the importance of secondary cratering to age constraints on planetary surfaces small craters less than one kilometer diameter can be primary craters produced by impact of interplanetary debris or they can be secondary craters produced by fallback of high velocity ejecta blocks from much larger but infrequent primary impacts the prevalent assumption over recent decades has been that primaries are most abundant so most small craters are independent random events and can be used for dating however recent results from europa and mars support the early theory that distant secondaries globally dominate the number of small lunar craters this would invalidate part of production functions that have been widely used for age dating crater excavation results in higher mean ejection velocities for smaller fragments resulting in a steeper size frequency distribution for secondary craters than is produced by the same size frequency distribution of interplanetary debris this review also discusses how small craters can sometimes be used to derive meaningful age constraints

radar imagery of mercury s putative polar ice 1999 2005 arecibo results abstract we present an updated survey of mercury s putative polar ice deposits based on high resolution 1 5 km imaging with the upgraded arecibo s band radar during 1999 2005 the north pole has now been imaged over a full range of longitude aspects making it possible to distinguish ice free areas from radar shadowed areas and thus better map the distribution of radar bright ice the new imagery of the south pole though derived from only a single pair of dates in 2005 improves on the pre upgrade arecibo imagery and reveals many additional ice features some medium size craters located within 3 of the north pole show near complete ice coverage over their floors central peaks and southern interior rim walls and little or no ice on their northern rim walls while one large 90 km crater at 85 n shows a sharp ice cutoff line running across its central floor all of this is consistent with the estimated polar extent of permanent shading from direct sunlight some craters show ice in regions that though permanently shaded should be too warm to maintain unprotected surface ice owing to indirect heating by reflected and reradiated sunlight however the ice distribution in these craters is in good agreement with models invoking insulation by a thin dust mantle comparisons with goldstone x band radar imagery indicate a wavelength dependence that could be consistent with such a dust mantle more than a dozen small ice features have been found at latitudes between 67 and 75 all of this low latitude ice is probably sheltered in or under steep pole facing crater rim walls although since most is located in the mariner unimaged hemisphere confirmation must await imaging by the messenger orbiter these low latitude features are concentrated toward the cold longitudes possibly indicating a thermal segregation effect governed by indirect heating the radar imagery places the corrected locations of the north and south poles at 7 w 88 35 n and 90 w 88 7 s respectively on the original mariner based maps

mercury radar imaging evidence for polar ice the first unambiguous full disk radar mapping of mercury at 3 5 centimeter wavelength with the goldstone 70 meter antenna transmitting and 26 antennas of the very large array receiving has provided evidence for the presence of polar ice the radar experiments conducted on 8 and 23 august 1991 were designed to image the half of mercury not photographed by mariner 10 the orbital geometry allowed viewing beyond the north pole of mercury a highly reflective region was clearly visible on the north pole during both experiments this polar region has areas in which the circular polarization ratio pt was 1 0 to 1 4 values 0 1 are typical for terrestrial planets such high values of have hitherto been observed in radar observations only from icy regions of mars and icy outer planet satellites

strain patterns in models of spreading gliding nappes three experiments have been carried out in order to study the progressive and finite strain in nappes where gravitational spreading and gliding occur together these experimental models were made from analogue materials able to collapse under their own weight on an inclined plane throughout most of the models the stretch trajectories in a vertical plane parallel to the flow show a low amplitude sigmoidal pattern consistent with a previous theoretical model other stretch trajectories exist at the front and back ends of each model the kinematic significance of all these trajectories is discussed in detail highest strain intensities are always found next to the base of the model strain paths calculated at different stages of flow indicate clearly that nowhere in the model is the motion simple there is a complex combination in time and space of simple and pure shear except at the base of the model where the motion approximates a simple shearing near the two parallel lateral sides of the model a lateral boundary effect can be observed from the strong curvature of the transverse markers surface grid and vertical layers strain paths have also been estimated in this complex zone where wrench shearing is superimposed upon vertical shortening and thrust shearing the geological implications of these experimental models are discussed in relation to recent theoretical studies and field work

the morphology of craters on mercury results from messenger flybys topographic data measured from the mercury laser altimeter mla and the mercury dual imaging system mdis aboard the messenger spacecraft were used for investigations of the relationship between depth and diameter for impact craters on mercury results using data from the messenger flybys of the innermost planet indicate that most of the craters measured with mla are shallower than those previously measured by using mariner 10 images mdis images of these same mla measured craters show that they have been modified the use of shadow measurement techniques which were found to be accurate relative to the mla results indicate that both small bowl shaped and large complex craters that are fresh possess depth to diameter ratios that are in good agreement with those measured from mariner 10 images the preliminary data also show that the depths of modified craters are shallower relative to fresh ones and might provide quantitative estimates of crater in filling by subsequent volcanic or impact processes the diameter that defines the transition from simple to complex craters on mercury based on messenger data is consistent with that reported from mariner 10 data

water exsolution from quartz implications for the generation of retrograde metamorphic fluids available data from the literature suggest that the solubility of h2o in nominally anhydrous minerals such as quartz should increase with increasing metamorphic grade if this is the case water should precipitate from the host mineral during cooling and uplift of regionally metamorphosed terranes calculations indicate that this mechanism could result in the release of significant quantities of h2o which would then be available to participate in retrograde hydration reactions and the formation of fluid inclusions

mercury full disk radar images and the detection and stability of ice at the north pole the first full disk radar images of mercury were obtained on august 8 and 23 1991 these images were constructed using the very large array vla in socorro new mexico to receive and map radar flux at 3 5 cm x band which was continuously transmitted from the 70 m jet propulsion laboratory deep space network antenna at goldstone california approximately 77 of the surface was imaged at resolutions as good as 150 km about half of the hemisphere photographed by mariner 10 was imaged as well as most of the hemisphere which has not previously been photographed at the time of the observations the north pole was visible and the feature with the highest same sense ss circular reflectivity in the images is near the nominal polar position the peak ss reflectivity of this feature was 7 9 and the circular polarization ratio throughout much of the feature is 1 our best estimate of the size of the feature is that its diameter is 350 km we interpret the feature to be indicative of the presence of ices because of the signal strength and polarization characteristics the ices must be very clean and were thus probably deposited in a relatively short time period the most likely place to find these ices is in any permanently shadowed areas in the polar regions which would be very cold in comparison to absolute reflectivities of other icy bodies and regions in the solar system the reflectivity of the north polar feature is slightly depressed this is probably because the ices do not fill an observational resolution cell i e there is incomplete areal coverage or are covered by a layer of dust or soil which absorbs some of the incoming and outgoing radar energy or both a covering layer would also protect any ices from erosion by energetic sources other prominent features on the unphotographed hemisphere correspond to positions where atmospheric sodium enhancements have been measured from earth this may indicate that these locations are large basins similar to the caloris basin where an atmospheric potassium enhancement has been measured which may be the result of increased degassing in the disrupted surface and near surface direct comparison of the radar echoes from our features with those from the caloris basin are hard to make as caloris was not imaged by us in a favorable geometry features on the photographed hemisphere are smaller and mostly associated with craters and crater complexes most notably the kuiper crater and its environs

staurolite growth and metamorphic zones in the kalak nappe complex of northeastern porsangerhalvoya north norway abstract four main thrust units have been differentiated in the kalak nappe complex of northeastern porsangerhalvoya all exhibit barrovian type prograde metamorphism although the metamorphic grade increases up the nappe pile the development of planar and non kinematic fabrics is the same in relation to the deformation sequence in each of the thrust sheets early porphyroblast growth preserves a planar fabric developed during burial as a result of overthrusting this was superseded by a randomly oriented and equidimensional non kinematic fabric which developed by recrystallization associated with increased fluid concentrations due to prograde metamorphic dehydration reactions these fabric developments have been preserved in textures associated with the prograde alteration of staurolite and muscovite porphyroblasts with increased fluid concentrations the shear strength of the rock declined and thrust propagation occurred the associated shear stresses caused rotational fabrics in some garnets and aligned mineral growth

