petalite postdating spodumene in pegmatite as a consequence of the 2 02 ga meteorite impact in the vredefort structure southern africa abstract rare element granitic pegmatites enriched in lithium are economically important hosts for the lithium aluminosilicates spodumene and petalite most spodumene and petalite crystallize from the pegmatite melt depending on pressure and temperature this paper reports the rare occurrence of petalite postdating the subsolidus stage in spodumene pegmatites within the 2 02 ga vredefort impact structure the largest oldest and most deeply eroded meteorite impact structure currently known on earth the studied spodumene bearing pegmatites postdate the deformation fabric in the surrounding amphibolites of the granitoid greenstone terrane impact related veins of fragment rich pseudotachylites cut across the pegmatites spodumene albite and quartz constitute the primary mineral assemblage of the pegmatites muscovite is late magmatic and contains inclusions of spodumene and albite during the subsolidus stage k feldspar replaced spodumene and albite during pervasive metasomatic alteration samples collected closer to the pseudotachylites show reaction textures between spodumene and quartz without the involvement of muscovite raman spectroscopy and microprobe analyses show that the reaction textures consist of quartz and petalite unlike spodumene petalite does not show a replacement by k feldspar the 40ar 39ar muscovite age of 2027 8 ma coincides with the timing of the meteorite impact and documents its effect on the spodumene pegmatites we suggest that the formation of petalite after spodumene was related to heating which was generated in the target rocks by the post shock thermal event thus the unique conditions associated with the meteorite impact can not only result in the formation of rare minerals in the target rocks but can also account for rare scenarios resulting in phases that would not otherwise form in nature

combination of precise leveling and insar data to constrain source parameters of the mw 6 5 26 december 2003 bam earthquake we used new precise leveling data acquired 40 days after the bam earthquake in combination with radar interferometry observations from both ascending and descending orbits to investigate static deformation associated with the 2003 bam earthquake we invert this geodetic data set to gain insight into the fault geometry and slip distribution of the rupture the best fitting dislocation model is a steeply east dipping right lateral strike slip fault that has a size of 11 by 8 km and strikes n2 w we find that such smooth geometry fits available geodetic data better than previously proposed models for this earthquake our distributed slip model indicates a maximum strike slip of 3 m occurring about 3 to 5 km deep the slip magnitude and depth of faulting taper to the north where the fault approaches the bam city inclusion of crustal layering increases the amount of maximum slip inferred at depth by about 4

sedimentary structures tentatively attributed to seismic events abstract fingerprints of earthquakes in the sedimentary record are as yet difficult to identify this is particularly true for failure phenomena dewatering slumping fault graded and turbidite structures that could have been triggered also by other kinds of energy input other promising candidates are strange biostratinomic features convex down stacking of shells current orientation in wave ripple accumulations as well as certain preservational features found in geosynclinal pelagic limestones but in every case verification of the seismic origin is still needed

order or chaos origin and mode of emplacement of breccias in floors of large impact structures abstract breccias in the crater floor of large impact structures are pseudotachylites sensu largo authigenic monomict and polymict clastic matrix breccias so called footwall breccias and impact melt breccias pseudotachylite bodies in the center of large impact structures e g vredefort dome south africa appear to have a random distribution and orientation but most dip steeply or vertically large bodies of pseudotachylite in the more distal sectors of the 200 km diameter sudbury structure have been interpreted as ring and terrace collapse features in the vredefort dome networks of randomly distributed pseudotachylite veins accompany large mother lode pseudotachylite dikes in general pseudotachylites in the floors of central parts of impact craters may form through explosive transfer of thermal shock energy in a process that could be termed flash replacement melting whereas pseudotachylites at large distances from the centers of large impact structure are believed to have formed through friction leading to partial or complete melting similar to the formation of tectonic pseudotachylites in smaller structures e g ries and slate islands clastic matrix breccias instead of pseudotachylites occur as the most common breccias in the crater floors they have a chaotic distribution pattern their dips are commonly also steep to vertical melt breccia dikes in the target rocks of the crater floor are associated with melt sheets that fill the lower part of the excavation cavity at vredefort erosion has removed the coherent melt sheet but melt breccia dikes vredefort granophyre in the crater floor are preserved they are characterized by a remarkably homogeneous chemical composition and are believed to represent the initial undifferentiated impact melt near the vredefort collar the granophyre forms more or less concentric dikes in the more central parts of the dome their orientation is more random but in places may be controlled by the archean fabric of the crater basement the offset dikes of the sudbury structure are associated with the sudbury igneous complex that represents in total or in part a differentiated impact melt sheet the dikes in many aspects are similar to vredefort granophyres but their interpretation as undifferentiated bulk melt is problematic on geochemical and structural grounds chemically these dikes are not homogeneous and in places they contain massive sulfide deposits that are unlikely components of an undifferentiated impact melt differentiation and precipitation of massive sulfides are slow processes compared with the presumably high energy and fast emplacement of a supposedly undifferentiated melt pseudotachylites of the central impact crater apparently are mainly produced in the compression phase of the impact process whereas clastic matrix breccias form during the uplift and crater modification phases impact melt breccia dikes contain pseudotachylite bearing inclusions and cut across pseudotachylite bodies such impact melt breccia dikes were probably emplaced during uplift and crater modification

