geometric and kinematic models for detachment folds with fixed and variable detachment depths abstract detachment folds are defined by competent rock units and are cored by incompetent units deformed internally above a detachment horizon we have developed two geometric models to constrain possible geometries and kinematic paths for ideal detachment folds the models each independently relate fold geometry to shortening and to detachment depth model assumptions include plane strain constant competent bedlength constant cross sectional area chevron fold geometry and no bed parallel shear outside the fold detachment depth is constant in one model but may vary in the other thus allowing evaluation of the implications for fold geometry and kinematics of fixed vs variable detachment depth detachment folds formed above a detachment unit of constant thickness constant detachment depth must be initially symmetrical and cannot grow with fixed hinges fixed arc length or a self similar geometry detachment folds formed above a detachment unit of variable thickness variable detachment depth must also be initially symmetrical but any one fold geometry can have a range of possible initial and final detachment depths kinematic paths for folds with fixed hinges fixed arc lengths migrating hinges variable arc lengths and selfsimilar geometries are all possible if detachment depth varies the change in detachment depth during deformation can be determined using the variable detachment depth model if either initial or final detachment depth is known the models demonstrate a wider range of variability in the geometry and kinematics of ideal detachment folds particularly for the variable depth model than is the case for ideal fault bend and fault propagation folds this variability limits the usefulness of simple geometric models for reconstructing the geometry of natural detachment folds balancing cross sections over a sufficient area and evaluating strain may compensate for these limitations

structural tests of diapir hypotheses in archean crust of ontario detailed structural maps of two granitoid complexes in the wabigoon subprovince are used to test three diapir hypotheses advanced in earlier papers the gneiss masses of and individual domes within the complexes fail the test for solid state diapirism the gneiss domes also fail the test for tensile bending caused by hypothetical magmatic diapirs in the subsurface an oval pluton located near the best exposed gneiss dome proves to be a synformal sheet rather than a funnel shaped magmatic diapir this pluton could be a syenite diorite phacolith emplaced into a concordant zone of dilation during the late upright folding of the gneiss mass about horizontal axes earlier tight folds were probably recumbent and south verging and like the gneissosity generated in a ductile shear regime with subhorizontal glide planes these observations have important implications for archean tectonics especially the relative horizontal displacement of large greenstone masses potentially allochthonous greenstone belts

impact induced frictional melting in ordinary chondrites a mechanism for deformation darkening and vein formation high speed friction experiments have been performed on the ordinary chondrites el hammami h5 s2 and sahara 97001 l6 s3 using an axial friction welding apparatus each sample was subjected to a strain rate of 103 to 104 s 1 which generated 250 to 500 m deep darkened zones on each sample cube thin section analyses reveal that the darkened areas are composed of silicate glass and mineral fragments intermingled with dispersed submicron size feni and fes blebs fracturing of mineral grains and the formation of tiny metallic veins define the extent of deformation beyond the darkened shear zone these features are not present in the original meteorites the shear zones and tiny veins are quite similar to certain vein systems seen in naturally deformed ordinary chondrites the experiments show that shock deformation is not required for the formation of melt veins and darkening in ordinary chondrites therefore the presence of melt veins and darkening does not imply that an ordinary chondrite has undergone severe shock deformation in fact high strain rate deformation and frictional melting are especially important for the formation of veins at low shock pressures

calcite twin widths and intensities as metamorphic indicators in natural low temperature deformation of limestone abstract twins in naturally deformed coarse grained limestones of the northern subalpine chain france show a direct relationship of twin width and intensity number of twins per mm to the peak metamorphism under diagenetic and anchimetamorphic conditions the transition from dominant thin twinning to dominant thick twinning occurs between a mean vitrinite reflectance ro of 1 60 and 2 60 or temperatures of between 150 and 200 c below the transition temperature increasing strain is apparently accommodated by adding new twins and above the transition temperature increasing strain is accommodated by twin widening similar transitions from thin to thick twinning with increasing metamorphism are also present in the helvetic alps the prealps and the central appalachian valley and ridge province these data for naturally deformed limestones support the interpretation from experimental studies of a temperature effect on twin width and intensity and indicate that the twinning in the northern subalpine chain occurred at or near peak metamorphism

stress tensor determination from the study of e twins in calcite a linear programming method abstract a new method for the determination of the stress tensor from calcite e twin lamellae has been discovered all e planes twinned and untwinned are taken into account to enable derivation of tensor characteristics the basic assumption is that the resolved shear stress a linear function of the unknown stress tensor components is greater respectively lower than the critical resolved shear stress for the twinned respectively untwinned planes the relevant inequality constraints can be solved using linear programming algorithms the application to experimental carrara marble and natural samples a goniatite from morocco has come up against one major difficulty polyphase deformation thus before applying this method it is necessary to operate some selection of the data one procedure based upon linear programming is proposed the results are consistent with those expected both for the experimental and natural cases for the carrara marble an estimate of the critical resolved shear stress t c for twinning gave a value around 300 400 bar the method can give in principle the stress tensor orientation and the relative magnitude of principal stresses simultaneously

magnetic susceptibility anisotropy strain and progressive deformation in permian sediments from the maritime alps france abstract the relationships among magnetic susceptibility anisotropy finite strain and progressive deformation have been studied in permian red shales and slates of the maritime alps southeastern france these rocks contain deformed reduction spots which serve as finite strain indicators the magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements the magnetic fabric changes progressively from oblate to prolate and back to oblate as deformation increases the quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined they differ between the less and more deformed areas perhaps due to a change in deformation mechanism accompanying an increase in metamorphism the relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

mid infrared spectroscopy of refractory inclusions cais in cv and co chondrites we present laboratory mid infrared absorption spectra 2 5 m to 16 0 m of powdered calcium aluminum rich inclusions cais and matrix separated from the carbonaceous chondrites allende cv3 2 vigarano cv3 3 and ornans co3 3 two groups of spectra with different features were found for the cai in the first group spectra are dominated by spinel pyroxene and sodalite nepheline where main features occur at 9 3 m 10 3 m and 11 3 m in the second group characteristic minerals are spinel and melilite with typical band maxima at 11 0 m and 12 3 m and a broad feature between 14 0 m and 15 0 m the position of the broad spinel feature probably depends on its iron content comparison of band positions in spectra from the cai components to observed circumstellar emission spectra indicates the potential occurrence of cai like material pyroxene and spinel rich features could occur in spectra of dust around the herbig ae star hd104237 the t tauri star hen3 600 and the post agb star r sge melilite and spinel rich components possibly appear in the spectrum of hd104237 hen3 600 04187 1927 r sge and the planetary nebula hb 12 there is also indication for a spinel component in dust from the herbig ae be star hd179218 the spectra of the agb stars r cas and aps show no features of cai type spinel

