viscosity of microlite bearing rhyolitic obsidians an experimental study to investigate the influence of microlites on lava flow rheology the viscosity of natural microlite bearing rhyolitic obsidians of calc alkaline and peralkaline compositions containing 0 1 0 4 wt water was measured at volcanologically relevant temperatures 650 950 c stresses 103 105 pa and strain rates 10 5 to 10 7 s 1 the glass transition temperatures t g were determined from scanning calorimetric measurements on the melts for a range of cooling heating rates based on the equivalence of enthalpic calorimetric and shear viscosity relaxation we calculated the viscosity of the melt in crystal bearing samples from the t g data the difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal bearing samples is interpreted to be the physical effect of microlites on the measured viscosity the effect of 45 vol crystals for both of these samples a connectivity factor is apparent with for the latter a close packing framework of crystals which is interpreted to influence the apparent viscosity

winding down the chicxulub impact the transition between impact and normal marine sedimentation near ground zero abstract the chicxulub impact led to the formation of a 200 km wide by 1 km deep crater on mexico s yucatan peninsula over a period of hours after the impact the ocean re entered and covered the impact basin beneath several hundred meters of water a suite of impactites were deposited across the crater during crater formation and by the resurge tsunami and seiche events that followed international ocean discovery program international continental scientific drilling program expedition 364 drilled into the peak ring of the chicxulub crater and recovered 130 m of impact deposits and a 75 cm thick fine grained carbonate rich transitional unit above which normal marine sedimentation resumed here we describe the results of analyses of the uppermost impact breccia suevite and the transitional unit which suggests a gradual waning of energy recorded by this local k pg boundary sequence the dominant depositional motif in the upper suevite and the transitional unit is of rapid sedimentation characterized by graded bedding local cross bedding and evidence of oscillatory currents the lower transitional unit records the change from deposition of dominantly sand sized to mainly silt to clay sized material with impact debris that decreases in both grain size and abundance upward the middle part of the transitional unit is interrupted by a 20 cm thick soft sediment slump overlain by graded and oscillatory current cross laminated beds the uppermost transitional unit is also soft sediment deformed contains trace fossils and an increasing abundance of planktic foraminifer and calcareous nannoplankton survivors the transitional unit as with similar deposits in other marine target impact craters records the final phases of impact related sedimentation prior to resumption of normal marine conditions petrographic and stable isotopic analyses of carbon from organic matter provide insight into post impact processes 13corg values are between terrestrial and marine end members with fluctuations of 1 3 timing of deposition of the transitional unit is complicated to ascertain the repetitive normally graded laminae both below and above the soft sediment deformed interval record rapid deposition from currents driven by tsunami and seiches processes that likely operated for weeks to potentially years post impact due to subsequent continental margin collapse events highly siderophile element enrichment at the top of the unit is likely from fine grained ejecta that circulated in the atmosphere for several years prior to settling the transitional unit is thus an exquisite record of the final phases of impact related sedimentation related to one of the most consequential events in earth history

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

repeated fracture and healing of silicic magma generate flow banding and earthquakes textures in an exceptionally preserved effusive rhyolite conduit at torfajokull iceland indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit rfh process anastomosing tuffisite veins filled by fine grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval welding of the particulate material during subsequent deformation led to thorough healing of veins allowing repeated fracture of the same body of magma we propose that the rfh process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions the rfh process may also form the flow banding that is nearly ubiquitous in obsidian

dynamic modeling suggests terrace zone asymmetry in the chicxulub crater is caused by target heterogeneity we investigate the cause of terrace zone asymmetry in the chicxulub impact crater using dynamic models of crater formation marine seismic data acquired across the crater show that the geometry of the crater s terrace zone a series of sedimentary megablocks that slumped into the crater from the crater rim varies significantly around the offshore half of the crater the seismic data also reveal that at the time of impact both the water depth and sediment thickness varied with azimuth around the impact site to test whether the observed heterogeneity in the pre impact target might have affected terrace zone geometry we constructed two end member models of upper target structure at chicxulub based on the seismic data at different azimuths one model representing the northwest sector had no water layer and a 3 km thick sediment layer the other model representing the northeast sector had a 2 km water layer above a 4 km sediment layer numerical models of vertical impacts into these two targets produced final craters that differ substantially in terrace zone geometry suggesting that the initial water depth and sediment thickness variations affected the structure of the terrace zone at chicxulub moreover the differences in terrace zone geometry between the two numerical models are consistent with the observed differences in the geometry of the terrace zone at different azimuths around the chicxulub crater we conclude that asymmetry in the pre impact target rocks at chicxulub is likely to be the primary cause of asymmetry in the terrace zone

the formation of peak ring basins working hypotheses and path forward in using observations to constrain models of impact basin formation abstract impact basins provide windows into the crustal structure and stratigraphy of planetary bodies however interpreting the stratigraphic origin of basin materials requires an understanding of the processes controlling basin formation and morphology peak ring basins exhibiting a rim crest and single interior ring of peaks provide important insight into the basin formation process as they are transitional between complex craters with central peaks and larger multi ring basins new image and altimetry data from the lunar reconnaissance orbiter as well as a suite of remote sensing datasets have permitted a reassessment of the origin of lunar peak ring basins we synthesize morphometric spectroscopic and gravity observations of lunar peak ring basins and describe two working hypotheses for the formation of peak rings that involve interactions between inward collapsing walls of the transient cavity and large central uplifts of the crust and mantle major facets of our observations are then compared and discussed in the context of numerical simulations of peak ring basin formation in order to plot a course for future model refinement and development

normal faulting origin for the cordillera and outer rook rings of orientale basin the moon 1 orientale basin is the youngest and best preserved large impact basin on the moon with at least four topographic rings contained within the topographic rim marked by the cordillera ring diameter 930 km its well exposed interior makes this basin a prime location to study basin formation processes forward mechanical modeling of basin ring topography shows that the outermost rings the cordillera ring cr and outer rook ring orr are large scale normal faults with displacements d of 0 8 to 5 2 km fault dip angles of 54 to 80 and vertical depth of faulting t between 19 and 37 km with most faults having t 30 5 km these faults and the distribution of maria inside the basin suggest that the transient crater important for determining many impact related characteristics such as projectile size was contained entirely within the orr and likely had a diameter between 500 and 550 km the difference in crustal thickness between the western and eastern sides of the basin is not a result of the basin forming event which indicates the formation of the hemispheric crustal thickness asymmetry was likely before the formation of orientale basin 3 68 to 3 85 ga

