ancient impactor components preserved and reworked in martian regolith breccia northwest africa 7034 abstract northwest africa nwa 7034 and paired stones represent unique samples of martian polymict regolith breccia multiple breccia subsamples characterized in this work confirm highly siderophile element hse re os ir ru pt pd contents that are consistently elevated e g os 9 3 18 4 ppb above indigenous martian igneous rocks mostly 187 os 188 os ratios of 0 119 0 136 record a wider variation than observed for all major chondrite types combined with the measured 187 re 188 os ratios of 0 154 0 994 the range in os isotope ratios indicates redistribution of re and os from originally chondritic components early in the history of the regolith commencing at 4 4 ga superimposed recent re mobility reflects exposure and weathering at or near the martian and terrestrial surfaces elevated os concentrations 38 0 and 92 6 ppb os superchondritic os hse ratios and 187 os 188 os of 0 1171 and 0 1197 measured for two subsamples of the breccia suggest the redistribution of impactor material at 1 5 1 9 ga possibly overlapping with a partial resetting event at 1 4 ga recorded by u pb isotope systematics in the breccia martian alteration of the originally chondritic hse host phases to form os ir rich nuggets and ni rich pyrite implies the influence of potentially impact driven hydrothermal systems multiple generations of impactor component admixture redistribution and alteration mark the formation and evolution of the martian regolith clasts and matrix of nwa 7034 and paired meteorites from the pre noachian until impact ejection to earth

forearc uplift and extension southern hawke s bay new zealand mid pleistocene to present the maraetotara plateau cape kidnappers area in southern hawke s bay new zealand lies in the forearc region of the obliquely convergent hikurangi margin where the pacific plate is presently subducting beneath the leading edge of the australian plate uplift accompanied by extensional faulting has been the dominant style of deformation in this area since mid pleistocene time principal strain directions and strain ratios determined from mesoscale fault arrays associated with major faults document the regional extension in the cape kidnappers area fault arrays record plane strain with northwest horizontal extension and vertical shortening in the maraetotara plateau area fault arrays record triaxial strain with roughly equal magnitudes of horizontal extension in two principal strain directions either east west and north south or northwest and northeast and vertical shortening the morphology seismicity and nature and distribution of active faults of the southern hawke s bay region suggest that the maraetotara plateau cape kidnappers area is the on land expression of an outer arc high which is developed between the accretionary wedge on the east and the forearc basin on the west the cause of uplift and extension in the maraetotara plateau cape kidnappers area may be either upper plate deformation related to structural evolution of the outer arc high and underlying backstop underplating on the hikurangi subduction zone or a combination of these processes the uplift and horizontal extension occurring in the maraetotara plateau cape kidnappers area contrast sharply with active deformation in adjacent parts of the forearc reflecting a distinct partitioning of strain in this part of the obliquely convergent plate boundary

plate tectonics and the archean earth if we accept that a critical condition for plate tectonics is the creation and maintenance of a global network of narrow boundaries separating multiple plates then to argue for plate tectonics dur

decline of giant impacts on mars by 4 48 billion years ago and an early opportunity for habitability the timing of the wane in heavy meteorite bombardment of the inner planets is debated its timing determines the onset of crustal conditions consistently below the thermal and shock pressure limits for microbiota survival and so bounds the occurrence of conditions that allow planets to be habitable here we determine this timing for mars by examining the metamorphic histories of the oldest known martian minerals 4 476 4 429 gyr old zircon and baddeleyite grains in meteorites derived from the southern highlands we use electron microscopy and atom probe tomography to show that none of these grains were exposed to the life limiting shock pressure of 78 gpa 97 of the grains exhibit weak to no shock metamorphic features and no thermal overprints from shock induced melting by contrast about 80 of the studied grains from bombarded crust on earth and the moon show such features the giant impact proposed to have created mars hemispheric dichotomy must therefore have taken place more than 4 48 gyr ago with no later cataclysmic bombardments considering thermal habitability models we conclude that portions of mars crust reached habitable pressures and temperatures by 4 2 gyr ago the onset of the martian wet period about 0 5 gyr earlier than the earliest known record of life on earth early abiogenesis by 4 2 gyr ago is now tenable for both planets the oldest known minerals from mars have no strong shock features indicating early cessation of giant impacts there according to microanalysis of zircon and baddeleyite grains in meteorites

