recrystallization and chemical changes in apatite in response to hypervelocity impact despite the wide utility of apatite ca5 po4 3 f cl oh in the geosciences including tracing volatile abundances on the moon and mars little is known about how the mineral responds to the extrem

the formation of large neoblasts in shocked zircon and their utility in dating impacts uranium lead u pb geochronology of individual shocked zircon grains has unique potential for dating bolide impact events neoblasts in granular textured zircon have been recognized as the shock related feature most effective at recording the impact age here we report the discovery of large neoblasts 5 100 mu m in dimension in shocked zircon at the sudbury impact structure canada the first report of in situ coarsely granular zircon from a terrestrial impact site other than the vredefort structure south africa the neoblast bearing sample was taken from a heterogeneous lithic clast rich igneous unit associated with the roof rocks of the impact melt sheet making this the first time a crater has been dated using neoblastic zircon from the upper part of its stratigraphy previous in situ discoveries of coarsely granular zircon at vredefort were all in impact generated mafic melt emplaced beneath the impact melt sheet electron backscatter diffraction analysis of the impact aged neoblasts indicates that the high pressure conditions inferred in the formation of many small neoblasts were not necessarily involved in the formation of these large ones their large size internal zonation and occurrence in a slowly cooling environment collectively suggest that large neoblasts at sudbury formed by relatively protracted post impact growth in shocked zircon incorporated into impact related melt based on insight from large neoblast growth in terrestrial settings we suggest that the ca 4 33 ga neoblasts recently reported in lunar zircon may imply a major basin forming event on the moon at that time new knowledge of the cratering environments in which large neoblasts form also raises the prospect of possibly linking ex situ granular zircon in lunar breccias with specific impact structures and thus better calibrating the lunar cratering record with radiometric ages

deformed monazite yields high temperature tectonic ages the deformation of monazite in the polymetamorphic sandmata granulite complex in india has been investigated by electron backscattered diffraction and sensitive high resolution ion microprobe shrimp quantitative microstructural analyses document the development of deformation twins in 100 001 and graphic 1 orientations low angle 10 boundary development associated with dislocation creep and the development of new grains due to dynamic recrystallization these data represent the first quantitative evidence of crystal plastic deformation of natural monazite shrimp u th pb analysis shows that the host monazite preserves discordant ages as old as 1666 28 ma along a trend from ca 1720 ma to ca 1000 ma with increasingly discordant ages recorded in zones of higher lattice distortion domains of recrystallized new grains within the monazite record a tightly clustered concordia age of 970 14 ma this age is interpreted to represent the timing of monazite dynamic recrystallization associated with deformation of the host protolith and is consistent with partial resetting and pb loss from domains deforming by dislocation creep the complex but systematic relationship between microstructure and age data in monazite provide the first direct evidence of pb isotope resetting during deformation the approach illustrates a new methodology for the dating of deformation events in high grade metamorphic rocks which are typically difficult to constrain 1 embed inline graphic 1 gif

fission track dating of haughton astrobleme and included biota devon island canada haughton astrobleme is a major extraterrestrial impact structure located on devon island in the canadian arctic archipelago northwest territories apatite grains separated from shocked precambrian gneiss contained in a polymict breccia from the center of the astrobleme yielded a fission track date of 22 4 million 1 4 million years before the present or early miocene aquitanian this provides a date for the impact event and an upper limit on the age of crater filling lake sediments and a flora and vertebrate fauna occurring in them a geologically precise date for these fossils provides an important biostratigraphic reference point for interpreting the biotic evolution of the arctic

cretaceous to early tertiary paleogeography in the hinterland of the sevier thrust belt east central nevada the cretaceous to early tertiary hinterland of the sevier thrust belt in east central nevada underwent two periods of sedimentary basin development the older early cretaceous episode is characterized by basins that formed coeval with deformation within a belt of contractional structures basins were characterized by periods of through flowing fluvial drainage the younger late cretaceous to eocene episode of basin development was characterized by internally drained alluvial basins that subsequently expanded into more regional lacustrine basins basal alluvial detritus indicates that basin inception occurred as a result of rapid development of topographic relief syn sedimentary tectonism was characterized by horizontal extension due to attainment of equilibrium topography in response to crustal thickening

