temporal and spatial patterns of sea level in inland basins recent events in the aral sea 1 we demonstrate in this paper that satellite altimeter data resolve the drop in the aral sea level during 1993 2000 of about 0 6 m per year resulting in a change of surface area from 35000 to 22000 km 2 and volume from 270 to 130 km 3 the sudden drop in the sea level of the northern basin on 04 21 1999 resulting from dam break up is also clearly resolved the temporal and spatial variability of sea level reveals response patterns which are characteristic for friction dominated shallow sea dynamics the combination of salinity and sea level data enables to identify the major events of environmental transition which are associated with the temporal variability in the total salt content 1 a peak in 1993 1994 and 2 an increasing trend in the last decade these events are indicative of an increase in the discharge of ground water but could also reveal overestimated salinity estimates from recent observations

quartz microstructures developed during non steady state plastic flow at rapidly decaying stress and strain rate abstract synseismic loading to very high stresses 0 5 gpa and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca 300 350 c near the lower tip of an inferred once seismically active crustal scale fault was proposed based on peculiar microstructures identified in rocks exposed over 100 km 2 in the sesia zone european western alps here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small scale shear zones sub basal deformation lamellae and arrays of elongate subgrains on the tem scale indicate an early stage of glide controlled deformation at high stresses distributed brittle failure is indicated by healed microcracks very fine grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep locally a fine grained foam microstructure indicates a final stage of static grain growth at low differential stress for the previously inferred peak stresses of about 0 5 gpa and given temperatures initial strain rates on the order of 10 10 s 1 are predicted by available flow laws for dislocation creep of quartz we emphasise the importance of short term non steady state deformation in the uppermost plastosphere underlying seismically active upper crust the related heterogeneous record of quartz is governed by the local stress history at constant temperature

garnets recording deep crustal earthquakes syn eclogite facies pseudotachylytes of the bergen arcs western norway were formed during co seismic faulting at a depth of 60 km during caledonian continent collision concurrent with fluid induced partial eclogitization of precambrian anorthositic granulites the pseudotachylytes contain the high pressure assemblage omphacite garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas rock volumes corresponding to outcrops of 100 m2 in area were shattered by pseudotachylytes associated with ultramylonites cataclasites and shear zones the granulite facies garnets unzoned and inclusion free away from the pseudotachylyte veins display planar and curved fractures brecciation and melting with decreasing distance from the veins sieve textured garnet developed by the disintegration of granulite facies garnet crystallization of eclogite facies mineral inclusions and welding together of garnet fragments fractured granulite facies garnets trend to more almandine rich compositions with variable grossular contents in the pseudotachylyte veins a second generation of minute 10 200 m euhedral or dendritic garnets has a higher femg ratio than the granulite facies garnet and a wide range in grossular content grs11 grs89 suggesting rapid growth and cooling fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness seismic faulting caused enhanced reaction rates during ongoing fluid induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density the sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation

transposed high pressure granulite fabrics cabo ortegal nw spain implications on the scales of deformation localization abstract high pressure granulite facies rocks of the bacariza formation cabo ortegal nw spain were syn metamorphically deformed at the contacts with the bounding units peridotite and eclogite massifs this enabled the formation of meter thick spectacular shear zones with reworked and transposed foliations and lineations the texturally stable mineral assemblage of the new fabrics records an intense ductile deformation of the mineral aggregate at temperatures of 700 800 c associated with amalgamation of eclogite high pressure granulitic rocks and ultramafic sheets in deep portions of a subduction channel the lattice preferred orientation of the main constituent minerals garnet augite amphibole plagioclase quartz and biotite discloses the active deformation mechanisms at the scale of the mineral grains and the relationships with the deformation at larger scales overprinting relationships of the metamorphic assemblages demonstrates that partitioning and deformation localization occurred at different scales under similar high grade conditions complete macroscopic transposition in the shear zones was complementary to meso and microscopic partitioning of deformation intensity and mechanisms between different lithological layers and mineral species

parting in giant garnets as an indicator of late brittle deformation at gore mountain warren county n y the barton garnet deposit contains large garnet crystals 2 5 25 cm disseminated through a gabbro derived garnet amphibolite these crystals are characterized by two sets of parting planes which intersect at about 80 and have a constant attitude throughout the amphibolite the results of x ray diffraction studies on the parting planes of the garnet crystals by the back reflection laue method and by x ray diffractometry show that the parting plane sets are not crystallographically controlled by the lattice of the garnet and that the garnets are not oriented in the amphibolite each parting set in the giant garnets and its corresponding parallel joint set in the amphibolite matrix are interpreted as brittle fractures which developed in response to a discrete tectonic episode

