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

observations at terrestrial impact structures their utility in constraining crater formation hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near surface volume of the target body local geology of the target area tends to be of secondary importance and the net result is that impacts of similar size on a given planetary body produce similar results this is the essence of the utility of observations at impact craters particularly terrestrial craters in constraining impact processes unfortunately there are few well documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time nevertheless observations of the terrestrial impact record have contributed substantially to fundamental properties of impact there is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling in particular from recent hydrocode models the terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that z models are at best first order approximations observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures there are however major observational gaps and cases e g vredefort where current observations and hydrocode models are apparently inconsistent it is perhaps time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s which were fueled by observations at terrestrial impact structures filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta

early fracturing and impact residue emplacement can modelling help to predict their location in major craters understanding the nature and composition of larger extraterrestrial bodies that may collide with the earth is important one source of information lies in the record of ancient impact craters some of which have yielded chemical information as to the impacting body many deeply eroded craters have no remaining melt sheet or ejecta yet may contain impactor residue within basement fractures the emplacement mechanism for fractionated siderophile residues is likely to be gaseous although melt droplets and some solid materials may survive for breccia and melt filled fractures to contain extraterrestrial material they must form very early in the impact process most current numerical models do not dwell on the formation and location of early major fractures although fractures in and around small craters on brittle glass exposed to hypervelocity impact in low earth orbit have been successfully simulated modelling of fracture development associated with larger craters may help locate impact residues and test the models themselves

automated recognition of valley lines and drainage networks from grid digital elevation models a review and a new method abstract the literature on valley and drainage line recognition from grid digital elevation models dems is reviewed two problems preventing the successful delineation of fully connected drainage networks of single line width are identified a the positioning of the ends of drainage networks and b the assignment of drainage directions to a dem in particular the assignment of drainage directions across flat areas and at closed depressions in the dem pits a new method for drainage network delineation is suggested which incorporates an alternative method for drainage direction assignment over a dem this method is found to produce realistic directions and performs well on two complex dems

paleomagnetism of two late pleistocene lake basins in colorado an evaluation of detrital remanent magnetization as a recorder of the geomagnetic field paleomagnetic studies of magnetite bearing lacustrine sediments from two late pleistocene sections age range about 12 000 23 000 14c years one at devlins park in the front range the other at triangle park in the northern end of the sawatch range colorado indicate that detrital remanent magnetization of fine grained silty clay sediments at both locations was capable of recording changes in the magnetic field over periods as short as 100 200 years whereas coarser grained silts in the triangle park section seem to be less efficient recorders of the geomagnetic field the remanence of the fine grained sediments contains an apparent inclination error of about 9 at both localities in addition locally operative depositional and or postdepositional processes have produced both intrabasinal and interbasinal systematic differences in the remanence records of up to about 30 in declination and 10 in inclination comparison of the directional data from devlins park with those from four other broadly contemporaneous north american lake basins reveals that although there are abundant minor and several major discrepancies there are general similarities in the longer wavelength features particularly of declination the differences in the records are attributable to depositional and postdepositional factors variable and secondary overprinting local field differences sampling techniques and incorrect stratigraphic correlations the overall similarity in the records probably reflects the influence of the geomagnetic field the greatest difficulty in determining the significance of the similarities and differences between the records from the various lakes is the present lack of precise age and stratigraphic control

a magnetization process of sediments laboratory experiments on post depositional remanent magnetization summary palaeomagnetic studies require a theory of magnetization mechanism of sediments and a method of estimating magnetic field intensity from their remanences this paper establishes a physical basis for the generation of the remanence in deep sea and lake sediments experimentally redeposition experiments have been carried out under centrifugal force in weak magnetic fields the centrifuging method produces post depositional remanent magnetization post drm in the compacted sediment and its remanence and susceptibility are compatible with those of natural sediments and reconstituted materials of other redepositional experiments three properties of the post drm have been deduced from the experiments 1 the efficiency of acquisition of post drm decreases with increase in density during the compaction process 2 the total post drm is equal to the sum of the partial post drm addition law and 3 time is not a substantial factor for alignment of the magnetic particles these results lead to the conclusions that the magnetic particles do not rotate steadily but in a series of steps and that the density change is the crucial factor giving rise to the post drm a mathematical formula representing the remanence record in sediments is proposed on the basis of the experimental results and the model the principal equation is expressed as an integral of the product of three parameters over time when sediments have been compacted the field intensity variation characteristic function of the sediment and the time derivative of the density change

