analysis of lake ice dynamics and morphology on lake el gygytgyn ne siberia using synthetic aperture radar sar and landsat 1 a time series of more than 450 combined ers 2 radarsat 1 and landsat 7 scenes acquired between 1998 and 2001 was analyzed to develop a fairly complete picture of lake ice dynamics on lake el gygytgyn ne siberia 67 5 n 172 e this 14 km3 lake partially fills a meteorite impact crater formed 3 6 million years ago and is home to a paleoenvironmental coring project the duration of lake ice cover and the onset of lake ice breakup are important both to interpretations of the archived sediment core record and to future drilling projects that will use the ice as a stable platform ice formation snowmelt and ice breakup likely occur in late october mid may and early july respectively these data were used to validate a one dimensional energy balance lake ice model which can now be used to hindcast paleoclimate based on core proxy information synthetic aperture radar sar backscatter from the lake ice also revealed unusual spatial variations in bubble content which were found to indicate the level of biological productivity in the sediments directly beneath the ice with the highest productivity located in the shallowest 0 10 m as well as the deepest 170 175 m regions of the lake seismic data indicates that the backscatter anomaly above the deepest water is collocated with the central peak of the impact crater 500 m below the surface several hypotheses are presented to explain this anomaly regardless of cause the fact that large spatial variations in biological productivity exist in the lake has important implications for selecting the locations of future sediment cores

pantasma evidence for a pleistocene circa 14 km diameter impact crater in nicaragua the circa 14 km diameter pantasma circular structure in oligocene volcanic rocks in nicaragua is here studied for the first time to understand its origin geomorphology field mapping petrographic and geochemical investigations all are consistent with an impact origin for the pantasma structure observations supporting an impact origin include outward dipping volcanic flows the presence of former melt bearing polymict breccia impact glass with lechatelierite and low h 2 o 10 km found in the americas

impact processes permafrost dynamics and climate and environmental variability in the terrestrial arctic as inferred from the unique 3 6 myr record of lake el gygytgyn far east russia a review lake el gygytgyn in far east russia is a 3 6 myr old impact crater lake located in an area that has never been affected by cenozoic glaciations nor desiccation the unique sediment record of the l

on vertical advection truncation errors in terrain following numerical models comparison to a laboratory model for upwelling over submarine canyons 1 submarine canyons which indent the continental shelf are frequently regions of steep up to 45 three dimensional topography recent observations have delineated the flow over several submarine canyons during 2 4 day long upwelling episodes thus upwelling episodes over submarine canyons provide an excellent flow regime for evaluating numerical and physical models here we compare a physical and numerical model simulation of an upwelling event over a simplified submarine canyon the numerical model being evaluated is a version of the s coordinate rutgers university model scrum careful matching between the models is necessary for a stringent comparison results show a poor comparison for the homogeneous case due to nonhydrostatic effects in the laboratory model results for the stratified case are better but show a systematic difference between the numerical results and laboratory results this difference is shown not to be due to nonhydrostatic effects rather the difference is due to truncation errors in the calculation of the vertical advection of density in the numerical model the calculation is inaccurate due to the terrain following coordinates combined with a strong vertical gradient in density vertical shear in the horizontal velocity and topography with strong curvature

dynamic friction and the origin of the complexity of earthquake sources abstract we study a simple antiplane fault of finite length embedded in a homogeneous isotropic elastic solid to understand the origin of seismic source heterogeneity in the presence of nonlinear rate and state dependent friction all the mechanical properties of the medium and friction are assumed homogeneous friction includes a characteristic length that is longer than the grid size so that our models have a well defined continuum limit starting from a heterogeneous initial stress distribution we apply a slowly increasing uniform stress load far from the fault and we simulate the seismicity for a few 1000 events the style of seismicity produced by this model is determined by a control parameter associated with the degree of rate dependence of friction for classical friction models with rate independent friction no complexity appears and seismicity is perfectly periodic for weakly rate dependent friction large ruptures are still periodic but small seismicity becomes increasingly nonstationary when friction is highly rate dependent seismicity becomes nonperiodic and ruptures of all sizes occur inside the fault highly rate dependent friction destabilizes the healing process producing premature healing of slip and partial stress drop partial stress drop produces large variations in the state of stress that in turn produce earthquakes of different sizes similar results have been found by other authors using the burridge and knopoff model we conjecture that all models in which static stress drop is only a fraction of the dynamic stress drop produce stress heterogeneity

obducted nappe sequence in the san juan islands northwest cascades thrust system washington and british columbia the san juan islands northwest cascades thrust system in washington and british columbia is composed of previously accreted terranes now assembled as four broadly defined composite nappes stacked on a continental footwall of wrangellia and the coast plutonic complex emplacement ages of the nappe sequence are interpreted from zircon ages field relations and lithlogies to young upward the basal nappe was emplaced prior to early turonian time 93 ma indicated by the occurrence of age distinctive zircons from this nappe in the sidney island formation of the nanaimo group the emplacement age of the highest nappe in the thrust system postdates 87 ma detrital zircons within the nappe the nappes bear high pressure low temperature hp lt mineral assemblages indicative of deep burial in a thrust wedge however several features indicate that metamorphism occurred prior to nappe assembly metamorphic discontinuities at nappe boundaries absence of hp lt assemblages in the footwall to the nappe pile and absence of significant unroofing detritus in the nanaimo group a synorogenic relationship of the thrust system to the nanaimo group is evident from mutually overlapping ages and by conglomerates of nanaimo affinity that lie within the nappe pile from the foregoing relations and broader cordilleran geology the tectonic history of the nappe terranes is interpreted to involve initial accretion and subduction zone metamorphism south of the present locality uplift and exhumation orogen parallel northward transport of the nappes as part of a forearc sliver and finally obduction at the present site over the truncated south end of wrangellia and the coast plutonic complex

maskelynite hosted apatite in the chassigny meteorite insights into late stage magmatic volatile evolution in martian magmas apatite hosting maskelynite and alkali maskelynite within regions interstitial to cumulus olivine differ compositionally from that found within melt inclusions in the chassigny martian meteorite feldspar glass compositions within the interstitial regions evolve along a high temperature crystallization path within the melt inclusions feldspar glass shows evolution to low temperatures extending into the subsolidus regime coupled with these differences in maskelynite compositions are differences in volatile abundance in apatite included within the maskelynite apatite found within the large olivine hosted polyphase melt inclusions is uniformly fluorapatite whereas that interstitial to cumulus olivine is chlor fluorapatite we propose that the differences in maskelynite and apatite compositions within the melt inclusion and interstitial regions arose primarily from different crystallization conditions melt inclusion maskelynite and apatite are consistent with nearly closed system buildup of magmatic volatiles during crystallization within the melt inclusions exsolution of a cl bearing fluid phase and retention of this fluid phase within the melt inclusions into the hydrothermal regime for the interstitial regions however the higher solidus temperatures of the interstitial melts the early crystallization of feldspar with significant ternary component and the cl rich nature of the apatite all suggest open system fluid migration through the cumulus pile ingress of cl rich h2o poor brine from a hotter less evolved portion of the magma plumbing system and interaction of this cl rich fluid with melt prior to the crystallization of feldspar such processes of fluid migration through a cumulus pile have been suggested on earth in layered intrusions and apatite is the primary recorder of this process

the role of strain heating in the evolution of megathrusts on the basis of elementary energy considerations and the nonlinear temperature dependence of experimental flow laws a model is developed for the interplay of thermal and mechanical processes along megathrust systems the model is based on the observation that thrust systems involving large portions of the lithosphere have a two part history during the early history of movement dry lower crustal upper mantle rocks are deformed in a shear zone where because of the nonlinear effect of temperature dependent diffusion strain heating rates are directly controlled by the initial thermal state that is heating is insignificant if initial temperatures are high but where temperatures are low large temperature increases are possible as a result strain heating acts as a thermal buffer such that upon arrival of upper crustal rocks at depths of 20 km or more the temperatures along the thrust would be controlled largely by the rheology of hanging wall rocks rather than the initial thermal state moreover the relatively high melting temperatures for dry lower crustal upper mantle rocks implies that the temperatures achieved along the thrust should be relatively high as thrusting progresses this early history is successively eliminated as wet upper crustal rocks are emplaced beneath the preheated hanging wall the granitic melts that characterize many natural megathrust systems are probably generated when these wet metasedimentary rocks move beneath the hanging wall either by hydrous fluxing of the hot hanging wall or by melting in the highest grade footwall rocks rapid uplift implied by both metamorphic studies and preservation of the inverted metamorphic sequence is interpreted as a late phase event created by tectonic pileup beneath the thrust thorough testing of the model will require thorough assessment of the time dependent problem however one dimensional steady state models together with energy considerations indicate that the general model is allowable the concept of thermal buffering by strain heating is important because it implies that megathrust systems are a natural system where metamorphic temperatures are controlled by rock rheology hence they represent real world systems where a thermal and mechanical system are intimately linked this rationale is used to extend the model by indicating a method in which metamorphic conditions along exhumed megathrusts might be used as a natural constraint on experimentally determined flow laws

three dimensional dynamic simulation of the 1992 landers earthquake the 1992 landers california earthquake magnitude 7 3 was modeled as the propagation of a spontaneous rupture controlled by a realistic prestress distribution with the use of a three dimensional finite difference method the dynamic rupture reproduces the general slip pattern used to compute the initial stress distribution and generates near fault ground motions at the surface similar to observations the simulated rupture propagates on the fault along a complex path with highly variable speed and rise time changing the magnitude and pattern of the stress significantly the method provides the framework to estimate earthquake rupture parameters from recorded seismic and geodetic data

thermal evolution of convergent plate margins thermal modeling and reevaluation of isotopic ar ages for blueschists in the franciscan complex of california modeling of heat flow through a wedge shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of ar isotopic ages and metamorphic progressions across blueschist belts if convergence rates are maintained at rates of a few centimeters per year or more material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where ar loss by diffusion and recrystallization effectively ceases maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider because of this difference in thermal history k ar and 40ar 39ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth application of this thermal modeling to the franciscan subduction complex of northern california indicates that the coherent blueschists in the eastern belt typically with 115 to 125 m y ar isotopic ages and the high grade blueschist blocks in the mud matrix melanges of the western portion of the central belt typically with 140 to 150 m y ar isotopic ages could both have formed during a single prolonged period of continuous convergence

