evidence for postseismic viscoelastic relaxation following the 1959 m 7 5 hebgen lake montana earthquake a 1983 releveling survey conducted by the national geodetic survey combined with previous surveys in 1923 1960 1967 and 1975 provides evidence for ongoing vertical deformation of a broad region diameter of 150 km surrounding the site of the 1959 hebgen lake earthquake deformation consists of relative uplift centered roughly on the coseismic fault and a smaller amplitude zone of relative subsidence south of the fault maximum observed elevation change during the postseismic period 1960 1983 exceeds 30 cm assuming that the entire uplift i e 1923 1983 occurred following the 1959 earthquake i e uplift measured between 1923 and 1960 was entirely postseismic the rate of vertical deformation appears to have decreased exponentially with a characteristic decay time of about 10 years the spatial pattern and time behavior of the observed movements are consistent with simple models of postseismic deformation following normal faulting in an elastic layer thickness of 30 40 km overlying a viscoelastic half space viscosity of 1019 pa s this model may also account for the anomalously large horizontal strain around the hebgen lake region measured by repeated trilateration surveys during the period 1973 1984 the releveling measurements in the hebgen lake region appear to provide the first observations of viscoelastic relaxation in the asthenosphere following an intraplate earthquake in the united states

structural asymmetry in martian impact craters as an indicator for an impact trajectory abstract impact crater formation is a highly dynamic and complex geological phenomenon methods of structural geology are capable of giving insights into deformation processes that occur during cratering while most deformation observed in craters exhibits a generally radial symmetry recent discoveries of non radial structural elements in the central uplifts of terrestrial craters have led to the suggestion that obliquely impacting projectiles can cause non radial deformation in the central subsurface area of the crater the lack of ejecta blankets as accepted indicators of an oblique impact on earth make it difficult to correlate the observed non radial structures with the impact direction therefore in this study three martian complex impact craters were selected that exhibit an oblique ejecta blanket and layered bedrock in the central uplift the central uplifts were structurally mapped using high resolution imaging science experiment hirise imagery and additional high resolution digital terrain models dtms structural data were evaluated and show a correlation between the orientation of at least two structural criteria and the direction of impact 1 the strike of upturned bedrock layers is on average perpendicular to the impact trajectory and 2 the majority of faults show a preferred trend parallel to the impact trajectory a fourth martian crater was examined as a counterexample which had a radial ejecta blanket and thus no indication of an oblique trajectory strike in this crater showed no preferred orientation these results will help to solidify the suggested correlation between non radial structural features in impact craters and the horizontal component of momentum transferred from an obliquely impacting projectile to the target

impact crater degradation on venusian plains in venusian plains impact craters without extensive low backscatter ejecta deposits are more likely to have low backscatter floors be embayed by volcanic deposits and exhibit fractures as compared to craters with extensive low backscatter ejecta we interpret these trends as evidence of ongoing degradation of low backscatter ejecta by aeolian activity weathering and volcanism using a crater age sequence based on extent of preservation of low backscatter ejecta together with monte carlo simulations we find that tectonic activity has extended over a longer time period than volcanism

temporal gravity and height changes of the yellowstone caldera 1977 1994 this paper describes the longest record of gravity measurements in the area of the yellowstone caldera wyoming the temporal gravity changes at the 12 gal 10 8 ms 2 precision level are compared with changes in heights from leveling and gps the gravity field decreased across the caldera from 1977 to 1983 during the uplift and attained a maximum decrease of up to 60 2 gal along the caldera axis the gravity field then reversed polarity to increasing values of up to 60 12 gal between 1986 and 1993 the ratio between height and gravity changes varied during the entire time but converged over the latter period following the free air gravity gradient general ground deformation deduced from leveling showed caldera wide uplift of 15 mm a during the period of gravity decrease then from leveling and gps subsidence of 25 mm a during the gravity increase

sediment sources and siltation in mountain reservoirs a case study from the central spanish pyrenees abstract based on a 137 cs derived chronology we reconstruct the depositional history of the barasona reservoir in the esera isabena basin central pyrenees spain most of the sediments were delivered to the barasona reservoir during flood events comparative mineralogical studies of the esera and isabena river channels and the barasona reservoir sediments help to identify sediment sources and areas of high sediment yield risk and to establish erosion and sediment transport processes in the watershed changes in the mineralogical composition of the sediments can be used to discriminate the relative contribution of the esera and isabena rivers during flood periods three main periods in the siltation history of the barasona reservoir were distinguished 1 from dam construction 1932 to the early 1950s the reservoir was flushed every year using the lower gates and average sediment accumulation was very low 1 5 cm yr 2 during the 1950s and 1960s the sedimentation rate increased and reached the highest values 15 cm yr 24 6 cm yr and 18 2 cm yr in the esera river mouth the northern and the southern areas of the reservoir respectively both changes in the exploitation regime no annual flushes and climate variability increased flood frequency and river inflow caused this increase in sediment delivery and accumulation 3 the third period was initiated by the enlargement of the dam in 1972 the new sedimentary dynamics in the reservoir caused a general decrease in the sedimentation rate 11 6 cm yr in the southern areas 10 cm yr in the esera mouth that was especially noticeable in the northern areas where the accumulation rate was reduced to 3 7 cm yr an increase in channel erosion processes during this period contributed to the transport of sediments to the inner areas of the reservoir

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

rainfall runoff and sediment transport relations in a mesoscale mountainous catchment the river isabena ebro basin this paper examines the relations between rainfall runoff and suspended sediment transport in the isabena basin during a quasi average hydrological year the isabena is a mesoscale river basin that drains a mountainous area comprising patches of highly erodible materials badlands the paper includes an analysis of the different hydrological and sedimentary responses of the catchment to a similar rainfall thirty four floods were studied with a very variable response observed runoff coefficients ranged from 0 32 to 33 the sedimentary response was also highly variable with maximum suspended sediment concentrations ssc oscillating between 0 1 and 90 g l 1 and flood sediment loads varying from 27 to 54 000 t per hydrological event most sediment load was concentrated in spring when competent floods occur frequently pearson correlation matrix and backward stepwise multiple regression indicate that the hydrological response of the catchment is strongly correlated with total precipitation event duration and rainfall of the previous days very low correlation was observed with rainfall intensity the relation between rainfall and sediment transport followed the same trend sediment variables e g total load and ssc were significantly correlated with variables such as total rainfall and rainfall over the previous days although the significance level was lower in comparison with the runoff related variables there was again no correlation between sediment variables and rainfall intensity on going research in the area suggests that apart from rainfall factors such as sediment availability in the badlands and accumulation of sediment in the channels influences the river s sedimentary response the non linear hydrosedimentary response is reflected in the wide range of runoff coefficients and sediment loads that have been observed in response to similar amounts of precipitation

the transition from complex craters to multi ring basins on the moon quantitative geometric properties from lunar reconnaissance orbiter lunar orbiter laser altimeter lola data united states national aeronautics and space administration nasa lunar reconnaissance orbiter lunar orbiter laser altimeter lola experiment nnx09am54g