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

modelling the lava dome extruded at soufriere hills volcano montserrat august 2005 may 2006 part i dome shape and internal structure lava domes comprise core carapace and clastic talus components they can grow endogenously by inflation of a core and or exogenously with the extrusion of shear bounded lobes and whaleback lobes at the surface internal structure is paramount in determining the extent to which lava dome growth evolves stably or conversely the propensity for collapse the more core lava that exists within a dome in both relative and absolute terms the more explosive energy is available both for large pyroclastic flows following collapse and in particular for lateral blast events following very rapid removal of lateral support to the dome knowledge of the location of the core lava within the dome is also relevant for hazard assessment purposes a spreading toe or lobe of core lava over a talus substrate may be both relatively unstable and likely to accelerate to more violent activity during the early phases of a retrogressive collapse soufriere hills volcano montserrat has been erupting since 1995 and has produced numerous lava domes that have undergone repeated collapse events we consider one continuous dome growth period from august 2005 to may 2006 that resulted in a dome collapse event on 20th may 2006 the collapse event lasted 3 h removing the whole dome plus dome remnants from a previous growth period in an unusually violent and rapid collapse event we use an axisymmetrical computational finite element method model for the growth and evolution of a lava dome our model comprises evolving core carapace and talus components based on axisymmetrical endogenous dome growth which permits us to model the interface between talus and core despite explicitly only modelling axisymmetrical endogenous dome growth our core talus model simulates many of the observed growth characteristics of the 2005 2006 shv lava dome well further it is possible for our simulations to replicate large scale exogenous characteristics when a considerable volume of talus has accumulated around the lower flanks of the dome model results suggest that dome core can override talus within a growing dome potentially generating a region of significant weakness and a potential locus for collapse initiation

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary the cretaceous paleogene boundary similar to 65 5 million years ago marks one of the three largest mass extinctions in the past 500 million years the extinction event coincided with a large asteroid impact at chicxulub mexico and occurred within the time of deccan flood basalt volcanism in india here we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction notably a single ejecta rich deposit compositionally linked to the chicxulub impact is globally distributed at the cretaceous paleogene boundary the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations for example darkness and cooling lead us to conclude that the chicxulub impact triggered the mass extinction

viscosity of the dome of mount st helens abstract a sample of dacite from the dome of mt st helens has been studied experimentally and found to have a viscosity that declines from about 10 15 at 800 c to 10 3 poise at 1500 c its yield strength declines from about 2 10 6 dynes cm 2 at 800 c to less than the precision of our measurements 10 4 dynes cm 2 above 1050 c the geometrical form of the dome is approximately consistent with these properties if the internal temperature of the dome is between 900 and 1000 c the melting behavior of the crystalline dacite has been determined between 1000 and 1225 c data on the composition of the liquid fraction and sizes and proportion of crystals have been used to derive a set of equations for predicting the effective viscosity of crystal bearing magmas below their liquidus temperatures although of the bulk composition of the magma has changed little the increasing crystallinity and viscosity appear to have influenced eruptive behavior of the volcano during its most recent periods of activity

geochemical variability of the yucatan basement constraints from crystalline clasts in chicxulub impactites the 65 ma old chicxulub impact structure with a diameter of about 180 km is again in the focus of the geosciences because of the recently commenced drilling of the scientific well yaxcopoil 1 chicxulub is buried beneath thick post impact sediments yet samples of basement lithologies in the drill cores provide a unique insight into age and composition of the crust beneath yucatan this study presents major element sr and nd isotope data for chicxulub impact melt lithologies and clasts of basement lithologies in impact breccias from the pemex drill cores c 1 and y 6 as well as data for ejecta material from the k t boundaries at la lajilla mexico and furlo italy the impact melt lithologies have an andesitic composition with significantly varying contents of al ca and alkali elements their present day 87sr 86sr ratios cluster at about 0 7085 and 143nd 144nd ratios range from 0 5123 to 0 5125 compared to the melt lithologies that stayed inside the crater data for ejecta material show larger variations the 87sr 86sr ratios range from 0 7081 for chloritized spherules from la lajilla to 0 7151 for sanidine spherules from furlo the 143nd 144nd ratio is 0 5126 for la lajilla and 0 5120 for the furlo spherules in an tchur nd tur sr diagram the melt lithologies plot in a field delimited by cretaceous platform sediments various felsic lithic clasts and a newly found mafic fragment from a suevite granite gneiss and amphibolite have been identified among the fragments from crystalline basement gneiss their 87sr 86sr ratios range from 0 7084 to 0 7141 and their 143nd 144nd ratios range from 0 5121 to 0 5126 the tnddm model ages vary from 0 7 to 1 4 ga pointing to different source terranes for these rocks this leads us to believe that the geological evolution and the lithological composition of the yucatn basement is probably more complex than generally assumed and gondwanan as well as laurentian crust may be present in the yucatn basement

rheological change and degassing during a trachytic vulcanian eruption at kilian volcano chaine des puys france magma ascent during silicic dome forming eruptions is characterized by significant changes in magma viscosity permeability and gas overpressure in the conduit these changes depend on a set of parameters such as ascent rate outgassing and crystallization efficiency and magma viscosity which in turn may influence the prevailing conditions for effusive versus explosive activity here we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive explosive transitions during vulcanian phases of the 9 4 ka eruption of kilian volcano chaine des puys france our results suggest that effusive activity at the onset of vulcanian episodes at kilian volcano was promoted by i rapid ascent of initially crystal poor and volatile rich trachytic magma ii a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization and iii efficient degassing outgassing in a crystal rich magma at shallow depths trachytic magma repeatedly replenished the upper conduit and variations in the amount of decompression and cooling caused vertical textural stratification leading to variable degrees of crystallization and outgassing outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles fractures and tuffisite veins fostering the formation of cristobalite in the carapace and talus regions build up of overpressure was likely caused by closing of pore space bubbles and fractures in the dome through a combination of pore collapse cristobalite formation sintering in tuffisite veins and limited pre fragmentation coalescence in the dome or underlying hot vesicular magma sealing of the carapace may have caused a transition from open to closed system degassing and to renewed explosive activity we generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity rapid microlite growth and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents we show that trachytes may erupt with a similar style e g vulcanian but at significantly higher ascent rates than their andesitic dacitic and rhyolitic counterparts this suggests that the periodicity of effusive explosive transitions at trachytic volcanoes may differ from that observed at the well monitored andesitic dacitic and rhyolitic volcanoes which has implications for hazard assessment associated with trachytic eruptions

physical properties of the 1980 mount st helens cryptodome magma physical properties of cryptodome and remelted samples of the mount st helens grey dacite have been measured in the laboratory the viscosity of cryptodome dacite measured by parallel plate viscometry ranges from 10 82 to 9 94 log10 pa s t 900 982 c and shrinkage effects were dilatometrically observed at t 900 c the viscosity of remelted dacite samples measured by the micropenetration method is 10 60 9 25 log10 pa s t 736 802 c and viscosities measured by rotational viscometry are 3 22 1 66 log10 pa s t 1298 1594 c comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water bearing melt demonstrates significant deviations between measured and calculated values this difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal bearing vesicular melt viscosities at low temperature assuming that the cryptodome magma of the 18 may 1980 mount st helens eruption was residing at 900 c with a phenocryst content of 30 vol a vesicularity of 36 vol and a bulk water content of 0 6 wt we estimate the magma viscosity to be 1010 8 pa s

