lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

low pressure granulite facies metamorphism in the larsemann hills area east antarctica petrology and tectonic implications for the evolution of the prydz bay area thermobarometric studies on various granulite facies areas along the prydz bay coast east antarctica 73 79 e 68 70 s show that at around 1100 ma during a late proterozoic orogeny the rocks of the larsemann hills suffered a lower pressure metamorphic peak than the surrounding areas along the prydz bay coast the rocks affected by this event include parts of the vestfold hills block plus all of the rauer group the larsemann hills and the munro kerr mountains the dykes in the south west corner of the vestfold hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south west 650 c 6 5 kbar collerson et al 1983 the rauer group was metamorphosed at 800 c and 7 5 kbar harley 1987a the larsemann hills at 750 c and 4 5 kbar and the munro kerr mountains probably at around 850 c and 5 kbar retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas followed by isobaric cooling at this depth this paper shows that the peak metamorphism in the larsemann hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow although other areas in prydz bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model the inferred pre metamorphic peak histories and the requirement of consistency with the larsemann hills make it unlikely that collision followed by erosion driven decompression is an appropriate model we suggest that the thermal regime of the crust in the larsemann hills region was controlled by a perturbation in the asthenosphere with magma invasion of the crust we suggest that the 500 ma event represented in prydz bay by granitic outcrops at landing bluff and by several k ar ages from the larsemann hills area was responsible for the final excavation of the terrane

deep crustal structure of the chicxulub impact crater we present the results of a wide angle seismic survey conducted over the chicxulub impact crater profile chicx a a1 is a chord across the offshore portion of the crater and chicx b f is an onshore offshore profile through the center of the crater we use travel times recorded by 32 ocean bottom seismograph and 19 land based receivers to model the crustal structure of the crater the primary feature in the shallow velocity structure is a low velocity tertiary basin the edge of which correlates with a steep gravity gradient in some regions of our survey directly beneath the tertiary basin at the center of the crater is a region of relatively low velocities and densities that is interpreted as a unit of melt rocks either suevite breccias with isolated melt pods or suevite breccias overlying a coherent melt sheet this unit has a thickness of 1 km and a diameter of 100 km central uplift with a diameter of 40 60 km is observed on chicx b f a lower limit on the vertical extent of uplift is 9 km shallow basement is also observed along the northwest portion of chicx b f associated with a prominent northwest trending high in the gravity field superimposed on a regional trend is moho uplift of 1 km near the center of the chicx a a1 profile with adjacent moho deepening of 1 25 1 5 km the moho topography may be related to deformation processes associated with the formation of the outer ring or to the excavation and collapse of the transient cavity

vredefort sudbury chicxulub three of a kind vredefort sudbury and chicxulub are the largest known terrestrial impact structures all have been cited as multi ring basins the available data indicate that all have some form of multiple ring attributes most commonly structural features chicxulub however is the only example with morphological ring features there are also commonalities in the structural and lithological features of vredefort and sudbury and it is possible to construct a generalized compilation of the character of 200 300 km diameter impact basins on earth it is not clear however that any of these structures had the original morphological characteristics of large lunar multi ring basins additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large scale cratering processes if this is not sufficient incentive for further studies the environmental effects of chicxulub aside the vredefort sudbury and chicxulub impact events are also the reason for

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

imaging of intrabasalt and subbasalt structure with full wavefield seismic tomography 1 the imaging of subsurface structure using conventional seismic reflection methods is problematic across areas where high velocity basaltic rocks are intercalated with sedimentary layers of low velocity in this study we use an approach that combines wavefield inversion of both normal incidence and wide aperture seismic data to constrain velocity structure within and beneath the basalt our field data are a two ship long offset profile acquired in the faeroe basin a conventional traveltime inversion of the wide angle data is performed to obtain a starting model for the wavefield inversion synthetic tests are used to explore resolution and to determine a strategy for inversion of the field data our final velocity model shows a high velocity basaltic layer underlain by an intermediate velocity layer that is presumably sedimentary the basalts appear highly heterogeneous in velocity and the degree of heterogeneity increases significantly with depth within the layer a prestack depth migration using the final velocity model reveals a number of new features despite the velocity heterogeneity within the basalts strong reflections are seen within and immediately beneath the basaltic section and these reflectors are more clearly imaged on the final depth migrated section than hitherto the transition from sediments to basalt does not appear to be sharp uniquely defined or locally subhorizontal the data suggest that velocity heterogeneity and the complexity of the transition from basalt to sediment may explain the difficulties of imaging beneath basalts west of britain wavefield tomography can resolve this complexity and hence improve both imaging and confidence of interpretation within and beneath basalts