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

geological and geophysical investigation of kamil crater egypt we detail the kamil crater egypt structure and refine the impact scenario based on the geological and geophysical data collected during our first expedition in february 2010 kamil crater is a model for terrestrial small scale hypervelocity impact craters it is an exceptionally well preserved simple crater with a diameter of 45 m depth of 10 m and rayed pattern of bright ejecta it occurs in a simple geological context flat rocky desert surface and target rocks comprising subhorizontally layered sandstones the high depth to diameter ratio of the transient crater its concave yet asymmetric bottom and the fact that kamil crater is not part of a crater field confirm that it formed by the impact of a single iron mass or a tight cluster of fragments that fragmented upon hypervelocity impact with the ground the circular crater shape and asymmetries in ejecta and shrapnel distributions coherently indicate a direction of incidence from the nw and an impact angle of approximately 30 to 45 newly identified asymmetries including the off center bottom of the transient crater floor downrange maximum overturning of target rocks along the impact direction and lower crater rim elevation downrange may be diagnostic of oblique impacts in well preserved craters geomagnetic data reveal no buried individual impactor masses 100 kg and suggest that the total mass of the buried shrapnel 100 g is approximately 1050 1700 kg based on this mass value plus that of shrapnel 10 g identified earlier on the surface during systematic search the new estimate of the minimum projectile mass is approximately 5 t

oblique impacts and peak ring position venus and chicxulub 1 the circular peak ring of the chicxulub impact crater is not concentric with the outer rim of the structure as defined by both gravity and seismic data although this has been attributed to an oblique impact and used to define the direction of impactor approach different groups have suggested radically different approach azimuths based on the same data sets we investigate the putative correlation between peak ring offset and impactor azimuth by examining a suite of 19 peak ring craters on venus for which the impactor s approach direction is known from their asymmetric external ejecta we find no statistically significant correlation between peak ring offset and the azimuth of the impactor s approach

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

a new method to determine the direction of impact asymmetry of concentric impact craters as observed in the field lockne on mars in experiments and simulations most impacts occur at an angle with respect to the horizontal plane this is primarily reflected in the ejecta distribution but at very low angle structural asymmetries such as elongation of the crater and nonradial development of the central peak become apparent unfortunately impact craters with pristine ejecta layers are rare on earth and also in areas with strong past or ongoing surface erosion on other planetary bodies and the structural analysis of central peaks requires good exposures or even on site access to outcrop however target properties are known to greatly influence the shape of the crater especially the relatively common target configuration of a weaker layer covering a more rigid basement one such effect is the formation of concentric craters i e a nested deeper inner crater surrounded by a shallow outer crater here we show that with decreasing impact angle there is a downrange shift of the outer crater with respect to the nested crater we use a combination of 1 field observation and published 3 d numerical simulation of one of the best examples of a terrestrial concentric impact crater formed in a layered target with preserved ejecta layer the lockne crater sweden 2 remote sensing data for three pristine concentric impact craters on mars with preserved ejecta layers further constraining the direction of impact as well as 3 laboratory impact experiments to develop the offset in crater concentricity into a complementary method to determine the direction of impact for layered target craters with poorly preserved ejecta layers

the smart 1 lunar impact the smart 1 spacecraft impacted the moon on 3rd september 2006 at a speed of 2 km s 1 and at a very shallow angle of incidence 1 the resulting impact crater is too small to be viewed from the earth accordingly the general crater size and shape have been determined here by laboratory impact experiments at the same speed and angle of incidence combined with extrapolating to the correct size scale to match the smart 1 impact this predicts a highly asymmetric crater approximately 5 5 26 m long 1 9 9 m wide 0 23 1 5 m deep and 0 71 6 9 m3 volume some of the excavated mass will have gone into crater rim walls but 0 64 6 3 m3 would have been ejecta on ballistic trajectories corresponding to a cloud of 2200 21 800 kg of lunar material moving away from the impact site the shallow messier crater on the moon is similarly asymmetric and is usually taken as arising from a highly oblique impact the light flash from the impact and the associated ejecta plume were observed from earth but the flash magnitude was not obtained so it is not possible to obtain the luminous efficiency of the impact event

internal crater modification on venus recognizing crater centered volcanism by changes in floor morphometry and floor brightness bright floored and dark floored craters on venus show systematic differences in their size distribution and apparent modification bright floored craters exhibit the following characteristics an interior radar brightness comparable to the youngest craters on venus a tendency toward smaller crater diameters and a broad range of crater elevations in contrast the dark floored craters are darker than pristine craters on venus are typically much larger and preferentially occur at lower elevations they also have larger floors than pristine craters of the same size and are similar in many respects to floor fractured craters on venus of the four proposed origins for dark crater floors these observations are most consistent with crater centered volcanism surface weathering or eolian deposition may contribute to floor darkening in some cases but neither of these mechanisms nor impact melts can independently explain the full range of observed modifications based on the measurements of this study volcanism has affected at least 6 13 of the impact craters on venus the estimated lava fill thicknesses are of the order of 100 500 m thus if these flows are representative of eruptions elsewhere in the venusian lowlands the implied average flux of plains forming volcanism on venus for the last 300 m y is 0 01 0 02 km3 yr

upper crustal structure of the yellowstone caldera from seismic delay time analyses and gravity correlations the 1978 yellowstone snake river plain seismic experiment provided detailed refraction data that were recorded across a two dimensional array of seismographs in yellowstone national park a delay time analysis was applied to 173 crystalline basement pg arrivals from these data to determine the three dimensional distribution of velocities and the layer configuration of the upper crust beneath the yellowstone caldera the p wave velocity structure of the caldera is characterized by a surface layer of combined sediments and rhyolite flows averaging 2 8 km s that range in thickness from 1 5 to 2 0 km adjacent to the caldera the crystalline upper crustal layer has a velocity of 6 05 0 01 km s but this layer decreases by 6 to 5 70 km s beneath the caldera and extends northeast 15 km beyond the caldera smaller zones of very low p velocity 4 0 km s a 30 velocity reduction compared to the 6 05 km s layer occur in the upper crust beneath the northeastern caldera rim and beneath the southwest caldera in the vicinity of the upper and midway geyser basins a three dimensional gravity interpretation based upon densities derived from the seismic model suggests that the regional gravity low of 60 mgal over the caldera correlates directly with 1 the surface layer of combined sediments and rhyolite flows 2 the low velocity 5 7 km s upper crustal layer and 3 the 4 0 km s low velocity zone beneath the northeastern caldera rim an interpretation of the seismic velocities and densities based on experimental data and theoretical models suggests that the 6 05 km s 2 70 g cm3 regional pg velocity is characteristic of crystalline basement rocks such as the precambrian granitic gneisses that are exposed north and south of the yellowstone caldera the 5 7 km s velocity layer 2 65 g cm3 is interpreted to be a 10 km thick hot body of granitic composition the 4 0 km s low velocity body 2 40 g cm3 beneath the northeastern caldera rim is associated with a local 20 mgal gravity anomaly with a range of interpretations from a large steam dominated system to a felsic body of 10 to 50 partial melt the postulated 4 0 km s low velocity layer beneath the southwestern caldera does not have a corresponding gravity signature and may reflect a zone of high seismic attenuation attributed to a saturated fracture zone this seismic velocity model of the yellowstone caldera suggests that the source of the quaternary silicic volcanic rocks corresponds to the 5 7 km s layer in the upper crust that appears to contribute the principal source of heat to the yellowstone hydrothermal systems the anomalous low velocity body in the northeastern caldera rim area may be a cupola of partial melt with an overlying vapor dominated carapace and may represent the waning stages of the third cycle of quaternary volcanism or the beginning of a new cycle of volcanism that will continue the northeastward progression of the silicic volcanism of the yellowstone snake river plain propagating system

crustal deformation and source models of the yellowstone volcanic field from geodetic data 1 geodetic observations comprising interferometric synthetic aperture radar insar global positioning system gps and precision leveling measurements are used to infer volume change in the subsurface associated with the dynamics of the yellowstone volcanic system we focus primarily on the yellowstone caldera and its related magmatic hydrothermal and fault systems it appears that known faults play a significant role in controlling crustal volume increases and decreases due to the migration of volcanic and hydrothermal fluids for example over 5 cm of subsidence from 1992 to 1995 is associated with source volume changes 6 10 km beneath the nw trending elephant back fault zone and a north trending fault cutting across the caldera furthermore we are able to image an episode of fluid intrusion near the northern edge of the caldera the intrusion is elongated in the north south direction and is parallel to the north trending volume decrease the primary intrusion and related hydrothermal activity occurred between 1996 and 2000 though the volume changes appear to have continued shallowed and changed shape between 2000 and 2002 there is evidence that the intrusive activity influenced extensional faults to the north of the caldera

