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

shatter cones at the keurusselka impact structure and their relation to local jointing shatter cones are the only distinct meso to macroscopic recognition criterion for impact structures yet not all is known about their formation the keurusselka impact structure finland is interesting in that it presents a multitude of well exposed shatter cones in medium to coarse grained granitoids the allegedly 27 km wide keurusselka impact structure was formed about 1150 ma ago in rocks of the central finland granitoid complex special attention was paid in this work to possible relationships between shatter cones and local as well as regionally occurring fracture or joint systems a possible shatter cone find outside the previously suggested edge of the structure could mean that the keurusselka impact structure is larger than previously thought the spacing between joints fractures from regional joint systems was influenced by the impact but impact induced fractures strongly follow the regional joint orientation trends there is a distinct relationship between shatter cones and joints shatter cones occur on and against joint surfaces of varied orientations and belonging to the regional orientation trends planar fractures pf and planar deformation features pdf were found in three shatter cone samples from the central most part of the impact structure whereas other country rock samples from the same level of exposure but further from the assumed center lack shock deformation features pdf occurrence is enhanced within 5 mm of shatter cone surfaces which is interpreted to suggest that shock wave reverberation at preimpact joints could be responsible for this local enhancement of shock deformation some shatter cone surfaces are coated with a quasi opaque material which is also found in conspicuous veinlets that branch off from shatter cone surfaces and resemble pseudotachylitic breccia veins the vein filling is composed of two mineral phases one of which could be identified as a montmorillonitic phyllosilicate the second phase could not be identified yet the original composition of the fill could not be determined further work is required on this material observed joints and fractures were discussed against findings from barringer impact crater they show that impact induced joints in the basement rock do not follow impact specific orientations such as radial conical or concentric

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

geochemistry of mineral water and gases of the razdolnoe spa primorye far east of russia abstract new isotopic and chemical data on the sodium bicarbonate water and associated gases from the razdolnoe spa located in the coastal zone of primorsky kray of the russian far east together with previous stable isotope data 18o d 13c allow elucidation of the origin and evolution of the groundwater and gases from the spa the water is characterized by low temperature 12 c tds 2 5 6 0 g l high contents of b 5 mg l and f 4 5 mg l and low contents of cl and so4 water isotopic composition indicates its essentially meteoric origin which may comply with an older groundwater that was recharged under different colder climatic conditions major components of bubbling gases are ch4 68 vol n2 28 and co2 4 the obtained values 13c and d for co2 and ch4 definitely indicate the marine microbial origin of methane thus the high methane content in the waters relates to the biochemical processes and presence of a dispersed organic matter in the host rocks based on the regional hydrogeology and the geological structure of the razdolnoe spa mesozoic fractured rocks containing na hco3 mineral water and gases are reservoir rocks a chemical composition of water and gases originates in different environmental conditions

venus the nature of the surface from venera panoramas images of the surface of venus obtained by the soviet venera 9 10 13 and 14 landers have been analyzed to provide a basis for understanding the nature of geologic processes operating there the four spacecraft landed in the beta phoebe region at median elevations in the upland rolling plains province the landing points are each separated by distances of more than a thousand kilometers the venera panoramas were digitized and transformed into various perspectives in order to facilitate analysis and comparison with other planetary surfaces bedrock is exposed at the venera 10 13 and 14 sites and is characterized by semicontinuous flat polygonal to subrounded patches up to several meters in width the bedrock surface is often dominated by subhorizontal to horizontal layered plates with thicknesses of several centimeters and abundant linear and polygonal vertical fractures angular to subangular layered to platy blocks in the 5 to 70 cm range dominate the venera 9 site and occur much less frequently at the other sites blocks appear to share many characteristics with the exposed bedrock and are interpreted to be largely derived from it soils particles 1 cm are abundant at the venera 9 10 and 13 sites but are uncommon at venera 14 features indicative of a strong eolian influence moats dunes wind tails are not observed a striking aspect of the venera landing sites is their extreme similarity despite separation distances of thousands of kilometers several hypotheses are considered for the origin of the bedrock surfaces and we investigate in detail the hypothesis that bedrock originated from surface lava flows in this interpretation the broadly platy nature of the surface is analogous to the rolling and undulating nature of terrestrial pahoehoe flows caused by the formation and deformation of a semisolid crust the layering is interpreted to be formed by a combination of upper thermal boundary layer formation and horizontal sheets formed by cooling and shearing during flow emplacement vertical fractures are attributed largely to joint patterns formed during cooling this interpretation made on the basis of surface morphology is consistent with venera 13 and 14 geochemical results which reported high potassium basalt and tholeiitic basalt compositions respectively if this interpretation is correct large regions of the beta phoebe area are likely to be characterized by lava flows the relative freshness of features observed by venera 14 suggests that some bedrock surfaces are geologically young or that erosion rates are low