geochemistry and metallogeny of ag pb zn veins in the purcell basin british columbia the mesoproterozoic purcell basin in canada contains three different types of veins 1 type 1 pb zn ag veins are composed of fe rich sphalerite pyrrhotite galena freibergite and pyrite with minor quartz gangue 2 type 2 pb ag cu au veins are characterized by galena pyrite freibergite and gold in a quartz gangue and 3 type 3 ag pb zn vein and replacement deposits contain fe poor sphalerite galena pyrite and freibergite in a quartz dolomite gangue that locally replaces host dolostone the sulfur isotope composition of type 1 2 and 3 vein and replacement deposits closely reflects their respective stratigraphic positions type 1 and 2 veins hosted in lower purcell sedimentary rocks have light sulfur isotope compositions 4 8 to 2 5 similar to diagenic pyrite disseminated in clastic sedimentary host rocks type 3 veins and the replacement deposits hosted in upper purcell supergroup rocks have heavy sulfur isotope compositions 8 3 17 2 indicative of thermo chemical reduction of marine sulfate in carbonate host rocks quartz from type 1 pb zn ag veins has 18 o smow values ranging from 7 to 17 whereas type 2 and 3 vein and replacement deposits have 18 o values ranging from 9 to 20 the range of 18 o values for type 1 2 and 3 veins is the result of mixing of metamorphic fluids with a 18 o value higher than 11 5 and an upper crustal fluid with a 18 o lower than 1 2 in equilibrium with the host rocks during proterozoic type 1 and mesozoic cenozoic type 2 and 3 hydrothermal events galena and freibergite lead isotope compositions plot in two groups consistent with previous studies type 1 pb zn ag veins have a nonradiogenic lead signature 206 pb 204 pb 16 322 16 435 and are considered proterozoic in age type 2 pb ag au cu veins and type 3 ag pb zn vein and replacement deposits have a radiogenic lead signature 206 pb 204 pb 17 842 19 347 and are interpreted to be mesozoic cenozoic in age hydrothermal sericite from the type 3 ptarmigan replacement deposit yields a 40 ar 39 ar age of 133 1 0 7 ma indicating a cretaceous age for type 3 deposits type 1 veins are interpreted to be the result of hydrothermal fluids generated during the proterozoic east kootenay orogeny whereas type 2 and 3 veins and replacement deposits are interpreted to be the result of late cretaceous paleocene tectonism

new observational evidence of global seismic effects of basin forming impacts on the moon from lunar reconnaissance orbiter lunar orbiter laser altimeter data 1 new maps of kilometer scale topographic roughness and concavity of the moon reveal a very distinctive roughness signature of the proximal ejecta deposits of the orientale basin the hevelius formation no other lunar impact basin even the just preceding imbrium basin is characterized by this type of signature although most have similar types of ejecta units and secondary crater structures the preservation of this distinctive signature and its lack in basins formed prior to orientale is interpreted to be the result of seismically induced smoothing caused by this latest major basin forming event intense seismic waves accompanying the orientale basin forming event preceded the emplacement of its ejecta in time and operated to shake and smooth steep and rough topography associated with earlier basin deposits such as imbrium orientale ejecta emplaced immediately following the passage of the seismic waves formed the distinctive roughness signature that has been preserved for almost 4 billion years

crustal fracturing and chert dike formation triggered by large meteorite impacts ca 3 260 ga barberton greenstone belt south africa the ca 3260 ma contact between the largely volcanic onverwacht group and overlying largely sedimentary fig tree group in the barberton greenstone belt south africa is widely marked by chert dikes that extend downward for up to 100 m into underlying sedimentary and volcanic rocks of the mendon formation onverwacht group in the barite valley area these dikes formed as open fractures that were filled by both precipitative fill and the downward flowage of liquefied carbonaceous sediments and ash at the top of the mendon formation spherules that formed during a large meteorite or asteroid impact event occur in a wave and or current deposited unit spherule bed s2 which widely marks the onverwacht fig tree contact and as loose grains and masses within some chert dikes up to 50 m below the contact four main types of chert dikes and veins are recognized type 1 irregular dikes up to 8 m wide that extend downward across as much as 100 m of stratigraphy type 2 small vertical dikes most 1 m wide which are restricted to the lower half of the mendon chert section type 3 small crosscutting veins most 50 cm across filled with precipitative silica and type 4 small irregular to bedding parallel to irregular veins mostly 10 cm wide filled with translucent precipitative silica type 2 dikes formed first and reflect a short lived seismic event that locally decoupled the sedimentary section at the top of the mendon formation from underlying volcanic rocks and opened narrow vertical tension fractures in the lower lithified part of the sedimentary section later seismic events triggered formation of the larger type 1 fractures throughout the sedimentary and upper volcanic section widespread liquefaction of soft uppermost mendon sediments and flowage of the liquefied sediments and loose impact generated spherules into the open fractures late stage tsunamis everywhere eroded and reworked the spherule layer the coincidence of crustal disruption dike formation spherule deposition and tsunami activity suggests that all were related to the s2 impact or impact cluster crustal disruption at this time also formed local relief that provided clastic sediment to the postimpact fig tree group including a small conglomeratic fan delta in the barite valley area remobilization and further movement of debris in the subsurface continued for some time locally the deposition of dense baritic sediments over soft dike materials induced remobilization of material in the dike causing foundering of s2 and 1 2 m of overlying baritic sediments into the dike spherule beds occur at the base of the fig tree group over wide areas of the barberton belt marking the abrupt change from 300 m y of predominantly anorogenic mafic and komatiitic volcanism of the onverwacht group to orogenic clastic sedimentation and associated felsic volcanism of the fig tree group this area never again returned to onverwacht style mafic and ultramafic volcanism but evolved 100 m y later into the kaapvaal craton these results indicate that this major transition in crustal evolution coincided with and was perhaps triggered by major impact events ca 3260 3240 ma