radial cracks around α quartz inclusions in almandine constraints on the metamorphic history of the oman mountains radiating tensional cracks around quartz inclusions in almandine have been observed in metapelite samples from the southeastern saih hatat tectonic window northeastern oman mountains these almandines show an inclusion rich glaucophane epidote and strongly deformed core with inclusions of different mineral phases the rim of the same almandines is inclusion poor and shows only quartz apatite zircon rutile and baal phosphates as inclusions quartz and apatite inclusions in the rim are single crystals often surrounded by radial cracks these radial cracks developed during uplift by the dilation of quartz 4 5 vol without a phase transformation subsequently these cracks were filled with kaolinite phengite si content 3 4 per formula unit p f u chlorite and fe oxides we calculated the appearance of radial cracks without phase transformation using the mathematical procedure of van der molen and van roermund 1 this calculation involves terms for thermal expansion isothermal compressibility and shear modulus for the example of quartz and almandine for the same p and t interval during a retrograde path published geothermobarometric estimates give pressures of between 1 0 and 2 0 gpa and temperatures of between 450 and 600 c for the peak conditions for these rocks of the saih hatat tectonic window on the basis of these p t data we calculated different retrograde p t paths in the quartz domain initiation of garnet fracturing is dependent on the p t starting conditions and the component of isothermal compression of the retrograde path the calculations yield a set of smooth monotonic curves whose exact position on the p t plane between 0 1 and 0 6 gpa and 40 and 500 c depends on the initial p t conditions and the component of isothermal compressibility of the retrograde p t paths this model can be used in general terms to estimate pressure and temperature for the following cases 1 if independent evidence such as petrological data allow the determination of the final pressure at which radial cracks appeared the initial inclusion pressure can be recalculated 2 if the initial inclusion pressure is known e g from petrological data the conditions of radial cracking can be calculated and the pair initial pressure final pressure leads to an estimate of the shape of the retrograde p t path as a function of its component of isothermal decompression in the example from the northeastern saih hatat tectonic window the late syntectonic growth of albite phengite kaolinite suggests that the final pressure for fracturing ranged between 0 4 gpa and 0 5 gpa at temperatures of 300 c these values correspond to high initial pressures of at least 2 0 gpa at a temperature of 550 c the following geodynamic model is suggested a regionally extended metamorphism led to the growth of inclusion rich garnets in the rocks from the northeastern saih hatat tectonic window at depths of about 30 km 0 1 gpa about 450 c continuing prograde metamorphism at a depth of more than 60 km with p 2 0 gpa and t 550 c affected a metapelite unit that is only exposed immediately south of as sifah village in this area clear rims of almandine grew around the older garnets and entrapped mainly quartz and apatite during uplift along a retrograde p t path with a large component of isothermal decompression radial cracks around quartz inclusions developed in the rims of almandines at a depth of about 12 km 0 4 0 5 gpa 300 c

metamorphic evolution of the sesia lanzo zone western alps time constraints from multi system geochronology the sesia lanzo zone is a polymetamorphic unit containing hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the alpine orogeny different parts of the unit record different stages on the p t deformation evolution allowing multi system isotopic studies to unravel the precise timing of the metamorphic history new rb sr white mica and u pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60 70 ma this substantially alters the common view of early to mid cretaceous eclogite facies metamorphism in this unit the new results are more consistent with the established geotectonic framework for the alpine orogeny since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension it is also more concordant with recent studies of other units that demonstrate post cretaceous high pressure metamorphism step heated 40ar 39ar analysis of phengites yields good plateaux giving ages older than the corresponding rb sr age such anomalously high ages indicate the presence of radiogenic argon rich fluids in the grain boundary network under the fluid pressure conditions acting during this high pressure metamorphic event the u pb sphene ages are variable in polymetamorphic rocks and show inheritance of older pb or sphene crystals into the high pressure event two monometamorphic assemblages yield concordant ages at 66 1 ma reflecting crystallisation of the eclogite facies assemblage the gneiss minuti complex gmc lies structurally below the eclogitic micaschists and its pervasive greenschist facies fabric yields tightly clustered rb sr white mica ages at 38 39 ma this greenschist event did not affect the majority of the emc the 40ar 39ar ages of micas formed at this time were very disturbed whereas micas surviving from an earlier higher pressure assemblage had their 40ar 39ar system reset the greenschist event did not strongly affect u pb systematics in hercynian age sphenes suggesting that the gmc did not uniformly suffer an eclogite facies metamorphism during the alpine cycle but was juxtaposed against the emc later in the orogeny this model still requires that the locus of deformation and metamorphism and possibly fluid flux moved outboard with time leaving the sesia lanzo basement as a shear bounded unreactive block within the orogenic wedge

