the global martian volcanic evolutionary history abstract viking mission image data revealed the total spatial extent of preserved volcanic surface on mars one of the dominating surface expressions is olympus mons and the surrounding volcanic province tharsis earlier studies of the global volcanic sequence of events based on stratigraphic relationships and crater count statistics were limited to the image resolution of the viking orbiter camera here a global investigation based on high resolution image data gathered by the high resolution stereo camera hrsc during the first years of mars express orbiting around mars is presented additionally mars orbiter camera moc and thermal emission imaging system themis images were used for more detailed and complementary information the results reveal global volcanism during the noachian period 3 7 ga followed by more focused vent volcanism in three tharsis elysium and circum hellas and later two tharsis and elysium volcanic provinces finally the volcanic activity became localized to the tharsis region about 1 6 ga ago where volcanism was active until very recently 200 100 ma these age results were expected from radiometric dating of martian meteorites but now verified for extended geological units mainly found in the tharsis montes surroundings showing prolonged volcanism for more than 3 5 billions years the volcanic activity on mars appears episodic but decaying in intensity and localizing in space the spatial and temporal extent of martian volcanism based on crater count statistics now provides a much better database for modelling the thermodynamic evolution of mars

high temperature mylonitization of quartzofeldspathic gneisses example from the schirmacher hills east antarctica abstract in the schirmacher hills most of the ductile shearing took place under high to medium grade amphibolite facies metamorphism the microstructure of the mylonites shows characteristic features of high temperature deformation and thus gives us an idea of deformation mechanisms of the constituent minerals at great crustal depth the variation in microstructure of the sheared rock is partly due to heterogeneity of the intensity of strain from domain to domain producing protomylonites orthomylonites and ultramylonites however a large part of the microstructural variation has resulted from syn to post tectonic recrystallization and grain growth of constituent minerals both quartz and feldspar have deformed by crystal plastic processes with dominant grain boundary migration the present aspect ratio of the feldspar grains is a result of various degrees of dynamic recrystallization along the grain boundary the ratio varies between 1 5 and 2 presence of exsolution lamellae in perthites and formation of myrmekite at the strained grains of k feldspar suggest diffusion assisted dislocation creep these mylonites are characterized by the presence of weakly strained or unstrained long quartz ribbons the development of quartz ribbons with the absence of significant strain suggests grain recovery and grain growth during high temperature mylonitization the growth of quartz ribbons took place by coalescing neighbouring grains both along and across the ribbon length at the ultramylonite stage the fine grained matrix of quartz and feldspar mostly accommodates the bulk strain

a multi step model for the origin of e3 enstatite chondrites it appears that the mineralogy and chemical properties of type 3 enstatite chondrites could have been established by fractionation processes removal of a refractory component and depletion of water in the solar nebula and by equilibration with nebular gas at low to intermediate temperatures approximately 700 950 k we describe a model for the origin of type 3 enstatite chondrites that for the first time can simultaneously account for the mineral abundances bulk chemistry and phase compositions of these chondrites by the operation of plausible processes in the solar nebula this model which assumes a representative nebular gas pressure of 10 5 bar entails three steps 1 initial removal of 56 of the equilibrium condensed phases in a system of solar composition at 1270 k 2 an average loss of 80 85 water vapor in the remaining gas and 3 two different closure temperatures for the condensed phases the first step involves a refractory element fractionation and is needed to account for the overall major element composition of enstatite chondrites assuming an initial system with a solar composition the second step water vapor depletion is needed to stabilize si bearing metal oldhamite and niningerite which are characteristic minerals of the enstatite chondrites variations in closure temperatures are suggested by the way in which the bulk chemistry and mineral assemblages of predicted condensates change with temperature and how these parameters correlate with the observations of enstatite chondrites in general most phases in type 3 enstatite chondrites appear to have ceased equilibrating with nebular gas at approximately 900 950 k except for fe metal which continued to partially react with nebular gas to temperatures as low as 700 k

early differentiation of the earth and the problem of mantle siderophile elements a new approach the long standing problem of the excess abundances of siderophile elements in the mantle can be resolved by considering an equilibrium core mantle differentiation in the earth at 3000 to 3500 kelvin this high temperature differentiation results in mantle siderophile element abundances that closely match the observed values some lithophile light elements could enter the core in this process as is necessary to account for its low density the abundances of siderophile elements in the mantle are consistent with the conclusion derived from the recent physical models that the earth was molten during accretion

the martian subsurface as a potential window into the origin of life few traces of earth s geologic record are preserved from the time of life s emergence over 3 800 million years ago consequently what little we understand about abiogenesis the origin of life on earth is based primarily on laboratory experiments and theory the best geological lens for understanding early earth might actually come from mars a planet with a crust that s overall far more ancient than our own on earth surface sedimentary environments are thought to best preserve evidence of ancient life but this is mostly because our planet has been dominated by high photosynthetic biomass production at the surface for the last 2 500 million years or more by the time oxygenic photosynthesis evolved on earth mars had been a hyperarid frozen desert with a surface bombarded by high energy solar and cosmic radiation for more than a billion years and as a result photosynthetic surface life may never have occurred on mars therefore one must question whether searching for evidence of life in martian surface sediments is the best strategy this perspective explores the possibility that the abundant hydrothermal environments on mars might provide more valuable insights into life s origins ancient hydrothermal deposits formed in the martian subsurface may be the best targets for finding evidence for ancient life on mars and clues about the origin of life on earth

deformation mechanisms in a high temperature quartz feldspar mylonite evidence for superplastic flow in the lower continental crust abstract microstructures and crystallographic preferred orientations in a fine grained banded quartz feldspar mylonite were studied by optical microscopy sem and tem mylonite formation occurred in retrograde amphibolite facies metamorphism interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity strain incompatibilities during grain sliding in the superplastic quartz feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar the rate of which probably controls the rate of superplastic deformation there is evidence for equal flow stress levels in the superplastic and crystal plastic domains in this case mechanism partitioning results in strain rate partitioning fast deformation in the superplastic bands therefore dominates flow and deformation is probably best modelled by a superplastic law if this deformational behaviour is typical shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress or at low differential stresses in case of fixed strain rates