geological observations of damage asymmetry in the structure of the san jacinto san andreas and punchbowl faults in southern california a possible indicator for preferred rupture propagation direction we present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the san andreas san jacinto and punchbowl faults in southern california the observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters a fault zone scale of meters to tens of meters and related geomorphologic features the observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters in all of those scales more damage is found on the side of the fault with faster seismic velocities at seismogenic depths the observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks the data are consistent with a preferred northwestward propagation direction for ruptures on all three faults if our results are supported by additional observations asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section the property of a preferred rupture direction can explain anomalous behavior of historic rupture events and may have profound implications for many aspects of earthquake physics on large faults

noise and target strength degradation accompanying shallow buried explosions ground shock from shallow buried explosions is commonly accompanied by acoustic noise analysis of records from a series of 10 ton chemical detonations in gulf coast sediments provides a basis for the first quantitative description of such noise during a cratering event the test series included shots at depths of 3 12 m leading to craters about 20 m in radius most of the records studied were taken within the eventual crater they include stress velocity and acceleration oscillations arising from reflections off the free surface or subsurface layers are evident in stress records high frequency oscillations attributed to reflections or scattering from smaller scale inhomogeneities are particularly striking in the acceleration records where peak accelerations were 10 100 km s2 oscillations with frequencies of 5 25 khz are obvious they exhibit exponential decay with the dominant frequency decreasing with time approximate values of q estimated from these accelerograms are 30 20 agreeing with q for similar materials derived from seismic techniques since seismic q s include both internal dissipation and scattering losses whereas explosive q s include internal dissipation and radiative losses the magnitude of scattering and radiative losses must be comparable the latter alone correspond to an effective q of about 300 so that internal dissipation dominates q in these poorly consolidated sediments acoustic fluidization may have occurred for a couple of milliseconds in the top 1 or 2 m of soil within the eventual crater by this we mean that the material was liable to flow if the threshold shear stress were exceeded this result is inferred from ground shock stress duration acoustic amplitude decay and initial overburden stress fluidization of such limited duration and extent would have no effect on crater growth just as would be expected for such a low yield event

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

shock melting of k feldspar and interlacing with cataclastically deformed plagioclase in granitic rocks at toqqusap nunaa southern west greenland implications for the genesis of the maniitsoq structure abstract folded sheets of mesoarchaean leucocratic plagioclase k feldspar mesoperthite bearing granitic rocks in the toqqusap nunaa area of the maniitsoq structure west greenland are characterised by their very fine grain sizes and microstructures without normal igneous or planar linear tectonic fabrics quartz forms equidimensional and branching ductilely deformed aggregates and bifurcating panels with protrusions constrictions and chains of ball shaped grains with healed radiating intergranular fractures plagioclase an10 20 was cataclastically deformed and comminuted whereas k feldspar and mesoperthite are devoid of cataclastic microstructures k feldspar forms dispersed highly irregular grains with numerous cusps and saddles indicating almost ubiquitous direct shock melting of this mineral it is commonly located along former fractures in plagioclase resulting in an interlaced feldspar microstructure with contact shapes indicating subsequent melting of plagioclase directly adjacent to k feldspar mesoperthite forms separate rounded and irregular grains with protrusions and cusped margins indicating crystallisation from melts some mesoperthite grains are texturally and compositionally heterogeneous and contain internal lenses of k feldspar and or plagioclase other mesoperthite grains comprise coarsened unzipped areas presumably due to localised fluid controlled dissolution reprecipitation processes the ternary feldspar precursor of the mesoperthite is interpreted as having crystallised from variably effectively mixed k feldspar shock melts and plagioclase contact melts direct melting of k feldspar but no whole rock melting requires shock metamorphism with a short lived temperature excursion to above the melting temperature of k feldspar 1300 c the presence of three different feldspar species and absence of chemical zonation magmatic mantling or metamorphic coronas furthermore hinders interpretations solely by means of endogenic magmatic or ultrametamorphic processes the feldspar microstructures with direct k feldspar melts in the leucocratic orthogneisses of the toqqusap nunaa area thus confirm that the recently discovered maniitsoq structure represents the deep crustal remnants of a giant mesoarchaean meteorite impact

origin of pseudotachylite in the lower witwatersrand supergroup vredefort dome south africa constraints from metamorphic studies small sub millimetre to centimetre thick pseudotachylite veins in metapelitic rocks of the lower witwatersrand supergroup in the collar of the vredefort dome are recrystallized to a biotite cordierite paragenesis indicating the attainment of lower amphibolite facies metamorphic conditions t 500 c following their formation the veins occur along ubiquitous anastomosing fracture networks displaying millimetre to centimetre scale displacements and truncate and incorporate fragments of an older ca 2 05 ga mid amphibolite facies peak metamorphic assemblage the post pseudotachylite metamorphism is attributed to the same event that caused highly variable annealing of ca 2 02 ga shock microdeformation features in rocks across the dome together with the disproportionately large volume of pseudotachylite in the rocks relative to that found in tectonic provinces and the absence of evidence of genetically related large scale shear or fault zones this metamorphic evidence suggests an origin for the pseudotachylite related to the 2 02 ga vredefort bolide impact event that produced the dome possible origins for the pseudotachylite include shock brecciation and melting post shock decompression melting and friction melting associated with the post impact high velocity excavation and modification phases of crater formation the post pseudotachylite metamorphism is attributed to the combined effects of an elevated pre impact crustal geotherm 25 c km related to the waning stages of the 2 05 ga peak metamorphism release of elastic strain energy from the impact shock event and contact effects beneath the impact melt body

