comparison of an optimized resistivity array with dipole dipole soundings in karst terrain wereporttheresultsofafieldcomparisonof2dresistivity data collected using both the traditional dipole dipole array and a new computer optimized array recently described in literature the study was conducted at a karst site in eastern pennsylvania computer simulations suggested that for a given line length the optimal array and the dipole dipole arraywouldbeequallyeffectiveatimagingshallowtargetsbut the optimal array would provide better resolution at depth ourfield test results showed that the two arrays imaged karst bedrock topography equally well when the full grid of 2d lines were combined and analyzed using 3d inversion however theoptimalarraywasabletoresolveacrosscuttingbedrock fracture system that was not visible in the dipole dipole data the existence of the fracture system was confirmed by drilling because the optimal array requires roughly three times as many measurements per line we conclude that the optimalarrayispreferabletotraditionaldipole dipolesoundings only when the slight improvement in resolution at depth ismoreimportantthanrapiddatacollection

linear filtering of vlf dip angle measurements the suggested interpretation technique is based on discrete linear filtering of vlf data the output of the described filtering results is expressed in terms of an equivalent current density at a specified depth that would cause the measured magnetic field the most practical six point filter gives an accuracy of 8 the filter is an extension of the commonly used fraser filter to process vlf dip angle data filtering the same data set for various depths gives an idea about the change of current density with depth areas with high current density correspond to good conductors the conductor dip can also be determined the use of the technique is illustrated on theoretical and field examples in all cases a good correlation with original models and other types of geophysical measurements was obtained as shown in the last example the filtering technique is also applicable in interpretation of other electromagnetic methods

application of seismic refraction tomography in karst terrane seismic refraction tomography field data were collected on several bridge foundation sites in pennsylvania in close proximity to geotechnical boring locations profiles determined from these field measurements were plotted against drilling data and these comparisons revealed the ability of seismic wave velocities to differentiate overburden soil from rock in addition foundation construction data were collected at each of the sites and compared with refraction test results determined prior to construction in particular top of rock revealed by an excavation and pile tip elevations at driving refusal were compared with refraction test results from these data it appears that seismic wave tomograms can characterize the soil rock interface and that it is possible to predict expected design pile lengths based upon a measured p wave velocity tomogram it can be concluded from these site comparisons that geophysical techniques such as seismic refraction tomography can provide important additional information to site characterization for bridge foundations in karst terrane however these techniques should not be viewed as a replacement but should be conducted during design stage site investigation to aid selection of borehole locations and other testing needs

pseudotachylite in impact structures generation by friction melting and shock brecciation a review and discussion abstract fault related pseudotachylites are generated as the result of brittle or brittle ductile deformation related to seismic faulting it is generally accepted among tectonic workers that while cataclasis plays a role in pseudotachylite formation friction melting is involved in addition pseudotachylite and pseudotachylite like breccias have been repeatedly described from the basement exposures of impact and cryptoexplosion structures and as veinlets in shocked meteorites and lunar samples the relevant literature is reviewed with regard to controversial usage of the term pseudotachylite and attention is drawn to several problematic aspects 1 the term pseudotachylite is currently used both as a purely descriptive term and with genetic implications 2 no unambiguous criteria for comparison and distinction of tectonically and impact generated pseudotachylites have been identified 3 it needs to be considered that in impact structures pseudotachylite could be generated by two distinct processes namely by shock brecciation melting and by friction melting pseudotachylite in impact structures could be the result of either of these two processes or of both 4 it is recommended to adhere more stringently when describing breccias from impact or cryptoexplosion structures to nomenclature accepted for the description of fault rocks in tectonic environments this would help to avoid confusion when discriminating between melt rocks pseudotachylite impact melt rock fragmental breccias cataclasite fragmental impact breccia and mylonitic rocks 5 no criteria for the distinction of melt bearing impact breccias impact melt rock and pseudotachylite have been established either in order to solve these problems close interaction between structural geologists and impact workers is warranted it is suggested that the understanding of breccias in impact structures would be improved by paying particular attention to the temporal relationships between different breccia types this would facilitate recognition of possibly different formational processes for impact produced melt and fragmental breccias and their distinction from pre and post shock deformation products