grail gravity observations of the transition from complex crater to peak ring basin on the moon implications for crustal structure and impact basin formation abstract high resolution gravity data from the gravity recovery and interior laboratory grail mission provide the opportunity to analyze the detailed gravity and crustal structure of impact features in the morphological transition from complex craters to peak ring basins on the moon we calculate average radial profiles of free air anomalies and bouguer anomalies for peak ring basins protobasins and the largest complex craters complex craters and protobasins have free air anomalies that are positively correlated with surface topography unlike the prominent lunar mascons positive free air anomalies in areas of low elevation associated with large basins the bouguer gravity anomaly profiles of complex craters are highly irregular with central positive anomalies that are generally absent or not clearly tied to interior morphology in contrast gravity profiles for peak ring basins 200 km to 580 km are much more regular and are highly correlated with surface morphology a central positive bouguer anomaly is confined within the peak ring and a negative bouguer anomaly annulus extends from the edge of the positive anomaly outward to about the rim crest a number of degraded basins lacking interior peak rings have diameters and gravity patterns similar to those of well preserved peak ring basins if these structures represent degraded peak ring basins the number of peak ring basins on the moon would increase by more than a factor of two to 34 the gravity anomalies within basins are interpreted to be due to uplift of the mantle confined within the peak ring and an annulus of thickened crust between the peak ring and rim crest we hypothesize that mantle uplift is influenced by interaction between the transient cavity and the mantle further mascon formation is generally disconnected from the number of basin rings formed and occurs over a wide range of basin sizes these observations have important implications for models of basin and mascon formation on the moon and other planetary bodies

shallow oceanic crust full waveform tomographic images of the seismic layer 2a 2b boundary 1 we present results of full waveform tomographic inversions of four profiles acquired over young intermediate and fast spreading rate oceanic crust the mean velocity depth functions from our study include a 0 25 0 30 km thick low velocity low gradient region beneath the seafloor overlying a 0 24 0 28 km thick high gradient region together these regions compose seismic layer 2a mean layer 2a interval velocities are 3 0 3 2 km s the mean depth to the layer 2a 2b boundary is 0 49 0 54 km and mean velocities within the upper 0 25 km of layer 2b are 4 7 4 9 km s previous velocity analyses of the study areas using 1 d ray tracing underestimate the thickness of the high gradient region at the base of layer 2a we observe differences in the waveform inversion velocity models that correspond to imaging of the layer 2a event regions with a layer 2a event have higher velocity gradients at the base of layer 2a intermittent high velocities which we interpret as massive flows are observed in the waveform inversion velocity models at 0 05 0 10 km below the seafloor bsf over 10 25 of the intermediate spreading profiles and 20 45 of the fast spreading profiles the high gradient region located 0 25 0 54 km bsf at the base of layer 2a may be associated with an increased prevalence of massive flows the first appearance of dikes lava dike transition zone or with increased crack sealing by hydrothermal products the upper portion of layer 2b which begins at 0 49 0 54 km bsf may correspond to sheeted dikes or the top of the transition zone of lavas and dikes

rock fluidization during peak ring formation of large impact structures large meteorite impact structures on the terrestrial bodies of the solar system contain pronounced topographic rings which emerged from uplifted target crustal rocks within minutes of impact to flow rapidly over large distances these target rocks must have weakened drastically but they subsequently regained sufficient strength to build and sustain topographic rings the mechanisms of rock deformation that accomplish such extreme change in mechanical behaviour during cratering are largely unknown and have been debated for decades recent drilling of the approximately 200 km diameter chicxulub impact structure in mexico has produced a record of brittle and viscous deformation within its peak ring rocks here we show how catastrophic rock weakening upon impact is followed by an increase in rock strength that culminated in the formation of the peak ring during cratering the observations point to quasi continuous rock flow and hence acoustic fluidization as the dominant physical process controlling initial cratering followed by increasingly localized faulting

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

magma fragmentation by rapid decompression the processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology observations of explosive eruptions1 3 as well as theoretical analyses of the underlying processes4 have raised the question of whether the rapid decompression of highly viscous vesicular magma that results from the collapse of a lava dome or volcanic edifice can in itself produce explosive magma fragmentation and pyroclast formation here we report the results of a laboratory investigation of pyroclast formation following rapid decompression samples of magma from the 1980 eruption of mount st helens when rapidly depressurized from initial pressures of up to 12 mpa and at temperatures in the range 750 825 c fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions moreover we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events

dynamics of seismogenic volcanic extrusion at mount st helens in 2004 05 the 2004 05 eruption of mount st helens exhibited sustained near equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick slip motion along the margins of the plug as it was forced incrementally upwards by ascending solidifying gas poor magma we formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma plug system and that of a variably damped oscillator modelled stick slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption and they imply that magma pressure never deviated much from the steady equilibrium pressure we infer that the volcano was probably poised in a near eruptive equilibrium state long before the onset of the 2004 05 eruption

structural uplift beneath the chicxulub impact structure models of the central structure of large impact craters are poorly constrained partly because of the lack of well preserved terrestrial examples and partly because of the extreme nature of impact events even large impact craters take only a few minutes to form during which time rocks from the deep crust move upward many kilometers interacting with impact melts and breccias before settling to their final position we construct a new model of central uplift beneath the chicxulub crater based upon a well constrained 3 d velocity model obtained by jointly inverting seismic traveltime and gravity data the input tomographic data set has good resolution and many rays cross the central uplift in many directions we use laboratory measurements to convert between velocity and density our velocity model possesses a high velocity zone near the crater center and velocity gradually decreases outside this zone we use regional refraction data to interpret these velocities in terms of a broad 80 km wide zone of structural uplift in which the central rocks originate from the lower crust and the surrounding rocks from the midcrust and upper crust this is in contrast with previous models in which the zone of central uplift is either 40 50 km or 150 km wide our interpretation is consistent with scaling laws yucatan basement lithology other velocity data observations at similar sized terrestrial craters and dynamic modeling of peak ring formation our model of the uplift at chicxulub can be used to help distinguish between competing models of effective target strength in numerical models of crater formation