seismic lg wave q tomography in and around northeast china 1 we investigate regional variations in the lg wave quality factor q in northeast china and its vicinity with a tomographic method digital seismic data recorded at 20 broadband stations from 125 regional events are used to extract lg wave spectra tomographic inversions are independently conducted at 58 discrete frequencies distributed log evenly between 0 05 and 10 0 hz we simultaneously invert for the lg wave q distribution and source spectra at individual frequencies without using any a priori assumption about the frequency dependence of the q model and source function the best spatial resolution is approximately 1 1 in well covered areas for frequencies between 0 4 and 2 0 hz the lg q shows significant regional variations and an apparent relationship with regional geology we use a statistical method to investigate the regional variations of lg q and their frequency dependence the average q0 1 hz lg q in the entire investigated region is 414 sedimentary basins are usually characterized by lower average q0 values from 155 to 391 while volcanic mountain areas have relatively high average q0 values from 630 to 675 lg q generally increases with increasing frequency however the frequency dependence has complex nonlinear features on a double logarithmic scale indicating that the commonly adopted power law relationship may be oversimplified in a broad frequency band the frequency dependence varies in different geological areas with larger variations seen at lower frequencies

three dimensional joint inversion of traveltime and gravity data across the chicxulub impact crater in 2005 an extensive new seismic refraction data set was acquired over the central part of the chicxulub impact crater allowing us to image its structure with much better resolution than before however models derived from traveltime data are limited by the available ray coverage and the nonuniqueness that is inherent to all geophysical methods therefore many different models can fit the data equally well to address these issues we have developed a new method to simultaneously invert traveltime and gravity data to obtain an integrated model to convert velocity to density we use a linear relationship derived from measurements on core from the chicxulub impact basin thus providing a reliable conversion equation that is typical for lithologies of the central part of this crater prior to utilizing the inversion on the observed data we have run a suite of tests to establish the optimum weighting between traveltime and gravity constraints using a synthetic model of central crater structure and the real experimental geometry these synthetic tests indicate which inversion parameters lead to the best recovery of subsurface structure as well as which parts of the model are well resolved we applied the method to all existing gravity data and to seismic refraction data acquired in 1996 and the new higher resolution seismic refraction data acquired in 2005 we favor the traveltime model wherever we have sufficient ray coverage and the joint model where we have no ray coverage

extraordinary rocks from the peak ring of the chicxulub impact crater p wave velocity density and porosity measurements from iodp icdp expedition 364 joint international ocean discovery program and international continental scientific drilling program expedition 364 drilled into the peak ring of the chicxulub impact crater we present p wave velocity density and porosity measurements from hole m0077a that reveal unusual physical properties of the peak ring rocks across the boundary between post impact sedimentary rock and suevite impact melt bearing breccia we measure a sharp decrease in velocity and density and an increase in porosity velocity density and porosity values for the suevite are 2900 3700 m s 2 06 2 37 g cm3 and 20 35 respectively the thin 25 m impact melt rock unit below the suevite has velocity measurements of 3650 4350 m s density measurements of 2 26 2 37 g cm3 and porosity measurements of 19 22 we associate the low velocity low density and high porosity of suevite and impact melt rock with rapid emplacement hydrothermal alteration products and observations of pore space vugs and vesicles the uplifted granitic peak ring materials have values of 4000 4200 m s 2 39 2 44 g cm3 and 8 13 for velocity density and porosity respectively these values differ significantly from typical unaltered granite which has higher velocity and density and lower porosity the majority of hole m0077a peak ring velocity density and porosity measurements indicate considerable rock damage and are consistent with numerical model predictions for peak ring formation where the lithologies present within the peak ring represent some of the most shocked and damaged rocks in an impact basin we integrate our results with previous seismic datasets to map the suevite near the borehole we map suevite below the paleogene sedimentary rock in the annular trough on the peak ring and in the central basin implying that post impact suevite covered the entire floor of the impact basin suevite thickness is 100 165 m on the top of the peak ring but 200 m in the central basin suggesting that suevite flowed downslope from the collapsing central uplift during and after peak ring formation accumulating preferentially within the central basin

new links between the chicxulub impact structure and the cretaceous tertiary boundary the 200 km diameter chicxulub structure1 3 in northern yucatan mexico has emerged as the prime candidate for the cretaceous tertiary k t boundary impact crater3 6 concentric geophysical anomalies associated with enigmatic occurrences of upper cretaceous breccias and andesitic rocks led penfield and camargo1 to suspect that this structure was a buried impact basin more recently the discovery of shocked quartz grains in a chicxulub breccia3 and chemical similarities between chicxulub rocks and k t tektite like glasses3 6 have been advanced as evidence that the chicxulub structure is a k t impact site here we present evidence from core samples that chicxulub is indeed a k t source crater and can apparently account for all the evidence of impact distributed globally at the k t boundary without the need for simultaneous multiple impacts or comet showers shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the k t boundary ejecta layer7 8 the chicxulub melt rocks that we studied contain anomalously high levels of iridium up to 13 5 parts per 109 also consistent with the iridium enriched k t boundary layer9 our best estimate of the crystallization age of these melt rocks as determined by 40ar 39ar analyses is 65 2 0 4 1 myr in good agreement with the mean plateau age of 64 98 0 05 myr recently reported10 furthermore these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity the only such episode consistent with40ar 39ar constraints is chron 29r which includes the k t boundary