analyzing the ages of south polar craters on the moon implications for the sources and evolution of surface water ice abstract surface water ice in the permanently shadowed polar regions of the moon has a patchy surficial distribution and is not found within all available cold trapping areas to date it is not well understood when the ice was delivered which has important implications for the surficial characteristics of the ice as well as for possible delivery mechanisms here we present absolute model ages for 20 south polar craters that host surface water ice providing maximum estimates of the ages of surface ice contained within these craters we quantify the amount of available cold trapping surface area that is occupied by water ice in order to examine the relationship between the patchiness of ice within each crater and the age of each host crater the majority of surface ice is contained in old craters 3 1 gyr where the majority of cold trapping area on the pole exists the ice is these ancient craters is very patchy in surficial distribution occupying siegler et al 2016 if true polar wander did occur on the moon then the ages of ice bearing craters presented here set an upper limit for the age of post true polar wander hydrogen emplacement of 4 1 0 1 gyr

new evidence for surface water ice in small scale cold traps and in three large craters at the north polar region of mercury from the mercury laser altimeter the mercury laser altimeter mla measured surface reflectance rs at 1064 nm on mercury most water ice deposits have anomalously low rs values indicative of an insulating layer beneath which ice is buried previous detections of surface water ice without an insulating layer were limited to seven possible craters here we map rs in three additional permanently shadowed craters that host radar bright deposits each crater has a mean rs value 0 3 suggesting that water ice is exposed at the surface without an overlying insulating layer we also identify small scale cold traps 0 3 and permanent shadows have biannual maximum surface temperatures 100 k we suggest that a substantial amount of mercury s water ice is not confined to large craters but exists within micro cold traps within rough patches and inter crater terrain

cretaceous ongonites topaz bearing albite rich microleucogranites from ongon khairkhan central mongolia products of extreme magmatic fractionation and pervasive metasomatic fluid rock interaction abstract ongonites were defined at their type locality at ongon khairkhan central mongolia as pristine magmatic topaz bearing albite quartz keratophyres with up to 4 wt f and containing phenocrysts of albite k feldspar quartz and rare mica and topaz hosted in a groundmass composed of the same minerals however detailed petrographic and sem eds studies indicate that these rocks underwent considerable subsolidus exchange with deuteric fluids as evidenced by the presence of albitic plagioclase ab 100 and end member orthoclase or 100 secondary li fe rich mica zinnwaldite enriched in rare metals sn w ta nb pitted feldspars containing fluid inclusions and disseminated fluorite the 120 ma old dyke rocks emplaced at a high structural level in the crust are strongly peraluminous leucogranites characterized by high al and alkalis that are also enriched in rb cs ga and ta depleted in mg ca zr ba sr and eu and have anomalous k rb rb sr zr hf and nb ta ratios compared to the average continental crust however the suite has nd isotopic ratios nd 120 1 similar to those of contemporaneous a type granites of the mongolian transbaikalian igneous province of the central asian orogenic belt the inferred primary 18 o 6 to 7 and pb isotopic values are consistent with a granitic parent magma and interaction with orthomagmatic fluids the ongonites and constituent minerals record 1 an extensive and protracted crystal fractionation history in part due to the presence of volatiles particularly f which depressed the solidus temperature of the felsic rocks and extended its duration of crystallization and 2 subsolidus exchange with fluids which includes late flux of heated meteoric water as indicated by modified whole rock 18 o values 0 5 to 2 7 the interaction of the ongonites with internally derived orthomagmatic fluids is considered to result in enrichment and or redistribution of several incompatible elements but not to have greatly modified their original major element chemistry which indicates that this suite represents the last stages of fractionation of a highly differentiated f rich granitic magma the escape of these evolved melts from the apical part of the underlying pluton is now represented by the ongonite dykes late stage magmatic water rich fluids enriched in incompatible elements including nb ta sn and w were responsible for the late to post magmatic alteration and associated w mineralization

late to post collisional brittle ductile deformation in the himalayan orogen evidences from structural studies in the lesser himalayan crystallines kumaon himalaya india abstract the chiplakot crystalline belt ccb is a broad shear zone located within the meta sedimentary zone of the lesser himalaya evidence from mesostructures and microstructures reveals development of this shear zone in a compressional ductile tectonic regime during continental collision in the himalaya by top to sw shearing along ne dipping shear surfaces the shear surfaces are parallel to both the upper and basal thrust planes of the ccb a hitherto unrecognized late to post collisional brittle ductile tectonic regime comprised of two discrete phases is established from dynamic analysis of the kink bands extensional shear fractures foliation boudinage competent layer boudinage quartz veins and associated flanking folds and brittle normal and reverse faults in the ccb kink bands foliation parallel slip and general shear were major consequences of this late to post collisional brittle ductile tectonic episode and were associated with the intrusion of a swarm of quartz veins quartz veins associated with flanking folds are developed at high angles with respect to the mylonitic foliation and parallel to the axial plane of the kink bands throughout the ccb the geometry of the flanking folds along the quartz veins is a consequence of the fracturing and not vice versa the precise shape of the folds depends on the nature of general shearing along the wall of quartz veins and various orientations to the shear direction

orbital eccentricity driven temperature variation at mercury s poles 1 due to mercury s extremely small obliquity variations in orbital eccentricity are likely to have been the dominant cause of changes in polar temperatures on mercury throughout the bulk of its history in the last 10 myr the eccentricity has varied between 0 1 and 0 3 over the past 100 myr eccentricity extremes of nearly zero to greater than 0 4 have likely been reached we calculate the temperature in the near polar region within 10 of the north pole as it varies in response to past eccentricities these calculations show periods with dramatically different temperatures diurnal amplitudes and long period thermal cycles all of which can have a dramatic effect on longitudinal variations in long term subsurface ice stability given forthcoming topographic data thermal models including long term eccentricity can help constrain when past ice might have been delivered to mercury

metamorphic and structural diachroneity in the finnmarkian nappes of north norway two periods of garnet growth gt1 and gt2 have been found in the finnmarkian nappes of north norway in the kolvik nappe the lowest nappe gt1 has preserved an s2 syntectonic spiral inclusion fabric in the olderfjord nappe an earlier s1 fabric and an interkinematic inter d1 d2 fabric have been preserved in gt1 whilst only the s1 fabric has been found in gt1 in the brennsvik nappe the highest nappe in each nappe gt2 overgrew a penetrative fabric s2 wrapped around gt1 in the kolvik nappe inclusion fabrics may be continuous from gt1 into gt2 but in the higher nappes there is a distinct break gt2 may have been partially syntectonic with d3 in the brennsvik nappe chemically gt1 in the kolvik nappe and in parts of the olderfjord and brennsvik nappes has antithetic fe mn zoning in all nappes xca and xmg are weakly zoned in gt1 xmg increases outwards and is greater in the higher nappes in gt1 suggesting higher nucleation temperatures in the olderfjord and brennsvik nappes gt2 is marked by increasing xca probably due to changing garnet plagioclase equilibria although the fe mg ratio remains constant xmg is higher in gt2 than gt1 basement rocks within the nappe pile have an early pre finnmarkian growth gt1 and a later finnmarkian growth gth correlated with gt2 on the basis of chemical zoning patterns the diachroneity of gt1 is ascribed to progressively earlier compared to the structural development cessation of overstepping of garnet forming reactions before peak metamorphism in the higher nappes resulting in earlier structural events being preserved

strike slip restoration of the barents sea caledonides terrane finnmark north norway the trollfjord komagelv t k fault is a major wnw ese trending dextral strike slip fault which cuts the caledonides of northern norway with a palaeomagnetically determined displacement of 500 1000 km although earlier interpretations suggested a pre caledonian age for the strike slip displacement our analysis shows that the fault movement was intimately associated with late caledonian thrusting the epizone high intermediate baric type metamorphism of the barents sea caledonides has been used to constrain their restored position relative to other units in the caledonides in northern scandinavia this indicates a postmetamorphic dextral displacement along the t k fault of 415 km the kalak nappe complex straddles the line of the t k fault but has not been cut by it despite this relationship it is argued that the kalak nappe complex was emplaced prior to strike slip fault movement north of the fault structures reflect blind thrusting and passive roof duplex development refolded by en echelon strike slip structures indicating that t k fault movement started after thrusting south of the fault structures in the gaissa thrust belt have been affected by strike slip movement although final thrust movements deformed the t k fault the kalak nappe complex north of the t k fault was carried passively by the strike slip block while to the south major thrusting occurred