differentiation of vesta implications for a shallow magma ocean abstract the dawn mission confirms earlier predictions that the asteroid 4 vesta is differentiated with an iron rich core a silicate mantle and a basaltic crust and supports the conjecture of vesta being the parent body of the hed meteorites to better understand its early evolution we perform numerical calculations of the thermo chemical evolution adopting new data obtained by the dawn mission such as mass bulk density and size of the asteroid we have expanded the thermo chemical evolution model of neumann et al 2012 that includes accretion compaction melting and the associated changes of the material properties and the partitioning of incompatible elements such as the radioactive heat sources advective heat transport and differentiation by porous flow to further consider convection and the associated effective cooling in a potential magma ocean depending on the melt fraction the heat transport by melt segregation is modelled either by assuming melt flow in a porous medium or by simulating vigorous convection and heat flux of a magma ocean with a high effective thermal conductivity our results show that partitioning of 26al and its transport with the silicate melt is crucial for the formation of a global and deep magma ocean due to the enrichment of 26al in the liquid phase and its accumulation in the sub surface for formation times t 0 1 5 ma a thin shallow magma ocean with a thickness of 1 to a few tens of km forms its thickness depends on the viscosity of silicate melt the lifetime of the shallow magma ocean is o 10 4 o 10 6 years and convection in this layer is accompanied by the extrusion of 26al at the surface resulting in the formation of a basaltic crust the interior differentiates from the outside inwards with a mantle that is depleted in 26al and core formation is completed within 0 3 ma the lower mantle experiences a maximal melt fraction of 45 suggesting a harzburgitic to dunitic composition our results support the formation of non cumulate eucrites by the extrusion of early partial melt while cumulate eucrites and diogenites may form from the crystallising shallow magma ocean silicate melt is present in the mantle for up to 150 ma and convection in a crystallising core proceeds for approximately 100 ma supporting the idea of an early magnetic field to explain the remnant magnetisation observed in some hed meteorites

chondritic late accretion to mars and the nature of shergottite reservoirs abstract mars is considered to have formed as a planetary embryo that experienced extensive differentiation early in its history shergottite meteorites preserve evidence for this history and for late accretion events that affected their mantle sources within mars here we report the first coupled 187re 187os 87sr 86sr highly siderophile element hse os ir ru pt pd re and major element abundance dataset for martian shergottites that span a range of mgo contents from 6 4 to 30 3 wt the shergottites range from picro basalt to basaltic andesite compositions have enriched to depleted incompatible trace element compositions and define fractional crystallization trends enabling the determination of hse compatibility for martian magmatism in the order os ir ru pt pd re this order of compatibility is like that defined previously for earth and the moon but the fractionation of strongly compatible os ir and ru appears to take place at higher mgo contents in martian magmas due to early onset of sulfide fractionation in general enriched shergottites have lower mgo contents than intermediate or depleted shergottites and have fractionated hse patterns re pd pt ru ir os and more radiogenic measured 87sr 86sr 0 7127 0 7235 and 187os 188os 0 140 0 247 than intermediate or depleted shergottite meteorites 87sr 86sr 0 7010 0 7132 187os 188os 0 127 0 141 osmium isotope compositions corrected for crystallization age define compositions that are implausibly unradiogenic in some enriched shergottites implying recent mobilization of re in some samples filtering for the effects of alteration and high re os through crystal liquid fractionation leads to a positive correlation between age corrected sr and os isotope compositions mixing between hypothetical martian crustal and mantle reservoirs are unable to generate the observed sr os isotope compositions of shergottites which require either distinct and discrete long term incompatible element depleted and enriched mantle sources or originate from hybridized melting of deep melts with metasomatized martian lithosphere using mgo regression methods we obtain a modified estimate of the bulk silicate mars hse composition of in ng g 1 0 4 re 7 4 pd 9 6 pt 6 2 ru 3 7 ir 4 os and a long term chondritic 187os 188os ratio 0 1312 this result does not permit existing models invoking high pressure and temperature partitioning of the hse instead our estimate implies 0 6 0 7 by mass of late accretion of broadly chondritic material to mars our results indicate that mars could have accreted earlier than earth but that disproportional accretion of large bodies and a relative constant flux of accretion of available materials in the first 50 100 ma of solar system led to the broad similarity in hse abundances between earth and mars

polymetamorphism in the schirmacher hills granulites east antarctica implications for tectonothermal reworking of an isobarically cooled deep continental crust abstract the precambrian basement of the schirmacher hills queen maud land east antarctica has evolved through multiple episodes of deformation and metamorphism the rocks have suffered at least five phases of deformation the imprint of the early deformation d 1 is preserved in some mafic isolated enclaves the second and the third deformations d 2 and d 3 are the dominant deformations of this area and produced isoclinal folds with transposition of earlier cleavages the later deformations d 4 and d 5 produced two sets of open upright folds detailed mineralogical textural mineral chemical studies and geothermobarometry on khondalite leptynite as well as different varieties of enderbite and mafic granulites have revealed that the rocks suffered two phases of metamorphism under granulite facies conditions followed by an amphibolite facies overprint m 1 is broadly coeval with d 1 only in mafic granulite enclaves within enderbitic gneiss and took place at ca 10 kbar 900 c the mafic magma parental to the enclaves probably crystallized at 11 2 kbar following post peak near isobaric cooling the mafic granulites were transported to shallower levels by the enderbitic magma m 2 recorded in all the lithologies occurred at ca 8 kbar 800 850 c and synchronous with d 2 post peak m 2 evolution of the rocks was characterized by a pressure temperature drop of 2 kbar and 200 c respectively and textures indicative of both cooling and decompression are preserved in different rocks the relative timing of the two however cannot be worked out m 3 synchronous with d 3 took place at 6 kbar 600 650 c and evolved hydrous fluid flux correlation with available structural and geochronological data shows that both m 1 and m 2 could be of grenvillian event m 3 could well be pan african age

