syntectonic development of a strain independent steady state grain size during mylonitization abstract michibayashi k 1993 syntectonic development of a strain independent steady state grain size during mylonitization tectonophysics 222 151 164 for a distance of greater than 50 km along the median tectonic line japan syntectonic recrystallization of quartz has resulted in the development of a strain independent steady state grain size within the core of the kashio shear zone that is also independent of rock type the mean grain size of quartz in recrystallized aggregates progressively decreases towards the shear zone core becoming stable at around approximately 37 m even though the strain increases within more strongly mylonitized rocks some variation of mean grain sizes may result from secondary effects due to hydrothermal activity associated with the median tectonic line despite the development of a steady state mean grain size individual grains were not stable as revealed by log normal grain size distributions consequently dynamic recrystallization and grain growth were competing processes that achieved a balance and hence resulted in the development of the steady state grain size the actual grain size that resulted probably depended on both the water content and strain rate as well as the flow stress approaching the centre of the shear zone mylonitic fabrics e g s c fabrics mica fish were established well before a steady state grain size was reached a steady state grain shape shape orientation and crystallographic fabrics may not be developed until even more mylonitization had occurred variable grain sizes result from heterogeneous deformation as a consequence of deformation partitioning even under a stable stress prior to the development of a steady state grain size consequently it should be determined whether a steady state grain size has been achieved within a shear zone before the grain size is used as a paleopiezometer

kinematics of mylonitic rocks along the median tectonic line akaishi range central japan abstract the ryoke belt along the median tectonic line mtl in the akaishi range is composed of quartz diorite granodiorite and metasedimentary rocks in amphibolite facies microstructural evidence reveals that the quartz diorite and most of the metasedimentary rocks in the study area suffered a ductile shear deformation or a mylonitizaton the shear surface is subhorizontal and the shear direction of the hanging wall trends n s based on the attitude of foliation and lineation of mylonitic rocks shear sense is deduced from microstructures such as mica fish and asymmetric tails on porphyroclasts they indicate relative southward displacement of the hanging wall comparison of deformation features in the described mylonites with those in fault rocks along the mtl distribution of the mylonites and kinematics of the mylonites and the mtl lead to the conclusion that the mylonites were mostly formed in a subhorizontal shear zone which was then exposed due to a dip slip component of faulting on the mtl

petrofabrics of high pressure rocks exhumed at the slab mantle interface from the point of no return in a subduction zone sivrihisar turkey the highest pressure recorded by metamorphic rocks exhumed from oceanic subduction zones is 2 5 gpa corresponding to the maximum decoupling depth mdd 80 10 km identified in active subduction zones beyond the mdd the point of no return exhumation is unlikely the sivrihisar massif turkey is a coherent terrane of lawsonite eclogite and blueschist facies rocks in which assemblages and fabrics record p t fluid deformation conditions during exhumation from 80 to 45 km crystallographic fabrics and other features of high pressure metasedimentary and metabasaltic rocks record transitions during exhumation in quartzite microstructures and crystallographic fabrics record deformation in the dislocation creep regime including dynamic recrystallization during decompression and a transition from prism slip to activation of rhomb and basal slip that may be related to a decrease in water fugacity during decompression 2 5 to 1 5 gpa phengite lawsonite and omphacite or glaucophane in quartzite and metabasalt remained stable during deformation and omphacite developed an l type crystallographic fabric in marble aragonite developed columnar textures with strong crystallographic fabrics that persisted during partial to complete dynamic recrystallization that was likely achieved in the stability field of aragonite p 1 2 gpa results of kinematic vorticity analysis based on lawsonite shape fabrics are consistent with shear criteria in quartzite and metabasalt and indicate a large component of coaxial deformation in the exhuming channel beneath a simple shear dominated interface this large coaxial component may have multiplied the exhuming power of the subduction channel and forced deeply subducted rocks to flow back from the point of no return

infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks infrared microspectroscopy has been applied to thin sections of various deformed and metamorphosed rocks in order to investigate water content of quartz in these rocks the broad ir band absorbance around 3400 cm 1 probably due to fluid inclusion molecular water h2o was used to calculate water contents deformed granitic rocks from the yanazawa kamimura area near the median tectonic line mtl showed an increase of water content in quartz from about 300 ppm to 2500 ppm toward the mtl with increasing degree of deformation metacherts from sambagawa metamorphic rocks asemigawa route showed a systematic decrease of water content in quartz from about 1000 ppm to 200 ppm with increasing metamorphic degree from chlorite garnet albite biotite to oligoclase biotite zones an archaean metachert from the napier complex granulite facies has only 40 ppm water comparing this with an inuyama unmetamorphosed chert sample having water contents of 3500 to 7000 ppm a starting point of metamorphism the systematic decrease of water in quartz with increasing metamorphic grade may extend from unmetamorphosed cherts to the highest grade metacherts water contents in high pressure metamorphic rocks in japan are mostly of the order of 500 ppm 300 700 ppm except for those from kurosegawa with a water content as high as 1700 ppm these results represent an exploratory analysis of intragranular water contents in quartz in various deformed and metamorphosed rocks further micro ft ir studies of the distribution of water in crustal rocks will provide a quantitative basis for examining the geochemical cycle of water in the earth s crust

well documented travel history of mesozoic pelagic chert in japan from remote ocean to subduction zone the mino tanba belt in southwest japan a segment of the cordilleran type orogenic chain of jurassic east asia is composed mainly of a middle upper jurassic subduction accretion complex in which triassic and lower jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with middle upper jurassic clastic rocks high resolution microfossil conodont and radiolaria research has identified very low average sedimentation rates of about 0 5 g cm 1000 yr in the chert units similar to those of modern pelagic sediments accumulated in open ocean environments judging from the low average sedimentation rate high purity of biogenic silica long duration of continuous deposition 50 m y and wide along strike extent 1000 km the bedded radiolarian cherts in the mino tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments accordingly the alleged origin in smaller marginal basins is untenable upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench the tectonic interlayering of these cherts and coarse grained terrigenous elastics is a secondary feature that was added through duplexing underplating in the subduction zone on the basis of the primary stratigraphy and field occurrence of triassic bedded chert in the mino tanba belt newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a cordilleran type bedded chert from its birth at a mid oceanic ridge to its demise at a subduction zone

grain boundary migration of quartz during annealing experiments at high temperatures and pressures with implications for metamorphic geology annealing experiments on agate quartz schist and metachert at 800 1000 c confining pressures of 400 and 800 mpa and annealing times of 6 0 10 3 6 105s revealed normal grain growth of quartz in the agate grain size increasing with time but the rate of grain growth decreasing with increasing grain size the boundaries of agate with quartz schist and metachert migrated into the agate at the expense of fine grained quartz in the agate the driving force for the migration appears to be the reduction of surface energy associated with removal of fine grained quartz in the agate assuming the activation energy as qm 11 kcal mol 1 a general expression for the relationship between velocity of boundary migration v and driving force p is where is the specific surface energy of quartz r is the gas constant and t is the absolute temperature the velocity is relatively fast at high temperatures on a geological time scale the expression assists a quantitative understanding of the microstructural development of quartz at metamorphic conditions