post impact structural crater modification due to sediment loading an overlooked process post impact crater morphology and structure modifications due to sediment loading are analyzed in detail and exemplified in five well preserved impact craters mjolnir chesapeake bay chicxulub montagnais and bosumtwi the analysis demonstrates that the geometry and the structural and stratigraphic relations of post impact strata provide information about the amplitude the spatial distribution and the mode of post impact deformation reconstruction of the original morphology and structure for the mjolnir chicxulub and bosumtwi craters demonstrates the long term subsidence and differential compaction that takes place between the crater and the outside platform region and laterally within the crater structure at mjolnir the central high developed as a prominent feature during post impact burial the height of the peak ring was enhanced and the cumulative throw on the rim faults was increased the original chicxulub crater exhibited considerably less prominent peakring and inner ring crater rim features than the present crater the original relief of the peak ring was on the order of 420 570 m currently 535 575 m the relief on the inner ring crater rim was 300 450 m currently 700 m the original bosumtwi crater exhibited a central uplift high whose structural relief increased during burial current height 101 110 m in contrast to the original height of 85 110 m whereas the surrounding western part of the annular trough was subdued more that the eastern part exhibiting original depths of 43 68 m currently 46 m and 49 55 m currently 50 m respectively furthermore a quantitative model for the porosity change caused by the chesapeake bay impact was developed utilizing the modeled density distribution the model shows that compared with the surrounding platform the porosity increased immediately after impact up to 8 5 in the collapsed and brecciated crater center currently 6 due to post impact compaction in contrast porosity decreased by 2 3 currently 3 to 4 5 due to post impact compaction in the peak ring region the lateral variations in porosity at chesapeake bay crater are compatible with similar porosity variations at mjolnir crater and are considered to be responsible for the moderate chesapeake bay gravity signature annular low of 8 mgal instead of 15 mgal the analysis shows that the reconstructions and the long term alterations due to post impact burial are closely related to the impact disturbed target rock volume and a brecciated region of laterally varying thickness and depth varying physical properties the study further shows that several crater morphological and structural parameters are prone to post impact burial modification and are either exaggerated or subdued during post impact burial preliminary correction factors are established based on the integrated reconstruction and post impact deformation analysis the crater morphological and structural parameters corrected from post impact loading and modification effects can be used to better constrain cratering scaling law estimates and impact related consequences

testing the resolution of a 3d velocity tomogram across the chicxulub crater abstract an integrated offshore onshore reflection and refraction experiment was shot across the chicxulub impact crater in 1996 the refraction data were previously inverted in 3d using first arrival travel time tomography a regularized inversion in which both data misfit and model roughness are minimized simultaneously was used to determine a smooth velocity tomogram across the inner crater region however the experimental geometry for the refraction data was irregular causing concern that this velocity model might not be well resolved in this paper we present a suite of checkerboard tests to investigate the lateral resolution of our velocity model the chicxulub crater is located partly onshore and partly offshore with its centre close to the yucatan coastline in mexico the shallow water limited acquisition of marine reflection data to distances of not closer than 25 km from the crater centre and the centre of the structure is imaged with refraction data only intriguing velocity anomalies were observed across the central crater region providing constraints on the lithological and structural form of chicxulub a high velocity body within the central crater is most likely to represent lower crustal rocks that were stratigraphically uplifted during the formation of this complex crater the concave shape of this stratigraphic uplift suggests clues to the mechanics of large crater collapse the rocks appear to have moved upward and outward an inward dipping zone of lowered velocity has been interpreted as delimiting the outer edge of a central zone of melt rich rocks the resolution tests presented here indicate that these observed velocity anomalies are likely to be real