empirical constraints on shock features in monazite using shocked zircon inclusions shock deformation microstructures in monazite have been systematically characterized for the first time in grains from the vredefort impact structure in south africa electron backscatter diffraction mapping has identified 12 unique orientations of monazite deformation twins including 7 orientations that have not previously been described in experiments or nature other shock features include planar deformation bands and strain free neoblasts which have been shown to date deformation shock twinned zircon inclusions within the deformed monazite require pressures of 20 gpa thus providing critical empirical constraints on formation conditions confirming a hypervelocity impact origin of the monazite microstructures the vredefort monazite grains described here represent the first case of using shocked mineral inclusions to empirically calibrate shock microstructures formed in the host mineral these results conclusively establish monazite as a recorder of shock deformation and highlight its use in identifying and dating impact structures

a pressure temperature phase diagram for zircon at extreme conditions abstract hypervelocity impact processes are uniquely capable of generating shock metamorphism which causes mineralogical transformations and deformation that register pressure p and temperature t conditions far beyond even the most extreme conditions created by terrestrial tectonics the mineral zircon zrsio4 responds to shock deformation in various ways including crystal plasticity twinning polymorphism e g transformation to the isochemical mineral reidite formation of granular texture and dissociation to zro2 sio2 which provide robust thermobarometers that record different extreme conditions the importance of understanding these material processes is two fold first these processes can mobilize and redistribute trace elements and thus be accompanied by variable degrees of resetting of the u pb system which is significant for the use of zircon as a geochronometer second some features described herein form exclusively during shock events and are diagnostic criteria that can be used to confirm the hypervelocity origin of suspected impact structures we present new p t diagrams showing the phase relations of zrsio4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints we present case studies to illustrate zircon microstructures formed in extreme environments and present electron backscatter diffraction data for grains from three impact structures mistastin lake of canada ries of germany and acraman of australia that preserve different minerals and microstructures associated with different shock conditions for each locality we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the p t history we outline a conceptual framework for a zircon based approach to extreme thermobarometry that incorporates both direct observation of high p and high t phases as well as inferences for the former existence of phases from orientation relationships in recrystallised products a concept we refer to here as phase heritage this new approach can be used to unravel the pressure temperature history of zircon bearing samples that have experienced extreme conditions such as rocks that originated in the earth s mantle and those shocked during impact events on earth and other planetary bodies

halogen and cl isotopic systematics in martian phosphates implications for the cl cycle and surface halogen reservoirs on mars abstract the cl isotopic compositions and halogen cl f br and i abundances in phosphates from eight martian meteorites spanning most rock types and ages currently available have been measured in situ by secondary ion mass spectrometry sims likewise the distribution of halogens has been documented by x ray mapping halogen concentrations range over several orders of magnitude up to some of the largest concentrations yet measured in martian samples or on the martian surface and the inter element ratios are highly variable similarly cl isotope compositions exhibit a larger range than all pristine terrestrial igneous rocks phosphates in ancient 4 ga meteorites orthopyroxenite alh 84001 and breccia nwa 7533 have positive 37 cl anomalies 1 1 to 2 5 these samples also exhibit explicit whole rock and grain scale evidence for hydrothermal or aqueous activity in contrast the phosphates in the younger basaltic shergottite meteorites 37 cl anomalies 0 2 to 5 6 phosphates with the largest negative 37 cl anomalies display zonation in which the rims of the grains are enriched in all halogens and have significantly more negative 37 cl anomalies suggestive of interaction with the surface of mars during the latest stages of basalt crystallization the phosphates with no textural major element or halogen enrichment evidence for mixing with this surface reservoir have an average 37 cl of 0 6 supporting a similar initial cl isotope composition for mars the earth and the moon oxidation and reduction of chlorine are the only processes known to strongly fractionate cl isotopes both positively and negatively and perchlorate has been detected in weight percent concentrations on the martian surface the age range and obvious mixing history of the phosphates studied here suggest perchlorate formation and halogen cycling via brines which have been documented on the martian surface has been active throughout martian history