factors in the preservation of coesite the importance of fluid infiltration the survival of coesite in ultrahigh pressure uhp rocks is most commonly attributed to rapid exhumation continuous cooling during uplift and inclusion in strong phases that can sustain a high internal over pressure during decompression exceptions to all of these criteria exist perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite we used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of uhp rocks in all cases oh concentrations in coesite are below the detection limit of 100 ppm h2o the silica phases surrounding coesite however show varying amounts of h2o this is most spectacularly observed in pyrope quartzites from the dora maira massif that contain at least three phases of silica replacing coesite also distinguished by varying color of cathodoluminescence cl palisade textured quartz 100 ppm h2o red violet cl mosaic quartz which is actually chalcedony up to 0 4 wt h2o yellow brown cl and a rare highly hydrated silica phase interpreted to be opal 7 wt h2o dark blue cl very similar signatures are observed in a grospydite xenolith from the roberts victor kimberlite the quartz replacing coesite in other uhp samples studied contains on the order of 500 ppm h2o or less and most measurements are under the detection limit of our technique we infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase the formation of hydrous silica phases such as chalcedony and opal however must take place at much lower temperatures after cracking of the host phase which allows external fluids to infiltrate delay of fluid infiltration to low temperatures where kinetics are slow even in the presence of water is the most critical factor in the preservation of coesite

the nature of crystalline silica from the tag submarine hydrothermal mound 26 n mid atlantic ridge silica occurs in abundance in a variety of hydrothermal samples from the trans atlantic geotraverse tag hydrothermal mound 26 n mid atlantic ridge the water content trace element chemistry and mineralogy of crystalline silica from 15 different samples have been examined by vibrational spectroscopy and probe microanalysis the samples are from shallow subsurface ferric iron oxyhydroxide silica deposits n 4 a fragment of an active white smoker chimney n 1 anhydrite bearing hydrothermal breccias n 2 pyrite silica breccias n 3 and silicified wall rock breccias n 5 length fast chalcedony occurs in association with variable quantities of ferric iron oxyhydroxides in hydrothermal breccias from the mound flanks within shallower subsurface chert samples and within white smoker chimney walls samples from the anhydrite zone contain textures which are suggestive of an origin involving replacement of anhydrite samples taken from tag 1 and 5 from below the anhydrite zone contain no chalcedony instead they contain subhedral quartz crystals which show oscillatory zoning in aluminium two types of crystalline silica namely type a and type b quartz are defined on the basis of the infrared spectra in the oh region from 3200 cm 1 to 3600 cm 1 the type a quartz occurs beneath the anhydrite zone at tag 1 and 5 we propose a model that relates specific varieties of crystalline silica to different thermal and chemical environments within the mound interior length fast chalcedony occurs in an outer low temperature envelope across the top and sides of the mound the common association between length fast chalcedony and ferric iron oxyhydroxides suggests that chalcedony crystallization is favoured where catalysis by ferric iron can occur the apparent suppression of fibrous silica at the expense of single quartz crystals with increasing depth is attributed to differing growth rates and degrees of supersaturation of silica bearing solutions with increasing temperature within the mound the transition from type a to type b single crystal growth is interpreted to occur at temperatures approaching 360 c due to decreasing solubility of aluminium in quartz so that aluminium is rendered unavailable for type a valence compensation

water distribution in low grade siliceous metamorphic rocks by micro ftir and its relation to grain size a case from the kanto mountain region japan abstract infrared microspectroscopy reveals that liquid like water is always present in polycrystalline quartz grains from low grade metamorphic cherts and shale in the chichibu group and the mikabu greenrock complex of the kanto mountains area the water distribution in these rocks is heterogeneous and is related to rock textures in the microcrystalline quartz o h species due possibly to si oh are generally present the liquid like water is also present at grain boundaries and or in sub micron fluid inclusions in the microcrystalline parts on the other hand in quartz grains larger than 20 m in diameter within veins only the liquid like water is present possibly as fluid inclusions trapped during quartz vein formation in the microcrystalline cherty matrix of the metamorphic rocks a general decrease in liquid like water content is observed with increasing metamorphic grade associated with an increase in grain size two representative grain shapes a cube and a regular tetradecahedron having 14 planes are used to estimate the surface area between grains per unit volume with the grain size d m 3 d for cubic and 2 37 d for tetradecahedron grain boundary volumes were then calculated assuming grain boundary widths from 0 5 to 20 nm and normalized by unit volume of the rock the measured ir data on low grade metamorphic cherty rocks fall closely on these curves with grain boundary width of around 10 nm for the both models assuming saturation of h2o at grain boundary the observed decrease in water content with increase in grain size can be rigorously explained by the decrease in grain boundary volume per unit volume these results suggest that liquid like water occurs mainly between grain boundaries in the microcrystalline quartz of low grade metamorphic rocks although this grain boundary water model is the first approximation and requires further details of quantitative grain boundary textures the present model can provide a new approach to understand water distribution in polycrystalline rocks

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

shearing during progressive retrogression in granitoids abrupt grain size reduction of quartz at the plastic brittle transition for feldspar the kashio shear zone chubu district japan affects the southeast margins of the ryoke granitic plutons as well as the ryoke metasedimentary rocks and is truncated by the median tectonic line near the shear zone centre coarse quartz grains within the granitic rocks have been dynamically recrystallized to quartz aggregates towards the pluton margins the grain size data show an abrupt reduction from various sizes 59 148 m to a steady state size around 35 m at approximately 400 m from the boundary of the granitic plutons without any tectonic boundary such as a fault being present this abrupt grain size reduction accompanies brittle deformation in feldspar that overprints earlier plastic deformation and develops strong mylonitic fabrics in contrast feldspar away from the pluton margins contains only weak microstructural evidence for plastic deformation and still locally preserves magmatic polycrystal structures amongst the inhomogeneously recrystallized quartz grains these features combined with an isochronological study suggest that the kashio shear zone began at high temperature 450 c as a ductile event that affected relatively wide regions in the ryoke granitic plutons before rapid cooling by ca 70 ma at this latter stage strong mylonitization at lower temperatures 300 c in a semiductile regime occurred in a narrow zone towards the shear zone centre here recrystallized quartz aggregates reached a steady state grain size by ca 60 ma that is cooling within the granitic plutons resulted in strain localization with the abrupt grain size reduction of quartz aggregates correlating with the plastic brittle transition of feldspar

implications of mylonitic microstructures for the geotectonic evolution of the median tectonic line central japan the median tectonic line mtl the most prominent onshore fault in japan demarcates the cretaceous hiji quartz dioritic gneiss of the ryoke belt on the west from the high p t type sambagawa metamorphic rocks on the east in the takato area toward the mtl the hiji gneiss grades into strongly mylonitized rocks characterized by grain size reduction of quartz in the mylonitic rocks the development of fluxion banding sm is remarkably influenced by the existence of k feldspar forming a myrmekitic intergrowth brittle microstructures indicative of truly cataclastic deformation are observed only in mylonitic rocks close to the mtl early deep level ductile deformation apparently gave way to shallower brittle deformation at a later stage the attitude of stretching lineations lm and asymmetric microstructures observed in the mylonites suggest that sinistral strike slip shearing with a subordinate component of vertical slip took place during mylonitization in mid cretaceous time the mylonitic rocks and their protolith supposed to have constituted the eastern limb of the shear zone were probably eroded out and lost by upheaval of the sambagawa belt relative to the ryoke belt

