yarrabubba a large deeply eroded impact structure in the yilgarn craton western australia abstract yarrabubba is a newly discovered impact structure situated within the complex granite greenstone terrain of the yilgarn craton shock metamorphic effects including shatter cones planar deformation features in quartz grains and pseudotachylites were found in deeply eroded archean granites near yarrabubba station southeast of meekatharra western australia aeromagnetic images reveal arcuate demagnetization features at diameters between 11 and 25 km which roughly correspond to the outcropping of the yarrabubba granite and are centered on a magnetic high halo around the barlangi granophyre 119 50 e 27 10 s rapid quench textures in the barlangi granophyre and its inter fingering relationships with pseudotachylites suggest that it is an impact melt that was injected into the yarrabubba granite and spread along fault discontinuities both the potassic yarrabubba granite and the felsic barlangi granophyre are atypical in the northern yilgarn as are the abundant fracturing and frictional melting within the local granitoids these anomalous geological features associated with shock metamorphic effects are indicative of a hypervelocity impact origin the age of the impact is uncertain but geological and geophysical relationships suggest that the yarrabubba structure was formed during the early proterozoic

hydrothermally altered impact melt rock and breccia contributions to the soil of mars samples of melt rock and breccia from 12 terrestrial impact craters were examined to identify alteration minerals and their conditions of formation the dominant assemblage in most cases is clay silica k feldspar zeolite indicating hydrothermal alteration at low pressures and temperatures of 100 300 c the clays are predominantly fe chlorites and smectites in most cases depleted in al and enriched in fe and mg relative to their source rocks alteration of impact glass is often complete while alteration of crystalling melt rock is limited to a few percent of the rock volume impact breccia is altered to only a small extent compared to the alteration of glass impact induced hydrothermal alteration is to be expected at martian impact sites if significant quantities of ground ice and or water are present the alteration products are likely to resemble the clays found at terrestrial craters al fe fractionation similar to that observed on earth is strongly suggested by the viking soil analyses the quantitative contribution of hydrothermal clays to the soil of mars is estimated based on a model which predicts the generation of the equivalent of a 61 m thick blanket of impact melt planet wide the inefficiency of melt production in small craters and the inefficiency of alteration in large craters limit the expected production of clay to a few percent of this figure clays produced by impact induced hydrothermal alteration may be expected to contribute to the fines on mars but this contribution is probably not dominant relative to other suggested sources

geophysical and structural criteria for the identification of buried impact structures with reference to australia the discovery of large asteroid impact structures likely and possible impact structures onshore and offshore the australian continent woodleigh 120 km 360 ma gnargoo 75 km lower permian upper cretaceous tookoonooka 55 65 km 125 ma talundilly 84 km 125 ma mount ashmore 100 km end eocene and warburton twin structures 400 km pre end carboniferous requires re examination of the diagnostic criteria used for their identification bouguer anomalies of established impact structures chicxulub 170 km 64 98 0 05 ma woodleigh impact structure and gnargoo probable impact structure display a unique structural architecture where pre impact structural ridges are intersected and truncated by the outer ring of the circular structure seismic reflection data outline circular central uplift domes basement plugs and rim synclines sharp circular seismic tomography anomalies indicate low velocity columns under both the woodleigh impact structure and warburton probable impact hinting at deep crustal fracturing deformed curved and clouded intra crystalline planar deformation features in quartz qz pdfs displaying miller indices 10 11 10 12 10 13 diagnostic of shock metamorphism abound around exposed established impact structures vredefort 298 km 2023 4 ma sudbury 250 km 1850 3 ma charlevoix 54 km 342 15 ma manicouagan 100 km 214 1 ma tookoonooka and talundilly deformed qz pdfs allow recognition of shock metamorphism in buried impact structures where original qz pdfs were bent recrystallized and or clouded during formation of the central uplift and hydrothermal activity triggered by the impact planar deformation in quartz can also occur in explosive pyroclastic units but are limited to boehm lamella brazil twins with single lamella sets 0001 it has been suggested that a class of microstructures in quartz referred to as metamorphic deformation lamella qz mdl occur in endogenic tectonic metamorphic terrains however no type locality has been established for qz mdl of non impact origin

mesoarchaean palaeoproterozoic stratigraphic record of the singhbhum crustal province eastern india a synthesis the mesoarchaean palaeoproterozoic stratigraphic record of the singhbhum crustal province eastern india implies sedimentation and volcanism in a changing tectonic scenario and thus assumes immense geological significance although efforts have been made by many researchers in the past several decades to summarize various geological aspects of the singhbhum crustal province a critical synthesis of various stratigraphic issues was long overdue the present contribution is an updated critical synthesis of the mesoarchaean palaeoproterozoic stratigraphic record of the singhbhum crustal province we have pointed out the problematic stratigraphic issues of the singhbhum crustal province that deserve careful scrutiny in order to gain better insights into the mode of stratigraphic sequence building

