mapping bedrock beneath glacial till using cdp seismic reflection methods this paper is a case history demonstrating the applicability of the common depth point cdp seismic reflection method to image bedrock beneath glacial till in northwestern iowa reflections from the base of the 40 m thick glacial till are clearly observable on field files at around 45 to 50 ms two way traveltime and possess a dominant frequency of around 100 hz the bedrock reflection is confirmed by drill data the seismic data are of sufficient quality to detect local bedrock topographic changes and to interpret discontinuities along the till bedrock interface finite difference synthetic seismograms substantiate the interpreted reflections and the diffraction signatures from faults observed on the field files at some locations along the seismic line intra till reflections are apparent on the field files these intra till features are on the order of tens of meters in length along the line traverse and reflections from them are not enhanced by common depth point processing intra till reflections could be indicative of gravels or other alluvial materials that may serve as local aquifers

anatomy of the chesapeake bay impact structure revealed by seismic imaging delmarva peninsula virginia usa 1 a 30 km long radial seismic reflection and refraction survey completed across the northern part of the late eocene chesapeake bay impact structure cbis on the delmarva peninsula virginia usa confirms that the cbis is a complex central peak crater we used a tomographic p wave velocity model and low fold reflection images constrained by data from two deep boreholes located on the profile to interpret the structure and composition of the upper 5 km of crust the seismic images exhibit well defined structural features including with increasing radial distance a collapsed central uplift a breccia filled moat and a collapsed transient crater margin which collectively constitute a 40 km wide collapsed transient crater and a shallowly deformed annular trough these seismic images are the first to resolve the deep structure of the crater 1 km and the boundaries between the central uplift moat and annular trough several distinct seismic signatures distinguish breccia units from each other and from more coherent crystalline rocks below the central uplift moat and annular trough within the moat breccia extends to a minimum depth of 1 5 km or a maximum of 3 5 km depending upon the interpretation of the deepest layered materials the images show 350 to 500 m of postimpact sediments above the impactites the imaged structure of the cbis indicates a complex sequence of events during the cratering process that will provide new constraints for numerical modeling

shear wave splitting and anisotropy from the aftershocks of the nahanni northwest territories earthquakes two large earthquakes ms 6 6 6 9 occurred in 1985 near the north nahanni river in the northwest territories canada the first on october 5 and the second on december 23 the earthquakes were followed by long aftershock sequences aftershocks between m 1 0 and 3 9 were recorded by five to eight three component stations during three field experiments in october 1985 and in january and september 1986 two distinct shear waves were generally observed in september 1986 the plane of polarization of the first arriving shear wave was well defined and averaged 156 8 measured clockwise from north the stress direction was inferred from the initial shear wave polarizations recorded at one station from a widespread aftershock distribution but was limited to no more than 10 km around it the delay between first and second arriving shear waves was up to 190 ms assuming that the two shear waves result from anisotropy and that the entire ray paths varying in length from 7 to 23 km contributed the maximum velocity anisotropy is 7 however the distribution of the anisotropy is not uniform the pressure axes determined from fault plane solutions for the two main shocks are nearly e w normal to the compressive direction found in the september 1986 field survey from shear wave splitting data four out of seven of the aftershocks for which focal mechanism orientations could be determined showed pressure axis orientations that agreed with the stress direction inferred from the initial shear wave polarizations from the field survey

proximal cretaceous tertiary boundary impact deposits in the caribbean trace element isotopic and mineralogic studies indicate that the proposed impact at the cretaceous tertiary k t boundary occurred in an ocean basin although a minor component of continental material is required the size and abundance of shocked minerals and the restricted geographic occurrence of the ejecta layer and impact wave deposits suggest an impact between the americas coarse boundary sediments at sites 151 and 153 in the colombian basin and 5 to 450 meter thick boundary sediments in cuba may be deposits of a giant wave produced by a nearby oceanic impact on the southern peninsula of haiti a approximately 50 centimeter thick ejecta layer occurs at the k t boundary this ejecta layer is approximately 25 times as thick as that at any known k t site and suggests an impact site within approximately 1000 kilometers seismic reflection profiles suggest that a buried approximately 300 km diameter candidate structure occurs in the colombian basin

azimuthal anomalies of short period p wave arrivals from nahanni aftershocks northwest territories canada and effects of surface topography abstract following two large earthquakes in the northwest territories of canada in 1985 and one in 1988 four field surveys were undertaken to study the extensive aftershock activity some of the seismographs employed recorded three component digital data which allowed the directions of approach of short period p wave arrivals to be analyzed at three stations observed azimuthal deviations reached as much as 40 from theoretically expected azimuths as computed from the inferred hypocenters for the events theoretical results are presented that support the hypothesis that the observed azimuthal deviations are caused principally by local topographic slopes in the vicinity of the recording instruments similar theoretical results indicate that effects of local surface topography on arrival azimuths of sh and sv waves are nearly an order of magnitude less than for p waves so that such effects will generally be unimportant in shear wave splitting studies

distinct phases of relative sea level changes in the central adriatic during the last 1500 years influence of climatic variations abstract we present a new sea level reconstruction for the past 1500 years based on biological indicators from the central adriatic islands biogenic littoral rims built by the coralline rhodophyte lithophyllum byssoides were found on the particularly exposed sites on the rocky coasts of the islands of vis ravnik and bisevo in croatia the presence of thick and well developed lithophyllum rims considered to be precise 10 cm sea level indicators points directly to the rising sea level environment biogenic rims were mapped measured and sampled for 14c dating the obtained results point to four phases of sea level changes the sea level was near stable from around 550 till 770 cal ad in the dark ages cold period dacp then during the medieval climate anomaly mca 770 till 1330 cal ad the sea level increased at a rate of 0 71 mm yr during the little ice age lia 1330 till 1640 cal ad it was near stable again later the sea level started to rise at a much higher rate particularly during the current warm period cwp these data were compared with local predictions derived from a glacio hydro isostatic models associated with the last glacial cycle if the isostatic eustatic component is separated this area seems to have almost stable tectonic conditions during the past 1500 years our results show that the large algal rims most likely grew during near stable sea level conditions that occurred during two relatively colder periods in the past 1500 years they also reveal that well developed up to 1 8 m wide upper levels of algal rims were formed during 300 years of stabilisation throughout the lia

guano derived δ 13 c based paleo hydroclimate record from gaura cu musca cave sw romania the 13c values of 23 unevenly spaced guano samples from a 17 cm long clay sediment profile in gaura cu musc cave gm in sw romania made it possible to preliminarily characterize the medieval warm period summer hydroclimate regime the beginning of the sequence ad 990 was rather wet for more than a century before becoming progressively drier after a brief yet distinct wet period around ad 1170 drier conditions with a possible shift from c3 to a mixed c3 dominated c4 type vegetation 2 lower 13c values prevailed for almost half a century before the climate became colder and wetter at the onset of the little ice age when bats left the cave the guano inferred wet and dry intervals from the gm cave are mirrored by changes in the color and amount of clay accumulated in the cave they also agree well with reconstructions based on pollen and charcoal from peat bogs and 13c and 18o on speleothems from other romanian sites overall these results indicate that the 13c of bat guano can provide a sensitive record of the short term coupling between local regional climate and the plant insect bat guano system