ftir microspectroscopy of ediacaran phosphatized microfossils from the doushantuo formation weng an south china abstract phosphatized microfossils from ca 580 ma from the doushantuo formation in the weng an region of south china were analyzed by fourier transform infrared ftir microspectroscopy for their chemical characterization two morpho types of phosphatized embryo like fossils megasphaera and megaclonophycus were analyzed together with algal fossils transmission ir spectra of the microfossils have absorption bands of around 2960 cm 1 and 2925 cm 1 indicating the presence of aliphatic hydrocarbon anti symmetric aliphatic ch 3 and aliphatic ch 2 and have an additional band of around 1595 cm 1 probably derived from aromatic moieties aromatic c c in addition ir microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo like fossils the embryo like fossils appear to show three types of ch 3 ch 2 peak height ratios r 3 2 and aromatic c c ch 2 peak height ratios r c c 2 values 1 high r 3 2 low r c c 2 type r 3 2 0 2 1 0 and r c c 2 0 2 2 low r 3 2 medium r c c 2 type r 3 2 0 2 0 6 and r c c 2 1 4 and 3 low r 3 2 high r c c 2 type r 3 2 0 2 0 6 and r c c 2 1 8 all three types are contained in both megasphaera and megaclonophycus raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration indicating that the organics were embedded in situ prior to thermal maturation the ir spectroscopic differences might reflect differences in original organic compositions of microorganisms and or immediate post mortem alteration this suggests that the origins of phosphatized embryo like fossils are more diverse than was previously recognized on the basis of their morphology a comparison of r 3 2 and r c c 2 values in the embryo like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo like fossils belong to animal embryo others are algae but none of them originate from bacteria

active faulting on the wallula fault zone within the olympic wallowa lineament washington state usa the wallula fault zone is an integral feature of the olympic wallowa lineament an 500 km long topographic lineament oblique to the cascadia plate boundary extending from vancouver island british columbia to walla walla washington the structure and past earthquake activity of the wallula fault zone are important because of nearby infrastructure and also because the fault zone defines part of the olympic wallowa lineament in south central washington and suggests that the olympic wallowa lineament may have a structural origin we used aeromagnetic and ground magnetic data to locate the trace of the wallula fault zone in the subsurface and map a quarry exposure of the wallula fault zone near finley washington to investigate past earthquakes along the fault we mapped three main packages of rocks and unconsolidated sediments in an 10 m high quarry exposure our mapping suggests at least three late pleistocene earthquakes with surface rupture and an episode of liquefaction in the holocene along the wallula fault zone faint striae on the master fault surface are subhorizontal and suggest reverse dextral oblique motion for these earthquakes consistent with dextral offset on the wallula fault zone inferred from offset aeromagnetic anomalies associated with ca 8 5 ma basalt dikes magnetic surveys show that the wallula fault actually lies 350 m to the southwest of the trace shown on published maps passes directly through deformed late pleistocene or younger deposits exposed at finley quarry and extends uninterrupted over 120 km

the rate of water penetration in experimentally deformed quartzite implications for hydrolytic weakening abstract in order to determine the rate at which water penetrates the grains of a quartzite we performed several types of experiments including water added deformation and hydrostatic experiments on vacuum dried samples and hydrostatic annealing experiments on previously deformed quartzites samples of heavitree quartzite with an average grain size of 200 m and intragranular water concentration of 1750 420 h 10 6 si as measured by ftir were vacuum dried at 800 c 6 pa for 12 h after vacuum drying the grains showed thermal cracks with an average spacing of 50 m and an intragranular water concentration of 240 60 h 10 6 si one constraint on the rate of water penetration into quartz was obtained by deforming vacuum dried samples with 0 3 wt 20 000 h 10 6 si water added at 800 c 1500 mpa and a strain rate of 10 6 s conditions at which as is quartzite undergoes climb accommodated dislocation creep the time between reaching experimental conditions and yielding of the sample was between 2 and 9 h the samples had yield stresses of 100 250 mpa values well below the yield stress of 1500 mpa for a vacuum dried sample deformation of the grains was homogeneous on both optical and tem scales indicating that the grains had fully equilibrated with the water related defect that is responsible for water weakening ftir measurements are consistent with this interpretation one of the deformed samples has an intragranular water concentration of 820 160 h 10 6 si and vacuum dried samples hydrostatically annealed for 3 or 10 h at the same conditions as the deformed samples have an average intragranular water concentration of 730 180 h 10 6 si to obtain a second constraint on the rate of water penetration hydrothermal annealing experiments were performed on portions of a sample of heavitree quartzite which were previously deformed without water added at 700 c 1200 mpa and 10 5 s and have a high 10 16 m 2 and homogeneous dislocation density two samples were annealed at 700 c 1500 mpa for 24 h one with 0 2 wt water and one with no water added the grains in the water added sample have a rim approximately 15 m wide with dislocation recovery microstructures indicating equilibration with the water related defect the sample with no water added has a homogeneous dislocation density throughout the grains which is similar to that of the original deformed sample the 15 m wide recovered rim suggests a penetration rate approximately an order of magnitude faster than the rate of oxygen diffusion under hydrothermal conditions the observed rate of penetration of the water related species is consistent with theoretical considerations and calculations which predict a diffusion rate for h 2 o that is an order of magnitude faster than the rate of hydrothermal oxygen diffusion we infer from our experiments that the rate of water penetration into quartz is consistent with diffusion of h 2 o

coupled trace element mobilisation and strain softening in quartz during retrograde fluid infiltration in dry granulite protoliths this study focuses on the retrograde rheological and chemical evolution of quartz and the behaviour of quartzites during retrograde metamorphism following dry high grade metamorphism at 750 c 7 kbar sem cl and la hr icp ms are applied to document quartz texture and chemistry respectively four generations of quartz were distinguished by sem cl qz1 qz2 qz3 and qz4 qz1 brecciated and partly dissolved old grains is enriched in b al and ti when compared with the other types qz2 formed during brecciation and partial dissolution of qz1 has low al contents 50 ppm but due to rutile inclusions variable ti contents when occurring in amphibolite 210 10 ppm but more consistent values when occurring in quartzites peak value 32 ppm qz3 dark grey luminescent quartz forming fluid migration channels fluid pathways has ti 5 ppm and al contents below 10 ppm and b 1 ppm qz4 comprises are group of quartz later than qz3 filling micron thick cracks and pods with very low luminescent quartz i e darker than qz3 the textural and chemical evolution of quartz in our study is explained by two major influxes of aqueous fluids during regional uplift and retrogression they facilitated rehydration and recrystallisation in the otherwise dry high grade quartzites the first introduction of aqueous fluids was associated with brecciation of the high grade quartz qz1 and dissolution precipitation of quartz qz2 ti in quartz geothermometry wark and watson contrib mineral petrol 152 6 637 652 gives 626 c in agreement with the retrograde pt path deduced from phase diagrams later fluid influx associated with scapolitisation of amphibolite caused localised recrystallisation qz3 and alteration of biotite to muscovite along mm wide fluid migration channels during subsequent deformation qz3 deformed plastically and recovered by subgrain rotation recrystallisation sgr resulting in a reduction of grain size whereas qz1 quartz formed micro faults qz2 was plastic but did not experience sgr to the same degree as qz3 quartz increased plasticity and recovery rates most likely relate to an increased h2o fugacity and the depletion in trace elements of the quartz lattice by promoting strain softening processes dislocation climb and recovery

