propagation characteristics of lg across the tibetan plateau abstract we present the results of a study on the propagation of lg within the tibetan plateau we analyze over 1200 seismograms for lg propagation from 185 local and regional earthquakes recorded at distances from 150 to 2000 km 52 of which are located within the tibetan plateau although the propagation of lg has been observed across most of asia lg has not been observed for paths crossing the tibetan plateau we analyze paths that cross the plateau boundaries and paths contained entirely within the tibetan plateau lg is generated within the tibetan plateau and can propagate to epicentral distances of at least 600 km for events located outside of the tibetan plateau lg is either absent or severely diminished for paths that cross both the himalayan and kunlun ranges confirming that the margins of the plateau weaken lg transmission based on the observations from our qualitative analysis we invert lg amplitudes for event station paths that are confined to the tibetan plateau to estimate the quality factor q the inversion yields a frequency dependent q function q f 366 37 f 0 45 0 06 0 5 f 16 h z similar observations in other areas indicate that frequency dependent apparent q within the tibetan plateau is well below that expected for a typical continental interior instead it has a frequency dependence that is characteristic of a tectonically active region

estimation of source parameters and scaling relations for moderate size earthquakes in north west himalaya abstract the scaling relation and self similarity of earthquake process have been investigated by estimating the source parameters of 34 moderate size earthquakes m b 3 4 5 8 occurred in the nw himalaya the spectral analysis of body waves of 217 accelerograms recorded at 48 sites have been carried out using in the present analysis the brune s 2 model has been adopted for this purpose the average ratio of the p wave corner frequency fc p to the s wave corner frequency fc s has been found to be 1 39 with fc p fc s for 90 of the events analyzed here this implies the shift in the corner frequency in agreement with many other similar studies done for different regions the static stress drop values for all the events analyzed here lie in the range 10 100 bars average stress drop value of the order of 43 19 bars for the region this suggests the likely estimate of the dynamic stress drop which is 2 3 times the static stress drop is in the range of about 80 120 bars this suggests the relatively high seismic hazard in the nw himalaya as high frequency strong ground motions are governed by the stress drop the estimated values of stress drop do not show significant variation with seismic moment for the range 5 10 14 2 10 17 n m this observation along with the cube root scaling of corner frequencies suggests the self similarity of the moderate size earthquakes in the region the scaling relation between seismic moment and corner frequency m o f c 3 3 47 10 16 n m s 3 estimated in the present study can be utilized to estimate the source dimension given the seismic moment of the earthquake for the hazard assessment the present study puts the constrains on the important parameters stress drop and source dimension required for the synthesis of strong ground motion from the future expected earthquakes in the region therefore the present study is useful for the seismic hazard and risk related studies for nw himalaya

first petrographic results on impactites from the yaxcopoil 1 borehole chicxulub structure mexico the icdp yaxcopoil 1 yax 1 borehole located 60 km south southwest of the center of the chicxulub impact structure intercepted an interval of allogenic impactites depth of 795 895 m petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations unit 1 795 922 m comprises an apparently reworked poorly sorted and graded fine grained clast supported melt fragment bearing suevitic breccia the interstitial material similar to units 2 and 3 is permeated by numerous carbonate veinlets units 2 823 846 m and 3 846 861 m are groundmass supported breccias that comprise green to variegated angular and fluidal melt particles the groundmass of units 2 and 3 comprises predominantly fine grained calcite altered alkali element ca and si rich cement as well as occasional lithic fragments unit 4 861 885 m represents a massive variably devitrified and brecciated impact melt rock the lowermost unit unit 5 885 895 m comprises highly variable proportions of melt rock particles mrp and lithic fragments in a fine grained carbonate dominated groundmass this groundmass could represent either a secondary ydrothermal phase or a carbonate melt phase or both units 1 and 5 contain well preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units all units show diagnostic shock deformation features in quartz and feldspar clasts our observations reveal that most felsic and all mafic mrp are altered they register extensive k metasomatism in terms of emplacement we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume whereby unit 1 was subsequently reworked by resurging water unit 4 represents a coherent impact melt body the formation of which involved a significant proportion of crystalline basement unit 5 is believed to represent an initial ejecta ground surge deposit

stability assessment and quantitative evaluation of h v spectral ratios for site response studies in kumaon himalaya india using ambient noise recorded by a broadband seismograph network the kumaon himalaya region in india has accumulated considerable seismic risk potential from moderate to high seismicity due to ongoing tectonic evolutionary processes to assess conditions of high seismic risk arising from local site effects at populated locations we applied the empirical horizontal to vertical h v spectral amplitude ratio method nakamura in quarterly reports of the railway technical research institute tokyo 30 25 33 1989 using ambient seismic noise recorded by a network of 32 digital broadband seismographs from june 2005 to june 2008 the data and the estimated parameters were subjected to stability tests to assess the effect of seasonal variations seasonal variations in the ambient noise data seemed to show a lesser effect on fundamental frequency estimates and a slightly greater effect on the peak h v amplitudes validation of the ambient noise results was done by complementary tests using h v ratios of local and regional earthquakes the peak corresponding to the fundamental resonance frequency is prominently present in both the ambient noise and the earthquake datasets the study locations showed distinct h v curve topologies corresponding to the effects of both velocity contrast at well defined frequencies and characteristic topographic effect around a certain frequency range the clearly indicated peaks in the h v curves possibly correspond to velocity contrasts created by weathered sediments overlying hard rock basements in rocky hills our study indicates high site responses at many populated locations near the surface trace of the seismically active main central thrust mct and other fault boundaries the fundamental resonance frequencies estimated from the site response studies at the 32 locations could be useful in preliminary site characterization ground motion prediction and seismic hazard analysis

impact lithologies and their emplacement in the chicxulub impact crater initial results from the chicxulub scientific drilling project yaxcopoil mexico the chicxulub scientific drilling project csdp mexico produced a continuous core of material from depths of 404 to 1511 m in the yaxcopoil 1 yax 1 borehole revealing top to bottom tertiary marine sediments polymict breccias an impact melt unit and one or more blocks of cretaceous target sediments that are crosscut with impact generated dikes in a region that lies between the peak ring and final crater rim the impact melt and breccias in the yax 1 borehole are 100 m thick which is approximately 1 5 the thickness of breccias and melts exposed in the yucatan 6 exploration hole which is also thought to be located between the peak ring and final rim of the chicxulub crater the sequence and composition of impact melts and breccias are grossly similar to those in the yucatan 6 hole compared to breccias in other impact craters the chicxulub breccias are incredibly rich in silicate melt fragments up to 84 versus 30 to 50 for example in the ries the melt in the yax 1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km thick carbonate platform in the target area small amounts of immiscible molten carbonate were ejected with the silicate melt and clastic carbonate often forms the matrix of the polymict breccias the melt unit appears to have been deposited while molten but brecciated after solidification the melt fragments in the polymict breccias appear to have solidified in flight before deposition and fractured during transport and deposition

thermal evolution and exhumation of the ladakh batholith northwest himalaya india high elevation low relief regions are recognised across the northwest himalaya including the ladakh and kohistan areas a homogeneity of uplift and cooling histories across these large spatial 360 km and temporal 35 ma scales would be remarkable here we examine the thermal evolution of the ladakh batholith northwest india since the early tertiary using a combination of amphibole geobarometry and apatite and zircon low temperature thermochronometry data to investigate the spatio temporal progression of emplacement uplift and exhumation the emplacement depth varies with distance north of the indus river and suggests that the southern part of the batholith 3200 m above mean sea level indicate significant exhumation of the southern margin of the ladakh batholith in the oligocene 26 ma while exhumation of the northern margin occurred later in the mid to late miocene 17 ma topography across the batholith was asymmetric throughout the oligocene and miocene and potentially only since the pliocene has the current amplitude and wavelength been established this extensive dataset from across the ladakh batholith does not support a generalised regional model of early rapid exhumation post india eurasia collision followed by steady and slow denudation to the present

hydrocode simulation of the chicxulub impact event and the production of climatically active gases we constructed a numerical model of the chicxulub impact event using the chart d squared csq code coupled with the analytic equation of state aneos package in the simulations we utilized a target stratigraphy based on borehole data and employed newly developed equations of state for the materials that are believed to play a crucial role in the impact related extinction hypothesis carbonates calcite and evaporites anhydrite simulations explored the effects of different projectile sizes 10 to 30 km in diameter and porosity 0 to 50 the effect of impact speed is addressed by doing simulations of asteroid impacts vi 20 km s and comet impacts vi 50 km s the masses of climatically important species injected into the upper atmosphere by the impact increase with the energy of the impact event ranging from 350 to 3500 gt for co2 from 40 to 560 gt for s and from 200 to 1400 gt for water vapor while our results are in good agreement with those of ivanov et al 1996 our estimated co2 production is 1 to 2 orders of magnitude lower than the results of takata and ahrens 1994 indicating that the impact event enhanced the end cretaceous atmospheric co2 inventory by at most 40 consequently sulfur may have been the most important climatically active gas injected into the stratosphere the amount of s released by the impact is several orders of magnitude higher than any known volcanic eruption and with h2o is high enough to produce a sudden and significant perturbation of earth s climate