venus concentrations of radar reflective minerals by wind abstract areas of high radar reflectivity and low emissivity on venus may be covered with windblown material rich in fe and ti such as ilmenite a mineral common in some basaltic lava flows experiments under venus like conditions show that dense mineral grains become concentrated on the surfaces of windblown sediments when subjected to the low winds typical for venus

surface modification of venus as inferred from magellan observations of plains in sedna planitia clear stratigraphic relations can be discerned among volcanic flow units young flows exhibit sar specific cross section values similar to fresh terrestrial basalt flows whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows total degradation of 1 m depth over 0 6 b y is inferred for the sedna area from radar signatures impact crater abundances and ejecta superposition relations with respect to volcanic flow units analyses of parabolic ejecta deposits associated with the crater stuart imply that the material is typically centimeters in thickness a relatively small fraction 10 of venusian impact craters exhibit prominent parabolic ejecta deposits these craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years the inferred dispersal rate 10 3 m yr is too low to produce the degradation of flows at sedna planitia and it is concluded that the dominant flow modification process is in situ weathering in addition elevation dependent weathering is inferred in western ovda regio where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations the inferred rate of generation of high reflection coefficient materials is no more than 10 2 m yr based on the inability of aeolian activity to cover high reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high reflectivity surfaces at high elevations surface modification rates on venus are orders of magnitude lower than on earth venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on mars but are comparable to the estimated rate of weathering and erosion of martian bedrock low surface modification rates imply that it will be possible to determine regional scale age variations on venus based on the degree of preservation of volcanic landforms and microwave signatures

an overview of venus geology the magellan spacecraft is producing comprehensive image and altimetry data for the planet venus initial geologic mapping of the planet reveals a surface dominated by volcanic plains and characterized by extensive volcanism and tectonic deformation geologic and geomorphologic units include plains terrains tectonic terrains and surficial material units understanding the origin of these units and the relation between them is an ongoing task of the magellan team

1 88 1 87 ga post kinematic intrusions of the central finland granitoid complex a shift from c type to a type magmatism during lithospheric convergence abstract the evolution of the svecofennian orogen spans from 1 95 ga to 1 80 ga with the main crust forming activity having taken place 1 90 1 87 ga ago subsequent deformation was largely concentrated to southern finland where transpressional structures indicate lithospheric convergence until 1 80 ga much of the svecofennian orogen in finland is occupied by the central finland granitoid complex cfgc that mainly consists of i type granodiorite and granite these 1 89 1 88 ga intrusive rocks are typically foliated and considered synkinematic a distinct suite of post kinematic undeformed or slightly foliated plutons crosscut the synkinematic rocks they are predominantly quartz monzonites and monzogranites and have been divided into three types according to their petrographic mineral and chemical characteristics compared to the calc alkaline synkinematic granitoids the post kinematic suite generally has an alkaline affinity with higher fe ti k ba zr and nb and lower mg ca and sr at a given sio 2 content the type 1 post kinematic plutons are peraluminous and contained a prominent sedimentary component in their source the types 2 and 3 plutons are marginally metaluminous to peraluminous the type 2 granites especially those in the western cfgc approach the classic 1 65 1 54 ga rapakivi granites of southern finland in their petrographic characteristics magmatic association tholeiitic mafic rocks and elevated contents of incompatible elements and hence have a type characteristics the type 3 plutons contain a pyroxene bearing margin type 3a or contain pyroxene throughout type 3b the type 3b rocks have c type charnockite magma type affinities and some plutons of this subgroup are alkaline the type 3a plutons appear to be transitional between the type 2 and type 3b plutons the synkinematic magmatic episode in the cfgc area probably involved partial melting of intermediate high k rocks in the lower crust with a magmatic addition from a mafic underplate this magmatic episode left a hornblende and biotite poor granulitic residue in the lower crust heat from the mafic underplate triggered partial melting of the granulite to produce the post kinematic magmatism and also introduced mafic material into the post kinematic magmas the post kinematic magmatism took place over quite a short time period ca 15 ma during overall lithospheric convergence it extended beyond the cfgc area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from c type to a type the post kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust partial melting of the mafic lower crust produced c type magmatism soon after the first melting episode the crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal a type magmas were formed in the western cfgc