enrichment of trace elements in garnet amphibolites from a paleo subduction zone catalina schist southern california the abundance p t stability solubility and element partitioning behavior of minerals such as rutile garnet sphene apatite zircon zoisite and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones the influence of these minerals on the composition of subduction related magmas has been inferred and disputed from inverse modelling of the geochemistry of island arc basalt or by experiment although direct samples of the dehydration partial melting region of a mature subduction zone have not been reported from subduction complexes garnet amphibolites from melanges of circumpacific and caribbean blueschist terranes reflect high t 600 c conditions in shallower regions such rocks record geochemical processes that affected deep seated high t portions of paleo subduction zones in the catalina schist a subduction zone metamorphic terrane of southern california metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta ultramafic rocks this mafic and ultramafic complex may represent either slab derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab mantle wedge contact or both the trace element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by inaa xrf electron microprobe and sem in order of increasing alteration from a probable metabasalt protolith three common types of garnet amphibolite blocks in the catalina schist are 1 non migmatitic clinopyroxene bearing blocks which are compositionally similar to morb that has lost an albite component 2 garnet amphibolite blocks which have rinds that reflect local interaction between metabasite metaperidotite and fluid and 3 migmatites that are extremely enriched in th hfse lree and other trace elements these trace element enrichments are mineralogically controlled by rutile garnet sphene apatite zircon zoisite and allanite alkali and alkaline earth elements are much less enriched in the solid assemblage and thus appear to be decoupled from the other elements in the inferred metasomatic process es the compositions of migmatitic garnet amphibolite blocks seem to complement that of average island arc tholeiite trace element metasomatism reflects fluid solid rather than melt solid interaction the metasomatic effects indicate that h2o rich fluid perhaps with a significant component of na al silicate and alkalis carried th u sr ree and hfse fractionations of lree in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex exotic lree deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment if the paleo subduction zone represented by the mafic and ultramafic complex of the catalina schist had continued its thermal and fluid evolution a selvage of similarly enriched rocks might have been generated along the slab mantle wedge contact between 30 and 85 km depth rocks affected by subduction zone metasomatism although rarely recognized at the surface could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments

detrital zircon constraints on terrane ages and affinities and timing of orogenic events in the san juan islands and north cascades washington detrital zircon geochronology of this report pertains to cretaceous orogeny in northwest washington an event that involved blueschist metamorphism and emplacement of nappes in the san juan islands northwest cascades thrust system and continental arc magmatism and associated barrovian metamorphism in the neighboring coast plutonic complex structurally low in the thrust system quartzose gneiss of the yellow aster complex yields an entirely precambrian suite of detrital zircons with an age pattern that is similar to that of ordovician miogeoclinal rocks and the outboard yukon tanana yreka and shoo fly terranes elsewhere in the cordillera midway in the nappe pile of the northwest cascades sandstone in the bell pass melange has a zircon age population of 110 ma an age that together with the spectrum of exotic materials associated with the sandstone suggests the melange was a major zone of dislocation during mid cretaceous nappe emplacement at a high level in the thrust system are nappes of the fida

accessory minerals and subduction zone metasomatism a geochemical comparison of two melanges washington and california u s a abstract the ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks which can only be studied indirectly in contrast the role of accessory minerals in lower t and p metasomatic processes within paleo subduction zones can be studied directly in subduction zone metamorphic terranes the gee point iron mountain locality of the shuksan metamorphic suite north cascades washington state is a high t melange of metamafic blocks in a matrix of meta ultramafic rocks this melange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the catalina schist santa catalina island southern california both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction some garnet amphibolite blocks from the gee point iron mountain locality display trace element enrichments similar to those in counterparts from the catalina schist some catalina blocks are highly enriched in th rare earth elements ree the high field strength elements ti nb ta zr and hf hfse u and sr compared to mid ocean ridge basalt morb and to other garnet amphibolite blocks in the same unit textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of th ree and hfse in blocks from both areas the mg rich rinds around blocks and the meta ultramafic matrix from both melanges are highly enriched in a large number of trace elements compared to harzburgites dunites and serpentinites evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace element enrichments data from the gee point and catalina melanges suggest that the accessory minerals titanite rutile apatite zircon and ree rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction related fluid rock interaction mobilization of incompatible elements and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid rich metamorphic environments in subduction zones

chromium isotope anomaly in an impactite sample from the el gygytgyn structure russia evidence for a ureilite projectile the 3 6 ma 18 km diameter el gygytgyn impact structure arctic russia is unique among the currently known terrestrial impact craters in that it is the only one that was formed in acid volcanic rocks previous analyses of impactites from el gygytgyn showed minor enrichments of the siderophile elements including ir which together with distinct cr enrichments gave rise to speculation that an achondritic projectile was involved we studied the major and trace element composition in samples from the new icdp drill core obtained near the center of the structure as well as the chromium isotopic composition of an impact glass sample collected on the surface several suevitic breccias from the upper part of the suevite sequence in the drill core show higher cr and ni contents compared with felsic volcanic rocks in the lower part of the core and from surface samples however it is difficult to unambiguously establish a meteoritic component from trace element data as input from rare mafic target rocks is a possibility in contrast the cr isotopic composition of the impact glass sample yielded a nonterrestrial e 54 cr value of 0 72 0 31 2 std err this negative e 54 cr is different from known carbonaceous chondrite values e 54 cr of 0 95 to 1 65 but is nearly identical to reported values for ureilites approximately 0 77 the value is however also within analytical error of eucrites approximately 0 38 and ordinary chondrites approximately 0 42 given the chemical signatures found in previous analyses of el gytgytgyn impactites and the similarity of our cr isotopic data to ureilites we suggest that the impacting asteroid could have been an f type asteroid of mixed composition similar to the recent almahata sitta fall in sudan

dynamic faulting studied by a finite difference method we have developed a finite difference method that is especially adapted to the study of dynamic shear cracks we studied a number of simple earthquake source models in two and three dimensions with special emphasis on the modeling of the stress field we compared our numerical results for semi infinite and self similar shear cracks with the few exact solutions that are available in the literature we then studied spontaneous rupture propagation with the help of a maximum stress criterion from dimensional arguments and a few simple examples we showed that the maximum stress criterion depended on the physical dimensions of the fault for a given maximum stress intensity the finer the numerical mesh the higher the maximum stress that had to be adopted a study of in plane cracks showed that at high rupture velocities the numerical results did not resolve the stress concentration due to the rupture front from the stress peak associated with the shear wave propagating in front of the crack we suggest that this is the reason why transonic rupture velocities are found in the numerical solutions of in plane faulting when the rupture resistance is rather low finally we studied the spontaneous propagation of an initially circular rupture two distinct modes of nucleation of the rupture were studied in the first a plane circular shear crack was formed instantaneously in a uniformly prestressed medium after a while once stress concentrations had developed around the crack edge the rupture started to grow in the second type of nucleation a preexisting circular crack became unstable at time t 0 and started to grow the latter model appeared to us as a more realistic simulation of earthquake triggering in this case the initial stress was nonuniform and was the static field of the preexisting fault

physical properties of the drill core from the el gygytgyn impact structure ne russia the el gygytgyn impact structure in northeast russia was drilled in 2008 2009 the 3 5 ma old structure has a rim to rim diameter of 18 km and is the only known impact structure that has been formed on a siliceous volcanic target the petrophysical rock and paleomagnetic properties including attempted reorientation of samples along the el gygytgyn drill core were analyzed physical properties such as bulk density porosity seismic velocity and electrical conductivity clearly showed the propagation of shock and the associated fracturing the grain density however was probably influenced by the postimpact hydrothermal activity and or the distribution of impact melt the highest values of electrical conductivity coincided with higher concentrations of particular metals as indicated by raschke et al 2012a the rock and paleomagnetic investigations showed iron titanium oxides with varying oxidation reduction states as the main magnetic fraction in the core samples and indicated them as carriers for remanent magnetization with few exceptions most samples showed normal polarity of characteristic remanent magnetization and confirmed that the impact occurred after the gauss gilbert approximately 3 596 ma reversal shallower inclinations than that expected for a 3 5 ma dipole field were probably due to impact related block movements and or compaction

three dimensional nonplanar simulation of the 1992 landers earthquake 1 we have simulated the rupture process of the 1992 landers california earthquake using a realistic nonplanar fault geometry including fault bends and branches through a three dimensional boundary integral equation method considering an external loading system due to local tectonics as well as a nonplanar fault structure we could reproduce the characteristic features of this earthquake such as spontaneous rupture transfer between faults pattern of slip distribution and propagation of rupture front without assuming any laterally heterogeneous distribution of parameters we could also reproduce seismic waveforms near the faults especially in the forward direction of rupture propagation where rupture directivity controlled by rupture transfer between faults significantly affects ground motion

petrography and geochemistry of impactites and volcanic bedrock in the icdp drill core d1c from lake el gygytgyn ne russia the 3 6 ma old and 18 km diameter el gygytgyn impact structure in ne siberia was drilled in 2008 09 by icdp international continental scientific drilling program a 517 m long core hole d1c was drilled into the outer flank of the central uplift structure with an overall core recovery of approximately 63 thereby approximately 315 m lake sediments and approximately 202 m impactites were recovered here we present a detailed petrographic and geochemical assessment of the impact breccia and bedrock sections in this core the 97 m long lower bedrock unit 517 420 m below lake floor blf consists of an ignimbrite in the overlying upper bedrock unit 420 390 mblf the core recovered a sequence of similar ignimbrite and several decimeters of mafic rocks we interpret these units as rocks that are located close to their former preimpact position but have been somewhat rotated due to collapse of the central uplift i e it represents parautochthonous basement from about 390 to 328 mblf occurs a suevite package with an impact melt poor clast dominated matrix and lithic and mineral clasts that cover the entire range of volcanic target rocks known from the el gygytgyn region all stages of shock metamorphism unshocked to melted were observed in clasts and in microclasts of the matrix of suevite from different depths immediately below this package at the contact to the underlying bedrock occurs a 1 m wide sheared zone within vitrophyric ignimbrite which we consider the actual crater floor the uppermost approximately 12 m from 328 316 mblf depth seem to comprise reworked suevite consisting of a mixture of sediments and suevite with more and on average stronger shocked minerals than found in the main suevite unit this includes a small component of glassy spherules and impact melt fragments toward the top of this unit lake sediments progressively become the dominant material in this section we assume that this unit contains a fallback component from the ejecta plume that was mixed with the first sediments of the postimpact crater lake and possibly some rocks that slumped off the inner crater wall similar to a thin layer at the base of the sediment section of borehole lb 5a recovered in lake bosumtwi ghana