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

rheology of granitic magmas during ascent and emplacement flow abstract considerable progress has been made over the past decade in under standing the static rheological properties of granitic magmas in the continental crust changes in h2o content co2 content and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid fluid mixture although the strengths of granitic magmas over the crystallization interval are still poorly constrained theoretical investigations suggest that during magma ascent yield strengths of the order of 9 kpa are required to com pletely retard the upward flow in meter wide conduits in low bagnold number magma suspensions with moderate crystal contents solidosities 0 1 0 3 viscous fluc tuations may lead to flow differentiation by shear enhanced diffusion ams and mi crostructural studies support the idea that granite plutons are intruded as crystal poor liquids 50 with fabric and foliation development restricted to the final stages of emplacement if so then these fabrics contain no information on the ascent verti cal transport history of the magma deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt resulting in a range of local macroscopic flow structures including layering crystal alignment and other mechanical instabilities such as shear zones as the suspension viscosity varies with stress rate it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear new theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved

review and comparison of models for describing non equilibrium and preferential flow and transport in the vadose zone abstract in this paper we review various approaches for modeling preferential and non equilibrium flow and transport in the vadose zone existing approaches differ in terms of their underlying assumptions and complexity they range from relatively simplistic models to more complex physically based dual porosity dual permeability and multi region type models a relatively simple dual porosity flow model results when the richards equation is combined with composite double hump type equations for the hydraulic properties to account for both soil textural matrix and soil structural fractures macropores peds effects on flow the simplest non equilibrium flow model a single porosity model which distinguishes between actual and equilibrium water contents is based on a formulation by ross and smettem soil sci soc am j 64 2000 1926 that requires only one additional parameter to account for non equilibrium a more complex dual porosity mobile immobile water flow model results when the richards or kinematic wave equations are used for flow in the fractures and immobile water is assumed to exist in the matrix we also discuss various dual permeability models including the formulation of gerke and van genuchten water resour res 29 1993a 305 and the kinematic wave approach as used in the macro model of jarvis technical description and sample simulations department of soil science swedish university of agricultural science uppsala sweden 1994 51 both of these models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures advantages and disadvantages of the different models are discussed and the need for inter code comparison is stressed especially against field data that are sufficiently comprehensive to allow calibration validation of the more complex models and to distinguish between alternative modeling concepts several examples and comparisons of equilibrium and various non equilibrium flow and transport models are also provided

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

the distribution and purity of anorthosite across the orientale basin new perspectives from moon mineralogy mapper data 1 the orientale basin is a multiring impact structure on the western limb of the moon that provides a clear view of the primary lunar crust exposed during basin formation previously near infrared reflectance spectra suggested that orientale s inner rook ring irr is very poor in mafic minerals and may represent anorthosite excavated from the moon s upper crust however detailed assessment of the mineralogy of these anorthosites was prohibited because the available spectroscopic data sets did not identify the diagnostic plagioclase absorption feature near 1250 nm recently however this absorption has been identified in several spectroscopic data sets including the moon mineralogy mapper m3 enabling the unique identification of a plagioclase dominated lithology at orientale for the first time here we present the first in depth characterization of the orientale anorthosites based on direct measurement of their plagioclase component in addition detailed geologic context of the exposures is discussed based on analysis of lunar reconnaissance orbiter narrow angle camera images for selected anorthosite identifications the results confirm that anorthosite is overwhelmingly concentrated in the irr comparison with nonlinear spectral mixing models suggests that the anorthosite is exceedingly pure containing 95 vol plagioclase in most areas and commonly 99 100 vol these new data place important constraints on magma ocean crystallization scenarios which must produce a zone of highly pure anorthosite spanning the entire lateral extent of the 430 km diameter irr

mid sized complex crater formation in mixed crystalline sedimentary targets insight from modeling and observation large impact crater formation is an important geologic process that is not fully understood the current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets real targets are rarely uniform for example the majority of earth s surface is covered by sedimentary rocks and or a water layer the ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process to advance understanding of the mechanics of crater collapse and the effect of variations in target properties on crater formation the first bridging the gap workshop recommended that geological observation and numerical modeling focussed on mid sized 15 30 km diameter craters on earth these are large enough to be complex small enough to be mapped surveyed and modelled at high resolution and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables in this paper we compare observations and numerical models of three 18 26 km diameter craters formed in different target lithology ries germany haughton canada and el gygytgyn russia based on the first order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid sized complex crater formation in a subaerial mixed crystalline sedimentary target we compared our results with interpreted geological profiles of ries and haughton based on detailed new and published geological mapping and published geophysical surveys our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre impact sedimentary cover in each case we conclude that the presence of an inner ring at ries and not at haughton is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post impact erosion for constant impact energy transient and final crater diameters increase with increasing sediment thickness

importance of pre impact crustal structure for the asymmetry of the chicxulub impact crater the final geometry of an impact crater can be significantly influenced by the geological and geomorphological structure of the impact site therefore it may not on its own provide accurate information regarding the direction and angle of impact

application of waveform tomography to marine seismic reflection data from the queen charlotte basin of western canada to obtain a higher resolution quantitative p wave velocity model 2d waveform tomography was applied to seismic reflection data from the queen charlotte sedimentary basin off the west coast of canada the forward modeling and inversion were implemented in the frequency domain using the visco acoustic wave equation field data preconditioning consisted of f k filtering 2d amplitude scaling shot to shot amplitude balancing and time windowing the field data were inverted between 7 and 13 66 hz with attenuation introduced for frequencies 10 5 hz to improve the final velocity model two different approaches to sampling the frequencies were evaluated the limited maximum offset of the marine data 3770 m and the relatively high starting frequency 7 hz were the main challenges encountered during the inversion an inversion strategy that successively recovered shallow to deep structures was designed to mitigate these issues the inclusion of later arrivals in the waveform tomography resulted in a veloci

full waveform tomographic images of the peak ring at the chicxulub impact crater peak rings are a feature of large impact craters on the terrestrial planets and are generally believed to be formed from deeply buried rocks that are uplifted during crater formation the precise lithology and kinematics of peak ring formation however remains unclear previous work has revealed a suite of bright inward dipping reflectors beneath the peak ring at the chicxulub impact crater and that the peak ring was formed from rocks with a relatively low seismic velocity new two dimensional full waveform tomographic velocity images show that the uppermost lithology of the peak ring is formed from a thin 100 200 m thick layer of low velocity 3000 3200 m s rocks this low velocity layer is most likely composed of highly porous allogenic impact breccias our models also show that the change in velocity between lithologies within and outside the peak ring is more abrupt than previously realized and occurs close to the location of the dipping reflectors across the peak ring velocity appears to correlate well with predicted shock pressures from a dynamic model of crater formation where the rocks that form the peak ring originate from an uplifted basement that has been subjected to high shock pressures 10 50 gpa and lie above downthrown sedimentary rocks that have been subjected to shock pressures of 5 gpa these observations suggest that low velocities within the peak ring may be related to shock effects and that the dipping reflectors underneath the peak ring might represent the boundary between highly shocked basement and weakly shocked sediments