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

petrogenesis of the post kinematic magmatism of the central finland granitoid complex i radiogenic isotope constraints and implications for crustal evolution post kinematic granitoids of the central finland granitoid complex introduction cfgc were emplaced into the palaeoproterozoic crust of central in many precambrian shield areas extensive palaeofinland shortly after the 1 89 ga peak of the svecofennian proterozoic granitoid magmatism marks the mature stage orogeny they are found as discordant generally non foliated plutons of crustal accretion that led to rapid growth of juvenile with aand c type granite characteristics and a bimodal mafic terranes around pre existing archaean cratons patchett felsic magmatic association conventional and ion microprobe et al 1981 van schmus bickford 1981 patchett u pb data show that the post kinematic magmatism commenced bridgwater 1984 huhma 1986 sadowski bettenin the northeastern cfgc at 1 885 ga and gradually moved court 1996 on a global scale these processes led to the to the west where the plutons are dated at 1 870 ga radiogenic establishment of the palaeoproterozoic supercontinent isotope nd sr pb composition of the post kinematic plutons nena northern europe north america gower et al indicates homogeneous initial ratios nd at 1 875 ga 0 sr 1990 rogers 1996 the scattered remnants of which sr 0 703 and stacey kramers type pb with low longcurrently constitute a substantial fraction of the conterm th u 2 these help to constrain the protolith history of tinental crust of the northern hemisphere the post kinematic magmatism nd and pb isotopic data for in the finnish part of the fennoscandian or baltic palaeoproterozoic granitoids pb isotopic data for associated sulphshield fig 1 the evolution of the 1 9 ga svecoides and nd and pb isotopic composition of the classic 1 6 ga fennian orogen involved several magmatic events e g finnish rapakivi granites reveal several distinct crustal domains in korsman et al 1999 emplacement of early orogenic the finnish svecofennian orogen and indicate that the central part 1 93 1 91 ga tonalites was followed by predominant of the svecofennian orogen consists of several terranes microsynorogenic 1 90 1 87 ga magmatism characterized by continents that were amalgamated in palaeoproterozoic times the i type tonalite granodiorite and granite and arc type cfgc region is the oldest 2 0 ga among these terranes volcanism at 1 84 1 81 ga late orogenic mainly stype granites were emplaced as a broad belt across southernmost finland these and associated mafic rocks have been ascribed to crustal shortening vaisanen

geographical distribution of 3he 4he ratios in japan implications for arc tectonics and incipient magmatism the 3he 4he and 4he 20ne ratios of 115 natural gas samples including various types of gases from volcanoes hot springs mineral springs water wells petroleum fields and natural gas fields were measured using a mass spectrometer the observed 3he 4he and 4he 20ne ratios range from 7 47 10 8 to 9 65 10 6 and from 0 26 to 1100 respectively the 3he 4he ratios reflect well the geotectonic structure of the japanese islands in northeastern ne japan there is a clear geographical difference in the 3he 4he ratios between the frontal arc forearc and volcanic arc back arc regions lower ratios were found in the trench side region and higher ratios in the back arc side this result suggests that the mantle derived helium in the volcanic arc region is associated with the diapiric uprise of a magma lower 3he 4he ratios in the frontal arc region may be due to radiogenic he produced by radioactive decay of u and th in the crustal and sedimentary rocks in southwestern sw japan there is no clear geographical contrast in the 3he 4he ratios some samples in the frontal arc region show quite high 3he 4he ratios the tendency of the 3he 4he ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from ne japan the high 3he 4he ratios observed in the frontal arc region in sw japan may be indicative of renewed or incipient magmatism due to a descending young and warm slab

the keurusselka impact structure finland impact origin confirmed by characterization of planar deformation features in quartz grains although the meteorite impact origin of the keurusselkaimpact structure central finland has been established on the basis of the occurrence of shatter cones no detailed microscopic examination of the impactites from this structure has so far been made previous microscope investigations of in situ rocks did not yield any firm evidence of shock features raiskila et al 2008 kinnunen and hietala 2009 we have carried out microscopic observations on petrographic thin sections from seven in situ shatter cone samples and report here the discovery of planar fractures pfs and planar deformation features pdfs in quartz and feldspar grains the detection and characterization of microscopic shock metamorphic features in the investigated samples substantiates a meteorite impact origin for the keurusselkastructure the crystallographic orientations of 372 pdf sets in 276 quartz grains were measured using a universal stage u stage microscope for five of the seven distinct shatter cone samples based on our u stage results we estimate that investigated shatter cone samples from the keurusselkastructure have experienced peak shock pressures from approximately 2 gpa to slightly less than 20 gpa for the more heavily shocked samples the decoration of most of the pdfs with fluid inclusions also indicates that these originally amorphous shock features were altered by postimpact processes finally our field observations indicate that the exposed surface corresponds to the crater floor it is however difficult to estimate the exact diameter of the structure and the precise amount of material that has been eroded since its formation

magellan initial analysis of venus surface modification initial magellan observations reveal a planet with high dielectric constant materials exposed preferentially in elevated regions with high slopes ejecta deposits extending up to 1000 kilometers to the west of several impact craters windblown deposits and features in areas where there are both obstacles and a source of particulate material and evidence for slow steady degradation by atmosphere surface interactions and mass movements

origin of carbon in fumarolic gas from island arcs abstract we have measured 3 he 4 he ratios 13c values and co 2 3 he ratios of fumaroles and hot spring gases in three volcanic areas in japan the 3 he 4 he ratios 13c values and co 2 3 he ratios vary from 02 05 ratm 9 7 to 2 7 and from 7 74 109 to 1 18 1011 respectively based on these data and those in the literature we present the first quantitative estimates of the upper mantle sediment and limestone including slab components in co2 of volcanic and hydrothermal fluids from subduction zones taking compiled c isotopic data of limestone and organic sediment assuming that the co 2 3 he ratio and 13c value of mid ocean ridge basalt morb are representative of the upper mantle component and that later isotopic and elemental fractionations are minimal it is possible to deconvole each fraction of c on the basis of simple mixing equations up to 20 of the carbon in high temperature volcanic gases in subduction zones is derived from a morb type source while the major fraction of the gases is attributable to co2 produced by decarbonation of subducted marine limestone and slab carbonate