blueschist facies metamorphism shear heating and p t t paths in subduction shear zones the low temperatures recorded by blueschist facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15 50 km in this paper steady state and transient pressure temperature time p t t paths followed by subducted material are calculated using analytical expressions presented by molnar and england 1990 supplemented by two dimensional numerical calculations in the absence of shear heating calculated steady state p t t paths are very cold intersecting 1 gpa 10 kbar at t 200 c for a wide range of subduction parameters for the case of constant shear stress in the subduction shear zone and typical thermal parameters steady state p t t paths intersect the blueschist facies for 10 60 mpa and 0 100 mpa for convergence rates of 10 and 3 cm yr 1 respectively blueschist facies metamorphism will not occur in rapid subduction zones if shear stresses exceed 60 mpa for the case of shear stress increasing linearly with depth steady state p t t paths intersect the blueschist facies when 1 9 p and 0 14 p for convergence rates of 10 and 3 cm yr 1 respectively subduction zone shear stresses may be considerably lower and possibly zero if blueschist terrains represent material metamorphosed during the early stages of subduction first 5 10 m y while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation subduction zone shear stresses of several tens of mpa at 35 km depth or a few percent of lithostatic pressure are consistent with a steady state model of franciscan metamorphism a progressive underplating model for the formation of the pelona schist inverted metamorphic gradient and surface heat flow measurements in the ne japan subduction zone

the moho in subduction zones abstract the moho in subduction zones exists in two distinct forms one associated with the subducting oceanic plate and second with the overriding plate the seismic expression of both forms is linked to the nature of a landward dipping low velocity zone lvz that has been detected in a majority of subduction zones about the globe and that approximately coincides with wadati benioff seismicity we review seismic studies that constrain the properties of the lvz in cascadia where it has been extensively studied for over a quarter century a model in which the lvz is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal layer 2 is consistent with available geological and geophysical data and reconciles previously conflicting interpretations in this model the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic layer 3 below the resulting seismic velocity contrast approaching 50 for s waves significantly overwhelms that of a weaker underlying oceanic moho at greater depths oceanic crust undergoes eclogitization in a top down sense leading to gradual disappearance of the lvz the large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled forearc mantle wedge pervasive serpentinization and free fluids reduce velocities within the wedge thereby diminishing erasing or even inverting the seismic contrast associated with the moho of the overriding plate this model is tested against observations of lvzs and forearc mantle structure worldwide

magnetostratigraphic results from impact crater lake el gygytgyn northeastern siberia a 300 kyr long high resolution terrestrial palaeoclimatic record from the arctic summary a 12 7 m long sedimentary record recovered from lake el gygytgyn located in a meteorite impact crater created 3 6 ma in late cretaceous igneous rocks on chukotka peninsula northeast siberia has been analysed for its palaeo and rock magnetic properties continuous high resolution 1 mm measurements of magnetic susceptibility yielded successions of pronounced lows and highs analyses of the rock magnetic properties by low and high temperature runs of magnetic susceptibility determination of hysteresis parameters as well as irm acquisition experiments yielded a dominance of psd pseudo single domain magnetite in intervals of high magnetic susceptibility whereas due to selective magnetite dissolution associated with anoxic lake water and or pore water conditions during times of enhanced deposition of organic matter haematite dominates within low susceptibility intervals in terms of mass percentage here however magnetic properties are still dominated by magnetite five ams accelerator mass spectromentry 14 c ages eight irsl infrared stimulated luminescense ages together with preliminary pollen data suggest that variations in magnetite content reflect climatic variability of the last 300 ka with low high susceptibilities representing cold warm climates this pattern caused by a complex system of deposition preservation or decomposition of organic matter and or magnetic minerals in turn can be correlated in detail to global climate archives such as the oxygen isotope records from greenland ice cores and marine sediments respectively thus the sedimentary sequence recovered from lake el gygytgyn represents the longest continuous terrestrial climate record now available from the arctic

el gygytgyn impact crater russia structure tectonics and morphology the 3 6 myr old el gygytgyn impact crater is located in central chukotka northeastern russia the crater is a well preserved impact structure with an inner basin about 15 km in diameter surrounded by an uplifted rim about 18 km in diameter the flat floor of the crater is in part occupied by lake el gygytgyn 12 km in diameter and surrounding terraces the average profile of the rim is asymmetric with a steep inner wall and a gentle outer flank the rim height is about 180 m above the lake level and 140 m above the surrounding area an outer ring feature on average 14 m high occurs at about 1 75 crater radii from the center of the structure el gygytgyn crater is surrounded by a complex network of faults the density of the faults decreases from the bottom of the rim to the rim crest and outside the crater to a distance of about 2 7 crater radii lake el gygytgyn is surrounded by a number of lacustrine terraces only minor remnants are preserved of the highest terraces 80 and 60 m above the present day lake level the widest of the terraces is 40 m above the current lake level and surrounds the lake on the west and northwest sides the only outlet of the lake is the enmivaam river which cuts through the crater rim in the southeast in terms of structure el gygytgyn is well preserved and displays some interesting but not well understood features e g an outer ring similar to those observed at a few other impact structures

strontium and neodymium isotope systematics of target rocks and impactites from the el gygytgyn impact structure linking impactites and target rocks the 3 6 ma el gygytgyn structure located in northeastern russia on the chukotka peninsula is an 18 km diameter complex impact structure the bedrock is formed by mostly high silica volcanic rocks of the 87 ma old okhotsk chukotka volcanic belt ocvb volcanic target rocks and impact glasses collected on the surface as well as drill core samples of bedrock and impact breccias have been investigated by thermal ionization mass spectrometry tims to obtain new insights into the relationships between these lithologies in terms of nd and sr isotope systematics major and trace element data for impact glasses are added to compare with the composition of target rocks and drill core samples sr isotope data are useful tracers of alteration processes and nd isotopes reveal characteristics of the magmatic sources of the target rocks impact breccias and impact glasses there are three types of target rocks mapped on the surface mafic volcanics dacitic tuff and lava of the koekvun formation and dacitic to rhyolitic ignimbrite of the pykarvaam formation the latter represents the main contributor to the impact rocks the drill core is divided into a suevite and a bedrock section by the sr isotope data for which different postimpact alteration regimes have been detected impact glasses from the present day surface did not suffer postimpact hydrothermal alteration and their data indicate a coherent alteration trend in terms of sr isotopes with the target rocks from the surface surprisingly the target rocks do not show isotopic coherence with the central chukotka segment of the ocvb or with the berlozhya magmatic assemblage bma a late jurassic felsic volcanic suite that crops out in the eastern part of the central chukotka segment of the ocvb however concordance for these rocks exists with the okhotsk segment of the ocvb this finding argues for variable source magmas having contributed to the build up of the ocvb

mesozoic evolution of exotic terranes and marginal seas western north america abstract regional terrane analysis has been combined with a global evaluation of plate kinematics to produce a new tectonic model for the mesozoic evolution of western north america and its associated marginal seas the model employs a two tiered data reliability ranking system to resolve conflicts within data sets the lower tier of the ranking system involves the assignment of a numerical rank to paleomagnetic and or paleobiogeographic data based on each data set s reliability the upper tier of the ranking system places the paleomagnetic and paleobiogeographic data in perspective by assigning a relative order of importance to different types of data the most reliable data are considered to be departure rift and arrival collision times that are tightly constrained by independent data sets evidence of subduction and or strike slip motion also ranks high in the master ranking system paleomagnetic and vertebrate paleobiogeographic data come next in the hierarchy followed by invertebrate and floral paleobiogeographic data application of this approach to a case study the baja british columbia controversy has resulted in a coherent model for the entire region results derived from the application of this modelling approach to the problem of the mesozoic assembly of western north america suggest that its evolution may be modeled using a minimum of 57 large terranes oblique convergence may have been the dominant tectonic process along the western north american margin during the mesozoic causing large scale latitudinal transport of many exotic terranes the motions of the terranes produced a number of marginal seas yukonia etc that were short lived but important components of western margin tectonics a combination of collision events side swiping episodes and changes in plate kinematics led to the initiation of several regional tectonic upheavals including the nevadan and sevier orogenies

evolution of the blueschist and greenschist facies rocks of sifnos cyclades greece the metamorphism on the island of sifnos is characterized by the eocene development of a coherent highpressure blueschist terrane and an early miocene greenschist facies overprint this study documents the metamorphic evolution of the blueschist assemblages still preserved in the northern parts of the island and their subsequent transformation into greenschists in the central and southern parts the oxygen isotope geothermometry is based on calibrations for quartz pyroxenes and magnetite matthews et al 1983a augmented by revised calibrations for the minerals muscovite qz mu 1 55 106 t 2 epidote qz ep 1 56 1 92 ps 106 t 2 and rutile qz ru 4 54 106 t 2 oxygen isotope analyses of minerals from the blueschist unit of northern sifnos give consistent fractionations which are independent of rock type an average temperature of 455 c was obtained although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred the minerals and whole rocks of the greenschists of central sifnos are systematically enriched in 18o relative to the blueschist assemblages chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of ca2 and co2 the petrologic isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18o co2 enriched aqueous solutions it is reasonable to assume that this is connected with the miocene magmatic activity observed throughout the cyclades the marbles separating the blueschist from the greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation the pressure of the blueschist metamorphism is estimated at 14 2 kbar corresponding to a depth of ca 50 km the structural style and stratigraphy of sifnos are suggestive of the subduction of a continental margin sequence it is clear that the considerable tectonic depression may be associated with continental collision and underthrusting

breaking of a single asperity rupture process and seismic radiation the problem of spontaneous shear rupture of a single circular asperity on an infinite fault plane is studied initially the fault plane is broken everywhere except at a circular asperity an applied displacement at infinity results in a stress concentration along the bounding edge of the asperity the frictional stress on the broken part of the fault plane is taken to be a constant once a point on the asperity breaks the stress there drops to the same value as on the main fault surface the rupture is started by relaxing the shear stress at a point on the asperity edge and is then allowed to propagate spontaneously using a critical stress level fracture criterion the rupture process is calculated numerically it is found that for asperities of constant strength the rupture first propagates around the edge of the asperity and then inward a phenomenon best described by the well known term of classic military maneuver the double encircling pincer movement in the appendix the expressions for the far field seismic radiation due to the rupture of such an asperity are derived it is shown that the nth cartesian component of the far field displacement at x t for p sv and sh waves using the notation of aki and richards 1980 is given by un x t dni 4 c2r s0 i3 t r c ds thus the far field pulses can be directly found from the stress drops on the fault plane this formula is also true for crack or dislocation problems the directivity function dni for displacement for the asperity problem is found to be that for the double couple modified by some factor in particular the fault plane is a nodal plane for sv waves for the rupturing of asperities on a finite fault these directivity functions are applicable only to the initial part of observed pulses at a receiver provided the receiver is not located on the fault plane outside the broken part of the main crack edge in which case it is inapplicable for all time