cataclastic deformation of garnet a record of synseismic loading and postseismic creep abstract microstructural evidence for episodic deformation of the uppermost plastosphere related to failure of the schizosphere in major seismic events has been previously reported from the sesia zone western alps with inferred extraordinarily high stresses and strain rates for the early stages of creep during stress relaxation at temperatures of about 300 350 c in some of these rocks crystals of the high strength mineral garnet embedded in a quartz matrix are intensely fragmented detailed optical electron microprobe and scanning electron microscopic study combined with electron backscatter diffraction techniques suggests that fragmentation of garnet took place quasi instantaneously at very high peak stresses subsequent deformation during stress relaxation caused displacement of the garnet fragments by frictional sliding controlled by dislocation creep in the surrounding quartz matrix in a final stage after restoration of a near lithostatic pore fluid pressure the garnet fragments were pulled apart and the opening cracks were filled by quartz and feldspar precipitated from the pore fluid this record appears to be consistent with the proposed model of synseismic loading and postseismic creep and the related pore pressure changes

thermomechanics of an extensional shear zone raft river metamorphic core complex nw utah abstract a detailed structural and microstructural analysis of the miocene raft river detachment shear zone nw utah provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes combined microstructural electron backscattered diffraction strain and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east along the transport direction compatible with observed finite strain markers and a model of necking of the shear zone microstructural evidence quartz microstructures and deformation lamellae suggests that the detachment shear zone evolved at its peak strength close to the dislocation creep exponential creep transition where meteoric fluids played an important role on strain hardening embrittlement and eventually seismic failure empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results indicate that the shear zone developed under stress ranging from 40 mpa to 60 mpa using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10 12 and 10 14 s 1 we suggest that a compressed geothermal gradient across this detachment which was produced by a combination of ductile shearing heat advection and cooling by meteoric fluids may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone

mechanical twinning of jadeite an indication of synseismic loading beneath the brittle plastic transition the orientation distribution of mechanically twinned jadeite crystals in a metagranite from the sesia zone western alps is evaluated to derive the orientation of the principal stress directions for a prominent stage of ductile deformation at ca 300 to 350 c that has been interpreted to reflect synseismic loading and postseismic creep by kuster and stockhert 1999 it is tested whether the orientation distribution can be used to obtain information on the magnitude of differential stress if the critical shear stress for twinning is known assuming that the critical shear stress of 140 mpa determined by kolle and blacic 1982 for mechanical twinning of clinopyroxene in the 100 001 system holds true for jadeite and assuming a homogeneous stress field as a first approximation differential stresses of the order of 1 2 gpa are inferred for metagranites with ca 15 jadeite and of 0 5 gpa for jadeitite with 80 of jadeite notwithstanding the uncertainty for the critical resolved shear stress for jadeite these unexpectedly high values are suspected to be due to a combination of the following effects 1 an inhomogeneous stress field in the polyphase material with curved stress trajectories 2 stress concentration in jadeite due to load transfer from the quartz matrix and 3 minor reorientation of the jadeite crystals in the flowing quartz matrix hence absolute values of differential stress cannot be derived from the orientation distribution of twinned jadeite in polyphase rocks notwithstanding the failure in deriving reliable absolute values a short period with exceptionally high stress must have occurred and is fully consistent with the microstructural record of other minerals it is proposed that the mechanical twinning of jadeite was caused by a short lived stress peak and flow at laboratory strain rates related to quasi instantaneous loading of the uppermost level of the ductile lower crust during brittle failure of the brittle upper crust in a major seismic event

plastic deformation and recrystallization of garnet a mechanism to facilitate diffusion creep elongate and deformed garnets from glenelg nw scotland occur within a thin shear zone transecting an eclogite body that has undergone partial retrogression to amphibolite facies at circa 700 c optical microscopy back scattered electron imaging electron probe microanalysis and electron back scatter diffraction reveal garnet sub structures that are developed as a function of strain subgrains with low angle misorientation boundaries occur at low strain and garnet orientations are dispersed around rational crystallographic axes across these boundaries towards high strain areas boundary misorientations increase and there is a loss of crystallographic control on misorientations which tend towards random in high strain areas a polygonal garnet microstructure is developed the garnet orientations are randomly dispersed around the original single crystal orientation some garnet grains are elongate and ca rich garnet occurs on the faces of elongate grains oriented normal to the foliation commonly the garnet grains are admixed with matrix minerals and where in contact with other phases garnet is well faceted we suggest that individual garnet porphyroclasts record an evolution from low strain conditions where dislocation creep and recovery accommodated deformation through increasing strain where dynamic recrystallization occurred by subgrain rotation to highest strains where recrystallized grains were able to deform by diffusion creep assisted grain boundary sliding with associated rotations