magellan radar performance and data products the magellan venus orbiter carries only one scientific instrument a 12 6 centimeter wavelength radar system shared among three data taking modes the synthetic aperture mode images radar echoes from the venus surface at a resolution of between 120 and 300 meters depending on spacecraft altitude in the altimetric mode relative height measurement accuracies may approach 5 meters depending on the terrain9s roughness although orbital uncertainties place a floor of about 50 meters on the absolute uncertainty in areas of extremely rough topography accuracy is limited by the inherent line of sight radar resolution of about 88 meters the maximum elevation observed to date corresponding to a planetary radius of 6062 kilometers lies within maxwell mons when used as a thermal emission radiometer the system can determine surface emissivities to an absolute accuracy of about 0 02 mosaicked and archival digital data products will be released in compact disk cdrom format

impact craters on venus initial analysis from magellan magellan radar images of 15 percent of the planet show 135 craters of probable impact origin craters more than 15 km across tend to contain central peaks multiple central peaks and peak rings many craters smaller than 15 km exhibit multiple floors or appear in clusters these phenomena are attributed to atmospheric breakup of incoming meteoroids additionally the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across ejecta is found at greater distances than that predicted by simple ballistic emplacement and the distal ends of some ejecta deposits are lobate these characteristics may represent surface flows of material initially entrained in the atmosphere many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid craters are absent from several large areas such as a 5 million square kilometer region around sappho patera where the most likely explanation for the dearth of craters is volcanic resurfacing there is apparently a spectrum of surface ages on venus ranging approximately from 0 to 800 million years and therefore venus must be a geologically active planet

mineral chemistry constraints on the evolution of the 1 88 1 87 ga post kinematic granite plutons in the central finland granitoid complex abstract a suite of post kinematic 1 88 1 87 ga silicic plutons crosscut 1 89 1 88 ga synkinematic granitoids in the central finland granitoid complex cfgc in south central finland the plutons range from biotite hornblende quartz monzonite to syenogranite and include pyroxene and olivine bearing varieties mineral chemical data on feldspars biotite amphibole pyroxenes olivine and oxides of the post kinematic plutons are presented the data are interpreted to show that these plutons register 1 a considerable range in pressure from 2 4 kbar amphibole barometry to 5 7 kbar olivine pyroxene barometry 2 temperatures mostly reflecting resetting during cooling 450 800 c quilf thermometry and 3 low fo2 log fo2 fmq 0 3 to 1 5 quilf equilibria in particular plutons with olivine and pyroxene bearing margins and amphibole dominated central parts record progressive oxidation and hydration upon cooling shifting from the quilf equilibrium toward kuilb the post kinematic granites can be considered post collisional in regard to compressional events in the cfgc and display many of the characteristics of the anorogenic 1 6 ga rapakivi granites further south they were presumably derived from a deep and dry crustal source like the rapakivi granites

magnetic characterization of non ideal single domain monoclinic pyrrhotite and its demagnetization under hydrostatic pressure up to 2 gpa with implications for impact demagnetization here we present a comprehensive magnetic characterization of synthesized non ideal single domain sd monoclinic pyrrhotite fe 7 s 8 the samples were in the form of a powder and a powder dispersed in epoxy non ideal refers to a powder fraction of predominantly sd size with a minor contribution of small pseudo single domain grains such non ideal sd pyrrhotite was found to be a remanence carrier in several types of meteorites carbonaceous chondrites snc which justifies the usage of synthetic compositions as analogous to natural samples data were collected from 5 to 633 k and include low field magnetic susceptibility 0 thermomagnetic curves major hysteresis loops back field remanence demagnetization curves first order reversal curves forcs alternating field and pressure demagnetization of saturation isothermal remanent magnetization sirm low temperature data such as zero field cooled and field cooled remanence datasets together with room temperature sirm cooling warming cycles as well as xrd and mossbauer spectra the characteristic besnus transition is observed at 33 k forc diagrams indicate interacting sd grains the application of hydrostatic pressure up to 2 gpa using nonmagnetic high pressure cells resulted in the demagnetization of the sample by 32 38 repeated cycling from 1 8 gpa to atmospheric pressure and back resulted in a total remanence decrease of 44 after 3 cycles pressure demagnetization experiments have important implications for meteorite paleomagnetism and suggest that some published paleointensities of meteorites with non ideal sd monoclinic pyrrhotite as remanence carrier may be lower limits because shock demagnetization was not accounted for

