effects of asteroid and comet impacts on habitats for lithophytic organisms a synthesis asteroid and comet impacts can have a profound influence on the habitats available for lithophytic microorganisms using evidence from the haughton impact structure nunavut canadian high arctic we describe the role of impacts in influencing the nature of the lithophytic ecological niche impact induced increases in rock porosity and fracturing can result in the formation of cryptoendolithic habitats in some cases and depending upon the target material an increase in rock translucence can yield new habitats for photosynthetic cryptoendoliths chasmoendolithic habitats are associated with cracks and cavities connected to the surface of the rock and are commonly increased in abundance as a result of impact bulking chasmoendolithic habitats require less specific geological conditions than are required for cryptoendolithic habitats and their formation is likely to be common to most impact events impact events are unlikely to have an influence on epilithic and hypolithic habitats except in rare cases where for example the formation of impact glasses might yield new hypolithic habitats we present a synthetic understanding of the influence of asteroid and comet impacts on the availability and characteristics of rocky habitats for microorganisms

impact craters and craterform structures in fennoscandia abstract sixty two nearly circular topographic morphological or geophysical structures in fennoscandia and estonia were studied to discover their origin the structures were divided into five classes fifteen were caused by meteorite impacts class a and a four of them lappajarvi janisjarvi mien and dellen contain large volumes of impact melt the age of the recognized impact craters varies from pre historic 3500 b c to 700 ma and their diameters from 0 04 km to 55 km the majority of the other structures are probable class b or possible class c craters for which there is not yet sufficient proof of impact origin there is increasing evidence that some of the large arcuate morphological or geophysical structures class e represent the deeply eroded scars of very old craters but owing to the lack of identified shock metamorphic features or impact generated rocks e g allogenic breccias they cannot so far be classified as impact craters so far no craterform structure or ejecta layer of archaean or early proterozoic age has been found in fennoscandia although statistically remnants of ancient cratering events should be found in the fennoscandian shield new ways of searching for these structures are proposed the impact cratering rate in fennoscandia is 2 4 10 14 km 2a 1 n 12 the very small craters are omitted which corresponds to about two events per 100 ma for the last 700 ma this is a minimum estimate and is higher than the global cratering rate

geophysical investigations of the lake lappajarvi impact structure western finland abstract a circular negative bouguer anomaly approximately 10 mgal in amplitude and 17 km in diameter is associated with the lappajarvi impact structure the related mass deficit is 440 10 11 kg there are irregularities in the anomaly pattern due to differentially faulted bedrock blocks and lateral density contrasts between bedrock shocked bedrock breccia suevite impact melt rock and overburden the minimum amount of impact melt rock was interpreted from the positive residual bouguer anomaly between the centre and the rim in the northwestern part of the crater as being 1 5 2 0 km 3 there are a few small aeromagnetic anomalies up to 200 nt at an altitude of 40 m within the crater which is mostly characterized by a lack of magnetized material strong magnetic anomalies a few thousand nt which are abundant in the surroundings disappear in the crater area the aero electromagnetic aem data show strong anomalies with inphase quadrature ratios close to 1 indicating moderately conducting material within the crater an electromagnetic wide band sampo em profile was measured across the annular depression between the centre and the rim of the crater three layers were interpreted the thickness of the uppermost layer mostly conductive overburden is about 100 200 m and the resistivity 60 170 m in the annular depression beneath the uppermost layer there is a 50 250 m thick layer of suevites and breccias with resistivity of about 17 50 m the resistive lowest layer follows the base of breccia and suevite some separate soundings were carried out near the centre of the crater according to one of these soundings there is a conductive layer of suevite and breccia with a resistivity of 30 40 m beginning at depth of 150 m beneath the exposed impact melt rock the results of geophysical interpretations such as the annular depression between the centre and the rim of the crater the small thickness diameter ratio of the geophysically anomalous part of the crater and the calculated amount of impact melt rock indicate that the lappajarvi structure belongs to the category of complex craters rhomboidal geophysical anomaly patterns in particular aem anomalies parallel to old bedrock structures suggest that the collapse of the transient crater and or subsequent crater modification were controlled by nw se and ne sw trending pre existing fracture and fault zones

plate motion on europa and nonrigid behavior of the icy lithosphere the castalia macula region we have developed a generalized quantitative technique for determining the finite pole of rotation between two rigid plates and use it to critically examine differing reconstructions of a region surrounding a prominent dark spot on europa castalia macula this region is located near the equator of europa s trailing hemisphere and has been suggested as a site where crustal convergence may have occurred previous reconstructions of the region have indicated that a ridge set and or a band like complex that define a collection of tectonic plates in the region accommodated surface contraction however a critical examination of the differences between these reconstructions has been complicated by the lack of a finite pole of rotation for the plates involved in either reconstruction we have applied our modeling technique coupled with a detailed examination of the morphology and cross cutting relationships involving this ridge set and band like complex to determine if a unique reconstruction exists for several tectonic plates in this region the cross cutting relationships involving the ridge set also allow us to test the general assumption that plates behave rigidly on europa assuming rigid behavior our results suggest that a unique reconstruction does exist indicating the ridge set accommodated surface contraction however analysis performed to test the assumption of plate rigidity indicates that one or more of the plates in the region did not behave rigidly this does not rule out surface contraction along the ridge set but does indicate that a component of nonrigid behavior must be considered

geological and geophysical studies of the agoudal impact structure central high atlas morocco new evidence for crater size and age since the discovery of shatter cones scs near the village of agoudal morocco central high atlas mountains in 2013 the absence of one or several associated circular structures led to speculation about the age of the impact event the number and the size of the impact crater or craters additional constraints on the crater size age and erosion rates are obtained here from geological structural and geophysical mapping and from cosmogenic nuclide data our geological maps of the agoudal impact site at the scales of 1 30 000 6 km 2 and 1 15 000 2 25 km 2 include all known occurrences of scs in target rocks breccias and vertical to overturned strata considering that strata surrounding the impact site are subhorizontal we argue that disturbed strata are related to the impact event three types of breccias have been observed two of them br1 2 and br2 could be produced by erosion sedimentation consolidation processes with no evidence for impact breccias while breccia br1 might be impact related the most probable center of the structure is estimated at 31 degrees 59 13 73 n 5 degrees 30 55 14 w using the concentric deviation method applied to the orientation of strata over the disturbed area despite the absence of a morphological expression the ground magnetic and electromagnetic surveys reveal anomalies spatially associated with disturbed strata and sc occurrences the geophysical data the structural observations and the area of occurrence of scs in target rocks are all consistent with an original size of 1 4 4 2 km in diameter cosmogenic nuclide data cl 36 constrain the local erosion rates between 220 22 m ma 1 and 430 43 m ma 1 these erosion rates may remove the topographic expression of such a crater and its ejecta in a time period of about 0 3 1 9 ma this age is older than the agoudal iron meteorite age 105 40 kyr this new age constraint excludes the possibility of a genetic relationship between the agoudal iron meteorite fall and the formation of the agoudal impact site a chronolgy chart including the atlas orogeny the alternation of sedimentation and erosion periods and the meteoritic impacts is presented based on all obtained and combined data