a centennial reappraisal of the vredefort pseudotachylytes shaken not stirred by meteorite impact major pseudotachylyte zones constitute a spectacular component of the renowned c 2 023 ga vredefort impact structure south africa but it has always been difficult to explain how they were formed in his original account in 1916 shand interpreted the pseudotachylyte as due to cataclasis and frictional heating but pointed out two enigmas that have remained since there were no associated major faults and the pseudotachylyte volumes he observed were far greater than in similar rocks located within faults elsewhere on earth new observations show that the vredefort pseudotachylyte zones were indeed formed by cataclasis and frictional heating not by faulting but owing to impact induced seismic shaking initiated around temporarily loosened blocks in dendritic fracture systems progressive cataclasis of such loose blocks by intense high frequency oscillations of the country rock at the beginning of the cratering process led to size reduction rounding and comminution and frictional melting of feldspars and biotite in the comminuted parts most pseudotachylyte was thus not injected from anywhere but produced in situ the process of seismic shaking is well known from impacts on the moon and asteroids terrestrial earthquakes and nuclear tests but has largely been overlooked in terrestrial cratering except in the theoretical concept of acoustic fluidization

searching for giant ancient impact structures on earth the mesoarchaean maniitsoq structure west greenland a 100km scale circular region in the archaean north atlantic craton centred at 651150n 511500w near maniitsoq town in west greenland comprises a set of highly unusual geological features that were created during a single event involving intense crushing and heating and are incompatible with crustal orogenic processes th presently exposed features of the maniitsoq structure were buried 20 25 km below the surface when this event occurred at c 3ga during waning convergent orogeny these features include a large aeromagnetic anomaly a central 35x50 km2 large area of comminuted quartzo feldspathic material regional scale circular deformation widespread random fractures with feather like textures intense fracture cleavage amphibolite granite matrix breccias unrelated to faulting or intrusions formation and common fluidisation of microbreccias abundant evidence of direct k feldspar and plagioclase melting superimposed on already migmatised rocks deformation of quartz by oc4 slip formation of planar elements in quartz and plagioclase and emplacement of crustally contaminated ultramafic intrusions and regional scale hydrothermal alteration under amphibolite facies conditions the diagnostic tools employed to identify impacting in the upper crust are inadequate for structures preserved deep within the continental crust nevertheless the inferred scale strain rates and temperatures necessary to create the maniitsoq structure rule out a terrestrial origin of the structure

pseudotachylite in impact structures generation by friction melting and shock brecciation a review and discussion abstract fault related pseudotachylites are generated as the result of brittle or brittle ductile deformation related to seismic faulting it is generally accepted among tectonic workers that while cataclasis plays a role in pseudotachylite formation friction melting is involved in addition pseudotachylite and pseudotachylite like breccias have been repeatedly described from the basement exposures of impact and cryptoexplosion structures and as veinlets in shocked meteorites and lunar samples the relevant literature is reviewed with regard to controversial usage of the term pseudotachylite and attention is drawn to several problematic aspects 1 the term pseudotachylite is currently used both as a purely descriptive term and with genetic implications 2 no unambiguous criteria for comparison and distinction of tectonically and impact generated pseudotachylites have been identified 3 it needs to be considered that in impact structures pseudotachylite could be generated by two distinct processes namely by shock brecciation melting and by friction melting pseudotachylite in impact structures could be the result of either of these two processes or of both 4 it is recommended to adhere more stringently when describing breccias from impact or cryptoexplosion structures to nomenclature accepted for the description of fault rocks in tectonic environments this would help to avoid confusion when discriminating between melt rocks pseudotachylite impact melt rock fragmental breccias cataclasite fragmental impact breccia and mylonitic rocks 5 no criteria for the distinction of melt bearing impact breccias impact melt rock and pseudotachylite have been established either in order to solve these problems close interaction between structural geologists and impact workers is warranted it is suggested that the understanding of breccias in impact structures would be improved by paying particular attention to the temporal relationships between different breccia types this would facilitate recognition of possibly different formational processes for impact produced melt and fragmental breccias and their distinction from pre and post shock deformation products

the finnefjeld domain maniitsoq structure west greenland differential rheological features and mechanical homogenisation in response to impacting the 35 by 50 km large mesoarchaean finnefjeld domain near maniitsoq in the north atlantic craton of southern west greenland constitutes the central part of the previously proposed deeply eroded maniitsoq impact structure with an age of 3 0 ga the finnefjeld domain is an exceedingly homogeneous quartzo feldspathic rock mass which superficially appears to be a late orogenic deep crustal intrusive granitoid pluton and which was described as such for decades however new observations confirm and qualify the first observations from 1962 of these rocks as cataclastic the finnefjeld domain is characterised by a highly unusual mixed rheological behaviour plagioclase displays brittle behaviour with cataclasis quartz was ductilely deformed and k feldspar was melted the deformation and homogenisation of the finnefjeld domain was caused by an intense event of heating and deformation which was coseismic in nature and comprised numerous increments of pure shear strain this type of intense brittle regional deformation and concomitant direct mineral melting in the deep crust is unknown from endogenic orogenic events and is ascribed to deep crustal effects of impacting c 2014 elsevier b v all rights reserved