kinematic analysis of a large scale leading edge fold lost river range idaho abstract the mahogany creek buck creek structure within the lost river range idaho is a large scale leading edge fold which developed in late paleozoic outer shelf stratigraphy during the sevier orogeny incremental strain histories determined from antitaxial fibrous pressure shadows are used to quantify temporal variations in the magnitude and orientation of elongation as a function of structural position around the leading edge of this northward and eastward propagating blind thrust plane strain is indicated by coaxial up dip extension on cleavage planes on the backlimb of the structure the bluebird mountain formation a calcareous sandstone has deformed by flexural flow whereas the forelimb of the structure records spin through a fixed steeply plunging incremental extension direction the flat limb ahead of the anticline exhibits top to the foreland simple shear in contrast the underlying surrett canyon formation a thick bedded limestone has experienced flexural flow localized within thin zones on both limbs of the anticline samples from the anticlinal hinge zone suggests kinematic partitioning with pin lines which were variably distributed within each unit and which were temporally transient finite elongations are greatest in the hinge and above the foreland flat in the bluebird mountain formation but are negligible in the surrett canyon formation except within the mechanically active interbeds of the hanging wall and within the footwall adjacent to the fault along the northernmost exposures strain histories in the hanging wall do not vary significantly along strike while the footwall varies from undeformed in the south to penetratively deformed in the north thus the strain data are consistent with self similar along strike fold development in the hanging wall during strike parallel ramp propagation forward thrust propagation may have been blunted by a combination of fold accommodated shortening and footwall deformation to the north once the fold had tightened and fold shortening required greater differential stress forward fault propagation resumed transporting the anticline onto the upper flat with little change in hanging wall fold geometry or strain distribution

the influence of grain size and grain size distribution on methods for estimating paleostresses from twinning in carbonates abstract this study examines the relative differential stresses around a minor thrust fault within the mountain city window tennessee u s a the fault zone developed within dolomite rocks and deformation took place by twinning fracturing pressure solution and the development of fine grained deformation zones grain size reduction is observed from undeformed wall rock to the center of the fault zone and occurred by dynamic recrystallization methods to determine paleostresses in naturally deformed rocks from twinning assume a single coaxial strain inducing event the recrystallization within the center of the fault zone removed the effects of earlier deformation so that the twinning more closely reflects a single coaxial deformation event late in the fault history two methods were used to estimate the relative differential stresses across the fault zone and the results show opposite trends towards the center of the fault zone the different results may be partially explained by the influence of grain size as only one of these methods considers the influence of grain size in addition the grainsize data from this fault zone demonstrate that the tendency for a grain size class to be twinned depends on the grain size distribution the grain size distribution may result in grain to grain stress concentrations that induce twinning thus grain size distribution should also be considered to achieve more accurate estimates of paleostresses

geophysical consequences of planetary scale impacts into a mars like planet all planetary bodies with old surfaces exhibit planetary scale impact craters vast scars caused by the large impacts at the end of solar system accretion or the late heavy bombardment here we investigate the geophysical consequences of planetary scale impacts into a mars like planet by simulating the events using a smoothed particle hydrodynamics sph model our simulations probe impact energies over two orders of magnitude 2 10 27 6 10 29 j impact velocities from the planet s escape velocity to twice mars orbital velocity 6 50 km s and impact angles from head on to highly oblique 0 75 the simulation results confirm that for planetary scale impacts surface curvature radial gravity the large relative size of the impactor to the planet and the greater penetration of the impactor contribute to significant differences in the geophysical expression compared to small craters which can effectively be treated as acting in a half space the results show that the excavated crustal cavity size and the total melt production scale similarly for both small and planetary scale impacts as a function of impact energy however in planetary scale impacts a significant fraction of the melt is sequestered at depth and thus does not contribute to resetting the planetary surface complete surface resetting is likely only in the most energetic 6 10 29 j slow and head on impacts simulated a crater rim is not present for planetary scale impacts with energies 10 29 j and angles 45 but rather the ejecta is more uniformly distributed over the planetary surface antipodal crustal removal and melting is present for energetic 10 29 j fast 6 km s and low angle 45 impacts the most massive impactors with both high impact energy and low velocity contribute sufficient angular momentum to increase the rotation period of the mars sized target to about a day impact velocities of 20 km s result in net mass erosion from the target for all simulated energies and angles the hypothesized impact origin of planetary structures may be tested by the presence and distribution of the geochemically distinct impactor material

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

calcite twinning constraints on stress strain fields along the mid atlantic ridge iceland calcite veins and amygdule fillings within basalts older than 0 7 ma are mechanically twinned and preserve a subhorizontal shortening strain that resulted from compression and shortening normal to the mid atlantic ridge on both sides of the plate boundary our sample suite includes 19 specimens 7 from the north american plate 4 veins 3 amygdule fillings and 12 from the european plate 9 veins 3 amygdule fillings 18 of which record ridge normal subhorizontal shortening five of the strain analyses two from the north american plate and three from the european plate have a high percentage of negative expected values and these secondary strain results record a ridge parallel shortening strain with plunges that vary parallel to the ridge axis averaged shortening strain magnitudes for the twinned calcite 2 5 european plate 6 02 north american plate and inferred differential stresses 48 mpa that caused the twinning are modest and are thought to represent regional tectonic conditions i e ridge push not local e g hotspot or glacial loading phenomena

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

mineralogy microtexture and composition of shock induced melt pockets in the los angeles basaltic shergottite analytical electron microscopy of shock features in the basaltic shergottite los angeles stone 1 reveals 1 shock recorded in the bulk sample and 2 localized pressure and temperature excursions that have generated melt pockets up to 4 mm in diameter bulk shock effects include microfaulting offsets 1 200 mm mosaicism deformed exsolution lamellae and planar fracturing in pyroxene undulose extinction in whitlockite mechanical twinning in titanomagnetite and ilmenite and the transformation of plagioclase to maskelynite 4 remnant reduced birefringence the pressure estimates for bulk shock are 35 40 gpa localized shock excursions have generated three types of discrete melt zones 0 07 1 3 mm to 3 0 3 5 mm apparent diameter possessing glassy to microcrystalline groundmasses these melt pockets are differentiated on the basis of size clast volume and degree of crystallization and vesiculation melt veins and melt dikelets emanate from the melt pockets up to 3 mm into the host rock but do not necessarily connect with other melt pockets the melt pockets were generated by pressure temperature excursions of 60 80 gpa and 1600 2000 c resulting in discrete melting of adjacent host rock minerals at grain boundary margins concentric zoning in the margins of clinopyroxenes coincides with a progressive reduction in birefringence as melt pockets are approached this suggests that the shock excursions were focused as point sources in the wake of the shock front that induced bulk damage

strain analysis from calcite e twins in the cameros basin nw iberian chain spain abstract calcite twins and c axes from sparry grains that fill micro veins were used to unravel the deformational history of the cameros massif an inverted mesozoic sedimentary basin located in the northwestern part of the iberian chain spain calcite c axis and microvein orientations indicate the existence of two extension directions nw se and ne sw during the basin formation probably related to the slip along two syn sedimentary faults calcite e twins were developed only in micro veins from the frontal ramp area i e in the northeast side of the basin where the stresses were higher during the basin inversion twin analysis yields three different orientations nw se e w and ne sw for the minimum horizontal extension direction i e s hmax under coaxial conditions which suggests that the basin inversion was developed through several thrusting episodes during which the frontal and lateral ramps of each one of these episodes are interchanged moreover the perpendicularity between c axes and the minimum extension directions suggests that the basin inversion was produced along the same faults that controlled the basin formation the minimum extension directions deduced from e twins are also consistent with the shortening directions established previously for the tertiary evolution of the iberian chain which suggest that the basin inversion involved several tectonic events during the neogene these results show the advantage of the combined c axis and e twins analysis to unravel deformational and kinematic histories