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

magnetic investigations of the west hawk deep bay and clearwater impact structures canada due to the effects of erosion tectonism and burial impact structures are often obscured or destroyed geophysical methods are increasingly being used in detecting the signatures of impact structures while gravity lows associated with impact structures are well understood associated magnetic anomaly lows are not in this study drill cores from three canadian impact structures were analyzed for rock magnetic properties and mineralogy in order to explain the magnetic anomaly lows associated with these structures samples from the drill cores were cut and measured for anisotropy of magnetic susceptibility ams and natural remanent magnetization nrm parameters drill cores from the twin impact craters of the clearwater structure exhibited different nrm characteristics and samples from their respective drill cores were subject to demagnetization by alternating field and thermal techniques the difference noted in their nrm characteristics was attributed to the acquisition of a viscous remanent magnetization vrm at depth in clearwater east at all three structures both magnetic susceptibilities and remanent magnetizations are well below regional values in impact generated breccias melt rocks shocked crystalline rocks and in postimpact sedimentary infill the processes of brecciation alteration shock and infill by nonmagnetic sediments contribute to the development of the magnetic lows however a significant contribution to the observed magnetic anomalies was found by first order forward modelling to arise from basement rocks beneath the impact structures this zone of reduced magnetization may be caused by the partial demagnetization of magnetite by the impact induced transient stress wave traveling away from the point of impact

chaos on europa the characteristics of chaos regions on europa suggest they may be sites of melt through from below they are wide ranging in size location and age the largest are hundreds of kilometers across most are similar to conamara with a matrix reminiscent of frozen slush and often rafts of preexisting crust edges are of two types ramps perhaps the tapering of crustal thickness to zero or cliffs where rafts appear to have broken clear from the shore the small features called lenticulae generally appear to be small chaoses with textured matrix and occasional rafts and many domes may be small chaoses raised by isostatic compensation following refreezing of the crust the extent of chaoses often appears to be limited by ridge systems with the coastline parallel and set back by a distance comparable to the width of the ridge system preexisting ridges often survive as causeways or chains of rafts boundaries of chaoses are apparently not controlled by preexisting cracks consistent with formation by a thermal rather than mechanical process ridges may thicken the crust such that melt through is more likely but not always between ridge systems subsequent cracks and ridges form across preexisting chaoses ranging from fresh cases with few cracks or ridges across them with paths somewhat jagged as they meander among rafts to heavily dissected examples isolated tilted raft like blocks surrounded by densely ridged terrain may be relics of former chaotic terrain thus two fundamental resurfacing processes have alternated over europa s geological history melt through at various places and times forming chaos terrain and tectonic cracking and dilation building densely ridged and banded terrain mapping of chaos features based on morphology at 200 m shows that they correlate albeit imperfectly with dark regions in global 2 km resolution mosaics except dark regions due to ridge margins or craters extrapolating from our mapping of the 5 of europa covered by appropriate images at least 18 of the surface of europa is fresh appearing chaos an additional 4 is slightly modified chaos and much more older chaotic terrain has been overprinted by tectonic structures considerable area has been available globally to accommodate the expansion of crust that occurs along extensional ridges and bands chaos ubiquity suggests that europan geology has been dominated by the effects of having liquid water under a very thin ice shell with chaos regions being widespread indicators of occasional zero shell thickness

strike slip duplexing on jupiter s icy moon europa agenor linea is a 1500 km long 20 30 km wide geologically young zone of deformation on jupiter s icy moon europa on the basis of recent galileo high resolution images we interpret agenor linea as a strike slip zone formed in three stages by a combination of lithospheric separation extension and dextral horizontal shear agenor linea exhibits excellent examples of strike slip duplexes in an icy lithosphere unobscured by vegetation and unaltered by erosion

ordovician impact crater at kardla hiiumaa island estonia abstract the buried and well preserved kardla crater in estonia southern fennoscandia 58 59 22 40e with a diameter of approximately 4 km was discovered in 1967 the subsurface structure of the crater has been studied by gravity anomaly measurements aeromagnetics and by more than 300 boreholes in the crater on the rim ring wall and in the surrounding area recent boreholes reaching 800 m have revealed that the crater is 540 m deep and has 100 m high central peak barely visible in the present topography the crater is filled with palaeozoic and quaternary deposits and is mostly low marshy land surrounded by a low ridge along the ring wall quaternary landforms such as eskers and post glacial beach ridges also complicate the topography in the buried sub quaternary bedrock relief the crater occurs as a roundish depression bordered with two 10 30 m high horseshoe shaped elevations the bedrock topography was slightly drumlinized by glacial excavation the crater formed 455 ma in composite stratigraphy middle and lower ordovician 20 m and cambrian 120 m sedimentary rocks overlying crystalline basement on the bottom of a shallow nearshore sea the subsurface structure of the crater is well preserved in vertical section from the top are 1 post impact cover of ordovician sedimentary rocks 15 100 m thick 2 allochthonous breccias filling the lower part of the crater and beds of fall out breccias and conglomerates sandstones and sandy limestones consisting of debris of reworked fall out breccia and wall rocks surrounding the crater 3 a body of autochthonous and subautochthonous breccias forming the bottom and the central peak of the crater and also remnants of its rim shocked rocks and minerals from autochthonous and allochthonous breccias have been revealed by optical microscopy

geophysical research on the kardla impact structure hiiumaa island estonia the 4 km wide and 500 m deep circular kardla impact structure in hiiumaa island estonia of middle ordovician age 455 ma is buried under upper ordovician and quaternary sediments to constrain the geophysical models of the structure petrophysical properties such as magnetic susceptibility natural remanent magnetization nrm density electrical conductivity porosity and p wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area the results were used to interpret the central gravity anomaly of 3 mgal and the magnetic anomaly of 100 nt and also the surrounding weak positive anomalies revealed by high precision survey data the unshocked granitic rocks outside the structure have a mean density of 2630 kgm 3 their shocked counterparts have densities of 2400 kgm 3 at a depth of 500 m increasing up to 2550 kgm 3 at a depth of 850 m porosity and electrical conductivity decrease but p wave velocity increases as density increases away from the impact point thus the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for kardla as in many other impact structures the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks a new geophysical and geological model of the kardla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks together with structural data from numerous boreholes an important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores

large impact crater in the barents sea seismic profiles in the central barents sea record a conspicuous structure exhibiting gross morphological and structural characteristics closely resembling those of large impact craters this feature named the mjolnir structure is 39 km in diameter and of jurassic early cretaceous age it probably formed by the impact of an asteroid or comet 0 7 2 5 km in diameter into a shallow sea underlain by 5 km of sedimentary section if the impact interpretation is correct mjolnir is one of the largest well preserved craters in the errestrial record

new way of processing near surface magnetic data the utility of the comprehensive model of the magnetic field in assembling near surface magnetic surveys e g airborne or marine data for regional geologic studies one often has a problem in properly merging different surveys without applying ad hoc leveling methods or warping the long wavelengths of individual data sets a new comprehensive long wavelength and long time span magnetic field model based on satellite and observatory data may at last resolve this problem the model the comprehensive model cm is developed by terry sabaka of raytheon itss nasa and nils olsen of the danish space research institute the present version of the model cm3 incorporates data from magnetic field satellites pogo 1965 1970 magsat 1979 1980 orsted 2000 to present and champ 2001 to present and magnetic observatory data from the early 1960s to 2002

