the current state of knowledge about shatter cones introduction to the special issue shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments the occurrence of shatter cones is the only accepted meso to macroscopic recognition criterion for impact structures shatter cones exhibit a number of geometric characteristics orientation apical angles striation angles sizes that can be best described as varied from case to case possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them a database of shatter cone occurrences has been produced for this introduction paper to the special issue of meteoritics and planetary science on shatter cones distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter with a few currently questionable exceptions all pertinent hypotheses of formation are presented and discussed several may be discarded in light of the most recent observations the branching fracture mechanism and the interference models proposed respectively by sagy et al 2002 and baratoux and melosh 2003 require further evaluation new observations experiments or theoretical considerations presented in this special issue promise an important step forward based on a renewed effort to resolve the enigmatic origin of these important features

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

the mjolnir marine impact crater porosity anomaly abstract we have developed a quantitative model for the porosity change induced by the mjolnir impact using density and seismic traveltime distributions and postimpact sediment deformation compared with the surrounding undisturbed platform sediments the porosity increased immediately after impact up to 6 3 in the collapsed and brecciated crater periphery whereas porosity decreased by 1 in the dense central uplift following impact the crater was progressively buried by a 2 2 5 km thick overburden which caused radially varying differential compaction among the disrupted sedimentary rocks this compaction has significantly affected the postimpact crater evolution and decreased the porosity anomaly to values of 2 5 in the periphery and 2 2 in the center high resolution seismic profiles reveal possible gas related seismic amplitude anomalies spatially related to the 13 33 km3 excess pore space in impact deformed strata along the crater periphery potential plays may include lower to middle jurassic sands structurally trapped by impact generated tilted fault blocks and the synimpact extensively fractured allochthonous breccia however the crater is a very high risk hydrocarbon prospect because reservoir properties may have been destroyed by the deep mesozoic burial followed by large scale cenozoic uplift and erosion nonetheless the mjolnir porosity anomaly provides new data about the postimpact crater deformation similar data are almost entirely absent from the terrestrial impact record

the first description and confirmation of the vista alegre impact structure in the parana flood basalts of southern brazil abstract the vista alegre structure centered at 25 57 s and 52 41 w has been recently proposed as a meteorite impact structure the 9 5 km diameter structure is located in the parana state of southern brazil within the parana basin which contains one of the largest and most extensive flood basalt provinces on earth the parana flood basalts belong to the serra geral formation and are temporally related to the opening of the south atlantic ocean having been dated at about 133 132 ma tholeiitic basalts dominate the western portion of parana state with some minor rhyodacites morphologically vista alegre has a prominent circular outline in the form of an incomplete ring of escarpments and an inner depression the presence of a central uplift is not obvious but it is inferred by the occurrence of deformed sandstone blocks near the center of the structure these sandstones are possibly related to the triassic piramboia formation and or to the cretaceous botucatu formation these units are normally at stratigraphic depths of about 700 800 m below the present surface in this portion of the parana basin the structure appears to be in an advanced erosion stage and its interior is occupied by a soil cover several meters thick extensively used for agriculture as a result there are limited outcrops in the interior of the structure all of polymict breccias some of them melt bearing we report the extensive occurrence of shatter cones in the form of fine grained rock clasts within the polymict breccias the shatter cone bearing breccias occur at different locations within the structure separated by several kilometers the nested shatter cones range in size from about 0 5 to 20 cm for individual cones and up to half a meter for complete assemblages the shatter cones formed in fine grained parana flood basalt and might be the first examples of shatter cones in such a rock type in addition planar deformation features pdfs were found in quartz grains within sedimentary rock clasts of the polymict breccia these findings confirm the impact origin of the vista alegre structure