shatter cone and microscopic shock alteration evidence for a post paleoproterozoic terrestrial impact structure near santa fe new mexico usa article i nfo field mapping morphologic description and petrographic analysis of recently discovered shatter cones within paleoproterozoic crystalline rocks exposed over an area n 5k m 2 located 8 km northeast of santa fe new mexico usa give robust evidence of a previously unrecognized terrestrial impact structure herein we provisionally name this the santa fe impact structure the shatter cones are composed of nested sub conical curviplanar and flat joint surfaces bearing abundant curved and bifurcating striations that strongly resemble the multiply striated joint surfaces msjs documented from shatter cones at vredefort dome the cones occur as a penetrative feature in intrusive igneous and supracrustal metamorphic rocks are unusually large up to 2 m long and 0 5 m wide at the base display upward pointing apices and have subvertical northeastward plunging axes that crosscut regional host rock fabrics key characteristics of superficially similar but non shock generated conical and striated features are inconsistent with the properties of the santa fe cones in thin section sub millimeter scale dark semi opaque to isotropic veneers on cone surfaces and veinlets within cone interiors closely resemble previously described shock induced melt features microscopic grain alteration restricted generally to within 1 mm of the cone surfaces includes random fractures fluid micro inclusions sericite replacement in feldspar rare kink bands in mica optical mosaicism and decorated planar fractures pfs and planar deformation features pdfs in quartz the pfs and pdfs are dominated by a basal 0001 crystallographic orientation which indicate a peak shock pressure of 5 10 gpa that is consistent with shatter cone formation regional structural and exhumation models together with anomalous breccia units that overlie and crosscut the shatter cone bearing rocks may provide additional age constraints for the impact event the observed shatter cone outcrop area suggests that the minimum final crater diameter of the santa fe impact structure was 6 13 km

aeolian features on venus preliminary magellan results magellan synthetic aperture radar data reveal numerous surface features that are attributed to aeolian or wind processes wind streaks are the most common aeolian feature they consist of radar backscatter patterns that are high low or mixed in relation to the surface on which they occur a data base of more than 3400 wind streaks shows that low backscatter linear forms long narrow streaks are the most common and that most streaks occur between 17 s to 30 s and 5 n to 53 n on smooth plains moreover most streaks are associated with deposits from certain impact craters and some tectonically deformed terrains we infer that both of these geological settings provide fine particulate material that can be entrained by the low velocity winds on venus turbulence and wind patterns generated by the topographic features with which many streaks are associated can account for differences in particle distributions and in the patterns of the wind streaks thus some high backscatter streaks are considered to be zones that are swept free of sedimentary particles to expose rough bedrock other high backscatter streaks may be lag deposits of dense materials from which low density grains have been removed dense materials such as ilmenite or pyrite have dielectric properties that would produce high backscatter patterns wind streaks generally occur on slopes 2 and tend to be oriented toward the equator consistent with the hadley model of atmospheric circulation in addition to wind streaks other aeolian features on venus include yardangs and dune fields the aglaonice dune field centered at 25 s 340 e covers 1290 km 2 and is located in an ejecta flow channel from the aglaonice impact crater the meshkenet dune field located at 67 n 90 e covers 17 120 km 2 in a valley between ishtar terra and meshkenet tessera wind streaks associated with both dune fields suggest that the dunes are of transverse forms in which the dune crests are perpendicular to the prevailing winds dunes on venus signal the presence of sand size 60 to 2 000 m grains the possible yardangs are found at 9 n 60 5 e about 300 km southeast of the crater mead although most aeolian features are concentrated in smooth plains near the equator the occurrence of wind streaks is widespread and some have been found at all latitudes and elevations they demonstrate that aeolian processes operate widely on venus the intensity of wind erosion and deposits however varies with locality and is dependent on the wind regime and supply of particles

aqueous fluids derived from a subducting slab observed high 3he emanation and conductive anomaly in a non volcanic region kii peninsula southwest japan abstract it has long been recognized that the kii peninsula in the southwest japan arc is peculiar in a non volcanic region indicated by the presence of high temperature hot springs high terrestrial heat flow and high 3 he content in hot spring gases geophysical and geochemical studies were carried out to understand the geotectonic environment in the southern part of the kii peninsula most of the measured 3 he 4 he ratios are similar or higher than air indicating wide spread incorporation of mantle derived helium into meteoric water a region with rather high 3 he 4 he ratios 4 r a on the west side of the omine mountains coincides with the occurrence of high temperature hot springs a deep crustal resistivity structure across the omine mountains was imaged by wide band magnetotelluric soundings a 2 d inversion with n s strike using both tm and te modes reveals two conductors one in the upper 3 7 km depth and the other in lower crust 25 35 km depth to the west of the omine mountains the distribution of microearthquakes and low frequency tremors and the existence of seismic reflectors indicate that the large conductor in the lower crust is related to aqueous fluids derived from the philippine sea plate the upper crustal conductive zone may also reflect the aqueous fluids trapped in the upper crust which are presumably derived from the subducting slab considering the occurrence of seismic events in the subducting slab beneath the southern kii peninsula the aqueous fluids generated by dehydration of the slab mantle could plausibly include morb type helium derived from the residual lithospheric mantle therefore the high temperature hot springs and high 3 he emanations in hot spring gases and other geotectonic events in the southern kii peninsula may be due to heat flux and mantle derived helium discharged from aqueous fluid in the upper crust