displacement features associated with fault zones a comparison between observed examples and experimental models abstract fault zones commonly consist of discontinuous surfaces or are composed of different kinds of en echelon fractures tension cracks r or p type fractures which give the brittle shear zone a width translation along these faults and related features can occur by sliding on the elementary planes and or by solution deposition processes with opening of transtension zones and possible dilatation of the shear zone in order to understand how fault planes develop and to investigate the mechanical conditions corresponding to natural configurations displacement along an analogue fault model was examined under conditions of direct shear the experimental fracture patterns were then compared with the natural features the structural elements of certain faults created by testing and showing transpression and transtension zones were found to be similar to natural domino structures bounded by elementary shears and could be compared with computed situations it can be inferred that stresses are reoriented inside the shear zone the angle between elementary fractures depends on their order of development transpression and transtension zones occur systematically and the shear zone undergoes dilatancy under low normal stresses

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

structural history of high pressure metamorphic rocks in the southern vanoise massif french alps and their relation to alpine tectonic events abstract a nappe of amphibolite facies metamorphic rocks of pre permian age in the southern vanoise massif the arpont schist has been affected by an alpine hp lt metamorphism the first mesoscopically recognizable deformation d 1 post dated the high pressure peak jadeitic pyroxene quartz glaucophane lawsonite and was associated with glaucophane epidote d 1 produced a flat lying schistosity and a nw trending glaucophane lineation and was probably associated with nappe displacement involving nw directed subhorizontal shear d 2 formed small scale folds and a foliation associated with chlorite albite the changing parageneses during the period pre d 1 to d 1 to d 2 suggest decreasing pressure so that the deformation appears to have been related to the uplift history rather than to the process of tectonic burial d 2 was followed by a static metamorphism green biotite chlorite albite possibly of lepontine age se directed backthrusting and folding d 3 and later differential uplift along steep faults took place under low grade conditions

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

slickenside kinematic indicators abstract a new classification of slickenside kinematic indicators is presented based on 61 criteria these slickensides have been subdivided into eleven major groups v or crescentic markings steps fractures trains of inclined planar structures trailed material asymmetric elevations deformed elements mineralogical crystallographic orientations asymmetric plan view features asymmetric cavities and asymmetric folds this classification constitutes a useful tool for geologists interested in the determination of the shear sense in fault surfaces bearing slickensides examples of application of this classification to natural fault surfaces at different scales are presented

fold interference pattern at the top of basement domes and apparent vertical extrusion of hp rocks ambin and south vanoise massifs western alps the ambin and south vanoise brianconnais basement domes located within the hp metamorphic zone were chosen for a reappraisal of the early tectonic evolution of the internal alps in their western segment finite strain analyses have shown that hp rocks were exhumed in two main successive stages more or less expressed according to their structural position in the massifs a partitioning of deformation from core to rim of the domes is thus evidenced the d1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds in the core of the ambin dome to a non coaxial regime with dominant horizontal shortening and n to nw movement direction the d2 stage well expressed in the upper part of the domes took place under low blueschist facies conditions and culminated under greenschist facies conditions it developed a retrogressive foliation and pervasive e verging shear zones at all scales that locally define major 2 tectonic contacts the d2 shearing is responsible for the development of spectacular f1 f2 fold interference structures recognised in the western part of the south vanoise dome in this area we interpret the steeply dipping hp fabric s1 defining the axial planes of the first generation of now upright folds f1 as linked to an early flat lying nappe edifice 1 subsequently steepened by the d2 shearing geometry and kinematic characteristics for the early d1 nappe forming event are strongly consistent with most evolutionary models assuming that oceanic to european and apulian crust derived nappes were initially exhumed during n to nw verging overthrusting events synthetic to the subduction they tend also to suggest that the penninic hp units may have suffered an early exhumation by low angle thrusting rather than by vertical extrusion