composition of impact melt particles and the effects of post impact alteration in suevitic rocks at the yaxcopoil 1 drill core chicxulub crater mexico petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m thick suevitic sequence at the chicxulub yax 1 drill core was performed the aim of this study is to determine the composition of the impact melt the variation between different types of melt particles and the effects of post impact hydrothermal alteration we demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post impact hydrothermal alteration most melt particles are andesitic in composition clinopyroxene rich melt particles possess lower sio2 and higher cao contents these are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks multi stage post impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles following backwash of seawater into the crater palagonitization of glassy melt particles likely caused depletion of sio2 al2o3 cao na2o and enrichment of k2o and feotot during an early alteration stage since glass is very susceptible to fluid rock interaction the state of primary crystallization of the melt particles had a significant influence on the intensity of the postimpact hydrothermal mass transfer and was more pronounced in glassy melt particles than in wellcrystallized particles in contrast to other occurrences of chicxulub impactites the yax 1 suevitic rocks show strong potassium metasomatism with hydrothermal k feldspar formation and whole rock k20 enrichment especially in the lower unit of the suevitic sequence a late stage of hydrothermal alteration is characterized by precipitation of silica analcime and na bearing mg rich smectite among other minerals this indicates a general evolution from a silica undersaturated fluid at relatively high potassium activities at an early stage toward a silica oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction

chicxulub impactites geochemical clues to the precursor rocks the 65 ma chicxulub impact structure mexico with a diameter of 180 km is the focus of geoscientific research because of its link to the mass extinction event at the cretaceous tertiary k t boundary chicxulub now buried beneath thick post impact sediments is probably one of the best preserved terrestrial impact structures known because of its inaccessibility only limited samples on the impact lithologies from a few drill cores are available we report major element and sr nd o and c isotopic data for chicxulub impact melt lithologies and basement clasts in impact breccias of drill cores c 1 and y 6 and for melt particles in the chicxulub ejecta horizon at the k t boundary in beloc haiti the melt lithologies with sio2 ranging from 58 to 63 wt show significant variations in the content of al ca and the alkalies in the melt matrix samples 13c of the calcite is about 3 o the 18o values for the siliceous melt matrices of y 6 samples range from 9 9 to 12 4 o melt lithologies and the black haitian glass have rather uniform 87sr 86sr ratios 0 7079 to 0 7094 only one lithic fragment displays 87sr 86sr of 0 7141 the sr model ages tsrur for most lithologies range from 830 to 1833 ma unrealistic negative model ages point to an open rb sr system with loss of rb in a hydrothermal process the 143nd 144nd ratios for all samples except one basement clast with 143nd 144nd of 0 5121 cluster at 0 5123 to 0 5124 in an nd sr diagram impactites plot in a field delimited by nd of 2 to 6 and sr of 55 to 69 this field is not defined by the basement lithologies described to occur as lithic clasts in impact breccias and cretaceous sediments at least one additional intermediate to mafic precursor component is required to explain the data

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

petrology of impact melt rocks at the chicxulub multiring basin yucatan mexico compositions and textures of melt rocks from the upper part of the chicxulub structure are typical of melt rocks at other large terrestrial impact structures apart from variably elevated iridium concentrations less than 1 5 to 13 5 0 9 ppb indicating nonuniform dissemination of a meteoritic component bulk rock and phenocryst compositions imply that these melt rocks were derived exclusively from continental crust and platform sediment target lithologies modest differences in bulk chemistry among samples from wells located approximately 40 km apart suggest minor variations in relative contributions of these target lithologies to the melts subtle variations in the compositions of early formed pyroxene and plagioclase also support minor primary differences in chemistry between the melts evidence for pervasive hydrothermal alteration of the porous mesostasis includes albite k feldspar quartz epidote chlorite and other phyllosilicates as well as siderophile element enriched sulfides suggesting the possibility that chicxulub like sudbury may host important ore deposits

platinum group elements in impactites of the icdp chicxulub drill core yaxcopoil 1 are there traces of the projectile this study presents results of platinum group element pge analyses of impactites from the yaxcopoil 1 yax 1 and yucatan 6 drill cores of the 180 km diameter chicxulub crater these are the main elements used for projectile identification they were determined by nickel sulfide fire assay combined with inductively coupled plasma mass spectrometry the concentration of pge in the samples are low the concentration patterns of the suevite samples resemble the pattern of the continental crust we conclude that any meteoritic fraction in these samples is below 0 05 a syn and post impact modification of the pge pattern from meteoritic toward a continental crust pattern is very unlikely the globally distributed fallout at the cretaceous tertiary k t boundary however has high pge concentrations therefore the lack of a significant meteoritic pge signature in the crater is not an argument for a pge poor impactor taking the results of three dimensional numerical simulations of the chicxulub event into account the following conclusions are drawn 1 the main fraction of the impactor was ejected into and beyond the stratosphere distributed globally and deposited in the k t boundary clay and 2 the low amount of projectile contamination in the yax 1 lithologies may reflect an oblique impact however the role of volatiles in the mixing process between projectile and target is not well understood and may also have played a fundamental role

attenuation of multiphase surface waves in the basin and range province part i lg and lg coda summary broad band data recorded by the lawrence livermore laboratory network are used to investigate whether or not lg q and lg coda q have similar values in the basin and range province of north america in order to retrieve spectral estimates of the broad band lg and lg coda signals a hybrid method is used which retains high fidelity at lower frequencies while strongly smoothing the spectral estimates and minimizing the number of spectral points at high frequencies a stochastic approach is used to model the lg signal and ambient noise and to analyse results from two station determinations of lg q in the basin and range the effects of ambient noise frequency dependent station site response and focusing defocusing on lg q inversion are analysed it is shown that after averaging spectral ratios determined from repeated observations over four two station paths a modest site effect will not cause significant errors in the resulting lg q relation using this inversion scheme and spectral ratio estimates averaged over those paths at frequencies between 0 2 and 2 5 hz we obtain the relation qlg f 267 f 56 f0 37 0 06 for the basin and range province further constraints on lg q were obtained from lg coda which due to the natural averaging effect of scattering is insensitive to effects of focusing defocusing to investigate the validity of the single isotropic scattering model for lg coda in the basin and range province we calculated ischrons for scatterers corresponding to later lg coda signals as required by the single scattering model it is found that when significant portions of these isochrons are oceanic lg coda amplitude decay becomes irregular and abnormally slow suggesting a strong scattering reflection effect caused by the rapidly changing crustal structure along the coast applying the single isotropic scattering model to lg coda records with isochrons mostly confined to the continental crust results in q f 275 f 26 f 0 36 0 03 which is nearly identical to our qlg relation this indicates that even in a complicated geological province a joint interpretation of the dynamic properties of lg and lg coda can be successful and fruitful inconsistent lg q and coda q relations found in earlier studies in the basin and range province may be attributed to different settings of parameter trade offs different seismic phases or different dynamic properties being used

ries and chicxulub impact craters on earth provide insights for martian ejecta blankets terrestrial impact structures provide field evidence for cratering processes on planetary bodies that have an atmosphere and volatiles in the target rocks here we discuss two examples that may yield implications for martian craters 1 recent field analysis of the ries crater has revealed the existence of subhorizontal shear planes detachments in the periphery of the crater beneath the ejecta blanket at 0 9 1 8 crater radii distance their formation and associated radial outward shearing was caused by weak spallation and subsequent dragging during deposition of the ejecta curtain both processes are enhanced in rheologically layered targets and in the presence of fluids detachment faulting may also occur in the periphery of martian impacts and could be responsible for the formation of lobe parallel ridges and furrows in the inner layer of double layer and multiple layer ejecta craters 2 the ejecta blanket of the chicxulub crater was identified on the southeastern yucatn peninsula at distances of 3 0 5 0 crater radii from the impact center abundance of glide planes within the ejecta and particle abrasion both rise with crater distance which implies a ground hugging erosive and cohesive secondary ejecta flow systematic measurement of motion indicators revealed that the flow was deviated by a preexisting karst relief in analogy with martian fluidized ejecta blankets it is suggested that the large runout was related to subsurface volatiles and the presence of basal glide planes and was influenced by eroded bedrock lithologies it is proposed that ramparts may result from enhanced shear localization and a stacking of ejecta material along internal glide planes at decreasing flow rates when the flow begins to freeze below a certain yield stress

configuration of the indian moho beneath the nw himalaya and ladakh 1 teleseismic receiver function analysis of seismograms recorded on a 700 km long profile of 17 broadband seismographs traversing the nw himalaya shows a progressive northward deepening of the indian moho from 40 km beneath delhi south of the himalayan foredeep to 75 km beneath taksha at the karakoram fault similar studies by wittlinger et al 2004 to the north of the karakoram fault show that the moho continues to deepen to 90 km beneath western tibet before shallowing substantially to 50 60 km at the altyn tagh fault the continuity of the indian moho imaged in the receiver functions reported here along with those of wittlinger et al 2004 suggest that in this part of the himalayan orogen the indian plate may penetrate as far as the bangong suture and possibly as far north as the altyn tagh

physical properties of rocks from the upper part of the yaxcopoil 1 drill hole chicxulub crater physical properties were determined in a first step on post impact tertiary limestones from the depth interval of 404 666 m of the yaxcopoil 1 yax 1 scientific well drilled in the chicxulub impact crater mexico thermal conductivity thermal diffusivity density and porosity were measured on 120 dry and water saturated rocks with a core sampling interval of 2 2 5 m nondestructive non contact optical scanning technology was used for thermal property measurements including thermal anisotropy and inhomogeneity supplementary petrophysical properties acoustic velocities formation resisitivity factor internal surface and hydraulic permeability were determined on a selected subgroup of representative samples to derive correlations with the densely measured parameters establishing estimated depth logs to provide calibration values for the interpretation of geophysical data significant short and long scale variations of porosity 1 37 turned out to be the dominant factor influencing thermal acoustic and hydraulic properties of this post impact limestone formation correspondingly large variations of thermal conductivity thermal diffusivity acoustic velocities and hydraulic permeability were found these variations of physical properties allow us to subdivide the formation into several zones a combination of experimental data on thermal conductivity for dry and water saturated rocks and a theoretical model of effective thermal conductivity for heterogeneous media have been used to calculate thermal conductivity of mineral skeleton and pore aspect ratio for every core under study the results on thermal parameters are the necessary basis for the determination of heat flow density demonstrating the necessity of dense sampling in the case of inhomogeneous rock formations