decrepitated uhp fluid inclusions about diverse phase assemblages and extreme decompression rates erzgebirge germany minute polyphase inclusions in garnet of quartzofeldspathic rocks saidenbachite from the saxonian erzgebirge germany contain microdiamond or graphite phlogopite quartz paragonite phengite and other minerals in minor amounts these inclusions are interpreted to represent an original dense coh silicate fluid trapped in crystallizing garnet at depths of 150 km inspection of the inclusion population in a single garnet by sem reveals two characteristic features i the shape of most inclusions indicates healed radial cracks in the host garnet and thus the buildup of a significant differential pressure p i e a contrast in pressure between the inclusion pi and the host mineral pe the mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of 750 50 c and pressures below 2 5 gpa ii the diverse types of inclusions appear to be randomly distributed in the garnet host thus the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet combining observations i and ii we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times and thus of distinct histories of pi as p at decrepitation is primarily controlled by inclusion size and shape applying a flow law for dislocation creep of garnet a low strength of garnet at 750 50 c for low geological strain rates is predicted thus differential pressure should have been kept low i e pi pe by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks to attain the differential pressure pi pe required for catastrophic brittle failure of the garnet host the decompression rate must have been extremely high as a robust lower bound a minimum exhumation rate on the order of 100 m year 1 is suggested which corresponds to ascent rates of magma

crystal plasticity of natural garnet new microstructural evidence scanning electron microscope sem orientation contrast images of mantle nodules show that garnets contain cellular domains of different crystallographic orientation electron backscatter diffraction shows small crystallographic mismatches

modeling of radial microcracks at corners of inclusions in garnet using fracture mechanics radial microcracks around inclusions in garnet are important indicators of ultrahigh pressure metamorphism and have been used to interpret changes in pressure as a function of temperature during decompression of metamorphic terrains we investigate the growth of microcracks from inclusion corners using a two dimensional fracture mechanics based boundary element method code and propose that microcracks emanating from inclusions are primarily controlled by inclusion shape and size and do not require high confining lithostatic pressure or significant decompression to initiate results suggest that inclusions 100 m diameter develop microcracks during modest decompression the magnitude of decompression required to initiate microcracks decreases with increased degree of faceting and increased inclusion size the potential for crack growth at each corner depends on the corner sharpness and the overall shape of the inclusion we integrate model results with petrographic observations to understand mechanisms and conditions of garnet microcracking during regional metamorphism

charge contrast imaging of fine scale microstructure and compositional variation in garnet using the environmental scanning electron microscope gaseous secondary electron gse imaging of eclogite garnets under the environmental scanning electron microscope esem at low chamber gas pressures reveals detailed image contrast patterns charge contrast images cci that are not present in back scattered or secondary electron images image intensity is a function of the amount of surface charge accumulation successful acquisition of cci depends on frame size and beam scan rate at a given chamber gas pressure and beam current images are obtained in a few seconds and are stable and reproducible cci patterns do not correlate with cracks or grain boundaries but do correspond closely to variations in major element composition both in the form of concentric growth zoning and branching linear features interpreted as cracks that have been healed by new garnet growth causes of cci are not yet well understood but may be related to variations in lattice defect density and their influence on charge trapping and dissipation these in turn influence the rate of charge build up at or very close to the specimen surface one interesting possibility is that cci images detect vacancies related to non homovalent coupled substitutions involving for example ree and hydroxyl so the method offers a way of imaging the distribution of these trace species in garnets the cci images are rich in microstructural detail and offer the potential for rapid high resolution low noise reconnaissance mapping of intragranular microstructure and compositional variation in both natural and synthetic garnets

the nature and evolution of the main central thrust structural and geochronological constraints from the sikkim himalaya ne india the main central thrust mct is a prominent continental scale fault within the himalaya its definition has been the topic of some debate in the literature after a brief consideration of the state of discussion to clarify the definition we use in this work we report features from the field to the microstructural scale of a particularly well exposed section in sikkim ne india the nature of the protoliths as well as the overlying and underlying rocks is characterized in terms of e nd the dates of motion on the fault are constrained using u pb geochronology of zircon and monazite from pegmatitic dikes that cross cut the deformation fabric it is found that the mechanism of deformation recorded in the fault zone rocks is different compared to that found in the overlying greater himalayan gh or the underlying lesser himalayan lh rocks the gh and lh have different protolith characteristics as well combined with existing data on p t history dates of metamorphism and cooling and exhumation rates of the gh and the lh our measurements show that major motion on this fault occurred before 20 ma at 450 700 c but after peak metamorphism of rocks 750 800 c in this zone isolated events occurred in this zone as late as 11 ma possibly in the brittle domain this underscores the pulsed nature of movement over an extended period on such major faults and the related difficulties in dating fault movement determination of the rates of movement and designating a fault plane as in or out of sequence within a propagating deformation front