comment on k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia by i t uysal s d golding a y glikson a j mory and m glikson earth planet sci lett 192 2001 218 289 abstract k ar isotopic ages presented by uysal et al for illitic clay minerals from drill core samples were interpreted to date the woodleigh impact event at 359 4 ma allegedly implicating woodleigh in the late devonian mass extinction however only very equivocal evidence is presented by uysal et al to support a link between clay mineral paragenesis and impact related features and the k ar ages reveal a distribution that is essentially a continuum between 308 and 364 ma the age computed by uysal et al is based on an average of the five oldest ages within this group which has no geological or statistical basis the stratigraphic age constraints considered by uysal et al to be consistent with this age are much weaker than acknowledged and the impact could have been much older than mid devonian the size of the woodleigh crater is poorly constrained and the subject of an ongoing controversy uysal et al s suggestion of 120 km diameter is probably overestimated by a factor of two in which case a link to any mass extinction is unlikely

morokweng impact structure northwest province south africa geophysical imaging and shock petrographic studies abstract the existence of a large near circular structure centred on the area around morokweng 23 32 e 26 31 s in the northwest province of south africa and possibly extending into southern botswana is inferred from aeromagnetic and gravity data results of surface geological studies support earlier suggestions that this feature could represent an impact structure samples from an autochthonous ironstone breccia exposure about 47 km to the west of the centre of this structure and several samples of allochthonous quartzite from south of the central part of the structure contain shock metamorphosed quartz with impact diagnostic planar deformation features pdfs the geophysical evidence is suggestive of an original size of this impact structure of at least 70 km but several annular anomalies outside of the study region could be interpreted as indicating a diameter of 300 340 km the age of this structure is only loosely constrained but is believed to be younger than the 2 25 2 5 ga transvaal supergroup

crustal structure of oaxaca mexico from seismic refraction measurements seismic refraction and gravity data have been analyzed to obtain a model of the compressional wave structure of the ocean to continent transition in the state of oaxaca in southwestern mexico crustal thickness on the continent at the latitude 18 n is 45 4 km based on reflected phases from the moho discontinuity the crust has been modeled with three layers with velocities of 4 3 to 4 6 5 0 to 5 7 and 6 85 to 7 0 km sec each with positive velocity gradient the crust thins to 10 km at the coast near pinotepa nacional where precambrian metamorphic rocks are exposed 45 km from the mid america trench offshore the oceanic structure consists of an 8 km thick crust with a normal crustal velocity structure spudich and orcutt 1980 the apparent dip of the subducting plate beneath western mexico is 10 on the oceanic side strong reflections suggest a minimum depth of 35 km for the lithosphere asthenosphere boundary the asthenosphere has a seismic velocity of 7 6 km sec and a thin lid in which the velocity is 8 6 km sec

response to critical comment on a j mory et al 2000 woodleigh carnarvon basin western australia a new 120 km diameter impact structure by w u reimold and c koeberl

integrated geophysical investigation of the st james fault complex a case study abstractwe noninvasively detected the characteristics and location of a regional fault in an area of poor bedrock exposure complicated by karst weathering features in the subsurface because this regional fault is associated with sinkhole formation its location is important for hazard avoidance the bedrock lithologies on either side of the fault trace are similar hence we chose an approach that capitalized on the complementary strengths of very low frequency vlf electromagnetic resistivity and gravity methods vlf proved most useful as a first order reconnaissance tool allowing us to define a narrow target area for further geophysical exploration fault related epikarst was delineated using resistivity ultimately a high resolution gravity survey and subsequent inverse modeling using the results of the resistivity survey helped to further constrain the location and approximate orientation of the fault the combined results indicated that the location of the fault trace needed to be adjusted 53 m

end cretaceous cooling and mass extinction driven by a dark cloud encounter abstract we have identified iridium in an 5 m thick section of pelagic sediment cored in the deep sea floor at site 886c in addition to a distinct spike in iridium at the k pg boundary related to the chicxulub asteroid impact we distinguish the contribution of the extraterrestrial matter in the sediments from those of the terrestrial matter through a co ir diagram calling it the extraterrestrial index f ex this new index reveals a broad iridium anomaly around the chicxulub spike any mixtures of materials on the surface of the earth cannot explain the broad iridium component on the other hand we find that an encounter of the solar system with a giant molecular cloud can aptly explain the component especially if the molecular cloud has a size of 100 pc and the central density of over 2000 protons cm 3 kataoka et al 2013 2014 pointed that an encounter with a dark cloud would drive an environmental catastrophe leading to mass extinction solid particles from the hypothesized dark cloud would combine with the global environment of earth remaining in the stratosphere for at least several months or years with a sunshield effect estimated to be as large as 9 3 w m 2 the dark cloud would have caused global climate cooling in the last 8 myr of the cretaceous period consistent with the variations of stable isotope ratios in oxygen barrera and huber 1990 li and keller 1998 barrera and savin 1999 li and keller 1999 and strontium barrera and huber 1990 ingram 1995 sugarman et al 1995 the resulting growth of the continental ice sheet also resulted in a regression of the sea level the global cooling which appears to be associated with a decrease in the diversity of fossils eventually led to the mass extinction at the k pg boundary