partial melt in the upper middle crust of the northwest himalaya revealed by rayleigh wave dispersion article i nfo seismic shear wave velocities are sensitive to the partial melts that should be present in the himalayan orogen if low viscosity channel flow is active at the present day we analyzed regional earthquakes in the western himalaya and tibet recorded on 16 broadband seismometers deployed across the nw indian himalaya from the indian platform to the karakoram range we used a multiple filter technique to calculate the group velocity dispersion of fundamental mode rayleigh waves and then inverted the dispersion records to obtain separate one dimensional shear wave velocity models for five geologic provinces the tibetan plateau ladakh arc complex indus tsangpo suture zone tethyan himalaya and himalayan thrust belt our velocity models show a low velocity layer lvl with 7 17 velocity reduction centered at 30 km depth and apparently continuous from the tethyan himalaya to the tibetan plateau this lvl shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile of the possible explanations for low velocity and low resistivity in the mid crust only the presence of melts or aqueous fluids or both satisfactorily explains both sets of observations elevated heat flow observed in the nw himalaya implies that if aqueous fluids are present in the mid crust then the mid crust is well above its solidus comparison of our results with laboratory measurements and theoretical models suggests 3 7 melt is present in a channel in the upper middle crust of the nw himalaya at the present day and the physical conditions to enable active channel flow may be present

is the transition impact to post impact rock complete some remarks based on xrf scanning electron microprobe and thin section analyses of the yaxcopoil 1 core in the chicxulub crater the transition from impact to post impact rocks in the yaxcopoil 1 core is marked by a 2 cm thick clay layer characterized by dissolution features the clay overlies a 9 cm thick hardground overlying a 66 cm thick crossbedded unit consisting of dolomite sandstone alternating with thin micro conglomerate layers with litho and bioclasts and the altered remains of impact glass now smectite the micro conglomerates mark erosion surfaces microprobe and backscatter sem analysis of the dolomite rhombs show an early diagenetic complex zoned idiomorphic overgrowth with mn rich zones possibly formed by hot fluids related to cooling melt sheet in the crater the pore spaces are filled with several generations of coelestite barite k feldpar and sparry calcite xrf core scanning analysis detected high mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements cr co fe and ni in the clay layer shocked quartz occurs in the crossbedded unit but is absent in the clay layer the basal paleocene marls are strongly dissolved and do not contain a basal paleocene fauna the presence of a hardground the lack of siderophile elements shocked quartz or ni rich spinels in the clay layer and the absence of basal paleocene biozones p0 and pa all suggest that the top of the ejecta sequence and a significant part of the lower paleocene is missing due to the high energy sedimentation infill a hiatus at the top of the impactite is not unexpected but there is nothing in the biostratigraphy geochemistry and petrology of the yaxcopoil 1 core that can be used to argue against the synchroneity of the end cretaceous mass extinctions and the chicxulub crater

structure and impact indicators of the cretaceous sequence of the icdp drill core yaxcopoil 1 chicxulub impact crater mexico as part of the icdp chicxulub scientific drilling project the yaxcopoil 1 yax 1 bore hole was drilled 60 km south southwest of the center of the 180 km diameter chicxulub impact structure down to a depth of 1511 m a sequence of 615 m of deformed cretaceous carbonates and sulfates was recovered below a 100 m thick unit of suevitic breccias and 795 m of post impact tertiary rocks the cretaceous rocks are investigated with respect to deformation features and shock metamorphism to better constrain the deformational overprint and the kinematics of the cratering process the sequence displays variable degrees of impact induced brittle damage and post impact brittle deformation the degree of tilting and faulting of the cretaceous sequence was analyzed using 360 core scans and dip meter log data in accordance with lithological information these data suggest that the sedimentary sequence represents a number of structural units that are tilted and moved with respect to each other three main units and nine sub units were discriminated brittle deformation is most intense at the top of the sequence and at 1300 1400 m within these zones suevitic dikes polymict clastic dikes and impact melt rock dikes occur and may locally act as decoupling horizons the degree of brittle deformation depends on lithology massive dolomites are affected by penetrative faulting while stratified calcarenites and bituminous limestones display localized faulting the deformation pattern is consistent with a collapse scenario of the chicxulub transient crater cavity it is believed that the cretaceous sequence was originally located outside the transient crater cavity and eventually moved downward and toward the center to its present position between the peak ring and the crater rim thereby separating into blocks whether or not the stack of deformed cretaceous blocks was already displaced during the excavation process remains an open question the analysis of the deformation microstructure indicates that a shock metamorphic overprint is restricted to dike injections with an exception of the so called paraconglomerate abundant organic matter in the yax 1 core was present before the impact and was mobilized by impact induced heating and suggests that 12 km 3 of organic material was excavated during the cratering process

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

deep resistivity structure of the northwest indian himalaya and its tectonic implications 1 long period magnetotelluric mt data were collected at 15 stations on a 250 km long profile in the northwest indian himalaya to study the structure of this continent continent collision zone two dimensional mt inversion was used to find a resistivity model that fit the data in the upper crust low resistivity was imaged on the limbs of the tso morari dome and may originate in serpentenization or zones of graphite the indus tsangpo suture was imaged as a sub vertical conductive structure that dips northeast and merges with a mid crustal conductor a north dipping zone of low resistivity is imaged at the top of the underthrust indian plate and is likely due to fluids expelled from the underthrust sedimentary rocks north of the indus suture zone this layer is located at 20 25 km depth with a conductance around 3000 s the resistivity of 5 10 m can be attributed to the presence of fluids likely partial melt or aqueous fluids this layer is underlain by a relatively resistive indian crust that extends from the high himalaya to north of the indus suture a decrease in deep resistivity was observed at the northern end of the profile and is similar to the structures observed further east in the region of the indepth study despite the smaller north south extent of the orogen in the indian himalaya

petrogenesis of an augite bearing melt rock in the chicxulub structure and its relationship to k t impact spherules in haiti geophysical anomalies on the yucatan peninsula define a buried circular structure with an approximate diameter of 180 km refs 1 3 these anomalies along with stratigraphic and petrological data including evidence for shock metamorphism have been used to interpret the structure as an impact crater4 this structure known as chicxulub is particularly interesting because it formed at or near the end of the cretaceous period in the geographical region where an impact is believed to have occurred in large part because of a thick ejecta deposit found on haiti5 glassy tektite like relics in this deposit6 9 are unusually calcic up to 31 wt cao ref 7 providing a further circumstantial link with the chicxulub structure which penetrates a carbonate and evaporite sequence7 8 here we strengthen this link by showing that a simple chemical relationship exists between the glasses and an augite bearing melt rock found within the chicxulub structure and argue that the composition of this melt rock could not easily have been produced by volcanic processes

chicxulub impact event is cretaceous paleogene boundary in age new micropaleontological evidence abstract high resolution and quantitative planktic foraminiferal biostratigraphy from two se mexico stratigraphic sections bochil guayal shows that the chicxulub related complex clastic unit ccu is synchronous with the ejecta rich airfall layer and the cretaceous paleogene k pg catastrophic mass extinction horizon in the el kef tunisia and caravaca spain sections the lowermost danian h holmdelensis subzone biozone p0 was identified in both sections in a thin dark clay bed just above the ccu proving that such bed is chronostratigraphically equivalent to the k pg boundary clay of the el kef stratotype these new micropaleontogical data confirm that the k pg impact event and the chicxulub impact event are the same one this contradicts the suggestion by others that the chicxulub impact predated the k pg boundary by about 300 ka

hydrocode simulations of chicxulub crater collapse and peak ring formation we use hydrocode modeling to investigate dynamic models for the collapse of the chicxulub impact crater our aim is to integrate the results from numerical simulations with kinematic models derived from seismic reflection and wide angle velocity data to further our understanding of the formation of large impact craters in our simulations we model the collapse of a 100 km diameter bowlshaped cavity formed in comprehensively fractured crustal material to facilitate wholesale collapse we require that the strength of the target be significantly weakened in the present model we achieve this using acoustic fluidization where strong vibrations produced by the expanding shock wave cause extreme pressure fluctuations in the target at times and positions where the overburden pressure is sufficiently counteracted the frictional resistance is reduced enabling the rock debris to flow our simulations produce a collapsed crater that contains most of the features that we observe in the seismic data at chicxulub in particular we observe a topographic peak ring formed as material that is originally part of the central uplift collapses outward and is thrust over the inwardly collapsing transient crater rim this model for peak ring generation has not been previously demonstrated by numerical simulations and predicts that the peak ring is composed of deeply derived material and that the stratigraphy within the peak ring is overturned c 2002 elsevier science usa

strong lg wave attenuation in the northern and eastern tibetan plateau measured by a two station two event stacking method 1 the regional seismic phase lg which involves shear wave reverberations in the crust is strongly attenuated along propagation paths in northern and eastern tibet robust estimates of the attenuation quality factor qlg in these regions are obtained using a two station two event stacking method for broadband vertical component lg data in the passband 0 2 1 0 hz we find average 1 hz qlg values qo of 79 4 in northern central tibet and 119 17 in eastern tibet for the 0 2 0 5 hz passband localized regions of eastern tibet have qo estimates ranging from 66 to 121 these results confirm and augment the spatial coverage of recent estimates of low qo values in north central and south central tibet and support the notion of widespread partial melting of the crust throughout the plateau