modeling the formation of the k pg boundary layer abstract in this paper we investigate the formation of the cretaceous paleogene k pg boundary layer through numerical modeling the k pg layer is widely agreed to be composed of meteoritic material and target rock from the chicxulub impact site that has been ejected around the globe and mixed with local material during final deposition the observed composition and thickness of the k pg boundary layer changes with azimuth and distance from the impact site we have run a suite of numerical simulations to investigate whether we can replicate the observational data with a focus on the distal k pg layer and the impact glasses at proximal sites such as beloc haiti previous models of the k pg ejecta have assumed an initial velocity distribution and tracked the ejecta to its final destination here we attempt to model the entire process from impact to the arrival of the ejecta around the globe our models replicate the observed ejecta thickness at proximal sites and the modeled ejecta is composed of sediments and silicate basement rocks in agreement with observational data models that use a 45 impact angle are able to replicate the total ejecta and iridium volume at distal sites and the majority of the ejecta is composed of meteorite and target sediments sub vertical impacts generate too little iridium and oblique impacts of 30 degrees generate too much however in contrast to observations models that involve ballistic transport of ejecta lead to ejecta thickness decreasing with increasing distance and are unable to transport shocked minerals quartz and zircon from the chicxulub basement rocks around the globe we suggest that much of the k pg ejecta is transported non ballistically and that the most plausible mechanism is through re distribution from a hot expanding atmosphere the results are important for future investigations of the environmental effects of the chicxulub impact

mexican site for k t impact crater research throughout the caribbean suggests that the geophysical anomalies in the yucatan first noted by penfield and camargo 1981 and called the chicxulub crater could be the site of the impact purported to have caused the k t extinctions a semicircular ring of sink holes known locally as cenotes which correlates with the geophysical anomalies has been identified and it is argued that the origin of the cenote ring is related to postimpact subsidence of the chicxulub crater rim if there is indeed a crater the region within the cenote ring corresponds to its floor and the crater rim diameter is probably larger than 200 km if confirmed as a site of impact the chicxulub crater would be the largest terrestrial impact crater known which is consistent with the uniqueness of the k t global catastrophe

differential melt scaling for oblique impacts on terrestrial planets abstract analytical estimates of melt volumes produced by a given projectile and contained in a given impact crater are derived as a function of impact velocity impact angle planetary gravity target and projectile densities and specific internal energy of melting applications to impact events and impact craters on the earth moon and mars are demonstrated and discussed the most probable oblique impact 45 produces 1 6 times less melt volume than a vertical impact and 1 6 and 3 7 times more melt volume than impacts with 30 and 15 trajectories respectively the melt volume for a particular crater diameter increases with planetary gravity so a crater on earth should have more melt than similar size craters on mars and the moon the melt volume for a particular projectile diameter does not depend on gravity but has a strong dependence on impact velocity so the melt generated by a given projectile on the moon is significantly larger than on mars higher surface temperatures and geothermal gradients increase melt production as do lower energies of melting collectively the results imply thinner central melt sheets and a smaller proportion of melt particles in impact breccias on the moon and mars than on earth these effects are illustrated in a comparison of the chicxulub crater on earth linked to the cretaceous tertiary mass extinction gusev crater on mars where the mars exploration rover spirit landed and tsiolkovsky crater on the moon the results are comparable to those obtained from field and spacecraft observations other analytical expressions and hydrocode simulations

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

crater features diagnostic of oblique impacts the size and position of the central peak using magellan data we investigated two crater characteristics that have been cited as diagnostic of oblique impacts an uprange offset of the central peak in complex craters and an increasing central peak diameter relative to crater diameter with decreasing impact angle we find that the offset distribution is random and very similar to that for high angle impacts and that there is no correlation between central peak diameter and impact angle accordingly these two crater characteristics cannot be used to infer the impact angle or direction

investigation of shuttle radar topography mission data of the possible impact structure at serra da cangalha brazil the serra da cangalha crater structure in northeast brazil 13 km in diameter has long been widely considered to be a confirmed impact structure based on reports of shatter cone findings only very limited field work has been carried out at this crater structure landsat thematic mapper tm and shuttle radar topography mission srtm data sets for the region around this crater structure are compared here with regard to their suitability to determine first order structural detail of impact crater structures the srtm data provide very detailed information regarding drainage patterns and topography a pronounced central ring of up to 300 m elevation above the surrounding area two comparatively subdued intermediate rings of 6 and 10 5 km diameter respectively and the broad complex crater rim of up to 100 m elevation can be distinguished in the serra da cangalha data the maximum cratering related regional deformation radial and concentric features seems to be limited to a radial distance of 16 18 km from the center of the structure a first comparison of macrostructural information from several impact structures with that from serra da cangalha does not yield firm trends but the database is still very small at this stage the varied nature of the target geology strongly influences the development of structural features in any impact event