forming europa s folds strain requirements for the production of large amplitude deformation abstract the surface of jupiter s iconic icy satellite europa is dominated by tectonic features that have formed in extension in contrast features that have formed in contraction appear to be rare thus there exists an apparent imbalance between the observed surface expression and relative magnitudes of extensional and contractional strain in this paper we reexamine the amount of strain that can be accommodated during fold formation on europa using a two dimensional finite element model that allows the examination of fold formation at large strains and under a broad range of initial conditions we find that in contrast to previous assessments relatively large strains can be accommodated by the development of relatively low amplitude folds under our nominal conditions creating peak to trough fold amplitudes of 200 m consistent with europa s astypalaea folds requires 7 contraction of the lithosphere decreasing either the strain rate or the ice grain size allows the formation of larger amplitude folds at smaller strains but we find no cases where 200 m amplitude folds form at less than 3 strain only if both of these conditions are met simultaneously do large amplitude folds form at contractional strains 3 the requirement of larger strains relative to previous analytical models results from the combined effects of an initial period of fold nucleation generally 1 2 contractional strain in which limited amplification occurs and smaller amplitude growth rates overall the requirement of several percent strain to produce even moderate amplitude folds is consistent with previous numerical models of single layer and lithospheric scale viscoelastic folding under terrestrial conditions because the formation of large amplitude folds requires relatively large strain small but significant amounts of contraction can be accommodated predominantly through uniform thickening of the lithosphere during the nucleation phase of fold growth without the formation of obvious surface deformation the accommodation of contractional strains via such pre folding lithospheric thickening provides at least a partial solution to europa s apparent strain imbalance

jackpine creek magnetic anomaly identification of a buried meteorite impact structure the meteorite impact process is quite a remarkable natural phenomenon when it occurs in a sedimentary basin it can create a trap generating structure rim and central uplift and reservoir rocks ejecta piles and breccia infill both at the same time

geophysical methods for kimberlite exploration in northern canada in 1991 the discovery of diamondiferous kimberlite in canada s northwest territories nwt precipitated the largest mineral staking rush in north america and sparked a remarkable exploration effort which extended across northern canada thirteen years later two diamond mines with estimated annual productions of 1 billion can are in operation a third is in permitting and several other projects are approaching development ekati mine operated by bhp billiton since 1998 produced 4 8 million carats from 4 2 million tons of ore in 2003 while the nearby diavik diamond mine yielded 2 7 million carats from 1 0 million tons of ore during the same period when diavik has achieved full production in 2004 canada will account for 15 of global diamond output by value the exploration programs which created this new industry relied heavily upon the efficient application of complementary geophysical techniques this paper overviews geophysical exploration methods currently used in diamond exploration in northe

geophysical signature of malingen the minor crater of the lockne malingen doublet impact structure malingen is the 0 7km wide minor crater associated to the 10 times larger lockne crater in the unique lockne malingen doublet the craters formed at 458ma by the impact of a binary asteroid related

fault offsets and lateral crustal movement on europa evidence for a mobile ice shell abstract right lateral structural offsets of 25 km have been identified on the icy galilean satellite europa these occur along dark lineaments oriented orthogonally to wedge shaped bands which are also 25 km wide wedge shaped bands are interpreted as dilated tension fractures which formed as crustal blocks or plates 50 100 km across separated and slipped past each other along flanking strike slip faults this style of deformation does not appear to be characteristic of other lineament types and with the exception of earth appears to be unique to europa together the subparallel wedge shaped b bands form a broad nw se trending belt 1500 km long and less than 500 km across near the anti jovian point this belt is interpreted as a major crustal fracture or rift zone with a pole of rotation determined by the strike slip faults near 47 s 144 w and an approximate ne sw direction of maximum tensile stress extension may have been areally compensated at agenor linea a bright band of possible compressional origin global expansion tidal distortion and nonsynchronous rotation do not explain the inferred minimum principal i e least compressive stress directions alternatively fracturing near the anti jovian point may be a result of i solid state convection in the lower ice crust possibly triggered by uneven heat flow from the silicate interior ii rotation of the icy shell about the sub and anti jovian points induced by latitudinal lithospheric thickness variations or iii preferential strain accumulation from the rest of the icy shell no significant distortion of the crustal blocks occurred during fracturing and rotation indicating that the icy crust was probably mechanically decoupled from the silicate interior in this region over the time scale of fracturing decoupling on a global scale is also likely the simple geometry of lineaments argues for formation in an icy lithosphere not a silicate one and other evidence for fracturing caused by nonsynchronous rotation stress is not compatible with a tidally locked silicate interior which we show is likely unless the ice shell rotates independently decoupling could have been due to either warm ice or liquid water near the base of the icy crust mechanical bounds on lithospheric thickness a few to 10 km lead to heat flow estimates that admit both possibilities but favor decoupling by liquid water

polar wander and surface convergence of europa s ice shell evidence from a survey of strike slip displacement two global issues regarding europa are addressed by a survey of strike slip faults first a common type of terrain that appears to represent convergent sites of surface removal which may help compensate for substantial widespread dilation along tectonic bands elsewhere thus helping resolve the problem of conserving global surface area is identified second evidence for polar wander may provide the first confirmation of that theoretically predicted phenomenon these results among others come from an extensive survey of strike slip faults over the portion of the surface where galileo images at 200 m pixel resolution were obtained for regional mapping purposes the images cover two broad swaths that run from the far north to the far south one in the leading hemisphere and the other in the trailing hemisphere among the faults that have been mapped are a fault 170 km long with a strike slip offset of 83 km the greatest yet identified on europa and a quasi circular strike slip fault that surrounds a 500 km wide plate which has undergone rotation as a rigid unit reconstruction of specific examples of strike slip reveals sites of lateral convergence because europa is unique in many ways these sites are not similar to compression features on other bodies which may explain why they had previously been difficult to identify the distribution of strike slip in both hemispheres when compared with predictions of the theory of tidal walking provides evidence for polar wander the crust of europa appears to have slid as a single unit relative to the spin axis such that the site on the crust that was previously at the north rotational pole has wandered probably during the last few million years to a location currently in the leading hemisphere about 30 away from the spin axis such polar wander probably also explains symmetry patterns in the distribution of chaotic terrain pits and uplift features

palaeomagnetism of the lappajarvi impact structure western finland abstract palaeomagnetic and rock magnetic results are presented for rocks from the lappajarvi meteorite impact structure western finland oriented samples were collected from the melt rocks in the central island of the crater and of the palaeoproterozoic svecofennian target rocks at the crater rim the natural remanent magnitization nrm of the melt rocks is generally weak and metastable during demagnetization the characteristic nrm component carried by pyrrhotite yields a palaeomagnetic pole latitude 55 4 n longitude 152 6 e dp 6 2 dm 8 7 dg 6 sites suggesting an age of 195 ma for the impact the results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations the palaeomagnetic age differs from the 40ar 3939ar age of 77 ma for the impact assuming that the 40ar 39ar age is correct the discrepancy between palaeomagnetic and radiometric ages is best explained by post impact and post cooling tilting of the melt layer the tilting 15 may be due to isostatic rebounds along joints in the melt impact remanence has also been isolated in a few target rocks at the rim this poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1 9 ga old svecofennian direction this interpretation is supported by the observation that the nrm intensity is enhanced in specimens having the impact overprint the palaeomagnetic results are consistent with earlier interpretations based on geophysical morphological mineralogical and geochemical data that the lappajarvi structure is a complex meteorite impact site less than 200 ma old

shock experiments in range of 10 45 gpa with small multidomain magnetite in porous targets physical properties of multidomain magnetite bearing porous pellets shocked up to 45 gpa were measured the results show general magnetic softening as a result of shock however a relative magnetic hardening trend and slight magnetic susceptibility decrease is observed with increasing pressure among shocked samples initially the shock also seems to cause a slight decrease in porosity but at higher shock pressures macroscopic porosity increases progressively in our pellets the microscopic porosity remains almost unchanged since our samples have distinctly higher initial porosity compared with samples used in previous studies our results may be representative for impacts into highly porous magnetite bearing sedimentary or volcanic rocks and are relevant to impacts into such target rocks on earth and mars