a terrestrial perspective on using ex situ shocked zircons to date lunar impacts deformed lunar zircons yielding u pb ages from 4333 ma to 1407 ma have been interpreted as dating discrete impacts on the moon however the cause of age resetting in lunar zircons is equivocal as ex situ grains in breccias they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons detrital shocked zircons provide a terrestrial analog to ex situ lunar grains for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons electron backscatter diffraction and sensitive high resolution ion microprobe u pb analysis of zircons eroded from the ca 2020 ma vredefort impact structure south africa show that complete impact age resetting did not occur in microstructural domains characterized by microtwins planar fractures and low angle boundaries which record ages from 2890 ma to 2645 ma an impact age of 1975 39 ma was detected in neoblasts within a granular zircon that also contains shock microtwins which link neoblast formation to the impact however we show that granular texture can form during regional metamorphism and thus is not unique to impact environments these results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance and then targeting specific age reset microstructures with the recognition that zircon can deform plastically in both impact and magmatic environments age resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon based impact chronologies for the moon earth or other planetary bodies

an u th he age for the shallow marine wetumpka impact structure alabama usa single crystal u th he dating was applied to 24 apatite and 23 zircon grains from the wetumpka impact structure alabama usa this small approximately 5 7 6 km impact crater was formed in a shallow marine environment with no known preserved impact melt thus offering a challenge to common geochronological techniques a mean u th he apatite and zircon age of 84 4 1 4 ma 2 was obtained which is within error of the previously estimated late cretaceous impact age of approximately 83 5 ma in addition helium diffusion modeling of apatite and zircon grains during fireball contact shock metamorphism and hydrothermal events was undertaken to show the influence of these individual thermal processes on resetting u th he ages in the wetumpka samples this study has shown that the u th he geochronological technique has real potential for dating impact structures especially smaller and eroded impact structures that lack impact melt lithologies

a middle late triassic 40ar 39ar age for the paasselka impact structure se finland abstract 40ar 39ar dating of recrystallized feldspar glass particles separated from clast rich impact melt rocks from the approximately 10 km paasselka impact structure se finland yielded a middle to late triassic ladinian karnian pseudo plateau age of 228 7 3 0 3 4 ma 2 this new age makes paasselka the first known triassic impact structure dated by isotopic methods on the baltic shield the new paasselka impact age is within uncertainty coeval with isotopic ages recently obtained for the lake saint martin impact structure in canada indicating a new middle to late triassic impact crater population on earth the comparatively small crater size however suggests no relationship between the paasselka impact and a postulated extinction event at the middle late triassic boundary

a carnian 40ar 39ar age for the paasselka impact structure se finland an update a recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the approximately 10 km paasselka impact structure in southeast finland was dated via 40ar 39ar step heating the newly obtained plateau age of 228 7 1 8 2 2 ma 2 mswd 0 32 p 0 93 is equal to the previously published pseudoplateau age of 228 7 3 0 3 4 2 for the impact event according to the current international chronostratigraphic chart and using the most recent published suggestions for the k decay constants a carnian late triassic age for the paasselka impact structure of 231 0 1 8 2 2 ma 2 is calculated and considered the most precise and accurate age for this impact structure the new plateau age for paasselka confirms the previous dating result but is based on its internal statistics much more compelling

phosphate ages in apollo 14 breccias resolving multiple impact events with high precision u pb sims analyses abstract the u pb systems of apatite and merrillite grains within four separate apollo 14 impact melt breccia samples were analysed by secondary ion mass spectrometry no systematic difference was identified between the 207 pb 206 pb ages of the apatites and merrillites a combined 207 pb 206 pb age of 3927 2 ma 95 conf is determined for three of these samples 14305 103 3926 4 ma 14306 150 3926 6 ma 14314 13 3929 4 ma by combining these data with the ages previously obtained for zircons in apollo 12 impact melt breccia fragments and the lunar meteorite sau 169 a weighted average age of 3926 2 ma 95 conf is obtained which is attributed to the formation of the imbrium basin an age of 3943 5 ma is determined for the fourth breccia 14321 134 which is similar to ages of 3946 15 ma and 3958 19 ma obtained from several older phosphates in 14305 103 and 14314 13 the weighted average of these three older ages is 3944 4 ma 95 conf this is indistinguishable to the age 3938 4 ma 2 obtained for a different apollo 14 impact melt breccia in a previous study after investigating likely sources for this older 3940 ma age we conclude that the humorum or serenitatis basin forming events are likely candidates the potential identification of two large impact events within 15 myrs has important implications for the rate of lunar bombardment around 3 95 3 92 ga this study demonstrates the importance of high precision age determinations for interpreting the impact record of the moon as documented in lunar samples