artificial generation of pseudotachylyte using friction welding apparatus simulation of melting on a fault plane a 150 m thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 r p m under an axial load of 330 kg for 11 s using friction welding apparatus scanning electron microscopy and electron microprobe analysis reveal that the melt layer comprises sub angular to rounded porphyroclasts of clinopyroxene feldspar and ilmenite 20 m diameter derived from the host metadolerite set within a silicate glass matrix thermal calculations confirm that melting occurred at the rock interface and that mean surface temperatures in excess of 1400 c were attained the fused layer shows many textural similarities with pseudotachylyte described from fault zones morphologically the fused layer consists of a series of stacks of porphyroclasts welded together by melt to form build ups oriented at right angles to the friction surface there is also evidence of gouging ploughing and plucking as well as transfer and adhesion of material having occurred between the rock faces the mean surface velocity attained by the metadolerite 0 24 m s 1 and duration of the experiment are comparable with velocities and rise times of typical single jerk earthquakes occurring during stick slip seismic faulting within brittle crust i e slip rates of 0 1 0 5 m s 1 for say 1 10s in these respects the experiment successfully simulated frictional fusion on a fault plane in the absence of an intergranular fluid power dissipation during the experiment was about mw m 2 comparable only to very low values for earthquakes e g 1 100 mw m 2 for displacement rates of 0 1 0 5 m s 1 at shear stresses of 100 1000 bars this indicates that melting on fault planes during earthquakes should be commonplace field evidence however does not support this contention either pseudotachylyte is not being recognized in exhumed ancient seismic fault zones or melting only occurs under very special circumstances

pulverized fault rocks and damage asymmetry along the arima takatsuki tectonic line japan abstract we present field and laboratory data on pulverized rocks at the hakusui kyo outcrop of the arima takatsuki tectonic line attl which is a dextral strike slip fault with 17 km displacement juxtaposing granite to the south against rhyolite to the north the majority of slip at the surface is localized to a clay rich gouge fault core 8 10 cm in width surrounded by a coarsening outwards fault breccias up to 3 m wide fault damage is highly asymmetric with respect to the slipping zone the granite south of the fault has a pulverized damage zone up to 200 m wide while the rhyolite to the north has only about 3 m wide non pulverized fault breccia the degree of pulverization in the granite decreases approximately logarithmically with normal distance from the slip zone the highly fractured pulverized rocks exhibit several distinct textural characteristics in thin section grains appear to be highly comminuted but the original grain shapes and margins are recognizable microfractures tend to be tensile in no preferred orientation grain fragments display little to no rotation and lack evidence of in situ shear consequently at macroscale the rocks appear to preserve original granitic textures despite being highly fractured and friable the observed pulverization and rock damage asymmetry are most consistent with generation mechanism involving ruptures on a bimaterial interface with statistically preferred propagation direction leading to damage primarily on the side with higher seismic velocity at depth this is supported by laboratory measurements of p wave ultrasonic velocities on intact samples which indicate that the granites have consistently higher velocity than the rhyolite with increasing confining pressure

half a century of progress in research on terrestrial impact structures a review abstract the author who investigated the wolfe creek australia in 1962 and edited two benchmark sets of readings on meteorite craters and possible astroblemes in 1977 and 1979 reviews the state of knowledge at the present time the text is concerned with terrestrial impact structures geological features without any consideration of extraterrestrial analogues a handful of definitive publications are drawn on to present the story of terrestrial impact in a single article the text covers historical aspects briefly the effect of target variations the paucity of human observation of such large scale events distinction from volcanic endogenous structures modification by geological processes the transience of the crater initially formed on the target and its subsequent modifications the global geographic distribution of the 174 structures now listed of which a number are dubious attributions their distribution in geological time many ages being known only known to wide limits maximum or minimum values their size distribution calculations of impact frequencies shock effects processes on impact the stages of formation impact into shallow marine and deep sea targets impacts on ice about which little is known and finally the input of impact into biotic extinctions in this last lengthy section the summaries of the conclusions of scientists researching impact on earth and palaeontologists researching biotic impact are set side by side it is concluded that if the recent foraminiferal evidence obtained by gerta keller and associates is taken at its face value the case of impact as a sole agent in extinction is non existent biotic extinction is clearly a complex process involving a number of causes in some cases it was staggered in time and different sets of organisms responded quite differently and surprisingly even in the same extinction event extraterrestrial impact may have been one of the causes in some cases but it may have been regional rather than global in its effects we may never know how much input it had into the record of biotic extinction on earth an enormous amount of new knowledge has arisen from detailed studies of this new family of remarkable geological structures