geochemical signatures of redepositional environments the namibian continental margin abstract trace metal abundances in marine sediments have been used extensively to interpret periods of elevated primary productivity and ancient ocean redox conditions however sediment reworking that results in post depositional oxidation such as bio irrigation or lateral sediment transport through oxic water can modify the primary geochemical signal of the sediment which in turn may impact paleo redox and or productivity interpretations in the case of sediments on the namibian continental margin ncm lateral transport and redeposition contribute to the accumulation of organic matter on the margin slope to better constrain the geochemical effects of lateral transport on the ncm we examined the trace metal signature including solid phase fe mo v ni cu and ag and pore water fe mo and v in surface sediments up to 25 cmbsf along a transect from shelf to slope through the primary shelf and secondary upper slope depositional zones of the margin despite varying bottom water redox conditions ranging from seasonally anoxic upper shelf suboxic shelf break and oxic upper slope each site has elevated total organic carbon contents average toc content of 9 1 3 2 and 6 8 wt respectively due to high surface water primary productivity and lateral transport of organic rich material from the shelf to the upper slope our results show that the solid phase contents of the productivity proxies ni cu and ag parallel the organic carbon accumulations largely irrespective of the local redox conditions in contrast the solid phase contents of v mo and fe respond to the local bottom water redox conditions at each site being enriched under strongly reducing conditions and less enriched under oxic bottom waters despite enhanced toc at all three investigated sites thus the enrichment of trace metal and toc at each site on the ncm can not only be used to reconstruct primary depositional bottom water redox conditions but also to identify zones of sediment redeposition using an offshore transect allows for the identification of intense lateral transport and redeposition of organic rich sediments that are occurring along the margin the relative concentrations of both redox sensitive and productivity related trace metals suggest that the decoupling of trace metals and organic carbon enrichments occasionally observed in the geological record could be explained by the process of lateral transport and redeposition

geophysical characterization of the chicxulub impact crater 1 geophysical data indicate that the 65 5 million years ago chicxulub impact structure is a multi ring basin with three sets of semicontinuous arcuate ring faults and a topographic peak ring pr slump blocks define a terrace zone which steps down from the inner rim into the annular trough fault blocks underlie the pr which exhibits variable relief due to target asymmetries the central structural uplift is 10 km and the moho is displaced by 1 2 km the working hypothesis for the formation of chicxulub is a 50 km radius transient cavity lined with melt and impact breccia formed within 10 s of the impact and within minutes weakened rebounding crust rose kilometers above the surface the transient crater rim underwent localized deformation and collapsed into large slump blocks resulting in a inner rim at 70 85 km radius and outer ring faults at 70 130 km radius the overheightened structural uplift collapsed outward buried the inner slump blocks and formed the pr most of the impact melt was ultimately emplaced as a coherent 3 km thick melt sheet within the central basin that shallows within the inner regions of the pr smaller pockets of melt flowed into the annular trough subsequently slope collapse ejecta ground surge and tsunami waves infilled the annular trough and annular basin with sediments up to 3 km and 900 m thick respectively testing this working hypothesis requires direct observation of the impactites within and adjacent to the pr and central basin

stratigraphic and sedimentological observations from seismic data across the chicxulub impact basin seismic data across the offshore half of the chicxulub impact crater reveal a 145 kmdiameter post impact basin to be a thickening of tertiary sediment which thickens by 0 7 sec from the basin margin to the basin center the basin existed long after the impact and was gradually infilled to its current flat surface a suite of seismic horizons within the impact basin have been picked on four reflection lines across the crater they reveal that the western and northwestern parts of the impact basin were filled first subsequently there was a dramatic change in the depositional environment indicated by an unconformable surface that can be mapped across the entire basin a prograding shelf sequence downlaps onto this unconformity in the eastern basin the seismic stratigraphic relationships suggest a marine regression with sedimentation becoming gradually more passive as sediments fill the eastern part of the impact basin the central and northeastern parts of the basin are filled last the onshore hole yaxcopoil 1 yax 1 which was drilled on the flanks of the southern basin has been projected onto the offshore seismic data to the west of the crater center using dates obtained from this onshore well and regional data approximate ages have been placed on the most significant horizons in the offshore seismic data our preliminary interpretation is that the western and northwestern basins were almost entirely filled by 40 ma and that the marine regression observed in the eastern basin is early miocene in age offshore seismic stratigraphic analyses and onshore data within yax 1 suggest that the early paleocene is highly attenuated across the impact basin the mesozoic section appears to be 1 km thicker offshore than onshore we calculate that given this offshore thickening the volume of mesozoic rocks that have been excavated melted or vaporized during impact is around 15 larger than expected from calculations that assume the offshore thickness is equal to that onshore this has significant consequences for any environmental calculations the current offset between the k t boundary outside and inside the crater is 700 m however infilling of basins with sediments is usually accompanied by subsidence and immediately following the impact the difference would have been smaller we calculate the original topographic offset on the k t boundary to have been between 450 and 700 m which is in agreement with depthdiameter scaling laws for a mixed target