suspended sediment transport in a highly erodible catchment the river isabena southern pyrenees abstract understanding and quantifying sediment load is important in catchments draining highly erodible materials that eventually contribute to siltation of downstream reservoirs within this context the suspended sediment transport and its temporal dynamics have been studied in the river isabena 445 km 2 south central pyrenees ebro basin by means of direct sampling and turbidity recording during a 3 year dry period the average flood suspended sediment concentration was 8 g l 1 with maximum instantaneous values above 350 g l 1 the high scatter between discharge and suspended sediment concentrations up to five orders of magnitude has not permitted the use of rating curve methods to estimate the total load interpolation techniques yielded a mean annual sediment load of 184 253 t y 1 for the study period with a specific yield of 414 t km 2 y 1 this value resembles those reported for small torrents in nearby mountainous environments and is the result of the high connectivity between the badland source areas and stream courses a fact that maximises sediment conveyance through the catchment floods dominated the sediment transport and yield however sediment transport was more constant through time than that observed in mediterranean counterparts this can be attributed to the role of base flows that entrain fine sediment temporarily stored in the channel and force the river to carry high sediment concentrations i e generally in the order of 0 5 g l 1 even under minimum flow conditions

sediment transport from continuous monitoring in a perennial mediterranean stream vericat d batalla r j 2010 sediment transport from continuous monitoring in a perennial mediterranean stream catena 82 2 77 86 sponsorship research project sediment export from large semi arid catchments measurements and modelling sesam funded by the deutsche forschungsgemeinschaft dgf

the impact of refraction correction on leveling interpretations in southern california recent investigations show that the refraction corrections in leveling are large but can be adequately modeled the corrections are proportional to the temperature differences and the square of the sight lengths changes in sight length with time have resulted in apparent crustal movements when non refraction corrected data were used applying refraction corrections to leveling data in southern california significantly alters the size and configuration of the southern california uplift uplift at palmdale and gorman lebec relative to los angeles bm v32 during the 1953 1978 time period is no greater than 10 cm the bulk of the motion about 6 cm occurs as localized motion along the san gabriel fault during the 1953 1964 time period this motion along the san gabriel fault and uplift of 8 cm directly north of the san fernando fault between 1955 and 1968 may represent precursors to the 1971 san fernando earthquake

experimental ejection angles for oblique impacts implications for the subsurface flow field a simple analytical solution for subsurface particle motions during impact cratering is useful for tracking the evolution of the transient crater shape at late times a specific example of such an analytical solution is maxwells z model which is based on a point source assumption here the parameters for this model are constrained using measured ejection angles from both vertical and oblique experimental impacts at the nasa ames vertical gun range data from experiments reveal that impacts at angles as high as 45 to the targets surface generate subsurface flow fields that are significantly different from those created by vertical impacts the initial momentum of the projectile induces a subsurface momentum driven flow field that evolves in three dimensions of space and in time to an excavation flow field during both vertical and oblique impacts a single stationary pointsource model specifically maxwells z model however is found inadequate to explain this detailed evolution of the subsurface flow field during oblique impacts because 45 is the most likely impact angle on planetary surfaces a new analytical model based on a migrating point source could prove quite useful such a model must address the effects of the subsurface flow field evolution on crater excavation ejecta deposition and transient crater morphometry

baseflow control on sediment flux connectivity insights from a nested catchment study in central mexico in order to assess the extent of sediment connectivity between uplands and lowlands and to quantify the processes of in channel deposition and remobilization measurements of suspended sediment fluxes were conducted in a nested rural catchment of the mexican volcanic belt data were collected over one year at three upland sites 3 to 12 km2 and two downstream stations 390 630 km2 our results show that a structural discontinuity in the catchment i e abrupt slope decrease at the junction between piedmonts and the alluvial plain from 2 to 10 to 0 1 could be compensated by functional continuity during floods direct conveyance of fine sediment to the outlet occurred when a high stream transport capacity was reached erosion of the streambed was observed on various occasions and accounted for up to 50 of the flux leaving the catchment during one event conversely temporary in channel storage was apparent on other occasions amounting to up to 52 of the flux recorded upstream during one storm these two distinct behaviours were approximately equally distributed along the rainy season and strongly driven by the extent of coupling between surface and subsurface water this work indeed highlights the role of baseflow spatial variations in determining the extent of lowland sediment conveyance riverbed erosional processes occurred when large differences in pre event baseflow values i e at least a twofold longitudinal increase were observed between the 5 km distant lowland stations our findings outline the importance of systematically taking into consideration the baseflow parameter in research focusing on fine sediment transport across scales

a deep low velocity body under the yellowstone caldera wyoming delineation using teleseismic p wave residuals and tectonic interpretation in previously published papers iyer 1975 iyer and stewart 1977 iyer 1979 evidence was presented for the presence of a large body of anomalous material with compressional velocities considerably lower than normal from the surrounding rock in the crust and upper mantle under yellowstone national park wyoming in this paper we give a detailed analysis of p residual data from the 26 element telemetered seismic array and three groups of portable stations operated by the u s geological survey in the yellowstone region using a variety of analytical techniques we delineate the shape of the low velocity body determine the velocity structure inside the body and interpret the nature of the material that constitutes the body we will also outline some tectonic implications of our model the spectacular surface geothermal phenomena for which yellowstone is famous result from deep seated volcanism the volcanic evolution of yellowstone is discussed by christiansen and blank 1972 and summarized by eaton and others 1975 briefly the geothermal manifestations in yellowstone are results of three cycles of quaternary volcanism each culminating in a devastating pyroclastic eruption and resulting in large collapse calderas and predominantly rhyolitic flows the oldest cycle ended about 1 9 m y ago forming a caldera from island park to perhaps the central part of the present yellowstone caldera the second cycle climaxed about 1 2 m y ago and was confined to the island park caldera the third cycle began soon after the second and is responsible for the 75 by 45 km yellowstone caldera as it is outlined today this caldera is the result of a catastrophic eruption approximately 600 000 yr ago the most recent episode of volcanism probably began about 150 000 yr ago in the western half of the caldera near old faithful geyser and the youngest flows in yellowstone about 70 000 yr old are probably associated with this episode

testing the resolution of a 3d velocity tomogram across the chicxulub crater abstract an integrated offshore onshore reflection and refraction experiment was shot across the chicxulub impact crater in 1996 the refraction data were previously inverted in 3d using first arrival travel time tomography a regularized inversion in which both data misfit and model roughness are minimized simultaneously was used to determine a smooth velocity tomogram across the inner crater region however the experimental geometry for the refraction data was irregular causing concern that this velocity model might not be well resolved in this paper we present a suite of checkerboard tests to investigate the lateral resolution of our velocity model the chicxulub crater is located partly onshore and partly offshore with its centre close to the yucatan coastline in mexico the shallow water limited acquisition of marine reflection data to distances of not closer than 25 km from the crater centre and the centre of the structure is imaged with refraction data only intriguing velocity anomalies were observed across the central crater region providing constraints on the lithological and structural form of chicxulub a high velocity body within the central crater is most likely to represent lower crustal rocks that were stratigraphically uplifted during the formation of this complex crater the concave shape of this stratigraphic uplift suggests clues to the mechanics of large crater collapse the rocks appear to have moved upward and outward an inward dipping zone of lowered velocity has been interpreted as delimiting the outer edge of a central zone of melt rich rocks the resolution tests presented here indicate that these observed velocity anomalies are likely to be real