cleavage development within a foreland fold and thrust belt southern pyrenees spain abstract in the southern pyrenees lithologically distinct cleavage fronts are each parallel to bedding and dip 20 towards the foreland pressure solution was the dominant mechanism of cleavage development the mudstone cleavage front is coincident with the 195 c paleoisotherm and is associated with a pressure solution strain of 5 a mechanical twin strain of 4 and a deviatoric stress magnitude of 65 mpa illite crystallinity measurements define a geothermal gradient of 15 c km 1 and indicate that the paleoisotherms are bedding parallel deviatoric stress magnitudes from calcite twins were regionally constant at 65 mpa and principal stress axes were perpendicular to cleavage temperature was the primary control on deformation micromechanisms and the position and orientation of the cleavage front within the foreland thrust wedge deformation below the cleavage front occurs predominantly by pressure solution which in conjunction with mechanical twinning and microfracturing produces a quasi plastic rheology stress magnitudes determined from mechanical twinning of carbonate grains and long term 10 6 10 76 y strain rates determined for regional folds and faults suggest an apparent macroscopic viscosity of 9 8 10 18 to 7 2 10 19 pa s for the lower thrust wedge above the cleavage front temperature pressure solution strain total strain and mesoscale deformation diminish the region of the thrust wedge above the 100 c paleoisotherm is characterized by large brittle faults with cataclastic fault zones and negligible grain scale deformation indicating an elastico frictional rheology

calcite e twins as markers of recent tectonics insights from quaternary karstic deposits from se spain abstract superficial pleistocene karstic coatings found in the boundary region between the iberian and betic chains spain are often offset by normal faults these travertines were sampled to investigate whether mechanical twins in calcite can develop at low paleodepths surprisingly twinned grains are very abundant in sparry grains which are found in the proximity of the fault planes mean twin densities 28 twins mm paleostress determinations based on the analysis of calcite e twinning yielded the same principal stress orientations as the fault population analysis i e a nw se extension tectonic regime therefore calcite e twin analysis is a useful method with which to establish the orientation of recent stress tensors calcite e twins develop under conditions of minimum overburden 300 m during fault slip the low differential stresses during the fault slip event suggest that the critical resolved shear stress on the twin plane must be lower than the expected mean value of 10 mpa or that local stresses experienced by rock in and near the fault zone exceed ambient stress conditions during fault slip

low amplitude evolution of break thrust folding abstract this paper proposes a mechanistic explanation for fault related flexural slip folds that form predominantly by fixed hinge folding i e break thrust folds it also presents a stepwise evolutionary history for these folds which contrasts with the model suggested for self similar migrating hinge fault propagation fold models both mechanical folding theory and rock fabric evidence suggest that folding in advance of a growing thrust fault can evolve in a five step process 1 initial sinusoidal buckling instability

detachment folds with fixed hinges and variable detachment depth northeastern brooks range alaska abstract detachment anticlines in the northeastern brooks range accommodated displacement above a detachment by buckling of a competent unit over an incompetent unit meso and microstructures in hinges and a lack of relict hinge structures in limbs suggest that these folds grew with fixed hinges the structural thickness of the incompetent unit beneath the folds detachment depth varies from less than to greater than the stratigraphic thickness a model in which incompetent unit thickness varies with fold area better approximates the geometry of the folds than does a more conventional constant depth model additional discrepancies between modelled and observed incompetent unit thickness and field observations suggest non plane strain and or transport of material through the boundaries of the fold in the plane of the cross section the results of this study suggest a typical evolutionary sequence for detachment folds in the northeastern brooks range which may be applicable elsewhere anticlines initiate as fixed hinge buckle folds rapid initial increase in anticlinal cross sectional area results in a decrease in incompetent unit thickness fold area begins to decrease with tightening beyond an interlimb angle of 90 dg decreasing fold area is accommodated through some combination of structural thickening of the incompetent unit transport of solid or dissolved material out of the plane of section transport of material through the boundaries of the fold in the plane of the cross section and or truncation by thrust faults

formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid we propose that mesosiderites formed when a 200 400 km diameter asteroid with a molten core was disrupted by a 50 150 km diameter projectile to test whether impacts can excavate core iron and mix it with crustal material we used a low resolution smoothed particle hydrodynamics computer simulation for 50 300 km diameter differentiated targets we found that significant proportions of scrambled core material and hence potential mesosiderite metal material could be generated for near catastrophic impacts that reduce the target to 80 of its original diameter and about half of its original mass the proportion of scrambled core material would be about 5 vol equivalent to 10 vol of mesosiderite like material the paucity of olivine in mesosiderites and the lack of metal poor or troilite rich meteorites from the mesosiderite body probably reflect biased sampling mesosiderites may be olivine poor because mantle material was preferentially excluded from the metal rich regions of the reaccreted body molten metal globules probably crystallized around small cool fragments of crust hindering migration of metal to the core if mantle fragments were much hotter and larger than crustal fragments little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally the rapid cooling rates of mesosiderites above 850 c can be attributed to local thermal equilibration between hot and cold ejecta very slow cooling below 400 c probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris we infer that our model is more plausible than an earlier model that invoked an impact at 1 km s to mix projectile metal with target silicates if large impacts cannot effectively strip mantles from asteroidal cores as we infer we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material this may help to explain why relatively few olivine rich a type and metal rich asteroids m type are known some s type asteroids may be scrambled differentiated bodies

forming a moon with an earth like composition via a giant impact in the giant impact theory the moon formed from debris ejected into an earth orbiting disk by the collision of a large planet with the early earth prior impact simulations predict that much of the disk material originates from the colliding planet however earth and the moon have essentially identical oxygen isotope compositions this has been a challenge for the impact theory because the impactor s composition would have likely differed from that of earth we simulated impacts involving larger impactors than previously considered we show that these can produce a disk with the same composition as the planet s mantle consistent with earth moon compositional similarities such impacts require subsequent removal of angular momentum from the earth moon system through a resonance with the sun as recently proposed

determination of deviatoric stress tensors based on inversion of calcite twin data from experimentally deformed monophase samples preliminary results abstract following previous successful paleo stress determinations from numerically generated aggregates and naturally deformed samples preliminary results about stress reconstructions based on inversion of calcite twin data from experimentally deformed monophase samples are reported this new study carried out on samples of carrara marble and crinoidic limestone from eastern france provides a test of the validity of stress reconstructions using inverse methods which had not been properly calibrated before the percentages of correlated twins are discussed for each stress tesnor and the range of uncertainties on the determination of stress orientations and differential stress magnitudes is evaluated

determining deviatoric stress tensors from calcite twins applications to monophased synthetic and natural polycrystals theoretically calcite twins analysis makes possible the full determination of the deviatoric stress tensor responsible for the formation of the twin lamellae that is the orientations of the principal stress axes and the three differential stress values i j may be calculated assuming conditions of constant resolved shear stress for twinning inverse method using nonlinear regression and robust estimation is proposed applications to monophased synthetic polycrystals that are generated numerically by solving the direct problem and to a natural sample from the quercy southwest of france have given good results comparison with the first method proposed by turner 1953 modified by nissen 1964 for principal stress orientations and by jamison and spang 1976 for differential stress values clearly emphasizes the considerable interest of this new method