geochemistry of mineral waters and associated gases of the sakhalin island far east of russia abstract isotopic and chemical data on the mineral water mud volcanoes fluid and associated gases from the biggest russian island sakhalin together with previous stable isotope data d 18 o dd 13 c allow elucidation of their origin and general evolution the water fluid circulation is mainly related to marine environment inducing three distinct types na hco 3 cl alkali carbonate groundwaters na cl hco 3 highly evolved saline and na cl mature groundwaters indicating different evolution chemical evolution of groundwater on sakhalin island demonstrated cation exchange and salinization as dominant evolutionary pathways isotopic composition of groundwaters varies from meteoric to metamorphic waters these metamorphic waters consist of water hydration from the clay and seawater are traced in fluids of yuzhno sakhalin mud volcano despite modification by mixing with meteoric waters and water rock interaction processes fault systems that define the areas of highly mineralized water circulation appear to play a major role in the co 2 migration to the surface and ch 4 generation the 13 c co 2 values have pointed that gas phase in high pco 2 waters mostly consists of mantle derived co 2 the carbon isotope signature of methane 13 c ch 4 and d ch 4 indicates its distinct origin which is specified by tectonics methane manifestation in the south of the sakhalin island is mainly related to thermogenic reservoirs as they are more often dislocate by tectonics and crossed by active and permeable faults the sources of biogenous methane in the north of sakhalin island is related to younger and shallower reservoirs and less affected by tectonic processes the determinations of 222 rn have allowed observing that maximal radon flux is associated with high pco 2 waters

wind interaction with falling ejecta origin of the parabolic features on venus unusual parabolic features associated with impact craters have been observed by magellan on venus a strong correlation exists between the orientation of the features and the zonal winds on venus we propose a quantitative model in which the parabolic features are produced by the interaction of the zonal winds with material ejected ballistically from the impact crater as the ejecta particles fall through the atmosphere the winds transport them downwind from their entry point smaller particles being transported a greater distance since the ejecta distribution is initially axially symmetric and smaller particles are thrown farther from the crater the winds blow the particles on the upwind side back upon one another leading to a pile up of material on the downwind side the winds disperse the ejecta particles and no pile up occurs the resulting thickness distribution on the venusian surface matches the observed parabolic features closely the dual parabolic feature associated with the crater carson is also explained by this model

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

deglaciation of fennoscandia abstract to provide a new reconstruction of the deglaciation of the fennoscandian ice sheet in the form of calendar year time slices which are particularly useful for ice sheet modelling we have compiled and synthesized published geomorphological data for eskers ice marginal formations lineations marginal meltwater channels striae ice dammed lakes and geochronological data from radiocarbon varve optically stimulated luminescence and cosmogenic nuclide dating this is summarized as a deglaciation map of the fennoscandian ice sheet with isochrons marking every 1000 years between 22 and 13 cal kyr bp and every hundred years between 11 6 and final ice decay after 9 7 cal kyr bp deglaciation patterns vary across the fennoscandian ice sheet domain reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins for example the ice sheet margin in the high precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and other ice marginal deposits in many fjords and coastal valleys retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern sweden and rapid retreat of the calving ice margin in the baltic basin our reconstruction is consistent with much of the published research however the synthesis of a large amount of existing and new data support refined reconstructions in some areas for example the lgm extent of the ice sheet in northwestern russia was located far east and it occurred at a later time than the rest of the ice sheet at around 17 15 cal kyr bp we also propose a slightly different chronology of moraine formation over southern sweden based on improved correlations of moraine segments using new lidar data and tying the timing of moraine formation to greenland ice core cold stages retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet with the lowest rates on the high elevation and maritime norwegian margin retreat rates compared to the climatic information provided by the greenland ice core record show a general correspondence between retreat rate and climatic forcing although a close match between retreat rate and climate is unlikely because of other controls such as topography and marine versus terrestrial margins overall the time slice reconstructions of fennoscandian ice sheet deglaciation from 22 to 9 7 cal kyr bp provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the fennoscandian ice sheet and are an important resource for testing and refining ice sheet models

a long term perspective on glacial erosion a rough estimate of glacial erosion in the precambrian basement is related to the total denudation of bedrock since its formation in the proterozoic the basis for the calculations is reference to surfaces of different age glacial erosion is responsible for merely polishing the northern shields major denudation occurred during the proterozoic and amounted to tens of kilometres during long periods the basement was protected by palaeozoic cover rocks it was re exposed successively during the mesozoic and tertiary with denudation amounting to 600 m at the most glacial erosion can generally be counted in tens of metres though with great variations 1997 by john wiley sons ltd

magnetic modelling as a tool in the evaluation of impact structures with special reference to the tvaren bay impact crater se sweden abstract until recently computer modelling of radial symmetric structures like impact craters has presented difficulties as a result of this such models are scarce in the literature in this study the magnetic structure of the marine tvaren bay impact crater 58 46 n 17 25 e south of stockholm is investigated with a sophisticated commercially available computer program developed for gravimetric and magnetic modelling of geological structures aeromagnetic data along three profiles across the crater have been used it is concluded that the method is an important supplement to other geological and geophysical investigations in estimating the shape depth and extension of the crater and the surrounding region of basement that was fractured by the impact