extensional deformation along the late precambrian cambrian baltoscandian passive margin the sarektjakka nappe swedish caledonides the structural evolution of a part of the late precambrian baltoscandian passive margin just before the inception of seafloor spreading is described recording the change from deformation by faulting to dominantly magmatic extension of the crust the allochthon of the scandinavian caledonides contains the imbricated passive margin of continental baltica overlain by various exotic terranes the sarektjakka nappe in the seve nappe complex which contains the outer parts of baltica s passive margin consists of sedimentary rocks occurring as screens between vendian 573 74 ma diabase dykes these dykes constitute 70 80 of the nappe and locally form sheeted dyke complexes the sarektjakka nappe largely escaped penetrative caledonian deformation and preserves igneous metamorphic and structural elements that are linked to the evolution of a pre caledonian rift to a passive continental margin extensional deformation before dyke emplacement is recorded by normal faults pull apart structures and folds unconformities dykes affected by brittle deformation and fluidization of sediments during dyke emplacement indicate close relations between the deposition of sediments extensional deformation and dyke emplacement the sarektjakka nappe is compared with other parts of the baltica s passive margin and its tectonic evolution is discussed

craters hosting radar bright deposits in mercury s north polar region areas of persistent shadow determined from messenger images 1 radar bright features near mercury s poles were discovered in earth based radar images and proposed to be water ice present in permanently shadowed areas images from messenger s one year primary orbital mission provide the first nearly complete view of mercury s north polar region as well as multiple images of the surface under a range of illumination conditions we find that radar bright features near mercury s north pole are associated with locations persistently shadowed in messenger images within 10 of the pole almost all craters larger than 10 km in diameter host radar bright deposits there are several craters located near mercury s north pole with sufficiently large diameters to enable long lived water ice to be thermally stable at the surface within regions of permanent shadow craters located farther south also host radar bright deposits and show a preference for cold pole longitudes thermal models suggest that a thin insulating layer is required to cover these deposits if the radar bright material consists predominantly of long lived water ice many small 10 km diameter and low latitude extending southward to 66 n craters host radar bright material and water ice may not be thermally stable in these craters for 1 gy even beneath an insulating layer the correlation of radar bright features with persistently shadowed areas is consistent with the deposits being composed of water ice and future thermal modeling of small and low latitude craters has the potential to further constrain the nature source and timing of emplacement of the radar bright material

generation and modification of finite strain patterns by progressive thrust faulting in the laksefjord nappe finnmark abstract using deformed conglomerates as finite strain markers a strain map of the base of the laksefjord nappe has been produced and is presented finite strains are predominantly of flattening type and are explained in terms of a variable simple shear superimposed on a constant pure shear strain patterns are examined in terms of their three dimensional geometry and are shown to be clearly related to the presence of the basal laksefjord thrust late brittle thrust faulting has modified early finite strain patterns and a progressive thrusting model is presented to explain present day strain patterns finite strains developed in a mid crustal sub horizontal shear zone at lower greenschist facies account for only a small proportion of total nappe translation

thermal stability of volatiles in the north polar region of mercury thermal models for the north polar region of mercury calculated from topographic measurements made by the mercury surface space environment geochemistry and ranging messenger spacecraft show that the spatial distribution of regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice messenger measurements of near infrared surface reflectance indicate bright surfaces in the coldest areas where water ice is predicted to be stable at the surface and dark surfaces within and surrounding warmer areas where water ice is predicted to be stable only in the near subsurface we propose that the dark surface layer is a sublimation lag deposit that may be rich in impact derived organic material

age constraints of mercury s polar deposits suggest recent delivery of ice abstract surface ice at the poles of mercury appears as several m thick deposits that are composed of nearly pure water we provide new age estimates of the surfaces of mercury s polar deposits from combined analyses of poisson statistics and direct observations of crater densities within permanently shadowed radar bright regions imaged by the messenger spacecraft these age estimates conservatively suggest that ice was delivered to mercury within the last 330 myr the geologically young ages suggest that the surfaces have been recently refreshed and this may be accomplished by the delivery of ice in a young impactor or impactors a single recent impactor is more consistent with the relative purity of the ice as suggested by the earth based radar observations in contrast to ice on mercury observations of the lunar poles are suggestive of a highly patchy distribution of surface frost the patchiness of lunar polar deposits is consistent with long exposure times to the space weathering environment given enough time the polar deposits on mercury may age into a more heterogeneous spatial distribution similar to that on the moon

how thick are mercury s polar water ice deposits an estimate is made of the thickness of the radar bright deposits in craters near to mercury s north pole to construct an objective set of craters for this measurement an automated crater finding algorithm is developed and applied to a digital elevation model based on data from the mercury laser altimeter onboard the messenger spacecraft this produces a catalogue of 663 craters with diameters exceeding 4 km northwards of latitude 55 a subset of 12 larger well sampled and fresh polar craters are selected to search for correlations between topography and radar same sense backscatter cross section it is found that the typical excess height associated with the radar bright regions within these fresh polar craters is 50 35 m this puts an approximate upper limit on the total polar water ice deposits on mercury of 3 1015 kg

the distribution and origin of smooth plains on mercury 1 orbital images from the messenger spacecraft show that 27 of mercury s surface is covered by smooth plains the majority 65 of which are interpreted to be volcanic in origin most smooth plains share the spectral characteristics of mercury s northern smooth plains suggesting they also share their magnesian alkali basalt like composition a smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope suggesting more ultramafic compositions an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation the knobby and hummocky plains surrounding the caloris basin known as odin type plains occupy an additional 2 of mercury s surface the morphology of these plains and their color and stratigraphic relationships suggest that they formed as caloris ejecta although such an origin is in conflict with a straightforward interpretation of crater size frequency distributions if some fraction is volcanic this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions smooth plains are widespread on mercury but they are more heavily concentrated in the north and in the hemisphere surrounding caloris no simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed a likely volcanic origin for some older terrain on mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large scale volcanic deposits rather than differences in crustal formational process

areas of permanent shadow in mercury s south polar region ascertained by messenger orbital imaging 1 radar bright features near mercury s poles have been postulated to be deposits of water ice trapped in cold permanently shadowed interiors of impact craters from its orbit about mercury messenger repeatedly imaged the planet s south polar region over one mercury solar day providing a complete view of the terrain near the south pole and enabling the identification of areas of permanent shadow larger in horizontal extent than approximately 4 km in mercury s south polar region all radar bright features correspond to areas of permanent shadow application of previous thermal models suggests that the radar bright deposits in mercury s south polar cold traps are in locations consistent with a composition dominated by water ice provided that some manner of insulation such as a thin layer of regolith covers many of the deposits

the thermal stability of water ice at the poles of mercury recent radar observations of mercury have revealed the presence of anomalous radar reflectivity and polarization features near its north and south poles thermal model calculations show that despite mercury s proximity to the sun the temperatures of flat low reflectivity surfaces at mercury s poles are not expected to exceed 167 kelvin the locations of the anomalous polar radar features appear to be correlated with the locations of large high latitude impact craters maximum surface temperatures in the permanently shadowed regions of these craters are expected to be significantly colder as low as 60 kelvin in the largest craters these results are consistent with the presence of water ice because at temperatures lower than 112 kelvin water ice should be stable to evaporation over time scales of billions of years

did the dalradian slides originate as extensional faults rocks of the dalradian supergroup1 2 of scotland and ireland were deposited in late proterozoic early cambrian times on what became the southern margin of the laurentian plate and underwent metamorphism and deformation when this margin was affected by the grampian orogeny in the late cambrian3 4 the resulting complex structure is still not fully understood and a continuing problem is presented by the origin and significance of the slides a series of ductile faults which lie sub parallel to bedding5 although numerous slides have been recognized within the dalradian terrain6 the term has seldom been used outside the british caledonides it is generally assumed that these slides are synmetamorphic and were formed during grampian deformation however a consideration of their geometry stratigraphic relationships and context now suggests that they may have been initiated as syndepositional extensional faults which were only modified and not formed during the grampian orogeny

glaciation on mercury accumulation and flow of ice in permanently shadowed circum polar crater interiors abstract radar bright deposits that coincide with regions in permanent shadow typically found within impact craters at and near the poles of mercury have been interpreted as being composed of water ice we investigate the dynamic properties of these solid water ice deposits glaciers with the goal of constraining their movement flow rates possible deformation and related structures and deposits as an end member we treat the extreme case of maximum ice accumulation where these deposits fill the crater to the shadow line an event we would only expect following maximum accumulation conditions such as a large cometary impact or a comet shower we find that given the extremely cold conditions and the limited thickness of glaciers even under the most favorable accumulation conditions glaciation is cold based ice flow velocities are very low 4 10 8 m yr and with the exception of a sublimation lag deposit glaciation is unlikely to leave any significant impact on the terrain an important accelerator of ice flow is found to be the contribution of lateral heat conduction from the surrounding extremely hot surface terrain 225 k average with up to 450 k maximums which for 55 km craters yields velocities of 10 3 m yr while for smaller 10 km craters velocities may exceed 1 m yr enough to leave a detectable ice deformation imprint such as drop moraines however given that the observed deposits are substantially below their maximum fill capacities these estimates are unlikely to be attained