true polar wander on europa from global scale small circle depressions the tectonic patterns and stress history of europa are exceedingly complex and many large scale features remain unexplained true polar wander involving reorientation of europa s floating outer ice shell about the tidal axis with jupiter has been proposed as a possible explanation for some of the features this mechanism is possible if the icy shell is latitudinally variable in thickness and decoupled from the rocky interior it would impose high stress levels on the shell leading to predictable fracture patterns no satisfactory match to global scale features has hitherto been found for polar wander stress patterns here we describe broad arcuate troughs and depressions on europa that do not fit other proposed stress mechanisms in their current position using imaging from three spacecraft we have mapped two global scale organized concentric antipodal sets of arcuate troughs up to hundreds of kilometres long and 300 m to approximately 1 5 km deep an excellent match to these features is found with stresses caused by an episode of approximately 80 degrees true polar wander these depressions also appear to be geographically related to other large scale bright and dark lineaments suggesting that many of europa s tectonic patterns may also be related to true polar wander

microscopic planar deformation features in quartz of the vredefort structure anomalous but still suggestive of an impact origin abstract the orientation and distribution of diagnostic shock produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the vredefort structure in south africa these features are widespread occurring in all but four samples the majority of the planar features have a basal 0001 orientation with a few higher pressure so called features and three features occurring near the center of the structure their relative distribution and preservation are however anomalous compared to those at other large terrestrial impact structures and recorded shock pressures calculated on the basis of planar feature orientation do not show a regular decrease radially outward as observed at other known impact structures the apparently anomalous development of planar features is interpreted as a result of post shock recrystallization if the effect of recrystallization of quartz which generally increases inward toward the core of the structure is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure then higher pressures are recorded in the core compared to the collar rocks no evidence of a previously reported additional post recrystallization shock event is found in this study although previous researchers report that pseudotachylite which may or may not be associated with the shock event cuts recrystallized quartzites in is concluded that although anomalous the evidence from planar features in quartz is still consistent with the vredefort structure being an erosional remnant of a large complex impact structure

a unified nomenclature for tectonic structures on the surface of enceladus abstract enceladus has experienced widespread and diverse tectonic deformation the diversity and nonuniform distribution of structures visible on the surface attests to a complicated and perhaps long lived tectonic history currently no fundamental classification scheme based on morphology exists for the tectonic structures on enceladus which limits analysis and discussion of tectonic structures and regional and global tectonic histories characterization and classification of structures on enceladus is an important first step in understanding its tectonic history here we propose a global morphologic i e nongenetic classification scheme for tectonic structures on enceladus five classes of tectonic structures on enceladus have been identified based on images and limited topographic data troughs scarps chasmata ridges and bands morphological variation of structures exists within these classes reinforcing the complicated tectonic history of this small body

the evil twin of agenor tectonic convergence on europa the dilemma of the surface area budget on europa is resolved by identification of sites of crustal convergence which have balanced the continual and common creation of new surface along dilational bands and pull aparts convergence bands are characterized by a distinctive albeit subdued morphology the prominent unusual lineament agenor is one of several examples we also find diametrically opposite agenor a similar bright linear feature surrounded by markings that allow reconstruction which shows it to be a convergence feature until recently identification of convergence sites was difficult because these features are subtle and do not exhibit structures like the himalayas or plate subduction familiar from convergence of thick solid crusts on terrestrial planets

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

lithospheric dilation on europa abstract lithospheric dilation on europa has occurred at ridges bands and various hybrid lineaments on a global scale over a large part of the geological age of the surface dilational ridges class 2 in the r greenberg et al 1998 icarus 135 64 78 taxonomy are elevated are usually a few kilometers across and may have a lineated or hummocky interior and a pronounced medial groove bands are lower and usually wider than class 2 ridges and may have a lineated interior with no prominent medial groove some lineaments have characteristics of both ridges and bands the character of class 2 ridges bands and hybrid forms suggests that they are dilational gaps in the lithosphere filled from below and that they constitute a morphological continuum with class 2 ridges and bands as end members these relationships may be explained by a model in which external forcing superimposes a secular dilation on the tidal cycle that opens and closes cracks each europan day resulting in incomplete closure with accumulation and possible extrusion of new ice fill where the lineament ultimately falls on the morphological continuum especially how much it is elevated above ambient terrain depends upon the ratio of daily secular dilation to the amplitude of the cyclic tidal separation we call this ratio the dilation quotient changes in the dilation quotient during the active life of the lineament will create variable lineament forms one driver for dilation is tidal walking of strike slip faults which dilates linked nonparallel cracks that process is prominent in the 800 km long strike slip fault astypalaea linea a subsurface liquid water ocean allows the decoupling needed for horizontal displacements and is the source for the ice that fills the dilated lineaments

physical properties of the yaxcopoil 1 deep drill core chicxulub impact structure mexico abstract the chicxulub structure in mexico one of the largest impact structures on earth was formed 65 ma by a hypervelocity impact that led to the large mass extinction at the k pg boundary the chicxulub impact structure is well preserved but is buried beneath a sequence of carbonate sediments and thus requires drilling to obtain subsurface information the chicxulub scientific drilling program was carried out at hacienda yaxcopoil in the framework of the international continental scientific drilling program in 2001 2002 the structure was cored from 404 m down to 1511 m through three intervals 794 m of postimpact tertiary sediments a 100 m thick impactite sequence and 616 m of preimpact cretaceous rocks thought to represent a suite of megablocks physical property investigations show that the various lithologies including the impactite units and the k pg boundary layer can be characterized by their physical properties which depend on either changes in fabric or on mineralogical variations the magnetic properties show mostly dia or paramagnetic behavior with the exception of the impactite units that indicate the presence of ferromagnetic probably hydrothermally deposited magnetite and pyrrhotite the magnetic fraction contributes mainly to enhanced magnetization in the impactite lithologies and in this way to the observed magnetic anomalies the shape and orientation of the magnetic grains are varied and reflect inhomogeneous fabric development and the influence of impact related redeposition and hydrothermal activity the chicxulub impact occurred at the time of the reverse polarity geomagnetic chron 29r and this finding is consistent with the age of the k pg boundary

evidence for separation across a gray band on europa abstract thynia linea a gray band on europa is found to be a 25 km wide and 900 km long region of lithospheric separation which has been infilled by relatively dark material roughly a dozen older features and the cuspate segments that form its outline appear to have been displaced across its width displacement azimuths indicate a best fit pole of opening near 79 s 200 w however displacement magnitude decreases toward either end of the gray band indicating that it is more akin to a tear in a nonrigid europan lithosphere opening was in response to nw se directed tensile stress in accord with the stress predictions of nonsynchronous rotation observations of thynia linea are consistent with a laterally mobile brittle lithosphere decoupled from ductile or liquid material below as previously suggested to account for opening of wedge shaped bands in europa s antijovian region lithospheric separation and contemporaneous emplacement of new material offers a possible volcano tectonic scenario for resurfacing europa if this process is ongoing resurfacing can be accomplished on a time scale consistent with the satellite s surface age if one gray band or zone of wedge shaped bands forms every 10 3 10 4 yr and becomes unrecognizable with age this would imply that features on europa brighten with age as through continuous deposition of frost onto the surface