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

seismic reflection analysis of the manson impact structure iowa our combined interpretation of new high resolution seismic reflection data and reprocessed but previously published industrial vibroseis data indicates that the manson impact structure iowa has an apparent crater diameter of 35 kin an annular trough diameter of around 21 km a shallow floor 0 6 0 7 kin and a central uplift that has a minimum diameter of 7 5 kin the two reflection lines are coincidentally located along an east west radial transect and are consmined by shallow drill information results from the two data sets are correlative both data sets were instnunental to the final interpretation due to the trade offbetween resolution and depth of energy penetration based on the combined interpretation structural uplift of the central peak is estimated to be around 2 8 kin onlapping seismic sequences are present at the eastern edge of the central uplift these seismic packages observed only in the high resolution line are interpreted to represent impact breccia or debris material that was shed from the central peak or dynamically transported from outside of the crater

a 2500 yr late holocene multi proxy record of vegetation and hydrologic changes from a cave guano clay sequence in sw romania we provide sedimentological geochemical mineral magnetic stable carbon isotope charcoal and pollen based evidence from a guano clay sequence in gaura cu musc cave sw romania from which we deduced that from 1230 bc to ad 1240 climate oscillated between wet and dry from 1230 bc to ad 1000 the climate was wetter than the present prompting flooding of the cave preventing bats from roosting and resulting in a slow rate of clay accumulation the second half of the medieval warm period mwp was generally drier the cave experienced occasional flash flooding in between which maternity bat roosts established in the cave one extremely wet event occurred around ad 1170 when fe mn and ti zr ratios show the highest values coincident with a substantial increase of sediment load in the underground stream the mineral magnetic characteristics for the second part of the mwp indicate the partial input of surface sourced sediments reflecting agricultural development and forest clearance in the area pollen and microcharcoal studies confirm that the overall vegetation cover and human land use have not changed much in this region since the medieval times

shear wave splitting and anisotropy in the charlevoix seismic zone quebec during the month of august 1984 an experiment was undertaken in the charlevoix seismic zone to test the hypothesis that shear wave splitting could be observed in a seismically active region seismograms recorded by digital 3 component seismographs located very close to the epicentres of three earthquakes showed shear wave splitting the amount of splitting was estimated to be 1 6 to 2 of the shear wave velocity from particle motion diagrams the average azimuth of the direction of polarization was 30 these observations can be explained by the presence of vertical cracks striking 30 east of north with a minimum crack density of e 0 03

a radiocarbon dated bat guano deposit from n w romania implications for the timing of the little ice age and medieval climate anomaly abstract there is considerable interest in the potential of bat guano as an alternative record of palaeoclimate in regions that are devoid of more commonly utilised archives in this study designed originally to evaluate the potential of cave hosted bat guano to preserve temporal variations in the flux of cosmogenic 36 cl it was found that the guano depositional history is strongly linked to climatic conditions radiocarbon measurements on a 2 7 metre long core of bat guano from m gurici cave n w romania indicate a maximum depositional age of 1195 ad for the base of the core deposition of the lowermost portion of the accumulation occurred during the medieval climate anomaly the cave roost was subsequently devoid of bats during a regional cold phase linked to the little ice age with bats returning when local temperatures increased the rate of guano accumulation then appears to increase in tandem with anthropogenic warming this indicates that bat occupation at this roost site in m gurici cave is strongly linked to regional climate variability with habitation during warm periods possibly associated with the abundance of insects upon which the bats feed comparison of large peaks in anthropogenic 14 c and 36 cl production associated with nuclear weapons testing indicates downward migration of 36 cl probably reflecting post depositional migration within the guano deposit elevated 36 cl cl at the top of the core in comparison with modern atmospheric values may indicate recycling of bomb 36 cl in vegetation therefore we show that while bat guano contains abundant atmospherically derived chloride it has severe limitations as a potential archive of atmospherically derived 36 cl a solar proxy because of post depositional mobility however separation of organically bound chloride or the use of an alternative cosmogenic isotope 10 be in bat guano may offer an unexploited solar proxy that contains contemporaneous environmental signals such as stable isotopes e g 13 c and pollen in association with radiocarbon dating

persistent positive north atlantic oscillation mode dominated the medieval climate anomaly the medieval climate anomaly mca was the most recent pre industrial era warm interval of european climate yet its driving mechanisms remain uncertain we present here a 947 year long multidecadal north atlantic oscillation nao reconstruction and find a persistent positive nao during the mca supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker nao conditions into the little ice age lia globally distributed proxy data suggest that this nao shift is one aspect of a global mca lia climate transition that probably was coupled to prevailing la nina like conditions amplified by an intensified atlantic meridional overturning circulation during the mca

stable carbon and hydrogen isotopes from bat guano in the grand canyon usa reveal younger dryas and 8 2 ka events we inferred climate change through the pleistocene holocene transition from 13 c and d values of bat guano deposited from 14 5 to 6 5 ka calendar ka in bat cave grand canyon arizona the 13 c and d values generally covaried indicating that regional late pleistocene climate was relatively cool and wet and early holocene climate gradually became warmer with increased summer precipitation until ca 9 ka at which time the onset of modern north american monsoon like conditions occurred during the younger dryas event 13 c values decreased whereas d values increased indicating a cool and possibly drier period we also observed a distinct isotopic anomaly during the 8 2 ka event at which time both 13 c and d values decreased the 13 c values abruptly increased at 8 0 ka suggesting a rapid change in atmospheric circulation and greater infl uence from convective storms originating from the south deposits of bat guano represent a largely untapped source of paleoenvironmental information that can provide continuous and long term continental archives of environmental change

