cannot account for the toc cyclicity for lower cycle the mo v and u content is low and little variable as is the intensity of the oxidation which om suffered from this indicates that variations in phytoplanktonic productivity may be held responsible for the cyclicity in steady and mildly reducing redox conditions in the upper cycle cyclicity also appears to depend on productivity but variations in the concentration of mo v and u and in the oxidation state of the om suggest the environment was temporarily more reducing it is proposed that larger amounts of h 2 s were released into marine bottom waters as a result of initial om decomposition forced the oxic anoxic boundary to rise in the water column and thus favoured om storage the main driving force for variations in the om concentration was the productivity of organic matter walled phytoplankton redox conditions of the depositional environment could have had a positive action but only by acting as a positive feedback effect

active faults and evolving strike slip basins in the marmara sea northwest turkey a multichannel seismic reflection study abstract the north anatolian fault is a 1200 km long transform fault forming the boundary between the anatolian and eurasian plates near its western end it strikes through the marmara sea and is responsible for the creation of three deep marine depressions the north anatolian fault system and the linked basins in the eastern marmara sea were studied using newly acquired multi channel seismic reflection data in the marmara sea the north anatolian fault consists of a main strand and a few subsidiary branches the main strand is made up of the ganos 15 km long central marmara 105 km and north boundary 45 km fault segments the north boundary fault joins in the east to the izmit segment which was ruptured during the m s 7 4 earthquake on 17 8 1999 the north boundary fault is highly oblique to the regional displacement vector which results in the generation of a symmetrical fault wedge basin this cinarcik basin is a wedge shaped deep marine depression south of istanbul it is about 50 km long up to 20 km wide and 1250 m deep and is filled with pliocene to quaternary sediments over 3 km thick the north boundary fault joins in the west to the transpressional central marmara fault around the restraining segment boundary the syntransform sediments are being deformed into a large anticlinorium the cinarcik basin started to form when the westward propagating north anatolian fault intersected a northwest trending pre existing fault zone during the pliocene and bifurcated into nw and sw trending segments forming a transform transform transform type triple junction dextral strike slip movement along the arms of the triple junction led to the development of the cinarcik basin the present fault geometry in the marmara region was achieved later during the mid pliocene following the termination of the triple junction geometry south of the main north anatolian fault system there is a second active strike slip fault in the marmara sea made up of several segments a highly asymmetrical fault bend basin comprising pliocene quaternary sediments over 2 5 km thick is forming along a releasing fault segment of this south boundary fault system

formation of garnet polycrystals during metamorphic crystallization garnet polycrystals may form throughout the metamorphic history of a rock starting at the earliest stages of garnet growth when closely spaced nuclei coalesce in mica schist from townshend dam vt electron back scattered diffraction ebsd analysis shows that garnet polycrystals possess two or more distinct lattice orientations separated by high angle boundaries 28 61 the minimum rotational displacements required to bring these lattice orientations into concordance with each other are commonly normal to the same low energy planes that occur as crystal faces of euhedral garnet there is no evidence for intracrystalline deformation and the polycrystals therefore probably represent individual garnet crystals that coalesced during growth the boundaries cross cut growth zoning and inclusion trails of the polycrystals indicating that early formed polycrystals once coalesced behave chemically and physically as single crystals statistical analysis of a 3d high resolution x ray computed tomographic data set of a large sample 912 cm3 of a townshend dam schist combined with microprobe and ebsd analyses of garnet are consistent with a high degree of clustering at all stages of garnet growth the formation and prevalence of polycrystals implies that garnet nuclei impinged on each other and coalesced and that coalescence was a common feature throughout garnet growth in the rock

development of garnet porphyroblasts by multiple nucleation coalescence and boundary misorientation driven rotations two types of garnet porphyroblast occur in the schneeberg complex of the italian alps type 1 porphyroblasts form ellipsoidal pods with a centre consisting of unstrained quartz decussate mica and small garnet grains and a margin containing large garnet grains orientation contrast imaging using the scanning electron microscope shows that the larger marginal garnet grains comprise a number of orientation subdomains individual garnet grains without subdomains are small 50 m faceted and idioblastic and have simple zoning profiles with ca rich cores and ca poor rims subdomains of larger garnet grains are similar in size to the individual small garnet grains type 2 porphyroblasts comprise only ellipsoidal garnet with small subdomains in the centre and larger subdomains at the margin each subdomain has its own ca high ca dropping towards subdomain boundaries garnet grains with or without subdomains all have the same ca poor composition at rims in contact with other minerals the compositional zonation patterns are best explained by simultaneous multiple nucleation followed by growth and amalgamation of individual garnet grains the range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porphyroblast margin than in the centre the difference between type 1 and type 2 porphyroblasts is probably related to the growth rate differential across the porphyroblast electron backscatter diffraction shows that small individual garnet grains are randomly oriented large marginal garnet grains and subdomain bearing garnet grains have a strong preferred orientation clustering around a single garnet orientation misorientations across subdomain boundaries are small and misorientation axes are randomly oriented with respect to crystallographic orientations the only explanation that fits the observational data is that individual garnet grains rotated towards coincident orientations once they came into contact with each other this process was driven by the reduction of subdomain boundary energy associated with misorientation loss rotation of garnet grains was accommodated by diffusion in the subdomain boundary and diffusional creep and rigid body rotation of other minerals quartz and mica around the garnet an analytical model in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz suggests that at the conditions of metamorphism the rotation required to give a strong preferred orientation can occur on a similar time scale to that of porphyroblast growth