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

long period seismicity during magma movement at volcan de colima during the period from february to september 2005 volcan de colima produced 30 vulcanian explosions of sufficient magnitude to produce pyroclastic flows of variable size with a total volume of at least 2 5 106 m3 swarms of long period events were associated with each event their duration ranging from about 6 h to 3 days and each swarm containing up to 886 events the characteristics of the swarms have been studied to understand the source mechanism and their relationship with the vulcanian explosions in total 12 548 long period events were analysed using various comparative and statistical methods patterns were not apparent in the data with no correlation between different properties of the swarms duration magnitude or frequency of occurrence of lp events and the magnitude of the associated vulcanian explosion whether recorded by seismicity volume of pyroclastics or altitude of the eruption column this along with other characteristics of the swarms such as the continuation of the swarm after the explosion with an increase in long period event amplitude in some cases suggests that the mechanism is not merely associated with the pressurization under an impermeable cap and resulting pressure differentials between adjacent volumes within the system it is more likely that the production of long period events is dominated by brittle fracturing on the margins of an ascending magma body a model is proposed whereby the unloading above the ascending magma column produced by a vulcanian explosion resulted in an increase in ascent rate reflected in the increasing amplitude of long period events the results reflect the complexity of non linear processes involved during magma ascent degassing crystallization and rupture of the impermeable plug during the vulcanian process at volcan de colima as at many volcanoes long period events represent a useful precursor for eruptive activity for monitoring this paper highlights some useful analyses that can be carried out which could illustrate certain characteristics of an eruptive episode a preliminary model is presented of the conduit processes at work during the cyclic extrusive and explosive activity during 2005

rheological properties of basaltic lavas at sub liquidus temperatures laboratory and field measurements on lavas from mount etna models of many magmatic processes require accurate data on the rheological properties of lava at sub liquidus temperatures laboratory measurements of the rheological properties of basalts erupted on mount etna in 1983 were made at various crystal concentrations in a specially designed furnace using a haake rotovisco viscometer attached to a spindle designed to eliminate slippage at the melt spindle interface measurements were made at strain rates between 0 3 and 5 s 1 over the range of eruption temperatures on mount etna 1084 1125 c at temperatures above 1120 c the 1983 lava behaves as a newtonian fluid at lower temperatures the lava is a thixotropic pseudoplastic fluid with a maximum yield strength of 78 pa at a temperature of 1087 c in view of its low yield strength over this temperature range the theological behaviour of the lava approximates to that of a power law fluid apparent viscosities at unit shear strain rates increase from 150 pa s at 1125 c to 3000 pa s at 1084 c unit strain rate apparent viscosities measured in the field using a rotating vane viscometer range from 1385 pa s to 1630 pa s these values are in close agreement with those measured at the same temperature in the laboratory and with those calculated theoretically from the physico chemical properties of the lava

experimental generation of volcanic pseudotachylytes constraining rheology volcanic systems are highly dynamic environments that comprise rocks and magmas which in the process of strain localisation e g in catastrophic flank collapse or dome building events are candidate materials for the occurrence of frictional melting and the formation of pseudotachylytes we evaluate the frictional behaviour of a plagioclase two pyroxene and glass bearing andesite and introduce an approach to constrain the rheology of frictional melts frictional slip at a rate of 1 3 m s under an axial stress of 1 5 mpa induces heating at a rate approximating 130 c per meter of slip and frictional melting of the andesite occurs at 1000 c which corresponds to a peak in shear stress with continuing displacement the shear stress decreases by 25 from the peak strength and stabilises while the temperature equilibrates to 1230 1290 c the shear stress for the system is congruent with a non arrhenian temperature dependent frictional melt rheology exhibiting a non newtonian viscosity evolving from approximately 104 2 to 103 4 pa s post experiment analysis show a gradation from 1 the host rock to 2 an extraordinary zone of unmelted yet viscously deformed material to 3 a thin outer region of chemically heterogeneous protomelts and to 4 an inner region of chemically homogeneous frictionally generated modified melt in the core of the slip zone we discuss the role and identification of frictional melting in volcanic systems and the implication of viscous remobilisation of magmatic plugs during frictional slip of glass bearing volcanic materials

the formation of peak rings in large impact craters large impacts provide a mechanism for resurfacing planets through mixing near surface rocks with deeper material central peaks are formed from the dynamic uplift of rocks during crater formation as crater size increases central peaks transition to peak rings without samples debate surrounds the mechanics of peak ring formation and their depth of origin chicxulub is the only known impact structure on earth with an unequivocal peak ring but it is buried and only accessible through drilling expedition 364 sampled the chicxulub peak ring which we found was formed from uplifted fractured shocked felsic basement rocks the peak ring rocks are cross cut by dikes and shear zones and have an unusually low density and seismic velocity large impacts therefore generate vertical fluxes and increase porosity in planetary crust

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

the rheology of two phase magmas a review and analysis abstract we consider the current state of our understanding of the rheology of two phase magmas that is suspensions of either bubbles or crystals in a viscous silicate melt the discussion is restricted to strain rates at which the suspending melt can be considered newtonian we start by considering the range of textures found in magmas and the bubble deformation and particle motions caused by shearing we then review proposed models for suspensions focussing on those functions of the form r f or f that have been most widely used to describe magmatic systems r is the relative apparent viscosity of the suspension is the volume fraction of the suspended phase is the driving stress and is the strain rate both theoretical and empirical methods are presented and then compared against the available analogue i e non magmatic and magmatic data the paper contains new data and significant re analysis of previously published data we present a new semi empirical constitutive model for bubble bearing magmas that is valid for steady and unsteady flow and large strains and strain rates this equation utilises a new parameter the capillarity cx that encapsulates the combined effect of shearing and unsteadiness on bubble suspensions we also present a new scheme for dealing with polydispersivity of bubble suspensions new data on the rheology of particle suspensions undergoing forced oscillations are presented these data show that the cox merz rule only holds for dilute particle suspensions 0 25 a re analysis of all available experimental data that relate rheology to particle aspect ratio provides distinct curves of maximum packing as a function of aspect ratio for smooth and rough particles with magmatic data lying on the curve appropriate for rough particles we analyse several rheological datasets of crystal bearing basaltic magmas and find that they are in good agreement with the constitutive equations derived from analogue data by contrast the same equations do not agree well with data for high viscosity haplogranitic melts this may be an effect of fracturing or viscous dissipation within these samples the paper concludes with a practical rheological recipes section giving a step by step method for calculating a constitutive equation for a two phase magmatic suspension and assessing its likely accuracy