historic and paleomagnetic secular variation and the earth s core dynamo process this paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth s magnetic field secular variation and its implications for the core dynamo process secular variation is defined in this paper as the general spatio temporal variation of the field secular change will refer to the first time derivative of the spatial field strict geomagnetic definition of secular variation we will emphasize here the long term 10 year or longer behavior of the historic secular variation hsv for this is the variation that is comparable with the paleomagnetic secular variation psv two important related topics the 1969 1970 geomagnetic jerk in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources are treated more fully in other reviews

detrital remanent magnetization viscosity theory of the lock in zone post depositional remanent magnetization pdrm becomes permanent within a lock in zone whose apparent time delay depth and time span thickness are related to the characteristic time constants of compaction and magnetic grain rotation two simple models of the pdrm lock in zone are described using yaskawa s idea of effective sedimentary viscosity and mooney s empirical relation for the viscosity of a concentrated suspension of spheres the impulse response of the lock in zone determines the fidelity of the paleomagnetic recording

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

planar deformation features in quartz from impact produced polymict breccia of the xiuyan crater china the 1 8 km diameter xiuyan crater is an impact structure in northeastern china exposed in a proterozoic metamorphic rock complex the major rocks of the crater are composed of granulite hornblendite gneiss tremolite marble and marble the bottom at the center of the crater covers about 100 m thick lacustrine sediments underlain by 188 m thick crater fill breccia a layer of polymict breccia composed of clasts of granulite gneiss hornblendite and fragments of glass as well as clastic matrix occurs near the base in the depth interval from 260 to 295 m an investigation in quartz from the polymict breccia in the crater fill units reveals abundant planar deformation features pdfs quartz with multiple sets of pdfs is found in clasts of granulite that consist of mainly quartz and feldspar and in fine grained matrix of the impact produced polymict breccia a universal stage was used to measure the orientation of pdfs in 70 grains of quartz from five thin sections made from the clasts of granulite of polymict breccia recovered at the depth of 290 m forty four percent of the quartz grains contain three sets of pdfs and another 40 contain two sets of pdfs the most abundant pdfs are rhombohedron forms of pf10 12g xf10 13g a ndr zf10 11g with frequency of 33 5 22 3 and 9 6 respectively a predominant pdf form of f10 12g in quartz suggests a shock pressure 20 gpa the occurrence of pdfs in quartz from the polymict breccia provides crucial evidence for shock metamorphism of target rocks and confirms the impact origin of this crater which thus appears to be the first confirmed impact crater in china

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

correlation of late pleistocene glaciolacustrine and marine deposits by means of geomagnetic secular variation with examples from northern new york and southern ontario abstract the geomagnetic secular variation record retained by glaciolacustrine and marine sediments at nine sites in northern new york and southern ontario provides a means for stratigraphic correlation of glacial deposits for the time period between about 12 600 to 9900 14c yr b p measurement of the depositional remanent magnetism of sediments deposited in glacial lake iroquois and the champlain sea has produced a geomagnetic secular variation curve that represents the time period immediately following deglaciation about 12 600 14 c yr b p the curve varies from about 358 to 344 declination and 51 to 61 inclination over approximately 180 valve years marine sediments of the champlain sea have preserved a record approximately 1500 yr long that varies from about 2 to 29 declination and 47 to 60 inclination these combined glacial paleomagnetic records may also correlate with those from glacial sequences beyond our study area the shape and amplitude of the secular variation record in glaciolacustrine and marine sediments from the western adirondack borderland show agreement with other glacial varve secular variation records and suggest possible correlations with secular variation curves from lake cores

