geology and petrology of enormous volumes of impact melt on the moon a case study of the orientale basin impact melt sea abstract lunar basin forming impacts produce enormous volumes 10 5 km 3 of impact melt all known basin forming impacts combined may produce 10 8 km 3 of impact melt 1 20th the volume of the lunar crust despite their volumetric importance the geology and petrology of massive deposits of impact melt on the moon have been little studied in part because most basin impact melt deposits are old and have been obscured or buried by subsequent impact cratering and mare infill we investigate the geology and model the petrology of fresh massive impact melt deposits in the relatively young 930 km diameter orientale basin models of impact melt production combined with geologic analyses based on new lola topographic data suggest that most of the impact melt 2 3 produced by the orientale forming impact occurs in a 15 km thick impact melt sheet better described as an impact melt sea 350 km in diameter with a volume of 10 6 km 3 we anticipate that the orientale melt sea has undergone large scale igneous differentiation since terrestrial impact melt sheets such as manicouagan sudbury and morokweng less than a tenth of the thickness and a hundredth of the volume of the orientale melt sea have differentiated we develop a model for the cumulate stratigraphy of the solidified orientale impact melt sea a modeled cumulate stratigraphy occurring below a quench crust and anorthositic fallback breccia with an 8 km thick layer of norite overlying a 4 km layer of pyroxenite and a basal 2 km thick layer of dunite produced by equilibrium crystallization of a homogenized melt sea consistent with vigorous convection in that melt sea is supported by remotely sensed norite excavated by the central peak of maunder crater from 4 km depth generally we predict that very large basin forming impacts including the south pole aitken spa basin forming impact produce melt seas with a cumulate stratigraphy similar to that of the orientale melt sea impact melt differentiation may explain apparently anomalous lithologies excavated in the spa basin interior we note that impact melt differentiates are slow cooled the orientale melt sea took on the order of 10 5 years to solidify and if meteoritic siderophiles fractionate into metal or sulfide layers may not be siderophile enriched therefore impact melt differentiates may pass for pristine highland plutonic rocks in the lunar sample suite these predictions can be tested with current and future mission data

small scale heterogeneity at the top of the lower mantle around the mariana slab abstract short period waveform data recorded at the western us seismic array for 14 mariana deep earthquakes near 19 n show two anomalous wave packets within about 30 s after the arrival of the direct p wave simply called p or p wave one at about 13 s called x1 phase and the other around 29 s called x2 phase we perform array analyses to locate the sources of these phases in the first step we measure arrival time slowness back azimuth and amplitude of these phases relative to p the amplitudes of the x1 and x2 phases correlate with each other for the individual events and vary more than an order of magnitude among the events which cluster in a region spanning less than 50 km the amplitude correlation suggests that the two waves have a similar origin and the large amplitude variation eliminates receiver side reverberation as the origin of the phases the x1 phase has a slowness and an arrival azimuth which are not distinguished from those of p waves across the entire array and has nearly constant delay times regardless of the focal depths from 567 to 605 km the x2 phase arrives as a more emergent wave packet with slowness and azimuth different from those of p in the second step we compute composite semblance coefficients for the cases of p to p and s to p single scattering near the foci the x1 and x2 phases are best interpreted as s to p scattered waves generated in the uppermost lower mantle north of the focal region based on three observations 1 highest composite semblance values 2 scatterer locations mutually consistent between the two event groups near the depths of 500 and 600 km and 3 a reasonable amount of elastic property anomalies required the x1 phase scatterer is determined at 19 8 0 3 n 145 7 0 3 e 710 30 km on the other hand the scatterer of the x2 phase appears to split into two objects one at 20 4 0 4 n 145 4 1 0 e 900 80 km and the other at 20 6 0 4 n 147 4 0 5 e 860 40 km although the geometries of the scatterers of x1 and x2 phases are not constrained horizontal or subhorizontal discontinuities are unlikely the changes in elastic properties associated with these heterogeneous objects probably occur within several kilometers according to their high efficiency at converting short period waves they are thus likely to represent sharp chemical variations in major element composition these objects are located within a thickened high velocity anomaly at the top of the lower mantle which has been determined by previous seismic studies a plausible tectonic interpretation of these objects is that they are fragments of former oceanic crust which are entrained in the pacific slab impinging on the more viscous lower mantle

constraints on central uplift structure from the manicouagan impact crater recent drilling operations at the 90 km diameter late triassic manicouagan impact crater of quebec canada have provided new insight into the internal structure of a complex craters central region previous work had indicated that the impact event generated a 55 km diameter sheet of molten rock of relatively consistent originally 400 m thickness floran et al 1978 the drilling data reveals melt sheet thicknesses of up to 1500 m with kilometer scale lateral and substantial vertical variations in the geometry of the crater floor beneath the melt sheet the thickest melt section occurs in a 1500 m deep central trough encircled by a horseshoe shaped uplift of precambrian basement the uplift constitutes a modified central peak structure at least part of which breached the melt sheet mineralogical and compositional segregation differentiation of the thicker melt sheet section coupled with a lack of fractionation in the thinner units shows that the footwall geometry and associated trough structure were in place prior to melt sheet solidification marked lateral changes in sub melt sheet basement relief support the existence of a castellated footwall that was created by high angle impact related offsets of 100s to 1000s of meters this indicates that deformation during the modification stage of the cratering process was primarily facilitated by large displacement fault systems this work suggests that manicouagan is a central peak basin with rings which does not appear to fit with current complex crater classification schemes

the manicouagan impact structure an analysis of its original dimensions and form manicouagan quebec 51 23 n 68 42 w is the most intensively studied large complex terrestrial impact structure in a predominantly crystalline target the ground truth data available from manicouagan have considerable potential for interpreting the subsurface characteristics of lunar impact structures of comparable morphology and size two contrasting hypotheses however exist for the preerosional form of manicouagan 1 a multi ring basin with a final diameter of 75 km and a transient cavity diameter of 30 45 km floran and dence 1976 and 2 an endogenically modified peak ring basin with a final diameter of 100 km and a transient cavity diameter of 80 km orphal and schultz 1978 both hypotheses are based largely on topographic data and interpret the prominent 65 km diameter annular moat as a graben like feature formed either 1 by collapse of the outer slope of the transient cavity rim or 2 by uplift of the final crater floor through post impact intrusive activity the present analysis suggests that this annular moat is primarily an erosional feature and that the original form of the impact structure cannot be determined unequivocally on the basis of present topography if all the available topographic geologic and geophysical data are considered however then an internally consistent interpretation of original dimensions is possible the distribution of shock deformation effects in the basement rocks of the present crater floor suggests an original transient cavity diameter of 60 km the 55 km diameter impact melt covered inner plateau provides a minimum estimate for the diameter of the final floor of the crater and the annular moat with its outliers of downdropped ordovician limestone is interpreted as marking the contact between the floor and the innermost slump blocks of the final rim there is no compelling evidence to regard the annular moat as a graben by analogy with fresh lunar and mercurian structures a floor width of 55 km suggests a final rim diameter of 86 95 km this is consistent with the topographic data from outside the annular moat and with the residual peripheral gravity low which suggests a final rim diameter of 85 95 km due to erosion it is difficult to assign manicouagan to a particular morphological class of impact structure but on the basis of the available data it is most likely that the preerosional form was that of a central peak crater or possibly a peak ring basin at the time of its formation manicouagan may have been dimensionally and possibly morphologically similar to the 96 km diameter lunar crater copernicus