the new nb p aplite at reinhardsrieth a keystone in the lateral and depth zonations of the hagendorf pleystein pegmatite field se germany abstract the hagendorf pleystein pegmatite field hppf is the largest pegmatite concentration in central europe which has been intensively studied for its mineralogy mainly at hagendorf south pegmatite stock it was not until recently that the lateral and depth zonations of the hppf could be clarified when the newly discovered tabular aplite at reinhardsrieth aplite has been found at transition into the so called barren zone devoid of rare metals thereby the theory of the derivation of the hppf felsic mobilizates as a result of a successive differentiation of a pegmatitic melt from the flossenburg granite has to be discarded from the structural and lithological point of view stock like well zoned pegmatites e g hagendorf south developed in the hinge zone whereas tabular aplites such as reinhardsrieth evolved along the limb zone of strongly folded basement rocks in the latter case the accommodation space was smaller and the cooling of the felsic mobilizates much faster giving rise to fine grained aplitic rather than coarse grained felsic mobilizates chilled margin effect during the incipient stages of the evolution in the hppf at reinhardsrieth a stronger impact of granitization can be determined than at the neighboring aplites and pegmatites based upon the ree fractionation a tripartite subdivision may be established for the various pegmatites and aplites in the hppf 1 barren zone minor amounts of lree in a system otherwise dominated by feldspar and quartz 2 marginal zone mainly fe al phosphates plus columbite fe 3 central zone fe zn li phosphates plus columbite fe the afore mentioned zonation shows an e w trend and a bilateral symmetric arrangement of the various areas there are significant indications of an oxidizing facies to substitute for a more reducing facies at the transition from the central into the marginal zone of the hppf the key mineral for the primary pegmatite mineralization in the marginal and central zones is columbite fe its mn fe ratios plotted vs the ta nb ratios allow for an assessment to what extent metamorphic and magmatic processes have contributed to the built up of the pegmatites and aplites in the hppf even the most primitive nigrine hosted columbites fit well into this pattern forming a lithochemical halo in the roof rocks of the pegmatites the heavy mineral aggregated of rutile ilmenite nigrine giving host to the columbite as armored relicts got released from the basement rock on top of the pegmatites into the stream sediments where they are operative as an ore guide the morphology of zircon in the stream sediments has been utilized as an economic ore indicator the zr silicate is also indicative of the temperature of formation and closely supports the tripartite zonation established above manganiferous apatite is a good depth of emplacement indicator for each aplite and pegmatite respectively the higher the feo mno content of apatite the shallower the depth of emplacement of its host pegmatite or aplite vivianite in turn marks the onset of the hydrothermal alteration of the pegmatites and aplites and its mn contents relate to the depth where alteration has taken place or emplacement of the secondary phosphates the higher the mn content the shallower the depth of alteration peculiar element ratios were selected to design some key diagrams so as to corroborate the field based mineralogical and geological results in terms of zonation of genetic economic geology as well as to cater for applied economic geology as proximity indicators and ore guides when exploring for rare element pegmatite

anatomy of a continental subduction zone the main mantle thrust in northern pakistan the himalayas form an ideal natural laboratory to study the deformation processes of continental crust during collision orogeny new information is presented concerning the structural evolution of the main mantle thrust zone in the himalayas of n pakistan in the region around nanga parbat the hanging wall lies at relatively high levels within the kohistan arc terrane which has been emplaced onto indian continental rocks this thrust probably originated as a breakback structure in the hanging wall to the pre collisional oceanic subduction zone the present hanging wall contains a shear zone of about 1 km width developed under amphibolite facies conditions simple shear dominant strains have developed new fabrics parallel to the main thrust zone however these structures are redeformed by discrete extensional and compressional shears within about 100 m of the thrust contact again developed under amphibolite facies conditions the footwall consists of an old basement complex the nanga parbat gneisses overlain by a distinct suite of metasediments now at amphibolite facies this cover assemblage of psammites pelites and marbles with local metabasites consistently lies directly against rocks derived from the kohistan arc along the mmt the absence of structures suggestive of consistent rheological contrasts within the cover assemblages suggests that the vast majority of the deformation features they contain were developed only once they experienced substantial tectonic overburdens prior to this the indian cover rocks will have been passively subducted beneath the kohistan arc until into amphibolite facies we discuss these inferences in terms of evolving shear zone width with time and comment on the implications for predicting the character of mid deep crustal shear zones particularly from seismic reflection profiles

imaging mercury s polar deposits during messenger s low altitude campaign images obtained during messenger s low altitude campaign in the final year of the mission provide the highest spatial resolution views of mercury s polar deposits images for distinct areas of permanent shadow within 35 north polar craters were successfully captured during the campaign all of these regions of permanent shadow were found to have low reflectance surfaces with well defined boundaries additionally brightness variations across the deposits correlate with variations in the biannual maximum surface temperature across the permanently shadowed regions supporting the conclusion that multiple volatile organic compounds are contained in mercury s polar deposits in addition to water ice a recent large impact event or ongoing bombardment by micrometeoroids could deliver water as well as many volatile organic compounds to mercury either scenario is consistent with the distinctive reflectance properties and well defined boundaries of mercury s polar deposits and the presence of volatiles in all available cold traps

structural and strain analyses of the middle part of the talassian alatau ridge middle asia kirgizstan abstract the middle part of the talassian alatau ridge is composed of upper precambrian lower paleozoic greenschists the usunakhmatian sheet which were overthrust on unmetamorphosed rocks of nearly the same age the talassian and kumishtagian sheets three phases of deformation can be recognized in the usunakhmatian sheet the most intensive of which was the late silurian early devonian d 2 deformation in the usunakhmatian sheet the axis of maximum extension is subhorizontal and parallel to f 2 fold axes r xz varies from 2 4 to 7 5 and k from 0 03 to 1 75 flattening is widespread but the strain of many samples is nearly plane strain in the talassian sheet r xz varies from less than 1 8 to 4 4 and k from 0 to 0 35 r xz and k values increase near large faults r xz values determined by quartz and chert grains were found to be linearly related a three stage strain history is related to d 2 deformation the earliest was simple shear parallel to the axial surfaces of folds following this the fold and thrust system was transformed by pure shear with an orogen parallel axis of extension some minor increments of strain are locally represented by pressure shadows around pyrite crystals

high resolution radar imaging of mercury s north pole abstract the recently upgraded arecibo s band 12 6 cm radar was used to make delay doppler images of mercury s north polar region where earlier observations had shown strong echoes from putative ice deposits in craters the image resolution of 1 5 3 km is a substantial improvement over the 15 km resolution of the older arecibo images j k harmon et al 1994 nature 369 213 215 the new observations confirm all the original polar features and reveal many additional features including several at latitudes as low as 72 75 n and several from craters less than 10 km in diameter all of the new features located on the mariner imaged side of the planet can be matched with known craters or other shaded areas we find the north pole to be located 65 km from the original mariner based pole and 15 km from the new mariner based pole of m s robinson et al 1999 j geophys res 104 30 847 30 852 the improved resolution reveals fine structure in the radar features and their respective host craters including radar shadowing highlighting by central peaks and rim walls rim terracing and preferential concentration of radar bright deposits in shaded southern floor areas the radar features high brightness circular polarization inversion c 1 25 and confinement to regions permanently shaded from direct sunlight are all consistent with volume scattering from a cold trapped volatile such as clean water ice the sizes and locations of most of the features show good agreement with the thermal model of a r vasavada d a paige and s e wood 1999 icarus 141 179 193 for insulated buried water ice although the problems of explaining radar features in small craters and the rapid burial required at lower latitudes suggest that other factors may be suppressing ice loss after emplacement

obliquely foliated dikes as deformed incompetent single layers the structures that develop in deformed competent single layers are well known in comparison to the results of the deformation of incompetent single layers it is argued that the oblique foliations within some sets of deformed igneous sheet intrusions are the most common indication that such sheets deformed as single layers with less competence than their country rocks previous workers have implicitly assumed this situation when interpreting examples of such foliated sheets as having sheared between rigid country rocks while still hot this is taken here as a special case of a much more general situation in which strains in the sheets are induced by strains of the more competent country rocks as in previous models the foliation within sheets is interpreted as being oblique to their contacts because of layer parallel shears along the sheets in a departure from earlier approaches such shears are assumed here to arise from the rotation of the sheets due to the bulk strains of their country rocks this idea is explored using the patterns of the structural movement paths traced by their poles during strain each type of bulk strain leads to a different pattern of structural movement paths as the rotation of the sheet contacts drives the rotation of the foliation between them their poles follow colinear but opposed movement paths and are taken to lie at different positions along what will be called apparent structural movement paths this working hypothesis is empirically tested using data from two field areas sets of apparent structural movement paths for foliated intrusive sheets in two areas are found to compare favorably with theoretical structural movement paths expected if the sheets deformed as incompetent single layers in homogeneously deforming country rocks the approach developed here can be used to determine empirically the homogeneity of strain of the rocks from which the data have been derived the orientations and relative values of the principal strains and an approximation of the shape of the strain ellipsoid however no attempt is made to quantify the amount of strain recorded in the field examples quoted here