highly siderophile element depletion in the moon abstract coupled 187 os 188 os and highly siderophile element hse os ir ru pt pd re abundance data are reported for apollo 12 12005 12009 12019 12022 12038 12039 12040 apollo 15 15555 and apollo 17 70135 mare basalts along with mare basalt meteorites la paz icefield lap 04841 and miller range mil 05035 these mare basalts have consistently low hse abundances at 2 10 5 to 2 10 7 the chondritic abundance the most magnesian samples have broadly chondrite relative hse abundances and chondritic measured and calculated initial 187 os 188 os the lower abundances and fractionated hse compositions of more evolved mare basalts can be reproduced by modeling crystal liquid fractionation using rock melt bulk partition coefficients of 2 for os ir ru pt and pd and 1 5 for re lunar mare basalt bulk partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions leading to increased hse compatibility the chondritic relative abundances and chondritic 187 os 188 os of the most primitive high mgo mare basalts cannot readily be explained through regolith contamination during emplacement at the lunar surface mare basalt compositions are best modeled as representing 5 11 partial melting of metal free sources with low os ir ru pd 0 1 ng g 1 pt 0 2 ng g 1 re 0 01 ng g 1 and s 75 g g 1 with sulphide melt partitioning between 1000 and 10 000 apollo 12 olivine pigeonite and ilmenite normative mare basalts define an imprecise 187 re 187 os age of 3 0 0 9 ga with an initial 187 os 188 os of 0 107 0 010 this age is within uncertainty of 147 sm 143 nd ages for the samples the initial os isotopic composition of apollo 12 samples indicates that the source of these rocks evolved with re os within 10 of chondrite meteorites from the time that the mantle source became a system closed to siderophile additions to the time that the basalts erupted similarity in absolute hse abundances between mare basalts from the apollo 12 15 and 17 sites and from unknown regions of the moon la paz mare basalts mil 05035 indicates relatively homogeneous and low hse abundances within the lunar interior low absolute hse abundances and chondritic re os of mare basalts are consistent with a late accretion addition of 0 02 wt of the moon s mass to the mantle prior to the formation of the lunar crust late accretion must also have occurred significantly prior to cessation of lunar mantle differentiation 4 4 ga to enable efficient mixing and homogenization within the mantle low lunar hse abundances are consistent with proportionally 40 times more late accretion to earth than the moon disproportional late accretion to the two bodies is consistent with the small 182 w excess 21 28 ppm measured in lunar rocks compared to the silicate earth

physical state of the very early earth abstract the earliest surface environment of the earth is reconstructed in accordance with the planetary formation theory formation of an atmosphere is an inevitable consequence of earth s formation the atmosphere near the close of accretion is composed of 200 300 bars of h2 and h2o and several tens of bars of co and co2 either by the blanketing effect of the proto atmosphere or heating by large planetesimal impacts a magma ocean is formed during accretion we can distinguish three stages for the thermal evolution of the magma ocean and proto crust stage 0 is characterized by a super liquidus or completely molten regime near the surface at this stage the surface of the earth is covered by a super liquidus magma ocean no chemical differentiation is expected during this stage once the energy flux released by planet formation decreases to the 200 w m2 level the super liquidus magma ocean then disappears within a time interval of 1 m y this is the transition from stage 0 to 1 stage 1 is characterized by a partially molten magma ocean in the magma ocean consisting of 20 30 partial melt heat transport is controlled by melt solid separation a type of compositional convection rather than thermal convection chemical differentiation of the mantle mainly occurs in this stage once the energy flux drops to the 160 w m2 level more than 90 of water vapor in the proto atmosphere condense to form the proto oceans several tens of bars of co and co2 remain in the atmosphere just after formation of the oceans water oceans are occasionally evaporated by large impacts after each such event recondensation of the ocean takes several hundred years although the surface is covered by a chilled proto crust it is short lived because of extensive volcanic resurfacing activity as well as meteorite impacts resurfacing this stage ends when the energy flux drops to 0 1 1 w m2 level the duration time of stage 1 is estimated to be several hundred million years the best estimate is about 400 m y stage 2 is characterized by solid state convection this stage continues to the present day one of the most important change on the proto earth is the transition from stage 1 to 2 which occurs several hundred million years after the earth formation long lived crust is formed only after this transition

evidence for extremely rapid magma ocean crystallization and crust formation on mars the formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood the mineral zircon is a powerful tool for constraining crust formation because it can be accurately dated with the uranium to lead u pb isotopic decay system and is resistant to subsequent alteration moreover given the high concentration of hafnium in zircon the lutetium to hafnium 176lu 176hf isotopic decay system can be used to determine the nature and formation timescale of its source reservoir1 3 ancient igneous zircons with crystallization ages of around 4 430 million years myr have been reported in martian meteorites that are believed to represent regolith breccias from the southern highlands of mars4 5 these zircons are present in evolved lithologies interpreted to reflect re melted primary martian crust 4 thereby potentially providing insight into early crustal evolution on mars here we report concomitant high precision u pb ages and hf isotope compositions of ancient zircons from the nwa 7034 martian regolith breccia seven zircons with mostly concordant u pb ages define 207pb 206pb dates ranging from 4 476 3 0 9 myr ago to 4 429 7 1 0 myr ago including the oldest directly dated material from mars all zircons record unradiogenic initial hf isotope compositions inherited from an enriched andesitic like crust extracted from a primitive mantle no later than 4 547 myr ago thus a primordial crust existed on mars by this time and survived for around 100 myr before it was reworked possibly by impacts4 5 to produce magmas from which the zircons crystallized given that formation of a stable primordial crust is the end product of planetary differentiation our data require that the accretion core formation and magma ocean crystallization on mars were completed less than 20 myr after the formation of the solar system these timescales support models that suggest extremely rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle which subsequently overturns resulting in decompression melting of rising cumulates and production of a primordial basaltic to andesitic crust6 7