a paleomagnetic and rock magnetic study of the manicouagan impact structure implications for crater formation and geodynamo effects we report rock magnetic and paleomagnetic data from the similar to 214ma manicouagan canada impact crater based on 25 widely distributed sites of impact melt and basement rocks collected at the surface as well as from boreholes drilled to depths 1 5km titanomagnetite and titanohematite carry the magnetic remanence in impact melts above 320m elevation and in most basement rocks impact melts below 320m contain solely titanomagnetite magnetic susceptibility and saturation magnetization proxies for titanomagnetite concentration increase more than tenfold toward the base of the thickest impact melt that underwent fractional crystallization the titanomagnetite enriched zone partially contributes to a 2000nt magnetic anomaly in the crater s center stepwise demagnetization reveals a single normal polarity magnetization component in all samples regardless of the magnetic phases present coeval lock in remanence times for titanomagnetite and titanohematite indicate that the titanohematite formed 570 degrees c during oxi exsolution the average paleomagnetic direction and intensity coincide well with 214ma reference values we find no evidence for an aberration of the geomagnetic field over the several thousands of years it took to cool a 481m thick portion of the impact melt body hence the energy released by the manicouagan impact that created one of the 10 largest known craters on earth provoked no measurable disturbance of the geodynamo magnetic anisotropy of clast free impact melts define magnetic lineation directions that are in places radially oriented with respect to the crater s center centrifugal flow of the melt within the evolving transient crater probably generated the fabric

crater floor topography and impact melt sheet geometry of the sudbury impact structure canada terra nova 00 000 000 2010 abstract based on high resolution topography data and the geometry of geological contact traces at the surface we determined the orientation of layer interfaces of the sudbury igneous complex sic our analysis provides for the first time quantitative structural evidence for the thickness variation of its layers which supports an impact melt origin of the sic the present crater floor topography varies up to 400 m over distances of hundreds of metres to a few kilometres and up to 1500 m over a distance of about 25 km crater floor depressions are spatially associated with economically important sulphide mineral deposits which points to a viable exploration strategy finally observed variations in the width of the thermal aureole imparted by the sic on its host rocks are due to primary thickness variations of the sic rather than post impact deformation

nd isotope mapping of grenvillian crustal terranes in the vicinity of the manicouagan impact structure abstract over 70 new nd isotope analyses are presented for the manicouagan area of the grenville province to estimate the crustal age of target rocks involved in the 214 ma manicouagan impact structure and to reconstruct the precambrian geological evolution of this crustal segment the rocks fall into two main groups samples from the archean aged gagnon terrane to the north and west of the impact give t dm ages averaging 2 70 ga samples from the manicouagan imbricate zone miz and other allochthonous lithotectonic domains to the south of the impact yield paleoproterozoic t dm ages averaging 2 01 ga for the miz and 1 86 ga for the southern domains these paleoproterozoic terranes are correlated with makkovik age crust in labrador that was heavily reworked by labradorian magmatism that increased in intensity southwards the target rocks involved in the impact event would have consisted almost entirely of the miz which formed a layer several kilometres thick overlying archean crust at depth

petrology and u pb geochronology of mafic high pressure metamorphic coronites from the tshenukutish domain eastern grenville province abstract a suite of mafic and ultramafic rocks from the tshenukutish domain eastern grenville province have been the subject of a detailed petrographic and geochemical study combined with u pb geochronology the tshenukutish domain is the highest tectonic level in the recently discovered eclogite bearing manicouagan imbricate zone the mafic and ultramafic rocks are characteristic of two of the lithotectonic segments in the domain the lac espadon segment and the baie du nord segment in the lac espadon segment cumulate textures and geochemical characteristics suggest that the units represent a layered mafic and ultramafic complex and an unrelated apatite rich fe ti mafic pegmatite nelsonite body in contrast the samples from the baie du nord segment represent fairly homogeneous fe ti gabbros and anorthosites the samples studied show abundant petrographic evidence for high pressure metamorphism for instance the widespread development of garnet clinopyroxene and kyanite bearing metamorphic assemblages estimates of the conditions of high pressure metamorphism in previous studies and melting textures in deformed and hydrated gabbro margins are in the range 750 900 c and 1600 1800 mpa the gabbroic end members from both segments suggest within plate tholeiite petrotectonic signatures ree patterns of gabbros and cumulates in both segments suggest fractionation from a tholeiitic source cumulate textures fractionation and inherited zircons suggest high crustal level emplacement of the magmatic protoliths in the tshenukutish domain in the lac espadon segment u pb ages of igneous zircon and baddeleyite give an age range of 1643 1629 ma for the emplacement of the layered complex the nelsonite in the les gives a poorly constrained igneous zircon age of 1596 126 99 ma in the baie du nord segment baddeleyite gives an age of 1170 5 ma for emplacement of the fe ti gabbros this is the youngest intrusion in the td and thus gives a maximum limit for the onset of grenvillian metamorphism ages of high p metamorphism are further constrained by zircon formed during partial melting of deformed and hydrated fe ti gabbro margins 1046 3 ma overgrowth of igneous zircon 1042 22 28 ma in the nelsonite saccharoidal zircon replacing baddeleyite 1030 10 7 ma and 1030 12 ma and new growth of equant zircon 1012 12 ma from the undeformed interior of the gabbros these ages indicate that the high p metamorphism of the tshenukutish domain was late grenvillian ottawan retrogression to amphibolite facies is constrained by the ages of titanite 1006 997 ma in deformed gabbro margins rutile ages range between 960 and 929 ma and are interpreted as cooling ages the difference between the ages of titanite and rutile suggest slow cooling of the tshenukutish domain after initial high pressure and temperature metamorphism and exhumation

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

evidence for a metastable olivine wedge inside the subducted mariana slab abstract an anomalous seismic velocity structure is revealed inside the subducted pacific plate at northern mariana near the deepest earthquake foci we observe anomalous later phases in the p wave coda for a small cluster of deep earthquakes and an anomalous differential p wave slowness between a few pairs of the earthquakes belonging to the cluster we interpret these observations with a 5 low velocity wedge which is nearly 25 km wide at the depth of 590 km and is truncated at 630 km the wedge is bounded by two steeply dipping 60 70 transition layers with the thickness of about 4 km the highly localized seismic anomaly inside the slab represents a wedge of metastable olivine embedded in dry ringwoodite deep earthquakes occur across only one of the boundaries of the metastable wedge with some of them outside the wedge the mechanism of deep earthquakes is discussed based on the spatial pattern of seismicity relative to the wedge

physical conditions producing slab stagnation constraints of the clapeyron slope mantle viscosity trench retreat and dip angles abstract recent seismic tomography has revealed various morphologies in the subducted lithosphere in particular significant flattening and stagnation of slabs around the 660 km boundary are seen in some areas beneath the northwestern pacific subduction zones we examined the cause of slab stagnation in terms of the clapeyron slope of the phase transformation from ringwoodite to perovskite magnesiowustite trench retreat velocity dip angles and high viscosity of the lower mantle based on two dimensional 2 d numerical simulations of thermal convection in particular we examined the conditions necessary for slab stagnation assuming a very small absolute value of the clapeyron slope which were proposed based on recent high pressure high temperature high p t experiments our calculations show that slabs tend to stagnate above the 660 km boundary with an increasing absolute value of the clapeyron slope viscosity jump at the boundary and trench retreat velocity and a decreasing initial dip angle stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high p t experiments and other geophysical observations combining buoyancy high lower mantle viscosity and trench retreat we found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation two main regimes underlie slab stagnation buoyancy dominated and viscosity dominated regimes in the viscosity dominated regime it is possible for slabs to stagnate above the 660 km boundary even when the value of the clapeyron slope is 0 mpa k