attenuation of lg in the western us using the usarray 1 the unprecedented breadth and density of the usarray allows us to resolve lateral variations of 1 hz lg attenuation to 0 5 degrees over much of the western united states we collected over 31 000 lg amplitudes from 547 events and 408 stations of the initial usarray footprint two dimensional tomographic inversion yielded q ranging from 60 to 550 with a variance reduction of 47 relative to the best fit constant q model q 155 q is low over most of the region with lowest values associated with recent volcanic activity and active shear zones high q is found in older stable crust notably the colorado and columbia plateaus while smaller islands of high q are associated with batholiths including the sierra nevada peninsular and bitterroot ranges the western mojave and okanogan highlands as well as other areas of competent crust such as the harcuvar range and two topographically bounded blocks in the basin and range

late cenozoic structure and correlation to seismicity along the olympic wallowa lineament northwestern united states discussion and reply the columbia plateau portion of raisz s 1945 olympic wallowa lineament owl is a geologically fascinating feature whose origin structural significance and age have been the subject of numerous geologic studies over the years because of its proximity to nuclear and hydroelectric facilities despite all the work that has been done some aspects of this complex feature are still not well understood therefore we recognize the need for the continued study of this feature and welcome new efforts that further our understanding however mann and meyer have overlooked an extensive body of published and publicly available data on the owl and this has resulted in numerous mistakes and erroneous interpretations conclusions in their paper with little new insight into the subject in the following sections we discuss some of the more significant problems with mann and meyer s paper based upon the existing literature and the results of our ongoing research

water loss and the origin of thick ultramylonites hydrolytic weakening has been suggested as a major process facilitating strain localization in line with many studies that found a positive correlation between water content and intensity of deformation we examine the role of water in the unusually thick ultramylonite of the el pichao shear zone northwestern argentina we used fourier transform infrared spectroscopy to measure water content in quartz and feldspar comparing ultramylonitic rocks to mylonites and weakly deformed rocks quartz and feldspar in ultramylonites contained half the water of weakly deformed rocks contrary to findings in previous studies we propose that the kilometer thick ultramylonite formed in three stages 1 localized deformation and recrystallization caused release of intracrystalline water to grain boundaries which promoted grain boundary sliding forming the ultramylonite 2 high pressure in the shear zone continuously expelled intercrystalline water to the surroundings drying the boundaries and leading to strain hardening and 3 water migrated to less deformed rocks causing hydrolytic weakening repeating the cycle and widening the ultramylonite

development of microboudinage an estimate of changing differential stress with increasing strain microboudin structure has so far been used in structural analysis as a reliable measure of plastic strain and recently it has been proposed to use it as a measure of relative differential stress this paper demonstrates that stress and strain can be simultaneously deduced if the stress and strain analyses are carried out in combination the analysed samples are quartz schists from four localities of different high p low t type metamorphic belts in japan the samples include microboudinaged competent piedmontite grains within a ductile quartz matrix the analyses strongly suggest that the estimated differential stresses tend to increase with increasing strain and that at the final stage of the microboudinage they reach a maximum

isc gem global instrumental earthquake catalogue 1900 2009 ii location and seismicity patterns abstract we present the final results of a two year project sponsored by the global earthquake model gem foundation the isc gem global catalogue consists of some 19 thousand instrumentally recorded moderate to large earthquakes spanning 110 years of seismicity we relocated all events in the catalogue using a two tier approach the ehb location methodology engdahl et al 1998 was applied first to obtain improved hypocentres with special focus on the depth determination the locations were further refined in the next step by fixing the depths to those from the ehb analysis and applying the new international seismological centre isc location algorithm bondar and storchak 2011 that reduces location bias by accounting for correlated travel time prediction error structure to facilitate the relocation effort some one million seismic p and s wave arrival time data were added to the isc database for the period between 1904 and 1970 either from original station bulletins in the isc archive or by digitizing the scanned images of the international seismological summary iss bulletin villasenor and engdahl 2005 2007 although no substantial amount of new phase data were acquired for the modern period 1964 2009 the number of phases used in the location has still increased by three millions owing to fact that both the ehb and isc locators use most well recorded ak135 kennett et al 1995 phases in the location we show that the relocation effort yielded substantially improved locations especially in the first half of the 20th century we demonstrate significant improvements in focal depth estimates in subduction zones and other seismically active regions and we show that the isc gem catalogue provides an improved view of 110 years of global seismicity of the earth the isc gem global instrumental earthquake catalogue represents the final product of one of the ten global components in the gem program and is available to researchers at the isc www isc ac uk website

rheology of a coaxial shear zone in the virginia blue ridge wet quartzite dislocation creep at 250 280 c abstract cambrian quartzite along the western margin of the blue ridge near front royal virginia records significant penetrative strain under subgreenschist facies conditions strain analyses of the quartzite indicate general flattening and octahedral shear strain values up to 1 25 x z up to 5 7 1 samples with x z as low as 1 5 1 have crystallographic preferred orientations that record a dominance of basal slip and patterns consistent with coaxial flattening in most samples quartz has undergone minor amounts of dynamic recrystallization with mean grain sizes 4 5 m locally the quartzite is brittlely deformed and subgrains and recrystallized bulges are present along fractures and extinction bands fluid inclusion planes and fluid inclusions along deformation lamellae are abundant and quartz ftir analyses yield average h 106si values of 1100 3300 indicating fluid flow and high intragranular water content during deformation higher strain samples generally record lower water contents which may indicate that fluids migrated to grain boundaries during deformation dissolution microstructures are rare in high strain samples and we interpret most strain to have been accommodated by dislocation creep thermal modeling of zircon low temperature thermochronology data coupled with quartz c axis opening angle thermometry suggest deformation temperatures were likely 250 280 c these deformation temperatures are lower than what is typically considered the brittle plastic transition in quartz we attribute localization of this high strain zone and dislocation creep at these temperatures to hydrolytic weakening and a relatively low strain rate estimated to be 10 15 s 1

the role of fluids in low temperature fault related deformation of quartz arenite abstract the microstructural and fluid histories in quartz arenite deformed in a regional scale fault zone were investigated to determine the role of fluids in the selection of grain scale deformation mechanisms using cathodoluminescence fluid inclusion microthermometry and stable isotope geochemistry of quartz cements three chemically distinct aqueous fluids were identified that affected the rock at different times during deformation the earliest deformation microstructures are distributed cataclasites which were cemented by reddish brown luminescing quartz precipitated from an aqueous cacl 2 brine fluid i once cemented brittle deformation continued but localized along discrete shear surfaces in narrow zones 20 1000 m wide and on discrete microfractures microveins associated with this phase were cemented with bluish green luminescing quartz precipitated from an aqueous cacl 2 brine fluid ii water channelized along dissolution surfaces and in pervasive microfracture networks that formed at this time promoted hydrolytic weakening and crystal plastic deformation especially near fluid pathways temperatures during deformation associated with fluids i and ii were 225 235 c as determined from fluid inclusion data in the later stages of deformation an influx of iron bearing aqueous fluid fluid iii greatly accelerated the rate of quartz dissolution and filled fractures stylolites and vugs with goethite the progression of deformation mechanisms indicated by the microstructures and their relationship to the different fluids show that the selection of grain scale deformation processes is affected not only by the presence of fluids but also by their chemistry

deformation thermometry based on quartz c axis fabrics and recrystallization microstructures a review abstract for quartz rich tectonites two types of deformation thermometer are currently commonly employed 1 the quartz c axis fabric opening angle thermometer that provides an estimate of deformation temperatures when fabrics were locked in during dislocation creep and dynamic recrystallization 2 the quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization a critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development however fabric opening angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening in this paper the development of these thermometers is reviewed and their potential sensitivities to competing factors such as temperature strain rate water weakening and in the case of opening angles 3d strain type are discussed examples of the application of these potential thermometers to naturally deformed quartz rich rocks are given and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology based thermobarometers are highlighted in the review attention is focused on problems associated with applying these thermometers to natural deformation and examples of such problems are discussed