the martian hemispheric dichotomy may be due to a giant impact mars is divided into two fundamentally different geological provinces of approximately hemispherical extent1 3 the more southerly province is heavily cratered contains relatively old geological units and superficially resembles the lunar and mercurian highlands the northern province is relatively lightly cratered and contains younger geological units including extensive plains volcanic edifices and volcanic calderas except for the tharsis and elysium regions and other large volcanoes most of the younger northern province consists of lowlands which lie an average of 3 km below the highlands lowlands occupy about one third of mars they are separated from the highlands by a distinct scarp or by a sloping transitional zone as much as 700km wide in which highland materials have been disrupted and partly replaced by lowland deposits fig 1 the transition is characterized by a variety of landforms unknown on other planets the highlands and lowlands are in isostatic equilibrium across the transitional zone4 no generally accepted explanation for the cause of the highland lowland dichotomy has been proposed although thinning of the lithosph re in the northern hemisphere by internal processes has been suggested5 large impacts which were a feature of early solar system history have also been suggested as an important cause of such early planetary surface heterogeneities3 6 7 subcircular lowlands such as argyre chryse hellas and isidis planitia and the plains south of tharsis occupy previously recognized impact basins8 10 we propose here that the largest expanse of lowlands also results from formation of a large impact basin early in mars history that has profoundly shaped the planet s surface

highly siderophile element constraints on accretion and differentiation of the earth moon system a new combined rhenium osmium and platinum group element data set for basalts from the moon establishes that the basalts have uniformly low abundances of highly siderophile elements the data set indicates a lunar mantle with long term chondritic highly siderophile element ratios but with absolute abundances that are over 20 times lower than those in earth s mantle the results are consistent with silicate metal equilibrium during a giant impact and core formation in both bodies followed by post core formation late accretion that replenished their mantles with highly siderophile elements the lunar mantle experienced late accretion that was similar in composition to that of earth but volumetrically less than approximately 0 02 lunar mass and terminated earlier than for earth

a fracture model for quartz ribbons in straight gneisses abstract polycrystalline composite quartz ribbons in straight gneisses of the grenville province are explained as dilatant quartz filled fractures formed parallel to a strong preexisting gneissosity the ribbons are thin 3 mm but long and continuous and have straight and truncating contacts with bordering grains of other minerals they are similar in morphology to polycrystalline ribbons interpreted in the literature as resulting from high strains in contrast little strain is required if the ribbons form as quartz filled fractures a monoclinic symmetry of the shapes of ribbons and inclusions can be explained by fracture interactions as they propagate the symmetry is consistent with a late extensional south side down sense of shearing which has been documented elsewhere in the grenville province

strong water isotopic anomalies in the martian atmosphere probing current and ancient reservoirs we measured maps of atmospheric water h2o and its deuterated form hdo across the martian globe showing strong isotopic anomalies and a significant high deuterium hydrogen d h enrichment indicative of great water loss the maps sample the evolution of sublimation from the north polar cap revealing that the released water has a representative d h value enriched by a factor of about 7 relative to earth s ocean vienna standard mean ocean water vsmow certain basins and orographic depressions show even higher enrichment whereas high altitude regions show much lower values 1 to 3 vsmow our atmospheric maps indicate that water ice in the polar reservoirs is enriched in deuterium to at least 8 vsmow which would mean that early mars 4 5 billion years ago had a global equivalent water layer at least 137 meters deep

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

quartz plastic segregation and ribbon development in high grade striped gneisses quartz microstructures and c axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high grade alem paraiba shear zone in southeastern brazil are documented cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons these ribbons are separated by feldspar rich domains devoid of quartz the stage at which individual stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode which is reflected by an abrupt increase in the smoothness of the ribbon boundaries this change is interpreted to represent a strain softening kink in the stress strain time path progressive ribboning is accompanied by strengthening of the c axis fabric z maximum indicative of continued plastic flow by basal glide operation of basal glide at these high temperature conditions 680 700 c is interpreted to be a consequence of relatively dry deformation conditions a model is then proposed for development of straight quartz ribbons in high grade striped gneisses where scattered quartz grains are continuously stretched and segregated by crystal plastic processes the small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons

quartz ribbons in high grade granite gneiss modifications of dynamically formed quartz c axis preferred orientation by oriented grain growth quartz ribbons form a well defined ls fabric in granitic gneisses sheared and metamorphosed in the upper amphibolite facies boundaries of quartz grains are smooth and sub perpendicular to ribbon walls suggesting post deformational growth whereas c axes display asymmetric girdle patterns consistent with the sense of shear indicated by mesoscopic fabrics subdivision of the microfabrics according to the proportion of ribbon length occupied by individual grains indicates that the c axes of relatively short grains exhibit across girdle pattern similar to that of the whole sample longer grains reproduce elements of the pattern of the short grains suggesting that oriented grain growth occurred grain boundary mobility which clearly ended in a post deformational static regime probably began during dynamic recrystallization owing to space limitations imposed by the feldspathic matrix grain growth in the ribbons ceased before the initial dynamic fabric was erased in summary this study shows that the c axes pattern of longer grains within quartz ribbons reproduces part of the dynamically formed pattern of the shorter grains reflecting an oriented grain growth which concluded in a static episode although possibly initiated in the dynamic regime