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

major and trace element geochemistry of co2 rich groundwater in the volcanic aquifer system of the eastern sikhote alin russia the largest co2 rich mineral water resource in the sikhote alin ridge of eastern russia at gornovodnoe village was studied the high pco2 groundwaters are cold 5 8 10 c with ph 5 9 6 5 tds varied from 1 2 to 2 7 g l and belong to ca mg ca na hco3 type new data on geology mineralogy hydrogeology and hydrogeochemistry in conjunction with isotope data of water and gas phases have provided a much better understanding of the origin of this distinctive groundwater it was found that this water is of meteoric origin but its unusual chemical composition is controlled by interactions of co2 rich groundwater and the aquifer materials the dissolved co2 gas makes the water slightly acidic at about ph 6 2 which increased the leaching of many trace elements from host volcanic rocks typically considered immobile at these ph values 3he 4he ratios and 13c indicate that mantle degassing is important as a source of deep exogenic fluids the cold co2 rich groundwater of the eastern sikhote alin ridge is the result of interactions between fresh groundwater of meteoric origin mantle gases and the host volcanic rocks it thus highlights connectivity between deep and shallow fluids along with deep fractures related to ancient terrane boundaries

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

helium and argon isotopes in rocks minerals and related ground waters a case study in northern switzerland abstract to address the problems related to the transfer of helium and argon isotopes from rocks into related aquifers the concentrations of u th li and k and helium and argon isotopes were measured in a sedimentary sequence from tertiary to permo carboniferous and in crystalline rocks in northern switzerland in addition to whole rock samples mineral separates have also been investigated the observed concentrations of 3 he 4 he and 40 ar in rocks minerals and groundwaters are compared to calculated values which would result from in situ radiogenic production in a closed system this comparison shows that the rocks and minerals have almost completely lost radiogenic in situ produced helium loss of 3 he and 4 he has been controlled by different retention capacities of the rock forming and accessory minerals and fillings therefore measured 3 he 4 he ratios in these rocks can differ from the calculated production ratios the deviation is generally less than a factor of 3 however in certain chemical sediments e g anhydrites elevated ratios of 3 he 4 he up to 15 10 8 which is 20 times the production ratio were observed it is postulated that 3 h the precursor of 3 he might be chemically bound in such sediments during the 12 3 years half life of 3 h and that the beta decay energy of 3 h is too small to liberate 3 he the calculated closed system concentrations of helium in groundwater exceed the measured ones by up to three orders of magnitude implying that the water rock system has not been a closed system since the time of sedimentation and that movable waters have removed helium to a discharge area and to the atmosphere stagnant old waters however could contain high concentrations of helium and supply it to younger movable waters aquitards with low permeability permian shales appear to be a more important source of helium isotopes than water bearing rocks permian sandstones high abundances of all parent elements in the shales almost complete loss of radiogenic helium and the enhanced production ratio of 3 he 4 he 7 2 10 8 in these rocks similar to the ratio observed in the neighbour permian aquifer 9 4 10 8 suggest the shale as a major source of helium isotopes for this aquifer and diffusion as a principal process for helium transfer from the stagnant porewaters in shales into the movable waters in sandstones some sedimentary rocks e g permian sandstones and shales contain more helium than could have been produced since the time of sedimentation while permian sandstones mainly have retained this excess rare gas component they have released radiogenic helium the difference in the retention of these two components most probably demonstrates that migration of radiogenic helium from damage tracks along crystal imperfections to grain boundaries is an important process controlling its loss the abundances of excess 3 he and 4 he in rocks cannot be predicted by in situ production calculations studies of rocks and minerals are required the average ratio of 40 ar 36 ar 385 in groundwaters collected from permian sediments from the weiach borehole is much higher than that in the aquifers from the crystalline basement below the sediments radiogenic 40 ar in permian sediments is mainly liberated by diagenetic water rock interactions the low contribution of radiogenic 40 ar in deep aquifers of the crystalline basement and the fact that chlorine concentrations in groundwaters from the basement are also much lower than in the overlaying permian aquifer eliminates a hypothetical upwards fluid flux from this study we envisage intrabasin sources for helium and argon isotopes in groundwaters with mainly lateral migration of waters through sedimentary layers at different rates and diffusion of helium from slower velocity older waters in more stagnant zones shales into flow passes with highervelocity younger waters in sections with higher permeability sandstones an external source for noble gases is not required in such a model