multistage evolution and variable motion history of agenor linea europa abstract europa the second galilean moon of jupiter is composed of a silicate mantle and an ice shell which overlies a supposed subsurface ocean the surface of europa is scarred with fractures varying in morphology dimensions and geometry we focus on agenor linea a 1500 km bright band that extends across europa s antijovian to trailing southern hemisphere agenor linea is morphologically a band like strike slip fault however it experienced at least three evolutionary growth phases marked by three zones of varying albedo each with a different geological history structures within the band material and kinematic indicators are consistent with formation through oblique left lateral dilation followed by normal dilation and right lateral strike slip movement or combinations thereof these motions produced a cumulative maximum right lateral offset of 29 5 km and a maximum band width of 34 km we interpret agenor linea to have formed primarily in response to the combined effects of nonsynchronous rotation stress and diurnal tidal flexing if so its orientation is optimal for right lateral oblique opening in the current global stress field consistent with its most recent kinematic behavior a small amount of offset of relatively young tension fractures that postdate cryospheric disruption by lenticulae suggest the possibility of ongoing activity along agenor in contrast to a previously published model in which strike slip duplexing was the primary process of band formation this study shows that initial dilation of three distinct morphological zones under disparate stress orientations preceded strike slip motion that resulted in both localized and distributed deformation features within the zones across the width of the band

rock magnetic and paleomagnetic study of the keurusselka impact structure central finland abstract there are 31 proven impact structures in fennoscandia one of the most densely crater populated areas of the earth the recently discovered keurusselka impact structure 62 08 n 24 37 e is located within the central finland granitoid complex which formed 1890 1860 ma ago during the svecofennian orogeny it is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism e g planar deformation features in quartz new petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented the suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility the presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization four different remanent magnetization directions were isolated 1 a characteristic svecofennian target rock component a with a mean direction of d 334 8 i 45 6 95 14 9 yielding a pole plat 51 1 plon 241 9 a95 15 1 2 component b d 42 4 i 64 1 95 8 4 yielding a pole plat 61 0 plon 129 1 a95 10 6 3 component c d 159 5 i 65 4 95 10 7 yielding a pole plat 21 0 plon 39 3 a95 15 6 and 4 component e d 275 5 i 62 0 95 14 4 yielding a pole plat 39 7 plon 314 3 a95 19 7 components c and e are considered much younger possibly neoproterozoic overprints compared with the components a and b the pole of component b corresponds with the 1120 ma pole of salla diabase dyke and is in agreement with the 40ar 39ar age of 1140 ma from a pseudotachylitic breccia vein in a central part of the structure therefore component b could be related to the impact and thus represent the impact age

distribution of chaotic terrain on europa the locations of chaotic terrain are mapped over all regions of europa for which we have adequate resolution 200 m or better and appropriate lighting geometry sun angle 35 from the local horizontal comprising 9 of the total surface of the satellite nearly 30 of the mapped area is occupied by chaotic terrain recognizable at 200 m resolution and sampling at higher resolution suggests that at least 10 more may be covered by small chaos features the largest contiguous area of chaotic terrain is 1300 km across chaotic terrain displays variations in freshness of appearance probably because of aging by fine scale tectonics resurfacing of previous chaotic terrain by larger scale tectonics or disruption by newer chaos are common chaos formation is not necessarily recent relative to tectonics both types of process appear to have gone on diachronously the size distribution shows no dominant or characteristic size and appears to have been controlled by competition from tectonics which has created terrain that occupies more than half of the surface of europa

gridding aeromagnetic data using longitudinal and transverse horizontal gradients with the minimum curvature operator the measurement of gradients of the earth s magnetic field from an airborne platform has been advocated for many years in 1954 wickerham proposed the use of what is essentially a gradient along the flight direction longitudinal gradient hood s 1965 suggestion was to measure the vertical gradient using a total field magnetometer mounted in a tail stinger and a second magnetometer in a towed bird below the tail stinger in fact the system finally built by hood s group at the geological survey of canada used two tail stingers separated by 2 m the advantages of this type of total field vertical gradiometry were advocated so convincingly that vertical gradiometers was also developed for helicopter surveying one of the key developments that allowed the collection of high quality vertical gradient data from a fixed wing aircraft was the development of aircraft compensation schemes for gradiometer arrays subsequent installations generally measured a vertical gradient and a transverse gradient perpendicular to the flight direction the longitudinal gradient is either calculated by taking the spatial derivative of a single sensor traverse or by averaging two wing tip sensors and subtracting this from a tail sensor marcotte et al 1992 give a number of examples where having both horizontal gradients can help in the interpretation of magnetic data they suggest that both gradients can be used to 1 determine whether the geology is two dimensional 2 estimate the strike direction and 3 help calculate the vertical gradient interestingly marcotte et al demonstrate that the vertical gradient derived from the horizontal gradients gives data with noise levels one tenth of that seen on the measured vertical gradients other advantages of measured horizontal gradients are that they can be used 1 to derive a total field that is free of diurnal effects 2 to derive the magnetic gradient tensor 3 to estimate the depth

strike slip fault evolution on europa evidence from tailcrack geometries secondary cracks are commonly produced at stress concentration points at the tips of slipping interfaces such as faults these so called tailcracks form an antisymmetric pattern at opposite tips of the fault with a fracture geometry that is a mechanical indicator of the sense of slip whether left lateral or right lateral i present descriptions of tailcracks along numerous strike slip faults on europa two distinct styles of strike slip faults are identified ridge like and band like the angles between faults and tailcracks are variable and are commonly less than the theoretical 70 5 angle that approximately characterizes many terrestrial examples involving fault surfaces that remain in contact during slip median tailcrack angles are lower for band like faults 30 than ridge like faults 52 in addition the sense of curvature of band like fault tailcracks is characteristically opposite to that of ridge like faults analytical models of stress orientations around strike slip faults on europa indicate that these effects result from dilation during strike slip motion band like faults characteristically underwent concurrent dilation and shearing but this dynamic coupling is not characteristic of ridge like faults the implication is that strike slip faulting was not a globally homogeneous process on europa and as a result the morphologies of strike slip faults are variable furthermore band like faults appear to corroborate the predictions of the tidal walking theory exhibiting predominantly right lateral motions in the southern hemisphere and left lateral motions in the northern hemisphere however ridge like faults do not obey this slip sense convention suggesting that the tidal walking theory may be appropriate for describing the evolution of dilational band like faults but that ridge like faults may have resulted from a different driving mechanism