coalescing microstructure and fabric transitions with ams data in deformed limestone implications on deformation kinematics abstract the microstructures of the heterogeneously deformed limestones of the kolhan group singhbhum craton sc eastern india are investigated to study deformation mechanism grain size and shape preferred orientation spo development the texture of the deformed rocks is analyzed by electron back scattered diffraction ebsd in order to determine crystallographic preferred orientation cpo development in calcite grains the c axis pattern in protomylonite and mylonite define girdle patterns developed perpendicular to local shear planes showing monoclinic symmetry the latter is interpreted in terms of a top to south shear which is consistent with the observed microstructural and fabric data from the deformed rocks the anisotropy of magnetic susceptibility ams studies are performed on 22 heterogeneously strained limestone specimens to document strain variation within the kolhan rocks the field fabric spo data measured from calcite and mica grains cal and mi and calcite cpo data are compared with observed variation in ams fabric intensity pj across the kolhan basin integration of these data sets help to establish presence of large internal thrust planes within the kolhan basin which form a part of the regional foreland vergent thrust system developed along the northern margin of the sc

planar lamellar substructures in quartz both single and multiple sets of lamellar features in quartz grains can be produced in shock experiments and are observed at known impact sites they can also be produced in static constant strain rate experiments and are observed at known tectonic and explosive volcanic sites the former are strictly planar and parallel and generally conform to rational crystallographic orientations in the quartz crystal structure the latter can also be planar and parallel but sometimes show slight to moderate curvature with lamellar bifurcations and variable lamellar widths their orientations show a variety of forms with in general a broader angular distribution than that of the impact generated lamellae deductions of physical conditions of deformation from optical morphology spacing multiplicity and orientation information alone remain somewhat conjectural full characterization by transmission electron microscopy tem appears to be necessary

basement structural architecture and hydrocarbon conduit potential of polygonal faults in the cooper eromanga basin australia abstract a thorough and complete understanding of the structural geology and evolution of the cooper eromanga basin has been hampered by low resolution seismic data that becomes particularly difficult to interpret below the thick permian coal measures as a result researchers are tentative to interpret the basement fault architecture within the basin which is largely undefined to provide a better understanding of the basement fault geometry all available two dimensional seismic lines together with 12 three dimensional seismic surveys were structurally interpreted with assistance from seismic attribute analysis the upper cretaceous cadna owie formation and top permian reflectors were analysed using a common seismic attribute technique incoherency that was used to infer the presence of faults that may have otherwise been overlooked detailed basement fault maps for each seismic survey were constructed and used in conjunction with two dimensional seismic data interpretation to produce a regional basement fault map large north northeast south southwest striking sinistral strike slip faults were identified within the patchawarra trough appearing to splay from the main northeast southwest striking ridge these sinistral north northeast south southwest striking faults together with field scale southeast northwest striking dextral strike slip faults are optimally oriented to have potentially developed as a conjugated fault set under a south southeast north northwest oriented strike slip stress regime geomechanical modelling for a regionally extensive system of cretaceous polygonal faults was performed to calculate the leakage factor and dilation tendency of individual faults faults that extend into lower cretaceous oil rich reservoirs with strikes of between 060 n and 140 n and a high to near vertical dip angle were identified to most likely be acting as conduits for the tertiary migration of hydrocarbons from known lower cretaceous hydrocarbon reservoirs into shallow cretaceous sediments this research provides valuable information on the regional basement fault architecture and a more detailed exploration target for the cooper eromanga basin which were previously not available in literature

feather features a low shock pressure indicator in quartz 1 feather features are a recently discovered type of microstructure in shocked quartz they occur as short parallel to subparallel lamellae with spacing similar to that of planar deformation features pdfs these lamellae are always found in combination with a planar fracture pf feather features are crystallographically controlled 0001 101 and 112 orientations are common while 103 and 102 orientations are lacking samples from the nordlinger ries and the matt wilson impact structures are compared with plane wave shock recovery experiments 16 gpa microscopic analysis of quartz grains with feather features show that their formation is linked to shearing along the associated pfs during shock deformation the generation of shear fractures with feather features in plane wave shock recovery experiments suggests a pressure range of 7 10 gpa although further constraints are needed for an upper and lower pressure limit the orientation of feather features in rock samples is suggested to be controlled by the direction of the principal axis of stress with sheared pfs at 45 angles to the axis and most feather feature lamellae aligned parallel to the axis 20 the majority of feather feature lamellae emanate from the pfs in the same direction and in experiments point in the direction from which the shock wave came on the basis of their appearance in shocked quartz grains from 26 impact craters and the current lack of reports of these features in endogenically deformed crustal rocks their uniqueness as a new type of impact induced planar microstructure alongside pdfs and pfs is proposed