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

shock experiments on pre heated α and β quartz i optical and density data abstract discs of single crystal quartz unheated and pre heated to 275 c and 540 c i e quartz and 630 c i e quartz were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to either 10 1 0 or 0001 refractive indices density and the orientation of planar deformation features pdfs were determined on the recovered quartz samples refractive indices of pre heated quartz are unaffected up to 25 gpa but density starts to decrease slightly up to this pressure above 25 gpa pre heating causes drastic variations refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously until complete isotropization is reached at 35 gpa in quartz shocked at 630 c refractivity drops discontinuously in the interval from 25 to 26 gpa and complete transformation to diaplectic glass is reached at 26 gpa density follows the trends demonstrated by the optical parameters with higher pre shock temperatures yielding lower density at a given shock pressure these results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature invariant lying at 25 gpa whereas the pressure limit for complete transformation decreases with increasing pre shock temperature from 35 to 26 gpa quartz shocked parallel to 0001 always has a higher density and refractivity than that shocked parallel to 10 1 0 indicating a significant influence of the structural anisotropy this is also evident from the distribution of pdf orientations pressures 25 gpa cause in quartz shocked parallel to 10 1 0 pdfs that are predominantly oriented parallel to 10 1 2 while quartz shocked to the same pressures but parallel to 0001 contains almost exclusively pdfs parallel to 10 1 3 pdf orientations in quartz shocked at ambient temperature parallel to 10 1 0 show the following characteristics i the frequency of 10 1 3 is high at 20 gpa declines with increasing pressure and finally is totally absent at 32 gpa ii 10 1 3 begins to occur at 25 gpa increases and finally is the only remaining orientation at 32 gpa and iii 10 1 2 and 11 2 2 are only present at 20 and 25 gpa in contrast quartz shocked at 630 c parallel to 10 1 0 displays a broad pdf distribution with indistinct maxima and 10 1 3 is totally absent the small structural difference between and quartz which is reflected in the pdf distribution should allow in principle evaluation of the pre shock temperature in naturally impacted metamorphic rocks the results substantiate that formation of shock effects in quartz is dependent on i shock pressure ii pre shock temperature and iii shock wave direction so far the latter two parameters have not been considered in the application of experimental results to impact sites

shape and structure of the salzach valley austria from seismic traveltime tomography and full waveform inversion summary a 3 km long wide angle seismic survey across the salzach valley in the eastern alps austria provides a detailed characterization of its structure reflection and refraction traveltime tomography with adaptive gridding was applied to obtain a minimum structure model more detailed images of the valley fill were derived by acoustic full waveform inversion a multiscale frequency domain method was applied to data in the 4 26 hz range to improve the resolution from several hundred metres to less than fifty metres the starting frequency is unusually low for the size of the survey and it provides additional control of the long wavelengths of the background model which mitigates the non linearities of the inversion the final image is controlled by independent constraints from resistivity data and shallow wells it shows a confined upper aquifer that overlies a 150 m thick unsaturated zone no strong evidence was found for consolidated sediments at the bottom of the basin this and the large maximum depth to bedrock of 480 m suggests that the valley fill was repeatedly removed during strong glacial periods exposing the bedrock to deep erosion

imaging the 2013 explosive crater excavation and new dome formation at volcan de colima with terrasar x time lapse cameras and modelling abstract the summit region of steep volcanoes hosting lava domes often displays rapid geomorphologic and structural changes which are important for monitoring the source region of hazards explosive crater excavation is often followed by new lava dome growth which is one of the most dynamic morphometric changes that may occur at volcanoes however details of these crater formations and the ensuing new dome growth remain poorly studied a common problem is the lack of observational data due to hazardous field access and the limited resolution of satellite remote sensing techniques this paper describes the destructive constructive crater activity at volcan de colima mexico which occurred between january and march 2013 the crater geometry and early dome formation were observed through a combination of high resolution terrasar x spotmode satellite radar images and permanently installed monitoring cameras this combined time lapse imagery was used to identify ring shaped gas emissions prior to the explosion and to distinguish between the sequential explosion and crater excavation stages which were followed by dome growth by means of particle image velocimetry the digital flow field is computed from consecutive camera images showing that vertical dome growth is dominant at the beginning the upward growth is found to grade into spreading and a lateral growth domain after approximately two months of gradually filling the excavated craters with new magma the dome overflows the western margin of the crater and develops into a flow that produces block and ash flow hazards we discuss and compare the observations to discrete element models allowing us to mimic the vertical and lateral growth history of the dome and to estimate the maximum strength of the bulk rock mass moreover our results allow a discussion on the controls of a critical dome height that may be reached prior to its gravitational spreading

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

compositional analysis of the orientale basin using full resolution clementine data some preliminary results clementine imaged the moon globally in eleven different wavelengths we used these data to construct a full resolution 250 m pixel multispectral image of the orientale basin using this image in conjunction with other data sets such as earth based spectra we mapped the compositional units of the basin to obtain insights into its formation and evolution the basin ejecta units are largely uniform in composition with the inner montes rook formation being slightly more mafic than the outer hevelius formation the inner and outer rook mountains display many outcrops of pure anorthosite these and other findings are consistent with a three layer model for the lunar crust consisting of a mixed zone megaregolith tens of kilometres thick overlying a layer of anorthosite above a basaltic layer

an expanded non arrhenian model for silicate melt viscosity a treatment for metaluminous peraluminous and peralkaline liquids we present new viscosity measurements for melts spanning a wide range of anhydrous compositions including rhyolite trachyte moldavite andesite latite pantellerite basalt and basanite micropenetration and concentric cylinder viscometry measurements cover a viscosity range of 10 1 to 10 12 pas and a temperature range from 700 to 1650 c these new measurements combined with other published data provide a high quality database comprising 800 experimental data on 44 well characterized melt compositions this database is used to recalibrate the model proposed by giordano and dingwell giordano d dingwell d b 2003a non arrhenian multicomponent melt viscosity a model earth planet sci lett 208 337 349 for predicting the viscosity of natural silicate melts the present contribution clearly shows that 1 the viscosity temperature relationship of natural silicate liquids is very well represented by the vft equation log a b t c over the full range of viscosity considered here 2 the use of a constant high t limiting value of melt viscosity e g a is fully consistent with the experimental data 3 there are 3 different compositional suites peralkaline metaluminous and peraluminous that exhibit different patterns in viscosity 4 the viscosity of metaluminous liquids is well described by a simple mathematical expression involving the compositional parameter sm but the compositional dependence of viscosity for peralkaline and peraluminous melts is not fully controlled by sm for these extreme compositions we refitted the model using a temperature dependent parameter based on the excess of alkalies relative to alumina e g ae sm the recalibrated model reproduces the entire database to within 5 relative