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

fourier transform infrared spectroscopy determinations of intragranular water content in quartz bearing rocks implications for hydrolytic weakening in the laboratory and within the earth abstract fourier transform infrared ftir spectroscopy has been used to measure intragranular water contents of quartz and feldspar within fine grained quartzites granite and naturally deformed mylonites calibrations and tests of methods developed for this application using natural and synthetic quartz standards indicate that hydrogen concentrations down to 30 ppm are detectable within apertured regions 100 200 m in diam and concentrations of 400 ppm measurable to an accuracy of 30 quartzite and novaculite starting materials used in previous studies of mechanical properties contain substantial intragranular water contents 2400 3900 ppm much larger than water concentrations commonly found in large clear natural crystals and more comparable to those of wet synthetic crystals however unlike molecular water within rapidly grown synthetic quartz most of this water is freezable resembling fluid inclusion water of natural milky quartz crystals likewise quartz and feldspar grains within granite and mylonites deformed at greenschist to upper amphibolite facies conditions contain large amounts of water from 500 to 8000 ppm as fine often submicron size fluid inclusions and ir spectra are dominated by a freezable o h absorption band comparisons between dry natural and wet synthetic quartz crystals have formed the basis for our understanding of hydrolytic weakening however mechanisms by which water related defects access dislocation cores within quartzites deformed in the laboratory and mylonites deformed within natural shear zones may differ from those within idealized natural and synthetic single crystals and their mechanical properties may not be directly comparable

the age and pb loss behaviour of zircons from the isua supracrustal belt as determined by ion microprobe ion microprobe uthpb analyses of zircons from a conglomeratic metavolcanic unit in the isua supracrustal belt of west greenland show that their magmatic age is3807 1ma and failed to detect any older zircon cores or xenocrysts low u areas within zircons at a 30 m scale lost only small amounts of radiogenic pb and only in the recent past whereas high u areas within grains lost much greater proportions of their pb both recently and as early as the archaean some areas in the zircons have concordant upb and thpb ages the ion probe results agree with earlier determinations for the isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses

solution precipitation of k feldspar in deformed granitoids and its relationship to the distribution of water abstract we have investigated k feldspar recrystallisation in granitoid mylonites within a ductile shear zone from the ryoke metamorphic belt sw japan fine grained k feldspar 20 m on average occurs in the matrix and in pull apart areas within fractured k feldspar porphyroclasts these fine grains are elongated and oriented parallel to the main foliation in the matrix and their grain surfaces observed with the scanning electron microscope are not smooth but rough due to the development of very fine

reversible water weakening of quartz abstract we have performed looped pressure stepping deformation experiments at the same temperature 800 c and strain rate 2 10 6 s 1 on a milky quartz crystal and a quartzite that deform by dislocation creep our results demonstrate that flow strengths at fixed water content depend on f h 2 o and that the dependence is reversible thus rheology of quartzite can be predicted if f h 2 o is known in addition the effect of f h 2 o on strength of quartzite is greater than that of the milky quartz single crystal the differing sensitivities are due to differing effects that water has on deformation and recovery within grain interiors and at or near grain boundaries these results indicate that as temperature increases with depth and the dominant recovery mechanism of dislocation creep changes from recrystallization at grain boundaries to intragranular recovery dependence of quartz creep on f h 2 o may also decrease

the wallula fault and tectonic framework of south central washington as interpreted from magnetic and gravity anomalies abstract the yakima fold and thrust belt yftb in central washington has accommodated regional mostly north directed deformation of the cascadia backarc since prior to emplacement of miocene flood basalt of the columbia river basalt group crbg the yftb consists of two structural domains northern folds of the yftb strike eastward and terminate at the western margin of a 20 mgal negative gravity anomaly the pasco gravity low straddling the north american continental margin southern folds of the yftb strike southeastward form part of the olympic wallowa lineament owl and pass south of the pasco gravity low as the wallula fault zone an upper crustal model based on gravity and magnetic anomalies suggests that the pasco gravity low is caused in part by an 8 km deep tertiary basin the pasco sub basin abutting the continental margin and concealed beneath crbg the pasco sub basin is crossed by north northwest striking magnetic anomalies caused by dikes of the 8 5 ma ice harbor member of the crbg at their northern end dikes connect with the eastern terminus of the saddle mountains thrust of the yftb at their southern end dikes are disrupted by the wallula fault zone the episode of ne sw extension that promoted ice harbor dike injection apparently involved strike slip displacement on the saddle mountains and wallula faults the amount of lateral shear on the owl impacts the level of seismic hazard in the cascadia region ice harbor dikes as mapped with aeromagnetic data are dextrally offset by the wallula fault zone a total of 6 9 km assuming that dike offsets are tectonic in origin the wallula fault zone has experienced an average dextral shear of 0 8 mm y since dike emplacement 8 5 ma consistent with right lateral stream offsets observed at other locations along the owl southeastward the wallula fault transfers strain to the north striking hite fault the possible location of the m 5 7 milton freewater earthquake in 1936

a natural example of crystal plastic deformation enhancing the incorporation of water into quartz abstract water content of quartz in and around a greenschist facies mylonitic shear zone located in the western adirondacks was analyzed by micro ftir spectroscopy the shear zone is within a pegmatitic dike which cuts across a granitic gneiss the thickness of the shear zone varies along strike from 15 cm wide and encompassing all of the pegmatite dike at its northern most exposure to 5 cm wide approximately 10 m south along strike microstructures including quartz ribbons and recrystallized grains indicate quartz and feldspar within the mylonite underwent dislocation creep infrared spectral analysis was carried out using a nicolet micro ftir on mylonitic quartz ribbons pegmatitic quartz and gneissic quartz a small aperture size 56 m by 50 m for the ir beam allowed optically clear regions of the quartz grains to be analyzed without any contribution from grain boundaries the smallest dimension of the quartz ribbons is 0 3 mm whereas the pegmatitic quartz has a grain size of 3 to 5 cm results show mylonitic quartz ribbons contain the most water 320 h 10 6 si average range of 50 to 1120 h 10 6 si pegmatite quartz contains much less water 30 h 10 6 si average range of 20 40 h 10 6 si and the gneissic quartz contained an intermediate amount 200 h 10 6 si average range of 20 to 870 h 10 6 si these data indicate that water was preferentially incorporated into the deformed quartz ribbons