hydrocode modeling of the sierra madera impact structure we present the first hydrocode simulations of the formation of the sierra madera structure west texas usa which was caused by an impact into a thick sedimentary target sequence we modeled sierra madera using the isale hydrocode and here we present two best fit models 1 a crater with a rim final crater diameter of 12 km in agreement with previous authors interpretations of the original structure and 2 a crater 16 km in diameter with increased postimpact erosion both models fit some of the geologic observational data but discrepancies with estimates of peak shock pressure extent of deformation and stratigraphic displacement remain this study suggests that sierra madera may be a larger crater than previously reported and illustrates some of the challenges in simulating impact deformation of sedimentary lithologies as many terrestrial craters possess some amount of sedimentary rocks in the target sequence numerical models of impacts into sedimentary targets are essential to our understanding of target rock deformation and the mechanics of crater formation

isotopic heterogeneity in the sudbury impact melt sheet abstract a unique terrestrial large impact melt sheet is preserved in the 1850 ma sudbury structure ontario we have undertaken a pb isotope investigation of the southern limb of the melt sheet termed the south range main mass the model initial pb isotope ratios 207 pb 204 pb m vary stratigraphically through the predominantly quartz monzogabbroic lower unit varying from 15 40 to 15 45 at the base to ca 15 35 at the top of the sequence lateral variations of similar range occur in basal lower unit samples over scales of less than 5 km the range of these variations is similar to those of locally exposed upper crustal target rocks and it is evident that the melt sheet has efficiently preserved inherited variability during the violent phases of crater formation superheated impact melts are expected to be well mixed mechanically therefore significant post impact melting of target rocks fallback material and entrained clasts is required to explain such heterogeneity the sudbury structure hosts world class ni cu pge sulphide ore deposits systematic variation in 207 pb 204 pb m occurs throughout sulphide ores within the creighton embayment from massive 15 42 15 45 to interstitial ca 15 40 15 41 and disseminated ca 15 39 sulphide linking the pb isotope composition of these ores to the immediately overlying lower unit stratigraphy a protracted sulphide segregation history is apparent massive sulphides segregated early prior to or during initial silicate crystallisation although the total time involved in sulphide accumulation spanned much of the crystallisation of the lower unit it is also shown that lateral variations in ni depletion throughout the main mass correlate with pb isotopes those segments with the strongest chalcophile element depletion signatures reflecting the accumulation of significant basal sulphides have high initial pb isotope values consistent with early sulphide segregation the characterisation of pb isotopic heterogeneity has therefore provided insights into the evolution and scales of mixing of the melt sheet with the identified chemical variability between melt cells having a significant influence on ore forming processes

evidence of impact melt sheet differentiation of the lunar south pole aitken basin the south pole aitken spa basin is the largest basin on the moon the basin forming impact likely melted the upper part of the mantle and formed an impact melt sheet impact melt of large terrestrial craters differentiated following a general magmatic differentiation sequence however it is still debated whether or not the spa melt sheet underwent differentiation to address this we investigated the vertical and lateral variations in mineral composition of the spa impact melt sheet area by analyzing the surface mineralogy including 277 post spa craters using remote sensing reflectance data we identified a 7 km thick high ca pyroxene rich layer and below that a 8 km thick low ca pyroxene rich layer both filled the entire impact melt sheet area 630 km in diameter considering the vast distribution of these two layers their thicknesses their feo abundances around 13 5 wt which are lower than most of the lunar mare basalt but within the estimated range of the differentiated spa impact melt and that the observed lithologies are consistent with a differentiation model we conclude that the spa impact melt was differentiated moreover based on the lunar magma ocean differentiation model al2o3 and cao abundances of the upper part of the mantle would be much lower after the mantle overturn if the spa impact occurred after overturn these elemental abundances of the initial impact melt would be much lower than that of the before overturn impact case and result in different solidified layers our observation matches only the after overturn case which may imply that the spa impact occurred after overturn

geology and stratigraphy of king crater lunar farside abstract clementine and photographic data sets have been used to investigate the crustal stratigraphy and geology of king crater on the lunar farside 120 e 5 5 n pre existing topographic regimes or stress fields dominate many structures in the crater which has excavated materials from depths of up to 14 km the upper crust in the area is noritic anorthosite grading to a more anorthositic signature with depth a possible batholithic intrusion is also present in a 15 km wide band extending from the southern crater floor to at least 50 km north of king and from near surface levels down to at least the excavation depth of the crater it is generally feldspathic but is cut by mafic dykes now visible in the north wall king also shows evidence for the presence of a cryptomare exposed in regions of the peaks and in dark halo craters within the ejecta blanket localized olivine bearing mineralogies are observed on the central peaks suggesting isolated pockets of troctolitic mineralogies to have been present at 8 to 14 km depths copious volumes of crystalline melt produced from the impact event cover king s floor to a maximum thickness of 30 60 m and have pooled in a number of natural depressions outside of the main crater the main pool in the pre existing a1 tusi crater has a minimum depth of 150 m domes on the crater floor are verified as nonvolcanic in origin and did not act as a source for any of the lava like materials in king

seismological and experimental constraints on metastable phase transformations and rheology of the mariana slab abstract a wedge with a low seismic wave velocity implying the presence of metastable olivine and the depression of the 660 km seismic discontinuity have been observed inside the mariana slab based on these seismic observations and numerical calculation of thermal structures of the slab we suggest that the phase transformations from olivine to ringwoodite and from ringwoodite to perovskite ferropericlase the post spinel transformation occur at the depth of 630 km and 550 c and at 690 km and 700 c respectively in the central coldest part of the mariana slab combining this information with recent experimental kinetic data we constrain the details of non equilibrium phase transformations and rheology of the mariana slab the observed depth of the metastable olivine inside the mariana slab can be explained by growth controlled olivine ringwoodite transformation under relatively dry condition such as the water content of about 150 wt ppm h2o on the other hand nucleation process controls the depth of the post spinel transformation in the mariana slab and therefore the observed depressions of the 660 km discontinuity should depend on both the overpressure needed for the nucleation and the negative clapeyron slope for the transformation when the overpressure for the nucleation is 0 5 gpa the observed depressions can be explained by the clapeyron slope of 0 7 mpa k grain size evolution and viscosity structures in the mariana slab are estimated based on these metastable phase transformations the slab weakening due to the grain size reduction is limited in about 40 km in width below the metastable olivine wedge at the depths from 630 to 690 km whereas the width of the weakened portion of the slab increases to more than 120 km at deeper than the 690 km depth after the post spinel transformation these viscosity structures are consistent with the behaviors of the mariana slab estimated from seismic tomography the slab vertically descends to the bottom of the transition zone and substantially deforms and thickens at the top of the lower mantle