structural lithic units in external orogenic zones abstract changes in types of thrust fault structures are typical of external orogenic zones and are inferred here to be related to thickness and facies changes in stratigraphie sequences obvious deformation style changes occur from external to intermediate thrust sheets similar changes in style are observed and most easily interpreted along strike within individual thrust sheets to correlate stratigraphic and structural changes stratigraphic sections are divided into structural lithic units the subdivisions are based on the dominant deformation styles vertically in individual cross sections the major variables in determining deformation style are controlling stratigraphic members within individual stratigraphic structural units and the stacking sequence of lithologie types into different structural units in many areas the critical taper of one major dominant member regionally controls where structural style changes will occur areal change in either internal composition or in stacking sequence causes lateral change in deformation style lithological changes in structural units are coincident with changes in frontal thrust ramp positions connected by major lateral ramps and with changes in thrust zone geometries imbricate thrusts in one area versus a duplex zone along strike they are also associated with changes in thrust shapes from listric to stepped and changes from fault bend fold to fault propagation fold structures controlling members may be either strong or weak fold dominated regions and thrust faulted regions without major shales or evaporites are usually controlled by strong controlling members weak controlling members are major evaporite horizons or thick shales a thorough understanding of stratigraphic facies realms therefore may allow predictions of types and changes in structural styles it can also be very useful in the interpretation of less well exposed thrust terranes

palaeostress estimation using calcite twinning experimental calibration and application to nature abstract by means of high temperature deformation experiments on calcite rocks of different grain sizes we have established the relationships between differential stress and twinning incidence twin density and volume fraction and the way that these relationships depend on grain size both twinning incidence and volume fraction increase with grain size at a constant level of differential stress log twin density is proportional to stress but is independent of grain size at a given grain size volume fraction of twinning increases non linearly with differential stress twinning activity measured by twinning incidence increases linearly with stress the behaviour of each of these parameters is independent of temperature strain and strain rate these relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks this is illustrated by a study of twinning activity in limestones close to thrust faults in the cantabrian zone of the variscan orogenic belt in northwestern spain high peak differential stresses were measured ranging from 150 to 250 mpa using the turner dynamic analysis technique however it is clear that shear stresses resolved parallel to the fault planes were very low the attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces

kinematics of decollement folding in the lost river range idaho decollement folding of massive 2 m thick carbonate beds of the scott peak formation within the willow creek anticline northern lost river range idaho was associated with the development of thin 0 25 m widely spaced deformation zones incremental strain histories determined from fibrous overgrowths in strain fringes from the deformation zones provide a basis for kinematic analysis of the anticline plane strain is indicated by coaxial up dip extension on cleavage xy planes and a lack of extension in the y direction in xz sections coaxial incremental strain histories characterize fold hinges and document pinned hinges during folding the sense of rotation on opposing limbs indicates that the fiber curvature records primarily the spin of layers through fixed moderately steep 66 12 dg l diverging extension directions the magnitude of the external rotation for each sample is typically less than the bedding dip suggesting that the external rotation also includes a component of internal rotation related to layer parallel shear toward the upper flat panel or the hinge pin line the magnitude of this internal rotation varies within and between layers

normal faults in thrust sheets pre orogenic extension post orogenic extension or both abstract in fold and thrust belts that experienced both pre orogenic and post orogenic extension it may be difficult to establish whether observed normal faults pre dated post dated or were synchronous with thrusting geometrical structural patterns may be insufficient to constrain the relative chronology of extensional and contractional deformations the systematic use of kinematic criteria makes it possible to unequivocally define the timing relationships of reverse and normal fault development and hence to correctly unravel complex structural evolutions kinematic analysis in the southernmost umbria marche apennines of italy where both normal and thrust faults are present revealed a history of repeated tectonic inversion characterised by two distinct stages of extension separated by an episode of folding and thrusting structural overprinting relationships observed at thrust normal fault intersections were useful for i removing sequentially younger deformations and hence ii separating and quantifying the effects of orogenic contraction from those of both pre orogenic and post orogenic extension

paleostress magnitudes associated with development of mountain belts insights from tectonic analyses of calcite twins in the taiwan foothills determinations of stress magnitudes based on inversion of calcite twin data from fault related folds in the taiwan foothills show a decrease of differential stresses during synfolding erosion and exhumation suggesting that paleodepth of burial largely controlled the levels of differential stresses sustained by rocks regional frictional conditions linked to shallow and or deep decollement tectonics probably also influenced stress magnitudes combination of the twin inversion technique with fracture analysis and rock mechanics data provides first order estimates of principal stress values related to the major pleistocene shortening event in taiwan these stress estimates are compared to previous stress estimates in fold thrust belts and to available data on differential stress magnitudes in foreland and hinterland domains of orogens at the scale of an entire orogenic system the most striking point is a decrease of differential stresses from the hinterland toward the foreland this decrease not only reflects tectonic stress attenuation away from the plate boundary but also suggests a major control by the depth of deformation differential stresses estimated from natural deformation consequently provide an independent support to the depth dependent strength and the frictional behavior of the upper continental crust deduced from laboratory experiments and stress measurements in deep boreholes

dust from collisions a way to probe the composition of exo planets abstract in order to link infrared observations of dust formed during planet formation in debris disks to mid infrared spectroscopic data of planetary materials from differentiated terrestrial and asteroidal bodies we obtained absorption spectra of a representative suite of terrestrial crustal and mantle materials and of typical martian meteorites a series of debris disk spectra characterized by a strong feature in the 9 0 9 5 m range hd23514 hd15407a hd172555 and hd165014 is comparable to materials that underwent shock collision or high temperature events these are amorphous materials such as tektites sio2 glass obsidian and highly shocked shergottites as well as inclusions from mesosiderites group a a second group bd 20307 beta pictoris hd145263 id8 hd113766 hd69830 p1121 and eta corvi have strong pyroxene and olivine bands in the 9 12 m range and is very similar to ultramafic rocks e g harzburgite dunite group b this could indicate the occurrence of differentiated materials similar to those in our solar system in these other systems however mixing of projectile and target material as well as that of crustal and mantle material has to be taken into account in large scale events like hit and run and giant collisions or even large scale planetary impacts this could explain the olivine dominated dust of group b the crustal type material of group a would possibly require the stripping of upper layers by grazing style hit and run encounters or high energy events like evaporation condensation in giant collisions in tidal disruptions or the involvement of predominantly icy water bodies the resulting mineral dust would originate mainly in one of the involved planetesimals this could allow attributing the observed composition to a specific body such as e g eta corvi

precision and accuracy of the calcite strain gage technique the twinned calcite strain gage technique allows the determination of strain magnitude and direction in mildly deformed sedimentary rocks in that the measurements required for the calculation are rather time consuming it is important to know the minimum number required to obtain a precision that is comparable with the accuracy of the result this paper documents a series of tests on three different limestones having different textures and porosities in order to determine the precision and accuracy of the technique as a function of the number of measurements and the number of perpendicular thin sections the tests show that for most purposes the best measurement strategy will be to make 25 measurements on each of two perpendicular thin sections the best determined principal strain axis is the one having a magnitude that is opposite in sign from the other two over the range of zero to 11 shortening the magnitude of the odd signed strain axis will than have a precision and accuracy of about 11 in magnitude and a precision and accuracy of about 6 to 8 in orientation differences in texture and porosity do not appear to have significant effects upon precision and accuracy

impact spherules as a record of an ancient heavy bombardment of earth the fossilized remnants of vaporized asteroids called spherules can be used to infer that the flux of asteroid impacts on earth 3 5 billion years ago was much greater than it is now the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets impact craters are the most obvious indication of asteroid impacts but craters on earth are quickly obscured or destroyed by surface weathering and tectonic processes1 earth s impact history is inferred therefore either from estimates of the present day impactor flux as determined by observations of near earth asteroids or from the moon s incomplete impact chronology2 3 4 asteroids hitting earth typically vaporize a mass of target rock comparable to the projectile s mass as this vapour expands in a large plume or fireball it cools and condenses into molten droplets called spherules5 for asteroids larger than about ten kilometres in diameter these spherules are deposited in a global layer spherule layers preserved in the geologic record accordingly provide information about an impact even when the source crater cannot be found1 here we report estimates of the sizes and impact velocities of the asteroids that created global spherule layers the impact chronology from these spherule layers reveals that the impactor flux was significantly higher 3 5 billion years ago than it is now this conclusion is consistent with a gradual decline of the impactor flux after the late heavy bombardment

