a continuum of stress strength and slip in the cascadia subduction zone movement of the down going oceanic plate in subduction zones is accommodated by earthquakes slow slip and free slip with increasing depth analysis of accompanying tremor reveals a continuum of slow slip events in the cascadia subduction zone which suggests that deep free slip of the subducted plate may cause stress to be gradually transferred up the plate interface towards the seismogenic zone

impact shock behavior of mg and ca sulfates and their hydrates abstract shock recovery experiments on mgso4 caso4 and their hydrates kieserite epsomite and bassanite were performed to investigate shock induced dehydration and decomposition at shock pressures up to 36 gpa the recovered solid samples indicated dehydration at pressures below 24 gpa but no clear evidence was found for possible decomposition of mgso4 and caso4 to produce mgo or cao as final products these sulfates and hydrates have been observed on the surface of mars and the present experimental results can be applied towards understanding the presence of surface water on mars and the recycling of water by impacts this finding that the sulfate hydrates undergo dehydration upon impact as well as the fact that the sulfates caso4 and mgso4 absorb moisture suggests the total amount of water on mars has remained almost unchanged since the time of formation of the planet

mechanisms of ringwoodite formation in shocked meteorites evidence from l5 chondrite dhofar 1970 the formation of the high pressure compositional equivalents of olivine and pyroxene has been well documented within and surrounding shock induced veins in chondritic meteorites formed by crystallization from a liquid or solid state phase transformation typically polycrystalline ringwoodite grains have a narrow range of compositions that overlap with those of their olivine precursors whereas the formation of iron enriched ringwoodite has been documented from only a handful of meteorites here we report backscattered electron images quantitative wavelength dispersive spectrometry wds analyses qualitative wds elemental x ray maps and micro raman spectra that reveal the presence of fe rich ringwoodite fa44 63 as fine grained 500 nm polycrystalline rims on olivine fa24 25 wall rock and as clasts engulfed by shock melt in a previously unstudied l5 chondrite dhofar 1970 crystallization of majorite magnesiowustite in the vein interior and metastable mineral assemblages within 35 m of the vein margin attest to rapid crystallization of a superheated shock melt 2300 k from 20 25 gpa to ambient pressure and temperature the texture and composition of bright polycrystalline ringwoodite rims fa44 63 mno 0 01 0 08 wt surrounding dark polycrystalline olivine fa8 14 mno 0 56 0 65 wt implies a solid state transformation mechanism in which fe was preferentially partitioned to ringwoodite the spatial association between ringwoodite and shock melt suggests that the rapidly fluctuating thermal regimes experienced by chondritic minerals in contact with shock melt are necessary to both drive phase transformation but also to prevent back transformation

coherent and subsequent incoherent ringwoodite growth in olivine of shocked l6 chondrites abstract pervasive replacement of original olivine grains by ringwoodite adjacent to a shock melt vein of yamato 791384 l6 chondrite was observed with a feg sem in these olivines we observed three distinct olivine ringwoodite textures arranged in a spatially successive arrangement from the wall of the shock melt vein into the olivine grains 1 polycrystalline ringwoodite 2 oriented several sets of ringwoodite lamellae and 3 oriented single lamellae tem images show that the polycrystalline and oriented several sets of lamellae parts consist of polycrystalline ringwoodite indicating that phase transformation from olivine to ringwoodite is controlled by incoherent growth mechanism on the other hand the set of oriented single lamellae consists of thin ringwoodite platelets

episodic slow slip events accompanied by non volcanic tremors in southwest japan subduction zone 1 episodic slow slip events have been recognized by means of tilt changes in the western shikoku area southwest japan the crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non volcanic deep tremor activities in this area observed tilt changes can be explained by slow slip events occurring around the source area of tremors in each episode the source of the slow slip event and tremor migrate simultaneously the spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate

variety and spatial heterogeneity of tectonic tremor worldwide 1 deep tectonic tremor occurs at various sites worldwide and the source characteristics are heterogeneous even at small scales in this study tremor sources were determined using data sets of seismic waveforms from various locations some of which are not well recognized as being a site of tremor activity the regions of interest are subduction zones at nankai and kyushu japan cascadia in western north america mexico southern chile and new zealand tremor locations are consistent with the geometry of subducting plates and tremor depth tends to be shallower where younger plate is subducting tremor duration shows a negative correlation with tidal sensitivity as reported previously in western shikoku in the nankai subduction zone the duration is also related to the width of the tremor zone in the dip direction of the subducting plate linear structures resembling striations are observed in hypocenter distributions in western shikoku and in the probability of short term migration direction these spatial heterogeneities in tremor activity may be controlled by the heterogeneous frictional properties of the downgoing slab due to long term plate subduction thereby providing information on the maturity of the plate interface

transformation textures mechanisms of formation of high pressure minerals in shock melt veins of l6 chondrites and pressure temperature conditions of the shock events the high pressure polymorphs of olivine pyroxene and plagioclase in or adjacent to shock melt veins smvs in two l6 chondrites sahara 98222 and yamato 74445 were investigated to clarify the related transformation mechanisms and to estimate the pressure temperature conditions of the shock events wadsleyite and jadeite were identified in sahara 98222 wadsleyite ringwoodite majorite akimotoite jadeite and lingunite naalsi3o8 hollandite were identified in yamato 74445 wadsleyite nucleated along the grain boundaries and fractures of original olivine the nucleation and growth of ringwoodite occurred along the grain boundaries of original olivine and as intracrystalline ringwoodite lamellae within original olivine the nucleation and growth of majorite took place along the grain boundaries or fractures in original enstatite jadeite containing assemblages have complicated textures containing particle like stringer like and polycrystalline like phases coexistence of lingunite and jadeite containing assemblages shows a vein like texture we discuss these transformation mechanisms based on our textural observations and chemical composition analyses the shock pressure and temperature conditions in the smvs of these meteorites were also estimated based on the mineral assemblages in the smvs and in comparison with static high pressure experimental results as follows 13 16 gpa 1900 c for sahara 98222 and 17 24 gpa 2100 c for yamato 74445

high pressure phases in a shock induced melt vein of the tenham l6 chondrite constraints on shock pressure and duration abstract the microtexture and mineralogy of a 580 m wide melt vein in the tenham l6 chondrite were investigated using field emission scanning electron microscopy and transmission electron microscopy to better understand the shock conditions the melt vein consists of a matrix of silicate plus metal sulfide grains that crystallized from immiscible melts and sub rounded fragments of the host chondrite that have been entrained in the melt and transformed to polycrystalline high pressure silicates the melt vein matrix contains two distinct textures and mineral assemblages corresponding to the vein edge and interior the 30 m wide vein edge consists of vitrified silicate perovskite ringwoodite akimotoite majorite with minor metal sulfide the 520 m wide vein interior consists of majorite magnesiowustite with irregular metal sulfide blebs although these mineral assemblages are distinctly different the pressure stabilities of both assemblages are consistent with crystallization from similar pressure conditions the melt vein edge crystallized at about 23 25 gpa and the vein interior crystallized at about 21 25 gpa this relatively narrow pressure range suggests that the melt vein either crystallized at a constant equilibrium shock pressure of 25 gpa or during a relatively slow pressure release using a finite element heat transfer program to model the thermal history of this melt vein during shock we estimate that the time required to quench this 580 m wide vein was 40 ms because the entire vein contains high pressure minerals that crystallized from the melt the shock pressure duration was at least 40 ms using a synthetic hugoniot for tenham and assuming that the sample experienced a peak shock pressure of 25 gpa near the impact site we estimate that the tenham parent body experienced an impact with collision velocity 2 km s based on a one dimensional planar impact model we estimate that the projectile size was 150 m in thickness