dynamic faulting under rate dependent friction we discuss the effects of rate dependent friction on the propagation of seismic rupture on active faults several physicists using burridge and knopoff s box and spring model of faulting have proposed that fault complexity may arise from the spontaneous development of a self similar stress distribution on the fault plane if this model proves to be correct it has important consequences for the origin of the complexity of seismic sources in order to test these ideas on a more realistic earthquake model we developed a new boundary integral equation method for studying rupture propagation along an antiplane fault in the presence of nonlinear rate dependent friction we study rupture dynamics of models with single and twin asperities in our models asperities are places on the fault with a higher value of prestress othewise all fault parameters are homogeneous we show that for models with such asperities a slip velocity weakening friction leads to the propagation of supersonic healing phases and to the spontaneous arrest of fracture if the prestress outside the asperities is low enough for models with asperities we can also observe narrow slip velocity pulses qualitatively similar to the so called heaton pulses observed in some earthquake accelerograms we also observe a complex distribution of stress after the rupture that depends on details of the initial distribution of asperities and on the details of the friction law

evaluating the influence of meteorite impact events on global potassium feldspar availability to the atmosphere since 600 ma potassium feldspar present in global mineral aerosol 30 by combining crater size and tectonic reconstructions we are able to provide a quantitative and self consistent assessment of changes to global potassium feldspar availability considerable differences in potassium feldspar availability following meteorite impact events are revealed different impact events generated dust containing different amounts of potassium feldspar differing levels of influence upon climate are hypothesized and should now be tested by looking at stratigraphic records of these events to reveal the sensitivity of climate to different dust mineralogy supplementary material figures showing palaeolatitude determinations using various methods and reconstruction models and estimates of alkali feldspar abundance and distribution across contemporary continental landmasses for meteorite impact events are available at https doi org 10 6084 m9 figshare c 4253312

petrologic structural and age relations of serpentinite amphibolite and blueschist in the shuksan suite of the iron mountain gee point area north cascades washington part of the shuksan blueschist terrane near iron mountain and gee point north cascades washington has associated serpentinite amphibolite barroisite schist blueschist rare eclogite and blackwall type metasomatic rock field petrographic and microprobe observations indicate that the amphibolite and barroisite schists were metamorphosed in contact with peridotite and suggest that the peridotite may have been a heat source the serpentinite and associated rocks are structurally concordant with the regional blueschists and have been overprinted by blueschist metamorphism isotopic dating gives metamorphic ages of 148 5 to 164 6 m y for the amphibolite and barroisite schist and 129 5 m y for nearby regional shuksan blueschists the origin of the serpentinite amphibolite blueschist assemblage is interpreted to be the result of sequential events in a subduction zone as subduction began oceanic crustal materials underwent high temperature metamorphism along the hot ultramafic hanging wall and were converted to amphibolites materials that were subducted later came in contact with a cooler hanging wall and recrystallized as blueschist this hypothesis may be applicable to the origin of similar rock associations in the franciscan terrane and other orogenic belts

counterclockwise p t t trajectory from the metamorphic sole of a neo tethyan ophiolite turkey abstract the kiziltepe ophiolitic thrust sheet in the bolkar mountains of turkey occurs between two subparallel ophiolite belts bounding the tauride carbonate platform and represents a remnant of the cretaceous neo tethyan oceanic lithosphere it is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra oceanic subduction zone in the neo tethys 92 90 ma blueschist facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing p t with cooling regional tectonic constraints suggest a maastrichtian age for the timing of this blueschist facies metamorphism sodic amphibole rich veins and crossite mg riebeckite rims on hornblende suggest that growth of blueschist facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries we infer a counterclockwise p t t trajectory during which metamorphism was accompanied succeeded by rapid uplift along the northern edge of the tauride belt in late cretaceous early tertiary time

the sudbury impact layer in the paleoproterozoic iron ranges of northern michigan usa a layer of breccia that contains fragments of impact ejecta has been found at 10 sites in the paleoproterozoic iron ranges of northern michigan in the lake superior region of the united states radiometric age constraints from events predating and postdating depo si tion of the breccia are ca 1875 ma and 1830 ma the major bolide impact that occurred at 1850 ma at sudbury ontario 500 700 km east of these sites is the likely causative event the michigan sites described here along with previously described sites in minnesota and ontario defi ne an extensive ejecta bearing deposit throughout the paleoproterozoic iron ranges of the lake superior region that we refer to as the sudbury impact layer the layer at the sites in michigan exhibits a range of thicknesses lithologic characters and sedimentary settings the diversity of rock types and internal stratigraphic details of the layer imply that several different processes of transport and deposition are represented but the detailed investigations needed to document them are incomplete many of the sites had been described and interpreted previously as products of common terrestrial processes but the presence of relict shock induced planar deformation features in quartz indicates that the breccia layer is in fact the product of an extra terrestrial impact at most localities this layer also contains relict fragments of altered devitrifi ed glass and or accretionary lapilli one immediate use of the impact layer is as an ultraprecise time line that ties together the well known stratigraphic sequences of the various geographically separated iron ranges the correlation of which has remained controversial for many decades the sudbury impact layer most commonly lies at a horizon that records a signifi cant change in the character of sediments across the region the impact layer marks the end of a major period of banded iron formation deposition that was succeeded by deposition of ficlastic rocks commonly black shales the impact may have produced regional if not global changes in the environment that resulted in this widespread synchronous change in sedimentation style

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

nature of the low velocity zone in cascadia from receiver function waveform inversion abstract a dipping low velocity zone lvz is a ubiquitous structural element of subduction zones worldwide in this study we map seismic attributes characterizing the lvz beneath the cascadia subduction zone from northern vancouver island to northern california using receiver function waveform inversion throughout this region the lvz is characterized by high v p v s ratios mean 2 77 strong s velocity contrasts 50 and thicknesses averaging 3 37 km the lvz is immediately underlain by a second weaker layer exhibiting moderate v p v s ratios mean 1 85 with mean thickness of 4 62 km we interpret the combined structure in terms of subducting oceanic crust based on classical structural petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores the lvz is identified with pervasively hydrated high porosity pillow basalts and sheeted dikes of layer 2 with possible contributions from sediments layer 1 fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above and a low porosity gabbroic mafic cumulate dominated layer 3 below

central ring structure identified in one of the world s best preserved impact craters seismic refraction and reflection data were acquired in 2000 and 2003 to study the morphology and sedimentary fill of the remote el gygytgyn crater chukotka northeastern siberia diameter 18 km these data allow a first insight into the deeper structure of this unique impact crater wide angle data from sonobuoys reveal a five layer model a water layer two lacustrine sedimentary units that fill a bowl shaped apparent crater morphology consisting of an upper layer of fallback breccia with p wave velocities of 3000 m s and a lower layer of brecciated bedrock velocities 3600 m s the lowermost layer shows a distinct anticline structure that by analogy with other terrestrial and lunar craters of similar size can be interpreted as a central ring structure the el gygytgyn crater exhibits a well expressed morphology that is typical of craters formed in crystalline target rocks

crystallographic preferred orientation of talc determined by an improved ebsd procedure for sheet silicates implications for anisotropy at the slab mantle interface due to si metasomatism

three dimensional simulation of a magnitude 7 75 earthquake on the san andreas fault simulation of 2 minutes of long period ground motion in the los angeles area with the use of a three dimensional finite difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7 75 earthquake along the 170 kilometer section of the san andreas fault between tejon pass and san bernardino maximum ground velocities are predicted to occur near the fault 2 5 meters per second and in the los angeles basin 1 4 meters per second where large amplitude surface waves prolong shaking for more than 60 seconds simulated spectral amplitudes for some regions within the los angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the san andreas fault

can alteration experiments on impact melts from el gygytgyn and volcanic glasses shed new light on the formation of the martian surface this investigation involved three specimens an altered felsic sample of impactite from the icdp el gygytgyn drill core d1c and two reference volcanics from iceland namely a rhyolitic glass and a basaltic glass the goal of this work was to better understand the alteration of impact melt and volcanic glass and to apply the results to an investigation of alteration processes below the surface of mars hydrothermal batch alteration experiments with the el gygytgyn sample showed formation of various silica phases such as cristobalite opal and quartz according to geochemical modeling zeolites were also expected but zeolite minerals already present in the impactite prior to the experiment may have masked possible experimental zeolite growth basaltic glass was altered to smectite talc and opal the accompanying numerical modeling gave results that were similar or comparable to the laboratory experiments rhyolitic glass was kinetically more stable than basaltic glass and showed only minor formation of calcite and feldspar during the 3 week experiment the study showed that closed system isochemical alteration of both siliceous and mafic glasses and melts results in the formation of smectites and zeolites therefore to link alteration features on mars to specific physical conditions the exact identity of the mineral phases present in surface rocks must be known moreover our simulations on closed system isochemical alteration showed that the fraction of zeolites and silica formed relative to smectite depends on the source mineral altered therefore fractions of these mineral groups present in the martian soil may be used to better predict source rock characteristics

petrography of impact glasses and melt breccias from the el gygytgyn impact structure russia the el gygytgyn impact structure 18 km in diameter and 3 6 ma old in arctic siberia russia is the only impact structure on earth mostly excavated in acidic volcanic rocks the late cretaceous volcanic target includes lavas tuffs and ignimbrites of rhyolitic dacitic and andesitic composition and local occurrence of basalt although the ejecta blanket around the crater is nearly completely eroded bomb shaped impact glasses redeposited after the impact event occur in lacustrine terraces within the crater here we present detailed petrographic descriptions of newly collected impact glass bearing samples the observed features contribute to constrain the formation of the melt and its cooling history within the framework of the impact process the collected samples can be grouped into two types characterized by specific features 1 pure glasses containing very few clasts or new crystals and which were likely formed during the early stages of cratering and 2 a second type which represents composite samples with impact melt breccia lenses embedded in silicate glass these mixed samples probably resulted from inclusion of unmelted impact debris during ejection and deposition after deposition the glassy portions continued to deform whereas the impact melt breccia inclusions that probably had already cooled down behaved as rigid bodies in the flow