impact melt rocks from the paasselka impact structure se finland petrography and geochemistry abstractrecently samples of allochthonous melt rocks from the v10 km and 1 9 ga paasselk impact structure se finland were obtained in this study we present a first detailed petrographic and geochemical description of clast rich paasselk impact melt rocks shock metamorphic features comprise shocked feldspar grains intensely shocked and toasted quartz marginally molten and recrystallized clasts thought to have been diaplectic quartz glass largely fresh and recrystallized feldspar glasses decomposed biotite flakes recrystallized fluidal silica glass originally probably lechatelierite in partially molten sandstone clasts all set into a glassy to cryptocrystalline melt matrix the degree of shock metamorphism of clasts suggests initial whole rock melting at peak shock pressures of 35 gpa and post shock temperatures of up to 1500 c glass components vary in geochemical composition corresponding to the mixed character of the crystalline target rock i e representing different monomineralic and mixed mineral melts feldspar glasses and the fresh glassy to cryptocrystalline melt matrix indicate that the paasselk melt rocks are not intensely altered the geochemical composition of the paasselk impact melt rocks is roughly consistent with the compositions of melt rocks from a number of impact structures located within the crystalline basement of the baltic shield

an early ordovician 40ar 39ar age for the 50 km carswell impact structure canada the formation age of the large 50 km carswell impact structure canada has been a matter of debate ever since its discovery five decades ago with proposed ages ranging from mesoproterozoic to early cretaceous here we present new 40ar 39ar data for aliquots of euhedral adularia separated from vesicles in an impact melt rock from the central uplift of the structure the analyses of the adularia yielded a statistically robust early ordovician crystallization age of 481 5 0 8 ma 2 mean square of weighted deviates 1 06 p 0 30 the most plausible explanation for the formation of vesicle filling adularia is through low temperature mineral precipitation during residual hydrothermal circulation that followed the impact as no other known major intrusive extrusive or thermal events have occurred in the carswell region in the phanerozoic the new age of the carswell impact structure overlaps within uncertainty with the most precise ar ar ages proposed for the l chondrite parent body breakup event but not with the age of the stratigraphic sequence from which the meteorites and micrometeorites from this event were recovered thus either the carswell impact represents a separate unrelated impact event or the dynamic evolution of the l chondrite parent body breakup is more complicated than presently understood and carswell represents one of the earliest and largest known impacts of this event on earth less

u th he dating of terrestrial impact structures the manicouagan example the accurate dating of meteorite impact structures on earth has proven to be challenging melt sheets are amenable to high precision dating by the u pb and 40ar 39ar methods but many impact events do not produce them or they are not preserved in cases where high temperature shock metamorphism of the target materials has occurred without widespread melting these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact however the u th he chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites we report u th he dates for accessory minerals from the manicouagan impact structure in quebec canada nine zircons from a melt sheet sample yield a weighted mean age of 213 2 5 4 ma 2se indistinguishable from the published 214 1 ma 2 u pb zircon age for the impact in contrast five apatites from this sample yield dates between 205 9 6 5 and 162 0 5 3 ma 2 indicating variable postimpact helium loss due to low temperature thermal disturbance preimpact titanite crystals from a shocked meta anorthosite sample yield two dates consistent with the impact age at 212 27 and 214 13 ma 2 and two younger dates of 189 6 6 9 and 192 2 9 8 ma 2 suggestive of postimpact helium loss these results indicate that u th he chronometry is a suitable method for dating impact events although interpretation of the results requires recognition of possible 4he loss related to reheating subsequent to impact