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

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

hornblende alteration and fluid inclusions in kardla impact crater estonia evidence for impact induced hydrothermal activity the well preserved krdla impact crater on hiiumaa island estonia is a 4 km diameter structure formed in a shallow ordovician sea about 455 ma ago into a target composed of thin 150 m unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites amphibolites and gneisses the fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals the most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater chloritic mineral associations suggest formation temperatures of 100 300 c in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150 300 c in allochthonous breccia the rather low salinity of fluids in krdla crater 13 wt nacleq suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the cater rim walls raised above sea level after the impact or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement the well developed hydrothermal system in krdla crater shows that the thermal history of the shock heated and uplifted rocks in the central crater area rather than cooling of impact melt or suevite sheets controlled the distribution and intensity of the impact induced hydrothermal processes

geology petrography shock petrography and geochemistry of impactites and target rocks from the kardla crater estonia the kardla crater is a 4 km wide impact structure of late ordovician age located on hiiumaa island estonia the 455 ma old buried crater was formed in shallow seawater in precambrian crystalline target rocks that were covered with sedimentary rocks basement and breccia samples from 13 drill cores were studied mineralogically petrographically and geochemically geochemical analyses of major and trace elements were performed on 90 samples from allochthonous breccias sub crater and surrounding basement rocks the breccia units do not include any melt rocks or suevites the remarkably poorly mixed sedimentary and crystalline rocks were deposited separately within the allochthonous breccia suites of the crater the most intensely shockmetamorphosed allochthonous granitoid crystalline derived breccia layers contain planar deformation features pdfs in quartz indicating shock pressures of 2035 gpa an apparent kenrichment and ca na depletion of feldspar and hornblende bearing rocks in the allochthonous breccia units and sub crater basement is interpreted to be the result of early stage alteration in an impact induced hydrothermal system the chemical composition of the breccias shows no definite sign of an extraterrestrial contamination by modeling of the different breccia units with hmxmixing the indigenous component was determined from the abundances of the siderophile elements cr co ni ir and au in the breccia samples no unambiguous evidence for the incorporation of a meteoritic component above about 0 1 wt chondrite equivalent was found

habitability of europa s crust the role of tidal tectonic processes jupiter s satellite europa has been identified as one of the most likely sites for life in the solar system the tidal tectonic processes that appear to have governed europa s geology seem to require interaction with an ocean under only a very thin crust providing a variety of evolving environmental niches the mutually dependent relationship between orbital evolution and tidal processes in turn controls europa s rotation heating and stress surface lineaments are correlated with global stress patterns demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude tidal driving of strike slip faulting indicates that cracks penetrate to a fluid layer which is possible only with a very thin ice crust the characteristic ridge sets that cover tectonic terrain are likely built by tidal pumping of fluid and slush to the surface on a daily basis widespread tectonic dilation creates new surface as material rises from below chaotic terrain has morphology and other characteristics indicative of melt through from below surface colorants correlate with locations such as along large scale ridge systems and around chaotic terrain where ocean water reached the surface this model implies that as a result of tides liquid water regularly bathed crustal cracks and surfaces with heat and whatever nutrients are included in the oceanic chemistry creating a variety of habitable environments the processes were recent and thus most likely continue today longer term evolution of environmental conditions provided the need for adaptation and opportunity for evolution

sunshading geophysical data using fractional order horizontal gradients the interpretation of linear features in geophysical imagery is of considerable importance because they can correspond to dykes faults or other geologic discontinuities one popular tool commonly used for their enhancement is sunshading which considers the data values as if they were a topographic surface that is illuminated by a light source or sun which has a specified azimuth and elevation the reflectance of the light by the surface is calculated and imaged features whose strike direction is orthogonal to the sun azimuth reflect the light strongly and are consequently enhanced while those lying parallel to the sun direction do not reflect the light and become less apparent as a result the reflectance algorithm uses the first horizontal gradients of the data calculated in two orthogonal directions horn 1982

tectonic processes on europa tidal stresses mechanical response and visible features abstract europa s orbital eccentricity driven by the resonance with io and ganymede results in diurnal tides 3 5 day period and possibly in nonsynchronous rotation both diurnal variation and nonsynchronous rotation can create significant stress fields on europa s surface and both effects may produce cracking patterns and time sequences of apparent tectonic features on europa include lineaments that correlate with both sources of stress if we take into account nonsynchronous rotation after initial crack formation by amounts ranging up to several tens of degrees for example the crosscutting time sequence of features in the cadmus and minos linea region is consistent with a combined diurnal and nonsynchronous tensile stress field as it evolves during tens of degrees of nonsynchronous rotation constraints on the rotation rate from comparing voyager and galileo images show that significant rotation requires 104yr but could be fast enough to have allowed significant rotation since the last global resurfacing even if such resurfacing was as recent as a few million years ago once cracking is initiated diurnal tides work cracks so that they open and close daily although the daily effect is small over 105yr double ridges could plausibly be built along the cracks with sizes and morphologies consistent with observed structures according to a model in which underlying liquid water fills the open cracks partially freezes and is extruded during the daily closing of the cracks thus several lines of observational and theoretical evidence can be integrated if we assume nonsynchronous rotation and the existence of a liquid water layer

astypalaea linea a large scale strike slip fault on europa abstract astypalaea linea is an 810 km strike slip fault located near the south pole of europa in length it rivals the san andreas fault in california and it is the largest strike slip fault yet known on europa the fault was discovered using voyager 2 images based upon the presence of familiar strike slip features including linearity pull aparts and possible braids and upon the offset of multiple piercing points fault displacement is 42 km right lateral in the southern and central parts and probably throughout pull aparts present along the fault trace probably are gaps in the lithosphere bounded by vertical cracks and which opened due to fault motion and filled with material from below crosscutting relationships suggest the fault to be of intermediate relative age the fault may have initiated as a crack due to tension from combined diurnal tides and nonsynchronous rotation according to the tectonic model of r greenberg et al 1998a icarus 135 64 78 under the influence of varying diurnal tides strike slip offset may have occurred through a process called walking which depends upon an inelastic lithospheric response to displacement alternatively fault displacement may have been driven by currents in the theorized europan ocean which may have created simple shear structures such as braids the discovery of astypalaea linea extends the geographical range of lateral motion on europa such motion requires the presence of a decoupling zone of ductile ice or liquid water a sufficiently rigid lithosphere and a mechanism to consume surface area

double ridges on europa accommodate some of the missing surface contraction crosscutting relationships of tectonic lineaments on europa record the history of surface deformation we mapped the displacement and orientation of older features crosscut by two types of lineaments bands and double ridges these measurements allow us to determine both the strike perpendicular and strike parallel displacement along investigated features double ridges record both ridge perpendicular contraction and expansion with a mean of 0 16 0 06 km needs space of contraction based on the analysis of 16 double ridges bands record expansion with a mean of 3 33 0 27 km for the six bands analyzed but with perpendicular displacement less than their apparent morphologic widths of 3 24 km the implied global surface strain for double ridges including those that expand and bands is 2 22 0 76 contraction and 7 60 3 7 expansion respectively double ridges thus may accommodate part of the surface expansion recorded by bands most current models for double ridges do not predict contraction the models that satisfy the observations for bands are slow spreading models cryovolcanism and folding

a high precision aeromagnetic survey near the glen hummel field in texas identification of cultural and sedimentary anomaly sources we present a case study of a high resolution aeromagnetic survey near the glen hummel oil field figure 1 the survey was originally flown to support a study of aeromagnetic methods for environmental hazard detection but it provides an opportunity to investigate various problems that arise when analyzing low level total field magnetic anomalies in an area of petroleum production

palaeoproterozoic age of the east baltic granulitic crust abstract the crust beneath the east european platform is one of the largest domains of precambrian crust in europe granulites of poorly defined ages make up a substantial part of this crust representative samples from the granulites in southern estonia are lree enriched and provide nd t dm model ages between 2 08 and 2 18 ga the equivalent nd for t 1 9 ga range from 0 2 to 2 0 these data are similar to those of the svecofennian crust in finland and sweden and suggest that the crustal material originated in the palaeoproterozoic rather than the archaean thus the known volume of juvenile palaeoproterozoic crust in europe is expanded considerably a new aspect is thereby introduced into the geodynamic interpretation of the ne european proterozoic orogenies