complex evolution of transient slip derived from precise tremor locations in western shikoku japan transient slip events which occur more slowly than traditional earthquakes are increasingly being recognized as important components of strain release on faults and may substantially impact the earthquake cycle surface based geodetic instruments provide estimates of the overall slip distribution in larger transients but are unable to capture the detailed evolution of such slip either in time or in space accompanying some of these slip transients is a relatively weak extended duration seismic signal known as nonvolcanic tremor which has recently been shown to be generated by a sequence of shear failures occurring as part of the slip event by precisely locating the tremor we can track some features of slip evolution with unprecedented resolution here we analyze two weeklong episodes of tremor and slow slip in western shikoku japan we find that these slip transients do not evolve in a smooth and steady fashion but contain numerous subevents of smaller size and shorter duration in addition to along strike migration rates of 10 km d observed previously much faster migration also occurs usually in the slab dip direction at rates of 25 150 km h over distances of up to 20 km we observe such migration episodes in both the updip and downdip directions these episodes may be most common on certain portions of the plate boundary that generate strong tremor in intermittent bursts the surrounding regions of the fault may slip more continuously driving these stronger patches to repeated failures tremor activity has a strong tidal periodicity possibly reflecting the modulation of slow slip velocity by tidal stresses

natural cati2o4 structured fecr2o4 polymorph in the suizhou meteorite and its significance in mantle mineralogy abstract the first natural occurrence of a high pressure polymorph of chromite has been discovered in the shock metamorphosed suizhou meteorite the composition of this high pressure polymorph is identical to that of the precursor chromite the raman spectrum of this polymorph is distinct from that of chromite synchrotron x ray diffraction analysis revealed that this polymorph has an orthorhombic cati2o4 type structure the cell parameters are a 9 462 6 a b 9 562 9 a c 2 916 1 a v 263 8 4 a3 z 4 space group bbmm and the density 5 63 g cm3 the numbers in parentheses are standard deviations on the last significant digits this polymorph is 11 5 denser than chromite the p t conditions for the phase transformation from chromite to the cati2o4 structured polymorph are estimated at 20 23 gpa and 1800 to 2000 c respectively this dense cati2o4 structured fecr2o4 phase could be a host phase for cr al fe mg and mn and other metallic elements in the deep earth

high pressure polymorphs in yamato 790729 l6 chondrite and their significance for collisional conditions shock pressure recorded in yamato y 790729 classified as l6 type ordinary chondrite was evaluated based on high pressure polymorph assemblages and cathodoluminescence cl spectra of maskelynite the host rock of y 790729 consists mainly of olivine low ca pyroxene plagioclase metallic fe ni and iron sulfide with minor amounts of phosphate and chromite a shock melt vein was observed in the hostrock ringwoodite majorite akimotoite lingunite tuite and xieite occurred in and around the shock melt vein the shock pressure in the shock melt vein is about 14 23 gpa based on the phase equilibrium diagrams of high pressure polymorphs some plagioclase portions in the host rock occurred as maskelynite sixteen different cl spectra of maskelynite portions were deconvolved using three assigned emission components centered at 2 95 3 26 and 3 88 ev the intensity of emission component at 2 95 ev was selected as a calibrated barometer to estimate shock pressure and the results indicate pressures of about 11 19 gpa the difference in pressure between the shock melt vein and host rock might suggest heterogeneous shock conditions assuming an average shock pressure of 18 gpa the impact velocity of the parent body of y 790729 is calculated to be 1 90 km s 1 the parent body would be at least 10 km in size based on the incoherent formation mechanism of ringwoodite in y 790729

a microstructural investigation of natural lamellar ringwoodite in olivine of the shocked sixiangkou chondrite abstract olivine adjacent to the shock produced veins of sixiangkou meteorite contains several sets of ringwoodite lamellae nucleated and grown along planar fractures in olivine these lamellae consist of two generations of crystallite aggregates of ringwoodite and show features of incoherent nucleation and growth of ringwoodite along fractures of olivine the lamellae are of zonal fe mg distribution characterized as fe rich rim and mg rich interior it indicates that the intracrystalline transformation of olivine to ringwoodite proceeds through fe mg interdiffusion during growth of ringwoodite the formation of the lamellae favors a fracture involved phase transformation mechanism firstly a thin layer of crushed or distorted olivine along a gliding plane of a fracture was produced and then transformed into the first generation of crystallite layer of ringwoodite subsequently the second generation of relatively coarse grained ringwoodite grew on both interfaces of the firstly formed thin layer of ringwoodite this study brings forward a new observation of intracrystalline olivine ringwoodite transformation in addition to other mechanisms since mechanical deformations resulting in planar fractures are common in olivine of terrestrial mantle rocks this proposed mechanism could play an important role in the subducting lithospheric slabs