garnets as open systems during regional metamorphism if a garnet fractures during metamorphism its interior will be in chemical communication with matrix phases solids and fluids and may participate in late prograde or retrograde metamorphic reactions the core of a growth zoned garnet is unlikely to be in equilibrium with matrix phases and may react locally in the vicinity of fractures or extensively resulting in dissolution and reprecipitation of the garnet interior and or production of new phases the results of petrologic techniques such as thermobarometry pressure temperature path calculations and geochronometry may be misinterpreted if sites of garnet and mineral inclusion compositional modification are not identified brittle fracturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions the conditions at which differential expansion of an inclusion relative to its host garnet are sufficient to cause radial cracking around the inclusion vary as a function of peak pressure retrograde pressure temperature path and thermoelastic properties of the inclusion and garnet inclusion induced fracturing is favored by nearly isothermal decompression a common metamorphic pressure temperature path other fractures may form as a result of propagation of an inclusion host boundary or by mechanisms external to the garnet

k ar age determinations on phengites from the internal part of the sesia zone western alps italy k ar age determinations have been carried out on various well defined phengite populations from a small area of the internal part of the sesia zone lower aosta valley there the rocks have suffered high pressure metamorphism attributed to early alpine subduction in the stability field of jadeite quartz p 15 1 kbar at t 550 50 c coarse grained phengites from well preserved high pressure parageneses and phengites re crystallized early during decompression at still elevated temperatures in the stability field of albite yield identical ages close to 80 ma in the most external sample high pressure phengites yield 72 ma the ages around 80 ma are interpeted as essentially undisturbed cooling ages they are not notably influenced by paragenesis chemical composition polymorphism grain size deformation and recrystallization at higher temperatures this part of the sesia zone has cooled down about 80 ma ago to below the blocking temperature for the k ar system of white mica some of the samples show pronounced retrogression of the high pressure assemblages especially formation of albite and fine grained phengite at the expense of jadeite and are affected by intense late deformation at temperatures of about 300 to 350 c estimated from the small grain size of dynamically recrystallized quartz the ages obtained from extremely kinked or sheared coarse high pressure phengites scatter considerably being partly higher partly lower than 80 ma the fine grained phengite fractions from these samples yield lowered ages down to 70 ma these values are interpreted as mixed ages resulting from variable mixtures of fragmented high pressure phengites and new phengite replacing jadeite the k ar data suggest that the new phengites have crystallized between 60 and 70 ma b p all phengites formed at low temperatures at the expense of jadeite reveal high si contents these range up to 6 9 for the coarser grained earlier generations and up to 6 7 for the very fine grained last generation relatively high pressures are required to stabilize phengites so high in si this implies that cooling had not been achieved by uplift and erosion obviously the thermal structure of the crust was still strongly perturbed by continuing subduction and thrusting during the late cretaceous

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

controls on the failure mode of brittle inclusions hosted in a ductile matrix plane strain deformation experiments were performed on elliptical inclusions of cohesive sand embedded within a slab of pitch with the aim of investigating the mode of fracturing of brittle inclusions within a ductile matrix under pure and simple shear the inclusions failed in three different modes tensile fracturing mode 1 shear fracturing mode 2a and extensional shear fracturing mode 2b jeffrey s 1922 theory of the flow of a viscous medium around an ellipsoidal body was applied to the experimental results to determine the principal tensile and compressive stresses within an inclusion and analyze the failure modes using griffith s criterion the analysis reveals that the aspect ratio r and the orientation u of the inclusion control the principal tensile and compressive stresses within it and in turn govern the mode of brittle deformation at a particular inclusion orientation the tensile stress increases whereas the compressive stress decreases monotonically with increasing aspect ratio of the inclusion the principal stresses also vary with inclusion orientation for a given aspect ratio but not monotonically the analysis delimits the fields of each mode of brittle deformation of inclusions in r u space under pure shear and simple shear q 2001 elsevier science ltd all rights reserved