evidence for distributed clockwise rotation of the crust in the northwestern united states from fault geometries and focal mechanisms paleomagnetic and gps data indicate that washington and oregon have rotated clockwise for the past 16 myr late cenozoic and quaternary fault geometries seismicity lineaments and focal mechanisms provide evidence that this rotation is accommodated by north directed thrusting and right lateral strike slip faulting in washington and sw to w directed normal faulting and right lateral strike slip faulting to the east several curvilinear nw to nnw trending high angle strike slip faults and seismicity lineaments in washington and nw oregon define a geologic pole 117 7 w 47 9 n of rotation relative to north america many faults and focal mechanisms throughout northwestern u s and southwestern british columbia have orientations consistent with this geologic pole as do gps surface velocities corrected for elastic cascadia subduction zone coupling large quaternary normal faults radial to the geologic pole which appear to accommodate crustal rotation via crustal extension are widespread and can be found along the lewis and clark zone in montana within the centennial fault system north of the snake river plain in idaho and montana to the west of the wasatch front in utah and within the northern basin and range in oregon and nevada distributed strike slip faults are most prominent in western washington and oregon and may serve to transfer slip between faults throughout the northwestern u s

the december 1872 washington state earthquake the largest historical earthquake in eastern washington occurred on 15 december 1872 we used modified mercalli intensity mmi assignments for 12 twentieth century earthquakes to determine attenuation relations for different regions in the pacific northwest mmi attenuation for propagation paths east and west of the cascade mountains differs significantly only for epicentral distances greater than about 225 km we used these attenuation relations and the mmi assignments for the 15 december 1872 earthquake to conclude that its epicentral region was east of the cascade mountains near lake chelan washington and most probably near the south end of lake chelan the intensity magnitude m i is 6 8 and moment magnitude m is 6 5 7 0 at the 95 confidence level manuscript received 23 october 2001

effect of confining pressure on dilatation recrystallization and flow of rock salt at 150 c microstructural evidence for fluid assisted dynamic recrystallization fadrx is widespread in naturally deformed rock salt however the principal experimental evidence for fadrx in salt has been obtained from stress relaxation experiments and it is unclear whether the process occurs during steady state dislocation creep and what its effect might be here we report deformation experiments performed on natural rock salt at a constant strain rate of 3 5 10 7 s 1 150 c and confining pressures pc of 3 30 mpa samples deformed at pc 3 mpa showed continuous work hardening and minor dilatation microstructurally they exhibited intergranular cracking plus slip band and subgrain structures indicative of dislocation glide creep electron backscatter diffraction analysis revealed a high frequency of boundaries with low angle misorientations in the range 5 10 in contrast samples deformed at pc 6 5 mpa showed work hardening followed by steady state flow at strains 6 7 these samples compacted slightly and crystal plastic deformation was accompanied by extensive fadrx with a predominance of high angle boundaries 30 50 over low angle boundaries we infer that fadrx is suppressed by dilatation at low pressures as a result of grain boundary disruption at pressures high enough to prevent dilatation however fadrx acts as a recovery mechanism counteracting work hardening the results offer a possible explanation for rheological variability seen in previous experiments conducted at pressures up to 30 mpa a simple rate model for diffusion and interface controlled fadrx indicates that fadrx should become increasingly important toward natural halokinetic conditions although the effect on flow stresses is likely to be small

an ftir study of hydrogen in anorthoclase and associated melt inclusions high resolution fourier transform infrared ftir spectroscopy has been used to document the presence of hydrogen to estimate its concentration and to document its oxygen speciation in anorthoclase crystals and associated melt inclusions from mount erebus antarctica synchrotron generated infrared radiation 100 to 1000 times brighter than globar generated infrared radiation permits rapid collection of maps that depict relative intensities of a chosen ftir band across the mapped area spectra and or compositional maps showing variations in water concentration were collected from anorthoclase megacrysts and melt inclusions in the megacrysts studies of anorthoclase megacrysts involved collection of spectra from three mutually perpendicular sections cut from the crystals ftir spectra of anorthoclase crystals are characterized by a broad absorption band at approximately 3200 cm sup 1 in the mid ir range the universal mass absorption coefficient for mid ir range feldspar spectra established by johnson and rossman 2003 was used for quantitative estimates of water concentrations in the feldspar crystals based on integrated area under the 3200 cm sup 1 band water concentration in the anorthoclase sample was approximately 126 ppm with an overall error of approximately 30 ftir spectra of melt inclusions are characterized by a broad asymmetric absorption band at approx 3550 cm sup 1 that was used tomore calculate total water concentration the absence of a band at 1630 cm sup 1 suggests that water in the melt inclusions occurs as oh sup rather than as molecular h sub 2 o absorption coefficients established by mandeville et al 2002 for h species in glass were used to calculate water concentrations in the melt inclusions melt inclusions in the mt erebus anorthoclase have water concentrations ranging from 0 12 to 0 39 wt with an overall error of approximately 15 the ratio of water in anorthoclase crystals to water in the melt from which the crystals formed based on this study and at these low melt water concentrations is approximately 1 10 however water concentration varies significantly from one melt inclusion to another possibly suggesting initial melt water heterogeneity maps of water concentration show that variations in water concentration within melt inclusions are associated with fractures that cut the melt inclusions and in some cases do not extend out into surrounding crystals or into crystal inclusions thin approx 50 micro m thick zones of elevated water concentrations on the boundaries of the crystals in contact with melt inclusions suggest that water has diffused into the crystals from the melt inclusions less

deformation of granitic rocks experimental studies and natural examples crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales advances in our understanding have come from a combination of field observations laboratory experiments and theoretical models what necessarily links all these investigations is a focus on process crystal plasticity dislocation creep is one of several grain scale deformation processes or mechanisms each process is dominant over a certain range of conditions in the crust has a particular form of the flow law and produces a characteristic set of microstructures characterizing the deformation mechanisms for different minerals over a range of conditions allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces the information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings iterations among field theoretical and experimental approaches are critical for making progress in our understanding the deformation process of crystal plasticity is the main focus of this volume crystal plastic deformation of the crust is in many ways more complex than that of the mantle because most crustal rocks are polyphase aggregates and the strengths of common crustal silicates such as quartz feldspar and mica are very different at a given set of conditions yield strength envelopes for the crust as shown schematically in figure 1 are commonly plotted using the dislocation creep flow law for quartz which inherently assumes that quartz is the controlling phase and undergoes steady state flow but is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite let alone a crustal plum pudding of regions and layers of different lithologies the crust

evolution in h 2 o contents during deformation of polycrystalline quartz an experimental study shear experiments were performed in a griggs type apparatus at 800 c and 1 5 gpa at a strain rate of 2 1 10 5s 1 using different starting materials i powder grain size 6 10 m of dry brazil quartz with 0 15 wt added h2o ii dry brazil quartz porphyroclasts grain size 100 200 m devoid of fluid inclusions embedded in the same fine grained powder and iii wet porphyroclasts grain size 100 200 m containing initially a high density of m scale fluid inclusions embedded in the same powder after hot pressing samples were deformed to large shear strains 3 to 4 5 in order for the microstructures and h2o distribution to approach some state of equilibrium the h2o content and speciation in quartz were analyzed by fourier transform infra red ftir spectroscopy before and after the experiments mechanical peak strength is generally lower in experiments with 100 hydrated matrix intermediate in experiments incorporating wet porphyroclasts with a proportion of 30 or 70 and highest in those with dry porphyroclasts all experiments with porphyroclasts show pronounced strain weakening and the strengths of most samples converge to similar values at large strain wet porphyroclasts are pervasively recrystallized during deformation while dry porphyroclasts recrystallize only at their rims and remain weakly deformed recrystallization of the initially fluid inclusion rich porphyroclasts results in a decrease in inclusion abundance and total h2o content while h2o content of initially dry clasts increases during deformation h2o contents of all high strain samples converge to similar values for matrix and recrystallized grains in samples with wet porphyroclasts shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free fluid phase these high porosity sites are sinks for collecting h2o in excess of the storage capacity of the grain boundary network of the recrystallized aggregate the h2o storage capacity of the grain boundary network is determined as a h2o boundary film of 0 7 nm thickness