moving boulders in flash floods and estimating flow conditions using boulders in ancient deposits boulders moving in flash floods cause considerable damage and casualties more and bigger boulders move in flash floods than predicted from published theory the interpretation of flow conditions from the size of large particles within flash flood deposits has until now generally assumed that the velocity or discharge is unchanging in time i e flow is steady or changes instantaneously between periods of constant conditions standard practice is to apply theories developed for steady flow conditions to flash floods which are however inherently very unsteady flows this is likely to lead to overestimates of peak flow velocity or discharge flash floods are characterised by extremely rapid variations in flow that generate significant transient forces in addition to the mean flow drag these transient forces generated by rapid velocity changes are generally ignored in published theories but they are briefly so large that they could initiate the motion of boulders this paper develops a theory for the initiation of boulder movement due to the additional impulsive force generated by unsteady flow and discusses the implications keywords

a 12 000 year record of vertical deformation across the yellowstone caldera margin the shorelines of yellowstone lake the 600 ka yellowstone caldera exhibits several signs of unrest the most evident of which is historic ground deformation including both uplift and subsidence we document deformation in the area of the southeastern caldera across 12 000 years using the postglacial shoreline terraces of yellowstone lake raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces with an accuracy of 0 5 m of about 11 recognizable terraces the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns shoreline ages are based on minimum and maximum limiting radiocarbon and obsidian hydration dates each terrace is interpreted as representing an episode of uplift 1 kyr 1 of the caldera interior and subsequent subsidence with little net volume change this cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystallization and or episodic trapping and release of magmatic fluids evolved during cooling and crystallization in a self sealing reservoir as hypothesized by dzurisin et al 1990 early postglacial shoreline deformation reveals substantial intracaldera subsidence possibly reflecting cooling and loss of trapped hydrothermal fluids extension along north south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin including downwarping and local faulting of hot weak intracaldera crust net deformation over the past 3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim relative to the extracaldera region this uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence the cause of this trend is undetermined but is most likely related to the effects of regional extension and long term cooling within the yellowstone caldera

crater features diagnostic of oblique impacts the size and position of the central peak using magellan data we investigated two crater characteristics that have been cited as diagnostic of oblique impacts an uprange offset of the central peak in complex craters and an increasing central peak diameter relative to crater diameter with decreasing impact angle we find that the offset distribution is random and very similar to that for high angle impacts and that there is no correlation between central peak diameter and impact angle accordingly these two crater characteristics cannot be used to infer the impact angle or direction

structural record of an oblique impact abstract most impactors strike their target at an oblique angle the common criterion for identifying craters formed by an oblique impact is the pattern of the ejecta blanket on earth however ejecta blankets are rarely preserved and other morphological structural or geophysical criteria are needed here we present structural details from the central uplift of the upheaval dome impact structure that are diagnostic of the kinematics during crater collapse and central uplift formation a characteristic imbrication of thrust slices towards the se the pattern of strata orientation within the central uplift dominant radial faults that accommodated nw se shortening and an elliptical bedding outline indicate that the displacement field during crater collapse has not been axial symmetric instead an additional lateral component roughly towards the se is preserved in the internal structure of the central uplift the structural asymmetries are largest in the core of the central uplift and disappear outwards thereby preserving the large scale circular shape of the main structural elements rim monocline ring syncline comparison with numerical models of oblique impacts suggests that the additional lateral displacement component reflects a downrange material transport during the initial stages of central uplift formation similar patterns are identified in other impact structures and may serve as general criteria for identifying the impact direction of deeply eroded impact structures in sedimentary targets

projectile remnants in central peaks of lunar impact craters unusual minerals observed in lunar craters were thought to originate from beneath the moon s surface numerical simulations show that rather than being vaporized much of the impactor material can survive in the crater implying that the unusual minerals come from the impactor and may not be indigenous to the moon

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

elliptical structure of the lunar south pole aitken basin abstract the south pole aitken basin sp a is the largest and oldest basin on the moon the basin has usually been interpreted to exhibit a degraded circular structure but here we demonstrate that the topography iron and thorium signatures of the basin are well described by ellipses with axes measuring 2400 by 2050 km and centered at 53 191 e topography abundances of iron thorium and the distribution of mare basalts are all elevated in the northern halves of the ellipses we also identify an outer topographic ellipse whose semiminor axis scales with the main topographic ellipse by approximately 2 taken together these data imply that the basin was created by an oblique impact along an azimuth of approximately 19 measured counterclockwise from longitude 191 e the geometry of the elevated central farside topography surrounding sp a suggests that it predates the impact the elliptical ring structures of sp a and their scaling relationships will help to understand the formation of large and elliptical basins elsewhere in the solar system this refined basin shape will also inform local geology geochemistry and geophysics of the region

patterns of topographic change in sub humid badlands determined by high resolution multi temporal topographic surveys abstract badlands are highly erodible landscapes with sparse vegetation and rapid runoff responses badland surfaces experience high erosion rates that may have a direct and marked influence on river channel networks and catchment scale sediment budgets erosion rates from badlands have been widely estimated by discrete observations of topographic change measured by erosion pins or profile meters recent developments in survey instrumentation provide the opportunity to build high resolution topographic models over multiple spatial scales at sub centimeter accuracy in this paper we demonstrate how reliable estimates of topographic variables and temporal change can be derived for badlands by repeat terrestrial laser scanning tls surveys undertaken at multiple temporal event to annual scales a total of seven tls surveys were obtained in an experimental sub humid badland located in the central pyrenees data analyses were conducted on two temporal scales a five rainfall events and b three long term spaced over an annual cycle scales our results show a clear erosional pattern for most of the badland at the annual scale i e annual net change at around 6 cm yr 1 aspect surface roughness and slope were significant predictors of topographic change although the sign and magnitude of the change differed at the event scale net topographic change at the event scale varied from 1 8 to 1 4 cm although these patterns could be also affected by swell shrink processes surface roughness was more important at controlling badland geomorphological processes at the event scale than the annual scale our observations suggest that longer term studies may underestimate the importance of surface roughness as a control on badland geomorphology at the annual scale the effect of aspect in the sign of the topographic change was removed and slope becomes more important erosion proved to be dominant on slopes higher than 100 overall the results indicate that coupling of appropriately scaled spatial and temporal data is critical to understand topographic changes and their drivers on badlands although the approach outlined in this paper was only applied to a relatively small area with careful survey design and application of new survey technology it could readily be upscaled to cover entire badland systems such surveys would permit detailed analysis of controls of sedimentological connectivity across badlands and their influence on channel networks and catchment sediment budgets

analyses of shocked quartz at the global k p boundary indicate an origin from a single high angle oblique impact at chicxulub abstract the precise cause and timing of the cretaceous paleocene k p mass extinction 65 ma ago remains a matter of debate many advocate that the extinction was caused by a meteorite impact at chicxulub mexico and a number of potential kill mechanisms have been proposed for this although we now have good constraints on the size of this impact and chemistry of the target rocks estimates of its environmental consequences are hindered by a lack of knowledge about the obliquity of this impact an oblique impact is likely to have been far more catastrophic than a sub vertical one because greater volumes of volatiles would have been released into the atmosphere the principal purpose of this study was to characterize shocked quartz within distal k p ejecta to investigate whether the quartz distribution carried a signature of the direction and angle of impact our analyses show that the total number maximum and average size of shocked quartz grains all decrease gradually with paleodistance from chicxulub we do not find particularly high abundances in pacific sites relative to atlantic and european sites as has been previously reported and the size distribution around chicxulub is relatively symmetric ejecta samples at any one site display features that are indicative of a wide range of shock pressures but the mean degree of shock increases with paleodistance these shock and size distributions are both consistent with the k p layer having been formed by a single impact at chicxulub one site in the south atlantic contains quartz indicating an anomalously high average shock degree that may be indicative of an oblique impact with an uprange direction to the southeast 45 the apparent continuous coverage of proximal ejecta in this quadrant of the crater however suggests a relatively high impact angle of 45 we conclude that some of the more extreme predictions of the environmental consequences of a low angle impact at chicxulub are probably not applicable