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process

vein quartz microfabrics indicating progressive evolution of fractures into cavities during postseismic creep in the middle crust discordant quartz veins in metamorphic rocks formed at temperatures similar to 300 c in an extensional tectonic regime the vein shape is characterized by an aperture to length ratio exceeding the maximum value feasible for purely elastic deformation by orders of magnitude non elastic ductile deformation of the host rock must have controlled widening of the arrested cracks fracture parallel shortening is low due to the low accumulated strain the microstructural record of this stage of deformation cannot be isolated in the polyphase host rock information is exclusively obtained from the microfabrics of the vein sealing quartz there the density of geometrically necessary dislocations derived from orientation maps created by electron backscatter diffractometry ebsd decreases markedly from the vein wall to the centre the gradient is attributed to progressive cavity formation and buckling of the vein walls during sealing early grown crystals at the vein margins record a prolonged deformation history compared to lately grown grains in the centre of the vein the gradient and the lack of significant crystal plastic deformation of the crystals in the vein centre show that deformation of the completed vein is insignificant compared to deformation during progressive opening and sealing in view of the precursory brittle deformation and the intense crystal plastic deformation at the vein margins the monogenetic veins are interpreted to record a single mid crustal stress transient related to the earthquake cycle fracturing is proposed to be a consequence of coseismic loading while progressive opening and sealing record the deformation during postseismic stress relaxation by creep

high resolution environmental reconstruction in sw peloponnese greece covering the last c 6000 years evidence from agios floros fen messenian plain a paleolimnological record from the central messenian plain southwestern peloponnese southern greece indicates rapid changes in the water level and chemistry of a transient lake on the flanks of the taygetos mountains during the last c 6000 years the analyses are based on diatoms as well as carbon and nitrogen isotopes from bulk sediments in a 7 5 m long sediment core retrieved from the drained fen of agios floros at the northern banks of the ancient river pamisos the sequence consists of fen peat in the uppermost section underlain by lacustrine sediments which are punctuated by two layers of clay with diatomaceous silt bands high accumulation rate is recorded in the oldest part of the section up to 23 mm yr particularly during two decadal long periods centered at c 5700 and c 5300 cal bp the diatom record reveals pronounced peaks in the planktonic taxon cyclotella distinguenda which correspond to the laminated sequences reflecting the rapid development of a deep lake with an open water

creep of garnet in eclogite mechanisms and implications abstract the rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high and ultrahigh pressure this uncertainty is relates to the discrepancy between experimental and field based observations of eclogite deformation and in particular the role of garnet to further investigate this important aspect we performed a textural and micro structural investigation applying optical microscopy element mapping and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite the results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at hp conditions the mylonites polycrystals are flattened parallel to the main high pressure foliation in the rock and individual grains were shortened by an average 15 although dislocation creep is commonly presumed to dominate garnet straining in eclogites no record of this mechanism was found instead the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution the observations provide compelling evidence for the role of fluids and syn tectonic porosity in the weakening of garnet a supposedly rigid eclogite component such weakening represents a crucial step in the fundamental feedback loop between fluid ingress metamorphism and near instantaneous competence loss in rocks undergoing deep subduction

the petrological significance of misorientations between grains misorientation analysis quantifies microstructural features in tectonites metamorphic and igneous rocks and allows hypotheses on their formation to be tested the misorientation between two lattices can be expressed by a rotation axis and rotation angle for lattices with symmetry it is conventional to take the minimum angle that enables one lattice to be rotated into the other for a group of lattice measurements two types of misorientation distribution can be calculated selecting random pairs of grains gives the random pair misorientation distribution selecting neighbouring pairs gives the neighbour pair misorientation distribution the forms of both distributions are visualised using histograms or cumulative frequency diagrams they are strongly influenced by any overall crystallographic preferred orientation and by intrinsic crystal symmetry in many rocks the random pair misorientation distribution and neighbour pair misorientation distribution are statistically significantly different quantified using the kolmogorov smirnov test differences between the random pair misorientation distribution and neighbour pair misorientation distribution imply that adjacent grains have physically interacted or are inherited from a precursor microstructure interactions include 1 reduction in surface energy by lattice alignment we show this may have occurred in garnet clusters in schist and olivine in a cumulate it is well known in metals and may be a common geological process 2 nucleation where those nuclei have influenced the orientation of adjacent nuclei 3 mechanical rotations of facetted grains in compacting crystal mushes so that faces become parallel 4 growth twinning inheritance includes 1 subgrain rotation recrystallisation in tectonites deforming by crystal plastic processes 2 mechanical and transformation related twinning 3 domainal microstructures e g where grains have formed from a few large original grains may give rise to spurious correlations when the orientation data cover more than one domain with this proviso misorientation analysis can be used to investigate many important microstructural processes