shock behavior of zircon phase transition to scheelite structure and decomposition abstract both single crystal and powdered specimens of zircon zrsio4 were shocked to peak pressures between 30 and 94 gpa using the gun method and specimens recovered were studied by means of x ray diffraction analysis transmission electron microscopy and infrared spectroscopy transformation to the scheelite structure started above 30 gpa and was completed above 53 gpa in the case of single crystal specimens tetragonal unit cell parameters of the scheelite type zrsio4 at room condition are measured to bea 4 7341 1 a c 10 51 1 a c a 2 219 2 andv 235 5 2 a3 which is smaller than that of the zircon type by 9 9 the recovered scheelite type zrsio4 reverts to the zircon type after rapid heating to 1200 c at room pressure this transformation from the zircon type to the scheelite type is unique in that it is fast displacive like but does not reverse tetragonal zro2 was detected as decomposition product in the single crystal specimen shocked to 94 gpa and further confirmed in a powdered specimen shocked to 53 gpa where enhancement of temperature is expected because of high porosity decomposition behavior of zircon observed in natural shock events is discussed on the basis of present experimental results

impact history of the apollo 17 landing site revealed by u pb sims ages secondary ion mass spectrometry sims u pb ages of ca phosphates from four texturally distinct breccia samples 72255 76055 76015 76215 collected at the apollo 17 landing site were obtained in

crystal chemistry of the monazite and xenotime structures monazite and xenotime the re p04 dimorphs are the most ubiquitous rare earth re minerals yet accurate structure studies of the natural phases have not been reported here we report the results of high precision structure studies of both the natural phases and the synthetic re p04 phases for all individual stable rare earth elements monazite is monoclinic p2 n and xenotime is isostructural with zircon space group 14 amd both atomic arrangements are based on 001 chains of intervening phosphate tetrahedra and re polyhedra with a reogpolyhedron in xenotime that accommodates the heavy lanthanides tb lu in the synthetic phases and a re09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements la gd as the structure transforms from xenotime to monazite the crystallographic properties are comparable along the 00i chains with structural adjustments to the different sizes of re atoms occurring principally in 00i there are distinct similarities between the structures that are evident when their atomic arrangements are projected down 001 in that projection the chains exist in 100 planes with two planes per unit cell in monazite the planes are offset by 2 2 a along 010 relative to those in xenotime in order to accommodate the larger light re atoms the shift of the planes in monazite allows the re atom in that phase to bond to an additional 02 atom to complete the re09 polyhedron

in situ la icp ms apatite and zircon u pb geochronology of the nicholson lake impact structure canada shock and related thermal effects abstract in situ laser ablation inductively coupled plasma mass spectrometry la icp ms has been deployed to determine the u pb ages of impact metamorphosed apatite and zircon associated with the 12 5 km diameter nicholson lake impact structure northwest territories canada the dated phases occur within both impact melt bearing breccias and clast laden impact melts a total of 84 laser ablation analyses from 57 apatite grains within seven rock samples yield a minimum refined lower intercept at 387 5 ma mswd 0 87 2 n 26 and maximum age with lower intercept of 1740 ma a total of 90 laser ablation analyses on 52 zircon grains from two rock samples yield a minimum intercept age of 384 8 ma mswd 1 3 2 n 22 with a maximum upper intercept age of 2679 14 ma and a second discordia with a 1740 ma upper intercept age the results are consistent with the target rocks comprising archean snow island suite 2 7 ga and paleoproterozoic nueltin plutonic suite 1 74 ga with the impact event occurring at approximately 385 ma the degree of resetting of inherited apatite can be related to its proximity to impact melt now partly devitrified glass within the host impact melt bearing breccias and clast laden impact melt rocks those grains in direct contact with melt bodies are reset while those occurring as inclusions in lithic or mineral clasts partly or wholly retain their original isotopic compositions the youngest impact reset apatite ages are most closely associated with the highest shock levels i e granular textured and thermally dissociated zircons which we relate to juxtaposition of apatite with superheated melt due to accumulated radiation damage prior to impact many of the relic zircons are metamict which facilitated enhanced pb diffusion and their resetting to the impact age granular zircons record impact ages while those exhibiting planar fractures or experiencing negligible shock record basement ages we link apatite and zircon geochronology to the textural and structural states of zrsio4 and its associated shock levels via field emission scanning electron microscopy and micro raman spectrometry