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

the first day of the cenozoic highly expanded cretaceous paleogene k pg boundary section from the chicxulub peak ring recovered by international ocean discovery program iodp international continental scientific drilling program icdp expedition 364 provides an unprecedented window into the immediate aftermath of the impact site m0077 includes 130 m of impact melt rock and suevite deposited the first day of the cenozoic covered by

seismological analysis of conduit dynamics in fragmentation experiments we simulate gas burst and volcanic explosions under controlled laboratory conditions inducing fragmentation of volcanic rocks by rapid depressurization a series of experiments were performed in a shock tube apparatus at room temperature and a pressure range of 4 to 20 mpa using argon ar gas and particles or pumice samples of different porosities the instrumentation of this system with high precision piezoelectric sensors enabled us to capture elastic waves and to recognize their characteristic signatures by relating these signals to physical processes in the wave field we have been able to characterize the conduit mechanism and the source dynamics we compare and discuss conspicuous features of the waveforms and frequency spectra of these experimental signals with those of volcanic origin despite the fact that these signals are different in amplitude resulting from different scale conditions our observations indicate that the physical processes that occur during simulated explosions and those that occur during volcanic eruptions yield comparable signatures in their respective records the effects of the source receiver configuration and resonance also have significant implications all this suggests that the physical processes e g pressurization and depressurization of a system involve a system response that causes similar distinctive effects independent of the system size reflecting its intrinsic dynamics these similarities imply that powerful constraints on the source mechanisms of volcanic seismicity can emerge from seismic investigations of experimental simulations of volumetric sources such constraints may yield significant advances in the understanding of volcanic conduit dynamics and in the interpretation of seismic unrest at volcanic centers

non newtonian rheological law for highly crystalline dome lavas volcanic eruption models are hampered by the lack of multiphase magmatic flow laws most rheological models estimate the viscosity of multiphase lavas via the einstein roscoe equation but this simplification cannot be used for high crystallinity and it does not consider the non newtonian strain rate dependence of viscosity we carried out parallel plate experiments on natural samples to simulate multiphase lava deformation under various stresses and strain rates multiphase lavas exhibit an important component of shear thinning and appear to invalidate the adequacy of einstein roscoe based formulations for highly crystalline lava rheology the remarkable singular dependence of viscosity on strain rate yields a novel universal rheology law at eruptive temperatures t i e log 0 993 8974 t 0 543 log our work reveals the importance of considering microcracking and viscous dissipation at very high strain rate 10 3 s 1 explaining the occurrence of seismic swarms along the conduit margins and consequently supporting plug like magma ascent models

petrological architecture of a magmatic shear zone a multidisciplinary investigation of strain localisation during magma ascent at unzen volcano japan shearing of magma during ascent can promote strain localisation near the conduit margins anymechanical and thermal discontinuities associated with such events may alter the chemical physicaland rheological stability of the magma and thus its propensity to erupt lava spines can record suchprocesses preserving a range of macroscopic and microscopic deformation textures attributed toshearing and friction as magma ascends through the viscous brittle transition here we use a multi disciplinary approach combining petrology microstructures crystallography magnetics and experi mentation to assess the evidence role and extent of shearing across a marginal shear zone of the1994 1995 lava spine at unzen volcano japan our results show that crystals can effectively moni tor stress conditions during magma ascent with viscous remobilisation crystal plasticity and com minution all systematically increasing towards the spine margin accompanying this we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex tural variations across the shear zone from symplectitic to granular rims towards the spine margin in addition the compaction of pores chemical and textural alteration of interstitial glass and mag netic variations all change systematically with shear intensity the strong correlation between thedegree of shearing crystal deformation and disequilibrium features together with distinct magneticproperties implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent this was accompanied by late stage or post emplacement fluid andgas induced alteration of the gouge as well as oxidation and glass devitrification understandingand recognising evidence for strain localisation during magma ascent may therefore be vital whenassessing factors that regulate the style of volcanic eruptions which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava domes

impact induced porosity and microfracturing at the chicxulub impact structure porosity and its distribution in impact craters has an important effect on the petrophysical properties of impactites seismic wave speeds and reflectivity rock permeability strength and density these properties are important for the identification of potential craters and the understanding of the process and consequences of cratering the chicxulub impact structure recently drilled by the joint international ocean discovery program and international continental scientific drilling program expedition 364 provides a unique opportunity to compare direct observations of impactites with geophysical observations and models here we combine small scale petrographic and petrophysical measurements with larger scale geophysical measurements and numerical simulations of the chicxulub impact structure our aim is to assess the cause of unusually high porosities within the chicxulub peak ring and the capability of numerical impact simulations to predict the gravity signature and the distribution and texture of porosity within craters we show that high porosities within the chicxulub peak ring are primarily caused by shock induced microfracturing these fractures have preferred orientations which can be predicted by considering the orientations of principal stresses during shock and subsequent deformation during peak ring formation our results demonstrate that numerical impact simulations implementing the dynamic collapse model of peak ring formation can accurately predict the distribution and orientation of impact induced microfractures in large craters which plays an important role in the geophysical signature of impact structures plain language summary the chicxulub crater mexico is widely known for its association with the extinction of the nonavian dinosaurs at the end of the cretaceous period the crater was first identified due to its gravitational and magnetic anomalies potential impact structures are often identified in part on the basis of geophysical anomalies most commonly including a circular gravity low gravity is slightly weaker at craters because the impact cratering process removes mass from the impact site in this study we examine the cause of the chicxulub gravity anomaly by combining observations from recent drilling of the crater geophysical data measured across the crater and numerical impact simulations we demonstrate that porosity in rocks beneath the crater floor is primarily accommodated by fracturing during the impact cratering process that the orientation of those fractures are consistent with predictions from numerical impact simulations and that impact induced porosity is one of the primary causes of gravity anomalies in large impact craters

the fragmentation threshold of pyroclastic rocks in response to rapid decompression porous magma may fragment explosively this occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles this occurs at a critical pressure difference between the bubbles and the surrounding magma in this study we have investigated this pressure threshold necessary for the fragmentation of magma here we present the first comprehensive high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity permeability crystallinity and chemical composition we exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated experimentally we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample our results show that the fragmentation threshold is strongly dependent on porosity high porosity samples fragment at lower pressure differentials than low porosity samples the fragmentation threshold is inversely proportional to the porosity of the other factors permeability likely affects the fragmentation threshold at high porosity values whereas chemical composition crystallinity and bubble size distribution appear to have minor effects the relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma

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

full waveform tomography for lithospheric imaging results from a blind test in a realistic crustal model summary a comprehensive validation of 2 d frequency domain acoustic wave equation tomography was undertaken in a blind test using third party realistic elastic wave equation data the synthetic 2 d wide angle seismic data were provided prior to a recent workshop on the methods of controlled source seismology the true model was not revealed to the authors until after the presentation of our waveform tomography results the original model was specified on a detailed grid with variable p wave velocity s wave velocity density and viscoelastic q factor structure designed to simulate a section of continental crust 250 km long and 40 km deep synthetic vertical and horizontal component data were available for 51 shot locations spaced every 5 km recorded at 2779 receivers spaced every 90 m evenly spread along the surface of the model the data contained energy from 0 2 to 15 hz waveform tomography a combination of traveltime tomography and 2 d waveform inversion of the early arrivals of the seismic waveforms was used to recover crustal p velocity structure from the vertical component data using data from 51 sources 1390 receivers and frequencies between 0 8 and 7 0 hz the waveform tomography result contained apparent structure at wavelength scale resolution that was not evident on the traveltime tomography result the predicted acoustic waveforms in the final result matched the original elastic data to a high degree of accuracy during the workshop the exact model was revealed over much of the model the waveform tomography results provided a good correspondence with the true model from large to intermediate wavelength scales with a resolution limit on the order of 1 km a significant near surface low velocity zone invisible to traveltime methods was correctly recovered the results also provided a high resolution image of the complex structure of the entire crust and the depth and nature of the crust mantle transition some inaccuracies were observed near the edges of the images due to the limited ray coverage inherent to the footprint of the survey geometry several aspects of the waveform tomography strategy were critical to the success of the acoustic method with realistic synthetic viscoelastic data i the accuracy of the starting model from traveltime tomography ii implementation in the frequency domain iii the use of complex valued frequencies to effect time damping of the data residuals iv the selection of a suitable subset of data and data frequencies v progressive inversion of low to high frequency components of the data vi initial pre inversion matching of the amplitudes between observed and modelled data and vii sufficient preconditioning of both the data and the update images combined these strategies were effectively equivalent to a multiscale approach that mitigated the non linearity of the seismic inverse problem during the inversion we carried out repeated forward modelling to ensure our modelled waveforms matched the observed data as closely as possible in both frequency and time domains the synthetic data set used in this paper provides a benchmark for future testing of modelling inversion and imaging algorithms for wide angle lithospheric imaging