calcite twins from microveins as indicators of deformation history abstract in order to investigate whether the evidence of the deformation history is preserved in the microvein calcite twinning microstructures several families of calcite veins have been studied from weakly deformed mesozoic limestones of the western iberian chain eastern iberian peninsula the maximum shortening directions established with this method correspond to the previously established paleostress evolution of this chain the reconstructed stress regimes evolve from predominantly compressional four compressional episodes with the maximum shortening direction oriented successively nw se n s ne sw and e w to extensional the established orientations of maximum shortening axes are consistent with the directions of paleostrains that have been independently deduced from stylolites and fault striations in the same region moreover the results clearly show that the e twins record some paleostress orientations that are not registered in the studied area by fault population analysis in this sense the e twins record older paleostress orientations than the small faults probably because the faults were reactivated several times and the twins were not modified since their formation the calculated values of twin strains 1 differential stress magnitudes 62 mpa and the existence of type i twins formed below 200 c confirm that this region was slightly deformed under low p t conditions

calcite twin morphology a low temperature deformation geothermometer abstract twinning of the e plane is the dominant crystal plastic deformation mechanism in calcite deformed below about 400 c calcite in a twin domain has a different crystallographic orientation from the host calcite grain so called thin twins appear as thin black lines when viewed parallel to the twin plane at 200 320 magnification under a petrographic microscope thick twins viewed in the same way have a microscopically visible width of twinned material between black lines calcite e twin width and morphology has been correlated with temperature of deformation in naturally deformed coarse grained calcite in this paper we present a compilation and analysis of data from limestones of the frontal alps france and switzerland and the appalachian valley and ridge and plateau provinces eastern united states to document this temperature dependence mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate below 170 c and thick twins dominate above 200 c above 250 c dynamic recrystallization is an important deformation mechanism in calcite mean twin intensity twin planes mm correlates negatively with temperature and a cross plot of twin intensity with twin width can yield information about both strain and temperature of deformation these relationships provide a deformation geothermometer for rocks that might otherwise yield little or no paleotemperature data

microstructure of calcite in the cm2 carbonaceous chondrite lon 94101 implications for deformation history during and or after aqueous alteration the microstructure of calcite in the cm2 carbonaceous chondrite lon 94101 has been characterized using electron backscatter diffraction ebsd analysis to reconstruct the parent body deformation history during and or after aqueous alteration the results suggest that at least two events of calcite crystallization have taken place during aqueous alteration and at least three episodes of deformation are recorded by the calcite the first event of calcite crystallization produced calcite grains scattered throughout the matrix and the second event formed a calcite vein via localized fluid flow the first episode of deformation is recorded in the crystallographic preferred orientations of the calcite grains and occurred via a directed stress probably induced by compaction in shallow crustal levels of the parent body the second episode of deformation is recorded in an e twin microstructure and it suggests a deformation induced via directed stress by impact processing also in shallow crustal levels the third episode of deformation generated subgrains in the calcite vein and in some calcite grains and fragmented and disrupted the calcite vein this could have been a result of a relatively forceful deformation event perhaps when the meteorite was released from its parent body this study shows that carbonate microstructures in carbonaceous chondrites is a powerful and versatile tool for reconstructing the history of deformation during and or after aqueous alteration

multiple scales of mechanical stratification and decollement fold kinematics sierra madre oriental foreland northeast mexico regional to outcrop scale stratigraphic relationships within mesozoic shallow marine carbonates and clastics at sierra del fraile anticlinorium in northeast mexico play an important role in controlling the deformation distribution strain history and kinematics during folding structural analyses of meso and microscopic deformation features from all stratigraphic and structural positions within the anticlinorium allow for the delineation of five regional scale lithotectonic units kilometer to decameter scale stacking patterns of the preorogenic succession and the meter scale stacking patterns of individual shallowing upward carbonate cycles are shown to influence deformation kinematics in predictable ways mesoscopic faults and cleavage patterns along with penetrative strain from anisotropy of magnetic susceptibility ams fabrics constrain fold kinematics as progressive limb rotation about pinned anticlinal and synclinal hinges with constant limb length and a mobile basal decollement horizon within the evaporites of the minas viejas formation that core the anticlinorium ams fabrics are shown to be a useful proxy for nascent penetrative strain orientations in sedimentary rocks macroscopic differences in strain suggest strong lithologic facies control responding to differences in bed thickness grain size and carbonate content at the regional scale variation in deformation between the lithotectonic units identifies decollement horizons coincident with lithotectonic boundaries in the minas viejas upper taraises and parras formations

strain partitioning of deformation mechanisms in limestones examining the relationship of strain and anisotropy of magnetic susceptibility ams abstract in order to investigate the relationship between rock strain and anisotropy of magnetic susceptibility ams strain partitioning and ams analysis was conducted at 35 sites from two stratigraphically adjacent paleozoic limestone units in the patterson creek and wills mountain anticlines in the central appalachian orogen of west virginia in addition anisotropy of anhysteretic remanent magnetization aarm was conducted on selected samples to examine the role of preferentially oriented magnetite on the ams fabric strain is partitioned into bed normal shortening due to compaction solution strain 35 0 shortening bed parallel shortening due to tectonic solution strain 13 3 shortening calcite twinning strain 5 8 shortening and grain boundary sliding 26 7 shortening the ams fabrics in the rocks were found to be a result of a complex interaction between rock lithology deformation mechanisms and strain magnitude although all the rocks have experienced the same deformation conditions six different ams fabrics are exhibited each of the different ams fabrics is a composite fabric resulting from the overprinting of three components 1 an inherent primary depositional ams fabric that is attributed to preferentially oriented phyllosilicates in the rock matrix 2 a diagenetic and or compaction ams fabric formed during burial that is due to preferentially oriented phyllosilicates in solution structures and in the rock matrix and 3 a tectonic ams fabric that was imparted on the rocks by layer parallel shortening deformation prior to folding and is also attributed to preferentially oriented phyllosilicates in solution structures and in the rock matrix as well as twinning of ferroan calcite

tracking progressive deformation of an orogenic wedge through two successive internal thrusts insights from structure deformation profile strain and vorticity of the main central thrust mct and the pelling munsiari thrust pt sikkim himalayan fold thrust belt abstract we compare and contrast the structural evolution of two successive internal thrusts the main central thrust mct and the pelling munsiari thrust pt from the crystalline core of the sikkim himalaya in the context of progressive deformation involving a footwall lesser himalayan duplex the pt is its roof thrust the mylonitic foliations in both the shear zones are overprinted by successive cleavages the youngest cleavage from the pt zone continues within the hangingwall mct sheet that records one additional cleavage possibly indicating time transgressive cleavage development the cleavages become steeper while their intensity decreases structurally higher up within individual thrust sheets the rs values and angular shear strain decrease from the mylonite zones to structurally higher up within the sheets both the shear zones are type ii with decelerating strain paths rs relationships and microstructures indicate thrust parallel stretch is greater than thrust perpendicular component both the shear zones record strain partitioning with lesser competent mylonite domains recording higher rxz and a greater proportion of simple shear than the protomylonite domains the shear zones recorded the growth of the duplex during progressive deformation that contributed to pure shear dominated general shear higher flattening strain and greater translation on its roof thrust the pt than the overlying mct