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

aeromagnetic anomaly modeling of central zone structure and magnetic sources in the chicxulub crater abstract the analytic signal method and forward modeling in three dimensions are applied to magnetic field data over the chicxulub crater in order to determine and document the main magnetic anomaly sources and crater structure in the central zone aeromagnetic data over the structure reveal three strong well defined concentric patterns with a central 40 km diameter zone of high amplitude anomalies magnetic anomalies are interpreted to be associated with the melt sheet upper breccias and central uplift which present three to four orders of magnitude contrasts with the surrounding carbonate units the limited number of magnetic property measurements apparent wide range in the remanent intensity and susceptibility in the upper breccias and possible yet unconstrained zones of hydrothermal systems have restricted our ability to determine the characteristics and distribution of major structural elements of the chicxulub crater the amplitude of the analytic signal produces maxima over magnetization contrasts independent of the direction of magnetization interpretation of maxima location and depth distribution is used in a second stage as a priori information in the construction of an input prism assemblage magnetic configuration and its properties for three dimensional forward modeling results indicate that magnetic sources extend to a radial distance of 45 km from the center of the structure with average depths ranging between 2 and 4 km the magnetic anomaly sources in the central structural uplift zone are located in the range from 3 5 to 8 km depth with dominant contributions from an apparent large body forming the basement uplift

size and morphology of the chicxulub impact crater the chicxulub impact in mexico has been linked to the mass extinction of species at the end of the cretaceous period from seismic data collected across the offshore portion of the impact crater the diameter of the transient cavity is determined to be about 100 km this parameter is critical for constraining impact related effects on the cretaceous environment with previous estimates of the cavity diameter spanning an order of magnitude in impact energy the offshore seismic data indicate that the chicxulub crater has a multi ring basin morphology similar to large impact structures observed on other planets such as venus

coeval 40ar 39ar ages of 65 0 million years ago from chicxulub crater melt rock and cretaceous tertiary boundary tektites 40 ar 39 ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180 kilometer subsurface chicxulub crater in yucatan mexico has yielded well behaved incremental heating spectra with a mean plateau age of 64 98 0 05 million years ago ma the glassy melt rock of andesitic composition was obtained from core 9 1390 to 1393 meters in the chicxulub 1 well the age of the melt rock is virtually indistinguishable from 40 ar 39 ar ages obtained on tektite glass from beloc haiti and arroyo el mimbral northeastern mexico of 65 01 0 08 ma mean plateau age for beloc and 65 07 0 10 ma mean total fusion age for both sites the 40 ar 39 ar ages in conjunction with geochemical and petrological similarities strengthen the recent suggestion that the chicxulub structure is the source for the haitian and mexican tektites and is a viable candidate for the cretaceous tertiary boundary impact site

chicxulub the third dimension of a multi ring impact basin the buried 65 ma chicxulub impact structure in mexico is the largest well preserved impact crater known on earth seismic reflection data have revealed chicxulub to be a multi ring basin it has the morphology of the largest impact craters in the solar system we use these seismic data to relate surface morphology and near surface structure to deeper deformation within the crust to provide the first high resolution look into the third dimension of a multi ring impact basin we observe three distinct topographic ring types crater rims peak rings and outer rings each is associated with a different style of deep deformation crater rims are the head scarp of the terrace zone formed during the collapse of the transient cavity the peak ring at chicxulub appears to have formed when the central uplift collapsed overthrusting and overriding the terrace zone the impact has affected the whole crust the outer rings at chicxulub are linked to whole crustal deformation in which the middle crust and lower crust have moved inward and downward strong reflections that dip craterward at 35 cut the entire crust and connect normal faulting in the sedimentary section with zones of downthrown moho at a crater radius of 35 55 km weakly developed exterior rings appear as thrust faults with small offsets these appear to be the progenitors of the more significant normally faulted outer rings in multi ring basins

ringed structural zones with deep roots formed by the chicxulub impact the chicxulub meteorite comet impact of 65 ma occurred on the present northeast coast of the yucatan peninsula of mexico and is considered by many to have caused a mass extinction at the end of the cretaceous period impact craters are traditionally described in terms of ring structures deeply rooted faults underlying four previously recognized ring zones of the chicxulub impact crater as revealed on new deep seismic reflection profiles demonstrate the catastrophic and diverse deformations that produced these zones the innermost annular zone at chicxulub coincides with a central peak ring the seismic data show that the peak ring is composed of low velocity 5 km s 1 chaotically reflective material interpreted as impact breccia lying on top of blocks of pre impact strata that were downdropped 4 6 km normal faults within the cretaceous strata and deep inward dipping reflections characterize two deformation zones at radial distances of 55 65 and 85 98 km these normal faults accommodated the collapse during the modification stage of crater development of large radial width of 5 25 km blocks of pre impact strata uplifted during the excavation stage the structures root in the lower crust at depths of 15 30 km blind thrusts indicated by monoclines in pre impact sedimentary rocks and deep inward dipping reflections within the crystalline basement mark the outermost ring of deformation these monoclines correlate with small topographic changes on the cretaceous tertiary boundary layer at radial distances of 120 135 km palinspastic restoration of coherent blocks of downdropped pre impact strata enabled improved estimates of the size and shape of the hole formed by the impact in a prominent reflector at 3 to 4 km depth this hole is slightly elliptical along a se nw major axis radius of 68 versus 63 km