development of successive generations of shear zones in the precambrian basement of schirmacher hills east antarctica abstract in the precambrian basement of the schirmacher hills ductile shear zones have developed at several stages over a wide time span of tectonothermal history the earliest shear zones were formed under granulite facies conditions producing well foliated gneiss the majority of the ductile shear zones developed under amphibolite facies conditions the sheared rock is a mylonite characterized by drastic grain refinement of the constituent minerals the shear zones under amphibolite facies conditions are of four broad categories an early set subparallel to the gneissic foliation an early and a late cross cutting types shear zones which developed within massive rocks and lastly those which developed at the contacts of strong rheological contracts the last phase of deformation produced discrete shear fractures all the different phases of ductile shearing were broadly synchronous with emplacement of quartzofeldspathic materials pegmatite was emplaced even along some of the discrete shear fractures the magnitude of shear strain could be measured from the orientations of newly formed foliations and also from the rotation of early foliations

mars in the aftermath of a colossal impact abstract the abundance of highly siderophile elements hses inferred for mars mantle from martian meteorites implies a late veneer lv mass addition of 0 8 wt with broadly chondritic composition late accretion to mars by a differentiated ceres sized 1000 km diameter object can account for part of the requisite lv mass and geochronological constraints suggests that this must have occurred no later than ca 4480 ma here we analyze the outcome of the hypothetical lv giant impact to mars with smoothed particle hydrodynamics simulations together with analytical theory results show that in general about 50 of the impactor s metallic core shatters into 10 m fragments that subsequently fragment into sub mm metallic hail at re accretion this returns a promising delivery of hses into martian mantle compared to either a head on and hit and run collision in both cases 10 myr a dense pre noachian co2 atmosphere should lower the escape efficiency of hydrogen by ir emission a more detailed hydrodynamic atmospheric model of this early hydrogen atmosphere is warranted to examine its effect on pre noachian mars

a colossal impact enriched mars mantle with noble metals once the terrestrial planets had mostly completed their assembly bombardment continued by planetesimals left over from accretion highly siderophile element hse abundances in mars mantle imply that its late accretion supplement was 0 8 wt earth and the moon obtained an additional 0 7 wt and 0 02 wt respectively the disproportionately high earth moon accretion ratio is explicable by stochastic addition of a few remaining ceres sized bodies that preferentially targeted earth here we show that mars late accretion budget also requires a colossal impact a plausible visible remnant of which is the emispheric dichotomy the addition of sufficient hses to the martian mantle entails an impactor of at least 1200 km in diameter to have struck mars before 4430 ma by which time crust formation was well underway thus the dichotomy could be one of the oldest geophysical features of the martian crust ejected debris could be the source material for its satellites

how martian araneiforms get their shapes morphological analysis and diffusion limited aggregation model for polar surface erosion abstract araneiforms are radially converging systems of branching troughs exhibiting fractal properties they are found exclusively in the southern polar regions of mars and believed to be result of multiple repetitions of cold co2 gas jets eruptions araneiform troughs get carved by the overpressurized gas rushing underneath a seasonal ice layer towards a newly created opening current work is an attempt to quantitatively analyze araneiforms patterns and model their formation mechanism araneiform shapes range from small and simple structures of one or two connected branches to intricate dendritic patterns several kilometers across the dendritic quality of most araneiforms are suggestive that they can be described in terms commonly applied to terrestrial rivers the main difference between rivers and araneiforms from the morphological point of view is that araneiform patterns are not directed or in any way influenced by the local gravitational gradient we have adapted and for the first time applied to martian araneiforms qualitative morphological analysis typically used for terrestrial rivers for several locations in the martian southern polar regions we have calculated the largest order of tributaries tributary densities and bifurcation ratios we have shown that the large and well developed araneiforms with tributary orders larger than 4 closely follow horton s law of tributary orders and have bifurcation ratio that falls well inside the range of terrestrial rivers we have implemented a two dimensional diffusion limited aggregation dla model that describes formation of dendrite shapes by mathematical probabilistic means we compared modeled dendrite shapes to the araneiform shapes observed in the martian polar regions and evaluated their similarity using the morphological analysis of araneiforms we showed that dla model can successfully recreate 2d shapes of different observed araneiforms to simulate the patterns that deviate from central symmetry it is required to modify the main governing parameter of dla model its two dimensional probability field an analog to the governing probability field in the case of araneiforms is the gas pressure in the cavity underneath the ice layer right before the gas starts moving towards the newly created escape vent thus modeling the creation of araneiform patterns with dla leads to better understanding of seasonal processes that create them

meteorite constraints on martian atmospheric loss and paleoclimate abstract the evolution of mars atmosphere to its currently thin state incapable of supporting liquid water remains poorly understood and has important implications for martian climate history martian meteorites contain trapped atmospheric gases that can be used to constrain both the timing and effectiveness of atmospheric escape processes in this paper measurements of xenon isotopes in two ancient martian meteorites alh 84001 and nwa 7034 are reported the data indicate an early episode of atmospheric escape that mass fractionated xenon isotopes culminated within a few hundred million years of planetary formation and little change to the atmospheric xenon isotopic composition has occurred since this time in contrast on earth atmospheric xenon fractionation continued for at least two billion years pujol et al 2011 such differences in atmospheric xe fractionation between the two planets suggest that climate conditions on mars may have differed significantly from those on archean earth for example the hydrogen escape flux may not have exceeded the threshold required for xenon escape on mars after 4 2 4 3 ga which indicates that mars may have been significantly drier than earth after this time

accretion of phobos and deimos in an extended debris disc stirred by transient moons mars has two small moons that may have formed in the aftermath of a giant impact simulations suggest that phobos and deimos accreted from the disperse outer region of the debris disc that was stirred up by short lived larger moons