tidal tectonic processes and their implications for the character of europa s icy crust 1 jupiter s satellite europa is covered by an icy crust likely overlying a deep global liquid water ocean the mutually dependent relationship between the orbit and tidal processes controls europa s rotation heating and stress observations of the surface and investigation of tidal tectonic processes indicate that the dominant types of surface terrain have been created by frequent repeated exposure of an underlying ocean to the surface surface lineaments including ubiquitous cycloidal chains of arcs features are correlated with tidal stress patterns demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude stratigraphy of tectonic features combined with models of how and where they formed demonstrates a moderate rate of nonsynchronous rotation tidal driving of strike slip displacement in which surface areas shear passed one another by daily 85 hours on europa tidal stresses suggests that cracks penetrate to a liquid layer the characteristic ridge sets that cover tectonic terrain may be built by tidal pumping of fluid and slush to the surface on a daily basis widespread tectonic dilation creates new surface as material rises from below in addition to those tectonic terrains nearly half of the surface is chaotic terrain with morphology and other characteristics indicative of melt through from below consistent with the tectonic indications of thin ice and plausible tidal heating rates formation of both chaotic and tectonic terrains has continually resurfaced the satellite while connecting the ocean to the surface surface colorants correlate with locations where ocean water reached the surface such as along large scale ridge systems and around chaotic terrain as a result of tides liquid water may have bathed crustal cracks and surfaces with heat transporting and mixing substances vertically such daily transported materials would include substances from the oceanic reservoir oxidants and fuels created at the surface as well as any organisms and their chemical products thus a variety of habitable environments likely exist in the crust deactivation of individual cracks after thousands of years due to the tidally driven nonsynchronous rotation constrains processes like ridge building and strike slip displacement and would require any living organisms to adapt to such change

kardla hiiumaa island estonia the buried and well preserved ordovician marine impact structure abstract the kardla marine impact structure estonia 58 58 n 22 46 e was formed at 455 ma upper ordovician in a shallow epicontinental sea some tens of kilometres from the land and erosion area the iron rich projectile about 200 m in diameter approached from the west at an angle of 30 45 the impactor penetrated about 50 m thick water layer and the sedimentary cover and exploded in the uppermost part of the crystalline basement a complex crater 4 km wide and about 500 m deep with a central uplift rising 130 m from the crater floor was formed the highest point of the rimwall is 110 m above the target level the rimwall is cut by at least two resurge excavated gullies the variable height of the rimwall obviously results from the obliqueness of the impact outside the crater an elliptical area was revealed 12 15 km in diameter with deformed sedimentary rocks below the target level the elliptical shape of this area may also be due to the oblique impact because the crater and its surroundings were buried directly after the impact the whole complex of impact related sediments is preserved there they are recovered by 160 wells six of which penetrate the entire complex of impact breccias inside the crater

new aeromagnetic data reveal large strike slip faults in the northern willamette valley oregon high resolution aeromagnetic data from the northern willamette valley oregon reveal large northwest striking faults buried beneath quaternary basin sediments several faults known from geologic mapping are well defined by the data and appear to extend far beyond their mapped surface traces the mount angel fault the likely source of the richter magnitude m l 5 6 earthquake in 1993 is at least 55 km long and may be connected in the subsurface with the gales creek fault 25 km farther northwest northeast of the mount angel fault a 60 km long northwest striking anomaly may represent a previously unrecognized dextral slip fault beneath the towns of canby and molalla vertical offsets along the mount angel fault increase with depth indicating a long history of movement for the fault dominantly northwest trending relatively straight faults consistent stepover geometries offset magnetic anomalies and earthquake focal mechanisms suggest that these faults collectively accommodate significant dextral slip the 1993 earthquake may have occurred on a left stepping restraining bend along the mount angel gales creek fault zone

a melt through model for chaos formation on europa the character of chaotic terrain on europa is consistent with its formation by the melting of a thin conducting ice shell from below tidal dissipation can provide adequate energy for such a process for example only a few percent of europa s predicted tidal heat spread over a region 200 km in diameter can lead to large melt regions within a few tens of thousands of years stronger more localized concentrations result in melt through in significantly shorter times i e a few hundred years the time scale for melt through is shorter than the time scale for the solid state viscous inflow of ice by several orders of magnitude in general modest concentrations of tidal heat can melt ice away faster than viscous inflow leading to melt through a mechanism to transmit these heat concentrations through the ocean is required for this model such heat transport could be the result of convective plumes in the ocean driven by seafloor volcanism or by the destabilization of a stratified ocean

conditions for the onset of plate tectonics on terrestrial planets and moons abstract plate tectonics on earth is driven by the subduction and stirring of dense oceanic lithosphere into the underlying mantle for such a regime to exist on any planet stresses associated with mantle convection must exceed the strength of the lithosphere this condition is sufficiently restrictive that plate tectonics currently operates only on earth and mantle convection in most terrestrial planets and moons is probably in a stagnant lid regime convective stresses on the lithosphere depend on the viscosity and velocity of underlying cold downwellings the lithospheric yield stress is controlled by its friction coefficient and elastic thickness the depth to the brittle ductile transition or bdt both convective stresses and the plate s yield strength depend critically on the size thermal state and cooling history of a planet accordingly here we use numerical simulations and scaling theory to identify conditions in which mantle convection leads to lithospheric failure for a range of conditions relevant to the terrestrial planets whereas earth is expected to be in a plate tectonic regime over its full thermal evolution the moon and mercury are expected to have always remained in a stagnant lid regime venus io and europa currently fall on the transition between the two regimes which is consistent with an episodic style of mantle convection for venus a tectonic component to deformation on io and the resurfacing history and lithospheric evolution of europa our results suggest that venus may have been in a plate tectonic regime in the past while stagnant now it is plausible that mars may have also been in an active lid regime depending on whether there was liquid water on the surface

international geomagnetic reference field its evolution and the difference in total field intensity between new and old models for 1965 1980 the total magnetic field near the surface of the earth is a sum of several constituent fields part of the total field consists of fields that are transient or rapidly varying these fields are caused either directly or indirectly by electric currents in the upper atmosphere and beyond the part of the total field that is more permanent arises from sources that are located inside the earth evidence suggests that this part has two principal constituents the main field and the crustal field

formation of cycloidal features on europa cycloidal patterns are widely distributed on the surface of jupiter s moon europa tensile cracks may have developed such a pattern in response to diurnal variations in tidal stress in europa s outer ice shell when the tensile strength of the ice is reached a crack may occur propagating cracks would move across an ever changing stress field following a curving path to a place and time where the tensile stress was insufficient to continue the propagation a few hours later when the stress at the end of the crack again exceeded the strength propagation would continue in a new direction thus one arcuate segment of the cycloidal chain would be produced during each day on europa for this model to work the tensile strength of europa s ice crust must be less than 40 kilopascals and there must be a thick fluid layer below the ice to allow sufficient tidal amplitude

euler deconvolution using differential similarity transformations of gravity or magnetic anomalies euler deconvolution of gravity and magnetic anomalies can be used to estimate the coordinates of a simple point source and the level of a constant background in the deconvolved field by testing a series of structural indices by using differential similarity transformations dsts joint estimations of the source coordinates the structural index and the coefficients of a linear background trend become possible dsts calculated from the derivatives of the interpreted field depend on the position of a chosen central point of similarity with respect to the source position on this basis techniques for euler deconvolution of anomalous fields are presented the theoretical formulations of dst and the deduced sets of equations for euler deconvolution are tested and verified on model and field examples of magnetic anomalies caused by one point and two point sources such as thin and thick dikes semi infinite and finite sills contacts steps faulted beds etc the possibility of an optimum estimation of the structural index can be used to define a second acceptance criterion along with the criterion of relative standard deviations the joint application of these two criteria ensures the selection of reliable results the suggested dst techniques are suitable for initial rapid estimations of the source type depth and plane location they may be applied to either profile or gridded data acquired from onecomponent measurements as well as from gradient or three component measurements additional data on the density or magnetization of the sources are not necessary