asymmetric signatures in simple craters as an indicator for an oblique impact direction in oblique impacts with an impact angle under 45 the bilateral shape of the distal ejecta blanket is used as the strongest indicator for an impact vector this bilateral symmetry is attenuated and is superimposed by radial symmetry towards the crater rim which remains circular for impact angles down to 10 15 the possibility that remnants of bilateral symmetry might still be present in the most proximal ejecta the overturned flap and the crater rim was explored with the intention of deducing an impact vector a model is presented that postulates bilateral patterns using proximal ejecta trajectories and predicts these patterns in the orientation of bedding planes in the crater rim this model was successfully correlated to patterns described by radial grooves in the proximal ejecta blanket of the oblique tooting crater on mars a new method was developed to detect structural asymmetries by converting bedding data into values that express the deviation from concentric strike orientation in the crater rim relative to the crater center termed concentric deviation the method was applied to field data from wolfe creek crater western australia bedding in the overturned flap implies an impactor striking from the east which refines earlier publications while bedding from the inner rim shows a correlation with the crater rim morphology

flood based analysis of high magnitude sediment transport using a non parametric method francke t lopez tarazon j a vericat d bronstert a batalla r j 2008 flood based analysis of high magnitude sediment transport using a non parametric method earth surface processes and landforms 33 13 2064 2077 sponsorship sesam catalan government european social fund

geomorphic agents versus vegetation spreading as causes of badland occurrence in a mediterranean subhumid mountainous area abstract previous evidence on the chief role of physical weathering on badland development in the vallcebre area south eastern pyrenees led to the study of the amounts of energy available for the main geomorphic agents and to analyze the results in the light of the patterns of vegetation cover and species distribution in order to explain both the occurrence of these badland forms and their preference for north facing shady hillslopes results showed that the weathering potential provided by freezing is about two orders of magnitude greater than rainfall kinetic energy and that the former strongly depends on hillslope aspect the study of vegetation demonstrated that vegetation is poorer in cover and species composition on shady hillslopes than on sunny ones spontaneous revegetation of these surfaces being restrained by cold thermal conditions rather than by water availability these results show that the occurrence of these badland areas depends on montane climatic attributes that control geomorphic and biological processes and that are different from those that promote the occurrence of badlands in dry mediterranean areas the preference for northern aspects provides evidence for these differences

variation in flow and suspended sediment transport in a montane river affected by hydropeaking and instream mining abstract the temporal and spatial variability of water and sediment loads of rivers is controlled by a suite of factors whose individual effects are often difficult to disentangle while land use changes and localised human activities such as instream mining and hydropeaking alter water and sediment transfer tributaries naturally contribute to discharge and sediment load of mainstem rivers and so may help compensate upstream anthropogenic factors the work presented here aimed to assess water and the sediment transfer in a river reach affected by gravel extraction and hydropeaking set against a backdrop of changes to the supply of water and sediment from tributaries discharge and suspended sediment transport were monitored during two average hydrological years at three cross sections along a 10 km reach of the upper river cinca in the southern pyrenees water and sediment loads differed substantially between the reaches the upper reach showed a largely torrential discharge regime controlled mainly by floods and had high but variable water and sediment loads the middle reach was influenced markedly by hydropeaking and tributary inflows which increased its annual water yield four fold suspended sediment load in this reach increased by only 25 compared to upstream indicating that dilution predominated in the lowermost section while discharge remained largely unaltered sediment load increased appreciably as a result of changes to sediment availability from instream mining and inputs from tributaries at the reach scale snowmelt and summer and autumn thunderstorms were responsible for most of the water yield while flood flows determined the magnitude and transport of the sediment load the study highlights that a combination of natural and human factors control the spatial and temporal transfer of water and sediment in river channels and that depending on their geographic location and effect size can result in marked variability even over short downstream distances

assessment of global positioning system measurements for studies of crustal deformation the u s geological survey has been using the global positioning system gps to perform repeated surveys of networks of different sizes since january 1986 the intersite distances from these various networks are 223 km for a single line connecting the palos verdes and vandenberg sites in southern california 30 50 km for a network of three sites south of the san francisco bay near loma prieta 9 30 km for a network located near hebgen lake montana 7 12 km for a network of sites in the vicinity of parkfield california and 240 m for a line located at the goldstone antenna complex in the mojave desert the lengths quoted do not include the intersite distances for the fiducial sites included in the solutions for orbit determination which can be up to 8000 km we have attempted to assess the long term precision and accuracy of determinations of relative site position for a range of line lengths this assessment has consisted of several different studies in assessing precision we have generally investigated repeatability usually about a best fit linear temporal trend the smallest fractional repeatability was yielded by the measurements on the palos verdes to vandenberg line the root mean square rms residual about the best fit line for these determinations was 6 mm 0 03 parts per million or ppm for the north component 11 mm 0 05 ppm for the east component and 40 mm 0 18 ppm for the vertical component for these determinations gps data from 3 4 consecutive days were combined to produce a single estimate of the vector separation we also used the day to day scatter from these measurements to perform an analysis of variance and found that these multiday determinations can be significantly more precise than estimates determined from a single observing session the worst results were obtained for the loma prieta network where we obtained rms residuals of 11 12 mm 0 3 0 4 ppm for north 14 18 mm 0 3 0 6 ppm for east and 18 21 mm 0 4 0 7 ppm for vertical we cannot definitely explain these poor results but we have evidence to indicate that this poor scatter is due to the short observing session 4 5 hours compared to 6 5 hours for the palos verdes to vandenberg results in assessing accuracy we compared the gps results to those from independent techniques at parkfield we compared the gps estimates of fault parallel motion to those from creep meters and alignment arrays and found excellent agreement within 1 2 mm yr 1 not only for the slip but also for the variation of slip along the san andreas fault we compared the loma prieta and hebgen lake line length estimates to those obtained from a geodolite a high precision laser distance measurement device that the u s geological survey has used for 20 years for studies of crustal deformation if we model the geodolite minus gps line length differences l as l l where l is the line length then we find 0 6 0 5 mm and 0 1 0 1 mm km 1 the uncertainties quoted are one standard deviation from this comparison we conclude that at this level of significance there is no detectable difference between lengths determined from geodolite and gps data finally we compared the gps estimates of the palos verdes to vandenberg line length to those obtained from very long baseline interferometry vlbi the mean difference between the six gps estimates of line length and a best fit line through the vlbi estimates was 1 mm and the rms difference was 12 mm 0 05 ppm close to the standard deviation of the gps measurements for this predominantly east west line

an assessment of the accuracy of the geodetic measurements that define the southern california uplift examination of the charge that the geodetic measurements which define the southern california uplift are seriously flawed by height dependent systematic errors indicates that this charge is unfounded our review of those factors designed to minimize the effects of rod error shows that systematic errors attributable to rod miscalibration are trivial and would tend to cancel during the particularly pertinent period 1955 1965 comparisons between the results of measurements in which the correlation between topography and signal is generally poor reveal large aseismic tilts in a number of places within and around the margins of the uplift especially significant in this context are the results of preuplift and postuplift levelings over routes characterized by diverse length topography and atmospheric conditions that produce closely matching temporally equivalent heights for a representative bench mark within the uplift statistical analyses of the short wavelength components of uplift signal and elevation along a frequently repeated survey line indicate variable correlations between the two however the spatial and temporal patterns of these correlations are inconsistent with their attribution to height dependent systematic errors statistically significant short wavelength correlations along this survey line are reasonably explained in part as due to real movement associated with differentially subsiding bench marks and there is a strong likelihood that this movement tends to dominate many of the correlations