pseudotachylite from the vredefort ring south africa and the origins of some lunar breccias pseudotachylite veins in the vredefort impact structure occur in rocks of widely varying composition and in stratigraphic units as thin as 10 m the compositions of the pseudotachylites however are the same as those of the rocks in which they occur with some systematic variations thus indicating in situ formation the textures of the pseudotachylites indicate that they formed predominantly by mechanical processes but temperatures were locally high enough to cause thermal metamorphism and possibly fusion the host rocks of the pseudotachylites are extensively mylonitized and show selective crushing of different mineral species greater resistance of quartz and feldspar to crushing partly but not entirely explains the systematic differences in composition between host rock and pseudotachylite shock features in the host rocks are generally of a low order but indicate pressures far in excess of those reasonably to be expected in crustal rocks deformed by terrestrial processes characteristics of the pseudotachylites breccias formed by one impact provide simple models for lunar breccias that may have undergone more than one cycle of brecciation

zircon microstructures record deformation history of shock and tectonically generated pseudotachylites a case study from the vredefort impact structure south africa

impact induced melting of archean granulites in the vredefort dome south africa i anatexis of metapelitic granulites mid crustal archean pelitic granulites in the vredefort dome experienced a static low p granulite facies overprint associated with the formation of the dome by meteorite impact at 2 02 ga heating and exhumation were virtually instantaneous with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 c and locally even exceeded 1350 c this led to comprehensive melting of the precursor archean granulite assemblages grt bt qtz pl ksp crd opx sil followed by peritectic crystallization of aluminous alkali feldspar crd spl crn sil parageneses and the segregation of small evolved biotite leucogranite bodies however at a distance of c 6 km from the centre pre impact rock features are largely preserved although partial replacement of garnet by symplectitic coronas of crd opx spl pl and biotite by orthopyroxene indicate that peak temperatures approached 775 50 c thin interstitial moats of k feldspar are closely associated with the orthopyroxene coronas they are interpreted as the remnants of low proportion partial melts generated by biotite breakdown both the textures and mineral compositional data support reduced equilibration volumes in these rocks which reflect rapid isobaric cooling following shock heating and exhumation the high temperatures and strong lateral thermal gradient are consistent with the modelled impact induced isotherm pattern for a 200 300 km diameter impact crater

nonlinear attenuation of s waves and love waves within ambient rock we obtain scaling relationships for nonlinear attenuation of s waves and love waves within sedimentary basins to assist numerical modeling these relationships constrain the past peak ground velocity pgv of strong 3 4 s love waves from san andreas events within greater los angeles as well as the maximum pgv of future waves that can propagate without strong nonlinear attenuation during each event the shaking episode cracks the stiff shallow rock over multiple events this repeated damage in the upper few hundred meters leads to self organization of the shear modulus dynamic strain is pgv divided by phase velocity and dynamic stress is strain times the shear modulus the frictional yield stress is proportional to depth times the effective coefficient of friction at the eventual quasi steady self organized state the shear modulus increases linearly with depth allowing inference of past typical pgv where rock over the damaged depth range barely reaches frictional failure still greater future pgv would cause frictional failure throughout the damaged zone nonlinearly attenuating the wave assuming self organization has taken place estimated maximum past pgv within greater los angeles basins is 0 4 2 6 m s 1 the upper part of this range includes regions of accumulating sediments with low s wave velocity that may have not yet compacted rather than having been damaged by strong shaking published numerical models indicate that strong love waves from the san andreas fault pass through whittier narrows within this corridor deep drawdown of the water table from its currently shallow and preindustrial levels would nearly double pgv of love waves reaching downtown los angeles

the global effects of impact induced seismic activity on fractured asteroid surface morphology abstract impact induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids in this study we use a series of linked seismic and geomorphic models to investigate the process in detail we begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid and we use this theory to model the global vibrations experienced on the surface of an asteroid following an impact these synthetic seismograms are then applied to a model of regolith resting on a slope and the resulting downslope motion is computed for a full range of impactor sizes next this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure showing how topographic erosion accumulates as a function of time and the number of impacts finally these results are applied in a stochastic cratering model for the surface of an eros like body same volume and surface area as the asteroid with craters formed by impacts and then erased by the effects of superposing craters ejecta coverage and seismic shakedown this simulation shows good agreement with the observed 433 eros cratering record at a main belt exposure age of 400 200 myr including the observed paucity of small craters the lowered equilibrium numbers loss rate production rate for craters less than 100 m in diameter is a direct result of seismic erasure which requires less than a meter of mobilized regolith to reproduce the near observations this study also points to an upper limit on asteroid size for experiencing global surface modifying seismic effects from individual impacts of about 70 100 km depending upon asteroid seismic properties larger asteroids will experience only localized regional seismic effects from individual impacts