shock metamorphic petrography and microraman spectroscopy of quartz in upper impactite interval icdp drill core lb 07a bosumtwi impact crater ghana standard and universal stage optical microscope and microraman spectroscopic examination of quartz from the upper impactite interval of the international continental scientific drilling program icdp lake bosumtwi crater drill core lb 07a demonstrates widespread but heterogeneous evidence of shock metamorphism in the upper impactite which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333 4 415 7 m quartz occurs as a major component within metasedimentary lithic clasts and as abundant isolated single crystal grains within matrix the noted quartz shock metamorphic features include phenomena related to a deformation such as abundant planar microstructures grain mosaicism and reduced birefringence b phase transformations such as rare diaplectic quartz glass and very rare coesite c melting such as isolated colorless to dark glassy and devitrified vesicular melt grains and d secondary post shock features such as abundant variable decoration of planar microstructures and patchy grain toasting common to abundant planar deformation features pdfs in quartz are dominated by 1013 equivalent crystallographic planes although significant percentages of 1012 and other higher index orientations also occur notably c 0001 planes are rare significantly the quartz pdf orientations match most closely those reported elsewhere from strongly shocked crystalline target impactites barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 gpa were widely reached pressures exceeding 40 45 gpa were more rare the relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz together with the nature and distribution of melt within suevite suggest a water or volatile rich target for the bosumtwi impact event

geochemistry and petrology of witwatersrand and dwyka diamictites from south africa search for an extraterrestrial component abstract diamictites are poorly sorted sediments characteristically carrying coarse grained clasts in a fine grained matrix they have generally been considered of glaciogenic or glaciomarine origin recently however it has been suggested that some massive tillite diamictite layers could represent impact breccias an earlier petrographic study of rock and mineral clasts from dwyka group diamictites revealed no evidence for shock metamorphism such as planar deformation features detailed geochemical studies of diamictite samples from the archean witwatersrand supergroup and the dwyka group of the mesozoic karoo supergroup from south africa are reported we studied the contents of the siderophile elements in these breccias as elevated abundances of such elements especially iridium could be indicative for an impact origin by use of coincidence spectrometry and other trace element analysis geochemical tracers of extraterrestrial components were sought however no enrichments of indicator elements for extraterrestrial components compared with ordinary continental crust were found thus neither geochemical nor petrographic evidence supports an impact origin of the diamictites from the dwyka group and the witwatersrand supergroup in south africa

k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia abstract woodleigh is a recently discovered impact structure with a diameter of 120 km and thereby represents the third largest proven phanerozoic impact structure known after morocweng and chicxulub k ar isotopic studies of fine grained authigenic illitic clay minerals

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

anatomy of black veins in zagami clues to the formation of high pressure phases abstract black melt veins in the martian basaltic meteorite zagami were investigated using analytical transmission electron microscopy with the goal to constrain the mechanisms and ptt conditions that result in the formation of high pressure phases therein our study reveals the existence of a number of high pressure minerals stishovite k and na ca rich hollandites akimotoite silicate ilmenite as well as amorphous grains of silicate perovskite composition crystals of a ferro magnesian silicate titanite with a low ca content have been identified for the first time the veins formed by rapid shear melting and solidified in extremely short times as a consequence the vein melt could not homogeneously mix and it partially preserves the chemical heterogeneity of precursor minerals accounting for the large diversity of high pressure minerals most of the tiny phases directly crystallized from the melt under high pressure at very high temperatures silicate perovskite probably amorphized upon decompression and evolved at its grain boundaries by solid state diffusion toward the lower pressure assemblage wustite stishovite not only do these observations help to understand the formation of high pressure phases in shock events but they also provide clues to the mineralogical complexity that can be expected in planetary mantles

roter kamm impact crater namibia geochemistry of basement rocks and breccias abstract the roter kamm crater in the southern namib desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts to better define the variety of target rocks and breccias we studied the petrography and chemical composition of a new suite of twenty eight basement and breccia samples from the roter kamm crater based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two layer target region an upper layer of gariep metasediments schist marble quartzite and sandstone overlying the crystalline basement of the namaqualand metamorphic complex the basement was also heavily intruded by coarse grained quartz veins and quartz and quartz feldspar pegmatites the clast population in the melt breccias indicates that impact induced melting involved mainly metasedimentary target rocks with rarely detected contributions from pegmatite and granite granodiorite three varieties of melt breccias can be defined 1 schistose 2 quartzitic melt breccias 3 true impact melt breccias these melt breccia types are chemically heterogeneous and even the impact melt breccias may have been produced in situ and not from a coherent melt body the shapes of the schistose melt breccias previously thought to be ejected impact breccias are most likely caused by erosion and these breccias are now interpreted to be locally derived the crater basement as exposed at the rim was structurally severely affected and at least locally considerable thermal energy was generated during formation of large volumes of cataclastic mylonitic and pseudotachylitic breccias analyses of mylonite and pseudotachylites from the crater rim as well as their respective host rocks show that these breccias were mainly formed from local material analyses of pseudotachylite like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar similar to their tectonically produced counterparts although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated several breccias compared to their host rocks do show evidence of severe chemical alteration chloritisation and sericitisation the presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact induced hydrothermal event in the crater area