contemporary vertical surface displacements in yellowstone national park relative vertical displacements and average relative vertical velocities have been determined for bench marks located throughout yellowstone national park ynp by comparing geodetic leveling measurements made in 1923 second order 1936 third order 1941 third order 1955 third order 1960 first order and 1975 1977 first order the 1923 1975 data indicate uplift greater than 400 mm 7 7 mm yr of an area approximately 15 20 km wide and at least 40 km long which is centered along the northeast trending axis of the 600 000 year old yellowstone caldera the maximum 1923 1975 vertical displacement in this area is 726 mm 14 0 mm yr the existence of a well defined and untilted yellowstone lake terrace implies that the 1923 1975 average vertical velocities cannot have been maintained for more than a few hundred years the 1955 1977 data from central ynp indicate that the northeastern caldera half has risen relative to the southwestern half at an average velocity of about 8 mm yr a similar bias is present in the 1923 1975 data but it is less pronounced the 1923 1960 and 1960 1975 data from northwestern ynp and a 200 mm depression in the 1923 1975 data near norris junction may be interpreted in terms of deformation associated with the 1959 hebgen lake earthquake and the 1975 yellowstone park earthquake a composite profile of the 1936 1977 and 1941 1977 average vertical velocity data which skirts the edge of the yellowstone caldera in northeastern ynp is most easily explained as the result of random leveling error and or bench mark instabilities the apparent youth of the 1923 1975 yellowstone uplift and its close association with the yellowstone caldera suggest that the most likely cause of the uplift is a recent not more than 500 years ago influx of molten material into the upper crust beneath the caldera

aseismic growth of durmid hill southeasternmost san andreas fault california our 2 27 km leveling line across and normal to the san andreas fault at durmid hill domed upward by 9 mm between september 1985 and december 1991 the flanks of the line tilted more than 1 rad yr implying that the uplift extends beyond the ends of the line and that absolute uplift of the crest of the hill may greatly exceed the measured rate of 1 5 mm yr trilateration measurements sampling 5 km of the fault zone within the hill from 1978 to 1992 yield a mean dextral slip rate of 2 4 0 3 mm yr and fault normal extension of 1 0 3 mm yr the inferred uplift rate during the 5 5 year leveling period as well as from geomorphic samples at 330 years and 35 000 years and from erosion of sedimentary strata younger than 738 000 years in monoclines abutting the fault at durmid hill is remarkably consistent at 1 to 4 mm yr and implies that the current processes may have persisted for millennia we investigate two mechanisms for the deformation that are defined by the symmetry of the observed doming transpression on a vertical or near vertical subsurface san andreas fault or slip on a shallow flat fault or detachment that terminates beneath durmid hill the resultant models suggest that uplift rates of 1 mm yr will result from 6 mm yr dextral shear along a vertical fault together with 1 mm yr horizontal convergence across that fault and this strain field is consistent with the paleoseismic record and our leveling data we conclude that interseismic processes account for most of the growth of durmid hill because the interseismic vertical deformation rates equal or exceed the long term vertical slip rate measured across the fault because dextral shear of 6 mm yr at durmid hill represents a small fraction of the 25 mm yr measured across the entire san andreas fault zone in coachella valley and because the san andreas fault at durmid hill may represent an earthquake nucleation zone the observed deformation processes may be of significant importance for understanding future earthquake rupture initiation or termination

atmospheric effects on ejecta emplacement and crater formation on venus from magellan the venus cratering record provides a unique environment for assessing the effects of both gravity and an atmosphere on impact crater formation this contribution uses surface signatures of energy partitioning as a framework for testing extrapolations from laboratory experiments and other planetary settings seven general conclusions can be drawn first the dense lower atmosphere of venus takes on the role of a low density target for bodies smaller than about 4 km in diameter air blasts created by cratering in the atmosphere create distinctive surface signatures that allow the derivation of an independent assessment of impactor energy at the limit of break up second dynamic pressures during entry of larger bodies will exceed their strength limit but may not prevent penetration of the atmosphere due to aerodynamic reshaping that minimizes the drag coefficient such a process may account for the formation of unusually small craters 1 3 km third the dense atmosphere of venus preserves signatures of early time cratering processes on the surface that are typically lost on atmosphere free surfaces such signatures not only provide another estimate of impactor energy but also include a distinctive record of the impactor i e comet versus asteroid in distinctive run out flows created before the crater has finished formation strong winds and turbulence associated with the atmospheric disturbance at later times create wind streaks behind topographic barriers fourth ballistic ejection of excavated debris occurs from craters on venus just as it does on planets without an atmosphere thereby underscoring the fundamental mechanical transfer of energy from impactor to target fifth ejecta emplacement is nonballistic due to the large dynamic forces acting on the advancing curtain and its constituent ejecta the outward moving ejecta curtain induces strong response winds that entrain ejecta and drive a ground hugging debris flow outward without returning to the cavity flow separation creates an overriding run out ejecta flow further sustained by atmospheric turbulence and identified as radar dark lobate lobes sixth radar dark parabolas are proposed to be late time fallout deposits created as the downrange fireball evolves aloft perhaps analogous to terrestrial tektite strewn fields and seventh the response of crater formation to the atmosphere is conversely expressed by a reduction in cratering efficiency as revealed by the unusually large central peak complexes and the unexpected diameter depth relations of craters hence surface ages may be significantly underestimated

trajectories and distribution of material ejected from the chicxulub impact crater implications for postimpact wildfires 1 the trajectories of low and high energy ejecta from the chicxulub impact crater have been computed using a numerical code and compared with analyses of debris deposited in k t boundary sections these calculations indicate that low energy ejecta produces centimeter to meter thick deposits within 4000 km of the point of impact although its azimuthal distribution depends on the incident angle of the projectile with the surface of the earth the high energy ejecta or vapor plume rises through the atmosphere and isotropically expands at the top of the atmosphere the velocity of this ejecta increases as it rises approximately 12 of the high energy ejecta is lost because it reaches escape velocities while 25 of the material reaccretes within 2 hours 55 reaccretes within 8 hours and 85 reaccretes within 72 hours this ejecta is distributed globally but it is concentrated around the chicxulub impact site and at the antipode corresponding to india and the indian ocean 65 million years ago it is also slightly smeared in a longitudinal direction because of earth s rotation because this debris does not reaccrete all at once the collisional shock heating of the atmosphere will be drawn out over a period of a few days and will occur in pulses this could be an important factor when calculating postimpact chemical reactions in the stratosphere the calculations also indicate the ejecta would have ignited wildfires on several continents around the world although the distribution of those fires depends partly on the trajectory of the projectile