co2 and the eruptibility of picrite and komatiite abstract a tholeiitic picrite erupted from the summit region of kilauea volcano hawaii in 1959 work by wright helz and schwindinger and anderson shows that the 1959 magma was a mixture of new hot mgo rich magma that rose through and mixed with cooler stored magma causing crystallization of abundant olivine phenocrysts co 2 dissolved in melt inclusions in olivine phenocrysts from the eruption of 1959 reveals that most of the olivines grew at pressures less than 1 kbar 100 mpa therefore the new magma probably was buoyant relative to stored magma at 1 kbar the mgo rich magma is characterized by a relatively dense melt and could only be buoyant if it contained more gas than the stored magma compositions of inclusions indicate that the gas was rich in co 2 because gas is highly compressible the mass fraction of co 2 rich gas that is required for magma buoyancy increases at greater pressures a critical pressure is that at which gassy mgo rich parental magma has the same density as stored magma if the parental magma invades stored magma at a sufficiently great pressure it will be negatively buoyant and its consequent descent will inhibit its eruption kilauea s degassing suggests that kilauean parental basalt may contain about 0 3 wt co 2 gassy picritic magma liquid with 15 wt mgo 0 3 bulk wt co 2 is buoyant with respect to degassed tholeiitic basalt 8 wt mgo liquid at pressures less than about 2 kbar for kilauea s 1959 picrite the inferred 1 kbar pressure at which new mgo rich magma invaded stored magma is consistent with its buoyant rise to the surface and eruption although the parental magma of kilauea is considered to have a picritic composition erupted picrite is uncommon particularly in kilauean summit eruptions beneath kilauea s summit a reservoir of intermittent magma storage extends downward to about 10 km corresponding to a pressure of about 2 7 kbar ryan has argued that the base of the magma storage reservoir is controlled by the transition region that separates porosity elimination from mineral compression the rarity of erupted picrite as a lava on kilauea probably reflects a combination of two factors 1 parental kilauean picritic magma probably has a co 2 content less than about 0 3 wt 2 most parental picritic magma enters the reservoir near its base where the pressure is sufficiently high to result in the bulk density of parental magma being greater than that of stored magma the new 1959 picritic magma ascended through an unusual route and probably first encountered stored magma at an unusually shallow level where the new magma was relatively buoyant because of the role of porosity the base of subvolcanic magma storage reservoirs is dependent only on pressure and rock strength therefore the base of reservoirs of stored magma beneath active volcanoes likely occurred between 2 and 3 kbar during ancient as well as modern times because gas free komatiitic magma is relatively dense it too is expected to be trapped beneath stored magma komatiitic magmas containing more than about 0 6 wt of co 2 however would be buoyant relative to stored degassed basalt melt even at the base of magma storage reservoirs the common occurrence of komatiitic lavas in archaean times may reflect a greater co 2 content of mantle derived archean parental magmas

microstructures deformation mechanisms and seismic properties of a palaeoproterozoic shear zone the mertz shear zone east antarctica abstract the mertz shear zone msz is a lithospheric scale structure that recorded mid crustal deformation during the 1 7 ga orogeny we performed a microstructural and crystallographic preferred orientation cpo study of samples from both mylonites and tectonic boudins that constitute relics of the terre adelie craton tac the deformation is highly accommodated in the msz by anastomosed shear bands which become more scattered elsewhere in the tac most of the msz amphibolite facies mylonites display similar cpo thermal conditions intensity of deformation and dominant shear strain preserved granulite facies boudins show both coaxial and non coaxial strains related to the previous 2 45 ga event this former deformation is more penetrative and less localized and shows a deformation gradient later affected by a major phase of recrystallization during retrogression at 2 42 ga both msz samples and granulite facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted cpo of minerals quartz feldspar biotite amphibole and orthopyroxene in particular we highlight the development of an uncommon cpo in orthopyroxene from weakly deformed samples characterized by 010 planes oriented parallel to the foliation plane 001 axes parallel to the stretching lineation and clustering of 100 axes near the y structural direction lastly we computed the seismic properties of the amphibolite and granulite facies rocks in the msz area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the msz our results reveal that i the low content of amphibole and biotite in the rock formations of the tac and ii the interactions between the cpo of the different mineralogical phases generate a seismically isotropic crust thus the seismic anisotropy recorded by the seismic stations of the tac including the msz must be due to mantle rather than crustal structures

relationships between o isotope equilibrium mineral alteration and rb sr chronometric validity in granitoids implications for determination of cooling rate a combined study of mineral o and rb sr isotopes was carried out for a number of mesozoic granitoids in china in order to compare the degree of o isotope equilibrium between coexisting minerals with the validity of mineral rb sr isochrons for granitoids a scrutiny of both o isotope geothermometry and rb sr internal isochron dating for corresponding minerals indicates that equilibrium o isotope fractionation between rb sr isochron minerals corresponds to geologically meaningful isochron ages if the variation in 87rb 86sr ratio is big enough to provide reasonably small uncertainties in age significant deviation of the rb sr isochron age from the actual age appears to depend on the difference in sr isotopic composition between an external fluid and the igneous minerals as a result o isotope disequilibrium is often caused by interaction between the rock and the external fluid that results in mineral alteration post magmatic alteration can cause isotope exchange between the minerals and an internally buffered fluid that is isotopically identical to the host rock the o isotope composition of coexisting minerals in studied samples changed principally due to a decrease in temperature both rb and sr concentrations and the sr isotope ratios of isochron minerals also changed due to the mixing of different sr reservoirs nevertheless the isochron age can remain unchanged if the mixing took place along the isochron chord between the internal fluid and the minerals from that newly altered minerals formed this provides an insight into the effect of internal and external fluids on the validity of mineral rb sr chronometry in addition an alternative approach is proposed to construct the cooling curve by a combined use of o isotope temperature and mineral isotope age for the granitoids of interest comparing with the traditional method using the empirical closure temperature for rb sr chronometry the proposed approach utilizes fewer variables with smaller uncertainties than the traditional way

dislocation creep of dry quartz small scale shear zones within the permian truzzo meta granite developed during the alpine orogeny at amphibolite facies conditions in these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250 750 m consistent with flow at low differential stresses fourier transform infrared ftir spectroscopy reveals very low water contents in the interior of recrystallized grains in the form of discrete oh peaks 20 h 10 6 si and very little broad band absorption 100 h 10 6 si the spectral characteristics are comparable to those of dry brazil quartz in ftir spectra magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening these ftir measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz it is suggested that dislocation creep of quartz in the truzzo meta granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions

does aftershock duration scale with mainshock size 1 it has been claimed that aftershock duration scales with the mainshock recurrence interval dieterich 1994 this implies a scaling between aftershock duration and mainshock magnitude to see if such a scaling exist we compare properties of aftershock sequences associated with mainshocks whose size span several magnitude units earthquake properties were examined along two northern california fault segments one is the calaveras fault another is a northern segment of the san andreas fault the first study area contains the rupture area of the m6 2 1984 morgan hill and the latter is adjacent the southern end of the m7 1 1989 loma prieta rupture both the morgan hill and the loma prieta aftershock activities lasted 1 3 108 seconds on the other hand aftershock activities of small to moderate mainshocks lasted about 1 2 106 seconds nevertheless there is no evidence for scaling between mainshock size and aftershock duration for moderate to small mainshocks it thus seems that large earthquakes because they break the entire seismogenic depth and interact with deeper parts of the crust trigger a relaxation mechanism that is inactive during aftershock activity due to moderate to small quakes and whose characteristic relaxation time is much longer than that of the brittle crust current seismicity models do not provide explanation for these observations

very early archean crustal accretion complexes preserved in the north atlantic craton the north atlantic craton contains very early archean supracrustal rocks orthogneisses and massive ultramafic rocks most units of supracrustal rocks are dominated by mafic volcanic rocks layered gabbros and banded iron formations bust some also contain abundant felsic volcanic sedimentary rocks quartzites and marbles some quartzites contain detrital zircons derived from rocks identical in age to felsic volcanic sedimentary rocks in these sequences ca 3800 ma and also from older ca 3850 ma sources the presence of the ca 3850 ma detrital zircons suggests that the supracrustal units containing them were deposited on or close to ca 3850 ma sialic crust the massive ultramafic rocks have chemical affinities to upper mantle rocks the voluminous suites of tonalitic gneisses are dominated by 3700 3730 ma bodies that intrude the supracrustal sequences but they also locally contain components with ages between 3820 and 3920 ma the diverse supracrustal units upper mantle rocks and ge 3820 ma components in the gneisses were tectonically interleaved in very early archean convergent plate boundaries giving rise to accretion complexes in the period 3700 3730 ma voluminous tonalitic magmas produced by partial melting of predominantly mafic rocks in the base of the accretion complexes were emplaced at higher levels forming juvenilemore continental crust and leaving behind a refractory lower crustal to upper mantle substrate less

a microstructural sequence of quartz schists in central shikoku south west japan abstract this paper describes a microstructural sequence of quartz schists metamorphosed chert in the asemi river region of the sambagawa metamorphic terrain in central shikoku southwest japan the asemi river region is divided into three areas on the basis of characteristics of microstructures of quartz schists observed under the optical microscope areas i ii and iii in ascending order of metamorphic grade microstructures in area i consist of finer equant equidimensional and polygonal quartz grains free from internal deformation features microstructures in area ii are characterized by oblate or elliptical grains with remarkable undulatory extinction surrounded by serrated grain boundaries microstructures in area iii consist mainly of coarser and equant grains without distinct internal deformation features the formation conditions of these microstructures are discussed in the light of recent experimental results

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks

three ingredients for improved global aftershock forecasts tectonic region time dependent catalog incompleteness and intersequence variability following a large earthquake seismic hazard can be orders of magnitude higher than the long term average as a result of aftershock triggering because of this heightened hazard emergency managers and the public demand rapid authoritative and reliable aftershock forecasts in the past u s geological survey usgs aftershock forecasts following large global earthquakes have been released on an ad hoc basis with inconsistent methods and in some cases aftershock parameters adapted from california to remedy this the usgs is currently developing an automated aftershock product based on the reasenberg and jones 1989 method that will generate more accurate forecasts to better capture spatial variations in aftershock productivity and decay we estimate regional aftershock parameters for sequences within the garcia et al 2012 tectonic regions we find that regional variations for mean aftershock productivity reach almost a factor of 10 we also develop a method to account for the time dependent magnitude of completeness following large events in the catalog in addition to estimating average sequence parameters within regions we develop an inverse method to estimate the intersequence parameter variability this allows for a more complete quantification of the forecast uncertainties and bayesian updating of the forecast as sequence specific information becomes available online material omori parameter confidence limits alternative omori fits taking into account b value variation larger tectonic regions and a longer temporal fitting window and synthetic test results

accreted oceanic materials in japan abstract the phanerozoic circum pacific orogenic belts contain numerous ocean derived materials accreted through plate converging processes japanese islands in particular display various kinds of oceanic materials of different origins including fragments of seamounts oceanic reef limestone morb like rocks and oceanic mantle and pelagic sediments the compilation of these rocks in many subduction complexes of late permian to the present led to following conclusions accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust i e layer 1 and layer 2 many fragments of seamount with alkali basalt 600 hot spot seamount 26 oceanic reef limestone 291 morb like basalt 200 and numerous cherts more than 1000 are recognized as ancient oceanic materials accreted to the japanese islands however gabbros and mantle materials of layer 3 and lower parts of the oceanic lithosphere scarcely occur in subduction accretion complexes except for a few examples of back arc basin or fore arc origin accretion occurs episodically in southwest japan oceanic materials were accreted intermittently in 1 a end permian 2 b middle late jurassic 3 c late cretaceous times 4 d at ca 50 ma and 5 e in miocene times while in northeast japan and hokkaido this occurred in b middle late jurassic c late cretaceous and f early cretaceous times in contrast to the general belief on accretion of younger oceanic plates the majority of japanese subduction accretion complexes were formed during the subduction of plates up to 160 ma old the accretionary events in end permian and middle late jurassic times coincide with northward collision of ancient island arcs oceanic rises or seamount chains of hot spot origin with the asian continent accretion relevant to subduction of older plates may be controlled by the collision subduction process of these topographic reliefs on an oceanic plate in addition the chronological coincidence with the continent collision amalgamation between the sino korean and siberian platforms and between the sino korean and yangtze blocks also implies collision induced voluminous supply of elastics from back arc regions and its contribution to the formation of huge accretionary complexes accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts this implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels with respect to the metamorphic grades for japanese subduction complexes oceanic materials have less than 1 vol in the zeolite facies 15 20 in the prehnite pumpellyite metagraywacke facies and ca 30 in the greenschist glaucophane schist facies and albite epidote amphibolite fades this relationship indicates that the major process for landward accretion of oceanic materials is not off scraping or sedimentary mixing at the trench but underplating subcretion at much deeper levels of a subduction zone

late cenozoic structure and correlations to seismicity along the olympic wallowa lineament northwest united states late cenozoic fault geometry structure paleoseismicity and patterns of recent seismicity at two seismic zones along the olympic wallowa lineament owl of western idaho northeast oregon and southeast washington indicate limited right oblique slip displacement along multiple northwest striking faults that constitute the lineament the southern end of the owl originates in the long valley fault system and western snake river plain in western idaho where it accommodates basin and range extension several kilometers west of the long valley fault system post middle miocene displacement along the owl has resulted in adjacent areas of uplift and subsidence the owl in northeast oregon consists of a wide zone of northwest striking faults and is associated with several large inferred pull apart basins the owl then emerges from the blue mountain uplift as a much narrower zone of faults in the columbia plateau known as the wallula fault zone wfz structural relationships in the wfz strongly suggest that it is a right slip extensional duplex an ash horizon that correlates with the mount st helens j composition 10 7 ka is displaced vertically 5 m by a normal fault along a segment of the wfz the intensity center for a damaging m 6 1 earthquake in 1936 is located in the wfz only a few kilometers northeast of this holocene fault other youthful appearing strike slip and thrust faults are located farther northwest along the owl and may pose a seismic hazard to adjacent population centers and critical facilities