platinum group elements in the morokweng impact structure south africa evidence for the impact of a large ordinary chondrite projectile at the jurassic cretaceous boundary radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in earth history e g the cretaceous tertiary k t and jurassic cretaceous j k boundaries the morokweng impact crater in south africa is coincident in age with the j k boundary however the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks meteorites are highly enriched in the platinum group elements pge which have been widely used as a tool for identifying the presence of a meteorite signature here we present new pge analyses from the morokweng impact melt sheet our data reveal high pge concentrations and high degree of pge correlation through the melt sheet regression analysis was used to determine the projectile pge signature and constrain input from the terrestrial target rocks the closest match to morokweng is the pge signature of ordinary l or ll chondrite meteorites which is broadly in agreement with the results of an earlier cr isotope study the results of these independent studies provide strong evidence that a large ordinary chondrite projectile struck the area of morokweng in the late jurassic

automatic geomorphic feature extraction from lidar in flat and engineered landscapes 1 high resolution topographic data derived from light detection and ranging lidar technology enables detailed geomorphic observations to be made on spatially extensive areas in a way that was previously not possible availability of this data provides new opportunities to study the spatial organization of landscapes and channel network features increase the accuracy of environmental transport models and inform decisions for targeting conservation practices however with the opportunity of increased resolution topographic data come formidable challenges in terms of automatic geomorphic feature extraction analysis and interpretation low relief landscapes are particularly challenging because topographic gradients are low and in many places both the landscape and the channel network have been heavily modified by humans this is especially true for agricultural landscapes which dominate the midwestern united states the goal of this work is to address several issues related to feature extraction in flat lands by using geonet a recently developed method based on nonlinear multiscale filtering and geodesic optimization for automatic extraction of geomorphic features channel heads and channel networks from high resolution topographic data here we test the ability of geonet to extract channel networks in flat and human impacted landscapes using 3 m lidar data for the le sueur river basin a 2880 km 2 subbasin of the minnesota river basin we propose a curvature analysis to differentiate between channels and manmade structures that are not part of the river network such as roads and bridges we document that laplacian curvature more effectively distinguishes channels in flat human impacted landscapes compared with geometric curvature in addition we develop a method for performing automated channel morphometric analysis including extraction of cross sections detection of bank locations and identification of geomorphic bankfull water surface elevation using the slope plotted along each channel floodplain cross section we demonstrate the ability to identify and measure the height of river banks and bluffs finally we present an example that demonstrates how extracting such features automatically is important for modeling channel evolution water and sediment transport and channel floodplain sediment exchange

shock metamorphic features and their zoning and orientation in the precambrian rocks of the manicouagan structure quebec canada abstract the manicouagan structure of impact origin is located in eastern quebec about 300 km north of the st lawrence river the complex is of triassic age the shock deformation of the more important rock forming minerals of the autochthonous or at least parautochthonous rocks within the structure is described four zones of shock metamorphism are delineated based on the deformation features of plagioclase 1 plagioclase without planar features 2 plagioclase with planar features 3 first occurrence of plagioclase with partial isotropization and 4 first occurrence of plagioclase with total isotropization these four zones are more or less concentric and indicate a general increase in the grade of shock metamorphism towards the centre of the structure however several small areas probably underlain by allochthonous rocks appear not to fit this general picture biotite and garnet show evidence of a shock orientation of their deformation features an impact origin for the structure is favoured the impact possibly triggering volcanism

experimental approach to generate shock veins in single crystal olivine by shear melting a shock experiment has been devised to produce large shear in a single crystal sample of olivine the recovered sample exhibits macroscopic shear faults resembling shock veins in ordinary chondrites examination with transmission electron microscopy reveals a high density of dislocations in the bulk olivine the shear faults appear as thin veins containing small grains of olivine and pockets of glass the microstructure and composition of the material in the veins point to fractional crystallization of a melt an order of magnitude calculation is consistent with the idea that the veins were produced by shear melting these results support the view that shock veins in meteorites are the result of shear heating rather than of pressure heterogeneities