formation of high pressure minerals in shocked l6 chondrite yamato 791384 constraints on shock conditions and parent body size we have studied the high pressure phases observed in yamato 791384 l6 chondrite host meteorite mainly consists of olivine pyroxenes and plagioclase glass mineral fragments observed in the veins and the vein margin region of the host meteorite were partially or totally transformed into high pressure phases ringwoodite majorite akimotoite naalsi3o8 hollandite and jadeite matrix of the shock vein contains majorite pyrope solid solution the spatial distribution indicates that high pressure phases are present in the shock veins and host rocks adjacent to the shock veins investigation of the high pressure phases revealed that in y791384 fragments and adjacent matrix were subjected to pressures around of 18 23 gpa and the vein experienced temperatures around 2300 c during the shock event ringwoodite lamellae were observed in the host olivine adjacent to the vein kinetic investigation for ringwoodite lamellar growth in olivine indicates that the shocked meteorite experienced the pressure for at least 4 s such long duration of the pressure pulse indicates that the size of the parent body was greater than 10 km

phenomenology of deep slow earthquake family in southwest japan spatiotemporal characteristics and segmentation 1 the phenomenology of deep slow earthquakes including low frequency tremors very low frequency earthquakes and short term slow slip events on the subducting plate interface in southwest japan is investigated on the basis of their spatiotemporal characteristics the belt like distribution of tremors is divided into segments bounded by gaps the repetition of tremor episode depends on the magnitude of the episode which is defined by the number of detected tremors within each episode major tremor episodes with a large number of detected tremors recur at an interval of approximately 6 months accompanying the short term slow slip events in large segments in small segments tremor episodes with small numbers of tremors have recurrence intervals of a few months some of them occasionally accompany the slight ground tilting caused by a small size slow slip event even in the same segment there exist different recurrence intervals according to the magnitude of the tremor episode the migration pattern varies greatly in initiation termination direction and speed however the migration direction has a general tendency for each segment on both sides of the gap the tremor starts next to the gap and propagates to the other side assuming all tremor episodes are caused by slow slip events the equivalent moment and slip are estimated from the number of detected tremor counts and the area of each tremor episode the equivalent slip history estimated from all tremor episodes is almost constant in each segment this indicates that tremor activity is a good proxy for slow slips

natural high pressure polymorph of merrillite in the shock veins of the suizhou meteorite abstract a new high pressure polymorph of merrillite with the structure of trigonal ca 3 po 4 2 was found in the shock produced veins of the suizhou meteorite where it coexists with ringwoodite majorite naalsi 3 o 8 hollandite and majorite pyrope garnet the crystallographic nature of this natural ca 3 po 4 2 phase was characterized by raman spectroscopy and x ray diffraction and all data compare favorably to the same data obtained from ca 3 po 4 2 synthesized at 14 gpa and 1400 c the cell parameters of this new high pressure mineral are a 5 258 1 angstroms and c 18 727 3 angstroms space group r 3m and density 3 447 g cm 3 where the number in parentheses are standard deviations in the last significant digits the natural occurrence of the ca 3 po 4 2 phase together with other high pressure minerals constrains the pressure of the shock veins at about 23 gpa the suizhou meteorite provides the first naturally occurring example of ca 3 po 4 2 polymorph