chicxulub central crater structure initial results from physical property measurements and combined velocity and gravity modeling the chicxulub crater in mexico is a nearly pristine example of a large impact crater its slow burial has left the central impact basin intact within which there is an apparently uneroded topographic peak ring its burial however means that we must rely on drill holes and geophysical data to interpret the crater form interpretations of crater structures using geophysical data are often guided by numerical modeling and observations at other large terrestrial craters however such endeavors are hindered by uncertainties in current numerical models and the lack of any obvious progressive change in structure with increasing crater size for this reason proposed structural models across chicxulub remain divergent particularly within the central crater region where the deepest well is only 1 6 km deep the shape and precise location of the stratigraphic uplift are disputed the spatial extent and distribution of the allogenic impact breccias and melt rocks remain unknown as do the lithological nature of the peak ring and the mechanism for its formation the objective of our research is to provide a well constrained 3d structural and lithological model across the central region of the chicxulub crater that is consistent with combined geophysical data sets and drill core samples with this in mind we present initial physical property measurements made on 18 core samples from the yaxcopoil 1 yax 1 drill hole between 400 and 1500 m deep and present a new density model that is in agreement with both the 3d velocity and gravity data future collation of petrophysical and geochemical data from yax 1 core as well as further seismic surveys and drilling will allow us to calibrate our geophysical models assigning a suite of physical properties to each lithology an accurate 3d model of chicxulub is critical to our understanding of large craters and to the constraining of the environmental effects of this impact

crosshole seismic waveform tomography ii resolution analysis summary in an accompanying paper we used waveform tomography to obtain a velocity model between two boreholes from a real crosshole seismic experiment as for all inversions of geophysical data it is important to make an assessment of the final model to determine which parts of the model are well resolved and can confidently be used for geological interpretation in this paper we use checkerboard tests to provide a quantitative estimate of the performance of the inversion and the reliability of the final velocity model we use the output from the checkerboard tests to determine resolvability across the velocity model such tests can act as good guides for designing appropriate inversion strategies here we discovered that by including both reference model and smoothing constraints in initial inversions and then relaxing the smoothing constraint for later inversions an optimum velocity image was obtained additionally we noticed that the performance of the inversion was dependent on a relationship between velocity perturbation and checkerboard grid size larger velocity perturbations were betterresolved when the grid size was also increased our results suggest that model assessment is an essential step prior to interpreting features in waveform tomographic images

shallow crustal structure of chicxulub impact crater imaged with seismic gravity and magnetotelluric data inferences about the central uplift summary the structure of the onshore portion of the chicxulub impact crater yucatan mexico has been studied with seismic gravity and magnetotelluric mt exploration methods a dispersion analysis of rayleigh waves along a 150 km long east west profile permitted the shallow tertiary cover to be imaged to a depth of 400 m three layers were mapped the thickness of the first two layers increase as we approach the sinkhole ring from the exterior of the crater consistent with the existence of a central basin those thicknesses change from 100 150 m outside the crater rim to more than 200 m in the terrace zone at the crater centre the first layer is again approximately 100 m thick and has a low shear wave velocity approximately 1 km s 1 its velocity is slightly larger in the rest of the profile outside the central basin we observed a third layer with a shear wave velocity larger than 1 8 km s 1 but its thickness could not be determined the velocity distribution along our profile increases monotonously with depth without low velocity layers the inferred inward slope of the two shallow layers immediately outside the central basin correlates well with a smooth gravity gradient in addition we present results of detailed gravity measurements along a profile that coincides in part with the line covered by the seismic experiment these gravity measurements were inverted to obtain a density model in this model which is of course non unique the steep gravity gradients are explained by the faults along which the slumping of the terrace blocks took place two of these may correspond to the faults observed at radial distances of 85 and 96 km in offshore seismic data finally we show results obtained using mt soundings they provide additional independent constraints on the structure of the chicxulub crater mt soundings at the centre of the crater show a sharper increase in resistivity than elsewhere in the impact structure this sharper increase in resistivity may be related to the uplifted basement of the structural high at a depth of 4 km only the mt soundings located over the central gravity high display this feature our three different data sets are independently consistent with the existence of a central structural high in the crater and support a twin peak shape for this high