fracturing of garnet crystals in anisotropic metamorphic rocks during uplift garnet crystals in granulite facies mylonites and gneisses from the morin shear zone in the grenville province quebec are characterized by pervasive closely spaced relatively straight tensile fractures aligned systematically normal to the mylonitic foliation and lineation the fractures developed preferentially in coarse grains 0 2 mm or in grains with a large aspect ratio 2 fractured segments of each garnet crystal have not been separated or filled with matrix minerals quartz and feldspar or with retrograde minerals biotite muscovite and chlorite this suggests that the garnets were fractured at shallow crustal depths ti300 c or 15 km for a thermal gradient zo c km we propose that the garnet fractures were formed by the response of anisotropic metamorphic rocks to a horizontal extension during uplift and cooling of the metamorphic terrane within the upper crust using a modified shear lag theory we can explain why tensile fracturing took place preferentially in stiff garnet rather than in felsic material and why the fractures are unequally spaced in a garnet grain our data suggest that the seouential tensile fractures of garnet grains record a progressive uplift process of metamorphic rocks within the upper crust 0 1997 elsevier science ltd

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

simulating coseismic deformation of quartz in the middle crust and fabric evolution during postseismic stress relaxation an experimental study abstract non steady state deformation and annealing experiments on vein quartz are designed to simulate earthquake driven episodic deformation in the middle crust three types of experiments were carried out using a modified griggs type solid medium deformation apparatus all three start with high stress deformation at a temperature of 400 c and a constant strain rate of 10 4 s 1 type a some are followed by annealing in the stability field of quartz for 14 15 h at zero nominal differential stress and temperatures of 800 1000 c type a b or by annealing for 15 h at 900 c and at a residual stress type a c the quartz samples reveal a very high strength 2 gpa at a few percent of permanent strain the microstructures after short term high stress deformation type a record localized brittle and plastic deformation statisc annealing type a b results in recrystallisation restricted to the highly damaged zones the new grains aligned in strings and without crystallographic preferred orientation indicate nucleation and growth annealing at non hydrostatic conditions type a c results in shear zones that also develop from deformation bands or cracks that formed during the preceding high stress deformation in this case however the recrystallised zone is several grain diameters wide the grains are elongate and a marked crystallographic preferred orientation indicates flow by dislocation creep with dynamic recrystallisation quartz microstructures identical to those produced in type a b experiments are observed in cores recovered from long valley exploratory well in the quaternary long valley caldera california with considerable seismic activity the experiments demonstrate the behaviour of quartz at coseismic loading type a and subsequent static annealing type a b or creep at decaying stress type a c in the middle crust the experimentally produced microfabrics allow to identify similar processes and conditions in exhumed rocks

elevated stresses and creep rates beneath the brittle ductile transition caused by seismic faulting in the upper crust 1 large faults in the brittle upper continental crust are envisaged to terminate near the brittle ductile transition zone at a depth that is a complex function of material properties temperature stress and strain rate here we investigate the effect of coseismic displacement along a fault in the upper crust on the adjacent middle and lower crust using a two dimensional numerical model of faulting within an elastic frictional viscous rheology results demonstrate that a major displacement event can cause a dramatic increase in differential stress and creep strain rates in the crust near and below the lower termination of the fault a transient deflection of the brittle ductile transition zone and a zone of elevated strain rates down to the base of the crust the perturbation decays over timescales consistent with postseismic relaxation the stresses and strain rates predicted in such models are in accord with those inferred from microstructural damage analysis of exhumed metamorphic rocks indicating short term deformation at decaying stress and strain rates

strength of two phase rocks a model based on fiber loading theory abstract in order to predict the strength of two phase rocks in terms of the component volume fraction the aspect ratio of the strong phase and the strength contrast between the strong and weak phases a simple model is developed according to a fiber loading theory modified for short fiber composites the analytical solution derived from elastic theory is found to hold in good approximation for rocks consisting of power law viscous minerals there is a good agreement between the predicted results and the experiments for two phase aggregates anhydrite halite calcite halite fe ag and clinopyroxene plagioclase the model is compared with several previous different models

refinements of shear lag model and its applications abstract since it was developed by cox cox h l 1952 the elasticity and strength of paper and other fibrous materials br j appl phys 3 72 79 the shear lag model slm or fiber loading theory has been widely used by material scientists and structural geologists as a powerful analytical method for analyzing mechanical interactions between hard and soft phases layers in composites and two phase rocks we found that several approximations in previous mechanical derivations of the model can be improved in terms of mechanical equilibrium and boundary conditions our refinements make the model more rigorous and elaborate applications of the improved model include 1 prediction of the axial tensile stress in strong inclusions embedded in a continuous weak matrix and consequently interpretation of extension fracture boundinage 2 modeling of the distribution of shear stresses in the weak matrix surrounding the strong inclusions and accordingly interpretation of the variations of dislocation density and of recrystallized grain size in composites and polyphase rocks and 3 prediction of mechanical properties of industrial composites and two phase rocks the analytical results of the improved model agree approximately with those obtained from both experimental investigations and numerical calculations