thermodynamic calculation of emplacement pressures for batholithic rocks california implications for the aluminum in hornblende barometer the aluminum in hornblende ah barometer has been widely used to estimate the emplacement pressure p of granitic rocks application of the barometer has remained controversial because the controls on the aluminum content of hornblende imposed by temperature t magma composition and fluid composition remain incompletely understood in this paper the p t conditions of equilibration for 19 rocks from the mesozoic batholiths of california are estimated by simultaneous evaluation of 1 the p sensitive reaction tremolite phlogopite 2 anorthite 2 albite 2 pargasite 6 quartz k feldspar and 2 previously published amphibole plagioclase thermometer equilibria p estimates for the batholiths range from 1 2 to 7 4 kbar and are consistent with independent geologic evidence the highest p samples 5 1 7 4 kbar are located near the san andreas fault furthermore it appears that regional p gradients determined by previous ah barometry studies are valid but that the absolute values of the p estimates will need to be revised somewhat 1 5 kbar i suggest that the method of this study provides a viable thermodynamics based alternative to conventional ah barometry that requires no a priori assumptions about t or bulk composition

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

shock deformation confirms the impact origin for the cerro do jarau rio grande do sul brazil structure cerro do jarau is a conspicuous circular morpho structural feature in rio grande do sul state brazil with a central elevated core in the otherwise flat pampas terrain typical for the border regions between brazil and uruguay the structure has a diameter of approximately 13 5 km it is centered at 30o12 s and 56o32 w and was formed on basaltic flows of the cretaceous serra geral formation which is part of the parana etendeka large igneous province lip and in sandstones of the botucatu and guara formations the structure was first spotted on aerial photographs in the 1960s ever since its origin has been debated sometimes in terms of an endogenous igneous origin sometimes as the result of an exogenous meteorite impact event in recent years a number of studies have been conducted in order to investigate its nature and origin although the results have indicated a possible impact origin no conclusive evidence could be produced the interpretation of an impact origin was mostly based on the morphological characteristics of the structure geophysical data as well as the occurrence of different breccia types extensive deformation silicification of the rocks within the structure in particular the sandstones and also on the widespread occurrence of low pressure deformation features including some planar fractures pfs a detailed optical microscopic analysis of samples collected during a number of field campaigns since 2007 resulted in the disclosure of a large number of quartz grains from sandstone and monomict arenite breccia from the central part of the structure with pfs and feather features ffs as well as a number of quartz grains exhibiting planar deformation features pdfs while most of these latter grains only carry a single set of pdfs we have observed several with two sets and one grain with three sets of pdfs consequently we here propose cerro do jarau as the seventh confirmed impact structure in brazil cerro do jarau together with vargeao dome santa catalina state and vista alegre parana state is one of very few impact structures on earth formed in basaltic rocks

insights into the morphology of the serra da cangalha impact structure from geophysical modeling forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources in this context we have developed 2 5 d gravity models of the serra da cangalha impact structure for the purpose of investigating geological bodies structures underneath the crater interpretation of the models was supported by ground magnetic data acquired along profiles as well as by high resolution aeromagnetic data ground magnetic data reveal the presence of short wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1 9 km whereas in the region beneath the central uplift it is raised to 0 5 1 km below the current surface these depths are also supported by 2 5 d gravity models showing a gentle relief for the basement beneath the central uplift area geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater as well as on shock induced modifications of target rocks the results showed that the morphology is consistent with the current observations of the crater and that serra da cangalha was formed by a meteorite of approximately 1 4 km diameter striking at 12 km s 1

age and tectonic setting of the camaguey volcanic intrusive arc cuba late cretaceous extension and uplift in the western greater antilles abstract ar ar ages determined on rocks and minerals from the camaguey area in central cuba provide age constraints on events that accompanied the northward migration of cuba into the caribbean region and its subsequent collision with the bahamas bank whole rock samples from the camujiro and piragua formations part of the camaguey volcanic sequence yielded ar ar ages of 74 72 ma distinctly younger than the 100 80 ma ages indicated by fossils in interlayered sedimentary rocks syenite and granodiorite in the camaguey batholith which cut these volcanic rocks yield generally similar ar ar ages of 75 72 ma for hornblende biotite and feldspar these ages are interpreted to reflect relatively rapid uplift and cooling of most of the volcanic intrusive arc additional constraints on the timing of this uplift are provided by ar ar ages of 71 75 ma for rhyolite rhyodacite domes of the la sierra formation and 52 ma for andesitic basalt of la mulata formation which appear to have been emplaced onto erosion s