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

tungsten isotope composition of the acasta gneiss complex high precision tungsten 182w 184w isotope measurements on well characterised mafic and felsic samples of the ca 3960 ma acasta gneiss complex agc northwest territories canada show radiogenic e182w values between 0 06 to 0 15 two ca 3600 ma felsic samples from this terrane have e182w 0 and are the oldest samples so far documented to have a w isotopic composition indistinguishable from that of the modern mantle the e182w data are correlated with e142nd roth et al 2014 and we attribute this variability to incomplete metamorphic homogenisation of the 3960 ma protolith with more recent material in a 3370 ma tectono thermal event notably the value of the positive e182w anomalies seen in the 3960 ma agc samples that are least affected by metamorphic homogenisation is comparable to that observed in other early archean rocks isua supracrustal belt greenland nuvvuagittuq supracrustal belt canada and the late archean kostomuksha komatiites karelia this demonstrates a globally constant signature we infer that the presence of a pre late veneer mantle represents the most straightforward interpretation of a uniform distribution of ew182 0 15 value in archean rocks of different ages we show that such a notion is compatible with independent constraints from highly siderophile element abundances in mafic and ultra mafic archean mantle derived rocks the absence of anomalous e182w and e142nd so far measured in samples younger than ca 2800 ma suggests progressive convective homogenisation of silicate reservoirs the downward mixing of an upper mantle rich in late delivered meteoritic material might account for these combined observations

tectonothermal history recorded in mafic dykes and enclaves of gneissic basement in the schirmacher hills east antarctica abstract there are successive generations of mafic dykes and other mafic and ultramafic bodies within the gneissic rocks of the schirmacher hills in east antarctica three major groups of events can be recognised by comparison of their internal structures with those of their charnockitic or granitic host and by identification of their successive stages of transformation by metamorphism and migmatization these events are 1 granulite facies metamorphism m 1 syntectonic with d 1 2 a second phase of granulite facies metamorphism m 2 and associated charnockitization syntectonic with transposition of d 1 foliation by isoclinal folding during d 2 and 3 emplacements of granites and pegmatites associated with amphibolite facies metamorphism m 3 broadly syntectonic with two generations of isoclinal folding d 3 the oldest mafic and ultramafic rocks occur as isolated enclaves and preserve a discordant d 1 foliation marked by metamorphic minerals characteristic of the first granulite facies metamorphism mafic bodies that postdate m 1 occur as dykes that were emplaced either 1 syn m 2 2 syn m 3 or 3 post m 3 it is suggested that m 1 m 2 and m 3 took place towards the end of the neo meso and palaeoproterozoic respectively

accretion timescale and impact history of mars deduced from the isotopic systematics of martian meteorites abstract high precision sm nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on mars associated with the formation of their mantle sources these data are used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic ages determined from the sm nd isotopic systematics of individual samples are strongly dependent on the assumed nd isotopic composition of the bulk planet this assumption is problematic given differences observed between the nd isotopic composition of earth and chondritic meteorites and the fact that these materials are both commonly used to represent bulk planetary nd isotopic compositions ages determined from the slope of 146sm 142nd whole rock isochrons are not dependent on the assumed 142nd 144nd ratio of the planet but require the sample suite to be derived from complementary contemporaneously formed reservoirs in this work we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a 143nd 142nd whole rock isochron and is also independent of any a priori assumptions regarding the bulk isotopic composition of the planet this expression is also applicable to mineral isochrons and has been used to successfully calculate 143nd 142nd model crystallization ages of early refractory solids as well as lunar samples this permits ages to be obtained using only nd isotopic measurements without the need for 147sm 144nd isotope dilution determinations when used in conjunction with high precision nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 6 ma because the sm nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the solar system recent thermal models however suggest that mars sized bodies cool rapidly in less than 5 ma after accretion ceases even in the presence of a thick atmosphere assuming these models are correct an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 ma late accretional heating of mars could either be associated with protracted accretion occurring at a quasi steady state or alternatively be associated with a late giant impact if this scenario is correct then accretion of mars sized bodies takes up to 60 ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation this further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed

heterogeneous delivery of silicate and metal to the earth by large planetesimals after the moon s formation earth experienced a protracted bombardment by leftover planetesimals the mass delivered during this stage of late accretion has been estimated to be approximately 0 5 of earth s present mass based on highly siderophile element concentrations in the earth s mantle and the assumption that all highly siderophile elements delivered by impacts were retained in the mantle however late accretion may have involved mostly large 1 500 km in diameter and therefore differentiated projectiles in which highly siderophile elements were sequestered primarily in metallic cores here we present smoothed particle hydrodynamics impact simulations that show that substantial portions of a large planetesimal s core may descend to the earth s core or escape accretion entirely both outcomes reduce the delivery of highly siderophile elements to the earth s mantle and imply a late accretion mass that may be two to five times greater than previously thought further we demonstrate that projectile material can be concentrated within localized domains of earth s mantle producing both positive and negative 182w isotopic anomalies of the order of 10 to 100 ppm in this scenario some isotopic anomalies observed in terrestrial rocks can be explained as products of collisions after moon formation

phase boundary mobility in naturally deformed high grade quartzofeldspathic rocks evidence for diffusional creep abstract grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern grenville province ontario canada suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep in these rocks quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions in the naturally deformed rocks however phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation i e in directions of inferred finite extension which implies that phase boundary motion and cusp formation occurred during deformation optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep however the morphology of quartz feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone we propose that phase boundary motion resulted from a diffusion assisted process involving dissolution at foliation parallel quartz feldspar phase boundaries mass transfer over length scales of the order of feldspar domain size 200 m or greater and precipitation at quartz feldspar phase boundary cusps this study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks it also has important implications for the natural rheological behavior of the mid and lower continental crust