stable isotope composition of cave guano from eastern borneo reveals tropical environments over the past 15 000 cal yr bp abstract insular southeast asia is a key driver for global atmospheric and oceanic circulation is a hotspot for biodiversity and conservation and is likely to have played a unique and important role in early human dispersals despite this partially due to its vast size and remote tropical location very few continuous palaeoenvironmental records exist especially in eastern borneo therefore we investigated 13 c and 15 n values and geochemistry of two cave guano deposits to reconstruct palaeoenvironments in eastern borneo firstly a profile was recovered from gomantong caves sabah reflecting a continuous deposit over 15 cal kyr bp secondly a profile was recovered from bau bau cave east kalimantan that ranged from 15 5 cal kyr bp the geochemical signature of each deposit confirmed the material to be ancient guano 13 c values revealed that a continuous dense rainforest persisted over at least the last 15 cal kyr bp around the gomantong site that was relatively insensitive to regional climate change by contrast 13 c values at bau bau indicate that although rainforest remained dominant in the record a significant drying occurred between 7 7 and 6 3 cal kyr bp with up to 25 grasses present although most regional models suggest that sea level rise and increased holocene insolation led to an increase in monsoonal moisture we find some evidence of more regional variability and that a reduction in monsoonal precipitation could have occurred however we cannot discount the implementation of an anthropogenic fire regime that opened the canopy allowing more grasses to occur

seismicity and faulting in the charlevoix zone of the st lawrence valley abstract continuous monitoring of the seismically active charlevoix zone since late 1977 has delineated the boundaries of the earthquake zone under the north shore and has enabled an association to be made with surface mapped faults that strike along the st lawrence valley and dip to the southeast within the active zone aseismic volumes are found under parts of the st lawrence river and another elongated zone of activity is found to the southeast on unmapped faults dipping under the river the previous suggestions that the activity is related to old rift faults which have been later weakened by a meteor impact would seem to be upheld

a three dimensional p wave velocity model for the charlevoix seismic zone quebec canada 1 a three dimensional p wave velocity model has been developed for the charlevoix seismic zone csz the csz is located along the st lawrence river 100 km northeast of quebec city canada and is one of the most active seismic zones in eastern north america five earthquakes with magnitudes equal to or exceeding 6 0 have occurred in the csz in historic time and around 200 earthquakes occur annually hypocenters are located in precambrian basement rocks basement rocks have been affected by numerous tectonic events including grenvillian collision iapetan rifting and meteor impact we performed a sequential tomographic inversion for p wave velocity structure based upon 3093 p wave arrivals from 489 earthquakes recorded by 12 stations high velocity is associated with the center of the impact crater the region of high velocity is surrounded by low velocities interpreted to be highly disrupted rocks an elongated high velocity region is present at midcrustal depths that trends parallel to the st lawrence river earthquakes avoid the high velocity body and separate into two bands one on either side of the feature larger earthquakes magnitude 4 have occurred along the northern edges of the high velocity region

borehole tilt measurements from charlevoix quebec an array of three borehole tiltmeters near quebec city in eastern canada is designed to study the tidal and secular response of the crust in the charlevoix seismic zone the objectives of this study of the first year of data from two boreholes of the array are to investigate the spatial coherency of the tidal observations and determine whether there are time variations in the tidal amplitudes and phases and to describe the main features of the secular tilt signal the tidal analysis was done using a modified version of the hycon harmonic analysis program with which the time varying tidal amplitudes and phases were determined by the sequential analysis of overlapping 2 monthly subsets of the data the admittance observed for the major semidiurnal m2 and diurnal o1 constituents varies by up to 10 and 30 respectively and is strongly correlated between boreholes comparison with admittance variations determined from two nearby tide gauges indicates a strong correlation in the amplitude fluctuations pointing to a predominantly marine loading source for the time varying tilt admittance differences of up to 20 in amplitude and 5 in phase were found between the mean m2 results determined from boreholes 1 and 2 located only 80 m apart indicating small scale distortion of the local tilt field by lateral inhomogeneities the secular tilt from both boreholes correlates strongly with transient and seasonal water table fluctuations suggesting the dominant influence of pore pressure effects on the nontidal tilt a preliminary estimate of the detectability of long term regional trends in tilt is 0 4 rad yr

evidence for chlorine recycling hydrosphere biosphere atmosphere in a forested wet zone on the canadian shield abstract the ability to measure environmental levels of 36 cl by accelerator mass spectrometry and 3 h by 3 he ingrowth mass spectrometry has made it possible to use the pulses of these two isotopes released into the atmosphere during nuclear weapons testing as tracers of cl and water movement in soils and groundwater the authors have investigated the movement of these tracers below a forested wet zone and have found that both are retarded to a differing extent in the near surface because of vegetative uptake and recycling adsorption by clay particles followed by slow release to the groundwater may also be significant the data accumulated in this region of near vertical recharge have gone a considerable distance towards explaining the anomalously low concentrations of 36 cl measured in the 5 laurentian great lakes as well as indicating possible mechanisms for large scale cl recycling in the atmosphere and biosphere identification of the near term non conservative behaviour of the cl is significant since such a phenomenon could introduce errors in many watershed calculations e g water residence times evaporation rates and mixing calculations

the seismic travel time drop in the charlevoix region from 1979 to 1980 evidence for aligned saturated cracks in the crust a significant drop in seismic travel times of up to 1 0 occurred in the charlevoix region between 1979 and 1980 possibly related to the m 5 0 aug 1979 earthquake in the vicinity a travel time drop of this magnitude could have been produced either by the closing of dry or saturated cracks in the upper crustal material or by the saturation of dry or partly saturated cracks however the anisotropy of travel time changes in this area supports the view that this travel time drop was caused by the closing of water saturated aligned vertical cracks in the crustal material three different crack directions with respect to north were resolved 0 or 90 in the precambrian rocks underneath the st lawrence river 18 or 72 in the shallow rocks 5 km of the charlevoix crater region and 35 or 125 in the paleozoic cover rocks crack closure would require a decrease in the pore volume of the rocks which would be expected to produce subsidence in the charlevoix area since repeated levellings restrict the vertical crustal motion during this time interval to less than 2 cm we conclude that either the effective aspect ratio of cracks is less than 0 0001 or the process of crack closure occurred in a number of unconnected regions more specifically the crack deformation would have to occur in isolated inclusions less than 1 km in diameter and no deeper than about 6 km the process of crack closure may have been triggered by the passage of seismic waves from the m 5 0 earthquake