the acquisition of post depositional detrital remanent magnetization in a variety of natural sediments summary post depositional detrital remanent magnetization pdrm is the primary means whereby many sediments acquire their palaeomagnetic signal we have studied the acquisition of this magnetization in a variety of natural sediments our technique involves determining the magnetic direction recorded by a sediment as a function of the water content present in the sediment when the sediment experiences a change in the direction of the applied magnetic field most of the sediments used in this study were collected wet from natural environments and were preserved in their original state until they were used in the experiments grain sizes were measured by the settling tube method which led to the determination of the clay silt and sand fractions in each sediment isothermal remanent magnetization acquisition studies indicated that the predominant magnetic carriers were magnetite in the pdrm acquisition studies two distinct modes of behaviour were found for sediments with a sand content less than 60 per cent the original direction of magnetization was preserved regardless of the water content such behaviour is not consistent with a theoretical model which assumes that at high water contents the magnetic carriers remain mobile within fluid filled voids and hence are able to realign along a new magnetic field direction for sediments with a sand content in excess of 60 remagnetization along a new magnetic field direction occurred as expected provided the sediments were sufficiently wet studies of natural sediments and corresponding samples of dried and reconstituted sediments have demonstrated that the magnetic characterization of a sediment can be reliably determined even for older desiccated sediments

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

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

precambrian tectonic history of the west australian craton and adjacent orogens this review is a synthesis of the precambrian geology of the west aus tralian craton and its fringing orogens it describes the evidence that has recently been reinvestigated and reinterpreted to reveal a long dynamic history of repeated aggregation and dispersal of crustal fragments cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time they are generally tabular bodies with great areal extent millions of square kilo meters and lithospheric thickness 200 400 km orogens are narrow steep zones of tectonic and metamorphic activity associated with the deposition of sediments and the intrusion of plutonic rocks they occur within or along the margins of continents and are generally a few hundred kilometers wide thousands of kilometers long and extend to the base of the litho sphere most orogens are initially associated with compressive tectonics crustal thickening and the transient formation of mountain belts cratons and orogens may survive for very long periods of time but as part of a thin crust on a dynamic mantle they are inherently mobile and travel as rafts for great distances around the surface of the earth they may be torn apart dispersed and reassembled with other continental fragments the boundaries of these crustal fragments once established remain zones of crustal weakness that may be reactivated as intra continental mobile belts or lines along which cratons are assembled and dismembered cf hoffman 1 988

late quaternary secular variation of geomagnetic declination in western new york a secular variation record of the geomagnetic declination has been obtained from glaciolacustrine varved clays and postglacial sediments sampled both from outcrop and from piston cores from lake ontario it appears to be uninterrupted and covers the interval 14 000 years b p 14c to the present the record for the interval 14 000 12 300 years b p consists of true declination and was obtained from samples collected from outcrop the record for the interval 12 500 years b p to the present consists of relative declination and was obtained from piston cores the secular variation record is internally consistent and is compatible with the chronology of events known to have occurred during and following the deglaciation of western new york use of the record for magnetic correlation within the study area is demonstrated

late quaternary paleomagnetic record of the goderich basin lake huron paleomagnetic analysis of two cores taken of the late quaternary sequence of the goderich basin lake huron resulted in high resolution declination inclination intensity of magnetization j magnetic susceptibility k and q ratio j k curves there was a consistent relative relationship of the features along the curves of all five parameters for both cores studied which confirms the reproducibility of the method and suggests that paleomagnetic curves can be used for time parallel correlation of the late quaternary sequence of the goderich basin