shocked monazite chronometry integrating microstructural and in situ isotopic age data for determining precise impact ages monazite is a robust geochronometer and occurs in a wide range of rock types monazite also records shock deformation from meteorite impact but the effects of impact related microstructures on the u th pb systematics remain poorly constrained we have therefore analyzed shock deformed monazite grains from the central uplift of the vredefort impact structure south africa and impact melt from the araguainha impact structure brazil using electron backscatter diffraction electron microprobe elemental mapping and secondary ion mass spectrometry sims crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal plastic deformation features including shock twins planar deformation bands and neoblasts shock twins were documented in up to four different orientations within individual monazite grains occurring as compound and or type one twins in 001 100 left 10 bar 1 right 110 left 212 right and type two irrational twin planes with rational shear directions in 0 bar 1 bar 1 and bar 1 bar 1 0 sims u th pb analyses of the plastically deformed parent domains reveal discordant age arrays where discordance scales with increasing plastic strain the correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event neoblasts in granular monazite domains are strain free having grown during the impact events via consumption of strained parent grains neoblastic monazite from the inlandsee leucogranofels at vredefort records a 207pb 206pb age of 2010 15 ma 2 n 9 consistent with previous impact age estimates of 2020 ma neoblastic monazite from araguainha impact melt yield a concordia age of 259 5 ma 2 n 7 which is consistent with previous impact age estimates of 255 3 ma our results demonstrate that targeting discrete microstructural domains in shocked monazite as identified through orientation mapping for in situ u th pb analysis can date impact related deformation monazite is therefore one of the few high temperature geochronometers that can be used for accurate and precise dating of meteorite impacts

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

experimental shock deformation in zircon a transmission electron microscopic study abstract in recent years apparently shock induced and thus impact characteristic microdeformations in the form of planar microdeformation features and so called strawberry granular texture have been observed in zircons in rocks from confirmed impact structures and from the k t boundary the nature of the planar microdeformations in this mineral is however still unknown and critical information is needed regarding the shock pressure range in which these deformation effects are produced we experimentally shock deformed two series of thin zircon zrsio 4 target plates cut perpendicular to the c axis at shock pressures of 20 40 and 60 gpa the recovered samples were characterized by optical and scanning electron microscopy in addition one sample series was studied by transmission electron microscopy tem microdeformation effects observed at 20 gpa include pervasive micro cleavage and dislocation patterns plastic deformation is indicated by a high density of straight dislocations in glide configuration the dominant glide systems are 010 micro cleavages induced by shear stresses during the compression stage occur mostly in the 100 planes the large density of dislocations at crack tips shows that plastic deformation was initiated by the micro cracking processs at 40 gpa the sample was partly transformed from the zircon z to a scheelite cawo 4 type s structure planar deformation features pdfs containing an amorphous phase of zircon composition are present in the not yet transformed zircon relics the phase with scheelite structure initiated in the 100 planes of zircon consists of thin 0 1 to several m bands that crosscut the zircon matrix the phase transformation is displacive martensitic and can be related by 100 z 112 s and 001 z s the scheelite structure phase is densely twinned with twins in the 112 plane the 60 gpa sample consists completely of the scheelite structure phase crosscutting and displacing relationships between twins and pdfs demonstrate that pdfs are formed in the zircon structure i e before the phase transformation to the scheelite structure occurred most likely at the shock front crystallographic orientations of optically visible planar features in zircon in comparison with orientations of planar defects at the tem scale suggest that the optically visible features are more likely planar microfractures than pdfs

structural evolution and sequence formation offshore south gabon during the tertiary abstract the present investigation of the tertiary succession offshore south gabon is based on seismic surveys of 1985 1989 vintage and wells covering the southern part of gabonese waters in general the tertiary basin off south gabon evolved during two structural stages during the paleocene and early middle eocene a trough like basin subsided slowly towards the west during this period there was a cyclic development of the succession which consists of up to four sequences mostly bounded by type 2 sequence boundaries the sequential changes were induced by eustatic sea level changes this pattern changed during the neogene when the evolution was characterized by continental crustal flexuring in early middle miocene times this resulted in strong subsidence of the western part of the basin and uplift of the eastern part consequently the eastern part was exposed to strong erosion and a thick middle miocene succession was deposited west of the shelf approximately the present day shelf especially on the slope the clinoformal and lens shaped middle late miocene succession which consists of four sequences bounded by type 1 sequence boundaries has been controlled by eustatic changes in sea level in addition to the overall subsidence pattern salt tectonics has modified the external and internal architecture of the sequences on a local scale canyon formation in connection with slumping results in the establishment of well developed fan systems on the lower part of the slope and on the basin floor area