the distribution of meteoric cl 36 in precipitation across europe in spring 2007 abstract the cosmogenic isotope 36 cl is a valuable tool for understanding many earth system processes but an improved knowledge of its spatial distribution at the earth s surface is critical for several applications meteoric 36 cl fallout reflects complex interactions between atmospheric production transport and deposition processes and predictive models require experimental validation this study investigates for the first time in a systematic way the spatial distribution of meteoric 36 cl on a continental scale within the european landmass using precipitation samples collected during spring 2007 36 cl cl ratios increase with distance inland and the new data exhibit a strong exponential relationship with 18 o values of precipitation r s 0 75 yielding a useful predictive framework for future studies precipitation events in central european regions are characterised by high 36 cl cl ratios with a maximum measured in lyon france of 746 134 10 15 the new data confirm models for the dependence of 36 cl fallout on latitude with the highest mean springtime fallout 53 6 atoms m 2 s 1 occurring in the 40 50 n latitudinal band with sharp decreases in fallout in high latitude regions and more gradual decreases towards the lower latitudes the 36 cl bomb pulse introduced by thermonuclear weapon testing predominantly in the 1950 s has persisted in the environment for c 50 years but the new data indicate that 36 cl fallout has now essentially returned to natural pre bomb values

shear wave splitting and anisotropy in the charlevoix seismic zone quebec in 1985 during the month of october 1985 a second experiment was undertaken in the charlevoix seismic zone to further test the hypothesis that shear wave splitting could be observed in a seismically active region the first experiment had been undertaken in 1984 and yielded only a very limited amount of data seismograms recorded by digital three component seismographs located very close to the epicentres of seven earthquakes showed shear wave splitting over 15 different paths the amount of wave variation varied from about 24 to 160 ms or from 0 4 to 8 7 of the wave velocity the largest value occurred over the shortest path of about 7 km for which essentially the whole path may be anisotropic leading to a crack density e of less than 0 09 for the rest of the data all with less than 3 shear wave velocity variation e varies from 0 005 to 0 03 if whole path anisotropy is assumed these values of e are not significantly different from those obtained in 1984 the average azimuth of the initial shear wave p

the anatomy of a buried complex impact structure the mjolnir structure barents sea analysis of the prominent seismic disturbance at the 40 km diameter mjolnir impact structure based on an extensive seismic reflection database shows that the observed broad brimmed bowl shaped disturbance was formed as a result of the impact of an asteroid or comet during volgian berriasian time 149 141 m seismic mapping exhibits and visualizes a 850 1400 km3 disturbed volume and analysis of several structural features within the disturbance provides insight into major cratering processes such as brecciation and excavation melting gravitational collapse of the transient crater and structural uplift a transient crater of 16 km in diameter and 4 5 km in depth is determined from transient and final crater dimensions we obtain an estimate of the degree of gravitational collapse of the order of 2 5 considerably larger than the average expected values for typical terrestrial craters the extensive collapse took place by low angle decollement surfaces at the periphery and by chaotic debris mass flows toward the center furthermore we estimate the mjolnir projectile to have been 0 9 3 km in diameter and that the physical impact released energy in the range of 2 4 53 1020 j corresponding to an earthquake of magnitude 8 3 the mjolnir impact is not associated with a significant mass extinction however dissipation of the energy released during the mjolnir impact was sufficient to have caused several short term near field perturbations such as large amplitude tsunami waves affecting the barents sea region and possibly adjacent areas in the arctic

solar activity during the last 1000 yr inferred from radionuclide records identification of the causes of past climate change requires detailed knowledge of one of the most important natural factors solar forcing prior to the period of direct solar observations radionuclide abundances in natural archives provide the best known proxies for changes in solar activity here we present two independent reconstructions of changes in solar activity during the last 1000 yr which are inferred from be 10 and c 14 records we analyse the tree ring c 14 data shcal intca104 from 1000 to 1510 ad and annual data from 1511 to 1950 ad and four be 10 records from greenland ice cores camp century grip milcent and dye3 together with two be 10 records from antarctic ice cores dome concordia and south pole in general the be 10 and c 14 records exhibit good agreement that allows us to obtain reliable estimates of past solar magnetic modulation of the radionuclide production rates differences between be 10 records from antarctica and greenland indicate that climatic changes have influenced the deposition of be 10 during some periods of the last 1000 yr the radionuclide based reconstructions of past changes in solar activity do not always agree with the sunspot record which indicates that the coupling between those proxies is not as close as has been sometimes assumed the tree ring c 14 record and be 10 from antarctica indicate that recent solar activity is high but not exceptional with respect to the last 1000 yr c 2006 elsevier ltd all rights reserved less

montagnais a submarine impact structure on the scotian shelf eastern canada the first impact crater in the ocean has been identified on the outer continental shelf off nova scotia canada the crater is well preserved and buried by 510 m of tertiary and quaternary marine sediments the crater is a circular structure at least 45 km wide and 2 7 km deep with a central structural uplift the uplift is at least 1 250 m and possibly 2 900 m in height its upper surface is 11 5 km across and it has a central pit 3 5 km wide in the center an exploratory oil well located near the center of the structure encountered cambro ordovician metamorphic rocks of the basement which are fractured with planar deformation features in quartz representative of pressures in the 8 to 12 gpa range the uplifted basement is covered by 552 m of breccia shock induced features are common they include planar lamellae in quartz and feldspars partial to total isotropization of silicates and the occurrence of mixed tectosilicate glasses some of which exhibit flow textures with total dissociation of the mafic components to ill defined aggregates these features are diagnostic of a 35 to 50 gpa shock level for parts of the breccia the top of the breccia is covered by a 40 5 m thick suevite zone two crystalline melt rock horizons 72 and 34 m thick are enclosed in the breccia even though the feldspars in the melt are labradorite to bytownite in composition the chemical composition of the rock is rhyolitic with 70 77 of sio 2 similar to the composition of the basement there is no obvious enrichment in siderophile elements except an increase in iridium 0 1 0 3 ppb the occurrence of a thick section of impact melt rocks and breccia capping the central uplift is a feature not recognized in other complex impact craters and may be a function of impact into a marine environment dating by 39 ar 40 ar indicates that the impact occurred at about 51 ma late early eocene and this is confirmed by biostratigraphic data from immediately overlying sediments the compositional and structural imprint left on the montagnais structure by the colliding body points to the impactor being most probably the nucleus of a comet

azimuthal variations in p wave travel times and shear wave splitting in the charlevoix seismic zone abstract a significant drop of up to 1 in the seismic p travel times recorded from controlled explosions occurred in the charlevoix seismic zone csz between 1979 and 1980 the travel time data exhibited variations not only with distance but also with azimuth these variations are interpreted in terms of extensive dilatancy anisotropy eda in 1984 and 1985 shear wave splitting was observed in the csz on micro earthquake seismograms under the assumption of eda the observations are interpreted in terms of a dominant crack orientation of about 36 and a crack density of around 0 05 to 0 02 the observations from the p and s waves are not incompatible with one another