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

depth dependent mode of tremor migration beneath kii peninsula nankai subduction zone 1 we investigated the migration mode of deep non volcanic tremor activity beneath kii peninsula southwest japan major tremor episodes are characterized by long term migration with a velocity of about 10 km day propagating along the strike of the subducting plate similar tremor migration in cascadia is accompanied by reverse propagation at speeds on the order of 100 km day and much faster slip parallel migration at speeds on the order of 1000 km day we systematically searched for migrating tremor with clear linearity in space and time as a result we found tremor migrations at speeds ranging from 1 to 60 km hr depending on the along dip position in the tremor zone the observed decrease in migration speed with increasing measurement time scale suggests that migration is controlled by a diffusion process the along strike migration at lower speeds including both forward and backward directions relative to the long term migration episode is concentrated at the updip side of the tremor zone whereas the faster slip parallel migration is distributed over the entire zone the long term migration seems to consist of and be excited by the propagation of along strike creep at the updip part the concentration of along strike migrating tremor sequences at the updip side may reflect the existence of abundant fluid that accumulates at the corner of the mantle wedge the faster slip parallel migrations represent projections of along strike fluctuations in slip pulse propagation controlled by striations along the plate interface

subaqueous volcanism in the etnean area evidence for hydromagmatic activity and regional uplift inferred from the castle rock of acicastello abstract the subalkaline rocks outcropping at the acicastello castle rock catania sicily and on its abrasion platforms are related to the oldest etnean volcanism 500 300 ka gillot p y kieffer g romano r 1994 the evolution of mount etna in the light of potassium argon dating acta vulcanol 5 81 87 here submarine lavas with pillows closely packed onto each other are associated with heterogeneous and poorly sorted volcaniclastic breccia levels with sub vertical sharp boundaries the present day attitude was previously interpreted as due to a local tilt di re m 1963 hyaloclastites and pillow lavas of acicastello mt etna bull volcanol 25 281 284 kieffer g 1985 evolution structurale et dynamique d un grand volcan polygenique stades d edification et activite actuelle de l etna sicile clermont ferrand iidoctorat etat tesi clermont ferrand ii or to the seaward sliding of the entire eastern etnean flank borgia a ferrari l pasquare g 1992 importance of gravitational spreading in the tectonic and volcanic evolution of mount etna nature 357 231 235 on the assumption of originally horizontal boundaries on the contrary our observations do not match the hypothesis of a significantly tilted succession and lead us to conclude that apart from the strong regional uplift the present castle rock exposure did not suffer any substantial change of its attitude

the effect of strain energy on growth rates during the olivine spinel transformation and implications for olivine metastability in subducting slabs we have studied growth kinetics during the transformation of mg1 8fe0 2sio4 san carlos olivine to its high pressure polymorphs wadsleyite phase and ringwoodite phase at 800 1200 c and nominal pressures of 16 20 gpa in experiments in which a large 500 600 m olivine single crystal contained in a matrix of either fine grained olivine or nacl was transformed reaction rims of wadsleyite ringwoodite form on the margins of the single crystal by incoherent grain boundary nucleation and interface controlled growth contrary to theoretical expectations the growth rate of these reaction rims decreases sharply as a function of time for instance at 1100 c and 18 gpa growth ceases on an experimental timescale after the rim width reaches 20 25 m in order to explain this observation we develop an elastic model based on the theory of a misfitting inclusion comparing the results of this model with the experimental data suggests that elastic strain energy which develops because of the large volume decrease associated with the transformation is responsible for the decreasing growth rates on the other hand experimental and theoretical results suggest that elastic strain energy is relatively unimportant when grains of the product phase are randomly dispersed and distinct reaction rims do not form this is the case when the nucleation rate is low the growth rate is fast and the reactant olivine is fine grained on a geological timescale in subducting lithosphere where the grain size of the olivine is large the growth rates of grain boundary nucleated reaction rims are likely to be controlled by viscoelastic relaxation therefore current kinetic models of olivine metastability in subducting slabs which are based on simple extrapolations of experimental data and on the assumption that the growth rate is constant at fixed temperature and pressure need to be reevaluated