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

radar mapping of mercury full disk images and polar anomalies a random code technique has been used at arecibo to obtain delay doppler radar images of the full disk of mercury anomalously bright features were found at the north and south poles the north polar feature is oblong 4 by 8 and offset from the pole the smaller south polar feature is mostly confined to the floor of the crater chao meng fu the polar locations and radar properties of these features indicate that they may be produced by volume scattering in ice the images also reveal a variety of more subdued reflectivity features ranging in size from hundreds to thousands of kilometers some of these appear to have an impact origin

kilometer scale topographic roughness of mercury correlation with geologic features and units we present maps of the topographic roughness of the northern circumpolar area of 30 mercury at kilometer scales the maps are derived from range profiles obtained by the 31 mercury laser altimeter mla instrument onboard the mercury surface space 32 environment geochemistry and ranging messenger mission as measures of 33 roughness we used the interquartile range of profile curvature at three baselines 0 7 km 34 2 8 km and 11 km the maps provide a synoptic overview of variations of typical 35 topographic textures they show a dichotomy between the smooth northern plains and 36 rougher more heavily cratered terrains analysis of the scale dependence of roughness 37 indicates that the regolith on mercury is thicker than on the moon by approximately a 38 factor of three roughness contrasts within northern volcanic plains of mercury indicate a 39 younger unit inside goethe basin and inside another unnamed stealth basin these new 40 data permit interplanetary comparisons of topographic roughness

patterns of stretch trajectories and strain rates within spreading gliding nappes abstract stretch trajectories in vertical flow planes through superficial nappes are thought to be specific for some classical mechanical models so that their recognition in the field is of particular interest in order to find a pattern of stretch trajectories related to a gravitational process a strain factorization is attempted from experimental scale models of nappes where spreading and gliding are combined the strain within this type of nappes is schematically considered as the simultaneous combination of two components a simple shearing component and a pure shearing component spatial and time distribution of these two components is computed from both time and spatial evolution of the strain pattern within the scale models a better understanding of stretch trajectories in spreading gliding nappes is consequently provided strain rates are also computed from scale models and the sudden increase of strain rate from top to bottom is explained finally a mechanical model of spreading gliding nappe is briefly discussed

inverted isograds at sulitjelma norway the result of shear zone deformation at sulitjelma norway there is a major inversion of metamorphic isograds beneath an inverted but undisrupted ophiolite the flysch like furulund schist in which the inverted isograds occur is also inverted and the early folds in it are downward facing the isograds cut across the axial surfaces of early folds and across the schistosity these relationships are explained as the consequence of metamorphism during the progressive development of a large overfold the inverted limb of the overfold is regarded as a major thick gently dipping shear zone separating the lower grade lower part of the caledonian allochthon below from the higher grade upper part of the allochthon above the association between stratigraphical inversion downward facing of syn schistosity folds and metamorphic inversion is explained by the progressive development of the shear zone it is suggested that the presence of such shear zones is a common feature of orogenic belts formed by continental collision

tin tantalum niobium mineralization in the penouta deposit nw spain textural features and mineral chemistry to unravel the genesis and evolution of cassiterite and columbite group minerals in a peraluminous system abstract the sn ta nb penouta granite is a highly evolved rare metal post kinematic leucogranite located in north western spain which was intermittently mined from roman times until 1985 when falling metal prices led to the closure of most mines in the country mineralization consists mainly of cassiterite cassiterite i and columbite group minerals cgm disseminated in the leucogranite that increase in abundance and crystal size towards the upper part of the granitic cupola especially in banded pegmatite like dikes upwards quartz veins and greisen zones associated with the wall rock mainly augen gneisses were developed containing coarse grained cassiterite cassiterite ii in contrast to nearly homogenous cassiterite chemistry cgm may show complex compositional zonation patterns in relation with compositional variations of the leucogranite taking into account the mineral chemistry and textural features of cgm two main groups were distinguished i the first one group i follows a general main trend from columbite fe to columbite mn and finally to tantalite mn which is in accordance with the evolution of the magma observed from the less to the more evolved zones of the leucogranite and from bottom to top of the body and responds to common fractionation patterns of decreasing nb ta ratios in the melt and ii a second group group ii with intermediate values of mn mn fe and ta ta nb due to local supersaturation effects of the melt during crystallization resulting in complex oscillatory zoned cgm crystals textural and chemical features of cassiterite and cgm support the magmatic origin for both minerals although cassiterite crystallized later in the sequence in both generations mineral chemistry evolution of cgm is in good agreement with a greater solubility of tantalite in the melt at the upper levels where besides the fluorine contents of apatite and muscovite were increased and the onset of fluorite crystallization occurred so the fluorine enrichment in the upper levels seems likely to be an important component to explain the mineral chemistry of the penouta granite although the same mineral chemistry scheme could also be explained in terms of white mica fractionation upwards furthermore the absence of typical textures originated by fluids in the cgm as well as the lack of these minerals in the quartz veins point to a low influence of the hydrothermal fluids in dissolution and recrystallization processes of the primary oxides

thrust tectonics a mechanistic view from the west and central alps abstract two modern models of thin skinned thrust tectonics by elliott 1976 and chapple 1978 are reviewed and found to be of restricted application following possible variations of surface and basal slope and based on published alpine cross sections eight elementary two dimensional mechanical models are set up these include the possibility of a weak basal layer a common feature of alpine thrust sheets and an externally applied subhorizontal compression six of these have two variants depending on if the maximum shear stress in the sheet attains the yield stress in which case there is permanent deformation and the sheet changes shape perhaps to that of a different elementary model the long ignored analysis by goguel 1948 of stresses in a relaxed plastic material at depth below an inclined surface is outlined stress components resulting from this analysis are used together with supplementary stresses derived by hafner 1951 to represent an externally applied subhorizontal compression to suggest ways in which decollement possibilities of the models can be investigated some specific examples are considered

bright and dark polar deposits on mercury evidence for surface volatiles measurements of surface reflectance of permanently shadowed areas near mercury s north pole reveal regions of anomalously dark and bright deposits at 1064 nanometer wavelength these reflectance anomalies are concentrated on poleward facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near surface water ice correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation impacts of comets or volatile rich asteroids could have provided both dark and bright deposits

investigating mercury s south polar deposits arecibo radar observations and high resolution determination of illumination conditions there is strong evidence that mercury s polar deposits are water ice hosted in permanently shadowed regions in this study we present new arecibo radar observations of mercury s south pole which reveal numerous radar bright deposits and substantially increase the radar imaging coverage we also use images from messenger s full mission to determine the illumination conditions of mercury s south polar region at the same spatial resolution as the north polar region enabling comparisons between the two poles the area of radar bright deposits in mercury s south is roughly double that found in the north consistent with the larger permanently shadowed area in the older cratered terrain at the south relative to the younger smooth plains at the north radar bright features are strongly associated with regions of permanent shadow at both poles consistent with water ice being the dominant component of the deposits however both of mercury s polar regions show that roughly 50 of permanently shadowed regions lack radar bright deposits despite some of these locations having thermal environments that are conducive to the presence of water ice the observed uneven distribution of water ice among mercury s polar cold traps may suggest that the source of mercury s water ice was not a steady regular process but rather that the source was an episodic event such as a recent large impact on the innermost planet