density and magnetic susceptibility of rocks from the lockne and tvaren marine impact structures the lockne and tv ren impact craters in sweden formed in a marine environment during the ordovician the contrast in density between the impact breccias and the surrounding target rock of these two craters is significantly lower than what has been found in craters formed in crystalline targets on land another marine target structure the estonian k rdla structure demonstrates intermediate contrast in impact breccia and target rock which we attribute to the interpreted shallowness of the sea at the k rdla impact site we conclude that the main cause for these low density contrasts is pore and fracture filling of calcite with subordinate quartz and fluorite calcite is the most abundant cement and its density differs most from that of fractured and brecciated bedrock with a low degree of cementation furthermore from the studied cases it is concluded that the target rock to impact rock contrast is generally the highest in craters formed on land in crystalline targets and the lowest in craters formed at sea while craters formed on land in sedimentary targets are intermediate the low density contrasts should decrease the negative gravity anomalies of marine craters

high resolution aeromagnetic mapping of volcanic terrain yellowstone national park high resolution aeromagnetic data acquired over yellowstone national park ynp show contrasting patterns reflecting differences in rock composition types and degree of alteration and crustal structures that mirror the variable geology of the yellowstone plateau the older eocene absaroka volcanic supergroup a series of mostly altered andesitic volcanic and volcaniclastic rocks partially exposed in mountains on the eastern margin of ynp produces high amplitude positive magnetic anomalies strongly contrasting with the less magnetic younger latest cenozoic yellowstone plateau group primarily a series of fresh and variably altered rhyolitic rocks covering most of ynp the yellowstone caldera is the centerpiece of the yellowstone plateau part of its boundary can be identified on the aeromagnetic map as a series of discontinuous negative magnetic anomalies that reflect faults or zones along which extensive hydrothermal alteration is localized the large volume rhyolitic ignimbrite deposits of the 0 63 ma lava creek tuff and the 2 1 ma huckleberry ridge tuff which are prominent lithologies peripheral to the yellowstone caldera produce insignificant magnetic signatures a zone of moderate amplitude positive anomalies coincides with the mapped extent of several post caldera rhyolitic lavas linear magnetic anomalies reflect the rectilinear fault systems characteristic of resurgent domes in the center of the caldera peripheral to the caldera the high resolution aeromagnetic map clearly delineates flow unit boundaries of pre and post caldera basalt flows which occur stratigraphically below the post caldera rhyolitic lavas and are not exposed extensively at the surface all of the hot spring and geyser basins such as norris upper and lower geyser basins west thumb and gibbon are associated with negative magnetic anomalies reflecting hydrothermal alteration that has destroyed the magnetic susceptibility of minerals in the volcanic rocks within yellowstone lake which is mostly within the yellowstone caldera aeromagnetic lows also are associated with known hydrothermal activity in the lake many of the magnetic lows extend beyond the areas of alteration and hot springs suggesting a more extensive currently active or fossil hydrothermal system than is currently mapped steep magnetic gradients suggesting faults or fractures bound the magnetic lows this implies that fractures localize the hot springs magnetic gradient trends reflect the mapped basin and range structural trends of north and northwest as well as northeasterly trends that parallel the regional trend of the snake river plain and the track of the yellowstone hot spot which follow the precambrian structural grain these trends are found both at small scales such as in hydrothermal basins and at more regional fault scales which suggests that the regional stress field and reactivated

the historical development of the magnetic method in exploration the magnetic method perhaps the oldest of geophysical exploration techniques blossomed after the advent of airborne surveys in world war ii with improvements in instrumentation navigation and platform compensation it is now possible to map the entire crustal section at a variety of scales from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale methods of data filtering display and interpretation have also advanced especially with the availability of low cost high performance personal computers and color raster graphics the magnetic method is the primary exploration tool in the search for minerals in other arenas the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications such as locating intrasedimentary faults defining subtle lithologic contacts mapping salt domes in weakly magnetic sediments and better defining targets through 3d inversion these new applications have increased the method s utility in all realms of exploration in the search for minerals oil and gas geothermal resources and groundwater and for a variety of other purposes such as natural hazards assessment mapping impact structures and engineering and environmental studies

segmented lineaments on europa implications for the formation of ridge complexes and bright bands we describe several segmented lineaments on europa s surface these lineaments are extensive stretching for 100s 1000s of km and have ridge complex or bright band morphologies the geometries of the segmented portions of these features are diagnostic of the remote normal and shear stress environment in which they formed and therefore constrain ridge complex and bright band formation mechanisms analysis of four ridge complexes indicates that they formed in a remote normal stress environment that was tensile and isotropic or nearly so and that these lineaments may have formed in a manner more analogous to bands on europa than to ridges the stress environment associated with these ridge complexes may also explain the anastomosing nature of their interior morphology analysis of two bright bands indicate that one formed in a remote normal stress environment that was tensile and the other was reactivated under a combination of remote tensile normal stress and remote sinistral shear stress aspects of the morphologies of these features also indicate that bright bands likely have complex deformation histories that can include multiple episodes of reactivation

fault induced perturbed stress fields and associated tensile and compressive deformation at fault tips in the ice shell of europa implications for fault mechanics secondary fractures at the tips of strike slip faults are common in the ice shell of europa large magnitude perturbed stress fields must therefore be considered to be a viable driving mechanism for the development of part of the fracture sequence fault motions produce extensional and compressional quadrants around the fault tips theoretically these quadrants can be associated with tensile and compressive deformational features i e cracks and anti cracks respectively accordingly we describe examples of both types of deformation at fault tips on europa in the form of extensional tailcracks and compressional anti cracks the characteristics of these features with respect to the plane of the fault create a fingerprint for the mechanics of fault slip accumulation when compared with linear elastic fracture mechanics lefm models of perturbed stress fields around fault tips tailcrack kink angles and curving geometry can be used to determine whether opening accompanies sliding motion kink angles in the 50e70 range are common along strike slip faults that resemble ridges and indicate that little to no opening accompanied sliding in contrast tailcrack kink angles are closer to 30 for strike slip faults that resemble bands with tailcrack curvatures opposite to ridge like fault examples indicating that these faults undergo significant dilation and infill during fault slip episodes anti cracks which may result from compression and volume reduction of porous near surface ice have geometries that further constrain fault motion history corroborating the results of tailcrack analysis the angular separation between anti cracks and tailcracks are similar to lefm predictions indicating the absence of cohesive end zones near the tips of europan faults hence suggesting homogeneous frictional properties along the fault length tailcrack analysis can be applied to the interpretation of cycloidal ridges chains of arcuate cracks on europa that are separated by sharp kinks called cusps cusp angles are reminiscent of tailcrack kink angles along ridge like strike slip faults cycloid growth in a temporally variable tidal stress field ultimately resolves shear stresses onto the near tip region of a growing cycloid segment thus resultant slip and associated tailcrack development may be the driving force behind the initiation of the succeeding arcuate segment hence facilitating the ongoing propagation of the cycloid chain 2005 elsevier ltd all rights reserved