cretaceous tertiary chicxulub impact angle and its consequences the chicxulub impact structure exhibits asymmetries in its geophysical signatures that closely resemble asymmetries produced by oblique impacts in laboratory experiments and recognized on planetary surfaces these asymmetric signatures suggest a trajectory for the chicxulub bolide from the southeast to the northwest at a 20 30 angle from the horizontal as a result biotic extinctions may have been most severe and catastrophic in the northern hemisphere geographic variation in the magnitude of the cretaceous tertiary k t fern spike and palynofloral extinctions are consistent with the proposed trajectory

gravity and magnetic field modeling and structure of the chicxulub crater mexico the chicxulub crater is an 180 km diameter peak ring crater based on drill hole logs and samples potential fields seismic reflection profiles and surface fracture patterns a structural cross section produced based on these constraints has the features expected for a large complex impact crater the bouguer gravity anomaly consists of a broad 90 km radius 30 mgal low with a central 20 km radius 20 mgal high and two 5 mgal concentric lows at 35 and 60 km radius the gravity anomaly is disrupted by large scale basement anomalies and possibly by large scale slumping and backwash erosion effects the magnetic field anomaly over the crater consists of three zones an outer zone from 45 to 90 km radius of low amplitude short wavelength anomalies with an irregular perimeter a middle zone from 20 to 45 km radius of high amplitude short wavelength anomalies slightly elongated nnw sse and an inner 20 km radius single large amplitude anomaly magnetic field modeling indicates that the melt pool averages 90 km in diameter and the melt volume in the crater is estimated at 20 000 km3 the melt pool size constrains the collapsed transient cavity diameter to 90 km gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but 40 km in diameter and underlies or protrudes into the melt pool the preliminary structural cross section indicates that the inferred peak ring is decoupled from the structural uplift the geometry and bouguer gravity signature of the crater indicate that no significant uplift of the moho or relaxation of the crater has occurred

peak ring formation in large impact craters geophysical constraints from chicxulub a seismic reflection and three dimensional wide angle tomographic study of the buried v200 km diameter chicxulub impact crater in mexico reveals the kinematics of central structural uplift and peak ring formation during large crater collapse the seismic data show downward and inward radial collapse of the transient cavity in the outer crater and upward and outward collapse within the central structurally uplifted region peak rings are formed by the interference between these two flow regimes and involve significant radial transport of material hydrocode modeling replicates the observed collapse features impact generated melt rocks lie mostly inside the peak ring the melt appears to be clast rich and undifferentiated with a maximum thickness of 3 5 km in the center fl 2000 elsevier science b v all rights reserved

3d seismic refraction traveltime tomography at a groundwater contamination site we have applied traveltime tomography to 3d seismic refraction data collected at hillair force base utah in an approximately 95 40 m area over a shallow 20 m groundwater contamination site the purpose of this study is to test the ability of 3d first arrival time data to characterize the shallow environment and aid remediation efforts the aquifer is bounded below by a clay aquiclude into which a paleochannel has been incised and acts as a trap for dense nonaqueous phase liquiddnaplcontaminants aregularizednonlineartomographicapproachwasappliedto187 877 first arrival traveltimes to obtain the smoothest minimumstructure 3d velocity model the resulting velocity model contains a velocity increase from less than 300 to 1500 m s in the upper 15 m the model also contains a north southtrendinglow velocityfeatureinterpretedtobethepaleochannel based on more than 100 wells in the area checkerboard testsshow7 5 10 mlateralresolutionthroughoutmostofthe model the preferred final model was chosen after a systematic test of the free parameters involved in the tomographic approach includingthestartingmodel thefinalvelocitymodel compares favorably with a 3d poststack depth migration and 2d waveform inversion of coincident reflection data while the long wavelength features of the model reveal the primary target of the survey the paleochannel the velocity model is likely a very smooth characterization of the true velocity structure particularly in the vertical direction given thesizeofthefirstfresnelzoneforthesedata

heat flow pattern at the chicxulub impact crater northern yucatan mexico abstract along an east west profile crossing the chicxulub impact structure in northern yucatan mexico curie depths were obtained from statistical spectral analysis of a grid of aeromagnetic data 256 km wide and 600 km long these depths were corrected for flight height and depth to the sea floor to determine the geothermal gradient assuming a temperature of 580 c for the curie temperature heat flow was then calculated from the geothermal gradients using a value of 2 67 w m k for the mean crustal thermal conductivity the results show a conspicuous heat flow high above on the impact basin in this location the heat flow is 80 mw m 2 approximately available offshore estimates of the depth to the crustal magnetic source bases on the northern yucatan platform and onshore heat flow determination on 8 shallow bore holes and in a 1511 m deep one support the existence of this major high heat flow anomaly associated with the impact crater this high heat flow might be related to the impact through 1 an uplift of the crystalline basement rocks in the center of the crater and 2 impact induced radioactive element concentration into the crust below the impact structure higher thermal conductivities at the lower crust might also play a key role available seismological and thermal property data are compatible with these mechanisms