the nature distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the vredefort dome south africa abstract the vredefort dome represents the uplift centre of a well known 2 00 ga old impact structure of unusually large magnitude shock features like shatter cones planar features and high pressure silica polymorphs are common the present study deals with the description mode of occurrence field distribution and post shock metamorphic alteration of coesite and stishovite which were poorly documented up to now these minerals occur as unusually large crystals in the quartzite of the wall rock in contact with very thin pseudotachylite veins in the pseudotachylite itself which can be interpreted as a friction and or a shock recovery melt fine needles of kyanite are ubiquitous it is proposed that these thin pseudotachylites a type formed during the transit of the shock wave they preceded the development of thick pseudotachylite and microbreccia veins b type which formed during the tensional period which immediately followed from comparison with the model of formation of high pressure polymorphs in porous sandstone it is suggested that higher pressure and stress was concentrated along the a type veins at the arrival of the shock front at this time the quartz was transformed into a high pressure phase which was probably poorly crystalline behind the shock front that is during the rarefaction the pressure was progressively released and the polymorphs crystallized from this initial high pressure phase stishovite crystallized first followed by coesite the high pressure conditions may have lasted an unusually long time due to the magnitude of the vredefort impact this long time probably about one second may account for the large size of the coesite and stishovite crystals most of the high pressure silica polymorphs are corroded to a variable degree by secondary quartz and preserved only in a restricted area of the impact structure this alteration is attributed to post shock metamorphism due to the temperature of the rock before impact plus the heat left after shock recovery some of the evidence put forward by authors against the impact model is discussed according to the facts arising from this investigation it is concluded that an origin by impact remains the simplest and most realistic model to explain the origin of the vredefort dome

experimental study on impact induced seismic wave propagation through granular materials abstract impact induced seismic waves are supposed to cause movements of regolith particles resulting in modifications of asteroidal surfaces the imparted seismic energy is thus a key parameter to determining the scale and magnitude of this seismic shaking process it is important to study the propagation velocity attenuation rate and vibration period of the impact induced seismic wave to estimate the seismic energy hence we conducted impact cratering experiments at kobe university using a 200 m glass beads target to simulate a regolith layer and measured the impact induced seismic wave using three accelerometers set on the target surface at differences ranging from 3 2 to 12 7 cm the target was impacted with three kinds of projectiles at 100 m s 1 using a one stage gas gun the propagation velocity of the seismic wave in the beads target was 108 9 m s 1 and the maximum acceleration gmax in the unit of m s 2 measured by each accelerometer showed good correlation with the distance from the impact point normalized by the crater radius x r irrespective of projectile type they also were fitted by one power law equation gmax 102 19 x r 2 21 the half period of the first peak of the measured seismic waves was 0 72 ms and this duration was almost consistent with the penetration time of each projectile into the target according to these measurements we estimated the impact seismic efficiency factor that is the ratio of seismic energy to kinetic energy of the projectile to be almost constant 5 7 10 4 inside the crater rim while it exponentially decreased with distance from the impact point outside the crater rim at a distance quadruple of the crater radius the efficiency factors were 4 4 10 5 for polycarbonate projectile and 9 5 10 5 for alumina and stainless steel projectiles

physics of crustal fracturing and chert dike formation triggered by asteroid impact 3 26 ga barberton greenstone belt south africa archean asteroid impacts reflected in the presence of spherule beds in the 3 2 3 5 ga barberton greenstone belt bgb south africa generated extreme seismic waves spherule bed s2 provides a field example it locally lies at the contact between the onverwacht and fig tree groups in the bgb which formed as a result of the impact of asteroid possibly 50 km diameter scaling calculations indicate that very strong seismic waves traveled several crater diameters from the impact site where they widely damaged onverwacht rocks over much of the bgb lithified sediments near the top of the onverwacht group failed with opening mode fractures the underlying volcanic sequence then failed with normal faults and opening mode fractures surficial unlithified sediments liquefied and behaved as a fluid these liquefied sediments and some impact produced spherules filled near surface fractures today represented by swarms of chert dikes strong impact related tsunamis then swept the seafloor p waves and rayleigh waves from the impact greatly exceeded the amplitudes of typical earthquake waves the duration of extreme shaking was also far longer probably hundreds of seconds than that from strong earthquakes dynamic strains of 10 3 occurred from the surface and downward throughout the lithosphere shaking weakened the onverwacht volcanic edifice and the surface layers locally moved downhill from gravity accommodated by faults and open mode fractures coast parallel opening mode fractures on the fore arc coast of chile formed as a result of megathrust events are the closest modern analogs it is even conceivable that dynamic stresses throughout the lithosphere initiated subduction beneath the onverwacht rocks

petrographic and geochemical evidence for an allochthonous possibly impact melt origin of pseudotachylite from the vredefort dome south africa abstract hypotheses proposed to explain the origin of pseudotachylite bodies formed during impact cratering include 1 frictional heating 2 shock loading 3 decompression or 4 drainage of impact melt into target rocks in order to differentiate among these processes we conducted detailed geochemical and petrographic analysis of the matrices in pseudotachylitic veins and dikes and of their respective wall rocks our analyses indicate that the chemical compositions of matrices locally deviate significantly from their immediate wall rocks and that assimilation of wall rock has substantially modified the pseudotachylite matrix compositions in places variable magnitudes of assimilation can be explained by the surface area of wall rock or its fragments in contact with melt as well as the initial temperature and cooling rate of the pseudotachylitic melt chemical trends observed can be explained either by admixture of an exotic melt component with immediate wall rock or by mixing of melts derived from local lithologies trends in the compositional deviation of centimetre to metre wide pseudotachylite dikes from their immediate wall rocks are consistent with the presence of a primary melt component having granitoid composition akin to the average composition of vredefort granophyre dikes within veins melt transport can be geochemically and petrographically traced for distances of centimetres to metres with the direction of melt transport from larger pseudotachylite veins toward smaller ones and into apophyses sulphide and silicate mineralogy indicates that the initial temperature of pseudotachylitic melt must have been at least 1200 1700 c collectively these characteristics point to an allochthonous origin of pseudotachylitic melt we advocate the possibility that impact melt from the initially superheated impact melt sheet contributed to the formation of pseudotachylite bodies at vredefort