impact related dike breccia lithologies in the icdp drill core yaxcopoil 1 chicxulub impact structure mexico petrographic descriptions of three dike breccia lithologies from drill core yaxcopoil 1 yax 1 are presented they occur within allochthonous units of displaced sedimentary megablocks of the chicxulub impact structure the suevitic dike breccias are the uppermost dike lithology they contain melt rock particles and melt injections into the dike groundmass shock features occur ubiquitously and indicate a strong thermal annealing flow textures suggest a highly energetic emplacement process possibly during the excavation stage as a ground surge related deposit the impact melt rock dikes are present in a strongly brecciated megablock interval as flow textured anastomozing veinlets of impact melt rock that were altered to clay minerals the melt impregnated a dolomitic host rock indicating a low viscosity and thus high initial temperatures brecciation of the impact melt rock dikes occurred while they were still below the glass transition temperature suggesting that dynamic conditions prevailed shortly after the emplacement process major element data indicates that the impact melt rock dikes differ in composition from the homogenized impact melt rock of chicxulub this could point to an emplacement during the late compression or early excavation stages of cratering the clastic polymict dike breccias are coeval with pervasive brittle fracturing of the host rocks they bear clasts including some crystalline basement and possible melt rock particles in a fine grained dolomite matrix with turbulent flow textures fabric and texture indicate a granular flow at ambient pressures such conditions could be envisaged for the excavation phase while the transient cavity grew and fractures opened

lateral variations of crustal seismic attenuation along the indepth profiles in tibet from lg q inversion 1 fourier spectra of crustal guided lg waves are collected to study seismic attenuation or q along the indepth profiles the 1 hz lg q q0 values are measured between many pairs of two stations and input to a back projection algorithm to obtain laterally varying q0 models along the indepth iii profile in central tibet the q0 model consists of low and nearly constant values of 90 these low values are consistent with abnormally high temperature as well as partial melts in the crust along the indepth ii profile in southern tibet the q0 values increase southward they are extremely low 60 over the northernmost segment but increase by a factor of 2 over a distance of 100 km between the indus yalong suture and the kangmar dome farther south and into the high himalayas q0 values are higher than 300 regional rayleigh waves observed along the northern indepth ii profile are used to infer a low velocity low q layer at midcrustal depths between 15 and 30 km in southern tibet the aqueous fluid trapped in the upper crust and a midcrust partial melting zone associated with the underthrusting of the indian lithosphere are the likely causes of the high lg attenuation in southern tibet low q rather then a systematic lg conversion along a dipping moho is the main cause of the previously observed lg blockage over paths crossing the southern tibetan boundary

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

radial transpression ridges a new structural feature of complex impact craters the gravity driven collapse of complex impact craters induces the mass transfer of large rock volumes in distal parts of a crater inward movements dominate whereas in the centre of an impact structure outward movements occur if the central uplift collapses the particle trajectory field is centro symmetric which signifies that the conditions for plane strain deformation are not fullfilled converging particle trajectories can be compensated either by a bulk thickening of inward sliding masses folding repetition of rock units along thrust faults plastic flow or by the formation of localised radial transpression ridges rtr at the edges of individual landslides in these transpression ridges material is uplifted to accommodate the converging mass flow different modes of uplift are possible including radial folding lateral overthrusting and the formation of positive flower structures a simple geometric model is used to estimate the amount of transpression thickening and bulk thickening of inward sliding masses on the base of volumetric considerations the existence of rtrs is confirmed by structural investigations at the siljan impact structure sweden as well as at other complex impact craters on earth

strong lg attenuation in the tibetan plateau the widespread existence of strong lg attenuation in the tibetan plateau is further demonstrated by analysis of lg spectra on many paths within the plateau and quantitative estimates of the lg attenuation from low frequency lg signals for new localized path geometries strong path length dependent shifts of the lg spectra to lower corner frequencies with increasing distance are observed across the plateau consistent with a low regional average 1 hz lg attenuation value q 0 of about 125 there are clearly lateral variations within the plateau found in this and other recent studies with localized areas having q 0 values of 60 90 low enough to eliminate high frequency lg energy over path lengths of just several hundred kilometers while some localized areas may have higher q 0 values of up to 147 or higher a q 0 value of 103 is found in south central tibet compatible with recent work by others for higher frequencies on very localized scales and values from 83 to 147 are found in eastern tibet the lowest q 0 estimates found in tibet tend to be in areas for which there is evidence of volcanism and or partial melting within the crust however the strong regional attenuation may have a contribution from scattering by small scale crustal heterogeneity the strong lg attenuation in tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges to seismic monitoring of the region for possible clandestine nuclear tests

magnetostratigraphy of the impact breccias and post impact carbonates from borehole yaxcopoil 1 chicxulub impact crater yucatan mexico we report the magnetostratigraphy of the sedimentary sequence between the impact breccias and the post impact carbonate sequence conducted on samples recovered by yaxcopoil 1 yax 1 samples of impact breccias show reverse polarities that span up to 56 cm into the post impact carbonate lithologies we correlate these breccias to those of pemex boreholes yucatan 6 and chicxulub 1 from which we tied our magnetostratigraphy to the radiometric age from a melt sample from the yucatan 6 borehole thin section analyses of the carbonate samples showed a significant amount of dark minerals and glass shards that we identified as the magnetic carriers therefore we propose that the mechanism of magnetic acquisition within the carbonate rocks for the interval studied is detrital remanent magnetism drm with these samples we constructed the scale of geomagnetic polarities where we find two polarities within the sequence a reverse polarity event within the impact breccias and the base of the post impact carbonate sequence up to 794 07 m and a normal polarity event in the last 20 cm of the interval studied the polarities recorded in the sequence analyzed are interpreted to span from chron 29r to 29n and we propose that the reverse polarity event lies within the 29r chron the magnetostratigraphy of the sequence studied shows that the horizon at 794 11 m deep interpreted as the k t boundary lies within the geomagnetic chron 29r which contains the k t boundary

spinel bearing spherules condensed from the chicxulub impact vapor plume formation of the giant chicxulub crater off mexico s yucatan peninsula coincided with deposition of the global ir rich cretaceous tertiary k t stratigraphic boundary layer at ca 65 ma the boundary is marked most sharply by abundant spherules containing unaltered grains of magnesioferrite spinel here we predict for the first time the sequential condensation of solids and liquids from the plume of vaporized rock expected from oblique k t impacts we predict highly oxidizing plumes that condense silicate liquid droplets bearing spinel grains whose compositions closely match those marking the actual boundary systematic global variations in spinel composition are consistent with higher condensation temperatures for spinels found at atlantic and european sites than for those in the pacific

seismological evidence for shallow crustal melt beneath the garhwal high himalaya india implications for the himalayan channel flow summary we investigate the seismic attenuation along a 160 km profile from the lower to the high himalaya in the garhwal region india through the analysis of lg waveforms from regional earthquakes recorded on 18 broadband seismographs with interstation spacing of 7 10 km lateral variability in attenuation is derived through the inversion of 36 two station q 0 lg q at 1 hz measurements using a global optimization scheme we observe a contrasting attenuation property in the two himalayan belts the lower himalaya has q 0 of 742 235 similar to those observed in the indian shield whereas the high himalaya is characterized by an unusually low q 0 value of 30 60 the seismic attenuation is also well correlated with the p wave teleseismic traveltime residual pattern faster arrival approximately 0 2 s at stations in the lower himalaya as compared to azimuthally independent time delay of 0 75 s for the high himalayan stations the high attenuation and low velocity in the high himalaya suggests that low viscosity and partial melt in the crust can be correlated to the presence of miocene leucogranite plutons in this himalayan belt a magmatic product of the indo asian collision and presumably evidence of a partial melting event this shallow low viscosity channel possibly connects the mid crustal channel beneath tibet

the suevite of drill hole yucatan 6 in the chicxulub impact crater the suevite breccia of the chicxulub impact crater yucatan mexico is more variable and complex in terms of composition and stratigraphy than suevites observed at other craters detailed studies microscope electron microprobe sem xrf have been carried out on a noncontinuous set of samples from the drill hole yucatn 6 y6 located 50 km sw from the center of the impact structure three subunits can be distinguished in the suevite the upper unit is a fine grained carbonate rich suevite breccia with few shocked basement clasts mostly altered melt fragments and formerly melted carbonate material the middle suevite is a coarse grained suevite with shocked basement clasts and altered silicate melt fragments the lower suevite unit is composed of shocked basement and melt fragments and large evaporite clasts the matrix of the suevite is not clastic but recrystallized and composed mainly of feldspar and pyroxene the composition of the upper members of the suevite is dominated by the sedimentary cover of the yucatan target rock with depth in well y6 the amount of carbonate decreases and the proportion of evaporite and silicate basement rocks increases significantly even at the thin section scale melt phases of different chemistry can be identified showing that no widespread homogenization of the melt took place the melt compositions also reflect the heterogeneity of the deep yucatan basement calcite with characteristic feathery texture indicates the existence of formerly pure carbonate melt the proportion of carbonate to evaporite clasts is less than 5 1 except in the lower suevite where large evaporite clasts are present this proportion constrains the amount of co2 and sox released by the impact event

seismic character of moho beneath the nw himalaya and ladakh inferred from regional earthquakes travel time data we study the uppermost mantle velocities and dip of indian moho beneath the nw himalaya and ladakh using 42 regional waveform data recorded on 15 seismographs along a 600 km long profile we use the two way travel time and interstation velocity methods the apparent pn and sn velocities beneath the nw himalaya are 8 08 0 04 and 4 64 0 07 km s for earthquakes occurring south of the profile downdip western indian shield and 8 70 0 13 and 4 76 0 12 km s for earthquakes from north updip western tibet similarly these velocities beneath ladakh are 7 18 0 07 and 4 32 0 05 km s for earthquakes due south downdip north indian shield and 8 50 0 10 and 4 39 0 12 km s for earthquakes due north updip western tibet these velocity variations constrain the moho dip at 2 4 0 14o beneath the nw himalaya and 6 6 0 54o beneath ladakh considering the varying dips along the profile we observe that the true pn 8 37 0 07 km s and sn 4 70 0 1 km s velocities are higher for the nw himalaya than for ladakh 7 73 0 08 and 4 33 0 09 km s the large variation in interstation pn velocity is observed between the station pairs near the indus zangpo suture zone due to steep dipping 7 1o to 6 26o of the indian moho in the himalaya region the interstation and average values of the velocities and moho dip are comparable whereas a variation is observed in different segments of the ladakh region the results show that the indian moho is underthrusting at a shallow angle 2 5o beneath the himalaya steepens abruptly 6 6o further north of the southern tibetan detachment and continues at a shallow angle 3 8o beneath ladakh