power law scaling of topographic depressions and their hydrologic connectivity topographic depressions areas of no lateral surface flow are ubiquitous characteristics of the land surface that control many ecosystem and biogeochemical processes high density of depressions increases the surface storage capacity whereas lower depression density increases runoff thus influencing soil moisture states hydrologic connectivity and the climate soil vegetation interactions with the widespread availability of high resolution lidar based digital elevation model ldem data it is now possible to identify and characterize the structure of the spatial distribution of topographic depressions for incorporation in ecohydrologic and biogeochemical studies here we use ldem data to document the prevalence and patterns of topographic depressions across five different landscapes in the united states and quantitatively characterize the probability distribution of attributes such as surface area storage volume and the distance to the nearest neighbor through the use of a depression identification algorithm we show that these probability distributions of attributes follow scaling laws indicative of a structure in which a large fraction of land surface areas can consist of high number of topographic depressions of all sizes and can account for 4 to 21 mm of depression storage this implies that the impacts of small scale topographic depressions in the landscapes on the redistribution of material fluxes evaporation and hydrologic connectivity are quite significant

platinum group element geochemistry of continental tholeiites analysis of the long range dyke swarm newfoundland canada abstract the rift related late proterozoic long range continental dykes of western newfoundland have platinum group element pge pd pt rh ru and ir determined concentrations that correlate with indicators of magma evolution e g mg mg mg 0 9 fe tot and magma alkalinity la sm suggesting that metamorphism had a minimal effect on the pge the low melting point lmp pge pd pt and rh and cu behaved incompatibly as magma evolved indicating that sulphide fractionation did not occur and that the dykes hold little economic promise hypotheses that augite a major fractionating phase has an affinity for the pge are not supported primitive rocks tend to have lower ir than evolved rocks and all samples display a negative high melting point hmp pge ru and ir anomaly on mantle normalized diagrams olivine and early chromite fractionation may have decreased the concentrations of these elements the long range dykes confirm that pd ir ratios and absolute lmp pge concentrations tend to decrease with increasing hot spot magma alkalinity the lower pge concentrations in most morb compared with hot spot basalts imply that there are at least two mantle reservoirs for the pge lithosphere overlying mantle sources continental or oceanic has little effect on pge concentrations the apparently similar source region pge concentrations for hot spot basalts suggest that differences in pge concentrations between morb and hot spot basalts reflect incomplete segregation of the pge into the core during its formation and rise of the homogeneously dispersed elements to upper mantle depths in deep mantle plumes

geophysical anomalies and quartz microstructures eastern warburton basin north east south australia tectonic or impact shock metamorphic origin abstract the eastern warburton basin northeast south australia features major geophysical anomalies including a magnetic high of near 200 nt centred on a 25 km wide magnetic low 30 000 km large eastern warburton basin the qz pe include multiple intersecting planar to curved sub planar elements with relic lamellae less than 2 m wide with spacing of 4 5 m qz pe are commonly re deformed displaying bent and wavy patterns accompanied with fluid inclusions u stage measurements of a total of 243 planar sets in 157 quartz grains indicate dominance of 10 12 10 13 and subsidiary 11 22 22 41 m 10 11 and x 51 61 planes transmission electron microscopy tem analysis displays relic narrow 1 m wide lamellae and relic non sub grain boundaries where crystal segments maintain optical continuity extensive sericite alteration of feldspar suggests hydrothermal alteration to a depth of 500 m below the unconformity which overlies the qz pe bearing warburton basin terrain the data are discussed in terms of a tectonic metamorphic deformation and b impact shock metamorphism producing planar deformation features qz pdf deformed qz pe are compared to re deformed qz pdfs in the sudbury vredefort manicouagan and charlevoix impact structures a 4 5 km uplift of the big lake granite suite during 298 295 ma is consistent with missing of upper ordovician to devonian strata and possible impact rebound the occurrence of circular seismic tomography anomalies below the east warburton basin the poolowana basin and the woodleigh impact structure signifies a potential diagnostic nature of circular tomographic anomalies

archaeo and paleosecular variation and long term asymmetries of the geomagnetic field this report summarizes progress made during the last four years in i obtaining records of the geomagnetic secular variation using paleomagnetic techniques ii analysis and modelling of the secular variation and iii the study of asymmetries in the time averaged field seen both as departures from the geocentric axial dipole model and as statistical differences between polarity states