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

absence of shock metamorphic effects in the bushveld complex south africa results of an intensive search abstract an extensive search was made for distinctive shock metamorphic effects in order to test the theory that the bushveld complex bc was formed by the impact of large extraterrestrial objects field and petrographie studies concentrated on two geological environments in which current impact cratering theories predict the formation and preservation of such shock metamorphic effects as shatter cones unusual fragmental or melt breccias and shock lamellae in quartz these are 1 pre bc quartzite xenoliths in the rooiberg group a thick sequence of acid lavas rhyolites and rhyodacites and related rocks proposed by earlier workers as a possible crater filling impact melt and 2 the locally observable contact between the rooiberg group and the underlying pre bc basement pretoria group a contact possibly corresponding to the original crater floor in established impact structures more than 320 samples of pre bc quartzite were collected and examined petrographically for shock metamorphic effects no megascopic shock metamorphic effects shatter cones or unusual breccias were observed in any outcrop in thin section no petrographic shock effects e g shock lamellae hi quartz and kink bands in biotite were observed in any sample the quartzites from both the basement 80 samples and the rooiberg xenoliths 245 samples are virtually undeformed cataclastic textures are rare and individual quartz grains show only normal metamorphic deformation lamellae bohm lamellae and single or multiple sets of fluid inclusions arranged along irregular planes which appear to be healed fractures although some of these parallel sets superficially resemble so called decorated shock lamellae universal stage measurements of their orientation demonstrate that they are totally different the absence of both megascopic and microscopic shock metamorphic effects in a large suite of carefully selected samples strongly indicates that impact models for the bc are not correct and that the bc is not an impact structure this conclusion also agrees with 1 the complex stratigraphy now recognized in the lower rooiberg group which is not consistent with an impact melt origin 2 the absence in the rooiberg group of any of the distinctive melt rich fragmentai breccias which dominate the crater filling deposits in established large impact structures and 3 the undeformed character of the basement rocks below the rooiberg group and the apparently concordant contact between them

microfabric study on the deformational and thermal history of the alpi apuane marbles carrara marbles italy marbles from different geometrical and structural positions within the alpi apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure c axis orientation and temperature analysis statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80 100 m to 250 300 m this is correlated with an equilibration calcite dolomite temperature from 360 380 c to 420 430 c two kinds of dynamically recrystallized microstructures have been investigated a first one exhibiting coarse grains 150 200 m with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size 40 50 m and predominantly straight grain boundaries these microstructural types associated with localized post thermal peak shear zones and meter to kilometer scale folds are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms dislocation creep associated with predominant grain boundary migration type b1 or subgrain rotation recrystallization type b2 these differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type b1 390 c than in type b2 370 340 c we have been able to relate the development of the different microfabric types to the successive stages of deformation of the alpi apuane metamorphic complex

new insight into cainozoic sedimentary basins and palaeozoic suture zones in southeast australia from ambient noise surface wave tomography 1 detailed images of rayleigh wave group velocity are derived from ambient seismic noise recorded by wombat a large rolling seismic array project in southeast australia group velocity maps sensitive to crustal structure exhibit low velocity anomalies in the presence of sedimentary basins and recent hot spot volcanism and high velocities in regions of out cropping metamorphic and igneous rocks distinct and well constrained patches of low velocity within the murray basin provide new insight into the spatial extent and possible composition of pre tertiary infra basins in a broader tectonic context our results show little evidence for the palaeozoic building blocks of the southeast australian continent that have been inferred from geological mapping and potential field data this may mean that apparent changes in basement terrane near the surface are not associated with major changes in composition at depth

planar deformation features and impact glass in inclusions from the vredefort granophyre south africa the vredefort granophyre represents impact melt that was injected downward into fractures in the floor of the vredefort impact structure south africa this unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite feldspathic quartzite arkose and granitic material with minor proportions of shale and epidiorite two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features quartz grains in other inclusions display a vermicular texture which is reminiscent of checkerboard feldspar feldspars range from large twinned crystals in some inclusions to fine grained aggregates that apparently are the product of decomposition of larger primary crystals in rare inclusions a mafic mineral probably biotite or amphibole has been transformed to very fine grained aggregates of secondary phases that include small euhedral crystals of fe rich spinel these data indicate that inclusions within the vredefort granophyre were exposed to shock pressures ranging from 5 to 8 30 gpa many of these inclusions contain small rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals including k feldspar quartz augite low ca pyroxene and magnetite the composition of this devitrified glass varies from inclusion to inclusion but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals in the case of granitoid inclusions melt pockets commonly occur at the boundaries between feldspar and quartz grains in metasedimentary inclusions some of these melt pockets contain remnants of partially melted feldspar grains these melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot 650 to 1610 oc impact melt that crystallized to form the vredefort granophyre