hydrothermal alteration in the core of the yaxcopoil 1 borehole chicxulub impact structure mexico petrographic electron microprobe and raman spectrometric analyses of yaxcopoil 1 core samples from the chicxulub crater indicate that the impact generated a hydrothermal system relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids the earliest calcite halite and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside hedenbergite aeg3fs 18 33 en32 11wo 47 53 after primary augite and widespread na k for ca metasomatic alkali exchange in plagioclase hydrothermal sphene apatite magnetite bornite as well as early calcite combined 3 to 8 vol were introduced with metasomatic feldspar a lower temperature regime characterized by smectite after probable primary glass secondary chlorite and other pre existing mafic minerals as well as very abundant calcite veins and open space fillings extensively overprinted the early hydrothermal stage the composition of early and late hydrothermal minerals show that the solution was chlorine rich cl f 10 and that its fe mg ratio and oxidation state increased substantially 4 to 5 logo2 units as temperature decreased through time the most altered zone in the impactite sequence occurs 30 m above the impact melt the lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at yaxcopoil 1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre existing saline brine and heat that traveled to the yaxcopoil 1 site from adjacent areas where the melt sheet was thicker limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the tertiary marine cover in addition scarce latest anatase stringers chalcopyrite and barite in vugs francolite after apatite and recrystallized halite are the likely products of limited post ydrothermal ambient temperature diagenesis or ocean and or meteoric water circulation

reconstruction of the chicxulub ejecta plume from its deposits in drill core yaxcopoil 1 formation conditions of suevite like impactites from an 100 m thick drill core sequence through the cretaceous tertiary chicxulub crater were reconstructed from empirical data obtained by petrologic and image analytical methods the temporal evolution of the cratering process from the initial stage of excavation to the collapse of the ejecta plume is evidenced by the petrographic characteristics and modal composition of the suevitic rocks including the size distribution and shape parameters of melt particles emplacement of the lowermost suevitic deposits likely started in the first minute after the impact by the passing ejecta curtain that interacted with the expanding ejecta plume these ejecta deposits were capped by a tongue of coherent impact melt that was transported outward from the crater center during the collapse of the central uplift 5 min after impact on top of this brecciated impact melt rock the collapsing ejecta plume deposited air fall suevites the basal air fall unit middle suevite may have been deposited due to a density current like clumping of hot debris with progressive cooling regions of the ejecta plume were entrained in its collapse that produced vapor condensates accretionary rims and different oxygen fugacities after cooling progressed atmospheric conditions began to reestablish over the crater and turbulence decreased supposedly after the first 10 min of initial ejecta plume collapse this led to a winnowing out of fine matrix material and distinct sorting however due to aquatic reworking only material that was deposited until 1 h after cessation of turbulent atmospheric conditions was retained

emplacement of cretaceous tertiary boundary shocked quartz from chicxulub crater observations on shocked quartz in cretaceous tertiary k t boundary sediments compellingly tied to chicxulub crater raise three problems first in north america shocked quartz occurs above the main k t ejecta layer second shocked quartz is more abundant west than east of chicxulub third shocked quartz reached distances requiring initial velocities up to 8 kilometers per second corresponding to shock pressures that would produce melt not the moderate pressure shock lamellae observed shock devolatilization and the expansion of carbon dioxide and water from impacted wet carbonate producing a warm accelerating fireball after the initial hot fireball of silicate vapor may explain all three problems

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

magnetic mineralogy of the yaxcopoil 1 core chicxulub core from the yaxcopoil 1 yax 1 hole drilled as a result of the chicxulub scientific drilling project csdp has been analyzed to investigate the relationship between opaque mineralogy and rock magnetic properties twenty one samples of suevite recovered from the depth range 818 894 m are generally paramagnetic with an average susceptibility of 2000 x 10 6 si and have weak remanent magnetization intensities average 0 1 a m the predominant magnetic phase is secondary magnetite formed as a result of low temperature 45000 x 10 6 si and a remanent magnetization of 77 5 a m magnetic mafic basement clasts are a common component in the yax 1 impactite sequence the high susceptibility and remanence in the mafic basement clasts are caused by the replacement of amphiboles and pyroxenes by an assemblage with fine 1 m magnetite ilmenite k feldspar and stilpnomelane replacement of the mafic minerals by the magnetic alteration assemblage occurred before impact similar alteration mechanisms if operative within the melt sheet could explain the presence of the high amplitude magnetic anomalies observed at chicxulub

impactites of the yaxcopoil 1 drilling site chicxulub impact structure petrography geochemistry and depositional environment the impact breccias encountered in drill hole yaxcopoil 1 yax 1 in the chicxulub impact structure have been subdivided into six units the two uppermost units are redeposited suevite and suevite and together are only 28 m thick the two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine grained top unit 3 23 m thick nue ardente and a coarse breccia unit 4 15 m thick below as such they consist of a mlange of clastic matrix breccia and melt breccia the pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain unit 5 24 m thick is a silicate impact melt breccia whereas unit 6 10 m thick is largely a carbonate melt breccia with some clastic matrix components unit 5 and 6 reflect an overturning of the target stratigraphy the suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris reaction of the super heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes this activity caused further brecciation of melt and melt fragments the fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the yax 1 drill site this is evidence that the fallback suevite deposit units 1 and 2 originally was much thicker unit 1 exhibits sedimentological features suggestive of suevite redeposition erosion possibly has occurred right after the k t impact due to seawater backsurge but erosion processes spanning thousands of years may also have been active therefore the top of the 100 m thick impactite sequence at yaxcopoil in our opinion is not the k t boundary

yucatan subsurface stratigraphy implications and constraints for the chicxulub impact much of the discussion about the effects of an end of cretaceous impact by a large extraterrestrial body in northwestern yucatan has been done in the context of limited and partly erroneous published data on the mesozoic stratigraphy of that area reexamination of cores and geophysical logs taken in several pemex wells has produced improved lithologic and biostratigraphic correlation of the jurassic to maastrichtian section across the northern yucatan peninsula these data suggest that major disturbance of strata by an impact would have been confined to within about 100 km of the proposed impact center near chicxulub the only unusual lithologic unit is polymict breccia which apparently was penetrated at or near the top of the cretaceous section in all the deep wells of northern yucatan this breccia in pemex wells yucatan 1 2 4 5a and 6 is composed predominantly of detrital dolomite limestone and anhydrite clasts set in dolomitized carbonate mud matrix which contains upper maastrichtian foraminifers these constituents mixed with fragments of altered glass or melt rock shocked quartz and feldspar and basement rock suggest an impact as the most likely origin for the breccia the timing of brecciation is poorly constrained by biostratigraphic data there is some evidence however that the breccia unit is overlain by about 18 m of uppermost maastrichtian marls suggesting an impact before the cretaceous tertiary boundary in addition there may have been more than one episode of breccia deposition

characteristics of lg attenuation in the tibetan plateau 1 lg a regional seismic wave comprised of multiple shear wave reverberations trapped in the crustal waveguide is important for magnitude estimation and source discrimination for monitoring nuclear testing treaties in stable continental regions lg propagates with a group velocity of about 3 5 km s and can often be observed at distances up to 4000 km to better understand the absence of high frequency lg arrivals for paths traversing the northern boundary of the tibetan plateau we investigate spatial variations of broadband 0 15 5 0 hz energy in the lg group velocity window 3 6 3 0 km s using regional waveforms recorded at the chinese digital seismic network station wmq vertical component seismograms are analyzed for 90 events with magnitudes of 4 4 mb 6 4 that occurred between 1987 and 1999 in the tibetan plateau and around its margins the lg amplitude spectra for events located near the northern margin of the plateau have apparent corner frequencies of 1 2 hz nearly identical to those for comparable size events at similar distances outside the plateau high frequency 1 hz lg energy recorded at wmq decreases rapidly as a function of source distance into the plateau a path length of 300 400 km within the northern plateau suffices to eliminate 2 5 hz lg energy for events in southern tibet with paths crossing the central portion of the tibetan plateau almost total lg extinction occurs even for energy in the low frequency band of 0 2 1 hz corresponding apparent corner frequencies of the lg amplitude spectra range between 0 2 and 0 4 hz the corner frequency shift is found to vary systematically with path length across the plateau linear regressions demonstrate that the shift in apparent corner frequency of lg amplitude spectra is negatively correlated with features of the tibetan plateau such as mean elevation along the paths or travel distance within the plateau above specified elevation thresholds the systematic variations in the amplitude and frequency content of energy in the lg window as a function of path length within the plateau indicate that strong crustal attenuation plays an important role in lg extinction for paths traversing central and northern tibet superimposed on any structural blockage effects associated with abrupt thinning of the crust near the northern boundary of the plateau spectral ratios of many event pairs along great circle paths give estimates of frequency dependent lg attenuation for paths crossing western central and eastern sectors of tibet the region of strong sn attenuation in northern central tibet also has strong lg attenuation with qlg for 1 hz on the order of qo 80 90 while in southern central tibet qo increases to about 316 and in eastern and western tibet qo is on the order of 120 200 for paths traversing the entire plateau the strong lg attenuation in northern central tibet is responsible for the so called lg blockage and may be associated with partial melting in the crustal low velocity layer in northern tibet our qlg values for tibet are significantly lower than most earlier estimates primarily as a result of not excluding blocked observations along with allowing for lateral variations within tibet index terms 7203 seismology body wave propagation 7255 seismology surface waves and free oscillations 7205 seismology continental crust 1242 keywords lg tibetan plateau nuclear event discrimination crustal attenuation lg blockage crustal partial melting