the paradox between low shock stage and evidence for compaction in cm carbonaceous chondrites explained by multiple low intensity impacts abstract petrographic analysis of eight cm carbonaceous chondrites eet 96029 lap 031166 lon 94101 met 01072 murchison murray sco 06043 que 93005 by electron imaging and diffraction and x ray computed tomography reveals that six of them have a petrofabric defined by shock flattened chondrules with the exception of murchison those cms that have a strong petrofabric also contain open or mineralized fractures indicating that tensional stresses accompanying the impacts were sufficient to locally exceed the yield strength of the meteorite matrix the cms studied span a wide range of petrologic subtypes and in common with rubin 2012 we find that the strength of their petrofabrics increases with their degree of aqueous alteration this correspondence suggests that impacts were responsible for enhancing alteration probably because the fracture networks they formed tapped fluid reservoirs elsewhere in the parent body two meteorites that do not fit this pattern are met 01072 and murchison both have a strong petrofabric but are relatively unaltered in the case of met 01072 impact deformation is likely to have postdated parent body aqueous activity the same may also be true for murchison but as this meteorite also lacks fractures and veins its chondrules were most likely flattened by multiple low intensity impacts multiphase deformation of murchison is also revealed by the microstructures of calcite grains and chondrule defined petrofabrics as revealed by x ray computed tomography the contradiction between the commonplace evidence for impact deformation of cms and their low shock stages most belong to s1 can be explained by most if not all having been exposed to multiple low intensity i e

a computer program for the calcite strain gauge technique abstract the calcite strain gauge technique has been used successfully to determine strains in slightly to moderately deformed limestones the technique involves the use of a universal stage to collect calcite twin orientation thickness and frequency data the data are reduced by a least squares method to give the orientations and magnitudes of the principal strains the purpose of this paper is to describe an easy to use computer program for the technique and an accompanying manual on collecting data for the strain gauge analysis the program calculates principal strain magnitudes and orientations total strain due to twinning j20 5 standard errors for the strains average twin width twin intensity and percent negative expected values in addition the program can plot stereonets with compression axes tension axes poles to e twin planes crystallographic c axes g axes average twin widths per twin set twin intensity per twin set and positive and negative expected values the stereonets may also be rotated all calculated orientation data may be saved in a computer file for later input into a stereonet contouring program the user may choose automatically to remove 20 of the twin sets with the largest deviation or manually to remove selected twin sets from the data being analyzed in order to clean the data twin sets that give positive or negative expected values may also be analyzed separately

ramifications of four dimensional progressive deformation in contractional mountain belts abstract studies of progressive deformation aim to identify and establish temporal sequences of structural stages each characterized by a suite of structures which form under specific rock and environmental conditions because of the natural spatial variation in rock type and environmental parameters in an orogen several structural stages may operate concurrently in different parts of an orogenic wedge at any given instant in time rocks experiencing different stages of deformation are bounded by deformation fronts as intrinsic and extrinsic conditions of deformation vary through time spatial migration of deformation fronts causes rocks to record temporal overprinting of structural stages in convergent orogens deformation fronts as mapped in their finite state are typically forelandward dipping due to the regional orogenic wedge geometry and thermal structure depending on the competing deformation processes deformation rate changes on either side of the front may cause deformation fronts to change orientation relief and surface area with time differences in migration rates of different deformation fronts could cause rearrangement of the sequence of structural stages with depth and laterally across the same mountain belt

calcite sealing in a fractured geothermal reservoir insights from combined ebsd and chemistry mapping fractures play an important role as fluid flow pathways in geothermal resources hosted in indurated greywacke basement of the taupo volcanic zone new zealand including the kawerau geothermal field over time the permeability of such geothermal reservoirs can be degraded by fracture sealing as minerals deposit out of transported geothermal fluids calcite is one such fracture sealing mineral this study for the first time utilises combined data from electron backscatter diffraction and chemical mapping to characterise calcite vein fill morphologies and gain insight into the mechanisms of calcite fracture sealing in the kawerau geothermal field two calcite sealing mechanisms are identified 1 asymmetrical syntaxial growth of calcite inferred by the presence of single twinned calcite crystals spanning the entire fracture width and 2 3d interlocking growth of bladed vein calcite into free space as determined from chemical and crystallographic orientation mapping this study also identifies other potential uses of combined ebsd and chemical mapping to understand geothermal field evolution including potentially informing on levels of fluid supersaturation from the study of calcite lattice distortion and providing information on a reservoir s history of stress strain and deformation through investigation of calcite crystal deformation and twinning patterns

geometric and numerical model of progressive limb rotation in detachment folds a simple area balanced geometric model of progressive limb rotation in detachment folds allows the quantification of displacement uplift and rotation and the assessment of their effect upon growth strata geometries translation of this geometric model into a velocity model of deformation allows the modeling of associated growth strata resulting from background sedimentation and local erosion transport and deposition we show that with constant displacement and sedimentation rate this model predicts a progressive unconformity indicating the onlap and eventual overlap of the fold limb caused by a decreasing uplift rate this decrease is a consequence of the kinematics of progressive limb rotation increases in displacement rate with sedimentation rate kept constant can lead to offlap and angular unconformities although subsequent onlap does not necessarily reflect a return to a lower displacement rate modeled growth strata geometries exhibit many of the features reported from the literature

intraplate paleostresses reconstructed with calcite twinning and faulting improved method and application to the eastern paris basin lorraine france this study deals with the reconstruction of intraplate paleostresses calcite twinning inversion provides paleostress directions and differential values even in poorly outcropping areas however existing methods are not well adapted to multiphase deformation cases and the calculation of stress values provides unreliable estimates modifications were brought to a calcite twinning inverse method and to the processing of results in a new twin data separation process the whole set of data is used to calculate each tensor the variability of yield stress value for twinning with strain hardening is taken into consideration a first order yield stress calcite grain size dependence function has been established with a sample showing various grain sizes this modified method of calcite twinning inversion was applied in the eastern paris basin lorraine france in this platform area measured fractures are preferentially trending ne sw and nw se at all scales the meso cenozoic paleostress succession reconstructed with fault slips is complex jurassic sedimentation occurred during the tethys rifting a mainly strike slip phase with r1 successively striking nnw and nne is attributed to the tertiary pyrenean orogenesis and is followed by wnw extension in the oligocene after local ene compression wnw compression was probably due to the alpine late miocene collision the calcite twinning inversion has confirmed these tectonic events and their succession order and the directions of stress axes were specified using grain size and strain rate sensitive yield stress values the differential stresses can be estimated with an uncertainty of few mpa they range between 20 and 30 mpa depending on the tectonic event the oligocene paleodepths of the sampled calcite estimated using differential stress values are structurally sensible thus the original calcite twinning inverse method has been improved complex paleostress successions such as in platform areas may be deciphered by combination with fault slip inversion even if additional improvement is possible this calcite twinning inverse method opens new ways to determine stress distributions in tectonic plates d 2004 published by elsevier b v