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

planar deformation features in shocked quartz a transmission electron microscopy investigation abstract we present a detailed investigation by transmission electron microscopy tem of the fine structure of planar deformation features pdf in quartz grains within which shock metamorphism has been detected optically and which originate from various sites slate islands la malbaie and manicouagan lake canada ries crater germany the toba caldera sumatra the vredefort complex in south africa and the k t boundary at raton basin colorado pdfs appear on tem micrographs as straight and very narrow tem reveals many more pdfs than are optically detected probably because only concentrations of such defects in narrow areas can induce an optically detectable contrast caution should therefore be exercised when applying criteria based on the optical observation of single or multiple sets of pdfs pdfs in samples from slate islands and raton basin exhibit a peculiar morphology a high density of very tiny bubbles precipitated on the dislocation lines comparison with the behavior of wet quartz strongly suggests that these materials experienced lengthy annealing at moderate pressure and temperature

k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia abstract woodleigh is a recently discovered impact structure with a diameter of 120 km and thereby represents the third largest proven phanerozoic impact structure known after morocweng and chicxulub k ar isotopic studies of fine grained authigenic illitic clay minerals

geophysical anomalies and quartz deformation of the warburton west structure central australia abstract this paper reports geophysical anomalies and intra crystalline quartz lamellae in drill cores from the warburton west basin overlapping the border of south australia and the northern territory the pre upper carboniferous 450 300 km large warburton basin north eastern south australia is marked by distinct eastern and western magnetic gravity and low velocity seismic tomography anomalies quartz grains from arenite core samples contain intra crystalline lamellae in carbonate quartz veins and in clastic grains similar to those reported earlier from arenites volcanic rocks and granites from the warburton east basin universal stage measurements of quartz lamellae in both sub basins define miller bravais indices of 10 12 and 10 13 in situ quartz lamellae occur only in pre late carboniferous rocks whereas lamellae bearing clastic quartz grains occur in both pre late carboniferous and post late carboniferous rocks the latter likely redeposited from the pre late carboniferous basement quartz lamellae in clastic quartz grains are mostly curved and bent either due to tectonic deformation or to re deformation of impact generated planar features during crustal rebound or and post impact tectonic deformation seismic tomography low velocity anomalies in both warburton west basin and warburton east basin suggest fracturing of the crust to depths of more than 20 km geophysical modelling of the cooper basin which overlies the eastern warburton east basin suggests existence of a body of high density 2 9 3 0 gr cm 3 and high magnetic susceptibility si 0 012 0 037 at a depth of 6 10 km at the centre of the anomalies in the warburton west basin a large magnetic body of si 0 030 is modelled below 10 km with a large positive gravity anomaly offset to the north of the magnetic anomaly in both the warburton east and warburton west the deep crustal fracturing suggested by the low velocity seismic tomography complicates interpretations of the gravity data universal stage measurements of quartz lamellae suggest presence of both planar deformation features of shock metamorphic derivation and deformed planar lamella the latter may be attributed either to re deformation of impact generated lamella impact rebound deformation or and post impact tectonic deformation the magnetic anomalies in the warburton east and west sub basins are interpreted in terms of 1 presence of deep seated central mafic bodies 2 deep crustal fracturing and 3 removal of devonian and carboniferous strata associated with rebound of a central uplift consequent on large asteroid impact further tests of the warburton structures require deep crustal seismic transects

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

surface wave tomography from ambient seismic noise of accelerograph networks in southern korea 1 an efficient and robust cross correlation technique for estimation of surface wave green s function between two locations is applied to a set of ambient noise data recorded by accelerograph networks in the southern korea we have estimated 1843 group velocities of rayleigh waves in the period band of 2 4 seconds from correlation pairs of 91 accelerograph stations relatively dense and uniform coverage of the pairs provides us with high resolution tomographic image of surface wave group velocities in the region the resulting group velocity distribution correlates well with the characteristic features of geological map of the region displaying low velocities for the sedimentary basins and regions of volcanic and metasedimentary rocks and high velocities for the regions where granitoid basement rocks outcrop the short period information from the accelerograph network can enhance resolution of the subsurface structures at depths within a few kilometers which may be predominantly responsible for simulation results of strong ground motions