geology of lonar crater india lonar crater india is one of the youngest and best preserved impact structures on earth the 1 88 km diameter simple crater formed entirely within the deccan traps making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the moon in this study we present a meter scale resolution digital elevation model geological map of lonar crater and the surrounding area and radiocarbon ages for histosols beneath the distal ejecta impact related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric subhorizontal noncylindrical fold axes at the crater rim the rim fold hinge is preserved around 10 15 of the crater although tearing in the rim fold is inferred from fi eld and paleomagnetic observations no tear faults are identifi ed indicating that large displacements in the crater walls are not characteristic of small craters in basalt one signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer parallel slip faults there is little fl uvial erosion of the continuous ejecta blanket portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules in particular in the eastern and western rim as well as in the depression north of the crater known as little lonar the emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow based on the preserved thickness distribution of the ejecta the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol and the lack of sorting and stratifi cation in the bulk of the ejecta the ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on mars

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

geology and impact features of vargeao dome southern brazil abstract vargeao dome southern brazil is a circular feature formed in lava flows of the lower cretaceous serra geral formation and in sandstones of the parana basin even though its impact origin was already proposed in the 1980s little information about its geological and impact features is available in the literature the structure has a rim rim diameter of approximately 12 km and comprises several ring like concentric features with multiple concentric lineaments the presence of a central uplift is suggested by the occurrence of deformed sandstone strata of the botucatu and piramboia formations we present the morphological structural characteristics of vargeao dome characterize the different rock types that occur in its interior mainly brecciated volcanic rocks bvr of the serra geral formation and discuss the deformation and shock features in the volcanic rocks and in sandstones these features comprise shatter cones in sandstone and basalt as well as planar microstructures in quartz a geochemical comparison of the target rock equivalents from outside the structure with the shocked rocks from its interior shows that both the bvrs and the brecciated sandstone have a composition largely similar to that of the corresponding unshocked lithologies no traces of meteoritic material have been found so far the results confirm the impact origin of vargeao dome making it one of the largest among the rare impact craters in basaltic targets known on earth

integrated 3 d model from gravity and petrophysical data at the bosumtwi impact structure ghana the bosumtwi impact structure of central ghana was drilled in 2004 as part of the international continental scientific drilling program icdp a vast amount of geoscience data is available from the pre site surveys and the actual drilling phase a 3 d gravity model was constructed and calibrated with the available data from the two icdp boreholes lb 07a and lb 08a the 3 d gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data and with the petrophysical data from the lb 08a and lb 07a core materials and the two borehole logs furthermore the model exhibits lateral density variations across the structure and refines the results from previous 2 5 d modeling an important new element of the 3 d model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling

the complex impact structure serra da cangalha tocantins state brazil abstract serra da cangalha is a complex impact structure with a crater diameter of 13 700 m and a central uplift diameter of 5800 m new findings of shatter cones planar fractures feather features and possible planar deformation features are presented several ring like features that are visible on remote sensing imagery are caused by selective erosion of tilted strata the target at serra da cangalha is composed of devonian to permian sedimentary rocks mainly sandstones that are interlayered with siltstone and claystones nne ssw and wnw ese striking joint sets were present prior to the impact and also overprinted the structure after its formation as preferred zones of weakness these joint sets partly controlled the shape of the outer perimeter of the structure and in particular affected the deformation within the central uplift joints in radial orientation to the impact center did not undergo a change in orientation during tilting of strata when the central uplift was formed these planes were used as major displacement zones the asymmetry of the central uplift with preferred overturning of strata in the northern to western sector may suggest a moderately oblique impact from a southerly direction buckle folding of tilted strata as well as strata overturning indicates that the central uplift became gravitationally unstable at the end of crater formation