constraining the pressure threshold of impact induced calcite twinning implications for the deformation history of aqueously altered carbonaceous chondrite parent bodies calcite twinning in carbonaceous chondrite meteorites can be used to reconstruct the deformation history and the parent body environment during and or after aqueous alteration but the shock pressure threshold at which the twins develop is unknown accordingly the aim of this study is to determine the magnitude of shock pressure that is needed to generate calcite twins this was done by measuring the depths of twinning beneath the resulting craters from experimental impacts in six calcite targets combined with hydrocode modelling of the peak pressures at the corresponding depths within the targets brecciation fracturing and calcite e twinning occur below the floors of all the craters and results from the hydrocode modelling show that the twins start to form at shock pressures of 110 to 480 mpa which is at least a factor of ten higher than the 10 mpa that is considered to be required to produce calcite twins in low strain rate terrestrial settings these pressures are equivalent to shock stage s1 as determined by olivine microstructures and consistent with calcite twinning in carbonaceous chondrites being a result of impact gardening in shallow levels of their asteroidal parent bodies

condensation evaporation melting and crystallization in the primitive solar nebula experimental data in the system mgo sio 2 h 2 to 1 0x10 super 9 bar and 1870 degrees c with variable oxygen fugacity

critical re evaluation of differential stress estimates from calcite twins in coarse grained limestone abstract comparison of two techniques to estimate differential stress magnitude based on calcite twinning reveals significantly different calculated values of differential stress for limestone experimentally and naturally deformed at low temperature this inequality may be due to fundamental differences between the rock type marble versus limestone and deformation temperatures above and below 200 c for samples used in technique calibration versus samples used in testing and application a technique that was empirically derived from marble experimentally deformed at 200 800 c yields values of differential stress that are factors of 2 to 9 times larger than maximum differential stress measured during experimental deformation of limestone at low temperature

the recent upper miocene to quaternary and present tectonic stress distributions in the iberian peninsula a general synthesis of the recent and present stress situation and evolution in the iberian peninsula was obtained from microstructural and seismological analysis the stress evolution was deduced from 1 fault population analysis fpa from 409 sites distributed throughout the iberian peninsula 2 paleostress indicators given by 324 stations taken from the bibliography and 3 seismic data corresponding to 161 focal mechanisms evenly spread in the studied region the application of fpa together with the determination of stress tensors and focal mechanisms for the whole iberian microplate has provided two main results 1 the iberian peninsula is undergoing a nw se oriented compression except for the northeastern part pyrenees ebro basin and iberian chain where it is n s to ne sw and the gulf of cadiz where it seems to be e w and 2 the main trends of the stress field have remained almost constant since the upper miocene the analysis performed by zones suggests the presence of local heterogeneities in the stress field

distribution of layer parallel shortening fabrics in appalachian foreland of new york and pennsylvania evidence for two non coaxial phases of alleghany orogeny abstract a structural interpretation of a part of the central and northern appalachian foreland uses the correlation of mechanical twinning solution cleavage crenulation cleavage pencils joints and deformed fossils such a correlation suggests that within the central appalachians the alleghanian orogeny consists of two major phases a deformation possibly as old as pennsylvanian herein called the lackawanna phase and a second deformation termed the main phase which is permian or younger in age the lackawanna phase affects mainly the eastern parts of the foreland such as the hudson river valley and pocono plateau while the main phase affects most of the valley and ridge and alleghany plateau the lackawanna phase is interpreted as the product of strike slip motion possibly between the avalon microcontinent and north america the main phase may record the final convergence of africa against north america and its accredited microcontinents end of article last page 1168

the lunar magma ocean reconciling the solidification process with lunar petrology and geochronology abstract the moon is thought to have originated with a magma ocean that produced a plagioclase flotation crust as solidification proceeded ages of anorthositic crust range over at least 200 million years the model for solidification presented here integrates chemical and physical constraints of lunar magma ocean solidification to determine 1 the final thickness of flotation crust generated by a fractionally solidifying magma ocean 2 the timescale of crystallization before plagioclase is a stable phase 3 the timescale of solidification after the formation of the plagioclase flotation crust and 4 the post overturn lunar mantle composition and structure we find that magma oceans of as much as 1000 km depth are consistent with creating an anorthositic crust 40 to 50 km in thickness solidification of the magma ocean prior to formation of the flotation crust may occur on the order of 1000 years and complete solidification would require additional ten to tens of millions of years reconciling these short model timescales with radiometric dates of crustal samples requires either a very late forming moon combined with finding older crustal ages to be incorrect or calling on tidal heating of the crust by the early earth to prolong solidification gravitationally driven overturn of cumulates during tidal heating provides a mechanism for creating the compositions and ages of the lunar mg suite of crustal rocks further we find that upon crystallization the moon likely began with an azimuthally heterogeneous gravitationally stable mantle after magma ocean cumulate overturn this result may help explain the experimentally determined origin of picritic glasses at similar depths but from different source materials

hydrogen isotope analyses of alteration phases in the nakhlite martian meteorites abstract secondary alteration phases such as carbonates smectite clays and fe oxides are found within the martian meteorites if these meteorites were seen to fall the assumption can be reasonably made that the secondary phases have a martian origin however for martian meteorite finds this is not the case deuterium hydrogen d h ratio analysis can be used to distinguish between terrestrial and martian secondary alteration phases the martian atmosphere is currently five times enriched in deuterium compared to terrestrial seawater producing a deuterium enrichment in the martian alteration phases large enough to be detected with modern ion microprobe techniques we aimed to measure the d h ratio of the iddingsite like alteration veins in the nakhlite martian meteorites nakhla mil 03346 mil 090030 and y 000593 to help confirm the martian origin of this material and determine the relative amounts of terrestrial contamination in each meteorite as an observed fall nakhla is nominally uncontaminated by terrestrial alteration and shows elevated d values in its alteration veins however even the highest of these values is much below that of the martian atmosphere indicating that terrestrial contamination probably combined with fractionation effects between the martian atmosphere and the alteration material have reduced the d values of these veins the speed of hydrogen isotope exchange in the nakhlite iddingsite like alteration veins is demonstrated by the purely terrestrial d values in the veins of nakhla 110 a thin section exposed to the terrestrial atmosphere since its preparation in 1998 mineralogical heterogeneity also affects the hydrogen isotope exchange rate in these veins the wide variation in d within the veins of nakhla and y 000593 appears to be due to the heterogeneous nature of the vein material where some phases exchange hydrogen with the atmosphere at a faster rate than others

joint analyses of calcite twins and fault slips as a key for deciphering polyphase tectonics burgundy as a case study abstract determination of regional paleostresses based on calcite twin analysis has been successfully applied for the first time in the burgundy platform france paleostress tensors obtained with this method are internally consistent along the profile studied moreover the same directions of paleostresses are independently inferred from fault striation analysis the results clearly show that the late mesozoic cenozoic evolution of the area has involved polyphase deformation including 1 1 nne ssw extension probably late mesozoic in age 2 2 n s compression related to a major orogenic event in the pyrenees and provence late eocene in age 3 3 e w extension related to the oligocene rifting event the rhine saone rift system 4 4 wnw ese compression related to the late miocene westward thrusting of jura calcite twin and striated fault analyses are two complementary tools for paleostress determination their combined application will allow accurate mapping of paleostress trajectories in intraplate tectonics setting moreover calcite twins can allow stress and paleostress determination when macroscopic features are not observable which is often the case in very weakly deformed areas and in drill holes