how vein sealing boosts fracture widening rates the buckling enhanced aperture growth mechanism for syn tectonic veins abstract the paper introduces the mechanism of buckling enhanced aperture growth for syn tectonic veins that formed in simple shear dominated kinematic frameworks in the middle or lower crust apart from the well understood concepts of fracture widening driven by effective tensile stresses buckling enhanced fracture aperture growth relates widening to active outward buckling of more viscous incipient cement layers precipitated as hydrothermal minerals for the pore fluid on the walls of juvenile syn tectonic veins driven by fracture parallel compressive creep strain in the host rocks thus the mechanism proposed here follows similar principles as tectonic folding although important differences exist inspired by the structural record of low aspect ratio veins exposed in hp lt metamorphic rocks cropping out on south evia island greece generic numerical models are calculated to study development of buckling instabilities in such incompletely cemented veins and their impact on aperture growth rates the models indicate 1 that aperture growth rates increase with increasing viscosity contrast between the host rocks and the cement layers 2 an increase in the thickness of the cement layers cause acceleration of aperture growth 3 that support of restraining forces at the vein tips offered by the host rocks against buckling of the cement layers cause fully compressive states of stress ahead the fracture tips and 4 that fracture aperture growth is possible against fully compressive fracture normal stresses the buckling enhanced vein aperture growth mechanism yields important implications for the maintenance and decay of fracture bound permeability and for the mechanical state of the middle and lower crust in seismically active regions

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

experimental evidence for wall rock pulverization during dynamic rupture at ultra high pressure conditions abstract the mechanisms triggering intermediate and deep earthquakes have puzzled geologists for several decades there is still no consensus concerning whether such earthquakes are triggered by brittle or ductile mechanisms we performed a deformation experiment on a synthetic lawsonite bearing blueschist at a confining pressure of 3 gpa and temperatures from 583 to 1 073 k after deformation the recovered sample reveals conjugated shear fractures garnet crystals are dissected and displaced along these narrow faults and reveal micro and nanostructures that resemble natural pulverization structures as well as partial amorphization formation of such structures at low confining pressures is known to require high tensile stresses and strain rates and is explained by the propagation of a dynamic shear rupture the absence of shearing in the pulverized wall rock is taken as evidence that these structures pre date the subsequent heat producing frictional slip in analogy to observations at low pressure we infer that the garnet structures in our experiment result from rapid propagation of a shear fracture even at the high pressure exerted on the sample and thus suggest that brittle deformation is possible at lower crustal to upper mantle depths

amphibolite facies pseudotachylytes in premosello metagabbro and felsic mylonites ivrea zone italy abstract pseudotachylytes solidified friction induced melts produced during seismic slip that formed under high temperature ambient conditions might retain information on earthquake mechanics beneath the brittle ductile elasto frictional viscous plastic transition in the crust thin sharply bounded pseudotachylytes and ultramylonites occur together inside the granulite facies premosello metagabbro and inside country felsic mylonites of the ivrea zone italian southern alps petrographic and microstructural data indicate that the studied pseudotachylytes and ultramylonites are coeval and developed under amphibolite facies 500 650 c metamorphic conditions the occurrence of foliated pseudotachylytes and of local remnants of cataclastic microstructures within ultramylonites suggests that pseudotachylytes might have represented planar heterogeneities for nucleation of ultramylonites the association of coeval pseudotachylyte and mylonites is interpreted as caused by cyclic stages of coseismic high strain rate transients related to downward propagation of seismic ruptures from the upper brittle crust followed by post seismic stress relaxation by ductile flow

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

sub critical fracture and associated retrogression of garnet during mylonitic deformation garnets from the alpine fault mylonites are fractured most fractures have a retrogressive fracture fill a small number have garnet fill a detailed sem study has been carried out to elucidate the mechanisms of fracture and associated retrogression wide fracture zones contain free floating islands of garnet in a retrogressive matrix electron channelling patterns show that segments of garnet across such fracture zones rarely show any relative rotations where rotations are observed they are small and systematic some large fractures show clearly that fracture and retrogression occurs by a replacement process fracture surface morphologies are indicative of surface dissolution a model is established where crack tips advance by a process of dissolution at a high stress crack tip whilst precipitation occurs as the tip and its associated stress field advance precipitation of retrogressive fracture fill occurs if fracture occurs out of the field of stability of garnet