the western alps from the jurassic to oligocene spatio temporal constraints and evolutionary reconstructions abstract despite extensive research in the last 150 years the regional tectonic reconstruction of the western alps has remained controversial the curved orogenic belt consists of several ribbon like continental terranes sesia austroalpine internal crystalline massifs brianconnais which are separated by two or more ophiolitic sutures piemonte valais antrona lanzo canavese high pressure hp metamorphism of each terrane occurred during distinct orogenic episodes at 65 ma in the sesia austroalpine at 45 ma in the piemonte zone and at 35 ma in the internal crystalline massifs it is suggested that these events reflect individual accretionary episodes which together with kinematic indicators and the speed and direction of plate motions provide constraints for the discussed reconstruction model the model involves a prolonged orogenic history that took place during relative convergence of europe and adria here considered as a promontory of the african plate the first accretionary event involved the sesia austroalpine terrane final closure of the piemonte ocean occurred during the eocene 45 ma and involved ultra high pressure uhp metamorphism of the piemonte oceanic crust incorporation of the brianconnais terrane in the accretionary wedge occurred thereafter possibly during or after subduction of the valais ocean in the late eocene 45 35 ma this subduction was terminated at ca 35 ma when the internal crystalline massifs i e the assumed internal parts of the brianconnais terrane were buried into great depths and underwent hp and uhp metamorphism

opposite vergence of nappes and crustal extension in the french italian western alps in the internal pennine zone of the french italian western alps dora maira monviso rocciavre massifs and queyras region the structural trends define an apparent dome geometry that is truncated to the east by the po basin this geometry is classically interpreted as a backthrusting system the kinematic history of the area has been investigated using structural metamorphic and geochronologic data on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions 1 the first deformation occurred under greenschist facies metamorphism it caused a regional scale foliation and an ew trending lineation that lie parallel to ductile deformation zones and thrust contacts sense of shear indicators associated with the lineation indicate contradictory top to the east and top to the west senses of shear from these it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes phengite ar ar data yield late eocene early oligocene ages for the greenschist facies metamorphism 2 the second deformation caused the widespread development of high angle shear bands and normal faults on opposite sides of the dora maira massif resulting in the apparent dome geometry of the massif a steep 50 80 network of west dipping normal faults is particularly well developed on the west side of the dora maira massif differences of 2 3 kbar and 100 150 c between juxtaposed monviso rocciavre and queyras nappes implies 8 km of vertical displacement along the monviso upper tectonic contact these normal faults formed in a more brittle regime only a post early oligocene age can be estimated from microstructural studies for these final movements these deformations postdate hp lt metamorphism and resulted in the juxtaposition of nappes of distinctly different origin age grade and p t t history a kinematic model is presented in which opposite vergence of nappes followed by large scale crustal extension occurred in response to uplift during the convergence between the european and south alpine plates this model may account for rapid uplift rapid unroofing and cooling of high pressure low temperature rocks thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the alps that may have led to the preservation of coesite bearing rocks in the southern part of the dora maira massif

onset of solid state mantle convection and mixing during magma ocean solidification energy sources involved in the early stages of planetary formation can cause partial or even complete melting of the mantle of terrestrial bodies leading to the formation of magma oceans upon planetary cooling solidification is expected to take place from the bottom upwards because of the steeper slope of the liquid adiabat with respect to the liquidus elkins tanton 2012 solomatov 2015 fractional solidification in particular can lead to the formation of a compositional layering that can play a fundamental role for the subsequent long term dynamics and evolution of the interior tosi et al 2013 plesa et al 2014 in order to assess to what extent primordial compositional heterogeneities generated upon magma ocean solidification can be preserved we investigate the cooling and solidification of a whole mantle magma ocean along with the conditions that allow solid state convection to start mixing the mantle before solidification has completed to this end we run 2 d numerical simulations in cylindrical geometry using the finite volume code gaia huttig et al 2013 we treat the liquid magma ocean in a parametrized fashion while we self consistently solve the conservation equations of thermochemical convection in the growing solid mantle accounting for pressure temperature and melt dependent rheology we consider two end member cases fractional crystallization where melt is instantaneously extracted into the overlying liquid leaving beneath a differentiated mantle and batch crystallization where melt remains in contact with the silicate matrix throughout solidification causing no differentiation by testing the effects of different cooling rates and rayleigh numbers we show that for a lifetime of the liquid magma ocean between 1 and 10 myr lebrun et al 2013 the onset of solid state convection prior to complete mantle crystallization is possible and that part or even all of the compositional heterogeneities generated upon fractionation can be erased by efficient mantle stirring figure 1 we discuss the consequences of our findings in relation to the early and long term evolution of compositional heterogeneities generated via fractional crystallization of magma oceans in terrestrial bodies with emphasis on mars thermochemical history