quantitative kinematic flow analysis from the main central thrust zone nw himalaya india implications for a decelerating strain path and the extrusion of orogenic wedges quantitative kinematic indicators from the main central thrust zone mctz in the nw himalaya have been used to characterize the type of flow during deformation different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation forming associated fringe folds these record the late stage brittle ductile flow and reveal that a strong pure shear component of deformation occurred throughout the mctz to characterize earlier deformation increments fabrics from highly deformed quartz ribbons were analyzed well developed shape and lattice preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the mctz investigations of the c axes patterns reveal a strong asymmetry at the top of the mctz whereas the samples from the base of the mctz show almost perfectly symmetrical type i crossed girdles deformation within the mctz probably started close to simple shear flow at higher temperatures which progressively became a more general shear during cooling and ended in a pure shear dominated flow during the final stages of brittle ductile deformation i e a decelerating strain path using the higher himalaya crystalline as an example a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested

near surface temperatures on mercury and the moon and the stability of polar ice deposits abstract in order to assess the thermal stability of polar ice deposits we present model calculated temperatures of flat surfaces and surfaces within bowl shaped and flat floored polar impact craters on mercury and the moon our model includes appropriate insolation cycles realistic crater shapes multiple scattering of sunlight and infrared radiation and depth and temperature dependent regolith thermophysical properties unshaded water ice deposits on the surface of either body are rapidly lost to thermal sublimation a subsurface water ice deposit is stable within 2 latitude of the moon s poles meter thick water ice deposits require billions of years to sublime if located in the permanently shaded portions of flat floored craters within 10 latitude of the poles of mercury and 13 latitude of the poles of the moon results for craters associated with radar features on mercury are consistent with the presence of stable water ice deposits if a thin regolith layer thermally insulates deposits at lower latitudes and within smaller craters a regolith cover would also reduce losses from diffusion ion sputtering impact vaporization and h ly and is implied independently by the radar observations permanently shaded areas near the moon s poles are generally colder than those near mercury s poles but the moon s obliquity history its orbit through earth s magnetospheric tail and its radar opaque regolith may limit the volume and radar detectability of ice deposits there

thrust related strain gradients and thrusting mechanisms in a chevron folded sequence southeastern australia an e directed imbricate fan thrust system within a chevron folded monotonous quartz rich turbidite succession shows marked gradients in strain from the upper to lower parts of individual thrust sheets upper parts of thrust sheets show low to moderate strains xz strains 30 1 tight inclined chevron folds ila 30 with predominantly w dipping axial surfaces intense phyllitic cleavage with a strong down dip mineral stretching lineation and long curved tapering quartz pressure shadows on pyrite such differences in fold geometry the intensity of the cleavage and the geometry of quartz fibres in pressure shadows require thrust sheet emplacement to be synchronous with the internal deformation of sheets thrust sheets show a change from an approximately constant volume plane strain coaxial deformation in their upper parts to non coaxial high strain deformation with incremental i between those expected for pure shear and simple shear i e 0 i 5 45 in their basal parts observed geometries and intensities reflect partitioning of strain and deformation mechanisms within individual thrust sheet volumes both during the initiation and propagation of subsurface detachment faults and with their emplacement to shallower crustal levels strain softening related to chevron folding and more intense cleavage development within a 500 m basal zone has controlled the emplacement of these turbidite dominated thrust sheets transport and internal deformation of the thrust sheets did not involve gravitationally induced spreading but required either a push from the rear or some form of underthrusting

illumination conditions of the lunar polar regions using lola topography abstract we use high resolution altimetry data obtained by the lunar orbiter laser altimeter instrument onboard the lunar reconnaissance orbiter to characterize present illumination conditions in the polar regions of the moon compared to previous studies both the spatial and temporal extent of the simulations are increased significantly as well as the coverage fill ratio of the topographic maps used thanks to the 28 hz firing rate of the five beam instrument we determine the horizon elevation in a number of directions based on 240 m resolution polar digital elevation models reaching down to 75 latitude the illumination of both polar regions extending to 80 can be calculated for any geometry from those horizon longitudinal profiles we validated our modeling with recent lunar reconnaissance orbiter wide angle camera images we assessed the extent of permanently shadowed regions psrs defined as areas that never receive direct solar illumination and obtained total areas generally larger than previous studies 12 866 and 16 055 km2 in the north and south respectively we extended our direct illumination model to account for singly scattered light and found that every psr does receive some amount of scattered light during the year we conducted simulations over long periods several 18 6 years lunar precession cycles with a high temporal resolution 6 h and identified the most illuminated locations in the vicinity of both poles because of the importance of those sites for exploration and engineering considerations we characterized their illumination more precisely over the near future every year a location near the shackleton crater rim in the south polar region is sunlit continuously for 240 days and its longest continuous period in total darkness is about 1 5 days for some locations small height gains 10 m can dramatically improve their average illumination and reduce the night duration rendering some of those particularly attractive energy wise as possible sites for near continuous sources of solar power

the thickness of radar bright deposits in mercury s northern hemisphere from individual mercury laser altimeter tracks the discovery of mercury s radar bright deposits has expanded our understanding of volatiles in the solar system key to deciphering the history and origin of the radar bright deposits is an estimate of the volume of radar bright material that in turn requires a measure of the average thickness of the deposits in this study we investigate changes in topography across radar bright deposits hosted in flat floored complex craters using individual edited mercury laser altimeter mla tracks we compare the difference in heights of radar bright regions and non radar bright regions of the crater floor and the difference of similarly sized and located regions in non radar bright craters and show that the two populations cannot be distinguished the similarity of topography in these two sets of craters allows an upper limit of 15 m to be placed on the thickness of the radar bright deposits

structures associated with superimposed inhomogenous shearing of basic dykes from finnmark norway abstract metadolerites within the olderfjord nappe of the kalak nappe complex of north norway show a wide range of structural relationships during d2 simple shear rotated the dykes from an initially orthogonal relationship towards parallelism with the sedimentary layering and boudinaged those dykes that rotated to within 25 of the sedimentary layering during d3 dykes which had been rotated to within 5 of the layering were folded those lying between 5 and 25 were boudinaged whilst those at angles between 25 and 75 had folds developed against their margins at higher angles of discordance little evidence of d2 or d3 deformation has been observed in other thrust sheets within the kalak nappe complex metabasites have generally been sheared into parallelism with the compositional banding by comparison with the olderfjord nappe the average d2 shear strain within the kalak nappe complex ranges between 5 and 10 the anomalous preservation of high angle discordant relationships in the olderfjord nappe reflects the preservation of a large scale low strain block mega boudin within the nappe samples taken from the mega boudin are more suitable for geochemical analysis for tectonic setting discrimination

in situ analyses of micas in the yashan granite south china constraints on magmatic and hydrothermal evolutions of w and ta nb bearing granites article i nfo article history most rare metal granites in south china host major w deposits with few or without ta nb mineralization however the yashan graniticpluton located inthe yichun areaof western jiangxiprovince south china hosts a major nb ta deposit with minor w mineralization it is thus important for understanding the diversity of w and nb ta mineralization associated with rare metal granites the yashan pluton consists of multi stage intrusive units including the protolithionite muscovite granite li mica granite and topaz lepidolite granite from the early to late stages bulk rock ree contents and la yb ratios decrease from protolithionite granite to li mica granite to topaz lepidolite granite suggesting the dominant plagioclase fractionation this variation together with increasing li rb cs and ta but decreasing nb ta and zr hf ratios is consistent with the magmatic evolution in the yashan pluton micas are protolithionite muscovite li mica and lepidolite and zircons show wide concentration ranges of zro2 h fo2 u o2 t ho2 y 2o3 and p2o5 compositional variations of minerals such as increasing f rb and li in mica and increasing hf u and p in zircon are also in concert with the magmatic evolution from protolithionite granite to li mica granite to topaz lepidolite granite the most evolved topaz lepidolite granite has the highest bulk rock li rb cs f and p contents consistent with the highest contents of these elements and the lowest nb ta ratio in mica and the lowest zr hf ratio in zircon ta nb enrichment was closely related to the enrichment of volatile elements i e li f and p in the melt during magmatic evolution which raised the proportion of non bridging oxygens nbos in the melt the rims of zoned micas in the li mica and topaz lepidolite granites contain lower rb cs nb and ta and much lower f and w than the cores and or mantles indicating an exotic aqueous fluid during hydrothermal evolution some columbite group minerals may have formed from exotic aqueous fluids which were originally depleted in f rb cs nb ta and w but such fluids were not responsible for ta nb enrichment in the yashan granite the interaction of hydrothermal fluids with previously existing micas may have played an important role in leaching concentrating and transporting w fe and ti ta nb enrichment was associated with highly evolved magmas but w mineralization is closely related to hydrothermal fluid thus these magmatic and hydrothermal processes explain the diversity of w and ta nb mineralizations in the rare metal granites