a geologic history of the north central appalachians part 3 the alleghany orogeny the north central appalachians occupy a critical position within the 3000 km long appalachian orogen lying southwest of the boundary between the central and northern appalachians cnab the one billion year long history of tectonic activity in eastern laurentia includes the creation and evolution of the appalachian orogen during the paleozoic and the mesozoic transformation of the orogen into a passive margin during pangea9s disassembly a most important ingredient in the evolution of the orogen was the alleghany orogeny which was driven by the convergence and collision between laurentia laurussia and west gondwana africa the alleghany orogeny in the central and southern appalachians was a decollement tectonism that involved a larger part of eastern laurentia than had the previous three orogenies the fundamental element was a very low angle thrust decollement that originated in mid crustal levels east of the presently exposed appalachians and rose westwardly to progressively higher levels in the upper crust and the supra crustal paleozoic section alleghany deformation was widely developed in the hanging wall block allochthon primarily in the form of thrust faults and fold and thrust structures both of which splayed upward from the basal decollement the youngest manifestations of the alleghany orogeny were northeast trending strike slip faults and dextral shear zones in the piedmont in the north central appalachians the exposed allochthon consists of two parts the sedimentary externides appalachian plateau and valley and ridge provinces and the crystalline externides reading prong blue ridge belt and piedmont province long thrust cored anticlines predominate in the sedimentary externides a widespread layer parallel shortening preceded the folding it is largely coaxial with the folding but extends considerably farther to the northwest toward the craton it is hypothesized that the folding developed in reverse order sequentially from the northwest to the southeast the crystalline externides are dominated by low angle thrust faults and upright folds trending east northeast the first order valley and ridge folds on the northwest side acted as a buttress and diverted the crystalline externides rocks north northwestwardly onto the topographic low area over the anthracite region this thrusting of the crystalline externides caused anthracitization of the coals within the pennsylvanian rocks there metamorphism and magmatism were significant events during the earlier phase of the alleghany orogeny in the southern appalachians whatever magmatism and medium to high grade metamorphism developed in the north central appalachians are in the covered internides to the southeast the alleghany orogeny of the north central appalachians occurred during the early permian erosion of anticlinal crests probably began as the folds grew with accumulation of this locally derived sediment in the intervening synclines a regional alluvial plain coalesced above the partially eroded externides structures as erosion of the pre alleghany highland and the alleghany hinterland mountains continued to the southeast spreading sediment to the northwest this erosion and northwest transport probably persisted with diminishing intensity throughout the remainder of the permian and into the mesozoic and changed only with the beginning of crustal extension during the late triassic

gravity and structure of the continental margins of southwestern mexico and northwestern guatemala geophysical measurements over the eastern end of the tehuantepec ridge and adjacent continental margins of southern mexico and northern guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces a positive free air gravity anomaly which extends northwestward along the outer continental shelf of guatemala curves abruptly landward in the gulf of tehuantepec the positive shelf anomaly is on trend with the positive anomaly of the nicoya peninsula costa rica and suggests that rocks genetically related to cretaceous rocks of the nicoya complex extend northwestward along the continental shelf to the gulf of tehuantepec a crustal and subcrustal cross section of the continental margin of guatemala constrained by gravity magnetic and seismic refraction data indicates that the rock strata causing the outer shelf gravity high dip landward consistent with imbricate thrusting of the oceanic crust beneath and into the continental margin a model crustal cross section of the continental margin of southern mexico north of the tehuantepec ridge shows a markedly different margin structure with a relatively small amount of continental accretion and a continental crustal block extending to within approximately 25 km of the trench axis

late jurassic age for the morokweng impact structure southern africa abstract a roughly 70 km diameter circular feature buried beneath the kalahari sands in south africa is revealed on regional aeromagnetic maps boreholes drilled into the centre of the structure intercept a 250 m thick sheet of quartz norite interpreted as an impact melt which overlies brecciated and shock metamorphosed basement granite zircons recovered from the quartz norite yield u pb ages of 145 0 8 ma and biotites provide ar ar ages of 144 4 ma these data provide strong evidence for the occurrence of a late jurassic impact crater the morokweng impact structure 100 m beneath the surface