the sudbury igneous complex viscous emulsion differentiation of a superheated impact melt sheet the sudbury igneous complex of ontario canada is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1 85 ga ago the transient cavity and melting zone reached the moho and instantly 2 min relaxed to form a more familiar large shallow crater holding a thick superheated 1700 c melt sheet covered by 2 km of breccia there is little about the resulting bimodal igneous complex that resembles crystallization of well known sheet like bodies of similar composition yet the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions suggesting an intimate common parentage from the surrounding crust this petrogenetic enigma is explained here as a natural unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion which we take as the high temperature equivalent of breccia a wide spectrum of viscously discrete interdispersed parcels of mafic and felsic liquids reflecting the compositional heterogeneity of the target crustal materials formed the emulsion within days to months the emulsion components separated according to their relative densities into a bimodal norite granophyre assemblage that formed the basic structure of the present sudbury igneous complex there is clear evidence of this emulsion in the earliest dikes i e offsets which likely give the earliest state of the nascent melt sheet immediately following emulsion separation the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer these convective motions homogenized and rapidly cooled the magma to the liquidus temperatures whereupon convection ceased the pattern of convection was pinned in place by the embayment topography of the crater floor which in turn played a pivotal role in directing sulfide deposition into the embayments all further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins there is clear evidence of solidification from the floor upward and the roof downward minimal differentiation and compositional modification took place throughout cooling and solidification nevertheless during the solidification stage granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying onaping formation became unstable and entered the melt sheet and the partially melted remnants collected at the interface between the norite and granophyre some interstitial melt from the norite also percolated upward and altogether the blocks and melt produced the distinctive chemical and physical characteristics of the unusual transition zone the sudbury melt sheet is in essence a full scale magmatic experiment the conditions of formation relative to any other large terrestrial magma are precisely known thus the clear lack of any significant modal layering the overall homogeneous nature of each unit and the lack of any significant chemical differentiation through crystal fractionation establish sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers

sedimentary pge signatures in the late triassic ejecta deposits from japan implications for the identification of impactor abstract positive platinum group element pge concentration and negative os isotope anomalies reported from a claystone layer in the upper triassic bedded chert succession of the sakahogi section mino belt central japan are thought to have been derived from an impact event stratigraphic variations and concentrations of pge were examined in the sakahogi section to determine the type of the impactor upper triassic claystone layers where pge anomalies have been newly discovered in bedded chert successions in southwest japan were also examined these include i the unuma section in the inuyama area mino belt ii the hisuikyo section in the kamiaso area mino belt and iii the enoura section in the tsukumi area chichibu belt radiolarian and conodont biostratigraphic data indicate that these claystone layers are of upper middle norian age reconstruction of bedded chert in these sections suggests that they originate from open ocean pelagic deep sea sediments deposited in the panthalassa ocean the relatively flat ci chondrite normalized patterns of the least mobile pges ir ru and rh and the ru ir ratio determined by linear regression analysis suggest that a chondritic impactor is the source of the pge anomalies preserved in claystone samples from the study sections although ru ir ratios cannot conclusively distinguish chondrites from iron meteorites the cr ir ratios of the claystone layers range from 10 4 to 10 5 clearly indicating contribution from chondritic materials the chondritic impactor of the suggested size 3 3 7 8 km in diameter implies that a large amount of debris and or climatically active gasses e g sulfur oxides would have been released from the impactor which would have had a marked effect on the environment

some mineral physics constraints on the rheology and geothermal structure of earth s lower mantle we explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of earth s lower mantle we show that mgsio3 perovskite is significantly stronger than mgo periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase periclase we calculate viscosities of the lower mantle for two cases 1 where periclase occurs as isolated grains and 2 where periclase occurs as continuous films using mineral physics data and models of two phase rheology we find that the effective viscosity for the former is about 10 1000 times higher than the latter we therefore suggest that the rheology of the lower mantle is structure and hence strain dependent leading to weakening at large strains due to the formation of continuous films of periclase overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient both mgsio3 perovskite and periclase have relatively small activation energies e g r t m with g 10 14 where r is the gas constant and t m is melting temperature and therefore the depth variation of viscosity is rather small even for a nearly adiabatic temperature gradient however the geothermal gradients consistent with the geodynamical inference of nearly depth independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood a superadiabatic gradient of up to 0 6 k km is also consistent with mineral physics and geodynamical observations

tectonics of complex crater formation as revealed by the haughton impact structure devon island canadian high arctic the results of a systematic field mapping campaign at the haughton impact structure have revealed new information about the tectonic evolution of mid size complex impact structures these studies reveal that several structures are generated during the initial compressive outward directed growth of the transient cavity during the excavation stage of crater formation 1 sub vertical radial faults and fractures 2 sub horizontal bedding parallel detachment faults and 3 minor concentric faults and fractures uplift of the transient cavity floor toward the end of the excavation stage produces a central uplift compressional inward directed deformation results in the duplication of strata along thrust faults and folds it is notable that haughton lacks a central topographic peak or peak ring the gravitational collapse of transient cavity walls involves the complex interaction of a series of interconnected radial and concentric faults while the outermost concentric faults dip in toward the crater center the majority of the innermost faults at haughton dip away from the center complex interactions between an outward directed collapsing central uplift and inward collapsing crater walls during the final stages of crater modification resulted in a structural ring of uplifted intensely faulted sub vertical and or overturned strata at a radial distance from the crater center of 5 0 6 5 km converging flow during the collapse of transient cavity walls was accommodated by the formation of several structures 1 sub vertical radial faults and folds 2 positive flower structures and chaotically brecciated ridges 3 rollover anticlines in the hanging walls of major listric faults and 4 antithetic faults and crestal collapse grabens oblique strike slip i e centripetal movement along concentric faults also accommodated strain during the final stages of readjustment during the crater modification stage it is clear that deformation during collapse of the transient cavity walls at haughton was brittle and localized along discrete fault planes separating kilometer size blocks

shock induced kelyphite formation in the core of a complex impact crater we present a compositional and textural analysis of shock induced microtextures in garnet porphyroblasts in migmatitic garnet cordierite biotite paragneisses from the centre of the vredefort impact structure south africa detailed imaging and major element analysis of deformation features in and adjacent to the garnet porphyroblasts record a complex heterogeneous distribution of shock effects at the microscale as the most competent silicate mineral in the assemblage with the highest hugoniot elastic limit and a wide pressure temperature stability field the porphyroblastic garnet preserves a more diverse shock deformation response compared to minerals such as quartz and feldspar which underwent more comprehensive shock metamorphism and subsequent annealing the garnet porphyroblasts display pre impact fractures that are overprinted by later intra granular hertzian and distinctive planar fractures associated with the impact event shock induced strain localization occurred along internal slip planes and defects including pre existing fractures and inclusion boundaries in the garnet symplectitic kelyphitic coronas commonly enclose the garnet porphyroblasts and inhabit intra granular fractures the kelyphite assemblage in fractures with open communication beyond garnet grain boundaries is characterized by orthopyroxene cordierite sapphirine conversely the kelyphite assemblage in closed off intra granular fractures is highly variable comprising spatially restricted combinations of a secondary garnet phase with a majoritic component al rich orthopyroxene sapphirine and cordierite the impedance contrast between garnet porphyroblasts and their inclusions further facilitated the formation of shock induced features al rich orthopyroxene coronas together the textural and mineralogical data suggest that these features provide a record of oscillatory shock perturbations initiated under confining pressure beneath the transient crater floor this occurred as the shocked rock volume underwent post shock expansion forming the core of the central uplift and was followed by variable textural re equilibration this study thus provides a microtextural and mineralogical perspective of the shock regime within confined crust immediately prior to and during central uplift formation