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

a schematic model of crater modification by gravity models are proposed to account for the formation of slump terraces central peaks peak rings and concentric rings in large impact structures crater slumping and central peak formation is attributed to the process of acoustic fluidization which endows a volume of the target surrounding the crater with a bingham plastic rheology for a short time after the impact the parameters of this bingham plastic model are remarkably constant throughout the solar system the bingham yield stress ranges from a maximum of 80 bars on mercury to a minimum of 7 bars on callisto the bingham viscosity is between 109 and 1010 p at the onset of central peak formation for all bodies studied although it appears to rise to 1011 p as crater diameter grows to a few hundred kilometers asymmetric or scarp bounded concentric rings are due to a fundamentally different process they form when the target planet s lithosphere fractures under stresses imposed by the flow of the asthenosphere toward the transient crater cavity lithosphere yield strengths of less than a few hundred bars are implied by observed ring development on the moon and callisto

stable isotopes in guano potential contributions towards palaeoenvironmental reconstruction in tabon cave palawan philippines abstract tabon cave is a key site for the understanding of modern human dispersals in the philippine archipelago and island southeast asia nestled in the karst landscape that borders the southwestern coast of palawan it has delivered the earliest confirmed homo sapiens remains in the philippines dating to the late pleistocene as far back as around 47 ka among other methods the broad characteristics of the environment in which these humans once lived may be drawn using stable isotope analysis of the rich guano deposits in the cave an approach that follows a growing number of studies indicating the potential of guano as a palaeoenvironmental archive 13 c values reveal the general prevalence of c 3 forest tempered by savannah woodland with grassland contributions both well before and slightly after a securely dated fireplace at 32 ka the lower interval would refer to ois 3 or older interglacial periods while the upper interval would refer to the transition either before or after the last glacial maximum no useful conclusions are drawn from 15 n results due to suspected ammonia fractionation pending future dating efforts for confirmation this preliminary study contributes to the development of an alternative and promising palaeoenvironmental proxy and hopes to shed further light on the prehistoric odysseys that took place across island southeast asia

the manson impact structure 40ar 39ar age and its distal impact ejecta in the pierre shale in southeastern south dakota the 40 ar 39 ar ages of a sanidine clast from a melt matrix breccia of the manson iowa impact structure mis indicate that the mis formed 73 8 0 3 million years ago ma and is not coincident with the cretaceous tertiary boundary 64 43 0 05 ma the mis sanidine is 9 million years older than 40 ar 39 ar age spectra of mis shock metamorphosed microcline and melt matrix breccia interpreted earlier to be 64 to 65 ma grains of shock metamorphosed quartz feldspar and zircon were found in the crow creek member upper campanian at a biostratigraphic level constrained by radiometric ages in the pierre shale of south dakota that are consistent with the 40 ar 39 ar age of 73 8 0 3 ma for mis reported herein

original size of the vredefort structure implications for the geological evolution of the witwatersrand basin abstract historically there have been a range of diameter estimates for the large deeply eroded vredefort impact structure within the witwatersrand basin south africa here we estimate the diameter of the transient cavity at the present level of erosion as 124 140 km based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted transvaal outliers taking erosion into account 6 km and scaling to original final rim diameter an estimate of close to 300 km for the rim diameter is obtained independent estimates of the final rim diameter based on an empirical relation of central uplift diameter to rim diameter spatial distribution of pseudotachylites and concentric large scale structural patterns give a similar estimate of close to 300 km for the original final rim diameter an impact structure of this size is expected to have had an original multi ring form at this size the vredefort impact structure encompasses the bulk of the witwatersrand basin which appears to owe its preservation to the vredefort impact in addition the vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area which in recent interpretations is believed to be a component in the creation of the world class gold deposits of the witwatersrand basin

avoiding pitfalls in shallow seismic reflection surveys acquiring shallow reflection data requires the use of high frequencies preferably accompanied by broad bandwidths problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll erroneous interpretation of processed airwaves and air coupled waves as reflected seismic waves misinterpretation of refractions as reflections on stacked common midpoint cmp sections and emergence of processing artifacts processing and interpreting near surface reflection data correctly often requires more than a simple scaling down of the methods used in oil and gas exploration or crustal studies for example even under favorable conditions separating shallow reflections from shallow refractions during processing may prove difficult if not impossible artifacts emanating from inadequate velocity analysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum

bedrock fracture parameters from the interpretation of well tides this paper presents a theory to account for the main features of the tidal response of an isolated plane fracture as revealed by the water level in an intersecting open borehole according to the theory the diurnal and semidiurnal tidal constituents both amplitude and phase of water level variations depend on the fracture aperture the orientation dip and strike and the radial extent of the fracture and the compressibility of the asperites the theory is applied to water level observations made in six open boreholes located in crystalline rock at a test site at the chalk river nuclear laboratories atomic energy of canada limited in chalk river ontario canada the boreholes were uncased and intersected many open fractures so that a definitive test of the theory could not be made however in four cases a fracture model could be found which fitted the observations within the experimental error