cathodoluminescence as a tool to discriminate impact melt shocked and unshocked volcanics a case study of samples from the el gygytgyn impact structure el gygytgyn chukotka arctic russia is a well preserved impact structure mostly excavated in siliceous volcanic rocks for this reason the el gygytgyn structure has been investigated in recent years and drilled in 2009 in the framework of an icdp international continental scientific drilling program project the target rocks mostly consist of rhyodacitic ignimbrites and tuffs which make it difficult to distinguish impact melt clasts from fragments of unshocked target rock within the impact breccia several chemical and petrologic attempts other than dating individual clasts have been considered to distinguish impact melt from unshocked volcanic rock of the targets but none has proven reliable here we propose to use cathodoluminescence imaging and spectrometry whose intensity is inversely correlated with the degree of shock metamorphism experienced by the investigated lithology to aid in such a distinction specifically impact melt rocks display low cathodoluminescence intensity whereas unshocked volcanic rocks from the area typically show high luminescence this high luminescence decreases with the degree of shock experienced by the individual clasts in the impact breccia down to almost undetectable when the groundmass is completely molten this might apply only to el gygytgyn because the luminescence in volcanic rocks might be due to devitrification and recrystallization processes of the relatively old cretaceous target rock with respect to the young impactites 3 58 ma the alteration that affects most samples from the drill core does not have a significant effect on the cathodoluminescence response in conclusion cathodoluminescence imaging and spectra supported by raman spectroscopy potentially provide a useful tool for in situ characterization of siliceous impactites formed in volcanic target

an unusual occurrence of coesite at the lonar crater india coesite has been identified within ejected blocks of shocked basalt at lonar crater india this is the first report of coesite from the lonar crater coesite occurs within sio2 glass as distinct 30 m spherical aggregates of granular coesite identifiable both with optical petrography and with micro raman spectroscopy the coesite glass occurs only within former silica amygdules which is also the first report of high pressure polymorphs forming from a shocked secondary mineral detailed petrography and nmr spectroscopy suggest that the coesite crystallized directly from a localized sio2 melt as the result of complex interactions between the shock wave and these vesicle fillings

slip complexity in earthquake fault models we summarize studies of earthquake fault models that give rise to slip complexities like those in natural earthquakes for models of smooth faults between elastically deformable continua it is critical that the friction laws involve a characteristic distance for slip weakening or evolution of surface state that results in a finite nucleation size or coherent slip patch size h models of smooth faults using numerical cell size properly small compared to h show periodic response or complex and apparently chaotic histories of large events but have not been found to show small event complexity like the self similar power law gutenberg richter frequency size statistics this conclusion is supported in the present paper by fully inertial elastodynamic modeling of earthquake sequences in contrast some models of locally heterogeneous faults with quasi independent fault segments represented approximately by simulations with cell size larger than h so that the model becomes inherently discrete do show small event complexity of the gutenberg richter type models based on classical friction laws without a weakening length scale or for which the numerical procedure imposes an abrupt strength drop at the onset of slip have h 0 and hence always fall into the inherently discrete class we suggest that the small event complexity that some such models show will not survive regularization of the constitutive description by inclusion of an appropriate length scale leading to a finite h and a corresponding reduction of numerical grid size

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

modeling dynamic rupture in a 3d earthquake fault model we propose a fourth order staggered grid finite difference method to study dynamic faulting in three dimensions the method uses an implementation of the boundary conditions on the fault that allows the use of general friction models including slip weakening and rate dependence because the staggered grid method defines stresses and particle velocities at different grid points we preserve symmetry by implementing a two grid row thick fault zone slip is computed between points located at the borders of the fault zone while the two components of shear traction on the fault are forced to be symmetric inside the fault zone we study the properties of the numerical method comparing our simulations with well known properties of seismic ruptures in 3d among the properties that are well modeled by our method are full elastic wave interactions frictional instability rupture initiation from a finite initial patch spontaneous rupture growth at subsonic and supersonic speeds as well as healing by either stopping phases or rate dependent friction we use this method for simulating spontaneous rupture propagation along an arbitrarily loaded planar fault starting from a localized asperity on circular and rectangular faults the shape of the rupture front is close to elliptical and is systematically elongated in the in plane direction of traction drop this elongation is due to the presence of a strong shear stress peak that moves ahead of the rupture in the in plane direction at high initial stresses the rupture front becomes unstable and jumps to super shear speeds in the direction of in plane shear another interesting effect is the development of relatively narrow rupture fronts due to the presence of rate weakening friction the solutions for the thick fault boundary conditions scale with the slip weakening distance do and are stable and reproducible for do greater than about 4 in terms of 2t t ax finally a comparison of scalar and vector boundary conditions for the friction shows that slip is dominant along the direction of the prestress with the largest deviations in slip rate direction occurring near the rupture front and the edges of the fault

revised ages of blueschist metamorphism and the youngest pre thrusting rocks in the san juan islands washington new ages of rocks in the san juan islands northwest washington significantly change our understanding of the evolution of the san juan islands thrust system re examination of foraminifera bearing mudstones at richardson on lopez island indicates a late aptian 112 115 ma not late albian 100 ma age as currently presented in the literature the age brackets of thrusting marked by these pre thrusting mudstones and 84 ma post thrusting sedimentary rocks span a much longer period than previously thought diminishing controls on rates of displacement in the thrust system and the timing of regional deformation in western washington new 40 ar 39 ar plateau ages of phengite in blueschist facies meta volcanic rock also at richardson are 124 0 7 ma 2 late barremian these blueschist facies volcanic rocks are in fault contact with the fossiliferous mudstones therefore the blueschist facies metamorphism at richardson previously inferred to be associated with the thrusting now appears to have occurred prior to thrusting further the ar ages demonstrate that blueschist facies fabric formed earlier than the thrust event and is therefore not directly useful in analyzing the thrusting kinematics the richardson 40 ar 39 ar age is similar to isotopic ages found in the eastern san juan islands and in the shuksan blueschist terrane in the northwest cascades and thus fits into an emerging regional age pattern of blueschist facies metamorphism during late jurassic early cretaceous up to barremian but not late albian cenomanian if this pattern is more broadly confirmed for the san juan islands all the blueschist facies metamorphism can be regarded as having formed in subduction zones elsewhere along the continental margin rather than in the anomalous setting of an on land thrust system as in the san juan islands resume de nouveaux ges pour les roches des iles san juan dans le nord ouest de l etat de washington changent profondement notre comprehension de l evolution du systeme de chevauchement san juan un reexamen des mudstones foraminiferes a richardson sur l ile lopez indique un ge aptien tardif 112 115 ma et non albien tardif 100 ma tel qu indique actuellement dans la litterature les plages d ges pour le chevauchement marquees par ces mudstones precedant le chevauchement et des roches sedimentaires 84 ma posterieures au chevauchement couvrent une periode beaucoup plus grande que stipulee anterieurement diminuant les controles sur les taux de deplacement dans le systeme de chevauchement et le moment de la deformation regionale dans l ouest de l etat de washington de nouveaux ges plateaux 40 ar 39 ar de la phengite dans des roches volcaniques au facies des schistes bleus aussi a richardson datent

roter kamm impact crater namibia geochemistry of basement rocks and breccias abstract the roter kamm crater in the southern namib desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts to better define the variety of target rocks and breccias we studied the petrography and chemical composition of a new suite of twenty eight basement and breccia samples from the roter kamm crater based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two layer target region an upper layer of gariep metasediments schist marble quartzite and sandstone overlying the crystalline basement of the namaqualand metamorphic complex the basement was also heavily intruded by coarse grained quartz veins and quartz and quartz feldspar pegmatites the clast population in the melt breccias indicates that impact induced melting involved mainly metasedimentary target rocks with rarely detected contributions from pegmatite and granite granodiorite three varieties of melt breccias can be defined 1 schistose 2 quartzitic melt breccias 3 true impact melt breccias these melt breccia types are chemically heterogeneous and even the impact melt breccias may have been produced in situ and not from a coherent melt body the shapes of the schistose melt breccias previously thought to be ejected impact breccias are most likely caused by erosion and these breccias are now interpreted to be locally derived the crater basement as exposed at the rim was structurally severely affected and at least locally considerable thermal energy was generated during formation of large volumes of cataclastic mylonitic and pseudotachylitic breccias analyses of mylonite and pseudotachylites from the crater rim as well as their respective host rocks show that these breccias were mainly formed from local material analyses of pseudotachylite like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar similar to their tectonically produced counterparts although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated several breccias compared to their host rocks do show evidence of severe chemical alteration chloritisation and sericitisation the presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact induced hydrothermal event in the crater area

lithostratigraphy of the impactite and bedrock section of icdp drill core d1c from the el gygytgyn impact crater russia in 2008 2009 the international continental scientific drilling program icdp obtained drill cores from the el gygytgyn impact structure located on the chukotka peninsula russia these cores provide the most complete geological section ever obtained from an impact structure in siliceous volcanic rock the lithostratigraphy comprises a thick sequence of lacustrine sediments overlying impact breccias and deformed target rock the interval from 316 m below lake floor blf to the end of the core at 517 m depth can be subdivided into four lithological sequences at 316 m depth the first mesoscopic clasts of shocked target rock occur in lacustrine sediments the growing abundance of target rock clasts with increasing depth and corresponding decrease of lacustrine sediment components indicate the extent of this transition zone to 328 m depth it constitutes a zone of mixed reworked impact breccia and lacustrine sediments volcanic clasts in this reworked suevite section show all stages of shock metamorphism up to melting the underlying unit 328 390 m depth represents a suevite package a polymict impact breccia with considerable evidence of shock deformation in a wide variety of volcanic clasts this includes fragments with quartz that exhibit planar fractures and planar deformation features pdf in addition at three depths several centimeter sized clasts with shatter cones were detected due to microanalytical identification of relatively rare microscopic impact melt particles in the matrix of this breccia this material can be confidently labeled a suevite also in this sequence three unshocked 1 m thick intersections of volcanic blocks occur at 333 83 351 52 and 383 00 m depths the upper bedrock unit begins at 390 74 m depth has a thickness of 30 15 m and represents a sequence of different volcanic rocks an upper part with basaltic composition from 390 74 to 391 79 m depth overlying a lower rhyodacitic part from 391 79 to 420 27 m depth this parautochthonous basement unit is only very weakly affected by the impact only one shocked quartz grain with two sets of pdf was recorded at 391 33 m depth the lower bedrock unit 420 89 517 09 m depth end of core is a brittly deformed rather homogeneous welded ignimbrite that in part can be considered a cataclasite the top three meters of this section are sheared which could represent pre impact tectonic deformation a 54 cm thick injection of polymict impact breccia occurs at 471 42 471 96 m depth