3d seismic analysis investigating the relationship between stratigraphic architecture and structural activity in the intra cratonic cooper and eromanga basins australia abstract this research uses four three dimensional 3d seismic surveys located in australia s largest onshore hydrocarbon province the intra cratonic cooper and eromanga basins to present an approach that extracts important structural and stratigraphic information from geophysical data that can then be used to define the tectonostratigraphic evolution of subsurface provinces the methodology consists of 1 analysing isopach maps 2 cross section interpretation of stratigraphic features erosional surfaces and faults and 3 constraining the evolution of fault activity most faults within this province are basement involved with high dip angles the primary fault set is ne sw striking with secondary sets striking n s e w and nw se these high angle faults most likely developed as normal faults before being reactivated by five of the six major tectonic events field scale nw se strike slip faults are prolific and can often be overlooked due to the low seismic resolution a close relationship between on lapping features and present day structural highs was found during each of the major structural events particularly within hydrocarbon rich permian stratigraphy inferring that present day structures were present throughout basin development and intermittently reactivated significant stratal package thinning and a high presence of on lapping features were associated with regional basement involved faults particularly along the gidgealpa merrimelia innamincka and murteree nappacoongee ridges initial structural trap development occurred during the early permian but was most significant during the late triassic hydrocarbon accumulations were unaffected by structural growth after the critical moment in the petroleum system 90 ma as the final period of fault activity was during the late cretaceous this research constrains the tectonostratigraphic evolution of the intra cratonic cooper and eromanga basins while detailing an approach that extracts and analyses important structural and stratigraphic information from geophysical data where outcrop is not accessible

magnetic fabrics rock magnetism cathodoluminescence and petrography of apparently undeformed bambui carbonates from sao francisco basin minas gerais state se brazil an integrated study abstract magnetic measurements were performed on apparently undeformed limestones and carbonate shales from 44 sites in nearly horizontal stratigraphic layers mainly from the basal units of the neoproterozoic bambui group in the southern part of the sao francisco basin rock magnetism cathodoluminescence transmitted and reflected light microscopy analyses reveal that there is a mix of ferromagnetic minerals mainly magnetite and pyrrhotite in most sites in some sites however the ferromagnetic minerals are magnetite and hematite fine grained pyrrhotite and pyrite accompany rare fine grained graphite and probably amorphous carbon in some of stylolites while pyrrhotite is also present as larger interstitial masses in coarse grained domains outside but close to the stylolites magnetic fabrics were determined applying both anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanence magnetization aar the aar tensor was less well defined than the ams fabric due to the low ferromagnetic mineral content the analysis at the individual site scale defines three ams fabric types the first type two sites shows k min perpendicular to the bedding plane while k max and k int are scattered within bedding plane itself this fabric is usually interpreted as primary sedimentary compactional typical of totally undeformed sediments the second type shows the three well clustered ams axes with k min still perpendicular to the bedding plane this fabric is the most important since it was found in the majority of the sites the third type two sites is characterized by well clustered k max in the bedding plane while k min and k int are distributed along a girdle the second and third fabric types are interpreted as combinations of sedimentary compactional and tectonic contributions at the earliest and at a slightly later stage of deformation respectively ams represents the contribution of all the rock forming minerals while aar isolates the contribution of remanence bearing minerals from the matrix minerals however rock magnetism shown that anhysteretic remanence only reaches grains with coercivity k max and a max respectively in general k max is well developed and follows the trend of the main regional thrusts fold axes and faults generated in the first deformational phase while a max follows both this trend and that of structural lineaments formed during the second deformational phase these deformation phases arose from the compression which occurred during the evolution of the brasilia fold belt during the last stages of the brasiliano event the magnetic fabrics of the apparently undeformed bambui limestones are typical of very weakly deformed sediments in which the depositional compaction fabric has been partly overprinted by a tectonic one with minimum susceptibility direction remaining perpendicular to bedding this result is in agreement with the textures given by the petrographic observations

evidence of an impact origin for the azuara structure spain some 50 km south of zaragoza northeast spain there is a tectonically peculiar area which is suspected to be a large impact site it has a morphologically conspicuous ring structure with a diameter of roughly 30 km and consists mainly of mesozoic sediments emerging from the ebro tertiary basin and partly of palaeozoic rocks from the iberian system the strata of the ring in general dip to the center and are highly folded and faulted intense and unusual deformation is indicated by abundant mixed and monomict breccias a single poorly exposed outcrop of a mixed breccia with sedimentary fragments exhibits shock metamorphic effects within quartz grains systems of crystallographically oriented microscopic planar features and planar fractures can be observed as well as kink bands in micas within the breccia matrix the age of the impact is estimated to be between lower cretaceous and miocene

the evolution of the onaping formation at the sudbury impact structure the 1 4 1 6 km thick onaping formation consists of a complex series of breccias and melt bodies lying above the sudbury igneous complex sic at the sudbury impact structure based on the presence of shocked lithic clasts and various glassy phases the onaping has been described as a suevitic breccia with an origin at least in part as fallback material recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases both as clast types and discrete intrusive bodies the nature of the onaping and that of other suevitic breccias overlying impact melt sheets is reviewed the relative thickness internal stratigraphic and lithological character and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the onaping the sudbury structure formed in a foreland basin and water played an essential role in the evolution of the onaping as indicated by a major hydrothermal system generated during its formation taken together observations and interpretations of the onaping suggest a working hypothesis for the origin of the onaping that includes not only impact but also the interaction of sea water with the impact melt resulting in repeated explosive interactions involving proto sic materials and mixing with pre existing lithologies this is complicated by additional brecciation events due to the intrusion of proto sic materials into the evolving and thickening onaping fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two thirds of the onaping