mechanical twin sets in calcite as markers of recent collisional events in a fold and thrust belt evidence from the reefal limestones of southwestern taiwan stress reconstructions were carried out based on inversion of calcite twin data from slightly deformed quaternary reefal limestones from sw taiwan results demonstrate that twinning reliably recorded the complex quaternary and present day stress fields in a young mountain range undergoing collisional shortening the stress regimes reconstructed at microscopic scale are predominantly compressional and reflect the main stages of the neotectonic evolution of the collision belt no twinning has been observed in a sample collected in the holocene platform of southern taiwan indicating that these formations did not undergo sufficient stress to initiate calcite twinning three different stress regimes have been recorded in the samples collected in pleistocene reef limestones that developed in relation to active fold systems of the western foothills these regimes appear to be homogeneous at the site scale and at the scale of sw taiwan the reconstructed ene and nw trending compressions only present in sw taiwan probably reflect local perturbations of the regional 105 trending compression the perturbation might be due to activation of a set of folded faulted blocks in the studied area a nw se extension also identified in these samples is related to extrados stretching due to fold development the results are consistent with independent stress data i e quaternary faulting borehole breakouts and focal mechanisms of earthquakes the analysis of mechanical twins in calcite rocks constitutes a powerful tool for tectonic analyses in fold and thrust belts

mid infrared spectroscopy of impactites from the nordlinger ries impact crater abstract this study is part of an effort to build a mid infrared database 7 14 m of spectra for mertis mercury radiometer and thermal infrared spectrometer an instrument onboard of the esa jaxa bepicolombo space probe to be launched to mercury in 2017 mercury was exposed to abundant impacts throughout its history this study of terrestrial impactites can provide estimates of the effects of shock metamorphism on the mid infrared spectral properties of planetary materials in this study we focus on the nordlinger ries crater in southern germany a well preserved and easily accessible impact crater with abundant suevite impactites suevite and melt glass bulk samples from otting and aumuhle as well as red suevite from polsingen were characterized and their reflectance spectra in mid infrared range obtained in addition in situ mid infrared spectra were made from glasses and matrix areas in thin sections the results show similar but distinguishable spectra for both bulk suevite and melt glass samples as well as in situ measurements impact melt glass from aumuhle and otting have spectra dominated by a reststrahlen band at 9 3 9 6 m bulk melt rock from polsingen and bulk suevite and fine grained matrix have their strongest band between 9 4 and 9 6 m there are also features between 8 5 and 9 m and 12 5 12 8 m associated with crystalline phases there is evidence of weathering products in the fine grained matrix such as smectites mercury endured many impacts with impactors of all sizes over its history so spectral characteristics observed for impactites formed only in a single impact like in the ries impact event can be expected to be very common on planetary bodies exposed to many more impacts in their past we conclude that in mid infrared remote sensing data the surface of mercury can be expected to be dominated by features of amorphous materials

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites

a hit and run giant impact scenario abstract the formation of the moon from the debris of a slow and grazing giant impact of a mars sized impactor on the proto earth cameron and ward 1976 lunar planet sci conf canup and asphaug 2001 nature 412 708 is widely accepted today we present an alternative scenario with a hit and run collision asphaug 2010 chem erde 70 199 with a fractionally increased impact velocity and a steeper impact angle

mantle material in the main belt battered to bits the complete or near complete differentiation of a chondritic parent body is believed to result in an object with an fe ni core a thick olivine dominated mantle and a thin plagioclase pyroxene crust compositional groupings of iron meteorites give direct evidence that at least 60 chondritic parent bodies have been differentiated and subsequently destroyed a long standing problem has been that our meteorite collections and apparently our asteroid observations as well show a great absence of olivine dominated metal free mantle material while the basaltic achondrites hed meteorites represent metal free pyroxene dominated crustal samples the isotopic and geochemical evidence implies that this class is derived from only one parent body perhaps vesta thus the meteoritic and perhaps astronomical evidence also suggests a great absence of crustal material resulting from the collisional disruption of numerous parent bodies

magnetic fabrics and petrofabrics their orientation distributions and anisotropies abstract magnetic fabric and other petrofabric anisotropies may be described by second rank tensors represented by ellipsoids for a homogeneous petrofabric that is adequately sampled a stereoplot of the orientation distribution of the tensors principal axes maximum intermediate and minimum should show three orthogonal concentrations the concentrations form some combination of shapes from circular clusters through partial girdles to full girdles the concentrations elliptical eccentricities are constrained by the symmetry of the sample orientation distribution i e l l s l s etc as well as the individual sample anisotropies the mean orientations of principal axes must be orthogonal just as with individual sample tensors this requires tensor statistics for their calculation jelinek 1978 furthermore elliptical confidence cones for the means should parallel principal planes preserving overall orthorhombic symmetry however in practice sub orthorhombic symmetry may arise from unrepresentative sampling but it may also be a useful indicator of multiple or heterogeneous petrofabrics in the case of magnetic fabrics the wide range in average susceptibility values and variation in magnetic mineralogy permit small numbers of high susceptibility samples to deflect the orientation of the tensor mean away from the majority of samples normalizing the samples by their bulk susceptibility overcomes this but the orientation of high susceptibility outliers may signify an event or subfabric of importance that we should not discard therefore stereoplots of both normalized and non normalized orientation distributions should be compared preferably also identifying the outliers it is important to distinguish the shape of the orientation distribution ellipsoid from the shape of the individual magnetic fabric ellipsoids the qualitative l s nomenclature is best replaced by tj where tj 1 oblate tj 1 prolate jelinek 1981 invariably the orientation distribution is described by an ellipsoid whose shape is more spherical than that of the individual sample anisotropy ellipsoids because the latter have scattered orientations furthermore the shape of the orientation distribution ellipsoid need bear no relation to the shape of individual sample ellipsoid anisotropies the concepts are illustrated with 1119 measurements of anisotropy of magnetic susceptibility ams from seven areas and with 188 measurements of anisotropy of anhysteretic remanence aarm from two areas

calcite twinning strain constraints on the emplacement rate and kinematic pattern of the upper plate of the heart mountain detachment two models that address the emplacement rate of the upper plate of the heart mountain detachment hmd have been advanced these are the catastrophic tectonic denudation model and the slow incremental continuous allochthon model we compared the two models by analyzing upper and lower plate rocks for a detachment related overprint of the older layer parallel laramide sevier calcite twinning strain fabric lower plate pilgrim limestone samples even those collected a few meters below the detachment show no evidence of a detachment related twinning strain overprint negative expected values nevs avg 8 3 of the laramide sevier layer parallel fabric the allochthonous upper plate carbonate rocks preserve the layer parallel laramide sevier shortening strain but the shortening axis e1 for each block occurs in different non ew and both subhorizontal and non subhorizontal orientations for the six upper plate blocks that preserve chaotic shortening axis orientations no evidence of any detachment related twinning strain overprint nevs avg 7 5 was found in the absence of any hmd related twinning overprint the upper plate allochthon motion must have been rapid enough to be accommodated by fracturing of upper plate rocks without additionally twinning any calcite we conclude that the emplacement of the upper plate of the hmd was not accompanied by an overprint of the laramide sevier twinning fabric and that models that require catastrophic rather than slow incremental emplacement rates are best supported by these data

determination of principal stress magnitudes using calcite twins and rock mechanics data abstract this paper proposes a new method for determining principal stress magnitudes based on the use of the deviatoric stress tensor determined from computer inversion of calcite twin data additional evaluation of the vertical stress weight of overburden or of the mean stress is enough to calculate the complete stress tensor we emphasize that the best reasoning consists of combining calcite twin analysis with rock mechanics data failure friction criteria because the determination of the six parameters of the absolute stress tensor is overconstrained