correlation of early cretaceous blueschists in washington oregon and northern california the protolith and metamorphic histories of early cretaceous blueschists that occur in washington oregon and california are remarkably similar these blueschists are the shuksan metamorphic suite of northwestern washington the condrey mountain schist of northern california and southern oregon and the pickett peak terrane of northern california and southwestern oregon we suggest that these bodies were coextensive during their formation dispersion of the blueschists occurred in late cretaceous time by lateral translation of up to 900 km along the continental margin in response to oblique plate convergence

correlation of the tonga formation and the chiwaukum schist north cascades washington implications for late cretaceous orogenic mechanisms the tonga formation of the north cascades washington consists of weakly deformed bedded feldspathic graywacke and shale metamorphosed at chlorite to staurolite grade analysis of protolith lithology rb sr isotopic signature and plutonic metamorphic and deformational evolution in the tonga formation suggests that this unit is correlative with the chiwaukum schist of the cascades crystalline core the tonga formation occurs on the flank of the late cretaceous orogen whereas high grade chiwaukum schist is exposed in the core of the orogen metamorphic pressure as reflected by the distribution of metamorphic minerals and thermobarometry increases continuously northward in the tonga formation from the chlorite zone 4 kbar to the staurolite zone 7 kbar and then to the kyanite zone 7 kbar in the chiwaukum schist the findings reported herein do not support the previously advanced concept that the cascades crystalline core represents the easton blueschist terrane that was converted into higher temperature metamorphic rock by thermal relaxation after cessation of subduction the results of this study suggest a relatively simple two stage metamorphic history for the chiwaukum schist represented by 90 93 ma shallow contact metamorphism overprinted by harrovian metamorphism that increased sharply in pressure from south west to northeast and reached peak conditions after 90 ma all events occurring in a plutonic magmatic arc setting

characterization and morphological reconstruction of the terny impact structure central ukraine the terny impact structure located in central ukraine displays a variety of diagnostic indicators of shock metamorphism including shatter cones planar deformation features in quartz diaplectic glass selective melting of minerals and whole rock melting the structure has been modified by erosion and subsequently buried by recent sediments although there are no natural outcrops of the deformed basement rocks within the area mining exploration has provided surface and subsurface access to the structure exposing impact melt rocks shocked parautochthonous target rocks and allochthonous impact breccias including impact melt bearing breccias similar to suevites observed at the ries structure we have collected and studied samples from surface and subsurface exposures to a depth of approximately 750 m below the surface this analysis indicates the terny crater is centered on geographic coordinates 48 13 n 33 52 e the center location and the distribution of shock pressures constrain the transient crater diameter to be no less than approximately 8 4 km using widely accepted morphometric scaling relations we estimate the pre erosional rim diameter of terny crater to be approximately 16 19 km making it close in original size to the well preserved el gygytgyn crater in siberia comparison with el gygytgyn yields useful insights into the original morphology of the terny crater and indicates that the amount of erosion terny experienced prior to burial probably does not exceed 320 m

arc parallel extension and fluid flow in an ancient accretionary wedge the san juan islands washington structural analysis of the lopez structural complex a major late cretaceous terrane bounding fault zone in the san juan thrust system reveals a sequence of events that provides insight into accretionary wedge mechanics and regional tectonics after formation of regional ductile flattening and shear related fabrics the area was crosscut by brittle structures including 1 southwest vergent thrusts 2 extension veins and normal faults related to northwest southeast extension and 3 conjugate strike slip structures that record northwest southeast extension and northeast southwest shortening aragonite bearing veins are associated with thrust and normal faults but only rarely with strike slip faults high pressure low temperature hp lt minerals constrain the conditions for brittle deformation to 20 km and

geophysical investigation of a suture zone the border ranges fault of southern alaska the border ranges fault separates structurally complex accreted cretaceous rocks from less deformed middle or late paleozoic and younger rocks in the cook shelikof basin of the five types of geophysical data used to investigate this fault gravity data give the clearest indication of its presence and crustal structure for at least 400 km along the fault gravity anomalies include a 20 to 30 mgal peak along the fault s upper plate and a 40 mgal trough along the lower plate the paired anomaly can be modeled satisfactorily by a simple step in a deep dense layer that lies within 3 km of the projected offshore location of the fault relatively low density rocks lie along the fault s lower plate to a depth of about 10 km and the upper part of the fault dips within 20 of vertical satellite altimetry data show that two circular geoid lows lie along the border ranges fault and coincide with lows in free air gravity data seismic refraction and seismic reflection data suggest that the large scale density anomalies that cause both types of lows must lie at depths greater than about 1 km within the margin three regional magnetic anomalies knik arm seldovia and shelikof terminate at the border ranges fault suggesting that the fault truncates obliquely rocks that lie along its northwest side six seismic reflection lines cross the border ranges fault but none of them shows reflections from it the absence of such reflections probably results from the fault s steep dip and from the presence of strong water bottom multiples in the data from the late jurassic until the early late cretaceous the magmatic arc near the cook shelikof basin was inactive and we infer that the predominant motion along the border ranges fault was strike slip resurgent late cretaceous magmatism was contemporaneous with uplift of rocks along the northwest side of the border ranges fault and with deformation of turbidite sequences in the fault s lower plate we propose that during the late cretaceous motion along the border ranges changed from strike slip to reverse cenozoic rocks near the fault show no evidence for post cretaceous fault movement

on the inference of absolute stress levels from seismic radiation abstract spudich p k p 1992 on the inference of absolute stress levels from seismic radiation in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 99 106 this paper determines the conditions under which it is possible to learn the absolute stress level at a subset of points on a fault from observation of the fault kinematics specifically the points on a rupturing fault can be divided into two groups those points at which the rake rotates during the rupture process and those points at which the rake does not rotate if it is true that sliding frictional traction is collinear with the instantaneous velocity of one side of a fault then at the former group of points having rotating rakes there is a unique absolute stress consistent with the motion while at the latter group of points having rakes that do not rotate the absolute stress is unspecified by the motion published dislocation solutions for the 1979 imperial valley 1986 north palm springs 1987 whittier narrows and 1989 loma prieta california earthquakes are examined for evidence of temporal rotations of rake at the hypocenters and largest asperities the 1986 north palm springs earthquake shows evidence of a possible rake rotation at the hypocenter this may imply a low absolute stress level

the 2011 expedition to the el gygytgyn impact structure northeast russia toward a new geological map for the crater area el gygytgyn is a 3 6 ma 18 km diameter impact crater formed in an approximately 88 ma old volcanic target in northeast siberia the structure has been the subject of a recent icdp drilling project in parallel to those efforts a russian german expedition was undertaken in summer 2011 to investigate the permafrost soil lake terraces and the volcanic rocks of the southern and eastern crater rim this provided the unique opportunity for mapping and sampling of the volcanic target rocks around a large part of this complex impact structure samples from 43 outcrops were collected and analyzed petrographically and geochemically the results were combined with earlier mapping outcomes to create a new geological map of this impact structure and its immediate environs at the scale of 1 50 000 compositions of our rock suites are compared with the lithologies of the 2009 icdp drill core the ignimbrite described as lower bedrock in the icdp drill core shows petrographically and chemically strong similarities to the rhyolitic and rhyodacitic ignimbrites observed on surface the suevite sequence exposed in the icdp drill core is a mixture of all observed target rocks at their respective proportions in the area in contrast to previous studies the calculated average target composition of el gygytgyn takes the contribution of the basic target rocks into consideration mafic and intermediate rocks approximately 7 5 and felsic rocks approximately 92 5

fracpaq a matlab toolbox for the quantification of fracture patterns abstract the patterns of fractures in deformed rocks are rarely uniform or random fracture orientations sizes and spatial distributions often exhibit some kind of order in detail relationships may exist among the different fracture attributes e g small fractures dominated by one orientation larger fractures by another these relationships are important because the mechanical e g strength anisotropy and transport e g fluids heat properties of rock depend on these fracture attributes and patterns this paper describes fracpaq a new open source cross platform toolbox to quantify fracture patterns including distributions in fracture attributes and their spatial variation software has been developed to quantify fracture patterns from 2 d digital images such as thin section micrographs geological maps outcrop or aerial photographs or satellite images the toolbox comprises a suite of matlab scripts based on previously published quantitative methods for the analysis of fracture attributes orientations lengths intensity density and connectivity an estimate of permeability in 2 d is made using a parallel plate model the software provides an objective and consistent methodology for quantifying fracture patterns and their variations in 2 d across a wide range of length scales rock types and tectonic settings the implemented methods presented are inherently scale independent and a key task where applicable is analysing and integrating quantitative fracture pattern data from micro to macro scales the toolbox was developed in matlab and the source code is publicly available on github and the mathworks fileexchange the code runs on any computer with matlab installed including pcs with microsoft windows apple macs with mac os x and machines running different flavours of linux the application source code and sample input files are available in open repositories in the hope that other developers and researchers will optimise and extend the functionality for the benefit of the wider community

geochemical studies of impact breccias and country rocks from the el gygytgyn impact structure russia the complex impact structure el gygytgyn age 3 6 ma diameter 18 km in northeastern russia was formed in 88 ma old volcanic target rocks of the ochotsk chukotsky volcanic belt ocvb in 2009 el gygytgyn was the target of a drilling project of the international continental scientific drilling program icdp and in summer 2011 it was investigated further by a russian german expedition drill core material and surface samples including volcanic target rocks and impactites have been investigated by various geochemical techniques in order to improve the record of trace element characteristics for these lithologies and to attempt to detect and constrain a possible meteoritic component the bedrock units of the icdp drill core reflect the felsic volcanics that are predominant in the crater vicinity the overlying suevites comprise a mixture of all currently known target lithologies dominated by felsic rocks but lacking a discernable meteoritic component based on platinum group element abundances the reworked suevite directly overlain by lake sediments is not only comparatively enriched in shocked minerals and impact glass spherules but also contains the highest concentrations of os ir ru and rh compared to other el gygytgyn impactites this is to a lesser extent the result of admixture of a mafic component but more likely the signature of a chondritic meteoritic component however the highly siderophile element contribution from target material akin to the mafic blocks of the icdp drill core to the impactites remains poorly constrained