structure and origin of australian ring and dome features with reference to the search for asteroid impact events abstract ring dome and crater features on the australian continent and shelf include a 38 structures of confirmed or probable asteroid and meteorite impact origin and b numerous buried and exposed ring dome and crater features of undefined origin a large number of the latter include structural and geophysical elements consistent with impact structures pending test by field investigations and or drilling this paper documents and briefly describes 43 ring and dome features with the aim of appraising their similarities and differences from those of impact structures discrimination between impact structures and igneous plugs volcanic caldera and salt domes require field work and or drilling where crater like morphological patterns intersect pre existing linear structural features and contain central morphological highs and unique thrust and fault patterns an impact connection needs to tested in the field hints of potential buried impact structures may be furnished by single or multi ring tmi patterns circular tmi quiet zones corresponding gravity patterns low velocity and non reflective seismic zones a examples of crater form and dome form features containing elements consistent with an impact origin though unproven include auvergne delamere fiery creek monte christo mount moffatt tanami east youngerina and tingha b examples of buried multi ring features of possible to probable impact origin include augathella balfour downs calvert hills camooweal green swamp well herbert ikybon river ilkurka lennis mclarty hills mount davies mulkara neale sheridan creek oodjuongari and renehan c examples of igneous plugs unrelated to impacts include the monto gabbro and numerous circular granitoid plugs such as windinie hills granite and yataga granodiorite d large circular structures such as mount ashmore and gnargoo are considered to have convincing structural deformation features warranting classification as probable impact structures the origin of very large circular tmi and gravity patterns such as of the diamantina river drainage feature coonamona anomaly and the multiple tmi ring pattern of the deniliquin booligal remain unresolved the advent of 40 m tmi grid coverage promises to further uncover ring and dome features such as the mclarty hills multi ring feature potentially increasing the inventory of ring structures on the australian continent compared with frequency distribution patterns of extra terrestrial impact structures worldwide the australian record displays a relatively common occurrence of large impact structures and relative depletion in small impact structures and craters this is explained by the better preservation of large structures at deep crustal zones as compared to the erosion of small craters

a record of ancient cataclysm in modern sand shock microstructures in detrital minerals from the vaal river vredefort dome south africa the record of terrestrial meteorite impacts is fragmentary because most impact structures and ejecta are removed by erosion or buried discovery of the missing impact record from hadean to present may be advanced through identifi cation of residual shocked detritus to evaluate which shocked minerals survive erosion and sedimentary transport we investigated modern sands from the vaal river in south africa where it crosses the 2 02 ga vredefort dome the largest terrestrial impact structure known to date shocked minerals were identifi ed in all sediment samples including from the vaal channel and tributaries within the structure in transmitted light detrital quartz preserves discontinuous decorated planar features previously identifi ed as brazil twins which are readily visible as bright continuous features in cathodoluminescence images detrital zircons preserve fi ve orientations of planar fractures pfs which can produce dramatically offset growth zoning and apparent rotation of subgrains other zircons contain fi lled fractures that may represent a new shock microstructure detrital monazite preserves four orientations of pfs and many grains contain oscillatoryzoned shocked zircon inclusions which thus represent shocked inclusions within shocked accessory grains zircon and monazite with granular texture were also identifi ed this study is proof of the concept that shocked minerals can be identifi ed in sediments up to 2 billion years after an impact event and it demonstrates their potential for preserving evidence of ancient impacts the recognition of a new geological repository for impact evi dence provides a means for identifying distal shocked detritus from eroded structures of any age and may be particularly relevant to early earth studies

geophysical anomalies and quartz microstructures eastern warburton basin north east south australia tectonic or impact shock metamorphic origin abstract the eastern warburton basin northeast south australia features major geophysical anomalies including a magnetic high of near 200 nt centred on a 25 km wide magnetic low 30 000 km large eastern warburton basin the qz pe include multiple intersecting planar to curved sub planar elements with relic lamellae less than 2 m wide with spacing of 4 5 m qz pe are commonly re deformed displaying bent and wavy patterns accompanied with fluid inclusions u stage measurements of a total of 243 planar sets in 157 quartz grains indicate dominance of 10 12 10 13 and subsidiary 11 22 22 41 m 10 11 and x 51 61 planes transmission electron microscopy tem analysis displays relic narrow 1 m wide lamellae and relic non sub grain boundaries where crystal segments maintain optical continuity extensive sericite alteration of feldspar suggests hydrothermal alteration to a depth of 500 m below the unconformity which overlies the qz pe bearing warburton basin terrain the data are discussed in terms of a tectonic metamorphic deformation and b impact shock metamorphism producing planar deformation features qz pdf deformed qz pe are compared to re deformed qz pdfs in the sudbury vredefort manicouagan and charlevoix impact structures a 4 5 km uplift of the big lake granite suite during 298 295 ma is consistent with missing of upper ordovician to devonian strata and possible impact rebound the occurrence of circular seismic tomography anomalies below the east warburton basin the poolowana basin and the woodleigh impact structure signifies a potential diagnostic nature of circular tomographic anomalies