implications from stereo derived topography of venusian impact craters using radargrammetry we have created high resolution topographic maps of 74 venusian craters including all bright floored craters over 12 km in diameter covered by magellan stereo imagery our trend for rim floor depths rf as a function of diameter d for bright floored craters in the volcanic plains is rf 0 345 0 05 d0 235 0 05 and for dark floored craters in the plains rf 0 5 0 1 d0 0035 0 07 rim heights rh for bright floored craters in the plains are rh 0 06 0 03 d0 4 0 2 d 15 km and for dark floored craters in the plains rh 0 027 0 015 d0 4 0 2 these trends indicate that bright floored craters 30 km in diameter are on average deeper than dark floored craters by 180 m from rim to floor and have a 140 m higher rim and at 90 km in diameter they are 380 m deeper from rim to floor and have a 220 m higher rim the bright and dark floored populations are different at a 99 confidence level for both rim floor depths and rim heights the interpretation most consistent with our data and previous work by others is that venusian craters with radar dark floors have been partially filled and had their ejecta blankets embayed by regional scale lava flooding when the topography is examined in conjunction with the imagery it is clear that many dark floored craters have been surrounded by lavas that rose nearly to the crater rim even though a substantial portion of the crater s ejecta blanket was retained most previous analyses of the resurfacing history of venus have relied on past interpretations that only a small percentage of venusian craters are embayed by exterior volcanism because most craters on venus have dark floors our data indicate that the majority of venusian craters have been surrounded and partially filled by postimpact lavas and consequently those previous analyses may have significantly underestimated the amount of volcanism on the venusian surface over the past few hundred million years rim floor depths for venusian craters are consistent with the inverse gravity trend observed for the terrestrial planets and they are 50 deeper than current estimates for complex craters on the earth unlike the other terrestrial planets neither terrain floor depths nor central structure heights increase with increasing crater diameter an interesting trend for which we have no explanation is that on venus the moon mars and ganymede central peaks generally rise to within a constant elevation relative to the surrounding terrain but that elevation is lower on the moon and mars than on venus and ganymede

tilt observations using borehole tiltmeters 2 analysis of data from yellowstone national park we have installed borehole tiltmeters at five sites in yellowstone national park wyoming and have used these instruments to measure the spatial variation of the amplitude and phase of the principal semidiurnal tide the measured tides vary both with position and azimuth and differ from the sum of the body tide and the ocean load by up to 50 the difference predicted by a finite element model constructed from seismic refraction and gravity data has a maximum value of only 12 although the discrep ancy between our observations and the model is only marginally significant at some sites the disagreement between the model and our observa tions is much larger than we observed using the same instruments at other sites and cannot be attributed to an instrumental effect we have been unable to modify the model to explain our results while keeping it consistent with the previous observations

low angle collision with earth the elliptical impact crater matt wilson northern territory australia nearly all meteorite impact craters on earth are circular however 4 of craters should be formed by impacts at angles lower than 12 from the horizontal which should result in elongated crater structures the crater forming process that produces elliptical shapes is poorly understood we document the first elliptical crater on earth that contains a central uplift and that provides insights into the mechanisms of crater formation at a critical threshold angle of 10 15 the dimensions of the proterozoic matt wilson impact structure northern territory australia are 7 5 by 6 3 km corresponding to an aspect ratio of 1 2 with its long axis trending northeast southwest the exposed crater floor shows a preferred stacking of thrust sheets within the central uplift and in the surrounding syncline indicating northeast southwest shortening and a material transport top to the sw this is consistent with an up range to down range motion of rock caused by remnant horizontal momentum transferred from the impacting projectile to the target this preferential deformation interferes with a radially oriented convergent material flow characteristic for crater collapse the matt wilson crater provides evidence for the usefulness of structural asymmetries as a diagnostic tool to infer impact vectors the new impact crater is confirmed by the presence of planar deformation features planar fractures in quartz grains and its structural inventory

large enigmatic crater structures offshore southern california summary digital mosaics of swath and conventional bathymetry data reveal large distinct near circular crater structures in the inner continental borderland offshore southern california two have maximum crater diameters that exceed 30 km and a third has a crater diameter of approximately 12 km all three features exhibit the morphology of large complex craters raised outer rim ring moat and central uplift yet their exact origin remains a mystery preliminary analyses of available seismic gravity and magnetic data over these structures reveal both similarities and distinct differences in geometry structure and geophysical signature to known impact sites all three crater structures however occur within the catalina terrane a highly extended volcanic and metamorphic province floored by catalina schist basement a likely alternative origin may thus involve explosive volcanism caldera collapse and resurgent magmatism and or possibly plutonism and schist remobilization associated with the catalina terrane no single model for crater formation whether impact caldera or pluton fully accounts for all of the present observations regarding the morphology internal structure and known geology of these near circular features timing of crater formation post dates the initial rifting and rotation of the western transverse ranges and appears to predate major right slip along the san clemente and san diego trough fault systems or approximately 18 to 16 ma regardless of their origin these complex craters represent some of the largest structures of their kind in western north america and provide a unique opportunity to better understand the development of unusual crater structures in a submarine environment

geomorphologic mapping of the lunar crater tycho and its impact melt deposits abstract using selene kaguya terrain camera and lunar reconnaissance orbiter camera lroc data we produced a new high resolution 10 m pixel geomorphological and impact melt distribution map for the lunar crater tycho the distal ejecta blanket and crater rays were investigated using lroc wide angle camera wac data 100 m pixel while the fine scale morphologies of individual units were documented using high resolution 0 5 m pixel lroc narrow angle camera nac frames in particular tycho shows a large coherent melt sheet on the crater floor melt pools and flows along the terraced walls and melt pools on the continuous ejecta blanket the crater floor of tycho exhibits three distinct units distinguishable by their elevation and hummocky surface morphology the distribution of impact melt pools and ejecta as well as topographic asymmetries support the formation of tycho as an oblique impact from the w sw the asymmetric ejecta blanket significantly reduced melt emplacement uprange and the depressed uprange crater rim at tycho suggest an impact angle of 25 45

ejecta formation and crater development of the mjolnir impact crater ejecta correlation is an important element in the analysis of crater formation and its influence on the geological evolution in this study both the ejecta distribution and the internal crater development of the jurassic cretaceous mjolnir crater 40 km in diameter located in the barents sea are investigated through numerical simulations the simulations show a highly asymmetrical ejecta distribution and underscore the importance of a layer of surface water in ejecta distribution as expected the ejecta asymmetry increases as the angle of impact decreases the simulation also displays an uneven aerial distribution of ejecta the generation of the central high is a crucial part of crater formation in this study peak generation is shown to have a skewed development from approximately 50 90 sec after impact when the peak reaches its maximum height of 1 1 5 km during this stage the peak crest is moved about 5 km from an uprange to a downrange position ending with a final central position which has a symmetrical appearance that contrasts with its asymmetrical development

regolith behaviour and physical weathering of clayey mudrock as dependent on seasonal weather conditions in a badland area at vallcebre eastern pyrenees abstract studies on badland areas in the vallcebre basin pyrenees suggest that erosion rates are controlled by the weathering rate of mudrocks to obtain the temporal pattern of physical weathering and its control on the erosive processes monitoring of regolith temperatures at different aspect depth and lithology and periodical determination of regolith moisture and bulk density were carried out changes in surface regolith have also been monitored by means of photographic techniques using an especially designed tripod finally the hydrological and erosive response of the regolith trough the seasons were studied using rainfall simulations the results obtained support and confirm previous knowledge on the role of frost action in mudrock weathering and the role of subsequent regolith development and wetness status on infiltration and runoff generation these seasonal changes of behaviour are so important that they raise serious questions about the validity of observations or experiments carried out in only one season this work confirms that erosion processes are controlled by regolith formation during winter and regolith depletion by storm rainfall erosion during summer spring and early autumn it also raises important points about annual trends of infiltration and erodibility that should be used as guidelines for modelling runoff and sediment production

investigating the lake bosumtwi impact structure insight from numerical modeling 1 the current topography of the lake bosumtwi crater and some of its structural dimensions have been determined by geophysical methods we combine these data with sophisticated numerical models to evaluate the cratering process itself for example melt and tektite generation as well as to test the modeling code the geophysical maps show some asymmetry in plan view with the main anomaly north of the crater center the simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions while the late stage is modeled for the vertical impact without any asymmetry we estimate the projectile size from scaling laws and then varying material properties reproduce a crater which is similar to the bosumtwi but too deep bulking allows us to reconcile differences between the model results and the observed topography shock melt estimates are in good agreement with the bosumtwi magnetic signature modeled distribution of tektites assumes an impact angle of 30 45 and an impact direction from the n ne the combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well suited drill sites besides the central uplift and the annular moat with a suggested thick breccia cover drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact induced asymmetries