evolution of the martian mantle inferred from the 187re 187os isotope and highly siderophile element abundance systematics of shergottite meteorites abstract shergottite meteorites are a suite of mafic to ultramafic igneous rocks whose parental magmas probably derived from the martian mantle in this study a suite of 23 shergottites spanning their known range in bulk compositions rb sr sm nd and lu hf isotopes were measured for 187re 187os isotopic systematics and highly siderophile element abundances hse including os ir ru pt pd re the chief objective was to gain new insight on the chemical evolution of the martian mantle by unraveling the long term hse budget of its derivative melts possible effects upon hses related to crustal contamination as well as terrestrial and or martian surface alteration are also examined some of the shergottites are hot arid desert finds their respective acetic acid leachates and residues show that both re and os display open system behavior during sample residence at or near the martian and or terrestrial surfaces in some meteorites the alteration effects can be circumvented by analysis of the leached residues for those shergottites believed to record robust re os isotopic systematics calculated initial 187os 188os are well correlated with the initial 143nd 144nd shergottites from mantle sources with long term melt depleted characteristics initial e143nd of 36 to 40 have chondritic initial 187os ranging from 0 5 to 2 5 shergottites with intermediate initial e143nd of 8 to 17 have a range in initial 187os of 0 6 to 2 3 which overlaps the range for depleted shergottites shergottites from long term enriched sources with initial e143nd of 7 are characterized by suprachondritic 187os values of 5 to 15 the initial 187os variations for the shergottites do not show a correlation with indices of magmatic differentiation such as mgo or any systematic differences between hot arid desert finds antarctic finds or observed falls the strong correlation between the initial e143nd and 187os in shergottites from approximately 40 and 0 to 7 and 15 respectively is assessed in models for mixing depleted mantle derived melts with ancient crust modeled to be similar to evolved shergottite in composition and with assimilation fractional crystallization these models show that the correlation is unlikely to result from participation of martian crust more likely this correlation relates to contributions from depleted and enriched reservoirs formed in a martian magma ocean at ca 4 5 ga these models indicate that the shergottite endmember sources were generated by mixing between residual melts and cumulates that formed at variable stages during solidification of a magma ocean the expanded database for the hse abundances in shergottites suggests that their martian mantle sources have similar hse abundances to the terrestrial mantle consistent with prior studies the relatively high hse abundances in both planetary mantles likely cannot be accounted for by high pressure temperature metal silicate partitioning at the bases of magma oceans as has been suggested for earth if the hse were instead supplied by late accretion this event must have occurred prior to the crystallization of the last martian magma ocean

unique meteorite from early amazonian mars water rich basaltic breccia northwest africa 7034 we report data on the martian meteorite northwest africa nwa 7034 which shares some petrologic and geochemical characteristics with known martian meteorites of the snc i e shergottite nakhlite and chassignite group but also has some unique characteristics that would exclude it from that group nwa 7034 is a geochemically enriched crustal rock compositionally similar to basalts and average martian crust measured by recent rover and orbiter missions it formed 2 089 0 081 billion years ago during the early amazonian epoch in mars9 geologic history nwa 7034 has an order of magnitude more indigenous water than most snc meteorites with up to 6000 parts per million extraterrestrial h2o released during stepped heating it also has bulk oxygen isotope values of 17o 0 58 0 05 per mil and a heat released water oxygen isotope average value of 17o 0 330 0 011 per mil suggesting the existence of multiple oxygen reservoirs on mars

mixing in flow perturbations a model for development of mantled porphyroclasts in mylonites abstract the presence of a rigid object such as a porphyroclast in simple shear flow induces a flow perturbation with a relatively simple geometry domains of closed and open flow lines are bounded by a separatrix that surrounds the porphyroclast the concept of a separatrix to describe heterogeneous flow gives rise to some new insights in the development of porphyroclasts with a mantle of recrystallized material in mylonites the final shape of the recrystallized mantle depends on separatrix shape but also on the degree of intersection of the mantle and the separatrix and therefore on relative mantle width this model can explain the coexistence of several types of mantled porphyroclast in a single mylonite sample the occurrence of porphyroclasts that lack wings at high strain values and the presence of symmetric winged mantled porphyroclasts in ductile shear zones

tungsten isotopic evidence for disproportional late accretion to the earth and moon examination of three lunar samples reveals that the moon s mantle has an excess of the tungsten isotope 182w of about 20 parts per million relative to the present day earth s mantle this suggests that the two bodies had identical compositions immediately following the formation of the moon and that the compositions then diverged as a result of disproportional late accretion of chondritic material to the earth and moon two papers published in this issue of nature present precise measurements of tungsten isotope composition in lunar rocks that are best explained by the earth and moon having had similar composition immediately following formation of the moon and then having diverged as a result of disproportional late accretion of material to the two bodies mathieu touboul et al found small 182w excess of about 21 parts per million relative to the present day earth s mantle in metals extracted from two kreep rich apollo 16 impact melt rocks while thomas kruijer et al measured tungsten isotopes in seven kreep rich whole rock samples that span a wide range of cosmic ray exposure ages and found a 182w excess of about 27 parts per million over the present day earth s mantle characterization of the hafnium tungsten systematics 182hf decaying to 182w and emitting two electrons with a half life of 8 9 million years of the lunar mantle will enable better constraints on the timescale and processes involved in the currently accepted giant impact theory for the formation and evolution of the moon and for testing the late accretion hypothesis uniform terrestrial mantle like w isotopic compositions have been reported1 2 among crystallization products of the lunar magma ocean these observations were interpreted to reflect formation of the moon and crystallization of the lunar magma ocean after 182hf was no longer extant that is more than about 60 million years after the solar system formed here we present w isotope data for three lunar samples that are more precise by a factor of 4 than those previously reported1 2 the new data reveal that the lunar mantle has a well resolved 182w excess of 20 6 5 1 parts per million 2 standard deviations relative to the modern terrestrial mantle the offset between the mantles of the moon and the modern earth is best explained by assuming that the w isotopic compositions of the two bodies were identical immediately following formation of the moon and that they then diverged as a result of disproportional late accretion to the earth and moon3 4 one implication of this model is that metal from the core of the moon forming impactor must have efficiently stripped the earth s mantle of highly siderophile elements on its way to merge with the terrestrial core requiring a substantial but still poorly defined level of metal silicate equilibration

primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of mars five main stages of radial and concentric structures formed around tharsis from the noachian through the amazonian as determined by geologic mapping of 24 452 structures within the stratigraphic framework of mars and by testing their radial and concentric orientations tectonic activity peaked in the noachian stage 1 around the largest center claritas an elongate center extending more than 20 in latitude and defined by about half of the total grabens which are concentrated in the syria planum thaumasia and tempe terra regions during the late noachian and early hesperian stage 2 extensional structures formed along the length of present day valles marineris and in thaumasia with a secondary concentration near warrego vallis radial to a region just to the south of the central margin of valles marineris early hesperian stage 3 radial grabens in pavonis syria ulysses and tempe terra and somewhat concentric wrinkle ridges in lunae and solis plana and in thaumasia sirenum memnonia and amazonis are centered northwest of syria with secondary centers at thaumasia tempe terra ulysses fossae and western valles marineris late hesperian early amazonian stage 4 structures around alba patera the northeast trending alignment of tharsis montes and olympus mons appears centered on alba patera stage 5 structures middle late amazonian represent the last pulse of tharsis related activity and are found around the large shield volcanoes and are centered near pavonis mons tectonic activity around tharsis began in the noachian and generally decreased through geologic time to the amazonian statistically significant radial distributions of structures formed during each stage centered at different locations within the higher elevations of tharsis secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout tharsis

formation of phobos and deimos via a giant impact abstract although the two moons of mars phobos and deimos have long been thought to be captured asteroids recent observations of their compositions and orbits suggest that they may have formed from debris generated by one or more giant impacts of bodies with 0 01 target mass recent studies have both analytically estimated debris produced by giant impacts on mars and numerically examined the evolution of circum mars debris disks we perform a numerical study smoothed particle hydrodynamics simulation of debris retention from giant impacts onto mars particularly in relation to a borealis scale giant impact e 3 10 29 j capable of producing the borealis basin we find that a borealis scale impact is capable of producing a disk of mass 5 10 20 kg 1 4 of the impactor mass sufficient debris to form at least one of the martian moons according to recent numerical studies of martian debris disk evolution while a borealis scale impact may generate sufficient debris to form both phobos and deimos further studies of the debris disk evolution are necessary our results can serve as inputs for future studies of martian debris disk evolution

highly siderophile element abundances in eoarchean komatiite and basalt protoliths plume derived mg rich volcanic rocks komatiites high mg basalts and their metamorphic equivalents can record secular changes in the highly siderophile element hse abundances of mantle sources an apparent secular time dependent enrichment trend in hse abundances from paleoarchean to paleoproterozoic mantle derived rocks could represent the protracted homogenization of a late veneer chondritic contaminant into the pre late veneer komatiite source to search for a possible time dependence of a late accretion signature in the eoarchean mantle we report new data from rare 3700 myr old mafic and ultramafic schists locked in supracrustal belts from the inukjuak domain quebec canada and the akilia association west greenland our analysis shows that some of these experienced hse mobility and or include a cumulate component touboul et al in chem geol 383 63 75 2014 whereas several of the oldest samples show some of the most depleted hse abundances measured for rocks of this composition we consider these new data for the oldest documented rocks of komatiite protolith in light of the late veneer hypothesis

evidence for dominant grain boundary sliding deformation in greenschist and amphibolite grade polymineralic ultramylonites from the redbank deformed zone central australia abstract microstructural and textural investigations by scanning sem and transmission electron microscopy tem techniques have been performed on samples taken across two quartzo feldspathic mylonite zones from the redbank deformed zone central australia one has been deformed at greenschist facies gs the second at amphibolite facies am conditions with increasing strain the rock type changes from protomylonite to mylonite to ultramylonite the protomylonites and mylonites consist of alternating quartz and polymineralic quartz feldspar bands at the highest strains a homogeneous fine grained polymineralic ultramylonite occurs shear zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites tem and sem studies of the ultramylonites reveal a rectangular to square grain shape a continuous alignment of grain and interphase boundaries across several grain diameters a grain size gs 0 5 m am 5 11 m less than the equilibrium subgrain size and open and void containing grain and interphase boundaries analysis of local textures by electron back scatter diffraction ebsd in the sem showed a very weak crystallographic preferred orientation cpo for the quartz the grain misorientation relationships are not consistent with dislocation creep being the dominant deformation mechanism all structures are of the type expected if grain boundary sliding processes had contributed significantly to the deformation consequently the deformation of such quartzo feldspathic rocks and by implication the rheology of the redbank deformed zone must have been controlled by the mechanical properties of these fine grained polymineralic ultramylonites deforming by grain boundary sliding processes this is in contrast to the pure quartz bands which deformed by dislocation creep mechanisms and were less important in the rheology of the redbank deformed zone

fault related rocks suggestions for terminology many traditional terms for fault related rocks have undergone recent dynamic metamorphism under high pressure discussions by various groups of specialists a generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic engineering and legal purposes such a framework is proposed here focusing on a rate of strain versus rate of recovery diagram and relating this framework to the products of brittle and ductile deformation along faults

a post accretionary lull in large impacts on early mars the timing and number of large impact basins on early mars are poorly constrained gravity and topographic analyses support a lull in basin forming impacts following the main stage of accretion

extreme ductility of feldspars from a mylonite parry sound canada abstract single feldspar crystals in mylonites from the grenville structural province of canada have accumulated extreme strains by ductile mechanisms the samples studied are from a deep crustal shear zone and were collected at parry sound ontario the mylonites were derived from granite and leucogabbro and the feldspar crystals originated in late syntectonic pegmatite dykes optical and transmission electron microstructures of microperthitic alkali feldspars show evidence of syntectonic and synchronous dislocation climb leading to recovery si al ordering and na k interdiffusion evidence for the operation of these processes is respectively ubiquitous subgrains development and coarsening of tweed texture and transition of monoclinic to triclinic k feldspar and exsolution domain reorganization these processes occurred concurrently with extreme straining of the host crystals by dislocation and diffusion mechanisms the effects of ductile deformation preserved in these crystals are quite different from those usually observed in feldspars the unusual microstructures are thought to be due to the conditions of high temperature and confining pressure which existed during deformation in concert with a deformation enhancing point defect chemistry possibly associated with incorporation into the crystals of a species of water