reply to comment on k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia

fluid inclusions planar elements and pseudotachylites in the basement rocks of the vredefort structure south africa abstract the basement rocks of the vredefort structure are characterized by high temperature static metamorphism in the course of which a violent shock event occurred quartz in these rocks contains numerous fluid inclusions that consist of almost pure co 2 commonly such fluid inclusions are aligned along planar elements in quartz that were formed during the shock event crystallographic orientations of the planar elements were used to estimate minimum shock pressures of 75 100 30 kbar the fact that fluid inclusions are situated on planar elements indicates that the co 2 gas must have been available immediately after the shock event measurements of the homogenization temperatures of the fluid inclusions in samples from the vredefort basement allow subdivision of the co 2 inclusions into three groups the main group type 1 representing nearly 99 of the co 2 inclusions homogenizes to liquid in the range 26 30 c and therefore has densities of between 0 6 and 0 7 g cm 3 inclusions of this most important group are generally situated on planar elements high density type 2 and low density type 3 inclusions occur isolated in coarse unshocked quartz grains without planar elements in post shock recrystallized quartz grains and in mafic minerals apparently unaffected by the shock event type 2 inclusions with densities of about 0 9 g cm 3 are believed to belong to an early generation which was formed during the granulite facies metamorphism predating the static high temperature vredefort metamorphism low density inclusions of type 3 homogenizing to the gaseous state could be explained as a very late generation of inclusions formed under near surface conditions on the basis of the observation that type 1 inclusions are located on shock induced planar elements the time of entrapment must have been immediately after the shock event and a minimum pressure for co 2 between 2 and 5 kbar can be determined for this time within the entire basement based on independent temperature estimations the observations that a multitude of type 1 inclusions has been found in a minute millimetre sized pseudotachylite vein produced by the shock event is explained by recrystallization of diaplectic glass with this knowledge of microstructures in the vredefort basement rocks it is suggested that the ubiquity of co 2 and the shock event are related to each other

woodleigh impact structure australia shock petrography and geochemical studies the large complex woodleigh structure in the carnarvon basin of western australia has recently been added to the terrestrial impact crater record many aspects of this structure are however still uncertain this work provides a detailed petrographic assessment of a suite of representative drill core samples from the borehole woodleigh 1 that penetrated uplifted basement rocks of the central part of this structure fundamental rock and mineral deformation data and high precision chemical data including results of pge and oxygen isotopic analysis are presented the sampled interval displays likely impact produced macrodeformation in the form of fracturing and breccia veining at the microscopic scale contrary to earlier reports that these breccias represent pseudotachylite friction melt or even shock shear produced pseudotachylitic melt breccia cannot be confirmed due to pervasive post impact alteration abundant planar deformation features pdfs in quartz in addition to diaplectic glass and partial isotropization are the main shock deformation effects observed confirming that woodleigh is of impact origin over the investigated depth interval the statistics of quartz grains with a variable number of sets of pdfs does not change significantly and the patterns of crystallographic orientations of pdfs in randomly selected quartz grains does not indicate a change in absolute shock pressure with depth either the value of oxygen isotopes for the recognition of meteoritic contamination as proposed by earlier woodleigh workers is critically assessed neither ina nor pge analyses of our samples support the presence of a meteoritic component within this basement section as had been claimed in earlier work

progressive deformation and superposed fabrics related to cretaceous crustal underthrusting in western arizona u s a abstract in the maria fold and thrust belt a newly recognized e trending cretaceous orogenic belt in the southwestern united states ductile thrusts large folds and superposed cleavages record discordant emplacement of crystalline thrust sheets across previously tilted sections of crust style of deformation and direction of thrusting are in sharp contrast to those of the foreland fold thrust belt in adjacent segments of the cordillera the net effect of polyphase deformation in the maria belt was underthrusting of paleozoic and mesozoic metasedimentary rocks under the proterozoic crystalline basement of north america the structure of the maria belt is illustrated by the granite wash mountains in west central arizona where at least four non coaxial deformation events d 1 d 4 occurred during the cretaceous sse facing d 1 folds are associated with s directed thrusts and a low grade slaty cleavage d 1 structures are truncated by the gently dipping hercules thrust zone d 2 a regional sw vergent shear zone that placed proterozoic and jurassic crystalline rocks over upturned paleozoic and mesozoic supracrustal rocks exposures across the footwall margin of the hercules thrust zone show the progressive development of folds cleavage and metamorphism related to thrusting d 3 and d 4 structures include open folds and spaced cleavages that refold or transect d 1 and d 2 folds the d 2 hercules thrust zone and a d 3 shear zone are discordantly crosscut by late cretaceous plutons