explosive volcanism in the tharsis region global evidence in the martian geologic record 1 a global equatorial set of layered deposits on mars has been reexamined with mars global surveyor data the stratigraphy morphology and erosional characteristics of units separated by thousands of kilometers are remarkably similar to each other and consistent with a widespread episodic subaerial source of fine grained material this study is the first to propose a genetic link between many units that have traditionally been considered on a local or regional level including the medusae fossae formation valles marineris interior deposits chaotic terrain terra meridiani and arabia materials mapping relationships suggest that all of these units are post noachian in age and formation may have continued until the recent past thinning of deposits with increasing distance from the tharsis rise suggests that much of these materials may be volcanic ash flows and air fall from explosive eruptions in the tharsis region analogous to those produced by terrestrial plinian eruptions contemporary martian winds would preferentially transport ash from an eruption plume near tharsis east or west along the equator depending on the season erosion has removed much of the original volcanic ash and only isolated layered stacks remain sometimes in topographic depressions that inhibit erosion modeling indicates that widespread dispersion of ash after an eruption is easily attainable in present or past atmospheric conditions consistent with the hypothesis that the far reaching deposits may be composed of ash from past explosive eruptions in the vicinity of tharsis

tectonic analysis and the evolution of a curved collision belt the hsuehshan range northern taiwan abstract detailed studies of fracture patterns and paleostress reconstructions based on analyses of fault slip data sets allow identification of four main patterns of compressional paleostress orientations in the cenozoic rocks of northern taiwan these patterns correspond to five successive tectonic events dominated by 1 n s 2 wsw ene 3 wnw ese 4 nw se and 5 wnw ese directions of compression the first two events are characterized by relatively minor structures in the foreland fold and thrust belt the first event probably reflects major left lateral shear along nne ssw trends which occurred in the adjacent eastern metamorphic belt during the middle late miocene these first two events do not affect the plio pleistocene in the localities studied in contrast the last three events created the major compressional structures of the foreland belt during the plio pleistocene collision in taiwan preliminary studies suggest that the magnetic fabric of the rocks has been significantly influenced by compressional deformation along wnw ese and nw se trends especially in the slate belt thus confirming the importance of these last three events on the basis of paleomagnetic work in progress the northernmost segment of the taiwan belt has been affected by a clockwise rotation of about 20 taking into account the distribution of successive compressional paleostress patterns we conclude that of the 55 angle of the sharply curved belt of northern taiwan about 20 probably correspond to actual clockwise rotation during the plio pleistocene whereas the remaining 35 reflect the initial shape of the curved plio quaternary belt the paleostress history of northern taiwan is examined in terms of geodynamic evolution and kinematic reconstructions the plio quaternary development of the collision between the luzon arc and eurasia followed a late miocene predominantly transform motion from south to north of the philippine sea plate relative to eurasia the succession of plio pleistocene paleostress patterns in northern taiwan is found to be consistent with the longitudinal shift from northeast to southwest and the transverse migration from the inner to the outer zones of compressional phenomena related to the evolution of oblique collision some major changes in compressional trends are also accounted for by variations in the kinematics of plate convergence the most recent charge comprises a counterclockwise change in the direction of philippine sea plate eurasia convergence

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

inclined layer silicate microstructures and their use as a sense of slip indicator for brittle fault zones abstract this short note introduces a new microscale sense of slip indicator for brittle fault zones the microstructure is developed within laminated quartz veins and consists of fault parallel bands of layer silicate minerals with their long axes and 001 planes consistently inclined in the transport direction like other microscale sense of slip indicators this microstructure may prove most useful in situations where mesoscale criteria are either lacking or poorly developed because of its intimate association with fault zone vein material this microstructure may also help in deciphering the motion history of reactivated faults