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

improvement in the fault boundary conditions for a staggered grid finite difference method the staggered grid finite difference method is a powerful tool in seismology and is commonly used to study earthquake source dynamics in the staggered grid finite difference method stress and particle velocity components are calculated at different grid points and a faulting problem is a mixed boundary problem therefore different implementations of fault boundary conditions have been proposed 1982 chose the shear stress grid as the fault surface however this method has several problems 1 fault slip leakage outside the fault and 2 the stress bump beyond the crack tip caused by s waves is not well resolved 1998 solved the latter problem via thick fault implementation but the former problem remains and causes a new issue displacement discontinuity across the slip is not well modeled because of the artificial thickness of the fault in the present study we improve the implementation of the fault boundary conditions in the staggered grid finite difference method by using a fictious surface to satisfy the fault boundary conditions in our implementation velocity or displacement grids are set on the fault plane stress grids are shifted half grid spacing from the fault and stress on the fictitious surface in the rupture zone is given such that the interpolated stress on the fault is equal to the frictional stress within the area which does not rupture stress on the fictitious surface is given a condition of no discontinuity of the velocity or displacement fault normal displacement or velocity is given such that the normal stress on the fault is continuous across the fault artificial viscous damping is introduced on the fault to avoid vibration caused by onset of the slip our implementation has five advantages over previous versions 1 no leakage of the slip prior to rupture and 2 a zero thickness fault 3 stress on the fault is reliably calculated 4 our implementation is suitable for the study of fault constitutive laws as slip is defined as the difference between displacement on the plane of z 0 and that of z 0 and 5 cessation of slip is achieved correctly

coesite in suevites from the chesapeake bay impact structure the occurrence of coesite in suevites from the chesapeake bay impact structure is confirmed within a variety of textural domains in situ by raman spectroscopy for the first time and in mechanically separated grains by x ray diffraction microtextures of coesite identified in situ investigated under transmitted light and by scanning electron microscope reveal coesite as micrometer sized grains 1 3 m within amorphous silica of impact melt clasts and as submicrometer sized grains and polycrystalline aggregates within shocked quartz grains coesite bearing quartz grains are present both idiomorphically with original grain margins intact and as highly strained grains that underwent shock produced plastic deformation coesite commonly occurs in plastically deformed quartz grains within domains that appear brown toasted in transmitted light and rarely within quartz of spheroidal texture the coesite likely developed by a mechanism of solid state transformation from precursor quartz raman spectroscopy also showed a series of unidentified peaks associated with shocked quartz grains that likely represent unidentified silica phases possibly including a moganite like phase that has not previously been associated with coesite

qualitative thermobarometry of inverted metamorphism in the pelona and rand schists southern california using calciferous amphibole in mafic schist abstract the rand and pelona schists consist of eugeoclinal rock types overlain by continental basement along the vincent chocolate mountains vcm faults both schists display inverted metamorphic zonation defined in part by a systematic variation in composition of calcic to sodic calcic amphibole in mafic schist structurally upward the compositional progressions include increase of total a1 a1iv and ti but decrease in the ratios of na na ca to a1 a1 si and nam4 to a1vi fe3 ti these variations imply that structurally high rocks belong to a lower pressure metamorphic fades series than those at depth this result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences amphibole composition is dependent not only on structural position i e p t but also upon bulk rock composition the important controls are whole rock mg mg fe2 mn and fe3 fe2 the greatest impact of these factors however is on the absolute values of na and al rather than their ratio thus interpretation of facies series is not seriously hindered by compositional variability sodic amphibole in epidote blueschists from the rand schist is extensively replaced by sodic calcic amphibole sodic calcic amphibole in the rand schist and pelona schist is itself rimmed by actinolitic amphibole similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation in the rand and pelona schists the sequence probably formed during burial

reconstruction of dynamic rupture process of an earthquake with constraints of kinematic parameters in this paper a simple approximate method that converts the distribution of kinematic rupture parameters on a fault into that of physical or dynamic parameters is proposed the method in which the static stress drop assumed to be equal to the dynamic stress drop is applied to investigate rupture dynamics of the 1979 imperial valley california earthquake m 6 5 the dynamic rupture process is reconstructed by using a propagating crack which is a process of expanding area of stress release if we know the amount of stress drop and rupture time at each point on the fault dynamic rupture process can be simulated we use the kinematic parameters previously obtained by using waveform inversion techniques with constraints of the kinematic parameters the dynamic rupture process of the event can be reconstructed and several dynamic parameters stress drop and strength excess the difference between strength and applied initial stress can be estimated at each point on the fault by applying this method to the 1979 imperial valley earthquake we found that the rupture initiated at the low strength excess part first and then expanded toward the north and ruptured a large size high stress drop region located in the north part of the fault and finally decelerated and stopped at the north edge where the strength is high and stress drop is low these features are well explained by barrier model das and aki 1977 and are quite similar to those obtained by quin in 1990 who investigated the dynamic rupture process by the monte carlo technique with a large number of trials since repeated calculation is unnecessary our method is very effective and practical compared with his method

exhumation of blueschists along a tethyan suture in northwest turkey a blueschist belt formed during the mid cretaceous 88 ma high pressure metamorphism of a passive continental margin sequence of shale siltstone quartzite and limestone occurs immediately south of the tethyan suture in northwest turkey a transect across the tethyan suture was mapped to constrain the exhumation of blueschist facies assemblages the presence of jadeite glaucophane and lawsonite in the blueschists indicates p t conditions of 20 kbar and 430 c the blueschists are tectonically overlain by a cretaceous oceanic accretionary complex showing an incipient blueschist metamorphism a peridotite slab with subvertical layering lies along a low angle fault contact over the blueschists and the accretionary complex isolated slices of early cretaceous 101 4 ma garnet amphibolites occur at the base of the peridotite the garnet amphibolites which show an incipient blueschist facies overprint have probably formed during the oceanic subduction in the foot wall of the subduction zone and later underplated the hanging wall blueschists and peridotites are abruptly truncated in the north by the steeply dipping suture fault that juxtaposes unmetamorphosed and slightly deformed triassic to jurassic sediments of the northern continent against the peridotite and the accretionary complex tectonic juxtaposition of the lower pressure accretionary complex and peridotites over the higher pressure blueschists implies the removal of a 45 km thick oceanic mantle sequence above the blueschists regional geological and geometric constraints suggest that the exhumation of the blueschists was achieved by a combination of two simultaneously acting mechanisms the first was the detachment of the upper crustal blueschist sequence from its basement in the subduction zone and its incorporation to the hanging wall followed by buoyant ascent the second mechanism is the progressive shallowing of the subducted continental lithosphere resulting in the thinning of the mantle wedge above the subducted slab the blueschists were partly exposed prior to the paleocene continent continent collision post collisional eocene 52 48 ma calc alkaline plutons that were emplaced in the blueschists and the overlying peridotite at 10 km depth are interpreted to have formed by crustal anatexis from advective heating by mantle melts these have probably been generated during the upwelling of the asthenosphere under the shallowing continental lithosphere

heterogeneous distribution of dynamic stress drop and relative fault strength recovered from the results of waveform inversion the 1984 morgan hill california earthquake the dynamic rupture process of the 1984 morgan hill california earthquake m 6 2 has been investigated on the basis of a three dimensional 3d dynamic shear crack model from previous waveform inversion results for this purpose a locking fracture criterion is introduced for rupture propagation from which a lower bound of the peak shear stress just before rupturing and hence of the relative fault strength has been estimated at each fault segment the distribution of static and dynamic stress drops has also been independently evaluated from that of fault slips by linear and nonlinear inversion procedures respectively the results show that large slips located from the hypocenter to about 10 km and 14 to 17 km along the strike at depths between 8 and 12 km result from local dynamic stress drops exceeding 40 and 140 bars respectively negative stress drops down to 15 bars are required to explain very low slips over a shallow fault section if a non negative stress drop condition is imposed somewhat larger slips are obtained there than those from the waveform inversion the above results suggest that there could be a zone of velocity strengthening frictional behavior in the shallow crust which may have arrested slip motion during rupture propagation the strength excess is found to be generally small but somewhat larger at a small zone that has delayed rupture propagation the dynamic rupture initiated from a small nucleus zone with a low stress drop propagated southeastward breaking the deeper fault section with high stress drop and then broke a relatively high strength zone after a short time of arrest with highest stress drop

frequency domain inversion of strong motions application to the 1992 landers earthquake we present a frequency domain inversion in which the observed earthquake strong ground motions are used to constrain the space time dependence of slip on a fault green s functions are numerically evaluated and the parameters describing the rupture are the local slip rupture time and rise time these parameters are simultaneously evaluated without additional constraints this procedure allows for large variations in the local rupture velocity the june 28 1992 landers earthquake mw 7 3 offers an exceptional opportunity to apply this technique to a major strike slip event we model the rupture evolution including local differences in slip durations and variations in rupture velocity our final results are in good agreement with other inversion studies geodetic and surface observations the main discrepancies occurred at depth and at the end of the johnson valley fault we show that a relatively low resolution could be an explanation for these differences rupture velocity and slip are extremely heterogeneous both along strike and with depth a moment of 0 90 1020 n m was found the slip distribution shows that this event consists of a series of regions of high slip subevents separated by regions of relative low slip approximately 50 of the moment was released on the homestead valley fault in this region of large slip the rupture velocity inferred by our inversion is well constrained and is equal to 3 0 km s at depth and 2 5 km s near the surface our inversion favors the hypothesis that the duration of the slip at each point of the fault is of the order of the duration of rupture of each subevent

petrography geochemistry and hf nd isotope evolution of drill core samples and target rocks from the el gygytgyn impact crater ne chukotka arctic russia el gygytgyn in northeast chukotka russia is a 3 6 ma 18 km diameter impact structure the impact crater was recently drilled in the framework of a project sponsored by the international continental scientific drilling program icdp target rocks at the el gygytgyn area are dominated by the felsic members of the late cretaceous okhotsk chukotka volcanic belt ocvb such a target lithology is unique among terrestrial impact craters thereby providing the opportunity to study shock metamorphism in siliceous volcanic rocks here we present a petrographic geochemical and isotopic study of the section of the drill core underneath the lacustrine sediments extending from 316 m to 517 m below the lake bottom blb the drill core stratigraphy includes 80 m of suevite and a cross section through a volcanic suite which consists of 1 a middle section 390 423 mblb with dominant felsic tuffs and a few mafic members and 2 a welded rhyolitic dacitic ignimbrite 423 517 mblb the melt fragments embedded in the suevite are interpreted as being impact related by comparison with impact glasses from the crater and in opposition to the target rock which does not include similar melts a suevitic dyke crosscuts the lower section of the core at the depth 471 40 mblb evidence for shock metamorphism is concentrated in the upper 10 m of the drill core and almost limited to the suevitic breccia the geochemical and isotope nd and hf composition of samples from the target and the drill core reveals relationships to the berlozhya magmatic assemblage bma arguing for similar source magmas the primitive upper mantle pum normalized trace element plot of rocks investigated here confirms a subduction related signature as previously proposed for rocks from both ocvb and bma