from granite to highly evolved pegmatite a case study of the pinilla de fermoselle granite pegmatite system zamora spain abstract the pinilla de fermoselle pegmatite is a cap like body with an asymmetrical vertical zoning from a granitic facies at the bottom to the upper contact with the metamorphic country rocks the granite grades imperceptibly into the pegmatite which includes three main zones with different degrees of enrichment in li f b p rb cs be sn the essential minerals are quartz feldspar al micas from the muscovite lepidolite series fe micas biotite and zinnwaldite tourmaline schorl elbaite rossmanite and fe mn phosphates apatite beryl cassiterite and cookeite are the most significant accessory minerals the trace elements li be and sr show similar trends in feldspar micas and tourmaline with an increase in the li and be contents and a decrease in sr from the granite to the most evolved pegmatitic zone similar trends are shown by rb cs and ba for micas and k feldspar rb and cs increasing gradually from the granite to most evolved pegmatitic zones simultaneously to the decrease of ba in tourmaline nb and ta contents increase upwards whereas zn contents decrease in the same way the mn contents increase until intermediate degrees of evolution and decrease again in the pinkish elbaite combined field petrographic and geochemical data are consistent with a fractional crystallization model from a granitic melt with a clear petrogenetic relationship between the underlying peraluminous granite and the pegmatite body k feldspar and particularly micas and tourmaline appear as good geochemical monitors using trace elements such as li rb be sr and ba which offer intriguing insights into the petrogenesis of pegmatites

images of surface volatiles in mercury s polar craters acquired by the messenger spacecraft images acquired by nasa s messenger spacecraft have revealed the morphology of frozen volatiles in mercury s permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits the images show extensive spatially continuous regions with distinctive reflectance properties a site within prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance indicating that the surficial ice was emplaced after formation of the underlying craters in areas where water ice is inferred to be present but covered by a thin layer of dark organic rich volatile material regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries the sharp boundaries indicate that the volatile deposits at mercury s poles are geologically young relative to the time scale for lateral mixing by impacts and either are restored at the surface through an ongoing process or were delivered to the planet recently

the early caledonian finnmarkian event reassessed in finnmark 40ar 39ar cleavage age data from nw varangerhalvoya n norway abstract the relative significance of early finnmarkian and late scandian caledonian deformation in n norway is uncertain early studies suggested pervasive finnmarkian deformation whilst later results indicated a restricted finnmarkian domain the present work suggests it was more widespread than accepted and that inter finnmarkian scandian deformation occurred 40 ar 39 ar dating of 2 6 and 6 11 m pelitic fractions from the lower to mid greenschist facies tanahorn nappe five samples base middle allochthon and the epizone lokvikfjellet and barents sea groups three samples north varanger region in the north scandinavian caledonides show slightly discordant spectra most spectra from the tanahorn nappe preserve possible evidence of an early caledonian event in the high temperature steps with recoil excess ar effects in the low temperature steps no pre caledonian relict component has been recorded the results indicate finnmarkian deformation continued to 460 ma with scandian reactivation at 425 415 ma from the north varanger region a strongly crenulated sample yielded plateau ages 444 442 ma means of combined young steps from weakly to uncrenulated samples gave 470 450 ma suggesting penetrative strike slip deformation occurred in the late finnmarkian to inter finnmarkian scandian period no scandian ages were recorded in the north varanger region reassessment of published data from the laksefjord nappe and gaissa thrust belt suggests they were affected by finnmarkian deformation

strain models within spreading nappes abstract internal strain within nappes and thrust sheets is especially important in the understanding of emplacement mechanisms in this paper strain patterns within spreading nappes are investigated theoretically and experimentally three types of spreading nappe are distinguished using various boundary conditions spreading gliding spreading and extruding spreading nappes each type is placed in its proper natural environment strain components and strain gradients are summarized from the use of scale models performed with analogue materials mathematical computations from experimental modelling allow the calculation of 1 1 the schistosity trajectories in vertical flow planes 2 2 the relationship between stretch and transport direction and 3 3 the spatial distribution of different types of strain ellipsoid these provide some tests which make it possible to demonstrate in the field whether nappes are or are not gravity induced finally some field examples from the central alps and the pyrenees are briefly discussed they provide good support to the strain models presented in the present paper

contrasting styles of tectonism and magmatism in the late archaean crustal evolution of the northeastern part of the ivisârtoq region inner godthabsfjord southern west greenland abstract varieties of late archaean gneisses recognized on the basis of field petrographic and chemical data include multiphase tonalitic granitic gneisses with abundant enclaves of amphibolite granodioritic gneisses largely devoid of enclaves leucocratic schlieric gneisses with pods of amphibolite and hornblendite and grey granitic gneisses with locally abundant enclaves of amphibolite the bulk of the gneisses are correlated with the nuk gneisses c 3000 2900 ma of outer godthabsfjord most of the gneisses were emplaced during a period of horizontal tectonic interleaving of thick slices of amitsoq gneisses c 3700 ma and younger malene supracrustal rocks mainly amphibolites older than c 2900 ma interleaving took place on tectonic slides during regional horizontal heterogeneous simple shear movements flattening strains modified ameralik amphibolite dykes in the amitsoq gneisses and gave rise to s fabrics banding and schistosity in the malene rocks the s fabrics facilitating emplacement of voluminous nuk gneisses mainly as broadly concordant sheets potash feldspar blastesis and localized partial melting occurred in some of the earlier phases of nuk gneisses and possibly the amitsoq gneisses but the timing of these processes in relation to this period of emplacement is uncertain high temperatures appear to have been sustained into the second period of deformation when large mainly upright domes and synforms some cuspate were superimposed on the broadly horizontal pile of amitsoq gneisses malene rocks and nuk gneisses tectonic fabrics are dominated by mineral and ribbing lineations coaxial with folds of this second period magmatic emplacement of sheets of granitic gneisses and pegmatites continued many of the sheets being injected coplanar with the second phase parasitic folds pegmatites and schlieric gneisses were generated by syntectonic and younger feldspar blastesis and partial melting the close of late archaean magmatism is marked by the post tectonic qorqut granite and pegmatite complex c 2500 ma the horizontal tectonic movements emplacement of gneisses generation of domes and synforms and syntectonic injection of younger granitic gneisses appear to have taken place in the interval between c 3000 2600 ma ago similar relations between late archaean tectonism and magmatism have been recognized elsewhere in the archaean block of greenland but syntectonic injection of granitic gneisses and transformation of older gneisses to schlieric types appear to be more widespread in this part of the archaean block in ivis rtoq

the development of flanking folds during simple shear and their use as kinematic indicators abstract we used a numerical finite element simulation to model the formation of flanking folds around rotating planar structures e g veins faults or dykes in a non linear viscous medium during ideal simple shear if the planar structure is much more viscous than its host it does not deform and flanking folds with no displacement along the structure develop their vergence is consistent with the overall sense of shear however if the planar structure is much less viscous than its host strain is concentrated within the structure and a secondary shear zone is developed in which slip is opposite to the overall sense of shear then flanking folds develop that have a vergence which is incompatible with the drag on structure if the deflection of markers is not clearly preserved then such flanking folds can be easily misinterpreted as shear bands indicating a wrong shear sense the deflection of the foliation in flanking folds is very similar to deformed asymmetric pull aparts and can therefore help to interpret these otherwise ambiguous shear sense criteria because our model ideal simple shear boundary conditions fails to describe back rotation of planar structures in rocks we speculate that shear band geometries are indicative for general shear

rare element geochemistry and mineral deposits geological association of canada short course notes 17 r l linnen and i m samson editors pp 341 geological association of canada ottawa ontario canada 2005 isbn 1 897095 08 2 price us 43 00 this volume of papers from a geological association of canada short course which was held in st catharines ontario in 2004 is a valuable resource for petrologists and economic geologists most of the volume deals with magmatic processes and mineral deposits that occur in or are related to