the internal structure and geodynamics of mars inferred from a 4 2 gyr zircon record combining u pb ages with lu hf data in zircon provides insights into the magmatic history of rocky planets the northwest africa nwa 7034 7533 meteorites are samples of the southern highlands of mars containing zircon with ages as old as 4476 3 0 9 ma interpreted to reflect reworking of the primordial martian crust by impacts we extracted a statistically significant zircon population n 57 from nwa 7533 that defines a temporal record spanning 4 2 gyr ancient zircons record ages from 4485 5 2 2 ma to 4331 0 1 4 ma defining a bimodal distribution with groupings at 4474 10 ma and 4442 17 ma we interpret these to represent intense bombardment episodes at the planet s surface possibly triggered by the early migration of gas giant planets the unradiogenic initial hf isotope composition of these zircons establishes that mars s igneous activity prior to 4 3 ga was limited to impact related reworking of a chemically enriched primordial crust a group of younger detrital zircons record ages from 1548 0 8 8 ma to 299 5 0 6 ma the only plausible sources for these grains are the temporally associated elysium and tharsis volcanic provinces that are the expressions of deep seated mantle plumes the chondritic like hf isotope compositions of these zircons require the existence of a primitive and convecting mantle reservoir indicating that mars has been in a stagnant lid tectonic regime for most of its history our results imply that zircon is ubiquitous on the martian surface providing a faithful record of the planet s magmatic history

the building of the central swiss alps an experimental approach abstract a kinematic model explaining the building of the central swiss alps is proposed from a review of various kind of data including structural geophysical petrological and geochronological data the eo alpine event is taken into account in order to determine the thermo mechanical evolution from the cretaceous to the late tertiary the stacking process of crustal wedges which took place during cretaceous time has led to the formation of a thickened crust with a very low geothermal gradient high pressure low temperature metamorphism this period was followed by a lull in tectonic activity during which thermal relaxation took place the major tertiary deformation is thought to result from horizontal compression of a thickened crust having a geothermal gradient higher than the average horizontal surface forces are due to the adriatic sub plate acting as a rigid indenter the model explaining the present day structure of the swiss alps emphasizes vertical extrusion and horizontal intrusion of the deepest highly ductile basement units several scale model experiments have been carried out in order to test this basic kinematic idea they display quite similar structures to those identified for the swiss alps striking features of the swiss alps such as their metamorphic and uplift history are discussed and compared with the timing of the tectonic evolution we conclude that the general process of vertical extrusion and horizontal intrusion can possibly explain the overall structure of the central swiss alps external crystalline massifs and the penninic zone

geologic history of martian regolith breccia northwest africa 7034 evidence for hydrothermal activity and lithologic diversity in the martian crust the timing and mode of deposition for martian regolith breccia northwest africa nwa 7034 were determined by combining petrography shape analysis and thermochronology nwa 7034 is composed of igneous impact and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact volcanic glass the brecciated clasts are likely lithified portions of martian regolith with some evidence of past hydrothermal activity represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4 4 ga one ancient zircon was hosted by an alkali rich basalt clast confirming that alkalic volcanism occurred on mars very early nwa 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water the clast size distribution is similar to terrestrial pyroclastic deposits we infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption s and or impact event s although the ancient ages of igneous components favor mobilization by impact s despite ancient components the breccia has undergone a single pervasive thermal event at 500 800 c evident by groundmass texture and concordance of 1 5 ga dates for bulk rock k ar u pb in apatite and u pb in metamict zircons the 1 5 ga age is likely a thermal event that coincides with rainout breccia lithification we infer that the episodic process of regolith lithification dominated sedimentary processes during the amazonian epoch the absence of pre amazonian high temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification

martian mantle mineralogy investigated by the 176lu 176hf and 147sm 143nd systematics of shergottites abstract chemical heterogeneities in the martian mantle are believed to result from the crystallization of a magma ocean in the first 100 million years of its history shergottite meteorites from mars are thought to retain a compositional record of such early differentiation and the resulting mineralogy at different depths the coupled 176 lu 176 hf and 147 sm 143 nd isotope systematics in 9 shergottites are used here to investigate these issues three compositional groups in the shergottites display distinct isotope systematics one group commonly termed as depleted is characterized by positive 176 hf i from 46 2 to 50 4 and 143 nd i from 36 2 to 39 1 another termed as enriched has negative 176 hf i 16 5 to 13 2 and 143 nd i 7 0 to 6 5 the third group is intermediate between the depleted and enriched groups with positive 176 hf i 30 0 to 33 4 and 143 nd i 16 9 together they describe mixing curves between 176 hf 177 hf 143 nd 144 nd lu hf and sm nd implying that they sample two distinct sources in the martian mantle all shergottites are characterized by sm nd source sample but lu hf source lu hf sample this decoupling can be explained by two successive partial melting episodes in the depleted shergottite source and localized in the martian upper mantle the genesis of shergottites can be modeled using non modal equilibrium partial melting in a source initially composed of 60 olivine 21 clinopyroxene 9 orthopyroxene and 10 garnet with degrees of partial melting of 8 8 and 3 9 respectively for the two successive events the enriched end member of the shergottite mixing curve is best modeled by late stage quenched residual melt resulting from the crystallization of a magma ocean the depleted shergottite source may be modeled as a mixture of cumulates and residual melt as convection in the martian magma ocean is expected to reduce the incompatible trace element heterogeneity in the final solidified layers consequently equilibrium crystallization is preferred to model the crystallization of the martian magma ocean the models that best explain the shergottite data are those where the magma ocean is at a depth of at least 1350 km in mars

insights into the martian mantle the age and isotopics of the meteorite fall tissint the recent witnessed fall of the meteorite tissint represents the delivery of a pristine new sample from the surface of mars this meteorite provides an unprecedented opportunity to study a variety of aspects about the planet s evolution using the rb sr and sm nd isotopic systems we determined that tissint a depleted shergottite has a crystallization age of 574 20 ma an initial e 143 nd 42 2 0 5 and an initial 87 sr 86 sr 0 700760 11 these initial nd and sr isotopic compositions suggest that tissint originated from a mantle source on mars that is distinct from the source reservoirs of the other martian meteorites the known crystallization ages geochemical characteristics ejection ages and ejection dynamics of tissint and other similarly grouped martian meteorites suggest that they are likely derived from a source crater up to approximately 90 km in diameter with an age of approximately 1 ma that is located on terrain that is approximately 600 million years old

petrological constraints on the eclogite and blueschistfacies metamorphism and p t t paths in the western alps phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high p rocks in the nappes of the western alps very high p eclogite facies including kyanite eclogites and coesite pyrope assemblages eclogite facies paragonite zoisite eclogites high t blueschist facies glaucophane garnet lawsonite assemblages and low t blueschist facies glaucophane lawsonite pumpellyite assemblages the blueschist facies eclogite facies transition is promoted chiefly by increasing t low bulk xmg and relatively low h2o the variety of assemblages and the heterogeneous approach to equilibrium observed in the alpine rocks are not only constrained by the intersection of the reaction surfaces in p t x space but also by the effect and timing of the processes which control kinetics i e pervasive deformation and fluid infiltration the faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures and times all may have induced h2o gradients and contributed to the heterogeneous distribution of deformation through a process of reaction enhanced ductility also mass transfer may have been an effective process in determining the type of high p assemblage in particular rock volumes as regards the p t t paths only the post climax histories are recorded well in the alpine nappes post eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early alpine cretaceous event in contrast more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early alpine event and the subsequent meso alpine palaeogene collision between the european and african plates in the western alps prevalent eclogite facies conditions were attained during the metamorphic climax of the early alpine subduction while blueschist facies recrystallization characterizes the early alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso alpine continental collision

transition from basement to cover a geometric model abstract a new simple geometric model is proposed that explains the possible relationships between superficial thrust sheets and deeper fold nappes it demonstrates that the simultaneous formation of these two types of structures at different levels of the crust is kinematically and geometrically possible the model is based on the difference of rheological behaviour between a lower tectonic unit that will suffer mainly a ductile deformation along a shallow dipping shear zone and an upper and rigid tectonic unit that will deform mostly by bedding parallel slip at constant bed length the difference in rheologic behaviour implies that the horizontal shortening that can be produced within the two units is unequal and therefore leads to a relative movement between them this movement takes place within a detachment level of variable thickness the most efficient layers to enable detachment will consist of ductile sediments like evaporites shales or flysch type sediments recent geophysical data have confirmed the view that compressional mountain belts are mainly the result of continental lithosphere subduction the deformation is essentially localized in the upper part of the down going continental crust this fundamental basally driven mechanism constrains the geometry and kinematics of the proposed model examples from the alps show that it can be applied to many cases where cover thrust sheets are directly related to basement fold nappes