jadeite formation in shocked ordinary chondrites abstract albitic feldspar in shocked ordinary chondrites yamato 791384 l6 and yamato 75100 h6 and albite recovered from static high pressure and high temperature synthetic experiments kubo et al 2010 were investigated with a transmission electron microscope tem subsequent to a conventional micro raman spectroscopy analysis to clarify albite dissociation reaction under high pressure and high temperature condition when jadeite forms from albite sio 2 phase as a residual phase of albite dissociation reaction should accompany jadeite from the stoichiometry however albitic feldspar in and adjacent to shock melt veins of the shocked chondrites dissociates into jadeite residual amorphous or poorly crystallized material having varied chemical compositions between jadeite and sio 2 phase tem observations of albitic feldspar in the shocked chondrites and albite recovered from the static high pressure and high temperature synthetic experiments show that jadeite crystallization is initiated by grain refinement of albite or albitic feldspar nucleation occurs along grain boundaries or at triple junctions of the fine grained albite crystal assemblage jadeite crystal starts to grow from the nucleus through grain boundary diffusion considering pressure condition recorded in the shock melt veins of the shocked chondrites stishovite is the most likely as a residual sio 2 phase accompanying jadeite high pressure and high temperature condition induced by a dynamic event is very short stishovite would be hardy formed through a dynamic event due to sluggish nucleation rate of stishovite compared with that of jadeite thus leading to induce heterogeneous and incomplete albite dissociation reaction albite dissociates into jadeite residual amorphous material

rapid continuous streaking of tremor in cascadia nonvolcanic tremor is a recently discovered weak seismic signal associated with slow slip on a fault plane and has potential to answer many questions about how faults move its spatiotemporal distribution however is complex and varies over different time scales and the causal physical mechanisms remain unclear here we use a beam backprojection method to show rapid continuous slip parallel streaking of tremor over time scales of several minutes to an hour during the may 2008 episodic tremor and slip event in the cascadia subduction zone the streaks propagate across distances up to 65 km primarily parallel to the slip direction of the subduction zone both updip and downdip at velocities ranging from 30 to 200 km h we explore mainly two models that may explain such continuous tremor streaking the first involves interaction of slowly migrating creep front with slip parallel linear structures on the fault the second is pressure driven fluid flow through structurally controlled conduits on the fault both can be consistent with the observed propagation velocities and geometries although the second one requires unlikely condition in addition we put this new observation in the context of the overall variability of tremor behavior observed over different time scales

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

stability of clinopyroxene at pressure temperature conditions of the transition region abstract the stability of diopside at conditions of 16 19 gpa and 1250 1900 c was determined using a 6 8 type multi anvil apparatus at pressures of 17 18 gpa diopside undergoes a decomposition reaction to an assemblage of casio3 perovskite ca mg sio3 garnet or ilmenite depending on temperature the clapeyron slope for the breakdown reaction download download full size image is 2 mpa k 1 when coupled with a re examination of clinopyroxene stability in other more complex peridotite systems the experimental data are consistent with a recent proposal that the 520 km discontinuity in the transition region is caused by the exsolution of casio3 rich perovskite from garnet rich in majorite component

maskelynite formation via solid state transformation evidence of infrared and x ray anisotropy we present the results of a combined study of shocked labradorite from the lonar crater india using optical microscopy micro raman spectroscopy nuclear magnetic resonance nmr spectroscopy high energy x ray total scattering experiments and micro fourier transform infrared micro ftir spectroscopy we show that maskelynite of shock class 2 is structurally more similar to fused glass than to crystalline plagioclase however there are slight but significant differences preservation of original preimpact igneous zoning anisotropy at infrared wavelengths x ray anisotropy and preservation of some intermediate range order which are all consistent with a solid state transformation from plagioclase to maskelynite

high pressure phases in shock induced melt of the unique highly shocked ll6 chondrite northwest africa 757 northwest africa 757 is unique in the ll chondrite group because of its abundant shock induced melt and high pressure minerals olivine fragments entrained in the melt transform partially and completely into ringwoodite plagioclase and ca phosphate transform to maskelynite lingunite and tuite two distinct shock melt crystallization assemblages were studied by fib tem analysis the first melt assemblage which includes majoritic garnet ringwoodite plus magnetite magnesiowustite crystallized at pressures of 20 25 gpa the other melt assemblage which consists of clinopyroxene and wadsleyite solidified at 15 gpa suggesting a second veining event under lower pressure conditions these shock features are similar to those in s6 l chondrites and indicate that nwa 757 experienced an intense impact event comparable to the impact event that disrupted the l chondrite parent body at 470 ma