metastability of enstatite in deep subducting lithosphere olivine and mg fe sio3 pyroxene the most abundant minerals in the earth s upper mantle are believed to transform to high pressure phases at 400 km depth1 5 the possible metastable persistence of olivine to greater depths in some subduction zones with consequences for the origin of deep focus earthquakes and the dynamics of subduction has been discussed extensively6 14 but the role of other mantle minerals has not been considered we report here an experimental study of the kinetic behaviour of the magnesian pyroxene enstatite mgsio3 at upper mantle conditions we find that whereas forsterite mg olivine mg2sio4 transforms rapidly to phase at 1 200 c and 16 gpa the transformation of enstatite to phase plus stishovite on the same timescale requires much higher temperatures at lower temperatures enstatite transforms directly to the ilmenite structure but only at pressures greater than 20 gpa enstatite should therefore persist metastably to greater depths than olivine in subduction zones transforming directly to the ilmenite structure the enstatite ilmenite transformation is accompanied by a large decrease in volume which should increase the stresses in subducting slabs change the buoyancy forces that drive subduction and might also contribute to the origin of deep focus earthquakes

dating the cooling of exhumed central uplifts of impact structures by the u th he method a case study at manicouagan abstract forty titanite grain fragments from 9 central uplift samples of metamorphosed anorthosite from the manicouagan impact structure were dated by the u th he technique a u th he central age of 207 1 6 4 ma 2 standard error se n 40 has been determined with 4 outlier ages removed the central age is refined to 208 9 5 1 ma 2 se both of these ages are within error of the previously determined u pb zircon age of 214 1 ma 2 derived from the impact melt manicouagan s central uplift formed due to rapid elevation from 7 10 km depth as part of the modification stage of the impact process which has facilitated the dating of its emplacement due to resulting rapid exhumation and cooling and closure of the u th he system in titanite correlation with the previous u pb zircon 214 1 ma impact melt crystallization age indicates that the u th he titanite dating technique offers a new approach to dating complex impact structures in the absence of viable melt sheets or other melt products the youngest ca 195 ma u th he dates preserved in some titanite fragments are synchronous with early jurassic rift induced lithospheric thinning and associated igneous activity that defines the central atlantic magmatic province camp the u th he titanite data from manicouagan indicate that the influence of this regional event may extend west of the previously proposed limit of camp activity

the manicouagan impact melt sheet evidence for isotopic homogenization with limited assimilation abstract the rb sr pb pb and sm nd isotope chemistry of impact generated melt and target rocks has been determined from the 214 ma 90 km rim diameter manicouagan impact structure of canada fifty seven samples were obtained from 8 field sites and 11 drill core sites across the impact melt sheet located on the 55 km diameter central island ile rene levasseur the results reveal that the impact melt though locally differentiated via fractional crystallization was isotopically homogenized during its formation following its derivation primarily from charnockites and mesocratic gneisses the isotopic signatures of the target rocks indicate that the protolith for the melt was the proterozoic manicouagan imbricate zone and that the underlying archean gagnon terrane was not involved in melt production or its subsequent modification via assimilation despite impact melt resting on archean lithologies in the southwest sector of the melt sheet limited assimilation of the footwall in this sector is attributed to the relatively rapid cooling of a thinner peripheral melt body

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

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

development of microporosity diffusion channels and deuteric coarsening in perthitic alkali feldspars turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the klokken syenite using sem and tem it is caused by the presence of myriads of tubular micro inclusions either fluid filled micropores or sites of previous fluid inclusions and is associated with coarsening of microperthite and development of sub grains micropores are abundant in coarsened areas in which porosities may reach 4 5 but are almost absent from uncoarsened pristine braind microperthite areas the coarsening is patchy and involves a scale increase of up to 103 without change in the composition of the phases low albite and low microcline or in the bulk composition of the crystal it occurs abruptly along an irregular front within individual crystals which retain their original shapes the coherent braid microperthite gives way across the front to an irregular semi coherent film perthite over a few m and then to a highly coarsened irregular patch perthite containing numerous small sub grains on scales of a few hundred nm in both phases the coarsening and micropore formation occured at a t 400 450 c and it is inferred to have been driven by the release of coherent strain energy low angle grain boundary migration being favoured by a fluid the patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin possibly arising in part through exsolution of water from the feldspar the sub grain texture and microporosity modify profoundly the permeability of the rock and greatly enhance the subsequent reactivity of the feldspars

on critical observations that constrain models of terrestrial hypervelocity impact craters impact structures in the crystalline rocks of the canadian shield range over two orders of magnitude in size and display morphologies recognized elsewhere in the solar system this contribution draws upon new examinations of drill core from canadian craters to reaffirm some relationships modify others and refine the transitions from simple to complex with central peak to peak ring structures these include recognizing the hyperbolic form of transient craters sharpening the allochthon parautochthon distinction and proposing new formulae for key relationships it emphasizes the role of dynamic tensile strength and the attenuation of tensile rarefaction waves in determining the size of both transient and final crater dimensions on earth depth d to diameter d ratios are not invariant at about 1 10 but change smoothly with size from 1 3 at brent through 1 5 to 1 10 in the largest that is d 0 4 d0 75 in craters in crystalline rocks the central peak grows at about uplift 0 175 d until at d about 28 km the uplift rises above the original surface then collapses to form a peak ring structure these relationships demonstrate the dominant role of gravity in attenuating tensile rarefaction waves and controlling transient crater depth and overall size relative to the volume shocked

water transport into the deep mantle and formation of a hydrous transition zone abstract the phase relations in the peridotite water system to the top of the lower mantle reveal that a certain amount of water can be transported and stored in the transition zone by various hydrous minerals resulting in formation of a hydrous transition zone results of preliminary experiments to clarify the effect of water on kinetics of the olivine wadsleyite transformation are given and show that water enhances the transformation kinetics significantly there are potential dehydration sites in the deep mantle four sites along the descending slabs depending on their temperature profiles i e the mantle wedge of the subduction zone mantle transition zone top of the lower mantle where hydrous ringwoodite and superhydrous phase b decompose and deep in the lower mantle around 1200 1500 km depth where decomposition of phase d g occurs we also expect one site of dehydration melting at the base of the upper mantle where the hot mantle plume hydrated in the transition zone can be dehydrated the transition zone can serve as a water absorber for the mantle scale water circulation system some of the recent seismic works including the p wave tomography images reveal the existence of low velocity anomalies in the transition zone suggesting possibly a wet transition zone elevation of the 410 km discontinuity and lack of clear evidence for a metastable olivine wedge in cold slabs may be consistent with the equilibrium phase transformation from olivine to wadsleyite in cold and wet slabs on the basis of our kinetic study of the transformation under wet conditions

seismic evidence for a metastable olivine wedge in the subducting pacific slab under japan sea abstract we apply a forward modeling approach to high quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting pacific slab beneath the japan sea our results show that a finger like anomaly exists within the subducting pacific slab below 400 km depth which has a p wave velocity 5 lower than the surrounding slab velocity or 3 lower than that of the normal mantle suggesting the existence of a metastable olivine wedge mow in the slab the mow top and bottom depths are 400 and 560 km respectively the mow is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary at 560 km depth the mow is located at about 25 km below the slab upper boundary most of the deep earthquakes are located in the mow our results favor transformational faulting as the mechanism for deep earthquakes