impact origin of the avak structure arctic alaska and genesis of the barrow gas fields geophysical and subsurface geologic data suggest that the avak structure which underlies the arctic coastal plain 12 km southeast of barrow alaska is a hypervelocity meteorite or comet impact structure the structure is a roughly circular area of uplifted chaotically deformed upper triassic to lower cretaceous sedimentary rocks 8 km in diameter that is bounded by a ring of anastomosing inwardly dipping listric normal faults 12 km in diameter a zone of gently outward dipping sedimentary country rocks forms a discontinuous ring of rim anticlines within the peripheral ring of normal faults beyond these anticlines the sedimentary rocks are almost flat lying basement consists of strongly deformed ordovician and silurian argillite density and acoustic impedance con rasts between the argillite and the overlying strata produce gravity and seismic reflection signatures that define a ring of anticlines around the disturbed zone and a structural high surrounded by an annular structural low at its center in the adjacent barrow gas fields the tops of the informally named neocomian pebble shale unit and the gas producing lower jurassic barrow sand local usage lie at average subsea depths of 488 m and 670 m respectively in the avak 1 well drilled on the central high the pebble shale and the barrow sand lie near the surface documenting more than 500 m of relative uplift at the high the cores in this well have steep dips 30 90 degrees mixed breccia with franklinian argillite clasts 10 and 90 m above basement quartz grains with shock mosaicism and multiple sets of shock lamellae oriented concussion fractures in sand size quartz grains and shatter cones resembling those found in the peripheral zones of well documented impact structures in addition above background levels o fractured quartz grains in barrow sand were found as far as 19 km beyond the rim of the avak structure data concerning the age of the avak structure are not definitive if submarine landslide deposits in the upper part of the aptian and albian torok formation in the subsurface 200 km to the east were triggered by the avak event then the avak meteorite struck a submerged marine shelf about 100 or 5 ma however the impact features found at avak shatter cones concussion fractures shock lamellae and shock mosaicism in quartz grains and widespread cataclasis characterize the distal zones of meteorite impact structures fused rocks plastic deformation and shock metamorphic minerals found in more proximal zones of impact structures are apparently missing these observations and the lack of avak ejecta in cuttings and cores from the torok formation and nanushuk group albian to middle cenomanian in surrounding test wells indicate that the impact event postdated these beds in this case the avak meteorite struck a late cretaceous or tertiary marine shelf or coastal plain between the cenomanian ca 95 ma and deposition of the basal beds of the overlying late pliocene and quaternary gubik formation ca 3 ma

timing of the most recent neoglacial advance and retreat in the south shetland islands antarctic peninsula insights from raised beaches and holocene uplift rates abstract the timing of the most recent neoglacial advance in the antarctic peninsula is important for establishing global climate teleconnections and providing important post glacial rebound corrections to gravity based satellite measurements of ice loss however obtaining accurate ages from terrestrial geomorphic and sedimentary indicators of the most recent neoglacial advance in antarctica has been hampered by the lack of historical records and the difficulty of dating materials in antarctica here we use a new approach to dating flights of raised beaches in the south shetland islands of the northern antarctic peninsula to bracket the age of a neoglacial advance that occurred between 1500 and 1700 ad broadly synchronous with compilations for the timing of the little ice age in the northern hemisphere our approach is based on optically stimulated luminescence of the underside of buried cobbles to obtain the age of beaches previously shown to have been deposited immediately inside and outside the moraines of the most recent neoglacial advance in addition these beaches mark the timing of an apparent change in the rate of isostatic rebound thought to be in response to the same glacial advance within the south shetland islands we use a maxwell viscoelastic model of glacial isostatic adjustment gia to determine whether the rates of uplift calculated from the raised beaches are realistic given the limited constraints on the ice advance during this most recent neoglacial advance our rebound model suggests that the subsequent melting of an additional 16 22 increase in the volume of ice within the south shetland islands would result in a subsequent uplift rate of 12 5 mm yr that lasted until 1840 ad resulting in a cumulative uplift of 2 5 m this uplift rate and magnitude are in close agreement with observed rates and magnitudes calculated from the raised beaches since the most recent neoglacial advance along the south shetland islands and falls within the range of uplift rates from similar settings such as alaska

cosmogenic nuclides during isotope stages 2 and 3 abstract changes in the concentration of cosmogenic nuclides measured in natural archives are related mainly to changes in the cosmic ray induced production rate in the atmosphere but also to the subsequent transport and deposition processes which depend on the geochemical properties of the nuclides a comparison of 10 be and 36 cl data from ice cores from greenland ice cores and 14 c data from tree rings and other archives provides the following information for the isotope stages 2 and 3 1 changes in the cosmogenic nuclide production rate on time scales longer than approximately 2 kyr agree well with paleomagnetic data derived from deep sea sediments in particular the geomagnetic excursions leading to weak fields around 40 kyr bp laschamp and 32 kyr bp mono lake can be clearly seen 2 production variations on shorter time scales contain information on solar variability as an example the 205 yr devries cycle already known from the analysis of 14 c tree ring data during the holocene is also present in the 10 be data of the time period 25 55 kyr bp 3 a comparison of 14 c calculated using 10 be data with the measured 14 c record points to a reduction of the global deep water formation during the younger dryas cold event 4 using information available on the production rates of cosmogenic nuclides in the past derived from paleomagnetic data the paleoprecipitation rate can be calculated this is crucial for establishing precise time scales

younger dryas to mid holocene environmental history of the lowlands of nw transylvania romania pollen micro charcoal and total carbon analyses on sediments from the turbuta palaeolake in the transylvanian basin of nw romania reveal younger dryas to mid holocene environmental changes the chronostratigraphy relies on ams 14c measurements on organic matter and u th tims datings of snail shells results indicate the presence of pinus and betula open woodlands with small populations of picea ulmus alnus and salix before 12 000 cal yr bp a fairly abrupt replacement of pinus and betula by ulmus dominated woodlands at ca 11 900 cal yr bp likely represents competition effects of vegetation driven by climate warming at the onset of the holocene by 11 000 cal yr bp the woodlands were increasingly diverse and dense with the expansion of quercus fraxinus and tilia the establishment of corylus and the decline of upland herbaceous and shrubs taxa the marked expansion of quercus accompanied by tilia between 10 500 and 8000 cal yr bp could be the result of low effective moisture associated with both low elevation of the site and with regional change towards a drier climate at 10 000 cal yr bp corylus spread across the region and by 8000 cal yr bp it replaced quercus as a dominant forest constituent with only little representation of picea abies carpinus became established around 5500 cal yr bp but it was only a minor constituent in local woodlands until ca 5000 cal yr bp results from this study also indicate that the woodlands in the lowlands of turbuta were never closed

interpretation of seismic reflection gravity and magnetic data across middle proterozoic mid continent rift system northwestern wisconsin eastern minnesota and central iowa seismic reflection gravity and magnetic data were used along four profiles to study the mid continent rift system between northwestern wisconsin and central iowa mafic lavas in medial horsts are characterized by strong laterally continuous reflections that indicate a stratigraphic package as much as 10 km thick mafic roots extending beneath the reflective sequence to mid crustal depths were defined from gravity modeling in northwestern wisconsin southeastern minnesota and central iowa a strong normally directed remanent magnetization was modeled for most lavas although some lavas deep in the section of northwestern wisconsin and central iowa were modeled as magnetically reversed the horsts are apparently bounded by faults that are vertical or dip moderately inwar several of these faults originated as growth faults in fault bounded grabens but these faults were transposed into reverse faults by a late stage compressional event the medial horsts are flanked by several half graben basins that contain as much as 5 km of sedimentary material inferred to be lithologically akin to the oronto and bayfield groups of michigan and wisconsin several synclinal basins characterized by 2 4 km of sedimentary rocks are developed atop the medial horsts when viewed regionally the four interpretive profiles show a marked asymmetry with regard to the off axis placement of the deep mafic roots and the positions of postulated growth faults this asymmetry is consistent with the rift propagating as a series of northeast striking en echelon half grabens separated by northwest striking scissor faults or accommodation zones