formation of pseudotachylitic breccias in the central uplifts of very large impact structures scaling the melt formation abstract the processes leading to formation of sometimes massive occurrences of pseudotachylitic breccia ptb in impact structures have been strongly debated for decades variably an origin of these pseudotachylite friction melt like breccias by 1 shearing friction melting 2 so called shock compression melting with or without a shear component immediately after shock propagation through the target 3 decompression melting related to rapid uplift of crustal material due to central uplift formation 4 combinations of these processes or 5 intrusion of allochthonous impact melt from a coherent melt body has been advocated our investigations of these enigmatic breccias involve detailed multidisciplinary analysis of millimeter to meter sized occurrences from the type location the vredefort dome this complex archean to early proterozoic terrane constitutes the central uplift of the originally 250 km diameter vredefort impact structure in south africa previously results of microstructural and microchemical investigations have indicated that formation of very small veinlets involved local melting likely during the early shock compression phase however for larger veins and networks it was so far not possible to isolate a specific melt forming mechanism macroscopic to microscopic evidence for friction melting is very limited and so far chemical results have not directly supported ptb generation by intrusion of impact melt on the other hand evidence for filling of dilational sites with melt is abundant herein we present a new approach to the mysterium of ptb formation based on volumetric melt breccia calculations the foundation for this is the detailed analysis of a 1 5 3 0 04 m polished granite slab from a dimension stone quarry in the core of the vredefort dome this slab contains a 37 5 dm3 breccia zone the pure melt volume in 0 1 m3 ptb bearing granitic target rock outside of the several decimeter wide breccia zone in the granite slab was estimated at 5 2 dm3 this amount can be divided into 4 6 dm3 melt 88 for which we have evidenced a limited material transport at maximum 20 cm and 0 6 dm3 melt 12 with at most grain scale material transport which we consider in situ formed shock melt the breccia zone itself contains about 10 dm3 of matrix melt assuming melt exchange over 20 cm at the slab surface between breccia zone and surrounding melt bearing host rock volume the outer melt volume is calculated to contain the same amount of melt as contained by the massive breccia zone meso and microscopic observations indicate melt transport is more prominent from larger into smaller melt occurrences thus melt of the breccia zone could have provided the melt fill for all the small scale ptb veins in the surrounding target rock extrapolating this melt capacity calculation for 1 m3 ptb bearing host rock shows that a host rock volume of this dimension is able to take up some 52 dm3 melt scaling up 1000 fold to the outcrop scale reveals that exchange between a host rock volume of 2 m radius around a 37 m3 breccia zone could involve some 10 m3 melt these results demonstrate that large melt volumes i e large breccia zones can be derived in principle from local reservoirs however strong decompression would have to apply in order to exchange these considerable melt volumes which would only be realistic during the decompression phase of impact cratering upon central uplift formation or locally where compressive regimes acted during the subsequent down and outward collapse of the central uplift

equations of state of the high pressure phases of a natural peridotite and implications for the earth s lower mantle abstract to help determine the chemical composition of the earth s mantle we characterised the high pressure mineral assemblage of an undepleted natural peridotite thought to be representative of the earth s upper mantle to 107 gpa using high resolution x ray diffraction at lower mantle conditions the peridotite transforms to the assemblage 76 2 mg0 88fe0 052 fe0 013 al0 12si0 94 o3 orthorhombic perovskite by volume at zero pressure 17 2 mg0 80fe0 20 o magnesiowustite and 7 1 casio3 perovskite the measured room temperature bulk properties of this high pressure assemblage together with a range of estimates of thermal properties of the constituent minerals appear to be inconsistent with seismological constraints on the density and bulk modulus of the lower mantle our results suggest that the lower mantle differs in bulk composition e g richer in iron mg 0 85 from current estimates for the upper mantle requiring some amount of segregation between the upper and lower mantle over geological history

crustal modelling of the ivrea verbano zone in northern italy re examined coseismic cataclasis versus extensional shear zones and sideways rotation abstract new field and microstructural observations in the ivrea verbano zone ivz northern italy reveal extensive post hercynian cataclasis indentation structures microtectonic escape structures microcataclastic injectites and occasional true pseudotachylytes the deformation occurred in situ and was governed by pure shear these observations are incompatible with prevailing crustal models of the ivz most of these models assume the presence of lower crustal originally flat lying extensional shear zones sideways block rotation and exposure of the petrological moho although the original structural observations from the central ivz suggested that the post hercynian deformation was cataclastic rather than shear dominated other already published information also contradicts the sideways rotation of a crustal block comprising the ivz and the adjacent serie dei laghi sdl after the early permian as assumed in the models the ivz and sdl have different detrital zircon populations and are separated by a steep shear zone and a transcurrent fault not rotated listric faults as assumed in the models rotation is also prevented by a flat lying zone of miarolitic cavities in the early permian baveno granite and by contemporaneous vertical basic dykes the widespread in situ cataclasis is interpreted as due to coseismic deformation by high frequency seismic waves generated by earthquakes along the insubric fault and exerting oscillatory compression dilation and torsion in a 10 km wide zone along the footwall of the insubric line the in situ post hercynian cataclasis absence of lower crustal extensional shear zones and lack of sideways rotation of the ivz and sdl call for a new and simpler crustal model of the region

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