the origin and significance of metamorphosed tectonic blocks in melanges evidence from sulawesi indonesia metamorphosed tectonic blocks or knockers are widespread but volumetrically minor constituents of many circum pacific melange belts due to the common lack of an exposed in situ provenance and to the seemingly chaotic field disposition of most block bearing melanges their origin and uplift history are problematic and controversial on the indonesian island of sulawesi a block bearing melange is overlain by an ophiolite nappe the base of which is characterized by a metamorphic sole sequence petrological geochemical and geochronological data indicate a direct genetic relationship between high grade tectonic blocks in the melange and amphibolites in the metamorphic sole amphibolite precursors to lower temperature blueschist assemblages are virtually ubiquitous in the tectonic blocks and subdivisions based on the nature of the overprinting relationships can be systematically correlated with block distribution patterns orientated subparallel to the strike of the melange belt it is suggested here that the high grade tectonic blocks originated in a thin thermally zoned metamorphic sheet welded to the oceanic hangingwall plate at the inception of subduction break up of this sequence at depth by tectonic erosion led to dispersal of fragments into a newly developed serpentinite melange wedge blocks experienced abrupt changes in p t x conditions due to a combination of hydration in the new fluid rich environment gradual cooling of the hangingwall over time and continuing underflow dragging sheared blocks deeper into the subduction zone prior to upflow blocks plucked from the hangingwall at different depths and at different times evidently experienced uplift in different flow channels resulting in block concentrations with p t t paths characteristic of their source and flow trajectory at systematically greater distances from the subduction zone hangingwall the elucidation of the origin and significance of tectonic blocks in sulawesi has important implications not only for the tectonometamorphic evolution of similar inclusions in other melange belts but also for models of the inception and early stages of subduction

elastic finite difference modelling and imaging for earthquake sources summary 2 d modelling and imaging of line double couple and spatially extended earthquake sources is demonstrated extrapolation of two component elastic wavefields is implemented using finite differences running the extrapolatioin forward in time produces synthetic seismograms running it backward in time produces an image of the source provided that the recording aperture is adequate the source location extent orientation and radiation pattern may be recovered from the data synthetic examples include a normal fault at the edge of a rift a reverse fault in a subduction zone and a line double couple in a cylindrical whole earth model

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

two types of ductile deformation regional metamorphic belt abstract the geometry of plastic strains of regional metamorphic rocks is divided into two types one is the uniaxial extension strain prolate and the other the biaxial shortening strain oblate from the plane strain simple shear to the uniaxial shortening geometry the uniaxial extension geometry is found in deformation of metamorphic rocks of the sambagawa ryoke kamuikotan and franciscan belts while the biaxial type is seen in deformation of the shimanto the alpine the appalachian and the caledonian metamorphic rocks the classification of the metamorphic belts by means of strain geometry and strain path is independent of the metamorphic facies series the difference in strain geometry of metamorphic belts is responsible mainly for the tectonic stress regime in metamorphism it is suggested that the biaxial type belongs to the collision type orogeny and the uniaxial type to the cordilleran type orogeny

clast size distribution and quantitative petrography of shocked and unshocked rocks from the el gygytgyn impact structure a variety of quantitative petrographic methods such as the study of clast size distribution csd have been used in the study of melt rocks and have for example led to validation of the melting origin of pseudotachylites in paleoseismic faults the estimate of the energy involved in melting and crushing processes the distinction between seismic and aseismic faults and understanding the crystallization process in volcanic rocks recently quantitative petrography was applied to distinguish impact melts from pristine mare basalts on the moon here we apply this approach to impact lithologies formed in a volcanic target the el gygytgyn structure an 18 km diameter and 3 58 ma old impact crater in north eastern chukotka arctic russia represents the only known impact crater on earth mainly excavated in siliceous volcanic rocks the structure was recently drilled in the framework of an icdp project providing fresh samples of suevite and other impact breccias which can be compared with samples from the unshocked target as the target is mostly composed of rhyolitic dacitic ignimbrites and tuffs impact melt clasts are almost indistinguishable from the unshocked volcanic clasts in the absence of shock evidence we show here that geometric characterization provides a reproducible technique for quantitative description of impact lithologies although the studied suevite reveals a high local variability in the evaluated geometric parameters an overall homogenization of these parameters occurs furthermore quantitative petrography allows the classification of unshocked to slightly shocked volcanic clasts included in the suevite

el gygytgyn impact crater chukotka arctic russia impact cratering aspects of the 2009 icdp drilling project the el gygytgyn impact structure in chukutka arctic russia is the only impact crater currently known on earth that was formed in mostly acid volcanic rocks mainly of rhyolitic with some andesitic and dacitic compositions in addition because of its depth it has provided an excellent sediment trap that records paleoclimatic information for the 3 6 myr since its formation for these two main reasons because of the importance for impact and paleoclimate research el gygytgyn was the subject of an international continental scientific drilling program icdp drilling project in 2009 during this project which due to its logistical and financial challenges took almost a decade to come to fruition a total of 642 3 m of drill core was recovered at two sites from four holes the obtained material included sedimentary and impactite rocks in terms of impactites which were recovered from 316 08 to 517 30 m depth below lake bottom mblb three main parts of that core segment were identified from 316 to 390 mblb polymict lithic impact breccia mostly suevite with volcanic and impact melt clasts that locally contain shocked minerals in a fine grained clastic matrix from 385 to 423 mblb a brecciated sequence of volcanic rocks including both felsic and mafic basalt members and from 423 to 517 mblb a greenish rhyodacitic ignimbrite mostly monomict breccia the uppermost impactite 316 328 mblb contains lacustrine sediment mixed with impact affected components over the whole length of the impactite core the abundance of shock features decreases rapidly from the top to the bottom of the studied core section the distinction between original volcanic melt fragments and those that formed later as the result of the impact event posed major problems in the study of these rocks the sequence that contains fairly unambiguous evidence of impact melt which is not very abundant anyway usually less than a few volume is only about 75 m thick the reason for this rather thin fallback impactite sequence may be the location of the drill core on an elevated part of the central uplift a general lack of large coherent melt bodies is evident similar to that found at the similarly sized bosumtwi impact crater in ghana that however was formed in a target composed of a thin layer of sediment above crystalline rocks

shocked rocks and impact glasses from the el gygytgyn impact structure russia the elgygytgyn impact structure is about 18 km in diameter and is located in the central part of chukotka arctic russia the crater was formed in volcanic rock strata of cretaceous age which include lava and tuffs of rhyolites dacites and andesites a mid pliocene age of the crater was previously determined by fission track 3 45 0 15 ma and 40ar 39ar dating 3 58 0 04 ma the ejecta layer around the crater is completely eroded shock metamorphosed volcanic rocks impact melt rocks and bomb shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event clasts of volcanic rocks which range in composition from rhyolite to dacite represent all stages of shock metamorphism including selective melting and formation of homogeneous impact melt four stages of shocked volcanic rocks were identified stage i 35 gpa lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant pfs and pdfs coesite and stishovite occur as well stage ii 35 45 gpa quartz and feldspar are converted to diaplectic glass coesite but no stishovite stage iii 45 55 gpa partly melted volcanic rocks common diaplectic quartz glass feldspar is melted and stage iv 55 gpa melt rocks and glasses two main types of impact melt rocks occur in the crater 1 impact melt rocks and impact melt breccias containing abundant fragments of shocked volcanic rocks that were probably derived from now eroded impact melt flows on the crater walls and 2 aerodynamically shaped impact melt glass bombs composed of homogeneous glass the composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target cobalt ni and ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks only the cr abundances show a distinct enrichment which points toward an achondritic projectile however the present data do not allow one to unambiguously identify a meteoritic component in the elgygytgyn impact melt rocks

three dimensional finite difference simulation of fault dynamics rectangular faults with fixed rupture velocity we analyze three dimensional finite difference solutions for a simple shear crack model of faulting to determine the effects of fault length and width on the earthquake slip function the fault model is dynamic with only rupture velocity fault dimensions and dynamic stress drop prescribed the numerical solutions are accurate for frequencies up to 5 hz and are combined with asymptotic results for shear cracks in order to characterize the slip function at higher frequencies near the hypocenter the slip velocity exhibits a square root singularity whose intensity increases with hypocentral distance at distances greater than the fault width w growth of the velocity intensity ceases and the slip function becomes nearly invariant with distance along the fault length closed form expressions are developed for the dependence of static slip s slip rise time tr and slip velocity intensity v on fault geometry along the center line of a long narrow fault at hypocentral distances exceeding w these expressions reduce to s w tr 0 5 w vr and v w 2 vr where is the dynamic stress drop the shear modulus and vr the rupture velocity the numerical results imply that uniform dislocation kinematic earthquake models in which slip is represented by a ramp time function will underpredict high frequency ground motion relative to low frequency ground motion a further implication of the numerical solutions is that the nature of inelastic processes at the advancing edge of a long fault will depend on fault width but will be independent of rupture length

petrology of impactites from el gygytgyn crater breccias in icdp drill core 1c glassy impact melt rocks and spherules el gygytgyn is a 18 km diameter 3 6 ma old impact crater in ne siberia international continental scientific drilling program el gygytgyn hole 1c was drilled on the frozen crater lake 2 3 km from the crater center to a final depth of 517 m below the lake floor petrographic and geochemical analyses of 26 drill core samples three impact melt rocks from the surface and seven glass spherules from surface deposits outside the crater are used to characterize the impactite inventory at el gygytgyn the bottom 98 m of hole 1c intersected monomict brecciated unshocked rhyolitic ignimbrite with minor intercalations of polymict breccia and mafic inclusions these lithologies are overlain by 89 m of polymict breccia whose components occasionally exhibit scarce low degree shock metamorphic features this unit is succeeded by 10 m of suevite that contains about 1 vol glassy impact melt shards 1 cm in size and a low amount of shock metamorphosed lithic clasts the suevite is capped by a reworked fallout deposit that constitutes a transition over 4 m into lacustrine sedimentation a higher abundance of shock metamorphosed lithic clasts and glass spherules some with ni rich spinel and admixture of an ultramafic component characterize this unit we tentatively interpret this impactite section as allochthonous breccia in the vicinity of el gygytgyn s central ring uplift the geochemical compositions of seven glass spherules from terrace deposits 2 km outside the crater and eight spherules from the reworked fallout deposit in hole 1c show far greater variability than the composition of impact melt shards and impact melt rocks some of these spherules also show strong enrichments in siderophile elements