subcontinental scale crustal velocity changes along the pacific north america plate boundary transient tectonic deformation has long been noted within approx100 km of plate boundary fault zones and within active volcanic regions but it is unknown whether transient motions also occur at larger scales within plates relatively localized transients are known to occur as both seismic and episodic aseismic events and are generally ascribed to motions of magma bodies aseismic creep on faults or elastic or viscoelastic effects associated with earthquakes however triggering phenomena and systematic patterns of seismic strain release at subcontinental 1 000 km scale along diffuse plate boundaries have long suggested that energy transfer occurs at larger scale such transfer appears to occur by the interaction of stresses induced by surface wave propagation and magma or groundwater in the crust or from large scale stress diffusion within the oceanic mantle in the decades following clusters of great earthquakes here we report geodetic evidence for a coherent subcontinental scale change in tectonic velocity along a diffuse 1 000 km wide deformation zone our observations are derived from continuous gps global positioning system data collected over the past decade across the basin and range province which absorbs approximately 25 per cent of pacific north america relative plate motion the observed changes in site velocity define a sharp boundary near the centre of the province oriented roughly parallel to the north northwest relative plate motion vector we show that sites to the west of this boundary slowed relative to sites east of it by 1 mm yr 1 starting in late 1999

scales of disequilibrium and rates of equilibration during metamorphism mounting evidence suggests that partial disequilibrium meaning disequilibrium for some elements but not for others may be a common but rarely detected phenomenon during metamorphic mineral growth even in ordinary prograde reactions that progress to completion detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements with strong temperature dependence under lower greenschist facies conditions fe and mg may equilibrate at hand sample scale whereas mn and ca may not equilibrate even at millimeter scale although mn may achieve hand sample scale equilibration under upper greenschist facies conditions ca and many trivalent cations e g rees may not do so until temperatures exceed those of the middle amphibolite facies even in the lower granulite facies some elements e g y yb show indications of disequilibrium at sub centimeter scales during garnet growth analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion the same mechanism known to govern porphyroblast crystallization in many metamorphic environments despite this importance to petrology few quantitative determinations exist of inter granular diffusion rates under metamorphic conditions using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the llano uplift u s a a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of al in fluid undersaturated systems this result and an earlier estimate for fluid saturated systems provide bracketing values for al diffusivity during metamorphism

controls on metamorphic equilibration the importance of intergranular solubilities mediated by fluid composition a dramatic demonstration of the role of intergranular solubility in promoting chemical equilibration during metamorphism is found in the unusual zoning of garnet in pelitic schist exposed at harpswell neck maine usa many garnet crystals have irregular patchy distributions of mn cr fe and mg in their inclusion rich interiors transitioning to smooth concentric zoning in their inclusion poor outer rims in contrast zoning of ca and y is comparatively smooth and concentric throughout we re assess the disputed origin of these zoning features by examining garnet growth in the context of the thermal and structural history of the rocks and by evaluating the record of fluid rock interaction revealed in outcrop scale veining and fluid inclusion assemblages the transition in the character of garnet zoning correlates with the onset of a synkinematic simple shear dominated phase of garnet growth and with a shift in the composition of the intergranular fluid from co2 rich to h2o rich compositional variations in garnet are therefore best explained by a two stage growth history in which intergranular diffusive fluxes reflect differences in the concentration of dissolved species in these two contrasting fluids interiors of garnet crystals grew in the presence of a co2 rich fluid in which limited solubility for mn and cr and perhaps fe and mg produced patchy disequilibrium overprint zoning while appreciable solubility for ca and y permitted their rock wide equilibration rims grew in the presence of an h2o rich fluid in which high intergranular concentrations for all elements except cr enabled diffusion over length scales sufficient for rock wide equilibration this striking example of partial chemical equilibrium during reaction and porphyroblast growth implies that thermal effects may commonly be subsidiary in importance to solubilities in the intergranular medium as determinants of length scales for metamorphic equilibration