stratal extension in thrust footwalls makran accretionary prism implications for thrust tectonics small scale stratal extension is commonly associated with thrust faults in the makran accretionary prism of southwest pakistan low angle normal faults dipping in the direction of transport relative to the thrust are common in gouge zones and duplexes and may also cut the footwall and hanging wall where they cause stratal extension these faults are probably riedel shears we have studied a broad zone of more complex extensional deformation beneath one bedding parallel thrust the zone disrupts earlier compressional structures and affects a significant volume of the footwall normal faults initiating in a rearward sequence branch off the thrust and cut down into the footwall merging at a lower stratigraphic level part of the thrust displacement was thereby transfe red to that level and fragments of the footwall were detached and transferred southward beneath the hanging wall the latter remained undeformed and accommodation structures are confined to the footwall these effects may have been caused by variations in sliding resistance along the fault if a thrust transfers part or all of its displacement down section along normal faults in this way before eventually cutting back up toward the surface it will detach a rootless fragment of footwall rocks from a position forward of the initial ramp through that horizon this process can be referred to as footwall plucking it can explain rootless basement slices along thrusts for example and it has important implications for section restoration techniques and the interpretation of thrust belts

ancient geologic events on mars revealed by zircons and apatites from the martian regolith breccia nwa 7034

dating deformation using rb sr in white mica greenschist facies deformation ages from the entrelor shear zone italian alps ages of deformation have been obtained by rb sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation since white micas commonly define deformation fabrics in medium grade metamorphic rocks these ages can be directly related to structural geometries with regional context this direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust it does require that the dated minerals attained isotopic equilibrium with the dominant sr reservoir at temperatures lower than the closure temperature this resetting was apparently achieved during dynamic recrystallization of white micas in greenschist facies metasediments and metagranitoid units in the western alps the results suggest that the sr isotopic composition of the new mica is buffered by the coexisting high sr phases calcite feldspar or epidote via the grain boundary network high strain rocks from the entrelor shear zone system of the western alps have yielded indistinguishable white mica rb sr ages along 30 km of individual and kinematically linked shear zones the age of the back thrusting event is constrained at 34 1ma the age yielded by the younger generation of synkinematically crystallized white micas this event was short lived involving at least 20 km of shortening in 1 m y or less an earlier variably overprinted component dating from 38 to 37 ma has been identified in the mica fabric but its kinematic significance is uncertain this method of dating strain fabrics offers a powerful tool for tectonic studies since isotopic resetting can be directly linked to structural geometries microstructural textures and pt conditions it allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust

the relationship between martian gravity and topography the relationship between gravity and topography of various regions of mars is used to estimate their effective elastic thicknesses te using direct measurements of line of sight velocity rather than spherical harmonic coefficients estimates of te vary from 70 km for tharsis 29 km for elysium to 14 5 km for the southern hemisphere and show that the thickness of the martian lithosphere increases with age as the radioactive isotopes of k th and u decay a simple parameterised model of the convective thermal history is used to estimate the temperature structure of the lithosphere and shows that the base of the elastic layer has a temperature of 300 50 c or similar to the value for terrestrial continents in both cases the rheology is probably affected by the presence of water the short wavelength behaviour of the gravity field allows the density of the rocks that form the topography to be estimated and gives values of about 3 0 mg m3 for tharsis and elysium this value is substantially greater than that of 2 7 mg m3 obtained for earth and is in agreement with estimates from snc shergottites nahklites chassigny meteorites of 3 3 mg m3 the density of the topography of valles marineris is only 2 35 mg m3 and suggests that ice may be present below the surface in the heavily bombarded southern hemisphere isostatic compensation occurs at wavelengths as short as 700 km which requires the effective compensation depth to be no more than 10 km the gravity field with wavelengths greater than 1500 km may be supported dynamically by a plume rising beneath the tharsis region the difference in temperature between the solidus and the present areotherm is less than 250 c so melt generation can occur in rising plumes