megaboudins and lateral extension along the leading edge of a crystalline thrust sheet hudson highlands new york u s a abstract megascopic lenses of crystalline rock along the leading edge of the hudson highland thrust sheet new york are boudins that formed as the result of lateral extension during late paleozoic compression these megaboudins range from 50 m on each side to 1 2 3 2 km and form a ne trending chain along the northwest side of the green pond outlier and hudson highlands the crystalline hudson highlands were first thrust northwestward onto shales and siltstones of the martinsburg formation during the middle ordovician taconian orogeny and later eroded leaving a ne trending elongate klippe along the leading edge during subsequent compression the klippe was laterally extended into a chain of fracture boudins with 3 3 the cleavage and bedding in the surrounding shales and siltstones wraps around the boudins and into the boudin gaps the cleavage and bedding wrap through 61 114 forming shallow to moderately ne plunging fold axes on the northeast side of the gaps and shallow to moderately se plunging fold axes on the southeast side of the gaps strike slip shear indicators at the boudin corners support the mass movement of shale and siltstone in the wider boudin gaps conjugate nw striking normal faults form cross strike grabens that juxtapose overlying silurian strata into the gap areas fragments of brecciated gneiss from the boudins are also transported into the gap areas along these cross strike normal faults the boudins dextral strike slip stretching faults along the southeastern margin of the boudins and asymmetric gap fill geometries are prime criteria for the recognition and characterization of lateral stretching in contractional fold and thrust belts

constraining the thickness of polar ice deposits on mercury using the mercury laser altimeter and small craters in permanently shadowed regions abstract radar bright deposits at the poles of mercury are located in permanently shadowed regions which provide thermally stable environments for hosting and retaining water ice on the surface or in the near subsurface for geologic timescales while the areal distribution of these radar bright deposits is well characterized their thickness and thus their total mass and volume remain poorly constrained here we derive thickness estimates for selected water ice deposits using small simple craters visible within the permanently shadowed radar bright deposits we examine two endmember scenarios in case i these craters predate the emplacement of the ice and in case ii these craters postdate the emplacement of the ice in case i we find the difference between estimated depths of the original unfilled craters and the measured depths of the craters to find the estimated infill of material the average estimated infilled material for 9 craters assumed to be overlain with water ice is 41 14 30 m where 1 standard error of the mean is reported as uncertainty reported uncertainties are for statistical errors only additional systematic uncertainty may stem from georeferencing the images and topographic datasets from the radial accuracy of the altimeter measurements or from assumptions in our models including 1 ice is flat in the bowl shaped crater and 2 there is negligible ice at the crater rims in case ii we derive crater excavation depths to investigate the thickness of the ice layer that may have been penetrated by the impact while the absence of excavated regolith associated with the small craters observed suggests that impacts generally do not penetrate through the ice deposit the spatial resolution and complex illumination geometry of images may limit the observations therefore it is not possible to conclude whether the small craters in this study penetrate through the ice deposit and thus case ii does not provide a constraint on the ice thickness for mercury s polar deposits we argue that case i of the small craters predating the emplacement of the ice deposits is more likely given other geologic evidence that suggests that these ice deposits are relatively young using the ice thickness estimates from case i to calculate the total amount of water ice currently contained in mercury s polar deposits results in a value of 10 14 10 15 kg this is equivalent to 100 1000 km 3 ice in volume this volume of water ice is consistent with delivery via micrometeorite bombardment jupiter family comets or potentially a single impactor

radar mapping of mercury s polar anomalies ground based radar observations of mercury have revealed unusually strong highly depolarized echoes from the north1 2 and south2 poles these anomalous echoes have been cited as evidence of polar ice deposits1 5 thermal studies3 5 suggest that the permanently shaded floors of large polar craters are cold enough to preserve water ice in a stable state over aeons in spite of mercury s proximity to the sun here we present high resolution radar maps of mercury s polar regions derived from delay doppler measurements we have resolved the north and south polar anomalies into numerous crater sized features and we have been able to identify the source craters for many of these features after making small corrections to the pole positions on the mariner 10 images the coincidence with crater locations together with other properties of the radar features are consistent with the polar ice hypothesis

the stratigraphy of a sheet of supracrustal rocks within high grade orthogneisses and its bearing on late archaean structure in southern west greenland an extensive sheet of presumed middle archaean supracrustal rocks within high grade orthogneisses in inner godthabsfjord includes a section 3 km thick with exceptional preservation of pillow structures in low k tholeiitic amphibolites unambiguously indicating way up the amphibolites are associated with pillow structured and pyroclastic komatiitic ultramafic schists dunitic harzburgitic sills a stratiform gabbro pyritous metacherts and quartzo feldspathic gneisses and schists with field and chemical characteristics suggesting they formed as acid intermediate tuffaceous sediments the sheet is tectonically overlain by older orthogneisses amitsoq with tracts and enclaves of the early archaean isua akilia supracrustal association it is underlain by younger orthogneisses and granites with tracts of older variably reworked amitsoq gneisses rocks of the higher level were emplaced on a tectonic slide which marks the upper boundary of the supracrustal sheet the original lower boundary with amitsoq gneisses in the lower level may also have been a slide which has been obscured by emplacement of younger orthogneisses composite dome in dome structures with intervening synclinal cusps were superimposed on the sub horizontal layer cake structure of the supracrustal sheet and the higher and lower levels of orthogneisses syntectonic granitic and amphibolite dykes kangilingua dykes were emplaced in parts of the synforms whilst granite emplacement and feldspar blastesis took place in the cores of some domes the dome and cusp system appears to be part of a regional series of late archaean steeply inclined to upright structures trending northeast in the godthabsfjord region in inner godthabsfjord the system evolved in response to buoyancy forces generated by injection of basaltic magmas and partial melting of orthogneisses at depth within a regime of regional compression associated perhaps with transcurrent intraplate motions

comparison of areas in shadow from imaging and altimetry in the north polar region of mercury and implications for polar ice deposits earth based radar observations and results from the messenger mission have provided strong evidence that permanently shadowed regions near mercury s poles host deposits of water ice messenger s complete orbital image and topographic datasets enable mercury s surface to be observed and modeled under an extensive range of illumination conditions the shadowed regions of mercury s north polar region from 65 n to 90 n were mapped by analyzing mercury dual imaging system mdis images and by modeling illumination with mercury laser altimeter mla topographic data the two independent methods produced strong agreement in identifying shadowed areas all large radar bright deposits those hosted within impact craters 6 km in diameter collocate with regions of shadow identified by both methods however only 46 of the persistently shadowed areas determined from images and 43 of the permanently shadowed areas derived from altimetry host radar bright materials some sizable regions of shadow that do not host radar bright deposits experience thermal conditions similar to those that do the shadowed craters that lack radar bright materials show a relation with longitude that is not related to the thermal environment suggesting that the earth based radar observations of these locations may have been limited by viewing geometry but it is also possible that water ice in these locations is insulated by anomalously thick lag deposits or that these shadowed regions do not host water ice

constraints on the volatile distribution within shackleton crater at the lunar south pole shackleton crater is nearly coincident with the moon s south pole its interior receives almost no direct sunlight and is a perennial cold trap making shackleton a promising candidate location in which to seek sequestered volatiles however previous orbital and earth based radar mapping and orbital optical imaging have yielded conflicting interpretations about the existence of volatiles here we present observations from the lunar orbiter laser altimeter on board the lunar reconnaissance orbiter revealing shackleton to be an ancient unusually well preserved simple crater whose interior walls are fresher than its floor and rim shackleton floor deposits are nearly the same age as the rim suggesting that little floor deposition has occurred since the crater formed more than three billion years ago at a wavelength of 1 064 nanometres the floor of shackleton is brighter than the surrounding terrain and the interiors of nearby craters but not as bright as the interior walls the combined observations are explicable primarily by downslope movement of regolith on the walls exposing fresher underlying material the relatively brighter crater floor is most simply explained by decreased space weathering due to shadowing but a one micrometre thick layer containing about 20 per cent surficial ice is an alternative possibility

multi textured garnets from a single growth event an example from northern norway textural relationships between porphyroblasts of biotite and garnet in metasediments in the nordkinn peninsula area of the finnmarkian caledonides of north norway are apparently complex there is evidence for two textural zones in both mineral phases and superficially the development of these appears to have overlapped at least in part in time and space this apparently complex porphyroblast growth history can be considerably simplified if only one period of garnet growth occurred and if different inclusion fabrics developed where garnet replaced biotite porphyroblasts and where it overgrew the matrix foliation the possibility that porphyroblasts with textural evidence for multiphase growth histories actually grew during a single crystallization event is of importance in the interpretation and elucidation of tectonometamorphic relationships