stress field along the ryukyu arc and the okinawa trough inferred from moment tensors of shallow earthquakes abstract we investigated the stress field along the ryukyu arc and the okinawa trough using regional nied mt and global harvard cmt moment tensor catalogs using the combined dataset of 4 years of data from the nied mt catalog and 24 years of data from the harvard cmt catalog we found an arc parallel extensional stress province along the forearc of the ryukyu arc and observed a detailed extensional stress field in the okinawa trough this arc parallel extension is observed in the entire region of the ryukyu arc except at its northeastern end this stress field is clearly separated from the backarc extensional stress field in the okinawa trough by the volcanic chain since it exists in a normal subduction region as well as in a region of weakly coupled subduction the effects of oblique subduction might not be enough to explain the formation of the arc parallel extension thus we consider that the backarc opening process should play an important role in this arc parallel extension along the okinawa trough an arc perpendicular extensional stress field is observed in the southeastern and central ryukyu arc while in the northeastern okinawa trough the direction of the extensional axis is oblique to both the arc normal direction and the direction of plate motion two dimensional mechanical interaction models which consider the interactions between the subducting plate forearc and backarc are inadequate because an arc parallel extensional stress field is observed in the entire ryukyu arc and the extension axis in the northeastern okinawa trough is oblique to the arc trend the oblique backarc stress may originate beneath the backarc or on the continental side of the backarc one of the simplest ways to explain this observation might be active rifting in the northeastern okinawa trough

rim uplift and crater shape in meteor crater effects of target heterogeneities and trajectory obliquity 1 we have analyzed the rim structure of meteor crater arizona in order to understand the mechanism of rim uplift in simple craters and the causes of the shape of polygonal impact craters for this purpose we systematically determined bedding orientation of the autochthonous crater wall and overturned flap and analyzed the kinematics of major radial faults we found that rim uplift correlates with the crater shape and increases in the corners of the crater the two main mechanisms of differential uplift are the formation of horizontal interthrust wedges leading to the doubling of strata in the rim and radial corner faults or tear faults that vertically displace bedrock the development of meteor crater s quadrangular shape is caused by more effective crater excavation flow parallel to major joint sets additionally we infer the impact direction with a newly developed technique the two corners model and review the arguments in favor of an oblique trajectory while the data set is ambiguous several indicators suggest an impact direction from the nnw we conclude that oblique impacts should have an effect on early cratering and excavation flow whereas target heterogeneities like joints start to play a prominent role in later stages when the stresses induced by the excavation flow are in the same order of strength as the material involved

target rocks impact glasses and melt rocks from the lonar impact crater india petrography and geochemistry the lonar crater india is the only well preserved simple crater on earth in continental flood basalts it is excavated in the deccan trap basalts of cretaceous tertiary age a representative set of target basalts including the basalt flows excavated by the crater and a variety of impact breccias and impact glasses were analyzed for their major and trace element compositions impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes comparable geochemical patterns of immobile elements e g rees for glass melt rock and basalt indicates minimal fractionation between the target rocks and the impactites we found only little indication of post impact hydrothermal alteration in terms of volatile trace element changes no clear indication of an extraterrestrial component was found in any of our breccias and impact glasses indicating either a low level of contamination or a non chondritic or otherwise iridium poor impactor

investigating the lake bosumtwi impact structure insight from numerical modeling 1 the current topography of the lake bosumtwi crater and some of its structural dimensions have been determined by geophysical methods we combine these data with sophisticated numerical models to evaluate the cratering process itself for example melt and tektite generation as well as to test the modeling code the geophysical maps show some asymmetry in plan view with the main anomaly north of the crater center the simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions while the late stage is modeled for the vertical impact without any asymmetry we estimate the projectile size from scaling laws and then varying material properties reproduce a crater which is similar to the bosumtwi but too deep bulking allows us to reconcile differences between the model results and the observed topography shock melt estimates are in good agreement with the bosumtwi magnetic signature modeled distribution of tektites assumes an impact angle of 30 45 and an impact direction from the n ne the combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well suited drill sites besides the central uplift and the annular moat with a suggested thick breccia cover drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact induced asymmetries