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

record of the ancient martian hydrosphere and atmosphere preserved in zircon from a martian meteorite how the atmosphere hydrosphere and surface materials interacted on early mars is poorly understood oxygen isotopic composition of zircon grains in a martian meteorite reveals a prolonged history of exchange between martian regolith and atmosphere

the state of stress in an overriding plate situated above a flat slab the andes of central peru in the andes of central peru that are situated above a flat dipping slab analysis of both structural data collected in the field and available focal mechanisms show that the quaternary and recent state of stress is characterized by n s trending extension in the western cordillera and along the pacific coast whereas e w trending compression prevails in the sub andean lowlands and at the contact between the nazca and south american plates the eastern cordillera deformations suggest an intermediate state of stress i e strike slip faulting characterized by e w trending compression and n s trending extension thus major fault zones striking nw se i e parallel to the central peruvian andes are normal with a sinistral component in the western cordillera e g cordillera bianca and reverse sinistral in the eastern cordillera e g cordillera de huaytapallana this state of stress displays similarities with that of southern peru and may be also interpreted as an effect of compensated high topography in this model the vertical stress zz increases with the topography and hmax is considered as fairly constant trending e w roughly parallel to the convergence on both edges of the andes tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then hmax is 2 allowing hmin striking n s to be 3 along the pacific coast tectonics is not compressional as expected and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep peru chile trench however a conspicuous difference occurs in the andes of central peru in respect to the southern peru andes compressional strike slip tectonics is observed in the eastern cordillera indeed the mean elevation of the eastern cordillera is 3 7 km whereas that of the western cordillera is 4 2 km allowing an intermediate state of stress to occur therefore in central peru a lower mean andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern peru superficial deformations related to the subduction of the aseismic nazca ridge are restricted to the coastal domain opposite to this ridge they support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting

regolith breccia northwest africa 7533 mineralogy and petrology with implications for early mars northwest africa 7533 a polymict martian breccia consists of fine grained clast laden melt particles and microcrystalline matrix while both melt and matrix contain medium grained noritic monzonitic material and crystal clasts the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars microbasaltic clasts vitrophyric and microcrystalline spherules and shards the clast laden melt rocks contain clump like aggregates of orthopyroxene surrounded by aureoles of plagioclase some shards of vesicular melt rocks resemble the pyroxene plagioclase clump aureole structures submicron size matrix grains show some triple junctions but most are irregular with high intergranular porosity the noritic monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths and cooled more slowly than rocks with zoned pyroxene or fine grain size noritic material contains orthopyroxene or inverted pigeonite augite calcic to intermediate plagioclase and chromite to cr bearing magnetite monzonitic clasts contain augite sodic plagioclase k feldspar ti bearing magnetite ilmenite chlorapatite and zircon these feldspathic rocks show similarities to some rocks at gale crater like black trout mara and jake m the most magnesian orthopyroxene clasts are close to alh 84001 orthopyroxene in composition all these materials are enriched in siderophile elements indicating impact melting and incorporation of a projectile component except for ni poor pyroxene clasts which are from pristine rocks clast laden melt rocks spherules shards and siderophile element contents indicate formation of nwa 7533 as a regolith breccia the zircons mainly derived from monzonitic melt rocks crystallized at 4 43 0 03 ga humayun et al 2013 and a 147sm 143nd isochron for nwa 7034 yielding 4 42 0 07 ga nyquist et al 2016 defines the crystallization age of all its igneous portions the zircon from the monzonitic rocks has a higher 17o than other martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration nemchin et al 2014 this record of protolith interaction with atmosphere hydrosphere during regolith formation before melting demonstrates a thin atmosphere a wet early surface environment on mars and an evolved crust likely to have contaminated younger extrusive rocks the latest events recorded when the breccia was on mars are resetting of apatite much feldspar and some zircons at 1 35 1 4 ga bellucci et al 2015 and formation of ni bearing pyrite veins during or shortly after this disturbance lorand et al 2015

early martian mantle overturn inferred from isotopic composition of nakhlite meteorites following the crystallization of a magma ocean the martian mantle probably underwent an overturning event but its initiation timing and geochemical consequences are poorly constrained isotopic data for martian meteorites and numerical simulations provide strong evidence for early overturning in the martian mantle