fe rich and k rich mafic spherules from slumped and channelized chicxulub ejecta deposits in the northern la sierrita area ne mexico spherule deposits commonly interpreted as ejecta from the chicxulub impact at yucatan mexico are present in many k t cretaceous tertiary sections geological mapping of the northern la sierrita area ne mexico revealed the presence of 1 multiple spherule deposits embedded in late maastrichtian marls which are folded or disaggregated breccia like they are up to 6 m thick locally present in two outcrop areas and show limited lateral continuity these deposits consist of mm cm sized spherical to drop shaped vesiculated spherules angular to filamentous ejecta fragments and abundant carbonate they are interpreted as primary ejecta fallout deposits that have been affected by subsequent local slumps slides liquefaction and debris flows welded components suggest an initial ground surge like ejecta dispersion mode 2 a spherule deposit 10 60 cm thick that constitutes the base of a channelized sand siltstone deposit at or close to the k t boundary and is characterized by wide lateral continuity it is of similar petrologic composition to deposit 1 though slightly enriched in terrigeneous detritus thus reflecting influx from proximal shelf areas it is interpreted to result from debris flows and turbidite currents though no size sorting and abrasion of ejecta has been observed petrological mineralogical and geochemical criteria suggest that ejecta components from both types of spherule deposits are similar and originated from the chicxulub impact with multiple deposits produced by subsequent remolding reworking and redeposition spherules and fragments have an fe 25 30 wt al mg rich and si poor 25 wt sio2 composition and are altered to chlorite and iron oxides though rare k rich mafic glass 50 wt sio2 5 8 wt k is also present they contain ti fe k rich schlieren fe mg rich globules and rare m sized metallic and sulfidic ni co rich inclusions carbonate as clasts and within spherules and fragments shows textures indicative of quenching and or liquid immiscibility although potential ejecta fractionation and alteration make accurate evaluation difficult this composition suggests an ejecta origin mainly from mafic lithologies and carbonaceous sediments in addition to a contribution from intermediate felsic rocks and the possibility of meteoritic contamination

geochemical and petrographic characteristics of impactites and cretaceous target rocks from the yaxcopoil 1 borehole chicxulub impact structure mexico implications for target composition we present major and trace element data as well as petrographic observations for impactites suevitic groundmass bulk suevite and melt rock particles and target lithologies including cretaceous anhydrite dolomite argillaceous limestone and oil shale from the yaxcopoil 1 borehole chixculub impact structure the suevitic groundmass and bulk suevite have similar compositions largely representing mixtures of carbonate and silicate components the latter are dominated by melt rock particles trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites in contrast major elements indicate a strong calcitic component in the impactites the siliceous end member requires a mafic component in order to explain the low sio2 content multicomponent mixing of various target rocks the high alteration state and dilution by carbonate complicate the determination of primary melt particle compositions however two overlapping compositional groups can be discerned a high ba low ta group and a high fe high zn and high hf group cretaceous dolomitic rocks argillaceous limestone and shale are typically enriched in u as br and sb whereas anhydrite contains high sr contents the oil shale samples have abundances that are similar to the north american shale composite nasc but with a comparatively high u content clastic sedimentary rocks are characterized by relatively high th hf zr as and sb abundances petrographic observations indicate that the cretaceous rocks in the yaxcopoil 1 drill core likely register a multistage deformation history that spans the period from pre to post impact contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins we have found no evidence in our samples from a depth of 1347 1348 m for the presence of melt breccia we favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event

extremely acid permian lakes and ground waters in north america evaporites hosted by red beds red shales and sandstones some 275 265 million years old extend over a large area of the north american mid continent1 they were deposited in non marine saline lakes pans and mud flats2 settings that are typically assumed to have been alkaline here we use laser raman microprobe analyses of fluid inclusions trapped in halites from these permian deposits to argue for the existence of highly acidic ph 1 lakes and ground waters these extremely acidic systems may have extended over an area of 200 000 km2 modern analogues of such systems may be natural acid lake and groundwater systems ph 2 4 in southern australia3 4 5 6 7 8 9 both the ancient and modern acid systems are characterized by closed drainage arid climate low acid neutralizing capacity and the oxidation of minerals such as pyrite to generate acidity the discovery of widespread ancient acid lake and groundwater systems demands a re evaluation of reconstructions of surface conditions of the past and further investigations of the geochemistry and ecology of acid systems in general

origin and emplacement of the impact formations at chicxulub mexico as revealed by the icdp deep drilling at yaxcopoil 1 and by numerical modeling we present and interpret results of petrographic mineralogical and chemical analyses of the 1511 m deep icdp yaxcopoil 1 yax 1 drill core with special emphasis on the impactite units using numerical model calculations of the formation excavation and dynamic modification of the chicxulub crater constrained by laboratory data a model of the origin and emplacement of the impact formations of yax 1 and of the impact structure as a whole is derived the lower part of yax 1 is formed by displaced cretaceous target rocks 610 m thick while the upper part comprises six suevite type allochthonous breccia units 100 m thick from the texture and composition of these lithological units and from numerical model calculations we were able to link the seven distinct impact induced units of yax 1 to the corresponding successive phases of the crater formation and modification which are as follows 1 transient cavity formation including displacement and deposition of cretaceous megablocks 2 ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity 3 deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity brecciated impact melt rock 4 collapse of the ejecta plume and deposition of fall back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification 5 continued collapse of the ejecta plume and deposition of the upper suevite 6 late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere 7 final phase of fall back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite and 8 return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions our results are compatible with a 180 km and 100 km for the diameters of the final crater and the transient cavity of chicxulub respectively as previously proposed by several authors and b the interpretation of chicxulub as a peak ring impact basin that is at the transition to a multi ring basin

the 2011 sikkim earthquake mw 6 9 of the himalaya estimates of earthquake source parameters and representability of attenuation characteristics abstract the empirical accelerograms of the september 18 2011 sikkim earthquake m w 6 9 have been used in the present study to estimate its source parameters and to identify a representative relation for the attenuation of peak ground acceleration with distance the spectra of high energy packets observed in the sh waves approximated by transverse components of accelerograms has been modeled using brune s model a two step procedure has been adopted for this purpose the estimated source parameters of the 2011 sikkim earthquake have been found to be as corner frequency f c 0 09 hz seismic moment m 0 2 43 0 5 10 26 dyne cm stress drop 50 4 9 28 bars and the moment magnitude m w 6 87 0 05 these estimated values have been found to be consistent with those of reported values obtained using different data sets and procedures the high frequency earthquake ground motions are governed by stress drop at the source therefore a value of stress drop estimated here is useful for the simulation of earthquake strong ground motions in the region the attenuation characteristics of observed peak ground acceleration as a function of distance have been compared with those of obtained using five empirical relations it has been found that a relation given by mcguire 1978 represents the attenuation characteristics of peak ground acceleration is relatively better this relation may be used for estimating the peak ground acceleration of similar magnitude for future earthquakes in the region this relation is also useful for the simulation of earthquake strong ground motions in the region using envelope technique the outcome of the present study is useful for the estimation of the seismic hazard of the sikkim region of the himalaya

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

osmium isotope constraints on the proportion of bolide component in chicxulub impact melt rocks the spatial distribution and amount of material transferred from the bolide involved in the cretaceous tertiary k t event to the target rocks at chicxulub is still poorly constrained in this study re os isotopic analyses of impact melt breccias and lithic clasts from the yaxcopoil 1 yax 1 borehole were used to determine the distribution and proportion of the bolide component in the target rocks because of the much greater concentration of os in chondritic meteorites compared to the target rocks little addition of the bolide component would be necessary to greatly perturb the os concentration and isotopic composition of target rocks hence this is a very sensitive means of examining bolide contributions to the target rocks for the examined suite of samples the initial 187os 188os ratios vary from 0 19 to 2 3 conservative mixing calculations suggest that the bolide component comprised as much as approximately 0 1 by mass of some samples most samples however have negligible contributions from the bolide no samples have os that is dominated by the bolide component so for this suite of samples it is impossible to fingerprint the chemical nature of the bolide using relative abundances of siderophile elements these results suggest that the bolide did not contribute a significant amount of material to the target rocks this may in turn indicate that most of the bolide was vaporized upon impact or otherwise ejected without mixing with the melt from the target