characterization of lava flow degradation in the pisgah and cima volcanic fields california using landsat thematic mapper and airsar data digital landsat thematic mapper tm observations multi frequency polarimetric airsar radar data microtopographic profiles derived from helicopter borne photography and field measurements were analyzed over the pisgah and cima volcanic fields and lavic lake playa mojave desert california surface radiance factors were extracted from tm data using radiative transfer procedures to model the atmosphere and surface simultaneously airsar data were calibrated to backscatter specific cross sections using corner reflectors deployed at a number of locations analyses focused on the use of key field sites to understand compositional and textural controls on tm band 5 1 55 to 1 75 m radiance factors and c 5 6 cm l 24 cm and p 68 cm band specific cross sections for the pisgah and lavic lake sites the tm data are explained using a linear mixing model of basalt and fine grained sediment clay to sand sized grains of clay minerals quartz feldspar carbonate and lithic fragments radiance factors with the addition of shadows for the rough a9a flow on the other hand airsar data are shown to be primarily controlled by surface roughness as there is a strong correlation between specific cross section and surface height standard deviation for the elongate flows at the cima volcanic field which exhibit a mix of pahoehoe and a9a properties we find a positive linear relationship between tm band 5 radiance factors and flow ages for the 3 m y age range represented specific cross sections remain constant until about 0 14 ma and then decrease with age up to about 0 56 ma on the other hand data for the two older flows 0 70 and 0 85 ma show increasing specific cross sections with increasing age we interpret the increase in tm band 5 radiance factors from 0 05 to 0 25 with increasing flow age and the decrease in specific cross sections 8 db change at 35 incidence angle for l band for flows up to 0 56 ma in age as due to smoothing by mass wasting and infill by fine grained aeolian deposits the increased specific cross sections for older flows are interpreted to be a consequence of surface roughness generated by fluvial dissection

microcomputed tomography and shock microdeformation studies on shatter cones one of the aspects of impact cratering that are still not fully understood is the formation of shatter cones and related fracturing phenomena yet shatter cones have been applied as an impact diagnostic criterion for decades without the role of shock waves and target rock defects in their formation having been elucidated ever we have tested the application of the nondestructive microcomputed tomography ct method to visualize the interior of shatter cones in order to possibly resolve links between fracture patterns and shatter cone surface features striations and intervening valleys shatter coned samples from different impact sites and in different lithologies were investigated for their ct suitability with a shatter cone in sandstone from the serra da cangalha impact structure brazil remaining as the most promising candidate because of the fracture resolution achieved to validate the obtained ct data the scanned specimen was cut into three orthogonal sets of thin sections scans with 13 m resolution were obtained ct scans and microscopic analysis unraveled an orientation of subplanar fractures and related fluid inclusion trails and planar fracture pf orientations in the interior of shatter cones planar deformation features pdf were observed predominantly near the shatter cone surface previously undescribed varieties of feather features ff in the form of lamellae emanating from curviplanar and curved fractures as well as an arrowhead like ff development with microlamellae originating from both sides of a pf were observed the timing of shatter cone formation was investigated by establishing temporal relations to the generation of various shock microscopic effects shatter cones are thus generated post or syn formation of pf ff subplanar fractures and pdf the earliest possible time for shatter cone formation is during the late stage of the compressional phase that is shock wave passage of an impact event

mass movements on venus preliminary results from magellan cycle 1 observations mass movements on venus seen in radar images acquired by the magellan spacecraft during its first mapping cycle are easily interpreted within the scheme commonly used to classify terrestrial landslides rock slumps rock and or block slides rock avalanches debris avalanches and possibly debris flows are seen in areas of high relief and steep slope gradients and are most abundant in the tectonic troughs that crisscross much of the equatorial region of venus many classes of regolith and sediment movements are not seen such features might be too small to resolve in the 75 meter per picture element radar images or their absence may reflect the relatively thin cover of fine sediments inferred from emissivity measurements and other observations venusian landslides like those found within the valles marineris on mars tend to come from escarpments typically higher than those on earth they appear to fall between the terrestrial and martian height to length trends they are also somewhat larger using length as a surrogate for volume than terrestrial subaerial landslides but smaller than their martian counterparts good morphologic analogs can be found in terrestrial volcanic slides both subaerial and submarine oversteepening of volcanic edifices by intrusion and subsequent lateral collapse appears responsible for shaping a number of large isolated volcanos on venus faulting and seismically induced accelerations are probably responsible for the majority of non volcanic mass movements the atmosphere may participate in promoting the movement of some of the landslide debris but environmental factors e g rainfall temperature cycling do not appear to play as dominant a role as they do on earth based on the types and locations of landslides seen in the magellan data it is possible to scale the terrestrial occurrence rate to venus if venus is as seismically and volcanically active as the earth than of order one major landslide i e discernable in magellan images or 5 10 km in runout distance should occur per year which careful re examination of magellan images acquired during later mapping cycles may be able to detect