recent earthquake prediction research in japan japan has experienced many major earthquake disasters in the past early in this century research began that was aimed at predicting the occurrence of earthquakes and in 1965 an earthquake prediction program was started as a national project in 1978 a program for constant monitoring and assessment was formally inaugurated with the goal of forecasting the major earthquake that is expected to occur in the near future in the tokai district of central honshu island the issue of predicting the anticipated tokai earthquake is discussed in this article as well as the results of research on major recent earthquakes in japan the izu earthquakes 1978 and 1980 and the japan sea earthquake 1983

iron age to medieval entomogamous vegetation and rhinolophus hipposideros roost in south eastern wales uk abstract karst cave systems are well developed in wales uk and in some instances constitute important bat roosts ogof draenen near blaenavon in south east wales is the most recent major cave discovery 1994 with already 70 km of passages explored spanning a vertical range of 148 m with the exception of one small chamber siambre ddu located directly above the main ogof draenen system very few bats have been noticed inside extensive accumulations of guano attributable to rhinolophus hipposideros are however found in parts of the ogof draenen system in places covering many square metres and sometimes building heaps 0 5 m thick these represent volumes not yet found in any other cave system in the british isles although the date of the abandonment of the main ogof draenen system as a bat roost remains unknown six radiocarbon dates on guano from ogof draenen place the occupation in the iron age to medieval period at least palynological analysis was undertaken on ten samples distributed through the cave comparisons were made with a moss polster and a lake mud sample from the area to provide a first approximation of the regional modern pollen rain and with two modern guano samples one from siambre ddu and one from agen allwedd cave 5 km to the north west to provide a temporal comparison with the fossil guano agen allwedd cave currently is one of the largest active roosts for lesser horseshoe bats in britain and lies close to the present northern limit of this endangered species in europe the main results are that 1 the cave appears to have been used both as a summer and a winter roost 2 most of the ogof draenen guano is formed within c 1600 14 c years and if the largest heap is continuous it has accumulated within 750 14 c years i e 0 16 mm yr 1 3 the fossil guano samples reflect a relatively closed oak forest with more abundant ivy hedera and holly ilex than at present 4 insect pollinated plants such as ilex acer hedera and impatiens glandulifera are over represented in the guano samples 5 in addition to the usual causes of bat roost decline pesticides pollution in the case of ogof draenen we may add entrance blocked by rock collapse and decline of the local forest cover as well as change in its composition

long term summer temperature reconstruction inferred from tree ring records from the eastern carpathians the first 1 000 year long carpathian tree ring width chronology was established based on living and subfossil stone pine pinus cembra l samples from an upper timberline forest located in calimani mts romania tree ring data were standardized using the regional curve standardization method in order to preserve the low and medium frequency climate signals the de trended index strongly correlates with summer mean temperature both at annual and decadal scales the calimani summer mean temperature anomalies were reconstructed for the period ad 1163 2005 applying the rescaling method this new climate proxy from the carpathians shows similar fluctuations to other north hemispheric temperature reconstructions but with periods of distinct differences the fingerprint of little ice age in the calimani area is visible between ad 1370 and 1630 followed by lagged cold decades in ad 1820 and 1840 the recent warming is evident only after the 1980s in our reconstruction

twelve years of geophysical research in the charlevoix seismic zone abstract various and extensive geophysical monitoring began in 1974 in the charlevoix seismic zone csz the historical record and a major earthquake in 1925 characterize this zone as the likely site of a future damaging earthquake i e m 5 5 or larger and the aim of the monitoring was to develop the capability of predicting such earthquakes the parameters studied included microseismicity seismic travel times electrical impedance vertical movement horizontal movement tilt gravity change and strain through water well level changes although no clear precursor was detected in the months before the largest seismic event that occurred during the observation period m 5 0 the multiparameter experiment provided new insights into the structure and the mechanics of this active region observations of several hundred well located microearthquakes have permitted the delineation of ancient rift faults that are now being activated by high horizontal compression the geographical extent of the zone is sharply defined by the microseismicity which occurs in an area of 35 km 80 km straddling the st lawrence river observation of p waves from accurately timed explosions and s waves from earthquakes support the hypothesis of extensive dilatancy anisotropy eda involving saturated parallel cracks in the crust the data have permitted an estimate of both crack density and the direction for the principal compressive stress in the region n 30 e changes in electrical parameters with periods of months to years were detected at five of the six magnetotelluric stations installed in the region changes in the polarization angle of the earth s electrical telluric field correlated well for seven years with calculated groundwater accumulation levels at one site electric impedance changes showed no clear correlation from station to station but changes at one station appear to correlate with a seismic p wave travel time change anomaly and with the disappearance of a negative gravity anomaly at the time of an unusual number of earthquakes in the range m 2 to 3 from mid 1976 to mid 1977 it is suggested that these correlated changes could be related to aseismic stress redistribution and fluid flow along one of the ne striking rift faults about two years before the m 5 0 earthquake in 1979 continuous monitoring of tilt surface and borehole and well water levels was carried out at a single site charlevoix geophysical observatory although there were unexplained near surface tilts of the order of the earth tide amplitude within two or three days before some earthquakes they were either not spatially coherent or were related to small unexplained temperature changes in the tiltmeter vault no intermediate term one week to one year precursors were seen against a noise level of 1 3 rad rms for near surface tilt observations and a noise level of 0 2 0 3 rad rms for borehole tilt measurements at a depth of 50 100 m the threshold of detectable tidal tilt admittance changes for the borehole tiltmeters was lowered to 2 but no significant changes were seen four coseismic changes in the levels of water in boreholes were observed all were drops in water level which were much larger than expected on the basis of elastic dislocation theory it is suggested that these could be explained either by the collapse of a dilatant zone around the respective hypocentres or by triggered aseisnuc movement on a nearby active fault borehole water level tides observed from 1980 to 1986 exhibit significant variations in amplitude and phase which are thought to be the result of time varying elastic properties in a nearby structure some constraints can now be placed on the long term deformation of the csz as a whole from tiltmeter levelling and horizontal surveys data from a levelling array observed from 1976 to 1986 puts a limit of 0 54 rad yr on the secular tilt rate at the observatory a further limit of 0 1 rad yr is provided by borehole tilt measurements over the period 1983 1986 a limit of 2 cm is placed on vertical movements northwest of the st lawrence river by levelling surveys in 1965 1978 1980 and 1982 no significant 90 confidence deviatoric strain has been detected across the csz from horizontal control surveys carried out in 1912 1919 1965 1978 and 1983 the maximum allowable long term strain rate across the csz from 1919 to 1983 is 0 16 strain yr

