the volcanic sedimentologic and paleolimnologic history of the crater lake caldera floor oregon evidence for small caldera evolution apparent phreatic explosion craters caldera floor volcanic cones and geothermal features outline a ring fracture zone along which mount mazama collapsed to form the crater lake caldera during its climactic eruption about 6 850 yr b p within a few years subaerial deposits infilled the phreatic craters and then formed a thick wedge 10 20 m of mass flow deposits shed from caldera walls intense volcanic activity phreatic explosions subaerial flows and hydrothermal venting occurred during this early postcaldera stage and a central platform of subaerial andesite flows and scoria formed on the caldera floor radiocarbon ages suggest that deposition of iacustrine hemipelagic sediment began on the central platform about 150 yr after the caldera collapse this is the minimum time to fill the lake halfway with water and cover the platform assuming present hydrologic conditions of precipitation and evaporation but with negligible leakage of lake water wizard island formed during the final part of the 300 yr lake filling period as shown by its 1 upper subaerial lava flows from 0 to 70 m below present water level and lower subaqueous lava flows from 70 to 500 m and by 2 lacustrine turbidite sand derived from wizard island that was deposited on the central platform about 350 yr after the caldera collapse pollen stratigraphy indicates that the warm and dry climate of middle holocene time correlates with the early lake deposits diatom stratigraphy also suggests a more thermally stratified and phosphate rich environment associated respectively with this climate and greater hydrothermal activity during the early lake history apparent coarse grained and thick bedded turbidites of the early lake beds were deposited throughout northwest southwest and eastern basins during the time that volcanic and seismic activity formed the subaqueous wizard island merriam cone and rhyodacite dome the last known postcaldera volcanic activity produced a subaqueous rhyodacite ash bed and dome about 4 240 yr b p the late lake beds with base of slope aprons and thin fine grained basin plain turbidites were deposited during the volcanically quiescent period of the past 4 000 yr deposits in crater lake and on similar caldera floors suggest that four stages characterize the postcaldera evolution of smaller 10 km in diameter terrestrial caldera lake floors 1 initial stage caldera collapse forms the ring fracture zone that controls location of the main volcanic eruptive centers and sedimentary basin depocenters on the caldera floor 2 early stage subaerial sedimentation rapidly fills ring fracture depressions and constructs basin floor debris fans from calderawall landslides 3 first stage subaqueous sedimentation deposits thick flat lying lake turbidites throughout basins while a thin blanket of hemipelagic sediment covers volcanic edifices that continue to form concurrently with lake sedimentation and 4 second stage subaqueous sedimentation after the waning of major volcanic activity and the earlier periods of most rapid sedimentation develops small sili ciclastic basin base of slope turbidite aprons and central basin plains renewed volcanic activity or lake destruction could cause part or all of the cycle to repeat

further evidence for an impact origin of the tsenkher structure in the gobi altai mongolia geology of a 3 7 km crater with a well preserved ejecta blanket the tsenkher structure in the gobi altai mongolia is a c 3 7 km diameter crater with a well preserved ejecta blanket it has been hypothesized to be either of impact or volcanic origin in our previous work observations during our 2007 expedition and related sample analyses give further support for an impact origin the evidence includes the presence of a structurally uplifted near circular rim surrounded by an ejecta blanket and abundant breccias some of which are melt a nd millimetre scale spherule bearing planar deformation features pdfs were found in one quartz grain in a breccia sample fe rich grains are found in a vesicular melt sample that is also characterized by elevated platinum group element pge abundances with respect to the sedimentary bedrock of the area approximately an order of magnitude noble gas analysis of one breccia sample yielded an elevated 3he 4he value of 5 0 0 2 10 6 although not conclusive alone these geochemical results are consistent with a contribution of meteoritic components a volcanic origin in particular a maar formation would require explanations for the unusual conditions associated with tsenkher including its large size occurring in isolation the structurally uplifted rim and the lack of a bedded base surge deposit a pronounced rampart structure observed at the eastern ejecta is also unusual for any volcanic origin 40ar 39ar dating of a vesicular melt sample gives an age of the tsenkher structure of 4 9 0 9 ma the rampart structure could provide insights into the formation of similar ejecta morphologies associated with numerous impact craters on mars less

the rock elm meteorite impact structure wisconsin geology and shock metamorphic effects in quartz the rock elm structure in southwest wisconsin is an anomalous circular area of highly deformed rocks 6 5 km in diameter located in a region of virtually horizontal undeformed sedimentary rocks shock produced planar microstructures pms have been identified in quartz grains in several lithologies associated with the structure sandstones quartzite pebbles and breccia two distinct types of pms are pres ent p1 features which appear identical to planar fractures pfs or cleavage and p2 features which are interpreted as possible incipient planar deformation features pdfs the latter are uniquely produced by the shock waves associated with meteorite impact events both types of pms are oriented parallel to specific crystallographic planes in the quartz most commonly to c 0001 11 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 2 end document 2 and r z 10 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 1 end document 1 the association of unusual structurally deformed strata with distinct shock produced microdeformation features in their quartz bearing rocks establishes rock elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact preliminary paleontological studies indicate a minimum age of middle ordovician for the rock elm structure a similar age estimate 450 400 ma is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks

the north sea silverpit crater impact structure or pull apart basin the silverpit crater consists of a set of concentric faults up to 20 km in diameter in the southern north sea although formerly described as the first impact structure to be recognized in the uk its spatial association with local tectonic and stratigraphic features suggests that it is unlikely to have been caused by a random extra terrestrial event this paper proposes a terrestrial origin for the silverpit crater as a palaeogene pull apart basin linked to strike slip faulting in the carboniferous basement faulting and thinning of permian evaporites underlying the crater acted as a buffer to basement extension and caused both a flexural collapse of the overlying triassic strata and circumferential listric faulting in the cretaceous succession reactive diapirism in lower jurassic shales may have contributed to the formation of a central uplift at base cretaceous level the paper compares the tectonic setting of two contemporaneous strike slip related structures in southern england bovey basin in devon and the compton valence structure in dorset with that of the silverpit crater and concludes by briefly considering whether a pull apart model can help to explain the formation of upheaval dome a similar putative impact structure in the canyonlands national park of utah

the tsenkher structure in the gobi altai mongolia geomorphological hints of an impact origin the tsenkher structure in the gobi altai region of mongolia was studied using a wide array of remote sensing data and field observations the structure has a shallow 3 6 km wide central depression bordered by a near circular ridge putative crater rim with breaches to the northwest the central depression is obliterated partially by fluvial infill through these breaches outside the ridge the western side is a rugged terrain but the eastern half is characterized by a concentric outer ridge that occurs at about one radius distance from the inner ridge although intrusion salt tectonics and maar crater hypotheses cannot be completely ruled out its morphology strongly implies an impact origin for the tsenkher structure if of impact origin it has a well preserved morphology and its position in the basin fills indicates that the formation may have occurred relatively recently sometime during the late tertiary or quaternary the outer ridge morphology is similar to rampart craters on mars whose formation has been attributed to fluidization of a water rich target layer and ejecta materials or to atmospheric entrainment and deposition of ejected materials however other hypotheses including erosional remnant of ejecta blanket erosional scarp of structural rim uplift multi ring or deeply eroded crater rim of a peak ring crater are also possible at this stage a complex fluvial and probable lacustrine history is envisaged within the tsenkher structure the structure is also associated with archaeology including paleolithic and bronze age remains

the structure and geomorphology of the dashgil mud volcano azerbaijan the large scale and small scale 10 m structure and geomorphology of the active dashgil mud volcano in central azerbaijan have been studied through historical records it is known that this mud volcano has major eruptions every 6 32 years the activity observed during two field trips in 1995 consisted of low energy venting and debouching of mud water oil and gas from three mud pools salses and about 20 individual cones gryphons located on the summit plateau of the mud volcano besides the bubbling mud pools ranging from 30 to 75 m across and the cones ranging from 0 8 to 3 m height a 200 m long string of about 10 sinter mounds was found on the summit plateau the internal structure of the mud volcano and the processes responsible for the observed dashgil geomorphology are discussed assuming the dashgil mud volcano to be continuously active with an average methane production similar to that observed in 1995 between the rare major eruptions we have conservatively estimated the dashgil mud volcano to produce an annual amount of at least 800 sm3 standard cubic metres of gas mainly methane which is emitted directly into the atmosphere this estimate indicates that the natural contribution of atmospheric greenhouse radiatively active gases from the world s terrestrial and submarine mud volcanoes is highly significant

geophysical and magneto structural study of the maâdna structure talemzane algeria insights on its age and origin the ma dna structure is located approximately 400 km south of algiers 33 19 n 4 19 e and emplaced in upper cretaceous to eocene limestones although accepted as an impact crater on the basis of alleged observations of shock diagnostic features such as planar deformation features pdfs in quartz grains previous works were limited and further studies are desirable to ascertain the structure formation process and its age for this purpose the crater was investigated using a multidisciplinary approach including field observations detailed cartography of the different geological and structural units geophysical surveys anisotropy of magnetic susceptibility paleomagnetism and petrography of the collected samples we found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies ones that might indicate the occurrence of a causative material which is at least in part identical geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface using soil magnetic susceptibility measurements the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite a primary magnetization is inferred from a stable remanence with both normal and reverse directions carried by these two minerals although this is supposed to be a chemical remagnetization its normal polarity nature is considered to be a pliocene component subsequent to the crater formation the pole falls onto the miocene pliocene part of the african apparent polar wander path apwp consequently we estimate then formation of the ma adna crater to have occurred during the time period extending from the late miocene to the early pliocene unfortunately our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones nor shocked minerals were found a dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis which can still be considered as plausible only a drilling may provide a definite answer