characterization and significance of shocked quartz from the woodleigh impact structure western australia we re examined the buried woodleigh structure in western australia which has been inferred to be a multi ringed 120 km diameter impact crater because the proposed size and possible synchronicity with one of the pre mesozoic mass extinction events has attracted controversy we undertook a detailed study of the petrology and mineralogy of a number of samples of core from the woodleigh 1 borehole that was drilled into the central uplift of the structure crystalline proterozoic basement rocks comprising granites and gneisses in the woodleigh 1 core contain minor brecciation in discrete veins and reveal clear evidence of shock metamorphism over the full extent of the core imaging of laboratory etched quartz showed that a large number of grains contain shock deformation lamellae microstructural and crystallographic analysis of these lamellae by tem showed that they are planar deformation features pdfs that have subsequently undergone annealing and water assisted recrystallization the available geological petrographic and mineralogical evidence suggest that woodleigh is an eroded impact crater that is nearer to 60 km than 120 km in diameter future drilling projects should better constrain the level of erosion and may reveal any preserved impact lithologies

large volumes of anhydrous pseudotachylyte in the woodroffe thrust eastern musgrave ranges australia a mylonitic thrust zone at least 1 5 km thick forms a sharp contact between granulite and amphibolite facies gneisses in the eastern musgrave ranges central australia the thrust dips gently to the south and is interpreted as an extension of the woodroffe thrust which was formed about 550 ma ago mylonites at the base of the thrust grade upwards into ultramylonites which pass abruptly into a pseudotachylyte bearing zone approximately 1 km thick containing approximately 4 of pseudotachylyte veining the orientation of the veins appears to be random pseudotachylytes occur only in the granulite facies rocks and their precursors are felsic pyroxene and or garnet granofelses rotated blocks of ultramylonite are present in some of the pseudotachylytes and some pseudotachylyte veins have been plastically deformed suggesting nearly contemporaneous semiductile and brittle behaviour the matrix of the pseudotachylyte shows spectacular examples of igneous quench microstructures especially skeletal and dendritic crystals of plagioclase and feathery pyroxene dendrites also present are glass devitrification microstructures spherulites evidence of liquid flow and partly melted residual grains with former glassy rims showing different optical properties from those of the surrounding isotropic material these features confirm that the pseudotachylyte formed by melting in anhydrous conditions the matrix of the pseudotachylyte veins is less siliceous than the host rocks owing to non equilibrium melting of pyroxene garnet and plagioclase the igneous assemblages of the melt notably the crystallization of pigeonite are consistent with rapid cooling from very high temperature 1000 c melting and quenching is probably due to very local short lived rises in temperature accompanied by dilation

critical comment on a j mory et al woodleigh carnarvon basin western australia a new 120 km diameter impact structure mory et al 1 claim to report a newly discovered basement cored multi ring impact structure and hint at a possible permian triassic boundary age for the impact they purport that this structure could be as large as 120 km in diameter this comment emphasizes that these authors indeed identi ed a potentially interesting structure but that the evidence presented is inconclusive and not well constrained it does not prove that woodleigh is of impact origin mory et al s 1 claims especially regarding the alleged size of this structure and correlation with the p tr boundary mass extinction have generated extensive public interest presentation of such an alleged discovery must adhere stringently to scienti c principles namely that convincing evidence is obtained before interpretation which itself should not go beyond the limits set by the data at hand this comment demonstrates that mory et al did not conform to this principle issue will be taken with the arguments of mory et al in the sequence according to the structure of their paper 1 regional geological setting

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

a case study of the reliability of multielectrode earth resistivity testing for geotechnical investigations in karst terrains abstract the thinly mantled karst areas of northeastern pennsylvania have been associated with a number of structural failures caused by sinkholes geotechnical investigations in these areas have primarily relied upon intrusive probe tests either through borings or air track drillings the silty clay soils overlying the carbonate bedrock have limited the use of nonintrusive ground penetrating radar and electromagnetic methods however multielectrode earth resistivity testing may be a suitable method for locating subsurface features associated with sinkhole formation in these areas the case study presented in this paper concerns a site in northampton county where sinkholes have been occurring at an accelerated rate due to the influence of construction the bedrock at the site is located between 1 m and approximately 10 m below the surface the soil is primarily a silty clay seventy resistivity lines using a variety of electrode and line spacings were conducted at the site using the data obtained from the earth resistivity tests 17 borings were located to assess the accuracy of the resistivity results comparisons between boring and resistivity results demonstrate that the resistivity tests provide good information concerning the depth to the irregular surface of the bedrock however use of the test to locate subsurface anomalies associated with sinkhole formation and to determine their size will require additional research the paper presents the results of the study and provides recommendations for geotechnical investigation in silty clay soils overlying shallow carbonate bedrock