raman spectroscopy of fe ti cr oxides case study martian meteorite eeta79001 raman spectral features of chromite ulvospinel magnetite ilmenite hematite and some of their solid solutions are presented although most fe ti cr oxides produce relatively weak raman signals compared to oxyanionic minerals sufficient information can be extracted from their spectra to identify the end member mineral phases as well as some information about compositional variations in solid solutions correlations between raman spectral features and mineral chemistry are used to interpret the raman data of fe ti cr oxides found during raman point count measurements on rock chips of martian meteorite eeta79001 as an analog to mars on surface planetary investigations in general ulvospinel magnetite and chromite end members are readily distinguished by their raman spectral patterns as are ilmenite and hematite in the low signal to noise s n spectra generally obtained from the raman point count procedure the position and shape of the strongest peak of fe ti cr oxides in the region 660 680 cm 1 a1g mode is the most useful for discriminating fe3 ti cr al substitutions in the magnetite ulvospinel ulvospinel chromite and chromite spinel series but minor peaks in the range 300 600 cm 1 also assist in discrimination these spectral features are useful for investigating the variability among fe ti cr al oxide solid solutions in natural samples in eeta79001 a martian basaltic meteorite most of the oxide grains as measured with the electron microprobe are ulvospinel chromian ulvospinel and chromite but ilmenite titanian chromite and titanomagnetite are also observed the fe ti cr oxides identified by raman point count include end member ilmenite low al chromite spinel solid solutions ulvospinel magnetite solid solutions and more complex chromite spinel ulvospinel magnetite solid solutions the latter exhibit a wide range of main peak positions and broadened peak widths that may reflect structural disorder as well one raman spectrum suggests end member magnetite and one spectrum from a different rock chip appears to be that of non terrestrial hematite reflecting local oxidizing alteration which has not been observed previously in this meteorite these results show that analyses done in an automated mode on the surface of an unprepared martian rock sample can provide useful constraints on the fe ti cr oxide mineralogy present and on compositional variations within those minerals including an indication of oxygen fugacity

shock induced transition of naalsi3o8 feldspar into a hollandite structure in a l6 chondrite natural naalsi3o8 hollandite was found in the tenham chondrite it occurs as aggregates of extremely fine grains of several tens of nanometers the hollandite phase seems to have formed from host feldspar in a solid state reaction during the shock event it is very sensitive to electron beam irradiation and easily becomes amorphous similar to mgsio3 perovskite contained in the same chondrite the existence of crystalline silicate perovskite and hollandite in tenham indicates that the peak pressure and the post shock temperature were much lower than previously believed

the nature of maskelynite in shocked meteorites not diaplectic glass but a glass quenched from shock induced dense melt at high pressures abstract maskelynite an important constituent of shocked meteorites once thought to be diaplectic plagioclase glass formed by shock induced solid state transformation our systematic investigation of shocked l chondrites and snc meteorites indicates that maskelynite does not contain inherited fractures or cleavage and shock induced fractures we found no evidence for models calling for melting that initiated in pdfs and affected the whole crystals maskelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene this is indicative of shock induced melting and quenching of the dense melt at high pressure thus erasing the inherited and shock induced fractures this was followed by relaxation of the dense plagioclase glass which induced the expansion cracks in pyroxene and olivine enrichment in potassium deviation from stoichiometry degradation of igneous zoning the presence of offshoots of maskelynite in pyroxene the lack of vesiculation in the melt pockets melt veins and molten mesostasis are clear evidence for melting and quenching under high pressure our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid state transformation but a dense quenched glass the duration of the shock pulse in natural events can be several orders of magnitude longer than in shock experiments since kinetic effects are crucial factors in promoting phase transitions vitrification and melting experimentally induced solid state vitrification of plagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite bearing natural samples