shock metamorphism investigations of quartz grains in clasts from impact breccia of the eyreville b drill core chesapeake bay impact structure usa abstract the chesapeake bay impact structure approximately 85 km in diameter has been drilled in 2005 2006 at eyreville virginia usa to a total depth of 1766 m in the drill cores the abundance of shock metamorphosed material is very variable with depth shocked mineral and lithic clasts as well as melt particles are most abundant in suevitic impact breccia section 1397 1451 m depth shocked quartz i e quartz grains with planar fractures and or planar deformation features and melt particles although rare are also dispersed in the exmore formation unit 444 867 m depth other lithologies in the eyreville drill cores show no clear evidence of shock metamorphism here we report on the investigations of 40 samples from the impact breccia section a total of more than 27 000 quartz grains were examined in about 200 clasts the abundance of highly shocked clasts tends to decrease with increasing depth crystalline clasts derived from the crystalline basement are commonly only slightly shocked contain generally 10 rel of shocked quartz grains the clasts of metamorphosed sediments show a low proportion of shocked quartz grains mostly 10 rel sedimentary clasts show a wide range of proportions of shocked quartz grains with several of them being highly shocked clasts most values between 0 and 40 rel conglomerates show the highest proportion of shocked quartz grains of all types of clasts up to 83 rel polycrystalline quartz clasts are also commonly highly shocked contain mostly between 10 and 40 rel of shocked quartz grains these hard nonporous clasts are possibly more liable to show evidence of shock the investigations suggest that the intensity of shock metamorphism is the result of several parameters such as original position in the target both horizontal and vertical and the properties of each lithology e g grain size porosity and amount of matrix according to the universal stage investigations the dominant orientations of planar deformation features in quartz are and also

geophysical survey of the proposed tsenkher impact structure gobi altai mongolia abstract we have performed forward magnetic and gravity modeling of data obtainedduring the 2007 expedition to the 3 7 km in diameter circular tsenkher structure mongolia in order to evaluate the cause of its formation extensive occurrences ofbrecciated rocks mainly in the form of an ejecta blanket outside the elevated rim of thestructure support an explosive origin e g cosmic impact explosive volcanism the hostrocks in the area are mainly weakly magnetic silica rich sandstones and siltstones a nearabsence of surface exposures of volcanic rocks makes any major volcanic structures e g caldera unlikely likewise the magnetic models exclude any large subsurface intrusivebody this is supported by an 8 mgal gravity low over the structure indicating a subsurfacelow density body instead the best t is achieved for a bowl shaped structure with a slightcentral rise as expected for an impact crater of this size in mainly sedimentary target thestructure can be either root less i e impact crater or rooted with a narrow feeder dykewith relatively higher magnetic susceptibility and density i e volcanic maar crater thegeophysical signature the solitary appearance the predominantly sedimentary setting andthe comparably large size of the tsenkher structure favor the impact crater alternative however until mineralogical geochemical evidence for an impact is presented the maaralternative remains plausible although exceptional as it would make the tsenkher structureone of the largest in the world in an unusual setting for maar craters introductiongeological settingthe tsenkher structure in the transition zone of thegobi desert and altai mountains fig 1 was recentlyproposed to have formed by a cosmic impact komatsuet al 2006 optical satellite imagery reveals a circularstructure with about 3 6 3 7 km apparent rim to rimdiameter fig 2a the structure is located within a10 20 km wide basin anked by low mountain ranges some mainly granitic intrusions exist in the region komatsu et al 2006 but the basin area in the vicinityof the structure does not show any obvious volcanicfeatures or outcrops the closest known volcanic featureis the 70 m high zurkh volcano 50 km ene of thetsenkher structure of cial development assistanceproject of the czech republic odapcr 2003 sedimentary rocks of probable paleozoic and mesozoicages are exposed in windows through quaternary373

distribution petrography and shock metamorphism of the ejecta of the ries crater in germany a review abstract the lithologies and distribution of the various ejecta formations from the ries crater germany are described they may be subdivided into 1 low shock level 10 gpa represented by polymict crystalline breccias fall out suevites crater suevite and tektites moldavites the ejecta from the mesozoic sedimentary column do not show effects of shock pressures exceeding 10 gpa and only basement gneisses were shock melted to yield suevite glasses it is therefore concluded that the maximum impact energy was transferred from the impactor to the target rocks at a depth of at least 1000 m below ground level where a complex of gneisses underlies shallower granitic intrusions the estimated depth of the transient crater was about 2700 m and its diameter was 12 13 km bunte breccia megablocks of sedimentary rocks and crystalline breccias and megablocks were ejected in ballistic trajectories and form the earliest ejecta deposits extending up to a radius of 22 km fall out suevites are understood to have been lofted from the transient crater by an expanding plume of vaporized rocks and deposited outside the crater as fluidized turbulent masses on top of the earlier ejecta in contrast the crater suevite never left the crater cavity it represents the main mass of suevite moldavites are understood to have been formed during the first encounter of the impactor with the ground by condensation from vaporized and ionizised tertiary sands covering the surface of the target area

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

shock metamorphic microfeatures in chert from the jebel waqf as suwwan impact structure jordan the petrographic investigation of a shocked chalcedony quartzine and quartz bearing allochthonous chert nodule probably upper cretaceous recovered from surficial wadi gravels in the inner parts of the central uplift of the approximately 6 km in diameter jebel waqf as suwwan impact structure jordan reveals new potential shock indicators in microfibrous spherulitic silica in addition to well established shock metamorphic effects in coarser crystalline quartz the microcrystalline chert groundmass exhibits a macroscopic dendritic and suborthogonal fracture pattern commonly associated with thin recrystallization bands that intersect the pre existing diagenetic chert fabric fibrous aggregates of quartzine spherulites in chalcedony quartzine quartz veinlets locally have a shattered appearance and show conspicuous curved fractures perpendicular to the quartzine fiber direction and parallel to 0001 that commonly trend subparallel to planar fractures pfs in neighboring shocked quartz quartz exhibits pfs feather features ffs and mainly single sets of planar deformation features pdfs parallel to the basal plane 0001 brazil twins and rarely additional pdfs parallel to 1013 shock petrography indicates shock pressures of 10 gpa and high shock induced differential stresses that affected the chert nodule the internal crosscutting relationships of primary diagenetic and impact related deformational features together with shockpressure estimates suggest that the curved fractures across quartzine spherulites might represent specific low to medium pressure shock metamorphic features possibly in structural analogy to basal plane pfs in quartz the dendritic suborthogonal fractures in the microcrystalline chert groundmass and recrystallization bands are likely related to impact induced shear deformation and recrystallization respectively and cannot be considered as definite shock indicators

left lateral sense offset of upper proterozoic to paleozoic features across the gobi onon tost and zuunbayan faults in southern mongolia and implications for other central asian faults abstract we present a preliminary study of upper proterozoic to paleozoic features adjacent to and offset across three faults in southern mongolia restoration of four offset stratigraphic units adjacent to the gobi onon fault indicates a left lateral sense offset of 70 95 km the tost fault has a left lateral sense offset of 95 125 km defined by three offset features the zuunbayan fault has seven offset features that display left lateral sense offsets of 185 235 km deformation on these faults apparently occurred during the mesozoic prior to the late cretaceous based upon the age of offset features the age of overlap sequences and inferred timing relationships with regional contractional structures although cenozoic overprinting and reactivation has occurred along parts of these faults thus complicating their restorations we present a testable hypothesis which predicts possible connections between these faults and faults within china

experimental constraints on shock induced microstructures in naturally deformed silicates abstract planar deformation features pdfs in various minerals have long been accepted as evidence of impact induced deformation the uniqueness of this association was challenged in the context of the k t boundary extinction debate after mosaicism and microstructures similar to pdfs were reported from the products of explosive volcanism as a result of this debate a significant volume of new experimental and observational data on the development of shock induced microstructures has become available over the last ten years the results reveal that factors such as pre shock temperature pulse duration and crystallographic orientation of target minerals to the shock wave have a primary influence on how these microstructures develop data from diamond anvil cell and high pressure friction experiments reveal that the same solid state amorphization process that produces shock induced pdfs at low temperatures also occurs at much lower strain rates in static experiments the experimental data indicate that the amorphization process is thermally activated and that the character of the resulting pdfs is a function of the applied strain rate shock induced amorphization occurs along those crystallographic planes that are most readily transformed to the high pressure phase during very short pulse durations and produces pdfs that are visible at the optical scale lower strain rate deformation produces tem scale amorphization with orientations that are more homogeneously distributed throughout the target mineral and produces no optically visible pdfs the data confirm the uniqueness of multiple intersecting sets of optically visible pdfs as a diagnostic indicator of hypervelocity impact the data also support the hypothesis that the amorphization process can occur at a wide range of strain rates and that the limiting pressure for the process is controlled by the phase stability of the target mineral under the applied loading conditions not by the hel the data also suggest that the onset pressure the maximum pressure and the pressure range for producing optically visible pdfs decrease with increasing temperature and decreasing strain rate as the pressure range for optically visible pdf formation decreases it is replaced by homogeneous amorphization as observed in the anvil cell experiments thus the uniqueness of multiple intersecting sets of optically visible pdfs to hypervelocity impact is not due to a unique process but rather to a specific set of loading conditions that produce an optically visible microstructure likewise single sets of microdeformations observed in volcanic rocks are produced by the same process but at different loading conditions that preclude the development of multiple intersecting sets of pdfs the data also indicate that shock mosaicism which occurs above the hel represents a plastic response of the target mineral to loading rates that are too large to be accommodated by crystal plastic mechanisms observational data for some naturally deformed samples from the k t boundary the vredefort dome and volcanic rocks along with the experimental observations are used to constrain the range of conditions under which the natural microstructures form and to understand the differences between microstructures produced by impact volcanic and other natural processes finally some possible mechanisms for producing the microstructures observed in volcanic rocks are proposed

granites of the southern mongolia carboniferous arc new geochronological and geochemical constraints the crust in southern mongolia is part of the central asian orogenic belt a vast accretionary orogen that records the opening and closure of the palaeo asian ocean in the late proterozoic to palaeozoic the crustal evolution of the region is revealed in basement inliers that also contain intrusion related porphyry ore bodies that are important mineral exploration targets the saykhandulaan inlier in southeast mongolia is a devonian carboniferous segment of island arc crust which is dominantly composed of extrusive and sedimentary lithologies but which also contains the oyut ulaan i type quartz monzonite intrusion a u pb zircon age for the oyut ulaan monzonite indicates emplacement at 330 0 0 5 ma to the east of the saykhandulaan inlier intrusive complexes dominate the neighbouring mandakh inlier new ages are presented for four of these plutons the bronze fox granodiorite 333 6 0 6 ma the narin hudag monzonite 333 2 0 6 ma the shuteen quartz monzonite 325 5 1 0 ma and the north mandakh granite 292 3 0 5 the intrusive bodies of the saykhandulaan and mandakh inliers have two distinct geochronological and geochemical associations 1 mid carboniferous i type monzonites that constitute the most easterly intrusive expression of the southern mongolia carboniferous arc and 2 early permian a type and peralkaline granites that represent a post orogenic phase of voluminous granite emplacement both groups are significantly younger than the nearby oyu tolgoi and tsagaan suvarga cu porphyry ore bodies which have previously been dated as early and late devonian respectively the new data presented here provide constraints on the timing of the transition from island arc magmatism to post collisional extension related magmatism in the region and possible controls on fertile and infertile granitoid intrusions with respect to cu au mineralisation

tectonics and hydrocarbon systems of the east gobi basin mongolia mapping in the east gobi basin supplemented by seismic data reveals a structural and burial history for basins adjacent to the zuunbayan and tsagaan els oil fields the tectonic framework was combined with available well and outcrop data to model the timing and magnitude of hydrocarbon generation five structural episodes are recognized 1 pre jurassic northeast directed shortening that formed the tectonic fabric 2 middle jurassic to early cretaceous rifting along northeast trends that formed the subbasins of the east gobi basin 3 late early cretaceous north south shortening and inversion on existing normal faults shortening caused left lateral and reverse displacements on northeast trending faults 4 middle cretaceous uplift and erosion followed by 5 east west shortening and right lateral movement on northeast faults folds formed by inversion over middle jurassicearly cretaceous normal faults modeling suggests that the bituminous member of the zuunbayan formation should be mature over large parts of the unegt and zuunbayan subbasins oil migrated from mature source areas toward several traps including the zuunbayan and tsagaan els fields modeling suggests that early oil 104110 ma was generated in the zuunbayan and tsagaan els area because of deep burial during the cretaceous although generation began in the early cretaceous peak generation in the unegt subbasin occurred between 100 and 90 ma generation continued at a decreasing rate up to the present day kerogen maturity and oil field production suggests that oil is the most likely product scoping calculations of hydrocarbon volumes generated indicate that the unegt basin may have generated as much as 86 billion boe

tertiary strike slip faulting in southeastern mongolia and implications for asian tectonics abstract geologic maps have long portrayed the late cretaceous recent geologic history of southeastern mongolia as tectonically quiescent we present new data based on outcrop observations that indicate the northeast trending east gobi fault zone egfz was reactivated in the cenozoic as a sinistral strike slip fault system inversions of cenozoic fault slip data imply that faulting was associated with north northwest subhorizontal shortening and east northeast subhorizontal extension we propose that faulting is tertiary in age based on published interpretations of seismic reflection data which reveal that the mid cretaceous 100 95 ma unconformity is deformed by strike slip faults and based on field observation of strike slip faults and fracture sets that cut upper cretaceous and cenozoic strata but lack evidence for neotectonic activity published seismicity maps also appear to argue against significant quaternary faulting within the egfz these new data may lend credence to published models proposing a middle miocene or older kinematic linkage between the egfz and the altyn tagh fault in china the recognition that the egfz has a history of left lateral displacement in both the early mesozoic and cenozoic means that currently available estimates of offset based on displaced paleozoic rocks constrain total offset only this reactivation history supports the notion that inherited lithospheric structures are important in controlling the location and thus modes of intracontinental deformation in asia as a function of collisional far field effects and evolving boundary conditions of the pacific margin

a 5500 year proxy climate and vegetation record from blanket mire at talla moss borders scotland a monolith of peat taken from an upland blanket mire at talla moss southern scotland was sub jected to peat humification and pollen analyses to produce both a proxy climate record and a vegetational history covering the last 5500 years while the peat showed little visible stratigraphy with no intimation of major peat humification changes colorimetric data indicate a markedly oscillatory climate record which is apparently largely independent of or out of phase with major vegetational changes the raw data imply particular wet shifts in climate at c 3455 bp c 2600 bp c 1930 bp c 1095 bp with a markedly wet or cool and wet episode commencing at c 540 bp these are central age estimates and should not be regarded as precise dates for the inferred climate shifts other wet shifts apparently date from c 3070 bp c 2265 bp and c 1700 bp although the first of these corresponds with pollen evidence for significant prehistoric human activity in the locality spectral analysis of the peat h

on the formation of collapse dolines a modelling perspective abstract collapse dolines are among the most striking surface features in karst areas although they can be the result of different formation mechanisms evidence suggests that large collapse dolines form due to chemical and mechanical removal of material at and below the level of groundwater we have applied a genetic model of a two dimensional fracture network to calculate the rate of dissolutional bedrock removal in the heavily fractured crushed zone intersecting a karst conduit in the phreatic zone to account for infilling and breakdown processes in the crushed zone two simple rules were added to the basic model 1 continuous infilling of dissolutionally created voids prevents fractures from growing beyond some limited aperture although the dissolution proceeds 2 discontinuous collapsing causes sudden closure of a fracture once some critical aperture has been reached both rules limit the transmissivity of the network and the related flow rates therefore the constant head difference between the input and the output points is sustained and the flow remains distributed over the entire crushed zone provided that restrictions posed by the two rules permit turbulent flow dissolution rates also remain high in the entire region high surface area of water rock contact and high dissolution rates result in high overall removal rates of rock from the crushed zone one of the necessary conditions for the formation of large closed depressions despite the fact that the model neglects some processes and dynamics that would increase the removal rate the results suggest that large closed depressions could form in the order of 1 million years

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

the circular uneged uul structure east gobi basin mongolia geomorphic and structural evidence for meteorite impact into an unconsolidated coarse clastic target abstract the uneged uul structure is a 10 km circular complex multi ridged domal feature in the unegt subbasin of the east gobi basin southeastern mongolia as revealed by remote sensing and recent field reconnaissance the central part of the uneged uul structure comprises a complex central peak of outward radiating curved ridges composed of stratigraphically uplifted greenschist facies basement schists surrounded by an annular moat the most prominent feature of the structure is a central annular ridge 3 km in diameter composed of pebble boulder conglomerates and gravels of the upper jurassic sharilyn formation surrounded by three outer domal ridges composed of lower cretaceous conglomeratic sandstones and gypsum clays jurassic conglomerates forming the main part of the central annular ridge show effects of severe internal deformation the original population of pebbles cobbles and boulders appears moderately displaced and mostly broken but nowhere aligned along shear planes or foliated primary sedimentary features such as cross lamination or imbrication have been obliterated we explain this penetrative brecciation as a result of dissipative shearing caused by a strong and rapid singular event that in magnitude was beyond the range of the common crustal tectonics recorded elsewhere in this region disrupted and chaotically distributed conglomeratic sandstone beds in the central annular ridge dip in highly variable directions on a local scale but show an apparent se nw trend of bedding plane alignment further outside the tilted and uplifted upper jurassic to lower cretaceous strata of the domal area are overlain by the flat lying upper cretaceous which stratigraphically constrains the timing of deformation at the uneged uul structure to most likely the early cretaceous endogenic formation models such as magmatism and salt gypsum or mud diapirism fail to explain the nature of the uneged uul structure the uneged uul structure bears a set of geomorphic and structural features resembling those at some eroded complex impact structures on earth morphologically similar central peaks are observed at the spider and matt wilson impact structures in australia the central annular ridge reminds of that at gosses bluff in australia the outer domal ridges might correspond to ring like features as known from tin bider in algeria we therefore cautiously propose that an impact may have produced the uneged uul feature causing structural uplift 1000 m of basement rocks at its center so far no convincing evidence for shock metamorphism could be proven by field work and petrographic analyses however it is likely that at the time of the deformation event the unconsolidated conglomerates were highly porous and possibly immersed in groundwater buffering the propagation of sudden stress reducing deformation further studies will be in order to unravel the nature of the uneged uul structure which should be considered a promising possible impact structure

resolving the richat enigma doming and hydrothermal karstification above an alkaline complex the richat structure sahara mauritania appears as a large dome at least 40 km in diameter within a late proterozoic to ordovician sequence erosion has created circular cuestas represented by three nested rings dipping outward from the structure the center of the structure consists of a limestone dolomite shelf that encloses a kilometer scale siliceous breccia and is intruded by basaltic ring dikes kimberlitic intrusions and alkaline volcanic rocks several hypotheses have been presented to explain the spectacular richat structure and breccia but their origin remains enigmatic the breccia body is lenticular in shape and irregularly thins at its extremities to only a few meters the breccia was created during karst dissolution and collapse internal sediments fill the centimeter to meter scale cavities alkaline enrichment and the presence of cretaceous automorphous neoformed k feldspar demonstrate the hydrothermal origin of these internal sediments and their contemporaneity with magmatism a model is proposed in which doming and the production of hydrothermal fluids were instrumental in creating a favorable setting for dissolution the circular richat structure and its breccia core thus represent the superficial expression of a cretaceous alkaline complex with an exceptionally well preserved hydrothermal karst infilling at its summit

planar deformation features in quartz from impact produced polymict breccia of the xiuyan crater china the 1 8 km diameter xiuyan crater is an impact structure in northeastern china exposed in a proterozoic metamorphic rock complex the major rocks of the crater are composed of granulite hornblendite gneiss tremolite marble and marble the bottom at the center of the crater covers about 100 m thick lacustrine sediments underlain by 188 m thick crater fill breccia a layer of polymict breccia composed of clasts of granulite gneiss hornblendite and fragments of glass as well as clastic matrix occurs near the base in the depth interval from 260 to 295 m an investigation in quartz from the polymict breccia in the crater fill units reveals abundant planar deformation features pdfs quartz with multiple sets of pdfs is found in clasts of granulite that consist of mainly quartz and feldspar and in fine grained matrix of the impact produced polymict breccia a universal stage was used to measure the orientation of pdfs in 70 grains of quartz from five thin sections made from the clasts of granulite of polymict breccia recovered at the depth of 290 m forty four percent of the quartz grains contain three sets of pdfs and another 40 contain two sets of pdfs the most abundant pdfs are rhombohedron forms of pf10 12g xf10 13g a ndr zf10 11g with frequency of 33 5 22 3 and 9 6 respectively a predominant pdf form of f10 12g in quartz suggests a shock pressure 20 gpa the occurrence of pdfs in quartz from the polymict breccia provides crucial evidence for shock metamorphism of target rocks and confirms the impact origin of this crater which thus appears to be the first confirmed impact crater in china

dating raised bogs new aspects of ams 14c wiggle matching a reservoir effect and climatic change abstract high resolution ams dating of holocene raised bog deposits engbertsdijksvenen the netherlands shows natural 14c variations wiggles which can be matched with the dendrochronological calibration curve comparison of our results with other conventionally dated peat cores and the 14c record shows an unexpected and as yet unreported reservoir effect and a marked and sudden wetting of the bog surfaces coinciding with the advent of a sporer type solar minimum the homeric minimum the potential of ams dating to vastly improve the accuracy of holocene chronologies is demonstrated

steinheim suevite a first report of melt bearing impactites from the steinheim basin sw germany the 3 8 km steinheim basin in sw germany is a complex impact crater with central uplift hosted by a sequence of triassic to jurassic sedimentary rocks it exhibits a well preserved crater morphology intensely brecciated limestone blocks that form the crater rim as well as distinct shatter cones in limestones in addition an impact breccia mainly composed of middle to upper jurassic limestones marls mudstones and sandstones is known from drilling into the impact crater no impact melt lithologies however have so far been reported from the steinheim basin in samples of the breccia that were taken from the b 26 drill core we discovered small particles up to millimeters in size that are rich in sio2 50 wt and al2o3 28 wt and contain particles of fe ni co sulfides as well as target rock clasts shocked and unshocked quartz feldspar limestone and droplet shaped particles of calcite the particles exhibit distinct flow structures and relicts of schlieren and vesicles from the geochemical composition and the textural properties we interpret these particles as mixed silicate melt fragments widely recrystallized altered and or transformed into hydrous phyllosilicates furthermore we detected schlieren of lechatelierite and recrystallized carbonate melt on the basis of impactite nomenclature the melt bearing impact breccia in the steinheim basin can be denominated as steinheim suevite the geochemical character of the mixed melt particles points to middle jurassic sandstones eisensandstein formation that crop out at the center of the central uplift as the source for the melt fragments

mutually constrained geophysical data for the evaluation of a proposed impact structure lake hummeln sweden abstract lake hummeln covers a 1 2 km wide near circular depression in the precambrian basement of the baltic shield in the depression more than 150 m thick cambrian and ordovician marine sediments are underlain by non volcanic breccia the only explanation that does not meet contradictions is that the structure was caused by impact but evidence of shock metamorphism is until now missing this study focuses on the geophysical characterisation and includes magnetic and gravimetric modelling constrained by information from resistivity measurements geological mapping and a drill core the hummeln structure shows both a gravimetric and a magnetic low the magnetic low is not broken by any short wavelength anomalies which would indicate the presence of remanent melt occurrences the diameter of the magnetic low is about twice that of the proposed crater depression and probably indicates fracturing both the magnetic and the gravimetric models show a structure that is consistent with the impact hypothesis resistivity measurements were carried out in the surroundings of the proposed crater the fractured region indicated by the magnetometry does not produce a detectable resistivity anomaly however resistivity measurements proved useful in investigating the tectonic fracture zones crossing the area in the vicinity of the structure following the impact model a reconstruction of the pre erosional structure resulted in an approximately 50 wider original crater than seen today

endogenous and nonimpact origin of the arkenu circular structures al kufrah basin se libya the twin arkenu circular structures acs located in the al kufrah basin in southeastern libya were previously considered as double impact craters the arkenu craters the acs consist of a ne arkenu 1 and a sw structure arkenu 2 with approximate diameters of about 10 km they are characterized by two shallow depressions surrounded by concentric circular ridges and silica impregnated sedimentary dikes cut by local faults our field petrographic and textural observations exclude that the acs have an impact origin in fact we did not observe any evidence of shock metamorphism such as planar deformation features in the quartz grains of the collected samples and the previously reported shatter cones are wind erosion features in sandstones ventifacts conversely the acs should be regarded as a paired intrusion of porphyritic stocks of syenitic composition that inject the nubia formation and form a rather simple and eroded ring dike complex stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite hematite horizons typical of iron oxide copper gold deposits their origin was nearly coeval with the development of silicified dikes in the surroundings plugs of tephritic phonolitic rocks and lamprophyres monchiquites inject the nubian sandstone along conjugate fracture zones trending nnw sse and ne sw that crosscut the structural axis of the basin

variation in 14c age of macrofossils and different fractions of minute peat samples dated by ams accurate 14c dating of peat samples is crucial if peat stratigraphies are to be used as historical archives to analyse the variation in 14c age within small peat samples the 14c age of different mire plant remains and peat fractions n 39 from 2 cm thick peat slices was determined using accelerator mass spectrometry the 14c ages of mosses were assumed to be the most accurate to verify this we also tested the possibility that uncertainties could be introduced by mosses at the mire surface reassimilating co2 produced during the decomposition of old peat the average 14c content of living mosses 110 5 110 9 absolute modern pm did not differ between threesphagnum species representing hummock carpet and lawn growing species their 14c contents were also identical within experimental limits to the current 14c content of the atmosphere depending on sampled depth and peat type the differences in calibrated 14c age between different fragments or fractions within specific 2 cm thick peat samples

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

impact into unconsolidated water rich sediments at the marquez dome texas the marquez dome leon county texas represents a 13 km diameter paleocene eocene impact structure formed in largely unconsolidated sediments in a near shore environment the present study is an analysis of samples from cores taken from boreholes drilled separately on the edge of the central uplift and in the surrounding annular basin the borehole drilled in the annular basin of the structure penetrated a sequence of interbedded sands silts and shales that is typical of the stratigraphy of the surrounding area in contrast the borehole drilled on the edge of the central uplift penetrated material that is relatively homogeneous in chemical composition and texture and may represent a mixture of sand silt clay and minor carbonate derived from deeper levels in the preimpact stratigraphy veins containing pseudotachylitic breccias are not found and are not expected in this environment because low strength target materials are not conducive to frictional melting similarly the low strength and unconsolidated nature of these target materials are not conducive to the formation of other types of typical impact breccias e g melt rocks or suevites the absence of such lithologies results either from explosive ejection of these materials caused by the water rich character of the target sediments or more probably from removal of these materials by deeper postimpact erosion than has been suggested previously planar deformation features pdfs were not found in quartz grains from any of these samples the scarcity of quartz grains with pdfs which have only been reported in rare impact breccias from the central uplift and the large amount of vertical displacement indicated for the central uplift of this structure may also be a consequence of the low strength of target materials

evidence from northwest european bogs shows little ice age climatic changes driven by variations in solar activity fluctuations in holocene atmospheric radiocarbon concentrations have been shown to be due to variations in solar activity analyses of both 10 be and 14 c nuclides cone rm that production ratechanges during

shock metamorphic features in amphiboles from the xiuyan crater of china amphibole bearing gneiss fragments are common in the impact breccias of the xiuyan crater china three kinds of amphibole bearing gneiss fragments with different shock metamorphic levels have been identified shock metamorphic features of amphiboles in these gneisses were investigated in situ by optical microscope electron microprobe raman spectroscopy and x ray diffraction amphiboles in the weakly shocked gneiss shock pressure less than 10 gpa basically remain intact amphiboles in the moderately shocked gneiss shock pressure range between 35 and 45 gpa show strong deformation reduced optical interference color and partial loss of oh in the strongly shocked gneiss shock pressure above 50 gpa amphiboles are completely melted and dendritic pyroxenes crystallize from the melt the formation of dendritic pyroxenes shows nearly complete loss of water in the amphibole melt at shock induced high temperature above 1 500 c the occurrence of both diopside and pigeonite dendrites crystallized in the same amphibole melt shows inhomogenous melt composition and rapid cooling of the melt

the mud volcanoes of pakistan marine geologic investigations on the arabian sea by bundesanstalt fur geowissenschaften und rohstoffe bgr in 1995 and 1998 and land expeditions in 1998 and 1999 to the coastal regions of the makran desert pakistan have extended the knowledge of the aerial distribution of mud volcanoes these structures rise from under compacted formations within the regional accretionary prism which is built by the subduction of the oceanic crust of the arabian sea and its km thick sedimentary load the occurrence of mud volcanoes is limited to the abyssal plain near the accretionary front to the coastal region of the makran desert and to a region in the interior of the desert to the south to southeast of the so called hinglay synform the location of mud volcanoes in pakistan is clearly tied to fault systems mud volcanoes are conspicuously absent on the lower slope of the accretionary prism where thick gas hydrate layers have developed the presence of large gas plumes emerging from the seafloor landward of the gas hydrate stability zone at water depths of less than 800 m points to a redirection of fluids from depth which might explain the absence of mud volcanoes along the lower slope

the first large meteorite impact structure discovered in the middle east jebel waqf as suwwan jordan triggered by re evaluation of a 1960s report on the regional geology of the northeastern border region of jordan and following landsat satellite image investigation a 5 5 km diameter complex circular structure was discovered in the central eastern region of the kingdom of jordan initial ground truthing revealed complex geological structures involving upper cretaceous and paleogene strata and including a prominent outer rim rising up to 60 m above the surrounding plain an intermediate ring of up to 20 m elevation within a ring syncline and a central zone of stratigraphically uplifted sedimentary strata characterized by intense macroscopic folding and faulting widespread cataclasis and locally mesoscopic cataclasis deformation ten sites with shatter cone development in fine grained sandstone or limestone have been mapped to date mostly in the outer parts of the central uplifted area this finding confirms that the jebel waqf as suwwan structure was formed as the result of the impact of an extraterrestrial projectile search for impact diagnostic micro deformation has been rather unsuccessful only 1 quartz grain with both planar deformation features and planar fractures has been detected in a sandstone sample to date the overall majority of the approximately 70 samples investigated by micropetrographic analysis consist of extremely fine grained chert siltstone or marly limestone cataclasis is widespread in chert and limestone also on the micro scale considering the severely limited amount of characteristic impact microdeformation and the stratigraphic situation within the central uplift it is likely that a relatively deep level of the central uplift is currently exposed the extensive drainage demonstrated for this region supports the conclusion that this impact structure could be quite deeply eroded especially as its geology involves some relatively soft lithologies marls limestones the age of this impact event is at present poorly constrained at post middle to lower eocene

meteorites from mongolia the mineralogy and composition of six mongolian meteorites were studied in some detail previously only limited information existed about these rocks and some were still unclassified the six meteorites include three ordinary chondrites and three irons the ordinary chondrite adzhi bogdo stone is a regolith breccia ll3 6 containing various types of clasts some of foreign origin embedded within a fine grained clastic matrix tugalin bulen h6 and noyan bogdo l6 meteorites are typical well metamorphosed ordinary chondrites adzhi bogdo iron has to be regarded as an ia iron meteorite like campo del cielo or canyon diablo although the sample studied had been heated to about 900 c 950 c some time in the past thus eradicating all original structural elements manlai is structurally closely related to the iic iron meteorites but based on its chemistry which does not fit into this group it is suggested that manlai is an anomalous iron meteorite the third iron sargiin gobi is certainly a normal member of the ia iron meteorites the concentrations and isotopic compositions of he ne and ar were measured for all meteorites and their gas retention ages and exposure ages are discussed

quality of ptrm acquisition in pyrrhotite bearing contact metamorphic limestones possibility of a continuous record of earth magnetic field variations abstract the rockmagnetic and palaeomagnetic signal in pyrrhotite bearing limestones of different contact metamorphic settings were investigated related to intrusions ranging from small sills to large magmatic complexes magnetic susceptibility the pyrrhotite magnetite ratio and thermal modelling serve as an investigative tool to define three metamorphic zonations a contact zone of a mixed magnetic assemblage and low susceptibilities a pyrrhotite bearing transitional zone where full thermoremanent magnetisations trms are acquired due to temperatures above the curie temperature of pyrrhotite t c po and a marginal zone containing pyrrhotite and magnetite generated at temperatures below t c po the fact that trms can consist of independent ptrms is successfully tested by modified thellier experiments it is shown that a metamorphic environment with low fluid circulation provides a scenario for the recording of independent ptrms multicomponent behaviour of the nrm residing in samples from the transitional zone can therefore be interpreted as a continuous record of earth magnetic field variations

combinations of processes responsible for martian impact crater layered ejecta structures emplacement 1 we utilized images and stereo derived topographic data acquired by the high resolution stereo camera hrsc and thermal emission imaging system themis images together with other data in order to study the geology of layered ejecta structures associated with relatively pristine martian impact craters the geomorphology and morphometric properties indicate their origin as complex combinations of a variety of impact processes the studied inner layered ejecta structures often exhibit ground hugging characteristics and many of them do not have topographic profiles expected from simple ballistic emplacement such profiles include ones that are plateau shaped or thickening outward we think that water rich fluidized flows driven by the momentum due to the impact and by gravity together with ballistic emplacement and vortex produced by the atmosphere ejecta curtain interaction were essential to the inner layered ejecta structure formation we hypothesize that the thinner outer layered ejecta structures were formed by various combinations of shockwave induced liquefaction of water rich near surface sediments ballistic emplacement of ejecta entraining water and strong winds expanding vapor vortex base surge related to the impact the contribution of each proposed layered ejecta structure formation mechanism should have been variable depending on the condition of the impact

structural evolution of the 40 km wide araguainha impact structure central brazil the 40 km wide araguainha structure in central brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation the dominant structural features preserved at araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity slumping of the transient cavity walls resulted in inward verging inclined folds and a km scale anticline in the outer ring of the structure the folding stage was followed by radial and concentric faulting with downward displacement of kilometer scale blocks around the crater rim the central uplift records evidence for km scale upward movement of crystalline basement rocks from the transient cavity floor and lateral moment of sedimentary target rocks detached from the cavity walls much of the structural grain in the central uplift relates to structural stacking of km scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks outward plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets each individual sheet records an early stage of folding and thickening due to non coaxial strains shortly before sheet imbrication we attribute this folding and thickening phase to the kilometer scale inward movement of the target strata from the transient cavity walls to the central uplift the outer parts of the central uplift record additional outward movement of the target rocks possibly related to the collapse of the central uplift an inner ring structure at 10 12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks

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

high resolution records of late holocene climate change and carbon accumulation in two north west european ombrotrophic peat bogs the peat stratigraphy plant macrofossils colorimetric humification pollen non pollen microfossils carbon nitrogen ratios of three replicate cores from a raised peat bog in the uk walton moss and a single core from a raised peat bog in denmark lille vildmose were examined in an attempt to investigate the relation between long term climate change and changes in species composition of the peat forming vegetation nine wet shifts identified in the monoliths from walton moss and lille vildmose mainly occurred during periods of increasing atmospheric c 14 content because fluctuations in holocene atmospheric radiocarbon concentrations have been shown to be due to variations in solar activity the little ice age lia climatic deteriorations registered in both peat bogs were probably driven by changes in solar activity climatic deteriorations in the three replicate cores from walton moss are not all consistently registered and may reflect the internal variability in the extent of hollow lawn hummock microforms within raised peat bog ecosystems the lowest carbon accumulation values for the walton moss monoliths between ca cal ad 1300 and 1800 and between ca cal ad 1490 and 1580 for lille vildmose occurred during the course of lia climatic deteriorations peat bog primary productivity may have been reduced during the lia owing to lower spring summer temperatures and shorter growing seasons for the peat forming plants species specific higher decomposition rates may have also caused the lower carbon accumulation rates in the peat bogs during the lia c 2002 elsevier science b v all rights reserved

solar activity during the last 1000 yr inferred from radionuclide records identification of the causes of past climate change requires detailed knowledge of one of the most important natural factors solar forcing prior to the period of direct solar observations radionuclide abundances in natural archives provide the best known proxies for changes in solar activity here we present two independent reconstructions of changes in solar activity during the last 1000 yr which are inferred from be 10 and c 14 records we analyse the tree ring c 14 data shcal intca104 from 1000 to 1510 ad and annual data from 1511 to 1950 ad and four be 10 records from greenland ice cores camp century grip milcent and dye3 together with two be 10 records from antarctic ice cores dome concordia and south pole in general the be 10 and c 14 records exhibit good agreement that allows us to obtain reliable estimates of past solar magnetic modulation of the radionuclide production rates differences between be 10 records from antarctica and greenland indicate that climatic changes have influenced the deposition of be 10 during some periods of the last 1000 yr the radionuclide based reconstructions of past changes in solar activity do not always agree with the sunspot record which indicates that the coupling between those proxies is not as close as has been sometimes assumed the tree ring c 14 record and be 10 from antarctica indicate that recent solar activity is high but not exceptional with respect to the last 1000 yr c 2006 elsevier ltd all rights reserved less

ash shutbah a possible impact structure in saudi arabia we have investigated the ash shutbah circular structure in central saudi arabia 21 degrees 37n 45 degrees 39e using satellite imagery field mapping thin section petrography and x ray diffraction of collected samples the approximately 2 1km sized structure located in flat lying jurassic tuwaiq mountain limestone has been nearly peneplained by erosional processes satellite and structural data show a central area consisting of dhruma formation sandstones with steep bedding and tight folds plunging radially outward open folding occurs in displaced younger tuwaiq mountain limestone formation blocks surrounding the central area but is absent outside the circular structure an approximately 60cm thick unique folded and disrupted orthoquartzitic sandstone marker bed occurring in the central area of the structure is found 140m deeper in undisturbed escarpment outcrops located a few hundred meters west of the structure with exception of a possible concave shatter cone found in the orthoquartzite of the central area other diagnostic shock features are lacking some quartz rich sandstones from the central area show pervasive fracturing of quartz grains with common concussion fractures this deformation was followed by an event of quartz dissolution and calcite precipitation consistent with local sea or groundwater heating the combination of central stratigraphic uplift of 140m concussion features in discolored sandstone outward dipping concentric folds in the central area deformation restricted to the rocks of the ring structure a complex circular structure of 2 1km diameter that appears broadly consistent with what one would expect from an impact structure in sedimentary targets and a possible shatter cone all point to an impact origin of the ash shutbah structure in fact the ash shutbah structure appears to be a textbook example of an eroded complex impact crater located in flat lying sedimentary rocks where the undisturbed stratigraphic section can be studied in escarpment outcrops in the vicinity of the structure less

late triassic sinistral shear in the east gobi fault zone mongolia abstract tectonic studies of the east gobi fault zone in southeastern mongolia reveal multiple distinct intracontinental deformation events postdating late paleozoic arc accretion and continental amalgamation metamorphic tectonites of the tsagan subarga and tavan har blocks previously mapped as precambrian basement comprise a sinistral shear zone dominated by steeply dipping northeast striking foliations field observations and petrographic analyses indicate that the protoliths of the metamorphic tectonites are paleozoic arc volcanic and sedimentary sequences 40ar 39ar step heating analyses of minerals from pre syn and late to post kinematic lithologies bracket the timing of ductile sinistral shear as late triassic the main phase of distributed deformation associated with cooling through amphibolite upper greenschist facies conditions occurred ca 225 ma and shear zone activity waned ca 210 ma cooling rates inferred from the 40ar 39ar data are on the order of 40 20 c myr 1 apparent differences for the two basement blocks may reflect subsequent differential uplift during late jurassic cretaceous rifting relatively rapid late triassic cooling suggests a transtensional component to the deformation and is coincident with core complex formation in northern china late triassic intraplate deformation in southeastern mongolia is likely the result of far field forces associated with collision between mongolian arcs and the siberian craton i e closure of the mongol okhotsk ocean and or collisions associated with closure of the paleotethys the ductile shear zone has been documented over 250 km along strike and has been modified by subsequent brittle deformation events in the mesozoic and cenozoic

geology of lonar crater india lonar crater india is one of the youngest and best preserved impact structures on earth the 1 88 km diameter simple crater formed entirely within the deccan traps making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the moon in this study we present a meter scale resolution digital elevation model geological map of lonar crater and the surrounding area and radiocarbon ages for histosols beneath the distal ejecta impact related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric subhorizontal noncylindrical fold axes at the crater rim the rim fold hinge is preserved around 10 15 of the crater although tearing in the rim fold is inferred from fi eld and paleomagnetic observations no tear faults are identifi ed indicating that large displacements in the crater walls are not characteristic of small craters in basalt one signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer parallel slip faults there is little fl uvial erosion of the continuous ejecta blanket portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules in particular in the eastern and western rim as well as in the depression north of the crater known as little lonar the emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow based on the preserved thickness distribution of the ejecta the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol and the lack of sorting and stratifi cation in the bulk of the ejecta the ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on mars

the dhala structure bundelkhand craton central india eroded remnant of a large paleoproterozoic impact structure the newly discovered dhala structure madhya pradesh state india is the eroded remnant of an impact structure with an estimated present day apparent diameter of about 11 km it is located in the northwestern part of the archean bundelkhand craton the pre impact country rocks are predominantly granitoids of 2 5 ga age with minor 2 0 2 15 ga mafic intrusive rocks and they are overlain by post impact sediments of the presumably 1 7 ga vindhyan supergroup thus the age for this impact event is currently bracketed by these two sequences the dhala structure is asymmetrically disposed with respect to a central elevated area cea of vindhyan sediments the cea is surrounded by two prominent morphological rings comprising pre vindhyan arenaceousargillaceous and partially rudaceous metasediments and monomict granitoid breccia respectively there are also scattered outcrops of impact melt breccia exposed towards the inner edge of the monomict breccia zone occurring over a nearly 6 km long trend and with a maximum outcrop width of 170 m many lithic and mineral clasts within the melt breccia exhibit diagnostic shock metamorphic features including multiple sets of planar deformation features pdfs in quartz and feldspar ballen textured quartz occurrences of coesite and feldspar with checkerboard texture in addition various thermal alteration textures have been found in clasts of initially superheated impact melt the impact melt breccia also contains numerous fragments composed of partially devitrified impact melt that is mixed with unshocked as well as shock deformed quartz and feldspar clasts the chemical compositions of the impact melt rock and the regionally occurring granitoids are similar the ir contents of various impact melt breccia samples are close to the detection limit 1 1 5 ppb and do not provide evidence for the presence of a meteoritic component in the melt breccia the presence of diagnostic shock features in mineral and lithic clasts in impact melt breccia confirm dhala as an impact structure at 11 km dhala is the largest impact structure currently known in the region between the mediterranean and southeast asia

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

deformational features and impact generated breccia from the sierra madera impact structure west texas deformational features from the sierra madera impact structure west texas were studied to estimate shock temperature and shock pressure values involved during its formation sandstone limestone and polymict impact generated breccia samples occur within the central uplift the sandstones host several shock related deformational features including toasted quartz grains and planar microstructures planar fractures and planar deformation features in quartz zircon mineral grains separated from the sandstone samples provide evidence of shock deformation in the form of planar fractures and granular surface textures both sandstone and carbonate within the central uplift contain shatter cones whereas polymict impact generated breccia samples contain shatter cone fragments and clasts of mixed lithology most high temperature features e g high temperature mineral phases and a melt sheet typically associated with less eroded structures of similar size are lacking however some small millimeter size possible devitrified glass clasts within polymict breccias occur at several locations deformational features within zircon mineral grains shatter cones and quartz grains with multiple planar microstructures record pressure and temperature conditions during the impact of 3 20 gpa and postshock temperatures between 350 and 1000 c deformational conditions during the impact event were almost certainly higher given that erosion has presumably removed most of the more severely shocked rocks

geophysical investigation of the beinn an dubhaich granite skye there has been a long standing controversy concerning the form of the beinn an dubhaich granite which intrudes cambro ordovician limestones and contains enclosures of limestone at outcrop we have undertaken a three part geophysical investigation with the aim of resolving some issues raised by previous workers our most significant finding results from the measurement of a gravity profile across beinn an dubhaich it reveals a negative anomaly of more than 2 mgal over the granite outcrop our preferred model to fit this anomaly is a steep sided granite stock extending down to about 1 km depth which implies that the limestone enclosures are roof pendants in a supplementary study we measured magnetic profiles across dykes in some of the limestone enclosures in order to determine the depth to the contact with the underlying granite however in the most clear cut cases we could only determine a minimum depth of 20 m and were unable to constrain the maximum depth in hindsight we suggest that microgravity surveying would be a better method for determining the thicknesses of the limestone roof pendants in the third part of the investigation we measured a tight grid of magnetic profiles to the northeast of the granite outcrop to test a previous suggestion that dyke like lobes of granite intrude pre existing fractures in the overlying limestones we located several large magnetic anomalies associated with outcropping doleritic dykes and concluded that these had not been correctly identified in the earlier work consequently the ground magnetic profiles acquired to date provide no valid evidence to support the idea of an interfingering contact between the limestones and the underlying granite

structure and evolution of upheaval dome a pinched off salt diapir upheaval dome canyonlands national park utah is an enigmatic structure previously attributed to underlying salt doming cryptovolcanic explosion fluid escape or meteoritic impact we propose that an overhanging diapir of partly extrusive salt was pinched off from its stem and subsequently eroded many features support this inference especially synsedimentary structures that indicate jurassic growth of the dome over at least 20 m y conversely evidence favoring other hypotheses seems sparse and equivocal in the rim syncline strata were thinned by circumferentially striking low angle extensional faults verging both inward toward the center of the dome and outward near the dome s core radial shortening produced constrictional bulk strain forming an inward verging thrust duplex and tight to isoclinal circumferentially trending folds farther inward circumferential shortening predominated radially trending growth folds and imbricate thrusts pass inward into steep clastic dikes in the dome s core we infer that abortive salt glaciers spread from a passive salt stock during late triassic and early jurassic time during middle jurassic time the allochthonous salt spread into a pancake shaped glacier inferred to be 3 km in diameter diapiric pinch off may have involved inward gravitational collapse of the country rocks which intensely constricted the center of the dome sediments in the axial shear zone beneath the glacier steepened to near vertical the central uplift is inferred to be the toe of the convergent gravity spreading system

14c ams wiggle matching of raised bog deposits and models of peat accumulation abstract high resolution accelerator mass spectrometer ams 14 c dates of selected plant macrofossils from the raised bog engbertsdijksvenen eastern netherlands show century scale wiggles analogous to the radiocarbon calibration curve we used three relative time scales viz based on depth mass and pollen concentration respectively to match the peat ams dates to the calibration curve this procedure is repeated for one conventionally dated core for each relative time scale realistic confidence intervals are calculated depth appears to be the best time scale for certain stratigraphical units of a core this justifies using depth for wiggle matching conventionally dated mire cores published by others even when 14 c errors are larger our evidence shows four major sources of 14 c variation for mire deposits compared to treerings 1 dating error due to sample composition this includes a reservoir effect demonstrated for many bulk peat samples 2 hiatuses causing a sudden leap of peat 14 c age 3 changing accumulation rates apparent from a break in the slope of the peat 14 c ages 4 sampling error these results shed doubt on the assumed continuities in theoretical peat accumulation models both mire stratigraphy and changing accumulation rates can be explained in qualitative hydrological terms more evidence for a previously discovered reservoir effect in bog deposits is presented though the physical biological mechanisms remain uncertain

reflections of pre and early agrarian human impact in the pollen diagrams of estonia high resolution pollen profiles based on 30 cores taken from estonian lake and mire deposits were used to reconstruct the extent and type of land use over most of the holocene biostratigraphical studies combining pollen records archaeological investigations conventional and accelerator mass spectrometry ams radiocarbon dates and numerical analysis were used to detect human impact on the landscape and vegetation indications of human activity were traced back to the mesolithic 9000 4900 cal bc and are reflected in the pollen diagram by a wide range of anthropogenic indicators which constitute nearly half of the total pollen sum together with high charcoal content human impact during the mesolithic is also indicated by the rarefaction analysis which shows a doubling of palynological diversity the neolithic 4900 1800 cal bc is regarded as a transitional period between the pre agrarian mesolithic and the agrarian bronze age 1800 500 cal bc changes in vegetation composition attributed to late mesolithic 6400 4900 cal bc land use were found in several of the sites investigated pollen records display the development and intensive use of a pastoral landscape in the middle neolithic 4150 3200 cal bc expressed in pollen diagrams as a drop in the frequencies of broad leaved trees and an increase in picea abies herb pollen and palynological diversity and in some cases increase in charcoal frequencies primitive tillage probably occurred during the middle neolithic at least in the alvar flat terrain with a thin soil cover on carbonaceous bedrock and coastal areas of estonia the earliest cerealia pollen finds are dated to ca 4000 cal bc and represented by avena type and hordeum type pollen during the bronze age crop cultivation played an important role in the farming economy the beginning of crop cultivation in estonia was almost simultaneous with southern sweden but more than 1000 years earlier than in finland the adoption of arable farming in estonia occurred much later than in southern sweden d 2004 elsevier b v all rights reserved

carbon 14 wiggle match dating of peat deposits advantages and limitations carbon 14 wiggle match dating wmd of peat deposits uses the non linear relationship between c 14 age and calendar age to match the shape of a series of closely spaced peat c 14 dates with the c 14 calibration curve the method of wmd is discussed and its advantages and limitations are compared with calibration of individual dates a numerical approach to wmd is introduced that makes it possible to assess the precision of wmd chronologies during several intervals of the holocene the c 14 calibration curve shows less pronounced fluctuations we assess whether wiggle matching is also a feasible strategy for these parts of the c 14 calibration curve high precision chronologies such as obtainable with wmd are needed for studies of rapid climate changes and their possible causes during the holocene copyright c 2004 john wiley sons ltd

the jebel hadid structure al kufrah basin se libya a possible impact structure and potential hydrocarbon trap abstract the jebel hadid structure that formed in the nubian sandstone of the southern al kufrah basin is a 4 7 km circular feature with a set of multiple concentric annular ridges the age of the structure is probably post early cretaceous to pre pliocene as the prevalent geological processes that usually account for the formation of circular structures magmatism diapirism sand volcanism karst dissolution glacial erosion can be widely excluded in this region we suggest that the jebel hadid structure might represent an eroded complex impact structure in particular the jebel hadid structure bears a strong morphological resemblance to the impact structure of tin bider in algeria general host rock properties may be compared to those at the upheaval dome impact structure usa numerous terrestrial impact structures are hosts to hydrocarbon reservoirs this investigation provides a first prospective approach based on remote sensing discussing the economic potential of a circular structure of suspected impact origin the still underexplored al kufrah basin is considered as a high potential area for future hydrocarbon exploration if we consider a possible impact origin of the jebel hadid structure this feature might provide local secondary pathways and traps for hydrocarbon migration accumulation in the nubian series and thus ought to be considered as a preferential site for oil and gas exploration

seismic reflection test on the granite of the skye tertiary igneous centre we have conducted a seismic reflection test over a short profile on the granite of the skye tertiary central intrusive complex from previous gravity modelling work it had been inferred that the granite is approximately 1 5 km thick and overlies basic rocks the seismic data indicate that the granite is at least 2 km thick at the test location reflection events of alternating polarity between depths of 2 1 and 2 4 km suggest that basic and acidic sheets are interlayered at the base of the granitic mass

coesite and quartz characteristic of crystallization from shock produced silica melt in the xiuyan crater article i nfo abundant coesite and quartz coexist with the shock produced silica glasses that retain the morphology of primary quartz in the gneiss clasts of polymict breccia from the xiuyan crater quartz occurs as idiomorphic needle like and spherulitic crystals and polycrystalline aggregates coesite occurs as granular or idiomorphic dendritic and needle like crystals and polycrystalline aggregates of stringers the occurrence and morphology of coesite and quartz characterize crystallization from an undercooled dense silica melt during decompression coesite firstly crystallizes from shock produced dense silica melt at high pressure as pressure decreases to the stability field of quartz quartz crystallizes from the melt whereas coesite remains metastable state a growth rate of 10 3 m s is evaluated for the crystallization of both coesite and quartz based on an estimation of the pressure duration of 10 ms in the coesite stability field for the crater higher pressure in silica melt could play a key role for promoting the rapid growth of coesite and quartz by reducing the viscosity of silica melt and the glass transition temperatures substantially the occurrence of both coesite and quartz embedded in silica glass provides unambiguous evidence for an impact origin of the crater 2010 elsevier b v all rights reserved

improving the time control of the subboreal subatlantic transition in a czech peat sequence by 14c wiggle matching to achieve an optimal time control for a late subboreal to early subatlantic peat sequence from pancavska louka in the czech republic different strategies are applied to convert a series of radiocarbon dates into a calendar time scale the methods of selection and preparation of the samples for ams 14c dating are presented the results of calibrating single radiocarbon dates are compared with a 14c wiggle match strategy as the accumulation rate of the peat was not constant the concentrations of arboreal pollen are used to estimate the accumulation rate changes and to correct for these changes the resulting time control represents the best solution for this peat sequence with the methods currently available

natural occurrence of reidite in the xiuyan crater of china the high pressure minerals of reidite and coesite have been identified in the moderately shock metamorphosed gneiss shock stage ii 3545gpa and the strongly shock metamorphosed gneiss shock stage iii 4555gpa respectively from the polymict breccias of the xiuyan crater a simple impact structure 1 8km in diameter in china reidite in the shock stage ii gneiss displays lamellar textures developed in parental grains of zircon the phase transformation of zircon to reidite likely corresponds to a martensitic mechanism no coesite is found in the reidite bearing gneiss the shock stage iii gneiss contains abundant coesite but no reidite is identified in the rock coesite occurs as acicular dendritic and spherulitic crystals characteristic of crystallization from shock produced silica melt zircon in the rock is mostly recrystallized the postshock temperature in the shock stage iii gneiss is too high for the preservation of reidite whereas reidite survives in the shock stage ii gneiss because of relatively low postshock temperature reidite does not occur together with coesite because of difference in shock induced temperature between the shock stage ii gneiss and the shock stage iii gneiss

the cretaceous messum igneous complex s w etendeka namibia reinterpretation in terms of a downsag cauldron subsidence model abstract the messum igneous complex mic lies within the ene trending zone of lower cretaceous 132 ma damaraland intrusive complexes in namibia intruded into pan african damara basement it is defined by a roughly circular structure 18 km in diameter the bounding ring fault exposed along the eastern sector encircling messum are the volcanic sequences of the goboboseb mountains comprising a lower basalt series tafelkop and tafelberg types followed with intervening basalts by four voluminous quartz latite ql eruptions goboboseb and springbok ql units inferred stages of development are a an initial very broad basaltic lava shield comprising the tafelberg and tafelkop basalts and messum crater basalts mcb possibly ponded in near vent lava lakes embedded within the lower basaltic sequence is a localised rhyolite dominated eruptive centre ca 5 km in diameter interpreted as a funnel caldera located towards the centre of the mic b downsagging extending northwards from messum following the goboboseb ql eruptions 2300 km3 ponding of overlying basaltic units c climactic springbok ql eruption 6300 km3 producing further downsag together with the inward radial dip of all volcanic units towards the mic ring fault initiation d cauldron subsidence emplacement of a granitoid suite forming the mic moat area between the ring fault and the core region e intrusion of gabbroic cone sheets into incompletely solidified granitic melts within the southeastern moat resulting hybridisation and magma mingling produced extensive development of heterogeneous granitoid and hybrid dioritic lithologies f cone sheet intrusions of the eastern gabbros into more highly solidified granitoids of the southeastern moat g intrusion of thick 1 2 km western gabbro cone sheets exhibiting local fine scale layering into solidified granitoids mainly within the western moat minor late stage granitic intrusions h 2 3 ma quiescent period followed by quartz and ne syenite intrusions and finally basanite dykes emplaced within the mic core accompanying differential uplift of the core uplift resurgence within the mic has accompanied intrusion of the moat granitoids and mafic cone sheets thereby juxtaposing volcanic and intrusive sequences phases of both subsidence and uplift have characterised the mic the nw scotland tertiary central igneous complexes and messum show evidence of a number of parallel developments but also important differences the mic differs markedly from caldera systems within the western usa and circum pacific messum is therefore suggested to represent a distinct class of intrusive extrusive central complex although probably common in large igneous provinces

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

formation of alpine mylonites and pseudotachylytes at the base of the silvretta nappe eastern alps optical microscopy and transmission electron microscopy tem have been used to investigate deformation induced microstructures in alpine pseudotachylytes mylonites and ultramylonites from the upper austro alpine silvretta nappe at the southwestern margin of the lower engadine window in the eastern alps of switzerland and austria the earliest alpine stage of deformation d1 produced mylonitic gneisses with dislocation glide being the dominant deformation mechanism in quartz in the second stage d2a mylonites were formed as a result of strain softening in quartz manifest by dynamic recrystallization and a lattice preferred orientation tem observations are used to characterize the initial stages of migration and rotation recrystallization pseudotachylytes were generated during d2b when strain hardening due to dislocation tangling was not outweighed by strain softening several generations of mylonites and pseudotachylytes are observed a model for the propagation of seismic fractures initiated in comparatively rigid amphibolites into an otherwise ductile environment is proposed for the pseudotachylytes as such they are formed as transient discontinuities in the mylonites d1 and d2a b are related to the eo alpine detachment of the silvretta nappe subsequent eo alpine metamorphism in the investigated area reached the stilpnomelane zone and this coincided with the development of ultramylonites from both the mylonites and pseudotachylytes during d3 and peak metamorphism the increased temperature combined with fluid infiltration from the overridden penninic strata favoured grain boundary sliding superplasticity as the dominant deformation mechanism during d3

detailed investigation of preserved maar structures by combined geophysical surveys the detection of completely preserved maar structures is important not only for underground mapping but also for paleoclimate research because laminated maar lake sediments may contain a very detailed archive of climate history objective evidence for the existence of such structures can only be provided by geophysics and boreholes the combination of gravity and magnetic ground surveys appears to be an excellent tool to detect and identify buried maar structures their prominent properties are an almost circular gravity minimum corresponding to a crater filled with limnic sediments of low density and a magnetic anomaly caused by a pyroclastic or basaltic body in the diatreme which indicates the volcanic character seismic measurements provide the most detailed information about the internal structure of the maar sediments zones of low seismic reflectivity and very low density represent sediments of the late maar lake period the early lake period is indicated by debris flow deposits and turbidites represented by seismic reflectors the seismic sections clearly reveal the bowl like structure of the maar outside this bowl like structure there are only a few reflections which represent the basement taking into account the shape of the gravity anomaly seismic information allows geometrical modelling of the maar structure optimal drilling sites can be selected based on the results of geophysical surveying comparing the results of combined geophysical surveys above two maar structures of different ages yields a marked similarity in their geophysical pattern

non impact origin of the crater like structures in the gilf kebir area egypt implications for the geology of eastern sahara several small crater like structures occur in gilf kebir region sw egypt it has been previously suggested that they could be the result of meteoritic impacts here we outline the results of our geological and geophysical survey in the area the proposed impact origin for these structures is not supported by our observations and analyses and we suggest an alternative interpretation the crater like structures in gilf kebir area are likely related to endogenic processes typical of hydrothermal vent complexes in volcanic areas which may reflect the emplacement of subvolcanic intrusives

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

late holocene climate change and human impact recorded in a south swedish ombrotrophic peat bog abstract a peat core from a raised bog in southern sweden was examined plant macrofossils pollen non pollen microfossils colorimetric humification carbon nitrogen ratios bulk densities loss on ignition to investigate the effects of climate change and human impact on the plant species composition and carbon accumulation of the peat forming vegetation 14c wiggle match dating was applied for fine resolution dating cooling at the start of the little ice age was reflected by a decline of thermophilous trees from the pollen record between ca cal ad 1275 1590 coinciding with increases in atmospheric 14c pointing to decreases of solar activity wolf and sporer minimum human impact also decreased later the effect of human impact was clearly visible in the form of a black brown decomposed peat layer the top of which marks a gap of more than 300 years artificial drainage probably caused secondary decomposition and oxidation of the peat from ca cal ad 1960 the water table of the peat bog was restored following construction of a road and raised bog vegetation regenerated

sedimentary record and tectonic implications of mesozoic rifting in southeast mongolia the east gobi basin of mongolia is a poorly described late jurassic early cretaceous extensional province that holds great importance for reconstructions of mesozoic tectonics and paleogeography of eastern asia extension is especially well recorded in the structure and stratigraphy of the unegt and zuunbayan subbasins southwest of saynshand mongolia where outcrop and subsurface relationships permit recognition of prerift synrift and postrift mesozoic stratigraphic megasequences within the synrift megasequence three sequences developed in response to climatic and rift related structural controls on sedimentation where best exposed along the northern margin of the unegt subbasin each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone resedimented ashes and basalt flows punctuate the synrift megasequence rifting began in the unegt subbasin prior to 155 ma with coarse alluvial filling of local fault depressions subsidence generally outstripped sediment supply and fresh to saline lacustrine environments expanding southward with time dominated the unegt zuunbayan landscape for much of latest jurassic early cretaceous time episodic faulting and volcanism characterized the basin system for the balance of the early cretaceous a brief period of compressional and or transpressional basin inversion occurred at the end of the early cretaceous prior to deposition of a widespread upper cretaceous overlap sequence the driver s of late jurassic early cretaceous extension remain uncertain because southeast mongolia occupied an intraplate position by the beginning of the cretaceous extension in the east gobi basin was coeval with collapse and extension of early mesozoic contractional orogenic belts along the northern and southern borders of mongolia and probably was a linked phenomenon strike slip faulting associated with collisions on the southern asian and mongol okhotsk margins likely also played a role in late mesozoic deformation of the east gobi region perhaps partitioning the gobi from apparently coeval large magnitude contractional deformation in the yinshan yanshan orogenic belt south of the study area in inner mongolia

proxy climate record for the last 1000 years from irish blanket peat and a possible link to solar variability recent research has shown that ombrotrophic mires can yield a proxy climate signal based on changes in the degree of peat humification 1 2 and that oceanic margin sites show the most sensitive record 3 we compare humification records for the last 1000 yr from two radiocarbon dated blanket peat profiles from western ireland and show several corresponding periods of climatic fluctuations periods favouring reduced peat decomposition suggestive of wetter and or cooler climatic conditions seem to coincide with periods of reduced sunspot activity and atmospheric14c anomalies whilst climatologists have continued to debate a link between solar variability on a century timescale and climate change the exact nature of that link remains elusive 4 7 the results from ireland demonstrate that data derived from mires could be relevant to the debate as to the extent of solar forcing in natural climatic variability and the curves shown provide a continuous record to add to previous evidence for the so called medieval optimum and little ice age 8 the record may imply that during the past millennium climatic change at the oceanic margin of the northeast atlantic largely corresponded to inferred variations in solar output

late holocene human impact and climate change recorded in a north swedish peat deposit abstract a peat core from a mire with poor fen lawns and ombrotrophic hummock strings in northern sweden was examined plant macrofossils pollen non pollen microfossils testate amoebae colorimetric humification carbon nitrogen ratios bulk densities loss on ignition to investigate the effects of climate change and human impact on the plant species composition and peat accumulation of the peat forming vegetation during the last 1700 yr 14 c wiggle match dating was applied for high precision dating the lappmyran region was dominated by pinus and picea forest changes in land use patterns and population density were visible in the pollen record of the regional vegetation juniperus and rumex acetosa type pollen indicated the presence of grazed land in the area between ad 1500 and 1950 the vegetation at the coring spot gradually became ombrotrophic the local plant and testate amoebae compositions reflect changes in surface wetness caused by changes in precipitation and the internal dynamics of the bog a wet interval was observed during the maunder minimum of solar activity the presence of drepanocladus fluitans indicates less ombrotrophic conditions during this period probably caused by the inflow of minerogenic water from upslope owing to increased precipitation the reconstructed water table shows agreement with precipitation measurements which were available since ad 1860

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

yallalie a buried structure of possible impact origin in the perth basin western australia an enigmatic buried structure located in mesozoic sedimentary rocks in the perth basin western australia was discovered in 1990 by ampol exploration the basin like yallalie structure centred on 30 26 40 3 s 115 46 16 4 e is circular in plan view and about 12 km in diameter high resolution seismic reflection profiles across the structure show a basin shaped area of chaotic reflec tions that extend to a depth of approximately 2 km below the surface the structure has sharp boundaries with surrounding faulted but otherwise relatively undisturbed rocks in the centre of the structure there is an uplifted area approximately 3 4 km across similar to those described from com plex meteorite impact structures the seismically defined structure coincides with a circular topo graphic depression and image processing of digital elevation data has allowed recognition of concentric and radial structures extending as far as 40 km from the centre of the depression gravity surveys show the structure to be associated with a positive gravity anomaly of about 30 gu aeromagnetic surveys have defined annular anomalies associated with the central uplifted section and possibly margins of the structure a search for siderophile element enrichments by neutron activation analysis in the rocks of the structure which would indicate the presence of a meteorite component proved negative quartz grains in cores that penetrate the structure show the development of prismatic cleavage fractures and irregular slightly curved planes formed by brittle fracture an allochthonous breccia of late cretaceous rocks occurs a few kilometres west of the western margin of the structure quartz grains from a thin veneer of tertiary sediments that drape the structure are essentially unde formed however multiple sets of closely spaced planar deformation features in quartz characteristic of highly shocked rocks have yet to be observed in the rocks of the yallalie structure and the allochthonous breccia the morphology of the yallalie structure determined from topographic and geophysical data suggests strongly that it is of impact origin geological and geochemical evidence is equivocal but is not inconsistent with this interpretation

suevite of the ries crater germany source rocks and implications for cratering mechanics suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement glass inclusions and a small amount of sedimentary clasts data are given of the modal composition of fall out suevites deposited at isolated points around the crater and crater suevite forming a layer below post impact lake sediments in the crater cavity fall out suevites contain aerodynamically shaped bombs which are absent in crater suevite taking into account not only large glass fragments and bombs but also the finer fractions the glass content of fall out and crater suevites amounts to 47 and 29 vol respectively crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks mainly granites and a lower series of gneisses and amphibolite based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined suevites contain on the average 46 igneous and 54 metamorphic clasts in constrast weakly shocked and unshocked crystalline ejecta of the ries structure consist of 82 igneous and 18 metamorphic rocks

the oyut ulaan volcanic group stratigraphy magmatic evolution and timing of carboniferous arc development in se mongolia abstract the palaeozoic mesozoic tectonic evolution of central asia including the vast terrane collage that makes up mongolia has been a topic of considerable debate the oyut ulaan volcanic group is a sequence of volcanic and sedimentary rocks in se mongolia that forms the southern part of the devonian permian saykhandulaan inlier fieldwork traverses and mapping have established four distinct formations in the oyut ulaan volcanic group that record the nature of arc activity in part of the central asian orogenic belt during the carboniferous physical volcanological and sedimentological characteristics of the four formations suggest three clear eruptive styles 1 periodic andesite volcanism in an actively eroding arc setting that also contained large rivers and swamps 2 highly effusive plateau andesite volcanism 3 explosive rhyolitic effusion geochemical analyses of volcanic lithologies suggest that the group represents subduction related mature continental arc volcanism geochemical results document an evolving magma system to which surface processes of the volcano sedimentary model may be linked magma pulses and replenishments are identified from variations in chemostratigraphy newly obtained zircon ages from the volcanic succession fix its emplacement eruption at 323 0 0 7 ma mid carboniferous or late mississippian a granite cobble from the lower part of the oyut ulaan volcanic group gives a u pb zircon age of 338 9 0 4 ma indicating that arc plutons were emplaced 10 ma prior to the oyut ulaan volcanism and were eroded soon after our work provides timing constraints for final accretion and continental assembly in se mongolia and also sheds light on the petrological development of a magmatic arc system within an evolving accretionary orogen

a numerical approach to 14c wiggle match dating of organic deposits best fits and confidence intervals c 14 wiggle match dating wmd of peat deposits uses the non linear relationship between c 14 age and calendar age to match the shape of a sequence of closely spaced peat c 14 dates with the c 14 calibration curve a numerical approach to wmd enables the quantitative assessment of various possible wiggle match solutions and of calendar year confidence intervals for sequences of c 14 dates we assess the assumptions advantages and limitations of the method several case studies show that wmd results in more precise chronologies than when individual c 14 dates are calibrated wmd is most successful during periods with major excursions in the c 14 calibration curve e g in one case wmd could narrow down confidence intervals from 230 to 36 yr c 2003 elsevier science ltd all rights reserved

ibn batutah a possible simple impact structure in southeastern libya a remote sensing study abstract the ibn batutah feature is a circular structure centered on 21 34 10 n and 20 50 15 e and located in the prefecture of the hamadat ibn batutah plateau southeastern libya it is a singular geological structure formed in nubian sandstone of most likely early cretaceous age remote sensing investigations based on landsat etm dual band l and c and dual polarization hh and hv radar sir c and srtm data reveal a morphological resemblance of the ibn batuta structure to other confirmed terrestrial simple impact structures the structure is in the form of a circular basin 2 5 km in diameter and 25 m deep surrounded by a low relief rim that rises to within 5 m of the surrounding terrains over much of its extent no endogenic geological processes such as magmatism diapirism karst dissolution and glacial or fluvial erosion can decisively explain the formation of the ibn batutah structure within a large area of flat lying mesozoic sandstones thus this paper proposes that the ibn batutah structure represents a simple impact structure verification of this hypothesis will require collection and analyses of rock samples from in and around the structure although the area is currently almost inaccessible for environmental and security reasons

effects of human impact and climate change during the last 350 years recorded in a swedish raised bog deposit a peat core from an ombrotrophic mire in central sweden was analysed for multi proxy indicators plant macrofossils pollen non pollen microfossils testate amoebae colorimetric humification carbon nitrogen ratios bulk densities loss on ignition to evaluate the relative contributions of climate change and human impact on vegetation and peat accumulation during the last c 350 years 14c wiggle match dating was applied for high precision dating testate amoebae assemblages were used to reconstruct past water table depths and compared with other proxies and instrumental climate data from the mid 18th century onwards changes in mire surface wetness were mainly caused by climate change precipitation and evaporation but the internal dynamics of the local bog vegetation may also have played a role in the recorded water table changes the human impact signal in the pollen data was compared with swedish population and land use data a link between climate change human impact openness of the vegetation and demographic change was found cold and wet periods show a decrease in human impact and open land indicators in the pollen data followed by an increase in death rate and or an increase in emigration