reply to comment on k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia

woodleigh impact structure australia shock petrography and geochemical studies the large complex woodleigh structure in the carnarvon basin of western australia has recently been added to the terrestrial impact crater record many aspects of this structure are however still uncertain this work provides a detailed petrographic assessment of a suite of representative drill core samples from the borehole woodleigh 1 that penetrated uplifted basement rocks of the central part of this structure fundamental rock and mineral deformation data and high precision chemical data including results of pge and oxygen isotopic analysis are presented the sampled interval displays likely impact produced macrodeformation in the form of fracturing and breccia veining at the microscopic scale contrary to earlier reports that these breccias represent pseudotachylite friction melt or even shock shear produced pseudotachylitic melt breccia cannot be confirmed due to pervasive post impact alteration abundant planar deformation features pdfs in quartz in addition to diaplectic glass and partial isotropization are the main shock deformation effects observed confirming that woodleigh is of impact origin over the investigated depth interval the statistics of quartz grains with a variable number of sets of pdfs does not change significantly and the patterns of crystallographic orientations of pdfs in randomly selected quartz grains does not indicate a change in absolute shock pressure with depth either the value of oxygen isotopes for the recognition of meteoritic contamination as proposed by earlier woodleigh workers is critically assessed neither ina nor pge analyses of our samples support the presence of a meteoritic component within this basement section as had been claimed in earlier work

integrated geophysical techniques for subsurface imaging of active deformation across the himalayan frontal thrust in singhauli kala amb india abstract non invasive geophysical techniques employed across the himalayan frontal thrust hft at singhauli in the north western frontal himalaya to understand shallow subsurface geological structures and their nature in the context of active tectonics the combination of high resolution ground penetrating radar gpr electrical resistivity tomography ert and multi channel analysis of surface waves masw techniques with the different acquisition parameters found to be beneficial in gaining high resolution images of shallow subsurface geological structures the contact zone of resistivity obtained between 134 m suggests the juxtaposition of dissimilar lithological units explained as two faults dipping to the northwest related to the hft the average separation between fault 0 and fault i is 12m which in general agreement with the trench excavation survey the gpr radargram over a length of 45m show offset reflectors at a horizontal distance of 12 15m close to the active deformation zone the horizontal reflectors abruptly truncate across two northerly dipping planes the 2d vs section masw1 2 exhibited dipping attitude of the litho units with different velocity zones in the northeast direction which are also consistent with the local geological setting prominent liquefaction feature identified in the ert section specifically with the 1 and 1 5m electrode spacing show explicit expression of its existence due to contrasting resistivity ranging between 100 and 450 m a strong correlation has been established between the field based geological observations carried out earlier in this area through a trench excavation survey with the present work using geophysical techniques the integrated approach found to be highly beneficial in those areas where contrasting sub lithological units exist in terms of their physical properties this study argues for judicious use of the ert gpr and masw techniques to delineate shallow subsurface geology across various active faults in the himalayan terrain to comprehend the active deformation related to large magnitude paleoearthquakes and earthquake hazard assessment

microscopic lamellar deformation features in quartz discriminative characteristics of shock generated varieties the appearance and orientation of microscopic lamellar deformation features in quartz have been examined in samples from different geologic environments lamellar deformation features from known shock and or impact environments are sharp parallel closely spaced and tend to extend in a continuous manner across entire grains they have specific well documented orientations and occur in multiple sets per grain lamellar deformation features from other geologic environments including tectonic and explosive volcanic only superficially resemble shock features they are generally less well defined slightly curved wider spaced and tend to extend in a discontinuous manner across parts of grains they have relatively random orientations only some of which correspond to known shock orientations and generally occur as single sets per grain on the basis of these observations we conclude that there is a characteristic set of microscopic deformation features produced in quartz by shock these so called planar features differ from microscopic lamellar deformation features produced by other dynamic processes therefore the case for the formation of characteristic planar features in quartz by natural processes other than impact is still not proven