hydrogen diffusivity in wadsleyite and water distribution in the mantle transition zone abstract the kinetics of the hydrogen diffusivity in synthesized polycrystalline wadsleyite was measured by ir spectroscopy in order to determine the diffusion coefficient of hydrogen in wadsleyite the major constituent mineral in the mantle transition zone with the composition of mg0 89fe0 11 2sio4 the hydration experiments were conducted at 15 16 gpa and temperature range from 900 to 1200 c with mg oh 2 as the water source by kawai type multi anvil apparatus the diffusion rate obtained here is considered to be an effective diffusion coefficient with the grain size of 9 m involving contributions both from the lattice diffusion and the grain boundary diffusion the temperature dependence of diffusion of hydrogen in polycrystalline wadsleyite was determined to be dh 9 6 10 6 exp 123 32 kj mol 1 rt at 15 gpa hydrogen diffusion rate in wadsleyite is roughly consistent with the average diffusivity of hydrogen in olivine in recent years several authors have suggested a possibility of the hydrous transition zone and the distribution and transport properties of water are still debated on the basis of water dependence of the electrical conductivity the water content in the mantle transition zone was estimated from the observed conductivity by using the nernst einstein relation the results obtained here show that the distribution of water should be quite heterogeneous throughout the mantle transition zone

friction melt distribution in a multi ring impact basin it is generally accepted that multi ring basins are the consequence of very large impacts but the mechanism by which they form is still a matter of contention1 2 most of what is currently known about multi ring basins is based on remote studies of the moon3 5 and to a lesser extent mars and mercury4 6 but at least two multi ring impact basins have been recognized on earth7 the sudbury8 canada and vredefort9 south africa impact structures providing an opportunity to study their properties directly here we describe the distribution of friction melt pseudotachylyte in the floor of the sudbury impact basin although the veins and dykes of pseudotachylyte decrease in both thickness and frequency of occurrence towards the basin periphery the greatest volumes of friction melt appear to define four rings around the central impact melt sheet field evidence indicates that the rings originated as zones of large displacement which facilitated localized frictional melting of the basin floor during the modification collapse stage of the cratering process by analogy we argue that the rings of other multi ring impact basins are also likely to be the remnants of such large displacement fault zones

high pressure high temperature rocks from the base of thick continental crust geology and age constraints from the manicouagan imbricate zone eastern grenville province the grenville province of the canadian shield is composed of crustal material with laurentian affinities involved in continental collision during the grenvillian orogeny between 1 19 and 0 98 ga in the eastern grenville province the manicouagan imbricate zone miz is composed of paleoproterozoic labradorian 1 6 ga and mesoproterozoic pinwarian 1 4 ga crustal segments that were metamorphosed under high pressure high temperature high p t conditions p 1800 mpa and 800 t 900 c and intruded by synmetamorphic gabbroic stocks and dykes and postmetamorphic granite during a 1 0 ga crustal shortening event in the grenvillian orogeny the gabbroic dykes display high p mineral assemblages and a withinplate tholeiite signature attesting to the intrusion of asthenospheric melts during the high p metamorphism the structural configuration of miz is that of a thrust stack lelukuau terrane overlain by an extensional assembly of slices tshenukutish terrane to the north lelukuau terrane overlies archean basement and paleoproterozoic supracrustal rocks of the gagnon terrane along a thrust contact tshenukutish terrane is tectonically overlain to the south by crustal slices composed of pre grenvillian and early grenvillian lithologic units that are correlative with those in miz but which were at midcrustal levels during the grenvillian orogeny low p segment of the eastern grenville province and were intruded by anorthosite and granitoids farther south a likely tectonothermal evolution of miz at 1 0 ga involves crustal shortening by imbrication in a postsubduction phase of continental collision followed by convective removal of the underlying thickened lithospheric mantle and the rise of hot asthenospheric material close to the base of the crust exhumation of miz likely occurred from the basal levels of the crust and was achieved by 1 nw directed thrusting over a crustal scale ramp archean basement of the gagnon terrane with coeval extension at the higher levels of the pile stage 1 extension and 2 a second extension event stage 2 extension coeval with emplacement of postmetamorphic granite at the boundary between miz and the low p segment

shock veins in the central uplift of the manicouagan impact structure context and genesis we describe the development of shock veins that penetrate the anorthositic central uplift of the manicouagan impact structure they occur as thin 2 5 mm wide linear micro fault systems that can be traced for several meters in length and which predominantly trend radially from the point of impact the shock veins are distinguished by the development of maskelynite along vein margins and stishovite in vein matrices which are otherwise absent in non veined regions these phases define a shock excursion of up to 30 gpa in contrast to bulk shock effects of 12 gpa defined by development of shatter cones planar fractures and planar deformation features in various minerals the shock veins at manicouagan share many similarities with vein systems developed in meteorites they also provide an in situ context with which to better understand meteoroid source and lofting conditions in addition to containing high pressure phases the shock veins exhibit evidence for high temperature partial melting of host silicate clasts with the generation of flow textured fragments and glasses the formation of microcrystallites and dendrites from some melts indicates rapid cooling we propose a two stage generation mechanism comprising an initial high pressure shock excursion estimated to last 0 5 s based on projectile size considerations followed by a longer high temperature pulse of a few seconds duration we suggest that the shock excursion is initiated by target heterogeneities that cause distortions in the hemispherically propagating shock front this results in radially oriented tearing and vein formation with shock amplification occurring via intra vein shock reverberation high speed displacement along the veins is driven by stress release on rarefaction which results in frictional melting via adiabatic heating

u th he dating of terrestrial impact structures the manicouagan example the accurate dating of meteorite impact structures on earth has proven to be challenging melt sheets are amenable to high precision dating by the u pb and 40ar 39ar methods but many impact events do not produce them or they are not preserved in cases where high temperature shock metamorphism of the target materials has occurred without widespread melting these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact however the u th he chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites we report u th he dates for accessory minerals from the manicouagan impact structure in quebec canada nine zircons from a melt sheet sample yield a weighted mean age of 213 2 5 4 ma 2se indistinguishable from the published 214 1 ma 2 u pb zircon age for the impact in contrast five apatites from this sample yield dates between 205 9 6 5 and 162 0 5 3 ma 2 indicating variable postimpact helium loss due to low temperature thermal disturbance preimpact titanite crystals from a shocked meta anorthosite sample yield two dates consistent with the impact age at 212 27 and 214 13 ma 2 and two younger dates of 189 6 6 9 and 192 2 9 8 ma 2 suggestive of postimpact helium loss these results indicate that u th he chronometry is a suitable method for dating impact events although interpretation of the results requires recognition of possible 4he loss related to reheating subsequent to impact

maximum depth of seismicity and thermal parameter of the subducting slab general empirical relation and its application the maximum depth of seismicity dm in subduction zones is limited by some physical mechanisms related to the rheological strength of slab material metastability conditions failure strength etc the main factors controlling these properties are temperature and pressure within the lithosphere sinking into the mantle these p t conditions may be coarsely assessed from thermal models of slab subduction the observed relationship between dm the age of the subducted lithosphere a and the convergence rate v can be used to test the theoretical models and reveal inconsistencies which may indicate changes in failure mechanisms with depth we analyze the relation between dm and the thermal parameter of the descending slab which is a product of the age of the subducted lithosphere and the vertical component of convergence rate v seismicity profiles across the subduction zones of mexico chile kamchatka kuriles japan sumatra new hebrides aleutians tonga and marianas are used to estimate dm the quasi linear part of the dependence dm 240 km and 2000 km corresponds to relatively young and slowly descending slab and is in general agreement with the critical temperature models of deep earthquakes for 2000 km which corresponds to the relatively older lithosphere subducting at a higher rate the dm f dependence is nonlinear the flattening of the empirical dm f curve in the range of 2000 km 3500 km is found to be a direct indication of the influence of the equilibrium ol sp phase transition on dm models invoking the metastable ol sp phase transition as a mechanism which controls the deepest seismicity cannot be definitely constrained by the results of this study the general dependence dm f may be applied as a standard relation between dm a and v for the analysis of deep earthquakes and seismotectonic studies of the subduction zones