integrated interpretation of physical properties of rocks of the borehole yaxcopoil 1 chicxulub impact structure 1 the borehole yaxcopoil 1 drilled within the chicxulub meteoritic impact structure mexico was completely cored from 404 to 1511 m through postimpact tertiary limestones underlain by impactites the impactites comprise impact melt rich suevitic breccia followed by megablocks of cretaceous limestones calcarenites dolomites and anhydrites measurements of porosity density and thermal parameters on 450 samples equidistant sampling complete depth range and of ultrasonic velocities and electric resistivity on 80 representative samples are used to investigate the physical properties of carbonate rocks and to study the influence of the impact experiments under elevated pressure calculations using frequency dependent biot gassmann theory and cross checking with borehole logs where available show that ultrasonic laboratory and sonic in situ data correspond sonic and electric quasi continuous logs are obtained from empirical correlations with thermal conductivity density and porosity and consideration of mineralogical composition and microstructure these data give constraints on interpretation and geophysical modeling of e g seismic and gravity data in the tertiary postimpact limestone section the rock fabric porosity influences the physical properties the upper boundary of the impactites is distinctly determined by the high inhomogeneity factor and anisotropy coefficient of thermal conductivity and by the temperature gradient from high resolution borehole temperature measurements all physical properties indicate that the upper part of the suevitic breccia can be distinguished from the lower suevite unit in the cretaceous megablocks a high variability of all properties particularly thermal conductivity density of solid material and temperature gradient due to the high variability in the mineral composition calcite dolomite anhydrite is observed

hot pressing in conduit faults during lava dome extrusion insights from mount st helens 2004 2008 abstract rhyodacitic volcanoes such as mount st helens msh soufriere hills mount unzen and mount pelee erupt spines mantled by layers of magma derived cataclasite and fault gouge msh produced seven lava spines from 2004 2008 composed of low porosity compositionally uniform crystalline dacite dome extrusion was attended by continuous drumbeat seismicity derived from faulting along the conduit margin at 0 5 1 km depth and evidenced by the enveloping gouge layers we describe the properties of the gouge derived fault rocks including laboratory measurements of porosity and permeability the gouge varies from unconsolidated powder to lithified low porosity low permeability fault rocks we reconstruct the subsurface ascent of the msh magma using published field observations and create a model that reconciles the diverse properties of the gouge with conditions in the conduit during ascent i e velocity temperature we show lithification of the gouge to be driven by hot pressing processes wherein the combination of elevated temperature confining pressure and dwell time cause densification and solid state sintering of the comminuted crystal rich glass poor gouge the degree of gouge lithification corresponds with residence time in the conduit such that well lithified materials reflect extended times in the subsurface due to slower ascent rates with this insight we suggest that gouge competence can be used as a first order estimate of lava ascent rates furthermore we posit gouge lithification which reduces porosity and permeability inhibits volcanic outgassing thereby increasing the potential for explosive events at spine producing volcanoes

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

volcanic dilemma flow or blow volcanic eruptions can be either slow outpourings of lava or explosive ejections of fragmented rock in his perspective dingwell discusses the physical and chemical properties of magma that lead to these two styles of eruption new experiments and computer simulations are pointing to the involvement of solidlike behavior of liquid magma when it makes a transition to a glass

the strength of heterogeneous volcanic rocks a 2d approximation abstract volcanic rocks typically contain heterogeneities in the form of crystals and pores we investigate here the influence of such heterogeneity on the strength of volcanic rocks using an elastic damage mechanics model in which we numerically deform two dimensional samples comprising low strength elements representing crystals and zero strength elements representing pores these circular elements are stochastically generated so that there is no overlap in a medium representing the groundmass our modelling indicates that increasing the fraction of pores and or crystals reduces the strength of volcanic rocks and that increasing the pore fraction results in larger strength reductions than increasing the crystal fraction the model also highlights an important weakening role for pore diameter but finds that crystal diameter has a less significant influence for strength to account for heterogeneity pores and crystals we propose an effective medium approach where we define an effective pore fraction p v p v p v g where v p and v g are the pore and groundmass fractions respectively highly heterogeneous samples containing high pore and or crystal fractions will therefore have high values of p and vice versa when we express our numerical samples more than 200 simulations spanning a wide range of crystal and pore fractions in terms of p we find that their strengths can be described by a single curve for a given pore diameter to provide a predictive tool for the strength of heterogeneous volcanic rocks we propose a modified version of 2d solution for the sammis and ashby 1986 pore emanating crack model a micromechanical model designed to estimate strength using microstructural attributes such as porosity pore radius and fracture toughness the model reformulated to include p and therefore crystal fraction captures the strength curves for our numerical simulations over a sample heterogeneity range relevant to volcanic systems we find that published experimental data for which the porosity and crystal fraction is known correlate well with our modelled curves adding validity to our approach we present herein a 2d analytical tool to estimate the strength of volcanic rock that can be used when the porosity crystal fraction and maximum pore diameter are known

diffraction tomography and multisource holography applied to seismic imaging seismic tomography is emerging as an imaging method for determining subsurface structure when the view angle coverage is limited and the scale of the medium inhomogeneities is comparable with the wavelength as is often true in geophysical applications the performance of ordinary ray tomography becomes poor other tomographic methods are needed to improve the imaging process here we study diffraction tomography and multisource holography and evaluate their performances for surface reflection profiling srp vertical seismic profiling vsp and cross hole measurements theoretical formulations are derived for two dimensional geometry in terms of line sources along a source line and line receivers along a receiver line the theory for diffraction tomography is based on the born or rytov approximation the performances of diffraction tomography and multisource holography are evaluated by examining the information coverage in the spatial frequency domain and by numerical examples multisource holography

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

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