evolution of the cloudy boundary layer during the autumnal freezing of the beaufort sea the beaufort and arctic seas experiment base occurred during september and october 1994 off the canadian beaufort coast in this paper we provide a description of the characteristics of the the cloudy boundary layer that were observed from a research aircraft during base and provide an interpretation for the coevolution of the atmospheric boundary layer and sea ice characteristics during the autumnal freezing of the beaufort sea observations from a total of 12 research flights are presented during the course of the experiment the underlying surface made a transition from late summer melt conditions to full fledged winter conditions all of the observed boundary layer cloud decks are associated with stable temperature profiles with temperature inversions occurring either above or within each of the cloud decks unlike observations of summertime boundary layer clouds no multiple layered clouds in the boundary layer were observed examination of the cloud phase indicates a transition from predominantly liquid phase clouds at the beginning of the experiment to predominantly crystalline clouds at the end of the experiment local interactions between the clouds and underlying surface were examined in several case studies including the effects of leads using a time series of soundings over a period of 5 days in conjunction with the the national center for environmental prediction analyses an air mass modification event was examined that began after the intrusion of warm air from the pacific ocean the evolution of the air mass is apparent in decreasing temperatures in the lower atmosphere and formation of a low level mixed phase cloud that evolves with time into a deeper cloud layer that is crystalline

toroda creek half graben northeast washington late stage sedimentary infilling of a synextensional basin the toroda creek half graben is one of several north northeast trending eocene extensional basins associated in time and space with the okanogan and kettle metamorphic core complexes in the southern omineca crystalline belt of washington and british columbia sedimentary rocks of the middle eocene klondike mountain formation are the youngest stratified nonvolcanic rocks in the toroda creek half graben these sedimentary rocks preserve details of the character of toroda creek half graben basin development in a humid temperate setting help constrain the timing of brittle faulting in the basin and provide insight into approximate rates of okanogan core complex uplift facies distributions and provenance determinations indicate that the klondike mountain formation accumulated in a localized basin there is no evidence that the klondike mountain formation strata in the toroda creek half graben were once part of a more widespread deposit that later was segmented by faulting into its present configuration klondike mountain sedimentary strata consist of sedimentary gravity dominated rock avalanche slide and debris flow deposits that accumulated on alluvial fans that delivered their sediments into either a single elongate lake or a series of possibly interconnected lakes provenance determinations and facies distribution patterns suggest that klondike mountain detritus primarily was derived from steep unstable highlands exposed in the footwall of the bodie mountain fault on the east side of the basin sediments at the southern end of the basin were shed from an upland created by a west northwest trending transfer fault granite creek fault rates of footwall uplift and or basin subsidence estimated from unroofing of plutonic and presumed mylonitic rocks suggest rapid and significant basin margin uplift

modeling damage and deformation in impact simulations numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters particularly in cases where the final pristine crater morphology is ambiguous or unknown in this paper we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results we demonstrate this with an example simulation of an impact into a terrestrial granite target that produces a 10 km diameter transient crater the results of the simulation are shown in terms of damage a scalar quantity that reflects the totality of fragmentation and plastic strain both total plastic strain the accumulated amount of permanent shear deformation regardless of the sense of shear and net plastic strain the amount of permanent shear deformation where the sense of shear is accounted for damage and plastic strain are both greatest close to the impact site and decline with radial distance however the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains plastic strain in brittle rocks is very heterogeneous however continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock large compared to the spacing between individual zones of sliding this paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones furthermore we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity

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