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

a schematic model of crater modification by gravity models are proposed to account for the formation of slump terraces central peaks peak rings and concentric rings in large impact structures crater slumping and central peak formation is attributed to the process of acoustic fluidization which endows a volume of the target surrounding the crater with a bingham plastic rheology for a short time after the impact the parameters of this bingham plastic model are remarkably constant throughout the solar system the bingham yield stress ranges from a maximum of 80 bars on mercury to a minimum of 7 bars on callisto the bingham viscosity is between 109 and 1010 p at the onset of central peak formation for all bodies studied although it appears to rise to 1011 p as crater diameter grows to a few hundred kilometers asymmetric or scarp bounded concentric rings are due to a fundamentally different process they form when the target planet s lithosphere fractures under stresses imposed by the flow of the asthenosphere toward the transient crater cavity lithosphere yield strengths of less than a few hundred bars are implied by observed ring development on the moon and callisto

seismic evidence for olivine phase changes at the 410 and 660 kilometer discontinuities the view that the seismic discontinuities bounding the mantle transition zone at 410 and 660 kilometer depths are caused by isochemical phase transformations of the olivine structure is debated combining converted wave measurements in east asia and australia with seismic velocities from regional tomography studies we observe a correlation of the thickness of and wavespeed variations within the transition zone that is consistent with olivine structural transformations moreover the seismologically inferred clapeyron slopes are in agreement with the mineralogical clapeyron slopes of the mg fe 2sio4 spinel and postspinel transformations

the evolution of the onaping formation at the sudbury impact structure the 1 4 1 6 km thick onaping formation consists of a complex series of breccias and melt bodies lying above the sudbury igneous complex sic at the sudbury impact structure based on the presence of shocked lithic clasts and various glassy phases the onaping has been described as a suevitic breccia with an origin at least in part as fallback material recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases both as clast types and discrete intrusive bodies the nature of the onaping and that of other suevitic breccias overlying impact melt sheets is reviewed the relative thickness internal stratigraphic and lithological character and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the onaping the sudbury structure formed in a foreland basin and water played an essential role in the evolution of the onaping as indicated by a major hydrothermal system generated during its formation taken together observations and interpretations of the onaping suggest a working hypothesis for the origin of the onaping that includes not only impact but also the interaction of sea water with the impact melt resulting in repeated explosive interactions involving proto sic materials and mixing with pre existing lithologies this is complicated by additional brecciation events due to the intrusion of proto sic materials into the evolving and thickening onaping fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two thirds of the onaping

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

grain growth kinetics of ringwoodite and its implication for rheology of the subducting slab abstract grain growth rates of mg2sio4 and mg0 9fe0 1 2sio4 ringwoodites have been studied under the conditions of the mantle transition zone at a pressure of 21 gpa and temperatures ranging from 1473 to 2023 k using a kawai type high pressure apparatus the grain growth rates were expressed by gn g0n k0texp h rt where g m is grain size at time t s and g0 is the initial grain size at t 0 with n 4 5 0 8 and h 414 kj mol and log k0 20 4 1 6 m4 5 s for mg0 9fe0 1 2sio4 ringwoodite and h 456 kj mol and logk0 20 2 1 4 m4 5 s for mg2sio4 ringwoodite using these kinetic parameters for grain growth we estimated the grain size of ringwoodite in the cold subducting slabs to be less than 100 m this suggests that the dominant deformation mechanism in the subducting slab is diffusion creep it is also likely that some cold slabs are softer than the warmer surrounding mantle

using the viscoelastic relaxation of large impact craters to study the thermal history of mars we simulate the long term deformation of martian craters and investigate the role of lower crustal flow in the evolution of surface and subsurface topography using the finite element method and a viscoelastic rheological model we model the deformation of more than 30 large craters and quasi circular depressions qcds in the diameter range of 200 500 km in both the northern lowlands and southern highlands we determine the most appropriate background heat fluxes that produce the current topography beneath the impacts at the crust mantle boundary ranges from 40 to 90 mw m 2 our study shows that a higher background heat flux leads to more relaxation at the surface and subsurface by applying various viscous creep parameters for hydrous and anhydrous rheologies we demonstrate that mars s interior is wet to a certain degree which is consistent with other estimates since craters and qcds are distributed fairly equally on the surface of the red planet this study provides a less regionally biased picture of the thermal history of early mars than in previous studies based on our results the ancient average background heat flux in the northern lowlands was higher than that of the southern highlands which could indicate that whatever process formed the crustal dichotomy had a thermal signature at least through the middle noachian

evidence for coeval late triassic terrestrial impacts from the rochechouart france meteorite crater 1 high temperature impact melt breccias from the rochechouart france meteorite crater record a magnetization component with antipodal normal and reverse polarities the corresponding paleomagnetic pole for this component lies between the 220 ma and 210 ma reference poles on the eurasian apparent polar wander path consistent with the 214 8 ma 40ar 39ar age of the crater late triassic tectonic reconstructions of the eurasian and north american plates place this pole within 95 confidence limits of the paleomagnetic pole from the manicouagan canada meteorite impact crater which is dated at 214 1 ma together these observations reinforce the hypothesis of a late triassic multiple meteorite impact event on earth

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

in situ x ray diffraction study of post spinel transformation in a peridotite mantle implication for the 660 km discontinuity abstract we present the phase relations in anhydrous cao mgo feo al2o3 sio2 pyrolite to examine the influence of compositional difference between pyrolite and mg2sio4 on the post spinel phase transformation it is shown that in the pyrolite system the transformation occurs at about 0 5 gpa lower pressure relative to mg2sio4 we have carried out several in situ x ray diffraction experiments on ringwoodite to mg perovskite ferropericlase and backward transformations and found that post spinel transformation boundary can be expressed as p gpa 0 0005 t k 23 54 using the gold equation of state by tsuchiya t tsuchiya first principles prediction of the p v t equation of state of gold and the 660 km discontinuity in earth s mantle j geophys res 108 2003 doi 10 1029 2003jb002446 the interval for coexisting ringwoodite and mg perovskite was found to be 0 1 0 5 gpa the discrepancy between our data and the depth of the seismic discontinuity global average 654 km is about 20 km at 1850 k based on the results of in situ measurements we confirmed that the difference in chemical composition between pyrolite and mg2sio4 cannot modify the clapeyron slope of the post spinel transformation using experimental data and assuming the average mantle temperature 1850 k at 660 km we can account for only a half of variations in the depth of the 660 km discontinuity in subduction zones and at hot spots an additional explanation for the observed seismological variations at the 660 km discontinuity is required and may reflect influence of other minor components or volatiles