earthquake geodesy and hazard monitoring catastrophic events such as major earthquakes and volcanic eruptions result when the earth s crust fails in response to accumulated deformation the accumulation of deformation results from ongoing processes such as aseismic deformation of subcrustal rock associated with relative plate motions or the ascent of magma through a volcanic plumbing system geodetic measurements document the crustal deformation leading to these failures and the deformation resulting from them providing unique insight into the physical processes involved by virtue of the space and time domains of ground deformation detected with geodetic measurements geodesy is a useful tool in monitoring active tectonic and volcanic regimes for a wide range of natural events including earthquakes aseismic fault motions and volcanic events geodetic measurements constrain physical models of the processes that cause such events because these geodetic studies probe the underlying processes leading to and culminating in catastrophic natural events many such studies are directly applicable to earthquake and volcano hazard assessment

cretaceous tertiary chicxulub impact angle and its consequences the chicxulub impact structure exhibits asymmetries in its geophysical signatures that closely resemble asymmetries produced by oblique impacts in laboratory experiments and recognized on planetary surfaces these asymmetric signatures suggest a trajectory for the chicxulub bolide from the southeast to the northwest at a 20 30 angle from the horizontal as a result biotic extinctions may have been most severe and catastrophic in the northern hemisphere geographic variation in the magnitude of the cretaceous tertiary k t fern spike and palynofloral extinctions are consistent with the proposed trajectory

groundwater control on the suspended sediment load in the na borges river mallorca spain abstract groundwater dominance has important effects on the hydrological and geomorphological characteristics of river systems low suspended sediment concentrations and high water clarity are expected because significant inputs of sediment free spring water dilute the suspended sediment generated by storms however in many mediterranean rivers groundwater dominance is characterised by seasonal alternations of influent and effluent discharge involving significant variability on the sediment transport regimes such areas are often subject to soil and water conservation practices over the centuries that have reduced the sediment contribution from agricultural fields and favour subsurface flow to rivers moreover urbanisation during the twentieth century has changed the catchment hydrology and altered basic river processes due to its flashy regime in this context we monitored suspended sediment fluxes during a two year period in the na borges river a lowland agricultural catchment 319 km 2 on the island of mallorca balearic islands the suspended sediment concentration ssc was lower when the base flow index i e relative proportion of baseflow compared to stormflow bfi was higher therefore strong seasonal contrasts explain the high ssc coefficient of variation which is clearly related to dilution effects associated with different groundwater and surface water seasonal interactions a lack of correlation in the q ssc rating curves shows that factors other than discharge control sediment transport as a result at the event scale multiple regressions illustrate that groundwater and surface water interactions are involved in the sedimentary response of flood events in the winter the stability of baseflow driven by groundwater contributions and agricultural and urban spills causes hydraulic variables i e maximum discharge to exert the most important control on events whereas in the summer it is necessary to accumulate important volumes of rainfall creating a minimum of wet conditions in the catchment to activate hydrological pathways and deliver sediment to the drainage network the bfi is also related to sediment delivery processes as the loads are higher with lower bfi corroborating the fact that most sediment movement is caused by stormflow and its related factors overall suspended sediment yields were very low i e 2 yr 1 at all measuring sites such values are the consequence of the limited sediment delivery attributable to soil conservation practices low surface runoff coefficients and specific geomorphic features of groundwater dominated rivers such as low drainage density low gradient steep valley walls and flat valley floors

the sediment budget of a highly dynamic mesoscale catchment the river isabena the paper presents the sediment budget of the isabena basin a highly dynamic 445 km2 catchment located in the central pyrenees that is patched by highly erodible areas i e badlands the budget for the period 2007 2009 is constructed following a methodology that allows the interpolation of intermittent measurements of suspended sediment concentrations and enables a subsequent calculation of sediment loads data allow specification of the contribution of each subbasin to the water and sediment yield in the catchment outlet mean annual sediment load was 235 000 t y 1 specific sediment yield reached 2000 t km 2 y 1 a value that indicates very high sedimentary activity especially in the case of villacarli and lascuarre subcatchments were most badlands are located the specific sediment yield obtained for the entire isabena is 527 t km 2 y 1 a high value for such a mesoscale basin results show that a small part of the area i e 1 controls most of the catchment s gross sediment contribution sediment delivery ratio ratio between sediment input from primary sources and basin export has been estimated at around 90 while in channel storage represents the 5 of the annual load on average the high connectivity between sediment sources i e badlands and transfer paths i e streamcourses exacerbates the influence of the local sediment production on the catchment s sediment yield a quite unusual fact for a basin of this scale

crustal deformation of the yellowstone caldera from first gps measurements 1987 1989 a global positioning system gps network was established in 1987 in order to measure crustal deformation across the yellowstone plateau centered on the yellowstone caldera and adjacent teton wyoming and hebgen lake montana fault zones forty four gps stations in the yellow stone network were re surveyed in 1989 and station coordinates using broadcast orbits were obtained these results were used to assess the precision and accuracy of the gps surveys and to identify areas of rapid crustal deformation on the yellowstone plateau the gps measured displacements between 1987 and 1989 were calculated relative to a station at the eastern edge of the 100 km wide network the relative horizontal displacements have an average magnitude of 15 10 mm the relative vertical displacements are generally less than 25 mm about an average near zero and with an uncertainty of 20 mm comparisons with leveling and trilateration measurements taken nearly simultaneously suggest that the accuracy of these gps measurements is at the level of the above uncertainties gps measured vertical displacements across the northern part of the yellowstone caldera and 1st order leveling results show up to 20 mm of subsidence between 1987 and 1989 this subsidence is part of a six year period of caldera deflation that began in 1985 in addition up to 66 mm of uplift is observed with gps at the northwest edge of the yellowstone caldera and may be associated with processes following an intense earthquake swarm in 1985 86 and with caldera subsidence

the shape and appearance of craters formed by oblique impact on the moon and venus we surveyed the impact crater populations of venus and the moon dry targets with and without an atmosphere to characterize how the 3 dimensional shape of a crater and the appearance of the ejecta blanket varies with impact angle an empirical estimate of the impact angle below which particular phenomena occur was inferred from the cumulative percentage of impact craters exhibiting different traits the results of the surveys were mostly consistent with predictions from experimental work assuming a sin2 dependence for the cumulative fraction of craters forming below angle on the moon the following transitions occur 45 degrees the ejecta blanket becomes asymmetric 25 degrees a forbidden zone develops in the uprange portion of the ejecta blanket and the crater rim is depressed in that direction 15 degrees the rim becomes saddle shaped 10 degrees the rim becomes elongated in the direction of impact and the ejecta forms a butterfly pattern on venus the atmosphere causes asymmetries in the ejecta blanket to occur at higher impact angles the transitions on venus are 55 degrees the ejecta becomes heavily concentrated downrange 40 degrees a notch in the ejecta that extends to the rim appears and as impact angle decreases the notch develops into a larger forbidden zone 10 degrees a fly wing pattern develops where material is ejected in the crossrange direction but gets swept downrange no relationship between location or shape of the central structure and impact angle was observed on either planet no uprange steepening and no variation in internal slope or crater depth could be associated with impact angle on the moon for both planets as the impact angle decreases from vertical first the uprange and then the downrange rim decreases in elevation while the remainder of the rim stays at a constant elevation for craters on venus 15 km in diameter a variety of crater shapes are observed because meteoroid fragment dispersal is a significant fraction of crater diameter the longer path length for oblique impacts causes a correlation of clustered impact effects with oblique impact effects one consequence of this correlation is a shallowing of the crater with decreasing impact angle for small craters