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

more evidence that the chicxulub impact predates the k t mass extinction yaxcopoil 1 yax 1 drilled within the chicxulub crater was expected to yield the final proof that this impact occurred precisely 65 myr ago and caused the mass extinction at the cretaceous tertiary k t boundary instead contrary evidence was discovered based on five independent proxies sedimentologic biostratigraphic magnetostratigraphic stable isotopic and iridium that revealed that the chicxulub impact predates the k t boundary by about 300 000 years and could not have caused the mass extinction this is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest maastrichtian zone cf1 and is corroborated by magnetostratigraphic chron 29r and characteristic late maastrichtian stable isotope signals these results were first presented in keller et al 2004 in this study we present more detailed evidence of the presence of late maastrichtian planktic foraminifera sedimentologic and mineralogic analyses that demonstrate that the chicxulub impact breccia predates the k t boundary and that the sediments between the breccia and the k t boundary were deposited in a normal marine environment during the last 300 000 years of the cretaceous

use of electromagnetic radiation for potential forecast of earthquakes a forecast of an earthquake should identify the first stages of earthquake development nucleation and stick slip we show that a forecast cannot be achieved by seismic measurements due to their high attenuation but can be obtained by judicial filtering of electromagnetic radiation results show that electromagnetic radiation emitted from fractures femr during the early stages of an earthquake is less attenuated than seismic measurements due to the high frequencies involved but could be cluttered by external noise based on our previous studies an analysis of femr constructed on a profile of individual pulses can remove part of the clatter

major and trace element compositions of melt particles and associated phases from the yaxcopoil 1 drill core chicxulub impact structure mexico melt particles found at various depths in impactites from the yaxcopoil 1 borehole into the chicxulub impact structure yucatan have been analyzed for their major and trace element abundances a total of 176 electron microprobe and 45 la icp ms analyses from eight different melt particles were investigated the main purpose of this work was to constrain the compositions of precursor materials and secondary alteration characteristics of these melt particles individual melt particles are highly heterogeneous which makes compositional categorization extremely difficult melt particles from the uppermost part of the impactite sequence are ca and na depleted and show negative ce anomalies which is likely a result of seawater interaction various compositional groupings of melt particles are determined with ternary and binary element ratio plots involving major and trace elements this helps distinguish the degree of alteration versus primary heterogeneity of melt phases comparison of the trace element ratios sc zr y zr ba zr ba rb and sr rb with compositions of known target rocks provides some constraints on protolith compositions however the melt compositions analyzed exceed the known compositional diversity of possible target rocks normalized ree patterns are unique for each melt particle likely reflecting precursor mineral or rock compositions the various discrimination techniques indicate that the highly variable compositions are the products of melting of individual minerals or of mixtures of several minerals small angular shards that are particularly abundant in units 2 and 3 represent rapidly quenched melts whereas larger particles 0 5 mm that contain microlites and have fluidal schlieric textures cooled over a protracted period angular shard like particles with microlites in unit 5 likely crystallized below the glass transition temperature or underwent fragmentation during or after deposition

frequency dependent lg attenuation in the indian platform we use seismograms from regional earthquakes recorded on digital seismographs in peninsular india to determine the frequency dependent q of lg for the indian platform we measure lg attenuation by determining the decay of spectral amplitudes with distance the available data suggest some spatial variation in attenuation but a much denser ray path coverage would be required to validate such observations we therefore combine all the measurements of overlapping regions that span both the shield and intervening terranes to obtain an average value of attenuation for the indian platform lg q 665 10 with the frequency exponent n 0 67 0 03 this average value of lg attenuation for the indian platform is similar to the average for other stable regions of the globe

petrography and composition of martian regolith breccia meteorite northwest africa 7475 the northwest africa nwa 7475 meteorite is one of the several stones of paired regolith breccias from mars based on petrography oxygen isotope mineral compositions and bulk rock compositions its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten other clast types include crystallized impact melt rocks evolved plutonic rocks possible basalts contact metamorphosed rocks and siltstones impact spherules and vitrophyre shards record airborne transport and accreted dust rims were sintered on most clasts presumably during residence in an ejecta plume the clast assemblage records at least three impact events one that formed an impact melt sheet on mars 4 4 ga ago a second that assembled nwa 7475 from impactites associated with the impact melt sheet at 1 7 1 4 ga and a third that launched nwa 7475 from mars 5 ma ago mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to 5 and 15 gpa the mild postshock heating that resulted from these shock pressures would have been insufficient to sterilize this water bearing lithology during launch magnetite maghemite and pyrite are likely products of secondary alteration on mars textural relationships suggest that calcium carbonate and goethite are probably of terrestrial origin yet trace element chemistry indicates relatively low terrestrial alteration comparison of mars odyssey gamma ray spectrometer data with the fe and th abundances of nwa 7475 points to a provenance in the ancient southern highlands of mars gratteri crater with an age of 5 ma and an apparent diameter of 6 9 km marks one possible launch site of nwa 7475

fine grained dust rims in the tagish lake carbonaceous chondrite evidence for parent body alteration the tagish lake carbonaceous chondrite consists of heavily aqueously altered chondrules cais and larger mineral fragments in a fine grained phyllosilicate dominated matrix the vast majority of the coarse grained components in this meteorite are surrounded by continuous 1 5 to 200 m wide fine grained accretionary rims which are well known from meteorites belonging to petrological types 2 and 3 and whose origin and modification is still a matter of debate texturally the fine grained rims in tagish lake are very similar throughout the entire meteorite and independent of the nature of the enclosed object they typically display sharp boundaries to the core object and more gradational contacts to the meteorite matrix compared to the matrix the rims are much more finegrained and characterized by a significantly lower porosity the rims consist of an unequilibrated assemblage of phyllosilicates fe ni sulfides magnetites low ca pyroxenes and forsteritic olivines and are except for a much lower abundance of carbonates very similar to the tagish lake matrix electron microprobe and synchrotron x ray microprobe analyses show that matrix and rims are also very similar in composition and that the rims differ significantly from matrix and bulk meteorite only by being depleted in ca x ray elemental mapping and mineralogical observations indicate that ca was lost during aqueous alteration from the enclosed objects and preferentially crystallized as carbonates in the porous matrix the analyses also show that ca is strongly fractionated from al in the rims whereas there is no fractionation of the ti al ratios our data suggest that the fine grained rims in tagish lake initially formed by accretion in the solar nebula and were subsequently modified by in situ alteration on the parent body this pervasive alteration removed any potential evidence for preaccretionary alteration but did not change the overall texture of the tagish lake meteorite

major and trace element characteristics of impactites from the yaxcopoil 1 borehole chicxulub structure mexico approximately 100 m of impactites were retrieved from the icdp borehole yaxcopoil 1 yax 1 located 60 km south southwest from the center of the chicxulub impact crater on the yucatan peninsula of mexico here we characterize and discuss this impact breccia interval according to its geochemical characteristics chemical analysis of samples from all five recognized breccia units reveals that the impactites are of heterogeneous composition with regard to both major and trace elements at the single sample 8 16 cm 3 scale this is primarily due to a strong mixing relationship between carbonate and silicate fractions however averaged compositions for suevitic units 1 to 3 are similar and the silicate fraction after removal of the carbonate component indicates thorough mixing and homogenization analysis of the green melt breccia horizon unit 4 indicates that it contains a distinct mafic component large brown melt particles in units 2 3 and 4 represent a mixture of feldspathic and mafic components with high cao abundances unit 5 shows the greatest compositional diversity with highly variable abundances of sio2 cao and mgo inter sample heterogeneity is the result of small sample size combined with inherent heterogeneous lithological compositions highly variable particle size of melt and lithic components and post depositional alteration in contrast to samples from the y6 borehole from closer to the center of the structure yax 1 impactites have a strong carbonate component elevated loss on ignition rb and cs contents in the upper two impactite units indicate strong interaction with seawater the contents of the siderophile elements including ni co ir and cr do not indicate the presence of a significant extraterrestrial component in the yax 1 impactites

crust and mantle beneath western himalaya ladakh and western tibet from integrated seismic data abstract to understand deep structure of the western himalaya ladakh and western tibet we conduct an integrated analysis of teleseismic body wave recordings from the linear array of 16 portable broadband seismographs along a profile running through the himalaya to the karakoram the database includes p and s receiver functions teleseismic p and s residuals and observations of shear wave splitting in sks the analysis reveals beneath the himalaya a cold region at a depth of several hundred kilometers this anomaly can be caused by remnants of tethys subduction and perhaps is unrelated to the ongoing process the present day process is reflected in velocities beneath the lesser himalaya intermediate between those of the crust and the upper mantle this observation can be explained by scraping off the ductile lower crust of the underthrusting indian plate and accumulation of the high velocity crustal material in the frontal region of the thrust zone a representative value of the crustal thickness at the profile is 65 km and at most stations there is no evidence for a low velocity layer at mid crustal depths which makes crustal channel flow unlikely seismic waves in the upper 200 km of the mantle are faster than in global iasp91 model and the structure includes a low velocity layer sandwiched between two high velocity layers we interpret the lower layer as an image of subducted mantle lithosphere of the indian plate shear wave splitting is different in the south and the north in the south the fast direction of anisotropy is normal to the trend of the himalaya and can be interpreted as an effect of the ne motion of the indian lithosphere in the north the fast direction is oriented e w and can be explained by the fabric left in the presently extinct subduction zones in eastern tibet a similar analysis of the p and s receiver functions reveals upper mantle with the s velocities about 5 lower than in the west the mantle in the vicinity of station lsa lhasa contains a low s velocity layer between 160 km and 230 km depths with a velocity reduction of 0 2 km s underlain by the lehmann discontinuity

the dimensions of the chicxulub impact crater and impact melt sheet the chicxulub impact crater which is the principal source of impact debris in cretaceous tertiary boundary sediments is currently buried by a 1 km thick sequence of carbonate sediments because the crater is not exposed for direct scrutiny its size has remained uncertain and in particular estimates ranging from 100 to 300 km have been made on the basis of gravity and magnetic field data in this study the physical properties of the k t boundary impact ejecta and chicxulub impact melt sheet have been used to test the geophysical estimates of the impact crater s size on the basis of the measured thickness of impact ejecta in haiti and north america the calculated volume of the impact melt sheet and the chemical and isotopic composition of the melt sheet the chicxulub impact crater is inferred to be 180 km in diameter and to contain a 3 to 7 km thick melt sheet and breccia sequence within a centrally located 100 km diameter region that corresponds to the limits of the transient cavity