terrestrial impact melt rocks and glasses abstract the effects of meteorite and comet impact on earth are rock brecciation the formation of shock metamorphic features rock melting and the formation of impact structures i e simple craters complex craters and multi ring basins large events such as the 65 ma chicxulub impact are believed to have had catastrophic environmental effects that profoundly influenced the development of life on earth in this review an attempt is made to summarize some of the voluminous literature on impact melting one important aspect of planetary impact provide some comments on this process and to make suggestions for future research the products of impact melting are glasses impact melt rocks and pseudotachylites our treatise deals mainly with the geological setting petrography and major element chemistry of melt rocks and glasses impact glasses in several petrographic aspects are similar to volcanic glasses but they are associated with shock metamorphosed mineral and rock fragments and in places with siderophile element anomalies suggestive of meteoritic contamination they are found in allogenic breccia deposits within fall back suevite and outside fall out suevite impact craters and as spherules in distal ejecta large events such as the k t boundary chicxulub impact are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common on a microscopic scale however impact glasses are commonly strikingly heterogeneous tektites are glasses ejected from craters over large distances they are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal sedimentary rocks four tektite strewn fields are known three of which can be tied to specific impact craters impact melt rocks form sheets lenses and dike like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks dikes of impact melt rocks also intrude the rocks of the crater floor they commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine to coarse grained chemically they are strikingly homogeneous but as with impact glasses exemptions to this rule do exist large and thick melt bodies such as the sudbury igneous complex sic are differentiated or may represent a combination of impact melt rocks sensu strictu and impact triggered deep crustal melts a concerted multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes especially of melting excavation and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater impact triggered pressure release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu strictu this controversial idea needs to be tested by a re evaluation of existing data and models be they based on field or laboratory research only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping in this review we summarize observations made on impact melt rocks and impact glasses in a number of north american brent haughton manicouagan new quebec sudbury wanapitei all in canada asian popigai russia zhamanshin kazakhstan two south african structures morokweng and vredefort the henbury crater field of australia and one european crater ries germany our tables listing major element chemical compositions of impact glasses and melt rocks however include also data from structures not dealt with in further detail

siderophile rich inclusions from the morokweng impact melt sheet south africa possible fragments of a chondritic meteorite an 870 m thick diameter 30 km melt sheet associated with the morokweng impact structure south africa contains cr rich silicate inclusions up to 30 mm in diameter as well as disseminated ni rich sulphides and oxides that commonly occur in mutual intergrowths although the silicate inclusions are highly altered the mineral chemistry and petrographic evidence along with broadly chondritic re os systematics and platinum group element pge signatures all support the view that the silicate inclusions represent relict fragments of an impactor with compositions similar to ordinary chondrites the ni rich sulphides and oxides are significantly enriched up to 35 chondrites in pge ni and cr although the bulk pge signature of the melt sheet is chondritic the sulphides and oxides display mm and sub mm scale fractionation of pge into different inclusion components a model is presented whereby pieces of the projectile react with the relatively more oxidising impact melt into which they fell resulting in oxidation and fractionation of the metal silicate fragments to form the inclusion assemblages this may be the first reported occurrence of a large melt sheet 300 km3 contaminated by swarms of discrete pebble sized fragments of asteroid material the high proportion of meteorite fragments in the morokweng melt sheet suggests either that the structure was formed by a relatively low velocity 18 km s impact event or that the angle of impact was optimum for retaining maximum amounts of meteoritic material within the crater

the metastable olivine wedge in fast subducting slabs constraints from thermo kinetic coupling abstract the effects of a strong non linear thermo kinetic coupling in the olivine spinel transition are investigated with a 2d model for fast subducting slabs the thermo kinetic coupling problem is solved in the lagrangian framework by the differential algebraic method and by using experimental input parameters narrow regions with metastable olivine are found in fast and cold slabs down to the 660 km discontinuity while the thermo kinetic coupling reduces the depth and width of the zone where olivine and spinel coexist in contrast to recent kinetic models both latent heat release and heat diffusion place important constraints on the structure of the metastable wedge both processes trigger the ongoing phase transformation in the cold slab interior a thermal instability over a short timescale 10 4 10 5 yr can occur due to a positive thermo kinetic feedback this effect is enhanced at greater depths the depth of the metastable wedge is found to be extremely sensitive to certain geophysical and thermodynamic parameters such as the lithospheric basal temperature slab width and the activation energy for diffusion variations in these parameters within the experimental uncertainties can cause significant changes in the depth of the metastable wedge hence predictions about the exact position of the metastable olivine wedge are rather difficult we propose that deep focus earthquakes may occur due to a thermal runaway caused by shear heating while the thermal instability is enhanced by the latent heat release associated with the olivine spinel phase transition

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

the influence of olivine metastability on deep subduction of oceanic lithosphere abstract the dynamics of subduction of oceanic lithosphere is mainly controlled by thermal buoyancy forces and mineralogy in self consistent models the influence of metastable olivine mo on the dynamics and evolution of subducting oceanic lithosphere is investigated we consider both phase boundaries olivine ol to spinel sp 410 km and sp to perovskite pv magnesiowustite mw 660 km treating the kinetics of the phase transition in a simplified way while models do not allow the formation of mo penetrate straight into the lower mantle slabs with a sufficient amount of mo flatten out at the 660 km boundary before penetrating at a later stage furthermore our models show that the effect of mo to slow down subduction velocities within the transition zone is rather moderate

transition from complex craters to multi ringed basins on terrestrial planetary bodies scale dependent role of the expanding melt cavity and progressive interaction with the displaced zone 1 the observed differences between complex craters and multi ringed basins are described and a combined transient cavity displaced zone melt cavity model is explored to account for the observations in this nested melt cavity model the increasing influence of the percentage of the target undergoing impact melting at the sub impact point with increasing size the differential melt scaling of cintala and grieve 1998 causes fundamental changes in the nature of the transient cavity its relationship to the displaced zone and its short term collapse behavior the transition from complex craters to two ring basins involves expansion of the melting front into the displaced zone formation of a two component excavation cavity and a concomitant radial expansion outward of the zone of maximum elastic rebound the short term modification stage is then dominated by the strength differences between the fluid melt in the inner cavity and rocks of the displaced zone the highly shocked rocks at the outer margin of the expanded parabolic melting front rebound to form the expanded peak ring moving upward and laterally inward easily displacing fluid melt filling the inner depression at larger sizes differential melt scaling causes the peak ring diameter to expand relatively more rapidly than the basin rim and upon collapse the increased volume of melt ponds in the rebounded melt cavity inside the expanding peak ring as the transient melt cavity further increases proportionally in size it penetrates through the base of the displaced zone with significant consequences the resulting modification stage now incorporates into the collapse process inward and upward movement along the base of the displaced zone listric failure occurs inward into the fluid melt cavity beginning at the edge of the melt cavity and extending out along the base of the displaced zone up to the base of the rim structural uplift at 1 5 crater radii this forms an additional outer ring and a resulting megaterrace modifying the radial ejecta on the collapsed rim to form a domical facies at multi ring basin scales the significantly deeper penetration that occurs in the expanding melt cavity accounts for the maximum crustal thickness decrease that occurs inside the peak ring in the final basin early onset and higher density of peak ring basins on mercury is predicted by higher mean impact velocity and differential melt scaling