experimental generation of shock induced pseudotachylites along lithological interfaces to understand the mechanism of formation of shock induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation shock recovery experiments were carried out on samples consisting of two distinct lithologies dunite and quartzite it was possible to generate melt veins of 1 6 m width along lithological interfaces at moderate shock pressures 6 to 34 gpa the magnitudes of displacement along the interface strain rate and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins the experimentally produced veins resemble natural s type pseudotachylites the geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness formation of melt pockets and injection veins sudden changes in vein orientation and sharp vein margins two types of melt were observed vesicle free and vesicular melts dense vesicle free melt rock is likely to represent high pressure melts vesicular melts were also generated during shock compression but they remained in a molten state during pressure release and continued shearing intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine the composition of the frictional melts ranges from olivine normative to enstatite normative and is thus largely determined by olivine melting the validity of the sequence of friction melting susceptibilities of rock forming minerals inferred from tectonically produced pseudotachylites is confirmed and can now be applied to ultra high strain rates during shock compression

suevite breccia of the ries impact crater germany petrography chemistry and shock metamorphism of crystalline rock clasts clasts of deep seated crystalline basement rocks in suevites of the ries crater germany were catalogued lithologically and classified with regard to their degree of shock metamorphism the sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior these clasts can be grouped into seven types of metamorphic and nine types of igneous rocks one hundred forty three clasts representing these lithologies were analyzed for major element bulk composition the fallout suevite contains on average 4 vol of crystalline basement clasts 0 4 vol of sedimentary rocks 16 vol of glass bodies some of them aerodynamically shaped and 79 vol of groundmass on average 52 of all crystalline clasts are from metamorphic sources and 42 are of igneous origin using the shock classification of stoffler 1974 8 of all crystalline clasts appear unshocked 10 gpa and 34 30 and 27 of clasts are shocked to stages i 10 35 gpa ii 35 45 gpa and iii 45 60 gpa respectively the bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations this indicates that the glasses originate by shock fusion of a similarly composed basement the crater suevite contains the same crystalline rock types that occur in the fallout suevites the bore hole nordlingen 1973 yields an average of 62 vol metamorphic and 38 vol igneous rocks the crater suevite differs from fallout suevites by a higher clast glass ratio by preponderance 65 95 of clasts shocked to stage i only and by the absence of aerodynamically shaped glass bodies the source of crystalline clasts and melt particles of suevites is a volume of rocks located deep in the crystalline basement to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 gpa deep burst impact model fallout suevites were ejected propelled by an expanding plume of vaporized rock and withdrew preferentially from this volume melt and highly shocked clasts leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt

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

use of small amplitude paleomagnetic fluctuations for correlation and dating of continental climatic changes abstract recent advances in field and laboratory methods have made possible the study of paleomagnetic fluctuations recorded with high resolution in lake sediments of relatively small amplitude and short period 102 years to 104 years foremost among these methods are sediment coring techniques that allow azimuthally oriented long cores 25 m to be raised with small orientation errors 3 and the use of cryogenic magnetometers to measure weakly magnetized m 10 9 a m2 sediment samples accurately with these methods records of inclination declination and relative paleointensity nrm arm have been obtained in north america that span the last 20 000 years substantial agreement in declination and inclination features is seen on a scale of 103 km in north america for the last 10 000 years these paleomagnetic features can be useful in determining the relative age of paleoclimatic features particularly in those situations where 14c dating is unreliable similar agreement is seen for the last 4000 years between relative paleointensity estimates obtained from pennsylvania lake sediments and absolute paleointensity estimates obtained from 14c dated lava flows and potsherds from the western united states similar paleointensity fluctuations to those occurring from 0 to 4000 yr b p in the western united states are observed for the last 12 500 yr b p in lake sediments from new york and pennsylvania such relative paleointensity records from lake sediments if independently confirmed considerably extend the range and resolution of paleointensity data and have important implications for the study of secular variation models variations in atmospheric radiocarbon concentration and the proposed relationships between geomagnetism and climate an absolute secular variation chronology for the varved sediments of elk lake minnesota can be used to date magnetically the secular variation records of unvarved sediments elsewhere in north america