measuring the seismic properties of tibetan bright spots evidence for free aqueous fluids in the tibetan middle crust seismic bright spots are commonly interpreted to mark fluid concentrations but their nature melt or aqueous is usually inferred only from circumstantial evidence of the geologic setting a band of bright spot reflections has been imaged by project indepth international deep profiling of tibet and the himalayas at about 15 km depth along 150 km of the northern yadong gulu rift southern tibet we use indepth three component wide angle seismic data to measure seismic velocities at the bright spot reflector and theoretical rock physics bounds to constrain the nature of the fluids merging of data from multiple bright spots allows us to use a one dimensional approximation travel time modeling yields average p and s velocities for the upper crust above the bright spots of 5 3 0 2 and 3 2 0 2 km s 1 respectively reflection amplitude variation with offset avo modeling constrains the p and s velocities of the bright spots to 3 0 0 8 and 1 6 0 8 km s 1 respectively multiple modeling procedures suggest these velocities are not model dependent our results imply that of the order of 10 volume of free aqueous fluids in the tibetan middle crust produces the observed bright spot reflections the presence of relatively large quantities of free aqueous fluids presumably mostly saline supercritical h2o does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus about 650 c and thus the maximum surface heat flow to 110 mw m 2 the observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern tibetan crust

shock induced devolatilization of calcium sulfate and implications for k t extinctions the devolatilization of calcium sulfate which is present in the target rock of the chicxulub mexico impact structure and dispersal in the stratosphere of the resultant sulfuric acid aerosol have been suggested as a possible mechanism for the cretaceous tertiary extinctions we measured the amount of so 2 produced from two shock induced devolatilization reactions of calcium sulfate up to 42 gpa in the laboratory caso 4 sio 2 casio 3 so 3 caso 4 cao so 2 1 2o 2 we found both to proceed to a much lower extent than calculated by equilibrium thermodynamic calculations reaction products are found to be 10 2 times those calculated for equilibrium upon modeling the quantity of sulfur oxides degassed into the atmosphere from shock devolatilization of caso 4 in the chicxulub lithographic section the resulting 9 10 16 to 6 10 17 g in sulfur mass is lower by a factor of 10 100 than previous upper limit estimates the related environmental stress arising from the resultant global cooling and fallout of acid rain is insufficient to explain the widespread k t extinctions

geochemistry of drill core samples from yaxcopoil 1 chicxulub impact crater mexico the chemical composition of suevites displaced cretaceous target rocks and impactgenerated dikes within these rocks from the yaxcopoil 1 yax 1 drill core chicxulub impact crater mexico is reported and compared with the data from the yucatn 6 y6 samples within the six suevite subunits of yax 1 four units with different chemical compositions can be distinguished a upper lower sorted and upper suevite depth of 795846 m b middle suevite depth of 846861 m c brecciated impact melt rock depth of 861885 m and d lower suevite depth of 885895 m the suevite sequence a b and d display an increase of the cao content and a decrease of the silicate basement component from top to bottom in contrast the suevite of y6 shows an inverse trend the different distances of the yax 1 and y6 drilling sites from the crater center 60 and 47 km respectively lead to different suevite sequences within the cretaceous rocks of yax 1 a suevitic dike depth of 916 m does not display chemical differences when compared with the suevite while an impact melt rock dike depth of 1348 m is significantly enriched in immobile elements a clastic breccia dike depth of 1316 m is dominated by material derived locally from the host rock while the silicate rich component is similar to that found in the suevite significant enrichments of the k2o content were observed in the yax 1 suevite and the impact generated dikes all impactites of yax 1 and y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover an anhydrite component in the impactites is missing yax 1 or negligible y6

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

high crustal seismic attenuation in ladakh karakoram abstract an analysis of the lg wave has been attempted in order to study the seismic attenuation q 0 along a profile from the southern edge of the northwestern himalaya to ladakh and karakoram using broadband waveform recordings of regional earthquakes the lateral variability in seismic attenuation is derived from the inversion of 23 two station q 0 measurements using a differential evolution global optimization scheme the q 0 value decreases northwards from 700 in himalaya and 400 beneath the indus zangbo suture izs to 70 in ladakh karakoram this suggests an efficient transmission of seismic waves beneath the himalaya and the izs and high attenuation under ladakh karakoram the q 0 values for northwestern himalaya and the izs are significantly 50 higher compared to their counterpart at 90 e while ladakh karakoram and southern tibet show comparable q 0 70 the high attenuation zone low q 0 is correlatable with increase in electrical conductivity and decrease in the midcrustal s velocity suggesting the possible presence of aqueous fluid partial melt in the ladakh crust

evidence for ocean water invasion into the chicxulub crater at the cretaceous tertiary boundary the possibility of ocean water invasion into the chicxulub crater following the impact at the cretaceous tertiary boundary was investigated based on examination of an impactite between approximately 794 63 and 894 94 m in the yaxcopoil 1 yax 1 core the presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval abundant occurrence of nannofossils of late campanian to early maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation the maximum grain size of limestone lithics and vesicular melt fragments and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite the upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater most likely by high energy currents association with ocean water invasion

fingerprinting the k t impact site and determining the time of impact by upb dating of single shocked zircons from distal ejecta abstract u pb isotopic dating of single 1 3 g zircons from k t distal ejecta from a site in the raton basin colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site data for the least shocked zircons are slightly displaced from the 544 5 ma primary age for a component of the target site white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65 5 3 0 ma such shocked and granular zircons have never been reported from any source including explosive volcanic rocks zircon is refractory and has one of the highest thermal blocking temperatures hence it can record both shock features and primary and secondary ages without modification by post crystallization processes unlike shocked quartz which can come from almost anywhere on the earth s crust shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers with u pb zircon dating it is now possible to correlate ejecta layers derived from the same target site test the single versus multiple impact hypothesis and identify the target source of impact ejecta the ages obtained in this study indicate that the manson impact site iowa which has basement rocks that are mid proterozoic in age cannot be the source of k t distal ejecta the k t distal ejecta probably originated from a single impact site because most grains have the same primary age

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet

crustal flow pattern beneath the tibetan plateau constrained by regional lg wave q tomography abstract as a prominent geophysical anomaly unusually high seismic wave attenuation is observed in the crust and upper mantle of the tibetan plateau particularly along its northern area theoretical and laboratory investigations show that the strong seismic attenuation can indicate high temperatures and partial melting which may decrease the viscosity of the material and cause it to flow thus seismic attenuation distribution may provide useful constraints to the crust flows if they exist using lg wave q tomography we construct a 0 05 10 0 hz broadband high resolution crust attenuation model for the tibetan plateau and its surrounding regions the maximum spatial resolution is approximately 1 0 1 0 in well covered areas and for frequencies between 0 05 and 1 5 hz this attenuation model reveals an apparent low qlg belt stretching along the northern and eastern tibetan plateau combining the lg wave q model with other geophysical data two possible crust flow channels are found in the tibetan plateau the main flow channel is from north to east and then turns to southeastern tibet along the western edge of the rigid sichuan basin while a second channel starts from southern tibet and crosses the eastern himalayan syntaxis

foraminiferal biostratigraphy and paleoenvironmental reconstruction at the yaxcopoil 1 drill hole chicxulub crater yucatan peninsula the yaxcopoil 1 yax 1 drill hole comprises cretaceous limestones and calcarenites the k p boundary cocktail unit including impact breccia and a danian marly clay layer overlain by calcareous marls the biostratigraphy paleobathymetry and environmental turnover across the k p interval were inferred after analyzing the planktic and benthic foraminiferal assemblages the cretaceous samples only contain a few poorly preserved planktic foraminifera of a middle campanian to maastrichtian age while low diversity benthic foraminiferal assemblages suggest a sufficient nutrient supply to the sea floor and a shallow neritic occasionally stressed environment the impact breccia and the redeposited suevite are overlain by a 46 cm thick dolomitic calcareous sandstone unit that contains scarce reworked planktic foraminiferal specimens this unit probably represents the uppermost part of the initial infill of the crater the uppermost centimeters of this unit are bioturbated and its top represents a hiatus that spans at least the g cretacea pv eugubina and part of the p pseudobulloides biozones this unit is overlain by a 3 4 cm thick marly clay layer that represents a condensed layer benthic foraminiferal assemblages suggest a low food supply to the sea floor and environmental instability during the deposition of the marly clay layer the increase in diversity of the assemblages indicates that the environmental conditions improved and stabilized from the g compressa biozone toward the a uncinata p2 biozone the danian planktic and benthic foraminiferal assemblages indicate a deeper probably bathyal environment

impact glasses in fallout suevites from the ries impact structure germany an analytical sem study t impact generated glasses from fallout suevite deposits at the ries impact structure have been investigated using analytical scanning electron microscopy approximately 320 analyses of glass clasts were obtained four glass types are distinguished on the basis of composition and microtextures type 1 glasses correspond to the aerodynamically shaped glass bombs studied previously by many workers major oxide concentrations indicate the involvement of granitic rocks amphibolites and minor al rich gneisses during melting type 2 glasses are chemically heterogeneous even within individual clasts with variations of several wt in most of the major oxides e g 57 70 wt sio2 this suggests incomplete mixing of 1 mineral derived melts or 2 whole rock melts from a wide range of lithologies aluminium rich clinopyroxene and fe mg rich plagioclase quench crystals are present in type 1 and 2 glasses respectively type 3 glasses contain substantial amounts of h2o 12 17 wt low sio2 50 53 wt high al2o3 17 21 wt and high cao 5 7 wt contents this suggests an origin due to shock melting of part of the sedimentary cover type 4 glasses form a ubiquitous component of the suevites based on their high sio2 content 85 100 wt the only possible protolith are sandstones in the lowermost part of the sedimentary succession calcite forms globules within type 1 glasses with which it develops microtextures indicative of liquid immiscibility unequivocal evidence also exists for liquid immiscibility between what are now montmorillonite globules and type 1 2 and 4 glasses indicating that montmorillonite was originally an impact melt glass clearly the melt zone at the ries must have incorporated a substantial fraction of the sedimentary cover as well as the underlying crystalline basement rocks impact melts were derived from different target lithologies and these separate disaggregated melts did not substantially mix in most cases type 2 3 and 4 glasses and carbonate melts