accretion rate of extraterrestrial matter iridium deposited 33 to 67 million years ago iridium measured in 149 samples of a continuous 9 meter section of pacific abyssal clay covering the time span 33 to 67 million years ago shows a well defined peak only at the cretaceous tertiary boundary in the rest of the section iridium ranges from a minimum concentration near 0 35 nanograms per gram in the paleocene to a maximum near 1 7 in the eocene between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years correction for terrestrial iridium leads to an extraterrestrial flux of9 3 nanograms of iridium per square centimeter per million years and an estimated annual global influx of 78 billion grams of chondritic matter consistent with recent estimates of the influx of dust meteorites and crater producing bodies with masses ranging from 10 13 to 1018 grams combining the recent flux of objects ranging in mass from 106 to 107 grams with the flux of 1014 to 1015 gram objects indicates that the number of objects is equal to 0 54 divided by the radius in kilometers to the 2 1 power periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years the predicted iridium maxima more than 30 times background are not present in the intervals associated with the cretaceous tertiary boundary or the tektiteproducing late eocene events

numerical simulations of the mjolnir marine impact crater 1 the 40 km diameter mjolnir crater in the barents sea was created by a meteoroid impact into a 400 m shallow sea about 142 ma we have used a multimaterial hydrocode with depth depending target strength to model numerically the cratering and early modification stages of the mjolnir impact our models are constrained by the observed crater morphology and structure the best results were obtained for a composite target strength structure very low strength for the upper 3 km of sedimentary rocks and gradual increase from 3 to 6 5 km depth before using strength values typical for granitic rocks at greater depths the low strength of the target sediments at shallow depths led to exceptionally intensive slumping and gravitational collapse of the order of 2 0 2 5 considerably larger than the average expected values for typical terrestrial craters as a result we obtain a crater collapse in between simple and complex where slumping and gravitational collapse counteract the crater floor uplift and lead to suppression and burial of the central high the mjolnir impact in a shallow sea was responsible for a major disturbance of the water column following the impact induced outward water surge large amplitude tsunamis were formed and the rapid resurge of seawater into the excavated crater transported large amounts of ejecta and crater wall material back into the crater accounting for the redestribution of ejecta material and extensive infilling

the mont laurier quebec earthquake of 19 october 1990 and its seismotectonic environment on 19 october 1990 at 07 01 ut a magnitude mblg 5 0 earthquake occurred approximately 9 km southwest of mont laurier quebec canada latitude 46 47 n and longitude 75 59 w this earthquake is the largest to have occurred in the western quebec seismic zone since the 1944 magnitude 5 6 cornwall massena earthquake the mont laurier earthquake was widely felt up to distances of 500 km and caused some minor damage in the epicentral region intensity modified mercalli mm v during the week following the earthquake a field survey recorded 23 aftershocks magnitudes were all less than ml 1 5 analysis of the mainshock and its aftershocks indicates focal depths in the range 10 to 12 km of the two east west oriented nodal planes given by the predominantly thrust mechanism of the mainshock the steep northerly dipping one probably represents the fault plane as indicated by the trend of aftershock hypocenters the last 10 yr of recording have confirmed that most western quebec earthquakes including the mont laurier earthquake occur in a northwest southeast trending zone inside the central metasedimentary cm belt with most focal depths varying between 7 and 25 km although some northwest trending structural features are known correlating these with the epicentral trend is premature the mid crustal hypocentral depths of many earthquakes the east west trend of the fault plane of the mont laurier earthquake and variations in regional focal mechanisms all suggest reactivation of deep structural features which may not have a surface expression inside the central metasedimentary belt most earthquakes occur along the eastern side where the activity seems to correlate with the labelle shear zone and the morin anorthosite body which may act as a stress concentrator the western side of the activity does not end with the central metasedimentary belt a fact that implies that even though the factors controlling seismicity lie predominantly within the central metasedimentary belt the adjacent geologic domains are also seismogenic

magnetotelluric interpretation of crustal and mantle structure in the grenville province summary accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust a magnetotelluric mt station bat in the grenville province of the precambrian shield in eastern canada has been in operation since 1975 for time dependency studies of electrical resistivity changes related to earthquakes the mt response of the station displays low skew with small to moderate anisotropy one dimensional inversion of the apparent resistivity and phase reveals two well defined conductors in the crust one at 10 km and the second at the base of the crust the latter has a resistivity less than 50 m these results are substantiated by three additional mt stations located up to 40 km distant data from other new mt stations and from stations previously published in the literature are compared with two dimensional computer model results and with the three dimensional analogue scale model results of dosso et al while additional data for periods less than 100 s would be desirable the results from a number of the mt stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the grenville the inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 m this may coincide with a seismic low velocity zone observed in the mantle under the canadian shield

meteoroid mayhem in ole virginny source of the north american tektite strewn field new seismic reflection data from chesapeake bay reveal a buried 85 km wide 1 5 2 0 km deep peak ring impact crater carved through upper eocene to lower cretaceous sedimentary strata and into underlying pre mesozoic crystalline basement rocks a polymictic late eocene impact breccia composed mainly of locally derived sedimentary debris determined from four continuous cores surrounds and partly fills the crater structural and sedimentary characteristics of the chesapeake bay crater closely resemble those of the miocene ries peakring crater in southern germany we speculate that the chesapeake bay crater is the source of the north american tektite strewn field

reflection seismic investigations of the siljan impact structure sweden reflection seismic measurements were carried out during 1985 in the area of the siljan impact structure of sweden a total profile length of 80 km was shot to get a clearer picture of the subsurface geometry of impact structures and the processes that formed them than could be obtained from surface observations or theoretical modeling studies standard stacked sections have been interpreted in terms of current impact models the transient crater diameter is found to be approximately 22 km considerably smaller than current estimates based on interpretation of surface geology the structural uplift is estimated to be 3 km at the 5 to 8 km level and the material flow necessary to support the uplift is mainly horizontal rather than vertical