constraints on titan s topography through fractal analysis of shorelines titan s north polar hydrocarbon lakes offer a unique opportunity to indirectly characterize the statistical properties of titan s landscape the complexity of a shoreline can be related to the complexity of the landscape it is embedded in through fractal theory we mapped the shorelines of 290 of the north polar titanian lakes in the cassini synthetic aperture radar dataset out of these we used a subset of 190 lake shorelines for our analysis the fractal dimensions of the shorelines were calculated via two methods the divider ruler method and the box counting method at length scales of 1 10 km and found to average 1 27 and 1 32 respectively the inferred power spectral exponent of titan s topography from theoretical and empirical relations is found to be 2 which is lower than the values obtained from the global topography of the earth or venus some of the shorelines exhibit multi fractal behavior different fractal dimensions at different scales which we interpret to signify a transition from one set of dominant surface processes to another we did not observe any spatial variation in the fractal dimension with latitude however we do report significant spatial variation of the fractal dimension with longitude a systematic difference between the dimensions of orthogonal sections of lake shorelines is also noted which signifies possible anisotropy in titan s topography the topographic information thus gleaned can be used to constrain landscape evolution modeling to infer the dominant surface processes that sculpt the landscape of titan

construction of type curves of geomagnetic secular variation for dating lake sediments from east central north america type curves of palaeomagnetic secular variations of declination and inclination through the last 12 000 radiocarbon years have been constructed by combining data from the minnesota lakes with paral

dynamic puddle delineation and modeling of puddle to puddle filling spilling merging splitting overland flow processes 1 surface microtopography affects overland flow infiltration soil erosion pollutant transport and other fundamental hydrologic and environmental processes across scales under the influence of surface depressions overland flow essentially features a series of progressive puddle to puddle p2p filling spilling merging and splitting processes the objectives of this study are to characterize puddles and their hierarchical relationships and model the microtopography controlled p2p processes we proposed a new modeling framework for simulating the p2p overland flow dynamics through cell to cell c2c and p2p routing for a set of puddle based units pbus in a well delineated cascaded p2p drainage system testing of the p2p model demonstrated its potential to improve overland flow modeling and hydrologic connectivity analysis by explicitly incorporating the hydrologic roles of depressions and quantifying the real microtopography controlled p2p dynamics

pge distribution in 2 7 ga layered komatiite flows from the belingwe greenstone belt zimbabwe the well preserved anderson s and onias s komatiite flows from the late archaean belingwe greenstone belt 2 7 ga zimbabwe were sampled in the saskmar drill hole anderson s flow possesses a 1 1 m thick b1 subzone and a clinopyroxene spinifex subzone whereas onias s flow has spinifex and cumulate layers typical of komatiite flows platinum group element pge abundances throughout these two flows provide constraints on the formation of layered komatiite flows all samples from both flows have similar chondrite normalized pge patterns that are enriched in rh pt and pd relative to os and ir which are attributed to large degrees 30 of partial melting of the upper mantle their pd ir ratios are highly variable 2 0 31 in onias s flow and 2 3 61 in anderson s flow pge variations within both flows except the spinifex a zone and b1 subzone of anderson s flow together with those of major and other trace elements are consistent with a model involving olivine fractionation the spinifex a zone of anderson s flow has low pd ir ratios 6 8 which occur in an interval having negative eu anomalies caused by hydrothermal alteration the pt fractionation from pd and rh is documented from their relationship to mgo contents behaviours of pd and rh are similar and more incompatible than pt at the early stage of relatively slow cooling but less incompatible at the late stage during the formation of komatiite flows pge in the b1 subzone of anderson s flow do not follow an olivine fractionation trend pd ir ratios in this subzone increase from 28 at the top to 61 at the base these unusually high values can be accounted for by late stage development of the subzone and strong fractionation

exogenic and endogenic breccias a discussion of major problematics abstract pseudotachylitic breccias impact melt rock ultra cataclasites and even ultramylonites may macroscopically be of very similar appearance even detailed optical microscopic work may not suffice to identify the actual type of melt rock studied the difficulties to distinguish different tectonically produced breccias fault rocks and impact breccias for which no uniformly accepted nomenclature exists are examined and the various formational processes friction melting or shock melting at the macro and micro scales are discussed special emphasis is placed on the occurrence of different and multiple breccia types within impact structures where not only impact induced friction melt and impact melt may occur but where tectonically produced fault rocks that are not related to the impact event may further complicate the breccia situation melting on shatter cone surfaces and its likely formation the proposed origin as distal impact breccia of certain diamictites and problems with carbonate breccias and caused by metamorphic overprint on breccias are discussed it is cautioned not too easily to reach generalized conclusions for example with regard to the usage of the a and b type pseudotachylite terminology areas in need of further breccia studies if possible in close collaboration between impact and tectonic researchers are emphasized