shock barometer using cathodoluminescence of alkali feldspar 1 color cathodoluminescence cl images of unshocked and experimentally shocked sanidine at pressures up to 40 1 gpa showed red violet emission below 20 0 gpa and blue emission above 20 0 gpa the phases in these shock recovered samples were identified as crystalline feldspar for red violet emitting areas and as diaplectic feldspar glass for blue emitting ones by micro raman spectroscopy cl spectra of these shocked sanidine have emissions at 330 380 and 400 420 nm of which intensities increase with an increase in shock pressure similar uv blue emissions were found in alkali feldspar and the glass in martian meteorites and ries crater impactite the deconvolution of these cl spectra provides the emission component at 2 948 ev assigned to shock induced defect center where this intensity correlates linearly with peak shock induced pressure on sanidine with little dependence on composition and structure the correlation gives quantitative values of the shock pressures experienced by the feldspar resulting in estimated shock pressures of martian meteorites and ries crater impactite the cl intensity of feldspar has a potential for a universal shock barometer with high spatial resolution 1 m and in a wide pressure range theoretically 4 5 40 1 gpa this leads to a breakthrough in understanding the impact histories on earth moon and mars

migration properties of non volcanic tremor in shikoku southwest japan 1 based on the precise locations of deep non volcanic tremor in southwest japan which show a depth dependent behavior we clarified characteristic properties of tremor migration major tremor swarms are mainly initiated at deep levels of the tremor zone corresponding to areas of frequent minor tremor and migrate up dip along the subduction interface in central and eastern shikoku tremor occurs in two clusters separated by a small gap with the timing of tremor activity in the clusters differing by 1 or 2 days the time evolution of tremor activity in western shikoku reveals that the migration front of tremor propagates continuously and radially even in the case of a tremor gap these findings indicate that the rupture front of slow slip events propagates continuously and radially from the deep levels through the area without tremor activity the occurrence of tremor may depend on the existence of inhomogeneous patches upon the plate interface

akimotoite in the tenham meteorite crystal chemistry and high pressure transformation mechanisms the transformation of pyroxene to its high pressure polymorph akimotoite mgsio3 ilmenite type structure is documented in the shock induced melt veins of the l6 tenham chondrite four textural relationships between pyroxene and akimotoite are observed in former pyroxene grains entrained in the shear melt vein and in pyroxene grains attached to the wall of the melt vein in one of the entrained enstatite grains the transformation to akimotoite is partial one third of the grain is transformed to a polycrystalline aggregate of akimotoite with a scalloped interface with enstatite akimotoite fe0 4mg1 24ca0 07na0 12al0 14 si1 963al0 037 o6 is slightly enriched in ca cao 1 74 al and na with respect to enstatite cao 0 71 narrow bands of polycrystalline akimotoite with three crystallographic orientations and a chemical composition similar to the surrounding enstatite intersect the other part of the grain in a second grain entrained in the shear melt vein tablets of akimotoite are interwoven with a pyroxene glass with a lower na and fe content and a higher mg content compared to the adjacent akimotoite this pyroxene glass is probably the product of mg fe sio3 perovskite amorphization polycrystalline akimotoite is also formed at the expense of enstatite at the vein wall of the shear melt vein in that case akimotoite is also enriched in ca cao 2 85 na nao 1 72 and al al2o3 4 14 all these observations suggest that akimotoite is mainly formed by solid state transformation of former pyroxenes with subsequent diffusion of calcium aluminum and sodium from the chondritic melt of the shear melt vein finally the first complete x ray diffraction pattern of natural akimotoite is presented

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

space time correlation of slip and tremor during the 2009 cascadia slow slip event 1 episodic tremor and slip ets involving transient deformations accompanied by emergent low frequency tremor occurs in subduction zones around the world ets events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes despite the clear association of tremor and slip the physical relationship between them is unresolved tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault or stress increases ahead of the propagating slow slip front previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models to address this we invert gps data from the august 2009 ets event in central cascadia for the space time evolution of fault slip rate we find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip rate this supports the first hypothesis that tremor asperities are loaded directly by slow slip rather than by stress increases ahead of the slip front and provides new insights into the mechanics of ets

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses