u pb isotopic study of relict zircon inclusions recovered from muong nong type tektites abstract isotopic compositions of u and pb were determined for relict zircon inclusions recovered from muong nong type tektites from the australasian and north american strewn fields these inclusions are generally opaque white with granular textures probably representing mechanical disaggregation and or local melting and recrystallization all zircons have strongly discordant u pb isotopic ratios and the isotopic ratios of most of the zircons are dominated by common pb this is interpreted as being the result of complete resetting of the isotopic system during the impact event and exchange with pb from the target sediments ages that may represent the target rocks were only retained by one sample representing a zircon that was extracted from a muong nong type georgiaite a value of 0 6 ga was obtained which is similar to the nd and sr model ages obtained for the north american tektites our results indicate that the u pb isotopic system in almost all zircons from tektites has been totally reset during the impact event this is in contrast to zircons extracted from impact melt rocks at some impact structures and or distal ejecta some of which are only partly reset

the dav and periadriatic fault systems in the eastern alps south of the tauern window alpine deformation of austroalpine units south of the tauern window is dominated by two kinematic regimes prior to intrusion of the main periadriatic plutons at 30 ma the shear sense was sinistral in the current orientation with a minor north side up component sinistral shearing locally overprints contact metamorphic porphyroblasts and early periadriatic dykes direct rb sr dating of microsampled synkinematic muscovite gave ages in the range 33 30 ma whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted 40ar 39ar age of 46 ma the transition from sinistral to dextral transpressive kinematics related to the periadriatic fault occurred rapidly between solidification of the earlier dykes and of the main plutons subsequent brittle ductile to brittle faults are compatible with n s to nnw sse shortening and orogen parallel extension antithetic riedel shears are distinguished from the previous sinistral fabric by their fine grained quartz microstructures with local pseudotachylyte formation one such pseudotachylyte from speikboden gave a 40ar 39ar age of 20 ma consistent with pseudotachylyte ages related to the periadriatic fault the magnitude of dextral offset on the periadriatic fault cannot be directly estimated however the jump in zircon and apatite fission track ages establishes that the relative vertical displacement was 4 5 km since 24 ma and that movement continued until at least 13 ma

changing patterns of exhumation and denudation in front of an advancing crustal indenter tauern window eastern alps the changing shape of indenting crustal blocks during northward motion of the adriatic microplate induced migration of miocene doming and orogen parallel extension of orogenic crust in the tauern window new structural and kinematic data indicate that initial shortening of the penninic nappe pile in the tauern window by upright folding and strike slip faulting was transitional to coeval north south shortening and east west extension the latter was accommodated by normal faulting at the eastern and western margins of the window retrodeforming these post nappe structures in map view yields a map view reconstruction of the orogenic crust back to 30 ma including the onset of pronounced indentation at 21 ma this model supports the notion that indentation involved approximately equal amounts of north south shortening and orogen parallel stretching and extrusion toward the pannonian basin as measured from the indenter tip to the european foreland in the north and austroalpine units in the east comparison of areal denudation of the orogenic crust before and after indentation indicate that erosion associated with upright folding was the primary agent of denudation whereas extensional unroofing and limited erosion along normal faults at the eastern and western ends of the tauern window accounted for only about a third of the total denudation

ocean drilling project hole 689b spherules and upper eocene microtektite and clinopyroxene bearing spherule strewn fields montanari et al 1993 reported a positive ir anomaly in the upper eocene sediments from ocean drilling program hole 689b on the maud rise southern ocean vonhof 1998 described microtektites and clinopyroxene bearing cpx spherules associated with the ir anomaly in hole 689b and suggested that they belong to the north american and equatorial pacific cpx strewn fields respectively we searched a suite of 27 samples taken through the spherule layer from hole 689b and we recovered 386 microtektites and 667 cpx spherules we studied the petrography of the microtektites and cpx spherules and determined the major element compositions of 31 microtektites and 14 cpx spherules using energy dispersive x ray analysis we also determined the minor element compositions of eight microtektites using instrumental neutron activation analysis we found that the peak abundance of cpx spherules is 2 cm below the peak abundance of the microtektites 128 7 m below sea floor which suggests that the cpx spherule layer may be slightly older 3 5 ka the microtektites are mostly spherical and are generally transparent and colorless they are similar to the north american microtektites in composition the biggest differences being their generally lower na2o and generally higher zr ba and ir up to 0 3 ppb contents we agree with vonhof 1998 that the hole 689b microtektites probably belong to the north american tektite strewn field we calculate that the number of microtektites 125 m cm2 at hole 689b is 52 this number is close to the concentration predicted by extrapolation of the trend of concentration vs distance from the chesapeake bay structure based on data from other north american microtektite bearing sites thus the north american strewn field may be at least four times larger than previously mapped the hole 689b cpx spherules range from translucent yellow to opaque black but most are opaque tan to dark brown they are generally spherical in shape and all are 125 m in diameter some contain ni rich spinels in addition to clinopyroxene microlites the cpx spherules are petrographically and compositionally similar to cpx spherules previously found in the northwestern atlantic ocean caribbean sea gulf of mexico equatorial pacific and eastern indian ocean the abundance and widespread geographic occurrence of these spherules suggest that the strewn field may be global in geographic extent assuming a global extent we estimate that there may be at least 25 billion metric tons of cpx spherules in the strewn field based on age size and geographic location we speculate that the 100 km diameter popigai crater in northern siberia may be the source of the cpx spherule layer

neodymium and strontium isotopic study of australasian tektites new constraints on the provenance and age of target materials nd and sr isotopic studies of australasian tektites provide information on the age and provenance of the target materials and allow us to characterize the target area and the impact process leading to tektite formation nd values of australites splash form indochinites and muong nong type indochinites are indistinguishable within analytical uncertainty and average 11 5 u depleted mantle nd model ages fall within the narrow range of 1490 1620 ma indicating that the source material was derived dominantly from a proterozoic crustal terrane sr values are variable and are correlated with the geographic location of the tektite samples analyses of four muong nong type or layered indochinites from a single locality in eastern thailand yield an isochron age of 167 12 ma 2 a correlation of rb sr fractionation with sr model ages indicates that the last major rb sr fractionation event experienced by the target materials occurred 175 15 ma ago we interpret this age as the time of deposition of sedimentary target rocks and consider the compositional layering observed in muong nong type tektites to reflect compositional variability inherited from jurassic sediments the nd and sr isotopic data provide evidence that all australasian tektites were derived from a single sedimentary formation with a narrow range of stratigraphic ages close to 170 ma we suggest that all of the australasian tektites were derived from a single impact event and that the australites represent the upper part of a melt sheet ejected at high velocity whereas the indochinites represent melts formed at a lower level in the target material which were distributed closer to the area of impact the impact site is inferred to be within an area of jurassic sedimentary bedrock which spans the geopolitical boundaries between northern cambodia southern laos and southeastern thailand

a detailed chronology of the australasian impact event the brunhes matuyama geomagnetic polarity reversal and global climate change a mechanism had been recently proposed to show how an impact event can trigger a geomagnetic polarity reversal by means of rapid climate cooling we test the proposed mechanism by examining the record from two high sedimentation rate 8 11 cm kyr deep sea sediment cores odp sites 767 and 769 from marginal seas of the indonesian archipelago which record the australasian impact with well defined microtektite layers the brunhes matuyama polarity reversal with strong and stable remanent magnetizations and global climate with oxygen isotope variations in planktonic foraminifera both odp cores show the impact to have preceded the reversal of magnetic field directions by about 12 kyr both records indicate that the field intensity was increasing near the time of impact and that it continued to increase for about 4 kyr afterwards furthermore the oxygen isotope record available from sediments at odp site 769 shows no indication of discernible climate cooling following the impact the microtektite event occurred in the later part of glacial stage 20 and was followed by a smooth warming trend to interglacial stage 19 thus the detailed chronology does not support the previously proposed model which would predict that a decrease in geomagnetic field intensity resulted from a minor glaciation following the impact event we conclude that the evidence for a causal link between impacts and geomagnetic reversals remains insufficient to demonstrate a physical connection

nucleation of ductile shear zones on dilatant fractures small shear zones in granitic rocks of the sierra nevada california and near roses rosas northeast spain display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation many zones have sharp nearly planar boundaries between their highly deformed interiors and the undeformed wall rock the roses shear zones narrow continuously to hairline fractures at their ends mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleation of the shear zones we suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes thereby promoting chemical alteration and or hydrolytic weakening

have distal impact ejecta changed through geologic time sand size spherules of former silicate melt have been identified in 10 thin early archean to paleoproterozoic layers that appear to be reworked ejecta from large impacts melt spherules are common in phanerozoic layers of distal impact ejecta but the aggregate thickness of spherules in early precambrian layers is generally 10 to 100 times greater and they display textures rarely seen in phanerozoic spherules these contrasts suggest secular variations in the nature of both impactors and target materials such as a progressive decrease in the size of impactors and a progressive increase in the fraction of earth s surface covered by continental crust

impact microcrater morphology on australasian microtektites scanning electron microscopy of 137 australasian microtektites and fragments from 4 sediment cores in the central indian ocean reveals more than 2000 impact generated features in the size range of 0 3 to 600 m 2 dominant crater number densities in m and sub m sizes therefore tektite producing impacts can lead to the generation of microcraters that mimic those found on lunar surface materials and for the lunar rocks to qualify as reliable cosmic dust flux detectors their tumbling histories and lunar surface orientations have to be known precisely

coesite and shocked quartz discovered in the australasian and north american microtektite layers samples from the north american australasian and ivory coast microtektite layers were examined for unmelted impact ejecta 125 m size coesite and shocked quartz identified by x ray diffraction were found at all the sites where a well defined north american microtektite layer had previously been documented coesite and shocked quartz were also foundin seven out of 33 cores from the australasian strewn field however no coesite or shocked quartz was found associated with the ivory coast microtektite layer stishovite was also found at three of the australasian sites all of the australasian cores containing coesite and shocked quartz were taken from within 2000 km of indochina which is believed by most to be the source area for the australasian tektites and microtektites six of the seven cores have the highest concentrations of microtektites ever determined for australasian microtektite bearing cores the seventh core did not contain any microtektites which may be consistent with the hypothesis that the microtektites were deposited in a raylike pattern the discovery of coesite and impact ejecta associated with both the north american and australasian microtektite layers provides further support for the hypothesis of terrestrial impact origin of tektites

the geodetic signature of the earthquake cycle at subduction zones model constraints on the deep processes recent megathrust events in tohoku japan maule chile and sumatra indonesia were well recorded much has been learned about the dominant physical processes in megathrust zones partial locking of the plate interface detailed coseismic slip relocking afterslip viscoelastic mantle relaxation and interseismic loading these and older observations show complex spatial and temporal patterns in crustal deformation and displacement and significant differences among different margins a key question is whether these differences reflect variations in the underlying processes like differences in locking or the margin geometry or whether they are a consequence of the stage in the earthquake cycle of the margin quantitative models can connect these plate boundary processes to surficial and far field observations we use relatively simple cyclic geodynamic models to isolate the first order geodetic signature of the megathrust cycle coseismic and subsequent slip on the subduction interface is dynamically and consistently driven a review of global preseismic coseismic and postseismic geodetic observations and of their fit to the model predictions indicates that similar physical processes are active at different margins most of the observed variability between the individual margins appears to be controlled by their different stages in the earthquake cycle the modeling results also provide a possible explanation for observations of tensile faulting aftershocks and tensile cracking of the overriding plate which are puzzling in the context of convergence compression from the inversion of our synthetic gnss velocities we find that geodetic observations may incorrectly suggest weak locking of some margins for example the west aleutian margin

on the petrology of brittle precursors of shear zones an expression of concomitant brittle deformation and fluid rock interactions in the ductile continental crust the inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies like fractures referred to as shear zone brittle precursors in the neves area western tauern window eastern alps although the structural control of these brittle precursors on ductile strain localization is well established the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate the present petrological study carried out on a brittle precursor of a shear zone affecting the neves metagranodiorite aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event the brittle precursor consists of a 100 500 m wide recrystallized zone with a host mineral controlled stable mineral assemblage composed of plagioclase garnet quartz biotite zoisite white mica pyrite plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of alpine metamorphism and fluid rock interactions concomitant with the brittle deformation phase equilibrium modelling of this garnet bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0 6 0 05 gpa 500 20 c and then grew along a prograde path to 0 75 0 05 gpa 530 20 c these amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area suggesting that both brittle and ductile deformation were active in the ductile realm above 500 c for a depth range between 17 and 21 km we speculate that the neves area fulfils most of the required conditions to have hosted slow earthquakes during alpine continental collision that is coupled frictional and viscous deformation under high fluid pressure conditions 450 c further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings

age and provenance of the target materials for tektites and possible impactites as inferred from sm nd and rb sr systematics sm nd and rb sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects tektites have large negative e nd 0 values and are uniform within each tektite group while the e sr 0 are large positive values and show considerable variation within each group chemical trace element and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust each tektite group is characterized by a uniform nd model age t chur nd interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites 1 1 15 ae for australasian tektites 2 1 91 ae for ivory coast tektites 3 0 9 ae for moldavites 4 0 65 ae for north american tektites and 5 0 9 ae for high si irghizites sr model ages t ur sr are variable within each group reflecting rb sr fractionation and in the favorable limit of very high rb sr ratios approach the time of sedimentation of the parent material which melted to form the tektites australasian tektites are derived from 0 25 ae sediments moldavites from 0 0 ae sediments ivory coast tektites from 0 95 ae sediments possible parent sediments of other tektite groups have poorly constrained ages our data on moldavites and ivory coast tektites are consistent with derivation from the ries and bosumtwi craters respectively irghizites are isotopically distinct from australasian tektites and are probably not related sanidine spherules from a cretaceous tertiary boundary clay have initial e nd 2 e sr 5 and are not derived from old continental crust or meteoritic feldspar they may represent a mixture of basaltic oceanic crust and sediments implying an oceanic impact these isotopic results are also consistent with a volcanic origin for the spherules

impact melts and glasses on mars 1 dark mobile materials on mars are widely considered to be volcanically derived windblown particulates such a conclusion is consistent with the large volcanic edifices on mars and the assumption that a volatile rich crust would not favor significant impact melt generation several post viking discoveries however provide new perspectives for reconsidering alternative origins not only for some of the mobile dark veneers but also certain materials at the pathfinder and viking landing sites first certain dark materials on mars do not exhibit the expected mafic absorption features in their reflectance spectra second the pathfinder mission revealed unexpected feldspathic compositions of vesicular blocks that lead to a preliminary conclusion that they represent andesites or basaltic andesites rather than basalts third recent discoveries of holocene to miocene impacts in argentine loessoid deposits provide new insights for the nature of melts generated from eolian sediments fourth global dispersal of impact melt products are now recognized in the terrestrial record throughout the cenozoic consequently the generation and preservation of impact glass on mars should be reconsidered possible occurrences include possible tektite like strewnfields downrange from recent oblique impacts dark avalanches emerging from backwasting cliffs of easily eroded unconformable deposits global dispersal of glassy ejecta from hesperian and amazonian craters of chicxulub scale and dark mobile veneers emerging from noachian craters we propose that the mobile dark materials comprising many of the named features of mars as well as some blocks at the pathfinder viking landing sites may represent impact melt products

misleading positions of geomagnetic reversal boundaries in eurasian loess and implications for correlation between continental and marine sedimentary sequences abstract the matuyama brunhes boundary mbb where the earth s magnetic field changed to its present orientation is one of the most frequently used time markers in continental quaternary stratigraphy although magnetic signals have been successfully recovered from loess deposits and an apparently coherent magnetostratigraphy seems to be in place a close examination of the published loess records from northern mid latitudes reveals significant uncertainty and variability for the position of the mbb in asia and part of central europe the mbb has been found in loess a glacial deposit this is in contrast to the well established palaeomagnetic records in deep sea sediments where the mbb is located in the interglacial oxygen isotope stage 19 this discrepancy has caused serious confusion in stratigraphic correlation between loess and marine sequences here we examine the causes of the discrepancy between the two sedimentary environments in their records of the mbb our approach includes a comparison of the mbb in loess and other continental records and an evaluation of the stratigraphic significance of the microtektites found around the mbb based on the occurrence of the mbb in an interglacial stage in lake and continental margin sequences and the correlative stratigraphic positions of the microtektites in marine sediments and loess we demonstrate that the measured positions of the mbb in loess are erroneous and misleading and its true occurrence was during the time of formation of the palaeosol corresponding to oxygen isotope stage 19 the most plausible cause for the apparent offset i e the mbb being found in the underlying loess instead is that the acquisition of remanent magnetisation in loess occurs at a certain depth substantially below the land surface the degree of such displacement is dependent on the lithology and structure of the deposits formed prior to the reversal and the time delay is estimated to be in the order of 103 104 years we show that other geomagnetic boundaries such as those of jaramillo and cobb mountain in loess have been similarly displaced though to a varying extent we suggest that the remanence acquisition in loess is closely related to the post depositional evolution in the structure of loessic materials which is in part influenced by regional climate conditions our conclusions on the displaced remanence acquisition have implications for loess chronology magnetostratigraphy as well as the geomagnetic interpretation of detailed magnetic signatures from loess

ivory coast microtektite strewn field description and relation to the jaramillo geomagnetic event abstract during the present study the ivory coast microtektite layer was found in cores from five equatorial atlantic sites bringing the total number of ivory coast microtektite bearing cores to eleven the strewn field appears to be restricted to between 9 n and 12 s latitude there is a general increase in the concentration of microtektites towards the bosumtwi crater which is generally thought to be the source of the ivory coast tektites the relationship between the onset of the jaramillo subchron and the ivory coast microtektite layer has been investigated in six cores a plot of the difference in depth between the base of the jaramillo subchron and the microtektite layer versus sediment accumulation rate was used to determine the average post depositional remanent magnetization pdrm acquisition depth and the age difference between the onset of the jaramillo subchron and the deposition of the microtektites assuming that the pdrm acquisition depth does not vary with sediment accumulation rate we find that the average pdrm acquisition depth is 7 cm and that the microtektites were deposited approximately 8 ky after the onset of the jaramillo subchron this indicates that the impact responsible for the ivory coast tektites and microtektites could not be causally related to the geomagnetic reversal at the base of the jaramillo subchron

high resolution record of the matuyama brunhes transition constrains the age of javanese homo erectus in the sangiran dome indonesia a detailed paleomagnetic study conducted in the sangiran area java has provided a reliable age constraint on hominid fossil bearing formations a reverse to normal polarity transition marks a 7 m thick section across the upper tuff in the bapang formation the transition has three short reversal episodes and is overlain by a thick normal polarity magnetozone that was fission track dated to the brunhes chron this pattern closely resembles another high resolution matuyama brunhes mb transition record in an osaka bay marine core in the sangiran sediments four successive transitional polarity fields lie just below the presumed main mb boundary their virtual geomagnetic poles cluster in the western south pacific partly overlapping the transitional virtual geomagnetic poles from hawaiian and canary islands lavas which have a mean 40ar 39ar age of 776 2 ka thus the polarity transition is unambiguously the mb boundary a revised correlation of tuff layers in the bapang formation reveals that the hominid last occurrence and the tektite level in the sangiran area are nearly coincident just below the upper middle tuff which underlies the mb transition the stratigraphic relationship of the tektite level to the mb transition in the sangiran area is consistent with deep sea core data that show that the meteorite impact preceded the mb reversal by about 12 ka the mb boundary currently defines the uppermost horizon yielding homo erectus fossils in the sangiran area

modes of orogen parallel stretching and extensional exhumation in response to microplate indentation and roll back subduction tauern window eastern alps the tauern window exposes a paleogene nappe stack consisting of highly metamorphosed oceanic alpine tethys and continental distal european margin thrust sheets in the eastern part of this window this nappe stack eastern tauern subdome etd is bounded by a neogene system of shear the katschberg shear zone system kszs that accommodated orogen parallel stretching orogen normal shortening and exhumation with respect to the structurally overlying austroalpine units adriatic margin the kszs comprises a 5 km thick belt of retrograde mylonite the central segment of which is a southeast dipping low angle extensional shear zone with a brittle overprint katschberg normal fault knf at the northern and southern ends of this central segment the kszs loses its brittle overprint and swings around both corners of the etd to become subvertical dextral and sinistral strike slip faults the latter represent stretching faults whose displacements decrease westward to near zero the kinematic continuity of top east to top southeast ductile shearing along the central low angle extensional part of the kszs with strike slip shearing along its steep ends combined with maximum tectonic omission of nappes of the etd in the footwall of the knf indicates that north south shortening orogen parallel stretching and normal faulting were coeval stratigraphic and radiometric ages constrain exhumation of the folded nappe complex in the footwall of the kszs to have begun at 23 21 ma leading to rapid cooling between 21 and 16 ma this exhumation involved a combination of tectonic unroofing by extensional shearing upright folding and erosional denudation the contribution of tectonic unroofing is greatest along the central segment of the kszs and decreases westward to the central part of the tauern window the kszs formed in response to the indentation of wedge shaped blocks of semi rigid austroalpine basement located in front of the south alpine indenter that was part of the adriatic microplate northward motion of this indenter along the sinistral giudicarie belt offsets the periadriatic fault and triggered rapid exhumation of orogenic crust within the entire tauern window exhumation involved strike slip and normal faulting that accommodated about 100 km of orogen parallel extension and was contemporaneous with about 30 km of orogen perpendicular north south shortening of the etd extension of the pannonian basin related to roll back subduction in the carpathians began at 20 ma but did not affect the eastern alps before about 17 ma the effect of this extension was to reduce the lateral resistance to eastward crustal flow away from the zone of greatest thickening in the tauern window area therefore we propose that roll back subduction temporarily enhanced rather than triggered exhumation and orogen parallel motion in the eastern alps lateral extrusion and orogen parallel extension in the eastern alps have continued from 12 to 10 ma to the present and are driven by northward push of adria

australasian microtektites impactor identification using cr co and ni ratios impactor identification is one of the challenges of large scale impact cratering studies due to the dilution of meteoritic material in impactites typically 1 wt the nature of the impactor that generated the australasian tektite microtektite strewn field i e the largest cenozoic strewn field similar to 15 of the earth s surface the youngest similar to 0 78 myr old on earth and the only one without an associated impact crater so far is an outstanding issue we identify a chondritic impactor signature in 77 australasian microtektites size range similar to 200 700 mm from within 3000 km from the hypothetical impact location in indochina similar to 17 degrees n 107 degrees e based on variations of cr co and ni interelement ratios in a co ni vs cr ni space 46 microtektites analyzed in this work by laser ablation inductively coupled plasma mass spectrometry and 31 from literature by means of neutron activation analyses with cr co and ni concentrations up to similar to 370 50 and 680 mu g g respectively despite substantial overlap in cr ni versus co ni composition for several meteorite types with chondritic composition chondrites and primitive achondrites regression calculation based on similar to 85 of the studied microtektites best fit a mixing line between crustal compositions and an ll chondrite however due to some scatter mainly in the cr versus ni ratios in the considered dataset an ll chondrite may not be the best fit to the data amongst impactors of primitive compositions eight high ni cr and five low ni cr outlier microtektites similar to 15 in total deviate from the above mixing trend perhaps resulting from incomplete homogenization of heterogeneous impactor and target precursor materials at the microtektite scale respectively together with previous evidence from the similar to 35 myr old popigai impact spherules and the similar to 1 myr old ivory coast microtektites our finding suggests that at least three of the five known cenozoic distal impact ejecta were generated by the impacts of large stony asteroids of chondritic composition and possibly of ordinary chondritic composition the impactor signature found in australasian microtektites documents mixing of target and impactor melts upon impact cratering this requires target impactor mixing in both the two competing models in literature for the formation of the australasian tektites microtektites the impact cratering and low altitude airburst plume models c 2017 elsevier ltd all rights reserved

nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions abstract in the neves area the pre alpine intrusive protolith of the zentralgneise unit tauern window eastern alps is well preserved in a kilometric scale low strain domain without pervasive alpine deformation it is compositionally heterogeneous consisting predominantly of granodiorites with lesser leucocratic granites and different generations of lamprophyres and aplites the intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes incipient alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities resulting in the development of several different types of shear zones fractures without alteration haloes initially accommodate slip entirely on the fracture itself with increasing deformation a foliation is progressively developed in the adjacent host rock eventually producing a single heterogeneous ductile shear zone with the typical sigmoidal foliation pattern strong layers aplite dykes and bleached alteration haloes developed to either side of precursor fractures localize shear on their boundaries to produce characteristic paired shear zones shearing is more evenly distributed within weak layers lamprophyres and quartz veins with a marked discontinuity in shear strain against the adjacent little deformed granodiorite shear zone development was accompanied by the formation of new fractures and quartz rich veins in the host rock which in turn also localized shear magmatic contacts fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations the kinematics of the overprinting strike slip shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis 1 here oriented ca 345 discontinuities of almost all orientations show shear reactivation even in the case of very low resolved shear stress indicating an overall viscous response of the system without a specific yield stress the geometry and kinematics of the shear zone network suggest that the overall deformation in low strain domains was close to coaxial during deformation along the shear zone network compatibility was maintained by fracturing with the development of new quartz veins and by distributed ductile deformation of the host rock especially within contractional domains of the shear network and at shear zone intersections deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities shear zone development in intact rock is always preceded by fracturing which localizes subsequent shear reactivation

the extraterrestrial impact evidence at the palaeocene eocene boundary and sequence of environmental change on the continental shelf we have identified clear evidence of an extraterrestrial impact within the onset of the carbon isotope excursion cie that defines the palaeocene eocene p e boundary hyperthermal event approx 56 ma from several sites on the eastern atlantic coastal plain and offshore we review and update the state of the evidence for an impact at the p e boundary including a k ar cooling age of the ejecta that is indistinguishable from the depositional age at the p e which establishes the ejecta horizon as an isochronous stratigraphic indicator at the p e immediately above the ejecta peak at the base of the coastal plain marlboro clay unit we identify a sharp increase in charcoal abundance coincident with the previously observed dramatic increase in magnetic nanoparticles of soil pyrogenic origin we therefore revisit the observed sequence of events through the p e boundary on the western atlantic coastal plain showing that an extraterrestrial impact led to wildfires landscape denudation and deposition of the thick marlboro clay whose base coincides with the spherule horizon and cie onset the sr ca ratio of the spherules indicates that the carbon responsible for the onset may be vaporized caco 3 target rock mixed with isotopically light carbon from the impactor or elsewhere crucially we do not argue that the impact was responsible for the full manifestation of the cie observed globally onset to recovery approx 170 kyr rather that a rapid onset was triggered by the impact and followed by additional carbon from other processes such as the eruption of the north atlantic igneous province such a scenario agrees well with recent modelling work though it should be revisited more explicitly this article is part of a discussion meeting issue hyperthermals rapid and extreme global warming in our geological past

north american microtektites are more oxidized than tektites iron oxidation states and coordination numbers have been determined by micro x ray absorption near edge spectroscopy xanes on the cores of a large group of microtektites from the australasian ivory coast and north american na tektite strewn field the north american microtektites used in this study have been collected from five sites at different distances from the source crater most have sio2 content between 70 and 80 wt accurate analysis of the pre edge peak energy position and integrated area allowed determination of fe3 fe2 fe3 ratios on all samples with an estimated error of 0 05 microtektites from the australasian and ivory coast strewn fields show low values of the fe3 fe2 fe3 ratios in fair agreement with tektites from the same strewn field in contrast microtektites from the north american strewn fields show a wide range of fe3 fe2 fe3 ratios from 0 02 to ca 0 61 comparison of fe oxidation state data with chemical composition do not show any relation between fe3 fe2 fe3 ratios and na ca or k contents thus suggesting that the high fe oxidation states are not the consequence of sea water alteration the difference between the fe oxidation state of tektites and microtektites from the north american strewn fields suggests that some factors in the formation of the north american microtektites were different than for the north american tektites and for microtektites in the other strewn fields previous fe oxidation state data on na tektites strongly suggest that the wide range in fe oxidation state we found on na microtektites is not related to lateral heterogeneity of the target rocks despite a correlation between microtektite oxidation state and distance from the source crater we maintain that fe oxidation state is not related only to the microtektite droplet flight distance this is in keeping with the fact that no significant variations in the fe oxidation state have been found in microtektites from the australasian strewn field even for australasian microtektites recovered in antarctica the fe oxidation state in north american microtektites could be explained by interaction of melt droplets with a h2o rich vapor plumes generated during the impact these data point out that some difference must exist between the thermal histories of microtektites and tektites from the na strewn field moreover microtektites from the na strewn field show also distinctively higher oxidation states than those from ivory coast or the australasian strewn fields

stress change before and after the 2011 m9 tohoku oki earthquake abstract megathrust systems hold important clues for our understanding of long and short term plate boundary dynamics and the 2011 m9 tohoku oki earthquake provides a data rich case in point here we show that the f net moment tensor catalog indicates systematic changes in crustal stress in the years leading up to the m9 due to the co seismic effect and for the last few years due to viscous relaxation we explore the match between imaged stress change and the perturbations that are expected from 3 d mechanical models of the visco elastic relaxation and afterslip effects of the m9 while these models were constructed based on geodetic and structural seismology constraints alone they match many characteristics of the seismicity inferred stress change this provides additional confidence in the modeling approach and new clues for our understanding of plate boundary dynamics for the japan trench the success of deterministic approaches for exploring crustal stress change also implies that joint inversions using stress from focal mechanisms and geodetic constraints may be feasible such future efforts should provide key insights into time dependent seismic hazard including earthquake triggering scenarios

geochronological constraints on the evolution of the periadriatic fault system alps fault rocks from various segments of the periadriatic fault system paf alps have been directly dated using texturally controlled rb sr microsampling dating applied to mylonites and both stepwise heating and laser ablation 40ar 39ar dating applied to pseudotachylytes the new fault ages place better constraints on tectonic models proposed for the paf particularly in its central sector along the north giudicarie fault oligocene e se directed thrusting 29 32 ma is currently best explained as accommodation across a cogenetic restraining bend within the oligocene dextral tonale pustertal fault system in this case the limited jump in metamorphic grade observed across the north giudicarie fault restricts the dextral displacement along the kinematically linked tonale fault to 30 km dextral displacement between the tonale and pustertal faults cannot be transferred via the peio fault because of both late cretaceous fault ages 74 67 ma and sinistral transtensive fault kinematics in combination with other pseudotachylyte ages 62 58 ma widespread late cretaceous paleocene extension is established within the austroalpine unit coeval with sedimentation of gosau group sediments early miocene pseudotachylyte ages 22 16 ma from the tonale pustertal jaufen and passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the eastern alps this event coincides with exhumation of the penninic units and contemporaneous sedimentation within fault bound basins

afterslip and viscoelastic relaxation model inferred from the large scale post seismic deformation following the 2010 mw 8 8 maule earthquake chile

iridium anomaly associated with the australasian tektite producing impact masses of the impactor and of the australasian tektites abstract recently published studies of ocean drilling project odp cores from near southeast asia revealed microtektite contents much higher than those in previously studied cores suggesting that ir contents might be enhanced in the tektite bearing horizons we determined a positive ir anomaly in odp core 758b from the ninetyeast ridge eastern indian ocean the peak ir concentration of 190 pg g was 2 the continuum level the net ir fluence is 1 8 0 5 ng cm 2 over the depth interval from 10 87 to 11 32 m a small additional peak also associated with microtektites contributes another 0 5 ng ir cm 2 concentrations of ir in core 769a show more scatter but a small ir enhancement is associated with the peak microtektite abundance our best estimate of the poorly constrained fluence is 1 3 0 7 ng cm 2 data on deep sea cores show that the microtektite fluence falls exponentially away from southeast asia the fluence dropping a factor of 2 in 400 km in southeast asia the trend merges with a roughly estimated mass fluence of 1 1 g cm 2 inferred from evidence of a melt sheet in northeast thailand integration of the inferred distribution yields a total mass of australasian tektites of 3 2 1016 g much higher than previous estimates assuming a similar fallout distribution for the impactor and a chondritic composition allows us to calculate its mass to be 1 5 1015 g about 3 orders of magnitude smaller than the minimum mass of the impactor responsible for the extinctions at the end of the cretaceous

crater ejecta scaling laws fundamental forms based on dimensional analysis a model of crater ejecta is constructed using dimensional analysis and a recently developed theory of energy and momentum coupling in cratering events general relations are derived that provide a rationale for scaling laboratory measurements of ejecta to larger events specific expressions are presented for ejection velocities and ejecta blanket profiles in two limiting regimes of crater formation the so called gravity and strength regimes in the gravity regime ejecta velocities at geometrically similar launch points within craters vary as the square root of the product of crater radius and gravity this relation implies geometric similarity of ejecta blankets that is the thickness of an ejecta blanket as a function of distance from the crater center is the same for all sizes of craters if the thickness and range are expressed in terms of crater radii in the strength regime ejecta velocities are independent of crater size consequently ejecta blankets are not geometrically similar in this regime for points away from the crater rim the expressions for ejecta velocities and thickness take the form of power laws the exponents in these power laws are functions of an exponent that appears in crater radius scaling relations thus experimental studies of the dependence of crater radius on impact conditions determine scaling relations for ejecta predicted ejection velocities and ejecta blanket profiles based on measured values of are compared to existing measurements of velocities and debris profiles

micromechanisms of shear zone propagation at the brittle viscous transition our investigation of progressively strained rock samples from the margins of greenschist facies shear zones utilizes a space for time approach to reveal how the mylonitic overprint of metapsammitic and pelitic host rocks at the cap de creus involved brittle fracturing we present a set of microscale observations indicating that microfractures formed immediately prior to or coevally with a fine grained mylonite microfracturing dominated early stages of strain localization on the scale of the shear zones on the microscale centimeter long fractures facilitated strain softening by allowing enhanced fluid access thereby accelerating the dynamic recrystallization of quartz and a metamorphic reaction of biotite as these two processes produce a polyphase matrix of small dislocation poor grains that eventually form an interconnected rheologically weak phase fractures become inactive this represents a strain dependent brittle viscous transition we outline this transition in a conceptual model for the rheological evolution of mid crustal shear zones

extraterrestrial component associated with australasian microtektites in a core from odp site 758b major and trace element analyses of 30 sediment samples from odp core 758b across the brunhes matuyama geomagnetic polarity reversal and the australasian microtektite bearing layer show a small but significant anomaly in the elemental abundances of ir co and ni the maximum of the anomaly between 10 89 and 11 00 m below seafloor correlates with the maximum abundance of microtektites iridium abundances which show the most significant anomaly reach 0 16 ppb which is evidence for an extraterrestrial component

fission track age of transantarctic mountain microtektites abstract we determined the fission track age of transantarctic mountain microtektites the plateau method yielded a formation age of 0 85 0 17 ma this age overlaps within error with that of the catastrophic impact that produced the australasian tektite microtektite strewn field ca 0 8 ma ago this provides further evidence that transantarctic mountain microtektites belong to the australasian tektite microtektite strewn field as previously suggested on the basis of geochemical evidence sr nd isotope systematics and poorly resolved radiometric data

preceding coseismic and postseismic slips of the 2011 tohoku earthquake japan 1 we estimated the spatial and temporal evolution of the preceding aseismic slip from january 2003 to january 2011 the coseismic slip of the tohoku earthquake and the postseismic slip after the earthquake based on global positioning system gps data time dependent analysis indicates aseismic slip offshore of miyagi and fukushima prefectures from 2004 associated with a series of subduction earthquakes that overlap the aseismic slip area these preceding aseismic and coseismic slip areas are centered between the centers of the coseismic and afterslip areas of the tohoku earthquake offshore of miyagi prefecture while they overlap the coseismic and afterslip areas of the tohoku earthquake off fukushima prefecture the timing of moment magnitude nine mw9 class earthquakes appears to be controlled by multiple preceding slip events smaller earthquakes and their afterslip the preceding aseismic and coseismic slip decreased the coupling rate off the tohoku coast suggesting the possibility that the preceding slip represented a precursive stage of the tohoku earthquake the afterslip of the tohoku earthquake occurred in an area where the coseismic slip was not large complementing the large coseismic slip zone the afterslip along iwate miyagi extends up to 80 km in depth and is currently the sole mechanism of strain release in this depth range the source region of the anticipated miyagi oki earthquake shows small postseismic slip after the tohoku earthquake reflecting the energy release at the time of the earthquake aftershock activity is roughly governed by an afterslip process

from geodetic imaging of seismic and aseismic fault slip to dynamic modeling of the seismic cycle understanding the partitioning of seismic and aseismic fault slip is central to seismotectonics as it ultimately determines the seismic potential of faults thanks to advances in tectonic geodesy it is now possible to develop kinematic models of the spatiotemporal evolution of slip over the seismic cycle and to determine the budget of seismic and aseismic slip studies of subduction zones and continental faults have shown that aseismic creep is common and sometimes prevalent within the seismogenic depth range interseismic coupling is generally observed to be spatially heterogeneous defining locked patches of stress accumulation to be released in future earthquakes or aseismic transients surrounded by creeping areas clay rich tectonites high temperature and elevated pore fluid pressure seem to be key factors promoting aseismic creep the generally logarithmic time evolution of afterslip is a distinctive feature of creeping faults that suggests a logarithmic dependency of fault friction on slip rate

indentation model of the eastern alps and the origin of the tauern window a lithospheric model scaled for density and viscosity was shortened obliquely in front of an indenter whose shape and orientation match those of the south alpine indenter deformation was initially localized into subparallel sets of conjugate transpressive thrusts these structures evolved into folds which were strongly amplified and converged closer together during ongoing shortening finally forming a series of tight antiforms at the same time this tightly folded structure locally accommodated significant orogen parallel extension attaining 100 in front of the leading edge of the indenter but only 15 on the scale of the entire model the internal architecture and fault pattern of the model in map view are very similar to those of the tauern window in the eastern alps emphasizing the importance of localization of folding and shortening rather than orogen parallel extension for its formation

deformation cycles of subduction earthquakes in a viscoelastic earth subduction zones produce the largest earthquakes over the past two decades space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes the short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle piecing together geodetic snapshots from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short term years and long term decades and centuries viscous behaviour of the mantle traditional views based on elastic models such as coseismic deformation being a mirror image of interseismic deformation are being thoroughly revised snapshots of subduction zones using space geodesy reveal that the viscous behaviour of the mantle controls crustal deformation requiring the revision of traditional elastic models for earthquake risk assessment space geodesy the use of satellites to monitor earth has revolutionized our view of crustal deformation between consecutive earthquakes however the brevity of such measurements means that studies must be done by comparing multiple subduction zones at different stages of the earthquake cycle by piecing together geodetic snapshots from the sumatra chile and cascadia subduction zones this review presents a unifying picture in which deformation is controlled by the viscous behaviour of the mantle in both the short term years and the long term decades to centuries

small scale ductile shear zones neither extending nor thickening nor narrowing abstract the length thickness and strain gradients of small scale 10 3 10 1 m thick ductile shear zones are pre determined by the presence of surface precursors e g fractures or compositional layers and associated fluid rock interaction fractures with their surrounding host rock damage zone and concurrent tectonic under pressure during dilation provide efficient fluid pathways and networks with diffusive fluid rock interaction at their margins the fluid induced gradational to zoned compositional haloes that symmetrically surround a fracture control the strain gradients of developing shear zones and result in a diversity of geometric types including single homogeneous to heterogeneous shear zones and paired shear zones as a consequence geochemical differences between shear zone and host rock may reflect fluid rock interaction during the precursor brittle history rather than during slip especially considering that shear zones with even a small component of stretch may be over pressured and therefore unable to drain fluids from the surrounding rocks support from field observation for this interpretation is given by 1 the lack of correlation between shear zone thickness and accumulated displacement and 2 the similar thickness of shear zones and locally unexploited alteration haloes surrounding fracture precursors most small scale shear zones in massive rocks granitoids are initially neither thickening nor narrowing with increasing strain this concept may also apply to more foliated rocks but does not necessarily hold for larger scale shear zones

mineralogy porosity and fluid control on thermal conductivity of sedimentary rocks summary estimates of thermal conductivity of the main non clay and clay sedimentary minerals are presented especially for the case of argillaceous ones these estimates are averaged estimates based on the interpretation of laboratory conductivity porosity and mineralogy measurements performed on small volumes of the order of 100cm3 of water saturated moist and air saturated samples the sampling is representative of the main sedimentary rocks sandstones carbonates evaporites and shales and it is composed of samples considered as structurally isotropic material first we experimentally verify the first order control of mineralogy porosity and fluid content on bulk conductivity and we demonstrate that such influences may be predicted accurately using a geometric mean model as long as isotropic samples are used then we interpret the data using an inverse method in order to estimate the average mineral conductivities of the main non clay and clay minerals which give the best fit to the individual laboratory measurements through the geometric mean model the analysis is based on measurements on 82 non clay samples and 29 clay samples taken on oil well cores or cuttings on outcrops and on artificially recompacted samples estimates of average mineral conductivities returned by the inversion process for the main non clay minerals are similar to generally admitted values 7 7 w m k for quartz 3 3 w m i k for calcite 5 3 w m l k for dolomite and 6 3 w m k for anhydrite values obtained for the clay minerals are systematically lower than those obtained for non clay ones they are of the order of 1 9 w m k for illite and smectite 2 6 w m k for kaolinite and 3 3 w m l k for chlorite these estimates combined with porosity and mineralogy data are used with the geometric mean model in order to verify its first order validity for predicting thermal conductivity of small volumes of porous isotropic sedimentary rocks the bulk conductivity is predicted with an accuracy of the order of f10 per cent for moist or water saturated samples and of the order of f20 per cent for air saturated samples

chemical analyses of bosumtwi crater target rocks compared with the ivory coast tektites abstract the bosumtwi crater ghana was excavated in phyllites and greywackes with subordinate microgranite dykes and quartz veins of the 2000 ma old lower birimian system with a granodiorite intrusion at pepiakese on its northeastern side new major and trace element analyses are presented for 7 phyllites 5 greywackes 2 microgranites 3 pepiakese intrusion rocks and 1 suevite using xrf and inna means and standard deviations were calculated using all available modern analyses for each element in the bosumtwi target rocks bosumtwi suevite glasses and ivory coast tektites good agreements between the means were found for the three groups with the suevite glasses and tektites having more limited compositional ranges than the target rocks least squares mixing between target rock types shows that the best fits to the tektite and suevite glass compositions require components of about a third or a quarter from the pepiakese intrusion and some extra silica derived from quartz veins as well as the metasediments the new data provide evidence for vapour phase fractionation of p2o5 and na2o in the tektites in addition to the previously reported pb and rb evidence for a meteoritic component in the tektites was found to be equivocal since the target rocks are probably a sufficient source of the meteorite indicator elements ni and ir

australasian microtektites in the south china sea and the west philippine sea implications for age size and location of the impact crater abstract microtektites from two deep sea cores in the south china sea and the west philippine sea are identified as belonging to the australasian tektite strewn field based on the morphology chronostratigraphic occurrence and geographical location of these microtektites the higher concentrations of microtektites 1000 cm2 in the marginal seas of the western pacific with the peak concentration in the south china sea support the hypothesis of a large impact crater in indochina these two new occurrences lead to a more precise dating of the impact event at 793 ka whereas the size of the australasian source crater on the indochina peninsula is estimated to be 90 1 16 km introduction the australasian strewn field covering at least one tenth of the earth s surface is the largest and the youngest 0 77 0 02 ma izett and obradovich 1992 of the four known tektite strewn fields microtektites belonging to the australasian strewn field have been found in more than 40 deep sea cores throughout much of the indian ocean the western equatorial pacific ocean and the sulu celebes and philippine seas glass

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

why calcite can be stronger than quartz 1 in the neves area eastern alps calcite forms asymmetric centimeter scale single crystal porphyroclasts in quartz mylonites developed during hydrous amphibolite facies metamorphism at 550 c under these conditions coarse calcite was clearly stronger than the surrounding polycrystalline dynamically recrystallized quartz matrix experimental results indicate that coarse calcite is less strain rate sensitive than wet quartzite consistent with an inversion in strength on extrapolation to natural strain rates for this to occur wet quartzite must be weak flowing at differential stress of 10 mpa the lack of high temperature twins showing bulging or recrystallization in calcite clasts is consistent with such low stresses during shear zone development under near peak metamorphic conditions the maximum effective viscosity ratio of coarse calcite to quartzite for these conditions is probably not large 10 however numerical modeling shows that ratios of around 2 are sufficient to maintain near rigid calcite clast behavior for power law rheology with stress exponents appropriate to quartz n 3 4 and coarse calcite n 6 the inversion in relative strength reflects the difference in influence of water on the crystal plastic flow of calcite and quartz water has a dramatic effect for quartz but little or no effect for calcite quartz rich rocks under hydrous amphibolite facies conditions in the middle to lower crust are therefore relatively weak in fact weaker than coarse calcite and flow at much lower stresses than dry quartz rich rocks at similar crustal levels

alpine metamorphism and veining in the zentralgneis complex of the sw tauern window a model of fluid rock interactions based on fluid inclusions abstract the granitic to tonalitic gneisses of the zentralgneis complex in the sw tauern window contain numerous discordant quartz biotite plagioclase veins occurring either as isolated or in parallel sets with a preferential orientation perpendicular to the main foliation of host rocks veins show a zonal distribution of minerals with plagioclase and biotite at the walls and quartz in the centre quartz contains abundant fluid inclusions fi mostly occurring along secondary planes all primary and most secondary inclusions three phase h2o co2 inclusions formed from a dominant fluid type of aqueous carbonic composition fi have been characterized by microthermometry laser raman and laser ablation optical emission spectroscopies which have confirmed their similarity also with respect to composition and density fluids display a somewhat unusual composition especially a wide range of rather high salinities 3 7 mol deduced from the final melting temperature of clathrate which covers a wide range from 4 2 to 4 0 c the volatile phase is dominated by co2 3 12 mol with small amounts of n2 the final homogenization temperature of these inclusions is generally 300 c but independently constrained trapping temperatures of 550 600 c yield to pressure estimates of 3 5 7 5 kbar highest pressures are in agreement with the p t conditions reached during the tauern metamorphism peak veining was triggered by hydrofracturing induced by pl bt producing devolatilization and probably decarbonation reactions in the adjacent gneisses and quartz precipitated in response to the significant pressure and or salinity fluctuations shown by fi during each microfracturing healing stage

geographic variation in australasian microtektite concentrations implications concerning the location and size of the source crater geographic variations in the concentration of australasian microtektites in 42 cores from the indian ocean western equatorial pacific ocean and the philippine celebes and sulo seas were used to predict the location of the australasian tektite source crater and to estimate its size the location that seems to best explain the geographic variations in microtektite concentrations is located in central cambodia at about 12 n latitude and 106 e longitude the diameter of the source crater is estimated to be between 32 and 114 km based on thickness of the microtektite layer at each site and distance from the predicted source area in cambodia the large range in estimated size of the source crater is due to a lack of knowledge about the value of the exponent that describes the radial decrease in ejecta thickness additional microtektite ejecta bearing sites closer to the source region could help resolve this problem

source of the australasian tektite strewn field a possible off shore impact site although there is a preponderance of evidence that tektites were formed by asteroid impacts on the earth no source crater has been found for the largest and youngest of the strewn fields the australasian strewn field we have examined a combined seasat geos 3 altimeter data set of sea surface heights in the northern portion of the australasian strewn field for negative gravity anomalies on the continental shelf and slope which might be related to the source crater for these tektites a large negative anomaly which we call the qui nhon slope anomaly is a sea surface depression of approximately 1 5 meters over an area of 100 km diameter centered at 13 47 n latitude and 110 37 e longitude it corresponds to a gravity anomaly of about 50 mgal a surface ship gravimeter track across most of the structure confirms the satellite measurements this site is on the continental slope in 1 to 3 km of water a sea floor topographic depression of more than 200 meters occurs in the southeast quadrant of the anomaly the center of the structure is 175 km offshore and is less than 300 km from the muong nong substrewn field and the prolific dalat site perhaps the largest concentration of tektites within the australasian field due to its location size and magnitude we propose this anomaly may be due to the impact structure that produced the australasian strewn field

effect of planetary rotation on distal tektite deposition on mars 1 the rapid rotation of mars creates a significant pseudoforce known as the coriolis force that greatly modifies the flight paths and subsequent deposition of distal ejecta ejecta with launch velocities of 3 km s and greater an accurate depiction of the effects of the coriolis force requires the integration of the coriolis terms directly into a series of spherical ballistic equations applied within a rotating reference frame the resulting landing positions of radially ejected particles tend to form distinct wrapping patterns that are focused to specific areas in the hemisphere opposite from the source crater around the pole for high latitude craters 45 in latitude and above and in an equatorial band for low latitude craters below 45 latitude consequently particular locations on the surface receive deposits in stages direct delivery of ejecta followed by deposits of higher velocity ejecta this staged deposition leads to regions of enhanced ejecta accumulations where meters of material collect millimeters would be predicted in nonrotational radial decay models thus high latitude craters could supply a considerable amount of distal products tektites and melts to polar locations perhaps contributing to the dark circumpolar materials