three sets of crystallographic sub planar structures in quartz formed by tectonic deformation abstract in quartz multiple sets of fine planar deformation microstructures that have specific crystallographic orientations parallel to planes with low miller bravais indices are commonly considered as shock induced planar deformation features pdfs 1 diagnostic of shock metamorphism using polarized light microscopy we demonstrate that up to three sets of tectonically induced sub planar fine extinction bands febs sub parallel to the basal and crystallographic planes are common in vein quartz in low grade tectonometamorphic settings we conclude that the observation of multiple 2 3 sets of fine scale closely spaced crystallographically controlled sub planar microstructures is not sufficient to unambiguously distinguish pdfs from tectonic febs

mechanics microstructure and ams evolution of a synthetic porphyritic calcite aggregate deformed in torsion in order to investigate the mechanical microstructural and ams evolution of porphyritic mylonites we made a synthetic aggregate composed of 70 fine calcite 3 mm in length defined a conspicuous

tomography of the alpine region from observations of seismic ambient noise we use correlations of the ambient seismic noise to study the crust in western europe cross correlation of 1 year of noise recorded at 150 three components broadband stations yields more than 3 000 rayleigh wave group velocity measurements these measurements are used to construct rayleigh group velocity maps of the alpine region and surrounding area in the 5 80 s period band in the 5 10 s period band the seismic noise recorded in europe is dominated by surface waves originating from the northern atlantic ocean this anisotropy of the noise and the uneven station distribution affect the azimuthal distribution of the paths where we obtain reliable group velocity measurements as a consequence our group velocity models have better resolution in the northeast direction than in the southwest direction finally we invert the resulting rayleigh wave group velocity maps to determine the moho depth our results are in good agreement with the result of the numerous active experiments in the alps and provide a continuous image of the alpine structure

experimental reproduction of tectonic deformation lamellae in quartz and comparison to shock induced planar deformation features planar features can develop in quartz during comparatively slow tectonic deformation and during very fast dynamic shock metamorphism despite their very different structural nature tectonically induced deformation lamellae have sometimes been mistaken as shock induced planar deformation features pdfs to understand the formation of deformation lamellae and to address the substantial differences between them and pdfs we have conducted deformation experiments on single crystals of quartz in a griggs type apparatus at a temperature of 800 c a confining pressure of 12 kbar and a strain rate of 0 7 1 1 x 10 6 the deformed samples were analyzed with transmission electron microscopy tem and compared to natural pdfs from the ries crater germany tem revealed that tectonic deformation lamellae are associated with numerous sub parallel curved subgrain walls across which the orientation of the crystal changes slightly the formation of deformation lamellae is due to glide and climb controlled deformation in the exponential creep regime in contrast the pdfs in shocked quartz from the ries are perfectly planar crystallographically controlled features that originally represented amorphous lamellae due to postshock annealing and hydrothermal activity they are recrystallized and decorated with fluid inclusions

low temperature intracrystalline deformation microstructures in quartz a review of numerous genetic interpretations of the individual low temperature intracrystalline deformation microstructures in quartz shows that there is no consensus concerning their formation mechanisms therefore we introduce a new purely descriptive terminology for the three categories of intracrystalline deformation microstructures formed in the low quartz stability field fine extinction bands feb wide extinction bands web and localised extinction bands leb the localised extinction bands are further subdivided into blocky bleb straight sleb and granular gleb morphological types a detailed polarised light microscopy study of vein quartz from the low grade metamorphic high ardenne slate belt belgium further reveals a series of particular geometric relationships between these newly defined intracrystalline deformation microstructures these geometric relationships are largely unrecognised or underemphasised in the literature and need to be taken into account in any future genetic interpretation based on our observations and a critical assessment of the current genetic models we argue that the interpretation of the pertinent microstructures in terms of ambient conditions and deformation history should be made with care as long as the genesis of these microstructures is not better confined

microscopic lamellar deformation features in quartz discriminative characteristics of shock generated varieties the appearance and orientation of microscopic lamellar deformation features in quartz have been examined in samples from different geologic environments lamellar deformation features from known shock and or impact environments are sharp parallel closely spaced and tend to extend in a continuous manner across entire grains they have specific well documented orientations and occur in multiple sets per grain lamellar deformation features from other geologic environments including tectonic and explosive volcanic only superficially resemble shock features they are generally less well defined slightly curved wider spaced and tend to extend in a discontinuous manner across parts of grains they have relatively random orientations only some of which correspond to known shock orientations and generally occur as single sets per grain on the basis of these observations we conclude that there is a characteristic set of microscopic deformation features produced in quartz by shock these so called planar features differ from microscopic lamellar deformation features produced by other dynamic processes therefore the case for the formation of characteristic planar features in quartz by natural processes other than impact is still not proven